Journal of
Materials Chemistry C
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Pyroelectric and electrocaloric materials
Cite this: J. Mater. Chem. C, 2013, 1,
23
Xinyu Li,a Sheng-Guo Lu,a Xiang-Zhong Chen,ab Haiming Gu,a Xiao-shi Qiana
and Q. M. Zhang*a
Published on 23 October 2012. Downloaded on 2/3/2023 12:07:31 PM.
Both the pyroelectric and electrocaloric effects originate from the cross-coupling between polarization and
temperature in insulating dielectrics. Although both effects have been studied for many decades for
various applications and large pyroelectric effect has been observed in many polar-dielectrics, it is only
very recently that a large electrocaloric effect (ECE) was obtained in ferroelectric ceramic thin films and
Received 26th September 2012
Accepted 26th September 2012
polymers, which revives the interest in the ECE. This review will summarize typical properties of
pyroelectric and electrocaloric materials, present figures of merit for both phenomena, examine the
relationship between the pyroelectric and electrocaloric effect. Moreover, we will also present
DOI: 10.1039/c2tc00283c
theoretical works, experimental results, and material modifications to achieve large responses in
www.rsc.org/MaterialsC
electrocaloric materials.
1
Introduction
The pyroelectric and electrocaloric effects originate from the
cross-coupling between polarization and temperature in certain
classes of insulating dielectrics. In simple terms, the pyroelectric effect refers to the polarization change caused by temperature change, while the electrocaloric effect is the reversed
process, i.e., a temperature change will be generated when there
is a polarization change. These effects can be related by the
Maxwell relations.1
Pyroelectricity was rst observed more than 2400 years ago,2
and has been studied for a long time.3–6 Pyroelectric materials
a
Department of Electrical Engineering and Materials Research Institute, The
Pennsylvania State University, University Park, PA 16802, USA. E-mail: qxz1@psu.edu
b
Department of Polymer Science & Engineering and Key Laboratory of Mesoscopic
Chemistry of MOE, School of Chemistry & Chemical Engineering, Nanjing
University, Nanjing, 210093, China
Xinyu Li is a Ph.D. candidate in
the
Electrical
Engineering
Department of The Pennsylvania
State University. He obtained
his B.E. degree in Department of
Materials Science and Engineering, Tsinghua University,
Beijing, China. His current
research focuses on the development of the electrocaloric
effect in organic materials, as
well as the design of solid-state
cooling devices based on the
phenomenon.
This journal is ª The Royal Society of Chemistry 2013
have found a wide range of applications,7 such as thermal
imaging, laser detectors, radiometers, infrared sensors, pollution monitor and tools for gas analysis, re alarms, and intruder
alarms.
Although the study on the electrocaloric effect (ECE) can be
dated back to 1930, when a very weak ECE was measured in
Rochelle salt,8 it was not until the 1960s that more studies on
the electrocaloric materials were carried out. However, the
small ECE observed in the second half of 20th century, where the
adiabatic temperature change DT was less than 2 C, makes it
not attractive for practical applications. The eld has been
revived since 2006, when Mischenko et al. reported a giant ECE
in ceramic thin lms in which DT of 12 C was observed at
226 C,9 and Neese et al. discovered a giant ECE near room
temperature in a class of ferroelectric polymer, which exhibits
both a large DT (>12 C) and isothermal entropy change DS
(>50 J kg1 K1).10 Electrocaloric materials with large ECE are
Sheng-Guo (David) Lu is
currently a senior research
scientist consultant at Strategic
Polymer Sciences and an adjunct
senior research associate at The
Pennsylvania State University.
Dr Lu’s interest includes functional inorganic materials, electroactive polymers, composites,
and applications as sensors,
actuators, and electrocaloric
refrigerators. He is a senior
member of IEEE and has over
100 publications.
J. Mater. Chem. C, 2013, 1, 23–37 | 23
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Journal of Materials Chemistry C
promising to realize compact cooling devices for next-generation refrigeration, temperature regulation and air-conditioning,
with high efficiency and environmentally friendliness.
This review will summarize main properties of typical pyroelectric and electrocaloric materials, including gures of merit
for both effects. In contrast to the study of pyroelectric materials
and their device applications, which is a relatively mature eld,
the understanding of electrocaloric materials is still at an early
stage. Hence, this review will devote most of the volume to
covering theoretical works and experimental results related to
the ECE. The relationship between the pyroelectric and electrocaloric effect will be discussed, and more importantly,
considerations for material modications to achieve large ECE
(with an emphasis on electrocaloric polymers) will be
presented.
2
Pyroelectricity and pyroelectric materials
2.1
Pyroelectricity
Pyroelectricity is the electrical response of an insulating
dielectric to a change in temperature. In most experimental
studies, the pyroelectric coefficient p is,
dD
X
p ¼
;
(1)
dT X ;E
Feature Article
where D is the electric displacement (in most pyroelectric
materials, D is approximately equal to the P, the polarization), T
is the temperature, X is the stress, and E is the electric eld.
Subscript X means the measurement is made in stress free
conditions.
Pyroelectric materials are usually polar materials in which
the lattice is non-centrosymmetrical. Among the 32 point group
symmetries of crystals, only 10 belong to the polar point group,
i.e., noncentrosymmetrical. They are: triclinic, 1 (C1); monoclinic, m and 2 (C2); orthorhombic, 2 mm (C2V); trigonal, 3 (C3)
and 3 m (C3V); tetragonal, 4 (C4) and 4 mm (C4V); hexagonal, 6
(C6) and 6 mm (C6V). In addition, pyroelectricity is allowed in
the two limiting Curie groups N (CN) and Nmm (CNV), which
represent symmetry of a textured material (e.g., fabric structure)
and that of an electrically poled polycrystalline material,
respectively.
Pyroelectricity was rst observed more than 2400 years ago
by the Greek philosopher Theophrastus,2 and has been studied
for a long time.3–6 To date, a large number of pyroelectric
materials have been reported.7,11–15 Examples include barium
strontium titanate (BST), triglycinesulphate (TGS), lithium
niobate (LNB), lead zirconatetitanate (PZT), lead magnesium
niobate-lead titanate (PMN-PT), strontium barium niobate
(SBN), and polyvinylidene uoride (PVDF).
Xiang-Zhong Chen received his
bachelor degree (2007) in Polymer Materials and Engineering.
He is pursuing his PhD degree in
Polymer Chemistry and Physics
from
Nanjing
University.
Currently, he is visiting Prof.
Qiming Zhang’s group at The
Pennsylvania State University.
His research focuses on energy
storage and conversion properties of PVDF-based electroactive
polymers.
Xiao-shi Qian studied materials
science and engineering at
Nanjing University, China, and
received his masters degree in
2010. He is pursuing his Ph.D.
degree in electric engineering
department and Dr Qiming
Zhang’s lab. He is conducting
research on the electrocaloric
effect in dielectric uids and
ferroelectric polymers, and
cooling devices using electrocaloric effect materials as
refrigerant.
Haiming Gu received his B.S.
and M.S. degree from Department of Electronics Engineering
and Institute of Microelectronics, Tsinghua University,
Beijing China. Currently, he is a
Ph.D. candidate at The Pennsylvania State University. His
research topics focus on
modeling and fabrication of
compact-size/micro-size solidstate cooling devices based on
electrocaloric effect.
Dr Qiming Zhang is Distinguished Professor of Electrical
Engineering
and
Materials
Science and Engineering of Penn
State University. His research
area is in novel electronic materials, especially so electronic
materials and ferroelectric based
materials, covering a broad
range of applications of solid
state electronic materials such as
electromechanical,
dielectric,
photonic and electro-optic, and
pyroelectric applications.
24 | J. Mater. Chem. C, 2013, 1, 23–37
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2.2
Journal of Materials Chemistry C
Figure of merit (FOM) for pyroelectric materials
Pyroelectric materials have found a number of applications in
areas such as thermal imaging, laser detectors, radiometers,
infrared sensors, pollution monitoring and gas analysis, re
alarms, and intruder alarms.7 For these devices, three gures of
merit are usually employed to evaluate a pyroelectric material
and its interface with the electronic readout circuit, which can
be a charge sensitive circuit or a voltage sensitivity circuit. They
are current response FOM Fi ¼ p for the charge sensitive
readout, voltage response FOM FV ¼ p/30 for the readout circuit
in voltage mode, and for detectors, the signal to noise ratio
pffiffiffiffiffi
(SNR) is a critical parameter and FOM FD ¼ p= 300 measures the
SNR of the sensor. Here, 30 and 300 are the real and imaginary
parts of permittivity, respectively. Some examples of these FOM
will be presented in the next section.
2.3
Secondary pyroelectric effect
The primary pyroelectric coefficient is dened as the pyroelectric
coefficient measured in a clamped condition px (strain x ¼
constant). As has been observed, pyroelectricity measured in a
clamped condition can be quite different from that measured
under stress free conditions (X ¼ constant) because of the piezoelectric contribution caused by the thermal expansion of the
material. The pyroelectric coefficient caused by the thermal
expansion is termed as the secondary pyroelectric coefficient. The
total pyroelectric coefficient measured at constant stress pX can be
expressed as16,17
x
X X,E E
pX
i ¼ pi + dijkcjklmalm,
(2)
where dXijk is the piezoelectric strain tensor under a constant
stress, cX,E
jklm the elastic stiffness tensor under constant stress and
electric eld, aElm the thermal expansion tensor under a constant
Table 1
electric eld. Hence the rst term in eqn (2) represents the
primary pyroelectric effect and the second term is the secondary
effect. As examples, Table 1 lists the primary and secondary
pyroelectric coefficients of several organic and inorganic materials measured at room temperature, along with their symmetry
groups. As can be seen, for some pyroelectric materials, the
secondary effect can be comparable or even larger than that of
the primary effect.
2.4
Pyroelectric materials
Large numbers of ferroelectrics have been investigated for
pyroelectric applications, which involve organic and inorganic
materials. On the organic material side, PVDF based polymers,
and side-chain liquid crystal polymers have been studied. On
the inorganic material side, single crystals, poled polycrystalline ceramics, and ceramic thin lms have been
explored. Near a continuous ferroelectric–paraelectric (FE–PE)
phase transition, an extremely large pyroelectric coefficient
may be obtained when under a DC bias electric eld. In
addition, a hybrid approach, ceramic–polymer composites, has
also been studied.
PVDF as well as many PVDF based copolymers and
terpolymers possess ferroelectricity. In most cases, these are
semi-crystalline polymers and the crystalline phase is in a
lamellar form. Polarization is the long range ordering of the
dipoles formed from the C–H/C–F bonds, which are
perpendicular to the backbone chains.18 PVDF does not show
a FE–PE phase transition before melting (180 C).19 The
copolymers of PVDF with triuoroethylene (TrFE >20 mol%)
display FE–PE phase transition before melting and hence at
room temperature show stronger pyroelectric coefficient.20
Table 2 lists the pyroelectric coefficients of the P(VDF-TrFE)
copolymers
at
different
compositions
at
room
temperature.21,22
Pyroelectric coefficients of several organic and inorganic materials13
Materials
(A) Nonferroelectrics
CdS (6 mm)
CdSe (6 mm)
ZnO (6 mm)
BeO (6 mm)
Tourmaline (3 m)
Li2SO4$2H2O (2)
(B) Ferroelectrics
LiNbO3 (3m)
LiTaO3 (3m)
NaNO2 (2 mm)
Pb5Ge3O11 (3)
Sr0.5Ba0.5Nb2O6 (4 mm)
Ba2NaNb5O15 (2 mm)
TGS (2)
PVDF (2 mm)
BaTiO3 (Nm)
Pb(Zr0.95Ti0.05)O3(Nm)
Pb(Zr0.52Ti0.48)O3(Nm)
Primary coefficient
(mC m2 K1)
Secondary coefficient
(mC m2 K1)
3.0
2.94
6.9
3.39
0.48
+60.2
1.0
0.56
2.5
0.01
3.52
+20.1
95.9
178
135
110.5
529
141.8
330
14
260
305.7
110
+12.9
+2.0
5.0
+15.5
21
+41.8
+60
13
+60
+37.7
+60
This journal is ª The Royal Society of Chemistry 2013
Measured coefficient
(mC m2 K1)
4.0
3.50
9.4
3.40
4.0
+80.3
83
176
140
95
550
100
270
27
200
268
50
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Journal of Materials Chemistry C
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Table 2
Feature Article
Room temperature pyroelectric properties of P(VDF-TrFE) copolymers and a LCP21–23
Materials
Fi ¼ p (mC m2 K1)
30
FV ¼ p/30 (mC m2 K1)
pffiffiffiffiffi
FD ¼ p= 300 (mC m2 K1)
P(VDF-TrFE) 50/50
P(VDF-TrFE) 56/44
P(VDF-TrFE) 70/30
P(VDF-TrFE) 75/25
P(VDF-TrFE) 80/20
PVDF
LCP
40
38
55
33
31
27
0.98
18
12
8
7.4
7
11
6.7
2
3
6.8
4
4.4
2.46
0.15
5.06
70
200
94
91
33.24
3.46
Liquid crystal side-chain polymers (LCP), where the polymer
architecture consists of a polymer backbone, exible spacers,
and mesogenic groups based on a cyanobiphenyl moiety, also
exhibit some pyroelectric response.23 One example is the polymethacrylates separated with alkyl chains of ve to seven
methylene groups from 40 -cyano-4-biphenyl mesogenic
moieties.
There are many reports on the pyroelectric coefficients for
bulk single crystals and ceramics, especially the Pb(Mg1/3Nb2/3)
O3–0.26PbTiO3(PMN-PT) and Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)
O3–PbTiO3(PIMNT) relaxor ferroelectric single crystals and
SrBaNb2O6(SBN) ceramics synthesized by the hot forging
method.2434 Single crystals demonstrate large pyroelectric
coefficients and FOMs as listed in Table 3.24–34
Thin lms are attractive for pyroelectric applications
because they can form texture or epitaxial structure, leading
to properties approaching that of single crystals. Thin lm
devices are also more compatible with the semiconductor
fabrication process, since many thin lms can be integrated
on Si substrates at temperatures as low as 600 C. In addition, it was also observed that the relaxor ferroelectric thin
lms exhibit much larger pyroelectric coefficients than
conventional perovskite structured thin lms.15 Table 4
presents pyroelectric coefficients and FOMs of several thin
lms.35–45
The ceramic–polymer composites combine polymeric properties such as mechanical exibility, formability, and low cost
with the ceramic properties such as large p and excellent
mechanical strength. These hybrids exhibit better pyroelectric
properties over the individual constituent phase while
Table 3
possessing excellent mechanical strength, formability, and
robustness of the polymer, which may someday be useful for IR
detectors without supporting substrates. Table 5 lists pyroelectric coefficients and FOMs of several ceramic–polymer
composites.46–52
3 Electrocaloric effect and electrocaloric
materials
3.1
General consideration of the electrocaloric effect
3.1.1 MAXWELL RELATIONS. In general, the Gibbs free energy
G for a dielectric material could be expressed as a function of
temperature T, entropy S, stress X, strain x, electric eld E and
electric displacement D in the form
G ¼ U TS Xixi EjDj,
(3)
where U is the internal energy of the system, the stress and eld
terms are written using Einstein notation where i runs from 1 to
6 and j is from 1 to 3. The differential form of eqn (3) is
dG ¼ SdT xidXi DjdEj,
Hence the entropy S and electric displacement Dj are,
vG
vG
S¼
; Dj ¼ ;
vT X ;E
vEj T;X
(4)
(5)
Eqn (5) leads to the Maxwell relation, linking the electrocaloric effect to the pyroelectric effect for a thermodynamically
reversible system,1
Pyroelectric properties of PMN-PT crystals and SBN ceramics at room temperature24–34
Materials
Fi ¼ p (mC m2 K1)
30
FV ¼ p/30 (mC m2 K1)
pffiffiffiffiffi
FD ¼ p= 300 (mC m2 K1)
PMN-0.13PT h111i poled
PMN-0.21PT h111i poled
Fe doped PMN-0.38PT
PMN-0.29PT
Mn doped PMN-0.38PT
Mn doped PMN-0.26PT
PIMNT (42/30/28)
SBN53/47 HF (t)a
SBN53/47 HF (k)a
3260
1790
568
1280
1620
1720
900
510
400
3107
961
310
515
688
660
702
980
468
1.0
1.8
1.8
2.4
2.3
2.6
1.28
0.52
0.85
1018
1059
394
710
2790
2994
760
120
263
a
t: parallel to pressing axis; k: perpendicular to pressing axis.
26 | J. Mater. Chem. C, 2013, 1, 23–37
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Table 4
Journal of Materials Chemistry C
Pyroelectric coefficients and FOMs of thin films35–45
Materials
Fi ¼ p (mC m2 K1)
30
FV ¼ p/30 (mC m2 K1)
pffiffiffiffiffi
FD ¼ p= 300 (mC m2 K1)
PbTiO3/Pt/Si
PZT 15/85/Pt/Si
PLT10-20/Pt/Si
Porous PLT/Pt/Si
LiNbO3 (006)/Pt/Si
PLT5-15/Pt/MgO
PLZT 7.5/8/92-20/80
PbSc1/2Ta1/2O3/sapphire
K0.89Na0.11Ta0.55Nb0.45O3/KTaO3
130–145
160–220
200–576
220
71
400–1300
360–820
6000
5200
180–260
200–230
153–550
90
(30)
100–350
193–260
6500
12000
0.50–0.81
0.70–1.10
2.3
2.4
2.4
1.1–13
1.4–4.2
0.92
0.43
43–91
86–156
55–466
232
130
214–1678
171–518
430
336
vS
vEj
vDj
¼
¼ pE ;
vT E;X
T;X
ðE (6)
DS ¼
0
and
vT
vE
T vD
TpE
¼
;
cE
cE vT E
¼
S
(7)
where cE is the heat capacity. Under a constant stress X, the
isothermal entropy change DS and adiabatic temperature
change DT of an ECE material as the E is changes from E1 to E2
can be expressed as1
E
ð2 vD
DS ¼
dE;
(8)
vT E
E1
E
ð2
DT ¼ T
E1
1 vD
dE;
cE vT E
(9)
Eqn (6) through eqn (9) indicate that in order to achieve a
large DS and DT, the dielectric materials should possess a
large pyroelectric coefficient over a broad electric eld range.
For ferroelectric materials, a large pyroelectric effect exists
near the FE–PE phase transition. It is also noted that a large
DT may be achieved even if DS is small when cE of a dielectric
material is small. However, for practical refrigeration applications, an ECE material should possess both a large DS
and DT.53
It is noted that in the temperature region including a rstorder FE–PE transition, eqn (8) should be modied to take into
account the discontinuous change of the polarization DP (and
DD) at the transition temperature, i.e.,
Table 5
vD
vE
;
dE DD
vT E
vT
(10)
It should be emphasized that one should be cautious in
using the Maxwell relation to deduce the ECE near rst order
FE–PE transitions. The hysteresis associated with rst order FE–
PE transition means the process is not thermodynamically
reversible and the Maxwell relation is derived based on reversible thermodynamic process.
Although a few studies on the ECE were conducted in which
direct measurement of DT and/or DS was made,54,55 most
experimental studies were based on the Maxwell relation where
the electric displacement D versus temperature T under
different electric elds was characterized. DS and DT were
deduced from eqn (8) and (9).
3.1.2 ECE DERIVED FROM THE LANDAU–DEVONSHIRE
PHENOMENOLOGICAL THEORY. The Landau–Devonshire (L–D)
phenomenological theory has been widely utilized to describe
the macroscopic phenomena that occur in the polar materials,
e.g. ferroelectric or ferromagnetic materials near their phase
transitions. Here the L–D phenomenological theory is used to
estimate the ECE of ferroelectrics. From the L–D theory, the
Gibbs free energy of a ferroelectric material can be written as
an expansion of the polarization P (for most ferroelectrics,
P z D) as1
1
1
1
G ¼ aP2 þ xP4 þ zP6 EP;
2
4
6
(11)
where a ¼ b(T T0), and b, x and z are temperature-indepen vG
dent phenomenological coefficients. From
¼ DS and
vT E
Pyroelectric coefficients and FOMs of ceramic–polymer composites46–52
Materials
Fi ¼ p (mC m2 K1)
30
FV ¼ p/30 (mC m2 K1)
pffiffiffiffiffi
FD ¼ p= 300 (mC m2 K1)
TGS-PVDF
PT-P(VDF-TrFE)
PCLT-P(VDF-TrFE)
PZT-PU
PZT-P(VDF-TrFE)
LiTaO3/P(VDF-TrFE) @70 C
90
40.7
56.5
90
92
137.5
27.3
57.3
15.1
24
29
20.7
3.3
0.71
3.74
3.75
3.2
6.6
—
28
113
22
93
67
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J. Mater. Chem. C, 2013, 1, 23–37 | 27
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Journal of Materials Chemistry C
Feature Article
at temperatures above FE–PE transition where a single P value
exists under an applied eld E, one obtains,
1
DS ¼ bP2 ;
2
(12)
and the adiabatic temperature change DT(¼TDS/cE) can be
obtained, i.e.,
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DT ¼
1
bTP2 ;
2cE
(14)
which yields a general form of the Maxwell relation,
vQ
vS
¼
:
vT V ;F
vV T;F
(15)
When the dimensional change is small, eqn (15) is reduced
to eqn (6).
Assuming the ECE lm has a volume U, an area Az in XY
plane (perpendicular to the lm thickness and the applied
28 | J. Mater. Chem. C, 2013, 1, 23–37
dQ ¼ AzdP + PdAz,
(16)
if dAz, the dimensional change, is not negligibly small.
So the unit volume entropy Ds ¼ DS/U is
E
ð2 (13)
Eqn (12) indicates that the entropy will be reduced when the
material increases its polarization or changes to a polar state
from a non-polar state when an external action, e.g. temperature, electric eld or stress, is applied. The entropy change and
temperature change are associated with the phenomenological
coefficient b and polarization, viz. proportional to b and P2. Both
parameters will affect the ECE values of the materials. A material with a large b and a large P will generate a large ECE entropy
change and temperature change near the FE–PE phase transition temperature.
As an example, the L–D phenomenological theory predicts
large ECE values in ferroelectric P(VDF-TrFE) polymers. P(VDFTrFE) 65/35 mol% copolymer, with b ¼ 3.5 107 J m C2 K1
and D ¼ 0.08 Cm2,56 will exhibit a DS ¼ 62 J kg1 K1. Making
use of its specic heat capacity cE ¼ 1.4 103 J kg1 K1,57 and
Curie temperature Tc ¼ 102 C,56 yields a DT ¼ 16.6 C. The large
DS and DT values suggest that a large ECE may be achieved in
ferroelectric P(VDF-TrFE) copolymers.
In addition, the heat of FE–PE phase transition can also be
used to assess the ECE (Q ¼ TDS) in a ferroelectric material at
temperatures above the FE–PE transition. For example, P(VDFTrFE) 68/32 mol% copolymer shows a heat of FE–PE transition
larger than 2.1 104 J kg1 (or DS z 56.0 J kg1 K1), which is
consistent with the prediction based on the phenomenological
theory that a large ECE can be obtained in P(VDF-TrFE) ferroelectric polymers.10,58,59
3.1.3 CONTRIBUTION OF SECONDARY PYROELECTRICITY TO THE
ECE. For polymeric materials, the secondary pyroelectric effect
can be signicant. In order to derive the thermodynamic
relations correctly, the secondary effect should be included in
the considerations. In a real experimental situation, it is the
force F, displacement R, voltage V, charge Q, temperature T,
and the total entropy S of the sample that are directly
measured. The general form of the elastic Gibbs energy, hence,
can be written as60
dG ¼ SdT R$dF QdV,
electric eld), the electric eld is along the z direction, and
let D ¼ P for ferroelectric polymers, then dQ should be
written as
Ds ¼
vQ
vT
E1
Az dE:
(17)
E;X
On the device side, the total dipole moment M of the sample
is related to the measured charge Q and polarization P as60,61
Q ¼ AP ¼ M/d.
(18)
1 vM
vP
¼A
and eqn (6), we have
Making use of
d vT V
vT V
1
0
V
ð2 P vd
C
B1
dV A:
Ds ¼ @ bP2 þ
(19)
2
d 2 vT V
V1
For freestanding lms, the mechanical boundary conditions
are Xi ¼ 0 (I ¼ 1, 2, and 3). Then the entropy change per unit
volume Ds in eqn (19) has the form62
1
0
E
ð2 P vd
C
B1
dE A:
Ds ¼ @ bP2 þ
(20)
2
d vT E
E1
Using (vd/d)/vT ¼ a and equation (E ¼ b(T Tc)P + gP3 for a
2 order phase transition), Ds becomes
nd
1
1
1
Ds ¼ bP2 aPE agP4 :
2
2
4
(21)
Taking the typical data of b ¼ 2.4 107 J m C2 K1,58,63 a ¼ 2
103 K1 (in the phase transition regime),64 and g ¼ 8.3 1011 J m5 C4,65 for a P(VDF-TrFE)55/45 mol% copolymer, it can
1
be estimated that bP 2 ¼ 5:88 104 J m3 K1 , the sum of the
2
last two terms on the right side of eqn (21) is 2.05 104 J m3
K1, where P ¼ 0.07 Cm2 and E ¼ 150 MV m1 are used. The
contribution of secondary pyroelectricity to the ECE is 26%. The
results indicate that for polymers, the thermal expansion has a
signicant impact on the ECE entropy change through the
interaction among the thermal expansion, electric eld, and
polarization.
Now if the polymer lms are xed to an inorganic substrate,
the boundary conditions become a1 ¼ a2, x1 ¼ x2 ¼ 0, X1 ¼ X2 s
0 (the lm surface is along the XY plane and Z is the lm
thickness direction). Using the electrostriction instead of
piezoelectricity,66–68 near the phase transition temperature, the
elastic Gibbs free energy can be written as
1
1
G ¼ bðT Tc ÞP2 þ gP4 EP ðX1 þ X2 Þ Q13 P2
2
4
1 2
s11 X1 þ X2 2 s12 X1 X2 :
2
(22)
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The equations of state could be expressed as11
E,T
X,T
dx1 ¼ dx2 ¼ (sE,T
11 + s12 )dX1 + 2Q13 P3dP3 ¼ 0,
(23)
and it can be deduced that
1
2a1 S1 þ 2a21 T
:
Ds ¼ bP2 þ
2
s11 þ s12
(24)
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Eqn (24) indicates that the clamping effect of the substrate
can change the ECE of polymer lms. Since for P(VDF-TrFE)
based polymers, the strain S1 ¼ Q13P2 > 0,69 the second term of
the right hand side of eqn (24) will enhance the ECE entropy
change in the clamped situation in addition to the contribution
of the eld induced phase transition.70
3.2
Fig. 2 Schematic illustration of the two most common crystalline chain
conformations in PVDF (a) tg+tg conformation and (b) all-trans conformation.
ECE measurement methods
Generally, ECE measurement methods can be categorized into
two groups: the indirect method, where entropy change DS and
temperature change DT are deduced using Maxwell’s relation;
and the direct method, where DS and/or DT are acquired from
specially designed calorimeters.
The indirect method is based on the Maxwell relation, shown
in eqn (6), from which the DS and DT of the ECE as the electric
eld changes from E1 to E2 can be deduced, i.e., eqn (8) and (9).
The indirect method has been widely used to characterize the ECE
since the 1960s,71 due to the fact that the apparatus measuring the
polarization as a function of temperature is quite universal and
relatively convenient to use. However, it should be noted that
since the Maxwell relation is derived for ergodic systems, one
needs to be cautious when using the indirect method to deducing
the ECE in material systems that are not thermodynamically
reversible such as the relaxor ferroelectric polymers.72
The direct method (measuring entropy change (extensive) or
temperature change (intensive)) can thus be divided into two
subgroups. Since there is no standard commercial equipment
for measuring the ECE, various calorimeters have been developed by several research groups to measure the electrocaloric
effect under different electric elds over a wide temperature
range. The commercial DSC system can be modied so that
electric elds can be applied to the sample, and the heat
absorbed or ejected by the sample is simultaneously recorded.28
A calorimeter using a heat ux sensor was developed by Zhang’s
group at Penn State,73 as shown in Fig. 1, where the heat
generated by the ECE of the sample is calibrated using the heat
Fig. 1 Schematic configuration of direct ECE measurement setup with heat flux
sensor.
This journal is ª The Royal Society of Chemistry 2013
generated by a standard reference resistor R, from which DS is
determined. By substituting the heat ux sensor with an
infrared sensor or thermal couple, the adiabatic temperature
change can be obtained using the same calorimeter.63 A high
resolution calorimeter using a miniature thermistor and weak
thermal link to the thermal bath was also developed for direct
ECE measurement in thin lms.63 Additionally, other apparatuses using multiple thermometers to measure the temperature
difference between the sample and background were also
reported.55,74 Factors that may inuence the accuracy of a direct
ECE measurement over a certain temperature range include the
sensitivity, signal-to-noise ratio and response time of the
thermal sensor, the stability of temperature controller, and the
ability of the system to provide good thermal contact during the
measurement.
3.3
Electrocaloric polymers
Very large ECE has been observed in PVDF-based polymers.
These materials can also be fabricated into thin lms (to
operate the cooling device in voltages normally used in
commercial cooling devices) and large size devices, these polymers are probably the most likely ECE materials to be developed
into commercial applications. This section will summarize the
main advancements achieved in this eld.
In PVDF based polymers, it has been reported that there exist
ve crystalline phases. The most interesting one is the ferroelectric b phase, in which, polymer chains adopt all-trans
(TTTT) conformation, where all the uorine atoms are on one
side of the chain, forming dipoles perpendicular to the chain
direction, as illustrated in Fig. 2.18,75,76 The ferroelectric b phase
will undergo a phase transition and turn into a paraelectric
phase, a phase consisting of a random sequence of trans-gauche
(TG) bonds, such as TGTG0 , and T3GT3G0 isomers. Correspondingly, the dipoles in the crystallites also change from an
ordered state to a disordered state. Additionally, with defect
modication such as high energy irradiation or copolymerizing
with a third bulky monomer, the normal ferroelectric polymers
can be converted into relaxor ferroelectrics, in which randomly
distributed nano-polar regions are embedded in a non-polar
matrix. When an external electric eld is applied, the random
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Fig. 3 (a) Isothermal entropy change DS and (b) adiabatic temperature change
DT, measured by indirect method, as a function of ambient temperature at
different electric fields for P(VDF-TrFE) 55/45 copolymer.
Fig. 4 Directly measured DT as a function of temperature under several electric
fields for non-stretched P(VDF-TrFE) 55/45 mol% copolymers and comparison
with that indirectly measured.
Feature Article
The PVDF-based polymers that have been studied include
P(VDF-TrFE)
copolymers,
high-energy-electron-irradiated
P(VDF-TrFE) copolymers, (P(VDF-TrFE-CFE)) terpolymers (CFE:
chlorouoroethylene), and P(VDF-TrFE) copolymer/P(VDFTrFE-CFE) terpolymer blends.
3.3.1 ECE IN THE NORMAL FERROELECTRIC P(VDF-TRFE)
COPOLYMER. In a ferroelectric copolymer, the FE–PE transition
corresponds to the order–disorder transition of the dipole
states, thus a large ECE associated with dipole rearrangement
would be generated near the FE–PE phase transition. P(VDFTrFE) 55/45 mol% copolymer was studied as an electrocaloric
material because its FE–PE phase transition is continuous.
Fig. 3 presents DS and DT at several electric eld levels
deduced by indirect measurement.10 As can be seen, the
copolymer exhibits a DS of more than 55 J kg1 K1 and DT of
more than 12 C under an electric eld of 209 MV m1 and at
temperatures around 80 C, which are the rst experimental
result demonstrating a giant ECE in polymers. Fig. 4 presents
DS and DT as a function of temperature measured under several
electric elds in P(VDF-TrFE) 55/45 mol% copolymer using the
direct method as well as the comparison between the ECE
results directly measured and calculated from the indirect
method.62 The data show that the ECE reaches a maximum at
the FE–PE transition, where a DT ¼ 12 C can be induced under
a 120 MV m1 electric eld. At temperatures away from the FE–
PE transition, DT drops rapidly, which is a characteristic feature
for normal ferroelectric materials. The comparison result
reveals that within the experimental error, the ECE deduced
from the Maxwell relation is consistent with that directly
measured, and also indicates that for a ferroelectric material at
temperatures above FE–PE transition, the Maxwell relation is
valid for deducing ECE.
3.3.2 ECE IN THE RELAXOR POLYMERS. Defect modication,
either through copolymerization with a bulky monomer such as
CFE or CTFE (chlorotriuoroethylene) to form a terpolymer or by
direct high-energy-electron irradiation on copolymer lms, can
convert the normal ferroelectric P(VDF-TrFE) polymer into a
ferroelectric relaxor, with a large room-temperature dielectric
constant, slim P–E hysteresis loop and disorder dipole states at
room temperature.66,77 Defect modication shis the operating
temperature of the ECE material to around room temperature,
and relaxor polymers have the potential to generate an even
larger ECE than the normal ferroelectric P(VDF-TrFE) copolymer.
The ECE in a dielectric is determined by the dipolar entropy
change DSp between polar and non-polar states, as shown in
eqn (25) and (26),
DT ¼
T
Sp ð0; TÞ Sp ðE; TÞ ;
CE
(25)
Tln U
Psat 2 ;
330 QCE
(26)
DTsat ¼
dipoles in either the paraelectric phase of normal ferroelectric
polymers or in the relaxor ferroelectrics, will reorient along the
electric eld direction, causing a structure change in the polymer as well as a temperature change as compensation for the
entropy change.
30 | J. Mater. Chem. C, 2013, 1, 23–37
where Sp(0,T) is the dipolar entropy when E ¼ 0, and Sp(E,T)
corresponds to the entropy of a dipole aligned state when
external electric eld E is applied, CE is the heat capacity, U is
the number of possible polar states, Psat is the saturated
polarization value, Q is the Curie constant in electrocaloric
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Fig. 5 (a) Dielectric constant vs. temperature measured at different frequencies
(b) directly measured ECE for the high energy electron irradiated P(VDF-TrFE) 68/
32% relaxor copolymer.
materials.78 For a dipolar disordered state at E ¼ 0, such as the
relaxor at temperatures above the broad dielectric constant
peak, Sp(0,T) is proportional to P2ln(U). Thus, through defect
modication to introduce local states and enhance the number
of available polar states, relaxor ferroelectrics have the potential
to realize a large ECE around room temperature.
3.3.2.1 Giant electrocaloric effect in the irradiated P(VDFTrFE) relaxor copolymers. It has been shown that by employing
high energy electron irradiation, the normal ferroelectric
P(VDF-TrFE) copolymer can be converted to a relaxor ferroelectric polymer which displays high dielectric constant (50 at
1 kHz), large reversible polarization change, and high electrostriction at room temperature.66 Fig. 5(a) presents the dielectric
constant of a high energy electron irradiated P(VDF-TrFE) 68/32
mol% copolymer, which has a broad dielectric constant peak
around room temperature and that peak position moves
progressively toward higher temperatures with frequency, a
characteristic feature of relaxor ferroelectrics. The high energy
electron irradiation breaks up long range polar-correlation in
the polymer, which stabilizes dipolar disordered states around
room temperature and generates local polar-states that may
enhance the ECE, as discussed earlier. The ECE of the irradiated
P(VDF-TrFE) 68/32 mol% copolymer measured near the broad
dielectric peak (33 C) as a function of electric eld is presented
in Fig. 5(b).63 Under a eld of 160 MV m1, an adiabatic
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Journal of Materials Chemistry C
temperature change DT ¼ 20 C and isothermal entropy change
DS ¼ 95 J kg1 K1 can be obtained. Such a large ECE response
shows promise for compact cooling applications with high
efficiency and cooling power, and may open up a new direction
for electrocaloric materials research.
3.3.2.2 ECE in relaxor ferroelectric P(VDF-TrFE-CFE) terpolymers. In the P(VDF-TrFE-CFE) relaxor ferroelectric polymers,
large ECE has also been observed. ECE of the P(VDF-TrFE-CFE)
59.2/33.6/7.2 mol% terpolymer directly measured at 30 C, as
presented in Fig. 6(a), showing a very large DT 16 C induced
under 160 MV m1 electric eld; besides the large ECE, several
relaxor ferroelectric polymers also display a nearly temperature
independent ECE as presented in Fig. 6(b).73 For a P(VDF-TrFECFE) 59.2/33.6/7.2 mol% relaxor ferroelectric terpolymer, the
ECE response is nearly temperature independent from 0 C to
45 C, which is in sharp contrast to that in normal ferroelectrics
where the ECE peaks at the FE-PE transition and displays strong
temperature dependence. Such a temperature independent ECE
over a broad range is attractive for practical cooling device
applications, especially for those requiring large temperature
spans between cold and hot ends. These results also reveal the
intricate roles played by the defects in tailoring the ferroelectric
response and its polar nano-structures to generate a large ECE
and its temperature response behavior. As has been shown
here, the temperature dependence of the ECE of the terpolymer
lms depends critically on the lm preparation conditions,
while the uniaxially stretched terpolymer lms show a
pronounced temperature dependence on the ECE, the nonstretched lms exhibit a nearly temperature independent ECE
from 5 C to 45 C. At 5 C and 55 C, the DT of the uniaxially
stretched lms is more than 15% smaller, compared with that
of the non-stretched lms. Such a difference is likely to be
caused by the changes in possible polar states and polarcorrelation length due to lm stretching.
It has also been observed that the ECE acquired in
terpolymer by direct and indirect methods are very different, as
presented in Fig. 7. The directly measured ECE from the relaxor
terpolymer is much larger than that deduced from the Maxwell
relation. Moreover, the directly measured ECE displays a much
weaker temperature dependence at E < 70 MV m1. The results
indicate that the Maxwell relation is not suitable to deduce ECE
for the relaxor ferroelectric polymers even at temperatures
above the broad dielectric constant maximum, since the
Maxwell relations are valid only for systems in thermodynamic
equilibrium (ergodic systems), while the relaxor ferroelectric
polymers are non-ergodic systems even at temperatures above
the dielectric constant maximum.72
3.3.2.3 ECE in the relaxor ferroelectric P(VDF-TrFE-CFE)
terpolymers/P(VDF-TrFE) copolymer blends. It is well known that
nano-composites such as polymer blends, which exploit the
merits of both the base polymer and the additive polymer, offer
a great opportunity to enhance and tailor the material properties.19,79–81 It has been recently reported that the blends of
P(VDF-TrFE-CFE) relaxor terpolymer with 10 wt% of P(VDFTrFE) exhibit a 30% increase in the adiabatic temperature
change compared with that of pure terpolymer, as shown in
Fig. 8.82 Both increased crystallinity in the blends and
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Fig. 6 Directly measured ECE in non-stretched P(VDF-TrFE-CFE) 59.2/33.6/7.2 mol% terpolymer (a) adiabatic temperature change DT vs. applied electric field at 30 C
and (b) adiabatic temperature change DT and isothermal entropy change DS vs. sample temperature under different applied fields, (c) adiabatic temperature change DT
and isothermal entropy change DS vs. sample temperature under different applied fields in stretched P(VDF-TrFE-CFE) terpolymer under a constant electric field of 100
MV m1.
conversion of the copolymer from a normal ferroelectric to a
relaxor lead to an increased relaxor polarization response and
an enhanced ECE. Another advantage of the blends over pure
terpolymer is that the Young’s modulus of the blends is
improved compared to the pure terpolymer.
3.4
Electrocaloric ceramic materials
3.4.1 BULK MATERIALS AND THICK FILMS. Many early ECE
studies were limited to ceramics in the low temperature range
32 | J. Mater. Chem. C, 2013, 1, 23–37
of 4–15 K where the ceramics have low specic heat and hence
may exhibit a large adiabatic temperature change.83 The largest
ECE effect was found in SrTiO3,84,85 which is not ferroelectric
but has a temperature-dependent dielectric constant. The
maximum DT is 0.3 C from the ECE was observed at 10 K.85
The ECE at low temperature was also studied in KTaO3 single
crystals.86
Aer 1960s, the ECE in many perovskite ceramics were
studied. It is well known that the properties of these ceramics
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Fig. 7 Temperature dependences of the directly measured DT of the terpolymer
under different measuring electric fields.
Fig. 8 (a) Adiabatic temperature change as a function of electric field at room
temperature (b) Adiabatic temperature changes as a function of sample
temperature at fixed electric field of 100 MV m1.
can be tuned by doping, through which normal ferroelectrics
(FE), anti-ferroelectrics (AFE), and ferroelectric relaxors can be
obtained, providing very good models to understand the key
factors that may inuence the ECE.
Substitution of B-cations in PZT by Sn (PSZT) allows tuning
of the temperature and composition ranges of the ferroelectric
and antiferroelectric phases.71,87,88 The largest ECE (i.e., the
change of entropy at the phase transition) appears in compositions where the both the AFE and FE reach their stability limit
at high temperature and a rst order transition from a PE to FE
can be induced electrically. On the other hand, in compositions
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Journal of Materials Chemistry C
where an electric eld-induced AFE to FE transition is observed,
the ECE is much smaller and changes slowly in a wide range of
temperature.89 The largest electrocaloric effect in bulk ceramics
(2.6 C) was measured for PSZT annealed at high temperature.
The magnitude of this effect strongly depends on microstructure. Large grain ceramics with small temperature and eld
ranges of AFE phase stability yield the largest electrocaloric
effect.90 La doped PZT thick lms were also examined and a
maximum temperature change of 8.5 C was observed under a
eld of 7.5 MV m1.91
In normal ferroelectrics, the sharp FE–PE transition usually
leads to a narrow temperature range in which a large ECE exists.
However, the ECE can be found in a wider temperature range in
ferroelectric relaxors. For example, PMN (Pb(MgNb)TiO3)
exhibits a diffused dielectric phase transition around 13 C,
which leads to a relatively large ECE near room temperature,
i.e., maintaining a DT 2.5 C in temperature range from 16 C
to 67 C under an electric eld of 9 MV m1.92 PMN-PT (PbTiO3)
with different compositions were also investigated in both
ceramics and single crystals.93–98
In lead scandium niobate (PSN) and its solid solution with
lead scandium tantalate (PST), the ordering of perovskite heterovalent ions in sublattice B can be tailored either by substitution or thermal treatment.99,100 A high degree of long-range
order leads to a high ECE maximum (DT ¼ 1.7 C) as well as a
high phase transition temperature, while a decreased order
causes a lower ECE (which occurs over a broader temperature
range) and diffuse phase transitions.
Some lead-free ceramics such as Ba0.73Sr0.27TiO3, Na0.5Bi0.5TiO3, and Ba0.3Na0.7Ti0.3Nb0.7O3 are also investigated for their
ECE.101–103 Although ECE is small, lead-free materials are
attractive because of their environmentally friendliness.
The ECE reported in all the bulk ceramic samples is below 3
C, which makes them unimpressive for practical applications.
The main reason is the low dielectric strength of bulk ceramics
and the difficult in fabricating bulk ceramic to thin thickness
(below 50 mm). By fabricating thick lms using an interdigitated
multilayer geometry, the lm thickness of each layer can be
reduced to 10 mm, which allows for application of high electric
elds. Multilayered PbSc0.5Ta0.5O3,104 BaTiO3105–108 were also
studied.
3.4.2 THIN FILMS. Thin ceramic lms allow for the application of high electric elds (>10 MV m1) and thus a high ECE
may be realized. Here we refer the thin lms as to those thinner
than 1 mm.
Mischenko et al.9 rst reported a temperature change of
12 C in 350 nm PbZr0.95Ti0.05O3 thin lm near Curie temperature of 222 C. A temperature change of about 11 C was also
observed in a 700 nm PbZrO3 thin lm near its phase transition
temperature at 235 C.109 However, the phase transitions in
these thin lms are rst-order transitions and the high ECE can
only exist in a narrow temperature range. Additionally, the
phase transition temperature is too high for applications near
room temperature. To obtain a large ECE at a wide range of
temperature near room temperature, La doped PZT thin lms
were explored and a DT 40 C was reported under the electric
eld of 120 MV m1 at 45 C.63
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Table 6
Feature Article
ECE properties for some organic and inorganic materials investigated
Material
Form
Experiment temp.
( C)
DT (K)
P(VDF-TrFE) 55/45 (ref. 10)
P(VDF-TrFE) 55/45 (ref. 72)
P(VDF-TrFE) 70/30 (ref. 118)
Irradiated P(VDF-TrFE) 68/32 (ref. 63)
P(VDF-TrFE-CFE) 59.2/33.6/7.2 (ref. 73)
SrTiO3 (ref. 84)
SrTiO3 (ref. 85)
KTaO3 (ref. 86)
PbZr0.455Sn0.455Ti0.0903 (ref. 71)
PbZr0.75Sn0.20Ti0.05O3 (ref. 90)
Pb0.97La0.02(Zr0.95Ti0.05)O3 (ref. 91)
PMN (ref. 92)
0.9PMN-0.1PT (ref. 93)
0.75PMN-0.25PT (ref. 94)
0.87PMN-0.13PT (ref. 98)
Pb(Sc0.5Nb0.5)O3 (ref. 99)
Pb(Sc0.5Ta0.5)O3–Pb(Sc0.5Nb0.5)O3
(ref. 100)
Pb(Mg0.5W0.5)0.5Ti0.5O3 (ref. 119)
Na0.5Bi0.5TiO3 (ref. 102)
Ba0.73Sr0.27TiO3 (ref. 101)
Ba0.3Na0.7Ti0.3Nb0.7O3 (ref. 103)
PST(PbSc0.5Ta0.5O3) (ref. 104)
BaTiO3 (commercial) (ref. 74)
PbZr0.95Ti0.05O3 (ref. 9)
Pb0.88La0.08Zr0.65Ti0.35O3 (ref. 63)
PbZr0.52Ti0.48O3 (ref. 120)
PbZrO3 (ref. 109)
0.9 PMN-0.1 PT (ref. 111)
0.93 PMN-0.07 PT (ref. 110)
0.67 PMN-0.33 PT (ref. 112)
0.65 PMN-0.35 PT (ref. 114)
SrBiTa2O9 (ref. 117)
PbSc0.5Ta0.5O3 (ref. 116)
Films
Films
Films
Films
Films
Ceramics
Single crystal
Single crystal
Ceramics
Ceramics
Thick lm
Ceramics
Ceramics
Ceramics
Ceramics
Ceramics
Ceramics
80
67
117
33
30
256
262
260
44–55
161
1.7
16–67
50
120–130
18
92
2 10
12.6
12
21
20
15.7
0.06
0.3
0.25
1.4
2.6
8.5
2.5
0.55
0.5
0.5
0.9
1–1.7
Ceramics
Ceramics
Ceramics
Ceramics
Multilayer
Multilayer
Thin lms
Thin lms
Thin lms
Thin lms
Thin lms
Thin lms
Thin lms
Thin lms
Thin lms
Thin lms
139
140
25
13
0.4
0.33
1
34 | J. Mater. Chem. C, 2013, 1, 23–37
DTDS
(J kg1)
62
781.2
95
80
1900
1256
2.5
6.25
0.45
0.1485
0.026
2.4 (3.5)
0.55
12
40
11.1
11.4
5
9
14.5
31
4.93
6.2
80
226
45
387
235
60
25
152
140
288
68
A DT of 9 K under 72.3 MV m1 in 0.93PMN-0.07PT thin lms
was observed at depolarizing temperature 18 C instead of the
temperature of 35 C, where the dielectric constant peak
appeared, suggesting a dipolar glass-relaxor phase transition in
this system.110 For 0.9PMN-0.1PT, the maximum DT of 5 C was
observed at 75 C, where a pseudocubic relaxor ferroelectric
transforms to a cubic paraelectric phase.111 The PMN-PT with
30%–35% PT are extremely interesting, because of the morphotropic phase boundary (MPB) can contribute additionally to
the ECE.112–115
Other thin lms such as PbSc0.5Ta0.5O3 relaxor with a wide
temperature range of ECE were also investigated. A DT ¼ 6.2 C
at 77.4 MV m1 was reported.116 SrBiTa2O9,117 a lead-free
perovskite–type bismuth layered oxide, was investigated where
a DT of 4.93 C at around 290 C was reported.
Table 6 summarizes the ECE properties for organic and
inorganic materials. For comparison, selected ECE results
reported in the literature on inorganic materials, especially
inorganic ferroelectric thin lms where very high voltage can be
applied, are also included. As can be seen, the relaxor ferroelectric polymers, because of their large DT and DS, broad
operation temperature range and easy scaling up for various
DS
(J kg1 K1)
7.9
50
6.17
94.8
2000
68.49
6.3
39.06
E (MV m1)
Measurement
method
209
120
300
160
150
0.8
0.7
1.56
3
3
112(75)
9
2.91
1.35
2
2
2.5
Indirect
Direct
Indirect
Direct
Direct
Direct
Direct
Direct
Direct
Direct
Indirect
Direct
Direct
DSC
Direct
Direct
Direct
2.3
5
2.4
1.5
1.38
30
77.6(50)
120
57.7
51(40)
89.5
72.3
60
74.7
60
77.4
Indirect
Indirect
DSC
DSC
Direct
Direct
Indirect
Direct
Indirect
Indirect
Indirect
Indirect
Indirect
Indirect
Indirect
Indirect
sized cooling devices, offer the most attractive ECE properties
for practical cooling device applications.
4
Concluding remarks
Pyroelectric effects in major polar materials are summarized,
including the effects in polymers, single crystals, ceramics, thin
lms, and liquid crystalline polymers. It is shown that the
pyroelectric effect experimentally measured oen includes both
the primary effect and the secondary effect (dimensional effect).
Three gures of merit are usually used to evaluate the properties
of the pyroelectric materials, which are necessary since besides
the material, a sensor system also consists of an electronic
readout circuit, which can be in either the charge detection
mode (FOM Fi ¼ p) or voltage detection mode (FOM FV ¼ p/30 ).
Also the signal-to-noise ratio is important
in sensor designs
pffiffiffiffiffi
which is related to the FOM FD ¼ p= 300 .
Compared with the pyroelectric materials, the study of ECE
is still in its early stage. We note that in general the ECE and
pyroelectric effect can be related through the Maxwell relation,
and hence most ECE studies have been based on this relation
(the indirect method). Since the Maxwell relation is valid only
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for systems and processes which are thermodynamically
reversible and many ECE are studied near rst order phase
transitions, one should be cautious when using the indirect
method to deduce ECE and interpret the experimental results.
Based on general thermodynamic considerations, a polarmaterial with a large polarization change and short polarcorrelation length (and hence a small Curie constant (or a large
b coefficient) in the L–D theory) will exhibit a large ECE. In
ferroelectrics, the large polarization change occurs near a FE–PE
transition and hence the largest ECE in the material will occur
near the FE-PE transition. Moreover, since the dipolar entropy is
basically directly related to the degree of dipolar disorder, by
increasing the number of polar-states in the disordered phase
can lead to a larger ECE. These considerations suggest that
relaxor ferroelectrics may exhibit larger ECE than the normal
ferroelectrics. Working with dielectrics near morphotropic
phase boundary may also lead to higher ECE. All of these have
indeed been experimentally observed. The advantage of
working with the relaxor ferroelectrics is that the large ECE can
occur over a broad temperature range compared with the
normal ferroelectric in which the large ECE occurs over a narrower temperature range about the ferroelectric transition.
Analogous to the pyroelectric effect, the secondary effect (the
dimensional change of the EC material) can also play an
important role.
The reports of large ECE in several materials systems create
great opportunity to exploit these materials for solid-state
cooling applications such as cooling of electronic components
and on-chip cooling, refrigeration and air-conditioning.
Compared with ceramic thin lms which are supported by a
substrate and in which large ECEs are obtained, polymer thin
lms with large ECE are more attractive for practical cooling
device applications since they can be fabricated into large size
devices with high breakdown eld. Moreover, polymer/nanoparticle composites can be very promising for device applications, due to the enhancement in both ECE and thermal
conductivity, which are equally important from cooling device
application point of view. In developing materials with large
ECE, one should also pay special attention to the possibility of
these materials and phenomena for practical applications. With
the advancement of fundamental understanding of ECE in
insulating dielectrics, further experimental studies of ECE
materials, as well as cooling device studies exploiting these ECE
materials, we expect that ECE materials possessing better
properties will be developed.
Acknowledgements
Xinyu Li, Xiao-shi Qian, Q. M. Zhang were supported by the US
DoE, Office of Basic Energy Sciences, Division of Materials
Science and Engineering under Award no. DE-FG02-07ER46410.
Sheng-Guo Lu and Xiang-Zhong Chen were supported by the
Army Research Office under Grant no. W911NF-11-1-0534.
Haiming Gu was supported by DOE SBIR Phase II, Contract no.
DE-SC0003340 (subcontract from Strategic Polymers, Inc.) We
thank Quinn Burlingame and Shan Wu for assistance in
preparing the manuscript.
This journal is ª The Royal Society of Chemistry 2013
Journal of Materials Chemistry C
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