Elastic properties, thermal stability, and thermodynamic parameters of MoAlB

MoAlB is the first and, so far, the only transition-metal boride that forms alumina when heated in air and is thus potentially useful for high-temperature applications. Herein, the thermal stability in argon and vacuum atmospheres and the thermodynamic parameters of bulk polycrystalline MoAlB were investigated experimentally. At temperatures above 1708 K, in vacuum and inert atmospheres, this compound incongruently melts into the binary MoB and liquid aluminum metal as confirmed by differential thermal analysis, quenching experiments, x-ray diffraction, and scanning electron microscopy. Making use of that information together with heat-capacity measurements in the 4--1000-K temperature range---successfully modeled as the sum of lattice, electronic, and dilation contributions---the standard enthalpy, entropy, and free energy of formation are computed and reported for the full temperature range. The standard enthalpy of formation of MoAlB at 298 K was found to be $\ensuremath{-}132\ifmmode\pm\else\textpm\fi{}3.2$ kJ/mol. Lastly, the thermal conductivity values are computed and modeled using a variation of the Slack model in the 300--1600-K temperature range.