Variation of the Oxidation Rate of Silicon Carbide with Water‐Vapor Pressure

Abstract

Chemically vapor deposited silicon carbide (CVD SiC) was oxidized at temperatures of 1000°-1400°C in H 2 O/O 2 gas mixtures with compositions of 10-90 vol% water vapor at a total pressure of 1 atm. Additional experiments were conducted in H 2 O/argon mixtures at a temperature of 1100°C. Experiments were designed to minimize impurity and volatility effects, so that only intrinsic water-vapor effects were observed. The oxidation kinetics increased as the watervapor content increased. The parabolic oxidation rates in the range of 10-90 vol% water vapor (the balance being oxygen) were approximately one order of magnitude higher than the rates that were observed in dry oxygen for temperatures of 1200°-1400°C. The power-law dependence of the parabolic oxidation rate on the partial pressure of water vapor at all temperatures of the study indicated that the molecular species was not the sole rate-limiting oxidant. The determination of an activation energy for diffusion was complicated by variations in the oxidation mechanism and oxide-scale morphology with the partial pressure of water vapor and the temperature.

Synthesis of Al4SiC4 powders via carbothermic reduction: Reaction and grain growth mechanisms

“…Exposing those silicon contained ceramics to oxygen containing atmosphere at high temperature leads to formation of a protective SiO 2 oxidation scale which can sufficiently prevent the further oxidation inwards [1,2]. However, the formed SiO 2 protective scale will be degraded significantly under certain atmospheric vapors as alkali vapor and/or water vapor due to the formation of volatile Si(OH) 4 phase [3]. For example, the oxidation kinetics of SiC ceramics increased with enhanced water vapor content and the oxidation rates were approximately one order of magnitude higher than the one observed in the dry oxygen in the 1200-1400 ℃ temperature range [4].…”

“…Concerning the reduction of Al 2 O 3 phase, it was considered as the main source of the Al 4 (Fig. 5).…”

“…It is worth noting that under high temperature oxidation condition, a dual protective layer consisting of inner mullite and outer alumina is formed on the Al 4 SiC 4 surface which can improve the oxidation resistance even in water vapor atmosphere [5,9,10]. However, the application of Al 4 SiC 4 is confined due to the difficulty in fabricating highly pure samples. Hence, most of reported research work focused on the synthesis of this ternary ceramic.…”

“…However, the formed SiO 2 protective scale will be degraded significantly under certain atmospheric vapors as alkali vapor and/or water vapor due to the formation of volatile Si(OH) 4 phase [3]. For example, the oxidation kinetics of SiC ceramics increased with enhanced water vapor content and the oxidation rates were approximately one order of magnitude higher than the one observed in the dry oxygen in the 1200-1400 ℃ temperature range [4]. Thus the application of the silicon-based non-oxide ceramic materials under severe conditions is challenged.…”

“…Hence, most of reported research work focused on the synthesis of this ternary ceramic. There are mainly three kinds of powder mixtures used to synthesize the Al 4 SiC 4 powders or bulk, i.e., (1) using aluminum, silicon, and carbon powders as raw materials [11][12][13], (2) using Al 4 C 3 and SiC powders as raw materials [14,15], and (3) using Al 2 O 3 , SiO 2 , and C as raw materials by carbothermic reduction [16][17][18][19][20]. The former two reactant mixtures require expensive and active metal powders or carbides as starting materials.…”