GB2084124A - Improved graphite X-ray tube target - Google Patents

SPECIFICATION Improved graphite X-ray tube target This invention pertains to improvements in rotary anode x-ray tube targets which are composed partially or substantially entirely of graphite. Those skilled in the rotating anode x-ray tube art recognized long ago that graphite targets have some advantages over conventional metal targets by virtue of graphite having low density, high thermal emissivity and good dimensional stability at high temperatures. Conventional x-ray targets are usually comprised of a tungsten or molybdenum substrate having a tungsten-rhenium alloy layer providing the focal spot track on which the electron beam of the tube impinges to produce x-rays. The density of tungsten is almost ten times as great as graphite and the density of molybdenum is almost five times as great. The most advanced x-ray diagnostic procedures require x-ray tube targets that have increasingly higher heat storage capacity and this requirement has been met to the extent possible by increasing the volume, that is, the thickness and diameter of metal targets.The great moment of inertia of a heavy metal target has made it difficult to accelerate the target from zero rotational velocity to 10,000 rpm in about two to four seconds and to decelerate it in about the same amount of time as is required in high capacity x-ray tubes. The great weight, concomitant moment of inertia and high temperatures of metal targets impose severe thermal and mechanical stress on the rotating anode bearings. Graphite targets have not been considered practical in high energy x-ray tubes up to the time the present invention was made. One reason is that the graphite dusts off or releases particulates and liberates occluded gases at the high temperatures and high vacuum prevailing in an operating x-ray tube. Dusting off has frequently caused electrical breakdown or flashover between the anode and cathode of the x-ray tube, thereby severely limiting the maximum operating voltage of the tube. Attempts have been made to overcome this problem by coating the graphite surface with metal carbides, typically molybdenum carbide, titanium carbide and tantalum carbide, but such coating techniques have not been successful in eliminating loss of particulate matter from the surface. Moreover, the desirably high thermal emissivity of graphite has been diminished invariably by the various sealant coatings which have been tried. As indicated earlier, the focal track layer of graphite-based targets is usually a refractory metal such as tungsten or, most commonly, tungstenrhenium alloy bonded to the substrate. It has been an all too frequent experience in prior art graphite target tubes, however, for the focal track layer to delaminate due to the absence of an effective carbon diffusion barrier between the graphite substrate and the tungsten-rhenium focal track layer. A consequence is that carbon diffuses into the alloy and forms tungsten carbide in the focal track region of the target. At the operating temperatures of the target, tungsten carbide is brittle and has a coefficient of expansion that is incompatible with the graphite and the W-Re alloy.Moreover, tungsten carbide (WC and W2C) melt at "C. Temperature in the focal spot exceed 3,100"C and the entire target body or substrate may obtain a temperature of "C in high capacity x-ray tubes. Thermal cycling increases the likelihood of delamination occurring. The present invention markedly reduces and, as a practical matter, overcomes the above-recited problems formerly occurring with use of graphite targets by subjecting the graphite substrate or target disk to pyrolytic carbon infiltration (PCI). This treatment results in deposition of a thin impervious coating of pyrolytic carbon on all internal and external surfaces of the graphite target substrate and effectively prevents escape of any adsorbed gases from the target in the high vacuum of an x-ray tube. Surface voids or microscopic cavities between graphite crystallites are coated with pyrolytic carbon to serve, not only as a sealant, but to increase the strength of the graphite body at its surface by coating any minute flaws, cracks or other potential failure sites. Very importantly, the pyrolytic carbon coating prevents loss of graphite particles through thermal cycling, rubbing, abrasion or other mechanical action. The coating does not diminish surface roughness which is desirable because a rough surface has betteremissivitythan a smooth surface. The pyrolytic carbon coating method requires starting with a graphite substrate from which surface contaminants and adsorbed gases have been driven off. This condition can be achieved by heating the substrate to at least do in a furance which has initially been pumped down to a fairly low vacuum to substantially eliminate oxygen after which hydrogen is fed through the furnace. This preliminary relatively high temperature treatment rids the target body of adsorbed gases and volatile contaminants that are inherent in graphite. The clean substrate is then subjected to pyrolytic carbon infiltration which, in general terms, involves heating the graphite substrate to about "C or higher in the presence of a hydrocarbon vapor which is preferably methane mixed with hydrogen at low pressure. The methane may be obtained by using natural gas which is rich in methane. Other hydrocarbons such as propylene and acetylene have been used to yield pyrolytic carbon as can be seen in patented prior art. The hydrocarbon reduces to atomic carbon which settles on the substrate to form a tenacious coating. The process is carried on for several hours and the length of time required will depend on the thickness of pyrolytic carbon layer that is desired. After the pyrolytic carbon coating is deposited, the tungsten-rhenium or other metal focal track layer may be deposited by any one of several known methods. A basically known chemical vapor deposition (CVD) method is preferred. Finally, the target is heat treated at very high temperature and cooled as is customary prior to it being assembled in the x-ray tube. How the foregoing and other more specific objects of the invention are achieved and the specific steps of the method and the parameters involved will now be discussed in greater detail in reference to the drawing. The present invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a transverse section of a typical rotating anode x-ray tube target, composed of a graphite substrate disk or body having a metallic focal track layer and which disk is subjected to pyrolytic carbon infiltration in accordance with the invention; Figure2 is a transverse section of a composite target having a graphite substrate and an interfacing refractory metal layer that has a metallic focal track layer deposited on it; Figure 3 is a diagram, simulating a photomicrograph of a typical graphite material section prior to it having been subjected to pyrolytic carbon infiltration; and Figure 4 shows the same sample cross section as in Figure 3 after it has been subjected to pyrolytic carbon infiltration. The typical rotating anode x-raytube target 10 depicted in Figure 1 comprises a graphite target body or disk 11 which is improved by being coated with pyrolytic carbon in accordance with the invention. An annular layer 12 of metal on one surface of the body or substrate 11 constitutes the focal track. In this particular design, the focal track layer extends axially along the periphery to an annular groove 13 which augments the physical grip between the metal focal track layer and the graphite substrate. The target in Figure 1 is shown as being mounted on the fragmentarily illustrated stem 14 of an x-ray tube rotor, not shown. The stem has a cylindrical portion 15 which extends through a corresponding hole 16 in the target body. The cylindrical portion terminates in a threaded portion 17 which is flat on its opposite sides to fit into a washer 18 that is brazed onto the target and has a non-round hole which is complementary to the shape of the threaded portion so the stem is effectively keyed into the washer. A nut 19 secures the threaded stem to the target body. Another type of target which may be improved by means of pyrolytic carbon infiltration is depicted in Figure 2. This is a metal and graphite composite target comprising a graphite substrate disk 20 which has a refractory metal disk 21 brazed to it. The braze joint is marked 22. Refractory metal disk 21 is commonly made of molybdenum although it may be tungsten or alloys of these metals. A typical tungsten-rhenium focal track layer 23 is bonded to layer 21 using well-known powder metallurgy techniques. The target has a central shouldered hole 24 for receiving a stem, not shown, which would extend from the anode rotor, not shown. The specific properties of the graphite used by makers of x-ray tube targets will depend upon the manner in which the graphite is formed and this will vary with different graphite manufacturers. The polycrystalline graphite customarily used for x-ray tube targets consists of graphite crystallites held together with a binder such as coal tar pitch which has been somewhat graphitized in the graphite forming process. The binder is relatively weak so, as mentioned earlier, graphite crystallites at the surface are not bound tightly and tend to dust off. Voids occur in the binder which result in liberation of gases in the high vacuum of the x-ray tube. Commercially available graphites are known to have various densities.The pyrolytic carbon infiltration (PCI) process described herein is used most advantageously with what might be characterized as medium density graphite in the range of 1.75 to 1.85 grams per cc. A type of graphite found to be desirable, by way of example and not limitation, is one that has a density of 1.82 grams per cubic centimeter which compares with a theoretical maximum density for graphite of about 2.26 grams per cubic centimeter. Graphite target substrates are generally machined by turning them from a cylindrical billet of graphite. Before the PCI treatment, the grain structure of the graphite resembles that in Figure 3 which simulates a magnified photomicrograph of a fragment of bulk graphite. The depicted yet untreated polycrystalline graphite consists of individual graphite crystallites such as those marked with the letter G with interstices or voids, marked V, held together by graphitized carbon binder C. There will be some voids within the body of the target and, as shown in Figure 3, there will be some on the substrate surface between adjoining graphite crystallites G.The graphite crystallites are characterized as being planar and they are thin compared with their length so at the surface they present plane edges which results in larger total surface area and increases reactivity for carbon deposition which is advantageous if the PCI coating process disclosed herein is used and is disadvantageous without it. The individual crystallites G in Figure 3, of course, have a fairly wellorganized atomic lattice in themselves which is typical of polycrystalline graphite. It will be evident, however, that where surface voids occur, the crystallites are not soundly bonded in the graphite mass in which case they may become segregated, especially under high temperature cycling, and produce the dusting off problem which the PCI treatment method disclosed herein eliminates. Before the graphite target body is subjected to pyrolytic carbon infiltration process, the target is pretreated to drive off any surface contaminants and adsorbed or trapped gases. Pretreatment is performed in a hydrogen furnace with which anyone skilled in the x-ray tube art would be familar so its construction need not be described. The graphite target body is supported in the furnace in a manner which allows the surface of the target body, include ing the hole for the rotor stem, to be exposed to the furnace ambient. The interior of the furnace is pumped down to a vacuum on the order of 10-3 torr to minimize the amount of oxygen in the interior of the furnace. The furnace is then purged by flowing hydrogen through it and the temperature of the furnace is raised. Then hydrogen is fed through the furnace at a constant rate. It has been found that a feed rate of 20 to 30 standard cubic feet (570 to 850 standard liters) per hour is satisfactory. Hydrogen pressure in the furnace is a variable which depends on the volume of the furnace chamber and the flow rate. Generally, a pressure of less than 1 torr will prevail. The purging and long term hydrogen feed process is carried out while the temperature of the target body is maintained, preferably in the range of "C to C. However, the decontamination pretreatment could be carried out at temperatures up to "C if the furnace permitted. The concurrent heating and hydrogen flow steps are carried on for at least an hour, preferably, but this time could be shortened or lengthened depending on the temperature of the furnace.Operating at below "C is not recommended. After the pretreatment is completed, the target body is subjected to the pyrolytic carbon infiltration or coating process. This is also done in a convention al gas flow furnace. While the furnace is being purged with hydrogen, the graphite target body is lowered to a temperature in the range of 1 C to Cwith a temperature nearer to the higher value in the range being preferred. Optimum temperature may have to be determined by making trial runs at a few temperatures since this temperature may differ with some types of graphite. While the graphite target body is maintained at a temperature within the range indicated above, a mixture of methane and hydrogen gases is flowed through the furnace. A ratio of two volumes of methane to one of hydrogen is desirable.A pressure range for the gas mixture of 1 to 3 torr should be maintained with about 2 torr being preferable. Exceeding 3 torr is undesirable. Stated in another way, the flow rate for methane may be about 40 standard cubic feet ( standard liters) per hour and for hydrogen, 20 standard cubic feet (565 standard liters) per hour. At the temperature prevailing in the furnace, the hydrogen and methane react upon contact with the hot graphite body in a manner which results in reduction of the carbon and carbon is deposited on the graphite target substrate. This deposited carbon also penetrates into the voids existing between the graphite crystallites at the surface of the target body and coats these crystallites. Of course, all surfaces of graphite crystallites which are exposed to the furnace ambient are similarly coated with pyrolytic carbon. The pyrolytic carbon infiltration process just described is carried on for a long time, typically up to about 35 hours, but the length of time will depend on the thickness of the pyrolytic carbon coating which is found to be satisfactoryforthe particular type of graphite out of which the target body is made. For a relatively high porosity and comparatively medium density graphite such as one having a density of about 1.82 grams per cc, an average coating thickness of about 8 to 10 micrometers of pyrolytic carbon was deposited after 35 hours of treatment at the temperature and glass flow rates indicated above. The desirability of using a relatively high porosity or medium density graphite for the target body can be appreciated by referring to Figure 4 which shows a simulated photomicrograph of the same region of the target body surface as it is depicted in Figure 3 after it has been coated with pyrolytic carbon. The pyrolytic carbon coating is designated generally by the reference letter P. Typically, a graphite crystallite such as the one marked 35 will have the thickest coating on a completely exposed surface marked 36. The depth of penetration of the pyrolytic carbon depends on the depth and width of the voids V. Thus, the thickness of the pyrolytic carbon layer at the bottom of a void, as where marked 37, is somewhat less than the thickness near the mouth of the void space or pore. This illustrates the advantage of using graphite which has a density somewhat less than theoretical maximum density. Higher porosity permits greater pyrolytic carbon infiltration. It has been observed that the pyrolytic carbon coating is tightly adherent, anisotropic and is com prised of very small graphite crystallites aligned with basal planes parallel to the local surface on which they have deposited. As explained, thickness is maximum at the outer surface and decreases a little with depth of penetration. With the process parameters indicated above, deposition of pyrolytic carbon at the rate of .017 micrometers per hour has been obtained. The pyrolytic carbon infiltrated graphite target bodies retain the high thermal emissivity properties of uncoated graphite. Because the carbon simply coats the interiors of the surface voids or pores instead of filling them, microscopic surface roughness is preserved. As is known, a rough surface has higher emissivity. By way of example, spectral thermal emittance of 0.82 at 2.0 micrometers wavelength has been observed. After the target body has been coated with pyrolytic carbon, the tungsten-rhenium alloy focal spot track is applied. Generally, the focal track layer 12 will have a thickness of up to about 0.03 of an inch (.76 of a mm). Delamination is believed to be the result of carbon at the substrate surface diffusing into the tungsten-rhenium alloy and reacting to form tungsten carbide which weakens the bond between the tungsten-rhenium layer and the substrate. It will be evident that if the concentration of rhenium is highest and tungsten lowest at the interface between the target body and tungsten-rhenium focal track layer, tungsten carbide production is likely to be minimized since pure rhenium constitutes the best barrier. There are several ways of applying the tungstenrhenium focal track layer such as vacuum evaporation, ion plating, sputtering, flash evaporation and chemical vapor deposition (CVD) known to those skilled in the x-ray tube art. The CVD process results in a good layer and permits obtaining a rhenium gradient through the layer with maximum concentration of rhenium at the graphite-to-focal track layer interface. A suitable CVD method is described in U.S. Patent 3,819,971. Generally, the CVD method for applying a tungsten-rhenium alloy involves a vapor phase mixture of metal halides and hydrogen which enters a high temperature reaction chamber and results in hydrogen reduction of the halide when the vapors approach the temperature of the target body. The metal deposits on the target body or substrate and the resulting hydrogen-halogen acid vapors are removed from the furnace chamber. The substates of either metal or graphite are normally held at a temperature in the range of 700"C to "C. The halides are eitherthe chlorides or fluorides of tungsten and rhenium. By having a higher concentration of rhenium in the vapor phase during the early part of the deposition process, the concentration of rhenium at the interface of the focal track layer and pyroltyic carbon coated surface can be increased so there is a good barrier layer against formation of tungsten carbide. As the process progresses, the proportion of tungsten halide is increased until, at the electron beam impact surface of the layer, the alloy usually consists of about 3% rhenium and 97% tungsten. Processing of a composite graphite and metal target body of the type shown in Figure 2 is similar, insofar as graphite treatment is concerned, to the process which has been described in reference to a substantially all-graphite target body shown in Figure 1. However, in the Figure 2 type of target, the pyrolytic carbon is first deposited on the entire graphite substrate and then removed only from the focal track area to permit subsequent deposition of the focal track tungsten-rhenium alloy 23. X-raytubes using graphite targets having pyrolytic carbon coating as described above, exhibit marked improvement in high voltage stability at anodecathode voltages more than twice as high as those which could be applied to plane graphite targets at the same high x-ray tube currents. By way of example, in a test arrangement which permitted going up tq 190,000 volts, the pyrolytic carbon targets were still voltage stable while uncoated graphite targets under the same conditions results in instability and flashover at about 80,000 volts. Although the physicai characteristics and the parameters for carrying out the pyrolytic carbon deposition process have been described in detail, such description is intended to be illustrative rather than limiting, for it is to be understood that some modifications and changes may be made by those skilled in the art. Hence, the true scope of the invention should be determined only by interpreting the claims which follow.

Carbon (Graphite) (C) Sputtering Targets Overview

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Carbon (Graphite) (C) General Information

Standard carbon graphite targets can be very difficult to sputter. Carbon has one of the lowest sputter yields of all elements which is attributed to its Sp2 microstructure as well as its anisotropic electrical characteristics. Due to open spaces in Carbon's structural lattice, sputter rates are low and the process is very time-consuming. Standard carbon graphite targets are typically produced by hot-pressing. These targets are generally highly porous and contain randomly oriented grains which results in different localized effects, contributing to the low sputter yield.

On the other hand, Pyrolytic Graphite sputtering targets are much more directional and may have the ability to sputter at higher rates. Pyrolytic Graphite targets are made by chemical vapor deposition (CVD) and are grown atom-by-atom. The resulting material has better thermal, electrical and chemical properties. Due to the nature of the deposition process by CVD, Pyrolytic Graphite material approaches the theoretical density and is essentially non-porous so outgassing occurs quickly.

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Carbon (Graphite) (C) Specifications

* This is a recommendation based on our experience running these materials in KJLC guns. The ratings are based on unbonded targets and are material specific. Bonded targets should be run at lower powers to prevent bonding failures. Bonded targets should be run at 20 Watts/Square Inch or lower, depending on the material.

* Suggested maximum power densities are based on using a sputter up orientation with optimal thermal transfer from target to the sputter cathode cooling well. Using other sputtering orientations or if there is a poor thermal interface between target to sputter cathode cooling well may require a reduction in suggested maximum power density and/or application of a thermal transfer paste. Please contact for specific power recommendations.

Notes:

• Bonding is recommended for these materials. Many materials have characteristics which are not amenable to sputtering, such as, brittleness and low thermal conductivity. Request more information, please click here.
• This material may require special ramp up and ramp down procedures. This process may not be necessary with other materials. Targets that have a low thermal conductivity are susceptible to thermal shock. Please click here for Ramp Procedure for Ceramic Target Break-in.