Self-Propagating Magnesiothermic Reduction Synthesis Of High Purity ZrB₂ Micro-Powders

In this paper, high purity ZrB₂ fine powders were successfully prepared by self-propagating magnesium reduction process using exothermic reactions in ZrO2-B₂O₃-Mg system. Combustion reaction mechanism, combustion process craft, microstructures and powder characteristic were studied with many advanced experimentations and equipments.According to the thermodynamics theories, reaction free energy and adiabatic temperature in ZrO2-B₂O₃-Mg system have been theoretically calculated and analyzed. Results of adiabatic temperature calculation showed that the adiabatic temperature of ZrO2-B₂O₃-Mg system without diluent was 3093K, and it reduced with improvement of diluent content. When the diluent MgO content was 0⁴5%,or the diluent ZrBB2 content was 0⁶2% the SHS processwould occer. Results of reaction free energy calculation showed that the redox reaction of B₂O₃ and Mg, the reaction of B and Zr, the reaction of ZrO₂, B₂O₃ and Mg would occer in temperature 400K²200K, because their reaction free energy were lower than zero, the redox reaction of ZrO₂ and Mg would occer in temperature 0¹600K . Based on thermodynamic theoretical calculation and kinetic theories, the micro reaction process of ZrO2-B₂O₃-Mg system was studied by differential scanning calorimetry analysis (DSC), quenching experimentation of combustion wave, microstructure analysis and chemical composition analysis. Differential scanning calorimetry analysis showed several medium reations were involved in the reaction of ZrO2-B₂O₃-Mg. B₂O₃ melted in 450℃, then Mg melted in 650℃, the three phase reaction was taken place from 730℃, ZrO₂ reacted with Mg at first, then was the redox reaction of B₂O₃ and Mg, finally was the synthetic reaction of B and Zr. Quenching experimentation showed the whole process could be divided into four stages: Stageâ… , B₂O₃ and Mg melted, the molten Mg seeped through molten B₂O₃ and ZrO2 grain group inner part under capillary function, thus formed a hollow ball. Molten Mg and B₂O₃, ZrO₂ grain assembled on the suface of the hollow ball. Stageâ…¡, ZrO₂ particles reacted with molten Mg...