A probe for the continuous on-line measurement of hydrogen dissolved in liquid aluminium has been developed using the perovskite proton conductor <i>CaZrO<sub>3</sub></i>-<i>In</i>, in conjunction with a <i>Zr,ZrH<sub>2</sub></i> solid state reference. The probe has been designed to an industrial prototype standard, and measurements were in good agreement with established (but cumbersome) hydrogen measurement methods such as the AISCAN analyser, LECO analysis, and the Straube-Pfeiffer technique. The electrolytic domain of <i>CaZrO<sub>3</sub></i>-<i>In </i>has been investigated using a novel double-cell arrangement to simultaneously fix the chemical potential of both hydrogen and oxygen at the electrodes of a pellet of the electrolyte. As predicted from the defect equilibria, the electrolyte makes the transition from the hydrogen ion conduction domain to the oxygen ion domain under conditions of low oxygen partial pressure and high hydrogen partial pressure. The <i>pO<sub>2</sub></i> corresponding to the <i>Zr,ZrO<sub>2</sub></i> equilibrium is low enough to locate <i>CaZrO<sub>3</sub></i>-<i>In</i> in the oxygen ion conduction domain. Sodium <i>b-</i>alumina was employed to measure the <i>pO<sub>2 </sub></i>developed under operating conditions at the sensor reference electrode by a sample zirconium containing dissolved oxygen. This was found to be many orders of magnitude higher than the <i>pO<sub>2</sub></i> corresponding to the <i>Zr,ZrO<sub>2</sub></i> equilibrium, and was five magnitude higher than the <i>pO<sub>2</sub></i> marking the <i>H<sup>+</sup></i>/<i>O<sup>2-</sup></i> conduction domain boundary for <i>CaZrO<sub>3</sub></i>-<i>In</i>, suggesting that <i>Zr, ZrH<sub>2</sub></i> may be suitable as a reference material. Hydrogen sensors manufactured using <i>CaZrO<sub>3</sub></i>-<i>In </i>as the solid electrolyte and a <i>Zr,ZrH<sub>2</sub></i> solid state reference were found to provide stable, reproducible <i>emfs </i>at constant temperature and <i>pH<sub>2</sub></i>, and showed full Nernstian response following a change in <i>pH<sub>2</sub></i> at the measuring electrode. The recently developed Current Reversal Mode can accurately determine the <i>emf </i>of a solid electrolyte sensor, and also provides additional information in the form of the sensor resistance. A detailed study has identified how the CRM parameters should be selected in order to make accurate measurements. The sensor resistance measurement was used to improve measurement accuracy and stability of an internally heated yttria stabilised zirconia oxygen sensor. When applied to a commercially available probe for measuring hydrogen dissolved in liquid aluminium it was found that erroneous <i>emf</i> readings, measured after prolonged use in the melt, were accompanied by a corresponding increase in the sensor resistance, opening up the possibility of using CRM as an on-line diagnostic tool.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:604069 |
| Date | January 2004 |
| Creators | Hills, M. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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