Patent Application
20090086920
1. Field of the Invention
The present invention relates generally to x-ray systems, devices, and related components. More particularly, embodiments of the invention relate to x-ray target assemblies that are manufactured using an electroforming process.
2. Related Technology
The x-ray tube has become essential in medical diagnostic imaging, medical therapy, and various medical testing and material analysis industries. An x-ray tube typically includes a cathode assembly and an anode assembly disposed within an evacuated enclosure. The cathode assembly includes an electron source and the anode assembly includes a target surface that is oriented to receive electrons emitted by the electron source. During operation of the x-ray tube, an electric current is applied to the electron source, which causes electrons to be produced by thermionic emission. The electrons are then accelerated toward the target surface of the anode assembly by applying a high-voltage potential between the cathode assembly and the anode assembly. When the electrons strike the anode assembly target surface, the kinetic energy of the electrons causes the production of x-rays. Some of the x-rays so produced ultimately exit the x-ray tube through a window in the x-ray tube, and interact with a material sample, patient, or other object.
Stationary anode x-ray tubes employ a stationary anode assembly that maintains the anode target surface stationary with respect to the stream of electrons produced by the cathode assembly electron source. In contrast, rotating anode x-ray tubes employ a rotating anode assembly that rotates portions of the anode's target surface into and out of the stream of electrons produced by the cathode assembly electron source. The target surfaces of both stationary and rotary anode x-ray tubes are generally angled, or otherwise oriented, so as to maximize the amount of x-rays produced at the target surface that can exit the x-ray tube via a window in the x-ray tube.
In an x-ray tube device with a rotatable anode, the target has previously consisted of a disk made of a refractory metal such as tungsten, and the x-rays are generated by making the electron beam collide with this target, while the target is being rotated at high speed. Rotation of the target is achieved by driving the rotor provided on a support shaft extending from the target. Such an arrangement is typical of rotating x-ray tubes and has remained relatively unchanged in concept of operation since its induction.
Because of the high melting point of the metals used to make x-ray targets, most x-ray targets are made using powder metallurgy. In powder metallurgy, the metal part is manufactured by pressing a powder and then sintering the powder to form the part. The part is then heated and forged to cause densification. In many cases, the powder is densified up to 97% a theoretical density.
Embodiments of the invention concern x-ray target assemblies that are manufactured using an electroforming process. The electroforming process can be used to manufacture various components of the anode assembly, including but not limited to, an x-ray target substrate, an x-ray target focal track, an x-ray target stem, a metal barrier layer on a metal x-ray target substrate, a metal barrier layer on a carbon x-ray target substrate, a metal barrier layer on a carbon x-ray target heat sink, or a metal layer that mechanically couples two or more additional components of the x-ray target assembly. The electroforming process can be used to manufacture x-ray targets with a unique design and/or improved material properties.
The electroforming process used to manufacture the one or more components of the x-ray target can by carried out by providing an electroforming apparatus that includes an electrolyte, a metal anode, and an electrically conductive cathode. The electrically conductive cathode includes (i) an intermediate x-ray target assembly upon which the metal is to be deposited and/or (ii) an electrically conductive mold for forming a component of an x-ray target assembly.
The x-ray target component (e.g., a substrate or focal track) is formed by submersing the cathode in the electrolyte and applying a voltage across the anode and the cathode to cause the metal from the anode to be electrodeposited on the intermediate x-ray target and/or the cathode mold. The electrodeposition is continued until a desired thickness of metal is formed.
The electroforming process of the invention can be used to deposit high melting point metals typically used in manufacturing high performance x-ray target assemblies. Examples of high melting point metals that can be used to manufacture components of an x-ray target assembly include, but are not limited to Mo, Ta, Re, W, Nb, V, Ir, Rh, Pt, and Pd.
The electrodeposition of high melting point metals is facilitated by the use of a molten salt electrolyte and high operating temperatures. Examples of suitable temperatures for carrying out electrodeposition of high melting point metals includes temperatures greater than about 500° C., more preferably greater than about 800° C., and up to 1000° C. Examples of suitable molten salts that can be used as electrolytes include, but are not limited to, sodium chloride, potassium chloride, sodium fluoride, potassium fluoride, and the like.
The electroformed component is then incorporated into an x-ray target assembly. The x-ray target assemblies of the invention typically include a substrate and a target surface such as a focal track. The target assembly can also include a x-ray target stem and/or barrier layers that separate two or more components of the x-ray target assembly. The barrier layer can be used to separate a carbon based substrate from the focal track material or from the heat sink. The barrier layer can also be used to provide a thermal barrier between a carbon heat sink and the x-ray target stem by reducing radiative heat.
The electroformed component can also be a metal layer that connects two or more other components of the x-ray target assembly together. For example, an x-ray target stem that is attached to the substrate using a fastener can be secured by applying a coating over the fastener and the substrate using the electrodeposition technique of the invention. The electroformed coating can be used in place of or in addition to braze washers that are typically used for this purpose.
The use of electroforming to manufacture one or more components of the x-ray target assembly has surprising and unexpected results in the performance of the x-ray target. Components manufactured using electrodeposition have superior microcrystalline properties compared to components made by powder metallurgy. The electrodeposited components have substantially 100% density. The high density results in very low porosity. The high density and low porosity is advantageous for a track material due to its ability to emit x-rays upon impingement of electrons. In addition, high density leads to increased strength, which allows the target assembly to be operated under more strenuous and thus higher performance conditions (e.g., greater than 650° C.).
Another significant advantage of the components manufactured using the electroforming process is the columnar microcrystalline structure that the process produces.
Another advantage of the targets manufactured according to the present invention is the thickness with which the highly ordered crystal lattice can be grown. The columnar microcrystalline structure can be grown to thicknesses of greater than 0.75 mm, more preferably greater than 1 mm, and most preferably greater than about 1.25 mm. Metal layers grown at these thicknesses can provide excellent bonding between layers and can provide a rigidity that avoids the situation where the metal layer delaminates and curls up. These results are in contrast to targets made using a CVD process, which are often limited to deposition depths of less than about 0.5 mm due to extremely slow deposition rates (often more than 20 times slower than the electroforming process of the present invention). The high deposition rates used in the present invention allow for greater thicknesses and give the deposited material a highly dense, highly ductile, and unique microcrystalline structure.
Surprisingly, targets manufactured using the process of the present invention have achieved high power rating during operation in an x-ray tube. The targets of the present invention can be used at track power rating of from about 60 kW to about 150 kW, more preferably 80 kW to about 125 kW, depending on target size (e.g., target with 200 mm diameter). These higher power ratings allow higher performance when used in an x-ray tube.
These and other advantages and features of the invention will become more fully apparent from the following description and appended claims.
In order that the manner in which the above-recited and other advantages and features of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
The present invention relates to the manufacture of x-ray target assemblies (i.e, the x-ray target anode) by electroforming one or more metal layers of the target. The present invention can be carried out on any type of x-ray target that includes metal layers made from high melting point metals, such as, but not limited to, refractory metals.
The anode assembly 100 is rotated by an induction motor, which drives stem 112.
In a typical x-ray tube, the anode and cathode assemblies are sealed in a vacuum envelope. The stator portion of the motor is typically provided outside the vacuum envelope. The x-ray tube can is enclosed in a casing having a window for the X-rays that are generated to escape the tube. The casing can be filled with oil to absorb heat produced as a result of x-ray generation.
The x-ray target illustrated in
While
The following provides a description of x-ray targets manufactured using an electroforming process. As described in more detail below, the electroforming process can advantageously be used to manufacture various metal components of the x-ray target assembly, including but not limited to the substrate, the focal track, the stem, barrier layers, and other metal layers used to strengthen and/or secure the components of the x-ray target assembly. X-ray target assemblies manufactured, at least in part, using the electroforming process have improved mechanical properties compared to target assemblies manufactured using powder metallurgy techniques. These improved properties are due to the unique microcrystalline structure of the metal layers deposited using electroforming. In addition, by electroforming the one or more metal components of the x-ray target, the target can be manufactured in unique steps that improve the target design and/or reduce the cost of manufacturing the x-ray target.
For purposes of this invention, the term “x-ray target assembly” or “assembly” includes x-ray target components (e.g., a substrate, a stem, or a carbon heat sink) that are “assembled” by both mechanical means (e.g., a fastener) and/or metallurgically (e.g., brazed or electrodeposited).
The electroforming process used to manufacture one or more metal components of the x-ray target is carried out by electrodepositing a metal using an electroforming apparatus.
The metals deposited using the electroforming process of the invention can be any metal suitable for use in manufacturing high performance x-ray targets. The metals used to manufacture high performance x-ray target are typically high melting-point metals having a melting point above about 1650° C. Examples include Mo, Ta, Re, W, Nb, V, Ir, and Rh. More preferably, the metal is a refractory metal selected from the group of tungsten, molybdenum, niobium, tantalum, and rhenium.
The metals used for electroforming can be provided in relatively pure form or alternatively they can be scrap metals which various amounts of contaminants. In several embodiments of the invention impure metals can be used as the anode metal since the electrodeposition process selectively deposits only the pure metal. Thus, the electrodeposition process of the invention can use cheaper, impure sources of metal while achieving very high purity electroformed components.
The electrodeposition is carried out until a desired thickness is reached. The time needed to reach a particular thickness depends on the rate of deposition. In one embodiment the deposition rate is in a range from about 5 micron/h to about 80 micron/h, more preferably in a range from about 25 micron/h to about 50 micron/hr. The thicknesses of the electroformed component is typically limited by the need for a practical duration. The rate of deposition using the electroforming process of the invention can yield thicknesses in a range from about 0.02 mm to about 5 mm, more preferably about 0.75 mm to about 5 mm, even more preferably about 1 mm to about 3.5 mm, and most preferably about 1.25 mm to about 3 mm.
In a preferred embodiment, the electroforming process is carried out at a relatively high temperature. Heating element 330 is used to control the temperature of the electrolyte 320 during deposition of the metal. Examples of suitable temperatures include temperatures greater than about 500° C., more preferably greater than about 800° C., and up to 1000° C. Electroforming performed at these temperatures reduces internal deposition stresses, which allows relatively thick layers of metal to be formed. In addition, deposition at these higher temperatures gives the metals smaller and more uniform grain sizes. In a preferred embodiment, the microcrystalline structure of the metal deposited at a high temperature is columnar.
The electrolyte used during the deposition process can be any electrolyte capable of acting as a medium to dissolve metal atoms from the anode and transfer the metal atoms to the cathode. In one embodiment, the electrolyte is a molten metal salt. Example of suitable salts include chlorides or fluorides of sodium or potassium or both. The salt can be made molten by applying heat using heating element 330 of electroforming apparatus 300.
During the metal deposition, the voltage across the anode and the cathode allows the metal atoms to be dissolved in the electrolyte and carried through the electrolyte to the cathode. The negative charge on the surface of the cathode causes the positively charged metal atoms in the electrolyte to be deposited. Electrodeposition occurs anywhere there is negatively charged surface in contact with the electrolyte.
The shape of the negatively charged surface of the cathode determines the shape of the electrodeposited metal layer. The cathode can be made to have almost any desired negatively charged surface. However, to maximize uniformity in the electrodeposited layer it is advantageous to avoid sharp corners and other fine points. In one embodiment, the electrically conductive surface area is provided by an intermediate x-ray target. For example, as described in more detail below with regard to
Alternatively, the electrically conductive cathode surface can be a form that provides a desired shape for making an x-ray target component but is then separated from the x-ray target component. For example, the form can be a carbon block that provides a desired shape for making an x-ray target substrate. The carbon block can then be removed and the electroformed substrate can be incorporated into an x-ray target assembly. For purposes of this invention, the term “electroforming” encompasses both a process where the “mold” or “form” is separated from the deposited metal and a process where the mold or form (e.g., a target substrate) remains attached to the deposited material and therefore becomes part of the finished x-ray target.
The shape of the deposited metal layer can also be controlled by masking a portion of the surface of the cathode using a non-conductive material. For example, where an intermediate x-ray target is used as the cathode, portions of the intermediate x-ray target can be masked with a chemically inert and non-conductive material to avoid coating that portion of the intermediate target. An example of a suitable non-conductive material is a ceramic material such as boronitride or borocarbide. Where a ceramic material is used, relatively lower temperatures can be used to ensure stability of the ceramic material in the electrolyte. Following electrodeposition, the mask is removed to yield an uncoated surface (i.e., uncoated with respect to the material being deposited in that particular deposition step).
In an alternative embodiment, the mask can be a conductive material that is used as a sacrificial mask. In this case the mask can be a graphite or other material that is coated during electrodeposition but the mask can be easily removed so as to not require extensive machining of the intermediate targets.
The shape of the electroformed component is also determined in part by the thickness of the deposited metal. The thickness is controlled by allowing electrodeposition to continue until the desired thickness of metal is achieved. The thickness of the electroformed component depends on the rate of deposition and the duration of deposition. The rate of deposition can depend on the electrolyte used, the type of metal being deposited, and the voltage applied by the electroforming apparatus. The electroforming process used in the present invention can be relatively fast as compared to other techniques such as chemical vapor deposition. Unlike some deposition techniques, the electroforming process of the invention can have sufficiently high deposition rates to achieve metal thicknesses suitable for making x-ray target substrates, x-ray target focal tracks, x-ray target stems, and other useful metal components of an x-ray target assembly. In one embodiment, the rate of deposition used in the method of the invention is in a range from about 5 microns/h to about 80 micron/h, more preferably in a range from about 25 micron/h to about 50 micron/h.
In one embodiment, the electrodeposition is used to deposit a composite metal or alloy. Using two or more different metals in the electroforming anode results in a uniform deposition of both metals. If desired, the concentration of the two or more metals can be varied throughout the deposition process to yield a layer with a continuously or semi-continuously variable composition (i.e., a graded composition). A graded composition can be used to ensure that certain alloying metals are placed closer to a surface or component interface where the alloying metal is more important. alternatively a graded alloying composition can provide a transition layer between two dissimilar layers, thereby improving the bonding between two dissimilar layers and reducing the likelihood of delamination.
The electroformed x-ray target component can be formed so as to have its final desired shape, or alternatively, the electroformed component can be further machined to have the shape and dimensions desired for incorporating the component into an x-ray target assembly.
The electroforming process of the invention is used to manufacture one or more components of an x-ray target assembly. Examples of suitable components of an x-ray target assembly that can be manufactured according to the present invention include, but are not limited to, the x-ray target substrate, the x-ray target focal track, the x-ray target stem, barrier layers incorporated into the x-ray target assembly, and other metal layers used to strengthen and/or secure the components of the x-ray target assembly.
The metal or metals electrodeposited to form substrate 404 can be any metals suitable for use as an x-ray target substrate. Examples of suitable material for forming a metal substrate include, but are not limited to, molybdenum and molybdenum alloys such as Mo—W, Mo—Re, or Mo—W—Re. The electrodeposition process can be used to form almost any desired composition so long as the composition includes materials that can be electrodeposited. If desired the substrate can be a composite material and/or a composite material with a graded composition of an alloying element. In one embodiment, the alloying element has a higher concentration at the surface where the substrate contacts another component (e.g., the focal track). For example, a substrate including Mo and W can have a higher percentage of W near the substrate track interface.
Advantageously the electroforming process of the invention can be used to form a relatively thick substrate. Examples of thicknesses that can be achieved in a relatively reasonable period are in a range from about 0.5 mm to about 5 mm.
Substrate 404 is typically formed to have an angled focal track location 408. Focal track location 408 is the location where a focal track material can be deposited for making an x-ray target focal track. Because electrodeposition tends to deposit a uniform thickness, in a preferred embodiment, block 402 has angled surface 412 that corresponds to focal track location 408. In an alternative embodiment, focal track location 408 can be made by machining target substrate 404 after it has been electroformed. The thickness of substrate 404 is determined by controlling the rate of deposition and the duration of deposition. Any focal track material can be deposited on focal track location 408 using any technique, including electroforming, CVD, or other known deposition techniques.
To retain carbon block 402 as a heat sink, the carbon material is typically selected so as to have a similar coefficient of thermal expansion as substrate 404. Matching the coefficient of thermal expansion of substrate 404 and carbon block 402 avoids the separation that can occur when materials of substantially different coefficients are cooled following electroforming. Alternatively, if it is desired to remove carbon block 402 after electrodeposition, the coefficients of thermal expansion can be selected to be different to facilitate separation. The coefficient of thermal expansion of metals and carbons useful for forming x-ray target components are known in the art and selecting similar or dissimilar coefficients is within the skill of those in the art.
A portion of the upper surface of carbon block 402 can remain uncoated as shown in
The substrate 404 manufactured according to the invention is incorporated into an x-ray target assembly. In one embodiment, the x-ray target assembly is incorporated into a rotating anode target that includes an x-ray focal track, a stem, and/or a carbon heat sink. These components of the x-ray target assembly can be manufactured or provided using techniques known in the art or alternatively, where a metal is used, the component can be provided by electroforming according to the present invention and as described herein.
The intermediate target assembly 400 can be particularly advantageous for use in rotating anode targets due to the ability to form a non-planar interface between substrate 404 and heat sink 402. As shown in
The use of the angled portion 412 of heat sink 402 forms a focal track location with a desired angle for depositing a focal track. In addition, the heat sink is evenly spaced from the focal track at the substrate-heatsink interface. This is in contrast to targets that are shaped in a way that is suitable for brazing a heat sink onto the substrate (e.g., substrate 504 shown in
Another advantage of the electroformed substrate 404 of intermediate x-ray target 400 is the use of a skirt 414 and cap 416. One limitation of rotating anode targets is the rotation speed at which the heat sink will begin to fail. For example an 8 inch graphite target manufactured using methods known in the art can currently be rotated at about 9,000 RPM without fracturing. Skirt 414 of substrate 404 extends vertically down the lateral side of heat sink 402 and protects heat sink 402 from fracturing. Skirt 414 can extend along the entire lateral side of heat sink 402 or a portion thereof. In a preferred embodiment, skirt 414 extends along at least about 50% of the lateral edge, more preferably at least about 80% and most preferably substantially the entire lateral edge. In one embodiment, skirt 414 can include a cap 416 that extends inward from the lateral edge near an exposed bottom surface of heat sink 402. Cap 416 extends around the bottom of heat sink 402 to help prevent heat sink 402 from debonding from substrate 404.
X-ray target assemblies that have substrates employing a skirt 414 can be rotated as substantially higher rotation speeds than a similar target that does not have a skirt. In one embodiment, the x-ray target is a rotating anode target having a skirt on the lateral edge of the heat sink and the target assembly can be rotated at rates of between 9,000 and 15,000 RPM, more preferably 10,000-12,000 RPM during use (for a target greater than 8 inches in diameter). Rotating the target at higher speeds improves thermal loading on the focal track, thereby distributing the heat and allowing longer and/or higher performance targets.
A ceramic nut 512 secures support member 506 to substrate 504 during the electroforming process. Ceramic nut 512 is made from a dielectric material such that no material is deposited on the portion of the surface of substrate 504 that is encapsulated by nut 512. A ceramic mask 514 can be used to cap the underside 508 of substrate 504 to prevent underside 508 from being coated with metal. However, if desired, mask 514 is not used and layer 510 extends onto the surface of underside 508. In such an embodiment, this portion of layer 510 can become part of the final x-ray target assembly or alternatively any undesired portion can be removed using known techniques such as grinding.
Electroformed metal layer 510 can be further processed to provide an x-ray target component with a desired shape.
In an alternative embodiment of the invention, an x-ray target focal track is electroformed on a carbon substrate.
Metal layer 612, which includes x-ray focal track 610, is formed on substrate 602 using an electroforming apparatus 300 (
The x-ray target focal track can also be manufactured to cover only a portion of the carbon substrate, thereby leaving a portion of the carbon substrate exposed. FIG. 6C shows carbon substrate 602 with a barrier layer 608 and a metal layer 612, which provides an x-ray target focal track 610. Barrier layer 608 and metal layer 612 are not coated on portion 614 of substrate 602. Leaving portion 614 uncoated allows good heat dissipation from substrate 602. A portion 616 of barrier layer 608 is coated onto substrate 602 to reduce heat dissipation near the center of the substrate. This configuration of the barrier layer 608 and metal layer 612 can be achieved by grinding an intermediate target as in
In an alternative embodiment, a target stem is manufactured using an electroforming process.
The electroforming process of the invention can be used to form metal layers on the substrate that function as a barrier layer or a metal layer used to strengthen and/or secure the components of the x-ray target assembly.
The barrier layers and strengthening metal layers can be electroformed independently or simultaneously with the electroformation of other layers of the x-ray target assembly. For example, in
The electroforming process can also be used to form layers that strengthen one or more components of the x-ray target assembly and/or secure two or more additional components of the x-ray target assembly.
In an alternative embodiment substrate 804 and heat sink 808 can be joined by brazing using a noble metal (e.g., platinum) rather than forming them using electrodeposition as described above with respect to
The x-ray target components manufactured using an electroforming process are incorporated into an x-ray target assembly. The x-ray target assembly includes at least a substrate and a target material having a configuration and composition suitable for emitting x-rays when impinged upon by an electron source. In a preferred embodiment the x-ray target includes a substrate, an x-ray target focal track, and a stem.
The substrate can have any shape suitable for use in an x-ray tube. To facilitate rotation in a rotating anode target, the substrate is preferably disk-like. The thickness of the substrate and shape is selected to maximize strength, heat dissipation, and ease of manufacturing while minimizing cost. In one embodiment, the substrate is substantially disk shaped and has a thickness in a range from about 10 mm to about 14 mm.
The substrate can be made from any electrically conductive material. Because the x-ray target is used as an anode in the x-ray tube, the substrate should be electrically conductive to allow a charge to be applied to the target surface. The need to provide electrical conductivity when used in an x-ray tube is advantageous for making and/or coating the substrate using electroforming according to the invention since electroforming also requires electrically conductive surfaces.
The material used in the substrate can be carbon, carbon composites, metals, alloys, or oxide-dispersed-strengthened refractory metal (ODS refractory metal). In a preferred embodiment, the primary refractory metal is Mo. Molybdenum-based substrates have yielded exceptionally good substrates for use in rotating anode x-ray tubes.
Metal substrates can be manufactured using any combination of suitable techniques including powder metallurgy, machine grinding, extrusion, etc. If a carbon substrate is used, the carbon substrate is provided as a block of graphite, carbon composite, or other suitable conductive material. The carbon substrate can be machined to have desired features for an x-ray target assembly.
The x-ray target track material can be any material that can emit x-rays when impinged upon by an electron source. Examples of suitable materials include tungsten and alloys of tungsten, such as tungsten rhenium alloys. Preferably the track material is formed using an electroforming process as described above. Electroformed target focal tracks have surprisingly been found to be much more ductile than focal tracks made from the same material and manufactured using other technique such as powder metallurgy or vacuum plasma spay process. Due to the improved ductility, the electroformed target focal track can be manufactured using less rhenium, which traditionally has been added to improve ductility. In one embodiment, the percent of rhenium in a tungsten based focal track is less than 5 wt %, more preferably less than about 1 wt % and most preferably substantially free of rhenium. It is believed that the improved ductility is due to the substantially 100% dense columnar microcrystalline structure achieved in focal tracks manufactured using the electroforming process.
The x-ray target assembly typically includes a stem portion. The stem is a component used to support the target and, in the case of a rotating anode target, the stem is the means by which an induction rotor causes rotation of the x-ray target assembly. The stem typically includes the same metals that can be used as a metal substrate material.
A heat sink is typically used where the substrate is metallic. The heat sink is typically a carbon-based structure positioned on the substrate so as to absorb heat generated from electrons impinging upon the focal track and thereby creating x-rays. Where a carbon substrate is used, the carbon substrate can function as a heat sink and a heat sink is therefore not necessary.
If the x-ray target assembly includes a heat sink separate from the substrate, the heat sink can be made of any thermoconductive material such as, but not limited to, graphite or thermally conductive carbon composite. During use, the heat sink absorbs thermal energy from the substrate and dissipates the heat. The heat sink can have any shape or size so long as the heat sink adequately dissipates heat and is suitable for rotating anodes. Typically the heat sink is disk-shaped to facilitate high speed rotation. The surface of the heat sink that faces the substrate can have a regular or irregular pattern of grooves to enhance the surface area that bonds with the substrate. In one embodiment, the pattern comprises concentric or phonographic grooves.
The heat sink can be brazed or otherwise bonded to the substrate. Examples of suitable brazing materials include Zr, Ti, V, Cr, Fe, Co, Ni, Pt, Rd, or Pd or alloys including these elements. However, it can be advantageous to avoid a braze, since the braze can be a source of delamination. In one embodiment, the substrate is electroformed to the heat sink so as to avoid the necessity of brazing the heat sink to the substrate.
The x-ray target assembly optionally includes a barrier material. The barrier layer can be made from a substantially pure metal or an alloy. Examples of suitable metals include Mo, Ta, Re, W, Nb, V, Ir, Rh, Pt, and Pd, and combinations of these. These compounds can also be used in combination with boron, silicon, nitrogen, or carbon in the form of metal borides, nitrides, silicides, carbides, or combinations of these.
The thickness of the barrier layer can depend on the desired use of the barrier layer. If the barrier layer is to provide added strength, a relatively thicker layer is desired. Where the barrier layer is used to prevent a chemical reaction between to components of the x-ray target during electroforming or another manufacturing process, the barrier layer can be made only as thick as necessary to prevent the chemical reaction. In one embodiment, the barrier layer has a thickness in a range from about 0.01 mm to about 2.5 mm, more preferably in a range from about 0.1 mm to about 1.5 mm, and most preferably in a range from about 0.25 mm to about 1.0 mm.
Of the components used to manufacture the x-ray target assembly, any number of components can be manufactured using electroforming so long as the component can be made from a metal or metal alloy suitable for electrodeposition. While there are many advantages to using as many electroformed components as possible, embodiments of the invention contemplate as few as a single component manufactured using an electroforming process.
The x-ray target assemblies of the present invention can advantageously be incorporated into an x-ray tube.
As is typical, a high-voltage potential is provided between assembly 100 and cathode 158. In the illustrated embodiment, cathode 158 is biased by a power source (not shown) to have a large negative voltage, while assembly 100 is maintained at ground potential. In other embodiments, the cathode is biased with a high negative voltage while the anode is biased with a high positive voltage. Cathode 158 includes at least one filament 164 that is electrically connected to a power source. During operation, electrical current is passed through the filament 164 to cause electrons, designated at 168, to be emitted from cathode 158 by thermionic emission. Application of the high-voltage differential between anode assembly 100 and cathode 158 then causes electrons 168 to accelerate from cathode filament 164 toward a focal track 114 that is positioned on a target surface of rotating assembly 100.
As electrons 168 accelerate, they gain a substantial amount of kinetic energy, and upon striking the target material on focal track 114, some of this kinetic energy is converted into electromagnetic waves of very high frequency (i.e., x-rays). At least some of the emitted x-rays, designated at 172, are directed through x-ray transmissive window 174 disposed in outer housing 152. Window 174 is comprised of an x-ray transmissive material so as to enable the x-rays to pass through window 174 and exit x-ray tube 150. The x-rays exiting tube 150 can then be directed for penetration into an object, such as a patient's body during a medical evaluation, or a sample for purposes of metals and chemical analysis and baggage inspection.
The high performance and/or larger diameters of the x-ray target assemblies of the present invention make the x-ray target assemblies of the invention particularly suitable for use in high performance devices such as CT-scanners. CT-scanners incorporating the x-ray tubes of the invention can achieve higher intensity x-rays that allow for higher resolution medical imaging and baggage inspection. Thus, the CT-scanners of the invention can be made to detect medical or material features that might not otherwise be possible with x-ray tubes having inferior performance.
The disclosed embodiments are to be considered in all respects only as exemplary and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing disclosure. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
