EP1771863A2

EP1771863A2 - Junction process for a ceramic material and a metallic material with the interposition of a transition material

Info

Publication number: EP1771863A2
Application number: EP05817907A
Authority: EP
Inventors: Stefano Libera; Eliseo Visca
Original assignee: Agenzia Nazionale per le Nuove Tecnologie lEnergia e lo Sviluppo Economico Sostenibile ENEA; Centro Sviluppo Materiali SpA
Current assignee: Agenzia Nazionale per le Nuove Tecnologie lEnergia e lo Sviluppo Economico Sostenibile ENEA; Centro Sviluppo Materiali SpA
Priority date: 2004-07-20
Filing date: 2005-07-20
Publication date: 2007-04-11
Also published as: CN101015024A; ITRM20040368A1; WO2006024971A2; WO2006024971A8; JP2008507465A; WO2006024971A3

Abstract

The invention refers to a method useful for obtaining junctions having high qualities of mechanical resistance and capabilities of heat conduction between materials with different physical properties, and in particular ceramic/metal junctions or ceramic/metal composites in which the different thermal expansion coefficient entails remarkable stresses in the interface both during the junction process and their industrial application. The issues solved with the proposed method are the metal's difficulty of wetting the surfaces to be coupled and the general low mechanical resistance to tensile stress of ceramics or ceramic compounds. The first issue is solved with the application of a Titanium-base alloy that, by combining with the ceramic at a surface level enables metal to wet the surface. The second issue is solved by increasing the specific surface of the ceramic or compound, machining it through long-pitch multi-start thread.

Description

JUNCTION PROCESS FOR A CERAMIC MATERIAL AND A METALLIC MATERIAL WITH THE INTERPOSITION OF A TRANSITION MATERIAL

DESCRIPTION

The present invention refers to a junction process for a ceramic material, in particular a graphite fibre composite, and a metallic material, in particular a Copper pipe, with the interposition of a transition material, preferably it also Copper or a Copper alloy. The invention finds a preferred application in the manufacturing of a divertor of a nuclear fusion reactor, in particular a Tokamak-type reactor.

In various fields of the art it is necessary to carry out junctions between materials exhibiting different chemico- physical properties and capable of withstanding high thermal stresses. In such junctions, the heat flows to which the materials into contact are subjected, both during the same manufacturing of the coupling and in the actual operation of the component, induce on such materials high mechanical stresses; this above all since, temperatures being equal, the different thermal expansion coefficient causes different deformations therein. Said issues are particularly felt in the field of nuclear fusion reactors, and specifically in the construction of divertors of so-called "Tokamak"-type reactors. As it is known to those skilled in the art, divertors are conductive heat exchangers apt to transfer heat from plasma to a coolant flowing into a metallic pipe internal to said divertor. Therefore, a Tokamak reactor divertor is the most thermally stressed component, with extremely high heat flows, specifically in the order of 20 MW/m². Hence, said metallic pipe should be shielded from direct contact with the plasma by an outer coating in ceramic refractory material such as Tungsten or graphite fibre composites (CFC) . In particular, divertors are generally divided into two zones: in a first zone, wherein a higher heat flow is provided, the metallic pipe is shielded by a Carbon-matrix composite (Carbon Fibre Composite: CFC) , whereas in a second zone, in which a lower heat flow is provided, the pipe is shielded by Tungsten (W) blocks or monoblocks .

As mentioned above, the junction between the metallic pipe ^arid the shielding ceramic materials should be such as to withstand the heat flows of manufacturing and to ensure an optimal heat exchange even after thousands of machine cycles. However, as mentioned above, owing to the different expansion coefficient, the heat flows induce remarkable mechanical stresses in the materials of the junction, considering that the differences in temperature between the plasma-contacting outer surface and the coolant reach even the 2000 ⁰C. E.g., the pipe, when made of Copper or Copper alloy, exhibits a thermal expansion coefficient

(TEC) equal to about 2OxIO^"6 at 500 ⁰C, whereas the refractory coating exhibits practically nil TEC.

Mechanical stresses induced by the different expansion of the materials of the junction can entail the formation of flaws, cracks or other mechanical damages compromising the component operation and the operation safety. In an attempt at overcoming these drawbacks, between the refractory material and the metallic pipe it is laid a suitable thickness of soft, so-called "transition" material. The function of such a transition material is that of compensating said significant differences between the thermal expansion coefficient of the refractory material and that of the metallic one, acting as a mechanical "bearing" therebetween so as to avoid the formation of said flaws or cracks.

However, it should be taken into account that transition material in general, and in particular Copper in molten form, is incapable of diffusing into the ceramic material, and especially into graphite, above all not succeeding, not even at high temperatures, to wet it in order to create between the two materials the continuity needed to obtain a good junction. In an attempt at overcoming said problem of the wettability of the refractory material by the transition material, it has been proposed the activation of the said refractory material by Titanium vaporization carried out with the technique known as Chemical vapour deposition. However, this junction manufacturing process is rather complex and costly.

Hence, the technical problem underlying the present invention is to provide a process for manufacturing a junction of the above-described type overcoming the drawbacks mentioned above with reference to the known art.

Such a problem is solved by a process according to claim 1. According to the same inventive concept, the present invention further refers to a heat exchanger, and in particular to a divertor, according to claim 34. Preferred features of the present invention are present in the dependent claims thereof. The present invention provides several relevant advantages. As it will be better understood from the following detailed description, the main advantage lies in that the invention allows to obtain a junction of high mechanical and thermal resistance by an effective and low-cost combination of steps.

Other advantages, features and the modes of employ of the present invention will be made apparent in the following detailed description of some embodiments thereof, given by way of example and without limitative purposes. Reference will be made to the figures of the annexed drawings, wherein:

* figures 1 and 2 show each a respective SEM (Scanning Electron Microscope) image of a Dunlop 678-type Cu//CFC junction manufactured with the process according to the present invention;

* figures 3 and 4 show each a respective SEM image of the junction manufactured with the process according to the invention.

The junction process according to the invention will hereinafter be described within the context of a preferred application thereof, and precisely the construction of a divertor of a Tokamak reactor.

As mentioned in the introduction, such a divertor envisages the manufacturing of a junction between an outer coating of refractory ceramic material and an inner metallic pipe by interposition of a transition material. The junction is carried out at a surface for coupling the refractory material, which in the case of the divertor is curved and of substantially cylindrical geometry just as it should receive said pipe. According to a preferred embodiment of the invention, the ceramic material is a graphite fibre compound (CFC) and the transition material is OFHC (Oxygen-Free High Conductivity) Copper.

As mentioned always in the introduction, the CFC compound is particularly suitable for the most thermally stressed zone of the divertor. A variant embodiment provides instead the use of Tungsten (W) as refractory material, suitable for the manufacturing of the less stressed portion of the divertor itself. The refractory material is manufactured in blocks or tiles to be applied onto the surface of the pipe through which the coolant flows.

In a first step of the junction manufacturing process, in the refractory material it is made a hole apt to receive said pipe. Then, the inner surface of the hole, i.e. said coupling surface, is machined to obtain therein a parallel multi- start thread, and preferably a 7-start one. Preferably, the thread has a depth comprised in a range of about 0.3- 1.0 mm, and even more preferably equal to about 0.6 mm. Then, the inner surface of the hole is prepared for the junction with the transition material by chemical cleaning, preferably carried out with acetone in a known ultrasound machine.

Then, the inner surface of the hole is subjected to drying, preferably carried out in air furnace at about 200 ⁰C. In a subsequent step of the process, the refractory material is subjected to degassing in a vacuum furnace, preferably with a vacuum higher than 10^"5 mbar and at a temperature higher than about 1350 ⁰C; this in order to eliminate any substances trapped in the structure of the refractory material that might interfere with the subsequent steps of the process.

According to the invention, the junction process then envisages a pre-brazing to be carried out with a brazing material or alloy, preferably a Titanium-Copper-nickel alloy, e.g. that produced by Wesgo. Such a pre-brazing is preferably carried out at a temperature comprised in a range of about 900-1200 ⁰C, and even more preferably equal to about 1050 ⁰C, for about 5 min under a vacuum higher than about 10^"5 mbar. The brazing alloy is provided in the form of foil and positioned within the hole of the refractory material so as to cover the entire zone involved by the junction. Then, the refractory material - brazing alloy assembly is subjected to a vacuum furnace treatment at a temperature suitable to form a compound between the two materials of the assembly, compound constituting just the activating agent of the subsequent coupling with the transition material. In the present embodiment said activating agent is a TiC compound, exhibiting great capability of diffusion into graphite.

At the end of the furnace treatment, the brazing alloy in excess, which is crystallized, is removed by conventional techniques. Then, the transition material can be positioned within the hole and the assembly is subjected to vacuum furnace treatment at a vacuum pressure of about 10^"6 mbar for about 5 min and at a temperature higher than the melting temperature of the transition material used. Hence, in the present case the furnace treatment is carried out at a temperature higher than 1083 ⁰C, which corresponds to the melting point of Copper. Moreover, the temperature, time and pressure of the furnace treatment are such as to bring the transition material to a viscosity enabling it to seep into all the tracks of the thread made in the hole and also into any porosity possibly present into the morphology of the refractory material . Finally, excess transition material is removed by a conventional machining until obtaining the desired thickness. Preferably, the final thickness of the transition material is comprised in a range of about 0.5 - 1.0 mm. Lastly, the 'tiles' or blocks of the refractory material treated as described hereto are coupled to the metallic pipe, in the present embodiment made of Copper alloy, through standard techniques such as brazing, HIPping (Hot Isostatic Pressing) , HRP (Hot Radial Pressing) or the like.

The fact that, according to the invention, the activation of the refractory material by brazing alloy is carried out separately and prior to the application of the transition material allows an optimal activation of the former and therefore an optimal coupling thereof to the transition material.

Therefore, in practice the process of the invention achieves the junction of a ceramic material with a transition material by the melting of the latter on a pre-brazed surface (PBC) .

Moreover, it should be taken into account that any type of junction process entails brazing or casting cycles at very high temperatures (in the order, e.g., of 1000⁰C) with very high residual stresses at the interface between the junction materials. Moreover, the refractory materials, and especially the graphite in composite form like the CFC, exhibit a low mechanical resistance. Hence, the junction process has to be such as to obtain a coupling in which the active surface is somehow increased.

Such a problem is solved in the present case by said thread. In fact, this solution, which is applicable even separately and independently from the invention subject matter of the independent claims, yields an effective increase of the active surface of the refractory material. Moreover, the proposed solution, based on making a thread, is much more inexpensive and in some cases more effective than the known solution envisaging the making, through high-quantity lasers, of superficial micro-holes. Moreover, the solution based on the making of a thread entails an additional advantage linked to the fact that many refractory materials, among which also graphite, are not isotropic. In fact, the use of multi-starts allows, active surface extensions being equal, to "lengthen" the pitch of the thread, avoiding to sever or anyhow interrupt into more sections the sturdier fibres of the refractory material, i.e. the longitudinal ones in the case of the CFC.

The process of the invention, in the abovedescribed embodiments, was qualified by using thermal tests such as to induce stresses comparable to those envisaged in the manufacturing and working steps.

In particular, the junction was heated in air up to 400 ⁰C and then swiftly cooled in water at room temperature. This treatment was repeated 30 times. In support of the qualification process, on the samples there were carried out metallographic investigations

(optical microscope and SEM) and non-destructive controls with pulse-echo ultrasound technique, demonstrating that the junction thus obtained is capable of withstanding these thermal shocks. Therefore, all the more so it will withstand less severe transients, such as those foreseen for the divertor components. Said tests were carried out on two types of CFC, in particular:

* two-dimensional CFC, i.e. with fibres oriented mainly in two directions (in particular the commercially available material known as "Dunlop 678 by Dunlop") ; and

* three-dimensional CFC, i.e. with a preparation of the matrix with fibres in the three directions, providing a composite much more uniform and with better mechanical and thermal characteristics (in particular the commercially available material known as "NB 31 by SECMA") .

Results on Dunlop 678-type Cu//CFC junction The results highlighted that this type of graphite composite, having been designed with a two-dimensional- type technique, actually exhibits a highly porous structure with cavities such as to require no intervention of a thread to obtain good junctions. The micrography and the images obtained by SEM investigation shown in figures 1 and 2 highlight the transition Copper capability of seeping into the natural interstices of this composite matrix. This ability is due above all to the presence of Titanium (TiC) allowing the wettability of the refractory material by the transition material .

Results on NB 31-type Cu//CFC junction

This graphite matrix is much more compact and void of cavities; to obtain an optimum-quality junction the active surface had to be increased through the multi- start thread process.

Also left-right crossed pitch thread were tested, yet the best result with regard to resistance was that with seven 1mm-staggered starts. Metallographic tests and ultrasound checks were carried out prior to and after the thermal stress cycles.

An ultrasound scanning on a transversal plane (C-scan) was performed, respectively prior to and after the water- immersed boresonic probe test. The C-scan is the depiction of the map of the amplitudes of the reflections obtained from the interfaces. From yielded images, it is inferred that the mapping exhibits the same distribution prior to and after the test, and that therefore no detaching is detected.

The sample micrography performed after the thermal tests are shown in figures 3 and 4.

The present invention has hereto been described with reference to preferred embodiments thereof. It is understood that there could be other embodiments afferent to ..the same inventive kernel, all falling within the protective scope of the claims set forth hereinafter.

Claims

1. A process for manufacturing a junction between a refractory ceramic material and a metallic element by- interposition of a transition material, comprising the steps of:

(a) applying a brazing material on a coupling surface of said refractory material, so as to obtain the formation, on said coupling surface, of an activating compound between said materials; and (b) subsequently to said step (a) , obtaining the coupling by melting of said transition material with said refractory material at said coupling surface activated by said brazing material.

2. The process according to claim 1, wherein said coupling surface has a substantially curved shape.

3. The process according to the preceding claim, wherein said coupling surface has a substantially cylindrical geometry.

4. The process according to any one of the preceding claims, wherein said refractory material comprises a graphite fibre composite.

5. The process according to the preceding claim, wherein said refractory material is CFC.

6. The process according to any one of the preceding claims, wherein said refractory material comprises

Tungsten.

7. The process according to any one of the preceding claims, wherein said step (a) provides that said brazing material be provided in the form of a foil.

8. The process according to any one of the preceding claims, wherein said step (a) is carried out in a vacuum furnace.

9. The process according to any one of the preceding claims, wherein said brazing material is an alloy of Titanium, Copper and nickel .

10. The process according to the preceding claim, wherein said step (a) is carried out at a temperature comprised in a range of about 900 - 1200 ⁰C.

11. The process according to the preceding claim, wherein said step (a) is carried out at a temperature equal to about 1050 ⁰C.

12. The process according to any one of the claims 9 to 11, wherein said step (a) is carried out at a pressure lower than about 10^"5 mbar.

13. The process according to any one of the preceding claims, wherein said activating compound is Titanium carbide (TiC) .

14. The process according to any one of the preceding claims, wherein said step (b) is carried out at a temperature such as to bring said transition material to a viscosity enabling it to seep into the porosities of said refractory material activated by said brazing material.

15. The process according to any one of the preceding claims, wherein said step (b) is carried out in a vacuum furnace.

16. The process according to any one of the preceding claims, wherein said transition material is Copper or a Copper alloy.

17. The process according to the preceding claim, wherein said transition material is OFHC ("Oxygen-Free High Conductivity") Copper.

18. The process according to claim 16 or 17, wherein said step (b) is carried out at a vacuum pressure of about 10^"6 mbar.

19. The process according to any one of the preceding claims, wherein said step (b) is carried out at a temperature higher than the melting point of said transition material .

20. The process according to the preceding claim when dependent from claim 16 or 17, wherein said step (b) is carried out at a temperature higher than about 1083 ⁰C.

21. The process according to any one of the preceding claims, wherein said transition material has, after said step (b) , a thickness comprised in a range of about 0.5 - 1.0 mm.

22. The process according to any one of the preceding claims, comprising, prior to said step (a) , a step of chemical cleaning of said coupling surface.

23. The process according to any one of the preceding claims, comprising, prior to said step (a) , a step of degassing of said refractory material .

24. The process according to any one of the preceding claims, comprising, prior to said step (a) , a step (α) of increasing the extension of said coupling surface.

25. The process according to the preceding claim, wherein said step (α) provides that a thread be made on said coupling surface.

26. The process according to the preceding claim, wherein said thread is multi-start.

27. The process according to the preceding claim, wherein said thread has seven starts.

28. The process according to any one of the claims 25 to 27, wherein said thread has a depth comprised in a range of about 0.3-1.0 mm.

29. The process according to the preceding claim, wherein said thread has a depth equal to about 0.6 mm.

30. The process according to any one of the preceding claims, comprising, subsequently to said step (a) and prior to said step (b) , a step of removing the brazing material in excess.

31. The process according to any one of the preceding claims, comprising, subsequently to said step (b) , a step of coupling with said metallic element carried out by a technique selected in a group comprising HIPping (Hot Isostatic Pressing) and HRP (Hot Radial Pressing) .

32. The process according to any one of the preceding claims, wherein said metallic element is made of a Copper alloy.

33. The process according to any one of the preceding claims, wherein said metallic element is a pipe.

34. A heat exchanger having a skirt apt to transmit heat, which skirt comprises a junction between a refractory material and a metallic element with the interposition of a transition material, which junction is obtained by the process according to any one of the preceding claims.

35. The heat exchanger according to the preceding claim, wherein said metallic element is a pipe apt to receive a coolant.

36. The heat exchanger according to claim 34 or 35, which is a divertor of a fusion reactor.

37. The heat exchanger according to the preceding claim, which is a divertor of a Tokamak reactor.

38. A Tokamak reactor comprising a divertor according to the preceding claim.