Spinel Particles Research Articles

High-Voltage LiNi1/2Mn3/2O4 Spinel: Cationic Order and Particle Size Distribution

Lithium–nickel manganese spinel, LiNi1/2Mn3/2O4, is capable to intercalate lithium reversible at a high voltage delivering a high specific energy when used as cathode material for lithium ion batteries. In this study, the effects of cationic order and particle size distribution on the lithium intercalation in high-voltage LiNi1/2Mn3/2O4 spinel are examined by the application of diffraction and spectroscopic techniques. At 400 °C, nonstoichiometric LiNi1/2Mn3/2O4-δ with a disordered spinel structure and particle size distribution between between 10 and 20 nm is obtained. 7Li NMR with ultrafast spinning rates and electron paramagnetic resonance spectroscopy show that Ni2+, Mn3+, and Mn4+ ions are nonuniformly distributed forming nanoscale domains with (Ni2+,Mn4+)-, (Ni2+,Mn4+,Mn3+)-, and (Mn3+,Mn4+) compositions, respectively, the entirely cubic spinel structure being preserved. By increasing the annealing temperature, the amount of Mn3+ decreases, and Ni2+ and Mn4+ tend to set up a long-range order. The pa...

Cycling Stability of 5V LiMn1.5Ni0.5O4 Spinel Particles Coated with a Thin Film Mixed Conductor

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The mechanism of oxide film formation on Alloy 690 in oxygenated high temperature water

Oxide films formed on Alloy 690 exposed to 290 °C water containing 3 ppm O 2 were investigated. It was found that Cr rich oxides form initially through solid-state reactions. Ni–Fe spinels gradually develop on surface layer by precipitation with increasing immersion time. Initially formed Cr rich oxides react with outwards diffusing Ni and Fe to form small spinel particles which then vanish gradually. An inner layer develops from oxide/matrix interface through inward diffusion of oxidant. Cr is preferentially oxidized and tends to dissolve into solution. The resultant inner layer consists of predominant NiO which cannot serve as a protective barrier layer.

Effects of Mg addition on foam stability of Al foams made by powder metallurgy route

Al foams were produced by applying a powder metallurgy route. Foam expansion, cell structures and foam stability of foamed samples were investigated. The results show that larger expansion and more homogeneous cell structures were achieved due to the presence of 1·0 wt-%Mg addition. Mg addition results in the formation of spinel particles through the reaction between Mg powder and alumina on Al powder. Spinel particles show good wetting with Al melt and are fully embedded into Plateau borders and cell walls. The liquid is tightly trapped within cell walls and Plateau borders in the presence of spinel particles, preventing cell walls from thinning further and improving foam stability significantly.

The effect of Tb 3+ doping on the structure and spectroscopic properties of MgAl 2O 4 nanopowders

In this paper, a modified sol–gel method was employed to prepare nanostructured MgAl 2O 4 spinel powders doped with Tb 3+ ions and thermally treated at 700 and 1000 °C for 3 h. The structural properties of the prepared at 700 and 1000 °C powders where characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). According to obtained XRD patterns the formation of single-phase spinels after calcination was confirmed. The XRD analyses demonstrated that the powders were single-phase spinel nanopowders with high crystallite dispersion. The Rietveld method was applied to calculate lattice parameters. The averaged spinel particle size was determined to be ∼10 nm for calcination at 700 °C and ∼20 nm at 1000 °C. The emission and excitation spectra measured at room and low temperature (77 K) for the samples calcined at 700 and 1000 °C demonstrated characteristic spectra of Tb 3+ ions. The effect of MgAl 2O 4:Tb 3+ grain sizes on luminescence properties was noticed.

Foamability of MgAl2O4 (Spinel)-Reinforced Aluminum Alloy Composites

A novel foamable aluminum alloy has been developed. It contains sub-micron-sized MgAl2O4 (spinel) particles that are generated in situ by a reaction of SiO2 with a molten Al-Mg alloy. The study involves an optimization of parameters such as Mg concentration, SiO2 particles size, and reaction time and shows that a composite containing MgAl2O4 particles as chief reinforcement in the matrix leads to effective foaming. Composites containing large sized transition phases and particle agglomerates in the matrix yield poor foam structure. The best foamable composite obtained contained 3.4 vol. pct of ultrafine (80 nm to 1 μm) MgAl2O4 particles uniformly distributed in an Al-Si alloy matrix. The corresponding metal foam contained 75 pct porosity and exhibited a uniform distribution of cells.

A new polishing compound for processing optical items, based on nanopowders of aluminum–magnesium spinel

One of the requirements imposed on polishing materials is that an isometric form must be present in the abrasive grain (the crystallite). This condition is satisfied by aluminum–magnesium spinel, MgAl2O4, which crystallizes in cubic symmetry, forming crystallites of octahedral habit, and possesses satisfactory hardness. This paper describes a method of obtaining nanosize particles of spinel by mechanical activation of a mix of hydroxides at the stage preceding heat treatment at temperatures of 700–800 °C, significantly lower than those used in known methods of synthesizing spinel. Results are given that show that spinel has satisfactory polishing ability.

Implementing Realistic Geometry and Measured Diffusion Coefficients into Single Particle Electrode Modeling Based on Experiments with Single LiMn2O4 Spinel Particles

Realistic geometry and diffusion coefficients were measured from a single particle LiMn2O4 electrode and implemented into a three-dimensional multiphysics simulation of a single particle, in order to demonstrate a novel approach to electrode material study. Dispersed particles were used, and electrochemical techniques and atomic force microscopy were performed on isolated single particles. Diffusion coefficients measured from both cyclic voltammetry and the potentiostatic intermittent titration technique ranged between 3.2 × 10−12 and 1.2 × 10−11 cm2/s, which was similar to values measured from thin film LiMn2O4 electrodes. The trend of diffusivity change over potential (versus lithium counter electrode) was similar to those observed from both composite cells and thin film electrodes. The measured diffusion coefficients were then used in simulation of discharge of the irregular particle, by importing the particle morphology into a finite element simulation, in order to simulate intercalation-induced stress generation. Simulation results showed a higher maximum stress generation due to altering diffusivity around the peak current potentials and high local stress concentration on the sharply indented surface area, suggesting that particle irregularities are important in studying both electrochemical performance and local failure mechanisms in cathode materials.

Double roles of aluminium ion on surface-modified spinel LiMn1.97Ti0.03O4

Spinel LiMn2O4 is one of the cathode active materials for the rechargeable lithium-ion batteries with the most potential, but suffers from a fast capacity fade when cycling at elevated temperatures (55 °C). Spinel LiMn1.97Ti0.03O4 coated with aluminum nitrate is heat-treated at various temperatures. The sample treated at low temperature (500 °C) exhibits porous Al2O3 layer coated on the surface, and little Al ion diffusing into the spinel structure. As for the sample treated at mid-temperature (700 °C), it exhibits double layers coated on the surface: the inner layer is LiAlxMn1.97−xTi0.03O4 solid solution and the outer layer is compact Al2O3 metal oxides. When the heat-treatment temperature increases to a high level (850 °C), nearly all the Al ion diffusing into the structure to form Al doped solid solution layer coated on the spinel. The relationship between the structure of the modified samples and their electrochemical properties is investigated. The galvanostatic charge–discharge results show that those surface-modified samples exhibit improved cycling performance compared to LiMn1.97Ti0.03O4; and the sample treated at mid-temperature delivers the best capacity retention, especially at 55 °C. The improved cycling performance is ascribed to the coated layers of Al2O3 and LiAlxMn1.97−xTi0.03O4 on the spinel particles. The Al2O3 layer could reduce the HF damage by scavenging of HF. While the LiAlxMn1.97−xTi0.03O4 layer could enhance the stability of the spinel structure due to the stronger Al–O bond; moreover, this coating layer having the same spinel structure with the LiMn1.97Ti0.03O4 core material would not hinder the lithium ion diffusion. The sample treated at mid-temperature has the double layers coated on the surface, and its Al2O3 coating becomes compact and thin which could effectively block the direct contact between the electrolyte and spinel particles as well as reduce the lithium ion diffusion distance in the inactive layer, leading to greatly improved capacity retention.

Zr, Y, Sb添加のZnOバリスタの課電劣化への影響

The effects of amount of Sb added to Bi-Mn-Co-Sb-Si-Cr-Ni-Zr-added ZnO varistors on the tolerance characteristics of electrical degradation were investigated. Furthermore, the effects of addition of both Zr and Y to Bi-Mn-Co-Sb-Si-Cr-Ni-added ZnO varistors (the same composition as one of commercial varistors) on the varistor voltage and leakage current were investigated. It was found that addition of Sb enhances the tolerance characteristics to the electrical degradation and the amount of Sb2O3 addition of approximately 1 mol% is the optimal. When the amount of added Sb increased, the amount of existence of a Zn2.33Sb0.67O4-type spinel particle increased, because the amount of existence of Zr compound increased similarly. It was suggested that Zr compound was also related to the electrical degradation characteristic. Varistor voltage decreases a little when the amount of added Zr increases in simultaneous addition of Zr and Y. On the other hand, the value of nonlinear index α hardly changed. Without Zr, the increase in the leakage current accompanying the increase in the amount of added Y, which has been reported conventionally, was not found in this study. However, adding Zr of approximately 0.2 - 0.45 mol%, the leakage current increased by adding Y more than 1 mol% and adding Zr of approximately 0.6 mol%, it decreased.

A mechanistic model to predict matte temperatures during the smelting of UG2-rich blends of platinum group metal concentrates

High matte temperatures can be related to numerous catastrophic furnace failures in the platinum group metal (PGM) industry where chromite-rich upper group 2 (UG2) concentrates are smelted. Chromite rich concentrates require high slag temperatures as well as sufficient mixing to suspend the chromite spinel particles in the slag and prevent settling in a so-called “mushy” layer consisting of a three phase emulsion of slag, matte and chromite particles. To achieve sufficient bath mixing and to melt and suspend chromite spinel build-up, high hearth power densities are utilised. However, high hearth power densities in conjunction with a heat-isolating concentrate layer, leads to high side wall heat fluxes which motivated the use of intensive cooling in the furnace side wall so that a slag freeze lining can be formed. If matte temperatures are above the slag liquidus temperature, any matte that comes into contact with the freeze lining can destroy the freeze lining. Moreover, if the matte temperature exceeds ca. 1500 °C, chemical thermodynamics indicate that matte has the ability to sulfidise MgO–Fe x O–Cr 2O 3 refractories, leading to rapid wear of refractories exposed to high temperature flowing matte. Models are derived for the concentrate-to-matte and slag-to-matte droplet heat transfer. Calculations using the derived models, physical properties and furnace operating conditions give realistic matte temperatures and show that matte temperatures rapidly increase as the concentrate bed becomes matte drainage rate limiting. It is shown that for each concentrate blend mean particle size and mineralogy, there is a maximum smelting rate above which the concentrate bed becomes rate limiting with regards matte drainage, thereby significantly contributing to matte preheating, prior to further heat absorption from the slag layer.

Ex-nitrate Co/SBA-15 catalysts prepared with calibrated silica grains: Information given by TPR, TEM, SAXS and WAXS

The aim of the present work was to check for the presence of an ordered array of Co-based nanoparticles in catalysts prepared by impregnation and by using the two-solvents technique with pentane and cyclohexane on a single SBA-15 batch (prepared by precipitation in strongly acidic conditions (HCl 2 mol L −1)). Cobalt nitrate is used as a precursor for an at. Co/Si ratio of 0.03. Solids are characterized after heat treatments in oxidizing (up to 700 °C, 2 °C/min, under O 2 10% in vol./Ar) and reducing (up to 1000 °C, 7 °C/min, under a H 2 10% in vol./He) conditions. After oxidation up to 700 °C, characteristic spinel diffraction peaks are observed in WAXS. The relative intensities of these peaks suggest that Co 3O 4 dominates when using impregnation whereas mixed Co 3– x Si x O 4 domains are formed when using the two-solvents technique. Broad and symmetrical when using pentane, the X-ray diffraction peaks are more asymmetric with cyclohexane. On a statistical scale, more asymmetric spinel particles are then formed with this solvent. Still with cyclohexane, a correlation diffraction peak at 0.6° in SAXS is observed. Two TPR peaks before 500 °C are associated with the reduction of Co 3O 4 particles into CoO and then metallic Co. Two other TPR peaks (a shoulder at 330–350 °C, a broad peak at 640–680 °C) indicate the presence of particularly small oxidized particles in strong interaction with silica (detected neither by XRD, nor by TEM). By reference to recently published data, additional reductions observed within the range 800–900 °C and 900–1000 °C are associated with cobalt silicate Co 2SiO 4 domains, respectively dispersed on the silica surface and bulk. In reduced samples, two main kinds of metallic particles are identified: (i) large and spherical particles (diameter >20 nm) growing outside silica grains and often covered by a silica layer. These particles are polytypic face-centered cubic with some hexagonal intergrowths, (ii) smaller particles (diameter <10 nm) which remain dispersed inside the silica grains and/or inside silica walls. In SAXS, there is no correlation peak but the diffusion background detected at the lowest angles can be associated with the diffusion due to the largest Co-based particles.

SrFe2O4 complex oxide an effective and environmentally benign catalyst for selective oxidation of styrene

The SrFe2O4 spinel type catalyst is synthesized by citrate gel method. The precursor and oxide are well characterized by various techniques such as TG–DTG, FT-IR, X-ray diffraction, SEM, and X-ray fluorescence. The crystallization temperature of the spinel particle prepared by citrate gel method is 600°C, which is lower than that of ferrite prepared by other methods. SrFe2O4 catalysts prepared by citrate gel method show high activity for styrene oxidation in the presence of H2O2 (30%) as an oxidizing agent and aqueous medium. GCMS analysis revealed that, during the course of reaction, the oxidative cleavage of CC double bond of styrene takes place selectively to give benzaldehyde selectivity up to 64mol%, whereas phenyl acetaldehyde selectivity 28mol% and others’ selectivity 8mol% as minor product. Solvents have marked influence on the product distribution in the selective oxidation of styrene; water seems to be the best solvent. The optimization and effect of various reaction conditions on the conversion of styrene and product distribution were also studied. The highest benzaldehyde yield was obtained at 70°C in water solvent over 0.1g of catalyst with 18h of reaction.

Petrographic and thermodynamic study of slags in EAF stainless steelmaking

A study of the typical characteristics of electrical arc furnace (EAF) slags in the production of the stainless steel (AISI 304L) was carried out. Twenty-eight slag samples were taken from 14 heats. Simultaneously with each slag sampling, the temperature of the steel was measured, and one steel sample was taken. The selected slag samples were studied both using SEM–EDS and light optical microscopy. Computational thermodynamics was used as a tool to predict the equilibrium phases in the top slag as well as the amount of these phases at the process temperatures. It was observed that, at process temperature, the stainless EAF slag generally consists of liquid oxides, spinel particles and metallic droplets. Under normal operation, the amount of spinel particles is 2–6 wt-%. In addition, the influence of the slag temperature, basicity, MgO content and Cr2O3 content on the amount of spinel precipitates and thereby on the foaming index of the top slag is also illustrated and discussed. More specifically, it was found that, within the compositional range of the slag samples, the critical parameter affecting the amount of solid spinel particles in the slag is the chromium oxide content.

Microstructural characteristics of the oxide scale formed on 304 stainless steel in oxygenated high temperature water

The microstructural characteristics of oxide scale formed on type 304 stainless steel in oxygenated high temperature water have been investigated. From outer to inner layer, the oxide scale consists of faceted spinel particles, irregularly shaped hematite particles and a compact layer of nano-sized spinels. Some outmost spinels formed on top of other particles are depleted in Cr, while the hematite particles tightly embedded into the inner layer contain more Cr in the inner than in the outer part. The inner nano-sized oxide grow inwards directly from the bottom of outer particles. The related oxidation mechanism is discussed.

MgAl2O4 Spinel-Stabilized Calcium Oxide Absorbents with Improved Durability for High-Temperature CO2 Capture

With efficient energy recovery, calcium-oxide-based absorbents that operate at elevated temperatures have an advantage over absorbents that operate at lower temperatures for CO2 capture from coal power plants. The major limitation of these absorbents is that the carbonation and decarbonation reactions of CaO and CaCO3 are far from complete or reversible. Rapid loss of CO2 capacity over many carbonation/decarbonation cycles is always observed because of severe absorbent sintering. We have found that this sintering effect can be effectively mitigated by properly mixing calcium oxide precursors with small rod-like MgAl2O4 spinel nanoparticles. A new class of CaO-based absorbents with much improved high-temperature durability was developed by wet physical mixing of calcium acetate with nano MgAl2O4 spinel particles followed by high-temperature calcination. CaO−MgAl2O4 (32 wt % spinel content) material provides 34 wt % CO2 capacity after 65 carbonation−decarbonation cycles (650 and 850 °C, respectively), corre...

Optimization of reaction conditions in selective oxidation of styrene over fine crystallite spinel-type CaFe 2O 4 complex oxide catalyst

The CaFe 2O 4 spinel-type catalyst was synthesized by citrate gel method and well characterized by thermogravimetric analysis, atomic absorption spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction and transmission electron microscopy. The crystallization temperature of the spinel particle prepared by citrate gel method was 600 °C which was lower than that of ferrite prepared by other methods. CaFe 2O 4 catalysts prepared by citrate gel method show better activity for styrene oxidation in the presence of dilute H 2O 2 (30%) as an oxidizing agent. In this reaction the oxidative cleavage of carbon–carbon double bond of styrene takes place selectively with 38 ± 2 mol% conversion. The major product of the reaction is benzaldehyde up to 91 ± 2 mol% and minor product phenyl acetaldehyde up to 9 ± 2 mol%, respectively. The products obtained in the styrene oxidation reaction were analyzed by gas chromatography and mass spectroscopy. The influence of the catalyst, reaction time, temperature, amount of catalyst, styrene/H 2O 2 molar ratio and solvents on the conversion and product distribution were studied.

Re-examining the effect of ZnO on nanosized 5 V LiNi 0.5Mn 1.5O 4 spinel: An effective procedure for enhancing its rate capability at room and high temperatures

Based on the known effectiveness of ZnO as a hypothetical coating, its ability to expand the rate capability of nanosized LiNi 0.5Mn 1.5O 4 was examined. The additive was characterized by X-ray photoelectron spectroscopy (XPS) analysis. The intensity of the 2p peak for Zn in the XPS concentration depth profiles remained unchanged over a long sputtering period; therefore, rather than forming a coating layer over the spinel particles, as commonly described in the literature, ZnO seemingly deposits as uniformly dispersed nanoparticles in the bulk material. TEM images were consistent with this alternative model for explaining where the oxide is located. ZnO-treated spinel cycled at low rates ( C/4) at room temperature was found to exhibit an increase in delivered capacity (more than a 10%) with good capacity retention. The increase was less marked at high cycling rates (8 C). Cells cycled at 50 °C exhibited similarly improved properties; however, the increase in capacity at low cycling rates was somewhat lower. These changes are explained in the light of the results obtained by treating the spinel with HF at room temperature: ZnO clearly hindered dissolution of the spinel. In fact, ZnO partially protects the spinel from the attack of HF traces in LiPF 6 based electrolyte during prolonged contact between the two. This protective effect, however, is insubstantial at high charge/discharge rates, where contact between the spinel and electrolyte is shorter.

X-ray diffraction studies on crystallite size evolution of CoFe 2O 4 nanoparticles prepared using mechanical alloying and sintering

Nanosized cobalt ferrite spinel particles have been prepared by using mechanically alloyed nanoparticles. The effects of various preparation parameters on the crystallite size of cobalt ferrite which includes milling time; ball-to powder weight ratio (BPR) and sintering temperature, were studied using X-ray diffractometer (XRD). Scherrer's equation was used to study the crystallite size evolution of the as-prepared materials. The results of the as-milled sample revealed that both milling time and BPR plays a role in determining the crystallite size of the milled powder. However, where sintering is involved, the sintering temperature results in grain growth, and thus plays a dominant role in determining the final crystallite size of the samples sintered at higher temperature (above 900 °C). From the vibrating-sample magnetometer (VSM) measurement it was observed that the coercivity of the as-milled samples without sintering is almost negligible, which is a type characteristic of superparamagnetic material. However, for the sintered samples, the saturation increases while coercivity decreases with increases sintering temperature.

Effect of Thermal Annealing on Electrical Degradation Characteristics of Sb&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-Bi-Mn-Co-Doped ZnO Varistors

The effects of thermal annealing on the electrical degradation of Sb2O3-Bi-Mn-Co-doped ZnO varistors were investigated. For samples with 0.04 mol% Sb2O3 or more, the nonlinearity index  of the voltage-current (V-I) characteristics after electrical degradation increased upon annealing. More-over, the value of  after electrical degradation was proportional to the full width at half maximum (FWHM) of the X-ray diffraction peak for Zn2.33Sb0.67O4-type spinel particles under various annealing conditions. The added Sb2O3 did not dissolve in the ZnO grains but became segregated at grain boundaries. Therefore, it is speculated that the increase in the FWHM of the spinel particles is due to an increase in the numbers of fine spinel particles at grain boundaries and triple points during annealing. Furthermore, it is suggested that the improvement in the electrical degradation upon annealing is due to a decrease in the mobility of oxide ions or Zn2+ ions owing to their being blocked by uniformly distributed fine spinel particles at grain boundaries.