Strength degradation and failure limits of dense and porous ceramic membrane materials

Thin dense membrane layers, mechanically supported by porous substrates, are considered as the most efficient designs for oxygen supply units used in Oxy-fuel processes and membrane reactors. Based on the favorable permeation properties and chemical stability, several materials were suggested as promising membrane and substrate materials: Ba0.5Sr0.5Co0.8Fe0.2O3−δ, La0.6−xSr0.4Co0.2Fe0.8O3−δ (x = 0, 0.02) and Ce0.9Gd0.1O1.95−δ. Although membranes operate at elevated temperatures, the ends of tubes in certain three-end concepts remain almost at room temperature. The current work concentrates on the failure potential of these membrane parts, where in a complex device also the highest residual stresses should arise due to differences in thermal expansion. In particular, sensitivity of the materials to subcritical crack growth was assessed since the long-term reliability of the component does not only depend on its initial strength, but also on strength degradation effects. The results were subsequently used as a basis for a strength–probability–time lifetime prediction.