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SYNTHESIS AND PROCESSING: GENERAL METHODS 34 The nanophase compacts are then formed in situ by collecting the powder scraped from the cold finger into a compaction device. The scraping and compaction process is again done under UHV conditions, after removal of the gas atmosphere, to guarantee the cleanliness of the particle surfaces and subsequent nanophase interfaces and also to minimize the amount of any trapped gases. Although the evaporation itself is done in an inert-gas atmosphere, UHV conditions for the production system are favorable, since gas leakage during the deposition should be reduced as much as possible because of the high reactivity of the small particles. However, if a reaction of the particles with a gas is desired, such as the oxidation of a metal to form an oxide ceramic, the gas can be easily added to the inert-gas atmosphere during or after the evaporation. Nanophase ceramics (TiO2, rutile), with greatly increased sinterability over normal coarser-grained ceramics and with no need for compaction or sintering additives, have recently been synthesized using this method (Siegel et al., 1988). This enhanced sinterability appears to result from a combination of the small and relatively uniform particle size available in the gas- condensation method and the clean particle surfaces, and resulting grain boundaries, maintained in the in situ consolidation process. Particle agglomeration has not been a problem in the synthesis of nanophase TiO2 (Siegel et al., 1988) and thus is not expected to be a significant problem in the processing of nanophase ceramics in general. A hint as to why this might be the case can be found in the particle morphology generated by the gas-condensation method. The fractal-like structure of the as-collected TiO2 powders indicates that fewer boundaries are formed between individual particles than are normally seen in the three-dimensionally compact particle agglomeration resulting from conventional ceramic processing methods. Hence, less energy is required in the nanophase processing method to break down this one-dimensional, tree-like structure during compaction than would normally be expected. The cleanliness of the small particle surfaces and the resulting high degree of activity lead to the rather dense nanophase ceramic compacts obtained with this method. RAPID SOLIDIFICATION PROCESSING A well-known method of producing ultrafine microstructures is by rapid quenching from the molten state. It has found its greatest utility in the rapid solidification of metals and alloys. The resulting microstructures exhibit high degrees of supersaturation. The finest microstructures are obtained at the highest cooling rates, corresponding to the highest undercoolings of the molten state. Large undercoolings also promote prolific nucleation and the formation of microgranular materials containing one or more phases. The same can be obtained by thermomechanical treatment of metastable phases. Techniques are available for producing melt-quenched powders in the size range from 1 to 10 µm by gas-and centrifugal-atomization methods. Electro
