Kinetic Monte Carlo Investigation of the Effects of Vacancy Pairing on Oxygen Diffusivity in Yttria-Stabilized Zirconia
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Kinetic Monte Carlo Investigation of the Effects of Vacancy Pairing on Oxygen Diffusivity in Yttria-Stabilized Zirconia Brian S. Good Materials and Structures Division NASA Glenn Research Center, Cleveland, OH, USA. ABSTRACT Yttria-stabilized zirconia’s high oxygen diffusivity and corresponding high ionic conductivity, and its structural stability over a broad range of temperatures, have made the material of interest for use in a number of applications, for example, as solid electrolytes in fuel cells. At low concentrations, the stabilizing yttria also serves to increase the oxygen diffusivity through the presence of corresponding oxygen vacancies, needed to maintain charge neutrality. At higher yttria concentration, however, diffusivity is impeded by the larger number of relatively high energy migration barriers associated with yttrium cations. In addition, there is evidence that oxygen vacancies preferentially occupy nearest-neighbor sites around either dopant or Zr cations, further affecting vacancy diffusion. We present the results of ab initio calculations that indicate that it is energetically favorable for oxygen vacancies to occupy nearest-neighbor sites adjacent to Y ions, and that the presence of vacancies near either species of cation lowers the migration barriers. Kinetic Monte Carlo results from simulations incorporating this effect are presented and compared with results from simulations in which the effect is not present. INTRODUCTION Zirconia-based materials, and yttria-stabilized zirconia (YSZ) in particular, are of interest for a variety of technological applications. Pure zirconia exists in a monoclinic structure below about 1100C, with a tetragonal phase stable above that temperature to about 2300C, and a cubic phase stable from there to the melting point [1]. These phase transitions limit the suitability of zirconia for high-temperature applications, particular those that involve frequent thermal cycling. The tetragonal and cubic phases can be stabilized via substitutional doping, with aliovalent cations such as Y3+ or Ca2+ replacing Zr4+ ions. The cubic phase can be fully stabilized via doping with Y3+, and the resulting yttria-stabilized zirconia remains in the same cubic fluorite phase from room temperature to the melting point. YSZ’s thermal stability and low thermal conductivity make it suitable for high-temperature applications such as thermal barrier coatings for turbine engine components. Oxygen diffusion takes place via diffusive oxygen ion hopping among vacancy sites, with the oxygen ion passing between two barrier cations. When zirconia is cation-doped, additional compensating oxygen vacancies are formed so as to maintain overall electrical neutrality. The addition of these vacancies increases the number of potential hopping sites, increases the diffusivity and the diffusive ionic conductivity, which can attain values large enough to make YSZ and related materials of interest for use as oxygen sensors, or as solid electrolytes for fuel cells. However, the oxygen diffusivity does not i
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