A thermodynamic study of polycrystalline tin dioxide sensors and antimony doped devices in the presence of combustible gases

A previous paper described preliminary results obtained by simultaneously monitoring changes in tin dioxide sensor resistance and temperature upon exposure to various reducing gas mixtures. Sensors were prepared by coating a small diameter platinum wire spiral with a sintered polycrystalline tin dioxide based bead to which an additional surface contact wire was attached enabling the semiconductor resistance to be measured. Changes in sensor temperature were monitored by incorporating the sensor coil into a conventional resistance bridge circuit. When the sensors were operated under oxygen depleted conditions, the introduction of reducing gases such as CO or H{sub 2} gave rise to considerable increases in resistance response of up to 4 orders of magnitude and this was matched by a significant cooling of the SnO{sub 2} bead. This is in direct contrast to sensor behavior in air where the presence of these reducing gases leads to a measurable increase in bead temperature and moderate changes in SnO{sub 2} resistance.