Patent Grant
8243884
The invention relates to an X-ray anode which comprises a coating which generates X-rays on bombardment with focused electrons and is joined to a support body. The support body comprises a strength-imparting region composed of a material having a strength at 500° C. of greater than 100 MPa.
In the generation of X-rays by bombardment of an anode material with a focused electron beam, about 99% of the radiant energy is converted into heat. The focal spot is therefore subjected to very high specific inputs of energy per unit area which are in the order of magnitude of from 10 to 100 MW/m2. This results in very high focal spot temperatures and in the case of pulsed electron bombardment of rotating X-ray anodes thermomechanical fatigue of the focal track. The limit of possible energy input is given by aging of the focal track combined with a progressive decrease in the dose performance and/or with the loss of the high-voltage stability of the tubes. To slow these effects, optimized removal of heat from the focal spot or the focal track is necessary.
The largest part by far of the radiation sources used in X-ray computer tomography are rotating X-ray anodes in which the energy of the electron beam brought into line focus is distributed around a ring, known as the focal track, by rotation of the anode at high speed. The energy introduced during recording of the image of up to some megajoules is firstly mostly temporarily stored in the X-ray anode and, in particular, given off to the surrounding cooling medium during the pause between recording of images by radiation, in the case of rotational anodes having a sliding groove bearing also by heat conduction into the bearing.
Rotating anodes according to the prior art comprise a coating which generates X-rays on bombardment with focused electrons, for example a coating composed of a tungsten-rhenium alloy, which is applied to a support body, for example a disk composed of a molybdenum-based material. A molybdenum-based material customary for this application is TZM having the composition Mo-0.5% by weight of Ti-0.08% by weight of Zr-0.04% by weight of C. Depending on the field in which the anode is used, a graphite body can be soldered onto the rear side of the metal disk in order to increase the heat storage capacity and radiation of heat. At the initial temperature for operation of the tube (about 40° C.), the thermal conductivities of W-10% by weight of Re, TZM and graphite are about 85, 125 and 135 W/m·K, respectively, but decrease significantly with increasing anode temperature.
In a new generation of X-ray tubes, known as rotary tubes, the anode is fixed as base to a tube which rotates as a whole and the anode is actively cooled on the rear side. The energy balance of the anode is dominated by the removal of heat into the cooling medium. Heat storage plays a minor role. DE 10 2005 039 188 B4 describes an X-ray tube having a cathode and an anode made of a first material, with the anode being provided on its first side facing away from the cathode with, at least in sections, a heat conducting element made of a second material which has a higher thermal conductivity than the first material in order to conduct away heat, where the second material has a thermal conductivity of at least 500 W/mK and the second material is made of titanium-doped graphite.
DE 10 2004 003 370 A1 describes a high-performance anode base for a directly cooled rotary tube, which base comprises a high-temperature-resistant material such as tungsten, molybdenum or a composite of the two materials, with the underside of the anode base in the region of the focal point track being shaped and/or another highly thermally conductive material being introduced or applied in this region in such a way that improved heat removal and thus a lower temperature gradient within this region of the material is obtained. Copper is mentioned as material having a high thermal conductivity.
There have been numerous approaches to improving the heat removal in rotating X-ray anodes in past years. Despite the excellent thermal conductivity of diamond at room temperature, diamond received little attention because of the sharply decreasing thermal conductivity at elevated temperatures and the conversion into graphite at T>1100° C. Thus, U.S. Pat. No. 4,972,449 proposed the use of a diamond layer intercalated between the coating and the support body. However, diamond also has a significantly lower coefficient of expansion than the adjacent materials, as a result of which stresses are induced in the composite body. Furthermore, the classical powder-metallurgical production route for X-ray anodes, namely the powder-metallurgical joining of focal track coating and support body, cannot be employed since the sintering process would lead to conversion of the diamond layer into graphite. X-ray anodes according to U.S. Pat. No. 4,972,449 can therefore only be produced by coating methods, for example CVD processes.
It is therefore an object of the present invention to provide an X-ray anode which has a support body having improved heat removal. A further object is to reduce the stresses in the composite of support body/coating.
The X-ray anode comprises a coating and a support body, with the support body comprising a strength-imparting region and also a region composed of a diamond-metal composite. The diamond-metal composite comprises diamond grains surrounded by binder phase(s). The binder phase(s) comprises/comprise a binder metal, preferably a binder metal based on copper, silver, aluminum and alloys of these materials, and also optionally up to 20% by volume of carbides. Varying the diamond content and binder phase content makes it possible to match the diamond-metal composite to the surrounding materials in terms of thermal conductivity and thermal expansion in such a way that tailored solutions for a wide variety of requirements are possible. A gradated structure of the diamond-metal composite in which the proportion of diamond is highest near the coating and decreases in the direction of the maximum heat flow can be advantageous. In this way, it is possible to achieve minimization of the stresses in the composite caused by different coefficients of thermal expansion of the materials used. Furthermore, diamond powder can be processed with a broad particle size spectrum. Preferred particle sizes are in the range from 50 to 400 μm, ideally from 100 to 250 μm. Apart from natural diamonds, it is also possible to process cheaper synthetic diamonds in this way. The preferred proportion by volume of the diamond grains is from 40 to 90% by volume, and that of the binder phase(s) is from 10 to 60% by volume. A diamond content of from 40 to 90% by volume ensures that the stresses in the composite are reliably reduced to a level which is not critical for use. Particularly advantageous diamond contents and binder phase contents are from 50 to 70% by volume and from 30 to 50% by volume, respectively.
The binder metal preferably comprises from 80 to 100 atom % of at least one matrix metal from the group consisting of Cu, Ag, Al, from 0 to 20 atom % of a metal having a solubility at room temperature in the matrix metal of less than 1 atom % and from 0 to 1 atom % of a metal having a solubility at room temperature in the matrix metal of greater than 1 atom %, balance production-related impurities. Alloying elements having a solubility at room temperature in the matrix metal of less than 1 atom % reduce the thermal conductivity to a small extent and can therefore be present in amounts of up to 20 atom %, while alloying elements having a solubility of greater than 1 atom % are restricted to 1 atom % because of their adverse effect on the thermal conductivity.
Good bonding between the diamond phase and metal phase is necessary in order to ensure a transition from the phonon conductivity of diamond to the electron conductivity of the binder metal. This can be achieved, for example, by formation of a carbidic phase located between the diamond phase and the metal phase. Studies have shown that even carbide films having a thickness of a few layers of atoms significantly improve the thermal conductivity. Carbide-forming elements which have been found to be useful are the metallic elements of groups 4b (Ti, Zr, Hf), 5b (V, Nb, Ta), 6b (Cr, Mo, W) of the Periodic Table and also B and Si. The weak carbide formers Si and B are particularly suitable. When the matrix metal is a carbide-forming element such as aluminum, the addition of further carbide-forming elements can be omitted. Furthermore, it is advantageous for the element forming the carbidic phase also to be present in the binder metal. Preference is given to the carbide-forming elements which have a solubility in the respective matrix metal of less than 1 atom %. If the solubility is greater, the thermal conductivity of the binder metal and thus that of the diamond-metal composite are again reduced. Preferred compositions of the binder metal are aluminum materials comprising from 0.005 to 3 atom % of one or more of the elements V, Nb, Ta, Ti, Zr, Hf, B, Cr, Mo, W and/or comprising from 0.005 to 20 atom % of Si.
On the basis of Ag, these are materials comprising from 0.005 to 5 atom % of one or more elements of the group Zr, Hf and/or from 0.005 to 10 atom % of one or more elements of the group V, Nb, Ta, Cr, Mo, W and/or from 0.005 to 20 atom % of Si. Particularly advantageous properties are achieved using Cu-based matrix metals which are alloyed with from 0.005 to 3 atom % of one or more elements of the group Ti, Zr, Hf and/or from 0.005 to 10 atom % of one or more elements of the group Mo, W, B, V, Nb, Ta, Cr, and/or from 0.005 to 20 atom % of B. Ag alloys with from 0.1 to 12 atom % of Si and Cu alloys with from 0.1 to 14 atom % of boron, balance usually impurities have been found to be particularly advantageous binder metals.
A particularly advantageous effect can also be achieved when coated diamond powders (metallic or carbidic layer) are used.
The use of the diamond-metal composite according to the invention makes it possible to conically widen the heat flow and thus increase the efficiency of active cooling in the case of actively cooled X-ray anodes. Comprehensive experiments on such X-ray anodes have shown that the solution according to the invention reduces the temperature to such an extent that the predicted low thermal conductivity of the diamond-metal composite at elevated use temperatures still does not have a function-limiting effect.
Since the diamond-metal composites according to the invention have limited mechanical properties such as tensile and compressive strength, fracture toughness and fatigue strength and can accordingly not be thermally cycled as free-standing structure under use conditions of X-ray anodes, the support body comprises not only the diamond-metal composite but also a strength-imparting region of a structural material which has a strength at 500° C. of greater than 100 MPa. The diamond-metal composite is protected against interfering deformation or initiation of cracks caused by centrifugal forces or thermomechanical stresses by the structural stiffness of the structural component. This makes it possible to optimize the diamond-metal composite firstly in respect of thermal conductivity, in particular by increasing the proportion of diamond. Secondly, the diamond-metal composite can be matched in terms of its thermal expansion to the structural material. In this way, the functions of the support body can be decoupled from firstly structural strength and rupture strength and secondly heat removal. Particularly suitable structural materials which may be mentioned are Mo, Mo alloys, W, W alloys, W—Cu composites, Mo—Cu composites, particle-reinforced Cu alloys and particle-reinforced Al alloys. As particularly advantageous molybdenum alloys, mention may be made of TZM (Mo-0.5% by weight of titanium-0.08% by weight of zirconium-0.04% by weight of C) and MHC (Mo-1.2% by weight of Hf-0.08% by weight of C).
The region of the diamond-metal composite can directly adjoin the coating. This is possible and appropriate when the temperature on the rear side of the coating can be reduced by the diamond-metal composite to such an extent that no damage to the material, for example melting of the binder phase(s) of the diamond-metal composite, occurs. If this is not the case, it is advantageous for the strength-imparting region composed of a structural material which is stable under use conditions, preferably molybdenum, tungsten or an alloy of these metals, to extend between the diamond-metal composite and the coating.
The diamond-metal composite is preferably arranged under that region of the coating in which heat arises due to the action of the electron beam. In the case of a rotating X-ray anode, this is the ring-shaped focal track. This gives preferred embodiments for the region of the diamond-metal composite, namely regions having an axially symmetrical geometry, for example a disk or a ring. The cross section is preferably approximately rectangular or trapezoidal.
Viewed in the direction of the maximum heat flow, it is also advantageous for the region of the diamond-metal composite to be followed by a further heat-removing region composed of a highly thermally conductive metal which can be given its final shape, in particular in respect of the construction of cooling structures, by means of conventional cutting machining processes. As highly thermally conductive metals, mention may be made of copper, aluminum, silver and alloys thereof. This heat-removing region is also preferably configured as a ring-shaped element or as a disk and firmly bonded to the diamond-metal composite and/or the strength-imparting region.
In the direction of maximum heat flow, the X-ray anode preferably has the following structure at least in the region of the maximum heat stress:
from 0.01 mm to 1 mm coating, from 0 to 4 mm strength-imparting region, from 2 to 15 mm region of the diamond-metal composite and from 0 to 10 mm heat-removing region. A minimum thickness of the coating of 0.01 mm can is necessary for X-ray-physical reasons. At coating thicknesses above 1 mm and/or a thickness of the strength-imparting region above 4 mm, the heat removal is reduced since the W—Re alloys which are customarily used and the structural materials available have a reduced thermal conductivity compared to the diamond-metal composite. It is particularly advantageous for the thickness of the coating to be from 0.2 to 0.4 mm and that of the strength-imparting region to be from 0.5 to 4 mm.
The inventive structure of an X-ray anode can be employed particularly advantageously, in particular, in the case of rotating anodes and when the rotating anode is in turn used as actively cooled bottom of a rotary tube. To achieve sufficient structural strength of the rotating anode, it is found to be useful for the center to be formed by only the structural material. Furthermore, it is advantageous for the region of the diamond-metal composite to be embedded as ring- or disk-shaped element in an appropriate depression of the strength-imparting region of the support body and thus be supported by the latter against mechanical stresses which occur. The structural material is advantageously firmly bonded on one side to the coating and on the other side to the diamond-metal composite. The firmly bonding of the structural component and the diamond-metal composite can advantageously be carried out in situ during the synthesis in suitable recesses in the strength-imparting region of the anode body (for example by pressure infiltration or by hot isostatic pressing). On the other hand, it is possible to synthesize the composite on its own and produce a body of suitable shape therefrom and then firmly bond this body to the structural component, for example by soldering or another known joining process.
To produce the diamond-metal composite, there are a number of available processes in which the binder metal is firmly bonded to the diamond either via the melt phase or via the solid phase. Via the melt phase, the processes advantageously proceed by means of pressure infiltration. Typical infiltration temperatures are about 100° C. above the respective melting point of the binder metal. Reactions with the diamond grain then may form the abovementioned carbide phases enveloping the diamond grains.
A particularly suitable production process comprises the following production steps:
In the production of the bond between diamond grain and binder metal in the solid phase, the bond between the diamond grain and the binder metal is formed by diffusion. The required diffusion paths can be achieved even at temperatures T of ˜0.5-0.8 Tm (Tm=melting point of the binder metal in degrees kelvin) and hold times of a few hours. Suitable processes are, for example, hot pressing and hot isostatic pressing of diamond/metal powder mixtures. Bonding is advantageously improved or accelerated by means of suitable coatings on the diamond grains. In the case of the solid-phase reaction, it is possible, with appropriate pretreatment of the diamond grains and selection of the consolidation conditions, to reduce the contents of additive materials by orders of magnitude or possibly dispense with these entirely, as a result of which the high thermal conductivity of the pure binder phase can largely be retained.
Combinations of the two reaction routes, for example brief passing through the melt phase under super-atmospheric pressure for pore-free backfilling of the diamond bed followed by a solid-state pressure diffusion phase at decreased temperatures, can also be advantageous, in particular for achieving high proportions of diamond in the composite.
A particularly suitable process comprises the production steps:
A further suitable process comprises the production steps:
Further processes, in particular processes for producing composites, e.g. gas-phase infiltration of the binder metal, are in principle also possible for producing the diamond-metal composite.
The invention is illustrated below by means of examples.
To produce the binder phase based on Cu, disks of the high-strength Mo alloy TZM (Mo-0.5% by weight of Ti-0.08% by weight of Zr-0.01 to 0.06% by weight of C) having a diameter of 50 mm and a thickness of 30 mm were produced by a conventional powder-metallurgical route via powder pressing/sintering/forging. A cylindrical depression having a diameter of 30 mm and a depth of 20 mm was machined into these disks. In the following working step, a diamond bed having an average particle diameter (determined by laser light scattering) of 150 μm was introduced in each case into the depression formed in this way and the ring-shaped depression was infiltrated with Cu alloys having the following compositions: Cu-0.5 atom % of B, Cu-2 atom % of B and Cu-8 atom % of B by gas pressure infiltration to produce the diamond-metal composite.
In addition, Nb-coated (layer thickness about 1 μm) diamond powder having an average particle diameter (determined by laser light scattering) of 150 μm was introduced into the ring-shaped depression and pure Cu in particulate form was positioned above it. Identical experiments were carried out using Cr-, Ti- and Mo-coated powders. The gas pressure infiltration was in each case carried out under an Ar protective gas atmosphere at 1100° C. and a gas pressure of 2 bar. The proportion by volume of diamond was about 55% in all specimens. The thermal conductivity of the Cu-diamond composites at 500° C. was in the range from 290 to 350 W/m·K.
To produce the binder phase based on Ag, disks as described in example 1 were produced. To produce the diamond-metal composite, a diamond bed having an average particle diameter (determined by laser light scattering) of 150 μm was in each case introduced into the depression and the ring-shaped depression was infiltrated with Ag alloys of the following compositions: Ag-0.5 atom % of Si, Ag-3 atom % of Si, Ag-11 atom % of Si and Ag-18 atom % of Si by gas pressure infiltration.
In addition, Nb-coated (layer thickness about 1 μm) diamond powder having an average particle diameter (determined by laser light scattering) of 150 μm was introduced into the ring-shaped depression and pure Ag in particulate form was positioned above it. Identical experiments were carried out using Cr-, Ti- and Mo-coated powders. The gas pressure infiltration was in each case carried out under an Ar protective gas atmosphere at 1000° C. and a gas pressure of 2 bar. The proportion by volume of diamond was about 55% in all specimens. The thermal conductivity of the Ag-diamond composites at 500° C. was in the range from 340 to 440 W/m·K.
To produce the binder phase based on Al, disks as described in example 1 were produced. To produce the diamond-metal composite, a diamond bed having an average particle diameter (determined by laser light scattering) of 150 μm was in each case introduced into the depression and the ring-shaped depression was infiltrated with Al materials of the following compositions: Al, Al-3 atom % of Si, Al-12 atom % of Si and Al-15 atom % of Si by gas pressure infiltration.
In addition, Nb-coated (layer thickness about 1 μm) diamond powder having an average particle diameter (determined by laser light scattering) of 150 μm was introduced into the ring-shaped depression and pure Al in particulate form was positioned above it. Identical experiments were carried out using Cr-, Ti- and Mo-coated powders. The gas pressure infiltration was in each case carried out under an Ar protective gas atmosphere at 700° C. and a gas pressure of 2 bar. The proportion by volume of diamond was about 55% in all specimens. The thermal conductivity of the Al-diamond composites at RT was in the range from 400 to 450 W/m·K.
A rotating anode -1- having a structure as shown in
A rotating anode -1- having a structure as shown in
A rotating anode -1- having a structure as shown in
The thermal conductivity measured on the resulting copper-diamond composite was 490 W/m·K (at 22° C.)
A rotating anode -1- having a structure as shown in
As reference anode for comparative tests in X-ray tubes, use was made of an anode for rotary tubes which had the same structure and was made according to the present-day state of the art but was backfilled with copper instead of diamond-metal composite.
All rotating anodes backfilled with diamond-metal composite as described in examples 4 to 7 displayed excellent use behavior when tested in rotary tubes under test conditions more severe compared to the present-day limiting load (increase in the electric power by 20% compared to the reference anodes according to the prior art) and showed a significantly slowed decrease in the X-ray dose over the test time compared to the reference anodes despite the increased load. The reduction in the roughening of the focal track, which is responsible for the decrease in the X-ray dose over the life of the anode, correlated in a good approximation with the relative increase in the thermal conductivity of the diamond-metal composite present in each case. In destructive analyses of the various anodes carried out after the end of the test, no damage to the bond between the strength-imparting component and the diamond-metal composite or within the latter between diamond grains and binder metal was observed.
| Number | Date | Country | Kind |
|---|---|---|---|
| GM583/2007 U | Sep 2007 | AT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/AT2008/000343 | 9/25/2008 | WO | 00 | 3/26/2010 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2009/039545 | 4/2/2009 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 6031285 | Nishibayashi | Feb 2000 | A |
| 6132812 | Rödhammer et al. | Oct 2000 | A |
| 7197119 | Freudenberger et al. | Mar 2007 | B2 |
| Number | Date | Country |
|---|---|---|
| 10128245 | Jun 2002 | DE |
| 102004003370 | Aug 2005 | DE |
| 0874385 | Oct 1998 | EP |
| 0898310 | Feb 1999 | EP |
| 00007735 | Jun 2000 | FR |
| 2002093355 | Mar 2002 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20100316193 A1 | Dec 2010 | US |
