ABSTRACT. AIN is a high thermal conductivity material and has received much interest in the electronics industry because of the need for smaller and more reliable integrated circuits and the need for higher voltage devices for power applications. Conventionally, AIN is fabricated by hot pressing, hot isostatic pressing and sometimes, pressureless sintering with post heat treatment. All these processes require high processing temperature and long processing time. The use of microwave energy as an alternative to conventional heating methods shows great promise. Microwave heating is instantaneous, selective, and volumetric. In addition, microwave sintering has been shown to have the potential of processing materials with improved properties over those conventionally processed.

In this work, we report results of high thermal conductivity AIN sintered by a pressureless microwave process at 1820^oC for 60 minutes. A thermal conductivity of 179 Wm^-1K^-1 was achieved. Further annealing in a conventional furnace at 1820^oC for 60 minutes increased the thermal conductivity to 205 Wm^-1k^-1. To our knowledge, this is the first thermal conductivity measurement that is reported for microwave processed AIN. The effect of sintering time on phase composition, thermal conductivity, and densification behavior was investigated. Microstructure studies and lattice parameter determination showed that thermal conductivity increases with increasing sintering time. We believe that this is primarily due to a decrease in oxygen content in the AIN lattice as more of the secondary phase forms in the grain junctions and getters oxygen. The results indicate that oxygen plays a more important role in affecting thermal conductivity of AIN than porosity and secondary phase contents when samples are close to full density.

This research was supported by the Office of Naval Research/Naval Research Laboratory, and in part by the Air Force Office of Scientific Research, Ceramics and Materials Program.

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