The Effect of Thermal History on the Structure of Chemically and Vapor Deposited Silver Films on Glass
The observation1 of silver agglomeration in second surface mirrors used for solar applications has emphasized consideration of the effect of thermal history on the optical properties of mirrors. Thermal history effects may arise from the processing of mirrors, the application of protective coatings, or from outdoor exposure. Mirrors may be subject to elevated temperatures (T≤400°C) for short periods of time, or to low temperatures (T≤60°C) for long (≤30 years) periods of time. Although a significant amount of work has been done on thermally driven agglomeration of silver films,2–7 most of these studies have been restricted to vapor deposited films on vitreous silica. Large area reflectors, such as those used in heliostats, will almost certainly be deposited by commercial chemical methods on substrates of soda-lime-silicate or other glasses which differ considerably from vitreous silica in composition and properties. The present study addresses the effect of this change in deposition technique and substrate on silver agglomeration. These problems were studied by optical and scanning electron microscopy, refleetometry, and x-ray diffraction. The results indicate that both the method used to deposit the silver and the type of glass affect the agglomeration process and the character of the reflective film.
KeywordsThermal History Silver Film Outdoor Exposure Silver Powder Float Glass
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