Molecular dynamics study of oxygen-ion diffusion in yttria-stabilized zirconia grain boundaries

This work focuses on understanding the oxygen-ion transport through the mixed Grain Boundaries (GBs) present in yttria-stabilized zirconia (YSZ), a common solid oxide fuel cells (SOFCs) electrolyte. The mixed GBs, having a disordered arrangement of atoms due to the merging of crystalline facets of the two adjacent grains, were found to be the most prominent in experimentally prepared samples. In this work, all GBs were constructed using the amorphization and recrystallization technique implemented via molecular dynamics. An in-depth study was conducted for a mixed GB present in our 8YSZ sample, formed by the planes (-511) and (21-1). Oxygen-ion self-diffusion was studied for a range of temperatures between 700 K and 2300 K and for samples having Y2O3 concentration between 4-14 mol%. The oxygen-ion diffusion in the prepared mixed GBs was then compared with the well-investigated high symmetry GBs, i.e., Σ = 5(310)/[001], Σ = 11(311)/[110], Σ = 13(510)/[001], another mixed GB formed by (20-1) and (100) planes and the single-crystal. In agreement with the previous studies, the optimum Y2O3 content for maximum oxygen self-diffusion in mixed GB was found to be 8 mol%. Furthermore, it was found that oxygen self-diffusion is hindered not only at the GB core but also inside grains having specific orientations. This is attributed to the formation of oxygen vacancy (VO) clusters indicated by Y3+ segregation and the subsequent lack of O-2 ions at the GB core. The collective analysis of this study indicates that at optimum Y2O3 concentration, specific grain orientations concerning the GB plane reduce the oxygen vacancy clustering at the grain interior regions close to GB, thereby resulting in the overall higher oxygen-ion diffusivity of the mixed GBs.