Additive sintering process in aluminium nitride ceramics

Calcium compounds and yttrium compounds are frequently used as additives to densify AlN ceramics. They react with oxygen impurities, such as Al2O3 in the raw AlN powder and in aluminate forms. A phase diagram is helpful in deciding the amount of additives to add, but the AlN sintering process doesn't come to equilibrium. In this paper, the authors investigate the differences between sintering AlN with CaCO3 and with Y2O3. With CaCO3 added, the calcium-aluminate liquid phase evaporates from the sintered AlN body at high sintering temperatures. The AlN thermal conductivity doesn't increase above a sintering temperature of 1700 degrees C because residual oxygen substitutes into the AlN grains. With Y2O3 added, AlN sinters below 1850 degrees C, as predicted by a phase diagram. With a large amount of Y2O3 added, 5 wt.%, thermal conductivity doesn't depend on residual oxygen. This is because yttrium doesn't evaporate from the sintered AlN body and residual oxygen substitutes into AlN grains at an early stage of the AlN sintering