Embodiments of the invention relate generally to x-ray tubes and, more particularly, to a method of fabricating x-ray tube components.
Traditional x-ray imaging systems include an x-ray source and a detector array. X-rays are generated by the x-ray source, pass through an object, and are detected by the detector array. Electrical signals generated by the detector array are conditioned to reconstruct an x-ray image of the object.
In general, the x-ray source is in the form of an x-ray tube that includes a vacuum housing enclosing an anode assembly and a cathode assembly. The cathode assembly includes an electron emitting filament that is capable of emitting electrons. The anode assembly provides an anode target that is spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode. In operation, electrons emitted by the cathode filament are accelerated towards a focal spot on the anode target by placing a high voltage potential between the cathode and the anode target. These accelerating electrons impinge on the focal spot area of the anode target. The anode target is constructed of a high refractory metal so that when the electrons strike, at least a portion of the resultant kinetic energy generates x-radiation, or x-rays. The x-rays then pass through a window that is formed within a wall of the vacuum enclosure, and are collimated towards a target area, such as a patient. As is well known, the x-rays that pass through the target area can be detected and analyzed so as to be used in any one of a number of applications, such as a medical diagnostic examination.
In general, only a very small portion—approximately one percent in some cases—of an x-ray tube's input energy results in the production of x-rays. In fact, the majority of the input energy resulting from the high speed electron collisions at the target surface is converted into heat of extremely high temperatures. This excess heat is absorbed by the anode assembly and is conducted to other portions of the anode assembly and to the other components that are disposed within the vacuum housing.
Because of the heat generated in the x-ray tube during operation, it is required that many components in the x-ray tube—such as the anode assembly (target and shaft), cathode cup, electron collector, etc.—be formed of a refractory material that is configured to withstand the high operating temperatures in the x-ray tube. Such refractory materials can include, for example, tungsten, molybdenum, and/or molybdenum alloys, such as molybdenum with additives of titanium, zirconium, and carbon (“TZM”).
Typically, such refractory x-ray tube components are manufactured via a press-sinter-forge (PSF) process, hot-pressing process, or hot isostatic pressing process. Such production processes have inherent drawbacks that cannot be overcome—with such drawbacks including achievable material density and process cycle time, according to the specific process employed. With respect to a PSF process, for example, the separate steps of pressing metal powders to form a compacted “green” shape” or “pre-form,” sintering the pre-form, and close-die forging the pre-form to form a final component, lead to an increased cycle time that is undesirable from a cost and business standpoint.
Therefore, it would be desirable to provide a process for manufacturing refractory x-ray tube components having a reduced cycle time. If would also be desirable for such a process to provide the components as near-net-shape components and as full density/near-full density material components.
Embodiments of the invention provide a method that overcomes the aforementioned drawbacks.
According to one aspect of the invention, a method of fabricating an x-ray tube component includes providing a powder into an electrically conductive die constructed to have a cavity shaped as the x-ray tube component being fabricated and simultaneously applying a mechanical pressure and an electric field to the die so as to cause sintering of the powder and thereby fabricate the x-ray tube component, wherein the electric field applied to the die generates heat internally in the die that is passed to the powder, so as to heat the powder responsive to the applied electric field.
According to another aspect of the invention, a method of fabricating an x-ray tube component useable in an x-ray tube includes providing a powder into an electrically conductive die, wherein the powder comprises one of a refractory metallic powder, a non-refractory metallic powder, and a ceramic powder, and wherein the die is constructed to have a cavity shaped as the x-ray tube component being fabricated. The method also includes compacting the powder into the electrically conductive die and prepping a volume about the die for a subsequent sintering operation, wherein prepping the volume comprises one of creating a vacuum environment about the die or introducing an inert or reducing gas about the die. The method further includes performing a field assisted sintering technology (FAST) process to sinter the powder and thereby fabricate the x-ray tube component.
According to yet another aspect of the invention, an x-ray tube component that is configured for use in an x-ray tube is fabricated by providing a powder into an electrically conductive die constructed to have a cavity shaped as the x-ray tube component being fabricated and simultaneously applying a mechanical pressure and an electric field to the die so as to cause sintering of the powder and thereby fabricate the x-ray tube component, wherein the electric field applied to the die generates heat internally in the die that is passed to the powder, so as to heat the powder responsive to the applied electric field.
Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings.
The drawings illustrate embodiments presently contemplated for carrying out the invention.
In the drawings:
Embodiments of the invention are directed to a process for manufacturing x-ray tube components. A field-assisted sintering technology (FAST) process, also known as spark plasma sintering (SPS), is employed to generate x-ray tube components, with the FAST process providing for a reduced cycle time in manufacturing the component(s), and with the component(s) being provided as near-net-shape components and as full density/near-full density material components.
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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, floppy discs, 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.
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According to embodiments of the invention, various components in x-ray tube 12, including refractory and non-refractory components, are manufactured using a field-assisted sintering technology (FAST) (i.e., spark plasma sintering (SPS) process). The FAST process employs a simultaneous application of pressure and an electric field to enhance atom mobility in a component being produced, with supplemental temperature being added to further increase mobility and reduce cycle time. The main characteristic of FAST is that a current is applied that directly passes through an electrically conductive die (e.g., graphite die), and optionally the powder of the component being fabricated (in case of an electrically conductive powder). Therefore, the heat applied for sintering is generated internally within the component, in contrast to the conventional hot pressing, where the heat is provided by external heating elements.
This facilitates a very high heating or cooling rate of up to 500 C/min (e.g., 100 C/min), hence the FAST process generally is very fast (e.g., within a few minutes). The general speed of the FAST process ensures it has the potential of densifying powders with nanosize particles or nanostructure while avoiding coarsening which accompanies standard densification routes. As such, the FAST process can produce x-ray tube components having full or near-full material density—thereby potentially improving the material properties of the components, such as toughness, fatigue growth crack rate (FGCR), modulus of elasticity, dielectric constant, and/or ductile brittle transition temperature (DBTT), as non-limiting examples. Beneficially, these improved material properties can improve life of the x-ray tube components, such as by increasing a life of the anode target based on a 2 to 4× reduction in FGCR.
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A pair of up and down pair of spacers 108 are positioned on opposing sides of the die 102, with the spacers 108 being supported by punch electrodes 110 and pressed thereby at a pressure of, for example, about 1 MPa against the die 102. The spacers 108 are configured as conductive members, and a current (pulse, DC or AC) generated from a current supply 112 is supplied to the spacers 108 and the die 102 via the punch electrodes 110. The die 102, the spacers 108, and the punch electrodes 110 are placed in a vacuum chamber 114 that provides an inert environment for performing of the FAST process.
Also included in system 100 is a temperature measuring device 116, such as a pyrometer, that functions to measure the temperature of the component being fabricated in the die 102 in a non-contact manner. A control unit 118 included in system 100 drives and controls the pulse current supply 112, the pressure applied by punch electrodes 110, and the functioning of temperature measuring device 116. The control unit 118 is configured to drive the punch electrodes 110 compress the spacers 108 with a predetermined amount of pressure.
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At STEP 126, a simultaneous application of pressure and an electric field is provided to the die 102 in performing of the FAST technique, with the applied pressure, displacement, and temperature of the power being monitored at STEP 128 till completion of the fabrication process at STEP 130, at which time a cool down of the finished component is performed. In performing STEP 126, pressure can be applied to the die 102 and powder compact by way of punch electrodes 110, for example, and the electric field can be provided by a power supply 112 that provides a DC, AC or pulsed power for example. The current that is applied passes through the die and is transferred to the powder of the component being fabricated. Therefore, the heat applied for sintering is generated internally within the component, so as to facilitate a very rapid heating or cooling rate (up to 1000 K/min) in the powder compact. The simultaneous application of pressure and current (and the rapid heating achieved thereby) serves to enhance atom mobility in the power compact being produced, so as to provide the capability of densifying the powder with nanosize or nanostructure, while avoiding coarsening which accompanies standard densification routes. As such, the FAST technique 120 can produce x-ray tube components having full or near-full material density—thereby potentially improving the material properties of the components, such as toughness, fatigue growth crack rate (FGCR), modulus of elasticity, dielectric constant, and/or ductile brittle transition temperature (DBTT), as non-limiting examples. While not shown in
According to embodiments of the invention, in employing technique 120 for example, mechanical pressure of up to 100 MPa can be applied along with a high current of up to 10,000 A, so as to create a high heating rate of up to 500 degrees Celsius per minute and generate temperatures of up to 2400 degrees Celsius. When providing these conditions in a vacuum or inert environment, high density (e.g., 96-99% relative density), near-net shape x-ray tube components can be fabricated at a fraction of the conventional press-sinter-forge (PSF) cycle time—with cycle times of 5 minutes being achievable with a FAST process.
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Beneficially, embodiments of the invention thus provide a FAST process that produces near-net-shape, full/near-full density material x-ray tube components, including refractory and non-refractory components. Fabrication of x-ray tube components via a FAST process provides a cost advantage due to efficient material utilization, single-piece flow, and significantly reduced cycle-time, as well as associated inventory improved material efficiency, cost, cycle-time, and inventory. Furthermore, fabrication of x-ray tube components via a FAST process potentially provides components of increased material density, so as to improve material properties such as toughness, FGCR, modulus of elasticity, dielectric constant, and/or DBTT—thereby prolonging the life of such x-ray tube components.
According to one embodiment of the invention, a method of fabricating an x-ray tube component includes providing a powder into an electrically conductive die constructed to have a cavity shaped as the x-ray tube component being fabricated and simultaneously applying a mechanical pressure and an electric field to the die so as to cause sintering of the powder and thereby fabricate the x-ray tube component, wherein the electric field applied to the die generates heat internally in the die that is passed to the powder, so as to heat the powder responsive to the applied electric field.
According to another embodiment of the invention, a method of fabricating an x-ray tube component useable in an x-ray tube includes providing a powder into an electrically conductive die, wherein the powder comprises one of a refractory metallic powder, a non-refractory metallic powder, and a ceramic powder, and wherein the die is constructed to have a cavity shaped as the x-ray tube component being fabricated. The method also includes compacting the powder into the electrically conductive die and prepping a volume about the die for a subsequent sintering operation, wherein prepping the volume comprises one of creating a vacuum environment about the die or introducing an inert or reducing gas about the die. The method further includes performing a field assisted sintering technology (FAST) process to sinter the powder and thereby fabricate the x-ray tube component.
According to yet another embodiment of the invention, an x-ray tube component that is configured for use in an x-ray tube is fabricated by providing a powder into an electrically conductive die constructed to have a cavity shaped as the x-ray tube component being fabricated and simultaneously applying a mechanical pressure and an electric field to the die so as to cause sintering of the powder and thereby fabricate the x-ray tube component, wherein the electric field applied to the die generates heat internally in the die that is passed to the powder, so as to heat the powder responsive to the applied electric field.
Embodiments of the invention have been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.