High temperature kinetics of solid boron gasification by B2O3(g) - Chemical propulsion implications

Abstract

New flow reactor measurements are reported of the intrinsic kinetics of the gasification of solid boron by each of these important vapors: B2O3(g), O2(g), CO2(g), and H2O(g) at surface temperatures between 1330 and 2050 K. For illustrative purposes, our data for the remarkably efficient B2O3(g)/B(s) reaction and the O2(g)/B(s) reaction are used to discuss the expected sequence of rate-controlling processes for the combustion of individual B(s) particles in air under typical ramjet conditions. A diagram of (log-) particle diameter vs (log-) chamber pressure is shown to be particularly useful for this purpose, as well as to display the onset of noncontinuum behavior and the locus of expected particle extinction due to passivation associated with the kinetically controlled onset of condensed B2O3 on the gas/solid interface. In this way we show that, whereas most previous boron particle combustion and extinction laboratory experiments have been performed in the regime of gas-phase diffusion control, under conditions of actual ramjet interest the gas/solid kinetics for the efficient B2O3(g)/B(s) reaction and the slower O2(g)/B(s) reaction, as well as noncontinuum transport effects, become rate limiting.