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

Oxygen isotope exchange experiments, (H2O)-O-18/(H2O)-O-16 (wet anneals) and O-18(2)/O-16(2) (dry anneals), were performed on single crystal samples of yttria-stabilized zirconia (YSZ) at a temperature of T=1073K 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 pH(2)O and oxygen partial pressure pO(2). Isothermal values of D* were found to depend on neither pO(2) nor pH(2)O (nor surface roughness). Isothermal values of k*, in contrast, displayed a strong dependence on pO(2) or pH(2)O; 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 pH(2)O=18mbar show a change in activation enthalpy at T approximate to 973K. The behavior of k* is discussed in terms of surface composition, surface area and surface reaction mechanisms.