Some practical points to consider with respect to thermal conductivity and electrical resistivity of ceramic substrates for high-temperature gas sensors

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Highlights

  • Both alumina and LTCC substrates cannot be considered as ideal electrical insulators.

  • The resistivity of alumina substrates and of LTCC is strongly temperature dependent.

  • The resistivity of alumina substrates may get affected if NO2 is in the ambience.

  • The thermal conductivity of alumina decreases strongly with temperature.

  • The thermal conductivity of LTCC decreases remains constant at lower values.

Abstract

In the field of gas sensors and high temperature flow sensors, ceramic thick-film-based structures play a key role, particularly in harsh environments. Their substrates have to be electrically insulating and chemically inert. Ceramic substrates, especially alumina ones, are often considered as ideal sensor substrates. However, neither their electrical resistivity nor their chemical inertness is ideal. Even the thermal conductivity is that temperature dependent that one has to consider it when modeling temperature profiles. In this contribution, measurements on the resistivity and the thermal conductivity between room temperature and 800 °C of two related materials used for gas sensors substrates – alumina and LTCC ceramics – are presented. Additionally, resistance data of uncoated interdigital structures when exposed to NO2 at around 300 °C indicate that reactions between substrate and NO2 may occur and have to be kept in mind.

Introduction

Metal oxide gas sensors as well as air flow sensors are often designed for operating temperatures above 300 °C. Therefore, the number of suitable substrate materials is limited, particularly, if the sensors are applied in harsh environments [1], [2], [3], [4]. Here, ceramic substrates are the materials of choice. Besides the chemical and mechanical stability, electrical insulation and thermal conduction properties of the applied substrates dominate the material selection. The substrates have to be highly electrically insulating and their resistivity, ρ, should not depend on the chemical species in the ambience. Also their thermal conductivity, λ, should be taken into account. To design gas sensors with a uniform temperature distribution in the area of the functional film (active area), a high thermal conductivity of the substrates is preferred.

For instance, if interdigital electrodes are applied on top of a substrate and the substrate is heated by a reverse-side heater as depicted in Fig. 1, the substrate has to homogenize the temperature distribution in the active area, on which the gas sensitive functional film is applied. However, to decouple two different heat sources, as it is the case for sensors measuring heat differences (calorimeter, flow sensors [5], [6], [7]), a low λ is preferred. In this case, the substrates manufactured in LTCC technology seem to be more suitable due to their lower thermal conductivity [8], [9], [10].

Whereas alumina is the most-used material in thick-film technology, increasingly often LTCC ceramics are used to build gas sensors. Therefore, this article, which discusses some practical aspects that should be considered when dealing with conductometric gas sensors, focuses on both materials.

Section snippets

Temperature dependency of thermal conductivity

A design process of a gas sensor begins with thermal calculations, very often supported by Finite Elements Analysis (FEA). Therefore, for proper calculations of thermal effects, knowledge of the temperature dependent thermal conductivity of the substrates, λ(T), is mandatory [5]. In this study, the thermal conductivity was determined with a laser flash device (LFA1000 Laser Flash, Linseis, Selb, Germany). We measured the thermal conductivity of pellets with a diameter of 10 mm and thicknesses up

Electrical resistivity at higher temperatures

Alumina is often considered as an ideal substrate for gas sensors due to its excellent electrical insulation capability at room temperature. However, at temperatures above 300 °C, the behavior of alumina can be far from this expectation.

To determine the specific resistance, ρ, samples in form of a parallel plate capacitor were prepared from different alumina and LTCC materials. Electrodes were screen-printed to form a capacitive sample as shown in the inset in Fig. 7. The samples were heated in

Chemical inertness of alumina substrates

When using ceramic substrates for conductometric or impedimetric gas sensors one neglects usually responses of the materials itself towards analyte exposition. To verify this, we investigated whether the resistance of uncoated interdigital electrodes on alumina substrates depends on some typical gas components (Fig. 8). Platinum interdigital electrodes (lines and spaces 100 μm) were screen-printed on the alumina substrates and impedance spectra were recorded during applying oxidizing or reducing

Conclusions

Some practical points that should be considered when applying ceramic substrates for high-temperature gas sensors are shown and discussed here. The temperature dependency of thermal and electrical conductivity of ceramic substrates is often neglected. For the design process, the lower thermal conductivity at high temperature leads to a less homogeneous temperature distribution in the active area, an effect that can be compensated by the heater design. This effect becomes especially obvious for

Acknowledgement

To obtain thermal conductivity data, LFA1000 Laser Flash analysis was kindly provided by Linseis, Selb, Germany.

Jaroslaw Kita received his MSc degree at the Department of Electronics, Wroclaw University of Technology (Poland) in 1998. In 2003, he received the PhD degree in electronics from Department of Microsystem Electronics and Photonics at the same university. In 2004, he joined the University of Bayreuth (Germany), Department of Functional Materials and he is now a permanent member and senior scientist. His main interests are in the field of LTCC technology (application of LTCC for gas sensors and

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Jaroslaw Kita received his MSc degree at the Department of Electronics, Wroclaw University of Technology (Poland) in 1998. In 2003, he received the PhD degree in electronics from Department of Microsystem Electronics and Photonics at the same university. In 2004, he joined the University of Bayreuth (Germany), Department of Functional Materials and he is now a permanent member and senior scientist. His main interests are in the field of LTCC technology (application of LTCC for gas sensors and microsystems) as well as in thick-film technique.

Andreas Engelbrecht received his diploma degree in material science from the University of Bayreuth, Germany, in 2013. Currently he is working as a PhD student at the Department of Functional Materials at the University of Bayreuth. His research focuses on electrochemical methods, especially suitable catalysts for electrochemical reduction of CO2.

Franz Schubert received his diploma degree in materials science from the University of Bayreuth, Germany, in 2012. At the moment he is working in the field of LTCC technology and thick-film technique as a PhD student at the Department of Functional Materials at the University of Bayreuth. His main interest is in the development of a new LTCC-based sensor for high temperature applications.

Andrea Groß (formerly Andrea Geupel) received her diploma degree in materials science from the University of Bayreuth, Germany, in 2008. Since then she is PhD student at the Department of Functional Materials at the University of Bayreuth. Her research interests are in dosimeter-type gas sensors, their applications and materials for exhaust after treatment systems, especially Lean NOx Traps.

Frank Rettig received his Dipl-Ing (FH) degree in technology of physics from the University of Applied Sciences in Ravensburg-Weingarten, Germany in 2000. In 2000, he joined the former Daimler Chrysler Research facilities in Friedrichshafen. In 2001 started to study material sciences at the University of Bayreuth. He received his diploma degree in 2003. After he finished his PhD at the Department of Functional Materials (University of Bayreuth), he joined Robert Bosch GmbH in Reutlingen, Germany.

Ralf Moos received the Diploma degree in electrical engineering in 1989 and the Ph.D. degree from the University of Karlsruhe, Karlsruhe, Germany, where he conducted research on defect chemistry of titanates. He joined Daimler in 1995 and worked in the serial development of exhaust gas aftertreatment systems. In 1997, he switched over to Daimler Research, Friedrichshafen, Germany. As a team leader gas sensors, he headed several projects in the field of exhaust gas sensing. In 2001, he was appointed head of the Department of Functional Materials (Chair) of the University of Bayreuth. His main research interests are materials, systems, and concepts for gas sensing and exhaust gas aftertreatment.

Selected papers presented at EUROSENSORS 2014, the XXVIII edition of the conference series, Brescia, Italy, September 7–10, 2014.

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