The electron beam in the X-ray tube is provided by a tungsten wire, which has been used since the electron tube and the incandescent bulb. Although other emitters were tested: dispenser cathodes, lanthanum and cerium hexaboride, thorium and rhenium doped tungsten, pure tungsten is still the best filament material. The filament is made of wire, which is wound into a spiral and inserted into a cup, which serves as a focusing element to form the necessary rectangular electron beam. The spiral is used to increase the surface area of the filament to maximize electron emission.
Tungsten wire is easily obtained and processed into a usable form. The wire shape is sturdy and durable, and maintains its shape while controlling stresses such as vibration and shock. X-ray tube manufacturers stabilize and strengthen the filament through a process called recrystallization. This changes the microstructure of the original fiber thread to a structure in which the ratio of the length to the diameter of the crystal structure is in the range of 3 to 6. Recrystallization is achieved by heating the wire very quickly to about 2600 degrees Celsius in a few seconds and holding it.
The common parameter of filament is life. When hot tungsten evaporates slowly from its surface, the higher the temperature, the greater the evaporation rate. Ideally, tungsten evaporates uniformly, but in fact it starts to form hot spots at the grain boundaries, which are visible as “notches”. Hot spots are more likely to evaporate tungsten, and the wire becomes thinner at these locations and eventually burns away. The higher the filament temperature, the more tungsten grains grow over time and the faster the slotting. In addition, if the cold filament allows high excitation inrush current, it will burn out faster due to overheating of the thinned spot.
Regarding the filament life, a wire quality reduction of about 10% is considered as the end of life. This means that the wire diameter has been reduced by 5.13%, and the filament has reached approximately 98% of its life. Many manufacturers believe that a 5% or 6% reduction in diameter is the end of life.
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