GB1601971A

GB1601971A - Manufacture of superconductors

Info

Publication number: GB1601971A
Application number: GB10533/78A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1977-03-16
Filing date: 1978-03-16
Publication date: 1981-11-04
Also published as: GB1601972A; DE2711496A1; GB1601973A; DE2711496C3; FR2384334A1; DE2711496B2; FR2384334B1

Description

(54) THE MANUFACTURE OF SUPERCONDUCTORS (71) We, SIEMENS AKTIENGESELLSCHAFT, a German company, of Berlin and Munich, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to the manufacture of superconductors.

Intermetallic superconductor compounds consisting of two elements, such as, for example NbXSn or V3Ga, which are of A3B type and have A15 metallic structure, have very good superconductor properties and are distinguished more particularly by a high critical magnetic field BC2' a high transition temperature Tc and a high critical current density Ie. Such compounds are therefore more particularly suitable for use in conductors for superconductor coils for generating strong magnetic fields. In addition, ternary compounds such as, for example, niobiumaluminium-germanium NbXAI(, 8Ge" are of particular interest. Since these compounds are generally very brittle, however, it is difficult to produce them in a form suitable, for example, for magnet coils.

There is known a process whereby superconductors comprising intermetallic compounds formed of two components can be produced in the form of long wires or strips.

This process serves for the production of so-called multi-core superconductors comprising a plurality of superconductor wires, for example of NbRSn or V7Ga. disposed in a normally conducting matrix, or comprising niobium or vanadium wires having surface layers consisting of the aforesaid compounds. In such a process an intermediate structure is arrived at in which a ductile element of the intermetallic compound to be produced is provided in wire-form, for example a niobium or vanadium wire, which wire is enclosed in a sheathing of a ductile metal matrix, for example of copper, which may also contain a small amount of the remaining element or elements of the intermetallic compound in the form of an alloy.

Alternatively, a plurality of such wires may be incorporated in the matrix. The intermediate structure thus obtained is then subjected to treatment for reducing its crosssection. In this way on the one hand a long wire such as is required for coils is obtained, while on the other hand the diameter of the wire cores consisting, for example, of niobium or vanadium, is reduced to a low value of the order of magnitude of about 30 ,um to 50 Fm or less in this treatment, and this is advantageous with regard to the superconducting properties of the finished superconductor. Also a good metallurgical combination is obtained by means of this process step between the wire core or cores and the matrix material by which the core or cores is or are surrounded, without any reactions occurring which might embrittle the superconductor. After the reduction of cross-section the remaining element or elements of the intermetallic compound are then diffused from the outside into the matrix material of the intermediate structure and the intermediate structure consisting of one or more wire cores and the surrounding matrix material is subjected to a heat treatment in such manner that the desired superconductor compound is formed by reaction of the core material with the remaining element or elements of the compound which are contained in the surrounding matrix. The element or elements of the superconductor compound contained in the matrix thereby diffuse through the matrix into the core material consisting of the other element of the compound and react with the latter element to form a layer consisting of the desired superconductor compound.

In superconductors formed by such a process, however, transition temperatures Tc are often obtained which are far below the theoretical value expected for the superconductor material. The degree to which such an effect is manifest, which effect is referred to as Tc degradation, is substantially dependent upon the manufacturing parameters of the superconductor, more particularly upon the heat treatment conditions.

The Tc degradation is also accompanied by a degradation of the critical current strength Ic, especially with high magnetic fields.

The reason for the occurrence of this degradation behaviour is believed to reside in stresses which are set up as a result of effects of different thermal expansion coefficients for the superconductor compound and for the matrix material by which the wires containing this compound are surrouflded, which occur during cooling from the diffusion temperature to the operating temperature of the superconductor. Thus, for example, the thermal expansion coefficient for a copper-tin-bronze at 16 . 10-6K-l is about twice as high as that of Nb or Nb3Sn which is about 7 . 10-6K-'. As a result pressure is exerted on the superconductor material by the matrix material at the operating temperature of the superconductor. The resultant reduction of the transition temperature Tc and of the critical current strength 1c of the superconductor can be reversed to some extent by subjecting the superconductor to an external elongation. However, if the elongation is too great the critical current strength again decreases. On variation of the external stress on the superconductor, therefore, the critical current strength passes through a maximum. In order to achieve an increase in the critical current strength of the superconductor, the latter must therefore be exposed to a predetermined tensile stress whose maximum value must not be exceeded. Such a value for the superconductors in a superconductor magnet coil is very difficult to set up because the danger always exists that an irreversible 1c degradation may occur with excessively high tensile stresses, for example of an order of magnitude of 16tic, which is due to damage to the superconductor material.

It would be desirable therefore to provide a process in which internal stresses due to different expansion coefficients in superconductors are reduced without any danger of damage to the superconductor.

According to the present invention there is provided a process for manufacturing a superconductor, wherein an intermediate structure, comprising at least one core body, made of at least one but not every component of a superconductor compound to be formed in the process, embedded in a matrix of material that has a total crosssectional area no greater than twice the total cross-sectional area of the said at least one core body and that is electrically normally conductive at an operating temperature at which the said superconductor compound is superconductive, is provided with an external shell consisting of an alloy containing one or more of the remaining components of the superconductor compound, and is subjected to heat treatment, during which the or each remaining component of the superconductor compound is diffused through the matrix material to the or each core body, such as to bring about formation of the said superconductor compound at the or each core body, and wherein the said external shell is removed after the heat treatment.

An embodiment of this invention can provide an advantage more particularly in that, owing to the low proportions of normally-conducting matrix material, the parts of the superconductor which are superconducting at operating temperatures are enclosed by very thin layers consisting of the matrix material. These thin layers are relatively easily deformable during cooling to operating temperature so that they can readily adapt themselves to the dimensions of the superconducting parts of the superconductor without being able to exert any relatively great pressure on the superconducting parts. Degradation effects in a superconductor embodying this invention are, therefore, correspondingly small.

In an embodiment of this invention there is provided an intermediate structure having a shell or outer layer consisting of an alloy of the remaining element or one of more of the remaining elements. This shell is removed after the heat treatment for the production of the superconductor compound. It may with advantage be chemically etched away.

In this way there can be obtained an advantageously small ratio of the crosssectional area of the matrix material to the total cross-sectional of the core body or bodies and hence to the cross-section of the superconducting area of those parts of the finished conductor which are superconducting at operating temperatures.

Reference will now be made by way of example to the accompanying drawing, in each of the Figures of which an intermediate structure for a process embodying the invention is diagrammatically illustrated.

The examples given below with reference to the Figures all concern the manufacture of NbSSn multifilament conductors, in which the superconductor intermetallic compound Nb3Sn is produced from a first element niobium and a second element tin by solid diffusion.

There is illustrated in cross-section in Figure 1 an intermediate structure 2 obtained in the manufacture of a Nb3Sn filament conductor which consists in known manner of a bronze matrix 3 having incorporated therein niobium filaments 4, of which only a few are shown in the Figure. The intermediate structure may already have been subjected to a treatment for the reduction of cross-section, but not to final heat treatment for the formation of Nb3Sn layers on the niobium filaments. The compressive stresses acting on the formed superconductor layers of such a multifilament or multi-core conductor may be limited, for example, by providing cross-sectional area of bronze which is sufficient to provide only the amount of tin which is required to be converted for the formation of the superconductor compound. In this case, the volume or cross-sectional area ratio a of bronze to niobium is 0.59/ . The quotient 13 takes account of the proportion by weight of tin in the bronze matrix which is converted.

If, for example, there is provided in the intermediate structure a bronze matrix containing 15% by weight of tin and if 2% by weight of tin is still present in the bronze after the diffusion heating to form the superconductor compound, then 13% by weight has been converted, that is to say, the quantity 13 then amounts to 0.13. In this case, a volume or cross-sectional area ratio a of 4.5:1 is obtained.

However, a reduction of the compression stresses exerted by the matrix material on the superconductor parts of such a superconductor is obtained in embodiments of the present invention by making the volume or cross-sectional area ratio a lower than 2:1. A corresponding example is described in the following.

Example 1 A tin-containing shell of relatively large thickness is provided on a wire-form intermediate structure with a copper or coppertin matrix having embedded niobium cores to be reacted with the niobium cores by means of a transport reaction.

The multifilament wire-form intermediate structure shown in Figure 2 therefore consists of a central portion 9 having an external diameter of 0.3 mm, which consists of a bronze matrix 3 with 1000 niobium cores 4 embedded therein. The ratio a of bronze cross-sectional area to niobium cross sectional area is advantageously 1.5:1. This wire-form intermediate structure is enclosed in a shell 10 of pure bronze having a thickness of 0.15 mm so that the ratio atOt of this body is in all equal to 4.5:1. After a diffusion heating at, for example, 750"C for 20 hours, the outer bronze shell 10 of the multifilament wire is chemically etched away so that the ratio a of the wire is again less than 2:1. The conductor thereby pro duced thus also has a relatively high core content and therefore a correspondingly high current-carrying capacity at high magnetic fields and high temperatures.

In Example 1 it has been assumed that the superconductor parts of the conductor produced are surrounded by normallyconducting material which has a higher thermal expansion coefficient than the superconductor material.

In the manufacturing processes of Example 1 it may be desirable for the temperature of the heat treatment for the formation of the superconductor compound to be kept as low as possible. This reason for this will be explained with reference to the following example, in which a cross-sectional area ratio a in accordance with the invention is not provided but from which the reasons for low heat treatment temperature can be understood.

Example 2 An intermediate structure consisting of 1000 niobium cores embedded in a bronze matrix having an external diameter of 0.3 mm and a ratio a of bronze to niobium of 3.5:1 is subjected to a diffusion heat treatment at 6300C for about 250 hours. In this time a Nb3Sn layer of a thickness of 1.5 Rm is formed around each core. The current density of the wire thus produced is not substantially higher at 5 teslas than in the case of a heat treatment at 750"C, which produces a similar layer thickness. Thus the current density of the conductor heattreated at 6300C is about 1.8 . 105 A/cm2, while that of the conductor heat-treated at 750"C is about 1.6. 105 A/cm2. However the quenching stresses are lower with the lower heat-treatment temperature so that the critical temperature Tc and the critical field strength BC2 are higher. However this means that the conductor heat-treated at 6300C has a higher critical current strength Ic at higher fields. At 15 teslas, therefore, the following critical current strengths comparatively occur: for the conductor - heat-treated at 630"C, Ic is equal to 0.2. 105 A/cm2, whilst Ic for the conductors heat-treated at 750"C is only 0.1 . 105 A/cm-.

While the examples concern manufacture of superconductors involving the formation of the superconductor intermetallic compound Nb3Sn, the present invention is equally suitable for all known superconductor compounds such as, for example. V3Ga or V3Si, which are formed bv heattreatment from two components each initially containing at least one element of the compound to be formed.

It will be noted that for such AB compounds the element of higher melting temperature is incorporated in the core body or bodies.

Claims

WHAT WE CLAIM IS:

1. A process for manufacturing a superconductor, wherein an intermediate structure, comprising at least one core body, made of at least one but not every component of a superconductor compound to be formed in the process, embedded in a matrix of material that has a total crosssectional area no greater than twice the total cross-sectional area of the said at least one core body and that is electrically normally conductive at an operating temperature at which the said superconductor compound is superconductive. is provided with an external shell consisting of an alloy containing one or more of the remaining components of the superconductor compound and is subjected to heat treatment, during which the or each remaining component of the superconductor compound is diffused through the matrix material to the or each core body, such as to bring about formation of the said superconductor compound at the or each core body. and wherein the said external shell is removed after the heat treatment.

2. A process as claimed in claim I, " he rein the said external shell is removed bv chemical etching.

t.. A process as claimed in claim l or 2, wherein the material of the matrix is an alloy which contains one or more of the remaining components of the said superconductor compound.

4. A process as claimed in any preceding claim, wherein the said superconductor compound is of the AXB type, where A is a first elementary component and B is a second elementary component of the compound, having A-15 crystal structure, the elementary component in the said at least one core body embedded in the matrix being that elementary component, A or B, of higher melting temperature.

5. A process as claimed in claim 4, wherein the said superconductor compound is Nb3Sn, V3Ga or V3Si.

6. A process of manufacturing a superconductor, substantially as hereinbefore described with reference to Example 1 or Examples 1 and 2, and to Figure 2 of the accompanying drawings.

7. A superconductor made by a process as claimed in any preceding claim.