Improvement of near-infrared (NIR) reflectivity and black color tone by doping Zn 2+ into the Ca 2 Mn 0.85 Ti 0.15 O 4 structure.

Inorganic black pigments with thermal barrier characteristics, Ca 2 Mn 0.85- x Ti 0.15 Zn x O 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 Ca 2 Mn 0.85 Ti 0.15 O 4 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 Ca 2 Mn 0.77 Ti 0.15 Zn 0.08 O 3.92 ( R NIR = 74.6%) became larger than that of Ca 2 Mn 0.85 Ti 0.15 O 4 ( 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 [MnO 6 ] octahedra and a decrease in the manganese ion concentration. The R NIR value of the Ca 2 Mn 0.77 Ti 0.15 Zn 0.08 O 3.92 pigment was also larger than those of the commercially available pigments ( R NIR < 53.0%). Therefore, Ca 2 Mn 0.77 Ti 0.15 Zn 0.08 O 3.92 has potential to be an inorganic black pigment for thermal shielding.