Preparation and Properties of Spinel Made by Vapor Transport and Diffusion in the System MgO‐Al ₂ O ₃

In a hydrogen atmosphere, MgO was vaporized by heating MgO (periclase) in the range 1500° to 1900° C, and the vapor products diffused into Al 2 O 3 (sapphire) to form a uniform outer layer of spinel on all surfaces. At 1900°C. a spinel layer 48 mils thick could be obtained in 16 hours. The rates of spinel formation were determined and the activation energy was calculated to be about 100 kcal. per mole. In the spinel layer, the lattice constant, Vickers hardness, and the refractive index varied from the outer surface to the inner boundary in a fairly uniform manner, indicating a continuous change in composition from 1MgO: 1Al 2 O 3 to 1MgO: 2–3Al 2 O 3 . In conversion to spinel a volume increase of 47% over that of the original sapphire took place. Transparent clear shapes of spinel such as disks and rods were obtained from clear sapphire and from translucent polycrystalline alumina. Clear rods of spinel with polished ends acted as thin lenses, round rods as convex lenses, and flat rods as planocylindrical lenses owing to the increase in refractive index from the outer surface to the inner central portion. Objects made of opaque polycrystalline alumina were also converted to spinel. The MgO periclase blocks were etched in the hydrogen atmosphere, and the vapor products of Al 2 O 3 entered the MgO to form tiny spinel droplets in an opalescent border. In an oxidizing atmosphere, spinel was formed only on the surface of the sapphire directly in contact with periclase, in the range 1500° to 1900°C.