Characterization of new and degraded mirrors with AES, ESCA and SIMS

Abstract

Samples representing each stage of second surface mirror production were examined with AES, ESCA and SIMS, in combination with ion sputtering. Finished mirrors exposed to the exterior environment were also analyzed. Significant weathering had already occurred on a 6 month old soda-lime-silicate glass that was intended for use in the Barstow solar pilot plant. Such weathering may adversely affect the quality of mirrors produced from this glass. Further glass-water reactions occur during the silvering step, which was demonstrated by the detection ofdeuterium into the glass on mirrors made from D 2O silver solutions. In addition, diffusion of silver into the glass matrix was found, even on freshly prepared mirrors. Diffusion effects were separated from sputter broadening by a comparison of depth profiles of wet process deposited silver on float glass and electron beam deposited silver on quartz. Copper and silver layers were intermixed to a greater extent than expected from published diffusion or miscibility data. The results are consistent with a model of copper and silver layers existing as microcrystalline islands, having dimensions smaller than the spatial resolution of the techniques used, and support recent SEM data. On degraded specimens, sulfur was strongly associated with the reacting edge of the copper films, while chloride was similarly associated with the silver films, implying that these elements are important corrosive agents. The source of the sulfur and chloride may be either the backing paint or airborne particulates. Chloride may also be left from the sensitization step. Initial degradation may involve attack from the paint side on the copper film as well as reaction at the silver-glass interface. Water appears to accelerate the degradation.