Frequency modulation of laser ultrasound transducer using carbon nanotube-coated polyethylene microsphere

(1)

polyethylene microsphere

Soo Won Kwon, Won Young Choi, Hyeong Geun Jo, et al.

Citation: The Journal of the Acoustical Society of America 147, EL351 (2020); doi: 10.1121/10.0000952 View online: https://doi.org/10.1121/10.0000952

View Table of Contents: https://asa.scitation.org/toc/jas/147/4 Published by the Acoustical Society of America

ARTICLES YOU MAY BE INTERESTED IN

Design and characterization of an enclosed coaxial carbon nanotube speaker

The Journal of the Acoustical Society of America 147, EL333 (2020); https://doi.org/10.1121/10.0001029

Source localization in the deep ocean using a convolutional neural network

Shape optimization of acoustic horns using the multimodal method

Estimation of seabed properties and range from vector acoustic observations of underwater ship noise The Journal of the Acoustical Society of America 147, EL345 (2020); https://doi.org/10.1121/10.0001089

Ship source level estimation and uncertainty quantification in shallow water via Bayesian marginalization The Journal of the Acoustical Society of America 147, EL339 (2020); https://doi.org/10.1121/10.0001096

Analysis of a wooden specimen's mechanical properties through acoustic measurements in the very near field The Journal of the Acoustical Society of America 147, EL320 (2020); https://doi.org/10.1121/10.0001030

(2)

Frequency modulation of laser ultrasound transducer using carbon nanotube-coated polyethylene microsphere

Soo WonKwon,Won YoungChoi,^a)Hyeong GeunJo,and Kwan KyuPark^b)

Department of Convergence Mechanical Engineering, Hanyang University, Seoul 04763, South Korea [email protected], [email protected], [email protected], [email protected]

Abstract: An ultrasound transducer was fabricated by dropping a multi-walled carbon nanotube solution containing a mixture of carbon nanotubes and ethoxyethanol directly on the surface of polyethylene microspheres. The frequency modulation depended on the diameter of the polyethylene microspheres. To investigate this relationship, three types of polyethylene microspheres with different diameters were used in simulations and experiments.

These specimens were attached to polydimethylsiloxane and glass plates. A comparison revealed that the 50lm diameter polyethylene spheres coated with carbon nanotubes had the highest ultrasound frequency. This work showed that smaller polyethylene microspheres generate higher ultrasound frequencies. ^V^C2020 Acoustical Society of America

[Editor: Juan Tu] Pages: EL351–EL356

Received:6 January 2020 Accepted:9 March 2020 Published Online:21 April 2020

1. Introduction

Small ultrasound transducers are used in biomedical applications¹and medical ultrasound imaging. Piezoelectric material has been used as a compact ultrasonic transducer. However, in order to manufacture piezoelectric materials with a small size, dicing has to be performed, which makes it difficult and expensive to produce ceramic piezoelectric material. To overcome this difficulty, piezoelectric micromachined ultrasonic transducers (PMUTs)² and capacitive micromachined ultrasonic transducers (CMUTs)³ that use microelectromechanical systems (MEMS) have been studied. However, when using PMUTs and CMUTs for ultrasound transducers, small diameters are required to produce high-frequency devices which are difficult to fabricate because of the MEMS process. There are also yield and uniformity issues along with electrical signal interfer- ence from electrical circuits.

The photoacoustic (PA)⁴method can generate ultrasonic waves through light absorption and thermal expansion. This method can produce a higher sound pressure than PMUTs and CMUTs and is used in biomedical applications. Recently, a PA device that used carbon nanotubes (CNTs) with high absorption characteristics was studied.^{5–9,12–14}

Laser-generated ultrasound transducers are also being studied.^{5–9,12–14} The laser- generated ultrasound transducer was developed by Scruby et al.⁵ in 1980. In this method, metal is irradiated by a laser and generates ultrasound through thermal expansion. Noimarket al.⁶performed an ultrasound characteristics comparison of a polydimethylsiloxane (PDMS) composite, gold nanoparticles, and multi-walled carbon nanotubes (MWCNTs). Chen et al.⁷ studied the improvement of the sound pressure conversion efficiency through the use of a PDMS-Au-CNT yarn structure and Siregar et al.⁸studied the laser heat process in relation to the CNT radius. Li et al.⁹characterized and analyzed the CNT diameter and length.

This paper proposes a laser ultrasound transducer that allows the resonance frequency to be adjusted by changing the diameter of the CNT-coated polyethylene microspheres. The frequency range was analyzed by irradiating CNT-coated polyethylene microspheres with three different diameters with a pulsed laser. These three different sizes of CNT microspheres were implemented on PDMS and glass plates.

The main objective of the present work was to modulate the transducer frequency by changing the size of the CNT-coated microspheres. When the CNT-coated microspheres were irradiated, the CNTs absorbed the laser energy and converted the optical energy into thermal energy. The thermal energy was transferred to the polymer microspheres, which repeatedly expanded and contracted with the laser pulse. Ultrasonic waves were generated by this mechanism. This system used MWCNTs, which have excellent absorption properties. This was an

a)ORCID ID: 0000-0002-8565-7823.

b)Author to whom correspondence should be addressed, ORCID ID: 0000-0001-5117-5853.

EXPRESSLETTERS

(3)

advantage for the ultrasonic transmitter. The polyethylene microspheres were used because this polymer has a high coefficient of thermal expansion (CTE), and polyethylene microspheres of a specific size are easy to obtain in the market.

There are two equations of a PA wave. One is the time domain equation, another is the frequency domain equation of PA wave. Equation(1)is the general PA wave equation,¹⁰

r² 1 v²

@²

@t²

p r;ð tÞ ¼ b jv²

@²T r;ð tÞ

@t² (1)

(r: the position of measurement point;t: time;v: speed of sound;b: thermal coefficient of volume expansion;j: isothermal compressibility;T: increasing temperature).

Erkol et al.¹¹ treated PA’s radial part equation as Gaussian, and the Fourier transform was used to find a solution in frequency domain Eq.(2),

pðr;xÞ ¼ p₀ 2p

1

vrexp s²_px² 2

ðR

0

r⁰exp r⁰² 2s²

exp ix

vðrr⁰Þ exp ix vðrþr⁰Þ

dr⁰

( )

(2) (x: frequency; p0: initial pressure;s: laser spot size;sp: pulse duration; R: Radius of the spherical object).

Using Eq.(2), a computer calculation for variable R shows that a small diameter spherical object leads to high frequencies in the PA.

It was assumed that the speed of the thermal expansion and contraction of these small microspheres would be faster than that of large microspheres. The speed difference between the thermal expansion and contraction values resulted in a frequency difference.

When an Nd:YAG laser was used to irradiate microspheres of different sizes, high- frequency ultrasonic waves were generated by the small microspheres [Fig. 1(a)]. As shown in Fig. 1(b), a group of large CNT-coated microspheres irradiated by the laser produced low- frequency ultrasound. Three types of CNT-coated polyethylene microspheres were selected, as shown in Figs. 1(d)–1(h). The CNT coating was good overall. The CNT-coated polyethylene microspheres were classified into three types according to their diameters: type I (polyethylene microsphere diameter: 45–53lm), type II (polyethylene microsphere diameter: 90–106lm), and type III (polyethylene microsphere diameter: 180–212lm). A scanning electron microscopy (SEM) photograph of a CNT-coated microsphere’s surface is shown in Fig.1(i).

Fig. 1. (Color online) Principle of frequency modulation: (a) high frequency with small microspheres and (b) low frequency with large microspheres, optical images of single and groups of CNT-coated polyethylene microspheres: single microsphere with diameters of (c) 50lm, (d) 100lm, and (e) 200lm, groups of microspheres with diameters of (f) 50lm, (g) 100lm, and (h) 200lm. (i) An expanded view of the SEM image of the surface of a CNT-coated microsphere.

EL352 J. Acoust. Soc. Am.147(4), April 2020 Kwonet al.

(4)

2. Simulation

In order to verify this method, a finite element analysis (FEA) was conducted using the commer- cial package Comsol Multiphysics^V^R (Version 5.3a, Comsol Inc., Burlington, MA). The CNT- coated microspheres were modeled, and the geometry used in the simulation was similar to that of a specimen used in the actual experiment. The CNT-coated microspheres were surrounded by a medium. Water was used for the medium, MWCNTs were used for the CNTs, and filled polyethylene was used for the microspheres. In this simulation, 7 ns of heat was applied to simulate the application of the pulse laser.

Three different models were used, depending on the diameter of the microspheres. The first model’s microsphere diameter was 50lm, the second was 100lm, and the third was 200lm.

The CNT thickness of all the models was 0.2lm. The thermal properties of MWCNTs and polymers have been studied by many researchers. The thermal properties of the MWCNTs used in these calculations are listed in Table1.

An acoustic pressure was generated by the difference in the microsphere’s thermal expansion and contraction. The pressure data for a 200lm diameter microsphere are shown in Fig.2(a).

The results of a frequency analysis using the fast Fourier transform (FFT) indicated that the center frequencies for 50, 100, and 200lm diameter were approximately 27.83, 14.97, and 7.2 MHz, respectively, as shown in Fig.2(b). A comparison of the data shows that a smaller diameter for the microsphere results in a higher frequency for the ultrasonic waves. Experiments were conducted based on these simulation results.

3. Experiment method

CNT-polymer composite coatings have been studied by many researchers. The most common coating techniques are the spin coating method,¹⁵spray coating method,¹⁶ chemical vapor deposition method,^12,14,17 and dip-coating method.¹⁸

In this experiment, CNTs were used to coat polyethylene polymer microspheres that were pre-made to control the size. Thus, most CNT-polymer coating methods that use a CNT and PDMS mixture were not suitable. Therefore, a new coating method was created. First, polyethylene microspheres (clear polyethylene microspheres, 960 kg/m³, Cospheric LLC, Goleta, CA) were spread on the bottom plate, which was a microscope slide. Second, a MWCNT solution (a graphitized MWCNT solution, 2.0 mg/ml, Bnb Materials Co., Ltd., Busan, Korea) was dropped onto the microspheres using a syringe. Third, when the MWCNT solution was dry, the second step was repeated. In order to reduce the coffee ring effect,^19–21 the microspheres were turned upside down, and the second and third steps were repeated several times.

In this experiment, the bottom plate was needed because the microspheres could not be fixed alone in the water. The bottom plates were made of transparent PDMS (Sylgard 184, Dow Corning Co., Midland, MI) and glass slides, and were each 1 mm thick. Type I, type II, and type III were fixed with adhesive (#75 spray adhesive, 3 M Co., Ltd., Maplewood, MN) to the bottom

Table 1. Thermal properties of MWCNTs and polyethylene.

Material Thermal capacityCp Thermal conductivityk Densityq Coefficient of thermal expansiona

MWCNT 63.8 J/(kg K) 55 W/(m K) 1.75 kg/m³ 2.110⁵K¹

Polyethylene 1900 J/(kg K) 0.38 W/(m K) 930 kg/m³ 1.510⁴K¹

Fig. 2. (a) Simulated acoustic signal (D¼200lm) and (b) FFT results for CNT-coated polyethylene microsphere.

EXPRESSLETTERS

(5)

plates in a circle shape with a diameter of 3 mm. These specimens were placed in a water tank toward the surface of the water.

The experimental schematic diagram is shown in Fig.3(a). The excitation laser source was an Nd:YAG laser (Minilite I, Amplitude, CA), which had a wavelength of 532 nm, pulse duration of 7 ns, and pulse repetition frequency of 10 Hz. The laser beam was reflected by a mirror (laser line mirror, Edmund Optics Inc., Barrington, NJ) installed at 45, which caused the laser beam to enter the water perpendicularly. The laser beam passed through the water and penetrated the CNT-coated polyethylene microspheres with a laser intensity of 6 mJ/cm². The CNTs absorbed the laser energy and generated heat. This heat was transferred to the polyethylene microspheres, which caused the polyethylene microspheres to expand and contract. This mechanism generated acoustic waves. A high-frequency hydrophone (HNR500, ONDA Corp., Sunnyvale, CA) was employed to measure the acoustic waves. It was installed at a slight incline 30 mm away from the CNT-coated polyethylene microspheres to prevent the laser from reaching it.

4. Experiment result

The acoustic waves generated by the CNT-coated polyethylene microspheres had different resonance frequencies depending on the diameter of the polyethylene microspheres. Regardless of the bottom plate material, the acoustic signals from the same types of transducers were similar, as shown in Fig. 3(b). When the bottom plate was glass, the FFT results indicated that the resonance frequencies of type I, type II, and type III were 4.01,. 3.42, and 2.93 MHz, respectively, as shown in Fig.3(c). The 3 dB bandwidths of type I, type II, and type III were 133%, 94.82%, and 96.55%, respectively. The 3 dB bandwidth was calculated using Eq.(3),

bandwidth¼f₂f₁ fc

fc¼f₁þf₂ 2

: (3)

When the bottom plate was PDMS, the resonance frequencies of the type I, type II, and type III transducers were 5.01, 4.52, and 3.48 MHz, respectively, as shown in Fig.3(d). The 3 dB bandwidths for type I, type II, and type III were 123%, 92.43%, and 94.83%, respectively.

Figures 3(c) and 3(d) show that a smaller diameter for the CNT-coated polymer microspheres resulted in a higher frequency for the ultrasound waves.

In the experiment, the polyethylene microspheres could not be placed in the water alone.

Therefore, glass and PDMS bottom plates with different CTE values were used (glass CTE: 9 ppm/K, PDMS CTE: 310 ppm/K). Adhesives were also used to attach the polyethylene microspheres to the

Fig. 3. (Color online) (a) Experimental schematic diagram and experiment signals: (b) acoustic wave data for type III, and FFT results for type I, type II, and type III; (c) under glass bottom plate condition (d) and PDMS bottom plate condition.

(6)

bottom plate. Simulation results showed that the center frequencies were approximately 27.83, 14.97, and 7.2 MHz for beads with diameters of 50, 100, and 200lm, respectively. In the experiment, the frequencies were 5.01, 4.52, and 3.48 MHz, as shown in Fig. 3(d). Heat energy converted by the absorption of the laser by the CNTs was transferred to the adhesive and bottom plate. The thermal expansion and tremors of the bottom plate and adhesive were assumed to affect the frequency change in the experiment. The simulation and experimental results showed the different frequency change rates, but both showed a tendency to generate higher frequencies as the beads became smaller in size.

In summary, three types of CNT-coated polyethylene microspheres with different diameters were selected for laser-generated ultrasound transducers. In the simulation, the modeling did not have a bottom plate, unlike the experiment to study the relationship between frequency and diameter size of CNT-coated polyethylene microsphere. However, since the experiment could not be conducted physically without a bottom plate, the material with different thermal coefficients and hardness differences were used as the bottom plate. FEA and experimental measurement results tended to be consistent. During these experiments and simulations, a relation was found to increase the resonance frequency and fractional bandwidth in the smallest diameter CNT-coated polyethylene microsphere condition. The results show that the smallest diameter CNT-coated polyethylene microspheres can be generated up to 5.01 MHz with a 133% bandwidth in the diameter range of 50–200lm. The laser generated ultrasonic transducer using CNT-coated polyethylene spheres will be useful for medical imaging applications that require different frequencies for each measurement site.

Acknowledgments

This study was funded by the National Research Foundation of Korea, grant-funded by the Korea government (MSIT) (Grant No. 2019R1F1A1062162).

References and links

1M. W. Schellenberg and H. K. Hunt, “Hand-held optoacoustic imaging: A review,”Photoacoust.11, 14–27 (2018).

2F. Akasheh, T. Myers, J. D. Fraser, S. Bose, and A. Bandyopadhyay, “Development of piezoelectric micromachined ultrasonic transducers,”Sens. Actuators, A111, 275–287 (2004).

3A. S. Ergun, Y. Huang, X. Zhuang, O. Oralkan, G. G. Yarahoglu, and B. T. Khuri-Yakub, “Capacitive micromachined ultrasonic transducers: Fabrication technology,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control 52, 2242–2258 (2005).

4M. Xu and L. V. Wang, “Photoacoustic imaging in biomedicine,”Rev. Sci. Instrum.77, 041101 (2006).

5C. B. Scruby, R. J. Dewhurst, D. A. Hutchins, and S. B. Palmer, “Quantitative studies of thermally generated elastic waves in laser-irradiated metals,”J. Appl. Phys.51, 6210–6216 (1980).

6S. Noimark, R. J. Colchester, R. K. Poduval, E. Maneas, E. J. Alles, T. Zhao, E. Z. Zhang, M. Ashworth, E. Tsolaki, A. H. Chester, N. Latif, S. Bertazzo, A. L. David, S. Ourselin, P. C. Beard, I. P. Parkin, I. Papakonstantinou, and A.

E. Desjardins, “Polydimethylsiloxane composites for optical ultrasound generation and multimodality imaging,”Adv.

Funct. Mater.28, 1–16 (2018).

7Z. Chen, Y. Wu, Y. Yang, J. Li, B. Xie, X. Li, S. Lei, J. Ou-Yang, X. Yang, Q. Zhou, and B. Zhu, “Multilayered carbon nanotube yarn based optoacoustic transducer with high energy conversion efficiency for ultrasound application,”

Nano Energy46, 314–321 (2018).

8S. Siregar, S. Oktamuliani, and Y. Saijo, “A theoretical model of laser heating carbon nanotubes,”Nanomaterials8, 580 (2018).

9J. Li, X. Lan, S. Lei, J. Ou-Yang, X. Yang, and B. Zhu, “Effects of carbon nanotube thermal conductivity on optoacoustic transducer performance,”Carbon145, 112–118 (2019).

10L. V. Wang and H. I. Wu,Biomedical Optics: Principles and Imaging(John Wiley & Sons, New York, 2012).

11H. Erkol, E. Aytac-Kipergil, M. U. Arabul, and M. B. Unlu, “Analysis of laser parameters in the solution of photoacoustic wave equation,”Proc. SPIE8581, 858136 (2013).

12H. W. Baac, J. G. Ok, T. Lee, and L. J. Guo, “Nano-structural characteristics of carbon nanotube-polymer composite films for high-amplitude optoacoustic generation,”Nanoscale7, 14460–14468 (2015).

13H. W. Baac, J. G. Ok, H. J. Park, T. Ling, S. L. Chen, A. J. Hart, and L. J. Guo, “Carbon nanotube composite optoacoustic transmitters for strong and high frequency ultrasound generation,”Appl. Phys. Lett.97, 234104 (2010).

14H. W. Baac, J. G. Ok, A. Maxwell, K. T. Lee, Y. C. Chen, A. J. Hart, Z. Xu, E. Yoon, and L. J. Guo, “Carbon-nanotube optoacoustic lens for focused ultrasound generation and high-precision targeted therapy,” Sci. Reports2, 1–8 (2012).

15B. Y. Hsieh, J. Kim, J. Zhu, S. Li, X. Zhang, and X. Jiang, “A laser ultrasound transducer using carbon nanofibers- polydimethylsiloxane composite thin film,”Appl. Phys. Lett.106, 021902 (2015).

16C. Park, S. W. Kim, Y.-S. Lee, S. H. Lee, K. H. Song, and L. S. Park, “Spray coating of carbon nanotube on polyethylene terephthalate film for touch panel application,”J. Nanosci. Nanotechnol.12, 5351–5355 (2012).

17T. Lee, J. G. Ok, L. J. Guo, and H. W. Baac, “Low f-number photoacoustic lens for tight ultrasonic focusing and free-field micro-cavitation in water,”Appl. Phys. Lett.108, 104102 (2016).

18S. Noimark, R. J. Colchester, B. J. Blackburn, E. Z. Zhang, E. J. Alles, S. Ourselin, P. C. Beard, I. Papakonstantinou, I. P. Parkin, and A. E. Desjardins, “Carbon-nanotube–PDMS composite coatings on optical fibers for all-optical ultrasound imaging,”Adv. Funct. Mater.26, 8390–8396 (2016).

EXPRESSLETTERS

(7)

19H. Hu and R. G. Larson, “Marangoni effect reverses coffee-ring depositions,”J. Phys. Chem. B110, 7090–7094 (2006).

20E. Tekin, P. J. Smith, and U. S. Schubert, “Inkjet printing as a deposition and patterning tool for polymers and inor- ganic particles,”Soft Matter4, 703–713 (2008).

21R. Tortorich and J.-W. Choi, “Inkjet printing of carbon nanotubes,”Nanomater.3, 453–468 (2013).