Pyrolytic Carbon Deposition on Graphitic Surfaces
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PYROLYTIC CARBON DEPOSITION ON GRAPHITIC SURFACES Ismail M. K. Ismail University of Dayton Research Institute c/o Phillips Laboratory/ RKFC, Edwards Air Force Base, CA 93523-5000, USA
ABSTRACT The present study outlines the importance of carbon active sites in controlling the kinetics of pyrolytic carbon deposition on a graphitized pitch fiber at 1025'C. Blockage of active sites with chemisorbed hydrogen retards deposition rates substantially. Removal of the chernisorbed hydrogen from the occupied sites raises deposition rates to the normal values noted on a fresh "clean" surface. The effect of surface activation prior to deposition is discussed. Activating the surface generates additional active sites and enhances the rates. However, not all the newly developed sites contribute to the kinetics. After the deposition of the first carbon layer, a fraction of those newly developed sites is instantaneously blocked and does not further contribute to subsequent deposition. The remaining fraction, along with~the original sites available before activation, keeps replicating during the remaining course of deposition. That is, the disappearance of one active site after carbon deposition is associated with the generation of a new site. This trend continues up to the deposition of 60 carbon layers.
INTRODUCTION The chemical vapor deposition (CVD) of pyrolytic carbon (pyro c) on carbon substrates is an important process for fabricating carbon composites which are used in many aerospace applications. The CVD process has two main advantages. First, by changing the experimental deposition conditions, one can obtain carbon composites having a wide spectrum of properties. Second, compared to other methods used for fabricating carbon composites, such as coal-tar or pitch impregnation, CVD is simpler, cleaner, and less expensive. In addition, it takes place in one step which operates at lower temperatures and pressures than the other industrial processes that are currently involved in fabricating carbon composites. Extensive studies on the CVD of pyro c have appeared in the literature, including the structure and properties of deposits [1-4], the relations between deposit properties and processing conditions [3,4], the kinetics and mechanism of deposition [1,3,5,6], and the characteristics of deposits [1,3,5,7-9]. The kinetics of deposition depends on the type of substrate, provided that other experimental parameters are fixed [101. For example, at 1025°C and 1 atm, the kinetics under a flow of 10% CH 4 in Ar was not the same for all substrates [10]. The results indicated that at least four categories of different substrates are possible: non-porous carbons with small surface areas, microporous carbons with large internal surface areas, nonporous carbons with large external surface areas, and activated carbons with considerable external and internal surface areas [10]. Thus, the total surface area (TSA) of substrates played a major role in CVD kinetics. The importance of carbon active surface area (ASA) to CVD kinetics, however, was
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