High resistivity materials are used for making resistance elements for heating devices, starters for electric motors, resistances used in precision measuring instruments, loading resistances and rheostats and filaments for incandescent lamps.
High resistivity materials are used in all such applications where a large value of resistance is required. If low resistivity materials were used for such applications the length of the wire would be too large which would increase to a large extent the overall size of the equipment.
Properties & Applications of High Resistivity Materials
Alloys of nickel, chromium, copper, iron, manganese are extensively used as resistance materials. These alloys have high specific resistance (about 25 times that of pure copper) and a very low-temperature coefficient of resistance (about one-twentieth of that of pure copper). In general the higher the specific resistance of the material, the shorter and stouter the resistance element with simple and stronger construction. The various materials used as high resistance materials are described below:
Though pure platinum is very good metal but its use is limited due to its high cost, high-temperature coefficient of resistance (0.003 Ω/Ω/per oC ). It is a heavy, grayish-white, non-corroding noble metal which is malleable and ductile, but poorly fusible and resistant to most chemicals. The properties of platinum are as follows:
Specific Weight — 21400 kg/ m3
Melting Point — 1775oC
Boiling Point — 4530oC
Thermal Coefficient of Expansion — 9 x 10-4 per oC
Resistivity — 0.1 µΩ-m
Platinum can be drawn into thin filaments and strips and is used in different electrical devices, for example, as the heating elements in electrical laboratory ovens and furnaces (at extra high temperatures above 1,300oC it begins to atomize), in platinum-platinum-rhodium thermocouples working at temperatures up to 1,600oC.
Multiple drawing of a bimetallic platinum-silver wire, after which the surface layer of silver is dissolved in nitric acid (that does not act on the platinum), is the way extra-thin elements of about one-micron diameter are made for suspending movable parts in electric meters and other sensitive instruments.
Other applications of platinum are insoluble anodes, thermocouples, electrical contacts, corrosion-resistant laboratory accessories, electrical furnace windings, catalytic gas igniters, grids in special-purpose vacuum tubes, etc.
Tungsten is a silver-white hard metal, obtained by a suitable chemical process from ore concentrates. The properties of tungsten are as follows:
Specific Weight — 20000 kg/m3
Melting Point — 3410oC
Boiling Point — 5900oC
Thermal Coefficient of Expansion — 4.4 x 10-6 /oC
Resistivity — 5.51 x 10-8 Ω-m
Temperature Coefficient of Resistance — 0.0045 Ω/Ω/oC.
The thinner the tungsten wire, the greater is its tensile strength. The tensile strength of forged bars 6 by 3 mm amounts to tens of kg/mm2, while that for thin filaments is as high as 300 – 400 kg/mm2.
Tungsten is one of the main materials used in electronics and vacuum engineering. It is used as the filament in incandescent lamps and as the electrodes, heaters, springs, catches and so on in electron X-ray and another kind of tubes.
This metal is very much like tungsten as to its appearance and properties as well as the way it is processed. The properties of molybdenum are as under:
Specific Weight — 10200 kg/m3
Melting Point — 2620°C
Boiling Point — 3700°C
Thermal Coefficient of Expansion — 5.3 x 10-6 per °C
Resistivity — 4.8 x 10-8 Ω-m
Temperature Coefficient of Resistance — 0.0047 Ω/Ω/ oC.
Molybdenum is useful as the target in X-ray tubes and as structural members in high-vacuum electron tubes because of its ability to form a tight seal with glass.
It is an important alloying element since it imparts hardness and corrosion resistance to ferrous and non-ferrous alloys containing it.
It is an alloy of 84 percent copper, 12 percent manganese and 4 percent nickel of a reddish-brown color. Its properties are:
Specific Resistance — 0.42 to 0.74 µΩ-m
Temperature Coefficient of Resistance – 0.00003 Ω/ Ω/oC
Thermo-emf against copper — about 5 µV/oC
Density – 8400 Kg/m3
Tensile strength – 40 to 55 kg/mm2
Maximum working temperature – Not over 60oC
It is the most widely used material for the manufacture of resistance boxes, standard resistances, and shunts.
Constantan is an alloy of 60 percent copper and 40 percent nickel. Its properties are:
Resistivity — 49 x 10-8 Ω-m
Temperature coefficient of resistance – Very small; may be either positive or negative.
Thermo-emf against copper – about 40 µV/oC
Density – 8900 kg/m3
Tensile strength – 40 to 50 kg/mm2
Maximum Working Temperature – 400 to 450oC
It is widely used in the manufacture of rheostats and heater resistors, provided their working temperature does not exceed 400 to 500 °C. Due to the generation of high thermo-emf, when in a junction with copper, it cannot be used in the manufacture of standard resistors for high precision instruments.
The use of this property of constantan is made in thermocouples for measuring temperature up to several hundred degrees.
Nichrome is an alloy of nickel and chromium. The properties of nichrome are:
Resistivity — about 1.0 µΩ-m
Temperature coefficient of resistance — about 0.00044 Ω/Ω/oC
Density — 8400 kg/m3
Tensile strength 70 kg/mm2
Working temperature up to 1000 °C
Nichrome is available in the form of a wire or tape and is widely used as the heating element in domestic appliances (such as the heater, electric iron, toaster, etc.) and furnaces with working temperatures up to 1000 °C.
Properties of High Resistivity Material
A high resistivity material besides possessing a high value of resistivity should also possess the following additional properties for reasons mentioned against each:
Low Temperature Coefficient: High resistivity materials are often used as shunts in electrical measuring instruments, in making wire-wound precision resistances and resistance boxes. For such precision applications, an important requirement is that the material of the element should have a negligible temperature coefficient of resistance as otherwise the accuracy of measurements will be affected.
High Melting Point: In applications like loading rheostats and starters for electrical motors the material of the resistance element should be able to withstand high temperatures for a long time without melting. The consideration of the resistance temperature coefficient in these cases is also important but comparatively higher values than those mentioned in the above paragraph are permissible. The consideration of high melting point is important also for resistance materials used in electrical heating devices like room heaters, furnaces, etc.
No Tendency for Oxidation: Materials used as high resistance elements in heating appliances should be able to withstand high temperature for a long time without oxidation. This is because if an oxide layer is formed on the heating element the amount of heat radiation will reduce.
Ductility: High resistance materials are required to be made in the shape of very thin wires in the case of precision wire-wound resistors and the shape of thick wires in the case of the elements used in ovens, heaters, starters, etc. High resistance materials to be used for such applications should, therefore, be capable of being drawn into wires of different sizes and further be capable of being coiled.
High mechanical strength: High resistivity materials to be used for applications, where the wire must be very thin, are required to have high tensile strength as otherwise, they may break during the drawing of the wire or during the assembly and subsequent operation.