Fracture test of thin sheet electrolytes for solid oxide fuel cells

The fracture stress of the electrolyte determines the limit of the mechanical integrity of electrolyte supported solid oxide fuel cells. However, the measurement of the fracture stress of thin sheet specimens typically leads to difficulties in experiment and analysis. In particular evaluation problems arise in bending tests due to the non-linear deformation behaviour. In addition to conventional bi-axial bending tests of 6ScSZ and 3YSZ electrolyte foils with a thickness of ∼100 μm a new method is introduced, which relies on ring-on-ring tests with composite specimens. By gluing the thin electrolyte foil on a steel substrate, non-linear deformation of the foils is avoided. The method allows the correlation of the fracture stress with a defined electrolyte portion under maximum stress and hence a prediction of failure stresses and probabilities for real-size cells. The results reveal a higher characteristic fracture stress and Weibull modulus for the 3YSZ foils compared to those of the 6ScSZ variant. In both electrolyte materials, failure of the foils appeared to be related to surface defects.