EFFECT OF CAO ON THE THERMAL-CONDUCTIVITY OF ALUMINUM NITRIDE

The effect of CaO on the thermal conductivity of aluminum nitride pres sureless sintered with 3 wt% Y2O3 as a sintering aid was investigated. Over the composition range of 0 to 2.0 wt% additions, CaO decreased t he thermal conductivity of the sintered parts by 10%. CaO doping rende red the secondary oxide phases more wetting and thus with a greater te ndency to penetrate along the grain boundaries. Furthermore, CaO segre gation to the grain boundaries was observed even on those grain bounda ries apparently free of secondary phases. These microstructural change s disrupted the connectivity of the high thermal conductivity AlN grai ns and were the main factors contributing to the decrease in the therm al conductivity of the ceramic parts. CaO additions to samples doped w ith SiO2 had the opposite effect, increasing the thermal conductivity. CaO removed SiO2 from the AlN grains and incorporated it into the oxi de second phases, most likely through charge-compensating substitution s Ca2+ + Si4+ for Y3+ and/or Al3+. Thus, AlN samples containing both S iO2 and CaO had higher thermal conductivity than those containing comp arable amounts of SiO2 alone.