Page 98 - Maxwell House

78 Chapter 2

increases reaching its absorption peak at frequencies = /2 around 11 GHz
where = 1/. Therefore, the water dipole relaxation time = 1/ = 1.45x10 −11 [s]
that is three order of magnitude higher than in metals. Clearly, the much higher inertia of dipoles
in comparison with free electrons in metals explains this phenomenon. Note that the absorbance
is broadband and significant around .

The green area of absorption resonances. This spectral region corresponds the frequencies
where the spring resonances 0, are revealed. There are three relatively narrow-banded
absorption peaks: comparatively week around 51 THz (microwave band), another one around
102 THz (near infrared), and the strong one around 430 THz (red light). The water as any other
dielectric becomes highly reflective near the resonance frequency, and the penetrating radiation
is rapidly dissipated.

Rose area of ultra-high frequencies. In this area () → 1 and () → 0, since dipoles are
′
′′

too inertial to response and form the polarization vector, the water becomes completely
transparent.
Practically all dielectric materials follow more or less the same frequency trail. The differences
are in plasma frequency, relaxation time, the location of absorption resonances and their
magnitude. For example, Teflon, one of the best low loss dielectric, has a very weak relaxation
peak around 1.15 – 1.4 THz (depending on production technology) and the strong absorption
peak at 6 THz.
Now we can start a tour through the world of more sophisticated and mainly not available from
natural materials. Continuous innovations bring in engineering practice probably every day
some new material with unique characteristics due to tremendously advance in solid-state
physics and chemistry. Recall semiconductor and fiber optic revolution, relatively recent
discovery of graphene and rapid development of nanotechnology. So keep your eyes widely
open.

2.6 FERRO-MATERIALS

2.6.1 Introduction
The prefix ferro, meaning iron, describes the property of conductive and nonconductive
materials that have a spontaneous magnetic or electric polarization that exists in the absence
of an external magnetic or electric field (sometimes both). The spatial vector of polarization
can be changed by the application of an external magnetic or electric field, respectively. Most
of the ferro-materials have one feature in common – pre-existing alignment of electric or
magnetic moments (sometimes both in magnetoelectric materials) in parallel. As a result, they
form a number of small areas called domains where all or almost all available moments line up,
i.e., the domain is fully saturated or close to saturation. Doing so they minimize their stored
energy and stable in time. The detailed exploration of electric and magnetic domain formation
is rather complicated and requires knowledge of crystallography and quantum mechanics that
is out of this book scope. The reader interested in more details should consult classic book [22].
We limit ourselves to the simplified phenomenological approach.

Note that mentioned above ferro-definition sounds strange for most ferroelectric materials
because often there are no traces of iron in them, but that is an unpleasant fact accepted in
technical and scientific literature.