Showing papers on "Cobalt published in 2000"

Spin-Dependent Tunneling in Self-Assembled Cobalt-Nanocrystal Superlattices

Abstract: Self-assembled devices composed of periodic arrays of 10-nanometer-diameter cobalt nanocrystals display spin-dependent electron transport. Current-voltage characteristics are well described by single-electron tunneling in a uniform array. At temperatures below 20 kelvin, device magnetoresistance ratios are on the order of 10%, approaching the maximum predicted for ensembles of cobalt islands with randomly oriented preferred magnetic axes. Low-energy spin-flip scattering suppresses magnetoresistance with increasing temperature and bias-voltage.

Inter-ligand reactions: in situ formation of new polydentate ligands

Abstract: Two ligands have been synthesized by derivatisation of cyanuric chloride: 6-(diethylamino)-2,4-disulfanyl-1,3,5-triazine (H2SSta) 1 and 6-(diethylamino)-2-hydroxo-4-sulfanyl-1,3,5-triazine (H2OSta) 2 have been characterised by X-ray crystallography, which shows intermolecular hydrogen bonding in the solid state, leading to dimers of 1 and ribbons of 2. On reaction with metal salts both ligands undergo oligomerisation reactions. Compound 1 reacts with nickel chloride to form a mononuclear complex, [Ni{(Sta)S(S2ta)}] 3. In 3 two triazine ligands have reacted, to form a tetradentate ligand in which two triazine rings are bridged by a sulfur group, with a co-ordinated disulfide group present on one ring and a co-ordinated thiolate on the second. Compound 2 reacts with cobalt(II) chloride to form a cage complex, [Co6NaO(OStaH)7{S(Ota)2}2(O2CPh)2(H2O)2] 4. This complicated structure contains two polydentate ligands formed by linking triazine groups through a bridging sulfur. The cage contains four cobalt(II) and two cobalt(III) sites which are assigned by bond length considerations. The compound [Co(OSta)3] 5 co-crystallises with 4, and its structure has also been determined.

Layered Hydroxide Metal Acetates (Metal = Zinc, Cobalt, and Nickel): Elaboration via Hydrolysis in Polyol Medium and Comparative Study

Abstract: Layered hydroxide metal acetates (metal = zinc, cobalt, and nickel) have been prepared by a new route belonging to the chimie douce method. This novel method involves the hydrolysis, in polyol medium, of in situ-formed complexes supposed to be alkoxyacetates. These layered hydroxide metal acetates present poorly ordered character, and their X-ray patterns have features typical of lamellar compounds with turbostratic disorder. Their chemical formula was established to be M(OH)2-x(CH3COO)x·nH2O with (x, n) = (0.42, 0.31), (0.38, 0.53), and (0.40, 0.63) for Zn, Co, and Ni, respectively. The layered hydroxide nickel acetate has a classical brucite structure with a random substitution of some hydroxyl groups by acetate groups. The layered hydroxide zinc and cobalt acetates are isomorphous with the hydrozincite structure, in which cations are located in both octahedral and tetrahedral sites. The acetate anion behaves as a unidentate ligand in LHS−Ni and LHS−Co, where LHS indicates layered hydroxide salt, and is...

Catalytic graphitization of carbon aerogels by transition metals

Abstract: Carbon aerogels and Cr-, Fe-, Co-, and Ni-containing carbon aerogels were obtained by pyrolysis, at temperatures between 500 and 1800 °C, of the corresponding aerogels prepared by the sol−gel metho...

Chemical Control of Superparamagnetic Properties of Magnesium and Cobalt Spinel Ferrite Nanoparticles through Atomic Level Magnetic Couplings

Abstract: A correlation between the electron spin−orbital angular momentum coupling and the superparamagnetic properties has been established in MgFe2O4 and CoFe2O4 spinel ferrite nanoparticles The contribution to the magnetic anisotropy from the Fe3+ lattice sites is almost the same in both nanocrystallites as neutron diffraction studies have shown a similar cation distribution in these two types of spinel ferrite nanoparticles Due to the strong magnetic couplings from Co2+ lattice sites, the blocking temperature of CoFe2O4 nanoparticles is at least 150 deg higher than the same sized MgFe2O4 nanoparticles Mossbauer spectroscopy studies demonstrate that the magnetic anisotropy of CoFe2O4 nanoparticles is higher than that of the same size MgFe2O4 nanoparticles These studies indicate that the superparamagnetic properties of nanoparticles can be controlled through chemically adjusting the magnetic anisotropy energy

Polymeric transition metal complexes and uses thereof

Abstract: Novel polymeric transition metal complexes of iron, cobalt, ruthenium, osmium, and vanadium are described. The polymeric transition metal complexes can be electron transfer mediators in enzyme-based electrochemical sensors. In such instances, transition metal complexes accept electrons from, or transfer electrons to, enzymes at a high rate and also exchange electrons rapidly with the sensor.

Oligomerisation of ethylene by bis(imino)pyridyliron and -cobalt complexes

Abstract: A series of bis(imino)pyridyliron and -cobalt complexes [[2,6-(CR=NAr)2C5H3N]MX2] (R=H, Me; M=Fe, Co; X=Cl, Br) 8-16 containing imino-aryl rings (Ar) with at least one small ortho substituent, as well as Ar=biphenyl and Ar=naphthyl, has been synthesised. Crystallographic analyses of complexes 9 (Ar = 2,3-dimethylphenyl), 13 and 14 (Ar= biphenyl; X= Cl or Br, respectively) reveal a distorted trigonal-bipyramidal geometry in the solid state. These complexes, in combination with methyl aluminoxane (MAO), are active catalysts for the oligomerisation of ethylene, yielding >99% linear alpha-olefin mixtures that follow a Schulz-Flory distribution. Iron ketimine (R = Me) precatalysts give the highest activities and a greater alpha-value than their aldimine (R = H) analogues. Cobalt precatalysts follow a similar trend, though their activities are almost two orders of magnitude lower than those of the corresponding iron catalysts. Ethylene pressure studies on cobalt precatalyst 15 reveal a first-order dependence on ethylene for both the rate of propagation and the rate of chain transfer, and a pressure independence of the alpha value.

Preparation and characterization of ZnO clusters inside mesoporous silica

Abstract: Wide band-gap semiconductor-zinc oxide nanoclusters have been prepared in the channels of MCM-41 materials by functionalizing the MCM-41 with ethylenediamine groups, absorbing zinc cations, and calcinating at high temperatures. The products have been characterized by XRD, TEM, EDS, nitrogen adsorption and desorption, and UV-vis and PL spectroscopies. ZnO clusters were mostly confined and dispersed in the pores of mesoporous hosts. No large ZnO particles on the external surfaces have been detected. A massive blue-shift in UV-vis absorption spectra has been observed and large band increase can be expected. The nature of the PL spectrum has been attributed to the defects related to oxygen vacancies. In addition, the assembly of cobalt, nickel, and copper oxides inside MCM-41 materials has also been tried by this scheme, but at the moment, only the cobalt oxide can be prepared with good results. Unfortunately, noble metals have usually grown into large particles on the outside surface of MCM-41 by this scheme, e.g., a lot of silver particles with sizes much larger than the pore diameter of MCM-41 host have been obtained. However, the explanation is not yet clear.

Nickel, cobalt, and their alloys

Abstract: This book is a comprehensive guide to the compositions, properties, processing, performance and applications of nickel, cobalt, and their alloys. It includes all of the essential information contained in the 20-volume ASM Handbook series. Includes new or updated coverage in the following areas: Expanded corrosion coverage including guidelines for selecting the best alloy for specific environments or applications Data sheets covering the compositions, specifications, applications and properties for dozens of the most commercially important heat, corrosion and wear-resistant nickel and cobalt alloys Recent advances in superalloy development, including coatings to extend high-temperature service life Unique characteristics of nickel and cobalt which allow them to be used in special-purpose applications, e.g., magnets, controlled-expansion devices, electronics, and implants for the human body Engineering applications for nickel and cobalt coatings produced by electroplating, electroforming, electroless coating, thermal spraying, and weld surfacing. Contents include: Nickel and Its Alloys: The Nickel Industry, occurrence, recovery and consumption Uses of nickel Wrought and Cast Corrosion-Resistant Alloys Cast Heat-Resistant Ni-Cr and Ni-Cr-Fe alloys Superalloys Special-Purpose Alloys Nickel Coatings Corrosion Behavior, including performance in specific environments, Stress-Corrosion Cracking and Hydrogen Embrittlement, and High-Temperature Corrosion Fabrication and Finishing, including forming, forging, powder metallurgy, heat treating, machining, welding and brazing, cleaning and finishing, and high-temperature coatings for superalloys Metallography, Microstructures, and Phase Diagrams Cobalt and Its Alloys: The Cobalt Industry, occurrence, recovery and consumption Uses of cobalt Cobalt-base alloys Wear behavior Corrosion Behavior Fabrication Characteristics Metallography, Microstructures and Phase Diagrams.

The reaction performances and characterization of Fischer–Tropsch synthesis Co/SiO2 catalysts prepared from mixed cobalt salts

Abstract: Catalysts prepared by mixed impregnation of cobalt(II) nitrate and cobalt(II) acetate displayed higher activity than the catalysts prepared from either mono-precursor at mild reaction conditions (1 MPa total pressure, H2/CO=2, T=513 K) of Fischer–Tropsch synthesis (FTS). X-ray diffraction (XRD) indicated that highly dispersed cobalt metal provided the main active sites on the catalyst prepared by mixed impregnation method. Through the mixed impregnation, different cobalt species were formed and their reduction performances were detected by the temperature-programmed reduction (TPR) and thermal gravimetric analysis. Transmission electronic microscopy (TEM) and FT-IR spectroscopy of adsorbed CO as probe molecule revealed that the presence of different sites associated with cobalt after the reduction of the catalysts with hydrogen at 673 K. It was assumed that the metal readily reduced from cobalt nitrate promoted the reduction of Co2+ to metallic state in cobalt acetate by H2 spillover mechanism during catalyst reduction process. The reduced cobalt from cobalt acetate was highly dispersed and remarkably enhanced the catalytic activity.

Blue CoAl2O4 particles prepared by the sol-gel and citrate-gel methods

Abstract: Bright blue CoAl2O4 particles were prepared by the sol−gel and citrate−gel methods using aluminum sec-butoxide, cobalt salts, and citric acid as oxides precursors. Both methods start from sols of the precursor alkoxides and salts, and involve formation of homogeneous solid intermediates, reducing atomic diffusion processes during thermal treatment. This important feature results in a substantial lowering of the time and temperature needed for the formation of the desired compounds. The stages of the formation of CoAl2O4, as well as the characterization of the resulting compounds were done using XRD, FTIR, UV−VIS, SEM, and TGA/DTA techniques. The structure, coloration, particle size, and temperature of formation of the resulting CoAl2O4 phases were found to depend on the precursors and methods used for preparation and the calcination temperature. The lowest temperature for preparation of the blue cobalt aluminate of about 700 °C was obtained using the citrate−gel method. This temperature is much lower than...

Desulfurization and novel sorbents for same

Abstract: Particulate sorbent compositions which are suitable for the removal of sulfur from streams of cracked-gasoline or diesel fuel are provided which have increased porosity, improved resistance to deactivation through the addition of a calcium compound selected from the group consisting of calcium sulfate, calcium silicate, calcium phosphate or calcium aluminate to the support system comprised of zinc oxide, silica and alumina having thereon a promotor wherein the promotor is metal, metal oxide or metal oxide precursor with the metal being selected from the group consisting of cobalt, nickel, iron, manganese, copper, molybdenum, tungsten, silver, tin and vanadium or mixtures thereof and wherein the valence of such promotor has been substantially reduced to 2 or less. Process for the preparation of such sorbent systems as well as the use of same for the desulfurization of cracked-gasolines and diesel fuels are also provided.

Ab-Initio XRPD Crystal Structure and Giant Hysteretic Effect (Hc = 5.9 T) of a New Hybrid Terephthalate-Based Cobalt(II) Magnet†

Abstract: A new terephthalate-based cobalt hydroxide, Co2(OH)2(C8H4O4), was synthesized by the hydrothermal method. Its crystal structure has been determined by ab-initio XRPD methods (monoclinic, C2/m, a = 19.943(1), b = 3.2895(1), c = 6.2896(3) A, β = 95.746(3)°) and fully refined by the Rietveld technique down to Rp = 0.15 for 9301 observed data (178 independent reflections). The terephthalates are coordinated and pillared directly to the cobalt hydroxide layers and thus a three-dimensional framework is formed. Because of the bonds with the terephthalates, two crystallographically inequivalent cobalt sites are found inside the hydroxide layers, with different octahedral orientations. Magnetic studies show that the intralayer exchange interaction between Co(II) ions is ferromagnetic but the whole system orders antiferromagnetically at 48 K with a metamagnetic transition above a threshold field of 0.2 T. The existence of conjugated π electrons in terephthalates explains the antiferromagnetic interactions between t...

Metal−Support Interactions in Co/Al2O3 Catalysts: A Comparative Study on Reactivity of Support

Abstract: Co/Al2O3 catalysts have been prepared with conventional impregnation and sol−gel methods to vary the chemical reactivity of the alumina support. The material system has been investigated with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance Fourier transform spectroscopy (DRIFT), Brunauer−Emmett−Teller (BET) method, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) (with thermogravimetric analysis (TGA) and differential thermal analysis (DTA)), and gas chromotography (GC) for the catalytic oxidation of CO. It had been found that the reactivity of the support changes the surface structure and chemical composition of catalysts significantly. When the metal−support interaction is weak, Co3O4 is a predominant surface phase (which is interfaced by a “cobalt surface phase”). With an increase in support reactivity, CoO and CoAl2O4 are found to be present on the surface. The cobalt content on the surface decreases upon the increase in suppor...

Red and blue luminescent metallo-supramolecular coordination polymers assembled through π–π interactions

Abstract: The use of π-stacking interactions to control the aggregation of photo-active metal centres is explored through the design of bis(2,2′;6′,2′′-terpyridyl) metal complexes functionalised with biphenyl ‘tails’. Aryl–aryl interactions control the aggregation of the metal complexes into polymetallic arrays in the solid state. Cobalt(II), ruthenium(II), nickel(II), copper(II), zinc(II) and cadmium(II) bis-ligand complexes and a mixed ligand ruthenium(II) complex have been structurally characterised. The solid-state structures are dependent on which units dominate the π-stacking. For cobalt, ruthenium, nickel and copper, biphenylene–biphenylene interactions lead to linear rod-like arrays, while for the group 12 d10 ions zinc and cadmium, biphenylene–pyridyl interactions lead to two-dimensional sheets. The addition of the biphenylene tail has favourable effects on the photophysical-properties of the complexes which exhibit room temperature red (ruthenium) or blue (zinc and cadmium) luminescence, both in solution and the solid state.

Metal–support interaction in Co/SiO2 and Co/TiO2

Abstract: Cobalt supported on silica and titania catalysts were investigated by XPS, TPR, TPD, XRD and TEM in order to elucidate the Co/support interactions and their effect on the dispersion and reducibility of cobalt. The reducibility of cobalt was studied both with TPR experiments, in which the temperature is raised at a steady rate, and with XPS after in-situ reduction treatments at constant temperature. Silica-supported catalysts were prepared with various Co loadings (2–27.5 wt.%) and they exhibited significant structural and morphological differences. A significant Co/titania interaction was found, while no conclusive proof of any interaction was found for Co/silica. The degree of interaction between cobalt and the support affected not only the response of cobalt to reduction, but also its dispersion. In fact, cobalt spreads on titania during reduction and tends to sinter on silica.

Photoinduced Ferrimagnetic Systems in Prussian Blue Analogues CIxCo4[Fe(CN)6]y (CI = Alkali Cation). 2. X-ray Absorption Spectroscopy of the Metastable State

Abstract: A CoFe Prussian blue analogue Rb1.8Co4[Fe(CN)6]3.3·13H2O was synthesized, which presents an important photomagnetic effect. The electronic structure and the local structure of the ground and of the excited states have been investigated. X-ray absorption spectroscopy measurements at the Co and Fe L2,3 edges and cobalt K-edge (XANES and EXAFS) evidence the local electronic transfer and the spin change of the cobalt ions induced by irradiation. We observed a 0.19 A increase of the Co−N bond length, associated with the transformation of CoIII low spin to CoII high spin. The CoII/CoIII ratio has been evaluated as a function of the irradiation time and revealed as an important parameter to understanding the bulk magnetic properties. The combined role of the diamagnetic FeII−CoIII pairs and hexacyanoferrate(III) vacancies is locally evidenced. This work is a new step in the understanding of the photoinduced electron transfer.

Manufacturing method for semiconductor metalization barrier

Abstract: A semiconductor metalization barrier, and manufacturing method therefor, is provided which is a stack of a cobalt layer and cobalt tungsten layer deposited on a copper bonding pad.

TPR and XPS study of cobalt–copper mixed oxide catalysts: evidence of a strong Co–Cu interaction

Abstract: In this work the results of a TPR and XPS investigation of CoxOy–CuO mixed oxides in the range of composition Co : Cu=100:0–8:92 are reported and compared. The final catalysts were obtained by thermal decomposition in air and N2 at 723 K for 24 h of single‐phase cobalt–copper hydroxycarbonates prepared by coprecipitation at constant pH. The Co : Cu=100 : 0 specimen calcined in air formed the Co2+[Co3]2O4 (Co3O4) spinel phase. The copper‐containing catalysts (Co : Cu=85 : 15–8 : 92) showed mainly two phases: (i) spinels, like Co2+[Co3+]2O4, Co 1-x 2+ Cu x 2+ [Co3+]2O4 and (ii) pure CuO, the relative amount of each phase depending on the Co : Cu atomic ratio. The results of the XPS study are consistent with the bulk findings and revealed the presence of Co2+, Co3+ and Cu2+ species at the catalyst surface. Moreover, the surface quantitative analysis evidenced a cobalt enrichment, in particular for the most diluted cobalt samples. The TPR study showed that the catalyst reduction is affected by a strong mutual influence between cobalt and copper. The reducibility of the mixed oxide catalysts was always promoted with respect to that of the pure Co3O4 and CuO phases and the reduction of cobalt was markedly enhanced by the presence of copper. Cobalt and copper were both reduced to metals regardless of the catalyst composition. On the other hand, the Co : Cu=100 : 0 specimen calcined in N2 formed, as expected, CoO. The initial addition of copper resulted in the formation of the Cu+Co3+O2 compound, besides CoO, up to a Co/Cu=1 atomic ratio at which the CuCoO2 phase was the main component. A further addition of copper led to the formation of CuCoO2 and CuO phases. The XPS results were in good agreement with these findings and the surface quantitative analysis revealed a less enrichment of cobalt with respect to the catalysts calcined in air. The TPR analysis confirmed that the reduction of the N2‐calcined catalysts was also remarkably promoted by the presence of copper. Also in this case cobalt and copper metal were the final products of reduction.

Electrochemical behaviour of cobalt in aqueous solutions of different pH

Abstract: A systematic study of the corrosion and passivation behaviour of cobalt in aqueous solutions of different pH was carried out. Open circuit potential measurements, polarization experiments and electrochemical spectroscopic (EIS) investigations were employed. The experimental results show that the metal surface is always covered by a native passive film which consists of CoO. The formation of the oxide film obeys a two-electron charge transfer process. The dissolution of the barrier film is controlled by the pH of the solution. In neutral and basic solutions the barrier film is stable. In these media a barrier film thickening with the formation of secondary layer is considered. In acidic solutions, the passive film is unstable and dissolves via a pure chemical process. The mechanism of the corrosion and passivation processes taking place at the electrode/electrolyte interface in the different solutions is discussed. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to confirm the electrochemical measurements and the suggested mechanisms.

Structural tuning of the magnetic behavior in spinel-structure ferrite thin films

Abstract: Epitaxial cobalt ferrite thin films provide a model system for understanding the magnetic properties of spinel structure ferrite thin films We demonstrate that the anomalous magnetic behavior, observed in our antiphase-boundary-free films, can be explained in terms of cation distribution and lattice distortions The magnetic anisotropy is a delicate balance between strain anisotropy, due to internal lattice strain or equivalent external strain effects, and magnetocrystalline anisotropy, due to cation distribution By annealing the cobalt ferrite thin films and hence varying the cobalt cation distribution among the tetrahedral and octahedral sites and the strain state of the films, we are able to tune the symmetry and the magnitude of the magnetic anisotropy