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

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Handbook of dielectric, piezoelectric and ferroelectric materials100

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field effect on the temperature anomalies of dielectric permittivity in PMN–PT

single crystals’, Phys. Rev. B., 2005 72 184104.

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piezoelectric response in Pb(Mg

1/3

2/3

–PbTiO

single crystals’, Solid State

Commun., 2006 137 16–20.

32 Ren W, Liu S F, and Mukherjee B K, ‘Piezoelectric properties and phase transitions

of <001> oriented PZN–PT single crystals’, Appl. Phys. Lett., 2002 80 3174–6.

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electromechanical actuators’, Mat. Res. Innovat., 1997 1 20–25.

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properties of perovskite materials at cryogenic temperatures’, J. Mater. Sci, 1998 34

469–473.

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single crystal PMN–PT with high energy conversion efficiency’, JFE Technical

Report, No. 06, pp 46–53, October 2005.

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101

4.1 Introduction

Medical ultrasound imaging technique has progressed considerably in the

recent years. With the improvement of ultrasound image resolution and a

low device price compared with computer tomography (CT), magnetic

resonance imaging (MRI) and X-ray imaging systems, the market share of

ultrasound imaging system is gradually increasing.

The applications of medical

ultrasonic imaging span a wide frequency range, from 1.5 to 30MHz, depending

on the organs to be imaged. For abdominal, obstetric and cardiological

applications, the frequency range is from 2 to 5MHz. For pediatric and

peripheral vascular applications, the range is from 5 to 7.5MHz. For imaging

small objects such as the eye and for many other emerging applications, such

as intracardiac and intravascular imaging, the frequency range is from 10 to

30MHz.

Ultrasound imaging technology has a long history since its modest

beginnings in the 1950s and the image quality of medical ultrasound has

improved vastly with linear and phased array transducer designs replacing

the mechanical 2D transducer used in the 1980s. There are many ultrasound

imaging systems using new technology with such advanced features as

compounded imaging, second harmonic imaging, contrast agents, color flow

Doppler and 3D imaging.

To improve the ultrasound imaging quality, many technologies have been

used for transducer fabrication, such as multilayer, piezo-material/polymer

composite,

single crystal, multi-row, fine pitch, and cMUT (capacitive

microfabricated ultrasound transducer).

Some technologies have already

been used for commercialized transducers and the ultrasound image quality

has progressed considerably because of this advanced technology; some new

technologies are still being developed for commercialization as cMUT.

Among these technologies, the single crystal transducer has directly

benefited from the recent development of relaxor-based materials. In 1997,

Park and Shrout

reported that (1 – x)Pb(Zn

1/3

2/3

–xPbTiO

(PZN–PT)

Piezoelectric single crystals for medical

ultrasonic transducers

S M R H I M, Humanscan Co. Ltd, Korea and M C SHIN

and S-G LEE, IBULe Photonics Co. Ltd, Korea

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Handbook of dielectric, piezoelectric and ferroelectric materials102

and (1 – x)Pb(Mg

1/3

2/3

–xPbTiO

(PMN–PT) single crystals exhibited

very high piezoelectric coefficients (d

> 2000 pC/N) and strain levels

(0.6%) with little hysteresis when crystals are poled along the <001> direction.

As a result, these materials have great potential for use in both actuator and

transducer applications.

5–8

These materials have excellent piezoelectric

properties near the morphotropic phase boundary (MPB) where the solid

solution changes structure from rhombohedral to tetragonal, near the

composition of 35% PT (x = 0.35).

In this chapter, an overview of the growth of PMN–PT single crystal and

the characterization of PMN–PT single crystals will be presented first. Then,

the disadvantages of Pb(Zr

1–x

(PZT) ceramics will be compared to the

advantages of PMN–PT single crystals. Finally, the commercialization of

PMN–PT single crystal transducers and the future of single crystal transducers

will be discussed in detail.

4.2 Piezoelectric single crystals

Single crystals of the solid solutions between complex lead perovskites and

lead titanate, with the general formula Pb(B

–PbTiO

= Mg

, Ni

, Sc

, etc.; B

= Nb

, Ta

, etc.), have been extensively studied

since Kuwata et al. reported the extremely high piezoelectric properties in

PZN–PT.

Among these single crystals, PZN–PT and PMN–PT exhibit an

ultra-high piezoelectric constant, d

, and a very large electromechanical

coupling factor, k

, when the crystals are poled along the <001> direction

which is different with the spontaneous polarization direction of <111> in

comparison with those of PZT ceramics.

Such excellent properties make

them attractive for both actuator and ultrasonic transducer applications.

4.2.1 Crystal growth

Because of the instability of perovskite PZN phase, it is difficult to synthesize

pure PZN–PT ceramic powder by solid-state reaction under normal conditions.

Therefore, PZN–PT crystal growth is performed with the component oxides

and the PbO flux. In contrast, PMN–PT ceramic powder can be synthesized

by the columbite method

and PMN–PT crystals can be grown from the

melt. Lee et al. first suggested the use of the Bridgman technique with sealed

Pt crucibles as an attractive alternative to flux growth.

Some of the defects

(e.g. dendrites, inclusions and voids) found in the PMN–PT crystals grown

by directly melting the component oxides can be caused by melt

inhomogeneity.

11–16

It may be difficult to form a homogeneous PMN–PT

melt from the component oxides because of the large differences in the

melting temperature and the density. For example, MgO is not only more

refractory than PbO (T

m-MgO

= 2852 °C, T

m-PbO

= 886 °C), but it is also much

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Piezoelectric single crystals for medical ultrasonic transducers 103

lighter (ρ

MgO

= 3.58 g/cm

, ρ

PbO

= 9.53g/cm

). In order to avoid melt

homogeneity problems, it is desirable to pre-synthesize charge material.

An accurate high-temperature phase diagram is very important for the

growth of large single crystals. Thermogravimetric/differential thermal analysis

(TG/DTA) was used to obtain a high-temperature phase diagram. A complete

pseudo-binary phased diagram between PZNT91/9 and PbO was established

by Dong and Ye.

The typical eutectic behavior is shown in Fig. 4.1(a). This

phase diagram helps to grow PZN–PT single crystals by the solution Bridgman

PbO (wt%)

100800604020

PZNT + PbO

Liquid

PZNT + liquid

PbO + liquid

1500

1300

1100

900

Temperature (K)

(a)

Liquidus curve (DTA data)

Solidus curve (EPMA data)

Solidus curve (from [17])

Solid

Liquid

0.0

Fraction of PT

(b)

0.60.2 0.4 0.8 1.0

1320

1315

1310

1305

1300

1295

1290

1285

1280

Temperature (°C)

4.1

High-temperature phase diagram of the PZNT–PbO

and PMN–

pseudo-binary systems.

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Handbook of dielectric, piezoelectric and ferroelectric materials104

method. The melting points of PMN–PT observed by DTA are given in Table

4.1. The measured melting points decreases incrementally from x = 0 (1319°C)

to x = 1.0 (1285°C). These results are very helpful in estimating where the

initial solid–liquid interface position would be during seeding, and in preventing

complete melting of the seed or powdered charge. After growth, the actual

initial interface position could be estimated through either optical inspection

or composition measurements. A pseudo-binary phased diagram between

PMN and PT was proposed by Zawilski et al., as shown in Fig. 4.1(b).

The growth of large PZN–PT and PMN–PT single crystals has mostly

been carried out using the following four methods: flux (using PbO flux),

Bridgman, solution Bridgman (using PbO flux) and abnormal grain growth.

A schematic diagram of Bridgman growth system is shown in Fig. 4.2.

The growth was generally carried out in either tapered Pt crucibles with seed

pockets or straight-walled Pt crucibles. In order to prevent the component

volatilization and to control the atmosphere, the crucible was sealed with a

lid by welding. The crucible preparation is very important in the Bridgman

growth of PMN–PT since PbO-containing melts can attack Pt at elevated

temperatures. Leaks of any size can be a problem, and large leaks can ruin

an experiment.

During growth, a vapor-filled region formed above the melt because the

charges, which are composed of packed powder, shrink on melting. Pressure

built up in this region at growth temperatures, and the upper ~2–3cm of the

crucibles typically swelled by several millimeters. The swelling did not

usually affect the growth results. However, small amounts of red/orange

Table 4.1

Melting points of the (1 –

)PMN–

PT system

Composition (

) Melting point (°C) Notes

0.00 1320

19,20

Both used sealed Pt crucibles and

arrived at the same value

1319

DTA using sealed Pt crucibles

0.10 1318

DTA using sealed Pt crucibles

0.20 1316

DTA using sealed Pt crucibles

0.30 1296

DTA using unsealed Pt crucibles in air

1309

DTA using sealed Pt crucibles

0.32 1271, 1288

DTA of flux grown crystal in unsealed

Pt pans under N

atmosphere (two

values were from the two peaks

observed)

0.35 1288

DTA using unsealed Pt crucibles in air

1297

DTA using sealed Pt crucibles

0.40 1284

DTA using unsealed Pt crucibles in air

1293

DTA using sealed Pt crucibles

1.00 1285

DTA using sealed Pt crucibles

1285

Phase diagram study using sealed Pt

crucibles

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Piezoelectric single crystals for medical ultrasonic transducers 105

materials were often deposited at the top of the crucibles, indicating some

leakage. The furnace gradient at the growth interface was ~20°C/cm. The

thermocouple was welded on the side of the crucible near the bottom to

control the position of the initial growth interface. Unidirectional solidification

was accomplished through either furnace or crucible translation. Typical

translation rates of 0.1~1mm/h were used.

After growth, the crystals were slowly cooled down to room temperature

to prevent cracking. Particular care was taken in the 200°C to room temperature

range (cooling rate of 5°C/h) where the material undergoes a phase transition

from cubic to tetragonal or rhombohedral phase (depending on composition).

The crucibles were not reusable, as the grown crystals could not be removed

without cutting away the crucible. The grown crystals had smooth and shiny

surfaces on which the grain structure of the Pt crucible was imprinted.

4.2.2 Characterization of the grown crystals

The Ti concentration increases along the growing direction. The Ti

concentration profile is very similar to the classic shape of the unidirectional

solidification of a completely mixed melt with no diffusion in the solid. The

equation describing the concentration profile of a system for the case of

complete mixing is

C = kC

(1 – f)

(k–1)

4.1

High-

temperature

zone

Low-

tempera-

ture zone

Measure

thermocouple

Supporter

Alumina process tube

Alumina plug

Heater

Pt crucible

Melt

Send

Crystal

Control

thermocouple

4.2

Schematic diagram of a Bridgman growth system.

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Handbook of dielectric, piezoelectric and ferroelectric materials106

where C is the concentration in the solid, k is the effective segregation

coefficient, C

is the initial melt concentration and f is the fraction of the

original melt that has solidified. Figure 4.3 shows compositional variation in

PMN–PT single crystal.

From the logarithmic plot, the effective distribution

coefficient (k) was estimated about 0.82 for C

= 30% in Fig. 4.3. These

results are very similar to those reported by Zawilski et al..

Figure 4.4 shows the X-ray diffraction (XRD) patterns obtained from

different positions of a PMN–PT single grown crystal, with the characteristic

tetragonal peak splitting taking place at higher Ti concentrations, in accordance

with the composition analysis. Therefore, because of the segregation, from

the bottom to the top of the crystal, the phase symmetry gradually changes

from rhombohedral to tetragonal with increasing Ti contents.

The polarization directions of domains are different between the

rhombohedral and tetragonal phases. Each phase has a specific domain

structure. Figures 4.5 and 4.6 show the domain configurations of the

rhombohedral and tetragonal PMN–PT crystals, respectively, observed by

optical microscopy, surface etching and scanning force microscopy (SFM).

In the rhombohedral phase, both regular tire-track patterns and irregular

fingerprint patterns were observed on the as-grown crystal plates. The tire-

track patterns were composed of {1 0 0} and {1 1 0} domain walls. The

irregular fingerprint patterns were antiparallel domains. In the tetragonal

crystals, three different types of etching area were observed. These patterns

consisted of 180° domains and 90° {1 1 0} domain boundary.

The best piezoelectric response was found in the rhombohedral PZN–PT

Solidified fraction

0.80.70.60.50.40.30.20.1

Molar concentration (%)

100

4.3

Compositional variations along the axial direction of a PMN–PT

crystal grown by the Bridgman method

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Piezoelectric single crystals for medical ultrasonic transducers 107

and PMN–PT single crystals poled long the <001> direction,

which is different

from spontaneous polarization direction. The <001> poled crystals show a

macroscopic 4mm symmetry and excellent piezoelectric properties. It is

also very important to have a complete set of elastic, piezoelectric and dielectric

constants. The data can be obtained by using a combined method involving

both pulse-echo and impedance resonance techniques.

A complete set of

data for <001> poled rhombohedral PMN–PT single crystals is given in

Table 4.2.

The uniformity of the crystals is very important for the performance of

transducer. It can be checked by measuring the chemical composition of the

crystal wafer and the dielectric and piezoelectric properties of the elements

of N channel, as shown in Fig. 4.7.

4.3 Single crystal transducers

4.3.1 Single crystals versus conventional PZT ceramics

One key method of improving ultrasound imaging is to increase the performance

of ultrasonic transducer. The vast majority of ultrasonic transducers utilize

polycrystalline piezoelectrics based on the composition Pb(Zr

1–x

(PZT).

These materials bring the benefit of relatively high electromechanical coupling

factor and piezoelectric constant and, therefore, display improved probe

bandwidth and sensitivity. Recent ultrasonic arrays for medical imaging

system are made from piezoelectric ceramics, such as PZT-5A, PZT-5H, etc.

2θ

6050403020

Ti 38.3%

Ti 36.9%

Ti 32.4%

Ti 29.6%

Ti 25.6%

4.4

XRD pattern from different vertical locations in a grown PMN–PT

crystal.

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Handbook of dielectric, piezoelectric and ferroelectric materials108

330.98 mm

0 mm

[010]

[100]

[010]

[100]

[010]

[100]

[110]

[1 1 0]

[010]

[110]

[010]

[100]

[010]

[100]

Upward

Downward

6.91 V

0 V

0 µm

60 µm

0 µm

60 µm

(a)

(b) (c)

(d)

(e)

(f)

(g)

[1 1 0]

4.5

Domain patterns of rhombohedral single crystal (PMN–25.6%PT)

observed at room temperature: (a) by SFM in the piezoresponse

mode (topology (left) and piezoresponse (right) are displayed);

(b) fingerprint pattern (180° domain wall) observed by SEM after

etching; (c) tire-track pattern (71° or 109° domain wall);

(d) magnification of (c); (e) spatial configuration of (d); (f) domain

pattern under a polarizing microscope; and (g) domain pattern under

a polarizing microscope after heat treatment.

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Piezoelectric single crystals for medical ultrasonic transducers 109

[110]

[1 1 0]

[010]

[1 1 0]

[110]

[100]

[110]

[1 1 0]

[110]

[010]

[100]

[010]

[100]

. . . . . . a–c domain wall

(a)

(b)

(e)

(f)

(g)

[1 1 0]

[ 1 10]

383.46 nm

0 mm

8.16 V

0 V

4.6

Domain patterns of tetragonal PMN–PT (PMN–38%PT) single

crystals at room temperature: (a) polarizing micrograph; (b) etching

pattern consisting of three different types of etching area; (c) etching

pattern of

–

domains; (d) etching pattern of

–

domains; (e) size

relation between

-domains and

-domains; (f) spatial configuration

of (d); and (g) piezoelectric mode SFM scan on (0 0 1)-oriented

platelet: topology (left) and normal piezoresponse (right) are

displayed.

–a

0 µm 30 µm

0 µm

30 µm

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