The gas-solid isotope exchange method using H20 ~8 with the continuously recording gravimetric technique was employed for the measurement of the oxygen tracer diffusion coefficient in single-crystal zirconia. In the temperature range investigated (1300-1600~ the tetragonal form of the oxide is stable. Both helium and hydrogen atmospheres were used, the latter to determine the effect of hydrogen on oxygen mobility in the solid. Oxygen diffusion was found to be strongly dependent on trivalent impurities in the oxide. The oxygen vacancy diffusivity was determined from tracer diffusivity measurements in specimens of different impurity levels. Chemical diffusion coefficients were also determined from the tracer diffusivities and compared with values obtained from corrosion rate data.Zirconia is a material with many applications (1). In spite of extensive research on oxygen diffusion in the low-temperature monoclinic form of ZrO2 (2-4) and in stabilized zirconia (5, 6), no data exist for the pure tetragonal form, which is stable in the temperature range 1300-1600~ Conductivity data (7-9) suggest an oxygen diffusion coefficient between 10 -7 and 10 _8 cm2/s in this temperature range (10), but inaccuracies in the transport number measurements (11) clouds the relation between conductivity and diffusivity. Pawel (12, 13) and Reed (14) deduced the chemical diffusivity of oxygen in zirconia scales during corrosion of zirconium or the zirconium-based alloy Zircaloy, assuming that anion diffusion is the rate-controlling step and that electron mobility is high. These estimates also depend on knowledge of the oxygen-to-zirconium ratio at the lower phase boundary of zirconia, which is not well established (15-20). Direct measurement of the tracer diffusivity is useful for assessing the accuracy of oxygen mobility estimates obtained from the conductivity and corrosion data. The objectives of the present study are to determine the oxygen diffusion coefficient in tetragonal zirconia and to assess the effect of hydrogen on oxygen mobility.The method employed is a combination of gas-solid isotope exchange and gravimetry. The weight increase of a specimen due to the reactionis followed with time. The weight gain data are converted to the average oxygen-18 enrichment of the solid and the oxygen tracer diffusion coefficient is obtained by fitting to the appropriate solution of Fick's second law. Specimens of three different concentrations of trivalent cation impurities are tested because of effect that these species have on the anion vacancy concentration, and hence on the diffusivity of oxygen. Experimental MethodThe experimental method is described in detail in Ref.(21) and a brief summary is given below. The specimen is hung from a continuously recording electrobalance in the hot zone of a vacuum resistance furnace (Fig. 1). The furnace tube consists of a molybdenum outer tube to maintain the vacuum on the outside in the presence of the atmospheric pressure gas flowing inside. The sole purpose of the vacuum system is to protect the tu...

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