Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J35/00—X-ray tubes
  • H01J35/02—Details
  • H01J35/04—Electrodes ; Mutual position thereof; Constructional adaptations therefor
  • H01J35/08—Anodes; Anti cathodes
  • H01J35/10—Rotary anodes; Arrangements for rotating anodes; Cooling rotary anodes
  • H01J35/105—Cooling of rotating anodes, e.g. heat emitting layers or structures

Definitions

• the invention relates to an X-ray anode, in particular a rotating anode, with high heat emissivity, with a base body made of high-melting metal or its alloys and a focal spot or focal path region made of high-melting metal which may differ from the base body, the X-ray anode at least on parts of the surface outside the focal path essentially has the metals titanium, zirconium and / or aluminum-containing oxidic coating.
• X-ray tube anodes only emit a fraction of the radiated energy in the form of X-rays. The rest is transferred to heat and has to leave the anode in the form of heat radiation. It has therefore been known for many years to improve the heat emissivity of X-ray anodes from high-melting metals by means of an oxidic coating (AT 337 314, DE-OS 22 01 979, DE-OS 24 43 354). These prior publications claim to increase the adhesion of the oxide layer on the surface of the base metal compared to the prior art and to increase the thermal emissivity of the anode surface by means of different oxide materials and production processes. It has been shown that the performance of layers produced in this way is limited in view of the continuously increasing requirements for X-ray anodes with regard to layer aging, heat radiation capability and the resistance to degassing (avoidance of electrical flashovers).
• the EU A2 0 172 491 describes in a further development an X-ray anode made of a molybdenum alloy with an oxide coating from a mixture of 40% - 70% titanium oxide, the rest of the stabilizing oxides from the group ZrO2, HfO, MgO, CeO2, La2O3 and SrO.
• this prior publication has in particular the task of melting the oxides into smooth, shiny, shimmering layers by means of economical processes.
• EU A2 0 244 776 relates essentially to the same subject of the invention.
• the invention relates to the pretreatment of the oxidic material before application to the X-ray anode using conventional spraying techniques.
• a mixture of 77% -85% titanium dioxide with 15-23% by weight calcium oxide is processed in a first process step into a powder with a homogeneous phase and then optionally mixed with other oxide powders by known spraying methods.
• Plasma spraying, sputtering processes, chemical and physical deposition processes from the gas phase or also the electron beam process are mentioned as coating processes for the oxide coating on the X-ray anodes made of refractory metals.
• An X-ray anode made of refractory metal is usually subjected to a degassing annealing at the end of the manufacturing process.
• the degassing annealing of the anode serves to avoid gas leaks and consequently highly undesirable flashovers between the electrodes when they are used in an X-ray tube in a high vacuum.
• the inventive teaching of this prior publication includes an advantageous coordination of the material composition of the oxide layer with regard to the annealing treatment after the coating of the X-ray anodes.
• These degassing anneals simultaneously serve for the final formation and melting of the oxide phase, ie the conversion into a state which cannot be achieved by an oxide application process such as the plasma spraying process alone.
• the layer composition according to the prior publication and the processes for their production do not meet the requirements sufficiently. Rather, when annealing the oxide layers according to this prior publication, there is the risk that at an annealing temperature at which the oxides melt into smooth, well-adhering layers, these are already so thin that the contour between coated and uncoated parts of the X-ray anode surface is undesirable, in Area of the focal path disappears to an intolerable extent. In addition, such oxide layers have an annoying gas phase formation at the required annealing temperatures.
• the object of the present invention is therefore to give the oxidic surface layer such a composition that, on its one hand, at least maintain the good adhesion properties between the oxide layer and the substrate and the good thermal emissivity properties of the layer which have been achievable so far when it is produced by customary application methods, including annealing treatment not be surpassed.
• the structural structure and the composition of the oxide layer should allow easier technical handling in the production of the layer, in particular with regard to smooth melting without annoying evaporation and unwanted flow of the oxide during the annealing treatment of the X-ray anode.
• the object is achieved in that the oxidic coating on the X-ray anode contains 1-20% by weight of silicon oxide and is a homogeneously melted phase.
• the oxide layer according to the invention applied to an X-ray anode made of high-melting metals has excellent adhesion, smooth surfaces and a high thermal heat coefficient ⁇ 0.80.
• the oxidic layer has the decisive advantage over the prior art that it is less liquid under otherwise comparable conditions during the required annealing treatment of the anode. that is, the melt toughness is higher in comparison with similar oxide layers without the addition of silicon oxide when melting during the annealing treatment.
• the contours between surface parts with and without oxide coating do not melt. There is only a comparatively small amount of evaporation and unwanted precipitation of oxide components on uncoated surface parts during the annealing process.
• layers with a desired surface roughness of approx. 20 ⁇ m (R T ) and the appearance of an orange peel can be achieved.
• X-ray rotary anodes are currently usually made from the refractory metals tungsten, molybdenum or molybdenum alloys, in particular the carbon-containing alloy TZM.
• the oxidic coating has the previously preferred oxide components zirconium oxide, calcium oxide and / or titanium oxide, for example in a ratio of 70: 10: 20.
• the calcium oxide can be partially or completely replaced by other stabilizing oxides known for such applications and can also be supplemented by small amounts of other, thermally stable compounds, such as borides and / or nitrides.
• the aforementioned oxide mixture can contain up to 10% by weight of aluminum oxide, primarily for reducing or controlling the melting temperature.
• the remaining part of the composition of the oxidic coating is silicon oxide according to the invention with a weight fraction of 1-20%, preferably 4-7%.
• the thickness of the oxide layer can vary between a few and a few thousand micrometers, depending on the deposition process.
• the known PVD and CVD processes in particular plasma CVD processes and sputtering processes, as well as flame spraying, plasma spraying and electron beam processes have proven themselves as deposition processes.
• the homogeneous phase in the oxidic coating is to be understood as a finely divided oxide mixture.
• the desired oxide layer structure and surface roughness can be achieved with good adhesion between the layer and the base material by means of annealing at temperatures between 1550 ° C and 1680 ° C and a glow time between 30 minutes and 1.5 hours achieve advantageous.
• the evaporation of oxide components begins to become noticeable at temperatures above approx. 1550 ° C. In the worst cases, it is therefore advisable to cover the focal path area during the annealing treatment or to perform a final cleaning, for example also grinding treatment of the focal path after the annealing treatment.
• the molybdenum alloy TZM with low carbon content tends to release carbon at temperatures above 1550 °.
• the released carbon forms volatile CO or CO2 in the oxide layer with the oxygen components of the oxide and causes the layer to age prematurely.
• An X-ray rotating anode consisting of the alloy Mo 5% by weight W has an approx. 2 mm thick W-Re layer in the focal path area.
• this anode surface is provided with an oxide layer according to the invention.
• a completely sintered and mechanically formed X-ray anode on the back of the anode to be coated is cleaned and roughened by means of sandblasting and, if possible, immediately afterwards coated with oxide powder under the usual process conditions by plasma spraying.
• the applied oxide powder has the following composition: 89% by weight of an oxide mixture of 72% by weight ZrO2, 8% by weight CaO, 20% by weight TiO2, further 5% by weight Al2O3 and 6% by weight Si-O2.
• the rotating anode coated in this way must be subjected to an annealing treatment in order to make it usable for use in X-ray tubes.
• the rotating anode both the base material and the layer material, is largely freed of gas inclusions and of contaminants which are volatile at higher temperatures, in order to prevent electrical flashovers as a result of the release of gas inclusions when the rotating anode is subsequently used in the high-vacuum X-ray tube.
• the degassing annealing takes place within a narrow temperature and time range, matched to the anode base material, in order to avoid undesired structural changes in the base material.
• the applied layer must also be treated within a very specific temperature and time range in order to achieve melting in the desired homogeneous phase and with a slightly nubbed surface structure (orange peel layer).
• annealing takes place at 1620 ° C for 65 minutes.
• the melted layer has the desired degree of blackening and the desired surface structure (orange peel).
• There is no uncontrolled flow of the melting oxide layer especially not in the transition area between coated and uncoated parts of the rotating anode surface. Insofar as gaseous oxides evaporate from the layer surface during the annealing process, these do not form a disruptive layer covering in the originally uncoated focal path area of the rotating anode.
• the rotating anode was then tested in an X-ray tube arrangement under practical conditions. It ran smoothly for several days within the required limit load.
• An X-ray rotating anode consisting of the alloy TZM has an approx. 2 mm thick W / Re layer in the focal path area. To increase the heat radiation capability, this anode surface is provided with an oxide layer according to the invention.
• a completely sintered and mechanically shaped X-ray anode is cleaned and roughened by means of sandblasting and, if possible, coated immediately afterwards under the usual process conditions by means of plasma spraying outside the focal path area.
• a molybdenum layer acting as a carbon barrier is applied and subjected to a reduction annealing in hydrogen at 1350 ° C. for 2 hours.
• a first oxide coating based on aluminum oxide-titanium oxide It is only this oxide layer that enables the blackening oxidic coating to be melted in the required quality.
• the final oxidic coating has the composition: 94% by weight of an oxide mixture of 72% zirconium oxide, 8% calcium oxide, 20% titanium oxide, and also 6% silicon oxide.
• the rotating anode coated in this way must be subjected to an annealing treatment in accordance with Example 1.
• Example 1 The rotating anode was then tested according to Example 1 in an X-ray tube test arrangement under practical conditions. There it ran trouble-free within the required limit load.

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• 1989
  • 1989-10-02 AT AT0227689A patent/AT394643B/de not_active IP Right Cessation
• 1990
  • 1990-09-27 EP EP90202558A patent/EP0421521B1/fr not_active Expired - Lifetime
  • 1990-09-27 DE DE59007689T patent/DE59007689D1/de not_active Expired - Fee Related
  • 1990-10-01 JP JP2263696A patent/JPH03127439A/ja active Pending
  • 1990-10-02 US US07/591,624 patent/US5157705A/en not_active Expired - Fee Related

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