ISSN:
1572-8838
Keywords:
oxygen sensors
;
thick film
;
zirconia
;
amperometric sensors
;
cracks
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Electrical Engineering, Measurement and Control Technology
Notes:
Abstract Thick film amperometric oxygen sensors have been constructed using an ink prepared from a powder of yttria-stabilized cubic zirconia with 150nm particle size printed onto an alumina substrate. At a fixed temperature these sensors display characteristics typical of an amperometric sensor. However, the limiting current varies with operating temperature (T) according to a T−3.1 dependence. This indicates that the diffusion barrier (electrolyte) becomes more restrictive as the temperature is raised. Cracks in the zirconia film (detected by SEM) arise from the differential thermal expansion coefficient of the thick film and the substrate. Diffusion of oxygen to the cathode occurs through the cracks which open and close as the temperature is, respectively, lowered and raised. Measurements at total gas pressures in the range 1–1000mbar indicate that the mode of diffusion is of the Knudsen-type up to 120mbar then tending towards mainly a bulk- type at 1000mbar. Crack dimensions and their relation to the variation of limiting current and temperature dependence for the region of bulk diffusion are treated theoretically. A good correlation with theory is obtained using the results from the two sources, namely, crack dimensions at room temperature and sensor characteristics in the bulk diffusion regime. This provides evidence for the mechanism proposed to explain both the magnitude and sign of the observed temperature coefficient. The value determined by microscopy for the differential coefficient of thermal expansion between the thick film and the substrate (α) was 1.1×10−6K−1. Also, characteristics of the sensor in the bulk diffusion region give a value for α/θ2 of 0.70×10−6K−1, where θ is the tortuosity factor of the slots. Hence, θ=1.2. This value is in agreement with the value determined by microscopy within the bounds of experimental uncertainty.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1003447325519
