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

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Handbook of dielectric, piezoelectric and ferroelectric materials800

• The magnitude of strain caused by an external electric field is dependent

on the angle between the directions of spontaneous polarization and

external field, and the development of texture reduces the misorientation

angle (Takenaka and Sakata, 1980). Figure 26.1 shows two adjacent

grains, A and B. The direction of spontaneous polarization is parallel to

that of the external electric field in grain A but not in grain B. Grain A

has the largest strain. Therefore, a larger strain is expected in textured

materials than materials with randomly oriented grains, because all grains

in the textured materials have the orientation of grain A.

• The difference in the magnitude of strain of two adjacent grains lowers

overall strain; grain B with a smaller strain suppresses the deformation

of grain A with a larger strain, resulting in a small overall strain. In

textured materials, the difference in the magnitude of strain of two adjacent

Table 26.1

Enhancement of piezoelectric properties by texture formation

Compounds Properties Random Textured References

0.5

(Na

0.85

0.15

)

0.5

TiO

(%) 0.295 0.402 Tani (1998)

(pC/N) –36.7 –57.4 Tani (1998)

(10

–3

V m/N) 7.21 11.2 Tani (1998)

94.5Bi

0.5

TiO

–5.5BaTiO

(pC/N) 110 200 Yilmaz

et al

(2003b)

0.675Pb(Mg

1/3

2/3

–

(pC/N) 580 1310 Kwon

et al

0.325PbTiO

(2005)

(pC/N) –195 –282 Sabolsky

et al

(2003)

(%) 0.673 0.755 Sabolsky

et al

(2003)

0.44

0.52

0.04

)

× (Nb

0.86

0.10

0.04

(pC/N) 300 416 Saito

et al

(2004)

, coupling factors;

, piezoelectric coefficients.

Electric field

Spontaneous

polarization

26.1

Direction of spontaneous polarization of grains A and B and that

of external electric field.

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Processing textured piezoelectric and dielectric ceramics 801

grains is small, and a large overall strain is expected. This results in

large piezoelectric coefficients.

• Another origin is attributed to engineered domain configuration (Kwon

et al., 2005). Higher piezoelectric constants along a certain direction

than along the direction of spontaneous polarization are expected for

materials with the engineered domain configuration (Wada et al., 1999).

For a tetragonal BaTiO

single crystal, for example, the piezoelectric

constant d

is higher when an electric field is applied along the [111]

direction than the [001] direction. In polycrystalline materials with

randomly oriented grains, the direction of the applied electric field with

respect to the crystallographic axes is dependent on the orientation of

each grain and d

of overall compact is the average value of d

for each

grain. In textured materials, an electric field is applied along a certain

direction, and the materials textured with the engineered domain

configuration are expected to have high piezoelectric properties.

26.2 Reactive-templated grain growth method

The methods to prepare textured ceramics are based on the conventional

ceramic processing processes: preparation of powder particles, consolidation

of the particles and sintering of the compacts. The techniques to develop

texture are (1) the application of pressure during heating, i.e. hot pressing

(Igarashi et al., 1978) and hot forging (Takenaka and Sakata, 1980) and (2)

the alignment of particles in green compacts. In the latter technique, the

particles are aligned during consolidation of powder particles by the application

of magnetic field (Bedoya et al., 2002; Makiya et al., 2003) or under flow

velocity gradient (Holmes et al., 1979). This chapter deals with the preparation

of textured materials by the flow velocity gradient, i.e. tape casting of slurries

containing anisometric particles by a doctor-blade process. When the slurry

passes under the blade, a flow velocity gradient develops. This gradient

causes the alignment of anisometric particles in concentrated slurries (Watanabe

et al., 1989). Thus, a cast sheet with aligned anisometric particles is obtained.

Sintered compacts are prepared from the cast sheet by a method similar to

that used for the fabrication of multilayer ceramic capacitors (Kahn et al.,

1988).

Anisometric particles are easily prepared by molten salt synthesis (Kimura

and Yamaguchi, 1987). Salt such as NaCl, KCl, Na

and K

is used

as molten salt. Starting oxides (and carbonates) are mixed with the salt and

heated at temperatures above the melting point of the salt. In the Bi

case, for example, Bi

and TiO

with the stoichiometric ratio are mixed

with KCl or a mixture of NaCl and KCl. The product phase (Bi

) is

formed by the reaction between starting oxides in molten salt. Prolonged

heating at high temperatures causes particle growth by Ostwald ripening

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Handbook of dielectric, piezoelectric and ferroelectric materials802

(Rahaman, 2003) and particles with a well-developed crystal habit are obtained.

When the compound has a highly anisotropic crystal structure, anisometric

particles are obtained; for the compounds belonging to the bismuth layer

structure and tungsten bronze families, plate-like and needle-like particles

are formed, respectively (Kimura and Yamaguchi, 1987). Figure 26.2 shows

the photomicrograph of plate-like Bi

particles prepared using molten

KCl. Generally speaking, the particle size and aspect ratio are determined

mainly by the heating temperature and the chemical species of the salt (Kimura

and Yamaguchi, 1987).

The anisometric particles are mixed with small equiaxed particles with

the same composition to make slurries (Horn et al., 1999; Duran et al.,

2000). In green compacts, the anisometric particles are dispersed in the

small equiaxed particles (matrix particles). Furthermore, they align with

specific particle orientation; for plate-like particles, their plate face is aligned

parallel to the sheet surface, and for needle-like particles, their needle axis is

aligned parallel to the casting direction. Sintering of green compact causes

densification and grain growth. In many cases, the anisometric particles

grow at the expense of matrix particles. Thus, the volume fraction of grains

with the desired orientation increases and textured materials are obtained.

Ideally, all the matrix particles are consumed. This process is called templated

grain growth (TGG), because the anisometric particles act as templates for

texture development (Messing et al., 2004).

The application of the TGG process to compounds with the perovskite

structure has been limited because of the lack of appropriate templates.

Platelike BaTiO

, SrTiO

and Ba

are prepared and used as templates

for <001>-textured Pb(Mg

1/3

2/3

–PbTiO

(Sabolsky et al., 2001a,b),

<001>-textured Bi

0.5

TiO

–BaTiO

(Yilmaz et al., 2003a) and <111>-

textured BaTiO

(Kimura et al., 2005) by the TGG process. However, SrTiO

and Ba

are non-piezoelectric and the presence of these particles in

10 µm

26.2

Photomicrograph of plate-like Bi

particles prepared in

molten KCl.

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Processing textured piezoelectric and dielectric ceramics 803

the sintered compacts reduces the piezoelectric constants (Yang et al., 1998;

Nakai and Fukuoka, 1999). It is desirable to obtain textured ceramics using

the template particles with the same composition as the matrix particles.

This is made possible by a reactive-templated grain growth (RTGG) process

(Tani and Kimura, 2006; Kimura, 2006).

Figure 26.3 shows the flowchart of the preparation of textured ceramics

by the RTGG process, using Bi

0.5

(Na

1–x

)

0.5

TiO

(BNKT) as an example.

At first, a precursor (reactive template) of a target compound is selected. The

reactive template must be anisometric and contain the elements of the target

compound; in the BNKT case, plate-like Bi

particles (Fig. 26.2) are

selected. Then, the reaction to form the target compound is designed, and

complementary compounds for the target compound are determined. In the

BNKT case, the reaction is

+ 2(1 – x)Na

+ 2xK

+ 5TiO

→ 8Bi

0.5

(Na

1–x

)

0.5

TiO

+ 4CO

26.1

and the complementary compounds are Na

and TiO

. The mixture

of reactive template and complementary compounds is mixed with solvent,

Solvent, binder, plasticizer, etc.

Equiaxed particles

(complementary

compounds)

Anisometric particles

(reactive templates)

Sintered compact

Calcined compact

Green compact

Sheet

Slurry

Cold isostatic pressing

sintering

Calcination

Cutting, lamination, pressing

Binder burn-out

Tape casting

Mixing

26.3

Flowchart of RTGG process.

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binder, plasticizer and other materials to form slurry. The slurry is tape cast

to form thin sheets in which plate-like Bi

particles are aligned with

their plate face parallel to the sheet surface (Fig. 26.4a). The sheets are cut,

laminated and pressed to form a green compact with the desired dimensions.

The compact is heated to remove organic ingredients and, most importantly,

to form the target compound (BNKT) by an in situ reaction [26.1]. Figure

26.4(b) schematically shows the structure of a calcined compact, and Fig.

26.5 shows the microstructure of a calcined BNKT compact. The large plate-

like and small equiaxed grains are present in the calcined compact. These

grains are BNKT. The plate-like grains are oriented with their plate face

parallel to the sheet surface.

It is well known that volume expansion is accompanied by the formation

of perovskite phase from the starting materials (Tsutai et al., 2001). This is

the situation for the BNKT case. The volume expansion reduces the density

of compact after calcination, and causes slow densification during sintering.

Therefore, the compact after calcination is subject to cold isostatic pressing

(CIP). By this procedure, the green compacts with about 60% of the theoretical

density are obtained.

Reactive template particles

Complementary

compounds

Matrix particles

Template particles

(a)

(b)

(c)

26.4

Structures of (a) green compact, (b) calcined compact and

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The compact after CIP is sintered at appropriate temperatures. Figure

26.4(c) schematically shows the structure of a sintered compact and Fig.

26.6 shows the microstructure development during sintering of BNKT. The

comparison of these microstructures with the microstructure of the calcined

compact (Fig. 26.5) indicates the growth of template grains. In the BNKT

case, the template grains are plate-like and have a rectangular cross-section.

Sintering increases mainly their thickness without changing cross-sectional

shape, and finally, the compact is composed of only plate-like grains. Figure

26.7 shows their X-ray diffraction patterns; the intensity of (100) and (200)

peaks increases as the sintering temperature is increased, indicating the

development of <100> texture. These results indicate that the BNKT template

grains with their <100> direction perpendicular to the compact surface grow

at the expense of randomly oriented matrix grains, leading to an increase in

the degree of orientation.

Because texture development is caused by the growth of template grains,

their relative amount and growth rate are important to obtain highly textured

compacts. The relative amount of template and matrix grains is determined

by the formation reaction of target compound. In the case of reaction [26.1],

template BNKT grains are formed by the diffusion of Na

O and K

O into

particles by reaction [26.2] (Seno and Tani, 1999):

+ 0.75(Na

+ K

)

→ 3Bi

0.5

(Na, K)

0.5

TiO

+ 1.25Bi

+ 0.75CO

26.2

produced by reaction [26.2] reacts with Na

, K

, and TiO

and forms equiaxed BNKT grains by reaction [26.3]:

1.25Bi

+ 1.25(Na

+ K

) + 5TiO

→ 5Bi

0.5

(Na, K)

0.5

TiO

+ 1.25CO

26.3

Casting direction

5 µm

Template particle

Matrix

26.5

Microstructure of calcined compact of BNKT.

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These grains are matrix grains. Therefore, the ratio of template to matrix

grains is 3:5.

It is impossible to increase the relative amount of template grains, but the

relative amount of matrix grains can be increased by adding the predetermined

amount of starting materials which are shown in reaction [26.4]:

+ (1 – x)Na

+ xK

+ 4TiO

→ 4Bi

0.5

(Na

1–x

)

0.5

TiO

+ 2CO

26.4

When the amount of template grains is designed to be 20%, the reaction is

expressed by [26.5]:

4Bi

+ 7Bi

+ 15(1 – x)Na

+ 15xK

+ 48TiO

→ 60Bi

0.5

(Na

1–x

)

0.5

TiO

+ 15CO

26.5

Template particle

Matrix

1000 °C

1050 °C

2 µm

1100 °C

26.6

Microstructures of sintered compact of BNKT.

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Processing textured piezoelectric and dielectric ceramics 807

26.3 Selection of reactive templates

The selection of reactive templates is important to prepare textured ceramics

by the RTGG process (West and Payne, 2003). The conditions required for

the reactive template are that (1) it contains all or a part of elements composing

the target compound, (2) its crystal structure is highly anisotropic and

anisometric particles can be obtained, (3) it has crystallographic relations

with the target compound and (4) the target compound is formed by the

unidirectional diffusion of complementary compound into the reactive template.

fulfills all these conditions required for the reactive template of

<100>-textured BNKT: (1) it contains Bi and Ti, (2) it has a layer structure

and plate-like particles can be easily obtained (Fig. 26.2), (3) it contains

perovskite blocks sandwiched by Bi

layers and the perovskite blocks

have a structure similar to that of BNKT (Jaffe et al., 1971) and (4) BNKT

is formed by the unidirectional diffusion of Na

O and K

O into Bi

and the structure of perovskite blocks is preserved (Seno and Tani, 1999).

Reactive templates other than Bi

are required for target compounds

containing elements other than Bi and Ti. The data reporting topotaxy are

useful for selecting the reactive templates. In the BaTiO

case, for example,

TiO

and Ba

are used for <110> and <111> texture, respectively

(Sugawara et al., 2001; Kimura, 2006). In the TiO

case, the topotaxial

relation of [001]

rutile

//[110]

BaTiO3

and [100]

rutile

//[111]

BaTiO3

has been reported

(Suyama et al., 1979). Furthermore, BaTiO

is formed by the unidirectional

diffusion of BaO into TiO

(Suyama and Kato, 1977), and needle-like TiO

particles with <001> parallel to the needle axis are commercially available

Intensity (arb. unit)

20 30 40 50 60

2θ/deg (Cukα)

1200 °C

1100 °C

1000 °C

900 °C

(100)

(110)

(111)

(200)

(210)

(211)

26.7

X-ray diffraction patterns of textured BNKT ceramics sintered at

various temperatures.

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(Fig. 26.8a). In the Ba

case, the relation of

(001) //(111)

Ba Ti O BaTiO

17 40 3

and

(010) //(110)

Ba Ti O BaTiO

17 40 3

has been reported (Krasevec et al., 1987;

Senz et al., 1998).

Plate-like Ba

particles with <001> perpendicular

to the plate face can be easily obtained by molten salt synthesis (Fig. 26.8b)

(Sugawara et al., 2001; Kimura et al., 2005).

Plate-like BaTiO

particles made by the Remeika method are one of the

candidates for the template of BaTiO

-containing materials (Sabolsky et al.,

2001a). The <100> direction lies perpendicular to the plate face and the

particles can be used as templates for <100> texture. The problem is their

low aspect ratio.

The plate-like particles with a high aspect ratio can be easily obtained by

molten salt synthesis for compounds belonging to the bismuth layer structure

family, Bi

, CaBi

, SrBi

, BaBi

and so forth.

However, these compounds contain Bi

as one of the constituents, and

this must be removed before use as the reactive template for compounds that

do not contain Bi. Sintering under vacuum is necessary for the preparation

of <100>-textured BaTiO

(Sugawara et al., 2004) using reaction [26.6]:

+ 3BaCO

→ 3BaTiO

+ 2Bi

26.6

(a)

(b)

500 µ m

10µm

26.8

Photomicrographs of (a) needle-like TiO

and (b) plate-like

particles.

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Processing textured piezoelectric and dielectric ceramics 809

formed by reaction [26.6] is removed from the product phase by

heating under vacuum because of the high vapor pressure of Bi

. The

reaction in molten salt is another route (Saito et al., 2004). Plate-like NaNbO

particles are prepared from plate-like Bi

2.5

3.5

particles, which is a

member of the bismuth layer structure family. At first plate-like

2.5

3.5

particles are prepared by molten salt synthesis and then

reacted with Na

in molten salt to form plate-like NaNbO

particles,

which are used as reactive templates for (Na,K)NbO

-based materials. The

reaction in molten salt is

4Bi

2.5

3.5

+ 3Na

→ 20NaNbO

+ 5Bi

+ 3CO

26.7

and Bi

produced is removed by washing the reaction product with acid.

A similar technique is applied to the preparation of plate-like SrTiO

particles

(Saito and Takao, 2006).

26.4 Factors determining texture development

The degree of orientation is determined by several factors. The most important

factor is the alignment of reactive template particles in the cast sheets, which

is determined by the motion of particles in the slurries. When the particles

are well dispersed in the slurries, the particles are easily aligned by tape

casting (Watanabe et al., 1989). However, when the particles are agglomerated

(coagulated) in the slurries, the alignment of particles is difficult (Sugawara

et al., 2001). For example, plate-like Ba

particles form agglomerates

with BaCO

, and these agglomerates have an equiaxed shape and have no

tendency to align by flow velocity gradient. The dispersion of particles is

controlled by several parameters (Pugh, 1994). In a single component system,

the control of particle dispersion is rather easy. However, the RTGG process

uses slurries containing multicomponents (reactive template and

complementary compounds), resulting in the possibility of heterocoagulation

(Hunter, 1993). The proper selection of dispersant is necessary to prepare

well-dispersed slurries. Therefore, the confirmation of particle orientation in

the cast sheets by X-ray diffraction is necessary before further processing.

In the RTGG process, calcined compacts are composed of template and

matrix grains as shown in Figs 26.4(b) and 26.5, and texture develops by the

growth of template grains at the expense of matrix grains (Fig. 26.6). Therefore,

the degree of orientation is determined by the relative amount of template

grains and their growth rate. In this sense, the situation is the same as that

occurred in the TGG process (Messing et al., 2004).

Because piezoelectric ceramics are subject to poling before use, their

density must be high; at least 95% of the theoretical density is desired.

Therefore, the sintered density as well as the degree of orientation is an

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