The present invention relates generally to x-ray systems, devices, and related components. More particularly, embodiments of the invention relate to brazed x-ray target assemblies that include an oxide dispersion strengthened (ODS) alloy substrate and a carbon-based heat sink and methods for manufacturing such x-ray target assemblies and related devices.
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, rotary anode x-ray tubes employ a rotary 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.
The operating conditions for X-ray tubes have changed considerably in the recent decades. Due to continuous demands from radiologists for higher power from X-ray tubes, more and more tubes are using composite rotating anodes with tungsten-rhenium as a focal spot layer, molybdenum alloy as a substrate, and brazed graphite as a heat sink.
Increasing the power levels of the x-ray tube is typically accompanied by an increase in the operating temperatures of the anode, which, if high enough, may result in deformation of the molybdenum alloy substrate. Deformation in the substrate can cause large stresses in the metallurgical bond between the alloy substrate and the graphite heat sink. Should the stress exceed a threshold value, a complete debond of the graphite heat sink can result. The magnitude of this stress imposes a limit on the maximum size, rotational speed and highest allowable temperature of the alloy substrate.
To overcome the problem with deformation of molybdenum alloy substrates at elevated temperatures, other Mo-bearing alloys have been considered by x-ray target manufacturers. Oxide dispersion strengthened (ODS) Mo alloys currently show promising performance for reducing deformation of the substrate at high temperatures.
The present invention provides an x-ray target assembly, a method for manufacturing an x-ray target assembly, an x-ray tube and a high performance CT-scanner as defined in the claims. Embodiments of the invention concern x-ray target assemblies for use as an anode in an x-ray device. The x-ray target assemblies of the embodiments have an oxide dispersion strengthened (ODS) refractory metal alloy substrate (e.g., ODS Mo alloy) that is bonded to a carbon-containing heat sink. Good bonding between the substrate and the heat sink is achieved by placing an oxide-free barrier layer between the ODS metal substrate and the heat sink. The oxide-free barrier layer advantageously minimizes or eliminates chemical reactions that would otherwise be possible between the dispersed oxides and the carbon-based heat sink during the manufacturing process. Preventing these undesired reactions while manufacturing the x-ray target assembly yields a device with improved bonding between the heat sink and the substrate, compared to devices manufactured without the barrier layer.
The present invention includes a method for manufacturing an x-ray target assembly having an ODS refractory metal substrate. In one embodiment, the method includes (i) providing an ODS refractory metal substrate (e.g., ODS Mo alloy), (ii) forming a barrier layer on the substrate, and (iii) brazing a carbon-containing heat sink to the barrier layer.
The substrates used in the x-ray targets of the invention can be made of any molybdenum or molybdenum alloy that has an oxide dispersed throughout to improve the thermal stability and/or strength of the alloy. In one embodiment, the primary metal component of the substrate is molybdenum or molybdenum alloyed with a proper amount of rhenium, titanium, zirconium, niobium, tantalum, hafnium, vanadium, or tungsten. The amount of alloying element can vary depending on solution strengthening effect of the element.
The ODS refractory metal alloy includes one or more oxides dispersed throughout the substrate to achieve the desired thermal stability and strength. Examples of suitable oxide compounds that can be dispersed in the refractory metal include, but are not limited to, lanthana (La2O3), yttria (Y2O3), ceria (CeO2), thoria (ThO2), and combinations of these. The oxide can be included in the refractory metal in a range from about 0.1 wt% to about 10 wt%, more preferably in a range from about 1 wt% to about 6 wt%, and most preferably in a range from about 2 wt% to about 4 wt% depending on production capability and performance requirements.
The barrier layer is a thin layer of metal coated on the substrate to prevent the oxides in the substrate from reacting with carbon in the heat sink during brazing. The material used to make the barrier layer is selected to be substantially free of the oxides in the substrate, and preferably free of any oxides. In one embodiment, the barrier layer can be substantially pure Mo, Ta, Re, W, Ti, Nb, V, Hf, Zr, or a Mo alloy of these. Alternatively, other applicable the barrier layer can be a carbide, nitride, boride, or silicide of these metals.
The barrier layer can be coated (i.e., formed) on the substrate using a deposition technique or a mechanical technique or other similar technique. In one embodiment the barrier layer is deposited on the substrate using plasma spray coating, salt-bath electrodeposition, electroplating, vacuum sputtering, melt evaporation, chemical vapor deposition, or a similar technique. Aternatively the barrier layer can be metallurgically bonded to the substrate using hot rolling, cold rolling, upset forging, hot isostatic pressing, cold isostatic pressing, or a similar technique.
The foregoing techniques allow the barrier layer to be made very thin. A thin barrier layer can be advantageous by minimizing the effect that the barrier layer has on the thermal stability and/or strength of the substrate, while still providing the desired barrier to reactions between the substrate and the carbon-containing heat sink. In the embodiments according to the invention, the thickness of the barrier layer (e.g., Mo) is 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 mm.
The carbon-containing heat sink is brazed to the substrate to form the target assembly. Any type of thermally conductive carbon-based heat sink can be used. Graphitic and/or composite carbon heat sinks are preferred for their durability and ability to conduct heat away from the substrate during use.
The heat sink can be brazed to the barrier layer using a braze that is compatible with the particular heat sink being used and the particular metals in the barrier layer. Examples of suitable brazes include, but are not limited to, Zr, Ti, V, Cr, Fe, Co, Ni, Pt, Rd, and Pd.
The x-ray target devices manufactured using the method of the present invention have surprisingly good bonding between the heat sink and the substrate compared to similar devices manufactured without a barrier layer. It is believed that the superior bonding in the x-ray target assemblies of the invention is achieved by reducing or eliminating the production of carbon oxide gases during the brazing process. The barrier layer keeps oxides in the ODS refractory metal away from the carbon heat sink during the brazing process, thereby preventing the oxides from reacting with carbon to form a gaseous carbon oxide. Preventing the formation of gases during brazing improves the quality of bonding.
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:
Reference will now be made to the figures wherein like structures will be provided with like reference designations. It is understood that the drawings are diagrammatic and schematic representations of exemplary embodiments of the invention, and are not limiting of the present invention nor are they necessarily drawn to scale.
The anode assembly 100 is rotated by an induction motor, which drives bearing sleeve 119 and bearing shaft 118. Bearing shaft 118 is connected to stem 112 and rotatably supported by bearings 120. Bearings 120 are housed in bearing housing 122, which supports the x-ray target assembly 100.
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.
Barrier layer 124 is positioned between substrate 110 and heat sink heat sink 116 to prevent chemical reactions between substrate 110 and heat sink 116 during brazing. The following describes the substrate, heat sink, and interlayers in more detail.
Substrate 110 can have any shape suitable for use in an x-ray tube. To facilitate rotation, 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. Substrate 110 can be made from any 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.
In addition to the refractory metal, the substrate includes one or more inert metal oxides that are dispersed as fine particles throughout the refractory metal. Examples of suitable metal oxides that can be dispersed in the refractory metal include lanthana, yttria, ceria, and thoria, with lanthana being preferred.
The metal oxide compounds can be included in the refractory metal in any amount that yields a strengthened refractory metal substrate. In one embodiment, the weight percent of the metal oxide (e.g., La) in the ODS refractory metal is in a range from about 0.1 wt% to about 10 wt%, more preferably in a range from about 1.0 wt% to about 6 wt%, and most preferably in a range from about 2 wt% to about 4 wt%.
The grain size of the ODS refractory metal and the particle size of the metal oxides can have an effect on the properties of the substrate. In one embodiment, the grain size is in a range from about 10 to about 50 micrometers and the average diameter of the oxide particles is in a range from about 0.05 micrometers to about 5 micrometers.
The ODS refractory metals can be manufactured using any suitable method. In one embodiment of the invention, the ODS refractory metal can be manufactured by using the following steps: (a) forming a slurry of molybdenum oxide and an aqueous solution of a metal salt selected from nitrates or acetates of lanthanum, cerium, thorium, or yttria; (b) heating the slurry in a hydrogen atmosphere to produce a powder of molybdenum and the oxides of the metal salt; (c) mixing and cold isostatically pressing the powder; (d) sintering the powder from step (c) in a hydrogen atmosphere to produce a sintered product; and (e) thermomechanically processing the sintered product to a total reduction in cross-sectional area of about 60% to about 80% or even higher to produce about a 93%-99% dense molybdenum alloy containing an oxide dispersion.
The ODS refractory metals are typically provided as a powder. Substrate 110 can be manufactured from the powders using known powder metallurgy techniques.
Additional examples of suitable ODS Mo alloys and methods for making the ODS Mo alloys are disclosed in
The heat sink 116 is a carbon-based structure positioned on the substrate 110 so as to absorb heat generated from electrons impinging upon focal track 114, to create x-rays. Heat sink 116 is made of a 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 barrier layer is a thin layer of a metal compound that separates the ODS refractory metal from the carbon-based heat sink. The barrier layer material is substantially free of the metal oxides that are present in the substrate and is preferably free of any oxides. The barrier layer can be made from a substantially pure metal. Examples of suitable metals include Mo, Ta, Re, W, Ti, Nb, V, Hf, Zr, 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. Where a metal carbide is used as the barrier layer, the surface of the substrate can be chemically etched before depositing the carbide so as to remove oxides from the surface of the substrate, thereby preventing the oxides from reacting with the carbide.
In one embodiment, the barrier layer is made very thin to minimize the effect that the barrier layer has on the overall mechanical and chemical properties of the substrate. Since the barrier layer does not include the dispersed oxides, the barrier layer does not have some of the same properties as the oxide containing portion of the substrate. By using a thin layer, the overall properties of the substrate can retain the strength and thermal properties of the substrate. In the embodiments according to the invention, the barrier layer has a thickness 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.
The braze layer is a layer of metal that bonds the surface of the carbon-containing heat sink to the substrate. The braze layer is formed by brazing (i.e., melting) a brazing material. At the interface of the heat sink and the braze layer, the braze layer takes the shape of the surface of the heat sink. In one embodiment, the heat sink has concentric rings or another grooved pattern that the braze layer fills to provide high surface area contact.
The braze layer is made from any brazing material compatible with brazing the carbon-based heat sink to the barrier layer. The brazing material is typically selected to have a melting point that is below that of the substrate. In one embodiment, the braze material and a component of the barrier layer form a diffused boundary. The diffused boundary can be formed by selecting a braze that melts at a temperature similar to at least one metal component in the barrier material. In one embodiment, the braze and the barrier material form a eutectic during brazing. Examples of suitable brazing materials include Zr, Ti, V, Cr, Fe, Co, Ni, Pt, Rd, and Pd.
Embodiments of the invention also include methods for manufacturing an anode assembly. Methods for manufacturing the anode assembly according to the invention generally include (i) providing an x-ray target substrate having a deposited track, (ii) forming a barrier layer on at least a portion of the substrate on the opposite side as the track, and (iii) brazing a carbon-containing heat sink to the substrate.
The substrate can be provided in powder form or as a manufactured and/or machined piece. In the case where the substrate is provided in powder form, the manufacturing process of the invention yields a solidified solid structure having the desired shape (e.g., a disk shaped solid).
The barrier layer is formed on at least a portion of the substrate. Preferably the barrier layer covers the entire surface where the heat sink is to be attached. If desired, the barrier layer can cover the entire substrate (e.g., a W or W-Re substrate).
The barrier layer is formed on the substrate by either depositing a barrier layer material or by mechanically bonding a thin layer of barrier material to the substrate. Examples of suitable materials that can be deposited or mechanically bonded to the substrate include pure metals, borides, nitrides, silicides, or carbides of Mo, Ta, Re, W, Ti, Nb, V, Hf, Zr, and combinations of these.
To deposit the barrier material on the substrate, any deposition technique compatible with the barrier layer materials can be used. Examples of suitable deposition techniques include plasma spray coating, salt-bath electrodeposition, electroplating, vacuum sputtering, melt evaporation, chemical or physical vapor deposition, or a combination of these. The deposition technique is carried out so as to deposit the barrier layer material in the desired location and with a desired thickness. Those skilled in the art are familiar with the use of the foregoing techniques to deposit thin or thick layers of metals.
Where a carbide is used as the barrier material, it can be advantageous to remove oxides from the surface of the substrate before depositing the metal carbide compound. Removing oxides prevents the carbides from forming carbon oxides during the deposition process. Carbon oxides typically form gases that can cause poor bonding between the substrate and the barrier layer. The oxides on the surface of the substrate can be removed using chemical etching techniques known in the art.
In an alternative embodiment, a mechanical technique can be used to form the barrier layer. Examples of mechanical techniques that can be used to form the barrier layer include hot rolling, cold rolling, upset forging, hot isostatic pressing, cold isostatic pressing, and combinations thereof. In this embodiment, the barrier material can be provided as a thin sheet that is pressed onto the substrate.
In one embodiment, the barrier layer can be formed while simultaneously forming the substrate from a powder ODS refractory metal. In this embodiment, a three-dimensional piece of barrier material (e.g., a can) is used to hold the powdered ODS refractory metal. The substrate is then formed inside the barrier material using a technique such as isostatic pressing. In this embodiment, the barrier material encapsulates the substrate. If desired, the barrier material can be machined or otherwise worked to remove a portion of the barrier material.
In a subsequent step, the carbon heat sink is brazed to the substrate (i.e., to the barrier layer on the substrate). In one embodiment, the braze can be Ti, Zr, or a compound thereof. The barrier layer material, braze material, brazing temperature, and thickness of the barrier layer are selected to ensure that the braze will form a melt and bond the heat sink to the barrier layer, while preventing oxides in the ODS metal from coming into contact with the carbon substrate. The braze and barrier material can be selected such that the brazing temperature is below the melting point of the barrier material. In one embodiment, the barrier material has a melting point in a range from about 1600 °C to about 2000 °C, alternatively in a range from about 1700 °C to about 1900 °C. In some cases the melting point of the barrier layer can be in a range between about 2000 °C and 3900 °C (e.g., for W or TaC or ZrC materials).
The type of barrier material used can also affect the temperature need during brazing. In one embodiment, the materials are selected to form a eutectic, thereby allowing for brazing at relatively low temperatures. Alternatively, if the barrier material is a boride, nitride, silicide, or carbide, the brazing temperature may need to be close to the melting point of the braze. In addition, the particular barrier material selected should be thermodynamically stable above the melting point of the braze. For example, where Zr is used as a braze, the barrier material can be selected to be stable at a temperature above 1850 °C, the melting point of Zr.
During the brazing process, the barrier material provides a physical barrier between the carbon atoms in the heat sink and the oxides in the ODS refractory metal of the substrate. During brazing, there is typically some diffusion of atoms at the boundary between the braze and other layers. By using a barrier layer that is substantially free of oxides, there are no oxides available to migrate through the braze to the boundary with the carbon heat sink when the braze is molten. Similarly, if carbon radicals diffuse from the boundary with the carbon heat sink to the boundary with the barrier layer, there will be no oxides that can form carbon oxide gases. By reducing and/or eliminating the formation of carbon oxide gases during brazing, the braze is able to form a good bond with the carbon heat sink and allows for a higher performance anode to be built.
The superior bonding of the carbon heat sink to the substrate allows for very large targets to be manufactured. In one embodiment, the target has a diameter of at least about 100 mm, more preferably at least about 150 mm and most preferably at least about 200 mm. Of course smaller diameters such as from 25-100 mm are also possible. Larger diameter targets are useful for making higher performance x-ray tubes.
The following provides an example of an x-ray target assembly and method according to one embodiment of the invention. An x-ray target assembly was manufactured from a substrate comprising an ODS Mo alloy. The ODS Mo alloy included 1 wt% La2O3 dispersed in Mo. The substrate was disk-shaped with an approximately 200 mm diameter. A W-Re track was deposited on the upper side of the substrate using a vacuum plasma spray process. A substantially pure Mo barrier layer of about 0.25 mm to 1 mm was deposited on the substrate using a vacuum plasma spray process.
A graphite heat sink with "record grooves" was brazed to the substrate using a Zr braze washer (thickness of ~ 0.25 mm).
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 anode 100 and cathode 208. In the illustrated embodiment, cathode 208 is biased by a power source (not shown) to have a large negative voltage, while anode 100 is maintained at ground potential. In other embodiments, the cathode is biased with a negative voltage while the anode is biased with a positive voltage.
Cathode 208 includes at least one filament 214 that is electrically connected to a high-voltage source. During operation, electrical current is passed through the filament 214 to cause electrons, designated at 218, to be emitted from cathode 208 by thermionic emission. Application of the high-voltage differential between anode 100 and cathode 208 then causes electrons 218 to accelerate from cathode filament 214 toward a focal track 114 that is positioned on a target surface of rotating anode 100.
As electrons 218 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 222, are directed through x-ray transmissive window 224 disposed in outer housing 202. Window 224 is comprised of an x-ray transmissive material so as to enable the x-rays to pass through window 224 and exit x-ray tube 200. The x-rays exiting the tube 100 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 materials analysis.
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 spectroscopy. Thus, the CT-scanners of the invention can be made to detect material features that might not otherwise be possible with x-ray tubes having inferior performance. In addition, the improved bonding of the heat sink to the substrate improves the durability of the device, thereby lowering operating costs and/or avoiding downtime for repairs or maintenance.
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.
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.