Segregation in zirconia: equilibrium versus non‐equilibrium segregation

Abstract

The present work considers the differences between the following two phenomena: equilibrium segregation, which is a thermodynamic phenomenon where the driving force is excess interfacial energy; and non-equilibrium segregation, which can be produced by phenomenological shocks applied to the solid surface. Fully stabilized cubic yttria-stabilized zirconia (YSZ) is used as the exemplar for these considerations. Equilibrium segregation in YSZ results in enrichment of the surface and near-surface layers in constituent elements, typically yttria and impurities. This segregation is an intrinsic material property and has an impact on the performance of zirconia at elevated temperatures. On the other hand, non-equilibrium segregation leads to a complex distribution of properties (structure and concentration gradients) that are determined by the experimental procedures used rather than being a material property. However, such non-equilibrium segregation can result in localized structural changes. The present paper also considers the effect of the gas phase on surface properties of metal oxides, including YSZ, and the surface dynamics of YSZ at temperatures below that required to reach equilibrium.

Atomistic simulations of surface segregation of defects in solid oxide electrolytes

“…The former divided by the mean bulk value of this ratio (0.22 for 10 mol.% YSZ and GDC) gives a surface dopant enrichment factor of 1.6. These values are close to those reported from X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and low-energy ion scattering (LEIS) measurements [1][2][3][4][5][6][7][8][9][10], which are summarized in Table 1. We also note that except for the LEIS measurements, the spatial resolutions of these experiments are limited to 20-40 Å and cannot resolve the detailed distribution of dopants in the first few atomic layers.…”

“…In a number of experimental studies, dopant cations (i.e. Y 3+ in YSZ and Gd 3+ in GDC) have been observed to segregate to the surfaces [1][2][3][4][5][6][7][8][9][10][11] and grain boundaries. Dopant segregation at the surface is important in fuel cell applications because it can affect the near-surface chemical reactions and ionic transport of the electrolytes.…”

Evolution of the electrochemical interface in high-temperature fuel cells and electrolysers

“…They tend to become covered by glassy phases consisting of SiO2, Na2O and other oxides, which may originate from the raw materials, the supplied gases or auxiliary components such as sealing glasses. Even in case of very clean single crystals a "mono-layer" of impurities of similar "glassy" composition is usually observed on the surface of yttria and or scandia stabilized zirconia (YSZ, ScSZ) 39,40 . Furthermore, dopants like Y2O3 in YSZ and Gd2O3 in ceria gadolinia oxide (CGO) segregate to the grain boundaries as well as to the interface between the bulk crystal and the glassy surface layer 39,41 .…”

Atomistic study of segregation and diffusion of yttrium and calcium cations near electrolyte surfaces in solid oxide fuel cells

“…At this temperature, migration of various types of cation defects, such as dopants and impurities (or additives) in the raw materials may be activated, resulting in surface segregation and change in surface chemical composition. dependence of the composition profile below the surface [17,[20][21][22][23]. Because the properties of in-plane surface diffusion mentioned above could be strongly affected by impurity levels or the chemical composition at the surface, better understanding of the segregation physics of cation defects in YSZ is essential.…”