Electrical and Magnetic Properties of Materials

Dr. Emmanuel Kwesi Arthur

Email: ekarthur2005@yahoo.com

Phone #: +233541710532

Department of Materials Engineering,

Kwame Nkrumah University of Science and Technology, Kumasi, Ghana

©2017

Course Code: MSE 455

1

Lecture Four

Example

2

Given, Polarization P = 10-6 Coul/sq-m

Find r for E = 50 kV/m

Calculate the Electric Charge Displacement, D

Example

3

Show that the relative dielectric constant of a barium titanate crystal, which, when inserted in a parallel plate condenser of area 10 mm x 10 mm and distance of separation of 2 mm, gives a capacitance of 10–9 F is 2259.

Electrical Terms

Electric Field is the ratio of a Voltage Drop to Distance over Which the Drop Occurs; to whit

and Current will Flow

Now as V Increases toward at Some Point the Dielectric will “Break Down”

mV

lVΕ

/units

mV

lVΕ flowibd

/units

Thus the Dielectric E-Field Strength

Examples

• r = 1.00059

• Ebd = 3 x 106 V/m (75 V/mil) For Air at Room

Conditions

6

Impurities in dielectrics

Single-crystals of wide-gap insulators are optically transparent (diamond, alumina)

Impurities in the band gap can lead to absorption of light with a specific frequency

Doping with shallow impurities can also lead to semiconducting behaviour of the dielectrics. This is favourable for high-temperature applications because one does not have to worry about intrinsic carriers (e.g. in the case of diamond or more likely SiC)

Dielectric material can be solid, liquid or gas

High vacuum can also be used as a dielectric

Solid dielectrics are most commonly use like glass, rubber, mica etc..

As a liquid dielectric material Transformer oil, cable oil, Capacitor oil, Vegetable oil etc can be used.

Gaseous dielectric materials are used for both as insulators and also as a cooling agents.

For example: Air, Hydrogen, nitrogen, Helium, Sulphur- dioxide, Propen, methane etc..

Types of Dielectric Material

I) Mica: It is inorganic mineral material made up of silicate of aluminiumwith silicate of soda, potash and magnesia. It is rigid, tough and strong.It has high dielectric strength and is not affected by moisture.It is widely used in irons, hot plates and toasters.

II) Glass: It is inorganic material made by the fusion of differentoxides like SiO2, ZnO and MgO.

It is Brittle and hard material and has good dielectric strengthIt is mostly used in the capacitors. Also used as dielectric tubes in radios and television.

III) Asbestos: It is naturally occurring material. In general it consist of magnesium silicate.It has low dielectric strength. It is used as insulating materialto prevent current flow in the outer body. It is widely used in

the form of the paper, tap, cloth etc.

Solid Dielectric Material:

IV) Rubber: It is organic polymer. It may be natural or synthetic. It has good electrical and thermal properties also it has good tensile strength. It is used for the insulating materials on electrical wires.

V) Ceramics: They are generally non-matalic inorganic compounds such as silicates, aluminates, oxides, carbides, borides etc.

Ceramics can be classified as: clay products, refractories, and glasses.

Ceramics are hard, strong and dense. They have exellent dielectric and mechanical properties. They widely used as insulators in switches, plug holders etc. They are also used as dielectric in capacitors.

2) Liquid Dielectric Material:

I) Mineral Insulating Oil : These oils are obtained from crude petroleum. These have good thermal stability.

They are used in Transformers as cooling and insulating material and also in Capacitors.

Transformer oil, cable oil and capacitor oil belong to the category of mineral insulating oil.

II) Synthetic Insulating Oil : Askarels, aroclors, sovol and savtol are a few synthetic oils that are widely used.

They are very much resistant to fire hazards.

Due to longer life and safety in operating condition, these oils are used as coolants and insulators in high voltage transformers in place of Transformer oil.

II) Miscellaneous Insulating Oil :

Vaseline, vegetable oils and silicon liquid belongs to these category. Silicon liquids has thermal stability upto 200 C and are very costly.

The dielectric strength of these oils are same as mineral oils so they are also used in the H.V transformers.

3) Gaseous Dielectric Material:I) Air : It is naturally available dielectric material.

Dielectric loss is practically zero. The dielectric constant of airlinearly increase with increase in pressure.

It is used as dielectrics in air condensers.

It can be used as an insulator only in low voltageapplications.

II) Nitrogen : It is important gaseous dielectric material. It prevent oxidation.It is used in cables and capacitors under pressure.

III) Sulphure Hexafluoride:It is formed by burning of Sulphure in fluorine atmosphere.It has superior cooling properties than air and nitrogen.

It is used in the transformers, electrical switches, voltagestabilizer and X-ray apparatus.

IV) Inert Gases: They are used in electronic tubes and dischargetubes as insulators.

Properties of Good Dielectric Material

It should have high resistivity to reduce the leakage

current.

It should have high dielectric strength.

It should have high mechanical strength.

It should have high fire resistance.

It should have low thermal expansion.

It should have high thermal conductivity.

It should have low dielectric loss.

It should have low water absorption quality.

Applications of Dielectrics

1. Capacitors

2. Transformers

3. Polymeric film

4. Electrolytic

5. Power and Distribution transformers

6. Other applications

All Done for Today

ElectricalCapacity

Ferro-electricity Ferro-electricity is defined as the spontaneous alignment of

electric dipoles in the absence of an external field.

The spontaneous polarization results from relative displacement of cations and anions from their symmetrical positions. Therefore, ferroelectric materials must posses permanent dipoles.

Examples of ferroelectric materials: BaTiO3, Rochelle salt (NaKC4H4O6.4H2O), potassium dihydrogen phosphate (KH2PO4), potassium niobate (KNbO3), lead zirconate titanate [Pb (ZrO3, TiO3)].

These materials have extremely high dielectric constants at relatively low applied field frequencies. Hence, capacitors made from ferroelectric materials are smaller than those made from other dielectric materials.

16

Piezoelectricity

Piezo-electricity is defined as conversion of electrical energy into mechanical strain and vice versa.

It arises due to polarization induced by an external force. Thus by reversing the direction of external force, direction of the established field can be reversed i.e. the application of an external electric field alters the net dipole length causing a dimensional change.

Application for these materials includes microphones, ultrasonic generators, sonar detectors, and mechanical strain gauges.

Examples: Barium titanate, lead titanate, lead zirconate (PbZrO3),ammonium dihydrogen phosphate (NH4H2PO4), and quartz.

17

Assignment

1. What is Ohm’s Law?

2. What is resistivity?

3. Briefly explain the band theory of electrical conduction.

4. What is Fermi energy?

5. Why are metals highly conductive?

6. Briefly explain the conduction mechanism in metals?

7. What is the difference between band structure of Cu and Mg?

8. How is the conductivity of metals affected by impurity level?

9. What is the role of dislocations on conductivity of metals?

10. Why does the metallic conductivity decrease with increasing temperature?

11. What is the typical band gap in semiconductors?

12. What is intrinsic semi conductivity?

18

Assignment

13. Show that the conductivity in intrinsic semi conductors,

14. What is extrinsic semi conductivity? Which factors control the conductivity in these semi conductors?

15. What are acceptor and donor levels?

16. Explain the atomic and band theory models of extrinsic semi conductivity.

17. What is the effect of temperature on extrinsic semi conductivity?

18. How does the carrier concentration in intrinsic semi conductors depend on temperature?

19. Name some compound semi conductors.

20. Calculate the electrical conductivity of intrinsic Si at 150 C. The carries concentration in Si at 150 C is 4 x 1019 m-3 and e = 0.06 m2/V-s and h = 0.022 m2/V-s.

19

What is an insulator? What is the reason that a material

behaves an insulator?

What is an insulator? What is the reason that a material

behaves an insulator?

Discuss factors that affect the electrical resistance of

materials.

Define a dielectric. Explain the common properties and uses

of dielectrics.

Describe with examples the three categories of dielectrics.

Describe the characteristic properties of ferroelectric

materials.

What are ferroelectric materials?

What is piezoelectricity?20

Explain the meaning and origin of piezoelectricity. Justify the

statement that “all ferroelectric crystals are piezoelectric, but all

piezoelectric crystals are not necessarily ferroelectric”.

How do temperature and impurities affect electrical resistivity of

metals?

What are the characteristics of dielectric materials?

How polarization takes place in dielectrics?

What are the possible polarization types in a dielectric?

What will happen to the charged stored in a capacitor, if a dielectric

material is inserted between the plates of a capacitor?

What are the main causes of electric breakdown of a dielectric?21

Explain, why:

(a) In actual practice, several thin layers of dielectric are used in

capacitors instead of a single thick layer?

(b) Oxygen free high conductivity copper is specified for bus bars?

(c) Manufactures usually specify maximum safe operating

temperature on electric motors?

(d) In case of dielectric materials, energy required for an electron

to cross the gap is large?

(e) The breakdown voltage does not increase in proportion to the

increase in thickness of dielectrics?

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23

Assignment

24

25

• Electrical conductivity and resistivity are:

-- material parameters-- geometry independent

• Conductors, semiconductors, and insulators...

-- differ in range of conductivity values-- differ in availability of electron excitation states

• For metals, resistivity is increased by

-- increasing temperature-- addition of imperfections-- plastic deformation

• For pure semiconductors, conductivity is increased by

-- increasing temperature-- doping [e.g., adding B to Si (p-type) or P to Si (n-type)]

• Other electrical characteristics

-- ferroelectricity-- piezoelectricity

Summary

QUESTIONS

26

Magnetic Properties and Magnetic Materials

27

Part Two

Issues to address Why study the magnetic properties?

What is magnetism?

What are the atomic reasons for magnetism?

Classification of Magnetic Materials

Magnetic Dipoles and Magnetic Moments

Magnetization, Permeability, and the Magnetic Field

Diamagnetic, Paramagnetic, Ferromagnetic, Ferrimagnetic, and Superparamagnetic Materials

Domain Structure and the Hysteresis Loop

The Curie Temperature

Applications of Magnetic Materials

Metallic and Ceramic Magnetic Materials28

Why do we study magnetic properties of Materials?

An understanding of the mechanism that explains the permanent magnetic behavior of some materials

May allow us to alter and

In some cases tailor the magnetic properties

Iron, some steels, and the naturally occurring mineral lodestone (Magnetite]

are wellknown examples of materials tat exhibit magnetic properties

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Why do we study magnetic properties of Materials?

Many of our modern technological devices rely on magnetism and magnetic materials-

Electrical power generators

Transformer,

Electric motors

Radio, television, telephones,

Computers, and

Components of sound and video reproduction systems.

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The Concept of “Fields”

A magnet has a ‘magnetic field’

distributed throughout

the surrounding space

Michael Faraday realized that ...

31

Magnetic Field Lines

Magnetic field is a force which is generated due to energy change in a volume of space

Magnetic field lines describe the structure of magnetic fields in three dimensions.

A magnetic field is produced by an electrical charge in motion e.g. current flowing in a conductor, orbital movement and spin of electrons.

The magnetic field can be described by imaginary lines as shown in the figure below for a magnet and a current loop.

The magnetic force is strongest near the poles where they come together.

32

Magnetic Field

Magnetic poles All magnetic materials have two poles- north pole and

south pole

Just as in electrostatics, like repels like and opposites attract

-N repels N, S repels S, N attracts S

Magnetic field lines Similar to electric field lines

The more closely spaced the lines, the more intense the field

Magnetic field lines point away from north poles and toward south poles, always form closed loops

The magnetic field, or strength of the magnet, is concentrated at the poles.

The field exists in all directions but decreases in strength as distance from the poles increases.

Field indicated by lines of force.

Unlike poles attract

Like magnetic poles repel

33

Magnetic Dipole

34

The Magnetic Field of the Earth Earth is a huge natural magnet.

The north pole of a magnet is the one that seeks the earth’s magnetic north pole.

The south pole is the one that is opposite the north pole.

When you use a compass, the north-pointing end of the needle points toward a spot near (but not exactly at) the Earth’s geographic north pole.

The Earth’s magnetic poles are defined by the planet’s magnetic field.

That means the south magnetic pole of the planet is near the north geographic pole.35

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The Magnetic Field of the Earth

The gauss is a unit used to measure the strength of a magnetic field.

The magnetic field of the Earth is very weak (0.5 gauss) compared with the strength of the field on the surface of the classroom ceramic magnets (1000 gauss).

Historical data shows that both the strength of the Earth’s magnetic field and the location of the north and south magnetic poles can switch places.

Today, the Earth’s magnetic field is losing approximately 7 percent of its strength every 100 years.

37

Induction by the Magnetic Field

Magnetizing an iron bar by induction.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Induction is the electric or magnetic effect of one body on another without any contact between them.

When an iron bar is placed in the field of a magnet, poles are induced in the iron bar.

The induced poles in the iron have polarity opposite from the poles of the magnet.

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Induction by the Magnetic Field

Permeability () is the ability of a material to support magnetic flux.

Relative permeability (r) compares a material with air. Ferromagnetic values range from 100 to 9000.

Magnetic shields use highly permeable materials to prevent external fields from interfering with the operation of a device or instrument.

Magnetic shieldaround a meter

movement.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.39

40

• Created by current through a coil:

Generation of a Magnetic Field -- Vacuum

I = current (ampere)

I

BN = total number of turns

= length of each turn (m)

B = magnetic field (tesla)

41

A magnetic field is induced in the material

Generation of a Magnetic Field --within a Solid Material

current I

B = Magnetic field (tesla)inside the material

r

0

Relative permeability (dimensionless)

B

Magnetic field StrengthIf a magnetic field, H, is generated by a cylindrical coil

(solenoid) of n turns and length l, H = nI/l (A/m)

Magnetic flux density, B: It is the magnitude of the field strength within a substance subjected to a field H

µ, called the permeability, is the measure of the degree to which a material can be magnetized.

In vacuum B = µoH. µo is the permeability of vacuum and is a universal constant. µo = 4π x 10-7(H/m). µr = µ/µo is the relative permeability.

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Magnetic Flux ΦMagnetic flux is defined as the number of lines of force flowing

outward from a magnet’s north pole.

Symbol: Φ

Units:

maxwell (Mx) equals one field line

weber (Wb) One weber (Wb) = 1 x 108 lines or Mx

Total flux Φ is 6 lines or 6 Mx. Flux density B at point P is 2 lines per square centimeter or 2 G.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.43

Flux Density B

Flux density is the number of lines per unit area of a section perpendicular to the direction of flux. Symbol: B

Equation: B = Φ / area

Flux Density Units Gauss (G) = 1 Mx/cm2 (cgs unit)

Tesla (T) = 1 Wb/meter2 (SI unit)

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Magnetization

45

Magnetic Susceptibility

46

Induction by the Magnetic Field Magnetic Permeability

Magnetic permeability is the ability to concentrate lines of magnetic force.

Ferromagnetic materials have high permeability.

Magnetic shields are made of materials having high permeability.

Symbol: r (no units; r is a comparison of two densities)

Magnetic permeability - The ratio between inductance or magnetization and magnetic field. It is a measure of the ease with which magnetic flux lines can ‘‘flow’’ through a material.

Magnetization - The total magnetic moment per unit volume.

Magnetic susceptibility - The ratio between magnetization and the applied field.

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Permeability: ( µ )

48

The Magnetic induction B is proportional to the applied Magnetic field intensity H.

H

B

HB

HB

Where µ permeability of a medium

49

Relative permeability µr

The ratio of permeability of medium to the permeability of free space is called relative permeability µr of the solid.

00

0

B

B

H

BH

B

r

r

50

Magnetization

Magnetization refers to the process of converting a non-magnetic material into a Magnetic material.

The intensity of Magnetization is directly related to the applied field H.

H

M

HM

HM

m

m

lity susceptibi magnetic

51

Magnetism on Microscopic Scale

Electrons can generate magnetism in three ways: i) As moving charges as current, ii) Due to their spin of electrons andiii) Due to their orbital rotation around a core.

The later two mechanisms (spin, orbital) are responsible for magnetic behavior in matter

When the electrons revolves around the nucleus Orbital magnetic moment arises, similarly when the electron spins, spin Magnetic moment arises.

eµlm

Origin of Magnetic Moment

Origin of magnetic dipoles: (a) The spin of the electron produces a magnetic field with a direction dependent on the quantum number ms. (b) Electrons Electrons orbiting around the nucleus create a magnetic field around the atom.

Spin & Obit

52

Spins in 3d Metals

53

All atoms have electrons, so you might think that all materials should be magnetic, but there is great variability in the magnetic properties of materials.

The electrons in some atoms align to cancel out one another’s magnetic influence.

While all materials show some kind of magnetic effect, the magnetism in most materials is too weak to detect without highly sensitive instruments.

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