High temperature performance of tungsten wire
The working temperature of tungsten filament used in incandescent lamps is often between 2300 and 2800°C. Generally, the greater the power of the bulb, the higher the working temperature of the filament. It can be seen that the working temperature of the filament far exceeds the recrystallization temperature of the tungsten filament. At this time , under the action of its own weight, the filament segment between the two hooks will droop. In severe cases, the filament may droop until it collides with the glass shell of the bulb.
For doped tungsten wires that are mixed with a small amount of potassium, silicon, and aluminum oxides during the powder metallurgy process of tungsten, although the content of silicon and aluminum in the final finished wire is only a few parts per million, the content of potassium is only Tens of parts per million, but the droop of the filament made of this doped tungsten filament can be greatly improved.
The reason is that there is a big difference in the recrystallized crystal structure of doped tungsten wire and undoped tungsten wire. The recrystallized crystal of undoped tungsten wire is basically an equiaxed crystal, while the recrystallized crystal structure of doped tungsten wire is a long strip of overlapping coarse grains.
From the perspective of high-temperature creep theory of metal materials, the recrystallized crystal structure of this thick and long overlap structure can greatly improve its ability to resist sagging at high temperatures. According to a series of transmission electron microscopy and Auger energy spectrometer research and analysis conducted in the 1970s, it was shown that the formation of the recrystallized crystal structure of the thick and long lap structure unique to the doped tungsten wire is closely related to the content of the doped tungsten wire.
Potassium is closely related. The trace amounts of potassium remaining in the doped tungsten strips form potassium bubble rows parallel to the wire axis during processing, which hinders the lateral growth of the grains during the recrystallization process, thus forming a thick and long overlap structure.
The sagging of incandescent lamp filaments is not only related to the content of added elements and processing technology in the doped tungsten filament, but also to the processing technology during the filament production process.
The tungsten wire retains a large amount of internal stress when it is drawn into finished filament, and when it is wound into filament, new internal stress of uneven deformation is generated on the cross section of the tungsten wire. These internal stresses must be completely eliminated before the filament is installed into the bulb. Otherwise, the filament will twist, deform, and sag when the bulb starts to ignite. Sagging of the filament will seriously reduce the luminous efficiency of the bulb.
