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SYNTHESIS AND PROCESSING: MORPHOLOGICALLY SPECIFIC METHODS. 44 By cooling the source and/or adding a divergent section, very large cluster species can be obtained. The pulsed-laser vaporization source is ideal for forming nonvolatile material clusters from dimers to 100 atoms. The helium or other inert carrier is pulsed over the rod source that is in turn pulsed by a laser, causing material to vaporize into the carrier gas. In the continuous thermal source the graphite oven (or tungsten oven) can be heated resistively or by electron bombardment. The supercritical solvent-extraction source allows for molecular species of low volatility to be transferred to the gas phase (J. M. Weare, D. R. Miller, and J. E. Crowell, private communication, 1988). This method has been used to grow films of silica, organic polymers, and Al2O3. In practice, the material of interest is dissolved in a supercritical fluid and expanded into a vacuum through a small capillary. The solvent evaporates, leaving the solute material to form clusters. Supercritical water is a convenient solvent for ceramic materials. One of the most important properties of a supercritical fluid, especially for making a wide range of cluster sizes, is the continuously variable solvating power of the fluid obtained by adjusting temperature, pressure, and composition (e.g., electrolytes). It is apparent that cluster properties are extremely important in the morphology and nature of the thin films produced by these methods, since the size of the clusters deposited is quite sensitive to the source temperature and pressure. Furthermore, by varying the composition of the solvent (e.g., by adding trace amounts of electrolytes), it is expected that the chemistry of the clusters may be altered. In addition, species may be added externally to the clusters prior to deposition by crossing the nozzle cluster beam with a second molecular beam. Catalytically active atoms such as Pt and Rh can be selectively added to the clusters by these methods. The extension of this technology to include excitation of gaseous species in corona-discharge free jets is being considered. DISPERSED-PHASE STRUCTURES The dispersed-phase composites listed in Table 4 have been prepared by chemical vapor deposition. In general, these materials have been deposited using conventional equipment and processes, with the exception that one or more additional reactant gases were added to the inlet gas stream. Some examples are taken from Lackey and coworkers (1987). Carbon + SiC. The structure and properties of pyrolytic carbon can be varied by alternating the deposition conditions. An even broader range of properties is afforded by the addition of a dispersed SiC phase. Carbon and SiC have little solid solubility in one another; carbon containing from 0 to 72 weight percent
