Thick-film amperometric zirconia oxygen sensors: influence of cobalt oxide as a sintering aid

Amperometric thick-film zirconia oxygen sensors are electrochemical devices in which the zirconia thick-film acts as both electrolyte and diffusion barrier. Their preparation involves temperatures of 1300?1400??C to sinter the thick-film and reduce the through-porosity to a sufficiently low value to restrict oxygen diffusion rates and hence sensor limiting currents. The resulting sensors normally require an operating temperature of 800??C to enable operation in the percentage oxygen concentration range. In this work sensors were prepared with and without doping with cobalt oxide. Thick-films were characterized using scanning electron microscopy to view a fracture edge and by plotting current?voltage curves of the prepared sensors operated at 600?800??C in oxygen concentrations up to 21% at atmospheric pressure. It was found that doping with cobalt oxide markedly increased the sintering rate and enabled a reduction of 100?110??C in sintering temperature for a given final through-porosity. As a result it was possible to operate doped sensors at a temperature around 200??C lower than otherwise identically-prepared undoped sensors. This is expected to have a beneficial effect on sensor life and reduce operating power.