Solvothermal and Hydrothermal Techniques

A solvothermal process is one in which a material is either recrystallized or chemically synthesized from solution in a sealed container above ambient temperature and pressure. The recrystallization process was discussed in Section 1.5.1. In the present chapter we consider synthesis. The first solvothermal syntheses were carried out by Robert Wilhelm Bunsen (1811-1899) in 1839 at the University of Marburg. Bunsen grew barium carbonate and strontium carbonate at temperatures above 200°C and pressures above 100 bar (Laudise, 1987). In 1845, C. E. Shafhautl observed tiny quartz crystals upon transformation of freshly precipitated silicic acid in a Papin s digester or pressure cooker (Rabenau, 1985). Often, the name solvothermal is replaced with a term to more closely refer to the solvent used. For example, solvothermal becomes hydrothermal if an aqueous solution is used as the solvent, or ammothermal if ammonia is used. In extreme cases, solvothermal synthesis takes place at or over the supercritical point of the solvent. But in most cases, the pressures and temperatures are in the subcritical realm, where the physical properties of the solvent (e.g., density, viscosity, dielectric constant) can be controlled as a function of temperature and pressure. By far, most syntheses have taken place in the subcritical realm of water. Therefore, we focus our discussion of the materials synthesis on the hydrothermal process. 

The physical properties of water at or near the critical point are dramatically different from those of the water typically used in aqueous reactions. At the critical point, water becomes far less dense (0.3 g/cm3), as the hydrogen bonding is 

Hydrothermal synthesis is often applied to the preparation of oxides. The synthesis of metal oxides in hydrothermal conditions is believed to occur in a two-step process. In the first step, there is a fast hydrolysis of a metal salt solution to give the metal hydroxides. During the second step, the hydroxide is dehydrated, yielding the metal oxide desired. The overall rate is a function of the temperature, the ion product of water, and the dielectric constant of the solvent. The two steps are in balance during the reaction. The hydroxide of the metal salt is favored by a high dielectric constant, while the dehydration of the metal hydroxide is favored by a low dielectric constant. Since the fast reaction is the first step, it is expected that as one approaches supercritical conditions, the rate of reaction increases. 

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