Optical Properties and Solar Selectivity of Inhomogeneous Metal-Insulator Coatings.

This thesis reports on coatings consisting of ultrafine metal particles produced by gas evaporation and by electron beam co-evaporation of metal and dielectric. Such coatings have optical properties that make them interesting as selective absorbers of solar energy. Various effective medium theories for the optical properties of inhomogeneous materials are derived from a unified framework. The limits of validity of the theories are discussed. Recent theoretical results on the bounds of the effective dielectric permeability were found to be valuable in the analysis of the experimental results. Ultrafine chromium and nickel particles were produced by gas evaporation. The coatings were characterized by electron microscopy and electron diffraction. Their optical properties could be explained by the Maxwell Garnett theory provided that the clustered nature of the coatings was taken into account. Cermet coatings, consisting of cobalt particles in aluminum oxide, were produced by simultaneous electron -beam evaporation from two sources. These coatings permit a detailed comparison with theory. The film structure was studied by a wide range of material characterization techniques, such as electron microscopy, electron diffraction, Auger electron spectroscopy, Secondary ion mass spectroscopy, Rutherford backscattering and field ion microscopy. Also electrical conductivity measurements gave information on the structure. The complex dielectric permeability of the Co -Al(,2)O(,3) coatings was determined, by a novel technique, from spectral reflectance and transmittance measurements. The optical properties were studied in the wavelength range 0.35-40 (mu)M. As far as I know, the complex dielectric permeability of cermets has not been measured before in the thermal infrared wavelength range. The optical properties could be reconciled with the Maxwell Garnett theory, for low volume fractions of cobalt in the films. For higher metal contents the experimental values departed more and more from this theory. The experimental results were also compared to the bounds on the effective dielectric permeability. Discrepancies were found in the infrared wavelength range. They are not understood at present. Starting from the measured dielectric permeability of Co-Al(,2)O(,3), the properties of a selective solar absorber was optimized. It was found that a Co-Al(,2)O(,3) coating (with 50-60 vol. % cobalt) with an Al(,2)O(,3) overlayer, when deposited onto a nickel substrate, showed a very high solar absorptance, (alpha)(,s), and a low hemispherical emittance, e(,H). These properties were also verified experimentally. The best of the Al(,2)O(,3)/Co-Al(,2)O(,3) selective solar absorbers on nickel had (alpha)(,s) = 0.94 and e(,H) = 0.10. Such good results were achieved without any grading of the coating composition.