Growth mechanism of AlN crystals via thermal nitridation of sintered Al₂O₃-ZrO₂ plates

The crystal growth of AlN from aluminum oxides was studied using a thermal nitridation method. Four types of aluminum oxides, sintered Al₂O₃, ZrO₂-containing sintered Al₂O₃, and a- and c-plane sapphires, were used as a source material. As observed, millimeter-sized AlN crystal grains were successfully grown from the ZrO₂-containing sintered Al₂O₃ only at temperatures ranging from 2223 to 2323 K. The growth mechanism, including the role of ZrO₂ additive, was discussed from a thermodynamic viewpoint. The following growth model was proposed: predominant nitridation of ZrO₂ in Al₂O₃ suppresses Al₂O₃ nitridation, and the ZrO₂-Al₂O₃ liquid phase forms, which promotes the formation of Al₂O(g) and Al(g) from Al₂O₃. These Al-based gases react with CN(g) and/or N₂(g) to form AlN crystals on the Al₂O₃-ZrO₂ plate.