Oxygen Chemical Diffusion Research Articles

A Chemical Diffusion‐Controlled Electrode Reaction at the Compact La1 − xSrxMnO3/Stabilized Zirconia Interface in Oxygen Atmospheres

In order to elucidate how the oxide‐ion electron mixed conduction in an oxide cathode affects the kinetics of the cathode reaction in solid oxide fuel cells, a compact layer was prepared as a working electrode on the electrolyte of 3 mole percent doped by a laser flash evaporation method, and electrochemical measurements were made as a function of oxygen partial pressures, , at 700 to 900°C. Contrary to the case of a porous electrode system, the electrode interface conductivity was found to increase with decreasing . With large anodic polarization, the oxide layer was found to be peeled off by oxygen evolution at the electrode/stabilized zirconia interface. The kinetics controlled by chemical diffusion of oxygen in the oxide layer were observed under cathodic polarization with Pa at 900°C. The oxide ion conductivity in was found to be proportional to , where aO is the oxygen activity in the oxide.

Existence of fluorite-type solid solutions in alkali metal-uranium-oxygen systems

The solubilities of Li 2O and Na 2O in UO 2 + x have been established for the first time. They form cubic solid solutions, Li y U 1 − y O 2 + x and Na y U 1 − y O 2 + x ( y = 0–0.2 and x ⪋ 0). The lattice parameters of these solid solutions can be expressed, respectively, as a function of x and y: a 0(nm) = 0.54704 − 0.0104x − 0.0495y and a 0(nm) = 0.54704 − 0.0102 x − 0.0345 y. Oxidation of these solid solutions in air up to 773 K gave cubic MO 2 + x and the kinetics of oxidation was found to be controlled by oxygen chemical diffusion similar to oxidation of UO 2.

Defect chemistry in metal oxides

Abstract The presentation gives a brief overview and description of point defects and extended defects that may be considered to arise from clustering or ordering of point defects. Defect chemistry and the relations between defect concentrations and transport properties are exemplified and illustrated by a review of nonstoichiometry, electrical conductivity, diffusion of cobalt and oxygen tracers, chemical diffusion, and high temperature creep of Co1-δO. Crystals are like people, it is the defects that make them interesting F.C. Frank

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.

Diffusion-induced grain-boundary migration during internal reduction of chromium-doped Al2O3

Abstract Diffusion-induced grain-boundary migration is observed during internal reduction of chromium-doped A12O3. Chemical diffusion of oxygen along grain boundaries is fast compared with volume diffusion; therefore the internal reduction front penetrates from the grain boundaries into the grains, leading to chromium precipitation and formation of a thin, coherently strained chromium-depleted oxide layer. Migration of the grain boundary to either side of the boundary occurs, while the global energy in the overswept area decreases owing to oxygen potential relaxation, precipitate coarsening and elastic strain relaxation. The role of multiple factors, namely the chemical composition of the starting mixed oxide, chemical composition inhomogeneities, crystallographic orientation of adjacent grains and of the boundary plane, kinetics and mechanism of the internal reduction itself, on the direction and velocity of boundary migration is experimentally studied and interpreted.

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.

Room-temperature oxygen diffusion in a-axis normal YBa 2Cu 3O x thin films

Near-surface oxygen chemical diffusion at room temperature in YBa 2Cu 3O x thin films with a-axis orientation normal to the substrate has been studied using four-probe DC conductivity measurements. The results show that under the same experimental conditions the oxygen out-diffusion coefficient is about two orders of magnitude higher in these films compared to those with the c-axis normal to the substrate. We also present data on oxygen out-diffusion enhancement in these films under ultraviolet (UV) light illumination.

Precise Determination of the Chemical Diffusion Coefficient of Calcium‐Doped Lanthanum Chromites by Means of Electrical Conductivity Relaxation

Chemical relaxation experiments were conducted on sintered samples of calcium‐doped lanthanum chromites by abruptly changing the oxygen partial pressure in the atmosphere and following the time change of conductivity. The re‐equilibration kinetics was analyzed by fitting the relaxation data to the solutions of Fick's second law for appropriate boundary conditions. The diffusion equation ignoring the effect of surface reaction failed to describe the transient behavior especially for the initial stage, while that taking the surface effect into account gave a satisfactory interpretation of the overall relaxation process and allowed a precise determination of the two kinetic parameters: oxygen chemical diffusion coefficient and surface reaction rate constant. The chemical diffusion coefficients increased with a decrease of the oxygen partial pressure due to the corresponding change in the concentration of the moving species. The activation energy was similar to that of oxygen vacancy diffusion coefficients in other monocrystalline perovskites, suggesting that the measured diffusion coefficients were attributable to lattice diffusion. The surface reaction rate constant increased with a decrease of the oxygen partial pressure similarly to the reported oxygen nonstoichiometry, which implies that the presence of oxygen vacancies plays an important role in the surface reaction kinetics.

Electrochemical intercalation of oxygen in lanthanum nickel oxide (La2NiO4+x)(0 .ltoreq. x .ltoreq. 0.145)

A recent electrochemical study of the isostructural compound La[sub 2]NiO[sub 4+x] has shown that x values as high as 0.25 can be achieved. The experiments were carried out by oxidation at constant potential for different times in order to synthesize a series of discrete compositions. Open-circuit voltages emphasizing the higher compositions were reported, but no data were presented for the deintercalation of oxygen on reduction and the question of the reversibility of oxygen intercalation was not addressed. In the course of similar experiments, the author's have found that both oxidation and reduction of La[sub 2]NiO[sub 4-x] are rapid and highly reversible in the composition range 0 [le] x [le] 0.145. The electrochemical data obtained for La[sub 2]NiO[sub 4+x] have been used to define the phase diagram, determine the thermodynamics of oxygen intercalation and to estimate the oxygen chemical diffusion coefficient in the single phase region. 21 refs., 2 figs.

High Temperature Oxygen Transport and Electrochemical Behavior of YBa2Cu3 O x

In this preliminary study, the high temperature oxygen transport and electrochemical behavior of was evaluated in a solid‐state electrochemical cell employing a yttria‐stabilized zirconia (YSZ) solid electrolyte. The oxygen chemical diffusion coefficient was measured by a solid‐state potentiostatic step technique and found to be at 800°C. The dc polarization behavior of electrodes deposited on YSZ electrolyte was governed primarily by an ohmic process in oxygen, air, and He environments in both the anodic and cathodic directions. The frequency dispersion of the electrochemical response obtained by ac impedance spectroscopy showed two depressed circular arcs in complex plane plots that are offset from the origin, indicative of a resistive path in the equivalent circuit model at high frequencies. It is proposed that the electrode poses the major impedance to the overall rate and dominates the electrochemical performance of the cell.

Oxygen chemical diffusion in strontium doped lanthanum manganites

The chemical diffusion coefficient of oxygen in (La 0.79Sr 0.20)MnO 3−δ and (La 0.50Sr 0.50)MnO 3−δ was measured in a solid state electrochemical cell using the potentiostatic step method. In the temperature range between 700°C and 860°C, the chemical diffusion coefficient of oxygen in (La 0.79Sr 0.20)MnO 3−δ was found to be in the range of 10 −8 to 10 −6 cm 2/s and also strongly dependent on the oxygen partial pressure between 0.21 atm and 10 −8 atm. For the case of the stoichiometric oxygen content, the activation energy for oxygen chemical diffusion in this material was determine to be about 70 kJ/mole. Similar measurements between 700°C and 840°C using samples of the composition (La 0.50Sr 0.50)MnO 3−δ yielded comparable diffusion coefficient values. Such rapid oxygen transport in these materials make them attractive for high temperature fuel cell electrode and gas separation membrane applications. They also exhibited unusually high values of the order of 10 3 to 10 4 for the thermodynamic enhancement factor indicative of narrow oxygen nonstoichiometry.

Kinetics of the electrode reaction at the CO-CO 2, porous Pt/stabilized zirconia interface

To elucidate the reaction kinetics of CO gas at the electrodes of zirconia sensors and solid oxide fuel cells, measurements were made on the electrochemical impedance spectra and steady-state polarization current at the porous Pt/stabilized zirconia (SZ) electrodes in the COCO 2 atmosphere at 600–1000°C. The impedance arcs were depressed semicircles, the centers of which were 45° below the abscissa. The eletrode impedance was expressed by a parallel circuit of resistance, R E , and Warburg impedance, Z E =A(1−i)ω − 1 2 . Here, A is related to the diffusion constant, D, of the diffusion process in the electrode reaction by A=kD − 1 2 , where k is constant. The P O 2 and temperature dependences of D agreed with those of the electronic conductivity of SZ. It was concluded that Z E is determined by the oxygen chemical diffusion in SZ in the relaxation at the closely contacted interface of the Pt particles/SZ. From the steady-state current-potential relationships, the rate equation in the CO 2 rich atmosphere was determined as i=k OP 1 2 CO a 1 2 0−k 0P 1 2 CO2 . It was shown that the rate determining reaction is CO ad(Pt)+O ad(SZ)→CO 2ad(Pt) at the triple phase boundary. Possible mechanism in the CO rich atmosphere was also discussed.

Oxygen diffusion in basalt and andesite melts: experimental results and discussion of chemical versus tracer diffusion

Chemical diffusion coefficients for oxygen in melts of Columbia River basalt (Ice Harbor Dam flow) and Mt. Hood andesite have been determined at 1 atm. The diffusion model is that of sorption or desorption of oxygen into a sphere of uniform initial concentration from a constant and semi-infinite atmosphere. The experimental design utilizes a thermogravimetric balance to monitor the rate of weight change arising from the response of the sample redox state to an imposed fO2. Oxygen diffusion coefficients are approximately an order-ofmagnitude greater for basaltic melt than for andesitic melt. At 1260° C, the oxygen diffusion coefficients are: D=1.65×10−6cm2/s and D=1.43×10−7cm2/s for the basalt and andesite melts, respectively. The high oxygen diffusivity in basaltic melt correlates with a high ratio of nonbridging oxygen/tetrahedrally coordinated cations, low melt viscosity, and high contents of network-modifying cations. The dependence of the oxygen diffusion coefficient on temperature is: D=36.4exp(−51,600±3200/RT)cm2/s for the basalt and D=52.5exp(−60,060±4900/RT)cm2/s for the andesite (R in cal/deg-mol; T in Kelvin). Diffusion coefficients are independent of the direction of oxygen diffusion (equilibrium can be approached from extremely oxidizing or reducing conditions) and thus, melt redox state. Characteristic diffusion distances for oxygen at 1260° C vary from 10-2 to 102 m over the time interval of 1 to 106 years. A compensation diagram shows two distinct trends for oxygen chemical diffusion and oxygen tracer diffusion. These different linear relationships are interpreted as supporting distinct oxygen transport mechanisms. Because oxygen chemical diffusivities are generally greater than tracer diffusivities and their Arrhenius activation energies are less, transport mechanisms involving either molecular oxygen or vacancy diffusion are favored.

Ionic transport in LiNbO3

The high temperature equilibrium conductivity (950 °C–1050 °C) of congruent LiNbO3 can be resolved into two components: an electronic portion that is dependent on the oxygen partial pressure and an ionic portion that is pressure independent. It is shown that the two components can be obtained from an analysis of the total equilibrium conductivity measured as a function of oxygen partial pressure. The ionic transport number (fractional ionic conductivity) thus obtained is compared with that obtained from an oxygen concentration cell measurement. The two techniques are found to be in excellent agreement, confirming the experimental validity of the defect chemistry method. From the temperature dependence of the ionic conductivity, the activation energy (138 kJ/mol [1.43 eV]) for the ionic transport is obtained. The results are in good agreement with the value previously obtained for the oxygen chemical diffusivity.

Monte Carlo study of tracer and chemical diffusion of oxygen in YBa2Cu3O6+2c.

Tracer and chemical diffusion of oxygen in YBa{sub 2}Cu{sub 3}O{sub 6+2{ital c}} has been studied by Monte Carlo simulations of an asymmetric next-nearest-neighbor Ising model. Striking differences between tracer and chemical diffusion are found and explained by the effect of the thermodynamic factor, which is determined also by Monte Carlo simulation. Especially for {ital c}=0.5, the effect of the thermodynamic factor is large. It is also found that tracer diffusion in the ordered orthorhombic phase of YBa{sub 2}Cu{sub 3}O{sub 6+2{ital c}} is strongly anisotropic, which is explained in terms of different diffusion mechanisms.

Variation of the chemical diffusivity of oxygen and viscosity of an andesite melt with pressure at constant temperature

The chemical diffusivity of oxygen and the viscosity of an andesite melt have been measured at pressures from 3.5 to 20 kbar and 1350°C. Oxygen chemical diffusivity and melt viscosity vary antithetically as a function of pressure. The melt viscosity increases from 1 bar to 3.5–5 kbar, passes through a maximum, and then decreases with further increase in pressure to at least 20 kbar. The chemical diffusivity of oxygen decreases from 1 bar to 5 kbar, passes through a minimum, increases from 5 to 10 kbar, and then remains essentially constant with increasing pressure. In all cases the chemical diffusivity of oxygen is larger than that predicted by the Eyring equation. The results, when combined with data for basalt melts, show that the relationship between the chemical diffusivity of oxygen and melt viscosity is not adequately expressed by the Eyring equation. The data suggest that there are three different relationships between oxygen chemical diffusivity and melt viscosity and that those relationships occur over specific viscosity ranges: (1) at high viscosity the chemical diffusivity of oxygen is related to melt viscosity by the Eyring equation; (2) at intermediate viscosities diffusivity increases more rapidly than the Eyring equation predicts; and (3) at low viscosity the chemical diffusivity of oxygen is constant and independent of melt viscosity. A model is presented that predicts oxygen chemical diffusivities from melt viscosities. The model diffusivities suggest that oxygen chemical diffusion is comparable in magnitude to divalent cation diffusivities in low-viscosity melts, but that oxygen diffuses much more slowly than do cations in high-viscosity melts.

Chemical diffusion in Calcia-stabilized zirconia ceramic

Chemical diffusion of oxygen in Calcia-stabilized zirconia (CSZ) with impurities of Al 2O 3, SiO 2, MgO, Fe 2O 3 and TiO 2 was studied by the non-steady state permeation and the desorption techniques at 960–1450°C. The chemical diffusion coefficient, D, was found to be inversely proportional to the relative deviation from stoichiometry, and to increase with increasing P O 2 . The time constant of equilibration processes with a surrounding atmosphere is increased as the concentration of the trapping centers is increased. The CSZ electrolyte free of Fe 2O 3 gives fast response in EMF.

A high temperature study of chemical diffusion in mixed thorium—cerium dioxide

High temperature gas—solid reactions in non-stoichiometric actinide and rare earth oxide systems involving oxidation or reduction over a limited range of oxygen-to-metal ratio are usually diffusion controlled in nature, especially when carried out under controlled oxygen potential conditions. Kinetic studies of such reactions can be of use in the elucidation of oxygen chemical diffusion coefficients. This paper presents a mathematical formulation for the treatment of kinetic data of such reactions and evaluation of chemical diffusion coefficients. Details of a high temperature reduction study carried out on thoria—20 mol% ceria solid solution employing a thermogravimetric technique in the temperature range 1000–1200°C are presented and oxygen chemical diffusion coefficients are also evaluated.

Oxygen chemical diffusion in three basaltic liquids at elevated temperatures and pressures

The diffusivity of oxygen has been measured in three basaltic liquids from 1280 to 1450°C and 4 to 21 kilobars using a solid media piston-cylinder apparatus. The measurements were done by monitoring the reduction of ferric iron in previously oxidized spheres of basalt melt. The compositions studied were olivine nephelinite, alkali basalt, and 1921 Kilauea tholeiite. The isobaric temperature dependence of oxygen diffusivity is adequately described by Arrhenius relationships for the three liquids studied. Arrhenius activation energies were determined at 12 kilobars for olivine nephelinite (62± 6 kcal/mole) and tholeiite (51 ± 4 kcal/mole) and at 4, 12, and 20 kilobars for alkali basalt (70 ± 7, 86 ± 6, and 71 ± 14 kcal/mole, respectively). The Arrhenius parameters for the three compositions define a compensation law which is indistinguishable from those for oxygen diffusion in simple silicate melts (DUNN, 1982) and for divalent cation diffusion in basaltic melts ( Hofmann, 1980). These results suggest that the principal species contributing to the total diffusivity of oxygen is the oxide anion (O 2−). The isothermal pressure dependence of oxygen diffusion is complex and quite different from that observed for cationic diffusion in silicate melts. All three compositions show a sharp decrease in oxygen diffusivity at approximately the same pressure as the change in the liquidus phase from olivine to pyroxene, but otherwise the pressure dependence can be described by Arrhenius type equations. The equations yield negative activation volumes for the olivine nehpelinite and the alkali basalt. The activation volumes determined for the tholeiite are near zero at low pressure and positive at high pressure. A negative activation volume represents a decrease in the average size of the principal diffusing species. The results of this study are consistent with a melt model which includes both continuous changes in the relative proportions of the various anionic species in the melt with pressure and the occurrence of anionic disproportionation reactions within narrow pressure ranges.