Structural Influence on the Thermal Conversion of Self‐Catalyzed HfB2/ZrB2 Sol–Gel Precursors by Rapid Ultrasonication of Oxychloride Hydrates

Abstract

Sol–gel precursors to HfB2 and ZrB2 are processed by high‐energy ultrasonication of Hf,Zr oxychloride hydrates, triethyl borate, and phenolic resin to form precipitate‐free sols that turn into stable gels with no catalyst addition. Both precursor concentration and structure (a sol or a gel) are found to influence the synthesis of the diboride phase at high temperature. Decreasing sol concentration increases powder surface area from 3.6 to 6.8 m2/g, whereas heat‐treating a gel leads to residual oxides and carbides. Particles are either fine spherical particles, unique elongated rods, and/or platelets, indicating particle growth with directional coarsening. Investigation of the conversion process to ZrB2 indicates that a multistep reaction is likely taking place with: (1) ZrC formation, (2) ZrC reacts with B2O3 or ZrC reacts with B2O3 and C to form ZrB2. At low temperatures, ZrC formation is limiting, while at higher temperatures the reaction of ZrC to ZrB2 becomes rate limiting. ZrC is found to be a direct reducing agent for B2O3 at low temperature (~1200°C) to form ZrB2 and ZrO2, whereas at high temperatures (~1500°C) it reacts with B2O3 and C to form pure ZrB2.