Reflectivity of silver and silver-coated substrates from 25°C to 800°C [for solar collectors]

A bench top facility was used to evaluate the reflectivity of several candidate solar thermal power system coating-substrate combinations in vacuum at elevated temperatures. Silver was selected as the reflective coating of choice, while copper, nickel, electroless nickel on copper, and 304 stainless steel were selected as substrates. Pure silver, with no coating at all, was also evaluated. An optically flat silver-coated sapphire substrate was used as a standard. All metal substrates were either metallurgically polished or diamond turned to a mirror finish prior to silver deposition. Silicon dioxide was used as a protective coating in most cases. Reflectivity measurements were made at room temperature in the visible range with a spectrophotometer, and at elevated temperatures up to 800°C with a helium-neon laser at 632 nm. Results from the high temperature reflectivity measurements are presented