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SYNTHESIS AND PROCESSING: GENERAL METHODS 28 sizes to less reactive micrometer sizes. The freeze-drying process for producing metal or ceramic powders is very simple in concept. It involves the dissolution of salts containing cations of interest, flash freezing of the resulting solution, and sublimation of the solvent, followed by conversion to the desired product. The conversion can be effected by calcining to the appropriate oxide or metal form. Alternatively, conversion may be to a compound--for example, by reacting with a carburizing gas to form a refractory carbide. The initial step of the process (that is, the dissolution of the salt or salts) produces a homogeneous solution that is retained during the low-temperature quenching process. This essentially locks in the cation distribution that had been present in the liquid solution. The homogeneity of the product is retained during the freeze-drying process, since little atomic movement occurs during the low-temperature sublimation process. Because most of the frozen solvent molecules are removed by sublimation, the resulting structure is a very open one. The resulting increased surface-to-volume ratio makes feasible lower calcining temperatures, which in turn leads to a finer particle size after the calcining operation. CHEMICAL VAPOR SYNTHESIS Submicron-sized powders have been produced by chemical vapor synthesis for many years, using flame or plasma torch reactors. Primary particles produced can range in size from as small as 6 nm to as large as 600 nm, with size controlled primarily by the flame or plasma temperature and rate of cooldown. Often these primary particles are cemented together to form aggregates, which can contain from 10 to as many as 1000 primary particles, depending on the substance and manufacturing conditions. Normally, conditions that produce small primary particles yield aggregates with many particles so that complete control of product morphology is not possible. With the exception of carbon black, flame reactors are normally employed to produce oxides--the largest volume commercial products being fumed silica and titanium dioxide. Fumed alumina, zirconium dioxide, and mixed oxides have been produced on a developmental basis. Plasmas permit temperatures well above those attainable with combustion or chemical flames and also allow independent control of the operating atmosphere. Diverse ceramic raw materials such as titanium diboride, carbides, and nitrides have been produced in these systems, and some of these operations are at the commercial prototype scale. Generally, chemical vapor synthesis requires moderately expensive volatile metal chlorides or fluorides as the raw material, but the simplicity of the process (e.g., product isolation) often compensates for this cost.
