Electrical Resistance Nickel Alloys

Heating element Alloys

Heating elements made of resistance heating alloys are widely used in small household appliances and many branches of industry, including large process heating systems, metal-treating industry furnaces, and kilns for firing ceramics.

Some heating elements in industrial furnaces are supposed to operate continuously at very high-temperature, reaching 1300 ⁰C and more.

Although resistivity is a very important factor, resistance heating alloys must also meet a number of other requirements, in order to provide an extended service life and avoid failure. Resistor alloys show:

• High, uniform electrical resistivity
• The reproducible temperature coefficient of resistivity
• Stable electrical properties
• High current carrying capacity
• Good tensile strength
• High peak operating temperature
• Resistance to contamination
• Immunization ability (metal forms a layer of protective oxides, preventing further corrosion)
• High heat resistance
• Creep-resistance and strength at high temperatures
• Absence of volatile components
• High emissivity
• Low thermal expansion
• Good resistance to thermal shock

Alloys with higher nickel content show relatively low changes in resistance with temperature.

Cooling Rate and Section Size – impact on the change in resistance

Change of resistance with temperature of nichrome alloys depends on section size and the cooling rate after the last production heat treatment. The smallest change occurs for slowly cooled heavy sections. The maximum change occurs for small sections, which cool rapidly.

Alloying additions:

Nickel alloys designed for heating elements are made of various metals and each one of them affects the final properties of an alloy.

The most important alloying additions are:

• Chromium – Main alloying addition. Increases resistivity, heat resistance, and creep resistance.
• Cerium – Small addition of 0.1% extends the lifetime of nichrome alloy heating elements almost 10 times and increases the peak operating temperature up to 1200 ⁰C.
• Iron – Dissolves well in nickel. Improves economy of an alloy. Reduces heat resistance.

Measuring and Regulating Electrical Resistance Nickel Alloys

Electrical resistance alloys are used in instruments and control equipment to measure and regulate electrical characteristics.

Meter resistors include:

• Resistance standards
• Precision resistors
• Measuring bridges

Nickel-containing alloys used for meter resistors are radio alloys and manganians, copper alloys consisting 2-22% of nickel.

Variable resistors include:

• Laboratory slide resistors
• Motor starting and control resistors
• Lighting controllers

Electrical resistance alloys for thermometers

Resistor alloys are used to transform electricity into heat, but can also be used to measure temperature. A thermocouple is an example of such measuring devices.

Thermocouple Nickel Alloys

A thermocouple is a sensor made of two dissimilar alloys, generating electromotive force (emf), which varies with temperature.

Although any combination of two dissimilar alloys will generate emf, thermocouple alloys need to meet other requirements, like the stability of emf, reproducibility, corrosion resistance, mechanical properties in high temperature etc.

Therefore nickel is in common use in thermocouple alloys. Thermocouple nickel alloys are:

• Constantan 55Cu-45Ni
• Chromel 89Ni-9.8Cr-1Fe-0.2Mn
• Alumel 94Ni-0.5Fe-2Al-2.5Mn-1Si
• Nicrosil 84Ni-14Cr-1.4Si
• Nisil 95Ni-4.4Si-0.15Mg
• Alloy 20 (82Ni-18Mo)
• Alloy 19 (99Ni-1Co)

Types of Nickel-alloy thermocouples

• Type J: iron-constantan. Sensitivity 50 µV/°C. Max temperature 750 °C. Oxidation and reduction resistant.
• Type K: chromel-alumel. Sensitivity 41 µV/°C. Max temperature1260 °C. Oxidation resistant.
• Type N: nicrosil-nisil. Sensitivity 39 µV/°C. Max temperature 1260 °C. Oxidation resistant.
• Type M: alloy 20 – alloy 19. Special use.
• Type E: chromel-constantan. Non-magnetic. Sensitivity 68 µV/°C. Max temperature 740 °C. Oxidation resistant.
• Type T: copper-constantan. Non-magnetic. Sensitivity 43 µV/°C. Max temperature 370 °C. Oxidation and reduction resistant.

Copper-Nikel Alloys

Many resistor alloys are composed of copper and nickel. Those two metals share desirable physical and mechanical properties and are resistant to corrosion in many media. What are the electrical properties of these metals?

Copper is a great electricity conductor and has a relatively low temperature coefficient of resistance. Nickel is very resistant to oxidation in high-temperature and show quite high resistivity and very high thermoelectric potential difference with copper. Cupronickels feature combinations of above properties.

One of the most important resistor alloys is constantan 55Cu-45Ni. It has the highest electrical resistivity, the lowest temperature coefficient of resistance and generates the highest electromotive force against platinum of any copper-nickel alloys.

Therefore, constantan is used both for electrical resistors and thermocouples.