Tungsten Bar and Doping Bubble

In the early 1900s, the United States developed a high temperature anti-drooping tungsten wire. Doping (semiconductor), intentionally introducing impurities into an extremely pure semiconductor to change its electrical properties. After many tests, it is found that its sagging resistance is due to a small amount of silicon, aluminum, potassium oxide in the tungsten oxide before reduction. Among them, the researchers proposed a "bubble statement." "Bubble statement" holds the view that the dopant is prone to have the doping effect in the first reduction process, and then form the doping bubble during the vertical melting. These bubbles will be stretched, annealed in follow-up processes. Then it will become small holes arranged in clusters, but these holes may have an impact on the cross motion of tungsten filament grain boundaries during the second recrystallization, or even hinder its movement, eventually leading to the tungsten wire forming a especial recrystallized grain structure, which would make wolfram filament have a high temperature sagging resistance.  Anti-sagging tungsten filament is mainly used for incandescent, fluorescent and other light bulb and the heat wire of tube. Anti-sagging tungsten wire of automobile and motorcycle lights is one of the most important applications. There are many methods to detect its anti-sagging performance, the most common ways include high-temperature creep test, high temperature bending drop method (V-shaped test), etc.

Through the preliminary calculation of the test, it can be concluded that the effective conductive cross section area of tungsten bar would significantly reduced if content of potassium increasingly. Because residual potassium will evaporate into gas in a sintering process, and then produce large amounts of adulterate bubble, resulting in a reduction of its effective conducting cross-sectional area. When potassium content is lower than 70ppm, change of density of adulterate bubbles is more pronounced, it would reduced rapidly as decreasing of potassium content, which will reduce the high temperature sagging resistance of tungsten filament.

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