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

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Handbook of dielectric, piezoelectric and ferroelectric materials890

room temperature. A quartz substrate was immersed into the colloidal solution,

raised up from it, and then air-dried. Figure 29.9 shows the photograph of a

KNbO

thin film produced on a glass substrate. A transparent and uniform

thin film of polycrystals is formed, as is apparent in the figure. Observation

by atomic force microsopy of the thin film before drying revealed a film that

comprised plate-shaped particles in the order of 100–300nm, in which the

nanosheets are coagulated. The film orientation was not confirmed clearly,

probably because the thermal disturbance at room temperature dominates

the orientation as a result of the overly small nanosheets in the solution.

Figure 29.10 shows a scanning electron microscopy (SEM) micrograph of

the thin film cross-section and Fig. 29.11 shows its XRD pattern. A crystalline

KNbO

thin film is thus obtained without heat treatment. The effects of the

immersion time on the film thickness are less significant because the nanosheets

in the colloidal solution are used for the formation of the thin film at the

beginning and the film showed little growth afterward. To control the film

thickness, the initial concentration of the solution should merely be adjusted.

Intensity/a.u.

200 400 600 800 1000

Raman shift/cm

–1

(a)

(b)

(c)

29.6

Raman spectra of (a) precursor K

NbO

F and (b) nano-KNbO

powders produced using the process described in the text, and (c)

colloidal solution.

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Room temperature preparation of KNbO

nanoparticles 891

The use of a colloidal solution with a concentration of approximately 1 mass

percent allows the formation of a thick film of 1–3µm after several hours of

immersion. Because heating is not required for crystallization, various materials

including glasses and plastics can be used as substrates.

Nb–O–Nb

29.7

Structural model of deduced nanosheets.

NbO

Perovskite nanosheet

KNbO

29.8

Schematic diagram of the reaction mechanism.

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Next, we attempted the fabrication of the KNbO

thin film using a K

NbO

single crystal. This process is also simple in that the K

NbO

F single crystal

is obtained using K

–KF as the flux which is placed on the substrate and

left under a saturated water-vapor atmosphere (a sealed container with water).

The K

NbO

F single crystal grows at temperatures lower than 800°C. Figure

29.12 shows the XRD pattern after the K

NbO

F single crystal was subjected

to water vapor processing. It is noteworthy that the water vapor processing

at room temperature causes the decomposition of the layered perovskite

NbO

F, which undergoes a structural change. Unlike the reaction in water,

the nanostructuring of particles does not occur in the water vapor processing.

The configuration of the original single crystal is retained. Because the

single crystal is plate-shaped, KNbO

obtained after water vapor processing

is also present as a thick film on the substrate. A feature of the resultant film

Glass

Glass + thin film

29.9

Photograph of transparent KNbO

thin film produced on a glass

substrate.

KNbO

thin film

Glass

substrate

10µm

29.10

SEM image of a KNbO

thin film cross-section.

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Room temperature preparation of KNbO

nanoparticles 893

is that the single-crystal KNbO

film grown is also oriented in the c-axis

direction because the oriented single crystal is used as a precursor. Evaluation

of nonlinear optical properties was carried out for the resultant film. Figure

29.13 shows a photograph of a single crystal film after irradiation with a

YAG : Nd laser of 1064 nm wavelength. A second harmonic generation

(SHG) is distinctly noted, indicating the possibility that a single-crystal film

produced at room temperature can be used in nonlinear optical devices.

20 40 60 80

2θ/deg.

Intensity/a.u.

: KNbO

: Halo pattern of

quartz substrate

29.11

X-ray diffraction pattern of a KNbO

thin film.

20 40 60 80

2θ/deg.

Intensity/a.u.

Crushed

single crystal

Thin film

Orthorhombic KNbO

(001)

(100)

(110)

(101)

(111)

(002)

(200)

(102)

(112)

(211)

(202)

(220)

(212)

(103)

(301)

29.12

X-ray diffraction pattern of a KNbO

film produced from a

NbO

F single crystal.

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Handbook of dielectric, piezoelectric and ferroelectric materials894

29.13

Photograph of a single crystal film on a glass substrate (left).

Micrograph shows a spot of SHG signal with half wavelength

(532 nm) of irradiated 1064 nm YAG: Nd laser (right).

2 mm

29.4 Conclusions and future trends

The solution process that we have developed is characteristic in that it uses

water, the most common solvent, and furthermore, the process proceeds

under very mild room temperature conditions. It must be emphasized that

the process takes place in a nearly closed system that uses neither high

temperatures nor high energy. It does not even emit a gas, making this a truly

‘environmentally friendly’ process. The uniqueness of the developed process

is further supported by the fact that perovskite KNbO

, which is useful as an

electroceramic material, can be synthesized in only one step, such that the

solution reaction is performed at room temperature, by selectively dissolving

a layered perovskite precursor, a solid in water. Therefore, to obtain highly

crystalline and morphologically controlled materials, neither electrochemical

processing nor additional processing such as heating is necessary, as required

by conventional solution processes, thereby ensuring an environmentally

sound and cost-effective process.

Furthermore, precursors that change to a perovskite structure through a

self-organizing process are not limited to K

NbO

F. In fact, our preliminary

study has revealed that layered perovskite Sr

CoO

Cl possesses an ordered

structure that selectively includes halogen ions between layers, similar to

NbO

F, and exhibits a selective interlayer dissolution process in an acidic

aqueous solution. Therefore, the soft chemical route of selective dissolution

from the layered perovskite precursors is a general and powerful tool for

low-temperature synthesis of perovskite materials.

29.5 References

1. K. Yamanouchi, H. Odagawa, T. Kojima and T. Matsumura, Electronic Letters, 33,

193 (1997).

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Room temperature preparation of KNbO

nanoparticles 895

2. K. Nakamura and Y. Kawamura, IEEE Trans. Ultrason. Ferroelect. Freq. Contr., 47,

750 (2000).

3. U. Fluckinger, H. Arend and H. R. Oswald, J. Am. Ceram. Soc., 56, 575 (1977).

4. K. Toda, N. Sakai, H. Ohnuma, Y. Aoyama, S. Tokuoka and M. Sato, Ext. Abstr.

(The 25th Symposium on Solid State Ionics in Japan 1999); The Solid State Ionics

Society of Japan, 2A9.

5. M. Dion, M. Ganne and M. Tournoux, Mater. Res. Bull., 16, 1429 (1981).

6. J. Gopalakrishnan and V. Bhat, Inorg. Chem., 26, 4299 (1987).

7. F. Galasso and W. Darby, J. Phys. Chem., 66, 1318 (1962).

8. L-S. Du, F. Wang and C. P. Grey, J. Solid State Chem., 140, 285 (1998).

9. A. F. Vik, V. Dracopoulos, G. N. Papatheodorou and T. Ostvold, J. Alloys Compd.,

321, 284 (2001).

10. A. M. Quittet, M. I. Bell, M. Karauzman and P. M. Raccah, Phys. Rev. B, 14, 5068

(1976).

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30.1 Introduction

So-called ferroelectric materials may be divided into two classes: ferroelectrics

and relaxors (Table 30.1) [1]. Typically, relaxors have at least one

crystallographic site that is occupied by two or more ions. Unlike ferroelectrics,

relaxors exhibit what is known as a diffuse transition: the real part of the

permittivity

′

is high but the maximum of

′

= ( )fT

is wide. In relaxors,

there is a strong influence of the frequency f of the electric field on the

′

= ( )fT

curves; when f increases,

′

decreases and the

′

maximum

temperature increases [1]. The latter refers to T

(not T

) owing to its large

variation with f. Concerning the

′′

= ( )fT

curve, when f increases, the

imaginary part of the permittivity

′′

increases and the

′′

maximum

temperature increases. The dielectric curve of a Pb(Mg

1/3

2/3

(PMN)

ceramic is a good example [2]. In many relaxors, the relationship between f

and T

can be described using the Vogel–Fulcher (VF) relationship [3, 4]:

Log = log –

( – )

mVF

kT T

30.1

where f

is the Debye frequency, T

the temperature of

′

max

at the given

frequency f, T

the static freezing temperature, E

the activation energy and

k the Boltzmann constant [5, 6]. In addition, there is a deviation from the

Curie–Weiss law (

′

= C/(T–T

)). The value of the Curie–Weiss temperature

is greater than that of T

. To date, various physical models have been

proposed to explain the properties of relaxors [7]: compositional fluctuations

and diffuse phase transition [8, 9], the superparaelectric model [10], the

nanostructure-octahedral model [11–17], the dipole-glass or the more recently

derived model [18, 19], the random-field model [20, 21], the domain-wall

model [22] and the random-layer model [23, 24]. Solid-state chemistry aspects

of lead-free relaxor ferroelectrics have also been developed [25, 26].

In addition to the usual applications for ferroelectrics, relaxors are of

great interest as dielectrics in capacitors and actuators [27]. Most relaxors

Lead-free relaxors

A SIMON and J RAVEZ, University Bordeaux1, France

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Lead-free relaxors 897

are lead-based ceramics such as PMN and derived compounds, or

Pb(M M )O

1/2 1/2

′′′

(M′ = Sc, In; M″ = Nb, Ta) with long-range polar order.

However, these compounds have the obvious disadvantages associated with

the volatility and toxicity of PbO. Therefore, much current research is directed

towards more environmentally friendly Pb-free relaxor materials. The present

chapter discusses some of the solid-state chemistry aspects of lead-free

ferroelectric relaxors. The selected compounds belong to either perovskite

or tetragonal tungsten bronze (TTB) families.

30.2 Lead-free relaxor ceramics derived from BaTiO

30.2.1 Relations between relaxor behaviour and ionic

substitutions in solid solutions

Barium titanate is of perovskite AMX

type structure with a 12-CN

(coordination number) site (Ba

) and a 6-CN one (Ti

). It presents three

phase transitions:

rhombohedral orthorhombic tetragonal

183K 268K 393K

→→→

cubic

Table 30.1

Some physical properties of ferroelectrics or relaxors of perovskite type

Ferroelectrics Relaxors

Octahedral site occupation – More than one different

cation

Composition heterogeneity – Nanoscopic scale

Polar region size Microdomains Nanodomains

Symmetry for

≤

Tetragonal, orthorhombic Macroscopically cubic

(or

) or rhombohedral (optical or X-ray

diffraction studies)

Ferroelectric–paraelectric Sharp Diffuse

transition

Frequency

dispersion of

′

not dependent on

′

decreases when

′

′′

for

≤

increases

(or

) ———————————— ———————————

′′

not dependent on

′′

increases when

increases

Frequency dependence of

not dependent on

increases when

(or

) increases

Thermal variation of

′

in Curie–Weiss law Deviation from

the paraelectric phase Curie–Weiss law

Thermal variation of

, at Sharp (1st order trans.) or Progressive decrease

(or

), on heating progressive (2nd order with a polarisation tail for

trans.) decrease at

;

≥

= 0 if

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The three low-temperature phases are ferroelectric. Numerous solid solutions

can easily be made by either cationic or anionic substitutions [28, 29].

Depending on both the substitution and the composition, different types

of behaviour were evidenced in binary systems:

• For homovalent substitutions on the A site (e.g. Ba

1–x

TiO

; A = Ca,

Sr) or for compositions very close to BT whatever the type of substitution,

a ferroelectric–paraelectric transition was observed at T

. The three phase

transitions were retained, as was the case for BaTiO

• For homovalent substitutions on the M site (e.g. Ba(Ti

1–x

; M = Zr,

Sn, Ce) the transition sequence was the same as that of BT for low

values of x (e.g. x ≤ 0.10, M = Zr) [30–33]. For greater values of x (e.g.

0.10 < x ≤ 0.27, M = Zr) there was only one rhombohedral, ferroelectric

→

cubic, paraelectric transition after the disappearance of the

orthorhombic and the tetragonal phases. The transition was second order

for x = 0.20 as T

= T

(Fig. 30.1). For the highest values of x (e.g. x >

0.27, M = Zr) there was only one broad peak of

′

at T

with a typical

relaxor frequency dispersion. There was also a deviation from the Curie–

Weiss law. Such properties are the dielectric attributes of a relaxor

behaviour. As an example, Fig. 30.2 shows the temperature dependences

of both

′

and

′′

and the corresponding deviation from the Curie–

Weiss law (T

> T

), for a ceramic with composition Ba(Ti

0.65

0.35

As x increases the value of T

dev.

– T

and the relaxor effect increase

0.1kHz

1kHz

10kHz

100kHz

1kHz

′

25 000

20 000

15 000

10 000

5000

50 100 150 200 250 300 350 400 450

(K)

50 150 250 350 450

(K)

0.002

0.001

0.000

′

30.1

Temperature and frequency dependences of

′

and temperature

dependence of 1/

′

, for a ceramic with composition Ba(Ti

0.80

0.20

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Lead-free relaxors 899

dev.

: temperature at which

′

starts diverging from the Curie–Weiss

law). Figure 30.3 illustrates how well the data log f vs. T

fit the Vogel–

Fulcher law (T

= 154 ± 10 K). Scaling dielectric characteristics differ

from those of lead complex perovskite relaxor [34].

′

20 000

15 000

10 000

5000

0.1 kHz

0.5 kHz

1 kHz

5 kHz

10 kHz

50 kHz

100 kHz

200 kHz

50 100 150 200 250 300 350

(K)

(a)

50 150 250 350 450

(K)

dev.

0.002

0.001

1 kHz

′′

2500

2000

1500

1000

500

50 100 150 200 250 300 350

(K)

(b)

0.1 kHz

0.5 kHz

1 kHz

5 kHz

10 kHz

50 kHz

100 kHz

200 kHz

30.2

Temperature and frequency dependences of

′

(a) and

′′

(b) and temperature dependence of

′

(a) for a ceramic with

composition Ba(Ti

0.65

0.35

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