Fast relaxation of stresses in solid oxide cells through reduction. Part I: Macro-stresses in the cell layers

To assess the risk of failure of various components in solid oxide cell (SOC) stacks, the temporal evolution of stresses from sintering and thermal gradients in the operating stacks must be known. In this work it is shown experimentally that the residual stresses in a solid oxide cell are relaxed and, in most cases, go to zero at the point of chemical reduction of the structurally dominant fuel electrode from NiO-YSZ to Ni-YSZ. This is essential for understanding and modeling the stresses during the SOC stack assembly and after. In part I, the in-plane macro-strain and stresses in each layer is determined by in-situ X-ray diffraction at different temperatures and during the chemical reduction. The stresses are also analyzed by a multilayer model of the cell. The relaxation of stresses is explained and attributed to so-called accelerated creep occurring in the nickel phase of fuel electrode.