AC Impedance Spectroscopy of CaF^sub 2^-doped AlN Ceramics

The electrical conductivity of CaF^sub 2^-doped aluminum nitride (AlN) ceramics was characterized at high temperatures, up to 500°C, by AC impedance spectroscopy. High thermal conductive CaF^sub 2^-doped AlN ceramics were sintered with a second additive, Al^sub 2^O^sub 3^, added to control the electrical conductivity. The effects of calcium fluoride (CaF^sub 2^) on microstructure and related electrical conductivity of AlN ceramics were examined. Investigation into the microstructure of specimens by TEM analysis showed that AlN ceramics sintered with only CaF^sub 2^ additive have no secondary phases at grain boundaries. Addition of Al^sub 2^O^sub 3^ caused the formation of amorphous phases at grain boundaries. Addition of Al^sub 2^O^sub 3^ to CaF^sub 2^-doped AlN ceramics at temperatures 200°C-500°C revealed a variation in electrical resistivity that was four orders of magnitude larger than for the specimen without Al^sub 2^O^sub 3^. The amorphous phase at the grain boundary greatly increases the electrical resistivity of AlN ceramics without causing a significant deterioration of thermal conductivity. [PUBLICATION ABSTRACT]