Direct Energy, 2018a

2 CAPACITORS AND PIEZOELECTRIC DEVICES 23
Part I
Survey of Energy Conversion
Devices
2 Capacitors and Piezoelectric Devices
2.1 Introduction
This chapter begins with a discussion of material polarization, and then it
discusses capacitors and piezoelectric devices. The next chapter discusses
pyroelectric devices and electro-optic devices. All of these devices are all
constructed from a thin dielectric layer, and operation of all of these devices
involves establishing a material polarization, charge build up, throughout
this dielectric material. In piezoelectric materials, mechanical strain causes
a material polarization. As with many energy conversion devices, piezoelectric
devices can work both ways, converting mechanical energy to electricity
or converting electricity to mechanical vibrations. In pyroelectric devices, a
temperature gradient causes the material polarization, and in electro-optic
devices, an external optical electric eld causes the material polarization.
Why start the discussion of energy conversion devices with a discussion
of capacitors? Capacitors are familiar to electrical engineers, and they are
energy storage devices. How do capacitors work? What are the components
of a capacitor? What materials are capacitors made out of? What are the
dierences between dierent types of capacitors such as mica capacitors
and electrolytic capacitors? In an introductory circuits course, a capacitor
is a device where the relationship between the current i and voltage v is
given by
i = C dv
dt
(2.1)
and the capacitance C is just a constant. The only dierence between one
capacitor and another is the capacitance value. In order to answer these
questions further, we need to go beyond this model. Through this study,
we will gain insights into piezoelectric devices, pyroelectric devices, and
electro-optic devices too.