An iron-based superconducting tape core powder and its application, and a method for preparing an iron-based superconducting tape

Abstract

The present invention belongs to the technical field of superconducting material processing engineering, and particularly relates to a core powder of an iron-based superconducting tape and its application, and a preparation method of an iron-based superconducting tape. By adding alkali metals (Li, Na) to the iron-based superconducting powder in the present invention, during the subsequent annealing process of the iron-based superconducting tape, Li, Na and FeAs react to form superconducting phases of LiFeAs or NaFeAs, enabling the FeAs existing at the grain boundaries to transform wetly into superconducting phases of LiFeAs or NaFeAs, achieving grain boundaries that can completely transmit superconducting current in the superconducting core of the iron-based superconducting tape, thereby realizing a microstructure with fully superconducting grain boundaries in the iron-based superconducting tape, solving the problems of the hindrance of the FeAs wetting phase to superconducting current and the restriction of grain boundaries existing in iron-based superconducting materials on superconducting current, and significantly improving the critical current density, current-carrying performance and practicality of the iron-based superconducting tape.

Description

Technical Field

[0001] The present invention belongs to the technical field of superconducting material processing engineering, and particularly relates to a core powder of an iron-based superconducting tape and its application, and a preparation method of an iron-based superconducting tape. Background Art

[0002] Iron-based superconductors have attracted strong attention from physicists and materials scientists since 2008. Compared with oxide high-temperature superconducting materials, iron-based superconductors have a simpler crystal structure, a larger coherence length, a smaller anisotropy, and a simpler preparation process. They are considered to have great application potential in the field of strong magnetic fields and have received attention in the international superconducting community. With the in-depth research, the high critical current characteristics of iron-based superconductors have gradually emerged. The prior art [Realization of practical level current densities in Sr 0.6 K 0.4 Fe2As2 tape conductors for high-field applications, Appl. Phys. Lett. 104(2014)202601] discloses that the critical transport current density of iron-based superconducting wires exceeds the practical threshold of 10 5 A / cm 2 . In subsequent work, people further optimized the preparation process of iron-based superconducting wires, and the transport performance of the wires continued to improve, showing good application potential.

[0003] Although high critical current densities have been obtained in both iron-based superconducting single crystals and thin films, the critical current density of iron-based superconducting tapes is still relatively low, which means that there is still a large room for improvement in the critical current density of iron-based superconducting tapes. In recent research [Strengthened proximity effect at grain boundaries to enhance inter-grain supercurrent in Ba 1-x K xIt was found in [Fe2As2 superconductors, Mater. Today Phys. 28 (2022) 100848] that there are impurity phases such as FeAs in the superconducting core of iron-based superconducting wires. The FeAs phase is a grain boundary wetting phase and almost all exists at grain boundaries. Moreover, the superconducting order parameter of the FeAs phase itself is relatively low, resulting in poor ability of superconducting current to pass through the grain boundaries containing the FeAs phase. Therefore, in the research on further improving the critical current density of iron-based superconducting tapes, removing the FeAs wetting phase at grain boundaries has been placed in an important position. As an intermediate phase for preparing superconducting precursor powder, the FeAs phase will inevitably appear during the synthesis process of the superconducting phase, and FeAs itself is relatively stable and it is difficult to completely remove it only through process control. Therefore, it is necessary to find a method to remove the FeAs wetting phase in iron-based superconducting wires, improve the connectivity between grains, thereby ensuring the effective area for superconducting current to pass through grain boundaries at the microstructural level, improving the critical current density of iron-based superconducting tapes, and making them have higher practical value.

[0004] Currently, the methods used to remove the FeAs phase in the superconducting core of iron-based superconducting tapes are mainly stepwise synthesis [High critical current density in textured Ba-122 / Ag tapes fabricated by a scalable rolling process, Scripta Materialia 99 (2015) 33–36], optimization of the precursor powder ratio [Strengthened proximity effect at grain boundaries to enhance inter-grain supercurrent in Ba 1-x K x Fe2As2 superconductors, Mater. Today Phys. 28 (2022) 100848] and other methods. These processes can only control the FeAs phase macroscopically and have no improvement effect on the FeAs phase at grain boundaries. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide an iron-based superconducting tape core powder and its application, and a preparation method of an iron-based superconducting tape. The present invention adds an alkali metal to the iron-based superconducting tape core powder, which can eliminate the FeAs wetting phase at the crystallization site of the iron-based superconducting tape.

[0006] To achieve the above purpose, the present invention provides the following technical solutions:

[0007] The present invention provides an iron-based superconducting tape core powder, comprising an iron-based superconducting powder and an alkali metal; the alkali metal includes lithium and / or sodium.

[0008] Preferably, the chemical composition of the iron-based superconducting powder is Ba 1-x K x Fe2As2, Sr 1-x K x Fe2As2, SmFeAsO 1-x F x or CaKFe4As4, where x is from 0 to 1.

[0009] Preferably, the atomic number of the alkali metal is 0.1-10% of the total atomic number of the metals in the iron-based superconducting powder.

[0010] Preferably, the preparation method of the iron-based superconducting powder is as follows: under the atmosphere of a protective gas, according to the chemical composition of the iron-based superconducting powder, the raw materials corresponding to the elements are mixed, and then grinding and heat treatment are carried out in sequence to obtain the iron-based superconducting powder.

[0011] Preferably, the particle size of the iron-based superconducting powder is 1-30 μm.

[0012] The present invention also provides the application of the iron-based superconducting tape core powder described in the above technical solution in the preparation of iron-based superconducting tapes.

[0013] The present invention also provides a preparation method of an iron-based superconducting tape, including the following steps:

[0014] Placing the iron-based superconducting tape core powder in a metal tube, blocking both ends of the metal tube, and performing drawing on the obtained tube-loading composite to obtain a single-core wire;

[0015] The iron-based superconducting tape core powder is the iron-based superconducting tape core powder described in the above technical solution;

[0016] After wrapping a high-strength metal strip around a single one of the single-core wires or a plurality of the single-core wires to form a composite sheathed wire, processing is carried out to obtain a high-strength single-core wire or a high-strength multi-core wire;

[0017] Rolling the single-core wire, the high-strength single-core wire or the high-strength multi-core wire to obtain a tape;

[0018] Performing vacuum annealing on the tape to obtain an iron-based superconducting tape.

[0019] Preferably, the temperature of the vacuum annealing is 100-1000 °C; the heat preservation time of the vacuum annealing is 0.1-50 h.

[0020] Preferably, the rolling is flat-roll rolling, cold pressing or hot pressing; the pass reduction rate of the rolling is 8-30%.

[0021] Preferably, the pass reduction rate of the drawing is 4-12%.

[0022] The present invention provides a core powder for an iron-based superconducting tape, comprising an iron-based superconducting powder and an alkali metal; the alkali metal includes lithium and / or sodium. By adding an alkali metal (Li, Na) to the iron-based superconducting powder in the present invention, during the subsequent annealing process of the iron-based superconducting tape, Li, Na and FeAs react to form a superconducting phase of LiFeAs or NaFeAs, so that the FeAs wetting phase originally existing at the grain boundaries is transformed into a superconducting phase of LiFeAs or NaFeAs, and grain boundaries capable of completely transmitting superconducting current are realized in the superconducting core of the iron-based superconducting tape, thereby realizing a microstructure of all-superconducting grain boundaries in the iron-based superconducting tape, solving the problem of the hindrance of the FeAs wetting phase to the superconducting current and the problem of the restriction of the grain boundaries existing in the iron-based superconducting material to the superconducting current, and significantly improving the critical current density, current-carrying performance and practicality of the iron-based superconducting tape.

[0023] In the heat treatment process of preparing the iron-based superconducting tape in the present invention, by optimizing the selection of the heat treatment temperature and time, the FeAs phase can fully react with the Li or Na metal element to form a superconducting phase of LiFeAs or NaFeAs. The present invention adopts different processing techniques according to different sheath materials and superconducting core compositions to meet different requirements, and improves the transmission performance of the iron-based superconducting tape by more than 10%. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a transmission electron microscope image of the grain boundaries of the iron-based superconducting tape prepared in Example 1 of the present invention;

[0025] Figure 2 It is a transmission electron microscope image of the iron-based superconducting tape with FeAs grain boundary wetting phase prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] The present invention provides a core powder for an iron-based superconducting tape, comprising an iron-based superconducting powder and an alkali metal; the alkali metal includes lithium and / or sodium.

[0027] Unless otherwise specified, the present invention has no special requirements for the sources of the raw materials used, and commercially available products well-known to those skilled in the art can be used.

[0028] The core powder for an iron-based superconducting tape provided by the present invention includes an iron-based superconducting powder. In the present invention, the chemical composition of the iron-based superconducting powder is preferably Ba 1-x K x Fe2As2, Sr 1-x K x Fe2As2, SmFeAsO 1-x F xor CaKFe4As4, where x ranges from 0 to 1, more preferably from 0.4 to 0.6; the particle size of the iron-based superconducting powder is preferably 1 - 30 μm, more preferably 10 - 20 μm, and most preferably 18 μm.

[0029] In the present invention, the preparation method of the iron-based superconducting powder is preferably: in an atmosphere of protective gas, according to the chemical composition of the iron-based superconducting powder, the raw materials corresponding to the elements are mixed, and grinding and heat treatment are carried out in sequence to obtain the iron-based superconducting powder.

[0030] In the present invention, the raw material corresponding to barium element in the chemical composition of the iron-based superconducting powder is preferably metallic barium chips, the raw material corresponding to samarium element is preferably metallic samarium chips and / or samarium fluoride, more preferably metallic samarium chips, the raw material corresponding to strontium element is preferably metallic strontium chips, the raw material corresponding to calcium element is preferably metallic calcium, the raw material corresponding to potassium element is preferably potassium blocks, the raw material corresponding to iron element is preferably iron powder and / or iron(III) oxide, and the raw material corresponding to arsenic element is preferably arsenic particles; the raw material corresponding to O element is preferably iron(III) oxide; the raw material corresponding to F element is preferably samarium fluoride.

[0031] In the embodiments of the present invention, the raw materials corresponding to the elements in the chemical composition of the iron-based superconducting powder are specifically metallic barium chips, potassium blocks, iron powder and arsenic particles, or metallic samarium chips, iron powder, arsenic particles, iron(III) oxide and samarium fluoride.

[0032] In the present invention, the mass purity of the metallic barium chips is preferably ≥99.50%, more preferably 99.50%; the mass purity of the potassium blocks is preferably ≥99.95%, more preferably 99.95%; the mass purity of the iron powder is preferably ≥99.99%, more preferably 99.99%; the mass purity of the arsenic particles is preferably ≥99.95%, more preferably 99.95%; the mass purity of the metallic samarium chips ≥99.50%, more preferably 99.50%.

[0033] In the present invention, the grinding is preferably ball milling; the rotation speed of the ball milling is preferably 200 - 800 rpm, more preferably 300 - 500 rpm; the time of the ball milling is preferably 10 - 30 h, more preferably 15 - 20 h; the equipment used for the ball milling preferably includes a ball milling tank and ball milling media; the ball milling media is preferably cemented carbide balls, more preferably tool stainless steel balls; the particle size of the powder obtained by the ball milling is preferably 50 - 100 μm, more preferably 60 - 80 μm.

[0034] Before carrying out the heat treatment, the present invention preferably further includes: placing the powder obtained by the ball milling in a metal tube and blocking both ends of the metal tube. In the present invention, the metal tube is preferably an Nb tube; the blocking is preferably carried out using copper plugs.

[0035] In the present invention, the protective gas is preferably argon; the purity of the argon is preferably 99 - 99.999%, more preferably 99.999%; the temperature of the heat treatment is preferably 100 - 1300 °C, more preferably 500 - 900 °C; the holding time of the heat treatment is preferably 0.1 - 50 h, more preferably 10 - 40 h.

[0036] In the present invention, the chemical reaction between elements is completed through heat treatment to generate a superconducting phase.

[0037] After the heat treatment, the present invention preferably pulverizes the superconducting powder obtained from the heat treatment to obtain the iron-based superconducting powder. The present invention has no special limitation on the pulverization process, and a pulverization process well-known in the art can be used.

[0038] The iron-based superconducting tape core powder provided by the present invention contains an alkali metal. In the present invention, the alkali metal includes lithium and / or sodium, preferably lithium or sodium; in order to introduce as little Li or Na as possible while ensuring the elimination of the FeAs phase, the atomic number of the alkali metal is preferably 0.1 - 10% of the total number of metal atoms in the iron-based superconducting powder, more preferably 0.5 - 5%; the alkali metal is preferably an alkali metal powder; the particle size of the alkali metal powder is preferably 1 - 5 mm, more preferably 3 mm.

[0039] In the present invention, the preparation method of the iron-based superconducting tape core powder is preferably to uniformly mix the iron-based superconducting powder and the alkali metal.

[0040] The present invention also provides the application of the iron-based superconducting tape core powder described in the above technical solution in the preparation of an iron-based superconducting tape.

[0041] The present invention also provides a method for preparing an iron-based superconducting tape, comprising the following steps:

[0042] Placing the iron-based superconducting tape core powder in a metal tube, sealing both ends of the metal tube, and drawing the obtained tube-filled composite to obtain a single-core wire;

[0043] The iron-based superconducting tape core powder is the iron-based superconducting tape core powder described in the above technical solution;

[0044] Or wrapping a high-strength metal strip around a single one of the single-core wires or a plurality of the single-core wires to form a composite sheathed wire, and then performing processing to obtain a high-strength single-core wire or a high-strength multi-core wire;

[0045] Rolling the single-core wire, the high-strength single-core wire or the high-strength multi-core wire to obtain a tape;

[0046] Performing vacuum annealing on the tape to obtain an iron-based superconducting tape.

[0047] In the present invention, the number of core materials of the iron-based superconducting tape is preferably ≥1, more preferably 5-7.

[0048] In the present invention, the core powder of the iron-based superconducting tape is placed in a metal tube, and both ends of the metal tube are sealed to obtain a tube-packed composite.

[0049] In the present invention, it is preferred to seal the metal tube after it is completely filled.

[0050] In the present invention, the metal tube is preferably a silver tube or a silver alloy tube, more preferably a silver alloy tube; the silver alloy tube is preferably a silver-tin alloy tube; the inner diameter of the metal tube is preferably 5 cm, the outer diameter is preferably 8 cm, and the length is preferably 10 cm; the sealing is preferably carried out with Nb plugs.

[0051] After obtaining the tube-packed composite, the present invention draws the tube-packed composite to obtain a single-core wire.

[0052] In the present invention, the diameter of the wire obtained by drawing is preferably 1-2 mm, more preferably 1.5-1.95 mm; the pass reduction rate of the drawing is preferably 4-12%, more preferably 5-10%; the number of drawing times is preferably 30-50 times, more preferably 35-40 times.

[0053] During the drawing process, the grains in the core of the iron-based superconducting tape form an axial texture.

[0054] After obtaining the single-core wire, the present invention wraps a high-strength metal strip around a single or multiple single-core wires to form a composite sheath wire, and then processes it to obtain a high-strength single-core wire or a high-strength multi-core wire.

[0055] In the present invention, the material of the high-strength metal strip is preferably Cu, Monel or stainless steel, more preferably stainless steel; the processing preferably includes drawing or wire rolling, more preferably drawing; the specific content of the drawing is as described above and will not be elaborated here; the present invention has no special limitation on the wire rolling, and the well-known wire rolling technology in the art can be used.

[0056] After obtaining the single-core wire, the high-strength single-core wire or the high-strength multi-core wire, the present invention rolls the single-core wire, the high-strength single-core wire or the high-strength multi-core wire to obtain a tape.

[0057] In the present invention, the thickness of the rolled strip is preferably 0.2 - 0.6 mm, more preferably 0.3 mm; the width of the rolled strip is preferably 3 - 6 mm, more preferably 4 - 5 mm; the processing rate per pass of the rolling is preferably 8 - 30%, more preferably 10 - 15%; the rolling is preferably flat roll rolling, cold pressing or hot pressing; the number of passes of the rolling is preferably 3 - 10 passes, more preferably 5 passes; the strip is preferably a C-axis texture strip.

[0058] After obtaining the strip, the present invention subjects the strip to vacuum annealing to obtain an iron-based superconducting strip.

[0059] In the present invention, the degree of vacuum of the vacuum annealing is preferably < 10 -3 Pa; the temperature of the vacuum annealing is preferably 100 - 1000 °C, more preferably 500 - 850 °C; the holding time of the vacuum annealing is preferably 0.1 - 50 h, more preferably 3 - 10 h.

[0060] After the vacuum annealing, the present invention cools the annealed strip in the furnace to room temperature.

[0061] In the present invention, by adding alkali metals (Li, Na) to the iron-based superconducting powder, during the annealing process, Li, Na and FeAs react to form superconducting phases of LiFeAs or NaFeAs, causing the FeAs originally present at the grain boundaries to transform towards the superconducting phases of LiFeAs or NaFeAs, achieving grain boundaries that can completely transmit superconducting current in the superconducting core of the iron-based superconducting strip, thereby realizing a microstructure with fully superconducting grain boundaries in the iron-based superconducting strip, effectively solving the problem of the restriction of grain boundaries on superconducting current in iron-based superconducting materials, and significantly improving the critical current density and practical potential of iron-based superconducting strips.

[0062] In the heat treatment process of preparing the iron-based superconducting powder in the present invention, by optimizing the selection of the heat treatment temperature and time, the components can fully react to form superconducting phases. According to different sheathing materials such as copper and stainless steel and different superconducting core components such as Ba 1- x K x Fe2As2, Sr 1-x K x Fe2As2, different processing techniques are adopted to meet different requirements such as high magnetic thermal stability or high mechanical strength, and to reduce costs and improve the transport performance of iron-based superconducting strips to varying degrees.

[0063] Next, the technical solutions in the present invention will be clearly and completely described in combination with the embodiments in the present invention, but they cannot be construed as limiting the protection scope of the present invention.

[0064] Example 1

[0065] In an argon atmosphere of 99.999%, metal barium chips (mass purity of 99.50%), potassium blocks (mass purity of 99.95%), iron powder (mass purity of 99.99%), and arsenic particles (mass purity of 99.95%) were accurately weighed according to the molar ratio Ba:K:Fe:As = 0.6:0.4:2:2, and then loaded into a ball mill jar and ball milled at 500 rpm for 10 h. The ball milling medium was tool stainless steel balls, and the particle size of the ball milled powder was 60 μm. The ball milled powder was loaded into an Nb tube, and both ends were sealed with copper plugs and then heat treated at 900 °C for 50 h to obtain Ba 0.6 K 0.4 Fe2As2 superconducting powder;

[0066] The obtained Ba 0.6 K 0.4 Fe2As2 superconducting powder was pulverized (the particle size after pulverization was 18 μm), and then 5% atomic ratio of Na metal blocks (particle size of 3 mm) was added and mixed evenly to obtain the precursor powder for preparing iron-based superconducting tapes. The precursor powder mixed with Na was filled into a silver-tin alloy tube with an inner diameter of 5 cm, an outer diameter of 8 cm, and a length of 10 cm; then both ends were sealed with Nb plugs respectively to obtain a silver-tin alloy tube-packed composite; the tube-packed composite was drawn 40 times to obtain a silver-tin alloy-clad single-core wire with a diameter of 1.95 mm, and the pass reduction rate was 10% for each pass; the wiped silver-tin alloy-clad single-core wire with a length of 5 cm was rolled 5 times with a rolling device at a pass reduction rate of 10% to obtain a single-core tape with a thickness of 0.3 mm and a width of 4.7 mm. Then it was annealed at 880 °C for 0.5 h in a vacuum (<10 - 3 Pa), and after the annealing furnace cooled to room temperature, a 5% Na-doped single-core Ba 0.6 K 0.4 Fe2As2 superconducting tape was obtained.

[0067] The superconducting transition temperature and critical current of the 5% Na-doped single-core Ba 0.6 K 0.4 Fe2As2 superconducting tape were measured respectively by a comprehensive physical property measurement system (PPMS system) and a low-temperature high-magnetic-field critical current test system. The superconducting transition temperature was 37 K, and the critical current density was greater than 150000 A / cm 2 (4.2 K, 10 T).

[0068] Example 2

[0069] In an atmosphere of 99.999% argon, metallic Sm chips (mass purity 99.50%), iron powder (mass purity 99.99%), arsenic particles (mass purity 99.95%), Fe2O3, and SmF3 were accurately weighed according to the molar ratio Sm:Fe:As:Fe2O3:SmF3 = 0.92:0.4:0.3:0.076, loaded into a ball mill jar, and ball milled at 500 rpm for 15 h. The ball milling medium was tool stainless steel balls. The particle size of the ball milled powder was 60 μm. The ball milled powder was loaded into an Nb tube, and both ends were sealed with plugs and then heat treated at 1100 °C for 30 h to obtain SmFeAsO 1-x F x (X=(0.2)) superconducting powder;

[0070] The obtained SmFeAsO 1-x F x superconducting powder was pulverized (particle size after pulverization was 18 μm), and then 3% atomic ratio of Li metal blocks (particle size 3 mm) was added and mixed evenly to obtain the precursor powder for preparing iron-based superconducting tapes. The precursor powder mixed with Li was filled into a silver-tin alloy tube with an inner diameter of 5 cm, an outer diameter of 8 cm, and a length of 10 cm. Then, both ends were respectively sealed with Nb plugs to obtain a silver-tin alloy tube-filled composite. The tube-filled composite was respectively drawn 40 times to obtain silver-tin alloy sheathed single-core wires with a diameter of 1.95 mm, and the pass reduction rate was 10% for each pass. One 5-cm long silver-tin alloy sheathed single-core wire wiped clean was rolled 5 times using a rolling device at a pass reduction rate of 5% to obtain a single-core tape with a thickness of 0.3 mm and a width of 4.7 mm. Then, it was annealed at 880 °C for 0.5 h in a vacuum (<10 -3 Pa), and after the annealing furnace cooled to room temperature, 3% Li-doped single-core SmFeAsO 1-x F x (X=(0.2)) superconducting tapes were obtained.

[0071] The superconducting transition temperature and critical current of the 3% Li-doped SmFeAsO 1-x F x (X=(0.2)) superconducting tapes were respectively measured by a comprehensive physical property measurement system (PPMS system) and a low-temperature high-magnetic-field critical current test system. The superconducting transition temperature was 44 K, and the critical engineering current density was greater than 300 A / cm 2 (4.2 K, 10 T).

[0072] Comparative Example 1

[0073] Iron-based superconducting tapes with a FeAs grain boundary wetting phase prepared by traditional processes. In an argon atmosphere of 99.999%, metal barium chips (mass purity of 99.50%), potassium blocks (mass purity of 99.95%), iron powder (mass purity of 99.99%), and arsenic particles (mass purity of 99.95%) were accurately weighed according to the molar ratio Ba:K:Fe:As = 0.6:0.4:2:2 and then loaded into a ball mill jar and ball milled at 500 rpm for 10 h. The ball milling medium was tool stainless steel balls. The particle size of the ball milled powder was 60 μm. The ball milled powder was loaded into an Nb tube, and both ends were sealed with copper plugs and then heat treated at 900 °C for 50 h to obtain Ba 0.6 K 0.4 Fe2As2 superconducting powder;

[0074] The obtained superconducting powder was pulverized (particle size after pulverization was 18 μm) and then filled into a silver-tin alloy tube with an inner diameter of 5 cm, an outer diameter of 8 cm, and a length of 10 cm; then both ends were respectively sealed with Nb plugs to obtain a silver-tin alloy tube-filled composite; the tube-filled composite was drawn 40 times to obtain silver-tin alloy sheathed single-core wires with a diameter of 1.95 mm, and the pass reduction rate was 10% for each pass; the wiped silver-tin alloy sheathed single-core wires with a length of 5 cm were rolled 5 times using a rolling device with a pass reduction rate of 10% to obtain a single-core tape with a thickness of 0.3 mm and a width of 4.7 mm, and then annealed at 880 °C under vacuum (<10 -3 Pa) for 0.5 h. After the annealing furnace cooled to room temperature, single-core Ba 0.6 K 0.4 Fe2As2 superconducting tapes were obtained.

[0075] The superconducting transition temperature and critical current of the single-core Ba 0.6 K 0.4 Fe2As2 superconducting tapes were measured respectively by a comprehensive physical property measurement system (PPMS system) and a low-temperature high-magnetic field critical current test system. The superconducting transition temperature was 37 K, and the critical current density was greater than 50000 A / cm 2 (4.2 K, 10 T).

[0076] Comparative Example 2

[0077] Fe-based superconducting tapes doped with excessive Na prepared by traditional processes. Under an argon atmosphere of 99.999%, metal barium chips (mass purity of 99.50%), potassium blocks (mass purity of 99.95%), iron powder (mass purity of 99.99%), and arsenic particles (mass purity of 99.95%) were accurately weighed according to the molar ratio Ba:K:Fe:As = 0.6:0.4:2:2, placed in a ball mill jar, and ball milled at 500 rpm for 10 h. The ball milling medium was tool stainless steel balls. The particle size of the ball milled powder was 60 μm. The ball milled powder was loaded into an Nb tube, and both ends were sealed with copper plugs and then heat treated at 900 °C for 50 h to obtain Ba 0.6 K 0.4 Fe2As2 superconducting powder;

[0078] The obtained Ba 0.6 K 0.4 Fe2As2 superconducting powder was pulverized (the particle size after pulverization was 18 μm), and then Na metal blocks with an atomic ratio of 30% (particle size of 3 mm) were added and mixed evenly to obtain the precursor powder for preparing the Fe-based superconducting tape. The precursor powder mixed with Na was filled into a silver-tin alloy tube with an inner diameter of 5 cm, an outer diameter of 8 cm, and a length of 10 cm; then both ends were sealed with Nb plugs respectively to obtain a silver-tin alloy tube-filled composite. The tube-filled composite was drawn 40 times to obtain a silver-tin alloy sheathed single-core wire with a diameter of 1.95 mm, and the pass processing rate was 10% each time; the wiped silver-tin alloy sheathed single-core wire with a length of 5 cm was rolled 5 times using a rolling device at a pass processing rate of 10% to obtain a single-core tape with a thickness of 0.3 mm and a width of 4.7 mm. Then, it was annealed at 880 °C for 0.5 h under vacuum (<10 -3 Pa). After the annealing furnace cooled to room temperature, a single-core Ba 0.6 K 0.4 Fe2As2 superconducting tape doped with 30% Na was obtained.

[0079] The superconducting transition temperature and critical current of the 30% Na-doped single-core Ba 0.6 K 0.4 Fe2As2 superconducting tape were measured respectively by a comprehensive physical property measurement system (PPMS system) and a low-temperature high-magnetic-field critical current test system. The superconducting transition temperature was 30 K, and the critical current density was 10000 A / cm 2 (4.2 K, 10 T).

[0080] Microscopic characterization test

[0081] The Fe-based superconducting tapes prepared in Example 1 of the present invention and the Fe-based superconducting tapes with FeAs grain boundary wetting phase prepared in Comparative Example 1 were tested by transmission electron microscopy. The results are shown in Figure 1 and Figure 2 respectively.

[0082] It can be seen from Figure 1 that in the present invention, by doping an appropriate amount of Na into the precursor powder, lattice fringes can be clearly observed both in the grains of the obtained superconducting core and in the materials between the grains, indicating that there is no amorphous material in the superconducting core. This is because the chemical reaction occurs between the FeAs wetting phase at the grain boundaries of the superconducting core of the tape and the doped metal Na due to Na doping, and the amorphous FeAs wetting phase that hinders the transmission of superconducting current is transformed into the NaFeAs superconducting phase with obvious lattice fringes, solving the problem of poor grain boundary connectivity caused by the grain boundary wetting phase from the microscopic structure, thus laying a foundation for significantly improving the critical current density of the iron-based superconducting tape and being conducive to promoting the practical application of the iron-based superconducting material.

[0083] It can be seen from Figure 2 that there is an amorphous material between the superconducting grains. This substance has no lattice fringes under the transmission electron microscope, and only Fe and As elements are detected by elemental analysis, which conforms to the characteristics of the FeAs wetting phase. The existence of this FeAs wetting phase hinders the transmission of superconducting current between the grains, reduces the effective superconducting transmission interface, and results in a lower critical current density of the iron-based superconducting tape prepared by this method.

[0084] Although the above embodiments have described the present invention in detail, they are only a part rather than all of the embodiments of the present invention. People can also obtain other embodiments based on these embodiments without creative efforts, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A preparation method of an iron-based superconducting tape, characterized in that, It includes the following steps: Put the iron-based superconducting powder and alkali metal into a metal tube, seal both ends of the metal tube, and perform drawing on the obtained tube-filled composite to obtain a single-core wire; Optionally, wrap a high-strength metal strip around a single one of the single-core wires or multiple of the single-core wires to form a composite sheathed wire, and then perform processing to obtain a high-strength single-core wire or a high-strength multi-core wire; Roll the single-core wire, high-strength single-core wire or high-strength multi-core wire to obtain a strip; Perform vacuum annealing on the strip to obtain an iron-based superconducting strip; The chemical composition of the iron-based superconducting powder is Ba 1-x K x Fe2As2 or Sr 1-x K x Fe2As2, where 0 < x < 1; the alkali metal includes lithium and / or sodium; The atomic number of the alkali metal is 0.5 to 10% of the total number of metal atoms in the iron-based superconducting powder.

2. The preparation method according to claim 1, wherein The temperature of the vacuum annealing is 850 to 1000 °C; the holding time of the vacuum annealing is 0.1 to 50 h.

3. The preparation method according to claim 1, characterized in that, The rolling is flat-roll rolling, cold pressing or hot pressing; the pass reduction rate of the rolling is 8 to 30%.

4. The preparation method according to claim 1, characterized in that, The pass reduction rate of the drawing is 4 to 12%.

5. The preparation method according to claim 1, characterized in that, The preparation method of the iron-based superconducting powder is: under the atmosphere of a protective gas, mix the raw materials corresponding to the elements according to the chemical composition of the iron-based superconducting powder, and perform powder grinding and heat treatment in sequence to obtain the iron-based superconducting powder.

6. The preparation method according to claim 5, characterized in that, The particle size of the iron-based superconducting powder is 1 to 30 µm.

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