**2.2 Sol-gel method**

108 Polycrystalline Materials – Theoretical and Practical Aspects

There are different methods of synthesis used in the field of Solid State Chemistry. Among them, one of the most used is that known as ceramic method. The ceramic method

of the grinding of the stoichiometric quantities of the corresponding starting compounds and subsequent heating by using furnaces at relatively high temperatures. However, it is not

The sol-gel method and, used more and more every day, the freeze-drying method, seem to offer purer materials by reducing the heating time and working at not so high temperatures.

The well known as ceramic method consists of the mixing and grinding of the starting compounds in stoichiometric quantities, being afterwards heated in furnaces at relatively high temperatures for relatively long dwelling times. Eventually, the desired material will

The main disadvantage of this method is the low homogeneity of the obtained product, which demands repeated procedures of intermediate homogenization and thermal treatment at high temperature. Thus, the method requires the grinding (intermittently) of the material during the heating treatment in order to minimize the drawback created by the interface (Figure 2) resulting of the reaction between the starting reagents. Figure 2 shows how that interface would be formed by A and B chemically combined. That interface increases its size as a function of the time of reaction, giving rise to a slower and less efficient transfer of A and B in opposite direction to meet and react. Thus, by grinding the grain formed by A, B, and AB, one will be able to increase the contact surface between A and B and therefore to make more efficient the reaction. We should just take into account that the solid state reactions are extremely slow, and that is why the high temperature and high dwelling times are required, but besides, a high contact surface between the reagents is desired to increase the velocity of the reaction. As a result, high crystalline materials are

Besides, other methods complete the list of synthesis processes of solid materials.

**2. Methods of synthesis** 

**2.1 Ceramic method** 

the most efficient method many times.

be obtained, with a smaller or higher grade of impurities.

obtained, which are not acceptable for catalytic applications.

 **An+**

corresponds to the interface created between them.

Fig. 2. Reaction between grains corresponding to An+ and Bm+. The cross-lined area

 **Bm+**

consists

The sol-gel method is a wet-chemical process widely used in the field of materials science. The starting point is the colloidal solution (*sol*) that acts as the precursor for an integrated network (*gel*) of either discrete particles or network polymers.

Perovskites can be synthesized starting from the oxides or salts of the different metals wanted to be part of the final structure by dissolving them in a solution containing a complexing agent (citric acid, e.g.) and heating it up to form gel. Immediately after, a drastic heating leads to the decomposition of the organic part and therefore to the formation of a fine and intimate distribution of the different metals in the resulting precursor, meaning this a high contact surface between them that allows a more efficient reaction at a relatively high temperature and a prudent dwelling time.
