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Materials for Lamp Filaments

The filament is an important part of incandescent lamp. The life of an incandescent lamp, depends on its filament. The filament material must have following merits-
  1. High melting point.
  2. Low vapour pressure.
  3. Free from oxidation in inert gas (i.e. argon, nitrogen etc.) medium at operating temperature.
  4. High resistivity.
  5. Low thermal coefficient of expansion.
  6. Low temperature coefficient of resistance.
  7. Should have high young modulus and tensile strength.
  8. Sufficient ductility so that can be drawn in the form of very thin wire.
  9. Ability to be converted in the shape of filament.
  10. High fatigue resistance against thermally induced fluctuating stresses.

List Materials used for Filament of Incandescent Lamp

Different type of material are used for making filament to f incandescent lamp. Some of these materials are listed below-
  • Carbon
  • Tantalum
  • Tungsten

Carbon

Properties of Carbon

  1. Resistivity at 20oC : 1000-7000 µΩ -cm
  2. Temperature coefficient of resistanceat 20oC: -0.0002 to -0.0008 /oC
  3. Melting point: 3500oC
  4. Specific gravity: 2.1gm /cm3

Uses of Carbon

  1. Used for making pressure sensitive resistors which are used in automatic voltage regulators.
  2. Used for manufacturing the carbon brushes, which are used in DC machines. These carbon brushes improve the commutation as well as reduce the wear and tear.
Note
  1. Commercial efficiency of carbon filament lamp is 4.5 lumens per watts or 3.5 watts per candle power
  2. To prevent the blackening of bulb, the working temperature is limited up to 1800oC.

Tantalum

Properties of Tantalum

  1. Resistivity at 20oC : 12.4 µΩ -cm
  2. Temperature coefficient of resistanceat 20oC: 0.0036/oC
  3. Melting point: 2900oC
  4. Specific gravity: 16.6 gm /cm3
Note Commercial efficiency of Tantalum filament lamp is 3.6 watts per candle power. Due to low efficiency it is not much used these days.

Tungsten

Tungsten is produced by very complicated processes from rare ores or from tungstic acids. Some facts about tungsten are listed below-
  1. Very Hard
  2. Resistivity is twice to aluminum
  3. High tensile strength
  4. Can be drawn in the form of very thin wire
  5. Oxidize very quickly in the presence of oxygen
  6. Can be used up to 2000oC in the atmosphere of inert gases (Nitrogen, Argon etc.) without oxidation.

Properties of Tungsten

Properties of tungsten are listed below-
  1. Specific weight: 20 gm/cm3
  2. Resistivity at 20oC: 5.65µΩ -cm
  3. Temperature coefficient of resistanceat 20oC: 0.005 / oC
  4. Melting point : 3410oC
  5. Boiling point : 5900oC
  6. Thermal coefficient of expansion: 4.3 / K

Uses of Tungsten

  1. Used for making filaments of incandescent lamp.
  2. As electrode in X- ray tubes.
  3. The great hardness, high melting and boiling points make it suitable for use as electrical contact material in certain applications. It is having high resistance for destructive forces produces during operation of electrical contacts.
Note
  1. The efficiency of tungsten filament lamp is about 12 lumen per watts.
  2. Operating temperature up to 2500oC.
  3. Do not have blackening effect on bulb like carbon filament have.
Due to above merits tungsten is preferred for making filament of incandescent lamp.

Compression of Filament Materials with respect to required Properties for Incandescent Lamp Filament

Sl No.PropertyCarbonTantalumTungsten
1Melting point (oC)350029003410
2Resistivity (µΩ-cm) at 20oC1000-700012.45.65
3Tensile strength (over 1650oC)lowestmediumHighest
4Blackening effectIt have blacking effect on bulbDoes not have blackening effect on BulbDoes not have blackening effect on Bulb
5Lumens efficiency (Lumens / watts)4.5 3.612
6Oxidation Resistance in inert gasLowestMedium Highest
7Thermal coefficient of expansion (/K)2 to 66.54.3
8Temperature coefficient of resistance (/oC)at 20oC-0.0002 to -0.00080.00360.005

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