SENSORDEVICES 2013 : The Fourth International Conference on Sensor Device Technologies and Applications
Development of Shear Horizontal Surface Acoustic Wave Sensor for Detecting Methanol Concentrations in a Direct Methanol Fuel Cell
Jun Kondoh Takuya Nozawa and Saburo Endo Graduate School of Science and Technology Graduate School of Engineering Shizuoka University Shizuoka University Hamamatsu-shi, Japan Hamamatsu-shi, Japan [email protected]
Abstract— A liquid phase sensor is realized by using a shear As the generation efficiency of the DMFC depends on the horizontal surface acoustic wave (SH-SAW). The SH-SAW concentration of MeOH, the concentration must be known. sensor can detect density and viscosity product, conductivity As the DMFC has an optimum concentration range, and relative permittivity of liquid. Sensor sensitivity depends monitoring of the MeOH concentration is required. The on a piezoelectric substrate used. When a 36° rotated Y-cut, X MeOH concentration is a function of sound speed, viscosity, propagating LiTaO3 is used for the substrate, high sensitive density, permittivity, and refractive index. The SH-SAW detection of electrical properties is possible. In this paper, the sensor can detect viscosity and density products, and SH-SAW sensor is applied to a methanol (MeOH) sensor. First, permittivity. The SH-SAW sensor, which is fabricated on frequency dependences of the SH-SAW sensor are discussed. 36° rotated Y-cut, X-propagating LiTaO (36YX-LiTaO ) The results indicate that the responses of high frequency 3 3 can detect the permittivity with high sensitivity [6]. In sensor agree with the perturbation theory. The application of the SH-SAW sensor is MeOH concentration detection in a previous research [6-9], frequency of the SH-SAW sensor direct methanol fuel cell (DMFC). As a formic acid is was fixed at 50 MHz. In this paper, the SH-SAW sensors produced during electrode reactions in the DMFC, real fuel with different frequencies are used. The MeOH solutions are becomes binary-mixture solutions of MeOH and formic acid. measured by changing the temperature. The experimental In this paper, the solutions are also measured. It is found that results are compared with the perturbation theory. Also, influence of the formic acid can be neglected by using high influence of a formic acid is discussed. frequency SH-SAW sensor. This means that the MeOH concentration is determined from phase shift. II. THEORY Detection mechanism of the MeOH concentration is Keywords-component; SH-SAW sensor; direct methanol fuel based on the electrical perturbation [10, 11]. A reference cel;, methanol senso; influence of sensor frequency liquid is assumed as nonelectrolyte and expressed as
I. INTRODUCTION ℓ ε . (1) Surface acoustic wave (SAW) devices have a wide range of applications not only in filters, resonators, and oscillators Here, ℓ , , and ε are dielectric constant, relative [1], but also in sensors [2] and actuators [3]. Propagating permittivity of the reference liquid, and dielectric constant of characteristics of a SAW depends on an adjacent medium free space, respectively. Electrical property of a sample on a SAW propagating surface. The SAW is mechanically solution is represented by a complex permittivity, ℓ′, as and electrically perturbed, when the adjacent medium follows. physically and chemically changes. When a liquid is loaded on the SAW propagating surface, the SAW is ′ influenced by liquid properties, such as density, viscosity, ℓ ε (2) conductivity, and permittivity. However, to realize a liquid-phase SAW sensor, it is necessary to use a shear horizontal SAW (SH-SAW) to avoid a longitudinal wave Here, is conductivity of liquid, is an angular frequency radiation into a liquid [4]. of the SH-SAW sensor, and √ 1. The prime (′) denotes A direct methanol fuel cell (DMFC) [5] is operated using parameter of the sample solution. The SH-SAW is a methanol (MeOH) solution. The reactions on the anode electrically perturbed the change from (1) to (2). The and cathode in the DMFC are as follows: velocity shift, ΔV/V, and attenuation change which is normalized by wave number, Δα/k, are expressed by the CH OH H O 6H 6e CO for the anode, following equations. 3 2 2 3 2 2 T O2 6H 6e 3H2O for the cathode. V K s '/ 0 r ' r r ' 0 P 2 (3) V 2 2 T 2 '/ r ' 0 P
Copyright (c) IARIA, 2013. ISBN: 978-1-61208-297-4 112 SENSORDEVICES 2013 : The Fourth International Conference on Sensor Device Technologies and Applications
2 T K s ' / r 0 P (4) k 2 2 T 2 ' / r ' 0 P