A comparative study on combustion synthesis of Ta-B compounds

A comparative study on the preparation of various tantalum borides (including Ta₂B, Ta₃B₂, TaB, Ta₅B₆, Ta₃B₄, and TaB₂) in the Ta-B system was experimentally conducted by self-propagating high-temperature synthesis (SHS) from the elemental powder compacts of their corresponding stoichiometries. Both combustion temperature and reaction front velocity increased and then decreased with increasing boron content in the powder mixture. The fastest flame front with a reaction temperature of 1732 °C and a propagation rate of 11.2 mm/s was observed in the sample of Ta:B = 1:1. The combustion temperature (1205 °C) and flame-front velocity (3.82 mm/s) for the powder compact of Ta:B = 2:1 were the lowest. According to the XRD analysis, single-phase TaB and TaB₂ were produced from the samples of Ta:B = 1:1 and 1:2, respectively. However, multiphase products were synthesized from the samples of other stoichiometries. In the final products from Ta-rich samples of Ta:B = 2:1 and 3:2, two boride phases, Ta₂B and TaB, along with a large amount of residual Ta were detected. The products yielded from boron-rich reactants of Ta:B = 5:6 and 3:4 were composed of TaB, Ta₃B₄, and TaB₂. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with the formation of TaB and TaB₂ by solid state combustion were determined to be 131.1 and 181.4 kJ/mol, respectively.