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

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Handbook of dielectric, piezoelectric and ferroelectric materials870

co-precipitation method also tend to have a microstructure with plate-like

anisotropic grains preferentially oriented parallel to the (00

10) plane. This

can clearly be seen on the X-ray diffraction (XRD) patterns in Fig. 28.13 in

which a considerable increase in the intensity of the (00

10) peak is observed

for the ceramic synthesized by the co-precipitation method. The preferential

grain orientation occurs when the sintering temperature and time are increased

to 1200°C and 8h, and it is a possible reason for a lower measured density.

The effects of the two different soft chemical methods on the grain size and

orientation of ceramics sintered at different temperatures for different times

are depicted by the SEM pictures in Fig. 28.14.

A comparison of the dielectric properties of SBT samples prepared by

different methods is given in Table 28.2. It shows the advantage of both

these soft chemical processes in improving the room temperature dielectric

constant as well as the dielectric maximum at the Curie temperature, T

when compared with solid-state synthesis. The soft chemical routes are therefore

the answer to the many shortcomings of the solid-state reaction method for

the preparation of SBT and related materials.

Typical ferroelectric behaviour is revealed in the SBT ceramics prepared

by the sol–gel method and sintered at 1000°C for 8h (SG1000(8)), 1200°C

for 4h (SG1200(4)) and 1200 °C for 8h (SG1200(8)), by the dielectric hysteresis

loops displayed in Fig. 28.15(a)–(c), respectively. The values of the remnant

polarization and coercive field are extracted and shown in Table 28.3. They

are compared with the values reported in the literature for ceramics prepared

from solid-state reaction processes, and for single crystals. An increase in

the remnant polarization (P

) and a decrease in the coercive field (E

) are

achieved with increasing sintering temperature and time. P

increases from

~3.40µC/cm

in SG1000(8) to 6.0µC/cm

in SG1200(4), and to 7.6 µC/cm

in SG1200(8). SG1200(4) and SG1200(8) show an E

of 23 and 22 kV/cm,

respectively, while SG1000(8) has the highest coercive field E

= 34.5 kV/

cm, and its polarization is only saturated at a field as high as ~100 kV/cm.

Figures 28.15(d)–(e) illustrate the ferroelectric hysteresis loops of the ceramics

synthesized by the co-precipitation method and sintered at 1200 °C for 4h

(PR1200(4)) and 1200°C for 8h (PR1200(8)), respectively. PR1200(4) and

PR1200(8) yield almost the same values of the P

and E

: 3.5 and 3.7 µC/

and 19.5 and 21 kV/cm, respectively. However, the P

values for these

ceramics obtained from the co-precipitation method are only half of those

observed in ceramics prepared by the sol–gel process and sintered under the

same conditions.

The lower P

values observed in the ceramics prepared by the co-precipitation

method can be attributed to the preferred orientation along the [00

10] direction

of the grains, as depicted in the XRD pattern of the PR1200(8) ceramic (Fig.

28.13b). The spontaneous polarization in SBT is known to be parallel to the

a-axis because the O–Ta–O chains in the a–b plane have high polarizability

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Novel solution routes to ferroelectrics and relaxors 871

10 20 30 40 50 60

2θ (degrees)

(a)

Intensity (a.u.)

1200 °C (8 h)

1200 °C (4 h)

1000 °C (8 h)

750 °C (4 h)

500 °C (2 h)

(115)

(111)

(113)

(006)

(200)

(00

10)

(119)

(208)

(220)

(20

10)

(11

13)

(315)

10 20 30 40 50 60

2θ (degrees)

(b)

Intensity (a.u.)

1200 °C (8 h)

1200 °C (4 h)

1000 °C (8 h)

750 °C (4 h)

(115)

(111)

(113)

(006)

(200)

(119)

(208)

(220)

(20

10)

(11

13)

(315)

(00

10)

Intensity (a.u.)

10 20 30 40 50 60

2θ (degrees)

28.13

XRD patterns of SBT ceramic samples prepared by (a) the sol–

gel process and (b) the co-precipitation method, and sintered at

different temperatures for different times. The inset in (b) is the XRD

pattern of the precursor powder obtained by the co-precipitation

method (reproduced with permission from

Chem. Mater.

2006, 18(2),

532–540).

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Handbook of dielectric, piezoelectric and ferroelectric materials872

and, hence, is responsible for the polarization [29]. Therefore, the polarization

measured in ceramics containing more grains with the polarization oriented

along the a-axis parallel to the field direction is larger than in those containing

mostly grains with the c-axis along the field direction (i.e. perpendicular to

the electrode surfaces) [56,57]. Moreover, the measured densities of the

SG1200(4) and SG1200(8) ceramics are much higher than those of PR1200(4)

and PR1200(8), which is another reason for their higher P

values. The

(a)

(d)

(b)

(e)

(c)

(f)

28.14

Scanning electron microscopy images of SBT ceramics

prepared by the sol–gel process (a–c) and the co-precipitation

method (d–e) and sintered at 1000°C for 8h (a) and (d), 1200°C for

4h (b) and (e), and 1200°C for 8h (c) and (f) (reproduced with

permission from

Chem. Mater.

2006, 18(2), 532-540).

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Novel solution routes to ferroelectrics and relaxors 873

relationship between the high preferential orientation, relatively low density

and low P

value observed in the PR1200(8) ceramic is consistent with the

previous report that excessive directional grain growth in SBT and BaBi

ceramics leads to a lower density resulting from a loosely packed microstructure

[43,58].

Overall, the sol–gel process produces ceramics with much better dielectric

and ferroelectric properties because it allows more homogeneous grain

distribution by reducing c-oriented grains in the materials, thus achieving

the purpose of ‘grain-control’. This method therefore shows promising potential

for depositing SBT thin films with minimized c-axis orientation and hence,

improved remnant polarization, which is a key performance interesting for

non-volatile memory applications.

28.5 Novel sol–gel route to ferroelectric Bi

and Bi

ceramics

28.5.1 Background

As discussed in Section 28.4, SrBi

(SBT) was found to be a promising

alternative to PZT-based materials because of its excellent fatigue resistance

and very good data retention over a long period of time [28,29]. However,

the relatively low remnant polarization and the higher processing temperature

of SBT may limit its application in FRAM [59]. This has led to research on

other lead-free ferroelectric materials with better properties, and Bi

(BIT) has emerged as a possible solution to the problem. BIT also belongs to

the Aurivillius family of compounds [33] with a layered structure in which

n perovskite-like (A

n–1

3n+1

)

2–

blocks alternate with (Bi

)

layers. In

BIT, three perovskite (Bi

)

2–

units are stacked with the bismuth–oxygen

layers, and the major component of spontaneous polarization is along the a-

Table 28.2

Comparison of the dielectric properties of SBT prepared by

different methods (reproduced with permission from

Chem. Mater.

2006,

18(2) 532–540)

Method

′

(100 kHz)

′

max

(100 kHz)

(°C)

Sol–gel* 207 900 330

Co-precipitation* 219 800 330

Single crystals [14]** 135 127 320

Single crystals [38]† 105 130 355

Single crystals [38]‡ 110 1325 355

Solid state reactions [35] ~100 ~500 315

This work.

Single crystals with dominant (001) orientation and about 1–3% of (113)

orientation.

†

Perfectly

-oriented single crystals.

‡Single crystals oriented in the

-plane.

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Handbook of dielectric, piezoelectric and ferroelectric materials874

axis (only a much smaller component along the c-axis). As a lead-free

ferroelectric with a high Curie temperature (T

= 675°C) and a high remnant

polarization (P

= 50µC/cm

) along the main polar crystallographic a-axis

[60], BIT appears to be a promising candidate for applications in piezoelectric

transducers and memory devices over a wide temperature range [55,61].

However, BIT, especially in the form of thin film, is known to suffer from

serious fatigue failure after only about 10

switching cycles [62]. The high

in BIT results in a large coercive field (E

) at room temperature, making

(mC/cm

)

–2

–4

–6

–100 –50 0 50 100

(kV/cm)

(a)

(kV/cm)

–2

–4

–6

–8

–50 –40 –30 –20 –10 0 10 20 30 40

(kV/cm)

(b)

(c)

(d)

(e)

(mC/cm

)

–2

–4

–6

–8

–10

–50 –40 –30 –20 –10 0 10 20 30 40

(mC/cm

)

–50 –40 –30 –20 –10 0 10 20 30 40

(kV/cm)

–2

–4

–6

(mC/cm

)

(kV/cm)

(mC/cm

)

–50 –40 –30 –20 –10 01020304050

–2

–4

–6

28.15

Ferroelectric hysteresis loops of SBT ceramics derived from the

sol–gel process and the co-precipitation method, and sintered at

different temperatures and times: (a) SG1000(8), (b) SG1200(4), (c)

SG1200(8), (d) PR1200(4), and (e) PR1200(8) (reproduced with

permission from

Chem. Mater.

2006, 18(2), 532–540).

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Novel solution routes to ferroelectrics and relaxors 875

the poling of the material difficult. To lower E

, T

must be decreased. Wolfe

and Newnham [63] found that substituting trivalent rare earth ions for Bi

in the perovskite slabs of the Bi-layered ferroelectrics has the effect of

decreasing T

. Takenaka and Sakata [64] studied the effect of the La

substitution for Bi

in BIT, i.e. Bi

4–x

(xBLT), and found that T

decreases with the increasing amounts of La

from 675°C for x = 0 to about

–150°C for x = 1.5.

In the wake of intensive research efforts, it was found that replacing some

of the Bi

ions in the perovskite slabs by the La

ions greatly improves the

properties of BIT and also lowers the processing temperature [64]. For instance,

thin films of Bi

4–x

(x = 0.75) were prepared at a relatively low

temperature of 650°C, which exhibit good fatigue endurance even after 3 ×

switching cycles [65]. With its lower processing temperature and higher

remnant polarization compared with SBT, Bi

3.25

0.75

stands as a better

candidate for FRAM application. A number of different techniques were

reported for the synthesis of BIT and BLT ceramics and thin films. The

metal-organic chemical vapor deposition, the metal-organic deposition and

the sol–gel process are among the most commonly used methods for preparing

the thin films of these materials. Solid-state reactions employed for the

synthesis of BIT powders require relatively high temperatures (>1100°C)

[66]. Several soft chemical routes were used for the preparation of BIT

ceramics, including the co-precipitation method [67,68], the molten salt

synthesis [69] and the sol–gel process [70,71]. In contrast, there have been

only a few reports on the soft chemical synthesis of BLT powders and ceramics.

Shen et al. [72] reported the sol–gel preparation of Bi

4–x

(x = 0.4)

powders and ceramics using ethylene glycol as a solvent. This method led to

the preparation of fine BLT powders at a calcination temperature of 750°C.

However, it required drying the sol at 95–105°C over a period of 5 days in

order to obtain the gel.

Following our recent work on the soft chemical synthesis of SBT ceramics

[73], as described in Section 28.4, we have synthesized Bi

(BIT) and

Table 28.3

Comparison of the ferroelectric properties of SBT ceramics synthesized

by the sol-gel process and the co-precipitation method with the literature data on

single crystals and ceramics prepared by the solid state reaction (reproduced with

permission from

Chem. Mater.

2006, 18(2) 532–540)

Samples Sol–gel method Co-precipitation Single Single Solid-state

crystals crystals reaction

1000(8) 1200(4) 1200(8) 1200(4) 1200(8) [14]* [39]** [44]

34.5 23 22 19.5 21 1 22 30

3.4 6.0 7.6 3.5 3.7 0.01 14 4.5

*Single crystals with dominant (001) orientation and about 1–3% of (113) orientation.

**Single crystals oriented in the

-plane.

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Handbook of dielectric, piezoelectric and ferroelectric materials876

4–x

(xBLT) ceramics by an improved sol–gel method based on

ethylene glycol. The flowchart in Fig. 28.16 outlines the procedure for the

synthesis of precursor solutions, gels and ceramics of BIT and BLT. Our

main objective was to study the effect of sol–gel processing parameters and

La-doping on the phase formation and dielectric properties of the BLT

compounds. It has been shown that, once again, ethylene glycol proves to be

a good solvent for the starting materials, and this new method allows the

synthesis of the xBLT powders and ceramics at relatively low temperatures

with good dielectric properties. With this new sol–gel method, the formation

of the pure Bi-layered phase has been achieved at a temperature as low as

600°C for BIT and xBLT with x = 0.1, 0.2 and 0.3, and 500°C for xBLT with

x = 0.4−0.75, after heat treatment of the gels for 2h. The X-ray diffraction

(XRD) patterns are shown in Fig. 28.17. The sols obtained are stable over a

period of 3–5 days, which points to the potential of this sol–gel process for

the deposition of the thin films of BIT and xBLT for device applications.

28.6 Future trends

The exciting field of soft chemistry will continue to attract considerable

attention in the area of materials science and engineering, more specifically

Bi(OAc)

ethylene glycol

TIAA

ethylene glycol

La(acac)

ethylene glycol

La–EG

solution

Ti–EG

solution

Bi–EG

solution

Bi-Ti-EG

solution

Bi-Ti-La

gel

BIT/BLT

ceramics

Calcination and

sintering

Solvent evaporation

on rotary evaporator

Dropwise addition and

reflux for 1/2h

Dropwise

addition and

reflux for 1h

Reflux for 1hReflux for 1h

~ 10ml acetic acid

reflux for 1h

28.16

Flowchart for the preparation of precursor solutions, gels, and

ceramics of Bi

(BIT) and Bi

4–

(BLT).

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Novel solution routes to ferroelectrics and relaxors 877

in the synthesis of thin and thick films. In addition to the intensive research

on dielectric and ferroelectric thin films initiated mainly for ferroelectric

random access memory (FRAM) and dynamic random access memory

(DRAM) device applications, recent emphasis is put more towards micro-

electro-mechanical systems (MEMS). The strong piezoelectric effect and

28.17

X-ray diffraction patterns of (a) BIT samples treated at different

temperatures, and (b)

BLT samples treated at 500°C.

TiO

Unknown phase

TiO

Intensity (a.u.)

1000 °C

900 °C

800 °C

700 °C

600 °C

500 °C

10 20 30 40 50 60

2θ (deg.)

(a)

Intensity (a.u.)

10 20 30 40 50 60

2θ (deg.)

(b)

0.75BLT

0.7BLT

0.6BLT

0.5BLT

0.4BLT

0.3BLT

0.2BLT

0.1BLT

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Handbook of dielectric, piezoelectric and ferroelectric materials878

high energy density of ferroelectric materials which result from their remarkably

high dielectric constant make them very important materials for MEMS

applications in the form of thin films [74]. PZT thin films deposited by

several techniques such as physical deposition (e.g. PLD), chemical vapor

deposition (e.g. MOCVD) and solution deposition (e.g. sol-gel process) are

currently being used in a number of MEMS devices such as micro-

accelerometers [75], force sensors [76], micropumps [74] and micromotors

[77]. The solution deposition has become the preferred method because of

its cost-effectiveness compared with the former two techniques, excellent

control on stoichiometry in multiple-cation systems, good homogeneity and

ability to make films with a thickness of several micrometers. Moreover,

solution deposition routes are simple and do not require, for example, vacuum

or reactor chambers.

The novel sol–gel routes that we have developed for the synthesis of

(1 – x)PMN–xPT (x = 0.10 and 0.35), SBT and BLT ceramics allow the

formation of stable sols which can have tremendous potential for the fabrication

of ferroelectric thin films for MEMS and FRAM applications. The room

temperature synthesized PMN–PT sols are promising candidates for the

deposition of thin and thick films with excellent piezo- and ferroelectric

properties that can replace the currently used PZT films in MEMS. On the

other hand, the sol–gel-derived SBT sol will offer the possibility of depositing

thin films with reduced c-axis orientation and thereby improved remnant

polarization, which will make them interesting for non-volatile memories.

The SBT and BLT thin films can be outstanding lead-free and fatigue-free

alternatives to the PZT films.

Single crystals of the (1 – x)PMN−xPT solid solution with compositions

near the morphotropic phase boundary (MPB) have shown some of the

highest piezoelectric properties, making them excellent materials for

electromechanical transducer applications [6]. The ceramics of (1 – x)PMN−

xPT are expected to be a viable alternative to the single crystals if their

piezo- and ferroelectric properties can be optimized. The novel sol-gel process

developed in this work has led to the 0.65PMN−0.35PT ceramics of high

density and good dielectric and ferroelectric properties. However, the

piezoelectric performance of the ceramics remains to be assessed. Moreover,

ceramics of other PMN−xPT compositions, especially those in the

rhombohedral phase but close to the MPB (x = 28–32%), need to be prepared

with a view to achieving the optimum piezoelectric response and

electromechanical properties (as referred to the performance of the single

crystals).

The ability to make the powders or particles of nanometric size represents

another feature of the new soft-chemical routes, which remains to be explored.

Ferroelectric nano-powders have been given a great deal of attention in the

past few years, not only because of the fundamental interest related to particle

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Novel solution routes to ferroelectrics and relaxors 879

size effects on the structure and properties of the materials, but also because

of the practical applications arising from the enhanced properties of the

nano-powders. For instance, ferroelectric BaTiO

-based fine particles have

been used as raw materials for electronic devices such as multilayered ceramic

capacitors (MLCC). The size dependence of the dielectric constant of BaTiO

particles reveals a maximum of 15000 with a particle size of about 68nm

[78] or a maximum of 5000 with a size of 140nm [79]. Since the ceramics

and single crystals of PMN−xPT, especially PMN−10PT, exhibit a very high

dielectric constant around room temperature, it is expected that the nano-

powders/particles of PMN−xPT can greatly improve the performance of

miniaturized MLCC, more than the BaTiO

counterpart does, with enhanced

capacitance and higher power density for micro-capacitors.

28.7 Acknowledgments

This work was supported by the Natural Science and Engineering Research

Council of Canada (NSERC) and the US Office of Naval Research (Grant

No. N00014-1-06-0166).

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