Optical characterization and modeling of black pigments used in thickness-sensitive solar-selective absorbing paints

Abstract

The performance of black pigments used in thickness-sensitive solar-selective absorbing paints for solar thermal collectors depends on the optical properties of the pigments. Knowledge of the intrinsic optical properties of most paint pigments is very limited. Pellets made from either FeMnCuO x or black carbon dispersed in KBr matrix have been used to determine the effective optical coefficients of the pigments. Scattering and absorption cross-sections were derived from reflectance and transmittance measurements at near normal angle of incidence in the wavelength range 0.3 to 2.5 μm. Volume concentrations of the pigments that gave linear dependencies for the coefficients were found to be between 0.053 and 0.53% (FeMnCuO x ) and 0.076 and 0.31% (black carbon). Subsequently, a four-flux model has been used for calculation of reflectance for thickness-sensitive spectrally-selective paints. The paints were obtained from the FeMnCuO x and black carbon pigments embedded in silicone and phenoxy resin, respectively. We have used the experimentally determined scattering and absorption cross-sections of the pigments as input to the four-flux theory and the calculations have been performed for different thicknesses and/or pigment volume fraction of the paints. The calculated reflectances were compared with experiments and the results for thick films fit well.