CoAl2O4 Pigment Research Articles

New insights on the optical properties and upconversion fluorescence of Er-doped CoAl2O4 nanocrystals.

In this study, Er-doped CoAl2O4 nanocrystals (NCs) were synthesized via co-precipitation. All the NCs were crystallized in the form of a single phase with a spinel structure and Er3+ ions replaced Al3+ ions in the formation of the CoAl2-xErxO4 alloy structure. The optical characteristics of the Er3+ ion-doped CoAl2O4 NCs were thoroughly investigated by analyzing both the UV-VIS and photoluminescence spectra, using the Judd-Ofelt theory. The effect of Er doping content on the luminescent properties of the CoAl2O4 pigment (using lasers emitting at wavelengths of 413 and 978 nm) has been studied. The values of Judd-Oflet intensity parameters (Ω2, Ω4, and Ω6) were determined from the absorption spectra using the least square fitting method. The J-O parameters were calculated and compared with those of other host materials; the values of the Ω2, Ω4, and Ω6 parameters decreased with an increase in Er concentration. This suggests that the rigidity and local symmetry of the host materials become weaker as the concentration of Er3+ ions increases. The highest value of the Ω2 parameter, when compared with Ω4 and Ω6, suggests that the vibrational frequencies in the given samples are relatively low. The upconversion fluorescence phenomenon was observed and explained in detail under an excitation wavelength of 978 nm when the excitation power was varied.

Study of the Synthesis Variables in the Preparation of CoAl2O4 Pigment Using Microwaves to Reduce Energetic Consumption

Due to the importance of optimizing the manufacture of ceramic pigments, motivated by the increase in prices of both raw materials and energy, and the need to control manufacturing parameters to obtain optimal conditions for the preparation of ceramic inks, two synthesis routes (traditional route and coprecipitation) and two calcination methods (traditional oven and microwave oven) are proposed to obtain the blue ceramic pigment CoAl2O4 with the aim of minimizing the use of mineralizers or flux agents and reducing energy consumption in its manufacturing. The pigments prepared were characterized by thermal analysis and structurally by XRD and SEM, with particle sizes below 300 nm observed. Finally, the colorimetric coordinates of glazed tiles with the pigments obtained were characterized. In all cases, the microwave-assisted synthesis increased the color intensity, considerably decreasing the temperature and calcination time, obtaining a particle size under 300 nm with a very narrow size distribution, and substantially improving the energy cost of its preparation and the color development of the final product. The viability of the combination of synthesis by coprecipitation and microwave calcination as a method of industrial preparation of ceramic pigments has been demonstrated.

Utilization of Sea Sand for Preparation of High-Performance CoAl2O4 Composite Pigments via a Cleaner Mechanochemistry Route

Exploitation and utilization of low-value natural mineral resources in a feasible way are promising under the background of green sustainable development. In this study, high-performance cobalt blue composite pigments with low cobalt consumption were facilely fabricated based on sea sand via a cleaner mechanochemistry route followed by a calcining process. It was revealed that cobalt blue nanoparticles were loaded on the surface and near edge areas of quartz in sea sand. The desirable blueness was related to the improved dispersion of cobalt blue nanoparticles as well as the strengthened reflection to blue light by quartz. In addition, the removal of existing Fe species in sea sand had a positive effect on the blue intensity of composite pigments. The prepared composite pigments based on original sea sand presented an excellent acid resistance and color performance, which were sufficient to satisfy the needs of the related fields. Therefore, this study is expected to develop a feasible and cleaner strategy for the direct value-added utilization of sea sand and propose a novel design for the construction of high-performance CoAl2O4 pigments with low cobalt consumption as well.

Mechanochemical preparation of low-cost cobalt blue composite pigments with good color and acid resistance based on desert sands

The development of a facile cleaner route is important for the preparation of low-cost cobalt blue pigments with high blue chroma. In this study, three desert sands were directly employed to prepare cobalt blue composite pigments via a cleaner CaCO3-assisted mechanochemical method followed by a calcining process. The color property, phase compositions, microstructure and acid resistance of composite pigments were studied in combination with the relevant characterization techniques. The results indicated that incorporation of desert sands improved the distribution of CoAl2O4 nanoparticles and reflected more blue light at 430 nm, and thus composite pigments presented excellent color properties (L* = 38.87, a* = −0.75, b* = −44.75) compared with the pure CoAl2O4 pigments and most reported pigments. Furthermore, the color property of the pigments was related to the chemical compositions of the original desert sands, and the color of the composite pigments could be adjusted by gradient acid etching of desert sands. Meanwhile, the obtained composite pigments exhibited excellent acid resistance and good application properties as painting ink on plain paper. Therefore, this study provided a promising strategy for the preparation of cobalt blue composite pigments with low production cost and cobalt content based on desert sands.

Preparation and characterization of supercritical fluid - fried (CoAl2O4) cobalt blue nano-pigment

ABSTRACT Cobalt blue pigments, which mainly comprise CoAl2O4, are widely synthesized as nano-scale particles using the coprecipitation method. Such ultrafine powders are necessary for the ceramics industry because of their significant effect on the color of glaze and bulk tiles. In this paper, cobalt blue pigments have been prepared using the coprecipitation method and then subjected to supercritical fluid drying. The differences in their physicochemical and optical properties are characterized using a series of analysis including X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive spectrometer (EDS), and CIE-L*a*b*colorimetry analysis. The results indicate that the CoAl2O4 pigments subjected to supercritical fluid drying exhibit a fine-scale structure, a crystallite size of approximately 52 nm at 1400°C, and an excellent stability at high temperatures. They also present a vivid blue color, with improved brightness and hue than those of other cobalt blue pigments. Furthermore, this study elucidates the promising potential of supercritical fluid drying for fabricating nano-sized CoAl2O4 pigments with excellent optical and physicochemical properties.

Preparation and coloring mechanism of MAl2O4/CoAl2O4/quartz sand (M = Ca or Ba) composite pigments

The doping of divalent metal ions is considered as a feasible strategy for reducing the consumption of expensive cobalt source and regulating the color performance of CoAl2O4 pigments. However, it may be controversial for formation of solid solution in the system of MAl2O4/CoAl2O4 (M = Ca or Ba) due to the bigger differences in ionic radius and coordination number between Ba2+ (or Ca2+) and Co2+, and thus the coloring mechanism of CoAl2O4 pigments is vague after incorporation of Ba2+ (or Ca2+). In this study, MAl2O4/CoAl2O4/quartz sand (M = Ca or Ba) composite pigments are fabricated using the quartz sand waste discarded from mineral processing as substrate. The effect of the added amount of Ba2+ (or Ca2+) on the color properties of composite pigments are investigated, and the optimum b∗ values of CaAl2O4/CoAl2O4/quartz sand and BaAl2O4/CoAl2O4/quartz sand reach −47.92 and −48.06, respectively. Furthermore, it reveals that the obtained pigments are composed of quartz, hexagonal CaAl2O4 or BaAl2O4, and cubic spinel CoAl2O4, and thus CaAl2O4 or BaAl2O4 cannot form solid solution with CoAl2O4 by replacing Co2+ with Ba2+ or Ca2+ owing to the significant differences in ionic radius and coordination number between Ba2+ (or Ca2+) and Co2+. The improvement in the color performance of CoAl2O4 pigments may be related to the intrinsic oxygen vacancy defect of CaAl2O4 or BaAl2O4, which promote CoAl2O4 to reflect more blue visible light at 430 nm.

Research on the low-temperature synthesis of cobalt aluminum spinel type blue pigments

In this paper, CuxCo1−xAl2O4 (x = 0.1–0.5) blue pigments have been successfully synthesized by combustion method at a low temperature (700 °C). Replacing Cu2+ for Co2+ in CoAl2O4 pigment enhanced the violence of the combustion reaction and the spinel-type precursor can be directly obtained. Therefore, it is conducive to synthesis of CoAl2O4 pigment at a low temperature. The intensity of blue hue of CuxCo1−xAl2O4 (x = 0.1–0.5) pigments decreases with increase of the Cu/Co ratio, and it increases with increase of the calcination temperature. When the “x” is the optimal value of 0.1, the CuxCo1−xAl2O4 pigment prepared at 700 °C exhibited excellent color performance similar to the previously reported CoAl2O4 pigment, which the chromatic value is L* = 39.51, a* = −16.67, b* = −21.58, C* = 27.27. After applying the pigments in transparent glaze and ceramic ink, the pigments exhibit high tinting strength and stability, indicating its potential application in ceramic decoration and ceramic ink-jet printing technology. This work provides a facile strategy of synthesis spinel-type powder at a low temperature.

Synthesis and color properties of the CoAl2O4/Al2O3 hybrid blue pigments with low cobalt contents

The CoAl2O4/Al2O3 hybrid pigments with low cobalt content were prepared by heterogeneous chemical precipitation and calcination methods. The structure and properties of the synthesized pigments were characterized by XRD, SEM, EDX, UV–Vis spectroscopy and colorimetry. Results showed that the hybrid pigments were consisted of the CoAl2O4 spinel and Al2O3 corundum, and the CoAl2O4 spinel evenly covered on the surface of Al2O3 corundum. With the increasing Co/Al mole ratio, the CoAl2O4 spinel contents in the hybrid pigments increased, and the blue components (− b*) of the pigments were elevated. Comparing with pure CoAl2O4 pigments, the cobalt content of hybrid pigments was lower, but its blue components (− b*) were exceeded to 40 when the Co/Al mole ratio reached up to above 0.10, which was closed to that of the commercial cobalt blue pigments. The synthesized pigments presented good chemical and thermal stability, high tinting strength. The vivid blue PU paint and resin, blue glaze with low cobalt content were obtained by mixing 5 wt% CoAl2O4/Al2O3 hybrid pigments. Therefore, the hybrid pigments could be used as candidate to substitute for the expensive commercial cobalt blue pigments because of its lower cobalt content and brilliant blue hue.

High-performance spherical urchin-like CoAl2O4 pigments prepared via microemulsion-hydrothermal-precipitation method

In this study, the spherical urchin-like CoAl2O4 pigment was prepared using a microemulsion-hydrothermal-precipitation (M-H-P) method. This method was used to prepare the high-performance CoAl2O4 pigment, which has good dispersibility, small particle size, narrow particle size distribution, and large BET surface area. Use of this method also successfully achieved the recycling of organic solvents. SEM and TEM results show that the CoAl2O4 pigment is the soft-aggregation of lamellar nanocrystals with a mean particle size of 1–1.2 μm. The urchin-like pigment exhibits better color performance than the previously reported CoAl2O4 pigment. After applying the pigment on a high-temperature transparent glaze, the pigment exhibits excellent coloration, indicating its potential application in ceramic decoration and ceramic ink-jet technology. This study provides a new method for preparing high-performance powders.

Preparation and characterization of encapsulated CoAl2O4 pigment and charge control agent for ceramic toner via suspension polymerization

Styrene-butyl acrylate resin encapsulated CoAl2O4 pigment and charge control agent (CCA) for ceramic toner were synthesized via suspension polymerization. The components of composite particles were investigated by fourier transform infrared spectroscopy (FTIR), energy dispersive spectrometer (EDS) and thermogravimetric analysis (TGA). The characterization results of scanning electronic microscopy (SEM) and laser particle size analyzer showed that the composite particles have good sphericity, the smaller particle size and narrower size distribution when the stirring speed, dosages of PVA, CoAl2O4 and CCA were 400 rpm, 1.0 g, 1.5 g and 0.05 g, respectively. The colorimetric properties of the composite particles are related with the content and dispersion of the pigment particles, the blue value (b*) and the color saturation (C*) of the composite particles increased with the CoAl2O4 content, when the amount of CoAl2O4 pigments was 1.5 g, the color performance was the best. In addition, the colormetric parameters of the calcinated composite sample (at 1000 °C) were better than those of the sample before calcination, and almost the same as those of the original CoAl2O4 pigment.

Formation and Coloring Mechanism of Typical Aluminosilicate Clay Minerals for CoAl2O4 Hybrid Pigment Preparation.

Different kinds of aluminosilicate minerals were employed to fabricate CoAl2O4 hybrid pigment for studying its formation and coloring mechanism. It revealed that the color of the obtained hybrid pigments was determined by the content of Al2O3 and lightness of clay minerals. The higher the Al2O3 content and the lightness of clay minerals, the better the color parameters of hybrid pigments. During the preparation of hybrid pigments, CoAl2O4 nanoparticles were confined to be loaded on the surface of the aluminosilicate minerals, which effectively prevented from the aggregation and the size increase of CoAl2O4 nanoparticles. What's more, aluminosilicate mineral might be an ideal natural aluminum source to compensate the aluminum loss due to the dissolution of Al(OH)3 at alkaline medium during precursor preparation, keeping an optimum molar ratio of Co2+/Al3+ for formation of spinel CoAl2O4 pigments in the process of high-temperature crystallization.

Synthesis and luminescence properties of Er-doped and Er/Yb-codoped CoAl2O4 pigments

The development of methods to obtain ceramic pigments with fluorescence characteristics or hyper-domain performance has attracted research interest. In this study, Er-doped and Er/Yb-codoped CoAl2O4 pigments were synthesized by co-precipitation. The influence of Er doping, Er/Yb codoping, and the doping contents on the colour, composition, and luminescence properties of CoAl2O4 pigments were investigated. Er doping did not significantly affect the phase composition and colour performance of the CoAl2O4 pigment. The upconversion effect was induced by excitation with a 950 nm laser. Er doping could enhance the upconversion luminescence intensity of the CoAl2O4 pigment. At a 1 mol.% Er doping content, all luminous intensities reached maximum values. The downconversion effect was observed after excitation with a 350 nm laser. The luminescence was blue, and slight differences in the luminescence intensity with different Er doping contents were observed. Yb2O3 was formed after Er/Yb was codoped at > 6 mol.% Yb doping content, thereby affecting the phase composition and colour performance of the CoAl2O4 pigment. The luminous intensity of red light (739 nm) was the strongest in all cases. The luminous intensities of all of the colours decreased to the minimum value at 9 mol.% Yb doping content compared with those at other doping contents. At 3 mol.% Yb doping content, the luminous intensities of all colours decreased to very low values (close to 0), and the quenching effect of Yb3+ was achieved using a 350 nm laser. Er/Yb-codoped CoAl2O4 pigments exhibited the upconversion luminescence effect under an infrared laser and inhibited visible-light reflection under an UV laser. This ceramic pigment can be developed further to obtain ceramic pigments with anti-counterfeit function.

Controlled synthesis of blue spherical CoAl2O4 pigment powder in Pickering emulsion assisted with a hydrothermal process

A spherical blue CoAl2O4 pigment powder commonly used for coloring ceramic products has successfully been synthesized by co-precipitation of Co2+ and Al3+ ions in Pickering emulsion assisted with a hydrothermal process, and the formation mechanism of the as-prepared powder has briefly been discussed. The effects of pH condition on the crystalline phase and color tone of the pigment powder were investigated at 280 °C by varying the pH value of the reaction system. Then, under an optimized pH condition to produce blue, the pigment powder was synthesized still hydrothermally at different temperatures (210, 240, 270 and 300 °C) to assess the effect of hydrothermal reaction temperature on the pigment particle morphology. The resulting pigment powders were characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), CIELAB colorimetric analysis (CIE-L∗a∗b∗), laser particle size analysis (LPS), and N2-adsorption measurement (BET). FE-SEM and TEM observations indicated that the pigment powders of spherical shape obviously were constructed by a core–shell double-layer structure, the radii of which all were ∼150 nm or so with a shell of ∼25 nm in thickness. XRD results demonstrated that the pigment powders that were obtained from the aforementioned method under a wide range of pH values invariably contained basically crystalline CoAl2O4 of high purity. CIE-L∗a∗b∗ data suggested that the pigment powders at 280 °C and pH ≤ 11.2 appeared different chromas of bluish colors compared withthat displaying black which was obtained at pH = 12.3. LPS and BET results showed that the CoAl2O4 pigment powder prepared under the urea concentration of 3.6040 g for 24 h at the hydrothermal temperatures of 240 °C exhibited a narrow particle size distribution, the specific surface area of which was evaluated to be ∼56.4 m2/g.

Environmental blue CoAl2O4 pigment co-doped by Zn2+ and Mg2+: synthesis, structure and optical properties

ABSTRACTNano-sized blue solid solutions ZnxMg0.5−xCo0.5Al2O4 (x = 0–0.5) have been synthesised by the Pechini method. Single-phase ZnxMg0.5−xCo0.5Al2O4 with crystallite size of ∼40 nm was identified by XRD measurement. The TG-DSC results indicated that the phase formation temperature of ZnxMg0.5−xCo0.5Al2O4 increased with the substitution of Zn2+/Mg2+ → Co2+ proceeding. The UV–vis spectra illustrated that the Zn0.3Mg0.2Co0.5Al2O4 pigment displayed the most intensive blue colour with the strongest absorbance appearing within the visible region. The FT-IR spectra suggested that the inversion degree of ZnxMg0.5−xCo0.5Al2O4 pigment reduces with the increase of Zn2+ rather than Mg2+, enabling to control the pigment colour by tuning the Zn2+ content. The FE-SEM images showed an irregular shaped morphology of ZnxMg0.5−xCo0.5Al2O4 crystal, different from the cubic-like morphology of CoAl2O4 crystal. The XPS results illustrated that the inversion of pure CoAl2O4 pigment is larger than that of Zn0.3Mg0.2Co0.5Al2O4. Both Zn0.2Mg0.3Co0.5Al2O4 and Zn0.3Mg0.2Co0.5Al2O4 show commercial potential in pigments application.

Optimization of Aqueous Nano Ceramic Ink and Printing Characterization for Digital Ink-Jet Printing

The advantage of ceramic ink-jet printing technology is the accurate and fast printing process of digital images for various products. For digital ink-jet printing applications, ceramic ink requires proper viscosity and surface tension, along with dispersion stability of the inorganic pigments. The purpose of this study is the formulation of an environment-friendly ceramic ink with a water-based system; using nano-sized CoAl2O4 pigment as a raw material, ink should have dispersion stability to prevent nozzle clogging during ink-jet printing process. In addition, the surface tension of the ceramic ink was optimized with the polysiloxane surfactant according to the surface tension requirement (20 - 45 mN/m) for ceramic ink-jet printing; by adjusting the viscosity with poly ethylene oxide, jetting behavior of the ceramic ink was investigated according to changes in the physical features through drop watcher measurement. Key words: Suspensions, Traditional ceramics, Ink-jet printing, Raw material, Aqueous ceramic ink

Effects of colouration mechanism and stability of CoAl2O4 ceramic pigments sintered on substrates

CoAl2O4 ceramic pigments produced by common methods have low chroma values, which limit their application in ceramic glazing, enamel glazing, and ceramic product printing. The colour stability of the ceramic pigments and changes upon high-temperature sintering are important factors, especially in 3D printing. In this study, CoAl2O4 ceramic pigments were synthesized using a co-precipitation method and both the colouration mechanism and stability of CoAl2O4 ceramic pigments were investigated. The Co content, Co ion valence, and coordination number were found to be important factors affecting the CoAl2O4 ceramic pigments. When the Co content was excessively high, either Co3+was produced or Co2+ entered the octahedron spaces of the spinel structure. The coordination field of the Co ions was consequently altered and the colour transitioned from blue to green. After sintering on substrates at 1300°C, the brightness and chroma of obtained CoAl2O4 coatings were superior to those of the pigment powders prior to sintering, and the colour of CoAl2O4 with a relatively high Co/Al ratio changed from green to blue. The latter pigment may have formed a partially inverse spinel structure upon sintering at a high temperature. The probability of Co2+ entering the octahedron spaces increased; concomitantly, the number of Co2+ in the tetrahedron spaces decreased, resulting in an increase in the coordination number of Co. In addition, Co3+ in the ceramic pigments was transformed into Co2+ by deoxidation reactions during the coating sintering process, thereby increasing the chroma of blue CoAl2O4 pigments.

Energy Efficiency in the Microwave-Assisted Solid-State Synthesis of Cobalt Aluminate Pigment

Abstract: Due to their rapid, selective, and volumetric heating, microwaves have been widely used in the past to enhance solid-state reactions as well as the synthesis of ceramic pigments. The aim of this work is to present a case study involving the preparation of blue CoAl2O4 pigment using different microwave applicators and generator frequencies, showing the advantages which can derive from a properly designed microwave reactor for the solid-state synthesis of such pigment. The results show that, when using a properly designed microwave applicator, the specific energy consumption can be significantly lowered compared to conventional heating techniques. Consistently with the data on the dielectric properties of precursors found in literature, the microwave processing of cobalt oxide and aluminum hydroxide mixtures resulted more favorable at 2.45 GHz than at 5.8 GHz microwave frequency.

Bright blue halloysite/CoAl2O4 hybrid pigments: Preparation, characterization and application in water-based painting

Bright blue CoAl2O4/halloysite hybrid pigments were prepared via combining chemical co-precipitation and calcination process. The obtained hybrid pigments were characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, high resolution transmission electron microscopy, and ultraviolet–visible spectroscopy. The introduction of halloysite not only decreased the calcining temperature for formation of spinel-type CoAl2O4, but also enhanced the color properties of CoAl2O4 pigments including hue, brightness and chroma. The hybrid pigments incoropated 60% halloysite presented excellent color properties (L* = 54.6, a* = −2.5, b* = −50.4, h* = 262.21, and C* = 47.35) and excellent environmental resistance tolerance toward acid, alkali and ultraviolet light. In particular, the hybrid pigments could be well dispersed into water-based paint with good suspension stability. It indicated that the hybrid pigments could be served as a promising pigment with bright blue to be applied in various fields such as ceramics, water based paints, inks, etc.

Spectral properties of the UV absorbing and near-IR reflecting blue pigment, YIn1-xMnxO3

Materials in the full miscibility range of YInO3 – YMnO3 were prepared and compared to CI Pigment Blue 28 (CoAl2O4 spinel) industrial pigments. The materials were analyzed by XRD, UV–Vis–NIR spectroscopy, SEM, Laser particle size analysis, and accelerated/outdoor weathering. The optimal composition, YIn0.8Mn0.2O3, is comparable to standard CoAl2O4 pigments, for example, having similar CIE color L* (∼34) and b* (∼-39) values, and increased a* (∼10) values in an acrylic coating. The potential utility of YIn0.8Mn0.2O3 as an industrial pigment was demonstrated by 5000 h of accelerated QUV weathering (ΔE* = 0.5) and 48 months of Cincinnati, Ohio outdoor exposure (ΔE* = 1.1) in which the performance was comparable or improved compared to CoAl2O4. The increased UV absorbance measured by thin film absorptivity, and decreased heat buildup determined from PVC spectral properties demonstrate that the YInO3 – YMnO3 materials offer a robust and ‘cool’ cobalt-free alternative to CoAl2O4 pigments.

Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization

In this work, CoAl2O4 pigments successfully were prepared via thermal decomposition method by employing green tea extract as precipitating agent and capping agent. The effects of preparation parameters such as: calcination temperature, surfactants, solvent and pH were investigated to reach the optimum conditions. The biosynthesized cobalt alumina spinel was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray energy dispersive spectroscopy (EDS), ultraviolet–visible (UV–Vis) spectroscopy and vibrating sample magnetometer (VSM). The photocatalytic activity of CoAl2O4 nanostructures was compared by degradation of methylene blue in aqueous solution under UV light irradiation. Also the hydrogen storage capacity of CoAl2O4 was measured by chronopotentiometry method. The capacity of CoAl2O4 in 1 mA current and 20 cycles obtained 1100 mh/g.