Scholarly article on topic 'Polymer-based VOC Sensor Module for Wireless Sensor Network System'

Polymer-based VOC Sensor Module for Wireless Sensor Network System Academic research paper on "Materials engineering"

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Procedia Engineering
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{polymer / cantilever / "frequency shift" / VOC / "wireless sensor network system"}

Abstract of research paper on Materials engineering, author of scientific article — N. Shiraishi, M. Kimura, H. Okada, Y. Ando

Abstract We have developed and evaluated a Polymer-based VOC (volatile organic compounds) sensor module to realize a low-cost wireless sensor network system for environmental monitoring. The polymer-based VOC sensor module consists of a polymer- based VOC sensor, a sensing circuit, a transmitter and a receiver. The polymer-based VOC sensor composed polycarbonate (PC) cantilevers with PVDF (polyvinylidene fluoride) piezoelectric films. We examined the change of resonance frequency in the presence of VOC vapor by setting up a resonant sensor evaluation system equipped with a VOC dilution flow system and temperature-controlled chamber. The resonant frequency of polymer-based VOC sensor shifted significantly downward during the toluene, octane and ethanol exposure. The resonant frequency was converted to I2C output signal by I2C-I/F and transmitted to receiver

Academic research paper on topic "Polymer-based VOC Sensor Module for Wireless Sensor Network System"

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Procedía Engineering 87 (2014) 600 - 603

Procedía Engineering

www.elsevier.com/locate/procedia

EUROSENSORS 2014, the XXVIII edition of the conference series

Polymer-based VOC sensor module for wireless sensor network

system

N. Shiraishia' 4 *, M.Kimura3' b, H. Okada8, c and Y. Andoc d

aNMEMS Technology Research Organization, Tsukuba 305-8564, Japan bShinshu University, Nagano 386-8567, Japan cNational Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8564, Japan dTokyo University of Agriculture and Technology, Tokyo 184-8588, Japan

Abstract

We have developed and evaluated a Polymer-based VOC (volatile organic compounds) sensor module to realize a low-cost wireless sensor network system for environmental monitoring. The polymer-based VOC sensor module consists of a polymer-based VOC sensor, a sensing circuit, a transmitter and a receiver. The polymer-based VOC sensor composed polycarbonate (PC) cantilevers with PVDF (polyvinylidene fluoride) piezoelectric films. We examined the change of resonance frequency in the presence of VOC vapor by setting up a resonant sensor evaluation system equipped with a VOC dilution flow system and temperature-controlled chamber. The resonant frequency of polymer-based VOC sensor shifted significantly downward during the toluene, octane and ethanol exposure. The resonant frequency was converted to I2C output signal by I2C-I/F and transmitted to receiver

© 2014PublishedbyElsevierLtd. This isanopenaccess article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the scientific committee of Eurosensors 2014 Keywords: polymer; cantilever; frequency shift; VOC; wireless sensor network system

1. Introduction

VOC (volatile organic compounds) sensors are widely used in human healthcare and environmental monitoring of chemicals for risk prevention. The MOS (metal oxide semiconductor) sensor, the most widely used sensor for VOC

* Corresponding author. Tel.: +81-29-875-7966; fax: +81-29-875-7967. E-mail address: nshiraishi@nmems.or.jp

1877-7058 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Peer-review under responsibility of the scientific committee of Eurosensors 2014 doi: 10.1016/j.proeng.2014.11.560

detection to date, consumes power at levels too high to permit practical use for real-time local monitoring. As an alternative, resonant mass sensors of vibrating cantilevers based on MEMS (microelectromechanical systems) are believed to have good potential for this purpose [1' 2]. Most resonant cantilever sensors so far reported have been made of silicon-based materials, namely, silicon, silicon nitride, or silicon oxide [1' 2]. Yet silicon-based cantilever sensors are invariably expensive to fabricate and have strong environmental impact during fabrication. This might limit their industrial use for real-time local monitoring. Polymer-based cantilevers offer advantages such as low cost and environmentally friendly fabrication process [3]. Furthermore, the polymer-based cantilevers have the potential of reactive layer-less VOC sensor, because the polymer cantilever of polymeric material adsorbs VOC in itself, whereas silicon-based cantilever sensors absolutely need reactive layer of PBD (polybutadiene) or TiO2 (Titanium dioxide). In this paper, a polymer-based VOC sensor module was first developed for wireless sensor network system. We then describe a fabrication of polymer-based VOC sensor. Finally, we examined the frequency shift of the polymer-based VOC sensor induced by the adsorption of VOC molecules using VOC dilution flow system.

2. Principle of VOC sensing using polymer-based VOC sensor module

Figure 1(a) shows the principle of VOC sensing with the polymer-based VOC sensor module. The module consists of a polymer-based VOC sensor, a sensing circuit, a transmitter and a receiver. The sensor consists of polycarbonate (PC) cantilevers and piezoelectric films of PVDF (polyvinylidene fluoride). The sensor is set onto an external vibrator over the sensing circuit composed of an oscillating circuit and I2C-I/F. When the driving signal from the oscillating circuit is applied to the external vibrator, the PC cantilevers vibrate at its resonant frequencies. The vibration powers of the PC cantilevers in resonance are transmitted to the PVDF piezoelectric films. Resonant mass sensor detects target VOC gases by measuring variations in the resonant frequency of the sensing structure due to mass loading. When the cantilever adsorbs the VOC gas molecules, the mass increases and the resonant frequency decreases. Hence, the VOC gas can be detected by measuring the resonant frequency shift. The resonant frequency is converted to I2C output signal and transmitted to receiver. Fig. 1 (b) shows photograph of the polymer-based VOC sensor module.

Fig. 1 (a) Schematic showing how the polymer-based VOC sensor module detects VOC gases; (b) Photograph of the polymer-based VOC sensor module

3. Development of polymer-based VOC sensor

We developed the polymer-based VOC sensor which consists of the PC cantilevers and the piezoelectric films of PVDF. The photograph in Fig. 2 (a) shows the entire image of the polymer-based VOC sensor at the completion of fabrication. The polymer-based VOC sensor set onto PZT vibrator and placed in package. Cu thin film and Au wire were adopted for the electric connection. Fig. 2 (b) shows enlarged micrograph of the PC cantilever. The PC cantilever length was 1500^m, width was 300^m and thickness was 100^m. The overlapped area between the PC

cantilever and PVDF piezoelectric film with upper and lower electrodes measured 100^m X 300^m. The thickness of the PVDF piezoelectric film is 9^m with Cu/Ni (10nm/70nm) electrode.

PC cantilever .

PVDF / piezoelectric film

2000|im

PZT ^ p vibrator \ Au wire

Cu thin film

Fig. 2 (a) External view of the polymer-based VOC sensor fixed on a PZT vibrator; (b) Enlarged micrograph of the fabricated PC cantilever

4. VOC detection by the polymer-based VOC sensor module

The module was installed in the evaluation system equipped with a VOC dilution flow system and temperature-controlled chamber to investigate the basic characteristic of the polymer-based VOC sensor module (Fig. 3). The PC cantilever in polymer-based VOC sensor oscillated at the 4th flexural vibration mode at 294 kHz. Fig. 4 (a) shows I2C output signal from the I2C-I/F of the polymer-based VOC sensor module. The carrier frequency of wireless communication was set to 315 MHz, which is the corticated license-free frequency range in Japan. Fig. 4 (b) shows the resonant frequency shift of the PC cantilever. The PC cantilever was continuously exposed to toluene, octane and ethanol for 180 seconds (gray zone in Fig. 4 (b)). The resonant frequency shifted significantly downward during the toluene, octane and ethanol exposure, reaching 54 Hz, 52 Hz and 15 Hz below baseline at the 3-minute point. Subsequently, in the absence of toluene, octane and ethanol, the resonant frequency shifted back up and returned to the baseline condition at the 6-minute point without heat treatment.

Fig. 3 Schematic view of our system for evaluating VOC sensitivity

Fig. 4 (a) I2C output signal from the I2C-I/F of the polymer-based VOC sensor module; (b) Resonant frequency shift of the PC cantilever. The frequency shift of the PC cantilever was recorded while injecting the chamber with pure nitrogen gas, followed by nitrogen diluted with toluene, octane and ethanol gases at a concentration of 5000 ppm (flow quantity of 100 sccm), followed by pure nitrogen.

5. Conclusions

We have developed a polymer-based VOC sensor module consisting of a polymer-based VOC sensor, a sensing circuit, a transmitter and a receiver. The polymer-based VOC sensor composed polycarbonate (PC) cantilevers with PVDF (polyvinylidene fluoride) piezoelectric films. We examined the change in resonance frequency in the presence of VOC vapor by setting up the polymer-based VOC sensor module evaluation system equipped with a VOC dilution flow system and temperature-controlled chamber. The resonant frequency of PC cantilever shifted significantly downward during the toluene, octane and ethanol exposure. The resonant frequency is converted to I2C output signal by I2C-I/F and transmitted to receiver.

Acknowledgements

The "Development of polymer-based vibration VOC sensors" is one of several research themes pursued in the "Green sensor network project (GSN-PJ)" run by the New Energy and Industrial Technology Development Organization (NEDO).

The authors would like to thank Mr. Kusano (Shinsyu University) and Ms. Sato (Shinsyu University) for their technical support.

References

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