Non-sag tungsten wire is also called doped tungsten wire, which is doped with the element of K (potassium) or other elements. It can be used as a filament in electric lamps.
The beneficial effects of doping to improve the creep resistance of tungsten wire were recognized as early as 1910, and doping was practiced henceforth. For example, tungsten oxide powder is doped with potassium-containing chemicals. Non-sag tungsten wire is unique in that it is a composite between two mutually insoluble metals, tungsten and potassium. The non-sag properties are attributed to longitudinal rows of sub-microscopic bubbles containing liquid and/or gaseous potassium.
Pure tungsten wire is not suitable to make filaments for incandescent lamps. Under typical operating conditions, the individual grains of the filament have the tendency to offset, or slide off with respect to each other. This causes the filament to sag and short out. A lamp made with such filaments will, therefore, fail prematurely.
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