Applications of Ferroelectric Ceramic Materials

Ferroelectric Ceramics : Processing,Properties & Applications

Ahmad Safari, Rajesh K. Panda, and Victor F. Janas

Department of Ceramic Science and Engineering,

Rutgers University, Piscataway NJ 08855, USA

Keywords:

Ferroelectricity, piezoelectricity, pyroelectricity, dielectrics, ceramics, single crystals,

thin films.

Abstract:

Ceramic materials and single crystals showing ferroelectric behavior are being used in many

applications in electronics and optics. A large number of applications of ferroelectric ceramics also

exploit properties that are an indirect consequence of ferroelectricity, such as dielectric,

piezoelectric, pyroelectric and electro-optic properties. This review introduces the basic principles

and characteristics of ferroelectric materials and lists various materials chosen from single

crystals, ceramics, polymers and ceramic-polymer composites which show ferroelectric behavior.

Many different applications arising from ferroelectricity and related phenomena in ceramics and

thin films have been discussed with their processing techniques.

1 : Introduction :

Ferroelectricity is a phenomena which was discovered in 1921. The name refers to certain

magnetic analogies, though it is somewhat misleading as it has no connection with iron (ferrum)

at all. Ferroelectricity has also been called Seignette electricity, as Seignette or Rochelle Salt (RS)

was the first material found to show ferroelectric properties such as a spontaneous polarization on

cooling below the Curie point, ferroelectric domains and a ferroelectric hysteresis loop. A huge

leap in the research on ferroelectric materials came in the 1950's, leading to the widespread use

of barium titanate (BaTiO3) based ceramics in capacitor applications and piezoelectric transducer

devices. Since then, many other ferroelectric ceramics including lead titanate (PbTiO3), lead

zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), and relaxor ferroelectrics like

lead magnesium niobate (PMN) have been developed and utilized for a variety of applications.

With the development of ceramic processing and thin film technology, many new applications

have emerged. The biggest use of ferroelectric ceramics have been in the areas such as dielectric

ceramics for capacitor applications, ferroelectric thin films for non volatile memories, piezoelectric

materials for medical ultrasound imaging and actuators, and electro-optic materials for data

storage and displays.

Applications of Ferroelectric Ceramic Materials http://www.rci.rutgers.edu/~ecerg/projects/ferroelectric.html

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In the past few decades, many books and reviews have been written explaining the concepts of

ferroelectricity in materials [1-12]. In this chapter, an effort is made to introduce the basic

principles governing ferroelectricity and list the various materials which exhibit these properties.

The processing of ferroelectric ceramics in general, with a few examples are described. Finally, a

few important applications of ferroelectric materials are briefly discussed.

2 : General properties of Ferroelectrics :

2.1 : Crystal Symmetry :

The lattice structure described by the Bravais unit cell of the crystal governs the crystal

symmetry. Though there are thousands of crystals in nature, they all can be grouped together

into 230 microscopic symmetry types or space groups based on the symmetry elements [13,14].

Most of the crystals possess symmetry elements in addition to the repetitions expressed by the

crystal lattice. The operation of any single symmetry element of the group leaves the pattern of

symmetry unchanged. In studying the physical properties of crystals, only the orientations of the

symmetry elements and not their relative positions are important. Hence, if only the orientations

of the symmetry elements are taken into account, then the macroscopic symmetry elements in

crystals reduce to a center of symmetry, mirror plane, 1-, 2-, 3-, 4- or 6- fold rotation axes and

1-, 2-, 3-, 4- or 6- fold inversion axes. A combination of these symmetry elements gives us the

macroscopic symmetry also called as point groups. It can be shown by the inspection of the 230

space groups that there are just 32 point groups. As shown in Table 1, the seven crystal systems

can be divided into these point groups according to the point group symmetry they possess.

Table 1 : Point groups for the seven crystal systems.

Crystal

Structure

Point Groups Centro-

Symmetric

Non-centrosymmetric

Piezoelectric Pyroelectric

Triclinic

_

1, 1

_

1

1

1


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Monoclinic

2, m, 2/m

2/m

2, m

2, m

Orthorhombic

222, mm2, mmm

mmm

222, mm2

mm2,

Tetragonal

_ _

4, 4, 4/m, 422, 4mm, 42m, (4/m)mm

4/m, (4/m)mm

_ _

4, 4, 422, 4mm, 42m

4, 4mm

Trigonal

_ _

3, 3, 32, 3m, 3m

_ _

3, 3m

3, 32, 3m

3, 3m

Hexagonal

_ _

6, 6, 6/m, 622, 6mm, 6m2, (6/m)mm

6/m, (6/m)mm

_ _

6, 6, 622, 6mm, 6m2

6, 6mm

Cubic

_

23, m3, 432, 43m, m3m

m3, m3m

_

23, 43m

------

The thirty-two point groups can be further classified into (a) crystals having a center of symmetry

and (b) crystals which do not possess a center of symmetry. Crystals with a center of symmetry

include the 11 point groups labeled centrosymmetric in Table 1. These point groups do not show

polarity. The remaining 21 point groups do not have a center of symmetry (i.e.

non-centrosymmetric). A crystal having no center of symmetry possesses one or more

crystallographically unique directional axes. All non-centrosymmetric point groups, except the 432

point group, show piezoelectric effect along unique directional axes. Piezoelectricity is the ability

of certain crystalline materials to develop an electrical charge proportional to a mechanical stress.

It was discovered by the Curie brothers in 1880. Piezoelectric materials also show a converse

effect, where a geometric strain (deformation) is produced on the application of a voltage. The

direct and converse piezoelectric effects can be expressed in tensor notation as,

Pi = d

ijk s

jk (Direct Effect) (1)

eij

= dkij

Ek (Converse Effect) (2)

where Pi is the polarization generated along the i- axis in response to the applied stress s

jk, and

dkij

is the piezoelectric coefficient. For the converse effect, eij is the strain generated in a

particular orientation of the crystal on the application of electric field Ei along the i-axis. [13]


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Out of the twenty point groups which show the piezoelectric effect, ten point groups (including 1,

2, m, mm2, 4, 4mm, 3, 3m, 6, and 6mm) have only one unique direction axis. Such crystals are

called polar crystals as they show spontaneous polarization.

2.2 : Spontaneous Polarization and Pyroelectric Effect :

The spontaneous polarization is given by the value of the dipole moment per unit volume or by

the value of the charge per unit area on the surface perpendicular to the axis of spontaneous

polarization. The axis of spontaneous polarization is usually along a given crystal axis. Although a

crystal with polar axes (20 non-centrosymmetric point groups) shows the piezoelectric effect, it is

not necessary for it to have a spontaneous polarization vector. It could be due to the canceling of

the electric moments along the different polar axes to give a zero net polarization. Only crystals

with a unique polar axis (10 out of 21 non-centrosymmetric point groups) show a spontaneous

polarization vector Ps along this axis. The value of the spontaneous polarization depends on the

temperature. This is called the pyroelectric effect which was first discovered in tourmaline by

Teophrast in 314 B.C. and so named by Brewster in 1824 [15]. The pyroelectric effect can be

described in terms of the pyroelectric coefficient p . A small change in the temperature D T, in a

crystal, in a gradual manner, leads to a change in the spontaneous polarization vector D Ps given

by,

D Ps = p D T (3)

Figure 1 shows the variation of the spontaneous polarization Ps with temperature for a BaTiO

3

ferroelectric crystal. An increase in the temperature leads to a decrease in the spontaneous

polarization BaTiO3 has a negative pyroelectric coefficient. The polarization suddenly falls to zero

on heating the crystal above the Curie point. [16]


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Fig. 1 : The temperature dependence of spontaneous polarization Ps for BaTiO

3 ferroelectric

crystal. [16]

2.3 : Ferroelectric Domains and Hysteresis Loop :

As described above, pyroelectric crystals show a spontaneous polarization Ps in a certain

temperature range. If the magnitude and direction of Ps can be reversed by an external electric

field, then such crystals are said to show ferroelectric behavior. Hence, all single crystals and

successfully poled ceramics which show ferroelectric behavior are pyroelectric, but not vice versa.

For example tourmaline shows pyroelectricity but is not ferroelectric.

Ferroelectric crystals possess regions with uniform polarization called ferroelectric domains.

Within a domain, all the electric dipoles are aligned in the same direction. There may be many

domains in a crystal separated by interfaces called domain walls. A ferroelectric single crystal,

when grown, has multiple ferroelectric domains. A single domain can be obtained by domain wall

motion made possible by the application of an appropriate electric field. A very strong field could

lead to the reversal of the polarization in the domain, known as domain switching. [17,18]

The main difference between pyroelectric and ferroelectric materials is that the direction of the

spontaneous polarization in ferroelectrics can be switched by an applied electric field. The

polarization reversal can be observed by measuring the ferroelectric hysteresis as shown in Fig. 2.

As the electric field strength is increased, the domains start to align in the positive direction

giving rise to a rapid increase in the polarization (OB). At very high field levels, the polarization

reaches a saturation value (Psat

). The polarization does not fall to zero when the external field is

removed. At zero external field, some of the domains remain aligned in the positive direction,

hence the crystal will show a remnant polarization Pr. The crystal cannot be completely

depolarized until a field of magnitude OF is applied in the negative direction. The external field


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needed to reduce the polarization to zero is called the coercive field strength Ec. If the field is

increased to a more

Fig. 2 : A Polarization vs. Electric Field (P-E) hysteresis loop for a typical ferroelectric crystal.

[12]

negative value, the direction of polarization flips and hence a hysteresis loop is obtained. The

value of the spontaneous polarization Ps

(OE) is obtained by extrapolating the curve onto the

polarization axes (CE).

2.4 : Curie Point and Phase Transitions :

All ferroelectric materials have a transition temperature called the Curie point (Tc). At a

temperature T > Tc the crystal does not exhibit ferroelectricity, while for T < T

c it is ferroelectric.

On decreasing the temperature through the Curie point, a ferroelectric crystal undergoes a phase

transition from a non-ferroelectric phase to a ferroelectric phase. If there are more than one

ferroelectric phases, the temperature at which the crystal transforms from one ferroelectric phase

to another is called the transition temperature. Early research work on ferroelectric transitions

has been summarized by Nettleton [19,20]. Figure 3 shows the variation of the relative

permittivity er with temperature as a BaTiO

3 crystal is cooled from its paraelectric cubic phase to

the ferroelectric tetragonal, orthorhombic, and rhombohedral phases. Near the Curie point or

transition temperatures, thermodynamic properties including dielectric, elastic, optical, and

thermal constants show an anomalous behavior. This is due to a distortion in the crystal as the

phase structure changes. The temperature dependence of the dielectric constant above the Curie


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point (T > Tc) in ferroelectric crystals is governed by the Curie-Weiss law :

e = e 0 + C/(T-T

o) (4)

Fig. 3 : Variation of dielectric constants (a and c axis) with temperature for BaTiO3. [21]

where e is the permittivity of the material, e 0

is the permittivity of vacuum, C is the Curie

constant and To is the Curie temperature. The Curie Temperature T

o is different from the Curie

point Tc. T

o is a formula constant obtained by extrapolation, while T

c is the actual temperature

where the crystal structure changes. For first order transitions To < T

c while for second order

phase transitions To = T

c. [5]

3 : Types of ferroelectric materials :

The types of ferroelectric materials discussed in this chapter have been grouped according to their

structure. The four main types of structures discussed include the corner sharing oxygen

octahedra, compounds containing hydrogen bonded radicals, organic polymers and ceramic

polymer composites.

3.1 : Corner Sharing Octahedra :

A large class of ferroelectric crystals are made up of mixed oxides containing corner sharing


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octahedra of O2-

ions schematically shown in Fig. 4. Inside each octahedron is a cation Bb+

where

'b' varies from 3 to 6. The space between the octahedra are occupied by Aa+

ions where 'a' varies

from 1 to 3. In prototypic forms, the geometric centers of the Aa+

, Bb+

and O2-

ions coincide,

giving rise to a non-polar lattice. When polarized, the A and B ions are displaced from their

geometric centers with respect to the O2-

ions, to give a net polarity to the lattice. These

displacements occur due to the changes in the lattice structure when phase transitions take place

as the temperature is changed. The formation of dipoles by the displacement of ions will not lead

to spontaneous polarization if a compensation pattern of dipoles are formed which give zero net

dipole moment. The corner sharing oxygen octahedra discussed in this chapter includes the

perovskite type compounds, tungsten bronze type compounds, bismuth oxide layer structured

compounds, and lithium niobate and tantalate.

3.1.1 : Perovskites :

Perovskite is a family name of a group of materials and the mineral name of calcium titanate

(CaTiO3) having a structure of the type ABO

3. Many piezoelectric (including ferroelectric)

ceramics such as Barium Titanate (BaTiO3), Lead Titanate (PbTiO

3), Lead Zirconate Titanate

(PZT), Lead Lanthanum Zirconate Titanate (PLZT), Lead Magnesium Niobate (PMN), Potassium

Niobate (KNbO3), Potassium Sodium Niobate (K

xNa

1-xNbO

3), and Potassium Tantalate Niobate

(K(TaxNb

1-x)O

3) have a perovskite type structure. Most of the above are discussed in detail

below.

(a) Barium Titatate (BaTiO3, BT)

Barium titanate (BaTiO3) has a paraelectric cubic phase above its Curie point of about 130° C. In

the temperature range of 130° C to 0° C the ferroelectric tetragonal phase with a c/a ratio of ~

1.01 is stable. The spontaneous polarization is along one of the [001] directions in the original

cubic structure. Between 0° C and -90° C, the ferroelectric orthorhombic phase is stable with the

polarization along one of the [110] directions in the original cubic structure. On decreasing the

temperature below -90° C the phase transition from the orthorhombic to ferroelectric

rhombohedral phase leads to polarization along one of the [111] cubic directions.

The spontaneous polarization on cooling BaTiO3 below the Curie point T

c is due to changes in the

crystal structure. As shown in Fig. 5 the paraelectric cubic phase is stable above 130° C with the

center of positive charges (Ba2+

and Ti4+

ions) coinciding with the center of


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Fig. 4 : (a) A cubic ABO3 (BaTiO

3) perovskite-type unit cell and (b) three dimensional network of

corner sharing octahedra of O2-

ions [12].

Fig. 5 : The crystal structure of BaTiO3 (a) above the Curie point the cell is cubic; (b) below the

Curie point the structure is tetragonal with Ba2+

and Ti4+

ions displaced relative to O2-

ions.

negative charge (O2-

). On cooling below the Curie point Tc, a tetragonal structure develops where

the center of Ba2+

and Ti4+

ions are displaced relative to the O2-

ions, leading to the formation of

electric dipoles. Spontaneous polarization developed is the net dipole moment produced per unit

volume for the dipoles pointing in a given direction [22].

Various A and B site substitutions in different concentrations have been tried to see their effect on

the dielectric and ferroelectric properties of BaTiO3. Sr

2+ substitutions to the A site have been

found to reduce the Curie point linearly towards room temperature. The substitution of Pb2+

for

Ba2+

raises the Curie point. The simultaneous substitution into both A and B sites with different

ions can be used to tailor the properties of BaTiO3. The effect of various isovalent substitutions on

the transition temperatures of BaTiO3 ceramic are shown in Fig. 6 [5, 23-25].

The dielectric properties of BaTiO3 are found to be dependent on the grain size [26-28]. Figure 7

shows the variation of dielectric constant with temperature for BaTiO3 ceramics with a fine (~ 1 m


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m) and coarse (~ 50 m m) grain size. Large grained BaTiO3 (³ 1 m m) shows an extremely high

dielectric constant at the Curie point. This is because of the formation of multiple domains in a

single grain, the motion of whose walls increases the dielectric constant at the Curie point. For a

BaTiO3 ceramic with fine grains (~ 1 m m), a single domain forms inside each grain. The

movement of domain walls are restricted by the grain boundaries, thus leading to a low dielectric

constant at the Curie point as compared to coarse grained BaTiO3 [29]. The room temperature

dielectric constant (er) of coarse grained (³ 10 m m) BT ceramics is found to be in the range of

1500-2000. On the other hand, fine grained ( ~1 m m) BT ceramics exhibit a room temperature

dielectric constant between 3500-6000. The grain size effect on the dielectric constant at room

temperature has been explained by the work of Buessem et. al. [30] and Arlt et. al. [31]

Buessem and coworkers proposed that the internal stresses in fine grained BaTiO3 must be much

greater than the coarse grained ceramic, thus leading to a higher permittivity at room

temperature. Arlt studied the domain structures in BT ceramics and showed that the room

Fig. 6 : The effect of isovalent substitutions on the transition temperatures of BT ceramic [5].


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Fig. 7 : The variation of the relative permittivity (er) with temperature for BaTiO

3 ceramics with

(a) 1 mm grain size and (b) 50 mm grain size. [29]

temperature er reached a peak value at a critical grain size of ~0.7 mm. He concluded that the

enhanced dielectric constant was due to the increased 90o domain wall density. The mobility of

the 90o domain walls in very fine grained BT is hindered and only less than 25 % of the e

r was

achieved.

As the BT ceramics have a very large room temperature dielectric constant, they are mainly used

multilayer capacitor applications. The grain size control is very important for these applications.

(b) Lead Titanate (PbTiO3, PT)

Lead titanate is a ferroelectric material having a structure similar to BaTiO3 with a high Curie

point (490° C). On decreasing the temperature through the Curie point a phase transition from

the paraelectric cubic phase to the ferroelectric tetragonal phase takes place.

Lead titanate ceramics are difficult to fabricate in the bulk form as they undergo a large volume

change on cooling below the Curie point. It is the result of a cubic (c/a = 1.00) to tetragonal (c/a

= 1.064) phase transformation leading to a strain of > 6%. Hence, pure PbTiO3 ceramics crack

and fracture during fabrication. The spontaneous strain developed during cooling can be reduced

by modifying the lead titanate with various dopants such as Ca, Sr, Ba, Sn, and W to obtain a

crack free ceramic. One representative modified lead titanate composition that has been

extensively investigated recently is (Pb0.76

Ca0.24

) ((Co0.50

W0.50

)0.04

Ti0.96

)O3 with 2 mol. %

MnO added to it. This composition has a decreased c/a ratio and Curie point of 255 oC). [32-35]


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(c) Lead Zirconate Titanate [Pb(ZrxTi

1-x)O

3, PZT]

Lead Zirconate Titanate (PZT) is a binary solid solution of PbZrO3

an antiferroelectric

(orthorhombic structure) and PbTiO3 a ferroelectric (tetragonal perovskite structure). PZT has a

perovskite type structure with the Ti4+

and Zr4+

ions occupying the B site at random. The PZT

phase diagram is shown in Fig. 8. At high temperatures PZT has the cubic perovskite structure

which is paraelectric. On cooling below the Curie point line, the structure undergoes a phase

transition to form a ferroelectric tetragonal or rhombohedral phase. In the tetragonal phase, the

spontaneous polarization is along the <100> set of directions while in the rhombohedral phase

the polarization is along the <111> set of directions. As shown in Fig. 9 most physical properties

such as dielectric and piezoelectric constants show an anomalous behavior at the morphotropic

phase boundary (MPB). The MPB separating the two ferroelectric tetragonal and orthorhombic

phases has a room temperature composition with a Zr/Ti ratio of ~ 52/48. PZT ceramics with the

MPB composition show excellent piezoelectric properties. The poling of the PZT ceramic (see

Section 4) is also easy at this composition because the spontaneous polarization within each grain

can be switched to one of the 14 possible orientations (eight [111] directions for the

rhombohedral phase and six [100] directions for the tetragonal phase). Below the Zr/Ti ratio of

95/5 the solid solution is antiferroelectric with an orthorhombic phase. On the application of an

electric field to this composition a double hysteresis loop is obtained. This is because of the strong

influence of the antiferroelectric PbZrO3 phase. [5]

In order to suit some specific requirements for certain applications, piezoelectric ceramics can be

modified by doping them with ions which have a valence different than the ions in the lattice.

Piezoelectric PZT ceramics having the composition at the MPB can be doped with ions to form

"hard" and "soft" PZT's. Hard PZT's are doped with acceptor ions such as K+, Na

+ (for A site) and

Fe3+

, Al3+

, Mn3+

(for B site), creating oxygen vacancies in the lattice [37, 38]. Hard PZT's usually

have lower permittivities, smaller electrical losses and lower piezoelectric coefficients. These are

more difficult to pole and depole, thus making them ideal for rugged applications. On the other

hand, soft PZT's are doped with donor ions such as La3+

(for A site) and Nb5+

, Sb5+

(for B site)

leading to the creation of A site vacancies in the lattice [39-42]. The soft PZT's have a higher

permittivity, larger losses, higher piezoelectric coefficient and are easy to pole and depole. They

can be used for applications requiring very high piezoelectric properties.


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Fig. 8 : The PZT phase diagram. [36]

Fig. 9 : The effect of composition on the dielectric constant and electromechanical coupling factor

kp in PZT ceramics. [5]

(d) Lead Lanthanum Zirconate Titanate ((Pb1-x

Lax)(Zr

1-yTi

y)1-x/4

O3

VB

0.25x O

3, PLZT)

PLZT is a transparent ferroelectric ceramic formed by doping La3+

ions on the A sites of lead

zirconate titanate (PZT). The PLZT ceramics have the same perovskite structure as BaTiO3 and

PZT. The transparent nature of PLZT has led to its use in electro-optic applications. Before the

development of PLZT, the electro-optic effect was seen only for single crystals. The two factors


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that are responsible for getting a transparent PLZT ceramic include the reduction in the

anisotropy of the PZT crystal structure by the substitution of La3+

and the ability to get a pore

free ceramic by either hot pressing or liquid phase sintering.

The general formula for PLZT is given by (Pb1-x

Lax)(Zr

1-yTi

y)1-x/4

O3V

B

0.25xO

3 and

(Pb1-x

Lax)1-0.5x

(Zr1-y

Tiy)V

A

0.5xO

3. The first formula assumes that La

3+ ions go to the A site and

vacancies (VB) are created on the B site to maintain charge balance. The second formula assumes

that vacancies are created on the A site. The actual structure may be due to the combination of A

and B site vacancies.

The room temperature phase diagram of PLZT system is shown in Fig. 10. The different phases in

the diagram are a tetragonal ferroelectric phase (FT), a rhombohedral ferroelectric phase (F

R), a

cubic relaxor ferroelectric phase (FC), an orthorhombic antiferroelectric phase (A

0) and a cubic

paraelectric phase (PC)

The electro-optic applications of PLZT ceramics depends on the composition. Figure 11 shows the

hysteresis loops for various PLZT compositions from the phase diagram. PLZT ceramic

compositions in the tetragonal ferroelectric (FT) region show hysteresis loops with a very high

coercive field (EC). Materials with this composition exhibit linear electro-optic behavior for E < E

C.

PLZT ceramic compositions in the rhombohedral ferroelectric (FR) region of the PLZT phase

diagram have loops with a low coercive field. These PLZT ceramics are useful for optical memory

applications.

Fig. 10 : Room temperature phase diagram of the PLZT system. The regions in the diagram are,

a tetragonal ferroelectric phase (FT); a rhombohedral ferroelectric phase (F

R); a cubic relaxor

ferroelectric phase (FC); an orthorhombic antiferroelectric phase (A

O); and a cubic paraelectric

phase (PC). [46]


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Fig. 11 : Representative hysteresis loops obtained for different ferroelectric compositions (a) FT

(b) FR (c) F

C and (d) A

O regions of the PLZT phase diagram. [46]

PLZT ceramic compositions with the relaxor ferroelectric behavior are characterized by a slim

hysteresis loop. They show large quadratic electro-optic effects which are used for making flash

protection goggles to shield them from intense radiation. This is one of the biggest applications of

the electro-optic effect shown by transparent PLZT ceramics. The PLZT ceramics in the

antiferroelectric region show a hysteresis loop expected from an antiferroelectric material. These

components are used for memory applications. [9, 10, 43-46]

(e) Lead Magnesium Niobate (Pb(Mg1/3

Nb2/3

)O3, PMN):

Relaxor ferroelectrics are a class of lead based perovskite type compounds with the general

formula Pb(B1,B

2)O

3 where B

1 is a lower valency cation (like Mg

2+, Zn

2+, Ni

2+, Fe

3+) and B

2 is a

higher valency cation (like Nb5+

, Ta5+

, W5+

). Pure lead magnesium niobate (PMN or Pb(Mg1/3

Nb2

/3)O

3) is a representative of this class of materials with a Curie point at -10° C. The main

differences between relaxor and normal ferroelectrics is shown in Table 2.

Relaxor ferroelectrics like PMN can be distinguished from normal ferroelectrics such as BaTiO3 and

PZT, by the presence of a broad diffused and dispersive phase transition on cooling below the

Curie point. Figure 12 shows the variation in the dielectric properties with temperature for PMN

ceramic. It shows a very high room temperature dielectric constant and a low temperature

dependence of dielectric constant. The diffused phase transitions in relaxor ferroelectrics are due

to the compositional heterogeneity seen on a microscopic scale. For


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Table 2 : Differences between normal and relaxor ferroelectrics. [47]

Property

Normal Ferroelectric Relaxor Ferroelectric

Dielectric temperature

dependenceSharp 1

st or 2

nd order

transition at Curie point Tc

Broad diffused phase

transition at Curie maxima

Dielectric frequency

dependence

Weak Frequency dependence Strong frequency dependence

Dielectric Behavior in

paraelectric range ( T > Tc)

Follows Curie - Weiss law Follows Curie - Weiss square

law

Remnant polarization (PR) Strong P

RWeak P

R

Scattering of light Strong anisotropy Very weak anisotropy to light

Diffraction of X-Rays Line splitting due to

deformation from paraelectric

to ferroelectric phase

No X-Ray line splitting

Fig. 12 : Variation of the dielectric properties of PMN with temperature. [11]


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example, there is disorder in the B site for Pb(Mg1/3

Nb2/3

)O3. The composition of Mg and Nb is

not stoichiometric in the microregions, leading to different ferroelectric transition temperatures

which broadens the dielectric peak.

The relaxors also show a very strong frequency dependence of the dielectric constant. The Curie

point shifts to higher temperatures with increasing frequency. The dielectric losses are highest

just below the Curie point Tc. For relaxors which have a second order phase transition, the

remnant polarization, Pr, is not lost at the Curie point but gradually decreases to zero on

increasing the temperature beyond Tc [47-50].

The most widely studied relaxor material is the PMN-PT solid solution system. The phase diagram

of PMN-PT is shown in Fig. 13. The addition of PT, which has a Curie point of 490oC, shifts the T

c

of the composition towards higher temperatures. The morphotropic phase boundary composition

(0.65 PMN and 0.35 PT) is piezoelectric in nature. Ceramics with this composition are excellent

candidates for piezoelectric transducers. Compositions with a Curie point near room temperature

(like 0.95 PMN and 0.10 PT) have very large dielectric constants (er > 20,000) which makes them

very attractive for multilayer capacitor and strain actuator applications. As opposed to

piezoelectric strain (see Section 2.1) the electrostrictive component of strain is proportional to the

square of the polarization. Writing the strain in tensor notation,

eij = Q

ijkl P

k P

l (5)

P = eo e

r E (6)

where e is the strain, 'Q' is the electrostrictive coefficient and 'P' is the polarization. In eq. (6) eo is

the permittivity of vacuum, er is the dielectric constant and 'E' is the electric field. As shown in

eqs. (5) and (6), the total strain obtained from electrostrictive effect is a (er)2[51].

3.1.2 : Tungsten Bronze type Compounds :

The tungsten bronze type ferroelectric crystals have a structure similar to tetragonal tungsten

bronze KxWO

3 (x<1). Lead niobate (PbNb

2O

6) was one of the first crystals of the tungsten bronze

type structure to show useful ferroelectric properties. The site occupancy formula for this type of

structure is given by (A1)2(A

2)4(C)

4(B

1)2(B

2)8O

30. Figure 14 shows the schematic of the

projection of the tungsten bronze type structure on the (001) plane. For lead niobate the B1 and

B2 sites are occupied by Nb

5+ ions. The open nature of the structure as compared to the


17 of 40 9/12/2011 4:13 PM

perovskite allows a wide range of cation and anion substitutions without loss of ferroelectricity. At

present, tungsten bronze family of oxide ferroelectrics, numbers more than 85.

Fig. 13 : Phase diagram for PMN-PT solid solution. [51]

Fig. 14 : Schematic diagram showing a projection of the tungsten-bronze structure on the (001)

plane. The orthorhombic and tetragonal cells are shown by solid and dotted lines respectively.

[12]

The ferroelectric crystals grown from solid solutions of alkali and alkaline earth niobates have

shown great potential for being used as a material for laser modulation, pyroelectric detectors,

hydrophones, and ultrasonic applications. The high Curie point (Tc = 560

oC for lead niobate) of

their compounds makes them suitable for high temperature applications.


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It is difficult to fabricate piezoelectric PbNb2O

6 ceramics because of the formation of a stable

nonferroelectric rhombohedral phase on cooling to room temperature. Rapid cooling from the

sintering temperature is used to prevent the formation of the rhombohedral phase. Another

problem associated with this type of materials is the large volume change due to phase

transformation on cooling below the Curie point, leading to cracking of the ceramic [52-53].

3.1.3 : Bismuth Oxide Layer Structured Ferroelectrics :

The two most important piezoelectric materials with the (Bi2O

2)2+

layer structure are bismuth

titanate (Bi4Ti

3O

12) and lead bismuth niobate (PbBi

2Nb

2O

9). As shown in Fig. 15, the structure of

PbBi2Nb

4O

9 consists of corner linked perovskite-like sheets, separated by (Bi

2O

2)2+

layers.

The plate like crystal structure of these compounds leads to highly anisotropic ferroelectric

properties. The ceramics fabricated from the (Bi2O

2)2+

layer compounds do not have very good

piezoelectric properties because of a very low poling efficiency. The piezoelectric properties have

been shown to be improved by grain orientation during the processing step. One fabrication

method involves the tape casting of plate like Bi4Ti

3O

12 and PbBi

2Nb

4O

9 powders. The powders

get aligned during the formation of the green tape. The orientation is further enhanced on

sintering. In the other method, the ceramic is hot forged leading to the orientation of the grains

along the forged direction. [54-56]

The bismuth oxide layer structured ferroelectrics may become important piezoelectric ceramics

because of their higher stability, higher operating temperature (Tc = 550-650

oC), and higher

operating frequency. These ceramics are mainly useful for piezoelectric resonators which need to

exhibit a very stable resonant frequency.

3.1.4 : Lithium Niobate and Tantalate :

Lithium niobate (LiNbO3) and lithium tantalate (LiTaO

3) have similar structures. As shown in Fig.

16 the LiNbO3 structure is actually a variant of the perovskite structure with a much more

restrictive arrangement. The ferroelectric behavior of LiNbO3 and LiTaO

3 was first discovered in

1949. Their crystals are very stable with very high Curie points of 1210° C and 620° C for LiNbO3

and LiTaO3 respectively. These compounds are mainly used in the single crystal form and have

found applications in piezoelectric, pyroelectric and electro-optic devices [57, 58].

3.2 : Compounds Containing Hydrogen Bonded Radicals :


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Several water soluble ferroelectrics usually made in the single crystal form, have hydrogen

bonded radicals in them.

Fig. 15 : One half of the tetragonal (4/mmm) unit cell of PbBi2Nb

2O

9. A denotes the perovskite

double layer (PbNb2O

7)2-

; B is a hypothetical PbNbO3 and C denotes the (Bi

2O

2)2+

layers. [36]


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Fig. 16 : Structure of Ferroelectric LiNbO3 and LiTaO

3. [36]

Potassium dihydrogen phosphate (KH2PO

4, KDP) has a tetragonal point group 42m. Hence this

crystal shows only piezoelectricity and no ferroelectric behavior at room temperature. On

decreasing the temperature below the Curie point (Tc = -150° C) it transforms into a ferroelectric

orthorhombic phase. KDP single crystals exhibit very good electro-optical and non-linear optical

properties [59, 60].

Triglycine sulfate (NH2CH

2COOH)

3H

2SO

4, TGS) has a ferroelectric monoclinic point group 2 at

room temperature. On heating the crystal above the Curie point(Tc = 49.7° C) the crystal

structure changes from the ferroelectric monoclinic point group to a centrosymmetric monoclinic

point group 2/m. The TGS crystals show good pyroelectric properties [61,62].

Rochelle salt (NaKC4H

4O

6 . 4H

2O, sodium potassium tantalate tetrahydrate) was the first

ferroelectric material to be discovered. Between the two Curie points of -18° C and +24° C,

Rochelle salt is ferroelectric with a monoclinic point group 2. In the non-ferroelectric region,

Rochelle salt has an orthorhombic point group 222 and hence it shows piezoelectric effect. Single

crystals of Rochelle salt are widely used for piezoelectric transducers [2].

These water soluble crystals are still used due to their superiority over other crystals in some

properties. Yet these crystals have many deficiencies such as weak ferroelectricity, low Curie

point, poor mechanical properties, and deliquescence. For these reasons, KDP, TGS and Rochelle

salt single crystals are being gradually replaced by piezoelectric ceramics.


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3.3 : Organic Polymers :

Polyvinylidene fluoride (PVDF, (CH2-CF

2)n) and copolymers of PVDF with trifluoroethylene

{P(VDF-TrFE)} have found applications as piezoelectric and pyroelectric materials. The

piezoelectric and pyroelectric properties of these polymers are due to the remnant polarization

obtained by orienting the crystalline phase of the polymer in a strong poling field. Hence the

piezoelectric and pyroelectric properties depend on the degree of crystallinity of the polymer and

the ferroelectric polarization of the crystalline phase [63, 64].

The piezo-polymers have some properties which make them better suited for use in medical

imaging applications. The density of these polymers is very close to that of water and the human

body tissues, hence there is no acoustic impedance mismatch with the body. The piezo-polymers

are also flexible and conformable to any shape. However, there are also some problems

associated with the piezoelectric polymers including the very low dielectric constant (K = 5-10)

which could lead to electrical impedance matching problems with the electronics. The dielectric

losses at high frequencies are very large for these piezopolymers. The polymers also have a low

Curie point and the degradation of the polymer starts occuring at low temperatures (70-100° C).

The poling efficiency is very low for polymer specimens with large thickness (>1mm).

3.4 : Ceramic Polymer Composites :

The drive for piezoelectric composites stems from the fact that desirable properties could not be

obtained from single phase materials such as piezoceramics or piezopolymers. For example, in an

electromechanical transducer, the desire is to maximize the piezoelectric sensitivity , minimize

the density to obtain good acoustic matching with water, and make the transducer mechanically

flexible to conform to a curved surface. Neither a ceramic nor a polymer satisfies these

requirements. The requirement can be optimized by combining the most useful properties of the

two phases which do not ordinarily appear together.

Piezoelectric composites are made up of an active ceramic phase embedded in a passive polymer.

The properties of the composite depend on the connectivity of the phases, volume percent of

ceramic, and the spatial distribution of the active phase in the composite. The concept of

connectivity developed by Newnham et al. [65] describes the arrangement of the component

phases within a composite. It is critical in determining the electromechanical properties of the

composite. Figure 17 shows the 10 different types of connectivities possible in a diphasic

composite. It is shown in the form A-B where 'A' refers to the number of directions in which the

active phase is self connected or continuous. 'B' shows the continuity directions of the passive

phase. The density, acoustic impedance, dielectric constant and the piezoelectric properties like

the electromechanical coupling coefficient kt change with the volume fraction of the ceramic.

Optimum material and piezoelectric parameters for medical ultrasound applications are obtained

for ~ 20-25 vol. % PZT ceramic in the composite. Many excellent reviews on the processing and

properties of piezoelectric / polymer composites have been written in the last few years [66, 67].

The ultrasound imaging application is described in detail in Section 6.1.


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4 : PROCESSING OF FERROELECTRIC CERAMICS :

The final electromechanical properties of ferroelectric ceramic components greatly depend upon

the processing conditions of the ceramic. Each step of processing has to be

Fig. 17 : Connectivity patterns for a diphasic solid A-B. The shaded parts represent the A phase

while the white parts show the B phase. [66]

carefully monitored and controlled to get the best product. A flowchart for a typical ferroelectric

oxide ceramic manufacturing process is shown in Fig. 18.

The raw materials (metal oxides or metal carbonates) are first weighed according to the

stoichiometric formula of the ferroelectric ceramic desired. The raw materials should be of high

purity. The particle size of the powders must be in the submicron range for the solid phase

reactions to occur by atomic diffusion.

The powders are then mixed either mechanically or chemically. Mechanical mixing is usually done

by either ball milling or attrition milling for a short time. Chemical mixing on the other hand is

more homogeneous as it is done by precipitating the precursors in the same container.

During the calcination step the solid phase reaction takes place between the constituents giving

the ferroelectric phase. For example for the calcination of PZT, the starting raw materials PbO,

TiO2 and ZrO

2 are mixed in the molar ratio of 2:1:1, pressed into lumps and then calcined in

ambient air at 800° C to obtain the perovskite phase. The calcining temperature is important as it

influences the density and hence the electromechanical properties of the final product. The higher

the calcining temperature, the higher the homogeneity and density of the final ceramic product.

However calcining PZT at T > 800° C could lead to lead loss, resulting in a detrimental effect on

the electrical properties. So proper calcination at the right temperature is necessary to obtain the

best electrical and mechanical properties [68, 69].

After calcining, the lumps are ground by milling. The green bodies should have a certain minimum


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density before they can be sintered. The desired shape and a minimum green density can be

provided by various techniques including powder compaction, slip-casting, and extrusion. The

choice of the method depends on the type of powder used, particle size distribution, state of

agglomeration, desired shape, and thickness of the part

Fig. 18 : Flowchart for the processing of ferroelectric ceramics.

After shaping, the green bodies are heated very slowly to between 500-600° C in order to remove

any binder present. The binder burnout rate should be £ 1-2 ° C/min in order to allow the gases to

come out slowly without forming cracks and blisters in the ceramic part. After the binder burnout

is over, the samples are taken to a higher temperature for sintering to take place. The sintering

temperature and time should be optimum for proper densification to occur without abnormal

grain growth. The sintering of oxide ceramics must be carried out in an oxidizing atmosphere or

in air. For lead containing piezoelectric ceramics (PZT, PbTiO3, PLZT, etc.) lead loss occurs at

temperatures above 800° C. In order to reduce the lead loss during sintering, the samples are

kept in a sealed crucible with a saturated PbO vapor in it.

The dipoles within a single domain have the same orientation. In ferroelectric ceramics with fine

grain sizes (< 1 m m) each grain is a single domain with the domain wall at the grain boundary. If

the grain size is larger (> 1 m m) then there could be multiple domains in a single grain. As

schematically shown in Fig. 19 (a), when the ferroelectric ceramic is cooled after sintering, it does

not show any piezoelectricity because of the random orientations of the ferroelectric domains in

the ceramic. Piezoelectric behavior can be induced in a ferroelectric ceramic by a process called

"poling". In this process a direct current (dc) electric field with a strength larger than the coercive

field strength is applied to the ferroelectric ceramic at a high temperature, but below the Curie

point. On the application of the external dc field the spontaneous polarization within each grain

gets orientated towards the direction of the applied field, as shown in Fig. 19 (b). This leads to a

net polarization in the poling direction. All the domains in a ceramic can never get fully aligned

along the poling axis because the orientations of the polarization is restricted by the symmetry.

For example, if the material has an orthorhombic perovskite structure then the polarization gets

oriented along one of the eight [111] directions. The higher the number of possible orientations,

the better is the poling efficiency. The reason for the good poling efficiency of PZT ceramic at the

morphotropic phase boundary (MPB) is because of the availability of 14 easy polarization

directions for this composition (eight along the [111] set of directions in an orthorhombic phase

and six along the [100] set of direction in the tetragonal phase) [29].


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Fig. 19 : Schematic of the poling process in piezoelectric ceramics: (a) In the absence of electric

field the domains have random orientation of polarization; (b) the polarization within the domains

aligns in the direction of the applied field.

5 : Applications of Ferroelectric Ceramics :

5.1 : Capacitors :

A capacitor consists of a dielectric material sandwiched between two electrodes. The total

capacitance for this device is given by

C = (e 0e r

A) / t (7)

where 'C' is the capacitance, e 0

is the permittivity of free space, e r is the relative dielectric

permittivity, 't' is the distance between the electrodes, and 'A' is the area of the electrodes.

To get a high volumetric efficiency (capacitance per unit volume) the dielectric material between

the electrodes should have a large dielectric constant, a large area and a small thickness. BaTiO3

based ceramics having a perovskite type structure show dielectric constant values as high as

15,000 as compared to 5 or 10 for common ceramic and polymer materials. The high dielectric

constant BaTiO3 ceramic based disk capacitors are simple to make and have captured more than

50% of the ceramic capacitor market.

The volumetric efficiency can be further enhanced by using multilayer ceramic (MLC) capacitors.

As shown in Fig. 20, the MLC capacitor structure consists of alternate layers of dielectric and

electrode material. Each individual dielectric layer contributes capacitance to the MLC capacitor as

the electrodes terminate in a parallel configuration. Hence the effective equation for capacitance

becomes,

C = (ne 0e r

A) / t (8


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Fig. 20 : Schematic of a typical multilayer ceramic (MLC) capacitor.

where n = number of dielectric layers. The advances in tape casting technology have made it

possible to make dielectric layers < 20 m m thick. This, combined with the use of a high dielectric

constant ceramic like BaTiO3, allows large capacitance values to be achieved in relatively small

volume capacitor devices. Many good reviews have been written on the state of the MLC capacitor

technology and the technical challenges it faces [9, 10, 70, 71].

MLC capacitors are made by the tape casting process. A slurry with a suitable binder/solvent

system is first made from the dielectric ceramic powder. Thin green sheets of the ceramic are

then made by the tape casting process. An ink consisting of an electrode (Pd, Ag-Pd, etc.) and

organics is screen printed on the dielectric sheets and then hundreds of sheets are stacked one

on top of the other. A low pressure at a temperature between ~ 50 and 70° C is applied to

laminate the sheets. These laminates are then diced to form a monolithic green MLC capacitor.

The binder removal is accomplished by heating the green body very slowly to a temperature of ~

300-400° C. The MLC capacitor is then sintered at a high temperature depending on the type of

ceramic. After applying the terminations for the internal electrodes of the MLC capacitors, the

capacitor is mounted on the electronic substrate by soldering.

The critical steps in the fabrication of the MLC capacitors include the formation of thin uniform

green sheets from the ceramic slurry. Any non-uniformity in the thickness could lead to dielectric

breakdown during operation of the device. The co-firing of the ceramic tapes and metal

electrodes should be compatible and should not lead to any reactions between the two.

The high dielectric constants of the relaxors materials such as PMN make them ideal for MLC

capacitor applications. Relaxors have various advantages over BaTiO3 based materials including

higher dielectric constants (e r » 25,000 vs. a maximum e

r of » 15,000 for BaTiO

3 based

ceramics). In addition, the temperature dependence of permittivity is much lower for relaxor

materials, with no microstructure control needed for getting a shallow peak as in BaTiO3. Much

finer grain sizes (£ 1m m) can be used for relaxor ferroelectrics as compared to BaTiO3 (2-5m m)

to get high dielectric constant with low temperature dependence. The electric field dependence of

the dielectric properties is also lower for relaxor dielectrics. Finally, the PbO based relaxors can be


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sintered at T < 1000° C, thus allowing the use of cheaper silver based electrodes instead of

palladium based electrodes used with BaTiO3 based dielectrics which sinter at much higher

temperatures.

In spite of various advantages of relaxor materials over BaTiO3 based dielectrics, they have some

inherent problems which have to be overcome before large scale production takes place. The

dielectric losses for relaxors are the highest just below the Curie point. The permittivity is also

highly frequency dependent in this temperature range. The processing of PbO based relaxor

powders leads to the formation of a low dielectric constant pyrochlore phase, leading to a

decrease in the relative permitivity of the dielectric. Also the relaxor ceramics are mechanically

weak and it is difficult to reproduce the same properties from batch to batch, and control of the

lead rich atmosphere is needed for proper sintering of the relaxor material [29].

5.2 : Applications of Ferroelectric Thin Films :

Ferroelectric thin films have attracted attention for applications in many electronic and

electro-optic devices. Some of the important ferroelectric materials being used for making thin

films include the perovskite type materials such as BaTiO3, PbTiO

3, Pb(Zr

xTi

1-x)O

3 and

Pb(Mg1/3

Nb2/3

)O3. Some of the non-perovskite materials being studied include Bi

4Ti

3O

12 and

(Pb,Ba)Nb2O

6. Applications of ferroelectric thin films utilize the unique dielectric, piezoelectric,

pyroelectric, and electro-optic properties of ferroelectric materials. Some of the most important

electronic applications of ferroelectric thin films include nonvolatile memories, thin films

capacitors, pyroelectric sensors, and surface acoustic wave (SAW) substrates. The electro-optic

devices being studied include optical waveguides and optical memories and displays.

5.2.1 : Ferroelectric Memories :

Semiconductor memories such as dynamic random access memories (DRAM's) and static random

access memories (SRAM's) currently dominate the market. However, the disadvantage of these

memories is that they are volatile, i.e. the stored information is lost when the power fails. The

non-volatile memories available at this time include complementary metal oxide semiconductors

(CMOS) with battery backup and electrically erasable read only memories (EEPROM's). These

non-volatile memories are very expensive. The main advantages offered by ferroelectric random

access memories (FRAM's) include non-volatile and radiation hardened compatibility with CMOS

and GaAs circuitry, high speed (30ns cycle time for read/erase/rewrite) and high density (4(m

m)2 cell size).

As described in Section 2.2, ferroelectric materials spontaneously polarize on cooling below the

Tc. The magnitude and direction of polarization can be reversed by the application of an external

electric field. The FRAM's made from ferroelectric thin films make use of this phenomena to store

data.

Data is stored by localized polarization switching in the microscopic regions of ferroelectric thin


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films. The FRAM's are non-volatile because the polarization remains in the same state after the

voltage is removed (as ferroelectrics have a non-linear hysteresis curve). The radiation hardness

of FRAM's allow for the use of devices containing these memories in harsh environments such as

outer space [72-75].

The fabrication of ferroelectric thin films is done by three main methods including physical vapor

deposition (PVD), chemical vapor deposition (CVD) and sol-gel processing. In PVD, precursors of

the desired film composition are vaporized and deposited on the substrate by one of the

sputtering techniques (i.e. rf magnetron sputtering, pulsed vapor deposition (PLD) etc.). This

process produces films of very high quality. The CVD technique uses volatile chemical precursors

which are vaporized on heated substrates. The advantages of this technique include high

deposition rates, pinhole free films and good stoichiometric control. The sol-gel method involves

the hydrolysis and condensation of organometallic precursors on the substrate. The deposited sol

is annealed at low temperatures to crystallize and densify the film. The main advantages of this

process include low temperature processing and low cost. Some other thin film deposition

techniques that are used include liquid phase epitaxy (LPE), epitaxial growth by melting (EGM),

evaporation methods, molecular beam epitaxy (MBE) and laser ablation [29, 76].

There are some critical processing parameters which have to be taken into consideration during

the fabrication of ferroelectric thin films. The coercive field (Ec) and remnant polarization (P

r)

govern the hysteresis behavior of ferroelectric materials. PZT thin films are used for FRAM's

because of its wide range of switching characteristics (hysteresis) obtained by changing the

composition and deposition method. Hence they have to be chosen depending on the properties

desired. The film thickness should be small to minimize the switching voltage. For PZT films with

a thickness of ~ 200-300 nm the switching can be done at 5V. The interface between the

substrate and the PZT film should be minimized to lower the formation of a non-ferroelectric

region at the interface. The bottom electrode material should not react with the PZT film at the

high temperatures of thin film deposition.

For good performance of the device, the ferroelectric thin film should fulfill certain requirements.

As the FRAM operates on the basis of polarization switching, the ferroelectric material should

have a large remnant polarization and a small coercive field. To obtain it, the PZT composition

and the thin film microstructure have to be optimized. One of the problems with ferroelectric

memories is the tendency to lose its ability to store data after a certain number of read/write

cycles. This phenomena is called fatigue. The PZT thin film FRAM's can withstand 1012

cycles but

a fatigue resistance of 1015

cycles is desired if the FRAM's are to replace semiconductor memories

completely. The length of time a ferroelectric memory can store information (without switching) is

called retention. The aging of polarization with time should be low and retention should be for at

least a few years.

The prospects of FRAM devices replacing the semiconductor memories in the near future are very

bright. A great deal of work still must be done to obtain the optimum composition and

microstructure of PZT thin films, and to study the interaction of the PZT film with the electrode

material. Finally, the storage device should be reliable.

5.2.2 : Electro-optic Applications :

The requirements for using ferroelectric thin films for electro-optic applications include an

optically transparent film with a high degree of crystallinity. The electro-optic thin film devices are


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of two types; one in which the propagation of light is along the plane of the film (optical

waveguides) and the other in which the light passes through the film (optical memory and

displays). These two types of applications are discussed below.

Ferroelectric Thin Film Waveguides :

An optical waveguide controls the propagation of light in a transparent material (ferroelectric thin

film) along a certain path. For the waveguide to work properly, the refractive index of the film

should be higher that that of the substrate. For light to propagate in the waveguide, the thin film

should be optically transparent. This can be achieved by fabricating the film under clean

conditions and aiming for a fine grain size with ultra-phase purity and high density. A great deal

of work has been done on making ferroelectric thin film waveguides from LiNbO3 and Li(Nb,Ta)O

3

using LPE, EGM, and MBE methods. PZT and PLZT thin films are even better candidates for optical

waveguide applications because of their large electro-optic coefficients [77-79].

Ferroelectric Thin Film Optical Memory Displays:

Ferroelectric thin films may replace the use of PLZT bulk ceramics for optical memory and display

applications. The advantages offered by thin films for display applications include a simplification

of the display device design and lower operating voltages as compared to PLZT ceramic devices.

Optical memories using PLZT thin films will also need lower operating voltages.

The material requirements for thin film optical memory and displays include large electro-optic

coefficients and/or strong photosensitivities for the film. PZT and PLZT thin films show a lot of

promise for these optical applications [29].

5.2.3 : Other Ferroelectric Thin Film Applications :

Thin Film Capacitors :

As described in Section 5.1 the high dielectric permittivity of ferroelectric ceramics such as

BaTiO3, PMN and PZT make them very useful for capacitor applications. The MLC capacitors have

a very high volumetric efficiency (capacitance per unit volume) because of the combined

capacitance of thin ceramic tapes (~ 10-20 m m) stacked one on top of the other. The volumetric

efficiency of the MLC capacitor can be further increased if the thickness of the ceramic sheets can

be made lower (< 10 m m). Thin film technology can be used to make dielectric layers as thin as

1 m m. BaTiO3 and PMN are the two important materials being looked at for thin film MLC

applications [80].

Pyroelectric Detectors :

Pyroelectricity is the polarization produced due to a small change in temperature. Single crystals


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of triglycine sulfate (TGS), LiTaO3, and (Sr,Ba)Nb

2O

6 are widely used for heat sensing

applications. The use of ferroelectric thin films for pyroelectric devices is advantageous because of

the high cost of growing single crystals and also the thin film geometry is convenient for device

design. PbTiO3, (Pb,La)TiO

3 and PZT have been widely studied for thin film pyroelectric sensing

applications [81-84].

Surface Acoustic Wave Substrates :

As shown in Fig. 21 surface acoustic wave (SAW) devices are fabricated by depositing interdigital

electrodes on the surface of a piezoelectric substrate. An elastic wave generated at the input

interdigital transducer (IDT) travels along the surface of the piezoelectric substrate and it is

detected by the output interdigital transducer. These devices are mainly used for delay lines and

filters in television and microwave communication applications.

Until recently, only piezoelectric single crystal substrates typically made from LiNbO3, or LiTaO

3

were used as SAW substrates. Surface acoustic wave propagation in PZT thin films have been

reported recently [85-86].

6 : APPLICATIONS OF PIEZOELECTRIC CERAMICS :

6.1 : Medical Ultrasound Applications :

Piezoelectric materials can be used for both active and passive transducer applications. In the

passive mode the transducer acts as a sound receiver i.e. there is conversion of sound energy

into an electrical signal. The converse piezoelectric effect permits a transducer to act as an active

sound transmitter. In the pulse echo mode, the transducer is used to perform both the active and

passive functions at the same time. A sound wave is propagated into the medium and a faint

echo is received back after a small time gap due to the acoustic impedance mismatch between

the interface materials. This principle is used in transducers for ultrasonic medical imaging

applications.


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Fig. 21 : Schematic representation of the generation, propagation and detection of surface

acoustic waves (SAW) on a piezoelectric substrate with interdigital electrode.

A lot of progress has been made in the last twenty years in the area of ultrasonic diagnostic

equipment. This is mainly because of the developments in the area of piezoelectric materials used

as the vibrator material and instrument electronics. Many reviews in this field have been

published in recent years [87-89].

The ultrasound imaging method is superior as the diagnosis can be done without the need of

cutting as in surgery. When compared to X-Ray imaging, it is safer at the low power levels used

in normal scans. It is also helpful in distinguishing between soft tissues which is not possible with

X-Rays. However, X-Ray and ultrasonic imaging can be said to be complementary to each other

as the X-Rays can be used to image the bones which is not possible in biomedical imaging.

As explained earlier, the principle of ultrasonic imaging is based on the pulse echo mode of

operation. The transducer is exited by an electrical signal which in turn produces a vibrational

pulse in the medium to be interrogated (in this case the body). If the ultrasonic wave encounters

an impedance in the direction of propagation, part of the energy is reflected back towards the

transducer. This reflected echo produces a voltage signal which is used to generate the image of

the internal organs and tissues in the body. The acoustic impedance difference between one

tissue to another is small so the vibrational pulse penetrates to larger depths and gives a good

imaging capability.

The applications of medical imaging span a wide frequency range (1.5-30 MHz) depending on the

organs to be imaged. For example the frequency range of 2-5 MHz is used for abdominal,

obstetrical and cardiological applications, 5-7.5 MHz for pediatric and peripheral vascular

applications and 10-30 MHz for intravascular, intracardiac and eye imaging. There is a wide

frequency range because one of the factors limiting the attainable resolution is the wavelength of

the radiation used to form the image.

Wavelength(l) x frequency(f) = velocity of the wave in the medium(v) (9)

As the velocity of sound is 1500 ms-1

in the human body, the resolution varies from l mm - 50mm

for the frequency range of 1.5-30 MHz. Thus there is a balance of good resolution obtained with


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high frequency and high penetration got with a lower frequency. The highest frequency that can

be used for a given application is limited by frequency dependent attenuation ( 0.5 dB/cm.MHz)

within the body [90].

When the ultrasonic wave hits an impedance, for example a tissue, then a fraction R of the

incident energy is reflected back.

R= ((Z2 - Z

1) / (Z

2 + Z

1))

2 (10)

where Z2 and Z

1 are the acoustic impedance of the transducer material and body respectively.

The acoustic impedance 'Z' of a medium is defined as the product of density of the medium and

the velocity of sound in the medium (Z=rc). The units of acoustic impedance is kg/m2.sec and is

called as Rayl. Soft PZT ceramics are widely used as transducers for medical imaging. They have

an acoustic impedance of 30 MRayl which is very high as compared to the human body with an

impedance of only 1.5 MRayl. Hence there is a reflection of the signal at the transducer / body

interface. The losses at the interface could be minimized by using matching layers which have an

impedance which is the geometric mean of impedances of the transducer material and the body.

6.2 : Other Piezoelectric Applications :

Gas Ignitors :

A typical design for a voltage generator for gas ignitors is shown in Fig. 22. It consists of two

oppositely poled ceramic cylinders attached end to end in order to double the charge available for

the spark. The compressive force has to be applied quickly to avoid the leakage of charge across

the surfaces of the piezoelectric ceramic.

The generation of the spark takes place in two stages. The application of a compressive force 'F'

on the poled ceramic (under open circuit conditions) leads to a decrease in the length by dLD. The

potential energy developed across the ends must be higher than the breakdown voltage of the

gap, for sparking to occur. When the spark gap breakdown occurs the second stage of energy

generation starts. The electric discharge across the gap results in a change from open circuit

conditions to closed circuit conditions with the voltage dropping to a lower level. The compliance

of the material increases and allows further compression of the ceramic by (dLE - dL

D) where dL

E

is the displacement that would have occurred if the force 'F' had been applied under short circuit

conditions. The combination of the strains from the open and short circuit conditions produce

more energy that can be dissipated in the spark. Usually PZT ceramic disks are used for this

application [11].

Displacement Transducers :

When force is applied to a long piezoelectric cantilever beam, one side is in tension while the

other side will be in compression. No electrical output can be obtained from this


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Fig. 22 : A piezoelectric spark generator.

homogenous body by bending. Bimorphs made with two halves of separate beams with an

electrode in between as well as on the top and bottom surfaces is shown in Fig. 23. If the beams

are poled in the opposite direction then on the application of a force 'F' the voltage generated on

the outer electrodes will be additive (Fig. 23 (a)). If the beams are poled in the same direction,

the additive output can be obtained by connecting the outer electrodes and the center electrode

as shown in Fig. 23 (b).

Conversely the application of a voltage to the bimorph causes it to bend. As shown in Fig. 24. the

application of an electric field causes one half of the cantilever beam to expand and the other half

to contract. If they are joined in the form of a bimorph it leads to a net displacement on the

application of an electric field [11].

Fig. 23 : Cantilever bimorphs with (a) a series connection and (b) parallel connection of beams.


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Fig. 24 : Schematic of a bimorph showing (a) two free halves of the bimorph; (b) teh free halves

in an electric field; (c) the two free halves shown in (a) are joined to form a bimorph.

Accelerometers :

An accelerometer is a device which gives an electrical output proportional to the acceleration. A

shear mode accelerometer is shown in Fig. 25. The transducer is a piezoelectric cylinder which is

poled along its axis but has its poling electrodes removed and the sensing electrodes applied to

its inner and outer surfaces. The cylinder is joined to the fixed central pole on the inside and a

cylindrical mass on the outside. When an axial acceleration takes place the cylinder is subjected

to a shear force between the outer mass and inner pole. Any motion in the radial direction does

not give any output as the d11

piezocoefficient for PZT ceramic accelerometer is zero. So the

device is highly directional [11].

Piezoelectric Transformers :

Low voltage to high voltage transformation can be done by using a piezoelectric plate. Fig. 26

shows a flat plate having electrodes on half of it larger face and on an edge. The region between

the larger face electrodes and the edge electrode are poled separately. A length mode resonance

is excited by applying a low AC voltage source between the larger face electrodes. The step up

voltage ration would be proportional to the ratio of the input to output capacitance and the

efficiency of the device. This principle has been used for making EHT transformers for miniature

television receivers [11].

Impact Printer Head :

Dot matrix impact printers driven by multilayer piezoelectric ceramic actuators have been

successfully produced on a large commercial scale. As shown in Fig. 27, the printing pin element

consists of a piezoactuator, a stroke amplifier operated on the lever principle and a printing wire.

When a pulse with a peak voltage of 150 V is applied to the piezoelectric actuator, the printing

wire moves by about 400 mm, making the tip of the wire to hit the paper through the ink ribbon.

The advantages of the dot matrix printer over the conventional electromagnetic drive printers


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includes a higher printing speed (double the speed because of the presence of 24 printing pin

elements) and half the power consumption as compared to the latter printer [91].

Fig. 25 : A shear mode accelerometer.

Fig. 26 : A piezoelectric transformer with the arrows indicating the poling directions.

Precision XYq Stage :

Mechanical stages with fine scale movements have been developed using piezo-actuators. They

are made up of a linear X-Y stage and a rotational q stage. The linear stage moves independently

in the X and Y directions using two piezo-actuators with active feedback control from precision

laser interferometeric position sensors. The performance of the XYq stage is given by : X-Y

displacement range of 30 x 30 mm and q -rotation of 270 mrad; a linear and angular resolution of

0.01 mm and 0.9 mrad respectively and a time response of < 30 msec. These stages are being

used for synchrotron radiation lithography and are expected to be used for Very Large Scale

Integration (VLSI) manufacturing in the future [92].


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Fig. 27 : Schematic of a printing pin element.

7 : SUMMARY :

Ceramic materials and single crystals showing ferroelectric behavior are being used in many

applications in electronics and optics. In this chapter, an effort has been made to introduce the

basic principles governing ferroelectricity and list various materials which exhibit these properties.

The processing of ferroelectric ceramics have also been described, as have been a number of the

key applications for ferroelectric materials. Finally, a number of applications of ferroelectric

ceramics exploiting properties that are an indirect consequence of ferroelectricity, such as

dielectrics, piezoelectricity, and pyroelectricity, are described.

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