**Method**

**Chapter 1**

**Provisional chapter**

**Advanced Ceramic Materials Sintered by Microwave**

**Advanced Ceramic Materials Sintered by Microwave** 

Processing of ceramic materials has also a strong impact in the quality of the consolidated body, as it plays a key role in the resulting microstructure and, as a consequence, in its final properties. Advanced ceramic materials are commonly processed as powders and densified via a high-temperature process. Traditional processing techniques include hot isostatic pressing, mold casting, and sintering in conventional ovens. As ceramics require very high processing temperatures compared to metals and polymers, these processes tend to be very energy intensive and result in higher production costs to the manufacturers. Therefore, new technologies known as nonconventional sintering techniques, such as microwave technology, are being developed in order to reduce energy consumption, while maintaining or even improving the characteristics of the resulting ceramic material. This novel and innovative technology aims at helping industrial sectors lower their production costs and, at the same time, lessen their environmental impact. On the other hand, it is interesting and necessary to know and explore the basic principles of microwaves to advance in the development of materials that demand, every day more, the different industrial sectors. This chapter presents the most recent advances of two materials

with a great industrial future: zirconia and lithium aluminosilicate.

**Keywords:** ceramic materials, microwave technology, microstructure, mechanical

High-temperature processes are required to consolidate ceramic powders, such as zirconia (Y-TZP), alumina, silicon carbide, and so on, in order to obtain full densification of the material. Sintering is a common material processing technique aimed at fulfilling this task.

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.78831

**Technology**

**Abstract**

**1. Introduction**

**Technology**

Amparo Borrell and Maria Dolores Salvador

Amparo Borrell and Maria Dolores Salvador

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78831

properties, advanced applications

### **Advanced Ceramic Materials Sintered by Microwave Technology Advanced Ceramic Materials Sintered by Microwave Technology**

DOI: 10.5772/intechopen.78831

Amparo Borrell and Maria Dolores Salvador Amparo Borrell and Maria Dolores Salvador

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78831

### **Abstract**

Processing of ceramic materials has also a strong impact in the quality of the consolidated body, as it plays a key role in the resulting microstructure and, as a consequence, in its final properties. Advanced ceramic materials are commonly processed as powders and densified via a high-temperature process. Traditional processing techniques include hot isostatic pressing, mold casting, and sintering in conventional ovens. As ceramics require very high processing temperatures compared to metals and polymers, these processes tend to be very energy intensive and result in higher production costs to the manufacturers. Therefore, new technologies known as nonconventional sintering techniques, such as microwave technology, are being developed in order to reduce energy consumption, while maintaining or even improving the characteristics of the resulting ceramic material. This novel and innovative technology aims at helping industrial sectors lower their production costs and, at the same time, lessen their environmental impact. On the other hand, it is interesting and necessary to know and explore the basic principles of microwaves to advance in the development of materials that demand, every day more, the different industrial sectors. This chapter presents the most recent advances of two materials with a great industrial future: zirconia and lithium aluminosilicate.

**Keywords:** ceramic materials, microwave technology, microstructure, mechanical properties, advanced applications

### **1. Introduction**

High-temperature processes are required to consolidate ceramic powders, such as zirconia (Y-TZP), alumina, silicon carbide, and so on, in order to obtain full densification of the material. Sintering is a common material processing technique aimed at fulfilling this task.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The fundamental principle behind sintering consists in the thermal activation of mass transfer mechanisms when exposing a powder compact, known as a "green" body, to a hightemperature process, at a dwell temperature below the melting point of the material. The main purpose of sintering is to obtain a dense and resistant body with properties as close as possible to those of a theoretical, fully dense solid. However, in some cases, sintering can also be employed to adjust some of the properties based on the performance requirements of the material by not reaching full consolidation, such as in porous materials.

**2. Microwave sintering technology**

ently in the presence of microwaves.

mechanical properties [17, 18].

Microwaves have been used since the 1960s for heating purposes, particularly for food- and water-based products. Industrially, the use of microwave energy has become increasingly important because it represents an alternative to traditional with high-temperature processes. For example, so far, it has been employed in wood drying, resin curing, and polymer synthesis. The growing interest in industrial microwave heating is due mostly to the reduction of production costs resulting from lower energy consumption and shorter processing times [6–8]. However, several aspects need to still be investigated as each material behaves differ-

Advanced Ceramic Materials Sintered by Microwave Technology

http://dx.doi.org/10.5772/intechopen.78831

5

The application of microwave heating has now expanded to material science and technology, beginning with process control and moving onto ceramic drying, powder calcination, and decomposition of gases with microwave plasma, in addition to powder synthesis [5]. Scientific interest on this powerful tool has been recorded in the study as there has been an increase of bibliographical entries for the term "microwaves" in the last decades because the applications of this technology have diversified enormously. In the last 25 years, research and development on the dielectric heating attributed to microwaves began with topics in chemical synthesis and material processing, such as reactive sintering of superconductors, magnetoresistors, nanomaterials production, vitreous phase formation, hydrothermal generation of zeolites, among others [9]. In this sense, one of the major areas for research and development

Microwave sintering is considered a relatively new ceramic material processing technique that differs significantly from conventional sintering methods due to the nature of the heat transfer mechanisms involved. Hence, microwave sintering is classified as a non-conventional sintering technique. This method presents itself as a fast, economical, and flexible processing tool. Some of the most important advantages against conventional sintering systems include lower energy consumption and production costs, reduction of processing times, higher heating rates, and, in some cases, even an improvement in the physical properties of the consolidated material [6, 12]. As a consequence, scientific interest in this novel technique has been developed progressively. In a general sense, microwave sintering increases the densification of the material at lower dwell temperatures when compared to conventional sintering [13, 14], employing shorter times and less energy [15, 16], and resulting in an improvement of the microstructure and

The first sinterability studies of ceramics by exposure to microwave energy were carried out on the so-called black ceramics, which are the compounds based on tungsten carbide (WC). Two of the main issues regarding sintering of these materials by conventional means are the high temperatures (>1500°C) and long dwell times that result in grain coarsening. For the first time, in 1991, J. P. Cheng showed that the WC/Co system could be sintered by microwave heating technology [19]. In his work, a commercial WC powder with a 6–12 mol% Co content was investigated, and an improvement in the mechanical properties was achieved when

of microwave heating involves sintering of ceramic powders [10, 11].

**2.1. Microwave sintering**

Two main types of sintering can be identified based on the nature of the process: liquid phase and solid phase. Even though the term liquid phase may suggest exceeding the melting point of the material, it is used to describe the addition of compounds with significantly lower melting points that aid in the consolidation of the main powder, which is regarded as the matrix phase and provides the main properties of the consolidated body. In this chapter, however, only solid phase sintering is considered.

Currently, innovative sintering methods are being explored and studied in order to modify densification mechanisms that may improve the microstructure and mechanical properties of sintered materials and also is very important to reduce time fabrication of these materials. Two main stages have been recognized during the sintering process: densification and grain growth [1]. The main purpose for modifying sintering mechanisms is to obtain relative densities close to theoretical values, while maintaining a controlled, but limited, grain growth [2]. Also, the optimization of the process by reducing the sintering time to decrease energy consumption and/or increasing heating rates is an important aspect that is being considered [3]. As a consequence, in order to improve the sintering process, novel non-conventional sintering methods have been investigated and developed.

Particularly, microwave sintering represents an interesting opportunity at consolidating advanced ceramic materials with a reduced processing time and energy consumption by utilizing electromagnetic radiation to provide high-enough temperatures that allow full densification of the material. The most important advantages of microwave sintering against conventional sintering methods are listed as follows [4, 5]:


This chapter reports on microwave material interaction, the basics of microwave processing, heating mechanisms, theoretical aspects in dielectric heating, and microwave systems for heating. The challenges in the field of microwave processing of advanced materials, such as zirconia and lithium aluminosilicate, have been discussed and studied from the point of view of different authors.

### **2. Microwave sintering technology**

### **2.1. Microwave sintering**

The fundamental principle behind sintering consists in the thermal activation of mass transfer mechanisms when exposing a powder compact, known as a "green" body, to a hightemperature process, at a dwell temperature below the melting point of the material. The main purpose of sintering is to obtain a dense and resistant body with properties as close as possible to those of a theoretical, fully dense solid. However, in some cases, sintering can also be employed to adjust some of the properties based on the performance requirements of the

Two main types of sintering can be identified based on the nature of the process: liquid phase and solid phase. Even though the term liquid phase may suggest exceeding the melting point of the material, it is used to describe the addition of compounds with significantly lower melting points that aid in the consolidation of the main powder, which is regarded as the matrix phase and provides the main properties of the consolidated body. In this chapter, however,

Currently, innovative sintering methods are being explored and studied in order to modify densification mechanisms that may improve the microstructure and mechanical properties of sintered materials and also is very important to reduce time fabrication of these materials. Two main stages have been recognized during the sintering process: densification and grain growth [1]. The main purpose for modifying sintering mechanisms is to obtain relative densities close to theoretical values, while maintaining a controlled, but limited, grain growth [2]. Also, the optimization of the process by reducing the sintering time to decrease energy consumption and/or increasing heating rates is an important aspect that is being considered [3]. As a consequence, in order to improve the sintering process, novel non-conventional sintering

Particularly, microwave sintering represents an interesting opportunity at consolidating advanced ceramic materials with a reduced processing time and energy consumption by utilizing electromagnetic radiation to provide high-enough temperatures that allow full densification of the material. The most important advantages of microwave sintering against

• materials with a finer (nanometric) microstructure with a high degree of densification and enhanced mechanical properties may be obtained due to the densification mechanisms

This chapter reports on microwave material interaction, the basics of microwave processing, heating mechanisms, theoretical aspects in dielectric heating, and microwave systems for heating. The challenges in the field of microwave processing of advanced materials, such as zirconia and lithium aluminosilicate, have been discussed and studied from the point of view

material by not reaching full consolidation, such as in porous materials.

only solid phase sintering is considered.

4 Sintering Technology - Method and Application

methods have been investigated and developed.

conventional sintering methods are listed as follows [4, 5]:

• shorter sintering time and lower energy consumption;

• flexible due to the possibility of processing *near-net-shape* materials.

• higher heating rates can be used;

involved;

of different authors.

Microwaves have been used since the 1960s for heating purposes, particularly for food- and water-based products. Industrially, the use of microwave energy has become increasingly important because it represents an alternative to traditional with high-temperature processes. For example, so far, it has been employed in wood drying, resin curing, and polymer synthesis. The growing interest in industrial microwave heating is due mostly to the reduction of production costs resulting from lower energy consumption and shorter processing times [6–8]. However, several aspects need to still be investigated as each material behaves differently in the presence of microwaves.

The application of microwave heating has now expanded to material science and technology, beginning with process control and moving onto ceramic drying, powder calcination, and decomposition of gases with microwave plasma, in addition to powder synthesis [5]. Scientific interest on this powerful tool has been recorded in the study as there has been an increase of bibliographical entries for the term "microwaves" in the last decades because the applications of this technology have diversified enormously. In the last 25 years, research and development on the dielectric heating attributed to microwaves began with topics in chemical synthesis and material processing, such as reactive sintering of superconductors, magnetoresistors, nanomaterials production, vitreous phase formation, hydrothermal generation of zeolites, among others [9]. In this sense, one of the major areas for research and development of microwave heating involves sintering of ceramic powders [10, 11].

Microwave sintering is considered a relatively new ceramic material processing technique that differs significantly from conventional sintering methods due to the nature of the heat transfer mechanisms involved. Hence, microwave sintering is classified as a non-conventional sintering technique. This method presents itself as a fast, economical, and flexible processing tool. Some of the most important advantages against conventional sintering systems include lower energy consumption and production costs, reduction of processing times, higher heating rates, and, in some cases, even an improvement in the physical properties of the consolidated material [6, 12]. As a consequence, scientific interest in this novel technique has been developed progressively.

In a general sense, microwave sintering increases the densification of the material at lower dwell temperatures when compared to conventional sintering [13, 14], employing shorter times and less energy [15, 16], and resulting in an improvement of the microstructure and mechanical properties [17, 18].

The first sinterability studies of ceramics by exposure to microwave energy were carried out on the so-called black ceramics, which are the compounds based on tungsten carbide (WC). Two of the main issues regarding sintering of these materials by conventional means are the high temperatures (>1500°C) and long dwell times that result in grain coarsening. For the first time, in 1991, J. P. Cheng showed that the WC/Co system could be sintered by microwave heating technology [19]. In his work, a commercial WC powder with a 6–12 mol% Co content was investigated, and an improvement in the mechanical properties was achieved when compared to conventional methods by utilizing sintering temperature between 1250 and 1320°C and dwell times of only 10–30 min. The relative density values were close to theoretical and a fine and homogeneous microstructure was observed, without the use of grain growth inhibitors. Also, the materials exhibited a higher resistance to corrosion and erosion [20].

The next step involved the processing of more traditional ceramic materials such as alumina and zirconia. Even though alumina behaves as a transparent material in the presence of microwaves, susceptors, which are materials with a high microwave absorbance, or dopants can be employed. Tian et al. were able to obtain 99.9% relative density values with an average grain size of 1.9 μm for MgO-doped Al<sup>2</sup> O3 sintered at 1700°C in a microwave oven [21]. Additionally, Katz and Blake were able to reach a densification of 99% for α-alumina with grain sizes between 5 and 50 μm after microwave sintering, where the total processing time was 100 min at a dwell temperature of 1400°C [22]. Transparent alumina materials have also been obtained via microwave processing at lower sintering temperature and shorter times [23].

In the case of nanometric yttria-stabilized zirconia (YSZ), microstructure and mechanical properties can be enhanced when processed via microwave sintering [24]. By application of hybrid heating with the aid of a susceptor, sintered materials with densities close to theoretical values can be obtained at temperatures 200°C below those employed in conventional sintering [25, 26]. Moreover, the grain size decreases considerably and hardness values are almost 2 GPa higher [18].

energy from one point to another. These properties govern the interaction of microwaves with materials and produce heating in some of them. Depending on the electrical and magnetic properties of the material, their interaction with microwaves can be classified as one of three

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7

• **Transparent**: Microwaves penetrate and are transmitted through the material completely with no energy transfer occurring (**Figure 2a**). These materials are known as low-loss

• **Opaque**: Microwaves are reflected with no penetration into the material and no energy transfer. These are known as conductors (**Figure 2b**). Metals are mostly considered to be

• **Absorbent**: Microwaves are absorbed by the material, and an exchange of electromagnetic energy occurs (**Figure 2c**). The amount of absorption depends on the dielectric properties

A fourth type of interaction known as mixed absorption has also been proposed. In this particular case, mixed or multi-phase materials with different degrees of microwave absorption are sought after. Most electrically insulating ceramics such as alumina, MgO, silica, and glasses are transparent to microwaves at room temperature, but, when heated above a certain critical

Other ceramics, such as SiC, are able to absorb microwave energy more efficiently at room temperature. Therefore, the addition of a microwave-absorbing second phase to ceramics that behave transparent at room temperature can greatly enhance the interaction of the system with microwaves allowing a hybrid heating of the material. In Section 5, a more in-depth

, they begin to absorb and couple more effectively with microwave radiation.

types [5]:

insulators.

of the material.

temperature *Tc*

opaque to microwave energy.

**Figure 1.** Electromagnetic spectrum diagram.

description of hybrid heating is given.

In the last 5 years, research on microwave sintering has also focused in the processing of ceramic composites to improve their functional as well as structural properties and extend its applications to several industrial sectors. Also, the design and optimization of current microwave ovens has also been an important research topic. These systems need to be adjusted to the characteristics of the material that is to be processed, since the behavior under a microwave field varies from one to another. Therefore, studying the fundamental principles and involved mechanisms in microwave energy conversion may allow the production of more energy-efficient ovens.

### **2.2. Microwave heating fundamentals**

Microwaves are a form of electromagnetic radiation that correspond to frequencies between 300 MHz (*λ* = 1 m) and 300 GHz (*λ* = 1 mm), as shown in **Figure 1**. Among their most important industrial applications are telecommunications and heating. The possibility to use microwave energy for heat generation was first discovered in the late 1940s, while tests were being carried out with magnetrons [8]. Consequently, the first microwave systems for food heating were developed. As research in microwave energy and its applications continued, uses expanded to industrial processes such as drying and curing. In the last few decades, sintering of materials with microwave radiation has also become an active field of investigation.

### *2.2.1. Interaction of microwaves with matter*

Microwaves, as any other type of electromagnetic radiation, have electrical and magnetic field components, amplitude, phase angle, and the ability to propagate, that is, to transfer Advanced Ceramic Materials Sintered by Microwave Technology http://dx.doi.org/10.5772/intechopen.78831 7

**Figure 1.** Electromagnetic spectrum diagram.

compared to conventional methods by utilizing sintering temperature between 1250 and 1320°C and dwell times of only 10–30 min. The relative density values were close to theoretical and a fine and homogeneous microstructure was observed, without the use of grain growth inhibitors. Also, the materials exhibited a higher resistance to corrosion and erosion [20].

The next step involved the processing of more traditional ceramic materials such as alumina and zirconia. Even though alumina behaves as a transparent material in the presence of microwaves, susceptors, which are materials with a high microwave absorbance, or dopants can be employed. Tian et al. were able to obtain 99.9% relative density values with an aver-

O3

Additionally, Katz and Blake were able to reach a densification of 99% for α-alumina with grain sizes between 5 and 50 μm after microwave sintering, where the total processing time was 100 min at a dwell temperature of 1400°C [22]. Transparent alumina materials have also been obtained via microwave processing at lower sintering temperature and shorter times [23]. In the case of nanometric yttria-stabilized zirconia (YSZ), microstructure and mechanical properties can be enhanced when processed via microwave sintering [24]. By application of hybrid heating with the aid of a susceptor, sintered materials with densities close to theoretical values can be obtained at temperatures 200°C below those employed in conventional sintering [25, 26]. Moreover, the grain size decreases considerably and hardness values are

In the last 5 years, research on microwave sintering has also focused in the processing of ceramic composites to improve their functional as well as structural properties and extend its applications to several industrial sectors. Also, the design and optimization of current microwave ovens has also been an important research topic. These systems need to be adjusted to the characteristics of the material that is to be processed, since the behavior under a microwave field varies from one to another. Therefore, studying the fundamental principles and involved mechanisms in microwave energy conversion may allow the production of more energy-efficient ovens.

Microwaves are a form of electromagnetic radiation that correspond to frequencies between 300 MHz (*λ* = 1 m) and 300 GHz (*λ* = 1 mm), as shown in **Figure 1**. Among their most important industrial applications are telecommunications and heating. The possibility to use microwave energy for heat generation was first discovered in the late 1940s, while tests were being carried out with magnetrons [8]. Consequently, the first microwave systems for food heating were developed. As research in microwave energy and its applications continued, uses expanded to industrial processes such as drying and curing. In the last few decades, sintering of materials with microwave radiation has also become an active field of investigation.

Microwaves, as any other type of electromagnetic radiation, have electrical and magnetic field components, amplitude, phase angle, and the ability to propagate, that is, to transfer

sintered at 1700°C in a microwave oven [21].

age grain size of 1.9 μm for MgO-doped Al<sup>2</sup>

6 Sintering Technology - Method and Application

almost 2 GPa higher [18].

**2.2. Microwave heating fundamentals**

*2.2.1. Interaction of microwaves with matter*

energy from one point to another. These properties govern the interaction of microwaves with materials and produce heating in some of them. Depending on the electrical and magnetic properties of the material, their interaction with microwaves can be classified as one of three types [5]:


A fourth type of interaction known as mixed absorption has also been proposed. In this particular case, mixed or multi-phase materials with different degrees of microwave absorption are sought after. Most electrically insulating ceramics such as alumina, MgO, silica, and glasses are transparent to microwaves at room temperature, but, when heated above a certain critical temperature *Tc* , they begin to absorb and couple more effectively with microwave radiation. Other ceramics, such as SiC, are able to absorb microwave energy more efficiently at room temperature. Therefore, the addition of a microwave-absorbing second phase to ceramics that behave transparent at room temperature can greatly enhance the interaction of the system with microwaves allowing a hybrid heating of the material. In Section 5, a more in-depth description of hybrid heating is given.

material by the various mechanisms. The properties of the material that are most important for the interaction are the permittivity *ε* for a dielectric material and the permeability *μ* for a magnetic material [27]. Considering that dielectric heating is the most relevant mechanism for ceramics, this description will only focus in aspects related to permittivity and properties

When microwaves penetrate the material, the electromagnetic field induces motion in the free and bound charges (electrons and ions) and in dipoles. The induced motion is resisted because it causes a departure from the natural equilibrium of the system, and this resistance due to frictional, elastic, and inertial forces leads to the dissipation of energy. As a result, the electric field associated with microwave radiation is attenuated, and heating of the material occurs.

The dielectric interaction between materials and microwave radiation can be described by

Both parameters play a critical role in the uniform heating of the material. The absorbed


The loss tangent, *tanθ*, is a term associated with the capacity of the material to be polarized and heat itself. In other words, these terms describe the microwave energy conversion into

The loss factor, *ε"*, measures the ability of the material to convert the incoming microwave energy into heat, and the dielectric constant, *ε'*, measures the polarizability of the material. In microwave material processing, maximum values for *ε"* in combination with mild values of

Both, *ε'* and *ε"*, depend on temperature and the frequency of the field. At low frequencies, all microwave energy is absorbed by the rotating movement of the dipoles and *ε'* reaches a maximum; however, there are no collisions because the displacement is very slow. At high frequencies, the material does not have enough time to respond to the oscillating electric field, and therefore, *ε'* reaches a minimum. The loss of energy caused by the collisions is represented by *ε"*. The key relies on finding a frequency for each material at which the absorption

= 2*f ε*<sup>0</sup> *ε*′

tan*θ*|*E*| 2

Advanced Ceramic Materials Sintered by Microwave Technology

http://dx.doi.org/10.5772/intechopen.78831

/*ε*′ (2)

) and is expressed accord-

(1)

9

power is the volumetric absorption of microwave energy (in W/m<sup>3</sup>

2

heat. The relationship describing the loss tangent is given by

where *ε"* = loss factor; *ε'* = dielectric constant, inherent to the material.

tan*θ* = *ε*′′

of energy (*ε'*) as well as the loss of energy (*ε"*) is high.

= 2*f ε*<sup>0</sup> *ε*′′

where *f* = frequency of the electric field and *E* = amplitude of the electric field.

that arise from it.

two main parameters [6, 28–30]:

ing to the following equation:

*ε'* are desired (**Figure 3**).

*P* = *σ*|*E*|

• depth of microwave penetration, D

• absorbed power, P

**Figure 2.** Material/microwave interaction representation classified according to their behavior: (a) transparent, (b) opaque, and (c) absorbent.

### *2.2.2. Microwave heating mechanisms*

In order to explain the interaction of absorbing materials with microwave radiation and the energy transfer that occurs during this interaction, several physical mechanisms have been proposed. These mechanisms include bipolar rotation, resistive heating, electromagnetic heating, and dielectric heating. Depending on the material, the response to incoming radiation can be attributed to one mechanism or a combination of several of them:


### **2.3. Theoretical aspects in dielectric heating**

The degree of interaction between the microwave electric and magnetic field components with the dielectric or magnetic material determines the rate at which energy is dissipated in the material by the various mechanisms. The properties of the material that are most important for the interaction are the permittivity *ε* for a dielectric material and the permeability *μ* for a magnetic material [27]. Considering that dielectric heating is the most relevant mechanism for ceramics, this description will only focus in aspects related to permittivity and properties that arise from it.

When microwaves penetrate the material, the electromagnetic field induces motion in the free and bound charges (electrons and ions) and in dipoles. The induced motion is resisted because it causes a departure from the natural equilibrium of the system, and this resistance due to frictional, elastic, and inertial forces leads to the dissipation of energy. As a result, the electric field associated with microwave radiation is attenuated, and heating of the material occurs.

The dielectric interaction between materials and microwave radiation can be described by two main parameters [6, 28–30]:

• absorbed power, P

*2.2.2. Microwave heating mechanisms*

8 Sintering Technology - Method and Application

opaque, and (c) absorbent.

described.

In order to explain the interaction of absorbing materials with microwave radiation and the energy transfer that occurs during this interaction, several physical mechanisms have been proposed. These mechanisms include bipolar rotation, resistive heating, electromagnetic heating, and dielectric heating. Depending on the material, the response to incoming radia-

**Figure 2.** Material/microwave interaction representation classified according to their behavior: (a) transparent, (b)

• **Bipolar rotation** occurs when electrically neutral polar molecules with positive and negatives charges are separated. Within a microwave field, these dipoles rotate in the direction of increasing amplitude. As a consequence of this rotation, friction among the molecules

• **Resistive heating** occurs in conductors or semiconductors with relatively high electrical resistivity. These materials possess free electrons or a high ionic content where the ions

• **Electromagnetic heating** takes place in materials with magnetic properties that are highly susceptible to external electromagnetic fields, such as those induced by microwave radiation. This type of heating can be described as magnetic pole rotation of the material analo-

• Finally, the fourth mechanism, **dielectric heating**, is a mix of bipolar rotations and resistive heating. In microwave sintering of ceramics, this is the predominant mechanism. In the next section, the principles of dielectric heating in microwave-absorbent materials are

The degree of interaction between the microwave electric and magnetic field components with the dielectric or magnetic material determines the rate at which energy is dissipated in the

tion can be attributed to one mechanism or a combination of several of them:

gous to the rotation of polar molecules in oscillating electrical fields.

arises generating heat uniformly throughout the material.

receive enough freedom so current can be generated.

**2.3. Theoretical aspects in dielectric heating**

• depth of microwave penetration, D

Both parameters play a critical role in the uniform heating of the material. The absorbed power is the volumetric absorption of microwave energy (in W/m<sup>3</sup> ) and is expressed according to the following equation:

$$P = \sigma^{\parallel} E^{\parallel} = 2\pi f \varepsilon\_0 \varepsilon^{\top} E^{\parallel} = 2\pi f \varepsilon\_0 \varepsilon^{\top} \tan \theta \left| E^{\parallel} \right|^2 \tag{1}$$

where *f* = frequency of the electric field and *E* = amplitude of the electric field.

The loss tangent, *tanθ*, is a term associated with the capacity of the material to be polarized and heat itself. In other words, these terms describe the microwave energy conversion into heat. The relationship describing the loss tangent is given by

$$
\tan \theta \le \varepsilon^\circ / \varepsilon^\circ \tag{2}
$$

where *ε"* = loss factor; *ε'* = dielectric constant, inherent to the material.

The loss factor, *ε"*, measures the ability of the material to convert the incoming microwave energy into heat, and the dielectric constant, *ε'*, measures the polarizability of the material. In microwave material processing, maximum values for *ε"* in combination with mild values of *ε'* are desired (**Figure 3**).

Both, *ε'* and *ε"*, depend on temperature and the frequency of the field. At low frequencies, all microwave energy is absorbed by the rotating movement of the dipoles and *ε'* reaches a maximum; however, there are no collisions because the displacement is very slow. At high frequencies, the material does not have enough time to respond to the oscillating electric field, and therefore, *ε'* reaches a minimum. The loss of energy caused by the collisions is represented by *ε"*. The key relies on finding a frequency for each material at which the absorption of energy (*ε'*) as well as the loss of energy (*ε"*) is high.

**Figure 3.** Relationship between factor loss and absorbed power at a frequency of 2.45 GHz and room temperature for some common materials.

A general explanation is based on a fundamental body, such as a grain particle, in its neutral state containing polarized molecules distributed in random positions. These molecules can easily be reoriented by the effect of an external electric field, as shown in **Figure 4**.

time generate heat throughout the whole material as a consequence of the characteristics of

**Figure 5.** Representation of the reorientation of the molecules in the presence of an alternating electric field, such as that

The second main parameter in microwave/material interaction is microwave penetration depth, *D*. This parameter determines the penetration depth at which the power is reduced by

> 2*f* √ \_\_\_ 2 *ε*′ (√

High frequencies in combination with high dielectric property values translate into superficial heating of the material, while low frequencies with small dielectric property values give

Based on the properties of materials, it is well known that those with a high conductivity and permeability present a lower penetration depth for a given frequency. The penetration depth of many materials oscillates around 1 μm, which means that heating tends to stay at the surface. If powders with a particle size of approximately that of *D* are employed, there is the

A microwave oven is composed of three main elements: (1) microwave source, which is in charge of generating the electromagnetic radiation, (2) transmission lines, which transmit the microwaves, and (3) a resonant cavity, which is where the interaction with matter takes place [28].

<sup>=</sup> \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *<sup>C</sup>*

\_\_\_\_\_\_\_\_\_\_ 1 + tan2 *θ* − 1)

\_\_1 2

Advanced Ceramic Materials Sintered by Microwave Technology

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11

(3)

dielectric heating.

induced by microwaves.

place to volumetric heating.

**2.4. Microwave systems for heating**

half and is expressed in the following manner:

*<sup>D</sup>* <sup>=</sup> <sup>3</sup> *<sup>π</sup>* \_\_\_\_\_\_\_\_\_\_\_\_\_ <sup>0</sup>

8.686*π*tan*θ* (

possibility to heat the whole surface directly and homogeneously.

*ε*′ \_\_ *ε*0) \_\_1 2

If the polarity of the electric field is changing constantly, molecules will modify their orientation accordingly in a very fast manner so as to align with the field (**Figure 5**) and, as a consequence, heat will be generated due to the friction among them and electrical resistive effects from unbound charges. The material heats up as a function of the absorbed energy during this process.

The main difference with respect to conventional sintering is the direction of heat flow [31], because in conventional sintering, heat is transferred from the surface of the material toward the inside due to the heating mechanisms involved. In contrast, in microwave sintering, in the presence of a strong electric field, molecules vibrate with the same intensity and at the same

**Figure 4.** Position of the molecules (a) in its natural state, and (b) with the application of an external electric field.

**Figure 5.** Representation of the reorientation of the molecules in the presence of an alternating electric field, such as that induced by microwaves.

time generate heat throughout the whole material as a consequence of the characteristics of dielectric heating.

The second main parameter in microwave/material interaction is microwave penetration depth, *D*. This parameter determines the penetration depth at which the power is reduced by half and is expressed in the following manner:

 $\pi\_P = \pi\_P = \pi\_P$ 

half and is expressed in the following manner:

$$D = \frac{3\pi\_0}{8.686\pi \tan\theta \left(\frac{\varepsilon'}{\varepsilon\_0}\right)^{\frac{1}{2}}} = \frac{\mathcal{C}}{2\pi f \sqrt[4]{2\pi} \left(\sqrt[4]{1+\tan^2\theta}-1\right)^{\frac{1}{2}}} \tag{3}$$

High frequencies in combination with high dielectric property values translate into superficial heating of the material, while low frequencies with small dielectric property values give place to volumetric heating.

Based on the properties of materials, it is well known that those with a high conductivity and permeability present a lower penetration depth for a given frequency. The penetration depth of many materials oscillates around 1 μm, which means that heating tends to stay at the surface. If powders with a particle size of approximately that of *D* are employed, there is the possibility to heat the whole surface directly and homogeneously.

### **2.4. Microwave systems for heating**

A general explanation is based on a fundamental body, such as a grain particle, in its neutral state containing polarized molecules distributed in random positions. These molecules can

**Figure 3.** Relationship between factor loss and absorbed power at a frequency of 2.45 GHz and room temperature for

If the polarity of the electric field is changing constantly, molecules will modify their orientation accordingly in a very fast manner so as to align with the field (**Figure 5**) and, as a consequence, heat will be generated due to the friction among them and electrical resistive effects from unbound charges. The material heats up as a function of the absorbed energy

The main difference with respect to conventional sintering is the direction of heat flow [31], because in conventional sintering, heat is transferred from the surface of the material toward the inside due to the heating mechanisms involved. In contrast, in microwave sintering, in the presence of a strong electric field, molecules vibrate with the same intensity and at the same

**Figure 4.** Position of the molecules (a) in its natural state, and (b) with the application of an external electric field.

easily be reoriented by the effect of an external electric field, as shown in **Figure 4**.

during this process.

some common materials.

10 Sintering Technology - Method and Application

A microwave oven is composed of three main elements: (1) microwave source, which is in charge of generating the electromagnetic radiation, (2) transmission lines, which transmit the microwaves, and (3) a resonant cavity, which is where the interaction with matter takes place [28].

The theoretical principle that governs each of the components is based on Maxwell Equations [30, 32]:

$$
\nabla \times \mathbf{E} = \frac{\partial \mathbf{B}}{\partial t'} \,\nabla \cdot \mathbf{B} = 0. \tag{4}
$$

incandescent and emits electrons by thermionic effect. The cylinder, connected to the positive pole, attracts the electrons. The whole setup is located between the poles of a powerful

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The open space between the plate and the cathode is referred to as interaction space. In this space, electric and magnetic fields interact to exert a force on the electrons. Given that an electric charge creates an electromagnetic field around it, all the electrons, moving in circles in the cavities, produce electromagnetic waves, in this case microwaves, perpendicular to their

Usually, for microwave heating applications, the frequency of the generated electromagnetic radiation is 2.45 GHz. This frequency corresponds to one of the so-called Industrial, Scientific and Medical (ISM) frequencies, which are free of utilization for these types of applications. The insertion of magnetrons in commercial microwave ovens for home use has translated in more economical sources of this frequency by allowing the fabrication of magnetrons in a large scale. Moreover, other ISM frequencies are also employed for heating applications, such as Bluetooth and WiFi [33]. The power generated by the magnetron can be controlled by

own displacement and with a frequency that depends on the size of the cavities.

changing the amplitude of the cathode's current or the intensity of the magnetic field.

**Transmission lines:** This element is responsible for transmitting the generated microwave radiation by the source to the main cavity. In low-power systems, transmission lines are usually coaxial cables. However, for high-frequency systems, the loss occurring in the coaxial cables is quite substantial. Therefore, circular or rectangular waveguides are necessary for

**Circulator:** This component provides protection to the source against possible unwanted load reflections. The circulator is capable of redirecting the microwave power that was not consumed by the material to be sintered toward a water load. This water load heats up avoiding

**Reflectometer:** This element measures the effective consumed power by the material sample to be heated. This information provides reliable information about the power consumed dur-

**Tuning system:** This element is fixed to the microwave oven and is employed to couple the microwave incident radiation to the cavity. Different types of tuners can be utilized. For example, the simplest one consists of an iris that couples the incident power directly to the cavity. More complex tuners are the three-stub adapter that allows a dynamic adaptation of

**Resonant cavity:** This is the microwave system nucleus, where the incident electromagnetic radiation heats and sinters the material. Cavity design is one of the most critical parts of microwave equipment for material processing. The temperature distribution within the material, which is heated by microwave radiation, is inherently linked to the distribution of the electric field inside the cavity. In material processing, resonant cavities with different mode configurations, including single-mode, multi-mode, and multi-mode with variable frequency,

that the reflected power gets back causing damages to the source.

electromagnet.

proper wave transmission.

ing the sintering of the sample.

the coupling process to the cavity.

are employed [32, 34].

$$
\nabla \times \mathbf{H} = \frac{\partial \mathbf{D}}{\partial t} + \mathbf{J}\_{\prime} \cdot \nabla \cdot \mathbf{D} = \rho \tag{5}
$$

where **E** = electric field vector; **B** = magnetic flux density vector; **H** = magnetic field vector; **J** = current density vector; **D** = electric flux density vector; *ρ* = charge density.

Maxwell equations are the physical laws that describe an electromagnetic field and its variations with time. The design of an efficient microwave system to process materials requires understanding of electromagnetic theory.

In the following paragraphs, a description of the different components that are part of a microwave system is given.

**Magnetron:** This is the most important part of a microwave source. This device transforms the electrical energy from the low-frequency electric grid into a high-frequency electromagnetic energy (microwaves). It consists of a metallic cylinder where a series of resonant cavities are disposed radially and communicated to a major central cavity, which has a titanium filament in its axis (**Figure 6**). The cylinder acts as an anode and the central filament as a cathode. The filament, which is connected to the negative pole of a continuous current source, becomes

**Figure 6.** Magnetron schematic showing all the elements required for generation of microwave radiation.

incandescent and emits electrons by thermionic effect. The cylinder, connected to the positive pole, attracts the electrons. The whole setup is located between the poles of a powerful electromagnet.

The theoretical principle that governs each of the components is based on Maxwell Equations

where **E** = electric field vector; **B** = magnetic flux density vector; **H** = magnetic field vector;

Maxwell equations are the physical laws that describe an electromagnetic field and its variations with time. The design of an efficient microwave system to process materials requires

In the following paragraphs, a description of the different components that are part of a

**Magnetron:** This is the most important part of a microwave source. This device transforms the electrical energy from the low-frequency electric grid into a high-frequency electromagnetic energy (microwaves). It consists of a metallic cylinder where a series of resonant cavities are disposed radially and communicated to a major central cavity, which has a titanium filament in its axis (**Figure 6**). The cylinder acts as an anode and the central filament as a cathode. The filament, which is connected to the negative pole of a continuous current source, becomes

**Figure 6.** Magnetron schematic showing all the elements required for generation of microwave radiation.

**J** = current density vector; **D** = electric flux density vector; *ρ* = charge density.

<sup>∂</sup>*<sup>t</sup>* , <sup>∇</sup><sup>∙</sup> **<sup>B</sup>** <sup>=</sup> 0. (4)

<sup>∂</sup>*<sup>t</sup>* <sup>+</sup> **J,** <sup>∇</sup><sup>∙</sup> **<sup>D</sup>** <sup>=</sup> *<sup>ρ</sup>* (5)

[30, 32]:

<sup>∇</sup><sup>×</sup> **<sup>E</sup>** <sup>=</sup> \_\_\_ <sup>∂</sup>**<sup>B</sup>**

12 Sintering Technology - Method and Application

<sup>∇</sup><sup>×</sup> **<sup>H</sup>** <sup>=</sup> \_\_\_ <sup>∂</sup>**<sup>D</sup>**

understanding of electromagnetic theory.

microwave system is given.

The open space between the plate and the cathode is referred to as interaction space. In this space, electric and magnetic fields interact to exert a force on the electrons. Given that an electric charge creates an electromagnetic field around it, all the electrons, moving in circles in the cavities, produce electromagnetic waves, in this case microwaves, perpendicular to their own displacement and with a frequency that depends on the size of the cavities.

Usually, for microwave heating applications, the frequency of the generated electromagnetic radiation is 2.45 GHz. This frequency corresponds to one of the so-called Industrial, Scientific and Medical (ISM) frequencies, which are free of utilization for these types of applications. The insertion of magnetrons in commercial microwave ovens for home use has translated in more economical sources of this frequency by allowing the fabrication of magnetrons in a large scale. Moreover, other ISM frequencies are also employed for heating applications, such as Bluetooth and WiFi [33]. The power generated by the magnetron can be controlled by changing the amplitude of the cathode's current or the intensity of the magnetic field.

**Transmission lines:** This element is responsible for transmitting the generated microwave radiation by the source to the main cavity. In low-power systems, transmission lines are usually coaxial cables. However, for high-frequency systems, the loss occurring in the coaxial cables is quite substantial. Therefore, circular or rectangular waveguides are necessary for proper wave transmission.

**Circulator:** This component provides protection to the source against possible unwanted load reflections. The circulator is capable of redirecting the microwave power that was not consumed by the material to be sintered toward a water load. This water load heats up avoiding that the reflected power gets back causing damages to the source.

**Reflectometer:** This element measures the effective consumed power by the material sample to be heated. This information provides reliable information about the power consumed during the sintering of the sample.

**Tuning system:** This element is fixed to the microwave oven and is employed to couple the microwave incident radiation to the cavity. Different types of tuners can be utilized. For example, the simplest one consists of an iris that couples the incident power directly to the cavity. More complex tuners are the three-stub adapter that allows a dynamic adaptation of the coupling process to the cavity.

**Resonant cavity:** This is the microwave system nucleus, where the incident electromagnetic radiation heats and sinters the material. Cavity design is one of the most critical parts of microwave equipment for material processing. The temperature distribution within the material, which is heated by microwave radiation, is inherently linked to the distribution of the electric field inside the cavity. In material processing, resonant cavities with different mode configurations, including single-mode, multi-mode, and multi-mode with variable frequency, are employed [32, 34].

The size of a single-mode resonant cavity must be in the order of one wavelength. Additionally, in order to maintain a resonant mode, these systems require a microwave source that allows frequency variations or that the cavity dynamically changes its size to couple the frequency of the microwaves. Generally, the distribution of the electromagnetic field in this type of cavity is well known. With an adequate cavity design, the microwave field may be focalized to a particular zone where the material sample can be sintered. An additional advantage for this type of cavity is the fact that the dielectric properties of the material can be monitored during sintering.

A plausible solution that materials scientists and engineers have developed consists on a hybrid method that combines direct microwave heating coupled with heat transfer coming from another material that surrounds the specimen to be sintered [37]. This system is an example of mixed absorption heating, with a high dielectric loss at both low and high

Advanced Ceramic Materials Sintered by Microwave Technology

http://dx.doi.org/10.5772/intechopen.78831

In this scenario, microwaves are absorbed by the material with highest dielectric losses at room temperature while microwaves propagate through the material with lower losses at room temperature. Heat and energy are transferred from the absorbing material to the transparent material. This type of heating makes use of a specific component known as a susceptor. This heating-aid element is the absorbent material and possesses a very high dielectric loss at room temperature, transmitting heat to the material to be sintered via conventional heat

dielectric properties, and inducing high dielectric losses, it is able to absorb microwave energy

This combined action, known as microwave hybrid heating, can be employed for fast sintering of compacted powders. In this particular case, the direction of heat flow in the specimen to be sintered occurs in two directions: from the surface to the nucleus due to the effect of the susceptor and from the nucleus to the surface once it is able to absorb microwave radiation

**Figure 7.** Sequence diagram of microwave hybrid heating for material sintering: (a) before exposure to microwave radiation, (b) susceptor heating under MW radiation, and (c) specimen to be sintered able to absorb MW energy giving

, changing its

15

transfer mechanisms. Once the material has heated sufficiently surpassing its *Tc*

[37]. A representation of a bidirectional hybrid heating can be seen in **Figure 7**.

temperatures.

and heat itself.

place to bidirectional hybrid heating.

Multi-mode cavities are able to maintain several modes simultaneously. The design of home microwave ovens is based on this type of cavity. The greater the size of the cavity, the higher the number of possible resonant modes. Hence, multi-mode cavities are larger than a wavelength, which contrast with the size of single-mode systems.

The presence of different resonant modes results in the existence of multiple hotspots inside the cavity. Local fluctuations in the electromagnetic field can result in overheating of certain areas. In order to minimize these hotspots, the electromagnetic field must be uniform. Field uniformity can be achieved by increasing the size of the cavity and varying the sample position dynamically, for example, with a rotating plate or stirrers. By increasing cavity size, the number of modes increases and, as a consequence, the heating patterns of each mode begin to superimpose and the stirrers or the plates change the distribution of the field inside the cavity.

### **2.5. Microwave hybrid heating: bidirectional heating**

One of the main issues associated with microwave sintering of materials is their initial microwave radiation absorption and heating. Most of the processing is carried out at a relatively low frequency of 2.45 GHz, which makes the initial heating of the material very difficult to control. Another important problem that may arise consists in the thermal instability that materials are prone to due to the changes in their properties, such as their dielectric constant, *ε'*. Variations in dielectric properties as a function of temperature may translate into poor temperature control and overheating of the specimen. Such behavior is present in several materials such as alumina and zirconia.

Temperature gradients that arise during heating can produce microcracking and an unequal distribution of resulting physical properties, such as density and hardness. Therefore, thermal insulators or coatings may be necessary to avoid the presence of these gradients. Nonetheless, these insulators can provoke the control loss of the temperature.

Ceramics tend to exhibit an abrupt increase in *ε"* as a function of increasing temperature. The temperature at which dielectric properties change is known as the critical temperature, *Tc* . Below *Tc* , at a given frequency, most ceramics are poor absorbers behaving as transparent materials and need to be heated by an external source. No mathematical relationship has been found that relates temperature to fundamental material properties, hence *Tc* values must be measured experimentally [35, 36]. This *Tc* can pose some problems when processing complex and large samples. Unless heated uniformly by an external source, localized hotspots can develop in the material. These spots begin to absorb microwave radiation before the rest of the material in phenomenon known as thermal runaway. As a consequence, this can lead to the fracture and/or warping of specimens. Thermal runaway can be limited by using uniform external heating and a homogeneous microwave field.

A plausible solution that materials scientists and engineers have developed consists on a hybrid method that combines direct microwave heating coupled with heat transfer coming from another material that surrounds the specimen to be sintered [37]. This system is an example of mixed absorption heating, with a high dielectric loss at both low and high temperatures.

The size of a single-mode resonant cavity must be in the order of one wavelength. Additionally, in order to maintain a resonant mode, these systems require a microwave source that allows frequency variations or that the cavity dynamically changes its size to couple the frequency of the microwaves. Generally, the distribution of the electromagnetic field in this type of cavity is well known. With an adequate cavity design, the microwave field may be focalized to a particular zone where the material sample can be sintered. An additional advantage for this type of cavity is the fact that the dielectric properties of the material can be monitored during sintering. Multi-mode cavities are able to maintain several modes simultaneously. The design of home microwave ovens is based on this type of cavity. The greater the size of the cavity, the higher the number of possible resonant modes. Hence, multi-mode cavities are larger than a wave-

The presence of different resonant modes results in the existence of multiple hotspots inside the cavity. Local fluctuations in the electromagnetic field can result in overheating of certain areas. In order to minimize these hotspots, the electromagnetic field must be uniform. Field uniformity can be achieved by increasing the size of the cavity and varying the sample position dynamically, for example, with a rotating plate or stirrers. By increasing cavity size, the number of modes increases and, as a consequence, the heating patterns of each mode begin to superimpose and the stirrers or the plates change the distribution of the field inside the cavity.

One of the main issues associated with microwave sintering of materials is their initial microwave radiation absorption and heating. Most of the processing is carried out at a relatively low frequency of 2.45 GHz, which makes the initial heating of the material very difficult to control. Another important problem that may arise consists in the thermal instability that materials are prone to due to the changes in their properties, such as their dielectric constant, *ε'*. Variations in dielectric properties as a function of temperature may translate into poor temperature control and overheating of the specimen. Such behavior is present in several

Temperature gradients that arise during heating can produce microcracking and an unequal distribution of resulting physical properties, such as density and hardness. Therefore, thermal insulators or coatings may be necessary to avoid the presence of these gradients. Nonetheless,

Ceramics tend to exhibit an abrupt increase in *ε"* as a function of increasing temperature. The temperature at which dielectric properties change is known as the critical temperature,

materials and need to be heated by an external source. No mathematical relationship has been

and large samples. Unless heated uniformly by an external source, localized hotspots can develop in the material. These spots begin to absorb microwave radiation before the rest of the material in phenomenon known as thermal runaway. As a consequence, this can lead to the fracture and/or warping of specimens. Thermal runaway can be limited by using uniform

found that relates temperature to fundamental material properties, hence *Tc*

, at a given frequency, most ceramics are poor absorbers behaving as transparent

can pose some problems when processing complex

values must be

length, which contrast with the size of single-mode systems.

14 Sintering Technology - Method and Application

**2.5. Microwave hybrid heating: bidirectional heating**

these insulators can provoke the control loss of the temperature.

materials such as alumina and zirconia.

measured experimentally [35, 36]. This *Tc*

external heating and a homogeneous microwave field.

*Tc*

. Below *Tc*

In this scenario, microwaves are absorbed by the material with highest dielectric losses at room temperature while microwaves propagate through the material with lower losses at room temperature. Heat and energy are transferred from the absorbing material to the transparent material. This type of heating makes use of a specific component known as a susceptor. This heating-aid element is the absorbent material and possesses a very high dielectric loss at room temperature, transmitting heat to the material to be sintered via conventional heat transfer mechanisms. Once the material has heated sufficiently surpassing its *Tc* , changing its dielectric properties, and inducing high dielectric losses, it is able to absorb microwave energy and heat itself.

This combined action, known as microwave hybrid heating, can be employed for fast sintering of compacted powders. In this particular case, the direction of heat flow in the specimen to be sintered occurs in two directions: from the surface to the nucleus due to the effect of the susceptor and from the nucleus to the surface once it is able to absorb microwave radiation [37]. A representation of a bidirectional hybrid heating can be seen in **Figure 7**.

**Figure 7.** Sequence diagram of microwave hybrid heating for material sintering: (a) before exposure to microwave radiation, (b) susceptor heating under MW radiation, and (c) specimen to be sintered able to absorb MW energy giving place to bidirectional hybrid heating.

### **2.6. Microwave sintering of zirconia**

Mechanical properties and microstructure of Y-TZP-sintered materials are strongly influenced by the degree of densification and grain nucleation that result due to the sintering process. This is, in turn, determined by the heating mechanisms that take place within the material. Current commercial sintering of ceramic materials is based on conventional heat transfer mechanisms: conduction, convection, and radiation. In this case, heat is generated from heating elements and a temperature gradient arises, as heat is transferred from the surface to the material's core. This method, however, requires long processing times. As a consequence, grain broadening occurs [38], which leads to a decrease in the final mechanical properties of the material [39]. It also requires a high-energy consumption to reach such high temperatures, which must also be maintained for long periods of time (around 2–4 h or more) if fully dense materials are desired.

In general, the study suggests that microwave sintering of zirconia can result in comparable mechanical properties and high degrees of densification comparable to those achieved with conventional sintering systems at lower dwell temperatures and significantly shorter sintering times [50–54]. Moreover, some studies have demonstrated that microwave-sintered specimens exhibit enhanced crystallinity [55] and improved mechanical properties [18, 49, 56].

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Over the past few decades, the lithium aluminosilicate (LAS) compositions have been extensively studied because it is very low or even negative thermal expansion compounds have found a wide application field including cookware, bakeware, electronic devices, telescope mirror blanks, ring-laser gyroscopes, and optically stable platforms [57]. Sintered negative thermal expansion materials have usually low mechanical strength because the expansion anisotropy causes microcracking. This is due to different extents of thermal expansion in different crystallographic orientations, which induces internal stress with temperature change. On the other hand, it has been reported by Pelletant et al. [58] that the microcracking depends on the grain size; therefore, an increase in the β-eucryptite grain size causes a progressive microcracking and consequently a more negative bulk of thermal expansion coefficient. Nevertheless, the usefulness of these thermal properties in the production of materials with null expansion has a wide range of potential engineering, photonic, electronic, and structural

β-Eucryptite is the most negative thermal expansion phase in the lithium aluminosilicate system, and therefore β-eucryptite has been thoroughly studied [60]. Compared with the number of studies of glass–ceramic materials, there are few studies in the literature, which deal with this system as a ceramic material in the solid state [61]. This is important because as far as possible, obtaining 100% theoretically dense materials in this system in solid state would improve the mechanical properties as such modulus of elasticity compared with glass-ceramic materials with similar thermal shock characteristics. In LAS system, the high temperatures required to fully densify ceramic powders result in large grain sizes due to Ostwald ripening when traditional sintering techniques are used [38]. This makes obtaining dense materials with nanometric and submicrometric grain sizes extremely difficult, and, as a consequence, the sintered materials do not achieve high mechanical properties. To overcome the problem of grain growth, non-conventional sintering methods have emerged as promis-

Spark plasma sintering (SPS) was reported in [62] as a non-conventional sintering technique for LAS materials that can lead to high relative dense ceramics with no or with very low amounts of a glassy phase. This technique is restricted to materials with disk forms of different diameters, whereas materials with a near-net-shape approach have still not been possible to obtain. Moreover, Vanmeensel et al. [66] reported that the temperature distribution inside the tool and specimen is not homogeneous during the spark plasma sintering technique, especially, for electrical insulating samples (such as LAS ceramics), due to temperature gradient existing between the border and the center of the sample in the intermediate and final stage of sintering. Other important factor to consider is the high-energy consumption of SPS

**2.7. Microwave sintering of lithium aluminosilicate**

applications [59].

ing techniques [62–65].

technique.

One advantageous and useful non-conventional method that can modify the densification mechanisms and results in faster processing of Y-TZP ceramics is microwave sintering [40]. The energy conversion of electromagnetic radiation into heat by the material itself due to the material's dielectric properties is the driving force for densification [41]. The rise in temperature is determined by the amount of energy absorbed in the process. The acceleration of diffusion mechanisms during sintering by the oscillating electric field has also been proposed by some authors to explain enhancement of the sintering process, in what is called a "microwave effect" [42]. Because it is a non-contact technique, the effects of differential sintering are minimized [43], which is another advantage over conventional sintering methods, where differential densification is an important problem that arises from the slow heating rates.

The dielectric loss factor of zirconia is quite different from those of other oxide and non-oxide ceramics. At a frequency of 2.45 GHz, zirconia does not couple adequately with microwaves at room temperature. The loss factor, ε", of Y-TZP at room temperature is similar to microwavetransparent materials, with a value of approximately 0.04 [25]. However, the dielectric loss increases tremendously with temperature, reaching a value of almost 100 at 1000°C. Therefore, zirconia can become a very absorptive material by raising its temperature. In order to achieve this, two different approaches can be found:


Previous reports [4, 18, 47] have demonstrated that with microwave sintering, highly dense materials can be obtained without a substantial grain coarsening because dwell time is considerably shorter and heating rates are quite high in comparison with conventional sintering [48]. Energy consumption is also significantly reduced as a consequence of the mechanisms involved in microwave heating and the abovementioned shortening of processing times. As a result, several advantages arise including improved mechanical properties and reduced environmental impact [5, 49]. This method may provide lower costs for professionals and customers maintaining or even improving the quality of the final product.

In general, the study suggests that microwave sintering of zirconia can result in comparable mechanical properties and high degrees of densification comparable to those achieved with conventional sintering systems at lower dwell temperatures and significantly shorter sintering times [50–54]. Moreover, some studies have demonstrated that microwave-sintered specimens exhibit enhanced crystallinity [55] and improved mechanical properties [18, 49, 56].

### **2.7. Microwave sintering of lithium aluminosilicate**

**2.6. Microwave sintering of zirconia**

16 Sintering Technology - Method and Application

this, two different approaches can be found:

Mechanical properties and microstructure of Y-TZP-sintered materials are strongly influenced by the degree of densification and grain nucleation that result due to the sintering process. This is, in turn, determined by the heating mechanisms that take place within the material. Current commercial sintering of ceramic materials is based on conventional heat transfer mechanisms: conduction, convection, and radiation. In this case, heat is generated from heating elements and a temperature gradient arises, as heat is transferred from the surface to the material's core. This method, however, requires long processing times. As a consequence, grain broadening occurs [38], which leads to a decrease in the final mechanical properties of the material [39]. It also requires a high-energy consumption to reach such high temperatures, which must also be maintained for long periods of time (around 2–4 h or more) if fully dense materials are desired.

One advantageous and useful non-conventional method that can modify the densification mechanisms and results in faster processing of Y-TZP ceramics is microwave sintering [40]. The energy conversion of electromagnetic radiation into heat by the material itself due to the material's dielectric properties is the driving force for densification [41]. The rise in temperature is determined by the amount of energy absorbed in the process. The acceleration of diffusion mechanisms during sintering by the oscillating electric field has also been proposed by some authors to explain enhancement of the sintering process, in what is called a "microwave effect" [42]. Because it is a non-contact technique, the effects of differential sintering are minimized [43], which is another advantage over conventional sintering methods, where differential densification is an important problem that arises from the slow heating rates.

The dielectric loss factor of zirconia is quite different from those of other oxide and non-oxide ceramics. At a frequency of 2.45 GHz, zirconia does not couple adequately with microwaves at room temperature. The loss factor, ε", of Y-TZP at room temperature is similar to microwavetransparent materials, with a value of approximately 0.04 [25]. However, the dielectric loss increases tremendously with temperature, reaching a value of almost 100 at 1000°C. Therefore, zirconia can become a very absorptive material by raising its temperature. In order to achieve

• With the aid of a susceptor, generally (SiC), as it has been described in the previous section.

• Employing conventional resistive elements to initially heat the zirconia until its Tc is

Previous reports [4, 18, 47] have demonstrated that with microwave sintering, highly dense materials can be obtained without a substantial grain coarsening because dwell time is considerably shorter and heating rates are quite high in comparison with conventional sintering [48]. Energy consumption is also significantly reduced as a consequence of the mechanisms involved in microwave heating and the abovementioned shortening of processing times. As a result, several advantages arise including improved mechanical properties and reduced environmental impact [5, 49]. This method may provide lower costs for professionals and

reached, and zirconia is able to interact with the microwave field by itself [46].

This method is the most commonly found in the study [25, 44, 45].

customers maintaining or even improving the quality of the final product.

Over the past few decades, the lithium aluminosilicate (LAS) compositions have been extensively studied because it is very low or even negative thermal expansion compounds have found a wide application field including cookware, bakeware, electronic devices, telescope mirror blanks, ring-laser gyroscopes, and optically stable platforms [57]. Sintered negative thermal expansion materials have usually low mechanical strength because the expansion anisotropy causes microcracking. This is due to different extents of thermal expansion in different crystallographic orientations, which induces internal stress with temperature change. On the other hand, it has been reported by Pelletant et al. [58] that the microcracking depends on the grain size; therefore, an increase in the β-eucryptite grain size causes a progressive microcracking and consequently a more negative bulk of thermal expansion coefficient. Nevertheless, the usefulness of these thermal properties in the production of materials with null expansion has a wide range of potential engineering, photonic, electronic, and structural applications [59].

β-Eucryptite is the most negative thermal expansion phase in the lithium aluminosilicate system, and therefore β-eucryptite has been thoroughly studied [60]. Compared with the number of studies of glass–ceramic materials, there are few studies in the literature, which deal with this system as a ceramic material in the solid state [61]. This is important because as far as possible, obtaining 100% theoretically dense materials in this system in solid state would improve the mechanical properties as such modulus of elasticity compared with glass-ceramic materials with similar thermal shock characteristics. In LAS system, the high temperatures required to fully densify ceramic powders result in large grain sizes due to Ostwald ripening when traditional sintering techniques are used [38]. This makes obtaining dense materials with nanometric and submicrometric grain sizes extremely difficult, and, as a consequence, the sintered materials do not achieve high mechanical properties. To overcome the problem of grain growth, non-conventional sintering methods have emerged as promising techniques [62–65].

Spark plasma sintering (SPS) was reported in [62] as a non-conventional sintering technique for LAS materials that can lead to high relative dense ceramics with no or with very low amounts of a glassy phase. This technique is restricted to materials with disk forms of different diameters, whereas materials with a near-net-shape approach have still not been possible to obtain. Moreover, Vanmeensel et al. [66] reported that the temperature distribution inside the tool and specimen is not homogeneous during the spark plasma sintering technique, especially, for electrical insulating samples (such as LAS ceramics), due to temperature gradient existing between the border and the center of the sample in the intermediate and final stage of sintering. Other important factor to consider is the high-energy consumption of SPS technique.

Microwave heating is a non-conventional sintering technique to solve the difficulties found with previous techniques such as SPS. The microwave technique was specially designed to fabricate ceramic LAS bodies with a high density, a very low glass proportion, and high mechanical properties (hardness and Young's modulus) [63]. An important characteristic associated to microwave process, it is possible to directly obtain materials with complex parts (*near-net-shape* components) directly in the microwave furnace without the application of pressure and without any carbon contamination. This supposes other significant advantage compared with the spark plasma sintering [64]. This point is essential in order to use this sintering technology where the final dimension of the sintered component has to be almost constant in order to reduce the final machining cost of nanocomposites.

**2.8. Advantages and disadvantages of microwave sintering technology**

homogenization and uniform heat distribution.

interesting alternative for the processing of advance ceramics.

**3. Conclusions**

During sintering process, the heating occurs by the three conventional heat transfer mechanisms: conduction, convection, and radiation. Conduction results by heat diffusion between surfaces in contact, for example, in walls inside the furnace that are in contact with the compact. Convective heat transfer occurs from the bulk flow of the gas in the furnace to the compact surface. Thermal radiation is emitted by high-temperature furnace elements and converted into electromagnetic energy that is transferred to the surroundings. The compact receives this electromagnetic energy causing it to heat up. Heat from radiation is, however, quite low, and most of the heating of the compact occurs by means of conduction and convection. Due to the nature of heat transfer mechanisms involved in this method, the surface of the material always heats first, and a temperature gradient between the compact surface and the interior of the material arises, resulting in heat flow from the surface to the bulk. As a consequence, considerably long dwell times (>2 h) are required in order to obtain a complete temperature

Advanced Ceramic Materials Sintered by Microwave Technology

http://dx.doi.org/10.5772/intechopen.78831

19

Another important sintering approach is pressure-assisted sintering, which consists in the external application of pressure during the heating process. Four main ways can be employed to apply pressure. The first one is hot pressing (HP), resulting from uniaxially applying pressure to the powder in a die. The second one is sinter forging, which is similar to hot pressing but without confining the sample in a die. The third one is called hot isostatic pressing (HIP), which consists in the isostatic application of pressure by means of a gas. The fourth one is spark plasma sintering (SPS) and flash sintering which is similar to HP but using a high heating rate. Pressure-assisted sintering enhances the rate of densification significantly relative to the coarsening rate [27]. However, an important disadvantage of pressure-assisted sintering is the high cost of production being only available for specific industrial applications that require specialized, high-cost components. Another limitation is that only simple shapes can be processed due to the use of dyes.

Currently, most commercial materials are processed by conventional sintering and SPS. One of the major drawbacks of these systems, particularly for ceramics, is the high-energy consumption required to reach such high temperatures and dwell times in order to obtain an adequate densification and mechanical properties. Therefore, new approaches on sintering of these materials need to be explored. For example, employing furnaces for heating components with small dimensions would not be energetically efficient. Hence, sintering systems with a focalized energy delivery to the material, such as microwave sintering, can decrease energy use significantly. Moreover, techniques must be flexible and allow for the processing of near-net-shape materials because complex and unique pieces are needed since shapes vary completely from one application to the next. Therefore, microwave sintering confirms as an

Currently, innovative sintering methods are being explored and studied in order to reduce energy consumption and production costs, as well as processing tools that allow modification of the densification mechanisms that may improve the microstructure and mechanical

Previous reports [63–65] confirmed the possibility of successfully obtaining well-densified β-eucryptite ceramics by using microwave sintering technology with glass-free at relatively low temperatures (1200°C) and very low energy consumed (<80 W). **Figure 8** shows the temperature profile and microwave-absorbed power during the sintering process of an LAS specimen [63]. The figure shows a microwave experiment with a resident time of the ceramic sample of 10 min around 1200°C. The LAS material is a good absorber of microwave radiation at 2.45 GHz, and this implies that the heating is homogeneously distributed throughout the material. The dilatometric data presented for the cryogenic temperature interval are essential in order to design these kinds of materials for space applications in which controlled and very low thermal expansion behavior are needed at very low temperatures. This is the case of mirror blanks in satellites, where exceptional thermal properties are demanded together with exceptional mechanical properties, that is, the β-eucryptite sample sintered at 1200°C shows Young's modulus of 110 MPa and a hardness of 7.1 GPa values [63]. Compared with other heating modes, conventional, and spark plasma sintering [64], the most important characteristics associated to microwave process are the rapid and volumetric heating, which improves the final properties of the materials.

**Figure 8.** Temperature profile and microwave absorbed power during the sintering process of the LAS specimen.

### **2.8. Advantages and disadvantages of microwave sintering technology**

During sintering process, the heating occurs by the three conventional heat transfer mechanisms: conduction, convection, and radiation. Conduction results by heat diffusion between surfaces in contact, for example, in walls inside the furnace that are in contact with the compact. Convective heat transfer occurs from the bulk flow of the gas in the furnace to the compact surface. Thermal radiation is emitted by high-temperature furnace elements and converted into electromagnetic energy that is transferred to the surroundings. The compact receives this electromagnetic energy causing it to heat up. Heat from radiation is, however, quite low, and most of the heating of the compact occurs by means of conduction and convection. Due to the nature of heat transfer mechanisms involved in this method, the surface of the material always heats first, and a temperature gradient between the compact surface and the interior of the material arises, resulting in heat flow from the surface to the bulk. As a consequence, considerably long dwell times (>2 h) are required in order to obtain a complete temperature homogenization and uniform heat distribution.

Another important sintering approach is pressure-assisted sintering, which consists in the external application of pressure during the heating process. Four main ways can be employed to apply pressure. The first one is hot pressing (HP), resulting from uniaxially applying pressure to the powder in a die. The second one is sinter forging, which is similar to hot pressing but without confining the sample in a die. The third one is called hot isostatic pressing (HIP), which consists in the isostatic application of pressure by means of a gas. The fourth one is spark plasma sintering (SPS) and flash sintering which is similar to HP but using a high heating rate. Pressure-assisted sintering enhances the rate of densification significantly relative to the coarsening rate [27]. However, an important disadvantage of pressure-assisted sintering is the high cost of production being only available for specific industrial applications that require specialized, high-cost components. Another limitation is that only simple shapes can be processed due to the use of dyes.

Currently, most commercial materials are processed by conventional sintering and SPS. One of the major drawbacks of these systems, particularly for ceramics, is the high-energy consumption required to reach such high temperatures and dwell times in order to obtain an adequate densification and mechanical properties. Therefore, new approaches on sintering of these materials need to be explored. For example, employing furnaces for heating components with small dimensions would not be energetically efficient. Hence, sintering systems with a focalized energy delivery to the material, such as microwave sintering, can decrease energy use significantly. Moreover, techniques must be flexible and allow for the processing of near-net-shape materials because complex and unique pieces are needed since shapes vary completely from one application to the next. Therefore, microwave sintering confirms as an interesting alternative for the processing of advance ceramics.

### **3. Conclusions**

Microwave heating is a non-conventional sintering technique to solve the difficulties found with previous techniques such as SPS. The microwave technique was specially designed to fabricate ceramic LAS bodies with a high density, a very low glass proportion, and high mechanical properties (hardness and Young's modulus) [63]. An important characteristic associated to microwave process, it is possible to directly obtain materials with complex parts (*near-net-shape* components) directly in the microwave furnace without the application of pressure and without any carbon contamination. This supposes other significant advantage compared with the spark plasma sintering [64]. This point is essential in order to use this sintering technology where the final dimension of the sintered component has to be almost

Previous reports [63–65] confirmed the possibility of successfully obtaining well-densified β-eucryptite ceramics by using microwave sintering technology with glass-free at relatively low temperatures (1200°C) and very low energy consumed (<80 W). **Figure 8** shows the temperature profile and microwave-absorbed power during the sintering process of an LAS specimen [63]. The figure shows a microwave experiment with a resident time of the ceramic sample of 10 min around 1200°C. The LAS material is a good absorber of microwave radiation at 2.45 GHz, and this implies that the heating is homogeneously distributed throughout the material. The dilatometric data presented for the cryogenic temperature interval are essential in order to design these kinds of materials for space applications in which controlled and very low thermal expansion behavior are needed at very low temperatures. This is the case of mirror blanks in satellites, where exceptional thermal properties are demanded together with exceptional mechanical properties, that is, the β-eucryptite sample sintered at 1200°C shows Young's modulus of 110 MPa and a hardness of 7.1 GPa values [63]. Compared with other heating modes, conventional, and spark plasma sintering [64], the most important characteristics associated to microwave process are the rapid and volumetric heating, which improves

**Figure 8.** Temperature profile and microwave absorbed power during the sintering process of the LAS specimen.

constant in order to reduce the final machining cost of nanocomposites.

the final properties of the materials.

18 Sintering Technology - Method and Application

Currently, innovative sintering methods are being explored and studied in order to reduce energy consumption and production costs, as well as processing tools that allow modification of the densification mechanisms that may improve the microstructure and mechanical properties of sintered materials. The main purpose for modifying sintering mechanisms is to obtain relative densities close to theoretical values, while maintaining a controlled, but limited, grain growth. Potential of microwaves in material processing has been identified several decades ago. However, owing to limited understanding of the phenomena, their use remained largely confined to only a few materials. Moreover, the overwhelming success of microwave in communication overshadowed its application in other areas. However, discrete attempts in material processing yielded many breakthroughs. In the last 65 years, the microwave processing of materials has become popular due to its potential advantages over the conventional techniques. Overall, microwave sintering is a very good alternative for sintering and consolidating commercial materials for structural applications due to the resulting finer microstructure, enhanced mechanical properties, and reduction in processing times and energy consumption.

[6] Presenda A, Salvador MD, Penaranda-Foix FL, Moreno R, Borrell A. Effect of microwave sintering on microstructure and mechanical properties in Y-TZP materials used for den-

Advanced Ceramic Materials Sintered by Microwave Technology

http://dx.doi.org/10.5772/intechopen.78831

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[13] Wang J, Binner J, Vaidhyanathan B, Joomun N, Kilner J, Dimitrakis G, et al. Evidence for the microwave effect during hybrid sintering. Journal of the American Ceramic Society.

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### **Acknowledgements**

The authors gratefully acknowledge the funding support of the Spanish Ministry of Economy and Competitiveness (JCI-2011-10498, IJCI-2014-19839, and RYC-2016-20915), the Generalitat Valenciana (GV/2014/009, GRISOLIA/2013/035, and GRISOLIAP/2018/168), Universitat Politècnica de València, and Dr. A. Presenda and Dr. R. Benavente for contributing to the research work described in this chapter.

### **Author details**

Amparo Borrell\* and Maria Dolores Salvador

\*Address all correspondence to: aborrell@upv.es

Institute of Materials Technology, Polytechnic University of Valencia, Valencia, Spain

### **References**


[6] Presenda A, Salvador MD, Penaranda-Foix FL, Moreno R, Borrell A. Effect of microwave sintering on microstructure and mechanical properties in Y-TZP materials used for dental applications. Ceramics International. 2015;**41**:7125-7132

properties of sintered materials. The main purpose for modifying sintering mechanisms is to obtain relative densities close to theoretical values, while maintaining a controlled, but limited, grain growth. Potential of microwaves in material processing has been identified several decades ago. However, owing to limited understanding of the phenomena, their use remained largely confined to only a few materials. Moreover, the overwhelming success of microwave in communication overshadowed its application in other areas. However, discrete attempts in material processing yielded many breakthroughs. In the last 65 years, the microwave processing of materials has become popular due to its potential advantages over the conventional techniques. Overall, microwave sintering is a very good alternative for sintering and consolidating commercial materials for structural applications due to the resulting finer microstructure, enhanced mechanical properties, and reduction in processing times and

The authors gratefully acknowledge the funding support of the Spanish Ministry of Economy and Competitiveness (JCI-2011-10498, IJCI-2014-19839, and RYC-2016-20915), the Generalitat Valenciana (GV/2014/009, GRISOLIA/2013/035, and GRISOLIAP/2018/168), Universitat Politècnica de València, and Dr. A. Presenda and Dr. R. Benavente for contributing to the research work

Institute of Materials Technology, Polytechnic University of Valencia, Valencia, Spain

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Amparo Borrell\* and Maria Dolores Salvador

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**Chapter 2**

**Provisional chapter**

**Sintering Processing of Complex Magnetic Ceramic**

**Oxides: A Comparison Between Sintering of** 

**Process of Top-Down Approach Synthesis**

**Sintering Processing of Complex Magnetic Ceramic** 

**of Top-Down Approach Synthesis**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78654

mechanically activated Ni0.5Zn0.5Fe2

**Abstract**

**1. Introduction**

Zhi Huang Low, Ismayadi Ismail and Kim Song Tan

Zhi Huang Low, Ismayadi Ismail and Kim Song Tan

**Oxides: A Comparison Between Sintering of Bottom-**

**Up Approach Synthesis and Mechanochemical Process**

**Bottom-Up Approach Synthesis and Mechanochemical** 

Sintering is a common synthesis method for the fabrication of ceramics. The widespread use of sintering for the production of complex ceramic oxide especially ferrites has led to a variety of investigations on the subject. Top-down approach synthesis like mechanochemical process has recently been suggested as a promising synthesis method for replacing bottom-up approach synthesis methods like sintering, questioning its necessity for thermal treatment at high temperature. Understanding of sintering mechanism is crucial in order to optimize and enhance the advantages of sintering, which cannot be replaced by other techniques. In general, ferrites with particular set of behaviors require a particular set of microstructural properties influenced by the sintering steps. The main objective of this chapter is to understand how the increase of sintering temperature affects the microstructural evolution, in order to develop a fundamental science understanding for the mechanism of sintering. In the second part of this chapter, presentation of experimental results on sintering of

netic, and optical properties was reported. Lastly, a comparative study between sintering (bottom-up approach) and mechanochemical (top-down approach) process is presented.

Sintering is one of the oldest material synthesis methods has existed for thousands of years. Since the introduction of controlled sintering process of ceramic, the methodology has gained

O4

**Keywords:** bottom-up approach, sintering, barium hexaferrite, Ni-Zn ferrite

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

nanoparticles and its effect on microstructural, mag-

DOI: 10.5772/intechopen.78654


### **Sintering Processing of Complex Magnetic Ceramic Oxides: A Comparison Between Sintering of Bottom-Up Approach Synthesis and Mechanochemical Process of Top-Down Approach Synthesis Sintering Processing of Complex Magnetic Ceramic Oxides: A Comparison Between Sintering of Bottom-Up Approach Synthesis and Mechanochemical Process of Top-Down Approach Synthesis**

DOI: 10.5772/intechopen.78654

Zhi Huang Low, Ismayadi Ismail and Kim Song Tan Zhi Huang Low, Ismayadi Ismail and Kim Song Tan

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78654

### **Abstract**

O8


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[55] Vasudevan R, Karthik T, Ganesan S, Jayavel R. Effect of microwave sintering on the structural and densification behavior of sol–gel derived zirconia toughened alumina

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[58] Pelletant A, Reveron H, Chêvalier J, Fantozzi G, Blanchard L, Guinot F, Falzon F. Grain size dependence of pure β-eucryptite thermal expansion coefficient. Materials Letters.

[59] Chen JC, Huang GC, Hu C, Weng JP. Synthesis of negative-thermal expansion ZrW<sup>2</sup>

[61] García-Moreno O, Fernández A, Khainakov S, Torrecillas R. Negative thermal expansion of lithium aluminosilicate ceramics at cryogenic temperatures. Scripta Materialia.

[62] García-Moreno O, Fernández A, Torrecillas R. Solid state sintering of very low and negative thermal expansion ceramics by spark plasma sintering. Ceramic International.

[63] Benavente R, Borrell A, Salvador MD, García-Moreno O, Peñaranda-Foix FL, Catala-Civera JM. Fabrication of near-zero thermal expansion of fully dense β-eucryptite

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24 Sintering Technology - Method and Application

substrates. Scripta Materialia. 2003;**49**:261-266

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Sintering is a common synthesis method for the fabrication of ceramics. The widespread use of sintering for the production of complex ceramic oxide especially ferrites has led to a variety of investigations on the subject. Top-down approach synthesis like mechanochemical process has recently been suggested as a promising synthesis method for replacing bottom-up approach synthesis methods like sintering, questioning its necessity for thermal treatment at high temperature. Understanding of sintering mechanism is crucial in order to optimize and enhance the advantages of sintering, which cannot be replaced by other techniques. In general, ferrites with particular set of behaviors require a particular set of microstructural properties influenced by the sintering steps. The main objective of this chapter is to understand how the increase of sintering temperature affects the microstructural evolution, in order to develop a fundamental science understanding for the mechanism of sintering. In the second part of this chapter, presentation of experimental results on sintering of mechanically activated Ni0.5Zn0.5Fe2 O4 nanoparticles and its effect on microstructural, magnetic, and optical properties was reported. Lastly, a comparative study between sintering (bottom-up approach) and mechanochemical (top-down approach) process is presented.

**Keywords:** bottom-up approach, sintering, barium hexaferrite, Ni-Zn ferrite

### **1. Introduction**

Sintering is one of the oldest material synthesis methods has existed for thousands of years. Since the introduction of controlled sintering process of ceramic, the methodology has gained

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

rapid growth and well established as one of the most trustable synthesis method for the production of complex ceramic oxides with desired properties [1]. Sintering is categorized as bottom-up approach synthesis as it involves the construction of nanostructures in materials atom-by-atom, layer-by-layer, from small to large sizes [2]. Since twentieth century, energy efficiency and productivity are two important factors in choosing a particular methodology [3]; therefore, top-down approach synthesis method like mechanochemistry has emerged as one of the most promising candidates to replace known current methodologies like sintering, questioning the necessity for thermal treatment at high temperatures. However, there are advantages of sintering that are irreplaceable by other methodologies. Sintering offers matter transport through diffusion while maintaining the stoichiometry of the ceramic material. Commonly, a single phase ceramic oxide with low porosity can be achieved by sintering of the material to a range of 50–80% of its melting point [1]. With an appropriate sintering temperature, the material does not melt, while atomic diffusion can be activated to achieve a dense, compact, and high crystallinity material, which is essential for the fabrication process. Although the optimization of sintering parameters to achieve complete phases of complex ceramic oxides is crucial; however, the fundamental knowledge behind sintering: the correlation between microstructural properties induced by the thermal activation of sintering, with important behaviors like magnetic and optical properties, is important for the understanding of sintering mechanism.

where *K* is the anisotropy constant, *V* is the volume of the crystal, and *Ө* is the angle between the easy axis and the direction of the field-induced magnetic field. When the grains are small in dimensions or below a critical size, they dissipate minimum energy, therefore, the energy required to create a new domain or shifting the domains is much higher than that required in maintaining the material as single domain. The effect of grain size changed some aspects of magnetic behavior of yttrium iron garnets [6]. Below a critical size, as the volume or size of the grains increases, grain size remains in the single-domain range, therefore, the *EA* value increases, the magnetocrystalline anisotropy energy becomes stronger and the coercivity (the energy required to change the direction of the magnetization) increases. After the grain size exceeds the critical size, intergranular domain walls are formed inside the grains, because the energy is not sufficient to maintain relatively big grains as single domains. Therefore, domain walls are created to reduce the overall energy of the system. Grain size has similar impact on the magnetic properties of hard ferrite, BaFe12O19, which has a critical size as well. Studies defined the critical size as the minimum grain volume that the anisotropy energy is able to overcome thermal agitation [7]. Another important magnetic behavior is the measure or ability of a material to sustain a magnetic field within the material when external field applied. This is known as the magnetic permeability. Magnetic permeability is strongly influenced by the presence of grain boundaries or amorphous surfaces, as they will act as impediments to domain wall movement. Bulk materials have fewer grain boundaries, therefore, higher the permeability. This phenomenon especially noticeable in ferrites as their grain boundaries are thicker [4]. The effect of sintering soaking time on the microstructural properties of nickel zinc ferrite was investigated [8, 9]. Grain size increases with increasing soaking time. The increase of grain size is the main factor that causes the increase of initial permeability. Bulk materials have a low-volume fraction of

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27

grain boundary as shown in **Figure 1**. Volume fraction can be represented by:

\_\_4 <sup>3</sup> *<sup>π</sup>R*<sup>3</sup> <sup>−</sup> \_\_4

<sup>3</sup> *πr* <sup>3</sup> \_\_\_\_\_\_\_\_\_ \_\_4

where *Vshell* is the volume fraction of the structurally disordered boundary region, *R* is the radius of a particle, and *r* is the radius of the core region of a particle. If we assume the thickness of the shell

**Figure 1.** Schematic representation of bulk and nanoparticles, and the definition of R, radius of a particle, and r, radius

<sup>3</sup> *<sup>π</sup>R*<sup>3</sup> <sup>×</sup> 100% (2)

*Vshell* =

of the core of a particle.

### **2. Microstructural aspects of complex magnetic ceramic oxides**

Microstructure of complex magnetic ceramic oxides consists of grains, grain boundaries, porosity, and defects structures. As complex as it is microstructural properties influence the behaviors of these complex ceramic oxides. For instance, microstructural properties like surface morphology, atomic arrangement, size and shape affect major macroscopic properties such as magnetic, optical, mechanical, electrical, and many other properties of complex ceramic oxides. These are known as the microstructural dependent behaviors of complex ceramic oxides. Nanomaterials exhibit unique behaviors compared to their bulk counterparts [4].

### **2.1. Size**

There are some important behaviors related to magnetic ceramic oxides, which are size dependent. For instance, magnetic properties and particle, grain, or crystallite size are relevant to each other. When the particles are in nano-size, the percentage of amorphous grain boundary volumes in material is high compared to particles in micron size. The presence of large volume fraction of amorphous phase in the material hinders the exchange interaction between magnetic moments. Therefore, small particles are likely to exhibit weak ferromagnetic, superparamagnetic, and paramagnetic behaviors. Small size polycrystalline nickel zinc ferrite dissipates minimum energy [5]. The magnetocrystalline anisotropy energy, *EA* for ferrite can be defined by the following equation:

$$E\_A = KV \sin^2 \Theta \tag{1}$$

where *K* is the anisotropy constant, *V* is the volume of the crystal, and *Ө* is the angle between the easy axis and the direction of the field-induced magnetic field. When the grains are small in dimensions or below a critical size, they dissipate minimum energy, therefore, the energy required to create a new domain or shifting the domains is much higher than that required in maintaining the material as single domain. The effect of grain size changed some aspects of magnetic behavior of yttrium iron garnets [6]. Below a critical size, as the volume or size of the grains increases, grain size remains in the single-domain range, therefore, the *EA* value increases, the magnetocrystalline anisotropy energy becomes stronger and the coercivity (the energy required to change the direction of the magnetization) increases. After the grain size exceeds the critical size, intergranular domain walls are formed inside the grains, because the energy is not sufficient to maintain relatively big grains as single domains. Therefore, domain walls are created to reduce the overall energy of the system. Grain size has similar impact on the magnetic properties of hard ferrite, BaFe12O19, which has a critical size as well. Studies defined the critical size as the minimum grain volume that the anisotropy energy is able to overcome thermal agitation [7].

rapid growth and well established as one of the most trustable synthesis method for the production of complex ceramic oxides with desired properties [1]. Sintering is categorized as bottom-up approach synthesis as it involves the construction of nanostructures in materials atom-by-atom, layer-by-layer, from small to large sizes [2]. Since twentieth century, energy efficiency and productivity are two important factors in choosing a particular methodology [3]; therefore, top-down approach synthesis method like mechanochemistry has emerged as one of the most promising candidates to replace known current methodologies like sintering, questioning the necessity for thermal treatment at high temperatures. However, there are advantages of sintering that are irreplaceable by other methodologies. Sintering offers matter transport through diffusion while maintaining the stoichiometry of the ceramic material. Commonly, a single phase ceramic oxide with low porosity can be achieved by sintering of the material to a range of 50–80% of its melting point [1]. With an appropriate sintering temperature, the material does not melt, while atomic diffusion can be activated to achieve a dense, compact, and high crystallinity material, which is essential for the fabrication process. Although the optimization of sintering parameters to achieve complete phases of complex ceramic oxides is crucial; however, the fundamental knowledge behind sintering: the correlation between microstructural properties induced by the thermal activation of sintering, with important behaviors like magnetic and optical properties, is important for the understanding

**2. Microstructural aspects of complex magnetic ceramic oxides**

exhibit unique behaviors compared to their bulk counterparts [4].

ferrite can be defined by the following equation:

Microstructure of complex magnetic ceramic oxides consists of grains, grain boundaries, porosity, and defects structures. As complex as it is microstructural properties influence the behaviors of these complex ceramic oxides. For instance, microstructural properties like surface morphology, atomic arrangement, size and shape affect major macroscopic properties such as magnetic, optical, mechanical, electrical, and many other properties of complex ceramic oxides. These are known as the microstructural dependent behaviors of complex ceramic oxides. Nanomaterials

There are some important behaviors related to magnetic ceramic oxides, which are size dependent. For instance, magnetic properties and particle, grain, or crystallite size are relevant to each other. When the particles are in nano-size, the percentage of amorphous grain boundary volumes in material is high compared to particles in micron size. The presence of large volume fraction of amorphous phase in the material hinders the exchange interaction between magnetic moments. Therefore, small particles are likely to exhibit weak ferromagnetic, superparamagnetic, and paramagnetic behaviors. Small size polycrystalline nickel zinc ferrite dissipates minimum energy [5]. The magnetocrystalline anisotropy energy, *EA* for

*EA* = *KV sin*<sup>2</sup> *Ө* (1)

of sintering mechanism.

26 Sintering Technology - Method and Application

**2.1. Size**

Another important magnetic behavior is the measure or ability of a material to sustain a magnetic field within the material when external field applied. This is known as the magnetic permeability. Magnetic permeability is strongly influenced by the presence of grain boundaries or amorphous surfaces, as they will act as impediments to domain wall movement. Bulk materials have fewer grain boundaries, therefore, higher the permeability. This phenomenon especially noticeable in ferrites as their grain boundaries are thicker [4]. The effect of sintering soaking time on the microstructural properties of nickel zinc ferrite was investigated [8, 9]. Grain size increases with increasing soaking time. The increase of grain size is the main factor that causes the increase of initial permeability. Bulk materials have a low-volume fraction of grain boundary as shown in **Figure 1**. Volume fraction can be represented by:

$$V\_{\text{odd}} = \frac{\frac{4}{3}\pi R^3 - \frac{4}{3}\pi r^3}{\frac{4}{3}\pi R^3} \times 100\% \tag{2}$$

where *Vshell* is the volume fraction of the structurally disordered boundary region, *R* is the radius of a particle, and *r* is the radius of the core region of a particle. If we assume the thickness of the shell

**Figure 1.** Schematic representation of bulk and nanoparticles, and the definition of R, radius of a particle, and r, radius of the core of a particle.

is approximately 5 nm, for nanoparticles (R < 100 nm), the volume fraction of this structurally disordered shell is large (>10%) while for bulk material, the volume fraction of the shell is not significant (<0.02%). As the particles undergo densification and coarsening through increasing sintering temperature, particle size increases from nano-sized to micron-sized. As a result, volume fraction of disordered grain boundary becomes less significant. Therefore, the pinning effects of this disordered boundary region on the domain walls motion is not significant for bulk materials.

### **2.2. Defects and porosity**

Porosity is another microstructural feature that has the pinning effect on the movement of the domain walls. Porosity is abundant in complex magnetic ceramic oxide because it cannot be eliminated by heat treatment. Heat treatment offers grain growth, densification, and boundaries expansion. However, many pores are swept over by grain boundaries and remain within large grain [4]. Porosity and grain size effects sometimes seem inseparable because grain growth and densification happen simultaneously. In case of magnetic properties, saturation magnetization is associated with the following equation [10]:

$$M\_s = \left(1 - p\right) \left4 \,\pi M\_o \tag{3}$$

field. From the equation, notice that the thickness of boundary region has strong influence on the control of the magnetic properties of ferrites because the thickness of the boundary regions can

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Attention has been paid to investigate synthesis techniques and their impacts on new materials, particularly nanostructured and nanocrystalline materials. Synthesis technique is strongly related to behaviors of the investigated nanomaterial because the chosen synthesis technique is responsible for tailoring the atomic and microstructure of the nanostructured material. Numerous published studies have improved our understanding of the effects of synthesis technique on the behaviors of complex magnetic ceramic oxide, especially technologically important hard and soft ferrites [5, 14–16]. Most of the significant findings show that the results are of limited significance unless the microstructures, chemical composition, defects, and atomic arrangement of the investigated ferrite are well-characterized. Generally speaking, the techniques of preparing ferrite are categorized into two: bottom-up and top-down approaches, as shown in **Figure 2**. Bottom-up approach synthesis is a ceramic powder processing approach that engages atoms, ions, molecules or particles as starting building blocks.

be altered simply by small amount of additives, impurities, or phase transformations.

**Figure 2.** Schematic representations of (a) bottom-up and (b) top-down approaches.

**3. Ceramic synthesis techniques**

where p is the porosity, M<sup>o</sup> is the magnetization extrapolated to zero porosity. Therefore, we can conclude that saturation magnetization is porosity dependent while coercivity is size dependent. Previous study proved that the independence of coercivity from porosity, while saturation magnetization and remanence are independent from grain size effect [10]. In addition to porosity, other defects such as cracks, inclusions, foreign phases, strains, as well as dislocations would alter the magnetic behaviors of ferrites. Defects act as energy wells have a strong pinning effect on the domain wall motion and thus require higher activation energy to detach [4].

### **2.3. Boundary region**

It is believed that boundary region possesses higher energy compared to volume defects. Therefore, boundary region is a highly reactive region, which allows nucleation of new phases. As nanostructured materials have higher surface-to-volume ratio, they are reactive compared to their corresponding bulk materials. In ceramic materials, boundary region between phases and grains governs many properties and processes, for example, as fracture strength, plastic deformation, conductivity, dielectric loss, and phase transformation. All materials have interfacial energy and tension that can be calculated by same thermodynamic formulation [11]. Boundaries act as sinks and sources for the formation of lattice imperfections, diffusion, and phase transformations when deformation occurs. Some behaviors of ceramic oxides such as coercivity and permeability are strongly related to their boundaries [12]. The direction of magnetic moments within the material could be changed easily when the pinning effects of the boundary regions is diminished. Apparent permeability can be expressed as following [13]: *μapp* <sup>=</sup> (1 <sup>−</sup> *<sup>p</sup>*) *<sup>μ</sup>* \_\_\_\_\_\_\_\_\_\_\_\_\_\_ *<sup>o</sup>*

$$\mu\_{app} = \frac{\left(1 - p\right)\mu\_o}{\left(1 + \frac{p}{2}\right)\left(1 + 0.75 \frac{t}{D} \frac{\mu\_o}{\mu\_o}\right)}\tag{4}$$

where *p* is the porosity, *D* is the average grain size, *t* is the effective thickness of boundary region, *μb* is the permeability of the boundary region, *μo* is the permeability free form the demagnetizing field. From the equation, notice that the thickness of boundary region has strong influence on the control of the magnetic properties of ferrites because the thickness of the boundary regions can be altered simply by small amount of additives, impurities, or phase transformations.

### **3. Ceramic synthesis techniques**

is approximately 5 nm, for nanoparticles (R < 100 nm), the volume fraction of this structurally disordered shell is large (>10%) while for bulk material, the volume fraction of the shell is not significant (<0.02%). As the particles undergo densification and coarsening through increasing sintering temperature, particle size increases from nano-sized to micron-sized. As a result, volume fraction of disordered grain boundary becomes less significant. Therefore, the pinning effects of this disordered boundary region on the domain walls motion is not significant for bulk materials.

Porosity is another microstructural feature that has the pinning effect on the movement of the domain walls. Porosity is abundant in complex magnetic ceramic oxide because it cannot be eliminated by heat treatment. Heat treatment offers grain growth, densification, and boundaries expansion. However, many pores are swept over by grain boundaries and remain within large grain [4]. Porosity and grain size effects sometimes seem inseparable because grain growth and densification happen simultaneously. In case of magnetic properties, satu-

*Ms* = (1 − *p*) 4 *Mo* (3)

conclude that saturation magnetization is porosity dependent while coercivity is size dependent. Previous study proved that the independence of coercivity from porosity, while saturation magnetization and remanence are independent from grain size effect [10]. In addition to porosity, other defects such as cracks, inclusions, foreign phases, strains, as well as dislocations would alter the magnetic behaviors of ferrites. Defects act as energy wells have a strong pinning effect on the domain wall motion and thus require higher activation energy to detach [4].

It is believed that boundary region possesses higher energy compared to volume defects. Therefore, boundary region is a highly reactive region, which allows nucleation of new phases. As nanostructured materials have higher surface-to-volume ratio, they are reactive compared to their corresponding bulk materials. In ceramic materials, boundary region between phases and grains governs many properties and processes, for example, as fracture strength, plastic deformation, conductivity, dielectric loss, and phase transformation. All materials have interfacial energy and tension that can be calculated by same thermodynamic formulation [11]. Boundaries act as sinks and sources for the formation of lattice imperfections, diffusion, and phase transformations when deformation occurs. Some behaviors of ceramic oxides such as coercivity and permeability are strongly related to their boundaries [12]. The direction of magnetic moments within the material could be changed easily when the pinning effects of the boundary regions is diminished. Apparent permeability can be expressed as following [13]:

(<sup>1</sup> <sup>+</sup> *<sup>p</sup>*\_\_

<sup>2</sup>)(<sup>1</sup> <sup>+</sup> 0.75 \_\_*<sup>t</sup>*

where *p* is the porosity, *D* is the average grain size, *t* is the effective thickness of boundary region,

*D μ*\_\_*o μb*)

is the permeability free form the demagnetizing

(4)

is the magnetization extrapolated to zero porosity. Therefore, we can

ration magnetization is associated with the following equation [10]:

*μapp* <sup>=</sup> (1 <sup>−</sup> *<sup>p</sup>*) *<sup>μ</sup>* \_\_\_\_\_\_\_\_\_\_\_\_\_\_ *<sup>o</sup>*

is the permeability of the boundary region, *μo*

**2.2. Defects and porosity**

28 Sintering Technology - Method and Application

where p is the porosity, M<sup>o</sup>

**2.3. Boundary region**

*μb*

Attention has been paid to investigate synthesis techniques and their impacts on new materials, particularly nanostructured and nanocrystalline materials. Synthesis technique is strongly related to behaviors of the investigated nanomaterial because the chosen synthesis technique is responsible for tailoring the atomic and microstructure of the nanostructured material. Numerous published studies have improved our understanding of the effects of synthesis technique on the behaviors of complex magnetic ceramic oxide, especially technologically important hard and soft ferrites [5, 14–16]. Most of the significant findings show that the results are of limited significance unless the microstructures, chemical composition, defects, and atomic arrangement of the investigated ferrite are well-characterized. Generally speaking, the techniques of preparing ferrite are categorized into two: bottom-up and top-down approaches, as shown in **Figure 2**. Bottom-up approach synthesis is a ceramic powder processing approach that engages atoms, ions, molecules or particles as starting building blocks.

**Figure 2.** Schematic representations of (a) bottom-up and (b) top-down approaches.

By combining or assembling these building blocks, nanoscale clusters, or corresponding bulk materials are formed. Top-down approach synthesis is a ceramic powder processing approach that begins with micro-structured materials. The approach utilizes mechanical, chemical, or other form of energy to perform structural decomposition to obtain nanoscale materials. Both approaches have its advantages and drawback. For instance, bottom-up approach synthesis such as chemical processes and solid-state routes are capable of producing fine nanocrystalline materials with high purity and homogeneity. However, they have disadvantages like not environmental friendly, high cost of chemical precursors, solvent evaporation, and necessity for thermal treatment at high temperature. On the other hand, top-down approach synthesis like mechanochemical process is considered as green process because it minimizes damage to the environment, fast, economical, and can effectively take nanostructure forms [2]. However, contaminations, defects, and damages that were induced into the material system need to carefully take into account for good material production [15].

**5. Sintering of mechanically activated ferrite powders**

heat treatment, typically with the use of a furnace to obtain the final product.

**6. Experimental results of sintering of mechanically activated soft** 

O4

existed as secondary phase at 600°C. The [121] peak shows the existence of secondary

O3

O4

sented in **Figure 3**. In view of the results obtained, the occurrence of [121] peak in 600°C spectrum indicated incomplete reaction between raw materials to form a single phase powder.

fused into the tetrahedral sites while Ni2+ ions occupied the octahedral sites. As the starting powders were mechanically activated by high-energy ball milling by SPEX is the modal name of the dual mixer machine. Which was specially modified to achieve high speeds (approximately 1725 rpm) for the effective production of nanostructured particles; this enables the

phase. The α-Fe<sup>2</sup>

after sintering from 600 to 1200°C are pre-

phase disappeared when the sintering

was formed as Zn2+ ions dif-

**ferrite Ni0.5Zn0.5Fe2**

O3

α-Fe<sup>2</sup> O3

phase α-Fe<sup>2</sup>

**6.1. Microstructural properties**

**O4**

X-ray Diffraction (XRD) spectra of Ni0.5Zn0.5Fe2

in the Ni0.5Zn0.5Fe2

O4

temperature was increased to 700°C. A complete Ni0.5Zn0.5Fe2

Top-down and bottom-up approaches have their own advantages and drawback as mentioned in the previous section. Conventional solid-state process is a bottom-up approach ceramic processing method that involves neither wet chemical reactions nor vapor phase interactions. There are two important processing steps that will affect the quality of the end product: starting powder preparation and heat treatment. The solid-state process is considered as the simplest synthesis route for various ferrites. In the starting powder preparation stage, high-purity raw materials would mix together according to the stoichiometric balance of the final product. This mixing process is being carried out by either dry or wet milling media for a certain period to produce a homogenous distributed starting powder. Then, the starting powder will undergo a

Sintering Processing of Complex Magnetic Ceramic Oxides: A Comparison Between Sintering…

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31

The conventional solid-state process is capable of producing advanced material with unique compositions such as refractory ceramics, glasses, and crystals. Previous research showed that conventional solid-state process was capable of producing particles between 100 nm and 1 micron [16]. However, conventional solid-state process may result in high synthesis temperature because diffusion reaction is limited under low temperature. Besides, this process may produce an incomplete reaction, which results in inhomogeneous products. Other issues of using this process are lack of control of the kinetics and the difficulties of producing desired end products [17]. In order to overcome the drawback of conventional solid-state process, the implementation of mechanical alloying in the starting powder preparation is recommended by many researchers. Apart from the practicality, mechanically activated starting powders exhibit nanostructures and high reactivity. Therefore, it provides an easy, fast, and economical option to produce the desired material. Previous studies showed that starting powder synthesized via mechanical alloying, had a relatively low sintering temperature for the formation of pure, single phase material [5, 7, 18, 19].

### **4. Ferrites**

Ferrites belong to a class of complex magnetic ceramic oxide. The crystal structure of ferrites can be observed as an interlocking network of cations and negatively charged divalent oxygen ions [4]. When a layer of oxygen ions is closely packed lines that connect the centers of these oxygen ions will form a network of equilateral triangles. The second layer of closely packed oxygen ions is arranged in such a way that the centers of these oxygen ions are superimposed with the centers of the equilateral triangles of the first layer. If a similar third layer repeats the same arrangement with the first layer, this arrangement is known as hexagonal close-packed structure in the type of "ababab" stacking sequence. On the other hand, if the third layer arranges in such a way that the centers of the oxygen lie directly over the centers of equilateral triangles adjacent to the ones used for hexagonal close-packed, this will produce a cubic closepacked with a stacking sequence of "abcabc." Then, ferrites are further categorized according to their molar ratio of Fe2 O3 to other oxide components (modifier oxide) present in the ceramic as presented in **Table 1**.


**Table 1.** Classification of ferrites according to variation in molar ratio of Fe<sup>2</sup> O3 to modifier oxide.

### **5. Sintering of mechanically activated ferrite powders**

By combining or assembling these building blocks, nanoscale clusters, or corresponding bulk materials are formed. Top-down approach synthesis is a ceramic powder processing approach that begins with micro-structured materials. The approach utilizes mechanical, chemical, or other form of energy to perform structural decomposition to obtain nanoscale materials. Both approaches have its advantages and drawback. For instance, bottom-up approach synthesis such as chemical processes and solid-state routes are capable of producing fine nanocrystalline materials with high purity and homogeneity. However, they have disadvantages like not environmental friendly, high cost of chemical precursors, solvent evaporation, and necessity for thermal treatment at high temperature. On the other hand, top-down approach synthesis like mechanochemical process is considered as green process because it minimizes damage to the environment, fast, economical, and can effectively take nanostructure forms [2]. However, contaminations, defects, and damages that were induced into the material system need to

Ferrites belong to a class of complex magnetic ceramic oxide. The crystal structure of ferrites can be observed as an interlocking network of cations and negatively charged divalent oxygen ions [4]. When a layer of oxygen ions is closely packed lines that connect the centers of these oxygen ions will form a network of equilateral triangles. The second layer of closely packed oxygen ions is arranged in such a way that the centers of these oxygen ions are superimposed with the centers of the equilateral triangles of the first layer. If a similar third layer repeats the same arrangement with the first layer, this arrangement is known as hexagonal close-packed structure in the type of "ababab" stacking sequence. On the other hand, if the third layer arranges in such a way that the centers of the oxygen lie directly over the centers of equilateral triangles adjacent to the ones used for hexagonal close-packed, this will produce a cubic closepacked with a stacking sequence of "abcabc." Then, ferrites are further categorized according

to other oxide components (modifier oxide) present in the ceramic

metal oxide. Example: BaO, SrO

Example: NiO, ZnO

O3

to modifier oxide.

**Modifier oxide Example**

BaFe12O19

Ni0.5Zn0.5Fe2

Fe3 (FeO4 )3

O4

carefully take into account for good material production [15].

**4. Ferrites**

30 Sintering Technology - Method and Application

to their molar ratio of Fe2

as presented in **Table 1**.

O3

**Fe2 O3**

Magnetoplumbite Hexagonal 6:1 Group IIA divalent

Spinel Cubic 1:1 Transition metal oxide.

**Table 1.** Classification of ferrites according to variation in molar ratio of Fe<sup>2</sup>

Garnet Cubic 3:5 Rare earth oxide Y3

**oxide**

 **to modifier** 

**Type Structure Molar ratio of** 

Top-down and bottom-up approaches have their own advantages and drawback as mentioned in the previous section. Conventional solid-state process is a bottom-up approach ceramic processing method that involves neither wet chemical reactions nor vapor phase interactions. There are two important processing steps that will affect the quality of the end product: starting powder preparation and heat treatment. The solid-state process is considered as the simplest synthesis route for various ferrites. In the starting powder preparation stage, high-purity raw materials would mix together according to the stoichiometric balance of the final product. This mixing process is being carried out by either dry or wet milling media for a certain period to produce a homogenous distributed starting powder. Then, the starting powder will undergo a heat treatment, typically with the use of a furnace to obtain the final product.

The conventional solid-state process is capable of producing advanced material with unique compositions such as refractory ceramics, glasses, and crystals. Previous research showed that conventional solid-state process was capable of producing particles between 100 nm and 1 micron [16]. However, conventional solid-state process may result in high synthesis temperature because diffusion reaction is limited under low temperature. Besides, this process may produce an incomplete reaction, which results in inhomogeneous products. Other issues of using this process are lack of control of the kinetics and the difficulties of producing desired end products [17]. In order to overcome the drawback of conventional solid-state process, the implementation of mechanical alloying in the starting powder preparation is recommended by many researchers. Apart from the practicality, mechanically activated starting powders exhibit nanostructures and high reactivity. Therefore, it provides an easy, fast, and economical option to produce the desired material. Previous studies showed that starting powder synthesized via mechanical alloying, had a relatively low sintering temperature for the formation of pure, single phase material [5, 7, 18, 19].

### **6. Experimental results of sintering of mechanically activated soft ferrite Ni0.5Zn0.5Fe2 O4**

### **6.1. Microstructural properties**

X-ray Diffraction (XRD) spectra of Ni0.5Zn0.5Fe2 O4 after sintering from 600 to 1200°C are presented in **Figure 3**. In view of the results obtained, the occurrence of [121] peak in 600°C spectrum indicated incomplete reaction between raw materials to form a single phase powder. α-Fe<sup>2</sup> O3 existed as secondary phase at 600°C. The [121] peak shows the existence of secondary phase α-Fe<sup>2</sup> O3 in the Ni0.5Zn0.5Fe2 O4 phase. The α-Fe<sup>2</sup> O3 phase disappeared when the sintering temperature was increased to 700°C. A complete Ni0.5Zn0.5Fe2 O4 was formed as Zn2+ ions diffused into the tetrahedral sites while Ni2+ ions occupied the octahedral sites. As the starting powders were mechanically activated by high-energy ball milling by SPEX is the modal name of the dual mixer machine. Which was specially modified to achieve high speeds (approximately 1725 rpm) for the effective production of nanostructured particles; this enables the

**Figure 3.** X-ray diffraction patterns of Ni0.5Zn0.5Fe2 O4 sintered from 600 to 1200°C.

formation of single phase at a lower sintering temperature. It is worth mentioning that the synthesis temperature for single phase Ni-Zn ferrite for refluxing method is between 950 and 1150°C [20]; sol–gel technique requires more than 1000°C [21]; co-precipitation method requires 550–1000°C [22]. The intensity of the Bragg peaks increased, and the peak widths decreased with increasing sintering temperature indicating the increase of crystallinity and particle size.

Structural information was obtained from Rietveld refinement. The increase of lattice parameters and unit cell volume was observed. As shown in **Figure 4**, as the sintering temperature increased, the unit cell volume expanded, and Zn2+ ions diffused into the interstitial sites; this was crucial for the reaction as interstitial diffusion is the most important lattice diffusion mechanism [1]. Further increasing of sintering temperature (>900°C), a decrease in lattice parameters and unit cell volume was observed. This could be due to the small amount of Zn2+ ions evaporated from the lattice [8]. This is because zinc has a low-boiling point of 907°C. Mechanically activated starting material has high lattice strain as defects and inhomogeneity could be introduced into the system. This is known as the second order stress, which it modifies the materials by one grain to another or from one part of the grain to another on a microscopic scale. There was also first order stress induced by milling. This type of stress modifies the material uniformly across the entire material [23], causing a macroscopic variation on the material. By increasing the sintering temperature, relaxation can be attained for macro and micro stresses induced during milling.

**Figure 5** shows the evolution of particle size, crystallite size, and morphological properties

**Figure 5.** Average particle size and crystallite size as a function of sintering temperature; the evolution of morphology is

shown in the inserted field emission scanning electron microscope (FESEM) micrographs.

observed for samples sintered at 600-, 700-, and 800°C. Phenomena such as rearrangement

with elevating sintering temperature. As a whole, bottom-up synthesis of

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acquires three stages of sintering. Initial stage of sintering can be

of Ni0.5Zn0.5Fe2

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**Figure 4.** Unit cell volume and lattice strain as a function of sintering temperature.

soft ferrite, Ni0.5Zn0.5Fe2

Sintering Processing of Complex Magnetic Ceramic Oxides: A Comparison Between Sintering… http://dx.doi.org/10.5772/intechopen.78654 33

**Figure 4.** Unit cell volume and lattice strain as a function of sintering temperature.

**Figure 3.** X-ray diffraction patterns of Ni0.5Zn0.5Fe2

32 Sintering Technology - Method and Application

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sintered from 600 to 1200°C.

formation of single phase at a lower sintering temperature. It is worth mentioning that the synthesis temperature for single phase Ni-Zn ferrite for refluxing method is between 950 and 1150°C [20]; sol–gel technique requires more than 1000°C [21]; co-precipitation method requires 550–1000°C [22]. The intensity of the Bragg peaks increased, and the peak widths decreased with increasing sintering temperature indicating the increase of crystallinity and particle size. Structural information was obtained from Rietveld refinement. The increase of lattice parameters and unit cell volume was observed. As shown in **Figure 4**, as the sintering temperature increased, the unit cell volume expanded, and Zn2+ ions diffused into the interstitial sites; this was crucial for the reaction as interstitial diffusion is the most important lattice diffusion mechanism [1]. Further increasing of sintering temperature (>900°C), a decrease in lattice parameters and unit cell volume was observed. This could be due to the small amount of Zn2+ ions evaporated from the lattice [8]. This is because zinc has a low-boiling point of 907°C. Mechanically activated starting material has high lattice strain as defects and inhomogeneity could be introduced into the system. This is known as the second order stress, which it modifies the materials by one grain to another or from one part of the grain to another on a microscopic scale. There was also first order stress induced by milling. This type of stress modifies the material uniformly across the entire material [23], causing a macroscopic variation on the material. By increasing the sintering temperature, relaxation can be attained for macro and micro stresses induced during milling.

**Figure 5.** Average particle size and crystallite size as a function of sintering temperature; the evolution of morphology is shown in the inserted field emission scanning electron microscope (FESEM) micrographs.

**Figure 5** shows the evolution of particle size, crystallite size, and morphological properties of Ni0.5Zn0.5Fe2 O4 with elevating sintering temperature. As a whole, bottom-up synthesis of soft ferrite, Ni0.5Zn0.5Fe2 O4 acquires three stages of sintering. Initial stage of sintering can be observed for samples sintered at 600-, 700-, and 800°C. Phenomena such as rearrangement of particles and necking structure can be observed at this stage. At the intermediate stage (900-, 1000-, and 1100°C), further increase of sintering provides sufficient thermal energy for nanoparticles to move closer. Grain boundaries are formed. However, the most significant observation for intermediate stage is the formation of interconnected pores. Finally, the sample sintered at 1200°C exhibited the final stage of sintering. Isolated pores are observed, and rigid crystal structure is visible. The coarsening and densification of particles are observed with increasing sintering temperature.

are in tangential contact. This is to activate the material transport mechanism through diffusion. During this process, necking structures are formed between particles. At intermediate stage, densification occurs and the pores shrink to reduce their cross-section. As a result, interconnected pores are formed at the boundary regions. Densification and coarsening continue to occur; eventually, the pores become unstable and isolated at the final stage of sintering [1].

High resolution transmission electron microscopy (HRTEM) is utilized to identify some unique features of each stage in terms of atomic arrangement, structural information, and defects like grain boundaries. In **Figure 7a**, a lattice spacing of 2.53 Å was measured for

such a way that they are in tangential contact. The contact points between particles are the material transport paths that allow diffusions to occur at early stage of sintering. In **Figure 7b**, it can be seen that the spheres begin to coalesce. The radius of the necking structure has reached a value of >0.50 of the particle radius. This indicated that at sintering temperature of

In **Figure 8a**, it can be observed that two particles were brought together, and they are undergoing deformation in response to surface energy reduction. Massive lattice diffusion and material transport occur between these particles. In **Figure 8b**, grains adopt the shape of polyhedron with multiple faces, and the edge of the particle appears to have a clean

800°C, the particles are near the end of an initial stage of sintering [1].

**Figure 7.** High resolution transmission electron microscopy (TEM) images for Ni0.5Zn0.5Fe2

(a) 600°C, and (b) 800°C (initial stage of sintering).

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nanoparticles sintered at

, corresponding to (113) lattice plane. A few particles rearrange themselves in

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Ni0.5Zn0.5Fe2

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### **6.2. Three stages of sintering**

The activation energy of particle growth of sintering is strongly related to the size evolution of the particles [24]. Size-dependent activation energy can be represented by the plot of log particle size (*D*) versus the reciprocal of absolute temperature (*1/T*) of Ni0.5Zn0.5Fe2 O4 [25]. Three distinct stages of sintering can be observed in **Figure 6**. Activation energy is the lowest at initial stage, indicating the particles are nano-sized, which exhibit relatively largest surface area. Small thermal energy is enough to initiate the particle growth. Through intermediate and final stages, particle size increases, therefore, the activation energy for particle growth increases hence higher thermal energy is required for the densification and coarsening mechanisms in sintering [26]. As a summary during initial stage, particles rearranged themselves so that they

**Figure 6.** Plot of log D versus the reciprocal of absolute temperature (1/T) of Ni0.5Zn0.5Fe2 O4 showing three stages of sintering.

are in tangential contact. This is to activate the material transport mechanism through diffusion. During this process, necking structures are formed between particles. At intermediate stage, densification occurs and the pores shrink to reduce their cross-section. As a result, interconnected pores are formed at the boundary regions. Densification and coarsening continue to occur; eventually, the pores become unstable and isolated at the final stage of sintering [1].

High resolution transmission electron microscopy (HRTEM) is utilized to identify some unique features of each stage in terms of atomic arrangement, structural information, and defects like grain boundaries. In **Figure 7a**, a lattice spacing of 2.53 Å was measured for Ni0.5Zn0.5Fe2 O4 , corresponding to (113) lattice plane. A few particles rearrange themselves in such a way that they are in tangential contact. The contact points between particles are the material transport paths that allow diffusions to occur at early stage of sintering. In **Figure 7b**, it can be seen that the spheres begin to coalesce. The radius of the necking structure has reached a value of >0.50 of the particle radius. This indicated that at sintering temperature of 800°C, the particles are near the end of an initial stage of sintering [1].

In **Figure 8a**, it can be observed that two particles were brought together, and they are undergoing deformation in response to surface energy reduction. Massive lattice diffusion and material transport occur between these particles. In **Figure 8b**, grains adopt the shape of polyhedron with multiple faces, and the edge of the particle appears to have a clean

**Figure 7.** High resolution transmission electron microscopy (TEM) images for Ni0.5Zn0.5Fe2 O4 nanoparticles sintered at (a) 600°C, and (b) 800°C (initial stage of sintering).

**Figure 6.** Plot of log D versus the reciprocal of absolute temperature (1/T) of Ni0.5Zn0.5Fe2

of particles and necking structure can be observed at this stage. At the intermediate stage (900-, 1000-, and 1100°C), further increase of sintering provides sufficient thermal energy for nanoparticles to move closer. Grain boundaries are formed. However, the most significant observation for intermediate stage is the formation of interconnected pores. Finally, the sample sintered at 1200°C exhibited the final stage of sintering. Isolated pores are observed, and rigid crystal structure is visible. The coarsening and densification of particles are observed

The activation energy of particle growth of sintering is strongly related to the size evolution of the particles [24]. Size-dependent activation energy can be represented by the plot of log

Three distinct stages of sintering can be observed in **Figure 6**. Activation energy is the lowest at initial stage, indicating the particles are nano-sized, which exhibit relatively largest surface area. Small thermal energy is enough to initiate the particle growth. Through intermediate and final stages, particle size increases, therefore, the activation energy for particle growth increases hence higher thermal energy is required for the densification and coarsening mechanisms in sintering [26]. As a summary during initial stage, particles rearranged themselves so that they

particle size (*D*) versus the reciprocal of absolute temperature (*1/T*) of Ni0.5Zn0.5Fe2

with increasing sintering temperature.

34 Sintering Technology - Method and Application

**6.2. Three stages of sintering**

sintering.

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showing three stages of

O4 [25].

**Figure 8.** High resolution TEM images for Ni0.5Zn0.5Fe2 O4 nanoparticles sintered at (a) 900°C, (b) 1100°C (intermediate stage of sintering), and (c) 1200°C (final stage of sintering).

crystalline surface, where amorphous phase diminishes at 1100°C. In the final stage of sintering (**Figure 8c**), a homogeneous atomic arrangement, with (113) lattice plane is formed.

anisotropy is enhanced. To change the orientation of magnetic moment, higher energy is required

temperature, (b) plot of M10kOe versus sintering temperature, (c) plot of coercivity versus sintering temperature.

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Sintering Processing of Complex Magnetic Ceramic Oxides: A Comparison Between Sintering…

favorable in terms of energy level [27]. In order to reduce the overall energy of the system, domain

In **Figure 10**, a red shift of optical property is observed with increasing sintering temperature. It can be seen that the increase of crystallite is accompanied with the decrease of optical bandgap values (red-shift). This is thought to be due to size-dependent quantum confinement effect. Quantum confinement effect can be observed when the crystallite size is in the same order as the wavelength of the electron. The energy level at the microscopic level can be

where *h* is the Planck constant, *k* is the wave factor (*k = 2π/λ*), *m* is the mass of electron. When

where *a* is the crystallite size of the material and *n* is an integer. Based on Eqs. 5 and 6, the value of wave vector *k* has an inversely proportional relationship with the crystallite size. The crystallite size increased with increasing sintering temperature resulting in decrease of wave vector *k* value. When we substitute n = 1, 2, 3, and so on, for Eqs. 5 and 6, the difference between two consecutive energy becomes smaller. Therefore, energy bandgap values decrease with increasing crystallite size. This phenomenon happens when the motion of electrons is restricted in a nano-scale size.

*<sup>λ</sup>* <sup>=</sup> \_\_\_ *n* coercivity increases

, high magnetocrystalline anisotropy is not

: (a) hysteresis loops at different sintering

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37

<sup>2</sup>*<sup>m</sup>* (5)

*<sup>a</sup>* (6)

to overcome this magnetocrystalline anisotropy energy. Therefore, below *Dc*,

the crystallite size is small, the wave vector *k* can be expressed as [28]:

with increasing average particle size. Above this *Dc*

**Figure 9.** Magnetic parameters of bottom-up synthesis Ni0.5Zn0.5Fe2

walls are created causing the coercivity to decrease.

*<sup>E</sup>* <sup>=</sup> (*hk*)<sup>2</sup> \_\_\_\_

*k* = \_\_\_ <sup>2</sup>*<sup>π</sup>*

described by the expression [28]:

### **6.3. Microstructural related properties of sintered Ni0.5Zn0.5Fe2 O4**

**Figure 9a** shows the M-H hysteresis loops of Ni0.5Zn0.5Fe2 O4 sintered at various temperatures. The magnetic parameters are extracted from hysteresis loops. All the samples sintered from 600 to 1200°C exhibited less slanting, narrow sigmoid hysteresis loop. This indicates that the preparation of raw powder with modified high-speed mechanical alloying increases the reactivity of nanoparticles. Ferromagnetic phase exists in the sample even at low sintering temperatures such as 600 and 700°C. **Figure 9b** shows the plot of maximum magnetization at 10 kOe, *M10kOe* against sintering temperature. In view of the results obtained, the *M10kOe* values increase with increasing sintering temperature. At low sintering temperature, small particles exhibit surface distortion due to the interaction of transition metal ions in the lattice with oxygen atoms, causing a reduction in the resultant magnetic moment. This phenomenon is normally predominant in ultrafine particles because of their large surface to volume ratio. This effect becomes less influential at high sintering temperature as particle size increases. Volume fraction of amorphous phase decreases with increasing sintering temperature. Thus, the exchange interaction between particles increases with increasing volume fraction of crystalline phase. As a result, strong ferromagnetic behaviors are strengthened with erect, narrow, and well-defined sigmoid hysteresis loops are observed with increasing temperature. Coercivity has an indirect relationship with particle size. At low sintering temperature, there are amorphous phase and defects like grain boundaries in the sample. Therefore, the magnetocrystalline anisotropy is low because the crystalline volume fraction is low. Below a critical size (*Dc* ≈ 90 nm), coercivity increases with average particle size. As the sintering temperature increases, the crystalline volume fraction increases, the magnetocrystalline Sintering Processing of Complex Magnetic Ceramic Oxides: A Comparison Between Sintering… http://dx.doi.org/10.5772/intechopen.78654 37

**Figure 9.** Magnetic parameters of bottom-up synthesis Ni0.5Zn0.5Fe2 O4 : (a) hysteresis loops at different sintering temperature, (b) plot of M10kOe versus sintering temperature, (c) plot of coercivity versus sintering temperature.

**Figure 8.** High resolution TEM images for Ni0.5Zn0.5Fe2

36 Sintering Technology - Method and Application

stage of sintering), and (c) 1200°C (final stage of sintering).

**6.3. Microstructural related properties of sintered Ni0.5Zn0.5Fe2**

**Figure 9a** shows the M-H hysteresis loops of Ni0.5Zn0.5Fe2

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crystalline surface, where amorphous phase diminishes at 1100°C. In the final stage of sintering (**Figure 8c**), a homogeneous atomic arrangement, with (113) lattice plane is formed.

magnetic parameters are extracted from hysteresis loops. All the samples sintered from 600 to 1200°C exhibited less slanting, narrow sigmoid hysteresis loop. This indicates that the preparation of raw powder with modified high-speed mechanical alloying increases the reactivity of nanoparticles. Ferromagnetic phase exists in the sample even at low sintering temperatures such as 600 and 700°C. **Figure 9b** shows the plot of maximum magnetization at 10 kOe, *M10kOe* against sintering temperature. In view of the results obtained, the *M10kOe* values increase with increasing sintering temperature. At low sintering temperature, small particles exhibit surface distortion due to the interaction of transition metal ions in the lattice with oxygen atoms, causing a reduction in the resultant magnetic moment. This phenomenon is normally predominant in ultrafine particles because of their large surface to volume ratio. This effect becomes less influential at high sintering temperature as particle size increases. Volume fraction of amorphous phase decreases with increasing sintering temperature. Thus, the exchange interaction between particles increases with increasing volume fraction of crystalline phase. As a result, strong ferromagnetic behaviors are strengthened with erect, narrow, and well-defined sigmoid hysteresis loops are observed with increasing temperature. Coercivity has an indirect relationship with particle size. At low sintering temperature, there are amorphous phase and defects like grain boundaries in the sample. Therefore, the magnetocrystalline anisotropy is low because the crystalline volume fraction is low. Below a critical size (*Dc* ≈ 90 nm), coercivity increases with average particle size. As the sintering temperature increases, the crystalline volume fraction increases, the magnetocrystalline

nanoparticles sintered at (a) 900°C, (b) 1100°C (intermediate

sintered at various temperatures. The

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anisotropy is enhanced. To change the orientation of magnetic moment, higher energy is required to overcome this magnetocrystalline anisotropy energy. Therefore, below *Dc*, coercivity increases with increasing average particle size. Above this *Dc* , high magnetocrystalline anisotropy is not favorable in terms of energy level [27]. In order to reduce the overall energy of the system, domain walls are created causing the coercivity to decrease.

In **Figure 10**, a red shift of optical property is observed with increasing sintering temperature. It can be seen that the increase of crystallite is accompanied with the decrease of optical bandgap values (red-shift). This is thought to be due to size-dependent quantum confinement effect. Quantum confinement effect can be observed when the crystallite size is in the same order as the wavelength of the electron. The energy level at the microscopic level can be described by the expression [28]:

$$E = \frac{(hk)^2}{2m} \tag{5}$$

where *h* is the Planck constant, *k* is the wave factor (*k = 2π/λ*), *m* is the mass of electron. When the crystallite size is small, the wave vector *k* can be expressed as [28]:

$$k = \frac{2\pi}{\lambda} = \frac{n\pi}{d} \tag{6}$$

where *a* is the crystallite size of the material and *n* is an integer. Based on Eqs. 5 and 6, the value of wave vector *k* has an inversely proportional relationship with the crystallite size. The crystallite size increased with increasing sintering temperature resulting in decrease of wave vector *k* value. When we substitute n = 1, 2, 3, and so on, for Eqs. 5 and 6, the difference between two consecutive energy becomes smaller. Therefore, energy bandgap values decrease with increasing crystallite size. This phenomenon happens when the motion of electrons is restricted in a nano-scale size.

**Figure 10.** Optical properties of Ni0.5Zn0.5Fe2 O4 nanoparticles sintered at different sintering temperature.

### **7. Comparative study of bottom-up and top-down approach synthesis**

Sample with similar particle size, synthesized via mechanochemical process with optimized parameters [29] is chosen as a candidate for this comparative study with two parameters were chosen, which were milling at 8 hours (top-down approach) and sintering synthesis at 900°C (bottom-up approach). **Figure 11** shows the XRD diffraction patterns milled at 8 hours and sintered at 900°C Ni0.5Zn0.5Fe2 O4 nanoparticles. Nanoparticles that milled 8 hours exhibit a superimposition of broad diffraction reflections on the broad diffraction maximum or "hump," indicating the presence of a highly disordered phase. Nanoparticles that sintered at 900°C exhibit a single phase pattern with sharp Braggs peaks.

**Figure 12.** shows the field emission scanning electron microscope (FESEM) micrographs and particle size distribution for nanoparticles synthesized by different synthesis approaches. As can be seen, nanoparticles that sintered at 900°C have a narrower size distribution compared to nanoparticles that milled at 8 hours. Commercial nanoparticles are uniform in size. Densification mechanism of sintering can be seen in **Figure 12b**. Small and large particles coexisted for both bottom-up and top-down approaches synthesized nanoparticles. However, particles with rigid and clear grain boundaries can be observed in sintered particles while top-down approach synthesized particles are agglomerated particles with randomly shaped boundaries.

**Figure 13** shows the M-H hysteresis loops of Ni0.5Zn0.5Fe2 O4 nanoparticles synthesized via different synthesis approaches. Nanoparticles that milled at 8 hours exhibited complex disordering in structure. Therefore, it possesses canted spin arrangement that has significant implications on its magnetism. The maximum magnetization at 10 kOe is lower compared to nanoparticles that sintered at 900°C. On the other hand, nanoparticles that sintered at 900°C exhibited low coercivity with high saturation magnetization (the magnetization at 10 kOe had saturated). This indicated that the formation of single phase nickel zinc ferrite that exhibits soft ferrite magnetic properties. The optical bandgap values were 1.39–1.30 eV for sintered at 900°C and milled at 8 hours nanoparticles, respectively. Both bottom-up and top-down approaches synthesized nanoparticles exhibit same order optical bandgap value. It is evident that optical bandgap is a size-dependent behavior. However, defects that induced during mechanochemical process reduced the optical bandgap value of nanoparticles that milled at

**Figure 12.** FESEM micrographs and particle size distribution for (a) milled at 8 hours and (b) sintered at 900°C.

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nanoparticles synthesized by different synthesis approaches.

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8 hours. This is attributed to structural disorder bandgap narrowing effect.

**Figure 11.** X-ray diffraction patterns of Ni0.5Zn0.5Fe2

Sintering Processing of Complex Magnetic Ceramic Oxides: A Comparison Between Sintering… http://dx.doi.org/10.5772/intechopen.78654 39

**Figure 11.** X-ray diffraction patterns of Ni0.5Zn0.5Fe2 O4 nanoparticles synthesized by different synthesis approaches.

**7. Comparative study of bottom-up and top-down approach synthesis**

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900°C exhibit a single phase pattern with sharp Braggs peaks.

**Figure 13** shows the M-H hysteresis loops of Ni0.5Zn0.5Fe2

and sintered at 900°C Ni0.5Zn0.5Fe2

**Figure 10.** Optical properties of Ni0.5Zn0.5Fe2

38 Sintering Technology - Method and Application

boundaries.

Sample with similar particle size, synthesized via mechanochemical process with optimized parameters [29] is chosen as a candidate for this comparative study with two parameters were chosen, which were milling at 8 hours (top-down approach) and sintering synthesis at 900°C (bottom-up approach). **Figure 11** shows the XRD diffraction patterns milled at 8 hours

a superimposition of broad diffraction reflections on the broad diffraction maximum or "hump," indicating the presence of a highly disordered phase. Nanoparticles that sintered at

**Figure 12.** shows the field emission scanning electron microscope (FESEM) micrographs and particle size distribution for nanoparticles synthesized by different synthesis approaches. As can be seen, nanoparticles that sintered at 900°C have a narrower size distribution compared to nanoparticles that milled at 8 hours. Commercial nanoparticles are uniform in size. Densification mechanism of sintering can be seen in **Figure 12b**. Small and large particles coexisted for both bottom-up and top-down approaches synthesized nanoparticles. However, particles with rigid and clear grain boundaries can be observed in sintered particles while top-down approach synthesized particles are agglomerated particles with randomly shaped

different synthesis approaches. Nanoparticles that milled at 8 hours exhibited complex disordering in structure. Therefore, it possesses canted spin arrangement that has significant implications on its magnetism. The maximum magnetization at 10 kOe is lower compared to nanoparticles that sintered at 900°C. On the other hand, nanoparticles that sintered at 900°C exhibited low coercivity with high saturation magnetization (the magnetization at 10 kOe had

nanoparticles. Nanoparticles that milled 8 hours exhibit

nanoparticles sintered at different sintering temperature.

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nanoparticles synthesized via

**Figure 12.** FESEM micrographs and particle size distribution for (a) milled at 8 hours and (b) sintered at 900°C.

saturated). This indicated that the formation of single phase nickel zinc ferrite that exhibits soft ferrite magnetic properties. The optical bandgap values were 1.39–1.30 eV for sintered at 900°C and milled at 8 hours nanoparticles, respectively. Both bottom-up and top-down approaches synthesized nanoparticles exhibit same order optical bandgap value. It is evident that optical bandgap is a size-dependent behavior. However, defects that induced during mechanochemical process reduced the optical bandgap value of nanoparticles that milled at 8 hours. This is attributed to structural disorder bandgap narrowing effect.

**Author details**

Zhi Huang Low<sup>1</sup>

Malaysia

**References**

Ni0.3Zn0.7Fe2

3787-3794

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2012;**258**(7):2679-2685

, Ismayadi Ismail<sup>1</sup>

ders. Boca Raton, Florida: CRC Press; 2010

\*Address all correspondence to: ismayadi@upm.edu.my

Universiti Putra, Malaysia, Serdang, Selangor Darul Ehsan, Malaysia

[1] Rahaman MN. Sintering of Ceramics. Boca Raton: CRC Press; 2007

\* and Kim Song Tan2

Sintering Processing of Complex Magnetic Ceramic Oxides: A Comparison Between Sintering…

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41

1 Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology,

[2] Sopicka-Lizer M. High-Energy Ball Milling Mechanochemical Processing of Nanopow-

[3] Holdren JP. Science and technology for sustainable well-being. Science. 2008;**319**:424-434 [4] Goldman A. Modern Ferrite Technology. 2nd ed. Pittsburgh, PA, USA: Springer; 2006 [5] Idza IR, Hashim M, Rodziah N, Ismayadi I, Norailiana AR. Influence of evolving microstructure on magnetic-hysteresis characteristics in polycrystalline nickel–zinc ferrite,

. Materials Research Bulletin. 2012;**47**(6):1345-1352

[6] Rodziah N, Hashim M, Idza IR, Ismayadi I, Hapishah AN, Khamirul MA. Applied surface science dependence of developing magnetic hysteresis characteristics on stages of evolving microstructure in polycrystalline yttrium iron garnet. Applied Surface Science.

[7] Shafie MSE, Hashim M, Ismail I, Kanagesan S, Fadzidah MI, Idza IR, Hajalilou A, Sabbaghizadeh R. Magnetic M–H loops family characteristics in the microstructure evolution of BaFe12O19. Journal of Materials Science: Materials in Electronics. 2014;**25**(9):

[8] Ismail I, Hashim M, Matori KA, Alias R, Hassan J. The transition from paramagnetic to

[10] Igarashi H, Okazaki K. Effects of porosity and grain size on the magnetic properties of

[11] Kingery WD. Plausible concepts necessary and sufficient for interpretation of ceramic grain-boundary phenomena: I, grain-boundary characteristics, structure, and electro-

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ferromagnetic states as influenced by evolving microstructure of Ni0.5Zn0.5Fe2

[9] Bera J, Roy PK. Effect of grain size on electromagnetic properties of Ni0.7Zn0.3Fe2

NiZn ferrite. Journal of the American Ceramic Society. 1977;**60**(1-2):51-54

static potential. Journal of the American Ceramic Society. 1974;**57**(1):1-8

of Superconductivity and Novel Magnetism. 2012;**25**(1):71-77

rite. Physica B: Condensed Matter. 2005;**363**(1-4):128-132

2 Advanced Imaging Centre, Malaysian Rubber Board, RRIM Sungai Buloh, Selangor,

**Figure 13.** M-H hysteresis loops of Ni0.5Zn0.5Fe2 O4 nanoparticles synthesized by different synthesis approaches.

### **8. Conclusions**

As most common approach for the fabrication of ceramic material, sintering shows some irreplaceable advantages. Sintering provides control on processing variables like sintering temperature, to achieve required microstructure for a particular set of properties. The synthesis temperature for single homogeneous phase can be lowered by mechanically activates the starting materials. Three stages of sintering mechanism can be observed in the experimental data of Ni-Zn ferrite. The observed evolutional relationship between microstructural, magnetic, and optical properties can be used to develop a useful framework for designing a sintering condition for final microstructure with desired properties. From the comparative study of top-down and bottom-up approaches carried out, we concluded that different synthesis methods produced ceramic materials with different behaviors. Top-down approach synthesis method has the ability to produce nanocrystalline particles, which then must be compacted without losing the refined microstructural properties, with high uniformity in terms of size, and morphological properties. This remains a challenge to this approach otherwise it is a versatile method. Bottom-up approach synthesis method is capable of producing particles with refined microstructures, which then high-purity single phase particles must be produced with particle size below 100 nm. This is relatively more difficult as single phase can only be achieved when sufficient heat energy is provided, and typically single phase particles are produced at high sintering temperature where particle growth is unavoidable.

### **Acknowledgements**

We would like to dedicate this chapter and show our gratitude to the late Assoc. Prof. Dr. Mansor Hashim from Universiti Putra Malaysia, Malaysia for sharing his pearls of wisdom with us during the course of this research.

### **Author details**

Zhi Huang Low<sup>1</sup> , Ismayadi Ismail<sup>1</sup> \* and Kim Song Tan2

\*Address all correspondence to: ismayadi@upm.edu.my

1 Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology, Universiti Putra, Malaysia, Serdang, Selangor Darul Ehsan, Malaysia

2 Advanced Imaging Centre, Malaysian Rubber Board, RRIM Sungai Buloh, Selangor, Malaysia

### **References**

**Figure 13.** M-H hysteresis loops of Ni0.5Zn0.5Fe2

40 Sintering Technology - Method and Application

**8. Conclusions**

growth is unavoidable.

**Acknowledgements**

with us during the course of this research.

O4

As most common approach for the fabrication of ceramic material, sintering shows some irreplaceable advantages. Sintering provides control on processing variables like sintering temperature, to achieve required microstructure for a particular set of properties. The synthesis temperature for single homogeneous phase can be lowered by mechanically activates the starting materials. Three stages of sintering mechanism can be observed in the experimental data of Ni-Zn ferrite. The observed evolutional relationship between microstructural, magnetic, and optical properties can be used to develop a useful framework for designing a sintering condition for final microstructure with desired properties. From the comparative study of top-down and bottom-up approaches carried out, we concluded that different synthesis methods produced ceramic materials with different behaviors. Top-down approach synthesis method has the ability to produce nanocrystalline particles, which then must be compacted without losing the refined microstructural properties, with high uniformity in terms of size, and morphological properties. This remains a challenge to this approach otherwise it is a versatile method. Bottom-up approach synthesis method is capable of producing particles with refined microstructures, which then high-purity single phase particles must be produced with particle size below 100 nm. This is relatively more difficult as single phase can only be achieved when sufficient heat energy is provided, and typically single phase particles are produced at high sintering temperature where particle

We would like to dedicate this chapter and show our gratitude to the late Assoc. Prof. Dr. Mansor Hashim from Universiti Putra Malaysia, Malaysia for sharing his pearls of wisdom

nanoparticles synthesized by different synthesis approaches.


[12] Dho J, Lee EK, Park JY, Hur NH. Effects of the grain boundary on the coercivity of barium ferrite BaFe12O19. Journal of Magnetism and Magnetic Materials. 2005;**285**(1-2):164-168

[26] Kang SJL. Sintering Densification, Grain Growth, and Microstructure. London, United

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[27] Skomski R, Sellmyer DJ. Intrinsic and extrinsic properties of advanced magnetic materi-

[28] Fox M. Optical Properties of Solids. 2nd ed. United States: Clarendon Press Oxford; 2010 [29] Low ZH, Chen SK, Ismail I, Tan KS, Liew JYC. Structural transformations of mechanically induced top-down approach BaFe12O19 nanoparticles synthesized from high crystallinity bulk materials. Journal of Magnetism and Magnetic Materials. 2017;**429**:192-202

Kingdom: Elsevier; 2005

als. ChemInform. 2006;**37**(47):1


[26] Kang SJL. Sintering Densification, Grain Growth, and Microstructure. London, United Kingdom: Elsevier; 2005

[12] Dho J, Lee EK, Park JY, Hur NH. Effects of the grain boundary on the coercivity of barium ferrite BaFe12O19. Journal of Magnetism and Magnetic Materials. 2005;**285**(1-2):164-168 [13] Rikukawa H. Relationship between microstructures and magnetic properties of ferrites containing closed pores. IEEE Transactions on Magnetics. 1982;**18**(6):1535-1537

[14] Ibrahim IR, Hashim M, Nazlan R, Ismail I, Wan Ab Rahman WN, Abdullah NH, Idris FM, Shafie MSE, Zulkimi MMM. Grouping trends of magnetic permeability components

[15] Suryanarayana C. Mechanical Alloying and Milling. New York: Marcel Dekker; 2004

[16] Benito G, Morales MP, Requena J, Raposo V, Vázquez M, Moya JS. Barium hexaferrite monodispersed nanoparticles prepared by the ceramic method. Journal of Magnetism

[17] Zahi S, Daud AR, Hashim M. A comparative study of nickel-zinc ferrites by sol-gel route and solid-state reaction. Materials Chemistry and Physics. 2007;**106**(2-3):452-456 [18] Hajalilou A, Hashim M, Ebrahimi-Kahrizsangi R, Kamari HM. Influence of evolving microstructure on electrical and magnetic characteristics in mechanically synthesized polycrystalline Ni-ferrite nanoparticles. Journal of Alloys and Compounds. 2015;**633**:

[19] Low ZH, Hashim M, Ismail I, Kanagesan S, Ezzad Shafie MS, Idris FM, Ibrahim IR. Development of magnetic B-H hysteresis loops through stages of microstructure evolution of bulk BaFe12O19. Journal of Superconductivity and Novel Magnetism. 2015;**28**(10):

[20] Yan W, Li Q, Zhong H, Zhong Z. Characterization and low-temperature sintering of

[21] Zahi S, Hashim M, Daud AR. Synthesis, magnetic properties and microstructure of Ni-Zn ferrite by sol-gel technique. Journal of Magnetism and Magnetic Materials. 2007;

[22] Chen DG, Tang XG, Wu JB, Zhang W, Liu QX, Jiang YP. Effect of grain size on the mag-

[23] Cullity BD, Stock SR. Elements of X-Ray Diffraction. 3rd ed. United Kingdom: Pearson,

[24] Phuoc TX, Chen R-H.Modeling the effect of particle size on the activation energy and ignition temperature of metallic nanoparticles. Combustion and Flame. 2012;**159**(1):416-419

[25] Jarcho M, Bolen CH, Thomas MB, Bobick J, Kay JF, Doremus RH. Hydroxylapatite synthesis and characterization in sense polycristalline forms. Journal of Materials Science.

process. Journal of Magnetism and Magnetic Materials. 2011;**323**(12):1717-1721

netic properties of superparamagnetic Ni0.5Zn0.5Fe2

nano-powders prepared by refluxing method. Powder Technology.

O4

nanoparticles by co-precipitation

O4

. Journal of Magnetism

in their parallel evolution with microstructure in Ni0.3Zn0.7Fe2

and Magnetic Materials. 2014;**355**:265-275

42 Sintering Technology - Method and Application

and Magnetic Materials. 2001;**234**(1):65-72

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Ni0.5Zn0.5Fe2

2009;**192**(1):23-26

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Prentice Hall; 2011

1976;**11**:2027-2035

O4


**Chapter 3**

**Provisional chapter**

**Sintering Temperature Effect on Microstructure and**

**and Magnetic Evolution Properties with Nano- and** 

The morphology and evolution of magnetic properties in multisample sintering (MSS) of

ship with their dependences on sintering temperature. Sintering is an important process in ferrite fabrication which involved the process of transforming a noncrystalline powder into a polycrystalline solid by heating process. Under the influence of heat, the surface area is reduced through the formation and growth of bond between the particles associated with reduction in surface energy. This makes the particles move closer, grains to form by the movement of grain boundaries to grow over pores, and results in decreasing the porosity and increasing the density of the sample. Technological applications, especially in electronics applications, require high-density nanostructured ferrites, integrated by sintering from nanoparticles. The evolution from low to high sintering temperature will result in the transition from disordered to ordered ferromagnetism behavior.

O12, YIG) and single-sample sintering (SSS) of nickel zinc ferrite

Fe5

O12, YIG), SSS nickel zinc ferrite

O12, YIG) and single-sample

, NZF) have been used as a studied

, NZF) were studied in detail, focusing on the parallel evolving relation-

O4

Fe5

, NZF), microstructure, magnetic properties

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.78638

**Magnetic Evolution Properties with Nano- and**

**Sintering Temperature Effect on Microstructure** 

**Micrometer Grain Size in Ferrite Polycrystals**

**Micrometer Grain Size in Ferrite Polycrystals**

Raba'ah Syahidah Azis,

**Abstract**

(Ni0.6Zn0.4Fe2

(Ni0.6Zn0.4Fe2

yttrium iron garnet (Y<sup>3</sup>

O4

material in this research work.

O4

**Keywords:** MSS yttrium iron garnet (Y<sup>3</sup>

Raba'ah Syahidah Azis,

Muhammad Syazwan Mustaffa and Nuraine Mariana Mohd Shahrani

Muhammad Syazwan Mustaffa and Nuraine Mariana Mohd Shahrani

http://dx.doi.org/10.5772/intechopen.78638

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Fe5

Multisample sintering (MSS) of yttrium iron garnet (Y<sup>3</sup>

sintering (SSS) of nickel zinc ferrite (Ni0.6Zn0.4Fe2

### **Sintering Temperature Effect on Microstructure and Magnetic Evolution Properties with Nano- and Micrometer Grain Size in Ferrite Polycrystals Sintering Temperature Effect on Microstructure and Magnetic Evolution Properties with Nano- and Micrometer Grain Size in Ferrite Polycrystals**

DOI: 10.5772/intechopen.78638

Raba'ah Syahidah Azis, Muhammad Syazwan Mustaffa and Nuraine Mariana Mohd Shahrani Raba'ah Syahidah Azis, Muhammad Syazwan Mustaffa and Nuraine Mariana Mohd Shahrani

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.78638

### **Abstract**

The morphology and evolution of magnetic properties in multisample sintering (MSS) of yttrium iron garnet (Y<sup>3</sup> Fe5 O12, YIG) and single-sample sintering (SSS) of nickel zinc ferrite (Ni0.6Zn0.4Fe2 O4 , NZF) were studied in detail, focusing on the parallel evolving relationship with their dependences on sintering temperature. Sintering is an important process in ferrite fabrication which involved the process of transforming a noncrystalline powder into a polycrystalline solid by heating process. Under the influence of heat, the surface area is reduced through the formation and growth of bond between the particles associated with reduction in surface energy. This makes the particles move closer, grains to form by the movement of grain boundaries to grow over pores, and results in decreasing the porosity and increasing the density of the sample. Technological applications, especially in electronics applications, require high-density nanostructured ferrites, integrated by sintering from nanoparticles. The evolution from low to high sintering temperature will result in the transition from disordered to ordered ferromagnetism behavior. Multisample sintering (MSS) of yttrium iron garnet (Y<sup>3</sup> Fe5 O12, YIG) and single-sample sintering (SSS) of nickel zinc ferrite (Ni0.6Zn0.4Fe2 O4 , NZF) have been used as a studied material in this research work.

**Keywords:** MSS yttrium iron garnet (Y<sup>3</sup> Fe5 O12, YIG), SSS nickel zinc ferrite (Ni0.6Zn0.4Fe2 O4 , NZF), microstructure, magnetic properties

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **1. Introduction of garnet and spinel ferrites**

Garnet ferrites with composition (A<sup>3</sup> B5 O12) structure have unique electromagnetic, magneto-optical, mechanical, and thermal properties [1]. Garnet ferrite comprises three crystallographic lattice (a, b, and c) sites. Among these lattice sites, the 24Fe3+, 16Fe3+, and 24R3+ ions occupy the [b] tetrahedral, (a) octahedral, and {c} dodecahedral sites, respectively, whereas oxygen ions are distributed to the interstitial sites [2]. However, R is the rare earth ions such as Ce, Gd, Y, and Nd. The general formula for rare earth garnets is R<sup>3</sup> Fe5 O12, whereas the ion distribution in rare earth garnets are [Fe<sup>2</sup> ](Fe3 ){R<sup>3</sup> }O12 represented tetrahedral, octahedral, and dodecahedral sites, respectively [3]. As for Ni-Zn ferrite, it has a spinel ferrite structure. Spinel ferrite crystallizes in the cubic structure. Spinels have the general formula M(Fe2 O4 ), where M is usually a divalent cation such as manganese (Mn2+), nickel (Ni2+), cobalt (Co2+), zinc (Zn2+), copper (Cu2+), or magnesium (Mg2+). The unit cell of spinel ferrites consists of 32 oxygen, 16 trivalent iron, and 8 divalent metal irons. The most important feature of the unit cell is that its array of oxygen ions leaves open two kinds of interstices, which can be filled by the metal ions. These interstices are referred to as tetrahedral or A sites and octahedral or B sites. The sintering process plays a prominent role in the fabrication of ceramics. Almost all ceramic bodies must be fired at elevated temperatures to produce a microstructure with the desired properties. This widespread use of sintering process has led to a variety of approaches to the subject. The criteria that should be met before sintering can occur are the mechanism for material transport and source of energy to activate and sustain this material transport. The relationship between microstructural properties with the effect of sintering temperature toward magnetic characteristics of MSS of yttrium iron garnet (Y<sup>3</sup> Fe5 O12, YIG) and SSS of nickel zinc ferrite (Ni0.6Zn0.4Fe2 O4 , NZF) is the focus of interest in this research work.

**2.2. Preparation and characterization of MSS of YIG**

the stoichiometric ratio based on Eq. 1:

Fe2 O3

(Fe2 O3

Eq. 2:

The starting raw powder materials of yttrium oxide (Y<sup>2</sup>

O3

Sintering Temperature Effect on Microstructure and Magnetic Evolution Properties…

which derived from mill scale (Curie separation technique) were mixed according to

5Fe2 O3 + 3Y<sup>2</sup> O3 → 2Y<sup>3</sup> Fe5 O12 (1)

The raw materials were mixed using an agate mortar for about 1 h. The mixing operation is necessary to combine the starting materials into a thoroughly homogeneous mixture. The mixing powder then was milled by using high-energy ball mill (SPEX8000D) with the ball to the powder weight ratio (BPR) of 10:1 for 9 h. After milling, polyvinyl alcohol with 1 wt.% PVA was added in the powder as a binder for giving strength to the pressed compact and was lubricated with 0.3 wt.% of zinc stearate. The mixture powder was pressed with 300 MPa into a toroidal shape. Then, the samples were sintered at different sintering temperatures from 500 to 1400°C for 9 h in air. The evolution of microstructural properties of the sample was determined by using a NovaNano 230 FESEM. The distribution of grain sizes was obtained by taking at least 200 different grain images and estimating the mean diameters of individual grains for each sample using J-image software. The magnetization studies were performed at room temperature using a LakeShore 7404 vibrating sample magnetometer with a maximum magnetic field of 11 kG. The variations of complex permeability were measured using an Agilent HP4291A Impedance Analyzer in the range of 1 MHz to 1.8 GHz.

**2.3. Preparation and characterization of SSS of nickel zinc ferrite (Ni0.6Zn0.4Fe2**

for each sample using J-image software. The saturation induction, *Bs*

The starting raw powder materials of nickel oxide (NiO), zinc oxide (ZnO), and iron oxide

0.6NiO + 0.4ZnO + Fe2 O3 → Ni0.6 Zn0.4 Fe2 O4 (2)

The mixed material was crushed by using a SPEX8000D HEBM machine at room temperature. The raw mixed powders were milled according to 10:1 ball to the powder weight ratio (BPR) for 6 h. The milled powder was granulated by using polyvinyl alcohol (1 wt.% PVA) as a binder and 0.1 wt. % zinc stearate was added as a lubricant. The previously granulated powder was then uniaxially pressed into toroidal form with pressure of 440 MPa. The single toroidal sample was repeatedly sintered from 600 up to 1200°C with an increment of 25°C under ambient air condition for 10 h. The evolution of microstructural properties of the sintered toroid was studied using a NovaNano 230 FESEM. The distribution of grain sizes was measured by taking at least 200 different grain images and estimating the mean diameters of individual grains

determined from a *B-H* hysteresis loop which was obtained via a Linkjoin Technology MATS 2010SD Static Hysteresis graph. The frequency variations of the complex permeability were measured using an Agilent HP4291A Impedance Analyzer in the range of 1 MHz to 1.8 GHz.

) (>99% purity, Alfa Aesar) were mixed according to the stoichiometric ratio based on

) (99.9% purity, Alfa Aesar) and

http://dx.doi.org/10.5772/intechopen.78638

47

**O4 , NZF)**

, and coercivity, *Hc*

, were

### **2. Brief overview of preparation methods**

### **2.1. Preparation of hematite (Fe2 O3 )**

About 100 g of mill scale was weighed using digital weighing balance. The mill scale was used as Fe2 O3 source for preparing YIG. The mill scale was crushed by wet milling process for 48 h to obtain the precise sized powder. The magnetic particles were poured into a glass tube filled with 90–100°C distilled water in the presence of 1 T external magnetic field. Due to the weak susceptibility of ferromagnetic particles, FeO (wustite) presumably would drop to the bottom of the tube, and the Fe<sup>3</sup> O4 (magnetite) and Fe2 O3 (hematite) would be attracted to the surface close to the poles. This separation was sorted out based on the Curie temperature of FeO, Fe2 O3 , and Fe3 O4 particles [4, 5]. The one that has been used for Fe2 O3 production is the bottom particles. The powder was oxidized using furnace at 500°C for 9 h in air. The yield of oxidation, Fe2 O3 , was sieving to obtain a fine powder and used as a raw material in preparing YIG.

### **2.2. Preparation and characterization of MSS of YIG**

**1. Introduction of garnet and spinel ferrites**

whereas the ion distribution in rare earth garnets are [Fe<sup>2</sup>

O4

Fe5

**2. Brief overview of preparation methods**

**O3 )**

the focus of interest in this research work.

drop to the bottom of the tube, and the Fe<sup>3</sup>

O3

, and Fe3

O4

O3

B5

neto-optical, mechanical, and thermal properties [1]. Garnet ferrite comprises three crystallographic lattice (a, b, and c) sites. Among these lattice sites, the 24Fe3+, 16Fe3+, and 24R3+ ions occupy the [b] tetrahedral, (a) octahedral, and {c} dodecahedral sites, respectively, whereas oxygen ions are distributed to the interstitial sites [2]. However, R is the rare earth

hedral, octahedral, and dodecahedral sites, respectively [3]. As for Ni-Zn ferrite, it has a spinel ferrite structure. Spinel ferrite crystallizes in the cubic structure. Spinels have the

nickel (Ni2+), cobalt (Co2+), zinc (Zn2+), copper (Cu2+), or magnesium (Mg2+). The unit cell of spinel ferrites consists of 32 oxygen, 16 trivalent iron, and 8 divalent metal irons. The most important feature of the unit cell is that its array of oxygen ions leaves open two kinds of interstices, which can be filled by the metal ions. These interstices are referred to as tetrahedral or A sites and octahedral or B sites. The sintering process plays a prominent role in the fabrication of ceramics. Almost all ceramic bodies must be fired at elevated temperatures to produce a microstructure with the desired properties. This widespread use of sintering process has led to a variety of approaches to the subject. The criteria that should be met before sintering can occur are the mechanism for material transport and source of energy to activate and sustain this material transport. The relationship between microstructural properties with the effect of sintering temperature toward magnetic characteristics of MSS

About 100 g of mill scale was weighed using digital weighing balance. The mill scale was

for 48 h to obtain the precise sized powder. The magnetic particles were poured into a glass tube filled with 90–100°C distilled water in the presence of 1 T external magnetic field. Due to the weak susceptibility of ferromagnetic particles, FeO (wustite) presumably would

O4

attracted to the surface close to the poles. This separation was sorted out based on the Curie

production is the bottom particles. The powder was oxidized using furnace at 500°C for 9 h

ions such as Ce, Gd, Y, and Nd. The general formula for rare earth garnets is R<sup>3</sup>

O12) structure have unique electromagnetic, mag-

](Fe3 ){R<sup>3</sup>

), where M is usually a divalent cation such as manganese (Mn2+),

O12, YIG) and SSS of nickel zinc ferrite (Ni0.6Zn0.4Fe2

source for preparing YIG. The mill scale was crushed by wet milling process

(magnetite) and Fe2

O3

particles [4, 5]. The one that has been used for Fe2

, was sieving to obtain a fine powder and used as a raw

Fe5 O12,

}O12 represented tetra-

O4

(hematite) would be

O3

, NZF) is

Garnet ferrites with composition (A<sup>3</sup>

46 Sintering Technology - Method and Application

general formula M(Fe2

of yttrium iron garnet (Y<sup>3</sup>

**2.1. Preparation of hematite (Fe2**

O3

temperature of FeO, Fe2

material in preparing YIG.

in air. The yield of oxidation, Fe2

used as Fe2

The starting raw powder materials of yttrium oxide (Y<sup>2</sup> O3 ) (99.9% purity, Alfa Aesar) and Fe2 O3 which derived from mill scale (Curie separation technique) were mixed according to the stoichiometric ratio based on Eq. 1:

$$\text{5Fe}\_2\text{O}\_3 + \text{3Y}\_2\text{O}\_3 \rightarrow 2\text{Y}\_3\text{Fe}\_5\text{O}\_{12} \tag{1}$$

The raw materials were mixed using an agate mortar for about 1 h. The mixing operation is necessary to combine the starting materials into a thoroughly homogeneous mixture. The mixing powder then was milled by using high-energy ball mill (SPEX8000D) with the ball to the powder weight ratio (BPR) of 10:1 for 9 h. After milling, polyvinyl alcohol with 1 wt.% PVA was added in the powder as a binder for giving strength to the pressed compact and was lubricated with 0.3 wt.% of zinc stearate. The mixture powder was pressed with 300 MPa into a toroidal shape. Then, the samples were sintered at different sintering temperatures from 500 to 1400°C for 9 h in air. The evolution of microstructural properties of the sample was determined by using a NovaNano 230 FESEM. The distribution of grain sizes was obtained by taking at least 200 different grain images and estimating the mean diameters of individual grains for each sample using J-image software. The magnetization studies were performed at room temperature using a LakeShore 7404 vibrating sample magnetometer with a maximum magnetic field of 11 kG. The variations of complex permeability were measured using an Agilent HP4291A Impedance Analyzer in the range of 1 MHz to 1.8 GHz.

### **2.3. Preparation and characterization of SSS of nickel zinc ferrite (Ni0.6Zn0.4Fe2 O4 , NZF)**

The starting raw powder materials of nickel oxide (NiO), zinc oxide (ZnO), and iron oxide (Fe2 O3 ) (>99% purity, Alfa Aesar) were mixed according to the stoichiometric ratio based on Eq. 2:

$$0.6\text{NiO} + 0.4\text{ZnO} + \text{Fe}\_2\text{O}\_3 \rightarrow \text{Ni}\_{a\delta}\text{Zn}\_{a4}\text{Fe}\_2\text{O}\_4\tag{2}$$

The mixed material was crushed by using a SPEX8000D HEBM machine at room temperature. The raw mixed powders were milled according to 10:1 ball to the powder weight ratio (BPR) for 6 h. The milled powder was granulated by using polyvinyl alcohol (1 wt.% PVA) as a binder and 0.1 wt. % zinc stearate was added as a lubricant. The previously granulated powder was then uniaxially pressed into toroidal form with pressure of 440 MPa. The single toroidal sample was repeatedly sintered from 600 up to 1200°C with an increment of 25°C under ambient air condition for 10 h. The evolution of microstructural properties of the sintered toroid was studied using a NovaNano 230 FESEM. The distribution of grain sizes was measured by taking at least 200 different grain images and estimating the mean diameters of individual grains for each sample using J-image software. The saturation induction, *Bs* , and coercivity, *Hc* , were determined from a *B-H* hysteresis loop which was obtained via a Linkjoin Technology MATS 2010SD Static Hysteresis graph. The frequency variations of the complex permeability were measured using an Agilent HP4291A Impedance Analyzer in the range of 1 MHz to 1.8 GHz.

### **3. Microstructure and magnetic properties of MSS yttrium iron garnet and SSS nickel zinc ferrite**

### **3.1. MSS of yttrium iron garnet (YIG)**

The samples undergo grain growth or the increase in size of grains when sintering at higher temperature. Sintering can be defined as removal of the pores between starting particles accompanied by shrinkage of the component combined with growth together and formation of strong bonds between adjacent particles [6, 7]. **Figure 1** shows the FESEM micrograph of samples sintered at different sintering temperatures. The micrograph of the sample sintered at 500, 600, and 700°C revealed that the sample encounters initial stage of sintering. The initial stage of sintering involves rearrangement of the powder particles and formation of strong bond or necks at the contact point between particles [8]. At 500 and 600°C, the sample showed nearly the same evolution trend, where the sample sintered at 500–600°C showed slight particle growth and rearrangement of the particles. After sintering at 700 and 800°C, the sample undergoes the formation of necks between particles. This can be noticed with the existence of dumbbell shape in the micrograph at these sintering temperatures. The red-dotted circles in **Figure 1** at 700°C indicate the necking structure between the particles. SEM micrograph of the samples sintered at 900, 1000, and 1100°C exhibits intermediate stage of sintering. Intermediate sintering is the stage where the size of the necks grows, the amount of porosity decreases substantially, and the particles move closer [9]. At this range of temperature, grain boundaries are formed and move so that some grains grew at the expense of others. Grain growth becomes increasingly active as the pore structure collapses. The pinning effect of the pore diminishes as they shrink and occupy less grain boundary area. Further sintering at 1200, 1300, and 1400°C corresponds to the final stage of sintering. The grains with the hexagonal structure are observed in this range of temperature. At this stage, the pores diminished and are slowly eliminated by diffusion of vacancies from the pores along the grain boundaries. The grain boundaries are regions of more open crystal structure than the grain themselves. Thus, the diffusion along grain boundary is more rapid. Reducing the grain boundary area by the grain growth lowers the energy of the system to a more stable state. From the results, it is believed that a mass transport mechanism started with atomic surface diffusion at relatively low temperature and continued to occur by the grain boundary diffusion, resulting in formation of necking, contact growth, pore elimination, and grain growth.

The magnetization versus magnetic field (*M-H*) curve of the sintered samples is measured at room temperature, as shown in **Figure 2**, and the corresponding saturation magnetization, *Ms* versus sintering temperature is given in **Table 1**. The saturation magnetization reached 0.597, 0.792, 0.259, and 0.069 emu/g at 500, 600, 700, and 800°C, respectively. These values could be related to the presence of weak ferromagnetic behavior of α-Fe<sup>2</sup> O3 and YFeO<sup>3</sup> and a significant amount of amorphous phase [10]. Moreover, such trend can also be associated with the mixture of disordered and ordered magnetism. The samples sintered from 500 to 800°C contained only weak magnetic phase as the magnetization in this temperature range is almost zero. It represents a very small amount of ordered magnetism in these samples. In addition, the smaller value of saturation magnetization in smaller grain size at this region temperature is attributed to the fine greater fraction of surface spins in the particles that tends to be canted with a smaller net moment. At 700 and 800°C, orthoferrites and hematite show

the weak ferromagnetic behavior. The weak ferromagnetism arises from the low symmetry of the magnetic unit cell, producing a spin-canted structure of Fe sublattices. The weak fer-

lelism. The increase in saturation magnetization for the sample sintered from 900 to 1100°C appears to be established by the initial formation of ferrimagnetic YIG phase at 900°C from the amorphous phase, while the sintering temperature at 1100°C shows only a single phase of YIG exist. This trend is characterized by a remarkable transformation from mixture of disordered and ordered to completely ordered arrangements of the magnetic moments in the sample.

is due to a slight disorder of the spin axis from exact antiparal-

Sintering Temperature Effect on Microstructure and Magnetic Evolution Properties…

http://dx.doi.org/10.5772/intechopen.78638

49

romagnetic behavior of α-Fe<sup>2</sup>

O3

**Figure 1.** FESEM micrograph of MSS of YIG sintered from 500 to 1400°C.

Sintering Temperature Effect on Microstructure and Magnetic Evolution Properties… http://dx.doi.org/10.5772/intechopen.78638 49

**Figure 1.** FESEM micrograph of MSS of YIG sintered from 500 to 1400°C.

**3. Microstructure and magnetic properties of MSS yttrium iron** 

The samples undergo grain growth or the increase in size of grains when sintering at higher temperature. Sintering can be defined as removal of the pores between starting particles accompanied by shrinkage of the component combined with growth together and formation of strong bonds between adjacent particles [6, 7]. **Figure 1** shows the FESEM micrograph of samples sintered at different sintering temperatures. The micrograph of the sample sintered at 500, 600, and 700°C revealed that the sample encounters initial stage of sintering. The initial stage of sintering involves rearrangement of the powder particles and formation of strong bond or necks at the contact point between particles [8]. At 500 and 600°C, the sample showed nearly the same evolution trend, where the sample sintered at 500–600°C showed slight particle growth and rearrangement of the particles. After sintering at 700 and 800°C, the sample undergoes the formation of necks between particles. This can be noticed with the existence of dumbbell shape in the micrograph at these sintering temperatures. The red-dotted circles in **Figure 1** at 700°C indicate the necking structure between the particles. SEM micrograph of the samples sintered at 900, 1000, and 1100°C exhibits intermediate stage of sintering. Intermediate sintering is the stage where the size of the necks grows, the amount of porosity decreases substantially, and the particles move closer [9]. At this range of temperature, grain boundaries are formed and move so that some grains grew at the expense of others. Grain growth becomes increasingly active as the pore structure collapses. The pinning effect of the pore diminishes as they shrink and occupy less grain boundary area. Further sintering at 1200, 1300, and 1400°C corresponds to the final stage of sintering. The grains with the hexagonal structure are observed in this range of temperature. At this stage, the pores diminished and are slowly eliminated by diffusion of vacancies from the pores along the grain boundaries. The grain boundaries are regions of more open crystal structure than the grain themselves. Thus, the diffusion along grain boundary is more rapid. Reducing the grain boundary area by the grain growth lowers the energy of the system to a more stable state. From the results, it is believed that a mass transport mechanism started with atomic surface diffusion at relatively low temperature and continued to occur by the grain boundary diffusion, resulting in formation of necking, contact growth, pore elimination, and grain growth.

The magnetization versus magnetic field (*M-H*) curve of the sintered samples is measured at room temperature, as shown in **Figure 2**, and the corresponding saturation magnetization,

a significant amount of amorphous phase [10]. Moreover, such trend can also be associated with the mixture of disordered and ordered magnetism. The samples sintered from 500 to 800°C contained only weak magnetic phase as the magnetization in this temperature range is almost zero. It represents a very small amount of ordered magnetism in these samples. In addition, the smaller value of saturation magnetization in smaller grain size at this region temperature is attributed to the fine greater fraction of surface spins in the particles that tends to be canted with a smaller net moment. At 700 and 800°C, orthoferrites and hematite show

could be related to the presence of weak ferromagnetic behavior of α-Fe<sup>2</sup>

 versus sintering temperature is given in **Table 1**. The saturation magnetization reached 0.597, 0.792, 0.259, and 0.069 emu/g at 500, 600, 700, and 800°C, respectively. These values

O3

and YFeO<sup>3</sup>

and

**garnet and SSS nickel zinc ferrite**

**3.1. MSS of yttrium iron garnet (YIG)**

48 Sintering Technology - Method and Application

*Ms*

the weak ferromagnetic behavior. The weak ferromagnetism arises from the low symmetry of the magnetic unit cell, producing a spin-canted structure of Fe sublattices. The weak ferromagnetic behavior of α-Fe<sup>2</sup> O3 is due to a slight disorder of the spin axis from exact antiparallelism. The increase in saturation magnetization for the sample sintered from 900 to 1100°C appears to be established by the initial formation of ferrimagnetic YIG phase at 900°C from the amorphous phase, while the sintering temperature at 1100°C shows only a single phase of YIG exist. This trend is characterized by a remarkable transformation from mixture of disordered and ordered to completely ordered arrangements of the magnetic moments in the sample.

**Figure 2.** Magnetization curve of MSS of YIG sintered from 500 to 1400°C.


**Table 1.** Saturation magnetization, *M<sup>s</sup> ,* and coercivity, *H<sup>c</sup> ,* of MSS of YIG sintered from 500 to 1400°C.

The sample sintered at 900°C exhibits the saturation magnetizations lower than the value of 26.8 emu/g, and at 1000°C, the saturation magnetization is close for the usual YIG ceramic, 26.8 emu/g [11]. This should be the contribution from the smallness of YIG present at 900°C and the basis of well-crystalline YIG with poor yttrium iron perovskite (YFeO<sup>3</sup> ) phase with high grain boundary content at 1000°C. The maximum saturation magnetization (27.074 emu/g) can be achieved at 1100°C. This value is higher as the amorphous phase is diminished due to the larger grain size and increasing bulk volume fraction of YIG. Thus, a strong interaction of magnetic moment within domains occurred due to exchange force. The powder sintered from 1200 to 1400°C shows a decreasing value of magnetization. The decrease of magnetization is

mainly connected with oxygen gas surface interaction at higher sintering temperature. For larger grains, magnetization can follow the easy magnetization directions in the single grains, and domains can be formed within the grains. Thus, the magnetization process is determined by the magnetocrystalline anisotropy of the crystallites. For very small grains, ferromagnetic

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51

**Figure 3.** Real permeability, *μ′,* of MSS of YIG sintered from 500 to 1400°C.

**Figure 4.** Loss factor, *μ″,* of MSS of YIG sintered from 500 to 1400°C.

Sintering Temperature Effect on Microstructure and Magnetic Evolution Properties… http://dx.doi.org/10.5772/intechopen.78638 51

**Figure 3.** Real permeability, *μ′,* of MSS of YIG sintered from 500 to 1400°C.

**Figure 4.** Loss factor, *μ″,* of MSS of YIG sintered from 500 to 1400°C.

The sample sintered at 900°C exhibits the saturation magnetizations lower than the value of 26.8 emu/g, and at 1000°C, the saturation magnetization is close for the usual YIG ceramic, 26.8 emu/g [11]. This should be the contribution from the smallness of YIG present at 900°C and

*,* of MSS of YIG sintered from 500 to 1400°C.

grain boundary content at 1000°C. The maximum saturation magnetization (27.074 emu/g) can be achieved at 1100°C. This value is higher as the amorphous phase is diminished due to the larger grain size and increasing bulk volume fraction of YIG. Thus, a strong interaction of magnetic moment within domains occurred due to exchange force. The powder sintered from 1200 to 1400°C shows a decreasing value of magnetization. The decrease of magnetization is

) phase with high

the basis of well-crystalline YIG with poor yttrium iron perovskite (YFeO<sup>3</sup>

*,* and coercivity, *H<sup>c</sup>*

**Figure 2.** Magnetization curve of MSS of YIG sintered from 500 to 1400°C.

**Table 1.** Saturation magnetization, *M<sup>s</sup>*

50 Sintering Technology - Method and Application

mainly connected with oxygen gas surface interaction at higher sintering temperature. For larger grains, magnetization can follow the easy magnetization directions in the single grains, and domains can be formed within the grains. Thus, the magnetization process is determined by the magnetocrystalline anisotropy of the crystallites. For very small grains, ferromagnetic

exchange interaction more and more forces the magnetic moments to align parallel, thus

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53

Complex permeability consists of two components: the real permeability, *μ′,* and loss factor, *μ″*. The room temperature of real permeability and loss factor of sintered samples are measured from 1.0 MHz to 1.8 GHz. The sample shows the *μ′* value is increased to a maximum value and then decreases rapidly to a very low value (**Figure 3**). The same trend of the loss factor is observed in **Figure 4**. The samples show the increment of permeability loss factor with the rise in frequency and attain the maximum value at particular frequency and decrease with further increase in frequency. From both figures (**Figures 3** and **4**), it can be seen that the *μ″* maximizes, while the *μ′* drops off at 10 MHz because of the occurrence of a magnetic resonance. The sample sintered at 500–800°C has low values of the real permeability and does not show the trend of permeability as same as the higher temperature because of the presence of amorphous phase. The increase of *μ′* value with sintering temperature after 900°C and above is due to the increasing of grain sizes which correspondingly reduce the grain boundaries inside the sample. As the crystallinity and phase purity increase with increasing sintering temperature, the magnetic mass is increased and makes the movement of the domain wall easier. The increase in the sintering temperature also results in the decrease of magnetic anisotropy by decreasing the internal stress and crystal anisotropy [12]. Hence, the hindrance of the movement of domain wall is reduced and increases the value of *μ′*.

The loss factor, *μ″,* is the imaginary part of initial permeability. The permeability loss factor arises due to the lag between the magnetization or flux induction and external applied field [6]. The main types of losses encounter in ferrites are the hysteresis loss, eddy current loss, and residual loss. Otsuki et al. [13] reported that the larger grain size increases the eddy current loss because of the easier movement of domain wall in the larger grain. The fraction of larger grains that are occupied with domain wall also increases hysteresis losses. Hysteresis loss can be minimized if one reduces the hindrance to the domain wall motion by reducing the pinning center to the domain wall movement such as volume fraction of pores, impurities and dislocations, and internal strain inside the sample. The hysteresis loss becomes less important in the high-frequency range due to spin rotation at higher frequency. The eddy current loss is important at higher frequency because of the circulating current induced in the sample due to the change of magnetic field which leads to the energy losses. However, in the polycrystalline YIG, the eddy current losses can be neglected due to high resistivity of YIG. Residual loss is also important in the highfrequency range. To reduce the residual loss, the complex permeability has to be made to

peak at the high frequency as possible by using fine-grained sample.

SEM micrographs for single-sample sintering (SSS) of Ni0.6Zn0.4Fe2

**O4 )**

The increased average grain size shows the microstructural evolution of the sample. The microstructural evolution can be described by adapting the stages of sintering which are three

O4

are shown in **Figure 5**.

**3.2. SSS of nickel zinc ferrite (Ni0.6Zn0.4Fe2**

major stages involved in this process:

impeding the magnetization to follow the easy direction of each individual grain.

**Figure 5.** FESEM micrograph of SSS of Ni0.6Zn0.4Fe2 O4 sintered from 600 to 1200°C.

exchange interaction more and more forces the magnetic moments to align parallel, thus impeding the magnetization to follow the easy direction of each individual grain.

Complex permeability consists of two components: the real permeability, *μ′,* and loss factor, *μ″*. The room temperature of real permeability and loss factor of sintered samples are measured from 1.0 MHz to 1.8 GHz. The sample shows the *μ′* value is increased to a maximum value and then decreases rapidly to a very low value (**Figure 3**). The same trend of the loss factor is observed in **Figure 4**. The samples show the increment of permeability loss factor with the rise in frequency and attain the maximum value at particular frequency and decrease with further increase in frequency. From both figures (**Figures 3** and **4**), it can be seen that the *μ″* maximizes, while the *μ′* drops off at 10 MHz because of the occurrence of a magnetic resonance. The sample sintered at 500–800°C has low values of the real permeability and does not show the trend of permeability as same as the higher temperature because of the presence of amorphous phase. The increase of *μ′* value with sintering temperature after 900°C and above is due to the increasing of grain sizes which correspondingly reduce the grain boundaries inside the sample. As the crystallinity and phase purity increase with increasing sintering temperature, the magnetic mass is increased and makes the movement of the domain wall easier. The increase in the sintering temperature also results in the decrease of magnetic anisotropy by decreasing the internal stress and crystal anisotropy [12]. Hence, the hindrance of the movement of domain wall is reduced and increases the value of *μ′*.

The loss factor, *μ″,* is the imaginary part of initial permeability. The permeability loss factor arises due to the lag between the magnetization or flux induction and external applied field [6]. The main types of losses encounter in ferrites are the hysteresis loss, eddy current loss, and residual loss. Otsuki et al. [13] reported that the larger grain size increases the eddy current loss because of the easier movement of domain wall in the larger grain. The fraction of larger grains that are occupied with domain wall also increases hysteresis losses. Hysteresis loss can be minimized if one reduces the hindrance to the domain wall motion by reducing the pinning center to the domain wall movement such as volume fraction of pores, impurities and dislocations, and internal strain inside the sample. The hysteresis loss becomes less important in the high-frequency range due to spin rotation at higher frequency. The eddy current loss is important at higher frequency because of the circulating current induced in the sample due to the change of magnetic field which leads to the energy losses. However, in the polycrystalline YIG, the eddy current losses can be neglected due to high resistivity of YIG. Residual loss is also important in the highfrequency range. To reduce the residual loss, the complex permeability has to be made to peak at the high frequency as possible by using fine-grained sample.

### **3.2. SSS of nickel zinc ferrite (Ni0.6Zn0.4Fe2 O4 )**

**Figure 5.** FESEM micrograph of SSS of Ni0.6Zn0.4Fe2

52 Sintering Technology - Method and Application

O4

sintered from 600 to 1200°C.

SEM micrographs for single-sample sintering (SSS) of Ni0.6Zn0.4Fe2 O4 are shown in **Figure 5**. The increased average grain size shows the microstructural evolution of the sample. The microstructural evolution can be described by adapting the stages of sintering which are three major stages involved in this process:

**i.** Initial stage sintering involves rearrangement of the powder particles and formation of a strong bond or neck at the contact points between particles [14].

slightly, and then drops sharply when the frequency gets to a certain high value, which is

The real permeability increase with sintering temperature can be attributed to increase in density and grain size with sintering temperature [20]. At higher sintering temperature, with increased grain size, a fewer number of grain boundaries would be present, and diminished grain boundary caused the existence of very mobile domain walls thus increasing the perme-

barrier to domain wall motion due to pinning of the wall. Besides that, the increase in sintering temperature results in a decrease of the magnetic anisotropy by decreasing the internal stress and crystal anisotropy, which reduces the hindrance to the motion of domain walls [17, 21–23]. The same trend is seen in the case of variation in loss factor, *μ″,* with respect to frequency as shown in **Figure 8**. The loss factor was observed first to remain constant with frequency, attain the maximum value at a particular frequency, and then decrease with increase in frequency. The loss factor increased with the increasing sintering temperature and grain size. When the grain size increases, more domain walls exist in the grain. Therefore, the domain walls can move easily in the larger grain. When the grain is large, which means a

o

http://dx.doi.org/10.5772/intechopen.78638

<sup>o</sup> = Constant (3)

Sintering Temperature Effect on Microstructure and Magnetic Evolution Properties…

. During grain growth, pores become fewer which act as

, confirm

55

called the cutoff frequency. The magnetic permeability and the cutoff frequency, *f*

O4

the Snoek's relation [19] by using Eq. 3:

*μ*<sup>i</sup> *f*

ability value of the Ni0.6Zn0.4Fe2

**Figure 6.** *B-H* hysteresis loop of SSS of Ni0.6Zn0.4Fe2

O4

sintered at 600 to 1200°C.


The developments of *B-H* hysteresis loops are shown in **Figure 6**. The hysteresis properties of polycrystalline nickel zinc ferrite (**Table 2**) are very sensitive to the structure and volume fraction of the complete phase, and also to the grain size. The trends of loops are discussed below:


**Figure 7** shows the real permeability, *μ′* results plotted against the frequency in the range of 1.0 MHz to 1.8 GHz. The real permeability remains almost unchanged at low frequency, rises slightly, and then drops sharply when the frequency gets to a certain high value, which is called the cutoff frequency. The magnetic permeability and the cutoff frequency, *f* o , confirm the Snoek's relation [19] by using Eq. 3:

**i.** Initial stage sintering involves rearrangement of the powder particles and formation of a

**ii.** Sample sintered from 600 to 1050°C shows an intermediate stage of sintering, where the size of the neck grows, the amount of porosity decreases substantially, and particles move closer leading to shrinkage of the component. Grain boundaries and grains are formed and move so that some grains grow at the expense of others. This stage continues while the pore channels are connected (open porosity) but is considered over when the

**iii.** Final stage sintering occurred in the sample sintered from 1075 to 1200°C. In this stage, the pores become closed and are slowly eliminated generally by diffusion of vacancies from the pores along grain boundaries with only a little densification of the component. The grain boundaries are regions of more open crystal structure than the grains themselves so that diffusion along them is more rapid. Grain size increases during this stage.

The developments of *B-H* hysteresis loops are shown in **Figure 6**. The hysteresis properties of polycrystalline nickel zinc ferrite (**Table 2**) are very sensitive to the structure and volume fraction of the complete phase, and also to the grain size. The trends of loops are discussed

**i.** 600–1000°C: narrowly bulging but linear-looking shape which consists of weak ferromagnetic phase + paramagnetic phase (amorphous phase) + superparamagnetic phase (crystalline phase and small particles). At this stage, the hysteresis shape is significantly dominated by paramagnetic phase because it does not show the properties associated with ordered magnetism. It shows very slight hysteresis with low saturation induction,

 indicating the low degree of crystallinity and a small amount of ferromagnetic phase. The magnetic moments begin to line up in the direction of applied field with a complex process such as domain growth, domain walls motion, and domain rotation [15–18]. **ii.** 1025–1125°C: slanted sigmoid shape which consists of moderate ferromagnetic phase + paramagnetic phase. As sintering temperature increased, paramagnetic states decreased, and at 1025°C, a moderately ferromagnetic state appeared. With further sintering, there is an increase in the volume fraction of grains, yielding more magnetic crystalline mass which would exhibit stronger ferromagnetism with negligible paramagnetic phase (amorphous phase) arising from nickel zinc ferrite phase formation. They have

values indicating that higher ferromagnetic phase crystallinity is formed [15–18].

**iii.** 1150–1200°C: erect, narrower, and well-defined sigmoid shape which consists of strong ferromagnetic phase. At this stage, strong ferromagnetism state was very dominant with negligible superparamagnetism and paramagnetism due to their high volume fraction of the complete Ni-Zn ferrite phase, high density, large grain size, and low amount of microstructure defects which allow easy domain wall movement in the magnetization

**Figure 7** shows the real permeability, *μ′* results plotted against the frequency in the range of 1.0 MHz to 1.8 GHz. The real permeability remains almost unchanged at low frequency, rises

strong bond or neck at the contact points between particles [14].

pores are isolated (closed porosity).

54 Sintering Technology - Method and Application

below:

*Bs*

higher *Bs*

and demagnetization process [15–18].

$$
\mu\_{\!\!\!\!/ \!} \!\!\!/ \_{o} \text{ = Constant} \tag{3}
$$

The real permeability increase with sintering temperature can be attributed to increase in density and grain size with sintering temperature [20]. At higher sintering temperature, with increased grain size, a fewer number of grain boundaries would be present, and diminished grain boundary caused the existence of very mobile domain walls thus increasing the permeability value of the Ni0.6Zn0.4Fe2 O4 . During grain growth, pores become fewer which act as barrier to domain wall motion due to pinning of the wall. Besides that, the increase in sintering temperature results in a decrease of the magnetic anisotropy by decreasing the internal stress and crystal anisotropy, which reduces the hindrance to the motion of domain walls [17, 21–23].

The same trend is seen in the case of variation in loss factor, *μ″,* with respect to frequency as shown in **Figure 8**. The loss factor was observed first to remain constant with frequency, attain the maximum value at a particular frequency, and then decrease with increase in frequency. The loss factor increased with the increasing sintering temperature and grain size. When the grain size increases, more domain walls exist in the grain. Therefore, the domain walls can move easily in the larger grain. When the grain is large, which means a

**Figure 6.** *B-H* hysteresis loop of SSS of Ni0.6Zn0.4Fe2 O4 sintered at 600 to 1200°C.


**Table 2.** Average grain size, saturation induction, *Bs ,* and coercivity, *Hc* , of SSS of Ni0.6Zn0.4Fe2 O4 sintered from 600 to 1200°C.

decrease in the number of grain boundaries; it will not strongly impede the eddy current flows. Thus, larger eddy current is induced [17, 23–25]. Usually, small grains with prominent grain boundaries lead to higher resistivity, the eddy current loss of which is negligible. The losses in ferrites are associated with rotational resonance and domain wall relaxation. Rotational resonance is usually observed at higher frequencies, while domain wall relaxation is observed at lower frequencies [12, 26, 27]. The major contribution to the magnetic losses in ferrites is due to hysteresis losses, which are based on the damping phenomena associated with spin rotations and irreversible wall displacement. In the high-frequency range, the hysteresis losses become less important because the wall displacement is mainly damped and the losses would be mos due to spin rotation [13, 22, 28]. Comparing **Figures 7** and **8**, it is observed that the off-resonance frequency region of *μ″* firstly occurred and was later followed by the *μ″*. The lag in this process was due to *μ'* being in phase with the exter-

sintered from 600 to 1200°C.

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57

O4

MSS YIG and SS NZF have been prepared via mechanical alloying technique. The samples were sintered at various sintering temperatures in order to study the influence of sintering temperatures on the microstructure and magnetic properties. Increasing sintering temperature will enhance the grain size with less grain boundaries. This extrinsically increases the real magnetic permeability, *μ′,* loss factor, *μ″,* and the saturation magnetization, *Ms*

higher magnetic permeability represents the high frequency losses due to the presence of

. The

reduces the coercivity,

nal field, whereas *μ″* was out of phase with the external field [29].

grain boundaries as impediments to domain wall motion. The large *Ms*

**4. Conclusion**

**Figure 8.** Loss factor, *μ″,* of SSS of Ni0.6Zn0.4Fe2

**Figure 7.** Real permeability, *μ′,* of SSS of Ni0.6Zn0.4Fe2 O4 sintered from 600 to 1200°C.

Sintering Temperature Effect on Microstructure and Magnetic Evolution Properties… http://dx.doi.org/10.5772/intechopen.78638 57

**Figure 8.** Loss factor, *μ″,* of SSS of Ni0.6Zn0.4Fe2 O4 sintered from 600 to 1200°C.

decrease in the number of grain boundaries; it will not strongly impede the eddy current flows. Thus, larger eddy current is induced [17, 23–25]. Usually, small grains with prominent grain boundaries lead to higher resistivity, the eddy current loss of which is negligible. The losses in ferrites are associated with rotational resonance and domain wall relaxation. Rotational resonance is usually observed at higher frequencies, while domain wall relaxation is observed at lower frequencies [12, 26, 27]. The major contribution to the magnetic losses in ferrites is due to hysteresis losses, which are based on the damping phenomena associated with spin rotations and irreversible wall displacement. In the high-frequency range, the hysteresis losses become less important because the wall displacement is mainly damped and the losses would be mos due to spin rotation [13, 22, 28]. Comparing **Figures 7** and **8**, it is observed that the off-resonance frequency region of *μ″* firstly occurred and was later followed by the *μ″*. The lag in this process was due to *μ'* being in phase with the external field, whereas *μ″* was out of phase with the external field [29].

### **4. Conclusion**

**Figure 7.** Real permeability, *μ′,* of SSS of Ni0.6Zn0.4Fe2

**Table 2.** Average grain size, saturation induction, *Bs*

56 Sintering Technology - Method and Application

1200°C.

O4

sintered from 600 to 1200°C.

*,* and coercivity, *Hc*

, of SSS of Ni0.6Zn0.4Fe2

O4

sintered from 600 to

MSS YIG and SS NZF have been prepared via mechanical alloying technique. The samples were sintered at various sintering temperatures in order to study the influence of sintering temperatures on the microstructure and magnetic properties. Increasing sintering temperature will enhance the grain size with less grain boundaries. This extrinsically increases the real magnetic permeability, *μ′,* loss factor, *μ″,* and the saturation magnetization, *Ms* . The higher magnetic permeability represents the high frequency losses due to the presence of grain boundaries as impediments to domain wall motion. The large *Ms* reduces the coercivity, *Hc* as the increasing the multidomain grain size. The magnetic hysteresis and complex permeability graph can be categorized into three distinct groups which represent the formation of a paramagnetic state to a moderate ferromagnetic state and then to a strong ferromagnetic state with microstructural changes at varied sintering temperatures.

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2002;**9**(1):87-90

scientific.net/MSF.846.395.

s41779-017-0126-7

optmat.2008.07.006

net/AMR.501.324

ferrite Ni0.3Zn0.7Fe2

materresbull.2012.03.007

tb12019.x

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[10] Musa MA, Azis RS, Osman NH, Hassan J, Dihom MM. Structural and magnetic properties of yttrium aluminum iron garnet (YAlG) nanoferrite prepared via auto-combustion sol–gel synthesis. Journal of the Australian Ceramic Society. 2018;**54**(1):55-63. DOI: 10.1007/

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O4

from mill scale using Curie

59

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Fe5

O12 via mechani-

### **Acknowledgements**

The authors are thankful to Putra Research Grants, Universiti Putra Malaysia and Fundamental research Grants (FRGS), Ministry of Science and Technology Malaysia (MOSTI) for financial assistance. Also thanks to the Department of Physics, Faculty of Science, UPM and the Materials Synthesis and Characterization Laboratories (MSCL), ITMA, UPM for the measurements facilities.

### **Author details**

Raba'ah Syahidah Azis1,2\*, Muhammad Syazwan Mustaffa<sup>1</sup> and Nuraine Mariana Mohd Shahrani2

\*Address all correspondence to: rabaah@upm.edu.my

1 Department of Physics, Faculty of Science, Universiti Putra Malaysia, UPM, Serdang, Selangor, Malaysia

2 Materials Synthesis and Characterization Laboratory (MSCL), Institute of Advanced Technology, Universiti Putra Malaysia, UPM, Serdang, Selangor, Malaysia

### **References**


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*Hc*

**Acknowledgements**

58 Sintering Technology - Method and Application

**Author details**

Selangor, Malaysia

**References**

Nuraine Mariana Mohd Shahrani2

 as the increasing the multidomain grain size. The magnetic hysteresis and complex permeability graph can be categorized into three distinct groups which represent the formation of a paramagnetic state to a moderate ferromagnetic state and then to a strong ferromagnetic state

The authors are thankful to Putra Research Grants, Universiti Putra Malaysia and Fundamental research Grants (FRGS), Ministry of Science and Technology Malaysia (MOSTI) for financial assistance. Also thanks to the Department of Physics, Faculty of Science, UPM and the Materials Synthesis and Characterization Laboratories (MSCL), ITMA, UPM for the measurements facilities.

1 Department of Physics, Faculty of Science, Universiti Putra Malaysia, UPM, Serdang,

2 Materials Synthesis and Characterization Laboratory (MSCL), Institute of Advanced

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Technology, Universiti Putra Malaysia, UPM, Serdang, Selangor, Malaysia

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with microstructural changes at varied sintering temperatures.

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O4 .

O4 single

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synthesized via mechanical alloying.

O4

O4

synthesized by co-

prepared using


**Section 2**

**Application**

**Section 2**
