Ferroelectric materials: Its
properties and application
Submitted by
RAHUL RAJ
Reg. No. – 182011521393008
Roll No. – 1920191050025
In the partial fulfillment for
the award of the degree of
MASTER OF SCIENCE
IN
PHYSICS
UNDER THE ESTEEMED GUIDANCE OF
Dr. Seema Sharma
DEPARTMENT OF PHYSICS
A.N.COLLEGE, PATNA
PATLIPUTRA
UNIVERSITY
Anugrah Narayan College, Patna
CERTIFICATE
This is to certify that the thesis entitled “FERROELECTRIC MATERIALS: ITS PROPERTIES
AND APPLICATIONS” is submitted by RAHUL RAJ (Roll No - 1920191050025) to this institute in partial fulfilment
of the requirement for the award of
the degree of Master of Science in Department
of Physics, is a bonafied record of the work carried out under my
supervision and guidance. It is further certified that no part of this thesis
is submitted for the award of any degree.
(Dr. Seema Sharma)
Patna
Supervisor
Date
– Department
of Physics
Acknowledgement
I
have great pleasure to express my sincere gratitude to my supervisor Dr. Seema Sharma, Department of Physics, A. N. College Patna,
for providing their valuable guidance, suggestions
and encouragement throughout the course of the dissertation. Her constant
superintendence kept my spirits high and allowed me to take this dissertation
to its best possible outcome. I would also like to thank to all professors to
help in my project.
I
owe my respect to all my professors for inspiring my interest in Physics and
all non-teaching staffs of the Department of Physics for their timely help and
cooperation.
Patna
Date
: (RAHUL
RAJ)
CONTENTS
1. Introduction …………………………….……. ( 7 – 8 )
2. Theory of ferroelectrics
……………………… ( 8 – 10 )
3. Ferroelectrics phase
transitions ………………. ( 10 – 14 )
3.1. Phase changes
…………………………… ( 11 - 12 )
3.2. How phase transition
works ………………( 12 - 14 )
4. Polarization in ferroelectric materials ………...
( 14 – 17 )
4.1. Hysteresis loop
…………………………… ( 15 - 17 )
5. Types of ferroelectric
materials ……………….. ( 18 – 22 )
5.1. Perovskite
ferroelectric materials …….……. ( 18 )
5.2. Ilmenite
ferroelectric materials ………….…. ( 18 )
5.3. Polymeric
ferroelectric materials ………... ( 18 – 22
)
6. Applications of ferroelectric
materials ………….. ( 23 – 25 )
7. Bibliography …………………………………….. ( 26 – 27
)
ABSTRACT
In
terms of advances in technology, especially electronic devices for human use,
there are needs for miniaturization, low power, and flexibility. However, there
are problems that can be caused by these changes in terms of battery life and
size. In order to compensate for these problems, research on energy harvesting
using environmental energy (mechanical energy, thermal energy, solar energy
etc.) has attracted attention. Ferroelectric materials which have switchable
dipole moment are promising for energy harvesting fields because of its special
properties such as strong dipole moment, piezoelectricity, pyroelectricity. The
strong dipole moment in ferroelectric materials can increase internal potential
and output power of energy harvesters. In this review, we will provide an
overview of the recent research on various energy harvesting fields using
ferroelectrics. A brief introduction to energy harvesting and the properties of
the ferroelectric material are described, and applications to energy harvesters
to improve output power are described as well.
List of figures
Figure 1. A graph depicting the relationship between
spontaneous polarization and
temperature.
Above Tc the material becomes paraelectric and has no
spontaneous polarization. ( 10 )
Figure 2.
Phase change owing to rise or fall of
temperature ( 13 )
Figure 3.
Phase change owing to rise or fall of
pressure ( 14 )
Figure 4. Ferroelectric (P-E) hysteresis loop ( 16 )
1. Introduction
Two of
the most important trends in recent electronic technology have been the size
reduction and functional improvement of mobile electronic devices. Mobile
electronic devices are small, portable, and contain a variety of information
that is instantly accessible at all times, including the ability to share and
communicate information wirelessly. These devices are becoming even smaller and
lighter so that they can be wearable or attachable to objects that can be used
daily, such as a watch, glasses, or clothes. All devices that are based on
microelectronic technology require a lot of external power supply due to their
increased functions, and batteries are the most important power source for
mobile electronic devices. However, batteries take up increasingly significant
parts of the overall device volume and weight as the electronic devices are
miniaturized. Moreover, battery technology is limited in energy capacity per
volume for supplying sufficient energy to a mobile electronic device.
Therefore,
many studies have been focused on reducing power consumption and designing
energy efficient devices to reduce the sizes but extend the lifetimes of the
batteries. Nevertheless, the batteries must be replaced or recharged after being
discharged, and this is an obstacle to realizing always-on wearable electronic
devices. In order to make up for this problem, we need to develop an energy
harvesting system that can harvest and reuse energy sources from the ambient
environment. Energy harvesters convert various environmental energy sources
such as mechanical stress, vibration, light, and heat, etc. to electrical
energy. Each energy source can be converted to electrical energy by each
coupled-physical phenomenon such as piezoelectric, triboelectric photovoltaic,
and thermoelectric (or pyroelectric) effects. The amount of output energy
obtained from piezoelectric effect is ~ 5.92 μW/cm2,
triboelectric effect is ~ 0.7 mW/cm2, photovoltaic effect is ~ 22.1
mW/cm2, and pyroelectric effect is 1.4 μW/cm2. The
working principle of each energy harvester is different, but basically,
electric current is generated by internal polarization or potential. Therefore,
increasing the polarization density is important for improving output power of
energy harvester. Conventional materials have limitation in increasing internal
polarization because of low polarization density. Moreover hardness of the
conventional materials hinders application to wearable devices. However
introducing novel materials with strong and permanent polarization,
ferroelectric materials, can overcome these limitations. Ferroelectric
materials have permanent dipole moments once electric field is applied, so
polarization density can be increased through the insertion of ferroelectric materials.
First, we will briefly describe the types of ferroelectric materials as well as
the basic theory of energy harvesting technologies. A material can be
either piezoelectric, pyroelectric or ferroelectric, only if its crystalline
symmetry is inherent. A basic principle due to Neumann is that any physical
property exhibited by a crystal must have at least the symmetry of the point
group of the crystal. Thus, the above properties, which are inherently
asymmetric, can only arise in asymmetric crystals.
2. Theory of Ferroelectrics :
Ferroelectric
materials can be defined as dielectric materials in which polarization remains
permanently, even after removing the applied electric field. Moreover, the
direction of the dipole moment can be switched by applying electric field.
Among the 32 crystal classes, 21 have non-centrosymmetric and 20 have direct
piezoelectricity among them, which forms polarization through mechanical
stress. Ten of the piezoelectric crystal classes have spontaneous electric polarization
and this electric polarization varies with temperature change which is called
pyroelectricity. Some of the pyroelectric materials are ferroelectric materials
whose polarization can be reversed by external electric field. Therefore,
ferroelectric materials have both pyroelectricity and piezoelectricity. In
all FE materials there is an accompanying non-linear behavior relating polarization
and electric field. Electric polarization is a measure of the average dipole
moment volume density; it is an indicator of how strong and wellaligned the
dipoles are in a material. When an electric field is applied to a FE material
the polarization rapidly increases before becoming linear, it is at this point
that extrapolation to the zero electric field determines the value of
spontaneous polarization.
The
ferroelectricity can be tested by measuring polarization as a function of
electric field. Ferroelectric materials have spontaneous polarization, and this
varies with external electric field, so in a polarization versus electric field
curve. However, the ferroelectricity is shown only after the phase transition
below a certain temperature, called Curie temperature (TC).
These
materials have perovskite structures, like BaTiO3, whose general
chemical formula is ABO3, where A and B atoms are cations. Normally
the A cation has radius of 1.2–1.6 Å and B cation has one of 0.6–0.7 Å. A atoms
are positioned at the cube corner and oxygen atoms are positioned at the face
center and form an octahedron surrounding the B atom which is positioned at the
body center. Under electric field, the position of B cation shifts, then the
geometrically unbalanced electrical charge forms a dipole moment.
The
ilmenite ferroelectric materials have the same chemical formula as perovskite
materials, ABO3, e.g. LiNbO3 and LiTaO3.
However, the A cation is too small to fill the position of the perovskite
crystal coordinate. Oxygen atoms comprise hexagonal close-packed layer, and A
and B atoms are positioned at the octahedron site between layers.
The
first discovered and the most representative polymeric ferroelectric material
is polyvinylidene fluoride. Polymers have long carbon backbone, so their
structure is complex and has a lot of configurations depending on whether the
neighboring carbon bonds are trans or gauche. Among the configurations of PVDF.
The β-phase has all trans configuration. The fluorine atoms have the strongest
electronegativity, resulting C–F polar bond so that PVDF molecule has dipole
moment in perpendicular direction to its carbon chain. However, the dipole
moments of the pristine polymer chains are not arranged in single direction, so
the net polarization is zero. Therefore, a strong electric field is required to
arrange the dipole moments of chains, which is called electrical poling.
Energy
harvesting utilizes various energy sources, including mechanical, thermal, and
solar energies. Each energy source can be converted to electrical energy
through each coupled-physical phenomenon, but basic principle is same: the
variation of the internal dipole moment or potential. Therefore, the
introduction of ferroelectric materials to energy harvesters can increase
dipole moments and potential in the devices due to the strong polarization in
the ferroelectric materials so that conversion efficiency can be enhanced.
Figure
1. A graph depicting the relationship
between spontaneous polarization and temperature. Above Tc the material becomes
paraelectric and has no spontaneous polarization.
Since
polarization is a vector, there is a direction associated with its magnitude,
groups of similarly oriented dipoles are known as a domain. Domain walls are
the boundaries between different dipoles, groups of dipoles aligned in varying
directions; much like a fence separates yards. The application of an external
electric field will change the direction of the polarization within a domain;
the dipoles will want to align with the applied electric field, leading to a
process known as domain switching. In some cases the presence of an internal
bias field due to defect dipoles can affect the field necessary for domain
switching resulting in anomalous behavior of the hysteresis loop.
3. Ferroelectric phase
transitions
In a
perfect ferroelectric crystal, the phase transition takes place at temperature 𝑇𝐶 which is called the Curie
temperature. Real (defected) crystals and ceramic samples are characterized by
a presence of built-in electric field and mechanical strains. In the case of
the real crystal, the temperature of the phase transition 𝑇′𝐶, is displaced to lower temperature and the
temperature of the maximum of ε(T)T𝑚 is displaced to higher value with respect to 𝑇𝐶.Thus, for the real
ferroelectric sample (not for an incipient ferroelectric), one has 𝑇′𝐶<T𝐶<T𝑚. Some investigators suppose by default that 𝑇′𝐶=T𝐶=T𝑚. Such a supposition is wrong
and can lead to an incorrect treatment of experimental results. In the case of
a thin film sample, the phase transition and the dielectric response of a
ferroelectric sample are affected by the size of the sample, what is treated as
a size effect. Experimental data obtained as a result of measurement of the
dielectric constant as a function of temperature can be used for finding the
Curie temperature and other parameters of the material. First-order phase
transitions exhibit a discontinuity in the first derivative of the Gibbs free
energy with respect to the thermodynamic variable. Second-order phase
transitions are continuous in the first derivative but exhibit discontinuity in
a second derivative of the Gibbs free energy with respect to a thermodynamic
variable . In the first-order phase transition, volume, entropy and
polarization of the crystal change discontinuously at the transition point. In
the second-order phase transition, the specific heat changes discontinuously,
‘whereas volume, entropy and polarization change continuously at the phase
transition point. In the first-order phase transition, the energy appearing as
latent heat in an infinitely narrow temperature range interval. Phase
transition is when a substance changes from a solid, liquid, or gas state to a
different state. Every element and substance can transition from one phase to
another at a specific combination of temperature and pressure.
Phase Changes
Each
substance has three phases it can change into; solid, liquid, or gas(1).
Every substance is in one of these three phases at certain temperatures. The
temperature and pressure at which the substance will change is very dependent
on the intermolecular forces that are acting on the molecules and atoms of the
substance(2). There can be two phases coexisting in a single
container at the same time. This typically happens when the substance is transitioning
from one phase to another. This is called a two-phase state(4). In
the example of ice melting, while the ice is melting, there is both solid water
and liquid water in the cup.
There
are six ways a substance can change between these three phases; melting,
freezing, evaporating, condensing, sublimination, and deposition(2).
These processes are reversible and each transfers between phases differently:
- Melting: The transition
from the solid to the liquid phase
- Freezing: The transition
from the liquid phase to the solid phase
- Evaporating: The
transition from the liquid phase to the gas phase
- Condensing:The transition
from the gas phase to the liquid phase
- Sublimination: The
transition from the solid phase to the gas phase
- Deposition: The transition
from the gas phase to the solid phase
How Phase Transition works
There
are two variables to consider when looking at phase transition, pressure (P)
and temperature (T). For the gas state, The relationship between temperature
and pressure is defined by the equations below:
Ideal
Gas Law:
PV=nRT
………….. (1)
van der
Waals Equation of State:
(P+a∗n2V2)(V−nb)=nRT
………….. (2)
Where V
is volume, R is the gas constant, and n is the number of moles of gas.
The
ideal gas law assumes that no intermolecular forces are affecting the gas in
any way, while the van der Waals equation includes two constants, a and b, that
account for any intermolecular forces acting on the molecules of the gas.
Temperature
Temperature
can change the phase of a substance. One common example is putting water in a
freezer to change it into ice. In the picture above, we have a solid substance
in a container. When we put it on a heat source, like a burner, heat is
transferred to the substance increasing the kinetic energy of the molecules in
the substance. The temperature increases until the substance reaches its
melting point. As more and more heat is transferred beyond the melting point,
the substance begins to melt and become a liquid. This type of phase change is
called an isobaric process because the pressure of the system stays at a
constant level.
Figure 2. Phase change owing to
rise or fall of temperature
Melting point (Tf)
Each
substance has a melting point. The melting point is the temperature that a
solid will become a liquid. At different pressures, different temperatures are
required to melt a substance. Each pure element on the periodic table has a
normal melting point, the temperature that the element will become liquid when
the pressure is 1 atmosphere.
Boiling Point (Tb)
Each
substance also has a boiling point. The boiling point is the temperature that a
liquid will evaporate into a gas. The boiling point will change based on the
temperature and pressure. Just like the melting point, each pure element has a
normal boiling point at 1 atmosphere.
Pressure
Pressure
can also be used to change the phase of the substance. In the picture above, we
have a container fitted with a piston that seals in a gas. As the piston
compresses the gas, the pressure increases. Once the boiling point has been
reached, the gas will condense into a liquid. As the piston continues to
compress the liquid, the pressure will increase until the melting point has
been reached. The liquid will then freeze into a solid. This example is for an
isothermal process where the temperature is constant and only the pressure is
changing.
Figure 3. Phase change owing to
rise or fall of pressure
4. Polarization in ferroelectric
materials
The nonlinear nature of ferroelectric materials can
be used to make capacitors with adjustable capacitance. Typically,
a ferroelectric capacitor simply consists of a pair of electrodes
sandwiching a layer of ferroelectric material. The permittivity of
ferroelectrics is not only adjustable but commonly also very high, especially
when close to the phase transition temperature. Because of this, ferroelectric
capacitors are small in physical size compared to dielectric (non-tunable)
capacitors of similar capacitance.
The spontaneous polarization of ferroelectric
materials implies a hysteresis effect which can be used as a memory
function, and ferroelectric capacitors are indeed used to
make ferroelectric RAM for computers and RFID cards. In
these applications thin films of ferroelectric materials are typically used, as
this allows the field required to switch the polarization to be achieved with a
moderate voltage. However, when using thin films a great deal of attention
needs to be paid to the interfaces, electrodes and sample quality for devices to
work reliably.
Ferroelectric materials are required by symmetry
considerations to be also piezoelectric and pyroelectric. The combined
properties of memory, piezoelectricity, and pyroelectricity make ferroelectric capacitors very useful, e.g. for sensor
applications. Ferroelectric capacitors are used in medical ultrasound machines
(the capacitors generate and then listen for the ultrasound ping used to image
the internal organs of a body), high quality infrared cameras (the infrared
image is projected onto a two dimensional array of ferroelectric capacitors
capable of detecting temperature differences as small as millionths of a degree
Celsius), fire sensors, sonar, vibration sensors, and even fuel injectors on
diesel engines.Another idea of recent interest is the ferroelectric tunnel
junction in which a contact is made up by nanometer-thick ferroelectric
film placed between metal electrodes. The thickness of the ferroelectric
layer is small enough to allow tunneling of electrons. The piezoelectric and
interface effects as well as the depolarization field may lead to a giant electroresistance
switching effect.
Yet another burgeoning application is multiferroics, where researchers are looking for ways to couple magnetic and
ferroelectric ordering within a material or heterostructure; there are several
recent reviews on this topic.
Catalytic properties of ferroelectrics have
been studied since 1952 when Parravano observed anomalies in CO oxidation rates
over ferroelectric sodium and potassium niobates near the Curie
temperature of these materials Surface-perpendicular component of the
ferroelectric polarization can dope polarization-dependent charges on surfaces
of ferroelectric materials, changing their chemistry.This opens the possibility
of performing catalysis beyond the limits of the Sabatier
principle. Sabatier principle states that the surface-adsorbates
interaction has to be an optimal amount: not too weak to be inert toward the
reactants and not too strong to poison the surface and avoid desorption of the
products: a compromise situation. This set of optimum interactions is usually
referred to as "top of the volcano" in activity volcano plots.On the
other hand, ferroelectric polarization-dependent chemistry can offer the
possibility of switching the surface — adsorbates interaction from
strong adsorption to strong desorption, thus a compromise
between desorption and adsorption is no longer needed.Ferroelectric
polarization can also act as an energy harvester. Polarization
can help the separation of photo-generated electron-hole pairs, leading to
enhanced photocatalysis.Also, due to pyroelectric and piezoelectric effects under varying temperature
(heating/cooling cycles) or varying strain (vibrations) conditions extra
charges can appear on the surface and drive various (electro) chemical
reactions forward.
Hysteresis
loop
a
cycle of alternating changes involving elastic, magnetic, or dielectric
hysteresis is called hysteresis loop. In order to understand the hysteresis
loop of ferroelectric materials, we deduced a novel model combined with
the electric field, the temperature and the stress as a united parameter for
describing the diversified hysteresis loops. This model indicated that the
hysteresis loop of ferroelectric materials can be determined by the saturation
of polarization, the coercive field and the electric susceptibility as well as
the equivalent energy field analytically.
Figure 4. Ferroelectric (P-E)
hysteresis loop.
Dielectric
hysteresis was discovered by Valasek and may be defined as an effect in a
dielectric material similar to the hysteresis found in magnetic materials. In
this case, the electric displacement D for one direction in the crystal
(parallel to the x axis) was not only determined by the
applied field E, but also depended on its previous values. At low electric
fields and at very high electric fields a ferroelectric behaves like an
ordinary dielectric with a high dielectric constant, but at the coercive
field Ec polarization reversal occurs and induces a large
dielectric non-linearity.
At zero
field (E0) the electric displacement within a single domain has two
values (-Pr and +Pr), representing the opposite
orientations of the spontaneous polarization. In a multiple-domain crystal
the average zero-field displacement can have any value between these two
extremes (-Pr < D < +Pr). Ferroelectric
materials have a spontaneous electric polarization ,
i.e.,
they naturally possess dipole moment that add up in the direction normal to a
flat surface to provide a net electrical polarization. While most materials can
be polarized by applying an external electric field, not all have a spontaneous
polarization (nonzero polarization even when the applied field is zero). When
induced polarization is linearly proportional to the applied external electric
field, it is called dielectric polarization and the materials that exhibit this
behavior are called dielectrics. The slope of the polarization curve is defined
as dielectric
permittivity
of the material. In some materials, however, the relationship between the
induced polarization and the external electric field is nonlinear. Such
materials are termed as paraelecric.
Function
of the external electric field as the slope of the polarization curve varies
with change in electric field. Ferroelectric materials, on the other hand,
demonstrate the peculiar polarization curve, which is characterized by a hysteresis
loop. It can be noted that the electric polarization of such materials is
dependent not only on the directionand magnitude of the instantaneous electric
field, but also on the history of polarization. The electric polarization of
ferroelectric materials has nonzero value when the applied electric field is
zero. Also, the direction of the polarization in ferroelectric materials
switches when the direction of the applied alternating electric field is
reversed. However, reversal is often accompanied by some hysteresis, which
leads to the phenomenon of ferroelectric hysteresis.
Most
ferroelectric materials exhibit a transition temperature (called Curie point),
where the spontaneous polarization of a ferroelectric material drops to zero.
Increasing the temperature above the Curie point causes the ferroelectric
material to transition into a nonferroelectric or paraelectric phase.
polarization vs electric field curves of a ferroelectric material before and
after the ferroelectric to paraelectric phase transition. Change in
polarization of a ferroelectric material with respect to temperatureat
different applied electric fields. Blue curve depicts polarization vs
temperature relationship, when no electric field is applied. Increasing the
temperature reduces the polarization, and when temperate is equal to the Curie
temperature, polarization reduces to zero. Curie temperature of the
ferroelectric materials is of special interest in the field of ferroelectric thermal energy
harvesting because the pyroelectric coefficient (gradient of polarization vs
temperature, dPdT) is high near Curie temperature.
Types
of ferroelectric materials
Perovskite
ferroelectric materials
These
materials have perovskite structures, like BaTiO3, whose general
chemical formula is ABO3, where A and B atoms are cations. Normally
the A cation has radius of 1.2–1.6 Å and B cation has one of 0.6–0.7 Å. A atoms
are positioned at the cube corner and oxygen atoms are positioned at the face
center and form an octahedron surrounding the B atom which is positioned at the
body center. Under electric field, the position of B cation shifts, then the
geometrically unbalanced electrical charge forms a dipole moment.
Ilmenite
ferroelectric materials
The
ilmenite ferroelectric materials have the same chemical formula as perovskite
materials, ABO3, e.g. LiNbO3 and LiTaO3.
However, the A cation is too small to fill the position of the perovskite
crystal coordinate. Oxygen atoms comprise hexagonal close-packed layer, and A
and B atoms are positioned at the octahedron site between layers.
Polymeric
ferroelectric materials
The
first discovered and the most representative polymeric ferroelectric material
is polyvinylidene fluoride (PVDF). Polymers have long carbon backbone, so their
structure is complex and has a lot of configurations depending on whether the
neighboring carbon bonds are trans or gauche. The β-phase has all trans
configuration. The fluorine atoms have the strongest electronegativity,
resulting C–F polar bond so that PVDF molecule has dipole moment in
perpendicular direction to its carbon chain. However, the dipole moments of the
pristine polymer chains are not arranged in single direction, so the net
polarization is zero. Therefore, a strong electric field is required to arrange
the dipole moments of chains, which is called electrical poling. In addition,
copolymer with trifluoroethylene (10–46%) helps the formation of β-phase.
Energy
harvesting utilizes various energy sources, including mechanical, thermal, and
solar energies. Each energy source can be converted to electrical energy
through each coupled-physical phenomenon, but basic principle is same: the
variation of the internal dipole moment or potential. Therefore, the
introduction of ferroelectric materials to energy harvesters can increase
dipole moments and potential in the devices due to the strong polarization in
the ferroelectric materials so that conversion efficiency can be enhanced.
Piezoelectric
effect is a coupling phenomenon of mechanical strain and electric charge
separation. When mechanical stress is applied to the materials which have
asymmetric crystal structures, the crystal structure is deformed, resulting in
a separation of the center of charges. The charge separation induces a dipole
moment that is proportional to stress or strain (direct piezoelectric effect).
Since this was first discovered by Pierre Curie and Jacques Curie in 1880 using
quartz and Rochell salt, many piezoelectric materials, such as PbZr0.52Ti0.48O3 (PZT),
BaTiO3 (BTO), ZnO, and PVDF have been studied and had their
piezoelectric constants measured.
Due to
the coupling effect of mechanical strain and electric charge separation in
piezoelectric effect, the piezoelectric effect has been exploited to convert
mechanical energy. Energy harvesting using the piezoelectric effect was first
introduced by Wang’s group using piezoelectric semiconducting ZnO nanowires.
Since then, a lot of research on piezoelectric energy harvesters, called
piezoelectric nanogenerators (PENG), has been reported. Many researchers have
attempted to enhance the output performance of PENG by designing new devices.
Among the various factors to increase output performance, the development of a
material with a high piezoelectric coefficient is the most important.
Ferroelectric
materials have piezoelectricity as well, and their piezoelectric coefficient is
relatively high (d33 of PMN-PT ferroelectric ceramic is 630
pC/N). Initially, dipole moments in ferroelectric material are randomly aligned
so it has neither polarization nor piezoelectricity. However, once strong
electric field is applied, dipole moments are aligned in a single direction and
piezoelectricity is formed. Therefore, PENGs made of ferroelectric materials
have been reported.
Ferroelectric
ceramics with perovskite structure have relatively high piezoelectric
coefficients, so PENGs made of piezoelectric ceramics, such as BTO, PZT, ZnSnO3,
and Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT),
show high output powers. However, ceramic is a rigid material so it should be
deposited as a thin film in order to be flexible. Park et al. reported a thin
film BTO based PENG in 2010. The MIM (metal–insulator-metal, Au/BTO/Pt)
structure was patterned with a ribbon structure (300 μm × 50 μm)
array by photolithography and gas-based ICP-RIE etching. The patterned MIM
structure was transferred onto a plastic substrate (Kapton film) using a
polydimethylsiloxane (PDMS) stamp. Finally, SU8 epoxy was spin-coated on MIM
structure and a metal grid was connected to the top and bottom electrodes.
The
PENG with BTO thin film on flexible substrate is driven by compressive force
and bending. Then, tensile stress is applied to BTO film. The deformation of
the BTO film by tensile stress generates piezoelectric polarization and induces
charge induction in electrode resulting in electrical current (Fig. 2c).
Figure 2d shows
the output current (~ 10 nA) and voltage (0.3 V) of flexible BTO PENG with
1350 MIM structure arrays by periodic bending and unbending. The BTO based PENG
shows the possibility of ferroelectric ceramic material for flexible and high
output energy harvesters through thin film deposition and the transferring
technique.
Ferroelectric-polymer
composite PENG
In
order to further enhance output power, other ferroelectric materials with
higher piezoelectric coefficients such as PZT and PMN-PT were used for energy
harvesters. The high-power PENGs with thin film ferroelectric materials were
integrated on flexible plastic substrate and utilized for bio-implantable
devices. Although the output power of PENGs successfully increased up to an
open-circuit voltage of 200 V and short-circuit current density of 150
μA/cm2, ferroelectric materials are deposited as thin film,
resulting in the limitation of output power and fabricating large area devices.
Moreover, rigid ferroelectric thin film cannot be used under strong force.
In
order to solve these problems, polymer supported ferroelectric powders in PENG
have been reported. Polymer matrix-ferroelectric powder composite has
advantages in large area fabrication due to easy fabrication process and
low-cost, high stress application, and mechanical durability. In particular, ZnSnO3 is
an ecofriendly and biofriendly lead-free piezoelectric/ferroelectric material
and used as a high-power energy harvester without electrical poling. The XRD
pattern indicates that the crystal structure of ZnSnO3 is
rhombohedral. The rhombohedral structure of ZnSnO3 is comprised
of two octahedral ZnO6 and SnO6. As illustrated,
each octahedron has three long bonds and three short bonds, so Sn and Zn atoms
are placed on a deviated position from the octahedron center, resulting in
non-centrosymmetry and ferroelectricity.
Polymeric
ferroelectric based PENG
The
ferroelectric powder-embedded polymer composite shows high output power and
mechanical stability but is not acceptable for low magnitude and frequency
input force. Poly(vinylidene fluoride) (PVDF) is one of the representative
ferroelectric polymers. Its copolymer
poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) has a high
piezoelectric coefficient of d31 = 25 pC/N, d33 = 40 pC/N
and its flexibility is promising for application in fully flexible, foldable,
twistable, and stretchable PENG. Previously reported PENGs comprised of plastic
substrate and metal electrode have limitation in flexibility and
stretchability. However, the semi-metallic two-dimensional carbon material,
graphene, is a promising electrode for flexible electrode due to its high
mechanical durability and elasticity (1 TPa).
Lee et
al. developed highly sensitive P(VDF-TrFE) PENG, which is comprised of PDMS
polymer substrate and P(VDF-TrFE) sandwiched with graphene electrodes. The
output voltage of the highly sensitive PENG made of P(VDF-TrFE) and graphene
electrodes were investigated and compared to PENG with PEN substrate under
application of sound wave (82–110 dB at 100 Hz) . P(VDF-TrFE) PENG
shows enhancement of voltage from 50 mV to 600 mV because of its
highly sensitive response to input sound wave. In contrast, the PENG on the PEN
substrate has no output signal at low power of sound wave (85–95 dB) but
increases from 10 mV to 22 mV at 100–110 dB.
Triboelectric
effect is the charge exchange between two materials through contact or rubbing
each other. Although the detailed mechanism of triboelectric effect remains
elusive, there are four possibilities: electron transfer, ion transfer,
material transfer, and mechano-chemistry. Triboelectric charging occurs by
complex of these four phenomena. Numerical prediction of triboelectric charging
is not possible yet because there are too many factors to determine
triboelectric effect, but triboelectric charge polarity is predictable using
triboelectric series.
Static
charges by triboelectric effect have been considered as disturbance to human
health and especially industry because the charges have an effect on electric
devices. Therefore, there have been efforts to prevent the triboelectric
effect. However, prof. Zhong Lin Wang’s group invented a new type of energy
harvester called a triboelectric nanogenerator (TENG) which exploits the
triboelectric effect in 2012. TENG extend energy harvesting field more widely
due to its simple structure, light weight, and high output power.
Controllable
charge transfer by ferroelectric polarization
In
TENG, there is charge transfer between two materials. Generally, the amount and
polarity of charge is determined by material properties, especially work
function. However, work function is hardly modulated, so controlling
triboelectric effect of existing material is very limited. Introduction of
ferroelectric material can control and increase triboelectric charging behavior
due to its switchable and controllable polarization.
Atomic
force microscopy (AFM) is a very good tool for investigating ferroelectric and
triboelectric behavior, because both electrical polarizing and characterizing
ferroelectric polarization are available. Lee et al. investigated triboelectric
behavior of P(VDF-TrFE) polymer affected by triboelectric polarization. First,
P(VDF-TrFE) surface was polarized by applying positive and negative bias
voltage using AFM tip. During the electrical poling process, the charges are
over-injected from the AFM tip, so the surface potential image at the initial
state polarity opposite to ferroelectric polarization. However, the
over-injected charges are discharged as time goes on and surface potential
shows ferroelectric polarization. After surface potential become stable, the
P(VDF-TrFE) surface was rubbed with AFM tip to investigate triboelectric
effect. The surface potential on the region which has each different direction
of ferroelectric polarization became enhanced after rubbing. Even at a stable
state, there are screen charges on the polarized region, and these charges are
transferred to the AFM tip during the rubbing process.
Applications of ferroelectrics
Conventional ferroelectric materials are normally used in sensors and
actuators, memory devices, and field effect transistors, etc. Recent progress
in this area showed that ferroelectric materials can harvest energy from
multiple sources including mechanical energy, thermal fluctuations, and light. Beginning
with the fundamentals of ferroelectric materials, Ferroelectric Materials for
Energy Applications offers in-depth chapter coverage of: piezoelectric energy
generation; ferroelectric photovoltaics; organic-inorganic hybrid perovskites
for solar energy conversion; ferroelectric ceramics and thin films in electric
energy storage; ferroelectric polymer composites in electric energy storage;
pyroelectric energy harvesting; ferroelectrics in electrocaloric cooling;
ferroelectric in photocatalysis; The bi-stable polarization of ferroelectrics
makes them useful for binary memory systems. There are volatile and
non-volatile memory devices in erasable semiconductor memories.
The nonlinear nature of
ferroelectric materials can be used to make capacitors with adjustable
capacitance. Typically, a ferroelectric capacitor simply consists of
a pair of electrodes sandwiching a layer of ferroelectric material. The
permittivity of ferroelectrics is not only adjustable but commonly also very
high, especially when close to the phase transition temperature. Because of
this, ferroelectric capacitors are small in physical size compared to dielectric
(non-tunable) capacitors of similar capacitance.
The spontaneous polarization of
ferroelectric materials implies a hysteresis effect which can be used
as a memory function, and ferroelectric capacitors are indeed used to
make ferroelectric RAM for computers and RFID cards. In
these applications thin films of ferroelectric materials are typically used, as
this allows the field required to switch the polarization to be achieved with a
moderate voltage. However, when using thin films a great deal of attention
needs to be paid to the interfaces, electrodes and sample quality for devices
to work reliably.
Ferroelectric materials are required
by symmetry considerations to be also piezoelectric and pyroelectric. The
combined properties of memory, piezoelectricity, and pyroelectricity make ferroelectric capacitors
very useful, e.g. for sensor applications. Ferroelectric capacitors are used in
medical ultrasound machines (the capacitors generate and then listen for the
ultrasound ping used to image the internal organs of a body), high quality
infrared cameras (the infrared image is projected onto a two dimensional array
of ferroelectric capacitors capable of detecting temperature differences as
small as millionths of a degree Celsius), fire sensors, sonar, vibration
sensors, and even fuel injectors on diesel engines.
Another idea is
the ferroelectric tunnel junction in which a contact is made up by
nanometer-thick ferroelectric film placed between metal electrodes. The
thickness of the ferroelectric layer is small enough to allow tunneling of
electrons. The piezoelectric and interface effects as well as the
depolarization field may lead to a giant electroresistance (GER) switching effect.
Yet another burgeoning application
is multiferroics, where researchers are looking for
ways to couple magnetic and ferroelectric ordering within a material or
heterostructure; there are several recent reviews on this topic.
Catalytic properties of
ferroelectrics have been studied when Parravano observed anomalies in CO
oxidation rates over ferroelectric sodium and potassium niobates near
the Curie temperature of these materials. Surface-perpendicular
component of the ferroelectric polarization can dope polarization-dependent
charges on surfaces of ferroelectric materials, changing their
chemistry. This opens the possibility of performing catalysis beyond the
limits of the Sabatier principle. Sabatier principle states that the
surface-adsorbates interaction has to be an optimal amount: not too weak to be
inert toward the reactants and not too strong to poison the surface and avoid
desorption of the products: a compromise situation. This set of optimum
interactions is usually referred to as "top of the volcano" in activity
volcano plots. On the other hand, ferroelectric polarization-dependent
chemistry can offer the possibility of switching the surface—adsorbates
interaction from strong adsorption to strong desorption, thus a
compromise between desorption and adsorption is no longer needed. Ferroelectric
polarization can also act as an energy harvester. Polarization
can help the separation of photo-generated electron-hole pairs, leading to
enhanced photocatalysis.Also, due to pyroelectric and piezoelectric effects
under varying temperature (heating/cooling cycles) or varying strain
(vibrations) conditions extra charges can appear on the surface and drive
various (electro)chemical reactions forward.
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