Preparation method of PbMo6S8-based superconducting wire

Abstract

The invention discloses a preparation method of a PbMo6S8-based superconducting wire. The method comprises the following steps: 1, mixing and grinding Pb powder, Mo powder and S powder raw materials to obtain mixed powder; secondly, the mixed powder is pressed into a blank, vacuum tube sealing and sintering are conducted, and mixed powder of intermediate-state Pb, Mo and MoS2 is obtained; 3, pressing the mixed powder of the intermediate-state Pb, Mo and MoS2 into a powder rod, loading the powder rod into a metal tube, and drawing to obtain a wire rod; and 4, sintering the wire rod at a phase forming temperature. High-purity and high-uniformity nanoscale precursor powder is prepared through vacuum tube sealing and sintering of elementary substance raw material powder, the density of a wire core wire is improved by combining powder rod pressing and tube filling, and rapid heating is adopted in the heat treatment phase forming process, so that appearance of a second phase is inhibited, formation of a superconducting phase is promoted, and the grain boundary connectivity is enhanced; and the critical current density of the PbMo6S8 superconducting wire is improved, and the method is suitable for the field of superconducting wire preparation.

Description

Technical Field

[0001] The present invention belongs to the technical field of superconducting material preparation, and particularly relates to a method for preparing PbMo6S8-based superconducting wire. Background Art

[0002] With the rapid development of superconducting magnet technology, the steady-state magnetic field strength is increasing day by day, which puts forward new requirements for all aspects of the superconducting materials used in the magnet. At present, the superconducting materials for high-field magnets are mainly the low-temperature superconducting materials NbTi and Nb3Sn with mature and stable processes. However, the intrinsic upper critical fields (H c2 ) of these two types of low-temperature superconducting materials are 18T and 25T respectively, making it impossible for them to be practically applied in high-field magnets above 30T which have received extensive attention at present. Although high-temperature superconducting materials can be applied in the form of insert coils to increase the magnetic field strength of the magnet, the preparation technology of high-temperature superconducting materials is still immature at present, and the preparation cost is still relatively high. Therefore, developing new superconducting materials for high-field magnets is of extremely important significance for the further development of superconducting magnet technology and promoting the practical application process of superconducting materials.

[0003] In the 1970s, Chevrel et al. first discovered superconductivity in A x Mo 6-x S6 compounds represented by the PbMo6S8 system. This is the first discovered ternary superconducting compound, and then this molybdenum-based chalcogenide compound is called the Chevrel phase. In this type of material, Mo atoms and S atoms form a lattice unit in the form of Mo6S8, and A-site elements such as Pb are located in the gaps of the Mo6S8 unit, playing a crucial role in the superconductivity of the whole system. Through systematic measurement, it is found that the superconducting transition temperature (T c ) of PbMo6S8-based superconducting materials is about 15K, and it has a high upper critical field (H c2 (0), 60T) and very small anisotropy, which all ensure the application of this type of material in superconducting magnets. Therefore, PbMo6S8-based superconducting materials are expected to become practical superconducting materials for the next generation of high magnetic fields.

[0004] At present, the current-carrying capacity of PbMo6S8-based superconducting wire needs to be further improved, and there is still a certain distance from practical application. Grain boundary weak connection is one of the main factors restricting the improvement of its current-carrying capacity, and the holes, microcracks in the wire core filaments and the enrichment of the second phase at the grain boundaries are the main reasons for the weak connection. Therefore, eliminating the holes and microcracks in the wire core filaments and suppressing the formation of the second phase are effective ways to prepare high-current-carrying PbMo6S8-based superconducting wires. However, in the preparation process of PbMo6S8 materials, MoS2 is the main second phase and is very easy to form. Therefore, while increasing the density of the core filaments, how to avoid the formation of the second phase MoS2 is the key to preparing high-current-carrying PbMo6S8-based superconducting wires. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a preparation method of PbMo6S8-based superconducting wire aiming at the deficiencies of the above-mentioned prior art. This method prepares high-purity and high-uniformity nano-sized intermediate Pb, Mo, and MoS2 mixed powders by vacuum tube sintering of elemental Pb powder, Mo powder, and S powder, combines powder pressing and rod packing into a tube to increase the density of the wire core filaments, and adopts rapid heating during heat treatment for phase formation, which suppresses the appearance of the second phase, promotes the formation of the superconducting phase, enhances the grain boundary connectivity, and significantly improves the critical current density of PbMo6S8 superconducting wires, solving the problem of insufficient current-carrying capacity of PbMo6S8-based superconducting wires.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is: a preparation method of PbMo6S8-based superconducting wire, characterized in that the method comprises the following steps:

[0007] Step 1: In a glove box protected by argon, select Pb powder, Mo powder, and S powder raw materials according to the atomic ratio of Pb:Mo:S in the target product PbMo6S8-based superconducting wire of 1:6:8 for mixing and grinding to obtain a mixed powder;

[0008] Step 2: Press the mixed powder obtained in Step 1 into a green body and vacuum seal it in a quartz tube for sintering to obtain a mixed powder of intermediate Pb, Mo, and MoS2;

[0009] Step 3: Press the mixed powder of intermediate Pb, Mo, and MoS2 obtained in Step 2 into a powder rod, pack it into a metal tube, and perform drawing to obtain a wire;

[0010] Step 4: Sinter the wire obtained in Step 3 at the phase formation temperature to obtain a PbMo6S8-based superconducting wire.

[0011] The preliminary study of the present invention found that in the process of preparing PbMo6S8 using a mixed powder of single elements Pb, Mo, and S, due to the low melting points of Pb and S, the PbMo6S8 system is in a semi-molten state during the sintering process, which can effectively promote element diffusion and is conducive to the uniform phase formation of PbMo6S8. Based on the conclusion of this study, the present invention uses a mixed powder of Pb powder, Mo powder, and S powder as the initial raw material, and prepares a uniform intermediate state Pb, Mo, and MoS2 mixed powder by sintering after vacuum sealing, which promotes the uniform phase formation of PbMo6S8 during subsequent phase formation sintering, and combines the powder pressing rod tube filling process to prepare PbMo6S8 superconducting wire, which greatly improves the wire tube filling density, increases the wire core density, and enhances the crystal connectivity. The present invention combines the advantages of the pre-situ method and the in-situ method by preparing a uniform intermediate Pb, Mo, and MoS2 mixed powder, thereby ensuring the purity of the superconducting phase and forming a PbMo6S8 phase with good connectivity in situ, and ultimately obtaining a high-performance PbMo6S8-based superconducting wire.

[0012] The above-mentioned method for preparing a PbMo6S8-based superconducting wire is characterized in that the sintering temperature in step 2 is 550°C to 850°C and the time is 6h to 48h. By controlling the sintering temperature and time, it is ensured that the Pb powder, Mo powder and S powder react to form a mixed powder of intermediate Pb, Mo and MoS2.

[0013] The above-mentioned method for preparing a PbMo6S8-based superconducting wire is characterized in that the mixed powder of intermediate Pb, Mo and MoS2 in step 2 is a uniform mixed powder of Pb, Mo and MoS2.

[0014] The above-mentioned method for preparing a PbMo6S8-based superconducting wire is characterized in that the metal tube in step three is a Mo tube or a Nb tube, and the metal tube containing the powder rod is placed in an outer sheathed monel alloy tube and then drawn.

[0015] The above-mentioned method for preparing a PbMo6S8-based superconducting wire is characterized in that in step 4, the wire is heated to the phase-forming temperature at a heating rate of 5°C / min to 10°C / min for sintering, or the wire is directly placed in a furnace with the phase-forming temperature for sintering. In the heat treatment process of the wire of the present invention, by introducing a rapid heating process, the abnormal growth of MoS2 is successfully avoided, the appearance of the second phase is suppressed, the MoS2 phase is effectively eliminated, the formation of the PbMo6S8 superconducting phase is promoted, the grain connectivity is further enhanced, and the critical current density of the PbMo6S8 superconducting wire is significantly improved.

[0016] The above-mentioned method for preparing a PbMo6S8-based superconducting wire is characterized in that, in step four, the phase formation temperature is 800°C to 1500°C, and the sintering time is 12h to 72h.

[0017] The present invention has the following advantages compared with the prior art:

[0018] 1. In the present invention, elemental Pb powder, Mo powder, and S powder are ground and mixed, and then vacuum sealed and sintered to prepare an intermediate Pb, Mo, MoS2 mixed powder with fine and uniform grain size. Using this mixed powder as the precursor powder for preparing PbMo6S8-based superconducting wire effectively avoids the agglomeration phenomenon during the mixing process of directly using raw material MoS2 powder, lays a foundation for the uniform phase formation of PbMo6S8 subsequently, and thus improves the current-carrying performance of the PbMo6S8-based superconducting wire.

[0019] 2. The present invention adopts a powder pressing and rod-packing process, which greatly improves the packing density of the wire, increases the density of the wire core filaments, enhances the crystal connectivity, and effectively solves the problems of low core filament density and poor grain connectivity during the preparation of PbMo6S8 wire by the traditional powder-in-tube method, thereby obtaining a PbMo6S8-based superconducting wire with a high core filament density.

[0020] 3. By introducing a rapid heating process during the heat treatment and phase formation of the PbMo6S8 superconducting wire, the present invention inhibits the abnormal growth of MoS2, effectively promotes the formation of the PbMo6S8 phase, improves the grain connectivity, and prepares a PbMo6S8 superconducting wire with a high superconducting phase content and high current-carrying performance.

[0021] 4. The preparation process of the present invention is simple, the required equipment is easy to obtain, the preparation difficulty is reduced, and it is easy to realize the application.

[0022] The technical solution of the present invention will be further described in detail below through the drawings and embodiments. Brief Description of the Drawings

[0023] Figure 1 It is a micrograph of the intermediate Pb, Mo, and MoS2 mixed powder prepared in Example 1 of the present invention.

[0024] Figure 2 It is a cross-sectional metallograph of the PbMo6S8-based superconducting wire prepared in Example 1 of the present invention.

[0025] Figure 3 It is an XRD pattern of the core filaments in the PbMo6S8-based superconducting wire prepared in Example 1 of the present invention.

[0026] Figure 4 It is a micrograph of the PbMo6S8-based superconducting wire prepared in Comparative Example 1 of the present invention.

[0027] Figure 5 This is the microscopic morphology diagram of the PbMo6S8-based superconducting wire prepared in Comparative Example 2 of the present invention.

[0028] Figure 6 This is the XRD pattern of the core wire in the PbMo6S8-based superconducting wire prepared in Example 2 of the present invention. Specific Embodiments

[0029] Example 1

[0030] This example includes the following steps:

[0031] Step 1: In a glove box protected by argon, according to the atomic ratio of Pb:Mo:S in the target product PbMo6S8-based superconducting wire of 1:6:8, select Pb powder, Mo powder, and S powder raw materials with a mass purity of more than 99.99%, and then pour them into a mortar for mixing and grinding for more than 30 minutes to obtain a mixed powder;

[0032] Step 2: Press the mixed powder obtained in Step 1 into a green body, with a pressing pressure of 10 MPa and a pressure holding time of 5 minutes, then vacuum seal it in a quartz tube and sinter it at 550 °C for 48 hours to obtain a uniform mixed powder of intermediate state Pb, Mo, and MoS2;

[0033] Step 3: Press the mixed powder of intermediate state Pb, Mo, and MoS2 obtained in Step 2 into a powder rod, place it in a clean Mo tube in a glove box protected by argon, and then place the Mo tube containing the powder rod into an outer cladding Monel alloy tube for drawing to obtain a PbMo6S8 wire;

[0034] Step 4: Heat the wire obtained in Step 3 in an argon atmosphere at a heating rate of 5 °C / min to 800 °C and sinter it for 72 hours to obtain a PbMo6S8-based superconducting wire.

[0035] Figure 1 This is the microscopic morphology diagram of the mixed powder of intermediate state Pb, Mo, and MoS2 prepared in this example. From Figure 1 It can be seen that by grinding and mixing elemental Pb powder, Mo powder, and S powder and then vacuum sealing and sintering, the present invention prepares a mixed powder of intermediate state Pb, Mo, and MoS2 with fine and uniform grain size, effectively avoiding the agglomeration problem of MoS2 during the direct use of MoS2, and laying a foundation for the uniform phase formation of subsequent PbMo6S8.

[0036] Figure 2 This is the cross-sectional metallographic diagram of the PbMo6S8-based superconducting wire prepared in this example. From Figure 2 It can be seen that the composite cladding of the PbMo6S8-based superconducting wire is complete and the deformation is uniform..

[0037] After detection, the PbMo6S8-based superconducting wire prepared in this example has a high superconducting phase content of PbMo6S8. As Figure 3 shown, its critical current density reaches 4×10 4 A / cm 2 or more.

[0038] Comparative Example 1

[0039] The difference between this comparative example and Example 1 is that in Step 1, Pb, Mo, and MoS2 powders are selected as raw materials, and the process of vacuum-sealing the mixed powder into a quartz tube for sintering after pressing the mixed powder into a green body in Step 2 is not carried out. Instead, the mixed powder is directly subjected to the powder pressing and rod packing process in Step 3.

[0040] Figure 4 is the micrograph of the PbMo6S8-based superconducting wire prepared in this comparative example. From Figure 4 it can be seen that there are a large number of agglomerated MoS2 in the core wire of the PbMo6S8-based superconducting wire, which is mainly caused by the agglomeration of the raw material MoS2 during the grinding and mixing process. In Example 1, no large lamellar MoS2 and agglomeration phenomenon are found in the intermediate-state Pb, Mo, and MoS2 mixed powder prepared from elemental Pb powder, Mo powder, and S powder. Its grain size is small and the uniformity is high, laying a foundation for the preparation of subsequent high-current-carrying wires.

[0041] Comparative Example 2

[0042] The difference between this comparative example and Example 1 is that in Step 3, instead of using the powder pressing and rod packing process, the traditional powder packing process is adopted. The mixed powder of intermediate-state Pb, Mo, and MoS2 is filled into a clean Mo tube, and then filled into an outer sheath Monel alloy tube for drawing; in Step 4, the heating rate is 2℃ / min.

[0043] Figure 5 is the micrograph of the PbMo6S8-based superconducting wire prepared in this comparative example. From Figure 5 it can be seen that there are a large number of holes and microcracks in the PbMo6S8-based superconducting wire prepared by the traditional powder packing process combined with conventional slow heating. This is mainly because the powder packing density of the traditional powder packing process is low. After the wire is drawn, the density of the core wire is not effectively improved. The formation of holes and microcracks seriously hinders the transmission of superconducting current. Therefore, compared with Example 1, the critical current density of the PbMo6S8-based superconducting wire prepared in this comparative example is greatly reduced.

[0044] Example 2

[0045] This example includes the following steps:

[0046] Step 1: In a glove box under argon protection, according to the atomic ratio of Pb:Mo:S in the target product PbMo6S8-based superconducting wire, which is 1:6:8, select Pb powder, Mo powder, and S powder raw materials with a mass purity of over 99.99%. Then pour them into a mortar and mix and grind for more than 30 minutes to obtain a mixed powder;

[0047] Step 2: Press the mixed powder obtained in Step 1 into a green body. The pressing pressure is 10 MPa, and the pressure holding time is 5 minutes. Then vacuum seal it in a quartz tube and sinter at 850 °C for 6 hours to obtain a uniform mixed powder of intermediate state Pb, Mo, and MoS2;

[0048] Step 3: Press the mixed powder of intermediate state Pb, Mo, and MoS2 obtained in Step 2 into a powder rod, put it into a clean Nb tube in a glove box under argon protection, and then put the Nb tube containing the powder rod into an outer sheath Monel alloy tube for drawing to obtain PbMo6S8 wire;

[0049] Step 4: Directly put the wire obtained in Step 3 into a furnace at a temperature of 1500 °C in an argon atmosphere for sintering to obtain PbMo6S8-based superconducting wire.

[0050] After testing, the core wire density of the PbMo6S8-based superconducting wire prepared in this example is significantly increased compared with Comparative Example 2.

[0051] Figure 6 This is the XRD pattern of the core wire in the PbMo6S8-based superconducting wire prepared in this example. From Figure 6 it can be seen that the diffraction peaks in this figure are basically the PbMo6S8 phase, and the presence of a second phase such as MoS2 is not observed. Thus, it can be seen that through the method of the present invention, a PbMo6S8 superconducting wire with a high core wire density and a high superconducting phase content can be prepared.

[0052] Example 3

[0053] This example includes the following steps:

[0054] Step 1: In a glove box under argon protection, according to the atomic ratio of Pb:Mo:S in the target product PbMo6S8-based superconducting wire, which is 1:6:8, select Pb powder, Mo powder, and S powder raw materials with a mass purity of over 99.99%. Then pour them into a mortar and mix and grind for more than 30 minutes to obtain a mixed powder;

[0055] Step 2: Press the mixed powder obtained in Step 1 into a green body. The pressing pressure is 10 MPa, and the pressure holding time is 5 minutes. Then vacuum seal it in a quartz tube and sinter at 750 °C for 24 hours to obtain a uniform mixed powder of intermediate state Pb, Mo, and MoS2;

[0056] Step 3: Press the mixed powder of the intermediate states Pb, Mo, and MoS2 obtained in Step 2 into a powder rod, place it in a clean Nb tube in a glove box protected by argon, and then place the Nb tube containing the powder rod into an outer sheath Monel alloy tube for drawing to obtain PbMo6S8 wire;

[0057] Step 4: Heat the wire obtained in Step 3 in an argon atmosphere at a heating rate of 10 °C / min to 1000 °C and sinter for 24 h to obtain a PbMo6S8-based superconducting wire.

[0058] After testing, the critical current density of the PbMo6S8-based superconducting wire prepared in this example is further improved. At 4.2 K and self-field, its critical current density reaches 10 5 A / cm 2 Above, at the same time, the superconducting phase content and the core wire density are significantly improved compared with Comparative Examples 1 and 2.

[0059] In summary, the preparation method of the present invention effectively reduces the pores and microcracks in the core wire of the wire, increases the core wire density, enhances the grain boundary connectivity, promotes the uniform phase formation of PbMo6S8, reduces the content of the second phase in the core wire, and is beneficial to obtaining a high-current-carrying PbMo6S8 superconducting wire.

[0060] The above is only a preferred embodiment of the present invention and does not impose any limitation on the present invention. Any simple modification, change, and equivalent change made to the above embodiments according to the technical essence of the invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing a PbMo6S8-based superconducting wire, characterized in that: The method comprises the following steps: Step 1: In an argon-protected glove box, Pb powder, Mo powder and S powder raw materials are selected and mixed and ground according to the atomic ratio of Pb:Mo:S of the target product PbMo6S8-based superconducting wire of 1:6:8 to obtain a mixed powder; Step 2: Press the mixed powder obtained in step 1 into a green body, and sinter it in a quartz tube under vacuum to obtain a mixed powder of intermediate Pb, Mo and MoS2; Step 3: Press the mixed powder of intermediate Pb, Mo and MoS2 obtained in step 2 into a powder rod, put it into a metal tube, and draw it to obtain a wire; Step 4: Sinter the wire obtained in step 3 at a phase forming temperature to obtain a PbMo6S8-based superconducting wire.

2. The method for preparing a PbMo6S8-based superconducting wire according to claim 1, characterized in that: The sintering temperature in step 2 is 550° C. to 850° C., and the sintering time is 6 h to 48 h.

3. The method for preparing a PbMo6S8-based superconducting wire according to claim 1, characterized in that: The intermediate mixed powder of Pb, Mo and MoS2 in step 2 is a uniform mixed powder of Pb, Mo and MoS2.

4. The method for preparing a PbMo6S8-based superconducting wire according to claim 1, characterized in that: The metal tube described in step 3 is a Mo tube or a Nb tube, and the metal tube containing the powder rod is placed in an outer sheathed monel alloy tube and then drawn.

5. The method for preparing a PbMo6S8-based superconducting wire according to claim 1, characterized in that: In step 4, the wire is heated to the phase forming temperature at a heating rate of 5°C / min to 10°C / min for sintering, or the wire is directly placed in a furnace body with the phase forming temperature for sintering.

6. The method for preparing a PbMo6S8-based superconducting wire according to claim 1, characterized in that: The phase forming temperature in step 4 is 800° C. to 1500° C., and the sintering time is 12 h to 72 h.

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