RU2182736C2 - Composite superconductor manufacturing process - Google Patents

Abstract

FIELD: electrical engineering;. SUBSTANCE: proposed process for manufacturing Nb₃Sn, compound based composite superconductors designed for devices operating in high-strength (over 10 T) magnetic fields at high current densities and low hysteresis losses includes following operations: formation of primary composite blank bar that has external fabric sheath and axial cylindrical niobium block incorporating longitudinally arranged doping element; deformation of primary composite blank bar with intermediate heat treatment until hexahedral-section bar is attained; cutting of hexahedral-section bar into metered lengths; reassembly into case made of copper alloy or copper doped with other conductor elements such as diffusion barrier of tantalum or niobium during this stage of process; deformation with intermediate heat treatment until desired wire diameter is attained; diffusion heat treatment at 600-750 C until superconducting Nb₃Sn compound is attained. Process is characterized in that for softening tantalum or niobium diffusion barriers composite superconductor is given intermediate heat treatment after every 50-99% of cold deformation at 600-795 C with external sheath of conductor heated up through 1 mm of its diameter within 90-5 s and exposed to this temperature for 3-10 s. EFFECT: provision for avoiding breakage of conductors due to softening of diffusion barriers. 2 cl, 2 dwg, 2 tbl

Description

 The invention relates to the field of electrical engineering and can be used in devices primarily designed to operate in magnetic fields above 10 T at high current densities and low hysteresis losses.

A known method of producing a composite superconductor based on an intermetallic compound Nb ₃ Sn ("bronze" technology), including the formation of a workpiece, an outer shell in the form of a bronze pipe, a niobium rod placed in it, deformation of the obtained composite with intermediate heat treatments to the required cross section, cutting of the formed wire on individual rods and further formation of the composite the required number of times by placing the rods in the outer shell in the form of a bronze pipe, deformations preparing a preform with intermediate heat treatments to a final wire diameter of the required size and performing final diffusion heat treatment to form the Nb ₃ Sn / 1 / compound.

The conductors obtained by this method have high values of the critical current, critical temperature, and upper critical field, but with an unsatisfactory degree of stabilization associated with low thermal conductivity and high matrix resistance. In the case of large magnetic systems, the introduction of an additional stabilizing metal is required, in most cases which is high-purity copper, which in turn requires the introduction of a tantalum barrier, and often a combined tantalum and niobium barrier, preventing the diffusion of tin atoms into stabilizing copper. In this case, the conductor is a multicomponent system of metals and alloys with various mechanical properties, and some of them at the same time have limited ductility. Typically, in such wires designed to operate in medium and high magnetic fields, up to five different metals and alloys are used (copper, niobium, tantalum, BrO13, NT-50), while some elements of the wire from the same material have different configurations : for example, a diffusion barrier (actually a thin-walled tube) and fibers are made of niobium. Moreover, in the process of manufacturing the wire, the structural changes occurring at the border of their contact with other materials are completely different. At the interface between the niobium diffusion barrier and copper, there may be a solid solution based on copper (α-phase) and a solid solution based on niobium (β-phase); on the border of niobium with the tantalum barrier - only a solid solution. At the boundary of the niobium fiber with the bronze matrix, the formation of the intermetallic compound Nb ₃ Sn is possible. Therefore, the thermomechanical processing modes for all of the listed materials cannot be the same, and first of all, the modes that correspond to the most non-plastic materials, in which case bronze is considered and applied, are considered and applied.

 During processing, bronze is much faster from the listed components of the structure undergoes mechanical hardening, its plastic properties drop to one percent or less. The combination of high strength and low ductility leads to the fact that the wire begins to break. As a result, there is a need for multiple annealing.

There is also known a method of manufacturing a stabilized composite superconductor based on Nb ₃ Sn, selected as a prototype, comprising forming a primary composite billet containing an outer shell of matrix material and an axial cylindrical block of niobium containing a longitudinally located alloying component, deformation with intermediate heat treatments at 450 ÷ 550 ^o C. primary composite preform to obtain a hexagonal rod, cutting hexagonal rod to length, reassembly in Cases of copper with the introduction of the diffusion barrier of tantalum or the combination of tantalum and niobium, deformation with intermediate heat treatments at 450 ÷ 550 ^o C to a final diameter of the wire and conducting diffusion heat treatment at 600 ÷ 750 ^o C, while the possibility of providing a joint deformation of a multicomponent composite superconductor system at all stages of manufacturing, to achieve the size of the finished wire is ensured by conducting intermediate anneals every 18 ÷ 25% cold deformation to a wire size of ~ 6 , 4 mm and after 30 ÷ 50% below this size for 1 hour at a temperature of 500 ^o C [2]. Annealing is usually carried out in batch furnaces, for example, in vacuum furnaces or chamber furnaces with a neutral atmosphere.

 The disadvantages of this method are the difficulties in ensuring a large construction length associated with the destruction of barrier materials from refractory metals at the final stage of production. The reason for this is the various cyclic hardening of copper and bronze, which constitute significant volumes of the conductor.

 Barrier materials, occupying an intermediate position between the matrix and stabilizing materials, is a stress concentrator, has a significant degree of deformation at the final stage of manufacture, and experiences cyclic alternating stresses. The hardening of copper, bronze and tantalum in a composite wire during the cyclic alternation of cold deformation and intermediate annealing is shown in figure 1. A similar picture is observed in the case of a combined diffusion barrier composed of refractory metals of tantalum and niobium.

 An object of the present invention is to eliminate wire breakage associated with the destruction of the diffusion barrier during the cold deformation of the composite.

The essence of the invention lies in the fact that the method of manufacturing a composite superconductor based on Nb ₃ Sn, selected as a prototype, comprising forming a primary composite billet containing an outer shell of matrix material and an axial cylindrical block of niobium containing a longitudinally located alloying component, deformation with intermediate heat treatments at 450 ÷ 550 ^o C. primary composite preform to obtain a hexagonal rod, cutting hexagonal rod to length, repeated charge y in the cases of copper and introducing a diffusion barrier of tantalum or the combination of tantalum and niobium, deformation with intermediate heat treatments at 450 ÷ 550 ^o C to a final diameter of the wire and conducting diffusion heat treatment at 600 ÷ 750 ^o C, enables the joint deformation of a multi-component system composite superconductor at all stages of manufacture to achieve the size of the finished wire due to the softening of the diffusion barriers of tantalum or niobium by introducing additional intermediate te rmo-processing of a composite superconductor, which is carried out for passage through every 50 ÷ 99.5% of cold deformation at a temperature of from 700 to 795 ^o C provided that 1 mm of the outer sheath of the wire is heated in diameter for 60 ÷ 5 s and the exposure time at this temperature is from 3 to 10 s in continuous continuous furnaces.

Modern technologies for the manufacture of multifiber wires based on the Nb ₃ Sn compound involve the use of bronze with a maximum tin content. Usually, alloys from the region of a saturated α-solution are used, containing from 13 to 14.5% (by weight) of tin. The lower limit of the formation of the liquid phase in these alloys is 798 ^o C, therefore, the proposed temperature range should not cause fusion of bronze.

The process of formation of the Nb ₃ Sn phase at the bronze-niobium interface occurs in time and begins at temperatures above ~ 300 ÷ 350 ^o C. The rate of formation of the Nb ₃ Sn phase at 375 ^o C is approximately 2.17 • 10 ^-5 μm / min (0.13 μm per 100 hours). With increasing temperature, the phase growth rate noticeably increases, but even at a temperature of 750 ^o C it is only 5.56 • 10 ^-4 μm / min (1 μm for 30 hours). Considering the short duration of the additionally introduced anneals and their limited number, the released phase volume, if any, is so minimal that it will not have any noticeable effect on the ductility of the conductor.

 In the NbTi alloy, under the conditions of the above temperature range of short-term annealing, return processes and initial stages of polygonization occur, which are not associated with the decomposition of the solid solution and do not lead to a decrease in the ductility of the conductor.

 Changes in the strength properties of the combined Nb / Ta barrier of the MKNOS-7225 conductor during short-term high-temperature anneals depending on the temperature and annealing time are presented in Table 1.

 Table 1 shows a noticeable decrease in the strength of the combined Nb / Ta barrier in the composite wire during short-term high-temperature annealing, which ensures further cold deformation of the composite without violating the integrity of the barrier and, as a result, without destroying the wire itself.

 Short-term annealing for a conductor of large building length or mass can be provided only in continuous furnaces of a long type (for example, feed-through and induction furnaces), which can ensure due to their design features (changing the drawing speed) minimizing the time required to heat up the material to the annealing temperature and annealing, providing high stability characteristics of the wire.

 Short-term annealing can be carried out in a vacuum, using a neutral (argon, helium, nitrogen) or reducing (hydrogen) atmosphere.

Case Studies
In FIG. Figure 2 shows a superconducting wire based on an intermetallic compound Nb ₃ Sn with a diameter of 0.81 mm, including 7225 fibers, with a volume fraction of stabilizing copper of 60% (MKNOS-7225-0.81). The route of heat treatments carried out in a continuous furnace with a reducing atmosphere of the finally formed wire is presented in Table 2. According to this technology, a batch of wire was released with a total mass of more than 200 kg in the absence of wire breakage.

 In table 2, the technological route includes two short-term high-temperature annealing aimed at softening the combined Nb / Ta barrier.

A new technical result of the proposed method for manufacturing a composite superconductor based on an Nb ₃ Sn intermetallic compound is to eliminate wire breakage due to softening of diffusion barriers made of tantalum and niobium during short-term high-temperature annealing, not associated with the formation of an intermetallic compound Nb ₃ Sn and the decomposition of an NbTi alloy solid solution at intermediate stages of manufacture.

Sources of information
1. "Metallurgy and technology of superconducting materials." Ed. S. Foner and B. Schwartz. Per. from English M .: "Metallurgy", 1987, p. 282.

2. "Bronze-route Nb ₃ Sn superconducting wires with improved J _c and reduced bridging", Advanced in Cryogenic Engineering (Materials), Vol. 36, Edited by RP Reed and FR Fikett, Plenum Press, New York, 1990 (p. 139-146) (prototype).

Claims (2)

1. A method of manufacturing a composite superconductor based on the Nb ₃ Sn compound, comprising forming a primary composite billet containing an outer shell of matrix material and an axial cylindrical block of niobium containing a longitudinally located alloying component, cold deformation with intermediate heat treatments at 450-550 ^o C primary composite billet to obtain a hexagonal bar, cutting the hexagonal bar into measured lengths, assembling into copper covers with the introduction of a diffusion barrier from tantalum or co combined from tantalum and niobium, cold deformation with intermediate heat treatments at 450-550 ^o C to the final wire diameter and diffusion heat treatment at 600-750 ^o C, characterized in that after each 50-99.5% cold deformation, additional intermediate heat treatments are introduced to pass at a temperature of from 600 to 795 ^o C and the exposure time at this temperature from 3 to 10 s, provided that 1 mm of the outer sheath of the wire is heated in diameter for 90-5 s.  2. The method according to p. 1, characterized in that the additional intermediate heat treatment of the wire of the composite billet is carried out in the passage in continuous furnaces of a long type.

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