US 3836807 A
An x-ray tube anode of the rotary type has a target surface formed by a thin layer of tungsten/rhenium alloy, supported by laminates comprising first a tungsten alloy lamination comprising at least 70 percent tungsten by weight and behind this a molybdenum alloy lamination, the anode being made by powdered metal technology so the three components are integrated with a porosity of low value. The target surface is resistant to roughening and the anode is substantially free from the development of cracks even when heavily loaded.
Description (OCR text may contain errors)
United States Patent [191 Schreiner et a1.
ROTARY ANODE FOR X-RAY TUBES- Inventors: Horst Schreiner, Numberg; Ernst Geldner, Rosstal; Helmut Dietz, Nurnberg, all of Germany Assignee: Siemens Aktiengesellschaft,
Munich, Germany Filed: Mar. 1, 1973 Appl. No.: 336,963
Foreign Application Priority Data Mar. 13, 1972 Germany 2212058 US. Cl. 313/330, 313/60 Int. Cl. H01j 35/10 Field of Search 313/60, 330
References Cited UNITED STATES PATENTS 12/1958 Schram 313/60 Sept. 17, 1974 3,710,120 1/1973 Friedel ..3l3/60 3,731,128 5/1973 Haberrecker ..3l3/60 Primary Examiner--l-lerman Karl Saalbach Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or FirmKenyon & Kenyon Reilly Carr & Chapin  ABSTRACT 8 Claims, 3 Drawing Figures BACKGROUND OF THE INVENTION To produce x-rays, an anode provides a target against which an electron stream, or cathode rays, are focused, the target producing the x-rays but at the expense of substantial heating of the target and anode. The efficiency of the target material in the production of x-rays is proportional to its atomic number, the high atomic number of tungsten, in combination with its high melting point, making it particularly suitable for the target of an x-ray tube.
Overheating and target surface damage results if the electron stream power, or tube current, is increased too much in an effort to obtain an increasing x-ray output. Therefore, rotary anodes are used having a disk-like central portion and a beveled peripheral target surface area which constantly presents a new target surface area as the anode rotates. The working surface area of a rotary anode may be said to resemble in reverse the inside contour of a dished saucer.
Heretofore such a rotary anode has been made in the form of a laminate having two laminations, the front lamination forming the target surface being a tungsten/rhenium alloy and being of substantial thickness, and the other lamination, or supporting or back-up layer, being a molybdenum alloy. The rhenium component retards target surface roughening, and like other elements which might be used instead, is expensive.
Such a prior art rotary anode also is subject to cracking when subjected to the high thermal stressing resulting from operation of the x-ray tube at high power, as exemplified by a power in the area of I kw. The front lamination is relatively thick, so the tungsten alloying component comprises an undesirably large portion of the anode.
SUMMARY OF THE INVENTION An object of the present invention is to provide a rotary x-ray anode that is an improvement on such prior art rotary anodes, particularly when operated under heavy loading.
According to the present invention, this object is attained by making the target surface layer thinner and using behind it two additional layers, the first of which is composed of either pure tungsten or a high tungsten alloy containing at least 70 percent by weight, the other or back or third layer comprising a molybdenum alloy.
BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the present invention are illustrated somewhat schematically by the accompanying drawings in which:
FIG. 1 is a cross section through a first form;
FIG. 2 in a corresponding manner illustrates a second form; and
FIG. 3, again in cross section, illustrates a third form.
DESCRIPTION OF THE PREFERRED v EMBODIMENTS In all three figures of the above drawings, the anodes are designated 11. They are all of circular configuration and each has a central mounting hole which is unnumbered because it is the usual rotary anode mounting arrangement. In each figure the target surface layer is 12, the second or intermediate anode layer is 13, and the final or back layer of lamination is 14, the same numerals being used in FIGS. 2 and 3 because the only difference is in the anode cross-sectional shape.
In accordance with the invention, the first or target surface layer 12 comprises a tungsten alloy consisting of from 30 to 20 percent by weight of at least one, or one or more, of a metal selected from the class consisting of zirconium, hafnium, niobium, tantalum, rhenium, .osmium, or iridium, in addition to the tungsten. These alloying elements, of the tungsten alloy, causes the target surface of the layer 12, or x-ray producing layer, to resist roughening and the development of micro-cracks in the path of the high energy electron beam focal spot which, as can be seen from the drawings, is located in the area marked 15. This focal point is at the beveled peripheral portion of the anode as required for the angular projection of the resulting x-rays. Preferably the tungsten alloy has the described alloying elements limited to a range of from 5 to 15 percent by weight, with the balance being tungsten. Of the possible alloying elements described, rhenium is the one usually used.
7 The second layer 13, or intermediate lamination, is either substantially pure tungsten or a tungsten alloy containing at least percent tungsten by weight. To enhance the cold ductility and heat resistance provided by pure tungsten, alloys maybe included, alloying elements such as niobium, tantalum and/or zirconium being useful for this purpose providing they are not used in excess of 30 percent by weight of the alloy.
The third or back layer 14 of the new anode consists of a molybdenum alloy with the alloying component being at least one, or one or more, metals selected from the class consisting of titanium, zirconium, hafnium, niobium, tantalum, tungsten and rhenium. These alloying elements are used in amounts of from 0.05 to 20 percent by weight, preferably 2 to 10 percent by weight, the balance being the molybdenum.
The thickness of the x-ray producing or target surface layer 12 may range from 0.05 to 1 mm., preferably from 0.1 to 0.7 mm.; the thickness of the intermediate layer lamination 13 may range from I to 5 mm., preferably from 1.5 to 4 mm.; and finally, the thickness of the back or third layer 14 may range from 2 to 6 mm., preferably from 2.5 to 4 mm. The total anode thickness at its peripheral portion which travels through the electron beam focal spot 15 may range from 5 to 12mm., preferably 6 to 8 mm., and from this it can be seen that the foregoing dimensional values have reference to this portion of the rotary anode, the central portion being substantially thicker as shown in FIGS. 1 and 2, or the anode thickness may be the same radially throughout its extent as indicated'by FIG. 2.
In all instances the rotary anode of this invention may be made by well-known powdered metallurgy techniques. The powdered metals are molded in a die by compressing three layers of the respective powdered metal components, one on top of the other, into a solid body or compact with a well defined beveled peripheral portion. The compacting pressures may range from 1,000 to 8,000 kg/cm Consolidation may be effected by prior art sintering. Thereafter in a hydrogen atmosphere or under vacuum, the consolidated sintered shape is heated to temperatures between l,400 and 1,800C. The heated shape is then compressed, such as by hot hammer forging, to give the anode blank a high final density, or in other words, to free the shape from pores as much as possible. The ultimately compacted or forged anode has its target surface area, which is the area defined by rotation of the anode through the area 15, surface ground to within the narrow tolerances required by x-ray tube anode target surfaces in general. The heated sintered shape may be removed from its protective environment for the short time required for the hammer forging.
Specific examples of the present invention are providedby the following, it being understood that the cross-sectional shapes may be as exemplified by FIGS. 1 through 3.
Example 1 A layer of powder consisting of a mixture of molybdenum powder and another metallic powder, such as tungsten, is first poured into a steel mold to a height of 6 mm., for the formation of the back layer 14. On top of this is poured a layer of pure tungsten 6 mm. high to form thesecond or middle layer 13. Above this a layer of a powder mixture of WRelO is poured to a thickness of 1.5 mm. for the formation of the x-ray producing layer or target surface 12. The three layers of powder are subjected to a pressure of 4,000 kg/cm to form a solid compact with well-defined edge or peripherally beveled portions. Next, sintering takes place in an atmosphere of hydrogen at a temperature of between 2,000 and 2,400C. for the duration of one hour. Following the sintering, the highest possible density is obtained by one or more compression operations, such as hammer forging, at temperatures of between l,500 and l,700C. The sintered part may be electrically inductively heated in a protective gas and then exposed to air for a short period during this compacting or forging. The final porosity of the anode is less than 0.3 percent.
Example 2 A WRelO powder alloy is first poured into a steel mold to a height of 1.5 mm. for the formation of the upper or target layer 12. On top of this layer of WNb3 powder is poured to a height of5 mm. for the formation of the second or middle layer 13. On top of this is poured a powder mixture of Mo and an additional metallic powder. e.g., hafnium, to a height of 6 mm., to form the back layer 14. The three layers of powder are subjected to a pressure of 3,000 kg/cm to form a compressed solid compact with well-defined edges or beveled portion. The sintering is carried out in two steps, a preliminary sintering in hydrogen at l,000C. for 30 minutes, and a high sintering in vacuum at 2,000 to 2,400C. for one hour. Following the sintering, the highest possible density is obtained by one or more compression operations at a temperature between l,400 and 1,700C., in a protective gas if possible. The final porosity of the rotary anode, possibly densified by hot hammering, is less than 0.3 percent.
Example 3 The procedure is as in example 2, except for the composition of the three layers, e.g., the x-ray-producing layer 12 consists of a 0.2 mm. thickness of a WRelS alloy powder, the middle layer 13 of L5 mm. of a WRe3 alloy powder, and the back layer 14 of 4 mm. of a MoTa5 alloy powder mixture.
Example 4 Procedure is as in example 1, but the molybdenum layer is poured only 4 mm. high and consists of a MoZr5 powder mixture. The second layer of pure tungsten powder is filled to a height of 8 mm. The third layer of powder consists of Wlr 0.5, the pouring height being 1 mm.
The rotary x-ray anode of this invention, because of the permissible thinness of the target layer 12, permits considerable saving of rhenium content of the overall anode structure 1, as compared to previously known composition anodes using a thicker WRe alloy as the x-ray-producing or target layer. With the known rotary x-ray anodes, the rhenium content of the x-rayproducing layer is limited to between 3 and 10 percent by weight, to save on cost. It is well known that a higher rhenium content, e.g., between 10 and 20 percent, leads to a less severe roughening of the x-ray-producing surface layer than rhenium contents below 10 percent of rhenium by weight. Because of the three-layer structure of the rotary anode according to the invention, rhenium contents of, for instance, 15 percent by weight are possible, while keeping the total rhenium content of the entire rotary anode smaller than does the prior art. The other elements which may be used instead of rhenium are also expensive.
What is claimed is:
1. An x-ray tube rotary anode comprising a plurality of integrated laminations of sintered powdered metal of which the front or first lamination is a tungsten alloy; wherein the improvement comprises said anode having a second lamination of tungsten or high-tungsten alloy having a thickness from about 1 to about 5mm. behind said first lamination, and a third lamination of a molybdenum alloy behind said second lamination.
2. The anode of claim 1 in which said first lamination is from about 0.05 to about 1 mm. thick.
3. The anode of claim 1 in which said third lamination is from about 2 to about 6 mm. thick.
4. The anode of claim 1 in which said first layer is from about 0.05 to about 1 mm. thick, said second layer is from about 1 to about 5 mm. thick and said third lamination is from about 2 to about 6 mm. thick, said anode having a beveled peripheral portion forming an electron beam focus target annulus and the latter being from about 5 to about 12 mm. thick overall.
5. The anode of claim 1 in which said first lamination is an alloy consisting of tungsten and from about 3 to about 20 percent by weight of one or more alloying metals selected from the class consisting of zirconium, hafnium, niobium, tantalum, rhenium, osmium, or iridrum.
6. The anode of claim 1 in which said second lamination is a tungsten alloy consisting of by weight of at least percent tungsten with the balance being one or more alloying metals selected from the class consisting of niobium, tantalum and zirconium.
7. The anode of claim 1 in which said third lamination is a molybdenum alloy consisting of molybdenum and from about 0.05 to about 20 percent by weight of one or more alloying metals selected from the class consisting of titanium, zirconium, hafnium, niobium, tantalum, tungsten, and rhenium.
8. The anode of claim 4 in which said first lamination is an alloy consisting of tungsten and from about 3 to about 20 percent by weight of one or more alloying metals selected from the class consisting of zirconium, hafnium, niobium, tantalum, rhenium, osmium, or iridium, said second lamination is a tungsten alloy consist-