Effect of Y2O3 content on sintering of aluminum nitride

Liquid-phase sintering of AlN with Y2O3 as an additive has been studied to control the formation of the secondary phases. Using the Y2O3-Al2O3 phase equilibria, three liquid-phase sintering compositions occurring at the eutectic points and one composition close to that of 3Y2O3.5Al2O3 (YAG) were chosen. Sintering was performed in the temperature range of 1750-1950 °C for up to four hours under a N2 atmosphere to optimize the sintering conditions for each composition. The relative density exceeded 95% for all the compositions sintered at 1850 °C and full densification was achieved for all four compositions sintered at 1900 °C. Furthermore, AlN sintered without Y2O3 at 1900 °C for 1 hour resulted in a high relative density of >99%. The high density obtained from material without additive suggests the formation of an Al-O-N liquid and that of yttrium aluminum oxynitride for those composition containing Y2O3. The latter transforms to various aluminates upon cooling to room temperature. The microstructure and assemblage of the secondary phase have a significant effect on the final thermal conductivity of the sintered AlN.