Direct Energy, 2018a

42 2.3 Piezoelectric Devices
In certain dielectric materials, an external mechanical stress induces a
local material polarization. The charge buildup of this material polarization
induces a material polarization in nearby atoms forming electrical domains
[23]. This piezoelectric eect can occur whether the original material is
crystalline, amorphous, or polycrystalline [23]. In noncrystalline materials,
this eect is necessarily nonlinear, so these materials are not well described
by Eqs. 2.14 or 2.19.
The nonlinear process of a material polarization of one atom inducing
a material polarization of nearby atoms causing the formation of electrical
domains is called ferroelectricity. Ferroelectric materials may be crystalline,
amorphous, or polycrystalline. We will see in the next chapter that
materials can be ferroelectric pyroelectric and ferroelectric electro-optic in
addition to ferroelectric piezoelectric. The ferroelectric eect is limited by
temperature. For many ferroelectric materials, these eects occur only below
some temperature, called the Curie temperature. When the materials
are heated above the Curie temperature, the ferroelectric eect goes away
[33]. The material polarization of a ferroelectric material may depend on
whether or not a material polarization has previously been induced. If the
state of a material depends on its past history, we say that the material has
hysteresis. Ferroelectric materials may have a material polarization even
in the absence of an external mechanical stress or electric eld if a source
of energy has previously been applied.
While the prex ferro- means iron, most ferroelectric materials do not
contain iron, and most iron containing materials are not ferroelectric. The
word ferroelectric is used as an analogy to the word ferromagnetic. Some
iron containing materials are ferromagnetic. If an external magnetic eld is
applied across a ferromagnetic material, an internal magnetic eld is set up
in the material. Ferromagnetic materials can have a permanent magnetic
dipole even in the absence of an applied magnetic eld. We can model an
electric dipole as a pair of charges. We can model a magnetic dipole as a
small current loop. Ferromagnetic materials exhibit hysteresis, and they
have magnetic domains where the magnetic dipoles are aligned.
Originally, a piezoelectric ferroelectric material has randomly aligned
electrical domains and no net material polarization, so it starts out as
neither piezoelectric or ferroelectric. The process of causing a material
to exhibit piezoelectricity and ferroelectricity is called poling. To pole
a material, place it in a strong external electric eld [23], for example,
across the poles of a battery, hence the term. Poling does not change
the atomic structure, so if the material was originally amorphous, it will
remain amorphous. During this process, electrical domains form, and these
domains remain even when the external eld is removed. A material that is