Strength degradation and failure limits of dense and porous ceramic membrane materials
Introduction
The production of oxygen by the use of ceramic mixed ionic–electronic conducting membranes at high temperatures1 can be an interesting cost-effective alternative to the cryogenic methods. Two possible industrial applications for such membranes are oxygen supply in Oxy-fuel power plants2 and the integration in catalytic membrane reactors.3, 4, 5, 6 The most promising membrane material due to its high oxygen permeation rates7, 8 is Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF). Due to its chemical instability against CO2, it can only be used in an Oxy-fuel process without flue gas recirculation, in a three-end concept where no contact between the flue gas and the membrane occurs and there is no presence of CO2.9 La0.6−xSr0.4Co0.2Fe0.8O3-δ (LSCF) has lower permeation rates than BSCF10, but is expected to be stable under CO2 containing atmospheres.11 Recent reports have also shown the potential of oxygen separation by monolithic doped/multidoped ceria membranes.12, 13, 14 In particular, gadolinium doped ceria (Ce0.9Gd0.1O1.95−δ, CGO) was suggested as oxygen transport material for syngas application.15
With respect to design, thin membrane layers supported by a porous substrate (Fig. 1) are considered as the most efficient solutions for such air separation units.16 The porous substrate should provide mechanical stability of the entire membrane structure, since the application-related loadings (due to thermal or chemical strains, pressure gradients between feed and permeate side, etc.) cannot be sustained by the thin membrane layer. In certain three-end membrane design solutions the ends of tubes exhibit temperatures close to room temperature (RT).17, 18
Long-term reliability of the component does not only depend on its initial strength, but also on strength degradation effects such as subcritical crack growth (SCG).19, 20 The associated correlation between stress rate and strength is important to evaluate the effect of thermally induced stresses at the beginning of the start-up and at the end of the shut-down process of the membrane unit and can be further used as a basis for a strength–probability–time (SPT) lifetime prediction for a cold-end of the tube and as an outlook basis to assess the failure relevance of transient and service stages. Stress–probability–time (SPT) diagrams21 incorporate the time dependence of strength into failure statistics.
The aim of the current study was to determine and compare elastic and fracture properties, to assess the sensitivity to subcritical crack growth, and based on those data to predict mechanical limits for application relevant membrane lifetimes for critical membrane parts exhibiting temperatures close to RT. The creep behavior, which is assumed to be the failure relevant criterion for the tube part exhibiting elevated temperatures, was not considered in this study.22, 23 The porosity is known to have a large effect on the mechanical properties and is therefore taken into account.
Section snippets
Fracture statistics
The statistical distribution and hence the failure probability as a function of stress is commonly described by a two-parameter Weibull distribution24, 25 for brittle materials:where σ0 and m are constants, termed characteristic strength and Weibull modulus, respectively. The failure probability is related to the tested effective volume or area, depending if volume or surface defects are failure relevant. The correlation for volume defects can be described using the relationship
BSCF specimens
A detailed description of the preparation of the dense Ba0.5Sr0.5Co0.8Fe0.2O3−δ specimens can be found in.22 Porous disc-shaped BSCF specimens were produced by tape casting. The powders, supplied by Treibacher Industrie AG Austria, were mixed with 20 wt% organic pore former (corn starch) and different volatile additives to form homogenous slurry. Afterwards they were tape cast using the doctor blade technique32 with a blade gap of 1.9 mm, and dried in ambient air. The discs were cut using a
Microstructure
Graphical analyses of porosity and grain size are compiled in Table 2. The grain sizes of porous specimens are assumed to be similar to their dense counterparts.
In case of dense BSCF (Fig. 3), average porosity and grain size yield similar values as reported by Huang et al.,38 who analyzed identically produced dense material. They reported 4.5% and 10 ± 4 μm for porosity and grain size, respectively. The microstructural investigation revealed a rather inhomogeneous porosity of the dense LSCF
Conclusions
Porous and dense BSCF, LSCF and CGO ceramics were investigated since they appear to be promising materials for oxygen transport membrane application. These materials were characterized with respect to their mechanical behavior and failure limits at room temperature, and the results can serve as their direct comparison. For a precise lifetime prediction, actual stresses which appear in the membrane should be the aim of further investigations. Elastic modulus of the materials investigated
Acknowledgements
The investigations presented in this work are funded by the European Union through the FP7 NASA-OTM Project (NMP3-SL-2009-228701) and by the Federal Ministry of Economics and Technology via the MEM-OXYCOAL Project (grant no. 0327803). The authors would like to express their gratitude to Ms. T. Osipova, Mr. R. Küppers and Mr. J. Mönch for the experimental support, and Dr. B. Rutkowski for scientific discussions.
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