Design and fabrication of acoustic matching layerfor lead-free ultrasonic flowmeter

Mei HOTATE, Daichi YOSHIDOME, Takahiro KOJIMA, Takuya HOSHINA,Hiroaki TAKEDA and Takaaki TSURUMI³

Nano-Phononics Lab., Graduate School of Science and Engineering, Tokyo Institute of Technology,2–12–1 Ookayama, Meguro, Tokyo 152–8552, Japan

We designed and fabricated acoustic matching layers made of a BaTiO3/epoxy 0­3 composite material for a lead-free ultrasonicflowmeter. Specific acoustic impedance of fabricated 0­3 composite was controlled by changing volume fraction of BaTiO3 fineparticles. When the volume fraction of BaTiO3 fine particles was 40 vol% or less, sufficiently uniform 0­3 composite materialsand various specific acoustic impedances of 2.6­6.9 © 106 kg/m2s were obtained. For improving an accuracy of a lead-freeultrasonic transducer with (K,Na)NbO3-based ceramics, a pair of acoustic matching layers with a specific acoustic impedance of6.35 © 106 kg/m2s and a thickness of 0.260mm were fabricated. By using the fabricated acoustic matching layers in a lead-freeultrasonic flowmeter, the amplitude of output voltage was improved by 35% compared to that using commercialized matchinglayers. Even in comparison with the commercialized ultrasonic flowmeter with PZT-based materials, the amplitude of outputvoltage with the fabricated composite matching layers was 83.1%. The lead-free ultrasonic flowmeter with the fabricatedcomposite matching layer had high accuracy of a flow rate error of 3.5% or less. From above results, the effectiveness of theoptimized composite matching layers was verified.©2015 The Ceramic Society of Japan. All rights reserved.

Key-words : Ultrasonic flow meter, Piezoelectric ceramics, Lead-free, Acoustic matching layer, Composite

[Received November 28, 2014; Accepted January 22, 2015]

1. Introduction

Lead zirconate titanate (Pb(Zr,Ti)O3) or PZT)-based materialsare the most widely used materials for piezoelectric actuators,sensors and transducers due to their excellent piezoelectricproperties.1)­5) Considering lead toxicity, it is recently desiredto replace PZT-based materials for environmental protection.Lead-free piezoelectric materials,6)­9),12) such as piezoelectricsingle crystals, e. g. langasite10) and gehlenite,11) and ferroelectricceramics with Perovskite-type structure,13)­23) tungsten bronzestructure and bismuth layer-structured ferroelectrics,24),25) havebeen reported. However, lead-free materials which exhibit piezo-electric performance exceeding that of PZT-based materials areundiscovered. To replace PZT-based materials, it is necessary todesign and use a lead-free material to meet the requirementsof each application. Several material systems are now beingreconsidered as potentially attractive alternatives to PZT forspecial applications. For example, the potassium sodium niobate((K,Na)NbO3 or KNN)-based ceramics exhibit low dielectricconstants, large coupling coefficient, and low density, all ofwhich have advantages for high frequency transducer applica-tions.6) Especially, their lower acoustical impedance and the factthat they are nontoxic are advantages for in water and medicalapplications.Ultrasonic flowmeter is a potential application using lead-free

materials. In an U-shaped ultrasonic flowmeter shown in Fig. 1, apair of piezoelectric transducers is installed on the outer surfaceof the pipe. Each transducer works alternatively as both trans-

mitter and receiver of ultrasonic signals. When the ultrasonicwave is transmitted toward the upstream side against the waterflow direction, longer propagation time is required (Tbackward).In contrast, when the ultrasonic wave is transmitted toward thedownstream side with the flow direction, the propagation timebecame less (Tforward). That is, the signal is delayed or speeded upby the moving water. The flow velocity v of water is representedby26)

v ¼ L

2

1

Tbackward� 1

Tforward

� �; ð1Þ

where, L indicates the length of the pipe. Thereby the flow rateof water can be estimated. As mentioned above, KNN-basedmaterials have lower acoustic impedance than that of PZT-basedmaterials. The lower acoustic impedance benefits the improve-ment of input signal amplitude into the water. Therefore, KNN-based materials are considered to be a potential materials used astransducers in an ultrasonic flowmeter. In preliminary tests, we

Fig. 1. Schematic illustration of U-shaped ultrasonic flow meter.

³ Corresponding author: T. Tsurumi; E-mail: ttsurumi@ceram.titech.ac.jp

‡ Preface for this article: DOI http://dx.doi.org/10.2109/jcersj2.123.P5-1

Journal of the Ceramic Society of Japan 123 [5] 317-321 2015 Paper

©2015 The Ceramic Society of Japan

DOI http://dx.doi.org/10.2109/jcersj2.123.317

317

observed output signals from a receiver installed in a lead-freeultrasonic flowmeter, which meant that KNN-based materialswere available for transducers in an ultrasonic flowmeter. How-ever, the output voltage is approximately 50% lower than thatusing PZT-based materials. The accuracy of flow rate measuredby a lead-free ultrasonic flowmeter is not satisfactory. Thisis because the electromechanical coupling coefficient k andmechanical quality factor Q of KNN-based material is smallerthan those of PZT-based materials. The unavoidable deference inpiezoelectric performance decreases the accuracy of flow rate.To make up for the amplitude of ultrasonic wave output using

lead-free piezoelectric ceramics, we focused on acoustic match-ing layers in the ultrasonic flowmeter. Acoustic matching layersare installed between piezoelectric transducer and water, whichplay the role in acoustic impedance matching.27) In general,acoustic impedance and thickness of matching layer in commer-cialized flowmeter are not precisely optimized because the ultra-sonic output amplitude using PZT is sufficiently-high withoutoptimizing acoustic matching layer. However, the optimization ofacoustic matching layer should reduce the reflection of ultrasonicwave at the interface between piezoelectric transducer and water.The accuracy of flow rate measured by lead-free ultrasonicflowmeter may improve by optimizing acoustic matching layer.In this study, we designed and fabricated acoustic matching layeroptimized for lead-free ultrasonic flowmeter. The matching layerswere made of a 0­3 composite material consisted of ceramicsparticles and epoxy resin for controlling its acoustic impedance.To widely control the acoustic impedance of 0­3 compositematerial, the ceramics particles preferably have high density.Moreover, the particles should have high dispersibility in theepoxy resin to obtain a uniform composite material. For thesereasons, we selected BaTiO3 nanoparticles with an averageparticle size of 100 nm as the raw material.

2. Design of acoustic matching layer

Plane ultrasonic wave entering on a straight boundary betweentwo media (1 and 2) will both reflect back and transmit to theadjacent medium. The pressure amplitudes of the reflected andtransmitted waves are controlled by the impedance ratio of thetwo media. Specific acoustic impedance Z is given by Z = μc,where μ is the density and c is the acoustic velocity. The expres-sions for the reflectance R and transmittance T for an incomingwave from the medium 1 are

R ¼ Z2 � Z1

Z1 þ Z2

� �2

ð2Þ

and

T ¼ 4Z1Z2

ðZ1 þ Z2Þ2; ð3Þ

respectively, where Z1 and Z2 are specific acoustic impedanceof media 1 and 2, respectively. Thus, the reflectance and trans-mittance are controlled by the acoustic impedance ratio of twomedia. When the Z2 is considerably smaller than the Z1, thereflectance approaches 1 and the transmittance approaches 0.28) Inthe case of our lead-free ultrasonic flowmeter, the specific acousticimpedances of KNN-based ceramics and water are 26.9 © 106 and1.5 © 106 kg/m2s, respectively. Since the specific acoustic imped-ance of PZT-based ceramics is 35.6 © 106 kg/m2s, KNN-basedceramics have advantages for transducer applications from thepoint of view of low acoustic impedance. However, even whenKNN-based ceramics are used, the difference in acoustic imped-ance between piezoelectric ceramics and water is not small and

89% of incoming wave reflects on the boundary between piezo-electric ceramics and water. An acoustic matching layer, whoseacoustic impedance is intermediate value between the piezo-electric ceramics and water, is essential for reducing the reflectionof ultrasonic wave. When intermediate layer B is inserted betweenmedia A and C as shown in Fig. 2, the transmittance of acousticintensity I from medium A to medium C can be described as28)­32)

T ¼ 4

( ffiffiffiffiffiffiffiZC

ZA

sþ

ffiffiffiffiffiffiffiZA

ZC

s !2cos2

2³L

­B

!

þ ZBffiffiffiffiffiffiffiffiffiffiffiffiZAZC

p þffiffiffiffiffiffiffiffiffiffiffiffiffiZAZC

ZB

s !2sin2

2³L

­B

!)�1

ð4Þ

where ZA, ZB, and ZC are respectively specific acoustic imped-ance of media A, B, and C, L is length of medium B, ­B isultrasonic wavelength in medium B. In the special conditionof ZB=(ZAZC)1/2 and L=­B/4, the transmittance is equal to 1,which indicates that ultrasonic wave losslessly enter the mediumC. Therefore, the specific acoustic impedance of acoustic match-ing layer should be the geometric mean of the specific acousticimpedances of piezoelectric ceramics and water. In the case ofKNN-based ceramics, the desired value of specific acousticimpedance is 6.35 © 106 kg/m2s. In addition, the thickness ofmatching layer should be set to ­B/4, considering the acousticvelocity in the matching layer.

3. Experimental procedure

KNN-based piezoelectric ceramics with a diameter of 10mm, athickness of 1.5mm, d33 = 289 pC/N, g33 = 21.3 © 10¹3 Vm/N,kt = 0.405, and Q = 88.6 were used as a lead-free piezoelectrictransducer. As an acoustic matching layer, a 0­3 composite mate-rial made of BaTiO3 fine particles with an average particle sizeof 500 nm and epoxy resin (bisphenol A-type) was fabricated asfollows. First, BaTiO3 fine particles and epoxy resin were mixedat 1500 rpm for 2min and air bubbles contained in the resin weredefoamed at 1500 rpm for 2min using a planetary centrifugalmixer (THINKY, ARE-310). Next, the mixed paste was placedinto a plastic mold and it was cured at preset temperature of 60°Cfor 24 h. The specific acoustic impedance of BaTiO3/epoxy 0­3composite was controlled by changing the volume fraction ofdispersed BaTiO3 fine particles. Finally, the 0­3 composite wascut and polished to a thickness of ­/4 (0.260mm). The cross-sections of the fabricated composite materials were observed by ascanning electron microscope (SEM, JEOL, JCM-6000).A pair of fabricated matching layers was attached to the piezo-

electric transducers in both sides of U-shaped ultrasonic flow-meter with petrolatum under constant pressure. The diameter ofthe pipe in the ultrasonic flowmeter was 10mm and the material

Fig. 2. Propagation of ultrasonic wave in media A, B, and C.

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318

of the pipe was Teflon. One transducer of a pair of transducerswas connected to a function generator (Agilent 33210A) and theother was connected to an oscilloscope (Agilent 54642A). Thepipe of the flowmeter was filled with water and input voltage wasapplied to one transducer using the function generator. The waveform of input voltage was 4 pulses of sine wave with a frequencyof 2MHz and the peak-to-peak voltage was 20V. Propagatingultrasonic wave through matching layers and water was receivedusing the other transducer and the output voltage of the trans-ducer was measured by the oscilloscope.

4. Results and discussion

To examine the relationship between the specific acousticimpedance and the volume fraction of BaTiO3 particles inBaTiO3/epoxy composite, we fabricated composite materialswith various volume fractions of BaTiO3 particles. Figure 3 indi-cates the cross-sections of the fabricated composite materials.BaTiO3 fine particles seemed to be well dispersed and no bubbleswere observed when the volume fraction of BaTiO3 fine particleswas 40 vol% or less. Sufficiently uniform 0­3 composite mate-rials were successfully obtained.The bulk densities of the composite materials were measured

by measuring the weight and size. Figure 4(a) shows a relation-ship between the density of composite and volume fraction ofBaTiO3 fine particles. The density increased almost linearly withincreasing the volume fraction of BaTiO3 particles because ofhigher density of BaTiO3. Moreover, the acoustic velocities in thecomposite materials were measured. For measuring the acousticvelocities, a pair of PZT transducers was attached on both sidesof the composite plate with petrolatum. One transducer of a pairof transducers was connected to the function generator and theother was connected to the oscilloscope. After applying inputvoltage with 4 pulses of sine wave to one transducer, propagatingultrasonic wave through the matching layer was received usingthe other transducer. From the propagation time of ultrasonicwave and the thickness of the matching layer, the acousticvelocity in the composite was estimated. Figure 4(b) shows arelationship between the acoustic velocity in composite and thevolume fraction of BaTiO3 fine particles. Unlike the density,the acoustic velocity in composite is almost independent of thevolume fraction of BaTiO3 fine particles. The specific acousticimpedance was estimated by the product of the density and theacoustic velocity. Figure 4(c) indicates a relationship between thespecific acoustic impedance of composite and the volume fractionof BaTiO3 fine particles. The specific acoustic impedance of com-posite increased with the volume fraction of BaTiO3 particles.By changing the content of BaTiO3 particles, we obtained 0­3composite materials with various specific acoustic impedances of

2.6­6.9 © 106 kg/m2s.As a mixture rule of the acoustic impedance of 0­3 composite

material, Reuss model has been reported.33) In this model, theelastic modulus K and rigidity coefficient G of 0­3 compositematerial are represented as

1

K¼ v1

K1

þ 1� v1K2

ð5Þ

and

1

G¼ v1

G1

þ 1� v1G2

; ð6Þ

where K1 and K2 are respectively the elastic modulus of particleand matrix, G1 and G2 are respectively the rigidity coefficient ofparticle and matrix, v1 indicates the volume fraction of particle.The acoustic velocity in the composite material is described as

c ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiKþ 4

3G

μ

vuuut: ð7Þ

Moreover, the density of 0­3 composite material is

μ ¼ μ1¯1 þ μ2ð1� ¯1Þ; ð8Þwhere μ1 and μ2 are respectively the density of particle andmatrix. Using Eqs. (5)­(8), the specific acoustic impedance canbe estimated. The solid lines shown in Fig. 4 are theoretical curve

Fig. 3. SEM images of BaTiO3/epoxy composites with various BaTiO3

fractions of (a) 10, (b) 20, (c) 30, and (d) 40 vol%.

Fig. 4. Density (a), acoustic velocity (b), and specific acoustic imped-ance (c) of BaTiO3/epoxy composites as a function of volume fraction ofBaTiO3 fine particles.

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calculated by the Reuss model. It was found that the density ofcomposite obeyed the mixture rule represented by Eq. (8). On theother hand, the sound velocity of composite obeyed the Reussmodel when the volume fraction of BaTiO3 fine particles waslower than 20 vol%, while it was not consistent with the Reussmodel when the volume fraction was higher than 20 vol%. TheReuss model requires homogenous structure, which means thatthe size of particles is enough smaller than the wavelength ofultrasonic wave and the particles are uniformly dispersed. Fromthe results shown in Fig. 3, the secondary particle size of BaTiO3

is micrometer order, which was enough smaller than the ultra-sonic wavelength of millimeter order. However, for the high-concentration composite, the dispersion of BaTiO3 particlesmight be insufficient.From the relationship between the specific acoustic impedance

of composite and the volume fraction of BaTiO3 fine particlesshown in Fig. 4(c), the optimum volume fraction was estimatedas 36.5 vol% for obtaining specific acoustic impedance of6.35 © 106 kg/m2s which was the geometric mean of the specificacoustic impedances of KNN-based ceramics and water. There-fore, a pair of matching layers was fabricated using 0­3 com-posite with a volume fraction of 36.5 vol%. Considering theacoustic velocity, the thickness of the matching layers was setto 0.260mm. These matching layers were attached on the surfaceof KNN transducers and these were installed in the flowmeter.Figure 5(a) shows the propagating waveforms through waterusing the ultrasonic flow mater with commercialized matchinglayers and optimized composite matching layers. When the opti-mized composite matching layers were used, the amplitude ofoutput voltage was improved by 35% compared to that using acommercialized matching layer which is just plate-shaped epoxyresin. The effectiveness of the optimized composite matchinglayers was verified. Even in comparison with the commercialized

ultrasonic flowmeter with PZT-based material, the amplitude was83.1% which provided enough measurement accuracy of flowrate, as shown in Fig. 5(b). In fact, we measured flow rate ofwater with various speed of 5­45ml/s using lead-free ultrasonicflowmeter. As the result, it was found that the lead-free ultrasonicflowmeter with the optimized matching layer has high accuracyof a flow rate error of 3.5% or less.

5. Conclusions

In this study, we designed and fabricated acoustic matchinglayers optimized for KNN-based lead-free ultrasonic flowmeter.To control the specific acoustic impedance of matching layers,the volume fraction of BaTiO3 fine particles in BaTiO3/epoxy0­3 composite material was changed. For improving an accuracyof a KNN-based lead-free ultrasonic transducer, the specificacoustic impedance of matching layer should be the geometricmean of the specific acoustic impedance of KNN-based ceramicsand water, and the length of acoustic matching layer should beone quarter of the ultrasonic wavelength in acoustic matchinglayer. Therefore, acoustic matching layers with a specific acousticimpedance of 6.35 © 106 kg/m2s and thickness of 0.260mmwere fabricated. When the optimized composite matching layerswere used in KNN-based lead-free ultrasonic flowmeter, theamplitude of output voltage was improved by 35% compared tothat using a commercialized matching layers. Furthermore, it wasfound that the lead-free ultrasonic flowmeter with the optimizedcomposite matching layer has high accuracy of a flow rate errorof 3.5% or less. From the above, the effectiveness of the opti-mized composite matching layers was verified.

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