Research On Interionic Potential Models And Their Applications In Solid Electrolyte GDC

With the aggravation of the energy crisis, solid oxide fuel cell (SOFC) becomes the hotspot in the new energy field. As the key component of SOFC, the performance of electrolyte determines the life and application of SOFC. It is significant to research the mechanical properties of SOFC at atom scale by molecular dynamic (MD) simulations.In this thesis, the ab initio pairwise potential in a concise function form is first proposed for ionic solids via quantum physical and quantum chemical calculations. Then, the pair potential of gadolinia-doped ceria (GDC) systems is derived. A bunch of MD simulation results prove the universal applications and portability of the newly proposed potentials. After that, empirical triplet-interactions are introduced by the violation of the elastic constants between pair potential and experimental data. Then the many-body interactions of CaO and MgO are evaluated based on this technique. Finally, combining the MD simulations with triplet-potential of CeO2 and the couple electrochemomechanical theory, the distributions of the vacancy, stress, and electrostatic potential crossing the electrolyte thickness are investigated versus various applied voltage and other adjustable parameters in detail. The main contents of the thesis are included as follows.First of all, the fundamental principle and main algorithms of MD method are illustrated, such as the boundary conditions, the calculation techniques of long-range, numerical integrator, etc. With these key components, current author develops the MD software and proposes some algorithms to improve the computational efficiency. The neighbor list algorithm for a parallelepiped box, in terms of the range of the interactions between atoms, can be adopted in NV and NP ensembles universally. In addition, a kind of fast MD technique combining the advantages of discrete potential function and interpolation is proposed to reduce the calculation cost. Based on the effective methods described above, parallel large-scale calculation of MD simulation becomes possible.Following reliable MD simulation software, a method combining the stress and strain fluctuation formula with the elastic bath method is proposed to calculate the elastic constants. The significance is based on one of the benefits of elastic bath method, where the thermal strains of materials can be amplified or attenuated by appropriate choice of elastic constants of the bath. Results indicate that the combination method shows much better convergence than its component technique separately.In MD simulations, the interionic potential is crucial to describe the structure properties, therefore a novel method to derive the ab initio pairwise interionic potentials for ionic solids. Based on the Chen-Mobius lattice inversion, the pairwise interactions between cations and anions can be evaluated from multiple virtual structures, while the quantum chemistry technique is adopted to derive the short-range potentials for uniform ions. For solid solution GDC system, the virtual structures of CeO2 and A2O3 (A=Ce, Gd) are provided to eliminate the influences between uniform ions. A series of pseudopotential total energies are obtained via ab initio calculations. Then the interionic pair potentials are determined by lattice inversion. The quality of the proposed potentials are verified by MD simulations on their static properties, doped concentrations and temperature dependence of lattice constants, mean-square displacements, pair correlation functions. Results are consistent with corresponding experimental data, showing that the new form is valid over a wide range of interionic separations.Then, some empirical three-body interactions are introduced to compensate the defects of purely pairwise potentials, such as unphysical Cauchy relation. Therefore the newly proposed potentials combining the advantages of both ab initio calculations and empirical fitting process can be regarded as the semi-empirical potentials, which are applicable for describing the structure properties of ionic solids. Besides, such potential can be employed to evaluate the evolution behavior of solid solution due to the uniform potential of the same species ions. Utilized the framework developed above, the many-body potentials for typical ionic solids, CaO and MgO are derived. The pair interionic potentials are first determined via ab initio calculations. Then the empirical three-body interactions are adopted to modify the mechanical properties from the violation between purely pairwise potential and experiment data. The static properties are in good agreement with experimental data. And the MD simulations results of pressure and temperature dependence of lattice constants and elastic constants are consistent with corresponding experimental data and ab initio calculation. Furthermore, the phase transition pressures of CaO and MgO are 68 and 400GPa, respectively, coinciding with the measure values. All of these prove the validity and reliability of the proposed potentials. In addition, the three-body interactions of CeO2 are provided to describe the mechanical properties accurately.According to the couple electrochemomechanical theory, current author extends the applications with adjustable elastic modulus and constant strain cases. Non-stoichiometry produces volumetric expansion. Such volumetric changes are not accommodated by appropriate deformation, stresses are generated. On one hand, non-stoichiometry induces compositional stresses. On the other hand, such stresses also affect the defect distribution. The coupling is significant under working conditions as solid electrolyte in solid oxide fuel cell. With the aid of MD simulations and valid potentials, the major parameters of couple theory, such as the coefficient of compositional expansion and elastic constants versus variable oxygen vacancy, are determined. The calculation results of planar GDC are taken as an example to show the distributions of the vacancy, stress, and electrostatic potential crossing the electrolyte thickness for different applied voltage and other adjustable parameters in detail. Strong coupling effects are detected in the case of the existence of alterable Young's modulus and constant strains. Such consequences establish solid theory foundation for the further investigation of the crack propagation and life analysis.