The addition of reactive elements can have a significant effect on the oxidation behavior of alumina- and chromia-forming alloys. A model has been developed to explain the effects associated with the addition of reactive elements that is based on the segregation of reactive-element ions to scale grain boundaries and the metal-oxide interface. Reactive-element ions use these interaces as pathways for diffusion from the metal substrate to the gas interface of the scale. The driving force for this outward diffusion is the oxygen potential gradient across the scale. Doping of the scale grain boundaries results in scale growth primarily by inward oxygen diffusion, while doping at the metal-oxide interface slows the growth of interfacial voids and thus improves scale adhesion.

Experimental observations in support of the dynamic-segregation theory to explain the reactive-element effect

Introduction, H.J.M. Bouwmeester and P.J. Gellings Principles of Electrochemistry, H. Gerischer Solid State Background, I. Abrahams and P.G. Bruce Interface Electrical Phenomena in Ionic Solids, J. Nowotny Defect Chemistry in Solid State Electrochemistry, J. Schoonman Survey of Types of Solid Electrolytes, T. Kudo Electrochemistry of Mixed Ionic-Electronic Conductors, I. Riess Electrodics, I. Riess and J. Schoonman Principles of Main Experimental Methods, W. Weppner Electrochemical Sensors, P. Fabry and E. Siebert Solid State Batteries, C. Julien Solid Oxide Fuel Cells, A. Hammou and J. Guindet Electrocatalysis and Electrochemical Reactors, C.G. Vayenas and S. Bebelis Dense Ceramic Membranes for Oxygen Separation, H.J.M. Bouwmeester and A.J. Burggraaf Corrosion Studies, J.H.W. de Wit and T. Fransen Electrochromism and Electrochromic Devices, G.C. Granqvist Index

The CRC handbook of solid state electrochemistry

Dense ceramic membranes for methane conversion

The kinetics of stoichiometry change of an oxide--a prototype of a simple solid-state reaction and a process of substantial technological relevance--is studied and analyzed in great detail. Oxygen incorporation into strontium titanate was chosen as a model process. The complete reaction can be phenomenologically and mechanistically understood beginning with the surface reaction and ending with the transport in the perovskite. Key elements are a detailed knowledge of the defect chemistry of the perovskite as well as the application of a variety of experimental and theoretical tools, many of them evolving from this study. The importance of the reaction and transport steps for (electro)chemical applications is emphasized.

How is oxygen incorporated into oxides? A comprehensive kinetic study of a simple solid-state reaction with SrTiO3 as a model material.

Publisher Summary This chapter reviews the recent developments in the area of mixed ionic-electronic conducting membranes for oxygen separation, in which the membrane material is made dense—that is, free of cracks and connected-through porosity, being susceptible only for oxygen ionic and electronic transport. Emphasis is on the defect chemistry, mass transport, and the associated surface exchange kinetics. The basic elements of mixed ionic and electronic transport through dense ceramic membranes are focused. The chapter discusses mixed-conducting acceptor-doped perovskite and perovskite-related oxides and gives examples to illustrate the fundamental factors determining the oxygen fluxes through dense ceramic membranes. A key factor in the possible application of oxygen ion conducting ceramics is that, for use as solid electrolyte in fuel cells, batteries, oxygen pumps or sensors, their electronic transport number should be as low as possible. Stimulated by the search for candidate materials for electrodes in solid oxide fuel cells (SOFC) and oxygen separation membranes, researchers have explored the possibility of introducing electronic conductivity in oxygen-ion conducting fluorite-type matrices by doping with multi-valent dopants.

Chapter 10 Dense ceramic membranes for oxygen separation

The steady-state demixing of initially homogeneous oxides in an oxygen potential gradient is studied theoretically and experimentally. In ternary oxides which are stable (i.e., not oxidized or reduced) in the oxygen potential range in question, the crystal between two different oxygen potentials is shifted with respect to the oxygen lattice system. If DA> DB≫ DO, the crystal is normally enriched in AO at the side of the higher oxygen potential. Solutions of the transport problem are presented and the predicted effects are verified experimentally. The following systems are studied: Complete oxide solid solutions (A, B)O; A-B-O systems with limited solid solution ranges and miscibility gaps; A-B-O systems with compound (e.g., spinel) formation.

H. Schmalzried

Papers

Multicomponent Oxides in Oxygen Potential Gradients