Preparation And Properties Of High Near-infrared Reflective Rare Earth Pigments

Due to the special physical and chemical properties of La YO₃ and La₂Ce₂O₇,they have been widely used in ceramic pigment and turbine blade coating,respectively.To our knowledge,the near-infrared reflection of LaYO₃ and La₂Ce₂O₇pigment has been less reported.The development trends of thermal insulating coating is inorganic pigments with high near-infrared reflectance and environmentally benign.A series of environmentally benign and high NIR reflective pigments of LaYO₃and La₂Ce₂O₇ were synthesized via the sol-gel method.The Bi³⁺doping and Bi³⁺/Tb³⁺co-doping LaYO₃ pigments were used to modifiy the drab white color,the substitutions of Pr,Tb and W/Fe for Ce in La₂Ce₂O₇ were studied to improve the properties of color.The resulting powders were characterized by X-ray diffraction,UV-vis-NIR spectrophotometer and CIE L*a*b*?1976?color space.The doping of Bi³⁺for La³⁺in LaYO₃ resulted in the color changes from light yellow to dark yellow and Bi³⁺/Tb³⁺for La³⁺in LaYO₃ varies from soft orange to dark orange,and the Tb ion was added to improve the red color.Meanwhile,the band gaps of pigments decreased from 4.52eV to 2.95eV?Bi-doped?and 2.15eV?Bi/Tb co-doped?,the coloring mechanism is based on the shift of the charge transfer band of O?2p?-La?5d?to higher wavelengths?red shifting?by introducing an additional electronic energy level of bismuth.Interestingly,the Bi-doped pigments possessed high NIR solar reflectance?>95%?in the range of 700-2500 nm.In contrast,the Bi/Tb co-doped decreased the NIR solar reflectance from 98.28%to 78.77%.The applicability studies of La_0.5Bi_0.5YO₃ and La_0.5Bi_0.25Tb_0.25YO₃ pigments with TiO₂base coat on galvanized sheet revealed good coloring performance with high NIR reflectance.Acid/alkaline tests indicated good chemical stabilities of the pigments under the extreme weather conditions.The substitutions of Pr,Tb and W/Fe for Ce in La₂Ce₂O₇ pigments possessed a disordered defect fluorite-type structure and showed good chemical stabilities in acid/alkaline tests.The investigation has demonstrated that the doping of Pr⁴⁺or Tb⁴⁺for Ce⁴⁺in La₂Ce₂O₇ resulted in the color changes from light yellow,soft orange to dark orange.Moreover,the band gaps of pigments decreased from 3.34eV to 2.37eV?Pr-doped?and 2.24e V?Tb-doped?.By doping La₂Ce₂O₇ with Pr⁴⁺and Tb⁴⁺ions,an additional 4f1 electronic energy level of Pr⁴⁺and Tb⁴⁺is introduced between the O^2-valence and the Ce⁴⁺conduction band,and the reduced band gap energies can be observed.Interestingly,the Pr⁴⁺doped pigment possessed high NIR solar reflectance?>72.47%?in the range 700-2500nm with the color of dark orange?L*=46.87 a*=13.5b*=13.4?.The substitution of Tb⁴⁺for Ce⁴⁺changed the color to dark orange?L*=51.35 a*=15.94 b*=17.57?and decreased the NIR solar reflectance slightly from99.11%to 87.41%.The applied studies of two kinds of pigments coating on the galvanized sheets exhibited nice colors with high NIR solar reflectance.The typical pigment La₂Ce_1.5W_0.25Fe_0.25O7+?possessed the high NIR solar reflectance of 82.05%with orange color?L*=77.21,a*=8.79,b*=35.80?.Also the influence of calcination temperature has been studied,the higher temperature resulted in enhanced color and slightly reduced reflectance properties.The acid/alkali resistance studies confirmed the pigments were chemically stable.In a word,all of the pigments with high NIR reflectance have exhibited application potentials.