Controlling phase separation of TaxHf1-xC solid solution nanopowders during carbothermal reduction synthesis

Synthesis of single-phase tantalum hafnium carbide (TaxHf1-xC, 0<x<1) solid solution nanopowders via carbothermal reduction (CTR) reaction is complicated due to the difference in reactivity of parent oxides with carbon and presence of a miscibility gap in TaC-HfC phase diagram below similar to 887 degrees C. These can lead to phase separation, ie, formation of two distinct carbides instead of a single-phase solid solution. In this study, nanocrystalline TaxHf1-xC powders were synthesized via CTR of finely mixed amorphous tantalum-hafnium oxide(s) and carbon obtained from a low-cost aqueous solution processing of tantalum pentachloride, hafnium tetrachloride, and sucrose. Particular emphasis was given to investigate the influences of starting compositions and processing conditions on phase separation during the formation of carbide phase(s). It was found that due to the immiscibility of Ta-Hf oxides and relatively fast CTR reaction, individual nano-HfC and TaC phases form quickly (within minutes at 1600 degrees C), then go through interdiffusion forming carbide solid solution phase. Moreover, the presence of excess carbon in the CTR product slows down the interdiffusion of Ta and Hf dramatically and delays the solid solution formation, whereas DC electrical field (applied through the use of a spark plasma sintering system) accelerates interdiffusion significantly but leads to more grain growth.