Blue pigments based on CoxZn1−xAl2O4 spinels synthesized by the polymeric precursor method

The Co_xZn_1?xAl₂O₄ system (x = 0; 0.1; 0.3; 0.5; 0.7; 0.9 and 1) was synthesized by the polymeric precursor method and characterized by the techniques XRD, TG-DTA, IR, UV–vis and colorimetry. The XRD patterns displayed the characteristic peaks of the spinel structure and a good crystallinity. The DTA curves showed an exothermic peak corresponding to the enthalpy of the transition taking place at about 700 °C. The infrared spectra displayed vibrations at about 650, 550, 540, 520, 500, 490 cm^?1, which were ascribed to the spinel structure. The UV–vis spectra presented three bands at 550, 580 and 620 nm attributed to the Co²⁺ spin transitions in tetrahedral sites. The colorimetric data point out the formation of blue pigments, corresponding to highly negative values of b¹. The lightness, coordinate L¹, increases with the heat treatment temperature. These facts reveal that Co_xZn_1?xAl₂O₄ is a promising system that can be employed to obtain ceramic blue pigments.     

Blue-violet ceramic pigments based on Co and Mg Co2−xMgxP2O7 diphosphates

Solid solutions of Co and Mg diphosphates with compositions Co_2?xMg_xP₂O₇ (x = 0, 0.1, 0.2, 0.3, 0.5, 0.7, 1.0, 1.5 and 1.8) have been prepared and characterized for the first time as alternative low-toxicity blue ceramic pigments. The compositions were prepared through the conventional coprecipitation route and calcined up to 1000 °C/2 h. Samples were characterized by thermal analysis, XRD, SEM/EDX, UV–vis-NIR spectroscopy and colour measurements (CIE-L^*a^*b^*). Isostructural Co_2?xMg_xP₂O₇ diphosphate solid solutions (monoclinic system and P21/c spatial group) formed successfully within the studied range of compositions, accompanied only by a minor quantity of residual Co or Mg orthophosphates (M₃(PO₄)₂). Interestingly, the obtained solid solutions developed nice blue-violet colourations even with high Mg doping after enamelling within double-firing (x = 1.5–1.8) and single-firing (x = 1.0–1.5) ceramic glasses. These optimal compositions containing a minimized Co amount (measured values around 7–16 wt%) could be therefore less toxic alternatives to the conventional Co₃(PO₄)₂ blue ceramic pigment.     

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%).     

Environmental-friendly yellow pigment based on Tb and M (M = Ca or Ba) co-doped Y2O3

A yellow inorganic ceramic pigment with general formula Y_1.86?xM_xTb_0.14O_3?x/2 (M = Ca and/or Zn) with x = 0.06, 0.32 and 0.64 were synthesized by a modified Pechini method. XRD, SEM and HRTEM/EDX analysis showed the formation of solid solution at 1300 °C when x = 0.06 and 0.32. The best b* yellow coordinates were obtained for Ca and Zn co-doped Y_1.86Tb_0.14O₃ samples. The intensity of the yellow colour in the samples is related to the presence of Tb⁴⁺ ions. Samples with higher concentration of Tb⁴⁺ ions lead to a better yellow colour. The chemical stability of these pigments was determinate in an industrial glaze. The glazing tests indicated that the powder samples with x = 0.06 and 0.32 fired at 1300 °C were stable in the glaze. These results make it a potential candidate for environmental friendly yellow ceramic pigment to be used in applications such as pigment for glazes or inkjet printers.     

Karrooite green pigments doped with Co and Zn: Synthesis,color properties and stability in ceramic glazes

The solid-state synthesis and stabilization of Co doped (Mg_1−xCo_xTi₂O₅), Zn doped (Mg_1−xZn_xTi₂O₅) and Co- and Zn-codoped karrooite solid solutions (Mg_0.8−xZn_0.2Co_xTi₂O₅ and (Mg_0.5Zn_0.5)_1−xCo_xTi₂O₅) were investigated. In addition, the optical spectra, color properties and technological performance of (Co,Zn)-karrooite compositions as new green ceramic pigments were also analyzed. XRD characterization revealed for the first time the high solid solubility of Zn²⁺ in MgTi₂O₅ karrooite at 1200 ºC (between 60 and 80 mol% per Mg or karrooite formula unit). In contrast, the reactivity and stabilization of karrooite phase decreased in the case of Co²⁺ doping. Interestingly, codoping with Zn²⁺ ions at high molar ratios (Zn:Mg ratio equal to 1:1) enhanced the reactivity and enabled the stabilization of (Co,Zn)-MgTi₂O₅ karrooite solid solutions, even with high Co²⁺ loadings (20 mol% per karrooite formula unit). The (Co,Zn)-MgTi₂O₅ pigments exhibited yellowish-green colors associated to Co²⁺ ions allocated in octahedral M1 and M2 sites of karrooite lattice, and becoming more intense and less yellow the higher the Co content. However, Zn²⁺ codoping produced less saturated green colors with similar green but lower yellowish hues. The obtained pigments were not stable enough within the tested ceramic glazes, giving rise to turquoise colorations due to cobalt leaching and incorporation into tetrahedral sites of the glassy phase. The stability of Co-karrooite green pigments was higher in a Ca- and Zn-enriched ceramic glaze (B) fired at a higher temperature (1050 °C).     

Ni-doped hibonite (CaAl12O19): A new turquoise blue ceramic pigment

A new structure for ceramic pigments was synthesized by a conventional solid state reaction process. It is based on Ni-doped hibonite, CaAl₁₂O₁₉, which assumes a turquoise-like blue colour similar to that of V-doped zircon. Hibonite is associated with anorthite, CaAl₂Si₂O₈, acting like a fluxing agent in order to lower the synthesis temperature, and with cassiterite, SnO₂, acting as a tin buffer to promote coupled Ni²⁺ + Sn⁴⁺ → Al³⁺ + Al³⁺ substitutions, in order to ensure the electric neutrality of the hibonite lattice. Since relatively low chromophore contents are required, this new system constitutes an interesting alternative to the common blue ceramic pigments based on cobalt aluminate spinel or vanadium-doped zircon, implying lower cost and environmental advantages. The pigments characterization was performed by X-ray powder diffraction, diffuse reflectance spectroscopy, CIELAB colorimetric analysis, and testing in ceramic glazes and bodies. The substitution of Al³⁺ by bigger ions, like Ni²⁺ and Sn⁴⁺, increases the cell volume compared to undoped hibonite and is responsible for the turquoise blue colour, as verified by UV–vis analysis. The chromatic mechanism is due to incorporation of Ni²⁺ in tetrahedral coordination, likely occurring at the site M3 of the hibonite lattice, where it partially substitutes the Al³⁺ ion. While this product shows a strong hue as a pigment, it is not stable after severe testing in glazes and attempts to improve its colouring performance are now under development.     

Catalytic properties in N2O decomposition of mixed cobalt–iron spinels

The effect of introduction of iron in the Co_3 − xFe_xO₄ on catalytic activity in N₂O decomposition was investigated. The spinel catalysts were characterized by XRD, SEM, RS, BET methods, work function measurements and Mössbauer spectroscopy. Introduction of iron in the Co_3 − xFe_xO₄ spinel catalysts at the level of x < 1 leads to preferential substitution of Fe³⁺ in tetrahedral sites, whereas for x > 1 also octahedral ones are substituted. A strong correlation between deN₂O activity (T_50%) and work function was observed showing that electronic factor controls the catalytic reactivity of Co–Fe spinels. The results revealed that the active centers for N₂O decomposition are cobalt ions and thus even a low level of their substitution by iron leads to substantial decrease of the deN₂O activity of the cobalt spinel.     

Solution combustion of spinel CuMn2O4 ceramic pigments for thickness sensitive spectrally selective (TSSS) paint coatings

A series of spinel-type CuMn₂O₄ ceramic pigments were prepared by a facile and low-cost sol-gel solution combustion method and used as cost-effective materials to fabricate thickness sensitive spectrally selective (TSSS) paint coatings by a convenient spray-coating technique. The chemical component, crystalline morphology, and optical property of the copper manganese oxide ceramic pigment could be accurately controlled by altering the annealing temperature. X-ray diffraction (XRD) analysis confirmed that the ceramic pigments annealed at 500 °C for 1 h coincided well with the XRD patterns of crystalline CuMn₂O₄ in the JCPDS database, and there were segregated phases of CuO and Mn₂O₃. Furthermore, the pure spinel CuMn₂O₄ phase could be achieved at 900 °C for 1 h. The copper manganese oxide ceramic pigments could serve as an effective pigment for fabricating the TSSS paint coating, and the TSSS paint coatings based on ceramic pigments calcined at 900 °C showed solar absorptance of 0.895–0.905 and thermal emittance of 0.186–0.310. In addition, the accelerated thermal stability test revealed that the TSSS paint coating exhibited good thermal stability when it was exposed to air at a temperature of 300 °C for 300 h. Hence, the fabricated TSSS paint coating could be used as a solar absorber coating in the low-to-mid temperature domain.     

Synthesis and optical properties of Ce0.95Pr0.05−xMxO2 (M = Mn,Si) as potential ecological red pigments for coloration of plastics

Ecological red pigments Ce_0.95Pr_0.05?xM_xO₂ (M = Mn, Si) have been synthesized by conventional solid-state route and characterized by X-ray diffractometer, scanning electron microscope and UV–vis spectroscopy. Mn⁴⁺/Si⁴⁺ was incorporated into the CeO₂–PrO₂ system to tune the color properties of the pigments by shifting the optical absorption edge. Si⁴⁺ substitution blue shifts the absorption edge of Pr-doped ceria and shows bright reddish brown color. Mn⁴⁺ substitution stabilizes the absorption edge and exhibits dark brown hue. The coloring mechanism is based on the shift of charge transfer band of CeO₂ to higher wavelength by co-substitution of Pr⁴⁺ and tetravalent metal ions in ceria. Si co-doped pigments possess smaller particles and hence exhibit more lightness compared to Mn co-doped samples. The reddish brown pigments exhibit very good coloring performance in polymer matrix. These Ce_0.95Pr_0.05?xM_xO₂ (M = Mn, Si) pigments have potential to be used as ecological red pigments for coloration of plastics.     

Ferroelectric properties of Pb(Zr0.52Ti0.48)O3–Bi3.25La0.75Ti3O12 ceramics

The ceramic samples of compound (1 ? x)Pb(Zr_0.52Ti_0.48)O₃–xBi_3.25La_0.75Ti₃O₁₂ (when x = 0, 0.03, 0.05, 0.07, 0.10, 0.15 and 0.20) were prepared by a solid-state mixed oxide method. X-ray diffraction analysis showed that complete solid solutions occurred for all compositions. Perovskite phase with tetragonal crystal structure and corresponding lattice distortion was observed. Scanning electron micrographs of sample surfaces showed equiaxed grains for all compositions. Ferroelectric measurements revealed that the addition of small amount of BLT (x = 0.03) showed high remanent polarization (～33.5 μC cm^?2) and low coercive field (～2.74 kV mm^?1). Further increasing BLT content could maintain ferroelectric properties of PZT–BLT ceramics. Based on this study, ferroelectric properties of this PZT–BLT ceramic system can be improved for being further used in ferroelectric memory applications.     

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.     

Crystal structure,thermal expansion and electrical conductivity of perovskite oxides BaxSr1−xCo0.8Fe0.2O3−δ (0.3 ≤ x ≤ 0.7)

Ba_xSr_1−xCo_0.8Fe_0.2O_3−δ (0.3 ≤ x ≤ 0.7) composite oxides were prepared and characterized. The crystal structure, thermal expansion and electrical conductivity were studied by X-ray diffraction, dilatometer and four-point DC, respectively. For x ≤ 0.6 compositions, cubic perovskite structure was obtained and the lattice constant increased with increasing Ba content. Large amount of lattice oxygen was lost below 550 °C, which had significant effects on thermal and electrical properties. All the dilatometric curves had an inflection at about 350–500 °C, and thermal expansion coefficients were very high between 50 and 1000 °C with the value larger than 20 × 10⁻⁶ °C⁻¹. The conductivity were larger than 30 S cm⁻¹ above 500 °C except for x > 0.5 compositions. Furthermore, conductivity relaxation behaviors were also investigated at temperature 400–550 °C. Generally, Ba_0.4Sr_0.6Co_0.8Fe_0‘2O_3−δ and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ are potential cathode materials.     

New pink ceramic pigment based on chromium (IV)-doped lutetium gallium garnet

Lutetium gallium garnet codoped with chromium and calcium (Ca,Cr:LGG) and lutetium gallium garnet with chromium as single dopant (Cr:LGG) have been studied. Samples with Ca_xCr_xLu_3−2xGa₅O₁₂ (x = 0, 0.05, 0.10, 0.15, 0.2, 0.4, 0.6,) and Cr_xLu_3−xGa₅O₁₂ (x = 0.15 and 0.6) compositions were prepared by solid state reaction and the fired samples (1250 °C/6 h) were characterised by XRD, lattice parameters determination, UV–vis–NIR spectroscopy, CIEL*a*b* measurements and SEM/EDX. Moreover, the samples with calcium were tested as ceramic pigments in a conventional glaze matrix. In the Ca,Cr:LGG system solid solutions incorporating both Cr(III) and Cr(IV) ions were obtained. Cr(IV) occupies basically dodecahedral sites substituting for Lu (III) and is the predominant oxidation state up to x = 0.20 composition, giving a pink colouration in the ceramic glaze matrix. In the Cr:LGG system, only Cr(III) enters in solid solution occupying octahedral positions and producing green shades.     

Microwave dielectric properties and cation distributions of Zn1-3xAl2+2xO4 ceramics with defect structures

The spinel-structured Zn_1-3xAl_2+2xO₄ (x = 0–0.2) ceramics having defective structures were synthesized using the molten salt method, and their microwave dielectric properties and cation distributions were assessed. The ²⁷Al solid-state nuclear magnetic resonance spectra of these ceramics demonstrate that they have an intermediate spinel structure in which the tetrahedral site occupancy increases from 0.03 to 0.64 as x increases. Moreover, crystal structure refinements suggest that cation vacancies are located at octahedral sites for x = 0.1 and 0.2. Based on these data, the introduction of cation vacancies at octahedral sites appears to enhance the preferential occupation of tetrahedral sites by Al³⁺. The ε_r of these ceramics slightly decreased from 8.5 to 8.2 with increasing x, while the Q·f value increased significantly, from 127,532 to 202,468 GHz, upon the introduction of cation vacancies. An intermediate spinel structure with preferential occupancy of tetrahedral sites by trivalent cations exhibits an enhanced Q·f value.     

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.     

Spinel precipitation in Sn4+-doped Co1−xO polycrystals with dispersed Co2+xSn1−xO4 particles

Co_1?xO–SnO₂ powders in molar ratio of 92:8 were reactively sintered at 1400 °C to form Co_1?xO–Co_2+xSn_1?xO₄ composite and then cooled in furnace or air quenched for secondary Co_2+xSn_1?xO₄ spinel precipitation from the Sn⁴⁺ doped Co_1?xO grains. Electron microscope observations indicated the secondary spinel to precipitate at grain boundaries when slowly cooled, but as parallel-epitaxial platelets within the Sn⁴⁺ doped Co_1?xO grains with a precipitate free zone near the grain boundary when air quenched. A process of thermal-mismatch induced {1 1 0} cleaving, taking advantage of cobalt vacancies, and spontaneous healing by oxidation precipitation accounts for the platy spinel precipitation within the grains. The precipitate free zone can be attributed to cobalt vacancy depletion, i.e. site saturation, near the grain boundary during rapid cooling in air. The spinel nanocrystals nucleated from cobalt vacancies in association with Sn⁴⁺ dopant have well-developed {1 1 1} habit plane in order to minimize the coherency strain energy.     

Encapsulated carbon black prepared by sol–gel-spraying: A new black ceramic pigment

As a new black ceramic pigment, encapsulated carbon black pigment has been prepared by a sol–gel-spraying method. The obtained pigment sintered at 900 °C for 2 h in air has a deep black hue (L* = 19), indicating carbon black can be fully covered. In the pigment, a dense coating layer on carbon black is formed due to the fast transformation from sol into gel by rapid extraction of solvent. The transparent silica phase spaces out the fine crystalline (zirconia or zircon), which permits to display the color of carbon black. This preparation method provides a way to prepare the encapsulated pigments. It will provide more colorful ceramic pigment applied in ceramic decoration by encapsulating.     

Hydrothermal synthesis of 3D NixCo1−xS2 particles/graphene composite hydrogels for high performance supercapacitors

In this work, three dimensional (3D) Ni_xCo_1−xS₂/graphene composite hydrogels with different Ni contents (denoted as Ni_xCo_1−xS₂/GH (x = 0, 0.31, 0.56, 0.66, 1)) have been synthesized by a simple one-step hydrothermal method and utilized as the active materials of supercapacitors. The as-prepared samples present a 3D interconnected porous network with the pore sizes in the range of several to tens micrometers. Interestingly, the Ni_xCo_1−xS₂ particles are uniformly located on the graphene network and the particle size is evolved from ∼50 nm to ∼1.5 μm with the increase of Ni content. The electrochemical measurements revealed that the specific capacitance, rate capability and cyclability of different Ni_xCo_1−xS₂/GH electrodes are strongly affected by their different Ni content. Among these, the 3D Ni_0.31Co_0.69S₂/GH composite has the highest specific capacitance of 1166 F/g at a current density of 1 A/g. Furthermore, a specific capacitance of 559 F/g can be still maintained at high current density of 20 A/g. After 1000 charge–discharge cycles at 5 A/g, the specific capacitance remains a high value of 755 F/g.     

Effects of K/Na ratio on the phase structure and electrical properties of 0.98(KxNa1−x)NbO3–0.02BiScO3 lead-free ceramics

Lead-free piezoelectric ceramics 0.98(K_xNa_1?x)NbO₃–0.02BiScO₃ (0.98 K_xN_1?xN–0.02BS) (x = 0.30–0.60) doped with 0.8 mol% Mn were prepared by conventional solid-state sintering. The effects of K/Na ratio on the phase structure and electrical properties of the Mn doped 0.98 K_xN_1?xN–0.02BS (0.98 K_xN_1?xN–0.02BS–Mn) were mainly studied. It is experimentally demonstrated that the electrical properties strongly depend on K/Na ratio in the 0.98 K_xN_1?xN–0.02BS–Mn ceramics and when x = 0.45 the ceramics exhibit optimum electrical properties: d₃₃ ～ 308 pC/N, k_p ～ 0.495, ?_r ～ 1577, tan δ ～ 0.028. These results show that the 0.98 K_xN_1?xN–0.02BS–Mn ceramic with x = 0.45 is a promising lead-free piezoelectric material.     

Hidden value in low-cost inorganic pigments as potentially valuable magnetic materials

Inorganic pigments are substances that develop colour in organic solids such as ceramics and glazes, and are usually a complex mixture of oxides, and relatively low-cost. Their chromatic properties have been extensively studied, yet surprisingly the magnetic and electrical properties of these economic and common materials have been neglected, despite the fact many are based on ferrite spinels. Therefore, we investigated these properties in commercial black and brown pigments, to assess their potential as magnetic materials. The brown and black pigments were found to be spinel ferrites, with estimated formulas of Fe_1.34Cr_0.62Mn_0.66Zn_0.22Ni_0.10Co_0.06O₄ and Fe_1.02Cr_0.97Co_0.57Mn_0.23Ni_0.21O₄, respectively. The brown pigment also contained a higher amount of SiO₂ compared to the black pigment (~7 mol% vs. ~2 mol%), which appeared as a second phase of crystalline quartz, and adversely affected its porosity, magnetisation and electrical ac conductivity, compared to the black pigment. However, both were very magnetic and very soft ferrites. The brown pigment had M_s=11.7 A m² kg⁻¹ and H_c of 1.5 kA m⁻¹, with a high electrical conductivity (σ) of 4×10⁻⁴−7×10⁻³ Ω⁻¹ m⁻¹ between 100 Hz and 1 MHz. The black pigment was equally magnetically soft, but had a much greater magnetisation and lower electrical conductivity, with M_s=18.7 A m² kg⁻¹, H_c=2.4 kA m⁻¹, and σ=5×10⁻⁶−8×10⁻⁵ Ω⁻¹ m⁻¹ between 100 Hz and 1 MHz.This work has revealed the potential hidden value of low-cost commercial inorganic pigments based on spinel ferrites as magnetic materials. This demonstrates their potential at low-cost alternative materials for applications such as in power supply transformers, switching materials and sensors, where soft magnetisation is especially important.