Rare earth magnetostrictive composite material and preparation method thereof

Abstract

The invention relates to a rare earth magnetostrictive composite material and a preparation method thereof, the rare earth magnetostrictive composite material is an Sm-Fe-Si magnetostrictive composite material, the chemical general formula of the magnetostrictive composite material is Sm < x > Fe < 2-y > Si < y >, xlt; 2, 0lt; yt; Yt; and 0.4 part. Fe in the Sm-Fe binary alloy is replaced by the Si element, and the content of each component in the Sm-Fe-Si ternary alloy is regulated and controlled, so that the interface resistance can be effectively improved, the content of a high-conductivity phase is reduced, and the resistivity is remarkably improved. When the content of the Sm element in the alloy is increased, precipitation of a metal Sm phase can be promoted, generation and expansion of cracks in a brittle phase can be inhibited due to existence of a ductile phase, and the macroscopic mechanical property of the material is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of magnetostrictive materials, and particularly relates to a rare earth magnetostrictive composite material and a preparation method thereof. Background Art

[0002] Magnetostrictive materials refer to a class of materials with magnetostrictive effects, that is, under the action of an applied magnetic field, the material undergoes a change in magnetization state, resulting in a phenomenon of size and volume change. Rare earth-iron alloy materials represented by Tb-Dy-Fe have advantages such as large magnetostrictive coefficient, high energy density, fast response speed, large magneto-mechanical coupling coefficient, and high energy conversion rate at room temperature. Therefore, such materials have been increasingly widely used and developed in the fields of sensors and actuators, etc., involving multiple fields such as military, aerospace, electronics, precision machining, and medicine.

[0003] The giant magnetostrictive effect of rare earth-iron alloy materials comes from the Laves phase RFe 2 (R is a rare earth element) compound. In traditional terbium dysprosium iron alloy (commercial grade Terfenol-D), the content of heavy rare earth elements Tb and Dy reaches 60% (mass percentage), and its resources are scarce and the price is expensive. To reduce the raw material cost, light rare earth elements such as Ce, Pr, Nd, Sm, etc. have been successively introduced into the Laves phase to replace part of Tb and Dy. In recent years, RFe containing light rare earths 2 Magnetostrictive compounds have been widely studied. These research topics mainly focus on the synthesis of compounds, magnetostrictive effects, and related magnetic properties. However, in practical applications, other properties of the materials also have an important impact on the overall function of magnetostrictive devices. For example, the resistivity of Laves phase rare earth alloy materials at room temperature is generally very low, and when the materials are used in an alternating magnetic field (especially a high-frequency magnetic field), large eddy current losses are easily formed, which in turn generates heat and causes the temperature to rise, resulting in a decrease in magnetostrictive performance and energy conversion efficiency. The intrinsic brittleness of Laves phase rare earth alloy materials makes them difficult to machine mechanically, and magnetostrictive devices are also prone to overall failure due to material fracture during service. The above problems limit their application scope in various fields.

[0004] Resin-bonded giant magnetostrictive composite materials can be prepared into materials with different shapes according to requirements, and their resistivity is significantly higher than that of alloy materials. Such bonded composite materials and preparation processes have been widely studied and explored. For example, Chinese invention patent ZL201210034284.7 (authorized publication number CN102569638B) discloses a bonded rare earth giant magnetostrictive material with a layered structure, highly <111> orientation, and high resistance and low loss characteristics, and a preparation method thereof. By regulating the magnetocrystalline anisotropy of the alloy, the alloy composition is designed as Tb x Dy 1-x Fey (0.4 < x ≤ 1, 1.9 ≤ y ≤ 1.95). Alloy particles with a particle size of 10 - 300 μm, a binder, a curing agent, and a process control agent are mixed, and then dynamically magnetically oriented under a magnetic field induction of 6000 - 12000 Oe; finally, it is cured and formed under a static magnetic field and in a water bath at 40°C - 60°C to obtain a laminated structure, highly <111> preferentially oriented bonded rare earth giant magnetostrictive material with a resistivity reaching 1.23 - 8.00 Ω·m. Another example is the preparation method of a bonded rare earth iron giant magnetostrictive material disclosed in Chinese Invention Patent ZL201210201667.9 (authorized publication number CN102766792B), and the composition of this material is (R x Dy 1-x )Fe y , where R is Tb or Sm, the range of x is between 0.25 - 0.9, and the range of y is between 1.5 - 2.0. Its preparation method includes the following steps: First, the metal raw materials are mixed in the above proportions and arc melted under high-purity argon protection to obtain an alloy ingot, and then homogenized and annealed at 700 - 950°C for 45 - 50 hours, cooled to room temperature in the furnace. After that, the alloy ingot is crushed and ground into powder with a particle size of 50 - 180 μm, and after compaction, secondary stress relief annealing is carried out at 300 - 500°C for 5 - 8 hours under high-purity argon protection. Finally, the powder is mixed with a binder, placed in a mold and pressed into shape, demolded and cured in a magnetic field for 36 - 48 hours, and finally a bonded rare earth iron giant magnetostrictive material is obtained. The present invention adopts a bonding method of secondary annealing and magnetic field orientation, significantly reducing the stress of alloy powder, significantly improving the magnetostriction, dynamic magnetostriction coefficient, and magnetomechanical coupling coefficient of the prepared rare earth giant magnetostrictive material. At the same time, the material has a high resistivity, improving the high-frequency characteristics of the material. However, the low density of the polymer will cause a significant increase in the volume of the bonded composite material, which is not conducive to the miniaturization of devices. The presence of the polymer also inevitably reduces the heat dissipation performance of the material and inhibits the magnetostrictive strain, resulting in a large loss of the magnetostrictive coefficient and energy density of the material. In addition, the preparation process of the bonded composite material is relatively cumbersome, and there are often problems such as powder oxidation and porosity.

[0005] Therefore, it is necessary to further improve the existing rare earth magnetostriction and preparation methods. Summary of the Invention

[0006] The first technical problem to be solved by the present invention is: In view of the above-mentioned current situation of the prior art, to provide a rare earth magnetostrictive composite material with high density, high resistivity, and good mechanical properties at the same time.

[0007] The second technical problem to be solved by the present invention is to provide a preparation method of a rare earth magnetostrictive composite material with high density, high resistivity and good mechanical properties at the same time.

[0008] The technical solution adopted by the present invention to solve the above first technical problem is as follows: A rare earth magnetostrictive composite material, which is a Sm-Fe-Si magnetostrictive composite material, and the chemical general formula of the magnetostrictive composite material is Sm x Fe 2-y Si y , where 0.8 < x < 2 and 0 < y < 0.4.

[0009] Preferably, x and y satisfy: 1 ≤ x ≤ 1.6 and 0.05 ≤ y ≤ 0.2.

[0010] The Sm-Fe-Si magnetostrictive composite material includes a main phase with magnetostrictive effect and a second phase. The main phase with magnetostrictive effect is Sm(Fe,Si) 2 phase, and the second phase is one or a combination of two or more of Sm phase, Sm-Fe phase, Sm-Si phase, and Sm-Fe-Si phase.

[0011] In the above chemical general formula, when the Si element replaces Fe in SmFe 2 , and after annealing treatment, optimized microstructure and properties can be obtained. On the one hand, Sm(Fe,Si) 2 solid solution can be formed in the alloy, which helps to enhance the scattering of electrons by the lattice and defects, etc. On the other hand, it can promote the precipitation of second phases such as Sm-Si, Sm-Fe, and Sm-Fe-Si in the alloy, that is, the generation of composite phases, which can effectively enhance the scattering effect of the interface on electrons and reduce the content of high-conductivity phases. Both of these aspects can effectively increase the resistivity of the alloy; at the same time, increasing the content of Sm element in the alloy can promote the precipitation of high-toughness metallic Sm phase, and the presence of the tough phase is more helpful to inhibit the generation and propagation of cracks in the brittle phase and improve the macroscopic mechanical properties of the material. However, if the content of Si or Sm element in the alloy is too high, the proportion of Sm(Fe,Si) 2 phase in the composite material will be significantly reduced, resulting in serious deterioration of the magnetostrictive performance. Therefore, the content of each element in the alloy needs to be strictly limited.

[0012] At a temperature of 300K, the absolute value of the magnetostrictive strain (λ ∥ -λ ⊥ ) under a 20kOe magnetic field is ≥ 902 ppm, where λ ∥ is the magnetostrictive strain parallel to the magnetic field direction, and λ ⊥is the magnetostrictive strain perpendicular to the magnetic field direction; the resistivity > 75 μΩ·cm; the flexural strength > 54 MPa. Thus, a magnetostrictive composite material with both high resistivity and good mechanical properties is obtained.

[0013] Specifically, the absolute value of the magnetostrictive strain (λ ∥ -λ ⊥ ) is 902 - 1860 ppm, the resistivity ≤ 340 μΩ·cm; the flexural strength ≤ 83 MPa.

[0014] The technical solution adopted by the present invention to solve the above second technical problem is: a preparation method of the rare earth magnetostrictive composite material as described above, characterized in that it successively includes the following steps:

[0015] (1) Prepare raw materials according to the chemical formula of Sm x Fe 2-y Si y ; where 0.8 < x < 2, 0 < y < 0.4;

[0016] (2) Melt the raw materials in step (1) under the protection of an inert gas to obtain an alloy with uniform composition;

[0017] (3) Anneal the alloy in step (2) to form a composite material containing a main phase with magnetostrictive effect and a second phase.

[0018] For the Sm-Fe-Si magnetostrictive composite material prepared by the above method, the main phase with magnetostrictive effect is the Sm(Fe,Si) 2 phase, and the second phase is one or a combination of two or more of Sm phase, Sm-Fe phase, Sm-Si phase, and Sm-Fe-Si phase.

[0019] In step (2), the alloy can be in the form of blocks, flakes, filaments, etc., and its form is not limited as long as the alloying elements are uniform.

[0020] Preferably, in step (3), annealing is carried out in a high vacuum environment or in an inert gas, the annealing temperature is 600 - 900 °C, and the annealing time does not exceed 7 days.

[0021] Compared with the prior art, the advantages of the present invention are: the rare earth magnetostrictive composite material adopts a chemical general formula of Sm x Fe 2-y Si ySm-Fe-Si magnetostrictive composite material. In this Sm-Fe-Si magnetostrictive composite material, Si element is used to replace Fe in the Sm-Fe binary alloy, and the contents of each component in the Sm-Fe-Si ternary alloy are regulated, which can effectively enhance the scattering effect of the lattice and the interface on electrons, reduce the content of the high-conductivity phase, and significantly increase the resistivity. When the content of Sm element in the alloy is increased, it can promote the precipitation of the high-toughness metallic Sm phase, and the presence of the tough phase is more conducive to inhibiting the generation and propagation of cracks in the brittle phase, improving the macroscopic mechanical properties of the material. After the aforementioned Sm-Fe-Si magnetostrictive composite material is heat-treated, it will spontaneously generate Sm(Fe,Si) 2 main phase and the second phase. Among them, the second phase is one or a combination of two or more of Sm phase, Sm-Fe phase, Sm-Si phase, and Sm-Fe-Si phase. The presence of the second phase hinders the crack propagation during the deformation process of the composite material and improves the mechanical properties of the material; Si element can dissolve in SmFe 2 phase, increasing the resistivity of the main phase. At the same time, the second phase has a higher resistivity and can provide more phase interfaces, thus more effectively hindering the conduction of free electrons, thereby increasing the resistivity of the composite material; Si doping does not destroy the Laves phase with magnetostrictive effect in the material, and the generated second phase is a metal alloy with a higher density, and the composite material is almost a fully dense structure, which not only ensures a large magnetostrictive strain but also can provide a high energy conversion density. Therefore, the magnetostrictive material obtained by the present invention has high resistivity, good mechanical properties, high density, and large magnetostrictive effect. In addition, when using the casting process to manufacture the composite material, the raw materials do not need to be refined into powder, avoiding disadvantages such as material oxidation, and the raw material and manufacturing costs are low, which is convenient for the application and popularization of the material. Description of the Drawings

[0022] Figure 1 SmFe of Comparative Example 1 of the present invention 2 alloy (a), Sm of Example 1 1.02 Fe 1.8 Si 0.2 alloy (b), and Sm of Example 2 1.6 Fe 1.8 Si 0.2 alloy (c) backscattered electron mode scanning electron micrographs;

[0023] Figure 2 SmFe of Comparative Example 1 of the present invention 2 alloy, Sm of Example 1 1.02 Fe 1.8 Si 0.2 alloy, and Sm of Example 2 1.6 Fe 1.8 Si0.2 Magnetostrictive strain (λ ∥ -λ ⊥ ) of the alloy at 300K as a function of magnetic field;

[0024] Figure 3 SmFe of Comparative Example 1 of the present invention 2 alloy, Sm of Example 1 1.02 Fe 1.8 Si 0.2 alloy, and Sm of Example 2 1.6 Fe 1.8 Si 0.2 Variation curve of resistivity of the alloy with temperature;

[0025] Figure 4 SmFe of Comparative Example 1 of the present invention 2 alloy, Sm of Example 1 1.02 Fe 1.8 Si 0.2 alloy, and Sm of Example 2 1.6 Fe 1.8 Si 0.2 Three-point bending stress-strain curve of the alloy. Detailed implementation mode

[0026] The present invention will be further described in detail below with reference to the accompanying drawings and examples.

[0027] In the following comparative examples and examples, the rare earth magnetostrictive material is a Sm-Fe-Si magnetostrictive composite material, and the purities (mass percentage contents) of the raw materials Sm, Fe, and Si used are all greater than 99%. The alloying method used is induction melting, the scanning electron microscope used is FEI Quanta FEG 250 produced by FEI Company of the United States; the comprehensive physical property measurement system used is PPMS-EverCool and PPMS-DynaCool produced by Quantum Design Company of the United States; the universal material testing machine used is Zwick / Roell Z1.0 produced by ZwickRoell Company of Germany. Those skilled in the art can also use other devices with the same functions known in the industry.

[0028] Comparative Example 1:

[0029] The preparation method of the SmFe 2 magnetostrictive material in this comparative example successively includes the following steps:

[0030] (1) Prepare raw materials according to the SmFe 2 chemical formula, with the Sm element being 5% in excess to make up for the loss caused by volatilization;

[0031] (2) Put the raw materials prepared in step (1) into an induction melting furnace, evacuate to a vacuum degree of 5.0×10 -2 Pa or less, fill with high-purity Ar gas, keep warm for 3 minutes after the temperature rises to complete melting of the raw materials, and then pour the alloy liquid into a copper mold to form an alloy ingot with uniform composition;

[0032] (3) After cutting, grinding, cleaning and drying the alloy ingot melted in step (2), put it into a quartz tube, evacuate to a vacuum degree of 1.0×10 -3 Pa or less, fill with high-purity Ar gas, and then seal the tube;

[0033] (4) Put the sealed quartz tube in step (3) into an annealing furnace, heat to 800 °C, anneal for 7 days, take it out and quench in cold water.

[0034] Use a scanning electron microscope to observe the microstructure of the SmFe 2 alloy after annealing in step (4), as Figure 1 shown in a. This alloy is composed of a SmFe 2 main phase with magnetostrictive effect and an almost negligible Sm phase, and their volume fractions are 99% and 1% respectively. Use a comprehensive physical property measurement system to test the magnetostriction vs. magnetic field curve of the SmFe 2 alloy at 300 K after annealing in step (4), as Figure 2 shown. The magnetostriction strain (λ ∥ -λ ⊥ ) of this sample at 20 kOe is -2194 ppm. Use a comprehensive physical property measurement system to test the resistivity vs. temperature curve of the SmFe 2 alloy after annealing in step (4), as Figure 3 shown. The resistivity of this sample at 300 K is 75 μΩ·cm. Use a universal testing machine to test the three-point bending stress-strain curve of the SmFe 2 alloy at 300 K after annealing in step (4), as Figure 3 shown. The bending strength of this sample is 54 MPa. The density of this alloy at room temperature is 8.46 g / cm 3 .

[0035] Example 1:

[0036] In this example, the preparation method of the Sm-Fe-Si magnetostrictive composite material successively includes the following steps:

[0037] (1) Prepare raw materials according to the Sm 1.02 Fe 1.8 Si 0.2 chemical formula;

[0038] (2) Put the raw materials prepared in step (1) into an induction melting furnace, evacuate to a vacuum degree of 5.0×10 -2 Pa or less, fill with high-purity Ar gas, keep warm for 3 minutes after the temperature rises to complete melting of the raw materials, and then pour the alloy liquid into a copper mold to form an alloy ingot with uniform composition;

[0039] (3) After cutting, grinding, cleaning and drying the alloy ingot melted in step (2), put it into a quartz tube, evacuate to a vacuum degree of 1.0×10 -3 Pa or less, fill with high-purity Ar gas, and then seal the tube;

[0040] (4) Put the sealed quartz tube in step (3) into an annealing furnace, heat to 800 °C, anneal for 7 days, take out and quench in cold water.

[0041] Use a scanning electron microscope to observe the microstructure of the Sm 1.02 Fe 1.8 Si 0.2 alloy after annealing in step (4), as Figure 1 shown in b. This alloy includes a main phase of Sm(Fe,Si) with magnetostrictive effect and a second phase composed of Sm 2 Si 5 and Sm(Fe,Si) 3 , and their volume fractions are 86%, 10% and 4% respectively. It can be considered that the Sm 3 Fe 1.02 Si 1.8 alloy spontaneously generates a second phase to form a Sm-Fe-Si magnetostrictive composite material. Use a comprehensive physical property measurement system to test the magnetostriction vs. magnetic field curve of the Sm 0.2 alloy after annealing in step (4) at 300 K, as 1.02 Fe 1.8 Si 0.2 shown. The magnetostrictive strain (λ Figure 2 ) of this sample at 20 kOe is -902 ppm. Use a comprehensive physical property measurement system to test the resistivity vs. temperature curve of the Sm ∥ Fe ⊥ alloy after annealing in step (4), as 1.02 Fe 1.8 Si 0.2 shown. The resistivity of this sample at 300 K is 340 μΩ·cm. Use a universal testing machine to test the three-point bending stress-strain curve of the Sm Figure 3 alloy after annealing in step (4) at 300 K, as 1.02 Fe 1.8 Si 0.2 shown. Figure 4As shown. The flexural strength of this sample is 58 MPa. The density of this alloy at room temperature is 8.04 g / cm 3 .

[0042] Example 2:

[0043] The preparation method of the Sm-Fe-Si magnetostrictive composite material in this example successively includes the following steps:

[0044] (1) Prepare raw materials according to the chemical formula of Sm 1.6 Fe 1.8 Si 0.2 ;

[0045] (2) Put the raw materials prepared in step (1) into an induction melting furnace, evacuate to a vacuum degree of 5.0×10 -2 Pa or less, fill with high-purity Ar gas, keep it warm for 3 minutes after the temperature rises to the complete melting of the raw materials, and then pour the alloy liquid into a copper mold to form an alloy ingot with uniform composition;

[0046] (3) After cutting, grinding, cleaning and drying the alloy ingot melted in step (2), put it into a quartz tube, evacuate to a vacuum degree of 1.0×10 -3 Pa or less, fill with high-purity Ar gas, and then seal the tube;

[0047] (4) Put the sealed quartz tube in step (3) into an annealing furnace, heat it to 800 °C, anneal for 7 days, take it out and quench it in cold water.

[0048] Use a scanning electron microscope to observe the microstructure of the Sm 1.6 Fe 1.8 Si 0.2 alloy after annealing in step (4), as Figure 1 c shown. This alloy includes a Sm(Fe,Si) 2 main phase with magnetostrictive effect and a second phase composed of Sm 5 Si 3 and Sm, and their volume fractions are: 73%, 14% and 13% respectively. It can be considered that the Sm 1.6 Fe 1.8 Si 0.2 alloy spontaneously generates a second phase to form a Sm-Fe-Si magnetostrictive composite material. Use a comprehensive physical property measurement system to test the magnetostriction vs. magnetic field curve of the Sm 1.6 Fe 1.8 Si 0.2 alloy at 300 K, as Figure 2 shown. The magnetostrictive strain (λ ∥ -λ ⊥) was -1017 ppm. The resistivity-temperature curve of the Sm 1.6 Fe 1.8 Si 0.2 alloy after annealing in test step (4) was measured using a comprehensive physical property measurement system, as shown in Figure 3 . The resistivity of this sample at 300 K was 134 μΩ·cm. The three-point bending stress-strain curve of the Sm 1.6 Fe 1.8 Si 0.2 alloy after annealing in test step (4) at 300 K was measured using a universal testing machine, as shown in Figure 4 . The flexural strength of this sample was 83 MPa. The density of this alloy at room temperature was 8.02 g / cm 3 .

[0049] In each of Examples 3 to 5 compared with the above Comparative Example 1, the difference lies in that: the chemical formulas of the raw materials prepared in step (1) are different. In Examples 6 and 7 compared with the above Examples 1 and 2, the difference lies in that: the chemical formulas of the raw materials prepared in step (1) are different. The alloy composition, constituent phases and their contents, and properties of the Sm-Fe-Si magnetostrictive composite material of the present invention are specifically shown in Table 1 below.

[0050] From the above Examples 1 to 7 and Comparative Example 1, it can be seen that by using Si element to replace Fe in the Sm-Fe binary alloy, regulating the contents of each component in the Sm-Fe-Si ternary alloy, and performing heat treatment, a Sm(Fe,Si) main phase with magnetostrictive effect and a second phase can be spontaneously generated in the alloy. The second phase is one or a combination of two or more of Sm phase, Sm-Fe phase, Sm-Si phase, and Sm-Fe-Si phase. At this time, at a temperature of 300 K, the absolute value of the magnetostrictive strain (λ 2 -λ ∥ ) under a 20 kOe magnetic field is ≥902 ppm, where λ ⊥ is the magnetostrictive strain parallel to the magnetic field direction, and λ ∥ is the magnetostrictive strain perpendicular to the magnetic field direction; the resistivity > 75 μΩ·cm; the flexural strength > 54 MPa. ⊥

[0051] In summary, the Sm-Fe-Si magnetostrictive composite material provided by the present invention has a high resistivity, good mechanical properties, large magnetostrictive strain and density, and is an ideal magnetostrictive material. The above-described embodiments have detailed the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, supplements, equivalent replacements, etc. made within the principle scope of the present invention should be included within the protection scope of the present invention.

[0052] Table 1 Alloy composition, constituent phases and their contents, and properties of Sm-Fe-Si magnetostrictive composite materials

[0053]

[0054] In the above table, Sm-rich refers to the phases composed of elements such as Sm, Fe, Si, and O, and the Sm content in these phases is higher than that in Sm(Fe,Si) 2 main phase, but due to its low content or too small area, it cannot be accurately calibrated by energy dispersive spectrometer

Claims

1. A rare earth magnetostrictive composite material, characterized in that: It is a Sm-Fe-Si magnetostrictive composite material, the chemical formula of which is Sm x Fe 2-y Si y , where 0.8 <x<2,0<y<0.4。 2. The rare earth magnetostrictive composite material according to claim 1, characterized in that: The x and y satisfy: 1≤x≤1.6, 0.05≤y≤0.

2.

3. The rare earth magnetostrictive composite material according to claim 1, characterized in that: The Sm-Fe-Si magnetostrictive composite material includes a main phase with magnetostrictive effect and a second phase, wherein the main phase with magnetostrictive effect is a Sm(Fe,Si)2 phase, and the second phase is one of a Sm phase, a Sm-Fe phase, a Sm-Si phase, and a Sm-Fe-Si phase, or a combination of two or more thereof.

4. The rare earth magnetostrictive composite material according to any one of claims 1 to 3, characterized in that: At a temperature of 300K, the magnetostrictive strain (λ ∥ -λ ⊥ ) is ≥902ppm, where λ ∥ is the magnetostrictive strain parallel to the magnetic field, λ ⊥ It is the magnetostrictive strain in the direction perpendicular to the magnetic field; resistivity>75μΩ·cm; bending strength>54MPa.

5. The rare earth magnetostrictive composite material according to claim 4, characterized in that: The magnetostrictive strain (λ ∥ -λ ⊥ ) absolute value is ≤1860ppm; resistivity is ≤340μΩ·cm; flexural strength is ≤83MPa.

6. A method for preparing the rare earth magnetostrictive composite material according to any one of claims 1 to 5, characterized in that: The following steps are included in sequence: (1) According to Sm x Fe 2-y Si y Chemical formula preparation raw materials, of which 0.8 <x<2,0<y<0.4; (2) melting the raw materials in step (1) under the protection of an inert gas to obtain an alloy with uniform composition; (3) Annealing the alloy prepared in step (2) to form a composite material comprising a phase having a magnetostrictive effect and a second phase.

7. The preparation method according to claim 6, characterized in that: In step (3), annealing is performed in a high vacuum environment or in an inert gas, the annealing temperature is 600-900° C., and the annealing time does not exceed 7 days.

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