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.