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

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Handbook of dielectric, piezoelectric and ferroelectric materials290

the ultra-high d

of 1100 pC/N,

and the domain size in this PZN crystal

was found to be around 1 µm.

They also reported that for [111]

poled PZN

single-domain crystal had a d

of just 83 pC/N,

similar to that of BaTiO

Therefore, it can be expected that when a domain size of around 1 µm is

induced in the [111]

poled BaTiO

crystals, high-performance lead-free

piezoelectrics with ultra-high piezoelectric constants over 1000 pC/N can be

created.

10.7 New challenge of domain wall engineering

using patterning electrode

In the previous section, it was clarified that in BaTiO

crystals with engineered

domain configuration the piezoelectric constants significantly increased with

increasing domain wall densities, i.e. decreasing domain sizes.

12,13

In the

poling treatments, the whole plane electrode was always used, and the minimum

domain size was limited to greater than 5 µm. Therefore, it is important to

establish a new poling method to induce much smaller domain sizes (below

5 µm) in the BaTiO

crystals. Urenski et al. reported that for the KTiOPO

single crystals, the periodic domain structure was successfully induced using

patterned electrodes.

The objective in this section is to prepare the 31

resonators of BaTiO

crystals with a high piezoelectric constant (d

) by

introducing finer engineered domain configurations. For this purpose, a new

poling method using a patterning electrode was proposed to induce the

engineered domain configuration with smaller domain sizes below 5 µm,

and the piezoelectric properties were investigated as a function of domain

size.

The patterning electrode with a gold strip line of 3 µm width per 6 µm

spacing parallel to the [1–10]

direction was prepared on the top surface

using a photolithography technique while the whole electrode was prepared

on the bottom surface. First, on the top surface of the resonator, a photoresist

(Kayaku Microchem, SU-8 3000) layer with 2 µm in thickness was coated.

Then, mask alignment, UV radiation and development were performed. After

this process, gold electrodes were sputtered on both surfaces with an area of

1.2 × 4.0 mm

. The fine engineered domain configuration was induced by

using the patterning electrodes at various electric fields (0–10 kV/cm) and

temperatures (20–140 °C). Figure 10.25 shows the desirable engineered domain

configuration for the tetragonal BaTiO

single crystals. This domain

configuration is composed of just two kinds of polarization along the [010]

and [100]

directions, as shown in Fig. 10.26. Moreover, a previous study

revealed that in the engineered domain configuration, if the engineered domain

configuration with average domain size of 3 µm was induced in BaTiO

crystals, the d

is expected to be –337.7 pC/N using equation (10.4). In

general, it is known that the d

value can be expressed as |2∗d

|. Thus, for

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Domain wall engineering in piezoelectric crystals 291

a d

of –337.7 pC/N, the d

is expected to be 675.4 pC/N. Thus, a poling

treatment was performed to induce the engineered domain configuration

with a domain size of 3 µm.

Figure 10.27 shows the temperature–electric field phase diagram used for

the new poling method using a patterning electrode. The phase transition

temperature from tetragonal to cubic was 132.2 °C for BaTiO

crystals. Thus,

first, the temperature increased to 140.0 °C without an E-field, and the

appearance of the optical isotropic state was confirmed. Then, the temperature

decreased slowly to 134.0 °C, and an E-field was applied along the [111]

direction at 134.0 °C. The patterning electrode prepared in this study was

composed of (1) a photoresist strip line of 3 µm width per 6 µm spacing and

(2) a gold electrode deposited on both the patterned photoresist and the

crystal surface. Thus, it should be noted that in the patterning electrode,

photoresist still remained between crystal surface and gold electrode. This is

because this photoresist is stable up to 250 °C. However, when an electric

field over 6 kV/cm was applied at 134.0 °C, photoresist was burned out with

[112]

[110]

[111]

10.25

Schematic of desirable finer domain configuration.

[112]

[110]

[111]

[010]

[100]

35°

[010]

[100]

(b) Neutral (110) domain wall(a) Charged (110) domain wall

10.26

Domain configuration with (a) 90° charged and (b) 90° neutral

domain walls.

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Handbook of dielectric, piezoelectric and ferroelectric materials292

a large leakage current over 300 µA. Thus, in this study, the electric field

applied at 134.0 °C was limited below 6 kV/cm.

To induce the engineered domain configuration in BaTiO

crystals, the E-

field induced phase transition from cubic to tetragonal phases above T

132.2 °C was required. Thus, the following poling method was applied. At

134.0 °C, an electric field was slowly applied to 2 kV/cm, and then rapidly

increased to 5.6 kV/cm. Without soaking at 5.6 kV/cm, the temperature

decreased to 50 °C at a cooling rate of –10 °C/min under the electric field of

5.6 kV/cm. Figure 10.28 shows the optical microscope photographs of

patterning electrode with a pattern width of 3 µm and the consequently induced

engineered domain configuration. In Fig. 10.28(b), the average domain size

was estimated to be around 3 µm. When the whole plane electrode was used

to induce the finer engineered domain configuration, the minimum domain

size was always limited to be above 5 µm. Therefore, using the pattering

electrode with a gold strip width of 3 µm, the engineered domain configuration

with a domain size of 3 µm was successfully induced in BaTiO

crystals.

This reveals that in the poling treatment of BaTiO

crystals, the patterning

electrode is a very useful technique.

Finally, for this 31 resonator with 3 µm domain width, the piezoelectric

properties were measured using the resonance–antiresonance technique. As

mentioned before, a d

of –337.7 pC/N was expected for the 31 resonator of

the engineered domain configuration with a domain width of 3 µm.

The

measured d

was –243.2 pC/N, and if assuming d

as |2∗d

|, the d

was

estimated to be 486.4 pC/N. This measured value of d

was only 70% of the

expected value of –337.7 pC/N. Thus, the origin of this lower than expected

Temperature (°C)

130 132 134 136 138 140

Cubic

Unknown

6kV/cm

Tetragonal

E-field (kV/cm)

10.27

Phase transition diagram as functions of temperature and E-

field for [111]

oriented BaTiO

crystals.

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Domain wall engineering in piezoelectric crystals 293

value of d

was investigated. Figure 10.29 shows the domain structures near

the top and bottom electrodes of the 31 resonator. The domain size near the

top patterning electrode (high-voltage side) was estimated to be 3 µm while

that near the bottom electrode (ground side) was 8–9 µm. The gradient domain

sizes from 3 to 8–9 µm along thickness direction ([111]

) were first observed.

As mentioned previously, when the whole plane electrode was used for the

poling treatment, the minimum domain size was limited to 5 µm, but there

was no gradient domain size.

12,13

This difference might originate from different

(a) (b)

: 3 µm

10.28

Photograph of (a) the patterning electrode with a gold

electrode width of 3 µm and (b) the poled domain structure with an

average domain size of 3 µm.

100 µm

[111]

[112]

[110]

(a) (b)

10.29

Domain structure of the 31 BaTiO

resonator near (a) the top

surface (high-voltage side) and (b) the bottom surface (ground side).

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Handbook of dielectric, piezoelectric and ferroelectric materials294

E-fields. For the homogeneous domain size of 5.5 µm using the whole plane

electrode, the applied E-field was 10.1 kV/cm. This E-field was almost twice

as high as that of 5.6 kV/cm used in this study. Urenski et al. reported that for

the introduction of the expected periodic domain structure similar to the

patterning electrode, a much higher E-field than the coercive E-field (E

)

was required.

Therefore, if the high E-field over 10.1 kV/cm is applied to

the 31 resonator using the patterning electrode with 3 µm width, it is expected

that a homogeneous domain size of 3 µm can be induced in the resonator. At

present, the patterning electrode without photoresist is designed for this

purpose. In the near future, we shall be able to induce much finer homogeneous

domain sizes in the resonator to prepare lead-free piezoelectrics with a higher

(over –500 pC/N). Moreover, applying a uniaxial stress field can help

reduce the E-field using the above patterned electrodes in the poling method.

This new approach will be discussed in the next section.

10.8 New challenge of domain wall engineering

using uniaxial stress field

It is well known that the ferroelectric phase transition is affected by temperature,

E-field and stress field. Thus, application of a uniaxial stress field to the

poling treatment in addition to temperature and E-field may reduce the value

of the E-field below 10 kV/cm over T

. In this section, the phase transition

behavior of the [111]

oriented BaTiO

crystals was investigated as functions

of temperature, uniaxial stress and E-fields. Moreover, on the basis of the

results, a new poling method for the BaTiO

crystals will be proposed using

control of temperature, uniaxial stress and E-fields.

In the new poling system, temperature, E-field and uniaxial stress-field

must be controlled independently. A new poling attachment was designed as

shown in Fig. 10.30. The temperature can be changed from –190 to 600 °C

while the E-field and uniaxial stress field are applied along the [111]

directions,

independently. Since the uniaxial stress field was applied using an z-axis

stage and a precise uniaxial pressure value cannot be measured, the moving

length (µm) of the z-axis stage from a position at contact with sample without

pressure was defined as apparent uniaxial stress field through this chapter.

First, the phase transition behavior of the BaTiO

crystals by temperature

only was investigated without E-field and uniaxial stress field by domain

observation under crossed-polarizers. It was clearly observed that from a

temperature above T

, with decreasing temperature, the paraelectric phase

changed to an intermediate phase with super-paraelectric state, and finally

changed to the ferroelectric phase with randomly oriented spontaneous

polarization, as shown in Fig. 10.31.

The crystal structure of the intermediate phase with super-paraelectric

state is still unknown, but under crossed-polarizers, the crystal remained

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Domain wall engineering in piezoelectric crystals 295

birefringent while being rotated, which suggested that this phase is not the

paraelectric cubic phase. Next, above T

, the phase transition behavior of the

BaTiO

crystals induced only by E-field applied along the [111]

direction

was investigated without uniaxial stress field by domain observation under

crossed polarizers. With increasing E-field, the paraelectric phase changed

to the intermediate phase with super-paraelectric state, and finally to the

ferroelectric tetragonal phase with three oriented polar directions along [100]

-axis stage

Rotation stage

Chamber

Heater

Slide glass

Sample

Ground

Cover glass

Uniaxial stress along

[111]

direction

10.30

Schematic new poling attachment with controlled temperature,

uniaxial stress field and E-field.

Temperature

LowHigh

10.31

Phase transition behavior of the BaTiO

crystals by temperature

without E-field and uniaxial stress field.

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Handbook of dielectric, piezoelectric and ferroelectric materials296

[010]

and [001]

, as shown in Fig. 10.32. It should be noted that at the

temperature of (T

+ 1.5) °C, the electric field over 10 kV/cm is required to

induce the tetragonal phase. Finally, above T

, the phase transition behavior

of the BaTiO

crystals induced only by uniaxial stress field applied along the

[111]

direction was investigated without an E-field. With increasing uniaxial

stress field, the paraelectric phase changed to the intermediate phase with

super-paraelectric state, and finally to the ferroelectric phase with randomly

oriented spontaneous polarization, as shown in Fig. 10.33. When a poling

Uniaxial stress-field over

HighLow

10.32

Phase transition behavior of the BaTiO

crystals by uniaxial

stress field without E-field at (

+ 1.5) °C.

E-field over

HighLow

10.33

Phase transition behavior of the BaTiO

crystals by E-field

without uniaxial stress-field at (

+ 1.5) °C.

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Domain wall engineering in piezoelectric crystals 297

treatment was performed using an E-field, a higher E-field was required with

increasing temperature above T

. On the other hand, when a poling treatment

was performed using a uniaxial stress field, a smaller uniaxial stress field

was enough for obtaining as fully poled state with increasing temperature

above T

. This is because the BaTiO

crystal is ferroelectric and ferroelastic,

which suggested that uniaxial stress field is quite effective for poling treatment.

The above discussion revealed that the phase transition behavior from

cubic to tetragonal by temperature and uniaxial stress field was quite similar

because of the formation of the ferroelectric phase with randomly oriented

spontaneous polarization. On the other hand, the phase transition behavior

by E-field leads to the ferroelectric tetragonal phase with three oriented

polar directions. However, the phase transition from the cubic to the

intermediate phase with super-paraelectric state induced by temperature,

uniaxial stress field and E-field shows completely the same behavior. These

results suggested that above T

, the combination of uniaxial stress and E-

fields might be more effective for the poling of ferroelectric crystals. Thus,

the E-field for a poling treatment above T

can be reduced by the combination

of uniaxial stress-field and E-field drives.

In this poling process, (T

+ 3.5) °C was chosen as the poling temperature.

This is because at this temperature, it is impossible to pole the BaTiO

crystals using only an E-field owing to electric breakdown. Thus, two kinds

of poling treatments at (T

+ 3.5) °C were performed as follows, i.e. (a) a

lower uniaxial stress field below 10 µm and then a higher electric field above

10 kV/cm, and (b) a higher uniaxial stress field above 10 µm and then a

lower electric field below 10 kV/cm. Poling treatment (a) was performed

first, i.e. an apparent uniaxial stress field of 9 µm was applied to induce the

intermediate phase only, and after that, an electric field of 14 kV/cm was

applied to induce the ferroelectric phase with the oriented polar direction. As

a result, an almost fully poled state was achieved in the [111]

poled BaTiO

crystals, as shown in Fig. 10.34. The piezoelectric properties were measured

from Fig. 10.34. Figure 10.35 shows the domain configuration of the [111]

poled BaTiO

crystals. An average domain size in Fig. 10.35 was over 50 µm,

and two kinds of 90° domain configuration were clearly observed. Poling

treatment (b) was performed next, i.e. an apparent uniaxial stress field of

17 µm was applied to induce the coexistence of the intermediate and the

ferroelectric tetragonal phases, and after that, an electric field of 9.5 kV/cm

was applied to induce the ferroelectric phase. As a result, an almost fully

poled state was achieved for the [111]

poled BaTiO

crystals as shown in

Fig. 10.36. The piezoelectric properties were measured from Fig. 10.36. The

domain configuration by the poling treatment (b) was completely the same

as shown in Fig. 10.35. On the basis of the two impedance curves, the

piezoelectric constants for the [111]

poled BaTiO

crystals were determined

as shown in Table 10.3, when the d

value was measured using a d

meter.

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Handbook of dielectric, piezoelectric and ferroelectric materials298

The d

along the [111]

direction was calculated to be 224 pC/N for BaTiO

single-domain crystal.

In this study, the d

was measured to be 230 pC/N,

which is consistent with the previously calculated value. This revealed that

the uniaxial stress field is very effective for the poling treatment above T

reduce the E-field below 10 kV/cm. This poling treatment can be universal

for all ferroelectric materials including PZT ceramics.

10.9 What is domain wall engineering?

To enhance piezoelectric property, we must consider two contributions, i.e.

intrinsic and extrinsic effects. The intrinsic effect is dependent on the unit

Frequency,

(kHz)

720700680660640620

–90

Phase, θ (deg.)

Impedance, |

| (Ω)

1000

100

10.34

Frequency dependence of the impedance and the phase for the

[111]

oriented BaTiO

single crystals poled using the poling

treatment (a).

500 µm

[111]

[11–2]

[–110]

10.35

Domain configuration after the poling treatment of (a).

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Domain wall engineering in piezoelectric crystals 299

cell with symmetry and chemical composition. In the earlier sections, we

discussed the optimum crystal structure and crystallographic direction for

the best engineered domain configurations. This discussion should be related

to the intrinsic contribution to piezoelectric property. On the other hand,

in the later sections, it was revealed that 90° domain wall regions with

distorted and structure graduation had ultra-high piezoelectric constants above

80 000 pC/N. This contribution is an extrinsic effect, and this technique is

called domain wall engineering. In this technique, we must prepare a domain

wall region fixed in the crystals, and an engineered domain configuration is

a technique to fix domain wall in the crystals. As a result, we can obtain a

composite of domain wall regions and normal domain region.

Thus, to obtain highly piezoelectric crystals, a combination of intrinsic

and extrinsic effects is required. To maximize the intrinsic contribution, we

should use the [001]

oriented orthorhombic crystals, and if this crystal has

phase transition from orthorhombic to tetragonal at around –10 to 0 °C, we

can expect enhancement of piezoelectric properties. In addition, to maximize

the extrinsic contribution, we should induce a domain size of around 150 nm

in the [001]

oriented orthorhombic crystals. Arlt et al. reported that in

10.36

Frequency dependence of the impedance and the phase for the

[111]

oriented BaTiO

single crystals poled using the poling

treatment (b).

Frequency,

(kHz)

640620 660 680 700 720

Phase, θ (deg.)

–90

Impedance, |

| (Ω)

100

1000

Table 10.3

Piezoelectric properties for the [111]

poled BaTiO

crystal using the

poling treatment of (a) and (b)

Poling

(pN/m

)

(pC/N)

(%)

(pC/N)

treatment

(a) 2114 8.7 –136 34 230

(b) 1983 8.6 –144 37 230

: measured by

meter.

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