D7.3 - Solid-State Potentiometric CO2-Sensor in Thick-Film Technology

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
D7 - Medical II
S. Wiegärtner, G. Hagen, D. Biskupski, J. Kita, R. Moos - Universität Bayreuth, M. Seufert, J. Norman, N. Jörns, A. Bolz - Corscience GmbH & CO. KG, Erlangen, C. Schmaus, A. Kießig - Siegert electronic GmbH, Cadolzburg (Germany)
650 - 653


Monitoring the CO2 concentration is useful in many applications, for example to examine the quality of ambient air, for automotive applications, or for breath analysis. Solid-state potentiometric gas sensors are inexpensive and provide many advantages like fast sensor response, high selectivity, or long term-stability.
A solid-state potentiometric CO2-Sensor with internal heater is fully manufactured in planar thickfilm technology. Nasicon is used as sodium conducting phase. An eutectic mixture of Li2CO3 and BaCO3 (molar ratio 1:2) serves as measuring electrode material, which has a lower affinity to water vapor. As reference electrode material, we use a mixture of Na2Ti6O13 and TiO2. The potential difference (the electromotive force emf) is picked off between two Au-grid electrodes.
The integrated heater on the backside of the sensor, which is realized as a screen-printed platinum structure, allows to operate the sensor as a standalone device at the preferred temperature of 525 °C.
Characteristic curves are obtained by measuring the electromotive forces while varying the CO2- concentration in a defined synthetic gas atmosphere (10 % O2, 0 % H2O, N2 balance). Their slopes are given by the electron transfer number n of the Nernst equation. All of the investigated sensors show an electron transfer number of about n = 2.1, which is near the theoretical value of n = 2. This proves that the sensor characteristics are in good agreement with the theoretical considerations. As the characteristic curves of the tested sensors show the same slope and offset parameters, we can point out a high reproducibility in fabrication.
The sensor temperature can be kept constant by the internal heating element, even in flowing gas atmospheres. Therefore, the produced sensors can be used for different applications. For example, it is possible to measure the CO2-concentration of human breath during inhaling and exhaling.
In an initial test, a person in- and exhaled a few times through a tube, in which such a sensor was mounted. During exhalation, the sensor showed concentrations up to 5 % CO2. Throughout the inhalation steps with ambient air, the measured concentration falls down to nearly 0 % CO2 every time. These results agree with the expected values.