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Electrostrictive Polymers for Mechanical Energy Harvesting

Micka€el Lallart, Pierre-Jean Cottinet, Daniel Guyomar, Laurent Lebrun

INSA de Lyon, LGEF, Villeurbanne, France

Correspondence to: D. Guyomar (E-mail: [email protected])

Received 3 November 2011; accepted 22 December 2011; published online 6 February 2012

DOI: 10.1002/polb.23045

ABSTRACT: This article reviews the developments in electro-

strictive polymers for energy harvesting. Electrostrictive poly-

mers are a variety of electroactive polymers that deform due

to the electrostatic and polarization interaction between two

electrodes with opposite electric charge. Electrostrictive poly-

mers have been the subject of much interest and research

over the past decade. In earlier years, much of the focus

was placed on actuator configurations, and in more recent

years, the focus has turned to investigating material proper-ties that may enhance electromechanical activities. Since the

last 5 years and with the development of low-power elec-

tronics, the possibility of using these materials for energy

harvesting has been investigated. This review outlines the

operating principle in energy scavenging mode and conver-

sion mechanisms behind this generator technology, high-

lights some of its advantages over existing actuator

technologies, identifies some of the challenges associated

with its development, and examines the main focus of

research within this field, including some of the potential

applications. VC 2012 Wiley Periodicals, Inc. J Polym Sci Part

B: Polym Phys 50: 523–535, 2012

KEYWORDS: actuators; dielectric properties; electrostrictive

polymers; energy harvesting; ferroelectricity; nanoparticles

INTRODUCTION The performance of energy harvesters is

directly linked to the efficiency of the mechanical–electrical con-

version within the active materials. For piezoelectric materials,

the efficiency of the conversion can be estimated with the help

of the coupling coefficient. For a given vibration mode, this coef-

ficient expresses the ratio of the converted energy to the input

one. Another key point for electroactive materials concerns the

easiness of their integration within the whole structure.1,2

For energy harvesters, bulk materials are widely used in the

form of ceramics or single crystals. Beyond various types of

materials, lead zirconate titanate ceramics (PZT) and lead-

based relaxor single crystals are of significant interest. PZT

ceramics are cost effective and available in various yet lim-

ited shapes and with a wide range of properties depending

on their composition. They exhibit medium coupling factors

of 70% at least for the longitudinal 33 mode of vibration.3,4

Single crystals of lead magnesium niobate-lead titanate or

lead zinc niobate-lead titanate have focused a lot of attention

as they exhibit coupling coefficients as high as 90%, close to

the theoretical values of 100%.5,6 Because of their high cou-

pling coefficients, these two types of materials seem to be

promising for the energy conversion; however, drawbacks

such as brittleness and high density may prevent their use

in some applications.

An alternative solution to the use of these bulk materials is

the use of electroactive polymers (EAPs). They present the

advantages of being easily processed in various and complex

shapes, easily deposited on large surfaces while being cost

effective and very light.7

EAPs are divided into two main groups:7

• Electronic EAPs: Dielectric EAP, electrostrictive graft elasto-

mers, electrostrictive papers, electroviscoelastic elastomers,

ferroelectric polymers, liquid crystal elastomers, and so forth.

•

Ionic EAPs: Carbon nanotubes, conductive polymers, elec-trorheological fluids, ionic polymer gels, ionic polymer me-

tallic composites, and so forth.

Electrostriction is generally defined as a quadratic coupling

between strain (S ij ) and polarization (P m):8,9

E m ¼ e0T mn:P n þ 2:Qklmn:T kl :P n

S ij ¼ s P ijkl :T kl þ Qijmn:P m:P n

&(1)

where s P ijkl is the elastic compliance, Qijkl is the polarization-

related electrostriction coefficient, e0T

jk is the inverse of the

linear dielectric permittivity, T kl is the stress and E m the elec-

tric field. Assuming a linear relationship between the polar-

ization and the electric field, the strain S ij and electric flux

density Di are expressed as independent variables of the

electric field intensity E k , E l , and stress T kl by the constitu-

tive relations according to the equation set:8,9

S ij ¼ M ijkl :E k :E l þ s E ijkl :T kl

Di ¼ eT ik :E k þ 2:M ijkl :E l :T kl

((2)

VC 2012 Wiley Periodicals, Inc.

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where s E ijkl is the elastic compliance under constant electric

field, M ijkl is the electric field-related electrostriction coeffi-

cient, and eT ik is the linear dielectric permittivity.

Because of their high deformation abilities and elasticity, EAPs

cover a wide range of application possibilities, as shown in

Figure 1, which depicts the typical strain–stress abilities of

conversion materials when compared with typical application

cases. In the same manner, the operating frequency of such

materials suits in a much better way with the mechanical fre-

quency contents of typical systems (Fig. 2).

Because of their low losses that make them a premium

choice for energy-harvesting purposes, this research will be

focused on electronic EAPs and more specifically on electro-

strictive elastomers. The ‘‘Energy-Harvesting Techniques’’

section provides an overview of the different methods for

harvesting energy using such materials. The ‘‘Increase of the

Dielectric Constant and of the Electrostrictive Coefficient

with the Help of Fillers’’ section lies in presenting methods

for enhancing the electromechanical responses of electro-

strictive polymers and thus their energy-harvesting abilities.

Micka€el Lallart graduated from the Institut National des Sciences Appliquees de Lyon (INSA

Lyon), Lyon, France, in electrical engineering in 2006 and received his PhD in electronics,

electrotechnics, and automatics from the same university in 2008, where he worked for the

Laboratoire de Genie Electrique et Ferroelectricite (LGEF). After working as a post-doctoral

fellow in the Center for Intelligent Material Systems and Structures (CIMSS) in Virginia Tech,

Blacksburg, VA, USA, in 2009, Dr. Lallart has been hired as an associate professor in the

LGEF. His current field of interest focuses on vibration damping, energy harvesting, and

structural health monitoring using piezoelectric, pyroelectric, or electrostrictive devices, as

well as autonomous, self-powered wireless systems.

Pierre-Jean Cottinet graduated from the Institut National des Sciences Appliquees de Lyon

(INSA Lyon), Lyon, France, in 2008. He received a PhD degree in Acoustics in 2008 from the

Institut National des Sciences Appliquees de Lyon (INSA), France, for his thesis on

electostrictive polymer for energy harvesting and actuation. During 2011, he was at the

Florida State University as a post-doctoral and working on buckypaper in High-Performance

Materials Institute (HPMI). Currently, he is an associate professor at INSA de Lyon, with

research interests concerning electroactive materials (polymers, CNT, etc.) and smart

structures.

Daniel Guyomar received a degree in physics from the Amiens University, Amiens, France,

an engineering and a doctor-engineer degree in acoustics from the Compiegne University,

France, as well as a PhD degree in physics from the Paris VII University, Paris, France. In

1982–1983, he worked as a research associate in fluid dynamics at the University of Southern

California, Los Angeles, CA. He was a National Research Council Awardee (1983–1984)

detached at the Monterey Naval Postgraduate School, California, to develop transient wave

propagation modeling. He was hired by Schlumberger in 1984 to lead several projects

dealing with borehole imaging, and then moved to Thomson Submarine activities in the

field of underwater acoustics. In 1992, Dr. Guyomar co-created the Techsonic Company,

which is involved in research, development, and production of piezoelectric and ultrasonic

devices. He is presently a full-time university professor at the Institut National des Sciences

Appliquees de Lyon (INSA), Lyon, France, where he manages the Laboratoire de Genie

Electrique et Ferroelectricite (LGEF). He also works as a consultant for several companies. His

present research interests include the field of piezo-material characterization, piezoactuators,acoustics, power ultrasonics, vibration control, and energy harvesting.

Laurent Lebrun graduated from the Ecole Nationale Superieure d’Ingenieurs de Caen,

France, in 1991. He received a PhD degree in acoustics in 1995 from the Institut National des

Sciences Appliquees (INSA), de Lyon, France, for his thesis on piezoelectric motors. During

2001, he was a visiting scientist at the Materials Research Institute of the Pennsylvania State

University, State College, PA, in the group of Prof. Tom Shrout. Currently, he is a professor at

INSA de Lyon, with research interests concerning electroactive materials (ceramics, single

crystals, and polymers) and smart structures.

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Then, the ‘‘Practical Consideration and Figure of Merit of the

Conversion’’ section discusses practical considerations such

as materials properties, circuit topologies, and so forth.

Finally, the ‘‘Application of Electrostrictive Polymer Genera-tors’’ section concerns with the description of potential

application.

ENERGY-HARVESTING TECHNIQUES

The aim of this section is to propose a review of the possible

techniques for harvesting the energy converted by the mate-

rials. Basically, three approaches can be considered for

energy scavenging from vibrations using electrostrictive

polymers:

• Electrostatic cycles, inspired from purely capacitive techni-

ques.10–14

• Electrostrictive cycles, using charge and discharge opera-

tions.9,15–17

• Pseudo-piezoelectric cycles, consisting in applying a bias

voltage on the material and working around this static

regime.18–20

Electrostatic-Derived Cycles

As electrostrictive polymers feature dielectric behaviors, it is

possible to consider harvesting schemes usually used in

purely capacitive approaches.

10–14

Typically, there are twocycles that can be envisaged for such techniques:

• Ericsson (voltage constrained) cycle, which consists in:

1. Stretching the polymer.

2. Applying the electric field.

3. Releasing the applied mechanical stress while maintain-

ing the electric field, leading to a decrease of the elec-

tric flux density.

4. Removing the electric field.

• Stirling (charge constrained) cycle, whose principles rely on:

1. Stretching the polymer.

2. Applying the electric field.

3. Releasing the applied mechanical stress under constant

electric flux density (open-circuit conditions), resultingin an increase of the electric field.

4. Removing the electric field.

The associated strain (S )–stress (T ) and electric displace-

ment (D)–electric field (E ) cycles are shown in Figure 3.

From this curves, it can be shown that the Stirling cycle per-

mits converting more energy than the Ericsson approach.

However, the latter allows a better control of the electric

field, ensuring that the maximal admissible value is never

reached. The energy balance for the considered techniques is

given in Table 1, where e refers to the permittivity, M to the

electrostrictive coefficient, and T 0 and E 0 to the maximal

stress and applied electric field, respectively.

Electrostrictive Cycles

However, other cycles than those already used in other

energy conversion systems may be considered, taking

advantage of the electrostrictive nature of the materials.

Many cycles have been described in ref. 16, and particularly:

• Constant electric field stretching and open-circuit release:

1. Stretching under a given electric field E 0.

2. Releasing in open circuit (constant charge).

3. Decreasing the electric flux density to the original

position.

FIGURE 2 Comparison of frequency contents of conversion

materials and typical applications.

FIGURE 1 Comparison of stress–strain abilities of conversion materials and typical applications.

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• Constant electric field stretching and release:

1. Stretching under constant electric field E 0.

2. Increasing electric field to E 1.

3. Releasing the applied stress.

4. Decreasing electric field to E 0.

• Open-circuit stretching and release:

1. Stretching in open circuit (constant electric flux density)

with an initial electric field E 0.

2. Increasing electric field to E 1.

3. Releasing in open circuit (constant electric flux density).

4. Decreasing electric field to E 0.

The associated energy cycles and energy balances are givenin Figure 4 and Table 2, respectively. In contrast to the previ-

ously discussed cycles (electrostatic based), the pure electro-

strictive cycles require nonzero electrical initial conditions

(leading to nonzero initial strain as well), therefore possibly

wasting energy due to the losses in the material. It can also

be noted that the constant field during stretching and releas-

ing phases and constant electric flux during stretching and

releasing phases use the same principles than the Ericsson

and Stirling cycles of electrostatic devices, respectively (see

previous section), except that initial electrical conditions are

zeros in the latter cases.

In addition, these cycles require driving the electrical condi-

tions of the materials for a significant time period, hencemaking them quite complex to implement in an autonomous,

self-powered fashion. A simpler way for harvesting energy

from electrostrictive polymers using diodes is proposed in

refs. 16 and 21. This system therefore permits harvesting

energy in a purely passive fashion, hence making its imple-

mentation quite easy. The principles of this device, depicted

in Figure 5, consist of providing energy to the polymer

when its voltage reaches V L (it is considered that the diode

threshold voltages are negligible) and harvesting when it

attains V H.

At the beginning of a new cycle, the material voltage is V H,

and the stress is increasing, resulting in a decreasing electric

field. Therefore, the two diodes are blocked, and the system

is in open-circuit condition. After a critical value of the

stress, the voltage V L is reached and the left diode conducts

so that the electric displacement increases with the stress,

yielding a provided energy density:

W prov ¼ E L 2ME LT 0 À e E H À E Lð Þ½ (3)

where E H and E L are the electric field values associated with

the voltages V H and V L. As the stress is released, the material

voltage increases and the left diode is blocked. When the

voltage reaches V H, the right diode conducts and energy is

extracted, until the stress is minimum. The harvested energy

density is given by:

W extr ¼ E H 2ME LT 0 À e E H À E Lð Þ½ (4)

leading to the energy density balance:16

W harv ¼ 2ME LT 0 À e E H À E Lð Þ½ E H À E Lð Þ (5)

FIGURE 3 Electrostatic energy-harvesting cycles considering Ericsson and Stirling cycles.

TABLE 1 Energy Balance Considering Electrostatic-Based

Harvesting Techniques

Ericsson16 Stirling16

Provided electrical

energy density

1

2e þ 2MT 0ð ÞE 2

0

1

2e þ 2MT 0ð ÞE 2

0

Extracted electrical

energy density

1

2e þ 4MT 0ð ÞE 2

0

1

2

eþ2MT 0ð Þ2

eE

20

Harvested energy

density

MT 0E 2

01 þ 2

M

eT 0

À ÁMT 0E

2

0


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If the strain is small, the value of T min can thus be approxi-

mated by

T min ¼e

2M

V H

V LÀ 1

(9)

Pseudo-Piezoelectric Cycles

The final family for harvesting energy from electrostrictive

materials consists of applying a bias electric field to the sam-

ple, as shown in Figure 7, allowing simpler operations than

charge and discharge cycles (with possibly reduced losses).

In this device, the constant electric field, supposed to be

much larger than the electric field generated by the vibra-

tion, allows the device to operate dynamically in a similar

fashion than piezoelectric materials. Starting from the linear-

ized electrostrictive equations:

S ¼ sT þ M E DC þ E ACð Þ2

D ¼ e E DC þ E ACð Þ þ 2M E DC þ E ACð ÞT (10)

with E DC and E AC the bias and generated electric fields and s

the elastic compliance of the material, the dynamic behavior

may be expressed as follows:

_S ¼ s _T þ 2M E DC þ E ACð Þ _E AC

_D ¼ e þ 2MT ð Þ _E AC þ 2M E DC þ E ACð Þ _T (11)

Considering that the electric field E AC can be neglected facing

the bias electric field E DC and as long as the stress magni-

tude remains small enough so that e >> 2MT , the expres-

sions turn to:

_S % s _T þ 2ME DC_E AC

_D % e _E AC þ 2ME DC_T

(12)

which are similar to those obtained when using piezoelectric

element, with an equivalent piezoelectric coefficient

d ¼ 2ME DC, which depends on the bias electric field.

The energy cycles (obtained without the previous assump-

tions) are depicted in the top of Figure 8 when considering

that energy is harvested on a purely resistive load (R in

Fig. 7), yielding a harvested energy density (considering

E DC << E AC and e >> 2MT ):20

W harv % 4pq

1 þ eqxð Þ2

!ME DCð Þ2

xT 20 (13)

where q denotes the load resistivity. The maximum energy

density is therefore given by:20

W max % 2pM 2

eE 2DCT 20 (14)

However, to increase the conversion efficiency, it is possible

to use a nonlinear approach similar to the ‘‘synchronized

switch harvesting on inductor’’ for piezoelectric elements,23–27

which consists of reversing the dynamic voltage each time the

displacement reaches a maximum or a minimum value,22

leading to the cycles depicted in the bottom of Figure 8, and

allowing a harvested energy density given by:

W harv % 4pq

1 þ eqxð Þ2 eqxð Þ3

1 þ eqxð Þ2 1 þ cð Þ

ep

eqx À cÀ Á2

e2peqx À 1

p

þ 1

24

35

Â ME DCð Þ2xT 20 ð15Þ

with c given as the voltage inversion factor.23

Although the losses of pseudo-piezoelectric working mode may

be smaller than electrostatic-based and electrostrictive cycles

as the system is working around a bias point (hence no charge

losses appear), the dynamic voltage across the load remains

AC, preventing the use of such a system to power up electronic

components that usually require DC voltage. In ref. 28, an archi-

tecture was proposed to allow a DC voltage output of electro-

strictive materials used as energy harvesters in pseudo-

FIGURE 6 Energy cycles for the passive electrostrictive energy-harvesting device (adapted from ref. 16).

FIGURE 5 Passive energy-harvesting circuit.FIGURE 7 Pseudo-piezoelectric energy-harvesting device.


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composites by filling the polymer with high K fillers or con-

ductive fillers. Currently, a variety of methods are available

to increase the dielectric permittivity of polymer materials.

These may be classified into two main groups: those involv-

ing composites and those based on new synthetic polymers.

The first approach concerns the dispersion of a filler into the

polymer matrix. The second strategy, on the other hand,

deals with the synthesis of new materials with tailored

characteristics.

In the first case, the content of fillers must be high (tenth of

percents) to obtain a significant increase of the compositedielectric constant. The main drawback is the large increase

of the Young’s modulus and consequently the loose of

flexibility.

On the contrary, the use of conductive fillers leads to an

increase of the dielectric constant at very low contents

(some percents) especially if the size of the fillers is very

small (some nm), as the required electric field is decreased

to obtain the same polarization than unfilled samples. As an

additional consequence, the variation of the Young’s modulus

is kept low. This filling must be done without reaching the

percolation threshold for with the composite becomes con-

ductive and without decreasing too much the breakdown

voltage. These two parameters not only depend on the fillersmorphology and size and on the polymer matrix but also on

the dispersion and the self-organization of the fillers within

the matrix.

Table 4 summarizes some results obtained by filling polyur-

ethane (PU) and P(VDF-TrFE-CFE) with conductive nanofil-

lers. In the same manner, the filling can be achieved using

conductive polymers dispersed within the dielectric matrix

leading to the development of all-polymer percolative sys-

tems. As an example, coated polyanilines have been dis-

persed in a terpolymer matrix in refs. 35 and 36.

Finally, to overcome the problem of agglomeration that can

exist when fillers are dispersed within the matrix and to bet-

ter control their spatial distribution, Huang and Zhang37

developed chemical bonding of the filler to the backbone of

the polymer matrix.

The different methods available for enhancing the dielectric

permittivity of polymers are listed in Table 5, which also

gives advantages and drawbacks of each technique. Random

composites represent readily applicable approaches suitable

for increasing the dielectric permittivity of elastomers. In the

long run, the challenge consists in synthesizing a new highlypolarizable polymer. All this research is necessary to achieve

new generations of electrostrictive polymers, operating at

lower electric fields.

PRACTICAL CONSIDERATION AND FIGURE OF MERIT OF THE

CONVERSION

Many of the specific material properties affect all the bulk

energy-harvester properties. In this section, the material

properties are enumerated and the mechanisms through

which they influence the microgenerator performance are

described to summarize the previous two sections. The rela-

tionships between all introduced system parameters are

charted in Figure 13.Maximum Electric Field

In the different configurations, the harvested energy is pro-

portional to the square of the applied electric field. Theoret-

ically, it is appealing to work with high electric fields to

convert more energy. However, there exist maximum elec-

tric fields (E max), which can be defined as the maximum

electric field strength that the sample can withstand

intrinsically without breaking down, that is, without experi-

encing failure of its insulating properties. The electric field

at which breakdown occurs depends on the respective

FIGURE 10 Energy cycles for the pseudo-piezoelectric DC energy-harvesting device (adapted from ref. 28).


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geometries of the polymers and the electrodes on which the

electric field is applied, as well as the rate of increase of

the electric field. Because materials usually contain minute

defects, the practical dielectric strength will be a fraction of the intrinsic dielectric strength of an ideal, defect-free,

material.

In real cases, the electric field breakdown of EAP varies

from 70 to 200 MV/m. It originates from various types

of phenomenon such as thermal effect, effect of internal

and surface discharges, and effect of path. Moreover, in

the case of real microgenerators, it is important to work

under moderate electric fields to avoid problems inher-

ent with high-voltage insulation and to limit the electric

loss.

FIGURE 11 Comparison of the maximum harvested energy density for each technique (electrostrictive cycles based on constant

electric field or constant electric displacement during stretching/release are not depicted as they are similar to Ericsson and Stir-

ling cycles).

FIGURE 12 Comparison of the normalized harvested energy

densities.

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Maximum Stress and Strain

The strain and stress capacities also come into account in

the development of a vibration-based microgenerator. These

parameters not only determine the maximum displacements

but also the maximum forces necessary to produce such

elongations. For example, applications that involve human

movement harvesting (Fig. 14) features low stress but high

deformations. Therefore, the material must be flexible (low

Young’s modulus) to avoid interference with the user [thus

minimizing the magnitude of forces and being transparent

with a low cost of harvesting (COH)]. Moreover, it must be

able to feature deformation of more than 50% while mini-mizing mechanical and dielectric losses to ensure high con-

version efficiency. However, for applications where only low

strain is available (few percents) but with high stress, it is

interesting to have materials with high Young’s modulus to

convert more energy, because the latter is proportional to

the squared stress (assuming a linear strain–stress relation-

ship), as depicted in Figure 14.

Frequency Bandwidth

Electrical and mechanical losses are varying with the fre-

quency. These characteristics must be considered during the

development of microgenerators to ensure an optimal extrac-

tion of the energy over a wide frequency bandwidth. Figure15 shows the Young’s modulus and mechanical losses as a

function of the frequency considering a constant strain in

the case of pure PU material. For frequencies below 100 Hz,

the losses in the material are limited; however, the losses

become higher for frequencies around 1 kHz, limiting the

operation of the system for frequencies below this threshold,

which correspond to the typical vibration frequency contents

(Fig. 2).

Concerning the electrical losses, it is well known that the

mechanisms of polarization strongly depend on the fre-

quency and tend to disappear when the latter is increased,

as shown in Figure 16, which depicts the variation of the

dielectric permittivity and losses versus frequency in the

case of P(VDF-TrFE-CFE) sample. For example, Maxwell–Wag-

ner type polarization is known to be active at the lowest fre-

quencies, which explains why a decrease in the dielectricconstant was observed when the measurement frequency

was increased. The effective loss tangent shows the highest

value for low frequencies due to the electric conduction.38,39

Losses then decrease with the frequency except for the fre-

quencies where polarization mechanisms disappear.33

Maximum Energy Harvesting

For every energy-harvesting technique presented in the

‘‘Energy-Harvesting Techniques’’ section, it can be seen that

the electrostrictive coefficient M appears to be an important

parameter to increase the scavenging abilities of the system.

Although the ‘‘Increase of the Dielectric Constant and of the

Electrostrictive Coefficient with the Help of Fillers’’ sectionprovides a description of the various methods available to

increase this coefficient, the goal of this part is to present a

figure of merits able to realize a comparison of the different

technique available for harvesting energy.

Ren et al.9 demonstrated that it is possible to harvest 22.4

mJ/cm3 using the so-called constant electric field stretching

and open-circuit release methods; however, in case of

pseudo-piezoelectric mode, the harvested energy is equal to

34 nJ/cm3.28 Hence, the energy-harvesting abilities using the

pseudo-piezoelectric behavior seems to be lower than the

electrostrictive cycle-based energy-harvesting approaches.

Although this could be considered a disappointing result,

one should keep in mind that these values were not obtainedfor the same excitation and polarization field. For a fair com-

parison between each technique, Lallart et al.20 proposed a

figure of merit of the scavenging abilities that is able to com-

pare the different methods used for energy harvesting. This

figure of merit consists of dividing the harvested energy den-

sity by the squared mechanical and squared electrical stim-

uli, therefore allowing a normalization of the energy with

respect to the external condition. The results obtained in

TABLE 4 Effect of Nanofillers on Material Properties

Polymer Fillers

Content

(vol %)

Dielectric

Constant

Frequency Measurement

of Permittivity (Hz) M 33 (m2 /V2) 10À15

Frequency

Measurement

of M 33 (Hz) Reference

PU No 6.8 0.1 À1 0.1 33

PU SiC 0.5 10.9 0.1 À2.5 0.1 34

PU CB 1 15.4 0.1 À4 0.1 33

P(VDF-TrFE-CFE) No 65 0.1 À1.1 0.1 20

P(VDF-TrFE-CFE) CB 1 95 0.1 À2.4 0.1 20

P(VDF-TrFE-CFE) PANI 23 2,000 100 À0.15 1 35

P(VDF-TrFE-CFE) PANI 12.7 600 100 À0.02 1 35

SiC, silicon carbide; CB, carbon black; PANI, polyaniline.

TABLE 3 Simulation parameters

Parameter Value

Elastic compliance s 2.5 Â 10À9 PaÀ1

Relative permittivity e / e0 50

Electrostrictive coefficient M 2 Â 10À18 V2 /m2

Inversion coefficient c 0.8


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ref. 20 demonstrate the validity of this new figure of merit

and the potential as tools in helping the development of effi-

cient microgenerators.

Passive Material

Portable applications are powered with lower voltages com-

patible with battery output. Hence, to generate the high elec-tric field (typically 5 V/lm) required for working in the

pseudo-piezoelectric behavior or to realize energy cycles, a

step-up voltage converter has to be a part of the generator

circuit as shown in Figure 17. Current challenges in the field

of energy harvesting using electrostrictive polymers concern

the development of systems able to ensure the generation of

high electric fields at a small energy cost. This can be

TABLE 5 Comparison Between the Different Methods for Enhancing the Dielectric Permittivity

Type of fillers Advantages Drawbacks

Random composite Dielectric High dielectric permittivity Large percentage of fil ler

Increase in elastic modulus

Conductive High dielectric permittivity for

low percentage of filler

Increase in conductivity

Decrease of maximum voltage possible to apply

Polymer blend No fillers Very high dielectric permittivity Process of realization complex

No problem of conductivity

No mechanical reinforcement

FIGURE 13 Relation between electrostrictive microgenerators

and properties of materials.

FIGURE 14 Illustration of the properties of young modulus for

different applications.

FIGURE 15 Mechanical properties of pure PU versus

frequency.

FIGURE 16 Dielectric properties of pure P(VDF-TrFE-CFE) ver-

sus frequency.

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realized by the hybridization of electrostrictive polymer with

other electroactive materials such as piezoelectric generators

able to deliver high voltage.

In summary, this section highlighted important material

properties and their relationships to microgenerators per-

formance. The presentation of a figure of merit able to

assess the performance of electrostrictive polymers in terms

of energy scavenging has also been introduced. This criterion

is related to the energy density per cycle per squared strain

magnitude and per squared bias or applied electric field,

allowing to evaluate the energy-harvesting abilities inde-

pendently from extrinsic parameters such as dimensions, ex-

citation, or bias electric field.

APPLICATION OF ELECTROSTRICTIVE POLYMER

GENERATORS

Virtually, any application where there is a need of electrical

energy is a potential application for electrostrictive polymer

generators. However, electrostrictive polymer power genera-

tion is much more competitive for some applications when

compared with others. For example, electrostrictive polymers

are well suitable for harvesting energy for human motion.

Natural muscle, the driving force for human motion, is typi-

cally of low frequencies and intrinsically linear, both charac-

teristics where electrostrictive polymers offer advantages.

Many other interesting generator applications exist for elec-

trostrictive polymers. Remote and/or wireless devices are

growing in use, and these devices can ideally harvest their

own energy to eliminate the need of battery replacement.Electrostrictive polymers are well suited for these applica-

tions if mechanical energy is available from oscillatory or vi-

bratory motions such as that might occur in portable devices

carried by people, animals, plane, and so forth.

CONCLUSIONS

The further development of electrostrictive polymers as a via-

ble micro-generators technology requires much work, and to

this end, numerous exciting challenges lie ahead. First, the de-

velopment of improved electrostrictive polymer is essential.

Continued research into the effect of the incorporation of

dielectric and dielectric fillers could lead to the simultaneous

enhancement of electromechanical activities and the reduction

of operating voltage. Such improvements would serve to

broaden application spectrum of these types of microgenera-

tors. For example, significant reduction of operating voltage

could enable applications of realistic autonomous system.

EAPs and in particular electrostrictive polymers were

reviewed in this article as exciting candidate materials for

the development of a new age of microgenerator material.

The different principles of operation of these materials were

presented. The advantages of electrostrictive polymers over

conventional microgenerator technologies in a number of

metrics, in addition to some of the unique characteristics,

which they can offer, were discussed. Electrostrictive poly-

mers were examined at both the material and the microgen-

erators configuration level. Important material parameters,

both mechanical and electrical, are highlighted and various

approaches to optimize these properties for the development

of superior energy harvester devices were addressed. Such

approaches included the incorporation of dielectric fillers

and conductive fillers. In conclusion, electrostrictive polymer

generators have been studied extensively under laboratory

conditions where they have shown promising performance.

However, in practical applications, they have not yet achieved

their full potential.

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