Sintering of aluminum nitride with Y2O3 by secondary phase composition control

Aluminum nitride ceramics are in high demand for applications involving substrate materials for high speed, high capacity, and complex integrated circuits requiring high thermal conductivity. Hot pressing and liquid-phase sintering are the most common processing techniques used to fabricate AlN ceramics. Liquid-phase sintering involves the use of additives to enhance the sinterability, and in the case of AlN, the thermal conductivity. The function of the additives is to react with an oxide layer on the powder surface and form a liquid phase. The presence of this liquid phase aids densification and the removal of oxygen from the AlN lattice, the latter results in significant thermal conductivity enhancement.Liquid-phase sintering of AlN with $ rm Y sb2O sb3$ as an additive has been studied to control the formation of the secondary phases. Using the $ rm Y sb2O sb3$-$ rm Al sb2O sb3$ phase equilibria, three liquid-phase sintering compositions occurring at the eutectic points and one composition close to that of $ rm3Y sb2O sb3 5Al sb2O sb3$ (Yttrium Aluminum Garnet, YAG) were chosen. Sintering was performed in the temperature range of 1750-1950$ sp circ$C for 1 hour under a N$ sb2$ atmosphere to optimize the sintering temperature for each composition. Full densification of the AlN was achieved for all four compositions sintered at 1900$ sp circ$C.Investigation of the sintering kinetics was performed in a AlN/BN powder bed and on a BN setter at 1900$ sp circ$C for up to 4 hours. The kinetic study results did not agree with the classical and recently proposed liquid-phase sintering models due to the different mechanisms of liquid phase formation, and a difference in the chemical composition and viscosity of the various secondary phases.The microstructural evolution of sintered AlN was studied by SEM and TEM. The thermal conductivity of the fully sintered materials was measured, and the highest value achieved was 148.5 W/mK from the YAG composition due to the absence of the secondary phase film at grain junctions and the superior lattice oxygen absorption of the secondary phase compound to the other compositions.A study of the post-sintering heat-treatment of AlN in various reducing atmospheres was performed to determine the influence on thermal conductivity. Two different schematic models of the reducing reactions were proposed. The results indicate that such a heat-treatment can improve the thermal conductivity of AlN up to nearly 200 W/mK. This is due to further oxygen removal from both the AlN lattice and secondary phase, simultaneously