High thermal conductivity ceramics from combustion synthesized AlN powder through microwave sintering and reheating

Combustion synthesized AlN powders were investigated for use as starting material for obtaining high thermal conductivity specimens by microwave sintering and reheating. Microwave sintering and reheating were carried out in a TE ₁₀₃ single mode cavity with an adjustable microwave power in the range 0-3 kW at 2.45 GHz. Densification was found to be a primary requirement to obtain a high thermal conductivity AlN. AlN powders with good sinterability are required to achieve a high densification. AlN powder with D ₅₀ of ∼6 μm was found to have a poor sinterability that with D ₅₀ of ∼3 μm or finer, a good sinterability. Oxygen content was found to be another important factor determining the thermal conductivity. The thermal conductivity can be enhanced from ∼130 to ∼155 W/m K when the oxygen content is reduced from 2.3 to 1.4 wt %. The thermal conductivity can be significantly improved by microwave reheating of the sintered specimen under a reducing atmosphere. This is considered to be due to enhanced removal of the second phases by the reducing atmosphere.