Sol-Gel Synthesis and Formation Mechanism of Ultrahigh Temperature Ceramic: HfB_2

摘要

Hafnium diboride (HfB_2) 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 HfB_2 formation investigated using several techniques. The results showed that while temperatures as low as 1300℃ could be used to produce HfB_2 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 HfB_2 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℃ for 2 h. The reaction mechanism involved boro/carbo-thermal reduction and the indications were that the formation of HfB_2 at 1300℃ is through the intermediate formation of an amorphous B or boron suboxides, although at higher temperatures more than one reaction mechanism may be active.