High Resistivity or Low Conductivity Conducting Material

Conducting materials with relatively high resistivity can produce a useful resistance in a practical wire length. Depending on their other properties, they may be used as filaments in an incandescent lamp or as heating elements in electric heaters, furnaces, space heaters and electric irons. Their resistivity is high compared with copper, but they still conduct far better than electrical insulators.

Required Properties in High Resistivity or Low Conductivity Conducting Material

A high resistivity or low conductivity conducting material should be selected for the needs of its application. Useful properties include:

• Sufficient resistivity to obtain the required resistance from a practical size of element.
• An operating-temperature limit above the intended service temperature.
• Enough mechanical strength for manufacturing and service.
• Enough ductility to be drawn into wire without cracking.
• Resistance to oxidation and other forms of corrosion in the operating atmosphere.
• An acceptable total material and manufacturing cost.
• Stable properties and adequate service life.
• Suitable flexibility or formability for the intended element shape.

The following sections describe several materials used for resistance, heating, filament or contact applications.

1. Tungsten
2. Carbon
3. Nichrome or Brightray B
4. Nichrome V or Brightray C
5. Manganin

Tungsten

Tungsten is extracted from tungsten-bearing ores and refined through several processing stages. Its main engineering characteristics include:

• High hardness, although pure tungsten can also be brittle.
• Room-temperature resistivity of about 5.3 µΩ-cm, roughly twice that of aluminium.
• High strength at elevated temperatures.
• Can be processed into very fine wire under controlled conditions.
• Oxidises rapidly at high temperature in the presence of oxygen.
• Can operate at temperatures approaching 2000^oC in a suitable vacuum or inert atmosphere, subject to the product design and manufacturer limits.

Properties of Tungsten

Representative room-temperature and phase-change properties of tungsten are listed below:

• Density: approximately 19.3 g/cm³
• Electrical resistivity: approximately 5.3 µΩ-cm near 20°C
• Temperature coefficient of resistance: approximately 0.0045 / ^oC near 20°C
• Melting point: approximately 3410^oC
• Boiling point: approximately 5900^oC
• Linear thermal expansion coefficient: approximately 4.5 × 10^-6 / ^oC near room temperature

Uses of Tungsten

1. Wire filaments in an incandescent lamp.
2. Targets and electrodes in X-ray tubes.
3. Electrical contacts that need high hardness, arc-erosion resistance and high-temperature capability.

Carbon

Carbon-based materials are widely used in electrical engineering. Electrical carbon is a family of graphite, electrographite, resin-bonded carbon and metal-graphite grades, each with different electrical and mechanical properties.

Properties of Carbon

• Resistivity: strongly grade-dependent; values can span several orders of magnitude
• Temperature coefficient of resistance: often negative for graphite near room temperature, but dependent on grade and range /^oC
• High-temperature behaviour: carbon generally sublimes rather than melts at atmospheric pressure, at temperatures above about 3500^oC
• Typical graphite density: approximately 2.1 g/cm³

Uses of Carbon

Carbon grades have the following applications in electrical Engineering:

1. Carbon-pile pressure-sensitive resistors in some automatic voltage regulators.
2. Carbon brushes for DC machines, selected to support commutation and limit wear at the sliding contact.
3. Historical filaments for an incandescent lamp, before tungsten became standard.
4. Electrical contacts and current-transfer components.
5. Carbon composition and film resistors.
6. Conductive elements in some battery cell designs.
7. Graphite electrodes for electric furnaces.
8. Electrodes for arc lighting and welding.
9. Specialised components in vacuum valves and tubes.
10. Contact and resistor parts in telecommunications equipment.

Nichrome or Brightray B

Composition of Nichrome or Brightray B

Properties of Nichrome or Brightray B

• Resistivity: approximately 110 µΩ-cm at 20°C
• Temperature coefficient of resistance: small and positive, with the exact value dependent on composition and temperature /^oC
• Melting range: approximately 1350^oC for a typical Ni-Cr-Fe grade
• Density: approximately 8.2 g/cm³
• Good oxidation resistance at elevated temperature

Uses of Nichrome or Brightray B

This alloy family is used in tubular heaters and electric irons, subject to the element temperature and atmosphere limits for the specified grade.

Nichrome V or Brightray C

Composition of Nichrome V or Brightray C

Properties of Nichrome V or Brightray C

• Resistivity: approximately 109 µΩ-cm at 20°C
• Temperature coefficient of resistance: approximately 0.0001 /^oC near room temperature
• Melting point: approximately 1400^oC
• Density: approximately 8.3 g/cm³
• Good oxidation resistance at elevated temperature

Uses of Nichrome V or Brightray C

Nichrome V is used for heating elements in electric heaters and furnaces where its temperature rating, atmosphere compatibility and element loading are suitable.

Manganin

Composition of Manganin

Properties of Manganin

• Resistivity: approximately 43 µΩ-cm at 20°C
• Temperature coefficient of resistance: approximately ±0.00001 /^oC between 20°C and 50°C
• Melting point: approximately 960^oC
• Density: approximately 8.4 g/cm³
• High long-term resistance stability within its specified operating range

Uses of Manganin

Manganin has the following applications in electrical Engineering:

1. Manganin is generally not used for electric heating elements or furnaces because oxidation can cause resistance drift above its recommended air-service temperature.
2. Its very low temperature coefficient of resistance, low thermal electromotive force against copper and long-term stability make it suitable for standard resistors, shunts and measuring instruments.