Improvement of near-infrared (NIR) reflectivity and black color tone by doping Zn2+ into the Ca2Mn0.85Ti0.15O4 structure

Abstract

Inorganic black pigments with thermal barrier characteristics, Ca₂Mn_0.85−xTi_0.15Zn_xO_4−x (0 ≤ x ≤ 0.10), were synthesized using a conventional solid-state reaction method in order to improve the blackness without decreasing the near-infrared (NIR) reflectance of a Ca₂Mn_0.85Ti_0.15O₄ pigment, which was previously reported by our group. The composition was optimized to provide both high blackness and NIR reflection characteristics. As a result, the NIR solar reflectance value (R_NIR) of Ca₂Mn_0.77Ti_0.15Zn_0.08O_3.92 (R_NIR = 74.6%) became larger than that of Ca₂Mn_0.85Ti_0.15O₄ (R_NIR = 71.7%), and the black color tone of the former (L* = 23.2, a* = +2.81, b* = +0.83, C = 2.93) was improved in comparison with that of the latter (L* = 24.4, a* = +4.30, b* = +2.72, C = 5.09). This improvement is caused by the introduction of strain into the [MnO₆] octahedra and a decrease in the manganese ion concentration. The R_NIR value of the Ca₂Mn_0.77Ti_0.15Zn_0.08O_3.92 pigment was also larger than those of the commercially available pigments (R_NIR < 53.0%). Therefore, Ca₂Mn_0.77Ti_0.15Zn_0.08O_3.92 has potential to be an inorganic black pigment for thermal shielding.