Fabrication of robust solar mirrors on polymeric substrates by physical vapor deposition technique

Abstract

Aluminium, first surface, plastic mirrors have been developed to be used as heliostats in concentrated solar power stations. The Al reflective layer, the adhesion of the reflective layer to the polycarbonate (PC) substrate and the protective over-coat have all been optimized in order to achieve a highly reflective, robust and durable light-weight mirror. The Al layers were deposited by means of magnetron sputtering and the change in reflectance was investigated as a result of variations in the sputtering conditions. The influence of the substrate cleaning process and the addition of an interfacial seed layer on the adhesion of the reflective layer were also systematically studied. A thick sputtered SiO2 film and a dip-coated transparent hard-coat resin were examined as protective coatings against weathering tests. All prepared samples were characterised using, adhesion and abrasion tests, spectrophotometry, nano-scratch, salt-spray tests and both accelerated and outdoor weathering.The optimal three-layer stack was found to be 15 nm of SiO2 as an adhesive layer, 80 nm of Al as the reflector and 4.5 μm dip-coated, hard-coat resin as the protective layer. The three-layer stack plastic mirror gave an overall total solar weighted specular reflectance of ≈88% which was maintained even after aggressive abrasion tests such as Bayer and Steel Wool, 40 days within a salt-spray chamber and after 5 months under accelerated weathering.