Effect of calcination temperature and Ti substitution on optical properties of (Fe,Cr)2O3 cool black pigment prepared by spray pyrolysis

Near-infrared (NIR) reflective Fe_1.5Cr_0.5O₃ black pigments were synthesized by spray pyrolysis, and the effect of calcination temperature and Ti substitution on the optical properties were investigated. As the calcination temperature increased from 500 °C to 800 °C, the prepared Fe_1.5Cr_0.5O₃ powder had α-Fe₂O₃ phase without any impurities and the NIR reflectance steadily increased. On the otherhand, the smallest chroma value was observed at 700 °C. Therefore, in terms of achieving high NIR reflectance with keeping good blackness, the optimal calcination temperature was determined to be 700 °C. Substitution of Ti corresponding to 5-15% of the total metal ions in Fe_1.5Cr_0.5O₃ largely affected the NIR reflectance and chroma value, but there was no significant change in the optical band gap. Ti substitution of 5 mol% could further improve the NIR reflectance of the Fe_1.5Cr_0.5 black pigment without significant loss of blackness, but above 10% or higher the NIR reflectance largely decreased due to the loss of crystallinity. Resultantly, Fe_1.4Cr_0.5Ti_0.1O_3+δ (5% Ti substituted black pigment) was confirmed to have improved heat-shielding performance compared to Fe_1.5Cr_0.5O₃ in a temperature rise test under near-infrared illumination.