Direct Energy, 2018a

2 CAPACITORS AND PIEZOELECTRIC DEVICES 31
Figure 2.4: Through-hole size capacitors.
cluding polystyrene, polycarbonate, polyester, polypropylene, Teon, and
mylar [22]. These capacitors often have good accuracy, temperature stability,
and leakage characteristics [22].
Not all capacitors have solid dielectrics. A vacuum is a dielectric. Capacitors
with a vacuum dielectric are used in applications which involve
high voltage or which require very low leakage [22]. Capacitors with liquid
dielectrics made of oil are used in similar situations [22]. Electrolytic
capacitors often have dielectrics which are a combination of solid materials
with liquid electrolytes. An electrolyte is a liquid through which some
charges can ow more easily than others. Electrolytic capacitors are polarized,
meaning that they have positive and negative terminals, so, similar to
a diode, the orientation of the capacitor in a circuit is important. Inside an
electrolytic capacitor is a junction of multiple materials. The initial application
of voltage in the factory chemically creates an oxide layer which is
the dielectric. Reversing the voltage will dissolve the dielectric and destroy
the capacitor. One advantage of electrolytic capacitors is that a small device
can have a large capacitance. However, they often have poor accuracy,
temperature stability, and leakage [22]. Also, electrolytic capacitors have a
nite lifetime because the liquid can degrade over time.
2.3 Piezoelectric Devices
Can we induce a material polarization in an insulator in a way that does
not involve applying a voltage? If so, then this method can charge a capacitor,
and we can discharge the capacitor as usual to produce electricity.
Any device that accomplishes this task is an energy conversion device.