CN111825444B - Method for introducing columnar defects into ex-situ high-temperature superconducting thin film - Google Patents

Abstract

The invention relates to a method for introducing columnar defects into an ex-situ high-temperature superconducting film, which mainly solves the technical problem that the energy level required for manufacturing the columnar defects by laser irradiation is very high. The invention creatively provides a method for introducing columnar defects through femtosecond laser irradiation in the phase forming intermediate process of a YBCO superconducting film strip. Namely, various types of irradiation in the vertical direction on the surface of an intermediate product are carried out in the preparation process, so that the defect of preferred orientation is generated, and further, the columnar defect is introduced into a final product. The practical application of the YBCO superconducting tape is improved.

Description

A method for introducing columnar defects in ex-situ high temperature superconducting thin films

technical field

The invention relates to a method for introducing columnar defects in an ex-situ high-temperature superconducting thin film, that is, in the preparation process, femtosecond laser irradiation in the vertical direction of the surface of an intermediate product is performed to generate defects of preferential orientation, and then in the final product The introduction of columnar defects belongs to the field of material design and processing.

Background technique

Yttrium barium copper oxide (YBa ₂ Cu ₃ O _7-δ , hereinafter referred to as YBCO) high-temperature superconducting thin films has always been a research hotspot in the field of practical superconductivity due to its excellent performance and potential price advantage. However, at present, the practical application of YBCO superconducting films still faces two major problems: First, there is a Josephson-type weak connection between the grain boundaries of the YBCO superconducting film, which leads to different current-carrying capacities inside the superconducting grains and grain boundaries, which limits the critical The improvement of the current density; second, the lower critical current density under the external magnetic field also limits the application of YBCO superconducting films in many fields, so it is necessary to improve its field performance.

As the second type of superconductor, YBCO exhibits a mixed state with magnetic flux vortex lines when the external magnetic field H is greater than the lower critical field, and the motion of the vortex lines under the Lorentz force leads to a sharp drop in the critical current density ( Jc ) of the superconductor . For the undoped YBCO superconductor: there is an intrinsic pinning center in the ab plane, J ( θ ) will have a strong peak at the H‖ab plane; for the c axis, due to the lack of the corresponding intrinsic pinning The Jc in the direction of the H‖c axis is much smaller; therefore, the critical current density in the undoped YBCO film exhibits a large anisotropy.

In order to improve the electrical transport performance of the YBCO superconducting film in an external magnetic field, the researchers introduced a magnetic flux pinning center in the YBCO superconducting film by artificial means to pin the movement of the magnetic flux vortex lines. Artificial pinning centers (APCs) are classified into: (1) zero-dimensional APCs, point defects such as oxygen vacancies and element substitutions in superconductors; (2) one-dimensional APCs, including linear dislocations or Artificially formed nanopillars; (3) 2D APCs, the stacking faults existing in the superconductor, nanosheets, twin boundaries and other defects that can become pinning centers; (4) 3D APCs, dispersed in the superconductor with Nanoscale heterogeneous particles dominated by pinning centers.

Methods for introducing artificial pinning centers include: (1) high-energy heavy particle beam irradiation of high-temperature YBCO superconductors to generate c -axis aligned line defects; (2) addition of second-phase nanoparticles, including BaMO ₃ (M=Zr, Hf, Sm) etc.) perovskite structure compounds; (3) depositing a layer of nanoparticles on the metal substrate and then epitaxially growing YBCO. Currently, adding second-phase nanoparticles is an effective means. However, whether the second-phase nanoparticles are added by in-situ or ex-situ methods, firstly, it will directly affect the preparation and growth of YBCO thin films, and the process parameters need to be adjusted in time. Secondly, the size and distribution of nanoparticles have a great influence on the performance of YBCO superconducting thin films. It is more sensitive to the preparation process. High-energy heavy particle beam radiation, as a pure and additive-free method, directly bombards the grown YBCO superconducting film with heavy particles to create columnar physical defects with small size and uniform distribution parallel to the c -axis. The research shows that the defects formed by irradiation can effectively increase the Jc in the H‖c axis direction, and improve the practical application potential value of YBCO coated conductors.

In the process of preparing YBCO superconducting tapes by metal-organic solution chemical method (TFA-MOD), the raw materials are prepared through precursor solution, low-temperature coating and pyrolysis, medium-temperature pre-sintering, high-temperature phase-forming sintering, oxygen absorption, and silver-plated protective layer. And packaging and other steps, and finally form a commercially available YBCO high temperature superconducting tape. During the whole process, the low temperature coating film will shrink by ~100% thickness from colloid to pyrolysis film; after pre-sintering at medium temperature, the film will shrink again by ~10%; finally, after high temperature sintering, the film will shrink by ~50% thickness again . In the method of introducing artificial pinning centers, the substrate surface modification occurs before the YBCO raw material coating step, the second-phase nanoparticles are added during the preparation of the YBCO precursor solution, and the current high-energy particle radiation directly acts on high temperature on the crystallized film. Usually high-energy heavy particle irradiation can effectively bombard columnar defects on the YBCO dense ceramic thin film sintered at high temperature, but the energy level required is very large, and the energy level is usually as high as GeV or more. By adjusting the particle beam energy level of the high-energy particle radiation, the defect size and density distribution in thin films can be directly and effectively controlled. This is currently difficult to achieve by other means. But high-energy particle irradiation also has its limitations, that is, the energy levels used to create columnar defects are high. If the particle irradiation method is applied to the large-scale industrial manufacturing of YBCO superconducting tapes, the equipment and energy costs are too high.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for introducing columnar defects in an ex-situ high temperature superconducting thin film, which mainly solves the technical problem that laser irradiation requires a high energy level to manufacture columnar defects. The idea of the present invention is to provide a pioneering method for introducing columnar defects by femtosecond laser irradiation in the intermediate process of YBCO superconducting thin film strip phase formation. That is, various types of irradiation in the vertical direction of the surface of the intermediate product are carried out during the preparation process, resulting in defects of preferential orientation, and then columnar defects are introduced into the final product, which belongs to the field of material design and processing.

Technical scheme of the present invention: a method for introducing columnar defects in an ex-situ high temperature superconducting film, comprising the following steps:

(1) Configure YBCO precursor solution: weigh the raw materials yttrium acetate, barium acetate and copper acetate, dissolve yttrium acetate and barium acetate in deionized water and trifluoroacetic acid and stir, and dissolve copper acetate in deionized water and propionic acid Stir; add methanol to the two solutions for three times of reduced pressure distillation, then mix, add methanol to volume, and prepare a YBCO precursor solution;

(2) Spin coating: spin coating the TFA-YBCO precursor solution on the Hassler alloy substrate on a spin coater to obtain a precursor film;

(3) Low-temperature treatment: The spin-coated film is placed in a muffle furnace for low-temperature pyrolysis, and oxygen is first introduced into room temperature deionized water, and then into the furnace, and the film undergoes low-temperature treatment in the furnace;

(4) Medium temperature treatment: The pyrolysis film treated at low temperature is subjected to medium temperature treatment in an atmosphere of oxygen and water vapor to obtain a medium temperature treatment film;

(5) Femtosecond laser irradiation drilling: place the low-temperature pyrolysis film or the medium-temperature treatment film horizontally, and place the surface coated with the raw material upward, and irradiate the hole with the femtosecond laser perpendicular to the film plane;

(6) High temperature treatment: The irradiated film is then subjected to high temperature heat treatment in a high temperature furnace, in which the atmosphere is nitrogen, oxygen and water vapor; then the temperature is lowered in nitrogen and oxygen atmosphere to obtain high temperature crystallization with irradiated columnar physical defects the YBCO film;

(7) Post-treatment: the high-temperature crystallization film is then subjected to oxygen absorption treatment, and is kept in pure oxygen for a period of time to obtain a yttrium barium copper oxide superconducting thin film strip.

In the step (1), yttrium acetate, barium acetate and copper acetate are weighed in a stoichiometric ratio of 1:2:3; the stirring time is 1-2 h, and the total cation concentration of the YBCO precursor solution is 2.5 mol/L .

In the step (2), the spin coating machine adopts a rotational speed of 3000-6000 rpm, and the spin coating time is 1 min.

In the step (3), the low temperature treatment is to reduce the furnace temperature from 350°C to 150°C within 40 minutes.

In the step (4), the medium temperature treatment is heat treatment at 600 °C for 10 min, the total pressure of water vapor and oxygen atmosphere is about 20-23 Pa, preferably the oxygen pressure is 13 Pa, and the water vapor pressure is 10 Pa.

In the step (5), the femtosecond laser energy is 4-10 μJ , preferably 5 μJ , and the spot diameter is about 2 μm .

In the step (6), the high temperature treatment is to raise the temperature from room temperature to 780°C at a heating rate of 25°C, and keep the temperature for 120 minutes, wherein the atmosphere during the first 60 minutes of holding time is a nitrogen-oxygen mixture with an oxygen content of 150 ppm (the pressure is 150 ppm). 1 atm), and the mixed gas was first introduced into a water bath at 35 °C and then into the muffle furnace; the atmosphere for the last 60 min of holding time was a dry nitrogen-oxygen mixture.

In the step (7) and the post-treatment, after the heat preservation in the step (6) is completed, the temperature is lowered to 450° C. with the furnace, and the temperature is maintained for 60 min, and the reaction atmosphere in this process is changed from a nitrogen-oxygen mixture to a pure oxygen atmosphere; After the heat preservation is completed, the temperature is lowered to room temperature with the furnace.

The pyrolyzed films with columnar defects are then sintered at high temperature to obtain high-temperature crystallized YBCO superconducting films with columnar defects. The femtosecond laser is irradiated perpendicular to the surface of the sample, and the columnar defects formed on the YBCO low-temperature pyrolysis film will also break down the film to form columnar defects perpendicular to the surface of the film.

The thickness of the YBCO low-temperature pyrolysis film used to manufacture columnar defects ranges from 100 to 3000 nm.

The size and density distribution of the columnar defects can be directly and effectively controlled by adjusting the energy and intensity of the femtosecond laser.

The beneficial effects of the present invention are as follows: defects are introduced in the intermediate link of the preparation of the YBCO superconducting thin film and its coated conductor, and it is found that the low-temperature pyrolysis film and the medium-temperature pre-fired film before the high-temperature crystallization step are relatively loose in structure, so the use of energy level The low femtosecond laser irradiation can bombard the film to create columnar defects, and finally form an effective artificial pinning center. very important meaning.

Description of drawings

Fig. 1 is the process flow diagram of the present invention.

Fig. 2 is a surface topography (SEM) of the low-temperature pyrolysis film obtained in Example 1 of the present invention after being perforated by a femtosecond laser.

Fig. 3 is a surface topography (SEM) of the medium-temperature treated film obtained in Example 1 of the present invention after being perforated by a femtosecond laser.

4 is a surface topography (SEM) of the perforated low-temperature pyrolysis film obtained in Example 1 of the present invention after high-temperature crystallization.

FIG. 5 is the surface topography (SEM) of the perforated medium temperature treated film obtained in Example 1 of the present invention after high temperature crystallization.

Detailed ways

The present invention is further described below through the following embodiments, and it should be understood that the following embodiments are only used to illustrate the present invention, but not to limit the present invention. Referring to Figure 1,

The preparation process of YBCO superconducting thin film by TFA-MOD method is as follows:

1. Weigh the raw materials yttrium acetate, barium acetate and copper acetate according to the stoichiometric ratio (1:2:3.3), dissolve the yttrium acetate and barium acetate in an appropriate amount of deionized water and excess trifluoroacetic acid and stir for 1-2 h , while copper acetate was dissolved in an appropriate amount of deionized water and excess propionic acid and stirred for 1-2 h. Methanol was added to the two solutions for three times of reduced pressure distillation, then mixed, and methanol was added to the volume to prepare a YBCO precursor solution with a total cation concentration of 2.5 mol/L.

2. Spin-coat the YBCO precursor solution with a total cation concentration of 2.5 mol/L on the surface of the Hassler alloy substrate at a speed of 3000-6000 rpm on a spin coater for 1 min.

3. The spin-coated film was put into a muffle furnace for low-temperature pyrolysis. Oxygen with a flow rate of 1.5 L/min was first introduced into deionized water at room temperature, and then into the furnace. The film was subjected to low-temperature treatment for 40 minutes in the furnace. The temperature dropped from 350 °C to 150 °C during this time.

4. The pyrolysis film treated at low temperature was heat-treated at 600 °C for 10 min in an atmosphere of 13 Pa oxygen and 10 Pa water vapor to obtain a medium temperature treated film.

5. The low-temperature pyrolysis film or the middle-temperature treatment film is placed horizontally, and the surface coated with the raw material is placed upward, and placed under the femtosecond laser to carry out vertical irradiation and perforation; the femtosecond laser energy is 5 μ J, and the spot diameter is 5 μ J. 2 μm . Figure 2 shows the surface topography of the low temperature pyrolysis film after perforation by femtosecond laser, and Figure 3 shows the surface topography of the medium temperature treated film after perforation by femtosecond laser.

6. The irradiated film is then heated from room temperature to 780°C at a heating rate of 25°C in a high-temperature furnace, and kept for 120 minutes. The atmosphere during the first 60 minutes of holding time is a nitrogen-oxygen mixture with an oxygen content of 150 ppm ( The air pressure was 1 atm), and the mixed gas was first passed into a water bath at 35 °C and then into the muffle furnace; the atmosphere for the last 60 min of holding time was a dry nitrogen-oxygen mixture. The surface topography of the perforated low temperature pyrolysis film after high temperature crystallization is shown in Figure 4, and the surface topography of the perforated medium temperature treated film after high temperature crystallization is shown in Figure 5.

7. After the above-mentioned heat preservation is completed, the oxygen absorption treatment is carried out, that is, the temperature is lowered to 450 ℃ with the furnace, and the heat preservation is carried out for 60 min. In this process, the reaction atmosphere is changed from a nitrogen-oxygen mixture to a pure oxygen atmosphere (the pressure is 1 atm); The temperature is lowered to room temperature with the furnace to obtain the yttrium barium copper oxide superconducting thin film strip after oxygen absorption.

Claims (7)

1. a method for introducing columnar defects in an ex-situ method high temperature superconducting film, is characterized in that: comprise the following steps: (1) Configure YBCO precursor solution: weigh the raw materials yttrium acetate, barium acetate and copper acetate, dissolve yttrium acetate and barium acetate in deionized water and trifluoroacetic acid and stir, and dissolve copper acetate in deionized water and propionic acid Stir; add methanol to the two solutions for three times of reduced pressure distillation, then mix, add methanol to volume, and prepare a YBCO precursor solution; (2) Spin-coating the TFA-YBCO precursor solution on the Hassler alloy substrate on a spin coater to obtain a precursor film; (3) Low-temperature treatment: The spin-coated film is placed in a muffle furnace for low-temperature pyrolysis, oxygen is first introduced into room temperature deionized water, and then into the furnace, and the film undergoes low-temperature treatment in the furnace; the low-temperature treatment is 40min The inner furnace temperature is reduced from 350 ℃ to 150 ℃; (4) Medium temperature treatment: The pyrolysis film treated at low temperature is subjected to medium temperature treatment in an atmosphere of oxygen and water vapor to obtain a medium temperature treatment film; the medium temperature treatment is heat treatment at 600 °C for 10 min; (5) Femtosecond laser irradiation drilling: place the low-temperature pyrolysis film or the medium-temperature treatment film horizontally, and place the surface coated with the raw material upward, and irradiate the hole with the femtosecond laser perpendicular to the film plane; (6) High temperature treatment: The irradiated film is then subjected to high temperature heat treatment in a high temperature furnace, in which the atmosphere is nitrogen, oxygen and water vapor; then the temperature is lowered in nitrogen and oxygen atmosphere to obtain high temperature crystallization with irradiated columnar physical defects the YBCO film; (7) Post-treatment: the high-temperature crystallization film is then subjected to oxygen absorption treatment, and is kept in pure oxygen for a period of time to obtain a yttrium barium copper oxide superconducting thin film strip. 2. the method for introducing columnar defects in the ex-situ high temperature superconducting film according to claim 1, is characterized in that: described step (1), yttrium acetate, barium acetate, copper acetate are by stoichiometric ratio 1:2 : 3.3 Weigh; the stirring time is 1-2 h, and the total cation concentration of the YBCO precursor solution is 2.5 mol/L. 3 . The method for introducing columnar defects in an ex-situ high temperature superconducting thin film according to claim 1 , wherein in the step (2), during spin coating, the rotation speed is 3000-6000 rpm, and the duration is 1 min. 4 . 4 . The method for introducing columnar defects in an ex-situ high temperature superconducting thin film according to claim 1 , wherein in the step (5), the femtosecond laser energy is 4-10 μJ . 5 . 5. the method for introducing columnar defect in ex-situ method high temperature superconducting thin film according to claim 4 is characterized in that: femtosecond laser energy is 5 μ J, and spot diameter is 2 μ m. 6 . The method for introducing columnar defects in an ex-situ high-temperature superconducting thin film according to claim 1 , wherein in the step (6), the high-temperature treatment is a high-temperature heat treatment at 780° C. for 120 minutes, and the temperature is maintained for 120 minutes. 7 . The atmosphere during the first 60 min holding time was a wet nitrogen-oxygen mixture with an oxygen content of 150 ppm in a water bath at 35 °C; the atmosphere during the last 60 min holding time was a dry nitrogen-oxygen mixture. 7 . The method for introducing columnar defects in an ex-situ high-temperature superconducting thin film according to claim 1 , wherein in the step (7), in the post-processing, a pure oxygen atmosphere is used, and the gas pressure is 1 atm. 8 . , the oxygen absorption treatment temperature is 450 ℃ for 60 minutes, and finally the temperature is naturally cooled.

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