Deposition kinetics of Al₂O₃ from AlCl₃-CO₂-H₂-HCl gas mixtures by thermal CVD in a hot-wall reactor

Abstract
The deposition rate of α-Al₂O₃ was measured at 0.1 and 1 bar and variable partial pressures of AlCl₃, CO₂, H₂ and HCl by means of a thermobalance. HCl additions proved to have a retarding effect on the deposition. From the temperature and the total pressure dependence of the deposition rate it was concluded that the process was determined by chemical reactions rather than gas diffusion. The rate increased with AlCl₃, CO₂ and H₂ partial pressures, decreased with rising HCl contents, and was independent of the CO pressure. The partial pressure dependences did not lead to simple reaction orders indicating the overlap of at least two parallel reactions. Two rate equations were deduced from the measurements to describe two parallel reaction paths, one equation being based on an assumed reaction mechanism involving the homogeneous watergas shift reaction and the subsequent heterogeneous formation of AlOCl(ad) as rate-determining intermediate. The second equation is purely empirical. Results of the numerical simulation based on these equations and heat and mass transport calculations are compared with the experimental observation.