Low-temperature synthesis of single-phase refractory metal compound carbides

The compound carbide of refractory metal had great potential applications in ultra-high temperature, biomedical and nuclear fields. In the present study, a novel method composed of vacuum carbothermal reduction followed by calcium treatment was proposed to synthesize single-phase compound carbide. It was found that the required temperature for the formation of single-phase Ta4HfC5 solid solution in liquid calcium was above 1573 K. Meanwhile, the single-phase binary carbides (NbTaC2), ternary carbides (TiNbTaC3, ZrNbTaC3), quaternary carbides (TiHfNbTaC4, ZrHfNbTaC4), pentanary carbides (TiZrHfNbTaC5) and hexanary carbides (TiZrHfVNbTaC6) were also synthesized by the current method. The formation mechanism of the single-phase compound carbide was discussed and two parameters TSS (based on the mixing enthalpy change and mixing entropy change) and delta a (based on the difference of lattice parameter) were proposed for predicting the formation ability of solid solution. It was found that the necessary conditions for formation of solid solution by current method were TSS < 1800 K and delta a < 3.2%. In addition, when the value of TSS was lower than 5.5 x 103.ln(1/delta a)-19 x 103, the singlephase compound carbides could be easily synthesized.