**7. Conclusion**

*Design and Manufacturing*

lower temperatures [45].

produced using SPS method [52].

**6.3 SPS of nanostructured thermoelectric ceramics**

Several studies have been dedicated towards investigating the effect of grain size on

BaTiO3-based piezoceramics is one of the most studied using SPS. It has been demonstrated that SPS technology is effective in stabilising the metastable BaTiO3 cubic phase and reducing the intergranular effects on permittivity and DC resistance [45]. Moreover, SPS samples have shown higher permittivity values typically below the Curie temperature (Tc) [47]. It has been demonstrated that at finer grain sizes, the dielectric constant at the transition temperature decreases and Tc shifts to

Lead-based piezoceramics have dominated the market of piezoelectric ceramics for a long time. However, their continued use is now questionable owing to the associated health risk especially during processing. Another major concern in the sintering of PZT piezoceramics (Pb(Zr,Ti)O3) is the high sintering temperatures which promote the vitalization of lead [48, 49]. Moreover, a number of the proposed alternative piezoceramic materials also contain highly volatile elements such as in (Na,K)NbO3 which makes their sintering ability quite poor. The use of SPS has enabled suppression of lead loss through rapid heating rate, lower sintering temperature and shorter sintering times [50]. In one study, Han et al. demonstrated that the use of SPS can lower the sintering temperature of a Pb (Zr0.52Ti0.42Sn0. 02Nb0.04)O3 piezoceramic by a substantial 200–300°C while maintaining a high relative density (>99%) [51]. In a separate study, a (Na0.535K0.485)1−xLix(Nb0.8Ta0.2) O3 (x = 0.02–0.07) ceramic with improved mechanical and electrical properties was

There is an assumption that the nonlinear response of piezoceramics is grain size dependent; this is understood to be the variation of functional properties under an external stimulus. The two major contributors to nonlinear response of piezoceramics are the intrinsic (i.e. the contribution of composition, crystal structure, etc.) and extrinsic (i.e. grain size, domain wall dynamics, etc.) contributors [53, 54]. This implies that a significant decrease in grain size has the potential to produce a notable modification of nonlinear response in piezoceramics. It therefore means the stability of piezoelectric properties may be improved by controlling the grain size.

The wide application of TEs has not been realised mainly owing to low conversion efficiencies. For instance, commercially available TE materials possess a low ZT of 1 and average conversion efficiency of ~5% [55]. In order to promote the practical applications of TEs, it is critical to synthesise TE materials with ZT values >1; a TE device with ZT = 3 operating between room temperature and 773 K would yield ~ 50% of the Carnot efficiency [56]. It is evident from previous reviews that the key strategy in the improvement of ZT values for TEs has been the increase in the seeback coefficient and reduction in thermal conductivity. However, no significant improvement in ZT values has been reported through the tuning of these properties. Theoretical predictions have shown that nanostructuring can enhance the seeback coefficient through modification of density of states and can reduce the thermal conductivity by selective scattering of phonons, resulting in good ZT values. It should be noted here that the TE properties of nanostructured materials also depend on the size and morphology of microstructural features; thus, microstructural engineering is key in the development of TE materials. In 2005, Yu et al. observed that the seeback coefficient and thermal and electrical conductivities are all significantly

dependent on grain size; this was confirmed on CoSb3 TE materials [57].

It has been proven that the main design principle for the future TEs is the use of nanostructured architectures. A number of approaches have been utilised in

the piezoelectric properties of BaTiO3 ceramics down to nanometric scale.

**116**

There is clear evidence that SPS technology and TSS methodology have yielded quite some progressive results in the production of functional nanoceramic materials. Moreover, the use of modified TSS methodology in SPS equipment has shown great potential for yielding nanostructured materials with minimum risk of grain growth. However, what still remains controversial is the consistency of the functional properties and reproducibility of the methodologies used. Thus this area of study still remains highly energised for a broader enquiry. Furthermore, for most functional ceramic materials, nanostructuring has yielded enhanced material properties through various mechanisms. Although there is still room for improvement, it remains a challenge to material scientists and engineers alike to explore further and develop a deeper understanding of the mechanisms involved which may help achieve large increases in critical functional properties. Some of the highlighted problems which might have contributed to the inconsistences in functional properties include variations in the starting green densities and the likelihood of powder agglomeration at these finer sizes. This leads to inhomogeneous temperature distribution in samples and variations in sintered densities which has direct impact on material properties.

In conclusion, for practical purposes most of these materials have to satisfy certain conditions for this to become a reality: the synthesis route should be scalable, high quality and low cost, materials should have the ability to form dense compact nanostructured materials which are amenable to subsequent processing such as machining/device integration and lastly the nanostructured products should demonstrate enhanced functional properties over their micron-sized counterparts. This points to exciting scientific opportunities for continued research in order to gain more quantitative understanding to allow the design and optimisation of processes in the development of functional ceramic materials.

*Design and Manufacturing*
