Lightweight rotating anode for X-ray tube

A rotating anode structure for an X-ray tube is provided, having a lightweight target anode. A carbon-carbon composite target substrate has constituents and weave geometries. A refractory metal focal track layer is deposited on the substrate to produce X-rays. An interlayer is disposed between the focal track layer and the substrate to relieve thermal expansion mismatch stresses between the carbon-carbon composite anode target substrate and the refractory metal focal track layer. The interlayer is a rhenium interlayer and the focal track layer is typically a tungsten-rhenium focal track layer.

TECHNICAL FIELD 
The present invention relates to X-ray tubes and, more particularly, to a 
carbon-carbon composite and coating therefor for X-ray rotating anode 
assemblies. 
BACKGROUND ART 
The X-ray tube has become essential in medical diagnostic imaging, medical 
therapy, and various medical testing and material analysis industries. 
Typical X-ray tubes are built with a rotating anode structure for the 
purpose of distributing the heat generated at the focal spot. The anode is 
rotated by an induction motor comprising a cylindrical rotor built into a 
cantilevered axle that supports the disc shaped anode target, and an iron 
stator structure with copper windings that surrounds the elongated neck of 
the X-ray tube that contains the rotor. The rotor of the rotating anode 
assembly being driven by the stator which surrounds the rotor of the anode 
assembly is at anodic potential while the stator is referenced 
electrically to ground. The X-ray tube cathode provides a focused electron 
beam which is accelerated across the anode-to-cathode vacuum gap and 
produces X-rays upon impact with the anode. 
In an X-ray tube device with a rotatable anode, the target typically 
comprises 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. High speed rotating 
anodes can reach 9,000 to 11,000 RPM. Rotation of the target is achieved 
by driving the rotor provided on a support shaft extending from the 
target. 
Operating conditions for 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, were both devoted to joining rotating 
anodes made from molybdenum and molybdenum-tungsten alloys to stems made 
from columbium and its alloys. Continuing increases in applied energy 
during tube operation have led to a change in target composition to TZM or 
other molybdenum alloys, to increased target diameter and weight, as well 
as to the use of graphite as a heat sink in the back of the target. Future 
computerized tomography (CT) scanners will be capable of decreasing scan 
time from a one second rotation to a 0.5 second rotation or lower. 
However, such a decrease in scan time will quite possibly require a 
modification of the current CT anode design. The current CT anode design 
comprises two disks, one of a high head storage material such as graphite, 
and the second of a molybdenum alloy such as TZM. These two concentric 
disks are bonded together by means of a brazing process. A thin layer of 
refractory metal such as tungsten or tungsten alloy is deposited to form a 
focal track. Such a composite substrate structure may weigh in excess of 4 
kg. With faster scanner rotation rates, heavy targets will increase not 
only mechanical stress on the bearing materials but also a focal spot sag 
motion causing image artifacts. 
It would be desirable then to replace the present CT target design with a 
lightweight design comparable in thermal performance, particularly suited 
for use in X-ray rotating anode assemblies. 
SUMMARY OF THE INVENTION 
The present invention provides for a lightweight target anode made of 
carbonaceous materials and a refractory metal focal track coating for use 
in CT scanners. Carbon-carbon composite substrates for an X-ray rotating 
anode are provided, replacing graphite in previous systems, having 
constituents and weave geometries that result in relatively high thermal 
expansion in the in-plane direction to accept the focal track material, 
high thermal conductivity through the thickness to meet focal track 
loadability requirements, and high mechanical strength to sustain 
rotational stresses. The present invention provides for a coating capable 
of joining the refractory metal of the focal track with the carbon-carbon 
composite x-ray anodes, to relieve thermal expansion mismatch stresses 
between the refractory and carbonaceous materials. 
In accordance with one aspect of the present invention, a rotating anode 
structure for an X-ray tube is provided, having a lightweight target 
anode. A carbon-carbon composite target substrate has constituents and 
weave geometries. A refractory metal focal track layer is deposited on the 
substrate to produce X-rays. An interlayer is disposed between the focal 
track layer and the substrate to relieve thermal expansion mismatch 
stresses between the carbon-carbon composite anode target substrate and 
the refractory metal focal track layer. The interlayer is a rhenium 
interlayer and the focal track layer is typically a tungsten-rhenium focal 
track layer. 
Accordingly, it is an object of the present invention to provide a 
carbon-carbon composite material for a CT rotating anode. It is a further 
object of the present invention to provide a focal track interlayer system 
for joining the carbon-carbon composite material to a refractory metal 
focal track. It is a yet another object of the present invention to 
provide such a composite having constituents and weave geometries that 
result in relatively high thermal expansion in the in-plane direction, to 
accept the focal track material. It is still another object of the present 
invention to provide such a composite having constituents and weave 
geometries that result in relatively high thermal conductivity through the 
thickness to meet focal track loadability requirements. Finally, it is an 
object of the present invention to provide such a focal track layer system 
capable of accommodating tensile overstress and reducing microcracking. 
Other objects and advantages of the invention will be apparent from the 
following description, the accompanying drawings and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention relates to X-ray tubes which employ a rotating anode 
assembly and a cathode assembly. The purpose of this invention is to 
provide a lightweight rotating anode, capable of accommodating faster 
scanner rotation rates. The lightweight target anode is preferably 
comprised of carbonaceous materials, such as carbon-carbon composites, and 
is a potential candidate to replace the relatively heavy brazed graphite 
anode design in current and future CT scanner systems. Carbonaceous 
material targets have at least comparable thermal performance, while 
achieving significant weight reduction, as compared to existing tube 
target products. 
Referring now to the drawings, FIG. 1 illustrates a typical prior art CT 
anode target 10. The current CT anode 10 design comprises two disks 12 and 
14. One disk 14 is of a high head storage material such as graphite, and 
the second disk 12 is of a molybdenum alloy such as TZM. These two 
concentric disks are bonded together by means of a brazing process. A thin 
layer of refractory metal such as tungsten or tungsten alloy is deposited 
to form a focal track 16. Such a composite substrate structure may weigh 
in excess of 4 kg. With faster scanner rotation rates, heavy targets will 
increase not only mechanical stress on the bearing materials but also a 
focal spot sag motion causing image artifacts. 
The present invention proposes tailored woven carbon-carbon composite 
structures or reinforced carbon-carbon composite felts, to replace the 
graphite material in existing CT scanner systems. Carbonaceous materials 
already have desirable thermal and mechanical properties for X-ray 
applications, such as high strength-to-weight ratio, strength retention 
and creep resistance over a wide temperature range, resistance to thermal 
shock, high toughness and high thermal conductivity. These properties are 
important in the CT anode design. The present invention proposes the use 
of weaving processes and technologies, well known in the art, applied to 
the carbonaceous material, to achieve lightweight anode structures. 
The through-the-thickness high conductivity of the carbonaceous substrate 
of the present invention is accomplished by a high fiber volume fraction 
of high strength and high modules fibers. Suitable materials include, for 
example, Amoco P-120 or K-1100 pitch based products. Vapor grown carbon 
fiber (VGCF) with thermal conductivity in excess of 1500 W/m K, high 
strength and stiffness, is one alternative material for the z-direction 
reinforcement. 
In the in-plane direction, the carbon-carbon composite is weaved using a 
low conductivity, low modulus fiber. Rayon precursor materials such as 
continuous fibers or fabrics are of relatively low strength, elastic 
modules, and thermal properties. These are typically parameters which 
result in a relatively high thermal expansion carbonaceous material. 
For CT applications, the carbon-carbon composite material is treated and 
provided with the proper volume of fibers to achieve at least the same 
thermal performance as brazed graphite. Fiber is weaved in the 
Z-direction, densified and heat treated, to achieve at least two times 
higher conductivity than that of graphite in the Z-direction, and an 
in-plane conductivity equal to or greater than that of graphite, using 
treating and weaving processes well known in the art. 
In order to secure the deployment of carbon-carbon composite in X-ray tube 
application, the development of an adherent, long life focal track system 
is required. Carbon-carbon composites, including tailored woven structures 
and carbon fiber felts, have a lower coefficient of thermal expansion 
(CTE) than focal track materials of refractory metals. The thermal 
expansion mismatch between the carbon-carbon composite substrate and the 
target focal track can result in severe processing or service stresses and 
subsequent focal track layer spallation. Consequently, existing focal 
track coating processes, while suitable for use with graphite anodes, are 
not capable of relieving the thermal expansion mismatch stresses between 
carbonaceous and refractory materials. 
The present invention proposes a focal track coating system which allows 
carbon-carbon composites to replace graphite materials in a CT anode 
structure, which can accommodate faster scanner rotation rates. 
In accordance with the present invention, the present target design of FIG. 
1 is replaced by a lighter weight substrate which is comparable in thermal 
performance to the present target. FIG. 2 is a cross-sectional 
illustration of a CT anode target 18 constructed according to the present 
invention. Graphite material is known to have high heat storage capacity 
and low density. Unfortunately, it has proven to be inadequate for larger 
diameter targets. Due to the low mechanical strength of graphite, larger 
diameter targets tend to burst under the effect of centrifugal force. 
In accordance with the present invention, therefore, other carbonaceous 
materials, such as carbon-carbon composites are provided to replace the 
present CT anode targets 10. As described above, these multi-directional 
carbon-carbon composites are tailored with thermophysical and mechanical 
properties, to increase their expansion coefficient in the in-plane 
direction and provide high thermal conductivity through the thickness. 
In FIG. 2, the anode target 18 is comprised of such a carbon-carbon 
composite 20. A thin layer of refractory metal such as tungsten or 
tungsten alloy, including tungsten-rhenium, is deposited to form a focal 
track 22. The preferred thickness of the refractory metal layer 22 is in a 
range of 200 to 500 .mu.m and its composition comprises 5-10% rhenium. To 
relieve thermal expansion mismatch stresses between the carbonaceous 
material 20 and the refractory metal of the focal track 22, the anode 
target 18 further comprises an interlayer 24. The interlayer 24 provides 
ductile transition between the carbonaceous material 20 and the focal 
track 22. 
In a preferred embodiment of the present invention, the interlayer 24 
comprises a rhenium interlayer, capable of providing high ductility, 
particularly when the interlayer is a thick interlayer, significantly 
greater than 10 .mu.m. In a further preferred embodiment of the present 
invention, the thickness of the rhenium interlayer is desired to be about 
50-100 .mu.m. This relatively thick ductile interlayer is able to 
accommodate tensile overstress due to thermal expansion mismatch with the 
substrate on cooling from the deposition temperature and to reduce 
microcracking of the focal track coating system during thermal cycling. 
An adherent focal track layer system on carbon-carbon composite materials 
is formed by any suitable method, such as low pressure plasma spraying 
(LPPS), chemical vapor deposition (CVD), or other satisfactory methods. 
However, in a preferred embodiment of this invention, LPPS is method for 
forming the adherent focal track layers, which layers comprise the top 
layer (typically tungsten-rhenium) and the interlayer (preferably 
rhenium). Chemical vapor deposition has a tendency to produce highly dense 
coatings. Simulated electron beam testing on CVD coated carbon-carbon 
composite specimens has demonstrated that these highly dense CVD coatings 
do not accommodate the thermomechanical stresses produced during thermal 
cycling, and suffer some degradation of the interface between the rhenium 
interlayer and the top layer. In contrast, LPPS coatings with a controlled 
porosity level below 2% not only outperform the CVD coatings under 
identical thermal cycling conditions, but are capable of withstanding the 
same thermal load as the existing graphite targets. 
In accordance with the present invention, a carbonaceous material is 
proposed for use in constructing lightweight rotating anode structures for 
X-ray tubes. Further, a focal track coating system is provided for such 
carbonaceous composite x-ray anodes, capable of relieving thermal 
expansion mismatch stresses between the carbonaceous material of the anode 
and the refractory metal of the focal track. The focal track layer system 
of the present invention proposes a double layer structure comprising a 
fine grained rhenium interlayer and a fine grained top layer made of 
tungsten-rhenium alloy. 
It will be obvious to those skilled in the art that various modifications 
and variations of the present invention are possible without departing 
from the scope of the invention, which provides carbon-carbon composites 
for CT targets. The carbon-carbon composite targets fabricated in 
accordance with the present invention have comparable or better thermal 
performance and 50% weight reduction, as compared to existing CT tube 
target products. 
The invention has been described in detail with particular reference to 
certain preferred embodiments thereof, but it will be understood that 
modifications and variations can be effected within the spirit and scope 
of the invention.