Fabrication of Nanocomposite Ceramics by Crystallization of Rapidly Solidified Eutectic Melts

Eutectic melt solidification is shown to avoid cracking during solidification by quenching an amorphous phase. Subsequent annealing results in micro/nanostructure ceramics. This strategy has been applied to HfO₂–Al₂O₃–GdAlO₃ and Y₂O₃–CaO–Al₂O₃ ternary systems that have deep eutectics. In most cases, ceramic materials cracks when they are solidified from melt due to the thermal stress accumulated on the grain boundaries, the large specific volume difference between the melt and the crystalline solid, etc. The main reason why this strategy works is that a eutectic composition yields an amorphous phase from the melts by rapid cooling and the amorphous phase enables to design crystallization without cracking by postannealing. Appropriate postannealing for the quenched amorphous enables to control the crystallization behavior from the amorphous phase, which yields nanostructured composites without cracking. For the HfO₂–Al₂O₃–GdAlO₃ case, the melt solidification and postannealing yields a nanocomposite with high transparency due to reduced scattering of 5–10 nm crystallites. For the Y₂O₃–CaO–Al₂O₃ case, a plate-shaped bulk composite is obtained without cracking by molding the melt and postannealing.