Mass transfer and kinetics of the chemical vapor deposition of SiC onto fibers
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Mass transfer and kinetics of the chemical vapor deposition of SiC onto fibers W. Jack Lackey, Sundar Vaidyaraman,a) Bruce N. Beckloff,b) Thomas S. Moss III,c) and John S. Lewisd) George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received 16 February 1996; accepted 22 September 1997)
An internally consistent set of data was generated for the chemical vapor deposition (CVD) of SiC from methyltrichlorosilane (MTS) and H2 at atmospheric pressure. A moving fiber tow was used as the substrate. Coating rates between 0.3 and 3.7 mmymin and deposition efficiencies between 24 and 48% were obtained for MTS and H2 flow rates in the range 30 to 200 cm3ymin and 300 to 2000 cm3ymin, respectively. The data were analyzed and found to be best fit under a mass transfer regime. Based on this fit, a value of the constant in the Chilton–Colburn j factor expression for a moving fiber tow was estimated to be 2.74 3 1026 with a standard deviation of 3.2 3 1027 . The efficiency of the reaction was found to decrease with increases in the total flow rate, indicating that the effect of the decreased residence time of reagents in the reactor was larger than the increase in the mass transfer coefficient. Finally, a comparison between the efficiencies for a stationary and a moving tow revealed that the moving tow had a higher efficiency, possibly due to a disruption of the boundary layer by the tow motion or due to the decrease in the canning of the moving tow.
I. INTRODUCTION
The fabrication of small diameter ceramic fibers for use in high performance ceramic- and metal-matrix composites has received a large amount of attention in recent years for industrial and defense structural applications. Of the various candidate materials considered, silicon carbide (SiC) has been presented as a useful material due to its high temperature strength, high elastic modulus, low density, and resistance to oxidation and creep. At present, commercially available SiC and oxide fibers made by melt spinning of polymeric precursors or sol-gel processes are lacking in terms of their high cost, loss of strength at high temperatures, and excessive creep. For example, the less expensive grade of Nicalon fibers degrades at ,1200 ±C due to nonstoichiometry.1 Chemical vapor deposition (CVD) offers a method by which stoichiometric SiC fibers may be produced which have high strength and chemical purity, along with low porosity. The CVD process may be utilized to produce SiC fibers from the deposition of SiC onto small diameter carbon fibers, in the form of a tow. For a deposit thickness of 5 mm onto a 5 mm diameter fiber, the properties of the composite fiber are controlled by those of SiC since SiC comprises 89% of the fiber volume. a) Current b)
address: Aircraft Braking Systems, Akron, Ohio. Current address: Texas Instruments, Dallas, Texas. c)Current address: Los Alamos National Laboratory, Los Alamos, New Mexico. d) Current address: University of Florida, Gaines
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