Solar Absorptivity and Thermal Emissivity of Aluminum Coated with Silicon Oxide Films Prepared by Evaporation of Silicon Monoxide

Abstract

The solar absorptivity (alpha) and the total normal and hemispherical emissivities (? (n) and ?) of vacuum deposited Al coated with silicon oxide films prepared by evaporation of SiO were determined. For Al coated with true SiO films evaporated at 5 x 10(-7) Torr and deposited at rates >30 A/sec, both and e increase but alpha/? decreases more rapidly with increasing SiO thickness. Such coatings were found to be very stable under simulated space conditions, but they should not be used as temperature control coatings since their alpha, ? and alpha/? values are for most applications too high. The proper way to produce silicon oxide films on Al for temperature control coatings is reactive evaporation of SiO in the presence of oxygen followed by an uv treatment in air. Aluminum surfaces coated with such films have predictable alpha values of 11.0%-11.5% which remain essentially independent of the silicon oxide thickness. By increasing the thickness of reactively deposited silicon oxide on Al from zero to 2.96 micro, ? increases from 0.017 to 0.53, and alpha/? decreases from about 5 to 0.2. Such coatings have been successfully used as temperature control surfaces on many satellites, and there are ample laboratory and flight data to assure their high stability in space environment.