Q.7 Write short note on Fero and Piezo electricity.
Related
Questions -
Q. Define Curie temperature for a
ferroelectric material.
(AKTU. 2011 - 12, 13 - 14)
Ans. Piezoelectricity:
-
If a mechanical stress is applied to the
crystal, the atoms are slightly displaced. The ionic displacements are
symmetrical about the symmetry centres in the centrosymmetric crystals and the
charge distribution inside the crystal is not changed appreciably by the applied
stress. In the case of acentric crystals, the ionic displacements are
asymmetric and the
electrical dipoles are produced in the crystal. This effect is known as
piezoelectric effect and these crystals are called piezoelectric crystals. Conversely
when an electric field is applied to a piezoelectric crystal, dipoles are
induced and the atomic displacements produce a mechanical strain. This is
called the inverse piezoelectric effect. The displacement varies periodically
when the alternating field is applied to a piezoelectric crystal. At a certain
frequency called resonance frequency, these displacements are in phase with the
applied field and hence are maximum. The frequency range at which resonance
occurs is mostly very sharp hence such crystals are very much used for
frequency controls in radio transmitters and in electronic clocks.
To obtain a relation between the
mechanical stress and the electric polarization produced in a piezoelectric
crystal, consider a crystal of width d placed between two metal plates A and B.
If the crystal is compressed by an applied mechanical stress p, a mechanical
strain s is produced in the crystal.
..........(1)
The polarization density P in the
crystal is proportional to the applied mechanical stress, therefore we have
P = hp, ..........(2)
where h is the
piezoelectric constant.
On the other hand if an electric
field E is applied to the crystal then the induced strain is proportional to E,
i.e.
sin = h¢E. ..........(3)
When both an external field E and a
stress p are applied to the crystal, the electric displacement is given by
D = e0E + P = e0E + hp ..........(4)
and the
internal strain
s = h¢E + p/Y. ..........(5)
Piezoelectric properties depend on
temperature. These properties vanish at a certain temperature at which the
crystal lattice is rearranged so that a centre of symmetry is formed.
Piezoelectric effect has numerous
practical applications. Piezoelectric transducers are used for measuring
rapidly varying pressures. Piezoelectric microphones are well known. Quartz
ultrasonic vibrators are the well known examples of the inverse piezoelectric
effect.
Ferroelectricity:
-
There are certain pyroelectric crystals in which one can
reverse the direction of polarization by applying a sufficiently intense
external field. Such crystals are called ferro-electric and this effect is
called the ferro-electric effect. All the ferroelectric crystals are
pyroelectric crystals but the reverse is not true.
The term ferroelectricity is
applied to the ferroelectric phenomenon. A ferroelectric crystal generally
consists of regions called domains within each of which all the molecular
dipoles are parallel. In adjacent domains the polarization is in different
directions. In the crystal as a whole the dipole moments of these domains are
randomly oriented resulting in zero dipole moment. When an external electric
field is applied these domains tend to align themselves in the field direction.
The polarization induced does not vary linearly with the applied electric field
E. The ferroelectric materials exhibit hysteresis phenomenon in a manner
similar to ferromagnetic materials. They often exhibit large susceptibility. As
the electric field is applied the total polarization increases rapidly until
the saturation value is reached. It is the position where all the domains are
parallel to the applied field. When the field decreases to zero, the
polarization does not return to zero. The polarization left is called residual
or remanant polarization. The coercive field required to reverse the
polarization
