MICROSTRUCTURAL EFFECTS ON THE THERMAL-CONDUCTIVITY OF POLYCRYSTALLINE ALUMINUM NITRIDE

Polycrystalline AlN bodies were made with a range of porosities from various AlN powders by sintering or hot-pressing. Thermal conductivity data for material produced without sintering aids showed a gradual, yet definite, porosity dependence with scatter similar to other property-porosity studies. The thermal conductivity-porosity data for AlN with sintering aids showed the existence of two distinct regions: (1) a higher-porosity region (greater than approximately 2%) which was similar to the data for material without sintering aids, and (2) a low-porosity region where grain boundaries were seen to dominate thermal conductivity. Auger spectroscopy was used to investigate fracture surfaces for dense CaO-doped materials. Thermal conductivity was observed to correlate to effective grain-boundary thickness, which was calculated from quantitative analysis of the Auger data. A model consisting of cubic grains in a continuous grain-boundary phase accurately describes these data.