In recent years, the importance of monitoring combustion processes for fuel
efficiency and pollution control has necessitated the development of reliable gas
sensors for high temperature in situ detection. Semiconductor gas sensors have
been developed for a wide range of reactive gases, but are typically nonselective
to any particular gas in gaseous mixtures. The objective of this research was to
develop fast-responding solid-state sensors for the selective detection of CO gas
at elevated temperatures, as well as to understand and characterize the
associated sensing mechanisms.
From preliminary dc electrical resistance measurements, the sensing
properties of various semiconducting oxides were investigated, and Mo03- and
Ti02-based systems were selected as candidate sensor materials. Mo03 was
found to display an ON/OFF-type sensing behavior. X-ray photoelectron
spectroscopy (XPS) results indicated that the sensitivity could be attributed to the surface reduction of Mo03 to the Mo02 phase.
The Ti02 sensor displayed a gradual drop in electrical resistance with
increasing CO concentration but suffered from H2 interference. Two different
mechanisms were proposed (based on recovery characteristics) to describe the
sensitivity of Ti02: a surface-controlled mechanism for CO, supported by XPS
data, and a bulk-diffusion controlled mechanism for H2. Through second phase
additions of 10 wt% Y203 and 10 wt% Al203, the sensor was made selective to
CO and H2, respectively with no interference from NOx. Fe and Pd catalyst
additions improved both the response time and the lower limit of CO detection.
A sensor device of (Ti02-10 wt% Y203)-5 wt% Pd was developed with
optimized geometry, electrode configuration, and processing conditions. A
prototype device was sucessfully tested in an automobile engine for over 50
cycles, responding quickly and reversibly under revving and idling conditions.
Immittance spectroscopy (IS) was employed to characterize the electrical
behavior of Ti02. By extracting the electrical parameters through lumped
parameter/complex plane analysis, the sensing behavior was found to be grain
boundary-controlled, with sensitivity determined by electrical barriers at the
intergranular contacts. Upon exposure to CO gas, these barriers are effectively
lowered, and the depletion region width decreases. The observed decrease in
electrical resistance and increase in grain boundary capacitance provided
evidence in support of this proposed mechanism.