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Sensors and Actuators A 159 (2010) 168–173

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

journa l homepage: www.e lsev ier .com/ locate /sna

novel NO2 gas sensor using dual track SAW device

hangbao Wena,b,∗, Changchun Zhuc, Yongfeng Jua, Hongke Xua, Yanzhang Qiua

Institute of Micro-nanoelectronics, School of Electronics and Control Engineering, Chang’an University, Xi’an, 710064, ChinaShaanxi Engineering and Technique Research Center for Road and Traffic Detection, Xi’an, 710064, ChinaInstitute of Vacuum Microelectronics, School of Electronics and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China

r t i c l e i n f o

rticle history:eceived 6 August 2009eceived in revised form 23 February 2010ccepted 14 March 2010

a b s t r a c t

A nitrogen dioxide (NO2) gas sensor using dual track surface acoustic wave (SAW) device was developedand fabricated in this paper. The dual track architecture and the MSCs were skillfully applied to thesensor. The external perturbations were eliminated by the dual track architecture, and the bulk acousticwave (BAW) was suppressed by Multistrip coupler (MSC). The input IDT apodized by Morlet wavelet

vailable online 18 March 2010

eywords:urface acoustic wave (SAW) deviceitrogen dioxide (NO2)as sensor

function can improve the side lobe rejection compared with uniform IDT. Utilizing Tungsten powders,hydrogen peroxide, absolute methanol and polyvinylpyrrolidone as raw materials, the micro-porousnetwork Tungsten trioxide (WO3) film sensitive to NO2 gas was fabricated in measurement acoustictrack of dual track SAW device. Experiments results confirm that the NO2 gas sensor using dual track SAWdevice has good response characteristics to different concentrations NO2 gas from 0.5 ppm to 10 ppm.

sens

ual trackungsten trioxide (WO3)

Furthermore, the NO2 gas

. Introduction

Nitrogen dioxide (NO2) is a major component of vehicle exhaust,s well as the main emissions of the thermal power and chemicalroduction process [1,2]. It is a main source of acid rain and photo-hemical smog, and is very harmful to the plant, building facilitiesnd production equipments [3,4]. Furthermore, NO2 is a toxic gastself, and causes some serious diseases of human being, such aseart failure, arrhythmia and other cardiovascular causes of death5]. Hence, the monitor and measurement of NO2 gas have greatignificance to decrease the damage level and keep humane heath.o far there have been lots of efforts in developing a variety ofO2 gas sensors such as semiconductor sensor [6], capacitive type

ensor [7], and surface acoustic wave (SAW) sensor [8]. In theseensors, SAW sensor is passive, small volume, and low price, whichs quite attractive in portable instruments and wide requirementf applications.

In SAW device, the SAW energy is confined into a zone closeo the piezoelectric crystals surface and is of a few wavelengths

hick [9,10]. The acoustic energy confinement makes the SAW sen-or very sensitive to any changes around environment. Differentechanisms of interaction between the measured and the acoustic

ropagation have been reported, such as the mass loading effects,

∗ Corresponding author at: Institute of Micro-nanoelectronics, School of Electron-cs and Control Engineering, Chang’an University, Xi’an, 710064, China.el.: +86 29 82339349; fax: +86 29 82339349.

E-mail address: wchbdn@163.com (C. Wen).

924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2010.03.012

or using dual track SAW device has good reproducibility and stability.© 2010 Elsevier B.V. All rights reserved.

and the changes of the elastic loading and the electrical conductiv-ity loading [11–15]. Most usually, the SAW velocity is affected bythe additional mass loading of gas molecules in acoustic track. Theperturbation analysis of additional mass loading has been given byAuld [15].

Currently, there are two main research directions in gas sensorsusing SAW device. One direction is concerned with the improve-ment of architecture and performance of sensor, such as lowinsertion sensor, strong anti-interference and high stability sen-sor. Although the SAW energy confined near surface region is animportant merit to SAW device serving as gas sensor, it also makesSAW sensor sensitive to temperature and other external factors.Furthermore, the bulk acoustic wave (BAW) and side lobe havea great impact on the performances of gas sensor, and then itcan result in some measurement errors. In order to solve theseproblems, some measures and schemes have been adopted inSAW gas sensors, such as dual delay line pattern using the dif-ference principle [16], Multistrip coupler (MSC) suppressing BAW[17], but these shortcomings cannot be solved simultaneously, andonly partial performances are improved. In addition, the sensi-tive film on the propagation track is responsible for the responseperformances of SAW device, thus the other direction devotesto studying and synthesizing new sensitive material or sensi-tive film. For the NO2 gas sensitive film, several materials have

been tried, such as SnO2, ZnO, and WO3 (Tungsten trioxide) film[18–20]. In order to improve the sensitivity of WO3 film, thesemethods require other doped substances and high temperature[18,20]. However, the operating temperature of SAW devices isusually less than 100 ◦C, thus these films required temperatures

Actuators A 159 (2010) 168–173 169

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C. Wen et al. / Sensors and

bove 100 ◦C are not suitable as the sensitive film of SAW gas sen-or.

Here, we develop a NO2 gas sensor using dual track SAWevice. The sensor consists of a dual track SAW device and theicro-porous network WO3 film sensitive to NO2. Because the

olyvinylpyrrolidone is applied to the preparation of WO3 film, aumber of micro-porous networks were formed in WO3 film. Thisicro-porous network WO3 film can physically absorb NO2 gas at

oom temperature by means of the mass loading effects. The dualrack architecture eliminates the external perturbations, and the

SC suppresses the BAW, and the input IDT apodized by Morletavelet function can improve the side lobe rejection. Hence, the

ensor has achieved good response characteristics and stability.he fabrications and principles, and tests of NO2 gas sensor usingual track SAW device are presented in this paper. The paper isrganized as follows. Section 2 is devoted to realizing the NO2 gasensor using dual track SAW device. The working principles of sen-or are presented in Section 3. In Section 4, the test system andhe response characteristics, and the reproducibility and stabilityf the NO2 gas sensor using dual track SAW device are analyzedespectively. Conclusions are drawn in Section 5.

. Fabrication of sensor

.1. Design of SAW device

There are a number of piezoelectric materials available to SAWensors. The choice depends on the operation frequency and theype of the device. Here, in order to decrease the insertion loss andhe size of sensor, 128◦ Y-X LiNbO3 crystals with high electrome-hanical coupling constant are selected as substrate materials ofAW sensor [13]. The velocity of the SAW propagating on the 128◦

-X LiNbO3 substrate is 3980 m/s, and the electromechanical cou-ling constant K2 is 5.5%.

In our previous works [21,22], we found that the smaller sideobe can be obtained if the IDT of SAW device is apodized byhe envelope of Morlet wavelet function. In this dual track SAWevice, the input IDT is apodized by the envelope of Morlet waveletunction without using uniform IDT. Furthermore, the dummy elec-rodes are used to eliminate the phase front distortion of wavesropagating through apodized input IDT, and the split electrodesre used to minimize acoustic reflections within the transducers.luminum IDTs, with uniform spacing and metallization ratio of0%, are directly deposited by conventional contact ultraviolet pho-olithography on the top of the 128◦ Y-X LiNbO3 substrate.

Fig. 1(a) shows the schematic diagram of dual track SAW device.he center frequency of device is 101.764 MHz. The IDT0 is an inputDT apodized by the envelope of Morlet wavelet function. The elec-rodes width and electrodes gap of input IDT are 4.888 �m. Theumbers of electrodes pairs are 128. The maximum acoustic aper-ure is 1.53 mm. The MSC1 and MSC2 are two MSCs with identicalesign parameters. Their electrodes width and electrodes gap of

nput IDT are 7.16 �m, and the numbers of electrodes pairs are03. Fig. 1(b) shows the local magnified picture of IDT0 and MSC1xamined by PIXERA (Model: PVC-100). IDT1 and IDT2 are two uni-orm output IDTs with identical design parameters. The electrodesidth and electrodes gap of input IDT are 4.888 �m. The numbers

f electrodes pairs are 3 and their maximum acoustic aperture is.521 mm. Area1 is the region between MSC1 and output IDT1, andrea2 is the region between MSC2 and output IDT2. The distance

etween MSC1 and output IDT1, and the distance between MSC2nd output IDT2 are 5 mm. The height of output IDT1 and outputDT2 is 3.5 mm. Hence, we can design the sensitive areas of Area1nd Area2 as 4.5 mm × 3.2 mm. The sound absorption material canuppress the unwanted energy reflected from the substrate edges.

Fig. 1. Schematic diagram (a), and the local magnified picture (b) of dual track SAWdevice.

2.2. Preparation of WO3 film

Most usually, the WO3 (Tungsten trioxide) film can be depositedby sputtering technique, chemical vapour deposition and sol–gelpreparation [23–25]. However, these methods are quite expensiveand require complicated measurement equipment, which are diffi-cult to meet the requirement of low price and wide applications. Inaddition, in order to improve sensitivity, it is necessary to elevatemeasurement temperature and add other doped metal, such as Au,Pt, Pd and Nb.

Here, we select Tungsten powders, hydrogen peroxide, absolutemethanol and polyvinylpyrrolidone as raw materials. The micro-porous network WO3 film is fabricated in Area1 of dual trackSAW device via the spray method. At the room temperature, 1.2 gpure Tungsten powders were dissolved in 15 mL 30 wt% hydro-gen peroxide under stirring. The white solution can be obtained byultrasonication for 4 h. The 15 mL absolute methanol was dropwiseadded to the white solution under stirring. The white flocculentprecipitate was filtrated after 12 h at room temperature, and thetransparent solution was obtained. The 0.15 g polyvinylpyrrolidonewas added to the transparent solution. Then the relatively viscoussolution was formed, which was very stable and able to be place fordays without precipitation. Subsequently, the solution was concen-trated to 15 mL at 393 K, and then was adder to the sprayer.

The film thickness can be controlled by controlling the volumeof spray chamber of sprayer. In the experiments, we found that therelatively thick film can be achieved by the sprayer with a largevolume of spray chamber, and the relatively thin film can be pro-duced with a small volume of spray chamber. When the volume ofspray chamber of sprayer was 0.6 mL, the thickness of WO3 film isapproximately 90 �m. The thickness should be a more optimizedfor this sensor. Moreover, we found that the response amplitude ofsensor will decrease gradually, and the insertion loss will increasevaried with the thickness increasing. If the film thickness is lessthan 90 �m, we have to reduce the volume of solution in the spray

chamber. However, the area of sensitive film will reduce due tothe solution surface tension, so the sensitivity of the sensor is alsodecrease.

170 C. Wen et al. / Sensors and Actuators A 159 (2010) 168–173

urface

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Fig. 2. Substrate surface before and after coating with WO3 film. (a) Free s

A thickness of 2.5 mm plastic sheet was selected as mask plate.square hole with the same size as the Area1 was made in theask plate center. Under the protection of mask, the solution was

prayed onto the region Area1. In this experiment, the distanceetween mask plate and the device surface keeps about 5 mm, andhe distance between the nozzle and mask plate is about 3 cm. Ashe liquid surface tension, this method is not entirely accurate inontrolling the size of the sensitive area, but can avoid the leakageffects underneath the mask layer in the photoresist and othersethod, and the device surface contamination and device damage.The sensor was put into the thermostat at 333 K for 1 h, and

hen the power of the thermostat was cut off. The tempera-ure of sensor gradually returned to room temperature after 10 h.he micro-porous network WO3 film was obtained on the Area1f dual track SAW device. Because the structure-directing agentolyvinylpyrrolidone is applied to the preparation of WO3 film, aumber of micro-porous networks are formed in WO3 film. Fur-hermore, when the weight ratio between polyvinylpyrrolidonend pure Tungsten powders is from 10% to 16%, we find the porousize of WO3 film is about 1 to 50 �m and the sensor has goodbsorption and desorption characteristics. When the porous sizef WO3 film is less than 1 �m, the sensor has a long recovery time.hen the porous size of WO3 film is larger than 50 �m, the sen-

or has a poor adsorption to NO2 gas. Especially, the sensor haso obvious absorption features when the number of porous size in00 �m is more than 50%. In this research, the weight ratio betweenolyvinylpyrrolidone and pure Tungsten powders is 12.5%. Fig. 2hows the substrate surface before and after coating with WO3 filmxamined by PIXERA (Model: PVC-100).

. Working principles of sensor

When the electronic signal is applied to the sensor in Fig. 1,wing to the inverse piezoelectric effects of substrate, the inputDT (IDT0) in SAW sensor excites not only SAW propagating alongrystals surface, but also BAW. The BAW has higher frequency thanAW, and can produce some ripples and spurious components inesponse curve [26]. The SAW and BAW excited by input IDT wille bidirectional transmission, and the MSC can fully transfer SAWithout affecting BAW to continue to propagate in the primitive

coustic track [13,27]. In this case, the SAW and the BAW launchedy input IDT can be separated into different tracks. Therefore, one

an spread some sound absorption materials at the terminal of BAWransmitting, and ultimately eliminate the BAW that interferes inhe performance of SAW gas sensor.

Two full transfer MSCs (MSC1 and MSC2) are symmetri-ally deposited on the two acoustic propagation tracks of input

of 128◦ Y-X LiNbO3 crystals; (b) substrate surface coated with WO3 film.

IDT0, respectively. The SAW and the BAW launched by theinput IDT (IDT0) can be separated by MSCs (MSC1 and MSC2),and the SAW can be transferred into down acoustic tracks.The output IDTs (IDT1 and IDT2) with same design parametersare symmetrically deposited on two output acoustic tracks ofMSCs (MSC1 and MSC2), respectively. The SAW separated byfull transfer MSCs (MSC1 and MSC2) can be received by theoutput IDTs (IDT1 and IDT2) via regions Area1 and Area2, respec-tively. Hence, two acoustic tracks IDT0 → MSC1 → Area1 → IDT1and IDT0 → MSC2 → Area2 → IDT2 are symmetrically formed on theboth sides of input IDT (IDT0).

Because the design parameters of one component are identicaland symmetric in this sensor, the sensitive film can be fabricatedon regions Area1 or Area2. Once one region is chosen as measure-ment region to absorb gas, the other region without sensitive filmcan be seen as reference region. Hence, the SAW propagation trackwith measurement region can be called measurement track, andthe other track with reference region can be called reference track.

In this work, the region Area 1 is coated with WO3film sensitive to NO2 gas, and then the acoustic trackIDT0 → MSC1 → Area1 → IDT1 is the measurement track. Theother region Area2 remains the free surface of piezoelectric sub-strate, and then the acoustic track IDT0 → MSC2 → Area2 → IDT2 isthe reference track.

The concentration of NO2 gas was measured by this dual trackSAW gas sensor, the principles are as follows.

Before the sensor was exposed to the concentration C1 attime t0, the output frequency MC1t0 of the measurement trackIDT0 → MSC1 → Area1 → IDT1 and the output frequency RC1t0 of thereference track IDT0 → MSC2 → Area2 → IDT2 can be measured andobtained by network analyzer. Hence, the difference frequencyDC1t0 between the measurement track and the reference trackshould be

DC1t0 =∣∣MC1t0 − RC1t0

∣∣ (1)

When the concentration of NO2 gas is C1 at time t1, the differ-ence frequency signal DC1t1 between the measurement track andthe reference track is

DC1t1 =∣∣MC1t1 − RC1t1

∣∣ (2)

Similarly, when the concentration of NO2 gas is C1 at arbitrarytime tn, the difference frequency signal DC1tn between the measure-

ment track and the reference track should be

DC1tn =∣∣MC1tn − RC1tn

∣∣ (3)

One can measure the difference valuesDC1t0 , DC1t1 , DC1t2 , DC1t3 , DC1t4 , . . . , DC1tn for the NO2 concen-

C. Wen et al. / Sensors and Actuators A 159 (2010) 168–173 171

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Fig. 3. Frequency spectrum characteristic of the dual track SAW device.

ration C1 at time t0, t1, t2, t3, t4, . . ., tn. Hence, the curve �C1tn − tn

hat is the difference frequency signal varied with time is obtained.Using the similar approach, ones can obtain n − 1 curves, which

re DC2tn − tn, DC3tn − tn, DC4tn − tn, . . . , DCntn − tn at concentra-ion C2, C3, C4, . . ., Cn, respectively.

Assuming that the WO3 film is sufficiently stable, the addi-ional loading mass of sensitive film affected by the environmentalnterference can be omitted. For the NO2 gas sensor using dualrack SAW device at time tn, if the external perturbation is ˛n

or the measurement track IDT0 → MSC1 → Area1 → IDT1, and thexternal perturbation should also be ˛n for the reference trackDT0 → MSC2 → Area2 → IDT2 due to the symmetrical architecturef this sensor. The final measurement value DCntn is the differencerequency between the measurement track and the reference track,hus the external perturbation can be removed.

For example, when the NO2 gas concentration is C1 at time tn, theeasurement error from temperature is ˛nT for the measurement

rack and the reference track, respectively. Under this condition,he output DC1tn of the NO2 gas sensor using dual track SAW devicehould be the difference value signal between the measurementrack and the reference track, i.e.,

C1tn =∣∣(MC1tn + ˛nT ) − (RC1tn + ˛nT )

∣∣ =

∣∣MC1tn − RC1tn

∣∣ (4)

From Eq. (4), one can find that the measurement error ˛nT

s removed. Hence, the NO2 gas sensor using dual track SAWevice can compensate the temperature or other perturbationsrom external environment to the measurement signal.

The frequency spectrum characteristic of this dual track SAWevice is measured by Advantest R3765CG network analyzer, ashown in Fig. 3. We can find that the dual track SAW device canuppress the BAW, and avoid higher side lobe and spurious compo-ents in the right side of the frequency response curve. In addition,he input IDT is apodized with the envelope of Morlet wavelet func-ion, thus side lobe suppression is better than that of uniform IDT13,28].

. Sensor testing

.1. Experimental setup

In order to evaluate the response of NO2 gas sensor using dual

rack SAW device, we design the experimental setup, as shown inig. 4. The experimental setup consists of a NO2 gas sensor usingual track SAW device in tight gas chamber, an air and NO2 gasupply part, NO2 gas exhaust and purification part, an Advantest3765CG network analyzer, and power supply part and signals

Fig. 4. Schematic diagram of experimental setup for testing the NO2 gas sensorusing dual track SAW device.

processing part. The volume of gas chamber is 4833 cm3. The gassupply part in the experimental setup can obtain 0.1 ml injectedgas, and the concentration of NO2 gas is 1680 ppm. Thus, the NO2gas concentrations from 34.7 ppb to 1680 ppm can be detected bythe experimental setup shown in Fig. 4.

At the beginning of test, the air is injected into the chamber for2 min. When the NO2 gas is infused into the chamber, the differ-ence frequency of the sensor reaches to a stable value. Due to themass loading effects, the WO3 film can absorb NO2 gas, and producedownward shift in the frequency of the measurement track. Thus,the difference frequency between the measurement track and thereference track can be obtained.

With this experimental setup shown in Fig. 4, all differencefrequencies can be recorded and processed for different gas con-centrations at different time periods. The changes in differencefrequencies can be attributed to the mass uptake of NO2 gas onthe WO3 film.

In the experiment, all of the gas concentration was achieved bymixing the NO2 gas sample and laboratory air. Thus, each end ofthe experiment, we will pass air into the chamber, and purge theNO2 gas from the chamber.

4.2. Response characteristics

When the NO2 gas at certain concentrations is injected into thechamber, the NO2 gas adsorbed in WO3 film will gradually reach toa steady state, and the output response of the sensor will reach toa stable value. Here, the working principle of the sensor is mainlybased on the mass loading effects. Hence, the output responses ofsensor are different due to the mass loading effects determined bythe different NO2 gas concentrations.

In order to observe the response characteristics of NO2 gas sen-sor using dual track SAW device, the NO2 gas concentrations at0.5 ppm, 1 ppm, 2 ppm, 3 ppm, 5 ppm, 8 ppm and 10 ppm are mea-sured, the response characteristics of sensor for NO2 gas at differentconcentrations is shown in Fig. 5. We can observe that the differ-ence frequency is a function of time at different concentrations, andthe difference frequency outputted by the NO2 gas sensor using

dual track SAW device increases with increasing NO2 gas concen-tration.

Furthermore, we found that the porous size of WO3 film hasa great influence on the response time and the recovery time ofthe NO2 gas sensor using dual track SAW device. When the porous

172 C. Wen et al. / Sensors and Actuators A 159 (2010) 168–173

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ig. 5. Response characteristics of sensor for NO2 gas at different concentrations.

ize of WO3 film is about 1–50 �m, the sensor has good responseime and the recovery time. When the porous size of WO3 films less than 1 �m, the sensor has a long recovery time as the gasesorption time become longer. When the porous size of WO3lm is larger than 50 �m, the sensor has a poor response as theO3 film has a weak adsorption to NO2 gas. The sensor has no

bvious response when the porous size of WO3 film is larger than00 �m.

.3. Reproducibility

The reproducibility is a very important index for sensor. Inase of environmental monitoring, the threshold limit value (TLV)or NO2 gas is 3–25 ppm, according to the safety standards giveny the American Conference of Governmental Industrial Hygien-

st (ACGIH). Here, we examine the reproducibility by this sensoresponse to the concentration of NO2 gas at 3 ppm in chamber. Allests are performed at same experimental conditions, and the testrocedure is repeated five times using the same sensor. The testesults are shown in Fig. 6.

Fig. 6 shows the NO2 gas sensor using dual track SAW device has

ood reproducibility in five times test. In addition, one can observehat the sensor reaches a stable state at approximately 240 s, andhese test curves are coincident approximately at 260 s.

Fig. 6. Reproducibility of sensor for concentration at 3 ppm.

Fig. 7. Error curve of sensor in 60 days.

4.4. Stability

In order to examine the stability of the sensor, the NO2 gas sensorusing dual track SAW device is placed into the gas chamber withNO2 gas concentration at 3 ppm. The experiment is performed for60 days, and the gas concentration is measured every 24 h.

Fig. 7 shows the error curve of the frequency shift outputtedby the NO2 gas sensor using dual track SAW device. In 60 days,the maximum measurement error is about 180 Hz, and the rela-tive change of frequency is 0.82%. Moreover, the response speed ofsensor has not changed, thus the sensor has good stability in longterm.

In addition, the sensitive film is one of the important compo-nents of the sensor, so its stability can affect the stability of thesensor. In order to examine the stability of WO3 film, we heat upthe gas chamber with NO2 gas concentration at 3 ppm, and theambient temperature of gas chamber rises from 25 ◦C to 80 ◦C in3 min. We measure that the change of frequency shift in measure-ment track (IDT0 → MSC1 → Area1 → IDT1) is 0.185 kHz due to thechange of temperature, and the change of frequency shift in refer-ence track (IDT0 → MSC2 → Area2 → IDT2) is 0.179 kHz due to thechange of temperature. The concentration error produced by themass loading of sensitive film approximates 0.83 × 10−3 ppm dueto the change of temperature. Hence, the frequency shift producedby WO3 film does not change a lot in comparison with the frequencyshift produced by SAW device due to environmental interference.It should be noted that the performance deviation produced bysensitive film must be measured and consider provided that thesensitive film is not stable to the temperature change, and otherenvironmental interference is relatively large.

5. Conclusions

In this paper, we have presented a NO2 gas sensor using dualtrack SAW device. The WO3 film sensitive to NO2 gas was syn-thesized and fabricated in measurement acoustic track of dualtrack SAW device. The sensor has some advantages over SAWsensor, i.e., the elimination of the external perturbations, thesuppression of the BAW and the improvement of the side loberejection. The experiment results demonstrate that the sensor has

good response characteristics, reproducibility and stability. Fur-thermore, the dual SAW gas sensor can be used to detect anygases, supposing that the corresponding sensitive film can befound.

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cknowledgements

This project is supported by the National Natural Science Foun-ation of China (Grant Nos.60806043 and 60876038), the Chinaostdoctoral Science Foundation (Grant No. 20090461278), thepecial Fund for Basic Scientific Research of Central Colleges,hang’an University (Grant No. CHD2009JC025), and the Key Labo-atory Foundation of Shaanxi Engineering and Technique Researchenter for Road and Traffic Detection.

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Biographies

Changbao Wen was born in Shanxi, China in 1976. He received his PhD degree inphysics electronics from Xi’an Jiaotong University, in 2007. From 1996 to 2001, hewas an electronic engineer in Zhong TiaoShan Corp. He has been a faculty memberof School of Electronics and Control Engineering, Chang’an University. His currentresearch interests focus on SAW device, sensor and signal processing.

Changchun Zhu was born in 1936. He is now PhD tutor and professor of Xi’anJiaotong University, and the director of Institute of Vacuum Microelectronics andMEMS. He has been engaged in research works on vacuum microelectronics andsemiconductor devices, SAW and C-nanometer tube technology, etc. in recent years,more than 300 papers have been issued in scientific publications and conferencesof homeland and abroad, and ten patents have been possessed. He has won eightawards above provincial/ministerial level and many other awards.

Yongfeng Ju was born in Shaanxi, China in 1962. He received his BS degree fromChongqing University, China, in 1989, MS degree from Xi’an Jiaotong University, in1994, and PhD degree from Chang’an University, in 2006. He is now a professor inSchool of Electronics and Control Engineering of Chang’an University. His currentresearch interests focus on traffic intelligent control engineering, electronic devicesand signal processing.

Hongke Xu was born in Shaanxi, China in 1963. He received his PhD degree fromChang’an University, China, in 2006. He is now a professor in School of Electronicsand Control Engineering of Chang’an University. His current research interests focus

on signal processing and traffic intelligent control engineering.

Yanzhang Qiu was born in Shaanxi, China in 1962. He received his BS degree fromChang’an University, China, in 1989. He is now an associate professor in School ofElectronics and Control Engineering of Chang’an University. His current researchinterests focus on signal processing, electrical and electronics devices.