JPH01107439A - Rotary anode for x rays tube - Google Patents

Abstract

PURPOSE: To enhance thermal shock resistance by a method wherein a carbon layer is deposited by decomposing hydrocarbon gases, preferably acetylene or propane, by active direction current glow discharge. CONSTITUTION: A carbon layer 4 is deposited by decomposing hydrocarbon gases, preferably acetylene or propane, by active direct current glow discharge. In this kind of deposition, hydrocarbon is decomposed under the action of high- energy particles during gas discharge at a coating production temperature within a temperature range lower than a temperature necessary for thermal decomposition. During this discharge, ionized molecules, radicals or atoms are accelerated as ions toward a graphite board 3 connected to a negative electrode, or collides into the board 3 as neutral particles energized with collision energy. As a result, a layer 4 made of high-density carbon is deposited on the surface to exhibit excellent adhesion properties. This can enhance thermal shock resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention is directed to a method consisting at least partially of graphite, which graphite is provided with a coating of carbon.
Concerning a rotating anode for an i-tube.

BACKGROUND OF THE INVENTION Modern X-ray diagnostic methods, such as computed tomography, require rotating anodes with as large a heat capacity as possible, and this can be achieved by increasing the size of the rotating anode.

In addition to this large heat capacity, it is also required to rapidly accelerate the single-rotation anode to a rotational speed of 16,000 r.mu.m in the shortest possible time, or to quickly brake it from this rotational speed.

Conventional rotating anodes are generally comprised of high melting point metals such as tungsten, molybdenum, or alloys thereof. However, since these materials have a very high specific gravity, the requirements for rapid acceleration and deceleration cannot be easily satisfied when using these materials in rotating anodes for computed tomography.

For manufacturing such rotating anodes, graphite has proven to be an effective material due to its low density. Furthermore, graphite has a much higher heat capacity and better thermal emissivity than high melting point metals.

There are two main types of graphite rotating anodes in practical use.

In the first type, the body of the rotating anode consists of graphite, which body is provided in the focal orbit region with a thin target layer of a high melting point metal, usually a tungsten-rhenium alloy. Advantageously, this target layer is applied by a special coating method, such as a chemical vapor deposition method.

In a second type of graphite rotating anode, the target layer is overlaid on a substrate made of a high melting point metal, such as molybdenum or a molybdenum alloy, and the substrate is coated with one or more sintered graphite particles outside the target layer. It is preferably joined to the part by brazing.

In the case of all graphite rotating anodes, it is a disadvantage that the machining of the sintered graphite causes wear debris in the form of fine graphite particles to become embedded in the porous surface. These buried fine graphite particles cannot be removed satisfactorily even by special cleaning methods, such as cleaning in an ultrasonic bath, and x! ! During operation of the rotating anode in the tube, dust-like "floating" occurs, i.e. fine graphite particles detach from the graphite surface, for example under the action of electrostatic forces or due to centrifugal force, and
Deposits inside the wire tube. As a result, there is a risk of 7 latchovers occurring in the x-ray tube, especially at voltages above 100 kV'. Moreover, due to its high porosity (usually about 20% of the volume), gases adsorbed on the graphite can be liberated into the vacuum of the X-ray tube, which leads to vacuum deterioration and thus also to floodover. leading to serious driving accidents.

In order to avoid this "flying up" and outgassing of the graphite rotating anode as much as possible, the graphite surface has been covered with various coating materials, thereby sealing it, so to speak.

DE 31 34 1913 A1 describes the coating of rotating graphite anodes with a coating of pyrolytic carbon, for example.

This coating is formed by decomposition of gaseous hydrocarbon compounds at temperatures of 1000°C to 1100°C. Although the high-voltage stability of such a coated rotating anode is indeed significantly increased, the thermal emissivity is however significantly lower compared to a rotating anode with an uncoated graphite surface;
This again leads to limitations on the loading capacity of such rotating anodes.

Another disadvantage of pyrolytically applied carbon layers is that high coating temperatures of more than 1000° C. are required at pressures of 10 to 1000 bar.

This means that in the case of graphite rotating anodes in which one or more graphite parts are brazed to a substrate made of a high-melting point metal, the graphite coating must be formed before the brazing with the substrate. The result is that the hydrogen liberated in the process of film formation is
This is because a high film formation temperature may lead to embrittlement of the solder material and, therefore, damage to the material bond.

A significant drawback of pyrolytically deposited carbon layers on sintered graphite is that due to the different expansion coefficients of the sintered graphite and pyrolytic carbon layers, it creates large stresses in the layer, which are particularly difficult to resist thermal shock. The reason is that it works against the sexes. Also, a decrease in the adhesion or cohesive performance of a layer due to mechanical effects will normally occur at lower loads if there is an initial stress in a layer than in the case of a stress-relieved layer.

[Problem to be Solved by the Invention] The present invention is composed of at least partially graphite, the graphite is provided with a coating of carbon, and the coated graphite surface has a thermal emissivity at least as large as that of the uncoated graphite. The object of the present invention is to provide a rotating anode for an X-ray tube, which has improved thermal shock resistance compared to known graphite rotating anodes coated with pyrolytic carbon. Furthermore, it is intended to enable the production of the coating as a final processing step, in particular after the final processing of the component and after the brazing of the metal-graphite joint.

This object is achieved according to the invention in that the carbon layer is deposited by decomposing a hydrocarbon gas, preferably acetylene or propane, in an activated direct current glow discharge.

[Effect] In this type of precipitation, the decomposition of the hydrocarbons occurs under the action of energetic particles in the gas discharge, at a temperature clearly below that required for pyrolysis (at least 1000'C).
That is, the film formation temperature is in the temperature range of approximately 200 to 650°C. Molecules, groups or atoms ionized in this discharge are either accelerated as ions towards the graphite substrate connected to the cathode or strike the substrate as neutral particles energized by collisions. As a result, a dense and adherent layer of carbon is deposited on the surface.

The layers thus analyzed have very fine quality, often partially amorphous structural features with grain sizes below 0.17 zm, and usually have hexagonal prism axes parallel to the direction of layer growth. It is characterized by a preferred orientation of microcrystallites. In contrast, thermally deposited carbon layers generally exhibit a flaky structure with a coarse composition under the precipitation conditions actually used on an industrial scale, and in some cases exhibit a granular structure under modified precipitation conditions. However, they also have a coarse structure compared to the structure of the carbon layer deposited according to the invention.

A clear distinction between the pyrolytically overlaid carbon layer and the inventively overlaid carbon layer is provided by the different X-ray diffraction of the two layers. The carbon layer deposited according to this invention has a significantly broadened (002)-X-ray diffraction reflection (half-width of 2@>0.6°) compared to sintered graphite, while the thermal The resulting carbon layer shows almost no broadening.

Due to the coating according to the invention, the detachment of graphite particles from the surface during rotation of the rotating anode is significantly reduced, even to the same extent as in the case of rotating graphite anodes covered with a pyrolytic carbon layer. .

Although the average value of surface roughness Ra is reduced by the coating according to the invention, the extent of the reduction is much smaller than in the case of the thermally deposited layer, which existed before the formation of the coating. Related to surface roughness. Pores with a diameter approximately equal to twice the layer thickness and open toward the surface are, for example, approximately enclosed.

According to this containment effect and reduction of surface roughness.

It was quite surprising that the coating according to the invention does not cause a reduction in thermal emissivity compared to uncoated graphite, and usually even a slight improvement occurs. On the other hand, with pyrolytic coatings according to the prior art, a clear decrease in emissivity occurs.

This surprising effect is due to the greater microscopic roughness of the carbon layer deposited according to the invention compared to the thermally deposited coating layer, which is explained by the scanning electron microscopy. Proven.

The layer thickness of the carbon coating based on this invention is 3 m to 7 m.
It is advantageous to have a range of .

Below 3 gm, it is generally not possible to achieve sufficient encapsulation of graphite particles in the coating layer that may be loosened due to pretreatment or chipped due to localized surface topography.

In the case of layer thicknesses exceeding 7 μm, the internal stresses of the carbon layer according to the invention are already noticeable, and these internal stresses, combined with the thermal stresses generated during operation of the rotating anode, can lead to delamination of the layer. There is.

Standard commercially available physical vapor deposition layer coating equipment can be used to deposit the carbon layer according to this invention.

In a particularly advantageous embodiment of this deposition method, the rate of carbon deposition is increased by auxiliary evaporation of the graphitic carbon with an electron beam gun.

A very rapid deposition of the carbon layer according to the invention is thus achieved without any adverse effects on the layer properties.

[Example] Next, drawings showing an example of a rotating anode based on the present invention,
The present invention will be explained in detail using graphs showing its thermal radiation characteristics and scanning electron micrographs of its structure.

The graphite composite rotating anode according to the present invention, as shown in FIG. It consists of a graphite section 3 brazed to the underside of the base body 1. This rotating anode described as an example is 127
mm diameter and a heat capacity of about 70 OkJ with a maximum operating temperature of 1200'C.

Coating production on the rotating anode was carried out in a conventional ion blating coating production apparatus equipped with two electron beam guns, but also equipped with one auxiliary anode (triode arrangement!!l), according to the method described below.

The graphite composite anode was pre-cleaned in an ultrasonic cleaning bath with the brazing completed. Subsequently, the composite anode was vacuum heat treated at 1350°C for 5 hours, which resulted in sufficient degassing of the graphite part of the rotating anode. After coating the metal surface, the composite anode was immediately transferred to the coating formation chamber. was placed inside. The film forming chamber was evacuated to a pressure of 10-3 Pa and simultaneously heated to about 5006C. During this process, the graphite part of the rotating anode is again degassed at the zero-order ion etching stage, and the surface of the graphite part is exposed to an IPa argon atmosphere by narrowing the exhaust pipe and applying a DC voltage of 5 kV. This resulted in atomic purification for 30 minutes, with simultaneous heating of the rotating anode to approximately 650'C.

Subsequently, a film formation process was introduced by feeding propane into a sustained glow discharge, the partial pressure of propane being approximately 50 to 80% of the total pressure of 0.5 to 2 Pa. . The glow discharge during coating formation was maintained with a workpiece voltage of -1000 to -3000V. The degree of ionization of the plasma was increased by applying a voltage of +30 to +1 oov to the auxiliary anode, so that a current density of about 0.1 mA/cm2 was achieved at the graphite surface. In addition, two graphite circular materials placed in the coating production chamber each have a weight of 2 to 4k.
It was evaporated by two electron beam guns with a capacity of W, thereby obtaining a carbon deposition rate of about 2 to 5 g/h.

After a coating time of 3 hours, the coating process was terminated. The rotating anode was then cooled in vacuum. The carbon layer deposited on the graphite surface of the rotating anode according to the invention had a layer thickness of 5 #Lm.

A graphite composite rotary anode coated according to the invention is then tested in a tube test stand with respect to its thermal emissivity and withstand high voltage strength, with the same dimensions and material composition but with one uncoated graphite section. Another comparison was made for two graphite composite rotating anodes with pyrolytically coated graphite sections.

To ascertain the heat load capacity, these rotating anodes were
It was exposed to irradiation with a tube voltage of kV and a tube current of 250 mA and an emission time of 6.4 seconds.

In order not to exceed the maximum operating temperature of 1200 °C, a rest period of 150 seconds is inserted between the individual irradiations in the case of a rotating anode according to the invention and an uncoated rotating anode, while a pyrolytic coating In the case of a rotary anode, the rest time had to be extended to 190 seconds in order not to exceed this temperature limit.

FIG. 2 shows the temperature φ time curves of three different rotating anodes during tube operation. The cooling curve of the uncoated rotating anode is designated by the symbol 1, the cooling curve of the pyrolytically coated rotating anode is designated by the symbol 2, and the cooling curve of the rotary anode coated according to the invention is designated by the symbol 3. . These rotating anodes were operated at 1 by cyclic tube operation under the above loading conditions.
It was heated to not exceed the maximum operating temperature of 200'C (190 seconds rest time) and the cooling curve was measured using a 1-spectroscopic pyrometer. Based on these cooling curves, approximately the same radiative cooling can be read for the anode coated according to the invention as for uncoated graphite, whereas the anode with pyrolytically coated graphite cools clearly slower. , this has the consequence that anodes with pyrolytically coated graphite work at significantly higher temperatures during tube operation.

Measurements of the thermal emissivity were carried out in a radiometry chamber on specimens made of graphite coated according to the invention, pyrolytically coated specimens and uncoated specimens.

The coating was formed under the same conditions as those for the rotating anode.

The following values were obtained at T=1300K.

Uncoated graphite: ε=0.78 Graphite coated with pyrolytic carbon: ε=0.63 Graphite coated with carbon according to the invention:
ε = O, aO These rotating anodes were also tested for high voltage strength on an X-ray tube test stand.

The tube voltage was increased to 180 kV for high voltage strength testing. Due to the frequency of floodovers that occur in this case, rotating anodes coated according to the invention and pyrolytically coated rotating anodes have a significantly improved high durability compared to uncoated rotating anodes. The voltage strength was confirmed.

In order to demonstrate the improved thermal shock resistance of rotating anodes coated according to the invention, bending strips made of graphite were coated pyrolytically and with S carbon according to the invention. The thermally deposited carbon layers had on average three times as much layer stress as the carbon layers deposited according to the invention.

From this it can be deduced that the carbon layer according to the invention has significantly improved thermal shock resistance.

In Fig. 3, the flaky structure of the carbon layer deposited by thermal analysis can be observed, and this structure is similar to the fine-quality partially amorphous structure of the carbon layer precipitated based on the present invention and shown in Fig. 4. are clearly different.

[Brief explanation of the drawing]

FIG. 1 is an axial cross-sectional view of an embodiment of a rotating anode according to the present invention, and FIG. 2 is a diagram showing cooling curves of the rotating anode shown in FIG. 1, the pyrolytically coated rotating anode, and the uncoated rotating anode. , 3rd
The figure is a 6000x scanning electron micrograph of a cross section of a pyrolytically coated graphite section, and FIG. 4 is a 10000x scanning electron micrograph of a cross section of a graphite section coated according to the invention. 1...Substrate 2...Focal trajectory coating 3...Graphite portion 4...Carbon layer procedural correction writing type) 1. Indication of the case: Patent application No. 63-1905042, title of the invention: Rotating anode 3 for X-ray tubes, relationship with the case by the person making the amendment: Patent applicant address: Leutste, Tyrol, Austria (no street address) Name: Metalwerk, Bransee , Gesellschaft, Mitsut, Beschrenchtel, Hafrang 4, Agent ■ 112 5. Date of amendment order Sent October 25, 1986 6
, Subject of amendment Column 7 of brief explanation of drawings in the specification, Contents of amendment

Claims (1)

Claims: 1) In a rotating anode for an X-ray tube consisting at least in part of graphite, which graphite is provided with a coating of carbon, the deposition of the carbon layer is carried out using a hydrocarbon gas, preferably acetylene or propane. A rotating anode for an X-ray tube, characterized in that the rotating anode for an X-ray tube is 2) The rotating anode consists of a graphite substrate, which substrate has, at least in the focal orbit region, a target layer made of rhenium and/or a tungsten-rhenium alloy and overlaid on the graphite substrate. rotating anode. 3) The rotating anode consists of a base made of a high melting point metal or a high melting point alloy, and this base is made of tungsten or tungsten.
2. The rotating anode of claim 1, wherein the anode is soldered to one or more graphite parts outside the target layer of rhenium alloy. 4) The rotating anode according to claim 1, wherein the rate of carbon deposition is increased by simultaneous evaporation of graphite using an electron beam gun. 5) The carbon film has a microcrystalline structure with a particle size of less than 0.1 μm, and 2
5. The rotating anode according to claim 1, characterized in that it has a (002)-X-ray diffraction reflection with a half width of Θ>0.6°. 6) The rotating anode according to claim 5, wherein the carbon coating has a layer thickness of 3 μm to 7 μm.