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

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Handbook of dielectric, piezoelectric and ferroelectric materials160

6.1

SSCG of BZT: growth of a (100) BZT seed crystal in a BZT

ceramic.

6.2

A BZT single crystal, bigger than 50 × 50 × 10 mm

in size, grown

by the SSCG technique.

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Development of high-performance piezoelectric single crystals 161

Figure 6.3 shows the polished surfaces of (a) 70PMN–30PT and (b) PMN–

PZT single crystals grown by the SSCG process. Before the annealing of

crystal growth, a Ba(Zr

0.1

0.9

single crystal was placed on the top of the

ceramic. During the heat treatment, the BZT seed crystals grew into the (a)

6.3

Polished surfaces of (a) 70PMN–30PT and (b) PMN–PZT single

crystals grown by SSCG process.

(a)

(b)

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Handbook of dielectric, piezoelectric and ferroelectric materials162

70PMN–30PT and (b) PMN–PZT ceramics, respectively. In spite of the

difference in chemical composition between the seed crystals and the ceramics,

the BZT seed crystals were chemically stable in the Pb-based ceramics and

thus acted as an effective seed single crystal. These results demonstrate that

piezoelectric single crystals of BZT, PMN–PT, and PMN–PZT with sizes up

to 40 or 50 mm could be successfully fabricated by the simple SSCG process.

One of the advantages of the SSCG method is that the net shape crystal

growth is possible, because the melting of ceramic precursors does not occur

during the crystal growth process. Figure 6.4 shows the growth of a ring-

shaped BZT single crystal. First a ring-shaped BZT powder compact was

prepared by uni-axial pressing of BZT powder and sintered. After the sintering

(a)

(b)

6.4

Growth of a ring-shaped BZT single crystal from a BZT seed

crystal at the bottom (a) and two BZT single crystal rings grown by

SSCG technique without mechanical drilling process (b).

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Development of high-performance piezoelectric single crystals 163

the ring-shaped BZT ceramic was placed on the top of a (100) BZT single

crystal seed and then heat-treated, as shown in Fig. 6.4(a). Figure 6.4(a)

shows the growth of a BZT single crystal from the seed crystal to the ring-

shaped ceramic. Since the shape of the growing BZT single crystal was the

same as that of the BZT ceramic, ring-shaped BZT single crystals were

successfully fabricated without mechanical machining of single crystal blocks

after the crystal growth step, as shown in Fig. 6.4(b). As another example of

the net shape crystal growth, Figure 6.5 shows the growth of a PMN–PT

single crystal from a PMN–PT ceramic which was mechanically machined

in a 2–2 composite form before the crystal growth step. Instead of difficult

machining of brittle PMN–PT single crystals, the sintered PMN–PT ceramics

were diced as shown in Fig. 6.5(a) and heat-treated for the crystal growth.

By using this technique piezoelectric single crystals can be prepared in a 1–

3 or 2–2 composite form, while avoiding the mechanical machining steps of

brittle single crystals.

6.3 Dielectric and piezoelectric properties of BZT,

PMN–PT, and PMN–PZT single crystals

6.3.1 SSCG Ba(Zr,Ti)O

single crystals

The addition of Zr in BaTiO

decreases the Curie temperature (the cubic–

tetragonal phase transition temperature, T

), but increases the tetragonal–

orthorhombic transition temperature (T

) and the orthorhombic–rhombohedral

transition temperature (T

Therefore, it is possible to stabilize any of the

four phases (tetragonal, orthorhombic, rhombohedral, or cubic) at room

temperature by adding appropriate amounts of Zr.

When the orthorhombic

or rhombohedral phase is stabilized at room temperature, the resulting single

crystals show high piezoelectric properties and are promising candidates for

high-performance and non-lead piezoelectrics.

Figure 6.6 shows the variation of dielectric constant

()

with temperature

of the (001) single crystal plates of BaTiO

, Ba(Zr

0.05

0.95

Ba(Zr

0.075

0.925

and Ba(Zr

0.1

0.9

. The (001) BaTiO

single crystal

did not show any phase transition at temperatures between room temperature

and its Curie temperature. In the curve of Ba(Zr

0.05

0.95

single crystal,

one phase transition (T

) between tetragonal phase and orthorhombic phase

was observed in the measurement temperature range. The Ba(Zr

0.075

0.925

and Ba(Zr

0.1

0.9

single crystals showed two peaks of the orthorhombic–

tetragonal (T

) and rhombohedral–orthorhombic (T

) phase transitions.

Since the Ba(Zr

0.075

0.925

single crystal had rhombohedral phase at room

temperature and its T

was close to room temperature, their dielectric and

piezoelectric properties were characterized. Table 6.1 shows the measured

dielectric and piezoelectric properties of Ba(Zr

0.075

0.925

single crystals.

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Handbook of dielectric, piezoelectric and ferroelectric materials164

(a)

(b)

6.5

A PMN–PT polycrystalline ceramic prepared for fabrication of 2–2

composite (a) and growth of a PMN–PT single crystal in a 2–2

composite form from a BZT seed crystal at the bottom (b).

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Development of high-performance piezoelectric single crystals 165

Compared with other Ba(Zr

1–x

single crystals with different values

of x, the Ba(Zr

0.075

0.925

single crystals were found to exhibit better

piezoelectric properties (because they are rhombohedral at room temperature

and close to the phase boundary between rhombohedral and orthorhombic

phases). The measured electromechanical coupling factor (k

) and piezoelectric

coefficient (d

) were about 0.9 and 1050 pC/N, respectively. These results

confirm that rhombohedral Ba(Zr

1–x

single crystals with appropriate

values of x are promising candidates for high-performance and non-lead

piezoelectrics.

BaTiO

Ba(Zr

0.05

0.95

Ba(Zr

0.075

0.925

Ba(Zr

0.1

0.9

(R–O)

~ 40°C

Temperature [°C]

1501251007550250

Dielectric constant [

]

50000

40000

30000

20000

10000

6.6

Variation of dielectric constant (at 1 kHz) with temperature:

BaTiO

, Ba(Zr

0.05

0.95

, Ba(Zr

0.075

0.925

and Ba(Zr

0.1

0.9

single

crystals.

Table 6.1

Dielectric and piezoelectric properties of

Ba(Zr

0.075

0.925

single crystals

4000

tan δ > 0.01

0.9

[pC/N] 1150

[x10

–12

/N] 7.5

[x10

–12

/N] 45

[GPa] 25

[Hz m] 1050

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Handbook of dielectric, piezoelectric and ferroelectric materials166

6.3.2 SSCG PMN–PT single crystals

Following the discovery of the outstanding piezoelectric properties of

PMN–PT and PZN–PT single crystals, a great deal of effort has been put in

the growth of these crystals by either the flux method or the Bridgman

method.

4–10

In the flux method it is difficult to grow large and oriented single

crystals, which are necessary for commercial applications. Material loss due

to the vaporization of volatile PbO is another disadvantage of the flux method.

Feigelson

first suggested that the Bridgman method could be used to grow

PMN–PT single crystals with sealed Pt crucibles as an alternative to flux

growth. However, the Bridgman method presents several problems in the

crystal growth of PMN–PT crystals with composition near the morphotrophic

phase boundary (MPB) composition, including: compositional variation along

the growth direction and leak of Pt crucibles.

7,8,10

In order to overcome these

problems, several options such as the modified Bridgman method and the

zone leveling method were suggested. Among them the SSCG method is one

of promising techniques for growing chemically uniform PMN–PT single

crystals, because complete melting of major components is not necessary. In

particular, for the growth of chemically uniform single crystals of more

complicated compositions such as doped PMN–PT, or relaxor–PMN–PT, the

SSCG technique has advantages over the conventional crystal growth methods.

Figure 6.7 shows the variation of dielectric constant

()

with temperature

of (001) single crystal plates of undoped 70PMN–30PT, Fe-doped PMN–PT,

Undoped 70PMN–30PT

~ 140°C;

~ 90°C

Fe-doped 70PMN–30PT

~ 140°C;

~ 85°C

Mn-doped 70PMN–30PT

~ 135°C;

~ 85°C

Temperature [°C]

200150100500

Dielectric constant [

]

30000

20000

10000

6.7

Variation of dielectric constant (at 1 kHz) with temperature:

undoped 70PMN–30PT, Fe-doped 70PMN–30PT, and Mn-doped

70PMN–30PT single crystals.

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Development of high-performance piezoelectric single crystals 167

and Mn-doped PMN–PT, grown by the SSCG technique (see Fig. 6.3a). The

undoped 70PMN–30PT single crystal has a Curie temperature (T

) of ~140 °C

and a transition temperature between rhombohedral and tetragonal phases

) of ~90 °C. T

and T

of Fe-doped PMN–PT single crystals were

140 °C and 85 °C, respectively. Mn-doped PMN–PT single crystals show a

of 135 °C and a T

of 90 °C. Table 6.2 shows the measured dielectric and

piezoelectric properties of undoped 70PMN–30PT, Fe-doped PMN–PT, and

Mn-doped single crystals. These results show that the SSCG technique has

been successfully applied to the fabrication of undoped as well as doped

PMN–PT single crystals.

6.3.3 SSCG PMN-PZT

single crystals

The excellent piezoelectric properties (k

> 0.9; d

> 2000 pC/N) of PMN–

PT and PZN–PT single crystals make it possible to develop the next generation

electromechanical devices such as ultrasonic transducers and actuators.

However, their disadvantages such as low T

, T

, and E

limit their use in

general piezoelectric applications. Therefore piezoelectric single crystals

with high T

, T

, and E

are desired and are being developed for more

general applications. Several research groups have already reported growth

and properties of high T

piezoelectric crystals of relaxor–PT systems, such

as Pb(Sc

1/2

–PbTiO

(PSN–PT),

Pb(Yb

1/2

–PbTiO

(PYbN–

PT),

Pb(In

1/2

–PbTiO

(PIN–PT),

BiScO

–PbTiO

(BS–PT),

and

PIN–PMN-PT.

26,27

Owing to high vapor pressure of PbO melt, however, the

conventional growth processes of flux and Bridgman methods have critical

limitations such as a complicated growth process, and chemically

inhomogeneous crystals, resulting in low yield and high cost. Some expensive

Table 6.2

Dielectric and piezoelectric properties of (a) undoped 70PMN–30PT,

(b) Fe-doped 70PMN–30PT, and (c) Mn-doped 70PMN–30PT single crystals grown

by the SSCG process

(001) PMN– (001) Fe–doped (001) Mn–doped

30PT PMN–30PT PMN–30PT

5000 4850 3700

tan δ > 0.01 > 0.02 > 0.02

[°C] 140 140 135

[°C] 90 85 85

0.9 0.91 0.89

[pC/N] 1500 1450 1080

[x10

–12

/N] 10.7 10.2 9.5

[x10

–12

/N] 54 60 46

[GPa] 19 17 22

[Hz

m] 840 800 900

35 40 30

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Handbook of dielectric, piezoelectric and ferroelectric materials168

raw materials such as In

and Sc

will also increase the production

cost.

PZT ceramics have been the mainstay for high-performance piezoelectric

applications. Compositionally PZT ceramics lie near the MPB between the

tetragonal and rhombohedral phases and the MPB compositions exhibit

anomalously high dielectric and piezoelectric properties. Relaxor–PZT ceramics

have a wide range of MPB compositions depending on the ratio of relaxor/

PZ/PT, as well as the type of relaxors. Therefore, it is expected that relaxor–

PZT single crystals will have a wide range of dielectric and piezoelectric

properties, and some relaxor–PZT single crystals will show much better

properties than relaxor-PT single crystals such as PMN–PT, PZN–PT, and

PIN–PT, if relaxor–PZT materials could be grown in single crystal form. In

contrast to relaxor-PT crystal growth, PZT and relaxor–PZT cannot be readily

grown in bulk single crystal form because of incongruent melting behavior

of PZT. During incongruent melting of PZT, PZT decomposes to a liquid and

a solid phase ZrO

. The solid phase ZrO

in the liquid phase prevents continuous

growth of PZT single crystals. Attempts to grow single crystals of PZT and

relaxor–PZT only resulted in crystallites too small to allow adequate property

measurements. Therefore, in order to grow large PZT or relaxor–PZT single

crystals, a melting step of PZT should be avoided in the crystal growth process.

In the PMN–PZ–PT system near the MPB compositions, the T

and T

of PMN–PZT single crystals vary with the ratio of PMN/PZ/PT approximately

in the ranges of 150 °C < T

< 350 °C and 50 ° C < T

< 300 °C. Therefore,

by adjusting the composition and achieving chemical uniformity, PMN–PZT

single crystals of high T

and T

can be obtained. As shown in Fig. 6.3(b),

a chemically homogeneous and fully dense PMN-PZT single crystal was

grown by the SSCG technique. Figure 6.8 shows the possibility of controlling

and T

of PMN–PZT single crystals by changing the ratio of PMN/PZ/

PT. In Fig. 6.8, the Curie temperatures of all PMN–PZT single crystals are

approximately 200 °C; however, their T

varies from 100 °C to 160 °C

depending on the ratio of PMN:PZ:PT. This result shows that PMN–PZT

single crystals of high T

and T

can be fabricated and their T

and T

can

be controlled.

Figure 6.9 shows the polarization curves of (001) single crystal plates of

PMN–30PT and PMN–PZT with electric field. The coercive electric field

) of (001) PMN–PZT single crystal plates was higher than 5 kV/cm,

which is almost twice as high as that of a (001) PMN–30PT single crystal

plate. In Table 6.3 the measured dielectric and piezoelectric properties of

PMN–PZT single crystals are summarized. Among the PMN–PZT single

crystals near the MPB, PMN–PZT-1 single crystals showed very well-balanced

dielectric and piezoelectric properties with T

≈ 200 °C, T

≈ 100 °C,

()

≈ 6000, k

≈ 0.92, d

≈ 2000, and E

≈ 5 kV/cm. The d

of (011)/<100>

PMN–PZT-1 single crystal plates reached about 1900 pC/N.

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Development of high-performance piezoelectric single crystals 169

(a)

= 90°C,

= 130°C

(b)

= 100°C,

= 196°C

(c)

= 115°C,

= 196°C

(d)

= 130°C,

= 195°C

(e)

= 145°C,

= 190°C

(f)

= 160°C,

= 188°C

(a) PMN–30 PT

Temperature [°C]

250200150100500

Dielectric constant [

]

50000

40000

30000

20000

10000

6.8

Variation of dielectric constants (at 1 kHz) of PMN–PZT single

crystals with temperature.

6.9

Plots of polarization vs. electric field: (a) PMN–30PT, (b) PMN–

PZT-1, and (c) PMN–PZT-2.

–10

–20

–30

–40

–50

Ploarization (µC/cm

)

–30 –20 –10 0 102030

Electric field (kV/cm)

PMN–30PT

(

= 2.5 kV/cm)

PMN–PZT-1

(

= 5.4 kV/cm)

PMN–PZT-2

(

= 5.2 kV/cm)

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