Sol–Gel Synthesis and Formation Mechanism of Ultrahigh Temperature Ceramic: HfB2

Hafnium diboride (HfB₂) powder has been synthesized via a sol–gel‐based route using phenolic resin, hafnium chloride, and boric acid as the source of carbon, hafnium, and boron, respectively, though a small number of comparative experiments involved amorphous boron as boron source. The effects of heat‐treatment dwell time and hafnium:carbon (Hf:C) and hafnium:boron (Hf:B) molar ratio on the purity and morphology of the final powder have been studied and the mechanism of HfB₂ formation investigated using several techniques. The results showed that while temperatures as low as 1300°C could be used to produce HfB₂ particles, the heat treatment needed to last for about 25 h. This in turn resulted in anisotropic particle growth along the c‐axis of the HfB₂ crystals yielding tube‐like structures of about 10 μm long. Equiaxed particles 1–2 μm in size were obtained when the precursor was heat treated at 1600°C for 2 h. The reaction mechanism involved boro/carbothermal reduction and the indications were that the formation of HfB₂ at 1300°C is through the intermediate formation of an amorphous B or boron suboxides, although at higher temperatures more than one reaction mechanism may be active.