Oxygen Isotope Transport Properties of Yttria‐Stabilized Zirconia (YSZ) in O2‐ and H2O‐Containing Atmospheres

Abstract

AbstractOxygen isotope exchange experiments, H218O/H216O (”wet” anneals) and 18O2/16O2 (”dry” anneals), were performed on single crystal samples of yttria‐stabilized zirconia (YSZ) at a temperature of T = 1073 K with subsequent determination of the oxygen isotope profiles in the solid by time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). Such experiments yielded oxygen tracer diffusion coefficients (D*) and oxygen tracer surface exchange coefficients (k*), from both the polished (smooth) and unpolished (rough) sides of single crystal samples, as a function of water partial pressure pH2O and oxygen partial pressure pO2. Isothermal values of D* were found to depend on neither pO2 nor pH2O (nor surface roughness). Isothermal values of k*, in contrast, displayed a strong dependence on pO2 or pH2O; k*wet was, in addition, 2–3 orders of magnitude higher than k*dry. Surprisingly, surface roughness had little effect on k*wet, whereas rough surfaces exhibited much higher k*dry values than smooth surfaces. Data for k*wet obtained as a function of temperature at pH2O = 18 mbar show a change in activation enthalpy at T ≈ 973 K. The behavior of k* is discussed in terms of surface composition, surface area and surface reaction mechanisms.