Synthesis of High Near-Infrared Reflective Black Pigment Based on YMn2O5

Y(Mn0.95M0.05)2O5 (M = Al, Fe, Ga, Ti, and Zr) samples were synthesized via a sol–gel method using citric acid to find a new near-infrared (NIR) reflective black pigment. Among these samples, the optical reflectance of Y(Mn0.95Fe0.05)2O5 and Y(Mn0.95Ga0.05)2O5 in the near-infrared region was found to be larger than that of YMn2O5. Then, the concentration of the dopant (Fe or Ga) was changed between 0 and 15%, and the resulting UV–Vis–NIR reflectance spectra were measured. As a result, the optical reflectance of the Fe-doped samples decreased in the near-infrared region, while that of the Ga-doped samples increased. Accordingly, Y(Mn1−xGax)2O5 (0 ≤ x ≤ 0.20) samples were synthesized, and the crystal structure, particle size, optical properties, and color of the samples were characterized. The single-phase samples were obtained in the composition range of 0 ≤ x ≤ 0.15, and the lattice volume decreased with increasing Ga3+ concentration. Optical absorption below 850 nm was attributed to the charge transfer transition between O2p and Mn3d orbitals, and the absorption wavelength of Y(Mn1−xGax)2O5 shifted to the shorter wavelength side as the Ga3+ content increased, because of the decrease in the Mn3+ concentration. Although the sample color became slightly reddish black by the Ga3+ doping, the solar reflectance in the near-infrared region reached 47.6% at the composition of Y(Mn0.85Ga0.15)2O5. Furthermore, this NIR reflectance value was higher than those of the commercially available products (R < 45%).