Abstract
The effect of CaO on the thermal conductivity of aluminum nitride pressureless sintered with 3 wt% Y2O3 as a sintering aid was investigated. Over the composition range of 0 to 2.0 wt% additions, CaO decreased the thermal conductivity of the sintered parts by 10%. CaO doping rendered the secondary oxide phases more wetting and thus with a greater tendency to penetrate along the grain boundaries. Furthermore, CaO segregation to the grain boundaries was observed even on those grain boundaries apparently free of secondary phases. These microstructural changes disrupted the connectivity of the high thermal conductivity AIN grains and were the main factors contributing to the decrease in the thermal conductivity of the ceramic parts. CaO additions to samples doped with SiO2 had the opposite effect, increasing the thermal conductivity. CaO removed SiO2 from the AIN grains and incorporated it into the oxide second phases, most likely through charge‐compensating substitutions Ca2++ Si4+ for Y3+ and/or Al3+. Thus, AIN samples containing both SiO2 and CaO had higher thermal conductivity than those containing comparable amounts of SiO2 alone.
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