Pyroelectric Materials and Devices for Energy Harvesting Applications

Abstract

This review covers energy harvesting technologies associated with pyroelectric materials and systems. Such materials have the potential to generate electrical power from thermal fluctuations and is a less well explored form of thermal energy harvesting than thermoelectric systems. The pyroelectric effect and potential thermal and electric field cycles for energy harvesting are explored. Materials of interest are discussed and pyroelectric architectures and systems that can be employed to improve device performance, such as frequency and power level, are described. In addition to the solid materials employed, the appropriate pyroelectric harvesting circuits to condition and store the electrical power are discussed.

Full text

Pyroelectric materials and devices for energy
harvesting applications
C. R. Bowen,*
a
J. Taylor,
b
E. LeBoulbar,
ab
D. Zabek,
a
A. Chauhan
c
and R. Vaish
c
This review covers energy harvesting technologies associated with pyroelectric materials and systems. Such
materials have the potential to generate electrical power from thermal fluctuations and is a less well
explored form of thermal energy harvesting than thermoelectric systems. The pyroelectric effect and
potential thermal and electric field cycles for energy harvesting are explored. Materials of interest are
discussed and pyroelectric architectures and systems that can be employed to improve device
performance, such as frequency and power level, are described. In addition to the solid materials
employed, the appropriate pyroelectric harvesting circuits to condition and store the electrical power are
discussed.
1. Introduction
Energy harvesting is currently a topic of intense interest as a
result of the growing energy demands of society and as a means
to create autonomous and self-powered systems. For example,
applications for energy harvesting devices include battery-free
wireless sensor networks that do not require maintenance or
replacement, with typical power requirements in the mWtomW
range.
1
Methods to scavenge ‘local’energy sources to generate
electrical power have been considered by many researchers;
examples include the use of piezoelectric materials and elec-
tromagnetic systems to convert mechanical motion into elec-
trical energy. Other forms of ambient energy sources include (i)
light, where photo-voltaics and water-splitting are being
considered, (ii) wind, including the creation of micro-turbines
and (iii) harvesting electromagnetic waves using antennas.
In the context of thermal energy harvesting, heat remains an
almost ubiquitous and abundant ambient source of energy that
is oen wasted as low-grade waste heat.
2
Thermoelectrics have
been widely used and considered as a means to convert
temperature gradients into electrical energy using the Seebeck
effect. A less widely researched area is ‘pyroelectric energy
harvesting’
3
in which temperature uctuations are converted
into electrical energy; although the potential to convert thermal
energy to electrical energy using ferroelectric materials has been
considered in the 1960's and 1970's.
4–10
This review provides an introduction to current and past
research on pyroelectric energy harvesting materials and
systems. A number of excellent introductions to pyroelectric
materials already exist, most of which concentrate on their use
for heat-sensing, infra-red detection, thermal imaging, re
alarms, gas analysers and pollution monitors.
11–15
Lang has also
provided a historical review of pyroelectrics
16
and Lubomirsky
et al. reviewed in detail the methods of pyroelectric measure-
ment.
17
In the context of pyroelectric harvesting, Lingham et al.
provided a recent review with an emphasis on nano- and micro-
scale systems
18
and Hunter et al.
2
provided a history of pyro-
electric related harvesting;
2
there is also a recent book on the
topic waste energy harvesting including both mechanical and
thermal energies.
19
This review will describe the concept of
pyroelectricity and the methods by which energy can be derived
from temperature uctuations. A comparison with piezoelectric
vibration energy harvesters will be made since all pyroelectric
materials are also piezoelectric. Potential materials and relevant
gures of merit for the selection of appropriate pyroelectric
materials for energy harvesting will be discussed. While pyro-
electric materials form the basis of the harvesting device there is
a need to condition and store the power generated by the
material and the types of electrical circuits employed for har-
vesting will be summarised. Finally, examples of novel pyro-
electric systems from macro- to nano-scale will be described.
This review will focus on papers specically related to energy
harvesting and the majority of the materials employed to date
are in solid form; liquid crystals
20
and molecular ferroelectrics
21
are not covered in this review although it would be of interest in
the future to consider the applicability of these materials for
harvesting applications.
A signicant amount of waste heat is lost as a by-product of
power, refrigeration, or heat pump cycles
22
and it has been
reported that in 2009 half of the energy consumed in the United
States was wasted as low-grade waste heat.
23,24
To harvest waste
heat thermoelectric materials have attracted interest, with a
a
Department of Mechanical Engineering, University of Bath, Bath, BA2 7AY, UK.
E-mail: c.r.bowen@bath.ac.uk
b
Department of Electrical and Electronic Engineering, University of Bath, Bath, BA2
7AY, UK
c
School of Engineering, Indian Institute of Technology Mandi, 175 001, Himachal
Pradesh, India
Cite this: DOI: 10.1039/c4ee01759e
Received 9th June 2014
Accepted 25th July 2014
DOI: 10.1039/c4ee01759e
www.rsc.org/ees
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Energy &
Environmental
Science
REVIEW
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online
View Journal

number of commercial supplies of thermo-electric generators
(TEGs).
25,26
Thermoelectric materials and systems generate
electrical power from temperature gradients (dT/dx) while
pyroelectric materials produce power from temperature uctu-
ations (dT/dt)
27
and have some similarities to the way in which
piezoelectric harvesters convert mechanical oscillations (ds/dt)
into electricity.
28
Pyroelectric materials are of interest since
under the correct conditions they have the potential to operate
with a high thermodynamic efficiency and, compared to ther-
moelectrics, do not require bulky heat sinks to maintain a
temperature gradient.
29
We will see in this review that pyro-
electric harvesters tend to operate at low frequency, typically
<1 Hz,
30
due to the slow temperature oscillations in systems of
large thermal mass and heat transfer inertia. Since temperature
oscillations are oen slow, efforts to transform a temperature
gradient into a time variable temperature include the use of
cyclic pumping.
31
The power consumed by the pumping process
can be a relatively small fraction of the harvested energy (<2%),
which can make the process feasible.
31–33
Naturally occurring
temperature changes for harvesting are rare but examples
include changes in ambient temperature, the human body,
34
exhaust gases and natural temperature variations
35
due to
convection and solar energy.
36
2. The pyroelectric effect
All pyroelectrics are polar materials and exhibit a spontaneous
polarization P
s
in the absence of an applied electric eld.
12
Examples of polarisation include that of ionically bonded
materials whereby the polarisation can be a consequence of the
crystal structure, while in crystalline polymers with aligned
molecular chains it can be due to the alignment of polarised
covalent bonds.
11
The presence of a spontaneous polarisation in
the material leads to the presence of a charge on each surface of
the material and free charges, such as ions or electrons, are
attracted to the charged surfaces of the material. The origin of
pyroelectric behaviour is understood from the behaviour of the
surface charge as the ambient temperature is changed and
assuming that the polarisation level is dependent on material
temperature.
12
If a pyroelectric is heated (dT/dt> 0) there is a decrease in its
level of spontaneous polarisation as dipoles within the material
lose their orientation due to thermal vibrations, see Fig. 1. This
fall in the polarisation level leads to a decrease in the number of
free charges bound to the material surface.
12
If the material is
under open circuit conditions the free charges remain at the
electrode surface and an electric potential is generated across
the material.
18
If the material is under short circuit conditions
an electric current ows between the two polar surfaces of the
material. Similarly, if the pyroelectric is cooled (dT/dt< 0) the
dipoles regain their orientation leading to an increase in the
level of spontaneous polarization, thus reversing the electric
current ow under short circuit conditions as free charges are
attracted to the polar surfaces.
Eqn (1) denes the relationship between pyroelectric charge
(Q), generated current (i
p
), rate of temperature change (dT/dt),
surface area of the material (A) and pyroelectric coefficient (p)
36
under short circuit conditions with electrodes that are orien-
tated normal to the polar direction.
ip¼dQ
dt¼pA dT
dt(1)
The pyroelectric coefficient of an unclamped material, under a
constant stress and electric eld, is dened by eqn (2),
ps;E¼dPs
dTs;E
(2)
where P
s
is spontaneous polarisation
37
and subscripts sand E
correspond to conditions of constant stress and electric eld
respectively. While the pyroelectric coefficient is a vector
quantity, the electrodes that collect the charges are oen
normal to the polar direction and so the measured quantity is
oen treated as a scalar.
14
The ability of small changes in
temperature to produce a pyroelectric current has been exploi-
ted for infra-red imaging and motion detection by body heat.
12
This small electric current can also be considered for energy
harvesting applications.
To maximise the pyroelectric current under short-circuit
conditions, clearly the pyroelectric should have a large surface
area, large pyroelectric coefficient and a high rate of tempera-
ture change. Eqn (1) implies that the generated current (but not
necessarily power) is independent of thickness and propor-
tional to area since the current is simply associated with the
surface charge, an observation made by Gaugain in 1859.
14
Since there is a requirement for a pyroelectric material to be
polar and exhibit a level of polarisation all pyroelectric mate-
rials are piezoelectric (pyroelectrics are a sub-class of piezoelec-
tric materials so that all pyroelectrics are piezoelectric).
However, not all piezoelectrics are pyroelectric since materials
such as quartz only become polarised as a result of a mechan-
ical stress. In ‘ferroelectric’pyroelectric materials the
Fig. 1 Temperature dependence of spontaneous polarisation P
s
and
pyroelectric coefficient dP
s
/dTof a ferroelectric material, adapted
from.
14
Energy Environ. Sci. This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science Review
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

orientation, and sign, of the spontaneous polarisation can be
switched by reversing the direction of the applied electric eld.
These ferroelectrics are a sub-class of pyroelectric materials so
that all ferroelectrics are both pyroelectric and piezoelectric.
Table 1 highlights some common pyroelectric materials which
will be described in more detail later in the review. In general,
ferroelectric materials have larger pyroelectric and piezoelectric
coefficients compared to non-ferroelectrics; however if a ferro-
electric is heated beyond the Curie temperature (T
C
)it
undergoes a phase transition where the spontaneous polariza-
tion and both the pyroelectric and piezoelectric behaviour
vanish.
Fig. 1 shows an example of the decrease of the spontaneous
polarisation of a ferroelectric with increasing temperature and
the corresponding pyroelectric coefficient (dP
s
/dT) which
decreases to zero at the Curie temperature (T
C
) where the
material is no longer pyroelectric. The pyroelectric coefficient
rises signicantly as the material begins to rapidly lose its
polarisation as it approaches T
C
. While the loss of piezoelectric
properties above the Curie temperature is a disadvantage for
vibration harvesters, the phase transition at the Curie temper-
ature has attracted some interest for pyroelectric harvesting
since the material has the potential to discharge a large amount
of electrical energy as the level of polarisation falls to zero.
38
An
obvious complexity of thermally cycling above and below the
Curie temperature is the need to re-polarise the ferroelectric on
cooling below T
C
; this is usually achieved by the application of
an electric eld.
2.1 Primary, secondary and tertiary pyroelectric coefficients
As discussed, a temperature change alters the degree of polar-
isation and leads to an electric current. The primary pyroelectric
effect is relevant to the condition of a perfectly clamped material
under constant strain
12
with a homogenous heat distribution
without an external eld bias. In many cases of measurement
and energy harvesting a secondary pyroelectric effect is present
since thermal expansion induces a strain that alters the electric
displacement via the piezoelectric effect; the complexity of
separating pyroelectric and piezoelectric responses has been
well documented by Lubmirsky et al.
17
Using tensor notation the
primary pyroelectric coefficient at constant strain (p
x
), i.e. in the
clamped condition, is related to the pyroelectric coefficient at
constant stress (p
s
)byeqn3:
11,14,39,40
p
s,E
¼p
x,E
+d
ij
c
E
ij
a
E
i
(3)
where d
ij
are the piezoelectric coefficients, c
ij
are elastic
constants and athe thermal expansion coefficient; the
subscript xcorresponds to the conditions of constant strain.
The term d
ij
c
E
ij
a
E
i
is called the secondary pyroelectric coefficient
and while it can be small in ferroelectrics
11
it can make a
signicant contribution to the overall pyroelectric coefficient.
As a result, measurements of pyroelectric coefficients must be
under taken under specic conditions while in energy harvest-
ing applications temperature, frequency, electrical and
mechanical boundary conditions may be less precise or even
vary with time. It is therefore important to dene the
mechanical and electrical boundary conditions of harvesters.
19
Data for a variety of pyroelectrics has been collected in an
excellent review by Li et al.
39
and Table 1 includes both the
primary and secondary pyroelectric coefficient for some
common materials as examples. For thin-lm materials
substrate clamping can reduce the pyroelectric response to a
small value compared to its unclamped value, e.g. in the case of
CdS and ZnO.
40
This may be a potential advantage of replacing
thin lms with a nano-rod geometry that is less likely to suffer
from substrate clamping; nano-scale pyroelectric materials will
be described later in the review.
41
Whatmore
11
also described an
interesting condition where the secondary effect can be
important if the material is subjected to an alternating heat ux
whose frequency matches the mechanical resonance frequency;
this has not been considered for energy harvesting applications.
Coupling a pyroelectric to an external structure which
undergoes large thermal deformations is also a potential
approach to enhance harvested energy. Chang et al.
42
examined
laminate structures with differing thermal expansion and stiff-
ness characteristics to enhance the contribution of the secondary
pyroelectric coefficient for energy harvesting, with signicant
improvement in overall pyroelectric coefficient depending on the
thickness ratios of the individual layers.
43
Lim described a
cantilever where the thermal expansion mismatch between a
pyroelectric and thin lm was used to generate energy.
44
Tertiary pyroelectricity, due to non-uniform heating is also
possible since non-uniform heating generates shear stresses
that result in polarization through the piezoelectric effect.
12
In
this case the current generated is dependent on the magnitude
of the temperature gradient.
11
Secondary and tertiary effects are
therefore potential routes for enhancing thermal harvesting
along with heat transfer enhancement or materials selection or
development, which will now be described.
3. Pyroelectric materials
For an unclamped pyroelectric material, the expressions for
charge and voltage generated and energy stored in a pyroelectric
material can be derived as follows. From eqn (1), the pyroelec-
tric current is independent of material thickness and only
depends on the effective area of the electrode. By integrating
eqn (1) with respect to time, the net charge developed due to a
temperature change (DT) is;
Q¼pADT(4)
As pyroelectric materials are typically dielectric in nature, the
equivalent capacitance (C) is given by;
C¼A3s
33
h(5)
where 3
s
33
is the permittivity in the polarisation direction at
constant stress. The open circuit voltage (V) and electric eld
(E
eld
) developed across the electrodes, from Q¼CV, can be
expressed as:
V¼p
3s
33
hDT(6)
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Review Energy & Environmental Science
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

Table 1 Comparison of pyroelectric materials and figures of merit. For pyroelectric coefficient, p,first and second values in parenthesis are primary and secondary coefficient respectively. SC
indicates single crystal
Material Ferroelectric p(mCm
2
K
1
)3
33
/3
0
c
E
(MJ m
3
K
1
)
T
C
(C)
k
therm
(W m
1
K
1
)
k
2
(T
hot
¼300 K)
k2¼p2Thot
cE3s
33
F
i
(10
10
)
(m V
1
)
Fi¼p
cE
F
v
(m
2
C
1
)
Fv¼p
cE3s
33
F
E
(10
11
)
(J m
3
K
2
)
FE¼p2
3s
33
F
0
E
(10
11
)
(m
3
J
1
)
F0
E¼p2
3s
33ðcEÞ2Reference
Triglycine sulphide SC
(TGS)
3280
(60, 330)
38 2.3 49 0.65 0.030408 1.21 0.362 233.13 4.41 14 and 18
Lead magnesium niobate-
lead titanate PMN-0.25PT
h111i(SC)
31790 2100 2.5 121 2.5
(PMN-0.34PT)
0.020688 7.16 0.039 172.40 2.76 53
Mn : BNT-BT h111i(SC) 3588 279 2.89 —— 0.014536 2.03 0.082 140.03 1.68 73
Strontium barium niobate
(SBN x¼0.5)
3550 400 2.3 125 0.6 0.011146 2.39 0.068 85.45 1.62 14 and 138
Lithium tantalate (LiTaO
3
)3176
(175, 1)
47 3.2 665 3.9 0.006982 0.55 0.132 74.47 0.73 11 and 18
PMN-0.25PT 3746 2100 2.5 0.003593 2.98 0.031 29.94 0.48 31
Lead zirconate titanate (PZT) 3380 290 2.5 200 0.8 0.006752 1.52 0.059 56.26 0.90 14 and 139
Sodium nitride (NaNO
2
)340 4 2.2 164 2.2 0.006163 0.182 0.514 45.20 0.93 14
Mn : BNT-BT h110i(SC) 3513 535 2.89 —— 0.005770 1.77 0.037 55.58 0.67 73
CSBN x¼0.15 3361 972 2.1 —— 0.002164 1.71 0.020 15.15 0.34 140
Mn : BNT-BT h001i(SC) 3380 835 2.89 —— 0.002028 1.31 0.018 19.54 0.23 73
P(VDF-TrFE) 80/20 331 7 2.3 135 0.14 0.002023 0.13 0.218 15.51 0.29 14
BNLKBT 3360 858 2.83 —— 0.001809 1.27 0.017 17.07 0.21 141
BNKBT 3325 853 2.88 —— 0.001457 1.12 0.015 13.99 0.17 141
P(VDF-TrFE) 50/50 340 18 2.3 49 0.14 0.001310 0.17 0.109 10.04 0.19 14
PVDF 327
(14, 13)
9 2.3 80 0.14 0.001194 0.10 0.147 9.15 0.17 11, 12 and 14
KNN-LT 3165 1230 2.63 —— 0.000285 0.62 0.006 2.50 0.04 141
KNN-LTS 3190 1520 4.48 —— 0.000180 0.42 0.003 2.68 0.01 141
Lithium niobate (LiNbO
3
)383
(95.9, +12.9)
28.7 2.32 1210 1.1 0.003507 0.35 0.141 27.12 0.50 12, 39, 77, 138,
142 and 143
Barium titanate (BaTiO
3
)3200
(260, +60)
1200 2.5 120 3.0 0.000452 0.80 0.008 3.77 0.06 12, 139, 142
and 144
Zinc oxide (ZnO) 79.4
(6.9, 2.5)
11 2.8 —147 0.000097 0.034 0.034 0.91 0.01 18, 142, 145
and 146
Aluminium nitride (AlN) 76–8 10 2.38 —140 0.000070 0.033 0.038 0.55 0.01 142 and 147
Cadmium sulphide (CdS) 74(3, 1) 10.3 1.82 —40 0.000029 0.022 0.024 0.18 0.005 12, 142, 148
and 149
Gallium nitride (GaN) 74.8 11 2.97 —150 0.000024 0.016 0.017 0.24 0.003 142, 150
and 151
Energy Environ. Sci. This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science Review
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

Efield ¼p
3s
33
DT(7)
It can be seen from eqn (6) that the voltage developed across
the electrodes is inuenced by the thickness of the material (h)
and is invariant with respect to the area of the electrodes. Since
the total energy (E) stored in a capacitor is 1/2CV
2
, this repre-
sents the amount of energy stored in the material at the end of
the temperature change and is expressed as:
E¼1
2
p2
3s
33
AhðDTÞ2(8)
3.1 Pyroelectric energy harvesting gures of merit
A variety of gures of merit (FOMs) have been derived for
materials selection based on consideration of the thermal and
electrical circuits employed.
11
The most common are based on a
pyroelectric sensor application for the generation of maximum
current or voltage for a given power input.
14
For infra-red
detection devices based on current responsivity (F
i
),
14
to maxi-
mise the pyroelectric current generated for a given energy input
the FOM is:
Fi¼p
cE¼p
rcp
(9)
where c
p
is the specic heat capacity (J kg
1
K
1
), rthe density
(kg m
3
) and c
E
is the volume specic heat (J m
3
K
1
). For a
high voltage responsivity (F
v
)
14
to maximize pyroelectric voltage
for an energy input the FOM is:
Fv¼p
cE3s
33 ¼p
rcp3s
33
(10)
The F
i
and F
v
FOMs are oen used for selection of materials
for heat and infra-red detection, but these are not to be
confused with energy harvesting applications where generated
energy or power is a key criterion as well as the overall efficiency
of the conversion of thermal energy to electrical energy.
For energy harvesting applications two pyroelectric based
FOMs have been proposed to date.
45,46
An electro-thermal
coupling factor has been dened to estimate the effectiveness of
thermal harvesting:
45
k2¼p2Thot
cE3s
33 ¼p2Thot
rcp3s
33
(11)
where T
hot
is the maximum working temperature. This FOM has
a direct inuence on the efficiency and electrical work obtained
during cyclic temperature oscillation cycles, which will be
described later. For many materials the value is low (<1%) and
examples are shown in Table 1. An energy harvesting FOM, F
E
,
has also been proposed as:
46
FE¼p2
3s
33
(12)
The energy harvesting FOM, F
E
, has been widely used for
materials selection and materials design
38,47–50
for pyroelectric
harvesting applications. Compared to the voltage (F
v
) and
current (F
i
) responsivity the harvesting FOM, F
E
, does not
include the heat capacity. It is worth noting that the static
denitions of the FOMs do not take into account the transient
nature of heat transfer and dielectric losses; FOMs that include
dielectric loss and diffusivity have been dened for pyroelectric
sensing
11
and may be of interest to adapt for harvesting
applications.
An alternative pyroelectric harvesting FOM that includes the
inuence of heat capacity can be derived from eqn (8). The
relationship between input enthalpy (H) and resulting temper-
ature change (DT)isH¼Ahc
E
DT; substituting this into eqn (8)
provides an F
'
E
energy harvesting FOM.
F0
E¼p2
3s
33ðcEÞ2(13)
A higher value of F
0
E
implies that a larger amount of energy is
converted by the material for a given enthalpy input.
3.2 Pyroelectric materials and selection for energy
harvesting
Properties and calculated FOMs are given in Table 1 for a variety
of pyroelectric materials, including both ferroelectric and non-
ferroelectric materials. It can be observed from Table 1 that
triglycine sulphide (TGS) is potentially an excellent material for
pyroelectric energy conversion purposes. TGS has the chemical
composition of (NH
2
CH
2
COOH)
3
H
2
SO
4
and crystals based on
the glycine group (NH
2
CH
2
COOH) are polar and exhibit very
high pyroelectric FOMs.
11
Despite its high performance, TGS
has attracted limited interest for harvesting applications,
possibly due to its low Curie temperature of 49 C.
51
It is also
water soluble, hygroscopic and relatively low strength.
Lead magnesium niobate –lead titanate (PMN-PT) single
crystals are relatively new materials that are being explored for a
number of transducer applications. The (1 x)Pb(Mg
1/3
Nb
2/3
)-
O
3
-xPbTiO
3
(1 xPMN-xPT) system is a family of relaxor based
ferroelectric compositions which are of interest for transducer
devices due to their ultra-high piezoelectric and pyroelectric
coefficients. The morphological phase boundary (MPB) for
PMN-xPT spans from (x¼) 30 to 38 mol% and this range is
characterised by a monoclinic phase in co-existence with a
rhombohedral (up to 32 mol%) or tetragonal phase (32 to 38
mol%).
52
These crystals have a relatively low Curie temperature
(121 C); while this can limit the material to relatively low
temperature operation it is useful to remember that the pyro-
electric coefficient rises near T
C
, Fig. 1. Due to their single
crystal nature, the materials are relatively expensive and can be
formed in limited shapes.
53
The high FOM and range of phase
transitions associated with these materials have led to interest
in this material for a number of pyroelectric harvesting
applications.
36,46,54–57
The lead zirconate titanate (PZT) family remains a widely
used commercial ceramic due to its relative ease of fabrication
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Review Energy & Environmental Science
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

in polycrystalline form and good piezoelectric properties, with a
range of ‘hard’and ‘so’composition with tailored properties.
This family of material has therefore attracted interest for
pyroelectric harvesting applications.
36,37,58–67
Improved pyro-
electric properties were recently reported in compositionally
graded PbZr
1x
Ti
x
O
3
, including high pyroelectric harvesting
FOM by Mangalam et al.
47
Lanthanum doped PZT relaxor-
ferroelectrics have also been used for harvesting applications
where doping increases resistivity and coupling coefficients.
68–70
Lead-free materials are of interest for environmental and
health concerns and manganese doped bismuth sodium tita-
nate-barium titanate (BNT-BT) single crystal is a potential
pyroelectric energy harvesting material. They have been exam-
ined for harvesting at a theoretical level
71
since the composi-
tions possess excellent piezoelectric and pyroelectric
coefficients and high Curie temperatures (T
C
> 200 C).
72
BNT
based ceramics can be difficult to pole due to their high elec-
trical conductivities and dielectric loss and to overcome these
shortcomings they are oen doped. For example the composi-
tion Mn : BNT-BT is a 94.6Bi
0.5
Na
0.5
TiO
3
-5.4BaTiO
3
single
crystal doped with Mn and the h111iorientation of this crystal
possesses the highest pyroelectric coefficient and gure of merit
of lead-free ferroelectric materials.
73
In addition to BNT, Ba
0.65
Sr
0.35
TiO
3
(BST) thin lms have also been examined specically
for improved energy harvesting FOMs.
74
Lithium tantalate (LiTaO
3
, LTO) single crystal is a commer-
cially important optical and ferroelectric material and has been
explored for pyroelectric applications. LiTaO
3
is thermally
stable with a high Curie temperature of 665 C. Although it
possesses a FOMs lower than TGS, it nds a wider range of
applications owing to its low dielectric loss and thermal and
physical stability. LiTaO
3
has been combined with cement for
pyroelectric harvesting from pavements
75
and has even been
used to generate large potential differences for the generation of
ion beams for nuclear fusion studies.
76
For high temperature
applications, LiNbO
3
is also of interest due to its high Curie
temperature (1210 C).
77
A family of potassium sodium niobate based materials,
(K
0.5
Na
0.5
)NbO
3
(KNN), was recently discovered as a high
performance substitute for lead-based piezoelectric materials.
78
It has been reported to have piezoelectric performance
approaching commercially available lead-based materials such
as PZT and PMN-PT ceramics. The KNN-LT [{(K
0.5
Na
0.5
)
0.96
-
Li
0.04
}(Nb
0.8
Ta
0.2
)O
3
] and KNN-LTS [{(K
0.5
Na
0.5
)
0.96
-
Li
0.04
}(Nb
0.84
Ta
0.1
Sb
0.06
)O
3
] compositions have improved
characteristics compared to pure KNN.
79,80
Nano-scale KNbO
3
has been considered for pyroelectric harvesting.
81
Non-ferroelectric pyroelectrics include the wurtzite based
materials such as AlN, GaN, CdS and ZnO which have relatively
low pyroelectric coefficients compared to the ferroelectric
materials (Table 1). Since these materials are not ferroelectric
they are oen used in single crystal form, such as epitaxailly
grown lms,
18
or as highly orientated structures to achieve the
desired polarisation. The manufacture of nanostructured
materials, such as
41
ZnO nanowires, will be discussed later.
These materials do, however, exhibit higher thermal conduc-
tivities compared to the ferroelectric materials (see Table 1)
allowing a more rapid change in temperature due to changes in
ambient temperature.
Thematerialsabovearetypically ceramic-like and are
therefore relatively high density, high stiffness and brittle. If
mechanical exibility and toughness is desirable a polymeric
pyroelectric material can be considered for energy harvest-
ing, such as polyvinylidine-diuoride triuoro-ethane P(VDF-
TrFE). The basic PVDF composition has three main phases [a,
band g] based on the trans (T) and gauche (G) chain
conformations. The bphase of PVDF has ferroelectric prop-
erties owing to a crystalline structure obtained by an all trans-
arrangement of the polymer chains that gives rise to a
permanent dipole that can produce pyroelectric effects. The
polarization is reversible owing to the rotating bonds along
the polymer chains. Pure PVDF has a poorer performance
compared to P(VDF-TrFE); examples of the properties and
FOM for PVDF based materials are shown in Table 1. PVDF
based materials have attracted interest for pyroelectric har-
vesting
29,37,38,50,82–87
and their advantage is that they can be
readily produced in thin lm form (for improved heat
transfer), are low cost, exible, are chemically resistant and
have high breakdown elds. The material is limited to rela-
tively low temperatures (80 C). Porous forms of PVDF have
been examined to optimise its performance for energy har-
vesting,
50
for example the introduction of porosity can reduce
both the permittivity of the material and the heat capacity,
which is clearly benecial for some FOMs.
Composite materials are also attracting interest in an effort
to combine high-activity ceramic ferroelectrics with a exible
and low permittivity matrix;
45
these have been examined for
pyroelectric detectors but the ‘composite’approach certainly
offer avenues for creating interesting materials for harvesting
applications.
14,88
The Ashby method can be used to graphically represent the
performance of pyroelectric materials. In Fig. 2a the axes of the
Ashby chart are specically chosen so that a line may represent
different FOMs. We have selected the harvesting FOM (F
E
),
coupling coefficient (k
2
) and alternative harvesting FOM (F
00
E
) for
a comparative analysis of materials. Since the F
E
FOM is simply
the y-axis, materials that are higher on the y-axis exhibit a
higher F
E
; it is clear that TGS and single crystal PMN-0.25PT
h111iexhibit high values. It is possible to rearrange the
coupling coefficient, k
2
(eqn (11)) to form an equation for a
straight line (y¼mx +c), where mis the gradient of the line, by
taking logs of each side to logp2
333¼logðcEÞþlogk2
Thot.
This is represented by the dashed line on Fig. 2a with a gradient
of unity (m¼1). Materials which lie on the same line have an
equal performance with respect to k
2
, those above the line have
a higher k
2
. For the alternative energy FOM, F
0
E
, it is possible to
rearrange the terms to logp2
333¼2 logðcEÞþlogðF0
EÞwhich is
indicated by the dotted line with a gradient of two. Fig. 2a
indicates that the best performing materials for F
0
E
are TGS
followed by PMN-25PT h111i, Mn : BNT-BT h111i, SBN, LiTaO
3
,
PMN-25PT ceramic, PZT ceramic and Mn : BNT-BT h111i
Energy Environ. Sci. This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science Review
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

respectively. From the lead-free materials, co-polymers of PVDF
and BNT based bulk ceramics have better performance
compared to the KNN family. The non-ferroelectric pyroelectric
materials (e.g. ZnO, GaN, AlN) have lower FOMs compared to
the ferroelectrics (Fig. 2a and Table 1).
The FOMs do not include information regarding the
operating temperature which, for ferroelectrics, in oen
related to the Curie temperature (Table 1). Fig. 2b shows
examples of source temperatures for harvesting and energy
generated; polymer based PVDF materials are generally at
lower temperatures (<100 C) compared to the ferroelectric
ceramics where the doped PZT family are of interest for
higher temperatures.
4. Pyroelectric cycles for energy
harvesting
A variety of thermal cycles exist
89,90
for pyroelectric harvesting
and the following is an overview of the different approaches and
will be followed by a discussion of potential electrical circuits to
implement such cycles. These pyroelectric cycles have been
examined in detail by Sebald et al.
90
4.1 Carnot cycle
Fig. 3a shows the polarisation-electric eld plot of a Carnot cycle
which has two adiabatic (Path 1–2, Path 3–4) and two isothermal
processes (Path 2–3, Path 4–1).
89
The rst adiabatic process is
the increase in electric eld (1–2); the maximum eld allowable
is related to the dielectric strength at the operating temperature.
This is followed by an isothermal decrease of the electric eld
(Path 2–3). The process proceeds with a further adiabatic
application of electric eld of opposite sign (Path 3–4) and an
isothermal decrease of electric eld (Path 4–1). The cycle is
considered to be the most efficient in terms of energy conver-
sion between two working temperatures.
90
The efficiency
between two reservoirs of hot T
h
and cold T
C
temperatures is:
hCarnot ¼1Tc
Th
(14)
The Carnot cycle for pyroelectric energy harvesting faces
signicant practical limitations, for example the need for
adiabatic temperature changes (electro-caloric) and two
isothermal paths. The maximum temperature variation realis-
able by applying electric elds is also limited to only a few
degrees.
89,90
The Carnot cycle, while impractical, is oen used
for comparative purposes to other cycles.
4.2 Resistive cycles
A simple approach to using pyroelectric energy harvesting is to
connect the material to a resistive electrical load and subject it
to a temperature change.
90
Such an approach has been exam-
ined by van der Ziel.
7
When the pyroelectric harvester operates
with an external resistance the change in temperature drives an
electric current, equivalent to the change in polarisation,
through a resistance; if the load resistance is large then the
output voltage is large.
89
Estimation of the optimum energy
harvested by simple resistive loading is oen based on a sinu-
osoidal variation of temperature and an optimal resistive load
depending of frequency and material permittivity (total capac-
itance). Based on this type of cycle, the conversion efficiency
relative to the Carnot cycle is:
hResistive ¼p
4k2hCarnot (15)
where, k
2
is the electro-thermal coupling factor (eqn (11)) at
temperature T
h
. Given the small k
2
values in Table 1 the
conversion efficiency is low but the electrical circuits that can be
employed are relatively simple.
Fig. 2 (a) Ashby diagram for relevant materials properties. Ferroelec-
tric indicated by solid circles and non-ferroelectric are open-circles.
(b) Examples of heat source temperatures and harvested energy for a
variety of materials.
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Review Energy & Environmental Science
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

As examples of resistive loaded systems, pyroelectric cells
fabricated using screen-printed PZT lms and commercial PVDF
lms were considered as power sources for autonomous
sensors.
37
Pyroelectric currents of 10
7
A and charges of 10
5
C
were achieved for 300–360 K temperature changes over 100s.
37
Guyomar et al.
91
developed a pyroelectric micro-generator using
PVDF lms that produced 0.32 mW for a temperature amplitude
of 7 K at 0.2 Hz. Increased power could be produced by
increasing the temperature amplitude, frequency and using
higher pyroelectric coefficient materials; e.g. PMN-PT single
crystals (see Table 1). Xie et al.
28
examined pyroelectric harvesters
using PMN-PT, PVDF and PZT with peak power densities of 0.33,
0.20 and 0.12 mWcm
2
respectively. Again, large areas and high
pyroelectric coefficients were advantageous. While the power
output levels are relatively low, the advantages of this approach
are that a large range of working temperatures is possible
31
and
it is easy to implement. Sebald et al.
31
considered harvesting
natural temperature variations due to temperature changes of
clothing as they moved from the inside to outside. Power peaks
up to 0.2 mW cm
3
were predicted with a mean power of 1 mW
cm
3
; thinner structures could provide faster temperature vari-
ations. The impact of sample geometry and operation frequency
has also been examined.
58,92
One limitation of using simple resistive loading and
applying no external electric eld is that if a ferroelectric is
subjected to temperatures above the Curie temperature there
will be depolarisation of the material (see Fig. 1). When the
ferroelectric is subsequently re-cooled below T
C
, in the absence
of an applied electric eld, there is no longer any net polar-
isation (and therefore no pyroelectric behaviour) since the
ferroelectric domains are now randomly aligned. For this
reason, while high pyroelectric activity takes place around the
ferroelectric to paraelectric phase transition (Fig. 1),
46
resistive
generators using ferroelectrics tend to be limited to tempera-
tures below the Curie temperature and transform only a fraction
of the available heat into electricity.
82
Fig. 3 Example cycles. (a) Carnot (b) synchronised electric charge extraction (SEC) and (c) synchronised switch damping on inductor (SSDI).
Adapted from ref. 89 and 90.
Energy Environ. Sci. This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science Review
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

4.3 Synchronised electric charge extraction (SECE)
The synchronised electric charge extraction process involves
extracting the charge generated when the maximum tempera-
ture is reached and the stored energy is at a maximum and
extracting the charge again when the temperature is a
minimum.
90,93
Fig. 3b shows the polarisation-eld paths in a
SECE cycle. During Path 1–2, the pyroelectric is heated resulting
in an increase in open-circuit voltage (eqn (6)). The electric eld
is then discharged and the eld reduced to zero under
isothermal conditions (Path 2–3). The pyroelectric is then
cooled under open circuit (Path 3–4) and then isothermally
discharged (Path 4–1). For most materials the coupling is weak
(k
2
1) and the efficiency can be simplied to:
h
SECE
¼k
2
.h
Carnot
(16)
It can be seen that the cycle is more efficient than resistive
cycle (eqn (15)) and has advantages in that voltage control is not
required and no pre-determination of working temperatures or
their control is necessary.
90
However, like the resistive cycle, the
main disadvantage associated with this harvesting technique is
the low efficiency due to the low k
2
values (Table 1).
4.4 Synchronised switch damping on inductor (SSDI) cycle
The SSDI approach for pyroelectric harvesting is based on an
approach initially developed for vibration damping. In this case
the voltage on the pyroelectric material is switched on an
inductor at each maximum or minimum temperature so that
the electric-eld is rapidly reversed at low loss. Fig. 3c shows the
basic process and the main difference to SECE is that the
electric eld in not reduced to zero but to an almost opposite
value. Starting from Point 1 the temperature of the material is
increased in open-circuit conditions and a positive electric eld
is developed (Path 1–2). This induces an electric eld E
M1
on the
material due to pyroelectric effect. Upon reaching the maximum
temperature, the material is subjected to an isothermal eld
inversion to E
m1
through an external voltage. The ratio of
inversion is known as inversion quality b¼E
m1
/E
M1
and for
perfect inversion b¼1.
90
In process 3–4, the temperature is then
reduced and the absolute value of electric eld increases. On
reaching point 4, the inversion process is repeated and the
process continues. For weakly coupled materials, the efficiency
relative to the Carnot cycle is:
hSSDI ¼k21þb
1bhCarnot (17)
The SSDI cycle has since been modied to a synchronised
switch harvesting on inductor (SSHI) cycle, but with similar
conversion levels. The type of circuit to implement a SSHI will
be described later in this review.
4.5 Olsen cycle
In this section a different approach is described that relies on
the fact that thermally modulating the polarisation of the
material also varies its dielectric constant and hence its
capacitance.
89
The potential of ‘thermo-dielectric’generation
has been considered for over 50 years
4,6,9,94
and the initial
history of this work is well described by Khodayari et al.
3
Olsen
proposed a working pyroelectric cycle between two different
polarization curves obtained at two distinct temperatures for a
material. This type of pyroelectric converter is an electric form
of heat engine and Olsen et al. presented a number of important
papers and patents on thermal cycles for harvesting;
83,84,89,96–101
it has also been examined in detail by a number of
researchers.
38,54,68,71,82,85,102–109
The role of the secondary pyro-
electric and phase transitions on such cycles has been well
reported by McKinley et al.
54
Olsen introduced a clockwise harvesting cycle dening
electrical work N
D
as the area between two isotherms in an
electric displacement –eld (D–E) diagram which is shown in
Fig. 4.
96
The approach is an electrical analogue of the Ericsson
heat engine cycle
83
and has two isothermal (1–2, 3–4) and two
isoelectric paths (2–3, 4–1) which span the area N
D
. The integral
of EdD around the closed cycle corresponds to the electric work
available aer heat absorption and rejection:
N
D
¼ÞEdD (18)
In essence, a ferroelectric capacitor is charged at a cold
temperature and then discharged rst by heating it to a higher
temperature and then by reducing the applied electric eld.
57
These steps act to effectively reverse the direction of conven-
tional polarisation-eld hysteresis loops which convert elec-
trical energy into heat.
95
With this type of harvesting cycle the
net trade-offof employing an external electrical eld is a higher
efficiency compared to most other cycles.
98
For identical
boundary conditions of heat source temperature, the Olsen
cycle with ferroelectric lead zirconate stannate titanate ceramic
Fig. 4 (a) Electric displacement versus electric field (D–E) for pyro-
electric material at two temperature (T
cold
and T
hot
), adapted from.
57,98
The material (capacitor) is charged isothermally during the cool
portion of its thermal cycle (1 /2) after which it is heated (2 /3). The
effect of the heating is to decrease the polarisation (displacement) of
the material and hence its dielectric constant and capacitance
releasing electrical energy isothermally into an external circuit (3 /4).
The device is finally cooled and returned to the beginning of the
cycle (4 /1).
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Review Energy & Environmental Science
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

(PZST) transforms up to ten times more thermal energy into
electricity than using high impedance resistors.
89
To generate
high levels of power from the Olsen cycle the ability to apply
large electric elds is advantageous, i.e. to maximise E
HIGH
in
Fig. 4, and therefore materials with a high dielectric strength
are desirable to avoid eld-induced crack propagation.
70
Table 2
summarises some of the proposed temperature ranges, electric
elds and resulting energy densities. While polycrystalline
ceramics have a dielectric strength of 3–4kVmm
1
, single
crystals can withstand electric elds up to 12–14 kV mm
1
, and
oriented thin-lms electric elds up to 60–80 kV mm
1
.
110
Polymer-based PVDF materials are attractive materials because
of their low cost, mechanical exibility (compliance) and they
have been used at electric elds up to 120 kV mm
1
;
111
for
example compare the electric elds employed for the PVDF
based systems with the ceramics in Table 2. While high electric
elds and temperature ranges are desirable they can cause high
leakage currents across the active material, degrading the
available energy.
38
The need to employ external electric eld leads to this cycle
being employed for larger energy harvesting systems, rather
than nano- or micro-scale devices; making is less suitable for
wireless power systems. The energy harvesting capability of
[001] oriented 68PbMg
1/3
Nb
2/3
O
3
-32PbTiO
3
(PMN-32PT) single
crystal was measured by Kandilian et al.
57
by successively
dipping the material in oil baths at temperatures 80 C and
170 C and cycling the electric eld between 2 and 9 kV cm
1
.
This energy density was 100 mJ cm
3
per cycle, corresponding
to 4.92 mW cm
3
. It was estimated 40% of this energy resulted
from the strain polarization due to the rhombohedral to
tetragonal phase transition. For a 0.90Pb(Mg
1/3
Nb
2/3
)O
3
-
0.10PbTiO
3
ceramic the harvested energy reached 186 mJ cm
3
for a 50 C temperature change and an electric eld change of
3.5 kV mm
1
; based on an operating frequency of 2 Hz with a
10 C temperature change a power level of 100 mW cm
3
was
considered feasible. The continuous application of electric eld
in such cycles enables re-polarization of the material as it cools
from above the Curie temperature, this is an additional
advantage compared to harvesting cycles that do not apply an
electric eld.
In addition to increasing the maximum applied eld, it is
also of interest to work near a phase transition where the
polarization is strongly inuenced by temperature varia-
tions.
57
This has been examined for a range of ferroelectric
materials that exhibit phase transitions such ferroelectric–
ferroelectric
110
and ferroelectric-to-paraelectric.
57
One disad-
vantage of this approach is that the working temperature
range of the harvester is restricted to the vicinity of the phase
transition in terms of both electric eld and temperature,
32
unlike the resistive cycle approach. The Olsen cycle is suited to
operation near the linear polarisation region
110
and performs
best with an external electric eld operating around the Curie
temperature.
111
Duetotherequiredtransientchangein
temperature, the adjustment of the right operation frequency
of pyroelectric cycles is crucial for increasing energy trans-
formation.
112
McKinley et al.
113
recently reported a novel
thermo-mechanical energy conversion cycle in which the
temperature increase in an Olsen-based cycle was combined
with a mechanical stress to further decrease the polarisation
level; such an approach can be used to generate power at lower
temperatures along with the ability to adapt to changing
thermal and mechanical conditions.
Table 2 Comparison of operating conditions and energy densities for Olsen type cycles
Material T
low
(C) T
high
(C) E
low
MV m
1
E
high
MV m
1
Energy density
N
D
Jm
3
/cycle Reference
PNZST ceramic 158 170 0.4 2.8 95 98
PNZST ceramic 145 175 0.8 3.2 300 89
PMN-PT 90/10 ceramic 35 85 0.5 3.5 186 56
PLZT 8/65/35 ceramic 25 160 0.2 7.5 888 68
KNTM ceramic 140 160 0.1 5 629 152
BNLT ceramic 25 120 0.1 11.2 1146 153
BNKT ceramic 25 110 0.1 5.2 1986 153
BNK-BST ceramic 20 160 0.1 4.0 1523 154
YBFO thin lm 258 27 0.1 4.0 7570 155
PZST 157 177 0.4 3.2 131 112
PZST 145 178 1.2 3.2 130 96
PZST 146 159 0.0 2.9 100 89
PZST 110 170 0.0 2.8 0.4 97
PZN-4.5PT 100 160 0.0 2.0 217 32
PZN-5.5PT 100 190 0.0 1.2 150 23
PMN-10PT 30 80 0.0 3.5 186 56
PMN-32PT 80 170 0.0 0.9 100 57
P(VDF-TrFE) 73/27 23 67 23.0 53.0 30 84
P(VDF-TrFE) 60/40 58 77 4.1 47.2 52 104
P(VDF-TrFE) 60/40 67 81 20.3 37.9 130 82
P(VDF-TrFE) 60/40 25 110 20.0 50.0 521 38
P(VDF-TrFE) 60/40 25 120 20.0 60.0 900 109
P(VDF-TrFE-CFE) 61.3/29.7/9 0 25 0.0 25.0 50 156
Energy Environ. Sci. This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science Review
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

In summary the Olsen cycle is attractive due to its efficiency
and ability to exploit phase transitions with large changes in
polarisation. However it is oen restricted to operation between
two specic temperatures and since its operation cycle is more
complex the approach has oen been considered for relatively
larger systems
82
and less work on small-scale power generation
for wireless sensor applications.
114
5. Harvesting from pyroelectric
devices: circuit implementation
In order for an energy harvesting system to be practical it is
necessary to extract and store the harvested electrical energy in
an external device for future access. This section summarises
the electronic circuitry currently available for this purpose
which has been examined by a number of researchers.
90,93,115,116
The material is presented in two subsections where the rst
section considers simple temperature cycling while the second
considers systems based on Olsen-type cycles where electrical
energy is input into each cycle.
5.1 Energy harvesting from pyroelectric devices using
temperature cycling alone
Fig. 5 shows the thermoelectric equivalent circuits of a homo-
geneous pyroelectric cell. The thermoelectric equivalent circuit,
Fig. 5a, represents the conversion of incident thermal power W
into a temperature change. The relationship between current,
pyroelectric coefficient, area and rate of change of temperature
can expressed by the electrical equivalent circuit in Fig. 5b that
includes the pyroelectric current source I, the electrical capac-
itance C
P
and resistance R
P
resulting in an output voltage V
p
.
The two parts of the process are functionally linked by eqn (1).
Due to the low frequency of the temperature cycle, the source
impedance tends to be dominated by C
p
, resulting in relatively
large open circuit voltages and this is a characteristic that
pyroelectric devices share with piezoelectric sources.
Having converted thermal energy into electrical energy
(charge) it is necessary to transfer the electrical energy to an
external load (e.g. resistor) or storage medium (e.g. capacitor)
using a suitable interface. The simplest possible interface is a
resistive load connected directly across the terminals of the
device, i.e. a resistive cycle. The pyroelectric current I,ows
through the resistor as the temperature is cycled and electrical
power is dissipated in the electrical load. Even under optimum
conditions, where the load resistor is matched to the output
impedance of the pyroelectric device, the efficiency of this
arrangement is low; for example van der Ziel
7
reported effi-
ciencies as low as 0.025%. In a variant of this idea, electrical
energy can be transferred to a storage device (such as a capacitor
or storage battery), for future discharge. Since pyroelectric
devices are bidirectional an alternating thermal drive generates
an alternating current and rectication is necessary to harvest
the energy. A typical arrangement, sometimes referred to as the
‘standard interface’is shown in Fig. 6.
91
The pyroelectric energy
source is represented by the equivalent circuit of Fig. 5b and
this is connected to a diode bridge (D1–D4) providing full-wave
rectication of the output voltage from the device. The har-
vested energy is stored on the capacitor C
L
and dissipated by the
resistive load represented by R
L
where the electrical time
constant R
L
C
L
is chosen to be signicantly longer than the
thermal cycling period to minimise any ripple on the output
voltage.
Although the arrangement shown in Fig. 6 is practical in that
pyroelectric energy can be extracted and stored for future use,
its efficiency is similar to that obtained by connecting a resistor
directly across the device. An improved method to extract
pyroelectric energy should recognise the characteristics of the
source, such as a current source terminated by a resistance and
capacitance in parallel (dominated by C
p
), and increase the
voltage developed across the load. For example, the open circuit
voltage developed by the source could be coupled to the load
using an amplier to increase the load voltage and hence
available power. However, ampliers require power and given
the small amount of energy generated, the efficiency of such a
system is likely to be too low for the method to be viable. A more
successful approach is based on a switched (i.e. non-linear)
arrangement in which charge is extracted at points of maximum
and minimum temperature, i.e. when the stored electrical
energy is also a maximum. This method has origins in vibration
Fig. 5 Thermoelectric equivalent circuits of a homogeneous pyro-
electric cell. The circuit on the left (a) represents the conversion of
incident thermal energy Winto a change in temperature, T. Equivalent
circuit (b) represents the conversion of current generated by the
pyroelectric process into an output voltage V
p
. The two parts of the
process are connected by the eqn (1) and C
T
,R
T
,C
P
and R
P
are the
thermal and electrical capacitances and resistances respectively.
Fig. 6 ‘Standard’interface circuit consisting a diode bridge full-wave
rectifier interfacing a pyroelectric source (equivalent circuit of Fig. 5b
consisting of I
p
,C
p
,&R
p
) and a load consisting of C
L
&R
L
.I
p
is
synchronised to the temperature variation and the time constant C
L
R
L
is chosen to be much longer than the period of the temperature cycle
to minimise the ripple on the output voltage.
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Review Energy & Environmental Science
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

damping of mechanical structures
117,118
and has been applied
extensively to energy harvesting from piezoelectric trans-
ducers.
119
Due to the similarities between piezoelectric and
pyroelectric transducers it is no surprise that these synchronous
methods, such as that in Fig. 3c, also work well with the latter
class of devices.
ThearrangementshowninFig.7isoneexampleofthe
nonlinear approach applied to pyroelectric harvesting.
119
Thiscircuitissimplythe‘standard’interface of Fig. 6 to
which has been added an inductor Lthat can be connected in
shunt across the source by closing the switch S.This
nonlinear process is called synchronised switch harvesting on
inductor (SSHI) charge extraction and allows the high
impedance, essentially capacitive nature of the source, to be
turned to advantage. When the switch is closed L
p
and C
p
form a parallel tuned circuit, driven by I
p
to function as a
forced oscillator with a resonant frequency u0¼1=ffiffiffiffiffiffiffiffiffiffi
LPCP
p.If
Sis closed at the peak of temperature cycle and assuming u
0
is much higher than the temperature cycling frequency, it is
possible to capture most of the energy generated by the
pyroelectric device. The voltage across the tuned circuit can
be many times larger than the natural open circuit voltage of
the pyroelectric device (depending on the quality factor of the
inductor and resistive losses) resulting in a signicantly
increased load voltage.
The harvested energy is temporarily stored as magnetic
energy in L
p
, the only limitation on this process being losses
in the circuit; these are principally due to the nite resistance
of the switch and the inductor windings. The approach has
much in common with the buck/boost power converter whose
high efficiency stems essentially from the ability of an
inductor to store energy ‘loss-lessly’as a magnetic eld.
Using this method, signicant improvements in efficiency
compared to the ‘standard’interface have been reported,
91
although there is scope for further circuit development to
determine the ultimate limitation on the enhancement
obtainable.
5.2 Energy harvesting from pyroelectric devices using the
Olsen cycle
Upgrading the pyroelectric equivalent circuit with storage
capability enables isoelectric voltage operations, similar to an
isobaric process in a thermodynamic Ericsson cycle.
89
It has
been suggested that under ideal conditions a pyroelectric
converter using the Olsen-type cycle shown in Fig. 4 could
achieve about 15% efficiency or about half the capability of a
theoretical Carnot cycle.
89
In order to achieve this performance
it is necessary to be able to charge and discharge the device as
efficiently as possible (Paths 1–2 and 3–4 of the cycle in Fig. 4b).
In ref. 99 it was suggested that if standard switched-mode
methods were employed an efficiency of 95–98% could be
achieved in this part of the process. These circuits use inductors
to provide magnetic energy storage and, as described in Section
5.1, the process is essentially loss-less.
6. Pyroelectric harvesting devices
6.1 Nanostructured and micro-scale materials and devices
For pyroelectric harvesters the operating frequency of the device
is oen small (typically much less than 1 Hz). In uid based
systems the frequency of operation is limited by heat transfer
between the pyroelectric and the working uid that is oscillating
between hot and cold sources. To increase the operating
frequency, radiative heat transfer at the nano-scale has been
examined by Fang et al.
120
Energy transfer by thermal radiation
between two semi-innite solids is almost instantaneous and can
be enhanced by several orders of magnitude from the conven-
tional Stefan–Boltzmann law if their separation is a distance
smaller than a characteristic wavelength, given by Wien's
displacement law. A device was analysed by modelling nano-
scale radioactive heat transfer between a pyroelectric that was
subjected to oscillation using piezoelectric pillars in a vacuum
between hot and cold aluminium surfaces. A device using 60/40
porous poly(vinylidene uoride-triuoroethylene) was predicted
to have a 0.2% efficiency and a 0.84 mW cm
2
electrical power
output for the cold (273 K) and hot sources (388 K). A pyroelectric
plate made from 0.9PMN-PT composite thin lms achieved a
higher efficiency (1.3%) and a larger power output (6.5 mW
cm
2
) for a temperature oscillation amplitude of 10 K at a
temperature of 343 K at a relatively high frequency of 5 Hz.
A simple approach to improve the rate of temperature
change and increase the pyroelectric current is to reduce the
thickness of the pyroelectric, such as using thin lms. Yang
et al.
87
demonstrated a exible hybrid energy cell for simulta-
neously harvesting thermal, mechanical, and solar energies. A
ZnO-poly(3-hexylthioohene) hetero-junction solar cell was used
for harvesting solar energy while a PVDF-based pyroelectric and
piezoelectric nano-generator was built on its bottom surface for
harvesting thermal and mechanical energies. A pyroelectric
coefficient of 44 mCm
2
K
1
was measured. Using a lithium-
ion battery to store the harvested energy the device could drive
LED devices. Pyroelectric ‘nano-generators’based on ZnO
nanowire arrays were reported by Yang et al.
41
A higher pyro-
electric voltage and current coefficients were determined for the
Fig. 7 Parallel synchronised switch harvesting on inductor (SSHI)
charge extraction interface circuit. When Sis closed L
p
and C
p
form a
parallel tuned circuit, driven by I
p
to function as a forced oscillator with
a resonant frequency u0¼1=ffiffiffiffiffiffiffiffiffiffiffi
LPCP
p.IfSis closed at the peak of
temperature cycle and assuming u
0
is much higher than the
temperature cycling frequency, it is possible to capture most of the
energy generated by the pyroelectric device.
Energy Environ. Sci. This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science Review
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

nano-generator compared to bulk and lm material; this was
thought to be due to the preferred orientation of the ZnO
nanowire array. Yang et al. also presented an active temperature
change sensor based on a PZT generator.
59
The system consisted
a PZT microwire on a thin glass substrate with electrical
contacts at its ends and packaged in polydimethylsiloxane
(PDMS). The ferroelectric wire was polarised at room tempera-
ture and the output current and voltage increased linearly with
the rate of temperature change and to demonstrate its potential
the harvested energy was used to power a liquid crystal display.
A pyroelectric generator based on PZT thin lm (175 mm thick)
exhibited a pyroelectric coefficient of approximately 800 mC
m
2
K
1
with a maximum power density of 0.215 mW cm
3
based on the open-circuit voltage and short-circuit current
density.
66
The power of the pyroelectric generator was used to
charge a lithium ion battery and a single output pulse could
charge a LCD (Fig. 8). Lead-free KNbO
3
81
nanowire/PDMS
polymer with Ag and indium tin oxide (ITO) electrodes as a
exible nano-generator have also been fabricated where the
output could be tuned by the electric eld due to changes in
ferroelectric domain orientation. The nanowires were grown by
the hydrothermal method and were approximately 150 nm
diameter. The bulk pyroelectric coefficient of KbNO
3
is 50 mC
m
2
K
1
while the effective coefficient of the nanowire–polymer
mixture was 8 mCm
2
K
1
due to the presence of the non-polar
PDMS; the proposed advantage of the PDMS was to provide
exibility.
At the micro-scale Hsiao et al.
63,121,122
have reported the
etching of pyroelectric surfaces and etching the electrode
structure to improve energy harvesting performance. This was
based on the observation that partially covered top electrodes
provide a higher current and voltage responsivity than a fully
covered electrode since it allows the pyroelectric layer to be in
closer contact with the heat source.
123
Hsiao et al. showed that a
meshed top electrode and trenched pyroelectric improved the
responsivity of the PZT. One issue is that using a thicker pyro-
electric element leads to a larger total heat capacity that reduces
the temperature change
63
for a specic energy input. The
purpose of trenching the PZT was to enhance the rate of
temperature change due to a lateral temperature gradient as a
result of the trenched architecture. A vortex-like electrode with a
deep structure was also produced
122
by sandblasting which
improved the harvested power by 11% compared to a fully
covered electrode.
The size of the pyroelectric element has also been used to
tailor the phase transition temperatures in ferroelectric nano-
wires, enabling a ‘giant’pyroelectric response.
124
It was shown
using Landau–Ginzburg–Devonshire phenomenological theory
that it is possible to tune the pyroelectric coefficient by
changing the nanowire radius and the nature of the
surrounding media e.g. template material, gas or gel, since the
ferroelectric-ambient interface determines the surface energy
properties. While the predicted efficiency of these nanoscale
materials was low at room temperature it was noted that as the
temperature decreased the efficiency tends to the Carnot cycle
efficiency, making it suitable for low temperatures, e.g. space
applications. Nanowires of GaN and ZnO have been examined
using rst principles-based density functional theory (DFT)
calculations and considered the size dependency of the piezo-
electric coefficients and a ‘giant piezoelectric size effect’was
identied;
125
it would be interesting to examine the effect of
scaling on pyroelectric coefficients and nano-structured mate-
rials. The reader is referred to a review by Lingam et al. for a
further discussion on nano/microscale pyroelectric harvesters.
18
6.2 Hybrid generators
Since all pyroelectrics are piezoelectric it is perhaps not
surprising that researchers have attempted to combine both
pyroelectric and piezoelectric harvesting. The generation of an
electric current under short circuit conditions or an electric
potential in open circuit conditions as a result of the change in
polarisation with a temperature change has analogies with
piezoelectric harvesting. Table 3 compares the relevant equa-
tions for a pyroelectric subjected to a temperature change (DT)
and piezoelectric subjected to a stress (Ds) with similarities in
the relationships between current, voltage and stored energy
between temperature change and applied stress under both
open and closed circuit conditions. Due to their similarities
there is interest for potential hybrid piezoelectric–pyroelectric
harvesting systems
87,126,127
whereby a combination of tempera-
ture change and stress is applied. In such systems care must be
taken to ensure the changes in polarisation are constructive and
enhance the power generation of the harvesting device.
62
As
discussed by Sebald et al., the frequencies of temperature and
vibration may be different and there is a need to optimise the
electronics for such a hybrid system.
90
Lee et al.
126
fabricated a stretchable, hybrid piezoelectric–
pyroelectric nano-generator based on a micro-patterned
Fig. 8 (a) Cyclic change in temperature of a PZT thin-film pyroelectric
generator and corresponding differential temperature (dT/dt). (b)
Output voltage and current of the generator after rectificaton by a full-
wave bridge circuit. (c) Enlarged single output voltage peak, where it is
used to drive a LCD in the region “2”. (d) Calculated electrical potential
distribution across the PZT film.
66
Reprinted with permission from
American Chemical Society. Copyright (2012) American Chemical
Society.
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Review Energy & Environmental Science
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

piezoelectric P(VDF-TrFE) polymer, micro-patterned PDMS-
carbon nanotube (CNTs) composite and graphene nanosheets,
see Fig. 9. The PDMS-CNT was used to make the device exible
and also serve as a robust electrode on the base of the device.
Graphene was used as a top exible electrode to allow a fast
temperature gradient on the device due to its high thermal
conductivity. The potential of the material harvest both
mechanical loads (s) and temperature changes (DT) was
examined. The total change in polarisation is expressed as:
DP¼ds+pDT(19)
Erturun et al.
62
examined combined harvesting using a
heating lamp directed at a vibrating beam. Both effects were
initially investigated independently and subsequently coupled.
In some cases the combination of beam vibration with thermal
cycling had a negative effect on scavenged energy and this
indicates the potential complexities in such an approach,
especially to differences in frequency of temperature and
mechanical oscillations. However, the use of ‘piezoelectric–
pyroelectric-harvesters’potentially offers an interesting method
of enhancing power.
6.3 Pyroelectric ‘systems’and active oscillators
Oak Ridge National Laboratory
128,129
designed a MEMS based
cantilever harvesting system based on a thermally cycled pyro-
electric capacitor that acts as a bimorph cantilever. The
bimorph operates between two surfaces, one heated by waste
heat and the other is a cold heat sink (Fig. 10). Proof masses are
placed at the cantilever tip to ensure good thermal contact to
the hot and cold surfaces. When the cantilever is heated it
deforms due to a thermal expansion mismatch between the
bimorph layers that leads to it contacting the cold surface,
making the structure cool and deform in the reverse direction
and then making contact to the hot surface. This cyclic defor-
mation leads to the cantilever alternately contacting the hot and
cold surfaces at the resonant frequency of the cantilever to
generate a pyroelectric current. The use of a MEMS approach
means that large arrays of devices could be used to increase
power and this interesting approach allows the device to
Table 3 Comparison of relevant equations for pyroelectric p¼dP
s
/dT(C m
2
K
1
) and piezoelectric systems d
ij
¼dP
s
/ds(C N
1
or C m
2
N
1
m
2
) and 3
T
33
is permittivity at constant stress
Pyroelectric Piezoelectric
Charge (Q)Q¼pADTQ¼d
ij
ADs
Short-circuit current (i¼DQ/Dt)i¼pA DT
Dti¼dijADs
Dt
Open-circuit voltage (V¼Q/C)V¼p
3T
33
hDTV¼dij
3T
33
hDs
Stored energy (1/2CV
2
)E¼1
2
p2
3T
33
AhðDTÞ2E¼1
2
dij2
3T
33
AhðDsÞ2
Fig. 9 (a) Schematic of stretchable, hybrid piezoelectric–pyroelectric nano-generator (b) image of device (c) location of devices on body (d)
piezoelectric output on application of strain and pyroelectric output on changing temperature.
126
Reprinted with permission of John Wiley
and Sons.
Energy Environ. Sci. This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science Review
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

potentially operate at high frequencies, up to 20 Hz or
higher.
2,130
Another approach to increase operating frequency uses
liquid-based switchable thermal interfaces to convert a spatial
temperature gradient into temporal temperature oscilla-
tions;
29,114
the system operates in an Olsen type cycle as shown in
Fig. 4. In this work a plate with a pyroelectric material oscillates
up and down between a high temperature source and a cold heat
sink and repeatedly makes thermal contact to undergo temper-
ature oscillations, Fig. 11. In the thermally conducting state, the
pyroelectric is pressed against the hot or cold surface using a
linear actuator and liquid droplets at the interface deform to
make them merge into a continuous thin liquid layer of low
thermal resistance. In the non-thermally conducting state, the
pyroelectric material is physically separated from the hot and
cold surfaces and the liquid on the pyroelectric interface exists
as discrete droplets. By creating a hydrophilic pattern on the
surfaces the rupture distance was reduced, thus reducing the
distance required and increasing the operating frequency. A
device was demonstrated at frequencies of the order of 1 Hz with
a power density of 110 mW cm
3
.
Huesgen et al.
131–133
presented a micro heat engine manu-
factured using silicon micro-technology based on a cavity lled
with a liquid–gas phase-change uid that develops a recipro-
cating motion between heat source and heat sink. A bistable
membrane was used that buckled during expansion and
contraction of a phase-change uid to generate upward and
downward motion between the heat source and sink. The
engine was self-starting and self-regulating with a temperature
dependent operation frequency (e.g. 0.7 Hz for a 37 K temper-
ature difference). The micro ‘thermo-mechanic-pyroelectric’
energy generator (mTMPG) had a measured power output of
3mW for a temperature difference of 79.5 K. An optimised
design was proposed based on a similar process with a power
output of 39.4 mW for the same temperature difference that
used h111iPMN-0.13PT single crystal (Fig. 12).
55
The potential
benets of pulsed heat transfer has been considered by McKay
et al.
134
and Carlioz et al. who have attempted to combine
piezoelectric materials with hard and somagnetic materials
whose attracting forces vary with temperature.
135
A micro-system consisting of a carbon nanotube lm inte-
grated with a PZT cantilever for harvesting light and thermal
energy was reported by Kotipalli et al.
65
The carbon layers served
as an efficient absorber of thermal radiation, resulting in a
shape change of the carbon layer and induced bending into the
piezoelectric cantilever. A power of 2.1 mW was generated from a
light intensity of 0.13 W cm
2
. A carbon nanotube lm –
cantilever was observed to generate both AC voltages due to self-
reciprocation of the cantilevers and also generate a DC
component as a result of the thermoelectric properties of the
carbon layer.
136
Finally, Zhang et al.
137
showed that a pyroelectric under solar
radiation can produce energy as a result of uctuations due to
Fig. 10 Side view of the pyroelectric bimorph cantilever showing the
cantilevered capacitor thin film layers. The energy harvester consisting
of a bi-material cantilever which alternately contacts hot and cold
surfaces and generates a current in the pyroelectric capacitor.
130
Reprinted with permission of S. R. Hunter.
Fig. 11 (a) Pyroelectric energy harvesting module. Electrode assembly
containing a pyroelectric material is actuated up and down and makes
alternating thermal contact with the heat source (hot side) and sink
(cold side) via switchable thermal interfaces.
114
Reprinted with
permission from Elsevier.
Fig. 12 Illustration of proposed mTMPG. The h111iPMN-0.13PT crystal
mechanically displaces up and down via a bistable membrane as it
repeatedly heats at the source and cool at the sink.
55
Reprinted with
permission AIP Publishing LLC.
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Review Energy & Environmental Science
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

wind to induce the temperature uctuations that is necessary to
exploit the pyroelectric effect. This inspired a pyroelectric-based
solar and wind harvester developed by Krishnan et al.
64
The
mode of operation is to concentrate solar radiation onto a
pyroelectric material to induce a large temperature change. To
create the necessary temperature change a wind turbine was
used to create optical modulation with a gearing system to
optimise the modulation period to ensure large temperature
changes. For a PZT prototype a maximum power density 421 mW
cm
3
was produced. One of the initial reports of using ferro-
electric materials to harvest solar radiation is by Hoh
6
who
considered a spinning space vehicle and the change in polar-
isation and dielectric constant of a ferroelectric with tempera-
ture. Solar power generation using pyroelectrics has been
considered by van der Ziel
7
who highlighted the need for
focussing of radiation for pyroelectric harvesting.
7. Conclusions
Compared to other forms of energy harvesting and thermal
harvesting such as thermoelectric generators, the use of pyro-
electric harvesting to generate electrical energy from tempera-
ture uctuations is less well studied. While the efficiencies can
be high for specic thermal and electric cycles, especially Olsen-
based cycles, the inability to induce high frequency temperature
uctuations currently limits the amount of power that can be
harvested, this is in contrast to mechanical oscillations where
mechanical vibrations over 10
2
Hz are relatively easy to
implement.
With regards to potential harvesting cycles, resistively
loading the pyroelectric element is relatively simple and can
operate in a range of operating environments and temperatures
although the material must clearly maintain its polarised
nature. The use a natural temperature uctuations to generate a
pyroelectric current as surface charges are released on heating a
pyroelectric is generally of low efficiency. Other methods, such
as employing the Olsen thermal cycle with corresponding
changes in capacitance and material phase changes can
increase both the efficiency and the quantity of the power
generated compared to simple resistive loading. Such systems
are oen designed to operate within specic temperatures and
electric eld ranges and as a result Olsen-type systems tend to
be designed to operate is specic locations and manufactured
from bulk materials for larger-scale harvesting systems, rather
than low power systems for wireless sensor systems. Limited
systems have employed the Olsen-type cycle at the micro to
nano-scale.
In an effort to improve power capability attempts to increase
the operational frequency are oen undertaken, such as the
generation of mechanical oscillations from a temperature
gradient. The creation of pyroelectric harvesting materials and
systems at the nano-scale may also offer opportunities for
operation at higher frequencies. This can be coupled with the
development of new materials with improved pyroelectric
coefficients especially for harvesting applications. Materials and
material architectures with improved heat transfer are of
interest to increase rates of temperature change or improved
FOMs. Composite material systems or design of materials with
high FOM to tune the pyroelectric response and mechanical and
thermal properties are also potential future avenues of research.
Since the pyroelectric effect originates from spontaneous
polarisation within the material, all pyroelectric materials are
also piezoelectric, therefore hybrid pyro-piezo harvesting
systems are of interest. In the design of such systems care must
be taken to ensure both harvesting mechanisms are working in-
phase to enhance power generation. Novel systems that use
thermal uctuations or thermal gradients to generate a
mechanical stress to enhance the secondary or tertiary pyro-
electric coefficients are also of interest.
The low efficiency of resistive and synchronised electric
charge extraction cycles and low frequency of operation may
oen result in pyroelectric harvesting being a less favourable
harvesting option compared to vibration harvesters or photo-
voltaics for low power applications. However in locations with
low levels of mechanical vibrations or light it is an intriguing
option to generate useable power. It can also be used to enhance
the power generation capability of mechanical energy harvest-
ing systems.
Acknowledgements
C. R. Bowen would like to acknowledge funding from the
European Research Council under the European Union's
Seventh Framework Programme (FP/2007–2013)/ERC Grant
Agreement no. 320963 on Novel Energy Materials, Engineering
Science and Integrated Systems (NEMESIS).
References
1 S. P. Beeby, M. J. Tudor and N. M. White, Energy harvesting
vibration sources for microsystems applications, Meas. Sci.
Technol., 2006, 17, R175–R195.
2 S. R. Hunter, N. V. Lavrik, S. Mostafa, S. Rajic and
P. G. Datskos, Review of pyroelectric thermal energy
harvesting and new MEMs-based resonant energy
conversion techniques, Proc. SPIE, 2012, 8377, 83770D.
3 A. Khodayari, S. Mohammadi and D. Guyomar, Pyroelectric
energy harvesting: fundamentals and applications, VDM
Publishing, 2011.
4 W. H. Clingman and R. G. Moore, Application of
ferroelectricity to energy conversion processes, J. Appl.
Phys., 1961, 32, 675–681.
5 E. Fatuzzo, H. Kiess and R. Nitsche, Theoretical efficiency of
pyroelectric power converters, J. Appl. Phys., 1966, 37, 510–
516.
6 S. R. Hoh, Conversion of thermal to electrical energy with
ferroelectric materials, Proc. IEEE, 1963, 51, 838–845.
7 A. van der Ziel, Solar power generation with pyroelectric
effect, J. Appl. Phys., 1974, 45, 4128.
8 J. E. Drummond, Dielectric power conversion, in Energy 10;
Annual Intersociety Energy Conversion and Engineering
Conference, Newark, 1975, pp. 569–575.
Energy Environ. Sci. This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science Review
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

9 J. D. Childress, Application of a ferroelectric material in an
energy conversion device, J. Appl. Phys., 1962, 33, 1793–
1798.
10 J. E. Drummond, V. Fargo, M. Briscoe and H. Reed,
Demonstration of a high power density electrocaloric heat
engine, Ferroelectrics, 1980, 27, 215–218.
11 R. W. Whatmore, Pyroelectric devices and materials, Rep.
Prog. Phys., 1986, 49, 1335–1386.
12 S. B. Lang, Pyroelectricity: From ancient curiosity to
modern imaging tool, Phys. Today, 2005, 58,31–36.
13 J. Erhart, Experiments to demonstrate piezoelectric and
pyroelectric effects, Phys. Educ., 2013, 48, 438–447.
14 S. B. Lang and D. K. Das-Gupta, Pyroelectricity:
Fundamentals and Applications, in Handbook of Advanced
Electronic and Photonic Materials and Devices, ed. H. S.
Nalwa, Academic Press, 2001, vol. 4, pp. 1–54.
15 S. B. Lang and S. B. Muensit, Review of some lesser-known
applications of piezoelectric and pyroelectric polymers,
Appl. Phys. A: Mater. Sci. Process., 2006, 85, 125–134.
16 S. B. Lang, Pyroelectricity: A 2300-year history, Ferroelectrics,
1974, 7, 231–234.
17 I. Lubomirsky and O. Stafsudd, Practical guide for
pyroelectric measurements, Rev. Sci. Instrum., 2012, 83,
051101.
18 D. Lingam, A. R. Parikh, J. Huang, A. Jain and M. Minary-
Jolandan, Nano/microscale pyroelectric energy harvesting:
challenges and opportunities, Int. J. Smart Nano Mater.,
2013, 4, 229–245.
19 L. B. Kong, et al.,Waste energy harvesting: Mechanical and
thermal energies, Springer, London, 1994.
20 R. G. Kepler, Piezolectricity, pyroelectricity and
ferroelectricity in organic materials, Annu. Rev. Phys.
Chem., 1978, 29, 497–518.
21 J. Li, Y. Liu, Y. Zhang, H. Caic and R. Xiong, Molecular
ferroelectrics: where electronics meet biology, Phys. Chem.
Chem. Phys., 2013, 15, 20786–20796.
22 F. Y. Lee, A. Navid and L. Pilon, Pyroelectric waste heat
energy harvesting using heat conduction, Appl. Therm.
Eng., 2012, 37,30–37.
23 I. M. McKinley, R. Kandilian and L. Pilon, Waste heat
energy harvesting using the Olsen cycle on 0:945Pb(Zn1/3
Nb2/3)O3–0:055PbTiO
3
single crystals, Smart Mater.
Struct., 2012, 21, 035015.
24 A. Navid, D. Vanderpool, A. Bah and L. Pilon, Towards
optimization of a pyroelectric energy converter for
harvesting waste heat, Int. J. Heat Mass Transfer, 2010, 53,
4060–4070.
25 Micropelt, http://www.micropelt.com/technology.php.
26 Marlow Industries, Inc., https://www.marlow.com.
27 D. Guyomar and G. Sebald, Pyroelectric/electrocaloric
scavenging and cooling capabilities in ferroelectric
materials, International Journal of Applied Electromagnetics
and Mechanics, 2009, 31,41–46.
28 J. Xie, X. P. Mane, C. W. Green, K. M. Mossi and K. K. Leang,
Performance of thin piezoelectric materials for pyroelectric
energy harvesting, J. Intell. Mater. Syst. Struct., 2010, 21,
243–249.
29 G. Cha and Y. Jia, High power density pyroelectric energy
harversting incorporating switchable liquid-based thermal
interfaces, in IEEE 25th International Conference on Micro
Electro Mechanical Systems (MEMS), 2012, pp. 1241–1244.
30 H. Maiwa, Y. Ishizone and W. Sakamoto, Thermal and
vibrational energy harvesting using PZT- and BT-based
ceramics, in IEEE, Applications of Ferroelectrics, Aviero,
2012, pp. 1–4.
31 G. Sebald, D. Guyomar and A. Agbossu, On thermoelectric
and pyroelectric energy harvesting, Smart Mater. Struct.,
2009, 18, 125006.
32 A. Khodayari, S. Pruvost, G. Sebald, D. Guyomar and
S. Mohammadi, Nonlinear pyroelectric energy harvesting
from relaxor single crystals, IEEE Trans. Ultrason.
Ferroelectr. Freq. Control, 2009, 56, 693–699.
33 A. A. Sklar, A numerical investigation of a thermodielectric
power generation system, PhD Dissertation, Georgia
Institute of Technology, 2005.
34 A. Potnuru and Y. Tadesse, Characterization of pyroelectric
materials for energy harvesting from human body, Integr.
Ferroelectr., 2014, 150,23–50.
35 A. K. Batra, S. Bhatacharjee and A. K. Chilvery, Energy
harvesting roads via pyroelectric effect: a possible
approach, Proc. SPIE, 2011, 8035, 803519.
36 P. Mane, J. Xie, K. K. Leang and K. Mossi, Cyclic energy
harvesting from pyroelectric materials, IEEE Trans.
Ultrason. Ferroelectr. Freq. Control, 2011, 58(1), 10–17.
37 A. Cuadras, M. Gasulla and V. Ferrari, Thermal energy
harvesting through pyroelectricity, Sens. Actuators, A,
2010, 158, 132–139.
38 A. Navid and L. Pilon, Pyroelectric energy harvesting using
Olsen cycles in puried and porous poly(vinylidene
uoride-triuoroethylene) [P(VDF-TrFE)] thin lms, Smart
Mater. Struct., 2011, 20, 025012.
39 X. Li, et al., Pyroelectric and electrocaloric materials, J.
Mater. Chem., 2013, 1,23–37.
40 J. D. Zook and S. T. Liu, Pyroelectric effects in thin lm, J.
Appl. Phys., 1978, 49, 4604–4606.
41 Y. Yang, et al., Pyroelectric nanogenerators for harvesting
thermoelectric energy, Nano Lett., 2012, 12, 2833–2838.
42 H. H. S. Chang and Z. Huang, Laminate composites with
enhanced pyroelectric effects for energy harvesting, Smart
Mater. Struct., 2010, 19, 065018.
43 H. H. S. Chang, R. W. Whatmore and Z. Huang, Pyroelectric
enhancement in laminate composites under short circuit
condition, J. Appl. Phys., 2009, 106, 014101.
44 J. Lim, Energy harvesting device using pyroelectric material,
US Pat., US2011/0169372A1, 2010.
45 G. Sebald, E. Lefeuvre and D. Guyomar, Pyroelectric energy
conversion: Optimisation principles, IEEE Trans. Ultrason.
Ferroelectr. Freq. Control, 2008, 55, 538–551.
46 G. Sebald, L. Seveyrat, D. Guyomar, B. Guiffard, L. Lebrun
and S. Pruvost, Electrocaloric and pyroelectric properties
of 0.75Pb(Mg
1/3
Nb
2/3
)O
3
-0.25PbTiO
3
single crystals, J.
Appl. Phys., 2006, 100, 124112.
47 R. V. K. Mangalam, J. C. Agar, A. R. Damodaran, J. Karthik
and L. W. Martin, Improved Pyroelectric Figures of Merit in
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Review Energy & Environmental Science
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

Compositionally Graded PbZr
1–x
Ti
x
O
3
Thin Films, ACS
Appl. Mater. Interfaces, 2013, 5, 13235–13241.
48 S. K. T. Ravindran, T. Huesgen, M. Kroener and P. Woias, A
self-sustaining micro thermomechanic-pyroelectric
generator, Appl. Phys. Lett., 2011, 99, 104102.
49 S. K. T. Ravindran, T. Huesgen, M. Kroener, and P. Woias, A
self-sustaining pyroelectric energy harvester utilizing
spatial thermal gradients, in Solid-State Sensors, Actuators
and Microsystems Conference, Beijing, 2011, pp. 657–660.
50 A. Navid, C. S. Lynch and L. Pilon, Puried and porous
poly(vinylidene uoride-triuoroethylene) thin lms for
pyroelectric infrared sensing and energy harvesting, Smart
Mater. Struct., 2010, 19, 055006.
51 G. M. Loiacono, A. Shaulov, D. Dorman, J. Jacco and
G. Kostecky, Crystal growth and electrical properties of
DTGFB/TGSe solid solutions, J. Cryst. Growth, 1982, 60(1),
29–32.
52 M. Marsilius, et al., Mechanical Connement: An Effective
Way of Tuning Properties of Piezoelectric Crystals, Adv.
Funct. Mater., 2012, 22, 797–802.
53 P. Yu, Y. Tang and H. Luo, Fabrication, property and
application of novel pyroelectric single crystals—PMN–PT,
J. Electroceram., 2010, 24,1–4.
54 I. M. McKinley and L. Pilon, Phase transitions and thermal
expansion in pyroelectric energy conversion, Appl. Phys.
Lett., 2013, 102, 023906.
55 S. K. T. Ravindran, T. Huesgen, M. Kroener and P. Woias, A
self-sustaining micro thermomechanic-pyroelectric
generator, Appl. Phys. Lett., 2011, 99, 104102.
56 G. Sebald, S. Pruvost and D. Guyomar, Energy harvesting
based on Ericsson pyroelectric cycles in a relaxor
ferroelectric ceramic, Smart Mater. Struct., 2008, 17, 015012.
57 R. Kandilian, A. Navid and L. Pilon, The pyroelectric energy
harvesting capabilities of PMN-PT near the morphotropic
phase boundary, Smart Mater. Struct., 2011, 20, 055020.
58 J. Xie, P. P. Mane, C. W. Green, K. M. Mossi and K. K. Leang,
Energy harvesting by pyroelectric effect using PZT, in Proc.
ASME Conf. Smart Materials, Adaptive Strucutre, Intelligent
Systems, Maryland, 2008, pp. 273–277.
59 Y. Yang, Y. Zhou, J. M. Wu and Z. L. Wang, Single micro/
nanowire pyroelectric nanogenerators as self-powered
temperature sensors, ACS Nano, 2012, 6, 8456–8461.
60 A. Cuadras, M. Gasulla, A. Ghisla and V. Ferrari, Energy
harvesting from PZT pyroelectric cells, in IMTC 2006
Intrumentation and Measurement Technology Conference,
Sorrento, 2006, pp. 1668–1672.
61 S. Dalola, V. Ferrari and D. Marioli, Pyroelectric effect in
PZT thick lms for thermal energy harvesting in low-
power sensors, Procedia Eng., 2010, 5, 685–688.
62 U. Erturun, R. Waxman, C. Green, M. L. Richeson and
K. Mossi, Energy scavenging combining piezoelectric and
pyroelectric effects, in ASME 2010 Conference on Smart
Materials, Adaptive Structures and Intelligent Systems,
Philadelphia, 2010.
63 C. C. Hsiao, A. Siao and J. Ciou, Improvement of
pyroelectric cells for thermal energy harvesting, Sensors,
2012, 12, 534–548.
64 S. Krishnan, D. Ezhilarasi, G. Uma and M. Umapathy,
Pyroelectric-based solar and wind energy harvesting
system, IEEE Trans. Sustainable Energy, 2014, 5,
73–81.
65 V. Kotipalli, et al., Light and thermal energy cell based on
carbon nanotube lms, Appl. Phys. Lett., 2010, 97, 124102.
66 Y. Yang, S. Wang, Y. Zhang and Z. L. Wang, Pyroelectric
Nanogenerators for Driving Wireless Sensors, Nano Lett.,
2012, 12, 6408–6413.
67 U. Erturun, C. Green, M. L. Richeson and K. Mossi,
Experimental analysis of radiation heat-based energy
harvesting through pyroelectricity, Journal of Intelligent
Material Systems and Structures, 2014, DOI: 10.1177/
1045389X14533432.
68 F. Y. Lee, S. Goljahi, I. M. McKinley, C. S. Lynch and
L. Pilon, Pyroelectric waste heat energy harvesting using
relaxor ferroelectric 8/65/35 PLZT and the Olsen cycle,
Smart Mater. Struct., 2012, 21, 025021.
69 T. K. Chin, et al., Direct thermal to electrical energy
conversion using 9.5/65/35 PLZT ceramics in the ergodic
relaxor phase, IEEE Trans. Ultrason. Ferroelectr. Freq.
Control, 2012, 59, 2373–2385.
70 F. Y. Lee, H. R. Jo, C. S. Lynch and L. Pilon, Pyroelectric
energy conversion using PLZT ceramics and ferroelectric-
ergodic relaxor phase transition, Smart Mater. Struct.,
2013, 22, 025038.
71 G. Vats, R. Vaish and C. R. Bowen, An analysis of lead-free
(Bi
0.5
Na
0.5
)0.915-(Bi
0.5
K
0.5
)0.05Ba0.02Sr0.015TiO
3
ceramic
for efficient refrigeration and thermal energy harvesting,
J. Appl. Phys., 2014, 115, 013505.
72 C. Peng, J. F. Li and W. Gong, Preparation and properties of
(Bi
1/2
Na
1/2
) TiO
3
–Ba (Ti, Zr) O
3
lead-free piezoelectric
ceramics, Mater. Lett., 2005, 59(12), 1576–1580.
73 R. Sun, et al., Pyroelectric properties of Mn-doped
94.6Na
0.5
Bi
0.5
TiO
3
-5.4BaTiO
3
lead-free single crystals, J.
Appl. Phys., 2014, 115, 074101.
74 K. Huang, et al., Signicant polarization variation near
room temperature of Ba0.65Sr0.35TiO
3
thin lms for
pyroelectric energy harvesting, Sens. Actuators, B, 2012,
169, 208–212.
75 S. Bhattacharjee, A. K. Batra and J. Cain, Energy harvesting
from pavements using pyroelectric single crystal and nano-
composite based smart materials, in First Congress of
Transportation and Development Institute (TDI), Chicago,
2011, pp. 741–750.
76 B. Naranjo, J. K. Gimzewski and S. Putterman, Observation
of nuclear fusion driven by a pyroelectric crystal, Nature,
2005, 434, 1115–1117.
77 V. A. Borisenok, A. S. Koshelev and E. Z. Novitsky,
Pyroelectric materials for converters of pulsed ionizing
radiation energy into electric power, Bull. Russ. Acad. Sci.:
Phys., 1996, vol. 60, pp. 1660–1662.
78 Y. Saito, et al., Lead-free piezoceramics, Nature, 2004, 432,
84–87.
79 A. Kumar, A. Sharma, R. Kumar, R. Vaish and
V. S. Chauhan, Finite element analysis of vibration energy
harvesting using lead-free piezoelectric materials: A
Energy Environ. Sci. This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science Review
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

comparative study, Journal of Asian Ceramic Societies, 2014,
2, 138–143.
80 A. Sharma, R. Kumar, R. Vaish and V. S. Chauhan,
Performance of K
0.5
Na
0.5
NbO
3
(KNN)-based Lead-free
Piezoelectric Materials in Active Vibration Control, Int. J.
Appl. Ceram. Technol., 2014, DOI: 10.1111/ijac.12231.
81 Y. Yang, et al., Flexible pyroelectric nanogenerators using a
composite structure of lead-free KNbO
3
nanowires, Adv.
Mater., 2012, 24, 5357–5362.
82 H. Nguyen, A. Navid and L. Pilon, Pyroelectric energy
converter using co-polymer P(VDF-TrFE) and Olsen cycle
for waste heat energy harvesting, Appl. Therm. Eng., 2010,
30, 2127–2137.
83 R. B. Olsen, D. A. Bruno, J. M. Briscoe and E. W. Jacobs,
High electric eld resistivity and pyroelectric properties of
vinylidene uoride-triuoroethylene copolymer, J. Appl.
Phys., 1985, 58, 2854–2860.
84 R. B. Olsen, D. A. Bruno, J. M. Briscoe and E. W. Jacobs,
Pyroelectric coversion cycle of vinylidene uroide-
triuoroethylene copolymer, J. Appl. Phys., 1985, 57, 5036–
5042.
85 L. Kouchachvili and M. Ikura, Pyroelectric conversion-
effects of P(VDF–TrFE) preconditioning on power
conversion, J. Electrost., 2007, 65, 182–188.
86 H. Nguyen, A. Navid and L. Pilon, Pyroelectric energy
converter using co-polymer P(VDF-TrFE) and Olsen cycle
for waste heat energy harvesting, Appl. Therm. Eng., 2010,
30, 2127–2137.
87 Y. Yang, et al., Flexible hybrid energy cell for simultaneously
harvesting thermal, mechanical, and solar energies, ACS
Nano, 2013, 7, 785–790.
88 A. K. Batra, A. Alomari, M. Thomas, M. Dokhanian and
D. Reynolds, Paint based pyroelectric thick-lms for
infrared and energy harvesting devices, Adv. Sci., Eng.
Med., 2014, 6, 708–711.
89 R. B. Olsen, D. A. Bruno and J. M. Briscoe, Pyroelectric
conversion cycles, J. Appl. Phys., 1985, 58, 4709–4716.
90 G. Sebald, E. Lefeuvre and D. Guyomar, Pyroelectric energy
conversion: Optimisation principles, IEEE Trans. Ultrason.
Ferroelectr. Freq. Control, 2008, 55, 538–551.
91 D. Guyomar, G. Sebald, E. Lefeuvre and A. Khodayari,
Toward heat energy harvesting using pyroelectric
material, J. Intell. Mater. Syst. Struct., 2009, 20, 265–271.
92 S. Mohammadi and A. Khodayari, Free energy harvesting
from ambient temperature variations, International
Journal of Mechanical and Materials Engineering, 2011, 6,
167–170.
93 D. Guyomar and M. Lallart, Energy conversion
improvement in ferroelectrics: application to energy
harvesting and self-powered systems, in IEEE Ultrasonics
Symposium (IUS), Rome, 2009, pp. 1–10.
94 J. E. Drummond, Dielectric thermal power converter,
US4220906 A, 1977.
95 R. A. Olsen, Ferroelectric conversion of heat to electrical
energy –A demonstration, J. Energy, 1982, 6,91–95.
96 R. B. Olsen, J. M. Briscoe, D. A. Bruno and W. F. Butler, A
pyroelectric energy converter which employs regeneration,
Ferroelectrics, 1981, 38, 975–978.
97 R. B. Olsen and D. D. Brown, High efficiency direct
conversion of heat to electrical energy - related
pyroelectric measurements, Ferroelectrics, 1982, 40,17–27.
98 R. B. Olsen and D. Evans, Pyroelectric energy conversion:
Hysteresis loss and temperature sensitivity of a
ferroelectric material, J. Appl. Phys., 1983, 54, 5941–5944.
99 R. B. Olsen, Pyroelectric energy converter and method, US
4647836 A, 1984.
100 R. B. Olsen, Pyroelectric energy converter element
comprising vinylidene uoride-triuoroethylene
copolymer, US 4620262 A, 1984.
101 J. L. Yu and M. Ikura, Direct conversion of waste heat to
electricity using pyroelectric conversion, International
Journal of Electrical Power & Energy Systems, 2007, 27, 407–
413.
102 H. Zhu, S. Pruvost, D. Guyomar and A. Khodayari, Thermal
energy harvesting from Pb[(Zn.sub.1/3][Nb.sub.2/
3]).sub.0.955][Ti.sub.0.045][O.sub.3] single crystals phase
transitions, J. Appl. Phys., 2009, 106, 124102.
103 S. Mohammadi and A. Khodayari, Pyroelectric energy
harvesting with thermodynamic-based cycles, Smart
Mater. Res., 2012, 2012, 160956.
104 M. Ikura, Conversion of low-grade heat to electricity using
pyroelectric copolymer, Ferroelectrics, 2002, 267, 403–408.
105 D. Vanderpool, J. H. Yoon and L. Pilon, Simulations of a
prototypical device using pyroelectric materials for
harvesting waste heat, Int. J. Heat Mass Transfer, 2008, 51,
5052–5062.
106 L. Kouchachvili and M. Ikura, Improving the efficiency of
pyroelectric conversion, Int. J. Energy Res., 2008, 32, 328–
335.
107 M. Ikura and R. Charbonneau, Pyroelectric conversion
system, US 6528898 B1, 2003.
108 R. C. Moreno, B. A. James, A. Navid and L. Pilon,
Pyroelectric energy converter for harvesting waste heat:
Simulations versus experiments, Int. J. Heat Mass Transfer,
2012, 55, 4301–4311.
109 R. B. Olsen and D. A. Bruno, Pyroelectric conversion
materials, in ECEC '86; Proceedings of the Twenty-rst
Intersociety Energy Conversion Engineering Conference, San
Diego, 1986, pp. 89–93.
110 D. Guyomar, S. Pruvost and G. Sebald, Energy harvesting
based on FE-FE transitions in ferroelectric single crystals,
IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 2008, 55,
279–285.
111 G. Cha and Y. S. Ju, Electric eld dependence of the Curie
temperature of ferroelectric ply(vinyldeneuoride-
triuoroethylene) co-polymers for pyroelectric energy
harvesting, Smart Mater. Struct., 2012, 21, 022001.
112 R. B. Olsen, D. A. Bruno, J. M. Briscoe and J. Dullea,
Cascaded pyroelectric energy converter, Ferroelectrics,
1984, 59, 205–219.
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Review Energy & Environmental Science
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

113 I. M. McKinley, F. Y. Lee and L. Pilon, A novel
thermomechanical energy conversion cycle, Appl. Energy,
2014, 126,78–89.
114 G. Cha and Y. S. Ju, Pyroelectric energy harvesting using
liquid-based switchable thermal interfaces, Sens.
Actuators, A, 2013, 189, 100–107.
115 D. Guyomar, G. Sebald, S. Pruvost, M. Lallart, C. Richard
and A. Khodayari, Energy harversting from ambient
vibrations and heat, J. Intell. Mater. Syst. Struct., 2009, 20,
609–624.
116 M. Amokrane, A. Baysse, B. Nogarede and M. Rguiti, Low-
voltage active diode rectier for pyroelectric harvesting
cell, in IECON 2012, 38th Annual Conference on IEEE
Industrial Electronics Society, Montreal, 2012, pp. 1055–
1060.
117 C. Richard, D. Guyomar, D. Audigier, and G. Ching, Semi
passive damping using continuous switching of a
piezoelectric device I, in Proc. SPIE Smart Struct. and Mat.
Conf., Passive Damping and Isolation, San Diego, 1999, vol.
3672, p. 104.
118 C. Richard, D. Guyomar, D. Audigier and H. Bassaler,
Enhanced semi passive damping using continuous
switching of a piezoelectric device on an inductor, in
Proc. SPIE Smart Struct. and Mat. Conf., Passive Damping
and Isolation, 2000, vol. 3989, p. 288.
119 A. Badel, D. Guyomar, E. Lefeuvre and C. Richard,
Efficiency enhancement of a piezoelectric energy
harvesting device in pulsed operation by synchronous
charge inversion, Journal of Intelligent Materials Systems
and Structures, 2005, 16, 889–901.
120 J. Fang, H. Frederich and L. Pilon, Harvesting nanoscale
thermal radiation using pyroelectric materials, J. Heat
Transfer, 2010, 132, 092701.
121 C. C. Hsiao, J. Ciou, A. Siao and C. Lee, Temperature eld
analysis for PZT pyroelectric cells for thermal energy
harvesting, Sensors, 2011, 11, 10458–10473.
122 C. C. Hsiao and A. Siao, Improving pyroelectric energy
harvesting using a sandblast etching technique, Sensors,
2013, 13, 12113–12131.
123 C. S. Wei, et al., Partial-electroded ZnO pyroelectric sensors
for responsivity improvement, Sens. Actuators, A, 2006, 128,
18–24.
124 A. N. Morozovska, E. A. Eliseev, G. S. Svechnikov and
S. V. Kalinin, Pyroelectric response of ferroelectric
nanowires: Size effect and electric energy harvesting, J.
Appl. Phys., 2010, 108, 042009.
125 R. Agrawal and H. D. Espinosa, Giant Piezoelectric Size
Effects in Zinc Oxide and Gallium Nitride Nanowires. A
First Principles Investigation, Nano Lett., 2011, 11, 786–790.
126 J. H. Lee, et al., Highly stretchable piezoelectric-pyroelectric
hybrid nanogenerator, Adv. Mater., 2014, 26, 765–769.
127 M. Goudarzi1, K. Niazi and M. K. Besharati, Hybrid energy
harvesting from vibration and temperature gradient by PZT
and PMN-0.25PT ceramics, Mater. Phys. Mech., 2013, 16,55–
65.
128 S. R. Hunter and P. G. Datskos, Mems based pyroelectric
thermal energy harvester, US Patent Pub. no. US
20120056504 A1, 2012.
129 S. Mostafa, et al.,Anitie element model of self-resonating
bimorph cantilever for fast temperature cycling in a
pyroelectric energy harvester, Mater. Res. Soc. Symp. Proc.,
2011, 1325, 159–164.
130 S. R. Hunter, et al., Development of MEMS based
pyroelectric thermal energy harvesters, Proc. SPIE, 2011,
8035, 80350V.
131 T. Huesgen, J. Ruhhammer, G. Biancuzzi and P. Woias,
Detailed study of a micro heat engine for thermal energy
harvesting, J. Micromech. Microeng., 2010, 20, 104004.
132 S. K. T. Ravindran, T. Huesgen, M. Kroener and P. Woias, A
self-sustaining pyroelectric energy harvester utilizing
spatial thermal gradients, in Solid-State Sensors, Actuators
and Microsystems Conference (Transducers), Beijing, 2011,
pp. 657–660.
133 T. Huesgen, T. Ravindran, S. Karanilam and P. Woias,
Micromechanical pyroelectric generator, WO 2012025137
A1, 2010.
134 I. S. McKay and E. N. Wang, Thermal pulse energy
harvesting, Energy, 2013, 57, 632–640.
135 L. Carlioz, J. Delamare and S. Basrour, Temperature
threshold tuning of a thermal harvesting switch, in Solid
State Sensors, Actuators and Microsystems Conference,
Denver, 2009, pp. 1385–1388.
136 Z. Gong, Y. He, Y. Tseng, C. O'Neal and L. Que, A
micromachined carbon nanotube lm cantilever-based
energy cell, Nanotechnology, 2012, 23, 335401.
137 Q. Zhang, A. Agbossou, Z. Feng and M. Cosnier, Solar
micro-energy harvesting with pyroelectric effect and wind
ow, Sens. Actuators, A, 2011, 168, 335–342.
138 T. Lukasiewicz, M. A. Swirkowicz, J. Dec, W. Hofman and
W. Szyrski, Strontium–barium niobate single crystals,
growth and ferroelectric properties, J. Cryst. Growth, 2008,
310, 1464–1469.
139 J. G. Webster and E. Halit, Measurement, Instrumentation
and Sensors Handbook, CRC Press, 2014.
140 J. Zhang, X. Dong, F. Cao, S. Guo and G. Wang, Enhanced
pyroelectric properties of Ca
x
(Sr
0.5
Ba
0.5
)
1x
Nb
2
O
6
lead-
free ceramics, Appl. Phys. Lett., 2013, 102, 102908.
141 S. T. Lau, et al., Lead-free ceramics for pyroelectric
applications, J. Appl. Phys., 2008, 103, 104105.
142 C. R. Bowen, H. A. Kim, P. M. Weaver and S. Dunn,
Piezoelectric and ferroelectric materials and structures for
energy harvesting applications, Energy Environ. Sci., 2014,
7,25–44.
143 G. Hyseni, N. Caka, and K. Hysen, Infrared thermal
detectors parameters: Semiconductor bolometers versus
pyroelectrics, WSEAS Transactions on circuits and systems,
2010, 9, pp. 238–247.
144 Y. He, Heat capacity, thermal conductivity, and thermal
expansion of barium titanate-based ceramics,
Thermochim. Acta, 2004, 419, 135–141.
Energy Environ. Sci. This journal is © The Royal Society of Chemistry 2014
Energy & Environmental Science Review
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online

145 U. ¨
Ozg¨
ur, et al., Thermal conductivity of bulk ZnO aer
different thermal treatments, J. Electron. Mater., 2006, 35,
550–555.
146 Z. X. Huang, Z. A. Tang, J. Yu and S. Bai, Thermal
conductivity of nanoscale polycrystalline ZnO thin lms,
Physica B, 2011, 406, 818–823.
147 Mintres Material Data Sheet, http://www.mintres.com/
pdf%5Cdia19.pdf, 2014.
148 M. Pecht, et al.,Electronic Packaging Materials and Their
Properties, CRC Press, 1999.
149 C. Glaw, K. Janiak, A. Kummrow, V. Penschke and
H. J. Eichler, Thermal relaxation times of bistable thin
lm elements, J. Nonlinear Opt. Phys. Mater., 1993, 2, 209.
150 D. S. Ginley and D. Cahen, Fundamentals of Materials for
Energy and Environmental Sustainability, 2012.
151 Y. Wu and J. Singh, Transient study of self-heating effects
in AlGaN/GaN HFETs: Consequence of carrier velocities,
temperature, and device performance, J. Appl. Phys., 2007,
101, 113712.
152 G. Vats, A. Chauhan and R. Vaish, Thermal energy
harvesting using bulk lead-free ferroelectric ceramics, Int.
J. Appl. Ceram. Technol., 2014, DOI: 10.1111/ijac.12214.
153 A. Chauhan, S. Patel, G. Vats and R. Vaish, Enhanced
Thermal Energy Harvesting Using Li, K-Doped
Bi0.5Na0.5TiO
3
Lead-Free Ferroelectric Ceramics, Energy
Technology, 2014, 2(2), 205–209.
154 G. Vats, R. Vaish and C. R. Bowen, An analysis of lead-free
(Bi0.5Na0.5) 0.915-(Bi0. 5K0.5) 0.05 Ba0.02Sr0.015TiO
3
ceramic for efficient refrigeration and thermal energy
harvesting, J. Appl. Phys., 2014, 115(1), 013505.
155 G. Vats, H. S. Kushwaha and R. Vaish, Enormous energy
harvesting andstorage potential in multiferroic epitaxial
thin lm hetrostructures: an unforeseen era, Materials
Research Express, 2014, 1(1), 015503.
156 H. Zhu, S. Pruvost, P. J. Cottinet and D. Guyomar, Energy
harvesting by nonlinear capacitance variation for a
relaxor ferroelectric poly(vinylidene uoride-
triuoroethylene-chlorouoroethylene) terpolymer, Appl.
Phys. Lett., 2011, 98, 222901.
This journal is © The Royal Society of Chemistry 2014 Energy Environ. Sci.
Review Energy & Environmental Science
Published on 18 August 2014. Downloaded on 19/08/2014 11:03:51.
View Article Online