MICROSTRUCTURE AND THERMAL-CONDUCTIVITY OF ALN(Y2O3) CERAMICS SINTERED IN DIFFERENT ATMOSPHERES

Thermal conductivity, microstructure and oxygen content in the AlN lattice of AlN (Y2O3) ceramics, made of different AlN powders and prepared by either pressureless sintering in a graphite-heated furnace (type I samples) or sintered in a graphite-free furnace (type II samples), have been investigated. In the type I samples the secondary phases are present as a continuous network along the AlN grain edges, whereas they form isolated lumps in the type II samples. For these two types of microstructure simple models are presented which allow calculation of thermal conductivity as a function of the oxygen content dissolved in the AlN lattice and as a function of microstructural parameters (amount of secondary phases, thickness of layers between neighbouring AlN grains). The calculated conductivities are in good agreement with the measured values, the deviations found are typically < 15 W/(m K). As a result, it is shown that the oxygen content dissolved in the AlN lattice has the dominating influence in AlN ceramics having an amount of secondary phases < 10 vol.% and amorphous layers with a thickness of typically 2-4 nm between neighbouring AlN grains.