Electrical and Electronics Engineering Material
3rd Semester
Course
Description:
This course deals with the
properties of Magnetic, Resistor, and Dielectric and Semiconductor materials
from the peripherals of electrical engine.
Course content:
Unit 1. Conducting
Material [8]
1.1 Description of commonly used
resistors, alloys of Nickel, Iron, Chromium, Aluminum.
1.2 Band structure of conductors,
energy gap
1.3 Electrical properties:
resistivity, conductivity, effect of temperature, concept of drift and mobility
1.4 Resistor alloys:
1.4.1 Alloys of Ni, Fe, Cr, Al
1.4.2 Mechanical characteristics
1.4.3 Industrial application
Unit 2. Magnetic
material [20]
2.1. Classification based on
ferrous material and non-ferrous material
2.2. Use and their characteristics
2.3. B-H characteristics
2.4. Hysteresis loop, eddy current
losses
2.5. Magnetic permeability and
susceptibility
2.6. Domain structure
2.7. Ferrous materials
2.7.1. Common ferrous materials and
their engineering characteristics
2.7.2. Industrial applications
2.7.3. Corrosion: cause, effect and
methods of prevention
2.8. Non-ferrous materials
2.8.1. Common non-ferrous materials
and engineering characteristics
2.8.2. Some non-ferrous alloy
(copper, aluminum, brass, bronze, silver, gold) and their Industrial
application
2.8.3. Carbon as an electrical
material, its product (brushes) and application
2.8.4. Chemical/corrosion
characteristics of some commonly used non-ferrous metals
Unit 3. Dielectric
materials [18]
3.1 Definition of dielectric,
macroscopic approach
3.2 Polarization, Dielectric
constant, Electric Dipole moment, Electronic polarization, Ionic polarization
3.3 Dielectric breakdown
· Dielectric
breakdown in gases
· Dielectric
breakdown in liquids
· Dielectric
breakdown in solids
3.4 Ferro electricity and
Piezo-electricity
3.5 Properties of some dielectric
materials
3.6 Insulating materials
3.7 Identification of insulating
materials in general uses and their characteristics
3.8 Electrical characteristics of
some insulating materials e.g. plastics, resign, porcelain, glass, fiber glass,
mica, oil, insulating varnishes, gases (SF6)
Unit 4.
Semiconductor materials [14]
4.1. Definition, elements of
semi-conductor materials, electrical nature.
4.2. Band structure of Group IV
materials, energy gap.
4.3. Atomic structure of silicon,
germanium
4.4. Formation of electron and hole
4.5. Electrical conduction in
semi-conductors
4.6. Intrinsic and Extrinsic
semiconductor, concept of doping
4.7. N type semiconductor
4.8. P type semiconductor
Elements from Atomic Number 1 to 30 and their symbol
 |
| Fig Shell configuration of Al |
Classification of Energy Bands
1. Valence Band
The electrons in the outermost shell are known
as valence electrons. These valence electrons contain a series of energy levels
and form an energy band known as valence band. The valence band has the highest
occupied energy.
2. Conduction Band
The valence electrons are not tightly held to
the nucleus due to which a few of these valence electrons leave the outermost
orbit even at room temperature and become free electrons. The free electrons conduct
current in conductors and are therefore known as conduction electrons. The
conduction band is one that contains conduction electrons and has the lowest
occupied energy levels.
3. Forbidden Energy Gap
The gap between the valence band and the
conduction band is referred to as forbidden gap. As the name suggests, the
forbidden gap doesn’t have any energy and no electrons stay in this band. If
the forbidden energy gap is greater, then the valence band electrons are
tightly bound or firmly attached to the nucleus. We require some amount of
external energy that is equal to the forbidden energy gap.
The figure below shows the conduction band,
valence band and the forbidden energy gap.
Electrical Properties of materials
To finalize the material for an engineering product / application, we should have the
knowledge of Electrical properties of materials. The Electrical properties of a material
are those which determine ability of material to be suitable for a particular Application.
Resistivity
Conductivity
Its
uint is ℧/meter.
Dielectric Strength
It
is the property of material which indicates the ability of material to
withstand at high voltages. Generally it is specified for to represent their operating voltage. A
material having high dielectric strength can withstand at high voltages. Generally,
it is represented in the unit of KV/cm. Dielectric strength of some insulating
materials are listed below-
Temperature Coefficient of Resistance
1.
R2 – R1 ∝ R1
2.
R2 – R1 ∝ t2 – t1
3.
Property
of material of conductor.
Drift
Current and Mobility:
When a
steady electric field E volts/ meter is applied to a metal, the electrons move
to the positive terminal of the applied voltage. In their way, they
continuously collide with the atoms and rebound in a random fashion. Each
collision being inelastic i.e. the electrons lose some kinetic energy. After
the collision, the electrons are accelerated and gain certain component of
velocity in the direction opposite to that of applied electric field (-E) and
lose their energy at the next collision. Thus the applied electric filed does
not stop collision and random motion but makes the electron to drift towards
the positive terminal. Consequently, the electrons gains average drift velocity
v in the direction opposite to that of the applied electric field. It is observed
that the drift velocity v is proportional to the applied electric field E. i.e.
V α E
Or, v = μe E
Where μe is
called the electron mobility and is expressed in m2/ V-s.
Thus, Electrical mobility is the ability of
charged particles (such as electrons or protons) to move through a medium in
response to an electric field that is pulling them. Or Mobility is defined as
the average particle drift velocity per unit electric field.
This steady
state drift velocity is superimposed on the random motion of free electrons
caused by thermal agitation. This steady flow of electrons in one direction
caused by the applied electric field constitutes an electric current, called
the drift current.
Alloy
An alloy is
an mixture of metals, or a metal combined with one or more other elements.
Alloys are
useful because the properties of the alloy are different from the properties of
the elements they are made from.
Alloys are
made by mixing two or more elements, at least one of which is a metal. This is
usually called the primary metal or the base metal, and the name of this metal
may also be the name of the alloy.
For example,
brass is a mixture of copper and zinc. Steel is an alloy of iron with carbon,
but other elements may also be added to change its properties.
Resistor Alloys
These are
mostly alloys of different metals which are also conductors of electricity. The
resistivity of these materials is more than the resistivity of materials which
are considered as good conductors. The special properties of these resistor materials
are:
i. Loss of
energy in overcoming resistance and
ii. The
energy so lost gets dissipated as heat energy.
e.g.
Nichrome, manganin, constantan etc.
Resistor
alloys are those material which is formed when a resistor is alloyed with other
different materials to achieve high resistivity per unit length and improve
other properties like corrosion and temperature coefficient as per required.
Resistance
wire is wire intended for making electrical resistors (which are used to
control the amount of current in a circuit). It is better if the alloy used has
a high resistivity, since a shorter wire can then be used. In many situations,
the stability of the resistor is of primary importance, and thus the alloy's
temperature coefficient of resistivity and corrosion resistance play a large
part in material selection.
When
resistance wire is used for heating elements (in electric heaters, toasters,
and the like), high resistivity and oxidation resistance is important.
Sometimes
resistance wire is insulated by ceramic powder and sheathed in a tube of
another alloy. Such heating elements are used in electric ovens and water
heaters, and in specialized forms for cooktops.
Mechanical Properties
Alloy steels
have a wide range of special properties, such as hardness, toughness, corrosion
resistance, magnetizability, and ductility. Nonferrous alloys, mainly
copper–nickel, bronze, and aluminum alloys, are much used in coinage.
Individual
pure metals may possess useful properties such as good electrical conductivity,
high strength, and hardness, or heat and corrosion resistance. Commercial metal
alloys attempt to combine these beneficial properties in order to create metals
more useful for particular applications than any of their component elements.
Steel, for
example, requires the right combination of carbon and iron (about 99% iron and
1% carbon) in order to produce a metal that is stronger, lighter, and more
workable than pure iron.
Industrial Application of Alloys
Alloys have been used in industries
for a long time. Few widely used applications are:
1. Stainless Steel is used in wire and
ribbon forms for applications, such as screening, staple, belt, cable, weld,
metalizing, catheter, and suture wire.
2. Alloys of Gold and Silver are used in
the preparation of jewelry. White Gold, which is an alloy of Gold, Silver,
Palladium, and Nickel is used as cheap alternative of Platinum. A wide
selection of alloys is used in welding applications by numerous industries.
3. Some alloys function as
corrosion-resistant materials and are used in moisture rich-environments.
4. High temperature alloys have been
used for many aerospace and petrochemical applications. In addition, they have
been used for welding wire, where elevated temperatures and harsh environments
are routinely encountered. These alloys have been used in applications where
corrosion resistance and high strength must be maintained at elevated
temperatures.
5. Magnetic alloys are used for magnetic
cores and dry reed switches. Quality control measures include magnetic testing
to maintain consistently high standards of uniformity and performance.
6. Alloys are also used to produce
internal and external leads.
7. Nickel-Chromium,
Nickel-Chromium-Iron, and Iron-Chromium-Aluminum alloys have been used for
high-temperature heating elements.
8. Some alloys are used as resistance
elements to control or measure electric current. Applications have included
wire-wound resistors, rheostats, potentiometers, and shunts.
9. Thermocouple alloys have found a
wide-range of use in temperature sensing and control.
10.
Alloys
are also used as thermostat metals, radio and electronic devices, precision
devises in aircraft controls, telecommunications, automotive applications.
Alloys of nickel, their
characteristics and uses:
a.
Nichrome
A non-magnetic alloy of nickel (75-78%)
and chromium (20-30%), manganese (1.5%) and iron (balance), is the most common
resistance wire for heating purposes because it has a high resistivity and
resistance to oxidation at high temperatures. When used as a heating element,
resistance wire is usually wound into coils.
1.
Its resistivity is high.
2.
Temperature coefficient of resistance is low
3.
Withstand high temperature for long time without melting or oxidizing.
4.
It is silvery- white in appearance and has good ductile, mechanical and thermal
properties.
It
is used in making electric irons, tubular heating elements, furnace and heating
elements.
b. Eureka:
This
is an alloy of nickel (40%) and copper (60%). It is silver like in appearance.
Properties:
1.
Very stable alloy with very high working temperature.
2.
Heat resisting properties are poor.
3.
It does not rust or corrode due to air, heat and moisture.
4.
Resistivity is 49 micro ohm cm
5.
Melting point is 1300oC.
It is used
for making different types of rheostats, resistance wires, resistance boxes,
arc lamps, motor starter, loading in and supporting wire for electric
filaments.
Chromium Alloys
Nichrome
(nickel)
Ferrochrome
(iron)
Chromium
Alloys
Properties
1. It is also very resistant to
corrosion and wear, is very durable and has a very high melting point
2. Resistance to oxidation make nichrome
a popular material for use in heating elements.
3. fireworks and explosives as an
electrical ignition
4. Nichrome alloys are known for their
high mechanical strength and their high creep strength.
Aluminium alloys, their
characteristics and uses:
1. Al–Li (2.45% lithium): aerospace applications, including the Space Shuttle
2. Alnico (nickel, cobalt): used for permanent magnets
3. Birmabright (magnesium, manganese): used in car bodies, mainly used by Land Rover cars.
4. Hiduminium or R.R. alloys (2% copper, iron, nickel): used in aircraft pistons
5. Hydronalium (up to 12% magnesium, 1% manganese): used in shipbuilding, resists
seawater corrosion
6. Italma (3.5% magnesium, 0.3% manganese): formerly used to make coinage of the
Italian lira
Properties
1. It is cheaper than copper.
2. It is lighter in weight.
3. It is second in conductivity.
4. For the same ohmic resistance, its cross section is about 1.27 times
that of copper.
5. At higher voltages, it causes lower corona losses.
6. As the diameter of the conductor is more, it is subject to greater
wind pressure due to which the swing of the conductor and sag will be greater.
7. Since the conductors are liable to swing, it requires larger cross
section.
8. As the melting point of the conductor is low, the short-circuit
current will damage it.
Alloys of Iron
1. Cast iron: Iron (96–98%), carbon (2–4%), plus silicon.
Metal
structures such as bridges and heavy-duty cookware.
2. Invar Iron (64%), nickel (36%), which explains its alternative names:
FeNi36 and 64FeNi.
Pendulum
clocks and scientific instruments that need to resist heat expansion.
3.
Steel (general) Iron (80–98%), carbon (0.2–2%), plus other
metals such as chromium, manganese, and vanadium.
Metal
structures, car and airplane parts, and many other uses.
4.
Steel (stainless) Iron (50%+), chromium (10–30%), plus smaller
amounts of carbon, nickel, manganese, molybdenum, and other metals.
Unit 2: Magnetic material
Magnetic material
Definition:
The bodies which exhibit the property of attracting iron are
generally termed as magnets and those bodies which gets attracted to magnets
are termed as Magnetic material. These are of types:
1. Natural
magnet
2.
Artificial magnet
The area
around a magnet upto which its influence is felt is called its magnetic field.
There are continuous curves in a magnetic field termed as magnetic lines of
force which travels from the north to the South Pole as in figure. These are
assumed to continue through the magnet to the point from which they have
emerged. Thus each line of force forms a closed path.
The total
number of lines of force in the magnetic field is called the magnetic flux. It
is denoted by a symbol ‘Φ’. The unit of magnetic flux is weber (Wb). Lines of
magnetic flux possess the following property:
1. They form
closed loops
2. They
always start from the North Pole and end in the South Pole and are then continuous
through the body of the magnet.
3. They
never intersect each other.
4. Lines of
magnetic flux that are parallel and in the same direction repel one another.
 |
| Figure 1: Magnetic lines of force of a bar magnet |
Flux density (B):
Magnetic flux density is defined
as the magnetic flux per unit area of a surface at right angles to the magnetic
field. This is also known as magnetic induction. Its symbol is B and measured
in weber per sq. meter or Tesla.
Hence,
Magnetic flux Φ = flux density (B) x surface area (A), or
Flux density, B= Φ/A, Wb/m2 or Tesla.
Magnetic field intensity / Magnetic field
strength:
The force experienced on a unit
(one weber) N- pole placed ‘d’ meters apart from another pole strength ‘m’ Weber’s
in a medium of relative permeability μr is given by
F= m/(4Π μo μrd²)
Newton
and, is known as the magnetic
field strength at that point. It is represented by H. its units are newton/
weber or ampere-turns/meter.
Hence the magnetic field
intensity at any point is defined as the force experienced by a unit north pole,
when placed at that point, i.e H= m/(4Π μo μrd²) newtons/ wb.
The force experienced by a pole of ‘m’ webers placed in a uniform magnetic field of intensity H newtons/weber, will be equal to mH newtons.
2.2 B-H characteristic / Hysteresis loop:
Now if the magnetizing current in the coil is reduced to
zero, the magnetic field circulating around the core also reduces to zero.
However, the coils magnetic flux will not reach zero due to the residual
magnetism present within the core and this is shown in the curve from point a
to point b.
To reduce the flux density at point b to zero we need to
reverse the current flowing through the coil, the magnetizing force which must
be applied to null the residual flux density is called a ‘coercive force’, this
coercive force reverses the magnetic field rearranging the molecular magnets
until the core becomes unmagnetized at point c. An increase in this reverse
current causes the core to be magnetized in the opposite direction and increasing
this magnetization current further will cause the core to reach its saturation point‘d’
on the curve.
This point is symmetrical to point b. if the magnetizing
current is reduced again to zero the residual magnetism present in the core
will be equal to the previous value but in reverse at point e.
Again reversing the magnetizing current flowing through the
coil this time into a positive direction will cause the magnetic flux to reach
zero, point f on the curve and as before increasing the magnetization current
further in a positive direction will cause the core to reach saturation at
point a. Then the B-H curve follows the path of a-b-c-d-e-f-a as the
magnetizing current flowing through the coil alternates between a positive and
negative value such as the cycle of an AC voltage. This path is called a
magnetic Hysteresis Loop.
Retentivity or Remanence:
The ability for a coil to retain some of its magnetism
within the core after the magnetization process has stopped is called
retentivity or remanence, while the amount of flux density still remaining in
the core is called residual magnetism.
Magnetic Permeability and
Susceptibility:
Permeability:
It is the measured of the ability of a material to support the
formation of magnetic field within itself. Hence, it is the degree of
magnetization that a materials obtain in response to an applied magnetic field
represented by . Its unit is H/m henry/ meter or N/m2.
Relative Permeability:
Relative permeability of the material µr is define as the
ration of force between two magnetic poles placed at a certain distance in air
to the force between them placed at the same distance in medium.
µr
= F air / F medium
Susceptibility:
Magnetic susceptibility is the degree of magnetization of a
material in response to an applied magnetic field. If magnetic susceptibility
is positive, then the material can be paramagnetic, ferro magnetic, or anti
ferro magnetic. In this case the magnetic field is strengthened by the presence
of a material.
Alternatively, if magnetic susceptibility is negative, the
material is diamagnetic. As a result the magnetic field is weakened in the
presence of the material. In a large class of materials, there exists an
approximately linear relationship between M and H. if the material is isotropic
(having same magnitude or properties when measured in different direction.)
then,
M=Xm
*H
where, Xm is called magnetic susceptibility.
Note:
A linear relationship between M and H also implies a linear relationship between B and H. In fact we can write, B= H where, = o (1+ Xm) is termed as magnetic permeability.
Hysteresis Loss:
During each AC cycle current flowing in the forward and reverse
direction magnetizes and demagnetizes the core alternatively. Energy is lost in
each hysteresis cycle within the magnetic core. Energy loss is dependent on the
properties (e.g. Coercivity) of particular core material and is proportional to
the area of the hysteresis loop (B-H) curve.
Whenever the core is subjected to an alternating magnetic field
the domain present in the material will change their orientation after every
half cycle. The power consumed by the magnetic domains for changing the
orientation after every half cycle is called Hysteresis Loss.
At B-H curves, on X-axis, even if the current becomes zero the
material is still containing some amount of flux, which is known as
Retentivity. The material has a property to retain some flux and to make that
flux zero, a coercive force is applied. That extra force that is applied is
nothing but the Hysteresis Loss.
Hysteresis loss is given by:
Ph = Kh f Bm^ k
Where, Ph= Hysteresis Loss in watt per m3 or per kg.
Bm = maximum flux density weber/m2
Kh = hysteresis coefficient
K = steinmetz coefficient
f = frequency of magnetization
Domain Structure:
A magnetic domain is region within a magnetic material in
which the magnetization is in a uniform direction. This means that individual
magnetic moments of the atoms are aligned with one another and they point in
the same direction. When cooled below a temperature called Curie temperatures
material spontaneously divides into many small regions called magnetic domains.
The magnetization within each domain points in a uniform direction, but the
magnetization of different domains may point in different directions.
Magnetic domain structure is responsible for the magnetic behaviors of ferromagnetic material like iron, nickel, cobalt. This includes the formation of permanent magnets and the attraction of the ferromagnetic material to a magnetic field. The region separating magnetic domains are called domain wall.
When an external field M is applied, say in horizontal
direction the domain having parallel orientation of spontaneous magnetization
grow in size . This condition is shown in fig
(a). The motion of domain wall is considered as stealing of the neighboring
dipoles from other domains and aligning them in the direction of external field
so that preferred domain increases in size. As the external field is increased
progressively, a stage is reached when the whole material become one single
domain giving rise to saturation of the magnetization. Thus the hysteresis
results due to the motion of domain wall are also due to domain rotation to
some extent.
Ferrous materials: Some common ferrous metals include alloy steel, carbon steel, cast iron and wrought iron. These metals are prized for their tensile strength and durability. Carbon Steel – also known as structure steel – is a staple in the construction industry and is used in the tallest skyscrapers and longest bridges. Ferrous metals are also used in shipping containers, industrial piping, automobiles, railroad tracks, and many commercial and domestic tools. Ferrous metals have a high carbon content which generally makes them vulnerable to rust when exposed to moisture. There are two exceptions to this rule: wrought iron resists rust due to its purity and stainless steel is protected from rust by the presence of chromium. Most ferrous metals are magnetic which makes them very useful for motor and electrical applications. The use of ferrous metals in your refrigerator door allows you to pin your shopping list on it with a magnet.
Steel Steel is made by adding iron to carbon which hardens the iron. Alloy steel becomes even tougher as other elements like chromium and nickel are introduced. Steel is made by heating and melting iron ore in furnaces. The steel can is tapped from the furnaces and poured into molds to form steel bars. Steel is widely used in the construction and manufacturing industries.
Carbon Steel Carbon steel has higher carbon content in comparison to other types of steel making it exceptionally hard. It is commonly used in the manufacturing of machine tools, drills, blades, taps, and springs. It can keep a sharp cutting edge.
Alloy Steel Alloy steels incorporate elements such as chromium, nickel and titanium to impart greater strength and durability without increasing weight. Stainless steel is important alloy steel made using chromium. Alloy steels are used in construction, machine tools, and electrical components.
Cast Iron Cast iron is an alloy made from iron, carbon, and silicon. Cast iron is brittle and hard and resistant to wear. It’s used in water pipes, machine tools, automobile engines and stoves.
Wrought Iron Wrought iron is an alloy with so little carbon content it’s almost pure iron. During the manufacturing process, some slag is added which gives wrought iron excellent resistance to corrosion and oxidation; however, it is low in hardness and fatigue strength. Wrought iron is used for fencing and railings, agricultural implements, nails, barbed wire, chains, and various ornaments.
Corrosion:
Defination
Corrosion is a natural process, which converts a refined metal to a more chemically-stable form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually metals) by chemical and/or electrochemical reaction with their environment.
Cause:
The chemical or electrochemical reaction between a material, usually a metal and its environment, that produces a deterioration of the material and its properties. Metals are usually present in the stable oxidized form as ores from which they are extracted. These extracted metals have tendency to react with their environment and form corresponding metal oxides. This process of formation of metal oxides leads to deterioration, and is called corrosion.
Fe -> Fe2+ + 2e-
Fe2+ + 4OH- + 2O2 -> 2Fe2O3.H2O (rust)
Why corrosion should be avoided?
Corrosion can lead to failures in plants infrastructure and machines which are usually costly to repair, costly in terms of last of contaminated product, in terms of environmental damage, and possibly costly in terms of human safety.
The consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation of equipment or structures are often more serious than the simple loss of a mass of metal. Failures of various kinds and the need of expensive replacements may occur though the amount of metal destroyed is quite small. Some of the major harmful effects of corrosion can be summarized as follows:
1. Reduction of metal thickness leading to loss of mechanical strength and structural failure or breakdown.
2. Hazards or injuries to people arising from structural failures or breakdown (e.g. bridges, cars, aircraft).
3. Loss of time in availability of profit making industrial equipment.
4. Reduced value of goods due to deterioration of appearance.
5. Contamination of fluids on vessels and pipes.
6. Perforation of vessels and pipes allowing escape of their contents and possible harm to the surroundings.
7. Loss of technically important surface properties of metallic components. These could include frictional and bearing properties ease of fluid flow over a pipe surface, electrical conductivity of contacts, surface reflectivity or heat transfer across a surface.
8. Mechanical damage to valves, pumps, etc. or blockage of pipe by solid corrosion products.
Corrosion prevention Techniques:
In most of the situations, metal corrosion can be managed, slowed, or even stopped by using the proper techniques.
Corrosion prevention can take a number of forms depending on the circumstances of the metal being corroded.
Corrosion prevention techniques can be generally classified into 6 groups:
1. Environmental Modifications
2. Metal selection and surface conditions
3. Cathodic protection
4. Corrosion inhibitors
5. Conditioning the metal.
i. Coating
ii. Plating
Nonferrous material:
A non-ferrous metal is a metal, including alloys, that does not contain iron (ferrite) in appreciable amounts. Generally more expensive than ferrous metals, non-ferrous metals are used because of desirable properties such as low weight (e.g. aluminium), higher conductivity (e.g. copper), non-magnetic property or resistance to corrosion (e.g. zinc). Some non-ferrous materials are also used in the iron and steel industries. For example, bauxite is used as flux for blast furnaces, while others such as wolframite, pyrolusite and chromite are used in making ferrous alloys.
Important non-ferrous metals include aluminium, copper, lead, nickel, tin, titanium and zinc, and alloys such as brass (zinc +tin). Precious metals such as gold, silver and platinum and exotic or rare metals such as cobalt, mercury, tungsten, etc.
Aluminum: Aluminum is a relatively soft, durable, lightweight, ductile, and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. It is nonmagnetic and does not easily ignite. Aluminum is easily cast, forged (can be turned into shape by heating and beating), machined and welded. It’s not suitable for high-temperature environments. Aluminium is almost always alloyed, which markedly improves its mechanical properties, especially when tempered (degree of hardness and elasticity). For example, the common aluminium foils and beverage cans are alloys of 92% to 99% aluminium. The main alloying agents are copper, zinc, magnesium, manganese, and silicon (e.g., duralumin) with the levels of other metals in a few percent by weight.
Some of the many uses for aluminium metal are in:
i. Transportation (automobiles, aircraft, trucks, railway cars, marine vessels, bicycles, spacecraft, etc.) as sheet, tube, and castings.
ii. Packaging (cans, foil, frame of etc.).
iii. Food and beverage containers, because of its resistance to corrosion.
iv. Construction (windows, doors, siding, building wire, sheathing, roofing, etc.)
v. A wide range of household items, from cooking utensils to baseball bats and watches.
vi. Street lighting poles, sailing ship masts, walking poles.
vii. Outer shells and cases for consumer electronics and photographic equipment.
viii. Electrical transmission lines for power distribution
ix. Super purity aluminium (SPA, 99.980% to 99.999% Al), used in electronics and CDs, and also in wires/cabling.
x. Heat sinks for transistors, CPUs, and other components in electronic appliances.
xi. Light reflective surfaces and paint.
Copper: Copper is red in color, highly ductile, malleable and has high conductivity for electricity and heat. The major applications of copper are electrical wire, roofing and plumbing, and industrial machinery. Copper is used mostly as a pure metal, but when greater hardness is required, it is put into such alloys as brass and bronze. For more than two centuries, copper paint has been used on boat hulls (bottom part) to control the growth of plants and shellfish. A small part of the copper supply is used for nutritional supplements and fungicides in agriculture. Machining of copper is possible, although alloys are preferred for good machinability in creating intricate parts. It’s also used in sheet roofing, cartridge cases, statutes, and bearings.
Lead: Lead is a soft, heavy, malleable metal with a low melting point and low tensile strength. It can withstand corrosion from moisture and many acids. Lead is widely used in electrical power cables, batteries, building construction and soldering.
Zinc: Zinc is a medium to low strength metal with a very low melting point. It can be machined easily, but heating may be required to avoid cleavage of crystals. Zinc is most widely used in galvanizing, the process of applying a protective zinc coating to iron or steel to prevent rust.
Tin: Tin is very soft and malleable, ductile with low tensile strength. It’s often used to coat steel to prevent corrosion. Tinplate steel is used to make tin cans to hold food. In the late 19th century, tin foil was commonly used to wrap food products, but has since largely been replaced by aluminum foil. Tin is can also be alloyed with copper to produce tin brass and bronze.
Brass: Brass is a metal alloy made of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties. Brass has higher malleability than bronze or zinc. The relatively low melting point of brass (900 to 940 °C, 1,650 to 1,720 °F, depending on composition) and its flow characteristics make it a relatively easy material to cast. By varying the proportions of copper and zinc, the properties of the brass can be changed, allowing hard and soft brasses. The density of brass is 8.4 to 8.73 grams per cubic centimeter. Brass is used for decoration for its bright gold-like appearance; for
applications where low friction is required such as locks, gears, bearings, doorknobs, ammunition casings and valves; for plumbing and electrical applications; and extensively in brass musical instruments such as horns and bells where a combination of high workability and durability is desired.
Bronze: Bronze is an alloy consisting primarily of copper, commonly with about 12% tin and often with the addition of other metals (such as aluminium, manganese, nickel or zinc) and sometimes non-metals or metalloids.
Various kinds of bronze are used in many different industrial applications.
Phosphor bronze is used for ships' propellers (rotating shaft with blades), musical instruments, and electrical contacts. Bearings are often made of bronze for its friction properties.
Aluminium bronze is very hard and is used for bearings and machine tool ways. Bronze is used in making sculptures. Different musical instruments are made using bronze. E.g. Bells, doubles bass,
piano, harpsichord, guitars etc. Bronze has also been used in coins; most “copper” coins are actually bronze, with about 4 percent tin and 1 percent zinc.
As with coins, bronze has been used in the manufacture of various types of medals for centuries, and are known in contemporary times for being awarded for third place in sporting competitions and other events.
Silver: Silver is an extremely soft, ductile and malleable transition metal, though it is slightly less malleable than gold. Silver has a brilliant white metallic luster that can take a high polish, and which is so characteristic that the name of the metal itself has become a color name.
1. It exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. The electrical conductivity of silver is the greatest of all metals, greater even than copper, but it is not widely used for this property because of the higher cost.
2. Silver is used in the manufacture of crystalline solar photovoltaic panels. Silver is also used in solar cells.
3. Silver is used in water purifiers to prevent bacteria and algae from growing in the filters. The silver catalyzes oxygen and sanitizes the water, replacing chlorination. Silver ions are added to water purification systems in hospitals, community water systems, pools and spas, displacing chlorination.
4. Silver, along with other optically transparent layers, is applied to glass, creating low emissivity coatings used in high-performance insulated glazing.
5. Silver and silver alloys are used in some high-quality musical wind instruments. Flutes, in particular, are commonly constructed of silver alloy or silver-plated, both for appearance and for the surface friction properties of silver. Brass instruments, such as trumpets and baritone horns, are commonly plated in silver.
6. In medicine, silver is incorporated into wound dressings and used as an antibiotic coating in medical devices. Wound dressings containing silver sulfadiazine or silver nanomaterial are used to treat external infections.
7. Silver is used in food coloring.
8. Engine bearings rely on silver. The strongest bearing is made from steel that has been electroplated with silver. Silver's high melting point allows it to withstand the high temperature of engines. Silver also acts like a lubricant to reduce friction between a ball bearing and its housing.
9. Small quantities of silver are used as contacts in electrical switches. Industrial strength switches use silver. 10. Silver is also used in dentistry. It is used by dentist as a replacement to the teeth or as the tooth cap.
Gold: Gold is chemical element which in its purest form, it is a bright, slightly reddish yellow, dense, soft, malleable, and ductile metal. Gold conducts electricity, does not tarnish (lose brightness, is very easy to work, can be drawn into wire, can be hammered into thin sheets, alloys with many other metals, can be melted and cast into highly detailed shapes, has a wonderful color and a brilliant luster (shiny glow).
2.2.1 Uses of gold:
1. Gold in Phones: Gold in connectors, switches and relay contacts allows phones to remain free of corrosion and are an important part of modern cell phones.
2. Gold in Dentistry: Modern uses of gold in dentistry are usually in the form of white gold or gold alloys and include areas such as bridges, fillings, crowns.
3. Gold in Computers: Gold can be found inside desktops and laptops in the form of edge connectors employed to mount microprocessor and memory chips onto the motherboard as well as plug-and-socket connectors used to attach cables. While silver and copper are better conductors, gold’s resistance to corrosion makes it the preferred choice.
4. Gold in Electronic Devices: Gold is present in almost all small electronic devices because of low voltages and currents are easily interrupted by corrosion or tarnish. So that means anything from GPS systems to tablets to calculators to alarm clocks contain gold. Even large appliances such as microwaves, washing machines, and TVs contain a hint of gold.
5. In addition to this gold is also used in the field of medicine for different uses like cancer treatment, making medical equipment, diagnosis, implants etc. 6. Gold is also used as the jewelry, investments in banking sector as well as for monetary purposes.
Carbon as an electrical material: Carbon arc welding was the first electric welding. In this process, fusion of metal is accomplished by the heat of an electric arc. No pressure is used and generally, no shielding atmosphere is utilized. Filler rod is used only when necessary. Although not used extensively these days, it has, nevertheless, certain useful fields of application.
Carbon brush: A brush is a device which conducts current between stationary wires and moving parts, most commonly in a rotating shaft. Typical applications include electric motors, alternators and electric generators. A carbon brush is a sliding contact used to transmit electrical current from a static to a rotating part, in a motor or generator, and as regards DC machines, ensure a spark-free commutation. Carbon brushes are maintained by brush-holders to be in permanent contact with the slip ring assembly or with the commutator. A carbon brush can be: 1. Made of one or more carbon blocks 2. Equipped with one or more shunts/terminals. The carbon brush plays an essential part in the operation of electrical machines.
To enable it to fulfill its function, we need to consider three types of parameters:
1. Mechanical
i. Slip ring and commutator surface conditions (roughness)
ii. Friction coefficient of carbon brush
iii. Vibration
iv. Carbon brush pressure on a slip ring or commutator
v. Brush holders
2. Electrical
i. Voltage drop (or contact drop)
ii. Commutation (DC machines)
iii. Distribution of current in the brush contact surface
iv. Current density
v. Resistivity
3. Physical and chemical (environment)
i. Humidity
ii. Corrosive vapors or gases
iii. Oils and hydrocarbons
iv. Dust Considering those parameters, together with technical information the most suitable carbon brush grade for required application can be selected.
Uses of Carbon: Carbon,
in various forms and in combination of other materials, is widely used in
electrical engineering. Electrical carbon materials are manufactured from
graphite and other forms of carbon. Carbon is having following applications in
Electrical Engineering.
1. For
making electrical contacts
2. For making filament of incandescent lamp
3. For making resistors
4. For making brushes for electrical machines
such as DC machines, alternators.’
5. For
making battery cell elements
6. For making carbon electrodes for electric
furnaces
7. Arc lighting and welding electrodes
8. For making component for vacuum valves and
tubes
9. For makings parts for telecommunication
equipment.
Corrosion of nonferrous metals:
Aluminium corrosion: Aluminium corrosion resistance is very good in untreated aluminium. Untreated aluminium has very good corrosion resistance in most environments. This is primarily because aluminium spontaneously forms a thin but effective oxide layer that prevents further oxidation. The most common types of aluminium corrosion are:
Galvanic corrosion: Galvanic corrosion may occur where there is both metallic contact and an electrolytic bridge between different metals. The least noble metal in the combination becomes the anode and corrodes. The most noble of the metals becomes the cathode and is protected against corrosion. In most combinations with other metals, aluminium is the least noble metal. Thus, aluminium presents a greater risk of galvanic corrosion than most other structural materials.
Pitting: For aluminium, pitting is by far the most common type of corrosion. It occurs only in the presence of an electrolyte (either water or moisture) containing dissolved salts, usually chlorides. The corrosion generally shows itself as extremely small pits that, in the open air, reach a maximum penetration of a minor fraction of the metal’s thickness. Penetration may be greater in water and soil. As the products of corrosion often cover the points of attack, visible pits are not clearly seen on aluminium surfaces.
Crevice corrosion: Crevice corrosion can occur in narrow, liquid-filled crevices. Significant crevice corrosion can occur in marine atmospheres, or on the exteriors of vehicles. During transport and storage, water sometimes collects in the crevices aluminium surfaces and leads to superficial corrosion (“water staining”). The source of this water is rain or condensation that is sucked in between the metal surfaces. Condensation can form when cold material is taken into warm premises. The difference between night and day temperatures can also create condensation.
Stress corrosion, which leads to crack formation, is a more special type of corrosion. It occurs primarily in high-strength alloys (e.g. AlZnMg alloys) where these are subjected to prolonged tensile stress in the presence of a corrosive medium. This type of corrosion does not normally occur in common AlMgSi alloy.
Copper corrosion: Copper
turns green because of a process known as oxidation which is the removal of
electrons from the substance. Specifically, copper turns green because of
something known as copper carbonate. This is the substance that is found on top
of copper–whether they be copper pipes, pennies, statues or anything else. So,
the cause of copper turning green is copper carbonate. Certain conditions can
cause copper to corrode when it is exposed to particular soils, including:
a. Abnormally
aggressive soils Action of stray direct currents (DC) flowing in the ground
b. Faulty design and workmanship
c. Certain conditions created by alternating
currents (AC)
d. Thermo galvanic effects Galvanic action
involving dissimilar materials.
e. Coupling of copper with aluminum or copper
with steel can lead to severe galvanic corrosion. Cyanides are also very
corrosive to copper pipe.
Silver corrosion: When silver is attacked by sulfur, the chemical reaction leaves the metal with a brown-black colour on the surface which does not result in much metal loss. The attack also results in black narrow cracks and pits. The rate of pitting attack is generally slow, but can increase if a stronger acid or salts are present. Salts catalyze the reaction, thus increasing the rate of attack. Rubber bands contain sulfur, which is why rubber leaves black stripes on the metal. The black product left after sulfur corrosion is the chemical corrosion of the silver metal, and not a stain. When the black product is removed by polishing, a thin layer of the base silver metal is actually being removed.
Corrosion of Brass: Brass is susceptible to stress corrosion cracking, especially from ammonia or substances containing or releasing ammonia. The problem is sometimes known as season cracking.
Corrosion of Bronze: Copper is the major component of bronze and it passes through several stages of oxidation in the presence of oxygen and an electrolyte such as rain water. Eventually, the copper in bronze forms a green patina (greenish layer on the surface) on its surface that prevents further degradation of the underlying metal. Bronze also decays in contact with sulfurous compounds and chlorine, such as those present in sea spray.
Homework: Solve the question of chapter 1 from
atleast five past question set.
Email me at : 071bel350@pcampus.edu.np
Unit 3:Dielectric Material
Definition of Dielectric:
The conductors are meant to conduct electricity with
as low loss( low resistance) as possible, few also require these current
carrying conductor at various high voltages to be insulated from any unwanted
contact and this is achieved by using dielectric materials. A dielectric
material (dielectric) 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 a conductor,
but only slightly shift from their average equilibrium position causing
dielectric polarization.
Because of the
dielectric polarization, positive charges are displaced towards the field and
negative charges shift in the opposite direction. This creates an internal
electric filed within the dielectric itself.
Macroscopic
approach: The relationship between the electric flux density at
a point in a material and electric field strength is given by:
D=
ɛ x E
Where
D= electric flux density, Cm-2
E
= electric field strength, Vm-1
ɛ = Permittivity of the
medium,
ɛ = ɛo =
8.854 x 10-12 Fm-1 for
free space
And
ɛ = ɛo
ɛr for other material where ɛr is
the relative permittivity of the material. When a voltage V is applied across
the parallel plate capacitor as shown in figure, charges +Qo and –Qo
will be induced on the surface of these plates based in electrostatic
induction. The capacitance if the parallel plate capacitor is then given by,
C0 =
Qo/Vo
When a dielectric material (medium) is inserted between these plates, then the charges in the surfaces of the plates get increased due to the appearance of the charge on the surface of the dielectric near to the capacitor plates.
In vacuum, the electric flux density is numerically equal to the surface charge i.e. per unit area
σ0
= Q0 = ɛ0V/d
Where d is the separation between the plates.
Similarly, when there is a dielectric medium inserted between the plates, the
above relationship becomes
σ = Q = ɛ0 ɛrV/d
The
relative permittivity can be defined as increase in charge stored on the
capacitor plates due to insertion of the medium compared to the charge without
medium.
ɛr
= Q/Q0 = C/C0
Increase
in charge density when dielectric medium is inserted compared with that without
dielectric is called polarization. i.e.,
P= σ – σ0 = Q - Q0
P=
ɛ0 ɛrV/d - ɛ0 V/d
or,P=
ɛ0 V/d (ɛr – 1)
or,
P= ɛ0 (ɛr – 1) E
or,
P= ɛ0 X E
Where
E= V/d electric field strength X = ɛr – 1 = dielectric susceptibility P=
polarization vector of the material
Polarization P can also
be defined as the induced charge within dielectric per unit area. P= Q’/ A From
the above relationship we can conclude that for the value of relative
permittivity equal to unity, there will be no polarization (P=0), hence, no
additional surface charge on the plate.
Electric
dipole moment
Electric dipole moment is simply the separation
between a positive and negative charge of equal magnitude. If Q is the
magnitude of charge and a represents the vector from negative charge to
the positive charge then,
p = Q a
In a region containing a positive charge (+q) a negative charge (-q) will have 0 net charge. Nonetheless, these regions will have net dipole moment if their charge center doesn’t coincide. In an atom when there is no applied field the center of mass of atom and the charge center coincide with each other. So according to the definition of electric dipole moment, within the atom is zero. But when an external field is applied the atomic center of mass of an atom is mostly due to the contribution from the nucleus. In the other hand, the charge center for positive charge is unchanged; the negative charge center will be shifted in the direction opposite to the direction of applied field. The separation of charge centers means that there is induced electric dipole moment, which is termed as polarization. Whenever an atom is said to be polarized it possesses an effective dipole moment.
The induced dipole
moments depend primarily on the electric field causing it. Induced dipole
moment is related to applied electric field as
pind =α E
Where α= polarizability
of the atom.
Since the polarization of a neutral atoms involves the displacements of electrons α is called electronic polarizability or coefficient of electronic polarization and the process is known as electronic polarization. Almost all atoms possess electronic polarizability as electrons are not rigidly fixed in atom.
Ionic Polarization
Ionic crystals like NaCl, KCl, LiBr etc. witness ionic polarization. The ionic crystals have distinctly distinguishable ions located at well-defined Lattice sites so that each pair of oppositely charged neighboring ion has a dipole moment. As shown in figure in the absence of applied filed there is no net polarization of the specimen because the dipole moments of equal magnitude lined up head to head and tail to tail. The dipole moment p+ in the positive x direction has the same magnitude as dipole moment p- in the negative x-direction giving zero net polarization. When external field is applied, chlorine cl- ion are pushed opposite to the direction of applied field and sodium Na+ ion in the direction of the field about their equilibrium positions. Consequently dipole moment p+ in the direction of +X increases to p’+ and that in the –X direction decreases to p’- and the net dipole moment is no longer zero. The average dipole moment per ion (p’+ - p’-) in the presence of electric field depends on the field. The ionic polarizability (αi) is defined in a term of local field experienced by the ions as:
pav = αi Eloc
Generally, αi is greater
than αe by a factor of 10 or more, which leads to ionic solids having large
dielectric constants. If Ni is the numbers of ions in the dielectric, then
polarization is given by: P= Ni pav =Ni αi Eloc
Electronic polarization
also persists in these solids due to the displacement of negative charge
centered by the applied field but in ionic solids their contribution is much
smaller and can be neglected.
Dielectric
constant:
The
dielectric constant (К) is the relative permittivity of a dielectric material.
Dielectric constant, property of an electrical insulating material (dielectric
material) equal to the ration of the capacitance of a capacitor filled with the
given material to the capacitance of an identical capacitor in a vacuum without
the dielectric material. The insertion of a dielectric between the plates of a
parallel plates capacitor always increase its capacitance or ability to store
opposite charges on each plate, compared with this ability when the plates are
separated by a vacuum. If C is the value of the capacitance of a capacitor
filled with given dielectric and Co is the capacitance of an identical
capacitor in a vacuum the dielectric constant symbolized by the Greek letter
Kappa, К is simply expressed as
К=
C/Co
Dielectric breakdown / electrical breakdown:
Electrical
breakdown or dielectric breakdown is a long reduction in the resistance of an
electrical insulator when the voltage applied across it exceeds the breakdown
voltage. This results in the insulator becoming electrically conductive.
A
defining property of dielectric material is not only its ability to increase
capacitance but also and equally important its insulating behavior. Dielectric
materials are widely used as insulating media between conductors at different
voltages to prevent ionization of air and current flashovers between
conductors. The voltage across conductors and hence within the dielectric
cannot be increased without limit. A voltage will be eventually reached when
substantial current starts flowing through the dielectric, which may lead to
what is called Dielectric Breakdown.
In
gases and liquids, the breakdown is not permanents damage due to the breakdown;
the maximum electric filed that can be applied to the insulating medium without
causing dielectric breakdown is called Dielectric Strength of the
material and is denoted by Ebr.
The
dielectric strength of solid depends on molecular structure impurities in the
material, geometry, nature of the electrodes, temperature, ambient condition,
duration and frequency of applied filed and is different in AC and DC
conditions. The ageing also affects the dielectric strength.
Dielectric Breakdown in
Gases (e.g. Carbon monoxide, air, hydrogen, methane, nitrogen, oxygen)
The breakdown in gases
dielectric begins with ionization caused by collision of electrons. When a
strong electric field is applied the accelerating free electrons acquire energy
greater than the ionization of the gas. Most mobile electrons start the
collision ionization. The breakdown is initiated with a spark discharge which will
later developed an arc discharge resulting in high current density and
dielectric short circuit. The dielectric strength of gaseous dielectrics
depends on the following factors:
i.
Pressure
ii.
Uniformity of applied electric field
iii. Polarity of electrodes
iv Frequency of applied field.
Liquid Dielectric
Breakdown:
The mechanism of
breakdown in liquid dielectric depends upon the purity of dielectric. There are
different theories explaining breakdown in liquid dielectrics:
1.
Colloidal theory: The breakdown (in contaminated dielectric)
occurs due to the formation of conducting bridges between the electrodes by
drop droplets of emulsified water and suspended particles. The time taken to
form the bridge depends on
i. Extent of contamination
ii. Shape of electrodes
iii.
Gap between the electrodes
2.
Bubble theory
(In technically pure
dielectrics) the breakdown is initiated by ionization of contained in the
liquid. All liquid dissolve a certain quantity of gas specially air. Bubbles of
gases are formed by one or more of the following mechanisms:
a.
Gas pocket in the electrode surface.
b. Electrostatic repulsion between the surface
tension of the liquid to form the bubble.
c.
Vaporization of liquid by Corona type discharge.
d.
Gaseous products of ionization of the molecule.
At the spot where the electric filed initiates
ionization of the gas the intensity of electric field rises sharply. The gas
will act as a conductive medium eventually leading final breakdown.
3. Breakdown due to Globules
When
an insulating liquid contains in suspension a globule of another liquid then
the breakdown can take place in an electric field due to instability of the
globule. When globules become unstable it elongates rapidly and at a certain
length the breakdown channels develops at the end of the globule. When the
channels propagate total breakdown is caused.
Breakdown
in Solid Dielectrics
The
phenomenon of breakdown in solids is the most complicated in one and least
understood. The following three kinds of breakdown are possible in solid
dielectrics:
1.
Electro thermal Breakdown. This type of breakdown is caused due
to heat produce by the dielectric loss which is proportional to the intensity
of the electrostatic field and the frequency because of the heat generated due
to the losses is greater than the heat dissipated and if the voltage is applied
for a long enough period then the dielectrics will become unstable to reach a
state of internal thermal equilibrium resulting in an electro thermal breakdown
of the dielectric. The following conditions help in causing the electro thermal
breakdown of the dielectric:
1.
Large thickness of the dielectric
2.
High temperature of the dielectric and the surrounding medium.
3.
Continuous application of high voltage
4.
Large dielectric loss.
2.
Purely Electrical Breakdown:
3.
Electrochemical Breakdown:
This
type of breakdown normally occurs when the temperature is very high and the
surrounding air has high humidity. The insulation resistance of a material is
reduced due to electrolytic processes developed in the dielectric materials.
This happens when a DC voltage is continuously applied to a dielectric whose
leakage current due to ionic conductivity is adequately large.
Insulating materials:
Electrical
insulating materials are defined as material which offers a very large resistance
to flow of current and for that region they are used to keep the current in its
proper path along the conductor. A large number of substances and materials may
be classified as insulators, many of which have to be employed in practice, as
no single substance or material can satisfy all the requirements involved in
the numerous and varied applications of insulators in electrical engineering.
Such requirements involve consideration of physical properties, reliability,
cost, availability, adaptability to machining operations etc. thus in some
applications the insulating material in addition to its function as an
insulator may have to act as a rigid mechanical support the conductor and may
be installed out of doors, in which case the insulating qualities must be
retained under all atmospheric conditions. In other cases extremely flexibility
is required. Again, in electric heaters the insulating materials must maintain
their insulating qualities over a wide range of temperatures extending in some
cases to 1100o C, and for radio purposes the
insulating qualities must be maintained up to very high frequencies. In
electrical machines and transformers the insulating materials applied to the
conductors are required to be flexible, to have high specific electric strength
(to reduce thickness to minimum) and ability to withstand unlimited cycles
heating and cooling.
Characteristics
of Good Insulating Materials:
A
good insulating material should possess the following characteristics:
1.
Large insulating resistance
2. High dielectric strength
3.
Uniform viscosity – it gives uniform electrical and thermal properties
4.
Should be uniform throughout – it keeps the electric losses as low as possible
and electric stresses uniform under high voltage differences.
5.
Least thermal expansion.
6.
When exposed to arcing should be non-ignitable/
7.
Should be resistant to oils or liquids, gas fumes acids and alkalis.
8.
Should have no deteriorating effect on the material, in contact with it.
9.
Low dissipation factor (loss tangent)
10.
High mechanical strength
11.
High thermal conductivity
12.
Low permittivity
13. High thermal strength.
14.
Free from gaseous insulation to avoid discharges (for solids and gases)
15.
Should be homogeneous to avoid local stress concentration.
16.
Should be resistant to thermal and chemical deterioration.
Classification
of insulating materials
The
insulating materials can be classified in the following two ways:
1.
Classification according to substances and materials.
2.
Classification according to temperature.
Classification
according to substances and materials:
I
Solids (inorganic and organic)
Mica,
wood, slate, glass, porcelain, rubber, cotton, silk, rayon, terylene, paper and
cellulose materials etc.
II Liquids (oils and varnishes)
Linseed oil, refined
hydrocarbon mineral oils, spirit and synthetic, varnishes etc.
III
Gases
Dry
air, carbon dioxide, argon, nitrogen etc.
Classification according to Temperature:
Plastics:
Properties:
a.
Light weight.
b. Low thermal conductivity.
c. A wide range of colours.
d. Resistance to deterioration by moisture.
e.
Low electrical conductivity.
Epoxy
Resign: Properties:
a.
They are of transparent light amber colour and have very little shrinkage.
b. As adhesive these material have shown
extremely high bond strength without the need for pressure for curing.
c. Other properties are as follows:
Dielectric
constant (60Hz) ……. 3.3- 5.4
Volume
resistivity, Ohm-cm……… 1014-1016
Power
factor ………. 0.008-0.03
Silicon
Resins: Properties:
1.
Outstanding in heat resistance and resistance to ozone.
2.
Possess good electrical insulating properties.
3. Other properties of silicon moulded resins
(glass fiber filled) are given below:
Dielectric constant, 60 Hz…….. 4-5
Dielectric strength (kV/mm) ………… 8 -12
Plastics:
Properties:
1.
Light weight
2.
Low thermal conductivity
3.
Resistance to deterioration by moisture
4.
Low electrical conductivity
Porcelain:
Properties:
a.
Dry process porcelain is suitable only for low voltage application and for use
in dry locations.
b.
Wet process porcelain is being tried in making high voltage porcelain.
c.
Casting process for porcelain is used for the large high voltage insulators
terminals of HV machines.
d.
Volume resistivity varies from 10.13 to 10.16 ohm-cm at room temperature.
Volume
resistivity will decrease rapidly with an increase in temperature. In fact,
ordinary porcelain enamel actually becomes a fairly good electrical conductor
at or above its firing temperature. e. The dielectric constant is in the range
of 6 to 12. f. Dielectric strength of ordinary porcelain enamels will range
from 200 to 500 volts per mil with an average of 4 to 6 mils total thickness.
Glass:
Properties:
a.
The pure quartz glass is transparent, free from air inclusions. And possesses
extremely high electrical and physical characteristics. The loss tangent is
very small and almost independent of temperature.
b.
The volume resistivity at 200o C is extremely high, 1x1016
to
1x1017
ohm
cm.
c.
The dielectric constant varies from 3.8 to 16.2
d.
At high frequencies and high temperature glass failures due to electric stress
usually occur because of electro thermal break down.
Fiber
glass:
Cellulose insulation materials cannot work beyond 130oC with stability. They can be substituted by fiber glass insulation for withstanding high temperature. Fiber glass free from alkalies and metal oxides possess good dielectric strength, insulation strength and durability. It has good tensile strength and better moisture resistance than cotton. It is resistant to chemicals and oils.
Fiber glass insulation is used for
a.
The backing of mica for low voltage insulation
b.
The insulation of enamel wire
c.
Making glass tapes used for the insulation of winding sin high working
temperatures.
Mica:
1. It is mineral composed
of silicate of aluminium, with silicate of soda potash and magnesia. It occurs
in the form of crystals which can easily be split into lamina.
2.
It is affected by oils.
3.
The resistivity of the mica at 25oCrnages
from about 1012 to 106 Ω
cm depending on inclusions etc. 4. The dielectric strength varies from 40 to
150 kV/mm thickness.
5.
It has high dielectric strength and low power loss.
6.
Its electrical properties are deteriorated with the presence of quartz and
feldspar.
Transformer
oil:
The
transformer oil, in general, possesses the following properties:
1.
Electric strength …… 40 Kv when applied for one minute.
2.
Dielectric constant …….2.2
3.
Resistivity… 1015 ohm cm
4.
Freezing point of oil for switches and circuit breakers.
Insulating
Varnish:
Properties:
1.
Heating resistance – long term service at 250oC
(482oF).
2.
Electrical properties sustained for extended periods.
3.
Mechanical properties- providing rigidity, bond ability, vibration protection.
4.
Arc and corona resistance- much greater than organic materials.
5.
UV and ozone resistance.
Gases
(SF6):
Properties:
1.
Sulphur hexafluoride is formed by burning sulphur in an atmosphere of fluorine.
2.
It is one of the heaviest gases. (5 times higher than air).
3.
It has remarkable high dielectric strength.
4.
Its dielectric strength increases at high pressure and may even become equal to
that of mineral transformer oil.
5.
Apparatus insulated with sulphur hexafluoride gas are higher in weight than
those insulated with liquid dielectric.
6.
It increases the interrupting capacity of circuit breakers.
Electrical
properties of insulating materials:
Insulating
resistance:
It
may be defined as the resistance between two conductors (or system of
conductors) usually separated by insulating materials. It is the total
resistance in respect of two parallel paths, one through the body and other
over the surface of the body. Insulation resistance is affected by the
following factors:
i.
It falls with increase in temperature.
ii.
The resistivity of the insulator is
considerably lowered in the presence of moisture.
iii.
It decreases with the increase in applied
voltage.
iv.
Resistivity
v.
Volume resistivity is the resistance between
opposite faces of a cube of unit dimension.
It
is usually measured in, mega ohm- centimeters. The volume resistivity of most
insulating material is affected by temperatures; the resistivity decreasing
with an increase of temperature, i.e. the temperature co efficient of
resistivity is uniform over the whole surface.
Dielectric strength
The
maximum electric field that a dielectric can withstand under ideal conditions
without breaking down is known as the dielectric strength of the material. The
dielectric strength of an insulating material decreases with the length of time
that voltage is applied. Moisture, contamination, elevated temperatures, heat
ageing, mechanical stress and other factors may also markedly decrease
dielectric strength to as little as 10 % of the short time values at standard
laboratory conditions. The value of dielectric strength is useful in comparing
insulating materials, determining the effect of environmental and operating
conditions, measuring uniformity, and controlling acceptance of the material.
Power factor:
Power
factor is a measure of the power loss in the insulation and should be low.it
varies with the temperature and usually increases with the rise in temperature
of the insulation. A rapid increase indicates danger.
Dielectric constant
(permittivity)
The property is defined as the
ratio of the electric flux density in the material to that produced in free
space by the same electric force. Power factor and dielectric constant at power
frequencies can be used to compare insulating materials and determine the
effect if environment and operating conditions. When measured at high voltage,
power factor and dielectric constant are useful in evaluating high voltage
insulation system.
Dielectric
loss:
The dielectric losses occur in all
solid and liquid dielectrics due to:
i.
Conduction current: The conduction current is due to
imperfect insulating qualities of the dielectric. It is in phase with the
voltage and result in a power (IR) loss in the material which is dissipated as
heat.
ii.
Hysteresis: Dielectric hysteresis is defined
as the lagging of the electric flux behind the electric force producing it so
that under varying electric forces, a dissipation of energy occurs, the energy
loss due to this is cause being called the dielectric hysteresis loss
The
dielectric loss is affected by the following factors:
i.
Presence
of humidity ….. it increases the loss
ii.
Voltage
increase….. it cause high dielectric loss
iii.
Temperature
rise … it normally increases the loss.
iv.
Frequency
of applied voltage…. The loss increase proportionally with the frequency of
applied voltage.
Ferro
electricity:
Ferroelectricity
is
the property of certain non-conducting crystals, or dielectrics, that exhibit
spontaneous electric polarization (separation of the center of positive and
negative electric charge,
making
one side of the crystal positive and the opposite side negative) that can be
reversed in direction by the application of an appropriate electric field.
Ferroelectricity is named by analogy with ferromagnetism, which occurs in such
materials as iron. Iron atoms, being tiny magnets, spontaneously align
themselves in clusters (group) called ferromagnetic domains, which in turn can
be oriented in a given direction by the application of an external magnetic
field.
Ferroelectricity
ceases in a given material above a characteristic temperature, called its Curie temperature, because the heat
agitates the dipoles sufficiently to overcome the forces that spontaneously
align them.
Piezoelectricity:
Piezoelectricity is the
electricity (generation of electrical potential) which is produced as a result
of piezoelectric effect of some materials when subjected to mechanical stress
or compression or expansion. This effect is reversible i.e. its converse effect
also exist. That means when some material (normally crystals and ceramics) with
the application of electrical potential may result in change in mechanical
character (produce compression or expansion or strain).
Only certain crystals exhibit piezoelectricity because the phenomenon requires a special crystals structure that which has no center of symmetry. The NaCl unit cell has cubic symmetry and there is no net polarization when there is no force applied and the specimen is still not polarized after it has been applied by external force as shown in figure.
Application of
Piezoelectricity:
·
Used as sources of voltage and power in
electric cigarette lighter, in equipment used in battle field, in piezoelectric
transformer etc.
·
As sensors in microphones, pickups in
guitar and other musical instruments, power monitoring in medical treatment
etc.
·
As actuators in loudspeakers, inkjet
printers, piezoelectric motors etc.
·
As frequency standard in quartz clock.
Properties
of Dielectric materials:
Dielectric
strength
Dielectric
strength is defined as the property of an insulator which enables it to
withstand continuous electric stress or the maximum electrical stress which it
will successfully withstand. The dielectric strength decreases with the increase
in the thickness of specimen, frequency, operating temperature and humidity
Dielectric
loss Insulation Dielectric constant Susceptibility:
In
electricity (electromagnetism), the electric susceptibility (x) is a
dimensionless proportionality constant that indicates the degree of
polarization of a dielectric material in response to an applied electric field.
Greater the electric susceptibility, the greater the ability of a material to
polarize in response to the field, and thereby reduce the total electric field
inside the material (and store energy). It is in this way that the electric
susceptibility influences the electric permittivity of the material and thus
influences many other phenomena in that medium.
Permittivity:
In
electromagnetism, permittivity or absolute permittivity is the measure of
resistance that is encountered when forming an electric field in a medium. In
other words, permittivity is a measure of how an electric field affects, and is
affected by, a dielectric medium. The permittivity of a medium describes how
much electric field (more correctly, flux) is 'generated' per unit charge in
that medium. More electric flux exists in a medium with a low permittivity (per
unit charge) because of polarization effects. Permittivity is directly related
to electric susceptibility, which is a measure of how easily a dielectric
polarizes in response to an electric field. Thus, permittivity relates to a
material's ability to resist an electric field (while, unfortunately, the word
stem "permit" suggests the inverse quantity).
Unit 4: Semiconductor Materials
Characteristics
of Semiconductors:
Semiconductors
possess the following characteristics:
1.
The resistivity is usually high.
2.
The temperature co-efficient of resistance is always negative.
3.
The contact between semiconductor and a metal forms a layer which has a higher
resistance in one direction than the other.
4.
When some suitable metallic impurity (e.g. Arsenic, Gallium etc.) is added to a
semiconductor, its conducting properties change appreciably.
5.
They exhibit a rise in conductivity in the increasing temperature, with the
decreasing temperatures their conductivity falls off, and at low temperatures
semiconductors become dielectrics.
6. They are usually metallic in
appearance but (unlike metals) are generally hard and brittle.
Elements of semiconductor materials:
Of
all the elements in the periodic table, eleven are semiconductors which are
listed below:
i.
Alloys: Mg3Sb2,
ZnSb, Mg2Sn,
CdSb, Alsb, InSb, GeSb
ii.
Oxide: ZnO, Fe3O4,
Fe2O3,
Cu2o,
CuO, BaO, CoO, Al2O3,
TiO2,
UO2,
Cr2O3,
WO2,
MoO3.
iii.
Sulphides: Cu2S,
Ag2S,
PbS, ZnS, CdS, HgS, MoS2.
iv.
Halides: AgI, CuI
v.
Selenides and Tellurides
PbS
is
used in photo conductive devices, BaO
in oxide coated cathodes, caesiumm antimonide in photomultipliers etc.
Energy gap in
semiconductor:
a. Semi-conductor
Semiconductor is that
material which has the conduction property in between conductor and insulator.
It means semi-conductor does not block the current as insulator does. For
example silicon, boron, etc. the reason for such type of conductor is the small
gap between the valance band and conduction band. Semi-conductors have
comparatively less free electron than the conductor.
Atomic structure of
Silicon and Germanium
Silicon:
Atomic
number =14
Number
of electrons =14
Number
of protons =14
Number of neutron= 14
Germanium:
Number
of electron =32
Number
of proton = 32
Number of neutron = 41
Electrons
and Holes:
Pure semiconductors are relatively good insulator as compared with metals though not nearly as good as a true insulator like glass. To be useful in semiconductor applications, the intrinsic semiconductor (pure undoped semiconductor) must have no more than one impurity atom in 10 billion semiconductor atoms. Figure below (a) shows four electrons in the valance shell of a semiconductor forming covalent bonds to four other atoms.
 |
| Fig: (a) Intrinsic semiconductor is an insulator having complete electron shell. |
 |
(b)However thermal energy can create few
electron hole pairs resulting in weak conduction.
All electrons of an atom
are tied up in four covalent bonds, pair of shared electrons. Electrons are not
free to move about the crystal lattice. Thus intrinsic, pure semiconductors are
relatively good insulators as compared to metals.
Thermal energy may
occasionally free an electron from the crystal lattice as in figure (b). This
electron is free for conduction about the crystal lattice. When the electron
was freed, it left an empty spot with a positive charge in the crystal lattice
known as a “hole”.
This hole is not fixed to
the lattice but is free to move about. The free electron and hole both
contribute to conduction about the crystal lattice. That is, the electron is
free until it falls into a hole. This is called recombination. If an external
electric field is applied to the semiconductor, the electrons and holes will
conduct in opposite directions. Increasing temperature will increase the number
of electron and holes, decreasing the resistance. This is opposite of metals,
where by increasing the collisions of electrons with the crystal lattice.
The number of electrons
and holes in an intrinsic semiconductor are equal. However, both carriers do
not necessarily move with the4 same velocity with the application of external
field. The drift velocity of electron is always more than the drift velocity of
hole.
Intrinsic semiconductor: A
pure semiconductor is called intrinsic semiconductor. Here no free electrons
are available since all the covalent bonds are complete. A pure semiconductor
therefore behaves as an insulator. It exhibits a peculiar behavior even at room
temperature or with rise in temperature. The resistance of a semiconductor
decreases with increase in temperature.
Extrinsic semiconductor:
In a pure semiconductor,
which behaves like an insulator under ordinary conditions, if small amount of
certain metallic impurity is added it attains current conducting properties.
The impure semiconductor
is then called impurity semiconductor or extrinsic semiconductor. The process
of adding impurity (extremely in small amounts, about 1 part in 108)
to a semiconductor to make it extrinsic (impurity) semiconductor is called Doping.
Generally following
doping agents are used:
i.
Pentavalent atom having five valence electrons (arsenic, antimony, phosphorus)
…. Called donor atoms.
ii.
Trivalent atoms having three valence electrons (gallium, aluminum, boron)……
called acceptor atoms.
With the addition of
suitable impurities to semiconductor, two type of semiconductor are:
i.
N-type semiconductor
ii.
P- type semiconductor
N-
type semiconductor:
The
presence of even a minute quantity of impurity can produce N-type
semiconductor. When an intrinsic (Pure) semiconductor is doped with pentavalent
(having 5 valence electron) impurity semiconductor now becomes impure or
extrinsic and known as N-type semiconductor. For e.g. an atom of Germanium
possesses four valence electron; when it is replaced in the crystal lattice of
the substance by an impurity atom of antimony (Sb) which has five valence
electrons, the fifth valence electron (free electron) produces extrinsic N-type
conductivity. Such an impurity into a semiconductor is called donor impurity
(or donor). The conducting properties of germanium will depend upon the amount
of antimony (impurity) added.
 |
| Doping of germanium by antimony |
Note: Even though N-type semiconductor has excess of electrons, still it is electrically neutral. It is so because by addition of donor impurity, number of electrons available for conduction purposes becomes more than the number of holes available intrinsically. But the total charge of the semiconductor does not change because the donor impurity brings in as much negative charge (by way of electrons) as positive charge (by way of protons).
Electron
flow:
Electron
flow in an N- type semiconductor is similar to electrons moving in a metallic
wire. The N-type dopant atoms will yield electrons available for conduction.
These electrons due to the dopant are known as majority carriers, for they are
in the majority as compared to the very few thermal holes. If an electric field
is applied across N-type semiconductor bar as in figure (a), electrons enter
the negative (left) end of the bar, traverse (travel across) the crystal
lattice, and exit at the right to the (+) battery terminal.
P-type semiconductor:
When
an intrinsic semiconductor is doped with trivalent (having three valence
electrons) impurities semiconductor now becomes impure or extrinsic and is
known as P-type semiconductor. Since this impurity atom has one valence
electron less than the semiconductor atom that it has replaced, the impurity
atom cannot fill all the interatomic bonds, and the free bond can accept an
electron from the neighboring bond; leaving behind a vacancy or hole. Such an
impurity is called an acceptor impurity (or acceptor).
Note:
Even though P-type semiconductor has excess of holes for conduction purposes,
as a whole it is electrically neutral for the same reasons as discussed
earlier.
 |
| Doping of germanium by boron |
Current
flow in P-type:
The
P-type dopant, an electron acceptor, yields localized regions of positive
charge known as holes. The majority carrier in a P-type semiconductor is the
hole. While holes form at the trivalent dopant atoms site, they may move about
the semiconductor bar. If an electric field is applied across the P-type
semiconductor bar as in figure (b), the positive battery terminal is connected
to the left end of the P-type bar. Electron flow is out of the negative battery
terminal through the P-type bar, returning to the positive battery terminal.
An electron leaving the positive (left) end of the semiconductor bar for the positive battery terminal leaves a hole in the semiconductor that may move to the right. Holes traverse (travel) the crystal lattice from left to right. At the negative end of the bar an electron from the battery combines with a hole, neutralizing it. This makes room for positive end of the bar toward the right. As holes move left to right, that is responsible for the apparent hole movement.
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