Patent Application
20080257939
This invention relates generally to X-ray tube for high voltage applications, and more particularly to a method of brazing uniform structures using a brazing foil.
X-ray systems are generally utilized in various applications for imaging in the medical and non-medical fields. For example, X-ray systems, such as radiographic systems, computed tomography (CT) systems, and tomosynthesis systems, are used to create internal images or views of a patient based on the attenuation of X-ray beams passing through the patient. Based on the X-ray beams, a profile of the patient is created. Alternatively, X-ray systems may also be utilized in non-medical applications, such as detecting minute flaws in equipment or structures and/or scanning baggage at airports. Typically, the X-ray system includes an X-ray tube that is utilized as the source of X-ray beams directed to a detector or film. The X-ray tube includes a cathode assembly and an anode assembly, which may be housed inside an evacuated tube. The X-ray system may operate at high voltages and temperatures, which affect the life expectancy of the X-ray tube.
Because of the voltages and temperatures involved, various problems may occur that cause the X-ray tube to fail. The failures may include electrical stresses, such as high voltage instabilities, surface flashovers, and other insulating failures that reduce the life expectancy of the X-ray tube. That is, the insulator of the X-ray tube may fail because of the electrical stresses. The failures may also include high voltage breakdown of the insulating material due to the insulating material getting too hot because heat transfer from hot surfaces is not properly controlled.
Brazing is a process for joining metal parts as well as metal-to-ceramic parts, often of dissimilar composition, to each other. Typically, a brazing filler metal that has a melting point lower than that of the base material parts to be joined is interposed between the parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the brazing filler metal. Upon cooling, a strong joint is formed.
The applied pressure brazing techniques commonly used to braze components together are unable to produce a reliable braze. Common problems are misalignment of brazed parts, braze joint thickness not being uniform and predictable, voids or holes forming in the braze part of the assembly, and overflow of braze material into undesirable regions of the assembly due to sensitivity to need for symmetric load application.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a braze joint that has uniform thickness. In particular, there is a need for a new technique to overcome the poor braze joint control methods of today, which can lead to thermal stresses and thermal runaway conditions causing X-ray tubes to fail in electrical or mechanical manner, in X-ray tubes from mal-formed brazes.
The above-mentioned shortcomings, disadvantages and problems are addressed herein, which will be understood by reading and studying the following specification.
This invention is embodied in a brazing method comprising the steps of interposing an interlayer in foil form between components to be joined, providing a gap setting surface, assembling the components in a stack consisting of gap setting surface and the components separated by said interlayer foil, applying a controlled load on the top of the stack, heating the assembly under suitable conditions to a temperature at which the interlayer melts and reacts with the components, and cooling the assembly to produce a structure with uniform joints having substantially uniform dimensions, uniform layer of braze material for thermal conduction, and substantially uniform strength.
In another aspect, the present invention provides an X-ray tube. The X-ray tube includes an anode assembly configured to emit X-ray beams and a cathode assembly configured to emit electrons towards the anode assembly. A gap setting surface positioned between the cathode cup electrodes and an insulator controls the distance between the two components.
Apparatus, systems, and methods of varying scope are described herein. In addition to the aspects and advantages described in this summary, further aspects and advantages will become apparent by reference to the drawings and by reading the detailed description that follows.
In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken in a limiting sense.
The cathode circuit 36 includes a cathode assembly 48, a cathode receptacle 52, and a cathode driving circuit (not shown). The cathode assembly 48 includes a cathode cup 54 that is located within the insert 34 whereas the cathode terminal board 50 and the receptacle 52 are located outside of the insert 34. The receptacle 52 is coupled to a high voltage generator and filament drive circuit (both of which are not shown). The cathode cup 54 is coupled to an arm 56, which extends from a base or shell 58. The shell 58 separates a vacuum cavity 60 of the insert 34 from an oil cavity 62 that surrounds the insert 34. High voltage leads 64 extend from one or more filaments 66 (only one is shown) in the cathode cup 54 through the arm 56 and the shell 58 to the terminal board 50. The terminal board 50 is coupled to the receptacle 52 via the high voltage connections 68.
Power is received by the filaments 66 from the receptacle 52 and under control of the driving circuit. The power is supplied from the receptacle 52 to the terminal board 50 via the high voltage connections 68. The power is then supplied from the terminal board 50 to the filaments 66 via the high voltage leads 64. Besides holding HV leads in place, the terminal board 50 may also be utilized to offer additional bleeding resistance for the filaments 66, when temporary over-voltage occurs.
The cathode circuit 36 also includes multiple voltage-clamping devices 70. The clamping devices 70 may be coupled in parallel with and are substantially higher in resistance than the filaments 66. As such, the voltage clamping devices 70 do not affect voltage regulation of the cathode circuit 36 and performance of the driving circuit. The voltage-clamping devices 70 include a first clamping device 72, a second clamping device 74, and a third clamping device 76. The first clamping device 72 is coupled to the leads 64.
The second clamping device 74 is coupled to the connections 68 between the terminal board 50 and the receptacle 52. The third clamping device 76 is coupled to the HV connections 68 within the receptacle 52. The clamping devices 70 have a predetermined resistance and prevent overvoltage transients from occurring between the leads 64 and the HV connections 68. The clamping devices 70 perform as insulators and as voltage limiters.
When voltage potential between high voltage elements, such as between the leads 64 and the HV connections 68, is below a predetermined voltage level the clamping devices 70 perform as insulators and isolate the elements 64 and 68 from each other. The clamping devices 70 prevent the flow of current between the high voltage elements 64 and 68. When the voltage between the high voltage elements 64 or 68 is greater than or equal to the predetermined voltage level the clamping devices 70 allow the flow of current between the high voltage elements 64 or 68. Thus, the clamping devices 70 prevent voltage potential between the high voltage elements 64 or 68 from exceeding the predetermined voltage level.
The clamping devices 70 may be of various types, styles, sizes, shapes, and may be formed of various materials. The clamping devices 70 may be in the form of varistors, feed through varistors, resistive jumpers, and bleeding resistors, and may be formed of a resistive material, a resistive epoxy, and a semi-conductor epoxy. The clamping devices 70 may be in the form of a terminal board formed of resistive material or may be in some other form known in the art. The clamping devices 70 may, for example, be annular or disk like in shape, as shown. In addition, any number of clamping devices 70 may be used throughout the imaging tube 30.
The clamping devices 70 may be coupled between any high voltage elements including between cathode filaments, a cathode grid, and a cathode common, such as filaments 66, cathode cup 54, and return lines of the filaments. The clamping devices 70 may be voltage-clamping devices or may perform as current limiting devices. Several examples of clamping devices are described below. The clamping devices 70 may be formed of oxide zinc, silicone carbide, some other material known in the art, or a combination thereof. The use of varistors limits high frequency high voltage transients due to quick response time of the varistors.
Although the clamping devices described above and in the following figures are shown in particular locations, these locations are for example purposes only. The clamping devices may be located elsewhere.
The cathode cup assembly 54 includes the cathode cup 54 having the filament 202. The filament 202 has a large focal spot end and a common end 206. Each of the ends 206 are coupled to a pair of high voltage extensions that extend through a pair of feed through insulators. The feed through insulators are coupled to the cup 54 via washers. The washers are brazed to the cup 54.
Method 700 includes selecting a cathode part 702, selecting an insulator 704, assembling the cathode part and insulator, and brazing the cathode part with insulator. It should be noted that the selecting of the cathode part 702 and insulator 704 can be done sequentially or in parallel since these actions are independent of each other.
Method 700 begins with action 702. In action 702 the cathode part is selected. The cathode part can be a cathode post, cathode extender, or any other part of a cathode assembly.
In action 704, an insulator is selected. The insulator may include various aspects and structures that are utilized to provide support for the cathode part and the gap setting surface or pins 70. The insulator is made of electrically insulated material, such as ceramic.
In action 706, the cathode part selected in action 702 and the selected insulator 704 are assembled. The assembling of these parts is more than mere stacking of cathode part and the insulator, the gap setting surface 408 is also included to assure a desired gap, the braze foil 404 is included to braze the parts together, and the controlled load are all combined to acquire a controlled braze.
In action 708, the braze process is performed on the assembled components. Once the brazing material is applied, the assembled component 400 is heated to just over the melting point of the brazing material. The heating permits the assembled component 400 to become sufficiently coated or wetted by the brazing material. Preferably, the assembled component 400 is heated over the melting point of the braze foil 404, more preferably to about 7% over the melting point. The temperature, however, should be much lower than the melting points of the cathode part and the insulator. Thus, heating the assembled component to slightly over melting point of the braze foil 404 will melt only the brazing alloy and not the cathode part or insulator. After the heating assembly 400 is cooled. Since the gap control surface creates a uniform plane each joint between the components has optimum geometry with uniform dimensions and substantially uniform strength.
Method 800 includes action for selecting components 802, selecting an interlayer foil 804, selecting a gap setting surface 806, assembling the items selected in actions 802 through 806, applying heat to the assembly 810, and cooling the heated assembly 812.
Method 800 begins with action 802 of selecting components that need to be brazed together. The components can be any metal or material that needs to be permanently attached to another metal or material. In imaging the components are a combination of cathode cup, cathode extender, ceramic material, plastic material, insulator, metal alloy, and metal composite. In action 804, the interlayer foil or braze foil is selected. The braze foil (304, 404, 506) can have any arbitrary thickness, and it can be chemically machined or laser cut into a shape such as shown, to match the geometry of a cathode cup or any other shape. In action 806, the gap setting surface is selected. In actions 808 through 812 the components are assembled, heated, and cooled to produce braze components. Once the brazing material is applied and the components are secured in place, the assembly is heated to just over the melting point of the brazing material. The heating permits the liquification of braze material (304, 404) and bonding with component parts.
A technique for brazing components for an x-ray tube is described. A technical effect of the technique is that the joint between the components has optimum geometry with substantially uniform dimensions and substantially uniform strength. Although specific embodiments are illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations.
In particular, one of skill in the art will readily appreciate that the names of the methods and apparatus are not intended to limit embodiments. Furthermore, additional methods and apparatus can be added to the components, functions can be rearranged among the components, and new components to correspond to future enhancements and physical devices used in embodiments can be introduced without departing from the scope of embodiments. One of skill in the art will readily recognize that embodiments are applicable to future heating devices, different braze foils, and new alloys and metals.
