Low-temperature combustion synthesis of CuCr2O4 spinel powder for spectrally selective paints

CuCr2O4 spinel powder with high quality black hue, investigated as solar-absorbing pigment for spectrally selective paint, was synthesized by an environmental friendly sol-gel combustion process using citric acid as the fuel and metal nitrates as oxidizers. Single-phase CuCr2O4 spinel crystals were obtained after heat treatment of the as-burnt powder at a low temperature (600 degrees C) and the average crystallite size of the CuCr2O4 powders increased with the calcining temperature. Morphological analysis of powders calcined at various temperatures was done by field emission scanning electron microscopy. CuCr2O4 powder calcined at 700 degrees C was chosen as pigment to fabricate thickness sensitive spectrally selective paint coatings by simple spray-coating technique. For the sake of comparison, the as-burnt powder composed of mixed metal oxides (i.e., CuO and Cr2O3) was also used as pigment. The results reveal that the spinel CuCr2O4 based paint coatings exhibit much higher spectral selectivity (alpha(s) = 0.88-0.91, epsilon(100) = 0.27-0.35) which is depending on the coating thicknesses than that of coatings using as-burnt powder as pigment (alpha(s) = 0.83-0.88, epsilon(100) = 0.60-0.66). The CuCr2O4-based paint coatings showed no visible degradation after 600 h of condensation test and the performance criterion value is 0.04, indicating that the coatings have excellent long term stability.