Cool and photocatalytic yellow ceramic pigments; from lead-tin to Cr doped scheelite pigments

The historical lead based pigments (XVII-XVIII century), the modern Ni,Sb-rutile (1939) and Pr-zircon (1961) yellow pigments as well as the novel yellow pigments based on Cr-scheelite and Mo-yttrium cerate pigments were synthesized and compared as semiconductors of high NIR reflectance (cool pigments) and with photocatalytic activity. All powders are wide band gap semiconductors (E_g=2.1–3.1?eV) with high NIR reflectance, ranging from 80% for Cr-Scheelite to 86% of Naples yellow, except Ni,Sb-rutile pigment that shows 55%. The low color performance of lead based pigments in alkyd paint and lead-free glazes along with its toxicity, discard these pigments as cool eco-friendly pigments. The Pr-zircon, Cr-scheelite and Mo-yttrium cerate pigments exhibit high NIR reflectance (R_NIR =?80–83%) and yellow coloration (b*?=?57.6–45.3) but only Pr-zircon and Cr-scheelite show color stability in alkyd paint and lead free glaze applications. Regarding to results of Orange II photodegradation test, lead based and Ni,Sb-rutile pigments inhibits the photolysis of the UV irradiation, acting as protecting agents from irradiation. The Mo-cerate shows a slight photocatalytic activity (t_1/2 =?301?min) and both Cr-scheelite and Pr-zircon pigments show a moderate activity (t_1/2 =?198 and 184?min respectively). Looking at a possible industrial application, the leaching test shows that the fluorides released by Pr-zircon (16.8?mg/g) as well as the Cr(VI) of Cr-scheelite (6?mg/g) should be treated and controlled. The Cr-scheelite solid solution Ca(Cr_xW_1-x)O₄ is stable to x?=?0.2 but its pigmenting capacity saturates at x?=?0.1.     

Physico-chemical properties of ceramic pigments for high-temperature application

Heat resistant coatings are required primarily for stacks, exhaust pipes, reactors, space crafts and similar equipments that are permanently or occasionally exposed to elevated temperatures. High-temperature coatings are generally based on silicone resin with ceramic and metallic pigments. In this study, iron oxide, cobalt oxide (thermo chromic compound) and aluminum oxide are used for the preparation of four new types of coloured pigments. The thermal resistant characteristics of these ceramic pigments were studied by differential thermal analysis, thermo gravimetric analysis and differential scanning calorimetric analysis. These ceramic pigments are found to be thermally stable up to 400 °C.     

New vanadium doped calcium titanate ceramic pigment

In this paper a new pink vanadium doped calcium titanate Ca(V_xTi_1−x)O₃ ceramic pigment in conventional ceramic glazes is obtained by ceramic route and characterized. The limit of solid solution is near by x = 0.2, higher amounts of vanadium crystallizes Ca₂V₂O₇ which dilute the real amount of saturated Ca(V_xTi_1−x)O₃ solid solution and diminish the intensity of colour. The unit cell parameter measurements of Ca(V_xTi_1−x)O₃ agrees with the substitution of Ti⁴⁺ by V⁵⁺ that is associated to a V⁵⁺-O charge transfer at 420 nm on UV-vis-NIR spectra of 5% glazed samples that explain the pink colour obtained. In order to avoid the limitation due to the suppressing of oxygen vacancies by high valence cation V⁵⁺ substitution in a Ti⁴⁺ site of CaTiO₃ perovskite for to preserve the charge neutrality of the lattice; Fe³⁺ and V⁵⁺ codoped samples Ca(Fe_xV_xTi_1−2x)O₃x = 0.1, 0.2 and 0.3 were prepared and show a brown colour fired 1000 °C, but 5% glazed do not produce colour indicating that iron codoping inhibits the pigmenting capacity of vanadium doped CaTiO₃ perovskite.     

Synthesis of black pigments containing chromium from leather sludge

The black ceramic pigments with spinel structure have been prepared by using Cr-rich leather sludge in this paper. The washed Cr-rich leather sludge calcined at 1100 °C for 1 h as chromium oxide precursor (named as CA) was mixed with an appropriate proportion of other industrial metallic oxides, followed synthesizing black ceramic pigment by sintering. Both non-washed and washed sludge fired at 1100 °C were characterized by X-ray fluorescence (XRF) in order to determine their chemical compositions and X-ray diffraction (XRD) analysis to confirm that CA mainly contains Cr₂O₃ crystal phase. The results show that CA could be used as a source of chromium to prepare black pigment. The crystalline phases of obtained pigments were characterized by XRD. Furthermore, the morphology as well as the composition of pigments was investigated by scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS). The color coordinates of pigments were examined and compared with the commercial pigments based on CIE-L* a* b* values measured using UV–vis spectroscopy. The obtained pigments sintered at 1200 °C with 35–55 wt% content of CA possess the excellent black spinel structure and color effect. Under optimized conditions, the pigment has low average spectral reflectance (7%).     

Improved coloristic properties and high NIR reflectance of environment-friendly yellow pigments based on bismuth vanadate

Herein, fine powders of yellow pigments based on BiVO₄ were successfully prepared in two crystalline forms, tetragonal zircon-structured and monoclinic scheelite-structured, through a ethylene glycol-assisted method subjected to a thermal treatment at two different temperatures. The obtained materials were brightly colored with different hues of yellow, exhibited high reflection in the near-infrared region and showed visible luminescence under excitation by UV light. The new preparation method had a considerable effect on chromatic properties of the prepared scheelite-structured BiVO₄ pigments. The sample with the most vivid and bright shade of yellow was found to have the L^*a^*b^* and L^*C^*_ab h^*_ab color coordinates of (87.28, 0.37, 91.53) and (87.28, 91.53, 89.79), which are indicative of exceptionally good chromatic properties superior and/or comparable to those of other inorganic yellow pigments, both commercially available and recently described. In addition, the NIR reflectance of this powder was very high (≥?80%).     

Ceramic pigments based on chromium doped alkaline earth titanates

In this paper M(Ti_1?xCr_x)O₃ with M=Mg, Ca, Sr, Ba and x=0.01, 0.03, 0.05, 0.1 compositions have been prepared by solid state reaction in order to analyze their reactivity, structure, limit of solid solution, stability and pigmenting properties in conventional ceramic glazes. Cr–MgTiO₃ crystallizes magnesium ilmenite MgTiO₃ (SG=R-3) with residual MgCr₂O₄ spinel that inhibits the Cr entrance in solid solution and do not show pigmenting properties. Cr–CaTiO₃ crystallizes orthorhombic perovskite (SG=Pbnm) pink solid solutions associated to Cr⁴⁺–Ti⁴⁺ substitutions which above x=0.03 unstabilizes in glazes and produce light green shades. Cr–SrTiO₃ crystallizes ideal cubic perovskite (SG=Pm3m) pink solid solutions that unstabilizes in glazes above x=0.01. Finally Cr–BaTiO₃ crystallizes tetragonal perovskite (SG=P4mn) with residual peaks of BaCrO₄ at x=0.1 showing pink solid solutions until x=0.05 that unstabilizes in glazes above x=0.03. All powders show aggregates (between 2–10 μm of size) of fine particles (between 200–1000 nm).     

Synthesis of CoAl2O4/Al2O3 nanoparticles for ceramic blue pigments

The demand for pigments for industrial ceramic ink-jet printing is increasing steadily. The main challenge is the availability of ceramic pigments comprised of small-sized particles in order to avoid clogging of the printer head nozzle. This work presents a novel way to prepare ceramic blue pigment as a shell/core structure of CoAl₂O₄/Al₂O₃ nanoparticles through a simple and low-cost process. In this process, the colour tone of nano-pigment can be tuned by the selection of the cobalt precursor/Al₂O₃ ratio and calcining temperature.     

Synthesis and properties of blue zirconia ceramic based on Ni/Co doped Ba0.956Mg0.912Al10.088O17 blue pigments

Novel blue pigments based on Ba_0.956Mg_0.912Al_10.088-xNi_xO₁₇ (0 ≤ x ≤ 0.3) and Ba_0.956Mg_0.912Al_10.088-xCo_xO₁₇ (0 ≤ x ≤ 0.3) solid solutions were successfully synthesized by solid state method. The XRD results confirmed the structure of the as-synthesized sample belongs to hexagonal β-alumina structure with the space group of P6₃/mmc. The d-d electron transitions in Ni²⁺/Co²⁺ tetrahedral sites in the visible light range are the reason for the blue colors. Then, the as-prepared blue oxides were sintered with ZrO₂ powders at 1400 °C to prepare blue zirconia ceramic materials. Based on XRD and SEM analysis, the pigment phase is stable after high-temperature sintering with ZrO₂, and a clear grain boundary is observed. The XCT results indicate the prepared blue ceramics are dense and very small pores are rarely distributed inside the blue zirconia ceramic body. Additionally, the mechanical properties of the fabricated blue ceramics were maintained compared to pure ZrO₂ ceramic.     

Iron and chromium doped perovskite (CaMO3 M = Ti,Zr) ceramic pigments,effect of mineralizer

Solid solutions Ca(D_xM_1?x)O₃ (M = Ti, Zr and D = Fe,Cr), have been studied as ceramic pigment in conventional ceramic glazes using 0.5 mol/mol of NH₄Cl as flux agent by solid state reaction and by ammonia coprecipitation route. Ca(Cr_xTi_1?x)O₃ compositions obtained without addition of NH₄Cl as mineralizer, produce pink color in glazes at low x but CaCrO₄ crystallizes when x increases, producing undesired green colors. The crystallization of chromates can be avoided using NH₄Cl as mineralizer, giving a complete solid solution that produce pink color in glazes at low x and dark blue shades at high x. Coprecipitated sample produce blue colors at low x and at low temperature than ceramic sample (1000 °C instead 1200 °C for CE sample). Cr⁴⁺ ion acts as red chromophore, but at higher x values (blue samples) Cr³⁺ ion entrance affects the color. Ca(Fe_xTi_1?x)O₃ system crystallizes perovskite CaTiO₃ and pseudobrookite Fe₂TiO₅ together with rutile as residual crystalline phase, glazed samples change from a yellow to a pink color associated to the increase of pseudobrookite with firing temperature. Ca(Fe_xTi_1?x)O₃ and Ca(Cr_xZr_1?x)O₃ systems crystallize perovskite CaZrO₃ and zirconia (ZrO₂) in both monoclinic and cubic polymorphs, but iron or chromium oxides are not detected in the powders. Coprecipitated sample stabilises cubic form. The solid solution is not reached completely in these samples and is not stable in glazes.     

Solubility of blue CoAl2O4 ceramic pigments in water and diethylene glycol media

The chemical stability of blue CoAl₂O₄ ceramic pigment in aqueous and non-aqueous suspension was investigated. Distilled water and diethylene glycol (DEG) were used as media. The isoelectric points (IEP) for the commercial and synthesised CoAl₂O₄ pigment were determined to be 4.9 and 8.5, respectively. The IEP shifts toward acidic pH values for the commercial pigment with respect to the synthesised pigment due to the existence of the quartz phase. In water, the cobalt ion concentration was low (2.5 mg dm⁻³), and this concentration did not change with time. However, the aluminium ion concentration was initially high and then decreased to ∼10 mg dm⁻³ at pH 9 and 11 due to the precipitation of Al³⁺ ions as an Al(OH)₃ phase. In DEG, the dissolution of pigment particles with higher ion concentrations (>800 mg dm⁻³) was more significant than that of the aqueous medium, implying that CoAl₂O₄ pigment has insufficient chemical stability in DEG medium with respect to aqueous suspensions.     

Green ceramic pigment based on chromium recovered from a plating waste

The development of pigments with new formulas, departing from lower-cost raw materials, is driven by market growth and competitiveness required by the ceramic sector. In this work, the application of a residue from a metallurgical plant, containing a high amount of chromium (III) in ceramics glazes was studied. Cr₂O₃ was successfully separated from chrome plating waste resulting in a suitable synthesize ceramic pigment, based on this chromophore. A washing treatment was applied to the residue followed by a calcination at 1000 °C prior to the usage as pigment. The adequate purity of Cr₂O₃ recovered from waste was confirmed by analytical techniques (XRF, XRD, DTA-TG, FTIR and SEM-EDX). The glasses, containing 5 wt% of recovered Cr₂O₃, were carried out using frits and transparent micronized glass for mono- and double firing processes. The materials were characterized by XRD and FTIR, and by the colorimetric parameters CIELab. The resulting glazes containing the waste are within the range of typical pigments based on Cr₂O₃ in silicate and aluminate glazes for single firing, showing dark green hues in comparison with the previous data obtained from commercial grade Cr(III) oxide. However the presence of small amounts of the other chromophores in the waste modifies the tonality, affecting the purity of the chroma by increasing darkness. The leaching tests performed on the glazes suggest that Cr(III) is stable in this ceramic matrix. This results show that glazing is a suitable process for the Solidification/Stabilization of this waste.     

The stability of gahnite doped with chromium pigments in glazes from the French manufacture of Sèvres

The French manufacture of Sèvres, famous for its production of fine porcelain artifacts, has been synthesizing pink pigments mainly composed of the spinel phase ZnAl_2?xCr_xO₄ with x varying from 0.25 to 0.41 since the middle of the 19th century. This kind of pigment is mixed with an uncolored frit to obtain decorations for porcelain artifacts. However, the pink color of the pigment is altered in a particular uncolored frit and a brownish color appears. The mechanism of this color change was investigated. Observations under a scanning electron microscope revealed the formation of a phase rich in Cr resulting from reactions between the uncolored frit and the pigment during firing. X‐ray diffraction combined with Rietveld refinements and X‐ray absorption near edge structure measurements at the Cr K‐edge showed that the new formed phase belongs to the same spinel phase ZnAl_2?xCr_xO₄ than the pigment, but with a higher Cr content x. We showed that its formation and thus the stability of the pigment is driven by the Al content in the uncolored frit.     

Synthesis,phase composition and characterization of Co-diopside ceramic pigments

Ceramic pigments in the system CaO – CoO – MgO – 2SiO₂ were synthesized through solid-state high temperature sintering at 1000, 1100 and 1200 °C. The starting compositions were determined from the stoichiometric mineral diopside, following the expression CaO.xCoO.(1-x)MgO·2SiO₂, where x = 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.5, 0.7 and 0.9. The effect of sintering temperature and initial cobalt concentration on the phase composition and colour were studied by powder X-ray diffraction analysis, scanning electron microscopy (SEM), infrared (FT-IR), ultraviolet–visible - (UV–Vis) and electron paramagnetic resonance (EPR) spectroscopy. Poly-phase ceramics were obtained depending on synthesis parameters, which in addition to Co-diopside, may contain wollastonite, Co-åckermanite – Ca₂(Co, Mg)Si₂O₇, Co-olivine – CoMgSiO₄, Co-monticellite – Ca(Mg, Co) SiO₄, and cristobalite. Quantitative ratios of detected mineral phases and the degree of cobalt isomorphic substitution were determined by applying Rietveld refinement approach. Trends and consistencies were outlined revealing that concentration of cobalt in the initial synthesis batch and its isomorphous incorporation in the crystal structure of diopside affect the structural perfection of the cation polyhedra. This fact obviously plays a key role in controlling the amount of the main constituent phase in the run-product. Ceramics of highest diopside content and lowest measured degree of structural distortion were only formed at 1200 °C and initial cobalt concentration in the range x = 0.2–0.3. These samples are pink in colour and have the highest value of red coordinate in the CIELab system due to Co²⁺ ions in octahedral coordination in diopside structure.     

Synthesis and characterization of (Pr,Ce)-ZrSiO4 ceramic pigments: The properties of the pigments and the effect of Ce

A series of (Pr, Ce)-ZrSiO₄ ceramic pigments were synthesized and characterized using XRD, SEM, EDS, XPS, XRF, colorimeter, and a UV-VIS-NIR spectrometer. The prepared pigments were mainly composed of the zircon phase and were well crystallized. Only a specific amount of Pr was incorporated into the ZrSiO₄ lattice. Compared with Pr, Ce was almost completely incorporated into the ZrSiO₄ lattice and was homogeneously distributed within the pigment particles. The dopant Ce reduced the amount of Pr dissolved in the ZrSiO₄ lattice and thus caused the b* values of the samples to decrease slightly. Meanwhile, the presence of Ce induced an apparent increase in red tone in the samples. The enhanced red tone resulted primarily from an increase in the absorption of light with wavelengths between 500 and 565 nm. High-temperature stability analysis demonstrated that it is feasible to improve the tone of Pr-ZrSiO₄ pigment by doping with Ce.     

Synthesis and study of a new class of red pigments based on perovskite YAlO3 structure

A red pigment has been prepared by substituting chromium ions in aluminum ion sites in YAlO₃ perovskite structure. In a first step, effect of various mineralizers on YAlO₃ formation has been investigated, which resulted in decrease of formation temperature down to 1400 °C. In the next step, a red pigment corresponding to YAl_1−yCr_yO₃ (y = 0.05), has been prepared by heating a mixture containing Y₂O₃, Al₂O₃ and Cr₂O₃ at 1500 °C for 6 h. Later, effect of the doped chromium amount on the pigment redness (a^*) has been studied. The highest redness has been obtained when y was 0.04(YAl_1−yCr_yO₃). Application of the prepared red pigment in low and high temperature glazes, demonstrated its high chemical and thermal stability.     

Brown ceramic pigments based on chromium(III)-doped titanite obtained by spray pyrolysis

Cr-doped titanite (CaTiSiO₅) pigments were synthesized through spray pyrolysis of aerosols generated from aqueous solutions containing colloidal silica, calcium chloride, titanium(IV) oxychloride and chromium(III) nitrate. This process yielded amorphous powders with spherical morphology and broad size distribution (<10 μm) after thermal decomposition at 600 °C. The titanite phase was obtained by further calcination at 800 °C without any addition of flux agents. The brown color of the pigments can be attributed mainly to the existence of Cr(IV) ions occupying both, octahedral positions of Ti(IV) and tetrahedral position of Si(IV), together with a small amount of Cr(III) present as Cr₂O₃. The optimum pigment obtained by this method corresponded to a Cr/titanite mole ratio of 0.04.     

Multifunctional superhydrophobic and cool coating surfaces of the blue ceramic nanopigments based on the heulandite zeolite

The heulandite zeolite was successfully used as the host structure for a new Co-based blue colorant ceramic pigment. The method was the ion exchange of Co and Zn cations with the host structure, which induced a heat-collapsed structure followed by a recrystallization process at 1200 °C/h. The synthesized blue pigment was coated by spraying on the surface of pre-sintered ceramic tile followed by firing at 980 °C/10min. For modification of its surface, ZnO nanoparticles (NPs) and commercial antifouling were used. The blue powder pigment and ceramic coated surface were characterized with XRD, Rietveld refinement, TGA-DTA, FE-SEM-EDS, BET-BJH, UV-VIS-NIR spectroscopy, CIEL*a*b* color measurement, water contact angle (WCA), and roughness analysis. The single-crystal Co-willemite has been achieved (b* = ?45 and a* = ?0.5). The blue pigment coated on the surface after being modified with ZnO NPs and antifouling revealed a superhydrophobic surface with WCA = 153.7°. Due to this, a low water sliding angle of less than 5° have achieved. The surface showed high thermal stability and adherence. The superhydrophobicity was mainly due to the micro-nanopatterned lotus leaf-like crystal structure and roughness of 3.15 μm on the surface. The surface showed 64% total reflectance from 700 to 2500 nm (UV and NIR), which resulted in a perfect candidate for a cool coating.     

Synthesis of Y(Al,Cr)O3 red pigments by co-precipitation method and their interactions with glazes

New red pigment based on the system YAl_1−yCr_yO₃ (y = 0.01–0.1) was synthesized by co-precipitation method. The precipitant was attained by mixing solutions of yttrium, aluminum and chromium nitrates, respectively, and addition of ammonia as the precipitator. The effects of chromium as dopant and glaze composition on the color shade of resulting pigments were studied. EDX analysis of the prepared pigment particles, which was embedded in glaze, showed the occurrence of reactions between some glaze constituents and pigment particles. Accordingly, a glaze which was enriched in Al₂O₃ and poor in ZnO was more suitable in point of achieving a reddish shade. The resulting pigments were characterized by using X-ray diffraction (XRD), SEM and UV–vis spectrophotometer.     

The effect of mineralizing additives on synthesis of ceramic pigments based on natural wollastonite

The mineralizing effect of additives of natural fluorine-bearing mineral topaz and boric acid on the synthesis of ceramic pigments with the diopside and anorthite structures based on natural wollastonite is studied. The brightest color range is obtained at 1100°C. It is established that topaz has obvious advantages over the traditional mineralizer, i.e., boric acid. The obtained ceramic pigments have good chromophore properties, are resistant at high firing temperature, and can be extensively used in ceramic production. __________ Translated from Steklo i Keramika, No. 1, pp. 21–24, January, 2006.