A piezoelectric sensor is a device that uses the piezoelectric effect, to measure changes in pressure, acceleration, temperature, strain, or force by converting them ...
Piezoelectricity
From Wikipedia, the free encyclopedia
Piezoelectricity is the
electric charge that accumulates in certain solid materials (such as
crystals, certain
ceramics, and biological matter such as bone,
DNA and various
proteins)
[1] in response to applied mechanical
stress. The word
piezoelectricity means electricity resulting from pressure. It is derived from the
Greek piezo (πιέζω) or
piezein (πιέζειν), which means to squeeze or press, and
electric or
electron (
ήλεκτρον), which means
amber, an ancient source of electric charge.
[2] Piezoelectricity was discovered in 1880 by French physicists
Jacques and
Pierre Curie.
[3]
The
piezoelectric effect
is understood as the linear electromechanical interaction between the
mechanical and the electrical state in crystalline materials with no
inversion symmetry.
[4] The piezoelectric effect is a
reversible process
in that materials exhibiting the direct piezoelectric effect (the
internal generation of electrical charge resulting from an applied
mechanical
force)
also exhibit the reverse piezoelectric effect (the internal generation
of a mechanical strain resulting from an applied electrical field). For
example,
lead zirconate titanate
crystals will generate measurable piezoelectricity when their static
structure is deformed by about 0.1% of the original dimension.
Conversely, those same crystals will change about 0.1% of their static
dimension when an external electric field is applied to the material.
The inverse piezoelectric effect is used in production of ultrasonic
sound waves.
[5]
Piezoelectricity is found in useful applications, such as the
production and detection of sound, generation of high voltages,
electronic frequency generation,
microbalances, to drive an
ultrasonic nozzle,
and ultrafine focusing of optical assemblies. It is also the basis of a
number of scientific instrumental techniques with atomic resolution,
the
scanning probe microscopies, such as
STM,
AFM,
MTA,
SNOM, etc., and everyday uses, such as acting as the ignition source for
cigarette lighters, push-start
propane barbecues, and
quartz watches.
History
Discovery and early research
The
pyroelectric effect, by which a material generates an
electric potential in response to a temperature change, was studied by
Carl Linnaeus and
Franz Aepinus in the mid-18th century. Drawing on this knowledge, both
René Just Haüy and
Antoine César Becquerel posited a relationship between mechanical stress and electric charge; however, experiments by both proved inconclusive.
[6]
The first demonstration of the direct piezoelectric effect was in 1880 by the brothers
Pierre Curie and
Jacques Curie.
[7]
They combined their knowledge of pyroelectricity with their
understanding of the underlying crystal structures that gave rise to
pyroelectricity to predict crystal behavior, and demonstrated the effect
using crystals of
tourmaline,
quartz,
topaz,
cane sugar, and
Rochelle salt (sodium potassium tartrate tetrahydrate). Quartz and Rochelle salt exhibited the most piezoelectricity.
A piezoelectric disk generates a voltage when deformed (change in shape is greatly exaggerated)
The Curies, however, did not predict the converse piezoelectric
effect. The converse effect was mathematically deduced from fundamental
thermodynamic principles by
Gabriel Lippmann in 1881.
[8] The Curies immediately confirmed the existence of the converse effect,
[9]
and went on to obtain quantitative proof of the complete reversibility
of electro-elasto-mechanical deformations in piezoelectric crystals.
For the next few decades, piezoelectricity remained something of a
laboratory curiosity. More work was done to explore and define the
crystal structures that exhibited piezoelectricity. This culminated in
1910 with the publication of
Woldemar Voigt's Lehrbuch der Kristallphysik (Textbook on Crystal Physics),
[10]
which described the 20 natural crystal classes capable of
piezoelectricity, and rigorously defined the piezoelectric constants
using
tensor analysis.
World War I and post-war
The first practical application for piezoelectric devices was
sonar, first developed during
World War I. In
France in 1917,
Paul Langevin and his coworkers developed an
ultrasonic submarine detector.
[11] The detector consisted of a
transducer, made of thin quartz crystals carefully glued between two steel plates, and a
hydrophone to detect the returned
echo.
By emitting a high-frequency pulse from the transducer, and measuring
the amount of time it takes to hear an echo from the sound waves
bouncing off an object, one can calculate the distance to that object.
The use of piezoelectricity in sonar, and the success of that
project, created intense development interest in piezoelectric devices.
Over the next few decades, new piezoelectric materials and new
applications for those materials were explored and developed.
Piezoelectric devices found homes in many fields. Ceramic
phonograph
cartridges simplified player design, were cheap and accurate, and made
record players cheaper to maintain and easier to build. The development
of the ultrasonic transducer allowed for easy measurement of viscosity
and elasticity in fluids and solids, resulting in huge advances in
materials research. Ultrasonic
time-domain reflectometers
(which send an ultrasonic pulse through a material and measure
reflections from discontinuities) could find flaws inside cast metal and
stone objects, improving structural safety.
World War II and post-war
During
World War II, independent research groups in the
United States,
Russia, and
Japan discovered a new class of synthetic materials, called
ferroelectrics, which exhibited piezoelectric constants many times higher than natural materials. This led to intense research to develop
barium titanate and later lead zirconate titanate materials with specific properties for particular applications.
One significant example of the use of piezoelectric crystals was
developed by Bell Telephone Laboratories. Following World War I,
Frederick R. Lack, working in radio telephony in the engineering
department, developed the “AT cut” crystal, a crystal that operated
through a wide range of temperatures. Lack's crystal didn't need the
heavy accessories previous crystal used, facilitating its use on
aircraft. This development allowed Allied air forces to engage in
coordinated mass attacks through the use of aviation radio.
Development of piezoelectric devices and materials in the United
States was kept within the companies doing the development, mostly due
to the wartime beginnings of the field, and in the interests of securing
profitable patents. New materials were the first to be developed —
quartz crystals were the first commercially exploited piezoelectric
material, but scientists searched for higher-performance materials.
Despite the advances in materials and the maturation of manufacturing
processes, the United States market did not grow as quickly as Japan's
did. Without many new applications, the growth of the United States'
piezoelectric industry suffered.
In contrast, Japanese manufacturers shared their information, quickly
overcoming technical and manufacturing challenges and creating new
markets. In Japan, a temperature stable crystal cut was developed by
Issac Koga.
Japanese efforts in materials research created piezoceramic materials
competitive to the U.S. materials but free of expensive patent
restrictions. Major Japanese piezoelectric developments included new
designs of piezoceramic filters for radios and televisions, piezo
buzzers and audio transducers that can connect directly to electronic
circuits, and the
piezoelectric igniter,
which generates sparks for small engine ignition systems (and gas-grill
lighters) by compressing a ceramic disc. Ultrasonic transducers that
transmit sound waves through air had existed for quite some time but
first saw major commercial use in early television remote controls.
These transducers now are mounted on several
car models as an
echolocation device, helping the driver determine the distance from the rear of the car to any objects that may be in its path.
Mechanism
Piezoelectric plate used to convert
audio signal to sound waves
The nature of the piezoelectric effect is closely related to the occurrence of
electric dipole moments in solids. The latter may either be induced for
ions on
crystal lattice sites with asymmetric charge surroundings (as in
BaTiO3 and
PZTs) or may directly be carried by molecular groups (as in
cane sugar). The dipole density or
polarization (dimensionality [Cm/m
3] ) may easily be calculated for
crystals by summing up the dipole moments per volume of the crystallographic
unit cell.
[12] As every dipole is a vector, the dipole density
P is a
vector field.
Dipoles near each other tend to be aligned in regions called Weiss
domains. The domains are usually randomly oriented, but can be aligned
using the process of
poling (not the same as
magnetic poling),
a process by which a strong electric field is applied across the
material, usually at elevated temperatures. Not all piezoelectric
materials can be poled.
[13]
Of decisive importance for the piezoelectric effect is the change of polarization
P when applying a
mechanical stress.
This might either be caused by a re-configuration of the
dipole-inducing surrounding or by re-orientation of molecular dipole
moments under the influence of the external stress. Piezoelectricity may
then manifest in a variation of the polarization strength, its
direction or both, with the details depending on: 1. the orientation of
P within the crystal; 2.
crystal symmetry; and 3. the applied mechanical stress. The change in
P appears as a variation of surface
charge density upon the crystal faces, i.e. as a variation of the
electric field extending between the faces caused by a change in dipole density in the bulk. For example, a 1 cm
3 cube of quartz with 2 kN (500 lbf) of correctly applied force can produce a voltage of 12500
V.
[14]
Piezoelectric materials also show the opposite effect, called the
converse piezoelectric effect, where the application of an electrical field creates mechanical deformation in the crystal.
Mathematical description
Linear piezoelectricity is the combined effect of
- The linear electrical behavior of the material:
- where D is the electric charge density displacement (electric displacement), ε is permittivity (free-body dielectric constant), E is electric field strength, and
.
- where S is strain, s is compliance under short-circuit conditions, T is stress, and
.
These may be combined into so-called
coupled equations, of which the
strain-charge form is:
[15]
In matrix form,
![\begin{align}
\{S\} &= \left [s^E \right ]\{T\}+[d^t]\{E\} \\
\{D\} &= [d]\{T\}+\left [ \varepsilon^T \right ] \{E\} \,,
\end{align}](https://upload.wikimedia.org/math/9/e/0/9e03316ded4916b21fe70c6284c97918.png)
where
![[d]](https://upload.wikimedia.org/math/5/5/b/55bbeb5fefa388fc850000a090efc0eb.png)
is the matrix for the direct piezoelectric effect and
![[d^t]](https://upload.wikimedia.org/math/d/7/d/d7d9045fa870eb6f063228c4ad6224b2.png)
is the matrix for the converse piezoelectric effect. The superscript
E indicates a zero, or constant, electric field; the superscript
T indicates a zero, or constant, stress field; and the superscript
t stands for
transposition of a
matrix.
Notice that the third order tensor
maps vectors into symmetric matrices. There are no non-trivial
rotation-invariant tensors that have this property, which is why there
are no isotropic piezoelectric materials.
The strain-charge for a material of the
4mm (C
4v)
crystal class (such as a poled piezoelectric ceramic such as tetragonal PZT or BaTiO
3) as well as the
6mm crystal class may also be written as (ANSI IEEE 176):
where the first equation represents the relationship for the converse
piezoelectric effect and the latter for the direct piezoelectric
effect.
[16]
Although the above equations are the most used form in literature, some comments about the notation are necessary. Generally,
D and
E are
vectors, that is,
Cartesian tensor of rank-1; and permittivity ε is Cartesian tensor of rank 2. Strain and stress are, in principle, also rank-2
tensors.
But conventionally, because strain and stress are all symmetric
tensors, the subscript of strain and stress can be re-labeled in the
following fashion: 11 → 1; 22 → 2; 33 → 3; 23 → 4; 13 → 5; 12 → 6.
(Different convention may be used by different authors in literature.
Say, some use 12 → 4; 23 → 5; 31 → 6 instead.) That is why
S and
T appear to have the "vector form" of six components. Consequently,
s appears to be a 6 by 6 matrix instead of a rank-4 tensor. Such a re-labeled notation is often called
Voigt notation. Whether the shear strain components
are tensor components or engineering strains is another question. In
the equation above, they must be engineering strains for the 6,6
coefficient of the compliance matrix to be written as shown, i.e.,
. Engineering shear strains are double the value of the corresponding tensor shear, such as
and so on. This also means that
, where
is the shear modulus.
In total, there are four piezoelectric coefficients,
,
,
, and
defined as follows:
where the first set of four terms corresponds to the direct
piezoelectric effect and the second set of four terms corresponds to the
converse piezoelectric effect.
[17]
For those piezoelectric crystals for which the polarization is of the
crystal-field induced type, a formalism has been worked out that allows
for the calculation of piezoelectrical coefficients
from electrostatic lattice constants or higher-order
Madelung constants.
[12]
Crystal classes
Any spatially separated charge will result in an
electric field, and therefore an
electric potential. Shown here is a standard dielectric in a
capacitor.
In a piezoelectric device, mechanical stress, instead of an externally
applied voltage, causes the charge separation in the individual atoms of
the material.
Of the 32
crystal classes, 21 are non-
centrosymmetric (not having a centre of symmetry), and of these, 20 exhibit direct piezoelectricity
[18] (the 21st is the cubic class 432). Ten of these represent the polar crystal classes,
[19]
which show a spontaneous polarization without mechanical stress due to a
non-vanishing electric dipole moment associated with their unit cell,
and which exhibit pyroelectricity. If the dipole moment can be reversed
by the application of an electric field, the material is said to be
ferroelectric.
- Polar crystal classes: 1, 2, m, mm2, 4, 4 mm, 3, 3m, 6, 6 mm.
- Piezoelectric crystal classes: 1, 2, m, 222, mm2, 4, 4, 422, 4 mm, 42m, 3, 32, 3m, 6, 6, 622, 6 mm, 62m, 23, 43m.
For polar crystals, for which
P ≠ 0 holds without
applying a mechanical load, the piezoelectric effect manifests itself by
changing the magnitude or the direction of
P or both.
For the non-polar, but piezoelectric crystals, on the other hand, a polarization
P
different from zero is only elicited by applying a mechanical load. For
them the stress can be imagined to transform the material from a
non-polar crystal class (
P =0) to a polar one,
[12] having
P ≠ 0.
Materials
Many materials, both natural and synthetic, exhibit piezoelectricity:
Naturally occurring crystals
The action of piezoelectricity in Topaz can probably be attributed to
ordering of the (F,OH) in its lattice, which is otherwise
centrosymmetric: Orthorhombic Bipyramidal (mmm). Topaz has anomalous
optical properties which are attributed to such ordering.
[22]
Bone
Dry
bone exhibits some piezoelectric properties. Studies of Fukada
et al. showed that these are not due to the
apatite crystals, which are centrosymmetric, thus non-piezoelectric, but due to
collagen.
Collagen exhibits the polar uniaxial orientation of molecular dipoles
in its structure and can be considered as bioelectret, a sort of
dielectric material exhibiting quasipermanent space charge and dipolar
charge. Potentials are thought to occur when a number of collagen
molecules are stressed in the same way displacing significant numbers of
the charge carriers from the inside to the surface of the specimen.
Piezoelectricity of single individual collagen fibrils was measured
using piezoresponse force microscopy, and it was shown that collagen
fibrils behave predominantly as shear piezoelectric materials.
[23]
The piezoelectric effect is generally thought to act as a biological force sensor.
[24][25] This effect was exploited by research conducted at the
University of Pennsylvania
in the late 1970s and early 1980s, which established that sustained
application of electrical potential could stimulate both resorption and
growth (depending on the polarity) of bone in-vivo.
[26]
Further studies in the 1990s provided the mathematical equation to
confirm long bone wave propagation as to that of hexagonal (Class 6)
crystals.
[27]
Other natural materials
Biological materials exhibiting piezoelectric properties include:
Synthetic crystals
Synthetic ceramics
Tetragonal unit cell of lead titanate
Ceramics with randomly oriented grains must be ferroelectric to exhibit piezoelectricity.
[29]
The macroscopic piezoelectricity is possible in textured
polycrystalline non–ferroelectric piezoelectric materials, such as AlN
and ZnO. The family of ceramics with
perovskite,
tungsten-
bronze and related structures exhibits piezoelectricity:
- Barium titanate (BaTiO3)—Barium titanate was the first piezoelectric ceramic discovered.
- Lead zirconate titanate (Pb[ZrxTi1−x]O3 0≤x≤1)—more commonly known as PZT, lead zirconate titanate is the most common piezoelectric ceramic in use today.
- Potassium niobate (KNbO3)
- Sodium tungstate (Na2WO3)
- Ba2NaNb5O5
- Pb2KNb5O15
- Zinc oxide (ZnO)–Wurtzite structure.
While single crystals of ZnO are piezoelectric and pyroelectric,
polycrystalline (ceramic) ZnO with randomly oriented grains exhibits
neither piezoelectric nor pyroelectric effect. Not being ferroelectric,
polycrystalline ZnO cannot be poled like barium titanate or PZT.
Ceramics and polycrystalline thin films of ZnO may exhibit macroscopic
piezoelectricity and pyroelectricity only if they are textured
(grains are preferentially oriented), such that the piezoelectric and
pyroelectric responses of all individual grains do not cancel. This is
readily accomplished in polycrystalline thin films.[16]
Lead-free piezoceramics
More recently, there is growing concern regarding the toxicity in lead-containing devices driven by the result of
restriction of hazardous substances directive
regulations. To address this concern, there has been a resurgence in
the compositional development of lead-free piezoelectric materials.
- Sodium potassium niobate ((K,Na)NbO3).
This material is also known as NKN. In 2004, a group of Japanese
researchers led by Yasuyoshi Saito discovered a sodium potassium niobate
composition with properties close to those of PZT, including a high
.[30] Certain compositions of this material have been shown to retain a high mechanical quality factor (
)
with increasing vibration levels, whereas the mechanical quality factor
of hard PZT degrades in such conditions. This fact makes NKN a
promising replacement for high power resonance applications, such as
piezoelectric transformers.[31]
- Bismuth ferrite (BiFeO3) is also a promising candidate for the replacement of lead-based ceramics.
- Sodium niobate NaNbO3
- Bismuth titanate Bi4Ti3O12
- Sodium bismuth titanate Na0.5Bi0.5TiO3
So far, neither the environmental impact nor the stability of supplying these substances have been confirmed.
III-V and II-VI semiconductors
A piezoelectric potential can be created in any bulk or
nanostructured semiconductor crystal having non central symmetry, such
as the Group III-V and II-VI materials, due to polarization of ions
under applied stress and strain. This property is common to both the
zincblende and
wurtzite crystal structures. To first order, there is only one independent piezoelectric coefficient in
zincblende, called e
14, coupled to shear components of the strain. In
wurtzite, there are instead three independent piezoelectric coefficients: e
31, e
33 and e
15. The semiconductors where the strongest piezoelectricity is observed are those commonly found in the
wurtzite structure, i.e. GaN, InN, AlN and ZnO. ZnO is the most used material in the recent field of
piezotronics.
Since 2006, there have also been a number of reports of strong
non linear piezoelectric effects in polar semiconductors.
[32]
Such effects are generally recognized to be at least important if not
of the same order of magnitude as the first order approximation.
Polymers
- Polyvinylidene fluoride
(PVDF): PVDF exhibits piezoelectricity several times greater than
quartz. Unlike ceramics, where the crystal structure of the material
creates the piezoelectric effect, in polymers the intertwined long-chain
molecules attract and repel each other when an electric field is
applied.
Organic nanostructures
A strong shear piezoelectric activity was observed in self-assembled
diphenylalanine peptide nanotubes (PNTs), indicating electric
polarization directed along the tube axis. Comparison with LiNbO
3
and lateral signal calibration yields sufficiently high effective
piezoelectric coefficient values of at least 60 pm/V (shear response for
tubes of ≈200 nm in diameter). PNTs demonstrate linear deformation
without irreversible degradation in a broad range of driving voltages.
[33]
Application
Currently, industrial and manufacturing is the largest application
market for piezoelectric devices, followed by the automotive industry.
Strong demand also comes from medical instruments as well as information
and telecommunications. The global demand for piezoelectric devices was
valued at approximately US$14.8 billion in 2010. The largest material
group for piezoelectric devices is piezocrystal, and piezopolymer is
experiencing the fastest growth due to its low weight and small size.
[34]
Piezoelectric crystals are now used in numerous ways:
High voltage and power sources
Direct piezoelectricity of some substances, like quartz, can generate
potential differences of thousands of volts.
- The best-known application is the electric cigarette lighter:
pressing the button causes a spring-loaded hammer to hit a
piezoelectric crystal, producing a sufficiently high voltage electric
current that flows across a small spark gap, thus heating and igniting the gas. The portable sparkers used to ignite gas stoves work the same way, and many types of gas burners now have built-in piezo-based ignition systems.
- A similar idea is being researched by DARPA in the United States in a project called Energy Harvesting, which includes an attempt to power battlefield equipment by piezoelectric generators embedded in soldiers'
boots. However, these energy harvesting sources by association have an
impact on the body. DARPA's effort to harness 1–2 watts from continuous
shoe impact while walking were abandoned due to the impracticality and
the discomfort from the additional energy expended by a person wearing
the shoes. Other energy harvesting ideas include harvesting the energy
from human movements in train stations or other public places[35][36] and converting a dance floor to generate electricity.[37]
Vibrations from industrial machinery can also be harvested by
piezoeletric materials to charge batteries for backup supplies or to
power low-power microprocessors and wireless radios.[38]
- A piezoelectric transformer
is a type of AC voltage multiplier. Unlike a conventional transformer,
which uses magnetic coupling between input and output, the piezoelectric
transformer uses acoustic coupling. An input voltage is applied across a short length of a bar of piezoceramic material such as PZT,
creating an alternating stress in the bar by the inverse piezoelectric
effect and causing the whole bar to vibrate. The vibration frequency is
chosen to be the resonant frequency of the block, typically in the 100 kilohertz
to 1 megahertz range. A higher output voltage is then generated across
another section of the bar by the piezoelectric effect. Step-up ratios
of more than 1000:1 have been demonstrated[citation needed]. An extra feature of this transformer is that, by operating it above its resonant frequency, it can be made to appear as an inductive load, which is useful in circuits that require a controlled soft start.[39] These devices can be used in DC-AC inverters to drive cold cathode fluorescent lamps. Piezo transformers are some of the most compact high voltage sources.
Sensors
Many rocket-propelled grenades used a piezoelectric
fuse. For example:
RPG-7[40]
The principle of operation of a piezoelectric
sensor
is that a physical dimension, transformed into a force, acts on two
opposing faces of the sensing element. Depending on the design of a
sensor, different "modes" to load the piezoelectric element can be used:
longitudinal, transversal and shear.
Detection of pressure variations in the form of sound is the most common sensor application, e.g. piezoelectric
microphones (sound waves bend the piezoelectric material, creating a changing voltage) and piezoelectric
pickups for
acoustic-electric guitars. A piezo sensor attached to the body of an instrument is known as a
contact microphone.
Piezoelectric sensors especially are used with high frequency sound
in ultrasonic transducers for medical imaging and also industrial
nondestructive testing (NDT).
For many sensing techniques, the sensor can act as both a sensor and an actuator – often the term
transducer
is preferred when the device acts in this dual capacity, but most piezo
devices have this property of reversibility whether it is used or not.
Ultrasonic transducers, for example, can inject ultrasound waves into
the body, receive the returned wave, and convert it to an electrical
signal (a voltage). Most medical ultrasound transducers are
piezoelectric.
In addition to those mentioned above, various sensor applications include:
- Piezoelectric elements are also used in the detection and generation of sonar waves.
- Piezoelectric materials are used in single-axis and dual-axes tilt sensing.[41]
- Power monitoring in high power applications (e.g. medical treatment, sonochemistry and industrial processing).
- Piezoelectric microbalances are used as very sensitive chemical and biological sensors.
- Piezos are sometimes used in strain gauges.
- A piezoelectric transducer was used in the penetrometer instrument on the Huygens Probe
- Piezoelectric transducers are used in electronic drum pads to detect the impact of the drummer's sticks, and to detect muscle movements in medical acceleromyography.
- Automotive engine management systems
use piezoelectric transducers to detect Engine knock (Knock Sensor,
KS), also known as detonation, at certain hertz frequencies. A
piezoelectric transducer is also used in fuel injection systems to
measure manifold absolute pressure (MAP sensor) to determine engine
load, and ultimately the fuel injectors milliseconds of on time.
- Ultrasonic piezo sensors are used in the detection of acoustic emissions in acoustic emission testing.
Actuators
Metal disk with piezoelectric disk attached, used in a
buzzer
As very high electric fields correspond to only tiny changes in the
width of the crystal, this width can be changed with better-than-
µm precision, making piezo crystals the most important tool for positioning objects with extreme accuracy — thus their use in
actuators. Multilayer ceramics, using layers thinner than
100 µm, allow reaching high electric fields with voltage lower than
150 V. These ceramics are used within two kinds of actuators: direct piezo actuators and
Amplified piezoelectric actuators. While direct actuator's stroke is generally lower than
100 µm, amplified piezo actuators can reach millimeter strokes.
- Loudspeakers: Voltage is converted to mechanical movement of a metallic diaphragm.
- Piezoelectric motors: Piezoelectric elements apply a directional force to an axle,
causing it to rotate. Due to the extremely small distances involved,
the piezo motor is viewed as a high-precision replacement for the stepper motor.
- Piezoelectric elements can be used in laser
mirror alignment, where their ability to move a large mass (the mirror
mount) over microscopic distances is exploited to electronically align
some laser mirrors. By precisely controlling the distance between
mirrors, the laser electronics can accurately maintain optical
conditions inside the laser cavity to optimize the beam output.
- A related application is the acousto-optic modulator,
a device that scatters light off soundwaves in a crystal, generated by
piezoelectric elements. This is useful for fine-tuning a laser's
frequency.
- Atomic force microscopes and scanning tunneling microscopes employ converse piezoelectricity to keep the sensing needle close to the specimen.[42]
- Inkjet printers:
On many inkjet printers, piezoelectric crystals are used to drive the
ejection of ink from the inkjet print head towards the paper.
- Diesel engines: High-performance common rail diesel engines use piezoelectric fuel injectors, first developed by Robert Bosch GmbH, instead of the more common solenoid valve devices.
- Active vibration control using amplified actuators.
- X-ray shutters.
- XY stages for micro scanning used in infrared cameras.
- Moving the patient precisely inside active CT and MRI scanners where the strong radiation or magnetism precludes electric motors.[43]
- Crystal earpieces are sometimes used in old or low power radios.
- High-intensity focused ultrasound for localized heating or creating a localized Cavitation can be achieved, for example, in patient's body or in an industrial chemical process
Frequency standard
The piezoelectrical properties of quartz are useful as a
standard of frequency.
- Quartz clocks employ a crystal oscillator
made from a quartz crystal that uses a combination of both direct and
converse piezoelectricity to generate a regularly timed series of
electrical pulses that is used to mark time. The quartz crystal (like
any elastic material) has a precisely defined natural frequency (caused by its shape and size) at which it prefers to oscillate, and this is used to stabilize the frequency of a periodic voltage applied to the crystal.
- The same principle is critical in all radio transmitters and receivers, and in computers where it creates a clock pulse. Both of these usually use a frequency multiplier to reach gigahertz ranges.
Piezoelectric motors
Types of piezoelectric motor include:
Aside from the stepping stick-slip motor, all these motors work on
the same principle. Driven by dual orthogonal vibration modes with a
phase difference of 90°, the contact point between two surfaces vibrates in an
elliptical path, producing a
frictional
force between the surfaces. Usually, one surface is fixed, causing the
other to move. In most piezoelectric motors, the piezoelectric crystal
is excited by a
sine wave
signal at the resonant frequency of the motor. Using the resonance
effect, a much lower voltage can be used to produce a high vibration
amplitude.
A stick-slip motor works using the inertia of a mass and the friction
of a clamp. Such motors can be very small. Some are used for camera
sensor displacement, thus allowing an anti-shake function.
Reduction of vibrations and noise
Different teams of researchers have been investigating ways to reduce
vibrations in materials by attaching piezo elements to the material.
When the material is bent by a vibration in one direction, the
vibration-reduction system responds to the bend and sends electric power
to the piezo element to bend in the other direction. Future
applications of this technology are expected in cars and houses to
reduce noise. Further applications to flexible structures, such as
shells and plates, have also been studied for nearly three decades.
In a demonstration at the Material Vision Fair in
Frankfurt in November 2005, a team from
TU Darmstadt in
Germany showed several panels that were hit with a rubber mallet, and the panel with the piezo element immediately stopped swinging.
Piezoelectric ceramic fiber technology is being used as an electronic damping system on some
HEAD tennis rackets.
[44]
Infertility treatment
In people with previous
total fertilization failure, piezoelectric activation of oocytes together with
intracytoplasmic sperm injection (ICSI) seems to improve fertilization outcomes.
[45]
Surgery
A recent application of piezoelectric ultrasound sources is piezoelectric surgery, also known as
piezosurgery.
[3]
Piezosurgery is a minimally invasive technique that aims to cut a
target tissue with little damage to neighboring tissues. For example,
Hoigne
et al.[46]
reported its use in hand surgery for the cutting of bone, using
frequencies in the range 25–29 kHz, causing microvibrations of
60–210 μm. It has the ability to cut mineralized tissue without cutting
neurovascular tissue and other soft tissue, thereby maintaining a
blood-free operating area, better visibility and greater precision.
[47]
Potential applications
In 2015, Cambridge University researchers working in conjunction with
researchers from the National Physical Laboratory and Cambridge-based
dielectric antenna company Antenova Ltd, using thin films of
piezoelectric materials found that at a certain frequency, these
materials become not only efficient resonators, but efficient radiators
as well, meaning that they can potentially be used as antennas. The
researchers found that by subjecting the piezoelectric thin films to an
asymmetric excitation, the symmetry of the system is similarly broken,
resulting in a corresponding symmetry breaking of the electric field,
and the generation of electromagnetic radiation.
[48][49]
In recent years, several attempts at the macro-scale application of the piezoelectric technology have emerged
[50][51]
to harvest kinetic energy from walking pedestrians. The piezoelectric
floors have been trialed since the beginning of 2007 in two Japanese
train stations, Tokyo and Shibuya stations. The electricity generated
from the foot traffic is used to provide all the electricity needed to
run the automatic ticket gates and electronic display systems.
[52]
In London, a famous nightclub exploited the piezoelectric technology in
its dance floor. Parts of the lighting and sound systems in the club
can be powered by the energy harvesting tiles.
[53]
However, the piezoelectric tile deployed on the ground usually harvests
energy from low frequency strikes provided by the foot traffic. This
working condition may eventually lead to low power generation
efficiency.
[54]
In this case, locating high traffic areas is critical for
optimization of the energy harvesting efficiency, as well as the
orientation of the tile pavement significantly affects the total amount
of the harvested energy. A Density Flow evaluation is recommended to
qualitatively evaluate the piezoelectric power harvesting potential of
the considered area based on the number of pedestrian crossings per unit
time.
[54]
In X. Li's study, the potential application of a commercial
piezoelectric energy harvester in a central hub building at Macquarie
University in Sydney, Australia is examined and discussed. Optimization
of the piezoelectric tile deployment is presented according to the
frequency of pedestrian mobility and a model is developed where 3.1% of
the total floor area with the highest pedestrian mobility is paved with
piezoelectric tiles. The modelling results indicate that the total
annual energy harvesting potential for the proposed optimized tile
pavement model is estimated at 1.1 MW h/year, which would be sufficient
to meet close to 0.5% of the annual energy needs of the building.
[54]
In Israel, there is a company which has installed piezoelectric
materials under a busy highway. The energy generated is adequate and
powers street lights, billboards and signs.
[55]
Tyre company
Goodyear
has plans to develop an electricity generating tyre which has
piezoelectric material lined inside it. As the tyre moves, it deforms
and thus electricity is generated.
[56]
Photovoltaics
The efficiency of a hybrid
photovoltaic cell
that contains piezoelectric materials can be increased simply by
placing it near a source of ambient noise or vibration. The effect was
demonstrated with organic cells using
zinc oxide
nanotubes. The electricity generated by the piezoelectric effect itself
is a negligible percentage of the overall output. Sound levels as low
as 75 decibels improved efficiency by up to 50 percent. Efficiency
peaked at 10 kHz, the resonant frequency of the nanotubes. The
electrical field set up by the vibrating nanotubes interacts with
electrons migrating from the organic polymer layer. This process
decreases the likelihood of recombination, in which electrons are
energized but settle back into a hole instead of migrating to the
electron-accepting ZnO layer.
[57][58]
See also
References
The Curies, however, did not predict the converse piezoelectric effect. The converse effect was mathematically deduced from fundamental thermodynamic principles by Gabriel Lippmann in 1881.[8] The Curies immediately confirmed the existence of the converse effect,[9] and went on to obtain quantitative proof of the complete reversibility of electro-elasto-mechanical deformations in piezoelectric crystals.A piezoelectric disk generates a voltage when deformed (change in shape is greatly exaggerated)
The nature of the piezoelectric effect is closely related to the occurrence of electric dipole moments in solids. The latter may either be induced for ions on crystal lattice sites with asymmetric charge surroundings (as in BaTiO3 and PZTs) or may directly be carried by molecular groups (as in cane sugar). The dipole density or polarization (dimensionality [Cm/m3] ) may easily be calculated for crystals by summing up the dipole moments per volume of the crystallographic unit cell.[12] As every dipole is a vector, the dipole density P is a vector field. Dipoles near each other tend to be aligned in regions called Weiss domains. The domains are usually randomly oriented, but can be aligned using the process of poling (not the same as magnetic poling), a process by which a strong electric field is applied across the material, usually at elevated temperatures. Not all piezoelectric materials can be poled.[13]Piezoelectric plate used to convert audio signal to sound waves
Of the 32 crystal classes, 21 are non-centrosymmetric (not having a centre of symmetry), and of these, 20 exhibit direct piezoelectricity[18] (the 21st is the cubic class 432). Ten of these represent the polar crystal classes,[19] which show a spontaneous polarization without mechanical stress due to a non-vanishing electric dipole moment associated with their unit cell, and which exhibit pyroelectricity. If the dipole moment can be reversed by the application of an electric field, the material is said to be ferroelectric.Any spatially separated charge will result in an electric field, and therefore an electric potential. Shown here is a standard dielectric in a capacitor. In a piezoelectric device, mechanical stress, instead of an externally applied voltage, causes the charge separation in the individual atoms of the material.
Ceramics with randomly oriented grains must be ferroelectric to exhibit piezoelectricity.[29] The macroscopic piezoelectricity is possible in textured polycrystalline non–ferroelectric piezoelectric materials, such as AlN and ZnO. The family of ceramics with perovskite, tungsten-bronze and related structures exhibits piezoelectricity:Tetragonal unit cell of lead titanate
Piezoelectric disk used as a guitar pickup
Main article: Piezoelectric sensorThe principle of operation of a piezoelectric sensor is that a physical dimension, transformed into a force, acts on two opposing faces of the sensing element. Depending on the design of a sensor, different "modes" to load the piezoelectric element can be used: longitudinal, transversal and shear.
As very high electric fields correspond to only tiny changes in the width of the crystal, this width can be changed with better-than-µm precision, making piezo crystals the most important tool for positioning objects with extreme accuracy — thus their use in actuators. Multilayer ceramics, using layers thinner than 100 µm, allow reaching high electric fields with voltage lower than 150 V. These ceramics are used within two kinds of actuators: direct piezo actuators and Amplified piezoelectric actuators. While direct actuator's stroke is generally lower than 100 µm, amplified piezo actuators can reach millimeter strokes.Metal disk with piezoelectric disk attached, used in a buzzer
A slip-stick actuator.Main article: Piezoelectric motorTypes of piezoelectric motor include:
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