Synthesis and color properties of the TiO2@CoAl2O4 blue pigments with low cobalt content applied in ceramic glaze

The TiO₂@CoAl₂O₄ complex blue pigments with low cobalt content were synthesized through calcinations of the precursor obtained from coprecipitating Co²⁺ and Al³⁺ to form Co‐Al LDHs (layered double hydroxides) on the surface of TiO₂ particles. The structure and the properties of the synthesized pigments were characterized by XRD, SEM, TEM, UV‐Vis spectroscopy, XPS, and colorimeter. The precursors of the blue TiO₂@CoAl₂O₄ complex pigments were consisted of LDHs shell layer encapsulated TiO₂ microsphere. After calcinations at 1100°C, the LDHs shell layer were absolutely transformed to the spinel CoAl₂O₄, and the pigments presented a core‐shell structure and uniform sphere morphology (the diameter of microsphere was about 780 nm). The absorption bands at around 547, 584, and 624 nm in the Uv‐Vis absorption spectra of the TiO₂@CoAl₂O₄ complex pigments were corresponded to the characteristic absorption bands of the spinel CoAl₂O₄, revealed the pigments with a bright blue hue. In addition, as the mass ratio of CoAl₂O₄/TiO₂ increased to 0.4, the blue component of the pigments reached to 27.89 and slight color variation with the increase in the CoAl₂O₄ content in a range, possessed low cobalt content and exhibited a stabile performance in commercial low‐temperature ceramic glazes. The XGT results showed that the TiO₂@CoAl₂O₄ complex pigments with low cobalt content presented bright color in ceramic glaze. Especially, the synthesized pigments reduced the usage and toxicity of cobalt, which were efficiency for economy and environmental protection.     

Flame spray pyrolysis synthesis of ceramic nanopigments CoCr2O4: The effect of key variables

Spinel structure CoCr₂O₄ was synthesized by the non-conventional method flame spray pyrolysis (FSP) and the traditional route solid state reaction, where the optical properties were evaluated. The influences of FSP conditions as pressure of dispersion air and ceramic load of the solution over optical properties were evaluated using a 2² full factorial design with one replica. The final products were applied in ceramic glazes to evaluate pigmenting power. Powders were characterized by X ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis-NIR spectroscopy and Colorimetry. Results show that ceramic pigments obtained by FSP have highest percent reflectance and brightness than solid state reaction powders; nevertheless, both pigments are adequate to ceramic application. Besides, experiments showed that ceramic load of the starting solution have a strong influence over particle properties.     

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.     

Effects of starch addition on the characteristics of CoAl2O4 nano pigments

Nanocrystalline cobalt aluminate spinel, CoAl₂O₄, was prepared via a microwave‐assisted solution combustion process applying various mixtures of urea, glycine, and starch as a novel mixed fuel. The effects of starch addition (0, 10, 20, and 30 wt%) on the physical characteristics (e.g. crystallite size and colour) of the blue nano pigments were also investigated. The resultant powders were characterised by means of X‐ray diffraction, scanning electron microscopy, electron dispersive X‐ray analysis, and CIE L*a*b* colour measurements. The presence of a CoAl₂O₄ spinel lattice after calcination of precursors at 600 °C was confirmed by X‐ray diffraction patterns, and the crystallite sizes were ca. 10–39 nm. Colorimetric data pointed to the formation of bright‐blue pigments at low levels of starch addition. Scanning electron microscope images showed that starch enrichment reduced the agglomeration and size of synthesised nanoparticles.     

A comparative study on citrate sol-gel and combustion synthesis methods of CoAl2O4 spinel

CoAl₂O₄ spinel was successfully synthesized by combustion synthesis method using glycine and urea by 1:1 molar ratio as fuels and sol-gel process using citric acid as a chelating agent. The as-synthesized powders were calcined at desired temperatures to obtain CoAl₂O₄ spinel as a single phase. X-ray diffraction, thermogravimetric, and differential thermal analysis results revealed that the formation of CoAl₂O₄ spinel in combustion method needs 300°C higher temperatures than those of sol-gel. Scanning electron microscopy and transmission electron microscopy analysis results revealed that “sol-gel spinel” had nanometric particle size which was smaller than those of “combustion spinel.” Temperature programed reduction with hydrogen and Fourier transform infrared spectroscopy results declared that there was a little residual cobalt oxide in combustion spinel while there is no oxide resided in “sol-gel spinel.” Consequently, the sol-gel method has more benefit in synthesizing spinel with sulfate precursors than combustion.     

Effective synthesis of green nano-sized ceramic pigments by co-doping Zn2+, Cr3+ and Sm3+ into the cobalt-aluminate

A green nano-sized ceramic pigment Co_0.6Zn_0.4Al_0.8Cr_1.2-xSm_xO₄ has been successfully prepared by doping ions Zn²⁺, Cr³⁺, and Sm³⁺ into the crystalline CoAl₂O₄ spinel and using the complex-gel method along with agar as complexing and combustion agent. The Infrared Spectroscopy, X-ray diffraction, Thermo-gravimeter, High resolution transmission electron microscopy, Automatic color reader, and UV Vis diffuse reflectance were applied to characterize the pigment power and its gel precursor. The results reveal that the introduction of Zn²⁺, Cr³⁺, and Sm³⁺ could change the occupation status of ions in the tetrahedral and octahedral framework of CoAl₂O₄ spinel, leading to the colorant variation of the blue cobalt pigment CoAl₂O₄, With increasing the content of Sm³⁺ ion, the reflection peak position of the pigments in visible spectrum appeared a red-shift, ie the color transition from blue green to yellow green, and the average reflectivity in the violet region decreased to 13.31%, and the band gap energy also changed from 3.47 to 3.20 eV. This illustrates the better UV absorption of this green pigment and can be used as UV shielding material. With the hydrochloric acid or sodium hydroxide solution treatment, this pigment was found to be durable in chemical stability.     

Deposition of colored inorganic coating layers on lamellar sericite surface and the pigmentary performances

The inorganic composite pigments, Fe₂O₃/sericite, Bi₂O₃/sericite, and CoAl₂O₄/sericite, were prepared by the homogeneous hydrolysis of metal salts, such as FeCl₃, Bi(NO₃)₃, and Co(NO₃)₂/Al(NO₃)₃. The deposition of the inorganic coating layers on sericite surfaces was explored by X-ray diffraction, scanning electron microscopy, Fourier transform infrared, and X-ray photoelectron spectroscopy. The inorganic coating layers were uniformly and tightly anchored at sericite surfaces via chemical bonding at the interface between the coating layer and the sericite substrate. The pigmentary performances of the inorganic composite pigments were analyzed by CIE, indicating that red, yellow, and blue colored pigments were achieved by coating Fe₂O₃, Bi₂O₃, and CoAl₂O₄ on sericite surfaces, respectively. The pigmentary performances of the inorganic composite pigments were significantly affected by the morphology and thickness of inorganic coating layers.     

Paramagnetic Ho2O3 nanowires,nano-square sheets,and nanoplates

The morphology control of holmium oxide (Ho₂O₃) nanostructures has rarely been reported. In the present study, uniform Ho₂O₃ nanowires, nano-square sheets, and nanoplates were synthesized via the hydrothermal method and were characterized using scanning electron microscopy, high-resolution transmission electron microscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis, ultraviolet–visible–near-infrared absorption, and X-ray photoelectron spectroscopy. The morphology was observed to be composed of different crystal phases and Ho complexes, but it changed to the same cubic Ho₂O₃ crystal phase upon high-temperature thermal treatment. Paramagnetic properties were obtained and discussed for the three different morphologies mentioned above. The newly reported morphologies and the physicochemical properties reported herein will widen the application scope of Ho oxide materials.     

Ultrasonic vibration-assisted laser engineering net shaping of ZrO2-Al2O3 bulk parts: Effects on crack suppression,microstructure, and mechanical properties

Laser additive manufactured zirconia-alumina ceramic (ZrO₂-Al₂O₃) parts demonstrate severe problems resulting from cracking and inhomogeneous material dispersion. To reduce these problems, we propose a novel ultrasonic vibration-assisted laser engineered net shaping (LENS) process for fabrication of bulk ZrO₂-Al₂O₃ parts. Results showed that the initiation of cracks and the crack propagation were suppressed in the parts fabricated by LENS process with ultrasonic vibration. For the parts fabricated without ultrasonic vibration, the sizes of cracks decreased with the increase of laser power. Scanning electron microscope analyses proved that the introduction of ultrasonic vibration was beneficial for grain refinement and uniform material dispersion. Due to the suppressed cracking, refined grains, and homogenized material dispersion, the parts fabricated with ultrasonic vibration demonstrated better mechanical properties (including higher microhardness, higher wear resistance, and better compressive properties), compared with the parts fabricated without ultrasonic vibration.     

Yb2O3 nanowires,nanorods and nano-square plates

Developing a new morphology and revealing new physicochemical properties are very important steps in industrial applications. In this study, ytterbium oxide (Yb₂O₃) nanowires, nanorods and nano-square plates were first synthesized by a facial hydrothermal method and a post-thermal treatment, and characterized by X-ray diffraction crystallography, Fourier transform-infrared, UV–visible-NIR absorption, photoluminescence, and X-ray photoelectron spectroscopy. The morphology and the microstructures were examined by scanning electron microscopy and high resolution transmission electron microscopy. As-prepared samples with different morphologies showed different crystal phases but changed to the same cubic Yb₂O₃ phase upon thermal treatment. Eu(III) and Tb(III) activator ions were also doped into Yb₂O₃ host materials and discussed.     

The preparation and characteristics of cobalt blue colored mica titania pearlescent pigment by microemulsions

Blue pearlescent pigment was obtained by coating microemulsion-synthesized CoAl₂O₄ nanoparticles onto mica titania. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) showed that the spherical CoAl₂O₄ spinel was around 20 nm and dispersion of the nanoparticles on the coated surface was uniform. EDS revealed that the coating rate and coating efficiency of Co was about 3.12% and 97%, respectively. The study results indicate that the optimum technology parameters were 1:1 CoO:Al₂O₃ molar ratio, and the coating ratio of CoAl₂O₄ was 3.7–4.6% by weight.     

Structural,electronic, elastic,optical and vibrational properties of MAl2O4 (M = Co and Mn) aluminate spinels

First principles density functional theory calculations were performed to study structural, electronic, elastic, optical and vibrational properties of CoAl₂O₄ and MnAl₂O₄ aluminate spinels. Computed ground state properties such as unit-cell parameter and oxygen positional parameter differ by less than 1% from previously available theoretical and experimental results. However, the bulk modulus differs by less than 4% difference from available theoretical and experimental values for CoAl₂O₄ and less than 10% for MnAl₂O₄. Zone-center phonon frequencies and the phonon spectrum along high symmetry direction together with the phonon density of states were calculated using supercell method. Bandgaps of CoAl₂O₄ and MnAl₂O₄ were obtained as 1.78 and 2.21 eV respectively. CoAl₂O₄ was found to be more ionic than the MnAl₂O₄ spinel. And quasi harmonic method was used to calculate the Debye temperature for the studied compounds.     

Red ceramic pigments of terbium-doped ceria prepared through classical and non-conventional coprecipitation routes

Reddish ceramic pigments based on cerianite compositions Ce_1−xTb_xO₂ (x = 0.1, 0.2 and 0.3) and Ce_0.9Tb_0.05Y_0.05O₂ have been prepared. Firstly, the optimal composition giving the best red colour was selected (Ce_0.9Tb_0.1O₂) from ceramic and chlorides coprecipitation samples fired at 1100–1500 °C. Secondly, the effect of using different classical and non-conventional coprecipitation routes on the synthesis and colouring performance of CeO₂-Tb red pigments was analyzed for the first time. In addition to classical coprecipitation we tested also the use of hydrothermal treatment and other more homogeneous coprecipitation methods with oxalates and urea. Homogeneous coprecipitation was also combined with ultrasonic or microwave irradiation. Samples were characterized by thermal analysis, XRD, SEM/EDX, optical spectroscopy and colour measurements. Interestingly, the optimization of synthesis methods enabled to obtain more homogeneous (single phase) CeO₂-Tb solid solutions at lower temperatures (400–1100 °C), exhibiting very nice reddish colourations associated to lower energy bandgaps (E_g below 2.30 eV). The obtained optimal compositions could be therefore alternative candidates as environmentally friendly reddish ceramic pigments.     

One-step hydrothermal synthesis of yttria-stabilized tetragonal zirconia polycrystalline nanopowders for blue-colored zirconia-cobalt aluminate spinel composite ceramics

A novel approach for the preparation of blue-color giving zirconia nanopowders by doping of 3?mol% Y₂O₃ through simple one-step hydrothermal process is proposed. A blue-color giving yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) powders were prepared by urea-based solution, zirconium acetate, CoCl₂·6H₂O and AlCl₃ precursors in hydrothermal vessel at 24?h, 150?°C and 20 bars. Based on the results, the synthesized blue-color giving Y-TZP nanopowders have entirely tetragonal structure as mono phase with 3.8?±?0.2?nm average grain size and (Y, Co, Al)_xZrO₂; x?≤?0.03?at. with chemical composition. Thermal treatment was also applied to synthesized Y-TZP powders at 1200?°C and 1450?°C to observe the color evolution. Only sharp blue was obtained in Y-TZP powders resulting the development of zirconia-cobalt aluminate spinel (ZrO₂-CoAl₂O₄) composite ceramic structure for both temperatures after heat-treatment. Herein, not only formation of CoAl₂O₄ but also incorporation of cobalt (Co) and aluminum (Al) into the Y-TZP grains plays a critical role on evolving of blue color. This synthesized Y-TZP nanopowders can be a good candidate for one-step production of blue-color sintered ZrO₂-CoAl₂O₄ spinel composite ceramics in numerous ceramic applications due to their superior structural and functional properties.     

Preparation of titanium dioxide/tungsten disulfide composite photocatalysts with enhanced photocatalytic activity under visible light

Titanium dioxide/tungsten disulfide (TiO₂/WS₂) composite photocatalysts were fabricated via a one-step hydrothermal synthesis process, using TiCl₄ as titanium source and bulk WS₂ as sensitizer. The morphology, structure, specific surface area and optical absorption properties of the composite photocatalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), specific surface area analyzer and ultraviolet-visible diffuse reflection spectrum (UV-vis DRS), respectively. The photocatalytic activity of as-prepared photocatalysts was evaluated by the degradation of methyl orange (MO) under illumination of 500W Xenon lamp. The results indicated that TiO₂/WS₂ composite photocatalysts possessed excellent photocatalytic activity, and ～95% of the degradation rate for MO was reached when molar ratio of WS₂ to TiO₂ was 0.004 and the irradiation time was 60 min. Moreover, the carrier trapping experiment and fluorescence spectra showed that ·O ₂ ^? was the key component in the photocatalytic degradation process and O₂ was reduced to be ·O ₂ ^? by the electrons from the conduction band of TiO₂ and WS₂ for the degradation of MO.     

Production of a stable and homogeneous colloid dispersion of nano CoAl2O4 pigment for ceramic ink-jet ink

Different types of nano-CoAl₂O₄ pigments were prepared by controlling concentration of cetyltrimethylammonium bromide (CTAB) and polyvinyl pyrrolidone (PVP) as double capping agents in a co-precipitation process. The prepared nano-CoAl₂O₄ pigments were characterized by TGA, UV spectroscopy, and XRD. The optimum synthesized nano pigment was well dispersed into de-ionized water to form the ink. Rheology, surface tension and DLS of the prepared ink were examined. The prepared ink was printed onto a ceramic substrate. The printing process was repeated 1, 3 and 5 times in order to evaluate variations in the optical properties by changing thicknesses of the printed film. Appearance of the printed image and morphology of the prepared nano-pigments were observed by SEM. Moreover, shape and size of the nano-particles in the prepared ink were investigated by TEM. The obtained results revealed that the ink-jet printing method can be used to produce a nano-film of pigments on the ceramic.     

Structural investigation and optical properties of cobalt aluminate pigments derived from thermal decomposition of mixed-metal nitrate co-crystals

In this study, the coloration mechanism of cobalt aluminate spinel materials derived by thermal decomposition of the co-crystallized mixed-metal precursors has been reported. The structural alteration of cobalt aluminates, along with the origins of chroma values at the different calcination temperatures from 600 °C to 1200 °C were thoroughly investigated by combining XRF, XRD, XAS, UV-VIS DRS, FT-IR, and FT-Raman techniques. In-depth analyses of structural information and corresponding optical properties suggested that the coloration can be modified by controlling Co²⁺/Co³⁺ contents and their site occupancies in the spinel structure by adjusting calcination temperatures and selecting the counter ions with appropriate oxidizing power. The interplays among (1) the oxidation process at low temperatures, (2) the deoxidation process at high temperatures, (3) the reduction in the degree of spinel inversion (toward the emerging of normal-spinel CoAl₂O₄ structure), and (4) the existence of the minor Al₂O₃ domains are found to be key imperatives for tailoring the color appearances of cobalt aluminate powders in the broad range from black, dark-green, green, greenish-blue, dark-blue, towards bright-blue. Herein, we reveal the correlation between the synthetic parameters and the structural features of the obtained spinel-based materials, which could further exhibit the crucial procedure to control the coloration of inorganic pigments systematically.     

Wettability and interfacial reactions of a low Hf-containing nickel-based superalloy on Al2O3-based,SiO2-based,ZrSiO4, and CoAl2O4 substrates

To understand the wetting behavior and interfacial phenomena between molten superalloys and ceramic materials, the wettability and interfacial reactions of a low Hf-containing Nickel-based superalloy on the Al₂O₃-based, SiO₂-based, ZrSiO₄, and CoAl₂O₄ substrates were studied using the sessile drop method at 1773 K. The wetting angles of the alloy on the Al₂O₃-based, SiO₂-based, ZrSiO₄, and CoAl₂O₄ substrates were 141.4°, 143.5°, 135.7°, and 128.4°, respectively. This indicated that the wettability of the alloy on the Al₂O₃-based substrate was comparable to that on the SiO₂-based substrate, and the wettability of the CoAl₂O₄ system was the best among the four systems. The microstructure characteristics of the interface implied that Hf has a strong tendency to react with ceramic substrates, even at low contents. Additionally, the interfacial reactions transformed the Al₂O₃-based, SiO₂-based, ZrSiO₄, and CoAl₂O₄ ceramic substrates into (Al₂O₃ + HfO₂), (Al₂O₃ + HfO₂), (Al₂O₃ + HfO₂ + ZrO₂), and (Al₂O₃ + HfO₂ + Co), respectively, which were in contact with the alloys. The experimental results demonstrated that the wettability of the system was governed by the properties of the reaction products.     

Synthesis and characterization of Er2O3 nanorods and nanosheets

It is very important to develop a new synthetic route for lanthanide metal oxides with novel morphologies and having different fundamental properties and application performances. Here, we report Er₂O₃ nanorods and nanosheets prepared by a facile hydrothermal method followed by post-thermal annealing treatment. Thermal treatment of Er₄O₂(OH)₈(HNO₃) and Er₂O₅H₄ was carried out to form the nanorods and nanosheets, respectively. Their physicochemical properties were evaluated by field-emission scanning electron microscopy, X-ray diffraction crystallography, high-resolution transmission electron microscopy, Fourier transform infrared (IR) spectroscopy, Ultraviolet-visible-near IR absorption spectroscopy, and X-ray photoelectron spectroscopy. The synthesis method and novel fundamental properties provide valuable information for the development of Er complexes and oxides.     

Hydrothermal assisted synthesis and hot-corrosion resistance of nano lanthanum zirconate particles

The nano La₂Zr₂O₇ (LZ) particles with pyrochlore microstructure were successfully synthesized from a mixture of La(NO₃)₃, Zr(NO₃)₄ and C₁₉H₄₂BrN (CTAB) using hydrothermal assisted (HTA) synthesis which consists of two steps: hydrothermal treatment and calcination. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectra (FT-IR) and X-ray diffraction (XRD) spectroscopy were employed to study morphologies and phase compositions. The results suggest that HTA process led to very rapid synthesis of nano LZ compared to the conventional solid reaction process. The particles produced by HTA synthesis have cubic shape and the distribution of its grain size is from 10 nm to 30 nm. The present work demonstrates that the nano La₂Zr₂O₇ produced via HTA synthesis which have better hot-corrosion resistance is an ideal material for thermal barrier coatings.