Open-Air SHS Toward Boron Carbide Formation: A Comparative Study of B2O3-Al-C and B2O3-Mg-C Systems

A comparative compositional screening of combustion behavior and phase formation during self-propagating high-temperature synthesis in B2O3–Al–C and B2O3–Mg–C mixtures was performed under the same open-air laboratory conditions. Twelve strongly carbon-rich formulations were examined. These formulations were not intended to reproduce the target stoichiometric reactions and are interpreted as an empirical screening matrix rather than as optimized stoichiometric compositions. In the individual SHS runs, the Mg-containing formulations produced higher recorded maximum apparent local combustion front temperatures and estimated apparent average front propagation velocities than the Al-containing formulations. Because each formulation was tested only once, these observations do not establish reproducible differences between the two systems. Qualitative X-ray diffraction analysis identified Al2O3, Al20B4O36, Al4B2O9, and residual Al in the aluminothermic products. MgO, Mg2B2O5, and Mg3B2O6 were identified in the magnesiothermic products. Weak reflections attributable to B4C were observed in selected compositions, whereas oxides and metal borates were the principal crystalline phases identified in both systems. This result indicates that the carbide-forming pathway was competitively disadvantaged under the investigated open-air SHS conditions. Thermodynamic calculations for the idealized reactions showed that the relative standard driving force depended on temperature and the phase states of the reactants and products. The final phase assemblages indicate competition between carbide formation and the formation of stable oxide and borate phases. Atmospheric oxidation may also have contributed to the oxide-rich products. The results provide a descriptive comparison of the two investigated formulation sets and identify compositional patterns associated with limited B4C formation under open-air SHS conditions.