Solar thermal electric power systems use large solar reflectors to concentrate sunlight to generate electricity. The economic viability of these systems depends on developing a durable, low-cost reflector. The goals for such a reflector are specular reflectance above 90% for at least 10 years under outdoor service conditions and a large-volume manufacturing cost of less than $10.8/m2($1.00/ft 2 ). Currently, the best candidate materials for solar reflectors are silver-coated, low-iron glass and silvered polymer films. Polymer reflectors are lighter in weight, offer greater system design flexibility, and have the potential for lower cost than glass reflectors. A promising lowcost reflector consists of a silvered polymer protected by an optically transparent alumina coating.The coating is deposited by an ion-beam-assisted physical vapor deposition (IBAD) technique.Samples of this reflector have maintained high optical performance in accelerated testing at the National Renewable Energy Laboratory for more than 3000 hours. Solar reflectors produced using this technique may represent an opportunity to bring solar power generation to reality.

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This article reports results from a study on the effect bulk impurities in MgO substrates may have when substrates are annealed to improve film microstructure and transport properties. DyBaCuO thin films are prepared by molecular beam reactive coevaporation on both annealed and unannealed MgO(100). While predeposition annealing has led to improved film quality for some groups, under our conditions just the opposite occurs. We point out the existence of Ca segregation to the MgO surface as a factor that needs investigation because Ca is a common contaminant in commercially supplied MgO. Atomic force microscope images show decoration of annealed substrates with submicron size particles that may be composed of the segregated Ca. These particles seem to underlie the distinct differences seen in the surface microstructure of films on annealed and unannealed substrates. In the case of annealed substrates a more random microstructure is promoted. Under identical growth conditions, outgrowths present in films on unannealed (mechanically polished) substrates are oriented in an ordered pattern. These findings suggest that the expected benefits from predeposition annealing can be negated when significant surface segregation of bulk contaminants occurs.

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