Synthesis and characterization of black pigments based on calcium manganese oxides for high near-infrared (NIR) reflectance

Calcium manganese oxide pigments (CaMnO 3 , Ca 2 MnO 4 , Ca 3 Mn 2 O 7 and Ca 4 Mn 3 O 10 ) were synthesized by applying a conventional solid-state reaction technique. The powders obtained were characterized by acquiring XRD, UV-Vis and NIR diffuse reflectance spectra and taking CIE L * a * b * chromatic coordinate measurements. Comparison of the UV-Vis and NIR spectra for the four samples indicated Ca 2 MnO 4 to be the best near-infrared reflective black pigment. To optimize the color and thermal barrier properties of Ca 2 MnO 4 , Ti 4+ was doped into the Mn 4+ sites and Ca 2 Mn 1− x Ti x O 4 (0 < x ≤ 0.40) samples were synthesized. The lattice volume increased as the Ti 4+ concentration was increased. When the number of the Ti 4+ ions ( x ) in the chemical formula surpassed 0.30, however, the color changed from black to dark brown, and an unknown phase was observed in the XRD patterns. The NIR solar reflectance of the Ca 2 Mn 1− x Ti x O 4 pigments increased as the amount of Ti 4+ was increased for the samples in the range 0 ≤ x ≤ 0.35, but plateaued for x > 0.35. These results indicated the Ca 2 Mn 0.85 Ti 0.15 O 4 composition to be optimum for providing both an appropriate black color and NIR reflecting characteristics. This black pigment displayed sufficient NIR solar reflectance ( R = 66.2%), a value higher than those of the commercially available pigments ( R < 50%). Ca 2 Mn 0.85 Ti 0.15 O 4 was synthesized as an inorganic black pigment with thermal barrier properties, and its NIR solar reflectance value was much greater than those of the conventional NIR reflective black pigments.