Aluminum reduction pots at the Pittsburgh ReductionCompany's (Alcoa's) plant i n 1889. Adapted from a photograph, courtesy of Alcoa. DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor Battelle Memorial Institute, nor any of their employees, makes any WUIUI~~, expnsrcd or implkd, or assumes any kgal liability or nsponribi#tr for the accuracy, c o f n~, or uwfulmss of any information, apparlbs, produd, or process dirdortd, or rtprtsents that its use wwM not infdnge privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government of any agency thereof, or Battelle Memorial Institute. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

The pH response of 8 weight percent yttria-stabilized zirconia sensors is examined over a temperature range of 373-573 K. Good pH response was found throughout the temperature range. The internal half-cell was discovered to be poised by oxygen, which permits some simplification in the calibration of the sensor. Activation energy measurements imply that the primary conduction process involves the oxide ion. An improved electrical]mechanical seal is also discussed.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.102.42.98 Downloaded on 2015-03-14 to IP Vol. 132, No. 2 STABILIZED-ZIR'2ONIA pH SENSORS ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.102.42.98 Downloaded on 2015-03-14 to IP

Activities in support of the Sensors Development Program for FY 1989 were conducted under three major tasks: Digital Signal Analysis (DSA), Development of the Reference Anode, and Field Test of the Reference Anode. DSA of current and voltage signals colleCted from PNL bench-scale laboratory cells suggest there may be correlations between certain quantification parameters derived from these data and the current density of the working anode. Although some data show a dependence on alumina concentration, additional effort is required to determine whether a correlation of DSA with this important cell operating parameter can be unequivocally shown. If successful, such a correlation would form a viable basis for a DSA-based alumina sensor. Digital current and voltage signals collected during the Prototype Inert Anode Test gave correlations involving current density that were similar to those obtained using data from the PNL bench-scale laboratory cells. This suggests a DSA-based sensor may work in a large-scale aluminum reduction cell if it can be shown to work in a bench-scale test. In other words, there may be no serious effects on DSA data resulting from scale-up considerations. Analog current and voltage data were generally stable throughout the Prototype Inert Anode Test and therefore did not signal the onset of the corrosion of the prototype anode that was observed in post-test analysis. This indicates that monitoring de signals may not be a suitable approach to determining, in every case, when an inert anode is undergoing degradation during its operation. Despite the magnitude of effort expended in FY 1989 to develop the reference anode, serious issues regarding its operation and durability remain. Problems were also encountered in trying to use the reference anode in the Prototype Inert Anode Test. Consequently, PNL recommended that further development of the reference anode be suspended. Future tests, including the Pilot Cell Test, will use an Al/Alz03 reference electrode. The Pilot Cell Test was postponed to FY !99D because of delays in obtaining the required materials and manufacture of the anodes from commercial vendors. v Electrochemical impedance data collected on inert anodes in PNL benchscale laboratory cells during the evaluation of the reference anode show a dependence on current density and alumina concentration that is consistent with the formation of a resistive reaction layer. The formation of a resistive reaction layer is not the only possible mechanism for the observed results. Alternative mechanisms are being considered based on separate experimental studies. However, an alumina sensor based on the dependency of the impedance data is not expected to viable because significant hysteresis was observed in these data. vi ACKNOWLEDGMENTS The authors wish to acknowledge the technical assistance of N.

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