Relocation of secondary phases and oxygen-related defect of AlN ceramics

Abstract

Aluminum nitride (AlN) is promising as a thermal management material due to its excellent thermal conductivity and high electrical resistance. In this work, we attempt to elucidate the effect of the residual oxygen of raw materials and secondary phases on the thermal conductivity of sintered AlN ceramics. We found that as the annealing temperature and time increase, the secondary Y-Al-O phases migrate toward the triple junction from the grain boundary, which enhances the thermal conductivity, as well as the diffusion-out effect of the oxygen-related defects. When the residual oxygen of the raw materials is present as Al2O3with Ohsymmetry (Õ1.9 at%), the secondary phases of the annealed AlN are found to be thermodynamically stable YAM (yttrium aluminum monoclinic) and YAP (yttrium aluminum perovskite), which is consistent with the Al2O3-Y2O3binary phase equilibria. However, if the residual oxygen is located within tetrahedral AlN lattice ((Formula presented) ), a kinetically stable YAG (yttrium aluminum garnet) is observed at a lower annealing temperature (1700 °C), transforming into thermodynamically stable phases at a higher temperature (1750 °C). At higher oxygen concentrations (O ∼ 4.2 at%), the secondary phases are found to be thermodynamically stable YAG and YAP. The migration of such secondary phases toward the triple junction from the grain boundary was systematically verified by a microstructural analysis. This type of migration of secondary phases, which effectively reduces the phonon boundary scattering, was also found to enhance the thermal conductivity of the sintered AlN along with the reduced scattering center ((Formula presented) ) by the diffusion-out effect.