JPH09213248A - Manufacture of carbon-carbon compound material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a substrate formed out of carbon-carbon compound material for a X-ray rotating electrode, which is provided with comparatively high thermal expansion in the plane inner direction in such a way as to accept focal point tracking material, a high thermal conductivity in the thickness direction in such a way as to be met with the load resistant condition of the focal point tracking material, and with the constitution and the weaving form through which high mechanical strength capable of withstanding rotating stresses. SOLUTION: The compound material 20 is furnished with the constituent and a weaving form through which comparatively high thermal expansion can be obtained in the plane inner direction in such a way as to accept material 16 for focal point tracking, the compound material 20 is also furnished with the constituent and the weaving form through which comparatively high thermal conductivity can be obtained in such a way as to be met with the load resistant condition of the material 16 for focal point tracking, and finally the compound material 20 is also furnished with the constituent and the weaving form through which comparatively high mechanical strength capable of withstanding rotating stresses.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to rotating X-ray tubes and, more particularly, to a new target substrate for an X-ray rotating anode assembly.

[0002]

X-ray tubes have become essential in the fields of medical diagnostic imaging, medical therapy, and various medical testing and material analysis. A typical x-ray tube is constructed with a rotating anode structure to distribute the heat generated in the focal spot. The anode is rotated by an induction motor, which comprises a cylindrical rotor mounted on a cantilevered shaft supporting a disk-shaped anode target and an X-ray tube containing the rotor. An iron core stator structure having a copper winding surrounding an elongated neck. The rotor of the rotating anode assembly, driven by the stator surrounding the rotor of the anode assembly, is at the anode potential, while the stator is electrically referenced to ground. The cathode of the X-ray tube produces a focused electron beam, which is accelerated in the vacuum gap between the anode and cathode to produce X-rays when it strikes the anode.

In an X-ray tube device having a rotatable anode, the target typically comprises a disc made of a refractory metal such as tungsten to rotate the target at high speeds. In the meantime, X-rays are generated by colliding the electron beam with this target. High speed rotating anodes can reach 9,000 to 11,000 RPM. The rotation of the target is performed by driving a rotor provided on a support shaft extending from the target.

The operating conditions of X-ray tubes have changed considerably in the last two decades. U.S. Pat. No. 4,119,261 issued Oct. 10, 1978 and U.S. Pat. No. 4,129,241 issued Dec. 12, 1978, are made of molybdenum and molybdenum-tungsten alloys. It is described that the rotating anode is connected to a shaft made of columbium and its alloy. Due to the constantly increasing energy applied during the operation of the X-ray tube, the composition of the target changed to TZM or other molybdenum alloys, which also increased the diameter and weight of the target, graphite as a heat sink behind the target. Changed to use. Future computed tomography (CT) scanners are expected to be able to reduce scan times from 1 second rotation to 0.5 second rotation. However, this reduction in scan time would most likely necessitate changes to the current CT anode design. The current design of CT anodes consists of two discs, one made of a material with a large heat storage capacity, such as graphite, the second one made of a molybdenum alloy such as TZM. ing. These two concentric discs are joined together by a brazing method. A thin layer of refractory metal, such as tungsten or a tungsten alloy, is deposited to form the focal track. The weight of such a composite substrate structure may exceed 4 kg. At higher rotational speeds of the scanner, the heavy target not only increases the mechanical stress on the bearing material, but also the sag movement of the focal spot, leading to image artifacts.

Therefore, the present design of the CT target is
It is desirable to replace it with a lightweight substrate that has comparable thermal performance.

[0006]

SUMMARY OF THE INVENTION The present invention has a relatively high thermal expansion in the in-plane direction to accommodate the focus track material and a high thermal conductivity in the thickness direction to meet the load bearing requirements of the focus track. , A carbon-carbon composite substrate for an X-ray rotating anode having a composition and a woven shape that will have high mechanical strength to withstand rotational stress.

In one aspect of the invention, a method of making a carbon-carbon composite used in making an X-ray tube target is used. The method includes the step of adjusting the physical properties of the carbon-carbon composite material to achieve the desired physical properties. For example, a very high thermal conductivity in the thickness direction of the carbon-carbon composite disc is desirable, and a high coefficient of thermal expansion in the in-plane direction of the carbon-carbon composite disc is desirable.

Accordingly, it is an object of the present invention to provide a carbon-carbon composite material for an X-ray rotating anode. Another object of the present invention is to provide such a composite having a composition and woven shape such that a relatively high thermal expansion occurs in the in-plane direction to receive the focal track material. Another object of the invention is to provide such a composite having a composition and woven shape such that a relatively high thermal conductivity in the thickness direction is obtained to meet the load bearing requirements of the focal track. Especially. Finally, it is an object of the present invention to provide such a composite having a composition and woven shape such that a relatively high mechanical strength is obtained to withstand rotational stresses such as in centrifugal load operation. .

Other objects and advantages of the present invention will be apparent from the following description of the drawings.

[0010]

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a rotating X-ray tube employing a rotating anode assembly and a cathode assembly. It is an object of the present invention to address the higher rotational speeds of scanners and improve rotating anode materials. Referring now to the drawings, FIG. 1 shows a typical conventional CT anode target 10
Is shown. The current design of the CT anode 10 is two disks 12
And 14 are included. One disc 12 is made of a material having a large heat accumulation amount such as graphite, and the second disc 14
Is made of a molybdenum alloy such as TZM. These two concentric discs are joined together by a brazing method. A thin layer of refractory metal, such as tungsten or a tungsten alloy, is deposited to form the focal track 16. Such a composite substrate structure weighs 4 kg.
May be exceeded. At higher rotational speeds of the scanner, the heavy target not only increases the mechanical stress on the bearing material, but also the slack motion of the focal spot, leading to image artifacts.

In accordance with the present invention, the current design of the target shown in FIG. 1 is replaced with a lighter weight substrate whose thermal performance is comparable to current targets. FIG. 2 is a cross-sectional view of a CT anode target 18 constructed according to the present invention. Graphite materials are known to have high heat storage capacity and low density. Unfortunately, graphite materials have been found to be unsuitable for larger diameter targets. Because of the low mechanical strength of graphite, targets with larger diameters tend to tear under the action of centrifugal force. Therefore, other carbonaceous materials such as carbon-carbon composites are preferred in the present invention. Such materials can replace the current targets 10 of CT anodes with the right adjustment of thermophysical and mechanical properties.

In FIG. 4, the anode target 18 is composed of a carbon-carbon composite 20. A thin layer of refractory metal, such as tungsten or a tungsten alloy, is deposited to form the focal track 16 as shown in FIG. Carbon-carbon composites include mechanical and thermal properties, including strength to weight ratio, strength retention and creep resistance over a wide temperature range, resistance to thermal shock, high toughness and high thermal conductivity. It has a unique combination of properties.

However, one disadvantage of high strength, high thermal conductivity carbon-carbon composite materials is their low coefficient of thermal expansion (CTE) compared to that of tungsten or tungsten alloys. Misalignment of thermal expansion between the carbon-carbon composite substrate and the focal track of the target can cause significant stress during processing or use, followed by flaking of the focal track layer. Therefore, according to the present invention, it has a relatively high thermal expansion in the in-plane direction to accommodate the focus track material and a high thermal conductivity in the thickness direction to meet the load bearing requirements of the focus track,
A carbon-carbon composite substrate 20 having a component and a woven shape that has high mechanical strength to withstand rotational stress has been developed.

The high conductivity in the thickness direction of the substrate of the present invention is
This is achieved by increasing the fiber volume fraction of the high strength and high modulus fibers. Suitable materials are, for example, products based on Amaco P-120 or K-1,100 pitch. Vapor grown carbon fiber (VGCF) with thermal conductivity of over 1,500 W / mK, high strength and stiffness is one alternative material for reinforcement in the z-direction.

Rayon precursor materials such as continuous fibers or fabrics were tested in the in-plane direction to reduce the coefficient of expansion shift between the tungsten and carbon-carbon composites.
Fibers based on rayon have relatively low strength, elastic modulus and thermal properties. These are typically parameters that result in a material with high expansion. Carbon woven fabrics made from rayon precursor fabrics also have a relatively high coefficient of expansion.

In the present invention, several carbon-carbon composite substrates were prepared and evaluated. The invention will be better understood from the following description of two examples of carbon-carbon composites having very promising thermal and mechanical properties. The first substrate has a 4-D woven structure and has highly conductive P-120 carbon fibers in the z direction and polycarbon G-5 fibers in the xy direction. The second woven structure is composed of a highly expanded Amaco WCA cloth layer pierced with P-120 fibers. In order to achieve high mechanical integrity, the dough layer is rotated to various angles before piercing,
A D structure was formed. Both carbon-carbon structures consist of 60% fiber volume fraction of P-120 fibers to achieve high thermal conductivity in the z-direction. In a preferred embodiment of the present invention, both fiber preforms were densified into a carbon-carbon composite mold having a high density of 1.85 by an impregnation, carbonization and graphitization process.

To ensure that the thermal performance of these composites was adequate, thermal conductivity and CTE tests were performed and the results are shown in FIGS. 3 and 4. FIG.
Shows the measured CTE data in the in-plane direction along with the values for a typical carbon-carbon material. 4D material is shown by line 22 and stabbed dough material is line 24
Indicated by Developed complex, especially WC on line 24
The material made of A cloth expands much more than the typical carbon-carbon woven material shown by line 26 made of PAN or pitch precursor carbon fiber.

The thermal conductivity of the 4-D material was measured only at room temperature in the z direction and was comparable to that of pierced fabrics. The thermal conductivity of the stabbed fabric is
Measurements in both directions within a temperature range of 20 ° to 1,500 ° are shown in FIG. In FIG. 4, the conductivity in the thickness direction of the pierced cloth shown by the line 28 is about three times larger than that of the graphite shown by the line 30, and the conductivity in the plane shown by the line 32 is shown by the line 30. It is shown to be comparable to the graphite shown. Furthermore, the conductivity in the thickness direction is
Even higher than the conductivity of the refractory alloy shown by lines 34 and 36 used in the target.

In order to determine the ability of the developed composite to withstand the rotational forces, a disk of 150 mm diameter, 20,000
A rotation test up to 0 RPM was performed. Both materials passed this test successfully. Carbon for CT target-
It will be apparent to those skilled in the art that various modifications and variations of the present invention that result in carbon complexes are possible without departing from the scope of the present invention. Carbon-carbon composite targets made in accordance with the present invention have comparable or even better thermal performance than existing target products for CT x-ray tubes, with a 50% weight loss.

Although the present invention has been described in detail with respect to certain preferred embodiments, it should be understood that various modifications and changes can be made within the scope of the present invention.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a conventional CT anode target.

FIG. 2 is a cross-sectional view of a CT anode target according to the present invention.

FIG. 3 is a graph showing the thermal expansion of carbon-carbon composites developed according to the present invention.

FIG. 4 is a graph showing the thermal conductivity of pierced carbon-carbon fabric according to the present invention compared to current X-ray substrates.

[Explanation of symbols]

 16 focus track 18 CT anode target 20 carbon-carbon composite substrate

Claims (4)

[Claims] 1. A method for producing a carbon-carbon composite used in producing an X-ray tube target, which has a very high thermal conductivity in a thickness direction of a disk of the carbon-carbon composite, And producing a carbon-carbon composite comprising the step of adjusting the physical properties of the material of the carbon-carbon composite so as to achieve a high expansion coefficient in the in-plane direction of the disk of the carbon-carbon composite. how to. 2. The method for producing a carbon-carbon composite according to claim 1, wherein the carbon-carbon composite disk has high mechanical strength. 3. The carbon-carbon composite disk of claim 1 having a composition and weave shape that has a relatively high in-plane thermal expansion to receive the focal track material. A method for producing a carbon-carbon composite. 4. The carbon-carbon composite disk is made of a component and a woven shape having a relatively high thermal conductivity in the thickness direction of the disk so as to meet the load bearing condition of the focal track. The method for producing the carbon-carbon composite according to claim 1, which has.