Ultrasonic Transducers and the High Frequency Properties of Passive Materials

The performance of ultrasonic transducers is limited by the large acoustic impedance mismatch between the load (tissue) and the piezoelectric element. Therefore, the acoustic pulse transmitted into the tissue exhibits a long ringdown that degrades the axial resolution. One method that is often employed to dampen the ringdown is to fix a lossy backing material.

This process leads to a shorter pulse and thus reduced sensitivity. A better method is to integrate an optimal front matching layer, which can reduce the pulse length without affecting the sensitivity. Two or three matching layers can be employed for such a purpose with different combinations of thickness and acoustic impedance [1]-[3]. A lens is usually incorporated in front of the matching layer(s) in order to collimate the ultrasound beam over a given distance. Normally, the acoustic impedance of the lens material is analogous to that of the tissue. A transducer’s overall performance depends critically on these passive materials, i.e., backing, matching and lens materials.

Ultrasonic transducers that operate at frequencies greater than 20 MHz are capable of providing higher resolution in both the lateral and axial directions, leading to better diagnosis of various diseases and innovative medical applications [4]. At present, the design and fabrication of high frequency transducers that operate between 30 and 100 MHz continues to be an engineering challenge [5]. Therefore, a better understanding of the high frequency properties of passive and active transducer materials is very important at the design stage.

A number of papers have been published on the characterization of active materials [6], [7]; however, experimental data on the acoustic properties of passive materials at high frequencies are rather limited. In this article, a set of such data is reported for several matching, backing and lens materials determined using an ultrasonic spectroscopic technique. Attenuation as well as acoustic impedance was measured in the frequency range between 25 and 65 MHz.

Materials and Methods

Ultrasonic spectroscopy has been extensively used in the characterization of solid materials [8]-[10]. When the angle of incidence is adjusted, the mode conversion effect enables the measurement of both shear wave and longitudinal properties [10]. In this case, the experimental arrangement used is illustrated in Figure 1.

A couple of transducers with a bandwidth of 80%, a center frequency of 50 MHz and an element diameter of 0.63 cm were employed. Since the ultrasonic attenuation in water is roughly 6 dB/cm at 50 MHz and increases with frequency, the high frequency components of the acoustic signal decrease in amplitude as the separation distance between the receiver and transmitter increases. Hence, it is important that the distance between the receiver and transmitter is made as small as possible and, simultaneously, enables adequate room for the sample to rotate. In the experiments here, the distance L in Figure 1 is set to 3 cm.

A Panametrics 200 MHz computer-controlled pulser/ receiver was employed to create a pulse with a damping value of 50 Ω and an energy of 1 /µJ. The output waveform from the receiving transducer was then sampled by a digital oscilloscope (Tektronix TDS 460A) via a 50 Ω coax cable measuring 1 m in length. The sampling rate was 10 Gs/second. For each waveform, the total sample length was 2500 points, and each waveform was transferred to a computer through a GPIB interface. In order to reduce random errors, each signal was averaged 64 times. Then, the amplitude Aw and the phase spectra φw of water were measured using the FFT from the output with the sample absent. Once the sample is in place, the trigger delay time was adjusted to offset the extra delay caused by the sample path length. Following this, the amplitude A and the phase φ of the output signal with the sample in place were obtained.

Experimental arrangement.

Figure 1. Experimental arrangement.

The attenuation αL and phase velocity CL of the longitudinal wave were determined using the following equation [10]:

(1)

 

(2)

 

Here, the attenuation of water, αw is 0.000271*f2 (dB/mm*MHz2) (f in MHz); Cw is water velocity (1480 m/s) [10]; T is the trigger delay time, f is frequency and d is the sample thickness. TL is the total transmission coefficient for the longitudinal wave, which is equivalent to the product of the two transmission coefficients of the acoustic wave from the sample to the water and from the water to the sample. When the wave is incident at an angle other than 0°, the mode conversion effect generates a shear wave. With the incident angle at the critical angle of the longitudinal wave, the phase velocity Cs and attenuation αs of the shear wave were determined using (3) and (4), respectively [10]:

(3)

 

(4)

 

where θi represents the incident angle, θ is the refractive angle of shear wave, and Ts is the total transmission coefficient of the shear wave. In this experiment, a computerized UNISLIDE rotary table (Velmex, Inc., Bloomfield, NY) was used to control θi and θ was calculated from Snell's law.

Optimized matching layers should have a well-characterized impedance and low attenuation. 0-3 composites of alumina (AI2O3) (Buehler Ltd., Lake Bluff, IL) in an epoxy matrix of EPO-TEK 301 (Epoxy Technology, Inc., Beller-ica, MA) fulfills this need. EPO-TEK 301 was selected for its low attenuation, low viscosity and long pot life. Alumina powder with a 3 µm particle size was chosen to reduce the attenuation and provide a reasonable level of loading. First, EPO-TEK 301 was characterized, and then increasing volume fractions of alumina particles were added so as to achieve impedances above 3.0 Mrayls. Using the EPO-TEK 301, the preferred amount of alumina was hand mixed in a 40 mm diameter sample holder.

The resultant mixture was degassed in a vacuum chamber at less than 10 mtorr and then cast between two mold released glass plates placed 0.5 mm apart, cured at room temperature overnight, and finally post cured for another hour at 65 °C in the oven. The specimens were then 25 mm in diameter and 0.5 mm thick, and the major surfaces were parallel and flat with a thickness difference of less than 1%.

The volume fraction of alumina (V) was established by the relationship V = (M/ρ)/(M/ρ+M'/ρ'), where M and M' are the mass of alumina and EPO-TEK 301, and ρ and ρ' represent the density of alumina and EPO-TEK 301, respectively. For these types of experiments, the alumina’s volume fraction differed between 1.5 and 30%. In order to check the consistency of the properties, five specimens were made for each volume fraction of alumina.

Backing materials were developed with the acoustic impedance ranging from 2 to 10 Mrayls. However, a high attenuation is required for this type of application. The most commonly used backing materials are Tungsten polymer composites. Tungsten powder with a particle size of less than 5 µm (Alfa Aesar, Ward Hill, MA) was chosen. Using the same process described for the AI2O3 powder-epoxy composites, a set of tungsten/EPO-TEK 301 composites was prepared. The maximum volume fraction of fine tungsten particles (less than 5 µ) added to the epoxy was about 25%. Five specimens were again prepared to check the consistency of the properties.

Experimental Results

A. Matching Material

The properties of alumina/EPO-TEK 301 composites at 30 MHz are shown in Figure 2. Adding alumina powder to epoxy results in increased density [Figure 2(a)], ultrasonic velocity [Figure 2(b)], and acoustic impedance [Figure 2(c)]; however, the attenuation shows a nonlinear variation [Figure 2(d)]. There is an attenuation peak between 7 and 9% volume fraction of alumina. A similar behavior was also noted experimentally by Grewe in alumina/Spurr epoxy composites [11]. It must be remembered that since the polymer was the key constituent by volume (greater than 70 volume percent), all the composites in this part of the study were believed to have a 0-3 connectivity, meaning each particle was surrounded by the polymer matrix [11].

ariation of the following material properties as a function of the volume fraction of alumina in EPO-TEK 301 (Epoxy Technology, Inc., Bellerica, MA) at 30 MHz: a) density, b) phase velocity, c) acoustic impedance and d) attenuation. Error bars represent standard deviations.

Figure 2. Variation of the following material properties as a function of the volume fraction of alumina in EPO-TEK 301 (Epoxy Technology, Inc., Bellerica, MA) at 30 MHz: a) density, b) phase velocity, c) acoustic impedance and d) attenuation. Error bars represent standard deviations.

Devaney and Levine [12] have suggested a model based on a self-consistent formulation of multiple-scattering theory to explain the elastic properties of such a two-phase composite. With the help of this model, the bulk modulus K and the shear modulus G of the composite are given by:

(5)

 

(6)

 

where K1, G1, K2 and G2 are the bulk modules and shear modulus of the matrix and particle, respectively. V1 and V2 are the volume fraction of the matrix and particle. The density ρ of a two-phase system is merely the volume-averaged density.

(7)

 

where ρ1 and ρ2 are the densities of the matrix and filler, respectively. The longitudinal velocity CL is related to the mechanical properties of the medium through

(8)

 

and the sheer velocity is given by

(9)

 

The acoustic impedance, phase velocity and density of the alumina/EPO-TEK 301 composites were measured from (5)-(9), and the results are illustrated in Figure 2(a)-(c) as dashed lines. Good agreement between the model and experimental results was observed. A number of other matching materials were also examined, and the measured properties at 30 MHz are summed up in Table 1. The materials cover a range of impedance between 2 and 6 Mrayls for applications in single element, array, composite and monolithic transducers.

B. Backing Material

Shown in Figure 3 are the variations of four properties as a function of the volume fraction of tungsten in tungsten/EPO-TEK 301 composites. This tungsten-epoxy composite system was observed to have interesting effects. At first, a relatively sharp fall in ultrasonic velocity takes place with the increase of tungsten content as shown in Figure 3(b). Since the density of the composites increases linearly with the addition of tungsten [Figure 3(a)], the net result is a monotonic increase in acoustic impedance [Figure 3(c)].

Interestingly, an attenuation peak was also observed between 7 and 9% volume fraction of tungsten [Figure 3(d)], similar to the alumina/EPO-TEK 301 composites. On the basis of the Denavey model, the acoustic impedance, phase velocity and density for the tungsten-loaded epoxies were calculated and the results are illustrated in Figure 3(a)-(c) as dashed lines. There was good agreement between the model and the experimental results.

Variation of the following material properties as a function of the volume fraction of tungsten in EPO-TEK 301 (Epoxy Technology, Inc., Bellerica, MA) at 30 MHz: a) density, b) phase velocity, c) acoustic impedance, and d) attenuation of longitudinal wave. Error bars represent standard deviations.

Figure 3. Variation of the following material properties as a function of the volume fraction of tungsten in EPO-TEK 301 (Epoxy Technology, Inc., Bellerica, MA) at 30 MHz: a) density, b) phase velocity, c) acoustic impedance, and d) attenuation of longitudinal wave. Error bars represent standard deviations.

Table 1 lists the acoustic properties of a number of other backing materials quantified at 30 MHz, including E-Solder 3022 and Able-bond 16 LV. These materials contain silver particles that are surrounded by an epoxy matrix. All the materials showed high attenuation, a prerequisite for acoustic backings. As would be predicted, the materials’ acoustic properties changed following the centrifuging process, which increased the silver’s volume fraction. A Beckman Model TJ-6 (Beckman Instruments, Inc., Palo Alto, CA) was used as the centrifuge. The material was centrifuged using a centripetal acceleration of 17.5 m/s2 (corresponding to 3000 rpm at a 7" radius) for a period of 10 minutes. This step seperated the material into two strata; the top layer included an unloaded epoxy. By removing the unloaded material from the top surface, the sample was lapped to thickness allowing the densest portion to be used for characterization.

Table 1. Acoustic properties of some passive materials at 30 MHz.

  Density
(* 103 kg/m3)
Longitudinal wave Shear wave
Velocity
(* 103 m/s)
Loss
(dB/mm)
Impedance
(MrayIs)
Velocity
(* 103 m/s)
Loss
(dB/mm)
Impedance
(Mrayls)
EPO-TEK 353 ND1 1.24       1.23   1.52
EPO-TEK 301-21 1.15       1.23   1.41
2038/34042 1.18       1.26   1.49
*Ablebondl6-ILV3 2.40
*Ablebondl6-ILV3 (2800 RPM centrifuge for 15 min) 3.66 1.02 1.0* 102 3.73      
*E-Solder 30224 2.59 2.11 40 5.46      
*E-Solder 30224 (3000 RPM centrifuge for 10 min) 3.20 1.85 1.1* 102 5.92      
Araldite GY508/HY9565 1.11 2.21 35 2.45      
Conap EN-4/EN-76 1.10 1.71 39 1.88      
Cas tall U-2521, Urethane, Shore D-407 1.08 2.11 32 2.28 - - -
TPX8 0.822 2.17 5.8 1.78 - - -
Rexolite9 1.06 2.34 1.1 2.57 - - -
Celazole9 1.28 3.43 1.8 4.39 1.47 7.2 1.88

1 Epoxy Technology, Inc., Bellerica, MA.
2 Insulcast, a division of American Safety Technologies, Inc., Roeeland, NJ.
3 Ablestik, a division of the National Starch and Chemical Company, Rancho Domínguez, CA.
4 Von Roll I sola USA, Inc., New Haven, CT.
5 CIBA Specialty Chemicals, Performance Polymers, Inc., Brewster, NY.
6 Conop, Inc., Olean, NY.
7 CastaJl, Inc., East Weymouth, MA.
8 Matsui Plastics, White Plains, NY.
9 Curbell Plastics, Glenshaw, PA.
* Electrically conductive.

C. Frequency Dependence of Acoustic Properties of Lens Materials

The frequency dependence of the acoustic properties of two prospective lens materials was calculated in the frequency range from 25 to 65 MHz. In Figure 4, the attenuation and phase velocity of two representative materials – Rexolite and Araldite (GY508/HY956) – (refer to Table 1 for manufacturer information) are plotted as a function of frequency. It was observed that the attenuation of Rexolite was too low (1.1 dB/mm at 30 MHz) and its velocity dispersion was also extremely small, while the attenuation of the Araldite (GY508/HY956) (35 dB/mm at 30 MHz) was too high and its velocity also showed a strong frequency dependence.

The frequency dependence of phase velocity and attenuation of longitudinal wave in Rexolite and Araldite (GY508/HY956) (Curbell Plastics, Glenshaw, PA).

Figure 4. The frequency dependence of phase velocity and attenuation of longitudinal wave in Rexolite and Araldite (GY508/HY956) (Curbell Plastics, Glenshaw, PA).

Summary and Conclusions

Using ultrasonic spectroscopy, the acoustic properties of a number of passive materials for ultrasound transducers were characterized at room temperature in the frequency range of 25 to 65 MHz. The alumina/EPO-TEK 301 and tungsten/EPO-TEK 301 composites were developed with varying volume fractions of particle loading. Experimental results showed a monotonic increase in the acoustic impedance as the particle volume fraction increased. An attenuation peak was found to take place at about 9% volume fraction of particles.

A number of key passive materials were also developed and measured in the frequency between 25 and 65 MHz. The quantified results demonstrated that higher attenuation corresponds to greater velocity dispersion. High frequency ultrasonic transducers can be designed using these measured passive material properties.

References

[1] G. Kossoff, "The effects of backing and matching on the performance of piezoelectric ceramic transducers," IEEE Trans. Son-ics Ultrason., vol. SU-13, no. 1, pp. 20-30, 1966.

[2] H. W. Persson and C. H. Hertz, "Acoustic impedance matching of medical ultrasound transducers," Ultrasonics, vol. 23, no. 3, pp. 83-89, 1985.

[3] K. K. Shung and M. J. Zipparo, "Ultrasonic transducers and arrays," IEEE Eng. Med. Biol., vol. 15, no. 6, pp. 20-30, 1996.

[4] F. S. Foster, C. J. Pavlin, K. A. Harasiewicy, P. A. Christopher, and J. H. Turnbull, "Advances in ultrasound biomicroscopy," Ultrasound Med. Biol., vol. 26, no. 4, pp. 1-27, 2000.

[5] G. R. Lockwood, D. H. T\irnbull, D. A. Christopher, and F. S. Foster, "Beyond 30MHz: Applications of high frequencies ultrasound imaging," IEEE Eng. Med. Biol., vol. 15, no. 6, pp. 60-70, 1996.

[6] F. S. Foster, L. K. Ryan, and D. H. TUrnbull, "Characterization of lead zirconate titanate ceramics for use in miniature high-frequency (20-80 MHz) transducers," IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 38, no. 5, pp. 446-453, 1991.

[7] M. J. Zipparo, K. K. Shung, and T. R. Shrout, "Piezoceram-ics for high-frequency (20 to 100 MHz) single-element imaging transducers," IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 44, no. 5, pp. 1038-1046, 1997.

[8] W. Sachse and Y. H. Pao, "On the determination of phase and group velocities of dispersive waves in solids," J. Appl. Phys., vol. 49, no. 8, pp. 4320-4327, 1978.

[9] J. Wu, "Determination of velocity and attenuation of shear waves using ultrasonic spectroscopy," J. Acoust. Soc. Amer., vol. 99, no. 5, pp. 2871-2875, 1996.

[10] H. Wang, W. Jiang, and W. Cao, "Characterization of lead zirconate titanate piezoceramic using high frequency ultrasonic spectroscopy," J. Appl. Phys., vol. 85, no. 12, pp. 8083-8091, 1999.

[11] M. G. Grewe, T. R Gururjia, T. R. Shrout, and R. E. Newn-ham, "Acoustic properties of particle/polymer composites for ultrasonic transducer backing applications," IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 37, no. 6, pp. 506-513, 1990.

[12] A. J. Devaney and H. Levine, "Effective elastic parameters of random composites," Appl. Phys. Lett., vol. 37, no. 4, pp. 377-379, 1980.

[13] M. G. Silk, Ultrasonic Transducers for Nondestructive Testing. Bristol, England: Adam Hilger Ltd., 1984.

Epoxy Technology, Inc

This information has been sourced, reviewed and adapted from materials provided by Epoxy Technology, Inc.

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