Dielectric Polarization

The process of producing electric dipoles, which are oriented along the field direction, is called polarization in dielectric. Dielectric polarization is also the displacement of charged particles under the action of the external electric field. Hence it is useful is making circuits, capacitors etc.

VARIOUS POLARIZATION PROCESSES:

1. Ionic polarization.
2. Orientational polarization.
3. Electronic polarization.
4. Interfacial/ Space charge polarization.

Ionic Polarization: It is observed when different atoms that comprise a molecule share their electrons asymmetrically, and cause the electron cloud to attract towards the stronger binding atom, the atoms acquire charges of opposite polarity and an external field acting on these net charges will tend to change the equilibrium positions of the atoms themselves, leading to the ionic polarization. The net dipole moment is zero in absence of external electric field. This type of polarization generally occurs in ionic crystals (such as NaCl, KCl, and LiBr).

Orientational Polarization: When an ionic bond is formed between two molecules by the transfer of some valence electrons, a permanent dipole moment will originate in them. This permanent dipole moment is equal to the product of the charges of the transferred valence electrons and the inter-atomic distance between them. In the presence of an electric field E, the molecules carrying a permanent dipole moment will orient to align along the direction of the electric field E. This process is called the dipolar or orientational polarization. This occurs only in dipolar materials possessing permanent dipole moments.

Electronic polarization:

• Arises due to displacement of positively nucleus and negatively charge electrons in opposite direction, when electric field is applied.
• Finally creating a dipole moment in the dielectric material.
• Exhibited by monoatomic gases.
• Proportional to volume of atoms and is independent of temperature.

Interfacial/ Space charge Polarization: It is present in dielectric materials which contain charge carriers that can migrate for some distance through the bulk of the material (via diffusion etc.). Such a distortion appears to an outside observer as an increase in the capacitance of the sample and may be indistinguishable from the real rise of the dielectric permittivity.

In dielectric materials, localized charges (ions and vacancies, or electrons and holes) can hop from one site to another site, which creates the hopping polarization. Similarly the separation of the mobile positive and negative charges under an electric field can produce an interfacial polarization.