Superconducting joint using exfoliated ReBCO

Abstract

According to a first aspect of the present invention, there is provided a method of forming a superconducting joint between ReBCO tapes. Two or more ReBCO tapes are provided, each having an exposed ReBCO region. A bridge is provided, comprising an exposed ReBCO layer and an oxygen-permeable backing on the exposed ReBCO layer. Each exposed ReBCO region is bonded to the exposed ReBCO layer of the bridge by heating to a first temperature (T1) in an environment where the partial pressure of oxygen is sufficiently low that the melting point of the ReBCO (T.sub.R) is less than the melting point of silver (T.sub.Ag), the temperature (T1) being between the melting point of the ReBCO (T.sub.R) and the melting point of silver (T.sub.Ag), (T.sub.R<T1<T.sub.Ag). The resulting joint is annealed at a second temperature (T2) which is less than the melting point of ReBCO (T.sub.R) (T2<T.sub.R), for a time (t), in an environment where the partial pressure of oxygen is sufficient to reoxygenate the ReBCO at the second temperature (T2).

Claims

1. A method of forming a superconducting joint between ReBCO tapes, the method comprising: providing two or more ReBCO tapes, each having an exposed ReBCO region; providing a bridge comprising a ReBCO layer directly bonded to a silver layer on one side of the ReBCO layer facing away from the two or more ReBCO tapes, and having exposed ReBCO on the other side of the ReBCO layer facing towards the two or more ReBCO tapes; bonding each exposed ReBCO region to the exposed ReBCO layer of the bridge by heating to a first temperature (T1) in an environment where the partial pressure of oxygen is sufficiently low that the minimum joining temperature of the ReBCO (T.sub.R) is less than the melting point of silver (T.sub.Ag), the temperature (T1) being between the minimum joining temperature of the ReBCO (T.sub.J) and the melting point of silver (T.sub.Ag), (T.sub.J<T1<T.sub.Ag); annealing the resulting joint at a second temperature (T2) which is less than the orthorhombic to tetragonal phase transition temperature of ReBCO (T.sub.R) (T2<T.sub.R), for a time (t), in an environment where the partial pressure of oxygen is sufficient to reoxygenate the ReBCO at the second temperature (T2).

2. A method according to claim 1, wherein the exposed ReBCO layer is a textured ReBCO layer.

3. A method according to claim 2, wherein providing the bridge comprises: providing a further ReBCO tape having a substrate, buffer stack, the ReBCO layer, and the silver layer; removing the substrate and buffer stack from the ReBCO tape.

4. A method according to claim 2, wherein providing the bridge comprises: providing a further ReBCO tape having the ReBCO layer, the silver layer, and a further silver layer on the opposite side of the ReBCO layer; removing the further silver layer from one side of the ReBCO layer.

5. A method according to claim 1, wherein providing the two or more ReBCO tapes, each having an exposed ReBCO region comprises, for at least one of the ReBCO tapes, chemically removing a silver layer from the ReBCO tape in order to form the exposed ReBCO region.

6. A method of forming a superconducting joint between ReBCO tapes, the method comprising: providing a first ReBCO tape which is an exfoliated ReBCO tape comprising a ReBCO layer directly bonded to respective silver layers on each side of the ReBCO layer; providing a second ReBCO tape comprising a ReBCO layer bonded to a silver layer on at least one side of the ReBCO layer; chemically removing the silver layer from the region to be joined on each of the first and second ReBCO tape in order to produce an exposed ReBCO region on each tape; bonding the exposed ReBCO regions to each other by heating to a first temperature (T1) in an environment where the partial pressure of oxygen is sufficiently low that the minimum joining temperature of the ReBCO (T.sub.J) is less than the melting point of silver (T.sub.Ag), the temperature (T1) being between the minimum joining temperature of the ReBCO (T.sub.J) and the melting point of silver (T.sub.Ag), (T.sub.J<T1<T.sub.Ag); annealing the resulting joint at a second temperature (T2) which is less than the orthorhombic to tetragonal phase transition temperature of ReBCO (T.sub.R) (T2<T.sub.R), for a time (t), in an environment where the partial pressure of oxygen is sufficient to reoxygenate the ReBCO at the second temperature (T2).

7. A method according to claim 6, wherein the second ReBCO tape comprises a buffer stack and substrate on the side of the ReBCO layer opposite the silver layer.

8. A method according to claim 6, wherein the second ReBCO tape comprises a further silver layer on the other side of the ReBCO tape.

9. A method according to claim 6, wherein the time (t) is less than 5 hours.

10. A method according to claim 6, wherein the time (t) is less than 1 hour.

11. A method according to claim 6, wherein the oxygen-rich environment is a pure oxygen environment.

12. A superconducting current carrier comprising two ReBCO tapes connected by a joint comprising a bridge; the bridge comprising a ReBCO layer directly bonded to a silver layer on one side of the ReBCO layer facing away from the two ReBCO tapes, and having exposed ReBCO on the other side of the ReBCO layer facing towards the two ReBCO tapes; wherein the exposed ReBCO layer is bonded to a ReBCO layer of each of the ReBCO tapes.

13. A superconducting current carrier comprising two ReBCO tapes connected by a joint, wherein at least one of the ReBCO tapes is an exfoliated ReBCO tape comprising a ReBCO layer directly bonded to respective silver layers on each side of the ReBCO layer, and wherein the ReBCO layers of each tape are bonded at the joint.

14. A field coil comprising a superconducting current carrier according to claim 12.

15. A field coil comprising a superconducting current carrier according to claim 13.

16. A method according to claim 1, wherein the time (t) is less than 5 hours.

17. A method according to claim 1, wherein the time (t) is less than 1 hour.

18. A method according to claim 1, wherein the oxygen-rich environment is a pure oxygen environment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic illustration of a ReBCO tape;

(2) FIGS. 2a to 2c are schematic illustrations of joints between ReBCO tapes;

(3) FIG. 3 is a schematic illustration of a process of exfoliating ReBCO tape;

(4) FIG. 4 is a schematic illustration of a process of forming a joint between ReBCO tapes;

(5) FIG. 5 is a schematic illustration of a process of forming an indirect joint between ReBCO tapes.

DETAILED DESCRIPTION

(6) Recent advances in the manufacture of ReBCO tapes have allowed long sections of ReBCO to be exfoliatedi.e. removed intact from the substrate layer. This process is illustrated in FIG. 3. A ReBCO tape 301 (shown here comprising a substrate 311, buffer stack 312, ReBCO layer 313 and silver layer 314, i.e. without the stabiliser layer) is fed into the process, and the substrate 311 and buffer stack 312 are peeled away to leave the exfoliated ReBCO layer 313 and silver 314 layers. A backing lamination 315 is applied to the silver layer to support the exfoliated ReBCO during the process. The backing lamination 315 may be removed after exfoliation. However, the backing lamination 315 improves the structural properties of the exfoliated ReBCO, as the silver layer is unlikely to be sufficiently mechanically robust for many applications. As the ReBCO layer 313 has come from a ReBCO tape, it will be fully textured and have similar superconducting properties to the original tape.

(7) If the exfoliated ReBCO were to be used as a standard current carrying tape, a second silver layer would be applied to the exposed ReBCO face, and the tape would be encapsulated in a stabiliser layer to form an exfoliated HTS tape. However, the exfoliated ReBCO with a single silver layer can be used to make an improved ReBCO-ReBCO joint. The silver layer 314 is permeable to oxygen, and the backing lamination 315 can be made from an oxygen permeable material such as silver (as it does not have the same structural constraints as the substrate 311). Therefore, if the exfoliated ReBCO is used in a ReBCO-ReBCO joint, then oxygen diffusion to the ReBCO will be significantly faster than for a joint between two substrated HTS tapes (i.e. ReBCO tapes with the substrate still attached). If the exfoliated ReBCO is to be used in one or more exfoliated tapes which are being joined, then the second silver layer and stabiliser layer may be applied to the tape, excluding the regions to be joined, in order to remove the need for the step of chemically removing the silver layer. An exfoliated tape is structurally different to a conventional ReBCO tape in that an exfoliated HTS tape comprises a ReBCO layer with a silver layer on each face, whereas a conventional HTS tape comprises a ReBCO layer with a silver layer on one face and an oxide buffer stack on the other face (which connects to the substrate).

(8) This can be achieved as a joint between two exfoliated HTS tapes, each comprising a ReBCO layer 413 and two silver layers 414, as shown in FIG. 4, i.e: Step 401: The silver layer is chemically removed from one side of the exfoliated HTS tapes 41a in the region to be joined (401a), or exfoliated HTS tapes 41b are provided with an incomplete silver layer on one side (401b), leaving the ReBCO in the region to be joined 42 exposed; Step 402: The exposed ReBCO regions 42 are brought into contact and heat treated in a low-oxygen environment (PO.sub.2<approx. 200 Torr) at a temperature T above the minimum joining temperature of the ReBCO T.sub.R (approx 800 C. at PO.sub.2=0.01 Torr) but below the melting point of silver T.sub.Ag (961 C. at standard pressure); Step 403: The joined HTS tapes are annealed in an oxygen-rich environment below the orthorhombic to tetragonal phase transition temperature (530 C. to 560 C. depending on PO.sub.2) of the ReBCO to restore oxygen to the ReBCO.

(9) Example temperatures given are for YBCO, and will be different for other rare earth elements.

(10) The annealing step will be significantly faster than for substrated HTS tapes. The exact timings will depend on the temperature and PO.sub.2 used, but times of less than 5 hours are achievable, and a process at 500 C. in pure oxygen at 1 atmosphere (760 Torr) will take approximately 1 hour. This is a considerable improvement over the 350 hours required by prior art methods.

(11) A joint can be formed between an exfoliated HTS tape and a substrated HTS tape by a similar method.

(12) As a further alternative, an indirect joint can be formed between any combination of exfoliated HTS tapes and substrated HTS tapes by using an exfoliated ReBCO with oxygen-permeable backing (i.e. just silver, or silver plus an oxygen permeable backing layer 315) as a bridge between the tapes to be joined.

(13) A method of forming an indirect joint between two substrated HTS tapes, each comprising a ReBCO layer 513, a silver layer 514, and a substrate 511 is shown in FIG. 5. Step 501: The silver layer is chemically removed from the substrated HIS tapes 51 in the region to be joined, leaving the ReBCO in the region to be joined exposed 52. Step 502: The ReBCO of the substrated HIS tapes 51 is brought into contact with the ReBCO of a section of exfoliated ReBCO with oxygen permeable backing 53, and the joint is heat treated in a low-oxygen environment (PO.sub.2<approx. 200 Torr) at a temperature T above the minimum joining temperature of the ReBCO T.sub.R (approx 800 C. at PO.sub.2<0.01 Torr) but below the melting point of silver T.sub.Ag (961 C.). Step 503: The joint is annealed in an oxygen-rich environment below the orthorhombic to tetragonal phase transition temperature (530 C. to 560 C. depending on PO.sub.2) of the ReBCO to restore oxygen to the ReBCO.

(14) The exfoliated ReBCO with oxygen-permeable backing may be provided with a protective silver layer on the side to be joined (for storage and transport), in which case this can be removed in step 501.

(15) The oxygen-rich environment in steps 403 and 503 is an environment with PO.sub.2 sufficient to cause reoxygenation of the ReBCO, e.g. with PO.sub.2 above atmospheric levels (0.21 atm=160 Torr) or with PO.sub.2 greater than 1 atm (=720 Torr). The minimum PO.sub.2 required will depend on the temperature used for the annealing step, with higher temperature requiring greater PO.sub.2.

(16) While the example of FIG. 5 shows just two tapes being joined in a linear configuration, it will be appreciated that multiple tapes may be joined in a variety of configurations.

(17) The chemical removal of the silver layer in step 401a or 501 may be achieved by any suitable means which would remove the solver layer without affecting the ReBCO. For example, a Kl+I:H.sub.2O solution can be used to dissolve the silver layer on tapes without a copper stabiliser. For tapes with a copper stabiliser, an FeCl.sub.3 solution can be used to dissolve the copper, and then an H.sub.2O.sub.2:NH.sub.4OH solution to remove the silver chloride formed by that reaction as well as the remaining silver layer.