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

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Handbook of dielectric, piezoelectric and ferroelectric materials810

important parameter. The densification rate is determined by the particle

size; powders with small particle sizes are usually desired. The size of template

grains is determined by the size of reactive template particles. The reactive

template particles are usually made by molten salt synthesis, and the synthesis

at high temperatures results in particles of large sizes. For example, the

diameter and thickness are about 10 and 0.5µm, respectively, for plate-like

particles, whereas the particle size of matrix particles is usually submicrometer.

Calcined compacts used in the RTGG process are composed of anisometric

template grains and equiaxed matrix grains. Large template grains act as

rigid inclusions in the densification process and hinder the densification of

the matrix phase (Belmonte et al., 1995; Rahaman, 2003; Ozer et al., 2006).

The degree of hindrance is proportional to the amount of template grains (De

Jonghe et al., 1986). Therefore, a small amount of template grains is desirable

from the viewpoint of densification. However, too small an amount results in

the limited development of texture. Densification and texture development

are in a trade-off relationship, and the optimum template amount is determined

by experiments. Usually, 5–20 volume % is selected.

Texture develops by the selective growth of template grains (Fukuchi

et al., 2002). When a large template grain is surrounded by small matrix

grains, the template grain is surrounded by concave grain boundaries (Fig.

26.9). The grain boundaries migrate towards the center of curvature, resulting

in the growth of template grain. The rate of grain boundary migration is

inversely proportional to the radius of curvature, which is proportional to the

matrix grain size. Therefore, it is desirable to use small matrix particles for

texture development. A small particle size is also desirable for densification.

When the matrix particles are formed by in situ reaction in green compacts

as in the BNKT case (reaction [26.3]), the selection of particle sizes of

starting materials is important. There is a certain compound that determines

the matrix particle size of the target compound. In the BNKT case, TiO

determines the particle size of matrix BNKT, and small TiO

is favorable for

texture development (Fuse and Kimura, 2006).

Template grain

Matrix grains

26.9

Migration of boundary between template and matrix grains.

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Processing textured piezoelectric and dielectric ceramics 811

The most widely used technique to increase grain boundary mobility is

the addition of a liquid phase (Rahaman, 2003). The liquid phase promotes

material transport by a solution-precipitation mechanism. Therefore, the use

of liquid phase is reported for Al

(Seabaugh et al., 1997), mullite (Hong

and Messing, 1999), Pb(Mg

1/3

2/3

–PbTiO

(Sabolski et al., 2001b) and

0.53

0.47

(Duran et al., 2000).

BNKT was the first material to which the RTGG process was applied

(Tani, 1998). In this system, a special mechanism is operative for the promotion

of selective growth of template grains. The shape of matrix grains is irregular

with curved surfaces, whereas template grains have flat (100) surfaces, as

shown in Fig. 26.10 (Fuse and Kimura, 2006). In this system, the surface

energy of (100) is lower than that of other faces (hkl). In general, the driving

force of grain growth is the reduction of total grain boundary energy which

is the product of boundary area and boundary energy per unit area (γ). In

ordinary cases, γ is almost the same for various (hkl) planes and the driving

force is caused by the reduction of boundary area. In the present BNKT case,

on the other hand, the reduction in γ is also accompanied as shown in Fig.

26.10. Therefore, the growth of template grains with low γ is favorable,

causing the selective growth of template grains (Fuse and Kimura, 2006).

The (Na,K)Nb

-based ceramics for lead-free piezoelectrics also have flat

grain boundaries and this driving force of grain growth is operative (Saito et

al., 2004).

26.5 Application to solid solutions

The RTGG method has two advantages; this method can develop texture in

compounds with an isotropic crystal structure, as already mentioned, and

can be applicable to solid solutions with complex compositions, as exemplified

for the systems Bi

0.5

(Na,K)

0.5

TiO

(Fukuchi et al., 2002), Bi

0.5

(Na,K)

0.5

TiO

–

Pb(Zr, Ti)O

(Abe and Kimura, 2002), Bi

0.5

(Na, K)

0.5

TiO

–BaTiO

(Kimura

et al., 2004), Bi

0.5

(Na,K)

0.5

TiO

–BiFeO

(Kato and Kimura, 2006) and

(Na,K,Li)(Nb,Sb,Ta)O

(Saito et al., 2004). In the titanate systems, the reactive

template is Bi

and the end member of the solid solutions is added into

the matrix phase except for Bi

0.5

(Na,K)

0.5

TiO

. The calcine compacts are

composed of template grains of BNKT and matrix phase containing both

BNKT and the end member compound. The ions in matrix grains must

diffuse into template grains to form textured ceramics. Figure 26.11 shows

the effect of the direction of material transport on the growth and disappearance

of template grains. When the materials of matrix phase diffuse into template

grains (Fig. 26.11a), the template grains grow and the matrix grains disappear,

resulting in the development of texture. When the diffusion direction is the

opposite (Fig. 27.11b), on the other hand, the template grains disappear and

textured materials cannot be obtained.

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Handbook of dielectric, piezoelectric and ferroelectric materials812

The diffusion direction is dependent on the chemical composition. For the

BNKT–PZT case, as an example, the PZT composition determines the diffusion

direction and therefore texture development as shown in Fig. 26.12 (Abe and

Kimura, 2002). In this case, the template grains are BNKT and matrix grains

are PZT. When PZT is Ti-rich, the components of PZT have a large diffusion

rate and diffuse into BNKT template grains. When PZT is Zr-rich, on the

other hand, the diffusivity of PZT components is small and the components

of BNKT diffuse into PZT grains, resulting in the disappearance of template

grains. In the Bi

0.5

(Na,K)

0.5

TiO

–BiFeO

case, the direction of material

(a)

(b)

200nm

500nm

Matrix grain attached to template grain

Matrix grain spreading over template grain

Template grain

(100)

(hkl)

Matrix grain

Material

transport

(c)

26.10

Growth of template grains at the expense of matrix grains.

Contact between template and matrix grains is formed (a). Matrix

grains spread over (100) face of template grains and incorporate with

template grain (b). Material transport from matrix (hkl) plane to

template (100) plane reduces total surface free energy, because γ

(100)

is much less than γ

(hkl)

for BNKT (c).

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Processing textured piezoelectric and dielectric ceramics 813

transport is determined by the chemical composition of template grains.

When the template grains are BNT, they diffuse into BiFeO

, resulting in the

disappearance of template grains. When the template grains are BKT, on the

other hand, the diffusivity of BKT is smaller than that of BiFeO

and BiFeO

diffuses into BKT template grains, resulting in the formation of textured

materials.

26.6 Conclusions and future trends

The control of texture is necessary to develop high-performance, lead-free

piezoelectric ceramics, because the performance of PZT-based materials is

quite excellent and it is difficult to develop lead-free materials with performance

comparable to or better than PZT by compositional design on its own. The

methods other than the RTGG process can be applicable to the preparation

(a) (b)

26.11

Effect of diffusion direction on the disappearance of (a) matrix

and (b) template grains. The arrows indicate the diffusion direction.

26.12

Effect of PZT composition on texture development in BNKT–

PZT solid solutions sintered at 1200 °C.

Degree of orientation

0.8

0.6

0.4

0.2

0 0.2 0.4 0.6 0.8 1

in PbZr

1–

PZ-rich

side

PT-rich side

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Handbook of dielectric, piezoelectric and ferroelectric materials814

of piezoelectric materials with anisotropic crystal structures. These methods

are hardly applicable to materials with the perovskite structure. The RTGG

process is the most convenient preparation method of such materials.

In the RTGG process, the reactive template of a target compound is selected,

and mixed with complementary compound(s) for the target compound to

make slurry. The slurry is tape cast to form thin sheets, in which reactive

template particles are aligned. A green compact made from the cast sheets is

calcined to cause a reaction between the reactive template and the

complementary compound(s). The calcined compact is composed of template

and matrix grains of the target compound. Sintering causes the growth of

template grains at the expense of matrix grains, resulting in the development

of texture.

The RTGG process was proposed in 1998 and research to this date is

mainly focused on the search for reactive templates because the compounds

that fulfill the requirement for reactive templates are limited. The performance

is determined by the microstructure of sintered compacts as well as by the

chemical composition. Texture is one of the elements of microstructure. The

optimization of all elements of microstructure, which includes density and

grain size as well as the uniformity of microstructure, is required. Because

the calcined compact is composed of template and matrix grains, it is necessary

to understand the densification behavior and microstructure development in

such a compact with a non-uniform microstructure. Furthermore, the change

from starting materials (reactive template and complementary compound(s))

to template and matrix grains must be fully understood to select starting

materials with optimum powder characteristics.

The application of the RTGG process is not restricted to piezoelectric

ceramics. The establishment of high performance through RTGG process is

reported for thermoelectrics (Tani, 2006).

26.7 References

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potassium titanate–lead zirconate titanate solid solutions made by reactive templated

grain growth method’, J. Am. Ceram. Soc., 85 (5) 1114–1120.

Bedoya C, Muller C, Jacob F, Gagou Y, Fremy M A and Elkaim E (2002), ‘Magnetic-

field-induced orientation in Co-doped SrBi

ferroelectric oxide’, J. Phys.: Condens.

Matter, 14 (45) 11849–11857.

Belmonte M, Moya J S and Miranzo P (1995), ‘Bimodal sintering of Al

/Al

platelet

ceramic composites’, J. Am. Ceram. Soc., 78 (6) 1661–1667.

De Jonghe L C, Rahaman M N and Hsueh C H (1986), ‘Transient stresses in bimodal

compact during sintering’, Acta Metall., 34(7) 1467–1471.

Duran C, Trolier-McKinstry S and Messing G L (2000), ‘Fabrication and electrical properties

of textured Sr

0.53

0.47

ceramics by templated grain growth’, J. Am. Ceram.

Soc., 83 (9) 2203–2213.

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Processing textured piezoelectric and dielectric ceramics 815

Fukuchi E, Kimura T, Tani T, Takeuchi T and Saito Y (2002), ‘Effect of potassium

concentration on the grain orientation in bismuth sodium potassium titanate’, J. Am.

Ceram. Soc., 85 (6) 1461–1466.

Fuse K and Kimura T (2006), ‘Effect of particle sizes of starting materials on microstructure

development in textured Bi

0.5

(Na

0.5

)TiO

’, J. Am. Ceram. Soc., 89 (6) 1957–1964.

Granahan M, Holmes M, Shulze W A and Newnham R E (1981), ‘Grain-oriented PbNb

ceramics’, J. Am. Ceram. Soc., 64 (4) C-68–C-69.

Holmes M, Newnham R E and Cross L E (1979), ‘Grain-oriented ferroelectric ceramics’,

Am. Ceram. Soc. Bull., 58 (9) 872.

Hong S H and Messing G L (1999), ‘Development of textured mullite by templated grain

growth’, J. Am. Ceram. Soc., 82 (4) 867–872.

Horn J A, Zhang S C, Selvaraj U, Messing G L and Trolier-McKinstry S (1999), ‘Templated

grain growth of textured bismuth titanate’, J. Am. Ceram. Soc., 82 (4) 921–926.

Hunter R J (1993), Introduction to Modern Colloid Science, Oxford, Oxford University

Press.

Igarashi H, Matsunaga K, Taniai T and Okazaki K (1978), ‘Dielectric and piezoelectric

properties of grain-oriented PbBi

ceramics’, Am. Ceram. Soc. Bull., 57 (9)

815–817.

Jaffe B, Cook, Jr. W R and Jaffe H (1971), Piezoelectric Ceramics, London, Academic

Press.

Kahn M, Burks D, Burn I and Schulze W A (1988), ‘Ceramic capacitor technology’, in

Levinson L M, Electronic Ceramics, New York, Marcel Dekker, 191–274.

Kato K and Kimura T (2006), ‘Preparation of textured Bi

0.5

(Na,K)

0.5

TiO

–BiFeO

solid

solutions by reactive-templated grain growth process’, J. Korean Ceram. Soc., 43 (11)

693–699.

Kimura T (2006), ‘Application of texture engineering to piezoelectric ceramics – a review’,

J. Ceram. Soc. Jpn., 114 (1) 15–25.

Kimura T and Yamaguchi T (1987), ‘Morphology control of electronic ceramic powders

by molten salt synthesis’, Adv. Ceram., 21, 169–177.

Kimura T, Takahashi T, Tani T and Saito Y (2004), ‘Crystallographic texture development

in bismuth sodium titanate by reactive templated grain growth method’, J. Am. Ceram.

Soc., 87 (8) 1424–1429.

Kimura T, Miura Y and Fuse K (2005), ‘Texture development in barium titanate and

PMN-PT using hexabarium 17-titanate heterotemplates’, Int. J. Appl. Ceram. Technol.,

2 (1) 15–23.

Krasevec V, Drofenik M and Kolar D (1987), ‘Topotaxy between BaTiO

and Ba

’,

J. Am. Ceram. Soc., 70 (5) C-193–C-195.

Kwon S, Sabolsky E M, Messing G L and Trolier-McKinstry S (2005), ‘High strain,

<001> textured 0.675 Pb(Mg

1/3

2/3

–0.325PbTiO

ceramics: templated grain growth

and piezoelectric properties’, J. Am. Ceram. Soc., 88 (2) 312–317.

Makiya A, Kusano D, Tanaka S, Uchida N, Uematsu K, Kimura T, Kitazawa K and

Doshida Y (2003), ‘Particle oriented bismuth titanate ceramics made in high magnetic

field’, J. Ceram. Soc. Jpn., 111 (9) 702–704.

Messing G L, Trolier-McKinstry S, Sabolsky E M, Duran C, Kwon S, Brahmaroutu B,

Park P, Yilmaz H, Rehrig P W, Eitel K B, Suvaci E, Seabaugh M and Oh K S (2004),

‘Templated grain growth of textured piezoelectric ceramics’, Crit. Rev. Solid State

Mater. Sci., 29, 45–96.

Nakai Y and Fukuoka S (1999), ‘Piezoelectric and mechanical properties of BaBi

ceramics’, Jpn. J. Appl. Phys. Part 1, 38 (9B) 5568–5571.

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Handbook of dielectric, piezoelectric and ferroelectric materials816

Newnham R E (2005), Properties of Materials, New York, Oxford University Press.

Ozer I O, Suvaci E, Karademir B, Missiaen J M, Carry C P and Bouvard D (2006),

‘Anisotropic sintering shrinkage in alumina ceramics containing oriented platelets’, J.

Am. Ceram. Soc., 89 (6) 1972–1976.

Pugh R J (1994), ‘Dispersion and stability of ceramic powders in liquid’, in Pugh R J and

Bergstrom L, Surface and Colloid Chemistry in Advanced Ceramics Processing, New

York, Marcel Dekker, 127–192.

Rahaman M N (2003), Ceramic Processing and Sintering, New York, Marcel Dekker.

Sabolsky M, James A R, Kwon S, Trolier-McKinstry S and Messing G L (2001a),

‘Piezoelectric properties of <001> textured Pb(Mg

1/3

2/3

–PbTiO

ceramics’, Appl.

Phys. Lett., 78 (17) 2551–2553.

Sabolsky E M, Messing G L and Trolier-McKinstry S (2001b), ‘Kinetics of templated

grain growth of 0.65Pb(Mg

1/3

2/3

)–0.35PbTiO

’, J. Am. Ceram. Soc., 84 (11) 2507–

2513.

Sabolsky E M, Trolier-McKinstry S and Messing G L (2003), ‘Dielectric and piezoelectric

properties of <001> fiber-textured 0.675Pb(Mg

1/3

2/3

–0.325PbTiO

ceramics’,

J. Appl. Phys., 93 (7) 4072–4080.

Saito Y and Takao H (2006), ‘Synthesis of Platelike {100} SrTiO

particles by topochemical

microcrystal conversion and fabrication of grain-oriented ceramics’, Jpn. J. Appl.

Phys., 45 (9B) 7377–7381.

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M (2004), ‘Lead-free piezoceramics’, Nature, 432, 84–87.

Seabaugh M M, Kerscht I H and Messing G L (1997), ‘Texture development by templated

grain growth in liquid-phase-sintered α-alumina’, J. Am. Ceram. Soc., 80 (5) 1181–

1188.

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0.5

(Na

0.87

0.13

)

0.5

TiO

polycrystals’, Ferroelectrics, 224 (1–4) 793–800.

Senz S, Graff A, Blum W and Hesse D (1998), ‘Orientation relationship of reactively

grown Ba

and Ba

TiSi

on BaTiO

(001) determined by X-ray diffractometry’,

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BaTiO

bulk ceramics by reactive templated grain growth’, J. Ceram. Soc. Jpn., 109

(10) 897–890.

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oriented barium titanate’, Ceram. Trans., 136, 389–406.

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and BaCO

’,

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and BaTiO

’, Chem. Lett.,

8, 987–988.

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forged Bi

ceramics’, Jpn. J. Appl. Phys., 19 (1) 31–39.

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structure’, J. Korean Phys. Soc., 32, S1217–S1220.

Tani T (2006), ‘Texture engineering of electronic ceramics by the reactive-templated

grain growth method’, J. Ceram. Soc. Jpn., 114 (5) 363–370.

Tani T and Kimura T (2006), ‘Reactive templated grain growth processing for lead free

piezoelectric ceramics’, Adv. Appl. Ceram., 105 (1) 55–63.

Tsutai S, Hayashi T, Hayashi S and Nakagawa Z (2001), ‘Reaction mechanism of BaTiO

from powder compacts, BaCO

and TiO

and expansion phenomena during the formation

process’, J. Ceram. Soc. Jpn., 109(12) 1028–1034 (in Japanese).

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Processing textured piezoelectric and dielectric ceramics 817

Wada S, Suzuki S, Noma T, Suzuki T, Osada M, Kakihana M, Park S E, Cross L E and

Shrout T R (1999), ‘Enhanced piezoelectric property of barium titanate single crystals

with engineered domain configuration’, Jpn. J. Appl. Phys. Part 1, 38 (9B) 5505–

5511.

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in the fabrication of grain-oriented bismuth titanate’, J. Am. Ceram. Soc., 72 (2) 289–

293.

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27.1 Introduction

Important ferroelectric or antiferroelectric oxide ceramics for dielectric,

ferroelectric, piezoelectric, electrostrictive and/or pyroelectric applications

are restricted to perovskite-type, tungsten bronze-type and bismuth layer-

structured compounds. One recent trend in the study on piezoelectric and/or

pyroelectric ceramic compounds is the use of lead-free materials. The other

trend is the use of grain orientation techniques as the ceramic fabrication

methods.

Recently, bismuth layer-structured ferroelectrics (BLSF), which form one

of the BO

octahedral ferroelectric groups, have been extensively studied in

the form of thin films because BLSF seem to be excellent candidate materials

for non-volatile FeRAM (ferroelectric random access memory) in various

commercial applications. For example, one BLSF, SrBi

shows very

high-quality characteristics with fatigue-free property for FeRAM.

The family of BLSF is a group of important ferroelectric oxide materials

for dielectric, ferroelectric, piezoelectric and pyroelectric applications because

BLSF such as Bi

(BIT), PbBi

(PBN), and Na

0.5

4.5

(NBT) are characterized

1–3

by their low dielectric constant ε

, high Curie

temperature T

, and large anisotropy in electromechanical coupling factors

or k

compared with those of PZT family. Furthermore, these

characteristics can be easily enhanced by a grain orientation technique such

as the hot-forging (HF) method.

4–6

Therefore, grain-oriented BLSF ceramics

seem to be a superior candidate for the lead-free or low-lead-content

piezoelectric materials

with high T

and anisotropic characteristics

7,8

, or for

pyroelectric sensor materials with a large figure of merit.

9,10

BLSF ceramics

can be used as stable and high-Q piezoelectric materials to be operated at

high temperatures and at high frequencies.

Piezoelectric activities of conventionally sintered BLSF ceramics, however,

are not as large as expected from those of the single crystals.

11,12

One reason

for the low piezoelectric activity in such ceramics may be due to the two-

Grain orientation and electrical properties

of bismuth layer-structured ferroelectrics

T TAKENAKA, Tokyo University of Science, Japan

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Grain orientation and electrical properties of bismuth 819

dimensional restriction on the permissible rotations of the spontaneous

polarization, in comparison with three-dimensionally permitted ones in

perovskite-type ferroelectrics. Thus, ferroelectric ceramics whose grain

crystallites have such lower symmetry as bismuth layer-type compounds

may have an essentially weak point in that a satisfactorily large remanent

polarization cannot be obtained by electric poling.

In order to solve this problem, new fabrication processes of piezoelectric

ceramics are desirable through which the texture of the ceramics can be well

controlled to give preferred grain orientations. There are usually two fabrication

methods for orientating the grains in oxide ceramics: one is a hot working

technique which makes use of the motion of dislocations in the grains and of

the slip in the grain boundaries at high temperatures, and the other is a

topotaxial reaction which is based on the shape of the powder of the raw

material.

The grain orientation technique, which is one of several ceramic preparation

methods with a controlled grain structure during the ceramic sintering process,

gives the ceramics an anisotropic single-crystal-like behavior. Of many hot-

working methods such as hot-forging, hot-rolling, hot-extrusion, and super

plastic deformation, the HF

method

4–6

seems to be the most advantageous

grain orientation technique in laboratories for lower symmetric materials

with a large anisotropic crystal structure such as the bismuth layer-structured

type and the tungsten bronze type. The HF samples have a higher plastic

deformation than those produced by hot-pressing. A slip between (Bi

)

layers and perovskite-like layers in the c-plane of BLSF ceramics occurs

during the hot-forging process and finally the c-axis is aligned almost parallel

to the forging axis with a well-oriented grain structure. The HF-BLSF ceramics

whose spontaneous polarization lies in the grain-oriented c-plane normal to

the forging axis display the piezoelectric and pyroelectric properties that are

enhanced by the grain orientation effects. Studies of the grain orientations in

piezoelectric and pyroelectric ceramics have been systematically carried out

for more than 30 years. The author has been able to obtain ceramics with a

high degree of preferred grain orientation by using HF in many BLSF which

have shown greatly enhanced dielectric, ferroelectric, piezoelectric and

pyroelectric properties.

This chapter describes the effects of grain orientation on electrical properties

such as dielectric, ferroelectric, piezoelectric, and pyroelectric properties of

some grain-oriented BLSF ceramics by the HF method, comparing them

with ordinarily fired (OF) non-oriented ones.

27.2 Bismuth layer-structured ferroelectrics (BLSF)

More than 50 years ago Aurivillius

discovered a family of bismuth

compounds with a layer structure, and to date more than 50 compounds in

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