Elsevier

Sensors and Actuators A: Physical

Volume 241, 15 April 2016, Pages 145-151
Sensors and Actuators A: Physical

Miniaturized ceramic DSC device with strain gauge-based mass detection—First steps to realize a fully integrated DSC/TGA device

https://doi.org/10.1016/j.sna.2016.02.011Get rights and content

Highlights

  • Miniaturized differential scanning calorimeter has been realized in LTCC technology.

  • Device of 11 mm × 40 mm × 1.3 mm contains oven and temperature sensors.

  • Strain gauge was integrated to determine masses during heating and cooling.

  • Fully integrated DSC/TGA device is envisaged.

  • Development based on FE modeling.

Abstract

An existing miniaturized differential scanning calorimeter chip (DSC Chip) with a size of only 11 mm × 40 mm × 1.3 mm and a thermal resolution, which is comparable with conventional devices, served as a basis for this study. Due to its small dimensions and the high stability of the Low Temperature Co-fired Ceramics-based material (LTCC), fast heating rates up to 850 °C/min can be realized. Focus of this study are first approaches to integrate a mass sensing system onto the DSC chip, which allows determining mass specific thermal properties as well as mass changes during thermal analysis. By FEM analysis, the optimum position of the strain gauges was identified and the sensitivity of the mass sensing system was estimated in advance. The strain gauges were screen-printed and post-fired onto the structure. First tests show promising results in a mass measurement range of 8–200 mg.

Introduction

Differential scanning calorimetry (DSC) is a widely used thermoanalytical technique to determine thermally activated processes that occur in materials during a defined temperature program. However, not all materials can be analyzed in conventional DSC devices, since aggressive materials or reaction products may cause permanent damage of these highly expensive apparatuses. High weight and large apparatus dimensions as well as high power consumption exclude those devices from mobile use—so far, only tabletop units are on the market.

These disadvantages motivated the development of a low-cost miniaturized differential scanning calorimeter chip (in the further article denoted as DSC chip). Due to its very low production costs, it can also be used as a single-use device. Manufacturing in Low Temperature Co-fired Ceramics (LTCC) technology perfectly meets the condition for both, a compact, three-dimensional design along with low production costs [1], [2], [3], [4]. This miniaturized ceramic DSC chip includes all parts that are necessary for DSC measurements, namely a screen‐printed heater (that serves as an oven), screen-printed temperature sensor structures for the measurement of reference and sample temperature, and a crucible. Both its sensitivity and its thermal resolution are comparable with conventional DSC devices [5], [6], [7]. Working temperatures up to 600 °C are possible. A photograph and further details of the entire system can be obtained from [8] and are shown also in the supplementary information.

However, a DSC device can only detect the amount of heat absorbed or released from the sample. Actual heat capacities or mass specific enthalpies cannot be measured since information about the sample mass is required for that purpose. Even if the initial sample mass can be detected with the help of an external scale, faults in analysis appear as soon as mass changes occur during measurement (e.g. by evaporation of the sample). Therefore, the integration of a mass sensing system to continuously measure the sample mass is the focus of the current study. Integrating a mass sensing functionality into the DSC chip would pave the way for a miniaturized thermogravimetry (TG) device, allowing not only for a more precise DSC analysis but also for an extremely miniaturized combined Simultaneous Thermal Analysis (STA or TG-DSC) device.

In this study, the integration of strain gauges onto the ceramic DSC chip was examined as one possibility for mass detection. This solution meets the demand for low production costs as well as good process integration. First test structures, which were designed using FEM analysis, contained simple rectangular-shaped strain gauges. Measured data indicate that even with such simple structures, promising results can be obtained in a mass range of 8–200 mg.

Section snippets

Miniaturized ceramic DSC chip

This section shall briefly introduce the working principle and the setup of the existing ceramic DSC chip. It follows the heat flux differential scanning calorimetry principle. As depicted in Fig. 1a, a typical heat flux DSC device comprises two sample holders located inside an oven, in which the sample and the reference are placed. The temperature difference between sample and reference is measured as a function of temperature by two temperature sensors. By means of the device specific

Design and fabrication of the thick-film strain gauge-based mass sensing system

If the DSC chip is attached to its chip holder, it can be considered as a cantilever beam with a fixation on its one end and a mass load on its other (free) end. In literature, various numbers of different methods to detect the mass load on a cantilever system, including optical as well as resonator-based systems, are described and well-investigated [16], [17], [18]. However, the focus on low production costs as well as the demand for a solution, which is compatible with the LTCC manufacturing

Results

A photograph of the test setup is shown in Fig. 8. According to the FEM model, both test structures were clamped on the connection area (fixation) as depicted in Fig. 8. Different masses were placed consecutively on the chip head (mass sample) and the electrical resistance of one of the two strain gauges on the top side of each test structure was measured (Keithley DMM 2700).

Fig. 9a shows the relative resistance change of the strain gauge, ΔR/R0, versus time for both test structures. The

Conclusions and outlook

First approaches on the integration of a strain gauge-based mass sensing system onto a simplified version of an existing ceramic DSC chip showed promising results. By means of even very simply designed, beam like strain gauge elements and a very simple electronical setup, mass loadings below 10 mg could be detected when four strain gauges on the structure were connected to a Wheatstone bridge. This set up does not only ensure an increased resolution compared to a single connected strain gauge

Acknowledgements

The presented results have been achieved in the scope of the project KF2116719WM2 (ZIM Innovation Programme), supported by Federal Ministry of Economic Affairs and Energy on the basis of a decision by the German Bundestag. The authors would also like to thank Ms. U. Kuhn from the Chair of Polymer Engineering (Prof. Altstädt), University of Bayreuth, for the DSC reference measurements.

Annica Brandenburg received her engineering diploma in 2012 from University of Bayreuth, Germany. Since 2012, she is a PhD student at the Department of Functional Materials at the University of Bayreuth. Her research interests are LTTC-based sensor systems.

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    Annica Brandenburg received her engineering diploma in 2012 from University of Bayreuth, Germany. Since 2012, she is a PhD student at the Department of Functional Materials at the University of Bayreuth. Her research interests are LTTC-based sensor systems.

    Jaroslaw Kita received his MSc degree at the Department of Electronics, Wroclaw University of Technology (Poland) in 1998. In 2003, he received the Ph.D. degree in electronics from Department of Microsystem Electronics and Photonics at the same university. Since 2004, he is working at the Faculty of Engineering Science, University of Bayreuth (Germany), Functional Materials Group. His main interests are in the field of LTCC technology (application of LTCC in gas sensors and microsystems) as well as in thick-film technique.

    Eberhard Wappler, Dipl.-Ing., studied chemical engineering science at the comprehensive University of Essen, Germany. He joint W. C. Heraeus GmbH, Hanau, Germany, in 1976, where he worked on Thermal and Elementary Analysis. 1987 he switched over to the Heraeus Med GmbH, responsible for quality engineering of medical infrared laser and since 1994 manager for the IT of Heraeus Med. 1983 he founded wsk analysis technology (now wsk Measurement and Data Technology) with two partners. wsk is focused on Thermal Analysis, Contact Resistance, material science and surface technology as well as software solutions. 2001 he left Heraeus Med to concentrate on the management of wsk.

    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 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. As a team leader gas sensors, he headed 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 interest are materials, systems and concepts for all kind of sensors.

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