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.     

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.     

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.     

Flame synthesis of carbon nanotubes and few-layer graphene on metal-oxide spinel powders

Multi-walled and single-walled carbon nanotubes (CNTs) and few-layer graphene (FLG) are grown directly on spinel powders using flame synthesis. CNT and FLG growth occurs via the decomposition of flame-generated carbon precursors (e.g., CO, C₂H₂, and CH₄) over nanoparticles (i.e., Ni, Co, Fe, and Cu) reduced from the solid oxide. The growth of CNTs is investigated on NiAl₂O₄, CoAl₂O_4, and ZnFe₂O₄, using counterflow diffusion flame and multiple inverse-diffusion flames (m-IDFs), while the growth of FLG is investigated on CuFe₂O₄ using m-IDFs. As shown by analytical electron microscopy techniques, Raman spectroscopy, and X-ray diffraction, substrate temperature and spinel composition play critical roles in the growth of both CNTs and FLG.     

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.     

Characterization of blue CoAl2O4 nano-pigment synthesized by ultrasonic hydrothermal method

Nano-sized CoAl₂O₄ pigments, which have received significant attention as a coloring agent in glaze and bulk tile compositions, were successfully synthesized by substituting mechanical stirring during hydrothermal process with ultrasonic irradiation. Difference in physicochemical and optical properties of the CoAl₂O₄ pigments prepared by an ultrasonic-assisted-hydrothermal method was characterized using simultaneous thermo-gravimetric and differential thermal analysis (TG–DTA), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), diffuse reflectance spectroscopy, CIELAB colorimetric analysis, and testing in ceramic glazes and bodies. The ultrasonic-assisted CoAl₂O₄ pigments present a narrow particle size distribution with vivid blue color, and better thermal stability, allowing their use for ceramic inks processed at high temperature. Application of ultrasonic irradiation during the hydrothermal process produces nano-sized powders with better physicochemical and optical properties.     

CoAl2O4 spinel catalyst for soot combustion with NOx/O2

Mesoporous and nanosized cobalt aluminate spinel with high specific surface area was prepared using microwave assisted glycothermal method and used as soot combustion catalyst in a NO_x + O₂ stream. For comparison, zinc aluminate spinel and alumina supported platinum catalysts were prepared and tested. All samples were characterised using XRD, (HR)TEM, N₂ adsorption–desorption measurements. The CoAl₂O₄ spinel was able to oxidise soot as fast as the reference Pt/Al₂O₃ catalyst. Its catalytic activity can be attributed to a high NO_x chemisorption on the surface of this spinel, which leads to the fast oxidation of NO to NO₂.     

Synthesis and electrochemical characterization of LiMn2O4 cathode materials for lithium polymer batteries

Spinel LiMn₂O₄ powders with sub-micron, narrow particle-size distribution, and phase-pure particles were synthesized at low temperatures from aqueous solution of metal acetate containing glyoxylic acid as a chelating agent by a sol-gel method. The effects of the calcination temperature and glyoxylic acid quantity on the physicochemical properties of spinel LiMn₂O₄ powders were examined with X-ray diffractometry (XRD), the Brunauer-Emmett-Teller (BET) method and scanning electron microscopy (SEM). Porous LiMn₂O₄ electrode was characterized electrochemically with charge/discharge experiments and A.C. impedance spectroscopy. The cycling performance of a Li/polymer electrolyte/LiMn₂O₄ cell has been discussed in terms of contact and interfacial resistance by A. C. impedance spectroscopy.     

Characterization and Catalytic Oxidation of Carbon Monoxide Over Supported Cobalt Catalysts

Three types of supported cobalt catalysts (CoO_x/SiO₂, CoO_x/TiO₂ and CoO_x/Al₂O₃) were prepared by incipient wetness impregnation with aqueous Co(NO₃)₂·6H₂O solution. The phase composition and the interactions of cobalt with supports under different calcined temperatures were investigated using thermogravimetry (TG), N₂-adsorption at −196 °C, X-ray diffraction (XRD), temperature-programmed reduction (TPR) and diffuse reflectance spectroscopy (DRS). Their catalytic activities towards the CO oxidation were further studied in a continuous flow micro-reactor. The results showed that the interaction of cobalt oxide with supports was much stronger in the kinds of Al₂O₃ and TiO₂, while no conclusive evidence of any interaction was found for SiO₂. Besides the crystalline Co₃O₄ which was formed in three supported catalysts, both high-temperature phases CoAl₂O₄ and CoTiO₃ spinel were also detected under XRD, DRS and TPR analysis. The degree of interaction between cobalt oxide and the support not only affected the surface area and reduction behavior of the catalysts, the catalytic activity toward the CO oxidation also affected simultaneously. As the CoAl₂O₄ and CoTiO₃ spinel formed, both the surface area and catalytic activity decreased significantly.     

Synthesis and characterization of MgAl2O4–ZrO2 composites

Different types of dense 5–97% ZrO₂–MgAl₂O₄ composites have been prepared using a MgAl₂O₄ spinel obtained by calcining a stoichiometric mixture of aluminium tri-hydroxide and caustic MgO at 1300 °C for 1 h, and a commercial yttria partially stabilized zirconia (YPSZ) powder as starting raw materials by sintering at various temperatures ranging from 1500 to 1650 °C for 2 h. The characteristics of the MgAl₂O₄ spinel, the YPSZ powder and the various sintered products were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface area, particle size analysis, Archimedes principle, and Vickers indentation method. Characterization results revealed that the YPSZ addition increases the sintering ability, fracture toughness and hardness of MgAl₂O₄ spinel, whereas, the MgAl₂O₄ spinel hampered the sintering ability of YPSZ when sintered at elevated temperatures. A 20-wt.% YPSZ was found to be sufficient to increase the hardness and fracture toughness of MgAl₂O₄ spinel from 406 to 1314 Hv and 2.5 to 3.45 MPa m^1/2, respectively, when sintered at 1600 °C for 2 h.     

CoAl2O4–mullite composites prepared by sol–gel processes

The formation of CoAl₂O₄–mullite composites from diphasic sol–gel precursors with 3:2 mullite composition doped with 1, 2 and 3 at.% Co²⁺ was studied by differential scanning calorimetry (DSC), X-ray diffraction and Rietveld structure refinement. The course of thermal reactions is dominated by the intermediate formation of two faint crystallized phases having different composition and activation energies. The former phase with smaller activation energy (822 kJ mol⁻¹) is attributed to cobalt-containing spinel structure and the latter with larger activation energy (about 1200 kJ mol⁻¹) to Al–Si spinel. With temperature increase Co-containing spinel transforms progressively in CoAl₂O₄, while Al–Si spinel forms mullite above 1100 °C. Mullite lattice parameters, Rietveld refinement data and the CoAl₂O₄/Co²⁺ ratio in annealed samples points out that the majority of cobalt is incorporated in CoAl₂O₄ and only about 0.6 at.% enters mullite structure or the glassy phase, or both.     

Mechanical characteristics of FeAl2O4 and AlFe2O4 spinel phases in coatings – A study combining experimental evaluation and first-principles calculations

FeAl₂O₄ and AlFe₂O₄ have similar spinel structures but different atomic arrangements, leading to different properties. In this study, mechanical properties and structures of the two spinel phases in composite coatings prepared by reactive plasma spraying of Fe₂O₃-Al composite powder were investigated by means of multi-mode atomic force microscopy, X-ray diffraction, and scanning electron microscopy. It was demonstrated that AlFe₂O₄ was stronger than FeAl₂O₄ with a higher electron work function (EWF). First-principles calculations were conducted to understand the difference in mechanical properties between the two phases. It was shown that FeAl₂O₄ and AlFe₂O₄ have “normal” and “inverse” spinel structures with different configurations of valence electrons: Fe²⁺_tet(Al³⁺)₂O₄ and Fe³⁺_tet(Fe²⁺Al³⁺)_octO₄. Compared to Fe²⁺_tet(Al³⁺)₂O₄, Fe³⁺_tet(Fe²⁺Al³⁺)_octO₄ has its O and Fe atoms in the octahedral sites forming covalent bonds, which increase the stability and strength of the crystal, corresponding to higher EWF and larger bulk, shear and Young's moduli.     

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.     

Selective Catalytic Reduction of NO by C3H6 over Co/Al2O3 Catalyst with Extremely Low Cobalt Loading

In order to reveal the optimum Co loading, the selective catalytic reduction of NO with C₃H₆ over Co/Al₂O₃ catalyst was studied in a systematic fashion by varying the amount of cobalt oxide. It was found that upon loading a small amount of cobalt oxide (namely 0.5 wt% on a Co metal basis), the combination between Co(II) acetate salt and a high-purity alumina provided an active catalyst in the presence of excess oxygen and water. TPR measurement showed the presence of Co species other than CoAl₂O₄ spinel in the most excellent performance catalyst, from which the active sites should be produced.     

Synthesis of MFe2O4 (M=Mg2+, Zn2+, Mn2+) spinel ferrites and their structural,elastic and electron magnetic resonance properties

Structural, elastic and electron magnetic resonance investigations of spinel ferrites with the formula MFe₂O₄ (M = Mg²⁺, Zn²⁺, Mn²⁺) synthesized by the sol-gel auto-combustion method are reported here. XRD patterns revealed the co-existence of secondary phases along with the ferrite phase. The lattice parameter (8.301?Å, 8.366?Å and 8.434?Å) was found to be varying according to the ionic radii of cations. As determined by scanning electron microscopy (SEM), ZnFe₂O₄ has a comparatively narrow distribution of grain sizes (1.3–3.8?µm) compared to those in MnFe₂O₄ (0.8–4.3?µm) and MgFe₂O₄ (0.3–4.8?µm). The estimated values of average crystallite sizes (17.5?nm, 21.3?nm and 23.3?nm) determined from the X-ray diffraction peaks are considerably less than the average grain sizes (1.3?µm, 1.6?µm and 2.7?µm) estimated from the SEM histograms. The vibrational frequencies in FTIR spectra are in the conformity with the cubic spinel structure and their variation supports the variation of lattice parameter. Equal values of Poission's ratio (0.35) were obtained for the three systems which represent the isotropic behaviour of spinel ferrite systems. The exceptional low value of Lande's g-parameter for ZnFe₂O₄ indicates the dominance of Fe³⁺–O–Fe³⁺ superexchange interaction. Though cation redistribution is possible in the present ferrite systems, the secondary phases existed in these ferrite systems are predominantly influencing the structural, elastic and electron magnetic resonance properties.     

Micro analytical and magnetic characterization of aluminum-iron spinel (FeAl2O4) synthesized by combustion reaction

Materials with spinel structures have the general stoichiometric formula AB₂O_4. Their constituent elements A and B determine specific properties, some of important technological interest, particularly related to magnetic behavior. In the present study Aluminum–Iron, single-phase spinel, rarely found in nature, has been synthesized via combustion reaction. The presence and amount of urea as combustion source has been varied in order to verify its influence on the microstructure and magnetic properties of the product material. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been used to characterize the synthesized powders. The microstructural analysis indicated that the sample synthesized without urea as fuel corresponds to iron and aluminum oxides, but mostly amorphous, with low intensity X-ray diffraction peaks corresponding to Hematite particles. The samples produced with urea displayed in their X-ray diffractogram well defined peaks allowing to index the crystallographic planes of the constitutive phases: spinel and magnetite. Among these, the sample synthesized with stoichiometric amount of urea indicate being constituted by single-phase FeAl₂O₄ spinel. However, the samples synthesized with excess and deficiency of urea showed, besides the production of the desired spinel, the formation of Fe₃O₄ have occurred. Magnetic measurements indicate that samples synthesized with urea behave as a ferromagnetic material, while samples produced without urea exhibits paramagnetic behavior.     

Enhanced cycling stability of LiMn2O4 cathode by amorphous FePO4 coating

The cycling performance of LiMn₂O₄ at room and elevated temperatures is improved by FePO₄ modification through chemical deposition method. The pristine and FePO₄-coated LiMn₂O₄ materials are characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. Their cycling performances are thoroughly investigated and compared. The 3 wt.% FePO₄-coated LiMn₂O₄ exhibits capacity losses of only 32% and 34% at room temperature and 55 °C, respectively, after 80 cycles, much better than those of the pristine material, 55% and 72%. The cyclic voltammograms at 55 °C reveal that the improvement in the cycling performance of FePO₄-coated LiMn₂O₄ electrodes can be attributed to the stabilization of spinel structures. The separation of FePO₄ between active materials and electrolyte and its interaction with SEI (solid electrolyte interphase) film are believed to account for the improved performances.     

Synthesis and cycling behavior of LiMn2O4 cathode materials prepared by glycine-assisted sol-gel method for lithium secondary batteries

Spinel-type LiMn₂O₄ powders having submicron, narrow particle-size distribution and excellent phasepure particles have been synthesized at low temperatures from metal acetate aqueous solution containing glycine as a chelating agent by a sol-gel method. The dependence of the physicochemical properties and cycling characteristics of the spinel LiMn₂O₄ powders on the various calcination temperatures has been extensively studied. It was found that the physicochemical properties of the LiMn₂O₄ powders could be controlled by simply varying the calcination temperature. Glycine-assisted LiMn₂O₄ powders have shown excellent rechargeability and delivered discharge capacity of 119 mAh/g for more than 150th cycles in Li/polymer electrolyte/liMn₂O₄ cells.     

Electrophoretic co-deposition of Fe2O3 and Mn1,5Co1,5O4: Processing and oxidation performance of Fe-doped Mn-Co coatings for solid oxide cell interconnects

Fe-doped Mn_1,5Co_1,5O₄ coatings on Crofer22APU were processed by an electrophoretic co-deposition method and the corrosion resistance was tested at 750 °C up to 2000 h.The “in-situ” Fe-doping of the manganese cobalt spinel was achieved by electrophoretic co-deposition of Mn_1,5Co_1,5O₄ and Fe₂O₃ powders followed by a two-step reactive sintering treatment. The effects on the coating properties of two different Fe-doping levels (5 and 10 wt.% respectively) and two different temperatures of the reducing treatment (900 and 1000 °C) are discussed. Samples with Fe-doped coatings demonstrated a lower parabolic oxidation rate and thinner oxide scale in comparison with both the undoped Mn_1,5Co_1,5O₄ spinel coating and bare Crofer 22 APU. The best corrosion protection was achieved with the combined effect of Fe-doping and a higher temperature of the reducing step at 1000 °C.     

MCr2O4 (M=Co,Cu, and Zn) nanospinels for 2-propanol combustion: Correlation of structural properties with catalytic performance and stability

The correlation between structure and activity of MCr₂O₄ nanospinels (M=Co, Cu, and Zn) synthesized by a sol–gel combustion method was investigated for the oxidation of 2-propanol. The catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), N₂ adsorption/desorption, temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Wide-angle XRD patterns show that the samples are pure spinel phases with cubic structure for CoCr₂O₄ and ZnCr₂O₄, and tetragonal structure for CuCr₂O₄. FTIR spectra confirmed the spinel structure of samples. The spinels were tested for total oxidation of 2-propanol as a model reaction for the catalytic combustion of oxygenated organic pollutants. ZnCr₂O₄ exhibited the highest activity and stability than the others toward the combustion of 2-propanol. The higher activity of ZnCr₂O₄ was ascribed to existence of excess surface oxygen on catalyst, active Cr³⁺–Cr⁶⁺ pair sites, and synergistic effect between ZnO and ZnCr₂O₄ confirmed by TPR and XPS techniques. The high stability of ZnCr₂O₄ and CuCr₂O₄ was explained by the existence of stable Cr⁶⁺ species on the surface of catalysts. The study showed that ZnCr₂O₄ could be used as a promising catalyst in the catalytic conversion of organic compounds.