Method for preparing two-dimensional La3Ni2O7 superconducting material based on supercritical CO2 stripping technology

Abstract

The invention belongs to the technical field of superconducting material preparation, and discloses a method for preparing a two-dimensional La3Ni2O7 superconducting material based on a supercritical CO2 stripping technology. The preparation method comprises the following steps: (1) grinding La3Ni2O7 until the particle size is less than or equal to 10 microns, then adding the La3Ni2O7 into a mixed solvent composed of water and absolute ethyl alcohol according to the volume ratio of 1: (1-2) to obtain a turbid liquid with the concentration of 1g / L, and then carrying out ultrasonic treatment for 60-240 minutes to obtain a dispersion liquid; (2) transferring the dispersion liquid obtained in the step (1) into a supercritical CO2 reaction device, injecting CO2, stirring and reacting for 4-6 hours under a supercritical condition, releasing the pressure to normal pressure after the reaction is finished, and naturally cooling to room temperature in air; and (3) carrying out centrifugal treatment on the reaction liquid obtained in the step (2), and then carrying out vacuum drying on supernate to obtain the two-dimensional La3Ni2O7 superconducting material. The two-dimensional La3Ni2O7 superconducting material prepared by the preparation method disclosed by the invention has an excellent superconducting property.

Description

Technical Field

[0001] The present invention belongs to the technical field of superconducting material preparation, and particularly relates to a method for preparing two-dimensional La3Ni2O7 superconducting material based on supercritical CO2 exfoliation technology. Background Art

[0002] The research on high-temperature superconducting materials has always been one of the most challenging frontier topics in the fields of condensed matter physics and materials science. In recent years, the discovery of nickel-based superconductor La3Ni2O7 has opened up a new direction for high-temperature superconducting research. This material has a unique Ruddlesden-Popper layered structure, which is composed of alternating LaO and NiO planes stacked along the c-axis. Its special electronic configuration (3d 7.5 ) is both similar to and significantly different from that of copper-based superconductors. Notably, the apical oxygen atoms between adjacent NiO2 layers in La3Ni2O7 may play a key role in its superconducting behavior, providing an important opportunity for exploring new high-temperature superconducting mechanisms.

[0003] However, there are still significant challenges in realizing the practical application of La3Ni2O7 materials: Firstly, the La3Ni2O7 prepared by traditional preparation methods is mainly in bulk form, and it is difficult to achieve the controllable preparation of single-layer or few-layer two-dimensional structures. The atomic-scale thickness facilitates integration into existing semiconductor processes, enabling the miniaturization of devices such as superconducting qubits and superconducting nanowire single-photon detectors. Secondly, the atomic-scale thickness of two-dimensional superconductors can significantly reduce the operating energy consumption and is suitable for ultra-low-power electronic devices. Thirdly, increasing the critical transition temperature of superconductors has always been an important goal in superconducting research, as it helps to expand the application scope of superconducting materials. However, existing preparation technologies cannot precisely control the lattice strain of the material, and strain engineering is considered an effective means to regulate the superconducting critical temperature. Summary of the Invention

[0004] In order to solve the deficiencies existing in the prior art, the purpose of the present invention is to provide a method for preparing two-dimensional La3Ni2O7 superconducting material based on supercritical CO2 exfoliation technology.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows: A method for preparing two-dimensional La3Ni2O7 superconducting material based on supercritical CO2 exfoliation technology, the steps are as follows: (1), Grind La3Ni2O7 to a particle size ≤ 10 μm, and then place it in a mixed solvent composed of water and absolute ethanol according to a volume ratio of 1∶(1~2) to obtain a suspension with a concentration of 1 g / L. Then, ultrasonically treat it for 60~240 min to obtain a dispersion; (2) Transfer the dispersion obtained in step (1) into a supercritical CO2 reaction device, inject CO2, and stir and react for 4 - 6 h under supercritical conditions. After the reaction, release the pressure to atmospheric pressure and naturally cool to room temperature in air; (3) Centrifuge the reaction solution obtained in step (2), and then take the supernatant for vacuum drying to obtain a two-dimensional La3Ni2O7 superconducting material.

[0006] Preferably, in step (2), the temperature of the supercritical conditions is 60 - 120 °C and the pressure is 8 - 18 MPa.

[0007] Preferably, in step (2), the stirring is magnetic stirring with a rotation speed of 200 - 500 rpm.

[0008] Preferably, to avoid sudden changes in the material structure, in step (2), release the pressure to atmospheric pressure slowly at a rate of ≤0.5 MPa / min.

[0009] Preferably, in step (3), the rotation speed for centrifugation is 5000 - 10000 rpm and the time is 5 - 30 min.

[0010] Preferably, in step (3), the temperature for vacuum drying is 60 - 100 °C and the time is 6 - 12 h.

[0011] The principle of preparing the two-dimensional La3Ni2O7 superconducting material in the present invention: First, due to the unique physicochemical properties of supercritical CO2, which combines the high diffusivity of a gas and the strong solubility of a liquid, supercritical CO2 penetrates into the lattice interstices of the material; Second, supercritical CO2 molecules have specific interactions with the lattice, selectively weakening the binding energy of specific crystal planes; Finally, under precisely controlled temperature and pressure conditions, efficient exfoliation along the weakened crystal planes is achieved. This process not only realizes the two-dimensional transformation of the material, but also induces lattice strain in the material at the atomic scale (from the Amam space group to the Fmmm space group structure transformation). Under the synergistic effect of strain induction and the quantum confinement effect of two-dimensional materials, the symmetry-breaking rehybridization of Ni 3d electron orbits occurs, forming a special electron density of states distribution, so that obvious superconducting signs can be observed in the exfoliated two-dimensional material.

[0012] Beneficial effects: (1) The two-dimensional La3Ni2O7 superconducting material prepared in the present invention has excellent superconducting properties; (2) Green environmental protection process innovation: Using supercritical CO2 as the reaction medium completely replaces traditional toxic organic solvents, and the process realizes zero wastewater discharge, meeting the principles of green chemistry; (3) Precise and controllable strain engineering: Precise control of lattice strain can be achieved by adjusting supercritical parameters, with high strain uniformity. The "peeling-strain" synchronous regulation technology is pioneered, effectively improving efficiency. (4) Advantages in industrial application: The process flow is simplified, and the energy consumption is reduced compared with traditional methods. (5) Outstanding scientific value: It provides an ideal platform for studying the strain-superconductivity correlation, opens up a new direction for the research of nickel-based oxide superconductivity, and the established parameter regulation model can be extended to other layered materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 Comparison of XRD characterizations of two-dimensional La3Ni2O7 superconducting material and raw material La3Ni2O7: (a) XRD pattern, and the right figure is a partial enlarged view of the left figure; (b) Results of crystal structure refinement, the left figure is the refinement result of raw material La3Ni2O7, and the right figure is the refinement result of two-dimensional La3Ni2O7 superconducting material; (c) Unit cell diagram obtained after crystal structure refinement of two-dimensional La3Ni2O7 superconducting material.

[0014] Figure 2 Transmission electron microscope (TEM) images of two-dimensional La3Ni2O7 superconducting material: (a) and (b) are TEM images at different magnifications, and (c) and (d) are enlarged TEM images of the regions indicated by the green and yellow rectangular frames in (b), respectively.

[0015] Figure 3 Atomic force microscope (AFM) images of two-dimensional La3Ni2O7 superconducting material: (a) and (c) are three-dimensional AFM images, and (b) and (d) are two-dimensional AFM images.

[0016] Figure 4 Results of in-situ high-pressure electrical transport tests: (a) Resistance-temperature (R-T) characteristic curves of samples of two-dimensional La3Ni2O7 superconducting material at different pressures, (b) R-T characteristic curve (red solid line) of two-dimensional La3Ni2O7 superconducting material at 26.1 GPa and the relationship between the first derivative of resistance and temperature (pink solid line). DETAILED DESCRIPTION OF THE INVENTION

[0017] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described below in conjunction with specific embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those skilled in the art without creative efforts based on the described embodiments belong to the protection scope of the present invention.

[0018] In the following examples, the raw material La3Ni2O7 was prepared according to the prior art (G. WANG et al. Pressure-InducedSuperconductivity In Polycrystalline La3Ni2O 7-δ . PHYS.REV. X 14,011040(2024)).

[0019] Example 1 A method for preparing two-dimensional La3Ni2O7 superconducting material based on supercritical CO2 exfoliation technology is as follows: (1) Grind La3Ni2O7 to an average particle size of 8 μm, and then place it in a mixed solvent composed of water and absolute ethanol according to a volume ratio of 1:1 to obtain a suspension with a concentration of 1 g / L. Then, ultrasonically treat it for 180 min to promote subsequent exfoliation and obtain a dispersion; (2) Transfer the dispersion obtained in step (1) into a supercritical CO2 reaction device, inject CO2, and react under supercritical conditions of a temperature of 100 °C and a pressure of 16 MPa with magnetic stirring (rotation speed of 300 rpm) for 4 h. After the reaction, slowly release the pressure to atmospheric pressure at a rate of 0.5 MPa / min and naturally cool it to room temperature in air to avoid sudden changes in the material structure; (3) Centrifuge the reaction solution obtained in step (2) at a rotation speed of 5000 rpm for 15 min, then take the supernatant and place it in a vacuum drying oven at 60 °C for drying for 10 h to obtain a two-dimensional La3Ni2O7 superconducting material.

[0020] Figure 1 Comparison of XRD characterizations of the obtained two-dimensional La3Ni2O7 superconducting material and the raw material La3Ni2O7: (a) XRD pattern, the right figure is a partial enlarged view of the left figure; (b) Crystal structure refinement results, the left figure is the refinement result of the raw material La3Ni2O7, and the right figure is the refinement result of the two-dimensional La3Ni2O7 superconducting material; (c) Unit cell diagram obtained after the crystal structure of the two-dimensional La3Ni2O7 superconducting material is refined. From Figure 1 (a), it can be seen that the diffraction peaks of the sample after supercritical CO2 treatment show a tendency to shift towards lower angles (indicating an increase in the interplanar spacing). This left shift of the peak position is due to the chemically induced tensile stress generated during the exfoliation process, and this stress causes anisotropic expansion of the material lattice. Further, Figure 1 (b) The structure refinement results show that after treatment with supercritical CO2 (SC CO2), the crystal symmetry of the sample changes significantly, and the space group changes from the original Amam orthogonal structure to FmmmSymmetry. This phase transition process is closely related to the lattice parameter reorganization induced by chemical stress. This strain engineering strategy can effectively regulate the superconducting transition temperature. Notably, the tensile deformation of the unit cell along a specific axis during the space group transition is in good agreement with the increased lattice plane spacing characterized by the left shift of the XRD peak position; Figure 1 (c) shows that the measured thickness of a single La3Ni2O7 layer is 10.1 Å.

[0021] Figure 2 Figure 1 is the transmission electron microscope (TEM) image of the obtained two-dimensional La3Ni2O7 superconducting material: (a) and (b) are TEM images at different magnifications, and (c) and (d) are the magnified TEM images of the regions indicated by the green and yellow rectangular boxes in (b), respectively. It can be seen from Figure 2 that: Figure 2 The regions indicated by the green and yellow rectangular boxes in (b) correspond to the layered structure of the sample. The high-magnification TEM images of these regions are Figure 2 (c) and Figure 2 (d) show that the material thickness there is very thin. In addition, it can be observed that the edges of the flakes exhibit a typical folded structure, which further confirms the two-dimensional material characteristics of the sample.

[0022] Figure 3 Figure 2 is the atomic force microscope (AFM) image of the obtained two-dimensional La3Ni2O7 superconducting material: (a) and (c) are three-dimensional AFM images, and (b) and (d) are two-dimensional AFM images. By analyzing the three-dimensional structure diagrams, namely Figure 3 (a) and Figure 3 (c), it can be clearly seen that the surface of the obtained sample is smooth and flat without obvious defects; further two-dimensional structure diagrams, namely Figure 3 (b) and Figure 3 (d) show height measurements that the prepared sample specifically exhibits a quasi-2D nanosheet structure with a lateral size in the micrometer range and a thickness of about 4.1 nm, while the measured thickness of a single La3Ni2O7 layer is 10.1 Å. It can be calculated that the exfoliated sample has approximately four atomic layers, corresponding to two unit cells along the c-axis. Figure 3 The results show that the material prepared in Example 1 exhibits an obvious layered structure, highlighting its two-dimensional characteristics, which is of great significance for integration into semiconductor processes to achieve device miniaturization and low power consumption.

[0023] In-situ high-pressure electrical transport tests were carried out in a diamond anvil cell (DAC) device with a sample chamber diameter of 300 µm. The van der Pauw four-probe method was used to measure the electrical transport properties of the sample, and cubic boron nitride (cBN) powder was used as the insulating layer to ensure the reliability of electrical measurements. Figure 4Results of in-situ high-pressure electrical transport tests: (a) Resistance-temperature (R-T) characteristic curves of the sample at different pressures, (b) R-T characteristic curve (red solid line) of the two-dimensional La3Ni2O7 superconducting material at 26.1 GPa and the relationship between the first derivative of resistance and temperature (pink solid line). Figure 4 (a) shows that as the pressure increases, the resistance gradually decreases and loses its dependence on temperature. At 60.0 GPa, the resistance is zero, indicating superconducting properties; Figure 4 (b) shows that in addition to the abnormal resistance transition similar to superconductivity observed at 77 K, an obvious thermodynamic derivative anomaly (TDW) similar to density wave (DW) behavior was also found near 135 K, indicating that the material exhibits obvious superconducting phenomena. However, in the traditional preparation method, the superconducting transition temperature of this material is about 50 K, indicating that the preparation method adopted in the present invention significantly improves the superconducting transition temperature of the material, which helps to expand the application range of superconducting materials.

Claims

1. A method for preparing two-dimensional La3Ni2O7 superconducting material based on supercritical CO2 exfoliation technology, characterized in that, The steps are as follows: (1) Grind La3Ni2O7 to a particle size of ≤10 μm, and then put it into a mixed solvent composed of water and absolute ethanol in a volume ratio of 1∶(1~2) to obtain a suspension with a concentration of 1 g / L. Then, ultrasonically treat it for 60~240 min to obtain a dispersion; (2) Transfer the dispersion obtained in step (1) to a supercritical CO2 reaction device, inject CO2, and stir and react for 4~6 h under supercritical conditions. After the reaction, release the pressure to atmospheric pressure and naturally cool to room temperature in air; (3) Centrifuge the reaction solution obtained in step (2), and then take the supernatant for vacuum drying to obtain a two-dimensional La3Ni2O7 superconducting material.

2. The method for preparing two-dimensional La3Ni2O7 superconducting material based on supercritical CO2 exfoliation technology according to claim 1, characterized in that: In step (2), the temperature of the supercritical conditions is 60~120 °C and the pressure is 8~18 MPa.

3. The method for preparing a two-dimensional La3Ni2O7 superconducting material based on a supercritical CO2 exfoliation technique according to claim 1, characterized in that: In step (2), the stirring is magnetic stirring with a rotation speed of 200~500 rpm.

4. The method for preparing a two-dimensional La3Ni2O7 superconducting material based on the supercritical CO2 exfoliation technology according to claim 1, wherein: In step (2), release the pressure slowly to atmospheric pressure at a rate of ≤0.5 MPa / min.

5. The method for preparing two-dimensional La3Ni2O7 superconducting material based on supercritical CO2 exfoliation technology according to claim 1, wherein: In step (3), the rotation speed of the centrifugation is 5000~10000 rpm and the time is 5~30 min.

6. The method for preparing a two-dimensional La3Ni2O7 superconducting material based on a supercritical CO2 exfoliation technique according to claim 1, wherein: In step (3), the temperature of the vacuum drying is 60~100 °C and the time is 6~12 h.

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