Diamond and Sapphire for Infrared Windows and Domes

摘要

Chemical-vapor-deposited diamond is being developed as a durable bulk window material and as a coating material. Clear windows up to 1 mm thick and 60 mm diameter have been produced. Domes up to 1.5 mm thick and 75 mm in diameter have been grown and polishing is in progress. Methods have been demonstrated for attaching diamond coatings to optical windows. At 10.6 mum wavelength, the absorption coefficient is as low as 0.1 to 0.3 cm~(-1), optical scatter is below 1percent, and emissivity at 500deg C is below 3percent. Microwave dielectric properties, thermal properties, and most mechanical properties (except strength) of chemical-vapor-deposited diamond are equivalent to those of Type IIa natural diamond. Sapphire is the most durable, commercial window material. It has marginal thermal shock resistance for the most demanding missions because its strength falls rapidly at elevated temperature. A study of sapphire led to the serendipitous discovery that its strength above 400deg C is limited by weakness in compression, not tension. Compression on the c-axis of the crystal produces twins on rhombohedral crystal planes. When twins intersect, fractures form and mechanical failure results. Understanding the failure mechanism makes possible a research program to try to improve high-temperature strength.