Temperature-stimulated transient responses of the conductance of SnO2
gas sensors, as exemplified by the Taguchi Gas Sensor (TGS), are
comprehensively studied. These responses are determined at many temperatures
for sensors exposed to several fixed concentrations of oxygen in nitrogen. The
dynamic response of conductance exhibits complex kinetics characterized by time
constants which range, depending on ambient conditions, from seconds to days.
Measurement results are analyzed in the light of a proposed model of
device behavior. This heuristic model is constructed by combining some
fundamental experimental observations with kinetic predictions of the barrier
layer theory of adsorption. The analyses result in identification of the
physical mechanisms responsible for the complex kinetics and long time
constants. We find that sensor conductance is controlled by an intergranular
potental barrier consequent to oxygen adsorption.
The barrier potential exhibits an Elovich-type rate kinetic and its
functional dependences on sensor temperature for several oxygen partial
pressures are determined. In addition, the long-term drift of the TGS results
from the diffusion of a native non-stoichiometric defect, an oxygen vacancy,
evoked by changes in temperature or ambient oxygen pressure.
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