Continuum mechanics simulations of NiO/Ni-YSZ composites during reduction and re-oxidation

Repeated reduction–oxidation (redox) cycles on Ni-based solid oxide fuel cells (SOFC) have been experimentally well investigated and are known to be detrimental to the thermomechanical stability of the composites, especially on anode supported structures. In the present work the mechanistic analysis of the internal factors leading to the dimensional changes and the thermomechanical instability have been addressed, to our knowledge for the first time, using continuum mechanics simulations. The two intertwined percolating phases, YSZ and NiO/Ni, interact and the driving force for the dimensional change arises from the volumetric change related to the phase change NiO ↔ Ni. The measurable change in bulk length is given by the ceramic YSZ backbone as a response to the stress created by the chemical strain. The different subprocesses described in the model for YSZ were elastic and anelastic expansion, diffusional creep, grain boundary sliding (GBS) and microcracking due to excessive stress. In the Ni/NiO phase, nonelastic strains in terms of diffusional and power law creep were implemented, and additionally for NiO deformation due to microcracking and/or pseudoplasticity. Semi-empirical correlations were employed for creep limiting grain growth of Ni and NiO, particle coarsening of Ni and particle growth in NiO during the oxidation. Seven experimental cases of high temperature redox dilatometry were simulated. The model shows good qualitative agreement with the measurements. The different processes of importance for the dimensional behaviour are discussed.