Nanophase Materials Assembled from Atomic Clusters
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application of this idea in récent years4"8 to the synthesis of a variety of nanophase metals and ceramics has built upon a considérable body of earlier research into the production of ultrafine particles by the gas-condensation method, and upon previously assembled knowledge of p o w d e r metallurgy and ceramics. Earlier research on the gas-condensation method and on the resulting atomic clusters9"11 defined the various parameters (primarily type of gas, gas pressure, and evaporation rate) that control the sizes of the clusters formed in the conventional g a s - c o n d e n s a t i o n m e t h o d used to synthesize nanophase materials. Some unique advantages of nanophase materials and the nanophase processing method itself are as follows: 1. The ultrafine sizes of the atomic clusters and their surface cleanliness allow conventional restrictions of phase equilibria and kinetics to be overcome during material synthesis by the combination of short diffusion distances, high driving forces, and uncontaminated surfaces and interfaces. 2. The large fraction of atoms residing in the grain boundaries and interfaces of thèse materials allow for interface atomic arrangements to constitute significant volume fractions of material, yielding novel materials properties. 3. A wide range of materials can be produced in this way, including metals and alloys, intermetallic compounds, ceramics, and semiconductors. It is also apparent that they can be formed to contain crystalline, quasicrystalline, or amorphous structures. 4. The possibilities for reacting, coating, and mixing in situ various types, sizes,
and morphologies of clusters create a significant potential for the synthesis of new multicomponent composites with nanometer-sized microstructures and engineered properties. Most research to date, nevertheless, has concentrated on single-phase metals and ceramics. This article describes the method of materials synthesis and processing that leads to ultrafine-grained polycrystalline metals and ceramics with mean grain sizes ranging from 5 to 25 nm and, accordingly, numerous high-angle grain boundaries. The grain boundary structures and morphologies that resuit in materials deeply metastable against grain growth are also considered. Finally, some of the spécial properties and processing characteristics resulting from the scaled-down size of the grains and the clean interfaces are reviewed, and the future of cluster-assembled nanophase materials is assessed.
Synthesis of Nanophase Materials A typical apparatus for the synthesis of n a n o p h a s e materials via gascondensation is shown schematically in Figure 1. It comprises an ultrahigh v a c u u m (UHV) System fitted w i t h two resistively h e a t e d e v a p o r a t i o n sources, a cluster collection device (liquid-nitrogen-filled cold finger) and scraper a s s e m b l y , a n d in situ compaction devices for consolidating the powders produced and collected in the chamber. Before making the powders, the UHV System is first evacuated by a turbomolecular p u m p to below 10"5
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