DAI Jiming 1, ZHANG Xiaozhen 1, ZHOU Xiaojian 1, LI Qiaoping 1,
ZHOU Chengzhi 1, CHEN Renhua 2, LIU Huafeng 1, 2, ZHONG Huofeng 3
(1. School of Materials Science and Engineering, Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China;2. Jiangxi Jinhuan Pigments Co., Ltd., Yichun 336000, Jiangxi, China;
3. Longnan Xinkun Inorganic New Materials Co., Ltd., Ganzhou 341799, Jiangxi, China)
Extended abstract:[Background and purposes] Yellow inorganic pigments are widely-used coloring and decorative functional materials. However, traditional yellow inorganic pigments contain high levels of poisonous heavy metals, such as Cd, Pb and Sb. Therefore, their use is strictly limited due to increasingly stringent global environmental protection requirements. For this reason, developing environmentally friendly, non-toxic, new inorganic yellow pigments has become an inevitable industry trend. Rutile tetragonal TiO2 with a stable crystal structure and promising thermal and chemical stability is an important host material of oxide ceramic pigments. Different colored pigments can be obtained by using the metal ion doping strategy to modulate the bandwidth and spectral properties of rutile TiO2. Two rutile pigments co-doped with Cr-Sb and Ni-Sb have attracted much attention, due to their excellent yellow color-rendering properties. However, the presence of small amounts of the toxic heavy metals Cr and Sb may still adversely affect the environmental safety of the pigments. In this work, a novel Ni-Nb co-doped rutile TiO2-based yellow pigment was synthesized with Nb5+ replacing Sb5+ as the counterbalance ion, by using the high-temperature solid-phase reaction method. The effects of calcination temperature on crystalline structure, optical properties, color-rendering performance and stability in the ceramic glaze of the synthesized Ti0.85Ni0.05Nb0.10O2 pigments were studied.[Methods] Rutile Ti0.85Ni0.05Nb0.10O2 yellow ceramic pigments were prepared by using the high-temperature solid-phase reaction method. The raw materials, including TiO2, Ni(NO3)2‧6H2O and Nb2O5, were firstly weighed according to the stoichiometric ratio and then well-mixed by using planetary ball milling. The powder mixture was dried and then calcined at 1050–1200 ℃ for 2 h to obtain the pigments. Phase structure of the pigments was identified by using a D8-Advance X-ray diffractometer. Thermogravimetric and differential thermal analyses (TG-DTA) were carried by using a STA449C comprehensive thermal analyzer. CIE L*a*b* color parameters of the pigments and colored glazes were measured by using a Ci7600 colorimeter. Color saturation (C*) was calculated according to the equation C*=[(a*)2+(b*)2]1/2. UV-visible and near-infrared reflectance spectra of the pigments were tested by using a Lambda 950 spectrophotometer in the range of 175–3300 nm. Morphology and elemental compositions of the pigment powders were analyzed by using a Hitachi SU8010 field-emission scanning electron microscope (SEM) attached with an IXRF Model 550i energy disperse spectrometer (EDS).[Results] Ni-Nb co-doping promoted the transformation of TiO2 from the anatase to the rutile phase, while single rutile crystal phase can be obtained after calcination at 1050–1200 ℃ for 2 h. As the calcination temperature increased, the L* value (lightness) of the pigment gradually decreased, while the b* value (yellowness) and C* value (color saturation) increased significantly. The a* value (redness/greenness) gradually increased from a negative value (≤1100 ℃) to a positive value (≥1150 ℃), maintaining a small absolute value. Accordingly, its color changed from pale greenish yellow to bright reddish yellow. The Ti0.85Ni0.05Nb0.10O2 pigment synthesized at 1200 ℃ exhibited the highest yellowness, with the values of chromatic parameters L*, a* and b* to be 75.0, 6.7 and 61.3, respectively. Spectral analysis showed that the Ti0.85Ni0.05Nb0.10O2 pigment had high reflectance only in the wavelength range of 570–590 nm, giving it a yellow color. The NIR reflectance reached 83% at about 900 nm waveband, where the energy of sunlight is most concentrated. At calcination temperatures of ≤1100 ℃, the submicrometer-sized pigments with good grain dispersion were obtained. When the temperature was increased to 1200 ℃, however, the pigment's grains grew to about 3 μm and obvious agglomeration occurred. All the pigments synthesized at 1050–1200 ℃ showed high stability and excellent color rendering performance in the transparent ceramic glaze.[Conclusions] Calcination temperature has significant influences on crystalline structure, spectral properties, color performance and morphology of Ti0.85Ni0.05Nb0.10O2 pigments. The Ni-Nb codoping promoted the phase transformation of TiO2 from anatase to rutile. Stable single-phase rutile pigment can be obtained by calcining at 1050–1200 ℃ for 2 h. The increase in calcination temperature from 950 ℃ to 1200 ℃ significantly enhanced the yellowness of the pigments, while their colors changed from greenish yellow to reddish bright yellow. This change was closely related to the crystalline structure transformation, lattice distortion and the generation of charge defects caused by the Ni-Nb codoping. The synthesized Ti0.85Ni0.05Nb0.10O2 yellow pigments had fine grains, especially when the calcination temperature was ≤1100 ℃, while submicron pigments with good grain dispersion were obtained. The pigments showed high stability and excellent color-rendering in the ceramic glaze, together with high NIR reflectance performance. The environmentally-friendly rutile pigment has potential applications in thermal insulation and decorative coatings, glazes and plastics. They are of great significance for promoting energy conservation and emission reduction in areas such as buildings and vehicles.
Key words: ceramic pigments; rutile structure; Ni-Nb co-doping; color-rendering performance; stability