Effect of powder characteristics on thermal oxidation of boron

Boron has been widely studied as an attractive fuel for air-breathing propulsion due to its high oxidation enthalpy. However, the metrics from many relevant published ignition and combustion experiments with different boron powders cannot be compared with one another because of variations in experimental methods and conditions. Conversely, thermoanalytical measurements performed with different powders used consistent procedures and thus are suitable for direct comparisons. Here, the effects of characteristics of boron powders on their oxidation are elucidated based on the available reports. The oxidation metrics included temperatures for oxidation onset and peaks of differential scanning calorimetry (DSC) and differential thermogravimetry (DTG) traces, mass gain at a certain temperature and an integrated oxidation heat release. The oxidation metrics were correlated with data on particle size, purity, and crystallinity. The experimental heating rates, gas flowrates, and oxygen partial pressures were also considered. All characteristic temperatures recovered from the thermal analysis exhibit a minimum for 0.5 – 1.5 µm particles. The identified oxidation metrics/powder parameter trends are distinct for the amorphous and crystalline boron. Generally, both the particle size and crystallinity affect the oxidation. The effect of the boron purity remains unclear. In the future, it is recommended to specify the type of impurity, which might clarify the influence of the composition of boron-based powders on their oxidation. The observed trends are interpreted semi-quantitatively assuming each boron particle is an aggregate with a reactive shell and an inert core.