Morphological effect of second phase on the thermal conductivity of AlN ceramics

The effect of the morphology of second phases in sintered microstructure on the thermal conductivity of AIN ceramics was investigated. When an Y2O3-doped AlN specimen was cooled down slowly at a rate of 3 degrees C/min after sintering at 1850 degrees or 1900 degrees C, the second phases were concentrated in the corners of the AlN grains by an increase of dihedral angle during cooling, On the other hand, the fast-cooled specimen at a rate of 60 degrees C/min showed a different structure of the second phases interconnected through the triple-grain junctions, The specimen with isolated second-phase morphology showed a higher thermal conductivity than those with interconnected second-phase morphology, The measured thermal conductivity of the specimens with different morphologies of the second phases agreed well with the calculated one derived from modeled microstructures. From the comparison of the measured and calculated thermal conductivity, it was shown that the thermal conductivity of the specimen with interconnected second-phase morphology decreased steeply with an increase of the amount of the second phases, assuming the content of lattice oxygen to be constant, However, the thermal conductivity of the specimen with isolated second-phase morphology was rather insensitive to an increase of the amount of the second phases.