**2. General processing methods**

Different types of colloids can be used to produce polymers or particles from which we can obtain a ceramic material: for example, sols (suspensions of solid particles in a liquid), aerosols (suspensions of particles in a gas) or emulsions (suspensions of liquid droplets in another liquid). The sol-gel chemistry is based on the hydrolysis and polycondensation of molecular precursors such as metal alkoxides M(OR)x, where M = Si, Sn, Ti, Zr, Al, Mo, V, W, Ce and so forth. The following sequence of reactivity is usually found as Si(OR)4 << Sn(OR)4 = Ti(OR)4 < Zr(OR)4 = Ce(OR)4 (Novak, B.M.,1993 ). Fig. 1 presents a schema of the procedures which one could follow within the scope of sol-gel processing. In the sol-gel process, the precursors for the preparation of a colloid consist of a metal or metalloid element surrounded by various ligands. A list of the most commonly used alkoxy ligands is presented in Tab. 1.


Table 1. Commonly used ligands in sol-gel process.

Fig. 1. Schematic of sol-gel processing.
