Measuring oxygen diffusion and oxygen surface exchange by conductivity relaxation

For materials that are predominantly electronic conductors and possess a materials property that changes with oxygen partial pressure then a ‘relaxation’ technique may be used to measure oxygen transport behaviour. Perhaps the simplest to implement is electrical conductivity relaxation. For many mixed conducting materials a change in the oxygen partial pressure will result in a corresponding change in the electrical conductivity of the material due to, primarily, a change in the concentration of charge carriers. If the ionic conductivity of a material is significantly less than its electronic conductivity then the time for this change to propagate throughout the material is almost exclusively controlled by the movement of ionic species. This change or ‘relaxation’ can be modelled and oxygen diffusion coefficients and surface exchange coefficients can be extracted from the data. This paper will discuss the measurement of oxygen transport in mixed-conducting ceramic oxides using the conductivity relaxation technique. Emphasis will be placed on the analysis of the ‘relaxation’ profile and the problems associated with determining oxygen diffusion coefficients and oxygen surface exchange coefficients simultaneously.