Combustion Synthesis of Mullite/Metal Boride Composites
Formation of composite materials composed of mullite (3Al2O3×2SiO2) and transition metal borides (NbB2 and TaB2) was studied by self-propagating high-temperature synthesis (SHS). Starting materials included not only metal oxides (Nb2O5 and Ta2O5) and boron oxide (B2O3) as the sources of metallic elements and boron, but also Al and Si powders as the reducing agents. The evolution of mullite from in situ formed SiO2 and Al2O3 and synthesis of NbB2 and TaB2 were investigated. The effect of excess Si addition was studied on the combustion temperature, flame-front propagation velocity, and phase composition of the final product. For formation of the NbB2/mullite composites, the combustion velocity about 2.5 mm/s and reaction temperature around 1500 oC decreased slightly as the Si content increased. However, a considerable decrease in combustion front velocity from 2.74 to 1.43 mm/s and in reaction temperature from 1600 to 1250 oC was observed for the production of the TaB2/mullite composites. The XRD patterns of the final products confirmed the role of excess Si in the improvement of silicothermic reduction of B2O3 and subsequent evolution of NbB2, TaB2, and mullite. The EDS analysis indicated an atomic proportion close to that of 3Al2O3×2SiO2 for the mullite grains synthesized in this study.
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