High performance sputtered Ni : SiO2 composite solar absorber surfaces

Abstract

A solar selective absorber can be prepared by dispersing suitably sized metallic particles in an insulating host matrix. Absorption is assisted by controlling optical interference within the composite film. Graded index thin films of metallic nickel in quartz (Ni:SiO2) were made by co-sputtering with metal volume fractions ranging from 10–90% from top (anti reflecting coating) to bottom (base layer) of the structure, to minimise optical interference peaks. The films are 100–170nm thick with an additional 70nm anti-reflection (AR) coating. Coatings of different thickness, metal volume fraction and compositional gradient were investigated. Substrates were Al and Cu and films were deposited on either the bare substrate or substrates coated with evaporated nickel. The influence of substrate choice on the optical properties was studied. Films with solar absorptance, α, in the range 0.90–0.96 and thermal emittance, ε,=0.03–0.14 were achieved. The dependence of these properties on thickness, film composition and gradient and substrate were determined. A computer programme which calculates the solar absorptance and thermal emittance based on the assumptions of both Maxwell Garnett and Bruggeman theories for metal fill factors below and above 0.3, respectively, was used to design the structure of the composite films. The theoretical results are not presented here.