Moulson A.J., Herbert J.M. Electroceramics: Materials, Properties, Applications

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The sintered product usually has a density higher than 95% of theoretical and

a crystallite size in the 5–30 mm range. Both an excess and a deﬁciency of PbO

result in inferior piezoelectric properties so that careful control of all aspects of

the manufacturing process is essential.

Electrodes are applied after any necessary machining to shape or ﬁnishing. For

the majority of applications a suitably formulated silver-bearing paint is ﬁred on

at 600–800 8C. In the case of thin pieces, Ni–Cr or gold electrodes can be applied

by sputtering or evaporation.

Poling takes place with the specimens immersed in transformer oil at 100–

150 8C and with an applied ﬁeld of 1–4 MV m

. In the case of BaTiO

the ﬁeld

must be maintained whilst cooling to some 50 8C below the Curie point. For PZT

the temperature and voltage are optimized to give the maximum piezoelectric

coeﬃcients without allowing the leakage current to reach levels that could result

in thermal runaway and electrical breakdown. Higher ﬁelds can be used if they

are applied as a succession of short pulses.

Corona poling can also be used. In this case voltages of order 10

V are applied

to either a single needle or an array of needles with their points a few millimetres

from the ceramic surface; the opposite side of the ceramic is earthed. In this way

a high ﬁeld is established in the ceramic. The advantage that corona poling oﬀers

is a diminished risk of electrical breakdown. This is because the poling charge

cannot be quickly channelled to a ‘weak spot’, as it could be if it were to be

carried on a metallic electrode.

6.4 Important Commercial Piezoceramics

Compositions, composites and devices have been developed for the four main

uses of piezoceramics:

1. the generation of voltages;

2. electromechanical actuation;

3. frequency control;

4. the generation and detection of acoustic and ultrasonic energy.

The speciﬁc requirements to optimize performance for the various applications

are discussed in the following sections.

6.4.1 Barium titanate

BaTiO

was the ﬁrst material to be developed as a piezoceramic and was used on

a large scale for application 4 above. For most commercial purposes it has been

362 PIEZOELECTRIC CERAMICS

TEAMFLY

Team-Fly

superseded by PZT, although its ‘lead-free’ composition is attractive from the

‘health and safety’ perspective.

The structural transitions which occur in BaTiO

(see Section 2.7.3) are

accompanied by changes in almost all electrical and mechanical properties. The

transition temperatures can be altered by substitutions on the A and B sites, and

for many applications it is necessary to move them away from the working

temperature ranges so that the associated large temperature coeﬃcients are

avoided. The cubic–tetragonal and orthorhomic–rhombohedral transitions occur

well away from normal working temperatures, but the tetragonal–orthorhombic

transition occurs close to them. The substitution of Pb and Ca for Ba lowers this

transition temperature and has been used to control piezoelectric properties

around 0 8C which is important for underwater detection and echo sounding.

The substitution of Zr or Sn for Ti raises both the tetragonal–orthorhomic and

orthorhomic–rhombohedral transitions. Raising the former to above the

working-temperature range yields enhanced piezoelectric properties provided

that the poling is eﬀected below this temperature range and that it is not

subsequently exceeded. In the past such compositions have found widespread use

as bimorphs for record-player pick-up cartridges.

Technically pure BaTiO

(Fig. 6.12), or BaTiO

doped with isovalent

substituents, generally has too high a loss at the high ﬁeld strengths (0.2–

0.4 MV m

) required to generate useful ultrasonic powers. The dielectric loss

arises largely from the movement of domain walls. The control of the high-ﬁeld

loss is therefore a matter of controlling domain wall movement.

As discussed above, the introduction of acceptor dopants leads to domain wall

clamping. Manganese reduces the low-ﬁeld loss (see Section 5.7.1) but has little

eﬀect at high ﬁelds. Co

substituted for Ti at the 1–2at.% level is particularly

eﬀective, as shown in Fig. 6.12. Compositions containing cobalt must be ﬁred in

a fully oxidizing atmosphere since Co

is easily reduced to Co

. This change in

oxidation state is accompanied by a change in colour from blue-black to green

and ultimately to yellow, and an almost complete loss of piezoelectric properties.

Since the polar axes in barium titanate and PZT (see Fig. 2.40(b) and Fig. 2.44)

are longer than the perpendicular axes, ceramics expand in the polar direction

during poling. The application of a high compressive stress in the polar direction

to a poled ceramic causes depoling since the 908 domains switch direction as a

result of the ferroelastic eﬀect and the polar directions of the crystallites become

randomized.

In BaTiO

less than 10% of 908 domains are permanently altered in their polar

direction by poling, whereas some 40%–50% of 908,718 and 1098 domains are

aﬀected in PZT compositions. It is therefore understandable that BaTiO

shows

a greater resistance than PZT to depoling by compressive stresses, and this

resistance is particularly strong in cobalt-doped material, especially after a

period of ageing. This further illustrates the clamping eﬀect of Co

ions on

domain walls (see Section 6.3.2). Although iron doping has a similar eﬀect in

IMPORTANT COMMERCIAL PIEZOCERAMICS 363

PZT it has not been found possible to obtain a PZT ceramic as resistant to high

pressures as BaTiO

. For this reason the use of cobalt-doped BaTiO

for

producing high acoustic powers has continued in some applications, despite its

inferior piezoelectric activity.

6.4.2 Lead zirconate^lead titanate (‘PZT’)

PbTiO

(Curie point, 495 8C) has a similar tetragonal structure to BaTiO

but

with a c axis approximately 6% longer than its a axis at room temperature. The

high internal stresses developed on cooling a sintered ceramic through Tc cause

disintegration. PbZrO

(Curie point 234 8C) is orthorhombic with a structure

similar to that of orthorhombic BaTiO

but is antiferroelectric, i.e. the dipoles

due to a displacement of the Zr

ions from the geometric centre of the

surrounding six O

ions are alternatively directed in opposite senses so that the

spontaneous polarization is zero.

The replacement of zirconium by isovalent hafnium has little eﬀect apart from

shifting the morphotropic boundary (see Fig. 6.7) to 52mol.% PbTiO

Replacement by tin causes a slight loss in piezoelectric activity, shifts the

morphotropic boundary to 42 mol.% PbTiO

and lowers the Curie point from

370 to 250 8C.

The isovalent A-site substituents barium, strontium and calcium lower the

Curie point and have a small inﬂuence on the morphotropic composition. At the

5–10 mol.% level they enhance the permittivity and piezoelectric properties.

0.94

0.06

0.47

0.53

for instance has a relative permittivity of 1300, a k

of 0.58

364 PIEZOELECTRIC CERAMICS

Fig. 6.12 Dependence of tan d on the applied ﬁeld strength for barium titanates: curve A,

technically pure barium titanate; curve B, barium titanate containing lead and 1–2 at.% Ti

replaced by Co

, unpoled; curve C as B but poled.

and a Curie point of 328 8C compared with values of 730, 0.53 and 386 8C for the

unsubstituted composition.

The mechanism whereby aliovalent substituents may aﬀect properties has

already been discussed. Doping PZT with iron restricts domain movement in

much the same way as cobalt in BaTiO

does and results in the low loss at high

ﬁelds necessary in the generation of high-energy vibrations. Uniaxial compressive

stresses below the level that cause depoling raise the permittivity and dissipation

factor in iron-doped PZT, presumably as a result of the response of the

additional 908 walls generated by the applied stress.

Figure 6.13 shows a plot of charge against compressive stress for donor-doped

PZT. The output is considerably in excess of what would be expected from the

piezoelectric coeﬃcient determined at low stress. The excess charge is to be

attributed to the switching of 908 domains. Up to a limiting stress X

, the charge

is reabsorbed on releasing the stress, indicating that the 908 domain wall

movement is reversible, but at higher stresses only a part is reabsorbed and the

low-stress piezoelectric coeﬃcient is correspondingly progressively diminished.

The longer the time a stress is applied, the greater is the charge released and the

smaller is the fraction of it that is reabsorbed. The extra output due to the

reversible movement of 908 walls is made use of in high-voltage generators used

for the ignition of gas by sparks (see Section 6.5.1).

Donor-doped PZTs have higher permittivities and d coeﬃcients than acceptor-

doped materials and are therefore more suitable for converting mechanical into

electrical vibrations. They have higher dissipation factors than acceptor-doped

materials and are therefore not as suitable for wave ﬁlters. If this were not the

case, their low ageing coeﬃcients would be an advantage.

IMPORTANT COMMERCIAL PIEZOCERAMICS 365

Fig. 6.13 Electric displacement as a function of compressive stress for donor-doped PZT: - - -,

output expected from a low-stress piezoelectric constant.

Low temperature coeﬃcients of the resonant frequency can be obtained by

substituting small amounts of an alkaline earth for lead in acceptor-doped

material and adjusting the ratio of zirconium to titanium, which governs the

ratio of tetragonal to rhombohedral phases in the ceramic. The ageing that

accompanies acceptor doping can be greatly reduced by substituting small

amounts (51%) of Cr or U on the B site, although the mechanism has not been

elucidated. The ions are probably present as Cr

and U

or U

and, since

they slightly reduce the permittivity and piezoelectric activity, it can be assumed

that they accelerate the orientation of the acceptor ion–oxygen vacancy dipoles

so that it is virtually completed during poling.

Some applications require low mechanical as well as low dielectric losses

combined with high piezoelectric activity. This seems to be best achieved by

substituting a mixture of B-site donors and acceptors such as Nb and Mg in 2:1

atomic proportions.

6.4.3 Lead-based relaxor piezoelectric and electrostrictive

ceramics

There are many factors which determine the suitability of a particular

piezoceramic for a given transducer application. As a general rule the higher

the Curie point (T

) the lower are the room temperature values of permittivity

and charge (or strain) coeﬃcient. However, the low transition temperatures,

which favour high piezoelectric coeﬃcients, carry with them the risk of depoling

during the processing steps, for example machining, electroding and soldering,

usually necessary to engineer the material into a device.

The lead-based perovskite relaxor ferroelectrics, Pb(B’B’’)O

, (see Section

5.7.2) have exceptionally high permittivity values and therefore, from Eq. (6.7)

are expected to be strongly electromechanically active and especially suited to

transducer applications.

The search for piezoceramics having optimum electromechanical properties

has been extended to cover MPB compositions of PbTiO

with various relaxors.

The search is assisted by using a relationship established between the Curie point

of the MPB composition and the tolerance factor (see Section 2.7.3) for the

relaxor component [10].

The article by S.-E. Park and T.R. Shrout [11] provides a valuable supplement

to the discussion.

Polycrystalline ceramics

The ternary diagram (Fig. 6.14) is essentially the PZT binary diagram extended

to take account of additions of a relaxor of the type Pb(B’ B’’)O

366 PIEZOELECTRIC CERAMICS

The system now shows two morphotropic phase boundaries, the one already

discussed on the PZ–PT join, and the other on the PT–Pb(B’B’’ )O

join.

Compositions close to that of the MPB (II) have attracted considerable interest

because of their potential for electromechanical applications. The properties of

some selected systems are given in Table 6.2 together with those for the ‘PZT’

system for comparison.

As discussed in Section 5.7.2, in the case of niobium-containing perovskites it

is necessary to avoid the formation of pyrochlore-type phases if reproducible and

optimum dielectric and piezoelectric properties are to be achieved. G. Roberts

et al. [13] developed a modiﬁcation of the B-site precursor (‘columbite’ process)

route to produce high quality PNN–PZT ceramics.

IMPORTANT COMMERCIAL PIEZOCERAMICS 367

MPB (II)

Fig. 6.14 Room temperature ternary showing structural phase-ﬁelds in the PZ–PT–relaxor

system.

(I)

Table 6.2 Piezoelectric properties of some ‘relaxor based’ compositions close to the MPB(II)

(data taken from [11])

Composition x at MPB(II) k

/pC N

/10

/8C

(1  x) PMN–xPT 0.33 0.73 690 5 160

(1  x) PST–xPT 0.45 0.73 655 4 205

(1  x) PSN–xPT 0.43 0.76 504 2.5 248

0.5 PNN–0.345 PT–0.155PZ

0.40 0.80 900 7 (90

)

(1  x) PZ–xPT

0.48 MPB (I) 0.64–0.75 220–600 1–3.4 195–365

PMN ¼Pb(Mg

1/3

2/3

; PST ¼Pb(Sc

1/2

; PSN ¼Pb(Sc

1/2

;

PNN ¼Pb (Ni

1/3

2/3

;PZ¼PbZrO

;PT¼PbTiO

(a) Taken from [12].

(b) Estimated.

Although the polycrystalline relaxor-based compositions have useful piezo-

electric characteristics, taking their properties overall into account they do not

oﬀer signiﬁcant advantages over the well established PZT system. Their high

electrostriction coeﬃcients make them attractive for certain actuator applica-

tions (see Section 6.5.2), but the potentially important advance has been the

production of single crystals.

Through additions of PbTiO

to the prototype relaxor Pb(Mg

1/3

2/3

the

temperature at which the permittivity peaks can be adjusted and, close to the

composition 0.9 PMN–0.1 PT, it occurs at around room temperature. This is a

composition suited to actuator applications. The dependence of electrostriction

on grain size (Fig. 6.15) [14] is believed to be linked to the dependence of the size

of the nanosized ordered regions on grain size (T. Mishima et al. [15]).

Single crystals

Eﬀorts to grow usefully sized PZT single crystals with the prospect of their

oﬀering signiﬁcantly enhanced electromechanical properties compared to those

of the polycrystalline counterparts, have not been successful. However, in the

case of the ‘relaxor-PT’ systems large single crystals can be ﬂux-grown (see

Section 3.11) and these do oﬀer exceptionally good electromechanical properties

(Table 6.3) which are, of course, dependent upon the ‘cut’ of the piece with

respect to the crystallographic axes of the single crystal.

368 PIEZOELECTRIC CERAMICS

Fig. 6.15 Electrostriction in 0.93 PMN 0.07 PT showing the dependence on grain size (after

T.R. Shrout et al. [14]).

6.4.4 Lead niobate

The ferroelectric polymorph of lead niobate (PbNb

) is metastable at room

temperature. Above 1200 8C the structure is tetragonal tungsten bronze. On

cooling slowly below 1200 8C it transforms to an orange-brown rhombohedral

form that is paraelectric at room temperature. Rapid cooling from 1200 to

700 8C, combined with additions such as 2 wt% ZrTiO

, allows the tetragonal

form to persist at lower temperatures. It is bluish-green in colour with a Curie

point at 560 8C below which it undergoes a small orthorhombic distortion in the

c plane and becomes ferroelectric. The tungsten bronze structure is typiﬁed by

certain alkali metal compounds of tungstic oxide, e.g. K

5.7

The spaces between the MO

(MW, Ta, Nb, Ti etc.) form tunnels. In the

section shown in Fig. 6.16 the largest tunnels, A2, have ﬁve O

ions round their

periphery. The somewhat smaller tunnels, A1, have four peripheral O

ions,

while the smallest, A3, have three. Lower-valency cations are located in sites in

these tunnels above and below the plane shown. The MO

groups provide two

distinguishable B sites shown as B1 and B2. The general formula for a tungsten

bronze is

½ðA1)

ðA2Þ

A3½ðB1Þ

ðB2Þ

O

The A3 sites are vacant unless small (r

¼50–70 pm) ions such as Li

or Mg

are present. A large number of compounds are known in which one in six of the

A1 and A2 sites is also vacant, giving rise to the simpler formula AB

of which

PbNb

is an example.

In the majority of ferroelectric tungsten bronzes the polar axes are parallel to

the A-site tunnels. PbNb

is an exception; below the Curie point it undergoes

an orthorhombic distortion in the plane perpendicular to the A-site tunnels as

indicated in Fig. 6.16. It possesses four possible polar directions and can be poled

successfully in ceramic form.

Although it is diﬃcult to obtain a ceramic with less than 7% porosity and the

piezoelectric coeﬃcients are smaller than those of BaTiO

, PbNb

has

advantages in certain applications. Its Q

value is approximately 11, which is

IMPORTANT COMMERCIAL PIEZOCERAMICS 369

Table 6.3 Electromechanical properties of some relaxor-based single crystals (data taken

from [11])

Crystal Cut e

/pC N

71{

Pb(Zn

1/3

2/3

111 900 0.38 83

001 3600 0.85 1100

PZN–8 mol.% PT* 111 2150 0.40 82

001 4200 0.94 2070

PMN–35 mol.% PT 001 3100 0.92 1240

*The MPB lies at 10 mol.% PT.

{

Eﬀective d-coeﬃcient.

advantageous in broad-band applications, it can be heated up to 500 8C without

losing its polarity and its transverse coeﬃcients are low (d

¼9pCN

1

). The

latter property favours low sideways coupling between detecting elements

formed side by side on the same piece of ceramic and results in a hydrostatic

coeﬃcient d

of useful magnitude (d

¼ d

þ 2d

). It also has the advantage of a

lower PbO vapour pressure during sintering so that guarding against lead loss is

less of a problem than with PZT.

The substitution of lead by barium enhances the piezoelectric properties which

peak when Pb/Ba  1. At this and higher barium contents the structure changes

to tetragonal with the polar axis parallel to the A-site tunnels. There is a

morphotropic boundary, similar to that found in PZT compositions, and peak

values of piezoelectric properties are found near the Pb

1/2

composi-

tion. d

rises to 220 pC N

and d

rises to 790 pC N

, while Q

increases to

300 and the Curie point falls to 250 8C; thus most of the features peculiar to

370 PIEZOELECTRIC CERAMICS

Fig. 6.16 Projection of the tungsten bronze structure in the c plane. The inset shows the polar

directions in orthorhombic PbNb

: . M; *O [P.B. Jamieson et al. (1968) J. Chem. Phys.

48, 5048–57].

PbNb

are lost. The properties are intermediate between those of BaTiO

and

PZT but there has been little attempt to exploit the material commercially.

6.4.5 Lithium niobate and lithium tantalate

Lithium niobate and lithium tantalate have similar properties and their crystal

structure is similar to that of ilmenite. It consists of corner-sharing MO

groups

(MNb or Ta) that share one face with an LiO

group and an opposite face with

an empty O

octahedron. Figure 6.17 shows diagrammatically the positions of

and M

ions between layers of hexagonally close-packed O

ions. The

and Li

ions are displaced from the centres of their octahedra in opposite

directions in the ferroelectric phases. When the polarization is reversed both

cations move in the same direction, the M

ion to the other side of the midplane

between the O

ions and the Li

ion through to the other side of the adjacent

plane. There are only two possible polar directions and they are at 1808 to

one another. As a consequence only limited piezoelectric activity can be induced

in ceramic preparations by poling.

Large crystals of both LiNbO

and LiTaO

can be grown by the Czochralski

method (see Fig. 3.11). Their principal properties are given in Table 6.5 below.

The high melting point of LiTaO

necessitates the use of an iridium crucible

when growing crystals. This in turn makes it necessary to have a nitrogen

atmosphere to prevent the formation of IrO

which is volatile at 1650 8C. As a

result the crystals are dark in colour and conductive as grown and must be

embedded in LiTaO

powder and annealed at 1400 8C in air to render them

colourless and of high resistivity. The LiTaO

powder prevents the volatilization

IMPORTANT COMMERCIAL PIEZOCERAMICS 371

Fig. 6.17 The position of Li

and M (Nb

,Ta

) ions relative to the O

planes (horizontal

lines) in LiNbO

and LiTaO

in the two ferroelectric states and the paraelectric state.