...................................................................................................................................................
Piezoelectricity
 |
Quartz crystal.
|
How
Piezoelectricity Works
Cesca
Fleischer
Feel the Squeeze: How Piezoelectricity Works to
Make Crystals Conduct Electric Current
Piezo what? It sounds like a lot to take in, but
it’s simple to understand.
The word piezoelectric originates from the Greek
word piezein, which literally means to squeeze or press.
Instead of squeezing grapes to make wine, we’re
squeezing crystals to make an electric current!
Piezoelectricity is found in a ton of everyday
electronic devices, from quartz watches to speakers and microphones.
In a nutshell:
Piezoelectricity is the process of using
crystals to convert mechanical energy into electrical energy, or vice versa.
Regular crystals are defined by their organized
and repeating structure of atoms that are held together by bonds - this is
called a unit cell.
Most crystals, such as iron have a symmetrical
unit cell, which makes them useless for piezoelectric purposes.
There are other crystals that get lumped
together as piezoelectric materials.
The structure in these crystals aren’t
symmetrical but they still exist in an electrically neutral balance.
However, if you apply mechanical pressure to a
piezoelectric crystal, the structure deforms, atoms get pushed around, and
suddenly you have a crystal that can conduct an electrical current.
If you take the same piezoelectric crystal and
apply an electric current to it, the crystal will expand and contract,
converting electrical energy into mechanical energy.
Types of Piezoelectric Materials
There are a variety of piezoelectric materials
that can conduct an electric current, both man-made and natural.
The most well-known, and the first piezoelectric
material used in electronic devices is the quartz crystal.
Other naturally occurring piezoelectric
materials include cane sugar, Rochelle salt, topaz, tourmaline, and even bone.
As piezoelectric technology started to take off
after World War I we began developing man-made materials to rival the
performance of quartz.
Man-made piezoelectric materials include:
PZT is made from lead zirconate titanate and can
produce more voltage than quartz with the same amount of mechanical pressure.
Barium Titanate is a ceramic piezoelectric
material that was discovered during World War II and is known for its
long-lasting durability.
Lithium Niobate is a material that combines
oxygen, lithium, and nobium together in a ceramic material that performs
similar to barium titanate.
How Piezoelectricity Works
We have specific materials that are suited for
piezoelectricity applications, but how exactly does the process work? With the
Piezoelectric Effect.
The most unique trait of this effect is that it
works two ways. You can apply mechanical energy or electrical energy to the
same piezoelectric material and get an opposite result.
Applying mechanical energy to a crystal is
called a direct piezoelectric effect and works like this:
1. A piezoelectric crystal is
placed between two metal plates. At this point the material is in perfect
balance and does not conduct an electric current.
2. Mechanical pressure is then
applied to the material by the metal plates, which forces the electric charges
within the crystal out of balance. Excess negative and positive charges appear
on opposite sides of the crystal face.
3. The metal plate collects these
charges, which can be used to produce a voltage and send an electrical current
through a circuit.
That’s it, a simple application of mechanical
pressure, the squeezing of a crystal and suddenly you have an electric current.
You can also do the opposite, applying an
electrical signal to a material as an inverse piezoelectric
effect.
It works like this:
1. In the same situation as the
example above, we have a piezoelectric crystal placed between two metal plates.
The crystal’s structure is in perfect balance.
2. Electrical energy is then
applied to the crystal, which shrinks and expands the crystal’s structure.
3. As the crystal’s structure
expands and contracts, it converts the received electrical energy and releases
mechanical energy in the form of a sound wave.
The inverse piezoelectric effect is used in a
variety of applications. Take a speaker for example, which applies a voltage to
a piezoelectric ceramic, causing the material to vibrate the air as sound
waves.
The Discovery of Piezoelectricity
Piezoelectricity was first discovered in 1880 by
two brothers and French scientists, Jacques and Pierre Curie.
While experimenting with a variety of crystals,
they discovered that applying mechanical pressure to specific crystals like
quartz released an electrical charge. They called this the piezoelectric
effect.
The next 30 years saw Piezoelectricity reserved
largely for laboratory experiments and further refinement.
It wasn’t until World War I when
piezoelectricity was used for practical applications in sonar.
Sonar works by connecting a voltage to a
piezoelectric transmitter. This is the inverse piezoelectric effect in action,
which converts electrical energy into mechanical sound waves.
The sound waves travel through the water until
they hit an object. They then return back to a source receiver.
This receiver uses the direct piezoelectric
effect to convert sound waves into an electrical voltage, which can then be
processed by a signal processing device.
Using the time between when the signal left and
when it returned, an object’s distance can easily be calculated underwater.
With sonar a success, piezoelectricity gained
the eager eyes of the military. World War II advanced the technology even
further as researchers from the United States, Russia, and Japan worked to
craft new man-made piezoelectric materials called ferroelectrics.
This research led to two man-made materials that
are used alongside natural quartz crystal, barium titanate and lead zirconate
titanate.
Piezoelectricity Today
In today’s world of electronics piezoelectricity
is used everywhere.
Asking Google for directions to a new restaurant
uses piezoelectricity in the microphone.
There’s even a subway in Tokyo that uses the
power of human footsteps to power piezoelectric structures in the ground.
You’ll find piezoelectricity being used in these
electronic applications:
Actuators
Actuators use piezoelectricity to power devices
like knitting and braille machinery, video cameras, and smartphones. In this
system, a metal plate and an actuator device sandwiched together a piezoelectric
material. Voltage is then applied to the piezoelectric material, which expands
and contracts it. This movement causes the actuator to move as well.
Speakers & Buzzers
Speakers use piezoelectricity to power devices
like alarm clocks and other small mechanical devices that require high quality
audio capabilities. These systems take advantage of the inverse piezoelectric
effect by converting an audio voltage signal into mechanical energy as sound
waves.
Drivers
Drivers convert a low voltage battery into a
higher voltage which can then be used to drive a piezo device. This
amplification process begins with an oscillator which outputs smaller sine
waves. These sine waves are then amplified with a piezo amplifier.
Sensors
Sensors are used in a variety of applications
such as microphones, amplified guitars, and medical imaging equipment. A
piezoelectric microphone is used in these devices to detect pressure variations
in sound waves, which can then be converted to an electrical signal for
processing.
Power
One of the simplest applications for
piezoelectricity is the electric cigarette lighter. Pressing the button of the
lighter releases a spring-loaded hammer into a piezoelectric crystal. This
produces an electrical current that crosses a spark gap to heat and ignite gas.
This same piezoelectric power system is used in larger gas burners and oven
ranges.
Motors
Piezoelectric crystals are perfect for
applications that require precise accuracy, such as the movement of a motor. In
these devices, the piezoelectric material receives an electric signal, which is
then converted into mechanical energy to force a ceramic plate to move.
Piezoelectricity and the Future
What does the future hold for piezoelectricity?
The possibilities abound.
One popular idea that inventors are throwing
around is using piezoelectricity for energy harvesting.
Imagine having piezoelectric devices in your
smartphone that could be activated from the simple movement of your body to
keep them charged.
Thinking a bit bigger, you could also embed a
piezoelectric system underneath highway pavement that can be activated by the
wheels of traveling cars.
This energy could then be used light stoplights
and other nearby devices. Couple that with a road filled with electric cars and
you’d find yourself in net positive energy situation.
Want to help move piezoelectricity forward into
the future? Autodesk EAGLE has a ton of free piezo libraries ready for use in
your next project. Try Autodesk EAGLE for free today!
Cesca
is a Stanford University graduate, with a Bachelor of Science in Science,
Technology, and Society -- interdepartmental major focusing in Product Design,
Technology and Organizational Management.
PCB
design made easy
Make anything
with EAGLE PCB design software. Professional power for every electronics
designer.
 |
PZT piezo ceramics used in ultrasonic sensors.
|
 |
Lithium niobate.
|
|
 |
Barium Titanate
|
 |
Pierre Curie with his wife Maria in his lab.
|
No comments:
Post a Comment