Patent Application
20100246774
Embodiments of the invention relate generally to x-ray tubes and, more particularly, to an x-ray tube incorporating a spiral groove bearing (SGB) therein.
X-ray systems typically include an x-ray tube, a detector, and a support structure for 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 emits data received, 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 an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes include a cathode and an anode located within a high-vacuum environment. The anode structure is typically supported by ball bearings and is rotated for the purpose of distributing the heat generated at a focal spot. Typically, an induction motor is employed to rotate the anode, the 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 an anode-to-cathode 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. This places stringent demands on the ball bearings.
A liquid metal bearing may be employed in lieu of ball bearings. Advantages of liquid metal bearings include a high load capability and a high heat transfer capability due to an increased amount of contact area as compared to a ball bearing. Advantages also include low acoustic noise operation as is commonly understood in the art. Gallium and alloys thereof are typically used as the liquid metal, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube.
Gallium tends to be highly reactive and corrosive. Thus, a base metal that is resistant to such corrosion is desirable. As such, a refractory metal such as molybdenum is typically used as the base material for an SGB, and spiral grooves are typically machined in the surface, as known in the art, in order to provide a pumping action to maintain the liquid metal in its desired location. Not only is such material resistant to corrosion, but it tends to be vacuum-compatible and thus lends itself to an x-ray tube application. However, one concern that may be encountered in the use of a liquid metal is that of ensuring adequate wetability of bearing surfaces with the liquid metal. When adequate wetability does not occur, the liquid metal does not completely fill the SGB and the SGB may not uniformly distribute the liquid metal throughout the gap during use, thus shortening the life of the x-ray tube.
Wetability may be negatively affected due to exposure of the base metal to air or moisture prior to and/or during assembly, causing an oxide layer to form thereon. The oxide layer, in turn, deteriorates the wetability of the surface of the part with the liquid metal. Known techniques have been employed to improve or maintain the wetability of the base material under these circumstances. One known technique includes firing the bearing surfaces at approximately 800° C. in hydrogen and then storing the parts in a oxygen-protective atmosphere, like nitrogen or argon, until use. Another known technique includes coating the bearing parts with a carbide, boride, or nitride using, for instance, a physical vapor deposition (PVD) technique.
Another known technique includes applying molybdenum as a diffusion barrier using PVD. However, although molybdenum may be employed when applying such a diffusion barrier using PVD, the base material of the diffusion barrier is typically identical to the base material. Alternatively, materials applied via PVD using materials that differ from the base material tend to be limited to 2000 nm thicknesses for proper application in order to avoid cracking due to thermal mismatch of the applied barrier and the base metal. The thermal mismatch may be mitigated to an extent by employing a coating having an expansion coefficient that is similar to the base metal. However, such solutions tend to limit the number of base metal/coating options. Further, because of the thickness limitation, such materials are precluded from post-machining, thus necessitating that the diffusion barrier be applied having thicknesses that fall within the desired final tolerances of the final part. Also, because of the thickness limitation, such solutions to improve wetability still necessitate that the base material be resistive to the corrosive effects of the liquid metal, such as molybdenum. However, molybdenum tends to be expensive, both as a base material, and in terms of machining and processing.
One technique for minimizing base material expense and improving functionality is to include the preferred base metal (i.e., molybdenum) only in regions that will contact liquid metal. An extension made of a less expensive material may then be brazed or otherwise attached thereto, the extension serving as a mechanical connection as support for an anode. In other words, as an example, a stationary center shaft may support a rotatable support structure having an anode attached thereto. The center shaft may be made entirely of the preferred base metal, or the cost thereof may be reduced by attaching a less expensive steel thereto via a braze or other attachment method, thus reducing the total amount of the preferred base metal. Such a design may result in cost savings because of the less expensive steel portion being used in lieu of the preferred base metal. However, cost savings achieved while using this technique are typically offset to an extent by the additional attachment processing, such as by attaching the extension thereto having a hermetic seal.
One drawback in the use of molybdenum is that molybdenum can form an intermetallic layer with gallium that is not stable at typical operating temperatures of an SGB. Thus, an intermetallic layer tends to form as a result of contact between a solid molybdenum surface and liquid gallium, acting as an abrasive if it tends to break down, or particulate, on contact between stationary and rotating parts, which can lead to early life failure of the SGB. Formation of the intermetallic layer is a function of temperature and follows Arrhenius aging principles as is known in the art.
Thus, an SGB may be built of molybdenum and having gallium as a liquid metal therein, and with proper handling and processing a SGB made as such may provide adequate performance for the life of the x-ray tube. However, a base metal of molybdenum tends to be costly, and an alternative SGB having a molybdenum coating only in regions of contact with gallium typically includes a costly braze step. Machining of molybdenum or a molybdenum coating includes additional costs as well, and an additional wetting step (i.e., firing in a hydrogen environment) is a costly processing step associated with a molybdenum-based SGB. Further, molybdenum forms an intermetallic that is unstable at typical operating temperatures and, as imaging applications tend toward an increase in power, operating temperatures likewise increase, thus accelerating the growth and formation of the molybdenum-gallium intermetallic layer.
Therefore, it would be desirable to have an apparatus and method that reduces total costs associated with fabricating and using an SGB.
Embodiments of the invention provide an apparatus and method that overcome the aforementioned drawbacks by providing a material on the surfaces of SGB components.
According to an aspect of the invention, an x-ray tube includes a cathode and a target assembly positioned to receive electrons emitted from the cathode. The target assembly includes a target, and a spiral groove bearing (SGB) configured to support the target. The SGB includes a rotatable component having a first surface and a first material attached to the first surface, a stationary component having a second surface and a second material attached to the second surface, the stationary component positioned such that a gap is formed between the first material and the second material, and a liquid metal positioned in the gap, wherein at least one of the first and second materials comprises tantalum.
In accordance with another aspect of the invention, a target assembly includes a shaft having a first material attached to an outer surface thereof, a sleeve configured to support a target and having a second material attached to an inner surface thereof, and a liquid metal positioned between the first material and the second material, wherein at least one of the first and second materials comprises tantalum.
According to yet another aspect of the invention, a method of manufacturing a target assembly for an x-ray tube comprising the steps of providing a shaft having an outer surface material and having an outer diameter, providing a sleeve having an aperture exposing an inner surface material of the sleeve, wherein a diameter of the inner surface material is greater than the outer diameter of the outer surface material, applying a first layer to the inner surface material, and applying a second layer to the outer surface material. The method further includes acid etching at least one of the first layer and the second layer to remove an oxide therefrom, attaching a target to one of the shaft and the sleeve, inserting the shaft into the sleeve to form a shaft sleeve assembly, and applying a liquid metal to one of the first layer and the second layer of the shaft sleeve assembly.
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 12 receives the signals from the detector 10 and generates an image corresponding to the object 8 being scanned. A computer 14 communicates with processor 12 to enable an operator, using an operator console 16, to control the scanning parameters and to view the generated image. That is, operator console 16 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 imaging system 2 and view the reconstructed image or other data from computer 14 on a display unit 18. Additionally, operator console 16 allows an operator to store the generated image in a storage device 20 which may include hard drives, flash memory, compact discs, etc. The operator may also use operator console 16 to provide commands and instructions to computer 14 for controlling a source controller 22 that provides power and timing signals to x-ray source 4.
X-rays 6 are produced when high-speed electrons are suddenly decelerated when directed from the cathode 36 to the anode 32 via a potential difference therebetween of, for example, 60 thousand volts or more in the case of CT applications. The x-rays 6 are emitted through radiation emission passage 28 toward a detector array, such as detector 10 of
Referring now to
As illustrated in
In one embodiment, center shaft 41 includes a radial projection 54 positioned in a radial cavity 56 of sleeve 42, and sleeve 42 may include a removable cap 58 configured to allow assembly of components. Radial projection 54 limits axial motion of sleeve 42 relative to center shaft 41, and, as illustrated, liquid metal 50 is also included between radial projection 54 and sleeve 42, and between cap 58 and center shaft 41. Radial projection 54 need not be limited in axial length, but may be extended in axial length to provide additional mechanical support of components. In one embodiment, radial projection 54 includes herringbone or helical grooves along an axial surface 55. In another embodiment, radial projection 54 extends over an entire axial length of sleeve 42 of bearing assembly 34. In this embodiment, radial projection 54 takes on a cylindrical shape and is positioned within a cylindrical aperture within sleeve 42. In one embodiment, center shaft 41 includes a cavity 60 passing therethrough and configured to pass a coolant therein. Cavity 60 may include a feed line 62 positioned therein to pass a coolant 64 into cavity 60 at an inlet 66 and then exit therefrom at an outlet 68. As such, coolant 64 enables heat generated from anode 32 of x-ray tube 4 to be extracted therefrom and transferred external to x-ray tube 4. In one embodiment, bearing assembly 34 includes a removable endcap 69.
Center shaft 41, sleeve 42, removable cap 58, and endcap 69 include respective materials or coatings 70, 72 positioned thereon to prevent corrosion of their base material, thus enabling less expensive base materials to be used therein, according to embodiments of the invention. As will be discussed, materials or coatings 70, 72 may be applied as coatings (such as in
Tantalum forms an intermetallic layer when in contact with gallium, but it is recognized that tantalum forms a much more high temperature stable intermetallic layer at elevated temperature when compared with molybdenum. Coatings 70, 72 thus comprise tantalum, according to an embodiment of the invention. In another embodiment, tantalum may be combined with another material such as a refractory metal that may include molybdenum or tungsten. Coatings 70, 72 may be applied, according to embodiments of the invention, by molten salt deposition, electroplating, chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma-enhanced PVD (PE-PVD), a laser-enhanced process (such as laser-enhanced net shaping known as LENS®, LENS® is a registered trademark of Sandia Corporation, Albuquerque, N. Mex.), cold spray, sputtering, and combinations thereof. Coatings 70, 72 may be applied in thicknesses selected according to process conditions and desired outcomes, yet each has specific benefits associated therewith. In one embodiment a part to be coated may be masked, as understood in the art, in order that the coating 70 or coating 72 may be applied via, for instance, CVD, to a specific location. In another embodiment, coatings 70, 72 may be applied via, for instance, CVD and then removed via machining or other known removal technique in areas where coatings 70, 72 are not desired.
Referring still to
In embodiments, coatings 70, 72 are applied to thicknesses up to 1 mm or thicker. Such processes may include plasma spray, molten salt deposition, LENS®, and cold spray. Because of the thicknesses capable from these processes, the processes likewise support a post-machining process according to the invention by enabling grooves to be cut from the applied material during post-machining Cold spray, for instance, may be used to apply coatings 70, 72 by propelling fine powder particles at high velocities using a compressed gas. The particles are relatively cold, so bulk reaction on impact is in solid state, and there is little to no oxidation. Because the particles typically do not melt during the process, there is relatively little shrinkage upon cooling of the base material. Molten salt deposition may be used to apply coatings 70, 72 to sufficient thicknesses as well. The process includes electrolytic deposition of tantalum in a molten salt mixture. The salt mixture, in embodiments of the invention and as understood in the art, may include NaCl, KCl, and the like. During deposition, as understood in the art, the parts are cathodically polarized and the molten salt typically includes a source of ions of the refractory metal. It is to be recognized that the processes described are but examples for application of coatings according to the invention, and that any number of coating processes may be employed for application of a coating according to the invention.
The LENS® process typically includes a laser consolidation process to impinge and heat a region of a base material to cause the base material to melt. Typically, heat is applied to a base material via one or more lasers sufficiently to cause the base material to melt, and a powdered material (such as a refractory metal) is simultaneously supplied through a feeder to the heated region. Thus, the added material melts and bonds with the underlying material. Because LENS® uses a powder that is fed during the process, the powder may comprise a varying degree of powder components in order to tailor the coating density through its thickness. In other words, as an example, for a tantalum coating on a stainless steel base material such as 304 or 316 stainless, the coating may be applied at the beginning of the process having a low concentration of tantalum and a high concentration of base material. As the process continues during application of the coating, the percentage or concentration of tantalum may be increased while that of the tool steel is decreased, and such change may continue until 100% tantalum is applied.
Other processes, as described above, may likewise be used to apply a graded structure according to embodiments of the invention. In one example, a graded coating may be applied using CVD, by applying multiple layers having varying percentages of materials therein. As is understood in the art, any of the processes described above that are capable of applying a coating or layer having a controlled amount of a mixture may likewise be employed to apply a graded coating through multiple layers by varying the concentrations of components therein, according to embodiments of the invention. In addition, one skilled in the art will recognize that the graded coatings applied may include not only two, but multiple components to apply any number of coatings, according to the invention.
As such, a material may be applied in graded layers of varying concentration of tantalum that results in a gradual change in the thermal expansion coefficient through the thickness of the coating. Because, in this example, the coating near the surface of the base material has a high concentration of base material, it has a thermal expansion coefficient similar to that of the base material. The gradations change to increasing levels of tantalum until 100% tantalum coating is achieved on the outermost portions of the coating. Thus, thermal mismatch is minimized in contiguous portions of the coating, while a desired outer surface has that of tantalum.
Electroplating and CVD may be employed to apply coatings having thicknesses greater than, for instance, 0.1 mm, such as from 0.1 to 2 mm in thickness or greater. Such processes typically support a post-machining process by enabling machining to be performed by cutting grooves entirely from the applied coating while avoiding the base material.
Coatings 70, 72 that include tantalum may be applied having the base material maintained at elevated temperature during the coating process in order to reduce compressive residual stresses in the coatings at operational temperature according to embodiments of the invention. Such an approach would enable a broader mismatch of expansion coefficients of the material being applied to the underlying base material, thus enabling selection of both base and coating materials that differ from one another. In other words, such an approach increases the options for base material/coating combinations based on other desirable product attributes, such as, but not being limited to, thermal conductivity, thermal coefficient of expansion, strength, toughness, cost (both raw materials and processing), and weldability/joinability.
In embodiments of the invention, coating processes may be combined. For instance, although PVD or PE-PVD may not in themselves result in a coating thickness that is sufficient to support a post-machining process, PVD/PE-PVD may be combined with other processes to enhance adhesion of the coatings 70, 72 while enabling low-cost processing and base material options as discussed above. For instance, a base material may first have a coating applied via PVD or PE-PVD, and then a second coating may be applied thereto via, for instance, molten salt deposition or LENS®, as examples, may have improved adhesion, thus coatings 70, 72 may each comprise both the first adhesion layer and the second coating material applied thereto.
According to another embodiment of the invention, materials 70, 72 may be preformed from a preferred secondary material or multiple secondary materials and attached to the base material through cladding, brazing, hydroforming, isostatic pressing, rollbonding, rollforming, coextrusion, interference fit, etc. Referring to
Though the preformed pieces 74-80 are shown as being brazed to their respective bearing components, one skilled in the art will recognize that the pieces 74-80 may be bonded or attached via any number of attachment means, such as by welding, soldering, and the like. In embodiments of the invention, the thicknesses of pieces 74-80 are selected to enable post-machining step(s) prior to assembly, and the thicknesses are selected for simplicity of machining, handling, and brazing and are approximately 0.5 mm or greater.
After attachment of pieces 74-80 as applied material, pieces 74-80 are post-machined to obtain desired thicknesses, tolerances, surface qualities, and the like, to obtain a final coating, illustrated as coatings 70, 72 in
Thus, according to embodiments of the invention, materials or coatings 70, 72, (or pieces 74-80 and 87 as illustrated in
Typically, tantalum may form an oxide layer during processing and during exposure to environmental oxygen, which can reduce wetability of the surface thereof. As with molybdenum, parts coated with tantalum could be hydrogen fired to improve wetability. However, hydrogen firing has been found to embrittle tantalum. Thus, according to an embodiment of the invention, a further process step includes reducing or removing an oxide layer on one or both coatings 70, 72 by selectively plasma etching (i.e., using ionized plasma gases) desired locations.
Referring now to
Accordingly, because materials or coatings 70, 72 prevent corrosion of the base materials to which they are applied, the base materials selected may be less expensive. And because of the flexibility in material choice, base materials may be selected having improved engineering properties, such as, but not being limited to, thermal conductivity, thermal coefficient of expansion, strength, toughness, cost (both raw materials and processing), and weldability/joinability.
According to an embodiment of the invention, an x-ray tube includes a cathode and a target assembly positioned to receive electrons emitted from the cathode. The target assembly includes a target, and a spiral groove bearing (SGB) configured to support the target. The SGB includes a rotatable component having a first surface and a first material attached to the first surface, a stationary component having a second surface and a second material attached to the second surface, the stationary component positioned such that a gap is formed between the first material and the second material, and a liquid metal positioned in the gap, wherein at least one of the first and second materials comprises tantalum.
In accordance with another embodiment of the invention, a target assembly includes a shaft having a first material attached to an outer surface thereof, a sleeve configured to support a target and having a second material attached to an inner surface thereof, and a liquid metal positioned between the first material and the second material, wherein at least one of the first and second materials comprises tantalum.
According to yet another embodiment of the invention, a method of manufacturing a target assembly for an x-ray tube comprising the steps of providing a shaft having an outer surface material and having an outer diameter, providing a sleeve having an aperture exposing an inner surface material of the sleeve, wherein a diameter of the inner surface material is greater than the outer diameter of the outer surface material, applying a first layer to the inner surface material, and applying a second layer to the outer surface material. The method further includes acid etching at least one of the first layer and the second layer to remove an oxide therefrom, attaching a target to one of the shaft and the sleeve, inserting the shaft into the sleeve to form a shaft sleeve assembly, and applying a liquid metal to one of the first layer and the second layer of the shaft sleeve assembly.
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.
The present application is a continuation-in-part of and claims priority to U.S. patent application Ser. No. 12/410,518 filed Mar. 25, 2009.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12410518 | Mar 2009 | US |
| Child | 12716617 | US |
