Fabrication of TaB2/mullite composites by combustion synthesis with excess silicon and B2O3 additions

Preparation of TaB2/mullite composites from a cost- and energy-effective mixture of reactants was conducted by self-propagating high-temperature synthesis (SHS). The sample stoichiometry of 1.18Ta2O5+(2.36x)B2O3+6Al+(2y)Si was formulated with x=1.0–1.3 and y=1.0–2.0 for the study of the effects of excess B2O3 (x>1.0) and Si (y>1.0) on combustion characteristics and product compositions. The synthesis reaction involved coreduction of Ta2O5 and B2O3 by Al and Si. The reaction exothermicity and combustion velocity increased slightly with increasing B2O3 but decreased considerably with Si. Formation of TaB2 and mullite was improved by adopting excess B2O3 and Si to compensate for their evaporation loss at high combustion temperatures up to about 1600 °C. The sample with x=1.3 and y=1.5 was shown to yield the optimum formation of TaB2 and mullite. Mullite grains with a tubing-like shape were produced from in situ formed SiO2 and Al2O3 and had an atomic composition close to that of 3:2 mullite (3Al2O3·2SiO2).