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The sol-gel process can be considered as a method for the preparation of non-metallic inorganic materials by a chemical route. This general definition, however, does not match completely the specifics of this process since the sol phase is considered to be indispensable. The preparation of precursors for inorganic materials, for example, ceramics by precipitation processes or chemical vapor reaction to synthesize powders has been known for long times and industrially used widely as, for example, the Bayer process for the production of alumina powders. In order to make this type of processes efficient, the precipitation takes place under thermodynamic conditions, under which no disturbance of nucleation and growth takes place in order to obtain well-defined precipitates easy to be filtered or processed. Another example is the hydrothermal process, for example, developed by Degussa, or the fabrication of zirconia, where the transport mechanisms for crystal growth under elevated pressure and temperature conditions and the formation of thermodynamically stable and well crystallized phases is easier than under low-pressure, low-temperature conditions. If nucleation and growth take place under conditions where the nuclei absorb surface charges, each nucleus grows independently and no aggregation takes place if the surface charges (related to the so-called Zeta-potential) are in the range where the resulting repulsing forces are strong enough to avoid agglomeration. Moreover, if the concentration is off the feet and the resulting concentration of particles in the liquid system is low enough to keep the distances in a range where the Stern’s potential2 does not switch into attraction, colloidal solution can be kept stable. |
