The various electrical conducting materials are discussed as follows: 1. Copper 2. Aluminium 3. Tungsten 4. Iron and Steel 5. Nickel 6. Tin 7. Lead 8. Alloys 9. Carbon and Graphite 10. Molybdenum 11. Platinum 12. Mercury 13. Non-Linear Conductors.
1. Copper:
Properties:
(i) Its colour is reddish.
(ii) It is highly malleable and ductile.
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(iii) It can be cast, forged, rolled, drawn and machined.
(iv) Mechanical working hardens it, but annealing restores it to soft state.
(v) When heated to a very high temperature, it vaporizes and burns with a characteristic flame.
(vi) Density at 20°C, g/cm3 (pure copper) …8.94
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(vii) Melting point, °C (pure copper)…1083
(viii) Thermal conductivity, cal/cm sec. °C (pure copper) …0.923
(ix) Electrical resistivity, ohm mm2/m (pure copper) …0.01682
(x) Temperature coefficient of resistivity, °C-1 (pure copper) …0.00411
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(xi) Its electrical conductivity is very sensitive to the presence of slight impurities in the metal. Its value for soft annealed copper is 58 x 10-4 ohm-1 cm-1 at 20°C.
(xii) Conductivity decreases when hard drawn into wires; annealing is therefore, necessary before the material can be used in electrical equipment.
(xiii) High purity copper is obtained by electrolytic refining. Traces (0.1%) of iron, silicon or phosphorus seriously reduces the conductivity of copper.
(xiv) It has low contact resistance.
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(xv) It has lowest resistance per unit volume.
(xvi) By addition of one percent cadmium the tensile strength of copper, is increased by 50% but conductivity however, decreases by 15 percent. Cadmium addition makes copper suitable for the use in long spans of thin cross-section which requires conductors of high mechanical strength.
(xvii) Hardness of copper, as determined by a ball impression test corresponds to from 35 to 37 kg/mm2 for soft copper and from 65 to 120 kg/mm2 for hard copper. Copper loses its hardness abruptly at 200°C.
(xviii) Copper when exposed to ordinary atmospheres becomes oxidised, turning to black colour, but the oxide coating is protective, and the oxidising process is not progressive. When exposed to moist air containing carbon dioxide, it becomes coated with green basic carbonate, which is also protective.
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(xix) At temperature above 180°C it oxidizes in dry air.
(xx) In the presence of ammonia it is readily oxidized in air and it is also affected by sulphur dioxide.
(xxi) Copper is not readily attacked at high temperatures.
(xxii) In ordinary atmospheres it is not subjected to appreciable corrosion.
(xxiii) It is corroded slowly by sea water.
(xxiv) It readily alloys with many other metals.
(xxv) The properties of copper are not affected by a rapid cooling after annealing or rolling.
Mechanical properties of soft annealed copper:
Yield point … 70 N/mm2
Tensile strength … 200 N/mm2
Vickers hardness … 400 N/mm2
Young’s modulus of elasticity … 120 kN/mm2
Brinell hardness …30-40.
Uses:
1. Soft copper is used to make an extensive variety of winding wires and conductors for producing cables.
2. Hard copper is used to make commutator segments and contact wire.
3. Copper conductors having a steel core are also employed for long span transmission lines, where a combination of high conductivity, small sag and minimum cross-section are desired. In such conductors, an insulating tape over the wire has to be provided in order to prevent the corrosion action of steel on copper.
2. Aluminium:
Properties:
(i) It is silver-white in colour.
(ii) It possesses high ductility.
(iii) It can be readily worked by rolling, drawing, spinning, extruding and forging.
(iv) It has relatively high thermal and electrical conductivities.
(v) The metal is always covered with a thin, invisible film of oxide which is impermeable and protective in character. Aluminium, therefore, shows stability and long life under ordinary atmospheric pressure.
(vi) Density at 20°C, g/cm3 (pure aluminium) …2.703
(vii) Melting point, °C (pure aluminium) …657
(viii) Thermal conductivity, cal/cm.sec.°C (pure aluminium) …0.503
(ix) Electrical resistivity, ohm-mm2/m (pure aluminium) …0.02828
(x) Temperature coefficient of resistivity °C-1 (pure aluminium) …0.00403
(xi) It has the lowest resistance per unit weight.
(xii) It has high contact resistance.
(xiii) It cannot be soldered easily but fluxes have been devised to make soldering easy. Mechanical clamping and screwing methods have also been developed.
(xiv) It poses no problems in cable manufacture and installation.
(xv) Pure aluminium is softer than copper and can be rolled into thin sheets down to 6 to 7 microns in thickness (so called foil).
(xvi) Because of its low mechanical strength, aluminium cannot be drawn into very fine wire.
(xvii) Exposure to atmosphere high in hydrogen sulphide or sulphur dioxide does not cause severe attack of aluminium at ordinary temperatures, and for this reason aluminium and its alloys can be used in atmospheres which would be rapidly corrosive to many other metals.
(xviii) Most organic acids and their water solutions have little or no effect on aluminium at room temperature, although oxalic acid is an exception and is corrosive.
(xix) Solutions of strong alkalies, potassium or sodium hydroxides dissolve aluminium rapidly.
(xx) Aluminium in the presence of water and limited air or oxygen rapidly coverts into aluminium hydroxide, a whitish powder.
(xxi) It can be worked in hot condition as well as in the cold state.
(xxii) Its strength can be improved by cold working.
(xxiii) Aluminium has a high reflectivity for light rays.
Mechanical properties of aluminium:
Yield point … 30 N/mm2
Tensile strength … 90 N/mm2
Hardness (vickers pyramid number) … 22
Young’s modulus of elasticity … 72 kN/mm2
Elongation … 45%.
Uses:
1. The steel reinforced aluminium conductors (A.C.S.R.) find extensive use in long transmission lines.
2. Aluminium conductors are particularly suitable for operation in very high ambient temperature.
3. It is used in domestic wiring, flexible wires, rotor bars of squirrel cage induction motors.
4. Due to its less density, aluminium wound machines have less weight, but they are not commercially successful.
Aluminium is gaining ever increasing application for a number of economic and engineering reasons, primarily the high demand for conductor materials which cannot be met by copper production alone. In replacing copper wires with aluminium wires, due account should be taken of their differences in resistivity, mechanical strength and density. The I2R loss = I2r (l/a) where r is the resistivity.
The resistivity of copper is 2.1 x 10-8 Wm at 75°C while for aluminium it is 3.4 x 10-8 Wm. The resistivity of aluminium being 3.4/ 2.1 = 1.6 times that of copper, for the same loss and length of conductor an aluminium conductor should have a cross-sectional area of 60 percent greater than that of copper conductor. However, since the density of copper is about 3.3 times that of aluminium, weight of aluminium conductor will equal 48 percent of weight of copper.
Aluminium was first used, instead of copper, in the armature and field windings of motors, generators etc., and in transformers built in Germany during 1914-18. Although it is claimed that such machines compete in efficiency and price with copper wound machines, aluminium is, however, at an obvious disadvantage wherever the space for windings is limited.
When using aluminium winding wires, machines have to be redesigned for larger slots to accommodate aluminium wire if it is to have the same resistance as the copper wire it replaces. Aluminium when adopted as a conductor material in small transformers, decreases the overall cost of transformer. But when used in large transformers it gives increased size and cost. Because of its lightness aluminium is being used for making tanks of large power transformers. Aluminium is widely used for making die cast rotor winding of squirrel cage induction motors.
Aluminium oxidises quickly in normal atmospheric conditions and acquires a thin film of oxide Al2O3, which effectively protects it from further oxidation. By reason of high melting point of aluminium oxide coating (of the order of 2000°C) and the rapidity with which a freely exposed aluminium surface becomes oxidized, aluminium wire cannot be soldered by conventional means. Now-a-days aluminium is successfully soldered by using special ultrasonic soldering irons.
If copper and aluminium are brought into contact galvanic corrosion takes place. If the point of contact becomes wet a local galvanic couple is formed between the two metals producing rather high e.m.f. The polarity of this couple causes the current to flow on the outside surface from aluminium to copper conductor and as a result the aluminium conductor may be damaged badly by corrosion. Therefore, all the points of contact for copper and aluminium should be protected from moisture by applying varnish etc. Aluminium is badly corroded even by very weak alkali solutions.
3. Tungsten:
(i) Resistivity of tungsten is 5 μ ohm cm which is twice as poor as that of aluminium.
(ii) It has highest melting point among metals and therefore, it is suitable for applications requiring high operating conditions such as lamps and valve filament lamps.
(iii) The great hardness and high boiling and melting point of tungsten coupled with its resistance to abrasion establish this metal as an outstanding material for electrical contacts in certain applications. It is extremely resistant to the destructive forces of arcing.
Typical operating conditions for tungsten contacts are:
Voltage A.C. or D.C. … upto 250 V
Current… upto 15 A
(iv) Tungsten contacts are used in battery ignition systems, vibrators and electric razors.
4. Iron and Steel:
(i) Steel is used as conductor rail in traction on account of its cheapness and rigidity.
(ii) Galvanized steel and iron wires which are usually used for earth conductor in the voltage distribution systems may also be used for phase conductors in rural areas where cheapness is the main consideration. Such lines generally have large voltage drops due to high resistance and inductance.
(iii) Manganese addition has a hardening effect on steel and manganese steel (about 13% manganese) has the further property of being practically non-magnetic.
(iv) Steel alloyed with chromium and aluminium is used for making starters, rheostats where lightness combined with robustness and good heat dissipation are important considerations.
(v) Cast iron is employed in the manufacture of “resistance grids” to be used in the starting of large D.C. motors.
5. Nickel:
This material is mostly used in the making of electrodes of thermionic valves and sparking plugs. It is also used to form the positive plate of nickel accumulator which has distinct advantages over the ordinary lead acid accumulator.
6. Tin:
It is mainly used in the manufacture of low current fuses.
7. Lead:
It is used to form-
(i) Cable sheaths and
(ii) The plates of lead acid accumulator.
Lead sheaths arc required to protect the insulation of the cable from effects of moisture.
Lead is damage by-
(i) Sea water
(ii) Acids
(iii) Lime
(iv) Newly made concrete tranches, and
(v) Chalk with water.
Properties of Lead Conductors:
(i) Specific weight … 11.48 g/cm3
(ii) Melting point … 327°C
(iii) Resistivity, … r = 0.222 W mm2/m
8. Alloys:
Alloy materials are used for-
(i) Making resistors for laboratory instruments and for laboratory standards where a high constancy of resistance is desirable.
(ii) Making heater and thermocouple elements.
The important alloys are-
(a) Constantan or Eureka
(b) German silver
(c) Manganin
(d) Nichrome.
9. Carbon and Graphite:
(i) Carbon is used in automatic voltage regulators for making the pressure sensitive pile resistors.
(ii) It is used in the manufacture of welding electrodes, fixed and variable resistors for light currents and contacts of certain classes of D.C. suitable gear which are subjected to arcing.
(iii) Carbon brushes reduce considerably the severity of sparking and the rate of commutation wear in electrical machines,
(iv) Its temperature co-efficient of resistance is negative.
10. Molybdenum:
Properties of Molybdenum Conductor Materials:
(i) Specific weight …10.2 g/cm3
(ii) Melting point … 2620°C
(iii) Boiling point … 3700°C
(iv) Thermal coefficient of expansion … 5.3 x 10-6 per degree
(v) Resistivity … r = 0.048 W mm2/m
(vi) Temperature resistance coefficient … α = 0.047 per degree.
Uses:
1. It is employed as the target in X-ray tube and structural member in high vacuum electron tubes because of its ability to form a tight seal with glass.
2. Used for high temperature application in refractory elements.
11. Platinum:
Platinum can be drawn into thin filaments and strips and employed in different electrical devices.
It is used as heating elements in ovens and furnaces.
It is employed as conductor in the following:
(i) Thermocouples,
(ii) Electrical contacts,
(iii) Corrosion resistance applications,
(iv) Grids in vacuum tubes and
(v) Catalytic gas igniters.
Properties of Platinum Conductor Materials:
(i) Specific weight … 21.4 g/cm3
(ii) Melting point … 1775°C
(iii) Boiling point … 4530°C
(iv) Thermal coefficient of expansion … 9 x 10-6 per degree
(v) Resistivity … r = 0.1 W mm2/m
(vi) Temperature resistance coefficient … α = 0.00307 per degree
In modern engineering Rhenium, Tantalum, Mobium and Nickel are called refractory metals. They are also employed in the manufacture of electrolytic capacitors with aluminium.
The melting point of these metals ore of significant importance are as follows:
Rhenium = 3000°C, Tantalum = 2850°C, Miobium = 1950°C and Nickel = 1455°C.
12. Mercury:
Mercury conductors are used in mercury rectifiers, and gas filled tubes, liquid contact material in electric switches, vacuum devices, mercury electrodes in instruments.
Properties of Mercury Conductors:
(i) Specific weight … 13.55 g/cm3
(ii) Boiling point … 357°C
(iii) Thermal coefficient of expansion … 182 x 10-6 per degree
(iv) Resistivity … r = 0.95 W mm2/m
(v) Temperature resistance coefficient … α = 0.00027 per degree
13. Non-Linear Conductors:
Some conducting materials do not obey ohm’s law and the resistance of such materials may vary with the applied voltage. Such materials are said to be possess non-linear resistance. There are other classes of materials in which resistance varies not only with applied voltage but also with polarity of the applied voltage.