Effect of extended annealing cycles on the thermal conductivity of AlN/Y2O3 ceramics

The effect of extended annealing cycles (up to 50 h at 1800°C) on the thermal conductivity of polycrystalline AlN, doped with 5 wt% Y2O3, has been studied. The microstructural evolution upon annealing has also been characterized in detail, using quantitative scanning electron microscopy (SEM) observation and energy dispersive X-ray analysis (EDX). As-sintered AlN/Y2O3 composites typically contained a dilute yttrium aluminate secondary phase well distributed and completely wetting the AlN grains. Upon annealing, the AlN matrix grains isotropically grew, while the grain-boundary yttrium aluminate phase tended to segregate to triple grain junctions. This segregation process produced a collapse of the grain-boundary film thickness, thus resulting in a completely different AlN microstructure dispersed with isolated yttrium aluminate grains. Equilibrium of the microstructural morphology was achieved after annealing times in the interval 5–10 h. As a consequence of microstructural changes, the thermal conductivity of the annealed AlN polycrystal exceeded that of the as-sintered material. A discussion is given about the variation of thermal properties in terms of both segregation to the triple-grain junctions of the intergranular Y2O3-phase and grain-growth of the bulk AlN grains.