EP2206136B1 - X-ray rotating anode plate, and method for the production thereof - Google Patents

Description

The invention relates to an x-ray rotary anode plate and a method for its production, wherein the x-ray rotary anode plate has a base body. This base body, which carries an applied layer or an inserted body of X-ray active material with the focal path, for example of a tungsten-rhenium alloy with 5 to 10 mass% rhenium, has the function to give the overall structure the necessary strength and the derive heat energy arising in the energetic conversion of electron radiation into X-ray radiation. In connection with the derivation of the heat energy, the material of the base body depends above all on such characteristic values as heat capacity, thermal conductivity, heat transfer and adaptation of the thermal expansion between or from X-ray-active material and base body.

The requirements for the thermal and mechanical load capacity of the X-ray rotary anode plates are constantly increasing. Currently, high power x-ray tubes can experience temperatures of over 3000 ° C in the electronic focal spot. For better energy distribution of the plate rotates at 9,000 min ^-1; are planned speed of 15,000 min ^-1 and more. With the same objective, the diameter of the rotary anode plate is already at 200 mm and planned 300 mm! The strength of the base body material must take this fact into account.

For a long time, X-ray rotary anode plates having a base body made of a molybdenum alloy, such as molybdenum with additions of titanium, zirconium and carbon ("TZM") have been known ( DE 33 03 529 A1 ). At high rotational speeds of the rotary anode plate, this disturbs the high density of the main component molybdenum of 10.2 g / cm ³ in the base body. Such X-ray rotary anode plates can reach a mass of more than 5 kg. Especially in computer tomographs, the rotation of the X-ray rotary anode plate in the X-ray tube is still superimposed by the rotation and translation of the entire system in which the X-ray tube is located, so that uncontrolled centrifugal forces occur in several directions. Not to be underestimated are also the immense material costs of the described metal constructions.

For a given heat capacity, the density and thus also the mass of graphite are lower, which is why also joined X-ray rotary anode plates have become known with a base body made of graphite ( DE 32 38 352 A1 ). Because of the layered microstructure of graphite its strength at high speeds is completely insufficient. This also applies to base bodies made of graphite, which were provided, for example, by means of vacuum plasma spraying with an X-ray-active layer of tungsten-rhenium.

The strengths of the base body both molybdenum-based and graphite are limited, so that there is a real risk of damage or destruction in the above-mentioned mechanical and thermal stresses in use.

Finally, base body for the application mentioned of fiber-reinforced graphite have become known. Carbon fibers are preferably used, wherein, via the spatial arrangement of the fibers or fiber braids, for example, an adaptation of the thermal expansion coefficient of the base body to that of the applied X-ray-active material (US Pat. DE 103 01 069 A1 ) or a high thermal expansion in the radial direction associated with a high thermal conductivity in the axial direction to be achieved ( DE 196 50 061 A1 ). Although the mentioned Kohtenstofffasern in the fiber direction have good thermal conductivity and very good strength properties, but perpendicular to these properties are orders of magnitude worse. In the latter technical solution, this anisotropy has been attempted to be limited by a three-dimensional weaving of the carbon fibers, but the material nevertheless remains anisotropic in the two-digit micrometer range.

A novel material based on carbon are the so-called carbon nanotubes ("carbon nanotubes" or "CNT"), whose technical development from the very beginning quite vividly in the section "Background of the invention" in the translation of the European patent DE 695 32 044 T2 wherein the invention described in this patent with chemically active functional layers on carbon nanotubes an entirely different subject of the invention as the present invention.

While in conventional graphite carbon atoms are arranged in a hexagonal arrangement planar in individual planes, in the carbon nanotubes such hexagonal arrangements are tube-like closed, resulting in excellent mechanical, electrical and thermal properties. As the syllable "nano" expresses, the diameters of these carbon nanotubes are in the nanometer range; Depending on the source, one speaks 0.4 nm to 50 nm or 100 nm.

The bulk density of these carbon nanotubes according to the manufacturer is in the order of 0.15 g / cm ³ , the material density is given as 1.3 g / cm ³ to 1.4 g / cm ³ , which is significantly lower than that of graphite. The strength is called a theoretical value of 45 GPa, which would be about 20 times that of steel and 200 times the above-mentioned base body material TZM. The theoretical thermal conductivity is 6000 W / mK and thus exceeds that of diamond by twice and that of metallic heat conductors by at least one order of magnitude.

Also, applications of carbon nanotubes in the context of X-ray tubes have become known. These are mostly carbon nanotubes in a strictly parallel orientation.

Thus, a cathode for an X-ray tube is known, in which to achieve a cathode surface of small dimensions, the carbon nanotubes are arranged on a plate with a negative potential and emit electrons as emitter to an opposite target of copper (Japanese Patent Kurzfassung 2005166565).

In another cathode ray tube cathode they are arranged behind a control grid and serve to realize a cathode with adjustable emission surface (Japanese Patent Abbreviation 2006086001).

There is also known a technical solution in which on an X-ray-active layer (ie on the electron impact surface) of X-ray anodes a "forest" or a "hair tufts" of vertical, parallel, good heat conductive carbon fibers is arranged, which are not expressly carbon nanotubes ( US 5,943,389 ). The purpose of this arrangement is to dissipate heat through these carbon fibers in addition to heat dissipation through the base body.

Furthermore, an X-ray anode is known, on the anode impact surface of which carbon nanotubes are preferably arranged in the form of a tissue in order to suppress the formation of secondary electrons and the formation of a plasma or the release of neutral gases ( WO 03/043036 A1 ).

Finally, as prior art for the present invention, base bodies for X-ray rotary anodes are known in which carbon fibers are preferably carbon nanotubes in copper ( DE 102005039187 ) or titanium ( DE 102005039188 ) are incorporated to improve heat dissipation. Copper has the disadvantage that its melting point is too low for high heat dissipation performance; Titanium, like copper, has the disadvantage that it tends to carbide at the operating temperature with the carbon present.

Recently, carbon nanoparticles with graphite structure and substantially spherical shape having a mean grain size of, for example 55 nm have become known (company publication of the company Auer-Remy GmbH, Hamburg "Nanopowders", position "C 1249YD 7440-44-0"). In addition to advantageous in the present context properties of carbon nanoparticles prepares for spherical particles with their dimensions in all axial dimensions the achievement of a substantially isotropic properties of the base body securing spatial distribution naturally less procedural difficulties in the preparation of raw materials for the shaping of the base body than in carbon nanotubes their axial extent.

Some of the carbides and nitrides which play a role in the present invention to increase strength already exist in X-ray rotary anodes used, but in a completely different function and without statements to the grain size.

Thus, among other compounds, carbides and nitrides of tantalum, niobium, molybdenum and tungsten have been used for erosion resistant, liquid metal lubricated mating couples between the rotating anode shaft and its bearings ( DE 69 121 504 T2 ).

Tantalum carbide, among other compounds, has been proposed as the backside coating of the rotating anode plate to improve the heat radiation ( DE 2 805 154 ).

Finally, among other compounds, molybdenum carbide and tungsten carbide have become known in arrangements having a plurality of layers for adjusting the coefficient of thermal expansion between the X-ray active layer and the base body (FIG. DE 10 2005 015 920 ).

The invention has for its object to provide an X-ray Rotary anode plate with a base body, which the above-mentioned requirements with respect to the temperature of the focal spot and the target speeds of X-ray machines Rotanodentellern by a lower mass, a corresponding thermal conductivity and a sufficient high temperature strength at least the same or to meet the lowest possible material costs of the base body and thereby able to remedy the deficiencies of the prior art. The object of the invention is furthermore to provide a rational production method for such an X-ray rotary anode plate consisting of the base body with X-ray active layer with necessary or advantageous intermediate layers between the two.

This object is solved by the invention described in the claims.

The preparation of the base body according to the invention with carbon nanotubes or of high-performance graphite and / or fiber graphite materials containing such carbon nanotubes is conventional and also with the aid of the latest powder technologies it is important to ensure that the structure of the carbon nanotubes is not destroyed.

A fundamental prerequisite for achieving the desired effects with respect to high-temperature strength, thermal conductivity and thermal expansion is the quasi-homogeneous distribution of the carbon nanotubes in the component in order to obtain a base body that is essentially isotropic in the submaxoscopic region, ie. an anisotropy degree of, for example, <1.2 (ratio of the maximum value to the minimum value when measured in the three spatial dimensions) in terms of strength, thermal conductivity and thermal expansion to achieve. Particularly favorable is a slightly angled shape of the individual carbon nanotubes.

According to the invention, the macroflexibility of the base material may be enhanced by the addition of high strength compounds such as oxides, nitrides, borides, carbides, silicides of tantalum, niobium, chromium, silicon, molybdenum, hafnium, boron and / or tungsten or mixtures thereof and fibers of these materials be increased. Also, mixtures of these compounds are conceivable. In the interest of the bond strength between the base body and X-ray active layer, the proportion of these substances in the axial direction can be varied, it is also advantageous if the optionally present proportion of graphite or graphite fibers to the X-ray active layer in favor of the proportion of Kohlenstoffnano tube and said strength-increasing substances decreases.

The base body can be provided with the X-ray-active layer according to the usual coating method, wherein for controlling the harmful carbon diffusion per se known diffusion barrier layers of rhenium, molybdenum, tantalum, niobium, zirconium, titanium or compounds and combinations of these metals as well as in more advantageous Embodiment of the invention, a binding layer, for example, by introducing rhenium or rhenium compounds, or carbides in the surface region of the base body, are arranged.

Of advantage may also be the introduction of a leveling layer, preferably of molybdenum or a molybdenum alloy between the diffusion barrier layer and the X-ray active layer. It serves, for example, to compensate for differences between the two aforementioned layers in terms of thermal expansion and / or ductility.

A particularly quickly realizable technical solution is the joining of a conventional X-ray rotating anode plate made of metal with the base body, the plate can be performed much thinner because of the better strength properties of the base body according to the invention as in the prior art, which helps to save mass and cost, in principle It should be noted that the reduced mass is advantageous not only in terms of material costs but also because of the lower centrifugal forces.

With the use of carbon nanotubes, a certain nanoporosity is normally to be expected, so that processing in the vacuum range with a residual atmosphere of protective gases or else the use of capped carbon nanotubes are advantageous.

The advantageous economic effects of the invention as a result of the innovative materials used are presented below as compared to a base body of TZM as prior art. Carbon nanotubes are industrially produced on a small scale. The price level is around 150 EUR / kg, with a comparison with a metallic base body according to the prior art, not only the lower kilogram price but also the lower density to book. The approximate results for a 200 mm X-ray rotary anode plate: TZM: 5.0 kg corresponding to 1000 EUR material costs Carbon nanotubes: 0.7 kg corresponding to 100 EUR material costs

The advantageous effects of the invention as a result of the production method according to the invention lie in the saving of expensive coating or confectioning process and the necessary investments, in minimizing the use of materials and in an increase in strength of the entire component.

The invention will be explained in more detail below with reference to six exemplary embodiments.

Fig. 1:

einen Röntgen-Drehanodenteller entsprechend Ausführungsbeispiel 1,

Fig. 2:

einen Röntgen-Drehanodenteller entsprechend den Ausführungsbeispielen 2 und 3,

Fig. 3:

einen Röntgen-Drehanodenteller entsprechend Ausführungsbeispiel 4 und

Fig. 4:

einen Röntgen-Drehanodenteller entsprechend Ausführungsbeispiel 5.

Show it:

Fig. 1:

an X-ray rotary anode plate according to Embodiment 1,

Fig. 2:

an X-ray rotary anode plate according to embodiments 2 and 3,

3:

an X-ray rotary anode plate according to Embodiment 4 and

4:

an X-ray rotary anode plate according to Embodiment 5.

Embodiment 1

The in Fig. 1 X-ray rotary anode plate shown in section consists of a base body 1.1 with 60% by mass of carbon nanotubes and 40% by mass of nano-graphite powder particles, to which a diffusion barrier layer 3.1 made of tungsten-rhenium tantalum, known per se, is used as a bonding layer 4.1 by vacuum plasma spraying. and the X-ray active layer 2.1 are applied. The diameter of the X-ray rotary anode plate is 120 mm, its thickness 15 mm.

The base body 1.1 is manufactured by the conventional methods of powder metallurgy and graphite processing by mixing the powders, pressing and heat treatment, under circumstances using the hot pressing method, in dimensions close to the final shape and finished by cutting shaping.

Commercially available uncoated carbon nanotubes and fine-grained nano-graphite powder particles of high purity are used, the former differing only slightly in length and diameter and should be shorter than 10 nm on average. Your longitudinal axis should deviate from the straight line if possible

After the vacuum plasma spraying, a diffusion bonding between the base body 1.1 and the layers is formed by a suitable heat treatment and the X-ray rotary anode plate is finished by the usual methods.

Embodiment 2:

The in Fig. 2 In contrast to exemplary embodiment 1, an X-ray rotary anode plate shown in section has a base body 1.2, which consists of commercially available carbon nanotubes with an addition of 20% by volume of tungsten carbide.

In the base body 1.2 is a depression corresponding to the course of the isotherms in the operating state according to patent application no. 10 2005 000 784 A1 introduced, which is filled by the X-ray active layer 2.2 of tungsten with 5 mass% rhenium. The diffusion barrier layer 3.2, which is also the bonding layer 4.2, consists in this case of tantalum and has a thickness of 0.2 mm, is adapted to the shape of the depression and, like the X-ray active layer 2.2, corresponds in function to the corresponding layers 2.1; 3.1 and 4.1 of embodiment 1. The same applies to the geometric dimensions of the X-ray rotary anode plate.

The preparation of the complete component with all the layers mentioned above takes place in this case after filling in a suitable mold in one operation by hot pressing by pulse current at 2400 ° C at a pressure of 40 MPa in a residual gas atmosphere of argon with a slight hydrogen content at a residual pressure of about 2 Pa.

The final production takes place according to the usual procedures.

Embodiment 3

An improvement in quality of the X-ray rotary anode plate produced according to Embodiment 2 is achieved as follows: The layer 2.2 is adjusted to the composition tungsten with 1 mass% rhenium. After prefabrication of the component, the disk bevel is cleanly ground and applied by vacuum plasma spraying an X-ray active layer of the composition tungsten with 5% by mass of rhenium with a thickness of 200 microns.

The final production takes place according to the usual procedures.

Embodiment 4

The in Fig. 3 X-ray rotary anode plate shown in section represents technologically and in terms of the manufacturing process, a transitional shape between a conventional X-ray rotary anode plate made of metal and the inventive solution, of course, all the necessary features of the invention are realized.

The base body 1.3 beveled at the outer edge towards the axis corresponds in composition and technologically to the base body 1.1 of embodiment 1. With this base body 1.3 is a finished metal body 5 made of a molybdenum-TZM alloy with an X-ray active layer 2.3 by diffusion bonding to the surface. 6 connected.

The excellent strength properties of the base body 1.3 with a content of carbon nanotubes make it possible despite the intended high speeds and operating temperatures, the metal body 5 much thinner and easier to perform than in X-ray Drehanodentellem metal with a graphite base body after State of the art. The diameter of the X-ray rotary anode plate is, as in the embodiments 1 and 2, also 120 mm; the total thickness is different than in the aforementioned embodiments, a total of 16 mm, namely 6 mm of the metal body 5 plus 10 mm of the base body 1.3.

Embodiment 5:

The FIG. 4 shows a layered base body of an X-ray rotary anode plate according to claims 4 and 5, wherein the cross-sectional shape of the bonding layer 4.4 and the X-ray active layer 2.4, as in the above embodiments 2 and 3 according to claim 19, in turn, an isotherm of the temperature distribution in the vicinity of the X-ray Layer follows during operation.

From bottom to top, i. towards the X-ray-active layer, the layers of the base body have the following composition:

Lower layer 1.41: single-walled carbon nanotubes and on average 30% silicon carbide by volume, the content of which within this layer preferably increasing from top to bottom.

• 142a:100 Masse-%t Nano-Graphitpulverteilchen,
• 142b: 50 % Masse-% Nano-Graphitpulverteilchen und 50 Masse-% einwandige Kohlenstoffnanoröhrchen,
• 142c: massive, vorgeformte Platte aus röntgentauglichem Graphit, welche zur Verbesserung der Wärmeleitfähigkeit des Basiskörpers 1 beiträgt.

For the intermediate layer 1.42, three alternative variants are provided:

• 142a: 100% by mass of nano-graphite powder particles,
• 142b: 50% mass% of nano-graphite powder particles and 50 mass% of single-walled carbon nanotubes,
• 142 c: massive, preformed plate made of X-ray-compatible graphite, which contributes to improving the thermal conductivity of the base body 1.

Upper layer 143: Single-walled carbon nanotubes with an average of 20% by volume of tungsten carbide, the content of which within this layer preferably increasing from top to bottom.

Between these three layers 141, 142 and 143 are each 80 micron thick base body bonding layers of molybdenum carbide.

In a bevel of the upper layer 143 of 10 ° with respect to horizontal, a recess corresponding to the mentioned course of an isotherm is incorporated in the region of the focal path. There are in the direction from bottom to top: a diffusion barrier layer 3.4 of 100 microns thickness of 40% by volume tantalum carbide and 60 volume 5 niobium carbide. Thereupon a bonding layer 4.4 made of molybdenum with 12% by weight of tungsten and finally the depression up to the level of the bevel filling up the half-active layer 2.4 made of tungsten with 6% by mass rhenium is arranged approximately to half the depth of the depression.

The typical dimensions of such an X-ray rotary anode plate are for example: diameter: 120 mm, thickness of the layers 1.41 and 1.42 per 6 mm and the layer 1.43 8 mm. The width of the depression with the layers 3.4 and 4.4 and the X-ray active layer 2.4 is 35 mm and its maximum depth measured from the beveled surface of 6 mm.

List of used reference numbers

1.1; 1.2; 1.3

base body

1:41; 1.42 (a, b, c); 1:43

Layers of the base body

2.1; 2.2; 2.3, 2.4

X-ray active layer

3.1; 3.2; 3.4

Diffusion barrier layer

4.1; 4.2; 4.4

bonding layer

5

metal body

6

solder layer

7

Base body bonding layer

Claims (25)

1. An X-ray rotating anode plate having a base, characterized in that this base comprising carbon nanotubes in a quasi-homogeneous spatial distribution, ensuring substantially isotropic properties of the base in the submacroscopic range.
2. The X-ray rotating anode plate according to claim 1, characterized in that the base comprises 10 mass % to 90 mass % of carbon nanotubes.
3. The X-ray rotating anode plate according to claim 2, characterized in that the base comprises 50 mass % to 70 mass % of carbon nanotubes.
4. An X-ray rotating anode plate according to any one of the preceding claims, characterized in that the proportion of carbon nanotubes in the axial direction varies steadily and/or in layers.
5. An X-ray rotating anode plate according to any one of the preceding claims, characterized in that, in order to increase stability and improve heat conductivity the following compounds are further added to the base: oxides, nitrides, borides, carbides, silicides of tantalum, niobium, chromium, silicon, molybdenum, hafnium, boron and/or tungsten, or mixtures thereof.
6. The X-ray rotating anode plate according to claim 5, characterized in that the compounds are present in particle sizes in the nanometer range.
7. The X-ray rotating anode plate according to claim 6, characterized in that the average particle size of the compounds is 40 nm to 200 nm.
8. An X-ray rotating anode plate according to any one of claims 5 to 7, characterized in that the compo umds constitute 4% by volume to 80% by volume of the base.
9. The X-ray rotating anode plate according to claim 8, characterized in that the compounds constitute 20% by volume to 40% by volume of the base.
10. An X-ray rotating anode plate according to any one of claims 5 to 9, characterized in that the proportion of the additives according to claim 5 varies steadily or in layers in the axial direction.
11. An X-ray rotating anode plate according to any one of the preceding claims, characterized in that the axial direction of the individual carbon nanotubes deviates from the straight line.
12. The X-ray rotating anode plate according to claim 11, characterized in that the axial direction of the individual carbon nanotubes has an angled course.
13. An X-ray rotating anode plate according to any one of claims 1 to 12 comprising carbon nanotubes, characterized in that the carbon nanotubes have multiple walls.
14. An X-ray rotating anode plate according to any one of the preceding claims, characterized in that the lengths and diameters of the carbon nanotubes do not differ by more than a factor of 10.
15. The X-ray rotating anode plate according to claim 14, characterized in that the lengths and diameters of the carbon nanotubes do not differ by more than a factor of 3.
16. The X-ray rotating anode plate according to claim 15, characterized in that the carbon nanotubes have a spherical shape.
17. An X-ray rotating anode plate according to any one of the preceding claims, characterized in that a diffusion barrier layer (3.1, 3.2, 3.4) made of rhenium, molybdenum, tantalum, niobium, chromium, tungsten, zirconium, or combinations of these metals or compounds thereof, is disposed between the base (1.1, 1.2) and an X-ray active layer (2.1, 2.2, 2.4), the diffusion barrier layer functioning both as a bonding layer (4.1, 4.2, 4.4) and compensating layer.
18. The X-ray rotating anode plate according to claim 17, characterized in that the diffusion barrier layer (3.1, 3.2, 3.3) on the X-ray rotating anode plate extends beyond the X-ray active layer.
19. An X-ray rotating anode plate according to any one of the claims 17 or 18, characterized in that the X-ray active layer (2.2, 2.4) fills a groove, the cross-sectional shape of which corresponds to an isotherm of the temperature distribution in the area surrounding the X-ray active region in the operational state.
20. An X-ray rotating anode plate according to any one of claims 17 to 19, characterized in that the qualitative and/or quantitative composition of the layers in the axial direction varies steadily or in layers.
21. An X-ray rotating anode plate according to any one of the claims 1 to 16, characterized in that the base (1.3) is joined to a metal body (5) formed by a conventionally know method carries an X-ray active layer (2.3).
22. A method for producing an X-ray rotating anode plate according to any one of claims 17 to 21, characterized in that the starting materials of the base and of the X-ray active layer are brought into an near net shape in a pressing mold in one operation, by simultaneously applying pressure, temperature and temporally varied electric currents, so as to be compressed to the final density, forming high-strength diffusion bonds between these starting materials.
23. The method according to claim 22, characterized in that the starting materials also include those for a bonding layer and/or a diffusion barrier between those for the base and the X-ray active layer, in addition to other intermediate layers.
24. A method according to any one of the claims 22 or 23, characterized in that the starting materials for the base layer according to claim 5 are introduced into a pressing mold as a preshaped body.
25. A method according to any one of claims 22 to 24, characterized in that, after finishing the near net shape, the plate bevel is machined, whereafter an X-ray active layer is applied using suitable methods.

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