Dielectrics and its applications

Presentation on

Dielectrics

PPT designed by:Mohammedi Limdiwala

Contents• Introduction• Basic terms• Polarization of Dielectric• Polarization method• Internal Field• Clausius-Mossotti Equation• Types of dielectric• Properties of good Dielectric• Application of Dielectric

Mohdlimdi23 - Dielectrics24 Nov '14 2

Introduction• Dielectric materials are also called as insulators. • In dielectric materials, all the electrons are tightly

bound to their parent molecules and there are no free charges

• Dielectrics are non-metallic materials of high specific resistance and negative temperature coefficient of resistance.

• They have permanentelectric dipole moment


Basic Terms• Electric Field : The region around the charge within which

its effect is felt or experienced is known as electric field.• Electric flux Ø : It is defined as the total number of electric

lines of force passing through a given area in the electric field.• Electric flux density : It is defined as the number of electric

lines of force passing normally through an unit area of cross section in the field.

• Dielectric Constant r : Determines he characteristics of a dielectric.

• Polarization vector P : It is the dipole moment per unit volume of the dielectric material.

E

D


Basic Terms• Electric susceptibility : It is he ratio of polarization to he

net electric field as modified by induced chargeson the surface of the dielectric.

e


Polarization of Dielectric• Dielectric polarization is the displacement of charged particles

under the action of the external electric field. Several microscopic mechanisms are responsible.

• They are of following types:– Electronic– Ionic– Orientation– Interfacial


Polarization of DielectricElectronic Polarization

• Electronic Polarization occurs due to the displacement of positively charged nucleus and negatively charged electrons in opposite directions, when an external electric field is applied, and thereby a dipole moment is created in the dielectric.

• It occurs only at optical frequencies (1015Hz)N

re

)1(0


Polarization of DielectricIonic Polarization

• Ionic polarization arises due to the displacement of -ive ions and +ive ions in opposite directions and it occurs in ionic solids, in the presence of electric field.

• This polarization occurs at frequency 1013 Hz.

Mmi

e 1120

2

- +

Cl Na

- +

x2 x1

Electric field++

+

++

+++

_

____

__

_1x 2x

anioncation


Polarization of DielectricOrientation Polarization

• In polar molecules the net dipole moment is negligibly small since all the molecular dipoles are oriented randomly when there is no EF. In the presence of the electric field these all dipoles orient them selves in the direction of field as a result the net dipole moment becomes enormous.

• It occurs at a frequency 106 Hz to 1010Hz.

Tk Bo 3

2

Without Field With Field


Polarization of DielectricInterfacial Polarization

• The space-charge polarization occurs due to the diffusion of ions, along the field direction, thereby giving rise to redistribution of charges in the dielectrics.

Without Field With Field


Internal Field• Internal field in a dielectric is the space and time average of the

electric field intensity acting on a particular molecule in the dielectric material.

• Evaluation : Consider a dielectric be placed between the plates of a parallel plate capacitor and let there be an imaginary spherical cavity around the atom A inside the dielectric. The internal field at the atom site ‘A’ can be made up of four components E1 ,E2, E3

& E4.


Internal Field

+ ++ ++ + + + + ++

_ _ _ _ _ _ _ __E

Dielectricmaterial

Spherical Cavity

A

__

_

__ ___

+ + + + + ++

+

+ ++

+ ++

+

_

_

___

___


Internal Field• Field E1: is the field intensity at A due to the charge

density on the plates.

• Field E2: is the field intensity at A due to the charge density induced on the two sides of the dielectric.

• Field E3: is the field intensity at A due to the atoms contained in the cavity, we are assuming a cubic structure, so E3 = 0.

01

PEE

02

PE


Internal Field• Field E4: is due to polarized charges on the surface of

the spherical cavity.

• Hence internal field I given by:

+ +

E

d r

p q

R

dA

r

A

+

++

+ +++

+++

___

___

__ __

_

_

drdA sin.2 2

oi

i

pEE

EEEEE

3

4321


Clausius-Mossotti Equation• Clausius-Mossotti Equation is given by:

21

3 0

r

reN


Types of Dielectric Materials• Dielectric materials can be divided into following groups:

– Solid Dielectrics - are of following types:• Mica – is inorganic material and is crystalline in nature.• Glass – is inorganic material made by fusion of different oxides.• Rubber – is a organic polymer, which can be natural or artificial.• Ceramic – is non-metallic organic compound such as silicates


Types of Dielectric Materials– Liquid Dielectric – includes following:

• Mineral Insulating Oils – obtained from crude petroleum & have high oxidation resistance.

• Synthetic Insulating oil – are very much resistant to oxidation & fire hazards.

• Miscellaneous Insulating oils – Vaseline, vegetable oils, silicon oils belongs to this.

– Gaseous Dielectric – includes• Air• Nitrogen• Sulphur hexafluoride• Inert gases


Properties of a Good Dielectric Material• High resistivity to leakage current.• High dielectric strength.• High mechanical strength.• High fire resistance.• Low thermal expansion.• Low dielectric loss.• Low water absorption quality.• High chemical inertness.


Application of DielectricCapacitors

• The major role of dielectric in a capacitor is to store energy.• Capacitors may be divided into:

– With gaseous dielectric.– With liquid dielectric.– With solid dielectric.


Application of DielectricTransformer

• The major role of dielectric in a transformer is to as insulators as well as cooling agent.

• Transformer may be divided into:– With gaseous dielectric.– With liquid dielectric.– With solid dielectric.


Thank You


Engineering

Dielectrics and its applications