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

Abstract

Calcium manganese oxide pigments (CaMnO₃, Ca₂MnO₄, Ca₃Mn₂O₇ and Ca₄Mn₃O₁₀) 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 Ca₂MnO₄ to be the best near-infrared reflective black pigment. To optimize the color and thermal barrier properties of Ca₂MnO₄, Ti⁴⁺ was doped into the Mn⁴⁺ sites and Ca₂Mn_1−xTi_xO₄ (0 < x ≤ 0.40) samples were synthesized. The lattice volume increased as the Ti⁴⁺ concentration was increased. When the number of the Ti⁴⁺ 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 Ca₂Mn_1−xTi_xO₄ pigments increased as the amount of Ti⁴⁺ was increased for the samples in the range 0 ≤ x ≤ 0.35, but plateaued for x > 0.35. These results indicated the Ca₂Mn_0.85Ti_0.15O₄ 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%).