Thick-film solid electrolyte oxygen sensors using the direct ionic thermoelectric effect

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Abstract

A screen-printed direct ionic thermoelectric oxygen sensor based on 8 mol% Y2O3 stabilized zirconia (YSZ) is presented. Within the device, a temperature gradient is applied to the YSZ film via an integrated heater, and the resulting thermovoltage is measured. Theoretical considerations on oxygen ion conductors predict a well-defined dependency of the thermopower on the oxygen partial pressure p(O2). Experimental results on the present thick-film device are shown to agree well with the theory. Besides being intrinsically geometry-independent, the present potentiometric sensor showed no cross-sensitivity to a variety of reducing and oxidizing gases that are commonly present in the exhaust. In addition, a very low temperature dependence of the thermopower was observed.

Introduction

In view of the tightening of emission regulations, the monitoring of the oxygen partial pressure p(O2) in the exhaust gas is a major issue in many combustion processes. The most common chemical sensor principle in this field, the Lambda probe, is based on a solid state electrochemical cell (more precisely a concentration cell, e.g. [1], [2], [3]). In the simplest set-up, an oxygen-ion conducting ceramic membrane of yttria-stabilized zirconia (YSZ) separates the exhaust gas from a reference gas. According to Nernst's equation, the chemical potential difference or oxygen partial pressure ratio between the exhaust (a priori unknown p(O2,exhaust)) and the reference (known p(O2,ref)) results in a sensor voltage.

One particular advantage of this potentiometric measurement principle is its intrinsic independency on geometry. In order to integrate the required reference atmosphere, however, manufacturing of these sensor elements is very complex. As a consequence, alternative resistive sensor concepts were investigated for various materials (e.g. [4], [5]). This sensor type utilizes the fact that the conductivity of metal oxide semi-conductors such as ceria or titanates changes as a function of p(O2) in a well-defined way. As a major advantage, the sensor resistance is measured without the need for a reference atmosphere. While reducing complexity of the sensor set-up, the measured resistance depends to a large extent on its geometry and may thus change its value during operation, for example due to thermal aging or the formation of small cracks.

As a novel sensor principle for gas detection, direct thermoelectric gas sensors are introduced in [6], [7]. In direct thermoelectric gas sensors, the Seebeck coefficient (i.e., the thermopower) of the material is a function of the analyte gas concentration, in contrast to indirect thermoelectric gas sensors, e.g. as described in [8], where the concentration-dependent heat quantity generated by the reaction enthalpy of an analyte generates a temperature difference which is measured by thermocouples.

As shown in [9], direct thermoelectric gas sensors combine geometry-independence with a less complex sensor set-up. Whereas Refs. [6], [7] apply n-type or p-type semi-conducting films as gas sensitive elements, the present contribution is concerned with a sensor based on a solid electrolyte like yttria-stabilized zirconia (YSZ), as originally suggested in [10]. A brief discussion of the underlying mechanism is also given.

In the present contribution, a planar thick film oxygen sensor based on this non-isothermal method is presented. As the core component, a screen-printed yttria-stabilized zirconia film was used. The device was evaluated with respect to its oxygen sensitivity, its temperature characteristics, and the cross-interference of various gaseous species.

Section snippets

Theory of the measurement principle

We have to distinguish carefully the thermoelectric power (thermopower, or Seebeck coefficient, often also denoted as absolute thermopower) ɛ and the electromotive force (emf) of a non-isothermal galvanic cell divided by the applied temperature difference. Whereas the first is a materials property, the latter is an experimental observable which contains the thermoelectric properties of the electrical contacts to the thermoelectric materials under investigation. For ionic or mixed conductors

Sensor set-up

The sensor setup shown in Fig. 2 was similar to that described in [9], where an electronic semi-conductor was used as the gas sensitive phase. In the present study, this semi-conducting layer was replaced by an ion conductor.

The functional layers of the sensor device were prepared by screen-printing. First, a paste of 8 mol% Y2O3 stabilized zirconia (TZ-8YS, Tosoh) mixed with an organic binder (Decoflux WB41, Zschimmer&Schwarz) was prepared and screen-printed on a high purity (99%) alumina

Results and discussion

Fig. 4 presents the oxygen sensor characteristics of the thermoelectric sensor device operated at 973 K. Each point in this curve was calculated from the slope values at one specific p(O2) as described in the previous section.

A sensitivity of approximately −50 μV/K per decade p(O2) was calculated from this curve. This value is in excellent agreement with the theoretical value of −49.6 μV/K per decade p(O2) as derived above. The thermopower at p(O2) = 0.3 bar and p(O2) = 1 bar were determined to be 438 

Comparison with the conventional Lambda probe

At first glance, the conventional Lambda probe seems to be advantageous compared with the direct thermoelectric ionic oxygen sensor. In addition to the heater electrodes, only two more contact leads are required, one at the reference side and one on the exhaust side.

The direct ionic thermoelectric oxygen sensor in its present form requires at least one additional contact for the temperature modulation. If one sets aside a common ground as shown in Fig. 2, even two contacts for the modulation

Conclusion and outlook

A direct thermoelectric sensor setup based on yttria-stabilized zirconia (YSZ) is presented. The device, which was entirely manufactured in thick-film technology, was successfully operated as an oxygen sensor. The sensor characteristics were found to agree very well with the theory and basic results on YSZ, both in terms of sensitivity and absolute values. In addition, the sensor presented high selectivity to oxygen against a set of common cross-interfering gases and a low temperature

Acknowledgement

Financial support by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) under grants Mo1060/7-1 and Ja648/14-1 is gratefully acknowledged.

Ulla Röder-Roith received her diploma degree in materials science from the University of Bayreuth, Germany, in 2005. She is now PhD student at the Chair of Functional Materials, University of Bayreuth. She works on direct thermoelectric gas sensors based on solid ionic conductors and on the electrochemical reduction of nitrogen oxides in automotive exhausts.

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Ulla Röder-Roith received her diploma degree in materials science from the University of Bayreuth, Germany, in 2005. She is now PhD student at the Chair of Functional Materials, University of Bayreuth. She works on direct thermoelectric gas sensors based on solid ionic conductors and on the electrochemical reduction of nitrogen oxides in automotive exhausts.

Frank Rettig received the Dipl.-Ing. (FH) degree in technology of physics from the University of Applied Sciences, Ravensburg-Weingarten, Germany, in 2000 and the Diploma degree in material sciences from the University of Bayreuth, Bayreuth, Germany, in 2003. He worked on his PhD degree at the Chair of Functional Materials, University of Bayreuth, dealing with direct thermoelectric gas sensors. His research interests were gas sensors and LTCC-technology. In his current affiliation at Robert Bosch GmbH in Reutlingen, Germany, he is involved in projects in the field of LTCC technology.

Timo Röder received his diploma in Chemistry in 2008 at the Justus-Liebig-University of Gießen, Germany. He is now PhD student in the research group of Prof. Janek in Gießen. His research areas include the thermoelectric properties of ionic solids and their use for thermoelectric gas sensors.

Jürgen Janek obtained his Dr. rer. nat. degree in Physical Chemistry at University of Hannover. Since 1999 he is head of the Institute of Physical Chemistry at the Justus-Liebig-University Gießen. His main research fields are: solid state thermodynamics, kinetics and electrochemistry, plasma chemistry, transport phenomena and materials for electrochemical energy technologies.

Ralf Moos received the Diploma degree in electrical engineering in 1989 and the PhD degree from the University of Karlsruhe, Karlsruhe, Germany, where he conducted research on defect chemistry of titanates. He joined DaimlerChrysler in 1995 and worked in the serial development of exhaust gas aftertreatment systems. In 1997, he switched over to Daimler-Chrysler Research, Friedrichshafen, Germany, where he conducted several projects in the field of exhaust gas sensing. Since 2001, he is head of the Chair of Functional Materials of the University of Bayreuth. His main research interests are materials, systems and concepts for gas sensing and exhaust gas aftertreatment.

Kathy Sahner received her German engineering diploma in materials science from the Saarland University, Germany, in 2002. At the same time, she also received the corresponding French degree from the European School of Materials Science and Engineering (EEIGM), France. During her PhD, she investigated of gas sensitive perovskites including mechanistic modeling. She then worked as a postdoctoral researcher at the University of Bayreuth, Germany, and at the Massachusetts Institute of Technology, USA, on various projects within the field of electroceramics and ion conductors. She now works at Robert Bosch R&D.

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