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

The performance of black pigments used in thickness-sensitive solar-selecti ve absorbing paints for solar thermal collectors depends on the optical pro perties of the pigments. Knowledge of the intrinsic optical properties of m ost paint pigments is very limited. Pellets made from either FeMnCuOx or bl ack carbon dispersed in KBr matrix have been used to determine the effectiv e optical coefficients of the pigments. Scattering and absorption cross-sec tions were derived from reflectance and transmittance measurements at near normal angle of incidence in the wavelength range 0.3 to 2.5 mum. Volume co ncentrations of the pigments that gave linear dependencies for the coeffici ents were found to be between 0.053 and 0.53% (FeMnCuOx) and 0.076 and 0.31 % (black carbon). Subsequently, a four-flux model has been used for calcula tion of reflectance for thickness-sensitive spectrally-selective paints. Th e paints were obtained from the FeMnCuOx and black carbon pigments embedded in silicone and phenoxy resin, respectively. We have used the experimental ly determined scattering and absorption cross-sections of the pigments as i nput to the four-flux theory and the calculations have been performed for d ifferent thicknesses and/or pigment volume fraction of the paints, The calc ulated reflectances were compared with experiments and the results for thic k films fit well. (C) 2001 Elsevier Science Ltd. All rights reserved.