Zirconium Oxide - Yttrium Oxide fO2/pH Electrode Behavior: The Role Of Temperature On Acid Durability, Charge Carrier Type And Nernstian Response

Abstract

Zirconium-Oxide ceramics stabilized with -8-wt% Yttrium-Oxide can be employed to sense pH in high temperature (>90°C) aqueous environments with an accuracy of ±0.05 pH log units (Lvov et. al., in press), and to sense the fugacity of oxygen (fO2) in low temperature (>230°C) gaseous environments with an accuracy of ±0.2 (fO2) log units. The major components, m the two commercially available yttria-stabilized ceramics are yttria (-8-wt%) and zirconia (-91-wt%) with minor amounts of Ti, Fe and U. The textural differences in the two ceramics produce significantly different emf vs. 10,000/T responses. Response error can be introduced by: the ionic contribution of the softening glass that may be present, the catalytic action of the Pt sensor components, and the presence of Ti and Fe in the ceramic, which has been shown (Merino et. al., 1996) to alter the oxygen diffusivity of the ceramic. The first type of ceramic contains a 3-dimensionally-continuous Ca-Al-Si feldspathic glass that acts as a sintering aid during manufacturing. The glass, which has a higher ionic conductivity than the zirconia ceramic, reduces the bulk resistivity and induces an error over the temperature ranges higher than the softening point of the glass. The glass also reduces durability of the ceramic. When the glass hydrates it produces zeolites, which grow primarily in the triple-grain-junctions of the ceramic, thus mechanically weakening the ceramic and generating electronic, ionic and mechanical stability problems. The second type of ceramic contains no grain boundary glass, but does contain discrete silicate and oxide phases (such as diopside, wollastonite, periclase, silica, etc.) in the triple-grain-junctions. Because there is no inter-granular glass, the type-two ceramic does have a greater bulk resistivity compared with the type one ceramic. In a gas-sensing configuration, resistivity will affect the minimum temperature of sensor operation. A sensor with a higher bulk resistivity must reach a higher minimum temperature before the sensor will sense oxygen. The work presented in this manuscript suggests that the same is true for the sensor in its aqueous pH configuration. In addition to the mechanical degradation, there are also chemical leaching issues with both ceramics. While zirconia is relatively unleachable in its pure form, the addition of yttria, while creating the necessary lattice defects, increases the vulnerability of the solid solution grains to acidic solutions. This would possibly create ceramic durability problems during long-term down-hole operation. The ceramics do function well as a sensor and can produce highly accurate results (with calibration) and if the durability issues are considered during sensor assembly, the ceramic sensor could be highly desirable for many high temperature geologic and industrial applications. (Manna et al., 2002 and Manna et al., 2001).