Deposition of silicon carbide using the chemical vapor composites process: Process characterization and comparison with
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P. Reagan and M. Robbins ThermoTrex Corporation, 85 First Avenue, P.O. Box 9046, Waltham, Massachusetts 02254-9046 (Received 14 September 1992; accepted 26 February 1993)
In this work, we explore the use of the chemical vapor composites (CVC) process to increase the rates of silicon carbide (SiC) growth on graphite substrates. Large SiC seed particles are used that are deposited by gravity-driven sedimentation. The results show that addition of large (dp = 28 ^ m ) SiC seed particles to a gas phase containing hydrogen and methyltrichlorosilane increases the deposition rate of SiC by amounts substantially higher than that expected from the addition of the particle volume alone. Insight into the mechanism of this deposition rate enhancement is obtained through analysis of SEM photographs of deposits. Growth rates and deposit structures are consistent with the trends predicted by the previously developed random-sphere model of simultaneous particle-vapor deposition (RASSPVDN), which is used here to interpret the data.
I. INTRODUCTION The ability to form ceramic materials by chemical vapor deposition (CVD) and related methods is now well established. Thin films, corrosion-, wear-, and oxidationresistant coatings, ceramic powders, and composites of a wide variety of materials, including oxides, nitrides, and carbides have been produced.1 Formation of materials for electronic device applications (usually thin films) is currently possible at acceptable rates for most materials. However, extension of gas-phase deposition techniques to the production of thick coatings for structural materials, such as radiant heat tubes, is often uneconomical due to low deposition rates. Composite manufacture by chemical vapor infiltration (CVI) of porous preforms is similarly limited, with deposition times of tens of hours or days required.2 Several new approaches to high-deposition-rate ceramic synthesis have received some attention recently. One example is thermal gradient-forced flow CVI, which uses temperature gradients and pressure to overcome diffusion limitations in porous preforms to increase composite formation rates.3 Another novel but less developed technique is the particle-precipitation-aided chemical vapor deposition (PPCVD) process developed by Komiyama and co-workers.4"7 In this CVD variant, fine particles (=SO.5 /xm diameter) nucleate in the gas phase and codeposit on the substrate to achieve enhanced growth rates. Cooling the substrate relative to the gas causes particle- and vapor-phase deposition to occur simultaneously. Komiyama et al. propose that reactions of J. Mater. Res., Vol. 8, No. 7, Jul 1993
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gas-phase species occur on a porous region at the surface of the growing deposit.6 The three-dimensional interface provides a surface area for heterogeneous CVD reactions that can be several orders of magnitude larger than the smooth surface of the original substrate. Increases in aluminum nitride deposition rates of up to two orders of magnitude are observed by th
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