US 3037142 A
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y 1962 R. GRIFFOUL ET AL 3,037,142
X-RAY GENERATOR TUBES Filed July 22, 1959 Fig.1 9
FOCAL TEMPERATURE TEMPERATURE P THE FOCAL RING TEMPERATURE F THE ANODE DISC I TEMPERATURE United States Patent 3,037,142 X-RAY GENERATOR TUBES Roger Grifioul, Sevres, and Antonius Schram, Montgeron,
France, assignors to Compagnie Generale de Radiologie, Paris, France, a French body corporate Filed July 22, 1959, Ser. No. 828,763 Claims priority, application France Mar. 15, 1956 6 Claims. (Cl. 313-330) The present invention relates to X-ray generator tubes, and more particularly to anodes for such tubes and a method for their manufacture, and is a continuation-inpart of our co-pending application Serial No. 645,174, filed March 11, 1957.
One of the principal problems encountered in X-ray generator tubes is the thermal dissipation from the anode of the tube, since almost the whole of the electrical energy is transformed into heat in the anode. The speed of cooling of the anode limits the rate of making successive exposures or exposures after viewing. For X-ray generator tubes with a revolving anode, for example, the cooling is principally effected by radiation from the anode, which is usually a massive disc of tungsten, and inadequate cooling is liable to cause fracture of the anode.
It is an object of the present invention to provide an anode for an X-ray generator tube which hasimproved thermal dissipation, whereby the speed of cooling of the anode is considerably increased.
The invention consists in constructing the anode of an X-ray tube of tungsten of which the external surface, including the target or focal area, is carburized into a thin surface layer composed substantially entirely of tungsten carbide of the formula W C and which is substantially free of tungsten carbide of the formula WC and also of free carbon.
In order that the invention may be more clearly understood reference will now be made to the accompanying drawing, in which:
FIGURE 1 is a diametrical elevational view of an X-ray generator tube having a rotating anode according to the present invention, I
FIGURE 2 is a plan view of the anode assembly of the tube in FIGURE 1, and
FIGURES 3 and 4 are explanatory graphs.
Referring to FIGURES l and 2, the X-ray generator tube comprises an evacuated glass envelope 1 within which is disposed the support 2 for the cathode system comprising the filament or filaments 3 in the fo'cussing member 4 which is located opposite to the conical portion of a tungsten disc 7 forming the anode of the tube. The anode 7 is mounted by the screw 8 on molybdenum rod 6 which is rotated by the rotor 5. By applying a high tension so that the cathode is negative and the anode is positive, a beam of electrons emitted from the cathode bombards the target area 10 and produces X-radiation. Due to the rotation of the anode, the electrons bombard successive areas of the annular zone forming the focal ring 9. The electron energy is transformed almost entirely into heat which rapidly heats the focal ring and the whole mass of the anode 7.
Such a tube structure is already known, but as will be hereinafter described, according to this invention the anode 7 is treated to carburize its entire surface, including the focal ring, into a thin surface layer composed sub- 3,037,142 Patented May 29, 1962 that of the focal area or ring 9 which, during the same time, passes from T to T as shown in the broken line curve. Finally, as shown in the chain line curve, the temperature of the anode disc 7 also rises during the exposure and passes from T to T T being lower than T The phenomenon of ageing of the anode 7 promotes a reduction of X-rays depending on the temperature of the focal area 10. It is thus obvious that it is of interest to start from a temperature T which is as low as possible. Now, it is often necessary to make several exposures very close together or an exposure after viewing. The possibility of this depends on the cooling of the anode of the tube.
For a given anode, cooled principally by thermal radiation, the speed of cooling is proportional to the thermal dissipation which follows the Stephen-Boltzmann law W=the heat radiated in watts.
e=the total emissivity of the surface at the temperature T. 5=constant=5.67.l0 w./cm. K
T=the temperature of the surface in K.
T =the ambient temperature in K.
S=the radiating surface in cm. (apparent surface).
For the temperature in question, T may be neglected compared with T For given T and S, it is therefore necessary to increase 5 if it is desired to improve W. Now, for tungsten 6 has moreor less the following values:
At 0 K 0.114 At l500 K 0.192 At 1700 K 0.222 At 2000 K 0.260 At 2500 K 0.303
A certain number of other processes have also been proposed which consist in a deposit of another material, increasing the actual surface or at the same time also the intrinsic emissivity. These processes, however, present serious disadvantages for X-ray generator tubes, which can easily be understood, because most of these processes are proposed for electronic tubes other than X-ray generator tubes. Thus, it has been suggested to deposit refractory carbides on the base, directly by eletrophoresis, or by depositing a metal oxide followed by a chemical or thermal treatment, or by depositing a mixture of carbides and metallic oxides. The sintering of mixtures of carbides of zirconium and metallic zirconium or other refractory carbides has also been suggested. Finally, the sintering of a powder of tungsten or another refractory metal has been suggested. When dealing with a deposit by electrophoresis, the adherence of the deposit is not good, and in all cases one is obliged to spare the focal ring of the anode in order not to reduce the efficiency of the X-ray production and also because most of these deposits will not support very high temperatures. These processes are neither reliable nor economical; it is very diflicult to obtain regular deposits only where they are required and not to liberate too much gas at high temperatures. There are classical processes for carburizing anodes of nickel, giving a very efficient cooling. But these anodes cannot operate at the temperatures usually met in Xray tubes.
The present invention eliminates these difficulties by avoiding the difliculties of a deposit by electrophoresis or sintering, and allows a considerable increase of the emissivity to be obtained from the surface of a tungsten anode.
According to this invention, the thermal emissivity of the anode is increased by carburizing the whole of the surface of the anode, including the focal ring, into a thin surface layer composed substantially entirely of tungsten carbide having the formula W C, and which is substantially free of tungsten carbide of the formula WC and of free carbon. The invention presents the advantage of great simplicity. In this way an increase in the thermal emissivity of the order of 100% is obtained without appreciable reduction of the X-radiation. This latter factor is explained by the carbide layer being thin and by partial or complete decarburation of the focal ring, produced by the high temperatures.
The process for obtaining the carburized anode according to this invention is carried out as follows. The anode after cleaning by any suitable means, such as sand blasting, is first submitted to a de-gassing operation which can conveniently be carried out by heating the disc at about 1300 C. for five minutes in a vacuum of mm. Hg. The disc is allowed to cool for a few minutes and then carburized in an atmosphere of a hydro-carbon gas, for example hexane, under a low pressure, for example 3040 mm. of Hg, while the anode is heated to a temperature of about 1040 C. to 1200 C., for a few minutes, for example for about ten minutes. This produces a carbide layer which is only a few microns thick.
After carburizing, the anode disc is submitted to a second operation by heating in a vacuum at a temperature of about 1600 C. This second operation is for the purpose of ensuring that the tungsten carbide will have the formula W C. After the initial carburizing the carbide consists both of W C and WC and may even contain some free carbon. This second operation heat treatment under vacuum effects the combination of the carbide WC and the carbon with the tungsten so that the final carbide layer is substantially entirely of the formula W C.
It is important that the carbide layer should consist substantially entirely of W C and that the layer should be thin. Tungsten carbide of the formula WC is brittle and becomes fractured and dislodged due to the large mechanical stresses in the anode caused by the high temperature differences between different parts of the anode. Any free carbon will become liberated from the anode with the liability of causing arcing and tube breakdown. It i only when the tungsten carbide layer consists substantially entirely of W C that this layer remains firmly adherent and highly heat emissive in the severe operating conditions experienced in X-ray generator tubes.
FIGURE 4 shows the improvement obtained according to the process of the present invention. The curve A is that for the cooling of a conventional anode and curve B shows the cooling for an anode in accordance with this invention.
Whilst a particular embodiment has been described it will be understood that various modifications may be made without departing from the scope of this invention. For example, instead of making the anode of a solid disc of tungsten, it may be made with a core of a different material, such as molybdenum.
1. An X-ray generator tube comprising an envelope containing a cathode and an anode disc spaced from said cathode and mounted for rotation within the envelope,
said anode disc being made of tungsten and having its surface carburized into a thin surface layer of the order of a few microns composed substantially entirely of tunsten carbide of the formula W C, said carbide layer being substantially free of tungsten carbide of the formula WC and also of free carbon.
2. An X-ray generator tube comprising an envelope containing a cathode and an anode disc made of tungsten and spaced from said cathode, said anode disc having a conical peripheral zone and being rotatably mounted within the envelope and connected with drive means for rotating it with its conical peripheral zone opposite to the cathode so that electrons emitted from said cathode will bombard an annular focal area around said conical zone as the anode disc is rotated, wherein the surface of the anode disc including the focal area is carburized into a thin surface layer composed substantially entirely of tungsten carbide of the formula W C and which is substantially free of tungsten carbide of the formula WC and also of free carbon.
3. In an X-ray generator tube, a rotatable anode disc made of tungsten and having a conical peripheral zone, defining an annular focal area, the entire surface of the anode disc including the focal area being carburized into a thin surface layer composed substantially entirely of tungsten carbide of the formula W C and which is substantially free of tungsten carbide of the formula WC and also of free carbon.
4. The method of making an anode for an Xray generator tube which consists in making an anode disc having a surface of tungsten, submitting the anode disc to a carburizing treatment to carburize the surface of the tungsten to tungsten carbide, and heating the carburized anode in a vacuum to convert substantially all the carbide into tungsten carbide of the formula W C, and substantially free of tungsten carbide of the formula WC and also of free carbon.
5. The method as claimed in claim 4, which consists in carburizing the anode disc by heating it to a temperature of between 1040" C. to 1200 C. for a period of a few minutes in a hydro-carbon gas, and then heating the carburized anode in vacuum at a temperature of about 1600 C. to convert substantially all the carbide into tungsten carbide of the formula W C, and substantially free of tungsten carbide of the formula WC and also of free carbon.
6. The method of making an anode for an X-ray generator tube, which consists in making an anode disc of tungsten, heating the anode to a temperature of about 1300 C. in a vacuum of at least 10** mm. of Hg for a few minutes, allowing the disc to cool, heating the disc to a temperature of between 1040 C. to 1200 C. for a few minutes in a hydro-carbon gas to carburize the surface of the tungsten, and then heating the carburized anode in a vacuum at a temperature of about 1600 C. to convert substantially all the carbide into tungsten carbide of the formula W C, and substantially free of tungsten carbide of the formula WC and also of free carbon.
References Cited in the file of this patent UNITED STATES PATENTS 1,852,265 Upp Apr. 5, 1932 1,953,813 Matsushima Apr. 3, 1934 2,658,844 Harbaugh Nov. 10, 1953 2,718,605 Fenner Sept. 20, 1955 2,721,156 Steuck Oct. 18, 1955 2,813,210 Zunick Nov. 12, 1957