Multi-Scale Study On The Mechanical Behavior Of Solid Oxide Fuel Cell Electrolyte Under The Coupled Mechanical And Electrochemical Field

The contradiction between the combustion of fossil fuels and environmental protection makes human beings face the dual challenges of economic and social sustainable development.Solid oxide fuel cells(SOFC s)have been widely concerned for their unique advantages of high energy conversion and low pollution.As one of the core components of the SOFCs,the performance of the solid electrolyte greatly determines the performance,service life and commercialization process.High concentration of oxygen vacancy is the key of high ionic conductivity of solid electrolyte.However,as a structural defect,excessive oxygen vacancy accumulation inevitably affects the mechanical strength of electrolyte.Especially for the electrolyte with crack defects in complex working environment,the stress concentration at the crack tip and the multiple physical fields coupling effect will cause further aggregation of oxygen vacancy defects,which affects the strength and toughness of electrolyte materials.In this paper,the mechanical behaviors of gadolinia-doped ceria(GDC)electrolyte at micro scale and macro scale are studied by using molecular dynamics and multi-scale methods.The influence of multiple field coupling effect on fracture toughness of GDC electrolyte is systematically analyzed.The main contents of this paper are as follows:Firstly,the basic theory and algorithms of molecular dynamics are described.The anisotropic deformation behavior of GDC solid electrolytes is investigated by the molecular dynamics method.When the uniaxial tensile is loaded on GDC,different phase transformation and fracture mechanisms are found in the different crystal orientations.The influence of temperature,loading rate and doping concentration on the phase transformation and fracture behavior in the different crystal orientations is further explored,which is in good agreement with the experimental results.Subsequently,the mechanical behavior of non-stoichiometric GDC electrolyte under working conditions is simulated by using molecular dynamics method.The influence of non-stoichiometry and oxygen vacancy defects on the strength and phase transformation in the different crystal orientations is explored.Beyond that,the linear chemical strain and coefficient of compositional expansion(CCE)are calculated and compared with the experimental results.Moreover,the effects of electric field on the mechanical and electrical properties of GDC electrolyte under electromechanical coupling loading are investigated.These results and reaction mechanism can provide theoretical guidance for the manufacture and use of electrolytes.Secondly,the multi-scale simulation method combined with molecular dynamics and finite element method is used to study the fracture behaviors of GDC electrolyte.By this multi-scale method,the actual load can be transferred from the finite element nodes to the molecular dynamics region,which can not only expand the scope of simulation,but also save computing resources,and has higher authenticity and reliability.The molecular dynamics method and the multi-scale method are used to study the mechanism of crack propagation in the different crystal orientations.The influence of oxygen vacancy defects and crack defects on the strength of the material is explored,and the phenomenon of stress-induced phase transformation at the crack tip is found.Finally,based on the electrochemomechanical coupling theory,the defect diffusion equation of finite element form is derived,and the influence of the stress field at the crack tip on the non-stoichiometry effect is calculated in detail.Combined with the multi-scale calculation method,a multi-scale calculation model that connects the macro and micro under multi-field coupling is established.The influence of multi-field coupling effects on the fracture toughness of GDC electrolyte under different loads and different oxygen partial pressures is calculated in details.These results lay a solid foundation for the performance degradation,failure and fracture analysis of GDC electrolyte,and even for the whole fuel cell SOFCs system.