**3. Processing of functional ceramic materials**

*Design and Manufacturing*

atmosphere (low *PO*<sup>2</sup>

performance [13].

**2.4 Thermoelectric ceramics**

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

Ceramic capacitors are widely produced as sintered thin plates in a reducing

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

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

*<sup>κ</sup>* <sup>=</sup> *<sup>S</sup>*<sup>2</sup> \_\_\_\_\_\_\_ *<sup>T</sup>*

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

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

*(κ<sup>e</sup> <sup>+</sup> <sup>κ</sup>l)<sup>ρ</sup>* (2)

). In previous studies, the particle size effects of BaTiO3 on

include BaTiO3-based dielectric and lead-based dielectrics.

based on a dimensionless figure of merit (ZT) as follows:

enhance the ZT using the tuning of carrier concentration alone.

*ZT* <sup>=</sup> *<sup>S</sup>*<sup>2</sup> \_\_\_\_*<sup>T</sup>*

**108**

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 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 the final products.
