Carbon-in-silica composite selective solar absorbers: a determination of composition and dielectric properties

Abstract

The Bruggeman and Maxwell-Garnett effective medium approximations have been used widely to investigate optical properties of many different composite materials. In most cases, the effective medium approximation assumptions are based on random unit cell models in which some metal particles are embedded in a dielectric medium. The shapes of the embedded particles can be varied between spherical, ellipsoidal and cylindrical shapes. A new and interesting structure of connected short chains of completely amorphous carbon intermixed with short chains of silica at nanoscale level has been observed recently. A generalised Bergman representation based on an arbitrary spectral density function is currently applied on these carbon-in-silica samples with a reasonable success of fitting between experiment and theory. The curve-fitting procedure adopted here has resulted in information such as volume fraction of carbon relative to silica, percolation threshold, the thickness and effective dielectric function of the composite layer.