Patent Publication Number: US-7903786-B2

Title: Apparatus for increasing radiative heat transfer in an X-ray tube and method of making same

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to x-ray tubes and, more particularly, to a textured surface applied to anode components of an x-ray tube. 
     X-ray systems typically include an x-ray tube, a detector, and a bearing assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then transmits 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 a computed tomography (CT) package scanner. 
     X-ray tubes include an anode structure comprising a target onto which the electron beam impinges and from which x-rays are generated. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode target. Because of the high temperatures generated when the electron beam strikes the target, the anode assembly is typically rotated at high rotational speed for the purpose of distributing heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. 
     Newer generation x-ray tubes have increasing demands for providing higher peak power. Higher peak power, though, results in higher peak temperatures occurring in the target assembly, particularly at the target “track,” or the point of electron beam impact on the target. Thus, for increased peak power applied, there are life and reliability issues with respect to the target. 
     In general, radiation heat transfer may be improved by treating a surface such that its emissivity is increased. One known technique includes treating the surface by defining a dense array of cavities beneath the surface that are each exposed to the outer surface via respective small apertures that are on the order of, for example, 10 microns in diameter. In such an arrangement, the cavities behave as black bodies and may have an emissivity of essentially 1.0 over their exposed area on the surface. Thus, the overall emissivity of an original surface may be proportionately improved, and the improvement may be quantified by assuming an emissivity of 1.0 over the effective aperture areas of the cavities and by assuming that the remaining surface area, without apertures, has an emissivity equal to that of the original surface. In other words, the overall surface emissivity may be estimated by assuming that the areas of the apertures have an emissivity of 1.0 and by assuming that the remaining areas without cavities have an emissivity of the original surface. Thus, the overall emissivity may be improved by several-fold over a surface having originally a low surface emissivity. 
     Such a technique may, in theory, be applied to a surface of an x-ray tube target as well. However, in order to achieve the desired black body characteristics as described, typically the cavities applied to the surface have a depth-to-diameter ratio that is approximately 2:1 or greater. And, due to the unique operating environment of an x-ray tube (i.e., high temperature, high voltage, and high vacuum environment), applying such a treatment to a target may result in other negative consequences that preclude such an application therein. 
     For instance, cavities having a depth-to-diameter aspect ratio of 2:1 or larger on the surface of an x-ray tube target may introduce high-voltage instability problems in an x-ray tube. Because of the high depth-to-diameter ratio, the thin walls of the cavities tend to be friable, or easily fragmented, and may serve as a particulate source. Furthermore, the cavities may also serve to retain solvents or other films that may be introduced during processing of the target. Such deep cavities may act as virtual sources of contaminants, making cleaning very difficult, and possibly introducing a new long-term failure mode into the x-ray tube. 
     Therefore, it would be desirable to have a method and apparatus to improve the emissivity of x-ray tube target anode components while maintaining high-voltage stability of the x-ray tube in which it is operating, good mechanical integrity, and simplicity in handling. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention provides an apparatus for improving thermal performance of an x-ray tube target that overcomes the aforementioned drawbacks. 
     According to one aspect of the invention, a target assembly for generating x-rays includes a target substrate, and an emissive coating attached to the target substrate, the emissive coating including a textured material including a plurality of granular protrusions arranged to increase gray body emissive characteristics of the target assembly above that of the target substrate. 
     In accordance with another aspect of the invention, an x-ray tube target includes a target substrate comprising one of Mo and alloys thereof, and treating a target substrate with an emissive coating comprising a plurality of protuberant granulations having an arrangement that increases a gray body emissivity from the target substrate above that of an untreated target substrate. 
     Yet another aspect of the invention includes an imaging system having an x-ray detector and an x-ray emission source. The x-ray source includes a cathode and an anode. The anode includes a target base material and an emissive coating attached to the target base material, the emissive coating includes a plurality of protuberant granulations configured to increase gray body emissive characteristics of the emissive coating above an emissivity of the target base material. 
     Various other features and advantages of the invention will be made apparent from the following detailed description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
       In the drawings: 
         FIG. 1  is a block diagram of an imaging system that can benefit from incorporation of an embodiment of the invention. 
         FIG. 2  is a cross-sectional view of an x-ray tube according to an embodiment of the invention and useable with the system illustrated in  FIG. 1 . 
         FIG. 3  is an illustration of a chamber and technique for applying a coating to a substrate according to an embodiment of the invention. 
         FIG. 4  is an illustration of a surface morphology formed according to an embodiment of the invention. 
         FIG. 5  is an illustration of a surface morphology formed according to an embodiment of the invention. 
         FIG. 6  is a graph showing plots illustrating emissivity measured on surfaces formed according to embodiments of the invention. 
         FIG. 7  is a pictorial view of a CT system for use with a non-invasive package inspection system that can benefit from incorporation of an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a block diagram of an embodiment of an imaging system  10  designed both to acquire original image data and to process the image data for display and/or analysis in accordance with the invention. It will be appreciated by those skilled in the art that the invention is applicable to numerous industrial and medical imaging systems implementing an x-ray tube, such as x-ray or mammography systems. Other imaging systems such as computed tomography systems and digital radiography systems, which acquire three-dimensional image data for a volume, also benefit from the invention. The following discussion of x-ray system  10  is merely an example of one such implementation and is not intended to be limiting in terms of modality. 
     As shown in  FIG. 1 , x-ray system  10  includes an x-ray source  12  configured to project a beam of x-rays  14  through an object  16 . Object  16  may include a human subject, pieces of baggage, or other objects desired to be scanned. X-ray source  12  may be a conventional x-ray tube producing x-rays having a spectrum of energies that range, typically, from 30 keV to 200 keV. The x-rays  14  pass through object  16  and, after being attenuated by the object  16 , impinge upon a detector  18 . Each detector in detector  18  produces an analog electrical signal that represents the intensity of an impinging x-ray beam, and hence the attenuated beam, as it passes through the object  16 . In one embodiment, detector  18  is a scintillation based detector, however, it is also envisioned that direct-conversion type detectors (e.g., CZT detectors, etc.) may also be implemented. 
     A processor  20  receives the analog electrical 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 . Moreover, the invention will be described with respect to use in an x-ray tube. However, one skilled in the art will further appreciate that the invention is equally applicable for other systems that include a target used for the production of x-rays. 
       FIG. 2  illustrates a cross-sectional view of an x-ray tube  12  incorporating an embodiment of the invention. The x-ray tube  12  includes a frame or casing  50  having an x-ray window  52  formed therein. The frame  50  encloses a vacuum  54  and houses an anode or target assembly  56 , a bearing cartridge  58 , a cathode  60 , and a rotor  62 . The target assembly  56  includes a target substrate  57  having a target shaft  59  attached thereto. X-rays  14  are produced when high-speed electrons are decelerated when directed from the cathode  60  to the target substrate  57  via a potential difference therebetween of, for example, 60 thousand volts or more in the case of CT applications. The electrons impact a target track material  86  at focal point  61  and x-rays  14  emit therefrom. The x-rays  14  emit through the x-ray window  52  toward a detector array, such as detector  18  of  FIG. 1 . To avoid overheating the target track material  86  by the electrons, the target assembly  56  is rotated at a high rate of speed about a centerline  64  at, for example, 90-250 Hz. 
     The bearing cartridge  58  includes a front bearing assembly  63  and a rear bearing assembly  65 . The bearing cartridge  58  further includes a center shaft  66  attached to the rotor  62  at a first end  68  of center shaft  66  and a bearing hub  77  attached at a second end  70  of center shaft  66 . The front bearing assembly  63  includes a front inner race  72 , a front outer race  80 , and a plurality of front balls  76  that rollingly engage the front races  72 ,  80 . The rear bearing assembly  65  includes a rear inner race  74 , a rear outer race  82 , and a plurality of rear balls  78  that rollingly engage the rear races  74 ,  82 . Bearing cartridge  58  includes a stem  83  which is supported by the x-ray tube  12 . A stator (not shown) is positioned radially external to and drives the rotor  62 , which rotationally drives target assembly  56 . In one embodiment, a receptor  73  is positioned to surround the stem  83  and is attached to the x-ray tube  12  at a back plate  75 . The receptor  73  extends into a gap  79  formed between the target shaft  59  and the bearing hub  77 . 
     The target track material  86  typically includes tungsten or an alloy of tungsten, and the target substrate  57  typically includes molybdenum or an alloy of molybdenum. A heat storage medium  90 , such as graphite, may be used to sink and/or dissipate heat built-up near the focal point  61 . One skilled in the art will recognize that the target track material  86  and the target substrate  57  may comprise the same material, which is known in the art as an all metal target. 
     In operation, as electrons impact focal point  61  and produce x-rays, heat generated therein causes the target substrate  57  to increase in temperature, thus causing the heat to transfer predominantly via radiative heat transfer to surrounding components such as, and primarily, frame  50 . Heat generated in target substrate  57  also transfers conductively through target shaft  59  and bearing hub  77  to bearing cartridge  58  as well, leading to an increase in temperature of bearing cartridge  58 . 
     Without an emissive coating or other anode assembly modification, target substrate  57  may have an emissivity of, for instance, 0.18. As such, radiative heat transfer from the target assembly  56  may be limited, thus contributing to an increased operating temperature of the bearing cartridge  58  and other components of the target assembly  56 . Thus, to reduce conductive heat transfer into bearing cartridge  58  and to increase the amount of radiative heat transfer to the surrounding components, an emissive coating  92  may be applied to an outer surface  93  of target shaft  59 . An emissive coating  97 , furthermore, may be applied to surface  99  of the target substrate  57  and an emissive coating  94  may also be applied to an outer circumference  95  of the target substrate  57 . Furthermore, an emissive coating  89  may be applied to the surface  91  of the target substrate  57 . 
     Furthermore, emissive coatings may be applied to other surfaces that are encompassed within frame  50  and typically radiatively exchange heat with the target assembly  56 . For instance, emissive coating  85  may be applied to frame  50  at outer circumference surface  84  or an emissive coating  81  may be applied on axial surface  88  of back plate  75 . Additionally, an emissive coating  98  may be applied to surface  69  of rotor  62 , or an emissive coating  67  may be applied to receptor  73  at surface  96 . And, although the emissive coatings  67 ,  81 ,  85 , and  98 , are illustrated over only a small portion of their respective surfaces, one skilled in the art will recognize that the emissive coatings  67 ,  81 ,  85 , and  98 , like emissive coatings  89 ,  94 , and  97 , may be applied over the entire respective surfaces to which they are applied. 
     In one embodiment, the emissive coatings  67 ,  81 ,  85 ,  89 ,  94 ,  97 , and  98  include a plurality of structures applied on their respective surfaces to enhance radiative heat transfer therefrom. Depending on the degree of enhancement desired, the surface textures can range typically from roughened surfaces to high aspect ratio cavity structures. The surface textures can be formed in the coating, in the base object, or in a bulk material that is metallurgically attached to the base object (i.e., attached via brazing, welding, and the like). Because an x-ray tube target typically operates at 1300° C. or above and because surface emissivity is a function of temperature, it is desirable to have a spectral emissivity at, for instance, 0.75 or above at a wavelength up to approximately 2000 nm. 
     Surface emissivity may be increased by applying grain-like or pyramid-like surface morphologies according to embodiments of the invention. The topographical evolution of thin films and coatings may be controlled during physical vapor deposition (PVD), chemical vapor deposition (CVD), low-pressure plasma spray (LPPS), thermal spray, cold spray, reactive brazing, and cladding, as examples. The morphologies may include granular protrusions or protuberant granulations having projections in the nanometer scale as illustrated in  FIGS. 4 and 5 . In one embodiment, the modification of the morphology of a PVD coating can be varied by controlling the rate of vapor flux, flux ionization, substrate temperature, processing pressure, substrate bias voltage, substrate rotation rate, processing atmosphere (e.g. Ar/N 2  ratio for nitride coatings), and the angle between the incoming vapor flux and the substrate surface. 
     As an example,  FIG. 3  illustrates a PVD chamber  100  and technique for applying an optimized high emissive coating according to an embodiment of the invention. Chamber  100  includes an electron gun  102  configured to emit an electron beam  104  toward a target  106  constructed of, for example, titanium. Target  106 , having a diameter of approximately 68.5 mm, is placed into a water-cooled crucible  108 . A gas distribution ring  110  having perforations  112  is positioned proximately to target  106  and is fed by a gas  114 . In one embodiment, gas  114  is nitrogen, and in another embodiment, gas  114  includes a combination of nitrogen and argon. An electrode  116  is positioned proximately to target  106  between target  106  and a substrate  118 . Electrode  116  is configured to discharge to target  106  when power is applied to electrode  116 . 
     In operation, substrate  118 , having a surface  120  upon which a coating is to be applied, is positioned at an angle θ with respect to target  106 . In this example, the angle θ is 6°, however a range of angles between 0° and 90° may be equally applicable, depending on other combinations of settings and parameters applied during the coating process. Prior to deposition, chamber  100  is pumped to a vacuum below 1E-5 torr. Substrate  118  is rotated during the process, and nitrogen, or a mixture of nitrogen and argon, is fed into chamber  100 . Electron gun  102  is configured to emit an electron beam of 0.5-0.75 A having a 18 kV accelerating voltage and scan target  106 . Gas  114  is caused to flow at 1000 sccm through ring  110 . The chamber pressure is maintained at approximately 3-4 mTorr. Electrode  116  is powered with approximately 100 A at 30 V. Thus, electron beam  104  vaporizes material from target  106 , which emits therefrom and is ionized by discharges from electrode  116  causing a flow of ionized vapor  122  to be present in chamber  100 . The ionized vapor condenses on surface  120  and forms, in this embodiment, a TiN coating thereon. During deposition, surface  120  of substrate  118  is maintained at approximately 450° C. and is maintained at an angle θ of approximately 6° with respect to target  106 . Substrate  118  is biased to approximately −125 V and is rotated at approximately 10 RPM. 
     According to an embodiment of the invention, growth of TiN may thus be formed by: 1) evaporation of Ti from the surface of target  106 , 2) ionization of Ti vapor and nitrogen by an ionization device  116 , 3) formation of TiN coating at the surface  120  of substrate  118 . 
     Thus, according to one embodiment, an optimal TiN coating is applied using chamber  100  and technique described above. However, one skilled in the art will recognize that the optimized TiN coating may be applied according to other combinations of processes, and the configuration and operating parameters described above are but one combination of conditions that will result in coatings according to embodiments of the invention. Thus, different morphology types (e.g., topography resembling pyramids, grains, ribbons, hillocks, or craters) can be produced by changing these processing conditions according to embodiments of the invention. The morphology types may be applied to the surface by randomly generating a variety of feature sizes having varying sizes and depths. 
       FIGS. 4 and 5  illustrate coatings that may be applied according to embodiments of the invention. Referring to  FIG. 4 , a granular structure  150  having nanometer scale protuberant granulations  152  that may be formed having an increased emissivity by applying TiN to the surface using the PVD process described above, according to an embodiment of the invention, but using an angle θ of 10°. However, according to this embodiment, the structure, though having an increased emissivity, is not optimized and may be further optimized by using the an angle θ of 6° as described above. Referring next to  FIG. 5 , an optimized coating having a granular structure  160 , with granulations  162  formed thereon, may be altered from that in  FIG. 4  by positioning the surface during a PVD process to receive the coating material according to the processes described above. In the illustrated embodiments, the emissivity of the surface is increased by altering the gray body characteristics thereof, and the granular sizes of the grain-like or pyramid-like surface morphologies range up to approximately 500 nm in size. 
     In general, assuming an opaque material, the emissivity is a function of wavelength and may be expressed as:
 
 E= 1− R   Eqn. 1,
 
where E is the emissivity and R is the reflectivity. As such, a measure of surface reflectivity may provide a good approximation to surface emissivity. Thus, surface emissivity may be estimated, as illustrated in  FIG. 6 , by measuring the reflectivity and applying Eqn. 1.  FIG. 6  is a graph showing plots illustrating emissivity using reflectivity data measured on surfaces formed according to embodiments of the invention. As a reference, curve  200  illustrates emissivity for a surface coating formed by positioning the surface to receive the coating material with a 90° angle and using the parameters as described above. Emissivity is increased, as compared with, for instance, the coating described with respect to curve  200 , for the coating shown in  FIG. 4  applied via the process described in  FIG. 3  using an angle θ of 10° off of parallel (curve  202 ) instead of 6°. Thus, although emissivity is increased for this embodiment, the emissivity may be further increased and optimized by setting the angle to 6° off of parallel (curve  204 ), resulting in a corresponding increase in emissivity, and resulting in the optimized surface texture illustrated in  FIG. 5 . As such, using the process parameters described above, an optimized surface emissivity may be obtained by varying, for instance, the angle θ, and at 6° the process is optimized. However, as discussed, other combinations of process parameters may be applied that equally result in the optimized surface coating illustrated in  FIG. 5 .
 
     Thus, referring to  FIG. 6 , at 1500 nm wavelength, curve  200  illustrates an emissivity of approximately 10% from the reference material, which is increased to approximately 40% for curve  202  and to 80% for curve  204 . As such, by applying Eqn. 1, application of a surface structure as illustrated in  FIG. 5  may result in an emissivity at 1500 nm wavelength improved from approximately 10% to 80% over emissivity of the surface without the surface structure. Note that TiN behaves differently for wavelengths below 700 nm because of its electronic band structure. Nevertheless, over all wavelengths the surface emissivity is increased. Further, although the coating illustrated in  FIG. 4  is indicated to have a lower emissivity than the optimized coating illustrated in  FIG. 5 , that illustrated in  FIG. 4  nevertheless represents a significant improvement over a non-coated surface and is, as such, considered an embodiment of the invention disclosed herein. That is,  FIG. 4 , like  FIG. 5 , illustrates a coating having a surface emissivity that is increased by applying grain-like or pyramid-like surface morphologies that include granular protrusions, or protuberant granulations, and having granular sizes ranging approximately to 500 nm in size. 
     Additionally, the coating applied need not be limited to TiN, but may include in general one of a nitride and a carbide. Further, the cation moiety may be any one of titanium, zirconium, hafnium, vanadium, niobium, tantalum and chromium, or a combination thereof and, when the emissive coating includes one of a nitride and a carbide, it may be applied via one of PVD and wet etching. And, although a PVD apparatus and process is described above, other apparatus and processes may be equally applicable in forming textured coatings according to this invention. For instance, sputtering, chemical vapor deposition (CVD), low-pressure plasma spray (LPPS), thermal spray, cold spray, reactive brazing, and cladding. In embodiments where the coating includes one of a nitride and a carbide, the emissive coating is deposited via one of electron beam physical vapor deposition, sputtering, and filtered arc evaporation onto the substrate, wherein the surface of the substrate has an angle of inclination between 0° and 90° to the vapor depositing source. Thus, referring as an example back to  FIG. 3 , in this embodiment the angle θ is 45° or less. 
     In an LPPS embodiment, surface emissivity may be improved, according to this embodiment, and such improvement may be quantified in terms of surface roughness. For example, textured coatings including tungsten (W), molybdenum (Mo), and alloys thereof such as Mo—TiC or Mo—ZrC, with a surface roughness greater than 9 micrometers RMS may be deposited using LPPS. Such coatings typically result in roughened granular protrusions that increase surface emissivity from that of a polished surface having typically an emissivity of 0.3, to approximately 0.7 or greater for textured surfaces with roughness of about 12 micrometers RMS. 
       FIG. 7  is a pictorial view of a CT system for use with a non-invasive package inspection system. Package/baggage inspection system  500  includes a rotatable gantry  502  having an opening  504  therein through which packages or pieces of baggage may pass. The rotatable gantry  502  houses a high frequency electromagnetic energy source  506  as well as a detector assembly  508  having scintillator arrays comprised of scintillator cells. A conveyor system  510  is also provided and includes a conveyor belt  512  supported by structure  514  to automatically and continuously pass packages or baggage pieces  516  through opening  504  to be scanned. Objects  516  are fed through opening  504  by conveyor belt  512 , imaging data is then acquired, and the conveyor belt  512  removes the packages  516  from opening  504  in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages  516  for explosives, knives, guns, contraband, etc. 
     According to one embodiment of the invention, a target assembly for generating x-rays includes a target substrate, and an emissive coating attached to the target substrate, the emissive coating including a textured material including a plurality of granular protrusions arranged to increase gray body emissive characteristics of the target assembly above that of the target substrate. 
     In accordance with another embodiment of the invention, an x-ray tube target includes a target substrate comprising one of Mo and alloys thereof, and treating a target substrate with an emissive coating comprising a plurality of protuberant granulations having an arrangement that increases a gray body emissivity from the target substrate above that of an untreated target substrate. 
     Yet another embodiment of the invention includes an imaging system having an x-ray detector and an x-ray emission source. The x-ray source includes a cathode and an anode. The anode includes a target base material and an emissive coating attached to the target base material, the emissive coating includes a plurality of protuberant granulations configured to increase gray body emissive characteristics of the emissive coating above an emissivity of the target base material. 
     The invention has 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.