Bell 301 material science unit iii

BY PRASHANT KUMAR

ASST. PROFESSOR

MITS GWALIOR

DIELECTRIC MATERIALS:

What are dielectrics ?

A dielectric (or dielectric material) is an electrical insulator that can be polarized by an applied electric field.

When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor , but only slightly shift from their average equilibrium positions causing dielectric polarization.

Because of dielectric polarization, positive charges are displaced toward the field and negative charges shift in the opposite direction. This creates an internal electric field that reduces the overall field within the dielectric itself

Basically, there are four mechanisms of polarization (1.Electronic or Atomic Polarization 2. Ionic Polarization 3. Dipolar or Orientation Polarization 4. Space charge polarization

Polarization is separation of opposite charges in external electric field)

Electronic or Atomic Polarization

This involves the separation of the centre of the electron cloud around an atom with respect to the centre of its nucleus under the application of electric field

Ionic Polarization

This happens in solids with ionic bonding which automatically have dipoles but which get cancelled due to symmetry of the crystals. Here, external field leads to small displacement of ions from their equilibrium positions and hence inducing a net dipole moment .

Dipolar or Orientation Polarization

This is primarily due to orientation of molecular dipoles in the direction of applied field which would otherwise be randomly distributed due to thermal randomization.

Interface or Space Charge Polarization

This involves limited movement of charges resulting in alignment of charge dipoles under applied field. This usually happens at the grain boundaries or any other interface such as electrode-material interface. (eg multiphase material)

Ions diffuse over considerable distance and redistribution occurs in presence of external field.

ORIGIN OF

MAGNETIS

MOMENT

ANGULAR

MOMENTUM OF

ELECTRON (L)

ELECTRON

SPIN

NUCLEAR SPIN

( very week effect)

ANGULA

R

DIPOLE

MOMEN

T

SPIN MAGNETIC

MOMENT S

eS

m

2L

eL

m

The spin of the electron produces a magnetic field with a direction dependent on the quantum number S .

Comparison of Dia, Para and Ferro Magnetic materials:

DIA PARA FERRO

1. Diamagnetic

substances are those

substances which are

feebly repelled by a

magnet.

Eg. Antimony, Bismuth,

Copper, Gold, Silver,

Quartz, Mercury, Alcohol,

water, Hydrogen, Air,

Argon, etc.

Paramagnetic substances

are those substances

which are feebly attracted

by a magnet.

Eg. Aluminium, Chromium,

Alkali and Alkaline earth

metals, Platinum, Oxygen,

etc.

Ferromagnetic substances

are those substances

which are strongly

attracted by a magnet.

Eg. Iron, Cobalt, Nickel,

Gadolinium, Dysprosium,

etc.

2. When placed in magnetic

field, the lines of force tend

to avoid the substance.

The lines of force prefer to

pass through the substance

rather than air.

The lines of force tend to

crowd into the specimen.

N S

S N S N

2. When placed in non-

uniform magnetic field, it

moves from stronger to

weaker field (feeble

repulsion).

When placed in non-


moves from weaker to

stronger field (feeble

attraction).

When placed in non-


moves from weaker to

stronger field (strong

attraction).

3. When a diamagnetic

rod is freely suspended in

a uniform magnetic field, it

aligns itself in a direction

perpendicular to the field.

When a paramagnetic rod

is freely suspended in a



parallel to the field.

When a paramagnetic rod

is freely suspended in a



parallel to the field very

quickly.

SN SN SN

4. If diamagnetic liquid

taken in a watch glass is

placed in uniform

magnetic field, it collects

away from the centre

when the magnetic poles

are closer and collects at

the centre when the

magnetic poles are

farther.

If paramagnetic liquid


placed in uniform


at the centre when the

magnetic poles are closer

and collects away from

the centre when the

magnetic poles are

farther.

If ferromagnetic liquid


placed in uniform


at the centre when the

magnetic poles are closer

and collects away from

the centre when the

magnetic poles are

farther.

5. When a diamagnetic

substance is placed in a

magnetic field, it is

weakly magnetised in the

direction opposite to the

inducing field.

When a paramagnetic



weakly magnetised in the

direction of the inducing

field.

When a ferromagnetic



strongly magnetised in

the direction of the

inducing field.

6. Induced Dipole

Moment (M) is a small

– ve value.

Induced Dipole Moment

(M) is a small + ve value.

Induced Dipole Moment

(M) is a large + ve value.

8. Magnetic permeability

μ is always less than

unity.

Magnetic permeability μ

is more than unity.

Magnetic permeability μ

is large i.e. much more

than unity.

7. Intensity of

Magnetisation (I) has a

small – ve value.

Intensity of Magnetisation

(I) has a small + ve value.

Intensity of Magnetisation

(I) has a large + ve value.

9. Magnetic susceptibility

cm has a small – ve value.

Magnetic susceptibility cm

has a small + ve value.

Magnetic susceptibility cm

has a large + ve value.

10. They do not obey

Curie’s Law. i.e. their

properties do not change

with temperature.

They obey Curie’s Law.

They lose their magnetic

properties with rise in

temperature.

They obey Curie’s Law. At

a certain temperature

called Curie Point, they

lose ferromagnetic

properties and behave

like paramagnetic

substances.(Follows

modified Curie’s Weiss

Law)Curie’s Law:

Magnetic susceptibility of a material varies inversely

with the absolute temperature.

I α H / T or I / H α 1 / T

Xm α 1 / T

Xm = C / T (where C is Curie constant)

Curie temperature for iron is 1000 K, for cobalt 1400 K

and for nickel 600 K.

I

H / T

As temperature

increases above the curie

temp (Tc) due to higher

thermal energy many

domains align randomly

to diminish the net

dipole moment resulting

in change in magnetic

behaviour from

ferromagnetic to

paramagnetic behaviour

FERROMAGNETIC MATERIAL IS MAGNETISED USING TWO MECHANISMS

DOMAIN

GROWTH/DOMAIN

WALL

MOMENT==domain that

are parallel or nearly

parallel to external m.

field will Grow in size at

the cost of other domain

ROTATION OF DOMAIN

MAGNETIC MOMENT==

magnetic moment of domain

can rotate in direction of

applied field

Hysteresis means “remaining” in Greek, an effect remains after its cause has

disappeared. Hysteresis, a term coined by Sir James Alfred Ewing in 1881, a

Scottish physicist and engineer (1855-1935), defined it as: When there are two

physical quantities M and N such that cyclic variations of N cause cyclic

variations of M, then if the changes of M lag behind those of N, we may say that

there is hysteresis in the relation of M to N".

STEPS

An initially unmagnetized material is subjected to a cycle of magnetization. The values of Magnetic flux density B and the magnetizing field H are calculated at every stage and a closed loop is obtained on plotting a graph between M and H as shown in the figure.

The point ‘O’ represents the initial unmagnetized condition of the material. As the applied field is increased, the magnetization increases to the saturation point ‘A’ along ‘OA’.

As the applied field is reduced, the loop follows the path ‘AB’. ‘OB’ represents the magnetic flux density remaining in the material when the applied field is reduced to zero. This is called the residual magnetism or remanence. The property of retaining some magnetism on removing the magnetic field is called retentivity.

OC represents the magnetizing field to be applied in the opposite direction to remove residual magnetism. This is called coercive field and the property is called coercivity.

When the field is further increased in the reverse direction the material reaches negative saturation point ‘D’.

When the field is increased in positive direction, the curve follows path ‘DEF’.

Retentivity - It is a material's ability to retain a certain amount of residual magnetic field when the magnetizing force is removed after achieving saturation. (The value of B at point c on the hysteresis curve.)

Residual Magnetism - the magnetic flux density that remains in a material when the magnetizing force is zero. Note that residual magnetism and retentivity are the same when the material has been magnetized to the saturation point. However, the level of residual magnetism may be lower than the retentivity value when the magnetizing force did not reach the saturation level.

Coercive Force - The amount of reverse magnetic field which must be applied to a magnetic material to make the magnetic flux density return to zero. (The value of H at point d on the hysteresis curve.)

Coercivity: the resistance of a magnetic material to changes in magnetization, equivalent to the field intensity necessary to demagnetize the fully magnetized material.

Permeability - A property of a material that describes the ease with which a magnetic flux density is established in the component.

PARAMETERS HARD MAGNET SOFT MAGNET

HYSTRESIS LOSS HIGH LOW

Coercivity HIGH LOW

Retentivity HIGH LOW

Ease of magnetization and

demagnetization

Very difficult easier

Magnetic Permeablity Very small Large

Magnetic Susceptiblity Very small Large

Application Permanent magnet Electro magnet

Bell 301 material science unit iii

Engineering