FR2593325A1 - Graphite rotating anode for X-ray tube - Google Patents

Abstract

The present invention relates to a graphite rotating anode for an X-ray tube, including a base body 2 of graphite carrying a target material 12, 15 intended to produce X-radiation. The quality of the bond between the target material 12, 15 and the graphite base body 2 is considerably improved, by comparison with the prior art, through the use of a silicon carbide layer 11.

Description

ROTATING GRAPHITE ANODE FOR RADIOGENIC TUBE
The present invention relates to a rotating graphite anode for an X-ray tube, and particularly relates to means intended to improve the quality of the connections between the graphite and the target material used to produce X-radiation.

 Graphite is commonly used in rotating anodes for modern X-ray tubes, where its function is essentially to increase the thermal radiation of the anode to ensure its cooling. This function is particularly important since the evacuation of the heat, accumulated by the anode following a charge, is generally carried out only by radiation.

 In X-ray tubes, the X-rays produced result from the bombardment, by an electron beam, of a material carried by the anode and having a high atomic number in order to favor the emission of X photons. of photons X is accompanied by a strong emission of heat, because the energy yield of the X-rays produced, i.e. the energy ratio of the X photons to the energy of the incident electrons, is of the around 1 96, the rest is transformed into heat. Also, the material subjected to electronic bombardment must also be a refractory material, having a high melting point and a good thermal conductivity. Among the materials which have these characteristics to varying degrees, we find for example tungsten, tantalum, tungsten being particularly in common use; these refractory materials with high atomic number, with high melting point and good thermal conductivity being called in the following description "target materials".

 The flow of incident electrons is focused on a small surface of the anode called focal point, this surface becoming the source of X-radiation. The temperature of the focal point is a first temperature limit of the anode, which in particular limits the instantaneous charge which can be applied to the anode.

 With the particular aim of increasing the limit of the instantaneous load, the manufacturers used rotating anodes.

The rotating anode has the shape of a disc, rotated on itself around its axis of symmetry. This makes it possible to scroll the target material under the electron beam, and the rotation of the disc generates on the target material a focal ring which itself represents a second temperature limit. The heat is distributed along the focal ring, and encompasses the entire anode disc before dissipating by radiation.

 Initially, the rotating anodes were homogeneous, that is to say formed from a single material capable of constituting a target material. One of the disadvantages of such anodes results from their high weight which makes their assembly fragile mechanically and which prevents them from being used at a high speed of rotation, thereby reducing the advantage provided by the rotation of the anode.

 This has led to the production of rotating anode discs of the composite type, comprising a base body covered with a target material, the base body being in a material lighter than the target material. Mention may be made in this context of the anode discs having a basic molybdenum body covered with a tungsten target material.

 To increase the heat capacity of the composite type anodes, they have been provided with graphite; a particularly interesting application being represented by a rotating anode comprising a base body directly in graphite covered with tungsten.

 The use of graphite as a refractory support is of great interest compared to other refractories, in particular because of its low density and its high thermal radiation power, its radiation coefficient approaching the black body; it also has excellent thermal conductivity.

 One of the problems posed by the use of graphite lies in the bond with tungsten. This defect, in addition to causing mechanical fragility of the rotating anode, impairs the transfer of heat between the target material and the graphite. The area of the target material subjected to electronic bombardment, that is to say the focal point and consequently the focal ring, represents the heat source, and it is particularly important to have a minimum thermal resistance between this heat source and graphite, which graphite has the function of removing this heat by radiation.

 The application of tungsten to graphite also leads to the formation of tungsten carbide which tends to increase the poor adhesion of tungsten to graphite, and to generate areas of greater thermal resistance.

 The target material, in tungsten for example, can be formed according to a layer obtained by a conventional deposition process, such as for example, gas phase deposition, by electrolysis, sputtering, etc.

 The target material can also be made up of a solid piece, joined to a basic graphite body by brazing; the connecting element between the target material, in tungsten for example, and the graphite then being formed by a solder element such as tantalum or zirconium.

 A significant drawback in this latter configuration is due to the difference between the coefficients of expansion of the solder element and of the graphite. Since graphite has a low mechanical resistance, the mechanical stresses between the brazing element and the graphite, due to temperature variations, tend to break the graphite in contact with the brazing element and thus cause irreversible degradation of the bond. tungsten graphite.

 In the case where the tungstem or target material is deposited on the graphite base body in a layer, by one of the above-mentioned deposition methods, the connecting element consists of a layer previously deposited on the graphite , called the intermediate layer; this intermediate layer consists of another refractory metal, such as rhenium in which the carbon has a very low solubility and thus prevents2 at least up to a certain temperature, (of the order of 15500C), the carburetion of tungsten. restriction as to the temperature is nevertheless of great importance, on the one hand in that it tends to limit the operating temperature of the anode, and on the other hand because the energy radiated by the anode being proportional at power 4 of temperature, it is at high temperature that the presence of graphite is best exploited.

 In addition, the intermediate layer constitutes a bonding layer, since the tungsten does not adhere directly to the graphite.

 The intermediate layer is commonly formed from Rhenium, but other materials such as iridium or hafnium carbide are also known for this purpose. All these materials are also chosen because they are conductors of electricity, in order to allow the flow of the anode current whose intensity corresponds to the intensity of the incident electron beam which bombards the target material.

 These materials known to be able to constitute an intermediate layer, have in common a serious defect, which lies in that they have a coefficient of expansion very different from that of tungsten and graphite, and that consequently, the mechanical cohesion of the tungsten-graphite, that is to say the adhesion, is weak.

This causes mechanical brittleness that is greater the higher the temperature, and consequently causes poor heat transfer from the tungsten to the graphite; the degradation of the anode being irreversible.

 This shows that the quality of the bond is the target material or tungsten and graphite is essential.

 The present invention relates to a rotating anode for an X-ray tube, which does not have the drawbacks mentioned above thanks to the use of a new connecting element, between tungsten and graphite, which promotes good mechanical strength of the tungsten assembly. -graphite even at very high temperature.

According to the invention, a rotating anode for an X-ray tube, comprising a first graphite part, a second part comprising a target material intended to produce X-radiation, the first and the second part being mechanically linked, is characterized in that it has a layer of carbide
Silicon at the junction between the first part and the second part.

 Silicon carbide is a poor conductor of electricity, and this characteristic is opposed to designating it as one of the materials capable of realizing the bond between the target material such as tungsten and graphite, in a rotating anode for X-ray tube. Nevertheless, tests have shown that by using it in a small thickness, of a few microns, the silicon carbide does not oppose the flow of the anodic current of the X-ray tube, while still constituting a good anti-diffusion barrier. from carbon to tungsten, even at higher temperatures than in the case where an intermediate layer constituted according to the prior art is used, in rhenium for example: silicon carbide having little deviation from Stoichiometry.

 It should also be noted that the silicon carbide can also be used in a thick layer, insofar as the rotating anode includes means for flowing the anode current, as is explained further in the description which follows.

 On the other hand, silicon carbide has a considerable advantage, compared to rhenium, in that its coefficient of expansion is close to that of tungsten and that of graphite and thus, on the one hand, the protection which it achieves against the carburetion of tungsten, and on the other hand the bonding of tungsten on graphite (via silicon carbide), are obtained in a lasting manner, even under the influence of strong thermal variations. In addition, silicon carbide associates well with graphite, and it has a high melting point and a low vapor pressure which make its use interesting in an X-ray tube.

The invention will be better understood thanks to the description which follows, given by way of nonlimiting example, and to the four appended figures among which:
- Figure 1 shows a rotary anode according to the invention, comprising a layer of silicon carbide deposited between a layer of target material and a graphite body;
- Figure 2 shows the rotary anode of Figure 1, and illustrates means of electrical contact between the target material layer and the graphite;
- Figure 3 shows the rotating anode of Figure 1 and illustrates the case where the target material has a massive structure;
- Figure 4 shows a rotary anode according to the invention, comprising a basic body of molybdenum on which graphite is attached.

 Figure 1 shows a rotating anode 1 according to the invention.

 The anode 1 comprises a base body 2 of graphite. The base body 2 has an axis of symmetry 3 along which it has a hole 4 intended, in a conventional manner, for the passage of a support axis (not shown).

 The base body 2 has a face 10, on which is deposited an intermediate layer 11 intended to make the connection between the graphite of the base body 2 and a target material intended to produce X-rays. Generally, we mean by "target material", any pure or composite material, or alloy, used to produce X-radiation under the effect of bombardment by an electron beam.

 The target material, for example tungsten, covers the intermediate layer 11 according to a second layer 12. In the nonlimiting example described, the base body 2 constitutes a first piece of graphite, and the second layer 12 constitutes a second piece of target material, these first and second parts 2, 12 being linked mechanically by means of the first layer 11 as an intermediate element.

 The layer 12 of tungsten is deposited for example by means of a vacuum plasma torch, but can also be deposited using a different process such as for example, gas phase deposition, electrolysis, molten salts, sputtering etc. .. As explained in a continuation of the description, the tungsten can also be constituted by a single solid part, arranged on the intermediate layer 11, and brazed. In the nonlimiting example described, the intermediate layer 11 and the tungsten layer 12 have substantially a crown shape, respectively 14, 15, centered on the axis of symmetry 3, but could just as well, independently of one another, cover a larger part of the base body 2; important being that the tungsten layer 12 has a width L equal to or greater than a second width L1 of a focal ring 13, generated in a conventional manner during the operation of the anode 1.

 According to a characteristic of the invention, the intermediate layer 11 is constituted by silicon carbide.

This intermediate layer 11 of silicon carbide can be deposited in a thin layer (less than 10 microns for example), or in a thick layer, using a conventional process such as:
- the deposition, in the gaseous phase of silicon which is fueled in contact with the graphite of the base body 2
- direct deposition of silicon carbide by chemical vapor deposition;
- deposition of silicon, in a bath of salts melted by electrolysis;
- by sputtering, vacuum evaporation, etc.

 As has already been explained, one of the drawbacks of silicon carbide resides in that it is a poor conductor of electricity, particularly at low temperature, and its use in a thick layer or greater than a thickness e of approximately 10 microns, can degrade the quality of the flow of the anode current of the X-ray tube (not shown) in which the rotating anode is used 1. For a thickness e less than 10 microns tests have shown that there was no no special precautions to be taken with regard to the problem of the flow of the anode current. In the case where the intermediate layer 1 1 of silicon carbide is deposited in a thicker layer, it is recommended to provide certain arrangements in order to respond to this problem, these arrangements also being compatible in the case where the silicon carbide is deposited in a thin layer.

 The purpose of these arrangements is to ensure electrical contact between the layer 12 of tungsten and the graphite of the base body 2; the latter being, in a conventional manner for a rotating anode, connected to the positive pole (not shown) of the high voltage supply of the X-ray tube, via, for example, its support axis (not shown).

 Different means can be used to make the electrical connection between the layer 12 of tungsten and the graphite of base body 2.

 FIG. 2 illustrates, by way of nonlimiting example, a particularly simple means of ensuring electrical contact between the layer 12 of tungsten and the graphite of the base body 2. This means consists in reserving on the graphite of the base body 2, under the layer 12 of tungsten, one or more zones 20 free of silicon carbide. In the nonlimiting example described, this is obtained in a particularly simple manner, by conferring on the crown 15 according to which the layer is deposited. 12 of tungsten, a width L greater than a third width L2 of the ring 14 of silicon carbide II; this width L2 of the silicon carbide ring 11 being substantially equal to or greater than the second width L1 of the focal ring 13, the layer 12 of tungsten is directly deposited on the graphite, in the zones 20 outside the focal ring 13.

 The significant advantage provided by silicon carbide comes from the fact that it has an expansion coefficient close to that of tungsten and that of graphite. It is mainly near the region where the temperature of the anode is highest during the operation of the latter, that this advantage takes all its importance, that is to say with respect to the focal ring 13; the drawbacks due to the carburetion of the tungsten and to the quality of its adhesion, which are avoided thanks to the silicon carbide, having much lower repercussions in the zones 20 situated outside the focal ring 13.

 The various methods previously mentioned, for the deposition of silicon carbide, allow in a simple manner, by means of covers for example (not shown), to reserve on the graphite of basic body, zones 20 where the silicon carbide does not is not filed.

 The process for producing the rotating anode 1 according to the invention may consist, for example, of depositing silicon, by igneous electrolysis in a bath of molten fluorides.

Thus, for example, a mixture of lithium fluoride, sodium fluoride and potassium fluoride with a composition close to that of ternary eutectics is melted at a temperature above 6000 under a protective atmosphere, and added with hexafluorosilicate of potassium, raw charged by bubbling
Silicon tetrafluoride.

 By electrolysis, between a soluble silicon anode and a cathode formed by the base body 2 of graphite, a deposit of silicon is obtained on the graphite of the base body 2.

 The deposited silicon is composed with the carbon of the base body 2 to give the layer 11 of silicon carbide.

 Since silicon carbide is a poor conductor of electricity, the deposition of silicon carbide is spontaneously limited to an adjustable thickness with the value of the potential difference imposed on the electrolysis circuit.

 On the layer 11 of silicon carbide, the layer 12 of tungsten is deposited, for example by means of a vacuum plasma torch: tungsten powder is sent into a neutral gas plasma which is sprayed onto the base body 2 to be covered, particularly on the layer 11 of silicon carbide.

 The tungsten deposits thus obtained can easily be doped by mixing powders of other materials, such as rhenium, tantalum, etc., with the tungsten powder.

 The deposition of the layer 12 of tungsten, by the plasma torch under vacuum, can be located with precision making it possible to cover only the region which will undergo the electron bombardment, that is to say the focal ring 13, where a region more extensive further comprising zones 20 free of silicon carbide.

 Figure 3 illustrates the case where the target material has a massive structure.

 In this case, the target material can consist in a conventional manner of solid, pure or alloyed target material, such as for example solid tungsten or an alloy of the latter, or alternatively a composite, for example tungsten-molybdenum, such as comprising tungsten (possibly alloyed) on the surface, and a support (not shown) in molybdenum as an undercoat.

 In the nonlimiting example of the description, the target material is formed according to a crown such that the crown 15 which in the example described is made of solid tungsten, and constitutes a second part.

 The layer 11 of silicon carbide having been previously deposited on the graphite of the base body 2, for example according to the method described above, the crown 15 of tungsten is applied to the layer 11 of silicon carbide on which it is soldered.

The brazing element can be of a conventional type such as zirconium for example, and constitutes a third layer 18 which makes the connection between the crown 15 made of tungsten and the layer 11 of silicon carbide. This third layer 18 as a brazing element is combined on the one hand with tungsten, and on the other hand is combined with silicon carbide and, optionally, with the graphite of the base body 2 if, as in the case above, zones 20 free of silicon carbide have been reserved on the graphite.

 In this configuration, one of the important advantages provided by the silicon layer 11 is that it avoids deterioration of the surface of the graphite, which in the prior art is in direct contact with the brazing element and is thus finds itself subjected to high mechanical stresses during the heating of the rotating anode; the coefficients of expansion of the brazing element, in zirconium for example, and graphite being different. In the anode according to the invention, on the contrary, the coefficient of expansion of the silicon carbide is close to that of graphite. On the other hand, since silicon carbide has better mechanical strength than graphite, the difference between the expansion coefficients of zirconium and of silicon carbide has no untoward consequences.

 FIG. 4 shows a version of the rotary anode according to the invention in which the first graphite part 25 is bonded by brazing to a second part 26 carrying the target material 27.

 In the nonlimiting example described, the second part 26 constitutes a second basic body of molybdenum, the first face 10 of which is covered by the target material 27; the target material being for example tungsten, secured to the second base body 26 by a conventional mechanical and thermal process.

 In this configuration, the first piece 25 of graphite mainly has the function of increasing the heat capacity of the rotating anode 1 and of promoting its thermal radiation.

 In the nonlimiting example of the description, the first piece 25 of graphite has the form of a ring centered on the axis of symmetry 3 of the second base body 26. The first piece 25 of graphite is secured to the second body of base 26 on a second face 28 thereof, opposite the first face 10 carrying the target material 27. The first piece 25 of graphite has a surface 29 by which it is applied to the second face 28, after a layer 11 of silicon carbide has been deposited on the surface 29, by one of the methods mentioned above.

 The brazing element constitutes a second layer 18 which, with the first layer 11 of silicon carbide, provides the graphite-molybdenum bond. The advantage provided by the layer II of silicon carbide consists, as in the previous example, in avoiding the deterioration of a surface layer of graphite.

 It should be noted that in this latter configuration, the layer 11 of silicon carbide can be a thin or thick layer without there being the need to provide zones free of silicon carbide, the problem of current flow anodic does not arise in this case.

 The present invention is applicable to any type of anode in which a graphite element has been incorporated.

Claims (12)

 1. Rotating anode for an X-ray tube, comprising a first piece (2, 25) of graphite and a second piece (12,15,26) comprising a target material (12,15,27) intended to produce X-radiation, the first part (2,25) and the second part (12,15,26) being mechanically linked, characterized in that the connection between the first part (2,25) and the second part (12,15,26) comprises a layer (11) of silicon carbide.  2. Rotating anode according to claim 1, characterized in that the target material (12,15,27) is tungsten.  3. Rotating anode according to claim 1, characterized in that the target material (12,15,27) is a tungsten-polybdenum composite.  4. Rotating anode according to one of the preceding claims, characterized in that the layer (11) of silicon carbide is deposited on the first piece (2,25) of graphite.  5. Rotating anode according to one of the preceding claims, characterized in that the layer (11) of silicon carbide is a thin layer.  6. Rotating anode according to one of claims 1 to 4, characterized in that the layer (11) of silicon carbide is a thick layer.  7. Rotating anode according to one of the preceding claims, characterized in that it comprises means (L1, L2,20) for ensuring electrical contact between the target material (2,15) and the graphite part (2) .  8. Rotating anode according to the preceding claim characterized in that the means (L1, L2.20) for ensuring the electrical contact between the target material (12.15) and the first graphite part (2) have zones (20) graphite free of silicon carbide.  9. Rotating anode according to one of the preceding claims, characterized in that the first piece (2) of graphite constitutes a base body carrying the target material (12,15).  10. Rotating anode according to the preceding claim, characterized in that the target material consists of a second layer (12) deposited on the base body (2) by a deposition process.  11. Rotating anode according to claim 9, characterized in that the target material is formed in a solid part (15) linked to the base body (2) by brazing.  12. Rotating anode according to one of claims 1 to 8, characterized in that the second part (26) constitutes a basic molybdenum body carrying the target material (27), the first part (25) in graphite being linked to the second part (26) by brazing.