Oxygen Chemical Potential Gradient Research Articles

Surface modification of La 0.3Sr 0.7CoO 3− δ ceramic membranes

The dependence of oxygen permeability of dense La 0.3Sr 0.7CoO 3− δ ceramics on membrane thickness indicates significant surface exchange limitations to the permeation fluxes, which suggests a possibility to increase membrane performance by surface activation. The cobaltite membranes with various porous layers applied onto the permeate-side surface were tested at 850–1120 K. Silver-modified La 0.3Sr 0.7CoO 3− δ membranes showed enhanced permeation at temperatures above 950 K; deposition of porous layers of PrO x and Pr 0.7Sr 0.3CoO 3− δ had no positive effect. The maximum oxygen permeability at 850–1120 K was observed in the case of porous La 0.3Sr 0.7CoO 3− δ layers with surface density about 10 mg cm −2. These results suggest that the surface exchange of lanthanum–strontium cobaltite membranes under an oxygen chemical potential gradient is limited by both oxygen sorption at the surface and ion diffusion through the surface oxide layers. Oxygen permeability of La 0.3Sr 0.7CoO 3− δ ceramics was found to increase with increasing grain size due to decreasing grain-boundary resistance to ionic transport.

Oxidation of Tin + 1AlXn ( n = 1 ­ 3 and X = C , N) : I. Model

In this, Part I of a two-part study, a generalized model for the oxidation of the ternary compounds, where and X is carbon and/or nitrogen, is proposed. In all cases, the oxidation products in the 800-1100°C temperature range are rutile in which some Al is dissolved, i.e., where and The oxidation occurs by the inward diffusion of oxygen and the outward diffusion of and ions through the layer. The C and N atoms are presumed to diffuse through the reaction layers and oxidize. The basic premises of the model are that the subjection of the layer to an oxygen chemical potential gradient results in its demixing, with the dissolving into the rutile at the low oxygen partial pressure and its precipitation as at the high partial pressure side. If extensive, the demixing results in the formation of layers of porosity, through which the ions cannot diffuse but the ions can. The resulting microstructures can be highly striated where three layers; an -rich layer, an -rich layer, and a porous layer repeat numerous times. Comparison with previously published results on the oxidation of leaves little doubt that dissolution of the Al in the reaction layer enhances the oxidation kinetics. This is most probably accomplished by an increase in the oxygen vacancy concentration. The fact that the oxide scales are not fully dense is also believed to play an important role in enhancing the oxidation kinetics. © 2001 The Electrochemical Society. All rights reserved.

Analysis of transport phenomena in yttrium-doped zirconia in an oxygen chemical potential gradient and in short-circuit condition—Reply to the discussion from G. Petot-Ervas and C. Petot

It is shown that the essential issue in the controversy related to the nature of the driving force in the case of yttrium-doped zirconia placed between short-circuited reversible electrodes exposed to different chemical potentials is the omission of the potential drop occurring at the metal/solid electrolyte interface due to charges at the interface.

Oxygen Permeation Through Cobalt-Containing Perovskites: Surface Oxygen Exchange vs. Lattice Oxygen Diffusion

The oxygen permeation fluxes from to across cobalt-containing perovskite ceramic membranes and were measured by gas chromatography as functions of oxygen chemical potential gradient, temperature, thickness, and catalytic activity on the surface. Power indexes for uncatalyzed and for were obtained when vs. was plotted as a straight line. The results clearly indicate an overall permeation process controlled by both surface oxygen exchange and bulk oxygen diffusion for uncatalyzed and Application of a thin layer of catalytically active on the feeding-gas surface of under the condition of a fixed atm and a varied not only increases remarkably the overall oxygen flux, but also changes a mixed control to a bulk diffusion control. This enables evaluation of the bulk transport properties of the mixed conductors. A coat of on the permeate side has little catalytic effect, especially at low range, due to the formation of a poorly conducting brownmillerite phase. The results explicitly show a higher activation energy for the surface exchange kinetics than for the ambipolar transport in the mixed conductors. The mechanism of the surface exchange is discussed, and an analytic expression that agrees well with the experimental results is obtained. © 2001 The Electrochemical Society. All rights reserved.

Transport Properties and Thermal Expansion of Sr0.97Ti1−xFexO3−δ (x=0.2–0.8)

Increasing iron concentration in perovskite-type solid solutions Sr0.97Ti1−xFexO3−δ(x=0.2–0.8) was found to decrease the unit cell volume and to increase the partial ionic and electronic conductivities as well as the thermal expansion. The oxygen permeation through Sr0.97(Ti,Fe)O3−δ membranes is limited by both bulk ionic conduction and surface exchange rates. Prolonged stabilization under oxygen chemical potential gradients suggests formation of ordered microdomains in the perovskite lattice. The activation energy for the ionic conductivity, evaluated from oxygen permeation and Faradaic efficiency data, is nearly independent of iron content, varying in the range 97–104 kJ/mol. Thermal expansion coefficients of Sr0.97Ti1−xFex O3−δ (x=0.4–0.8) vary from (11.7–13.8)×10−6 K−1 at temperatures of 300–720 K to (16.6–27.0)×10−6 K−1 at higher temperatures. In comparison with the Sr(Ti, Co)O3−δ system, strontium titanates ferrites possess somewhat poorer transport properties, but lower thermal expansion.

Comparison of Power Densities and Chemical Potential Variation in Solid Oxide Fuel Cells with Multilayer and Single‐Layer Oxide Electrolytes

Several multilayer and single‐layer mixed conducting oxide structures are compared for their use as electrolytes in solid oxide fuel cells. Detailed calculations of the power density characteristics and the variation of the oxygen chemical potential gradient as a function of the external load and thickness of the oxide layers are provided. Engineering implications of the analysis in terms of designing efficient as well as mechanically and chemically stable fuel cells with a layered electrolyte structure are also provided.

Optimization of ionic transport through mixed conducting oxide ceramics

Ceramics exhibiting both electronic and ionic conductivities can be obtained by doping highly oxygen conducting matrices with multivalent cations. These materials allow oxygen transport to occur under open circuit conditions driven by an oxygen chemical potential gradient only. In the present work, equations are developed which express this oxygen flux as a function of environmental and material variables. For small P O 2 gradients, the oxygen flux is maximized when t O 2- = t e- = 0.5. To treat the case of significant P O 2 gradients, a more general analysis has been carried out using a model material system, ZrO 2-Y 2O 3-MO 2, where M is Ce or Ti. The extrinsic electron conductivity due to the multivalent dopant exhibits a maximum when plotted either as function of temperature or P O 2 . As a result, an inflexion point is observed at a critical oxygen pressure, P c O 2 in the curve describing oxygen flux through the material as a function of the environmental P O 2 . Below P c O 2 , the oxygen flux rapidly approaches a saturation value, J max O 2- . Both P c O 2 and J max O 2- are shown to be functions of temperature and material composition. The variation of P O 2 through the material is also discussed.

Modeling of transient and steady-state demixing of oxide solid solutions in an oxygen chemical potential gradient

A numerical technique for predicting the kinetics of demixing of oxide solid solutions in an oxygen chemical potential gradient is presented. The unique advantage of this technique is that it permits handling of transient state conditions, variable diffusion coefficients, and variable boundary conditions. Solutions obtained by this technique agree very well with the steady-state kinetics predicted by analytical methods.

Morphological changes on growing cobaltous oxide surface exposed to an oxygen chemical potential gradient

Morphological changes on growing cobaltous oxide surface exposed to an oxygen chemical potential gradient

Determination of Transport Properties of Alumina Oxide Scale

By measuring the potential difference between the two interfaces of an alumina scale developed on a FeCrAl alloy, or by applying an electric field during its growth, values of ti, σi, and σe along with their temperature dependence were determined. A residual potential, V0, which was measured between the two oxide interfaces, is related to the oxygen chemical potential gradient on the scale. Alumina growth is controlled by oxygenion diffusion (by either V″ or O″i), and the transport properties are modified by the amount and distribution of impurities.