Kinetic model of single boron particle ignition based upon both oxygen and (BO)$_n$ diffusion mechanism

A comprehensive ignition model for single boron particles in an oxygenated environment containing O$_2$ and H$_2$O is developed. Microcharacteristics of the boron oxide layer on the surface of boron particles at elevated temperatures are studied. Two typical distributions of species inside the surface oxide layer are detected. One is composed of three layers [B$_2$O$_3$, (BO)$_n$, and B$_2$O$_3$], while the other is composed of two layers [(BO)$_n$ and B$_2$O$_3$], both according to the order from the internal to external side of the layer. In the model development, two submodels, submodel I and submodel II, are developed with regard to two different observed species distributions in the surface oxide layer. For submodel I, it is assumed that both (BO)$_n$ and O$_2$ are the governing species diffusing into the liquid oxide layer. For submodel II, only (BO)$_n$ is the governing species. These two submodels are combined into a new bi-directional model consisting of four key kinetic processes: evaporation of the liquid oxide layer, global surface reaction between oxygen from the environment and boron, reaction between the inner boron core and oxygen, and global reaction of boron with water vapor. The ignition time predicted by the model is in good agreement with previous experimental data.