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

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Handbook of dielectric, piezoelectric and ferroelectric materials500

Cu-embedded multilayer transformers are now being developed. Figure

16.18 illustrates an experimental setup for sintering Cu-electrode-embedded

multilayer transformers in a reduced N

atmosphere. Figure 16.19 shows co-

fired multilayer transformers with pure Ag (right) and Cu (left) electrodes

sintered at 900 °C. The transformer performance will be reported in a successive

paper.

16.8 Summary and conclusions

• There are three loss origins in piezoelectrics: the dielectric, elastic and

piezoelectric losses. The 180

and non-180

domain wall motions contribute

primarily to the extensive dielectric and elastic losses, respectively.

• Heat generation occurs in the sample uniformly under an off-resonance

mainly due to the intensive dielectric loss, while heat is generated primarily

at the vibration nodal points via the intensive elastic loss under a resonance.

In both cases, the loss increase originates from the extensive dielectric

loss change with electric field and/or stress.

• In a ‘hard’ piezoelectric PZT, the mechanical quality factor Q

for the

mode is only 1/5 of that of the k

mode. We integrated the loss

anisotropy with an FEM software code and obtained more accurate

simulation results for the piezo-transducer designing.

• Actuator materials: doping rare-earth ions into PZT–Pb(Mn,X)O

(X =

Sb, Nb) ceramics increases the maximum vibration velocity up to 1m/s,

(s)

PbO (s)

Al (s)

UHP N

= 10

–6

atm.

16.18

Experimental setup for sintering Cu-electrode-embedded

multilayer transformers in a reducing N

atmosphere.

16.19

Multilayer co-fired transformer with hard PZT and Cu (left) or

pure Ag (right) electrode, sintered at 900 °C (Penn State trial

products).

23.83 58.87

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Loss mechanisms and high-power piezoelectric components 501

which corresponds to one order of magnitude higher energy density than

conventionally commercialized piezo-ceramics. To obtain high-power

density/high-vibration velocity materials, domain wall immobility/

stabilization via the positive internal bias field seems to be essential,

rather than the local domain wall pinning effect.

• Transducer materials: the Sb, Li and Mn-substituted 0.8Pb(Zr

0.48

0.52

–

0.16Pb(Zn

1/3

2/3

–0.04Pb(Ni

1/3

2/3

ceramics showed the value

of k

= 0.57, Q

= 1502 (planar mode), d

= 330 pC/N,

εε

= 1653

and the maximum vibration velocity = 0.58 m/s at 31-mode. Low-

temperature sinterable ‘hard’ piezoelectrics were also synthesized based

on the Sb, Li and Mn-substituted ceramics of 0.8Pb(Zr

0.5

–

0.16Pb(Zn

1/3

2/3

–0.04Pb(Ni

1/3

2/3

, by adding CuO and Bi

giving rise to k

= 0.56, Q

(31-mode) = 1023, d

= 294 pC/N, ε

/ε

1282 and tanδ = 0.59%, and vibration velocity 0.41 m/s.

• Cu embedded multilayer piezo-transformers were trial-manufactured under

a low temperature sintering process (900 °C for 2 hours).

16.9 Future trends

Since the above conclusions have been derived only from a limited number

of PZT-based soft and hard piezoelectrics, it is too early to generalize these

conclusions. Further investigations are highly required, including:

• determination of the three (dielectric, elastic and piezoelectric) losses

for various piezoceramics;

• determination of loss anisotropy for piezoceramics;

• loss origin clarification through dynamic domain observations, and the

modeling of loss mechanisms in piezoelectrics;

• high-power density piezoelectric actuators and transducer developments.

16.10 Acknowledgement

Part of this research was supported by the Office of Naval Research through

the grant no. N00014-96-1-1173 and N00014-99-1-0754.

16.11 References

1. K. H. Haerdtl, Ceram. Int l., 8, 121–127 (1982).

2. T. Ikeda, Fundamentals of Piezoelectric Materials Science (Ohm Publication Co.,

Tokyo, 1984), p. 83.

3. N. Setter ed., Piezoelectric Materials in Devices (2002).

4. K. Uchino and S. Hirose, IEEE-UFFC Trans., 48, 307–321 (2001).

5. N. Bhattacharya and K. Uchino, IEEE-UFFC Trans. (2006) [on review].

6. J. Zheng, S. Takahashi, S. Yoshikawa, K. Uchino and J. W. C. de Vries, J. Amer.

Ceram. Soc., 79, 3193–3198 (1996).

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Handbook of dielectric, piezoelectric and ferroelectric materials502

7. N. Uchida and T. Ikeda, Jpn. J. Appl. Phys., 6, 1079 (1967).

8. K. Uchino, Piezoelectric Actuators and Ultrasonic Motors (Kluwer Academic Publ.,

Boston, 1997), p. 197.

9. M. Umeda, K. Nakamura and S. Ueha, Jpn. J. Appl. Phys., 38, 3327–3330 (1999)

10. S. Hirose, M. Aoyagi, Y. Tomikawa, S. Takahashi and K. Uchino, Proc. Ultrasonics

Intl 95, Edinburgh, pp. 184–187 (1995).

11. S. Tashiro, M. Ikehiro and H. Igarashi, Jpn. J. Appl. Phys., 36, 3004–3009 (1997).

12. S. Takahashi and S. Hirose, Jpn. J. Appl. Phys., 32, 2422–2425 (1993).

13. K. Uchino, J. Zheng, A. Joshi, Y. H. Chen, S. Yoshikawa, S. Hirose, S. Takahashi

and J. W. C. de Vries, J. Electroceramics, 2, 33–40 (1998).

14. J. Ryu, H. W. Kim, K. Uchino and J. Lee, Jpn. J. Appl. Phys., 42, No. 3, 1–4 (2003).

15. Y. Gao, K. Uchino and D. Viehland, J. Appl. Phys., 92, 2094–2099 (2002).

16. K. Uchino, Ferroelectric Devices (Marcel Dekker, Inc., New York, 2000), p. 63.

17. Park, S.-H., S. Ural, C.-W. Ahn, S. Nahm and K. Uchino, Jpn. J. Appl. Phys., 45,

2667–2673 (2006).

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17.1 Introduction

–ZnO–Nb

(BZN)-based pyrochlore ceramics are lead-free dielectrics

with high permittivity, low loss and low sintering temperatures. BZN dielectric

ceramics were first developed in the 1970s by Chinese researchers for low

sintering temperature multilayer ceramic capacitors (MLCC) (ZP Wang et

al., 1985; Li et al., 1986). However, the very complex chemical compositions

and unknown complicated phase structures frustrated the early attempts to

make these promising low-sintering dielectrics in practical applications for

MLCC. In the late 1980s, BZN compositions were picked up and studied by

Yan and coworkers in Bell Lab (MF Yan et al., 1990; Ling, 1990). Their

studies involved much simplified chemical compositions using a two-step

powder process technology by mixing together calcined powders of BZN

and Bi

–NiO–Nb

(BNN). The resulting phase structures were still

complex and unclear. From the late 1980s up to now, Yao’s group has been

studying the Bi

–ZnO–Nb

(BZN) system and related dielectric materials

systematically. From their early studies (DH Liu et al., 1993; H Wang et al.,

1994a,b, 1995, 1996a,b, 1997; XL Wang et al., 1997; Cai et al., 1994a,b, c;),

the cubic pyrochlore structure was first recognized as a main phase in this

system in the compositions around Bi

2–2x

2–x

(x = 0.45–0.55), while

another main phase Bi

(Zn

2/3

4/3

with low symmetry in this system was

identified as orthorhombic then monoclinic pyrochlore. The PDF file 54-971

for (Bi

1.5

0.5

)(Zn

0.5

1.5

cubic pyrochlore and 54-972 for Bi

(Zn

1/3

2/3

)

pyrochlore were added to the International Center of Diffraction

Data. The continuous investigation has been focused on the structure–property

relations including phase diagram, phase equilibrium and dielectric property

optimization. Those fundamental works provide a useful understanding of

the BZN material system and accelerate the studies for dielectric properties

improvements and potential applications.

With the recent development in low-temperature co-fired ceramic (LTCC)

devices, the demands for new materials that can be co-fired with base metal

Bismuth-based pyrochlore dielectric

ceramics for microwave applications

HONG WANG and X YAO,

Xi’an Jiaotong University, China

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electrodes at lower temperatures (normally less than 1000 °C) have significantly

increased in the past decade. Bismuth-based pyrochlore dielectrics thus attracted

more and more attention due to their excellent dielectric properties and

lower sintering temperatures and have become promising candidates for

LTCC and microwave passive components since late 1990s (Cann et al.,

1996; Mergen et al., 1996, 1997). High-performance BZN-based temperature

stable dielectrics with low sintering temperature below 940 °C were developed

for multilayer ceramic capacitors (MLCC) (M Chen et al., 1998; Du et al.,

2001). Recent publications reported on the formation, stability, processing

windows and crystallographic characterization in this system as well as

successful manufacturing of prototype devices including LC filters and LTCC

components (H Wang, 1999a, 2004a; Randall, 2003; Zanetti, 2004). The

high tunability found in BZN cubic pyrochlore thin films makes this material

a new candidate for microwave tunable devices which may replace the

conventional (Ba

1–x

)TiO

thin film (Ren et al., 2001). Potential applications

of BZN dielectrics include their use in MLCC, tunable filters, phase shifters,

and electrically steerable antennas.

This chapter aims to review the highlights of the bismuth-based pyrochlore

dielectrics developed so far and discuss the strategy for tailoring both structure

and performance towards applications.

17.2 Crystal structures in the BZN system

The general formula of oxide pyrochlores can be written as A

O′ with

four crystallographically non-equivalent ions which are A (site 16d), B (site

16c), O (site 48f), and O′ (site 8b) (Subramanian, 1983). The space group of

an ideal pyrochlore structure is

Fd3m O

–

and there are eight formula

units per unit cell (Z = 8). Figure 17.1 shows the schematic of 1/4 unit cell

of pyrochlore structure. The pyrochlore can be regarded as a derivative

structure from a defective fluorite structure with anion vacancies on 8a sites.

Table 17.1 gives the atomic coordinate data of an ideal cubic pyrochlore.

Owing to the existence of vacancies on 8a site, the 48f anions thus have a

balance shift towards the two neighboring B cations. The A cations (usually

with ~1 Å ionic radius) are eight coordinated and are located within

scalenohedra (distorted cubes) that contains two equally spaced O′ anions at

a slightly shorter distance from the central cations (A

′

). The smaller B

cations (~ 0.6 Å ionic radius) are six coordinated and are located within

trigonal antiprisms (distorted octahedral, BO

) with all the six anions at

equal distances from the central cation. Thus the pyrochlore structure can be

described as a 3D network with the corner-sharing BO

octahedra and the

eight coordinated A cations (A

′

) locating in the interstices of BO

network (see Fig. 17.2).

The main crystal structures in the BZN ternary system were revealed as a

cubic pyrochlore (α)

Fd3m O

–

, Z = 8 and a low symmetry pyrochlore (β),

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Bismuth-based pyrochlore dielectric ceramics 505

which was indexed as orthorhombic first, then determined as monoclinic

zirconolite-like pyrochlore (C2/c –

) (H Wang et al., 1994b; XL Wang et

al., 1997; Levin et al. 2002b).

17.2.1 Crystal structure of cubic pyrochlore

The cubic pyrochlores in the BZN ternary system were first observed in the

region of Bi

2/3

4/3

4+3x/2

(1 ≤ x ≤ 10/6), Bi

8/3–x

4/3

6+x+2

(1 ≤ x ≤

10/6), and Bi

2–2x/3

2–x/3

(1.5 ≤ x ≤ 1.8) (DH Liu et al., 1993). It was

found to exist in a wide region around the composition of Bi

1.5

ZnNb

1.5

(H Wang et al., 1994b, 1996a; XL Wang et al., 1997), while the ideal pyrochlore

composition Bi

(Zn

1/3

2/3

)

was identified as a low symmetry pyrochlore

(H Wang et al., 1996b, 1997; XL Wang et al., 1997).

The structure of Bi

1.5

ZnNb

1.5

cubic pyrochlore was studied using X-

ray diffraction (H Wang et al., 1994b, 1995). By comparing the theoretically

B [

16c

]

A [

16d

]

O [

48f

]

O′ [

]

Vacancy [

]

17.1

Schematic of pyrochlore structure (1/4 unit cell).

Table 17.1

Pyrochlore (A

O′) structural data (origin on the B site)

Ion Location Site symm. Coordinates

16A 16d D

(0,0,0;0,1/2,1/2;1/2,0,1/2;1/2,1/2,0) +

1/2,1/2,1/2; 1/2,1/4,1/4;1/4,1/2,1/4;1/4,1/4,1/2

16B 16c D

0,0,0;0,1/4,1/4;1/4,0,1/4;1/4,1/4,0

48O 48f C

,1/8,1/8;–

,7/8,7/8;1/4–

,1/8,1/8;3/4+

,7/8,7/8

1/8,

,1/8;7/8,–

,7/8;1/8,1/4–

,1/8;7/8,3/4+

,7/8

1/8,1/8,

;7/8,7/8,–

;1/8,1/8,1/4–

;7/8,7/8,3/4+

′

8b T

3/8,3/8,3/8;5/8,5/8,5/8

for regular octahedra: 0.3125 (5/16)

for regular cube: 0.375 (3/8)

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Handbook of dielectric, piezoelectric and ferroelectric materials506

calculated X-ray diffraction patterns with the observed ones, the chemical

formula of this material was determined as a stuffed (Bi

1.5

0.5

)(Zn

0.5

1.5

pyrochlore with disordered cation distribution (H Wang et al., 1995). The

lattice parameter was refined by the Rietveld method as a = 10.555 Å.

Combining with Raman spectroscopy, the site occupation of

(Bi

1.5

0.5

)(Zn

0.5

1.5

was found to occur in such a way that Zn

is apt

to occupy the B site first and then enters the A site after the B site is fully

occupied (H Wang et al., 2003).

Levin et al. (2002a) have carried out a careful structural investigation on

the Bi

1.5

ZnNb

1.5

composition using combined electron, X-ray and neutron

powder diffraction techniques.

Their results showed small amounts of ZnO

existing in addition to the main cubic pyrochlore phase of Bi

1.5

ZnNb

1.5

The single pyrochlore phase forms at the composition Bi

1.5

0.92

1.5

6.92

Rietveld refinements using neutron powder diffraction data confirmed an

average pyrochlore structure A

O’ (

Fd3m O

–

; a = 10.5616(1) Å)

with both Bi and Zn mixed on the A-sites. Refinements also revealed significant

local deviations from the ideal pyrochlore arrangement which were caused

by apparent displacive disorder on both the A and O′ sites. The best fit was

obtained with a disordered model in which the A-cations were randomly

displaced by ~0.39 Å from the ideal eight-fold coordinated positions. The

refined structural parameters of (Bi

1.5

0.5

)(Zn

0.5

1.5

is presented in

Table 17.2.

17.2

Crystal structure of the BZN cubic pyrochlore (BO

network, 䊉 A

site; octahedral: B site; 䊊 O atom).

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17.2.2 Crystal structure of monoclinic zirconolite-like

structure

The low-symmetry phase in BZN system was identified as orthorhombic

first and then as monoclinic pyrochlore by using X-ray diffraction technology

on ceramic powder (XL Wang et al., 1997; H Wang et al., 2001a). Attempts

to grow the single crystal of this low-symmetry phase were not successful

because this β phase is incongruent (H Wang et al., 2001b). Levin et al.

(2002b) used electron, X-ray and neutron diffraction to investigate the β

phase in detail and obtained useful structural information of Bi

2/3

4/3

composition. The crystal structure of Bi

2/3

4/3

(see Fig. 17.3) was

thus determined as a monoclinic zirconolite-like structure (space group C2/

c –

, a = 13.1037(9) Å, b = 7.6735(3) Å, c = 12.1584(6) Å, β = 101.318(5)°].

According to the structural refinement using neutron diffraction data, Nb

preferentially occupies the six-fold coordinated sites in octahedral sheets

parallel to the (001) planes, while Zn is statically distributed between the

two half-occupied (5+1)-fold coordinated sites near the centres of six-membered

rings of the [Nb(Zn)O

] octahedral. The Nb/Zn cation layers alternate along

the c-axis with Bi-layers, in which Bi cations occupy both eight- and seven-

fold coordinated sites.

16.2.3 Non-stoichiometric pyrochlores and local

crystal chemistry

The pyrochlore with the formula A

is often written as A

O′ to

distinguish the oxygen atoms in the two different networks of BO

and A

O′.

The ‘defect’ pyrochlores form easily since the network of A

O′ can be partially

occupied or even completely absent. In the bismuth-based pyrochlores, the

composition Bi

1.5

ZnNb

1.5

was shown to be a two-phase mixture of pyrochlore

and ZnO. The single-phase cubic pyrochlore structure was obtained at the

composition Bi

1.5

0.92

1.5

6.92

where A sites were assumed to be occupied

by a disordered mixture of Bi

, Zn

and vacancies (Levin et al., 2002a).

Table 17.2

Room temperature structural parameters of (Bi

1.5

0.5

)(Zn

0.5

1.5

pyrochlore (Melot

et al.

, 2006)

Atom Site Occ.

xyzU

Bi/Zn 96

0.125/0.035 0.4689(1) 0.5174(2) 0.5174(2) 1.55(3)

Nb/Zn 16

0.750/0.250 0 0 0 1.32(2)

O′ 96

0.0767 0.3443(2) 0.3443(2) 0.3770(4) 2.2(1)

O48

1 0.31996(4) 1/8 1/8 2.20*

U11

, 3.30(3);

U22

U33

1.64(2);

U23

, 0.90(2). SG (space group).

Fd3m

(No. 227,

origin 2)

= 10.5555(5) Å,

= 1.872. Anisotropic parameters are presented for O.

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The single phase cubic pyrochlores were found mostly in the non-stoichiometric

compositions, such as Bi

1.50

0.45

(Zn

0.46

1.54

, Bi

1.50

0.45

(Zn

0.48

1.52

1.55

0.42

(Zn

0.49

1.51

, Bi

1.60

0.40

(Zn

0.53

1.47

, and Bi

1.63

0.35

(Zn

0.53

1.47

, with Zn

being partially absent on A sites while the B sites

being fully occupied by Nb

and Zn

(Vanderah et al., 2005). The structural

study using Raman spectroscopy revealed that the site occupation of Bi

1.5–

1+2y

1.5

7–y

(–0.3 ≤ y ≤ 0.3) could be explained as Bi

1.5–2y

0.5+2y

(Zn

0.5

1.5

7–y

with Zn

occupying the B site first and then entering the A

site after the B site was fully occupied (H Wang et al., 2003; Du et al.,

2004a). The ZnO deficiency compared to Bi

1.5

ZnNb

1.5

was also confirmed

by stoichiometric study on the compositions Bi

3+y

2–x

3–y

14–x–y

(–0.11 ≤

y ≤ 0.14, –0.03 ≤ y ≤ 0.31) (Tan et al., 2005). Local crystal chemistry and

structure diffused scattering on (Bi

1.5–α

0.5–β

)(Zn

0.5–γ

1.5–δ

7–1.5α–β–γ–2.5δ

and (Bi

1–x

)(M

III

(M = Fe

, In

) were carefully studied by using

electron diffraction technique (Withers et al., 2004; Y Liu et al., 2006, 2007;

Somphon et al., 2006). The fundamental underlying crystal chemistry of

bismuth-based pyrochlore is based on a strong local displacive disorder in

the A

O′ network which gives rise to the dielectric relaxation in the cubic

bismuth-based pyrochlores (Withers et al., 2004; Vanderah et al., 2005; Y

Liu et al., 2007). The displacive disorder in the (Bi

1.5

0.5

)(Zn

0.5

1.5

17.3

Crystal structure of the monoclinic zirconolite-like phase. (BO

network, 䊉 A site; Octahedral: B site; 䊊 O atom)

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(Bi

1.5

0.5

)(Zn

0.5

1.5

and (Bi

1.5

0.5

) (Zn

0.5

1.5

pyrochlores was

studied by time-of-flight neutron powder diffraction (Melot et al., 2006).

Although the precise nature of the disorder in the three pyrochlores is quite

similar, the reported dielectric constants of the three compounds are related

to the extent of local displacement, and (Bi

1.5

0.5

)(Zn

0.5

1.5

with the

largest extent of local atomic displacement of A and O′ is reported to have

the highest dielectric constant.

17.3 Phase equilibrium and phase relation of

BZN pyrochlores

17.3.1 Phase equilibrium and phase diagram

The phase equilibrium, phase formation, and phase relations in the Bi

–

ZnO–Nb

ternary system were investigated (H Wang et al., 1998; Kim

et al., 2002; Tan et al., 2005; Vanderah et al., 2005). The ceramic samples in

a triangular area (see Fig. 17.4) around the cubic pyrochlore phase

1.5

ZnNb

1.5

with the compositions of (Bi

2–3x

)(Zn

2–x

(0 ≤ x ≤

2/3), (Bi

2–x

)(Zn

(5–x)/15

(10+x)/15

)

7–0.3x

(0 ≤ x ≤ 2), (Bi

2–2x

)(Zn

(1–x)/

(2+x)/3

)

7–x

(0 ≤ x ≤ 1) and some other compositions around the ‘ideal’

pyrochlore composition Bi

(Zn

1/3

2/3

were selected and prepared by

conventional powder processing technique (H Wang et al., 1998). Quenching

technology was adopted to keep the high-temperature phase of the samples

after they reached the equilibrium. Based on X-ray diffraction analysis and

thermal analysis, the phase formation was studied from 550 °C to 1050 °C

with a temperature increment of 50 °C. The isothermal phase diagrams have

been obtained. The pure α phase region and β phase region were determined

in different temperatures, while the α–β co-existing phase was found to exist

between the two single phases region. The phase diagrams of pyrochlores in

the Bi

–ZnO–Sb

(Miles et al., 2006), Bi

–NiO–Nb

(Valant et

al., 2005) and Bi

–ZnO–Ta

(Khaw et al., 2007) ternary systems have

also been elaborated.

17.3.2 Phase formation of cubic pyrochlore and

monoclinic pyrochlore

The phase formation study was carried out by X-ray diffraction (XRD)

technique and thermal analysis (XL Wang et al., 1997; H Wang, 1998).

The

results show that Bi

, ZnO and Nb

do not react at 450 °C in the BZN

system. X-ray diffraction lines of the 24Bi

ZnO phase were observed at

500 to 550 °C. The diffraction lines of Bi

disappeared at 550 °C while

those of 24Bi

ZnO increased. This indicates that Bi

reacts with ZnO

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