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, uniformly mixing CuS, Mo and MoS2 powder, uniformly mixing with fused salt KCl, and carrying out heat treatment to obtain mixed powder; 2, the mixed powder is subjected to water washing, acid pickling and drying in sequence, and Mo6S8 powder is obtained; 3, the Mo6S8 powder is subjected to heat treatment to remove oxygen; 4, putting into a metal tube containing a central lead rod, and drawing into a wire rod; and 5, sealing in a quartz tube in vacuum, heating and sintering. According to the method, high-purity nano-sized CuMo6S8 powder is prepared by adopting a molten salt method to serve as precursor powder, and a central lead rod diffusion method is combined, so that a central Pb element is diffused to form a PbMo6S8 phase, generation of holes and a second phase is avoided, the number of grain boundaries and grain connectivity are effectively increased, an effective pinning center is formed, the current-carrying capacity of the PbMo6S8 superconducting wire is improved, and the service life of the PbMo6S8 superconducting wire is prolonged. The method is suitable for the field of preparation of superconducting magnets and the like under low temperature and high magnetic fields.

Description

Technical Field

[0001] The invention belongs to the technical field of superconducting materials, and in particular relates to a method for preparing a PbMo6S8-based superconducting wire. Background Art

[0002] With the rapid development of superconducting magnet technology, the steady-state magnetic field intensity is increasing day by day, which puts forward new requirements for the performance of superconducting materials used in magnets. At present, the preparation of high-field magnets mainly uses low-temperature superconducting materials NbTi and Nb3Sn, but their intrinsic upper critical field ( H c2 ) are relatively low, at 18T and 25T, respectively, making them difficult to manufacture for high-field magnets with magnetic field strengths exceeding 30T. High-temperature superconducting materials are currently primarily used in the form of intercalated coils to increase the magnetic field strength of magnets. Therefore, the development of new superconducting materials for high-field magnets is of vital importance for the further development of superconducting magnet technology and the advancement of the practical application of superconducting materials.

[0003] In the 1970s, Chevrel et al. first discovered superconductivity in molybdenum-sulfur compounds. Within this system, the PbMo6S8 superconducting material exhibits the highest superconducting transition temperature, 15K. Furthermore, its upper critical field, at 4.2K, approaches 60T, significantly favoring its application at low temperatures and high fields. Furthermore, this superconducting material exhibits low anisotropy and low production costs, facilitating the winding and industrial production of superconducting magnets. Therefore, PbMo6S8-based superconducting materials are expected to become the next generation of practical superconducting materials for high-magnetic-field applications.

[0004] However, during the phase formation process of the PbMo6S8 superconducting wire core powder, a second phase, MoS2, easily appears and exists at the grain boundaries, which can easily lead to weak grain boundary connections and affect the transmission of superconducting current. Furthermore, during the reaction process, after the raw materials have completely reacted, holes are easily formed in the core wire, reducing the connectivity between the grains. Furthermore, the magnetic flux pinning of PbMo6S8 superconducting materials is surface pinning. Therefore, reducing the PbMo6S8 grain size, increasing the number of grain boundaries, and forming effective magnetic flux pinning centers are effective means to improve the current carrying capacity of PbMo6S8 superconducting wires. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to address the shortcomings of the above-mentioned prior art and provide a method for preparing a PbMo6S8-based superconducting wire. This method uses a molten salt method to prepare high-purity nano-sized CuMo6S8 powder as a precursor powder. Combined with a central lead rod diffusion method, the central Pb element diffuses directly into the interstitial positions of the Mo6S8 to form a PbMo6S8 phase. This not only lowers the phase formation temperature but also effectively reduces the formation of voids and secondary phases, effectively increases the number of grain boundaries and grain connectivity, forms effective pinning centers, and improves the current-carrying capacity of the PbMo6S8 superconducting wire, thus solving the problem of poor current-carrying capacity of existing PbMo6S8 superconducting wires.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: 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, CuS, Mo, and MoS2 powders are uniformly mixed according to the atomic ratio of Cu:Mo:S in CuMo6S8 to obtain a mixed powder, and then the mixed powder is uniformly mixed with molten salt KCl in a mass ratio of 1:2, and heat-treated in an argon atmosphere to obtain a mixed powder containing KCl and CuMo6S8; Step 2: The mixed powder obtained in step 1 is washed with water, acid-washed, and dried in sequence to remove the molten salt KCl and Cu atoms in the mixed powder, respectively, to obtain a nano-sized Mo6S8 powder with a mass purity higher than 99%; Step 3: heat-treating the Mo6S8 powder obtained in step 2 to remove adsorbed oxygen; Step 4: The Mo6S8 powder after deoxygenation in step 3 is placed into a metal tube containing a central lead rod, and then drawn into PbMo6S8 wire; Step 5: The PbMo6S8 wire obtained in step 4 is vacuum sealed in a quartz tube and heated and sintered to obtain a PbMo6S8-based superconducting wire.

[0007] The present invention first adopts a molten salt method to mix raw material powder with molten salt KCl and then heat treat it to obtain a mixed powder containing KCl and CuMo6S8, and then washes it with water and pickles to remove the molten salt KCl and Cu atoms to obtain high-quality, pure, nano-sized Mo6S8 powder; then, the Mo6S8 powder is used as a precursor powder, deoxidized, and loaded into a metal tube containing a central lead rod, and drawn to prepare a PbMo6S8 wire; by utilizing the diffusion of the Pb element during the heat treatment process, the Pb element is directly inserted into the interstitial position of the Mo6S8 unit to form a PbMo6S8 phase, effectively avoiding holes caused by raw material consumption during the reaction process and a second phase caused by incomplete reaction; and then, the PbMo6S8-based superconducting wire is obtained by heating and sintering. The method of the present invention not only effectively reduces the phase formation temperature, obtains nano-sized PbMo6S8 grains, increases the number of grain boundaries, forms effective magnetic flux pinning centers, and improves its magnetic flux pinning ability, but also concentrates the holes at the center of the wire and forms a dense superconducting layer around the center of the wire, effectively reducing holes and microcracks, and enhancing grain connectivity, thereby improving the current carrying capacity of the PbMo6S8 superconducting wire.

[0008] The aforementioned method for preparing a PbMo6S8-based superconducting wire is characterized in that, in step three, the Mo6S8 powder is heat-treated in an argon-hydrogen atmosphere to remove oxygen. Typically, the heat-treatment temperature for deoxidation is 800°C, and the holding time is 3 hours. Heat-treating the Mo6S8 powder in an argon-hydrogen atmosphere to remove oxygen prevents the adverse effects of oxygen on the superconducting properties of the PbMo6S8.

[0009] The above-mentioned method for preparing a PbMo6S8-based superconducting wire is characterized in that the lead rod in the metal tube containing the central lead rod in step 4 is located at the center of the metal tube.

[0010] The above-mentioned method for preparing a PbMo6S8-based superconducting wire is characterized in that the heating and sintering temperature in step five is 600°C to 1200°C, and the time is 5h to 72h.

[0011] Compared with the prior art, the present invention has the following advantages: 1. The present invention adopts a molten salt method to prepare CuMo6S8 powder with fine grain size, high purity and good uniformity as a precursor powder. The central Pb element is directly inserted into the interstitial position of Mo6S8 through diffusion to form a PbMo6S8 phase, which effectively reduces the phase formation temperature of PbMo6S8 and avoids the holes generated by the phase formation reaction and the second phase caused by incomplete reaction, thereby improving the purity of the PbMo6S8 phase.

[0012] 2. The present invention adopts a molten salt method combined with an acid washing process to prepare nano-sized Mo6S8 powder, and then obtains a nano-sized PbMo6S8 phase, which increases the number of grain boundaries, forms effective magnetic flux pinning centers, and further improves the current-carrying capacity of the PbMo6S8 superconducting wire, laying the foundation for its application in low temperature and high magnetic field.

[0013] 3. 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 implement and apply.

[0014] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a microscopic morphology of the Mo6S8 powder prepared in Example 1 of the present invention.

[0016] Figure 2 This is a cross-sectional metallographic image of the PbMo6S8-based superconducting wire prepared in Example 1 of the present invention.

[0017] Figure 3 2 are XRD patterns of the PbMo6S8-based superconducting wire core wires prepared in Example 1 of the present invention and Comparative Example 1.

[0018] Figure 4 This is a cross-sectional microscopic morphology of the PbMo6S8-based superconducting wire core wire prepared in Comparative Example 2 of the present invention.

[0019] Figure 5 This is a microscopic morphology of the PbMo6S8-based superconducting wire core wire prepared in Comparative Example 3 of the present invention.

[0020] Figure 6 This is a microscopic morphology of the PbMo6S8-based superconducting wire core wire prepared in Example 2 of the present invention.

[0021] Figure 7 This is the XRD pattern of the PbMo6S8-based superconducting wire core wire prepared in Example 3 of the present invention. DETAILED DESCRIPTION

[0022] Example 1 This embodiment includes the following steps: Step 1. In an argon-protected glove box, according to the atomic ratio of Cu:Mo:S in CuMo6S8 of 1:6:8, select CuS, Mo and MoS2 powders with a mass purity of more than 99.99% and mix them evenly to obtain a mixed powder. Then, mix the mixed powder with molten salt KCl according to a mass ratio of CuMo6S8:KCl of 1:2, pour the mixture into a mortar and grind for more than 30 minutes, and then sinter at 850°C in an argon atmosphere for 60 hours for heat treatment to obtain a mixed powder containing KCl and CuMo6S8; Step 2: The mixed powder obtained in step 1 is washed with water, acid-washed, and dried in sequence to remove the molten salt KCl and Cu atoms in the mixed powder, respectively, to obtain a nano-sized Mo6S8 powder with a mass purity higher than 99%; Step 3: heat-treating the Mo6S8 powder obtained in step 2 at 800°C in an argon-hydrogen atmosphere to remove adsorbed oxygen; Step 4: The Mo6S8 powder after deoxygenation in step 3 is placed into a metal tube containing a central lead rod, and then drawn into a PbMo6S8 wire; the lead rod in the metal tube containing the central lead rod is located at the center of the metal tube; Step 5: The PbMo6S8 wire obtained in step 4 is vacuum sealed in a quartz tube and heated and sintered at 1000° C. for 12 hours to obtain a PbMo6S8-based superconducting wire.

[0023] Figure 1 The microscopic morphology of Mo6S8 powder prepared in this example is shown in FIG. Figure 1 It can be seen that the grain size of the Mo6S8 powder is less than 500 nm, which is nanometer size.

[0024] Figure 2 This is a cross-sectional metallographic image of the PbMo6S8-based superconducting wire prepared in this embodiment. Figure 2 It can be seen that a dense superconducting layer exists in the PbMo6S8-based superconducting wire, indicating that the method of the present invention utilizes the diffusion of the Pb element to gather all the holes at the center of the wire, forming a dense superconducting layer in the wire, and preparing a PbMo6S8-based superconducting wire.

[0025] Comparative Example 1 The difference between this comparative example and Example 1 is that the process of heat-treating and deoxidizing the Mo6S8 powder in step 3 is not performed, and the Mo6S8 powder obtained in step 2 is directly subjected to the tube filling and drawing processes in step 4 and the tube sealing and heating sintering processes in step 5.

[0026] Figure 3 The XRD patterns of the PbMo6S8-based superconducting wire core wires prepared in Example 1 and Comparative Example 1 of the present invention are shown in FIG. Figure 3It can be seen that the Mo6S8 powder in Comparative Example 1 was not subjected to heat treatment for deoxidation, so the formed PbMo6S8 phase contained a MoO2 phase. However, in Example 1, after the Mo6S8 powder was subjected to heat treatment for deoxidation, the diffraction peak of MoO2 disappeared, and a single PbMo6S8 phase was obtained, indicating that oxygen has an important influence on the superconductivity of the PbMo6S8 superconducting material. The formation of a high-content PbMo6S8 phase is ensured by heat treatment for deoxidation of the Mo6S8 powder.

[0027] Comparative Example 2 The difference between this comparative example and Example 1 is that the conventional powder filling method is adopted to directly fill the mixed powder of Mo6S8 powder and Pb powder into the metal tube for the tube filling and drawing in step 4 and the tube sealing and heating sintering process in step 5.

[0028] Figure 4 This is a cross-sectional microscopic morphology of the core wire of the PbMo6S8-based superconducting wire prepared in Comparative Example 2 of the present invention. Figure 4 It can be seen that there are obvious large holes in the core wire of the PbMo6S8-based superconducting wire formed by the reaction of Pb powder. Figure 2 and Figure 4 By comparison, it can be seen that in this embodiment, the Pb element is placed in the form of a lead rod at the center of the wire, so that the holes formed after the Pb element diffuses and reacts completely are concentrated in the center of the wire, greatly enhancing the connectivity between the grains, and forming a dense superconducting layer around them. This avoids the problem of leaving holes in the core wire superconducting layer after the Pb element completely reacts during the sintering process of traditional powder-filled tubes, resulting in a decrease in core wire density and poor grain connectivity.

[0029] Comparative Example 3 The difference between this comparative example and Example 1 is that steps 1 to 3 are not performed, and the mixed powder of Mo and MoS2 is directly loaded into a metal tube containing a central lead rod to carry out the tube loading and drawing processes in step 4 and the tube sealing and heating sintering processes in step 5.

[0030] Figure 5 The microscopic morphology of the core wire of the PbMo6S8-based superconducting wire prepared in this comparative example is shown in FIG. Figure 5 It can be seen that in the process of preparing PbMo6S8-based superconducting wire using a mixed powder of Mo and MoS2 as precursor powder, incomplete reaction is likely to occur, resulting in a large amount of unreacted MoS2 remaining in the core wire of the PbMo6S8-based superconducting wire, affecting the purity of the PbMo6S8 superconducting phase, and thus not conducive to improving the current-carrying performance of the PbMo6S8-based superconducting wire.

[0031] Example 2 This embodiment includes the following steps: Step 1. In an argon-protected glove box, according to the atomic ratio of Cu:Mo:S in CuMo6S8 of 1:6:8, select CuS, Mo and MoS2 powders with a mass purity of more than 99.99% and mix them evenly to obtain a mixed powder. Then, mix the mixed powder with molten salt KCl according to a mass ratio of CuMo6S8:KCl of 1:2, pour the mixture into a mortar and grind for more than 30 minutes, and then sinter at 850°C in an argon atmosphere for 60 hours for heat treatment to obtain a mixed powder containing KCl and CuMo6S8; Step 2: The mixed powder obtained in step 1 is washed with water, acid-washed, and dried in sequence to remove the molten salt KCl and Cu atoms in the mixed powder, respectively, to obtain a nano-sized Mo6S8 powder with a mass purity higher than 99%; Step 3: heat-treating the Mo6S8 powder obtained in step 2 at 800°C in an argon-hydrogen atmosphere to remove adsorbed oxygen; Step 4: The Mo6S8 powder after deoxygenation in step 3 is placed into a metal tube containing a central lead rod, and then drawn into a PbMo6S8 wire; the lead rod in the metal tube containing the central lead rod is located at the center of the metal tube; Step 5: The PbMo6S8 wire obtained in step 4 is vacuum sealed in a quartz tube and heated and sintered at 1200° C. for 5 h to obtain a PbMo6S8-based superconducting wire.

[0032] Figure 6 This is the microscopic morphology of the PbMo6S8-based superconducting wire core prepared in this embodiment. Figure 6 It can be seen that the PbMo6S8-based superconducting wire forms a phase quickly and evenly, there are no large holes in the core wire, and the core wire is relatively dense; further testing found that the superconducting transition temperature of the PbMo6S8-based superconducting wire is 13K.

[0033] Example 3 This embodiment includes the following steps: Step 1. In an argon-protected glove box, according to the atomic ratio of Cu:Mo:S in CuMo6S8 of 1:6:8, select CuS, Mo and MoS2 powders with a mass purity of more than 99.99% and mix them evenly to obtain a mixed powder. Then, mix the mixed powder with molten salt KCl according to a mass ratio of CuMo6S8:KCl of 1:2, pour the mixture into a mortar and grind for more than 30 minutes, and then sinter at 850°C in an argon atmosphere for 60 hours for heat treatment to obtain a mixed powder containing KCl and CuMo6S8; Step 2: The mixed powder obtained in step 1 is washed with water, acid-washed, and dried in sequence to remove the molten salt KCl and Cu atoms in the mixed powder, respectively, to obtain a nano-sized Mo6S8 powder with a mass purity higher than 99%; Step 3: heat-treating the Mo6S8 powder obtained in step 2 at 800°C in an argon-hydrogen atmosphere to remove adsorbed oxygen; Step 4: The Mo6S8 powder after deoxygenation in step 3 is placed into a metal tube containing a central lead rod, and then drawn into a PbMo6S8 wire; the lead rod in the metal tube containing the central lead rod is located at the center of the metal tube; Step 5: The PbMo6S8 wire obtained in step 4 is vacuum sealed in a quartz tube and heated and sintered at 600° C. for 72 hours to obtain a PbMo6S8-based superconducting wire.

[0034] Figure 7 The XRD pattern of the PbMo6S8-based superconducting wire core prepared in this embodiment is as follows: Figure 7 It can be seen that the main phase of the core wire of the PbMo6S8-based superconducting wire is the PbMo6S8 phase. Calculation shows that the superconducting phase content exceeds 95%. This shows that the technical solution of the present invention can be used to prepare a PbMo6S8-based superconducting wire with a high superconducting phase content.

[0035] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent variation made to the above embodiment based on the essence of the invention technology shall still fall within the scope of protection 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, CuS, Mo, and MoS2 powders are uniformly mixed according to the atomic ratio of Cu:Mo:S in CuMo6S8 to obtain a mixed powder, and then the mixed powder is uniformly mixed with molten salt KCl in a mass ratio of 1:2, and heat-treated in an argon atmosphere to obtain a mixed powder containing KCl and CuMo6S8; Step 2: The mixed powder obtained in step 1 is washed with water, acid-washed, and dried in sequence to remove the molten salt KCl and Cu atoms in the mixed powder, respectively, to obtain a nano-sized Mo6S8 powder with a mass purity higher than 99%; Step 3: heat-treating the Mo6S8 powder obtained in step 2 to remove adsorbed oxygen; Step 4: The Mo6S8 powder after deoxygenation in step 3 is placed into a metal tube containing a central lead rod, and then drawn into PbMo6S8 wire; Step 5: The PbMo6S8 wire obtained in step 4 is vacuum sealed in a quartz tube and heated and sintered to obtain a PbMo6S8-based superconducting wire.

2. The method for preparing a PbMo6S8-based superconducting wire according to claim 1, wherein: In step 3, the Mo6S8 powder is heat-treated in an argon-hydrogen atmosphere to remove oxygen. Typically, the heat-treatment temperature for deoxidation is 800° C. and the holding time is 3 hours.

3. The method for preparing a PbMo6S8-based superconducting wire according to claim 1, wherein: In the metal tube containing the central lead rod described in step 4, the lead rod is located at the center of the metal tube.

4. The method for preparing a PbMo6S8-based superconducting wire according to claim 1, wherein: The heating and sintering temperature in step 5 is 600° C. to 1200° C., and the sintering time is 5 h to 72 h.

Images