The Crack Growth Behavior In Solid Electrolyte Membrane Under Mechanical-electrochemical Loading

With the increasingly severe environmental pollution and energy shortage, SolidOxide Fuel Cells (SOFCs) have attracted extensive attention for their high powerefficiency and low pollution. As the key component, electrolyte is especially thefocus of research. The conductivity of the electrolyte is due to the presence of theoxygen vacancies in the material. In the complicated coupled-fields, the mechanicalproperties of the electrolyte are inevitably affected by oxygen vacancies. Especiallyfor the electrolyte with crack, the distribution of oxygen vacancies would be affected,and then the fracture toughness of the material. In the thesis, molecular dynamics(MD) simulation and multi-scale method are introduced to study the mechanicalproperties of GDC (gadolinia-doped ceria) systematically, and the main contents areincluded as follows:First, the basic theory and algorithms of molecular dynamics are described, andthe potentials of GDC are presented. The diffusion behaviors of oxygen ions in theelectrolyte are studied by using md simulations. According to the simulated resluts,the diffusivity of oxygen ions is influenced obviously by temperature, dopingconcentration, external load and grain boundary, meanwhile, these specialphenomenons are explained from micromorphology.Secondly, the deformation behaviors of GDC under uniaxial tensile loading aresimulated by means of md method. The phase transformations of GDC are observedduring the tension process by analyzing the crystal configurations. On this basis, theeffects of temperature and doping concentration on Young’s modulus and the fracturestrength of GDC are investigated, the simulated and experimental results agree well.Then, the GDC model with crack is studied by md simulation. Based on thesimulated results, the fracture mode and distribution of stress field are similar to themacroscopic fracture. During the whole process, localized plastic deformation anddislocation emission can be observed at the early stage, but the failure modes ofdifferent models are all brittle fracture. Besides, the effect of doping concentration onthe fracture behavior is studied.Finally, the mechanical-electrochemical coupled field is introduced. The crackpropagation behavior of non-stoichiometry GDC is studied by multi-scale method.The concentration of oxygen vacancies caused by non-stoichiometry effect increaseswith the gradient of oxygen partial pressure increases. Moreover, the coupling effectbetween the stress field near crack tip and concentration of oxygen vacancies isanalysed, and the fracture toughness of GDC is calculated with the coupling effect.