Rate of Sic Deposition from Methyltrichlorosilane and Influence of HCL Addition
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ABSTRACT Suitable feedgas compositions for monophase SiC deposition were determined starting from methyltrichlorosilane-hydrogen-argon mixtures at 1 bar total pressure and H2 /MTS ratios between 50 and zero. At temperatures from 900 to 1400'C, only a ratio of 1 at pMrs = 18 mbar gave the desired result. The deposition rate of SiC was determined as a function of MTS and HCl partial pressures, of gas flow rate, and of temperature at a H2 /MTS ratio of 1. Two different rate equations were found at [MTS]/[HCI] _>5.8 (region I) and < 5.8 (region II), respectively: Jsic(I) = k' [MTS][HC1]-l jsic(II) = k"[MTS][HC1]-3 , where brackets denote gas concentrations. The apparent activation energy was 380 kJ/mol common to both rate equations. A mathematical model taking into account temperature fields at reactor wall and substrate, heat and mass transport in the flowing gas, and heterogeneous reaction rates jsic(I) and (II) was developed and found to be in very good agreement with the experimental results. INTRODUCTION Chemical vapor deposition of silicon carbide plays an important role to improve the mechanical properties and the oxidation resistance of fibre-reinforced ceramic materials by coating the fibres before embedding them into the matrix and by filling up part of the open pores of the composite through chemical vapor infiltration. Both of these processes can only be carried out efficiently under conditions where the heterogeneous deposition reaction is rate-determining rather than mass transport in the gas phase. The determination of the rate laws governing the chemical reactions is therefore essential to select the most suitable conditions for fibre coating or pore infiltration with SiC. Previous investigations have delivered a complex and in parts contradictory picture of the deposition kinetics of SiC: In contrast to thermodynamic predictions [1], most gas compositions chosen as reactants do not yield pure SiC but Si+SiC or, less frequently, C+SiC [15]. For kinetic investigations, a precursor gas mixture has to be selected to deposit pure SiC throughout the tests. The influence of gas partial pressures and temperature on the deposition kinetics has been described by several authors, but with little agreement [1,3,4,6,7,8]. The precursor most frequently used has been methyltrichlorosilane (MTS), mostly mixed with hydrogen. The deposition rate seems to rise with MTS partial pressure, but the reaction order has been found to be zero [6,8] or one half [3] or one [9,10]. The influence of H2 on the rate seems to be low [4], while a retarding effect of HCI has been reported [8,11,121 with widely varying reaction orders [8]. The activation energies calculated from the temperature dependence of the reaction rate vary between 30 [13] and 400 kJ/mol [14]. The present investigation had the aim to determine - on the basis of MTS - the gas 63
Mat. Res. Soc. Symp. Proc. Vol. 363 ©1995 Materials Research Society
compositions suitable to deposit SiC free of Si or C, and to measure the rate of deposition as a function of reactant
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