Reaction mechanism of self-propagating magnesiothermic reduction of ZrB₂ powders

Fine zirconium diboride (ZrB₂) powders with high purity were successfully prepared by combustion synthesis through magnesiothermic reduction process in Mg–B₂O₃–ZrO₂ system. The reaction mechanism was investigated by differential thermal analysis and quenching experiment. The results show that the whole magnesiothermic reduction process includes three stages: first, molten B₂O₃ and Mg formed above the temperature of 650 °C, and glassy B₂O₃ and solid ZrO₂ particles were coated on the surface of the molten Mg; thus, the hollow balls can be formed when the molten Mg was exuded under capillary function. Second, ZrO₂ particles reacted with molten Mg to form Zr and MgO with dissolution–precipitation mechanism, which released a large amount of heat to induce the diffusion reaction between B₂O₃ and Mg to form B and MgO. Last, Zr reacted with B to form ZrB₂ grains. The preparation of ZrB₂ by self-propagating synthesis in Mg–B₂O₃–ZrO₂ system is a solid–liquid-liquid reaction.