(458d) Heterogeneous Oxidation of Boron Powders and Its Role in Ignition and Combustion

Despite significant thermodynamic benefits, applications of boron combustion are limited, primarily, due to its low bulk burn rates. It has been suggested that the rate limitations of boron ignition and combustion are due to relatively slow kinetics of the associated heterogeneous reactions. However, such reactions presently are poorly characterized. Experimentally, they are hard to quantify because of the complex surface morphology of boron powders containing agglomerates of partially fused nano-sized primary particles. In this study, an approach enabling quantitative description of boron oxidation is proposed, presenting each boron agglomerate as a spherical particle with a porous reactive shell and impenetrable inert core. This approach is validated experimentally, enabling one to use this reaction geometry to develop an explicit model for boron oxidation. Kinetic parameters of this model are determined using low-temperature, low-heating rate thermo-analytical experiments. For this interpretation, the reactive shell is assumed to consist of packed, spherical, mono-sized primary particles. The primary particle size is determined based on thermo-gravimetric measurements combined with analyses of specific surface areas of the powders tested. Using the particle size and thickness of the reactive shell enables one to assess the total reactive surface area, where oxidation occurs so that the rate of oxidation per unit of boron surface is obtained. The model is valid until a critical oxide layer thickness forms on the surface of the reacting boron nano-particles that changes the distribution of oxide, total reacting surface area and particle geometry. It is thus suitable to describe reactions leading to boron ignition. Interestingly, at high temperatures typical of full-fledged boron combustion, oxide layers are expected to volatilize, once again exposing boron surface to external gaseous oxidizer. Thus, the same oxidation process may be governing both low- and high-temperature reactions of boron. At high temperatures, however, it is projected that the complex morphology of boron agglomerates is lost and the molten boron is expected to form spherical droplets. The surface area of the molten burning droplets can therefore be estimated accounting for the size of the original agglomerates. The kinetics of boron oxidation recovered from the thermo-analytical measurements is then used to predict the reaction rates observed in combustion experiments. Comparison of these predictions and measured particle burn rates for boron will be presented and discussed.