Zuo-Guang. Ye Advanced Dielectric Piezoelectric and Ferroelectric Materials: Synthesis, Characterisation and Applications
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130
5.1 Introduction
Relaxor-based single crystals, such as Pb(Zn
1/3
Nb
2/3
)O
3
–PbTiO
3
(PZNT)
and Pb(Mg
1/3
Nb
2/3
)O
3
–PbTiO
3
(PMNT), have been recently found to offer
significantly high performance, with electromechanical coupling (k
33
>90%)
and piezoelectric coefficients (d
33
> 2000 pC/N) that outperform the PZT
polycrystalline ceramics (k
33
~75%, d
33
~ 400–600 pC/N), as shown in Fig.
5.1, making them promising candidates for medical ultrasonics, sonar
transducers and solid-state actuators.
1–7
However, their relatively low Curie
temperature (T
C
~130–170 °C) will limit their applications in transducers in
which thermal stability is anticipated in terms of dielectric and piezoelectric
property variation and depolarization as a result of postfabrication processes.
These usage will be further restricted by T
R-T
– the rhombohedral to tetragonal
phase transformation temperature, which is significantly lower than the Curie
temperature T
C
,
owing to a strongly curved morphotropic phase boundary
(MPB). As can be seen in Fig. 5.2, a generic phase diagram for relaxor–PT
single crystal systems depicting a strongly curved MPB, showing the T
R-T
is
far below the Curie temperature, especially at the MPB composition, where
the high piezoelectric property is expected.
In addition to the thermal environments, piezoelectric crystals used in
electromechanical devices are subjected to an electric field and/or mechanical
stress, usually the external factors will deteriorate the piezoelectric properties
stability owing to the field and/or stress and/or temperature-induced
rhombohedral (or monoclinic) phase to tetragonal phase transformation.
8,9
Figure 5.3 shows the electromechanical coupling factor k
33
of 0.67PMN–
0.33PT (abbreviated PMNT33; the same notation is used throughout the
chapter) crystals as a function of electric field and temperature, in which one
can see that the coupling factor increases slightly with increasing temperature,
and then drops sharply when the crystal in the electric field or thermally
induced tetragonal phase.
9
Figure 5.4 gives the piezoelectric coefficient d
33
as a function of temperature and dc bias, exhibiting the same tendency as
5
High-performance, high-T
C
piezoelectric crystals
SJ ZHANG, Pennsylvania State University, USA, J L U O,
TRS Technologies, Inc., USA, and D W SNYDER and
T R SHROUT, Pennsylvania State University, USA
WPNL2204
High-performance, high-T
C
piezoelectric crystals 131
that observed for electromechanical coupling factor. The phase transition is
found to occur at 5 kV/cm DC bias when the temperature is around 70 °C,
while it is only 2
kV/cm at 90 °C, indicating the piezoelectric behavior
depends strongly on both temperature and electric field. As shown in Fig.
5.5, the ferroelectric phase transition temperature (usage temperature range,
above which the piezoelectric activity degrades owing to the phase
transformation) for rhombohedral PMNT single crystals, decreases with
Curie temperature (°C)
(a)
400 500300200100
1.0
0.8
0.6
0.4
0.2
0.0
Electromechanical coupling
k
33
k
33
~90–95% for single crystals
Polycrystalline ceramic
d
33
> 2000pC/N for single crystals
Polycrystalline ceramic
Curie temperature (°C)
(b)
500400300200100
Piezoelectric
d
33
(pC/N)
1000
800
600
400
200
0
5.1
Electromechanical coupling
k
33
and piezoelectric
d
33
of relaxor–PT
single crystals, compared with polycrystalline piezoelectric ceramics
as a function of Curie temperature; the crystal systems exhibit
significantly higher values.
WPNL2204
Handbook of dielectric, piezoelectric and ferroelectric materials132
increasing dc bias field.
10
Figure 5.6 gives the phase diagram of the PMNT
system as a function of dc bias, in which the tetragonal phase region is found
to be expanded with increasing dc bias, further limiting the usage temperature
range for applications.
10
Higher T
C
with straightening MPB compositions is desirable and projected
PMNT phase diagram
Projected phase diagram
Temperature (°C)
Relaxor MPB PT
Dopant
Rhomb
Tetra
Cubic
30°C
50°C
70°C
90°C
105°C
DC bias (kV/cm)
1086420
1.00
0.95
0.90
0.85
0.80
0.75
0.70
Electromechanical coupling factor
k
33
Tetragonal phase
Rhombohedral phase
5.2
A generic phase diagram for relaxor–PT crystal systems,
depicting a strongly curved MPB; the dotted line is the projected
phase diagram for modified systems.
5.3
Electromechanical coupling factor
k
33
as a function of
temperature and dc bias for PMNT33 crystals (after ref. 9).
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High-performance, high-T
C
piezoelectric crystals 133
in the PMNT systems, as shown in Fig. 5.2, which will broaden the application
temperature usage range by 10–20
°C. Among all the relaxor–PT systems,
Pb(In
1/2
Nb
1/2
)O
3
–PbTiO
3
(PINT), Pb(Sc
1/2
Nb
1/2
)O
3
–PbTiO
3
(PSNT) or their
ternary systems with PMN end member were found to possess relative high
Curie temperatures near the MPB composition. For these systems, T
C
is in
the order of 260–310
°C, but T
R-T
is in the range of only 50–120 °C,
11–30
30°C
50°C
70°C
90°C
105°C
DC bias (kV/cm)
1086420
Piezoelectric coefficient (pC/N)
3000
2000
1000
0
5.4
Piezoelectric coefficient as a function of temperature and dc bias
for PMNT33 single crystals.
DC bias (kV/cm)
20151050
100
80
60
40
20
Ferroelectric phase transition temperature (°C)
PMNT27 X’tal
d
33
= 1100 pC/N
PMNT29 X’tal
d
33
= 1600 pC/N
PMNT33 X’tal
d
33
= 2000 pC/N
5.5
The ferroelectric phase transition temperature for the
rhombohedral PMNT single crystals as a function of bias (after ref.
10).
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Handbook of dielectric, piezoelectric and ferroelectric materials134
their poor performance at elevated temperature is still a barrier in many
applications.
In this chapter, three approaches were used to enhance the temperature
usage range of rhombohedral single crystal perovskites. First, A- or B-site
dopant modifications (~1–5
mol%) were made to the PMNT system to examine
the effect on the MPB curvature. This approach was selected because of the
commercial availability of PMNT single crystals using Bridgman growth.
Second, crystal growth and characterization of compositions based on
PSNT and PINT systems, with emphasis on the Pb(Yb
1/2
Nb
1/2
)O
3
–PbTiO
3
(PYNT) systems,
which having the highest T
C
over 330 °C among the
relaxor–PT systems.
31–39
Third, the crystal growth of new bismuth based
BiScO
3
–PbTiO
3
perovskites, which offers significantly higher T
C
s on the
order of 400
°C, greatly increasing the temperature range for transducer
applications.
40–47
For applications that require even higher temperatures ( >400 °C), for
example: the operation of probes within Venus’s 450
°C atmosphere, coal-
fired electric generation plants or the nuclear plants could benefit from high-
temperature sensors for health monitoring of furnace components and reactor
systems.
48
Considering the Curie temperature for most perovskite systems is
lower than 500 °C, this chapter will present an introduction of non-perovskite
piezoelectric single crystals, including LiNbO
3
, La
3
Ga
5
SiO
14
, GaPO
4
and
ReCa
4
O(BO
3
)
3
(Re = rare earth element) piezoelectric crystals, for the potential
of application at ultra-high temperature.
no dc bias
4 kV/cm
10 kV/cm
16 kV/cm
20 kV/cm
PT composition (%)PMN PT
35302520
Temperature (°C)
200
150
100
50
0
MPB
T
F–F
decreases with dc bias
T
C
increases with dc bias
5.6
Phase diagram for the PMNT single crystals under different dc
bias fields (after ref. 10).
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High-performance, high-T
C
piezoelectric crystals 135
5.2 Background on the growth of relaxor–PT
single crystals
Relaxor ferroelectrics are generally lead-based perovskite compounds with
the general formula Pb(B
I
B
II
)O
3
, where B
I
and B
II
are two different ionic
species. Myl’nikowa and Bokov firstly reported the growth of relaxor single
crystals in the late 1950s and single crystals of Pb(Mg
1/3
Nb
2/3
)O
3
(PMN),
Pb(Zn
1/3
Nb
2/3
)O
3
(PZN), Pb(Ni
1/3
Nb
2/3
)O
3
(PNN), Pb(Co
1/3
Nb
2/3
)O
3
(PCN)
were successfully grown by the solution growth technique.
49–51
PZNT single
crystals were obtained by the flux method and reported in 1982
52
and in
1989, Shrout et al. successfully synthesized single crystals in the PMNT
system from high-temperature solution.
53
In the late 1990s, systematic studies
on the flux growth (including the modified flux-Bridgman method) of large
size PZNT single crystals and Bridgman growth of PMNT crystals were
carried out.
54–71
For the growth of PZNT crystals, common fluxes are
PbO (Pb
3
O
4
) and/or a mixture of PbO and B
2
O
3
.
69–71
It was also reported
that PZNT crystals can be obtained by top-seeded solution techniques but
with less success because of lead evaporation.
72,73
In order to reduce the
defects in the PMNT crystals grown by the modified Bridgman method,
which include macroscopic inclusions and the composition uniformity, seeded
single crystals oriented along the crystallographic directions [001], [110]
and [111] were applied during the growth. It was found that good quality
PMNT single crystals could be obtained when grown along [111] and [110]
directions, but were difficult to grow along the [001] direction because of the
anisotropic growth rate, where the [111] direction was found to be more
competitive.
60,66,67
Other crystal fabrication techniques, such as solid-state
single crystal growth (SSCG), may provide ‘single crystal-like’ ceramics of
the PMNT system as a cost-effective means to further achieve market
dominance.
74
5.3 Modification of PMNT single crystals
It has been known for some time that there is a correlation between the
Curie temperature of perovskite ABO
3
ferroelectrics and the Goldschmidt
tolerance factor. The tolerance factor t for a perovskite is given by the
expression:
75
t
rr
rr
=
+
2( + )
a
O
b
O
where r
a
is the ionic radius of A site cations, r
b
is the ionic radius of B site
cations and r
O
is the ionic radius of oxygen (perovskite stability is maximized
for tolerance factors between 0.8 and 1.05). A correlation between the end
member tolerance factor and the Curie temperature of the MPB composition
WPNL2204
Handbook of dielectric, piezoelectric and ferroelectric materials136
was proposed based on the data known for the ABO
3
–PbTiO
3
solid solution
systems.
76
As the tolerance factor decreases, the Curie temperature of the
MPB composition increases. According to this assumption, A-site cations
with smaller ionic radius (when compared with the Pb
2+
ionic radius) or B-
site cations with larger ionic radius (when compared with the Ti
4+
ionic
radius) were selected as the doping elements for the modifications of the
PMNT systems and the preliminary results show that the doping on the A site
or B site can enhance the usage temperature range of PMNT30 by 10–20
°C
at a doping level of 1–5
mol%.
As shown in Fig. 5.7, the Curie temperature of an A-site modified PMNT30
crystal was found to keep constant while the ferroelectric phase transition
increased by 15
°C when compared with the pure counterpart, meanwhile,
both the Curie temperature and ferroelectric phase transition temperatures
for a B-site modified PMNT30 were found to shift upward by 10–15
°C
when compared with the pure crystals. The piezoelectric coefficient for both
A-site and B-site modified crystals were found to exhibit comparable values
of 1200–1800
pC/N, with slightly improved coercive field.
10
PMNT30-
T
c
= 138°C;
T
R–T
= 90°C
PMNT30-A
T
c
= 138°C;
T
R–T
= 110°C
PMNT30-B
T
c
= 150°C;
T
R–T
= 109°C
Temperature (°C)
180180180909090
40k
35k
30k
25k
20k
15k
10k
5k
0
Dielectric permittivity
5.7
Dielectric property as a function of temperature for A- or B-site
modified PMNT30, measured at 1, 10 and 100 kHz frequencies,
compared with the pure counterpart.
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High-performance, high-T
C
piezoelectric crystals 137
5.4 Relaxor–PT systems with high Curie
temperature
Table 5.1 lists most of the lead-based relaxor–PT systems, among which the
Pb(Sc
1/2
Nb
1/2
)O
3
–PbTiO
3
(PSNT), Pb(In
1/2
Nb
1/2
)O
3
–PbTiO
3
(PINT) and
Pb(Yb
1/2
Nb
1/2
)O
3
–PbTiO
3
(PYNT) were found to possess relative high Curie
temperatures near their MPB compositions.
39,77–79
Henceforth, numerous
researchers focused on single crystals of these binary systems or ternary
systems with PMN, in order to achieve high performance over a wide
temperature usage range.
11–39
PSNT single crystals with MPB compositions were successfully grown
by the high-temperature solution method, as reported by Yamashita and
coworkers
11,15
and Bing and Ye.
12–14
PSNT33 crystals were reported to possess
a high Curie temperature of T
C
~206 °C, 30–50 °C higher than PMNT and
PZNT systems; however, the electromechanical coupling factor and
dielectric constant were found to be only k
33
= 72% and 960, respectively,
due to the poor quality of the crystals. Ternary systems
Pb(Sc
1/2
Nb
1/2
)O
3
–Pb(Mg
1/3
Nb
2/3
)O
3
–PbTiO
3
(PSMNT)
23,24
and
Pb(Sc
1/2
Nb
1/2
)O
3
–Pb(Zn
1/3
Nb
2/3
)O
3
–PbTiO
3
(PSZNT)
28,29
were grown by the
flux method or solution Bridgman method. A T
C
of ~200 °C with a dielectric
constant of about 2600 were found for the grown single crystals, but no
piezoelectric properties was reported. Moreover, it was found to be difficult
to grow PSNT crystals directly from the melt due to the poor perovskite
stability; only small crystals with dimension in the order of few millimeters
could be obtained.
Single crystals of PINT and their electromechanical properties were
investigated and reported in last few years.
16–22
PINT28 crystals were grown
by the high-temperature solution method and the Curie temperature was
reported to be 260
°C with the ferroelectric phase transition temperature
Table 5.1
Morphotropic phase boundaries in perovskite Pb(B
I
B
II
)O
3
–PT systems
Binary system PT content
T
C
(°C)
T
R-T
(°C)
on MPB at MPB at MPB
(1–
x
)Pb(Zn
1/3
Nb
2/3
)O
3
–
x
PbTiO
3
(PZNT)
x
≅ 0.09 ~180 95
(1–
x
)Pb(Mg
1/3
Nb
2/3
)O
3
–
x
PbTiO
3
(PMNT)
x
≅ 0.33 ~150 80
(1–
x
)Pb(Ni
1/3
Nb
2/3
)O
3
–
x
PbTiO
3
(PNNT)
x
≅ 0.40 ~130 /
(1–
x
)Pb(Co
1/3
Nb
2/3
)O
3
–
x
PbTiO
3
(PCNT)
x
≅ 0.38 ~250 /
(1–
x
)Pb(Sc
1/2
Nb
1/2
)O
3
–
x
PbTiO
3
(PSNT)
x
≅ 0.43 ~260 100
(1–
x
)Pb(Sc
1/2
Ta
1/2
)O
3
–
x
PbTiO
3
(PSTT)
x
≅ 0.45 ~205 /
(1–
x
)Pb(Fe
1/2
Nb
1/2
)O
3
–
x
PbTiO
3
(PFNT)
x
≅ 0.07 ~140 /
(1–
x
)Pb(In
1/2
Nb
1/2
)O
3
–
x
PbTiO
3
(PINT)
x
≅ 0.37 ~320 120
(1–
x
)Pb(Yb
1/2
Nb
1/2
)O
3
–
x
PbTiO
3
(PYNT)
x
≅ 0.50 ~360 160
(1–
x
)Pb(Mg
1/2
W
1/2
)O
3
–
x
PbTiO
3
(PMWPT)
x
≅ 0.55 ~60 /
(1–
x
)Pb(Co
1/2
W
1/2
)O
3
–
x
PbTiO
3
(PCWPT)
x
≅ 0.45 ~310 /
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Handbook of dielectric, piezoelectric and ferroelectric materials138
T
R-T
around 100 °C. The electromechanical coupling factor in sliver mode
′
k
33
was found to be 78% and the longitudinal one was estimated to be 85%
at room temperature, which was almost independent of temperature up to
200
°C.
17,18
The enhanced temperature stability of a PINT crystal may be
ascribed to the crystal’s composition located away from its MPB as well as
its high T
C
.
19
The same research group also reported the growth of PINT
crystals by the solution Bridgman method. The composition of the grown
crystals was found to vary along the growth direction owing to the composition
segregation – the same issue encountered in Bridgman-grown PMNT crystals.
The T
C
and T
R-T
were found to be 300 °C and 120 °C, respectively, for
PINT31 crystals.
20
PINT single crystals were also grown directly from the
melt using the modified Bridgman method and reported to possess high
piezoelectric coefficients d
33
~2000 pC/N with an electromechanical coupling
k
33
~ 94%, comparable to the PMNT and PZNT systems, while exhibiting a
higher T
C
of 200 °C, but no information about the temperature usage range
(i.e. T
R-T
) of this crystal was reported.
22
Single crystals in the ternary
Pb(In
1/2
Nb
1/2
)O
3
–Pb(Mg
1/3
Nb
2/3
)O
3
–PbTiO
3
(PIMNT) system were also grown
using the solution and/or modified Bridgman techniques, a transition
temperature on the order of 200
°C with dielectric constant of ~4000 and
piezoelectric coefficient d
33
of ~2000 pC/N were reported, however, low
ferroelectric phase transitions, T
R-T
, on the order of 80–110 °C were observed.
Furthermore, large compositional segregation was found along the crystal
boules, similar to that found in PINT and PMNT systems.
16
PYNT with the MPB composition (PT content around 50%) was found to
possess a T
C
of ~360 °C, the highest among all the lead-based relaxor–PT
systems (see Table 5.1) and similar to commercial PZT5A ceramics.
Piezoelectric single crystals in the PYNT system were grown using the flux
method. Perovskite PYNT solid solutions were prepared using the ‘columbite
process’, which consists of presynthesis of the B-site precursor YbNbO
4
prior to reaction with PbO and TiO
2
, in order to stabilize the perovskite
phase.
33,35,80,81
Figure 5.8 presents the dielectric permittivity and dielectric
loss for [001] oriented PYNT45 crystals, in which one can see that there are
two dielectric anomalies, located at 325
°C and 160 °C, corresponding to the
Curie temperature T
C
and the rhombohedral to tetragonal phase transition
temperature T
R-T
, respectively. The increased T
R-T
temperature gives a
broadened temperature usage range and also stabilizes the property temperature
dependence as reported.
34
Bipolar polarization loops were measured on [001] oriented PYNT single
crystals with various PT contents at 1
Hz and ±40 kV/cm electric field. As
shown in Fig. 5.9, the shape of the P-E hysteresis loops changes from slim
to square with increasing PT content, revealing that the PYNT single crystals
transfer from relaxor-like behavior to normal ferroelectric behavior with
increasing PT content. The polarization hysteresis measured at 1
Hz and
WPNL2204
High-performance, high-T
C
piezoelectric crystals 139
±10 kV/cm field for PZNT4.5 and PMNT33 single crystals were also given
in Fig. 5.9 for comparison. The coercive field was found to be ~13
kV/cm for
PYNT46, much higher than those of the PZNT and PMNT crystals (which
are found to be 3.5 and 2.2
kV/cm, respectively).
35
Figures 5.10 – 5.12 show the electromechanical and piezoelectric properties
PYNT45 crystal
T
C
= 325°C
T
R–T
= 160°C
1k-100kHz
Temperature (°C)
400 500300200100
0.4
0.2
0.0
Dielectric loss
Dielectric permittivity
14000
12000
10000
8000
6000
4000
2000
0
5.8
Dielectric properties as a function of temperature for the PYNT45
single crystal, measured at 1, 10 and 100
kHz. (after ref. 34).
PYNT20
PYNT37
PYNT45
PZNT4.5
PMNT33
Electric field (kV/cm)
–40 –20 0 20 40
–40
Polarization (µC/cm
2
)
–20
0
20
40
5.9
Polarization hysteresis for the PYNT single crystals with PT
content at 0.20, 0.37 and 0.46, respectively, and compared to
PZNT4.5 and PMNT33 crystals.
WPNL2204