Oxygen Partial Pressure Conditions Research Articles

Role of Fluorite Formation in Orientation Selection in Sol-Gel Derived Pb(Zr,Ti)O3 Films on Pt Electrode Layers

AbstractWe present evidence that the microstructure of the metastable fluorite phase Pb2(Zr,Ti)2O6, which forms during the pyrolysis treatment of sol-gel PZT layers by homogeneous nucleation throughout the film, has a larger impact on orientation selection than has previously been realized. Elaborate TEM studies demonstrate that by varying the pyrolysis time, temperature and duration, the crystallinity of the transient fluorite phase can be greatly influenced. Pyrolysis at 350°C leads to the formation of a well crystallized fluorite phase with a cubic structure (Fd3m space group) and a= 10.5-10.6 Å. Rapid crystallization of the transient fluorite phase is attributed to the low oxygen partial pressure conditions, prevailing during burnoff of acetate groups originating from the lead starting compound. A possible connection between the fluorite formation kinetics and orientation selection in the layers is discussed.

Impedance Studies of Oxygen Exchange on Dense Thin Film Electrodes of La0.5Sr0.5CoO3 − δ

Solid‐state electrochemical cells with dense oriented thin film electrodes of (LSCO) were prepared on (100) surfaces of single‐crystal disks of yttria‐stabilized zirconia (YSZ) by the pulsed laser deposition technique. Oxygen exchange at the electrodes was studied with alternating current impedance spectroscopy under various temperature and oxygen partial pressure conditions. Three distinctive features were observed in the impedance spectra from high to low frequency corresponding to contributions from the ionic conduction of the YSZ electrolyte, ionic transfer at the LSCO/YSZ interface, and the oxygen exchange on the LSCO electrode surface. An equivalent circuit model of the electrode process is used to fit the impedance data. The time constant for the oxygen surface exchange was derived from the impedance simulation. The surface chemical exchange coefficients, , were calculated from the time constants as a function of temperature and . is at T = 700°C and . The activation energy at is ≈ 1.1 eV. The interfacial conductivity data were also derived from the impedance simulations as a function of temperature and . The activation energy for the interfacial transport at is ≈ 1.6 eV. © 2000 The Electrochemical Society. All rights reserved.

Effect of the environment on alpha-Al2O3 (0001) surface structures

We report that calculating the Gibbs free energy of the alpha-Al2O3 (0001) surfaces in equilibrium with a realistic environment containing both oxygen and hydrogen species is essential for obtaining theoretical predictions consistent with experimental observations. Using density-functional theory we find that even under conditions of high oxygen partial pressure the metal-terminated surface is surprisingly stable. An oxygen-terminated alpha-Al2O3 (0001) surface becomes stable only if hydrogen is present on the surface. In addition, including hydrogen on the surface resolves discrepancies between previous theoretical work and experimental results with respect to the magnitude and direction of surface relaxations.

Mechanical activation conditions of the Fe 2O 3 and V 2O 3 mixture powders in order to obtain a nanometric vanadium spinel ferrite

Co-milling of iron and vanadium oxides allows to obtain an intimate oxides mixture at a nanoscale, similar to a coprecipitate elaborated by soft chemistry. Reduction of such a mixture in the same temperature and oxygen partial pressure conditions (500°C and 10 −25 Pa) as the soft chemistry products leads to a nanometric vanadium ferrite with the only spinel phase. The characterization of the powders is achieved by X-ray diffraction (XRD), scanning electron microscopy, infrared (IR) spectrometry, thermogravimetry and calorimetry. Homogeneity of grain size and chemical composition is reached if the initial oxides have similar grain size.

Water pollution abatement by catalytic wet air oxidation in a trickle bed reactor

Catalytic wet air oxidation of an aqueous phenol solution was carried out in a trickle bed reactor at mild conditions of temperature (120–160°C) and oxygen partial pressure (0.6–1.2 MPa). Tests were performed at space times up to 1 h. A commercially available copper oxide supported over γ-alumina was used as catalyst. The results show that the phenol conversion, the chemical oxygen demand (COD) reduction and the distribution of oxidation products are considerably affected by the temperature, whereas the oxygen partial pressure only has minor influence. The process exhibits high selectivity towards the production of carbon dioxide. Oxalic, acetic and formic acids were found to be the main partial oxidation products but also traces of maleic and malonic acids and quinone-like products, such as dihydric phenols and benzoquinones, were detected. First order with respect to the phenol concentration and one half order with respect to the oxygen partial pressure was found for both phenol disappearance and COD reduction. The apparent activation energy was calculated to be 85 kJ/mol for phenol oxidation and 76 kJ/mol for COD abatement. These values favourably compare with those cited in the literature for intrinsic kinetics, which indicates minimal mass transfer limitations in the trickle bed reacting system used in this study.

Fabrication of YBCO coated conductors using biaxially textured Pt 0.66Pd 0.34 tapes as their substrates

Y 1Ba 2Cu 3O 7− δ (YBCO) superconducting films were grown on nonmagnetic and biaxially well textured Pt 0.66Pd 0.34 tapes. The Pt 0.66Pd 0.34 tapes were cube-textured by rolling and recrystallization. Good textures of Pt–Pd tapes were obtained only with certain concentrations of Pd and rolling temperatures. The buffer layers, CeO 2/YSZ/CeO 2, were deposited by e-beam evaporations. Good textures of buffer layers were obtained only under certain conditions of oxygen partial pressure. The buffer layers with smooth surfaces were free of cracks. The YBCO films deposited on the buffer layers by a sputtering method were biaxially well-textured with 87 K of zero resistance temperature.

Three-phase reactors for environmental remediation: catalytic wet oxidation of phenol using active carbon

Wet oxidation of phenol aqueous solutions was carried out in a fixed bed reactor operating in trickle flow regime. Mild conditions of temperature (140°C) and oxygen partial pressure (1–9 bar) were used. Three active carbons and one commercially available supported copper catalyst were tested as catalytic material. Previous studies demonstrated that active carbon gives higher phenol conversion than conventional oxidation catalysts, although significant loss of active carbon due to combustion was also found. In the present study, the combustion of the active carbon during the process is highly reduced by lowering the oxygen partial pressure from 9 to 2 bar, maintaining an acceptable phenol conversion. The comparison of the performance of three different active carbons shows that their physical and chemical characteristics largely influence on the phenol conversion achieved.

Protonic Conduction in SrY0.5Ta0.5O3-Based Ceramics at Elevated Temperatures

The perovskite-type-oxide solid solution SrY0.5+xTa0.5+xO3−δ was synthesized and its properties were investigated. The single phase character of the samples was confirmed by X-ray diffraction when 0 ≤ x ≤ 0.02, while lines from the impurity phase SrY2O4 appeared in patterns of x ≥ 0.03. The conductivities of SrY0.52Ta0.48O3−δ were about an order of magnitude higher than those of SrY0.50Ta0.50O3. The results of electrochemical measurements such as emf measurements of gas concentration cells, isotope effect in conductivity, and oxygen partial pressure dependence of conductivity showed that SrY0.52Ta0.48O3−δ exhibited pure protonic conduction in reducing atmospheres and p-type electronic conduction under high oxygen partial pressure conditions.

Interface fabrication of YBa2Cu3Oy ramp-edge junction with PrBa2(Cu1−xCox)3Oy barrier layer in enhanced oxidizing atmosphere

Thin film growth and fabrication process for YBa2Cu3Oy/PrBa2(Cu1−xCox)3Oy/YBa2Cu3Oy ramp-edge type junctions have been studied. Preferable conditions to maintain a stable ramp surface were investigated by comparative measurements of the surface morphology of ramp-edge structures of epitaxial bilayers of SrTiO3/YBa2Cu3Oy annealed at various conditions of temperature and oxygen partial pressures. The stability of the ramp-edge surfaces was discussed in terms of the equilibrium reaction of YBa2Cu3Oy, and the degradation of surfaces by damages through the fabrication process was concluded to be the main factor for the decomposition at high temperatures. A two-step etching technique and annealing with atomic oxygen activated by microwave enabled the ramp surfaces to be maintained quite stable even at the deposition temperature of barrier layers. The ramp surface of YBa2Cu3Oy annealed under 4×1014 cm−2 s−1 activated oxygen flux gave no evidence of decomposition at 800°C except for a slight decrease in thickness in the YBa2Cu3Oy layer. In ECR (electron cyclotron resonance)-processed junctions, resistively shunted junction (RSJ) type current–voltage characteristics were observed and the magnetic field dependence of the critical current showed a Fraunhofer pattern.

Kinetics of catalytic oxidation of aqueous sodium sulfite

A gas-lift bubble column was used to investigate the kinetics of the reaction between oxygen and aqueous solutions of sodium sulfite in the presence of cobalt sulfate catalyst. Reaction orders have been determined for the sulfite, catalyst, and oxygen under high and low oxygen partial-pressure conditions.

Mass spectroscopy of recoiled ion investigation of electrode segregation effects during the initial stages of SrBi2Ta2O9 film growth

The thermodynamic stability of Pt/Ti/SiO2/Si, Pt/TiO2/SiO2/Si, Pt/Ta/SiO2/Si, Ir/SiO2/Si, and RuO2/SiO2/Si electrode layers has been studied using mass spectroscopy of recoiled ions (MSRI), a new technique which provides monolayer-specific in situ surface analysis under conditions of modest oxygen partial pressures required for oxide film growth using sputter-deposition. Results of the initial stages of growth of SrBi2Ta2O9 films on the various substrates listed above are presented also. It is found that the initial growth of SrBi2Ta2O9 films strongly depends on the composition of the bottom electrode surface, on diffusion and segregation processes, and the presence of reactive species on the electrode surface.

High temperature electrocatalytic properties of the SOFC air electrode La 0.8Sr 0.2MnO 3/YSZ

The electrocatalytic properties of the air electrode La 0.8Sr 0.2MnO 3 in solid oxide fuel cells were examined by using complex impedance spectroscopy under the conditions of oxygen partial pressure, P(O 2), between 1 and 10 − 4 atm and temperatures, T, between 800 and 1000°C. The electrodes, O 2/La 0.8Sr 0.2MnO 3/YSZ, were prepared by co-firing at 1350, 1400, 1435, and 1500°C in air. In the measurements at 800–1000°C, σ E was nearly proportional to P(O 2) 1/2 at high P(O 2)s and was proportional to P(O 2) at low P(O 2)s. In the region where σ E was proportional to P(O 2), σ E showed nearly the same value irrespective of the different electrode microstructures, suggesting that the electrode reaction was controlled by O 2 gas diffusion. In the region where σ E∝ P(O 2) 1/2, the relationship between σ E and P(O 2) was interpreted by the model that the electrode reaction was controlled by the surface diffusion of adsorbed oxygen atoms, O ad, from the electrode surface to the electrode/electrolyte boundary. The results obtained in the present work meant that La 2Zr 2O 7, formed on the La 0.8Sr 0.2MnO 3 surface, disturbed the surface diffusion of O ad from the La 0.8Sr 0.2MnO 3 surface to the three phase boundary of O 2, La 0.8Sr 0.2MnO 3, and Y 2O 3 stabilized ZrO 2 (YSZ). Increasing the co-firing temperature, the surface composition of the electrode material changed, and hence the oxygen adsorption process might be changed.

Reactions between a zirconia-based electrolyte and LaCoO 3-based electrode materials

Thermodynamic calculations on high temperature reactions between an YSZ electrolyte and LaCoO 3 cathode materials identify the limiting conditions of temperature and oxygen partial pressure ranges required to prevent formation of resistive reaction products. The time dependence of the fraction reacted was evaluated under isothermal conditions and as a function of temperature. Theoretical models for both diffusion or phase boundary controlled reactions fail to fit the experimental data. This failure is probably related to loss of contact at the LaCoO 3 | La 2Zr 2O 7 interface, when significant oxygen pressures are built up inside compact reacting mixtures of YSZ and LaCoO 3.

Experimental set up for determining the temperature—oxygen partial pressure conditions during synthesis of spinel oxide nanoparticles

Nanometric spinel oxide powders, Fe 3− x M x O 4, where M is a transition element, have been synthetized by soft chemistry. This method generally leads to a non-stoichiometric phase, Fe 3− x M x O 4+δ where δ is the deviation from stoichiometry so that further annealing at low temperatures around 450°C and low oxygen partial pressure around 10 −25 Pa given by N 2/H 2/H 2O gas mixtures is required: this enables a stoichiometric compound to be obtained and a nanometric size to be maintained. The complete set up consisting of a gas mixer, a thermogravimetric apparatus and a preparative furnace is described. Some results concerning the conditions of temperature and oxygen partial pressure leading to stoichiometry are given in the case of vanadium ferrites containing or without Co.

Spectroscopic Studies of High‐Temperature Processes in Oxides

AbstractSpectroscopic studies can provide important information on physico‐chemical processes in solids, like, e.g., order‐disorder processes, the point defect structure and local defect arrangements, and heterogeneous as well as homogeneous chemical reactions of solids. Results will be reported relating to these topics from experiments that have been performed under in situ conditions, i.e. at high temperatures and under defined conditions of oxygen partial pressures. The examples have been chosen from high‐temperature studies using optical spectroscopy and Mössbauer spectroscopy.

Highly selective oxidative dehydrogenation of ethane to ethene over layered complex metal chloride oxide catalysts

Complex metal chloride oxides consisting of bismuth, alkali, alkaline earth, chlorine, and oxygen, were synthesized, characterized structurally, and tested as catalysts for the oxidative dehydrogenation of ethane to ethene with molecular oxygen. The catalysts were prepared by high-temperature solid-state reaction of appropriate mixtures of bismuth chloride oxide, bismuth oxide, alkali chloride, and alkaline earth chloride. We found that the catalysts containing strontium as the alkaline earth constituent and potassium as the alkali constituent were highly active and selective for the oxidative dehydrogenation of ethane. SrBi3O4Cl3, which is a fundamental phase of the catalysts, was characterized by single and double chlorine sheets in its layer structure. The catalyst having the composition of KSr2Bi3O4Cl6 showed an extremely high ethene selectivity, more than 90%, even under high oxygen partial pressure conditions and at high molar ratios of oxygen to ethane, and gave 70% yield of ethene at 640°C under an optimized feed gas composition.

A study on the La 1 − xSr xMnO 3 oxygen cathode

The impedance responses and current-overpotential characteristics were studied for oxygen reducing cathodes, stoichiometric La 1 − x Sr x MnO 3 ( x = 0.16 − 0.20) in solid oxide fuel cells, under varying conditions of temperature (700–900 °C) and oxygen partial pressure (1 − 10 −4 atm) for two distinctly different electrode morphologies (porous and dense structure). An electrode densified at the interface with the yttria-stabilized zirconia solid electrolyte was more efficient in this study. The reaction at the finely porous LSM proceeds via the triple phase boundary and ineffectively uses the mixed conductivity property of the material. Reaction mechanisms for both cases (porous and dense structure) are discussed and compared with the literature. A new, two-layered cathode structure is proposed.

96/00675 The decomposition flame of hydrazine in inert porous media

Combustion studies of single free-falling coal-water fuel (CWF) droplets were conducted in a drop-tube laminar-flow furnace at high heating rates. Most experiments were conducted with predried CWF agglomerates consisting of micronized (3.5 μm mean size) and beneficiated bituminous coal. Agglomerates of known initial size, in the range of 100–600 μm, were burned in air or pure oxygen at furnace temperatures between 1300 and 1500 K. Combustion of CWF agglomerates impregnated with calcium magnesium acetate (CMA), which is being considered as a sulfur capture agent, was also examined under the same conditions. Observations on the devolatilization and char combustion behavior of CWF agglomerates, formed after the evaporation of water, were conducted using pyrometric and cinematographic techniques. Complete time-temperature histories of burning agglomerates were obtained with a three-color near-infrared optical pyrometer, where distinct phases of volatile and char combustion were observed and analyzed. The volatile flame temperatures and the char combustion temperatures in air exceeded the furnace gas temperature by as much as 1000 and 600 K, respectively. The char combustion phase is the most prominent since it accounts for 75%–85% of the burnout time. The volatile combustion phase is also important because of the resulting high-temperature radiant flames. Overall burnout times were between 150 and 600 ms, for the range of agglomerate sizes tested. Char combustion for most agglomerates was controlled by boundary layer diffusion of oxygen. While most experiments involved predried agglomerates, a limited number of experiments were performed with CWF droplets to monitor the time required for water evaporation and subsequent heatup of the resulting agglomerates. Under the oxygen partial pressure and temperature conditions of this study water evaporation took approximately 25% of the total CWF furnace residence time. The swelling behavior of the agglomerates was studied and swelling factors in the neighborhood of 1.12 were determined. Although the bituminous CWF, studied herein, was found to swell and form cenospheric structures during heatup and devolatilization, the addition of CMA catalyst inhibited swelling of the agglomerates. However, the chars containing CMA exhibited occasional splitting or fragmentation during the volatile combustion stage.

Defect Chemistry of “BaCuO2” II. Transport Properties and Nature of Defects

Abstract The charge transport properties of "BaCuO2" with 88:90 (Ba :Cu) cation ratio were characterized by thermopower, electrical conductivity and ionic transport number measurements in a wide range of temperature and oxygen partial pressure conditions. The nature of carriers is always represented by small polarons due to self-trapping of the electronic holes generated by the oxygen non-stoichiometry equilibrium. Some anomalies in carrier mobility as a function of temperature are shown not to be related to incomplete ionization of oxygen atoms on interstitial sites

Defect Chemistry of " BaCuO2" I. Oxygen Non -Stoichiometry, Cation Molecularity and X-ray Diffraction Determinations

Abstract “BaCuO2” is the key intermediate in the synthesis of the Ba2YCu3O7−δ superconductor. Its very complex crystal structure is able to accommodate a large change in oxygen content. Oxygen non-stoichiometry of "BaCuO2" materials with 1:1 and 88:90 (Ba :Cu) molecularity has been investigated by polythermal X-ray powder diffraction coupled with isobaric-isothermal gravimetry determinations under different temperature and oxygen partial pressure conditions [300≤ T≤ 820 °C, 1 ≥ P(O2) ≥ 3 • 10-3 atm]. The 1:1 composition does not give well reproducible results, thus suggesting its polyphasic nature, at least in part of the investigated range. The results for the 88:90 ≅ 0.98 (Ba :Cu) composi­ tion are well reproducible and show that the material is single phase. Ba0.98CuO1.98 + δ is oxygen over-stoichiometric in the whole investigated [T, P(O2)] range, with a maximum value δ∼0.21. A Rietveld X-ray profile fitting is in agreement with previous single-crystal data. The trend of δ vs. P(O2) is consistent with the presence of oxygen interstitial defects on (possibly different) crystallographic sites.