Relaxation of stresses during reduction of anode supported SOFCs

To assess the reliability of solid oxide fuel cell (SOFC) stacks during operation, the stress field in the stack must be known. During operation the stress field will depend on time as creep processes relax stresses. This work reports further details on a newly discovered creep phenomenon, accelerated creep, taking place during the reduction of a Ni-YSZ anode.

This relaxes stresses at a much higher rate (~×104) than creep during operation. Thus, the phenomenon of accelerated creep during reduction has to be considered both in the production of stacks and in the analysis of the stress field in a stack based on anode supported SOFCs. Accelerated creep has previously been studied in experiments with simultaneous loading and reduction.

The hypothesis for the phenomenon centers around a significant softening of the Ni phase, which amongst other should lead to a significant relaxation of internal stresses in the Ni(O)-YSZ microstructure. The internal residual stresses can be anticipated due the different thermal contractions of the two phases from the sintering temperature to the reduction temperature.

It was thus concluded that with the recorded high creep rates, the stresses in a cell at the time of reduction should decrease significantly over minutes. In this work these internal stresses are measured in-situ before and after the reduction by use of X-ray diffraction. This is done by determining the elastic micro-strains (correlating to the stresses), which are assessed from the widening of the Bragg peaks.

This enables us to determine the stresses in the different phases locally inside the microstructure of the composite Ni(O)-YSZ anode. Furthermore, the residual stresses have been modeled during cool-down from the reduction temperature. The stresses have been assessed by use of a combination of a 3D microstructural reconstruction by FIB-SEM, a microstructural finite element model and analytical homogenization considerations.

A significant decrease of stresses is observed through the reduction as predicted, which partly confirms the hypothesis for the accelerated creep. Also, a significant relaxation of stresses to lower temperatures (~300°C) was also found. This was confirmed by the models, but is however not consistent with previous recorded coefficients of thermal expansion.