Gas Evaporation and Ultrafine Particles

Vapor phase nucleation can occur in a dense vapor cloud by multibody collisions. The nucleation can be encouraged by passing the atoms to be nucleated through a gas to provide the necessary collisions and cooling for nucleation. These particles have a size range of 1 to 100 nm (10 to 1000 A) and are called ultrafine particles or ultrafine clusters. The process of forming the particles is called gas evaporation (Ref 117, 118, 119). The size and size distribution of the particles depend on the gas density, the gas species, the evaporation rate, and the geometry of the system. When these particles deposit on a surface the resulting film is very porous and can be used as an optical radiation trap; for example, "black gold" infrared radiation bolometer films, germanium film solar absorber coatings (Ref 120) and low secondary electron emission surfaces (Ref 121). The particles themselves are used for various powder metallurgical processes, such as low-pressure, low-temperature sintering (Ref 122). Ultrafine particles of reactive materials are very pyrophoric because of their high surface area.

Evaporation into a plasma causes the ultrafine particles to have a negative charge and to be suspended in the plasma near walls where they can grow to appreciable size (Ref 123, 124, 125). Ultrafine particles of alloys can be formed by evaporation from a single source, or they can be evaporated from separate sources and nucleated in the gas. Ultrafine particles of compounds can be formed by having a reactive gas present during nucleation or by decomposing and reacting precursor gases in an arc or plasma.

Buckminster Fullerenes. Recent gas evaporation techniques have allowed the formation of the Buckminster fullerenes (C60 and C70 "Buckeyballs"), a newly discovered form of the carbon molecule that resembles the shape of a soccer ball (Ref 126, 127). The synthesis involves arcing two pure graphite electrodes in a partial vacuum containing helium. The carbon "soot" that forms contains from 3 to 40% fullerenes, depending on the experimental conditions. The fullerenes are extracted from the soot by dissolving them in boiling benzene or toluene, and then this extract is vacuum dried.

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