Direct Energy, 2018a

44 2.3 Piezoelectric Devices
made more easily into dierent shapes such as cylinders and spheres [33].
However, the materials used often have lower meltingtemperatures, higher
temperature expansion coecients, and are more brittle [33]. Crystalline
materials, such as quartz, have the advantages of being harder and having
a higher melting temperature [33].
2.3.5 Applications of Piezoelectricity
A number of electrical components involve piezoelectricity. When a voltage
is applied across a piece of piezoelectric material, it mechanically bends and
deforms. When the voltage is released, it springs back at a natural resonant
frequency. This material can be integrated with a feedback circuit
to produce oscillations at a precise frequency. Electrical oscillators of this
type are often made from crystalline quartz. A more recent application
is the piezoelectric transformer. These devices are used in the cold cathode
uorescent lamps which are used as backlight for LCD panels [23, p.
289]. The lamps require around a thousand volts to turn on and hundreds
of volts duringuse. Transformers made of magnets and coils can achieve
these high voltages, but piezoelectric transformers are much smaller, small
enough to be mounted on a printed circuit board. A traditional transformer
involves a pair of coils, and it converts AC electricity to magnetic
energy to AC electricity at a dierent voltage. Similarly, a piezoelectric
transformer also involves multiple energy conversion processes. In such a
device, AC electricity is converted to mechanical vibrations and then to
AC electricity at a dierent voltage. Energy is conserved in these devices,
so they can produce high voltages with low currents. Figure 2.10 shows
a piezoelectric transformer that can convert an input of 8 to 14 V to an
output up to 2 kV [35]. Figure 2.11 shows an example of some small piezoelectric
circuit components. Startingin the upper left and goingclockwise,
a microphone, ultrasonic transmitter and receiver, vibration sensor, and
oscillator are shown.
Eciency of energy conversion devices is hard to discuss because every
author makes dierent assumptions. However by any measure, eciency of
a commercial piezoelectric device is low, often 6% or less [36]. Due to this
low eciency, many piezoelectric devices are used as sensors. Regardless
of this low eciency, other devices are used for energy harvesting. For
example, one train station embedded piezoelectric devices in the platforms
to generate electricity. Piezoelectric devices also have been used to convert
the energy from the motion of a uid or from wind directly to electricity
[36].
There is interest in usingpiezoelectric devices for biomedical applica-