Formation of MgO‐B4C Composite via aThermite‐Based Combustion Reaction

Abstract

The combustion synthesis of MgO‐B4C composites was investigated by coupling a highly exothermic Mg‐B2O3 thermite reaction with a weakly exothermic B4C formation reaction. Unlike the case of using Al as the reducing agent, the interaction between Mg and B2O3 depends on the surrounding inert gas pressure due to the high vapor pressure of Mg. The interaction changes from one involving predominantly gaseous Mg and liquid B2O3 to one involving liquid Mg and liquid B2O3 as the pressure increases. At low inert gas pressure, the initiation temperature is found to be just below the melting point of Mg (650°C). As the inert gas pressure increases, the vaporization loss of reactants is reduced, and this in turn increases the combustion temperature, which promotes greater grain growth of the product phases, MgO and B4C. The particle size of B4C increased from about 0.2 to 5 μm as the pressure changed from 1 to 30 atm.