ELECTRICAL PROPERTIES OF DIELECTRICS - Advanced Signal Integrity for High-Speed Digital Design - page 252

+

+

Cl −

−

Cl −

Na +

Na +

E

Na +

Cl −

−

+ +

Na +

Sphere
of hydration

Water
Molecule

(a) (b)

Figure 6-4 (a) In the absence of an external electric field, polar molecules and ions will
tend to arrange themselves so that the total polarity is neutral. Example depicts the positive
sodium ion associated with and surrounded by the polar water molecule. (b) When an
external electric field is applied, the positive and negative ions will be displaced relative
to each other, inducing ionic or molecular polarization.

oriented toward the positive sodium and that the hydrated ion maintains a net
positive charge. When this solution is exposed to an external electric field,
the positive ions are displaced with respect to the negative ones, resulting in
physical migration—and separation—of the oppositely charged hydrated ions,
as shown in Figure 6-4b. If the poles of the applied field represent an electron
sink and source, and if the applied potential is sufficiently high, oxidation
and reduction (redox) reactions occur at the poles and there is net current in
the circuit. In the absence of the physical requirements for electron transfer
(i.e., redox reactions), only displacement occurs in response to the applied
field. In a solid material (such as NaC1), the individual ions are not able
to dissociate. However, when an electric field is applied, the electrons
associated with the molecule will be displaced with respect to the Na
and C1 nuclei over subatomic distances creating a dipole moment. This is
called ionic polarizability , is denoted as α i , and is also known as molecular
polarizability . For a detailed discussion, see the topics by Elliott [1993] and
Pauling [1948].

6.1.4 Relative Permittivity

The discussion above was primarily for instructional purposes. In general, the
polarizability of a material is not calculated directly, because it would require
precise knowledge of the atomic dimensions for each atom or molecule in the
material. For practical applications, the phenomenon is measured indirectly by
looking at the behavior of the dielectric permittivity versus frequency.