Estudiante Bachiller

Preparative Methods
3.1 INTRODUCTION
The interest in the properties of solids and the development of new materials has given rise to the development of a huge variety of methods for preparing them. The method chosen for any solid will depend not only on the composition of the solid but also on the form it is required in for its proposed use. For example, silica glass for fibre optics needs tobe much freer of impurities than silica glass used to make laboratory equipment. Some methods may be particularly useful for producing solids in forms that are not the stable form under normal conditions; for example, the synthesis of diamond employs high pressures. Other methods may be chosen because they favour the formation of unusual oxidation states, for example, the preparation of chromiumdioxide by the hydrothermal method, or because they promote the production of fine powders or, by contrast, large single crystals. For industrial use, a method that does not employ high temperatures could be favoured because of the ensuing energy savings. In the preparation of solids, care usually has to be taken to use stoichiometric quantities, pure starting materials, and to ensure that thereaction has gone to completion because it is usually not possible to purify a solid once it has formed. We do not have space here to discuss all the ingenious syntheses that have been employed over the past few years, so we shall concentrate on those that are commonly used with a few examples of techniques used for solids with particularly interesting properties. The preparation of organic solid statecompounds and polymers is not covered because, generally, it involves organic synthesis techniques which is a whole field in itself, and is covered in many organic textbooks. It is difficult to impose a logical order on such a diverse subject. The chapter begins by considering the most basic, and most commonly used, method of preparing solids, the ceramic method: this grand title disguises thefact that it simply means grinding up the reactant solids and heating them hard until they react! We then go on to look at refinements of this method, and ways of improving the uniformity of the reaction and reducing the reaction temperature. The following sections on microwave heating and combustion synthesis discuss alternative methods of inducing solid state reactions. Later sections concentrateon less well-known methods of preparing inorganic solids, such as using high pressures and gas-phase reactions. We also consider some methods used to produce particularly pure solids which are important in the semiconductor industry and the preparation of single crystals.

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3.2 HIGH TEMPERATURE CERAMIC METHODS 3.2.1 DIRECT HEATING OF SOLIDS
The simplest andmost common way of preparing solids is the ceramic method, which consists of heating together two non-volatile solids which react to form the required product. This method is used widely both industrially and in the laboratory, and can be used to synthesize a whole range of materials such as mixed metal oxides, sulfides, nitrides, aluminosilicates and many others—the first high-temperaturesuperconductors were made by a ceramic method. To take a simple example we can consider the formation of zircon, ZrSiO4, which is used in the ceramics industry as the basis of high temperature pigments to colour the glazes on bathroom china. It is made by the direct reaction of zirconia, ZrO2, and silica, SiO2 at 1300°C: ZrO2(s)+SiO2(s)=ZrSiO4(s) The procedure is to take stoichiometric amounts of thebinary oxides, grind them in a pestle and mortar to give a uniform small particle size, and then heat in a furnace for several hours in an alumina crucible (Figure 3.1). Despite its widespread use, the simple ceramic method has several disadvantages. High temperatures are generally required, typically between 500 and 2000°C, and this requires a large input of energy. This is because the coordination...