Optical properties of higher and lower refractive index composites

The focus of attention in this study was the choice of material for optically solar selective coatings on the basis of their optical constants. A computer program which calculate the optical constants, solar absorptance alpha and thermal emittence epsilon of the selective on the assumption of both the Maxwell Garnett and Bruggeman theories for the metallic volume fraction below and above 0-3 respectively, was used to design the structure of the composite films. Two systems of composite thin films of metal and dielectric were investigated experimentally, fabricated by (define it) RF and (define it) DC sputter coater. One system consist lower refractive index (Ni:SiO/sub 2/) and other has higher refractive index (V:Al/sub 2/O/sub 3) composites in the range of 0.3 mu m spectrum. These films were fabricated on infrared reflective substrates such as nickel plated copper. For comparison and verification, tungsten, cobalt and chromium based composites were also investigated which validated the concept of the choice of material in selective coatings. It was observed that high refractive index composites have lower reflective properties by choosing suitable metallic volume fraction (0.7) in dielectric and AR coating (70 nm). Higher value of imaginary part of refractive index, k, is responsible for higher absorption by a factor alpha = 2 pi k/lambda. Solar absorptance of 0.98 and 0.96 was achieved by simulation and experimental findings with less than 0.05 thermal emittance for 200 nm thick composites. It was also investigated that higher values of both n and k are more suitable in selective coatings. (author)