59Co NMR study in Co–Fe alloys/Co magnetite composites

Iron–cobalt compounds containing 20–30 at.% Fe with unusual crystallographic structure have been observed in metal-oxide composite synthesized in an aqueous media at 120–140°C. The oxide is a cobalt-containing magnetite. The metallic component is found in two crystallographic structures depending on the preparation conditions: a b.c.c. (α-Fe) structure and an α-Mn, also called Re₂₄Ti₅, structure (space group I-43m). The α-Mn-like CoFe alloy is encountered for the first time and transforms into the b.c.c. structure during annealing at temperatures as low as 160°C. In the b.c.c. structure itself, the cobalt and iron atoms present a chemical short-range order totally different from the regular B2 phase of the ordered bulk CoFe compound. This particular order is similar to the one encountered in high vacuum co-deposited thin films. For annealing temperatures larger than 400°C, the metallic component loses some Fe and transforms into nearly pure f.c.c. cobalt. This study shows that chemical as well as physical low temperature preparation techniques favor original CoFe ordered phases not achievable by conventional metallurgy.     

Thermal analysis of W-free Co–(Ni)–Al–Mo–Nb superalloys

CuAl layered double hydroxide (CuAl-LDH) was synthesized by co-precipitation. Sodium phenyl phosphate (SPP) and sodium dodecyl sulfate (SDS) are used to modify CuAl-LDH for preparing CuAl-(SPP)LDH and CuAl-(SDS)LDH, which were incorporated into epoxy resin (EP) to obtain EP/CuAl-(SPP)LDH and EP/CuAl-(SPP)LDH nanocomposites. The results indicate that SPP and SDS are intercalated into the interlayers of CuAl-LDH, and CuAl-(SPP)LDH has larger layer spacing than CuAl-(SDS)LDH. The thermal stability and flame-retardant performances of EP/CuAl-(SPP)LDH nanocomposites were better than those of EP/CuAl-(SDS)LDH composites. Compared with those of EP/4CuAl-(SDS)LDH nanocomposites, the peak heat release rate (PHRR) of EP/4CuAl-(SPP)LDH nanocomposites is reduced 25.8% and 55.6%, and peak smoke production rate (PSPR) value of EP/4CuAl-(SPP)LDH nanocomposites is reduced 27.6% and 46.2%, value of EP/4CuAl-(SPP)LDH nanocomposites is reduced 27.6% and 46.2%, respectively. The improved flame retardancy and smoke suppression performances of EP/CuAl-(SPP)LDH nanocomposites were attributed to the combination of copper compounds and SPP, promoting the formation of swollen, continuous and compact char layers on the surface of EP nanocomposites during combustion, eventually restraining the decomposition of EP nanocomposites.

     

Spontaneous near-substrate composition modulation in electrodeposited Fe–Co–Ni alloys

Conventional and reverse depth profile analysis of electrodeposited Fe–Co–Ni alloys was performed by secondary neutral mass spectrometry (SNMS). It was found that the reverse sputtering method gave a much better depth resolution at the vicinity of the substrate. The reverse SNMS spectra showed that the deposition of Fe–Co–Ni alloys starts with the formation of an Fe-rich zone followed by an increase in Co concentration, then the nickel content increases and a steady-state alloy composition is achieved. At high current density, the initial depth pattern reproduces itself twice before the composition becomes stable. It was concluded that the varying depth profile is a consequence of the anomalous nature of the codeposition of the alloy components, the depletion of the electrolyte with respect to the metal salts, and the dependence of the intensity of the hydrogen evolution on the deposit surface composition.     

Phase diagram and crystal chemistry of the La–Ca–Co–O system

The phase diagram of the La–Ca–Co–O system at 885 °C in air has been determined. The system consists of two materials that have interesting thermoelectric properties, namely, the misfit layered thermoelectric oxide solid solution, (Ca,La)₃Co₄O₉, and Ca₃Co₂O₆ which consists of 1D chains of alternating CoO₆ trigonal prism and CoO₆ octahedra. The reported La₂CaO₄ and the Ca-doped (La,Ca)₂CoO_4−z phases were not found at 885 °C. As a result of the absence of these phases, the phase diagram is significantly different from that reported at 1100 °C. Small solid solution regions of (La_1−xCa_x)₂O_3−z (0 ≤ x ≤ 0.08), (Ca_1−xLa_x)₃Co₄O₉ (0 ≤ x ≤ 0.07), and (La_1−xCa_x)CoO_3−z (0 ≤ x ≤ 0.2) were established.     

Kinetics of thiophene hydrodesulfurization over a supported Mo–Co–Ni catalyst

In this study, the kinetics of thiophene (TH) hydrodesulfurization (HDS) over the Mo–Co–Ni-supported catalyst was investigated. Trimetallic catalyst was synthesized by pore volume impregnation and the metal loadings were 11.5 wt % Mo, 2 wt % Co, and 2 wt % Ni. A large surface area of 243 m²/g and a relatively large pore volume of 0.34 cm³/g for the fresh Mo–Co–Ni-supported catalyst indicate a good accessibility to the catalytic centers for the HDS reaction. The acid strength distribution of the fresh and spent catalysts, as well as for the support, was determined by thermal desorption of diethylamine (DEA) with increase in temperature from 20 to 600 °C. The weak acid centers are obtained within a temperature range between 160 and 300 °C, followed by medium acid sites up to 440 °C. The strong acid centers are revealed above 440 °C. We found a higher content of weak acid centers for fresh and spent catalysts as well as alumina as compared to medium and strong acid sites. The catalyst stability in terms of conversion as a function of time on stream in a fixed bed flow reactor was examined and almost no loss in the catalyst activity was observed. Consequently, this fact demonstrated superior activity of the Mo–Co–Ni-based catalyst for TH HDS. The activity tests by varying the temperature from 200 to 275 °C and pressure from 30 to 60 bar with various space velocities of 1–4 h^?1 were investigated. A Langmuir–Hinshelwood model was used to analyze the kinetic data and to derive activation energy and adsorption parameters for TH HDS. The effect of temperature, pressure, and liquid hourly space velocity on the TH HDS activity was studied.     

Recent progress in nanocrystalline Sm–Co based magnets

Sm–Co alloys are the most promising candidates for high temperature applications in advanced power systems owing to their high Curie temperature and high thermal stability of the magnetic performance. The recently developed nanocrystalline Sm–Co based magnets exhibit great potentials for magnetic performance enhancement and are expected to enlarge applications to services under extreme conditions. However, there have been few comprehensive reviews on the development of the nanocrystalline Sm–Co magnets so far. The efforts in this article are paid to review the recent progress in both experimental and modeling studies on the nanocrystalline Sm–Co magnets. Particularly, the latest advances in nanostructuring technologies, doping modulation, data-driven composition design and strategies for enhancement of magnetic properties have been introduced and evaluated. Finally, new challenges and opportunities regarding the future development of high-performance nanocrystalline Sm–Co based magnets are proposed.     

Co–Cu–La catalysts for selective CO2 hydrogenation to higher hydrocarbons

Copper and lanthanum promoted cobalt catalysts for CO₂ hydrogenation to higher hydrocarbons are described. The catalysts were prepared by the self-propagating high-temperature synthesis followed by alkaline leaching. They are active in CO₂ hydrogenation at 200 °C under 10 bar pressure (CO₂ : H₂ = 1 : 3) with selectivity to C₂₊ alkanes up to 39%; no alkenes and alcohols are formed under these experimental conditions.     

Novel Co/graphene oxide and Co/nanoporous graphene catalysts for Fischer–Tropsch reaction

For the first time, nanoporous graphene and graphene oxide sheets have been synthesized and used as supports for preparation of Co/graphene-based catalysts to evaluate their efficiency in Fischer–Tropsch synthesis and for comparison with the performance of Co/Al₂O₃ to study the effects of the carbon supports on the reaction. Outstanding results were obtained compared with the alumina counterpart. Application of nanoporous graphene yielded heavier hydrocarbons compared with the Co/Al₂O₃ catalyst, possibly due to the high surface area and intrinsic properties of the carbon nanostructures as effective hydrogen carriers. Use of graphene oxide and nanoporous graphene supports also resulted in high CO₂ selectivity. However, the graphene-supported catalysts displayed lower C₁–C₄ hydrocarbon selectivity compared with the Al₂O₃ catalyst.     

Electrochemical deposition and corrosion stability of Zn–Co alloys

Electrochemically deposited Zn–Co alloys under various deposition conditions were investigated using anodic linear sweep voltammetry for phase structure determination, scanning electron microscopy for surface morphology analysis, atomic absorption spectroscopy for determination of chemical composition, and polarization measurements and open circuit potential measurements for determination of corrosion properties. The influence of deposition current density, temperature, and composition of deposition solution on the phase structure and corrosion properties of Zn–Co alloys was studied. It was shown that the ratio of cobalt to zinc ions in the plating bath strongly affects the chemical content and phase structure, as well as corrosion stability, of Zn–Co alloys. Zn–Co alloys deposited from plating baths with the lowest and the highest ratios of cobalt and zinc ions exhibited the lowest corrosion rate.     

Thermodynamics of oxygen solutions in titanium-containing Fe–Co melts

Performed for the first time, the thermodynamic analysis of oxygen solutions in titanium-containing Fe–Co melts showed that the deoxidizing power of titanium with increasing cobalt content of the melt first decreases, reaches a minimum at a cobalt content of 20%, and then increases. The titanium contents [%Ti]* at equilibrium points between the oxide phases TiO₂, Ti₃O₅, and Ti₂O₃ were determined. The curves of the oxygen solubility in titanium-containing iron–cobalt melts pass through a minimum, which shifts toward lower titanium contents with increasing cobalt content of the melt. Further alloying with titanium leads to an increase in the oxygen concentration of the melt so that the higher cobalt content of the melt, the steeper the increase in the oxygen content after the minimum as titanium is added to the melt.     

X-ray Fluorescence Characterization of Co/Ni–Cr Structures

A procedure was developed for the determination of the elemental composition and thickness of double-layer Co/Ni–Cr films on Polikor by X-ray fluorescence. Correction factors taking into account mutual interferences of elements in the system under study were calculated. The density of the film-forming materials was determined experimentally. The performance characteristics of the procedure were estimated.     

Catalytic oxidation of NO over Mn–Co–Ce–Ox catalysts: effect of reaction conditions

Manganese–cobalt–cerium oxide (Mn–Co–Ce–Ox) catalysts were synthesized by the co-precipitation method and tested for activity in low-temperature catalytic oxidation of NO in the presence of excess O₂. With the best Mn–Co–Ce mixed-oxide catalyst, approximately 80 % NO conversion was achieved at 150 °C and a space velocity of 35,000 h^?1. The effect of reaction conditions (reaction temperature, volume fractions of NO and O₂, gas hourly space velocity (GHSV), and catalyst stability) was investigated. The optimum reaction temperature was 150 °C. Increasing the O₂ content above 3 % results in almost no improvement of NO oxidation. This catalyst enables highly effective removal of NO within a wide range of GHSV. Furthermore, the stability of the Me–Co–Ce–Ox catalyst was excellent; no noticeable decrease of NO conversion was observed in 40 h.     

Surface Reorganization on Electrochemically-Induced Zn–Ni–Co Spinel Oxides for Enhanced Oxygen Electrocatalysis

Herein, we highlight redox-inert Zn²⁺ in spinel-type oxide (Zn_XNi_1−XCo₂O₄) to synergistically optimize physical pore structure and increase the formation of active species on the catalyst surface. The presence of Zn²⁺ segregation has been identified experimentally and theoretically under oxygen-evolving condition, the newly formed V_Zn−O−Co allows more suitable binding interaction between the active center Co and the oxygenated species, resulting in superior ORR performance. Moreover, a liquid flow Zn–air battery is constituted employing the structurally optimized Zn_0.4Ni_0.6Co₂O₄ nanoparticles supported on N-doped carbon nanotube (ZNCO/NCNTs) as an efficient air cathode, which presents remarkable power density (109.1 mW cm⁻²), high open circuit potential (1.48 V vs. Zn), excellent durability, and high-rate performance. This finding could elucidate the experimentally observed enhancement in the ORR activity of Zn_XNi_1−XCo₂O₄ oxides after the OER test.     

Effect of synthesis method on oxygen adsorption/desorption properties of La–Sr–Co–Fe–O perovskite-type oxide

There has been a growing interest in utilizing La–Sr–Co–Fe–O perovskite-type oxide for efficient high temperature oxygen adsorption applications and oxygen removal process. In this paper, we focus our attention on the analysis of the determinants of the synthesis methods of La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) powders for the oxygen adsorption/desorption applications. To this aim, LSCF powders were successfully synthesized by different synthesis routes using polymerized complex and citrate methods. The effects of synthesis methods on the structure, particle size, specific surface area, oxygen adsorption/desorption kinetics, and oxygen uptake capacities of LSCF perovskite-type oxides were investigated. The oxygen adsorption/desorption capacities and kinetics of the LSCF oxides increase with increasing (1) the temperature from 700 to 900 °C and (2) the surface area observed at a given temperature. Collectively, the experimental observations suggest that particle sizes may play an important role in oxygen uptake capacities and adsorption/desorption kinetics.     

Thermal parameters determination of Co–Al–W as-cast alloy homogenization by DTA analysis

The γ–γ′ Co-based superalloys are newly developed class of refractory alloys which may replace commercial Ni-based superalloys owing to their favorable properties at high temperature. In case of new Co-based superalloys, the heat treatment aims to obtain microstructure composed of appropriate volume fraction of small cuboidal γ′-Co₃(Al,W) precipitates within the γ-Co matrix. However, due to a high tendency to interdendritic segregations of alloying elements, the alloys based on Co–Al–W system should be normally homogenized before further steps of heat treatment (solutionizing and aging). In this study, thermal analysis was applied for determination of temperature range for primary heat treatment of the Co–9Al–9W (at.%). The differential thermal analysis (DTA) measurements were carried out on the thermal analyzer NETZSCH STA 449 F3 Jupiter. On the base of obtained results, respectively, solvus of γ′ phase and solidus temperatures were determined, as well as the thermal range of Co₃W (DO₁₉) phase precipitation. As a consequence, the heat treatment without homogenizing (only solution and aging) was proposed as a most suitable way to obtain beneficial microstructure.     

Phase Equilibria and Crystal Structure of Complex Oxides in the La–Sr–Co–Ni– System

Phase equilibria in the La–Sr–Co–Ni–O system were studied in air at 1100°. The samples for the study were synthesized by the standard ceramic and citrate processes. The limiting solubility and structure of La_1-xSr_xCo_1-yNi_yO_3- and (La_1-xSr_x)₂Co_1-yNi_yO₄ solid solutions were determined by Xray powder diffraction analysis. La_1-xSr_xCo_1-yNi_yO₃- solid solutions with 0 x 0.5 have a distorted rhombohedral perovskitelike structure (R c space group). An increase in the strontium concentration reduces the rhombohedral distortions, and the compounds with x < 0.5 have an ideal cubic structure (Pm3m space group). (La_1-xSr_x)₂Co_1-yNi_yO₄ crystals have a tetragonal K₂NiF₄ type unit cell (I4/mmm space group). The relationships between unit cell parameters and compositions were obtained for singlephase La_1-xSr_xCo_1-yNi_yO_3- and (La_1-xSr_x)₂Co_1-yNi_yO₄ samples. The existence regions of La_1-xSr_xCo_1-yNi_yO₃- and La_1-xSr_x)₂Co_1-yNi_yO₄ solid solutions were distinguished on P–T phase diagrams.     

Giant magnetoresistance in Co–Ag granular films prepared by electrodeposition

Preparation of granular magnetic Co–Ag films produced by pulsed electrodeposition from a chloride bath containing both cobalt ions and a low concentration of silver has been investigated. Deposition of cobalt on Ag is performed by a double pulse method. Combining in situ electrochemical and microgravimetric measurements, the kinetics of silver and cobalt reduction are presented. The thickness and deposition rates are monitored using an electrochemical quartz crystal microbalance (EQCM) during the growth of each material. Magnetic measurements have shown a superparamagnetic behavior in agreement with the existence of very small cobalt particles. Giant magnetoresistance (GMR) of ∼2% at room temperature is observed for the Co₅Ag₉₅ sample.     

Heterogeneous Fenton degradation of azo dye 4BS over Co–Mn–Fe ternary hydrotalcites

This work described the application of Co–Mn–Fe hydrotalcites (Co–Mn–Fe LDHs) as heterogeneous catalysts for Fenton reaction process. The Co–Mn–Fe LDHs were synthesized by the co-precipitation method and were characterized by X-ray diffractometry (XRD), infrared spectroscopy (IR), zeta potential, and BET surface area measurement. The catalytic activity of different kinds of hydrotalcites was evaluated by the degradation of Direct Scarlet 4BS (4BS). The Co–Mn–Fe LDHs showed the best catalytic performance among four catalysts. The presence of the Mn²⁺/Mn³⁺ at surface of catalyst could accelerate the reduction of Co³⁺–Co²⁺, and then increased in the catalytic activity of the Co–Mn–Fe LDHs. The effect of initial pH, catalyst dosage, dye concentration, and reaction temperature on the degradation of 4BS had been investigated. Radical reaction mechanism was proposed by the addition of radical scavenger. The degradation kinetic followed pseudo-first-order model. The activation energy of Co–Mn–Fe LDHs was determined to be 37.3 kJ/mol. The catalytic activity of Co–Mn–Fe LDHs was maintained after four cycles of reaction, which proved the reusability of catalyst. Finally, the possible reaction mechanisms involved in the heterogeneous Fenton system were proposed.     

Bimetallic Co–Cu polyaniline composites: Structure and electrocatalytic activity

Bimetallic Co–Cu polyaniline composites were produced by oxidative polymerization of aniline, with ammonium peroxydisulfate and hydrogen peroxide as oxidizing agents. Co(II) and Cu(II) chlorides were introduced into the polymer by the in situ method. It was found that the phase constitution of the composites is affected by their synthesis conditions and content of both metals in them. The electrocatalytic activity of the composites in the electrohydrogenation of p-nitroaniline in an aqueous-alcoholic-alkaline medium of the catholyte was studied and found to exceed that of composites synthesized with the use of H₂O₂ and evaporation of the solvent.