Slow Crack Growth Analyses of Oxygen Transport Ceramic Membranes

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1023-JJ08-03

Slow Crack Growth Analyses of Oxygen Transport Ceramic Membranes Nagendra Nagabhushana1, Jing Zhang2, Thangamani Nithyanantham1, and Sukumar Bandopadhyay1 1 Department of Mining and Geological Engineering, University of Alaska Fairbanks, Fairbanks, AK, 99775 2 Department of Mechanical Engineering, University of Alaska Fairbanks, Fairbanks, AK, 99775 ABSTRACT Perovskite-type oxides are promising materials with potential use as dense membranes for oxygen separations. We herein report slow crack growth (SCG) studies of La0.2Sr0.8Fe0.8Cr0.2O3-δ (LSFCO) perovskite membranes as Oxygen Transport Membranes (OTM). Two sample batches of perovskite were tested to investigate the effect of temperature, specific chemical environments, and loading rate on flexure strengths, using four-point bending tests. The first batch was examined at room temperature in air. The second batch was soaked in a N2/air atmosphere at 1000oC for 1 hour prior to application of load. Loading rates varied from 0.00005 mm/s to 0.01 mm/s. Flexural strength data indicate that the examined OTM material showed little susceptibility to SCG at room temperature in air. However, the sample is susceptible to SCG in a N2/air environment at 1000 oC. Also, the experiments demonstrate flexural-strength rate dependency, with strength increasing with the loading rate. The observed phenomena are explained by the decomposition and microstructural transitions in the perovskite. The results provide important information about OTM mechanical degradation, which is valuable for future OTM design applications. INTRODUCTION Perovskites have attracted considerable attention for use as electrodes in high temperature solid oxide fuel cells, oxygen separators, and catalytic membrane reactors for conversion of methane to syngas [1, 2]. So far, much of the available literature has focused on membrane synthesis and characterization. There are a few reports on the mechanical properties of the OTM membranes. Lee et al.[3] investigated the fracture strength of La0.6Sr0.4Co0.2Fe0.8O3-δ using fourpoint bend tests. The fracture strength of the membrane increased with sintering temperature or density. Chou et al. [4] measured the biaxial flexure strength of La1-xSrxCo0.2Fe0.8O3 (x = 0.2ñ 0.8). The strength depended on strontium amount, dropping from 160 MPa (at lower strontium content of 0.2 and 0.4) to 40 MPa (higher strontium content of 0.6 and 0.8). Overall, experimental data on the strength of these oxides are still relatively few. In this paper, we study the slow crack growth (SCG) of La0.2Sr0.8Fe0.8Cr0.2O3-δ (LSFCO) membranes as a function of temperature, environment and stress rate. These membranes are potential candidate materials for the Oxygen Transport Membranes (OTM). The perovskites are evaluated for strength and reliability through four-point bending tests. The fracture surfaces are analyzed by X-ray diffraction (XRD) and Scanning Electron Microscope (SEM). A finite element model (FEM) is also developed to study the stress distributions during bending test. The ob