Phase control during synthesis of nanocrystalline ultrahigh temperature tantalum-hafnium diboride powders

Ta1-xHfxB2 material is attractive for various aerospace applications. In this study, 2 low-cost approaches were adopted to synthesize nanocrystalline Ta0.5Hf0.5B2 solid solution and related composite powders. The first was based on carbothermal reduction reaction (CTR) of intimately mixed tantalum-hafnium-boron oxide(s) and carbon obtained from aqueous solution processing of TaCl5, HfCl4, B2O3, and sucrose as precursors. It was found that when using this method, due to the low solubility of each other for Ta2O5 and HfO2 and the difference in reactivity of those 2 oxides with carbon (as well as B2O3), individual TaB2 (-rich) and HfB2 phases always form separately. Those borides tend to remain phase separated due to the slow inter-diffusion between them. However, it was observed that addition of copper catalyst noticeably accelerates the inter-diffusion and the solid solution formation. The second approach was based on alkali metal reduction reaction, in which TaCl5 and HfCl4 are directly reacted with sodium borohydride (NaBH4). This method yields a single phase Ta0.5Hf0.5B2 solid solution nanopowders in one step at much lower temperatures (e.g., 700 degrees C) by avoiding the oxides formation and the associated phase separation of individual borides as observed in the CTR-based process.