Equilibrium Oxygen Content Research Articles

High-temperature phase equilibria in the oxide systems SrFe1−xGaxO2.5–SrFe1−xGaxO3 (, 0.1, 0.2)

X-ray powder diffraction, Raman scattering, equilibrium oxygen content and conductivity data are combined to construct high-temperature phase diagrams (T vs. δ) for the oxide systems SrFe1−xGaxO2.5–SrFe1−xGaxO3, where x=0, 0.1, and 0.2. Depending on oxygen content and gallium doping, three different structural types are shown to exist at high temperatures including cubic perovskite-, cubic brownmillerite-, and orthorhombic brownmillerite-type structures. Substitution of gallium for iron is shown to extend the oxygen content and temperature limits where the cubic brownmillerite-type phase is stable.

Equilibrium and non-equilibrium states of theYBCO–O2 system

The conditions (temperature, pressure and time) for attaining equilibrium with oxygen fordifferent YBCO materials are reported. The equilibrium oxygen content was used in orderto determine more exact values for the enthalpies and entropies of interaction ofYBa2Cu3Oy with oxygen. Internal equilibrium in YBCO is proposed and is ableto explain the effects of ageing and low temperature annealing onTc. Homogeneity and transitions to superconductivity of ceramics having equilibrium andnon-equilibrium oxygen contents are discussed.

Conductivity and carrier traps in La 1− xSr xCo 1− zMn zO 3− δ ( x=0.3; z=0 and 0.25)

Measurements of the equilibrium oxygen content, electrical conductivity and thermopower in the perovskite-like solid solution La 0.7Sr 0.3Co 1– z Mn z O 3− δ ( z=0 and 0.25) as a function of the temperature and oxygen partial pressure are used to determine the temperature dependence of the conductivity and thermopower at different values of the oxygen deficiency. A model for a hopping conductor with screened charge disproportionation is applied for the data analysis in combination with trapping reactions of n- and p-type carriers on local oxygen vacancy clusters and manganese cations, respectively. Changes in the ratio of n-type to p-type mobility are due to variations in oxygen vacancy concentration and manganese content, while the energetic parameters governing charge disproportionation of the trivalent cobalt cations and formation of vacancy associates are shown to be essentially invariable. These calculated charge carrier site occupancies are used to model temperature variations of the electrical properties in La 0.7Sr 0.3Co 1− z Mn z O 3− δ in favorable correspondence with experimental observations.

Macro-cracking in melt-grown YBaCuO superconductor induced by surface oxygenation

We have shown that oxygenation of the melt-grown YBa2Cu3O7/Y2BaCuO5 single grain superconductors is associated with the formation of macro-cracks parallel to the a/b-plane. The origin of these cracks is associated with tensile stress induced into the oxygenated surface layer due the c-lattice parameter shortening. The critical strain calculated according to Thoules's model for the cracking of brittle films on an elastic substrate suggests that the macro-cracking starts much earlier than the formation of the homogeneous layer of equilibrium oxygen content at the surface.

High-temperature equilibrium between YBa2Cu3-xCoxO6+δ solid solutions and oxygen

The equilibrium oxygen content of YBa2Cu3-xCoxO6+δ (0.2 ≤x ≤ 0.8) solid solutions was determined by coulometric titration at temperatures in the range 600–850°C and oxygen pressures 1 ≤po 2 ≤ 105 Pa. The partial enthalpy and entropy of oxygen dissolution were evaluated as a function of composition. The thermodynamic functions were found to change sharply near the high- and low-oxygen limits of the solid solution. The results are interpreted in terms of a quasi-chemical model considering three inequivalent oxygen sites in the basal plane.

Oxygen thermodynamics and ion conductivity in the solid solution La1−xSrxCoO3−δ at large strontium content

The equilibrium oxygen content was measured in the model system and important oxygen permeable material La1−xSrxCoO3−δ, where x=0.6, in the temperature range 650–900 °C and oxygen partial pressure range between 10−5 and 1 atm. The data were utilized to obtain changes in the partial entropy and enthalpy of oxygen in the solid as a function of the oxygen content. It is shown that the initially cubic perovskite undergoes to a phase transition to a tetragonal structure at δ >0.3. The oxygen permeation of L0.4Sr0.6CoO3−δ at 700–900 °C is found to be controlled by bulk solid state processes. The activation energy equals about 0.8 eV at high oxygen pressure and small oxygen nonstoichiometry. Increasing oxygen deficiency results in a rapid increase in the activation energy. In combination with thermodynamic data, these changes can be explained as resulting from the intrinsic spatial inhomogeneouty in oxygen vacancy distribution which varies both with temperature and oxygen nonstoichiometry. It is shown that, when the oxygen deficiency increases at constant temperature, the oxygen vacancies form locally ordered microdomains (clusters), which eventually results in a transition of the cubic perovskite structure to the tetragonal structure. The oxygen ion conductivity depends strongly on the development of the ordering.

Intercalation thermodynamics and chemical diffusion of oxygen in the solid solution YBa2Cu3−xCoxO6+δ

The equilibrium oxygen content as a function of the temperature and oxygen pressure was measured for the solid solution YBa2Cu3−xCoxO6+δ, where x=0, 0.2, 0.4, 0.6, 0.8, by using coulometric titration in the temperature range 600–850°C and oxygen pressures between 10−5 and 1.0 atm. The change in the partial molar enthalpy and entropy of the intercalated oxygen was determined at different oxygen and cobalt contents. The oxygen chemical diffusion was studied by thermogravimetric relaxation in the oxygen-controlled atmosphere. The thermodynamic data were employed to determine how the chemical diffusion coefficient, the thermodynamic factor and the random-diffusion coefficient depend on oxygen content in specimens with different cobalt concentration. The oxygen intercalation thermodynamics and diffusivity results provide evidence of ordering phenomena on a microscopic scale in the basal plane of the tetragonal solid solution YBa2Cu3−xCoxO6+δ. A model for the oxygen diffusion is suggested to explain the large difference between the random and tracer diffusion coefficients in YBa2Cu3O6+δ

High-temperature thermodynamics of oxygen equilibrium of solid solutions YBa 2Cu 3− xZn xO 6+ δ with gas phase

The equilibrium oxygen content was measured in the solid solution YBa 2Cu 3 x Zn x O 6+ δ , where x=0, 0.1, 0.2, 0.3, in the temperature range 600–825°C and oxygen pressure between 10 −5 and 1.0 atm. The concentration dependencies of the partial molar enthalpy Δ H°( δ) and entropy Δ S°( δ) of oxygen were determined. A decrease of both Δ H°( δ) and Δ S°( δ) at δ<0.2 is observed. This behavior is interpreted as evidence that participation of the apex O4 oxygen in the gas↔solid equilibrium occurs with exhaustion of the O1 and O5 oxygen positions in the basal plane.

Thermodynamics of the movable oxygen and conducting properties of the solid solution YBa2Cu3-xCoxO6+δ at high temperatures

High precision coulometric measurements of the equilibrium oxygen content in the solid solution YBa2Cu3-xCoxO6+δ, where x=0, 0.2, 0.4, 0.6 and 0.8, were carried out using a double-cell technique in the temperature range 600 – 850 °C and at oxygen pressure varying between 10−5 and 1 atm. The data were employed to determine the partial molar enthalpy and entropy of the movable oxygen depending on δ and x. The electrical conductivity and thermopower were also measured in the same range of the external parameters, and their dependence on the oxygen concentration was determined at different cobalt content. The data reveal several types of oxygen sites participating in the gas-solid equilibrium. The behavior of thermodynamic functions is indicative of the partial ordering of the complex species which form the structural layer Cu1-xCoxOδ with variable content of oxygen and cobalt. It was shown that replacement of copper by cobalt does not result in appearance of the electronic charge carriers. The behavior of the thermopower and electric conductivity was explained with a narrow band model. The energy change with δ and x of the p-band, which dominates the conductivity, was found to follow the respective change in the oxygen partial enthalpy. Thus, electronic carriers in the layered structure of the cuprate are strongly influenced by the labile oxygen ions.

97/04558 Preparation and gasification of a Thailand coal-water fuel: Ness, R. O. et al. Proc. Annu. Int. Pittsburgh Coal Conf., 1996, 13, (1), 443–448

In response to an inquiry by the Department of Mineral Resources (DMR) in Thailand, the Energy and Environmental Research Center (EERC) prepared a four-task program to assess the responsiveness of Wiang Haeng coal to the temperature and pressure conditions of hot-water drying (HWD). The results indicate that HWD made several improvements in the coal, notably increases in heating value and carbon content and reductions in equilibrium moisture and oxygen content. The equilibrium moisture content decreased from 37.4 wt% for the raw coal to about 20 wt% for the HWD coals. The energy density, determined at 500 cP, indicates an increase from 4450 to 6650 Btu/lb by hydrothermal treatment. Raw and HWD coal were then gasified at various mild gasification conditions of 700 C and 30 psig. The tests indicated that the coal is probably similar to other low-rank coals and will produce high levels of hydrogen and be fairly reactive.

The oxidation and carburization of FeMnAl alloys in a carbon-containing atmosphere

The morphology and reaction process of two austenitic FeMnAl alloys in the carbonaceous atmosphere were studied in this work. The austenitic FeMnAl alloys is internally oxidized and carburized in a carbon monoxide atmosphere in the temperature range from 900 to 1070 °C. The internal oxide is a mixture of MnAl2O4 and Al2O3, and the carbide is perovskite carbide with the formula (Fe,Mn)3AlC0.5. Oxidation and carburization of studied FeMnAl alloys in the CO atmosphere were proposed to occur in the sequence of CO dissociation, transportation of C and O atoms, internal oxidation, carbide formation and decomposition. During internal oxidation, Al2O3 was formed first by selective oxidation of Al. The Al2O3 particle then acted as the nucleus of the spinel oxide formation. MnAl2O4 grew from the Al2O3 particles to the finally observed morphology as the oxygen content around the Al2O3 particle became favourable for the formation of MnAl2O4. The carbide was formed as the C content in the matrix exceeded the solubility of the alloy and it decomposed as the oxygen content around the carbide higher than the equilibrium oxygen content of Al2O3.

Oxygen Nonstoichiometry of (Nd2/3Ce1/3)4(Ba2/3Nd1/3)4Cu6O16+x

The equilibrium oxygen content of (Nd2/3Ce1/3)4(Ba2/3 Nd1/3)4Cu6O16+x was measured by coulomeric titration in the temperature range 550-820°C and at oxygen pressures between 10-3 and 1 atm. From the results, the partial molar enthalpy and entropy of oxygen were calculated as functions of the oxygen content. The data were explained based on a point defects model and the equilibrium constants of the redox and charge disproportionation reactions involved were derived. A phase transition near the point x ≈ 1.83, which nominally corresponds to one trivalent copper cation per elementary unit, was observed.

Thallium solubility range in Tl2 –yBa2Can– 1CunO2n+ 4 –xsuperconductors

The Tl2 –yBa2Can– 1CunO2n+ 4 –xphases corresponding to n= 1, 2 and 3 have been synthesized at 860–880 °C under sealed-tube conditions and characterized by wet chemical analysis, X-ray diffraction and magnetic susceptibility. A lower Tl solubility limit of (2 –y)= 1.25 was found for all n, but the upper limit decreased from 1.89 for n= 1 to 1.80 for n= 2 and 1.65 for n= 3. Under our controlled synthetic conditions, which allowed variation of the Tl concentration under similar temperatures and oxygen partial pressures, the equilibrium oxygen content was found to vary with the change in Tl concentration so as to keep a constant oxidation of the CuO2 sheets. Over the entire Tl solubility ranges, the superconductive onset temperatures were nearly constant: Ton= 117 ± 1 K for n= 3, Ton= 105 ± 1 K for n= 2, and Ton= 86 ± 1 K for n= 1. The superconductive properties improve sensitively with chemical homogenization, and homogenization was achieved most easily for n= 2. Processing of Tl-2212 superconductors with a controlled Tc under sealed-tube conditions appears to be feasible.

Model for the oxygenation of the YBa2Cu3O7−x 90K superconductor

A model incorporating the thermodynamic equilibrium oxygen content, oxygen in-diffusion, and oxide phases for the oxygenation of the YBa2Cu3O7−x is described. For sintered polycrystals, grain growth and the resulting grain size distribution are included. The model is used to calculate the volume percent of each oxide phase for several processing conditions of sintered specimens and is compared with available results on the quantification of phases present. Such a comparison indicates that as the concentration of impurity phases increases so does the concentration of the less oxygenated 123 phases, suggesting that impurities coating 123 grains act as oxygen indiffusion barriers. The model is also used to investigate the uniformity of the oxygen content in large twinned and detwinned single crystals such as have been used for measuring superconductivity parameters.

Oxygen desorption and structure in CaBaLaCu 3O x

The thermodynamics and kinetics of oxygen desorption in the tetragonal superconductor CaBaLaCu 3 O x are examined. Equilibrium oxygen contents are determined by thermogravimetric analysis (TGA) for temperatures from 50 to 900°C. The results demonstrate that CaBaLaCu 3 O x ( x ≤6.85) loses oxygen more slowly than YBa 2 Cu 3 O x ( x ≤7) in flowing O 2 as the temperature is increased. The resistance to oxygen loss is linked to a higher enthaply for oxygen desorption in CaBaLaCu 3 O x of Δ H =1.74 eV/atom O (compared to Δ H ∼ 1.1 eV/atom O in YBa 2 Cu 3 O x ), as measured by differential scanning calorimetry (DSC). If the superconductor is sufficiently depleted of oxygen, the half-order reflections observed in transmission electron microscopy (TEM) diffraction patterns disappear. This result confirms that oxygen ordering is responsible for unit cell doubling. Finally, upon reducing CaBaLaCu 3 O x completely — to metallic Cu and elemental oxides — a two step reduction reaction occurs. It appears that the oxidation states of Cu and Ba are first reduced; then this reaction is followed by the conversion of Ba (OH) 2 to BaO.

Superconducting and magnetic properties as a function of the oxygen content in the Bi 2Sr 2CaCu 2O 8+δ phase

The oxygen content in the compound Bi 2Sr 2CaCu 2O 8+δ (2212) may be varied from δ=0.1 to δ=0.4. This variation influences the physical properties as observed in other high-T c systems (Y 1Ba 2Cu 3O 7−δ, Y 2Ba 4Cu 7O 15+δ, Tl 2Ba 2Cu 2O 6+δ). It determines the hole doping p ( Cu 2+ p) which can be related to the critical temperature T c, the Meissner fraction and the normal-state susceptibility. In this work on the Pb-free Bi-2212 system, we present the diagram of the critical temperature T c in the (p,T) plane for 10 -5 ⩽ p ⩽ 10 2 bar (O 2) and 300 ⩽ T ⩽ 840 °C. The equilibrium oxygen content is frozen by quenching the samples into liquid gallium. The sharp Meissner diamagnetic transitions vary between 50 and 94 K and the Meissner fraction increases with T c. The highest T c (optimum hole concentration) is obtained at T Quench = T 0 + a log 10P( O 2) with T 0=870 ° C, a=60 °C and P(O 2) in bar. The slope of the normal-state susceptibility d χ/dT, which is strongly negative for the oxidized samples, increases with progressing reduction and vanishes for the optimum doping.

Lanthanum doping of YBa2Cu3Ox

A wide variety of cation substitutions have been tried in the 1-2-3 compound, YBa2Cu3Ox, in the hope that its superconducting properties can be improved. While significant improvement has not been achieved, the response of the material to such substitutions can still be helpful in understanding its defect chemistry. In this paper, we report on measurements of the equilibrium oxygen content, by thermogravimetry, and the electrical conductivity of YBa2−zLazCu3Ox, with z = 0, 0.1, and 0.2. The results are consistent with the view that the stoichiometric composition for this system, YBa2Cu3O6, is a highly acceptor-doped material.

Experimental and Computational Characterization of Alloy–Spinel–Corundum Equilibrium in the System Ni–Co–Al–O at 1873 K

AbstractThe three-phase equilibrium between alloy, spinel solid solution and α-Al2O3 in the Ni–Co–Al–O system at 1873K has been fully characterized as a function of alloy composition using both experimental and computational methods. The equilibrium oxygen content of the liquid alloy has been measured by suction sampling and inert gas fusion analysis. The oxygen potential corresponding to the three-phase equilibrium has been determined by emf measurements on a solid state galvanic cell incorporating (Y2O3)ThO2 as the solid electrolyte and Cr+Cr2O3 as the reference electrode. The equilibrium composition of the spinel phase formed at the interface between the alloy and alumina crucible was measured by electron probe microanalysis (EPMA). The experimental results are compared with the values computed using a thermodynamic model. The model uses values for standard Gibbs' energies of formation of pure endmember spinels and Gibbs' energies of solution of gaseous oxygen in liquid nickel and cobalt available in t...

Oxygen content control for as-deposited YBa2Cu3Ox thin films by oxygen pressure during rapid cooling following laser deposition

As-deposited epitaxial YBa2Cu3Ox thin films have been prepared by ArF excimer laser deposition followed by a rapid cooling with controlled oxygen pressure. The best film after the rapid cooling has a Tc with a zero resistance of 88 K and a Jc (77 K) of 5.5×105 A/cm2. It has been observed that the c-lattice parameter decreases, that is, the oxygen content x increases from ∼6 to 7, with increasing oxygen pressure during the rapid cooling. The relation between x and oxygen pressure is almost consistent with that for a powder sample being in a thermal equilibrium. We conclude that the oxygen content in the as-deposited films is determined by the thermal equilibrium oxygen content during the cooling process.

Biological solubilization of untreated north dakota lignite by a mixed bacterial and a mixed bacterial/fungal culture

Four microbial cultures, two pure and two mixed, were examined for their abilities to solubilize chemically treated (oxidized) lignites, thermally treated (wet-carbonized) lignite, and untreated lignites. Extensive solubilization of oxidized lignites and limited solubilization of untreated North Dakota lignite was observed by three of the four cultures tested. Solubilization of wet-carbonized (a technique to reduce equilibrium moisture and oxygen contents) lignite was not demonstrated. The increase in solubilization correlated with the increase in the oxygen content of lignite and the pH of culture broths. These results also suggest that microbial solubilization of coal may involve nonlignin degrading organisms capable of producing alkaline conditions in the presence of coal.