Views: 1 Author: Site Editor Publish Time: 2022-04-21 Origin: Site
Coiled tungsten wire is undoubtedly the most critical part of producing incandescent lamps. The resistivity of tungsten filament is 5.3*10^-8. Tungsten has a high melting point, high resistivity, good strength and low vapor pressure. It is the best material for making incandescent filaments among all pure metals. However, tungsten is hard and brittle, making it difficult to process. When the current is heated to a certain temperature through the tungsten wire, the resistance value of the tungsten wire increases to a certain value (generally, the resistance value of the metal wire increases with the increase of temperature). The resistance of this material should be 1370℃-2000℃ at normal temperature, but this resistance value will change when the cross-sectional area and length of the tungsten wire changes. In 1909, Coolidge invented the processing technology of tungsten wire, which played a decisive role in the production and promotion of incandescent light bulbs, and its basic principle has been used to this day.
Except for a small amount of tungsten wire used as a heating material for high-temperature furnaces, heaters of electronic tubes and reinforcing ribs of composite materials, most of tungsten wires are used to make filaments of various incandescent lamps and tungsten halogen lamps and electrodes of gas discharge lamps. For the tungsten wire or tungsten rod used as the cathode of the gas discharge lamp, in order to reduce its electron work function, 0.5-3% thorium must be added, which is called tungsten-thorium wire. Since thorium is a radioactive element that pollutes the environment, cerium is used instead of thorium to make tungsten-cerium wire or tungsten-cerium rod. But the evaporation rate of cerium is high, so tungsten cerium wire or tungsten cerium rod can only be used for low-power gas discharge lamps.
Once the tungsten wire is recrystallized after being used at a high temperature, it becomes very brittle, and it is easily broken under impact or vibration. In some electric light source products that require high reliability, in order to prevent the filament from breaking, 3-5% rhenium is often added to the doped tungsten wire, which is called tungsten-rhenium wire, which can reduce the tungsten brittle transition temperature to Room temperature or below. This is a very peculiar rhenium effect. So far, no element has been found to replace rhenium and produce the same effect in tungsten.
Tungsten has good acid and alkali resistance at room temperature, but it is easily oxidized in humid air, so thin tungsten wire cannot be stored in a humid environment for too long. In addition, tungsten starts to react with carbon to form tungsten carbides at around 1200 °C, so pay attention to this problem in the hydrogen-burning treatment of the filament, otherwise the tungsten will react with the graphite lubricant on the surface, and the filament will become brittle and break.