Here is a list of popular electrical materials.
1. Materials of Low Resistivity and High Conductivity:
These materials should have the least possible electrical conductivity. They are employed for making conductors for all kinds of windings required in electrical machines, apparatus and devices, as well as for transmission and distribution of electrical energy.
Requirements of Materials of High Conductivity:
The materials of high conductivity should meet the following requirements:
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1. The highest possible conductivity.
2. The least possible temperature coefficient of resistivity.
3. Adequate mechanical strength.
4. Reliability and drawbility.
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5. Adequate resistance to corrosion.
6. Solderability.
7. Durability and low cost.
8. Flexibility and abundance.
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The above requirements vary with the purpose. Any impurity, whether metallic or non- metallic, increases their resistivity. Even a metal impurity having less resistivity will increase the resistivity of a particular metal conductor if the two metals form a solid solution. The reason for this is that addition of slightest impurity produces imperfections in the crystal lattice. The influence of various impurities on the resistivity of conductors depends on the metal they contaminate.
The effect produced by metal impurities on the resistivity of a given metal is dependent on the nature of the alloy formed.
The high conductivity materials most extensively used for electrical conductors in COPPER. Weldability and solerrability are the most important properties of copper.
A material of high conductivity is obtained by electrolytic refining of copper to obtain a product from 99.90 to 99.99 percent purity. The purest electrolytic copper has high conductivity but the standard grade has a slightly lower conductivity comparatively due to presence of impurities in it.
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Copper-base alloys containing tin, cadmium, beryllium and certain other metals are generally called bronzes and are also used as high conductivity materials. Another kind of copper-base alloy finding wide application in electrical engineering is brass, an alloy of copper with zinc.
Second to copper as a conductor material is aluminium. The resistivity of aluminium as well as its mechanical strength, depend on its purity and hardness (or annealing). Pure aluminium is softer than copper.
Characteristics of Copper Alloys (Brass and Bronzes) Brass:
(i) It contains 60% copper and 40% zinc.
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(ii) High tensile strength.
(iii) Lower conductivity than copper.
(iv) Easily shaped by pressing.
(v) Fairly resistant to corrosion.
(vi) Easily drawn into wires.
(vii) Easily welded and soldered.
Widely used as a current carrying and structural material in plug-points, socket-outlets, switches, lamp holders, fuse holders, knife switches, sliding contacts for starters and rheostats etc.
Bronze:
(i) It contains 90% copper and 10% tin.
(ii) Phosphor bronze is an alloy of ordinary bronze with a small amount of phosphorus. It finds use in propellers, pump rods etc.
(iii) Manganese bronze is an alloy of ordinary bronze and ferromanganese. It can be forged and rolled and has a strength equal to mild steel.
(iv) Aluminium bronze is an alloy of 90% copper and 10% aluminium and has a tensile strength of 3 kg/mm2.
(v) Lower conductivity than copper.
Beryllium bronze is used for making current carrying springs, sliding contacts, knife switch blades etc. Cadmium bronze is employed for contacting conductors and commutator segments.
2. Materials for Lamp Filaments:
The following materials are used for lamp filaments:
1. Carbon:
Commercial efficiency of carbon filament lamp is 4.5 lumens per watt or 3.5 watts per candle power.
To prevent the blackening of bulb, the working temperature is only 1800°C.
2. Tantalum:
(i) Resistivity … ρ = 12.4 μΩ cm
(ii) Temperature coefficient of resistance … α = 0.0036 per degree
(iii) Melting point … 2900°C
(iv) Specific gravity … 16.6
Its efficiency is about 1.6 watts per candle power. Due to low efficiency it is not much used these days.
3. Tungsten:
The efficiency of tungsten filaments which are worked at 2000°C in an evacuated bulb is 12 lumens per watt.
Procedure to Prepare Tungsten Filaments:
The preparation of tungsten filaments includes the following steps:
1. Chemically pure tungsten oxide is reduced at red heat in an atmosphere of hydrogen to metallic tungsten in the form of grey powder.
2. The powder is pressed in steel moulds into small bars under hydraulic pressure.
3. The bars are then heated in the furnace upto 1100°C in the presence of hydrogen whereby the articles sinter together. This imparts mechanical strength to the bars.
4. The mechanical strength is improved further by heating (electrically) the bars almost to melting point.
5. The bars are then hammered or rolled at red heat to make them ductile and finally drawn into filaments.
To improve the efficiency of the bulb, it is filled with an inert gas, organ with a small percentage of nitrogen. To minimise the convection currents produced by molecules of the gas in bulbs, the filaments is wound into close spiral and suspended horizontally in the form of a circular arc. In high wattage bulbs necks are provided. This is done to avoid blackening of the bulbs as the convection currents carrying particles from the filaments blacken the neck only without impairing the candle power in the direction below the horizontal.
The efficiency of gas filled “coiled-coil” tungsten filament lamps is about 25 lumens per watt or 0.6 watts per candle power and thus these lamps are called “half watt” lamp.
3. Materials Used for Transmission Lines:
The leading and most important material used for transmission lines is copper since it has high conductivity and high tensile strength. Aluminium is used for a large extent especially with a steel core for high voltage line.
The choice of materials depends upon:
(i) Cost of materials
(ii) Required electrical properties
(iii) Local conditions, and
(iv) Required mechanical strength.
Other materials employed for transmission lines are:
1. Cadmium copper materials.
2. Copper weld materials.
3. Phosphor bronze materials.
4. Galvanised steel materials.
5. Galvanised iron.
6. Steel cored aluminium materials.
7. Steel cored aluminium materials.
4. Stranded Conductors:
Stranded conductors are the conductors made of thin wires of small cross-section and bunched together. They are flexible, not rigid, and can be coiled very easily. They eliminate to a large extent the risk of breaking through insulation.
The following points are worth noting about stranded conductors:
(i) A stranded conductor is made by twisting the wires, called the strands, together to form layers.
(ii) The wires of each layer are laid in helical fashion round the preceding layer. The process is called ‘stranding’.
(iii) Generally stranding is done in opposite directions for successive layers. This means, if the wires of one layer are twisted in left hand direction, the next layer of wires will be twisted in the right hand direction, and so on.
(iv) A standard stranding consists of 6 wires around one wire, then 12 wires around the previous 6, then 18 wires around 12, then 24 wires around 18 and so on. The number of layers to be provided depends upon the number of wires to be provided. The centre wire is not counted as a layer.
Instead of single, 3 or 4 stranded wires may also be put in the centre and over them layers may be formed. If 3 stranded wires are put in the centre, 9 wires will be in the first layer, then 15 wires in the second layer and so on. The increase in the number of wires in each successive layer is 6 in each of the above cases.
In the Table 7.5 is given the formula which gives the number of wires for a particular layer, total number of wires in a particular stranded conductor and the diameter of the stranded conductor.
The stranded conductors are expressed as follows- 7/2.24, 19/2.50, 37/2.06, and so on.
First number (i.e., 7, 19, 37, etc.) indicates … Total no. of wires.
Second number (i.e., 2.24, 2.50, 2.06, etc.) indicates … Diameter of each wire in mm.
Let us consider 19/2.50 stranded conductor-
No. of wires at the centre = 1
No. of wires in the first layer = 6
No. of wires in the second layer = 12
Total no. of layer, n = 2
Total no. of wires in the strand = 1 + 3n (1 + n)
= 1 + 3 x 2 (1 + 2) = 1 + 18 = 19
Overall diameter of the stranded conductor
= (1 + 2n)d = (1 + 2 x 2)2.50
= 5 x 2.50 = 12.50 mm
Fig. 7.3 shows the cross-section of 19/2.50 stranded conductor, Fig. 7.4 and 7.5 show circular stranded conductors and compact circular stranded conductors respectively. The former require more insulating and protective materials than those required by the latter. Overall dimensions of compacted conductors become less as is evident from the Fig. 7.5.
Circular stranded conductors are normally used for single phase system.
Compact circular stranded conductors are employed in the manufacture of cables.
5. Bimetals:
A thermostatic bimetal element is based on the theory that metals expand on heating and contract on cooling. It comprises two strips made of different metals with different coefficients of expansion. The strips are welded together lengthwise.
When the element is heated it bends in such a way that metal with higher coefficient is on the outside of the arc formed and that with smaller coefficient on the inside. When the element is cooled it bends in other direction.
Some of the commonly used materials for making bimetallic strips are:
Iron, nickel, constantan … High coefficient of expansion.
Alloy of iron and nickel … Low coefficient of expansion.
Uses:
Bimetallic strips are used in making relays, regulators. They may be used for overload protection of electric motors, etc.
6. Electrical Contact Materials:
The contact materials play a significant role in electrical machinery and appliances.
The selection and description of various types of contact materials may be given as follows:
Selection of Contact Materials:
The successful operation of electrical contact is a function of general factors, the most important of which are:
1. Contact resistance.
2. Contact force.
3. Voltage and current.
1. Contact Resistance:
A primary function of nearly all contacts is to carry electric current. Therefore, they must offer as little resistance as possible to the flow of current. This factor is of greatest importance at very low voltages. The total resistance of a pair of contact is the sum of the resistance of the contact materials and the resistance at the interface or friction between the two contacts.
The specific resistance of the contact materials is normally low compared to resistance at the interfaces. Resistance at the interface is caused primarily by the very small area through which the current passes from one contact to another. So called flat surfaces have many small projections which prevent complete or total contact between the making contacts. Consequently the contact resistance under a given face is relatively independent of the total area of the contact face.
The contact resistance may vary markably because of surface contamination caused by chemical compounds formed from the contact materials themselves. The most important and troublesome of these are the chemical compounds formed by heat and erosion which result in oxide, chloride, carbonate, sulphide and sulphate films. The contact resistance is increased by the imposition of those relatively non-conducting films in the actual area of contact.
2. Contact Force:
The contact resistance of a specific pair of contacts is directly related to the force applied to the contacts in the closed position. The total force should always be as high as possible, although it is recognised that this must be compatible with the type of mechanism, as well as high performance and economic factors.
Quite frequently the force can be utilized more effectively by operating a radius-faced contact against a flat contact. Contact forces in most applications range from a fraction of 1 gram to as high as 1 kilogram.
3. Voltage and Current:
The performance and life of contact materials are intimately related to the voltage and current the contacts must make and break.
Contacts operating in D.C. circuits are subjected to material transfer, i.e., transfer of metal from face of one contact to the face of its making contact. This results in a buildup of mound on one contact and hole or crater in the making contact. The amount and directions of transfer, whether from positive to negative or vice versa depends upon the particular phenomenon encountered in the electrical circuit.
Material transfer in A.C. circuit is not usually encountered within the frequency range about 25 to 400 cycles because the arc is extinguished as the current passes through the first zero point in the cycle and polarity of each contact is constantly changing.
Other Factors Affecting Contact Performance:
In addition to foregoing three major factors there are several others, which, on occasion may be equally important, they are:
1. Contact bounce chatter.
2. Frequency of operation.
3. Speed of contact operation.
4. Type of electrical load.
5. Medium in which contacts operate.
7. Electrical Carbon Materials:
Electrical carbon materials are manufactured from graphite and other forms of carbon such as coal etc. They all consist of practically pure carbon.
Carbon claims the following fields of application:
1. For making brushes for electrical machines.
2. Contacts and resistors.
3. Carbon electrodes for electric-arc furnaces.
4. Battery cell elements.
5. Component for vacuum valves and tubes.
6. As arc light and welding carbons.
7. Microphone powders, membranes and other components for telecommunication equipment.
Electrical carbon products are manufactured from:
(i) Coke obtained from coal, coal tar residues, petroleum residues and peat.
(ii) Anthracites and so called thermo-anthracites obtained by heat treating anthracites at 900 to 1200°C.
Carbon black intended for the manufacture of electrical carbon products is selected for an ash content within the limit of 0.06 to 0.15 percent. Carbon black is produced by thermal decomposition of acetylene (called acetylene black) and by partial composition of hydrocarbon gases and liquids.
The following operations are carried out in the manufacture of most of electrical carbon products:
1. Grinding the raw carbon materials.
2. Mixing the powdered carbon with a binding agent such as coal tar pitch or coal tar.
3. Moulding the requisite articles from the resulting compound.
4. Baking the articles – It imparts to the articles adequate mechanical strength and hardness.
Arc Light Carbons:
These are generally made from coke, graphite and carbon black. They are cylindrical in shape. They are often made with a core of softer material; this core evaporates faster than the surrounding material and thus stabilizes the light output of the arc.
Carbon electrodes for electric-arc furnaces and electro-chemical processes are of following three kinds:
1. Baked
2. Graphitized
3. Self-Baking.
Baked electrodes are produced from:
(i) Foundry coke
(ii) Anthracite, and
(iii) Thermo-anthracite.
If they are to be machined cemented-carbide tipped tools shall be used as they have high hardness.
Graphitized electrodes are more expensive than baked electrodes as they require an additional graphitizing heat treatment in the course of manufacture.
These electrodes entail the following advantages:
(i) Less brittleness.
(ii) Lower resistivity.
(iii) Greater chemical resistance.
Self-baking electrodes differ in that they bake in the course of operation in a furnace, as they burn. The furnaces in which such electrodes are used are filled with special steel boxes into which the electrode mass is charged. This makes it possible to use electrodes upto 1.0 m in diameter with alternating current and upto 2.5 m in diameter with direct current. By using special blocks of carbon instead of steel boxes, the electrode diameter can be as great as 4.5 m and even greater.
Carbon Resistors:
These are produced with nominal resistance ratings ranging from 1 to 1 x 1012 ohms.
The carbon resistors may be of the following two types:
(i) Film type resistors.
(ii) Solid type resistors.
In the film type resistors a film of carbon serves as the resistance element. In the solid type resistors the same purpose is served by a solid rod of special material consisting of carbon and inorganic binding agents.
Film type resistor may be fixed or variable. The latter allows the resistance to be varied within definite limits.
Solid type resistors only serve as fixed resistors.
8. Soft and Hard Solders:
Solder is a readily fusible alloy used to join the surfaces of metals.
There are two types of solders:
1. Soft solder
2. Hard solder.
Composition and Properties:
A soft solder is usually a lead-tin mixture. The tensile strength of solder is greatest with 72.5 percent of lead, but this alloy is not sufficiently fusible to be used for general soldering. Any of the alloys that contain above 70 percent of lead have a melting point too high to be used in ordinary soldering with a copper tool, although they may be used with a steel tool. The chief uses to which such alloys are put in are coating of iron or steel sheets for roofing, filling of hollow castings etc.
The alloy that contains about 67 percent of lead is used for plumber’s work. The alloys containing 55 to 60 percent of lead melt from about 215 to 230°C; and are sufficiently fusible and freely flowing for ordinary soldering, that which contains 58 percent of lead is used to considerable extent for soldering joint in electric wiring because it is considered as easily flowing solder.
But by far the favourite alloy with those who use solder is that which contains 50 percent of lead and 50 percent of tin; it is known commonly as “half and half”. This alloy melts rapidly, flows freely and presents a bright surface when the joint is finished. It can be used for every purpose to which soft solder is applicable, except the “wiping joints”, as it is technically called in plumbing.
2. Hard Solder:
Hard solder is used for joining such metals as copper, silver and gold and alloys such as brass, gunmetal etc. When applied to copper, iron, brass and similar metals the operation is termed brazing, whereas when applied to precious metals it is termed silver soldering.
Hard solder are classified as:
1. Spelters
2. Silver solders.
3. Copper solders.
These are presented with compositions, melting points and uses in the Tables 7.7, 7.8 and 7.9.
Note:
Soft solders are used in making electrical connections in electronic devices and hard solders in power apparatus.
9. Thermocouple Materials:
A thermocouple is based on ‘Seebeck effect’. According to this ‘effect’ when different metals and alloys are welded or soldered at a junction and if temperature difference exists at the junction the e.m.f. is developed called electromotive force. The magnitude of e.m.f. is of the order of few microvolts per degree temperature difference between the two junctions.
The possible thermocouples are given below:
1. Constantan-iron couple
2. Constantan-copper couple
3. Copel (Cu = 56%, Ni = 44%) couple
4. Alumel (Ni = 95%, A1 = 2%, Si = 2%, Mg = 1%) couple
5. Cromel (Ni = 90%, Cr = 10%) couple
6. Copper copel couple
7. Platinum and platinum rhodium couple
8. Cromel alumel couple 1000°C
9. Iron copel thermocouple 350°C
10. Cromel copel thermocouple 60°C.
10. Magnet Wire:
It is also called magnetising coil, magnetic coil or field coil. It is an insulated wire and carries current. It is usually of circular cross-section (diameter varying from 0.0193 to 0.35 cm) though square and rectangular wires are also used. It is a copper wire covered with cotton and enamel. Where the temperature is not high, enamelled wire insulated with vinyl acetate resin varnish is used.
The dielectric strength of resin insulation is 1000 volts per mm. Nylon enamel is used at times when the temperatures are high (upto 200°C). Sometimes wire is insulated with glass fibre and coated with silicon resin. It is used for winding electromagnets, transformers, field coils for motors and generators etc.
11. Piezoelectric Materials:
A piezoelectric material is one in which an electric potential appears across of a crystal if the dimensions of the crystals are changed by the application of a mechanical force. This potential is produced by the displacement of external charges. The effect is reversible, i.e., conversely, if a varying potential is applied to the proper axis of the crystal, it will change the dimensions of the crystal thereby deforming it. This effect is known as piezoelectric effect.
a. Elements exhibiting piezoelectric qualities are sometimes known as electro-resistive element.
b. Common piezoelectric materials are:
i. Ammonium dihydrogen phosphate’,
ii. Rochelle salts;
iii. Lithium sulphate;
iv. Dipotassium tartrate;
v. Potassium dihydrogen phosphate;
vi. Quartz;
vii. Ceramics A and B.
There are two main groups of piezoelectric crystals:
1. Natural Crystals- such as quartz and tourmaline.
2. Synthetic Cyrstals- such as Rochelle salt, lithium suplphate, dipotassium tartrate, etc.
Working of a Piezoelectric Device:
A typical mode of operation of a piezoelectric device employed for measuring varying force applied to a simple plate is shown in Fig. 7.44. The magnitude and polarity of the induced charge on the crystal surface is proportional to the magnitude and direction of the applied force. The charge at the electrode gives rise to voltage (E), given by-
Advantages and Disadvantages of Piezoelectric Devices/Transducers:
Advantages:
1. High frequency response.
2. Small size.
3. High output.
4. Rugged construction.
5. Negligible phase shift.
Disadvantages:
1. Output affected by changes in temperature.
2. Cannot measure static conditions.
Applications:
These transducers find the following fields of application:
1. Accelerometer.
2. Pressure cells.
3. Force cells.
4. Ceramic microphones.
5. Phonograph pick-up
6. Cartridges.
7. Industrial cleansing apparatus.
8. Under-water detection system.
Note:
Transducer is a device which converts the energy from one form to another.
Comparison between Piezoelectricity and Ferroelectricity:
Piezoelectricity:
1. The crystal is polarised by the application of an external stress.
2. The phonomenon occurs in non-centrosymmetrical crystals, occurs in all the 20 crystals.
3. All piezoelectrics are surely not ferroelectric (e.g., tourmaline is piezoelectric, but not ferroelectric at all).
Ferroelectricity:
1. The source of polarisation is the dipole interaction energy itself.
2. The phenomenon occurs in Non-centrosymmetrical crystals, it occurs only in 10 crystals, namely, those which provide a favourable axes of polarity.
3. All ferroelectrics are piezoelectric.
The piezoelectric coefficient is the ratio of the set-up charge to the stress applied along a crystallographic axis. The ferroelectrics have very large piezoelectric coefficients.
12. Pyroelectric Materials:
Some piezoelectric crystals are symmetric structurally in such a fashion that they are spontaneously polarised in the absence of an electric field. The permanent dipoles inside the crystal produce an electric field which is usually marked by charges on the surface of the crystal or twinning inside the crystal.
Owing to the fact that the polarisation due to permanent dipoles present inside an insulation depends on the temperature, as such, a change in the temperature of the crystal produces a change in polarisation which can be detected. Thus, it is called the pyroelectric effect.
It has been observed that crystals belonging to 10 out of the 20 piezoelectric crystals exhibit this property.
Piezoelectric effect is represented by:
∆P = λ ∆T
where, ∆P = Change of polarisation,
λ = Pyroelectric constant, and
∆T = Change in temperature.
The polarisation direction of a pyroelectric crystal can be reversed by the application of a sufficiently intense external field, in that case the crystal is said to be ferroelectric.
Tourmaline and polyvinylidene fluoride are pyroelectric materials.
These materials are used for the following purposes:
(i) As detectors for faulty system traps in industry.
(ii) As infrared detector.
(iii) For building insulation.
13. Thermoelectric Materials and Devices/Transducers:
Two dissimilar metal conductors when jointed at the ends and the two junctions kept at different temperatures, then a small e.m.f. is produced in the circuit. The magnitude of this voltage depends upon the materials of conductors and the temperature difference between the two junctions. This thermoelectric effect is used in thermocouples for the measurement of temperature.
Any number of combination of metals may be used.
Two commonly employed combinations are:
1. Iron and constantan (an alloy of copper and nickel).
2. Chromel (an alloy of chromium and nickel) and alumel (an alloy of aluminium and nickel).
Thermocouple:
In its simplest form a thermocouple consists of two dissimilar metals or alloys which develop e.m.f. when the reference and measuring junctions are at different temperatures. The reference junction or cold junction is usually maintained at some constant temperature, such as 0°C.
Fig. 7.45 shows a simple circuit of a thermocouple and the temperature measuring device. In many industrial installations the instruments are equipped with automatic compensating devices for temperature changes of the reference junction, thus eliminating the necessity of maintaining this junction at constant temperature.
Table 7.15 gives the composition, useful temperatures range and temperature versus e.m.f. relationship for some commercial thermocouples.
14. Photoelectric Devices/Transducers:
Principle of Operation:
The photoelectric transducers operate on the principle that when, light strikes special combination of materials then following may result:
(i) Electrons may flow.
(ii) A voltage may be generated.
(iii) A resistance change may take place.
Applications:
These transducers are used in:
1. Control engineering.
2. Precision measuring devices.
3. Exposure meters used in photography.
4. Solar batteries as sources of electric power for rockets and television, counting machines etc.
5. Satellites used in space research.
Classification:
Photoelectric transducers may be grouped as follows:
1. Photoemissive cell;
2. Photoconductive cell;
3. Photovoltaic cell.