**6. The use of SPS technology in the synthesis of functional ceramic materials**

SPS technology provides an alternative and more effective route for grain refinement of ceramic materials. As discussed earlier, conventional sintering is ineffective in refinement of ceramic materials owing to the excessive grain growth of fine powders at elevated temperatures [20–22]. The TSS methodology was developed to mitigate this problem, and it provides a low-cost and effective route for grain refinement. However, this method requires very long isothermal holding times to effect sintering without grain growth. Therefore, it might not be suitable for high production rates (commercial purposes); in addition the prolonged holding times are likely to increase the energy costs. Moreover, there are ceramics that require pressure-assisted sintering to impart the required strength to the component during sintering. The shortcomings above can be minimised with the use of SPS technology which has been briefly discussed in the previous section.

**115**

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

The SPS technology has been successfully used in the production of nanometric functional ceramic materials. The discussion below presents some examples where SPS has been utilised to obtain bulk ceramic nanomaterials. The discussion is not meant to be exhaustive but to impart some critical in-depth knowledge on the

The magnetic properties of ferrites (magnetic ceramics) are structure sensitive and are affected by a number of factors such as phase composition, crystallite size and shape and the quantity of heterogeneities [39]. It is therefore critical to use a synthesis method capable of producing superior magnetic properties through control of structural homogeneities. The use of SPS technology is thought to promote ordering of spatial positions of magnetic moments of metal ions in the composite crystal lattice through pressure-assisted sintering [40]. The result is the formation of new magnetic phases through a pressure-induced transformation of the nanocrystalline phases [39]. It must be noted however that the mechanism of this transformation in SPS is not well developed for most of the ceramic ferrites. Papynov and co-workers studied the magnetic properties of nanostructured ferrites using SPS technology (α-Fe2O3 and α-Fe2O3-Fe3O4 composite) [39]. The authors established that the value of magnetisation increases significantly with increasing sintering temperature and reached a value of 10.2 emu/g at 1100°C (equivalent to a tenfold increase). This was attributed to changes in the crystalline phase and to a

lesser extent growth of ferrite grains which may affect magnetisation.

In their work Gaudisson et al. [41] consolidated a nanosized magnetic powder into a high-density solid at 750°C for 15 min to a final grain size range of 150 nm. In a separate study, a nanostructured Co-ferrite was shown to be sensitive to heating rate under the same sintering temperatures and times in SPS. A higher heating rate (80°C/min) maintained a finer grain size of 70 nm than a lower heating rate (15°C/min) which produced a grain size of 290 nm for two powders which were processed at the same sintering temperatures and times (2 min at 600°C followed by 5 min at 500°C) [41]. Ultrafine, highly dense yttrium iron garnet (YIG) was produced by SPS treatment at 750°C for 15 min at 100 MPa in wide contrast to the typical parameters used in conventional sintering which requires higher sintering

A SrFe12O19 hexaferrite with a grain size of 400 nm was obtained using SPS at

with conventional sintering at 1240°C for 2 h which produced a density of 4.83 g/cm3 and a grain size double that of the SPS-produced material [43]. Harder magnetic properties were obtained from the SPS-produced ferrite. In another study, harder magnetic properties were obtained for an SPS-sintered Ba-hexaferrite owing to limited grain growth; grains of 100–150 nm were obtained in comparison to

The grain size effect on the macroscopic functional properties of piezoceramics has been widely researched. There are however very few studies that correlate grain size and property stability. It has been shown that improved performance, high permittivity miniaturised devices can be obtained by microstructural control such as grain size and homogeneity [45]. Arlt and co-workers have shown the strong dependency of BaTiO3 functional properties on the microstructure and grain size [46]. Moreover, large grain sizes are detrimental to the mechanical strength of ceramic-based devices.

. There was a big contrast

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

synthesis procedures of a few selected case studies.

**6.1 SPS of nanostructured magnetic ceramics**

times (typically 1350°C for a few hours) [42].

**6.2 SPS of nanostructured piezoceramics**

1100°C for 5 min with a maximum density of 5.15 g/cm3

conventional sintering which produced a grain size of 1.5–8 μm [44].

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

The SPS technology has been successfully used in the production of nanometric functional ceramic materials. The discussion below presents some examples where SPS has been utilised to obtain bulk ceramic nanomaterials. The discussion is not meant to be exhaustive but to impart some critical in-depth knowledge on the synthesis procedures of a few selected case studies.

#### **6.1 SPS of nanostructured magnetic ceramics**

*Design and Manufacturing*

comparison to their micrometric counterparts.

methodology as a grain refinement process.

T2 range of 1150–850°C for BaTiO3 and 750–870°C for the ferrite with an isothermal holding time of 20 h. For the BaTiO3, the best result achieved showed a grain size of 35 nm and was sintered at 950°C in T1 and at 900°C in T2 for 2 h. On the other hand, a grain size of 200 nm was achieved for the ferrite sintered at 850°C in T1 and 800°C for 6 h in T2. In comparison to the result of Kim and Han discussed earlier, the use of a lower sintering temperature enabled the formation of BaTiO3 with a finer grain size. To elucidate this point, Karaki et al. [33] obtained a mean grain size of 1.6 μm with a relative density of 98.3% starting with a BaTiO3 of average particle size of 100 nm, cold compacted at 200 MPa. A TSS profile almost similar to that of Kim and Han et al. consists of a heating rate of 10°C/min to T1 (1230–1340°C) held for 1 min and a cooling rate of 30°C/min to T2 (1150–1200°C) held for 2, 4, 5 and 20 h. The best result was obtained with a sintering profile consisting of T1 (1320°C) and T2 (1150°C) with a 15 h holding time. There is a clear indication from the above results that BaTiO3 ceramics with high sintered density are obtainable at lower sintering temperature (≤1000°C) for both T1 and T2. Some of the authors observed that the piezoelectric coefficient is strongly influenced by the grain size [11]. Nanometric BaTiO3 ceramics possess superior piezoelectric coefficients in

As mentioned earlier, the current work is not meant to be exhaustive but to give a qualitative insight on the research covered so far. **Table 1** gives a summary of some of the successful work carried out and highlights some critical aspects in the TSS

Specific particularities were observed in each of the sintering cycles above. The majority of the authors compared their results with those obtained by conventional sintering and the TSS methodology resulted in superior grain refinement. The initial powder features such as particle size, microstructural homogeneity and green density are quite critical in the success of the TSS process. In the majority of cases, the difference between the T1 and T2 temperatures is <1500°C, and a larger holding time in T2 allowed smaller grain sizes to be obtained. The TSS method has also been

shown to improve the material properties (both mechanical and physical). In some cases the use of dopants was effective in inhibiting grain growth. Although the TSS approach has shown great success in effecting grain refinement, the very long isothermal holding times at T2 might not be suitable for commercial purposes. The SPS technology offers an alternative route for grain refinement, and results in **Table 1** show that it is more effective for materials such as YSZ ceramics. The SPS process generally can achieve grain refinement over shorter time periods.

**6. The use of SPS technology in the synthesis of functional ceramic** 

which has been briefly discussed in the previous section.

SPS technology provides an alternative and more effective route for grain refinement of ceramic materials. As discussed earlier, conventional sintering is ineffective in refinement of ceramic materials owing to the excessive grain growth of fine powders at elevated temperatures [20–22]. The TSS methodology was developed to mitigate this problem, and it provides a low-cost and effective route for grain refinement. However, this method requires very long isothermal holding times to effect sintering without grain growth. Therefore, it might not be suitable for high production rates (commercial purposes); in addition the prolonged holding times are likely to increase the energy costs. Moreover, there are ceramics that require pressure-assisted sintering to impart the required strength to the component during sintering. The shortcomings above can be minimised with the use of SPS technology

**114**

**materials**

The magnetic properties of ferrites (magnetic ceramics) are structure sensitive and are affected by a number of factors such as phase composition, crystallite size and shape and the quantity of heterogeneities [39]. It is therefore critical to use a synthesis method capable of producing superior magnetic properties through control of structural homogeneities. The use of SPS technology is thought to promote ordering of spatial positions of magnetic moments of metal ions in the composite crystal lattice through pressure-assisted sintering [40]. The result is the formation of new magnetic phases through a pressure-induced transformation of the nanocrystalline phases [39]. It must be noted however that the mechanism of this transformation in SPS is not well developed for most of the ceramic ferrites.

Papynov and co-workers studied the magnetic properties of nanostructured ferrites using SPS technology (α-Fe2O3 and α-Fe2O3-Fe3O4 composite) [39]. The authors established that the value of magnetisation increases significantly with increasing sintering temperature and reached a value of 10.2 emu/g at 1100°C (equivalent to a tenfold increase). This was attributed to changes in the crystalline phase and to a lesser extent growth of ferrite grains which may affect magnetisation.

In their work Gaudisson et al. [41] consolidated a nanosized magnetic powder into a high-density solid at 750°C for 15 min to a final grain size range of 150 nm. In a separate study, a nanostructured Co-ferrite was shown to be sensitive to heating rate under the same sintering temperatures and times in SPS. A higher heating rate (80°C/min) maintained a finer grain size of 70 nm than a lower heating rate (15°C/min) which produced a grain size of 290 nm for two powders which were processed at the same sintering temperatures and times (2 min at 600°C followed by 5 min at 500°C) [41]. Ultrafine, highly dense yttrium iron garnet (YIG) was produced by SPS treatment at 750°C for 15 min at 100 MPa in wide contrast to the typical parameters used in conventional sintering which requires higher sintering times (typically 1350°C for a few hours) [42].

A SrFe12O19 hexaferrite with a grain size of 400 nm was obtained using SPS at 1100°C for 5 min with a maximum density of 5.15 g/cm3 . There was a big contrast with conventional sintering at 1240°C for 2 h which produced a density of 4.83 g/cm3 and a grain size double that of the SPS-produced material [43]. Harder magnetic properties were obtained from the SPS-produced ferrite. In another study, harder magnetic properties were obtained for an SPS-sintered Ba-hexaferrite owing to limited grain growth; grains of 100–150 nm were obtained in comparison to conventional sintering which produced a grain size of 1.5–8 μm [44].

#### **6.2 SPS of nanostructured piezoceramics**

The grain size effect on the macroscopic functional properties of piezoceramics has been widely researched. There are however very few studies that correlate grain size and property stability. It has been shown that improved performance, high permittivity miniaturised devices can be obtained by microstructural control such as grain size and homogeneity [45]. Arlt and co-workers have shown the strong dependency of BaTiO3 functional properties on the microstructure and grain size [46]. Moreover, large grain sizes are detrimental to the mechanical strength of ceramic-based devices.

Several studies have been dedicated towards investigating the effect of grain size on the piezoelectric properties of BaTiO3 ceramics down to nanometric scale.

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 lower temperatures [45].

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 produced using SPS method [52].

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.

#### **6.3 SPS of nanostructured thermoelectric ceramics**

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

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developing nano-inclusions which are effective in reducing the lattice thermal conductivities [16]. Such methods include in situ dispersion of partially oxidised nanoparticles in matrix [58], endotaxial nanoprecipitates [59, 60] and embedded nano-inclusions [61, 62]. The SPS technology has been utilised in fabricating highly dense and fine-grained TEs [63]. Nanocomposite grains are believed to be effective in scattering phonons with a broad wavelength which enhances the functional properties of TEs [55]. By nanostructuring a wide variation in ZT values ranging from 0.4 to 1.7 has been obtained for nanocomposites with similar composition. A ZT of about 1.5 at 390 K was achieved for a (Bi,Sb)2Te3 nanocomposite produced by a combination of melt spinning of single elements followed by SPS sintering. Another Bi0.52Sb1.48Te3 nanocomposite material had a ZT ~ 1.56 [64, 65]. It has also been proven that by combining mechanical alloying and SPS sintering, one can

achieve high ZT of 1.5 at 700 K in AgPbmSbTem+2 nanocomposite [66].

Most of the bulk TE materials with highest ZT values are fabricated through the SPS process. Bi2Te3 compounds have been produced with ZT values ranging from 0.7 to 1.8 in the SPS [67]. The reason for a wide range of ZT values has been attributed to varying initial green densities which is key in determining the inner temperature of the sample. Moreover, powder aggregates can lead to inhomogeneous

There is clear evidence that SPS technology and TSS methodology have yielded

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

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

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

distribution of temperature.

which has direct impact on material properties.

in the development of functional ceramic materials.

**7. Conclusion**

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

developing nano-inclusions which are effective in reducing the lattice thermal conductivities [16]. Such methods include in situ dispersion of partially oxidised nanoparticles in matrix [58], endotaxial nanoprecipitates [59, 60] and embedded nano-inclusions [61, 62]. The SPS technology has been utilised in fabricating highly dense and fine-grained TEs [63]. Nanocomposite grains are believed to be effective in scattering phonons with a broad wavelength which enhances the functional properties of TEs [55]. By nanostructuring a wide variation in ZT values ranging from 0.4 to 1.7 has been obtained for nanocomposites with similar composition. A ZT of about 1.5 at 390 K was achieved for a (Bi,Sb)2Te3 nanocomposite produced by a combination of melt spinning of single elements followed by SPS sintering. Another Bi0.52Sb1.48Te3 nanocomposite material had a ZT ~ 1.56 [64, 65]. It has also been proven that by combining mechanical alloying and SPS sintering, one can achieve high ZT of 1.5 at 700 K in AgPbmSbTem+2 nanocomposite [66].

Most of the bulk TE materials with highest ZT values are fabricated through the SPS process. Bi2Te3 compounds have been produced with ZT values ranging from 0.7 to 1.8 in the SPS [67]. The reason for a wide range of ZT values has been attributed to varying initial green densities which is key in determining the inner temperature of the sample. Moreover, powder aggregates can lead to inhomogeneous distribution of temperature.
