Processing and Thermal Conductivity of Bulk Nanocrystalline Aluminum Nitride

Producing bulk AlN with grain sizes in the nano regime and measuring its thermal conductivity is an important milestone in the development of materials for high energy optical applications. We present the synthesis and subsequent densification of nano-AlN powder to produce bulk nanocrystalline AlN. The nanopowder is synthesized by converting transition alumina (delta-Al2O3) with <40 nm grain size to AlN using a carbon free reduction/nitridation process. We consolidated the nano-AlN powder using current activated pressure assisted densification (CAPAD) and achieved a relative density of 98% at 1300 degrees C with average grain size, <(d)over bar>similar to 125 nm. By contrast, high quality commercially available AlN powder yields densities similar to 75% under the same CAPAD conditions. We used the 3-omega method to measure the thermal conductivity, kappa of two nanocrystalline samples, 91% dense, (d) over bar = 110 nm and 99% dense, (d) over bar = 220 nm, respectively. The dense sample with 220 nm grains has a measured kappa = 43 W/(m.K) at room temperature, which is relatively high for a nanocrystalline ceramic, but still low compared to single crystal and large grain sized polycrystalline AlN which can exceed 300 W/(m.K). The reduction in k in both samples is understood as a combination of grain boundary scattering and porosity effects. We believe that these are finest (d) over bar reported in bulk dense AlN and is the first report of thermal conductivity for AlN with <= 220 nm grain size. The obtained kappa values are higher than the vast majority of conventional optical materials, demonstrating the advantage of AlN for high-energy optical applications.