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Published September 1998 | public
Journal Article

Measurement of oxygen fugacities under reducing conditions: Non-Nernstian behavior of Y_2O_3-doped zirconia oxygen sensors


A calibration procedure is presented for the use of a Y_2O_3-stabilized zirconia (YSZ) oxygen sensor in 1 atm gas-mixing furnaces in the temperature range 1200–1500°C and 0–8 orders of magnitude below the iron-wüstite (IW) buffer. Corrections to the Nernst equation were obtained by measuring apparent oxygen fugacities of gases in equilibrium with graphite (equilibrated with pure CO vapor), Cr + Cr_2O_3, and Ta + Ta_2O_5. Under reducing conditions, fO_2s calculated using the ideal form of the Nernst equation are erroneously high, by <0.1 log units at IW but by nearly three log units for Ta-Ta_2O_5 at 1000°C. The deviations between measured emfs and those calculated assuming Nernstian behavior of the electrolyte in the oxygen sensor reflect mixed ionic-electronic conduction. Measured emfs under reducing conditions are readily corrected for this effect via experimentally determined values of P_θ, the oxygen fugacity at which electronic conduction constitutes half of the total conductivity. For the oxygen sensors used in this study, log P_θ(± 0.20,lσ)3.70(±0.72)-32.95 ± 1.15 X10^3 T(K). Even under conditions more reducing than a gas of solar composition (f_(O2) = 10^(−18) at 1200°C), YSZ oxygen sensors can be used to determine absolute values of the oxygen fugacity to within ±0.2 log units.

Additional Information

© 1998 Elsevier Science Ltd. Received 9 June 1997. Revised 8 June 1998. Accepted 8 June 1998. Available online 20 January 1999. This work was supported by NASA grants NAGW-3212, NAG5-6181, and NAG5-4318. Discussions with E. Kolaczyk and S. Song concerning the uncertainty in determining the intersection of two lines were very helpful and the reviews of J. S. Huebner and an anonymous reviewer lead to significant improvements. A. M. Davis and S. B. Simon are thanked for their help with the SEM and microprobe.

Additional details

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