Effects of reduction of the Al-Y-O containing secondary phases during sintering of AlN with YF3 additions

The effect of reduction of the Al-Y-O containing secondary phases forming AIN and YN during sintering of AlN(YF3) embedded in AIN powder has systematically been studied The sintering was performed in a graphite furnace at 1880 degrees C. With increased reduction of the secondary phase, the following effects were observed (a) the chemical composition of the secondary phase changed in the order YAG --> YAP --> YAM --> Y2O3 --> YN; (b) reduction of the amount or removal of the secondary phase, (c) decreased sinterability of AIN when mole than 5 wt% YF3 was added, (d) reduction in the lattice oxygen content, and (e) increased thermal conductivity. Included in the paper is a discussion of a possible mechanism for the I eduction of the secondary phase, which we propose is controlled bl, the diffusion of gas species (CO/CO2 and/or Al2O) through the powder bed and the distance between the AIN samples and a graphite sour ce, as well as a proposed optimal sintering programme for AIN with Al-Y-O containing phases.