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

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Handbook of dielectric, piezoelectric and ferroelectric materials70

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Handbook of dielectric, piezoelectric and ferroelectric materials72

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3.1 Introduction

Piezoelectric single crystals based on Pb(Mg

1/3

2/3

)

1–x

(PMN–PT)

and Pb(Zn

1/3

2/3

)

1–x

(PZN–PT) offer large improvements in piezoelectric

strain and electromechanical coupling coefficient for actuator and transducer

applications [1, 2]. Over the past several years, much work has been done to

demonstrate the advantages crystals can provide for a variety of military and

domestic applications, including medical ultrasound transducers, naval sonar

transducers, high-precision actuators for adaptive optics and many other

potential applications [2–8]. Currently, crystal growers are focusing on PMN–

PT single crystal because it can be grown by solidification from a melt using

the Bridgman method. Although growth of PMN–PT by the Bridgman method

has already achieved larger crystals and resulted in cost reductions over flux

growth, refinement of Bridgman is ongoing, including increased crystal

diameter, increased longitudinal and radial uniformity, reduction of defects

and <001>-oriented growth. Further cost reductions and uniformity

improvements are also expected by applying zone melting growth to level

the composition along the crystallization direction in this solid-state solution

system.

In this chapter, the progress at TRS on the crystal growth of relaxor

piezoelectric single crystals is presented with the emphasis on Bridgman and

zone-melting growth of PMN–PT. Meanwhile the dynamic performance under

different frequency and electric bias conditions is discussed to provide useful

information for device design, in which the low ferroelectric–ferroelectric

phase transition temperature and low coercive field of single crystals are the

main issues. The broad range of actuator and transducer applications developed

or under development at TRS are reviewed, showing that much higher

performance over the conventional piezoelectric ceramics have been

demonstrated. Future trends in crystal growth and applications are discussed

briefly as well.

Recent developments and applications of

piezoelectric crystals

W S HACKENBERGER, J LUO, X JIANG,

K A SNOOK and P W REHRIG, TRS Technologies USA

and S ZHANG and T R SHROUT,

Pennsylvania State University, USA

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Handbook of dielectric, piezoelectric and ferroelectric materials74

3.2 Crystal growth and characterization of relaxor

piezoelectrics

3.2.1 Flux growth of PMN–PT and PZN–PT

PMN–PT and PZN–PT crystals were initially grown by the flux method,

which demonstrated ultra-high piezoelectric coefficients and strain levels

with low hysteresis, leading to a new era of relaxor ferroelectric materials

studies [2, 9]. In this process, oxide raw materials including high-purity

, MgCO

(or ZnO), Nb

and TiO

were weighed according to the

desired molar ratio with excess Pb

as a flux. The mixed powders were

usually loaded into a platinum crucible, which was sealed to minimize PbO

volatilization from the solution. The crystallization process was driven by

slow cooling (1–5°C/h) after the solution was held at soak temperature

(1100–1200 °C) for several hours in the furnace. Although high-quality PMN–

PT and PZN–PT crystals have been grown from flux for quite a long time,

limitations such as small crystal size, slow crystal growth rate and complicated

post-growth processing make flux growth unattractive for large-scale

commercial production of these crystals. The exploration of effective crystal

growth technologies for relaxor piezoelectric single crystals has been pursued

extensively. This section reviews the recent progress at TRS on Bridgman

and zone-melting growth of relaxor piezoelectric crystals.

3.2.2 Recent progress in Bridgman growth of

PMN–PT crystal

It has been shown that PMN–PT has a more stable perovskite phase and

exhibits less of a tendency to form the parasitic pyrochlore phase compared

with PZN–PT [10, 11]. Pre-synthesizing PMN–PT and using sealed Pt crucibles

to prevent volatilization of PbO above 1250°C, PMN–PT single crystals

have been grown directly from the melt using the Bridgman method [12, 13].

Compared with flux growth, the Bridgman technique can grow large single

crystal boules along specific crystallographic orientations with much higher

efficiency, which can potentially reduce the cost of PMN–PT crystal to be

competitive with high-quality piezoelectric ceramics used in high-end

applications.

Through several years’ effort on raw material synthesis, crystal growth

orientation control and crystal diameter enlargement, significant progress

has been made in Bridgman growth of PMN–PT. High-quality and large

PMN–PT crystals without macro-defects have been successfully grown along

different crystallographic orientations, including <111>, <110> and <001>.

Since PMN–PT crystal exhibits the highest piezoelectric response along the

<001> orientation [2], successful and repeatable <001>-oriented growth

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Recent developments and applications of piezoelectric crystals 75

produces highly uniform (001) wafers for the first time, regardless of the

axial composition segregation, and greatly facilitates crystal element fabrication.

By scaling-up crystal size, the cost of PMN–PT crystals has been reduced by

several times in recent years.

Owing to the large difference in melting point and density between each

oxide, especially between PbO (T

= 886°C, ρ = 9.53g/cm

) and MgO (T

= 2852°C, ρ = 3.58g/cm

), it is usually difficult to get a homogeneous

perovskite phase by directly mixing the oxides without the presence of a

minor amount of the pyrochlore phase [10]. Pre-synthesized PMN–PT using

a modified columbite precursor method has been proved to be the most

efficient way to eliminate the pyrochlore phase [4].

Crystal growth is usually performed in either straight or tapered Pt crucibles.

After the oriented PMN–PT crystal seed and synthesized PMN–PT powder

or ceramic pellets are charged into the Pt crucible, it is sealed with a Pt lid

to prevent the PbO evaporation at high temperature. Resistance-heated multi-

zone furnaces with soaking temperature higher than 1380°C and the axial

temperature gradient larger than 20°C/cm are used for the growth. Several

thermocouples are usually placed touching the Pt crucible to monitor the

melt temperature during the growth. Unidirectional solidification is

accomplished by slowly lowering the Pt crucible through the temperature

gradient with a translation speed less than 2mm/h.

Large PMN–PT crystals roughly along the <111> orientation can usually

be grown by the self-seeded Bridgman process (growth without the presence

of a single crystal seed), since <111> is the fastest growth direction. By

developing seeded Bridgman growth and applying different growth speeds

with precise temperature profile control, high quality PMN–PT crystals along

all the three major crystallographic orientations, <111>, <110> and <001>,

have been successfully grown with no major obstacles. Figure 3.1 shows the

crystal boules grown along <001>, which are usually 40–50mm in diameter

and about 100mm in length with light green color. Also shown are macro-

defect-free (001) wafers cut from these boules.

To evaluate the composition distributions in Bridgman grown crystals,

long bar-shape samples were cut from each boule and the main elements, Pb,

Mg, Nb and Ti in the PMN–PT system were profiled quantitatively along the

boules by electron probe micro-analysis (EPMA). Except for Pb, all other

elements exhibited compositional segregation during growth, with effective

segregation coefficients of Nb and Mg larger than 1 while that of Ti is

smaller than 1. Figure 3.2 compares the composition profile (Ti concentration)

in <111>- and <001>-oriented crystal boules grown under similar conditions

and shows no significant difference in composition gradient between growth

directions. The composition profiles of Ti were simulated by a well-established

equation used for describing the compositional segregation behavior in solid

solution systems during the normal solidification process [15]:

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Handbook of dielectric, piezoelectric and ferroelectric materials76

3.1

Photographs of crystal boules grown along <001> (a) and macro-

defect-free (001) wafers (b) cut from them.

(a)

(b)

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Recent developments and applications of piezoelectric crystals 77

ckc g

–1

= (1 – )

3.1

where k

is effective segregation coefficient, C

and C

are the solute

concentration in the crystal and the starting melt, and g is the solidified

fraction. By simulation, the estimated effective segregation coefficients of Ti

in these two runs were both around 0.83.

By observing the double side polished slices cut along the PMN–PT

crystal growth direction, usually three different regions can be identified in

each boule by the change of transparency and spontaneous domain structure.

In the lower part of the boule, where crystallization occurred first, the crystal

is semi-transparent and uniform with no visible ferroelectric domains. In the

middle part, the crystal is very cloudy due to light scattering by large and

irregular-shaped visible domains. In the upper part, the crystal becomes

suddenly quite transparent with only 90° and 180° domains observed. Long

bars have been cut along the growth direction from as-grown boules and

then sliced into small squares or rectangular plates with the large pair of

faces in (001) and with the aspect ratio between width (length) and thickness

larger than 10:1. These plates were poled (10 kV/cm at room temperature)

and the piezoelectric coefficient (d

) was measured individually by direct

observation of strain as a function of electric field using a modified Sawyer–

Tower circuit and linear variable differential transducer (LVDT) driven by a

lock-in amplifier (Stanford SR830). For the longitudinal variation of d

along the whole boule usually two peaks show up around the two boundaries

between the three parts of the boule. Figure 3.3 compares the piezoelectric

property (d

) variation

with the composition distribution measured by EPMA,

which shows the region between these two peaks roughly corresponds to a

SB-BIII-01 (111)-seeded

SB-BII-12 (001)-seeded

Simulation,

= 0.83

0 0.2 0.4 0.6 0.8 1

Fraction of solidified melt

0.50

0.45

0.40

0.35

0.30

0.25

0.20

Ti concentration

3.2

Composition profiles of Ti in <111>- and <001>-oriented crystal

boules.

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Handbook of dielectric, piezoelectric and ferroelectric materials78

PT (PbTiO

) concentration of 31–35% (15–20% of the total boule length),

and is basically consistent with the broad MPB region between rhombohedral

and tetragonal phases proposed by Guo et al. [16,17]. Studies show that the

mixed phases, including metastable ferroelectric monoclinic (FE

) and

ferroelectric orthorhombic (FE

) phases, are possibly present in this region

[16–18].

(001) wafers sliced from <111>-, <110>- and <001>-oriented PMN–PT

crystal boules have been cut into 5 × 5 × 0.5mm

plates so that the variation

of piezoelectric and dielectric properties within the wafers cut from differently

oriented boules could be evaluated (see Fig. 3.4). Piezoelectric coefficient,

, was measured by the same strain vs. electric field measurement as

mentioned above. Meanwhile, dielectric constant, K, was measured by a

Hewlett Packard 4194 impedance analyzer. For the wafers cut from <111>-

and <110>-oriented boules, two rectangular areas, which have all of their

edges along [001], were selected to evaluate property uniformity. Area A and

B are the same size (25 × 15mm

); however, A is closer to the morphotropic

phase boundary (MBP) than B, since the slice was cut inclined to the crystal

growth direction. As shown in Table 3.1, in both <111>- and <110>-oriented

growth, area B has higher property uniformity than area A in both d

and K.

Comparison of the A (or B) areas between the <111>- and <110>-oriented

growth shows that the latter obviously has better uniformity. Very high-

property uniformity has been achieved in (001) wafers of <001>-oriented

Simulated Ti distribution

Measured Ti distribution

SB-BII-05

(0.75, 0.31)

(0.89, 0.35)

3500

3000

2500

2000

1500

1000

500

(pC/N)

0.0 0.2 0.4 0.6 0.8 1.0

Normalized position from the cone

0.60

0.55

0.50

0.45

0.40

0.35

0.30

0.25

0.20

Ti concentration

3.3

Comparison of piezoelectric coefficient variation with Ti

concentration distribution along the same PMN–PT boule.

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Recent developments and applications of piezoelectric crystals 79

growth (cut perpendicular to the growth direction). The coefficient variation

in a whole (001) wafer was no more than ±2% for both piezoelectric coefficient

and dielectric constant, which shows significant advantage of the <001>-

oriented growth.

In summary, large high-quality PMN–PT crystals along the <111>-, <110>-

and <001>-orientations have been successfully grown by the seeded Bridgman

method. A broad MPB region covering 15–20% of the whole length of the

boule was identified with PT concentration from 31% to 35%, by

characterization of longitudinal variation of both piezoelectric properties

Composition gradient

(001) flat

(001) wafer with an (001) flat inclined to the axis

Growth orientation: (111)

(a)

Composition gradient

(001) flat

(001) wafer with an (001) flat on the side

Growth orientation: (110)

(b)

3.4

The cutting of (001) wafer and

mode plates from the boules

with different orientations.

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