Empa Activities 09/10 Information, Reliability and Simulation Technology
Non-destructive insight into SOFCs and materials Visual testing and light microscopy are the dominant methods for assessing fuel cells before assembly to stacks. But these techniques cannot unveil all the critical defects in the cells. While thermography allows a fast screening for hidden flaws, X-ray radiography and local tomography deliver high resolution images of the defects. The X-ray based methods can be applied to compare defect areas before and after durability tests in order to evaluate their criticality.
Peter Wyss, Erwin Hack, Peter Holtappels, in collaboration with HTCeramics (CH)
A key requirement for the solid oxide fuel cell (SOFC) technology is the understanding of the degradation and ageing phenomena. Non-destructive evaluation (NDE) methods can assist in identifying “hidden defects” in cells before assembly. Degradation can then be related to structural changes in the cell and the defect areas. A combination of thermography, radiography and tomography was applied to identify pores and other irregularities in cells from HTceramix SA with a size of 80 x 80 x 0.25 mm3. We performed impulse thermography in transmission using an InSb focal plane array camera (Cedip Jade III MW) with a lateral resolution of better than 0.7 mm. X-ray scanning over an angular range was used to explore the detection limit. We used ±60º for local tomography (panoramic tube VISCOM TEP 9225) and ±20º for radiography (sub micrometer spot size tube VISCOM TXD 9160). Filtering of radiographic scans equalizes the average density over all angles and enhances the visibility of small flaws. The thermographic overview indicated a spot of high thermal conductivity in one cell (Fig. 1). High resolution radiography scans at different angles (Fig. 2)
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Fig. 2: Local radiography. Defects are located in the electrolyte (A) and in the anode layer (B).
show voids and cracks in the electrolyte (A) and dense spots in the anode layer (B). Local tomography (Fig. 3) separates these flaws in the depth of the cell. The flaw detection limits were 40 µm for local tomography and 20 µm for radiography.
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Fig. 3: Local tomography. Volume slices at two depths from surface.
NDE techniques can be used to characterize significant defects and flaws without destruction of the specimen. They allow pre- and post-test analysis and thus can contribute to the understanding of degradation and durability. The application in quality control and production is still considered limited, since a single technique hardly reveals all desired information at the required resolution.
Support: EU-IP Real-SOFC, SBF Links: www.empa.ch/abt173 > X-ray, ultrasound, imaging
Fig.1: Thermography, fast overview. Left: immediately, right: 20 msec after flash. White frame: X-ray field of view in Figures 2 and 3. Contact: erwin.hack@empa.ch Reference: P. Wyss, E. Hack, P. Holtappels, Fuel Cells, Vol. 09, 907– 910 (2009)
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