## 2 Processing of Ceramics with Aurivillius Type Structure  
Ceramic method is the most used route for the processing of the ceramics with Aurivillius type structure. It is ideal because of its low cost and the possibility of obtaining high amounts of material. The synthesis is carried out by calcination at relatively high temperatures of precursors containing the structure components, mainly oxides and carbonates. The ceramics are usually obtained by sintering in air of pressed green pellets. A major drawback for this method is to have dense ceramics with adequate grain size. Its lamellar crystal growth makes the compaction of the precursor ceramic powder more difficult and the reduced mass transport during sintering leads to porous ceramics (higher than 10% of porosity) [6], which makes difficult their uses in piezoelectric applications. An approach to solve this problem is to use alternative synthesis methods to obtain precursors with a homogenous particle distribution characterized by small sizes and isotropic shapes. It will provide a better packing of the green pellet and a higher reactivity that allows the reduction of the processing temperature. Some of these alternative methods are detailed in the following sections. 1. _Hydrothermal synthesis._ In this technique, the combination of temperature and high pressure produced by water solutions in an autoclave allows the synthesis of materials with special characteristics in terms of small grain sizes (even in the nanometer scale) and particular and controlled shapes. The application to Aurivillius structure materials synthesis has been deeply explored in several works. Depending on variables such as time or temperature (<300 \({}^{\circ}\)C) [7], use of different mineralizers and concentrations [8] or starting materials [9], single phase materials with different sizes and shapes from the nanometric to micrometric scale can be synthesized. Figure 1: Prototype of the Aurivillius structure as a function of the number of pseudo-perovskite layers (taken from [2]). They give place to ceramics with relatively high densities, as shown in Figure 2[10] with good ferro/piezoelectric properties [7; 11]. 2. _Sol-gel_. In this process, the solution evolves gradually through hydrolysis and polycondensation reactions towards the formation of a gel-like network to form a colloid. The mixture at practically atomic level favors the reaction to synthesize precursors at low temperatures, allowing the control of particle size and shape. Thus, different compositions with Aurivillius-type structure were synthesized, at temperatures that can range from 525 to 700 \({}^{\circ}\)C [12; 13; 14; 15; 16] to have particles in the nanometric scale, as shown in Figure 3. Dense ceramics with high remnant polarization (24.15 \(\mu\)C/cm\({}^{2}\)) values can be achieved by this route in Bi\({}_{4}\)Ti\({}_{3}\)O\({}_{12}\) based ceramics [17]. 3. _Coprecipitation_. This is a synthesis route from a solution, in which the addition of a precipitant agent or the solvent evaporation reduces the solubility of the precursors, causing their precipitation. It allows a mixture of the components at low scale, favoring their reactivity in the calcination steps. Single phase materials are obtained at temperatures ranging from 700-800 \({}^{\circ}\)C, to have particles with sizes at the nanometric scale [18; 19; 20; 21]. Ceramics with high densities (>97%) can be accomplished at low temperature (875 \({}^{\circ}\)C) by sintering ceramic precursors obtained by this route [18]. 4. _Mechanochemical activation/mechanosynthesis_. High energy milling produces synthesis reactions (mechanosynthesis) or changes in the size and reactivity of the precursors (mechanochemical activation) that allow the reduction of the processing temperatures. It is accomplished by the high number of defects produced by the milling. Some Aurivillius compositions have been mechanosynthesized, mainly in planetary mills.