Lecture
High-resistance alloys. High-resistance alloys are conductor materials whose ρ values under normal conditions are not less than 3⋅10–7 Ohm⋅m. They are used in the manufacture of electrical measuring instruments, standard resistors, rheostats and electric heaters. Among the large number of materials used for these purposes, the most common in practice are copper-based alloys – manganin and constantan, as well as chromium-nickel and iron-chromium-aluminum alloys.
Manganin is a copper-based alloy for electrical measuring instruments and standard resistors (composition and properties are given in Table 2.1). Manganin has a yellowish tint and draws well into thin wire down to a diameter of 0.02 mm.
To obtain a low αρ and high stability of resistance over time, manganin is subjected to heat treatment – annealing at 350…550°C in vacuum, followed by slow cooling and additional long-term holding at room temperature.
Constantan is an alloy of copper and nickel (Table 2.1). Constantan is readily workable; it can be drawn into wire and rolled into strip of the same dimensions as manganin.
Constantan is used for the manufacture of rheostats and electric heating elements in cases where the operating temperature does not exceed 400…450°C.
When heated to a sufficiently high temperature, an oxide film forms on the surface of constantan, which has electrically insulating properties (oxide insulation). Constantan wire coated with such insulation can be wound tightly, turn to turn, without special insulation between the turns, provided that the voltage between adjacent turns does not exceed 1 V. Rheostats, for example, are manufactured in this way. Oxidizing constantan wire requires rapid (no more than 3 s) heating to a temperature of 900°C followed by cooling in air.
Chromium-nickel alloys (nichromes) (Table 2.1) are used for manufacturing heating elements for electric furnaces, hot plates, soldering irons, etc.
The high heat resistance of nichrome is explained by the resistance of this alloy to oxidation in air at high temperatures. Resistance at high temperatures in air is explained by the close values of the linear thermal expansion coefficients of the alloys and their oxide films. Therefore, the latter do not crack or separate from the alloy.
The service life of heating elements can be increased by embedding the coils in a solid inert medium such as clay-chamotte, which protects them from mechanical action and impedes the access of oxygen.
The oxide films on the surface of nichrome have small and stable contact resistances over a wide temperature range, even at low contact forces. Thanks to this, thin ductile nichrome wire is used for manufacturing miniature high-resistance variable resistors with good technical characteristics.
|
Alloy |
Resistivity, µOhm⋅m |
Temperature coefficient of resistivity, αρ⋅106, K–1 |
Maximum operating temperature, °C |
|
Manganin (80% Cu, 12% Mn, 2% Ni) |
0.42…0.48 |
5…30 |
100…200 |
|
Constantan (60% Cu, 40% Ni) |
0.48…0.52 |
– (5…25) |
450…500 |
|
Chromium-nickel alloys Kh15N60 (55…61% Ni, 15…18% Cr, 1.5% Mn, remainder – Fe) Kh20N80 (75…78% Ni, 20…23% Cr, 1.5% Mn, remainder – Fe) |
1.0…1.2
1.0…1.1 |
100…200
100…200 |
1000
1100 |
Thermocouple alloys. Due to the simplicity of manufacture, most thermocouples are made from metallic components. The most commonly used are:
Thermocouples can be used to measure the following temperatures:
− platinum-rhodium – platinum up to 1600°C;
− copper – constantan and copper – copel up to 350°C;
− iron – constantan, iron – copel and chromel – copel up to 600°C; − chromel – alumel up to 900–1000°C.
Among the thermocouples used in practice, the chromel – copel thermocouple has the highest thermo-EMF for a given temperature difference.
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