A PROCESS FOR THE MANUFACTURE OF INTRINSICALLY MULTIFILAMENT A-15 SUPERCONDUCTORS AND SUPERCONDUCTORS OBTAINED WITH SUCH PROCESS

Abstract: The process serves to obtain A-15 superconductors of the general formula A₃B and envisages a plurality of strips (4) being "sandwiched" between two sheets (2) of material A, the strips being disposed parallel to one another and spaced with a predetermined separation transversely of the strips (2), connecting the three layer composite A/B/A to a layer (15) of copper and then winding the multi-layer sheet A/B/A/Cu thus obtained around an element (10) of copper and introducing the coil (11) thus obtained into a copper container (12) and plastically working the blank (13) thus obtained to form a conductor which is twisted and then subjected to a heat treatment for diffusion at a temperature such as to prevent the formation of liquid phase, until the compound A₃B is obtained by interdiffusion. The material A is Nb or V or an alloy contain­ ing these and the material B is Al, Sn, Ge, Ga, Si, or an alloy containing one or more of these elements.

Description: A PROCESS FOR THE #kNUFACTURE OF INTRINSICALLY MULTI FILAMENT A-15 SUPERCONDUCTORS AND SUPERCONDUCTORS OBTAINED WITH SUCH PROCESS The present invention relates to a process for the production of multifilamentary superconducting conductors of the A-15 type in which the superconductor material is a compound of the chemical formula A3B (where A = Nb, V or their alloys: and B = Al, Sn, Ge, Ga, Si or their alloys) embedded in a matrix of copper for stabilisation. The present invention further relates to a superconducting conductor produced with this method. As is known, the commercially available superconducting materials can be divided into two groups: Nb-Ti alloys (ductile) for application in magnetic fields up to 9 T, and intermetallic A-15 compounds (fragile) for magnetic fields greater than 9 T. The Nb-Ti multifilamentary superconductors are obtained by assembling together a plurality of monofilaments each of which is constituted by an Nb-Ti superconducting element embedded in a copper matrix. The whole composition of monofilaments thus obtained is inserted into a further copper matrix (for example constituted by a hollow copper container) and the thus obtained composite is transformed by plastic working (for example by drawing) until its transverse section has been reduced to the required dimensions therefore obtaining, because of the contraction consequent on the elongation of the blank, a wirelike conductor. In the case of A-15 superconductors the plastic working of the compound A3B is not possible; on the contrary, this superconductor compound can be obtained only by means of an interdiffusion process between A and B caused by the heat treatment of the finished conductor, which conductor, in the three best known manufacturing processes, can be constituted respectively by: i) a bronze matrix rich in the element B (that is to say copper alloy - B) containing filaments of the element A, in the case of the "bronze route" process; ii) a matrix of copper containing filaments constituted by a mixture of the elements A and B finally sub-divided, in the case of the "powder metallurgy" process; iii) a matrix of copper containing filaments constituted by intimately connected elements A and B, in the case of the "thin sheet metallurgy" process. Whatever the process selected, the manufacture of A-15 multifilamentary strip conductors always requires the formation of monofilaments. Manufacturing monofilaments is laborious and expensive in that it is divided into a series of operations which are difficult to automate, such as: drawing of the hexagonal monofilament, cutting the hexagons to length, degreasing, pickling, rinsing, drying and assembling. Moreover, monofilaments made with the described method can become damaged andZor broken during the drawing operations, interrupting the continuity of the filaments and leading to a reduction in the useful section of the cable for the passage of current. The object of the present invention is that of providing a process for obtaining multifilanentary superconductors of the A-15 type stabilised with copper, free from the disadvantages described above, which can be quickly and economically performed and automated simply. The said object is achieved by the present invention in that it relates to a process for obtaining multifilamentary superconductors of the A-15 type, comprising a plurality of filaments constituted by a superconductor material of the general chemical formula A3B embedded in a copper matrix, characterised by the fact that it comprises in combination the following phases: : - preparation of a composite sheet which can be wound in a spiral and comprising two sheets of material A with a plurality of strips (4) of material B sandwiched between them, the strips being located alongside one another, spaced with a predetermined separation and extending transversely between the longer sides of the sheets; - connecting the three layer composite sheet A/B/A to a sheet of copper (Cu) and obtaining an A/B/A/du multi-layer sheet; - - preparation of a coil by winding the said multilayer sheet A/B/A/Cu around a cylindrical copper element with a sheet of the material A disposed in contact with the surface of this latter; ; - mounting the said coil within a copper container in such a way as to obtain a blank, and working the blank by plastic deformation thereof to obtain a conductor of predetermined dimensions and having transverse dimensions less than that of the blank and a length greater than the blank; - twisting the said conductor with a predetermined pitch; - heating the said conductor in an inert environment to a temperature lying within a range of values such as to avoid the formation of liquid phase but sufficient to cause an interdiffusion between the materials A and 3 to obtain the compound A3B. For a better understanding of the present invention there will now be given a non limitative description of an embodiment, with reference to the attached drawings, in which: Figures 1 and 2 illustrate two different elements obtained at the end of different respective phases of the process according to the present invention; and Figure 3 illustrates a phase in the process of the invention. The process according to the present invention serves to form a multifilamentary super conducting cable of the A-15 type stabilised with copper, not illustrated. The superconductor material of the type A-15 is constituted by an intermetallic compound of the general chemical formula A3B where A represents niobium (Nb) or vanadium (V) or an alloy containing one of these elements, and where B represents aluminium (A1), tin (Sn), germanium (Ge), gallium (Ga) or silicon (Si) or an alloy containing one or more of the elements. Like all intermetallic compounds, the compound A3B has great fragility, whilst both the metals A and the metals B, with the exception of Ge and Si, have a satisfactory workability. Such a cable is obtained, according to the invention, by means of a series of simple operations or successive phases, all easily automatable, which will now be described in detail. With reference to Figure 2, a composite sheet 1 of determined length is first of all prepared, which includes two substantially rectangular sheets 2 made from a material A (A=Nb or V or their alloys) and having a thickness tA, which "ssudwich" between them a plurality of substantially rectangular strips 4 made from a material B (B=A1, Si, Sn, Ge, Ga or their alloys) and having a thickness t3 and a transverse width b. The sheets 2 are in the form of strips delimited by opposite major sides 5 and opposite minor sides 6. The strips have a length substantially equal to the width of the sides 6 of the sheets 2, are positioned side by side and parallel to one another and disposed transversely with respect to the sides 5, and are spaced with a predetermined separation . The strips 4 can be deposited and fixed onto a face 3 of one of the sheets 2 facing the other sheet 2 by means of any known surface deposition method such as, for example, electro-deposition, electrophoresis, plasma spray, ionic implantation, evaporation under vacuum, plating or cathodic spraying (sputtering). in a preferred embodiment of the present invention the strips 4 are mechanically inserted between the. sheets 2 by means of the operation illustrated in Figure 3; wmithl reference to Figure 3, between the sheets 2 of material A is interposed a substantially rectangular sheet 7 made from the material 3 and of thickness t31 having a width greater than that of the sheet 2 and the same length. The sheet 7 is provided, transversely of its longer sides 8, parallel to the sides 5, with a plurality of substantially rectangular windows 9 formed through it by stamping in regular succession, positioned parallel to one another and spaced from one another with a predetermined constant separation; the windows 9 have a length greater than the width 6 of the sheets 2, a width equal to p and are spaced with a separation equal to b; in this way it is sufficient to cut the edges of the sheet 7 projecting from the sheets 2 ill correspondence with the sides 5 to obtain the composite three layer sheet A/B/A indicated with the reference numeral 1 in Figure 2. In fact, the portions of the sheet 7 delimited by the windows 9, in this way constitute the strips 4 of the composite sheet 1 and remain separated from one another. After having obtained the A/B/A composite 1 with the method described above and illustrated in Figure 3, or else with any other method such as those previously discussed, it is connected by the face of one of the sheets 2 to a sheet of copper 15 of the same length and width and of thickness tCu The A/B/A/Cu multi-layer sheet 1 a thus obtained (Figure 1) is wound tightly, substantially without any play, around a cylindrical copper element 10 in such a way as to form a coil 11. This winding is performed starting from the sheet 2 of material A left free from the sheet 15, in such a way as to position the sheet 2 of material A in contact with the surface of the element 10. Subsequently the coil 11 thus obtained is mounted within the interior of a substantially cylindrical container 12 of copper, preferably constituted by an open ended sleeve, into which the coil 11 is inserted coaxially, substantially without any radial play. There is thus obtained a blank 13, illustrated in Figure 1 , which is composed of a plurality of parallel strips 4 disposed in several concentric rings, each ring of strips 4 being enclosed in anarmular matrix of material A constituted by the spirally wound sheets 2, and the strips 4 being separated by spaces 14 of a width equal to the separation ; ; the matrix constituted by the sheets 2 is in turn enclosed in an annular matrix of copper constituted by the spirally wound sheet 15 the whole being contained in a further copper matrix constituted by the cylindrical element 10 and by the annular container 12 within which the coil 11 is enclosed. The blank 13 is at this point worked by plastic deformation in a known way, for example by drawing, until there is obtained from lt r by reduction of the transverse section and consequent elongation, a conductor (not illustrated) having the desired dimensions which will obviously have transverse dimensions less than those of the blank 13 and 2 length greater than that of the blank. Supposing that the final diameter of the element 1Q, after the plastic deformationS is d11 the transverse reduction undergone by the blank 13 with be R = wed1, a non-dimensional ratio which is unvaried even taking into consideration, rather than the diameter d of the element 10, the diameter D of the element 12; in these conditions, with simple geometric considerations, it is obtained that the new transverse dimensions of the strips 4, which constitute the filaments of the conductor, will be JbR and ¯gR. The conductor obtained by plastic working o± the blank 13 is then twisted in a known way, with a pitch of predetermined value, sufficient to dispose the filaments constituted by the strips 4 in such a way as to guarantee the magnetic de-coupling between them, and is then subjected to one or more heat treatments in a known manner, performed in a vacuum or an inert gas atmosphere at a temperature lying in a range of values such as to avoid the formation of liquid phase, and in particular the fusion of the material B (which could cause the formation of a con tinuous layer of B by liquid diffusion between the strips 4), but sufficient to cause an interdiffusion between the materials A and B of the sheets 2 and the strips 4, which as is known occurs predominately in a radial sense, so as to obtain the formation of the intermetallic compound of the formula A3B. If the heat treatment is performed in the correct manner the strips 4 of material 3 all transform integrally into superconductor filaments of material A3B. Because of the plastic deformation to which the com posite 13 is subjected the cavities 14 are further filled with material A of the deformed sheets 2 and therefore, at the end of the operations described, there is obtained a superconductor in which a plurality of A-15 filaments of material A3B of substantially rectangular section and disposed in a ring on a series of substantially concentric rings is embedded in a series of substantially annular matrices of material A, which are in their turn embedded in substantially annular matrices of copper which, together with the further matrices of copper constituting the core and the outer ring of the conductors (elements 10 and 12) allows an excellent stabilisation of the supercOn¯ ducting filaments. The matrix of material A, in which these latter are embedded, further allows, as is known, a minimisation in the losses in the conductors due to possible variable components in the magnetic field and/or in the current, in that it behaves as a resistive barrier between the superconductor filaments. This resistive barrier can be improved and increased by adding to the material A an element, such as tantalum (tea) which increases the electrical resistivity of it without detracting from the superconductor properties of the compound A3B. By means of the process described there is therefore obtained a '1cable" constituted by a single intrinsic al#y multifilamentary superconducting conductor, in which the plurality of superconductor filaments are derived not from an assembly operation of previously prepared monofilaments, but from the peculiar internal structure of the blank 13 and, in particular, of the coil 11. This allows one operation (that of assembly of the monofil aments) to be eliminated, which operation is laborious, of high cost and is difficult to automate, and provides a process in which all the phases can be easily and economically automated, involving only operations which can be effected easily. The invention will now be illustrated with a particular example, relating to the manufacture of a superconducting cable of Nb3 Al (A = Nb; B = Al) The starting materials were constituted by a sheet of copper of 100 Fm in thickness, by two sheets of niobium of 100 my in thickness and a sheet of aluminium of a thickness of 20 Fm; on this latter were formed a regular succession of windows 9 where b = 4mum, p = imam. After having inserted the sheet of Al between the two sheets of Nb the triple sheet Nb/Al/Nb was coupled to a sheet of Cu and the assembly wound on a copper cylinder of diameter equal to 6 mm. The coil obtained, having a diameter of 29 mm, was inserted into a cylindrical container of copper having a diameter of 40 mm. The thus obtained composite was worked plastically by drawing to obtain a wire having a diameter of 0.4 mm, which was twisted with a pitch of 2 cm. The wire was then treated under vacuum at 600 0C for 24 hours and subsequently at 850 C for two hours. X-ray tests confirmed the presence of the phase Nb3Al, whilst a scanning electron microscope detected that the superconductor filaments were formed with a good separation from one another in a matrix of non-reacted niobium. A short sample of the multifilamentary superconductor thus formed was subjected to superconductivity tests; the critical temperature was nearly 160 K, whilst the current density, critical at 6 Tesla, 4.2 K, was equal to 2.10A/Cm

Claims: CLAIMS 1. A process for obtaining multifilamentary superconductors of the A-15 type, comprising a plurality of filaments of a superconductor material of the general chemical formula A3B embedded in a matrix of copper, characterised by the fact that it comprises the fpllow- ing phases in combination: : - preparation of a composite sheet (1) which can be wound in a spiral and comprising two sheets (2) of material A with a plurality of strips (4) of material B sandwiched between them, the strips being located alongside one another, spaced with a predetermined separation and extending transversely between the longer sides (5) of the sheets (2); - connecting the said composite sheet (1) to a sheet of copper to obtain an A/B/A/Cu multi-layer sheet (lay; - preparation of a coil (11) by winding the said A/B/A/cu rnulti-layer sheet (lea) around a cylindrical copper element (10) with the surface of this latter being located in contact with a sheet (2) of material A: : - mounting the said coil (11) within a copper container (12) in such a way as to obtain a blank (13), and working the blank (13) by plastic deformation thereof to obtain a conductor of predetermined dimensions and having transverse dimensions less than those of the blank (13) and a length greater than the blank; - twisting the said conductor with a predetermined pitch; - heat treatment of the said conductor in an inert environment at a temperature lying in a range of values such as to avoid the formation of liquid phase but sufficient to cause an interdiffusion between the materials A and B to obtain the compound A3B. 2. A process according to Claim 1, characterised by the fact that the said heat treatment takes place under vacuum. 3. A process according to Claim 1, characterised by the façt that the said heat treatment takes place in an inert gas atmosphere. 4. A process according to any of the preceding Claims, characterised by the fact that the material A is niobium, vanadium or alloys of niobium and vanadium, and by the fact that the material B is aluminium, tin, germanium, gallium, silicon or alloys containing at least one of the said elements. 5. A process according to any preceding Claim, characterised by the fact that the said strips (4) of material B are obtained by a surface deposition process effected directly onto one face (3) of one of the said two sheets (2) of material A. 6. A process according to any of Claims from 1 to 4, characterised by the fact that the said strips (4) of material B are obtained by inserting a sheet (7) of material B between the said sheets (2) of material A to form a sandwich, there having been formed in the sheet (7) of material B a plurality of substantially rectangular windows (9) transversely disposed with respect to the longer sides (8) of the said sheet (7) and with predetermined regular separation, the width of the said sheet (7) being greater than that of the sheets (2) of material A, and cutting off the edges of the said sheet (7) projecting from the said sheets (2) of material A. 7. Multifilamentary superconductors of the A-15 type, characterised by the fact that they have been obtained by means of the process according to one of Claims from 1 to 6 8. Multifilamentary superconductors of the A-15 type comprising a plurality of filaments of superconductor material of the formula A3B stabilised with copper, where A = Nb, V or their alloys and B = Si, Sn, Al, Ge, Ga or their alloys, characterised by the fact that the said filaments are of substantially rectangular section and are disposed in substantially concentric rings disposed within respective substantially annular matrices of material A in which the said filaments are embedded and which are in their turn embedded in respective first substantially annular matrices of copper, the said matrices of material A being able to serve as a resistive barrier between the said superconductor filaments, the whole being contained in a second matrix of copper.

Inventors: CERESARA, SERGIO, SACCHETTI, NICOLA
Applicants: E.N.E.A., COMITATO NAZIONALE PER LA RICERCA E LO SVILUPPO DELL'ENERGIA NUCLEARE E DELLE ENERGIE ALTERNATIVE, EUROPA METALLI - LMI S.P.A.
Application year: 1984
Main classification: H01L39
IPC codes: H01L39