Red ceramics pigments had pure bright color and intensive tinting capacity, which had been widely applied in the decoration fields of ceramic body, glaze and inkjet printing. Chromium (Cr) doped α-Al2O3 materials had drawn great attention due to its pure bright color, excellent thermal and chemical stability, which were known as "ruby" or "ruby solution" and could be used as red ceramics pigments. Various synthesis methods had been reported for preparing Cr doped α-Al2O3 red pigments, which included hydrothermal method, Pechini method and combustion method. However, the above-mentioned methods had many disadvantages, such as poor color performance, complicated operation and high equipment requirements. Hence, Cr doped α-Al2O3 red pigments were prepared by novel non-hydrolytic sol-gel (NHSG) method using anhydrous aluminum chloride (AlCl3) and chromic chloride (CrCl3) as raw materials, anhydrous ethanol as solvent and Al powders as reducing agents, respectively. The effects of doping concentration, Al powders addition amount, precursor concentration and heat treatment temperature on the phase composition and coloration properties of pigments were investigated via X-ray diffraction (XRD), the automatic whiteness meter based on Commission International de I'Elcairage (CIE) system and ultraviolet and visible spectrophotometry (UV-Vis). In addition, the morphology, structure and particle size of prepared Cr doped α-Al2O3 pigments were studied via field-emission scanning electron microscope (FE-SEM), high resolution transmission electron microscope (HRTEM), and laser particle analyzer. XRD patterns showed that the relative intensity of the sample diffraction peak was gradually weakened, and the angle of diffraction peak was gradually decreased with the increase of doping concentration in the range of 0%~5% (atom fraction). Further, it could be found that the cell parameters and cell volume of sample gradually enlarged with boosted of Cr3+ doping concentration via Rietveld refinement, indicating that Cr3+ were successfully doped into α-Al2O3 lattice. The a*value of pigments increased gradually and then decreased with the increase of doping content. When the addition contents of Cr3+ were 4%, the samples exhibited good color property (a*=11.44). In addition, aluminum powder had strong reducibility, which could reduce CrCl3 to CrCl2. After non-hydrolysis polycondensation reaction, the raw materials could be uniformly mixed, which was beneficial to enhance doping efficiency. The formation of pure phase α-Al2O3 was not affected after adding Al powder. As the amount of Al powder increased, the L* value of the pigments first decreased and then increased, and the a* value first increased and then decreased. When n(Al)/n(CrCl3) was 6:1, the a* value of samples boosted to 12.45. The precursor concentration did not affect synthesized pure phase of α-Al2O3. The L* value of the synthesized pigments fluctuated in the range of 59.42~66.29, and the b* value fluctuated in the range of -2.34 to -3.46 with the increase of the precursor concentration. The a* value of the pigments increased first and then decreased with the increase of the precursor concentration. When the precursor concentration was 0.4 mol·L-1, the pigments exhibited excellent color property (a* = 14.32). Calcination temperature played an important role in the synthesis of pigments and color property. XRD and DTA-TG results showed that the samples synthesized by NHSG method were composed of α-Al2O3 and γ-Al2O3 at 1050 ℃. With the increase of the calcination temperature, γ-Al2O3 was gradually transformed into α-Al2O3. As the temperature increased to 1500 ℃, the samples were pure α-Al2O3 phases and had high crystallinity. The a* value of the pigments gradually increased and then tended to level off, as well as color saturation (Cab*) gradually increased with the increase of the heat treatment temperature. UV-Vis result indicated that the prepared samples had a strong reflectivity in the red light area (650 to 760 nm), indicating that the prepared Cr doped α-Al2O3 pigments exhibited pink hue. The synthesized Cr doped α-Al2O3 samples at 1100 ℃ were composed of agglomerate particles with a primary particle size of about 100 nm, which exhibited an irregular shape. It was found that the size of Cr doped α-Al2O3 pigments also grually increased with the elevation of the heat treatment temperature via SEM images. When the heat treatment temperature enhanced to 1500 ℃, the particles tended to be partially sintered. TEM and HRTEM images showed that the morphology of the sample was irregular and the lattice stripes had no obvious defects, indicating that the prepared pigment at 1200 ℃ had a higher degree of crystallinity. The laser particle analyzer showed that the particle size of the prepared pigment at 1200 ℃ exhibited a unimodal distribution and narrow particle size distribution, where D10 was 2.17 μm, D50 was 7.85 μm and D90 was 15.9 μm, The D90 of pigment particles was less than 20 μm, which could better meet the requirements of ceramic decoration. The excellent color performance (L*=66.29, a*=14.32, b*=-3.24, Cab*=14.68) of Cr doped α-Al2O3 pigments with small particle sizes (D50=7.85 μm) could be obtained via NHSG method, making it be potential candidate in the application of industrial ceramic decoration. © 2021, Youke Publishing Co., Ltd. All right reserved.
