**1. Introduction**

Over the past few decades, functional ceramics have played a significant role in advanced technologies owing to their unique thermal, electrical, magnetic, opto-electrical, superconducting and gas-sensing properties. As such, functional ceramics have become the frontiers for advanced technologies such as information technology, medical technology, energy transformation, storage, supply and manufacture technology. For instance, functional ceramics are widely used for electronic applications as they can operate at high power and high frequencies, at high temperatures and harsh conditions. Their capability to combine properties such as electrical insulation and magnetism, which is not possible with metals, gives them an additive advantage.

Functional ceramics are produced from chemically synthesised powders in the form of oxides, nitrides, carbides and borides mainly through a powder metallurgy route. The properties of functional ceramics are microstructure sensitive, and microstructural features such as grain size, composition, homogeneity and grain boundary constituents are critical to their performance and reliability. The processing route dictates the final microstructural features obtained; thus, the choice/ design of a processing route is key in material functionality. For instance, fine grain size has been experimentally proven to amplify functional material properties such as electrical conductivity, thermal conductivity, piezoelectric and ferroelectric properties [1]. It must be mentioned though that the fabrication of dense nanostructured functional ceramics by conventional sintering methods is quite challenging owing to the uncontrollable high grain growth rates [2]. This explains the shift in research focus towards nanostructured functional materials in the past few years.

In recent years, spark plasma sintering (SPS) technology has proven its capability to fabricate fine-grained microstructures possessing superior properties for a wide range of materials [2]. This method is increasingly being applied in the production of functional ceramic materials. It is against this background that the present chapter is aimed at giving an insight on the progress made so far and, furthermore, how the resulting microstructures and properties align with the required functions. It is imperative that a background on the various applications of functional ceramics be given prior to a detailed discussion on the SPS sintering methodologies.

### **2. The most popular functional ceramics and their applications**

Functional ceramics are materials tailored to possess exceptional properties (electrical, thermal, optical, piezoelectric and magnetic properties) by controlling the composition and microstructures [3]. These materials are being utilised in a broad range of applications owing to the distinct advantages they offer in comparison to metals. The list below is not meant to be exhaustive but to give a qualitative review on the applications of the most popular functional ceramic materials.

#### **2.1 Piezoceramics**

Piezoelectric ceramic materials couple electrical and mechanical responses in their functioning and are widely used for electromechanical sensors and actuators. These materials normally produce an electrical response in the form of either a voltage or charge proportional to the applied stress when subjected to a mechanical force. Conversely, an applied voltage can be converted into mechanical energy such as in piezoelectric motors and sound-/ultrasound-generating devices. Piezoelectric materials are widely used in dynamic applications which include mechanical impact, ignition systems, vibration suppression and sensing [4]. Typical examples of piezoelectric materials include crystalline quartz, barium titanate (BaTiO3), vanadium niobate and lead zirconate titanate (PZT) [3]. In recent years, research focus on lead-free piezoelectric materials has been intensified aimed at replacing lead-based materials in electronic devices for the sake of human health and preservation of the natural environment [5].

Piezoelectric materials are produced as multilayered components consisting of electrode-ceramic stacks which can be simple/complex shapes. Various techniques have been developed to fabricate the piezoelectric ceramics without conducting post-processing. These include injection moulding [6, 7], embossing [8] and fused deposition method [9]. The powder injection moulding (PIM) process has

**107**

*Fabrication of Fine-Grained Functional Ceramics by Two-Step Sintering or Spark Plasma…*

received much attention owing to its ability to produce complex-shaped, microsized PZT components with minimum damage to the sintered ceramic [10]. The multilayered components are subsequently co-sintered at temperatures less than the melting point of the electrodes typically 1200–1300°C. However, lower sintering temperatures are preferred to avoid damaging the inner electrodes in the stack. It has been observed that the piezoelectric coefficient which directly influences the performance of piezoceramics is strongly influenced by the grain size [11]. Despite the extensive studies carried out over the past decades on the grain size effects on the physical properties of these materials, there are still major controversies on the dependence of piezoelectric and ferroelectric properties on the grain size [12]. There are a number of discrepancies in the existing literature which will be

Magnetic ceramic materials are extensively used in electronics and information communication fields [13]. They are generally classified as 'soft' and 'hard' magnets where soft implies large magnetic fields cannot be generated on the outside, whereas in the case of hard magnets, a magnetic field is generated around the magnet itself. Two broad groups of materials are widely used in the industry, i.e. metal magnetic materials and complex oxide containing trivalent iron ion (ferrites)

Magnetic materials are generally used in the form of multilayer core of rolled thin plates or in the form of dust core [13]. At high frequencies, most metallic magnets tend to lose their magnetic properties (permeability and magnetic flux density) due to low electrical resistivity. On the other hand, ferrites (ceramic magnets) show higher electrical resistivity and smaller eddy current loss at high frequencies; hence, they are more widely used in alternating magnetic fields in comparison to metal magnetic materials. The hard ferrite is used extensively as permanent magnets for speakers and motors. One of the critical magnetic characteristics required for high-frequency materials is high permeability and is defined as the ratio between the magnetic flux density, *B*, and magnetic field, *H*, as follows:

Permeability is a structure sensitive characteristic and is strongly affected by the microstructure of the sintered material. There are two general compositions used for oxide magnetic materials, spinel type (MeFe2O3) and garnet type (Me3Fe5O12); typical examples include MnFe2O4 and Y3Fe5O12, respectively. Owing to the complex compositional nature of these oxides, a powder metallurgy route is normally employed for the production of oxide magnets. The microstructure and compositional control are quite critical elements of the magnetic properties of the final products. Further, magnetic properties of materials have been shown to change from those of multidomain to those of single-domain structure as the grain size is reduced below a critical size [14]. The introduction of fine-grained sintered magnetic materials has opened some opportunity for new potential applications as

In the last few decades, the rapid development of modern communication devices such as cellular telephones, antennas and global positioning systems has

*<sup>H</sup>* (1)

*DOI: http://dx.doi.org/10.5772/intechopen.86461*

discussed later in this chapter.

magnetic materials (referred to as ceramic magnets).

*μ* = \_\_*<sup>B</sup>*

well as complexity on basic research [14].

**2.3 Dielectric ceramics**

**2.2 Magnetic ceramics**

*Fabrication of Fine-Grained Functional Ceramics by Two-Step Sintering or Spark Plasma… DOI: http://dx.doi.org/10.5772/intechopen.86461*

received much attention owing to its ability to produce complex-shaped, microsized PZT components with minimum damage to the sintered ceramic [10]. The multilayered components are subsequently co-sintered at temperatures less than the melting point of the electrodes typically 1200–1300°C. However, lower sintering temperatures are preferred to avoid damaging the inner electrodes in the stack. It has been observed that the piezoelectric coefficient which directly influences the performance of piezoceramics is strongly influenced by the grain size [11]. Despite the extensive studies carried out over the past decades on the grain size effects on the physical properties of these materials, there are still major controversies on the dependence of piezoelectric and ferroelectric properties on the grain size [12]. There are a number of discrepancies in the existing literature which will be discussed later in this chapter.

#### **2.2 Magnetic ceramics**

*Design and Manufacturing*

methodologies.

**2.1 Piezoceramics**

vation of the natural environment [5].

Functional ceramics are produced from chemically synthesised powders in the form of oxides, nitrides, carbides and borides mainly through a powder metallurgy route. The properties of functional ceramics are microstructure sensitive, and microstructural features such as grain size, composition, homogeneity and grain boundary constituents are critical to their performance and reliability. The processing route dictates the final microstructural features obtained; thus, the choice/ design of a processing route is key in material functionality. For instance, fine grain size has been experimentally proven to amplify functional material properties such as electrical conductivity, thermal conductivity, piezoelectric and ferroelectric properties [1]. It must be mentioned though that the fabrication of dense nanostructured functional ceramics by conventional sintering methods is quite challenging owing to the uncontrollable high grain growth rates [2]. This explains the shift in research focus towards nanostructured functional materials in the past few years. In recent years, spark plasma sintering (SPS) technology has proven its capability to fabricate fine-grained microstructures possessing superior properties for a wide range of materials [2]. This method is increasingly being applied in the production of functional ceramic materials. It is against this background that the present chapter is aimed at giving an insight on the progress made so far and, furthermore, how the resulting microstructures and properties align with the required functions. It is imperative that a background on the various applications of functional ceramics be given prior to a detailed discussion on the SPS sintering

**2. The most popular functional ceramics and their applications**

Functional ceramics are materials tailored to possess exceptional properties (electrical, thermal, optical, piezoelectric and magnetic properties) by controlling the composition and microstructures [3]. These materials are being utilised in a broad range of applications owing to the distinct advantages they offer in comparison to metals. The list below is not meant to be exhaustive but to give a qualitative review on the applications of the most popular functional ceramic materials.

Piezoelectric ceramic materials couple electrical and mechanical responses in their functioning and are widely used for electromechanical sensors and actuators. These materials normally produce an electrical response in the form of either a voltage or charge proportional to the applied stress when subjected to a mechanical force. Conversely, an applied voltage can be converted into mechanical energy such as in piezoelectric motors and sound-/ultrasound-generating devices. Piezoelectric materials are widely used in dynamic applications which include mechanical impact, ignition systems, vibration suppression and sensing [4]. Typical examples of piezoelectric materials include crystalline quartz, barium titanate (BaTiO3), vanadium niobate and lead zirconate titanate (PZT) [3]. In recent years, research focus on lead-free piezoelectric materials has been intensified aimed at replacing lead-based materials in electronic devices for the sake of human health and preser-

Piezoelectric materials are produced as multilayered components consisting of electrode-ceramic stacks which can be simple/complex shapes. Various techniques have been developed to fabricate the piezoelectric ceramics without conducting post-processing. These include injection moulding [6, 7], embossing [8] and fused deposition method [9]. The powder injection moulding (PIM) process has

**106**

Magnetic ceramic materials are extensively used in electronics and information communication fields [13]. They are generally classified as 'soft' and 'hard' magnets where soft implies large magnetic fields cannot be generated on the outside, whereas in the case of hard magnets, a magnetic field is generated around the magnet itself. Two broad groups of materials are widely used in the industry, i.e. metal magnetic materials and complex oxide containing trivalent iron ion (ferrites) magnetic materials (referred to as ceramic magnets).

Magnetic materials are generally used in the form of multilayer core of rolled thin plates or in the form of dust core [13]. At high frequencies, most metallic magnets tend to lose their magnetic properties (permeability and magnetic flux density) due to low electrical resistivity. On the other hand, ferrites (ceramic magnets) show higher electrical resistivity and smaller eddy current loss at high frequencies; hence, they are more widely used in alternating magnetic fields in comparison to metal magnetic materials. The hard ferrite is used extensively as permanent magnets for speakers and motors. One of the critical magnetic characteristics required for high-frequency materials is high permeability and is defined as the ratio between the magnetic flux density, *B*, and magnetic field, *H*, as follows:

$$
\mu = \frac{B}{H} \tag{1}
$$

Permeability is a structure sensitive characteristic and is strongly affected by the microstructure of the sintered material. There are two general compositions used for oxide magnetic materials, spinel type (MeFe2O3) and garnet type (Me3Fe5O12); typical examples include MnFe2O4 and Y3Fe5O12, respectively. Owing to the complex compositional nature of these oxides, a powder metallurgy route is normally employed for the production of oxide magnets. The microstructure and compositional control are quite critical elements of the magnetic properties of the final products. Further, magnetic properties of materials have been shown to change from those of multidomain to those of single-domain structure as the grain size is reduced below a critical size [14]. The introduction of fine-grained sintered magnetic materials has opened some opportunity for new potential applications as well as complexity on basic research [14].

#### **2.3 Dielectric ceramics**

In the last few decades, the rapid development of modern communication devices such as cellular telephones, antennas and global positioning systems has energised research in microwave dielectric materials [15]. Dielectric ceramics are materials used widely in advanced electronic devices such as capacitors and microwave resonators. They are classified into two broad groups based on their dielectric properties. High-quality factor materials are characterised by linear changes in polarisation with applied electric field. This group is dominated by titanate-based materials which normally sinter at temperatures higher than 1100°C; typical examples include TiO2, MgTiO3, CaTiO3 and SrTiO3 [13]. This group is characterised by a dielectric constant ε*r* of less than 1000. The second group is characterised by materials possessing a dielectric constant ε*r* higher than 1000. Typical examples include BaTiO3-based dielectric and lead-based dielectrics.

Ceramic capacitors are widely produced as sintered thin plates in a reducing atmosphere (low *PO*<sup>2</sup> ). In previous studies, the particle size effects of BaTiO3 on dielectric properties have been carried out with several models of the critical size of ferroelectricity being proposed. Reliability study results have shown that the dielectric layer should be pore-free with fine grain sizes (typically 0.8 μm) for enhanced performance [13].

#### **2.4 Thermoelectric ceramics**

Thermoelectric (TE) ceramic materials can directly convert heat energy to electric energy due to thermoelectric effects [16]. TEs provide an alternative environmentally friendly energy conversion technology which is compact, high reliability, has no pollutants and is feasible over a wide temperature range. The majority of thermoelectric devices operating near room temperature are based on Bismuth telluride (Bi2Te3) and its alloys. These materials have been produced by a variety of methods which include powder metallurgy techniques such as hot pressing (HP), SPS, Bridgman and zone melting and high-pressure sintering methods. Recent studies have shown that grain refinement of Bi2Te3-based alloys can greatly enhance thermoelectric performance [16]. The performance of thermoelectric materials is based on a dimensionless figure of merit (ZT) as follows:

$$\text{ZTT} = \frac{\text{S}^2 \delta T}{\kappa} = \frac{\text{S}^2 T}{(\kappa\_\epsilon + \kappa\_l)\rho} \tag{2}$$

**109**

in the final products.

**4. Spark plasma sintering technology**

*Fabrication of Fine-Grained Functional Ceramics by Two-Step Sintering or Spark Plasma…*

It is apparent that the increasing demand for ceramic materials in more advanced technological applications has resulted in greater need for improved properties and reliability of functional materials [2, 13]. The fabrication process plays a critical role in final material characteristics. In other words, the properties of ceramic materials are dictated by the microstructure which is a function of the processing method utilised. Thus microstructures can be tailored through fabrication processes to produce desired properties. In the past few decades, there has been a wide acceptance among powder metallurgists that the quality and reliability of ceramic materials are largely dictated by utilising powders of controlled purity, particle size and size distribution, shape and degree of agglomeration. The characteristics of starting powders are determined by their production method of which a variety of methods are available for the production of ceramic materials. The processing methods are broadly classified into solid-state processing (e.g. mechanical alloying, self-propagating high-temperature synthesis (SHS), laser ablation) and solution chemistry (e.g. sol-gel, polymer pyrolysis, hydrothermal methods) [2]. The chemical processing methods are generally more expensive than solid-state methods but offer more strict control of the powder characteristic [13]. The choice of a powder processing route will therefore largely depend on the production cost

third generation, some new concepts such as band structure engineering by doping, reduction in lattice thermal conductivity, nanostructuring and all-scale hierarchical architecturing and quantum confinement effects have been introduced to enhance

*DOI: http://dx.doi.org/10.5772/intechopen.86461*

**3. Processing of functional ceramic materials**

and its capability to achieve desired powder characteristics.

Chemical methods involving chemical reactions under carefully controlled conditions normally result in ultrafine nanometric powders (<100 nm) with a narrow particle size distribution [13]. The main attraction in using nanometric powders is their ability to sinter at lower temperatures (typically <0.5 Tm); this is in accordance with Hering's law discussed in the next section. The diffusion distance during sintering is drastically shortened in nanostructured powders. Moreover, an enhancement of material properties is expected owing to a reduction in the flaw size, and a higher density of highly disordered interfaces is also attained at nanometric particle size range. On the other hand, powders produced by mechanical methods possess a wide particle size distribution which may lead to higher packing density in the green body. However, this advantage is far outweighed by the difficulty in microstructural control during sintering as large grains grow uncontrollably at the expense of the smaller grains, thus making grain size control impossible. However, it is important to underline that as particle size decreases, below ~0.5 μm, particles become more difficult to handle and tend to agglomerate resulting in nonuniform consolidation of powders. Thus the use of nanopowders requires proper control and handling to ensure high-quality properties are attained

In 1906, Bloxam filed the first patent on the successful consolidation of powder using the SPS technology [20]. Steady progress was made in the mid-1980s into the 1990s. The SPS technology sinters in a conducting die with a simultaneously applied mechanical pressure and DC pulses which allows for simultaneous densification

Seebeck coefficients [16].

where *S*, *δ*, *κ*, *ρ* and *T* represent the Seebeck coefficient (*S*), electrical conductivity (δ), thermal conductivity, resistivity and absolute temperature, respectively.

Thermal conductivity of TE materials consist of two parts: lattice thermal conductivity (κ*l*) and electronic thermal conductivity (κ*e*). In principle, a high ZT is obtained by large values of both seeback coefficient and electrical conductivity, while thermal conductivity (κ) is minimised to maintain the temperature difference (*T*) producing the Seebeck coefficient [17, 18]. However, this requirement contradicts the Wiedemann-Franz law which requires the electronic part of thermal conductivity to be proportional to electrical conductivity, and the Pisarenko relation limits the simultaneous enlargement of α and δ [19]. This makes it difficult to enhance the ZT using the tuning of carrier concentration alone.

Over the years, a number of strategies have been adopted to enhance the power factor and reduce thermal conductivity of TEs. This has resulted in the development of three generations of TEs over the 200-year period since their discovery in 1821. The development history has been characterised by achieving high ZTs > 2.0 through new concepts and technologies. The first TE generation devices are characterised by ZT ~ 1.0 operating at power conversion efficiencies of 4–5% [17]. In the 1990s the introduction of nanostructures increased the ZT values by about 70% to ZT ~ 1.7, and the power conversion efficiencies can be expected to be 11–15%. In the third generation, some new concepts such as band structure engineering by doping, reduction in lattice thermal conductivity, nanostructuring and all-scale hierarchical architecturing and quantum confinement effects have been introduced to enhance Seebeck coefficients [16].
