[0021] The rotary anode for an X-ray tube, designated in its entirety by 10 and only partially shown in the figures, comprises a base body 12 made of ceramic and a combustion zone 14 made of tungsten, which emits X-rays when irradiated by electrons radiation.
[0022] Due to the higher energy density in the operation of new X-ray tubes, the substrate 12 requires a material that is both resistant to high temperatures and has a sufficiently high mechanical strength at high temperatures so that a rotational frequency of 300 to 400 Hz can be maintained. A suitable class of materials satisfying these conditions are ceramics composed of silicon carbide or silicon carbide/diboride composites.
[0023] In order to improve the fixation of the combustion zone 14 on the base body made of this material, an intermediate layer 16 is formed between the combustion zone 14 and the base body 12 during the production of the rotary anode 10 , which intermediate layer 16 is made of high-temperature-resistant tungsten carbide and silicided Tungsten composition. The intermediate layer 16 is formed here by a heat-promoted diffusion process between the metallic tungsten of the combustion zone 14 and the ceramic of the base body. An illustration of a clearly defined intermediate layer 16 is thus shown schematically - in reality, the material compositions of the base body 12 and the combustion zone 14 transition nested within one another in a continuous interface region.
[0024] Various possibilities exist for forming this connection between the combustion zone 14 and the base body 12 .
[0025] During diffusion welding, first the tungsten ring or the tungsten segment in the form of the combustion zone 14 to be formed is placed on the base body 12 and fixed under mechanical pressure. By heating the composite to 1650-2000° C., the desired diffusion process occurs between the tungsten and the ceramic, whereby the intermediate layer 16 is formed and the combustion zone 14 is firmly fixed on the base body 12 .
[0026] Alternatively, orbital friction welding can also be used to form the rotating anode 12 . Here, a tungsten sheet in the form of segments is applied to the base body 12 . The base body 12 is then firmly clamped together with the tungsten sheet into the friction welding device and pressed against the corresponding friction surfaces. Frictional heat is caused by the orbital movement on both sides, which heats the connecting region of the combustion zone 14 to 1600-2000° C., so that the fixed connection of the combustion zone 14 to the base body is also achieved by forming an intermediate layer 16 comprising tungsten carbide and tungsten silicide. superior.
[0027] In addition to the joining methods already described, regeneration methods for producing the rotating anode 10 can also be used. An example of this is selective laser welding. Here, the combustion zone 14 is formed continuously by a bed of tungsten powder. For this purpose, the base body 12 is rotatably mounted on the powder bed and initially coated with a thin layer of tungsten powder, which is first preheated by means of relatively low-power laser radiation in order to improve the adhesion properties. The laser power is then increased, wherein the tungsten particles are sintered and the intermediate layer 16 is formed by diffusion between the tungsten and the base body 12 . After one layer is sintered, the base body 12 is lowered and tungsten powder is reapplied and the laser treatment is repeated until the desired burn zone thickness is achieved.
[0028] Another regeneration method for making the rotating anode 10 is electric field activated sintering or plasma spark sintering. For this sintering method, which is similar to hot pressing, the powder or raw material to be sintered is pre-compacted in a negative mold And make it in contact with graphite electrons. By means of a hydraulic press, pressure is applied to the material throughout the sintering process. At the same time, the graphite electrons are charged with several thousand amps of direct current at a few volts. The direct current conducted directly through the material to be sintered generates heat due to the ohmic resistance in the material from which it is heated up to the sintering temperature.
[0029] In this method, both the combustion zone 14 and the base body 12 can simultaneously consist of the corresponding tungsten or SiC/diboride powder. However, it is also possible to use and sinter preformed model bodies for the base body 12 and the combustion zone 14 . In all cases, a diffusion process between the tungsten and the ceramic also occurs here, which leads to the formation of the intermediate layer 16 .
[0030] An additional advantage of the last described regeneration method is that it enables complex shaped combustion bands 14 to be formed also on curved or other complex shaped surfaces. At the same time, a regenerated microstructure can be introduced into the combustion zone 14 in order to reduce intrinsic stress and control the fracture structure.
[0031] Particularly rapid production of the rotary anode 10 is achieved here in particular by electric field-activated sintering, which has a cycle time of 2-4 h. A short sintering time is especially important because holding too long at the sintering temperature can cause large grains to grow and recrystallize in the tungsten of the burn zone 14, which can impair its lifetime.
