Page 259 - Electrical Properties of Materials

Piezoelectricity, pyroelectricity, and ferroelectricity 241

high power densities are passed through the ceramic windows of klystrons
or magnetrons. Recent work with high-power lasers has shown that dielec-
tric breakdown still occurs at optical frequencies. In fact the maximum power
12
available from a solid-state laser is about 10 W from a series of cascaded
neodymium glass amplifier lasers. The reason why further amplification is not
possible is that the optical field strength disrupts the glass laser material.

10.12.3 Discharge breakdown
In materials such as mica or porous ceramics, where there is occluded gas, the
gas often ionizes before the solid breaks down. The gas ions can cause surface
damage, which accelerates breakdown. This shows up as intermittent sparking
and then breakdown as the test field is increased.

10.13 Piezoelectricity, pyroelectricity, and ferroelectricity

These are three classes of dielectrics, each having some interesting properties
arising from their crystal structure. Their names come from Greek and Latin.
Piezo means to press, pyr means fire, and ferrum means iron. Unfortunately,
ferroelectricity is a misnomer. It has nothing to do with iron. It got its name
from the similarity of its properties to ferromagnetism.
Piezoeloectricity is the widest class. Next come pyroelectrics and then fer-
roelectrics. Their interrelationship is shown in Fig. 10.14 in a Venn diagram.
All ferroelectrics are pyroelectrics, and all pyroelectrics are piezoelectric.
What is the difference between them? Very roughly: ferroelectrics have per-
manent dipole moments, pyroelectrics are sensitive to heat, and all of them
are sensitive to mechanical displacement. The one that has been known about
the longest is pyroelectricity; it was found in tourmaline, a natural crystal. Its
property to attract bits of straw and wood was noted by Theophrates some
25 centuries ago. Piezoelectricity was discovered by Pierre and Jacques Curie
in the 1880s whereas ferroelectricity is hardly a century old, discovered by
Valasek in the 1920s.

10.13.1 Piezoelectricity

Let us start with the piezoelectric effect. This is easy to describe in a few words:
a mechanical strain will produce dielectric polarization and, conversely, an ap-
plied electric field will cause mechanical strain. Which crystals will exhibit
this effect? Experts say that out of the 21 classes of crystals that lack a centre

Piezoelectrics Pyroelectrics Ferroelectrics
Fig. 10.14
Relation between the piezoelectric,
pyroelectric, and ferroelectric
materials.