MEASUREMENT OF OXYGEN FUGACITIES UNDER REDUCING CONDITIONS: NON-NERNSTIAN BEHAVIOR OF Y2O3-DOPED ZIRCONIA OXYGEN SENSORS - A REVIEW

Abstract

A calibration procedure is presented for the use of a Y2O3-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-wustite (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 + Cr2O3, and Ta + Ta2O5. Under reducing conditions, fO2s 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-Ta2O5 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 Pθ, the oxygen fugacity at which electronic conduction constitutes half of the total conductivity. For the oxygen sensors used in this study, logPθ(+/-0.20,1σ)=3.70(+/-0.72)-(32.95+/-1.15)x103T(K).Even under conditions more reducing than a gas of solar composition (fO2 = 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.