Structural Influence on the Thermal Conversion of Self‐Catalyzed HfB 2 / ZrB 2 Sol–Gel Precursors by Rapid Ultrasonication of Oxychloride Hydrates

Sol–gel precursors to HfB 2 and ZrB 2 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 m 2 /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 ZrB 2 indicates that a multistep reaction is likely taking place with: (1) ZrC formation, (2) ZrC reacts with B 2 O 3 or ZrC reacts with B 2 O 3 and C to form ZrB 2 . At low temperatures, ZrC formation is limiting, while at higher temperatures the reaction of ZrC to ZrB 2 becomes rate limiting. ZrC is found to be a direct reducing agent for B 2 O 3 at low temperature (~1200°C) to form ZrB 2 and ZrO 2 , whereas at high temperatures (~1500°C) it reacts with B 2 O 3 and C to form pure ZrB 2 .