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

Abstract

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.

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