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

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Handbook of dielectric, piezoelectric and ferroelectric materials120

possible for the PMN–PT probe to use a lower Tx frequency (the center

frequency of induced pulse on the probe from the imaging system) for better

penetration. This is very useful for harmonic imaging especially in cardiac

application. If the Tx frequency is lower, the Rx frequency (the center frequency

of received pulse by the probe from the human body) of harmonic component

is higher, resulting in a better resolution. Therefore, a broad bandwidth enhances

both penetration and resolution.

Table 4.4 gives a summary of pulse echo characteristics of PZT and

PMN–PT single crystals after averaging all of the elements. The center

frequencies of the PMN–PT probe are slightly higher than that of the PZT

probe. The echo amplitude of the PMN–PT probe is about 2dB higher than

that of the PZT probe. The –6dB fractional bandwidth of the PMN–PT probe

is about 30% broader than that of the PZT ceramic probe.

Figure 4.19 shows B-mode harmonic images of PZT and PMN–PT single

crystal probes using ACCUVIX XQ (Medison Co., Ltd). The system settings

of the both cases are the same except the Tx frequency. The Tx voltages of

both are the same and the Tx frequencies are slightly different because the

bandwidths of the two probes are different. The B-mode sensitivity of the

PMN–PT probe is higher than that of the PZT ceramic probe and the contrast

and resolution of the PMN–PT probe are also better than those of the PZT

Table 4.4

Summary of pulse echo characteristics of PZT and PMN–PT probes

Properties Relative Center –6 dB –20 dB –6 dB –20 dB

sensitivity frequency bandwidth bandwidth ringdown ringdown

[dB] [MHz] [%] [%] [µs] [ µs]

PZT 0 5.0 68 94 0.312 0.736

PMN–PT +2.1 5.9 98 130 0.189 0.462

4.19

Comparison of harmonic B-mode images of 6MHz phased array

with a fine pitch (150µm) (a) PZT probe vs. (b) PMN–PT probe.

(a) (b)

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Piezoelectric single crystals for medical ultrasonic transducers 121

probe. These results are consistent with the two-way pulse echo data in Table

4.4. If the system condition optimization can be done, the image quality of

the PMN–PT probe will be even better.

4.3.3 Future of medical ultrasound transducer using

single crystal

Multilayer single crystal probe

The electrical impedance mismatch between the small elements and the

cable or system is somewhat serious, especially for low center frequency

(< 4MHz) applications. In the case of a fine pitch transducer (150–200µm),

the electrical impedance of the element is about 800–1000Ω and this can

result in a significant mismatch to the cable and system (50–100Ω). A typical

solution to this electrical mismatch problem is to use buffer electronics or a

multilayer structure to reduce the electrical impedance of the element. The

electrical impedance of a 3MHz phased array using a double PMN–PT

single crystal is only 25% of a single layer PMN–PT phased array.

Figure

4.20 shows the cross-section of a double PMN–PT single crystal wafer. Each

layer thickness of the double layer is 0.2mm and the total thickness is

0.4mm. Table 4.5 summarizes the electrical properties of PZT ceramics,

single layer PMN–PT and double layer PMN–PT single crystals.

Figure 4.21 shows the electrical impedance of single layer and double

layer PMN–PT phased array elements. The impedance at 3MHz of the double

layer PMN–PT phased array is very close to the impedance of the cable with

85Ω. This means that the sensitivity loss of a double layer PMN–PT phased

array is much smaller than that of a single layer PMN–PT phased array.

Figures 4.22 and 4.23 show the B-mode harmonic image and color Doppler

4.20

Cross-section of a double layer PMN–PT single crystal.

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Handbook of dielectric, piezoelectric and ferroelectric materials122

Table 4.5

Measured electrical properties of PZT ceramics, and single layer and

double layer PMN–PT single crystals

Properties PZT Single layer Double layer

PMN–PT PMN–PT

Density [kg/m

] 7800 8000 8000

3800 5500 24 860

1250 1100 6390

tanδ 2<1 <1

′

0.74 0.82 0.862

′

[m/s] 4118 3180 2848

[MRayls] 32.1 25.6 23.0

Single layer

Multilayer

Frequency (MHz)

876543210

Impedance real & imaginary (Ω)

–1200

–900

–600

–300

300

600

800

1200

4.21

Electrical impedance of single layer and multilayer PMN–PT

phased array elements.

4.22

Comparison of harmonic B-mode images of 3MHz phased array

acquired by (a) a single layer PMN–PT probe and (b) a double layer

PMN–PT probe.

(a) (b)

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Piezoelectric single crystals for medical ultrasonic transducers 123

image of single layer PMN–PT and multi-layer PMN–PT probes, respectively.

The B-mode and color Doppler sensitivity of the multilayer PMN–PT probe

is higher than that of the single layer PMN–PT probe. Because of the lower

electrical impedance of the double layer PMN–PT single crystal, a higher

sensitivity is obtained compared with the single layer PMN–PT single crystal.

This means that the heat generation on the double layer PMN–PT probe

surface is reduced when the same voltage is applied to the probe.

Multilayer/polymer composite single crystal phased array

To produce a better image, broader bandwidth and higher sensitivity are

required. To increase the bandwidth, the piezomaterial/polymer composite is

one of the solutions. In the case of PZT ceramics, the electromechanical

coupling factor of PZT/polymer composite is higher than that of PZT ceramics

only because of the decreasing stiffness of ceramics. However, the dielectric

constant of the PZT/polymer composite is lower than that of PZT ceramics

because of the polymer having a low dielectric constant. If the dielectric

constant is low, the electrical impedance is high and then the sensitivity is

also low because of the electrical impedance mismatch between element and

cable. Therefore, we have to increase the dielectric constant in order to

increase the sensitivity of the probe.

Figure 4.24 shows the cross-section of a multilayer PMN–PT single crystal/

polymer composite wafer with 80% single crystal volume fraction. Table 4.6

summarizes the mechanical and electrical properties of PZT ceramics, and

single layer and multilayer/polymer composite PMN–PT single crystals. The

clamped dielectric constant of the multilayer/polymer composite PMN–PT

single crystal is almost four times that of 3203HD PZT ceramics. The

electromechanical coupling factor of the PMN–PT single crystal/polymer

(a) (b)

4.23

Color Doppler mode images of 3MHz phased array acquired by

(a) a single layer PMN–PT probe and (b) a double layer PMN–PT

probe.

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Handbook of dielectric, piezoelectric and ferroelectric materials124

composite is almost the same as that of the single layer PMN–PT single

crystal. Moreover, the acoustic impedance of the multilayer/polymer composite

PMN–PT is much lower than that of PZT and the single layer PMN–PT

single crystal. This means the acoustic impedance mismatch between

piezomaterial and human body is improved and it is possible to use comparably

low acoustic impedance matching layer with about 6–7MRayls, compared

with that of ceramic with about 8–10 MRayls.

Figure 4.25 shows the electrical impedance (without cable) of the elements

of single layer, multilayer, and multilayer/polymer PMN–PT single crystal.

The electrical impedance of the multilayer/polymer at 3.5MHz is only 40%

of that of the single layer PMN–PT single crystal. Figure 4.26 shows the

frequency spectra and waveforms of single layer, multilayer and multilayer/

polymer composite PMN–PT single crystal probes. The –6dB bandwidth

and sensitivity of multilayer/polymer composite PMN–PT probe is about

4.24

Cross-section of a multi-layer PMN–PT/polymer composite

wafer.

Table 4.6

Electrical and mechanical properties of 3203HD PZT ceramics, and

multilayer and multilayer/polymer composite PMN–PT single crystals

Properties 3203HD Single layer Multilayer/composite

Density [kg/m

] 7800 8000 8000

3800 5500 11 500

1250 1100 4000

tanδ [%] 2 <1 <1

′

0.74 0.82 0.82

′

[m/s] 4100 3200 2700

[MRayls] 32 26 19

PMN-PT

Polymer

30 µm

100 µm

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Piezoelectric single crystals for medical ultrasonic transducers 125

20% broader and 3dB higher than those of the single layer PMN–PT probe,

respectively. The measured acceptance angle of the multilayer/polymer PMN–

PT probe matches well the theoretical value.

4.4 Conclusions and future trends

The excellent piezoelectric properties of the single crystal improved the

performance of ultrasonic medical transducers. To provide better image quality

of recent imaging systems, the number of channels is increased and the

mismatch between probe and cable or system should be increased. To reduce

this electrical impedance mismatch, high dielectric materials and the multilayer

technique are being developed by many researchers. In particular, the multilayer

technique is very useful for a multi-row probe that is used for multi-focusing

and 3D image. However, the bandwidth of the multi-row probe is narrower

than the normal 1D probe because of the complicated electrical connection.

However, broad bandwidth is necessary for harmonic imaging in phased

array and when a single crystal is used for a multi-row probe, we can obtain

a bandwidth as broad as that using a normal 1D probe.

In recent years the capacitive microfabricated ultrasound transducer (cMUT)

has undergone intensive development and created interest as an alternative

to conventional piezoelectric ultrasound probes. Moreover, the cMUT has

advantages over a 2D array for 3D imaging and smart probe for SOC (system-

Single layer Multilayer Multilayer/composite

Frequency (MHz)

87654321

–1000

–800

–600

–400

–200

200

400

600

800

Impedance (Ω)

4.25

Electrical impedance (without cable) of the elements of single

layer, multi-layer, and multi-layer/polymer PMN–PT single crystals.

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Handbook of dielectric, piezoelectric and ferroelectric materials126

on-chip) or POC (point-of-care). Even though many things need to be improved

for commercialization, especially low sensitivity comparing with the normal

probe, cMUT has a very attractive future as a medical ultrasound probe.

4.26

(a) Frequency spectra and (b) waveforms of single layer, multi-

layer, and multi-layer/polymer PMN–PT single crystal probes.

Single layer Multilayer Multilayer/composite

Frequency (MHz)

(a)

876543210

–40

–30

–20

–10

Amplitude (dB)

Time (µs)

(b)

3.02.52.01.51.00.5

Output voltage (V)

–5

–4

–3

–2

–1

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Piezoelectric single crystals for medical ultrasonic transducers 127

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Piezoelectric single crystals for medical ultrasonic transducers 129

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