Oxygen Chemical Diffusion Coefficient Research Articles

Oxygen diffusion in orthopyroxene TG study

Oxygen partial pressure is supposed, by analogy with olivines, to influence the kinetics of the Fe-Mg exchange reaction in orthopyroxene. It has been demonstrated for olivines that the Fe−Mg interdiffusion coefficient is dependent on P(O2), according to D Fe−Mg ∝ P(O2)∼1/6 [1−3]. By means of thermogravimetric analyses performed at different P(O2) on orthopyroxene grains from a volcanic rock it was possible to detect a certain degree of non-stoichiometry which is function of P(O2).Oxygen moves into or out of the orthopyroxene lattice in response to a compositional gradient. Therefore, in orthopyroxene too, the Fe/Mg interdiffusion and hence the kinetics of the Fe−Mg intracrystalline exchange should be affected by P(O2). The oxygen chemical diffusion coefficients at P(O2)∼5⋅10−19atm were calculated at ∼400, 500 and 600°C. It was also verified on the orthopyroxene from the TPK-30F granulite that, at the operating conditions normally used for single-crystal annealing experiments, oxygen quickly responds to a chemical potential gradient in order to maintain the system in equilibrium conditions.

Chemical diffusion of oxygen in tin dioxide

Tin(IV) oxide is widely used in the Taguchi sensor for the detection of redox-active gases. The performance of the sensor critically depends on oxygen transport parameters since perceptible values of the chemical diffusion coefficient of oxygen cause severe drift effects. The present work investigates the kinetics of oxygen exchange by two independent methods, an EPR relaxation technique and the well-known conductivity relaxation technique. The experiments were performed on nominally undoped single crystals which contained traces of iron and aluminum. In the temperature range from 700 to 1000°C the chemical diffusion coefficient is obtained as D δ =0.02 cm 2 s −1 exp(−0.9 eV/ kT). The results of the relaxation experiments were complicated by additional processes occurring on different time scales. A doping independence assumed, we conclude that the kinetics of drift processes are determined by the surface exchange reaction of oxygen on SnO 2 rather than by the chemical diffusion of oxygen.

Determination of The Oxygen Chemical Diffusion Coefficient in Perovskites by a Thermogravimetric Method

A thermogravimetric method has been used for the determination of the oxygen chemical diffusion coefficients in La1–xSrxMnO3+δ; x=0; 0.05; 0.10; 0.15 (LSM). A temperature range of 700–1000°C was studied. The chemical diffusion coefficient varies between 1.6⋅10–13 and 1.8⋅10–10cm2s–1 for the samples in the temperature range studied. The activation energy for oxygen chemical diffusion was determined to be 190–280 kJ mol–1 for the LSM samples. The magnitude of the chemical diffusion coefficients of the LSM samples does not depend on the strontium site occupation factor.

Re-analysis of defect equilibria and transport parameters in Y 2O 3-stabilized ZrO 2 using EPR and optical relaxation

The defect chemistry and transport properties of single-crystalline 9.5 mol% Y 2O 3-stabilized ZrO 2 selectively doped with 3d transition metal or rare earth ions, are analyzed. In-situ and ex-situ EPR (ESR) and optical absorption spectroscopy are applied to quantify the concentrations of the redox-active ions in specific valence states as a function of temperature and oxygen partial pressure, as well as to measure the chemical diffusion coefficients of oxygen. Relevant self-consistent defect equilibrium constants (oxygen incorporation, dopant ion ionization) and transport coefficients (mobilities of electrons, holes, and oxygen vacancies) are extracted. The analysis uses the significance of trapping effects for the thermodynamic factor of oxygen, and the fact that the activity coefficient of oxygen vacancies, even though non-trivial, does not depend on pO 2 or minor redox-active impurities.

The chemical diffusivity of oxygen in liquid iron oxide and a calcium ferrite

Measurements have been made of the chemical diffusion coefficient of oxygen in liquid iron oxide at temperatures from 1673 to 1888 K and in a calcium ferrite (Fe/Ca = 2.57) at temperatures from 1573 to 1873 K. A gravimetric method was used to measure the oxygen uptake during the oxidation of the melts by oxygen or CO2-CO mixtures. The rate was shown to be controlled by mass transfer in the liquid melt. The chemical diffusivity of oxygen in liquid iron oxide at oxygen potential between air and oxygen was found to be 4.2±0.3 × 10−3 cm2/s at 1888 K. That in iron oxide at oxidation state close to iron saturation was established to be given by the empirical expression log D=−6220/T + 1.12 for temperatures between 1673 and 1773 K. For the calcium ferrite (Fe/Ca=2.57) at oxygen potential between air and oxygen, the diffusivity of oxygen was found to be given by log D=−1760/T−1.31 for temperatures between 1673 and 1873 K.

Tracer diffusion and chemical diffusion of oxygen in acceptor doped SrTiO3

Oxygen tracer and oxygen chemical diffusion coefficients have been determined for single crystalline SrTiO3 under defined temperature (f), oxygen partial pressure (p(O2)), and acceptor (Fe) dopant concentration (mFe) conditions. Oxygen tracer diffusion results were obtained (873K≤T≤1173K,p(O2)=105Pa) by means of 18O isotope exchange with subsequent analysis of the 18O in-diffusion profiles by secondary ion mass spectrometry (SIMS). In the case of chemical diffusion an in situ and spatially resolved, optical relaxation technique was applied (673K≤T≤973K,10Pa≤p(O2)≤105Pa). The dopant concentration in both experiments was varied between 4.3×1018cm−3≤mFe≤4.9×1019cm−3. The evaluation of (ex situ) tracer and (in situ) concentration profiles are shown to be in excellent agreement with defect chemical calculations. In contrast to the tracer diffusion coefficients (D∗) the chemical diffusion coefficients (Dδ) are sensitive to ionic–electronic (short-range) defect interactions (internal buffer effects influence the thermodynamic factor) caused by valence changes of the redox-active Fe-doping.

Chemical diffusion of oxygen in thoria–urania solid solution

The chemical diffusion coefficients of oxygen were measured for the (Th 1− y , U y )O 2+ x with uranium content y of 1, 3 and 5 mol% in the temperature range of 1213–1313 K under the oxidizing or reducing atmosphere. The magnitude of activation energies obtained for oxygen diffusion was in good agreement with the migration energy which is calculated based on interstitialcy mechanism by Colbourn. With increasing uranium content, the chemical diffusion coefficient increased and the activation energy for oxygen diffusion decreased slightly in the present experimental conditions. The excess oxygen were soluble into the (Th 1− y , U y )O 2+ x sample, so that the interaction energy between the higher valence uranium ions and interstitial oxygen ions might increase with decreasing uranium content. The effect of oxygen partial pressure on both the chemical diffusion coefficient and the lattice parameter of (Th 1− y , U y )O 2+ x could not be found. The lattice constant decreased linearly with uranium content, which indicated that the relation between these parameters obeyed Vegard’s law.

Electrical conductivity and chemical diffusivity of NiAl2O4 spinel under internal reforming fuel cell conditions

NiAl2O4 spinel was formed by solid state reaction. Its electrical conductivity was measured in the temperature range of 680–940 °C and under various oxygen-rich environments, as well as under reducing conditions. From the temperature dependence of the conductivity, the activation energies for conduction increase for decreasing oxygen partial pressures. From the partial oxygen pressure dependence, the defect structure of the material was analysed. The conductivity change with respect to PO2 can be attributed to singly and doubly ionized nickel vacancies. The chemical diffusivity of the oxide was determined by conductivity relaxation upon abrupt change in PO2 in the surrounding atmosphere. The oxygen chemical diffusion coefficient is of the order of magnitude of 10−4 cm2 s−1.

Study and modelling of oxygen diffusion in YBa2Cu3O7- under isothermal conditions

This paper presents a study of oxygen diffusion in YBa2Cu3O7- (YBCO) using isothermal thermogravimetric analysis. Comparing the results of a solution of Fick's second law to the experimental data, it has been possible to calculate an expression for the chemical diffusion coefficient of oxygen in YBCO. The good agreement between the theoretical and experimental data suggests that the diffusion rate is not determined by the formation of a highly oxygenated layer on the outer surface of the sample. The analysis depends on a finite element model developed to study the diffusion process. The model both calculates the fractional weight change of the sample during the isothermal oxygenation stage and determines the change in distribution of oxygen within the sample during the oxygenation process. The treatment presented provides a convenient basis for preliminary analyses of any proposed oxygenation procedure, to check oxygenation times and the final distribution of oxygen within the sample.

Chemical diffusion of oxygen in dense, textured (Bi,Pb) 2Sr 2Ca 2Cu 3O x

Dense polycrystalline (Bi,Pb) 2Sr 2Ca 2Cu 3O y specimens were fabricated by sinter forging. The specimens exhibited excellent phase purity and strong texture of c-axes, which were preferentially aligned parallel to the forging direction. The chemical diffusion coefficient of oxygen, D̃, measured in pure O 2 could be represented as D̃ ab ≈1.78×10 −3 exp(−0.96 eV/ kT), where the subscript ab refers to diffusion within the a– b-plane. The measurements suggested that D̃ ab ≫ D̃ c , where the subscript c refers to diffusion in the c-direction.

Synthesis, conductivity and oxygen diffusivity of Sr 2Fe 3O x

Sr 2Fe 3O x samples were synthesized by solid-state reaction at 1200°C in atmospheres with various oxygen partial pressures. X-ray and neutron powder diffraction experiments were conducted to determine the crystal structure. Electrical conductivity was measured by the four-probe method at elevated temperatures in atmospheres with various oxygen partial pressures. The oxygen chemical diffusion coefficient was determined by the conductivity relaxation method. Neutron powder diffraction showed that the Sr 2Fe 3O x material has a layered structure with an orthorhombic unit cell with lattice parameters a=11.122 Å, b=18.98 Å, c=5.585 Å, and V=1178.97 Å 3. The total conductivity of Sr 2Fe 3O x increases with increasing temperature and oxygen partial pressure; at 800°C in air, the conductivity is ≈0.5 Ω −1 cm −1. Oxygen chemical diffusion coefficient increases with increasing temperature. The coefficient at 900°C ≈1.2×10 −5 cm 2 s −1, is more than one order of magnitude higher than that of the mixed-conducting Sr 2Fe 2CoO x material. The activation energy for oxygen diffusion in Sr 2Fe 3O x , derived from the oxygen chemical diffusion coefficient, is ≈0.9 eV in air.

Preparation of Perovskite‐Type La1 − xSrxMnO3 Films by Vapor‐Phase Processes and Their Electrochemical Properties: II. Effects of Doping Strontium to on the Electrode Properties

Complex alternating current impedance and steady‐state polarization measurements have been conducted on dense and thin and film electrodes and porous‐sintered and electrodes in air at elevated temperatures between 873 and 1273 K, in order to study the reaction mechanism of oxygen reduction at the electrode of a solid oxide fuel cell. By fitting impedance spectra to an appropriate equivalent circuit, the chemical diffusion coefficient of oxygen and interfacial reaction resistance of the and film electrodes were determined. The chemical diffusion coefficient was scarcely affected by Sr doping, while the interfacial reaction resistance considerably decreased by Sr doping. Steady‐state polarization behavior of the porous‐sintered was dramatically improved by doping Sr, while those of the dense film were almost unchanged by Sr doping. These results suggested that the electrochemical reduction of oxygen at the porous electrode takes place around the triple phase boundary (TPB), and the reaction rate is controlled by the surface reactions close to the triple phase boundary region.

Studies on the kinetics of oxidation of Pu yTh 1− yO 2− x ( y = 0.2, 0.3 and 0.7) in air

Oxidation of sintered thorium-plutonium mixed oxide pellets was studied by thermogravimetry in the temperature range 400–1200 K. The kinetics of oxidation was found to be controlled by oxygen chemical diffusion with activation energies in the range 25–50 kJ mol −1. Oxygen chemical diffusion coefficients are reported for Pu 0.3Th 0.7O 1.94.

Oxygen nonstoichiometry in mixed-conducting SrFeCo 0.5O x

The electrical transport properties of SrFeCo 0.5O x were studied in various oxygen partial pressure ( p o 2 ) environments by conductivity experiments. Oxygen stoichiometry was determined by thermogravimetric analysis, while oxygen chemical diffusion coefficient was derived from time relaxation experiments by abruptly changing the p o 2 in the surrounding atmosphere and monitoring specimen weight change as a function of time. The structure of SrFeCo 0.5O x was determined by X-ray powder diffraction. Conductivity results showed that SrFeCo 0.5O x is a mixed conductor with high combined electronic and oxygen ionic conductivities. At 950 °C in air, total electrical conductivity is ≈23 S cm −1 and the ionic transference number is ≈0.4. In high- p o 2 environments, interstitial oxide ions and holes are the predominant charge carriers in SrFeCo 0.5O x . We found that oxygen transport in SrFeCo 0.5O x is primarily bulk controlled, and X-ray diffraction revealed that the SrFeCo 0.5O x system has an oxygen-deficient perovskite-like layered structure. Its structure can be fit by Rietveld analysis with an orthorhombic unit cell of lattice parameters a = 11.112, b = 19.12, c = 5.528 A ̊ , and cell volume of 1174.67 Å 3.

Measurement of 2000–2100°C oxygen diffusion coefficients in hypostoichiometric UO 2

The chemical diffusion coefficient of oxygen in UO 2− x has been measured using molten Zircaloy held in a UO 2 crucible to establish the concentration driving force. The oxygen-to-uranium distribution set up by this method was determined by scanning electron microscopy/image analysis of the number density and area fraction of uranium metal precipitates that form when UO 2− x disproportionates upon cooling. Oxygen diffusivities were determined by fitting the numerical solution of Fick's second law to the O/U profiles. The diffusion coefficients are (3.3±0.5)×10 −8 m 2/s at 2000°C and (3.6±0.8)×10 -8 m 2/s at 2100°C.

ＳＩＭＳによるセラミックス中の酸素拡散の測定

One of the most interesting features of secondary ion mass spectrometry (SIMS) is its ability to detect isotope ratio in a small area of a solid state sample. It enables to study isotope diffusion properties in solids. Experimental setup and data analysis for isotope exchange/SIMS measurement are introduced focusing on its application to oxygen transport study in ceramic materials. Fast diffusion paths and preferred surface reaction sites are visualized by sector type SIMS. Oxygen vacancy diffusion coefficient and chemical diffusion coefficient of nonstoichiometric oxides are also estimated from the isotope diffusion coefficient and thermodynamic data.

Critical Analysis of Potentiostatic Step Data for Oxygen Transport in Electronically Conducting Perovskites

The solid‐state potentiostatic technique is a convenient and versatile tool for studying oxygen transport in electronically conducting perovskites. Furthermore, a systematic analysis of the relaxation data helps elucidate the mechanistic nature of the prevalent oxygen rate process. As a case in point, we describe potential step measurements on 90% dense samples of , focusing per se on the assumptions underlying the technique and the interpretation of results. Careful analysis of the current‐relaxation data pointed to the presence of high‐diffusivity paths in the samples. It also indicated that the overall transport rate is controlled by an oxygen exchange reaction at the grain boundaries. This complication prevented unambiguous assessment of the chemical diffusion coefficient in this material. In addition, the analysis showed that wide disagreements in the oxygen chemical diffusion coefficients reported in the literature for doped perovskites may be attributed to differences in the sample quality (e.g., density, grain size, and distribution) and the measurement technique employed.

Determination of oxygen diffusivity in tetragonal YBa 2Cu 3O x using a solid state electrochemical method

A solid state electrochemical technique is used to determine the oxygen chemical and component diffusion coefficients in the tetragonal phase of YBa 2Cu 3O x . The measurements are performed at 850 °C, as a function of oxygen partial pressure and oxygen nonstoichiometry. The sample is held in a calcia-stabilized zirconia titration cell. An oxygen ionic current pulse is applied to the cell, and the EMF signal is monitored as a function of time. The diffusion coefficients are evaluated by analyzing the time dependence of the EMF, during and after the current pulse. It is found that the oxygen chemical diffusion coefficient increases weakly with x, with an average value of D ̃ ∼ 5 · 10 −5 cm 2/s at T = 850 °C. The oxygen component diffusion coefficient is found to be independent of nonstoichiometry, with a value of D o ∼ 5 · 10 −5 cm 2/s.

Determination of chemical diffusion coefficient of SrFeCo 0.5O x by the conductivity relaxation method

A conductivity relaxation experiment has been conducted on an SrFeCo 0.5O x sample by abruptly changing the oxygen partial pressure in the atmosphere and monitoring the change of conductivity as a function of time. The re-equilibrium process obeys Fick's second law. By fitting the relaxation data to the solution of the diffusion equation with appropriate boundary conditions, we could determine the oxygen chemical diffusion coefficient and the activation energy. The oxygen diffusion coefficient is 8.9 × 10 −7 cm 2/s at 900 °C and it increases with increase in temperature. Measured activation energy is 0.92 eV, which is slightly lower than that of other oxides in the system SrFe 1 − x Co x O y .

High temperature thermodynamics and oxygen permeability of PrBa 2Cu 3O 6 + x

The pressure-composition isotherms for oxygen in PrBa 2Cu 3O 6 + x were measured by means of coulometric titration in the temperature range 550–825 °C and at oxygen pressures between 10 −3 and 1 atm. The composition dependences of thermodynamic potentials reveal substantially non-ideal behavior of the ensemble of the oxygen ions in the basal plane. The oxygen ion conductivity of the PrBa 2Cu 3O 6 + x compound was measured by a permeation technique in the temperature range 600–850 °C and at oxygen pressures between 0.08 and 1.00 atm. The data were employed for calculation of the oxygen chemical diffusion coefficient. The activation energy of the oxygen conductivity and oxygen diffusivity was found to depend linearly on the oxygen content, U = 2.46 − 1.57 x (eV), in the integral 0.35 < x < 0.60 which was explained by interaction of movable oxygen ions.