Reflectance and durability of Ag mirrors coated with thin layers of Al(2)O(3) plus reactively deposited silicon oxide.

A technique for producing adherent and well-protected front surface Ag mirrors with an evaporated Al(2)O(3) + SiO(x) overcoat and an evaporated Al(2)O(3) underlayer is described. A thin Al(2)O(3) layer promotes adhesion between the Ag film and its substrate. The optimum thickness of each outer layer necessary to give good adhesion and protection to the Ag surface with minimal loss in reflectance due to ir absorption was found to be about 300 A for the Al(2)O(3) layer and between 1000 A and 2000 A for the SiO(x) film. Ag surfaces coated in this fashion retained a normal incidence reflectance in excess of 95% over the wavelength region from 450 nm to the far ir, even when exposed to harsh sulfide and humidity environments. Calculations demonstrate the advantage of using protected front surface Ag in comparison to more durable metal reflectors, such as Al or Rh, in terms of high reflectance and low visible region polarization. However, a very pronouncedrestrahlen reflectance and absorption effect in the thicker SiO(x) film is responsible for a calculated drop in reflectance from 98.5% to about 65% at lambda = 8.1 mum and 45 degrees incidence for the Ag + Al(2)O(3) + SiO(x) coating.

Thermal emissivity and solar absorptivity of aluminum coated with double layers of aluminum oxide and silicon oxide.

A technique using evaporated Al coated with double layers of Al(2)O(3) and silicon oxide to produce surface films having low solar absorptivity (alpha) and high total normal and hemispherical emissivities (epsilonN and epsilon) is described. High vacuum evaporation with an electron gun was used for preparing undecomposed films of Al(2)O(3) and SiO(2). alpha and epsilonN were determined from reflectance measurements made in the wavelength region from 0.2micro to 50micro. epsilon was measured calorimetrically by a transient thermal method. alpha of all Al + Al(2)O(3) + silicon oxide film combinations was determined to be about 0.12. The greatest increase in epsilonN and epsilon was obtained when Al was first coated with Al(2)O(3) about lambda/4 thick at 10micro and then overcoated with 2000 A to 4000 A of silicon oxide. With such film combinations alpha/epsilon values of less than 0.2 could be readily achieved. Surface films of this type were found to be extremely stable during simulated solar uv irradiation.

Solar absorptivity and thermal emissivity of aluminum coated with silicon oxide films prepared by evaporation of silicon monoxide.

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.

Reflectance, Solar Absorptivity, and Thermal Emissivity of SiO(2)-Coated Aluminum.

The reflectance, solar absorptivity (alpha), and the total normal and hemispherical emissivity (epsilonNu and epsilon) of evaporated aluminum coated with SiO(2) films of various thicknesses were determined. High vacuum evaporation with an electron gun was used for preparing uv transparent undecomposed films of SiO(2) up to thicknesses of more than 3.5 micro Because of their hardness, chemical stability, and excellent adherence, evaporated SiO(2) films were found to be very suitable as protective layers for aluminum front surface mirrors, especially if high reflectance in the uv is required. alpha of SiO(2)-coated Al was determined to be about 11 % and to be essentially independent of the SiO(2) thickness, whereas epsilonNu and epsilon increased with increasing oxide thickness, and reached values of 0.62 and 0.55, respectively, for a SiO(2) thickness of 3.75 micro. Films of this type are, therefore, suitable as surface layers for controlling the temperature of satellites in orbit. Ultraviolet irradiation in vacuum at one and five times the equivalent solar energy decreased the uv and visible reflectance of SiO(2)-coated Al. The effect of this reflectance decrease on alpha/epsilon and on the temperature of an orbiting satellite is discussed.