direction
is not so large as to cause electric breakdown of the crystal. The larger the
value of coercive field the better the residual polarization is attained by the
ferroelectric.
As the temperature of a
ferroelectric exceeds a certain value, its ferroelectric properties vanish and
it becomes an ordinary polar dielectric. This temperature Tc is called Curie temperature. It is typical for
each material. The point marking the transition between ferroelectric phase and
polar electric phase is called the Curie point. In certain cases, such as
Rochelle salt, there are two Curie points (i.e., ferroelectric properties
vanish with decreasing temperature also). The dielectric constant k is the
vicinity of the Curie point is given by the relation
..........(6)
where C is
the constant called the Curie constant, T0 the
Curie - Weiss Temperature, which is close to the Curie temperature TC and k0 is
the contribution due to the electronic polarization. This relation is known as
Curie - Weiss law. It shows that just above the critical temperature k is very
large.
One of the best known ferroelectric
materials is barium titanate (BaTiO3), which is often used in capacitors where
small size and large capacitance are required. Because of their low spontaneous
polarizabilities ferroelectric crystals are frequently used in computers as
memory cells. There are several groups of ferroelectric crystals. The examples
are :
Rochelle salt NaK (C4 H4 O6) 4H2O; Potassium
dihydrogen phosphate KH3PO4; Barium titanate Ba Ti O3 ; Guanidine compounds such as C(NH2)3 Al(SO4)2 .
6H2O.
Para electric materials have a small
positive susceptibility due to polarization in electric field. In fact all
dielectric materials (except vacuum) are paraelectrics.
There is no dielectric which has
negative susceptibility. Thus there is no existence of dia-electric. It is
because the polarization in the direction opposite to the polarizing field is
not observed in any case.
Q.8. How the polarization and dielectric constant
depend on frequency of applied field.
Related
Questions -
Q. Explain the behaviour of dielectric in
an alternating field. (AKTU. 2011 - 12)
Q. What do you mean by dielectric dispersion.
Ans. When a dielectric material is subjected to an alternating electric
field, the polarization components follow the field reversals. The total
polarization depends on the ability of dipoles to orient themselves in the
direction of the field. When the frequency of alternating voltage increases,
the value of permittivitymr of a polar
molecule at first remains constant but after a certain critical frequency, it
starts decreasing.
The relaxation frequencies of
different polarization mechanisms are different. When the frequency of the
alternating electric field matches the relaxation frequency of a particular
polarization mechanism. The absorption of energy from the applied field is
maximum. As per frequency of the applied field becomes more than the relaxation
frequency of a particular polarization mechanism, then that particular
polarization mechanism is stopped. In audio frequency region, all types of
polarization respond. If the frequency of the alternating voltage is much
higher than the relaxation frequency of the dipoles, the dipoles can not keep
in phase with the field. The orientation polarization is effective at low
frequencies and can not follow the field reversals at microwave frequencies. In
infra-red region, the ionic polarization fails to follow the field reversals
and in this region only electronic polarization contributes. In optical region,
the electron clouds follow the field reversals. If n be the refractive index of
the dielectric, then in the optical region
er = n2
It may be
concluded that the ionic and electronic polarizations are dominant in infra-red
and ultraviolet region, whereas the orientation and space-charge polarizations
contribute at radio or microwave frequencies. This situation is shown in Fig.
The polarization and consequently
the dielectric constant depend on the frequency of applied field. This
phenomena is called dielectric dispersion.
Q.9 Define dielectric loss and its dependens on
frequency.
Ans. Dielectric Loss Or Loss Tangent: -
Let
a parallel plate capacitor of capacitance C0 be
connected to an alternating source of e.m.f.