RU2088991C1 - Method of production of superconductor based on compound a-15 - Google Patents

Abstract

FIELD: devices intended for operation on alternating current and in quick-varying magnetic fields. SUBSTANCE: superconductor based on compound A-15 with an alloyed matrix is obtained by forming of composite, its deforming with conducting of intermediate annealings and final heat treatment for production of inter- metallic compounds. The alloying addition is selected from materials with a specific resistance of not lower than 8k10^-8 Ohm.m at +20 C in the form of rods evenly distributed over the volume of the composite at its assembly. EFFECT: simplified procedure and reduced cosy, enhanced specific resistance of matrix. 14 cl, 2 dwg

Description

 The invention relates to the field of electrical engineering and can be used in devices designed to operate on alternating current and in rapidly changing magnetic fields.

A known method for producing a composite superconductor based on an intermetallic compound A-15, for example, Nb ₃ Sn, including deformation to the required cross section with intermediate annealing of a workpiece containing a deformable bronze copper-tin matrix and niobium; further diffusion annealing of the obtained wire, as a result of which a composite superconductor is formed, which is a system of superconducting fibers based on Nb ₃ Sn in a bronze matrix (see UK patent N 1280583, H1K, H01V 11/00 05/05/72).

However, the superconductors thus obtained have an initial small value of the transverse electrical resistivity, which decreases during diffusion heat treatment due to depletion of the bronze matrix on tin, due to the formation of the Nb ₃ Sn compound on niobium fibers. Therefore, the use of these composites to create superconducting magnetic systems designed to operate in alternating fields is inefficient, since in a matrix with a low resistance in the direction transverse to the axis of the wire, an alternating magnetic field will induce transverse eddy currents flowing through the superconducting phase and closing through the matrix, providing thus the electrical connection between the superconducting fibers, which is a source of loss. In addition, Foucault eddy currents will be induced within the matrix material, which will lead to additional losses.

There is also known a method for producing a composite superconductor with an alloyed matrix material based on compound A-15, for example Nb ₃ Sn, comprising forming a composite containing niobium in a zinc alloyed deformable bronze copper-tin matrix, the alloying element being introduced into the charge composition, from which bronze is obtained alloy; then deformation of the composite to the required cross section with intermediate annealing; final heat treatment to form the Nb ₃ Sn intermetallic compound during the diffusion interaction between niobium and matrix material (see Wada H, Kimura M. Tachikawa K. Superconducting Properties of the Composit-processed Nb ₃ Sn Superconductor with the Cu-Sn- Zn Matrix J. Mater. Sci. 1978, v. 13, pp 1943-1950 "prototype").

The introduction of alloying additives in the copper-tin mixture ultimately contributes to the acceleration of the formation of Nb ₃ Sn intermetallic layers, increases critical characteristics, and improves the operational properties of composite superconductors.

 However, the existing alloying method does not allow to obtain a composite material with the highest possible electrical resistivity while maintaining high superconducting properties and the economical manufacture of conductors. This is due to the fact that alloying additives, which can significantly increase the value of the electrical resistivity of the matrix, have limited solubility in bronze in the solid state and lead to limited deformability of the composite due to the presence of precipitates of the second phases with increased hardness and low ductility. At the same time, it is difficult to process the composite by the methods of hot pressing and cold rolling or drawing, and it requires a large expenditure of energy and time for intermediate annealing, which is necessary to remove the hardening caused by the deformation of the composite; significant wire breakage and, accordingly, low yield.

 The aim of the invention is to increase the electrical resistivity of the matrix material while reducing cooperative losses when working on alternating current and in rapidly changing magnetic fields in a composite superconductor based on compound A-15, as well as simplifying the manufacture of the conductor.

This goal is achieved by the fact that in the known method of manufacturing a composite superconductor based on the compound A-15, for example Nb ₃ Sn, with a doped matrix material, which includes the steps of forming a composite by placing niobium in a doped bronze copper-tin matrix, deforming the composite to the required transverse section with an intermediate annealing, diffusion heat treating the resulting product to form a Nb ₃ Sn compound, the dopant which is carried out in the form of a rod s cylindrical, hexagonal or other shape, uniformly distributed over the volume of the composite workpiece during its assembly. The dopant is selected from materials with a specific electrical resistance at 20 ^o C not lower than 8 • 10 ^-8 Ohm • m and having a diffusion coefficient in the matrix material not lower than 10 ^-13 cm ² / s at a temperature of 600 800 ^o C, for example, from such like nickel, iron. Due to the diffusion of the alloying element from the rods into the matrix material, for example, bronze, copper 13 wt. tin, the volume of bronze is saturated with a partially soluble dopant, and the precipitates of the second phases (for example, the intermetallic non-ductile compound (Cu _1-y Ni _y ) ₂ Sn with a variable content of Ni and Cu depending on the distance to the Ni boundary (bronze than the closer, the more nickel) is not an obstacle, since the deformation limit of the conductor was completed earlier in the stages prior to diffusion heat treatment.

 A significant difference from the prototype is the exclusion of the alloying operation of the mixture, which allows to maximize the solubility of tin in bronze (13.5 wt. Or 7.7 at. Sn for bronze Cu-Sn), resulting in an increase in the current carrying capacity of the obtained superconductor, simplifies the manufacture of a bronze ingot.

 It is also important that a composite is subjected to deformation to the required cross section, the matrix component of which does not contain an alloying element dissolved in it, which reduces the number of intermediate anneals for the entire period of composite deformation, since the plasticity resource of bronze and the composite as a whole is increased; in addition, alloying inserts, for example of nickel, iron, have good deformability.

 The elimination of direct contact of the alloying insert with the bronze matrix, achieved by introducing a protective copper barrier around the insert, minimizes the diffusion interaction of the alloying material with the bronze matrix material, which can occur during the heating of the composite billet for pressing or during intermediate heat treatments.

 In FIG. 1 and FIG. 2 shows the cross-sections of wires with Ni alloy inserts, differing from each other in the geometry of the insert placement along the wire cross section. (Nb fibers not shown).

 Example 1

To obtain a composite superconductor based on Nb ₃ Sn with a matrix alloyed with nickel, the initial assembly of bimetal was carried out: the niobium rod was placed in bronze Cu-13 wt. Sn case is cylindrical in shape, and then the assembly was subjected to deformation by hot pressing and cold drawing with intermediate annealing and profiling on a hexagon with a key size of 6.3 mm. Subsequently, when assembling a 121-fiber billet, hexagonal composite niobium-bronze rods with a turn-key size S 6.3 mm in an amount of 108 pcs were again placed in bronze Cu 13 wt. Sn case, and 13 nickel rods of size S 6.3 mm were evenly distributed between niobium-bronze hexagons (Fig. 1). Then, the composite billet was deformed by hot pressing and cold drawing using intermediate heat treatments in the same way as in the case of a bimetallic billet.

The resulting hexagon (1) of size S 6.3 mm contained 108 niobium and 13 nickel fibers in a bronze matrix. Of 121 such hexagons, the third assembly was assembled again in Cu 13 wt. Sn a bronze case and the above method was turned into a wire with a diameter of 0.5 mm, containing 13068 niobium [2] and 1573 nickel fibers [3] in a bronze matrix [4] The finished wire is subjected to heat treatment at a temperature of 720 ^o C for 72 h, in the process of which there was a diffusion of nickel into the bronze matrix, and tin from the matrix into niobium fibers. As a result of this process, an Nb ₃ Sn compound was formed, as well as an intermetallic compound containing nickel, copper, and tin.

 Example 2

 The initial bimetallic composite was formed as in Example 1. Then, a 121-fiber composite preform was assembled, which contained 121 niobium-bronze composite inserts, profiled onto a hexagon with a key size S 6.3 mm and bronze Cu 13 wt. Sn case. This billet, as previously bimetallic, was turned into a hexagon with a key size S 6.3 mm (1).

Having taken 108 such hexagons, as well as 13 composite copper-nickel hexagons, S 6.3 mm in size, they were placed in bronze Cu-13 wt. Sn case, and the assembly obtained as described previously was turned into a wire with a diameter of 0.5 mm, containing 13068 niobium (2) fibers and 13 copper-nickel inserts (3) ((4) copper protective barrier) in a bronze matrix (5). The heat treatment of the conductor was carried out according to the regime of 750 ^o C 48 hours

Using the proposed method of alloying a material of a matrix of a superconductor based on compound A-15 provides the following advantages compared to the existing method (prototype):
a) the possibility of increasing the electrical resistivity of the matrix material while maintaining high superconducting properties of the conductor, which is especially important to reduce cooperative losses when the superconductor is operating on alternating current and in rapidly changing magnetic fields;
b) improving the deformability of the composite at all stages of the manufacture of the conductor, as well as simplifying the manufacture of the conductor, since doping of the matrix charge is excluded;
c) a decrease in the number of intermediate anneals for removing hardening from bronze, because due to the fact that the material of the composite matrix at all stages of the manufacture of the wire does not contain an alloying additive, the bronze plasticity resource is increased; for the same reason, wire breakage decreases, and the suitable output increases accordingly.

Claims (2)

1. A method of producing a superconductor based on compound A-15 with a doped matrix, comprising forming a composite, deforming it to a given cross section with intermediate annealing, and final heat treatment to form type A-15 intermetallic compounds, characterized in that, in order to increase the electrical resistivity the matrix material, as well as to simplify and reduce the cost of manufacturing, the dopant matrix is selected from materials having a resistivity not less than 8 • 10 ^{- 8} Ohm • m at +20 ^o C, coo wearing defined amount of additive material rods, distributed uniformly over the cross section of the composite, the number of which is determined by the diffusion coefficient of the dopant material and the bronze matrix during the final heat treatment parameters.  2. The method according to p. 1, characterized in that, in order to limit the diffusion interaction of the dopant material with the matrix material before the final heat treatment, the bar of dopant material is surrounded by a copper barrier.

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