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 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 B₂O ₃ 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 B ₂O₃ at low temperature (∼1200°C) to form ZrB ₂ and ZrO₂, whereas at high temperatures (∼1500°C) it reacts with B₂O₃ and C to form pure ZrB₂.