A comparative study on combustion synthesis of Ta-Si compounds

A comparative study on the preparation of tantalum silicides (including TaSi2, Ta5Si3, Ta2Si, and Ta3Si) in the Ta-Si system was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of corresponding stoichiometries. For the powder compacts of Ta:Si = 5:3 and 2:1, upon ignition a planar combustion front traversing the entire sample was easily achieved even without prior heating. In contrast, a preheating temperature of 300 °C was required for the samples of Ta:Si = 3:1 and 1:2 to establish the propagation of a planar reaction front in a self-sustaining manner. It was found that the flame-front propagation velocity and combustion temperature were increased by increasing the preheating temperature and sample compaction density. Among the test samples of different compositions, the reactant compact of Ta:Si = 5:3 exhibited the highest flame speed, followed sequentially by the powder compacts of Ta:Si = 2:1, 3:1, and 1:2. The variation of combustion temperature with starting stoichiometry of the reactant compact was in a manner consistent with that of the flame-front velocity. According to the XRD analysis, a complete conversion yielding single-phase disilicide TaSi2 was achieved from the sample compact of Ta:Si = 1:2. The silicides phases Ta5Si3 and Ta2Si were obtained in relatively pure form through combustion reactions of the powder compacts with Ta:Si = 5:3 and 2:1, respectively. However, a poor degree of phase transformation was observed in the case of Ta:Si = 3:1, which produced a multiphase product composed mostly of Ta2Si and Ta5Si3, along with small amounts of Ta3Si and elemental Ta. In addition, based upon the temperature dependence of combustion wave velocity established in this study, the activation energies associated with combustion synthesis of Ta5Si3, Ta2Si, and TaSi2 were determined to be 105.2, 65.5, and 153.8 kJ/mol, respectively.