If it is true that one good fortune outweighs a hundred bad fortunes, then the discovery of tungsten filament drawing technology may serve as a vivid confirmation of this. The method of tungsten filament production, which suffered so many failures, was a turning point in the development of powder metallurgy technology.

History

Incandescent light bulbs as we know them appeared in 1904. Prior to that, the source of light in the lamps had been coal electrodes For more than a quarter of a century the search for a suitable material had been on, ever since Swan in 1878 first presented in Newcastle his invention of coal lamps of eight and sixteen candle brightnesses,. At the end of the nineteenth century, von Welsbach made a metallic filament from osmium (t°pl = 2700° C). Osmium lamps were 6 times more efficient and brighter than carbon lamps, but the price of osmium, an element of the platinum group, made such lamps extremely expensive.

In 1903, von Bolton of Siemens & Halske suggested tantalum with a t°p of 2996 °C for filaments. Tantalum lamps were 15% more efficient than osmium lamps and even began to be introduced into production, but could not compete with the tungsten filament that appeared a year later. Tungsten filament lamps had squeezed all other lamps out of the market by 1911. The luminous efficiency of tungsten was twice that of osmium, and at high voltage almost four times that of osmium. Modern fluorescent lamps with a tungsten cathode are almost 9 times more efficient and brighter than the osmium ones of the late 19th century.

Development of tungsten filament technology

For a long time, because of the considerable brittleness of tungsten at normal temperature, it was not possible to draw a sufficiently thin filament. At the beginning of the 20th century, Siemens-Halske attempted to apply the drawing technology developed for tantalum. However, tungsten was not malleable enough for this. Later an original method was patented, when in a hydrogen atmosphere an electric arc was excited between tungsten electrodes to melt the tungsten billet in a graphite crucible, covered inside with tungsten powder. Droplets of molten tungsten Ø 10 mm and up to 30 mm long were obtained, which were further processed after cooling. According to another patent, tungsten powder was mixed with an organic paste, the resulting mass was squeezed through dies and annealed in an inert atmosphere. A rather thin tungsten filament was obtained. One of the promising developments was as follows. Carbon filament was heated in an atmosphere of hydrogen and _WCl4 vapor. On the surface of the thread precipitated tungsten that sintered with carbon in the form of white carbide WC. The filament was redispersed in a stream of hydrogen, which "washed out" the carbon, leaving a filament of pure tungsten, similar to the filament obtained by extrusion. All these technologies tried to protect tungsten as much as possible from oxidation when heated, because otherwise the granular structure of the filament made it very brittle. And in 1909 the American Coolidge managed to find the ideal mode of temperature and mechanical processing of tungsten without the use of filler.

Modern production

Tungsten powder of high purity is the starting material. Special mills grind the feedstock to a fine powder under a nitrogen atmosphere to avoid surface oxidation of the friction-heated particles. The blanks are then pressed in steel molds at a pressure of 5-25 ^kg/mm2. If the raw material is of poor quality, the billet becomes brittle. To avoid this, an easily oxidizable organic component is added to the raw material. The next step is sintering. The pressed billets, also called stacks, have a specific weight of about 2/3 of the density of tungsten metal, so they are sintered at a very high temperature. They are placed between water-cooled contacts and in an atmosphere of dry _H2 an electric discharge is passed through, heating them almost to the melting point. This increases the size of crystalline grains, and the billet itself reaches 95% of the density of the casting. The billet is forged at 1200-1500°C. In a special mill the sintered rods are squeezed with a hammer, thinning them by 12% each time. The tungsten grains are elongated and thus acquire a fibrillar structure. After forging, the filament is pulled through diamond screens. The diameter of the resulting filament is ~13 microns.

Percentage composition

Advantages of tungsten filament

Tungsten is the most refractory of the metals, its t° of melting is +3422°C. Tungsten filament is heat-resistant, has a minimum coefficient of thermal expansion, it has a very high electrical resistance and light output, high resistance to temperature creep, good thermal conductivity.

Disadvantages

Tungsten is one of the rare elements in the Earth's crust. The difficulty of obtaining it in its pure form and capriciousness of tungsten in processing - all this affects the cost of tungsten filament.

Applications

Tungsten filament is in demand in electrical engineering and electronics because of the low elasticity of tungsten vapor at high operating temperatures up to 2500 °C. Its refractoriness and exceptional light output make it indispensable in incandescent lamps, kinescopes and other vacuum tubes. Tungsten filament (GOST 19 671-91) of VA, VCh, VRN grades is designed not only for spirals and filament bodies. It is used for production of spiral and non-spiral cathodes of electronic devices and parts of semiconductor devices, loop heaters, grids, targets of X-ray tubes. From the brand BPH make inputs, traverses and parts that do not require tungsten alloy.

Buy at the best price

Evek GmbH stocks a wide selection of high quality tungsten filament and tungsten wire of various grades at a price that will satisfy any customer. We offer good conditions for wholesale and retail customers. If necessary, you will be consulted by experienced managers. All products are certified Quality is guaranteed by strict compliance with technological norms of production. Delivery - in the shortest terms. At the wholesale orders preferential discounts are provided.