Relationship Between Structure And Ionic Conduction Of Grain Boundary In YSZ

Yttria-stabilized zirconia (YSZ) is a widely used electrolyte material in solid oxide fuel cells (SOFCs) due to its high ionic conductivity and stability at high temperature. It has been found out that the ionic conductivity in grain boundaries (GBs) is2to3orders of magnitudes lower than that in grain interior. Using the single crystal YSZ can improve the conductivity to some extent, while it is difficult and expensive to fabricate. Consequently, YSZ for practical application in SOFCs is polycrystalline. In order to analyze the relationship between the detailed structures of the grain boundary and its ionic conductivity, the conductivities of different grain boundaries have been measured, the segregation and distribution of defects and clusters have been analyzed, and the influence of cluster distribution on the conductivities of grain boundaries has been discussed in this paper. Besides, the grain boundaries are described by the coincident site lattice model.Firstly, the resistances per area and activation energies of {111} and {110} twist grain boundaries in YSZ have been compared. It has been found out that the activation energies of {111} twist grain boundaries increase and then decrease with∑value, and the resistances per area increase slower with∑value. While both the activation energies and the resistances per area of the {110} boundaries decrease and then increase.Secondly, the formation energy, segregation energy of oxygen vacancies (Vo"), yttria ions (Yzr’),the formation energy, binding energy and segregation energy of their next near neighbor (Yzr’Vo") and (2Yzr’Vo") clusters, and the difference of segregation energy of (Yzr’Vo’’ and YZr’in the vicinity of {111}∑3,∑7and∑21GBs were calculated, and the distribution of the clusters has been speculated. It has been found out that, both isolated defects and their clusters are easily segregated in the vicinity of GBs, while the defects are most easily segregated in the vicinity of∑3GB, and the distribution of clusters is inhomogeneous. In the vicinity of∑7and∑21GBs, the segregation and distribution of defects and clusters is quite homogeneous. The segregation in∑7GB is easier than in E21GB. Besides, when the yttria ions locate in the coincident sites, the clusters in the vicinity of E7and∑21GBs are more stable.Lastly, according to the results of experiences and calculations, we can guess that three factors should be considered due to their influences on the ionic conductivity of GBs:the mismatch, the dopant segregation and the distribution of clusters. The increasing mismatch can improve the diffusion of oxygen vacancies, the dopant segregation and the gather of clusters in the vicinity of GBs can set back the diffusion of oxygen vacancies, while in the vicinity of {111}twist GBs.the inhomogeneous distribution of clusters Can weaken the hinder of oxygen vacancies.