Preparation of TaB/TaB2/mullite composites by combustion synthesis involving aluminothermic reduction of oxide precursors

Abstract

TaB/TaB2/mullite composites were fabricated by self-sustaining combustion involving aluminothermic reduction of two oxide precursors, Ta2O5 and B2O3. Reactant mixtures were formulated by different combinations of Ta2O5–B2O3–Al–SiO2 with or without addition of boron. Exothermic reduction of Ta2O5 and B2O3 by Al provides sufficient heat to sustain the synthesis reaction in the SHS mode. The increase of SiO2 improved the mullite formation, but reduced the combustion temperature and velocity due to the dilution effect of SiO2 and the endothermic mullitization process. The phase of tantalum borides formed depended on the content of Al and the addition of boron. Specifically, the TaB/mullite composite with a small amount of TaB2 was produced from an Al-deficient sample of Ta2O5–2B2O3–6Al–2SiO2. For a boron-added sample of Ta2O5–2B2O3–6Al–2SiO2–3B, it was found that TaB2 prevailed and the resulting product was a TaB2/mullite composite. The SHS-derived mullite grains exhibit a thick whisker shape with a large aspect ratio.