Cobalt Layers Research Articles

The Influence of Diffusion Barriers on the Capacitance Properties of Composite Anodes with Si–CuSi–Cu Composition

Film structures composed of several identical modules [Co(x)/a-Si(100)/Cu(70)], where the x value varies from 2 to 10 nm, are investigated as an anode material for liquid lithium-ion batteries. It is shown that relatively thick cobalt layers are effective diffusion barriers for silicon and copper atoms, as well as for lithium ions. This leads to a decrease in the capacitance characteristics of the films under study. Thin cobalt layers (less than 3 nm) with a network structure, on the contrary, increase the capacitance characteristics of the anode films. This is caused by more uniform distribution of lithium ion drift channels that are formed at the initial stage of electrochemical cycling of electrodes over their volume.

Low frequency vibrations observed on assemblies of vertical multiwall carbon nanotubes by Brillouin light scattering: determination of the Young modulus

Assemblies of vertical multiwall carbon nanotubes, (VCNTs), have been synthesized by coupling dewetting of cobalt or nickel ultrathin layers and plasma enhanced chemical vapor deposition. Electronic microscopies revealed well defined micrometer length nanotubes with inner radius of 3–4 nm and outer radius of 8–9 nm. Similar structural qualities have been revealed by Raman measurements. Dynamic behaviour of these VCNTs assemblies have been studied by means of Brillouin light scattering technique. The measured inelastic light scattering from VCNTs is attributed to bending vibrations of the nanotubes. The observed frequencies on both assemblies, considered as dense effective media, are compatible with an effective Young modulus of 850 GPa.

Long-Range Charge Transport in Diazonium-Based Single-Molecule Junctions.

Thin layers of cobalt and ruthenium polypyridyl-oligomers with thicknesses between 2 and 8 nm were deposited on gold by electrochemical reduction of diazonium salts. A scanning tunneling microscope was used to create single-molecule junctions (SMJs). The charge transport properties of the Au-[Co(tpy)2]n-Au (n = 1-4) SMJs do not depend markedly on the oligomer length, have an extremely low attenuation factor (β ∼ 0.19 nm-1), and do not show a thickness-dependent transition between two mechanisms. Resonant charge transport is proposed as the main transport mechanism. The SMJ conductance decreases by 1 order of magnitude upon changing the metal from Co to Ru. In Au-[Ru(tpy)2]n-Au and Au-[Ru(bpy)3]n-Au SMJs, a charge transport transition from direct tunneling to hopping is evidenced by a break in the length-dependent β-plot. The three different mechanisms observed are a clear molecular signature on transport in SMJs. Most importantly, these results are in good agreement with those obtained on large-area molecular junctions.

Application of titanium dioxide barrier layers for the ferromagnetic/ferroelectric multiferroics formation

The layered multiferroics Co/PZT were obtained by ion-beam sputtering-deposition method, where PZT is a ferroelectric ceramic based on lead titanate zirconate of the composition PbZr0.45Ti0.55O3 with a thermostable plane-parallel ferroelectric/ferromagnet interface. Using cross-sectional scanning electron microscopy (SEM), we studied the interface of a cobalt layer up to several micrometers thick with a thick ceramic substrate of lead zirconate titanate. It has been shown that the use of a titanium dioxide barrier layer of TiO2 instead of PZT allows quality improvement of the interface by reducing the duration of ion-beam planarization of the ferroelectric substrate, and also to eliminate the formation of intermediate chemical compounds. Based on the data of X-ray phase analysis (XRD), it was concluded that the TiO2 layer is amorphous. Magnetoelectric measurements have shown that the use of titanium dioxide instead of PZT under appropriate planarization modes can increase the low-frequency magnetoelectric effect to 5 mV/(cm∙Ое), compared with structures with a sputtering planarizing layer of PZT, where the magnitude of the low-frequency magnetoelectric effect is 2 mV/(cm∙Оe). These results allow us to improve the characteristics of these structures when used as sensitive elements in devices for formation – processing of information and magnetic field sensors based on the magnetoelectric effect.

Воздействие некоторых сложных хемостимуляторов и модификаторов на термооксидирование InP

Воздействие некоторых сложных хемостимуляторов и модификаторов на термооксидирование InP

Boosting multiple photo-assisted and temperature controlled reactions with a single redox-switchable catalyst: Solvents as internal substrates and reducing agent

An alternative and economically viable process for the synthesis of β-aryl enals, enones and the aryl amines has been developed by partial oxidation of ethanol, isopropanol and N, N-dimethyl formamide (DMF). The formation of β-aryl enals, enones and the aryl amines was catalyzed by a mixed metal oxides layer of cobalt and chromium supported on halloysite nanotubes, designated as CoCr2O4-HNT. The CC and CN bond formation reactions were found to be influenced by temperature and the nature of base. The condensation of aldehyde with in situ generated acetaldehyde by ethanol oxidation forming β-aryl enals occurred selectively at 120 °C. The partial oxidation of isopropanol to acetone and its condensation with aldehydes forming β-aryl enones occurred at room temperature. Increase in temperature caused the liberation of hydrogen gas from isopropanol and allowed the reversible reduction of aldehydes to alcohols. Increase in temperature in isopropanol and increase in base concentration in ethanol causes the selective reduction of aldehydes to alcohols. Besides being active for the Claisen-Schmidt type of reactions and the aryl halides amination process, the synthesized catalyst was also found to be highly active for the photocatalytic oxidation of benzyl alcohols in absence of any external oxidizing agent. The positive holes (h+) generated at the Co(II) site as evident from EPR analysis was considered to be responsible for high photocatalytic activity of the material reducing the recombination rate of holes and electrons (e−). Density Functional Theory calculations were performed to understand the mechanism of ethanol oxidation to acetaldehyde.

Flexible Anode Catalysts for Direct Hydrazine Fuel Cells

In this study we report a facile approach for fabrication of flexible anode catalysts consisting of a fiber-shaped cobalt deposited on the polyimide (PI) surface for direct hydrazine fuel cells (DHFCs). At first, a thin copper layer has been deposited on the PI surface using electroless metal plating method. Then, the fiber-shaped cobalt coating has been plated by the electrochemical deposition method from the electrolyte containing CoSO4, NaOH and N-(2-Hydroxyethyl) ethylenediamine. For comparison, the cobalt layer that has the smooth structure has been deposited on the flexible PI surface via electroless deposition using morpholine borane as a reducing agent.The morphology and composition of the prepared flexible catalysts have been characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The activity of catalysts has been investigated with respect to the oxidation of hydrazine in an alkaline medium.The results of electrochemical measurements showed that the deposited cobalt layer that has a fiber-shaped structure on the flexible PI surface exhibits an enhanced electrocatalytic activity towards the oxidation of hydrazine as compared with that for cobalt catalyst that has a smooth structure. Moreover, the prepared flexible catalysts seem to be a promising anode material for DHFCs.

3D Composite Nickel-Cobalt Oxides on Sintered Metal Fiber Mesh for Electrocatalytic and Pseudocapacitive Applications

Enhancement of specific surface area is known to improve performance of electrocatalytic system by intensifying the oxygen evolution (OER) process. In this work, nickel-cobalt oxides were formed on Bekipor ST 20AL3 3D stainless steel mesh, achieving a large surface area and high electric conductivity. Layer-by-layer potentiostatic electrodeposition was carried out in one solution by applying different potential values for individual cobalt and nickel oxide layer formation to ensure uniform distribution on each mesh filament. Hydroxide coatings were then thermally treated at 473–1073 K to obtain spinel-type oxides. X-ray diffraction analysis indicated three oxides—NiO, Co3O4 and NiCo2O4—as the main products obtained after annealing at 473–773 K. Surface morphology investigated by scanning electron microscopy showed a uniform coverage of each filament with lamellar nickel-cobalt oxides. The electrochemically active surface area of the films was determined by employing cyclic voltammetry of a ferricyanide/ferrocyanide redox system, which revealed a significant impact of annealing on the surface activity. Electrocatalytic performance tests indicated that annealing at 673 K significantly enhanced nickel-cobalt activity in OER, with overpotential of 536 mV at 10 mA cm−2, while further treatment had a detrimental effect. The coating was able to maintain this activity during long-term stability tests with no signs of surface damage. Galvanostatic charge/discharge measurements revealed a complex charge storing mechanism of nickel-cobalt oxide coatings, combining properties of batteries and supercapacitors. The highest energetic parameters achieved in this work were demonstrated by a coating annealed at 473 K (specific capacitance 769 F g−1, specific energy 312 W h kg−1, and specific power 750 W kg−1), along with great stability, maintaining 98% of its initial specific capacitance after 300 cycles.

Intercalation Synthesis of Cobalt Silicides under Graphene Grown on Silicon Carbide

The process of formation of cobalt silicides near the graphene-silicon carbide interface by intercalation of single-layer graphene grown on the 4H- and 6H-SiC(0001) polytypes with cobalt and silicon is studied. The experiments were carried out in situ in ultrahigh vacuum. The analysis of the samples is performed by high-energy-resolution photoelectron spectroscopy using synchrotron radiation, low-energy electron diffraction, and also Raman spectroscopy, atomic-force and kelvin-probe microscopies. The thicknesses of the deposited cobalt and silicon layers is varied to 2 nm, and the sample temperature, from room temperature to 1000°C. Co and Si atoms deposited on heated samples is found to penetrate under graphene and are localized between the buffer layer and the substrate, which leads to a transformation of the buffer layer into additional graphene layer. It is shown that the result of intercalation of the system with cobalt and silicon is the formation under two-layer graphene of a Co–Si solid solution and silicide CoSi coated by the surface Co3Si phase. It is shown that the thickness and the composition of the formed silicide films can be changed by varying the amount of the intercalated material and the order of their depositions.

Magnetic Properties and Spin-Wave Excitations in Co/Mn Multilayers

Co/Mn multilayers were prepared using evaporation technique under ultra-high vacuum conditions and their structural and magnetic properties were studied. The temperature dependence of the spontaneous magnetization [Formula: see text] was recorded for different cobalt layer thickness ([Formula: see text]) ranging from 19[Formula: see text]Å to 70[Formula: see text]Å. Bloch’s law describes satisfactory the thermal change of magnetization for all samples. In addition, the change of the spin-wave (SW) parameter [Formula: see text] versus [Formula: see text] was discussed. The torque magnetometry was utilized to extract the effective anisotropy ([Formula: see text]) and the interlayer coupling constant ([Formula: see text]) was determined from the in-plan M–H loops. An updated model based on SW theory was applied to describe the [Formula: see text] variation and to calculate the approximate values of exchange interactions for samples with different Co-layer thicknesses.

Alternate Layers of Cobalt Doped Tungsten Oxide and Reduced Graphene Oxide Composite as Electrode Material for Supercapacitor

Composites consist of layers of cobalt doped tungsten oxide and reduced graphitic oxide was prepared using jet nebulized spray pyrolysis technique. A mixture of ammonium tungstate and cobalt acetate solution was coated on glass substrate at 380 °C using jet nebulized spray pyrolysis technique. Spray technique is used to form the reduced graphene oxide (rGO) layer over the first layer and again ammonium tungstate and cobalt acetate solution coated over the surface of rGO and then calcined in nitrogen atmosphere at 450 °C for 2 h. The composite were characterized using XRD, SEM, PL, CV and EIS studies. The SEM and XRD reveal the layered structure. Cyclic voltammetry reveals the high specific capacitance 500 Fg-1 at 5 mV s-1 scan rate for 2 M Na2SO4 as electrolyte. Layered structure of CoWO4/rGO/CoWO4 composite enhances the specific capacitance. EIS gives the electrical series resistance of 7.9 Ω. The layered CoWO4/rGO/CoWO4 composite can be suitable electrode materials for supercapacitor.

Magnetization reversal in Co/GGG/YIG/GGG(1 1 1) nanoheterostructures: Interlayer magnetic coupling and orange peel effect

Interlayer magnetic coupling has been studied in the epitaxial system consisting of few nanometer thick magnetic layers of cobalt and yttrium iron garnet (YIG) separated by non-magnetic spacers of gadolinium gallium garnet (GGG). The samples were grown by laser molecular beam epitaxy on GGG(1 1 1) substrates. The layer morphology and crystal structure were characterized by atomic force microscopy, X-ray and electron diffraction techniques. MOKE and XMCD methods were applied for element selective study of the magnetization reversal in Co and YIG layers. For the Co/YIG/GGG heterostructures it has been found that magnetization loops of YIG and Co exhibit the same values of coercive field and are of the same shape indicating strong magnetic coupling between Co and YIG layers. In opposite, when YIG and Co are separated by a 1 nm thick GGG spacer layer, the coercive field of Co becomes very different from that of YIG. Moreover the center of YIG loop gets shifted in field whereas the sign and value of this shift depend on the current magnetic state of the Co layer. The magnetic interaction between Co and YIG can be interpreted in the terms of 2–3 Oe magnetic field induced in the YIG layer by the adjacent Co layer. This magnetic field is likely caused by the “orange peel” effect arising due to granular structure of Co layer.

Studies on solid particle erosion behaviour of D-Gun sprayed WC-Co, Stellite 6 and Stellite 21 coatings on SAE213-T12 boiler steel at 400 °C temperature

Erosion wear of boiler tubes occurs from room temperature to high temperature and is accelerated by oxidation and hot corrosion above 400 °C temperature. The demand for hard coatings compatible in improving erosion, oxidation and hot corrosion resistance while maintaining the desired mechanical properties of the base material is increasing with the advancement of technology. In this work, WC-12Co coating, Stellite 6 coating and Stellite 21 coatings have been deposited on SAE213-T12 boiler steel using a Detonation gun (D-gun) spray method, and their erosion performances have been evaluated at 400 °C temperature using an air jet erosion tester. The erosion tests have been carried out using the alumina erodent particles at 30° and 90° impact angles. The surfaces of the as-sprayed coatings and the specimens eroded at 400 °C temperature were analysed using a scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscope (EDS). Stellite 21 coating has shown comparatively lower erosion rate than the Stellite 6 and WC-Co coatings at 400 °C temperature attributed to the formation of protective oxides of chromium and cobalt at 400 °C. The effects of microhardness and temperature on the erosion behaviours of the coatings at 400 °C temperature are discussed. The oxide layers of chromium and cobalt and accumulation of hard phases of nickel, molybdenum and carbon contributed in the protective behaviour of the deposited Stellite 6 and Stellite 21 coatings at 400 °C. The oxidation enhanced erosion wear is observed at 400 °C temperature for all the coatings.

Bulk defects induced coercivity modulation of Co thin film based on a Ta/Bi double buffer layer

This paper reports the effect of Ta/Bi double buffer layer on the magnetic properties of Co thin film. By inserting a Bi layer into the Ta/Co interface of Ta/Co/Ru, the coercive field is increased by about 6 times. Moreover, a further increase of coercivity is observed for the Ta/Bi/Co/Ru film after 450 ℃ annealing. X-ray photoelectron spectroscopy (XPS) reveals that the Bi atoms diffuse to film surface through the cobalt layer. Positron annihilation spectroscopy (PAS) indicates that the diffusion of Bi atoms promotes the formation of vacancies, which may increase the density of bulk defects in the film and enhance the pinning strength of the magnetic domains of Co. Besides, high-resolution transmission electron microscopy (HRTEM) results confirm the better crystallization of Co layer with a uniform orientation due to the Bi diffusion, which increases the energy barrier for nucleation and movement of the reversal domain. These two reasons lead to the increase of coercivity simultaneously. This finding provides a feasible avenue for the tunable coercive field by the defect engineering.

Влияние диффузионных барьеров на емкостные свойства композитных анодов состава Si-CuSi-Cu

As an anode material for liquid lithium-ion batteries, film structures consisting of several identical modules [Co(x)/a-Si(100)/Cu(70)], where the value of x varied from 2 to 10 nm, have been studied. It is shown that relatively thick cobalt layers are effective diffusion barriers for silicon and copper atoms, as well as lithium ions. This leads to a decrease in the capacitive properties of the studied films. Thin cobalt layers (less than 3 nm) with a network structure, on the contrary, contribute to an increase in the capacitive properties of anode films. This is due to a more uniform distribution in their volume of drift channels of lithium ions, which are formed at the initial stage of electrode cycling.

Интеркаляционный синтез силицидов кобальта под графеном, выращенным на карбиде кремния

Abstract The process of formation of cobalt silicides near the graphene-silicon carbide interface by intercalation of single-layer graphene grown on the 4 H - and 6 H -SiC(0001) polytypes with cobalt and silicon is studied. The experiments were carried out in situ in ultrahigh vacuum. The analysis of the samples is performed by high-energy-resolution photoelectron spectroscopy using synchrotron radiation, low-energy electron diffraction, and also Raman spectroscopy, atomic-force and kelvin-probe microscopies. The thicknesses of the deposited cobalt and silicon layers is varied to 2 nm, and the sample temperature, from room temperature to 1000°C. Co and Si atoms deposited on heated samples is found to penetrate under graphene and are localized between the buffer layer and the substrate, which leads to a transformation of the buffer layer into additional graphene layer. It is shown that the result of intercalation of the system with cobalt and silicon is the formation under two-layer graphene of a Co–Si solid solution and silicide CoSi coated by the surface Co_3Si phase. It is shown that the thickness and the composition of the formed silicide films can be changed by varying the amount of the intercalated material and the order of their depositions.

Электроимпульсное плазменное спекание нанопорошков WC – 10 Co с различным содержанием углерода, полученных методом плазмохимического синтеза

The features of high-speed sintering of WC – Co nanopowders with various contents of excess carbon (colloidal graphite) were studied. To obtain powders, a process was used that included plasma-chemical and low-temperature syntheses and a chemical-metallurgical method of applying ultrathin cobalt layers by precipitation from a solution of salts. The consolidation of powder materials was carried out by the method of high-speed Spark Plasma Sintering. It was found that an increase in the concentration of free carbon (colloidal graphite) has the greatest effect on the shrinkage and sintering rate at the stage of intense shrinkage of WC-Co nanopowders. It is shown that an increase in the carbon content in the composition of nanopowders leads to a decrease in the value of sintering activation energy at the stage of intense shrinkage.It has been established that the process of nanopowder compaction at the intense shrinkage stage is determined by the intensity of the plastic flow and the grain boundary diffusion of cobalt. It is shown that the mechanism of plastic deformation of the γ-phase based on cobalt corresponds to the Coble diffusion creep. It was found that an increase in carbon content leads to decreased in activation energy at the intense shrinkage and does not significantly affect at stage III of sintering where decrease in the shrinkage intensity is observed. It was shown that a decrease in the sintering activation energy is due to a decrease in the tungsten concentration in the γ-phase.

Magnetic and magneto-optical properties of assembly of nanodots obtained from solid-state dewetting of ultrathin cobalt layer

An assembly of randomly organized cobalt nanoparticles were obtained by solid-state dewetting of a 3 nm-thick cobalt layer. Vibrating sample magnetometry and Brillouin light scattering techniques were used to investigate both their static and dynamic behaviors with respect to the initial native cobalt layer. The measurements obtained from the assembly of the obtained nanodots were analyzed by means of shape anisotropy contribution. The Brillouin spectra revealed an unusual reversed Stokes/anti-Stokes line height asymmetry comparing to that observed on the native layer. The effective optical properties of the nanodots, combined with the relation between mean field and inside-dot field allowed explaining the observed reversed height asymmetry. The assembly of nanodots behave as an effective magnetic and optical medium where these properties can be tuned by the elaboration process.

Cobalt Intercalation of Graphene on Silicon Carbide

In this paper, we studied cobalt intercalation of single-layer graphene grown on the 4H-SiC(0001) polytype. The experiments were carried out in situ under ultrahigh vacuum conditions by high energy resolution photoelectron spectroscopy using synchrotron radiation and low energy electron diffraction. The nominal thicknesses of the deposited cobalt layers varied in the range of 0.2–5 nm, while the sample temperature was varied from room temperature to 800°C. Unlike Fe films, the annealing of Co films deposited on graphene at room temperature is shown to not intercalate graphene by cobalt. The formation of the graphene–cobalt–SiC intercalation system was detected upon deposition of Co atoms on samples heated to temperatures of above ~400°C. Cobalt films with a thickness up to 2 nm under graphene are formed using this method, and they are shown to be magnetized along the surface at thicknesses of greater than 1.3 nm. Graphene intercalation by cobalt was found to be accompanied by the chemical interaction of Co atoms with silicon carbide leading to the synthesis of cobalt silicides. At temperatures of above 500°C, the growth of cobalt films under graphene is limited by the diffusion of Co atoms into the bulk of silicon carbide.

Iron doping influence on interlayer magnetic coupling and magnetoresistance in Co/MoFe/Co films

Abstract The interlayer exchange coupling and magnetotransport properties have been investigated in trilayer structures where two coupled symmetric Co layers were separated by a spacer. The trilayers have the following general structure: Co(dCo)/X(dspacer)/Co(dCo), where X stands for used spacer (pure Mo or codeposition Mo and Fe with chemical composition Mo0.8Fe0.2), dCo and dspacer are the thicknesses of the cobalt and spacer layers. Two samples sets were fabricated by molecular beam epitaxy: (i) wedge-like: the both Co wedges are similar (0–3 nm) and directed along one the side of the substrate while the spacer wedge (also 0–3 nm) was grown along perpendicular direction; (ii) uniform in thickness Co(3 nm)/X(0.7 nm)/Co(3 nm), exhibiting in-plane magnetization configuration and the strongest antiferromagnetic coupling. Magnetization processes were investigated using polar and longitudinal magneto-optical Kerr effect (MOKE) for studies of out-of-plane and in-plane magnetization configurations, respectively. With increase of dMo, the interlayer exchange coupling alternates between ferromagnetic and antiferromagnetic exhibiting as high as 0.3 T coupling field HIEC of antiparallel alignment for dMo = 0.7 nm. Iron doping results in about 3.5 times decrease of the HIEC, measured at first antiferromagnetic coupling range for both perpendicular and in-plane magnetization configurations. The oscillating dependence of HIEC on dspacer is analyzed in the frame of a Ruderman–Kittel–Kasuya–Yosida model. The giant magnetoresistance (GMR) effect has been investigated in the current in plane geometry. Relation between MOKE and GMR results is discussed.