Silver, mirror degradation

The state-of-the-art mirror system now in use is a glass second-surface silver mirror backed with copper and paint, as shown in Fig. 2. For this system, the characterization and study of the glass/silver, silver/copper, and copper/paint interfaces before and after various stages of use are clearly required to understand the multilayer mirror stack. The methods of characterization outlined in Sec. 2.4 of Ref. 3, especially those of ISS, XPS, AES, and SIMS, are clearly applicable to this problem. In ter facial degradation reactions may begin at the silver/glass interface... 

Silver, because of its high solar specular reflectance is the best candidate for mirror surfaces in the space station solar dynamic power system(8). Silver is also the most reactive material known toward atomic oxygen. Uncoated silver rapidly degrades on exposure (Fig. 6). P2 coated silver mirrors in contrast, show only a slight loss of solar specular reflectance on exposure of up to one week. The fractional loss of the solar reflectance extrapolated to 1000 hours of ashing time (about forty years in LEO) was only 0.025. 

All telescopes suffer from this thermal background, depending on the temperature of the telescope and its optics. In practice, telescopes with clean and freshly applied mirror coatings (such as silver) have emissivities >1% per surface at wavelengths beyond 1/rm. Of course as the optics degrade with time, dirt, etc. theemissivity will grow. 

We conclude that we are observing reaction at the polymer-silver interface that results in reflectance loss. This interface reaction could result from degradation of the bulk polymer or its additives, initiation at the polymer/mirror interface, or reaction with the ambient atmosphere. 

We have observed mirror failure due to physical delamination and chemical reaction (a) at the mlrror/backing interface apparently due to inadequate protection from the atmosphere by the backing and, (b) at the polymer/mirror Interface due to reaction with a light-sensitive antioxidant or possibly with the degrading polymer. For both silver and aluminum mirrors, interfacial reactions were only observed at the back surface of the mirror regardless of how the mirror was put down or assembled. 

Placing the reflecting surface in vacuum eliminates degradation of the mirror s reflectance, thus permitting high-quality (silver or aluminum with reflectance p — 0.91-0.96) first-surface mirrors to be used instead of anodized aluminum sheet metal or thin-film reflectors (p = 0.80-0.85) typical of the external reflector designs. 

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