Charged-defect transport induced stress in mixed ionic-electronic conducting ceramic membranes

Abstract

The electro-chemo-mechanical response within mixed-conducting ceramic membranes is characterized using numerical and analytic solutions. The approach is based on computational modeling using extensions of the Nernst-Planck-Poisson(NPP) formulation. In addition to diffusion and migration contributions, the Nernst-Planck fluxes are extended to incorporate the contribution of hydrostatic-stress gradients. The local stress gradients are the result of atomic-scale strain and distortion within the crystal lattice structure associated with changes in the local stoichiometry of the crystal lattice. This research is generally concerned with strontium-doped lanthanum cobaltites and ferrites (LSCF). The material is a mixed ionic and electronic conductor, with good oxide-ion conduction. An important aspect of the research is to understand the coupled effects of defect transport and stress, especially in oxygen-separation membranes. Varying the environmental oxygen pressure on the membrane surfaces induces transient responses of the charge-carrying-defect concentrations, electrostatic potential, and hydrostatic stress. The material properties for LSCF, including mechanical properties, thermodynamic properties, and charged-defect diffusivities, are based on previously published experimental data. Initially, the stresses that develop on account of chemical expansion are determined using one-dimensional, analytic stress solutions for planar and tubular membranes. The electrochemical and stress analyses are then extended to 2-D, where the NPP and elasticity equations are solved in an electrochemical assembly using the finite element method. The predicted chemically induced stresses are useful for the structural design of membranes and membrane assemblies.