GB2073945A

GB2073945A - Anode disk for a rotary-anode X-ray tube and method of manufacturing same.

Info

Publication number: GB2073945A
Application number: GB8110225A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1980-04-05
Filing date: 1981-04-01
Publication date: 1981-10-21
Also published as: DE3013441C2; FR2480033A1; JPS56146358U; GB2073945B; DE3013441A1; FR2480033B1

Abstract

Known rotary-anode X-ray tubes in which a graphite body is soldered to a heavy-metal body forming the rotary anode, have the drawback that the graphite body is damaged by the thermal and mechanical stresses occuring during operation. A heavy-metal body 9 is accordingly provided with a circumferential rim 22 which encircles at least a proportion of the graphite body 10. Between the inner side of the rim 22 and the outer side of the graphite body 10 a layer of solder is provided. It has been found that the graphite body is thus better protected against damage. Moreover, the soldering process is simplified in that the heavy-metal body can be situated below the graphite body during soldering to form a cup containing the solder thus preventing solder contamination of the electron impact surface of the anode. <IMAGE>

Description

SPECIFICATION Anode disk for a rotary-anode X-ray and method of manufacturing same The invention relates to an anode disk for a rotary-anode X-ray tube which comprises a rotationally-symmetrical heavy-metal body which is connected to a drive shaft in the assembled condition of the X-ray tube and a ring shaped graphite body which is connected to the heavy-metal body, for example by means of a layer of solder, and to a method of manufacturing said anode disk.

An anode disk of the latter kind is known from DE-AS 20 61 007. The graphite body therein serves to increase the thermal capacity of the anode disk and also to increase the thermal emissivity. Because the drive shaft is also connected to the heavy-metal body, the graphite body does not provide a supporting function. However, it has been found that during operation of an X-ray tube which is provided with such an anode disk, parts of the graphite body come loose from the graphite body due to shearing, so that the tube becomes useless.

The present invention has for an object to construct an anode disk of the kind set forth so that the risk of the graphite body becoming completely or partly detached from the heavymetal body as a result of thermal loading, is reduced.

According to the invention there is provided an anode disk for a rotary-anode X-ray tube which comprises a rotationally-symmetrical heavy-metal body which is connected to a drive shaft in the assembled condition of the X-ray tube and a graphite body which is connected to the heavy-metal body, characterized in that the heavy-metal body includes a ring-shaped rim which faces the graphite body, said ring-shaped ring encircling in a supporting relationship at least a proportion of the outer peripheral surface of the graphite body.

When the graphite body is connected to the heavy-metal body by means of a layer of solder, the ring-shaped rim of the heavy-metal body which faces the graphite body, is formed with an inner diameter which is larger than the outer diameter of the graphite body, the layer of solder also extending between the ring-shaped rim and the outer peripheral surface of the graphite body.

Tests have demonstated that such an anode disk exhibits a greater tolerance of thermal loading than said known anode disk. This can be explained as follows: in the known anode disk, those surfaces of the heavy-metal body and the graphite body which are connected to one another by soldering or thermal shrinking are flat. During the connection operation, both bodies have approximately the same temperature. After the connection, both bodies cool down; the heavy-metal body then shrinks more than the graphite body, at least if it consists basically of a body of molybdenum provided with a tungsten layer, because of its higher thermal expansion coefficient. In the layers of both bodies which adjoin the connection layer, stresses occur which may cause the graphite body or parts thereof to become detached under the effects of shear.

In the construction of the anode disk in accordance with the invention, after the connection process, the ring-shaped rim will exert an increasing compressive stress, via the connection layer or interface, on the outer edge of the graphite body as cooling progresses. As opposed to tensile forces, compression forces can be taken up to a substantial degree by a graphite body without breaking. These compression forces which decrease during operation of the X-ray tube due to the heating of the anode disk, tend to reduce the risk that the graphite disk will shear away from the anode.

The heavy-metal body to which the drive shaft is connected in accordance with the invention, takes up the forces transmitted by the drive shaft during operation, so that the graphite body is not mechanically loaded thereby.

The height of the rim of the heavy-metal body need not be equal to the thickness of the graphite body. Because the shearing forces occur notably in the region of the graphite body adjacent the solder layer during cooling, it is sufficient if only this region is encircled by the rim whose height, therefore, need only amount to a fraction of the thickness of the graphite body.

The inner diameter of the ring-shaped rim may be substantially equal to, or a few tenths of a millimeter more than, the outer diameter of the outer edge of the graphite encircled thereby. The mechanical compression forces are then transmitted to the graphite body via the connection layer.

It is not necessary either that the inner wall of the ring-shaped rim extends parallel to the axis of rotation of the anode disk. It may be desirable for the inner wall of the rim to extend at an angie of 30 with respect to the axis, because the notch stresses will then be less.

A particular advantage of an anode disk in accordance with the invention consists in that the soldering process can also be performed very simply. According to the soldering methods known thus far (see for example DE-AS 20 61 007), the heavy-metal body is situated over the graphite body during the soldering process. If the solder layer becomes too hot, the solder will flow from the gap between the two bodies in isolated locations, thus wetting not only the graphite body but also the upper surface of the heavy-metal body due to molecular forces. These residues not only lower the melting temperature in the vicinity of the @ocel path of the electron beam, but also reduc X-ray efficiency due to the generally lower atomic number of the solder material.

In accordance with the invention, the sol- dering process for an anode disk on the heavy-metal body and by subsequently ar- ranging the graphite body on the solder after which a soldering process is performed Thus, during the soldering process the graph- ite body is situated above the heavy-metal disk.

The ring-shaped rim of the heavy-metal 3isk which is in the lower position during soldering prevents escape of solder. The solder rises only in the gap formed between the inner of the ring-shaped rim and the outer edg- the graphite disk as a result of molecular forces, thus filling this gap.

However, this advantage is achieved only if the solder cannot escape in any other manne; for example, through a shaft mounting hole ir the centre of the heavy-metal disk. According to a further method of manufacturing an an- ode disk in accordance with the invention, the heavy-metal body is formed by a bowl-shaped body without a central aperture and wlhich is when soldered to the graphite body, the heavy- metal body and possibly the graphite k,ody then being provided with a central bore the soldering process. Because the h y- metal body thus forms a bowl during solder- ing, the solder layer cannot run off. Subse- quently, the heavy-metal body is provided with the central bore required for the drive shaft.The graphite body may be provided in advance with a bore through which the drive shaft is passed when the rotary anode X-r tube is assembled. However, it may alterna- tively be formed simultaneously with the bo@e in the heavy-metal body.

In a further embodiment in accordan@e witl the invention, the heavy-metal body is pre- vided with a centrally mounted stud which projects into the ring-shaped graphite body and whose outer diameter is only slightly Esss than the inner diameter of the ring-shaped graphite body. The stud which is centrally situated within the heavy-metal body, together with the outer rim of the heavy-metal bcdv thus complete the form of a ring-shaped bowl which can also be used to prevent the escape of the solder layer. The stud at the same time serves for the centering of the graphite body.

Embodiments of the invention will now described, by way of example, with reference to the accompanying diagrammatic drawings, of which: Figure 1 shows a rotary anode X-rny hubs including an anode disk in accordance witl the invention, Figures 2, 3 and 4 are sectional views o various embodiments of an anode disk in accordance with the invention.

The reference numeral 1 in Fig. 1 denotes the glass envelope of a rotary anode X-ray tube which supports a cathode arrangement 3 at one end and an anode arrangement 4 at its other end. The cathode arrangement comprises a shielding unit 5 to which the cathode head 6 in which the filament or filaments (not shown) are arranged, is secured. The filament voltages are applied via the leads 15, 16 and 17 which also carry the cathode high voltage - pstential .

The anode arrangement 7 comprises a rotor with a drive shaft 8 to which an anode disk 9, is connected. The anode disk 9 comprises a hea-"y-metal body 13 which is connected to the drive shaft 8. To achieve this, the upper end of the drive shaft is provided with a thread which is engaged by a threaded clamping plate 11 which, in the tightened condition, presses the heavy-metal body 13 against a ower abutment plate 12 connected to the drive shaft 8. The anode disk, shown in a sectional view in the drawing, also comprises a graphite body 10 which is soldered to the lower surface of the heavy-metal body 13, as shown with the drive shaft 8 or with the abutment 12. The anode high voltage is applied via connections 18.

Fig. 2 shows a suitable embodiment of the anode disk (not to scale). The heavy-metal body 13 consists basically of a body of molybdenum which is provided with a layer of tungsten or a tungsten alloy at least in the vicinity of the focal path 19 of an incident electron beam. It may have an outer diameter of, for example, 120 mm and a thickness of 9 mm. On its lower face, as shown in Fig. 2, the heavy-metal body is formed with a surface which extends perpendicularly with respect to the rotation or symmetry axis and which is bounded at the outer edge by a ring-shaped rim 22 and at the inner edge by a stud 23 formed in the heavy-metal body 13.If the heavy-metal body were to be rotated through l 80 about a horizontal axis, an annular bowl would then be provided on the face then facing upwards, the bottom of the bowl being formed by the surface 20, and its side walls being formed respectively at the outer edge by the rim 22 and at the inner edge by the stud 23.

It has been found that a height of only 1.5 mm for the outer rim can be effective for an embodiment in accordance with the invention.

The ring-shaped rim may, however, be higher than 1.5 mm. The central stud 23 which is centrally arranged with respect to the axis of rotation 21 and in which there is provided a bore 24 for the drive shaft 8 (Fig. 1), should have the same height as the ring-shaped rim 22.

The e flat face 25 of a rotationally-symmetri cal graphite body 10 is connected to the surface 20 of the heavy metal body, and the inner and outer peripheral surfaces of the body 10 which are parallel to the axis of rotation 21, are connected respectively to the stud 23 and to the ring-shaped rim 22, all by means of a solder layer 26. The graphite body 10 may be made of a very fine-grained graphite manufactured by pressing and sintering, for example, the quality 5890 marketed by Deutsche Carbone AG, which has a density of 1.85 g/cm3, a bending strength of 65 N/mm2 and a thermal expansion coefficient (at 100"C) of 4.2 X 10-6/K.The outer diameter of the graphite body 10 is smaller than the inner diameter of the ring-shaped rim 22, for example, 1.2 mm smaller, so that a solder layer having a thickness of 0.6 mm remains between the ring-shaped rim 22 and the outer periphery of the graphite body 1 0. The inner diameter of the graphite body 10 is larger than the outer diameter of the stud 23, by a small amount, for example, at the most 0.2 mm, so that before soldering the inner periphery of the graphite body bears on the stud 23 with only a small amount of transverse play and is thus effectively centred thereby.

For the connection of the heavy-metai body to the graphite body 10, firstly the heavymetal body is inverted, thus taking up the lower position (i.e. turned 180 about a horizontal axis with respect to the position shown in Fig. 2), so that the ring-shaped bowl formed by the surface 20 in conjunction with the ring-shaped rim 22 and the stud 23, is situated at the top. In this ring-shaped bowl there is arranged a solder disk of zirconium which has a thickness of 0.3 mm and whose inner diameter is larger than the inner diameter of the graphite body and whose outer diameter is smaller than the outer diameter of the graphite body, the difference being, for example, each time about 4 mm. The graphite body 10 is arranged thereon. This assembly is subsequently heated in an evacuated space, for example, by high frequency heating.At a temperature of approximately 1 500 C, a eutectic molten pool is formed (for example, as known from DE-AS 21 1 5 896 and the corresponding U.K. Patent Specification No.

1,383,557) which connects the heavy-metal body to the graphite body. In the case of high frequency heating (at 550 kHz), the solder first rises in the gap between the outer periphery of the graphite body 10 and the ringshaped rim 22, because the outer region (notably the ring-shaped rim) is heated more in the case of high frequency heating than the regions situated further inwards. After some time, the solder also rises in the gap between the stud 23 and the inner periphery of the graphite body 10, after which the supply of energy is terminated and the solder solidifies, its surface then forming a meniscus 27 in the gap between the graphite body 10 and the ring-shaped edge 22.

In this soldering process, the escape of solder is rendered hardly possible, so that the inherent unbalancing and contamination of the heavy-metal body can be precluded to a great extent. If the heating operation is interrupted somewhat too late, the consequences will not be detrimental as in the known soldering processes carried out hitherto. However, an excessively long duration for the supply of energy may have an adverse effect on the reliability of the soldered connection and should therefore be avoided, because carbides may then be formed.

The anode disk shown in Fig. 3, in which corresponding parts are denoted by the same reference numerals as those used in Fig. 2, differs from the anode disk shown in Fig. 2 in that the heavy-metal body whose upper surface (electron impact surface) is bevelled in known manner (angle of inclination with respect to the horizontal from approximately 5 to 20 ), has a uniform thickness except in the vicinity of the ring-shaped rim 22 and the stud 23, so that the surface 20' facing the graphite body 1 0 is not longer flat, as in the anode disk shown in Fig. 2, but also inclined.

The upper boundary surface of the graphite body 10 in the embodiment of Fig. 3 must then have a corresponding shape. The stud 23 must then be higher than the ring-shaped rim, but its end face is again situated approximately in a plane which is perpendicular to the axis of rotation 21 and in which the lower end face of the ring-shaped rim 22 is also situated. Soldering is performed in inverted configuration, in the same manner as described for the embodiment of Fig. 2.

Fig 4 shows an anode disk 1 3 during an intermediate stage of manufacture. The heavymetal body comprises a flat lower surface 20" which is bounded by the ring-shaped rim 22, and at this stage is not provided with either a bore or a stud at its centre. As in the case of Figs. 2 and 3, the disk 1 3 is first inverted, and in the ring-shaped bowl formed by the ring-shaped rim 22 and the surface 20", now directed in readiness for the soldering operation, there are arranged a solder disk, preferably made of zirconium, and the cylindrical graphite body 10 which may be provided with a central bore. After the soldering operation, the heavy-metal body 1 3 is provided (denoted by broken lines) with a bore 24' which serves for connecting the anode disk to the drive shaft. If the graphite body 10 does not already comprise such a central bore, this bore is also provided at this time through the graphite body. If the bore through the graphite body 10 has a diameter which does not exceed that of the bore through the heavymetal body 13, the drive shaft will also bear on the graphite body 10 in the assembled condition, but the graphite body will then only be exposed to compression forces which can readily be taken up thereby.

Claims

1. An anode disk for a rotary-anode X-ray tube which comprises a rotationally-symmetrical heavy-metal body which is connected to a drive shaft in the assembled condition of the X-ray tube and a graphite body which is connected to the heavy-metal body, character sized in that the heavy-metal body includes a ring-shaped rim which faces the graphite body, said ring-shaped rim encirling in a supporting relationship at least a proportion of the outer peripheral surface of the graphite body.

2. An anode disk as claimed in Claim 1, characterized in that the inner diameter of the ring-shaped rim is larger than the outer diameter of the graphite body, and the heavy-metal body and the graphite body are interconnected by means of a layer of solder which is also present between the ring-shaped rim and the outer peripheral surface of the graphite body.

3. An anode disk as claimed in Claim 2, characterized in that the heavy-metal body comprises a centrally arranged stud which projects into the ring-shaped graphite body and whose outer diameter is only slightly smaller than the inner diameter of the ringshaped graphite body.

4. An anode disk as claimed in any preceding claim, characterized in that the heavymetal body is formed of molybdenum and is provided with a layer of tungsten alloy on the surface which is remote from the interface between the heavy-metal body and the graphite body.

5. An anode disk as claimed in Claim 2, or in Claim 3 or Claim 4 when dependent on Claim 2, characterized in that the solder layer essentially consists of zirconium.

6. An anode disk for a rotary-anode X-ray tube, substantially as herein described with reference to any one of the Figures in the accompanying drawings.

7. The method of manufacturing an X-ray tube anode disk as claimed in Claim 2 or any one of Claims 3, 4 and 5 when dependent on Claim 2, including the steps of forming a said heavy-metal body so that the surface to be situated adjacent a said graphite body is in the form of a bowl-like container, mounting said heavy-metal body with said surface directed upwardly so as to contain any liquid when present, applying a disk of solder and then a said graphite body to said upwardly directed containing surface, and heating the assembly in an inert atmosphere so as to melt said solder and thereby to bond said graphite block to said heavy-metal body.

8. The method of manufacturing an X-ray tube anode disk as claimed in Claim 2, substantially as herein described with reference to any one of Figs. 2, 3 and 4 of the accompanying drawings.