Optical Properties and Solar Selectivity of Metal-Insulator Composites

We derive a number of theoretical expressions for the average dielectric permeability of composite materials from a model which treats scattering from random unit cells chosen so as to represent the basic microstructure. The limits of validity of these expressions are given. The general bounds on the average dielectric permeability which hold irrespective of microstructure are presented. The theoretical expressions are used to explain the measured optical properties for two types of samples: Co-Al2O3 produced by coevaporation, and Ni-pigmented anodic Al2O3. Applications of these coatings for efficient photothermal conversion of solar energy are discussed in detail.