Thermal-Shock-Resistant Sapphire by Microstructural Design

The proposed study will evaluate the effectiveness of combining polishing and doping to reduce twin nucleation sites and to prevent twin boundary movement in sapphire to improve its thermal shock resistance. Doped and undoped single-crystal sapphire will be grown by the Czochralski method. The crystal surfaces will be mechanically and chemically polished. Dopant diffusion into the surface layer will be performed on doped and undoped samples to introduce compressive stresses on the surface. C-axis compression tests will be performed at 600¿C and 800¿C to determine the effects of surface polishing and surface doping on the formation and growth of rhombohedral twins. The interaction of twin boundaries with local structures on the surface and inside the crystal will be studied via transmission electron microscopy. The additiveness of the proposed strengthening mechanisms will be established. The effect of dopant on the optical properties will be assessed. Infrared transmission and emittance of both undoped and doped samples will be measured to determine the effect of surface doping, precipitate size, and concentration on the IR transparency and the scatter.