RU2124775C1 - Method for producing long high-temperature superconducting parts - Google Patents

Abstract

FIELD: electrical engineering; manufacture of long stranded superconductors. SUBSTANCE: method involves deformation of complex blank built up of sheath and desired number of gaging parts of cut deformed blank to obtain long conductor, its high-temperature treatment at 830-835 C for producing superconducting phase of desired composition and structure in conductor strands, followed by production of non-homogeneous insulating coat of high oxygen permeability on conductor sheath by covering sheath surface with organometallic composition of metal concentration of 25-40 g/l metal mixture followed by low-temperature heat treatment at 350-400 C; then product is wound and subjected to additional heat treatment at 835-840 C. Critical properties of parts obtained don not need further correction of design. EFFECT: enlarged functional capabilities of products obtained. 1 dwg, 1 tbl, 1 ex

Description

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

Conductors based on high-temperature superconducting compounds are obtained by the method of "powder in a pipe", which consists in filling a metal shell (pipe) with ceramic powder of a high-temperature superconducting compound or semi-finished product, deformation of the obtained workpiece to the required dimensions, during which the ceramic core is densified, and high-temperature heat treatment is carried out for the formation in the core of the superconducting phase of the required composition without structural defects (cracks, pores, etc.), reducing critical properties, for example, I _k , critical current.

 The use of high-temperature superconductors in various electrical products, such as magnetic coils, implies their bending deformation (winding, twisting, etc.), which should not lead to a drop in critical properties. In addition, in the manufacture of products, it is necessary to use electrical insulation, for example, between coil turns [1, 2].

 In the manufacture of high-temperature superconducting products, braiding materials, for example, from aluminum oxide or electrical insulation coatings, obtained by applying ceramic powder mixtures with an organic binder to the subsequent burning of organics, or using other electrical insulation materials and coatings, are used as electrical insulation. However, braided aluminum oxide materials (and others, laid between the turns of the coil) provide only mechanical contact with the conductor shell, which can impede heat transfer between the cryogenic medium and the superconductor, and also have a large thickness of about 100 μm, which significantly increases the dimensions of the electrical device. In addition, these insulating materials suggest their one-time use, that is, when the number of coil turns changes, they are destroyed, which sharply reduces the reliability of the insulation. Coatings obtained by applying ceramic powder mixtures with an organic binder to the conductor sheath followed by burning out the organics have a significant thickness (more than 20-40 microns), uneven thickness and, most importantly, low adhesion to the conductor sheath, which makes it difficult to adjust the design electrical products.

Closest to the proposed technical solution is the "Method for producing long conductors based on high-temperature superconducting compounds" [3], including the formation of the workpiece in the form of a sealed sheath of metal or alloy, filling it with powder of the superconducting compound or semi-finished product, deformation of the obtained workpiece to the required size, application on the shell of a blank of a solution of an organometallic compound based on carboxylic acids containing zirconium, aluminum, yttrium (concentrations of metal or a mixture of metals - 50 g / l), low-temperature heat treatment in air or inert gas in the temperature range 400-650 ^o C, repetition of the cycle: applying a solution of organometallic compounds, low-temperature heat treatment the required number of times, each time from a solution containing various metals, product winding and high-temperature heat treatment at 840 ^o C. As a result of the described operations, the product (magnetic coil) is obtained from a conductor in an insulating coating. Such an insulating coating has a small thickness, reliably insulates the conductor sheath uniformly in thickness, and has increased adhesion to the sheath.

The disadvantage of the prototype method is the impossibility of adjusting the design of the product, for example, changing the number of turns of the coil, since at 840 ^° C a structure is formed in the thick single core ceramic core that practically does not allow the conductor to bend.

 It is obvious that if a conductor is subjected to deformation in an insulating coating, then the coating must also withstand the maximum possible deformation.

It should be noted that changing the place of high-temperature heat treatment or its mode, for example, carrying it out at a lower temperature immediately after deformation, does not unambiguously solve the problem of obtaining a product with an adjustable design with high critical properties. By changing the place and mode of high-temperature heat treatment, it is possible to adjust the product design, but it is not always possible to obtain the same critical properties as on the product obtained without structural adjustment, that is, when high-temperature heat treatment is carried out at a higher temperature at the final stage of the technology. This is due to the fact that no matter how resistant conductors to bending deformations based on high-temperature superconducting compounds are, these are metal-ceramic composite materials, and if the critical bending radius is slightly exceeded (when winding electrical products from such conductors or adjusting their design), critical characteristics (for example, I _to - critical current).

 Therefore, it is advisable, for example, to obtain a conductor using the “powder in pipe” method, conduct high-temperature heat treatment, cover the conductor with electrical insulation, wind the coil (solenoid), if necessary, adjust the number of turns in the coil and if the critical bending radius is exceeded when adjusting or winding the product , be able to conduct additional high-temperature heat treatment to "heal" cracks formed in the ceramic core of the conductor.

 For this, it is necessary to have an insulating coating having both a small thickness while maintaining the insulating properties, uniformity along the length of the conductor, good adhesion to its shell, and increased permeability to oxygen, since high-temperature heat treatment involves the presence of oxygen exchange between the heat-treating medium and the ceramic core. In addition, such a coating must withstand the maximum possible bending deformation.

The requirement of increased oxygen permeability is especially important for multicore conductors, which, due to the large number of cores, for example 19, 37, 61, 703, are more resistant to single-core bending deformations without loss of critical properties. Naturally, for multicore conductors (for example, 703 cores with respect to 1), significant permeability of the insulating coating for oxygen is required during high-temperature heat treatment carried out on an insulated conductor. The technical task of this invention was to increase the permeability of the insulating coating with oxygen, to increase its resistance to bending deformations while maintaining insulating properties, small thickness, uniformity along the length of the conductor and the required adhesion to its shell on a multicore long-length conductor, resistant to deformations (due to the structure of the ceramic core) to bend, which allows to obtain products on the basis of such a conductor, to correct their design and increase I _to by products adjusted to design to the maximum values of I _to obtained on products made without structural adjustment.

The problem is solved in that in the prototype method, which includes forming a preform in the form of a metal shell, filling it with powder of a superconducting compound or a semi-finished product, deforming the resulting preform to the required size, applying a solution of an organometallic compound based on carboxylic acids containing zirconium, aluminum to the surface of the shell , yttrium, winding and heat treatment, after the workpiece is deformed to the required dimensions, it is cut into measured parts and a complex workpiece is formed, for which placed the required number of deformed portions dimensional preform in a metal sheath is deformed complex preform to the desired dimensions, the high-temperature heat treatment is carried out at 830-835 ^o C, then the complex envelope blanks applied organometallic compound solution on the basis of carboxylic acids comprising zirconium, aluminum, yttrium, the concentration of the metal, metal mixture of 25-40 g / l, low temperature heat treatment is carried out at a temperature of 350-400 ^o C, and then make the winding products and spend additional th high-temperature heat treatment at a temperature of 835-840 ^o C.

Obtaining a multicore conductor allows (in comparison with a single-core conductor, the prototype method) to use large bending deformations during winding of the product without violating the integrity of the ceramic core and creating structural defects in it (for example, cracks) that cannot be healed during subsequent high-temperature heat treatment. The application of a solution of an organometallic compound with a concentration of metal, a mixture of metals 25-40 g / l in combination with low-temperature heat treatment at 350-400 ^o C allows you to get an amorphous insulation coating from non-stoichiometric oxides of the metals used or their mixture, which, along with a small thickness while maintaining the insulating properties uniformity along the length of the conductor required by adhesion to the shell of the latter, a new quality - increased (relative to the prototype method) oxygen permeability, providing acid tional multifilamentary exchange between the ceramic core and the high-temperature annealing medium. In addition, such a coating has an inhomogeneous structure and is able to relax the stresses arising from bending of the conductor into the heterogeneity of the structure (discontinuities, pores, cracks, channels) in the coating.

The formation of a non-homogeneous coating with increased oxygen permeability during low-temperature annealing occurs due to the thermal decomposition (pyrolysis) of metal-containing organic compounds (carboxylates, Me (COOH) _n , where, for example, Me - zirconium, aluminum, yttrium) in accordance with the reaction
Me (COOH) _n -t ---> Me _x O _y + CO ₂ + H ₂ O
with the formation on the conductor shell of a thin (0.1-0.3 μm) film of amorphous non-stoichiometric metal oxide. The resulting oxide non-conductive coating has an adjustable thickness of 0.1-6 μm (depending on the number of cycles) while maintaining electrical insulation properties and satisfactory contact between the coating and the conductor, in addition, it has a new property with respect to the prototype — the inhomogeneity of the structure due to the inhomogeneity of the structure of non-stoichiometric oxides obtained, i.e., the presence of heterogeneity (discontinuities, pores, cracks, channels permeable to oxygen, into which stress relaxation occurs).

Comparison of the proposed method with the prototype method shows that the distinctive features of this method are: high-temperature heat treatment at a temperature of 830-835 ^o C after deformation of a complex workpiece obtained by placing measured parts of a cut deformed workpiece in a metal or alloy shell, the use of solutions of organometallic compounds metal concentration of 25-40 g / l, followed by low-temperature heat treatment at a temperature of 350-400 ^o C and conducting the integral stage of the additional high-temperature heat treatment at a temperature of 835-840 ^o C.

Carrying out these operations in the described sequence led to the appearance of a new technical result: a new structure of a multicore ceramic core providing reliable contact between crystallites and resistance to bending deformations, and a new structure of an insulation coating providing reliable insulation, small thickness, uniformity in length, and required adhesion to the sheath and increased resistance to bending deformations (the critical bending radius of a multicore conductor in an insulator decreases by more than 10 times coating), as well as the increased permeability of such a coating for oxygen, which expands the scope of use of the conductor and makes it possible to adjust the design of the products over a wide range while maintaining the I _k in the product of the adjusted design at level I _{to the} product made without adjustment.

 Example.

Using the powder in pipe method, a composite billet was obtained: a shell (silver pipe) filled with Bi-2223 bismuth ceramic powder, which was deformed by drawing to a diameter of 1.18 mm, then cut into 19 parts each 2 m long and a complex billet was formed, why 19 measured parts of the deformed billet were placed in a silver pipe with a diameter of 8 mm with a wall thickness of 0.8 mm, then the resulting complex billet was deformed from a diameter of 8 mm to a diameter of 3.01 mm by drawing with a degree of deformation of 5% per pass. Further deformation from a diameter of 3.01 mm to a thickness of 0.25 mm was carried out by flat rolling with a degree of deformation of 5% per pass. As a result of the operations described above, a flat stranded conductor with a length of more than 50 m was obtained. Then, the resulting conductor was cut into 8.5 m pieces each and processed according to three schemes: coil winding - high temperature heat treatment at 840 ^° C (1), high temperature heat treatment at 832 ^° C C - coil winding (2), high temperature heat treatment at 832 ^o C - coil winding - additional high temperature heat treatment at 837 ^o C (3).

Using all three schemes, electrical insulation was applied by coating the conductor sheath with a solution of an organometallic compound containing zirconium carboxylates of a concentration of 25 g / l, followed by low-temperature heat treatment at 370-395 ^o C in an argon stream. If scheme (1) was used, the coating was applied immediately after the final deformation of the complex workpiece; if scheme (2) was used, the coating was applied after high temperature heat treatment; if scheme (3) was used, the coating was applied after the first high temperature heat treatment. High-temperature heat treatments at 832 ^o C and 840 ^o C were carried out for a total time of 200 hours each, additional high-temperature heat treatment at 837 ^o C for 50 hours.

 Bending tests with kink showed that samples with a coating having increased permeability to oxygen can withstand up to 7 cycles of deformation without noticeable peeling of the coating. The study of the microstructure showed (see the drawing) that the coating has an unevenness of no more than 3% of the average value in thickness, it adheres well to the silver shell, that is, it has strong diffusion contact with it, which ensures effective heat exchange with the environment.

Also, a mixture of aluminum carboxylates with an aluminum content of 25 g / L and a mixture of yttrium carboxylates with a yttrium content of 25 g / L were applied to the conductor’s shell, low-temperature heat treatment was carried out in an argon stream at a temperature of 350-355 ^o C using aluminum carboxylates and a temperature of 355-360 ^o C when using yttrium carboxylates. Cycle: application of a solution of an organometallic compound — low-temperature heat treatment was repeated 15 times, the coating thickness was 6 μm.

In addition, a mixture of zirconium carboxylates with a zirconium content of 40 g / l, a mixture of aluminum carboxylates with an aluminum content of 40 g / l, a mixture of yttrium carboxylates with a yttrium content of 40 g / l were applied to the surface of the conductor, low-temperature heat treatment was carried out in an argon stream at a temperature of 370- 400 ^o C when using zirconium carboxylates, a temperature of 350-355 ^o C when using aluminum carboxylates and a temperature of 355-360 ^o C when using yttrium carboxylates. The use of metal carboxylates with a yttrium, aluminum, zirconium content of 40 g / l made it possible to reduce the number of coating cycles to 12 with the same thickness of 6 μm.

Also, the coating layers were applied using organometallic compounds containing both zirconium and yttrium concentration in a mixture of metals 25 g / l, containing both zirconium and aluminum concentration in a mixture of metals 25 g / l, containing both aluminum and yttrium concentration in a mixture of metals 25 g / l, as well as containing both zirconium and aluminum, and yttrium concentration of a mixture of metals 25 g / l. The use of mixtures of carboxylates of these metals allowed to reduce the pyrolysis temperature to 350-365 ^o C, and the number of coating cycles to reduce to 10 with the same thickness of 6 microns.

In addition, the coating layers were applied using organometallic compounds containing both zirconium and yttrium concentration of a mixture of metals 40 g / l containing both zirconium and aluminum concentration of a mixture of metals 40 g / l containing both aluminum and yttrium concentration of a mixture of metals 40 g / l, as well as containing both zirconium and aluminum, and yttrium concentration of a mixture of metals 40 g / l. The use of mixtures of carboxylates of these metals allowed to reduce the pyrolysis temperature to 350-365 ^o C, and the number of coating cycles to reduce to 8 with the same thickness of 6 microns.

 Bending tests with kinks of all the coatings described above showed that samples with a coating having increased permeability to oxygen withstand up to 7 cycles of deformation without noticeable peeling of the coating. The study of the microstructure showed that the coating has a thickness unevenness of no more than 3% of the average value, is well adjacent to the silver shell, that is, it has strong diffusion contact with it, which ensures effective heat exchange with the environment.

 The results shown in the table (see the text of the example below) are achieved using these metals when combined in a mixture and for each of the metals in a different range of indicated concentrations and temperatures.

 When studying the insulating properties of the above coatings with a thickness of 6 μm using an ampervoltmeter (P-386), no conductive sections were detected along the entire length of the conductor.

 The critical currents were measured by the standard 4-pin method according to the criterion of 1 μV / cm at 77 K in an intrinsic magnetic field.

The table shows the comparative characteristics of coils based on a 19-wire conductor, depending on the manufacturing scheme. High-temperature heat treatments at 832 ^° C and 840 ^° C were carried out for a total time of 200 hours each, additional high-temperature heat treatment at 837 ^° C for 50 hours.

 Bending tests with kink showed that samples with an inhomogeneous coating with increased oxygen permeability can withstand up to 7 cycles of deformation without noticeable peeling of the coating. The study of the microstructure showed (see the drawing) that the coating has an unevenness of no more than 3% of the average value in thickness, it adheres well to the silver shell, that is, it has strong diffusion contact with it, which ensures effective heat exchange with the environment.

Also, the coating layers were applied using organometallic compounds containing both zirconium and yttrium concentration in a mixture of metals 25 g / l. The use of a mixture of zirconium and yttrium carboxylates allowed to reduce the pyrolysis temperature (low temperature heat treatment) to 350-365 ^o C, and the number of coating cycles to reduce to 10 with the same thickness of 6 μm.

 When studying the insulating properties of the above coatings with a thickness of 6 μm using an ampervoltmeter (P-386), no conductive sections were detected along the entire length of the conductor.

 The critical currents were measured by the standard 4-pin method according to the criterion of 1 μV / cm at 77 K in an intrinsic magnetic field.

 The table shows the comparative characteristics of coils based on a 19-wire conductor, depending on the manufacturing scheme.

The table shows that the use of the proposed coating on a conductor with the required structure of the ceramic core allows using scheme (2), that is, when it is possible to adjust the design of the product, but the values of I _{k are} reduced, additional heat treatment is carried out, scheme (3) is implemented and the value of I _{k is} not less than when implementing scheme (1), when such an adjustment is not provided.

 The presented data allow us to conclude that, from a multicore conductor with the required structure of a ceramic core in an inhomogeneous insulating coating with increased permeability to oxygen, it is possible to obtain products of a correct design with critical properties of products in which the design was not adjusted.

Literature
1. A. Otto, L.J. Mazur, E. Podtbur, D. Delhi and others. Multicore composite ribbons Bi-2223 made of a metal precursor. IEEE Transactions on Applid Superconductivity, no. 3, No. 1, March 1993, p. 915-922.

 2. P. Haldar, J. G. Hai Chun Il, J. A. Rice, L. P. Motovidlo and others. Production and properties of high-temperature tapes and magnets made of Bi-2223 superconductors in a silver shell. IEEE Transactions on Applid Superconductivity, no. 3, No. 1, March 1993, p. 1127-1130.

 3. A. D. Nikulin, A. K. Shikov, E. V. Antipova, I. I. Akimov. A method of obtaining long conductors based on high-temperature superconducting compounds. The decision to grant a patent for an invention according to the application N 95100565/07 (001048) is a prototype.

Claims (1)

A method for producing long-length high-temperature superconducting products, including forming a preform in the form of a metal shell, filling it with a powder of a superconducting compound or a semi-finished product, deforming the resulting preform to the required size, followed by applying a solution of an organometallic compound based on carboxylic acids containing zirconium, aluminum, yttrium to the surface of the shell, and carrying out winding and heat treatment, characterized in that after deformation of the workpiece to the required dimensions they are cut into measured parts and a complex workpiece is formed, for which the required number of measured parts of the deformed workpiece is placed in a metal shell, the complex workpiece is deformed to the required dimensions, high-temperature heat treatment is carried out at 830 - 835 ^o C, then a solution of the organometallic compound is applied to the shell of the complex workpiece based on carboxylic acids containing zirconium, aluminum, yttrium, metal concentrations, metal mixtures 25 - 40 g / l, conduct low-temperature heat treatment at 350 - 400 ^o C, then produce the product and conduct additional high-temperature heat treatment at 835 - 840 ^o C.

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