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).