| dc.description.abstract | Solid oxide fuel cell (SOFC) usually operates at a high temperature with a good fuel flexibility and a high efficiency. However, the coefficient of thermal expansion mismatch between ceramic electrodes and metallic components under this high temperature gives rise to a large amount of stresses in the cell stack. In order to predict the durability and reliability of a SOFC stack, a comprehensive three-dimensional finite element analysis (FEA) model based on a stack design being developed at the Institute of Nuclear Energy Research (INER) was constructed. The model was designed for using mica sealant in a compressive sealing design. The first objective of this study is using FEA to calculate the thermal stress distribution in a three-cell planar SOFC stack with a compressive mica sealing design under cyclic thermal loading and to investigate the effects of the applied assembly load on the stress distribution. Simulation results indicate that an applied compressive load of 0.6 MPa could eliminate the bending deformation of the PEN and frame leading to a well joined structure. For a greater applied load, the critical stresses in the glass-ceramic and mica sealants were increased. The glass-ceramic and mica sealants might fail for an applied compressive load of 6 MPa. In this regard, a 0.6 MPa compressive load might be an optimal assembly load. When the SOFC stack was subjected to cyclic thermal loading, the stress accumulation behavior was observed at the inner corners of the frames. The critical stress in the metallic interconnect/frame was increased with increasing number of operating cycles. However, the critical stresses in the PEN, nickel mesh, mica gasket, and glass-ceramic sealant barely changed with cycle number.
The second objective of the current study is to make a comparison of the stress distributions between two sealing designs, namely the rigid and compressive seals. Changing a rigid type of glass-ceramic sealant to a compressive type of mica gasket would influence the stress distribution, especially in the PENs. The critical stress in the PEN was decreased at room temperature but significantly increased at operation temperature. Such difference in the stress distribution could be ascribed by the difference in the constrained conditions at the interface of connecting components under various sealing designs.
The third objective of this work is to assess the structural reliability of the PEN subjected to such a high thermal stress by measuring the flexural strength of two anode materials, namely Slip-41 and Slip-48. The Slip-48 anode material has a higher flexural strength than does the Slip-41 anode material. The difference in the strength might be caused by their different porosity. Fractography analysis results indicate the Slip-48 anode has a smoother fracture surface and a less amount of defects, compared to the Slip-41. Both anode materials have no obvious temperature dependence on the flexural strength. By importing the mechanical properties of the Slip-48 anode material into stress analysis, the critical stress in the PENs at steady-operation stage would exceed its strength. Therefore, the strength of the tested anode materials needs to be improved for future use in the given SOFC stack design. | en_US |