Cobalt Segregation Research Articles

Temperature-induced atypical oxidation in Sm2(Co, Fe, Cu, Zr)17 alloy systems: Dependency on oxygen-induced elemental migration

This paper addresses temperature and/or oxygen-induced chemical segregation of elements in the growth of oxide scale on Sm2(Co, Fe, Cu, Zr)17. Whereas, oxidation at T ≤ 573 K leads to primarily iron and cobalt segregation and subsequently their oxides, that at > 573 K, yields to largely CuO because of copper segregation. Only combination of higher sample temperature and environmental oxygen are found to have driven copper from alloy interior to its surface. Moreover, oxidation mechanism is considerably slowed down at higher temperature and is attributed to semi passivating surface copper oxide formation, impeding passage of oxygen to layers underneath to it.

Key Role of Interfacial Cobalt Segregation in Stable Low-Resistance Composite Oxygen-Reducing Electrodes.

The development of efficient and stable oxygen-reducing electrodes is challenging but vital for the production of efficient electrochemical cells. Composite electrodes composed of mixed ionic-electronic conducting La1-xSrxCo1-yFeyO3-δ and ionic conducting doped CeO2 are considered promising components for solid oxide fuel cells. However, no consensus has been reached regarding the reasons of the good electrode performance, and inconsistent performance has been reported among various research groups. To mitigate the difficulties related to analyzing composite electrodes, this study applied three-terminal cathodic polarization to dense and nanoscale La0.6Sr0.4CoO3-δ-Ce0.8Sm0.2O1.9 (LSC-SDC) model electrodes. The critical factors determining the performance of the composite electrodes are the segregation of catalytic cobalt oxides to the electrolyte interfaces and the oxide-ion conducting paths provided by SDC. The addition of Co3O4 to the LSC-SDC electrode resulted in reduced LSC decomposition; thus, the interfacial and electrode resistances were low and stable. In the Co3O4-added LSC-SDC electrode under cathodic polarization, Co3O4 turned wurtzite-type CoO, which suggested that the Co3O4 addition suppressed the decomposition of LSC and, thus, the cathodic bias was maintained from the electrode surface to electrode-electrolyte interface. This study shows that cobalt oxide segregation behavior must be considered when discussing the performance of composite electrodes. Furthermore, by controlling the segregation process, microstructure, and phase evolution, stable low-resistance composite oxygen-reducing electrodes can be fabricated.

Serrated flow in nickel base super alloy 720Li at 350 and 450 °C: Effect of chromium and cobalt segregation at dislocation core

The tensile flow behavior of nickel base super alloy 720Li is investigated in the temperature range of 25 to 720 °C. At 350 and 450 °C, 720Li alloy exhibits serrated flow and the cause for the same is solute segregation at the dislocation core. From the energy filter transmission electron microscopy (EF-TEM) examination, it is evident that chromium segregation at dislocation core at 350 °C and chromium and cobalt segregation at dislocation core at 450 °C are primarily responsible for serrated flow in 720Li alloy.

Growth and characterization of ultrathin cobalt ferrite films on Pt(111)

CoFe2O4 thin films (5 nm and 20 nm thick) were grown by oxygen assisted molecular beam epitaxy on Pt(111) at 523 K and subsequently annealed at 773 K in vacuum or oxygen. They were characterized in-situ using Auger Electron Spectroscopy, Low-Energy Electron Diffraction, Scanning Tunneling Microscopy and Conversion Electron Mössbauer Spectroscopy. The as-grown films were composed of small, nanometric grains. Annealing of the films produced an increase in the grain size and gave rise to magnetic order at room temperature, although with a fraction of the films remaining in the paramagnetic state. Annealing also induced cobalt segregation to the surface of the thicker films. The measured Mössbauer spectra at low temperature were indicative of cobalt ferrite, the both films showing very similar hyperfine patterns. Annealing in oxygen or vacuum affected the cationic distribution, which was closer to that expected for an inverse spinel in the case of annealing in an oxygen atmosphere.

Characterization of the elemental distribution of superalloy composite powders by micro beam X-ray fluorescence and laser-induced breakdown spectroscopy

During the mechanical mixing of many types of powders, mechanical alloying will occur between powders of different components and particle sizes, accompanied by a series of complex physical and chemical changes. In this study, micro beam X-ray fluorescence (μ-XRF) and laser-induced breakdown spectroscopy (LIBS) were used to analyze the surface composition distribution and depth profile of cobalt and superalloy composite powders mixed for different ball-milling times. In addition, the variation of the content of the metal composite powders from the surface to the interior was determined. X-ray diffraction, scanning electron microscopy combined with energy-dispersive X-ray spectroscopy, and inductively coupled plasma atomic emission spectrometry were also used to characterize the morphology, structure, and the average content of the composite metal powders. The effects of the ball-milling time on the morphology, composition distribution, and microstructure of the composite metal powder were studied. Increasing ball-milling time led to improvement of the dispersion of cobalt powder in the superalloy composite. Fine cobalt particles were gradually attached to the surface of large superalloy particles by the mechanical alloying process. The cobalt contents determined by μ-XRF were clearly higher than the designed values owing to cobalt enrichment on the surface of the powder particles. Depth distribution analysis by LIBS also indicated the occurrence of cobalt segregation in the outer layer of composite particles with an enrichment thickness of normally less than 10 μm.

Magnetic properties of cobalt doped ZrO2 nanoparticles: Evidence of Co segregation

We synthesized pure and Co-doped (6.25%–12.5% at.) ZrO2 nanopowders in order to study their magnetic properties. We analyzed magnetic behavior as a function of the amount of Co and the oxygenation, which was controlled by low pressure thermal treatments. As prepared pure and Co-doped samples are diamagnetic and paramagnetic respectively. Ferromagnetism can be induced by performing low pressure thermal treatments, which becomes stronger as the dwell time of the thermal treatment is increased. This behavior can be reversed, recovering the initial diamagnetic or paramagnetic behavior, by performing reoxidizing thermal treatments. Also, a cumulative increase can be observed in the saturation of the magnetization with the number of low pressure thermal treatments performed. We believe that this phenomenon indicates that cobalt segregation induced by the thermal treatments is the responsible for the magnetic properties of the ZrO2-Co system.

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.

Reaction intermediate/product-induced segregation in cobalt–copper as the catalyst for hydrogen generation from the hydrolysis of sodium borohydride

Bimetallic cobalt–copper catalysts for the hydrolysis of NaBH4are unstable over multiple cycles because of surface borate-induced cobalt segregation.

The influence of CO adsorption on the surface composition of cobalt/palladium alloys

Segregation induced by the adsorption of gas phase species can strongly influence the composition of bimetallic surfaces and can therefore play an important role in influencing heterogeneous catalytic reactions. The addition of palladium to cobalt catalysts has been shown to promote Fischer Tropsch catalysis. We investigate the adsorption of CO onto bimetallic CoPd surfaces on Pd{111} using a combination of reflection absorption infrared spectroscopy and medium energy ion scattering. The vibrational frequency of adsorbed CO provides crucial information on the adsorption sites adopted by CO and medium energy ion scattering probes the surface composition before and after CO exposure. We show that cobalt segregation is induced by CO adsorption and rationalise these observations in terms of the strength of adsorption of CO in various surface adsorption sites.

Synthesis and characterization of bulk cobalt-doped ZnO and their thin films

(ZnO)1−x(Co3O4)x≤0.05 thin-film series were deposited in vacuum Ar–O2 and N2 environment using the pulsed-laser deposition (PLD) technique. Bulk pellets were prepared by the solid-state reaction technique by mixing ZnO and Co3O4 powder and further utilized for deposition of thin films. XRD patterns of pellets exhibit homogeneous ZnCoO phase formation with no other impurity phases. Single-phase (002) oriented ZnCoO thin films were formed without any cobalt segregation. The cobalt core peaks confirmed the existence of +2 valence state of cobalt in thin-film samples, and the binding energy difference between the cores peaks showed the homogeneous doping of cobalt in ZnO matrix. The magnetic ordering was achieved in thin films samples which were deposited in vacuum and N2 ambience.

Interplay of dopant, defects and electronic structure in driving ferromagnetism in Co-doped oxides:TiO2,CeO2 and ZnO

A comprehensive study of the defects and impurity (Co)-drivenferromagnetism is undertaken in the oxide semiconductors:TiO2, ZnO andCeO2. The effect ofmagnetic (Co2+) andnon-magnetic (Cu2+) impurities in conjunction with defects, such as oxygen vacancies (Vo), have been thoroughly investigated. Analyses of the x-ray diffraction (XRD) and x-rayphotoelectron spectroscopy (XPS) data reveal the incorporation of cobalt in the lattice,with no signature of cobalt segregation. It is shown that oxygen vacancies are necessary forthe ferromagnetic coupling in the Co-doped oxides mentioned above. The possibleexchange mechanisms responsible for the ferromagnetism are discussed in light of theenergy levels of dopants in the host oxides. In addition, Co and Cu co-dopedTiO2 samples are studied in order to understand the role of point defects in establishing roomtemperature ferromagnetism. The parameters calculated from the bound magneticpolaron (BMP) and Jorgensen’s optical electronegativity models offer a satisfactoryexplanation of the defect-driven ferromagnetism in the doped/co-doped samples.

Preparation of a multilayered composite catalyst for Fischer–Tropsch synthesis in a micro-chamber reactor

A 20 wt% Co/SiO2 catalyst grafted on a stainless steel substrate was used for Fischer–Tropsch synthesis (FTS) in a micro-chamber reactor. The fabrication of the catalyst was a complex process involving several steps. The first one, the pre-treatment of the AISI316L substrate through a plasma assisted chemical vapour deposition process (PACVD), allowed the formation of a silicon based polymer. This thin polymeric film was thermally degraded into a silicon oxide bonding layer. The coating of this bonding layer with sol–gel precursor of the Co/SiO2 catalysts was done with a spray coating process. The sequential combination of these steps led to a multilayered catalyst with a complex morphology. This material demonstrated high activity under FTS reaction. Activity and selectivity were higher under micro-chamber conditions than in a classical fixed-bed catalytic test. This was attributed to the increase of mass and heat transfers within the micro-device. Moreover distribution of metallic masses within the catalyst showed significant evolutions post FTS, including steel compound migrations and possible cobalt segregation.

Effect of Ru on LaCoO3 perovskite-derived catalyst properties tested in oxidative reforming of diesel

The oxidative reforming of diesel over Co/La2O3 and Ru–Co/La2O3 catalysts derived from LaCoO3 perovskite precursors was studied. Physicochemical characterization by XPS, TPR and XRD revealed that the incorporation of Ru to LaCoO3 produces changes in LaCoO3 evidenced by a smaller size of the LaCoO3 particles and cobalt segregation on the LaCoO3 surface. The modifications in the structure of LaCoO3 induced by the addition of Ru directly affect the dispersion and morphology of Co particles developed under the reaction. The active phases derived from pretreatment of perovskites evolve differently with time on stream, being observed that the presence of a greater proportion of perovskite phase in the Ru/LaCoO3 sample produces an increase in catalyst stability. TPD-MS analysis also indicates that bulk oxygen release from the Ru–Co/La2O3 sample could improve its catalytic behaviour. The characterization of used samples reveals that improvements in the cobalt surface concentration and Co–La2O3 interactions contribute to the better catalytic stability of the Ru–Co/La2O3-derived catalyst.

Structural and magnetic property of Co-doped–ZnO thin films prepared by pulsed laser deposition

Co-doped–ZnO thin films with the hexagonal zincite structure (wurtzite) have been synthesized on (0 0 6) Al 2O 3 sapphire substrate by pulsed laser deposition. Properties of the synthesized films are studied and the relation of the properties to its structural is reported. Single phase and highly textured Co-doped–ZnO thin films are successfully grown at relatively low temperature, 100 °C. Only single zincite (0 0 2) phase is formed and no cobalt segregation is detected. The structural, magnetic, electrical and optical properties of the deposited films were analyzed. Magnetic ordering was observed for all the films and the magnitude of the ordering is strongly dependent on the substrate temperature during deposition. Semiconductors properties of the carrier concentration and resistivity were also studied by Hall effect measurement. Both films deposited at 100 °C and 600 °C still preserve its semiconductor electron carrier density at the range of 10 21 cm −3. However, the resistivity increased with the increasing deposition temperature.

Investigation of the cobalt distribution in TiO2:Co thin films

We report a detailed study of the morphology of TiO2:Co films grown on both LaAlO3 (001) and SrTiO3 (001) substrates by pulsed laser deposition. The films are optically transparent and ferromagnetic at room temperature with a magnetic moment of 1.7±0.4 μB/Co. Plan view transmission electron microscope studies show clear evidence of cobalt segregation with the clustering more pronounced for higher cobalt concentrations. Films grown on SrTiO3 substrates show the presence of a thin surface rutile (111) layer, into which the cobalt appears to migrate. These results are supported by x-ray structure determination and resistivity measurements.

Effect of different collector dose rates on cobalt segregation

Nkana Concentrator is the most important mineral processing unit of Zambia Consolidated Copper Mines (ZCCM) (soon to be privatised) which contributes about 65% of cobalt concentrates treated at the Nkana and Chambeshi Cobalt plants to produce high purity cobalt metal. However, it has of late been experiencing problems with the segregation of the bulk concentrates which contain an appreciable amount of copper which overloads the performance of the cobalt tankhouses. In an effort to improve the copper/cobalt separation at segregation stage, a number of new collectors have been put on trial tests with a view of replacing the traditional Sodium Ethyl Xanthate (SEX) which has been in use since the sixties. The intention is to possibly isolate a collector that will give improved recoveries and grades of both copper and cobalt to their respective concentrates at different flotation stages. In this work, a series of laboratory testworks were conducted using Senkol 26, a Xanthic ester, Senkol 45, a blend of alkyl dithiophosphate and Xanthates and Senkol 700, a thionocarbomate type, which were supplied by SENMIN of South Africa on Nkana Concentrator feed materials. The feed materials are from four different sources namely; Mindola, Chibuluma, South Ore Body (S.O.B.) and Central shafts. The tests were carried out to compare the performance of the collectors based on their dose rates and later their performance compared to the collector currently being used at Nkana Concentrator. SEX at the dose rates of 20 and 30 g/t gave lower cobalt loses of 17 and 11.2 %TCo respectively, whilst grades were also very low at 0.25 and 0.19 %TCo respectively. The Senkol collectors exhibited poor separation of cobalt from copper at all dose rates except for Senkol 26 which gave loses of 8 %TCo at grade 0.14 %TCo at 50 g/t of collector which was much better than the SEX. This work has also shown that there is no incentive in increasing the dose rate of SEX beyond 30 g/t as there was no improved separation of cobalt from copper at an optimum depressant dosage and pH. On the other hand, the separation performance of Senkol 26 improved with increasing collector dose rate. Senkol 45 exhibited an increase in the of cobalt lost to the copper concentrate as the dose rate of the collector increased while no significant changes were noted with Senkol 700.

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.

Structure, Composition, and Morphology of Electrodeposited CoxFe1 − x Alloys

We have investigated the structure, composition, and morphology of alloys prepared on copper substrates under potentiostatic electrodeposition conditions currently used to produce composition modulated alloys. alloys with x = 1 to 0 have been obtained by simple control of the ratio in the bath solutions using the characteristic behavior of the codeposition of the metals. The alloy composition was determined by energy dispersive spectroscopy. X‐ray analysis of the structure of deposits indicates texture with coexistence of phases. For higher iron concentrations the alloys are mostly body‐centered‐cubic with (110) preferred orientation. For higher cobalt concentrations the alloys become mostly hexagonal with (0001) preferred orientation. Scanning electron microscopy was used to analyze the deposit morphology. Except for alloy concentrations of about x = 0.7, where a nonuniform character of deposits with partial cobalt segregation has been observed, the Co‐Fe alloys have a rather uniform composition and morphology in the cathodic potentials used for electrodeposition.

A resistance ratio method for alloy homogeneity control

A simple and rapid method has been developed for the homogeneity control of binary alloys in which the alloying element is in solid solution. This is based on the well-known resistance ratio method for the characterization of high-purity metals, but using resistances determined under liquid nitrogen rather than liquid helium. Resistances are measured by the four-point method using a sample holder for up to eight wire samples, connected to a high precision resistance meter. The sensitivity of the resistance ratio to the composition depends on the matrix and alloying elements, but can be high for dilute alloys, decreasing as the concentration increases. It is also influenced by lattice defects and impurities, which prevent absolute measurements of the composition. Two examples of use are given: cobalt segregation was detected in casting of V-1% Co alloy and was confirmed by neutron activation analysis, and homogeneity checks were carried out on a series of CuCr alloys containing 8 to 60 ppm Cr with a precision of 4 ppm Cr (standard deviation) or better.

Cobalt segregation in the LiNi 1−yCo yO 2 solid solution: A preliminary 7Li NMR study

LiNi 1−yCo yO 2 materials exhibit a layered structure built from alternate sheets of (Ni,Co)O 6 and of LiO 6 octahedra sharing edges. They are particularly interesting as positive electrode in rechargeable lithium batteries. Single-pulse 7Li NMR spectra allow to distinguish between the Li + ions with only cobalt as their first 3d neighbors and those with at least one Ni 3+ (LS t 2 6e 1). The small amount of excess Ni 2+ ions (t 2 6e 2) known to be present in the materials with small y causes an alteration of the spectra. Li + ions with only cobalt as their first cationic neighbors are detected even for small cobalt contents, which shows the existence of a cobalt segregation while X-rays show that no phase separation occurs.