The transient response of ZrO2 oxygen sensors to step changes in gas composition

Abstract

The transient voltage response of ZrO2 oxygen sensors was examined following step changes in gas composition. The experiments were performed on a laboratory flow reactor at 600° C. Composition changes between (a) 100% and (b) 1% O2 in N2 produced response curves whose symmetry varied between composition steps (a) from low-to-high oxygen and (b) from high-to-low oxygen. This difference is due to the logarithmic dependence of sensor voltage on oxygen partial pressure. Corresponding oxygen partial pressure-time curves, derived from experimental voltage via the Nernst equation, are symmetric with respect to the direction of composition changes. Abrupt transitions are found in voltage-time curves at 600° C following step changes of reactive gases; e.g. from O2/N2 mixtures to CO/N2, H2/N2 or D2/N2 mixtures. These voltage-steps represent transitions in stoichiometry of the surface boundary layer on the ZrO2 sensor. Delay times before the transition also reflect reaction stoichiometry. Response times with O2/CO, O2/H2 and O2/D2 follow trends predicted by the kinetic theory of gases. A limited number of experiments were performed to examine the relationships between sensor response and sensor catalytic activity. Poorer oxidation catalytic activity parallels slower response characteristics.