3515_Ch 6_Dielectric Properties of Materials_ M A Islam

DIELECTRIC PROPERTIES OF

MATERIALS

Dr. Mohammad Aminul Islam

Assistant Professor

Dept. of EEE

International Islamic University Chittagong

Chapter 6

Ancient times

1745 first condensor constructed by Cunaeus and Musschenbroek

And is known under name of Leyden jar

1837 Faraday studied the insulation material,which he called the dielectric

Middle of 1860s Maxwell’s unified theory of electromagnetic phenomena

= n2

1887 Hertz

1897 Drude

1847 Mossotti

1879 Clausius

Lorentz-Lorentz

Internal field 1912 Debye

Dipole moment M A Islam, EEE, IIUC

The dynamic range of Dielectric Spectroscopy

Dielectric spectroscopy is sensitive to relaxation processes

in an extremely wide range of characteristic times ( 10 5 - 10 -12 s)

Broadband Dielectric Spectroscopy

Porous materials and colloids

Clusters Single droplets and pores

Glass forming liquids

Macromolecules

10-2 10-4 100 102 104 106 108 1010 1012

Time Domain Dielectric Spectroscopy

f (Hz) 10-6

Water

ice

M A Islam, EEE, IIUC

Broadband Dielectric Spectroscopy

Cells

Time Domain Dielectric Spectroscopy

f (Hz)

Dielectric response in biological systems

Dielectric spectroscopy is sensitive to relaxation processes

in an extremely wide range of characteristic times ( 10 5 - 10 -11 s)

H

H3N+ — C — COO-

R

Ala Asp Arg Asn

Cys Glu Gln His

Ile Leu Lys Met

Phe Ser Thr Trp

Tyr Val

10-1

P

-

N+ Head group

region

Lipids

Proteins Water

- Dispersion

DNA, RNA

-Dispersion - Dispersion - Dispersion

Tissues

ice

Amino acids

103 102 0 105 106 107 108 109 1010 104 101 1011 1012 1013 1014


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. In

addition, the forbidden energy band gap (e.g.) for dielectric

materials is more than 3eV.

Not possible for the electrons in the valence band to excite to

the conduction band, by crossing the energy gap, even with

normal voltage or thermal energy.

Dielectrics are non-metallic materials of high specific

resistance and negative temperature coefficient of resistance

Dielectric Materials


Active and Passive Dielectrics

The dielectric materials can be classified into active

and passive dielectric materials.

i. Active dielectrics

When a dielectric material is kept in an external

electric field, if it actively accepts the electricity, then it is

known as active dielectric material. Thus, active dielectrics

are the dielectrics, which can easily adapt themselves to

store the electrical energy in it.

ii. Passive dielectrics

Passive dielectrics are the dielectrics, which restrict

the flow of electrical energy in them. So, these dielectrics

act as insulators.

Examples: All insulating materials such as glass,

mica, rubber etc.,


Basic Definitions in Dielectrics

Electric Field

The region around the charge within which its effect is

felt or experienced is known as electric field.

The electric field is assumed to consist of imaginary electric

lines of force. These lines of force originate from the positive

charges and terminate to the negative charges .

Electric field strength or electric field intensity (E)

Electric field strength at any point is defined as the force

experienced by an unit positive charge placed at the point. It is

denoted by ‘E’.

‘q’ - magnitude of the charge in coulombs

‘f’ - force experienced by that charge in Newton,

electric field strength (E)

. Its unit is Newton / Coulomb (or) volt / metre. M A Islam, EEE, IIUC

Electric flux

It is defined as the total number of electric lines of force passing

through a given area in the electric field. (Emanated from the positive

charge). Unit: Coulomb

Electric flux density or electric displacement

vector (D) It is defined as the number of electric lines of force passing

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

Its unit is Coulomb / m2


Permittivity Permittivity is defined as the ratio of electric displacement vector (D)

in a dielectric medium to the applied electric field strength (E).

Mathematically the permittivity is, .Its unit is Farad /metre

The permittivity indicates the degree to which the medium can resist

the flow of electric charge and is always greater than unity.

Dielectric Constant

The dielectric constant or relative permittivity of a material

determines its dielectric characteristics. It is the ratio of the

permittivity of the medium and the permittivity of free space


Electric Polarization Consider an atom. We know that it is electrically neutral.

Furthermore, the centre of the negative charge of the electrons

coincides with the positive nuclear charge, which means that

the atom has no net dipole moment.

However, when this atom is placed in an external electric

field, the centre of the positive charge is displaced along the

field direction while the centre of the negative charge is

displaced in the opposite direction.

When a dielectric material is placed inside an electric field,

such dipoles are created in all the atoms inside.


Polarizability () When the electric field strength ‘E’ is increased, the strength of the

induced dipole is also increased. Thus, the induced dipole moment is

proportional to the intensity of the electric field.

Polarization vector

The dipole moment per unit volume of the dielectric material is

called polarization vector.

‘ ’ - average dipole moment per molecule and

‘N’ - number of molecules per unit volume

polarization vector is given by,

Unit: Coulomb / m2


Relation between P,0 , r and E

The polarization ‘P’ is related to the electric flux density D as,

D = 0E + P

Since D = 0r E, the above relation becomes,

0 r E = 0E + P

(or) P = 0r E 0E

i.e. P = 0 ( r 1 )E

Electric susceptibility

The polarization P is proportional to the applied electric field

intensity E and it is in the same direction of ‘E’ It can be written as,


P E (or) P = 0 e E

Various Polarization mechanisms in

Dielectrics Dielectric polarization is the displacement of charged

particles under the action of the external electric field. Several

microscopic mechanisms are responsible for electric polarization.

Four types of microscopic polarization mechanisms. Electronic polarization

Ionic polarization

Orientation polarization and

Space-charge polarization.


i. Electronic 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.

The induced dipole moment µ = eE

where e = electronic polarizability.

Monoatomic gases exhibit this kind of polarization, Electronic

polarizability is proportional to the volume of the atoms and is independent

of temperature.

The electronic polarizability = e = 4e0R3 ( Farad.m2) where R

is the radius of the atom.


Nucleus Nucleus

Sphere of

electronic

charge

+Ze Ze Original Position

Field direction

Displaced

Equilibrium

position

x

Fig. (a) Position of +ve and –ve charges in an atom without

field (b) Position of +ve and –ve charges in an atom with

field


ii. Ionic Polarization Ionic polarization arises due to the displacement of -ve

ions and + ve ions in opposite directions and it occurs in

ionic solids, in the presence of electric field. The

displacement is independent of temperature. Example : NaCl

crystal

-

+

Cl Na

-

+

x2 x1

Fig. (a) Without field (b) With field


iii. Orientation Polarization

The orientation polarization arises due to the presence of polar

molecule in the dielectric medium.



Explanation:

In the case of a CH3Cl molecule, the positive and negative

charges do not coincide. The Cl- has more electro negativity than

hydrogen. Therefore, the chlorine atoms pull the bonded electrons

towards them more strongly than hydrogen atoms. Therefore, even in the

absence of field, there exists a net dipole moment.

Now, when the field is applied, positive portion align along the

direction of field and negative portion align in the opposite direction of the

field. This kind of polarization is called as orientation polarization.

This depends on temperature; when temperature is increased,

the thermal energy tends to randomize the alignment


Space-Charge 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



Explanation Without the application of external field, the ions are orderly

arranged as shown in the

Fig.

Now, when the field is applied, the ions diffuse with respect to

the direction of applied field. Thus the polarization occurs, known as

space charge polarization.

Normally, this type of polarization occurs in ferrites and

semiconductors and will be very small.


The Clausius–Mossotti relation is analyzed the relationship between the

dielectric constants of two different media, and the indices of refraction.

The Clausius–Mossotti law applies to the dielectric constant of a dielectric that

is perfect, homogeneous and isotropic. It is the second of the following three

equalities:

Clausius–Mossotti Relation


THANK YOU

3515_Ch 6_Dielectric Properties of Materials_ M A Islam

Documents

Transcript of 3515_Ch 6_Dielectric Properties of Materials_ M A Islam