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Fabrication and Characterization of Nanocrystalline TiO2-polymer Composite Matching Layers J. Zhu1, W. Cao1, B. Jiang1, D. S. Zhang2, H. Zheng2, Q. Zhou3 and K. K. Shung3 1

Material Research Institute, The Pennsylvania State University, University Park, PA 16802 2 Chemat Technology, Inc., Northridge, CA 91324 3 Department of Biomedical Engineering, University of Southern California and NIH Resource on Medical Ultrasonic Transducer Technology, Los Angeles, CA 90089 Abstract: A new nanocrystalline TiO2-polymer composite with high volume particle loading has been developed for matching layers of ultra high frequency medical ultrasonic transducers. The composite consists of 25 nm size TiO2 particles and Epoxy resin with the acoustic impedance reaching as high as 7.19 MRayls. Ultrasonic spectroscopy methods are used to measure the ultrasonic velocity of the composite thin film with thickness ~8 microns. It is found that the acoustic impedance can be increased by annealing the composite at higher temperatures. Our measured results show that this new composite has good acoustic impedance characteristics that are needed for matching layers of ultra-high frequency transducers.

to fabricate such homogeneous thin matching layers using sub-micron particles because there will be a lot of trapped air bubbles, which is worse for smaller particles. Therefore, proper matching layer materials with good acoustic characteristics are urgently needed for the development of ultra high frequency medical imaging transducers. In this paper, we report a new TiO2 nanocrystalline -polymer composite with high volume particle loading. The acoustic properties were measured using two different ultrasonic methods based on the quarter wavelength matching principle and the T-matrix technique. TiO2-polymer nanocomposite coatings are deposited on silicon and glass substrates, which posed some challenges in the property characterization. II. MATERIAL PRAPARATION AND CHARACTERIZATION

I. INTRODUCTION High frequency ultrasonic transducers are being used more and more in medical imaging. They can provide better resolution in both the axial and lateral directions, so as to further improve the capability of diseases diagnosis and monitoring of medical treatments [1]. Quarter wavelength matching layers between PZT and the imaging medium are critical for the performance of high sensitivity broad bandwidth transducers [2]. Composites consisting of high acoustic impedance powder and low acoustic impedance polymer are commonly used for matching layer material because one can tune the acoustic impedance by varying the volume ratio of the constituents [3]. However, higher frequency transducer needs very thin matching layers. For transducer of 100MHz, the required matching layer is around 7 micron thick. It is very difficult

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High quality TiO2 ٛ nanopowder with 30% rutile and 70% anatase was acquired from Degussa in Germany with average particle size in the range of 20-40 nm. Powder, Epoxy (EPO-TEK 301) and Ethanol were mixed at a molar ratio of 1:0.036:5.92. Colloidal sol forms hybrid composite solution together with matrix polymer and other additives. The mixture was ultrasonicated for 10-30 min to get homogeneous distribution using an ultrasonic horn. The viscous paste bottle was put into ice water to remove the heat resulting from the ultrasonic dispersion. Then, the paste was coated on glass or silicon substrates by the doctor-blade technique, producing a thin film with thickness ranging from 8-15 µm. The ethanol evaporated at room temperature after a few minutes. The films were strongly bounded to

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2007 IEEE Ultrasonics Symposium

ratio peak frequency:

the substrates. Atomic Force microscope (AFM) image of the hybrid composite thin layer coating on substrate are shown in figure 1.

(4)

v = 4d f 3 max

where d3 is the thickness of the film.

Figure 2. Pulse signal transmitted through thin layer composite on a thick glass substrate immersed in water.

Figure 1. AFM micrograph of TiO2-polymer nano-composite.

Execution of this method becomes more difficult when the substrate become thinner. The T-matrix method using continuous or tone burst ultrasonic waves is therefore used to treat such situation. For a multilayer structure shown in figure 3, the continuous wave form in each layer is:

To measure the acoustic properties of thin TiO2-polymer film, two methods based on ultrasonic quarter wavelength matching principle and T-matrix principle are used. The quarter wavelength method is applicable when the substrate is relatively thick so that the transmission wave within the thin film layer can be separated from the successive waves reflected from the substrate/water interface. From analyzing the wave propagation in the structure shown in figure 2, the FFT of signals passing through the substrate with thin layer composite, and substrate only are respectively given by [5]

y j = Aj e

1 1 − R31 R32 e −i 2 k3d 3

i (ωt + k j x )

 Aj  A    = Tj, j+1  j+1 = 1 * B  Bj+1 2Z j  j   (6) (Zj +Zj+1)eikj+1(l j+1 −l j ) (Zj −Zj+1)e−ikj+1(l j +1−l j )  Aj+1    (Z −Z )eikj+1(l j+1 −l j ) (Z −Z )e−ikj+1(l j+1 −l j ) Bj+1 j j+1    j j+1

[ ]

(1)

H ( f ) = T12T21 e −i ( k 2 − k1 ) d 2 U ( f ) (2) where T and R are transmission and reflection coefficients, respectively and U ( f ) denotes the FFT of the incident wave. The amplitude ratio is given by − (α 3 −α1 ) d 3

G ( f ) T23T31 e = H( f ) T21 1 − R31 R32 e − 2α 3d3 e −i 2 β 3d3

+ Bje

(5) The transmission matrix of the coefficients Aj and Bj at each boundary, the total transmission matrix [T] and the amplitude ratio of the outgoing and the incident waves are, respectively,

G ( f ) = T12T23T31 e −i ( k 2 −k1 ) d2 e −i ( k3 −k1 ) d 3 *U ( f )

i ( ωt − k j x )

(3)

[T ] = [T1, 2 ][T2,3 ][T3,1 ]

(7)

A4 1 = A1 T11

(8)

Based on our calculation results, the substrate usually dominates the receiving outgoing signal because it is much thicker than the film. The thin composite layer is only a perturbation, which becomes more significant as the film is getting

If the acoustic impedance of composite coating is between that of the substrate and water, we can get the wave velocity v from the amplitude

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tone-burst experiments varies between 10MHz to 125MHz with a frequency step of 50 kHz. The received through signal was amplified by 5052PR and sampled using a TDS 460A oscilloscope, then downloaded to a personal computer where Fast Fourier Transforms were performed by using MATLAB. The FFT results G ( f ) for substrate only, and H ( f ) for film on substrate are obtained and used to calculate the spectrum ratios, which are shown in figure 5. The obtained signal from the T-matrix method is presented in figure 6. One can see a clear phase shift in figure 6 when the composite film is deposited onto the silicon substrate. Finally, all measured results from the two methods are summarized Table I.

thicker or the substrate is getting thinner. Therefore, the T-matrix method can be used when the thickness of the film is comparable to that of the substrate.

Figure 3. Continuous wave transmitted through thin layer composite on thin substrate immersed in water.

III. EXPERIMENT RESULTS 1.65

The experiment setup shown in figure 4 is similar to the one described in Ref [6]. An 8 micron thick thin layer TiO2-polymer nanocomposite was deposited onto a thick glass plate and another film on a thin silicon plate. The nanocomposite films were later dried in the oven at 20 oC, 50 oC, 80 oC, 110 oC and 140 oC, respectively. The heating time at each temperature is one hour. The density of sample was measured after each heat treatment.

1.6 1.55

Ratio

1.5 1.45 1.4 1.35 1.3 1.25 1.2 4

4.5

5

5.5

6

6.5 Frequency

Figure 5. Spectrum Ratio

7

G( f ) H( f )

7.5

8

8.5

9 7

x 10

for composite

treated at different temperature. (Blue: 20 oC; Green: 50 o C; Red: 80 oC; Light Blue: 110 oC; Purple:140 oC).

Heat treatment can further increase the density and hardness of the composite so long as the temperature is within the tolerance of the polymer. There are two main contributions to this effect. One is due to further curing of the polymer, which increases its elastic stiffness, and the other is to burnout any residues from the sol-gel process. For comparison, all measured results are given in Table I. One can see that the heat treatment has great influence on the acoustic properties of the composite. At room temperature, the sound velocity is 1953m/s, and it goes up almost linearly for the temperature range investigated. After curing at 140 oC, the velocity reaches 2344m/s.

Figure 4. Configuration of experiment system.

The ultrasonic tests were conducted in a deionized water tank. One paired E9934 100 MHz immersion type ultrasonic transducers were used and the transmitting transducer was driven by a 5052PR 200MHz pulser/receiver (Broadband ultrasonic pulse signal) and an AWG2021 arbitrary waveform generator (Tone-burst single frequency wave). The frequency used for the

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Table I: Measured acoustic properties of the TiO2-polymer nanocomposite after curing at different temperatures for 1 hour. 20 oC 50 oC 80 oC 110 oC 140 oC Velocity (m/s) 1953 2031 2207 2221 2344 3 Density (kg/m ) 2715 2892 2965 3032 3068 Acoustic Impedance(MRay) 5.30 5.87 6.54 6.73 7.19

suited for making the matching layers of ultra-high frequency medical imaging transducers.

0.9

Silicon Silicon with TiO 2 Coating

This work was sponsored by the NIH under grant # P41 EB2182-10.

0.6 Amplitude

REFERENCES: [1] J. M. Cannata, T. A. Ritter, W. Chen, R.H.Silverman, K. K. Shung, “Design of efficient, broadband single element ultrasonic transducers for medical imaging applications,”IEEE Trans. Ultras. Ferroelect. Freq. Cont., Vol. 50, no. 11, 2003, pp. 1548-1556. [2] K. K. Shung, M. Zipparo, “Ultrasonic transducer and arrays,” IEEE Trans. Eng Med. Biol., Vol 15, 1996, pp.20-30. [3] Q. F. Zhou, J .H. Cha, Y. Huang, R. Zhang, W. Cao, J. M. Cannata, K. K. Shung, “P3Q-4 Nanocomposite Matching Layers for High Frequency Ultrasound Transducers”, IEEE Ultrasonics Symposium Proceedings, 2006, pp. 2365-2368 [4] Dongshe Zhang, Jonathan A. Downing, Fritz J. Knorr, Jeanne L. McHale “Room-Temperature Preparation of Nanocrystalline TiO2 Films and the Influence of Surface Properties on Dye-Sensitized Solar Energy Conversion” J. Phys. Chem. B, 110 (43), 2006, pp.21890 -21898. [5] H. Wang, W. Cao, “Characterizing Ultra-Thin Matching Layers of High-Frequency Ultrasonic Transducer Based on Impedance Matching Principle”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 51, no. 2, 2004, pp. 211-215. [6] H. Wang, T. A. Ritter, W. Cao, K. K. Shung, “Passive materials for high-frequency ultrasound transducers”, Proceedings of SPIE, Vol 3664, 1999, pp. 35-42

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0.0 20

30 F re qu enc y (M Hz) 40

50

Figure 6. Received signals through pure silicon plate and silicon plate with thin layer composite.

The density was measured by the Archimedes’s principle using Xylene liquid. One can see from Table I that the density also increases monotonically although it is not as linear and not as drastic compared to the velocity increase. IV. SUMMARY AND DISCUSSIONS A new TiO2-polymer nanocomposite with high volume particle loading and high acoustic impedance has been fabricated and characterized. Two ultrasonic methods have been implemented for measuring the acoustic properties of the composite films. The quarter wavelength spectroscopy works well for the thin film on a thick substrate, which is simple and quick, while the T-matrix method is more time consuming but more accurate when the film thickness is comparable to that of the substrate. We have demonstrated that eat treatment can further increase the acoustic impedance of the composite. For this nanocomposite, the highest acoustic impedance is greater than 7 MRayls, which is well

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