Durability Of Mirrors Research Articles

Durability Assessment of Silvered Glass Mirrors Exposed in Two Different Desert Sites Suitable for CSP Installations

The durability of thick solar glass mirrors has been evaluated in this study by exposing samples at two potential exposure sites. Samples have been exposed for a period of 18 months at different orientations (North, South, East, and West) to evaluate the impact of orientation on the durability. The samples performance has been evaluated by measuring the specular reflectance of the glass samples before and after an appropriate cleaning process. In addition, the contact angle and the surface energy have been analysed. Obtained results show that mirror durability is very specific to the environmental conditions of the exposure site.

Development and growth of corrosion features on protected silver mirrors during accelerated environmental exposure.

Various layer schemes have been developed to protect thin film silver mirrors from tarnish and corrosion. However, the mechanisms by which these additional layers improve mirror durability are not fully understood. Accelerated environmental exposure testing of protected silver mirrors was used to investigate the effects of layer composition on the mechanisms of corrosion feature development and growth. Two model mirror coatings were analyzed in which the composition of the base layer below the silver and the adhesion layer above the silver were varied. Large circular corrosion features formed preferentially along the silver-chromium interfaces, regardless of where this interface was located within the layered structure of the mirror. The corrosion features originated at coating defects in the dielectric protection layer, and their growth was likely driven by the oxidation and chloridation of both silver and chromium at the Ag-Cr interfaces.

Effect of intermediate layers on atomic layer deposition-aluminum oxide protected silver mirrors

This work investigates intermediate materials deposited between silver (Ag) thin-film mirrors and an aluminum oxide (AlOx) barrier overlayer and compares the effects on mirror durability to environmental stresses. Physical vapor deposition of various fluorides, oxides, and nitrides in combination with AlOx by atomic layer deposition (ALD) is used to develop several coating recipes. Ag-AlOx samples with different intermediate materials undergo aggressive high-temperature (80°C), high-humidity (80%) (HTHH) testing for 10 days. Reflectivity of mirror samples is measured before and after HTHH testing, and image processing techniques are used to analyze the specular surface of the samples after HTHH testing. Among the seven intermediate materials used in this work, TiN, MgAl2O4, NiO, and Al2O3 intermediate layers offer more robust protection against chemical corrosion and moisture when compared with samples with no intermediate layer. In addition, results show that the performance of the ALD-AlOx barrier overlayer depends significantly on the ALD-growth process temperature. Because higher durability is observed in samples with less transparent TiN and NiO layers, we propose a figure of merit based on post-HTHH testing reflectivity change and specular reflective mirror surface area remaining after HTHH testing to judge overall barrier performance.

Environmental durability of protected silver mirrors prepared by plasma beam sputtering.

Various overcoat layers have been developed to protect silver mirrors from tarnish and corrosion. However, the mechanisms by which these protective layers improve mirror durability are not fully understood. Mixed flowing gas exposure was used to investigate the corrosion behavior of plasma beam sputtered silver mirrors with different adhesion layer materials. A small amount of nickel in the adhesion layer had a significant impact on the silver-dielectric interface. Additionally, lateral transport of silver was found to be an important factor in the corrosion process. Better stability at all layer interfaces is suggested to improve mirror durability.

An investigation of the durability of front surface Ag mirrors using oxide protecting layers

This paper describes the calculation of the reflection of front surface Ag mirrors and the investigation of the adhesion and durability of these mirrors deposited on the BK-7 glass surface. Cr, Al 2O 3, SiO x and Y 2O 3 underlayers and Y 2O 3, Al 2O 3, SiO x protecting layers were investigated. Layers were deposited by thermal evaporation and e-beam gun methods. Calculated and measured spectral characteristics are presented. All mirrors were subjected to the climo-mechanical tests and the results of durability of mirrors are presented. The following layer combinations BK-7/Cr/Ag/Al 2O 3 and BK-7/Al 2O 3/Ag/Al 2O 3/SiO x gave the best climo-mechanical characteristics and reflection of R > 96%for γ = 450–1100nm. Some layer combinations like BK-7/Al 2O 3/Ag/Al 2O 3 gave better spectral characteristics but durability was not satisfactory for practical application.

Silvered polymer reflectors

Silvered polymers are being studied as reflectors for concentrators of solar radiation because of their potential for significant cost reduction compared with current glass mirror technology. Thin flexible silvered polymer films form excellent mirrors whose initial performance exceeds the long-range goal of a reflectance greater than 90% with a beam spread of a few mrad. Ongoing real-time weathering tests show that mirror performance has been maintained outdoors for 1 yr; accelerated tests indicate the durability is probably longer than 1 yr. It is demonstrated that stabilizers are needed in an acrylic polymer to maintain the reflectance of the silver and a set of effective stabilizers has been identified. The permanence of the stabilizers may limit mirror durability. Polymeric stabilizers and stabilizers that do not absorb u.v. light are being studied to improve mirror durability and optical performance.

Durability of silver-glass mirrors in moist acid vapors

The durability of glass second-surface mirrors is a major concern in the outdoor environment where they are used in solar energy applications. A simple durability test using moist HCl, H 2O and SO 2 vapors on 22 different types of mirrors was found to correlate well with experience on mirrors outdoors and with the expected durability of the mirrors. The mechanism of mirror failure was investigated, and it was found that pores and other defects in the protective paint of the mirror are probably the sites at which failure begins for most commercial mirrors.