The operating temperature of tungsten wire in bulb for incandescent lamps is usually between 2300 and 2800 ° C. Generally, the greater the power of the bulb, the higher the operating temperature of the filament. It can be seen that the operating temperature of the filament is far higher than the recrystallization of tungsten wire in bulb At this time, under the action of its own weight, the filament will sag between the two hooks. In severe cases, the filament can sag until it touches the glass bulb of the bulb.

For the doped tungsten wire in bulb doped with a small amount of potassium silicon aluminum oxide in 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 It is only a few tens of millionths, but the sag of the filament made of this kind of doped tungsten wire in bulb can be greatly improved. The reason is that the recrystallized crystal structure of doped tungsten wire in bulb is very different from that of undoped tungsten wire in bulb. The recrystallized crystals without doping tungsten wire in bulb are basically equiaxed crystals, while the recrystallized crystal structure of doped tungsten wire in bulb is coarse grains overlapping each other in the form of strips.

From the perspective of the high-temperature creep theory of metal materials, the recrystallized crystal structure of this thick and long overlapping 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, the formation of the recrystallized crystal structure of the thick and long overlapping structure unique to the doped tungsten wire in bulb is similar to that of the doped tungsten wire in bulb. Potassium contained in the bulb is closely related. The trace amount of potassium remaining in the doped tungsten strip forms a potassium bubble column parallel to the filament axis during processing, which hinders the lateral growth of the grain during the recrystallization process, thus forming a thick and long overlapping structure.