Nicolas Richet

Thermo-Chemo-Mechanical Modelling of Mixed Conductors in Transient Stages

This study deals with the modelling of the mechanical behaviour accounting for the expansion induced by the oxygen diffusion in MIECs membrane during semi-permeation transient stage. A dedicated model of chemical expansion and its numerical implementation is used to study the relationship between the mechanical stress and the oxygen flux. The impact of the ratio between oxygen bulk diffusion and surface exchange kinetics on mechanical stress in transient stage is discussed. At last, the need of a compromise between the oxygen flux performance and the mechanical reliability is underlined.

Modelling of the oxygen transport through MIEC membrane in transient stage

The transient stage is critical due to the stress induced by the chemical and thermal strain. In order to predict this strain, the oxygen activity field through the membrane needs to be known. Usually, the membrane is divided into three zones: the bulk where diffusion takes place and the two surfaces where exchanges between atmosphere and membrane take place. Oxygen bulk diffusion is well described by the Wagner theory. A consensus has not yet emerged regarding the surface exchange models proposed in the literature. Moreover, these models describe the permanent state, and cannot be extended to the transient stage. A new macroscopic surface exchange model which allows computing transient stage is proposed. This model assumed that the oxygen flux is governed by the association/dissociation of adsorbed oxygen and by the high energetic cost of oxygen reduction/oxidation. Then, the balance of transient specie only present on the surface is introduced to account for these two phenomena. The oxygen activity fields predicted by the proposed model are in agreement with the measures of chemical potential drop between the membrane and the atmosphere in permanent state. Transient stage measured during isothermal expansion test is partially reproduced.

A MACROSCOPIC MODEL OF THE THERMO-CHEMO-MECHANICAL BEHAVIOUR OF MIXED IONIC AND ELECTRONIC CONDUCTORS

This paper suggests a macroscopic model describing the thermo-chemo-mechanical behaviour of ceramic dense membrane for oxygen separation application. This work takes in account to oxygen permeation and strain induced by stoichiometry variation with working conditions. This model, developed within the traditional framework of phenomenological approach, is based on the assumption of strain partitions and requires only three state variables: oxygen activity, temperature and total strain. Oxygen bulk diffusion and surface exchanges are described thanks to the thermodynamic approach developed by Onsager. While many works focused on semi-permeation induced strain, the proposed model also includes the temperature effect on chemical expansion. Strains predicted by the proposed model are validated thanks to experimental test on La0.8Sr0.2Fe0.7Ga0.3O3-δ. Implemented in F.E.A code Abaqus, this model permits studying the design and the process management effects such as chemical shocks on the membrane reliability.