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

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Handbook of dielectric, piezoelectric and ferroelectric materials820

the group have been investigated; a large number have been found to be

ferroelectric.

14–20

Crystallographic studies of bismuth-layered oxides have

been made by many workers,

13–20

the dielectric properties have been examined

in detail by Subbarao,

and ferroelectric and piezoelectric properties have

been studied by some Japanese scientists.

2,3,7–8

The bismuth layer-structured oxides (BLSO) have a generalized chemical

formula of A

m–1

3m+3

where A is mono-, di-, or trivalent ions or a

mixture of them and B is tetra-, penta-, or hexavalent ions, or a mixture of

them, and the integer m takes any value from 1 to 5. Examples of their

crystal structures are shown in Fig. 27.l, where A shows (A

m–1

3m+1

)

which consists of (m – 1) layers of perovskite-like units (B) and is sandwiched

between (Bi

) layers (C). Bismuth titanate, Bi

(BIT) (Fig. 27.1b),

is a prototype of many bismuth layer structure oxides with the simplest

chemical composition (m = 3) and has a high Curie point of 680°C.

11,12

BLSO have BO

octahedra like perovskite, pyrochlore, and tungsten-

bronze structures and many of them are ferroelectric. An electrically neutral

condition gives the following formula, (m – 1)N(A) + m N(B) = 6m, where

N(A) and N(B) indicate the mean valencies of the A and the B ions, respectively.

However, BLSO have a great variety of available combinations of A and B

ions, respectively. For example, the basic BLSF are Bi

(BIT) (m = 3),

PbBi

(PBN) (m = 2), Bi

TiNbO

(BTN) (m = 2), Na

0.5

4.5

(c)

BaBi

Ti OBi Ti O

(b)

Pb O

(a)

-axis

5Å

27.1

Schematic drawings of (a) PbBi

(PBN) (

= 2),

(b) Bi

(BIT) (

= 3) and (c) BaBi

(BBT) (

= 4) structures

(one-half of the unit cell). A: The perovskite layer of (Bi

)

2–

BIT, B: a unit cell of the hypothetical perovskite structure BiTiO

BIT, C: the layer of (Bi

)

(after Aurivillius

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Grain orientation and electrical properties of bismuth 821

(NBT) (m = 4), PbBi

(PBT) (m = 4), and Pb

(P2BT)

(m = 5). Many derivative solid solutions have also been studied, mainly in

grain-oriented piezoelectric and pyroelectric ceramics.

Among a great number of the BLSF compounds, the following solid

solution systems were selected in this chapter on the basis of the results of

the electrical properties:

[BIT]

1–x

(Na

1/2

)

[PNBT–100x]

1–x

(Na

1/2

)

[PNC–100x]

(Na

1/2

)

1-x

[NCBT–100x]

8,22

(x = 0: NBT

; x = 1: CBT)

m–3+x

4–x

m–x

3m+3

(m = 2 [SBTT2(x)], x = 1~2; m = 3

[SBTT3(x)], x = 0~2)

24,25

where [ ] shows the abbreviation of the respective systems.

Among a great number of the BLSF compounds, the Na

0.5

4.5

(NBT) (m = 4) ceramic displays excellent piezoelectric properties.

2,23

On the

other hand, substituted perovskite compounds by Ca for A-site ions are

expected to have large anisotropic piezoelectric properties because the calcium

ionic radius is the smallest in the A-site ion group of the perovskite compounds.

Due to an enhanced anisotropy, the electromechanical coupling factor k

in the radial or transverse mode of 24 mol% calcium-modified PbTiO

(PT) ceramics is almost negligible,

in contrast with the large coupling

factor k

or k

in the thickness expansion or longitudinal expansion mode,

resulting in a very large anisotropy in the coupling factor, k

or k

. The

Ca-modified NBT, namely, (Na

1/2

)

1–x

[NCBT], ceramics

can be expected to show the most excellent piezoelectric and/or pyroelectric

properties.

Figure 27.2 shows the phase relation between well-known bismuth layer-

structured ferroelectrics such as Bi

TiTaO

(BTT) (m = 2), SrBi

(SBT)

(m = 2) and Bi

(BIT) (m = 3), including a perovskite compound

SrTiO

. The general formula of phase relation in Fig. 27.2 is as follows:

m–3+x

4–x

m–x

3m+3

(m = 2, x = 1~2; m = 3, x = 0~2)

where x corresponds to the amount of Ta ions in the layer structure and

1 ≤ x ≤ 2 for m = 2 and 0 ≤ x ≤ 2 for 3 ≤ m ≤ 5. Hereafter the formula is

abbreviated to SBTTm (x), especially the series of m = 2 and 3 are abbreviated

to SBTT2(x) (1 ≤ x ≤ 2) and SBTT3(x) (0 ≤ x ≤ 2).

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Handbook of dielectric, piezoelectric and ferroelectric materials822

27.3 Grain orientation and HOT-Forging (HF)

method

27.3.1 Ceramic sample preparations

OF (non-oriented) ceramic samples were prepared by the conventional ceramic

technique. Reagent grade oxide or carbonate powders of Bi

, PbO, Na

CaCO

, TiO

, and Nb

with 99+% purity were used as the starting materials.

To prepare the perovskite type (Na

1/2

)TiO

powders for the PNC system,

raw materials of CeO

, Na

, and TiO

were pre-calcined in a reduced

atmosphere using carbon graphite powders. The materials mixed using ball

milling were calcined at 850°C for 2h. After calcining, the ground and ball-

milled powders were pressed into a disc of 20mm in diameter and about

10mm in thickness. Pressed discs were finally sintered at 1000–1250°C for

1–2h in air.

= 2

= 3

= 4

= 5

BIT

SrBi

SBT

–3

> = 3

–2

–1

TaO

–3

> = 12

–1

–2

> = 12

TiTaO

BTT

SrBi

SBT

TiTa

TaO

SrBi

TaO

= ∞

Perovskite structure

SrTiO

27.2

The phase relation between well-known bismuth layer-

structured ferroelectrics such as Bi

TiTaO

(BTT) (

= 2), SrBi

(SBT) (

= 2) and Bi

(BIT) (

= 3) including a perovskite

compound SrTiO

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Grain orientation and electrical properties of bismuth 823

27.3.2 Grain orientation by hot-forging method

Grain-oriented samples were prepared by the HF method as described in

detail elsewhere.

Cylinder bodies 15mm in diameter and about 30–40mm

in height were pre-pressed in a steel die at a pressure of about 800kg/cm

and were used as starting bodies in the HF process. The pre-pressed cylinder

(green body) was sandwiched between two platinum plates to prevent the

sample from reacting with alumina plungers. A uniaxial compressive load of

about 200kg/cm

for the area of π (15/2)

was applied along the thickness

of the sample (the forging axis) using oil pressure. After ordinary firing at

1000–1200°C for 2h, the sample was gradually pressed by two alumina

plungers, since abrupt pressing will produce cracks in the sample. Then a

constant pressure and a maximum temperature were maintained for a variable

time. A lower temperature and abrupt pressing will produce cracks in the

sample. Four conditions are most critical: T

, the maximum temperature; P

the maximum total pressure; t

, the soaking time when the maximum pressure

is maintained; and t

, the annealing time. A degree of deformation in HF

ceramics is expressed in terms of an area ratio γ, and a thickness contraction

rate λ; i.e. γ = S

, and λ = (h

– h

)/h

, where S

and S

are cross-section

areas, and h

, h

thickness, of pre-sintered and hot-forged samples, respectively.

The value of h

is estimated by a relation of h

= (1 – α)h

, where h

is a

thickness of the green body and α is a linear shrinkage of the ordinarily fired

pellet which was sintered at the same temperature. The plastic deformations,

namely γ and λ, depend heavily on the forging temperature and the composition

of the sample. For example, HF BLSF ceramics showed an area ratio, λ, of

about 2–10, and a thickness contraction rate, λ, of about 50–90%, respectively.

A measured density of 7.78g/cm

in HF samples was 97% of the X-ray

density 8.04g/cm

, and it increased to 99% if further hot-pressing was applied

after the hot-forging.

27.4 Grain orientation effects on electrical

properties

27.4.1 Grain orientation and microstructure

The crystalline structure was confirmed by X-ray powder diffraction analysis

using a Ni-filtered CuKα radiation at a scanning speed of 1deg/min. Evaluation

of the HF samples was performed for the grain orientation factor, F, obtained

from X-ray diffraction patterns compared with that from the powder diffraction

patterns of the OF sample using the Lotgering method.

The texture of HF

samples was observed with a scanning electron microscope (SEM). For

preparing samples, ceramics were polished with successively finer

carborundum, were finished with 3 µm diamond paste, and then were thermally

etched at 1000–1100°C for 30min.

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Handbook of dielectric, piezoelectric and ferroelectric materials824

In the growth of BIT single crystals,

11,12

it is known that the c-plate grows

into a mica-like shape. Furthermore, Ikegami and Ueda

reported that grain

boundaries in OF BIT ceramics are of nearly rectangular in shape and fine

stripes are observed in each grain, always parallel to the length of each

rectangular grain boundary, which will be related closely to the crystal structure

with Bi

layers. Therefore, when X-rays are reflected by the plane

perpendicular to the forged direction of the ceramic, (00l) reflections are

expected to increase and others to decrease compared with those of OF

ceramics. Figure 27.3 shows X-ray diffraction patterns, reflected by the

perpendicular plane (b), (c) and by the parallel plane (d), respectively, in the

HF samples; reflected by the c-plate of a single crystal (a), and by OF

ceramics (e).

As shown in Fig. 27.3, (00l) reflections increase in the perpendicular

plane (b), (c) whose patterns resemble that of the c-plate of a single crystal

10 20 30 40 50 60 70 80

2 θ (deg) (cuKα)

(a)

(b)

(c)

(d)

(e)

(004)

(006)

(008)

(0010)

(0012)

(0014)

(0016)

(0018)

(0022)

(0020)

(0016)

(0012)

(0024)

(0026)

(0028)

(006)

(008)

(117)

(326)

(111)

(117)

(020)

(028)

(220)

(131)

(137)

(040)

(337)

(240)

(004)

(006)

(008)

(111)

(115)

(117)

(020)

(0014)

(028)

(220)

(034)

(137)

(3111)

(043)

(337)

(240)

27.3

X-ray diffraction patterns for Bi

(BIT), for (a) the

-plate of

a single crystal, (b) the perpendicular plane (non-polished), (c) the

perpendicular plane (polished) and (d) the parallel plane of hot-

forged ceramics, and (e) the ordinarily fired ceramics.

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Grain orientation and electrical properties of bismuth 825

(a), and (00l) reflections in the parallel plane (d) almost disappear, in comparison

with OF ceramics (e). Thus, in the HF ceramics, the grains are found to be

oriented with c-axes along a forged direction. The degree of grain orientation,

F, can be estimated by comparing the X-ray diffraction patterns with those

of non-oriented (OF) ceramics as follows.

First, a ratio p of the sum of the

intensities I (00l) of the (00l) reflections to the sum of the intensities I (hkl)

including (00l) is calculated over the range between 10° and 84° of 2θ.

Namely,

I hkl

(00 )

()

27.1

As the degree of orientation is increased, the value of p is increased from p

(a value for a non-oriented case) to 1 (a value for a completely oriented

case). Hence, if F is defined as

–

1 –

27.2

F varies from 0 (non-oriented) to 1 (completely oriented) and F may be used

as a measure of the grain orientation. In this way, F is estimated as 1, 0.98

and 0.95 for Fig. 27.3(a), (b) and (c), respectively, and F increased as the

area ratio γ increased (for example, γ = 8 in Fig. 27.3b and 27.3c).

The HF NCBT-5 sample was almost smoothly deformed at temperatures

of 1100–1150°C, and a thickness contraction rate λ of about 90% was obtained

with good reproducibility. The grain orientation factor F of the Mn-doped

NCBT system was more than 90%. The F values of the respective samples

are given in Table 27.1, with other related values.

27.4.2 Dielectric properties

The anisotropies in the electrical properties of HF samples were measured

by applying an electric field perpendicular [⊥] and parallel [//] to the forging

axis, that is, the electrodes of the HF samples

6,7

were adhered to the faces to

which were applied electric fields perpendicular and parallel to the forging

axis, respectively. Electrodes for studying the temperature dependence of

dielectric properties were made with fired-on silver–palladium (Ag-Pd) paste.

The temperature dependence of the dielectric constant, ε

, and dielectric

loss tangent, tan δ, was measured at 1 or 10MHz by an automated dielectric

measurement system with a multi-frequency LCR meter (YHP 4275A) in

the temperature range of room temperature to 800°C.

The dielectric constants ε

[⊥] and ε

[//] of HF BIT ceramics are

approximately 160 and 130, respectively, at room temperature. The εs [⊥]

increases steeply above 500°C and reaches about 1170 at ca. 680°C, which

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Handbook of dielectric, piezoelectric and ferroelectric materials826

Table 27.1

Elastic, piezoelectric, and dielectric (EPD) constants of the OF and HF (Na

1/2

)

1–

(NCBT–100

) ceramics

Sample NBT + Mn (0.1) NCBT-5 + Mn (0.1) NCBT-5 + Mn (0.2)

HF OF HF OF HF OF

Grain orientation factor

0.98 / 0.91 / 0.93 /

Measured density ρ

(g/cm

) 7.29 7.01 6.92 6.88 7.20 6.65

Curie temperature

(°C) 660 658 665 680 – 677

Dielectric constant

εε

149 140 134 148 132 135

Coupling factor (%)

32.5 14.7 35.6 16.1 40.0 18.8

2.8 3.3 2.75 2.85 2.34 2.39

Frequency constant (Hzm)

2170 2000 2260 1950 2290 1950

2130 1990 2180 1980 2170 1970

Piezoelectric constant

(10

–12

C/N) 33.7 15.6 34.9 18.3 38.3 20.8

2.80 3.49 2.61 3.15 2.17 2.58

(10

–3

V m/N) 25.6 12.6 29.4 13.9 32.8 17.4

2.12 2.81 2.20 2.40 1.86 2.16

(10

–15

/N) 600 60 742 109 988 205

Elastic constant (10

–12

/N)

8.17 9.13 8.11 9.84 7.85 10.2

7.56 9.01 7.58 9.32 7.38 9.72

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Grain orientation and electrical properties of bismuth 827

corresponds to the Curie temperature of single crystal. On the other hand, ε

[//] depends to some degrees on temperature over the whole temperature

range and has a small peak of 270 at ca. 680°C. Dielectric constant ε

[OF]

of OF ceramics shows an intermediate value between ε

[⊥] and εs [//]. The

symmetry of BIT is monoclinic below the Curie temperature of 675 °C.

11,12

In a BIT single crystal,

the dielectric constants ε

and ε

are about 100 and

200, respectively, at room temperature, and their temperature dependences

show sharp peaks of about 1750 at the Curie point, while εc has a value of

100 at room temperature and shows no peak up to 750°C. The dielectric

constants εs [⊥] and εs [//] of HF ceramics have similar temperature

dependences to those of ε

(or ε

) and ε

of a single crystal, respectively, so

that a large anisotropy in the dielectric constant was observed, the ratio of ε

[⊥] to ε

[//] being about 5 at the Curie point.

Figure 27.4 shows the temperature dependence of ε

and tan δ of the HF

PNC-10 ([⊥] and [//]), together with that of the OF sample for the sake of

comparison. A remarkable anisotropy in ε

[⊥] and ε

[//] is observed. The

ratio of ε

[⊥] to ε

[//] increases with temperature rise and reaches a maximum

at T

. In our previous paper

it has been shown that the ratio ε

[⊥]/ε

[//] can

be considered to be proportional to the degree of grain orientation F and has

various values from 4 to 12 at T

depending on the magnitude of F. In our

present measurement on PNC-10, for example, this ratio is found to be 2.0

at room temperature and 26.5 at T

, with F = 0.97. This suggests that PNC

solid solution can be an attractive member of a BLSF family because of its

large anisotropy.

Dielect, const, ε

(×10

)

0 100 200 300 400 500 600 700

Temp. (°C)

PNC–10

Pressure=forging axis

(⊥) (//)

Electrodes

at 1 MHz

(⊥)

(//)

(⊥)

tan δ (%)

27.4

Temperature dependence of ε

and tan δ for the HF PNC-10,

compared with those of the OF samples.

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Handbook of dielectric, piezoelectric and ferroelectric materials828

Figure 27.5 shows the T

of the (Na

1/2

)

1–x

(M = Ba, Ca,

Pb, Sr, and K

1/2

) (NXBT) system as a function of the amount (x) of the

substituted ions. The T

, except the case of M = Ca (NCBT), decrease as the

concentration, x, of the substituted ions increases. On the other hand, the T

of M = Ca (NCBT) increases as the Ca concentration, x, increases. This

suggests

that the T

is related to the change of the lattice distortion b/a as

mentioned above. The T

of CaBi

CBT (x = 1: Ca substituting for

1/2

) reaches a high of 785°C.

Figure 27.6 shows the Curie temperature T

of SBTT2 (x) and SBTT3 (x)

as a function of Ta concentration (x). The T

of the BIT (x = 0) ceramic in

SBTT3 is 680°C, and becomes lower gradually with the increasing amount

of Ta concentration (x). In other words, the ferroelectricity becomes weak

with the increasing amount of Sr and Ta ions, compared with that of the BIT

ceramic. The reason why the T

becomes lower is the increase of the amount

of modified SrTiO

, which has a paraelectric phase at room temperature, for

the perovskite-like unit in BLSF. The T

of the SBTT2 is higher than those

of the SBTT3. On the other hand, the T

of SBTT4(0) (m = 4, SBTi ) is lower

than those of the SBTT3(0) (BIT). It is thought that this tendency of the T

may be caused by the effects of the Bi

layer.

27.4.3 Resistivities

Temperature dependence of resistivity,

, was measured by using a high-

resistance meter (YHP 4329A and 4339B). Resistivity measurements were

Curie temperature

(°C)

800

700

600

500

400

NBT

0.0 0.2 0.4 0.6 0.8 1.0

Substituted ions (mol)

NXBT system

[KBi]

1/2

27.5

Curie temperature,

, of the (Na

1/2

)

1–

(M = Ba,

Ca, Pb, Sr and K

1/2

) (NXBT) system as a function of the amount

(

) of the substituted ions.

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Grain orientation and electrical properties of bismuth 829

made on unpoled specimens. In general, the conductivity of most dielectric

materials increases with increasing temperature. It is essential for the high-

temperature poling process that the conductivity of the material should

somehow be suppressed, otherwise it becomes very difficult for a large

electric field to be built up across the material at a high temperature. Also,

piezoceramics having a high Curie temperature or large coercive field such

as BLSF or PbTiO

must have high resistivity at high temperatures to avoid

thermal breakdown during the poling process.

Figure 27.7 shows the temperature dependence of the resistivity ρ of the

1–x

(Na

1/2

)

[PNC–100x]

system as a function of the reciprocal

absolute temperature. The resistivities of PNC-2 and PNC-5 are 10

or 10

times larger than those of conventional piezoceramics such as PZT and

BaTiO

. The resistivity at room temperature has a very high value of about

Ω cm. Even at 150°C, it ranges from 10

to 10

Ω cm. In Fig. 27.7, the

amount of Ce

in PBT + CeO

(0.11 wt.%) corresponds to that of Ce

PNC-2. PNC solid solution is more effective for realizing high resistivity

than using CeO

as an additive. It seems to suggest that the trivalent cerium

ion Ce

contributes to suppress the increase of electrical conductivity.

As shown in Fig. 27.8, the resistivity ρ [//] of hot-forged samples in the

parallel direction to the forging axis is higher than ρ [⊥] in the perpendicular

direction. We find the value of ρ [OF] for ordinarily fired ones midway

between ρ [//] and ρ [⊥]. The parallel resistivity ρ [//] is about five times as

large as the perpendicular resistivity ρ[⊥] over the range of measurements

that is, ρ[//]/ρ[⊥] = ρ[⊥]/ρ[//] ≅ 5, where σ[⊥] and σ[//] are conductivities.

Curie temperature,

(°C)

900

800

700

600

500

400

300

200

100

SBTi (

= 4)

BIT

SBTT3

SBTT2

SBTa (

= 2)

at 1 MHz

0 0.5 1 1.5 2

Ta concentration (

)

27.6

Curie temperature

of SBTT2 (

) and SBTT3 (

) as a function

of Ta concentration (

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