Patent Application
20160203939
The invention relates to a method of making an X-ray tube anode arrangement, the resulting anode arrangement and an X-ray tube including such an anode arrangement.
X-ray tubes include an anode. High energy electrons impact the anode driven by a voltage drop between the anode and a cathode. A small part of the energy of the high energy electrons is converted into X-rays. The rest of the energy must be removed by cooling.
This cooling may be carried out using a heat spreader. The overall heat transfer coefficient is important. Therefore, heat spreader materials with a high thermal conductivity are necessary to keep the anode temperature as low as possible. As the power of the X-ray tube rises, high temperatures cannot be avoided. It is still however necessary to keep the anode temperature low enough to prevent evaporation or melting of the anode material.
Thus, the environment of the anode in an X-ray tube can be very harsh-temperatures can be high and x-ray intensities also very high. It is necessary to maintain good anode function in this harsh environment. In particular the harsh environment does not simply include high temperatures of 800° C. or higher but also high temperature gradients.
Conventional heat spreader materials are copper or silver which are selected because of their high heat conductivity. However, such materials can have thermal expansion mismatches with the anode material which can lead to high stresses.
Selection of alternative materials is not easy, since it is not sufficient to simply choose another heat spreader material. It is also necessary to fix the anode securely to the heat spreader in such a way that the anode remains firmly attached with a low thermal resistance even in the challenging environment of an X-ray tube, and suitable ways of fixing the anode have not been previously identified. Accordingly, X-ray tubes continue to use copper or silver heat spreaders.
According to the invention, there is provided a method of manufacturing an X-ray tube anode arrangement, comprising: providing an anode of rhodium, molybdenum or tungsten; providing a heat spreader of a composite of molybdenum and/or tungsten; mounting the anode on the heat spreader with a layer of joint material therebetween; and bonding the anode to the heat spreader with the joint material.
The applicants have found that in this way it is possible to implement an x-ray anode arrangement with good heat sinking properties which is also highly reliable. In particular the anode arrangement can withstand high temperatures, high temperature gradients, fast temperature changes, extremely high radiation and extremely high electric field, while maintaining good high vacuum properties.
The anode arrangement can withstand rapid high power switches significantly better than with a conventional anode on a conventional silver or copper heat spreader.
Note that the term “composite” may include a mixture or an alloy as well as other forms such as a laminate. The term “alloy” here is not intended to suggest that copper and molybdenum and/or tungsten dissolve in each other.
The heat spreader may be a composite of molybdenum and copper or tungsten and copper.
The step of bonding the anode to the heat spreader may include diffusion bonding the anode to the heat spreader. The joint material may be gold.
Experiments presented below demonstrated excellent results with such an approach.
The joint material may be a thin layer of thickness 5 to 200 μm.
As an alternative to diffusion bonding, the step of bonding the anode to the heat spreader involves brazing the anode to the heat spreader, i.e. softening the joint material by greater heat than used for diffusion bonding.
Suitable joint materials for brazing include an alloy of silver and copper, an alloy of silver, copper and palladium, an alloy of gold and copper or an alloy of gold, copper and nickel.
In another aspect, the invention relates to an X-ray tube anode arrangement, comprising:
Such an X-ray tube anode arrangement may have excellent reliability and be capable of high power operation.
The joint layer may be a layer of gold of thickness 5 to 200 μm.
Alternatively, the joint layer may be an alloy of silver and copper, or an alloy of silver, copper and palladium, an alloy of gold and copper or an alloy of gold, copper and nickel.
In a yet further aspect, the invention may relate to an X-ray tube having an X-ray tube anode arrangement as discussed above.
An example of the invention will now be described with reference to the accompanying diagrams, in which:
Referring to
The heat spreader 4 is made of an alloy of molybdenum and copper, having an alloy composition chosen so that the thermal expansion coefficient matches the thermal expansion coefficient of rhodium.
The inventors have tested a number of different approaches to fixing the anode to the heat spreader. Good results have been obtained using a diffusion bond of gold. Thus, a bonding layer of joint material 10, in this case of gold, is provided between the anode 2 and the heat spreader 4 to firmly fix the anode to the heat spreader.
The layer of joint material may have a thickness of 5 to 200 μm, in embodiments 10 to 100 μm for example 50 μm.
A layer of corrosion resistant material 12 is provided on the rear of the heat spreader to avoid corrosion of the heat spreader 4. The corrosion resistant material 12 may be, for example, gold.
The cooling arrangement 6,8 is formed by a pair of concentric tubes 6,8, an outer tube 6 around an inner tube 8. The end of the tubes are closed with the corrosion resistant material 12 on heat spreader 4. A coolant, for example deionised water, is used to transport heat in the cooling arrangement.
In use, water is pumped along the inner tube 8 in the direction indicated by arrows, then flows across the corrosion resistant material 12 on heat spreader 4 where it takes heat from the heat spreader 4 and then is removed along a flow path between the inner 8 and outer 6 tubes. A circuit for the coolant is completed by a pump, filter, heat exchanger and stock barrel, which cools and recirculates the water.
To manufacture the X-ray tube anode arrangement, the anode 2 and heat spreader 4 are brought together with the bonding layer in the form of a sheet of joint material 10 between them. The anode 2 is then diffusion bonded to the heat spreader 4 by heating under pressure, but not to a temperature where the gold melts. This creates a diffusion bond.
In a specific embodiment, the diffusion bonding was carried out at a temperature between 700° C. and 950° C., for example 800° C., for between 15 minutes and 200 minutes, for example 120 minutes (two hours) in a forming gas atmosphere. The pressure used may be 10 bar to 500 bar, for example 80 bar; higher pressures may also be used. There is a trade off between temperature and time and higher temperatures may be used, for example, for shorter periods of time.
Finite element analysis has been carried out to calculate the plastic deformation of the resulting anode arrangement in use compared with a comparative example of the same anode attached to a silver heat spreader. See the results presented in
Referring to
In contrast, referring to
The joint in the anode arrangement in particular could withstand:
Further, a section through a joint as illustrated in the photomicrogrpah of
Prototype X-ray tubes were made with the new anode construction and these were able to withstand an increase in the number of power switches before tube failure of a factor of 2 compared with the comparative example. The invention accordingly delivers surprisingly good results in terms of improved tube life and performance.
Thus, the inventors have discovered how to reliably bond anodes of rhodium, molybdenum or tungsten to produce reliable joints in the extreme operating conditions of an X-ray tube.
In alternative embodiments, different anode materials are used, in particular the anode may be of molybdenum or tungsten.
As an alternative to diffusion bonding, brazing may also be used. In such a case, a metal layer of copper silver alloy or palladium copper silver alloy may be used. Such alloys are commercially available as “Cusil” or “Pacusil” respectively.
Before brazing, it may be advantageous to coat either the anode, the heat spreader or both with a thin layer of nickel or gold plate before brazing.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15150816.5 | Jan 2015 | EP | regional |
