High temperature-stability of (Pb 0.9 La 0.1 )(Zr 0.65 Ti 0.35 )O 3 ceramic for energy-storage applications at finite electric field strength

Abstract

[http://www.sciencedirect.com/science/article/pii/S1359646217302439] Temperature stability is one of the key factors for energy storage application of dielectric capacitors especially under stringent environmental conditions. In this work, we report that the (Pb0.9La0.1)(Zr0.65Ti0.35)O3 ceramic exhibits small variation of energy density (< 15%) over a wide temperature range (24 °C ~ 83 °C) at low field strength (E < 25 kV/cm). Further TEM observation and phase field simulations suggest that it can be attributed to continuous formation and growth of polar nanoregions with temperature and electric field, resulting in high-temperature stability for dielectric permittivity and energy density. Our finding may have implications for developing dielectric energy-storing devices with high thermal reliability.

Full text

Regular Article
High temperature-stability of (Pb
0.9
La
0.1
)(Zr
0.65
Ti
0.35
)O
3
ceramic for
energy-storage applications at finite electric field strength
Jinghui Gao
a
, Yongbin Liu
a
, Yan Wang
a
,DongWang
a,b,
⁎, Lisheng Zhong
a
, Xiaobing Ren
a,c
a
State Key Laboratory of Electrical Insulation and PowerEquipment and Multi-disciplinary MaterialsResearch Center, Frontier Institute of Scienceand Technology, Xi'anJiaotong University,Xi'an
710049, China
b
Center of Microstructure Science, Frontier Institute of Science and Technology, Xi'an Jiaotong University, Xi'an 710049, China
c
Ferroic Physics Group, National Institute for Materials Science, Tsukuba, 305-0047, Ibaraki, Japan
abstractarticle info
Article history:
Received 27 March 2017
Received in revised form 3 May 2017
Accepted 8 May 2017
Available online xxxx
Temperature stability is one of the key factors for energy storage application of dielectric capacitors especially
under stringent environmental conditions. In this work, we report that the (Pb
0.9
La
0.1
)(Zr
0.65
Ti
0.35
)O
3
ceramic
exhibits small variation of energy density (b15%) over a wide temperature range (24 °C ~ 83 °C) at low field
strength (Eb25 kV/cm). Further TEM observation and phase field simulations suggest that it can be attributed
to continuous formation and growth of polar nanoregions with temperature and electric field, resulting in
high-temperature stability for dielectric permittivity and energy density. Our finding may have implications
for developing dielectric energy-storing devices with high thermal reliability.
© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords:
Dielectrics
Energy storage
Ferroelectric ceramics
Ferroelectric domains
Phase transformations
Dielectric material, which exhibits polarization when applying
external electric field, enables the storage of electric energy and thus
plays an important role in the storage and manipulation of electric
power. Although the energy storage devices (e.g. supercapacitors,
rechargeable batteries, fuel cells etc.) with high energy density have
been drawing significant attention [1], dielectric energy-storing device,
on the other hand, is capable of fast delivering the electric energy.
Owing to high power density, the dielectricmaterials have the potential
to be utilized on power pulse devices, hybrid electric vehicles, portable
electronic devices and so on [2,3], and have also triggered wide research
interests [4–19].
Most of the dielectric energy-storing devices are designed for high
electric field strength applications, aiming to achieve high level of ener-
gy density [20–27], and the associated materials (e.g. polymers, ferro-
electrics and antiferroelectrics etc.) should be able to withstand large
electric field, resulting in an elevated voltage level [2–27]. For example,
the lowest voltage is as high as sub-kilo volt even for a thin film speci-
men with thethickness down to several micron-meters [25].Neverthe-
less, high voltage level restricts the application of energy-storing
material as wearable or portable electronic devices with miniaturized
dimension and high integration. Hence, it is necessary to develop the di-
electric energy storage materials within a finite electric field strength.
Our earlier work proposed an approach to enhance the energy density
in Ba(Ti,Sn)O
3
(BTS) system through elevating its permittivity using a
so-called tricritical transition [28,29]. However, although the energy
density for such a material is higher than most of ferroelectric systems
with the same field strength (E= 10 kV/cm), it is poor in temperature
stability for energy storage performance due to large thermal variation
of dielectric permittivity [29]. This deteriorates its thermal reliability,
which reduces the endurance with ambient temperature change as
well as device temperature rise. It is of great importance to broaden
the usage temperature for the application of dielectric materials in ener-
gy storage devices, which enables the designing of reliable devices.
It is well-known thatenergy storage performance highly relies on di-
electric permittivity. Thus, the relaxor ferroelectric material, which has
high thermal stability of dielectric permittivity [30], could be a potential
candidate. In this manuscript, we study the temperature-dependence of
energy density for a relaxor (Pb
0.9
La
0.1
)(Zr
0.65
Ti
0.35
)O
3
(PLZT) ceramic.
The result indicates that it exhibits a high degree of temperature-stabil-
ity for energy density at low electric field (Eb25 kV/cm). Further trans-
mission electron microscopic observation uncovers its microstructure
origin, and a phase field simulation has also been employed to under-
stand the underneath mechanism. Our results may provide guideline
for developing energy storage dielectric materials with excellent ther-
mal stability.
The (Pb
0.9
La
0.1
)(Zr
0.65
Ti
0.35
)O
3
relaxor ferroelectric ceramics were
fabricated by using the conventional solid-state sintering method from
the raw chemical powders of PbO, La
2
O
3
,ZrO
2
,TiO
2
. Calcination and
Scripta Materialia 137 (2017) 114–118
⁎Corresponding author.
E-mail addresses: wang_dong1223@mail.xjtu.edu.cn (D. Wang),
lszhong@mail.xjtu.edu.cn (L. Zhong), ren.xiaobing@nims.go.jp (X. Ren).
http://dx.doi.org/10.1016/j.scriptamat.2017.05.011
1359-6462/© 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Contents lists available at ScienceDirect
Scripta Materialia
journal homepage: www.elsevier.com/locate/scriptamat

final sintering were performed at the temperature of 1000 °C (3 h) and
1250 °C (3 h) in oxygen atmosphere. We used thin column specimens
with different dimensions for energy density test (Φ= 7.31 mm, d =
2 mm) and dielectric coefficients measurement (Φ= 20.6 mm, d =
2.6 mm). In order to measure the energy density of the samples, a Pre-
mier II ferroelectric test system (Radiant Technologies Inc.) was
employed to detect the polarization-electric field (P-E) loops at 10 Hz.
Further temperature-dependence of energy storage properties were
measured by using the same test system equipped with a heating cham-
ber. Dielectric permittivity and loss tangent at 10 Hz were measured
with a broad frequency dielectric spectrometer (Concept 80,
Novocontrol Technologies Inc.), andthe applied AC voltage is 1 V in am-
plitude. Moreover, the evolution of microstructure with temperature
was observed by using a JEM 2100F transmission electron microscope
(TEM) in combination with a heating specimen stage (Gatan 652). Fur-
thermore, the underlying mechanism was studied by phase field simu-
lation. Domain structure is described by spatial distribution of
spontaneous polarization P=(P
1
,P
2
,P
3
). The total free energy of the
system includes the following physically distinctive terms (local chem-
ical free energy f
bulk
, coupling term f
couple
caused by doping, long range
elastic interaction f
elas
, static electric interaction f
elec
, external electric
field f
elappl
and domain wall energy of gradient term f
grad
):
F¼FP;cðÞþFP;φðÞþFPðÞ
¼∫VfbulkdV þ∫VfcoupledV þ∫Vfel as þfelec þfelappl þfgrad

dV ð1Þ
Temporal evolution of the spontaneous polarization field can be ob-
tained by solving the time-dependent Ginzburg-Landau (TDGL) equa-
tion:
dPix;tðÞ
dt ¼−MδF
δPix;tðÞ
;i¼1;2;3ð2Þ
where Mis the kinetic coefficient and tis time. In the expression of free
energy (Eq. (1)), the chemical free energy f
bulk
is the temperature-de-
pendence term, which can be described as follows:
fbulk ¼A0
1T−T0

∑
i¼1;2;3
P2
iþA11 ∑
i¼1;2;3
P4
iþA12
2∑
i;j¼1;2;3;i≠j
PiPj

2
þA111 ∑
i¼1;2;3
P6
iþA112 ∑
i;j¼1;2;3;i≠j
P2
iP4
j

þA123 P2
1P2
2P3
3
 ð3Þ
where A
1
0
,A
11
,A
12
,A
111
,A
112
and A
123
are Landau expansion coefficients
and Tis the temperature for the materials system. The detailed method
and coefficient values can be found in Ref. [31]. Therefore, the micro-
structures at different temperature can be calculated by solving evolu-
tion equation Eq. (2).
In order to detect the energy density, we measured the P-E hystere-
sis loops for the relaxor ferroelectric material PLZT10/65/35 ceramics.
As a comparison, we also performed the same measurement for other
ferroelectric systems including modified PbZrTiO
3
(PZT) ceramics and
poly (vinylidene fluoride) (PVDF). In particular, the PVDF is one of the
most investigated ferroelectric polymer materials for energy storageap-
plication [2,3,9].Fig. 1(a) shows P-E loops for these specimens, and it
can be seen that the PLZT10/65/35 ceramic exhibits higher polarization
value compared with other selected materials. High polarization value
for PLZT10/65/35 ceramic is an indication for larger electric energy
density.
We then calculated the energy density for each selected specimen
from its P-E curve. It is well-known that the energy density u
e
can be de-
scribed as u
e
=∫EdD, where Eand Dare field strength and electric dis-
placement respectively. Such an integration can be carried out for P-E
curve of either charging (with the increase of E) or discharging (with
the decrease of E). We then use the one from the discharging P-E
curve, which is an indication of energy density that can be released
from dielectric material. We measure the P-E loops for each specimen
at a series of field strength, and the corresponding energy densities
are shown in Fig. 1(b). Thesespecimens show the similar trend that en-
ergy densities are enhanced with the increasing of electric field
strength. However,PLZT10/65/35 ceramic exhibits higher value of ener-
gy density compared with other selected materials at electric field of E
b25 kV/cm. Although u
e
for PLZT is far below the one for other dielec-
trics at high field region (E≈6000 kV/cm) [2], it may still be found po-
tential applications on the energy storage devices on low electric field
occasion.
The inset of Fig. 1(b) further shows the electric field-strength-de-
pendence of dielectric permittivity for the PLZT10/65/35 ceramic. Al-
though the electric field strength is believed to be the dominant factor
for energy density, dielectric permittivity ε
r
also plays an important
role on the energy-storing properties of the material, since the energy
density can be expressed as u
e
=ε
0
∫Ed(ε
r
E). The permittivity has been
evaluated from the derivative of polarization with respect to electric
field from the discharging P-E curve. It should be noticed that such a dif-
ferential permittivity involves the ferroelectric domain switching (or
polarization reversal) under relatively large electric field, and it usually
shows discrepancy in value with permittivity measured by impedance
analyzer (Fig. 2(a)) which only caused by domain wall motion under
weak electric field [32]. It can be seen from the inset of Fig. 1(b) that di-
electric permittivity decreases with the increasing of electric field. Such
a phenomenon has also been observed for other ferroelectric energy-
storing materials at higher electric field strength [2,25], and can thus
be considered as a general trend. Being different from the conventional
dielectric material with linear dielectric response, ferroelectric mate-
rials, including relaxor PLZT ceramics, exhibit dielectric nonlinearity
with respect to external field [33]. Several models have been proposed
Fig. 1. (a) The polarization-electric field (P-E) loops at the field strength of E= 25 kV/cm for PLZT10/65/35 ceramic, PZT-based ceramics and PVDF polymer. (b) The field-strength-
dependence of energy density for the tested materials. PLZT 10/65/35 shows large value in u
e
. The inset shows the differential dielectric permittivity decreases with increase of field
strength, suggesting the permittivity has ·higher impact in low field region.
115J. Gao et al. / Scripta Materialia 137 (2017) 114–118

to interpret such a phenomenon [34–38], and it is generally accepted
that the nonlinear dielectric response can be ascribed to the decrease
of extrinsic dielectric response due to disappearance of domain walls
with the enhancement of electric field [39–45]. The result suggests
that the impact of permittivity reduces with the increase of electric
field strength, and it seems that the dielectric permittivity is the domi-
nant factor for energy density at low electric field.
We then measured the temperature variation of dielectric permittiv-
ity for PLZT10/65/35 ceramic. Temperature-dependence of ε
r
and tanδ
at 10 Hz have been shown in Fig. 2(a). It can be seen that ε
r
changes
sluggishly with temperature. In order to further evaluate the tempera-
ture-dependence of energy density, we measured the P-E loops for
PLZT10/65/35 ceramic at a series of temperatures (as shown in Fig.
2(b)), and the corresponding energy density can thus be calculated
from the integration of these curves. Fig. 2(c) shows the change of ener-
gy density as a function of temperature. The comparison of its tempera-
ture variation rate between PLZT and the previously-reported tricritical
BTS have been shown in the inset. And it is found that the energy
density keeps unchanged in the measured temperature range, and its
varies by b15% from 24 °C to 83 °C, which is much lower compared
with BTS. This indicates that the PLZT10/65/35 ceramic exhibits excel-
lent temperature stability for energy storage properties at low electric
field. On the other hand, we detected the temperature-dependence of
energy loss which can be evaluated by calculating the area between
the charging and discharging P-E curves. As shown in the inset of Fig.
2(d), all of the losses with different field strength slightly change with
temperature and thevalues are below 60 mJ/cm
3
. The associated energy
density efficiency (Fig. 2(d)), which refers to the proportion of
discharging energy density in total energy density, also shows sluggish
change with temperature and the value can reach up to 70% ~ 82%. It
should be noted that although the energy storage properties for PLZT-
based materials (including its energy density, efficiency, temperature
variations etc.) have been evaluated by previous investigations [23–
25], the measurement at low electric field (with Eb30 kV/cm) is still
lacking. Our results suggest that the relaxor ferroelectric PLZT10/65/35
ceramic exhibits excellent temperature stability and reduced loss in
Fig. 3. Microstructure evolution of PLZT10/65/35 ceramic with temperature. The dark field TEM images at (a) 20 °C, (b) 50 °C and (c) 100 °C.
Fig. 2. (a)Temperature-dependenceof dielectric permittivity andloss tangent at 10 Hz for PLZT10/65/35 ceramic. (b) Polarization (P)- electric field (E) loops for PLZT10/65/35 at different
temperatures. (c) Temperature-dependence of energy density for PLZT10/65/35 ceramic showing an excellent thermal reliability, and the inset shows its comparison with tricritical
ferroelectric material Ba(Ti
0.895
Sn
0.105
)O
3
(BTS). (d) Temperature-dependence of efficiency for PLZT10/65/35 ceramic, and the inset shows thetemperature evolution of loss.
116 J. Gao et al. / Scripta Materialia 137 (2017) 114–118

low electric field region, and thus has the potential to be used as the en-
ergy storage devices on the finite voltage occasions (e.g. portable or
wearable electronics).
The underlying mechanism for the temperature stability of PLZT10/
65/35 ceramic can be interpreted by observing its microstructure evolu-
tion. We performed the TEM observation for the specimen and the
change of domain pattern has been monitored. It is well-known that
the polar nanoregions (PNRs) play an important role on the properties
of relaxor ferroelectric materials [37,38,46]. In order to observe PNRs
in PLZT10/65/35 ceramic, dark field images were taken with the objec-
tive aperture displaced to (111) reflection with [011] beam incidence. It
canbeseenfromFig. 3(a) that the microstructure exhibits a mottled do-
main pattern at 20 °C, indicating a typical microstructure feature of
PNRS for relaxors. When the temperature rises to 50 °C and 100 °C, as
shown in Fig. 3(b) and (c), the mottled domain patterns still remain, in-
dicating the existence of PNRs at the elevated temperatures. Therefore,
the gradual change for PNRs of relaxor ferroelectrics may facilitate a
thermal reliability for the energy storage properties of PLZT10/65/35
ceramic.
The physical mechanism of high energy density over wide tempera-
ture range in PLZT relaxor ferroelectrics can be attributed to the contin-
uous formation and growth of PNRs under temperature and electric
field. Fig. 4(a) shows the temperature dependence of polar domain
structures upon the temperature changing through phase field simula-
tions. The color and arrow directions of the microstructure show differ-
ent polar domain states. Our calculations based on Eqs. (1)–(3) can help
us to obtain the microstructure evolution at different temperature upon
cooling (Fig. 4(a)), which show that the polar domain gradually grow
upon cooling and all the polar domains are confined to a nanoscale be-
cause of the existence of local polarization (local field effect) caused by
randomly distributed defects. The phase transition behavior in PLZT x/
65/35 changes from paraelectric to ferroelectric to paraelectric to
relaxor with the increasing of composition x [47,48].Thelocalfield ef-
fect produces preferred PNRs formation position and limits the growth
of long-range-ordered ferroelectric phase. The coexistence of PNRs
and paraelectrics over a wide temperature range (above the freezing
temperature T
f
≈240 K) [47,48] can be viewed as the origin of wide
temperature operation range of energy storage in relaxor ferroelectrics.
Fig. 4(b) shows the polarization-electric field hysteresis loop of relaxor
ferroelectrics and related polar domain evolution at 20 °C through
phase field simulations. The P-E loop shows a narrow hysteresis loop
which may cause the high-density energy conversion efficiency. Insets
show the related polar domain evolution with electric field. When the
electric field increases, the system transforms from a multi polar do-
mains (different polardirection as shown by the colors and arrow direc-
tions) to almost one polar domains (most polar domains align to the
same direction and show yellow color) with remnant polar domains
stabilized by dopants. When the electric field is removed, the system
trend to return to its origin state of multi polar domains microstruc-
tures. This can be considered as the origin of large energy density and
high efficiency for relaxor ferroelectric material PLZT at low electric
field.
In conclusion, excellent temperature stability for the energy storage
property has been found in the (Pb
0.9
La
0.1
)(Zr
0.65
Ti
0.35
)O
3
(PLZT10/65/
35) ceramic. The variation of energy density is limited within a low
level (b15%) over a wide temperature range between 24 °C and 83 °C,
and the coefficient can reach to 70% ~ 80% at low electric field of E
b25 kV/cm. The reason for this temperature-stable energy storage per-
formance can be ascribed to the gradual change of PNRS with tempera-
ture suggested by TEM observation. Further phase field simulation
indicates that continuous formation and growth of PNRs are responsible
for excellent temperature stability of PLZT relaxor ferroelectrics. Our re-
sults may shed light on developing energy-storing dielectrics with large
temperature stability.
Acknowledgement
The authors gratefully acknowledge the support of the National Key
Research and Development Program of China (Grant Nos.
2016YFB0701302 and 2014CB644003), the National Natural Science
Foundation of China (Grant Nos. 51471127, 51671156) and National Ba-
sic Research Program of China (Grant Nos. 51431007, IRT13034, and
51321003), J.G. acknowledges State Key Laboratory of Electrical Insula-
tion and Power Equipment (EIPE16311) for financial support.
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