POLYMER COMPOSITION

Abstract

A polymer composition for impregnating a high temperature superconductor (HTS) coil, the composition comprising: a polymer resin, a plurality of particles of a first filler material, and a plurality of particles of a second filler material; wherein the median particle size of the second filler material is less than the median particle size of the first filler material. The polymer composition may be used to prepare a polymer impregnated HTS coil having a predetermined turn-to-turn spacing. A property of the polymer composition may also be modified, for example, the coefficient of thermal contraction and/or resistivity of the composition. Also disclosed is a polymer impregnated HTS coil and a method for preparing the coil.

Claims

1. A polymer composition for impregnating a high temperature superconductor (HTS) coil, the composition comprising: a polymer resin, a plurality of particles of a first filler material, and a plurality of particles of a second filler material; wherein the median particle size of the second filler material is less than the median particle size of the first filler material.

2. The polymer composition of claim 1, wherein the aspect ratio of the particles of a first filler material is above about 0.80.

3. The polymer composition of claim 1 or 2, wherein the particles of the first filler material are substantially spherical or substantially cubic.

4. The polymer composition of any one of claims 1-3, wherein the median particle size of the first filler material is about 5 μm to about 50 μm.

5. The polymer composition of any one of claims 1-4, wherein the median particle size of the first filler material has a standard deviation of about 40% of the particle size.

6. The polymer composition of any one of claims 1-5, wherein the maximum particle size of the second filler material is less than the median particle size of the first filler material.

7. The polymer composition of any one of claims 1-6, wherein the maximum particle size of the second filler material is less than about 3 μm.

8. The polymer composition of any one of claims 1-7, wherein the first filler material and the second filler material are independently selected from the group consisting of a material having a low coefficient of thermal contraction and an electrically conductive material.

9. The polymer composition of any one of claims 1-8, wherein the first filler material is diamond.

10. The polymer composition of any one of claims 1-9, wherein the particles of the first filler material comprise a coating of an electrically conductive material.

11. The polymer composition of claim 10, wherein the electrically conductive material comprising the coating is titanium.

12. The polymer composition of any one of claims 1-11, wherein the polymer composition further comprises a plurality of particles of a third filler material, and wherein the median particle size of the third filler material is less than the median particle size of the first filler material.

13. The polymer composition of claim 12, wherein the maximum particle size of the third filler material is less than the median particle size of the first filler material.

14. The polymer composition of claim 12 or 13, wherein the maximum particle size of the third filler material is less than about 3 μm.

15. The polymer composition of any one of claims 1-11, wherein the polymer composition further comprises a plurality of particles of a third filler material, and wherein the median particle size of the third filler material is substantially equal to the median particle size of the first filler material.

16. The polymer composition of any one of claims 12-15, wherein the third filler material is selected from the group consisting of a material having a low coefficient of thermal contraction and an electrically conductive material.

17. The polymer composition of claim 8 or 16, wherein the material having a low coefficient of thermal contraction is selected from the group consisting of silica, quartz, carbon powder, diamond, amorphous diamond, boron nitride, alumina, aluminium nitride, zinc oxide, zirconium oxide, magnesium oxide and mixtures of any two or more thereof.

18. The polymer composition of claim 8 or 16, wherein the electrically conductive material is selected from the group consisting of metals, metal oxides, carbon and mixtures of any two or more thereof.

19. The polymer composition of claim 18, wherein the metals or metal oxides are selected from the group consisting of nickel, chromium, copper, silver, gold, aluminium, titanium, oxides thereof and mixtures of any two or more thereof.

20. The polymer composition of claim 18, wherein the carbon is selected from the group consisting carbon powder, carbon black, carbon fibres, carbon nanofibres, graphite, graphene and mixtures of any two or more thereof.

21. The polymer composition of any one of claims 12-20, wherein the first and second filler materials have a low coefficient of thermal contraction and the third filler material is electrically conductive.

22. The polymer composition of any one of claims 12-21, wherein the first filler material is diamond, the second filler material is alumina and the third filler material is copper flakes.

23. The polymer composition of any one of claims 12-20, wherein the first filler material is titanium coated diamond, the second filler material is alumina and the third filler material is diamond.

24. The polymer composition of any one of claims 1-23, wherein the polymer resin is selected from the group consisting of epoxies, polyimides, polyethylenes, polyacrylates, polyurethanes and combinations of any two or more thereof.

25. The polymer composition of any one of claims 1-24, wherein the polymer resin is an epoxy.

26. A polymer impregnated HTS coil comprising: a winding component comprising an HTS material, wherein the coil is impregnated with the polymer composition of any one of claims 1-25.

27. A method of producing a polymer impregnated HTS coil, the method comprising the steps of: a) providing a winding component comprising an HTS material, b) applying the polymer composition of any one of claims 1-25 to the winding component, c) winding the coated winding component obtained from step b) into a coil, and d) curing the coil obtained from step c) to provide the polymer impregnated HTS coil.

28. The polymer impregnated HTS coil of claim 26 or the method of claim 27, wherein the winding component further comprises one or more co-wind materials.

29. The polymer impregnated HTS coil or method of claim 28 wherein the one or more co-wind materials are independently selected from the group consisting of copper, copper alloys, silver, titanium, steel and nickel-molybdenum alloys.

30. The polymer impregnated HTS coil of any one of claims 26-29 or the method of any one of claims 27-29, wherein the HTS material is a REBCO tape.

31. Use of a polymer composition of any one of claims 1-25 for preparing a polymer impregnated HTS coil having a predetermined turn-to-turn spacing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] The invention will now be described with reference to the Figures in which:

[0073] FIG. 1 is a cross section of an HTS coil impregnated with a polymer composition that does not contain gauging particles.

[0074] FIG. 2 shows cross sections of a polymer impregnated HTS coil comprising gauging particles. FIG. 2A is an overall cross section of the coil. FIG. 2B is a cross section through the coil near the inner diameter. FIG. 2C is a cross section through the coil near the middle of the winding. FIG. 2D is a cross section through the coil near the outer diameter of the winding.

[0075] FIG. 3 is a graph of contact resistivity vs concentration of copper for three test coils.

[0076] FIG. 4 is a graph of critical current performance of a polymer impregnated HTS coil, comprising gauging particles and a material having a low coefficient of thermal contraction, upon repeated thermal cycling.

[0077] FIG. 5 is a graph of the contact resistivity vs concentration of titanium-coated diamonds for four test coils.

[0078] FIG. 6 is a graph of contact resistivity vs temperature for four test coils.

DETAILED DESCRIPTION OF THE INVENTION

[0079] The present inventors have surprisingly determined that certain filler materials, when incorporated into a polymer composition, are useful for setting the turn-to-turn spacing of a polymer impregnated HTS coil.

[0080] Accordingly, in one aspect, the present invention provides a polymer composition comprising: a polymer resin, a plurality of particles of a first filler material, and a plurality of particles of a second filler material; wherein the median particle size of the second filler material is less than the median particle size of the first filler material. More particularly, the polymer composition is useful for impregnating an HTS coil.

[0081] The first filler material is included in the polymer composition for setting the turn-to-turn spacing of the resulting polymer impregnated HTS coil, i.e. as a gauging particle. Advantageously, the first filler material has a larger median particle size than the median particle size of any additional filler materials included in the polymer composition. Those persons skilled in the art can select the particle size of the first filler material to provide the desired turn-to-turn spacing of the polymer impregnated HTS coil.

[0082] In some embodiments, the median particle size of the first filler material is about 5 μm to about 100 μm, or about 5 μm to about 50 μm, or about 5 μm to about 40 μm, or about 5 μm to about 30 μm, or about 9.5 μm to about 50 μm, or about 9.5 μm to about 40 μm, or about 9.5 μm to about 29 μm, or about 25.2 μm, or about 25 μm, or about 20 μm, or about 12.5 μm , or about 12.4 μm.

[0083] Preferably, the particles of the first filler material have a narrow particle size distribution. For example, the particle size may have a standard deviation from the median particle size of about 50% of the particle size, or about 40% of the particle size, or about 30% of the particle size, or about 25% of the particle size, or about 20% of the particle size, or about 15% of the particle size, or about 10% of the particle size, or about 5% of the particle size. In some embodiments, the particle size has a standard deviation from the median particle size of less than about 50% of the particle size, or less than about 40% of the particle size, or less than about 30% of the particle size, or less than about 25% of the particle size, or less than about 20% of the particle size, or less than about 15% of the particle size, or less than about 10% of the particle size, or less than about 5% of the particle size. In some embodiments, the median particle size of the first filler material has a standard deviation of about 10 μm, or about 9 μm, or about 8 μm, or about 7 μm, or about 6 μm, or about 5 μm, or about 4 μm, or about 3 μm, or about 2 μm, or about 1 μm.

[0084] Preferably, the particles of the first filler material have a high aspect ratio, i.e. an aspect ratio approaching 1. For example, the particles of the first filler material may have an aspect ratio above about 0.80, or above about 0.85, or above about 0.90, or above about 0.95, or above about 0.99, or an aspect ratio of 1. In some embodiments, the particles of the first filler material are substantially spherical or substantially cubic. In some embodiments, it is preferred that the particles of the first filler material are substantially cubic. For example, when the first filler material is an electrically conductive material, or when the particles of the first filler material comprise a coating of an electrically conductive material.

[0085] The polymer composition comprises at least one additional filler material, i.e. a second filler material. The polymer composition may comprise any number of additional filler materials, for example, a third filler material, a fourth filler material, and so on, provided that the median particle size of any additional filler materials is substantially equal to or less than the median particle size of the first filler material.

[0086] In some embodiments, each additional filler material has a median particle size less than the median particle size of the first filler material. Preferably, each additional filler material has a maximum particle size less than the median particle size of the first filler material. In some embodiments, the maximum particle size of each additional filler material is at least about 10% less than the median particle size of the first filler material, or at least about 20% less than the median particle size of the first filler material, or at least about 30% less than the median particle size of the first filler material, or at least about 40% less than the median particle size of the first filler material, or at least about 50% less than the median particle size of the first filler material, or at least about 60% less than the median particle size of the first filler material, or at least about 70% less than the median particle size of the first filler material, or at least about 80% less than the median particle size of the first filler material, or at least about 90% less than the median particle size of the first filler material. In those embodiments wherein there is more than one additional filler material, the maximum particle size of each additional filler material may be selected independently of the particle size of any other additional filler material. In some embodiments, the maximum particle size of each additional filler material is less than about 5 μm, or less than about 4 μm, or less than about 3 μm, or less than about 2 μm, or less than about 1 μm, or less than about 0.5 μm.

[0087] In some embodiments, the polymer composition comprises a first filler material, a second filler material, and a third filler material, wherein the second filler material has a median particle size less than the median particle size of the first filler material and the third filler material has a median particle size substantially equal to the median particle size of the first filler material.

[0088] Any of the filler materials according to the present invention may be a functional material that modifies a property of the polymer composition, for example, the coefficient of thermal contraction and/or electrical conductivity of the polymer composition. Alternatively, any of the filler materials may be an inert material.

[0089] In some embodiments, at least one filler material is a material having a low coefficient of thermal contraction. Such a material may be used to modify the coefficient of thermal contraction of the polymer composition, and preferably match or approximately match the coefficient of thermal contraction of the polymer composition to that of the other components of the HTS coil. Advantageously, this may reduce delamination of the coil at low temperatures. Materials having a low coefficient of thermal contraction that are suitable for use in the present invention include, for example, silica, quartz, carbon powder, diamond (such as amorphous diamond), boron nitride, alumina, aluminium nitride, zinc oxide, zirconium oxide and magnesium oxide. The invention is not, however, limited thereto, and other materials may be used. Advantageously, the material having a low coefficient of thermal contraction may have a relatively good thermal conductivity. In some embodiments, the material having a low coefficient of thermal contraction is diamond (such as amorphous diamond), alumina or magnesium oxide.

[0090] In some embodiments, at least one filler material is an electrically conductive material. Electrically conductive materials may be included in the polymer composition to modify the resistivity of the polymer composition. Advantageously, those persons skilled in the art can select a suitable electrically conductive material to achieve the desired turn-to-turn resistivity in the resulting polymer impregnated HTS coil. Those persons skilled in the art will appreciate that turn-to-turn resistivity will be affected by, for example, the particle size of the electrically conductive material, the concentration of the electrically conductive material, the concentration of any non-conductive material(s), and the turn-to-turn spacing of the coil.

[0091] Suitable electrically conductive materials include, for example, metals and metal oxides such as chromium, nickel, copper, silver, gold, aluminium, titanium, oxides thereof and mixtures of any two or more thereof; and carbon, which may be in the form of, for example, carbon powder, carbon black, carbon fibres, carbon nanofibres, graphite, graphene and mixtures of any two or more thereof. In some embodiments, the metals or metal oxides are selected from the group consisting of copper, silver, gold, aluminium, oxides thereof and mixtures of any two or more thereof. In some embodiments, the metals or metal oxides are selected from the group consisting of copper, silver, gold, aluminium and oxides thereof. The invention is not, however, limited thereto, and other materials may be used. For example, those persons skilled in the art will appreciate other non-ferromagnetic metals may be useful in the invention. In some embodiments, the electrically conductive filler material is copper powder, copper flakes or silver coated copper flakes.

[0092] In some embodiments, at least one filler material is a material having a low coefficient of thermal contraction and at least one filler material is an electrically conductive material. In some embodiments, the first filler material is a material having a low coefficient of thermal contraction and the second filler material is an electrically conductive material. In some other embodiments, the first filler material and the second filler material are materials having a low coefficient of thermal contraction and a third filler material is an electrically conductive material.

[0093] Certain filler materials may also modify multiple properties of the polymer composition. For example, carbon powder has a low coefficient of thermal contraction and is electrically conductive.

[0094] Any of the filler materials according to the present invention may comprise a coating.

[0095] Accordingly, the particles of the first filler material may comprise a coating of an electrically conductive material. Suitable electrically conductive materials include, for example, metals and metal oxides such as chromium, nickel, copper, silver, gold, aluminium, titanium, oxides thereof and mixtures of any two or more thereof. However, those persons skilled in the art will appreciate other electrically conductive materials, such as other non-ferromagnetic metals, may be useful in the invention. Preferably, the electrically conductive material is titanium. Preferably, the first filler material is a material having a low coefficient of thermal contraction coated with an electrically conductive material. In some embodiments, the first filler material is titanium coated diamond. In some embodiments, the polymer composition comprises a first filler material that is titanium coated diamond, a second filler material that is alumina, and a third filler material is diamond. Advantageously, the variation in resistivity with temperature of a composition comprising particles of a filler material comprising a coating of an electrically conductive material may be reduced compared with that of a composition comprising particles of a filler material consisting of an electrically conductive material. In some embodiments, the resistivity of the composition comprising particles of a filler material coated with an electrically conductive material does not substantially change with temperature.

[0096] Suitable coated filler materials may be prepared using various techniques known to those skilled in the art including, but not limited to, electroless plating and vapour deposition.

[0097] In some embodiments, the particles of at least one filler material comprise an electrically conductive material coated with a different electrically conductive material. For example, in some embodiments, at least one filler material is silver coated copper.

[0098] In some embodiments, the median particle size of the first filler material comprising a coating of an electrically conductive material is about 5 μm to about 100 μm, or about 5 μm to about 50 μm, or about 5 μm to about 40 μm, or about 5 μm to about 30 μm, or about 9.5 μm to about 50 μm, or about 9.5 μm to about 40 μm, or about 9.5 μm to about 29 μm, or about 25.2 μm, or about 25 μm, or about 20 μm, or about 12.5 μm, or about 12.4 μm.

[0099] Those persons skilled in the art can select the amount of each filler material to be included in the polymer composition. Those skilled persons may take into account the nature of the filler material and the desired properties of the polymer composition and resulting polymer impregnated HTS coil. The total amount of filler materials included in the polymer composition may be, by weight of the polymer composition, in the range of about 10% to about 70%, for example, in an amount of about 10%, or about 15%, or about 20%, or about 25%, or about 30%, or about 35%, or about 40%, or about 45%, or about 50%, or about 55%, or about 60%, or about 65%, or about 70%.

[0100] In some embodiments, the polymer composition comprises a first filler material in an amount, by weight of the polymer composition, of about 10% to 50%, or about 20% to about 40%, or about 30%. In some embodiments, the polymer composition comprises a filler material having a low coefficient of thermal contraction in an amount, by weight of the polymer composition, of about 1% to 40%, or about 10% to about 30%, or about 20%. In some embodiments, the polymer composition comprises a filler material having a low coefficient of thermal contraction in an amount, by weight of the polymer composition, of about 30% to about 70%, or about 40% to 60%, or about 50%. In some embodiments, the polymer composition comprises an electrically conductive filler material in an amount, by weight of the polymer composition, of about 1% to about 40%, or about 5% to about 30%, or about 10% to about 20%.

[0101] Those persons skilled in the art will appreciate that the polymer resin may be any thermosetting polymer resin that is suitable for impregnating an HTS coil. Preferred polymer resins have a long pot life, high Young's modulus and cryogenic serviceability. The pot life of the polymer resin should be sufficiently long to allow winding of a full coil without appreciable curing of the resin. In some embodiments, a polymer resin having a low curing temperature, for example, less than 60° C., is used. Advantageously, such use may minimise damage to other components of the coil during the curing process. For instance, when a coil is prepared with a low temperature solder, a polymer resin having a low curing temperature may be selected to avoid destabilising the solder joints during the curing process. Suitable polymer resins include, for example, epoxies, polyimides, polyethylenes, polyacrylates, polyurethanes and combinations of any two or more thereof. In some embodiments, the polymer resin is selected from the group consisting of epoxies, polyimides, polyethylenes, polyacrylates and polyurethanes. In a preferred embodiment, the polymer resin is an epoxy resin, for example, a primary amine cured bisphenol-A/bisphenol-F blend epoxy resin, such as CTD-521.

[0102] The polymer composition of the present invention may be used to prepare a polymer impregnated HTS coil. The coil comprises a winding component that is formed into the shape of a coil such as a single pancake coil, a double pancake coil or a racetrack coil. The polymer composition is impregnated in the coil such that it forms layers between the turns of the winding component resulting in a composite sandwich structure.

[0103] The winding component comprises an HTS material. Suitable HTS materials include, for example, rare earth barium copper oxide (REBCO) superconductor materials; bismuth, thallium or mercury-based superconductor materials (for example, BSCCO, TBCCO and HBCCO); other cuprate-based superconductor materials; magnesium diboride and Fe-based superconductor materials. Preferably, the HTS material is a REBCO material containing, for example, yttrium, samarium, neodymium, gadolinium or a combination thereof, such as YBa.sub.2Cu.sub.3O.sub.7-δ (YBCO) or GdBa.sub.2Cu.sub.3O.sub.7-δ (GdBCO). The invention is not, however, limited thereto, and other HTS materials may be used.

[0104] The coil may further comprise one or more co-wind materials. Suitable co-wind materials include, for example, copper, copper alloys (such as brass), silver, titanium and steel (for example, stainless steel), or an alloy, such as nickel-molybdenum alloys (for example, Hastelloy C276).

[0105] The HTS material and, when present, the co-wind material may be in any geometry suitable for forming a coil such as a cable, strip, tape or wire. In some embodiments, the HTS material and, when present, the co-wind material each independently have a substantially planar surface transverse to the winding direction of the coil such that the interface between layers is substantially planar. Generally, high aspect materials are preferred. In some embodiments, the HTS material and, when present, the co-wind material each independently have a width-to-thickness ratio of at least 10.

[0106] The polymer impregnated HTS coil of the present invention may be prepared by conventional “wet winding” methods. For example, the polymer composition of the invention may be applied to the surface of the winding component. The coated winding component may then be wound into a coil and the polymer cured to provide the polymer impregnated HTS coil. In some embodiments, the steps of applying the polymer composition to the surface of the winding component and winding the coil are performed concurrently.

[0107] The following non-limiting examples are provided to illustrate the present invention and in no way limit the scope thereof.

EXAMPLES

[0108] In the following examples, unless stated otherwise, the HTS coils were prepared by wet winding a commercially supplied 4 mm width REBCO superconductor tape electroplated with 20 μm of copper on both sides, coated with a CTD-521 resin (supplied by Composite Technology Development). CTD-521 resin is a primary amine cured bisphenol-A/bisphenol-F blend epoxy resin. The resin is supplied in two parts, A and B, that are mixed prior to use. The resin may be filled by the supplier. For example, CTD-521-A20 is filled to 20% by weight with alumina powder having a maximum size less than 1 μm.

[0109] Unless stated otherwise, the amorphous diamond used in the following examples had a median particle size of 12.5 μm with a standard deviation of approximately 5 μm.

[0110] Silver coated copper flakes were used as an electrically conductive material. Silver coating minimises surface oxidation of the copper. The copper flakes had a maximum particle size of less than 3 μm.

Example 1: Comparison of Polymer Compositions With and Without Gauging Particles

[0111] Two polymer compositions were prepared to evaluate the effects of gauging particles on the turn-to-turn spacing of an HTS coil impregnated with the composition. The first polymer composition contained gauging particles. The composition was prepared from a CTD-521-A20 resin, copper flakes having a maximum particle size of 3 μm, and amorphous diamond particles having a median particle size of 12.5 μm with a standard deviation of approximately 5 μm. The diamond particles were added to the bisphenol (part A) of the epoxy in an amount of 60% by weight of diamonds to part A of the epoxy and uniformly dispersed. The second polymer composition was a comparative example that did not contain gauging particles. The comparative polymer composition was prepared from CTD-521-A20 and copper flakes having a maximum particle size of 3 μm. SEM was used to image HTS coils impregnated with each polymer composition. FIG. 2 shows cross sections of the HTS coil impregnated with the first polymer composition. The white regions are conductor and the black regions are epoxy. FIG. 2A shows a cross section of a coil that has been wound using the first polymer composition. FIGS. 2B-2D show that at the randomly selected locations near the centre, middle and outer regions of the coil, the turn-to-turn spacing is set by the size of the diamond particles. FIG. 1 shows a cross section of the HTS coil impregnated with the comparative polymer composition that does not contain gauging particles. Small flecks can be seen in the epoxy regions, which are predominately the copper flakes or alumina. Comparison of the two coils demonstrates that the uniformity of turn-to-turn spacing is significantly improved in the HTS coil impregnated with the polymer composition comprising gauging particles.

Example 2: Evaluation of Different Gauging Particles

[0112] Once the gauging effect of epoxy resins comprising gauging particles was observed and measured, a simpler technique was developed and used to verify the gauging effect for different size gauging materials. Initially, a metallic tape was used to dry wind a coil of known inner diameter (ID). The tape had a uniform thickness and similar dimensions to those of the superconductor. Once the required number of turns were wound, the outer diameter (OD) of the coil was measured. The dry coil was then unwound. The following formula can be used to calculate the thickness of each coil turn, which is also the thickness of the metallic tape:

[00001]$\mathrm{Coil}\mathrm{turn}\mathrm{thickness}=\frac{0.5\mathrm{OD}-0.5\mathrm{ID}}{\mathrm{Number}\mathrm{of}\mathrm{turns}}$

[0113] The same piece-length of metallic tape was then used to wind an epoxy impregnated coil. The coil was wound using the same process as the dry-wound coil, but with the additional step of painting an epoxy resin filled with gauging material onto the surface of the metallic tape immediately prior to winding. Once the full length of tape had been wound onto the coil, the OD of the epoxy impregnated coil was measured, and the same formula used to calculate the average turn thickness. The turn thickness now includes both the tape thickness and the epoxy thickness. The difference between the two outer diameters must therefore give the epoxy thickness, which is in turn set by the size of the gauging material.

[0114] Six coils were wound with different sizes and types of gauging materials and examined using this method to verify the gauging concept. The results are shown in Table 1. Coil 1 and coil 2 were wound without any additional filler material for reference. Coil 3 used 6 g of diamonds as a filler with a nominal D.sub.50 of 9.7 μm, D.sub.5 of 8 μm and D.sub.95 of 12 μm. The coil achieved an inter-layer thickness close to the nominal D.sub.50 of the diamond filler. Coil 4 was wound using an alumina filler with a D.sub.95 of 30 μm. Other particle size parameters were unknown. The coil achieved gauging of 24.7 μm. Similarly, addition of an alumina filler with a D.sub.95 of 15 μm as in coil 5 achieved gauging of 10.7 μm. Addition of additional filler particles with D.sub.95 smaller than the first (15 μm) gauging material as in coil 6 achieved the same gauging as coil 5, demonstrating that the turn-to-turn spacing is set by the largest particle size.

TABLE-US-00001 TABLE 1 Copper tape Dry coil Epoxied Turn Coil thickness Number OD coil OD ID Added spacing number [μm] of turns [mm] [mm] [mm] Epoxy filler(s) [μm] Coil 1 102.6 200 65.71 68.45 24.59 CTD-521 None  7 A20 26.8 g Coil 2 101.8 200 65.60 68.54 24.87 CTD-521 None  7.4 A20 26.8 g Coil 3 102.7 160 57.87 60.81 24.97 CTD-521 Diamonds 6 g  9.2 A20 26.8 g D.sub.50 9.7 μm Coil 4 101.0 200 65.5 75.18 24.9 CTD-521 Alumina 6 g 24.7 A20 26.8 g D.sub.95 30 μm Coil 5 101.4 200 65.42 69.70 24.84 CTD-521 Alumina 6 g 10.7 A20 26.8 g D.sub.95 15 μm Coil 6 102.3 200 65.81 70.11 25.85 CTD-521 1. Alumina 6 g, 10.8 A20 26.8 g D.sub.95 15 μm 2. Alumina 12 g D.sub.95 5 μm 3. Silica 4 g D.sub.95 1 μm

[0115] The second set of coil winding examples followed the inverse process. The design of the coil, i.e. the ID, OD, conductor thickness and number of turns, was specified. Since the turn-to-turn thickness of the coil is now fixed, the thickness of the resin layer required to achieve the turn-to-turn thickness is also fixed. The size of the gauging particle may now be determined, and hence used to fill an epoxy resin. Table 2 shows the details of five coils co-wound with superconductor and titanium tapes using this process. The number of turns reported in the table refers only to the superconductor. Coils 7, 8, 9 and 10 used amorphous diamonds for gauging with a D.sub.50 of 12.4 μm, and a D.sub.5 and D.sub.95 of 8 and 18 μm, respectively. Coil 11 used amorphous diamonds for gauging with a D.sub.50 of 25.2 μm, and a D.sub.5 and D.sub.95 of 18 and 37 μm, respectively. In all cases the copper was copper flake particle with major particle length D.sub.50 of 2 μm.

TABLE-US-00002 TABLE 2 First OD OD filler Material Number ID required obtained First particle Second Coil thickness of turns [mm] [mm] [mm] Epoxy filler size [μm] filler Coil 7, 88 μm SC  72  30.6  60.56  60.83 (7) CTD-521 Diamonds 12.4 Copper Coil 8 88 μm Ti  60.79 (8) A20  9 g 15.5 g 20.1 g Coil 9, 88 μm SC 100.4  48.22  88.56  88.45 (9) CTD-521 Diamonds 12.4 Copper Coil 10 88 μm Ti  88.61 (10) A20  9 g 15.5 g 20.1 g Coil 11 81 μm SC  93 247 287.82 288 (11) CTD-521 Diamonds 25.2 Copper 88 μm Ti A20 39 g 37 g 87.2 g

[0116] The diameters of the coils were measured with traditional Vernier callipers; a measurement error of +/−0.05 mm was expected for every diameter measurement. This error, for a coil of 200 turns corresponds to a +/−0.2 μm error in the calculation of the resin thickness.

[0117] The superconducting coils were wet co-wound with superconductor tape and titanium tape of similar thickness. The coil turn thickness in the calculation includes the superconductor (SC) thickness, the titanium thickness and two layers of epoxy resin. The resin thickness reported in the table refers to a single resin layer between the copper cladding of the HTS and the titanium.

Example 3: Evaluation of Electrically Conductive Material Containing Polymer Compositions

[0118] When a superconducting coil is wound, it is possible to calculate the resistivity by performing a sudden discharge measurement. This method involves operating a coil at a current below its critical current (I.sub.C) and allowing the magnetic field produced by the coil to stabilise. The coil is then open circuited, causing the magnet current to dissipate as heat through the coil winding turn-to-turn resistance. The rate at which the magnetic field at the centre of the coil decays is used to calculate the turn-to-turn resistance. The method for calculating resistivity is outlined in Wang et al. Supercond Sci Technol 2013, 26, 1-6.

[0119] To evaluate the use of electrically conductive filler particles for modifying the resistivity of an HTS coil, a series of small polymer impregnated test coils were prepared with varying amounts of copper filler particles. The coils had an ID of 25 mm and an OD of 45 mm comprising 50 turns of superconducting wire. The coils were co-wound with a titanium wire (0.175 mm). The polymer composition for these coils was prepared from a CTD-521-A20 epoxy with 15 g of part A, 5.1 g of part B and 9 g of 12.5 μm amorphous diamond powder, to which copper flakes were added in an amount of 4.5 g for test coil 12, 13.5 g for test coil 13 and 18 g for test coil 14. The turn-to-turn resistivity of the coils was calculated by the method outlined in Wang et al. Supercond Sci Technol 2013, 26, 1-6. The results, as shown in FIG. 3, indicate the turn-to-turn resistivity of a polymer impregnated HTS coil may be modified by varying the amount of copper flakes added to the polymer composition.

Example 4: Validation of Coil Performance

[0120] The thermal stability of a coil comprising gauging particles and a material having a low coefficient of thermal contraction was evaluated by subjecting two test coils, coil 13 and coil 15, to repeated thermal cycles and observing whether thermal degradation had occurred. Coil 13 was the same coil comprising 13.5 g copper from Example 3. Coil 15 was a larger coil having an ID of 247 mm, and an OD of 288 mm. Coil 15 was impregnated with a CTD-A521-A20 epoxy prepared with 65 g part A, 22.1 g part B, 39 g amorphous diamond powder and 61.1 g of copper flakes.

[0121] The superconducting performance of a superconductor coil was measured by performing a critical current test. The critical current is the current at which the coil or conductor transitions from being a superconductor to a normal conductor. Far below the critical current, a superconductor has zero resistance meaning that current can be injected into the superconductor without measuring any voltage. As the superconductor approaches its critical current, the voltage obeys a power law behaviour with increased current. If damage occurs to a superconductor for thermal or any other reasons, it is observable immediately by reduction in the critical current performance, or by a reduction in the sharpness of the transition between superconducting and normal states.

[0122] No significant degradation was observed in coil 13 or coil 15 upon repeated thermal cycles.

[0123] FIG. 4 shows the critical current curves of coil 13 for three successive thermal cycles between room temperature and 77 K. The coil is said to transition between the superconducting and normal states when it exceeds the voltage demarked by the dashed line.

Example 5: Polymer Compositions Comprising Titanium-Coated Diamond Particles

[0124] A series of small coils were wound using an epoxy filled with varying amounts of coated and uncoated diamond particles. The coils were wound using 100 g of CTD-521-A20, which as noted above comprises 20% by weight of 1 μm alumina powder (a second filler material). The coils comprised 50 turns co-wound with titanium ribbon having a thickness of 83.6 μm and REBCO superconductor having a thickness of 90 μm. The coils are detailed further in Table 3. The contact resistivity of each of the coils was assessed at 40 K. An approximately linear relationship between the concentration of titanium coated diamonds and the contact resistivity was observed, as shown in FIG. 5. The contact resistance of coils 16, 17 18 and 19 was assessed at different temperatures and shown to have little variation between temperatures (FIG. 6).

TABLE-US-00003 TABLE 3 ID OD First filler First filler Third Third filler Third filler Coil [mm] [mm] First filler d.sub.50 (μm) weight (g) filler d.sub.50 (μm) weight (g) Coil 16 25.13 48.36 Ti coated 20 um 130 — — — diamond Coil 17 25.11 47.68 Ti coated 20 um  20 Uncoated 20 um 80 diamond diamond Coil 18 25.36 47.9 Ti coated 20 um 100 — — — diamond Coil 19 25.1 47.8 Ti coated 20 um  60 Uncoated 20 um 40 diamond diamond

[0125] It is not the intention to limit the scope of the invention to the above mentioned examples only. As would be appreciated by a skilled person in the art, many variations are possible without departing from the scope of the invention as set out in the accompanying claims.