Infrared Transparent Nanocrystalline Alumina

This SBIR program will produce nanocrystalline, dense a-Al2O3 samples with high IR transparency by utilizing novel processing techniques starting with properly seeded transition alumina powders. Alpha alumina does not exist naturally in the environment and is synthesized by a number of various processes. Many of these processes generate wide particle size distributions and are generally agglomerated. If a homogeneous microstructure is not achieved, higher sintering temperatures are required to eliminate the porosity, compromising the fine grain size of the final product. The addition of seed particles, which are isostructural with a-Al2O3, provide low energy sites for heterogeneous nucleation to occur and reduce the energy barrier required for nucleation. Uniform microstructures of 100 nm or less a-Al2O3 can be achieved after the phase transformation with proper seeding. Gel-casting will be used to produce complex ogive shapes with green densities > 55%, which will be subsequently sintered to full density at lower temperatures than die pressed samples. A rapid microwave sintering technique will demonstrate that fine nanostructures can be achieved by increasing the densification kinetics and reducing the time at which grain growth can occur.