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Via the famous X-ray bremsstrahlung, the X-rays originate at the point where the electrons penetrate the anode material and heat is the byproduct. Of all refractory metal materials, tungsten has the highest melting point and lowest vapor pressure, therefore tungsten has been deployed as a target material in nearly all medical X-ray tube anodes since 1909. Because molybdenum has the high thermal conductivity and the fast heat dissipation, the application of molybdenum based tungsten target can contribute to increase heat storage capacity and decrease crack formation of the pure tungsten target. Furthermore, comparing with pure molybdenum, TZM alloy has higher recrystallization temperature and better mechanical characteristic. Rhenium increases the ductility of tungsten, reduces thermo-mechanical stress and increases anode service life thanks to a slower roughening of the anode surface (always accompanied by a dose reduction). Hence, the anode plates of rotating anode tubes usually include a 1 to 2 mm thin layer of tungsten-rhenium alloy deposited onto a plate of molybdenum or TZM.
The power of stationary anode X-ray tubes is restricted due to the impact of temperature on the focal spot. The larger the focal spot, the greater the loadability of the anode but also the greater the geometric unsharpness of X-ray images created. This dilemma can be solved by the introduction of the structure of rotating anode X-ray tubes, where the rotating electronic focal spot forms the focal path on the anode surface during the rotation of anodes. The heat generated is distributed on this loop and the heat per focal path unit area is decreased. As a result, image resolution is improved by high power of rotating anode X-ray tubes.  
With the improvement of clinical medicine, both physicians and patients have been expecting diagnostic image equipments with features of strong power and structural robustness. As the critical components of X-ray tubes, anode targets are expected with the characteristics of strong power and longevity subsequently. Our company has made breakthroughs in material science via some crucial know-how in powder metallurgy. The homogeneous distribution of alloy layers and the high degree of alloying in anodes produced by us will result in lower wear rate, longer lifetime, stronger power and smaller focal spot during usage.
Apart from rotation speed increase of rotating anodes, measures regarding target itself as below can be adopted to improve the anode power:

      1. Black coating of the anode plate: after black coating, the heat emission coefficient will be improved close to 1, implying the heat emitted can double under the same anode temperature than that emitted from the anode without black coating, resulting in a higher radiographic power, a shorter cooling time and an acceleration of single load and serial load.

      2. Introduction of graphite blocks: as the specific heat of graphite is around 10 times higher than that of tungsten and the superior radiation coefficient is close to which is better than that of molybdenum, also the coefficient of thermal expansion slightly lower than tungsten and molybdenum, graphite block in the design significantly increases the heat storage capacity of the anode, while requiring only a tiny increase in overall anode weight.

      3. Increase of the diameter of anode target: the bigger the target diameter, the higher the heat storage capacity and the faster the heat dissipation. Nevertheless, the simple increase of diameter will be restricted by the harassment caused by the mechanical strength and the balance. We are capable of the mass production of anodes with diameter of 102mm. Meanwhile, we are developing anodes with diameter of 150mm, which have been planned to be released in 2014.

     4.Elimination of the mechanical stress of rotating anodes: tungsten and molybdenum are brittle in cold condition. When releasing the first exposure with a high tube current and high tube voltage, extremely high temperature increases occur within the anode disk. This generates high mechanical stresses which can result in the formation of cracks. To prevent such heat cracks, thin radial slots are produced in different anode disk designs and preliminary heat treatments are introduced in production.

      5.Decrease of the target angle α: owning to power p≈1/sinα, the decrease of target angle can not only improve the image resolution but also increase the pulse power. Nevertheless, the smaller the target angle, the smaller the radiation field, and therefore the target angel should not be set too small.

Our company is able to manufacture stationary anodes and rotating anodes (TZM based tungsten target, TZM based tungsten/rhenium target, TZM based tungsten/rhenium target with black coating as well as TZM based tungsten/rhenium target brazed with graphite)to satisfy the demands from different customers. We are one of the best in the manufacture of tungsten/rhenium targets among all Chinese rivals.
  
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