The piezoelectric effect is the tendency of some materials to change their
dimensions when an electrical current is applied through them. In the direction
that the current flows, they increase in length. In the other two perpendicular
dimensions, their length shrinks. The following video, used courtesy of
NASA, demonstrates this phenomena.
The Curie brothers first observation was the charge of crystals when
mechanical stress is applied. Rochelle salt's properties were observed by
Pockels in 1894 (Germany). In 1921 Valsek saw the loop of the polarized Rochelle
salt and its electrical field axis. Later in 1935 Busch and Scherrer from
Switzerland discovered what was later known as the second family of
piezoelectrics. After 1917 they found a commercial use for piezoelectrics,
they could use an ultrasonic submarine detector, sonar. In 1946 they found
they could observe the piezoelectric effect on more than one single crystal
material which now saved much more money. Presently piezoelectric materials are most
commonly used in AC circuits. They are used to create a small
vibration, perhaps resulting in the production of sound or a spark for a lighter.
However, The movement of individual Piezo actuators when used in a D.C.
circuit can be compounded by placing them in series.
They can further be magnified into a bending action much the same way that
bimetallic thermometers bend enormously when their materials ever slightly
change in length. Static operation, even holding heavy loads, does
not consume power.
The way that PZT works is using a combination of chemical effects, physical
effects, and geometry. PZT crystallites are centro-symmetric cubic
(isotropic) before poling and after poling exhibit tetragonal symmetry (anisotropic
structure)
(1).
When an electric field is applied (and the crystal becomes "poled"), a variety of things happen. First, the Zirconate
Titanate is pulled toward the positive potential. The lead crystal around it
will change in dimensions to accommodate the new position of the Zirconate
Titanate. As a result, there is a distortion that causes growth in the
Dimensions aligned with the field and a contraction along the axes normal to the
electric field.
This, magnified to the molar level, is what causes the piezoelectric effect.
In the following original Bryce animation created by Jamie Burch, you will see a PZT crystal
initially un-poled. The electrical field will gradually be applied until the crystal
is completely polarized. Then, the electrical field will slack off, and
the crystal will revert to its original un-poled state.
There are many other types of piezoelectric materials, but they all operate
on the same principal.
Most of the concepts for discrete actuator piezo-elements were developed
during the peak of research activities in the middle of the 20th century or
short thereafter. Multi-layer stack actuators, an actuator design yielding
higher performance and still widely used today, was also developed during that
period. However, the past five years has shown a boom in the piezo-market
economy. Many new innovations on piezoelectrics have created an atmosphere
of double digit annual growth. The reason is that piezoelectrics play a
large part in wireless communications. For other examples of how piezoelectrics are used, reference Component One.
NASA: LaRC Morphing Project
http://science.nasa.gov/headlines/y2001/ast01mar_1.htm
Sensor Technology Limited
http://sensortech.ca/acoutrans.html
http://sensortech.ca/stack.htm
http://www.sensortech.ca/igniter.html
Physik Instrumente Tutorials #4
http://www.physikinstrumente.com/tutorial/4_4.html
Physik Instrumente Tutorials #15
http://www.physikinstrumente.com/tutorial/4_15.html
Smart Materials: IntelliMat!
http://www.smartmaterials.info/materials/history/piezoelectricity.html
The Infoshop
http://www.the-infoshop.com/study/bc8031_piezoelectric_crystals.html
