**Application**

**Chapter 4**

**Provisional chapter**

**Conventional Sintering Effects on the Microstructure**

**Conventional Sintering Effects on the Microstructure** 

DOI: 10.5772/intechopen.78652

**and Electrical Characteristics of Low-Voltage Ceramic**

**and Electrical Characteristics of Low-Voltage Ceramic** 

Conventional, free or pressure less sintering is the simplest technique which involves heating of a powder compact, previously prepared at ambient temperatures, without applying any external pressure. It can be conducted with various box furnaces or tube furnaces under different atmospheres (oxidizing, reducing, inert, and vacuum). Through the use of this method, a highly applicable varistor can be mass produced. Varistors are of a particular interest for modern surge protection of over-voltage. Nowadays, ZnO ceramic varistors are most favorable in electronic industry due to their excellent electrical characteristics and high energy handling capabilities. By optimizing the method during sintering process, the number of potential barriers formed can be controlled thus improving the capability of the low-voltage varistor. **Keywords:** conventional sintering, microstructure, electrical properties, low-voltage

Sintering or firing of ceramic materials is the heat treatment to provide the energy to the ceramic powder particle to bond together to remove the porosity exist from compaction

> © 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.

Wan Mohamad Ikhmal Wan Mohamad Kamaruzzaman,

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Kamaruzzaman, Maria Fazira Mohd Fekeri and

Wan Mohamad Ikhmal Wan Mohamad

**Varistor**

**Varistor**

Mohd Sabri Mohd Ghazali, Muhamad Syaizwadi Shaifudin, Wan Rafizah Wan Abdullah,

Mohd Sabri Mohd Ghazali, Muhamad Syaizwadi Shaifudin, Wan Rafizah Wan Abdullah,

Maria Fazira Mohd Fekeri and Muhamad Azman Zulkifli

Muhamad Azman Zulkifli

**Abstract**

varistors

**1. Introduction**

http://dx.doi.org/10.5772/intechopen.78652

### **Conventional Sintering Effects on the Microstructure and Electrical Characteristics of Low-Voltage Ceramic Varistor Conventional Sintering Effects on the Microstructure and Electrical Characteristics of Low-Voltage Ceramic Varistor**

DOI: 10.5772/intechopen.78652

Mohd Sabri Mohd Ghazali, Muhamad Syaizwadi Shaifudin, Wan Rafizah Wan Abdullah, Wan Mohamad Ikhmal Wan Mohamad Kamaruzzaman, Maria Fazira Mohd Fekeri and Muhamad Azman Zulkifli Mohd Sabri Mohd Ghazali, Muhamad Syaizwadi Shaifudin, Wan Rafizah Wan Abdullah, Wan Mohamad Ikhmal Wan Mohamad Kamaruzzaman, Maria Fazira Mohd Fekeri and Muhamad Azman Zulkifli

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.78652

**Abstract**

Conventional, free or pressure less sintering is the simplest technique which involves heating of a powder compact, previously prepared at ambient temperatures, without applying any external pressure. It can be conducted with various box furnaces or tube furnaces under different atmospheres (oxidizing, reducing, inert, and vacuum). Through the use of this method, a highly applicable varistor can be mass produced. Varistors are of a particular interest for modern surge protection of over-voltage. Nowadays, ZnO ceramic varistors are most favorable in electronic industry due to their excellent electrical characteristics and high energy handling capabilities. By optimizing the method during sintering process, the number of potential barriers formed can be controlled thus improving the capability of the low-voltage varistor.

**Keywords:** conventional sintering, microstructure, electrical properties, low-voltage varistors

### **1. Introduction**

Sintering or firing of ceramic materials is the heat treatment to provide the energy to the ceramic powder particle to bond together to remove the porosity exist from compaction

© 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.

stages. The sintering process involves strengthening of powder compact by heating to a high temperature. During sintering process, the ceramic powders of the separate particles disperse to the neighboring powder particles. The sintering process reduces the surface energy of the particles by decreasing their vapor-solid interfaces. The pores take places in the disc/pellet where it diminishes, resulting in densification of the compact ceramic powders, and increases its mechanical properties. The porosity will be decrease from the effect of sintering temperature and time. Sintering will be improved if a liquid phase takes part in the process and a long time and high temperature are needed for the dispersion happens in solid state.

which involves the final densification of the ceramic material at high temperature. Sintering process of varistor ceramic was commonly achieved at three stages. At the first stages, a liquid phase is formed due to the dispersion and the homogeneous distribution of the dopants and contribute to the grain growth at the second stage. For the second stage at the end the of the process which is for in the beginning of the last stage, the grain growth, crystallization of the secondary and spinel phase, formation of the potential barriers, and retraction of the liquid phase from the two grain boundaries to the triple junctions are taking place [6]. The sintering process gives a microstructure with conductive ZnO grains and improves the grain boundaries with additive. The sintered varistor pellets was then silver paste for electrical characteristics. This processing method is still preferred in varistor industries due to their low cost of production, processing viability, only need basic instrumentation tools for preparation of varistors, reduce risks and hazards leads to the attraction of this processing method.

Conventional Sintering Effects on the Microstructure and Electrical Characteristics…

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67

**2. Tailoring the properties of grain and grain boundaries during the** 

As one of the most widely used electronic tool in the world, designing and tailoring the microstructure of ceramic play a big role in determining their properties for a specific application. Nowadays, the usage of modern ceramic can be seen almost virtually in any modern devices. In fact, without the existence of ceramic, there would not be a \$2 trillion global industry today [7]. The only reason it manages to come this far is none other because of its capability to fit in various emergence of new products. Thus, researching and discovering a better method to tailor the properties of ceramic on the atomic level are of a great importance to future generation of technology. The focused of this topic will be dealing on the subject of tailoring the properties of grain and grain boundaries of ZnO ceramic varistor by reviewing previous

Zinc oxide is an interesting and useful material, which produces various applications such as optical devices, sensors, FET devices, SAW devices, and varistor by a few processing procedure. Between all of them, the effects of varistor with using zinc oxide have become interesting nowadays [8]. Zinc oxide based varistor (ZnO) is one of material electronic semiconductor ceramic that have properties high energy absorption which capable to defend electronic device from excess voltage flow that sent to the electronic components that will cause breakdown [9]. Commonly, sintered zinc oxide ceramics produce their own microstructure with numerous grains and grain boundaries. The microstructure unit consists of grain−grain and boundary−grain, which is 3-dimensional series and parallel connection and it is distributed into entire bulk. Pure zinc oxide will show a linear voltage (V)–current (I) connection that obey the Ohm's Law [10]. However, the main features of ZnO possess the attribute of nonlinear current-voltage (I-V) with a grain size suitable for enhancement to improve the breakdown voltage of a varistor. By decreasing the grain size below 10 μm the material become qualified to be applied in high voltage application while larger grain size (>30 μm) is fitting

**fabrication process**

related studies.

for low-voltage application [10].

Sintering can result in high-strength bonds, particularly in ceramic materials with a crystalline structure. Sintering is the final step in the ceramic fabrication process where it will provide ceramic powders with density. The sintering operation is carried out in many stages such as heating up, annealing at specified temperature and cooling. The atmosphere, temperature and duration need to be chosen carefully for ceramic materials in order to provide a ceramic material with particular characteristics. The required characteristics of ceramic material are needed to design processing methods that will provide this required properties. The aim of sintering process is to increase the mechanical strength of the material and to prevent deformation and cracking of samples. Sintering of ceramic powder compacts will undergo several significant changes including chemical reactions in the solid state such as decomposition, oxidation and phase transformations. The sintering proceeds in different ways for different ceramic materials to provide densification of ceramic powder compact to improve the properties of the material [1].

Fabrication of varistor ceramics is normally achieved via conventional solid-state ceramic fabrication method by applying sintering temperature. Varistor is a solid state electronic ceramic component used to protect electronic devices against overvoltage surges. Varistor are of particular interest to modern surge protection, which commonly made from zinc oxide. The application of ZnO varistor as high or low-voltage varistors is related to the presence of potential barriers and improves their microstructure, which can be controlled during sintering [2]. The sintering temperature has a prominent effect on the electrical characteristics of varistor ceramics where the process will contribute to the formation of a multi-phase microstructure and promoting the formation of potential barriers and also gives rise to a distinctive microstructure of ZnO varistor ceramics [3, 4]. ZnO varistor ceramics with minor additions of other oxides exhibit nonlinear electrical characteristic and, therefore are widely used as varistors devices to protect electronic equipment against overvoltage [5]. Conventional preparation of varistors are preparing of powder by weighing, milling, mixing and spray drying the milled of different metal oxide materials. After that, for electrical characterization the powder is pressed into disc-shapes (pellets) form with predetermined thickness in order to obtain a desired application. Finally, the pressed powder, which is the green pellets, will be expose to heat treatment by using conventional sintering at different sintering temperature, time and atmosphere, in order to improve their microstructure and electrical properties for desired application and method.

The sintering temperature of ceramic compact powder will transforms into a dense body with varistor characteristics. The physical properties are mostly developed during sintering, which involves the final densification of the ceramic material at high temperature. Sintering process of varistor ceramic was commonly achieved at three stages. At the first stages, a liquid phase is formed due to the dispersion and the homogeneous distribution of the dopants and contribute to the grain growth at the second stage. For the second stage at the end the of the process which is for in the beginning of the last stage, the grain growth, crystallization of the secondary and spinel phase, formation of the potential barriers, and retraction of the liquid phase from the two grain boundaries to the triple junctions are taking place [6]. The sintering process gives a microstructure with conductive ZnO grains and improves the grain boundaries with additive. The sintered varistor pellets was then silver paste for electrical characteristics. This processing method is still preferred in varistor industries due to their low cost of production, processing viability, only need basic instrumentation tools for preparation of varistors, reduce risks and hazards leads to the attraction of this processing method.

stages. The sintering process involves strengthening of powder compact by heating to a high temperature. During sintering process, the ceramic powders of the separate particles disperse to the neighboring powder particles. The sintering process reduces the surface energy of the particles by decreasing their vapor-solid interfaces. The pores take places in the disc/pellet where it diminishes, resulting in densification of the compact ceramic powders, and increases its mechanical properties. The porosity will be decrease from the effect of sintering temperature and time. Sintering will be improved if a liquid phase takes part in the process and a long

Sintering can result in high-strength bonds, particularly in ceramic materials with a crystalline structure. Sintering is the final step in the ceramic fabrication process where it will provide ceramic powders with density. The sintering operation is carried out in many stages such as heating up, annealing at specified temperature and cooling. The atmosphere, temperature and duration need to be chosen carefully for ceramic materials in order to provide a ceramic material with particular characteristics. The required characteristics of ceramic material are needed to design processing methods that will provide this required properties. The aim of sintering process is to increase the mechanical strength of the material and to prevent deformation and cracking of samples. Sintering of ceramic powder compacts will undergo several significant changes including chemical reactions in the solid state such as decomposition, oxidation and phase transformations. The sintering proceeds in different ways for different ceramic materials to provide densification of ceramic powder compact to improve the proper-

Fabrication of varistor ceramics is normally achieved via conventional solid-state ceramic fabrication method by applying sintering temperature. Varistor is a solid state electronic ceramic component used to protect electronic devices against overvoltage surges. Varistor are of particular interest to modern surge protection, which commonly made from zinc oxide. The application of ZnO varistor as high or low-voltage varistors is related to the presence of potential barriers and improves their microstructure, which can be controlled during sintering [2]. The sintering temperature has a prominent effect on the electrical characteristics of varistor ceramics where the process will contribute to the formation of a multi-phase microstructure and promoting the formation of potential barriers and also gives rise to a distinctive microstructure of ZnO varistor ceramics [3, 4]. ZnO varistor ceramics with minor additions of other oxides exhibit nonlinear electrical characteristic and, therefore are widely used as varistors devices to protect electronic equipment against overvoltage [5]. Conventional preparation of varistors are preparing of powder by weighing, milling, mixing and spray drying the milled of different metal oxide materials. After that, for electrical characterization the powder is pressed into disc-shapes (pellets) form with predetermined thickness in order to obtain a desired application. Finally, the pressed powder, which is the green pellets, will be expose to heat treatment by using conventional sintering at different sintering temperature, time and atmosphere, in order to improve their microstructure and electrical properties for desired

The sintering temperature of ceramic compact powder will transforms into a dense body with varistor characteristics. The physical properties are mostly developed during sintering,

time and high temperature are needed for the dispersion happens in solid state.

ties of the material [1].

66 Sintering Technology - Method and Application

application and method.

### **2. Tailoring the properties of grain and grain boundaries during the fabrication process**

As one of the most widely used electronic tool in the world, designing and tailoring the microstructure of ceramic play a big role in determining their properties for a specific application. Nowadays, the usage of modern ceramic can be seen almost virtually in any modern devices. In fact, without the existence of ceramic, there would not be a \$2 trillion global industry today [7]. The only reason it manages to come this far is none other because of its capability to fit in various emergence of new products. Thus, researching and discovering a better method to tailor the properties of ceramic on the atomic level are of a great importance to future generation of technology. The focused of this topic will be dealing on the subject of tailoring the properties of grain and grain boundaries of ZnO ceramic varistor by reviewing previous related studies.

Zinc oxide is an interesting and useful material, which produces various applications such as optical devices, sensors, FET devices, SAW devices, and varistor by a few processing procedure. Between all of them, the effects of varistor with using zinc oxide have become interesting nowadays [8]. Zinc oxide based varistor (ZnO) is one of material electronic semiconductor ceramic that have properties high energy absorption which capable to defend electronic device from excess voltage flow that sent to the electronic components that will cause breakdown [9]. Commonly, sintered zinc oxide ceramics produce their own microstructure with numerous grains and grain boundaries. The microstructure unit consists of grain−grain and boundary−grain, which is 3-dimensional series and parallel connection and it is distributed into entire bulk. Pure zinc oxide will show a linear voltage (V)–current (I) connection that obey the Ohm's Law [10]. However, the main features of ZnO possess the attribute of nonlinear current-voltage (I-V) with a grain size suitable for enhancement to improve the breakdown voltage of a varistor. By decreasing the grain size below 10 μm the material become qualified to be applied in high voltage application while larger grain size (>30 μm) is fitting for low-voltage application [10].

There are several factors, which contribute to controlling specific desired microstructure properties of ZnO ceramic varistor such as the temperature of sintering, hold time, addition of impurities (dopant), and so on [11]. By selecting a suitable combination of these factors during the fabrication process will an acceptable result is produced. For this topic, tailoring the microstructure of ceramic through a conventional method will be discussed and explained. The fabrication of ceramic generally begins with ceramic powder is processed into a compact form and pass through a heat treatment (sintering) where the structure starts to significantly change [12]. Changes include phase transformation and chemical reaction such as oxidation and decomposition. In different term, sintering is a diffusional process that occurred when the temperature of the material is increase to half or three quarters of its melting temperature [13]. Frankly, sintering can be considered as one of the major step in developing a desired outcome of a ceramic since during this process the densification begins to take place. The step is crucial due to density affect desirable properties such as dielectric constant and mechanical strength [14]. Detailed explanation on densification will be further explained later in the subtopic.

its function of dispelling unwanted composition, sometimes the process is also called as a purification process. The process will begin by increasing the temperature inside the furnace slowly until it reached the designated values usually above 500°C and below 900°C in an average spent of 2 hours [19]. After the heating process is finished, the inside furnace is cooled down slowly and turned off at specific temperature. Point to be noted is the temperature used in this step does not exceed the value of sintering temperature of the

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The process will then proceed with remilling the mixed powder while adding binder for the purpose of increasing the mechanical strength of green ceramic body. Binder such as polyvinyl alcohol (PVA) are commonly added in a small quantity and mix together using mortar and pestle [20]. There is supposedly an ideal method suggested i.e. through the use of spry-dry system. However, the disadvantages are it requires longer time, difficult to clean the mobile parts and mostly applied for a large amount of mixed powder. Thus, using agate-mortar is not only simpler but also proves to be fully functional. Selecting a suitable binder is also a key in producing a good pellet with adequate electrical properties. A good binder must yield high density and high fired strength, which are essential to increase the electrical properties of the ceramic varistor [21]. PVA is extensively chose in various study mainly due to its high affinity for adsorption reaction when reacts with

The following procedure is pressing where the finalized mixed powder is pressed under a high pressure forming a pellet or disk. The thickness of pellet depends upon the application to be tested. If for the purpose of producing an applicable low voltage ceramic varistor, the thickness is under 1.5 mm while above is for high voltage application. After the pellet is formed, then it will be sintered with the goal of converting a compact porous powder into a well-structured grain form possessing the desired mechanical and electrical properties. The optimum range of temperature used in the process is still debatable since many studies revealed different result when testing different substance under different temperature. But, the range of the best temperature generally falls between 800 and 1200°C [4, 22]. The sintering procedure sometimes continue to pre-sintering step where the products are once again sintered under the same condition due to the previous step unable to produce the desired grain structure or have some defects. By repeating the process, the structure will be further enhance

Conventional sintering technique can be considered as one of the commercially accepted method presently. Although there are other better methods such as sol–gel, hydrothermal and co-precipitation but this method is much more simple, faster and easier to use while ensuring

Microstructure is defined as structure of material that is extremely small in size. The structure of material can only be observed by using 25× magnification of microscope. The importance of understanding the microstructure lies in its capability to affect physical properties of material,

later procedure.

dispersed oxide particle in water.

a decent quality output.

result in superior mechanical and macro properties.

**2.2. Microstructure and electrical properties**

### **2.1. Process of powder through solid-state route**

Necessity to understand the importance of an appropriate techniques in the powder preparation is derived from a fact that the step will determine the outcome properties of the finished product. The process usually begins by setting a specific ratio of chemical composition without any presence of impurities with smallest grain size possible. By selecting a smaller and better developed material, the capability of the powder to produce a desired microstructure will significantly increase. Several studies have suggested the method of applying particle growth technique as well as disintegrating the grained materials as means to create smaller size powder [15]. However, such technique mostly required the researcher to spend large amount of money to obtained specific equipment with no real capability for mass production.

Previously, a technique consisting of ball mills are generally selected to fulfill the role of crushing the powder into fine form. Not only the technique is easier to operate, but the mechanism is also relatively simple. Moreover, it has wider control on the powder distribution by choosing a proper size and shape of the mill balls [16]. Nowadays, with the advance of technology the technique has been further develop and a better technique known as high energy ball milling has been introduced. The new technique is more adaptable and able to deal with lower particle size unlike the previous version where it can only function up till the micron size particle. By constantly colliding the particles of powder with the mill balls and bowl will result in mechanical energy, which promotes the phase reaction among the reactant reducing the size of particles to the size of nano. The use of the technique will also ensure the mixed oxide powder are evenly and homogenously mix up. Reportedly, the usage of high energy ball milling is a promising approach for the starting mixture of powder in the varistor ceramic preparation [17].

The mixed powder will later undergo calcination process where the materials are subjected to high temperature for the purpose of removing humidity and gases [18]. Due to its function of dispelling unwanted composition, sometimes the process is also called as a purification process. The process will begin by increasing the temperature inside the furnace slowly until it reached the designated values usually above 500°C and below 900°C in an average spent of 2 hours [19]. After the heating process is finished, the inside furnace is cooled down slowly and turned off at specific temperature. Point to be noted is the temperature used in this step does not exceed the value of sintering temperature of the later procedure.

The process will then proceed with remilling the mixed powder while adding binder for the purpose of increasing the mechanical strength of green ceramic body. Binder such as polyvinyl alcohol (PVA) are commonly added in a small quantity and mix together using mortar and pestle [20]. There is supposedly an ideal method suggested i.e. through the use of spry-dry system. However, the disadvantages are it requires longer time, difficult to clean the mobile parts and mostly applied for a large amount of mixed powder. Thus, using agate-mortar is not only simpler but also proves to be fully functional. Selecting a suitable binder is also a key in producing a good pellet with adequate electrical properties. A good binder must yield high density and high fired strength, which are essential to increase the electrical properties of the ceramic varistor [21]. PVA is extensively chose in various study mainly due to its high affinity for adsorption reaction when reacts with dispersed oxide particle in water.

The following procedure is pressing where the finalized mixed powder is pressed under a high pressure forming a pellet or disk. The thickness of pellet depends upon the application to be tested. If for the purpose of producing an applicable low voltage ceramic varistor, the thickness is under 1.5 mm while above is for high voltage application. After the pellet is formed, then it will be sintered with the goal of converting a compact porous powder into a well-structured grain form possessing the desired mechanical and electrical properties. The optimum range of temperature used in the process is still debatable since many studies revealed different result when testing different substance under different temperature. But, the range of the best temperature generally falls between 800 and 1200°C [4, 22]. The sintering procedure sometimes continue to pre-sintering step where the products are once again sintered under the same condition due to the previous step unable to produce the desired grain structure or have some defects. By repeating the process, the structure will be further enhance result in superior mechanical and macro properties.

Conventional sintering technique can be considered as one of the commercially accepted method presently. Although there are other better methods such as sol–gel, hydrothermal and co-precipitation but this method is much more simple, faster and easier to use while ensuring a decent quality output.

### **2.2. Microstructure and electrical properties**

There are several factors, which contribute to controlling specific desired microstructure properties of ZnO ceramic varistor such as the temperature of sintering, hold time, addition of impurities (dopant), and so on [11]. By selecting a suitable combination of these factors during the fabrication process will an acceptable result is produced. For this topic, tailoring the microstructure of ceramic through a conventional method will be discussed and explained. The fabrication of ceramic generally begins with ceramic powder is processed into a compact form and pass through a heat treatment (sintering) where the structure starts to significantly change [12]. Changes include phase transformation and chemical reaction such as oxidation and decomposition. In different term, sintering is a diffusional process that occurred when the temperature of the material is increase to half or three quarters of its melting temperature [13]. Frankly, sintering can be considered as one of the major step in developing a desired outcome of a ceramic since during this process the densification begins to take place. The step is crucial due to density affect desirable properties such as dielectric constant and mechanical strength [14]. Detailed explanation on densification will be further

Necessity to understand the importance of an appropriate techniques in the powder preparation is derived from a fact that the step will determine the outcome properties of the finished product. The process usually begins by setting a specific ratio of chemical composition without any presence of impurities with smallest grain size possible. By selecting a smaller and better developed material, the capability of the powder to produce a desired microstructure will significantly increase. Several studies have suggested the method of applying particle growth technique as well as disintegrating the grained materials as means to create smaller size powder [15]. However, such technique mostly required the researcher to spend large amount of money to obtained specific equipment with no real capability for mass

Previously, a technique consisting of ball mills are generally selected to fulfill the role of crushing the powder into fine form. Not only the technique is easier to operate, but the mechanism is also relatively simple. Moreover, it has wider control on the powder distribution by choosing a proper size and shape of the mill balls [16]. Nowadays, with the advance of technology the technique has been further develop and a better technique known as high energy ball milling has been introduced. The new technique is more adaptable and able to deal with lower particle size unlike the previous version where it can only function up till the micron size particle. By constantly colliding the particles of powder with the mill balls and bowl will result in mechanical energy, which promotes the phase reaction among the reactant reducing the size of particles to the size of nano. The use of the technique will also ensure the mixed oxide powder are evenly and homogenously mix up. Reportedly, the usage of high energy ball milling is a promising approach for the starting mixture of powder in the varistor ceramic

The mixed powder will later undergo calcination process where the materials are subjected to high temperature for the purpose of removing humidity and gases [18]. Due to

explained later in the subtopic.

68 Sintering Technology - Method and Application

production.

preparation [17].

**2.1. Process of powder through solid-state route**

Microstructure is defined as structure of material that is extremely small in size. The structure of material can only be observed by using 25× magnification of microscope. The importance of understanding the microstructure lies in its capability to affect physical properties of material, which is metals, ceramics and composite and also polymers. Such physical properties include strength, toughness, ductility, corrosion resistance, temperature behavior, and hardness or wear resistance [23]. Moreover, it is important to carefully decide the scale of magnification during conducting observation on microstructure since the characteristic of material microstructural may have a huge distinct when observed from various length scales. In ZnO-based varistor, the microstructure refers to the grain and grain boundaries.

arrangement. The movement of the particle will cause the microstructure to shrink contributing to the overall increase in the density. Moreover, the stage also leads the particles to form necks between one another as shown in **Figure 1** as the interparticle contact is increased. The first stage is assumed to finish when the extent of neck growth of particle reach to 0.4 and 0.5

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The intermediate stage starts immediately right after the end of the first when the pores of the powder have achieved their equilibrium configuration. Although the particles have begun to develop at this point, the overall density is still low with the pores are mostly linked to one another. Thus, in the second stage the densification will cause the length of cross section between the pores to significantly reduce which eventually result in the pores develop into an unstable state and break away. The second stage can in fact be regards as the major stage out of the three. With the particles are fully individualize, the final stage will take place. At this section, the sintering process generally covers the elimination of isolated pores present in the powder increasing the total density to its theoretical value. Furthermore, the growth of grains is also reach its crucial step at this point where larger grains will exponentially increase by

The importance of densifying the green bodies of ceramic varistor lies in the formation of continues 3D structure for further selected application. The mechanisms, which are generally responsible for densification, are migration of grain boundaries and diffusion of grain boundaries where the first oversee the last stage of the whole sintering process. Migration of grain boundary refers to the movement of boundary, which separates different grain body through diffusion of atoms from one body to another. Several factors act as the driving force impacting the movement such as strain and elastic energy. The second mechanism of grain boundary diffusion will further densify the ceramic varistor until it reaches highest density

Additionally, the whole densification process can also be seen in three different scales i.e. global, microstructure and atomic scale. Through global scale it shows the densification process which occurred because of surface energy minimization which leads to grain boundaries replacing solid–gas interface. The second scale of microstructure focused on the differences in pressure and concentration gradient due to the presence of vacancies that act as a driving force for the transfer of mass. Finally, the atomic scale reveals the condition of all atoms either in a convex or concave surfaces where there is higher concentration of atoms on the surface of

**Figure 1.** Diagrammatic depiction of (a) powder compact, (b) partial densification of neck growth and (c) fully densified

of its total radius.

sacrificing smaller grains [27].

capable by the mixed materials [28].

neck growth [26].

Nowadays, all of the electrical and electronic devices have varistor's help. ZnO-based varistor is usually used due to its ability to surge protection from overvoltage current. This is because, ZnO provide an electrostatic potential that act as a barrier between the grains in the sintered body of an electronic tools. Generally, the production of ZnO based varistor is prepared by the addition of additive, which is needed to improve the efficiency varistor for further application [12]. Through the sintering process, ZnO based varistor will form a polycrystalline structure that consist of semiconductor ZnO grains after sintering process [9].

Typically, ZnO is a material that controlled by grain boundaries. It is expected that the properties of the samples will be modified due to the many defect present. Microstructurally, the doped-ZnO samples consist of a very high conductive n-type ZnO grains that is surrounded by an electrically insulating regions of grain boundary. Increasing the sintering temperature up to limited temperature will cause the average size of grain gradually increased. This will reduce the discontinuity between the grains that happen when the microstructure became more compact with less grain boundaries. Due to the increasing of sintering temperature, larger driving forces for internal atomic diffusion enhance the grain growth and pore elimination [5].

In general, the structure of the grains, grain boundaries morphology, density and also distribution of second phase are some of the factors influenced the electrical properties of ZnO such nonlinear coefficient (*α*), breakdown field (*Eb* ), leakage current density (*J L* ), and barrier height (*ϕ<sup>b</sup>* ) [11, 24].The mechanical, magnetic piezoelectric and electrical properties of ceramic also will improve if the grain size is smaller which also can help to enhance the application of ceramics [5]. The parameter of the sintering process such as temperature and hold time is really important in getting grain structure. In order to form ceramic with good varistor characteristic, a homogeneous distribution of dopant and correct concentration of oxygen is necessary, as the conductivity of zinc oxide depends on oxygen defect in the structure [11]. Methods that involve during sintering process are important to investigate in order to achieve the solids microstructure and final properties [25].

### **2.3. Densification**

Density is defined as the amount of substance that occupies a defined volume at stated pressure and temperature. In the production of ceramic varistor, density is one of the essential component, which requires a special concern. Without a good control on the development of density, the material will not be able to achieved its desired performance [25]. If we take a look generally on the densification step occurred during sintering, the process can be considered to be divided in three stages i.e. initial, intermediate and final stage. During the first stage, when the particles of powder are exposed to sintering force it begins to rotate and slide into a stable arrangement. The movement of the particle will cause the microstructure to shrink contributing to the overall increase in the density. Moreover, the stage also leads the particles to form necks between one another as shown in **Figure 1** as the interparticle contact is increased. The first stage is assumed to finish when the extent of neck growth of particle reach to 0.4 and 0.5 of its total radius.

which is metals, ceramics and composite and also polymers. Such physical properties include strength, toughness, ductility, corrosion resistance, temperature behavior, and hardness or wear resistance [23]. Moreover, it is important to carefully decide the scale of magnification during conducting observation on microstructure since the characteristic of material microstructural may have a huge distinct when observed from various length scales. In ZnO-based

Nowadays, all of the electrical and electronic devices have varistor's help. ZnO-based varistor is usually used due to its ability to surge protection from overvoltage current. This is because, ZnO provide an electrostatic potential that act as a barrier between the grains in the sintered body of an electronic tools. Generally, the production of ZnO based varistor is prepared by the addition of additive, which is needed to improve the efficiency varistor for further application [12]. Through the sintering process, ZnO based varistor will form a polycrystalline

Typically, ZnO is a material that controlled by grain boundaries. It is expected that the properties of the samples will be modified due to the many defect present. Microstructurally, the doped-ZnO samples consist of a very high conductive n-type ZnO grains that is surrounded by an electrically insulating regions of grain boundary. Increasing the sintering temperature up to limited temperature will cause the average size of grain gradually increased. This will reduce the discontinuity between the grains that happen when the microstructure became more compact with less grain boundaries. Due to the increasing of sintering temperature, larger driving forces for internal atomic diffusion enhance the grain growth and

In general, the structure of the grains, grain boundaries morphology, density and also distribution of second phase are some of the factors influenced the electrical properties of ZnO

also will improve if the grain size is smaller which also can help to enhance the application of ceramics [5]. The parameter of the sintering process such as temperature and hold time is really important in getting grain structure. In order to form ceramic with good varistor characteristic, a homogeneous distribution of dopant and correct concentration of oxygen is necessary, as the conductivity of zinc oxide depends on oxygen defect in the structure [11]. Methods that involve during sintering process are important to investigate in order to achieve

Density is defined as the amount of substance that occupies a defined volume at stated pressure and temperature. In the production of ceramic varistor, density is one of the essential component, which requires a special concern. Without a good control on the development of density, the material will not be able to achieved its desired performance [25]. If we take a look generally on the densification step occurred during sintering, the process can be considered to be divided in three stages i.e. initial, intermediate and final stage. During the first stage, when the particles of powder are exposed to sintering force it begins to rotate and slide into a stable

) [11, 24].The mechanical, magnetic piezoelectric and electrical properties of ceramic

), leakage current density (*J*

*L*

), and barrier

varistor, the microstructure refers to the grain and grain boundaries.

structure that consist of semiconductor ZnO grains after sintering process [9].

pore elimination [5].

70 Sintering Technology - Method and Application

height (*ϕ<sup>b</sup>*

**2.3. Densification**

such nonlinear coefficient (*α*), breakdown field (*Eb*

the solids microstructure and final properties [25].

The intermediate stage starts immediately right after the end of the first when the pores of the powder have achieved their equilibrium configuration. Although the particles have begun to develop at this point, the overall density is still low with the pores are mostly linked to one another. Thus, in the second stage the densification will cause the length of cross section between the pores to significantly reduce which eventually result in the pores develop into an unstable state and break away. The second stage can in fact be regards as the major stage out of the three. With the particles are fully individualize, the final stage will take place. At this section, the sintering process generally covers the elimination of isolated pores present in the powder increasing the total density to its theoretical value. Furthermore, the growth of grains is also reach its crucial step at this point where larger grains will exponentially increase by sacrificing smaller grains [27].

The importance of densifying the green bodies of ceramic varistor lies in the formation of continues 3D structure for further selected application. The mechanisms, which are generally responsible for densification, are migration of grain boundaries and diffusion of grain boundaries where the first oversee the last stage of the whole sintering process. Migration of grain boundary refers to the movement of boundary, which separates different grain body through diffusion of atoms from one body to another. Several factors act as the driving force impacting the movement such as strain and elastic energy. The second mechanism of grain boundary diffusion will further densify the ceramic varistor until it reaches highest density capable by the mixed materials [28].

Additionally, the whole densification process can also be seen in three different scales i.e. global, microstructure and atomic scale. Through global scale it shows the densification process which occurred because of surface energy minimization which leads to grain boundaries replacing solid–gas interface. The second scale of microstructure focused on the differences in pressure and concentration gradient due to the presence of vacancies that act as a driving force for the transfer of mass. Finally, the atomic scale reveals the condition of all atoms either in a convex or concave surfaces where there is higher concentration of atoms on the surface of

**Figure 1.** Diagrammatic depiction of (a) powder compact, (b) partial densification of neck growth and (c) fully densified neck growth [26].

concave than convex. The movement on this scale can be seen as a flow of atom from higher place (higher concentration) to the lower region with the upper region having more energy and mobility [29].

Furthermore, the processed materials are mostly enhanced via inhibition of the grain for it to develop while reducing the processing time and energy required to complete the process. The application of microwave technology is not really something new in the field of processing and material science. Its applications are actually widely applied in various field such as calcination, drying of ceramic and decomposition of gaseous species. Processing materials it is only limited to only 2000 ceramics with the use of microwave, polymeric materials, semiconductors and inorganic. The advantages of this sintering technique are great microstructure control, improved the material mechanical properties, the product have no limit geometry and reduce the manufacturing cost due to low temperature, energy used and processing time. Microwave sintered sample also reported that hardly reveal any development and cobalt does not exhibit any dissolution of tungsten while there are nearly 20% dissolved in cobalt binder phase in the conventional sintering. The researcher also found that sample that sintered in microwave always showed improvement in mechanical properties compared to the conven-

Conventional Sintering Effects on the Microstructure and Electrical Characteristics…

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73

Spark Plasma Sintering or in a more complex Pulsed Electric Current Sintering (PECS) is a new technology in the field of metals, ceramics and composite fabrication starting from powders. With the nanostructured features, it has the potential of densifying powders while avoid it become rough which follow the densification routes [34]. This spark plasma sintering mechanism has been investigated in the 1960s and began to be used in metal powder compressed. But there is no wider use of it since the price of the equipment are very expensive coupled with inferior efficiency in sintering. To heat the specimens, the use of pulsed direct current is commonly used in these systems. SPS consist of several parts of uniaxial pressure machine where the water-cooled punches also work as electrodes, a pulsed DC generator a water cooled reaction chamber, position, pressure and temperature regulation system. The relatively low homogenous temperature and short duration required for this technique because it is really suitable for the preservation and nanocrystalline densification feature in the ceramics material. Nowadays SPS is widely used due to the possibility in performing a fast consolidation of ceramic that tough to sinter and composite ceramics during decreased

Zinc oxide (ZnO) ceramic materials are commonly used for overvoltage protection in electronic industry. ZnO varistor ceramic is nonlinear electrical component and high energy handling capabilities. Low-voltage varistor are now highly demand for surge protection in electronic devices with fast response, highly nonlinear current–voltage properties and energy absorption capabilities. The performance of low-voltage ZnO varistors can be improve by increasing the grain size, which allows the decreased grain boundary per unit volume and improves the nonlinear electrical characteristics. Low-voltage varistors are improved when their thickness is decrease to increase the size of ZnO grains. However the strength and energy absorption

tional sintered one [34].

temperature [33].

**3. Low-voltage ZnO-based varistor**

*2.4.3. Spark plasma sintering*

Up to date, several researches have shown the relationship between sintering temperature and how it affects the densification of ceramic varistor. Such example includes decreasing in sintering temperature cause an increase in pores, which directly decrease the density and vice versa [8, 30, 31]. The truth is each material has its own properties that cause this kind of situation to happen. Thus, for every material present and included in the production of ceramic varistor it requires an elaborate investigation to determine their specific characteristics before any conclusion can be made.

### **2.4. Sintering technology**

Sintering is the densification of powder compact with the help of thermal treatment. It is also the key for processing the ceramic and powder metallurgical [32]. Sintering can be divided by two categories, which is conventional sintering and advanced sintering. Advanced sintering included spark plasma sintering (SPS), hot pressing sintering and microwave sintering. Unfortunately, some of the technique produce different final product that might not help the economy due to it possessing a non-viable property. Thus, the conventional method is considered to be more appealing for the purpose of mass producing ceramic product since it has lower cost maintenance. For conventional method, minimum grain growth can be controlled by maximization the last density that determine by the heating curve. By controlling the procedure of the heating curve, high densification of grain size can be controlled [25].

### *2.4.1. Conventional sintering*

Conventional sintering technology is the simplest form in sintering that also known as pressureless sintering. It only involves heating of the powder compact after prepared at ambient temperatures without any external pressure applied during the process. Nanostructured ceramic materials that have dense properties normally acquire nanopowder that have undergo pressing process, which is done through a pressure assisting method. The pressure assisted method includes hot pressing, sinter forging, hot isostatic pressing, and others [33]. Hot pressing technique can also use to produce the mixture of two or more types of metals powder base product that can be improved the mechanical properties. When using the hot pressing method, some of the ceramic materials are found to be densified even at lower temperature when compared to conventional method. The benefits in using hot pressing sintering technique are firstly improving the densification kinetic and limited of grain development, where disadvantages are the end product have limited geometry and equipment needed highly in cost [34].

### *2.4.2. Microwave sintering*

Generally, it has been 3 decades since the microwave sintering of ceramics have been introduced. Respectively, it has some superiority, which is fast processing and heating selective. Furthermore, the processed materials are mostly enhanced via inhibition of the grain for it to develop while reducing the processing time and energy required to complete the process. The application of microwave technology is not really something new in the field of processing and material science. Its applications are actually widely applied in various field such as calcination, drying of ceramic and decomposition of gaseous species. Processing materials it is only limited to only 2000 ceramics with the use of microwave, polymeric materials, semiconductors and inorganic. The advantages of this sintering technique are great microstructure control, improved the material mechanical properties, the product have no limit geometry and reduce the manufacturing cost due to low temperature, energy used and processing time. Microwave sintered sample also reported that hardly reveal any development and cobalt does not exhibit any dissolution of tungsten while there are nearly 20% dissolved in cobalt binder phase in the conventional sintering. The researcher also found that sample that sintered in microwave always showed improvement in mechanical properties compared to the conventional sintered one [34].

### *2.4.3. Spark plasma sintering*

concave than convex. The movement on this scale can be seen as a flow of atom from higher place (higher concentration) to the lower region with the upper region having more energy

Up to date, several researches have shown the relationship between sintering temperature and how it affects the densification of ceramic varistor. Such example includes decreasing in sintering temperature cause an increase in pores, which directly decrease the density and vice versa [8, 30, 31]. The truth is each material has its own properties that cause this kind of situation to happen. Thus, for every material present and included in the production of ceramic varistor it requires an elaborate investigation to determine their specific characteristics before

Sintering is the densification of powder compact with the help of thermal treatment. It is also the key for processing the ceramic and powder metallurgical [32]. Sintering can be divided by two categories, which is conventional sintering and advanced sintering. Advanced sintering included spark plasma sintering (SPS), hot pressing sintering and microwave sintering. Unfortunately, some of the technique produce different final product that might not help the economy due to it possessing a non-viable property. Thus, the conventional method is considered to be more appealing for the purpose of mass producing ceramic product since it has lower cost maintenance. For conventional method, minimum grain growth can be controlled by maximization the last density that determine by the heating curve. By controlling the pro-

cedure of the heating curve, high densification of grain size can be controlled [25].

Conventional sintering technology is the simplest form in sintering that also known as pressureless sintering. It only involves heating of the powder compact after prepared at ambient temperatures without any external pressure applied during the process. Nanostructured ceramic materials that have dense properties normally acquire nanopowder that have undergo pressing process, which is done through a pressure assisting method. The pressure assisted method includes hot pressing, sinter forging, hot isostatic pressing, and others [33]. Hot pressing technique can also use to produce the mixture of two or more types of metals powder base product that can be improved the mechanical properties. When using the hot pressing method, some of the ceramic materials are found to be densified even at lower temperature when compared to conventional method. The benefits in using hot pressing sintering technique are firstly improving the densification kinetic and limited of grain development, where disadvantages are the end product have limited geometry and equipment

Generally, it has been 3 decades since the microwave sintering of ceramics have been introduced. Respectively, it has some superiority, which is fast processing and heating selective.

and mobility [29].

any conclusion can be made.

72 Sintering Technology - Method and Application

**2.4. Sintering technology**

*2.4.1. Conventional sintering*

needed highly in cost [34].

*2.4.2. Microwave sintering*

Spark Plasma Sintering or in a more complex Pulsed Electric Current Sintering (PECS) is a new technology in the field of metals, ceramics and composite fabrication starting from powders. With the nanostructured features, it has the potential of densifying powders while avoid it become rough which follow the densification routes [34]. This spark plasma sintering mechanism has been investigated in the 1960s and began to be used in metal powder compressed. But there is no wider use of it since the price of the equipment are very expensive coupled with inferior efficiency in sintering. To heat the specimens, the use of pulsed direct current is commonly used in these systems. SPS consist of several parts of uniaxial pressure machine where the water-cooled punches also work as electrodes, a pulsed DC generator a water cooled reaction chamber, position, pressure and temperature regulation system. The relatively low homogenous temperature and short duration required for this technique because it is really suitable for the preservation and nanocrystalline densification feature in the ceramics material. Nowadays SPS is widely used due to the possibility in performing a fast consolidation of ceramic that tough to sinter and composite ceramics during decreased temperature [33].

### **3. Low-voltage ZnO-based varistor**

Zinc oxide (ZnO) ceramic materials are commonly used for overvoltage protection in electronic industry. ZnO varistor ceramic is nonlinear electrical component and high energy handling capabilities. Low-voltage varistor are now highly demand for surge protection in electronic devices with fast response, highly nonlinear current–voltage properties and energy absorption capabilities. The performance of low-voltage ZnO varistors can be improve by increasing the grain size, which allows the decreased grain boundary per unit volume and improves the nonlinear electrical characteristics. Low-voltage varistors are improved when their thickness is decrease to increase the size of ZnO grains. However the strength and energy absorption capabilities of the thin ZnO varistor are very poor due to its small volume [35]. In addition to grains size and the additives, sintering temperature is an important parameter in the manufacture process of varistor-based ceramics. Low-voltage ZnO varistors are now being used for surge protection in integrated circuits and in automobiles. The electrical properties of low-voltage ZnO varistors are based on their composition and microstructure. Optimizing the process, the composition and microstructure of conventional varistors are used to achieve low-voltage varistor. Therefore, it is important to find a new method to fabricate high performance varistors without reducing their thickness.

**3.1. Barium titanate and calcium manganite as additive**

applications in electronic industry. Doping with BaTiO3

very attractive from both fundamental and applied perspectives.

titanium oxide (TiO2

tion. As barium titanate (BaTiO<sup>3</sup>

grain growth [42]. BaTiO3

presence of large BaTiO3

structure CaMnO3

Additive of CaMnO<sup>3</sup>

only ZnO grain and CaMnO3

properties of the varistor since BaTiO3

coefficient of resistivity (PTCR) sensors. The used of calcium manganite (CaMnO3

reveals that the heavily ZnO doping on the BaTiO3

A new processing technique in the production of low-voltage ZnO varistor are now being investigated for overvoltage protection in low-voltage electronic due to highly demand. The breakdown voltage (varistor voltage) is directly proportional to the number of ZnO grains in series between the electrodes, therefore, it can be achieve by decreasing the thickness of the disc/pellet or increase the size of ZnO grains. However, the thin ZnO varistor are weak, thus by using additives or improve their processing technique are important to optimize the performance of low-voltage varistor ceramics. In low-voltage varistor, a grain growth-enhancer

is one of the members of perovskite (ABO<sup>3</sup>

has significant effect due to the rich variety of physical properties such as high-temperature superconductivity and colossal magnetoresistance observed in these compounds makes them

Perovskite oxides have attracted much attention due to their structure properties formed by substitution make it outstanding functional materials which is exhibit various properties and one of the important usages of perovskite oxides is in the capacitor application because of their excellent dielectric properties [43]. The combination of varistor-capacitor characteristics makes it a promising material in the field of overvoltage protection of electronic devices. The

to control the microstructural development of the ceramic. According to previous research

pose of capacitor-varistor integration [44]. BaTiO3 is a prototypical ferroelectric material with a tetragonal distortion characteristic of the cubic perovskite structure. The ferroelectric distortion is facilitated by the large size of the Ba cation. Barium titanate is a good candidate for a variety of applications due to its excellent dielectric, ferroelectric, and piezoelectric properties [45]. It is extensively used in the electronic industry as capacitor and positive temperature

tor is extensively studied due to their unique properties that make them attractive in enhanc-

alkaline earth metal such as Ca, Sr., Ba and Pb, has been the subject of intense research during the last decade and it has a significant effect on the microstructure of ZnO varistor ceramics

ing additive, exhibit voltage-limiting electrical properties while the combination of perovskite

in order to produce low-voltage varistors. The combination of ZnO with perovskite manganite gives multifunctional properties for low-voltage electrical characteristics with large

as intergranular layer [46].

with the microstructure of ZnO varistor ceramics is simple consisting of

on the microstructure of ZnO varistor ceramics shows a good properties

ing the performance of the existing materials. Perovskite manganite AMnO<sup>3</sup>

[4]. In addition, the varistors prepared from ZnO with CaMnO3

) consist of TiO2

) is mostly used and can influence the degree of non-linearity of conduc-

Conventional Sintering Effects on the Microstructure and Electrical Characteristics…

grains on the ZnO microstructures will greatly improve the electrical

as the doping of ZnO based varistor possess the ability

) as additive material to produce low-voltage varis-

ceramic are very interesting for the pur-

, its addition can attribute to the formation of

on ZnO based varistor ceramics

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75

) family that has wide

, where A is an

perovskite as the only form-

In low-voltage ZnO varistors, the most influence additives are titanium oxide (TiO<sup>2</sup> ) which can greatly improve the grain growth of ZnO, thus is commonly used as a grain growth enhancing additive to produce low-voltage ZnO varistors. But, the doping of TiO2 reduces the degree of nonlinearity [36]. The degree of nonlinearity (*α*) is used to explain the characteristics of varistor ceramics with excellent surge withstanding capabilities. The coefficient α is the measure of efficiency of the device, the higher its values the more is the effectiveness of device in protecting a circuit from overvoltage [37]. The nonlinearity strongly depends on the microstructure and directly affects their electrical properties that can be adjusted by the means of sintering process. The performance of microstructure and electrical characteristics of varistor ceramic can be improve by adding additives by thermal treatment. A unique properties of grain boundaries is formed in the ceramics during sintering and they are responsible for determining the nonlinear electrical characteristics of varistor component. The chemical composition, sintering temperature, sintering time, heating and cooling rates are variables that can be adjust fundamentally to control the electrical performance of ZnO varistors [38].

The sintering temperature reaction between ZnO and additives lead to the formation of different phases in the ZnO grain boundary and the nonlinear properties are ascribe to the formation of potential barriers at the ZnO grain boundaries. The performance of ZnO-based components is sensitive to the presence of additive even though their amount is very small and the processing environment has significant effect on the microstructure of varistor ceramics. Development of specific microstructure at varying sintering condition in ZnO based varistor ceramics will determine its electrical characteristics especially at varistor voltage of the ceramic device, since it is directly related to the grain size and grain boundary of ZnO varistor ceramics. Therefore, the temperature at which these reactions take place will lead to different grain sizes, and different electrical properties will be obtained when fabricating the varistor device [39]. Industrially, varistor manufacturing is commonly by the conventional solid-state preparation method and ZnO varistors were manufactured through a high-temperature reaction called sintering. A dense varistor product was normally obtained through the sintering process, since the varistor performance depends on the final sintered microstructures, the sintering process must be carefully carried out. For sintering, the varistor powder needs to be hard-pressed to ceramic discs/pellets and should be heated at a temperature in the range of 1100–1250°C [40, 41]. The improvement of ZnO varistor with excellent electrical properties and high energy handling capability can be obtain through grain size control by using nanosize-doped ZnO powder and manage the excess of grain growth by step sintering process.

### **3.1. Barium titanate and calcium manganite as additive**

capabilities of the thin ZnO varistor are very poor due to its small volume [35]. In addition to grains size and the additives, sintering temperature is an important parameter in the manufacture process of varistor-based ceramics. Low-voltage ZnO varistors are now being used for surge protection in integrated circuits and in automobiles. The electrical properties of low-voltage ZnO varistors are based on their composition and microstructure. Optimizing the process, the composition and microstructure of conventional varistors are used to achieve low-voltage varistor. Therefore, it is important to find a new method to fabricate high perfor-

In low-voltage ZnO varistors, the most influence additives are titanium oxide (TiO<sup>2</sup>

enhancing additive to produce low-voltage ZnO varistors. But, the doping of TiO2

can greatly improve the grain growth of ZnO, thus is commonly used as a grain growth

the degree of nonlinearity [36]. The degree of nonlinearity (*α*) is used to explain the characteristics of varistor ceramics with excellent surge withstanding capabilities. The coefficient α is the measure of efficiency of the device, the higher its values the more is the effectiveness of device in protecting a circuit from overvoltage [37]. The nonlinearity strongly depends on the microstructure and directly affects their electrical properties that can be adjusted by the means of sintering process. The performance of microstructure and electrical characteristics of varistor ceramic can be improve by adding additives by thermal treatment. A unique properties of grain boundaries is formed in the ceramics during sintering and they are responsible for determining the nonlinear electrical characteristics of varistor component. The chemical composition, sintering temperature, sintering time, heating and cooling rates are variables that can be adjust fundamentally to control the electrical performance of ZnO

The sintering temperature reaction between ZnO and additives lead to the formation of different phases in the ZnO grain boundary and the nonlinear properties are ascribe to the formation of potential barriers at the ZnO grain boundaries. The performance of ZnO-based components is sensitive to the presence of additive even though their amount is very small and the processing environment has significant effect on the microstructure of varistor ceramics. Development of specific microstructure at varying sintering condition in ZnO based varistor ceramics will determine its electrical characteristics especially at varistor voltage of the ceramic device, since it is directly related to the grain size and grain boundary of ZnO varistor ceramics. Therefore, the temperature at which these reactions take place will lead to different grain sizes, and different electrical properties will be obtained when fabricating the varistor device [39]. Industrially, varistor manufacturing is commonly by the conventional solid-state preparation method and ZnO varistors were manufactured through a high-temperature reaction called sintering. A dense varistor product was normally obtained through the sintering process, since the varistor performance depends on the final sintered microstructures, the sintering process must be carefully carried out. For sintering, the varistor powder needs to be hard-pressed to ceramic discs/pellets and should be heated at a temperature in the range of 1100–1250°C [40, 41]. The improvement of ZnO varistor with excellent electrical properties and high energy handling capability can be obtain through grain size control by using nanosize-doped ZnO powder and manage the excess of grain

) which

reduces

mance varistors without reducing their thickness.

74 Sintering Technology - Method and Application

varistors [38].

growth by step sintering process.

A new processing technique in the production of low-voltage ZnO varistor are now being investigated for overvoltage protection in low-voltage electronic due to highly demand. The breakdown voltage (varistor voltage) is directly proportional to the number of ZnO grains in series between the electrodes, therefore, it can be achieve by decreasing the thickness of the disc/pellet or increase the size of ZnO grains. However, the thin ZnO varistor are weak, thus by using additives or improve their processing technique are important to optimize the performance of low-voltage varistor ceramics. In low-voltage varistor, a grain growth-enhancer titanium oxide (TiO2 ) is mostly used and can influence the degree of non-linearity of conduction. As barium titanate (BaTiO<sup>3</sup> ) consist of TiO2 , its addition can attribute to the formation of grain growth [42]. BaTiO3 is one of the members of perovskite (ABO<sup>3</sup> ) family that has wide applications in electronic industry. Doping with BaTiO3 on ZnO based varistor ceramics has significant effect due to the rich variety of physical properties such as high-temperature superconductivity and colossal magnetoresistance observed in these compounds makes them very attractive from both fundamental and applied perspectives.

Perovskite oxides have attracted much attention due to their structure properties formed by substitution make it outstanding functional materials which is exhibit various properties and one of the important usages of perovskite oxides is in the capacitor application because of their excellent dielectric properties [43]. The combination of varistor-capacitor characteristics makes it a promising material in the field of overvoltage protection of electronic devices. The presence of large BaTiO3 grains on the ZnO microstructures will greatly improve the electrical properties of the varistor since BaTiO3 as the doping of ZnO based varistor possess the ability to control the microstructural development of the ceramic. According to previous research reveals that the heavily ZnO doping on the BaTiO3 ceramic are very interesting for the purpose of capacitor-varistor integration [44]. BaTiO3 is a prototypical ferroelectric material with a tetragonal distortion characteristic of the cubic perovskite structure. The ferroelectric distortion is facilitated by the large size of the Ba cation. Barium titanate is a good candidate for a variety of applications due to its excellent dielectric, ferroelectric, and piezoelectric properties [45]. It is extensively used in the electronic industry as capacitor and positive temperature coefficient of resistivity (PTCR) sensors.

The used of calcium manganite (CaMnO3 ) as additive material to produce low-voltage varistor is extensively studied due to their unique properties that make them attractive in enhancing the performance of the existing materials. Perovskite manganite AMnO<sup>3</sup> , where A is an alkaline earth metal such as Ca, Sr., Ba and Pb, has been the subject of intense research during the last decade and it has a significant effect on the microstructure of ZnO varistor ceramics [4]. In addition, the varistors prepared from ZnO with CaMnO3 perovskite as the only forming additive, exhibit voltage-limiting electrical properties while the combination of perovskite structure CaMnO3 with the microstructure of ZnO varistor ceramics is simple consisting of only ZnO grain and CaMnO3 as intergranular layer [46].

Additive of CaMnO<sup>3</sup> on the microstructure of ZnO varistor ceramics shows a good properties in order to produce low-voltage varistors. The combination of ZnO with perovskite manganite gives multifunctional properties for low-voltage electrical characteristics with large nonlinear coefficients, which is suitable for semiconductor electronic and magnetoelectric devices due to magnetotransport properties of polycrystalline multi-phase ceramic [47]. The influence of perovskite CaMnO<sup>3</sup> as the only additives in the microstructure of ZnO varistor ceramics shows a significant effect on the electrical characteristics of low-voltage ZnO based varistor and with the new formulation for low-voltage ceramic varistor containing CaMnO3 as varistor former in spinel phase and doping elements of rare-earth also shows a potential to be used as doping low-voltage varistor [48]. The new generation of varistor that introduced perovskite as additive and as varistor former, make this device less use of additives as compared to first generation, which is use bismuth oxide as a varistor former [49–52].

CaMnO3

CaMnO3

The effect of BaTiO<sup>3</sup>

**Figure 2.** SEM micrographs of ZnO-BaTiO3

titanium oxide (TiO2

sist of TiO2

size of BaTiO3

as the additive improves the microstructure with the support of sintering process.

at sintering temperature in (a) 900°C, (b) 1100°C and (c) 1300°C.

Conventional Sintering Effects on the Microstructure and Electrical Characteristics…

by segregate of

77

as the doping of ZnO

. The varis-

concentration

act in inverse

. The varistor sintered

con-

) grains on the ZnO microstructures will

http://dx.doi.org/10.5772/intechopen.78652

percentage. It is well known that

The sintering temperature influences the microstructure of ZnO-CaMnO<sup>3</sup>

**3.3. Effects of ZnO + perovskite on the electrical properties**

of the decrease of ZnO grain size with increasing of BaTiO3

greatly improve the electrical properties of the varistor since BaTiO3

The incorporation of large barium titanate (BaTiO3

at the grain boundaries. The present of TiO2

for ZnO varistor is much better with BaTiO<sup>3</sup>

the effect of titanium oxide. The used of BaTiO<sup>3</sup>

In addition, the perovskite manganite CaMnO3

produce low-voltage varistor.

dopants at grain boundaries with the increase in sintering temperature.

based varistor possess the ability to control the microstructural development of the ceramic.

tor voltage is enhanced with the increase of the number of active grain boundaries because

at 1300°C decreases the varistor voltage due to the homogeneous microstructure of grain boundaries and increasing the grain size compared to varistor sintered at temperature 1250°C that possess high varistor voltage. The breakdown voltage of current–voltage characteristics

by increasing the grain size, which allows the decreased grain boundary per unit volume and improves the nonlinear electrical characteristics. The addition of perovskite structure BaTiO3 is attribute to the formation of potential barriers at the grain boundaries where the large grain

to the increase in nonlinearity of ZnO varistor, since it more dominant in comparison with

a suitable range of varistor voltage with the conventional sintering technology in order to

of the ZnO varistor system. It presents a good electrical properties for low-voltage varistor

content as the additive and the varistor voltage increase significantly with BaTiO<sup>3</sup>

size are increase it lowering the varistor voltage with the increase in of BaTiO3

since nonlinear coefficient (*α*) increases with the addition of BaTiO3

on the electrical properties can be seen clearly with the increase of BaTiO3

) are commonly used to produce low-voltage varistor, since BaTiO3

will greatly increase the grain size and the present of Barium will contribute

in the perovskite structure BaTiO3

. For low-voltage ZnO varistors it can be improve

as an additive for grain growth will produce

as an additive changes the breakdown voltage

it will increase the grain size but restrict the nonlinear properties. When the grain

### **3.2. Effects of ZnO + perovskite on the development of microstructure**

The further improvement of the electrical characteristics is associated to the ability to control the microstructural development in the ceramic materials. The used of barium titanate (BaTiO3 ) as an additive on the microstructure and grain growth in the ZnO varistor ceramics shows a significant effect, where it contains titanium oxide (TiO<sup>2</sup> ) which has mostly used as grain growth enhancer and can influence the nonlinear coefficient of varistor. BaTiO<sup>3</sup> is a ceramic material with a characteristic of the cubic perovskite structure and facilitated by the large size of the Ba cation. The displacement of atoms in BaTiO3 as a function of an external electric field will induce to a nonlinear behavior. ZnO-BaTiO<sup>3</sup> -based varistor ceramic sintered at 1300°C enhances their grain size and improves microstructural uniformity. The microstructure consists of two phase which is ZnO grain (primary phase) and inter-granular phase with concentration of BaTiO3 solid solution in the ZnO grain boundaries. The BaTiO3 as additive increase the grain size of ZnO compared to the sample without BaTiO3 at the same sintering temperature. From the microstructure, the ZnO grains reveal high concentration of additives with BaTiO3 element. The distribution of the chemical elements is homogeneous except near the grain boundaries where the solid solutions are located. The inhomogeneity is characterized by a strong concentration in the grain boundaries, which contain of excess BaTiO3 in ZnO microstructure. The secondary phase is located near triple-grain junctions and nodal points in the grain boundaries with the high concentration of the additives. The competition between dissolution and segregation of the BaTiO3 into the grain boundaries of ZnO are present and this chemical and physical reaction depends on the sintering temperature and amount of concentration between them. From **Figure 2**, the microstructure of ZnO doped with BaTiO3 is shown to be larger as the sintering temperature is increase from 900 to 1300°C.

Additive of calcium manganite (CaMnO<sup>3</sup> ) on the ZnO based varistor reveals the presence of ZnO as dominant in the microstructure and the secondary phase formed at the grain boundaries and also at the triple point junction which consist of CaMnO3 as varistor former for grain growth. The ionic radii of Mn2+ is larger than ZnO2+ ions, therefore, it segregated at the grain boundaries as secondary phases. However this phase reduces when the sintering temperature was increased to 1300°C due to the reactive melting of CaMnO3 . The non-uniformity of the grain structure of ZnO-CaMnO3 based varistor ceramics are reduce when the sintering temperature are increase which a uniform grains are present and free from abnormal grain growth by doping of CaMnO3 as an additive. ZnO ceramics doped with perovskite phase of Conventional Sintering Effects on the Microstructure and Electrical Characteristics… http://dx.doi.org/10.5772/intechopen.78652 77

**Figure 2.** SEM micrographs of ZnO-BaTiO3 at sintering temperature in (a) 900°C, (b) 1100°C and (c) 1300°C.

CaMnO3 as the additive improves the microstructure with the support of sintering process. The sintering temperature influences the microstructure of ZnO-CaMnO<sup>3</sup> by segregate of CaMnO3 dopants at grain boundaries with the increase in sintering temperature.

### **3.3. Effects of ZnO + perovskite on the electrical properties**

nonlinear coefficients, which is suitable for semiconductor electronic and magnetoelectric devices due to magnetotransport properties of polycrystalline multi-phase ceramic [47]. The

ceramics shows a significant effect on the electrical characteristics of low-voltage ZnO based varistor and with the new formulation for low-voltage ceramic varistor containing CaMnO3 as varistor former in spinel phase and doping elements of rare-earth also shows a potential to be used as doping low-voltage varistor [48]. The new generation of varistor that introduced perovskite as additive and as varistor former, make this device less use of additives as com-

The further improvement of the electrical characteristics is associated to the ability to control the microstructural development in the ceramic materials. The used of barium titanate

grain growth enhancer and can influence the nonlinear coefficient of varistor. BaTiO<sup>3</sup>

ceramic material with a characteristic of the cubic perovskite structure and facilitated by the

at 1300°C enhances their grain size and improves microstructural uniformity. The microstructure consists of two phase which is ZnO grain (primary phase) and inter-granular phase with

temperature. From the microstructure, the ZnO grains reveal high concentration of additives

the grain boundaries where the solid solutions are located. The inhomogeneity is character-

microstructure. The secondary phase is located near triple-grain junctions and nodal points in the grain boundaries with the high concentration of the additives. The competition between

this chemical and physical reaction depends on the sintering temperature and amount of concentration between them. From **Figure 2**, the microstructure of ZnO doped with BaTiO3

ZnO as dominant in the microstructure and the secondary phase formed at the grain bound-

growth. The ionic radii of Mn2+ is larger than ZnO2+ ions, therefore, it segregated at the grain boundaries as secondary phases. However this phase reduces when the sintering tempera-

temperature are increase which a uniform grains are present and free from abnormal grain

ized by a strong concentration in the grain boundaries, which contain of excess BaTiO3

shown to be larger as the sintering temperature is increase from 900 to 1300°C.

aries and also at the triple point junction which consist of CaMnO3

ture was increased to 1300°C due to the reactive melting of CaMnO3

) as an additive on the microstructure and grain growth in the ZnO varistor ceramics

solid solution in the ZnO grain boundaries. The BaTiO3

element. The distribution of the chemical elements is homogeneous except near

pared to first generation, which is use bismuth oxide as a varistor former [49–52].

**3.2. Effects of ZnO + perovskite on the development of microstructure**

shows a significant effect, where it contains titanium oxide (TiO<sup>2</sup>

large size of the Ba cation. The displacement of atoms in BaTiO3

increase the grain size of ZnO compared to the sample without BaTiO3

electric field will induce to a nonlinear behavior. ZnO-BaTiO<sup>3</sup>

as the only additives in the microstructure of ZnO varistor

) which has mostly used as

as a function of an external


into the grain boundaries of ZnO are present and

) on the ZnO based varistor reveals the presence of

based varistor ceramics are reduce when the sintering

as an additive. ZnO ceramics doped with perovskite phase of

is a

as additive

in ZnO

is

at the same sintering

as varistor former for grain

. The non-uniformity of

influence of perovskite CaMnO<sup>3</sup>

76 Sintering Technology - Method and Application

(BaTiO3

concentration of BaTiO3

dissolution and segregation of the BaTiO3

Additive of calcium manganite (CaMnO<sup>3</sup>

the grain structure of ZnO-CaMnO3

growth by doping of CaMnO3

with BaTiO3

The incorporation of large barium titanate (BaTiO3 ) grains on the ZnO microstructures will greatly improve the electrical properties of the varistor since BaTiO3 as the doping of ZnO based varistor possess the ability to control the microstructural development of the ceramic. The effect of BaTiO<sup>3</sup> on the electrical properties can be seen clearly with the increase of BaTiO3 content as the additive and the varistor voltage increase significantly with BaTiO<sup>3</sup> . The varistor voltage is enhanced with the increase of the number of active grain boundaries because of the decrease of ZnO grain size with increasing of BaTiO3 percentage. It is well known that titanium oxide (TiO2 ) are commonly used to produce low-voltage varistor, since BaTiO3 consist of TiO2 it will increase the grain size but restrict the nonlinear properties. When the grain size are increase it lowering the varistor voltage with the increase in of BaTiO3 concentration at the grain boundaries. The present of TiO2 in the perovskite structure BaTiO3 act in inverse since nonlinear coefficient (*α*) increases with the addition of BaTiO3 . The varistor sintered at 1300°C decreases the varistor voltage due to the homogeneous microstructure of grain boundaries and increasing the grain size compared to varistor sintered at temperature 1250°C that possess high varistor voltage. The breakdown voltage of current–voltage characteristics for ZnO varistor is much better with BaTiO<sup>3</sup> . For low-voltage ZnO varistors it can be improve by increasing the grain size, which allows the decreased grain boundary per unit volume and improves the nonlinear electrical characteristics. The addition of perovskite structure BaTiO3 is attribute to the formation of potential barriers at the grain boundaries where the large grain size of BaTiO3 will greatly increase the grain size and the present of Barium will contribute to the increase in nonlinearity of ZnO varistor, since it more dominant in comparison with the effect of titanium oxide. The used of BaTiO<sup>3</sup> as an additive for grain growth will produce a suitable range of varistor voltage with the conventional sintering technology in order to produce low-voltage varistor.

In addition, the perovskite manganite CaMnO3 as an additive changes the breakdown voltage of the ZnO varistor system. It presents a good electrical properties for low-voltage varistor with large nonlinearity coefficients [47] and surpasses the results as reported by using ZnO-Bi2 O3 based and ZnO-Pr6 O11 based varistor [53, 54]. The low-voltage nonlinearity originates as a result of higher concentration of manganese present at the grain boundary layer regions, being charge compensated by zinc vacancies [47]. The effect of sintering temperature on microstructure and electrical properties of low voltage varistor ceramics fabricated from a mixture of ZnO with CaMnO3 perovskite gives a broad idea to researcher for their further research on production of low-voltage varistor [55]. The effect sintering temperature at certain composition of this additives can exhibit a voltage-limiting in the electrical properties of ZnO varistor. The varistor sintered at 1200°C provided low varistor voltage per thickness of the ZnO ceramics for low voltage varistor [4].

boundary per unit volume improving the nonlinear parameters. Moreover, the low-voltage ZnO varistor ceramics can be improves by using suitable additives such as Barium Titanate and Calcium Manganite since it exhibits perovskite structure where these materials possess

,

,

79

ceramics used in low voltage varis-

, Wan Mohamad Ikhmal Wan Mohamad Kamaruzzaman<sup>1</sup>

Conventional Sintering Effects on the Microstructure and Electrical Characteristics…

http://dx.doi.org/10.5772/intechopen.78652

the ability to control the microstructure development during sintering process.

and Muhamad Azman Zulkifli<sup>1</sup>

1 Advanced Nano Materials (ANoMa) Research Group, Nano Research Team, School of Fundamental Science, Universiti Malaysia Terengganu, Kuala Nerus, Terengganu, Malaysia 2 School of Ocean Engineering, Universiti Malaysia Terengganu, Kuala Nerus, Terengganu,

3 Institute of Biodiversity and Sustainable Development, Universiti Malaysia Terengganu,

[1] Sutharsini U, Thanihaichelvan M, Singh R. Two-step sintering of ceramics. In: Sintering

[2] Souza FL, Gomes JW, Bueno PR, Cassia-Santos MR, Araujo AL, Leite ER, Varela JA. Effect of the addition of ZnO seeds on the electrical proprieties of ZnO-based varis-

[3] Anas S, Mangalaraja RV, Poothayal M, Shukla SK, Ananthakumar S. Direct synthesis of varistor-grade doped nanocrystalline ZnO and its densification through a step-sintering

[4] Lakin II, Zakaria A, Abdollah Y, Umaru D. Effect of sintering temperature on micro-

[5] Norailiana AR, Azmi BZ, Ismayadi I, Hashim M, Ibrahim IR, Nazlan R, Mohd Idris F. Parallel development of microstructure and electrical properties in doped-zinc oxide.

[6] Anas S, Mahesh KV, Maria MJ, Ananthakumar S. Sol-gel materials for varistor devices. In: Sol-Gel Materials for Energy, Environment and Electronic Applications. Cham:

tors. Digest Journal of Nanomaterials and Biostructures. 2015;**10**(1):189-197

Journal of Materials Science and Surface Engineering. 2017;**5**(2):528-532

Mohd Sabri Mohd Ghazali1,3\*, Muhamad Syaizwadi Shaifudin1

of Functional Materials. Rijeka, Croatia: InTech; 2018

technique. Acta Materialia. 2007;**55**(17):5792-5801

structure and electrical properties of ZnO+ CaMnO3

tors. Materials Chemistry and Physics. 2003;**80**(2):512-516

\*Address all correspondence to: mohdsabri@umt.edu.my

**Author details**

Malaysia

**References**

Wan Rafizah Wan Abdullah<sup>2</sup>

Kuala Nerus, Terengganu, Malaysia

Springer; 2017. pp. 23-59

Maria Fazira Mohd Fekeri1

### **4. Overview**

The preceding chapter in this book has presented the best available knowledge the conventional sintering effects as a driving force on the microstructure and electrical characteristics of low-voltage ceramic varistor. The aim of this chapter is to provide sufficient knowledge related to sintering technology that has been used for ceramic varistors fabrication industry. Almost the past century there has been a discovery of ceramic varistors and a few decade later, a varistor with simple formulation of ZnO-Bi2 O3 based varistor was successfully fabricated in industries and start from that, varistor have been growing, whereas, ZnO-perovskite introduced. At the heart of this magnificent semiconductor device is the sintering technologya way of heat treatment to make the ceramic varistor become compacted and less porosity.

The methodology in this chapter present low-voltage ZnO based varistor and its additives. The discussion part elaborates the recent studies related to microstructure and electrical properties of ZnO-perovskite based varistor as compared in citation to first generation and second generation, which are ZnO-Bi2 O3 based and ZnO-Pr6 O11 based varistor; respectively. The discussion concludes with driving force through sintering process in solid-state route, the desired low voltage ZnO-Bi2 O3 and ZnO-perovskite based varistor with favorable nonlinearity coefficient, α, is successfully fabricated.

### **5. Conclusion**

The effects of conventional sintering on the microstructure and electrical properties of lowvoltage ceramic varistor in this chapter are describe based on their useful properties which are determined by their properties of grain and grain boundaries during the fabrication process. The processing technique through solid-state route shows a significant effect with sintering process in the microstructural development of ZnO varistor ceramics. The densification of sintered ceramic varistor can be controlled by using different sintering technology in order to improve their microstructure and electrical properties especially for production of low-voltage varistor. It was also determined that a steady increase in sintering temperature and time until certain limitations results in larger size of grains which in turn will decrease the grain boundary per unit volume improving the nonlinear parameters. Moreover, the low-voltage ZnO varistor ceramics can be improves by using suitable additives such as Barium Titanate and Calcium Manganite since it exhibits perovskite structure where these materials possess the ability to control the microstructure development during sintering process.

### **Author details**

with large nonlinearity coefficients [47] and surpasses the results as reported by using ZnO-

as a result of higher concentration of manganese present at the grain boundary layer regions, being charge compensated by zinc vacancies [47]. The effect of sintering temperature on microstructure and electrical properties of low voltage varistor ceramics fabricated from a

research on production of low-voltage varistor [55]. The effect sintering temperature at certain composition of this additives can exhibit a voltage-limiting in the electrical properties of ZnO varistor. The varistor sintered at 1200°C provided low varistor voltage per thickness of the

The preceding chapter in this book has presented the best available knowledge the conventional sintering effects as a driving force on the microstructure and electrical characteristics of low-voltage ceramic varistor. The aim of this chapter is to provide sufficient knowledge related to sintering technology that has been used for ceramic varistors fabrication industry. Almost the past century there has been a discovery of ceramic varistors and a few decade

cated in industries and start from that, varistor have been growing, whereas, ZnO-perovskite introduced. At the heart of this magnificent semiconductor device is the sintering technologya way of heat treatment to make the ceramic varistor become compacted and less porosity. The methodology in this chapter present low-voltage ZnO based varistor and its additives. The discussion part elaborates the recent studies related to microstructure and electrical properties of ZnO-perovskite based varistor as compared in citation to first generation and

The discussion concludes with driving force through sintering process in solid-state route, the

The effects of conventional sintering on the microstructure and electrical properties of lowvoltage ceramic varistor in this chapter are describe based on their useful properties which are determined by their properties of grain and grain boundaries during the fabrication process. The processing technique through solid-state route shows a significant effect with sintering process in the microstructural development of ZnO varistor ceramics. The densification of sintered ceramic varistor can be controlled by using different sintering technology in order to improve their microstructure and electrical properties especially for production of low-voltage varistor. It was also determined that a steady increase in sintering temperature and time until certain limitations results in larger size of grains which in turn will decrease the grain

O3

O3

based and ZnO-Pr6

and ZnO-perovskite based varistor with favorable nonlinear-

O11 based varistor [53, 54]. The low-voltage nonlinearity originates

perovskite gives a broad idea to researcher for their further

based varistor was successfully fabri-

O11 based varistor; respectively.

Bi2 O3

based and ZnO-Pr6

78 Sintering Technology - Method and Application

mixture of ZnO with CaMnO3

**4. Overview**

ZnO ceramics for low voltage varistor [4].

later, a varistor with simple formulation of ZnO-Bi2

O3

second generation, which are ZnO-Bi2

ity coefficient, α, is successfully fabricated.

desired low voltage ZnO-Bi2

**5. Conclusion**

Mohd Sabri Mohd Ghazali1,3\*, Muhamad Syaizwadi Shaifudin1 , Wan Rafizah Wan Abdullah<sup>2</sup> , Wan Mohamad Ikhmal Wan Mohamad Kamaruzzaman<sup>1</sup> , Maria Fazira Mohd Fekeri1 and Muhamad Azman Zulkifli<sup>1</sup>

\*Address all correspondence to: mohdsabri@umt.edu.my

1 Advanced Nano Materials (ANoMa) Research Group, Nano Research Team, School of Fundamental Science, Universiti Malaysia Terengganu, Kuala Nerus, Terengganu, Malaysia

2 School of Ocean Engineering, Universiti Malaysia Terengganu, Kuala Nerus, Terengganu, Malaysia

3 Institute of Biodiversity and Sustainable Development, Universiti Malaysia Terengganu, Kuala Nerus, Terengganu, Malaysia

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**Chapter 5**

**Provisional chapter**

**Nanostructured Pure and Doped Zirconia: Synthesis**

**Nanostructured Pure and Doped Zirconia: Synthesis** 

Zirconia is a multifunctional material with potential applications in wide domains. Rareearth doped zirconia and stabilized zirconia yield interesting properties based on the phase transitions induced by the sintering conditions. Zirconia nanopowders were prepared by hydrothermal technique. Synthesis methods of zirconia with various rare earths are discussed here. An overview of the sintering of zirconia-based ceramics is presented

**Keywords:** sintering, solid oxide fuel cell, optical ceramics, hydrothermal synthesis

alumina [14] makes it to be employed in miniature optical devices. Two crystallographic trans-

such as monoclinic to tetragonal [15] at ~1170°C and tetragonal to cubic [16] at ~2370°C. The high temperature tetragonal and cubic forms are stabilized with different elements, such as

FSZ (fully stabilized zirconia), can be observed with concentration of 8% Yttria [18]. Stabilized

© 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.

) is one of the materials well known for multifunctional applications [1–8].

are refractories [9], oxygen sensors [10],

until room temperature, which is named

with its higher birefringence than

diffusivity, structural [12], and biomedical

between room temperature and its melting point (~2715°C)

DOI: 10.5772/intechopen.81323

**and Sintering for SOFC and Optical Applications**

**and Sintering for SOFC and Optical Applications**

Mythili Prakasam, Sorina Valsan, Yiying Lu, Felix Balima, Wenzhong Lu, Radu Piticescu and

Mythili Prakasam, Sorina Valsan, Yiying Lu, Felix Balima, Wenzhong Lu, Radu Piticescu

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

in particular for SOFC and sensors and optical applications.

Commonly employed application domains of ZrO2

Mg, Ca, Sc, Ce, and Y [17]. Existence of Cubic ZrO2

applications [13] due to its high strength and toughness. ZrO2

and fuel cell membranes [11] due to the high O2

formations are experienced by ZrO2

http://dx.doi.org/10.5772/intechopen.81323

Alain Largeteau

and Alain Largeteau

**Abstract**

**1. Introduction**

Zirconia (ZrO2

### **Nanostructured Pure and Doped Zirconia: Synthesis and Sintering for SOFC and Optical Applications Nanostructured Pure and Doped Zirconia: Synthesis and Sintering for SOFC and Optical Applications**

DOI: 10.5772/intechopen.81323

Mythili Prakasam, Sorina Valsan, Yiying Lu, Felix Balima, Wenzhong Lu, Radu Piticescu and Alain Largeteau Mythili Prakasam, Sorina Valsan, Yiying Lu, Felix Balima, Wenzhong Lu, Radu Piticescu and Alain Largeteau

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.81323

### **Abstract**

Zirconia is a multifunctional material with potential applications in wide domains. Rareearth doped zirconia and stabilized zirconia yield interesting properties based on the phase transitions induced by the sintering conditions. Zirconia nanopowders were prepared by hydrothermal technique. Synthesis methods of zirconia with various rare earths are discussed here. An overview of the sintering of zirconia-based ceramics is presented in particular for SOFC and sensors and optical applications.

**Keywords:** sintering, solid oxide fuel cell, optical ceramics, hydrothermal synthesis

### **1. Introduction**

Zirconia (ZrO2 ) is one of the materials well known for multifunctional applications [1–8]. Commonly employed application domains of ZrO2 are refractories [9], oxygen sensors [10], and fuel cell membranes [11] due to the high O2 diffusivity, structural [12], and biomedical applications [13] due to its high strength and toughness. ZrO2 with its higher birefringence than alumina [14] makes it to be employed in miniature optical devices. Two crystallographic transformations are experienced by ZrO2 between room temperature and its melting point (~2715°C) such as monoclinic to tetragonal [15] at ~1170°C and tetragonal to cubic [16] at ~2370°C. The high temperature tetragonal and cubic forms are stabilized with different elements, such as Mg, Ca, Sc, Ce, and Y [17]. Existence of Cubic ZrO2 until room temperature, which is named FSZ (fully stabilized zirconia), can be observed with concentration of 8% Yttria [18]. Stabilized

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.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

zirconia in particular with Yttria called YSZ is widely used for a variety of applications such as thermal barrier material, and with additional optical characteristics such as transparency/ translucency, they are used as windows [19] for anvil cells, infrared windows, laser host materials, armor applications, optical lenses, tooth-like esthetics, thermal insulating transparent windows, scratch-resistant electronics, bar scanners and high pressure sodium, and mercury halide lamps. Initially, YSZ single crystals [20] were widely known for its use as artificial gemstones and high thermal shock behavior. Nevertheless, owing to the advantages [21] of polycrystalline transparent ceramics in terms of time, cost, size, shape, and mechanical strength have recently been studied to replace single crystals. Due to the inherent birefringence, additional light scattering will be experienced in addition to the grain boundaries. In this chapter, we have focused our attention on the application of ZrO<sup>2</sup> for SOFC and optical transparent ceramics. Synthesis of rare earth doped ZrO2 nanopowders by hydrothermal method are discussed.

### **2. Zirconia phases**

The phase diagram of zirconia is very well known in the literature. The polymorphism of zirconia is presented in the scheme (**Figure 1**) below:

The different phases of pure zirconia are clearly identified in **Figure 2**. Indeed, without stabilization with yttria, zirconia is present in monoclinic form until a temperature of 1170°C, where it leaves room for the tetragonal phase. The cubic phase, meanwhile, can be obtained only from a temperature of 2370°C.

It is also noted that stabilization with yttria at 3 and 8% facilitates the organization of the zirconia crystals in the tetragonal phase, which is close to the cubic phase while avoiding the unstable monoclinic phase. The concentration of phase is stabilized partially with 3% of Y2 O3 , usually called partially stabilized zirconia (PSZ) (3YSZ), where both the monoclinic and tetragonal phases coexist. When the concentration of Y2 O3 reaches 8%, all the monoclinic phases are converted into tetragonal called as fully stabilized zirconia (FSZ) (8YSZ). Cubic phase is obtained with increasing Y2 O3 concentration, and the temperature required for densification is too high. It is known that the crystalline phase is easier to achieve than conventional crystal structures, but it is not possible to transform the phase into a monoclonal phase in the case of pure ZrO2 . It follows therefore that the more the zirconia is stabilized and the more it will be possible to obtain a phase that will have the desired optical properties.

The transition from tetragonal to monoclinic phase is done with a volume increase of about 4% leading to microcracking that drastically affects the mechanical properties. Therefore, the stabilization of cubic or tetragonal phase on larger temperature ranges is required for safety application. The tetragonal or cubic phase can be stabilized (e.g., the temperature of the "*с* → *t*"

earth oxides, etc. The doping elements affect not only the structural and mechanical proper-

classified into different systems as specified by Haberko et al. [22]: systems forming cubic solid

–MeO2

–MeO (Me = Mg, Cr, Co, Cu), ZrO2

Nanostructured Pure and Doped Zirconia: Synthesis and Sintering for SOFC and Optical…

http://dx.doi.org/10.5772/intechopen.81323

87

domain: HfO<sup>2</sup>

ties but also optical and electrical behavior. Synthetically, the binary system of ZrO2

O3

–Me2 O3

–ZrO2

(Me = Th, Ce); systems with the formation

, TiO2

–ZrO2

**,** and other rare

may be

(Me = Fe,

, etc.; and

transition may be decreased) by inducing such additives as MgO, CaO, Y2

domain: ZrO2

(Me = Fe, Mn), ZrO2

of other types of solid solutions in the rich ZrO2

**Figure 2.** Phase diagram of zirconia with yttria stabilization.

solutions in the rich ZrO2

–Me3 O4

Cr, La,), ZrO2

$$\begin{aligned} \text{ZrO}\_{2} \text{monoclinic} \xleftarrow[\text{X}000 - 1100 \text{°C}]{1000 \text{°C}} \text{ZrO}\_{2} \text{ tetpropagation} \xleftarrow[\text{X}000 - 1100 \text{°C}]{1000 \text{°C}} \text{ZrO}\_{2} \text{ or } \begin{aligned} \text{ZrO}\_{2} \text{ (} \text{at } \text{g} \text{-} \text{l)} \text{mol} \text{ or } \begin{aligned} \text{ZrO}\_{2} \text{ (} \text{at } \text{g} \text{-} \text{l)} \text{mol} \text{ or } \begin{aligned} \text{ZrO}\_{2} \text{ (} \text{at } \text{g} \text{-} \text{l)} \text{mol} \end{aligned} \end{aligned} \end{aligned} $$

**Figure 1.** Different crystalline phases of zirconia with respect to temperature.

Nanostructured Pure and Doped Zirconia: Synthesis and Sintering for SOFC and Optical… http://dx.doi.org/10.5772/intechopen.81323 87

**Figure 2.** Phase diagram of zirconia with yttria stabilization.

zirconia in particular with Yttria called YSZ is widely used for a variety of applications such as thermal barrier material, and with additional optical characteristics such as transparency/ translucency, they are used as windows [19] for anvil cells, infrared windows, laser host materials, armor applications, optical lenses, tooth-like esthetics, thermal insulating transparent windows, scratch-resistant electronics, bar scanners and high pressure sodium, and mercury halide lamps. Initially, YSZ single crystals [20] were widely known for its use as artificial gemstones and high thermal shock behavior. Nevertheless, owing to the advantages [21] of polycrystalline transparent ceramics in terms of time, cost, size, shape, and mechanical strength have recently been studied to replace single crystals. Due to the inherent birefringence, additional light scattering will be experienced in addition to the grain boundaries. In this chapter, we have focused

nanopowders by hydrothermal method are discussed.

The phase diagram of zirconia is very well known in the literature. The polymorphism of

The different phases of pure zirconia are clearly identified in **Figure 2**. Indeed, without stabilization with yttria, zirconia is present in monoclinic form until a temperature of 1170°C, where it leaves room for the tetragonal phase. The cubic phase, meanwhile, can be obtained

It is also noted that stabilization with yttria at 3 and 8% facilitates the organization of the zirconia crystals in the tetragonal phase, which is close to the cubic phase while avoiding the unstable monoclinic phase. The concentration of phase is stabilized partially with 3% of Y2

usually called partially stabilized zirconia (PSZ) (3YSZ), where both the monoclinic and tetrag-

are converted into tetragonal called as fully stabilized zirconia (FSZ) (8YSZ). Cubic phase is

is too high. It is known that the crystalline phase is easier to achieve than conventional crystal structures, but it is not possible to transform the phase into a monoclonal phase in the case of

O3

. It follows therefore that the more the zirconia is stabilized and the more it will be

concentration, and the temperature required for densification

for SOFC and optical transparent ceramics. Synthesis

reaches 8%, all the monoclinic phases

O3 ,

our attention on the application of ZrO<sup>2</sup>

86 Sintering Technology - Method and Application

only from a temperature of 2370°C.

obtained with increasing Y2

pure ZrO2

zirconia is presented in the scheme (**Figure 1**) below:

onal phases coexist. When the concentration of Y2

O3

**Figure 1.** Different crystalline phases of zirconia with respect to temperature.

possible to obtain a phase that will have the desired optical properties.

of rare earth doped ZrO2

**2. Zirconia phases**

The transition from tetragonal to monoclinic phase is done with a volume increase of about 4% leading to microcracking that drastically affects the mechanical properties. Therefore, the stabilization of cubic or tetragonal phase on larger temperature ranges is required for safety application. The tetragonal or cubic phase can be stabilized (e.g., the temperature of the "*с* → *t*" transition may be decreased) by inducing such additives as MgO, CaO, Y2 O3 **,** and other rare earth oxides, etc. The doping elements affect not only the structural and mechanical properties but also optical and electrical behavior. Synthetically, the binary system of ZrO2 may be classified into different systems as specified by Haberko et al. [22]: systems forming cubic solid solutions in the rich ZrO2 domain: ZrO2 –MeO (Me = Mg, Cr, Co, Cu), ZrO2 –Me2 O3 (Me = Fe, Cr, La,), ZrO2 –Me3 O4 (Me = Fe, Mn), ZrO2 –MeO2 (Me = Th, Ce); systems with the formation of other types of solid solutions in the rich ZrO2 domain: HfO<sup>2</sup> –ZrO2 , TiO2 –ZrO2 , etc.; and systems with no interactions between the components, e.g., ZnO–ZrO2 , Al2 O3 –ZrO2 . Zirconia ceramic materials are known as important candidates for functional and structural applications. Stabilized Y2 O3 –ZrO2 ceramics (YSZ) is the most common solid electrolyte used in various applications as oxygen sensors or fuel cells in automotive industry, metallurgical, glass and cement industries, gas pumps for removing oxygen traces from the gases used in special industrial processes, and fuel cells. Their utilization opened a new way for optimization of oxygen (air)/fuel ratios and made automotive and industries more environment friendly due to its adequate level of oxygen ion conductivity and desirable stability in both oxidizing and reducing atmospheres [23]. In principle, these sensors use the Nernst voltage generated by the difference of two different ion concentrations (with different partial pressures) on the sides of an electrolyte, which generate an electrical potential. This voltage is proportional to the natural logarithm of the ratio of the two different ion concentrations according to the Nernst equation:

$$
\Delta II = \frac{k\_9 T}{c\_o} = \ln \frac{c\_1}{c\_2} \tag{1}
$$

zirconia (YTZP) ceramics have lower activation energy for the ion conduction opening the field for their utilization at lower temperatures [28]*.* A comprehensive review with respect to the structure, chemistry, design and selection of materials, underlying mechanisms, and performance of each SOFC component, which opens up the future directions toward pursu-

Zirconia co-doped with different rare earth elements has been intensively studied during recent period due to the versatility of these materials in various optoelectronic devices and biomaterials. Some examples are summarized below. A single step, rapid microwave driven solution

Zirconia doped with selected trivalent rare earth oxides was successfully obtained by a complex polymerization method and may be considered promising candidates for white light-emitting

of Eu3+ doping and annealing on the morphology, crystal structure, and fluorescence properties of the resultant nanocrystals were investigated. Nanocrystals with tetragonal or cubic structure may find potential applications as the raw material for producing the transparent ceramics with efficient fluorescence properties [32]. Zirconium oxide powders doped with terbium, synthesized by hydrothermal route from a highly basic solution, were used to determine the role of the basic agent (NaOH, KOH, or LiOH) utilized to carry out the hydrothermal synthesis on their morphology, crystalline structure, photoluminescent, or cathodoluminescent properties [33]. Scandia-stabilized zirconia powder (ScSZ) was synthesized by a microwave-hydrothermal method. The structure of the ScSZ powder changed from a tetragonal to a cubic phase, and accordingly, the powder conductivity was increased from 90.55 to 120.56 mS/cm by the

synthesized by coprecipitation method, showing no toxicity and possessing good antibacterial ability [35]. The thermal stability of zirconia up to very high temperatures explains also its intensive use in energy generation applications as coatings or sintered bulk pieces. Thermal barrier coatings (TBCs) have proved to be a key technology in thermal stability, and their use to achieve surface temperature reduction of the underlying super alloys surpass all other achievements in the field of material technologies that have taken place in last three decades [36].

The technique most often used to prepare zirconia powders is the sol-gel route. The sol-gel process makes it possible to manufacture an inorganic polymer by simple chemical reactions and at a temperature close to room temperature. The synthesis is carried out from precursors. They are either liquid or solid and are mostly soluble in common solvents. The simple chemical reactions at the base of the process are triggered when the precursors are brought into contact with water. To prepare the pure zirconia powders [37–41], the precursor used is

Another technique used in the synthesis is coprecipitation [44]. It is a simultaneous precipitation of two substances, and it is used for the preparation of 8YSZ. To do this, we must precipitate Zr4+ and Y3+. The synthesis of the powders can also be obtained by pyrolysis of spray

:Eu3+ nanocrystals were synthesized by hydrothermal technique. Effects

Nanostructured Pure and Doped Zirconia: Synthesis and Sintering for SOFC and Optical…

CO3

: Eu3+nanophosphors [30].

http://dx.doi.org/10.5772/intechopen.81323

89

) during the microwave-hydrothermal

, 70% diluted in n-PrOH) [42]. Sol-gel synthesis can also

nanoparticles NPs were

ing SOFC research, was recently proposed in [29].

introduction of the mineralizer solutions (KOH + K<sup>2</sup>

**2.2. Zirconia nanopowder synthesis**

zirconium n-propoxide (Zr(OC3

processing [34]. Un-doped and rare earth (Dy and Ce)-doped ZrO2

 H<sup>7</sup> )4

be carried out to obtain zirconia powders doped with 3% of Yttria [43].

applications [31]. ZrO2

combustion technique was used to obtain luminescent, cubic ZrO2

where kB is the Boltzmann constant (= 1.38 × 10–23 J/K), T is the absolute temperature in K, e0 is the elementary charge (1.602 × 10−19 C), and ci is the ion concentration on the two sides of the solid electrolyte in mol/kg. The mechanism of zirconia partial oxygen pressure sensors is basically described below. At temperatures higher than a certain activation value depending on the composition and structure, zirconia partly dissociates to produce oxygen ions, which can be transported through the material when a voltage is applied. Due to this process, zirconia behaves like a solid electrolyte for oxygen. If two different oxygen pressures exist on either side of a zirconia material, the Nernst voltage can be measured across that element. Generally, ZrO2 -8 mol%Y2 O3 (YSZ) solid electrolytes exhibiting a conductivity of about 0.1 Ώ−<sup>1</sup> cm−<sup>1</sup> at 1000°C and about 3 × 10−<sup>5</sup> Ώ−<sup>1</sup> cm−<sup>1</sup> at 400°C corresponding to an activation energy of 96 kJ mol−<sup>1</sup> are used.

### **2.1. Rare earth doped zirconia phases**

To improve the ionic conductivity and sensors' quality factors in a large temperature range, different approaches were proposed: partial or total replacement of Y<sup>2</sup> O3 with Sc2 O3 having a maximum corresponding to the composition (Y0.5Sc0.5)\*0.3 Zr0.7O1.85. The main limitation of this approach is the decrease of conductivity observed with holding time due to the structural modifications [24]**.** The development of planar sensors using ceramic membranes and multipackaging technology in place of classical bulk sintered materials shows the ability to increase the efficiency of the thermal transfer but has limited effect on the ionic conductivity of the material itself [25]**.** In this case, the technology is the main limiting factor, since complex additives are required to control the dispersibility of the ceramic powders [26]. Reducing the diffusion and transport distances using nanocrystalline membranes and thin films, Kosacki et al. found that nanocrystalline YSZ thin films with mean grain sizes in the range 10–200 nm materials exhibited two-three orders of magnitude increase in conductivity compared to polycrystalline and single crystalline materials [27]. An activation energy in the range 0.85 ± 0.05 eV for bulk conductivity with a corresponding grain boundary conductivity of 1.0 ± 0.1 e for nanocrystalline 2–3 mol% Y2 O3 doped ZrO2 ceramics with average grain particle in the range 35–50 nm was reported. It was also reported that yttria doped tetragonal zirconia (YTZP) ceramics have lower activation energy for the ion conduction opening the field for their utilization at lower temperatures [28]*.* A comprehensive review with respect to the structure, chemistry, design and selection of materials, underlying mechanisms, and performance of each SOFC component, which opens up the future directions toward pursuing SOFC research, was recently proposed in [29].

Zirconia co-doped with different rare earth elements has been intensively studied during recent period due to the versatility of these materials in various optoelectronic devices and biomaterials. Some examples are summarized below. A single step, rapid microwave driven solution combustion technique was used to obtain luminescent, cubic ZrO2 : Eu3+nanophosphors [30]. Zirconia doped with selected trivalent rare earth oxides was successfully obtained by a complex polymerization method and may be considered promising candidates for white light-emitting applications [31]. ZrO2 :Eu3+ nanocrystals were synthesized by hydrothermal technique. Effects of Eu3+ doping and annealing on the morphology, crystal structure, and fluorescence properties of the resultant nanocrystals were investigated. Nanocrystals with tetragonal or cubic structure may find potential applications as the raw material for producing the transparent ceramics with efficient fluorescence properties [32]. Zirconium oxide powders doped with terbium, synthesized by hydrothermal route from a highly basic solution, were used to determine the role of the basic agent (NaOH, KOH, or LiOH) utilized to carry out the hydrothermal synthesis on their morphology, crystalline structure, photoluminescent, or cathodoluminescent properties [33]. Scandia-stabilized zirconia powder (ScSZ) was synthesized by a microwave-hydrothermal method. The structure of the ScSZ powder changed from a tetragonal to a cubic phase, and accordingly, the powder conductivity was increased from 90.55 to 120.56 mS/cm by the introduction of the mineralizer solutions (KOH + K<sup>2</sup> CO3 ) during the microwave-hydrothermal processing [34]. Un-doped and rare earth (Dy and Ce)-doped ZrO2 nanoparticles NPs were synthesized by coprecipitation method, showing no toxicity and possessing good antibacterial ability [35]. The thermal stability of zirconia up to very high temperatures explains also its intensive use in energy generation applications as coatings or sintered bulk pieces. Thermal barrier coatings (TBCs) have proved to be a key technology in thermal stability, and their use to achieve surface temperature reduction of the underlying super alloys surpass all other achievements in the field of material technologies that have taken place in last three decades [36].

### **2.2. Zirconia nanopowder synthesis**

systems with no interactions between the components, e.g., ZnO–ZrO2

tions. Stabilized Y2

K, e0

O3 –ZrO2

88 Sintering Technology - Method and Application

<sup>Δ</sup>*<sup>U</sup>* <sup>=</sup> *kB <sup>T</sup>* \_\_\_

that element. Generally, ZrO2

activation energy of 96 kJ mol−<sup>1</sup>

**2.1. Rare earth doped zirconia phases**

ity of 1.0 ± 0.1 e for nanocrystalline 2–3 mol% Y2

of about 0.1 Ώ−<sup>1</sup> cm−<sup>1</sup>

is the elementary charge (1.602 × 10−19 C), and ci


are used.

different approaches were proposed: partial or total replacement of Y<sup>2</sup>

O3

at 1000°C and about 3 × 10−<sup>5</sup> Ώ−<sup>1</sup> cm−<sup>1</sup>

ceramic materials are known as important candidates for functional and structural applica-

ous applications as oxygen sensors or fuel cells in automotive industry, metallurgical, glass and cement industries, gas pumps for removing oxygen traces from the gases used in special industrial processes, and fuel cells. Their utilization opened a new way for optimization of oxygen (air)/fuel ratios and made automotive and industries more environment friendly due to its adequate level of oxygen ion conductivity and desirable stability in both oxidizing and reducing atmospheres [23]. In principle, these sensors use the Nernst voltage generated by the difference of two different ion concentrations (with different partial pressures) on the sides of an electrolyte, which generate an electrical potential. This voltage is proportional to the natural logarithm of the ratio of the two different ion concentrations according to the Nernst equation:

*e*0

where kB is the Boltzmann constant (= 1.38 × 10–23 J/K), T is the absolute temperature in

sides of the solid electrolyte in mol/kg. The mechanism of zirconia partial oxygen pressure sensors is basically described below. At temperatures higher than a certain activation value depending on the composition and structure, zirconia partly dissociates to produce oxygen ions, which can be transported through the material when a voltage is applied. Due to this process, zirconia behaves like a solid electrolyte for oxygen. If two different oxygen pressures exist on either side of a zirconia material, the Nernst voltage can be measured across

To improve the ionic conductivity and sensors' quality factors in a large temperature range,

a maximum corresponding to the composition (Y0.5Sc0.5)\*0.3 Zr0.7O1.85. The main limitation of this approach is the decrease of conductivity observed with holding time due to the structural modifications [24]**.** The development of planar sensors using ceramic membranes and multipackaging technology in place of classical bulk sintered materials shows the ability to increase the efficiency of the thermal transfer but has limited effect on the ionic conductivity of the material itself [25]**.** In this case, the technology is the main limiting factor, since complex additives are required to control the dispersibility of the ceramic powders [26]. Reducing the diffusion and transport distances using nanocrystalline membranes and thin films, Kosacki et al. found that nanocrystalline YSZ thin films with mean grain sizes in the range 10–200 nm materials exhibited two-three orders of magnitude increase in conductivity compared to polycrystalline and single crystalline materials [27]. An activation energy in the range 0.85 ± 0.05 eV for bulk conductivity with a corresponding grain boundary conductiv-

O3

particle in the range 35–50 nm was reported. It was also reported that yttria doped tetragonal

doped ZrO2

<sup>=</sup> ln*<sup>c</sup>* \_\_1 *c*2

ceramics (YSZ) is the most common solid electrolyte used in vari-

, Al2 O3 –ZrO2

is the ion concentration on the two

at 400°C corresponding to an

with Sc2

ceramics with average grain

O3

having

O3

(YSZ) solid electrolytes exhibiting a conductivity

. Zirconia

(1)

The technique most often used to prepare zirconia powders is the sol-gel route. The sol-gel process makes it possible to manufacture an inorganic polymer by simple chemical reactions and at a temperature close to room temperature. The synthesis is carried out from precursors. They are either liquid or solid and are mostly soluble in common solvents. The simple chemical reactions at the base of the process are triggered when the precursors are brought into contact with water. To prepare the pure zirconia powders [37–41], the precursor used is zirconium n-propoxide (Zr(OC3 H<sup>7</sup> )4 , 70% diluted in n-PrOH) [42]. Sol-gel synthesis can also be carried out to obtain zirconia powders doped with 3% of Yttria [43].

Another technique used in the synthesis is coprecipitation [44]. It is a simultaneous precipitation of two substances, and it is used for the preparation of 8YSZ. To do this, we must precipitate Zr4+ and Y3+. The synthesis of the powders can also be obtained by pyrolysis of spray aerosol (spray-pyrolisis). This process involves injecting the spray containing the precursor solution into a combustion chamber where the particles are quickly ignited. This technique makes it possible to obtain zirconia powders doped with Yttria [45] and in particular doped with 8YSZ [46]. It is also possible to use a hydrothermal route to synthesize zirconia powders [47]. Hydrothermal synthesis allows the production of crystalline fine powders to deagglomerate. These qualities are suitable for the preparation of fine oxide/oxide composites by simultaneous synthesis of the two phases. The last technique that can be used is a homogeneous precipitation method of zirconium oxychloride, yttrium, urea, which is used as a precipitating agent, and polyacrylic acid, which is used as a dispersing agent [48].

Although many laboratory-scale reactions can be scaled up to economically produce large quantities of materials, the laboratory-scale reaction parameters may not be linearly related to that of large-scale reaction. The synthesis parameters such as temperature, pH, reactant concentration, and time should be ideally correlated with factors such as supersaturation, nucleation and growth rates, surface energy, and diffusion coefficients in order to ensure the reproducibility of reactions. A comparison between the main procedures used for the synthesis of doped zirconia materials is presented in **Table 1** from the point of view of scalability.

Nanostructured Pure and Doped Zirconia: Synthesis and Sintering for SOFC and Optical…

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91

The main advantages of the hydrothermal synthesis are one step process for powder synthesis or oriented ceramic films; minimized consumption energy; closed-flow system; relatively high deposition rate; products with much higher homogeneity than solid state processing; products with higher density than gas or vacuum processing (faster growth rate); and versatility: oxides, nonoxides, organic/biologic materials, and hybrid materials with different

Different theoretical and empirical models for solid state sintering were used for modeling the density of sintered zirconia nanomaterials using classical pressing and sintering technology, however being limited by the complexity of the structural modifications during the compaction process. A study on the influence of the synthesis and processing parameters on the ion conduction of YTZP nanomaterials and the characteristics of gauges for pressure sensors was

treatment of the precursor suspensions in a Teflon autoclave for various times at temperatures around 250°C using ammonia as mineralizing agent were used in the sintering studies.

ing the microstructure and properties described before were used (**Figure 3**) to obtain com-

The sintered bulk material was obtained by pseudo-biaxial pressing at 100 MPa followed by sintering in air. To eliminate the chemically bonded water, powders have been additionally attrition milled for 2 hours in acetone before addition of sintering additives. The optimal sintering parameters were estimated from the dynamic sintering curves obtained by the heating stage microscopy (**Figure 4**). Two shrinkage intervals can be clearly observed, the first from room temperature to approximately 550°C related to thermal decomposition of binders (polyvinyl alcohol PVA) and the second at 1400°C corresponding to sintering, with a total shrinkage of 28% at 1400°C. Compacts with densities higher than 96% of the theoretical and

doped with 3.5 mol% Y2

were used. YTZP powders obtained via the hydrothermal procedure hav-

O3

) obtained by hydrothermal

/g) and pycnometric density in the

morphologies may be obtained [49].

performed [50]. YTZP powders (ZrO2

pact materials via two methods:

range 5–5.2 g/cm3

**3. Classical sintering of rare earth doped zirconia**

Powders with very high specific surface area (195–200 m<sup>2</sup>

• Bulk material by pseudo-axial pressing and sintering.

grain sizes around 200 nm have been obtained.

• Tape casting of membranes followed by drying and sintering.

### *2.2.1. Synthesis methods for RE doped ZrO2 , with accent on hydrothermal synthesis*

The so-called triangle synthesis, properties, and applications must be fully exploited to obtain assessed materials for specific applications. The properties of nanostructured materials depend on the atomic structure, composition, microstructure, defects, and interfaces, which are controlled by thermodynamics and kinetics of the synthesis. Different synthesis routes for manufacturing of nanomaterials were proposed. Generally, they may be classified as physical, chemical, and combined routes. Other classification considers the top-down approach from the macroscale to the nanoscale or conversely by assembly of atoms or particles using the bottom-up approach. Chemical reactions for material synthesis can be done in solid (conventional synthesis route), liquid, or gaseous state. For solid state reactions, diffusion of atoms depends on the temperature of the reaction, and transport across grain boundaries and grain growth at elevated temperature reactions may lead to solids with large grain size. Compared to solid-state synthesis, diffusion in the liquid or gas phase is typically and advantageously many orders of magnitude larger than in the solid phase; thus, the synthesis of nanostructured materials can be achieved at lower temperatures, reducing the detrimental grain growth.


**Table 1.** A comparison between main synthesis routes for obtaining doped zirconia materials.

Although many laboratory-scale reactions can be scaled up to economically produce large quantities of materials, the laboratory-scale reaction parameters may not be linearly related to that of large-scale reaction. The synthesis parameters such as temperature, pH, reactant concentration, and time should be ideally correlated with factors such as supersaturation, nucleation and growth rates, surface energy, and diffusion coefficients in order to ensure the reproducibility of reactions. A comparison between the main procedures used for the synthesis of doped zirconia materials is presented in **Table 1** from the point of view of scalability.

The main advantages of the hydrothermal synthesis are one step process for powder synthesis or oriented ceramic films; minimized consumption energy; closed-flow system; relatively high deposition rate; products with much higher homogeneity than solid state processing; products with higher density than gas or vacuum processing (faster growth rate); and versatility: oxides, nonoxides, organic/biologic materials, and hybrid materials with different morphologies may be obtained [49].

### **3. Classical sintering of rare earth doped zirconia**

Different theoretical and empirical models for solid state sintering were used for modeling the density of sintered zirconia nanomaterials using classical pressing and sintering technology, however being limited by the complexity of the structural modifications during the compaction process. A study on the influence of the synthesis and processing parameters on the ion conduction of YTZP nanomaterials and the characteristics of gauges for pressure sensors was performed [50]. YTZP powders (ZrO2 doped with 3.5 mol% Y2 O3 ) obtained by hydrothermal treatment of the precursor suspensions in a Teflon autoclave for various times at temperatures around 250°C using ammonia as mineralizing agent were used in the sintering studies.

Powders with very high specific surface area (195–200 m<sup>2</sup> /g) and pycnometric density in the range 5–5.2 g/cm3 were used. YTZP powders obtained via the hydrothermal procedure having the microstructure and properties described before were used (**Figure 3**) to obtain compact materials via two methods:

• Bulk material by pseudo-axial pressing and sintering.

**Synthesis route Solid-state** 

Additional steps Calcinations,

Composition control

Environmental impact

**process**

*2.2.1. Synthesis methods for RE doped ZrO2*

90 Sintering Technology - Method and Application

milling

**Coprecipitation Hydrothermal Sol-gel Spray** 

*, with accent on hydrothermal synthesis*

No Calcinations,

milling

Poor Good Excellent Medium Excellent

High Moderate Low High Moderate

Morphology control Poor Medium Good Medium Good Particle size (nm) >1000 >100 10–100 >10 >10 Hard agglomerates Medium High Low Medium Low Impurities (%) 0.5–1 Max. 0.5 Max. 0.5 0.1–0.5 0.1–0.5

aerosol (spray-pyrolisis). This process involves injecting the spray containing the precursor solution into a combustion chamber where the particles are quickly ignited. This technique makes it possible to obtain zirconia powders doped with Yttria [45] and in particular doped with 8YSZ [46]. It is also possible to use a hydrothermal route to synthesize zirconia powders [47]. Hydrothermal synthesis allows the production of crystalline fine powders to deagglomerate. These qualities are suitable for the preparation of fine oxide/oxide composites by simultaneous synthesis of the two phases. The last technique that can be used is a homogeneous precipitation method of zirconium oxychloride, yttrium, urea, which is used as a precipitating

The so-called triangle synthesis, properties, and applications must be fully exploited to obtain assessed materials for specific applications. The properties of nanostructured materials depend on the atomic structure, composition, microstructure, defects, and interfaces, which are controlled by thermodynamics and kinetics of the synthesis. Different synthesis routes for manufacturing of nanomaterials were proposed. Generally, they may be classified as physical, chemical, and combined routes. Other classification considers the top-down approach from the macroscale to the nanoscale or conversely by assembly of atoms or particles using the bottom-up approach. Chemical reactions for material synthesis can be done in solid (conventional synthesis route), liquid, or gaseous state. For solid state reactions, diffusion of atoms depends on the temperature of the reaction, and transport across grain boundaries and grain growth at elevated temperature reactions may lead to solids with large grain size. Compared to solid-state synthesis, diffusion in the liquid or gas phase is typically and advantageously many orders of magnitude larger than in the solid phase; thus, the synthesis of nanostructured materials can be achieved at lower temperatures, reducing the detrimental grain growth.

agent, and polyacrylic acid, which is used as a dispersing agent [48].

Scalability Industrial Industrial Demonstration Demonstration R&D

Calcinations, milling

**Table 1.** A comparison between main synthesis routes for obtaining doped zirconia materials.

**pyrolysis**

No

• Tape casting of membranes followed by drying and sintering.

The sintered bulk material was obtained by pseudo-biaxial pressing at 100 MPa followed by sintering in air. To eliminate the chemically bonded water, powders have been additionally attrition milled for 2 hours in acetone before addition of sintering additives. The optimal sintering parameters were estimated from the dynamic sintering curves obtained by the heating stage microscopy (**Figure 4**). Two shrinkage intervals can be clearly observed, the first from room temperature to approximately 550°C related to thermal decomposition of binders (polyvinyl alcohol PVA) and the second at 1400°C corresponding to sintering, with a total shrinkage of 28% at 1400°C. Compacts with densities higher than 96% of the theoretical and grain sizes around 200 nm have been obtained.

**Figure 3.** Schematic flow sheet for obtaining of YTZP materials for sensors application.

The effective ionic conductivity of YTZP bulk materials was measured using impedancemetry measurements**.** The contributions of bulk and grain boundaries on the total ionic conductivity were calculated from the impedance spectra of samples. The results on the activation energy of ionic conduction are presented in **Table 2** below.

higher than of single crystals of similar composition. This effect increases with decreasing

O3

**Microstructure Activation energy of ionic** 

Nanostructured Pure and Doped Zirconia: Synthesis and Sintering for SOFC and Optical…

(grain growth

Various sintering techniques [51] have been employed till date for the fabrication of YSZ transparent ceramics. Most prevalently used techniques are hot isostatic pressing [52], spark plasma sintering (SPS) [53], and microwave sintering [54]. Different approaches have been employed for obtaining transparent ceramics [55] of YSZ either by means of dopant, high pressure, two-step load application procedure, or pre compaction followed by vacuum sintering and hot isostatic pressing. Kim et al. [56] studied the effects of the sintering conditions of dental zirconia ceramics on the grain size and translucency by comparing the microwave sintering and classical sintering. Jiang et al. [57] analyzed the effects of sintering temperature and

O3

tion of zirconia containing ceramic matrix composites by microwave processing. Tamburini

O3

) dental ceramic because 3% Y2

**conductivity (kJ/mol)**

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93

86

. Fang et al. [58] showed enhanced densifica-

O3

grain sizes and may practically neglect large micrometric grain sizes.

7.5 625 Polycrystalline 110 3 524 Polycrystalline 97.5

4 603 Polycrystalline 90 3 Single crystal 84

inhibitor))

**3.1. New sintering methods of rare earth doped zirconia**

**Table 2.** Activation energy of ionic conductivity in YTZP nanomaterials.

**Figure 4.** Dynamic sintering curves of YTZP nanopowders.

3 393 Polycrystalline (0.25% Al2

**Grain sizes (nm)**

**Y2 O3**

**YTZP**

 **mol% in** 

particle size (40 and 90 nm) on the 3YSZ (YSZ with 3% Y2

gives higher mechanical strength than 8% Y2

The model developed suggest that grain boundaries increase the total ionic conductivity of yttria-doped zirconia due to a "short circuit effect," leading to an apparent conductivity Nanostructured Pure and Doped Zirconia: Synthesis and Sintering for SOFC and Optical… http://dx.doi.org/10.5772/intechopen.81323 93

**Figure 4.** Dynamic sintering curves of YTZP nanopowders.


**Table 2.** Activation energy of ionic conductivity in YTZP nanomaterials.

higher than of single crystals of similar composition. This effect increases with decreasing grain sizes and may practically neglect large micrometric grain sizes.

### **3.1. New sintering methods of rare earth doped zirconia**

The effective ionic conductivity of YTZP bulk materials was measured using impedancemetry measurements**.** The contributions of bulk and grain boundaries on the total ionic conductivity were calculated from the impedance spectra of samples. The results on the activation energy

The model developed suggest that grain boundaries increase the total ionic conductivity of yttria-doped zirconia due to a "short circuit effect," leading to an apparent conductivity

of ionic conduction are presented in **Table 2** below.

92 Sintering Technology - Method and Application

**Figure 3.** Schematic flow sheet for obtaining of YTZP materials for sensors application.

Various sintering techniques [51] have been employed till date for the fabrication of YSZ transparent ceramics. Most prevalently used techniques are hot isostatic pressing [52], spark plasma sintering (SPS) [53], and microwave sintering [54]. Different approaches have been employed for obtaining transparent ceramics [55] of YSZ either by means of dopant, high pressure, two-step load application procedure, or pre compaction followed by vacuum sintering and hot isostatic pressing. Kim et al. [56] studied the effects of the sintering conditions of dental zirconia ceramics on the grain size and translucency by comparing the microwave sintering and classical sintering. Jiang et al. [57] analyzed the effects of sintering temperature and particle size (40 and 90 nm) on the 3YSZ (YSZ with 3% Y2 O3 ) dental ceramic because 3% Y2 O3 gives higher mechanical strength than 8% Y2 O3 . Fang et al. [58] showed enhanced densification of zirconia containing ceramic matrix composites by microwave processing. Tamburini et al. [59] reported on high pressure SPS, whereas Casolco et al. [60] used a traditional die set-up and two-step load application procedure. Klimke et al. and Krell et al. [14, 61] used CIP followed by HIP, whereas Tosoh [62] Corporation and their team used sintering aid such as TiO2 to obtain transparent 8YSZ (YSZ with 8% Y2 O3 ). The addition of TiO2 is reported to decrease the mechanical strength [63] of 8YSZ transparent ceramics with low transparency.

with high ultimate compressive strength >3.5 GPa and inelastic strain around 8% due to the transformation toughening. At higher temperatures, the high dislocation density induced by the flash sintering conditions improves the plasticity of the sintered ceramics and retards the

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95

Nowadays, materials such as glasses, polymers, or single crystals are used for applications requiring good optical properties such as laser, lenses, camera domes, and much more.

However, transparent ceramics are an interesting alternative to the aforesaid materials. Indeed, they have a greater ease of development of large complex parts, good mechanical properties (good resistance to thermal shocks and fractures), low thermal expansion, good thermal conductivity, and good tenacity. They are already used for various applications.

Transparent ceramics can be classified into two different forms based on their crystalline structure: the cubic structure and the noncubic structure. The cubic transparent ceramics is one of the widely reported in the literature. Manufacture of noncubic transparent ceramics is not an easy task, which can be explained as below. Depending on the type of structure, there will be the problem of birefringence occuring at the grain boundaries. For the noncubic structure, the material is said to be anisotropic. In this case, we observe two indices of refraction: the ordinary index and the extraordinary index. This extraordinary ray is going to make our material birefringent, and it will be necessary to control very closely the growth of the grains, due to the diffusion of

The methods of fabrication of transparent ceramics are numerous such as hot pressing, hot isostatic pressing, vacuum sintering, microwave sintering, and spark plasma sintering.

Until to date, there are no reports focusing on yielding 3YSZ and 8YSZ transparent ceramics by analyzing sintering parameters and their influence in yielding transparency by SPS sintering. Here, we report on yielding 3YSZ transparent ceramics containing tetragonal phases without addition of any dopants or high-pressure technique. Tetragonal phased zirconia has interesting mechanical strength due to its large refractive index and high dielectric constants. In order to obtain transparent ceramics, it is necessary to have maximum density and minimum porosity in the orders of <0.01 vol% in the final sintered body. The aforesaid is achieved due to the interplay of various sintering parameters (SP) such as sintering temperature, dwell time, heating/cooling rate, pressure, and temperature of pressure application. We have optimized the sintering parameters and demonstrated the possibility of obtaining transparent 3YSZ through reactive sintering during spark plasma sintering favorizing cubic phase formation with Y segregation around the grain boundaries. We demonstrated for the first time the presence of tetragonal and cubic phases in transparent ceramics of 3YSZ and 8YSZ obtained

*3.2.2. Rare earth-based zirconia-based transparent ceramics by spark plasma sintering*

by SPS. The experimental details and results are presented in the following sections.

**3.2. Classification of transparent materials and applications of transparent ceramics**

cracks nucleation and propagation [65].

*3.2.1. The manufacture of transparent ceramics*

light caused by the ceramics with birefringence.

Zirconia powders doped with 3 mol% Y2 O3 and co-doped with 3 mol% Y2 O3 –6 mol% CeO2 have been selected for preliminary sintering tests using a field assisted method [64]. The hydrothermally synthesized powders after were mixed with a solution containing 6 wt% polyvinyl alcohol as binder and spray dried using a LabPlant spray drier system (air speed of 3.5 m/s at evacuation and feeding rate of 617 ml/h using a 0.5 mm nozzle). Rapid analysis by optical micrographs of all investigated samples revealed the presence of rounded particles with sizes ranging from a few microns to tens of microns. The powder morphology was maintained after the heat treatment for 2 hours at 500°C to remove the PVA binder, and they have been further used in FAST sintering tests, using a thermal mechanical simulator—Gleeble 3800, with fully integrated digital closed-loop control thermal and mechanical testing system makes highly accurate process control possible. The tests were conducted with variation of different key parameters, such as pressure, maximum temperature, and holding time, where the temperature range applied was 1100–1300°C, with pressure range of 75–125 MPa and holding time of 120–240 second.

The relative density of the sample was calculated to 99.47%, which was under the sintering condition at 1300°C and 125 MPa, with 25°C/S heat rate and 120 Sholding time. It may be observed that no open porosity exists, which indicates that a fully dense bulk material was achieved (**Figure 5**).

Flash sintering is also a new sintering method that attracted significant attention for rapid densification of ceramics at low sintering temperatures, allowing to retain the fine grains and control the dielectric and mechanical properties. Flash sintering of yttria-stabilized zirconia at temperatures <600°C with a constant heating rate of 25°C/min leads to dense ceramics

**Figure 5.** SEM micrographs of sintered samples (a) ZrO2 -3Y and (b) co-doped with 3Y-6CeO2 -ZrO2 .

with high ultimate compressive strength >3.5 GPa and inelastic strain around 8% due to the transformation toughening. At higher temperatures, the high dislocation density induced by the flash sintering conditions improves the plasticity of the sintered ceramics and retards the cracks nucleation and propagation [65].

### **3.2. Classification of transparent materials and applications of transparent ceramics**

Nowadays, materials such as glasses, polymers, or single crystals are used for applications requiring good optical properties such as laser, lenses, camera domes, and much more.

However, transparent ceramics are an interesting alternative to the aforesaid materials. Indeed, they have a greater ease of development of large complex parts, good mechanical properties (good resistance to thermal shocks and fractures), low thermal expansion, good thermal conductivity, and good tenacity. They are already used for various applications.

### *3.2.1. The manufacture of transparent ceramics*

et al. [59] reported on high pressure SPS, whereas Casolco et al. [60] used a traditional die set-up and two-step load application procedure. Klimke et al. and Krell et al. [14, 61] used CIP followed by HIP, whereas Tosoh [62] Corporation and their team used sintering aid such

decrease the mechanical strength [63] of 8YSZ transparent ceramics with low transparency.

have been selected for preliminary sintering tests using a field assisted method [64]. The hydrothermally synthesized powders after were mixed with a solution containing 6 wt% polyvinyl alcohol as binder and spray dried using a LabPlant spray drier system (air speed of 3.5 m/s at evacuation and feeding rate of 617 ml/h using a 0.5 mm nozzle). Rapid analysis by optical micrographs of all investigated samples revealed the presence of rounded particles with sizes ranging from a few microns to tens of microns. The powder morphology was maintained after the heat treatment for 2 hours at 500°C to remove the PVA binder, and they have been further used in FAST sintering tests, using a thermal mechanical simulator—Gleeble 3800, with fully integrated digital closed-loop control thermal and mechanical testing system makes highly accurate process control possible. The tests were conducted with variation of different key parameters, such as pressure, maximum temperature, and holding time, where the temperature range applied was 1100–1300°C, with pressure range of 75–125 MPa and

The relative density of the sample was calculated to 99.47%, which was under the sintering condition at 1300°C and 125 MPa, with 25°C/S heat rate and 120 Sholding time. It may be observed that no open porosity exists, which indicates that a fully dense bulk material was

Flash sintering is also a new sintering method that attracted significant attention for rapid densification of ceramics at low sintering temperatures, allowing to retain the fine grains and control the dielectric and mechanical properties. Flash sintering of yttria-stabilized zirconia at temperatures <600°C with a constant heating rate of 25°C/min leads to dense ceramics



O3

O3

and co-doped with 3 mol% Y2

). The addition of TiO2

is reported to

–6 mol% CeO2

O3

to obtain transparent 8YSZ (YSZ with 8% Y2

Zirconia powders doped with 3 mol% Y2

94 Sintering Technology - Method and Application

holding time of 120–240 second.

**Figure 5.** SEM micrographs of sintered samples (a) ZrO2

achieved (**Figure 5**).

as TiO2

Transparent ceramics can be classified into two different forms based on their crystalline structure: the cubic structure and the noncubic structure. The cubic transparent ceramics is one of the widely reported in the literature. Manufacture of noncubic transparent ceramics is not an easy task, which can be explained as below. Depending on the type of structure, there will be the problem of birefringence occuring at the grain boundaries. For the noncubic structure, the material is said to be anisotropic. In this case, we observe two indices of refraction: the ordinary index and the extraordinary index. This extraordinary ray is going to make our material birefringent, and it will be necessary to control very closely the growth of the grains, due to the diffusion of light caused by the ceramics with birefringence.

The methods of fabrication of transparent ceramics are numerous such as hot pressing, hot isostatic pressing, vacuum sintering, microwave sintering, and spark plasma sintering.

### *3.2.2. Rare earth-based zirconia-based transparent ceramics by spark plasma sintering*

Until to date, there are no reports focusing on yielding 3YSZ and 8YSZ transparent ceramics by analyzing sintering parameters and their influence in yielding transparency by SPS sintering. Here, we report on yielding 3YSZ transparent ceramics containing tetragonal phases without addition of any dopants or high-pressure technique. Tetragonal phased zirconia has interesting mechanical strength due to its large refractive index and high dielectric constants. In order to obtain transparent ceramics, it is necessary to have maximum density and minimum porosity in the orders of <0.01 vol% in the final sintered body. The aforesaid is achieved due to the interplay of various sintering parameters (SP) such as sintering temperature, dwell time, heating/cooling rate, pressure, and temperature of pressure application. We have optimized the sintering parameters and demonstrated the possibility of obtaining transparent 3YSZ through reactive sintering during spark plasma sintering favorizing cubic phase formation with Y segregation around the grain boundaries. We demonstrated for the first time the presence of tetragonal and cubic phases in transparent ceramics of 3YSZ and 8YSZ obtained by SPS. The experimental details and results are presented in the following sections.

In the present study, 3YSZ and 8YSZ nanopowders (Tosoh Corporation) with average crystallite size ~20 nm average particle diameter 0.3 μm were used for the fabrication of 3YSZ and 8YSZ transparent ceramics. Spark plasma sintering (SPS) experiments were performed with DR. SINTER LAB Spark Plasma Sintering system, Model SPS-515S-FUJI. The experiments were performed under a vacuum of 10 Pa with the pulse sequence for the SPS applied voltage of 12:2 (i.e., 12 ON/2 OFF). 1 g of powder was used for each experiment. The experiment was carried out in a graphite mold with inner diameter of 10 mm and external diameter of 25 mm. The internal of the graphite die was covered with carbon foil (Papyex). The mold was covered with carbon fiber felt to limit the loss of heat radiation. Due to the usage of pyrometer, the temperature was first increased to 600°C within 3 min without regulation and then increased to a range of temperatures from 1150–1400°C with different heating rate (RH) ranging from 2.5 to 100°C/min and with 20 min dwell time. Uniaxial pressures ranging from 40 to 100 MPa were applied at room temperature (TR) and sintering temperature (TS ), and their significances have been analyzed. The cooling rates (RC) and RH were maintained equal in all the experiments. Then, the ceramics were ground and polished to a thickness of 1 mm with optical finishing. Powder X-ray diffraction (XRD) analysis was performed with a PANalytical X'Pert MDP diffractometer with θ-θ Bragg Brentano configuration, with a backscattering graphite monochromator for K<sup>α</sup> Cu radiation working at 40 kV and 40 mA. Temperature dependent XRD has been performed using a powder diffractometer (PANalytical X'Pert Pro) equipped with a high-temperature chamber Anton Paar HTK16 (1600° C) measuring with K<sup>α</sup> Cu radiation. The temperatures of analyses used were from room temperature until 1400°C. The density was measured by the Archimedes method in distilled water. The microstructure was observed by a scanning electron microscope (Joel 840 SEM) on fractured surface without polishing. The optical transmittance spectrum was measured by using a double beam spectrophotometer (Varian Cary 5000) at a range of between 200 and 7000 nm for a sample thickness of 1.5 mm.

It has to be mentioned that so far the ceramics of pure monoclinic ZrO2 did not yield transparency, whereas the samples of 3YSZ were translucent and that of 8YSZ are well transparent. In order to study the behavior of ZrO2 with three different compositions, all the samples were treated under same conditions, i.e., sintering temperature = 1200°C, heating/cooling rate = 2.5°, 5°, and 10°C/min, dwell time = 20 min, pressure applied = 100 MPa, and point of pressure application at the start of sintering cycle and the other being only during the dwell time. Though transparency was obtained for the pure ZrO2 , the sample was dense under the following conditions: sintering temperature = 1200°C, heating/cooling rate = 2.5°C/min, dwell time = 20 min, pressure applied = 100 MPa, and point of pressure application = only during the dwell time. The translucent sample of 3YSZ was obtained at sintering temperature = 1200° C, heating/cooling rate = 2.5°C/min, dwell time = 20 min, pressure applied = 100 MPa, and point of pressure application = only during the dwell time/beginning of sintering cycle. The transparent sample of 8YSZ was obtained at sintering temperature = 1200°C, heating/cooling rate = 2.5°C/min, dwell time = 20 min, pressure applied = 100 MPa, and point of pressure application = only during the dwell time.

The XRD analysis in **Figure 6(a)**, which shows stabilized zirconia, shows that two samples have the same chemical composition. The initial one-phase powder monoclinic (Baddeleyte) was expected, since it is at room temperature and there is no addition of stabilizer. In addition,

**Figure 6.** XRD analyses of sintered compacts of (a) pure zirconia, (b) 3YSZ, and (c) 8YSZ at 1200°C for 20 min with

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different heating rates.

Nanostructured Pure and Doped Zirconia: Synthesis and Sintering for SOFC and Optical… http://dx.doi.org/10.5772/intechopen.81323 97

In the present study, 3YSZ and 8YSZ nanopowders (Tosoh Corporation) with average crystallite size ~20 nm average particle diameter 0.3 μm were used for the fabrication of 3YSZ and 8YSZ transparent ceramics. Spark plasma sintering (SPS) experiments were performed with DR. SINTER LAB Spark Plasma Sintering system, Model SPS-515S-FUJI. The experiments were performed under a vacuum of 10 Pa with the pulse sequence for the SPS applied voltage of 12:2 (i.e., 12 ON/2 OFF). 1 g of powder was used for each experiment. The experiment was carried out in a graphite mold with inner diameter of 10 mm and external diameter of 25 mm. The internal of the graphite die was covered with carbon foil (Papyex). The mold was covered with carbon fiber felt to limit the loss of heat radiation. Due to the usage of pyrometer, the temperature was first increased to 600°C within 3 min without regulation and then increased to a range of temperatures from 1150–1400°C with different heating rate (RH) ranging from 2.5 to 100°C/min and with 20 min dwell time. Uniaxial pressures ranging from 40 to 100 MPa were

been analyzed. The cooling rates (RC) and RH were maintained equal in all the experiments. Then, the ceramics were ground and polished to a thickness of 1 mm with optical finishing. Powder X-ray diffraction (XRD) analysis was performed with a PANalytical X'Pert MDP diffractometer with θ-θ Bragg Brentano configuration, with a backscattering graphite mono-

has been performed using a powder diffractometer (PANalytical X'Pert Pro) equipped with a

temperatures of analyses used were from room temperature until 1400°C. The density was measured by the Archimedes method in distilled water. The microstructure was observed by a scanning electron microscope (Joel 840 SEM) on fractured surface without polishing. The optical transmittance spectrum was measured by using a double beam spectrophotometer (Varian Cary 5000) at a range of between 200 and 7000 nm for a sample thickness of 1.5 mm.

ency, whereas the samples of 3YSZ were translucent and that of 8YSZ are well transparent.

were treated under same conditions, i.e., sintering temperature = 1200°C, heating/cooling rate = 2.5°, 5°, and 10°C/min, dwell time = 20 min, pressure applied = 100 MPa, and point of pressure application at the start of sintering cycle and the other being only during the dwell

following conditions: sintering temperature = 1200°C, heating/cooling rate = 2.5°C/min, dwell time = 20 min, pressure applied = 100 MPa, and point of pressure application = only during the dwell time. The translucent sample of 3YSZ was obtained at sintering temperature = 1200° C, heating/cooling rate = 2.5°C/min, dwell time = 20 min, pressure applied = 100 MPa, and point of pressure application = only during the dwell time/beginning of sintering cycle. The transparent sample of 8YSZ was obtained at sintering temperature = 1200°C, heating/cooling rate = 2.5°C/min, dwell time = 20 min, pressure applied = 100 MPa, and point of pressure

The XRD analysis in **Figure 6(a)**, which shows stabilized zirconia, shows that two samples have the same chemical composition. The initial one-phase powder monoclinic (Baddeleyte) was expected, since it is at room temperature and there is no addition of stabilizer. In addition,

high-temperature chamber Anton Paar HTK16 (1600° C) measuring with K<sup>α</sup>

It has to be mentioned that so far the ceramics of pure monoclinic ZrO2

time. Though transparency was obtained for the pure ZrO2

In order to study the behavior of ZrO2

application = only during the dwell time.

Cu radiation working at 40 kV and 40 mA. Temperature dependent XRD

), and their significances have

Cu radiation. The

did not yield transpar-

, the sample was dense under the

with three different compositions, all the samples

applied at room temperature (TR) and sintering temperature (TS

chromator for K<sup>α</sup>

96 Sintering Technology - Method and Application

**Figure 6.** XRD analyses of sintered compacts of (a) pure zirconia, (b) 3YSZ, and (c) 8YSZ at 1200°C for 20 min with different heating rates.

**Figure 7.** SEM analysis of pure zirconia at sintering temperature of 1200°C for 20 min; Heating and cooling rate: 5°C/min.

**Figure 8.** SEM analyses of zirconia 3YSZ at a sintering temperature of 1200°C for 20 min and at heating and cooling rates of (a) 5°C/min and (b) 10°C/min.

temperature of 1200°C, for heating and cooling rates of 5° and 10°C/min, the cubic phase is obtained. The conversion to the cubic phase is induced by the segregation of the Y in the grain boundaries, which were probably caused due to the slow heating rate employed during the

**Figure 10.** Transmittance spectrum of pure zirconia stabilized with yttria in (a) UV-visible range and (b) near infrared

**Figure 9.** SEM analyzes of zirconia 8YSZ at a sintering temperature of 1200°C for 20 min and at heating and cooling rates

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The spectra of **Figure 10** show similar results. Indeed, the samples that have a maximum transmittance in the UV-visible near IR are the same as those in the IR, and the transmittance

Zirconia stabilized with higher % yttria and the more the ceramic has high transmittance. In addition, in the case of a zirconia partially stabilized and fully stabilized, the effect of a higher or lower heating rate has a significant impact. Indeed, the lower the heating rate, the more the material has a high maximum transmittance. It is clearly evident from **Figure 10** that the transmittance corresponding to the rate of transmission is 2.5°C/min, which shows high

transmittance, with transmittance of >50% in the visible and >65% in the near IR [66].

varies just slightly between the two spectra and it depends on the samples.

sintered at 1200°C for a dwell time of 20 min with different heating rates.

sintering cycle (**Figures 7–9**).

of (a) 5°C/min; (b) 2.5°C/min; and (c) 10°C/min.

pure zirconia after sintering at 1200°C always has a monoclinic phase because its stabilization and sintering temperature is too low for the transition to a tetragonal phase. However, we note that the characteristic peaks of the monoclinic phase disappear more and more, which shows the beginning of a transition to a tetragonal phase. The analysis in **Figure 6(b)** shows that the three samples of 3YSZ contain the same chemical elements. The initial powder of 3YSZ has a monoclinic phase that tends to become tetragonal. For stabilized zirconia with 3% Yttria at sintering temperature of 1200° C, for heating and cooling rates 5 and 10°C/min, the tetragonal phase is obtained. However, a slight peak is observed for the 3YSZ at a heating and cooling rate of 10° C/min, which must correspond to a chemical reaction or to the fact that the corresponding heating rate is not low enough, which does not allow time for the material to change correctly of phases. The analysis in **Figure 6(c)** shows that the three samples of 8YSZ contain the same chemical elements. The initial powder of 8YSZ has a mixture of tetragonal and cubic phase that tends to become cubic. For stabilized zirconia with 8% Yttria at sintering Nanostructured Pure and Doped Zirconia: Synthesis and Sintering for SOFC and Optical… http://dx.doi.org/10.5772/intechopen.81323 99

**Figure 9.** SEM analyzes of zirconia 8YSZ at a sintering temperature of 1200°C for 20 min and at heating and cooling rates of (a) 5°C/min; (b) 2.5°C/min; and (c) 10°C/min.

**Figure 7.** SEM analysis of pure zirconia at sintering temperature of 1200°C for 20 min; Heating and cooling rate: 5°C/min.

**Figure 8.** SEM analyses of zirconia 3YSZ at a sintering temperature of 1200°C for 20 min and at heating and cooling rates

pure zirconia after sintering at 1200°C always has a monoclinic phase because its stabilization and sintering temperature is too low for the transition to a tetragonal phase. However, we note that the characteristic peaks of the monoclinic phase disappear more and more, which shows the beginning of a transition to a tetragonal phase. The analysis in **Figure 6(b)** shows that the three samples of 3YSZ contain the same chemical elements. The initial powder of 3YSZ has a monoclinic phase that tends to become tetragonal. For stabilized zirconia with 3% Yttria at sintering temperature of 1200° C, for heating and cooling rates 5 and 10°C/min, the tetragonal phase is obtained. However, a slight peak is observed for the 3YSZ at a heating and cooling rate of 10° C/min, which must correspond to a chemical reaction or to the fact that the corresponding heating rate is not low enough, which does not allow time for the material to change correctly of phases. The analysis in **Figure 6(c)** shows that the three samples of 8YSZ contain the same chemical elements. The initial powder of 8YSZ has a mixture of tetragonal and cubic phase that tends to become cubic. For stabilized zirconia with 8% Yttria at sintering

of (a) 5°C/min and (b) 10°C/min.

98 Sintering Technology - Method and Application

**Figure 10.** Transmittance spectrum of pure zirconia stabilized with yttria in (a) UV-visible range and (b) near infrared sintered at 1200°C for a dwell time of 20 min with different heating rates.

temperature of 1200°C, for heating and cooling rates of 5° and 10°C/min, the cubic phase is obtained. The conversion to the cubic phase is induced by the segregation of the Y in the grain boundaries, which were probably caused due to the slow heating rate employed during the sintering cycle (**Figures 7–9**).

The spectra of **Figure 10** show similar results. Indeed, the samples that have a maximum transmittance in the UV-visible near IR are the same as those in the IR, and the transmittance varies just slightly between the two spectra and it depends on the samples.

Zirconia stabilized with higher % yttria and the more the ceramic has high transmittance. In addition, in the case of a zirconia partially stabilized and fully stabilized, the effect of a higher or lower heating rate has a significant impact. Indeed, the lower the heating rate, the more the material has a high maximum transmittance. It is clearly evident from **Figure 10** that the transmittance corresponding to the rate of transmission is 2.5°C/min, which shows high transmittance, with transmittance of >50% in the visible and >65% in the near IR [66].

### **4. Conclusion**

The current chapter deals with the various fabrication methodologies and synthesis of rare earth doped zirconia that can be employed for applications in areas, including catalysis, glassmaking, metallurgy, optoelectronics, batteries, and coatings for extreme environments. During recent years, it has been reported that using mixed rare earth oxides as dopant may strongly improve the functional properties of the matrix such as increasing thermal shock resistance of zirconia-based thermal barrier coatings (TBCs) and improve ionic conductivity of solid oxide fuel cells (SOFCs) by surface segregation mechanisms. Various powder synthesis methodologies with an accent on hydrothermal powder synthesis is discussed. The feasibility of obtaining the sintering compacts of rare earth oxides and co-doped rare earth oxides both from the commercial and hydrothermal synthesis by rapid sintering methods such as spark plasma sintering is demonstrated. The role of rare earth oxides on sintering and in the point of view of applications is evident from the current work for the zirconia. Further investigations are necessary to validate the role of co-doped rare earth oxides for thermal barrier coatings and in SOFCs. The aforesaid is being investigated as a part of the project "MONAMIX" and will be reported elsewhere later.

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### **Acknowledgements**

The authors duly acknowledge the following funding agencies for the present work through


### **Author details**

Mythili Prakasam1 \*, Sorina Valsan3 , Yiying Lu1,2, Felix Balima1 , Wenzhong Lu2 , Radu Piticescu3 and Alain Largeteau1

\*Address all correspondence to: mythili.prakasam@icmcb.cnrs.fr

1 CNRS, University of Bordeaux, ICMCB, UMR 5026, Pessac, France

2 Huazhong University of Science and Technology (HUST) School of Optical and Electronic Information, Wuhan, China

3 National R&D Institute for Nonferrous and Rare Metals, Pantelimon, Romania

### **References**

**4. Conclusion**

100 Sintering Technology - Method and Application

will be reported elsewhere later.

**Acknowledgements**

innovation program

**Author details**

Mythili Prakasam1

Information, Wuhan, China

Radu Piticescu3

University of Science and Technology.

\*, Sorina Valsan3

and Alain Largeteau1

\*Address all correspondence to: mythili.prakasam@icmcb.cnrs.fr

1 CNRS, University of Bordeaux, ICMCB, UMR 5026, Pessac, France

The current chapter deals with the various fabrication methodologies and synthesis of rare earth doped zirconia that can be employed for applications in areas, including catalysis, glassmaking, metallurgy, optoelectronics, batteries, and coatings for extreme environments. During recent years, it has been reported that using mixed rare earth oxides as dopant may strongly improve the functional properties of the matrix such as increasing thermal shock resistance of zirconia-based thermal barrier coatings (TBCs) and improve ionic conductivity of solid oxide fuel cells (SOFCs) by surface segregation mechanisms. Various powder synthesis methodologies with an accent on hydrothermal powder synthesis is discussed. The feasibility of obtaining the sintering compacts of rare earth oxides and co-doped rare earth oxides both from the commercial and hydrothermal synthesis by rapid sintering methods such as spark plasma sintering is demonstrated. The role of rare earth oxides on sintering and in the point of view of applications is evident from the current work for the zirconia. Further investigations are necessary to validate the role of co-doped rare earth oxides for thermal barrier coatings and in SOFCs. The aforesaid is being investigated as a part of the project "MONAMIX" and

The authors duly acknowledge the following funding agencies for the present work through

**1.** Grant agreement 692216 SUPERMAT from European Union's Horizon 2020 research and

**2.** Grant agreement ERAMIN 2 COFUND-Research & Innovation program on raw materials

**3.** Part of collaboration in photonics between University of Bordeaux and Huazhong

, Yiying Lu1,2, Felix Balima1

2 Huazhong University of Science and Technology (HUST) School of Optical and Electronic

3 National R&D Institute for Nonferrous and Rare Metals, Pantelimon, Romania

, Wenzhong Lu2

,

to foster circular economy-"MONAMIX"- "ANR-17-MIN2-0003-03"


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**Chapter 6**

**Provisional chapter**

**On the Performance of Carbon Nanotubes on Sintered**

**On the Performance of Carbon Nanotubes on Sintered** 

used in both technical and biomedical applications due to their combination of excellent chemical, physical, and mechanical properties. Pressureless sintering (PLS), reaction bonding (RB), hot pressing (HP), hot isostatic pressing (HIP), and spark plasma sintering (SPS) are the sintering methods more commonly used. The high brittleness, the low fracture toughness, and low thermal stability that possess these ceramics are its Achilles heel for numerous engineering applications. The incorporation of a second phase such as carbon nanotubes (CNTs) into the ceramic matrix has been attempted to overcome these drawbacks but the obtained results are still controversial considering that the homogeneous dispersion of CNTs and the interfacial bonding between two different ceramic materials remains as a difficult task leading to little or even no improvement in mechanical properties. Besides, the role of CNTs in the sintering of ceramic materials is not clear in the scientific literature taking into account parameters such as materials used and particularly inconsistencies in dispersion and mixing of the CNTs. We discuss how the CNTs can affect the sintering behavior and microstructural evolution of alumina and

**Keywords:** alumina, zirconia, carbon nanotubes, sintering, fracture toughness

Carbon nanotubes (commonly abridged as CNTs) are structures of nanometric dimension built up entirely by atoms of carbon and they have a high Young's modulus with good flexibility and

> © 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.

) ceramic monoliths and their combination are

DOI: 10.5772/intechopen.78542

**Alumina-Zirconia Ceramics**

**Alumina-Zirconia Ceramics**

Miguel Humberto Bocanegra-Bernal,

Miguel Humberto Bocanegra-Bernal,

http://dx.doi.org/10.5772/intechopen.78542

O3

zirconia ceramics and the combination of them.

**Abstract**

**1. Introduction**

The alumina (Al2

Alfredo Aguilar-Elguezabal, Armando Reyes-Rojas, Carlos Dominguez-Rios and Armando Garcia-Reyes

Alfredo Aguilar-Elguezabal, Armando Reyes-Rojas, Carlos Dominguez-Rios and Armando Garcia-Reyes

) and zirconia (ZrO2

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

### **On the Performance of Carbon Nanotubes on Sintered Alumina-Zirconia Ceramics On the Performance of Carbon Nanotubes on Sintered Alumina-Zirconia Ceramics**

DOI: 10.5772/intechopen.78542

Miguel Humberto Bocanegra-Bernal, Alfredo Aguilar-Elguezabal, Armando Reyes-Rojas, Carlos Dominguez-Rios and Armando Garcia-Reyes Miguel Humberto Bocanegra-Bernal, Alfredo Aguilar-Elguezabal, Armando Reyes-Rojas, Carlos Dominguez-Rios and Armando Garcia-Reyes

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.78542

### **Abstract**

The alumina (Al2 O3 ) and zirconia (ZrO2 ) ceramic monoliths and their combination are used in both technical and biomedical applications due to their combination of excellent chemical, physical, and mechanical properties. Pressureless sintering (PLS), reaction bonding (RB), hot pressing (HP), hot isostatic pressing (HIP), and spark plasma sintering (SPS) are the sintering methods more commonly used. The high brittleness, the low fracture toughness, and low thermal stability that possess these ceramics are its Achilles heel for numerous engineering applications. The incorporation of a second phase such as carbon nanotubes (CNTs) into the ceramic matrix has been attempted to overcome these drawbacks but the obtained results are still controversial considering that the homogeneous dispersion of CNTs and the interfacial bonding between two different ceramic materials remains as a difficult task leading to little or even no improvement in mechanical properties. Besides, the role of CNTs in the sintering of ceramic materials is not clear in the scientific literature taking into account parameters such as materials used and particularly inconsistencies in dispersion and mixing of the CNTs. We discuss how the CNTs can affect the sintering behavior and microstructural evolution of alumina and zirconia ceramics and the combination of them.

**Keywords:** alumina, zirconia, carbon nanotubes, sintering, fracture toughness

### **1. Introduction**

Carbon nanotubes (commonly abridged as CNTs) are structures of nanometric dimension built up entirely by atoms of carbon and they have a high Young's modulus with good flexibility and

© 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.

good thermal and chemical stability being visualized as a graphene sheet that has been rolled into a tube with hemispherical caps at one or both ends [1, 2]. It is well known that the sp<sup>2</sup> -sp2 covalent carbon-carbon bonding is one of the strongest existing in nature, which in turn leads to exceptional material properties as a consequence of their symmetric structure. In the scientific literature, many researchers have reported mechanical properties of CNTs that exceed those of any previously existing materials [3] attracting an intense interest from the scientific community, as well as from industry. Nanotubes along with graphene are currently the subject of several papers per day. Undoubtedly, the discovery of CNTs [4] has aroused greatest interest as a potential reinforcing agent for different composite materials [5, 6] in order to impart stiffness, strength, and toughness considering their outstanding intrinsic physical properties and low density.

between the two different materials (CNTs and ceramic matrix) [5, 11]. In spite of this, many interesting works have been carried out to improve the mechanical properties of ceramic

On the Performance of Carbon Nanotubes on Sintered Alumina-Zirconia Ceramics

during 3 min with SPS as densification method obtaining a fracture toughness of 9.7 MPa m1/2 being nearly three times that of nanocrystalline alumina (3.5 MPa m1/2); however, the results have not been reproduced up to now. In fact, these results were refuted by Wang et al. [16] who reported that CNT-alumina composites are highly contact damage-resistant and also showed that a more reliable single edge V-notched beam test could reveal no enhanced toughening, refuting therefore the claims of high toughness by Zhan et al. [16] in reference to the fracture toughness technique used. In other ceramic system prepared by HP, a mixture of MWCNTs and nano-SiC powders were reported by Ma et al. [17] being the dispersion of the MWCNTs very poor. However, an increase in both the bending strength and fracture tough-

The above mentioned are few examples where both SWCNTs and MWCNTs have been used as reinforcement agents and the results have been some controversial and contradictory. To reinforce ceramic matrices, there are different kinds of CNTs available and there has been much documented research reporting the incorporation of SWCNTs and MWCNTs into the ceramic matrices in order to convert them into tough, strong, electric, and thermal conductive materials [18, 19]. Indeed, approximately 88% of the reported cases used the readily available and economically feasible MWCNTs as a reinforcement agent in comparison to SWCNTs. Regardless of this, it is noteworthy that the problems to achieve homogeneous distribution of CNTs as well as the related problems to the reproducible preparation of ceramic composites with improved mechanical properties can be considered as key obstacles. This difficulty to disperse CNTs into the ceramic matrices has led to explore and to develop efficient and economical processing methods that enable homogeneous dispersion of different types of CNTs in ceramic hosts. Those dispersion methods are referred to colloidal, sol-gel, and electrophoretic deposition processing techniques that are of paramount importance as methods to directionally emplace the CNTs while reducing the energy demands for the manufacture of the final product. Nonetheless, nowadays the main attention is paid to ultrasonic, plasma techniques, and other physical techniques in combination to the use of surfactants, functionalizing, and debundling agents of distinct nature including elemental substances, metal and organic salts, mineral and organic acids, oxides, inorganic and organic peroxides, organic sulfonates, polymers, dyes, natural products, biomolecules, and coordination compounds in

order to produce ceramic nanocomposites with excellent mechanical properties [20].

Although currently there are several processes to manufacture CNTs containing alumina- and zirconia-based ceramic nanocomposites, some of them with a range of controversial results, it is evident that MWCNTs are preferably used as the reinforcement compared with SWCNTs [18, 21]. With the purpose to achieve nanostructures with outstanding mechanical properties, undoubtedly the nature of the different available CNTs and their processing conditions are of paramount importance and must therefore be considered. Commercially a great variety of CNTs are available with different conditions of preparation and subsequent treatment, which in turn and considering the sintering route adopted for the densification of the ceramic composite, could be the main reason for the controversy of results on their mechanical properties.

O3 + 10 vol% SWCNTs at 1150°C

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109

composites. Indeed, Zhan et al. [15] prepared 100% dense Al2

ness was obtained with a carbon nanotube content of around 10 vol%.

Taking into account that ceramic oxides such as aluminum oxide (Al2 O3 ), zirconium oxide (ZrO2 ), and a combination of them (alumina toughened zirconia (ATZ) and zirconia toughened alumina (ZTA)) are used in both structural and biomedical applications, it is of paramount importance to impart in these composites outstanding mechanical properties. In spite of combining the high strength and toughness of the tetragonal zirconia with the excellent hardness of the alumina ceramic, the low fracture toughness of brittleness of the alumina-based ceramics is still the main issue [7]. To overcome this weakness, the reinforcement in oxide ceramics can be carried out by short and long fibers. However, the CNTs due to their outstanding properties are fascinating materials as a reinforcement agent [5] and commonly two major structural forms of CNTs are known to exist as follows: single walled carbon nanotube (SWCNT) bundles and multi-walled carbon nanotubes (MWCNTs) [8]. Some experimental measurements have indicated that SWCNTs have Young's moduli ranging from 1 to 5 TPa, meanwhile MWCNTs have an average value of 1.8 TPa and a very similar relative density value [9, 10].

Nonetheless, the reduced size and dimensionality of CNTs lead to form complex networks of aggregates and bundles within ceramic composites [11]. In this context, this aggregation state plays an important role in defining the mechanical, electrical as well as thermal properties of the ceramic composites.

### **2. CNTs into the ceramic matrix during sintering**

Although the role of CNTs in the sintering and microstructural evolution of ceramic composites is not completely clarified in the literature, these can be processed using the regular processing route and then densified mainly by pressureless sintering (PLS), hot pressing (HP) sintering, and spark plasma sintering (SPS) [12, 13]. Regardless the sintering method used, the scientific literature has reported significant improvement of mechanical properties but the results obtained with CNTs reinforced alumina- and zirconia-based ceramics remain controversial and can be observed that the wide scattered and highly debatable could be arise from different testing techniques used [3]. Unfortunately, today there is very limited experimental data on whether the final distribution of CNTs within microstructure of composites is mainly achieved during the powder phase processing or the sintering process [14]. Two main challenges in the processing of CNTs as a reinforcement agent in ceramics remain the heel of Achilles: a homogeneous dispersion of CNTs in matrix materials and the interfacial bonding between the two different materials (CNTs and ceramic matrix) [5, 11]. In spite of this, many interesting works have been carried out to improve the mechanical properties of ceramic composites. Indeed, Zhan et al. [15] prepared 100% dense Al2 O3 + 10 vol% SWCNTs at 1150°C during 3 min with SPS as densification method obtaining a fracture toughness of 9.7 MPa m1/2 being nearly three times that of nanocrystalline alumina (3.5 MPa m1/2); however, the results have not been reproduced up to now. In fact, these results were refuted by Wang et al. [16] who reported that CNT-alumina composites are highly contact damage-resistant and also showed that a more reliable single edge V-notched beam test could reveal no enhanced toughening, refuting therefore the claims of high toughness by Zhan et al. [16] in reference to the fracture toughness technique used. In other ceramic system prepared by HP, a mixture of MWCNTs and nano-SiC powders were reported by Ma et al. [17] being the dispersion of the MWCNTs very poor. However, an increase in both the bending strength and fracture toughness was obtained with a carbon nanotube content of around 10 vol%.

good thermal and chemical stability being visualized as a graphene sheet that has been rolled into a tube with hemispherical caps at one or both ends [1, 2]. It is well known that the sp<sup>2</sup>

covalent carbon-carbon bonding is one of the strongest existing in nature, which in turn leads to exceptional material properties as a consequence of their symmetric structure. In the scientific literature, many researchers have reported mechanical properties of CNTs that exceed those of any previously existing materials [3] attracting an intense interest from the scientific community, as well as from industry. Nanotubes along with graphene are currently the subject of several papers per day. Undoubtedly, the discovery of CNTs [4] has aroused greatest interest as a potential reinforcing agent for different composite materials [5, 6] in order to impart stiffness, strength, and toughness considering their outstanding intrinsic physical properties and low density.

), and a combination of them (alumina toughened zirconia (ATZ) and zirconia toughened alumina (ZTA)) are used in both structural and biomedical applications, it is of paramount importance to impart in these composites outstanding mechanical properties. In spite of combining the high strength and toughness of the tetragonal zirconia with the excellent hardness of the alumina ceramic, the low fracture toughness of brittleness of the alumina-based ceramics is still the main issue [7]. To overcome this weakness, the reinforcement in oxide ceramics can be carried out by short and long fibers. However, the CNTs due to their outstanding properties are fascinating materials as a reinforcement agent [5] and commonly two major structural forms of CNTs are known to exist as follows: single walled carbon nanotube (SWCNT) bundles and multi-walled carbon nanotubes (MWCNTs) [8]. Some experimental measurements have indicated that SWCNTs have Young's moduli ranging from 1 to 5 TPa, meanwhile MWCNTs

Taking into account that ceramic oxides such as aluminum oxide (Al2

have an average value of 1.8 TPa and a very similar relative density value [9, 10].

**2. CNTs into the ceramic matrix during sintering**

Nonetheless, the reduced size and dimensionality of CNTs lead to form complex networks of aggregates and bundles within ceramic composites [11]. In this context, this aggregation state plays an important role in defining the mechanical, electrical as well as thermal properties of

Although the role of CNTs in the sintering and microstructural evolution of ceramic composites is not completely clarified in the literature, these can be processed using the regular processing route and then densified mainly by pressureless sintering (PLS), hot pressing (HP) sintering, and spark plasma sintering (SPS) [12, 13]. Regardless the sintering method used, the scientific literature has reported significant improvement of mechanical properties but the results obtained with CNTs reinforced alumina- and zirconia-based ceramics remain controversial and can be observed that the wide scattered and highly debatable could be arise from different testing techniques used [3]. Unfortunately, today there is very limited experimental data on whether the final distribution of CNTs within microstructure of composites is mainly achieved during the powder phase processing or the sintering process [14]. Two main challenges in the processing of CNTs as a reinforcement agent in ceramics remain the heel of Achilles: a homogeneous dispersion of CNTs in matrix materials and the interfacial bonding

(ZrO2

108 Sintering Technology - Method and Application

the ceramic composites.


O3

), zirconium oxide

The above mentioned are few examples where both SWCNTs and MWCNTs have been used as reinforcement agents and the results have been some controversial and contradictory. To reinforce ceramic matrices, there are different kinds of CNTs available and there has been much documented research reporting the incorporation of SWCNTs and MWCNTs into the ceramic matrices in order to convert them into tough, strong, electric, and thermal conductive materials [18, 19]. Indeed, approximately 88% of the reported cases used the readily available and economically feasible MWCNTs as a reinforcement agent in comparison to SWCNTs. Regardless of this, it is noteworthy that the problems to achieve homogeneous distribution of CNTs as well as the related problems to the reproducible preparation of ceramic composites with improved mechanical properties can be considered as key obstacles. This difficulty to disperse CNTs into the ceramic matrices has led to explore and to develop efficient and economical processing methods that enable homogeneous dispersion of different types of CNTs in ceramic hosts. Those dispersion methods are referred to colloidal, sol-gel, and electrophoretic deposition processing techniques that are of paramount importance as methods to directionally emplace the CNTs while reducing the energy demands for the manufacture of the final product. Nonetheless, nowadays the main attention is paid to ultrasonic, plasma techniques, and other physical techniques in combination to the use of surfactants, functionalizing, and debundling agents of distinct nature including elemental substances, metal and organic salts, mineral and organic acids, oxides, inorganic and organic peroxides, organic sulfonates, polymers, dyes, natural products, biomolecules, and coordination compounds in order to produce ceramic nanocomposites with excellent mechanical properties [20].

Although currently there are several processes to manufacture CNTs containing alumina- and zirconia-based ceramic nanocomposites, some of them with a range of controversial results, it is evident that MWCNTs are preferably used as the reinforcement compared with SWCNTs [18, 21]. With the purpose to achieve nanostructures with outstanding mechanical properties, undoubtedly the nature of the different available CNTs and their processing conditions are of paramount importance and must therefore be considered. Commercially a great variety of CNTs are available with different conditions of preparation and subsequent treatment, which in turn and considering the sintering route adopted for the densification of the ceramic composite, could be the main reason for the controversy of results on their mechanical properties.

Under this assumption, it could be asserted that the different types of CNTs and the amount of these, added to the ceramic matrix, can lead to strong variations of the mechanical properties and affinity to the ceramic matrix as a consequence of their tubular structure, the number of the roller graphene sheets, diameter, length and their crystallinity linked to the number and nature of surface defects, and surface chemistry [23, 34]. All these aspects in turn will produce microstructural variations and fracture behavior of ceramic nanocomposites manufactured under different densification methods even with the same percentage of CNTs content within the ceramic matrix and using the same sintering technique. **Figure 1** shows fracture surfaces corresponding to an alumina ceramic with additions of 0.1 wt% of four different kinds of MWCNTs and sintered by SPS at 1500°C 3 min.

Knowing that the CNTs addition significantly retards grain growth during sintering [16, 19, 22, 25, 26] due to the pinning of matrix grains by the presence of CNTs, it is curious to observe from **Figure 1** the growth grain in alumina for low CNT contents and dispersed into the ceramic matrix under same conditions. These results reflect the unexpected effect that CNTs

could have on the microstructural evolution, affecting mainly the mechanical properties what contrasts with the results reported in the literature [11, 27–29]. On the other hand, it is expected that low levels of CNTs can be easier to disperse into the comparison with high levels of them. Likewise, the presence and distribution of the CNTs within the ceramic matrix induces a combination of fracture mode in each composite where the CNTs as the second phase could be responsible for altering the fracture modes [18]. Conversely, high levels of both MWCNTs and SWCNTs induce to an undesirable agglomeration state which is detrimental to achieve high densification by the formation of clusters owing to van der Waals forces [30] and the concentration of reinforcement at certain point leading to worsening of overall mechanical properties [11] such as can be illustrated in **Figure 2** for zirconia toughened alumina (ZTA)

**Figure 2.** SEM of fracture surfaces of ZTA with additions of (a) 10 vol% MWCNT, and 10 vol% SWCNT (b), sintered at

On the Performance of Carbon Nanotubes on Sintered Alumina-Zirconia Ceramics

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111

For the same nanotubes content in **Figure 2**, generally the MWCNTs can be homogeneously dispersed (**Figure 1a**) meanwhile denser agglomerates of SWCNTs were formed (**Figure 2b**).

CNT content and Zhang et al. [32] in alumina ceramics with additions of 1, 3, and 5 vol% MWCNTs. In view of experimental evidences, such as **Figure 1** [33] and **Figure 2** [34], and other more reported in the scientific literature (for example Tables 1 and 2 in Ref. [18]) showing a broad spectrum of final densities and mechanical properties using different methods of purification and dispersion of CNTs into the several ceramic matrix systems, a too high CNT content must be avoided with the purpose to achieve a good dispersion by means of conventional and economic methods, since the use of sophisticated techniques can make the

Taking into account that intrinsic factors of CNTs such as diameter, length, nature of surface defects, orientation, mechanical strength, and affinity with the ceramic matrix have a strong influence on the microstructure and grain growth of the ceramic composites, the effective and optimum

O3

with variable

Similar observations have been reported by Zhang et al. [31] in CNTs-Al2

process expensive and its industrial scaling more difficult.

**reinforcement agents in ceramics**

**3. Requirements for optimum performance of CNTs as** 

with additions of 10 vol% of CNTs.

1520°C during 1 h, showing agglomeration of SWCNT in (b).

**Figure 1.** Scanning electron microscopy (SEM) micrographs of fracture surfaces of Al2O3 with additions of 0.1 wt% of four different kinds of MWCNTs. (a) MWCNT 1, outer diameter 50–80 nm, length 10–30 μm; (b) MWCNT 2, outer diameter 10–20 nm, length 10–30 μm; (c) MWCNT 3, outer diameter < 8 nm, length 10–30 μm; and (d) MWCNT CIMAV, outer diameter 10–70 nm, length 120–160 μm sintered by SPS at 1500°C 3 min. Figures (b) and (c) from the paper: SWCNTs versus MWCNTs as reinforcement agents in zirconia- and alumina-based nanocomposites: which one to use, Bocanegra-Bernal et al. [24]. © 2016 Scrivener Publishing LLC. With permission.

On the Performance of Carbon Nanotubes on Sintered Alumina-Zirconia Ceramics http://dx.doi.org/10.5772/intechopen.78542 111

**Figure 2.** SEM of fracture surfaces of ZTA with additions of (a) 10 vol% MWCNT, and 10 vol% SWCNT (b), sintered at 1520°C during 1 h, showing agglomeration of SWCNT in (b).

could have on the microstructural evolution, affecting mainly the mechanical properties what contrasts with the results reported in the literature [11, 27–29]. On the other hand, it is expected that low levels of CNTs can be easier to disperse into the comparison with high levels of them. Likewise, the presence and distribution of the CNTs within the ceramic matrix induces a combination of fracture mode in each composite where the CNTs as the second phase could be responsible for altering the fracture modes [18]. Conversely, high levels of both MWCNTs and SWCNTs induce to an undesirable agglomeration state which is detrimental to achieve high densification by the formation of clusters owing to van der Waals forces [30] and the concentration of reinforcement at certain point leading to worsening of overall mechanical properties [11] such as can be illustrated in **Figure 2** for zirconia toughened alumina (ZTA) with additions of 10 vol% of CNTs.

For the same nanotubes content in **Figure 2**, generally the MWCNTs can be homogeneously dispersed (**Figure 1a**) meanwhile denser agglomerates of SWCNTs were formed (**Figure 2b**). Similar observations have been reported by Zhang et al. [31] in CNTs-Al2 O3 with variable CNT content and Zhang et al. [32] in alumina ceramics with additions of 1, 3, and 5 vol% MWCNTs. In view of experimental evidences, such as **Figure 1** [33] and **Figure 2** [34], and other more reported in the scientific literature (for example Tables 1 and 2 in Ref. [18]) showing a broad spectrum of final densities and mechanical properties using different methods of purification and dispersion of CNTs into the several ceramic matrix systems, a too high CNT content must be avoided with the purpose to achieve a good dispersion by means of conventional and economic methods, since the use of sophisticated techniques can make the process expensive and its industrial scaling more difficult.

### **3. Requirements for optimum performance of CNTs as reinforcement agents in ceramics**

**Figure 1.** Scanning electron microscopy (SEM) micrographs of fracture surfaces of Al2O3 with additions of 0.1 wt% of four different kinds of MWCNTs. (a) MWCNT 1, outer diameter 50–80 nm, length 10–30 μm; (b) MWCNT 2, outer diameter 10–20 nm, length 10–30 μm; (c) MWCNT 3, outer diameter < 8 nm, length 10–30 μm; and (d) MWCNT CIMAV, outer diameter 10–70 nm, length 120–160 μm sintered by SPS at 1500°C 3 min. Figures (b) and (c) from the paper: SWCNTs versus MWCNTs as reinforcement agents in zirconia- and alumina-based nanocomposites: which one to use,

Under this assumption, it could be asserted that the different types of CNTs and the amount of these, added to the ceramic matrix, can lead to strong variations of the mechanical properties and affinity to the ceramic matrix as a consequence of their tubular structure, the number of the roller graphene sheets, diameter, length and their crystallinity linked to the number and nature of surface defects, and surface chemistry [23, 34]. All these aspects in turn will produce microstructural variations and fracture behavior of ceramic nanocomposites manufactured under different densification methods even with the same percentage of CNTs content within the ceramic matrix and using the same sintering technique. **Figure 1** shows fracture surfaces corresponding to an alumina ceramic with additions of 0.1 wt% of four different kinds of

Knowing that the CNTs addition significantly retards grain growth during sintering [16, 19, 22, 25, 26] due to the pinning of matrix grains by the presence of CNTs, it is curious to observe from **Figure 1** the growth grain in alumina for low CNT contents and dispersed into the ceramic matrix under same conditions. These results reflect the unexpected effect that CNTs

Bocanegra-Bernal et al. [24]. © 2016 Scrivener Publishing LLC. With permission.

MWCNTs and sintered by SPS at 1500°C 3 min.

110 Sintering Technology - Method and Application

Taking into account that intrinsic factors of CNTs such as diameter, length, nature of surface defects, orientation, mechanical strength, and affinity with the ceramic matrix have a strong influence on the microstructure and grain growth of the ceramic composites, the effective and optimum utilization of CNTs in composite applications depends strongly on the ability to disperse CNTs homogeneously throughout the matrix to obtain a good interfacial bonding which is required to achieve an efficient load transfer across the CNT-matrix interface as a primary condition for improving the mechanical properties of ceramic composites [35]. As a consequence of high van der Waals force, surface area and high aspect ratio of CNTs (most notorious in SWCNTs), inevitably self-aggregation occurs and therefore, the improvement of dispersion has become a challenge to maximize the properties of CNTs [36, 37]. **Figure 3** shows the fracture surface of alumina with additions of 0.5 wt% of SWCNTs where bundles of CNTs located intergranularly are evident impeding the densification of the ceramic composite during the sintering at high temperatures. Nevertheless, the characterization of the dispersion of CNTs within the microstructure in the sintered composites is often based on the visual observation of micrographs obtained from scanning electron microscopy (SEM). It is of paramount importance to quantify the quality of distribution of CNTs in the microstructure of the sintered samples to understand the broad properties that the CNTs can offer as reinforcement agents in the ceramic nanocomposites [14, 30].

Regardless of dispersion method used, it is indisputable that the quantity, location, and distribution of CNTs in the ceramic matrix play an important role in the sintering of the composites producing compounds with a varied range of mechanical properties stressing that the dispersion of CNTs is not an easily controllable and reproducible process and, therefore, that the final properties can depend in first instance on the route followed for their dispersion into the ceramic matrix, as well as the sintering route chosen. **Figure 4** illustrates the fracture surface of an alumina ceramic doped with 0.5 wt% of MWCNTs that are located parallel to the fracture surface indicating a poor bonding to the ceramic matrix not contributing to the improvement of the fracture toughness by the absence of toughening mechanisms such as crack branching, pull out, and crack deflection.

Recent work [38] reports for a same content of CNTs (0.1 wt%), a wide range of grain sizes and fracture toughness values, with the hardness remaining practically the same in alumina ceramic composites prepared by PLS, hot isostatic pressing (HIP), and sintering + hot isostatic pressing (sinter + HIP) routes. In all cases, the dispersion of CNTs and mixture preparation was performed under same conditions and the substantial difference observed in the final results could be explained by the different sintering kinetics of the three techniques applied in that work [38]. According to Orsolya [14], at temperatures higher than 1500°C and long sintering times, large amount of mass diffusion takes place facilitating a significant rearrangement of the nanotubes as well, while for SPS the sintering is completed in short time, where it is expected less and most likely short range rearrangements of the CNTs aggregates formed during the powder phase processing. However, it is well known that PLS and HP are the techniques that commonly require high temperatures, which could induce in some cases to the partial destruction of CNTs at these temperatures. Based on this, these sintering techniques have been replaced long ago by SPS, due to the damage of the CNTs by the higher temperatures and longer sintering times attained in those conventional methods.

of the CNTs to the ceramic matrix affecting the mechanical properties.

**Figure 4.** Scanning electron microscopy (SEM) micrograph of fracture surfaces of Al2O3 with additions of 0.5 wt% of MWCNTs sintered at 1520°C under atmospheric pressure with graphite powder as powder bed. Note the poor bonding

On the Performance of Carbon Nanotubes on Sintered Alumina-Zirconia Ceramics

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113

Considering the pros and cons of the different sintering methods used in the manufacture of ceramic nanocomposites, undoubtedly that SPS so far only facilitates the fabrication of simple geometries such as discs, rings, and cylinders, but the manufacture of more complex geometries is still in the development stage implying that on an industrial scale, PLS remains as the main sintering method adopted when complex geometries must be manufactured, mentioning that this method has been continuously perfected to obtain the best properties in the final compounds without compromising the integrity of nanotubes at high temperatures. Thus, intimate interfacial bonding between CNTs and ceramic matrix by an optimum dispersion of CNTs to achieve toughening of ceramic nanocomposite by a specific sintering method are the

main key points for preparing CNTs reinforced oxide ceramics [39].

**Figure 3.** Scanning electron microscopy (SEM) micrograph of fracture surfaces of Al2O3 with additions of 0.5 wt% of SWCNTs sintered at 1520°C under atmospheric pressure with graphite powder as powder bed.

utilization of CNTs in composite applications depends strongly on the ability to disperse CNTs homogeneously throughout the matrix to obtain a good interfacial bonding which is required to achieve an efficient load transfer across the CNT-matrix interface as a primary condition for improving the mechanical properties of ceramic composites [35]. As a consequence of high van der Waals force, surface area and high aspect ratio of CNTs (most notorious in SWCNTs), inevitably self-aggregation occurs and therefore, the improvement of dispersion has become a challenge to maximize the properties of CNTs [36, 37]. **Figure 3** shows the fracture surface of alumina with additions of 0.5 wt% of SWCNTs where bundles of CNTs located intergranularly are evident impeding the densification of the ceramic composite during the sintering at high temperatures. Nevertheless, the characterization of the dispersion of CNTs within the microstructure in the sintered composites is often based on the visual observation of micrographs obtained from scanning electron microscopy (SEM). It is of paramount importance to quantify the quality of distribution of CNTs in the microstructure of the sintered samples to understand the broad properties that the

Regardless of dispersion method used, it is indisputable that the quantity, location, and distribution of CNTs in the ceramic matrix play an important role in the sintering of the composites producing compounds with a varied range of mechanical properties stressing that the dispersion of CNTs is not an easily controllable and reproducible process and, therefore, that the final properties can depend in first instance on the route followed for their dispersion into the ceramic matrix, as well as the sintering route chosen. **Figure 4** illustrates the fracture surface of an alumina ceramic doped with 0.5 wt% of MWCNTs that are located parallel to the fracture surface indicating a poor bonding to the ceramic matrix not contributing to the improvement of the fracture toughness by the absence of toughening mechanisms such as

**Figure 3.** Scanning electron microscopy (SEM) micrograph of fracture surfaces of Al2O3 with additions of 0.5 wt% of

SWCNTs sintered at 1520°C under atmospheric pressure with graphite powder as powder bed.

CNTs can offer as reinforcement agents in the ceramic nanocomposites [14, 30].

crack branching, pull out, and crack deflection.

112 Sintering Technology - Method and Application

**Figure 4.** Scanning electron microscopy (SEM) micrograph of fracture surfaces of Al2O3 with additions of 0.5 wt% of MWCNTs sintered at 1520°C under atmospheric pressure with graphite powder as powder bed. Note the poor bonding of the CNTs to the ceramic matrix affecting the mechanical properties.

Recent work [38] reports for a same content of CNTs (0.1 wt%), a wide range of grain sizes and fracture toughness values, with the hardness remaining practically the same in alumina ceramic composites prepared by PLS, hot isostatic pressing (HIP), and sintering + hot isostatic pressing (sinter + HIP) routes. In all cases, the dispersion of CNTs and mixture preparation was performed under same conditions and the substantial difference observed in the final results could be explained by the different sintering kinetics of the three techniques applied in that work [38]. According to Orsolya [14], at temperatures higher than 1500°C and long sintering times, large amount of mass diffusion takes place facilitating a significant rearrangement of the nanotubes as well, while for SPS the sintering is completed in short time, where it is expected less and most likely short range rearrangements of the CNTs aggregates formed during the powder phase processing. However, it is well known that PLS and HP are the techniques that commonly require high temperatures, which could induce in some cases to the partial destruction of CNTs at these temperatures. Based on this, these sintering techniques have been replaced long ago by SPS, due to the damage of the CNTs by the higher temperatures and longer sintering times attained in those conventional methods.

Considering the pros and cons of the different sintering methods used in the manufacture of ceramic nanocomposites, undoubtedly that SPS so far only facilitates the fabrication of simple geometries such as discs, rings, and cylinders, but the manufacture of more complex geometries is still in the development stage implying that on an industrial scale, PLS remains as the main sintering method adopted when complex geometries must be manufactured, mentioning that this method has been continuously perfected to obtain the best properties in the final compounds without compromising the integrity of nanotubes at high temperatures. Thus, intimate interfacial bonding between CNTs and ceramic matrix by an optimum dispersion of CNTs to achieve toughening of ceramic nanocomposite by a specific sintering method are the main key points for preparing CNTs reinforced oxide ceramics [39].

### **4. Conclusions**

Many attempts have been made to improve the mechanical properties of ceramics through incorporating CNTs taking advantage of its mechanical and physical properties combined with their low density and judging from the results of several researchers, where unfortunately most of them have been disappointing for toughening, since very little or no increase in toughening upon introduction of either single- or multi-walled carbon nanotubes into alumina-zirconiabased ceramics has been shown. During the last decade, the CNTs and their processing and dispersion methods have been intensively studied. However, the controversial results reported could be arise from different dispersion techniques, sintering processes, and finally testing techniques used for their characterization. On the other side, there is the controversy to choose between the different kinds of CNTs to reinforce ceramic matrices to improve the mechanical properties. Another debatable question is to define the proper amount of CNT content to obtain a ceramic composite with improved mechanical properties, considering that even using the same contents (vol% or wt%) as well as the same type of CNTs into the ceramic matrix, the expected values of fracture toughness, and/or hardness could differ from each other favorably or unfavorably, particularly for higher concentrations of CNTs. Therefore, the meticulous selection of a specific kind of CNT as a reinforcement agent for a determined ceramic must be carried out after experimental trials, since the prediction of results starting from raw material known such as ceramic powders and CNTs (even knowing its diameter, length, and agglomeration state) is not an option, considering as above mentioned that dispersion of CNTs is nowadays considered by a sector of scientific community as a process very difficult to control and to reproduce and with base of this, the final properties will be different for each specific ceramic composition.

**Author details**

**References**

Carlos Dominguez-Rios<sup>1</sup>

Miguel Humberto Bocanegra-Bernal1

Nanotecnologìa, Chihuahua, Mexico

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On the Performance of Carbon Nanotubes on Sintered Alumina-Zirconia Ceramics

and Armando Garcia-Reyes2

1 Centro de Investigaciòn en Materiales Avanzados, CIMAV S.C., Laboratorio Nacional de

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[8] Qian D, Liu WK, Ruoff RS.Load transfer mechanism in carbon nanotube ropes. Composites Science and Technology. 2003;**63**:1561-1569. DOI: 10.1016/S0266-3538(03)00064-2

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\*Address all correspondence to: miguel.bocanegra@cimav.edu.mx

2 Interceramic, Department R and D, Chihuahua, Mexico

, Armando Reyes-Rojas<sup>1</sup>

http://dx.doi.org/10.5772/intechopen.78542

/Al2 O3 nano-

laser CO-vaporization.

,

115

Concluding, the development of CNT-based ceramic nanocomposites is a promissory subject but still with many difficulties and challenges and there is a lack of sufficient knowledge to systematically improve properties over traditional ceramic composites or their monoliths with notable enhancements. As can be pointed out by Curtin and Sheldon [40], *"the traditional interplay of careful processing and evaluation, coupled with mechanistic assessment of properties, remains a valid paradigm at the nanoscale and should be assiduously applied to future research in CNT-composite systems."* In other words, the optimum dispersion of CNTs into liquids could be achieved by mechanical (physical) or chemical methods, taking into account the amount and the type of CNTs to be dispersed as well as the optimal concentrations of aqueous surfactant solution and sonication time to contribute to the efficiently dispersion of CNTs.

### **Acknowledgements**

The authors wish to express their appreciation to Wilber Antúnez for SEM work.

### **Conflict of interest**

The authors declare that there is no conflict of interest.

### **Author details**

**4. Conclusions**

114 Sintering Technology - Method and Application

**Acknowledgements**

**Conflict of interest**

Many attempts have been made to improve the mechanical properties of ceramics through incorporating CNTs taking advantage of its mechanical and physical properties combined with their low density and judging from the results of several researchers, where unfortunately most of them have been disappointing for toughening, since very little or no increase in toughening upon introduction of either single- or multi-walled carbon nanotubes into alumina-zirconiabased ceramics has been shown. During the last decade, the CNTs and their processing and dispersion methods have been intensively studied. However, the controversial results reported could be arise from different dispersion techniques, sintering processes, and finally testing techniques used for their characterization. On the other side, there is the controversy to choose between the different kinds of CNTs to reinforce ceramic matrices to improve the mechanical properties. Another debatable question is to define the proper amount of CNT content to obtain a ceramic composite with improved mechanical properties, considering that even using the same contents (vol% or wt%) as well as the same type of CNTs into the ceramic matrix, the expected values of fracture toughness, and/or hardness could differ from each other favorably or unfavorably, particularly for higher concentrations of CNTs. Therefore, the meticulous selection of a specific kind of CNT as a reinforcement agent for a determined ceramic must be carried out after experimental trials, since the prediction of results starting from raw material known such as ceramic powders and CNTs (even knowing its diameter, length, and agglomeration state) is not an option, considering as above mentioned that dispersion of CNTs is nowadays considered by a sector of scientific community as a process very difficult to control and to reproduce and with base of this, the final properties will be different for each specific ceramic composition.

Concluding, the development of CNT-based ceramic nanocomposites is a promissory subject but still with many difficulties and challenges and there is a lack of sufficient knowledge to systematically improve properties over traditional ceramic composites or their monoliths with notable enhancements. As can be pointed out by Curtin and Sheldon [40], *"the traditional interplay of careful processing and evaluation, coupled with mechanistic assessment of properties, remains a valid paradigm at the nanoscale and should be assiduously applied to future research in CNT-composite systems."* In other words, the optimum dispersion of CNTs into liquids could be achieved by mechanical (physical) or chemical methods, taking into account the amount and the type of CNTs to be dispersed as well as the optimal concentrations of aqueous surfac-

tant solution and sonication time to contribute to the efficiently dispersion of CNTs.

The authors wish to express their appreciation to Wilber Antúnez for SEM work.

The authors declare that there is no conflict of interest.

Miguel Humberto Bocanegra-Bernal1 \*, Alfredo Aguilar-Elguezabal1 , Armando Reyes-Rojas<sup>1</sup> , Carlos Dominguez-Rios<sup>1</sup> and Armando Garcia-Reyes2

\*Address all correspondence to: miguel.bocanegra@cimav.edu.mx

1 Centro de Investigaciòn en Materiales Avanzados, CIMAV S.C., Laboratorio Nacional de Nanotecnologìa, Chihuahua, Mexico

2 Interceramic, Department R and D, Chihuahua, Mexico

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## *Edited by Malin Liu*

Sintering technology is an old and extensive technology in many areas, and it has been used especially in ceramic fabrication. This book covers many fields, for example, the development of different sintering technologies in recent years, such as spark plasma sintering, flash sintering, microwave sintering, reaction and laser sintering, and so on, and also some special ceramic material fabrication methods and applications, such as carbon nanotubes mixed with alumina and zirconia ceramics, pure and doped zirconia, ZnO ceramic varistors, and so on.

Published in London, UK © 2018 IntechOpen © lavendertime / iStock

Sintering Technology - Method and Application

Sintering Technology

Method and Application