Patent Application
20100135466
The invention relates generally to x-ray tubes and, more particularly, to a method and apparatus for creating an improved braze in x-ray tube components.
X-ray systems typically include an x-ray tube, a detector, and a gantry to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then processes the received data, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes include a rotating anode structure for the purpose of distributing heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed.
X-rays may be produced when high-speed electrons are decelerated when directed from the cathode to the target substrate via an electrical potential difference therebetween. The electrons impact a target track material at a focal point and x-rays emit therefrom. The x-ray tube includes a frame, typically constructed from stainless steel, and has an x-ray window formed therein. The x-ray window is typically made from beryllium, or other low-atomic number material. X-rays are emitted through the beryllium window toward a detector array. The x-ray tube may also include a collector attached to a perimeter of the beryllium window at an interface, wherein the collector absorbs stray electrons from the cathode or the target and conducts heat away from the interface. The collector is typically attached to the beryllium window using a brazing process.
Brazing and soldering are commonly used methods for joining two parts involving heating a fusible material, causing it to melt and wet both parts, and allowing the material to cool and bond the parts. However, diffusion and alloying between the fusible material and the parts being joined may result in formation of brittle intermetallic compounds, sometimes referred to as Laves phase intermetallics, which can weaken the braze joint and cause early life failure of the x-ray tube.
Therefore, it would be desirable to have a method for joining two parts, such as a beryllium window and an electron collector, having improved integrity and hermeticity therebetween.
The invention provides an apparatus and method of brazing a first material to a second material having improved integrity and hermeticity therebetween.
According to an aspect of the invention, a bonded assembly includes a member, and a substrate comprising beryllium, the substrate configured to be bonded to the member. The bonded assembly includes a first barrier applied to a surface of the substrate, a second barrier applied to a surface of the first barrier, a bonding material disposed between the second barrier and the member, and wherein the second barrier is configured to prevent dissolution of the first barrier into the bonding material.
In accordance with another aspect of the invention, a method of fusibly joining a substrate and an object includes providing a substrate comprising beryllium, providing an object configured to be fusibly joined to the substrate, depositing a first barrier layer onto a surface of the substrate, wherein the first barrier layer is configured to prevent formation of intermetallics at an interface between the object and the substrate, and depositing a second barrier layer onto a surface of the first barrier layer. The method further includes interposing a fusible material between the second barrier layer and the object, and heating the assembly to a temperature sufficient to create a fuse joint between the substrate and the object via the fusible material.
According to yet another aspect of the invention, an x-ray tube includes a window comprising beryllium, a collector configured to be braze joined to the window, a first diffusion barrier applied to a surface of the window, the first diffusion barrier configured to prevent formation of Laves phase intermetallics, a second diffusion barrier applied to a surface of the first diffusion barrier, and a braze material disposed between the second diffusion barrier and the collector.
Various other features and advantages will be made apparent from the following detailed description and the drawings.
The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention.
In the drawings:
As shown in
A processor 20 receives the signals from the detector 18 and generates an image corresponding to the object 16 being scanned. A computer 22 communicates with processor 20 to enable an operator, using operator console 24, to control the scanning parameters and to view the generated image. That is, operator console 24 includes some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus that allows an operator to control the x-ray system 10 and view the reconstructed image or other data from computer 22 on a display unit 26. Additionally, console 24 allows an operator to store the generated image in a storage device 28 which may include hard drives, flash memory, compact discs, etc. The operator may also use console 24 to provide commands and instructions to computer 22 for controlling a source controller 30 that provides power and timing signals to x-ray source 12.
X-ray tube 12 includes an electron collector 53 attached to frame 50. Electron collector 53 includes a window 58 proximate a passageway 52. Collector 53 is typically made of copper and may be joined via a braze or solder joint 66 to window 58. Window 58 is typically made of a low atomic number metal, such as beryllium, to allow passage of x-rays therethrough with minimum attenuation. Joint 66, referred to above as a braze or solder joint, may be formed or fused via either a braze or solder process according to embodiments of the invention. Thus, although the embodiments described below may be referred to as using a braze process, one skilled in the art will recognize that the joints and assemblies described herein may be formed via soldering or brazing, and that such terms in no way imply or limit a temperature at which the joint or assembly is formed. Thus, the terms braze and solder may interchangeably and equally refer to joining or fusing two or more materials by heating an additional material therebetween.
In operation, target 56 is spun via a stator (not shown) external to rotor 62. An electric current is applied to filament 55 via feedthrus 77 to heat filament 55 and emit electrons 67 therefrom. A high-voltage electric potential is applied between anode 56 and cathode 60, and the difference therebetween accelerates the emitted electrons 67 from cathode 60 to anode 56. The electrons 67 impinge the target 57 at the target track 86 and x-rays 69 emit therefrom and pass through passageway 52. During operation, stray electrons from cathode 60 may strike collector 53, thereby increasing the temperature in joint 66.
A solder or braze assembly 100 is illustrated in
First diffusion barrier layer 104 may be applied to substrate 102 using chemical vapor deposition, physical vapor deposition, electroplating, and the like. Second diffusion barrier layer 106 may likewise be applied to first diffusion barrier layer 104 using chemical vapor deposition, physical vapor deposition, electroplating, and the like. In the illustrated embodiment, silver is selected as the first diffusion barrier 104 material because, at or below 760° C., silver and beryllium do not form brittle Laves phase intermetallics at an interface therebetween. However, copper-silver based alloys with liquidus temperatures below 760° C. such as Cusiltin-10 would tend to dissolve the silver within first diffusion barrier 104, thus forming Laves phase intermetallics between the copper constituent of the material 108 and the substrate 102. As such, the second diffusion barrier 106, which may be gold or nickel, is included to prevent the silver from dissolving in the braze alloy. Thus, at or below 760° C., neither gold nor nickel will interact with the silver diffusion barrier or interact with the braze alloy to form Laves phase intermetallics at the interfaces therebetween.
An assembly 120 is illustrated in
First diffusion barrier 124 may be applied to the surface 123 of substrate 102 using chemical vapor deposition, physical vapor deposition, electroplating, and the like. Second diffusion barrier 126 may likewise be applied to the surface 125 of first diffusion barrier 124 using chemical vapor deposition, physical vapor deposition, electroplating, and the like. Aluminum is selected as a first diffusion barrier 124 material because, at or below 644° C., aluminum and beryllium do not form brittle Laves phase intermetallics at the interface between the two materials. Second diffusion barrier 126, which may include a transition metal such as cobalt or nickel, will not form brittle Laves phase intermetallics at the aluminum interface nor at the material 108 interface.
Assembly 140 is illustrated in
Referring now to
According to an embodiment of the invention, a bonded assembly includes a member, and a substrate comprising beryllium, the substrate configured to be bonded to the member. The bonded assembly includes a first barrier applied to a surface of the substrate, a second barrier applied to a surface of the first barrier, a bonding material disposed between the second barrier and the member, and wherein the second barrier is configured to prevent dissolution of the first barrier into the bonding material.
In accordance with another embodiment of the invention, a method of fusibly joining a substrate and an object includes providing a substrate comprising beryllium, providing an object configured to be fusibly joined to the substrate, depositing a first barrier layer onto a surface of the substrate, wherein the first barrier layer is configured to prevent formation of intermetallics at an interface between the object and the substrate, and depositing a second barrier layer onto a surface of the first barrier layer. The method further includes interposing a fusible material between the second barrier layer and the object, and heating the assembly to a temperature sufficient to create a fused joint between the substrate and the object via the fusible material.
According to yet another embodiment of the invention, an x-ray tube includes a window comprising beryllium, a collector configured to be braze joined to the window, a first diffusion barrier applied to a surface of the window, the first diffusion barrier configured to prevent formation of Laves phase intermetallics, a second diffusion barrier applied to a surface of the first diffusion barrier, and a braze material disposed between the second diffusion barrier and the collector.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
