## 2 Processing of Ceramics with Aurivillius Type Structure
### Texture  
Another alternative to have dense ceramics with Aurivillius structure is to synthesize the precursors by the classical ceramic or an alternative method and then prepare the ceramics by pressure-assisted sintering. There are three main routes for this: hot-forging, hot pressing and spark plasma sintering. A common characteristic for all these routes is that a relatively high pressure (typically in the order of MPa) is supplied to the green samples at high temperatures, in such way that the thermal and mechanical energy applied favors the closure of porosity. In the hot-forging technique, a uniaxial pressure is applied to a sample without lateral restrictions [39]. It was one of the first alternative techniques that were used to process highly dense ceramics with bismuth layered structure. Densities higher than 97% are achieved by this method. A difference between hot-forging and hot pressing is that in this late method, the lateral restriction limits the expansion of the ceramics during sintering [40]. Highly dense ceramics (>99%) at low temperatures can be achieved [41; 42]. Spark Plasma Sintering also provides a combination of thermal and mechanical energy for sintering. The main difference with hot-pressing and hot-forging is that heating is produced by pulsed electrical currents that heat the material (or alternatively the graphite die used) due to Joule effect [43]. As examples, densities higher than 95% have been obtained for CaBi\({}_{2}\)Nb\({}_{2}\)O\({}_{9}\) and Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\) compositions by SPS, with grain sizes depending on the initial precursors and SPS conditions in terms of temperature (1000-1150 \({}^{\circ}\)C) and time [44; 45] (Figure 6). A common characteristic to these methods is that the pressure aligns the platelets grains with the largest faces in perpendicular to the applied pressure (meaning c-axis orientation in parallel to the applied pressure). This gives ceramics with a high level of (00l) texture and thus with anisotropic properties. As the ferroelectric direction lies in the a-b plane in these materials, the samples measured in perpendicular to the pressure will have an enhancement of the ferro-piezoelectric properties with respect to the other direction and to the traditionally isotropic sintered ceramics. As examples, the remnant polarization values are 37, 9.3 and 3.3 \(\upmu\)C/cm\({}^{2}\) for the samples in the perpendicular, parallel direction and for ordinary sintering without pressure, respectively, in hot forged Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\)[39]. Bi\({}_{3}\)TiNbO\({}_{9}\) ceramics prepared by hot-pressing have d\({}_{33}\) coefficients: 14-15, 3-4 and 7 pC/N for perpendicular, parallel and ordinary fired samples [46]. In CaBi\({}_{2}\)Nb\({}_{2}\)O\({}_{9}\) ceramics prepared by SPS (Figure 6), these values are 19.5, 0.2 and 7.5 pC/N [44]. Figure 5: Optical micrographs of polished and thermally etched surfaces of a ceramic are hot-pressed at 900 \({}^{\circ}\)C–1 h and correspondingly recrystallized at 1100 \({}^{\circ}\)C–1 h (taken from [28]). The first experiments leading to the Aurivillus textured ceramic were carried out at the end of the 1970s and the 1980s (with methods still in use today) based on the simultaneous application of high temperature and pressures and were improved by other techniques. Very good results are given by the (reactive) template grain growth after alignment of the anisometric particles by different methods, mainly tape casting or extrusion based molding [47; 48]. In the process, large platelet-like particles are aligned in a matrix of powder with the target composition. During the sintering, densification and grain growth with a (00l) preferred orientation occurs. The Lotgering factor is often used to characterize the level of orientation achieved. A value of 0 denotes a random material, while a value of 1 denotes a perfect (00l) orientation. One of the first works that applied this technique for Aurivillius materials was reported by [PERSON] et al. [49].