Abstract:
A vacuum housing for an x-ray tube has a wall provided with a protective layer at least in an area near a focal spot of the x-ray tube that arises in the operation of the x-ray tube. In order to increase the lifespan, the protective layer of a material which can be one or more of niobium, zirconium, hafnium, vanadium, tantalum, chromium, molybdenum, tungsten, their alloys, AlN and/or gas turbine protective layers. The protective layer is applied to the wall by plasma spraying.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention concerns a vacuum housing for an x-ray tube having a wall provided with a protective layer at least in an area near a focal spot of the x-ray tube that arises in the operation of the x-ray tube, as well as a method for production of such a vacuum housing.  
         [0003]     2. Description of the Prior Art  
         [0004]     Particularly in the case of rotating anode x-ray tubes designed to save space, thermoshock stresses of the vacuum housing can occur in the vicinity of the focal spot. These stresses result from a secondary electron cloud that expands centrally from the focal spot with the beginning of the x-ray emission, The energy density of such a secondary electron cloud when incident on the inside of the vacuum enclosure can be so high that, between the cooled outside of the vacuum housing wall and the inside, a temperature difference occurs that leads to tensile stresses upon cooling and to thermoshock cracks or fissures given large stresses. Attempts to achieve relief by reduction of the wall thickness of the vacuum housing in the critical region, and thus to achieve a reduction of the thermoshock stress in the material does not lead to the desired results. Since a high local operating temperature and a high atmospheric pressure act on the cover of the evacuated housing, the material is no longer stable and begins to creep given a significant wall thickness reduction.  
         [0005]     In German OS 199 14 825 and German OS 100 23 356, the inside of the vacuum housing is provided with a layer having a thermal absorption coefficient that is increased relative to the uncoated material of the vacuum housing, The optical properties of the inside of the vacuum housing should be changed to the affect that thermal radiation is absorbed better, and consequently the waste heat introduced in the anode upon the x-ray beam generation is dissipated faster. Unfortunately, such coatings do not offer protection against a thermoshock stress.  
         [0006]     In order to increase the thermoshock resistance, and thus to increase the lifespan of the vacuum housing, it is proposed in German OS 196 12 220 to fashion the wall area that is adjacent the focal spot of the x-ray tube in operation of the x-ray tube from composite material formed of at least two layers. Its inner layer should exhibit a lower thermal expansion factor and/or a lower modulus of elasticity than the material of the outer layer. Feritic iron alloys or austenitic stainless steels are suited for the outer layer. Copper, nickel or at least alloys containing copper or nickel, in particular nickel-based alloys (known as compensation alloys with low thermal expansion factors) that contain iron, nickel and cobalt, as well as alloys comprising iron and nickel and also iron and palladium, are proposed as material for the inner layer. The wall of the vacuum housing can be the outer layer of the composite material that is provided with the inner layer serving as a protective layer.  
         [0007]     With increasing extent of the stress level and the stress amount by the x-ray beam, however, the danger of thermal fatigue cracks (which is associated with a decreasing lifespan of the vacuum housing or of the x-ray tube) also increases with this type of coating.  
       SUMMARY OF THE INVENTION  
       [0008]     An object of the present invention is to provide a vacuum housing of the type initially described that exhibits a long lifespan in spite of increasing extent of the stress level and of the stress amount by the secondary electrons, as well as to provide a method to produce such a vacuum housing.  
         [0009]     According to the invention, this object is achieved by a vacuum housing for an x-ray tube having a protective layer formed on a material selected from the groups consisting a niobium, zirconium, hafnium, vanadium, tantalum, chromium, molybdenum or tungsten, and their alloys, AlN, and gas turbine protective layers.  
         [0010]     By applying a protective layer composed of such a material, a functional separation is achieved in a first inner material region that is “sacrificed” and fatigued and can form thermoshock cracks, and in a second, outer material region or the base material that is not so highly stressed and therefore is not deformed and furthermore remains vacuum-sealed. As a “deformation protective layer”, in comparison with the base material, this protective layer should consequently compensate the largest part of the thermoshock fatigue but be relatively thermoshock resistant in performing this function. The stressed materials ensure this function and thus increase the lifespan of the vacuum housing.  
         [0011]     It has bee established that in particular the partial pressure of the coating material plays a significant role because the temperatures in the stress zone are high. The protective layer material should therefore preferably exhibits a smaller partial pressure than the base material (that is in, particular, an austenitic stainless steel) and thus exhibit a better temperature resistance.  
         [0012]     In particular, it has been established that the protective layer materials should exhibit the following property combination for a high lifespan under the stresses that occur in a vacuum housing; lower thermal expansion factor, lower E-modulus (abbrev. For modulus of elasticity) and lower partial pressure than the wall or base material.  
         [0013]     Due to the small thermal expansion factors, smaller mechanical thermal stresses ensue. Austenitic stainless steel has, for example, a high thermal expansion factor of 16·10-6/K. The thermal expansion factors of niobium, molybdenum, tantalum and tungsten are in the range of 4 to 6.5·10-6/K. Due to the smaller thermal expansion factor, the volume increase of the protective layer at the surface is smaller given temperature influence, and thus the danger of the separation of slip planes and the creation of cracks decreases.  
         [0014]     The materials preferably also exhibit a ductile deformation property. Given fatigue cracks, particles are formed that could break away from the material surface, which would disadvantageously affect the stress resistance of the tube. These particles are structural constituents that are released given crack formation, and the stress concentration can increase by the release of such particles.  
         [0015]     In particular niobium and/or its alloys are considered as preferred examples for this property combination. Niobium possesses the most advantageous combination of low partial pressure at increased operation temperature, small thermal expansion factors, small E-modulus and thus high ductility and low elastic limit. In contrast to the known materials nickel and cobalt, niobium exhibits a partial pressure and a low E-modulus which advantageously account for the temperature resistance and the ductility properties.  
         [0016]     In addition to niobium, further transition metals from the 4th, 5th and 6th neighboring groups of the periodic table of the chemical elements and/or their alloys are considered as coating materials, in particular zirconium, hafnium, vanadium, tantalum, chromium, molybdenum or tungsten.  
         [0017]     Tantalum and vanadium behave relatively similar to niobium and likewise belong to the preferred materials for a protective layer. However, relative to tantalum niobium exhibits the advantage that it is in comparison less significantly embrittled, which decreases the danger of particle formation when thermoshock cracks form. Relative to vanadium, niobium exhibits a lower partial pressure and thus a higher temperature resistance.  
         [0018]     Molybdenum and tungsten exhibit lower partial pressure, a lower thermal expansion and an excellent heat conductivity, however, in comparison to niobium the danger of embrittling is greater. This is also true for tungsten-based alloys that, in comparison to pure tungsten exhibit a higher partial pressure and thus a better temperature resistance.  
         [0019]     Furthermore, AlN is suitable for a protective layer. AlN has a good heat conductivity, which advantageously affects the protective layer properties and thus the lifespan of the vacuum housing.  
         [0020]     Materials that are used for gas turbine protective layers are also suited for protective layers. They noticeably increase the lifespan of the vacuum housing. They are preferably comprised of a metallic layer made up of NiCrAlY or CoCrAlY with or without additives of rhenium, on which an oxidic coating is applied.  
         [0021]     Gas turbine protective layers exhibit an advantageous lower partial pressure in comparison to austenitic stainless steel given temperatures up to 1,100° C., such that they are particularly suited as protective layer materials.  
         [0022]     In addition to the particular material selection for the protective layer, the location of the coating also plays a role for the increase of the lifespan. It has been established that in particular the face of the opening of the shaft facing the anode space is stressed in a vacuum housing that has an anode chamber and a cathode chamber and a shaft connecting both these chambers, with the primary electron beam striking the anode through the shaft. For low material usage, in order to noticeably increase the lifespan preferably only the face of this opening is coated. It is sufficient to coat the face of the opening partially or in sections, and in particularly to coat only half, meaning in a semicircle on the side facing the radiation window.  
         [0023]     For very high stresses, the face of the opening can also be completely coated, in the sense of all the way around.  
         [0024]     The layer thickness or the protective layer preferably is 0.5 to 3 mm, with layer thicknesses of 0.5 to 0.7 mm giving quite good results.  
         [0025]     As is known from German OS 196 12 220, the protective layer can be part of a two-layer composite material that is used in the wall of the vacuum housing, however the coating can be more economically applied on the base material with a coating method.  
         [0026]     In the method according to the invention, the protective layer composed of the aforementioned material is applied on the wall by plasma spraying. In this method, a powder is melted in a plasma flame and accelerated and applied on the carrier material. For coating with niobium and/or its alloys, vacuum plasma spraying (VPS) is the most economical and also advantageous method in order to prevent niobium from entering into a brittle connection with another element. In particular, atmospheric plasma spraying (APS) is suitable for coating with gas turbine protective layers.  
         [0027]     The desired protective layer thickness between 0.5 and 3 mm can be best applied by plasma spraying. In addition, the location of the coating can be arbitrarily limited with this method, which also makes possible the coating of only half the face of the opening. Moreover, this delimitation allows the diameter of the shaft with regard to the electron beam deflection to be not significantly reduced, which is an important criterion.  
         [0028]     The soldering known from German OS 196 12 220 is also suitable as an application method, preferably soldering of a protective sleeve (“thermosleeve”). Plasma spraying is preferred, however, because soldering is expensive due to the necessary non-deformation for a material termination, and the risk of embrittling of the solder connection (solder joint) exists. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0029]      FIG. 1  is a longitudinal section through a vacuum housing of an x-ray tube.  
         [0030]      FIG. 2  is a plan view of a non-coated shaft opening face of the cover with thermoshock cracks.  
         [0031]      FIG. 3  is a detailed view of  FIG. 2 , wherein half of the opening face is provided with a niobium protective layer.  
         [0032]      FIG. 4  is a stress-number curve for an austenitic stainless steel.  
         [0033]      FIG. 5  is a partial section through a vacuum housing of an x-ray tube according to the prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]      FIG. 5  shows a vacuum housing  1  (here a vacuum housing of a high-capacity x-ray tube for computed tomography) according to the prior art. A cathode arrangement  3  that includes a thermionic cathode  3   c  disposed in a focusing groove  3   a  of a cathode cover  3   b  is mounted in an insulator  2 . Emanating from cathode  3   c  is an electron beam E (indicated dashed) that strikes a focal spot BF on the incident surface  4   a  of a rotating anode  4 . The vacuum housing  1  is provided with a beam exit window (formed, for example, from beryllium) via which, in the operation of the x-ray tube, the x-ray beam E (the central and edge rays of which are indicated dashed and designated with ZS and RS) originating from the focal spot BF exits. Overall, the vacuum housing  1  is formed of a chamber  6  (anode chamber) containing the rotating anode  4  and a chamber  7  (cathode chamber) containing the cathode arrangement  3 , that are connected with a shaft  8 .  
         [0035]     When, upon activation of the x-ray tube, the secondary electron stream begins to flow, and the vacuum housing  1  is subject to a thermoshock-like stress in its region adjacent to the focal spot BF. In order to increase the thermoshock resistance and to prevent thermoshock cracks, the wall  9  of the vacuum housing in the stressed region is provided with a protective layer  10 .  
         [0036]     Using a somewhat modified embodiment of a vacuum housing  1  which, however, is comprised of the same components as the vacuum housing according to  FIG. 5 ,  FIG. 1  clarifies the region of the hole face  11  (endangered by the secondary electrons) of a shaft  8  arranged on a cover  12  of the vacuum housing  1 , as well as the location of the coating. For identification, reference characters corresponding to the components of  FIG. 5  are used.  
         [0037]     The shaft  8  should separate the cathode chamber  7  from the anode chamber  8 . In this manner, the cathode  3  is protected from the heat radiation of the entire anode plate and, due to the spatial separation, improves the stress resistance. Moreover, such a shaft  8  is an advantageous location to apply a deflection coil for the electron beam.  
         [0038]     The opening of the shaft  8  in the cover  12  of the x-ray tube, i.e. the face  11  of the opening of the shaft  8 , is particularly stressed upon operation of the x-ray tube. Upon each scan, a secondary electron cloud, the power density of which can be estimated to be at least 100 W/cm 2  is incident on this opening face  11 . At the beginning of the scan, a thermoshock ensues that fatigues the material in the opening face  11 . The consequences are thermoshock cracks TR in the opening face  11  in the junction to the cover  12 , as the exposure in  FIG. 2  clarifies.  FIG. 2  is a microscopic exposure of the opening face  11  (in a viewing direction shown in  FIG. 1  with an arrow BL) that shows the degree of the thermoshock cracks if the opening face  11  were not coated. The formation of the thermoshock cracks is dependent not on the duration of the stress, but rather only on the amplitude of the stress level and the number of stresses.  
         [0039]     To reduce the thermoshock stress of the region of the wall of the vacuum housing  1  adjacent to the focal spot BR, the regions of the vacuum housing particularly at risk of material fatigue are provided with a protective layer  10 . Particularly at risk is the facing region  11  of the shaft  8 , and here in particular the opening facing half  13  that points in the direction of the beam output window  5 .  
         [0040]     The detail view of  FIG. 3  shows an x-ray tube with a vacuum housing  1  with potential  14  and cover  12 . At the shaft  8 , half of the opening face  11  has been coated with a thermal fatigue protective layer  10 , namely on side  13  or the semicircle in the direction of the beam output window  5 , which is particularly at risk.  
         [0041]     The protective layer  10  that, in particular can be comprised of niobium, was applied with a vacuum plasma spray method and here exhibits a thickness of 0.5-0.7 mm. Niobium has proven to be particularly preferred for a protective layer. The reasons for this are that niobium is a very soft and ductile metal. Moreover, given very high temperature values it has a very small partial pressure or vapor pressure. The thermal expansion factor is very small, which makes the material more resistant to thermal fatigue than the wall material made of stainless steel. The ductility leads to only a very low probability that particles are released given structure fatigue.  
         [0042]     The following explanations further explain the functioning of the protective layer.  
         [0043]     The protective layer already compensates the largest part of the temperature difference. The mechanical stresses of the base material provided with a protective layer are thereby less high. This already leads to a substantial improvement of the present lifespan.  
         [0044]     It should be considered that a lifespan expectancy of a factor of 4 can be achieved in steels given a reduction of the stress by approximately 30%. This connection between the stress change and the break or the breaking load number of operations given specific cyclical stresses is explained using the stress-number diagram for austenitic stainless steel according to  FIG. 4 .  
         [0045]     The cycle number until the break depends on the −4th power of the mechanical stress. Given the reduction of the stress σ 1  by approximately 30% to a stress σ 2 , a lifespan extension results of  
           log   ⁡     (       σ   2     /     σ   1       )       /     log   ⁡     (       N   B2     /     N   B1       )         =           -   1     /   4     ⇒     
     ⁢       log   ⁡     (     1   /   0.7     )       /     log   ⁡     (       N   B2     /     N   B1       )           =           -   1     /   4     ⇒           ⁢   0.6196     =           ⁢     
     ⁢       log   ⁡     (       N   B2     /     N   B1       )       ⇒       N   B1     ≈       N   B2     /   0.24       ⇒       N   B1     ≈     4   ⁢     N   B2                   
        whereby N B1 , N B2  are the corresponding breaking load number of operations at the stresses σ 1 , σ 2 .        
 
         [0047]     This means that the breaking load number of operations N B1  is increased by approximately a factor of 4 given a reduction of the stress by 30%. However, an increase of the breaking load number of operations by a factor of 2 is already a great achievement in a stress range of approximately 20,000 exposures.  
         [0048]     Without the inventive coating with the stressed protective layer materials, thermoshock cracks result in the stressed areas. Examinations show the features of a stepped tear progression that exhibit an appearance similar to vibration bands given fatigue breaks. These vibration bands are created upon every scan or a plurality of scans. The start of the crack was in the face of the opening of the shaft of the cover, which is thus the most-stressed location, as is also shown in  FIG. 2 . Further examinations of the half of the hole face coated with niobium using surface parallel grindings showed the function of the protective layer. Only the protective layer exhibited cracks, while the base material without cracks exhibited a negligibly small crack.  
         [0049]     The invention can also be employed in x-ray tubes known as single-pole (unipolar) x-ray tubes in which the vacuum housing and the anode are at a common potential. The invention also can be applied in tubes known as bipolar x-ray tubes in which the vacuum housing is at a potential that is between that of the anode and that of the cathode. Although the invention is described herein in the example of rotating anode x-ray tubes, fixed anode x-ray tubes also can be provided with an inventive vacuum housing.  
         [0050]     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.