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

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Handbook of dielectric, piezoelectric and ferroelectric materials680

′

8000

6000

4000

2000

4000

3000

2000

1000

BaTiO

@Al

300 350 400 450 500

Temperature (K)

(a)

tan δ

0.05

0.04

0.03

0.02

0.01

BaTiO

@Al

300 350 400 450 500

Temperature (K)

22.6

Thermal dependences of (a)

′

(real part of the permittivity) and

(b) tan δ (dielectric losses) of BaTiO

@Al

and standard BaTiO

ceramics at 100kHz.

22.7

Scanning electron microscopy (SEM) micrograph of BST–MgO

composite of polished surface showing uniform distribution of BST

and MgO grains (Agrawal

et al

., 2004).

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Interface control in 3D ferroelectric nanocomposites 681

tunability 38% and ε′ ~ 220 close to the room temperature at ±80kV/cm at

100kHz). However, the addition of MgO in BST shifted the Curie temperature

from 2 to –52°C as shown in Fig. 22.8.

More recently, a straightforward and simple way to produce well-densified

ferroelectric ceramic composites with a full control of both architecture and

properties using spark plasma sintering (SPS) was proposed (Elissalde et al.,

2007). In the SPS process, the combination of high pressure, pulsed electrical

current and plasma generation leads to efficient heat transfer (Nygren and

Zhen, 2003; Anselmi-Tamburini et al., 2005a,b; Chen et al., 2005). The

heating source is not external as the electric current applied passes through

the conductive pressurised die containing the powder. Consequently very

high heating rates were reached. SPS consolidation enhances the sintering

kinetics and temperature and the processing time can be significantly reduced

compared to conventional sintering. Thus, the interdiffusion of cations from

the different layers is limited, allowing the preservation of clean interfaces.

Highly densified BST/MgO/BST multilayer ceramics (97%) were obtained

via SPS (Fig. 22.9). Suppression of interdiffusion between the ferroelectric

and the non-ferroelectric phases was possible. The composite exhibited

dielectric losses well below 1% while keeping the Curie temperature of the

ferroelectric part unchanged (Fig. 22.10).

A multilayer design had already been investigated for tunable microwave

devices in the case of Ba

0.5

TiO

/MgO thin film composites (Jain et al.,

2003). Rutherford backscattering (RBS) analysis and SEM observations

22.8

Temperature and frequency dependences of dielectric constant

and loss factor of Ba

0.6

0.4

TiO

–MgO composite with 40:60 wt ratio

(Agrawal

et al

., 2004).

10kHz

100kHz

1MHz

Dielectric constant

200

175

150

125

100

–250 –200 –150 –100 –50 0 50 100

Temperature (°C)

0.05

0.04

0.03

0.02

0.01

0.00

Dielectric loss

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Handbook of dielectric, piezoelectric and ferroelectric materials682

BST

MgO

BST

50 µm

22.9

SEM micrograph of the fracture surface of sandwiched BST/

MgO/BST ceramic obtained by SPS.

0% MgO

1.5% MgO

2.9% MgO

0% MgO

1.5% MgO

2.9% MgO

700

600

500

400

300

200

′

100 200 300 400 500

(K)

(a)

′

12 000

10000

8000

6000

4000

2000

0 100 200 300 400 500

(K)

(b)

tan δ

0.030

0.025

0.020

0.015

0.010

0.005

22.10

Dielectric permittivity (a) and losses (b) of SPS-sintered BST/

MgO/BST stack as a function of the thickness ratio MgO/BST.

MgO

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Interface control in 3D ferroelectric nanocomposites 683

revealed the annealed multilayer film as a 3-0 connectivity composite having

micro-MgO single crystals dispersed in a three-dimensionally connected

BST matrix. The dielectric data, given only at room temperature as a function

of frequency, showed reduced dielectric loss values. Improvement of both

the dielectric losses and quality factor was also reported in sputtered BST@SiO

multilayers (Reymond et al., 2004).

It is worth noting that improvement in these materials in terms of loss

tangent reduction is limited simply because of the ferroelectric materials’

intrinsic losses. A number of studies on bulk microwave ceramics have

shown the possibility of estimating the unavoidable intrinsic microwave

losses using the extrapolation from far IR range (Petzelt and Kamba, 2003).

Petzelt et al. (2003) demonstrated that measurement of dielectric response in

a very wide frequency range including low and high frequencies, microwaves,

submillimetre and IR region is very helpful in understanding the origin of

the dielectric losses in ferroelectric ceramics (Zurmülhen et al., 1995; Petzelt

et al., 2003; Kamba et al., 2004). The main contribution to the dielectric

losses has an extrinsic origin (lattice disorder, point defects, grain boundaries,

etc.). Intrinsic losses are due to phonon-related absorption. In the frequency

range 500–1200GHz, the measured losses are assumed to be of intrinsic

origin and can be described using the phonon parameters defined by R-FTIR

(Fourier transform infrared spectroscopy) measurements near the phonon

resonance. Determination and fitting of the dielectric losses frequency

dependence would then allow extrapolating and predicting the minimum

loss expected down to the microwave range.

22.3.2 Concentration gradients in ceramics

Temperature-stable BaTiO

-based dielectric materials have found extensive

application in the fabrication of multilayer ceramic capacitors (MLCC). To

comply with the X7R specification (Hennings and Rosenstein, 1984), the

main objective is to modify the ferroelectric properties by broadening the

dielectric constant–temperature profile. The relative dielectric constant must

not change by more than ±15% from the room temperature value, over a

temperature range from –55 to 125°C. Small grain size and chemical

inhomogeneity were found to be the main features important in obtaining

such X7R dielectric materials (Rawal et al., 1981). The most popular example

is the gradual increase of ionic substitution on the titanium site of BaTiO

grains from the centre to the surface of each grain. This graded structure,

usually called core–shell structure, is obtained during the processing of the

ceramics through a radial chemical gradient from the core of the grains to the

grain boundary. The local gradient within the shell leads to a distribution of

the Curie temperature. A smearing of the thermal variation of the ferroelectric

properties of BaTiO

(BT) is thus achieved with a highly stable dielectric

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Handbook of dielectric, piezoelectric and ferroelectric materials684

susceptibility. The literature is rich in studies based on both solid state chemistry

and materials science. Hennings and Rosenstein (1984) studied BaTiO

materials sintered with addition of the pseudophase ‘CdBi

’. A complex

core–shell microstructure was determined and the chemical inhomogeneity

was shown to emerge during a process of reactive liquid phase sintering.

Chemical substitutions using oxides such as NiO, ZrO

and Nb

are well

known to modify the BaTiO

ferroelectric properties by controlling grain

size and flattening the transition peak. According to several authors, the

transition temperatures in BaTiO

were also found to be modified by the

ZrO

addition. It was shown that an accumulation of ZrO

at the grain

boundaries leads to suppression of grain growth during sintering which flattened

dielectric response (Maurice, 1984; Armstrong et al., 1989). Depending on

the sintering temperature, ZrO

particles at the grain boundaries or core–

shell grains composed of a pure BT core surrounded by a shell of Zr-modified

material were observed. Flat permittivity in the temperature range 30–

125 °C and significantly reduced dielectric losses were reported. The dielectric

results were discussed on the basis of the microstructural considerations.

Sintering characteristics in the BaTiO

–Nb

–Co

system have been

studied in order to understand the formation mechanism and the stability of

the core–shell microstructure according to the donor/acceptor ratio (Hennings

and Schreinemacher, 1994; Chazono and Kishi, 2000). Yoon et al. (2002)

also studied the core–shell structure in donor (Nb)-rich BaTiO

and acceptor

(Mg)-rich BaTiO

compositions (Fig. 22.11). The grain-coarsening behaviour

during BaTiO

sintering is strongly affected by the donor/acceptor ratio and,

as a result, different dielectric performances are observed. Donor-rich ceramics

exhibit fine-grained microstructure and stable dielectric constant with

temperature. In contrast, a coarse-grained structure is observed in acceptor-

rich ceramics. In this last case, the peak value of the dielectric constant can

be explained by the undoped core region of the grain and the dopant

concentration of the shell.

The sintering behaviour of BT in terms of grain coarsening is modified by

the nature of the dopant. The determination of the dopant’s diffusion mechanism

(diffusion through liquid phase or solid state bulk diffusion) is of main

importance to control both the core–shell structure and the grain growth

during the sintering process. The dielectric performance is closely related to

such microstructural features. It becomes clear that full control of the interfaces,

during both the synthesis and the sintering steps, is essential to obtain tunable

dielectric properties.

Improved control of interfaces in ceramic with compositional gradient

A tentative design on the nanoscale was recently proposed with X7R dielectric

materials prepared by adding Nb

–Co

nanometre oxides to BT powder

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Interface control in 3D ferroelectric nanocomposites 685

0.5 µm

(a)

1234 56789101112

Position

(b)

Atomic ratio (%)

(per (Ba + Ti + Nb + Mg))

22.11

(a) TEM image of the core–shell structure of 6.2. Nb–2.8Mg–BT,

(b) scanning TEM energy dispersive X-ray spectroscopy (STEM/EDS)

results for the Nb and Mg concentrations at the points indicated in

(a) (Yoon

et al

., 2002).

(Yuan et al., 2004). The enhanced dielectric constant and depressed dielectric

losses obtained were attributed to a more uniform distribution of nanometre

dopants in BT grains with as a result the formation of core–shell structure

with thin grain shell. However this work was not supported by SEM or TEM

investigations.

The multidisciplinary approach, described above in the case of ferroelectric–

non-ferroelectric composites, was successfully applied to ceramics with gradient

concentration. Limited interdiffusion between the core and the shell can be

achieved in dense ceramics thanks to a full control of both the design during

synthesis and sintering. This is illustrated by one of the lastest improvements

on the fabrication of dielectric ceramics with locally graded structure proposed

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Handbook of dielectric, piezoelectric and ferroelectric materials686

by Buscaglia et al. (2006a). Their approach consists of the coating of BT

particles with a shell made of SrTiO

or BaZrO

through a precipitation

process (Fig. 22.12). Coagulation of SrTiO

/BaZrO

nanocrystals on the BT

cores was then obtained. Controlled sintering of the as-obtained core–shell

structure was possible thanks to the use of SPS which allows limited

interdiffusion during the thermal treatment (Fig. 22.13). The dielectric

investigations indicate that the dielectric constant can be tuned by adjusting

both the size of the core and the composition of the material via the amount

of coating material. The well-known grain size effect of the BT core on the

final dielectric properties was also considered. Effectively, broadening and

flattening of the dielectric constant–temperature profile arises when the grain

size of the BT is reduced below 1µm. Besides the specific X7R requirements,

continuous advances in the field of Multilayer Ceramic Capacitors (MLCCs)

towards miniaturisation have spectacularly renewed the interest focused on

size effect in nanocrystalline ceramics. Indeed, the need to reduce the layer

thickness while increasing the total number of layers entails a drastic decrease

of ferroelectric grain size (Randall, 2001; Buscaglia et al., 2006b).

22.4 Summary and future trends

The dielectric properties of ferroelectric ceramics were shown to be highly

sensitive to the various interfaces, particularly to charged defect localisation

at grain boundaries. When the materials were prepared in the form of graded

100 nm

22.12

TEM image of a typical BaTiO

@SrTiO

particle (Buscaglia

., 2006a).

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Interface control in 3D ferroelectric nanocomposites 687

materials and composites, this became even more drastic and thus raised

some technological drawbacks. We have reviewed such drawbacks in the

contexts of composites for telecommunications and graded ceramics for

multilayer ceramic capacitors. In both cases, improved control of interfaces

could be achieved through both the softening conditions of processing and

sintering. Soft chemistry routes have been used to prepare core–shell grains

as starting building blocks for composites of well-controlled architecture.

This was useful to decrease the dielectric losses, which are an intrinsic

limitation of ferroelectric ceramics.

Such a control of the nanoscale architecture of ferroelectric-based composites

will be helpful to achieve improved functionality. The first example of this

is the intimate mixing of ferroelectric cores with magnetic nanograins, thus

leading to the coexistence of ferroelectricity and magnetism in a given bulk

composite (Mornet et al., 2007). The next steps will be the tuning of interface

parameters in such composites, thus allowing for a better understanding of

grain boundaries contribution to the effective dielectric parameters in ceramics.

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Handbook of dielectric, piezoelectric and ferroelectric materials688

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