Abstract: A method for producing a transparent polycrystalline magnesia-alumina spinel body having high ultraviolet transmissivity and high strength and hardness characteristics is provided. A closed porosity body is formed from a spinel powder preferably having at least minimum purity characteristics. The closed porosity body can be formed by techniques including hot pressing and pressureless sintering. The closed porosity body is subjected to a hot isostatic pressure treatment to reduce residual porosity. The resulting body has high transmissivity of electro-magnetic radiation in the range between about 0.2 microns and about 6 microns and is resistant to abrasion or erosion, has high strength, and does not substantially deteriorate after exposure to ultraviolet light.

Abstract: In accordance with the invention there is provided a heat exchanger for transferring heat from the exhaust gases of a turbine engine to the intake air for the engine, the heat exchanger comprising a rotatable cylindrical ceramic body having axially extending passages therethrough for passage of the exhaust gases in one direction on one diametric half of the body and for passage of the intake air in the other direction on the other diametric half of said body, one axial end of the body thereby being at a higher temperature than the other axial end of the body, an annulus of ceramic bordering a cylindrical surface of the body and having a radially extending annular flange bonded to the cylindrical surface of the body at the low temperature axial end thereof, there being an annular gap between the ceramic annulus and the cylindrical surface of the body at the high temperature axial end thereof and an annulus of resilient sealing material in the annular gap.