Gas-Phase Kinetic Modeling of the AlCl[sub 3] Decomposition in the AlCl[sub 3]-CO[sub 2]-H[sub 2]-HCl System for a Hot-Wall CVD Reactor

Abstract

For a specific set of conditions applied in experimental hot-wall reactor investigations for depositing ␣-Al 2 O 3 from precursor gas mixtures containing AlCl 3 , kinetic modelling of the gas-phase reactions was carried out on the basis of rate parameters collected or estimated by Catoire and Swihart. Calculations are presented for upward flow rates of 200, 20, 0.41, and 0.08 slh, a vertical tube reactor ͑27 mm id͒ with a 450 mm isothermal zone ͑1050°C͒ and with varying concentrations of AlCl 3 , CO 2 , H 2 , and HCl in the feed gas at a total pressure of 0.1 bar. The residence times of the gas in the isothermal zone were 0.1, 1, 48, and 250 s. The results show that AlCl 3 decomposes by reacting with CO 2 and H 2 to form AlOCl as the major intermediate species. The water-gas shift reaction, CO 2 + H 2 = CO + H 2 O, is found to take place very slowly at short residence times and much faster at higher ones. The H 2 O generation increases with the AlOCl content in the gas phase. A comparison of the kinetic values with calculated equilibrium compositions shows that for residence times typically used in technical equipment for chemical vapor deposition ͑CVD͒, the gas-phase reactions are kinetically controlled at the temperature and pressure selected. The results may be helpful in understanding the complex reaction mechanisms involved in high-temperature CVD of alumina.

Atmospheric pressure chemical vapor deposition mechanism of Al2O3 film from AlCl3 and O2

“…For example, a recent kinetic study on the AlCl 3 /CO 2 /H 2 system identified AlOCl as a major gas phase intermediate which acts as a catalyst to the water-gas shift reaction [7]. Association reactions of the intermediates yield clusters that diffuse to the substrate surface to form the film or serve as nucleus for the formation of particles [23,24].…”

“…A commonly used CVD method of producing alumina film is done by reacting AlCl 3 vapor with CO 2 and H 2 at temperatures ranging from 700 to 1000 1C at reduced pressures [5,6]. In this system, the water-gas reaction (CO 2 +H 2 -H 2 O+CO) determines the rate of film formation [6][7][8]. It is critical to control the H 2 O content in the gas mixture because too much water would lead to a powdery deposit due to the rapid reaction of AlCl 3 with water [5].…”

Integrated Approach for the Development of Advanced, Coated Gas Turbine Blades

“…[28] Important adsorbed intermediate species suggested are AlHO 2 (ad) and AlOCl(ad). Tan et al [29] investigated the homogeneous gas phase kinetics based on the set of rate parameters presented by Catoire and Swihart [30] involving 92 reversible reactions among 32 species. Tan et al [29] suggested AlOCl as the major rate determining intermediate species in the gas phase.…”

“…Tan et al [29] investigated the homogeneous gas phase kinetics based on the set of rate parameters presented by Catoire and Swihart [30] involving 92 reversible reactions among 32 species. Tan et al [29] suggested AlOCl as the major rate determining intermediate species in the gas phase. Nitodas and Sotirchos, [28] though considering adsorbed AlOCl(ad), did not consider this species taking part in the homogeneous gas phase reactions.…”

“…However, details in the deposition rate are not described well since the chemical model used gives a too simplified picture of the actual reaction mechanisms. The homogeneous gas phase chemistry can probably be reasonably well described, [29,30] but the surface chemistry is not well understood at all so far.…”