Prospects of usinsg boron powders as fuel. II. Influence of aluminum and magnesium additives and their compounds on the thermal behavior of boron oxide

Physicochemical changes occurring in an oxide shell of boron particles during heating are extremely important for the oxidation and combustion of boron, including those in solid propellant compositions. The $\mathrm{Al}_2\mathrm{O}_3$, $\mathrm{MgO}$, $\mathrm{MgF}_2$, $\mathrm{Al}$, and $\mathrm{Mg}$ impurities were experimentally discovered in an oxide layer on boron particles obtained by various methods. The goal of this work is to determine the effect of these impurities on the thermal behavior of boron oxide, with particular attention paid to evaporation of $\mathrm{B}_2\mathrm{O}_3$. The temperature and thermal effects of reactions between the components are thermoanalytically determined, and the processes of dehydration, melting, and evaporation of boron oxide are analyzed in detail. The enthalpy of evaporation of boron oxide, starting at a temperature above $1300^\circ$C is experimentally determined, and is equal to $347\pm3$ kJ/mol. An interaction is observed between magnesium fluoride and boron oxide at a temperature of about $1000^\circ$C with a mass loss corresponding to the content of magnesium fluoride and the formation of gas-phase boron fluoride. It is established that boron dissolved in oxide has virtually no effect on the evaporation of the boron oxide melt, while the addition of $\mathrm{Al}_2\mathrm{O}_3$ or $\mathrm{MgO}$ significantly increases its thermal stability. Based on the analysis of the results obtained, an assumption is made about the effect of impurities on boron activity in the processes of boron ignition and combustion.