Development and Characterization of a Miniaturized Flame Ionization

Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 168 (2016) 1378 – 1381

30th Eurosensors Conference, EUROSENSORS 2016

Development and Characterization of a Miniaturized Flame Ionization Detector in Ceramic Multilayer Technology for Field Applications C. Lenza*, H. Neuberta, S. Zieschea, J. Försterb, C. Kochb, W. Kuipersb, M. Deilmannb, D. Jurkowc a

Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Winterbergstraße 28, Dresden 01277, Germany b KROHNE, Ludwig-Krohne Straße 5, Duisburg 47058, Germany c VIA electronic, Robert-Friese Straße 3, Hermsdorf 07629, Germany

Abstract A new type of a miniaturized flame ionization detector (μFID) for industrial and environmental field applications is presented. It is fabricated in Low Temperature Cofired Ceramics (LTCC) using multilayer technology. The developed solution integrates the fluidic structure with feed pipes, reaction chamber and exhaust and the electrical structure with electrodes for ignition and measurement as well in a monolithic ceramic component. The novel μFID was characterized using a varying methane concentration in a nitrogen sample gas. An absolute sensitivity of 9.4 mC/gC combined with a reduced gas consumption of 20 ml/min hydrogen in comparison to conventional FID systems were achieved. © Published by Elsevier Ltd. This © 2016 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Flame ionisation detector; μFID; Ceramic multilayer technology; LTCC; Field application; Portable detector

*

Corresponding author. Tel.: +49 351-2553-7511; fax: +49 351-2553-7600 . E-mail address: [email protected]

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

doi:10.1016/j.proeng.2016.11.383

C. Lenz et al. / Procedia Engineering 168 (2016) 1378 – 1381

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1. Introduction The flame ionization detector (FID) is the most common method for the quantification of hydrocarbons due to its very low detection limit (< 10-10 gC/s), high sensitivity (15 mC/gC), linearity and low cross-sensitivity to inorganic substances [1]. For this reason, the FID is the preferred detector for the hydrocarbon detection in gas chromatography (GC) and total hydrocarbon analyzers (THA). The measurement principle is based on the chemical ionization of hydrocarbons in a hydrogen flame and the detection of the resulting ion current in an electric field [1]. In conventional FIDs, the combustion gases hydrogen and air are typically supplied by nozzles with flow rates of 30 ml/min and 300 ml/min respectively. Due to its outstanding characteristics the FID is particularly well-suited for emission tracing or monitoring of potentially explosive atmospheres in sewage systems. It enables the gas forming (fermentation) control far below the lower explosion limit and would thus provide a higher protection level of civil infrastructures. However, suchlike true field applications require consequent miniaturization, high reliability, as well as reduced gas consumption in particular. Former studies have already introduced miniaturized FIDs with reduced gas consumption and appropriate sensor properties [2,3,4,5]. The most promising design of a micro-FID (μFID) has a planar arrangement and an opposite-nozzle micro-burner, integrated in a (8 x 8 x 0.5) mm³ glass-silicon-glass micro system [5]. The layout of the nozzles provides a stable counter-current diffusion flame by suppling the hydrogen and oxygen from opposite directions. Intermixture and reaction of the combustion gases occurs in a limited area of small velocities (stagnationpoint). This ensure low heat losses and long residence time of the combustion gases for total combustion as well as the sample gas for high ion yield. In spite of the promising characteristics, the design presented in [5] indicates an extensive fabrication process and thermal stress issues due to the combination of different materials. For that reason, the aim of our study was the development, fabrication and characterization of a μFID in ceramic multilayer (MLC) technology. This alternative microsystem technology [6] provides outstanding thermal and chemical robustness and the integration of mechanical, fluidical, and electrical micro-structures in a monolithic ceramic body [7,8]. In addition, fabrication in the panel-level process supports cost-effective high volume manufacturing at a high yield. Thus, this technology promises a significant advantage over the mentioned glass-silicon-glass design by simplifying the fabrication process and minimizing thermomechanical mismatch. 2. Design and Experimental 2.1. Design of a μFID in MLC technology

Fig. 1. Design of the ceramic μFID.

Figure 1 depicts the design of the novel ceramic μFID-chip with overall dimensions of (15 x 15 x 2.5) mm³. The hydrogen flame burns in the combustion chamber (C), which is symmetrically integrated in the ceramic body. Two opposite nozzles orthogonally pass into the chamber. Both are connected to an inlet, one for hydrogen (NH) and sample gas and the other for oxygen (NO). Two outlets at both ends of the combustion chamber minimize back pressure of the combusted gas. Measuring electrode (ME) and counter electrode (CE) are located at the bottom and at

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the top side of the combustion chamber. Two ignition electrodes (IE) are arranged below the nozzles. All integrated electrodes have feedthroughs (vias) to connecting pads at the chip-surface. PTC temperature sensors at the chip-surface determine the temperature at the fluidic and electrical connections and monitor the flame. 2.2. Fabrication of the LTCC-based ceramic μFID The ceramic μFID according to Figure 1 was fabricated using 4 inch LTCC green tapes, which allow for parallel fabrication of 16 μFIDs in one batch. The μFID consists of 12 layers, which are separately structured in the green state. First, the green tapes are geometrically structured (channels, combustion chamber, via-holes) by punching and laser ablation. Next, the metallization structures are deposited (vias, electrodes, terminations) by thick-film processes screen-printing and stencil-printing using co-fire compatible metal pastes. Afterwards, the green tapes are precisely stacked together and are uniaxially laminated. Subsequently, the structured laminate is co-fired in a box-furnace. In the post-firing process, the temperature sensors are deposited by using a PTC-paste. Finally the μFIDs were separated by wafer dicing and glass capillary tubes were assembled at the inlets and outlets. 2.3. Measurement Setup Figure 2a shows the fluidic and electrical interfaces of the ceramic μFID. For connecting them, the μFID is mounted on a PCB. Wires connect the contact pads of the μFID to the pads on the PCB (Figure 2b). The polarization voltage Up is supplied by a variable voltage source (Up max = ±100 V). A transformer circuit supplies the ignition voltage. The measurement unit for the ion current provides a maximum resolution of ΔI > 60 fA. Gases are supplied via tubing to the glass capillary tubes of the μFID. The hydrogen and oxygen flows are regulated by mass flow controllers (MFC). The sample gas, which is a variable mixture of nitrogen N2 and 1 % methane CH4 in N2 is also controlled by MFCs and is added to the hydrogen flow before it is fed into the μFID.

a

b

Fig. 2. (a) Experimental setup for the ceramic μFID; (b) Assembled ceramic μFID in LTCC after the integration in the measurement adapter.

3. Results First, we determined the minimum gas flows for a stable operation of the μFID. A hydrogen gas flow of 20 ml/min, an oxygen gas flow of 10 ml/min, and 6 ml/min for the sample gas were minimally required for stable operation. Next, we measured the total current Itotal as a function of the polarization voltage Up in the range from 0 V to 100 V at a constant CH4 concentration of csample = 0.9 % in the sample gas flow and compared it to the leakage current Ileak at pure nitrogen in the sample gas. The difference Itotal - Ileak is the pure ion current Iion which is a measure of the total hydrocarbon amount in the sample gas (Figure 3a). A saturation of the measurement signal occurs at Up = 20 V. Operating at this point leads to high sensitivity and low leakage current. For the next measurements the CH4-concentration in the sample gas csample was varied from 0.0 % to 1.0 % under a constant Up = 20 V and constant mass flows. Figure 3b depicts the measured Iion over the CH4 concentration.


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An absolute sensitivity of 9.4 mC/gC, a minimum detection limit (MDL) of 5.3 ·10-9 gC/s and a sufficient linearity were determined. These sensor properties enable the developed μFID for the announced application.

a

b

Fig. 3. (a) Ion current and leakage current as a function of the polarization voltage Up at 10 ml/min O2 flow, 20 ml/min H2 flow and 6 ml/min sample flow; (b) Influence of the CH4 concentration on the ion current at 10 ml/min O2 flow, 20 ml/min H2 flow, 6 ml/min sample flow and a polarization voltage Up = 20 V.

4. Conclusion A novel ceramic, miniaturized flame ionization detector (μFID) using LTCC and ceramic multilayer technology was presented. The combustion gas consumption is slightly reduced compared to commercial FIDs (20 ml/min H2 compared to 30 ml/min). Therefore this is a promising detector in particular for field applications and a comprehensive monitoring of sewage systems. A further reduction of the combustion gas consumption may requires an improved intermixture of the combustion gases in the combustion chamber and a reduced heat loss of the whole system. Our approach is to reduce the cross-sections of the nozzles and to reduce the heat transfer over the chip surface by optimizing the chip geometry of the μFID. The measured absolute sensitivity of the fabricated μFID is 9.4 mC/gC. It enables e. g. the monitoring of the atmosphere in sewage systems regarding the formation of explosives gas mixtures below the lower explosion limit. The future integration of a guard electrode to minimize the leakage current may improve the minimum detection limit (MDL) of the ceramic μFID, which would distinguish this system also for portable GC-detectors. Our future work will deal with these goals. Acknowledgements The research on the miniaturized FID in the ceramic multilayer technology is part of the project “FIDEX Autonomer Mikroflammenionisationsdetektor für den Explosionsschutz in zivilen Kanalisationsnetzen” and is financially supported by the German Bundesministerium für Bildung und Forschung BMBF (# 13N13271). References [1] H.H. Hill, D.G. McMinn, Detectors for Capillary Chromatography, J. Chemical Analysis 121 (1992) 7-21. [2] S. Zimmermann, S. Wischhusen, J. Müller, Micro flame ionization detector and micro flame spectrometer, J. Sensors and Actuators B 63 (2000) 159-166. [3] M. Wu, R. Yetter, Development and analysis of a LTCC micro stagnation-point flow combustor, J. Micromech. and Microeng. 18-12 (2008). [4] W.J. Kuipers, J. Müller, A planar micro-flame ionization detector with an integrated guard electrode, J. Micromech.and Microeng. 18 (2008). [5] W. Kuipers, J. Müller, Characterization of a microelectromechanical systems-based counter-current flame ionization detector, J. Chromatography A 1218 (2011) 1891-1898. [6] Yoshihiko Imanaka, Multilayered Low Temperature Cofired Ceramics (LTCC) Technology, Springer, Boston, 2005. [7] U. Partsch, C. Lenz, S. Ziesche, C. Lohrberg, H. Neubert, T. Maeder, LTCC-Based Sensors for Mechanical Quantaties, Proc. 48th International Conference on Microelectronics, Devices and Materials, MIDEM, Otočec (2012). [8] C. Lenz, S. Ziesche, U. Partsch, H. Neubert, Technological Investigation of LTCC-Based Micro-electro-mechanical Systems (MEMS) to improve reliability and accuracy, Proc. EMPC, Grenoble (2013).