Near-infrared reflective Mg0.5Co0.5-xMnxAl2O4 black pigment synthesized by spray pyrolysis: Optimization of composition and calcination temperature

Abstract

Co/Mn co-substituted MgAl2O4 pigments were spray pyrolysis, and their crystal structure and optical properties were characterized with changing the Co/Mn ratio and synthesis temperature. Regardless of the Co/Mn ratio, the resulting pigments showed spinel structured MgAl2O4 phase without any impurity phase. The synthesized Mg0.5Co0.5Al2O4 and Mg0.5Mn0.5Al2O4 showed a dark blue and brown color due to the visible light absorption by the 4A2(F) → 4T1(P) transition of Co2+ and the d-d transition of Mn2+, respectively. The black colored pigment was achieved by simultaneous substituting Co2+ and Mn2+ in the Mg2+ tetrahedral site in MgAl2O4. The optical band gap of the Mg0.5Co0.5-xMnxAl2O4 pigment decreased linearly from 3.08 eV (x = 0) to 2.46 eV (x = 0.4) as the Mn2+ content increased. As a result, Mg0.5Co0.1Mn0.4Al2O4 uniformly absorbed the entire visible light and had a black color (L* = 28.10, C* = 1.68). In terms of obtaining high near-infrared reflectance and good blackness, the optimal calcination temperature of Mg0.5Co0.1Mn0.4Al2O4 was determined to be 900 °C. The optimized Mg0.5Co0.1Mn0.4Al2O4 black pigment had a good LiDAR reflectance (38% at 905 nm) and excellent chemical stability, and showed improved heat shielding performance compared to carbon black using in black vehicles.