SOLUTIONS D- 04 Engineering MATERIALS

SOLUTIONS D- 04 ENGINEERING MATERIALS. (JUNE 2003)

1. a. C Dielectric materials.

b. A Over head transmission lines.

c. A Copper and zinc.

d. A Ductility

e. A Pentavalent

f. A Hard magnetic materials.

g. B Unity.

h. A Extrinsic semiconductor.

2a. Plastic materials can be classified into thermoplastic and thermosetting plastics.

Thermoplastic materials:- The properties of these plastic materials do not change considerably if they are melted and then cooled and solidify. They can be repeatedly melted or dissolved in various solvents. They are more elastic, less brittle and do not lose elasticity when subjected to prolonged heating. They are less apt to age thermally. Many of them possess extraordinary high insulating properties and are water repellent. They are polymers of linear structure, i.e. their molecules are elongated and are thread like. This, type of structure is fusible, soluble, highly plastic, capable of forming thin flexible threads and films. Examples are Polytetra Flouroethylene (P.T.F.E. or Teflon), Polyvinyl Chloride (P.V.C.).

Thermosetting Plastic Materials:- They undergo great changes when subjected to high temperatures for quite sometime. They are said to be baked and no longer can melt or be dissolved. They are less elastic, more brittle and lose their elasticity when subjected to prolonged heating. They are used, when an insulation is to withstand high temperatures without melting or losing its shape and mechanical strength. Thermosetting plastic substances are space-polymers and the molecules branch off in various directions during polymerisation. This structure makes them very rigid, poorly soluble, fusible and incapable of forming elastic threads and films. Examples are Phenol formaldehyde (Bakelite), Epoxy resins.

2b. Examples of the natural insulating materials are cotton, rubber, wood, mica.

3a. Fuse is a protective device, which consists of a thin wire or strip. This wire or strip is placed with the circuit it has to protect, so that the circuit-current flows through it. When this current is too large, the temperature of the wire or strip will increase till the wire or strip melts thus breaking the circuit and interrupting the supply.

A fuse material should possess the following properties;-

Low resistivity – This means, thin wires can be used, which will give less metal vapour after melting of the wire. Less metal vapour in the arc gives lower conductivity and thus makes quenching of arc easier.

Low conductivity of the metal vapour itself.

Low melting point- This means that the temperature of the fuse material for normal currents stays at a low value.

3b.

3a.

n – type semiconductor:- When small amount of pentavalent impurity is added to a pure semiconductor providing a large number of free electrons in it, the extrinsic semiconductor thus formed is known as n- type semiconductor. The addition of pentavalent impurities such as arsenic and antimony provide a large number of free electrons in the semiconductor crystal. Such impurities, which produce n- type semiconductors, are known as donor impurities because each atom of them donates one free electron to the semiconductor crystal.

p - type semiconductor:- When small amount of trivalent impurity is added to a pure semiconductor providing a large number of holes in it, the extrinsic semiconductor thus formed is known as p- type semiconductor. The addition of trivalent impurities such as gallium and indium provide a large number of holes in the semiconductor crystal. Such impurities, which produce p- type semiconductors, are known as acceptor impurities because each atom of them creates one hole, which can accept one electron.

4a. An atom consists of a positively charged nucleus surrounded by a group of negatively charged electrons. The nucleus is at the centre of the atom and electrons are established in definite states, orbits, energy- levels. The electrons in the outermost orbit of an atom are called valence electrons. It is these electrons that are responsible for determining the physical, chemical and electrical properties of the material.

On the basis of atomic structure the materials are classified as

1) When the No. of valence electrons of an atom is less than 4 i.e. half of maximum permissible 8 electrons, the material is a metal or conductor. Example Cu, Mg and Al have 1,2 and 3 valence electrons respectively.

2) When the No. of valence electrons of an atom is more than 4 , the material is a insulator. Example As, Se and Bromine have 5,6 and 7 valence electrons respectively.

3) When the No. of valence electrons of an atom is 4 i.e. exactly half of maximum permissible 8 electrons, the material is a semiconductor. Example Carbon, Silicon and Germanium have 4 valence electrons each.

4b. I) Lithography- It is one of the processes which is carried out during manufacture of integrated circuits. SiO₂ layer is thermally grown on the wafer surface. The pattern is transferred from mask to the oxide layer as follows - A photo resist liquid is uniformly applied over the oxide layer and dried. Photo resist is a material, which changes its solubility in certain organic solvents, when exposed to ultra violet light. Non-polymerised parts are then dissolved using organic solvents. The oxide layer below these parts is now etched using net chemical etchant. By, this process, the pattern is transferred from the mask to the oxide. During next diffusion, the layer of SiO₂serves as a mask.

2) Superconductivity- The resistivity of most metals increases with increase in temperature and vice-versa. There are some metals and chemical compounds whose resistivity becomes zero when their temperature is brought near 0⁰Kelvin (-273⁰C). At this stage such metals or compounds are said to have attained superconductivity. Example – mercury becomes superconducting at approximately 4.5 Kelvin (-268.5⁰C). Superconductivity, was discovered by Heike Kamerlingh Onnes. The transition from normal conductivity to superconductivity takes place almost suddenly; it occurs over a very narrow range of temperature about 0.05⁰K. The temperature at which the transition takes place from the state of normal conductivity to that of superconductivity is called transition temperature.

5a) I)Soft magnetic materials -have small enclosed area of hysteresis loop, high permeability, high saturation value, low eddy current losses which is achieved by using laminated cores, less residual magnetism. Soft magnetic materials are used for the construction of cores for electrical machines, transformers, electromagnets and reactors.

Hard Magnetic materials-have a gradually rising magnetization curve with large hysteresis loop area and hence large energy losses. They have high value of retentivity and high value of coercivity and low permeability. To saturate the hard magnetic materials, a high magnetizing force is required. Hard magnetic materials have the property of storing a considerable amount of magnetic energy after magnetization and retaining the same for a long time. Due to this property they are used in the manufacture of permanent magnets.

B-H curves for soft and hard magnetic materials

ii) Brass- Is an alloy of copper and zinc with 60% copper and 40% zinc. Its properties are-

1. Its electrical resistivity is 7.0X 10^-8 ohm-m, which is higher than the copper resistivity.

2. It is ductile and can be drawn into different shapes.

3. It melts at 890⁰C.

4. Its specific gravity is 3.3.

5. It has got an excellent corrosion resistance.

6. It has got good mechanical properties.

Uses:- Brass is used as a structural and current carrying material in power switches, plugs, sockets, lamp holders, fuse holders, knife switches, sliding contacts for starters and rheostats, wave guide components.

6) Electrical properties of an insulating material are:

Insulation resistance-is the property, by the virtue of which, a material resists flow of electrical current. It should be high as possible. Insulation resistance is of two types: I) Volume resistance; ii) Surface resistance.

The resistance offered to the current, which flows through the material is called volume resistance. The resistance offered to the current, which flows over the surface of the insulating material is called surface resistance. Factors that affect the insulation resistance are-temperature variations, exposure to moisture, voltage applied, aging.

Dielectric Strength- is therefore the minimum voltage which when applied to an insulating material will result in the destruction of its insulating properties. It can also be defined as the maximum potential gradient that the material can withstand without rupture. This value is expressed in volts or kilovolts per unit thickness of the insulating material. This value is greatly affected by the conditions under which the material is operated. Factors affecting the dielectric strength are temperature, humidity.

Dielectric Constant- Every insulating material has got the basic property of storing charge (Q), when a voltage (V) is applied across it. The charge is proportional to the voltage applied i.e. Q a V, or Q = CV. Where C is called the capacity or capacitance of the material across which the voltage is applied. Every insulating material behaves as a capacitor. Capacitance is different for different insulating material. The property of insulating materials that causes the difference in the value of capacitance, with the physical dimensions remaining the same is called dielectric constant or permittivity (Î).

Dielectric loss and loss angle: When a perfect insulation is subjected to alternating voltage, it is like applying alternate voltage to a perfect capacitor. In a perfect capacitor the charging current would lead the applied voltage by 90⁰exactly. This means that there is no power loss in the insulation. In most insulating materials this is not the case. There is a definite amount of dissipation of energy when an insulator is subjected to alternating voltage. This dissipation of energy is called dielectric loss. Factors affecting dielectric loss are – Frequency of applied voltage, humidity, temperature rise and voltage.

The dielectric phase angle is q and d = 90⁰ - q is the dielectric loss angle as shown in the fig. below.

7) (i) Rubber: The different types of rubber materials are- Natural rubber, Hard rubber and Synthetic rubber.

Natural Rubber – Natural rubber is extracted from milky sap collected from special trees. Water is then evaporated. Additives like sulphur, oxidation inhibitors like aromatic amino compounds, softeners like vegetable oil and fillers like carbon black and zinc oxide are added to it. It is vulcanized by adding sulphur and heating it. Vulcanization improves heat and frost resistance of rubber, making it mechanically stronger. The permittivity and power factor varies depending on the sulphur content and temperature change.

Properties – This rubber is moisture repellent and has good insulating properties. It has good abrasion resistance.

Applications – It is used for the manufacturing of protective clothing such gloves, boots. It is used as an insulation covering for wires and cables.

Hard Rubber – Hard rubber is obtained by addition of more sulphur and by extended vulcanization.

Properties – It has good electrical properties. Water absorption is less. Maximum permissible operating temperature is 60⁰ C. It can not be continuously exposed to Sun as it is harmful. It has high tensile strength.

Applications – Hard rubber is used for construction of storage battery housing, panel boards, bushings of various types. It is also used as jacketing material for cables.

Synthetic Rubber – The different types of synthetic rubber are

(a) Butadiene rubber – its properties are greater resistance to ageing and oxidation, lower tensile and tear strength, lower water absorption, higher heat conductivity.

(b) Butyl rubber – its properties are excellent resistance to vegetable oils and alcoholic solvents, but it is easily attacked by petroleum oils and greases. It has high resistance to ozone, high thermal and oxidation stability but poor tensile strength. It is used as insulation for wires and cables.

(c) Chloroprene Rubber (Neoprene Rubber) – It has better resistance to thermal ageing, oxidation, sunlight and gas diffusion. These rubbers have better thermal conductivity and more flame resistance. They exhibit better adhesion to metals. They possess better resistance to attack by solvents like mineral and vegetable oils but poor resistance to aromatic hydrocarbon liquids. They are inferior in mechanical properties like tear and tensile strength and abrasion resistance. Neoprene rubber is used as insulating material for wires and cables. It is also used as jacketing material for cables.

(d) Chlorosulphonated Polyethylene (Hypalon) – It has better electrical properties, high resistance to degradation when exposed to high temperature and oxidation. It can be operated at temperatures as high as 150⁰ C. It has poor solvent resistance to hydrocarbons. It is mechanically less tough. It is used as insulating material for wires and cables and also as jacketing material for cables.

(e) Silicon Rubber – It has high thermal conductivity. Its tensile strength is low, has good flexibility at low temperatures and resistant to ozone, oxidation and severe atmospheric conditions. It can be used over a wide range of temperatures from –100⁰to 150⁰C. Silicon rubber is used as insulating material for wires and cables, in the manufacture of moulded parts, as an insulating tape and coating material.

(ii) Mica and Mica Products – Mica is an inorganic material. It is one of the best insulating materials available. From the electrical point of view, mica is of two types – Muscovite mica and Phologopite mica.

Muscovite mica – Chemical composition is KH₂Al₃(SiO₄)₃. The properties are

i) Strong, tough and less flexible

ii) Colourless, yellow, silver or green in colour

iii) Insulating properties are very good

iv) Abrasion resistance is high

v) Alkalies do not affect it

Uses – Muscovite mica is used where electrical requirements are severe. Because of high dielectric strength, it is used in capacitors. It is also used in commutators due to high abrasion resistance.

Phologopite mica – Chemical composition is KH(MgF)₈MgAl(SiO₄)₃. The properties are

i) Amber, yellow, green or grey in colour

ii) Greater structural stability, being tougher and harder than muscovite mica, less rigid

iii) Resistant to alkalies, but less to acids

iv) Greater thermal stability than that of muscovite mica

Uses – It is used when there is greater need of thermal stability as in domestic appliances like irons, hotplates, toasters.

Mica products

(i) Glass bonded mica – Ground mica flakes and powdered glass when moulded makes glass bonded mica. This material is impervious to water and chemically stable. This is used in high humidity and high ambient temperatures.

(ii) Mica paper – Mica is broken into small particles in aqueous solution. Out of this sheets of mica paper are produced which are used as insulation for armature and field coils of rotating machines.

(iii) Manufactured mica – Mica flakes held together with adhesives is called manufactured mica. It is used in commutators, electrical heating devices, motor slot insulation, transformers, etc.

(iii) Glass and glass products – Glass is an inorganic material made by the fusion of different metallic oxides. The properties of glass are –

i) It is transparent, brittle and hard

ii) Glass is insoluble in water and usual organic solvents

iii) It has low dielectric loss, slow ageing and good mechanical strength

iv) It is susceptible to destruction when sudden and high temperature cycles are applied

Uses – Glass is used in moulded insulating devices such as electrical bushings, fuse bodies, insulators. Glass is used as a dielectric in capacitors. Radio and television tubes, electrical laminated boards also make use of glass

Glass Products

a) Silica glass or fused quartz – Silica when heated to a temperature of fusion and then cooled is known as silica glass. This material has good electrical properties, low coefficient of expansion and high resistivity.

b) Borosilicate glass or Pyrex – This glass requires 28 per cent of boron oxide along with other oxides. They resist the effect of chemicals and moisture better than other glasses.

c) Fibre glass insulation – This is capable of withstanding high temperature. For most applications fibre glass is impregnated with materials like synthetic resins or with mineral oil. They possess good electrical and mechanical properties and sufficient flexibility to be moulded into required shapes.

d) Epoxy glass – It is made by joining glass fibre layer with a thermosetting compound. It is immune to alkalies and acids and is used in PCB making, terminal holders and instrument cases.

(iv)Ceramics – Ceramics are materials made by high temperature firing treatment of natural clay and certain inorganic matters. The properties of ceramics are –

i) Ceramics are hard, strong and dense.

ii) Ceramics are not affected by chemical action except by strong acids and alkalies.

iii) Stronger in compression than in tension.

iv) Excellent dielectric properties.

v) Stable at high temperatures.

Uses – The capacity to withstand high temperature, immunity to moisture, good electrical properties make ceramics valuable for the use in different types of insulators, transformer bushing pins, fuse holders, plugs and sockets.

Main ceramic materials are

(a) Porcelain – It is used in low frequency applications due to high dielectric loss factor. It has low electrical resistivity.

(b) Steatite – These are low dielectric loss porcelains, used for variable capacitor coils, switches, resistor shafts and bushings. It is also used for low voltage and high frequency applications.

(c) Alumina – It has high mechanical and dielectric strength. It also has high electrical resistivity and low dielectric losses. It is chemically stable and retains its properties over a wide range of temperature and frequencies. It is used for circuit breakers, spark plugs, resistor cores, integrated circuits and power transistors.

(d) Zirconia – It has poor thermal conductivity and shock resistance. It is available with calcium or yttrium. It is used for high temperature heating elements.

8A) Extensive use, is made of iron-silicon alloy called silicon steel for relatively strong alternating magnetic fields generally used in transformers, electrical rotating machines, reactors, electromagnets and relays. Silicon sharply increases the electrical resistivity of iron thus decreasing the iron losses due to eddy currents. It increases the permeability at low and moderate flux densities but decreases it at high densities. Addition of silicon to iron reduces the hysteresis loss. The magnetostriction effect is also reduced. A, steel with more than 5% silicon may be too hard and brittle to be easily workable. The introduction of silicon in steel was an important development. In the past, iron was used as the core material in the form of sheets, the material gradually deteriorated due to repeated heating and cooling. This difficult overcome by using silicon sheet steel as core material. High silicon sheet steel with silicon of about 4% is used in magnetic circuits of power transformers, which operate at flux densities of about 1 Wb/m². The cores of rotating electrical machines are slotted with the teeth having small cross sectional areas. The flux density in the teeth is much higher, of the order of 1Wb/m². To obtain high flux density in the case of high silicon steel, the magnetising current necessary is very large. Also punching slots in high silicon steel would be difficult because it is harder and more brittle.

8b. An alloy of two or more metals of low melting point used for base metals is known as soldering. The alloy used for joining the metals is called solder. The most common solder is composed of 50% tin. Its melting point is about 185⁰C. Many commercial solders, contain larger percentage of lead and some antimony with less tin, as the electrical conductivity of lead is only about half that of tin. For soldering flux is to be used. Solders are of two types- Soft solders and Hard solders. Soft solders are composed of lead and tin in various proportions. Hard solders may be any solder with a melting point above that of lead-tin solders. Soft solders are used in electronic devices and hard solders in power apparatus for making permanent connections.

9 (i) Soft ferrites and their applications: These are non- metallic compounds consisting of ferric oxide and one or two bivalent metal oxides such as Nickel oxide, Manganese oxide or Zinc oxide. These have resistivity of the order of 10⁹ ohm-cm, which reduces eddy current losses at high frequency. The magnets made out of it have high coercive force and square hysteresis loop. Magnetic permeability of these materials is as high as 10,000 to 30,000. These materials are fabricated into shape such as E, U, I, beads and self shielding pot cores.

Applications – High frequency power transformers operating at 10 to 100 kHz, pulse transformers upto 100’ MHz, adjustable air gap inductors, recording heads make use of cores made of soft ferrites.

(ii) PVC and its applications – This is obtained from polymerisation of vinyl chloride in the presence of a catalyst like peroxides at about 50⁰ C. The properties like mechanical strength, porosity, flexibility, moisture absorption and electrical properties can be changed by adding certain materials. PVC has good mechanical and electrical properties. It is hard and brittle. It resists flame, most solvents and sunlight. It is non-hygroscopic. PVC is widely used as insulation and jacketing material for wires, cables. PVC films, tapes and sheets are used as insulation for dry batteries and conduit pipes.

10a)The desirable properties of an insulation material are:

1.Very good dielectric strength as that of mica (upto 80 kV/mm at 25⁰C).

2. Volume and surface resistivity equal to that of sulphur.

3. Good mechanical strength like that of steel.

4. Very high crushing resistance (as that of granite).

5. Easy of machining.

6.Good fire proofing qualities (as that of silica).

7.It should have very high chemical inertness.

8. It should have good water proofing qualities similar to that of paraffin wax.

10b) Dielectric loss and loss angle: When a perfect insulation is subjected to alternating voltage, it is like applying alternate voltage to a perfect capacitor. In a perfect capacitor the charging current would lead the applied voltage by 90⁰exactly. This means that there is no power loss in the insulation. In most insulating materials this is not the case. There is a definite amount of dissipation of energy when an insulator is subjected to alternating voltage. This dissipation of energy is called dielectric loss. Factors affecting dielectric loss are – Frequency of applied voltage, humidity, temperature rise and voltage.

The dielectric phase angle is q and d = 90⁰ - q is the dielectric loss angle as shown in the fig. below.

11) (i) Permeability : It is defined as the capability of the material to conduct flux. It is defined as the ratio of magnetic flux ‘B’ in a medium to the magnetic flux intensity ‘H’ at the same location in the medium, i.e. m = B/H, where B is plotted against H, a curve is obtained, called magnetization curve or B-H curve. The permeability of any material is not a constant. The permeability at low value of H is called initial permeability. The common core materials such as low carbon steel, silicon steel have low initial permeability.

(ii) Dielectric constant: It is also known as ‘Permittivity’. Every insulating material possesses an electrical capacitance. The capacitance of such unit depends upon dimensions and kind of dielectric placed between the capacitor plates. The capacitance of a parallel plate capacitor may be calculated from the formula C = Î A/t where Î is the permittivity of the material in F/m, A = Area of the plates and t = thickness of dielectric. Thus permittivity Î of a material is a measurement of its ability to form an electrical capacitance of the insulating material, the dimensions of the capacitor being taken equal. Dielectric constant or permittivity is not a constant but varies with temperature and frequency.

(iii) Corrosion: The process of constant eating (destruction) up of metals (from the surface) by the surrounding is called as corrosion. The metals are corroded when exposed to the atmosphere. The metals are generally converted into their oxides. This oxide covers the surface of the metal, which results in the destruction of the metal. Rusting of iron is the most common example of corrosion in which iron makes iron oxide with reaction with the oxygen of the atmosphere. The iron oxide covers the surface in the form of brownish powder. Therefore the conducting material should be corrosion resistant.

(iv) Contact resistance: This is measured as the voltage drop from tail to tail of the mated contacts with specified current flowing through the contact and is specified in milliohms. The total resistance offered by a contact depends upon the bulk resistivity of the contact material, actual surface area and mechanical wear of the contacts and environmental effects. To obtain low contact resistance it is necessary that the contact must be a good conductor of electricity. The contacts should have good mechanical resistance against wear due number of mechanical operations. The contacts are made using alloys, which have combination of mechanical properties, very good electrical and thermal conductivity and resistance to corrosion. Commonly used alloys are Beryllium copper, phosphor bronze, spring brass and low leaded brass. Plating of the contact metal is done to prevent deterioration of contacts mechanically and chemically and to obtain good surface conductivity. Commonly used plating materials are silver, gold, copper, nickel and tin.

SOLUTIONS D- 04 ENGINEERING MATERIALS (DEC 2003)

1. a. A Sum of the number of protons and neutrons.

b. B Brass.

c. B Low resitivity conducting materials.

d. B Bakelite.

e. D Light- weight permanent magnets.

f. C Temperature.

g. C 100 and 1000 components.

h. C 4electrons.

2a) On the basis of energy band materials are classified as insulators, conductors, and semiconductors.

Insulators: Substance like wood, glass, which do not allow the passage of current through them are known as insulators. The valence band of these substances is full whereas the conduction band is completely empty. The forbidden energy gap between valence band and conduction band is very large (8ev) as shown in the fig (a). Therefore a large amount of energy, i.e. a very high electric field is required to push the valence electrons to the conduction band. This is the reason, why such materials under ordinary conditions do not conduct at all and are designated as insulators.

Conductors: Substances like copper, aluminium, silver which allow the passage of current through them are conductors. The valence band of these substances overlaps the conduction band as shown in fig (b). Due to this overlapping, a large number of free electrons are available for conduction. This is the reason, why a slight potential difference applied across them causes a heavy flow of current through them.

Semiconductors: Substances like carbon, silicon, germanium whose electrical conductivity lies in between the conductors and insulators are known as semiconductors. The valence band of these substances is almost filled, but the conduction band is almost empty. The forbidden energy gap between valence and conduction band is very small (1ev) as shown in fig ( c ). Therefore comparatively a smaller electric field is required to push the valence electrons to the conduction band. This is the reason, why such materials under ordinary conditions do not conduct current and behaves as an insulator. Even at room temperature, when some heat energy is imparted to the valence electrons, a few of them cross over to the conduction band imparting minor conductivity to the semiconductors. As the temperature is increased, more valence electrons cross over to the conduction band and the conductivity of the material increases. Thus these materials have negative temperature co-efficient of resistance.

Energy Band Diagrams

2b) Superconductors - The resistivity of most metals increases with increase in temperature and vice-versa. There are some metals and chemical compounds whose resistivity becomes zero when their temperature is brought near 0⁰Kelvin (-273⁰C). At this stage such metals or compounds are said to have attained superconductivity. Example – mercury becomes superconducting at approximately 4.5 Kelvin (-268.5⁰C). Superconductivity, was discovered by Heike Kamerlingh Onnes. The transition from normal conductivity to superconductivity takes place almost suddenly; it occurs over a very narrow range of temperature about 0.05⁰K. The temperature at which the transition takes place from the state of normal conductivity to that of superconductivity is called transition temperature. Superconductors are used for producing very magnetic fields of about 50 Tesla. Magnetic energy can be stored in large superconductors and drawn as required to counter the voltage fluctuations during peak loading. Superconductors can be used to perform logic and storage functions in computers. As there is no, I²R losses in a superconductor, so power can be transmitted through the superconducting cables without any losses.

3a) Conducting materials are classified as low resistivity materials and high resistivity materials.

Low resistivity materials: The conducting materials having resistivity between 10^-8 to 10^-6 ohm-m come under this category and are used in transmission and distribution lines, transformers and motor windings. Properties:

a) Low temperature coefficient: For minimum variations in voltage drop and power loss with the change in temperature, these materials should have low temperature coefficient.

b) Sufficient mechanical strength: These materials must withstand the mechanical stresses developed during its use for particular applications.

c) Ductility: The material to be used for conductors must be ductile so that it can be drawn and moulded into different sizes and shapes.

d) Solderability: The conducting materials are required to be joined and the joint must have minimum contact resistance. These materials must have a good solderability.

e) Resistance to corrosion: The material should have a high resistance to corrosion so that it should not be corroded when used in different environmental conditions.

High resistivity materials: The conducting materials having resistivity between 10^-6 to 10^-3 ohm-m come under this category and are used for making resistance elements for heating devices, precision instruments, rheostats etc. Properties:

a) Low temperature coefficient: For minimum variations in voltage drop and power loss with the change in temperature, these materials should have low temperature coefficient.

b) High melting point: These materials, which are used as heating elements should have high melting point.

c) Ductility: The material to be used for conductors must be ductile so that it can be drawn and moulded into different sizes and shapes.

d) Oxidation resistance: The material should have a high oxidation resistance so that it should get oxidised when used in different environmental conditions.

e) High mechanical strength: These materials must withstand the mechanical stresses developed during its use for particular applications.

3b) When the heater element is Nichrome r = 100x 10^-8 W-m.

a = p (0.3 x10^-3)² m²

a = 7.069 x 10^-8m²

R =30 W

R = r Į = 30 = 100 x 10^-8 x Į

a 7.069 x 10^-8

Į = 2.12 m.

4) The electrical properties of an insulating material are :

Insulation resistance- The property by the virtue of which, a material resists flow of electrical current. It should be high as possible. Insulation resistance is of two types: I) Volume resistance; ii) Surface resistance.

The dielectric phase angle is q and d = 90⁰ - q is the dielectric loss angle as shown in the fig.

5a) Classification of insulating materials on the basis of their limiting safe temperatures for use:

Class	Maximum working temperature	Materials or Combination of materials
Y	90⁰C	Cotton, silk, paper, press board, wood, PVC with or without plasticiser, vulcanised natural rubber etc.
A	105⁰C	Cotton, silk and paper when impregnated or immersed in a liquid dielectric such as oil.
E	120⁰C	Materials possessing a degree of thermal stability allowing them to be operated at temperature 15⁰C higher than class A materials.
B	130⁰C	Mica, glass fibre, asbestos, etc. with suitable bonding substances.
F	155⁰C	Mica, glass fibre, asbestos, etc. with suitable bonding substances as well as other materials, not necessarily inorganic, which by experience or accepted test can be shown to be capable of operation at 155⁰C.
H	180⁰C	Materials such as silicon elastomer and combinations of materials, such as mica, glass fibre, asbestos etc, with suitable bonding substances such as appropriate silicon resins.
C	above 180⁰C	Mica, porcelain, glass and quartz with or without an inorganic binder.

5b) Chemical properties-

Chemical resistance: Presence of gases, water, acids, alkalies and salts affects different insulators differently. Chemically a material is a better insulator if it resists chemical action. Hygroscopicity: Many insulators come in contact with the atmosphere during manufacture or operation or both. Moisture affects the electrical properties of the insulator.

Effect of contact with other materials: Insulation remains in contact with different types of materials like air, gases, moisture, conducting materials and structural materials. This can adversely affect the insulating properties.

Ageing: Ageing is the long time effect of heat, chemical action, and voltage application.

These factors decide the natural life of an insulator.

6a) Plastics are materials which, consists of organic substances of high molecular weight and are capable of being formed into desired shape during or after their manufacture. The organic substances are called resins polymers and are derived from natural gas, petroleum etc. The polymers are mixed with other materials to modify their properties. There are two types of plastics: thermoplastics, thermosettings.

6b) (i) Teflon: It is obtained by the polymerisation of tetrafloourethylene.

Properties:

1.It has good electrical, mechanical and thermal properties.

2.It can tolerate very, high temperature, without damage.

3.Dielectric constant does not change with time, frequency and temperature.

4.Its insulation resistance is very high.

5.It is highly resistant to water absorption.

6.It melts at 327⁰C.

7.Its maximum useable temperature is 300⁰C.

Applications: Teflon is used as dielectric material in capacitors. It is used as covering for conductors and cables, which are required to operate at high temperature. It is used as a base material for PCB’s.

(ii) Bakelite: It is most common type of phenol formaldehyde. It is hard, thermosetting, dark coloured material. Its main properties are:

1. Its dielectric constant is 4.1.

2. Its dissipation factor is 0.001.

3. Its dielectric strength is 400 V/mil.

4. Volume resistivity is 10¹³ ohm-cm.

5. Maximum useable temperature is 300⁰F.

6. Its water absorption is 0.13%in 24 hours.

Applications: It is widely used for moulded parts such as lamp holders, terminal blocks, instrument cases and small panels.

7a) Ferromagnetic Materials:- These are materials in which magnetic dipoles interact in such a manner that they tend to line up in parallel. A ferromagnetic substance consists of a number of regions or domains, which are spontaneously magnetized. The direction of magnetization varies from domain to domain. The resultant magnetization is zero or nearly zero. The relative permeability is very high. The ferromagnetic materials are widely used in industries. Ex. Iron, nickel, cobalt.

Paramagnetic Materials:- The atoms of these materials contain permanent magnetic dipoles. Individual dipoles are oriented in random fashion such that resultant magnetic field is zero or negligible. For these materials relative permeability is slightly greater than unity and it is independent of magnetizing force. Ex. Chromium chloride, chromium oxide, manganese sulphate, air.

Diamagnetic Materials:- These are the materials whose atoms do not carry permanent magnetic dipoles. If an external magnetic field is applied to a diamagnetic material, it induces a magnetization in the direction opposite to the applied field intensity. For these materials the relative permeability is negative. These are hardly used as magnetic material in electrical/ electronic engineering applications. Ex. Aluminium oxide, copper, gold, barium chloride.

7b) Hard Magnetic materials-have a gradually rising magnetization curve with large hysteresis loop area and hence large energy losses. They have high value of retentivity and high value of coercivity and low permeability. To saturate the hard magnetic materials, a high magnetizing force is required. Hard magnetic materials have the property of storing a considerable amount of magnetic energy after magnetization and retaining the same for a long time. Due to this property they are used in the manufacture of permanent magnets. Some of the magnetically hard materials are rare earth cobalt, carbon steel, tungsten steel, cobalt steel, alnico and hard ferrites.

8) Brass- Is an alloy of copper and zinc with 60% copper and 40% zinc. Its properties are-

7. Its electrical resistivity is 7.0X 10^-8 ohm-m, which is higher than the copper resistivity.

8. It is ductile and can be drawn into different shapes.

9. It melts at 890⁰C.

10. Its specific gravity is 3.3.

11. It has got an excellent corrosion resistance.

12. It has got good mechanical properties.

Applications: Brass is used as a structural and current carrying material in power switches, plugs, sockets, lamp holders, fuse holders, knife switches, sliding contacts for starters and rheostats, wave- guide components.

Bronze: alloy of copper and tin.

1. This alloy is very hard and brittle.

2. Its corrosion resistance is better than brass.

3. It is ductile and can be drawn into different shapes.

4. In bronzes, which are used as electrical conductor, the content of tin and other metal is usually low as compared to the bronzes, which are used for mechanical applications.

Applications: It is used for making structural elements for equipments. It is also used for making current carrying springs, sliding contacts, knife switches.

Glass – Glass is an inorganic material made by the fusion of different metallic oxides. The properties of glass are –

i) It is transparent, brittle and hard

ii) Glass is insoluble in water and usual organic solvents

iii) It has low dielectric loss, slow ageing and good mechanical strength

iv) It is susceptible to destruction when sudden and high temperature cycles are applied

Glass Products

b) Borosilicate glass or Pyrex – This glass requires 28 per cent of boron oxide along with other oxides. They resist the effect of chemicals and moisture better than other glasses.

c) Fibre glass insulation – This is capable of withstanding high temperature. For most applications fibre- glass is impregnated with materials like synthetic resins or with mineral oil. They possess good electrical and mechanical properties and sufficient flexibility to be moulded into required shapes.

d) Epoxy glass – It is made by joining glass fibre layer with a thermosetting compound. It is immune to alkalies and acids and is used in PCB making, terminal holders and instrument cases.

Asbestos: It is inorganic fibrous material. Two types of asbestos are available.

Chrysotile asbestos: It is hydrated silicate of magnesium.

Its specific gravity varies between 2-2.8.
It is highly hygroscopic.
It has high dielectric losses and dielectric strength.

4. The melting temperature is 1525⁰C.

Amphibole asbestos: It is found in Africa and Alaska.

1. Its fibres cannot be woven easily as the fibres are too soft or too hard and brittle.

2. It possesses good tensile strength.

3. It is highly hygroscopic.

4. Its electrical properties are poorer.

Uses: Asbestos is used in low voltage work as insulation in the form of rope, tape, cloth and board. It is impregnated with liquid or solid resin in all such applications to improve its mechanical and electrical properties. It is used as insulator in wires and cables under high temperature conditions, as conductor insulator and layer insulator in transformer, as arcing barrier in switches and circuit breakers.

9a) Electrical conductivity of semiconductor changes appreciably with temperature variations. At absolute zero, it behaves as an insulator. At room temperature, because of thermal energy, some of covalent bonds of the semiconductor break. The breaking of bonds sets those electrons free, which are engaged in the formation of these bonds. This results in few free electrons. These electrons constitute a small current if potential is applied across the semiconductor crystal. This shows the conductivity for intrinsic semiconductor increases with increase in temperature. In case of extrinsic semiconductors, addition of small amount of impurities produces a large number of charge carriers. This number is so large that the conductivity of an extrinsic semiconductor is many times more than that of an intrinsic semiconductor at room temperature. In n - type semiconductor all the donors have donated their free electrons, at room temperature. The additional thermal energy only serves to increase the thermally generated carriers. This increases the minority carrier concentration. A temperature is reached when number of covalent bonds that are broken is large, so that number of holes is approximately equal to number of electrons. The extrinsic semiconductor then behaves like intrinsic semiconductor.

9b) Germanium:- It is one of the most common semiconductor material used for the application in electronics. The atomic number is 32. The number of electrons in the first, second, third and fourth orbit are 2, 8, 18and 4. It is clear that germanium atom has four valence electrons in the outermost orbit. It is known as tetravalent element. The germanium atoms are held together through covalent bonds. The forbidden gap in this material is very small 0.7ev. So small energy is sufficient to lift the electrons from valence to conduction band.

Silicon:- Silicon is another most commonly used semiconductor. Its atomic number is 14. The number of electrons in first, second and third orbit are 2, 8 and 4. The silicon atoms are also having four valence electrons and are known as tetravalent element. The various silicon atoms are held together through covalent bonds. The atoms of silicon are arranged in orderly pattern and form a crystalline structure. The forbidden energy gap in this material is quite small i.e. 1.1ev. It also needs small amount of energy to lift the electrons from valence to conduction band.

Atomic structure of Silicon

Energy Band Diagram of Silicon

10a)(i) Fuses: For rewirable fuses, lead-tin and copper-tin alloy is used. For cartridge and HRC fuses silver is used because of their low melting point and low resistivity.

(ii) Solder: For soft solders tin, lead (in fifty fifty ratio) alloy and for hard solders silver and copper- zinc alloy (in various proportions) are used because of their low melting points, good electrical conductivity.

Bimetals: Alloy of iron and nickel with low value of coefficient of thermal expansion are used as one element and metals like nickel, iron, constantan, brass etc., which, have high value of coefficient of thermal expansions are used as the other element.

10b) Thermocouples: Are used for the measurement of temperature. When two wires of different metals are joined together an emf exits across the the junction which dependent on the types of metals or alloys used and also directly proportional to the temperature of the junction. Depending on the range of temperature to be measured, proper materials are to be chosen for a thermocouple. If one junction called the cold junction is held at a known constant temperature, the emf produced becomes measure of the temperature of the other junction. The emf produced by a thermocouple is very small but it can be measured with reasonable accuracy by a sensitive moving coil millivoltmeter, which can be calibrated in terms of temperature. Some of the materials used for thermocouples are copper/constantan, iron/constantan.

11) (i) pnp transistor: A transistor has three terminals. In a pnp transistor has two blocks pf p- type semiconductors which are separated by a thin layer of n-type semiconductor. A pnp transistor circuit is shown in the fig below. The emitter base junction is forward biased while the collector base junction is reverse biased. The forward biased voltage is V_EBis quite small, whereas the reverse biased voltage V_CB is considerably high. As the emitter base junction is forward biased a large number of holes (majority carriers) in the emitter (p- type) are pushed towards the base. This constitutes the emitter current I_E. When these holes enter p- type material (base), they tend to combine with electrons. Since the base is lightly doped and very thin, only a few electrons (less than 5%) combine with electrons to constitute the base current I_B. The remaining holes (more than 95%) diffuse across the thin base region and reach the collector space charge layer. These holes then come under the influence of the negatively biased p- region and are attracted or collected by the collector. This constitutes collector current I_C. Thus, it is seen that almost the entire emitter current flows into the collector circuit. The emitter current is the sum of collector current and base current i.e. I_E = I_C+ I_B.

pnp transistor circuit

(ii) Hysteresis loop: In a ferromagnetic material, the flux density B increases when external applied magnetic field H to it is increased. When the saturation arrives, the increase in flux ceases even though H may be increased. This has be shown by OS in the fig. If the external field is gradually reduced, the original curve OS is not retraced but follows curve SR. The external field H is reduced to zero but B does not reduce to zero i.e. the material remains magnetised. The value of R flux density is called remanent flux density or residual magnetism. In order to demagnetise the material completely, external magnetic field must be reversed and when it reaches the value OC in reverse direction, it is seen that B is zero. This applied reverse magnetising force, which causes B to become zero is called coercive force. Further increase of H in reverse direction will now increase in B in reverse direction and again at the point S saturation occurs. The residual magnetism in reverse direction is represented by OR and to neutralise it H must be increased in positive direction to the value OC. Further increase in H will again magnetise the material and saturation will occur at S. The above property is characteristic of magnetic behaviour of the ferromagnetic material. When the material is taken through one complete cycle of magnetisation, it traces a loop called hysteresis loop. When a material is subjected to cyclic changes of magnetisation, the domains change the direction of their orientation in accordance with H. Work is done in changing the direction of domains, which leads to production of heat within the material. The energy required to take the material through one complete cycle of magnetisation is proportional to the area enclosed by the loop.

S

Hysteresis loop

(iii) Factors, which change the resistivity of conducting materials: -

Temperature- The electrical resistance of most metals increases with increase of temperature while those of semiconductors and electrolytes decreases with increase of temperature.

Alloying- Alloying is another factor, which affects the resistivity of a material. By the addition of some impurity to a metal, its resistivity can be changed. Alloys have higher resistivity than that of pure metal.

Mechanical stressing-When a material undergoes a mechanical treatment, its resistivity changes due to mechanical distortion of the crystal structure.

Age Hardening- Age hardening increases the resistivity of an alloy.

(iv) Aluminium: Properties

1) Pure aluminium is silver white in color.

2) It is a ductile metal and can be put to a shape by rolling, drawing and forging.

3) It melts at 655⁰C.

4) It is resistant to corrosion.

5) Its tensile strength is 60MN/m².

6) It can be alloyed with other elements.

7) Annealing can soften it.

8) It has a higher thermal conductivity.

Applications: Aluminium is widely used as conductor for power transmission and distribution. It is used in overhead transmission lines, busbars, ACSR conductors etc.

Copper: Properties

1) It is reddish brown in color.

2) It is malleable and ductile and can be cast, forged, rolled, drawn and machined.

3) It melts at 1083⁰C.

4) It easily alloys with other metals.

5) Electrical resistivity of copper is 1.7x10^-8 W-m.

6) Tensile strength for copper is 210 MN/m².

7) It is highly resistant to corrosion.

8) It is a non-magnetic material.

Applications: - Copper is used in conductor wires, coil windings of generators and transformers, cables, busbars etc.

SOLUTIONS D- 04 ENGINEERING MATERIALS (JUNE 2004)

1. a. B Silver

b. B Superconductivity

c. A Copper

d. A Insulators

e. A Dielectrics

f. B Curie point.

g. B Hard magnetic materials.

h. A P-type semiconductor.

2a) Materials, which are commonly used in electrical and electronics engineering, can be classified as conductors, insulators, semiconductors.

Conductors: Are the material, which allow the current to pass through them. These have very low electrical resistance and are available in a large variety having different properties. Also the number of valence electrons is less than four. The, valence- band and the conduction- band overlap each other. Examples are copper, brass, aluminium, silver, gold, bronze, etc.

Semiconductors: Are the materials, which possess the electrical resistivity in between that of conductors and insulators. They are used for the manufacture of diodes and transistors. Also the number of valence electrons is equal to four. There is a small forbidden energy gap of about 1eV between the conduction and the valence band. Examples: germanium, silicon, selenium, etc.

Insulators: Are the materials, which do not allow the current to pass through them without any appreciable loss. They have very high electrical resistance and are also available in a large variety to cover different applications. Some of the specific insulating materials are used for the purpose of storing of an electrical energy and are called dielectric materials such as mica, ceramic, paper etc. These materials are used as a dielectric in capacitors. Also the number of valence electrons is more than four. The energy gap between valence and conduction band is very large about 15eV. Examples: Mica, rubber, ceramics, glass, etc.

2b) Advantages of aluminium as compared copper as a conductor of electricity: The electrical conductivity of aluminium is next to that of copper. Its resistivity is 2.8X10^-8 ohm-m, i.e. about 1.6 times higher than copper. Its density is 2.68 which means that aluminium is much lighter than copper. Its melting point is 655⁰C. Like copper, it can be easily drawn into thin wires. Aluminium is soft metal but when alloyed with some other materials like magnesium, silicon or iron, it acquires higher mechanical strength and can be used for overhead transmission lines. Like copper, aluminium also forms an oxide layer over its surface when exposed to atmosphere and that layer prevents the material from further oxidation and acts as a resistance layer to corrosion.

Disadvantages: As aluminium is a soft material, there is always a possibility of loose contacts. Due, to the insulating property of aluminium oxide formed on the surface, it is difficult to solder aluminium wires. However for applications like winding of electrical machines and transformers, it is difficult to substitute aluminium for copper. This is because aluminium wires have lower tensile strength than that of copper.

3a)

Thermoplastic materials: The properties of these plastic materials do not change considerably if they are melted and then cooled and solidify. They can be repeatedly melted or dissolved in various solvents. They are more elastic, less brittle and do not lose elasticity when subjected to prolonged heating. They are less apt to age thermally. Many of them possess extraordinary high insulating properties and are water repellent. They are polymers of linear structure, i.e. their molecules are elongated and are thread like. This, type of structure is fusible, soluble, highly plastic, capable of forming thin flexible threads and films. Examples are Polytetra Flouroethylene (P.T.F.E. or Teflon), Polyvinyl Chloride (P.V.C.).

Thermosetting Plastic Materials: They undergo great changes when subjected to high temperatures for quite sometime. They are said to be baked and no longer can melt or be dissolved. They are less elastic, more brittle and lose their elasticity when subjected to prolonged heating. They are used, when the insulation is to withstand high temperatures without melting or losing its shape and mechanical strength. Thermosetting plastic substances are space-polymers and the molecules branch off in various directions during polymerisation. This structure makes them very rigid, poorly soluble, fusible and incapable of forming elastic threads and films. Examples are Phenol formaldehyde (Bakelite), Epoxy resins.

3b) (i) Superconductivity- The resistivity of most metals increases with increase in temperature and vice-versa. There are some metals and chemical compounds whose resistivity becomes zero when their temperature is brought near 0⁰Kelvin (-273⁰C). At this stage such metals or compounds are said to have attained superconductivity. Example – mercury becomes superconducting at approximately 4.5 Kelvin (-268.5⁰C). Superconductivity, was discovered by Heike Kamerlingh Onnes. The transition from normal conductivity to superconductivity takes place almost suddenly; it occurs over a very narrow range of temperature about 0.05⁰K. The temperature at which the transition takes place from the state of normal conductivity to that of superconductivity is called transition temperature.

(ii) Resistivity: The resistance R of a wire having cross-sectional area A and length L have the relationship, - R a L and R a 1/A ; resulting Ra L/A or R = r L / A ; Where r is the constant of proportionality and is called resistivity of a material and is defined as the resistance between the two opposite faces of a meter cube of that material. The unit of resistivity is ohm-m. Factors affecting resistivity are temperature, alloying, mechanical stressing, ageing.

4a) Electrical properties of an insulating material are:

The dielectric phase angle is q and d = 90⁰ - q is the dielectric loss angle.

4b) An insulator is designed to withstand certain amount of heat. But when an insulator is overheated, dielectric losses will increase. Also overheating will affect the various important properties as electrical properties, mechanical strength, hardness, viscosity, solubility etc.

5) (i)Brass - Is an alloy of copper and zinc with 60% copper and 40% zinc. Its electrical resistivity is 7.0X 10^-8 ohm-m, which is higher than the copper resistivity. It is ductile and can be drawn into different shapes. It melts at 890⁰C.Its specific gravity is 3.3.It has got an excellent corrosion resistance. It has got good mechanical properties. Brass is used as a structural and current carrying material in power switches, plugs, sockets, lamp holders, fuse holders, knife switches, sliding contacts for starters and reheostats, wave- guide components.

Bronze - Alloy of copper and tin. This alloy is very hard and brittle. Its corrosion resistance is better than brass. It is ductile and can be drawn into different shapes. In bronzes, which are used as electrical conductor, the content of tin and other metal is usually low as compared to the bronzes, which are used for mechanical applications. It is used for making structural elements for equipments. It is also used for making current carrying springs, sliding contacts, knife switches.

(ii) Thermocouples: Are used for the measurement of temperature. Depending on the range of temperature to be measured, proper materials are to be chosen for a thermocouple. If one junction called the cold junction is held at a known constant temperature, the emf produced becomes measure of the temperature of the other junction. The emf produced by a thermocouple is very small but it can be measured with reasonable accuracy by a sensitive moving coil millivoltmeter, which can be calibrated in terms of temperature. Some of the materials used for thermocouples are copper/constantan, iron/constantan, nickel/ nickelchromium.

(iii) P – N junction: When a p- type semiconductor is suitably joined to an n-type semiconductor the contact surface so formed is called p-n junction. All the semiconductor devices contain one or more p-n junction. P-N junction is fabricated by special techniques namely growing, alloying and diffusion methods. The p-type semiconductor is having negative acceptor ions and positively charged electrons. The n-type semiconductor is having positive donor ions and negatively charged electrons. When the two pieces are joined together and suitably treated they form a p-n junction. The moment they form a p-n junction, some of the conduction electrons from n-type material diffuse over to the p- type material and undergo electron – hole recombination with the holes available in the valence band. Simultaneously holes from p-type material diffuse over to n- type material and undergo hole-electron combination with the electrons available in the conduction band. This process is called diffusion. When a p-n junction is connected across an electric supply, the junction is said to be under biasing. The potential difference across the p- n junction can be applied in two ways, namely- forward biasing and reverse biasing. When the positive terminal of a dc source is connected to p-type, and negative terminal is connected n-type semiconductor of a p-n junction, the junction is said to be in forward biasing. When the positive terminal of a dc source is connected to n-type, and negative terminal is connected p-type semiconductor of a p-n junction, the junction is said to be in reverse biasing. With forward bias, a low resistance path is set up in the p-n junction, and hence current flows through the circuit. With reverse bias, a high resistance path is set up and no current flows through the circuit. This property is best suited for rectification of ac into dc.

p-n Junction

(iv) Fuse materials: Fuse is a protective device, which consists of a thin wire or strip. This wire or strip is placed with the circuit it has to protect, so that the circuit-current flows through it. When this current is too large, the temperature of the wire or strip will increase till the wire or strip melts thus breaking the circuit and interrupting the supply.

A fuse material should possess the following properties;-

Low conductivity of the metal vapour itself.

Low melting point- This means that the temperature of the fuse material for normal currents stays at a low value. Originally lead was used as fuse material because of its low melting point. But as the resistivity of lead is high, thick wires are required. For rewirable fuses alloys of tin and lead or tinned copper wires are commonly used. In cartridge fuses silver and silver alloys are used in fuses of lower ratings and copper alloys are used in fuses of higher ratings.

6) Insulating materials, on the basis of their physical and chemical structure may be classified in various categories as follows:

1.Fibrous materials: They are derived from animal origin or from cellulose, which is the major solid constituent of vegetable plants. The majority of materials are from cellulose. This includes paper, wood, card- board, cotton, jute and silk.

2. Impregnated fibrous material: The fibrous materials are impregnated with suitable impregnated oil, varnish, and epoxy - resin to improve its thermal, chemical and hygroscopic properties.

3. Non-resinous materials: Solid or semisolid insulations which are directly available in nature and are organic based come under this class. These materials are mineral waxes, asphalts, bitumen and chlorinated naphthalene.

4. Insulating liquids: Apart from working as insulation, they fulfil other important requirements like they offer good heat dissipation media, they used for extinguishing arcs in certain applications like circuit breakers. They include vegetable oils, fluorinated liquids, mineral insulating oils and synthetic liquids.

5. Ceramics: Are materials made by high temperature firing treatment of natural clay and certain inorganic matters. They are used as dielectric in capacitors, as insulators etc.

6. Mica and mica products: It is an inorganic mineral and one of the best natural insulating materials available. Mica is used as a dielectric in capacitors, as insulator. Some of the mica products are glass- bonded mica, synthetic mica, mica paper, manufactured mica.

7. Asbestos and asbestos products: These are strong and flexible fibres. It finds extensive use in electrical equipment as insulation because of its ability to withstand very high temperatures. Some of the asbestos products are: asbestos roving, asbestos paper, asbestos tapes and asbestos cement.

8.Glass: Glass is an inorganic material made by the fusion of different metallic oxides. It is normally transparent, brittle and hard. Glass finds its use in electrical industry because of its low dielectric loss, slow ageing and good mechanical strength. Glass is used in electrical- bushings, fuse bodies, insulators, radio and television tubes.

9. Natural and synthetic rubber: Natural rubber is obtained from the milky sap of rubber trees. It finds limited applications because it is rigid when solid, sticky when warm and gets oxidised, when exposed to atmosphere. Synthetic rubber are of various types such as butadiene rubber, butyl rubber, chloroprene and silicon rubber which are obtained by the polymerisation. Synthetic rubber, are used as insulating material for wires and cables. It is also used as jacketing material for cables.

10. Insulating resins and their products: Plastic or resins are of two types – one derived from plant and animals the other synthetic obtained from chemical reactions. Natural resins are used as binder material. It is used as thickening agent for manufacture of mineral insulating oils. Synthetic resins are used as insulation, manufacture of switches and instrument mountings, electrical bushings, radio and television cabinets etc.

11.Laminates, adhesives, enamels and varnishes: Laminates are multiple, thin layers or sheets of insulating materials like that of mica, paper, cloth, glass etc, bonded together. Adhesives, is a class of material compositions required to carry out bonding between two or more solid surfaces. Adhesives are used in the manufacture of laminated boards, coil winding cylinders, rods, tubes and special shaped insulators. Enamel is a fusible insulated coating of some organic base material, which is generally applied on conducting surface. Enamel finds extensive use in coating wires used for the windings of low rated motors, transformers, various types of instruments, etc. Varnish is a liquid, which when applied to a surface dries resulting in hard shining coating which is resistant to air and water. Lacquer is used for protecting wood and metal surface from external weather conditions.

7a) Soft and Hard magnetic materials: All ferromagnetic materials are divided into two broad groups soft and hard magnetic materials.

Materials, which have, a steeply rising magnetization curve, relatively small and narrow hysteresis loop and consequently small energy losses during cyclic magnetization are called soft magnetic materials. Soft magnetic materials are therefore employed in building cores for use in alternating magnetic fields. Examples are nickel-iron alloy and soft ferrites. Fig below shows a typical narrow hysteresis loop for soft magnetic materials.

Magnetic materials, which have a gradually rising magnetisation curve, large hysteresis loop area and large energy losses for each cycle of magnetisation, are called hard magnetic materials. Such materials are used for making permanent magnets. Examples are carbon steel, tungsten steel alnico. Fig below shows a broad hysteresis loop for hard magnetic materials.

Hysteresis curves for soft and hard magnetic materials

7b) (i) Corrosion : The process of constant eating (destruction) up of metals (from the surface) by the surrounding is called as corrosion. The metals are corroded when exposed to the atmosphere. The metals are generally converted into their oxides. This oxide covers the surface of the metal, which results in the destruction of the metal. Rusting of iron is the most common example of corrosion in which iron makes iron oxide with reaction with the oxygen of the atmosphere. The iron oxide covers the surface in the form of brownish powder. Therefore the conducting material should be corrosion resistant.

(ii) Bimetals: A bimetal is made of two metallic strips of unlike metal alloys with different coefficients of thermal expansion. At a certain temperature the strip will bend and actuate a switch or a lever of a switch. The bimetal can be heatedly directly or indirectly. When heated the element bends so that the metal with the greater coefficient of expansion is on the outside of the arc formed while that with smaller coefficient is on the inside. When cooled the element bends in the other direction. Alloys of iron and nickel with low coefficients of thermal expansion are used as one element of the bimetallic strip. The other element consists of materials having high values of thermal expansion. Examples are iron, nickel, constantan, brass etc. Bimetallic strips are used in electrical apparatus and in devices such as relays and regulators.

8a) Soft ferrites and their applications: These are non- metallic compounds consisting of ferric oxide and one or two bivalent metal oxides such as Nickel oxide, Manganese oxide or Zinc oxide. They have the resistivity of about, 10⁹ ohm-cm, which reduces eddy current losses at high frequency. The magnets made out of it have high coercive force and square hysteresis loop. Magnetic permeability of these materials is as high as 10,000 to 30,000. These materials are fabricated into shape such as- E, U, I, beads and self - shielding pot cores.

Applications – High frequency power transformers operating at 10 to 100 kHz, pulse transformers up to 1000 MHz, adjustable air gap inductors, recording heads make use of cores made of soft ferrites.

8b) (i) Eddy currents: Magnetic materials placed in alternating magnetic fields also have eddy currents induced in them. This is because the material is subjected to rate of change of flux linkages and in accordance with Faraday’s Law of electromagnetic induction, emfs are induced in the material causing currents, called eddy currents, to flow in the material. These currents cause loss of energy. This results in the heating up of the material. Eddy current loss is proportional to the square of the frequency and the square of the thickness of the material and inversely proportional to the resistivity of the material. In order to reduce this eddy currents loss thin sheets called laminations are used instead of solid core.

(ii) Magnetostriction: When ferromagnetic materials are magnetized a small change of dimensions of the material takes place. There is a small extension with corresponding reduction of cross-section of the crystals of which the material is made. When subject to rapidly alternating magnetic fields there is a rapid and continuous extension and contraction of the material. This is called magnetostriction. Magnetostriction is the major cause of hum in transformers and chokes.

(iii) Permeability – It is defined as the capability of the material to conduct flux. It is defined as the ratio of magnetic flux ‘B’ in a medium to the magnetic flux intensity ‘H’ at the same location in the medium, i.e. m = B/H, where B is plotted against H, a curve is obtained, called magnetization curve or B-H curve. The permeability of any material is not a constant. The permeability at low value of H is called initial permeability. The common core materials such as low carbon steel, silicon steel have low initial permeability.

9a) Diamagnetic Materials: These are the materials whose atoms do not carry permanent magnetic dipoles. If an external magnetic field is applied to a diamagnetic material, it induces a magnetization in the direction opposite to the applied field intensity. For these materials the relative permeability is negative. These are hardly used as magnetic material in electrical/ electronic engineering applications. Examples- Aluminium oxide, copper, gold, barium chloride.

Paramagnetic Materials: The atoms of these materials contain permanent magnetic dipoles. Individual dipoles are oriented in random fashion such that resultant magnetic field is zero or negligible. For these materials relative permeability is slightly greater than unity and it is independent of magnetizing force. Ex. Chromium chloride, chromium oxide, manganese sulphate, air.

Ferromagnetic Materials: These are materials in which magnetic dipoles interact in such a manner that they tend to line up in parallel. A ferromagnetic substance consists of a number of regions or domains, which are spontaneously magnetized. The direction of magnetization varies from domain to domain. The resultant magnetization is zero or nearly zero. The relative permeability is very high. The ferromagnetic materials are widely used in industries. Ex. Iron, nickel, cobalt.

9b) Alloy of two or more metals of low melting point used for base metals is known as soldering. The alloy used for joining the metals is called solder. The most common solder is composed of 50% tin. Its melting point is about 185⁰C. Many commercial solders, contain larger percentage of lead and some antimony with less tin, as the electrical conductivity of lead is only about half that of tin. For soldering flux is to be used. Solders are of two types- Soft solders and hard solders. Soft solders are composed of lead and tin in various proportions. Hard solders may be any solder with a melting point above that of lead-tin solders. Soft solders are used in electronic devices and hard solders in power apparatus for making permanent connections.

10) (i) Mica and Mica Products – Mica is an inorganic material. It is one of the best insulating materials available. From the electrical point of view, mica is of two types – Muscovite mica and Phlogopite mica.

Muscovite mica – Chemical composition is KH₂Al₃(SiO₄)₃. The properties are

i) Strong, tough and less flexible

ii) Colourless, yellow, silver or green in colour

iii) Insulating properties are very good

iv) Abrasion resistance is high

v) Alkalies do not affect it

Uses – Muscovite mica is used where electrical requirements are severe. Because of high dielectric strength, it is used in capacitors. It is also used in commutators, due to high abrasion resistance.

Phlogopite mica – Chemical composition is KH(MgF)₈MgAl(SiO₄)₃. The properties are

i) Amber, yellow, green or grey in colour

ii) Greater structural stability, being tougher and harder than muscovite mica, less rigid

iii) Resistant to alkalies, but less to acids

iv) Greater thermal stability than that of muscovite mica

Uses – It is used when there is greater need of thermal stability as in domestic appliances like irons, hotplates and toasters.

Mica products

(iii) Manufactured mica – Mica flakes held together with adhesives is called manufactured mica. It is used in commutators, electrical heating devices, motor slot insulation, transformers, etc.

(ii) Teflon: It is obtained by the polymerisation of tetrafloourethylene.

Properties: It has good electrical, mechanical and thermal properties. It can tolerate very, high temperature, without damage. Dielectric constant does not change with time, frequency and temperature. Its insulation resistance is very high. It is highly resistant to water absorption. It melts at 327⁰C.Its maximum useable temperature is 300⁰C.

Uses: Teflon is used as dielectric material in capacitors. It is used as covering for conductors and cables, which are required to operate at high temperature. It is used as a base material for PCB’s.

(iii) Rubber: The different types of rubber materials are- Natural rubber, Hard rubber and Synthetic rubber.

Natural Rubber – Natural rubber is extracted from milky sap collected from special trees. Water is then evaporated. Additives like sulphur, oxidation inhibitors like aromatic amino compounds, softeners like vegetable oil and fillers like carbon black and zinc oxide are added to it. It is vulcanised, by adding sulphur and heating it. Vulcanization improves heat and frost resistance of rubber, making it mechanically strong. The permittivity and power factor varies depending on the sulphur content and temperature change.

Properties – This rubber is moisture repellent and has good insulating properties. It has good abrasion resistance.

Applications – It is used for the manufacturing of protective clothing such gloves, boots. It is used as an insulation covering for wires and cables.

Hard Rubber – Hard rubber is obtained by addition of more sulphur and by extended vulcanization.

Properties – It has good electrical properties. Water absorption is less. Maximum permissible operating temperature is 60⁰ C. It can’ t be continuously exposed to sun as it is harmful. It has high tensile strength.

Applications – Hard rubber is used for construction of storage battery housing, panel boards, bushings of various types. It is also used as jacketing material for cables.

Synthetic Rubber – The different types of synthetic rubber are

(a) Butadiene rubber – its properties are greater resistance to ageing and oxidation, lower tensile and tear strength, lower water absorption, higher heat conductivity.

(d) Chlorosulfonated Polyethylene (Hypalon) – It has better electrical properties, high resistance to degradation when exposed to high temperature and oxidation. It can be operated at temperatures as high as 150⁰ C. It has poor solvent resistance to hydrocarbons. It is mechanically less tough. It is used as insulating material for wires and cables and also as jacketing material for cables.

(iv)Asbestos: It is inorganic fibrous material. Two types of asbestos are available.

Chrysotile asbestos: It is hydrated silicate of magnesium. Its specific gravity varies between 2-2.8. It is highly hygroscopic. It has high dielectric losses and dielectric strength. The melting temperature is 1525⁰C.

Amphibole asbestos: It is found in Africa and Alaska. Its fibres cannot be woven easily as the fibres are too soft or too hard and brittle. It possesses good tensile strength. It is highly hygroscopic. Its electrical properties are poorer.

11) (i) Cold rolled grain oriented steel: The grain orientation of silicon steel is obtained by a special technique called cold rolling. Sheet steel obtained in this process is called Cold Rolled Grain Oriented Steel. CRGO silicon steel is widely used for making transformer cores. The magnetising current required by transformers using CRGO steel is low. If CRGO sheet steel is used as core material for rotating machines, the core will be assembled from a large number of sections but this still will not result in grain orientations completely parallel with the flux path because of the circular nature of cores of rotating machines. Also assembling the core from sections of sheet steel will make the construction difficult and expensive.

(ii) Alnico: is an alloy of iron, cobalt, nickel and small amount of aluminium and copper. Properties are -

Saturation flux density is 1.2 Wb/m2.
More expensive than Alni, but magnetic properties are better than Alni.
Available in different grades, each having some different properties.
Hard and brittle, so it cannot be machined but has to be cast to shape and finished by grinding.
Magnets made with this alloy are smaller in size and lighter in weight than that made with tungsten steel.
The hysteresis loop is more rectangular.
Thermal and mechanical process, modify the crystal size and shape and thereby alter the shape of the hysteresis curve.

Uses: Alnico magnets find applications in loudspeakers, microwave devices, motors, generators, separators, vending machines and communication devices.

SOLUTIONS D- 04 ENGINEERING MATERIALS (DEC 2004)

1. a. C Magnetic materials.

b. D Four valence electrons.

c. C One.

d. A Ferromagnetic materials.

e. C Temperatures.

f. B Slightly more than unity.

g. D Determining whether the semiconductor is p or n type, determining the carrier concentration, calculating the mobility.

h. A Copper, manganese and nickel.

2a) Factors, which change the resistivity of conducting materials: -

Temperature- The electrical resistance of most metals increases with increase of temperature while those of semiconductors and electrolytes decreases with increase of temperature.

Mechanical stressing-When a material undergoes a mechanical treatment, its resistivity changes due to mechanical distortion of the crystal structure.

Age Hardening- Age hardening increases the resistivity of an alloy.

2b) When the heater element is Nichrome r = 100x 10^-8 W-m.

a = p (0.4 x10^-3)² m²

a = 12.6 x 10^-3m²

R = 40W

R = r l = 40 = 100 x 10^-8 x l

a 12.6 x 10^-3

l = 5 m.

3a) A large number of metals become superconducting below a temperature, which is the characteristic of the particular metal. They have zero resistivity and the temperature at which this change takes place is called superconducting transistion temperature. Metals, which are good conductors at room temperature like gold, silver, tin, do not exhibit superconducting properties. Whereas metals and compounds which have superconducting properties at certain temperatures are insulators at room temperature.

It is possible to destroy superconductivity by the application of a strong magnetic field. When, the magnetic field exceeds a certain critical value, the superconducting state disappears, the magnetic field penetrates the material and electrical resistance is restored.

3b) Copper: Properties

8) It is reddish brown in color.

9) It is malleable and ductile and can be cast, forged, rolled, drawn and machined.

10) It melts at 1083⁰C.

11) It easily alloys with other metals.

12) Electrical resistivity of copper is 1.7x10^-8 W-m.

13) Tensile strength for copper is 210 MN/m².

14) It is highly resistant to corrosion.

8) It is a non-magnetic material.

Applications: - Copper is used in conductor wires, coil windings of generators and transformers, cables, busbars etc.

Aluminium: Properties

9) Pure aluminium is silver white in color.

10) It is a ductile metal and can be put to a shape by rolling, drawing and forging.

11) It melts at 655⁰C.

12) It is resistant to corrosion.

13) Its tensile strength is 60MN/m².

14) It can be alloyed with other elements.

15) Annealing can soften it.

16) It has a higher thermal conductivity.

Applications:- Aluminium is widely used as conductor for power transmission and distribution. It is used in overhead transmission lines, busbars, ACSR conductors etc.

4a) When a magnetic material is subjected to a magnetic field, the magnetic dipoles get oriented in a particular direction and the material is magnetized, this process is called magnetization. Similarly, when a dielectric material is subjected to an electric field the dipoles of the material get oriented into a particular direction under the effect of the electric field. The material is said to be polarized and the phenomenon as polarization.

4b) Glass: - It is an amorphous substance. It consists of silicates and in some cases borates and phosphates. Properties

1) It has high resistivity & dielectric strength at ambient temperature.

2) Temperature coefficient is –ve and very large.

3) Tensile strength is low.

4) Coefficient of thermal expansion considerably varies with composition.

5) It is susceptible to destruction when used in high and low temperature cycle.

6) Surface resistivity falls considerably when exposed to moisture.

Applications:- Moulded glass is used in electrical bushings, fuse bodies, insulators. It is used as dielectric in capacitors. It is used in the manufacture of radio and television tubes, electrical lamps, and laminated boards. It is used to make optical fibers used in optical communications.

Cotton:- This is base material for insulating fibres. Properties of cotton can be improved by

Impregnating with varnish. Properties

1) It is hygroscopic. Moisture absorption is 70%.

2) It has low dielectric strength.

3) Its resistivity changes with moisture content.

4) It can be used upto 110⁰C.

5) Its density is 1.54gm/cm³.

Applications:- It is used as insulating material for armature winding of small and medium sized machines, small transformers, coils and chokes. Cotton covered wires are used for winding of small magnet coils.

5a) Constantan or Eureka { (55-60%) Cu, (45-40%)Ni}

German Silver (an alloy of CU, Zn, Ni)

Manganin (86% Cu, 2% Ni, 12% Mn)

Nichrome (61% Ni, 15% Cr, 24% Fe)

5b) Ferroelectricity: - Ferroelectric materials have a high dielectric constant, which is non-linear i.e., it depends to a considerable extent on the intensity of the electric field. Such materials exhibit hysterisis loops, i.e. the polarization is not a linear function of applied electric field. If the center of gravity of the positive and negative charges in a body does not coincide in the absence of an applied electric field, the substance has an electric dipole moment and is said to be spontaneously polarized. Such a substance is called ferroelectric. It contains small regions, which are polarized in different directions, even in the absence of an electric field. When the temperature exceeds a certain value called the Curie point, the substance loses its ferroelectric properties. Ex. Rochelle salt, Potassium dihydrogen phosphate, Barium titanate.

Piezoelectricity:- Piezoelectricity provides us a means of converting electrical energy to mechanical energy and vice versa.

When an electric field is applied to a substance it becomes polarized, the electrons and nuclei assume new geometric positions and the mechanical dimensions of the substance are altered. This phenomenon is called electrostriction. The reverse effect i.e. production of polarization by the application of mechanical stresses can take place only if the lattice has no center of symmetry, this phenomenon is known as piezoelectricity. Ex. Rochelle salt, Quartz, Barium titanate.

6a) Electrical contact materials: - A number of elements in their pure form such as copper, molybdenum, nickel, palladium, platinum, silver and tungsten are most make and break contact materials. Alloys of the above mentioned elements are also used for electrical contacts. Silver is an important contact material. Copper added to silver reduces the cost of the contact material. Whereas a combination of tungsten and silver results in a contact material having the advantages of the individual metals. A silver tungsten contact material will have high thermal and electrical conductivity. Copper contacts are used in control relays, motor starter switches and tap changers. Copper contacts may be used for currents (a.c or d.c) upto 500A and voltages (a.c or d.c) upto 600V. Contacts made of silver and silver alloys are widely used. Silver has better resistance to oxidation compared to copper and can be used for voltages (a.c or d.c) upto 600V and direct currents upto 50A and alternating currents upto 200A. Such contacts are used in all types of industrial applications, relays, generator cut outs, thermal overload devices and thermostatic control.

6b) Dielectric Constant or Permittivity: - Every insulating material possesses an electrical capacitance. The capacitance of such unit depends upon dimensions and kind of dielectric placed between the capacitor plates. The capacitance of a parallel plate capacitor may be calculated from the formula C =Î A/t where Î is the permittivity of the material in F/m, A = Area of the plates and t = thickness of dielectric. Thus permittivity Î of a material is a measurement of its ability to form an electrical capacitance of the insulating material, the dimensions of the capacitor being taken equal. Dielectric constant or permittivity is not a constant but varies with temperature and frequency.

Dielectric loss angle: - when an insulating material is subjected to alternating voltage, some of the electric energy is absorbed by the insulation and is dissipated as heat. Energy absorbed by the material in unit time is called dielectric loss. A perfect dielectric has a current, which leads the voltage by 90⁰, but the practical dielectric material has a current, which leads the voltage by less than 90⁰. The dielectric phase angle is q and d = 90⁰ - q is the dielectric loss angle.

7a) The electrical conductivity of semiconductor changes appreciably with temperature variations.

At absolute zero, it behaves as an insulator. At room temperature, because of thermal energy, some of the covalent bonds of the semiconductor break. The breaking of bonds sets those electrons free, which are engaged in the formation of these bonds. This results in few free electrons. These electrons constitute a small current if potential is applied across the semiconductor crystal. This shows the conductivity for intrinsic semiconductor increases with increase in temperature. In case of extrinsic semiconductors, addition of small amount of impurities produces a large number of charge carriers. This number is so large that the conductivity of an extrinsic semiconductor is many times more than that of an intrinsic semiconductor at room temperature. In n - type semiconductor all the donors have donated their free electrons, at room temperature. The additional thermal energy only serves to increase the thermally generated carriers. This increases the minority carrier concentration. A temperature is reached when number of covalent bonds that are broken is large, so that number of holes is approximately equal to number of electrons. The extrinsic semiconductor then behaves like intrinsic semiconductor.

7b) n – type semiconductor:- When small amount of pentavalent impurity is added to a pure semiconductor providing a large number of free electrons in it, the extrinsic semiconductor thus formed is known as n- type semiconductor. The addition of pentavalent impurities such as arsenic and antimony provide a large number of free electrons in the semiconductor crystal. Such impurities, which produce n- type semiconductors, are known as donor impurities because each atom of them donates one free electron to the semiconductor crystal.

p - type semiconductor:- When small amount of trivalent impurity is added to a pure semiconductor providing a large number of holes in it, the extrinsic semiconductor thus formed is known as p- type semiconductor. The addition of trivalent impurities such as gallium and indium provide a large number of holes in the semiconductor crystal. Such impurities, which produce p- type semiconductors, are known as acceptor impurities because each atom of them creates one hole, which can accept one electron.

8) Diamagnetic Materials:- These are the materials whose atoms do not carry permanent magnetic dipoles. If an external magnetic field is applied to a diamagnetic material, it induces a magnetization in the direction opposite to the applied field intensity. For these materials the relative permeability is negative. These are hardly used as magnetic material in electrical/ electronic engineering applications. Example: Aluminium oxide, copper, gold, barium chloride.

Paramagnetic Materials:- The atoms of these materials contain permanent magnetic dipoles. Individual dipoles are oriented in random fashion such that resultant magnetic field is zero or negligible. For these materials relative permeability is slightly greater than unity and it is independent of magnetizing force. Example: Chromium chloride, chromium oxide, manganese sulphate, air.

Ferromagnetic Materials:- These are materials in which magnetic dipoles interact in such a manner that they tend to line up in parallel. A ferromagnetic substance consists of a number of regions or domains, which are spontaneously magnetized. The direction of magnetization varies from domain to domain. The resultant magnetization is zero or nearly zero. The relative permeability is very high. The ferromagnetic materials are widely used in industries. Example: Iron, nickel, cobalt.

Ferrimagnetic Materials:-These are materials in which, unequal magnetic dipoles are lined up antiparallel to each other. The permeabilities are of the order that for ferromagnetic materials under ordinary conditions. The magnetic characteristics of ferrimagnetic materials are similar to those of ferromagnetic materials, as they show spontaneous magnetization below a certain temperature. Example: Magnetite, nickel, ferrite.

9a) Silicon Iron alloy:- Pure iron has low resistivity, which results in higher eddy current losses. These losses can be minimized by increasing the resistivity of the material, which is achieved by adding 1 to 4 % of silicon to iron. Silicon increases the electrical resistivity of iron. It reduces hysteresis loss. The magnetostriction effect is also reduced.

Silicon Iron alloy is used in the form of thin sheets called laminations. These laminations are used in transformers, small machines and large turbo- generators.

Nickel Iron alloy:- A group of iron alloys containing between 40 to 90 % nickel have much higher permeabilities at low flux densities and lower losses than ordinary iron. The important alloys are permalloy and mumetal. Mumetal has lower permeability but higher resistivity. Addition of small amounts of other elements to nickel iron alloys improves their magnetic properties.

Nickel Iron alloy is widely used in transformer cores and loading coils for telephone circuits, instrument transformers, for magnetic circuits of instruments, for magnetic screens of electronic equipments.

9b) Working of npn transistor:- An npn transistor is shown in the fig. The emitter base junction is forward biased while the collector base junction is reversed biased. The forward biased voltage V_EB is quite small, whereas the reverse biased voltage V_CB is considerably large.

As the emitter base junction is forward biased, a large number of electrons (majority carrier) in the emitter (n-type region) are pushed towards the base. This constitutes the emitter current I_E. When these electrons enter the p-type material (base), they tend to combine with holes. Since the base is lightly doped and very thin, only a few electrons (less than 5%) combine with the holes to constitute base current I_B. The remaining electrons (more than 95%) diffuse across the thin base region and reach the collector space charge layer. These electrons then come under the influence of positively biased n- region and are attracted or collected by the collector. This constitutes the collector current I_C. Thus, it is seen that almost the entire emitter current flows into the collector circuit. The emitter current is the sum of the collector, and base current. I_E =I_C + I_B.

npn Transistor Circuit

10a) Doping: - The process by which an impurity is added to semiconductor is known as doping. A semiconductor to which an impurity at controlled rate is added to make it conductive is known as an extrinsic semiconductor. The purpose of adding impurity in the semiconductor crystal is to increase the number of free electrons or holes to make it conductive. If pentavalent impurity is added to a pure semiconductor a large number of electrons will exist in it. If a trivalent impurity is added a large number of holes will exist in the semiconductor.

10b) Factors affecting permeability and hysterisis loss: - If the initial permeability is high, the hysterisis loss is low and vice versa. The permeability and the hysterisis loss depend upon the physical condition and chemical purity of the sample. The crystals of a ferromagnetic material when cold worked experience deformation as a result of which the material has very poor magnetic properties. Due to the internal strains on the domains, greater magnetic field is required to give a definite magnetization. Therefore the permeability decreases and the hysterisis loss is increased. A material, which has suffered magnetic damage due to cold work, may be heated to a sufficiently high temperature when the magnetic properties will be restored. The highest magnetic permeability and the lowest hysterisis loss that can be obtained are limited by the impurity content of the materials. Impurities affect the regular geometric pattern of the crystal and are harmful to the magnetic properties. The main impurities in the magnetic materials used for transformer cores and electrical machinery are carbon, sulphur, oxygen and nitrogen. Carbon is most detrimental and the amount of carbon is kept to a low value of 0.01% in commercial materials.

11a) Classification of materials:- On the basis of energy band materials are classified as insulators, conductors, semiconductors.

Insulators:- Substance like wood, glass, which do not allow the passage of current through them are known as insulators. The valence band of these substances is full whereas the conduction band is completely empty. The forbidden energy gap between valence band and conduction band is very large (8ev) as shown in fig. (a). Therefore a large amount of energy, i.e. a very high electric field is required to push the valence electrons to the conduction band. This is the reason, why such materials under ordinary conditions do not conduct at all and are designated as insulators.

Conductors:- Substances like copper, aluminium, silver which allow the passage of current through them are conductors. The valence band of these substances overlaps the conduction band as shown in fig. (b). Due to this overlapping, a large number of free electrons are available for conduction. This is the reason, why a slight potential difference applied across them causes a heavy flow of current through them.

Semiconductors:- Substances like carbon, silicon , germanium whose electrical conductivity lies in between the conductors and insulators are known as semiconductors. The valence band of these substances is almost filled, but the conduction band is almost empty. The forbidden energy gap between valence and conduction band is very small (1ev) as shown in fig. ( c). Therefore comparatively a smaller electric field is required to push the valence electrons to the conduction band. This is the reason, why such materials under ordinary conditions do not conduct current and behaves as an insulator. Even at room temperature, when some heat energy is imparted to the valence electrons, a few of them cross over to the conduction band imparting minor conductivity to the semiconductors. As the temperature is increased, more valence electrons cross over to the conduction band and the conductivity of the material increases. Thus these materials have negative temperature co-efficient of resistance.

Energy Band Diagrams

11b) Germanium:- It is one of the most common semiconductor material used for the application in electronics. The atomic number is 32. The number of electrons in the first, second, third and fourth orbit are 2, 8, 18and 4. It is clear that germanium atom has four valence electrons in the outermost orbit. It is known as tetravalent element. The germanium atoms are held together through covalent bonds. The forbidden gap in this material is very small 0.7ev. So small energy is sufficient to lift the electrons from valence to conduction band.

Atomic structure of Silicon

Energy Band Diagram of Silicon