HfO2-based electrolyte potentiometric oxygen sensors for liquid sodium

Abstract

The use of potentiometric sensors for the on-line monitoring of oxygen in molten sodium in fast breeder reactors has been studied since the 1970s. However, their lifetime and the reproducibility of the results are inadequate for commercial development. In this study, the performance of the sensor using yttria-doped hafnia is studied and compared with results obtained with thoria- and zirconia-based electrolytes. The yttria-doped hafnia sensor tested, in the form of a tube, exhibits a very reproducible signal and remarkable stability under very low oxygen levels (ca. 0.02 ppm) and low temperatures (200–300 °C), with a lifetime of ca. two months. Consequently, this electrolyte can be viewed as a promising material for sensing oxygen in molten sodium. The main sources of errors observed in the application of these sensors are reviewed under severe conditions at low temperatures. Discrepancies between the calculated electromotive force (emf) of the cell and the slope of the theoretical Nernst law and the experimental values are analyzed and hypotheses are discussed in order to explain the deviations from the expected values. For the zirconia-based sensor, an original interpretation of the sensor’s response to a change in oxygen concentration, obtained at low temperatures, is proposed, involving peroxide ions.