The filament is made of tungsten, which is a typical refractory metal with a melting point as high as 3410 degrees Celsius. But as a filament, in order to have good color rendering and color temperature, the working temperature is usually as high as 3000K. At this temperature, even if it is tungsten, its sublimation effect cannot be ignored. The tungsten on the filament will sublimate into steam, and the sublimated tungsten The steam will sublime on the glass shell at a relatively low temperature. If you look carefully, you will find that the long-used incandescent bulb glass will turn black. In fact, the sublimated tungsten vapor-deposits on the glass.
Under the effect of continuous sublimation, the filament will gradually become thinner, and because of the manufacturing process of the filament itself, it is not guaranteed that the thickness is uniform everywhere. As a result, the working temperature of the finer and denser parts will be higher, and the sublimation effect will be more obvious. , Forming a positive feedback-the hotter the place becomes thinner faster, the thinner the place becomes hotter. If it continues, there will always be a place that is weak enough to withstand the thermal shock of the lamp when it is not started, pulls apart under thermal stress, and quickly becomes thinner and melts before it breaks.
In short, it is the thinning of the filament caused by thermal sublimation.
This is the fate of incandescent lamps. Is there a way to extend life? Yes, in order to extend the life of incandescent lamps, people invented the halogen lamp, a new type of incandescent lamp. The general incandescent lamp is filled with nitrogen or evacuated. Halogen lamps are filled with halogen elements. The common ones are iodine (iodine tungsten lamp) and bromine (bromine tungsten lamp). What do they do?
The difference between halogen bulbs and other incandescent lamps is that the glass shell of the halogen lamp is filled with some halogen gas (usually iodine or bromine). Its working principle is: when the filament heats up, the tungsten atoms are evaporated and turned back to the glass. The direction of the tube wall moves. When it approaches the glass tube wall, the tungsten vapor is cooled to about 800°C and combines with halogen atoms to form tungsten halide (tungsten iodide or tungsten bromide).
The tungsten halide continues to move to the glass tube, and then returns to the oxidized filament. Because tungsten halide is a very unstable compound, it will decompose into halogen vapor and tungsten when heated, so that tungsten is on the filament again. Deposited to make up for the evaporated part. Through this regenerative cycle process, the service life of the filament is not only greatly extended (almost 4 times that of incandescent lamps), but also because the filament can work at a higher temperature, higher brightness, higher color temperature and Higher luminous efficiency.
The basic light-emitting principle of halogen tungsten lamps is the same as that of incandescent lamps, and they are both heat radiation sources. The difference is that the tungsten halogen lamp is filled with a special working gas, which is composed of 95% mixed gas (dibromomethane and krypton) and 5% high-purity nitrogen. These gases create tungsten halogen in the halogen bulb. cycle.
The specific process is that after the tungsten in the filament volatilizes, it will move to a place with a lower temperature, and then combine with Br2 at the tube wall to form WBr2; and at a higher temperature, WBr2 will decompose again, and the generated W will return to the filament Above, Br returns to the working gas, which is the process of the entire halogen tungsten cycle. Through this tungsten halogen cycle, the tungsten on the filament will not gradually volatilize, the filament will be burnt due to the "hot spot" effect, and it will not be blackened due to the deposition of tungsten on the bulb shell, and its life will be greatly extended.