Aero acoustic levitation to unveil the solidification of complex oxides: Case study of the system Al2O3–ZrO2

Abstract

Binary or multi-component phase mixtures are of particular technical importance. Despite the wide field of application of Al2O3 and ZrO2 composites, the literature reveals contradictions about the liquidus region. Containerless processing avoids typical problems of conventional techniques like melt contamination or lack of suitable container materials. Composition ranging from 0–80 mol% ZrO2 were melted and solidified in an aero acoustic levitator. A high-speed camera recorded the crystallization, allowing a direct correlation with the observed microstructures. Due to homogeneous nucleation, all samples exhibited significant undercooling before crystallization. The nucleation temperature of the different compositions roughly followed the slope of the liquidus line. Undercooling, solidification kinetics, and local reheating played a considerable role in microstructure formation. A lamellae eutectic microstructure was created over a large compositional range from 36–48 mol% ZrO2. On the hypoeutectic side, the effect was attributed to solidification within the coupled zone. However, hypereutectic samples also exhibit a zirconia-rich eutectic as well. We assume impaired ZrO2 nucleation or growth favors alumina precipitation from the melt. Aero acoustic levitation proved to be an excellent technique for studying complex high melting oxide systems, enabling a better understanding of their crystallization process.