The various high resistivity materials (including alloys) are described below:
1. Tungsten:
(i) Hard metal.
(ii) Resistivity twice that of aluminium.
(iii) Can be drawn into very thin wires (for making filaments). The thinner the tungsten wire, the greater is its tensile strength.
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(iv) In the atmosphere or inert gas (Nitrogen, Argon etc.) or in vacuum, tungsten can be readily worked at temperatures like 2000°C and even higher.
(v) Oxidises very quickly in the presence of oxygen even at temperature of a few hundred degree centigrade.
Uses:
I. This material is used in electronic and vacuum engineering. Used as filament in lamps, electrodes, heaters, springs etc. It is also used in electron, X-ray and other kinds of tubes.
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II. The tungsten filament is made in straight, coiled or coiled-coil form. The straight filament has tendency to sag if the lamp is operated in horizontal position. Coiled filament concentrates the light. Coiled-coil filament is used when an extremely concentrated light source is needed like in projectors.
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 acting forces.
Properties of Tungsten Conductor:
(i) Specific weight … 20 g/cm3
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(ii) Melting point … 3410°C
(iii) Boiling point … 5900°C
(iv) Thermal coefficient of expansion … 4.4 x 10-9 per degree
(v) Resistivity … ρ = 0.055 Ω mm2/m
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(vi) Temperature coefficient of resistance … α = 0.005
The tungsten is produced by very complicated processes from rare ores or from a tungstic acid.
2. Carbon:
(i) ρ = 1000 to 7000 μ ohm cm, α = – 0.0002 to – 0.0008
(ii) Melting point = 3500°C
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(iii) Density 1.7 to 3.5.
Uses:
1. Carbon is used in automatic voltage regulators for making the pressure sensitive pile resistors.
2. It is used in the manufacture of welding electrodes, fixed and variable resistors for light current and contacts of certain classes of D.C. switch gear.
Carbon brushes reduce considerably the severity of sparking and rate of commutation wear in electrical machines.
3. Nichrome or Brightray B:
Composition:
Ni = 60%, Cr = 15%, Fe = 25%.
Properties:
(i) Resistivity … ρ = 1.10 μΩ cm
(ii) Temperature coefficient of resistance … α = 0.0002 per degree
(iii) Specific gravity … 8.24
(iv) Melting point … 1350°C
Uses:
It is used in making of tubular heaters and electric irons.
4. Nichrome V or Brightray C:
Composition:
Ni = 80%, Cr = 20%
Properties:
(i) Resistivity … ρ = 40 m ohm cm
(ii) Temperature coefficient of resistance … α = 0.0001 per degree
(iii) Specific gravity … 8.4
(iv) Melting point … 1400°C
(v) Possess high resistance to oxidation and temperature effects.
Uses:
It is employed in the making of heating elements and furnaces.
5. Manganin:
Composition:
Cu = 84%; Mn = 12%; Ni = 4%.
Properties:
(i) Resistivity … ρ = 44 μΩ cm
(ii) Temperature coefficient of resistance … α = ± 0.0015 per degree
(iii) Specific gravity … 8.19
(iv) Melting point … 1020°C.
Uses:
It is used in instrument shunts and standard resistance coils.
6. Constantan or Eureka:
Composition:
Ni = 40%; Cu = 60%.
Properties:
(i) Resistivity … ρ = 44 μΩ cm
(ii) Temperature coefficient of resistance … α = 0.00002 per degree
(iii) Specific weight … 8.9
(iv) Melting point … 1300°C
(v) Heat resisting properties – poor.
Uses:
It is used for making starters, field regulator resistances; wires for resistance boxes and thermocouples.
7. German Silver or Nickel Silver or Electrum:
Composition:
Cu = 50%; Zn = 20%; Ni = 30%.
Properties:
(i) Resistivity … ρ = 30 μΩ cm
(ii) Temperature coefficient of resistance … α = 0.0004 per degree
Uses:
It is used in electrical measuring instruments.
8. Nirosta:
Composition:
Cr = 20%; Ni = 10%; Si = 0.05%; C = 0.15%; Mn = 0.25%.
Properties:
(i) Resistivity … ρ = 73 μΩ cm
(ii) Specific gravity … 7.86
(iii) Melting point … 1400°C.
Uses:
It is particularly suitable where exceptional corrosion resistance and heat resistance are of significant importance.
9. Fechral:
Composition:
Mn = 0.7%; Ni = 0.6%; Cr = 12%; Al = 3.5%; Fe = 83.2%.
Properties:
(i) Specific gravity …7.1
(ii) Temperature coefficient of resistance … α = 10 to 12 x 10-5 per degree
(iii) Maximum temperature … 850°C
(iv) Tensile strength … 700 N/m2
(v) Ultimate elongation … 10-15%
10. Chromal:
Composition:
Mn = 0.7%; Ni = 0.6%; Cr = 23%; Al = 4.5% and Fe = 71.2%
Properties:
(i) Specific gravity … 6.9
(ii) Temperature coefficient of resistance … 6.5 x 10-5 per degree
(iii) Maximum temperature … 1200°C
(iv) Tensile strength … 800 N/mm2
(v) Ultimate elongation … 10-15%
Materials of high resistivity may also be classified according to their purpose as follows:
1. Materials Used for Precision Work:
An important requirement imposed on high resistivity materials intended for use in precision electrical instruments and for making standard resistances is stability of resistance over the period of time (no tendency to age) and during fluctuations of temperature. The latter implies that the material should have a low resistance temperature coefficient.
The thermoelectric motive force resulting from contact of material with copper
should be minimum so as not to introduce errors into measurements. Cost is not of much importance for these materials. The most important material used for this class is ‘Manganin’. Nickel present in it serves to reduce thermo-e.m.f. of contact with copper to a very low value of about 1.0 microvolt per degree.
2. Materials Used for Heating Devices:
The primary requirements for high temperature resistance alloys intended for use in electric furnaces and heating devices is high working temperature. This requirement is satisfied by a material which has a sufficiently high melting point and is either non-corrosive or forms a surface layer of dense, high melting oxide protecting it from further corrosion.
‘Platinum’ is an incorrodible material with a high melting point (1710°C). Because of its high cost, platinum is used in laboratory electric furnaces with a working temperature of 1300°C.
The most extensively used high working temperature resistance materials are alloys of nickel, chromium and iron called Nichrome and alloys of aluminium, iron and chromium. The quality of these alloys, especially the working temperature strongly depends upon the chromium content. The presence of chromium ensures a high melting point of oxide coating.
The resistivity of Nichrome varies from 1.1 to 1.27 ohm per m and mm2. Nichrome is available as round wire and strip, cold drawn with an oxidized surface and also hot rolled with scale covered surface. The optimum working temperature for Nichrome wire is 900° to 1000°C.
3. Materials Used for Rheostats:
The resistance materials used in making rheostats can have a large thermo-e.m.f. and a large resistance temperature coefficient. But this material should meet special requirements such as a high permissible working temperature and low cost, the latter being dictated by the fact that these materials are required in large quantities in widely used devices and equipment where large changes in resistances are allowed. The principal alloy in this group is ‘Constantan’. A voltage (maximum) of one volt per turn should be used when designing constantan wire rheostats.