High performance Al-N cermet solar coatings deposited by a cylindrical direct current magnetron sputter coater

High efficiency Al-N cermet solar selective coatings have been designed using a numerical computer model and deposited experimentally. In numerical modeling calculations, Bruggeman approximations of dielectric function for composite materials were employed. Aluminum is used as a metallic component and AlON as a ceramic component in cermet. Numerical calculation results show that the double cermet layer film structure has the highest photothermal conversion efficiency for the Al-AlON cermet solar selective coatings. The optimized film has a solar absorptance of 0.958 and a hemispherical emittance of 0.035 at 80 degrees C. The optimized film yields two cermet layers with metal volume fractions of 0.143 and 0.275, and with layer thicknesses of 43 and 80 nm, going from the antireflection coating to the infrared reflector layer. In the optimized double cermet layer film the solar radiation is efficiently absorbed internally and by phase interference. Thermal loss is also efficiently reduced by using thin cermet layers with such low metal volume fractions, which are substantially transparent in the main wavelength region of blackbody radiation within the temperature range of interest, 50-200 degrees C. The predicted film structure has been used as a guide for the experimental deposition. The Al-N cermet solar selective coatings with double cermet layer film structure have been deposited onto batches of solar collector tubes using a commercial-scale cylindrical dc magnetron sputter coater. Two Al-N cermet solar absorber layers are deposited by de reactive sputtering in a gas mixture of argon and nitrogen. The different metal volume fractions in the cermet layers are achieved by changing the reactive nitrogen gas flow rate, while the sputtering current is fixed. A solar absorption of 0.96 and a normal emittance of 0.08 at 80 degrees C have been achieved for deposited Al-N cermet solar coatings. The baking process, for Ih at 400 degrees C in vacuum, hardly affects solar absorptance, however the emittance reduces about 0.01 at 80 degrees C. These high-performance low-cost Al-N cermet solar collector tubes may be a less-polluting replacement for black chromium solar collectors produced by electrochemical methods. (C) 1999 American Vacuum Society. [S0734-2101(99)05205-7].http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000082596600069&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=8e1609b174ce4e31116a60747a720701Materials Science, Coatings & FilmsPhysics, AppliedSCI(E)16ARTICLE52885-28901