Segregation Of Cobalt Research Articles

Segregation of nickel, cobalt and iron from limonite: recoveries, fundamentals and in situ reduction comparison

ABSTRACT This paper describes a study of the segregation process in which nickel, cobalt and iron are extracted as volatile metal chlorides from a limonite laterite and collected as ferronickel onto carbon within a single step, through addition of 5% CaCl2 and 5% coke to calcined limonite at 1123 and 1173 K. Segregation of limonite yielded maximum nickel, cobalt and iron recoveries of 78%, 23% and 1.1%, respectively within ferronickel containing 57% nickel. All recoveries were quantified using rate equations. Estimations from evolved gas and recycle analyses revealed that segregation reduction required 40–45% of the stoichiometric hydrogen requirement and <0.1% moisture from calcine. Ferronickel recovery and grades were comparable when using 5% coke for segregation and chloride-free in situ reduction. Comparison of carbon consumption clarified the roles of the Boudouard reaction and the water-gas reaction during segregation. Segregation and in situ reduction (from this and other work) were compared as potential processes for concentration of ferronickel from limonite. Segregation appeared to have significant advantages because at lower temperatures, it produced a larger grained, higher grade ferronickel, more amenable to physical concentration. Cet article décrit une étude du processus de ségrégation dans lequel le nickel, le cobalt et le fer sont extraits sous forme de chlorures métalliques volatiles à partir d'une limonite latéritique et captés sous forme de ferronickel sur du carbone, en une seule étape, par addition de 5% de CaCl2 et 5% de coke à la limonite calcinée à 1123 et 1173 K. La ségrégation de la limonite a produit des récupérations maximales de nickel, de cobalt et de fer de 78%, 23% et 1.1%, respectivement, dans le ferronickel contenant 57% de nickel. On a quantifié toutes les récupérations à l'aide d'équations de vitesse. Les estimations des analyses de gaz dégagé et de recyclage ont révélé que la réduction de la ségrégation nécessitait de 40 à 45% de la demande stœchiométrique d'hydrogène et <0.1% d'humidité du calcinat. La récupération et les teneurs du ferronickel étaient comparables lorsqu'on utilisait 5% de coke pour la ségrégation et une réduction in situ sans chlorure. La comparaison de la consommation de carbone a clarifié les rôles de la réaction de Boudouard et de la réaction eau-gaz lors de la ségrégation. On a comparé la ségrégation et la réduction in situ (à partir de ce travail et d'autres travaux) en tant que processus potentiels de concentration de ferronickel à partir de la limonite. La ségrégation semblait présenter des avantages significatifs parce qu'à des températures inférieures, elle produisait du ferronickel à grain plus gros et de qualité supérieure, plus propice à la concentration physique.

Adsorbate-Induced Segregation of Cobalt from PtCo Nanoparticles: Modeling Au Doping and Core AuCo Alloying for the Improvement of Fuel Cell Cathode Catalysts.

Platinum, when used as a cathode material for the oxygen reduction reaction, suffers from high overpotential and possible dissolution, in addition to the scarcity of the metal and resulting cost. Although the introduction of cobalt has been reported to improve reaction kinetics and decrease the precious metal loading, surface segregation or complete leakage of Co atoms causes degradation of the membrane electrode assembly, and either of these scenarios of structural rearrangement eventually decreases catalytic power. Ternary PtCo alloys with noble metals could possibly maintain activity with a higher dissolution potential. First-principles-based theoretical methods are utilized to identify the critical factors affecting segregation in Pt–Co binary and Pt–Co–Au ternary nanoparticles in the presence of oxidizing species. With a decreasing share of Pt, surface segregation of Co atoms was already found to become thermodynamically viable in the PtCo systems at low oxygen concentrations, which is assigned to high charge transfer between species. While the introduction of gold as a dopant caused structural changes that favor segregation of Co, creation of CoAu alloy core is calculated to significantly suppress Co leakage through modification of the electronic properties. The theoretical framework of geometrically different ternary systems provides a new route for the rational design of oxygen reduction catalysts.

The Role of Oxidized Carbides on Thermal-Mechanical Performance of Polycrystalline Superalloys

Oxidized MC carbides which act as main crack initiation sites in a polycrystalline superalloy under thermal-mechanical fatigue (TMF) conditions at 850 °C were studied. Microstructural observations in the TMF tested specimens were compared to findings from bulk samples exposed isothermally in air at 850 °C for 30 hours in the absence of any external applied load. Carbides were found to oxidize rapidly after exposure at 850 °C for 30 hours resulting in surface eruptions corresponding to oxidation products, from where micro-cracks initiated. Plastic deformation due to volume expansion of the often porous oxidized carbides led to high dislocation densities in the adjacent matrix as revealed by controlled electron channeling contrast imaging. The high dislocation density facilitated the dissolution kinetics of γ′ precipitates by segregation and diffusion of chromium and cobalt along the dislocations via pipe diffusion, resulting in the formation of soft recrystallized grains. Atom probe tomography revealed substantial compositional differences between the recrystallized grains and the adjacent undeformed γ matrix. Similar observations were made for the TMF tested alloy. Our observations provide new insights into the true detrimental role of oxidized MC carbides on the crack initiation performance of polycrystalline superalloys under TMF.

The effect of chromium and cobalt segregation at dislocations on nickel-based superalloys

The segregation of solutes at dislocations in a polycrystalline and a single crystal nickel-based superalloy is studied. Our observations confirm the often assumed but yet unproven diffusion along dislocations via pipe diffusion. Direct observation and quantitative, near-atomic scale segregation of chromium and cobalt at dislocations within γ' precipitates and at interfacial dislocations leading to the partial or complete dissolution of γ' precipitates at elevated temperatures is presented. Our results allow us to elucidate the physical mechanism by which pipe diffusion initiates the undesirable dissolution of γ' precipitates.

Polyol Synthesis of Cobalt–Copper Alloy Catalysts for Higher Alcohol Synthesis from Syngas

Novel catalysts for the selective production of higher alcohols from syngas could offer improved pathways towards synthetic fuels and chemicals. Cobalt–copper alloy catalysts have shown promising results for this reaction. To improve control over particle properties, a liquid phase nanoparticle synthesis based on the polyol method was selected to synthesize Co2.5Cu particles, which were then supported onto a variety of metal oxide supports (Al2O3, SiO2, TiO2, ZrO2). The catalysts were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy before and after catalytic testing in a flow reactor at 250 °C and 40 bar. The results show alloyed phases were obtained using the polyol method, resulting in selectivity towards higher alcohols, as high as 11.3% when supported on alumina. Segregation of cobalt and the formation of cobalt carbide were observed in the catalysts after catalytic testing, which may limit performance compared to the desired alloy phase.

Microstructure and Magnetic Properties of NdFeB Sintered Magnets Diffusion-Treated with Cu/Al Mixed Dyco Alloy-Powder

AbstractWe investigated the microstructural and magnetic property changes of DyCo, Cu + DyCo, and Al + DyCo diffusion-treated NdFeB sintered magnets. The coercivity of all diffusion treated magnet was increased at 880ºC of 1stpost annealing(PA), by 6.1 kOe in Cu and 7.0 kOe in Al mixed DyCo coated magnets, whereas this increment was found to be relatively low (3.9 kOe) in the magnet coated with DyCo only. The diffusivity and diffusion depth of Dy were increased in those magnets which were treated with Cu or Al mixed DyCo, mainly due to comparatively easy diffusion path provided by Cu and Al because of their solubility with Ndrich grain boundary phase. The formation of Cu/Al-rich grain boundary phase might have enhanced the diffusivity of Dy-atoms. Moreover, relatively a large number of Dy atoms reached into the magnet and mostly segregated at the interface of Nd2Fe14B and grain boundary phases covering Nd2Fe14B grains so that the core-shell type structures were developed. The formation of highly anisotropic (Nd, Dy)2Fe14B phase layer, which acted as the shell in the core-shell type structure so as to prevent the reverse domain movement, was the cause of enhancing the coercivity of diffusion treated NdFeB magnets. Segregation of cobalt in Nd-rich TJP followed by the formation of Co-rich phase was beneficial for the coercivity enhancement, resulting in the stabilization of the metastable c-Nd2O3phase.

Deactivation and stability of K-CoMoSx mixed alcohol synthesis catalysts

Potassium-promoted cobalt molybdenum sulfide (K-CoMoSx) mixed alcohol synthesis catalysts were operated from 118 to 3969h for the purpose of studying catalyst deactivation. Continuous and discontinuous sulfiding with H2S and methyl sulfides was considered. Fresh and discharged catalysts were analyzed via X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). When continuously sulfided, selectivity was maintained for thousands of hours. Without sulfur cofeed, catalysts exhibited a change in selectivity from alcohols to hydrocarbons over periods of hundreds of hours. Sulfur deprivation resulted in oxidization, carburization, and coking of the catalyst surface and segregation of cobalt into crystalline Co9S8. It is suggested that in the absence of a sulfiding agent, the catalyst surface becomes more acidic (oxidized) promoting dehydration of alcohols (selectivity change) and coking (blocking active sites). Reintroduction of H2S may reverse oxidation on non-coked surfaces. Proper sulfur maintenance may render catalysts operable for years without need of regeneration or replacement.

Surface Segregation and Chromium Deposition and Poisoning on La0.6Sr0.4Co0.2Fe0.8O3- δ Cathodes of Solid Oxide Fuel Cells

La0.6Sr0.4Co0.2Fe0.8O3- δ, (LSCF) perovskite oxide is one of the most common cathode materials used in solid oxide fuel cells (IT-SOFCs), but vulnerable to the chromium deposition and poisoning in the presence of gaseous chromium species from metallic interconnect. In this work, the fundamental relationship between the surface segregation and Cr deposition and poisoning of LSCF cathodes is studied on dense LSCF bar samples. The detailed SEM and FIB-EDS analysis results clearly indicate the segregation of strontium and cobalt, forming individual SrO and CoOx particles on the LSCF surface after annealing at 800 °C for 96 h. The results indicate that Cr deposition occurs preferentially on the segregated SrO particles on the LSCF surface.

Formation of double ring patterns on Co2MnSi Heusler alloy thin film by anodic oxidation under scanning probe microscope

Double ring formation on Co2MnSi (CMS) films is observed at electrical breakdown voltage during local anodic oxidation (LAO) using atomic force microscope (AFM). Corona effect and segregation of cobalt in the vicinity of the rings is studied using magnetic force microscopy and energy dispersive spectroscopy. Double ring formation is attributed to the interaction of ablated material with the induced magnetic field during LAO. Steepness of forward bias transport characteristics from the unperturbed region of the CMS film suggest a non equilibrium spin contribution. Such mesoscopic textures in magnetic films by AFM tip can be potentially used for memory storage applications.

Surface oxidation of NiCo alloy: A comparative X-ray photoelectron spectroscopy study in a wide pressure range

Oxidation of NiCo alloy has been studied under two pressure regimes, 5×10−10 and 5×10−1bar, by X-ray photoelectron spectroscopy (XPS). The aim of this work is to investigate the synergetic effect between the two alloy components during the initial stages of oxidation. The results showed that at low oxygen pressure, segregation and preferential oxidation of cobalt takes place, while oxidation of nickel is largely suppressed. The species dominating the surface is CoO but small amount of metallic cobalt still remains even after prolonged oxidation at 670K. At 0.5bar O2 pressure, alloy oxidation was found to be temperature depended. From 420K to 520K, cobalt is completely transformed to CoO and the Ni:Co atomic ratio at the surface approaches a minimum, similar to the observations at low pressure regime. However, at higher temperatures (from 520K to 720K), nickel is re-segregated on the surface, in the expense of cobalt, while CoO is further oxidized to Co3O4. At this temperature range formation of mixed Ni–Co–O spinel-like oxides is probable as supported by the characteristic modifications of the Ni 2p3/2 photoelectron peak and the increase of the Ni:Co atomic ratio.

A comparative in situ XPS study of PtRuCo catalyst in methanol steam reforming and water gas shift reactions

In the present study the oxidation state and the surface composition of a ternary PtRuCo catalyst were investigated under methanol steam reforming (SRM) and water gas shift (WGS) reaction conditions at 570 K. Ambient pressure X-ray photoelectron spectroscopy (APPES) was applied in situ at 0.5 mbar, while simultaneously monitoring the catalytic activity of the sample by on-line mass spectrometry. Non-destructive depth profile measurements performed under SRM reaction conditions over a polycrystalline PtCo foil, were also used to obtain detailed depth-resolved information. The results showed that surface segregation of cobalt and modification of its oxidation state occurs when switching from SRM to WGS reaction conditions. Evidence of ionic Pt was found only during WGS reaction, while Ru was mainly present in the metallic state. The results clearly demonstrate the dynamic response of the PtRuCo catalytic surface to the reaction atmosphere.

Synthesis and characterization of divalent cobalt-substituted mesoporous aluminophosphate molecular sieves and their application as novel heterogeneous catalysts for the oxidation of cycloalkanes

Thermally stable divalent cobalt-substituted hexagonal mesoporous aluminophosphate, CoHMA molecular sieves were hydrothermally synthesized and systematically characterized by several analytical and spectroscopic techniques. XRD, TEM, ED, ICP-AES, TGA-DTA, N 2 sorption, DR-UV–vis, XANES, and NH 3-TPD studies showed isomorphous substitution of divalent cobalt in a tetrahedral framework of a mesoporous aluminophosphate matrix. Furthermore, the blue color of CoHMA, before and after thermal treatment, further confirmed the tetrahedral environment of divalent cobalt in the matrix. As a result, the CoHMA catalyst showed excellent activity for the oxidation of cycloalkanes, that is, cyclohexane, cyclooctane and cyclododecane, under mild reaction conditions. Unlike the many other cobalt-based heterogeneous catalysts reported so far, CoHMA does not show any dislodgement or segregation of cobalt with calcination or any other postsynthesis treatments. In this study, the performance of CoHMA was also compared with the cobalt-containing microporous aluminophosphate, that is, CoAPO-5 with AFI topology, as well as with both microporous and mesoporous analogues of cobalt silicate molecular sieves that have MFI or MCM-41 structures.

Modification of WC hard alloy by compressive plasma flow

The effect of compressive plasma flows of nitrogen on the mechanical properties, phase and element composition of a WC hard alloy (6 wt.% of Co) has been investigated in this work. Compression plasma flows were obtained using a quasistationary plasma accelerator. X-ray diffraction analysis, Auger electron spectroscopy, scanning electron microscopy and X-ray microanalysis were used as investigation techniques.It was found that the whole depth of the modified layer amounted to 15 μm. The phase composition analysis showed that plasma treatment resulted in the transformation of a WC phase into a W2C one. The formation of a graphite phase was also found in the surface layer. The element composition analysis revealed surface segregation of carbon and cobalt. The findings also showed that microhardness of the hard alloy increased 1.7 times after treatment.

Transport and electrocatalytic properties of La0.3Sr0.7Co0.8Ga0.2O3?? membranes

Incorporation of gallium into the perovskite lattice of La0.3Sr0.7CoO3−δ leads to increasing unit cell volume and to decreasing thermal expansion, total conductivity and oxygen permeability. At 973–1223 K, the oxygen permeation fluxes through La0.3Sr0.7Co0.8Ga0.2O3−δ ceramics with 96.5% density are determined by the bulk ionic conduction and surface exchange rates. The total conductivity of La0.3Sr0.7Co0.8Ga0.2O3−δ, predominantly p-type electronic, exhibits an apparent pseudometallic behavior due to oxygen losses on heating, whereas the p(O2) dependencies of the conductivity and Seebeck coefficient suggest a small-polaron mechanism of hole transport. The average thermal expansion coefficients in air are 15.9×10−6 K−1 at 360–710 K and 27.9×10−6 K−1 at 710–1030 K. On decreasing oxygen pressure down to 4–30 Pa at 973–1223 K, perovskite-type La0.3Sr0.7Co0.8Ga0.2O3−δ transforms into a brownmillerite-like modification, whose electrical properties are essentially p(O2) independent. Further reduction results in the decomposition of the brownmillerite into a multiphase oxide mixture at p(O2)=8×10−10–3×10−4 Pa, and then in the segregation of metallic cobalt. Due to surface-limited oxygen transport, La0.3Sr0.7Co0.8Ga0.2O3−δ membranes are, however, kinetically stable under an air/CH4 gradient up to 1223 K. The conversion of dry methane in model membrane reactors increases with oxygen permeation flux and temperature, but yields high CO2 concentrations (>90%), indicating a dominant role of complete CH4 oxidation on the membrane surface.

Cobalt promoted fused iron catalyst for ammonia synthesis

The effect of cobalt on the properties of cobalt modified iron fused catalyst for ammonia synthesis has been investigated by scanning electron microscopy, Mössbauer spectroscopy, X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRFS) and Auger electron spectroscopy (AES). Introduction of cobalt changed considerably the sample behavior during activation and ammonia synthesis. The most active sample contained approximately 5.5 wt.% Co. Activation energies for the Co/Fe catalyst and iron catalyst were similar. The surface segregation of cobalt and iron has been postulated. This hypothesis was confirmed by an AES study of the model iron catalyst and the cobalt modified fused iron catalyst. Cobalt caused restructurization of iron to the Fe(111) face. This effect increased nitrogen adsorption and decreased hydrogen and ammonia adsorption and hence increased the catalytic activity of the Co/Fe catalyst.

Magnetic AC susceptibility study of the cobalt segregation process in melt-spun Cu–Co alloys

Temperature and frequency-dependent AC susceptibility has been used to characterize Cu 90Co 10 melt-spun ribbons, about 15 μm thick, in order to see to what extent this technique yields information about the segregation of cobalt in this alloy. The interpretation of the results includes, as a prerequisite, a transmission electron microscopy (TEM) characterization and makes use of previous field-dependent magnetization data on the same samples. Due to their different dynamical magnetic properties, the large intergrain precipitates, the small intragrain aggregates and the remaining Cu–Co solid solution, previously detected in these alloys, are independently observed by AC susceptibility as ferromagnetic, superparamagnetic and spin-glass species. Contrary to other, mostly local, microstructural characterization techniques of use with nanostructured materials, the AC susceptibility yields information about the whole sample. Furthermore, unlike the measurement of the temperature-dependent magnetization which is the magnetic technique mostly used until now, the results are basically independent of the thermal history. The correlation between microstructure and magnetic properties is illustrated by a scheme which includes magnetization, AC susceptibility and TEM data.

Ternary iron-cobalt silicide fabricated by ion beam synthesis

Ternary silicide layers were fabricated by the implantation of iron and cobalt into (100) silicon wafers. Two sets of samples with different iron to cobalt ratios were prepared, with cobalt being implanted first followed by iron in the first set, and the implantation order being reversed for the second set. In all cases a total Fe + Co dose of 5 × 10 17 cm −2 was used. The structural properties of the synthesised layers were assessed by Rutherford backscattering spectrometry (RBS) and cross-sectional transmission electron microscopy (XTEM). Our results indicate that for the samples where cobalt was implanted first epitaxial layers are formed after implantation. However, when iron was implanted first the layers are non-crystalline. For the samples with iron implanted frrst no significant improvement in crystal quality was observed with increasing anneal temperature. However, for those samples where cobalt was implanted first we observed a strong dependence of the crystal quality on the annealing temperature. For the samples with cobalt doses ≥ 2.5 × 10 17 cm −2, where cobalt was implanted first, segregation of iron and cobalt within the synthesised layer is observed.

Studies on the storage of electrochemically impregnated nickel/cadmium cells

The storage procedure commonly used for space-quality nickel cadmium cells is to keep the fully discharged cells in the shorted condition. These cells use chemically impregnated positive and negative electrodes and maintain good capacity during such storage. By contrast, space-quality nickel/cadmium cells developed in-house with electrochemically impregnated electrodes exhibit a fall in capacity. It is suggested that segregation of cobalt is the cause of capacity reduction after shorter-mode storage.

Oxygen adsorption on the Co50Ni50(111) surface

Oxygen adsorption on the Co50Ni50(111) surface at RT has been studied by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The uptake rate and the work function variations show the different stages of linear initial adsorption, oxide nucleation and oxide thickening. Chemisorption of oxygen induced a segregation of cobalt. Heating at 327°C caused a further cobalt segregation which reduced Ni2+ to Ni0 and finally islands of cobalt oxide were formed.

Analysis of surface segregation in Co-Ru alloy using Auger electron spectroscopy

The surface of Co-Ru specimens of 20.7, 5.4 and 1.5 at% Ru was investigated in the temperature range of 700–1200 K by Auger electron spectroscopy. In the quantification the monolayer segregation model was adopted. Variations of the cobalt concentration with temperature at equilibrium showed segregation of cobalt with a heat Q = 19.4 ± 3.5 kJ mol -1. The results are discussed in relation to theoretical predictions existing in the literature.