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Title: Solid state sensors for gas monitoring and control
Author: Hills, M.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2004
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A probe for the continuous on-line measurement of hydrogen dissolved in liquid aluminium has been developed using the perovskite proton conductor CaZrO3-In, in conjunction with a Zr,ZrH2 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 CaZrO3-In 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 pO2 corresponding to the Zr,ZrO2 equilibrium is low enough to locate CaZrO3-In in the oxygen ion conduction domain. Sodium b-alumina was employed to measure the pO2 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 pO2 corresponding to the Zr,ZrO2 equilibrium, and was five magnitude higher than the pO2 marking the H+/O2- conduction domain boundary for CaZrO3-In, suggesting that Zr, ZrH2 may be suitable as a reference material. Hydrogen sensors manufactured using CaZrO3-In as the solid electrolyte and a Zr,ZrH2 solid state reference were found to provide stable, reproducible emfs at constant temperature and pH2, and showed full Nernstian response following a change in pH2 at the measuring electrode. The recently developed Current Reversal Mode can accurately determine the emf 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 emf 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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available