Chiang Mai Journal of Science

Near-infrared (NIR) reflectance of cobalt ferrite (CoFe₂O₄) black pigment has been significantly enhanced by doping with Al₂O₃, MgO and CaO&MgO derived from dolomite. The CoFe₂O₄ pigment was synthesized by a conventional solid-state reaction of Co₃O₄ and Fe₂O₃ at 1100 ^oC. Various amounts of dopants including Al₂O₃, MgO and dolomite which yielded MgO&CaO upon calcination were added into starting raw materials to investigate doping effect. Substitutions of Al³⁺ for Fe³⁺, and Mg²⁺ and/or Ca²⁺ for Co²⁺ in the CoFe₂O₄ structure were manipulated to find an optimum condition that yielded high NIR reflectivity while its black color hue was maintained. Crystal structure, particle morphology and properties of the synthesized pigments were characterized by an XRD, SEM, UV-Vis-NIR spectrophotometer and CIE 1976 L*a*b* colorimetric method. The pure CoFe₂O₄ black pigment exhibited color lightness L* and NIR reflectance of 19.59 and 19.5%, respectively. The incorporation of the MgO and Al₂O₃ dopants resulted in a significant increase of the NIR reflectance to 47.6%. Similarly, a significantly enhanced NIR reflectance of 39.8% was obtained at the optimum doping content of Al₂O₃ and MgO&CaO derived from dolomite. The synthesized pigments exhibited high NIR reflectance that was comparable to the commercial reflective black pigments. Doping the CoFe₂O₄ with an inexpensive dolomite mineral not only enhanced its NIR reflectance but would also reduce the synthesis cost.