## 2 Processing of Ceramics with Aurivillius Type Structure
### Texture
They proved that only with a 5% of template particles, high densities (97%) and Lotgering factors (\(\sim\)0.96) were obtained by sintering at temperatures higher than 1000 \({}^{\circ}\)C. The anisotropy of grain-boundary energies drives grain growth in BiT and the grain rotation gives the texture. With the templated grain growth from tape-casting, the temperature increases the achieved orientation [50]. The piezoelectric properties of the textured ceramics increase when they are measured in the ferroelectric a-b plane. Thus, [PERSON] et al. have reported [51] an increase from 20 to 30 pC/N in Nb-doped Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\) ceramics in random and textured ceramics with Lotgering factors \(>\)0.96, as shown in Figure 7. Similar results have been published by other authors in different compositions with this structure. Thus, [PERSON] et al. reported a d\({}_{33}\) increase from 15 to 30 pC/N in Na\({}_{0.5}\)Bi\({}_{4.5}\)Ti\({}_{4}\)O\({}_{12}\)[52]; [PERSON] et al. achieved an increase from 12.6 to 25 pC/N in Mn-doped CaBi\({}_{4}\)Ti\({}_{4}\)O\({}_{15}\) ceramics [53]. Figure 6: SEM images of polished and thermally etched surfaces for (**a**) perpendicular; (**b**) parallel to the SPS pressing direction; (**c**) conventionally Sintered CaBi\({}_{2}\)Nb\({}_{2}\)O\({}_{9}\) (taken from [44]). Besides tape-casting techniques, there are other methods for texturing Aurivillius-type ceramics. For instance, [PERSON] et al. used the Rolling extended method to prepare Ca\({}_{0.85}\)(LiCe)\({}_{0.075}\)Bi\({}_{4}\)Ti\({}_{4}\)O\({}_{15}\) (LCCBT) textured ceramics with a templated grain growth [54]. In this technique, a clay-like paste containing the template and the matrix particles is forced to pass through a pair of rollers that produce the alignment of the anisometric particles. Lotgering factors of 0.93 were achieved. The d\({}_{33}\) value measured in the a-b plane is 29.5 pC/N, while for random ceramics it is 18 pC/N. [PERSON] et al. used a method called Superplastic-Deformation-Induced directional dynamic ripening to have grain-oriented Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\) ceramics [55]. They take advantage of the superplasticity properties of nanostructured materials before the \(T_{g}\) temperature of grain growth. By a subsequent SPS process, the authors were able to orientate the grains and promote their growth at 1000 \({}^{\circ}\)C. Lotgering factors of 0.985 were accomplished, with d\({}_{33}\) values of 17.6 pC/N. It is clear that the texture increases the piezoelectric properties if they are measured in the appropriate direction. However, the texture obtained by any of these methods is detrimental for the simultaneous appearance of good mechanical and ferroelectric properties. The resulting microstructure with the grain alignment favors the crack propagation along grain boundaries and through the a-b plane. The mechanical properties were shown to worsen for several Aurivillius phases [56]. Besides, the direction with the largest piezoelectric activity is the same as the maximal conductivity [57], which makes the unavoidable poling process and the application of potential devices at high temperature more difficult.