OPTICAL PROPERTIES OF CERMET MATERIALS

Cermet films can show a spectral selectivity which makes them well suited for efficient photothermal conversion of solar energy. Sufficiently fine-grained materials can be theoretically understood from effective medium theories. Two formulations apply depending on the microstructure topology ; the Maxwell Garnett theory is valid for metallic particles embedded in an insulating host, whereas the Bruggeman theory holds for a random mixture of metallic and insulating particles. We discuss extensions to treat size dependent electron scattering, shape and orientation effects, and the role of local aggregation. The theories are applied to several types of cermets : discontinuous metal films, gas evaporated coatings, coevaporated and cosputtered layers, electrodeposited films, and electrolytically and integrally coloured anodic aluminium oxide coatings. Successes and failures of the theoretical models are discussed and remaining uncertainties are pointed out. One general conclusion is that effective medium theories are very useful for understanding the optical properties both qualitatively and quantitatively. Another general result is that lack of sufficiently accurate sample characterization is still hampering further progress for several types of technologically interesting cermets