Simple description with polarisation curves and applications of dielectrics, paraelectrics and ferroelectrics.

1. - Dielectrics are materials which can be polarised in the
presence of an electric field.
- In the presence of the field, there is NO flow of charges,
but the charges slightly shift from their equilibrium
position.
- The positive ends align towards the negative and the
negative ends align towards the positive of the field. This
is called dielectric polarisation.
- Dielectrics and insulators are the same, but the words are
used in different situations.

2. - The measure of
polarisation of a material is
for a given electric field is
called ‘electric
susceptibility’ , χe
- Given by the formula
Where εo is permittivity
of free space, P is
polarisation density
and E is the strength of
the applied field.
Note: Polarisation
density P is defined as the
average dipole moment
per unit volume of the
material
P=εoχeE

3. Applications of dielectrics
 They are mainly used in capacitors as the charge storage
material between the metallic plates. This also prevents
the plates from coming in direct contact.
 Used in dielectric resonator antenna, for producing and
receiving microwave signals.
 Mineral oil is used as a dielectric and as a cooling
material in transformers.
 Used as insulator coating for wires and conducting
materials.

4. Paraelectrics
 Materials which get polarised in the presence of an
electric field but in a non linear pattern.
 For a dielectric the polarisation density baries linearly
with the field, which is not the case as in paraelectrics.
 When placed under an electric field the electron cloud
in molecules of the paraelectric gets distorted. The
level of this distortion is non linear w.r.t the field.
 E.g. SiO2, Al2O3 etc

5. In paraelectrics like Tantalum oxide (Ta2O5), when the
eletric field is applied, the central Ta ion moves from its
eqbm. position (resulting in polarisation) but returns back
when the field is removed.
Above: Comparison of dielectric (left) and paraelectric polarisation
(right). P is polarisation density and E is electric field strength

6. Applications of paraelectrics
 Used in timer devices and as signal filter materials.
 Used for refrigeration applications since it has been
found that loss of polarisation when the field is
removed leads to cooling.

7. Ferroelectric Materials
 Materials that get polarized in the presence of a field and
retain the polarization even after the removal of the field.
 E.g BaTiO3 , where the central Ti ion moves from its eqbm.
position on applying a field, but stays at the changed
position even after the field is removed. This results in the
retention of polarization.
 The magnitude of the change in position is much more
than that of the change seen in paraelectrics.

8. Ferroelectric Polarisation
- Left: The polarisation curve with
changing electric field for a
ferroelectric material. Note that the
material retains the polarisation
even when the E is zero.
- The direction of polarisation can
be reversed just by applying an
electric field in the opposite
direction.
- Such a type of a curve is called a
Hysterisis curve.
- Ferroelectrics become a
paraelectric when heated above a
certain temperature called the
Curie Temperature. (This idea is
similar to that in ferromagnetics)

9. Applications of ferroelectrics
 Used to make capacitors with tunable capacitance.
 Used in ferroelectric RAM, which stores a lot more data
in lesser space and is also more reusable.
 Used in RFID tags (Radio Frequency Identity) to tag
various items especially in retail businesses

10. Presentation by
Joel Cornelio
jcorn360@gmail.com