DE3134196A1 - "TARGET FOR A TURNING ANODE TUBE AND METHOD FOR THE PRODUCTION THEREOF" - Google Patents

Description

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Target for a rotating anode x-ray tube and its method of manufacture

. The invention relates to improved targets for rotating anode x-ray tubes, which consist partially or essentially entirely of graphite. ■ ■

The one who works in the field of rotating anode X-ray tubes A person skilled in the art has long ago recognized that graphite targets compared to conventional targets made of metal some Offer advantages because graphite has a low density, high thermal emissivity and good dimensional stability has high temperatures. Conventional targets for x-ray tubes usually consist of a tungsten or molybdenum substrate with a tungsten-rhenium alloy layer, which forms the focal path on which the electron beam of the tube for generating X-rays impinges. the The density of tungsten is nearly ten times that of graphite, and the density of molybdenum is nearly five times high. The most modern X-ray diagnostic procedures require X-ray tube targets with increasingly higher heat capacity, and this requirement has been increased by increasing the size of the

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Volume, i.e. the thickness and diameter of the metal targets, possible extent. The great moment of inertia

• a heavy metal target has made it difficult to move it from zero rotation speed to approximately 10,000 rpm

• 2 to 4 seconds to accelerate, and it is about the same To slow down the period as required for high-capacity x-ray tubes. The great weight that accompanying moment of inertia and the high temperatures of the metal targets make for the bearings of the rotating anode represent significant thermal and mechanical stress.

"*" ·· By the filing date of the present invention, graphite targets were not considered practical in high energy x-ray tubes. One reason is that the graphite is dust or macroparticles releases and occluded gases in the high temperatures and the high one that is in operation X-ray tube, releasing prevailing vacuum. The traffic jam ben often has an electrical breakdown or flashover between the anode and cathode of the X-ray • tube, which significantly limited the tube's maximum operating voltage. There were attempts undertook to solve this problem by coating the graphite surface with metal carbides, typically with molybdenum carbide, Titanium carbide and tantalum carbide, however, such coating techniques have in view the elimination of losses in particulate! Material from the surface proved unsuccessful. aside from that Without exception, the various insulating coatings that were investigated reduced the desired high thermal emissivity of graphite.

As stated earlier, is the focal orbit layer of graphite-based targets usually a high melting point

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Metal, such as tungsten, or, most commonly, a tungsten-rhenium alloy bonded to the substrate. However, it was the case with graphite target X-ray tubes. An all too common experience in the art that the focus web layer delaminates due to the absence of an effective carbon diffusion barrier between the graphite substrate and the tungsten-rhenium focus web layer. One consequence of this is that carbon diffuses into the alloy and forms tungsten carbide in the focal path area of the target. At the operating temperatures of the target, tungsten carbide is brittle and has a coefficient of expansion that is incompatible with graphite and the W-Re alloy. In addition, tungsten carbide (WC and WC) melts at 280 ° C. The temperature in the focal spot exceeds 3100 ° C, and the entire target body or the target substrate can reach a temperature of 1500 ⁰ C in X-ray tubes of high capacity. Cyclic thermal changes increase the likelihood of delamination occurring.

The present invention reduces the problems previously identified with the use of graphite targets and overcomes them in a practical manner by subjecting the graphite substrate or target disk to pyrolytic carbon (PCI) penetration. This treatment results in a deposition of a thin, impenetrable coating _: of pyrolytic carbon on all internal and external surfaces of the graphite target substrate and effectively prevents any adsorbed gases from escaping from the target in the high vacuum of an X-ray tube. Surface ticks or pores or microscopic holes between the graphite crystallites are coated with pyrolytic carbon, which is not only used as a

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Sealing agent, but also to increase the strength of the graphite body on its surface by coating any small cracks, cracks or other possible imperfections. It is very important that the Coating of py ro Iy ti apparent carbon a loss of graphite particles due to cyclic thermal changes, Rubbing, abrasion or any other mechanical effect prevented. The coating reduces the surface roughness not what is desirable, since a rough surface has a better emissivity than a smooth surface owns.

The process of coating with pyrolytic carbon requires that of a graphite substrate It is assumed from the surface of which impurities and adsorbed gases have been removed or driven off. This state can be achieved by heating the substrate to at least 1800 ° C. in an oven that is closed to Start evacuated to a fairly good vacuum to essentially eliminate oxygen, followed by hydrogen is passed through the furnace. This preliminary treatment at a relatively high temperature frees the Target body of adsorbed gases and volatile impurities that are present in graphite.

'The clean substrate is then subjected to the penetration of pyrolytic carbon, which is generally said heating the graphite substrate to about 1000 ° C or higher in the presence of a vaporous hydrocarbon, which is preferably methane mixed with hydrogen at low pressure. The methane can be obtained by using natural gas rich in methane. Other hydrocarbons, such as propylene and acetylene, were used to

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Achievement of py ro Iy ti seem carbon used like this can be found in the patent literature of the prior art. The hydrocarbon reduces to atomic Carbon that deposits on the substrate to form a tough coating. The procedure will take place over several Hours carried out, and the time required will depend on the thickness of the desired pyrolytic layer Depend on carbon.

After the layer of pyrolytic carbon has been deposited, the tungsten-rhenium or a another metal focal path layer can be deposited by any of several known methods fundamentally known process for chemical vapor deposition (CVD) is preferred. In the end it will be the target is heat treated at a very high temperature and cooled, as is usual, before it is placed in the x-ray tube is mounted.

Like the preceding and other more distinctive items of the invention and what characteristic steps of the method and parameters are involved will now be explained in more detail and with reference to the drawings.

Fig. 1 is a cross section of a typical target of a Rotary anode X-ray tube, consisting of a graphite substrate disc or a graphite substrate body with a focal path layer made of metal, which disc is subject to pyrolytic carbon penetration according to the present invention Invention is subject;

Fig. 2 is a cross section of a composite target with a graphite substrate and one with the boundary

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surface-connected refractory metal layer that includes a metal focal path layer deposited thereon;

Fig. 3 is a diagram showing a photomicrograph of a typical graphite material section simulated / before being exposed to pyrolytic carbon penetration has been; and

Fig. 4 shows the same sample cross section as in Fig. 3, after the sample had been subjected to pyrolytic carbon injection.

The typical target 10 of a rotating anode x-ray tube shown in FIG. 1 contains a graphite target body or disk 11, which is improved in that it is according to the present invention is coated with pyrolytic carbon. An annular layer 12 of metal on one Surface of the body or of the substrate 11 forms the Focus orbit. In this particular embodiment, the focal trajectory layer extends axially along the periphery up to an annular groove 13 which provides the physical grip between the metal focal sheet layer and reinforced the graphite substrate.

The target in Fig. 1 is drawn on the fragmentary Axis 14 of an x-ray tube rotor, not shown, is shown mounted. The axis has a cylindrical one Part 15 which extends through a corresponding hole 16 in the target body. The cylindrical part ends in a threaded part 17 at its opposite ends is flat to "fit" into a washer 18 soldered onto the target and a non-round hole which is complementary to the shape of the threaded portion so that the axis is effectively in the threaded portion '

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is wedged. A threaded ring 19 secures the threaded axle to the target body.

Another type of target that is produced by .pyrolytic Carbon can be improved is depicted in FIG. This is one composed of metal and graphite Target which contains a graphite substrate disc 20, which has a disc 21 made of high-melting-point material soldered onto it Has metal. The solder joint is denoted by reference number 22. The disc 21 made of high-melting Metal is usually made from molybdenum, although they can also be made from tungsten, or from alloys of these metals can be. A typical tungsten-rhenium focus orbit. layer 23 is bonded to layer 21 using well known powder metallurgy techniques. That . Target has a centrally stepped hole 24 for receiving. an axis (not shown), which differs from the (not shown) Anode rotor extends forth.

The distinguishing properties of the graphite used by the manufacturers of X-ray tube targets are derived from it depend on how the graphite has been made, and this will be done with the different graphite manufacturers vary. The usually used for the targets of X-ray / Polycrystalline graphite used in tubes consists of graphite crystallites, which are bonded with a binding agent such as coal tar pitch, something that has been graphitized during the graphite production process, are held together »The binder is relatively weak, so that the graphite crystal, as already mentioned above, is not firmly bonded to the surface and tend to be dusty. Pores occur in the binder, which lead to the release of gases in the high vacuum of the X-ray tube lead »It is known.

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that commercially available graphites have different densities exhibit. The pyrolytic carbon penetration (PCI) method described here will be most advantageous applied with a graphite classified as a medium density graphite ranging from 1.75 to 1.85 g / cm is marked. One type of graphite found to be desirable, given as an example, and none A limitation is such a density of 1.82 g / cm, which with a theoretical maximum density for graphite of about 2.26 g / cm2 can be compared.

Graphite target substrates are usually machined by turning a cylindrical graphite rod. Before the PCI treatment, the grain structure of the graphite is reminiscent of the structure in FIG. 3, which is an enlarged Simulated photomicrograph of a piece of graphite chunk. The one shown, but untreated polycrystalline graphite consists of individual graphite crystallites, such as those marked with the letter G, with cavities or pores, which are marked with the letter V, and which are held together by a graphitized carbon binder C. will. There will be some pores within the body of the target and, as shown in Figure 3, some pores on the substrate surface between adjacent graphite crystallites G be present. The graphite crystallites are marked as planar, and they are thin compared to their length, so that they are flat on the surface Edges show what leads to a larger total surface and increases the reactivity for the carbon deposition, which is beneficial when using the PCI coating process disclosed here applies, and what is disadvantageous without using the method according to the invention. The A-

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The individual crystallites G in FIG. 3 naturally have a fairly well structured atomic lattice, which is typical for polycrystalline graphite. It is apparent, however, that where pore occur on the surface, the crystallites are not properly bonded in the graphite mass, in which case they are segregated ^kön-: NEN, especially under cyclic thermal changes at high temperature, and thereby can lead to the dust problem which the PCI treatment method disclosed herein eliminates. ■

¹ ^ ■ Before the graphite target body is subjected to the Eindringyerfahren of pyrolytic carbon, treating the target for disposal and any for expelling impurities and the surface of adsorbed or trapped gases, above. The pretreatment is carried out in a hydrogen furnace known to those skilled in the X-ray tube art, so its construction need not be described. The graphite target body is installed in the furnace in such a way that the entire surface of the target body, including the hole for the rotor axis, is exposed to the furnace atmosphere. The interior of the furnace is pumped to a vacuum on the order of 10 torr to minimize the amount of oxygen inside the furnace. The furnace is then purged by flowing hydrogen and the temperature of the furnace increased. Then hydrogen is passed through the furnace at a constant rate. It was found,; that a discharge rate of 570 to 850 standard liters per hour is satisfactory. The hydrogen pressure in the furnace is a variable that depends on the volume of the furnace chamber and the flow rate. In general, a pressure of

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less than 1 torr prevail. The process of rinsing and the long-term hydrogen supply is performed while the temperature of the target body is preferably kept in the range of 180O ⁰ C to -1900 ⁰ C. However, the decontamination pretreatment could be carried out at temperatures of up to 250O ⁰ C if the furnace allows this. The simultaneous steps of heating and bubbling hydrogen are carried out for a period of at least 1 hour, preferably, but this time can be shortened or lengthened depending on the temperature of the furnace. Operation at temperatures below 1800 ° C is not recommended.

After the pretreatment is finished, the target body is subjected to the pyrolytic carbon penetration or the coating process. This is also done in a conventional gas flow furnace. While the furnace is purged with hydrogen, lowering the temperature of the graphite target body to "in a range of 1000 ⁰ C to 1100 ° C, wherein a temperature is preferably in the range, which is closer to the upper value. The ' optimum temperature can be determined by performing experiments at a few temperatures, as this temperature can differ with some types of graphite. While maintaining the graphite target body at a temperature within the above range, a mixture of methane and hydrogen gas is allowed to flow through the furnace. A ratio of two volumes of methane to one volume of hydrogen is desirable. A gas pressure range of 1 to 3 torr should be maintained, with about 2 torr being preferred. Pressure in excess of 3 Torr is undesirable. Different

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said, the flow rate for methane can be around 1130 standard liters per hour and for hydrogen are 565 standard liters per hour. At the temperature in the furnace the hydrogen and methane react on contact with the hot graphite body in a way that leads to one Reduction of the carbon results, and carbon is deposited on the graphite target substrate. This secluded Carbon also penetrates into those present between the graphite · crystallites on the surface of the target body Pores and coats these crystallites. It goes without saying that all surfaces are made of graphite crystallites,. exposed to the furnace atmosphere are similarly coated with pyrolytic carbon.

The process of pyrolytic carbon penetration just described takes a long time, typically up to about 35 hours, however the length of time will depend on the thickness of the pyrolytic carbon layer which are deemed to be satisfactory for the particular type of graphite from which the target body is made - is found. For a relatively high porosity and a graphite of comparatively medium density, such as one with a density of about 1.82 g / cm, was found after 35 hours of treatment at the above Temperature and the gas flow rates an average, borne coating thickness of about 8 to 10 ym pyrolytic Carbon deposited.

That the use of a graphite of relatively high porosity or medium density is very desirable for the target body, can be seen from Fig. 4, which a simulated photomicrograph of the same area of the target body surface as shown in FIG.

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is formed, however, after being with pyrolytic carbon was coated. The pyrolytic carbon layer is generally designated by the letter P. Typically a graphite crystallite such as that indicated by the reference numeral 35 becomes the thickest coating on a fully exposed surface indicated by the reference numeral 36. The depth of penetration of the pyrolytic carbon depends on the depth and the width of the pores V. So is the thickness of the pyrolytic layer Carbon at the bottom of a pore (denoted by reference numeral 37) is slightly lower than the thickness in the Proximity of the outer opening · of the pore space or the pore. this explains the advantage of using graphite, which has a density slightly lower than that theoretically maximum density is. A higher porosity allows an improved Ingress of py-roly ti schäm carbon.

It was observed that the pyrolytic carbon layer is tightly adherent and anisotropic, and consists of very small graphite crystallites that connect to the base surfaces are oriented parallel to the near surface on which they were deposited. As already explained, the thickness has a maximum on the outer surface and decreases a little with the depth of penetration. With the The above process parameters were a deposition of pyrolytic carbon at a rate of 0.017 µm per hour. The graphite target body with penetrated pyrolytic carbon retain the high thermal emissivity properties of uncoated graphite. Because the carbon is simply the inside of the surface gaps or pores instead of filling it in, it retains a microscopic surface roughness. As is known

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a rough surface has a higher emissivity. For example, there was a spectral thermal emission of 0.82 observed at 2.0 μm wavelength.

After the target body has been coated with pyrolytic carbon, the focus path is made of tungsten-rhenium alloy upset. Generally, the focal web layer 12 will have a thickness of up to about 0.76 mm. The delamination is believed to be the result of sticking to the substrate surface the WoIfram-Rhenium alloy diffusing in, and under Formation of tungsten carbide, which is the bond between the tungsten-rhenium layer and the substrate weakens, reacting Is carbon. It can be seen that when the concentration of rhenium is highest and of tungsten at the highest Interface between the target body and the tungsten-rhenium focal path layer lowest is the tungsten carbide production probably drops to a minimum, since pure rhenium is the best barrier.

There are several ways to get the tungsten-rhenium orbit layer apply, such as vacuum evaporation, ion plating, Sputtering, Flash Evaporation, and Chemical Vapor Deposition (CVD) known to those skilled in the X-ray tube arts are known. The CVD process leads to a good layer and enables a rhenium gradient to be achieved through the layer with a maximum concentration of rhenium at the graphite-to-focal trajectory layer interface. A suitable CVD process is described in US Pat. No. 3,819,971. In general, the CVD process relates a vapor phase mixture of metal halides and hydrogen to apply a tungsten-rhenium alloy entering a high temperature reaction chamber

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and results in hydrogen reduction of the halide when the vapors approach the temperature of the target body. The metal deposits on the target body or substrate and the resulting hydrogen-halogen acid vapors are removed from the furnace chamber. The substrates of either metal or graphite are normally maintained at a temperature in the range from 700 ⁰ C to 1000 ° C. The halides are either the chlorides or fluorides of tungsten and rhenium. By keeping a higher concentration of rhenium in the vapor phase during the previous part of the deposition process, the concentration of rhenium at the interface of the breakpoint web layer and the pyrolytic carbon coated surface can be increased so that a good barrier against the formation of Tungsten carbide is present. As the process proceeds, the amount of tungsten halide is increased until the alloy at the electron beam impact surface of the layer is usually about 3% rhenium and 97% tungsten.

Machining a composite graphite and metal target body of the type shown in Fig. 2, as far as the graphite treatment is concerned, is similar to the method with reference to a target body, consisting essentially only of graphite, shown in FIG has been described. However, in the target of the type of Fig. 2, the pyrolytic carbon becomes first on the whole. Graphite substrate deposited and then only removed from the focal trajectory area for a subsequent deposition the tungsten-rhenium alloy 23 on the focal trajectory enable.

X-ray tubes that use graphite targets, the one

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Layer of pyrolytic carbon as described above contain, show a significant improvement in their high voltage stability at anode-cathode voltages of more than double that applied to flat graphite targets at the same high X-ray tube currents can be. For example, in a test set-up that made it possible to test up to a voltage of 190 00 volts to go, the targets with pyrolytic carbon 'carbon still voltage stable while not coated Graphite targets under the same conditions lead to instability and flashover at around 80,000 volts. "

Although the physical properties and parameters for the implementation of the deposition process of pyrolytic It seems that carbon has been described in detail, this is supposed to be. ' However, the description does not represent a restriction but only serves for explanation, as it goes without saying that that modifications and changes can be made by one skilled in the art. It should therefore be the framework of the invention Application can only be determined by the interpretation of the attached claims.

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Claims (14)

ιαίη 1.28. Aug 1981 Kasoorsf rosso 41 Um./Dr.Sb./he. PATENT ADVOCATE _T. . , _n , ". ^. " _Κε Telephone (0611) 235555 ΤθΙθκ 04-16759 mapat d Postsdiock account. 282420-602 Frankfurt / M. Bank account: 225/0339 Deutsch © Bank AG, Frankfurt / M. 8755-15XR-1800 GENERAL ELECTRIC COMPANY 1 River Road
Schenectady, NY / USA - Claims Target for a rotating anode X-ray tube, characterized in that it is a substrate wafer (11) made of graphite with a coating of pyrolytic carbon includes. 2. Target according to claim 1, characterized in that the density of the graphite is in the range of 1.75 to 1.85 g / cm ³ . 3. Target according to claim 1, characterized in that that the density of the graphite is about 1.82 g / ciu. 4. Target disk according to claim 1, characterized in that it _{comprises an annular layer (12) of WoIfram-Rhenium alloy p} which forms an X-ray focal path and which is bonded to the pyrolytic carbon coating. 5. Target according to claim 1, characterized in that dass.es a disc (21) made of high schiöel pikeperch Metal bonded to a graphite disc (20) to protect the surfaces coated with pyrolytic carbon, bo ti α ο «e unlike the interface of the panes to be exposed to radiant heat, with one opposite the interface Surface of the disc (21) made of refractory metal has an annular layer (23) of tungsten-rhenium alloy bonded to it which forms an X-ray focal path. 6, method of treating one made of graphite •. Target substrate of an X-ray tube to prevent the Release of graphite particles from the substrate surfaces after installation in an X-ray tube, as a result of which the graphite target substrate, which is essentially free from surface impurities and adsorbed gases, is down to a temperature in the range. heated from 100O ⁰ C to 1100 ° C in an essentially oxygen-free atmosphere, and at the same time a mixture of a gaseous hydrocarbon and gaseous. Hydrogen passes over the heated graphite substrate in order to induce a reaction through the heat of the substrate, in which carbon is released from the hydrocarbon as pyrolytic carbon, which is deposited on the exposed surfaces of the substrate, and the gas flow and temperature conditions are determined Maintained until a layer of the desired thickness of '<* "' pyrolytic carbon is applied to the substrate. 7. The method according to claim 6, characterized in that the gaseous hydrocarbon Is methane. 8. Process for treating a graphite crystallite and Target substrate of an X-ray tube containing binder material to prevent the release of particles ■ - / 3 - BATH ORIGINAL Oo c · » aa oo 0 β OO d O * * » e β ο ο ο a » from the substrate surfaces after installation in an X-ray tube, the substrate having very small surface irregularities, representing the pores, and having a density which is less than the theoretical maximum density, characterized in that the graphite substrate is used to drive off adsorbed gases and other impurities by heating the same in a chamber in the absence, pretreated by oxygen to a temperature of at least up to 1800 ° C, and then the substrate to deposit a coating of pyrolytic carbon on its surfaces by heating it in a chamber from which the oxygen has been removed by purging, down to a temperature in the range of at least 100O ⁰ C to 110O ⁰ C, and while simultaneously passing a gaseous mixture of methane and hydrogen over the substrate in order to bring about a reaction by the heat of the substrate, at what carbon from the methane than pyrolytic carbon is released, which deposits on the surfaces of the substrate, and the gas flow and temperature conditions are maintained until a layer of the desired thickness of pyrolytic hydrocarbon is applied to the substrate, treated. 9. The method according to claim 8, characterized in that that the flow rate of hydrogen during the pretreatment is in the range of 570 to 850 standard liters per hour. 10. The method according to claim 8, characterized in that the temperature of the target is maintained between 1800 ° C and 1900 ⁰ C during the pretreatment, ■ ■ BATH ORIGINAL 11. The method according to claim 8, characterized in that that the volume ratio of methane gas to hydrogen gas 2: 1 for treatment for deposition of the pyrolytic carbon. 12. The method according to claim 11, characterized in that that the pressure range for the gaseous methane and hydrogen mixture is at one value between .1 to 3 Torr is held. 13. The method according to claim 8, characterized in that indicates that during the pyrolytic carbon deposition treatment, the flow rate of methane, for example. 1130 standard liters per hour and the flow rate of hydrogen is about 565 standard liters per hour. 14. The method according to any one of claims 8 to 13, characterized in that the treatment . is carried out for at least 35 hours for the deposition of pyrolytic carbon. BATH ORIGINAL