Black Cool Pigments for Urban Heat Island (UHI) Control: from Cr-Hematite to Mn-Melilite

Abstract

Black cool pigments are very interesting for its application in asphalt urban pavements and building floors for moderate the urban heat island effect (UHI) and improving air conditioning energy efficiency. Cool black pigments based on Cr doped hematite Fe2O3 (trigonal, R-3c), hexagonal perovskites YMnO3 (hexagonal, P63cm) and Sr4CuMn2O9 (trigonal, P321) and melilite Sr2(Mg0.5Mn0.5)Ge2O7 (tetragonal, P-421m) with high NIR reflectance synthesized by ceramic and coprecipitation method, are analyzed and compared from color yield in alkyd paint, ceramic glazes and porcelain stoneware, NIR reflectance, bandgap and photocatalytic activity on Orange II substrate. Sr4CuMn2O9 black powders show the nearest hue h to the reference carbon black and the highest NIR reflectance (51%). All pigments show high NIR reflectance in all tested applications. The Fe1.2Cr0.8O3 pigment shows good behavior in the free ZnO glaze and also in porcelain stoneware, YMnO3 and Sr4CuMn2O9 pigments are compatible with low temperature glazes, but Sr2(Mg0.5Mn0.5)Ge2O7 pigment loses the black color even in low temperature glazes. Sr4CuMn2O9 pigment shows moderate photoactivity on Orange II (t1/2=216 min) and the Fe1.2Cr0.8O3 pigment also shows some activity (t1/2=329 min).