Effect of calcination temperature and dolomite or Al2O3 doping on properties of NIR–reflective CoFe2O4 black pigment

CoFe2O4 NIR-reflective black pigment has been synthesized by solid-state reaction of cobalt oxide (Co3O4) and hematite (Fe2O3) at a temperature range of 900-1200°C to investigate the effect of reaction temperature on pigments’ properties. Various amounts of mineralizers such as CaO and MgO derived from dolomite mineral (CaMg(CO3)2) and Al2O3 were incorporated to investigate their effects on crystalline phase and physical properties of the pigments. Stoichiometric proportion of raw materials including Co3O4, hematite, dolomite and Al2O3 with a chemical formula of (CoO)1-2x(CaMg(CO3)2)x⋅(Al2O3)y(Fe2O3)1-y, where x, y = 0.25 and 0.5, were homogeneously mixed by wet-ball milling in acetone for at least 5 h. After filtering and drying, the mixture were calcined in ambient atmosphere at 900, 1000, 1100 and 1200°C for 3 h. The synthesized products were washed with de-ionized water for 3 times, dried at 105˚C, ground into fine powder and finally sieved through a 325-mesh screen. XRD analysis of the undoped products revealed the presence of pure spinel CoFe2O4 at calcination temperatures of 1000°C and higher. At 900°C, trace amount of unreacted Fe2O3 was observed. The CIE 1976 L*a*b* colorimetric analysis revealed a gradual change of color hue from dark brown for the precursor (L*=35.99) to black at 1100°C (L*=19.59), and turned to dark grey at 1200°C (L*=26.74). The NIR reflectance calculated at a wavelength rage of 700 to 2500 nm following the ASTM E903 standard gradually decreased from 26.2% after calcination at 900°C to 24.4 and 17.5% after calcination at 1000 and 1100°C, respectively, and slightly increased to 19.6% after calcination at 1200°C. Doping with Al2O3 resulted in a slight shift of XRD peaks towards higher diffraction angles upon the increase of Al2O3 content as a result of substitution of Fe3+ site with a smaller Al3+. On the other hand, dolomite doping resulted in a gradual shift towards lower diffraction angles upon the increased dolomite content due to substitution of Co2+ site with Mg2+ and Ca2+. The Al2O3 and dolomite doping resulted in a change of color hue from deep black to lighter black, and became dark brown at high dolomite doping content. The NIR reflectance gradually increased from 17.5% for the undoped sample to 22.5% for the sample doped with 0.5 molar ratio of Al2O3 (i.e. CoO⋅(Al2O3)0.5(Fe2O3)0.5), and to 24.5 and 45.6% for the samples doped with dolomite at molar ratio of 0.25 (i.e. (CoO)0.5(CaO:MgO)0.25⋅Fe2O3) and 0.5 (i.e. (CaO:MgO)0.5⋅Fe2O3), respectively. It has been demonstrated that NIR reflectance of the CoFe2O4 black pigment can be enhanced by doping with mineralizers with only a slight decrease in blackness.