Rational design of SOFC cathodes with hierarchical porosity (coll. LEPMI) (a) FIB-SEM tomography of cathodes films obtained by Electro Spray Deposition (top: LSCF film, bottom: 60:40 %vol LSCF:CGO film). (b) FEM model of a nanoporous column. The model accounts for oxygen surface exchange (at the column surface and in the nanopores) and for oxygen bulk diffusion. The model compares favorably with Electrochemical Impedance Spectroscopy measurements on the cathodes characterized by FIB-SEM tomography. (c) Design guidelines from the FEM model to tune the CGO and nanoporosity content (T=500°C / average nanopore size 60nm / 15% macroporosity). Experimentally, an Area Specific Resistance (ASR) of 0.35 ohm.cm² was achieved at 550°C by the 60:40 % LSCF:CGO film. Performance could be pushed further by increasing further the SOFC content but unfortunately such films lacked mechanical adhesion with electrolyte.
Strength of hierarchically porous ceramics : X-ray nanotomography and discrete element simulations (coll. University of Washington)
(a) SEM top view of a freeze-cast sample (plane perpendicular to the freeze-casting direction) showing colonies of pores aligned with one another. (b-d) Generation and utilization of a numerical microstructure obtained from a nanotomagraphy reconstruction with the discrete element method. (b) X-ray nanotomography of a freeze-cast sample (c) Discrete microstructure matched with the 3D image. (d) Simulation of a longitudinal crushing test showing the displacement of particles in the (x) direction, transverse to the loading axis (z). From Roussel, D. Strength of hierarchically porous ceramics: discrete simulations on X-ray nanotomography images, accepted for publication in Scripta Mat (2015).
Functional properties of LSM/YSZ porous SOFC cathodes by FIB-SEM tomography (coll. University of Washington)
3D representations of the phase connectivities. At the higher sintering temperature, the LSM increases its connectivity while the pore phases becomes markedly less connected. The YSZ phase is fully connected for both samples and appears to coarsen at the higher sintering temperature. From Lichtner, A. Z. et al. Dispersion, connectivity and tortuosity of hierarchical porosity composite SOFC cathodes prepared by freeze-casting. J. Eur. Ceram. Soc. 35, 585–595 (2015).
Toughness of porous ceramics : discrete element simulation
Evolution of the number of brocken bonds during a numerical tensile test of a pre-cracked plate. The sample shown has a relative density of 0.72. The top right image shows the localized damage occurring ahead of the crack-tip just before failure of the sample. From Jauffres, D., Martin, C. L., Lichtner, A. & Bordia, R. K. Simulation of the toughness of partially sintered ceramics with realistic microstructures. Acta Mater. 60, 4685 (2012).
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