RU2276417C1 - Method for producing long-measuring composite wire based on high-temperature superconducting compounds - Google Patents

Abstract

FIELD: production of long-measuring composite wires based on high-temperature superconducting compounds.

SUBSTANCE: proposed method includes formation of multiconductor billet by filling silver sheath with bismuth ceramic powder, deformation of multiconductor billet obtained to desired size by drawing without heating at deformation degree per pass ranging between 0.5 and 20%, cutting of deformed billet into metered sections, assembly of multiconductor billet by disposing desired number of metered parts of deformed multiconductor billet in sheath made of reinforced silver based alloy, extrusion of multiconductor billet at temperature ranging between 130 and 280 °C and drawing coefficient of 4 to 30, rolling in the open without heating at deformation ratio per pass between 1 and 50%, and thermomechanical treatment under preset conditions. Sequential compression of ceramic core raises density of critical current.

EFFECT: improved geometry of conductors, enhanced wire mechanical properties and resistance, reduced heat conduction of sheath.

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Description

The invention relates to the field of technical superconductivity, in particular to a technology for producing long composite multicore wires based on high-temperature superconducting (HTSC) compounds designed to create electrical products.

It is known that multicore wires based on HTSC compounds are obtained by the "powder in a pipe" method, which includes filling ceramic powder into a metal shell, deforming the obtained single-core preform to the required size, cutting it into measured parts, and assembling multi-core preform by placing the required amount of these measuring parts, deformation of a multicore blank and heat treatment in several stages with intermediate deformations between them (thermomechanical processing) [1]. In the case of filling in a metal shell, for example, ceramic powders, deformation is carried out in order to obtain the required wire size and the maximum possible sealing of the core before thermomechanical processing (TMT), which is carried out in order to form a superconducting phase in the ceramic core of the required composition and structure. When using ceramic powders, deformation is carried out by drawing and rolling, which do not allow reaching the required density of the ceramic core.

Also known are methods for producing wires based on HTSC compounds by the "powder in a pipe" method based on metal powders, however, when using metal powders, it is difficult to obtain a wire with a critical current density above 500 A / cm ² [2].

Closest to the proposed technical solution is a method for producing a stranded conductor [3] - a prototype, which includes obtaining a single-core workpiece by filling metal powder into a silver shell, extruding the obtained single-core workpiece to the required dimensions at a temperature of from 300 to 600 ° C and an extrusion coefficient of up to 800 , cutting a deformed workpiece into measured parts, assembling a multicore workpiece by placing in the silver shell of a multicore workpiece the required number of measured parts normed monozhilnoy preform multifilamentary extrusion billet at a temperature of from 300 to 600 ° C and a drawing ratio to the value of 800, rolling at a temperature of from 300 to 600 ° C in a controlled atmosphere (argon), oxidation, thermo-mechanical processing.

In the process of deformation by extrusion, the maximum possible core compaction of mono- and multicore preforms occurs when using the main deformation methods (drawing, rolling, extrusion) currently used, however, in the case of using metal powders after deformation, oxidation is carried out (metals are converted to oxides), at which core decompression, and TMT (the superconducting phase of the required composition and structure is already formed in a ceramic oxide core).

This method has several significant disadvantages:

- the use of metal powders requires the introduction of an oxidation operation of these powders before TMT, and this significantly complicates the process (an additional operation is introduced in a controlled atmosphere - oxidation of the core with oxygen diffusing through the conductor shell, but during which core softening also occurs); in addition, the difficulties in obtaining a stoichiometric superconducting compound in the core are obvious, and with TMT, there are additional difficulties in obtaining the required core structure, which leads to a significant decrease in the critical current density;

- extrusion at high temperatures (from 300 to 600 ° C) with large drawing ratios - up to 800 significantly complicates the process; extrusion can take place with a vertical and horizontal arrangement of the workpiece and the resulting wire, in both cases, when extruding large diameter workpieces with large drawing coefficients, it is necessary to provide equipment for receiving wires with high speeds, which are determined by the speed of the press equipment,

- conducting warm rolling at high temperature (from 300 to 600 ° C) in a controlled atmosphere (argon) also complicates the process and reduces its safety,

- the use of a silver sheath of multicore blanks does not allow to increase the mechanical properties of wires and electrical resistance of the sheath of multicore wires, which narrows the field of use of wires.

An object of the invention is to increase the critical current density by sequentially (from operation to operation) sealing the ceramic core and improving the geometry of the cores, increasing the mechanical properties of the wire and the electrical resistance of the sheath, reducing the thermal conductivity of the sheath and simplifying the method.

The problem is solved in that in the prototype method, which includes filling the powder into the silver shell of the core workpiece, deformation of the obtained core workpiece to the required dimensions, cutting the deformed workpiece into measured parts, assembling the multi-core workpiece by placing the required number of measured parts of the deformed core workpiece in a metal shell stranded billet, extrusion, rolling and HMT, the following is proposed: powder of bismuth ceram is poured into the shell of a single-stranded billet They deform the single-core workpiece by drawing at room temperature, that is, without heating, with a degree of deformation per pass of 0.5 to 20%, the multi-core workpiece is assembled by placing measured parts of the deformed single-core workpiece in a shell made of a hardened silver-based alloy, extrusion the obtained stranded billet is carried out at a temperature in the range from 130 to 280 ° C and with a draw coefficient of 4 to 30, rolling is carried out at room temperature in air with a degree of deformation of 1 pass up to 50%, after which thermomechanical treatment is carried out, including several stages of heat treatment at a temperature of from 820 to 850 ° C, for a time that ensures the formation of a superconducting phase in the ceramic core of the required composition and structure, with intermediate deformations between the stages of heat treatment with a degree of deformation per passage from 5 to 30%.

In the process of these operations, a sequential compaction of a multicore long wire occurs, the geometry of the cores improves, which ensures an increase in the critical current of the wire. The resulting wire also has improved mechanical properties and increased sheath electrical resistance.

Filling ceramic powder into the sheath makes it possible to obtain material close to the superconducting chemical composition in the core of the wire already at the initial stage of the wire. In the process of deformations (drawing, extrusion) and TMT, the ceramic core gradually consolidates. In the case of the use of metal powders, similar processes occur in the core, however, during oxidation to obtain material close to superconducting in the chemical composition of the wire core, significant deconsolidation of the core occurs (evident when oxygen passes into the core through a wire sheath with a thickness of 0.4 up to 0.5 mm). After oxidation, the compaction of the ceramic core already occurs only during thermal treatment, which includes, as a rule, only a few (2-3, and at most up to 4) intermediate deformations, which is insufficient for the required compaction of the ceramic core, and an increase in the number of intermediate deformations at TMT is impractical due to the violation of the structure, texture of the ceramic core and the geometry of the wire. This is one of the main reasons for the low critical currents of wires based on metal powders.

The deformation obtained at the previous stage of the single-core workpiece by drawing at room temperature with a degree of deformation per pass of 0.5 to 20% ensures the production of a single-core wire with a compacted ceramic core of the required shape and size, which greatly simplifies the process, makes it more stable due to the lack of significant temperature gradient and safe.

The use of a multicore preform of a silver-based alloy as a shell provides an increase in the mechanical properties of the wires and electrical resistance of the shell (in order to prevent current flowing through it, leading to local overheating of the conductor and its exit from the superconducting state), as well as reducing the thermal conductivity of the shell. The latter is necessary to reduce heat influx into the helium temperature zone (4.2 K) when using HTSC materials as current leads operating in a temperature gradient of 4.2 K - 77 K.

The deformation of a multi-strand workpiece by extrusion at a temperature of 130 to 280 ° C and an extrusion coefficient of 4 to 30 greatly simplifies the process, makes it stable (the temperature gradient decreases significantly), safe, and provides a multi-strand long wire with a ceramic core close in chemical composition to superconducting material, the desired shape and size. In addition, upon deformation of a multi-core preform by extrusion, further densification of the ceramic core also occurs. Moreover, the extrusion at a temperature of from 130 to 280 ° C, that is, lower than in the prototype, allows to obtain wires from a stranded billet in a shell made of a hardened alloy based on silver, assembled from mono-core in a shell made of silver.

With a decrease in the coefficient of drawing from 800 to 4-30, the probability of disturbance in the geometry of the cores sharply decreases, which subsequently favors an increase in the critical current.

Rolling without heating in air and with a degree of deformation per pass of 1 to 50% provides wires of the desired shape and size (for example, flat), mainly in thickness, with the required geometry of the core and greatly simplifies the process compared to rolling at a temperature of 300 up to 600 ° C in a controlled atmosphere. In addition, during rolling, further core compaction occurs.

TMT, which includes several stages of heat treatment at a temperature of from 820 to 850 ° C with intermediate strains between them with a degree of deformation per pass of 5 to 30%, provides further core compaction and the formation of a superconducting phase of the required composition and structure in it, which makes it possible to obtain a superconducting wire with high current-carrying characteristics.

During deformation of a single-core workpiece by drawing with a degree of deformation per pass of less than 0.5%, the geometric dimensions of the wire are violated, a wave-length along the length of the wire appears, and when drawing with a degree of deformation per pass of more than 20%, the integrity of the sheath occurs, resulting in the formation of small cracks and growth until the complete destruction of the sheath, which leads to rupture of the wire.

Extrusion at temperatures below 130 ° C upon receipt of a wire from a single-core workpiece in a silver sheath and a multi-core workpiece in a sheath of a hardened silver-based alloy leads to cracking of the workpiece up to the integrity of the ceramic cores due to a decrease in the ductility of the sheath material.

When the extrusion temperature is increased above 280 ° C, the geometry of the ceramic cores is violated due to a decrease in the strength characteristics of the sheath material - ceramic cores are thinned in some places along the length of the core and thickening of ceramic cores in other places along the length of the core.

Extrusion with a drawing coefficient less than 4 is not enough and requires an increase in the number of extrusion operations and, consequently, an increase in the total time of deformation of the stranded workpiece to the required size. Extrusion with a coefficient of extrusion of more than 30 at selected temperatures leads to a violation of the geometry of ceramic cores associated with a difference in the mechanical properties of extrudable materials, which has a significant effect on the deformation of materials at high degrees of deformation.

Carrying out rolling with heating, that is, at a temperature above room temperature, is impractical, since the material with the ceramic core, which is at this stage in the form of a powder (in the prototype, a metal core) undergoes deformation. In addition, on the one hand, with the used degrees of deformation per pass (from 1 to 50%), there is no need to conduct deformation with heating in order to increase the ductility of the rolled materials (as in the prototype method), on the other hand, an increase in the rolling temperature can lead to an increase in ductility of only the sheath and a violation of the geometry of ceramic cores due to a decrease in the strength characteristics of the material of the sheath, this can lead to thinning of ceramic cores in some places along the length of the core and thickening of ceramic cores to each other × locations along the length of wires, that always leads to a decrease in the critical current.

When rolling with a degree of deformation per pass of less than 1%, the geometric dimensions of the wire are violated, a wave-length appears along the length of the wire, and when rolling with a degree of deformation per pass of more than 50%, the sheath ruptures: from small cracks to its complete destruction, which leads to rupture of the wire .

Carrying out TMT at a temperature below 820 ° C and above 850 ° C and a degree of deformation per pass of less than 5% and more than 30% does not allow the formation of a superconducting phase of the required composition and structure in the ceramic core, in particular with a degree of deformation per pass of less than 5% at intermediate deformation does not occur in the laying of crystallites that make up the ceramic core, in the desired direction - the direction of the predominant current flow, and when the degree of deformation per pass is more than 30%, the geometry of the ceramic core . With a decrease in the temperature of the thermo-magnetic material below 820 ° C, the formation of a superconducting phase in the ceramic core does not occur. With an increase in the temperature of TMT above 850 ° C, a large amount of the liquid phase forms, which flows out of the shell (for example, through pores and microcracks), which leads to a violation of the integrity of the shell, violation of the stoichiometry of the ceramic core, and a sharp deterioration in the critical characteristics of the superconductor.

Carrying out these operations in the described sequence and under the indicated modes led to a new technical result: an increase in the critical current density due to the sequential compaction of the ceramic core, improved core geometry, increased mechanical properties of the wire and electrical resistance, reduced thermal conductivity of the sheath, and simplification of the method.

An example implementation. Silver metal ampoules (tubes 1000 mm long, 10 mm in diameter with a wall thickness of 1 mm - shells of monogin blanks) were filled with bismuth ceramic powder (Bi-2223) based on a final fill factor of 25 mm wire core. Further, the obtained single-core preforms were deformed by drawing at room temperature with a degree of deformation per pass of 10%, after which multicore preforms were formed by placing multicore preforms from the shells of the hardened alloy Ag + 1.1% weight. Sn dimensional parts of deformed single-core blanks in an Ag shell. As the shells of multicore blanks used pipes made of hardened alloy Ag + 1.1% weight. Sn (diameter 16 mm with a wall thickness of 1 mm, length 50 mm). In each of the shells of a multicore blank with a diameter of 16 mm from a hardened alloy Ag + 1.1% weight. Sn was placed in 217 measured parts of the deformed single-core blanks in a silver shell with a diameter of 0.82 mm. Further, multi-stranded blanks Ag (sheath of a single-core) - Ag + 1.1% weight. Sn (sheath of a multicore preform) was extruded with an extrusion coefficient of 4 and 30 at temperatures of 130 and 280 ° C. Then, all the materials obtained after extrusion were rolled without heating in air with a degree of deformation per pass of 15%. After that, on all the wires obtained, TMT was carried out in two stages at temperatures of 820 ° C and 850 ° C for a total time of 200 hours with intermediate rolling with a degree of deformation per pass of 12% to the final thickness of the wires based on (Bi-2223): 0, 2-0.3 mm.

The critical current in the wires was measured by the standard four-point method according to the criterion of 1 μV / cm.

On all the wires obtained by the proposed method, the critical current density (critical current, referred to the area of the superconducting core) is not less than 10 times higher than on the best wires obtained using metal powder, and not less than 5.5% higher than on wires obtained on the basis of ceramic powders without the use of extrusion, which characterizes the advantage of the proposed method.

Information sources

1. P. Haldar, L. Motovidlo. Processing High Critical Current Density Bi-2223 Wires and Tapes. The Journal of The Minerals and Materials Society (JOM), vol. 44, No. 10, October 1992, p. 54-58.

2. W. Gao, S.-C. Li et al. Synthesis of Bi-Pb-Sr-Ca-Cu Oxide / Silver Superconducting microcomposites by Oxidation of Metallic Precursors, Physica C 161 (1989), 71-75.

3. C.L. H. Thieme, D. Daty et al. High Strain Warm Extrusion and Warm Rolling of Multiflamentary Bi-2223 Metallic Precursor Wire. Advances in Cryogenic Engineering (Materials), vol. 44, Edited by Balachandran et al., Plenum Press, New York, 1998, pp. 533-540 is a prototype.

Claims (1)

A method for producing a lengthy composite wire based on high-temperature superconducting compounds, including forming a single-core workpiece by pouring powder into a silver shell, deforming it to the required dimensions, and cutting into measured parts, assembling a multi-core workpiece from the obtained measured parts by placing them in a metal shell, extruding it, rolling and thermomechanical treatment, characterized in that powder of bismuth ceramics is poured into the shell of the single-core workpiece, deformation of the single-core the billet is carried out by drawing without heating with a degree of deformation per pass from 0.5 to 20%, a hardened alloy based on silver is used as the shell material of a multicore blank, extrusion is carried out at a temperature of from 130 to 280 ° C and with a drawing coefficient from 4 to 30, rolling is carried out without heating with a degree of deformation per pass from 1 to 50%, thermomechanical processing is carried out in several stages at a temperature of from 820 to 850 ° C with intermediate strains with a degree of deformation per pass from 5 to 30%.