A substrate of nickel coated with a thin layer of silver and further coated with a thin protective layer of silicon dioxide has been found to constitute an optically reflective substrate/coating system that is suitable for use at high temperature in vacuum. This system was developed originally for use on space solar dynamic and solar thermal power systems and, in particular, on secondary solar-radiation concentrators, which are expected to operate at temperatures in the range of 400 to 800 °C. This substrate/coating system might also be applicable to hot optical components of terrestrial solar electric-power systems, solar-powered wastewater-treatment systems, solar-powered fiber-optic illumination systems, and laboratory optical instruments.
The predecessors to this substrate/coating system include a pure silver substrate diamond-turned to a mirror finish, a pure copper substrate coated with silver, and a single-crystal sapphire substrate coated with silver. After heating to 450 °C, the pure silver substrate exhibited severe grain-boundary growth, which rendered the surface diffuse. After heating to 420 °C, the copper-substrate/silver system exhibited diffusion of silver into the copper, rendering the surface layer silver poor. The sapphire/silver system exhibited excellent reflectivity characteristics when heated to 530 °C, but this system was deemed impractical because sapphire would be difficult to form into the shape of a secondary concentrator.
Silver has long been the reflective coating material of choice for use at room temperature. The present SiO2-on-Ag-on-Ni substrate/coating system retains its reflectivity at high temperature, at least partly because with nickel as the substrate, little or no silver diffuses into the substrate. Moreover, the nickel substrate can readily be machined to a desired shape and can be either metallurgically polished or diamond-turned to a mirror finish.
Initial samples of this substrate/coating system were fabricated by polishing nickel substrates to a mirror finish, followed by electron-beam evaporation (alternatively, sputtering could be used) to deposit silver to a thickness of 1,000 Å, followed by deposition of SiO2 to a thickness of 1,000Å. During a test of the first sample, the specular reflectivity was found not to change appreciably as the sample was heated from room temperature to almost 500 °C (see figure). Microscopic examination of a sample that had been heated to 798.5 °C revealed many small grains - too small to affect the reflectivity.
The smallness of the grains may be a fortuitous result of the thinness of the silver layer. It has been conjectured that the sizes of the grains might increase with the thickness of the silver. Thus, there might be an upper limit to the desired silver thickness, beyond which the specular reflectivity could be degraded by the formation of larger grains. Apparently, the upper limit is greater than the 1,000 Å of the initial samples. Optionally, one might select greater thicknesses of silver to promote the formation of larger grains in order to obtain diffusely reflective high-temperature coating/substrate combinations for use in optical components (e.g., integrating spheres) for capturing and diffusing light at high temperatures.
This work was done by Donald A. Jaworske of Lewis Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Materials category,or circle no. 107 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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