Dresden 2017 – wissenschaftliches Programm
MM 14: Topical session: Interface-Controlled Microstructures: Mechanical Properties and Mechano-Chemical Coupling - Electro- and mechno-chemical coupling
MM 14.3: Vortrag
Montag, 20. März 2017, 16:15–16:30, BAR 205
Grain boundary diffusion of ions and electrons through a film or scale — •Markus Tautschnig, Nicholas Harrison, and Michael Finnis — Imperial College London, London SW7 2AZ, UK
A model for ionic and electronic grain boundary transport through thin films, scales or membranes with columnar grain structure is introduced. The grain structure is idealized as a lattice of identical hexagonal cells - a honeycomb pattern. Reactions with the environment constitute the boundary conditions and drive the transport between the surfaces. Time-dependent simulations solving the Poisson equation self-consistently with the Nernst-Planck flux equations for the mobile species are performed. The model is used to interpret alumina membrane oxygen permeation experiments. The simulation results provide a complete description of the measurements and insight into the microscopic processes underpinning the oxygen permeation of the membrane. Most notably, the hypothesized transition between p-type and n-type ionic conductivity of the alumina grain boundaries as a function of the applied oxygen gas pressure is observed in the simulations. The range of validity of a simple analytic model for the oxygen permeation rate, similar to the Wagner theory of metal oxidation, is quantified by comparison to the numeric simulations. The coupling between the defect fluxes and the stress state at the grain boundaries is discussed. The three-dimensional model we develop here is readily adaptable to problems such as transport in a solid state electrode, or corrosion scale growth.