Field of the Invention

The present invention relates to an interlayer for joining materials of dissimilar coefficient of thermal expansion (CTE).

Background

A divertor is a device within a tokamak plasma vessel which allows for removal of waste material and power from the plasma while the tokamak is operating. The waste material naturally arises as particles diffuse out from the magnetically confined plasma core. To confine the plasma, tokamaks utilise magnetic fields. However, particles slowly and randomly diffuse out, and eventually impact one of the divertor surfaces, which are configured to withstand the high flux of ions.

Figure 1 shows a cross section of a tokamak plasma chamber, illustrating one possible configuration of the divertor. The divertor surfaces 101 are at the top and bottom of the plasma chamber 100. The plasma chamber first wall (i.e. plasma facing components) includes the inner wall 102 of the chamber, and also includes baffles 103 located close to each divertor surface, for directing the plasma at the divertors, and covers 104 over field coils internal to the plasma chamber.

The high heat flux and erosion experienced by the surface layer of a divertor requires a material that can stand up to those conditions. A common choice is a refractory metal having a melting point over 1850°C, e.g. titanium or vanadium, more preferably over 2000°C, e.g. molybdenum, or tungsten. However, refractory metals are generally brittle, so the cooling elements of the divertor which contain a coolant under pressure are commonly made from an alternative material which is tougher (hereafter a “thermally conductive material”), such as copper.

This presents a problem - the thermal expansion coefficient of the thermally conductive material will be very different to that of the refractory metal, and there will be significant heat flux and temperature changes at the join between them. This means that a common failure mode of a divertor is damage due to stresses at this join. The stresses at the join between the thermally conductive material and the refractory metal can be mitigated by providing an interlayer - a layer of material having a thermal expansion coefficient intermediate between that of the refractory metal and that of the thermally conductive material. The interlayer may be a composite material comprising both the refractory metal and the thermally conductive material, and may be graded such that the proportion of each material varies linearly through the interlayer, to provide a more gradual change in thermal expansion coefficient and other material mechanical and thermal properties.

Figure 2 shows the grading profile of exemplary known interlayers. Each interlayer 201 joins a high thermal expansion material 202 to a low thermal expansion material 203. In the example of a divertor for a tokamak, the high thermal expansion material may be copper and the low thermal expansion material may be tungsten. The interlayer depth “d” is the distance across the interlayer from the high thermal expansion material to the low thermal expansion material. Within this disclosure, the depth d will be measured in arbitrary units such that the transition between the high thermal expansion material 202 and the interlayer 201 is at d=0, and the transition between the low thermal expansion material 203 and the interlayer 201 is at d=1. It will be appreciated that any description relying on this definition of d can be transformed into any other scale for measuring the distance d by suitable mathematical operations, and this definition does not limit the thickness of the interlayer to 1 of any particular unit.

The “linear” interlayer 210 has a thermal expansion coefficient which decreases linearly with distance across the interlayer. While this is shown as a continuous decrease, it may be the result of e.g. a laminated composite with differing proportions of high thermal expansion and low thermal expansion layers in the different regions, with the value plotted on the graph being the effective bulk thermal expansion coefficient over a small distance.

The “stepped” interlayer 220 has a thermal expansion coefficient which decreases in a stepwise fashion with distance across the interlayer, with a linear decrease between each step. Each of these could be effectively implemented by lamination of thin films of each material, or by powder grading. However, lamination results in bonding layers through the interlayer which are parallel to it, and may be prone to failure depending on the materials being joined. Powder grading can suffer from similar failures in stepwise interlayers, at the interface between layers of different grading.

While the above has been written in the context of a divertor for a tokamak, such interlayers are also useful in other contexts where there is a need to join two materials with different thermal expansion coefficients where temperature variations occur. The particular case of a refractory metal joined to a thermally conductive material is also relevant on the “first wall” (i.e. plasma facing surface) of a plasma chamber such as a tokamak, and to other applications where a high heat flux and high erosion are expected, such as rocket exhausts.

Summary

According to an aspect of the invention, there is provided a method of manufacture, comprising: providing a first component comprising a first material; forming a first structure in the first material at an initial surface of the interlayer using a subtractive manufacturing technique, wherein the first structure comprises at least one new surface which faces away from the initial surface at an angle of less than 90 degrees; forming a second component comprising a second material, the second component comprising a second structure formed from the second material which conforms to the first structure and fills at least the space between the first structure and the initial surface; wherein the first structure and the second structure are shaped such that separating the first and second structure requires deforming one or both of the first or second structure.

According to another aspect of the invention, there is provided a method of manufacturing an interlayer comprising a first material and a second material. The interlayer has a first side and a second side opposite the first side. A block of a first material is provided, and a first structure is formed from the block of first material using a subtractive manufacturing technique. The first structure comprises at least one surface which is not in contact with the first side and faces towards the first side at an angle of less than 90 degrees. A second structure is formed from the second material, such that the second structure conforms to the first structure and fills the space between the first structure and the second side. The first structure and the second structure together form the interlayer, and the first structure and second structure are shaped such that separating the first and second structure requires deforming one or both of the first or second structure.

Brief Description of the Drawings

Figure 1 shows a cross section of a tokamak plasma chamber;

Figure 2 shows the grading profile of exemplary known interlayers;

Figure 3 illustrates a construction for producing a composite interlayer;

Figures 4A and 4B illustrate example mechanical keying features for an interlayer;

Figures 5A, 5B, 5C and 5D illustrate an exemplary interlayer which may be formed by laser ablation;

Figure 6 shows two possible grading profiles for interlayers;

Figure 7 shows approximations to the grading profiles of Figure 6.

Detailed Description

A construction for producing a composite interlayer is illustrated in Figure 3. In step 310, a block 301 of a first material is provided. This block may be part of one of the components to be joined by the interlayer (e.g. a block protruding from a surface of that component), or may be provided separately (for later attachment to that component). In step 320, a first structure 302 is formed from the block by a subtractive manufacturing process (e.g. etching, laser ablation, machining). In step 330 a second material (e.g. copper) is filled into that first structure 302 (e.g. by casting, powder sintering, or other methods as described later) to form a second structure 303 which, together with the first structure, forms the completed interlayer. The first structure extends to the first side of the interlayer (i.e. the bottom side in the figure), and the second structure extends to the second side of the interlayer (i.e. the top side).

The subtractive manufacture in step 320 is chosen and operated to provide a complex shape for the first structure, and in particular a shape which provides mechanical keying with the second structure when it is formed. Additionally, the subtractive manufacture process may provide a rough surface to the first structure (improving bonding to the second material) and/or introduce “structural grading” by controlling the cross section of the first structure as will be described later.

Mechanical keying is provided by including features such that, even in the case of complete failure of the bonding between them, the first and second structures could not be pulled apart without deforming one or both of them. For example, this may be achieved by having “overhangs” in the first structure as shown in Figure 4A, which shows some example overhanging structures 401 , 402, 403. In this figure, each overhanging structure has a part 410 where the surface of the first structure faces “down” towards the first side of the interlayer, or equivalently away from the second surface (i.e. towards the component the first material of the interlayer is attached to, surface of the first structure intended for such attachment). This may be any generally downward facing surface - i.e. at an angle of less than 90 degrees to the first surface (or bulk of the first material), more preferably less than 45 degrees, more preferably less than 30 degrees, more preferably less than 10 degrees, more preferably directly downward facing i.e. parallel to the first surface, or parallel but facing away from the second surface. The downward facing surface may be part of a curved surface where the tangent plane to the surface has such an angle to the bulk of the first material. In some examples, this may include the first structure having through holes as shown in Figure 4B, for example structures 404, 405, such that the second structure (once formed) loops through the through holes 420 and it would not be possible to separate the two structures without breaking one or both structures.

Where the interlayer is formed by subtractive manufacture into a first component, i.e. cutting into an already formed component rather than providing a separate block to form the interlayer, this arrangement may be equivalently defined as the newly formed downward facing surface (i.e. “new surface”) facing away from an initial surface of that component (i.e. the surface which the subtractive manufacturing cuts in to) at an angle of less than 90 degrees to the initial surface, more preferably less than 45 degrees, more preferably less than 30, more preferably less than 10 degrees, more preferably directly downwards facing i.e. parallel to the initial surface.

T o simplify design of the first structure, it may be designed as several smaller units which are tiled across the plane of the interlayer, and then the resulting tiling is printed to form the first structure. The first structure may be a periodic structure, e.g. where the tiles are the same and can tile periodically, or it may be a non-periodic structure, e.g. to provide different types of structure in different parts of the interlayer. For example, the first structure may be configured such that one region has improved keying features for additional structural stability, and another region has a different structural grading profile to account for differences in expected heat load across the interlayer.

Example subtractive manufacture processes that may be used include laser subtractive processes (e.g. laser ablation), mechanical subtractive processes, and chemical etching.

Where laser subtractive processes are used, this may be done with multiple laser passes at different angles, to create a first structure which would have sufficient mechanical keying. Figure 5A shows an exemplary first structure which can be produced via a relatively simple laser ablation process, with three sets of parallel cuts into the first structure, where each set is rotated about the vertical axis relative to the other sets, such that the first and third set are each rotated by 45° from the second set (in opposite senses). Figures 5B, 5C, and 5D show side views of the structure, with figure 5B being from the side indicated (showing the second set of cuts) and figures 5C and 5D being views along the diagonal from the indicated corner showing the first and third set of cuts. The resulting structure has the properties described above for good mechanical keying.

Mechanical subtractive processes may also be used, i.e. the removal of material by mechanical processes, such as sawing, or milling. Due to the complexity of the structures required for keying, the machining is best performed on a multi-axis e.g. at least 5 axis system, which can create the undercuts needed for good keying. Alternatively, a machining system with fewer degrees of freedom may be specially configured to perform the cuts required to remove the appropriate material.

Chemical etching may be performed and controlled via the use of masks, and techniques for forming undercuts with chemical etches as known in the art. For example, photolithography may be used to apply an etch-resistant mask in the form of a series of lines, that may be parallel or at angles to one another. Chemical or electrochemical etching of the unmasked surfaces by chemical attack of the surface can be used to create rounded undercuts leaving a structure with keying features such as pillars or columns that are wider at the top than their base. Repetition of the masking, and etching process can be used to form complex, stacked etched geometries that replicate a controlled gradient structure.

The above processes may be performed in multiple stages or combined in order to produce the structures required. For example, laser ablation or machining may be used to create initial cuts into which a chemical etchant is introduced to undercut the first material into spaces which the laser or machining tools would not be able to reach.

The second material may be filled into the first structure by any suitable method. Examples include casting, providing the second material as a powder that is then sintered, or providing the second material as a solid which is forced to conform to the first structure via the application of pressure and heat.

Casting involves providing the second material as a liquid, which then floods the spaces left by the first structure and is allowed to solidify in place. Casting is appropriate where the melting point of the second material is less than the melting point of the first material, to avoid damage to the first structure during casting.

Where the second material is provided as a powder, this will similarly be made to fill the spaces left by the first structure, and can then be sintered to form the second structure by any suitable technique, e.g. hot isostatic pressing (HIP), cold isostatic pressing, vacuum sintering, uniaxial hot pressing, vacuum uniaxial hot pressing, or field assisted sintering technique (FAST) pressing.

Where the second material is provided as a solid, this may be forced to conform to the second structure by the application of pressure and heat - i.e. causing a plastic deformation of the second material such that it conforms to the first structure and forms the second structure. This is performed at a temperature at which the second material has a lower yield strength than the first material (and the first and second material are selected such that such a temperature exists). This may be performed at a pressure sufficient to plastically deform the second material but not to plastically deform the first material, or at a higher pressure - which would allow some plastic deformation of the first material. With any of the above techniques, bonding may be improved by first providing a thin layer of the second material on the upper face of the first structure, e.g. by the application of a foil, by plasma spraying, by chemical vapour deposition (CVD), or other suitable technique. This layer may not cover the entirety of the upper face - e.g. where chemical etching is used to form overhangs, the difficulty in applying the layer underneath the overhangs may be greater than the benefit provided. However, techniques may be used to ensure good coverage of the thin layer of the second material, e.g. by providing CVD or plasma spraying sources which can provide the second material at an angle, to allow coverage of the complex structure of the first layer.

Additionally, with any of the above techniques, they may be performed in a way which forms the component the interlayer is joined to in situ on the interlayer, or vice versa. For example, when the second material is cast or provided as a powder, this may be done in a mould such that the portions beyond the interlayer form the desired component. Where the second material is provided as a solid and forced to conform by pressure, this solid may be the component which the interlayer is intended for.

The first structure may be configured to have a specific cross section at each depth through the interlayer, e.g. as a proportion of the area of the interlayer. In this way the required grading for the interlayer can be achieved structurally. For example, to achieve the linear grading 210 shown in Figure 2, the first structure may be configured such that the cross section of the first structure varies linearly from 100% of the area of the interlayer at the depth d=0, to 0% of the area of the interlayer at the depth d=1 . As the average coefficient of thermal expansion at a given depth into the interlayer is an average of the coefficient of thermal expansion of each material, weighted by the cross section of each material at that depth, this linear variation of the cross section of the first material results in a linear variation of the coefficient of thermal expansion through the interlayer, as required.

Figure 6 shows two proposals for alternative interlayer gradings. Each interlayer 601 joins a high thermal expansion material 202 to a low thermal expansion material 203. The first exemplary interlayer is a “sigmoid” interlayer 610, which follows a sigmoid function, i.e. a function where the slope tends to zero at each of the material layers 202, 203, and which has exactly one inflection point. This low slope at the joins between the interlayer and each material ensures that these regions have only low stress when thermal expansion occurs, and that the stress is instead in the central regions of the interlayer where material bonding is likely to be stronger.

The second exemplary interlayer 620 follows a function such that the slope of the function approaches zero monotonically towards the low thermal expansion material, i.e. the magnitude of the slope is always steady or decreasing as the function moves towards the low thermal expansion material. This results in the steepest changes in thermal expansion coefficient being adjacent to the high thermal expansion material, which provides favourable performance as the high thermal expansion material and the interlayer at that interface will generally be more ductile and compliant than the low thermal expansion material and the interlayer at the corresponding interface, and therefore able to accommodate higher stresses without failure.

One example function for the second exemplary interlayer would be a polynomial function of the form a = £ ₌₀ a _k d ^k , where a is the coefficient of thermal expansion (CTE) (averaged over a small thickness, in the case of composite materials), d is the depth through the interlayer (i.e. the distance from one of the materials being joined), and ak are numerical coefficients. By selection of the coefficients ak, a profile can be determined such that slope of the resulting function approaches zero monotonically at least within the range of the interlayer (0<d<1 , as defined previously). For example, the interlayer may follow a square function (where n=2) or a cubic function (where n=3).

As a more general example, the cross section of the first structure (and thus the coefficient of thermal expansion) may decrease monotonically through the thickness of the interlayer from the first side to the second side.

While the exemplary interlayers shown in Figure 6 are idealisations, approximations to those structures will still give considerably better results than simple linear or stepped interlayers. In the following description, “average CTE” of a region of the interlayer means the weighted average of the CTE of each material in the interlayer, weighted by the volume fraction of the material in that region.

Similar to the difference between the linear interlayer and stepped interlayer in Figure 2, a stepped equivalent to the interlayer functions described with reference to Figure 6 may be created. For the below discussion, consider an interlayer divided into N steps along its thickness, with each step taking up 1/N of the thickness of the interlayer, where N is at least 4 (to allow distinction over a stepped interlayer). The average CTE of each step has a value between the CTEs of the materials to be joined, and the average CTEs of each step decrease across the interlayer from the high thermal conductivity surface (connected to the material with high CTE) to the low thermal expansion surface (connected to the material with low CTE).

For an approximation to the sigmoid interlayer, the difference in average CTE for adjacent steps closer to the edge of the interlayer (i.e. closer to the high or low thermal expansion surfaces) will be less than the difference in average CTE for adjacent steps towards the midpoint of the interlayer.

For an approximation to the second exemplary interlayer, the difference in average CTE for adjacent steps will be greater the closer those steps are to the high thermal expansion surface.

While the above is described in terms of an interlayer constructed in discrete steps, it also applies to other constructions of an interlayer. As a first example, an interlayer which perfectly follows the functions shown in Figure 6 would also have such a variation of average CTE for N regions defined along the thickness of the interlayer, as would various intermediate approximations between the exact function and the stepped interlayer.

The particular case of N=4 is shown in Figure 7, which demonstrates this approximation for the interlayer functions 610 and 620. The interlayer is divided into 4 sections, from d=0 to d=1/4, d=1/4 to d=1/2, d=1/2 to d=3/4, and d=3/4 to d=1. The average value of each section is shown for each interlayer (711 , 712, 713, 714 for the interlayer function 610, from high to low CTE, and 721 , 722, 723, 724 for the interlayer function 620, from high to low CTE). As can be seen from the figure, the differences between these averages have the relationships described above.

The N=4 case will hold for any interlayer sufficiently different from the linear interlayers to show the desired improvements. The advantage will increase for interlayers that obey closer approximations to the smooth functions discussed above, but this will be a trade off with structural requirements of the interlayer (e.g. providing sufficient strength to keying features) and practicality of manufacture (e.g. there may be a need for a minimum cross section of e.g. 5%, 2%, or 1% of the interlayer area as the ideal grading function tends to zero, because the resulting structure would otherwise not be achievable).

Larger values of N in the above approximation provide more accurate approximations to the sigmoid or polynomial ideals, as appropriate. For example, N may be 5, 6, 8, or 10, with the relationships of the average CTE for each of the N layers being as described above.

Where the above discusses average CTE, it should be noted that this is linearly related to the fraction of the interlayer within the region which is composed of the first material. In particular, is the volume fraction of the first material, CTEi is the coefficient of thermal expansion of the first material, and CTE2 is the coefficient of thermal expansion of the second material. As such, any relationship between aCTE in different regions of the interlayer apply equivalently to the volume fraction of the first material in that region (and thereby to the average cross section of the first material in that region). In particular, in the case where the first material is the low thermal expansion material, the average cross sectional area in each region will decrease with distance from the first surface, and the relationship between differences in average cross sectional area will be the same as the relationships between differences in aCTE as defined above. The coefficient of thermal expansion at a depth d can be similarly calculated, and is equivalent to the CTE of an infinitesimally thin layer at that depth - i.e. CTE _d = k^CTE + (1 - /c)(CTE ₂ ) where CTEd is the CTE at a depth d, and k is the cross sectional area fraction of the first material at that depth.

One particular example use case for such interlayers is in attaching cooling apparatus, e.g. heat sinks and heat exchangers, to components expected to undergo high heat flux. In such examples the interlayer typically connects a high thermal expansion material (e.g. copper) of the cooling apparatus to a low thermal expansion material (e.g. tungsten or other refractory metal) of the component undergoing high heat flux. A particular example of such a component is a divertor of a tokamak plasma chamber, as described in the background.

Example materials for the interlayer include the case where the first and second material are both metals, e.g. the first material is a refractory metal or an alloy thereof, and the second material is copper or an alloy thereof. The refractory metals are those elemental metals having a melting point above 1850°C, which includes niobium, molybdenum, tantalum, tungsten, rhenium, and titanium.