## 2 Processing of Ceramics with Aurivillius Type Structure
Thus, Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\) has been obtained after 15 h of milling [22], CaBi\({}_{4}\)Ti\({}_{4}\)O\({}_{15}\) after 30 h [23], or SrBi\({}_{4}\)Ti\({}_{4}\)O\({}_{15}\) after 20 h [24]. In other cases, the synthesis temperature is decreased by obtaining an amorphous powder after milling [25; 26; 27]. The high reactivity of the precursors allows sintering of ceramics with low porosity at relatively low temperatures. A further increase in density can be achieved by hot pressing or by processes of recrystallization after hot pressing [28] that allows the control of the microstructure and texture (Figure 5). 5. _Molten salt synthesis_. It is based on the use of a molten salt mixed to the starting oxide/carbonates precursors to favor the dissolution-precipitation processes assisted by liquid phases. Final products are obtained at lower temperatures than by the classical ceramic route. The most commonly used salts are NaCl, KCl, Na\({}_{2}\)SO\({}_{4}\), K\({}_{2}\)SO\({}_{4}\) and a mixture of them at eutectic points that reduce the melting temperature. Depending on the used flux and the precursors, the synthesis conditions can vary. Thus, mixing salts with precursors obtained by hydrolysis or coprecipitation, instead of oxides and carbonates, Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\) and Bi\({}_{3}\)TiNbO\({}_{9}\) can be obtained at 600 and 700 \({}^{\circ}\)C, respectively [29; 30]. When oxides and carbonates are used, higher temperatures (800-1100 \({}^{\circ}\)C) are usually needed to synthesize Aurivillius-type powders with different compositions [31; 32; 33]. These high temperatures favor the growth of large plate-like particles that facilitates the processing of textured ceramics as will be explained later. The use of eutectic KCl/NaCl mixtures allows the reduction of synthesis temperatures (800 \({}^{\circ}\)C-2 h) [34] leading to small particles as the ones observed in Figure 4 that improve the Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\) photocatalytic activities. 6. _Microwave assisted synthesis_. With the application of microwaves at high frequency, the associated electromagnetic fields will heat materials with high dielectric losses. Alternatively, the heating of susceptors with high microwave absorption can be achieved. The main characteristic for this process is the high heating rate and short times that inhibit grain growth. The synthesis of BiGdWO\({}_{6}\), BiYWO\({}_{6}\) or Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\) in only 10 min has been reported [35; 36]. Temperatures of 900-1100 \({}^{\circ}\)C have been measured in those articles. This technique is also used in combination with other routes previously described, as molten salts or hydrothermal [37; 38] to further improve the synthesis of Aurivillius precursors. Figure 4: SEM micrographs of Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\) powder prepared by the molten salt synthesis method at 800 \({}^{\circ}\)C (taken from [34]). Figure 3: SEM image of nanometric particles of Bi\({}_{2}\)Sr\({}_{2}\)TiNb\({}_{2}\)O\({}_{12}\) prepared by sol-gel (taken from [14]). Figure 2: SEM photograph of Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\) pellets sintered at 900 \({}^{\circ}\)C from hydrothermal synthesized precursor. (Taken from [10]).