CoO System Research Articles

Thermodynamic modeling of the CeO2–CoO nano-phase diagram

A nano-phase diagram of the CeO2–CoO system was modeled thermodynamically with experimental data available in the literatures. The surface energies of CeO2 and CoO unavailable in the literatures were estimated reasonably on the thermodynamic basis. Butler’s model was used to describe the surface energy and the surface composition of the solution phases and then the nano interaction parameters on the particle radius were assessed through the multiple linear regression method. A consistent set of optimized interaction parameters in the present system was derived for describing the Gibbs energy of liquid, fluorite, and halite solution phases as a function of particle radius. The eutectic temperatures calculated in the present work interpreted well the experimental data for the unusual low sintering temperature of the nanoparticles with the tri-modal particle size distribution. Furthermore, with the aid of the present result, the microstructure evolution in the CGO–CoO system during the nanoparticle sintering was described reasonably. It is concluded that the present modeling will be a good guide for the condition of the liquid phase sintering to obtain the rapid densification of the nanoparticles at lower temperatures.

Improving the Magnetic Properties of Co–CoO Systems by Designed Oxygen Implantation Profiles

Oxygen implantation in ferromagnetic Co thin films is shown to be an advantageous route to improving the magnetic properties of Co-CoO systems by forming multiple nanoscaled ferromagnetic/antiferromagnetic interfaces homogeneously distributed throughout the layer. By properly designing the implantation conditions (energy and fluence) and the structure of the films (capping, buffer, and Co layer thickness), relatively uniform O profiles across the Co layer can be achieved using a single-energy ion implantation approach. This optimized configuration results in enhanced exchange bias loop shifts, improved loop homogeneity, increased blocking temperature, reduced relative training effects and increased retained remanence in the trained state with respect to both Co/CoO bilayers and O-implanted Co films with a Gaussian-like O depth profile. This underlines the great potential of ion implantation to tailor the magnetic properties by controllably modifying the local microstructure through tailored implantation profiles.

Consistent LDA’ + DMFT approach to the electronic structure of transition metal oxides: Charge transfer insulators and correlated metals

We discuss the recently proposed LDA’ + DMFT approach providing a consistent parameter-free treatment of the so-called double counting problem arising within the LDA + DMFT hybrid computational method for realistic strongly correlated materials. In this approach, the local exchange-correlation portion of the electron-electron interaction is excluded from self-consistent LDA calculations for strongly correlated electronic shells, e.g., d-states of transition metal compounds. Then, the corresponding double-counting term in the LDA’ + DMFT Hamiltonian is consistently set in the local Hartree (fully localized limit, FLL) form of the Hubbard model interaction term. We present the results of extensive LDA’ + DMFT calculations of densities of states, spectral densities, and optical conductivity for most typical representatives of two wide classes of strongly correlated systems in the paramagnetic phase: charge transfer insulators (MnO, CoO, and NiO) and strongly correlated metals (SrVO3 and Sr2RuO4). It is shown that for NiO and CoO systems, the LDA’ + DMFT approach qualitatively improves the conventional LDA + DMFT results with the FLL type of double counting, where CoO and NiO were obtained to be metals. Our calculations also include transition-metal 4s-states located near the Fermi level, missed in previous LDA + DMFT studies of these monoxides. General agreement with optical and the X-ray experiments is obtained. For strongly correlated metals, the LDA’ + DMFT results agree well with the earlier LDA + DMFT calculations and existing experiments. However, in general, LDA’ + DMFT results give better quantitative agreement with experimental data for band gap sizes and oxygen-state positions compared to the conventional LDA + DMFT method.

Microstructural Design of Bioinert Composites in the ZrO2–Y2O3–CeO2–Al2O3–CoO System

It is shown that a scientifically sound approach to each stage of producing ZrO2-based bioinert implants (from the synthesis of starting powders to their sintering) is a necessary condition for promoting the optimum structure and high mechanical properties. Conditions for producing bioinert implants with regular, laminar, and highly porous microstructures are found. The research results serve as a scientific basis for microstructural design of various bioinert implants in the ZrO2–Y2O3–CeO2–Al2O3-CoO system.

Experimental phase diagram determination and thermodynamic assessment of the Gd 2O 3–CoO system

New phase diagram data and a thermodynamic assessment of the Gd 2O 3–CoO system using the CALPHAD approach are presented, giving liquidus data and mutual solid solubilities of Co in Gd 2O 3 and Gd in CoO. The thermodynamic model parameters for the ternary Gd–Co–perovskite phase and for the mutual solid solubilities of Co in Gd 2O 3 and Gd in CoO are optimized to reproduce these new experimental data, as well as phase diagram data from literature. The Gd 2O 3–CoO phase diagram is refined based on the results of experiments using combined differential thermal analysis and thermogravimetry, scanning electron microscopy and X-ray diffraction techniques.

Control of Magnetism in Cobalt Nanoparticles by Oxygen Passivation

We report on the preparation of ferromagnetic cobalt nanospheres with antiferromagnetic oxide capping layer and its implication for the variation in magnetic property. The hcp cobalt nanospheres were prepared by thermal decomposition of cobalt carbonyl in the presence of organic surfactants. The spherical nanoparticles thus prepared were oxidized to grow antiferromagnetic layers of varying composition and thickness on top of cobalt spheres. High-resolution transmission electron microscopy confirmed growth of Co3O4 in one case and CoO in another case. Strong exchange anisotropy and enhanced coercive field was observed due to the core−shell structure in the Co−CoO system. On the other hand only a marginal improvement was seen in the Co−Co3O4 system. A low-temperature paramagnetic behavior was also observed that is interpreted in the framework of crystal defects in the oxide shell.

Thermodynamic possibilities and constraints for pure hydrogen production by a nickel and cobalt-based chemical looping process at lower temperatures

The reduction of nickel and cobalt oxides by hydrogen, CO, CH 4 and model syngas (mixtures of CO + H 2 or H 2 + CO + CO 2) and oxidation by water vapour has been studied from the thermodynamic and chemical equilibrium points of view. Attention was concentrated not only on convenient conditions for reduction of the relevant oxides to metals at temperatures in the range 400–1000 K, but also on the possible formation of undesired soot, carbides and carbonates as precursors for carbon monoxide and carbon dioxide formation in the steam oxidation step. Reduction of nickel and cobalt oxides (NiO, CoO and Co 3O 4) by hydrogen or CO at such temperatures is feasible. The oxidation of Ni and Co by steam and simultaneous production of hydrogen is thermodynamically the more difficult step at temperatures of 400–900 K. For the Ni–NiO and Co–CoO systems, the formation of corresponding Ni/Co–ferrite or Ni/Co aluminum spinel could be used for a higher hydrogen equilibrium yield. Only such Ni–NiO and Co–CoO systems with the support of ferrite and aluminum spinel formation could be suitable systems for chemical looping production of hydrogen by the chemical looping redox process. Oxidation of mixed Ni/Co–Fe metals or alloys by steam without segregation caused by preferential oxidation of Fe is critical for the ferrites. For processes based on Ni/Co aluminum spinel, reduction to metals is the critical part of the cyclic process. Under strongly reducing conditions, at high CO concentrations/pressures, formation of nickel carbide (Ni 3C) before cobalt carbide Co 2C is thermodynamically favored. Pressurized conditions during the reduction step with CO/CO 2 containing gases enhance the formation of soot and carbon containing carbide and/or carbonate compounds.

Quantitative analysis of X-ray absorption spectra using a 2D map representation

A promising possibility for the quantitative analysis of X-ray absorption near edge structure (XANES) spectra of nanosized electrode materials is demonstrated. We used a 2D map representation technique, which utilizes the values of the first derivatives of the absorbance with respect to the inserted Li + content plotted over the two-dimensional space defined by the inserted Li + content (mole) versus photon energy (eV) as a single map. The technique was applied to XANES spectra of the Li y CoO system in the first Li + insertion reaction for determining the structural and electronic variations associated with the change in Li + content. The obtained show that the intensities of two peaks at 7725 and 7711 eV increased with the Li + content and the difference of intensity change of these two peaks carried out for successive couples of spectra yielded the largest changes at 1.05 and 1.98 mol of Li content. This approach for quantitative analysis of XANES without using conventional simulation techniques enable us to interpret X-ray absorption spectroscopy (XAS) as a quantitative analytical technique with greater confidence.

Enhancement of the exchange-bias onset temperature in a columnar nanocrystalline Ni80Fe20∕Co3O4 thin film

Typically, exchange coupling can only occur when ferromagnetic spins couple to antiferromagnetic spins below the Néel temperature. We present results on a Ni80Fe20∕Co3O4 thin film that is composed of ∼10nm diameter nanocrystalline columns. Field cooling the film reveals low-temperature exchange-bias hysteresis loop shifts that are the same magnitude as those measured in a similar Ni80Fe20∕CoO system, although the bulk magnetocrystalline anisotropy is a factor of 5 smaller. The coercivity and exchange-bias loop shift exhibit the same temperature dependence with individual “blocking” temperatures. Most surprising is that the onset temperature of the exchange-bias is nearly four times higher than the bulk Néel temperature.

Extended solubility of CoO in ZnO and effects on magnetic properties

The metastable solid solubility extension of CoO in ZnO (wurtzite) was investigated in precursor-derived powders as well as in thin films grown on sapphire substrates by pulsed laser deposition. A maximum solubility of 30% Co2+ in ZnO was achieved in the powders. Transmission electron microscopy (TEM) of the films revealed them to have grown epitaxially and retained up to nearly 40% CoO in solid solution, but some Co2+ precipitated as rock-salt. The temperature dependence of the metastable solubility limit in the ZnO–CoO system was assessed and is discussed in terms of the relevant thermodynamic factors. The magnetic properties of n-type conductive Zn0.79Co0.2Al0.01Ofilms were studied, yielding evidence of a ferromagnetic phase with a TC of 25 K and a second, magnetically ordered, phase with positive exchange and arguably a TC of ∼250 K. Connections between the properties and microstructural observations in high resolution TEM are proposed.

Method to study temperature and stress induced magnetic transitions

A new method called magnetic transition spectrum (MTS) is described for studying magnetic phase transitions. The MTS method is an electronic method that monitors the dynamics of the micromagnetic structure as a function of temperature, stress, or any other perturbation that can cause a sudden variation in flux inside the magnetic material. It is based on the same principle upon which the well-known and established Barkhausen method is based, namely, Faraday’s law. However, instead of applying a magnetic field as in the Barkhausen method, temperature or stress is the external “force.” The efficacy of the MTS method is illustrated by studying magnetic transitions in magnetic shape memory alloys. The MTS method is simple to implement and is equally applicable for studying magnetic transitions in other systems, such as, for example, dynamics of exchange anisotropy, using the Co–CoO system, by cooling the sample across the Néel temperature. In general, it can be used to study magnetic phase transitions driven by any external influence that would cause an abrupt change in the micromagnetic state of the sample (for example, change in temperature, pressure, etc.).

Electrochemical crystal growth and properties in Ln 2O 3–MnO–CoO (Ln=La, Ho) system

We have developed a known approach of electrochemical deposition to grow successfully neutron sized single crystals of pure and mixed compounds in the Ln 2O 3–MnO–CoO system. Reported in detail, it is a growth technique with a seed of LnAO 3+ x (Ln=La, Ho; A=Co, Mn) single crystals up to 2 cm 3 in volume using the 2.2Cs 2MoO 4–MoO 3 mixtures as solvent. The best crystals were grown in the temperature range 1050–1100 °C to reveal FWHM ∼0.1° of the rocking curves. A primary crystallization temperature range for LaCo 1− x Mn x O 3 ternary oxides was found to be 830–1170 °C. But, quasi-equilibrium growth was seen only for the temperature interval 1000–1050 °C. In turn, much more stable growth and wide primary crystallization field were registered for single crystals of HoCo 1− x Mn x O 3 solid solution. The grown crystals accept mostly a cubic-like shape. However, weekly developed (1 1 1) type faces could be recognized in some cases. Here, we also report on studies of crystal field excitations and present the data of magnetic susceptibility of the electrochemically grown crystals.

Primary crystallization fields, growth features and properties of rare earth and barium-based cobaltates

High quality single crystals of LnBaCo 2O 5+ x ( 0 < x < 1 ) (Ln=Pr, Eu, Gd, Tb, Dy) were grown from overstoichiometric flux melt. In contrast, caused by significant flux melt creep and corrosion of alumina crucibles single crystals of these cobaltites with Y or Ln=La, Nd, Sm and Ho cannot be grown using similar temperature range and flux compositions. To solve this problem comprehensive studies of the phase primary crystallization fields in the corresponding quasi-ternary phase diagrams have been undertaken. For most of Ln 2O 3–BaO–CoO systems co-crystallization of several new phases was found. Bulk single crystals of a new cobaltate family LnBaCo 4O 7 (Ln=Eu, Gd, Y, Dy, Tb and Tm) were grown. Comparative data on magnetic susceptibility of the grown crystals are presented.

Sub-liquidus co-crystallization in the Ln 2O 3–BaO–CoO system: growth of large LnBaCo 2O 5+x (Ln=Eu, Gd, Tb, Dy) single crystals

An original approach to flux growth has been developed to provide for the first time large and high-quality single crystals of oxygen deficient LnBaCo 2O 5+ δ (Ln=Eu, Gd, Tb, Dy, Tb 0.9Dy 0.1). The data from DTA studies of both the BaO–CoO binary eutectic and temperature of crystal nucleation as well as analysis of more than 100 runs of sub-liquidus co-crystallization allow one to infer the tentative ( T–x) phase diagram of a quasi-binary cut in the Ln 2O 3–BaO–CoO system with the estimated primary crystallization field for the LnBaCo 2O 5+ δ (Ln-112) phase. As a result, in successful growth runs several crystals of a parallelepiped shape and up to 120 mm 3 in volume were grown. The dependence of the crystal shape on the rare earth ion is discussed. For most flux melts close to the ternary eutectic compositions of the corresponding quasi-binary cuts, co-crystallization of the layered cobaltite phase and a new one of LnBaCo 4O 7 (Ln-114) was found in a wide range of temperatures. Data on the twinned crystal structure of the optimally oxidized ( δ = 0.5 ) orthorhombic Ln-112 crystals and crystal characterization by chemical composition, magnetic susceptibility and resonance photoemission spectroscopy are discussed.

Various strategies to tune the ionic/electronic properties of electrode materials.

This Perspective highlights, through several snapshot examples, the importance of electrochemically-driven redox reactions in tuning the electronic/ionic as well as magnetic properties of 3d-metal-based inorganic compounds through a careful control of the metal oxidation state. Although such redox reactions usually imply the electron-ionic duality, they can be extended to insulating compounds (LiFePO(4)) or semiconductors (CoO) as long as we can combine electrochemistry at the nanoscale to reduce diffusion and migration limitations, and provide the compounds with electrons through metallic coating techniques. A thorough investigation of the composition-structure-property relationships of the Li(x)CoO(2) system, through the assembly of LiCoO(2)/Li electrochemical cells has led to the identification of the CoO(2) phase, whose property and stability are discussed in terms of cationic-anionic redox competition, thus bearing some similarity with the high T(c) cuprate superconductors. Such a d-sp redox competition could have structural and electronic consequences. Encouraged by the recently reported superconductivity in Na(x)CoO(2);yH(2)O phase, the room temperature Li(x)CuO(2) phase diagram was reinvestigated through Li-driven electrochemical reactions. A solid solution domain was unravelled but superconductivity was not evident. With Cu-based materials such as Cu(2.33)V(4)O(11), we have shown the feasibility of a new reversible Li electrochemically-driven copper extrusion/insertion process, owing to the enhanced copper diffusion within the structure.

CeO2—CoO Phase Diagram.

Phase equilibria in the CeO2−CoO system at temperatures above 1500°C were investigated. The microstructures and the phase compositions of the DTA (differential thermal analysis) samples and the quenched solid pellets were analyzed using SEM (scanning electron microscope), EDX (energy dispersive X-ray), and WDX (wavelength dispersive X-ray). A eutectic reaction was found at 1645 ± 5°C. The eutectic point was calculated to be at 82 ± 1.5 mol% CoO. The eutectic phases were the CeO2-rich phase (containing <5 mol% CoO) and the CoO-rich phase (containing ∼0.5 mol% CeO2). At 1580°C, the solubility of CoO in CeO2 was ∼3 mol%.

Synthesis and characterization of in situ grown magnetoelectric composites in the BaO–TiO–FeO–CoO system

Magnetoelectric composite materials with mixed spinel cobalt ferrite–cobalt titanate and perovskite barium titanate as co-existing phases have been grown in situ by solid state reactions over a wide region of compositions of the BaO–TiO–CoO–FeO system. An estimate of phase formation was obtained by XRD. The magnetoelectric properties of the composites in relation to their composition and synthesis process have been assessed. The magnetoelectric (ME) conversion factor has been measured by a dynamic method as a function of magnetic field and has been found to vary from 3.0081 to 5.5886 mV/cm Oe at room temperature depending on the synthesis procedure. Typical dependance of ME conversion factor on magnetic field shows that the magnetic saturation occurs at low stimulation and the samples are best suitable in responding to relatively weak magnetic field.

CeO2−CoO Phase Diagram

Phase equilibria in the CeO2−CoO system at temperatures above 1500°C were investigated. The microstructures and the phase compositions of the DTA (differential thermal analysis) samples and the quenched solid pellets were analyzed using SEM (scanning electron microscope), EDX (energy dispersive X‐ray), and WDX (wavelength dispersive X‐ray). A eutectic reaction was found at 1645 ± 5°C. The eutectic point was calculated to be at 82 ± 1.5 mol% CoO. The eutectic phases were the CeO2‐rich phase (containing &lt;5 mol% CoO) and the CoO‐rich phase (containing ∼0.5 mol% CeO2). At 1580°C, the solubility of CoO in CeO2 was ∼3 mol%.

Thermodynamic assessment of the Co-O system

The thermodynamic properties of CoO, Co3O4, and the liquid phase were assessed, and an optimized set of parameters of Gibbs energy functions is proposed. The two stable solid oxides, CoO and Co3O4, were both treated as stoichimetric compounds. The paramagneticantiferromagnetic transition of CoO is well represented by a magnetic ordering model. The Co3O4 spinel phase was described as a normal spinel at room temperature and with cation redistribution at high temperatures. A high-temperature anomaly of Co3O4 was interpreted as a normal-inverse spinel transition. An ionic two-sublattice model was used to model the liquid phase. A calculated phase diagram is presented, and values for the thermodynamic properties are compared with experimental data.

Coercive Force of Co–CoO Thin Films

A Co–CoO system was studied to obtain high coercive materials. Co–CoO thin films were sputtered at various oxygen gas flow rates by dual ion-beam sputtering. The Co was deposited onto glass substrates at room temperature and 650 K by argon ion-beam sputtering and simultaneously oxidized by oxygen gas from an assist ion gun. As the oxygen gas flow rate increases, there appear Co, the Co–CoO mixed phase, CoO, and Co2O3 in a stepwise manner. The coercive force increases sharply in the Co–CoO mixed phase region. However, the maximum coercive force at room temperature is small, about 600 Oe. At low temperatures, the addition of the CoO phase to Co causes a significant increase of the coercive force. The perpendicular coercive force of the Co–CoO film is larger than the in-plane coercive force. The perpendicular coercive force is about 17000 Oe at about 10 K. This is due to the magnetic anisotropy of the antiferromagnetic microcrystalline CoO below the Néel temperature of about 220 K. The CoO (111) plane is preferably oriented and the spin axis of CoO stands against the plane, which leads to a strong perpendicular magnetic anisotropy. There is a possibility that the coexistence of the ferromagnetic phase and the antiferromagnetic phase significantly increases the coercive force.