Synthesis and formation mechanism of HfB₂ ultrafine powders with low oxygen via flocculating settling assisted process and carbo/borothermal reduction

In this study, ultrafine HfB₂ powders with low oxygen were synthesized by a flocculating settling process which yielded ceramic precursors and subsequent carbo/borothermal reduction of the precursors. The liquid phase precursor method can achieve uniform mixing of components at the molecular level through multiple complexation reactions, and then realize the carbo/borothermal reduction reaction at a lower temperature to obtain ultrapure HfB₂ powders. The as-resulted quasi-spherical HfB₂ powders under the optimum conditions (atomic molar ratio M:B:C = 1:2.8:10) calcined at 1500 °C for 1 h have an average particle size of 205 nm and an oxygen content of 0.097 wt.%. Detailed analysis of the phase evolution of precursors shows that the formation of HfB₂ particles is a mass diffusion mode from the external to internal HfO₂ cores. We reveal that below 1300 °C, HfC is not an intermediate product of HfB₂ powder during the transition of precursors. Instead, HfC was formed as a by-product at high temperatures in the carbo/borothermal reduction process. The proposed formation mechanism of HfB₂ is completely different from the traditional two-step transformation method. After the sintering of the ultrafine powders, the HfB₂ ceramics show a relative density of 96.1% and superior mechanical properties compared to other works. Furthermore, by simply replacing the initial metal source, chlorinated group IV and V transitional metals (Ti, Zr, Ta, Nb) can also convert into high-purity and ultrafine diborides. This work shows that flocculating settling assisted carbo/borothermal reduction has potential in lot size production of various high-purity and ultrafine boride powders.