Synthesis and formation mechanism of nanocrystalline ZrB₂–Al₂O₃ composite powders via an amorphous precursor

ZrB₂–Al₂O₃ composite powders were synthesized at 1100 °C using a novel ZrB₂ precursor and Al powders as raw materials. The final ZrB₂–Al₂O₃ composite powders consisted of submicron Al₂O₃ and nanosize ZrB₂ (50–100 nm) particles, which were homogeneously mixed in microscale. Combined with thermodynamic calculation and experiment results, the formation mechanism of ZrB₂–Al₂O₃ composite powders was proposed as follows: ZrB₂ precursor first decomposed into ZrO₂ and amorphous B₂O₃. Aluminothermic reduction of ZrO₂ and B₂O₃ generated Zr and B atoms and the coproducts Al₂O₃, and then, a series of reactions between Zr atoms, B atoms and Al took place to form ZrB₂ and Al₃Zr. Then, ZrB₂, Al₂O₃ and Al were obtained through a liquid–solid reaction between Al₃Zr and B₂O₃, which is the limiting step in the conversion process. When the Al₃Zr was exhausted, the reaction between Al, ZrO₂ and B became the main reaction to obtain ZrB₂ and Al₂O₃.