The paper describes the results of our studies on material
processing, design, fabrication and characterization of two kinds of
multi-functional solid state sensor in thick-film and thin-film technology.
The first sensing device is based on Cr3+ doped perovskite ferroelectric
metal oxide Ba1-xSrxTiO3 (x = 0-1) and is
fabricated using thick-film technology. Thick films of porous Ba1-xSrxTiO3 exhibit a change in dielectric constant
with respect to temperature variations, while their electrical conductivity
is a function of humidity. Hence, temperature and humidity can be
simultaneously detected by measuring the film capacitance and the film
resistance respectively, without incurring crosstalk. The response times of
the thick-film sensor to humidity and temperature are 30 seconds and
60 seconds respectively. The second sensor is based on thermally evaporated
n-type semiconducting tungsten trioxide (WO3) thin films with a
thickness of about 150 nm. It utilizes water physisorption on grain
surfaces of WO3 at ambient temperatures as well as O3
chemisorption at elevated temperatures higher than 200 °C. Humidity
and O3 can therefore be detected by operating the sensor at
different temperature levels. The response times of the thin-film sensor to
humidity and O3 are 10 seconds and 30 seconds respectively. In
addition, the thin-film Pt heater at the backside of the sensor substrate
serves also as a thermometer measuring the temperature of the sensor
substrate. Cross-sensitivities among the three measurands, temperature,
humidity and gas, are negligible.
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