EFFECT OF Ni DOPING AND SYNTHESIS TEMPERATURE ON THE PROPERTIES OF NIR-REFLECTIVE ZnFe_2O_4 BLACK PIGMENTS

摘要

NIR-reflective Ni-doped ZnFe2O4 black pigments with the general formula of Zn(1-x)NixZnFe(2)O(4) have been synthesized by a conventional solid-state reaction of ZnO, Fe2O3 and NiO. The effect of Ni doping was studied by adding various amounts of NiO, with x ranging from 0 to 1. Proportional amounts of ZnO, Fe2O3 and NiO were homogeneously mixed together by means of wet milling, followed by filtration and drying. Then, the mixtures were calcined at 1000 and 1100 degrees C for 3 h. XRD analysis of the undoped pigments revealed the presence of pure ZnFe2O4 in all synthesis conditions. The addition of NiO dopant into the ZnFe2O4 caused a gradual shift of the diffraction peaks toward higher angles due to the substitution of Zn2+ by the smaller Ni2+ to form the Zn(1-x)NixFe(2)O(4). Upon the increase of NiO doping content and calcination temperature, the color hue of the pigments changed from light brown to brownish black, and a gradual reduction of the NIR reflectance was observed. For the black pigments which were synthesized at 1100 degrees C, the band-gap energy changed from 2.05 eV for ZnFe2O4 (x=0) to 1.65 eV for NiFe2O4 (x=1). At the optimum synthesis condition, resulting in black color hue and high NIR reflectance, the Zn0.5Ni0.5Fe2O4 pigment synthesized at 1100 degrees C had high NIR reflectance of 55.3%.