There are many factors that affect the thermal conductivity of AIN ceramics. In addition to sintering aids, the atmosphere, pressure, type of crucible, sintering temperature, holding time, and other process parameters during sintering also have a significant impact on thermal conductivity. The starting point for selecting additives and process conditions is: 1. It is conducive to removing oxygen impurities and purifying the lattice; 2. The second phase is isolated in the microstructure (not forming a continuous phase at the grain boundary) or completely eliminated.
One way to improve thermal conductivity is to change the additives and process conditions to eliminate the second phase. Kuramoto et al. used tricalcium aluminate 3CaO·Al2O3 (CaA) as an additive. The function of this additive is also to form a liquid phase to promote particle rearrangement, dissolution-precipitation, and grain growth; when the temperature rises to 1800℃, it reaches a density, and its thermal conductivity is 90W/(m·K). At this time, Ca is enriched at the intersection of the grain boundary triangle. If it is heat-insulated at 1800℃, due to the volatilization of the liquid phase and the decrease in Ca content, the material becomes translucent and the thermal conductivity increases to 175W/(m·K). The tricalcium aluminate added in this process has a good effect on removing oxygen impurities in the AIN lattice.
Ueno et al. reported a method of eliminating oxide grain boundary phases using a carbon thermal nitriding process. After sintering the AlN blank with 5% (wt) and 10% (wt) Y2O3 in N2 at 1800℃, it was placed in a graphite crucible and heat-treated in N2 at 1900℃. With the increase of the holding time, the grain boundary phase decreased sharply, and the grains gradually increased. The reason is that in the mixed atmosphere of carbon and nitrogen, the grain boundary phase YAG (Al5Y3O12) or YAM (Al2Y4O9) reacted with carbon and nitrogen for nitridation reduction, and YN was generated in addition to AlN and CO. At high temperatures, YN has high volatility, so Y2O3 also decreases with the generation and volatilization of YN. The surface YN can be treated with water after sintering. The thermal conductivity of this AlN ceramic is as high as 266 W/(m·K).
Control the appropriate amounts of additives to reduce the second phase to an isolated phase or eliminate it, and the thermal conductivity will increase. For example, when CaC2-Y2O3 composite additives are used, with a mass fraction of 3%, hot pressing and sintering at 1800℃ for 4h, AIN ceramics with a thermal conductivity of up to 228 W/(m·K) can be obtained; but when the mass fraction is 2%, the thermal conductivity is as high as 187 W/(m·K).
Controlling the appropriate reducing atmosphere pressure can reduce the oxygen content in the AIN lattice and increase the thermal conductivity. Udagawa et al. hot-pressed and sintered AIN samples with 2% and 4% CaO (mass fraction) added at 1900℃ in a nitrogen atmosphere for 9h; when the gas pressure increased from 100 kPa to 700 kPa, the thermal conductivity of the sintered sample increased from 200W/(m·K) to 266W/(m·K), the reason being that the oxygen content decreased significantly; but further increasing the gas pressure caused the thermal conductivity to drop rapidly again due to the increase in oxygen content.
Edgetech can provide aluminum nitride ceramic products with thermal conductivity ranging from 170-250W/mk.