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Mechanisms of tin oxide gas sensor response


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Lee, Andrew Peter 2001 , 'Mechanisms of tin oxide gas sensor response', PhD thesis, University of Tasmania.

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Tin oxide gas sensors are widely used for the detection of combustible gases in
oxygen-rich atmospheres. Adsorbed oxygen species withdraw electron density
from the surface of the Sn02 , increasing its electrical resistance. At elevated
temperatures, around 400 °C, combustible analyte gases displace or react with
adsorbed oxygen, increasing Sn02 surface electron density and thus decreasing its
electrical resistance. Sensor resistance has been found to vary non-linearly with
combustible gas concentration in a manner that has not been satisfactorily
explained despite thirty years of research into the sensing mechanism.
The operating temperature of tin oxide gas sensors is critical information in
studies of their response mechanism, yet has seldom been reported accurately in
the literature. In the current work, a new method of determining sensor
temperature radiometrically has been developed and used to determine the surface
temperature of two types of Figaro tin oxide sensors. The operating voltagetemperature
relationships for these sensors were found to be pseudo-linear and are
reported as T = 103 V+214±3 K for the Figaro TGS813 sensor with its base
removed, T = 101 V+224±5 K for the TGS813 with its base attached, and T =
106 V+238±5 K for the Figaro TG52611 sensor. These results indicate that sensor
temperatures are significantly higher than most previously reported estimates. Investigations of TGS2611 sensor response reveal that oxygen exhibits nearly
ideal (Langmuir) adsorption behaviour on these 5n0 2-based gas sensors. An
equation for the response of these devices to oxygen has been developed from a
combination of accepted adsorption and electrical conduction theories. Fits of this
equation to low and high sensor temperature oxygen response curves confirm previous findings regarding the speciation of adsorbed oxygen, ie at temperatures
below — 170 °C, oxygen adsorbs non-dissociatively, while above this temperature
it adsorbs dissociatively. From the temperature-dependent response of a
TGS2611 sensor operating in air, enthalpies of adsorption have been calculated
for non-dissociative (AH = -35.4 kJ mol -1 ) and dissociative (AH = -126.7 kJ mold )
oxygen adsorption. These values are characteristic of physisorption and
chemisorption of oxygen to the surface respectively.
A combination of infrared studies and measurements of sensor resistance for a
TGS2611 sensor have shown that n-alkanes adsorb competitively with oxygen
onto the sensor surface. A competitive adsorption (Hinshelwood) mechanism is
thus proposed for the response to combustible gases, using the previously
developed oxygen response equation as a basis. The sets of equations
representing this model are too difficult to solve implicitly, so their validity has
been demonstrated using Monte Carlo-type computer simulations of sensor
response to single n-alkanes and binary mixtures of these gases. Detailed
information has been acquired about the adsorption and kinetic behaviour of
oxygen and n-alkanes on tin oxide sensors, including the influence of alkane chain
length on static and dynamic temperature responses.
The research in this thesis represents the first satisfactory explanation, in terms of
heterogeneous adsorption and catalysis theory, of many aspects of sensor
response, including the influence of oxygen on sensor resistance, the non-linear
analyte response behaviour, the characteristic analyte resistance/temperature
profiles, and the complex response of analyte mixtures.

Item Type: Thesis - PhD
Authors/Creators:Lee, Andrew Peter
Keywords: Gas detectors, Tin oxide
Copyright Holders: The Author
Copyright Information:

Copyright 2001 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s)

Additional Information:

Thesis (PhD)--University of Tasmania, 2001. Includes bibliographical references

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