CS231677B1 - A method of modifying the initial multi-fiber superconducting conductor blanks - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for the preparation of NbzSn multi-fiber superconducting conductors for producing high-performance superconducting magnets, in particular for magnetic induction areas above 8 T. 3 of its diameter, filled with tin bronze or pure copper. The volume fraction of the niobium tube with the filling is 20 to 50% by volume of the matrix.

Description

The invention relates to a method of treating starting semi-finished products of multi-filament superconducting Nb^Sn conductors intended for the production of superconducting magnets with high parameters, especially for the range of magnetic inductions above 8 T.

Multifilament superconducting conductors Nb^Sn are currently the most progressive type of superconducting conductors. The well-known method of their production, the so-called bronze method, consists in extrusion pressing and gradual drawing of a system of niobium fibers placed in a basic matrix of tin bronze. The most important element of the entire system forming the conductor are the niobium fibers, which after reactive diffusion annealing become the carrier of superconductivity. Diffusion of tin from the tin bronze matrix forms a layer of the superconducting phase Nb^Sn on the surface of each fiber.

By analyzing the physical-metallurgical conditions of tin diffusion into niobium fibers, optimal parameters for the formation of superconducting Nb^Sn layers were found, at which the maximum values of the critical current of the entire conductor are achieved. In conductors exhibiting these high parameters, tin diffusion did not transform the niobium fiber into the Nb^Sn phase throughout the entire cross-section, but the inner part of the fiber remained unreacted and is composed of the original niobium.

For the actual technique of manufacturing and operating superconducting magnets, it is necessary that each superconducting conductor from which the magnet is wound is so-called stabilized, i.e. in its cross-section, on the surface or in the core and/or in several strips or veins, it contains a non-superconducting, but highly conductive metal under operating conditions at 4.2 K - most often very pure metal.

For technological reasons, it is usually necessary to place a stabilizing layer of |high-purity copper in the matrix of the initial semi-finished product at the beginning of the entire production process, i.e. before extrusion. Because during technological processing and especially

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- 2 during the final diffusion annealing, the required properties of the copper stabilization layer inevitably become completely degraded - by the diffusion of tin from the bronze matrix - the copper is separated from the bronze matrix by a so-called barrier, formed by a metal that effectively prevents the diffusion of tin. Tantalum is most often used for this purpose. The introduction of this barrier into the matrix of the initial semi-finished product not only brings with it technological problems and complicates the production process, but in addition, this barrier occupies a certain proportion of the effective cross-section of the superconducting conductor.

The above-mentioned shortcomings are eliminated by the method of treating the initial semi-finished products of multi-filament superconducting conductors Nb^Sn according to the present invention, intended for the production of superconducting magnets with high parameters, especially for the area of magnetic induction above 8 T. The essence of the invention lies in the fact that a niobium tube, the wall thickness of which is 0.05 to 0.3 of its diameter, is filled with tin bronze or pure copper. The volume fraction of all niobium tubes with filling is 20 to 50% of the volume cross-section of the matrix.

The method according to the invention solves the problem of a complete semi-finished product of a stabilized multi-filament superconducting conductor Nb^Sn, containing in addition to the basic matrix of tin bronze and niobium fibers also a proportion of stabilizing copper so that the initial niobium semi-finished products will be formed by tubes, which will simultaneously have the function of a barrier, separating the two other components, tin bronze and swords from each other. The method is aimed at increasing the cross-sectional efficiency of the superconducting conductor, i.e. to a relative increase in the critical current density, related to the entire cross-section of the conductor. The solution is based on the physical metallurgical and technological principles of multi-filament superconducting conductors based on niobium - tin, both for the purpose when the Nb^Sn fiber contains a niobium core in the final form, and for the arrangement when the superconducting conductor needs to be stabilized with a layer of pure copper and then separated from the basic matrix of tin bronze. Another advantage of the invention is that by omitting the tantalum barrier, the manufacturing process is reduced in cost and significantly simplified.

Example

An example of an embodiment according to the present invention are types of multi-filament superconducting Nb^Sn conductors, listed with their basic parameters in the following table under points B and C. For comparison of properties, three types of conductors with the same diameter of 0.5 mm in three alternative embodiments are presented here:

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A - classic stabilization method using a tantalum barrier/

B - by the method of niobium tubes with a bronze core in a copper tube/

C - by the method of niobium tubes with a copper core in a bronze matrix

In all the types listed, the same cross-sectional ratio of bronze to niobium of 2:1 is assumed. In types B and C, the same wall thickness of the niobium tube of 2 yam is used, which is half the diameter of the fibers of the type A conductor.

Average Average Share components Type^ cores pipes Number in the cross-section of the conductor conductors (Nb, Cu, ( Nb ) fibers / % / Cu-Sn) / /im / / /im / That Cu Cu-Sn Nb A . 4 -- 2821 12 33.34 36.4 18.2 B 17.8 21.8 351 ^a·» 33.34 44.4 22.2 C 5 9 1295 — 13 58 29 of the said share components in cross-section.of.the.conductor it follows, that the conductor type

B can be produced with the same proportion of copper to the total cross-section of the conductor as in type A. The saved proportion of cross-sectional area for the tantalum barrier is used to increase the proportion of niobium and the corresponding proportion of bronze in the cross-section of the conductor. The slightly increased labor intensity of this type of conductor is balanced by the fact that the number of fibers in the conductor can be lower with a higher and more effectively reacted niobium cross-section. The copper matrix of the type B conductor is in direct contact with the fibers; this increases the effect of dynamic stabilization and better protects the conductor during the transition to the normal state. The type B conductor is therefore advantageous for the construction of DC superconducting magnets for strong fields.

In a type C conductor, it is possible to significantly reduce the ratio of the copper cross-section to the total cross-section of the conductor to the extreme limit necessary to protect the conductor during the transition from the normal state. The obtained cross-section is again used to increase the proportion of niobium and the proportional proportion of bronze in the cross-section of the conductor. In this case, it is possible to achieve thin fibers in order to reduce losses during slow, time-varying processes. The bronze matrix, with its relatively high resistivity, simultaneously suppresses eddy currents. A type C conductor is advantageous for the construction of superconducting magnets with a time-varying strong magnetic field.

The increased cross-section of niobium and the superconducting Nb^Sn phase formed after the reaction mean a higher critical current/ and thus a higher average critical current density in type 3 and C conductors than is possible

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- 4 to achieve in type A conductors of the classic design with a tantalum barrier.

Both type B and C conductors can be twisted, similar to how Nb^Sn multi-strand superconducting conductors in the classical design are shortened, to limit eddy currents induced by extraneous, time-varying fields.

The wall thickness of the niobium tubes cannot be reduced too much so that the superconducting Nb^Sn layer formed during the final operation - reactive diffusion annealing - reaches the required size and also so that tin does not diffuse through this wall into the copper; it is obvious that the proportion of the component that will be placed in the tube is limited.

Therefore, depending on the required proportion of stabilizing copper in the cross-section of the entire superconducting conductor, two variants can be chosen: in the case of a requirement for a lower proportion of stabilizing copper, this is placed as a core in niobium tubes, the remaining matrix is formed by tin bronze; otherwise, the niobium tubes are filled with tin bronze and the pure copper forms the basic matrix into which the niobium tubes are placed. To achieve the effective cross-section of the resulting superconducting layer Nb^Sn relative to the basic matrix and with regard to the technological and physical metallurgical conditions of the production process, the thickness of the niobium tubes used is 0.05 to 0.3 of their diameter and the proportion of these semi-finished products (tubes filled with bronze or copper inside) is 20% to 50% of the total cross-section of the matrix.

For the production of conductors intended for DC superconducting magnets, starting semi-finished products are used - niobium tubes filled with tin bronze.

For the purpose of producing conductors intended for superconducting magnets with time-varying fields, starting semi-finished products - niobium tubes filled with copper - are preferably used.

Claims (1)

SUBJECT OF THE INVENTION 231 677 - Method for treating precursors of niobium tin multi-stranded superconducting conductors made of tin bronze, niobium and pure copper by extrusion, characterized in that the niobium tube, the wall thickness of which is 0.05 to 0.3 mm in diameter, is filled with tin bronze or pure copper, the cross-sectional volume of all filler niobium pipes being 20 to 50% by volume of the matrix cross-section.