Formation of NbB2/mullite composites by combustion synthesis involving metallothermic reduction of Nb2O5 and B2O3

NbB2/mullite composites were fabricated by self-propagating high-temperature synthesis (SHS) involving metallothermic reduction of Nb2O5 and B2O3 by Al and Si. Two reactant mixtures formulated as 13/11Nb2O5+26/11B2O3+6Al+xSi with x=2.0–3.0 and 13/9Nb2O5+52/27B2O3+52/27B+6Al+ySi with y=2.0–3.0 were prepared to study the effect of in situ formed SiO2 on combustion behavior and phase conversion. For comparison, test samples with pre-added SiO2 were employed and they were composed of 6/5Nb2O5+B2O3+14/5B+6Al+2SiO2, within which Al was the only reducing agent. When compared with combustion of the SiO2-added sample, the SHS process of the Si-adopted sample was more exothermic and effective in reduction of B2O3. Moreover, metallothermic reduction of B2O3 was greatly improved for the samples with excess Si and their final products were NbB2/mullite composites with negligible minor phases. However, due to incomplete reduction of B2O3 in the SiO2-added sample, the resulting product consisted of mullite, three boride phases (NbB, Nb3B4, and NbB2), and an amorphous compound.