Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-7, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
Recent Advances In Piezoelectric Nano generators
In Energy Harvesting Applications
BinoyBera
Abstract:Recently, the nanogenerators which can
convert the mechanical energy into electricity by
using piezoelectric materials have exhibited great
potential in microscale power supply and sensor
systems. In this paper, i provide a comprehensive
review of the research progress in piezoelectric
nanogenerators with different material. The
fundamental piezoelectric theory and typical
piezoelectric materials are firstly reviewed. After
that, the working mechanism, modeling, and
structure design of different types of piezoelectric
nanogenerators were discussed. Then the recent
progressof piezoelectric nanogenerators was also
reviewed. Finally, i also discussed the potential
application and future development of the
piezoelectric nanogenerators.
Keywords:Piezoelectricity, Nano-Generator, ZnO,
Lead Zirconium Titanate (PZT), Polyvinylidene
Fluoride (PVDF), P(VDF-TrFE)nanowires.
1. Introduction
Electrical power is most often generated at power
stations by electromechanical generators through
chemical combustion or nuclear fission, geothermal
power and kinetic energy of flowing water. In
recent years, with the surge of wireless
microelectromechanical
systems
and
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nanoelectromechanical system devices, there is
increasing demand for clean and efficient power
generation for the self-powering of these devices
from ambient energy sources, such as thermal
gradient, solar, mechanical vibration, and bio-fluid.
Piezoelectricity, i.e., the conversion of mechanical
energy to electrical signals, is one of the most
versatile phenomena to power small scale
electronic devices from the device environment. In
particular, the piezoelectric method for power
generation from harvesting mechanical energy,
such as the body movement, muscle stretching,
acoustic/ultrasonic wave, etc., has attracted a great
deal of attention for self-power/wireless charging,
and controllability of the output power.1–6 Power
generation through ambient energy harvesting has
several potentials, such as in sensor network
devices that observe an environment and assemble
useful data about the environment. These are
employed in situations where human interactions
are impossible. Hundreds, even thousands of tiny
devices should be placed in some locations, such as
an office building or the ocean floor, or even within
a living organism, to monitor certain variables.
Depending on the situations in which these
networks are placed, supplying power for these
devices might be an incredibly difficult task.
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Energy harvesting or energy scavengingis the
process of extracting small amount of energy from
ambient environment through various sources of
energy. The available energy for harvesting is
mainly provided by ambient light (artificial and
natural lighting), ambient radio frequency, thermal
sources and mechanical sources. Reduction in size
and energetic demands of sensors, and the low
power consumption trend in CMOS electronic
circuitry opened novel research lines on battery
recharge via available power sources. Harvesters
can be employed as battery rechargers in various
environments, such as industries, houses [7,8], the
military (as for unmanned aerial vehicles [9]) and
handheld or wearable devices [10–15]. The possibility
to avoid replacing exhausted batteries is highly
attractive for wireless networks (Wireless Sensor
Networks [16]), in which the maintenance costs due
to battery check and replacement are relevant.
Another emerging field of application is
biomedical systems, where the energy could be
harvested from an off-the-shelf piezoelectric unit
and used to implement drug delivery systems [17] or
tactile sensors [18–20]. Recent research also includes
energy conversion from the occlusal contact during
chewing by means of a piezoelectric layer [17,21] and
from heart beats [22].
We can classify the main energy harvesting
technologies by the hierarchy shown in Figure 1.
Motion harvester systems can be structured as
follows: the harvester collects inputs through its
frame, directly connected to the hosting structure
and to the transducer; at the end of the system
chain, a conditioning circuit manipulates the
electrical signals.
Figure 1.Hierarchy of main energy harvesting technologies.
The possibility and the effectiveness of extracting
energy from human activities has been under study
for years [23]. As a matter of fact, continuous and
uninterrupted power can potentially be available:
from typing (~mW), motion of upper limbs (~10
mW), air exhalation while breathing (~100 mW),
walking (~W) [24,25] (Figure 2), and in this work we
review state of the art of motion based energy
harvesting.
Among available motion based harvesting
techniques, piezoelectric transduction offers higher
power densities [26] in comparison to electrostatic
transduction (which also needs an initial
polarization). Also, piezoelectric technologies are
better suited than electromagnetic ones for MEMS
implementation, because of the limitations in
magnets miniaturization with current state-of-theart microfabrication processes [27].
Piezoelectric nanogenerators are very promising
and offer the possibility of performing this
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incredible task of supplying power these wireless
devices. Recent advances in piezoelectric
nanogenerators open many doors for power
generation through ambient energy harvesting for
practical applications.3,29–30 The
use of piezoelectric nanogenerators to capitalize on
the vibrations surrounding the device is one method
that has observed a dramatic increase in use for
power generation. The active materials in
piezoelectric nanogenerator have crystalline
structures with the ability to effectively transform
mechanical strain energy into electrical charges.
This property gives these active materials the
ability to absorb even very minute mechanical
energy from their surroundings, usually ambient
vibration, and transform it into electrical signals
that can be used to power other devices.6 This
paper discusses the recent advances in power
generation through piezoelectric nanogenerators as
well as the future goals that must be achieved to
find their way into everyday use.
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Figure 2. Estimation of available power that could be harvested during human activities (Adapted from [28]).
2. Piezoelectricity and Transduction
Principle
Piezoelectric effect is a unique property of certain
crystals where they will generate an electric field or
current if subjected to physical stress.The direct
piezoelectric effect was discovered by brothers
Pierre Curie and Jacques Curie in 1880. After that,
the same effect was observed in reverse, where an
imposed
electric field on the crystal will put stress on its
structure.
The piezoelectric effect is based on the
fundamental structure of a crystal lattice. Certain
crystalline structures have a charge balance with
negative and positive polarization, which neutralize
along the imaginary polar axis. When this charge
balance is perturbed with external stress onto the
crystal mesh, the energy is transferred by electric
charge carriers creating a current in the crystal.
Conversely, with the piezoelectric effect an
external charge input will create an unbalance in
the neutral charge state causing mechanical stress.
The connection between piezoelectricity and
crystal symmetry are closely established. The
piezoelectric effect is observed in crystals without
center of symmetry, and the relationship can be
explained with monocrystal and polycrystalline
structures.
In a monocrystal (Figure 3) the polar axes of all of
the charge carriers exhibit one-way directional
characteristics. These crystals demonstrate
symmetry, where the polar axes throughout the
crystal would lie unidirectional even if it was split
into pieces.
Figure 3.Monocrystal
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Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-7, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
Instead, a polycrystal (Figure 4) is characterized by
different regions within the material with different
polar axes. It is asymmetrical because there is no
point at which the crystal could be cut that would
leave the two remaining pieces with the same
resultant polar axes.
In order to attain the
piezoelectric effect, the polycrystal is heated to the
Curie point along with strong electric field. The
heat allows the molecules to move more freely and
the electric field forces the dipoles to rearrange in
accordance with the external field (Figure 5).
Figure 4.Polycrystal
Figure 5. (a) Polarizations; (b) Surviving Polarity.
As a result, the material possesses piezoelectric
effect: a voltage of the same polarity as of the
poling voltage appears between electrodes when
the material is compressed; and opposite polarity
appears when stretched. Material deformation takes
place when a voltage difference is applied, and if
Imperial Journal of Interdisciplinary Research (IJIR)
an AC signal is applied the material will vibrate at
the same frequency as the signal.
Piezoelectricity is governed by the following
constitutive equations, which link the stress , the
strain , the electric field E and the electrical
induction D:
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where
is the Young’s modulus,
is
the piezoelectric coefficient and
is the
clamped permittivity. The same relationship can be
written in other three forms, depending on the
couple of variable (among T, S , E and D ) chosen
to be independent . The superscript indicates a
constant electric field (which corresponds for
example to a short circuit condition, where ), as
well as the superscript stands for a condition of
constant strain.
For each couple of constitutive equations there is a different piezoelectric coefficient, defined as:
An important parameter is the electromechanical coupling factor ,
, which describes the conversion
between mechanical and electrical energy. It can be written in terms of coefficients of the material:
The efficiency of energy conversion, η is described, at resonance, as follows:
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Vol-2, Issue-7, 2016
ISSN: 2454-1362, http://www.onlinejournal.in
where, k2 is the coupling factor as defined in Equation (4) and Q is the quality factor of the generator .
To understand how the electrical quantities ( and ) are related to the mechanical ones (force and displacement ),
the particular case of a piezoelectric disk can be considered. In this case, from Equation (1) the following
relationships can be obtained :
In which the featuring quantities are the restoring force of the piezoelectric material, its stiffness when it is
short-circuited ,
the displacement z, the force factor α, the voltage across the electrodes V and the
outgoing current
, and the clamped capacitance
following approximations:
, These equations are derived considering the
and the featured quantities can be written as:
where, A and H are the section and thickness of the piezoelectric disk. In a more generic case of a mechanical
stress in direction pand an induced electric field in direction i, the open-circuit voltage of a piezoelectric device
can be written as follows:
Assuming that the voltage coefficient
electrodes.
is constant with the stress, and where is the gap between the
3. Piezoelectric Materials
Until now, several kinds of materials have
exhibited piezoelectricity including both natural
and synthetic materials, which are listed in Table 1.
Among them, piezoelectric ceramics, crystals, and
polymers were most developed and useful
piezoelectric materials. Piezoelectric ceramics
Imperial Journal of Interdisciplinary Research (IJIR)
usually refer to polycrystalline materials that
consisted of irregular collective small grains and
are prepared through the solid-state reaction and
sintering process. Under the poling electrical field,
the disordered spontaneous polarization in
piezoelectric ceramics can be realigned and keep
the remnant polarization after the removal of
external field. As a result, the piezoelectric
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ceramics can exhibit macropiezoelectric property.
Piezoelectric crystals, which refer to singlecrystalline materials, are usually unsymmetrical in
structure and therefor exhibit piezoelectric
property.
The
piezoelectric
ceramics
exhibit
high
piezoelectric constant and permittivity and can be
prepared into designed architectures, which makes
them suitable for the application in high-power
energy transducer and wideband filters. However,
the poor mechanical quality factor, high electrical
loss, and low stability of the piezoelectric ceramics
limited their application in high-frequency devices.
Comparatively, the natural piezoelectric crystals
such as quartz exhibit lower piezoelectric
properties and dielectric constant. Moreover, they
are limited in size due to the cuts of crystals.
However, the mechanical quality factor and
stability of quartz crystals are relatively higher than
ceramics.Therefore quartz crystals are always used
in high-frequency filters, transducers, and other
standard frequency controlling oscillators. Besides
the quartz crystals, the high-quality perovskite
piezoelectric single crystals such as the
Pb(A1/3B2/3)O3-PbTiO3 (A = Zn2+, Mg2+; B =
Nb5+) with much higher piezoelectric constant
(𝑑33 ~ 2600 pC/N), electromechanical coupling
coefficient (𝑘33 ~ 0.95), and strain (>1.7%) have
also been obtained since 1997 . These single
crystals are new-generation piezoelectricmaterials
for high performance piezoelectric devices and
systems including ultrasoundmedical imaging
probes, sonars for underwater communications, and
sensors/actuators. However, the size and shape of
the piezoelectric single crystals are difficult to be
preciously controlled during the growth process,
which limit the practical application in many fields
such as the microscaled actuators and composite
metamaterials. Furthermore, the piezoelectric
polymers such as polyvinylidene fluoride (PVDF)
with high flexibility, low density, and resistance as
well as relatively higher piezoelectricity voltage
constant (𝑔) have also attracted much attention in
recent years . Unlike the piezoelectric ceramic and
crystals, the intertwined long-chain molecules in
polymers attract and repel each other when an
electric field is applied. PVDF has exhibited great
potential in the application of acoustic ultrasound
measurements,
pressure
sensors,
and
ignition/detonations. However, the relatively low
piezoelectric strain constant (𝑑) of PVDF limited
the application in transducers.
4. Applications of
PiezoelectricMaterials
Nowadays, piezoelectric materials have been
widely used in the industrial, manufacturing,
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automotive industry, and medical instruments as
well as information and telecommunication fields,
and so forth. According to the operation mode of
the piezoelectric devices, the application of
piezoelectricmaterials can
be classified as follows.
(a) Sensor:Through the direct piezoelectric effect,
the piezoelectric materials can be used for the
detection of pressure variations in longitudinal,
transversal, and shear modes.The most commonly
used application of piezoelectric sensors is in the
sound form, such as the piezoelectric microphones,
piezoelectric pickups in acoustic-electric guitars,
and detection of sonar waves. Moreover, the
piezoelectric sensors can also be used with highfrequency field such as the ultrasonic medical
imaging or industrial nondestructive testing. In
addition, the piezoelectric sensors were also
employed in piezoelectric microbalance and strain
gauges.
(b) Actuator: On contrary to piezoelectric sensors,
the working of actuators is usually based on the
reverse piezoelectric effect to induce tiny changes
in the width of the piezoelectric materials by
applying high electric fields. Due to the relatively
high precision of the width changes, the
piezoelectric actuators are always used in accurate
positioning. For example, the piezoelectric motors
with high accuracy have already been used in
optical devices, transportation and aerospace
techniques, robots, medical devices,
biology, and nanomanipulation fields, such as the
atomic force microscopes (AFM), scanning
tunneling microscopes (STM), autofocusing
camera lens, inkjet printers, CT/MRI scanners, and
X-ray shutters.
(c)
Frequency
ControllingDevice:
Crystal
oscillator is an electronic oscillator circuit that uses
themechanical
resonance
of
a
vibrating
piezoelectric crystal to create an electrical signal
with a very precise frequency. The frequency can
be used to provide a stable clock signal for digital
integrated circuits. Moreover, the piezoelectric
materials have also been used in high-frequency
resonators and filters, such as the surface acoustic
wave devices and film bulk acoustic resonators.
(d) High Voltage and Power Sources: By applying
the
external
mechanical
stimulates,
the
piezoelectric ceramic or crystals can generate
potential differences with thousands of volts in
amplitude. Therefore, piezoelectric materials can
be used as high voltage and power sources. The
most commonly application is the piezoelectric
ignition/sparkers such as the cigarette lighters.
Moreover, the piezoelectric materials have been
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employed for energy harvesting applications. For
example, the energy from human movements and
vehicle movements in public places can be
harvested and converted into electricity for lighting
the lamps. Recently, themicroscale energy
harvesters were developed for harvesting the
smallscale mechanical energies by using the
piezoelectric nanomaterials, which is called
“piezoelectric nanogenerators.”The nanogenerators
can be used for charging the batteries or directly
driving some low-power microdevices.
5. piezoelectric nanogenerators and its
importance
In nanoscience and nanotechnology, developing a
novel wirelessnano-scale system, i.e. the
integration of nanodevices, functionalcomponents
and the power source, is of critical importance
forreal-time
and
implantable
bio-sensing,
environmental monitoringand portable electronics.
These wireless nano-systemsrequire their own
power sources despite their small size and
lowpower consumption. There are two ways of
achieving wirelessnano-systems. One is to use a
battery. Even if the battery hashuge capacitance, it
has a limited lifetime, and miniaturization
ofdevices limits the size of the battery, resulting in
short batterylifetime. Therefore, the main challenge
relies on the long-lifetime,small-sized and possibly
lightweight batteries. In addition, thebattery must
be
recharged
occasionally.
Consequently,
theminiaturization of a power package and selfpowering of thesenanosystems are some key
challenges for their possible applications.For
biomedical applications, it is important to consider
thetoxicity of the materials that compose batteries.
The otherapproach is to generate electrical power
through harvesting theambient energies.17 Energy
harvesting from the ambient forpowering a
nanosystem
is
very
important
for
its
independent,wireless and sustainable operation. A
piezoelectric nanogeneratoris a promising approach
for this application.Energy harvesting in our living
environment is a feasibleapproach for powering
micro-/nanodevices and mobile electronicsdue to
their small size, lower power consumption,
andspecial working environment. Nanomaterials
have uniqueadvantages for energy conversion,
including solar cells, piezoelectricnanogenerators,
thermoelectric cells, etc.The type ofenergy
harvested depends on the applications. For
mobile,implantable and personal electronics, solar
energy may not bethe best choice because it is not
available in many cases whenwhich the devices are
used. Alternatively, mechanical energy,including
vibrations, air flow, and human physical motion,
isavailable almost everywhere at all times, which is
called randomenergy with irregular amplitudes and
Imperial Journal of Interdisciplinary Research (IJIR)
frequencies. Piezoelectricnanogeneration is a novel
technology that has been developed for harvesting
this type of energy using piezoelectric
nanostructurearrays.
Nanogenerators can be used in areas that require a
foldable orflexible power source, such as implanted
biosensors in muscle orjoints, and have the
potential of directly converting biomechanicalor
hydraulic energy in the human body, such as flow
ofbody fluid, blood flow, heartbeat, and contraction
of the blood
vessels, muscle stretching or eye blinking, into
electricity topower the body-implanted devices.
Heart beat-driven flexiblenanogenerators can serve
as ultrasensitive sensors for realtimemonitoring of
the human-heart behavior, which might beapplied
to medical diagnostics as sensors and measurement
tools,
and confirming the feasibility of power conversion
inside a biofluidfor self-powering implantable and
wireless nanodevices andnanosystems in a biofluid
and
any
other
type
of
liquid.
Nanogeneratorsconvert the sound (noise or speech,
and even music)that always exists in everyday life
and the environment intoelectrical power.
Nanogenerators would be viable candidates tomeet
the world’s energy demands and efforts are
continued notonly for powering nanosystems but
also for powering micro-/nano-electronic devices.
Strong enough electrical power generatedthrough
nanogenerators has been used to continuouslydrive
a commercial liquid crystal display (LCD), light
upa commercial light-emitting diode (LED) and
laser diode (LD)that confirm the feasibility of using
nanogenerators for poweringmobile and personal
electronics.
6. Different materials as piezoelectric
nanogenerator
6.1 Lateral ZnO Nanowire Array
Because of the piezoelectric property of the ZnO
NW, the stress results in a piezoelectric field along
the length, which causes a transient charge flow in
the external circuit. The Schottky contact at the
bonded ends can regulate the charge flow. As a
result, the bending and releasing of the single-wireNG gives rise to an alternating flow of the charges
in the external circuit. In this work, the power
output has been scaled up with the integration of
hundreds of thousands of horizontally aligned
NWs, which was made by a scalable sweepingprinting-method that is simple, cost-effective, and
highly efficient.
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Device making procedure: The method consists of
two main steps. In the first step, the vertically
aligned NWs are transferred to a receiving
substrate to form horizontally aligned arrays. The
major components of the transfer setup are two
stages (Figure 7a). Stage 1 has a flat surface that
faces downward and holds the vertically aligned
NWs; stage 2 has a curved surface and holds the
receiving substrate. Polydimethylsiloxane (PDMS)
film on the surface of stage 2 is used as a cushion
layer to support the receiving substrate and
enhances the alignment of the transferred NWs.
The radius of the curved surface of stage 2 equals
the length of the rod supporting the stage, which is
free to move in circular motion (Supporting
Information Figure S1). In the second step,
electrodes are deposited to connect all of the NWs
together.
Vertically aligned ZnO NWs on Si substrates were
synthesized using physical vapor deposition
method. The dense and uniform NWs have the
length of ~50 μm, diameter of ~ 200 nm, and
growth direction along the c-axis.
FIGURE 6. Fabrication process and structure characterization of the HONG. (a) Experimental setup for
transferring vertically grown ZnO NWs to a flexible substrate to make horizontally aligned ZnO NW arrays
with crystallographic alignment. (b) SEM image of as-grown vertically aligned ZnO NWs by physical vapor
method on Si substrate. (c) SEM image of the as-transferred horizontal ZnO NWs on a flexible substrate. (d)
Process of fabricating Au electrodes on horizontal ZnO NW arrays, which includes photolithography,
metallization, and lift-off. (e) SEM image of ZnO NW arrays bonded by Au electrodes. Inset: demonstration of
an as-fabricated HONG. The arrowhead indicates the effective working area of the HONG.
A small piece of Si substrate with grown ZnO NWs
was mounted onto stage 1 (Figure 6a) and a piece
of Kapton film with the thickness of 125 μmwas
attached to stage 2 (Figure 6a). The distance
between the receiving substrate and NWs was
precisely controlled to form a loose contact
between the two. The receiving substrate then
counterclockwise swept across the vertical NWs
arrays, which were detached from Si substrate and
aligned on the receiving substrate along the
direction of sweeping due to the applied shear force
(Figure 6a). The as-transferred NWs are presented
in Figure 8c with an estimated average density of
1.1 × 106 cm-2. The length variation is probably
due to the fact that not all of the NWs were broken
off at the roots.
Next, the evenly spaced electrode pattern over the
aligned NWs
was
first defined using
photolithography and then followed by sputtering
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300 nm thick Au film (Figure 7d). After lifting off
the photoresist, 600 rows of stripe-shaped Au
electrodes with 10 μmspacing were fabricated on
top of the horizontal NW arrays (Figure 6e). Au
electrodes form Schottky contacts with the ZnO
NWs, which are mandatory for a working NG.
Approximately 3.0 × 105 NWs in an effective
working area of 1 cm2, as pointed by an arrowhead
in Figure 6d (inset), are in contact with electrodes
at both ends. Finally, a PDMS packaging over the
entire structure can further enhance mechanical
robustness and protect them device from invasive
chemicals.
Working principle and application: The working
principle of the HONG is illustrated by the
schematic diagrams in Figure 7(a,b). NWs
connected in parallel collectively contribute to the
current output; NWs in different rows connected in
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serial constructively improve the voltage output.
The same growth direction of all NWs and the
sweeping printing method ensure that the
crystallographic orientations of the horizontal NWs
are aligned along the sweeping direction.
Consequently, the polarity of the induced
piezopotential is also aligned, leading to
amacroscopic potential contributed constructively
by all of the NWs (Figure 8b).
FIGURE 7. Working principle and output measurement of the HONG. (a) Schematic diagram of HONG’s
structure without mechanical deformation, in which gold is used to form Schottky contacts with the ZnO NW
arrays. (b) Demonstration of the output scaling-up when mechanical deformation is induced, where the “(” signs
indicate the polarity of the local piezoelectric potential created in the NWs. (c) Open circuit voltage
measurement of the HONG. (d) Short circuit current measurement of the HONG. The measurement is
performed at a strain of 0.1% and strain rate of 5% s-1 with the deformation frequency of 0.33 Hz. The insets
are the enlarged view of the boxed area for one cycle of deformation.
To investigate the performance of the HONG, a
linear motor was used to periodically deform the
HONG in a cyclic stretching-releasing agitation
(0.33 Hz). The open-circuit voltage (Voc) and the
short-circuit current (Isc) were measured with
caution to rule out possible artifacts. At a strain of
0.1% and strain rate of 5% s-1, peak voltage and
current reached up to 2.03 V and 107 nA,
respectively. Assuming that all of the integrated
NWs actively contribute to the output, the current
generated by a single NW is averaged to be ~ 200
pA; and the voltage from each row is ~ 3.3 mV in
average. Considering the size of the working area
of the nanogenerator (1 cm2) (Figure 1e, inset), a
peak output power density of ~ 0.22 μW/cm2 has
been achieved, which is over 20-fold increase
compared to our latest report based on a more
complex design. For nanowires with the diameter
of ~ 200 nm, the power volume density is ~11
mW/cm3, which is 12-22 times of that from PZT
based cantilever energy harvester. The durability
test and further characterization were performed,
which prove the stability and robustness of the
HONGs (Supporting Information Figure S3).
Voltage linear superposition test verified the
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proposed working principle of the HONGs
(Supporting Information Figure S4).
Further scaling up the power output is expected to
be technically feasible. If NWs can be uniformly
and densely packed as a monolayer over the entire
working area, and all can actively contribute to the
output, the maximum power area density is
expected to reach ~ 22 μW/cm2. The power
volume density is anticipated to be improved up to
~ 1.1 W/cm3. With 20 layers of such NW arrays
stacked together, the power area density would be
boosted up to ~ 0.44 mW/cm2.
The performance of the HONG is affected by strain
and strain rate. For a given strain rate (5% s-1), an
increase in strain leads to a larger output (Figure
8a,b). Likewise, at a constant strain (0.1%), the
output is proportional to the strain rate (Figure
8c,d). Beyond a certain strain and strain rate,
saturation of the magnitude occurs, probably due to
the converse piezoelectric effect, which is the strain
created by the piezopotential and it is opposite to
the externally induced strain. It is noticed that 0.1%
strain is sufficient to induce effective output, which
is much smaller than the 6% fracture strain of the
ZnO NW predicted theoretically.
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FIGURE 8. Performance characterization of the HONG with increasing strain and strain rate. (a) Open circuit
voltage measurement of the HONG with increasing strain at a given strain rate of 5% s-1. (b) Short circuit
current measurement of the HONG with increasing strain at a given strain rate of 5% s-1. (c) Open circuit
voltage measurement of the HONG with increasing strain rate at a constant strain of 0.1%. (d) Short circuit
current measurement of the HONG with increasing strain rate at a constant strain of 0.1%. For all
measurements, the mechanical deformation frequency is fixed at 0.33 Hz.
FIGURE 9. Application of the electric energy generated by the HONG to drive a commercial light emitting
diode. (a) The electric output measured after a full wave rectifying bridge. Signals of negative signs are
reversed, as pointed by the arrowhead. Inset: Schematic of the chargingdischarging circuit for storing and
releasing the energy generated by the HONG, respectively. (b) Image of a commercial LED, which is
incorporated into the circuit. (c) Image of the LED in dim background before it was lit up. (d) Image of the LED
in dim background at the moment when it was lit up by the energy generated from the HONG.
Storing the generated energy and driving functional
devices are extremely important steps toward
practical applications of the nanogenerator. In this
work, they were accomplished by using a chargingdischarging circuit with two consecutive steps
(Figure 9). The circuit function is determined by
the status of a switch (Figure 9a inset). The switch
is at position A for energy storage achieved by
charging capacitors. Upon charging completion, the
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switch is switched to position B for energy
releasing to power a functional device, such as a
light emitting diode.
7.2 Nano-generators using PZT
Lead zirconatetitanate (PZT) has been used for
piezoelectric energy generation at the macro-scale.
PZT
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nanofibres are found to have a higher piezoelectric
voltage constant than semiconducting nanowires
due to their inherent polar crystal structure and
high dielectric value, and they can be synthesized
with a very high aspect ratio. However, bulk PZT
and its thin films are extremely fragile, and are not
useful for energy generation under alternating
loads. They have been found to be very sensitive to
high frequency. The problem of fragility, however,
disappears for high aspect ratio nanostructures .
Chen et al. demonstrated the possibility of
harvesting piezoelectric energy using PZT
nanomaterials. PZT nanofibres with a length and
diameter of 500 μm and 60 nm respectively were
laterally aligned on platinum, fine wire
interdigitated electrodes, and packaged using a soft
polymer of polydimethylsiloxane (or PDMS) on a
silicon substrate. Under the application of periodic
stress, this nano-generator produced a voltage of
1.63 V and a power of 0.03 μW at a load resistance
of
6
MΩ
Figure
11
shows
the
schematicarrangement of the PZT nanofibre-based
nano-generator,
the
piezoelectric
voltage
generation process and measured voltage generated
as a result of applied force.
Figure 10. Schematic arrangement of a PZT nanofibre-based nano-generator, the distribution of forces for
piezoelectric voltage generation and voltage generated as a result of applied force (clockwise) .
Xu et al.have demonstrated how epitaxially-grown
PZT nanowire arrays could be used for high output
piezo-energy harvesting and the possibility of using
such energy harvesters for mobile electronic
devices. A single array of such nanowires grown at
230oC produced a peak output voltage of ~0.7 V
and a current density of 4 μAcm- 2, with an average
power density of 2.8 mWcm-3. The alternating
current generated was rectified and stored, and
used for lighting a commercial laser diode.
Wu et al. have reported on a textile nano-generator
built using PZT nanowires that could be used
forwearable and self-powered devices. A generator
thus built could generate an output voltage of 6 V
andproduce a current of 45 nA. The nano-generator
was built cost effectively and was demonstrated to
Imperial Journal of Interdisciplinary Research (IJIR)
light acommercial LCD and power a ZnO nanowire
UV sensor for the quantitative detection of UV
light.
7.3 Nano-generators using barium titanate
The most recent material to be reported for
piezoelectric power generation is perovskite
BaTiO3, which is not only piezoelectric but also
ferroelectric. Park et al. have demonstrated the use
of BaTiO3 thin films on a flexible substrate for the
conversion of mechanical energy into electrical
energy for the first time. They used radio frequency
magnetron sputtering to deposit BaTiO3 thin films
under an electric field of 100 kV/cm on a
Pt/Ti/SiO2 substrate. The ribbon-structured thin
films were transferred onto a flexible substrate
using standard microfabrication and lithographic
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printing
techniques,
and
connected
by
interdigitated electrodes. By applying a periodic
bending force, the nano-generator produced an
output current density of 0.19μA/cm2 and a power
density of ~7 mW/cm3. Figure 11 gives the
schematic of the fabrication procedure of the nanogenerator.
Figure 11. Schematic illustration of the process for fabricating a flexible BaTiO3 nano-generator on plastic
substrates .
The analysis of piezoelectric potential distribution
was done for the thin film. The results obtained are
shown in Figure 12. It was found that when
stretched from both ends, the potential increased
from the bottom of the thin field (at 0 V), which is
connected to the substrate, to a maximum of 0.529
V at the topmost layer.
Figure 12. The calculated piezoelectric potential distribution inside the BaTiO3 thin film. A pure tensile strain
is assumed to exist in the thin film when the substrate is bent over 90° with a radius of 1.0 cm. The piezoelectric
potential difference inside the BaTiO3 thin film is 0.529 V .
Imperial Journal of Interdisciplinary Research (IJIR)
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7.5 Nanowire-based flexible P(VDF-TrFE)
nanogenerator
Among the many functional materials, the
poly(vinylidene
fluoride-cotrifluoroethylene)
[P(VDF TrFE)] polymer have both the pyroelectric
and piezoelectric properties, which make it ideal
material for fabricating a hybrid energy cell.
Nanostructures of P(VDF-TrFE) are particularly
attractive for energy harvesting due to geometrical
effect, improved mechanical properties and
sensitive
to
small
deformations.Currently,
electrospinning
process
based
on
electrohydrodynamic deformation have been
presented to produce P(VDF-TrFE) fibers, but this
involves specialized equipment, and high voltages
of nearly 10 kV that electrically pole the fibers and
the throughput of the process is again quite low.
Development of large scale P(VDF-TrFE)
nanogenerator with a low-cost fabrication method
has remained a major challenge. In this paper, we
demonstrate a high performance flexible P(VDFTrFE) nanogenerator based on P(VDF-TrFE)
nanowire array synthesized by a costeffective
template-wetting technique. The piezoelectric and
pyroelectric output electric signals of the flexible
hybridnanogenerator were measured respectively,
and output voltages were successfully integrated
together. As a demonstrated application, the output
electricity was used to power a largescaleliquid
crystal display screen.
Figure 13 shows the schematic fabrication progress
of the flexible P(VDF-TrFE) nanogenerator. The
manufacturing of the hybrid nanogenerator starts
by dissolving P(VDF-TrFE) powder with a molar
ratio of 70/30 (KunshanHisense Electronics Co.
Ltd) in N,N-dimethylformamide (DMF) solvent to
form a solution with a concentration of 10wt%.
Then a thin layer of P(VDF-TrFE) film with a
thickness of 4-5µm was spin coated onto a flexible
Au-coated kapton substrate (Figure 13a), which has
been carefully cleaned in acetone, ethanol and
deionized (DI)-water, and finally deposited with
100nm Au conductive layer. Following the
evaporation of the solvent, a anodic aluminum
oxide (AAO) nanoporous templates (Shanghai
Shangmu Technology Co. Ltd) was pressed against
the P(VDF-TrFE) film under a slight pressure. The
AAO/sample was then thermally maintained at a
temperature of 170º for 1h, which is higher than the
melting point of P(VDF-TrFE), resulting in the
formation of nanowires within the nanoporous, as
shown in Figure 20b. After annealing at 120º for
another 1h to improve the crystallinity of the
material, a slow cooling the sample to room
temperature to cure the nanowires. The free
standing P(VDF-TrFE) nanowire array was
obtained after dissolving the AAO template in 2 M
NaOH solution in water, as shown in Figure 13c.
Then a thin layer of PMMA was spin-coating at
1000 rpm for 30 seconds
onto top of the nanowires to avoid the short circuit
of the device (Figure 13d). Finally, by spinning a
conducting polymer PEDOT: PSS as the top
flexible electrode and applying a voltage of 50
MVm-1 to align the molecular dipoles along the
height direction of the P(VDF-TrFE) nanowires,
multiple flexible hybrid nanogenerator were
successfully fabricated, as shown in Figure 13e.
The device is mainly composed of three layers:
Au/Kapton, which acts as the bottom electrode,
P(VDFTrFE) nanowire array as the piezoelectric
and pyroelectric material and PEDOT: PSS
conducting polymer as the top electrode.
Fig.13: The schematic fabrication progress of the flexible P(VDFTrFE) nanogenerator.
Imperial Journal of Interdisciplinary Research (IJIR)
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After dissolving the AAO nanoporous template, the
polymeric crystallinity of the free standing P(VDFTrFE) nanowire array can be characterized by
Fourier Transform Infrared (FT-IR) spectrum in the
wave number range of 750- 1500cm-1 (Figure
14a). The P(VDF-TrFE) nanowire array show
discernable FTIR peaks at 850, 1288 and 1400 cm1 bands corresponding to the β crystalline
phase.AX-ray diffraction (XRD) measurement
were also carried out at 2θ angles ranging from 10°
to 30° to confirm the results obtained by the FTIR
measurements, as shown in Figure 14b. The
P(VDF-TrFE) nanowire array show the peak at
19.9°, corresponding to the overlapping of (110)
and (200) reflections, is attributable to the β
ferroelectric phase.
Fig. 14: (a) The FTIR spectra of the P(VDF-TrFE) nanowires. (b) XRD result of β phase (110/200) of P(VDFTrFE) nanowires.
Researchers first measured the output voltages of
the P(VDF-TrFE) hybrid nanogenerator for
harvesting mechanical energy. The hybrid NG
device was regularly deformed by a linear motor
with periodical bending and unbending motions, as
shown in Figure 15a. When the motor moved
10mm with an average
speed of 20mm/s, the hybrid NG device generates a
positive voltage of 5.6V upon the bending states,
and a corresponding negative output pulse is
measured upon the releasing states. The bottom
inset of Figure 15b-i show the magnified output
signal. To verify that the output signal was indeed
generated from the piezoelectric effect, a widely
accepted polarity switching test was conducted.
Piezoelectric output pulses with opposite sign were
obtained, as shown in Figure 15b-ii. We also
measured the output voltage of the hybrid NG
device under different bending speed varying from
4mm/s to 20mm/s.
The measured results in Figure 15(c) shows the
output voltages increase with the bending speed,
indicating that the output performance depends on
the bending strain rate at the fixed strain. This
behavior can be attributed to the incensement of
accumulated charges due to the quite fast electron
flows during fast bending and unbending
Imperial Journal of Interdisciplinary Research (IJIR)
motions.Furthermore, the hybrid NG device
exhibits good mechanical robustness and stability,
and the voltage amplitudes show only a slight
fluctuation after 1 hour (Figure 15d), indicating
that our hybrid NG device can be applied to harsh
mechanical conditions.
8. Recent progress of piezoelectric
nanogenerator
Now researchers are made flexible nanogenerator
(NG) that is is fabricated with a poly(vinylidene
fluoride) (PVDF) film, where deoxyribonucleic
acid (DNA) is the agent for the electroactive βphase nucleation. Denatured DNA is co-operating
to align the molecular -CH2/-CF2 dipoles of PVDF
causing piezoelectricity without electrical poling.
The NG is capable of harvesting energy from a
variety of easily accessible mechanical stress such
as human touch, machine vibration, football
juggling, and walking. The NG exhibits high
piezoelectric
energy
conversion
efficiency
facilitating the instant turn-on of several green or
blue light-emitting diodes. The generated energy
can be used to charge capacitors providing a wide
scope for the design of self-powered portable
devices.
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Fig. 15: DNA assisted PVDF nanogenerator
Scientists already knew about some differences
between the two groups of materials. Ceramics
have higher piezoelectric coefficients but their
stiffness can make them unstable under mechanical
vibration. In contrast, polymers have smaller
piezoelectric responses but great flexibility. They
found the two types also differ in how they respond
to stress- or strain- driven excitations, with
ceramics showing more promise for strain-driven
nanogenerators while polymers perform better
under stress.
“This is great for people looking to design a
nanogenerator for a particular application under a
particular mechanical driving scenario because then
you can choose the material that will perform best,”
researchers are now looking at also incorporating
both polymers and ceramics into the same
composite. This will combine the most useful
characteristics of both materials for a given
application. “The authors have made a detailed
analysis about the [effects of] driving mechanism
and frequency on the performance of
nanogenerators,” says Wang. “The results are
exciting for designing and optimizing the
performance of nanogenerators.”
9. Future Development of
Nanogenerators
Although there have been numerous research
works about the fabrication, performance, and
application of piezoelectric nanogenerators, the
Imperial Journal of Interdisciplinary Research (IJIR)
following listed several crucial issues still required
to be further improved:
(i) Increase of output power density.
(ii) The integration packaging of energy storage
unit with the nanogenerators.
(iii) Optimization on harvesting efficiency of
mechanical energy from various working
conditions.
(iv) Optimization of electromechanical conversion
efficiency through structural design.
(v) Long-term stability, mechanical strength, and
chemical stability of the nanogenerators. Moreover,
there are also some problems still required to be
solved for the application of nanogenerators, which
arelisted as follows:
(i) The structural design to guarantee the long-term
stability and mechanical strength of the active
chemical sensors.
(ii) The harvesting method of mechanical energy to
generate stable output voltage for the active
sensors,which is crucial for the accuracy of the
sensing results.
(iii) The integration and packaging of
nanogenerator and sensing unit for the selfpowered systems.
(iv) Temperature drift during the sensing process
for active/self-powered sensors.
(v) The integration of active or self-powered
system with date processing and transmitting
systems.
10. Summary and conclusion
Piezoelectricity is naturally available in certain
ceramics and crystals. Certain polymers have also
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been used for piezo-energy harvesting. We had
explored the various materials in use to date for
energy harvesting at the micro- and nano-scales.
Although ZnO is the most widely used crystal
material for this purpose, some ceramics and
polymers have also been used for the same
purpose. PZT is the most widely used ceramic
material, while a polymer material PVDF has been
used too. BaTiO3 is the most recent material being
used for piezo-energy harvesting. The relative
merits and de-merits of different materials as seen
through different research works are discussed
below.
Ceramic materials like PZT and BaTiO3 have high
piezoelectric sensitivity and coupling coefficients.
They are available commercially at low cost and in
a variety of designs. However, stability is an issue
for them, as they suffer from the loss of
polarization
with
continued
usage.
The
piezoelectric properties are also strongly dependent
on the operating temperatures. Electrical charge
separation can occur not just from mechanical
deformation but with temperature changes as well.
These are brittle substances; hence, they cannot
withstand mechanical deformation for long.
Piezoelectric single crystal materials (ZnO) are
easily synthesized in the required sizes and shapes,
and very economically too. They have a high
piezoelectric coefficient and electromechanical
coupling. ZnO is highly tensile and can thus
undergo huge mechanical deformations for a long
period of time. The piezoelectric properties are not
temperature-dependent and so can be operated in
higher temperature environments. Polymers, on the
other hand, are not inherently polarized and
undergo polarization with the application of an
external field in a special environment. However,
because of their properties of being light-weight,
flexible and biocompatible, they are increasingly
seen as potential wearable and foldable energy
harvesters for various biomedical applications and
are attracting research. ZnO nanowires had been
the most widely used material for nano-energy
harvesting. We found that, throughout the reported
research work done up until now and referenced
above for nano-energy harvesting, ZnO holds the
following advantages over other materials:
•
ZnO
is
a
versatile
metal
oxide
semiconductormaterial
with
inherent
piezoelectricity andsemiconducting properties.
•
The
piezoelectric
properties
improve
fornanostructures as compared to bulk ZnO.
•
Synthesizing
one-dimensional
ZnO
nanostructurescan be done economically at room
temperature.
• These nanostructures can not only withstand
hugedeformations, but so too can their
Imperial Journal of Interdisciplinary Research (IJIR)
mechanicalproperties - like resilience and tensility–
improvewith size reduction.
• They can be synthesized on any substrate and
cangenerate piezopotentials under any type of
vibration.
Here we have demonstrated PZT nanowires that
could be used for wearable and self-powered
devices. A generator thus built could generate an
output voltage of 6 V and produce a current of 45
nA. The nano-generator was built cost effectively
and was demonstrated to light a commercial LCD
and power a ZnO nanowire UV sensor for the
quantitative detection of UV light.
The nano-generator made of barium titanate
produced an output current density of 0.19 μA/cm2
and a power density of ~7 mW/cm3.
11. ACKNOWLEDGMENTS
BinoyBera would like to thank Dr. Shankar Narayan
Patra for his constant support and inspiration and
guidance.
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