Combustion synthesis of pink chromium-doped alumina with excellent near-infrared reflective properties

Abstract

Pink chromium-doped alumina, Al1.95Cr0.05O3, was prepared by microwave assisted combustion synthesis, starting from a stoichiometric mixture of Al(NO3)3·9H2O, Cr(NO3)3·9H2O and urea. No additional annealing was required, as the X-ray diffraction pattern of the sample indicated that Al1.95Cr0.05O3 was the single crystalline phase, having an average crystallite size of 63nm. The resulted powder had a pink colour (L*=83.1, a*=4.2, b*=−2.9) and a total solar reflectance of 69.2%. The pigment had a BET surface area of 1.8m2/g, which corresponds to an equivalent particle size of 838nm. NIR reflective properties of Al1.95Cr0.05O3 were tested by painting an aluminium sheet with a water-based acrylic coating containing 20% mass pigment. The dry film had a thickness of 150µm and a total solar reflectance of 61.7%. The surface temperature during irradiation with a 250W IR lamp indicated a very similar behaviour to that of a white TiO2-based coating. All these features highly recommend pink Al1.95Cr0.05O3 as an effective NIR-reflective (cool) pigment.