Module X : Specialty Materials
Dielectrics & insulating materials – Characteristics; Ceramics –
Mica and glass; Magnetic materials – basis of magnetism – Soft
and hard magnetic materials; Composites : Classification –
Particulate, fibrous and laminated composites – Hybrid
composites – Application of composites in electrical and
electronic components; Semiconductors – Extensive and
intensive; Metallic solids –Characteristics
Dielectrics
Electrically insulating materials
Materials which are used to prevent loss of electricity through certain parts in an electrical system
Electrical insulating materials having an electrical conductivity of < 10-6 mho per cm and they act as
stores of electrical charges
In these materials, it is almost not possible to excite the electrons from the valence band to
conduction band by an applied field
Generally dielectrics are also called as insulators, thereby poor conductors of electricity. However
they allow movement of some electrons at abnormally high temperatures, causing a small flow of
current
• Solid Dielectrics – Ceramic, Plastic, Mica, Glass and Ebonite – hard rubber
• Dielectric Liquid – Distilled Water
• Dielectric Gas – Dry Air, vacuum, nitrogen and helium
All dielectric materials are insulating materials
The difference between a dielectric and an insulator lies in their applications
If the main function of non-conducting material is to provide electrical insulation, then they are
called as insulator
On the other hand, if the main function of non-conducting material is to store electrical charges then
they are called as dielectrics
The dielectric materials can be classified into • 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
Ex : Piezoelectric materials
• Passive dielectrics
Passive dielectrics are the dielectrics, which restrict the flow of electrical energy in them. So,
these dielectrics act as insulators
Ex : All insulating materials such as glass, mica, rubber etc.,
Properties
• Generally, the dielectrics are non-metallic materials of high resistivity
• They have a very large energy gap (more than 3eV)
• All the electrons in the dielectrics are tightly bound to their parent nucleus
• As there are no free electrons to carry the current, the electrical conductivity of dielectrics is
very low
• They have negative temperature coefficient of resistance and high insulation resistance
The choice of dielectrics depends on mechanical, electrical, chemical and thermal properties
Characteristics of dielectrics
1. Resistivity
It is the reciprocal of conductivity (capacity of a material to conduct the electric current) because
a good resister possesses low conductivity or high resistivity
Based on the measurement of flow of direct current into the insulation after electrification of 1
min
Have insulation resistance ranging from 109 to 1020 ohm.cm at room temperature
In general, low insulation resistance materials deteriorates very fast under weathering conditions
2. Dielectric polarization
When an electric field is applied to a dielectric material, there is a change or displacement that
takes place within the material through induced dipole, called dielectric polarization
This displacement takes place in such a way that negative charges
move towards positive electrode and vice versa
Since the charges are not free to move, they do not flow
but the interior of the body becomes non - equipotential
3. Dielectric constant
It is a measure of polarisation of the dielectrics
The capacity of a material to store electric charge in the presence of an electric field
Ratio of the electrical capacity of the condenser containing the material to the capacity of the
same conductor with the material replaced by vacuum
(Or) Dielectric constant = Absolute permittivity (ε) / Permittivity of free space (εo )
εr =ε/εo
Permittivity represents polarizable nature of material
Its unit is farad/metre
4. Dielectric strength
The electrical strength that must be applied to cause
disruptive effect or discharge or accumulate current
through the body of a material
5. Dielectric losses
Caused due to absorption of electrical energy (under an alternating current gives rise to
dissipation of electrical energy in the insulating material) and by the leakage of current through
the insulating material (as a result of conduction) – electric shock during rainy season
A measure of the power loss in the insulation and it should be low
Very important in high voltage cables
7. Heat and temperature resistance
Some heat is always generated even in insulators used at low T
Hence insulators used must be capable of resisting the heat as well as temperature increase
8. Thermal aging
The insulators become weak as well as less serviceable because of working at elevated T for long
times or in contact with O2 or air
A good insulator should have less thermal aging
9. Mechanical strength
A measure of tensile strength and compressive strength of an insulator
It depends on –
• High T and humidity (should be non – hygroscopic)
• Viscosity in case the dielectric is a liquid
• Porosity (less porous, otherwise moisture enters into pores)
• Least thermal expansion and contraction
• Chemically inert to action of oils, solvents, acid, alkalis, etc.,
Selection of a dielectric material
• Should have high value of dielectric constant. It determines the capacity to develop charges on
its surface due to polarization
• Should have low dissipation factor – lower the dissipation factor, smaller is the dielectric loss
• Should have sufficiently high dielectric strength – higher the strength, greater is the voltage per
unit thickness (it can withstand before breakdown)
• Should have high insulating resistance – higher the insulating resistance, lower is the leakage
current through the dielectric
Applications of Dielectric Material
• These are used for energy storage in capacitors
• To enhance the performance of a semiconductor device, high permittivity dielectric materials
are used
• Dielectrics are used in Liquid Crystal Displays
• Ceramic dielectric is used in Dielectric Resonator Oscillator - the frequency determining element, to
produce signals with excellent signal stability (used widely in today's electronic warfare, missile, radar and
communication systems)
• Barium Strontium Titanate thin films are dielectric which are used in microwave tunable
devices providing high tunability and low leakage current
• Parylene (polymer with benzene ring attached with c=c on either sides) is used in industrial
coatings acts as a barrier between the substrate and the external environment
• In electrical transformers, mineral oils are used as a liquid dielectric and they assist in the
cooling process
• Castor oil is used in high-voltage capacitors to increase its capacitance value
• Electrets, a specially processed dielectric material acts as electrostatic equivalent to magnets
Insulating materials – materials in which electrical conduction does not occur
Classification
Insulating materials
Gaseous
Flexible
Liquid
Hard
Heat insensitive
(paper, mica, MgO)
Thermosetting
Solid
Thermoplastic
Rigid (glass, mica,
porcelain,
thermosetting)
Based on the function, insulating materials are classified into –
Thermal insulators and Electrical insulators
I. Gaseous insulating materials
Ex : Air, N2, H2, CO2
Dielectric air provides insulation between the over head transmission lines very cheaply
The life of an electrical equipment is reduced by the O2 present in air, which leads to oxidation
Similarly, if air is used in conjugation with an oil, then oxidation of oil causes sludge formation
which leads to loss of mechanical stability of electrical equipment
H2 is used to reduce windage losses in rotating machinery
N2 is used in place of air – O2 causes oxidation
CO2 is used in flexed capacitors and as pre impregnant of oil filled high voltage cables and
transformers
II. Liquid insulating materials
Used for eliminating the air or other gases or heat transfer purposes
Generally used in conjugation with solid insulants
Classified according to the temperature range in which they are used
i) Mineral oils (HC oils)
Used in the temperature range of – 50 oC to 110 oC
The electrical properties and resistance to thermal expansion are influenced by the presence of
non – hydrocarbon compounds – O, N, S, etc
Widely used in transformers, circuit breakers (an electrical switch designed to protect an
electrical circuit from damage caused by overcurrent/overload or short circuit), switch gears (a
device that is used to switch, control and protect the circuits as well as devices), capacitors, etc
Circuit breakers
Switch gears
Arc resitance
ii) Askarels – synthetic fire resistant materials
Non inflammable synthetic insulating liquid used in the temperature range of – 50 oC to 110 oC
Chlorinated HC are the most widely used dielectrics because of their excellent arc resistance (the
ability of an insulating material to withstand a high voltage electric arc and resist the formation
of a conducting path along its surface), high dielectric strength, low dielectric constant (4-6) and
small dielectric loss at ordinary T
iii) Silicone fluids
Used in the T range of – 90 oC to 220 oC, non conductive to metals upto 200 oC
Disadv : broken down into H2, C, SiC and SiO2, can’t be used as switch gear oils
iv) Fluorinated liquids
Used in the T range of – 50 oC to 200 oC
Non inflammable and highly chemically stable
Provide much better heat transfer from the windings and magnetic circuits in comparison with
HC and silicone oils
Disadv : highly volatile and get deteriorated in presence of moisture
v) Vegetable oils
The oldest insulating liquids and used in the T range of – 20 oC to 100 oC
Drying oils (linseed, tung oil, etc) are suitable in the formulations of insulating varnishes (used in
transformers & motor coils - machine which is used to change the energy from electrical to
mechanical)
Non drying oils are used as plasticisers in insulating resin composites
III. Solid insulating materials
Classified according to the T up to which they can be used
Limiting temperature – T up to which insulating properties can be retained
i) Power and press boards
Made from wood cellulose Ex : paper, card boards
Paper is used as an insulation in multilayers in the form of oil impregnated from moisture by a
lead sheath, generally used at voltages above 15,000 V
Vulcanized fibre sheet or board – treating paper with a solution of ZnCl2 and then pressing to get
a thickness of 1.5 – 25 mm
This board has good mechanical strength when impregnated with diphenyls and vegetable oils
ii) Fibrous insulators
Ex : silk, cotton, wool, jute, rayon, nylon, fibre glass
These are used for their high mechanical strength, flexibility, durability with extreme finesse and
easy processing
Disadv : low dielectric strength and moisture absorption
But inorganic fibrous materials such as glass fibre and asbestos (hydrated aluminium silicate) can
be used at high T, up to 180 oC
Varnish glass cloth has been employed in winding of those electric machines which work at high
T
iii) Varnished Cambrics – linen or cotton fabric with varnish or insulating oil
Used as a series of thin layers
But they need protection from moisture and generally used at low voltages only
iv) Flexible insulating materials
a) Natural resins – Rosins (varnish manufacturing), Shellac (good adhesive property), Amber
(manufacturing instruments), etc
b) Rubber compounds
For specific purposes, they are changed from plastic form to elastic compound by
combination with S
Synthetic rubber are also used for their chemical resistance and dielectric strength
Ex : Silicone rubber, butadiene rubber, chloroprene rubber
v) Synthetic resins
Extremely flexible, high tear strength and tensile strength
a) Polyethylene
An important insulating material for high frequency applications
Possesses dielectric constant of 2.3 and its low electrical losses are so good at radio frequencies,
less hygroscopic
Available in the form of powder, sheet, film, rod, pellet, tube and foam
Disadv : can’t withstand high T and melts at 110 oC, but cross linking PE is used for high voltage
application
Its stiffness is a handicap, Degraded by oxidation
Undergoes cracking during service because of actions of greases and HC
Uses
As radio frequency insulator in radio, television and communication circuit cables
In power cables and submarine cables, etc
b) Teflon
An ideal dielectric material with dielectric constant of 2 – 2.2
The most thermally and chemically stable insulator
Because of its stiffness, it must be used in thin layers
Withstand high T above 200 oC than any other semi flexible insulation
Available in the form of tapes, rods, sheets, tubing's and moulds
Electrically stable up to 327 oC (m.pt)
Can be readily machined but can be moulded with difficulty
Uses
As capacitor dielectrics and insulating material for almost all kinds of windings
Heat resistant materials are made by combining Teflon with glass cloth
vi) Rigid insulators
a) Glass – amorphous material, a mixture of silicates, phosphates, borates and other materials
b) Ceramics
Properties of electrical insulation
I. Electrical properties
a) Resistivity
Even the best electrical insulating materials allow some current to pass when a voltage is applied
due to presence of ions (polarization) but conductivity is small when compared to a metal
The resistivity of a insulating materials decreases as the T increases
b) Surface resistivity
There are two types of conductance
i) Bulk conductance – conductance offered by the entire insulating material. It is large
ii) Surface conductance – conductance across its surface. It is small, except water and ionizable
impurities are present
To avoid surface conductivity, the surface path length of an insulator is increased by
making the surface into series of ridges and valleys
c) Dissipation factor
When electric current is passed through an electric cable, some current will flow across the
insulating material called leakage current
This leakage current will be opposed by the insulating material so that heat will be produced
which will be dissipated into the atmospheric air
This heat loss is measured by a parameter called dissipation factor
For an ideal insulating material, it should be as low as possible
II. Thermal conductivity
Heat generated in an electrical equipment must be dissipated by thermal conductivity to avoid
overheating and damage to equipment
In general, dielectrics are poor conductor of heat therefore, it is necessary to add filler to an
electric insulating material to dissipate the heat
For ex., inorganic fillers are added to organic insulators to increase thermal conductivity
Liquid and gaseous insulators are used as coolants. Ex : transformer oil, H2, He, SF6 in
transformer to protect electric arc
III. Chemical properties
Dielectrics should be resistance to oils, liquids, gas fumes, acid and alkalis
Sometimes, acid fumes are generated
To withstand it, epoxy resins insulators are used
Organic dielectrics are flammable. In order to prevent flammability due to short circuit, these
dielectrics are filled with Antimony trioxide or other fillers
Dielectrics should be resistance to water
Reactions with water can degrade/destroy any insulating materials
IV. Mechanical properties
Flexibility, tear & tensile strength, shear strength & abrasive resistance are the most important
mechanical properties required by dielectrics
For electrical machines, dielectrics should have sufficient mechanical strength to withstand
vibration
For cables, flexibility is very important
Ceramics
Means burnt materials
Made from burning clay materials (inorganic silicates, metallic oxides and their combinations –
carbides, borides and nitrides)
Ceramics are any inorganic, non metallic solids produced by burning at elevated T
Hard, strong, dense and brittle
Either crystalline or amorphous and thermoplastic in nature
Possess excellent dielectric and mechanical properties
Dielectric constant varies between 4 – 10
Components
A fine grained or plastic portion – imparts plasticity and workability – clay
A crystalline or non plastic portion – contributes mechanical strength – silica
A flux or glassy material – helps in bonding and cementing the ingredients together - Feldspar
Classification of ceramics
Based on their composition, ceramics are classified as Oxides, Carbides, Nitrides, Sulfides (MoS2),
Fluorides ( KAlSi3O8 or NaAlSiO3 or CaAl2Si2O8), etc.
Based on application
Traditional ceramics are made
from three basic components:
clay, silica (flint) and feldspar
Engineering ceramics consist of
highly pure compounds of
aluminium oxide (Al2O3),
silicon carbide (SiC) and
silicon nitride (Si3N4)
Based on application
Glasses
Glasses are a familiar group of ceramics –
containers, windows, mirrors, lenses, etc
They are non-crystalline silicates containing
other oxides, usually CaO, Na2O, K2O and Al2O3
which influence the glass properties and its colour
Clay products
It is found in great abundance and popular
because of ease with which products are made
Clay products are mainly two kinds – structural products
(bricks, tiles, sewer pipes) and
white- wares (porcelain, chinaware, pottery, etc.)
Abrasive ceramics
These are used to grind, wear, or
cut away other material
The prime requisite for this group of materials
is hardness or wear resistance in addition to
high toughness (hardness is expressed in mho scale, Diamond - 10)
As they may also exposed to high temperatures, they need to exhibit some refractoriness
Diamond, silicon carbide, tungsten carbide, silica sand, aluminium oxide / corundum are some
typical examples of abrasive ceramic
Refractories
They have capacity to withstand high
temperatures without melting or decomposing;
and their inertness in severe environments
Thermal insulation is also an important functionality
Cements
cement, plaster of paris (CaSO4)2.H2O) and lime come under this group of ceramics
The characteristic property of these materials is that when they are mixed with
water, they form slurry which sets subsequently and hardens finally
Thus it is possible to form virtually any shape
They are also used as bonding phase, for example between construction bricks
of refractories
Cement
Plaster of paris
Lime
Applications of ceramic materials
• Alumina (Al2O3) - is used in many applications such as to contain molten metal, where material
is operated at very high temperatures under heavy loads, as insulators in spark plugs, and in
some unique applications such as dental and medical use. Chromium doped alumina is used for
making lasers
• Aluminium nitride (AlN) - because of its good electrical insulation and high thermal
conductivity, it is used in many electronic applications such as in electrical circuits operating at
a high frequency. It is also suitable for integrated circuits
• Diamond (C) - is the hardest material known in nature. It has many applications such as
industrial abrasives, cutting tools, abrasion resistant coatings, etc. it is also used in jewellery
• Lead zirconium titanate (PZT) - is the most widely used piezoelectric material, and is used as
gas igniters, ultrasound imaging, in underwater detectors
• Silica (SiO2) - Silica-based materials are used in thermal insulation, abrasives, laboratory
glassware, etc. it also found application in communications media as integral part of optical
fibers. Fine particles of silica are used in tires, paints, etc.
• Silicon carbide (SiC) - one of best ceramic material for very high temperature applications. It is
used as coatings on other material for protection from extreme temperatures. It is also used as
abrasive material. It is used as reinforcement in many metallic and ceramic based composites.
It is a semiconductor and often used in high temperature electronics. Silicon nitride (Si3N4) has
properties similar to those of SiC but is somewhat lower, and found applications in such as
automotive and gas turbine engines
• Titanium oxide (TiO2) - mostly found as pigment in paints. It also forms part of certain glass
ceramics. It is used to making other ceramics like BaTiO3
• Titanium boride (TiB2) - exhibits great toughness properties and hence found applications in
armour production. It is also a good conductor of both electricity and heat
• Uranium oxide (UO2) - mainly used as nuclear reactor fuel. It has exceptional dimensional
stability because its crystal structure can accommodate the products of fission process
• Yttrium aluminium garnet (YAG, Y3Al5O12) - has main application in lasers (Nd-YAG lasers)
• Zirconia (ZrO2) - used in producing many other ceramic materials. It is also used in making
oxygen gas sensors, as additive in many electronic ceramics. Its single crystals are part of
jewellery
Mica
An inorganic mineral compound
Group of minerals which are physically and chemically similar –
silicates of alumina with silicates of soda potash and magnesia
Exhibits two dimensional sheet or layer structure
General formula – XY2-3Z4O10 (OH, F)2
X – K, Na, Ba, Cs, (H3O), (NH4)
X – Na – common mica
Ca – brittle mica
Y – Al, Mg, Fe2+, Li, Cr, Mn, V, Zn
Lepidolite
Z – Si, Al, Fe3+, Be, Ti
Ex : Muscovite – KAl2 (Si3 Al) O10 (OH, F)2
Lepidolite - K(Li,Al)3(Al,Si,Rb)4O10(F,OH)2
Phlogopite - K2Mg6(Si6Al2O20)(OH)4
Phlogopite
Muscovite
General properties
• Crystalline in nature
• Can be split into very thin flat sheets
• Rigid, tough and strong
• Excellent naturally occurring insulating material
• Dielectric constant varies from 5 – 7.5
• Since it is a sheet mineral, it can be split into strong, flexible film having high T resistance and
electrical insulation property
• Chemically inert, elastic dielectric, flexible, hydrophilic and light weight
Mica products
1. Mica sheets/micanites
Produced by applying shellac (a yellow natural resin produced by the lac insect – polyhydroxy
organic acid + wax) on either sides of the mica to form a sheet
Paper/cloth is glued on one side/both sides of the sheet to increase the tensile strength of sheet
mica
Used in electrical equipment and applications in the form of washers, spacers, tubes, etc
2. Mica tapes/wrappers
Very thin materials, which can be impregnated with resin
Good dielectric strength, thermal conductivity
Low dissipation factor
Used in insulating high voltage coils, motors and other areas of rotating machines
Used for making of fire resistant cable
3. Mica paper
Since mica is crystalline in nature, it lacks flexibility
To improve its flexibility, mica is broken into small pieces in aqueous solution and then formed
into thin sheet by applying heat and pressure
It is then rolled in the form of paper
Has excellent insulating property – both thermal and electrical
High T resistant, up to 1,000 oC
High tensile strength
Has good resin penetration and air permeability
Used in transformers, capacitors
4. Glass bonded mica
Consists of glass and mica in the ratio from 40:60 to 60:40
Prepared by heating a mixture of powdered glass and grounded mica to a plastic state (600 oC)
and then compressed/moulded
Has the lowest tensile strength and the highest thermal conductivity among glass and glass
ceramics
Applications of mica
• The worlds largest mica deposits are found in igneous, sedimentary and metamorphic rocks
found in Bihar and Nellore districts
• Sheet mica has been used as electrical condensers as insulation sheets between commutator
segments or in heating factors
• Sheets of Muscovite are applied in optical instruments
• Lepidolite is used for manufacture of warmth resistant glass
• Phlogopite is used for spark plugs
• Mica capacitors are low loss capacitors which are used where the high frequency is required
and their value does not change much over time
• Electrical components - includes transistors, where mica can be used to amplify particular
signals and block out others. Also commonly used as insulation in power diodes,
semiconductors and rectifiers. Mica can help to fully insulate semiconductors from their
chassis, helping to dissipate heat and keep the components cool
• It is one of the important ingredient in makeup and various cosmetics. It gives a shimmery
effect and adds sparkle
• To have attractive background of your scrapbook mix water and mica powder and spray it on
the scrapbook. Mix in oil paint to give extra painting effects
Mica products
Mica plates
Mica sheet
Capacitor
Cosmetics
Mica tube
Paints
Glass
Glass is a super cooled liquid of infinite viscosity
∆, 1650 𝑜𝐶, 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑓𝑢𝑟𝑛𝑐𝑒
Quarz/SiO2
→
colourless liquid
cooled rapidly
→
clear colourless, glassy mass
(super cooled liquid) Quartz glass
Quartz glass is used for manufacture of laboratory apparatus, it can withstand rapid changes of T
Amorphous, hard, brittle and transparent substance
Chemically inert, absorb, reflect and transmit light
Has no definite melting point
Defined physically as a hard, rigid, under cold, brittle, non crystalline substance having no
definite m.pt and sufficiently high viscosity to prevent crystallisation
Chemically as a fused mixture of silicates, alkali and alkaline earth compounds and other glass
constituents such as CaO, MgO, SnO2, BaO, PbO
High electrical insulation and low thermal conductivity
Capacity of absorbing decorative colours without loss of transparency
Widely used in electrical industries because of its low dielectric loss and slow ageing
Important glasses for insulating purposes
1. Lead/Flint glass
Prepared by mixing Lead oxide, Silica and alkali with Lead content as high as 92%
Properties
High refractive index, ratio of velocity of light in vacuum to velocity in a specific medium
Low hardness, high dissipation factor
High density (8), low softening temperature
Uses
Used in optical work because of their high index
of refraction and high dispersion
Used in manufacture of electric bulbs, neon tubing
and radiators because of high electric resistance
Used as a shield to protect from X-rays and 𝛄 rays in
medical and atomic fields respectively
2. Borosilicate/Pyrex/Jena glass
Prepared by fusing Silica (80%) and B2O3 (10-20%) with Alumina (3%), K2O (3%) and Na2O
Properties
Possess low thermal coefficient of expansion and high chemical resistance (get better result by
replacing Na2O by more B2O3 & Alumina)
Also possess very high softening points and excellent resistivity (shock proof)
Uses
Used in manufacture of baking dishes (kitchen wares), laboratory glassware, insulators and
washers
Manufacture of pharmaceutical containers like syringes, vials etc, because of their chemical
inertness
A good material in fabricating slides and lenses for microscopes, telescopes and other optical
devices
Modern high powered flash lights and studio spotlights
Applications
3. Glass wool
A fibrous wool like material
Composed of intermingled fine threads or filaments of glass using a binder
Obtained by forcing molten mass of glass through small orifices
The glass wool formed in this way is carried away by a conveyor belt on which it is hurled
Properties
Non-combustible, fire proof, low electrical conductivity and heat proof material
Does not absorb moisture and water, High tensile strength (18 times that of steel)
Chemically resistant to a number of chemicals
Uses
Used for electrical, heat and sound insulation purposes
Ex : domestic and industrial appliances like oven, motors, vacuum cleaners, insulating metal
pipelines, walls and roof of houses
Used for filtration of corrosive liquids such as acids and alkalies
Manufacturing fibre glass by blending with resins
Applications
Fibre glass
4. Vitreosil/99.5% Silica glass
Prepared by heating pure sand (SiO2) to its melting point (> 1750 oC)
Can’t be shaped easily because of high viscosity of the glass and bubbles formation due to
absence of flux. The final product is translucent
When heated for a long time above its m.pt, a transparent glass known as clear silica glass is
obtained
Uses
Used for chemical plants, chemical lab wares and for electrical insulating materials in electrical
heaters and furnaces (heat is applied by induction heating of metal)
Magnetic materials
Those materials which are attracted by magnets and can be magnetized easily
Show the existence of magnetic field around them when they are magnetized
Ex : Pure Iron, Nickel-Iron ally, Cobalt steel, Chromium steel
Basis of magnetism
Magnetism is associated with the electrons in the atoms
Atoms are believed to be tiny bar magnets or magnetic dipoles each having North & South poles
In the unmagnetized state (randomly oriented), tiny atomic magnets are oriented randomly
Magnetic effect of one atom cancels other atom therefore, material as a whole is non magnetic
In the horizontally oriented state, N pole of one atom faces S pole of another
Each row of atoms form a thin magnet and many parallel horizontal rows form a single composite
magnet
If the applied magnetic field is removed –
Some magnetic materials lose their magnetism due to change in magnetic orientation
ie horizontal to random
Some materials do not lose their magnetism but magnetism is lost either by heat or physical
shock
Curie temperature - temperature beyond which a magnetic material loses its magnetic power
For Iron, Curie T is 799 oC
Classification of magnetic material
Classification of magnetic material
Based on relative permeability
Ferro
magnetic
Para
magnetic
Diamagnetic
Based on application
Soft magnetic
materials
Hard magnetic
materials
Permeability – the measure of the ability of a material to support the formation of a magnetic
field within itself
A degree of magnetization that a material obtains in response to an applied magnetic field
Diamagnetic materials
The relative permeability is slightly less than unity
When a substance is placed in an electric field, it induces a charge in the orbital motion of the
electron and produces a temporary magnetism in the substance
When the substance is taken away from the external field, the substance loses its magnetic
property
The net magnetic moment is zero in diamagnetic substance because when an external field, H is
applied to a diamagnetic substance then the magnetic moment of electrons is aligned to
the opposite direction of the applied field
Diamagnetic Substances produce negative
magnetization, χm
Ex : Cu, Ag, Au, etc
Paramagnetic materials
The relative permeability is slightly greater than unity
Each atom in a magnetic material possesses a permanent dipole moment due to incomplete
cancellation of electron spin and orbital magnetic moment
Therefore in the absence of an electrical field, orientation of magnetic moment is random
They freely rotate and give rise to para magnetism
Produce positive magnetization
Ex : Al, Cr, Mo, Ti, Zr– they are not used in electrical engineering field
Ferromagnetic materials
The relative permeability is greater than unity and depends on the applied field strength
Materials which can retain their magnetism even after the magnetic filed is removed
Produce very strong magnetization
Ex: Fe, Co, Ni, Cr and their alloys
However at one point or temperature the ferromagnetic
materials tend to lose its magnetic properties
(Curie point or Curie Temperature)
Antiferromagnetic materials
The magnetic moments are aligned in opposite directions
and are equal in magnitude
In presence of H, these materials are weakly magnetized
in the direction of the field
Ex : MnO, FeO, CoO, NiO
Soft magnetic materials
Easy to magnetize but lose their magnetism quickly and completely when the applied filed is
removed
Therefore they can be magnetized and demagnetized easily
Used in electric motors, generators, transformers, telephone receivers, radars, etc.,
Ex : soft iron, wrought iron, Iron-Silicon alloy, Iron-Nickel alloy, Ferrites (FeO with oxides of Co, Ni)
Hard magnetic materials
Materials which retain their magnetism even after removal of the external field
Used to make permanent magnets
Used in loud speakers, electrical measuring instruments
Ex : carbon steel, Tungsten steel
Pressed/sintered metal powder magnets –
compressing metal particles inside a solid mass at a
T < m.pt
B – induced magnetic field, H – applied magnetic field
Semiconductors
Electrical conductivity of a material is classified into insulators, conductor and semiconductor
based on the distance between valence band and the conduction band (energy gap – amount of
energy a valence electron must have in order to jump from valence to conduction band)
Fermi level - the highest energy level that an electron can occupy at the absolute 0 T
It lies between the valence band and conduction band because at absolute 0 T, the electrons are
all in the lowest energy state
Due to lack of sufficient energy at 0 Kelvin, the Fermi level can be considered as the sea of
fermions (or electrons) above which no electrons exist
Differences between conductor, semiconductor and insulator
Conductor
Semiconductor
insulator
Easily conducts electric current Conductivity lies between Does not conduct electrical
conductor and insulator
current
under
normal
conditions
Single element materials – Cu, Pure intrinsic state is neither a
Valence e- are tightly bound to
Ag, Au, etc
good conductor nor insulator
the atom. Therefore few free
Single element SC – Sb, As, B,
e- are available for conduction
Po, Te, Si, Ge (4 valence e-)
Once valence electron is Compound SC – Gallium Most
insulators
are
arsenide, Indium phosphide, compounds rather than single
loosely bound to the atom
Therefore, a small amount of SiC, Gallium nitride, SiGe
element materials
energy is required to free the
electron from the atom
Ex : Glass, mica, rubber
Since they overlap, electron in The band gap is small so that The gap is crossed only when a
the valence band move freely when a valence e- absorbs a very high voltage is applied
into the conduction band
photon, it crosses the gap
Types of semiconductor
I. Intrinsic semiconductor
Extremely pure elements like Si, Ge, Se having 4 valence e- in their atom
Their forbidden gap energy is about 1 ev
Also called an undoped semiconductor or i-type semiconductor
Have crystalline structure in which each atom forms four covalent bonds by sharing with four
neighbouring atoms
As the temperature of the semiconductor is increased, the e- gain more thermal energy and thus
break free from their shell
The process of ionization of the atoms in the crystal lattice creates a vacancy in the bond
between the atoms
The position from which the e- gets excited leaves a hole which is occupied by a nearby e- and
this process continues
The e- gets excited goes to conduction band thus leaving a hole in valence band
In this way, the hole travel from one atom to another
In the intrinsic SC, the number of free e- is equal to the number of holes
ne- = nh = ni
When the T of an intrinsic SC is 0 K, it behaves like an insulator, T > 0 K, the e- gets excited
Similarly, when an electric field is applied across an intrinsic SC at room T, e- in conduction band
moves to the anode
While the positive hole in the valence band moves to the cathode
Hence current in an intrinsic SC consists of simultaneous movement of conduction band e- and
valence band hole in opposite direction
II. Extrinsic semiconductor
The conductivity of SC can be improved by introducing a small number of atoms called impurities
The process of adding impurity atoms to the pure SC is called Doping
Generally, only 1 atom in 107 is replaced by a dopant
The addition of dopant reduces the energy gap thereby allowing more e- to flow from valence to
empty conduction band
By appropriate doping, conductivity may be increased by 10,000 times
Based on the nature of doping agent, the extrinsic SC is further classified into –
a) n-type extrinsic semiconductor
A pure SC – tetravalent (Si or Ge) is doped with a pentavalent impurity (P, As, Sb, Bi), 4 e- are
used to form a bond with Si and the 5th e- remains loosely bound to the donor atom itself
This e- is easily excited from valence band to conduction band when light is supplied
Each P atom donates one free e-. The number of free e- depends on the amount of impurity
(donor) added to the Si
A small amount of impurity (P) generates millions of free e-
Though n-type SC has large number of free e-, but these e- are given by dopant, which is
electrically neutral
Therefore, the total electric charge is also neutral
b) p-type extrinsic semiconductor
A trivalent impurity (B, Ga, In, Al) is added to an intrinsic SC, (Si), it is called p-type semiconductor
B has 3 valence e-, they form 3 covalent bonds with the neighbouring 3 Si atoms leading a hole with the
4th Si atom
This shows each B atom accepts one e- to fill the hole. Therefore, the dopant is called acceptor
A small addition of impurity provides millions of holes
Similar to n-type, p-type is also neutral
The hole travels to adjacent Si atom leaving a new hole there
Here the conduction is due to excitation of a hole
Although the intrinsic SC is a pure SC it is not used for practical manufacturing as has low
conductivity. The number of free charge carriers is less hence it has higher resistance to
conduction of charges
Whereas an extrinsic SC has greater conductivity as it has a number of free charge carriers.
Hence external SC are preferred for practical manufacturing of SC components and devices
Applications of semiconductor
• Temperature sensors are made with semiconductor devices
• They are used in 3D printing machines. Used in microchips and self-driving cars
• Used in calculators, solar plates, computers and other electronic devices
• Transistor and MOSFET (metal–oxide–semiconductor field-effect transistor – to switch electric
signals) used as a switch in Electrical Circuits are manufactured using the semiconductors
Metallic solids
Metallic solids such as Cu, Al, Fe are formed by metal atoms
Described as a uniform distribution of atomic nuclei within a ‘sea’ of delocalized eAtoms are held together by metallic bonding which give rise to high thermal and electrical
conductivity, metallic lustre, malleability and hardness
Bonding in metallic solids is different from other solids
Valence e- are delocalized providing a strong cohesive force that holds the atoms together
Electron is not bonded with any particular atom
Very little energy is needed to remove e- from a solid metal
When an electrical potential is applied, the e- can migrate through the solid toward the positive
electrode, thus producing high electrical conductivity
Therefore metallic solids consist of positive metal ions (kernels) in a cloud of valency e- (e- cloud)
The positive ions will tend to repel one another, but are held together by the negatively charged
electron cloud
Na – 1s22s22p63s1
Properties of Metallic solids
Many of the characteristic properties of metals are attributable to the non-localized or freeelectron character of the valence electrons
Electrical conductivity
Most metals are excellent electrical conductors because the electrons in the electron sea are free
to move and carry charge
Under the influence of electric field, delocalized valence e- move readily and conduct electricity
throughout the metal from one end to the other (towards positive charge anode)
Conductivity decreases with rise in T, because of increased thermal vibrations of the metal atoms
cause scattering of e- thereby obstructing free flow of eThermal conductivity
Metals conduct heat because the free electrons are able to transfer energy away from the heat
source and also because vibrations of atoms move through a solid metal as a wave
When one end of a metallic substance is heated, the kinetic energy of the electrons in that area
increases. These electrons transfer their kinetic energies to other electrons in the sea via
collisions
Therefore heat travels from hotter to colder part of the metal
Malleability and Ductility
When an ionic crystal (NaCl) is beaten, it is broken into many smaller pieces. Because the atoms
in the crystals are held together in a rigid lattice that is not easily deformed
In the case of metals, the sea of electrons in the metallic bond enables the deformation of the
lattice. Therefore, when metals are beaten, the rigid lattice is deformed and not fractured
Metallic Luster
When a beam of light falls on the surface of the metal, the electric field of light oscillates the e(at a wavelength) present on the metal surface
Wavelength of e- is absorbed by the light and remaining light is transmitted
Therefore metal surface exhibits shining appearance
High Melting and Boiling Points
The strong attraction between atoms in metallic bonds makes metals strong and gives them high
density, high melting point, high boiling point, and low volatility
The exceptions to this include zinc, cadmium, and mercury due to completed ns2 configuration
Mercury is a liquid under ordinary conditions and has a high vapour pressure
The metallic bond can retain its strength even when the metal is in its melt state
For example, gallium melts at 29.76oC but boils only at 2400oC. Therefore, molten gallium is a
non volatile liquid
Composites
A material which is composed of two or more materials at a microscopic scale and have
chemically distinct phases
Heterogeneous at a microscopic scale but statically homogeneous at macroscopic scale
Constituent materials have significantly different properties
Components which are made to form composites are mutually insoluble and form distinct
phases
properties of composites are determined by properties, amount, shape and size of the
constituents
Ex : Reinforced Concrete Cement (RCC) – steel is reinforced into concrete mixture to give strength
Ball – rubber is embedded between cellulose material (cotton) to give more strength to rubber
Natural composites
Peanut husk, banana skin
Wood – a strong and flexible
cellulose is surrounded
by soft protein, collagen
Why do we need composite materials?
• Strength
• Stiffness
• Toughness
• Corrosion resistance
• Wear resistance
• Reduced weight
• Fatigue life
• Thermal/Electrical insulation and conductivity
• Acoustic insulation
• Energy dissipation
• Attractiveness, cost
•Tailorable properties
Advantages
• High strength, stiffness, toughness
• Low electrical conductivity, thermal expansion
• Corrosion and oxidation resistance
• Reduced weight
• Applications
• Electronic circuit boards (PCB)
• Marine, aeronautical applications
• High temperature refractories
Constituents of composites
Composite
Dispersed phase
Matrix
Metal
Ceramic
Polymer
Carbon
and
graphite
Fibre
Particulates
Flakes
Whiskers
Matrix
• Contribute bulk of the composite, encloses the composite
• Binds the dispersed phase
• Distribute the applied external load to the dispersed phase
• Protects the dispersed phase from chemical action, abrasion from environment
• Prevents the propagation of brittle cracks due to its plasticity and softness
Requirements
• Ductile and corrosion resistant
• High bonding strength between matrix phase and disperse phase
Dispersed phase
• Stronger and harder, determines the internal structure of composite
• Contribute desired properties
• Responsible for load carrying and transfer strength to matrix
I. Fibre reinforced composites
It consists of filament (high tensile strength/specific gravity), matrix and bonding agent (binds
disperse phase to matrix)
It has high aspect ratio, stiffness and specific modules
Shape, orientation and composition of fibre decide property of composites
Fibre
Inorganic (high modulus, high
thermal stability)
Organic (low density, flexibility and
elasticity)
Polyester
Aramid
Carbon
Natural
(cotton,
jute, hemp)
Glass
Ceramic
Boron
Metallic
fibre
Examples for Fibre reinforced polymer (FRP)
1. Glass fibre reinforced polymer
The earliest known fibre
The polymer can be either thermoplastic or thermosetting
Glass fibre is used for improving the characteristics of polymer matrices – nylon, polyester, etc.,
Possesses lower density, high tensile strength, resistance to corrosion and chemicals
Applications
Automobile parts, storage tanks, plastic pipes, etc.,
Disadvantages
Prone to breakage when subjected to high tensile stress for long time
2. Carbon fibre reinforced polymer
Dispersed phase – graphite
Gives excellent resistance to corrosion
Lighter density, stiffest fibers known
Properties of graphite remain unchanged even at very high T
Applications
Structural components of aircraft and helicopters, bicyle, motorcycle
Sports materials – tennis racket
Musical instruments strings
Disadv
Highly expensive
3. Aramid fibre reinforced polymer
Aramid is a synthetic fiber made from the polymer aromatic polyamide
It was first introduced in the 1960s as a meta-aramid and later as para-aramid
Meta-aramid bonds are not aligned but are rather in a zigzag pattern and therefore will not
develop the higher tensile strength of para-aramid bonds
The trademark names of aramid fibers are Kevlar (para) and Nomex (meta)
Properties
A high heat, fire resistance and strong synthetic fibre
Density is less than glass and granite fibres
High tensile strength, high modulus, low weight and non conductive fibre
Good resistance to abrasion
But sensitive to acids, alkali and UV radiation
Applications
Making of helmets, hockey sticks, tennis strings and asbestos replacement
4. Alumina reinforced metal (matrix) composite
Improved stiffness, abrasion resistance and creep resistance
Used for making components of turbine engines
Offers good compressive strength rather than tensile strength in automobiles
5. Metal reinforced composites
Dispersed phase – alloys of Al, Mg, Cu, Ti with 20 -50 % of C, SiC, B, etc.,
Used at high T, possess high strength
Easily handled unlike glass fibre
Used in aerospace and new engine applications
Steel wire is the most extensively used fibre
6. Ceramic (SiC) reinforced composites
Known as Cermites (ceramic + metal)
Ex : TiC embedded in a matrix of a metal – Co or Ni
Ceramic refractory carbide (WC) with Co or Ni
Applications
As cutting tools for hardened steel
As both matrix and dispersed phase are refractory, they can withstand high T generated due to
friction during cutting action on the hard materials
Flakes
Thin, flat solid having 2D geometry, placed in a matrix
Ex : mica and glass
Flakes provide –
Uniform mechanical properties in the plane of the flakes
Higher strength, flexural modulus (tendency of a material
to undergo bending)
Higher dielectric strength and heat resistance
Better resistance to penetration by liquid and vapour
Lower cost compared to fibre
Often used in place of fibre
Metal flakes that are in close contact with each other in polymer can conduct
electricity and heat while mica and glass flakes resist both
Whiskers
A thin strong filament or fibre made by growing as crystals or single crystal grown with
nearly zero defects
High L/D ratio and strongest known material but its usage is restricted due to its cost and
difficulty in incorporation
Usually discontinuous and short fibres of different cross section
Ex : Graphite, SiC in ceramic matrix and SiC in Al (used in automobiles)
Properties depend on distribution, orientation and geometry of whisker void content
Smaller void content gives low susceptibility to weathering
Higher void content increases thermal and insulating properties
Types of reinforcement
Reinforcement
Particle
Large
Dispersion
particle
strengthened
Fiber
Continuous
long
(aligned)
Structural
Discontinu
ous (short)
Aligned
Laminates
Randomly
oriented
Sandwich
panels
I. Particulates
Small pieces of hard solid material (metallic/non-metallic) ranging from microscopic to
macroscopic
Distribution is uniform throughout making it isotropic (material properties same in all directions)
composite
Function
Increase surface hardness
Cause constraint (retard) on plastic deformation
Increases performance at elevated temperature,
abrasion resistance and strength
Thermal and electrical conductivities are modified
Decrease shrinkage and friction
Decrease cost of composite
Ex : Laminate composite : Capacitors – alternating layers of an insulator and a conductor
Conductivity in parallel plates (Al) and insulation in perpendicular direction (mica)
Particulates can be mixed with matrix in the form of metal (Al alloy), polymer (rubber), Ceramic
(concrete)
Examples
a. Large particle (1 – 50 𝜇𝑚)
b. Dispersion strengthened ( < 0.1 𝜇𝑚)
i) Ceramites – ceramic particle (strong and brittle) in a metal matrix (WC & TiC + Co/Ni)
Used for cutting tools for hardened steel
ii) Vulcanized rubber - natural rubber with S and
other curatives
Strengthened with 20 – 50 nm carbon black particles
iii) Concrete : matrix – cement and water
Dispersed phase is sand and pebbles
The function of sand pebbles is to increase the modulus of matrix, decrease permeability and
ductility of matrix
b. Dispersion strengthened ( < 0.1 𝜇𝑚)
Can be carbides, oxides, borides
Ex : sintered Al powder (fine Al2O3 in pure Al matrix) (applying a powdered
material into a solid by heating below m.pt
Ductile matrix + hard particle using powder metallurgy, composite is formed
This oxide prevents grain growth and movement of dislocations at the boundaries or through
them and produces high strength, high creep resistance and insensitivity to high-temperature
exposure
Particles distribution in a matrix
II. Fibre reinforcement
a. Continuous
The fibres are long and strength of the composite depends upon the direction of the fibre in
which they are aligned
If oriented to one direction ie longitudinal direction, more strength will be present along this
direction
b. Discontinuous
Fibres are short. Generally fibres are randomly arranged
Ex for fibres : polymer, metal, ceramic
Mechanical properties are isotropic
Have low strength than continuously arranged fibres
Woven, aligned and randomly oriented
III. Structural composites
a. Laminar
Composed of 2D sheets or panels that have a preferred high strength direction such as
plywood
Layers are stacked and cemented together such that the orientation of high strength direction
varies with each successive layer
b. Sandwich panel
Consists of two strong outer sheets called faces separated by a layer of less dense
material, called core
Ex : Foam material
Advantages
Light weight and large bend stiffness
Hybrid composites
Two or more fibres are incorporated within a single matrix
Between two fibres one is generally organic and another is inorganic in nature
Classification
Based on the possible interactions connecting the two fibres
Class I - Hybrid materials are those that show weak interactions between the two phases – van
der waals, H-bonding, electrostatic attraction
Class II - Connected by strong chemical interactions between them –covalent bond
Advantages
Inorganic clusters or nanoparticles with specific optical, electronic and magnetic properties can
be incorporated in organic polymer matrices
Applications
Scratch resistance coating with hydrophobic or anti fogging properties (chemicals that prevent
condensation of water)
Nanocomposite devices for electronic and opto electronic applications – LED, photodiodes, solar
cells, gas sensors, etc.,
Fire retardant materials for construction industry (epoxy polymer or aromatic compound with
hetero atom) + fibre (Glass, Aramide)
Composite electrolyte in solid state Lithium batteries
Dental filling materials
Recreational products – skating shoe, base ball shafts, bicycle frames
Space shuttle
Applications of composites in electronics and electrical industries
1. A PCB substrate must have good dielectric performance ie, it must insulate the conductive
layers from one another by blocking electrical conductivity, to minimize electrical signal loss,
crosstalk between conductive layers and noise
The majority of PCBs are made with E-glass/epoxy
Resin alternatives include vinyl ester and polyester, for commodity boards, and cyanate ester,
polyimide and bismaleimide triazine (BT) for more demanding, elevated-temperature
applications
2. Electromagnetic shielding
The effect of electromagnetic shielding is
to reduce the electromagnetic field effect
in a certain area generated by some
radiation sources, and to effectively control
the harm caused by electromagnetic radiation
from one area to another
Coating fillers with magnetic materials or incorporation of magnetic mat. in the polymer matrix
Carbon – carbon composites have good shield effectiveness
3. Risk from electromagnetic radiation
If human beings are exposed to the EM waves, the network of veins in high risk
organs such as eyes might be affected due to heat build-up in the eyes by the EM
waves which could not be easily dissipated
The use of low-resistance conductor material has a reflection and guiding effect on
electromagnetic energy flow and within the conductor material
It create the current and magnetic polarization which is opposite with the source
of electromagnetic field, thereby reduce the effect of radiation source in
electromagnetic field, by shielding effectiveness
Ex: carbon-polymer composite materials is used to protect people and electronic equipment
from exposure to electromagnetic radiation
4. Conductive mechanism of composite conductive polymer
With the increase of the concn. of conductive filler, the conductivity of the polymer increases
slowly
When the concentration reaches a certain value, the conductivity increases sharply, the polymer
becomes a conductor, and the filler concentration continues to increase but conductivity has not
changed much
The conductivity filler concentration at which the conductivity changed abruptly is called the
'diafiltration threshold’
when the content of the conductive filler reaches the 'diafiltration threshold', the conductive
particles contact each other to form an infinite network
The formation of conductive channels, carriers can freely move within the system thereby
making the composite conductive
5. Electrical switching and insulation
Properties that composite materials have include :
• Dielectric strength
• High thermal conductivity
• Low electrical conductivity for insulation
• Electromagnetic shielding effectiveness
• Heat resistance
• Low coefficient of thermal expansion
• Durability to withstand repeated use without a decrease in performance
• Moisture resistance for safety and durability
• Sound baffling for quieter operation
6. Wearable electronics – worn by a person for memory communication and senses
Ex : smart watches/chips
• Graphene/CNT polymer composites are widely being used to make wearable electronics
• Silver nanofillers in elastomer composite used in wearables
7. Electronic sensors
Carbon black polymer odour and flavour sensors for detecting vapours
Used for environmental monitoring to check air quality,
crime prevention such as bomb detection, quality control
Reinforcing phase: Dispersed carbon black particles
Reinforcing medium : Polystyrene
8. Satellite electronics mounted on composite panel
9. Lightning harvester
Graphene based composite technology is used to manufacture ultra-long cables - of circa 8 miles
in length
These ultra-long cables would have a highly-conductive coating of graphene - effectively making
them lightning rods which can reach up into the clouds
Clouds contain a massive amount of energy, in the form of static electricity, or the difference in
voltage between the bottom of a cloud and the ground
Lightning occurs when this voltage difference builds up to such an extent that electricity leaps
across this gap
The highly-conductive graphene coating on a GC (ground check cable ) composite cable (held
aloft by weather balloons) is used to harness electricity from clouds
As Electricity flows - even the extremely large bursts from lightning strikes - would travel down
the graphene-coated cable into a super-capacitor array, which could then release electricity into
the power grid in a controlled way