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Tungsten Filaments

Tungsten Filaments for electric glow lamps may be made by one of a number of distinct processes, viz.:
  1. Mechanical drawing of rods or wires of the pure metal. Finely divided tungsten pressed into the form of wire may be drawn slowly through an orifice surrounded by a small coil maintained at a temperature of 2000° to 2200° C. If the rate at which the wire travels is properly regulated, the tungsten forms a single homogeneous elongated crystal, the rate of growth of which equals the forward movement of the wire. The mean diameter of the particles is about 10-5 cm., and X-ray examination shows the crystalline form to be cubic.
  2. Compression of a paste of tungsten powder with a suitable organic compound, carbonisation of the crude filament, and finally sintering and shaping the product. This is probably the chief method now in application.
  3. Compression of colloidal tungsten without the use of a binding material. The requisite colloidal solution is obtained by making an electric arc between tungsten electrodes under water,5 by heating the powdered element alternately in acids and alkalies, or by the reduction of pure tungsten trioxide by means of potassium cyanide, this method being dependent upon the formation of an oxynitride.
  4. By a method of substitution or exchange, consisting of the introduction of a glowing metal- or carbon-filament into an atmosphere of tungsten oxychloride vapour and excess of hydrogen. Metallic tungsten is deposited upon the filament which thus consists of a core of carbon (or metal) enclosed in a tube of tungsten. According to the conditions, the carbon may be partially or completely replaced by the tungsten, and homogeneous filaments are readily obtained. The reactions which take place may be symbolised as follows:
      1. WO2Cl2 + 2C + H2 = 2HCl + 2CO + W,
      2. b. WOCl4 + C + 2H2 = 4HCl + CO + W.
      1. WO2Cl2 + 3H2 = 2HCl + 2H2O + W,
      2. WOCl4 + 3H2 = 4HCl + H2O + W.
A number of other methods and improvements upon the above have been worked out. Reference to the patent literature upon the subject is made below.

The use of tungsten for glow lamp filaments depends on its high melting-point and comparative non-volatility at high temperatures. The tungsten powder used in the above preparations is usually obtained by reduction of tungsten trioxide to which has been added a little thorium nitrate, so that the filament generally contains thorium oxide to the extent of from 0.7 to 10 per cent. In the absence of thoria, rapid crystal growth occurs on burning the filament, which generally has a fibrous structure, and the crystal boundaries extending across the filament cause weakness and liability to fracture. The presence of thoria, which segregates at the boundaries between the crystal grains, greatly reduces the tendency to crystal growth and so increases the durability of the filament. It has been shown that during the burning reduction of the oxide to thorium takes place. If the filament consists of a single elongated crystal, as when prepared by the first process described above, the possibility of such crystal growth is eliminated. In the ordinary evacuated tungsten lamp a temperature of about 2130° C. is obtained with safety, but if a higher temperature is reached, the tungsten begins to volatilise and condenses as a black deposit on the glass, which greatly reduces the efficiency of the lamp. It has been found that the dimming effect of this deposit may be overcome by coating the filament, previous to burning, with a layer of certain salts; for example, sodium chloride, sodium phosphate, potassium cyanide, or calcium fluoride. On passing the current the salt sublimes and condenses in a non-crystalline condition on the wall of the lamp, the tungsten sublimate is decolorised and its power of light absorption diminished. The effect does not appear to be due to any chemical action, but to the formation of solid solutions of tungsten in salt which have a considerably lower light absorption than the coherent metal films. The volatilisation of the tungsten may also be greatly reduced by filling the lamp with an inert gas such as nitrogen or argon under diminished pressure, usually about half an atmosphere. Higher temperatures may safely be reached with such lamps, and powerful illumination is obtained by this means.

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