Mechanisms oxide-electrolyte

Various mechanisms for electret effect formation in anodic oxides have been proposed. Lobushkin and co-workers241,242 assumed that it is caused by electrons captured at deep trap levels in oxides. This point of view was supported by Zudov and Zudova.244,250 Mikho and Koleboshin272 postulated that the surface charge of anodic oxides is caused by dissociation of water molecules at the oxide-electrolyte interface and absorption of OH groups. This mechanism was put forward to explain the restoration of the electret effect by UV irradiation of depolarized samples. Parkhutik and Shershulskii62 assumed that the electret effect is caused by the accumulation of incorporated anions into the growing oxide. They based their conclusions on measurements of the kinetics of Us accumulation in anodic oxides and comparative analyses of the kinetics of chemical composition variation of growing oxides. 

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

Scheme 17. Mechanism for Electrolyte Oxidation Coupled with Spinel Disproportionation and Mn + Dissolution...

Scheme 18. Mechanism for Electrolyte Oxidation on a Eully Charged Cathode Surface...

The inorganic membranes had until the late nineties received fairly little attention for applications in gas separation. This has mainly been due to their porous stmcmre, and therefore lack of ability to separate gas molecules. Within the group of inorganic membranes there are however the dense metallic membranes and the solid oxide electrolytes these are discussed separately in Section 4.3.5. With reference to Section 4.2, the possible transport mechanisms taking place in a porous membrane may be summarized as in Table 4.4 below, as well as the ability to separate gases (+) or not (—). Recent findings [29] have however documented that activated Knudsen diffusion may take place also in smaller pores than indicated in the table. 

Furan is, as a 7r-electron rich compound, easily oxidized electrolytically. In acetonitrile the oxidation potential of furan is comparable to that of anisole [35]. In methanol the oxidation is an ECEC mechanism in which 2,5-dihydro-2,5-dimethoxyfuran is formed [166] in the Clauson-Kaas reaction ... 

We have analyzed here a variety of adsorption data obtained while investigating the adsorption at oxide/water vapour interface, and oxide/electrolyte interface. That analysis summarized our extensive research conducted during the past few years, and concerning the model of adsorption on oxide surfaces. Our analysis shows, that only the model of energetically heterogeneous surface can be a proper basis for a successful theoretical description of adsorption at water vapour/oxide, and oxide/electrolyte interfaces. It is also demonstrated, that a simultaneous analysis of adsorption isotherms and heats of adsorption may lead to a new level of understanding the mechanism of adsorption in those systems. 

The point defect model of passivity and its breakdown is a variant of the penetration mechanism [51]. The transport of cations from the metal surface to the oxide-electrolyte interface corresponds to an inward movement of cation vacancies Vm+- This inward transport of Vm+ is supported by their high concentration at... 

A discussion will be given of electronic currents through sandwich structures of the type tantalum (or other substrate metal)/oxide film/metal counterelectrode. The thickness of the oxide film has varied from 25 to 5000 A. The counterelectrode has usually been deposited on the oxide by evaporation, but pressure contacts, mercury droplets, and electroless plating have also been used. The behavior of the system metal/oxide/electrolytic solution is more difficult to interpret and little can be added to a previous article. Even with the simple metal/insulator/metal system there is disagreement about which mechanisms control the current under the various conditions of temperature, thickness, and field. However, recent work has clarified the picture with regard to the choice of mechanisms, and experimental results are beginning to accumulate. Some effects, such as the negative resistance, which has been observed with films which have been subjected to a preliminary breakdown, can be explained only very tentatively. 

Atmospheric corrosion is an electrochemical process and its rate is governed the anodic and cathodic partial reactions taking place at the metal-electrolyte and oxide-electrolyte interfaces. The electrochemical mechanism of atmospheric corrosion resembles that of corrosion in aqueous solution, with two important differences firstly, the corrosion products stay on the surface, rather than being swept away by the electrolyte and, secondly, the electrolyte periodically evaporates during dry periods, then reforms during wet periods, when the metal is exposed to high humidity. 

Chebotin VN, Brainin MI, Soloviova LM, Pakhnutov lA, Lukach YS (1986) About the theory of solid oxide electrolytes electroreduction. Mechanism and kinetics of the process undo-galvanostatic blocking cathode. Elektrokhimiya (Rus) 22 158-163... 

On the basis of these ideas, Macdonald and eo-workers [13,14] developed their model of passivity and its breakdown involving the action of vacancies within the passive layer. It is assumed that cation vacancies migrate from the oxide-electrolyte to the metal-oxide interface, whieh is equivalent to the transport of cations in the opposite direetion. If these vacaneies penetrate into the metal phase at a slower rate than their transport through the oxide, they accumulate at the metal-oxide interface and finally lead to a loeal eoneentration. The related voids lead to stresses within the passive film and its final breakdown. The inward diffusion or migration of eation vacaneies is affeeted by the incorporation of Cl ions at the oxide-electrolyte interface aceording to the following mechanism The concentration c of metal ion V, and vacancies Fq2 are determined by the equihbrium of the Schottky pair formation at the oxide-eleetrolyte interface [Eq. (3)], which causes an inverse dependenee of their eoneentrations [Eq. (4)]. 

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