Mixed ionic-electronic conductive (MIEC) perovskite material La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF6428) is a promising candidate for the cathode in intermediate temperature solid oxide fuel cells (IT-SOFCs). Understanding the three dimensional (3D) microstructural characteristics of such a material is crucial to its application because they predominately determine the performance and durability of the porous cathodes and hence of the SOFCs. They affect the overall cathode kinetics and thus the electrochemical reaction efficiency, as well as the mechanical properties, which dominate the lifetime of SOFCs. It is necessary to balance the trade-off between the electrochemical performance, which is improved by high porosity and minimal sintering, and the ability to withstand mechanical constraints, which is improved by the opposite. To date LSCF6428 has been widely investigated on subjects of microstructure-related electrochemical performance, while little work has been reported on the mechanical properties and their correlation with the 3D microstructures. The main purpose of this research was to study the mechanical properties (i.e. elastic modulus, hardness and fracture toughness) of LSCF6428 cathode films and bulk samples fabricated by high temperature sintering, and to evaluate the effect of 3D microstructural parameters on elastic modulus, and the Poisson's ratio where applicable, by means of both experimental and numerical methods. Room-temperature mechanical properties were investigated by nanoindentation of porous bulk samples and porous films sintered at temperatures from 900 to 1200 °C. A spherical indenter was used so that the contact area was much greater than the scale of the porous microstructure. The elastic modulus of the bulk samples was found to increase from 33.8 to 174.3 GPa and hardness from 0.64 to 5.32 GPa as the porosity decreased from 45 to 5 vol% after sintering at 900 to 1200 °C. Densification under the indenter was found to have little influence on the measured elastic modulus. The residual porosity in the nominally dense sample was found to account for the discrepancy between the elastic moduli measured by nanoindentation and by impulse excitation. Based on the optimisation of a commercial LSCF6428 ink formulation, crack-free films of acceptable surface roughness for indentation were also prepared by sintering at 900 to 1200 °C. It was shown that reliable measurements of the true properties of the films were obtained by data extrapolation provided that the effects from both surface roughness and substrate were minimised to neglected levels within a certain range of indentation depth to film thickness ratio (which was 0.1 to 0.2 in this study).The elastic moduli of the films and bulk materials were approximately equal for a given porosity. Based on the crack length measurements for Berkovich-indented samples, the fracture toughnesses of bulk LSCF6428 were determined to increase from 0.51 to 0.99 MPa·m1/2, after sintering at 900 to 1200 °C. The microstructures of films before and after indentation were characterised using FIB/SEM slice and view technique and the actual 3D microstructure models of the porous films were reconstructed based on the tomographic data obtained. Finite element modelling of the elastic modulus of the resulting microstructures showed excellent agreement with the nanoindentation results. The 3D microstructures were numerically modified at constant porosity by applying a cellular automaton algorithm based method, so that the influence on elastic modulus of factors other than porosity could be evaluated. It was found that the heterogeneity of the pore structure has a significant influence on the elastic properties computed using mechanical simulation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:656576 |
| Date | January 2014 |
| Creators | Chen, Zhangwei |
| Contributors | Atkinson, Alan; Giuliani, Finn |
| Publisher | Imperial College London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10044/1/24705 |
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