Zuo-Guang. Ye Advanced Dielectric Piezoelectric and Ferroelectric Materials: Synthesis, Characterisation and Applications

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Handbook of dielectric, piezoelectric and ferroelectric materials110

Because of the small array element sizes, a very high relative dielectric

constant and high coupling factor are necessary in modern ultrasonic array

applications. The most important parameters in ultrasonic array are k

, d

and g

. The piezoelectric coefficient d

measures the strain developed per

applied field and is a measure of the effectiveness of the material as a driver

or transmitter. The constant g

is the electric field generated per applied

stress and is a measure of the effectiveness when used as a receiver. The

materials with higher coupling coefficients k

can give better bandwidths

and pulse response in medical applications.

To overcome the disadvantages of conventional PZT ceramics, the

multilayer/polymer composite technology is introduced into transducers.

The current performance status of a multilayer/polymer composite transducer

is almost the same as a normal single crystal transducer using a single layer,

but it is very difficult for a multilayer/polymer composite PZT transducer to

get high manufacturing yields.

The piezoelectric properties of PZT ceramics have shown little improvement

over the past two decades. On the other hand, the PZN–PT and PMN–PT

single crystals exhibit an ultra-high piezoelectric constant, d

, and a very

large electromechanical coupling factor, k

, in comparison with those of

PZT ceramics, making them attractive for both actuator and ultrasonic

transducer applications.

Many research results concerning medical transducers using single crystals

have been reported since the mid-1990s.

28–31

Saitoh et al. reported the first

images of B-mode and color Doppler images obtained by 3.5MHz 96 channels

phased array made from a PZN–PT single crystal.

The two-way sensitivity

of PZN–PT transducer is 6dB higher than that of conventional PZT transducer

and –6dB fractional bandwidth is 30% broader. Because the dielectric constant

Table 4.2

Elastic, dielectric, and piezoelectric data set of rhombohedral PMN–PT

crystals

Properties Values Properties Values

εε

()

6110 10

0.51

εε

()

1717 11

0.31

(10

–12

C/N) 1658 12

0.58

(10

–12

C/N) –894 13

(10

–12

/N) 56.2

(10

–12

C/N) 150.6 14

(10

–12

/N) –22.1

(10

–3

V m/N) 30.6 15

(10

–12

/N) –29.7

(10

–3

V m/N) –16.5 16

(10

–12

/N) 60.6

(10

–3

V m/N) 9.9 17

(10

–12

/N) 15.2

0.92 18

(10

–12

/N) 16.3

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Piezoelectric single crystals for medical ultrasonic transducers 111

(b)

(c)

0.25*15.8*0.4

: resonance freq.

: anti-resonance freq.

123 456789101112131415

Location

Atomic %

100

(a)

1, 2, 3

10, 11, 12

7, 8, 9

4, 5, 6 13, 14, 15

7000

6000

5000

4000

3000

2000

1000

Dielectric constant

0 1020304050

Channel no.

01020304050

Channel no.

5000

4000

3000

2000

1000

Frequency (kHz)

4.7

Uniformity of PMN–

PT single crystals: (a)

composition variation

check; (b) dielectric

constant; and (c)

resonance, anti-

resonance frequencies

for elements of N-

channel.

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Handbook of dielectric, piezoelectric and ferroelectric materials112

of the current PZT (>6000) is higher than that of previous PZT (<4000) and

is similar to that of single crystals, at the present, the sensitivity of single

crystal transducer is almost the same as PZT. Table 4.3 compares the mechanical

and electrical properties of the piezoelectric single crystals and PZT ceramics.

Although the dielectric constant of new PZT is higher than that of PMN–PT

single crystals, the coupling factor of new PZT ceramics is lower than that

of single crystals. Therefore, the performance of the transducer using new

PZT is lower than that of a single crystal transducer.

Because the electromechanical coupling factor of single crystals is higher

than that of PZT ceramics, the fractional bandwidth of single crystal transducers

is broader than that of PZT transducers. However, the Curie temperature of

single crystal is lower than that of PZT. The low Curie temperature can cause

depoling during the fabrication processes such as curing the bonding epoxy,

electrical connection, dicing process and so on. Figure 4.8 shows the

temperature dependence of the dielectric constant of a single crystal and a

PZT ceramic. To overcome this problem, a great deal of effort has been put

into increasing the Curie temperature of the PMN–PT and PZN–PT crystals

and into searching for new materials with a higher T

4.3.2 Commercialization of single crystal transducers

The major advantage of single crystal transducers is broad bandwidth and

high sensitivity compared with PZT ceramic transducer. One single crystal

Table 4.3

Mechanical and electrical properties of piezoelectric single crystals and

PZT ceramics

Properties PZT-5H 3203HD

(1)

PZT–5K

(2)

PZN–7%PT PMN–33%PT

Density 7500 7800 8200 8200 8000

[kg/m

]

Dielectric 3400 3800 6200 6500 5500

constant at

1kHz

Dielectric 2 2.4 2 <1 <1

loss [%]

Curie 190 225 160 167 140

temperature

[°C]

0.75 0.75 0.75 0.94 0.93

′

0.68 0.69 0.63 0.83 0.83

0.505 0.55 0.55 0.63

[pC/N] 593 650 870 2400 2000

(1) CTS Co. Ltd. (http://www.ctscorp.com).

(2) Morgan Electroceramics Co., Ltd (http://www.morganelectroceramics.com).

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Piezoelectric single crystals for medical ultrasonic transducers 113

transducer with broad bandwidth can substitute two different frequency PZT

transducers. Figure 4.9 gives the frequency spectra of 2.5 and 4MHz PZT

transducers and a 3MHz single crystal transducer, indicating this advantage

PZN–PT

PMN–PT

3203 HD

New PZT

Temperature (°C)

200180160140120100806040

50000

40000

30000

20000

10000

Dielectric constant

4.8

Temperature dependence of dielectric constant of PMN–PT,

PZN–PT, and PZTs.

2.5 MHz PZT 4.0 MHz PZT 3.0 MHz single crystal

Frequency (MHz)

7654321

Amplitude (dB)

–60

–70

–80

–90

–100

4.9

Frequency spectra of 2.5 and 4MHz PZT transducers vs. a 3 MHz

single crystal transducer.

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Handbook of dielectric, piezoelectric and ferroelectric materials114

of single crystal transducer with a broader bandwidth. The 3MHz single

crystal transducer was commercialized by Humanscan in 2003.

The process of transducer fabrication using single crystals differs from

that using PZT. There are many steps required for making transducers, including

acoustic stack fabrication, layer bonding, dicing, electrical connection, acoustic

lens fabrication and so on. Among these processes, the dicing process for

single crystal is appreciably different from that for PZT ceramics. Figure

4.10 shows diced elements of ordinary PZT, PMN–PT single crystal using

conventional PZT dicing process, and PMN–PT prepared using an optimized

dicing process. The element width is 120 µm and the kerf is 35µm. When a

conventional PZT dicing process is used on a single crystal, considerable

chipping occurs. A soft bonded and small grit size blade is necessary for

single crystal. The dicing speed and temperature control are also the most

important parameters for the dicing of single crystal.

Single crystals have low-frequency constants and, therefore, to make the

same frequency transducer, the single crystal has to be thinner than PZT

ceramic. The relatively low mechanical strength of single crystal is a serious

problem in making a high-frequency (>7 MHz) transducer. Another problem

for single crystal transducers is the composition uniformity across the wafer.

(a)

(b)

(c)

4.10

Diced element of (a) PZT; (b) PMN–PT single crystal using PZT

process; and (c) PMN–PT single crystal using optimized process.

0.05 mm

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Piezoelectric single crystals for medical ultrasonic transducers 115

Figure 4.11 shows the dielectric constant variation of 64 channels of PMN–

PT, PZN–PT and two PZT transducers. The variation of the dielectric constant

of the single crystal element is larger than that of PZT ceramics, but the

uniformity of single crystal has been improved compared with previous

results.

Because poor uniformity of single crystals is a serious problem in

the commercialization of transducers, useful and accurate non-destructive

inspection methods is developed such as using optical methods and dot

electrodes.

At the present, for the evaluation of uniformity of a single

crystal, the values of the dielectric constant, Curie temperature and composition

ratio are used.

Phased array

Phased array transducers using single crystals have been studied during the

last 10 years.

30, 32, 37–39

Figure 4.12(a) shows a 2.6MHz 64 channel phased

array presented at the 2003 IEEE UFFC conference

and Fig. 4.12(b) shows

a PMN–PT single crystal used in phased array and linear array. The crystal

thickness of 3.0MHz phased array is 0.4mm and that of the 7.5MHz linear

array is 0.13mm. Because the fractional bandwidth of single crystal transducers

is broader than that of PZT transducers, harmonic mode imaging exhibits

some advantages, which include a better lateral resolution, lower sidelobes

and less noise in the near field than conventional ultrasound imaging. However,

the price paid is a slightly worse axial resolution resulting from a reduced

bandwidth.

Therefore, single crystal transducers having broad fractional

4.11

Uniformity of dielectric constant of PMN–PT and PZN–PT

crystals and two PZT ceramics.

Element number

6050403020100

3203 HD

PMN–PT single crystal

PZN–PT single crystal

7000

6000

5000

4000

3000

Dielectric constant

New PZT

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Handbook of dielectric, piezoelectric and ferroelectric materials116

bandwidth are more suitable for harmonic imaging, especially in cardiac

applications.

The current PZT transducer has a limited bandwidth of the order of 80%.

However, the PMN–PT single crystal transducer shows about 100% fractional

bandwidth. Because the sensitivity of the single crystal transducer is higher

than that of the PZT transducer, the color Doppler performance of single

crystal transducer is better. The sensitivity of the PMN–PT transducer is

5 dB higher than that of PZT transducer. Figure 4.13 shows a two-way

spectrum from phased array using PZT ceramics and PMN–PT single crystals.

Harmonic mode cardiac images using PZT transducers are and PMN–PT

single crystal transducer are shown in Fig. 4.14. Figure 4.15 shows color

Doppler mode images of the four chamber view of an adult heart using PZT

and PMN–PT transducers.

High-frequency linear array

Attenuation of ultrasound in a transducer may occur because waves have

been scattered by finite-size grains or by dislocations in a solid. Another

cause of attenuation is the unequal thermal conduction and expansion of

neighboring grains due to their axes being rotated with respect to each other.

For these reasons, single crystals exhibit much lower sound attenuation at

high frequencies than the same materials in polycrystalline form.

Since

single crystal has a lower frequency constant than PZT ceramics, if the

frequency is the same, the thickness of single crystal is thinner than that of

PZT. In the case of 7.5MHz linear array, the thickness of PZT is about

180µm, while that of single crystal is 130µm. Moreover, because single

crystal has a low mechanical strength, cracks occurs easily during the dicing

process. Figure 4.16 shows a B-mode image and a color Doppler image

using a PMN–PT linear array.

4.12

(a) 2.6MHz 64 channel phased array using PMN–PT single

crystal and (b) PMN–PT single crystal used in phased array and

linear array.

(a) (b)

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Piezoelectric single crystals for medical ultrasonic transducers 117

Fine pitch phased array

The state-of-the-art transducers call for smaller and smaller elements, such

as fine pitch array, to improve image quality. A fine pitch increases element

directivity for spatial compounding and reduces harmonic frequency grating

lobe levels.

A 2.5MHz phased array with a fine pitch (170µm) using triple layer PZT

and three matching layers was reported.

According to recent research results,

the sensitivity of triple layer PZT phased array is almost the same as that of

single layer single crystal phased array.

The 6MHz phased array with a

0123456

–40

–30

–10

–20

01 23 4 5

–1.0

–0.5

0.5

1.0

0.0

Time (µs)

Frequency (MHz)

Output voltage (V)

Amplitude (dB)

01 2 34 56

–40

–30

–10

–20

01 23 4 5

–1.0

–0.5

0.5

1.0

0.0

Time (µs)

Frequency (MHz)

Output voltage (V)

Amplitude (dB)

(a)

(b)

4.13

Two-way spectrum of phased array using (a) conventional PZT

and (b) PMN–PT single crystal.

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Handbook of dielectric, piezoelectric and ferroelectric materials118

4.14

Harmonic mode images using (a) PZT and (b) PMN–PT single

crystal transducer.

(a)

(b)

(a)

(b)

4.15

Color Doppler mode images of (a) PZT and (b) PMN–PT single

crystal transducer.

4.16

(a) B-mode image and (b) Doppler image of 7.5MHz linear array

using PMN–PT single crystal.

(a)

(b)

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Piezoelectric single crystals for medical ultrasonic transducers 119

fine pitch (150 µm) using PMN–PT single crystal was announced.

Figure

4.17 shows a diced element surface having 0.15mm pitch and 0.03mm kerf

using PMN–PT single crystal. Figure 4.18 gives the frequency spectra of

PZT and PMN–PT probes, respectively. The –6dB low-frequency and high-

frequency edges of PZT probe are about 3.3 and 6.7MHz, respectively. The

low edge of the PMN–PT probe (3.0MHz) is similar to, but the high-frequency

edge is about 2MHz higher than that of the PZT probe. This means that it is

4.17

Diced element surface having 0.15mm element pitch and

0.03mm kerf.

PZT probe

PMN–PT probe

Frequency (MHz)

121086420

–40

–30

–20

–10

Amplitude (dB)

4.18

Frequency spectra of a PZT probe vs. a PMN–PT probe.

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