Effective Sintering Aids for Low-temperature Sintering of AlN Ceramics

A disappearing sintering aid was used to promote densification during the initial and middle stages of sintering and to be removed in gaseous form from the specimens during the final stage of sintering. From thermodynamic consideration such as assessment of Gibbs free energy change of formation of Al₂O₃ compounds including metal-oxide and evaluation of the vapor pressure of metal-oxide, Li₂O is expected to become a disappearing sintering aid for AlN sintering. Doping with Li₂O resulted in densification of AlN ceramics with Y₂O₃ and CaO additives by sintering at a firing temperature of 1600 °C. The amount of Li₂O in the specimens decreased by volatilization at temperatures higher than 1300 °C, and its amount was at a level of several ppm after firing at 1600 °C for 6 h. Low-temperature densification of AlN specimens by addition of Li₂O also caused the improvement of thermal conductivity and mechanical strength of sintered specimens. Present results indicate that a Li₂O addition is effective for AlN sintering. Furthermore, LiYO₂ was also used as a new sintering aid instead of Li₂O and Y₂O₃, and the results of thermal conductivity and mechanical strength are shown.