Improvement of near-infrared (NIR) reflectivity and black color tone by doping Zn2+ into the Ca2Mn0.85Ti0.15O4 structure
Inorganic black pigments with thermal barrier characteristics, Ca2Mn0.85−xTi0.15ZnxO4−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 Ca2Mn0.85Ti0.15O4 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 (RNIR) of Ca2Mn0.77Ti0.15Zn0.08O3.92 (RNIR = 74.6%) became larger than that of Ca2Mn0.85Ti0.15O4 (RNIR = 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 RNIR value of the Ca2Mn0.77Ti0.15Zn0.08O3.92 pigment was also larger than those of the commercially available pigments (RNIR < 53.0%). Therefore, Ca2Mn0.77Ti0.15Zn0.08O3.92 has potential to be an inorganic black pigment for thermal shielding.