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

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Handbook of dielectric, piezoelectric and ferroelectric materials90

for cryogenic wavefront correction due to the excellent cryogenic properties

of single crystal actuators [4, 38].

3.4.2 Single crystal actuators for rotorcraft flap control

Single crystal piezoelectric actuators for rotorcraft flap control have drawn

attention because of the fact that single crystal piezoelectrics have high

strain energy density [5, 34]. As an example, comparison between single

crystal stack actuators and commercially available PZT stacks (PI 151) has

been investigated for full scale flap control. Figure 3.13 shows the free angle

displacement vs. blocking force for PI 151 and PMN–PT single crystal stack

actuators. The required angle displacement-blocking force line for a rotor in

hover at 400 RPM actuating a 915mm (36in) flap is given in Fig. 3.13 as

reference. A flap displacement increase of 16% is reached using PMN–PT

single crystal actuators compared with those obtained for the 2X–frame

actuator using a commercially available ceramic stacked actuator. PI 151

400 RPM 36′ Flap

PI 151 (475 DC bias ± 725 V)

Single crystal (475 DC bias ± 725 V)

36″ Flap, 400 RPM

Single Crystal

PI151

0 20 40 60 80 100 120 140 160

Block moment (lbs-in.)

Angular displacement (deg.)

Stack

actuator

Trailing

edge flap

3.13

Free angular displacement/block force diagram for single crystal

and PI 151 ceramic stack actuators for trailing edge flap control. The

relationship between displacement and force for a rotor in hover at

400 RPM actuating a 36 in. flap is given as reference.

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Recent developments and applications of piezoelectric crystals 91

stack actuators would provide approximately 5° flap deflection output, while

single crystal stack actuators would provide 5.8°. Therefore, when driving

both actuators at their maximum capability, the single crystal stack actuator

will provide flap deflections 0.8° higher than PI 151. This would allow the

actuator system to withstand higher aerodynamic force conditions, or to

maintain those with a simpler amplification mechanism. More comparison

between single crystal and PZT actuators with various configurations including

tube actuators, shear actuators and benders for flap control can be found in

ref. [5].

3.4.3 Single crystal piezoelectric actuators for cryogenic

actuation

Precise cryogenic actuation is required in many NASA space applications

[35]. Some recent research efforts have reported on magnetostrictive cryogenic

actuation with superconducting coils by Energen, ferroelectric cryogenic rod

(or stack) actuators using ceramics by Xinetics, and single crystal stack

actuators and single crystal in-plane cryogenic actuators by TRS. Among

them, the superconducting technology does not yet allow mature

magnetostrictive cryogenic actuation, and the fabrication process, cost and

volume of actuators are not comparable with its piezoelectric counterpart.

Piezoelectric actuation is well known for its high-precision displacement

control, high force output, quick response, low power consumption and low

profile. However, most piezoelectric materials lose 75% of their piezoelectric

performance at temperatures less than 77 K, giving rise to extremely small

strain and actuation at cryogenic temperatures [35]. Fortunately, the recently

invented single crystal piezoelectrics exhibit significantly higher piezoelectric

performance at both room temperature and cryogenic temperatures [4], e.g.

of single crystal piezoelectrics (PMN–PT or PZN–PT) at 30K is about

equal to the d

of PZT-5A at room temperature, indicating promising cryogenic

actuation using single crystal piezoelectrics.

PMN–PT stack actuators, flextensional actuators and a piezomotor were

developed for cryogenic actuation. Figure 3.14 shows d

vs. temperature for

a 5 × 5 × 4 mm

single crystal stack actuator. It is noticed that the effective

at 30K is about 400pC/N. The single crystal flextensional actuator (Fig.

3.15) retained >60% of its room temperature stroke at 75K. For the cryogenic

linear motor test, we placed the PMN–PT single crystal piezoelectric motor

into a chamber full of liquid nitrogen. A thermo-couple was used to monitor

the chamber’s temperature. Experimental measurements showed that at

–200°C, the motor still worked properly with a slightly decreased speed [5].

All these results suggest that single crystal piezoelectric actuators hold

importance for cryogenic actuation applications.

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Handbook of dielectric, piezoelectric and ferroelectric materials92

3.5 Single crystal piezoelectric transducers

3.5.1 Medical transducers

Ultrasound transducers first began their use in medical investigation in the

first half of the twentieth century, and their reach has since broadened

extensively past the original gastrointestinal, obstetric and cardiac imaging

performed in the 1950s. The quality of imaging has greatly improved, providing

quantitative measurements of the developing fetus, as well as investigation

of blood flow and tumor detection. Data processing techniques have given

rise to Doppler and power Doppler imaging, harmonic imaging and

elastography. These, in combination with increased insonifying frequencies,

have opened new applications in ophthalmology, intravascular imaging,

dermatology and orthopedics.

Nearly all medical imaging transducers operate using a plate design, where

array elements are patterned from a piezoelectric plate that is located between

2500

2000

1500

1000

500

(pC/N)

0 50 100 150 200 250 300 350

Temperature (K)

3.14

Cryogenic

of a PMN–PT stack actuator.

3.15

Cryogenic displacement of a flextensional actuator.

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Recent developments and applications of piezoelectric crystals 93

acoustic matching layers and a backing layer. The piezoelectric material

provides the conversion between the electrical and acoustic domains. The

bulk of commercial systems utilize the ceramic PZT, which offers relatively

high width extensional (

′

) electromechanical coupling for array transducers.

Many of these piezoelectrics, however, are not ideal for pulse-echo operation

because they exhibit a low ‘figure of merit’ g

, due to the low charge

coefficient d

(= d

+ 2d

) and the high permittivity of the piezoelectric

material. Composites provide a partial solution to this problem. In particular,

composites utilizing 1–3 connectivity, as shown in Fig. 3.16, provide the

highest coupling (longitudinal length extensional, k

) and therefore have

the greatest potential for high bandwidth [6, 8]. An increase in the hydrostatic

parameters is observed, as shown in Table 3.2, as well as a decrease in

mechanical Q which allows for better detection of sharp pulses.

Single crystals offer further advantages for medical imaging, through

their increased anisotropy. The electromechanical coupling of PZN–PT and

PMN–PT single crystals is as high as 94% in the <001>, which can be

utilized in the composite structure. The resulting wide bandwidth, over 130%

[6], can be used for a variety of applications including sub-harmonic or

second harmonic imaging, Doppler and coded excitation for increased

penetration.

Piezoelectric

Polymer

filler

Table 3.2

Comparison of hydrostatic coefficients for pulse-echo operation of PZT

and PMN-PT materials

Piezoelectric Relative

permittivity (V m/N) (pC/N) (nm

/N)

PZT ceramic 1800 0.0025 40 100

PZT 1–3 composite 54 0.056 27 1536

PMN–PT single crystal 4436 0.002 80 160

PMN–PT composite 340 0.037 111 4115

3.16

Piezoelectric–polymer composite exhibiting 1–3 connectivity.

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Handbook of dielectric, piezoelectric and ferroelectric materials94

The increase in d

also outweighs the increase in permittivity, providing

a figure of merit more than three times that of PZT composite [39, 40]. This

directly improves the signal to noise ratio, providing either deeper penetration

or lower excitation amplitudes. This is particularly important in emerging

high-frequency ultrasound imaging applications in areas such as ophthalmology,

dermatology, intravascular ultrasound, small animal imaging and laparoscopic

procedures. Attenuation in tissues is highly dependent on frequency, and is

currently one of the primary limitations in imaging systems above 5–10

MHz.

Imaging resolution is proportional to pulse length, which is dependent

upon both frequency and bandwidth. However, higher frequency imaging is

limited in depth by attenuation, which is also proportional to frequency.

Harmonic imaging provides a solution by transmitting at one frequency and

receiving at a harmonic of that frequency, resulting in a high frequency

signal that only travels half the distance of a traditional pulse-echo signal.

Bandwidth is important in achieving adequate signal-to-noise ratio (SNR)

for both the transmitted and received signals, as illustrated in Fig. 3.17.

A 4.9MHz transducer utilizing single crystal was made to minimize this

signal loss. A single element transducer with an aperture of 18mm and 56%

volume fraction was manufactured. Utilization of two matching layers and

heavy backing helped to improve bandwidth to 132%, as shown in Fig. 3.18.

Measurement of a harmonic signal was performed using an excitation signal

consisting of a 64-cycle, sinusoidal tone burst at 4.5MHz. Sonazoid

microbubbles in a water bath were excited, and acted as the fundamental and

harmonic echo source. Acquisition was performed at 25MHz, and the signals

were averaged. From the spectrum shown in Fig. 3.19, it can be seen that the

second harmonic (at 9MHz) was only 17.5dB down from the fundamental

signal.

Signal loss

Spectral magnitude

Signal loss

3.17

Comparison of transducer bandwidth (dotted line) effect on

power available at fundamental and harmonic frequencies (solid

lines). Wider bandwidth provides less loss in signal generation and

reception while also allowing for wider bandwidth in each frequency

range.

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Recent developments and applications of piezoelectric crystals 95

3.5.2 Sonar transducers

Sonar applications require significantly more demanding acoustic performance

characteristics and environmental considerations. Sonar systems have ranges

from meters to kilometers, thus even at low-frequency operation of 2–500

kHz, high acoustic source levels are often used. Additionally, the variations

in temperature, hydrostatic pressure and the chemistry of seawater complicate

construction and performance standardization.

Tonpilz transducers (pistons with head and tail masses) are standard in

sonar applications operating near 100kHz or below. Figure 3.20 shows the

general design of a tonpilz element, of which a series are generally used to

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Frequency (MHz)

–57

–60

–65

–70

–75

–80

–82

Amplitude (dB)

3.18

Frequency spectrum from harmonic imaging transducer, based

on impulse response.

Fundamental

Second

harmonic

024681012

Frequency (MHz)

–45

–50

–55

–60

–65

–70

–75

–80

Amplitude (dB)

3.19

Spectrum showing fundamental and harmonic echoes from

Sonazoid obtained using single crystal transducer excited at 4.5MHz.

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Handbook of dielectric, piezoelectric and ferroelectric materials96

form an array. The stress bolt along the length is applied to prevent tension

in the piezoelectric. The motor section consists of piezoelectric rings connected

mechanically in series and electrically in parallel. An approximation to the

resonance frequency of such a system is given as [41]:

⋅

3.3

where E

is the Young’s modulus of the piezoelectric material, A is area of

the stack, l is length, and M

and M

are the masses of the head and tail,

respectively. It should be noted that the effective Young’s modulus of the

stack, including stress bolt and joints, provides a more exact approximation.

Exploration in both the military and industrial sectors is shifting towards

small, unmanned systems utilizing a wider array of sonar detectors and

sources. As detection technology becomes more sophisticated, there has also

been an increase in the complexity of towed sonar arrays. Both of these

applications have seen an increased demand for both larger bandwidth devices,

as well as smaller packages that maintain current drive capabilities.

For these reasons, there has been a move towards single crystal ferroelectrics.

In particular, the effective Young’s modulus is approximately one-quarter

that of traditional Navy Type III ceramics, resulting in a comparable decrease

in stack length when maintaining the frequency, as can be seen in Eq. 3.3.

The length of the piezoelectric stack is also generally larger than the width,

which allows the overall stack to operate in a near-33 mode. The high effective

of 90% results in a high bandwidth comparable to current piezoelectric-

magnetostrictive hybrid transducers. A high source level is achievable with

single crystal due to its high d

of 2200 pC/N. The large strain results in

high acoustic power relative to ceramic materials, and the low dielectric

losses (below 0.01) provide efficient transfer of energy between the transducer

and system.

3.20

Cross-section of typical tonpilz transducer.

Tail mass

Motor section

Head mass

Insulators

Stress bolt

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Recent developments and applications of piezoelectric crystals 97

Single crystal PZN–PT and PMN–PT are ‘soft’ materials and can depole

under high fields due to their low coercive fields (less than 0.4MV/m). The

majority of high-field ac excitation is done under bias to eliminate depoling

effects. Under bias, these materials offer significantly higher energy densities

(= )

rms

kEε

than Type III ceramics. PMN–PT shows an energy density of

8.7 kJ/m

compared with 0.6 kJ/m

for the ceramic [7]. Given higher energy

densities, much lower power is necessary for operation. Untethered vehicles

such as unmanned underwater vehicles (UUVs) for underwater oil or mineral

mining, cable inspection, oceanographic research or defense can benefit

from longer deployment schedules and greater coverage. Other sonar

applications include sonobuoys, towed arrays and hull-mounted sonar.

3.6 Conclusions and future trends

By seeded Bridgman growth, TRS has successfully grown large, high quality

PMN–PT crystals along <111>-, <110>- and <001>-orientations. Lateral

uniformity analysis indicates that the property variation in Bridgman-grown

crystals is strongly related not only to the growth direction, but also to the

sample cutting direction. Cutting perpendicular to the composition gradient,

which is always along the crystal growth direction, will give much less

property variation. Since (001) wafers cut from <001>-oriented boules are

perpendicular to the composition gradient, less than ±2% variation of both

the piezoelectric and dielectric properties has been achieved in the whole

wafer. However, for PMN–PT solid solution system, the composition

segregation inevitably exists in Bridgman growth, resulting in an

inhomogeneous composition distribution and the variations of dielectric and

piezoelectric properties along the growth orientation. It was demonstrated

that zone-melting growth is an effective technique to level the composition

variation and improve the property uniformity along the growth direction.

Currently, the Bridgman method is becoming a mature technique to produce

large PMN–PT crystals. In the future, the crystal growth techniques, such as

the modified zone-melting or the continuous feeding Bridgman growth [42],

which potentially reduce or even eliminate the composition segregation in

the as-grown crystals, should be further developed. Adapting these techniques

to large PMN–PT crystal growth can further improve the wafer-to-wafer

uniformity and reduce the cost of crystal production.

Device design based on PMN–PT piezoelectric crystal often involve high

electric field, high frequency, high temperature or any combination of these.

DC bias also must often be applied to keep the crystal from depoling. It was

found that, for PMN–33%PT crystals, frequency dispersion of dielectric

permittivity occurs when the temperature is below 86°C and over 151°C,

while the dielectric permittivity is frequency independent in the temperature

range of 86–151°C. A transition between the normal ferroelectric and relaxor

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Handbook of dielectric, piezoelectric and ferroelectric materials98

states is indicated by the appearance of a frequency dispersive shoulder

slightly below the T

and an abrupt disappearance of the frequency dispersion

at lower temperature range. No evident frequency-dependent dielectric behavior

was found for tetragonal PMNT39. It is evident that the T

of PMN–PT

crystals increases with external DC bias field, while the ferroelectric phase

transition, T

, shifts downward, limiting the temperature usage range of the

crystals.

Based on the full study of piezoelectric and dielectric properties, TRS has

demonstrated the merit of PMN–PT crystal over conventional PZT ceramics

for several types of crystal-based actuators for adaptive optics and cryogenic

actuations. TRS has also demonstrated the merits of crystals for high sensitivity

and broadband sonar and medical transducer applications. It is also realized

that the main obstacles for the broad range of applications of PMN–PT

crystals are its low T

, T

and E

. The new generation electric devices demand

the development of high-performance crystals for expanded operating domains,

such as high temperature, high drive field and/or high drive stress. Composition

modification of PMN–PT crystal by A-site or B-site substitution can be a

feasible approach for industry to enhance the performance and expand the

operating domains of PMN–PT, since the melt growth techniques developed

for pure PMN–PT crystal growth can be adapted to these materials with

further optimization. New binary or ternary systems, which have stable

perovskite phase so that the crystals can be directly grown from their melt,

should also been explored for high-temperature applications.

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Recent developments and applications of piezoelectric crystals 99

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