Asymmetric metal film and preparation method thereof

Abstract

The invention belongs to the technical field of metal filtering membranes, and particularly discloses an asymmetric metal membrane and a preparation method thereof. The asymmetric metal film provided by the invention comprises a film layer, a supporting layer and a transition net layer arranged between the film layer and the supporting layer, wherein the precision of the film layer is larger than or equal to 0.05 mu m and smaller than or equal to 150 mu m, the precision of the transition net layer is larger than the precision of the supporting layer, and the aperture ratio of the supporting layer to the film layer is (300-2000): 1. According to the asymmetric metal film provided by the invention, the film layer structures are defined as the film layer, the transition net layer and the supporting layer, the precision relation of the structures of all the layers and the aperture ratio of the supporting layer to the film layer are defined, and a synergistic effect is achieved among the specific film layer structures; the filter element has the advantages of high precision, high porosity, high strength and low filtering pressure difference, is suitable for large-flux filtering and has excellent filtering performance.

Description

Technical Field

[0001] The present invention relates to the technical field of metal filtration membranes, and in particular to an asymmetric metal membrane and a preparation method thereof. Background Art

[0002] With the rapid development of membrane science and technology, membrane separation, as a new and efficient separation technology, has been widely applied in many fields such as energy, petrochemical industry, biology, hygiene and medicine, environment, metallurgy, and food. Currently, common membrane materials mainly include metals, ceramics, and polymer organic materials. Porous metals have high mechanical strength and good processing, welding, and sealing properties, which are superior to ceramics and organic materials in many fields.

[0003] Porous metal membranes are mainly divided into symmetric metal membranes and asymmetric metal membranes. Most of the metal membranes used in industrial production are symmetric metal membranes. The higher the precision of the symmetric metal membrane, the lower the porosity and air permeability, which affects the filtration effect. Currently, the highest precision of the symmetric metal membrane can only reach 0.5 microns, but its flux is also extremely low, greatly reducing the filtration efficiency. At present, some researchers have also conducted in-depth research on asymmetric metal membranes, but there are still many problems in the application of existing asymmetric metal membranes. For example, if the existing asymmetric metal membrane wants to achieve high separation precision, finer metal powders will be used, resulting in a decrease in the porosity of the prepared metal membrane and a large flow resistance, thus reducing the flow rate during filtration. There are also problems that the film layer and the support layer of the existing asymmetric metal membrane are not firmly combined, and the film layer will fall off during backwashing. In summary, it is of great significance to provide a metal membrane with high precision, high porosity, high strength, and low filtration pressure difference. Summary of the Invention

[0004] Aiming at the problem that the existing metal membranes cannot balance the performance of high precision, high porosity, high strength, and low filtration pressure difference, the present invention provides an asymmetric metal membrane and a preparation method thereof. The asymmetric metal membrane provided by the present invention optimizes the membrane layer structure into a membrane layer, a transition mesh layer, and a support layer, and defines the precision relationship of each layer structure and the pore size ratio between the support layer and the membrane layer. The specific membrane layer structures achieve a synergistic effect, making the prepared asymmetric metal membrane have excellent filtration performance.

[0005] To solve the above technical problems, the technical solution provided by the present invention is:

[0006] The first aspect of the present invention provides an asymmetric metal membrane, including a membrane layer, a support layer, and a transition mesh layer disposed between the membrane layer and the support layer;

[0007] wherein, 0.05μm ≤ the precision of the membrane layer < the precision of the transition mesh layer < the precision of the support layer ≤ 150μm, and the pore size ratio of the support layer to the membrane layer is (300 - 2000):1.

[0008] During the research process, the inventors found that when directly preparing the support layer on a high-precision and thin film layer, if the support layer is too thick, the filtration pressure difference of the asymmetric metal membrane is large, resulting in a reduction in flux; if the support layer is too thin, not only is the preparation process complex, but also due to its too low strength, the asymmetric metal membrane may rupture during high-flux filtration. Therefore, the inventors introduced a transition mesh layer with moderate precision between the film layer and the support layer. This layer can not only buffer the filtration pressure of the asymmetric metal membrane and prevent the film layer from being damaged, but also increase the strength of the asymmetric metal membrane. The specific structure of the transition mesh layer realizes a pore size gradient transition in the asymmetric metal membrane, further improving the porosity of the asymmetric metal membrane, reducing the filtration pressure difference of the metal membrane, and enabling the asymmetric metal membrane to possess comprehensive properties such as high strength, high precision, high porosity, large flux, and low filtration pressure difference.

[0009] In conventional asymmetric metal membranes, when the pore size ratio between the support layer and the film layer is too large, the film layer is embedded in the support layer, affecting the separation effect; when the pore size ratio between the support layer and the film layer is too small, not only will the filtration flux of the filtrate be reduced, but also a large filtration pressure difference will be caused. The addition of the transition mesh layer can also avoid the problem of affecting the filtration effect due to the inappropriate pore size ratio between the film layer and the support layer during conventional preparation. By constructing a structure with a pore size gradient transition in the film layer, the separation precision and flux are balanced, improving the filtration effect. In addition, on the premise of ensuring the strength of the asymmetric metal membrane, the present invention significantly improves the porosity of the asymmetric metal membrane, reduces the pressure difference of the asymmetric metal membrane, and increases the flux by optimizing the pore size ratio between the support layer and the film layer, thereby improving the filtration effect.

[0010] The asymmetric metal membrane provided by the present invention optimizes the film layer structure into a film layer, a transition mesh layer, and a support layer, and defines the precision relationship of the structures of each layer and the pore size ratio between the support layer and the film layer. A synergistic effect is achieved between the specific film layer structures, while improving the filtration precision and porosity of the asymmetric metal membrane, reducing the filtration pressure difference, increasing the filtration flux, and ensuring the overall structural strength of the asymmetric metal membrane.

[0011] Preferably, the thickness of the film layer is 20μm - 150μm, and the precision is 0.05μm - 0.5μm.

[0012] The present invention further defines the thickness and precision of the film layer. The film layer has the characteristics of high precision and thin thickness. The high-precision film layer can achieve precise filtration; its relatively thin thickness can also reduce the filtration pressure difference of the asymmetric metal membrane, thereby increasing the filtration flux and filtration performance of the asymmetric metal membrane.

[0013] Preferably, the thickness of the transition mesh layer is 50μm - 150μm, and the precision is 10μm - 30μm.

[0014] Preferably, the thickness of the support layer is 800μm - 3500μm, and the precision is 30μm - 150μm.

[0015] The present invention further defines the thickness and precision of the transition network layer and the support layer, which is beneficial to further improve the porosity of the asymmetric metal membrane, reduce the filtration pressure difference of the asymmetric metal membrane, and improve the filtration effect.

[0016] Further preferably, the asymmetric metal membrane is in the form of a circular tube.

[0017] Further preferably, the outer diameter of the asymmetric metal membrane is Φ20mm - Φ200mm.

[0018] It should be further noted that the asymmetric metal membrane can be an outer - filtration asymmetric metal membrane or an inner - filtration asymmetric metal membrane.

[0019] The second aspect of the present invention provides a method for preparing the above - mentioned asymmetric metal membrane, including the following steps:

[0020] Step 1: Spread the first metal powder on one side of the wire mesh and press the first metal powder at 0.5MPa - 5MPa to obtain a film - layer precursor.

[0021] Step 2: Press the second metal powder at 2MPa - 13MPa on the other side of the wire mesh to obtain an asymmetric metal - membrane precursor.

[0022] Step 3: Keep the asymmetric metal - membrane precursor at 315℃ - 325℃ for the first heat preservation, and then keep it at 1300℃ - 1310℃ for the second heat preservation to obtain the asymmetric metal membrane.

[0023] Wherein, the mesh number of the second metal powder < the mesh number of the first metal powder < the mesh number of the wire mesh.

[0024] Currently, the main methods for preparing asymmetric metal membranes are the sintering - forming of the support layer and the process of coating the film layer and then sintering. These methods have problems such as complex preparation, long sintering time, and high cost. Moreover, the support layer and the film layer need to be sintered twice. Since it is difficult to fully match the process parameters such as temperature, time, and cooling rate in the two sinterings, there are problems such as voids in the microstructure of the sintering neck and discontinuous grain growth, resulting in weak connection of the sintering neck and low strength. When the inside of the metal membrane is blocked, back - blowing easily causes the film layer to fall off. Therefore, it is of great significance to study a method for preparing an asymmetric metal membrane to improve the comprehensive performance of the asymmetric metal membrane with high strength, high precision, high porosity, large flux, and low filtration pressure difference.

[0025] The preparation method of the asymmetric metal film provided by the present invention is to lay the first metal powder and the second metal powder on both sides of the wire mesh respectively, and then press them under a specific pressure, which can control the formation of a three-layer structure with a specific precision of the asymmetric metal film. It can not only improve the precision of the asymmetric metal film, but also increase the porosity of the asymmetric metal film while ensuring the strength of the asymmetric metal film, reduce the pressure difference of the asymmetric metal film, and thus improve the flux.

[0026] The present invention further controls the mesh number relationship among the first metal powder, the second metal powder and the wire mesh. As an intermediate transition layer, the wire mesh can prevent the first metal powder from embedding into the second metal powder, making the boundary between the film layer and the support layer clear; and the mesh numbers of the first metal powder and the second metal powder are both smaller than that of the wire mesh. During the pressing process, both the first metal powder and the second metal powder will form an embedding in the mesh structure of the wire mesh. During the sintering process, the powders embedded in the wire mesh are connected through sintering necks, strengthening the structure of the asymmetric metal film and improving its strength.

[0027] The present invention adopts a process of two-stage heating and heat preservation and one-stage sintering and forming. The asymmetric metal film precursor is heat-preserved and sintered under two specific temperature conditions, which can make the sintering necks generated between the first metal powder and the second metal powder grow uniformly and sufficiently, realize the tight combination between the metal powders, and greatly improve the strength of the asymmetric metal film.

[0028] It should be further noted that the wire mesh is a metal wire mesh.

[0029] Preferably, in step 1, the mesh number of the first metal powder is 325 - 550 meshes.

[0030] By limiting the particle size of the first metal powder, the precision of the asymmetric metal film can be further improved.

[0031] Preferably, in step 1, the first metal powder is at least one of SS304, SS304L, SS316, SS316L, SS317LN, 904L, 321, Ni 200, 316Ti, Monel 400, Inconel 600, Inconel 625, Hastalloy B, HastalloyC22, Hastalloy C276 and Hastalloy X.

[0032] Preferably, in step 1, the mesh number of the wire mesh is 500 - 1000 meshes.

[0033] The preferred mesh number of the wire mesh can ensure that part of the first metal powder and the second metal powder are embedded in the grid structure of the wire mesh, thereby improving the strength of the asymmetric metal film.

[0034] Preferably, in step 1, the thickness of the first metal powder after pressing is 20 μm - 150 μm.

[0035] Preferably, in step 1, the thickness of the first metal powder before pressing is 100 μm - 500 μm.

[0036] By defining the thickness of the first metal powder before and after pressing, the precision of the asymmetric metal film can be further ensured.

[0037] It should be further noted that in step 1, the pressing is carried out by an arc rolling mill.

[0038] Preferably, in step 2, the mesh number of the second metal powder is 100 - 200 mesh.

[0039] By defining the mesh number of the second metal powder, the precision of the support layer can be reduced, thereby increasing the porosity of the asymmetric metal film and reducing the filtration pressure difference.

[0040] Preferably, in step 2, the second metal powder is at least one of SS304, SS304L, SS316, SS316L, SS317LN, 904L, 321, Ni 200, 316Ti, Monel 400, Inconel 600, Inconel 625, Hastalloy B, Hastalloy C22, Hastalloy C276 and Hastalloy X.

[0041] Preferably, in step 2, the thickness of the second metal powder after pressing is 800 μm - 3500 μm.

[0042] It should be further noted that in step 2, the pressing is isostatic pressing.

[0043] Preferably, in step 3, it further includes: heating the asymmetric metal film precursor to 315 °C - 325 °C at a rate of 5 °C / min - 10 °C / min.

[0044] Preferably, in step 3, the time for the first heat preservation is 1 h - 2 h.

[0045] Preferably, in step 3, the temperature is raised from 315 °C - 325 °C to 1300 °C - 1310 °C at a heating rate of 2 °C / min - 4 °C / min.

[0046] Preferably, in step 3, the time for the second heat preservation is 8 h - 12 h.

[0047] It should be further noted that in step 3, the heating, heat preservation and sintering treatment are carried out under vacuum conditions.

[0048] By limiting the specific heating rate, holding temperature, and holding time during two heating and holding processes and one sintering and forming process, it is beneficial to make the sintering necks grow uniformly and fully in the inlaid part of the first metal powder and the second metal powder in the wire mesh, thereby improving the strength of the asymmetric metal film.

[0049] The asymmetric metal film provided by the present invention has the advantages of high precision, high porosity, high strength, low filtration pressure difference, and large flux. Its precision can be as low as 0.05 μm, the pore size ratio of the support layer to the film layer reaches 300 - 2000:1, the porosity > 50%, the circumferential tensile strength is 12610 N, and the flexural strength is 2460 N. Description of the Drawings

[0050] Figure 1 It is a gas permeability detection diagram of the metal films prepared in Example 1, Comparative Example 1, and Comparative Example 2 of the present invention. Detailed Embodiments

[0051] In order to make the objectives, technical solutions, and advantages of the present invention more clear, the following further details the present invention in conjunction with embodiments and drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0052] Example 1

[0053] This example provides an asymmetric metal film, including a film layer, a support layer, and a transition mesh layer disposed between the film layer and the support layer;

[0054] The thickness of the film layer is 50 μm, and the precision is 0.5 μm;

[0055] The thickness of the transition mesh layer is 100 μm, and the precision is 20 μm;

[0056] The thickness of the support layer is 2350 μm, and the precision is 150 μm.

[0057] This example provides a method for preparing the above asymmetric metal film, including the following steps:

[0058] Step 1: Cut a 600 - mesh 316L metal wire mesh into a rectangle with a length and width of 1850 mm and 189 mm respectively, place it in a mold, take 450 - mesh SS316L metal powder and spread it evenly on the wire mesh with a spreading thickness of 100 μm, and press it with an arc rolling machine at a pressure of 5 MPa. The thickness of the metal powder is 50 μm to obtain a film layer precursor;

[0059] Step 2: Make the film layer precursor into a cylindrical shape and place it on a tensioning mold. Fill the other side of the wire mesh in the film layer precursor with 100-mesh SS316L metal powder to a filling thickness of 2600 μm, and then press it with an isostatic pressing device at a pressure of 13 MPa. The thickness of the metal powder is 2350 μm to obtain an asymmetric metal film precursor;

[0060] Step 3: In a vacuum atmosphere baking furnace, heat the asymmetric metal film precursor at a heating rate of 5 °C / min to 320 °C, hold for 1 h, and then heat it at a heating rate of 2 °C / min to 1300 °C and hold for 12 h to obtain an asymmetric metal film.

[0061] Example 2

[0062] This example provides an asymmetric metal film, including a film layer, a support layer, and a transition mesh layer disposed between the film layer and the support layer;

[0063] The thickness of the film layer is 75 μm and the precision is 0.05 μm;

[0064] The thickness of the transition mesh layer is 150 μm and the precision is 10 μm;

[0065] The thickness of the support layer is 3500 μm and the precision is 100 μm.

[0066] This example provides a method for preparing the above asymmetric metal film, including the following steps:

[0067] Step 1: Cut a 1000-mesh 316L metal wire mesh into a rectangle with a length and width of 1850 mm and 189 mm respectively, put it into a mold, take 550-mesh SS316L metal powder and spread it flat on the wire mesh to a spreading thickness of 95 μm, and press it with an arc rolling machine at a pressure of 0.5 MPa. The thickness of the metal powder is 75 μm to obtain a film layer precursor;

[0068] Step 2: Make the film layer precursor into a cylindrical shape and place it on a tensioning mold. Fill the other side of the wire mesh in the film layer precursor with 200-mesh SS316L metal powder to a filling thickness of 3700 μm, and then press it with an isostatic pressing device at a pressure of 2 MPa. The thickness of the metal powder is 3500 μm to obtain an asymmetric metal film precursor;

[0069] Step 3: In a vacuum atmosphere baking furnace, heat the asymmetric metal film precursor at a heating rate of 10 °C / min to 315 °C, hold for 2 h, and then heat it at a heating rate of 4 °C / min to 1310 °C and hold for 8 h to obtain an asymmetric metal film.

[0070] Example 3

[0071] This embodiment provides an asymmetric metal film, which includes a film layer, a support layer, and a transition mesh layer disposed between the film layer and the support layer;

[0072] The thickness of the film layer is 20 μm, and the precision is 0.1 μm;

[0073] The thickness of the transition mesh layer is 50 μm, and the precision is 30 μm;

[0074] The thickness of the support layer is 800 μm, and the precision is 30 μm.

[0075] This embodiment provides a method for preparing the above asymmetric metal film, which includes the following steps:

[0076] Step 1: Cut a 316L metal wire mesh with 800 meshes into a rectangle with a length and width of 1850 mm and 189 mm respectively, place it in a mold, take 500-mesh SS316L metal powder and spread it evenly on the wire mesh, with a spreading thickness reaching 300 μm, and press it with an arc rolling mill at a pressure of 2 MPa. The thickness of the metal powder is 20 μm to obtain a film layer precursor;

[0077] Step 2: In a vacuum atmosphere roasting furnace, form the film layer precursor into a cylindrical shape and place it on a tensioning mold. Fill the other side of the wire mesh in the film layer precursor with 150-mesh SS316L metal powder, with a filling thickness of 950 μm, and then press it with an isostatic pressing device at a pressure of 8 MPa. The thickness of the metal powder is 800 μm to obtain an asymmetric metal film precursor;

[0078] Step 3: Heat the asymmetric metal film precursor to 325 °C at a heating rate of 6 °C / min, hold for 2 h, and then heat it to 1300 °C at a heating rate of 3 °C / min and hold for 10 h to obtain an asymmetric metal film.

[0079] Comparative Example 1

[0080] This comparative example provides a symmetric metal film with a precision of 0.5 μm. The specific preparation method is as follows:

[0081] Step a: Take 350-mesh SS316L metal powder and press it at a pressure of 13 MPa to form a metal film blank with an outer diameter of φ50 mm, a thickness of 2.5 mm, and a length of 1000 mm;

[0082] Step b: Fire the metal film blank in a vacuum atmosphere. Using a continuous heating method, with a heating rate of 10 °C / min, gradually heat it to 1350 °C, hold for 12 h, and after the firing is completed, obtain a symmetric metal film.

[0083] Comparative Example 2

[0084] This comparative example provides a coated asymmetric metal film with an accuracy of 0.5 μm. The specific preparation method is as follows:

[0085] 1. Preparation of the support layer: Take 316L metal powder with a mesh size of 180, and perform cold isostatic pressing at a pressure of 5 MPa to form a metal film blank with an outer diameter of φ50 mm, a thickness of 2.3 mm, and a length of 1000 mm.

[0086] 2. Precursor of the asymmetric metal film: Fire the metal film blank in a vacuum atmosphere. Use a continuous heating method with a heating rate of 5 °C / min, gradually heat up to 1400 °C, and hold for 10 h to complete the firing.

[0087] 3. Precoating of the film layer: Thoroughly stir and mix metal powder with a mesh size of 500 and 5000 g of DSA (the mass ratio of metal powder to DSA is 1:160) to form a transition agent. Coat the transition agent on the support layer at a speed of 2 m / s, with a coating thickness of 0.2 mm, and control the coating pressure at 100 kPa to evenly coat the transition agent on the inner layer of the support layer to form a precursor of the asymmetric metal film.

[0088] 4. Firing of the precursor of the asymmetric metal film: Fire using a two-step heating method. Use a hydrogen atmosphere for heating. After heating up to 300 °C at a heating rate of 5 °C / min, hold for 5 h; then heat up to 1120 °C at a heating rate of 5 °C / min and hold for 3 h to complete the firing, obtaining a coated asymmetric metal film.

[0089] Prepare the asymmetric metal films provided in Examples 1 - 3 of the present invention into asymmetric metal films with an outer diameter of 50 mm and a length of 100 mm.

[0090] Detect the porosity and gas permeability of the metal films prepared in Example 1 and Comparative Examples 1 - 2, and detect the ring tensile strength and flexural strength mechanical energy of the metal films prepared in Example 1 and Comparative Example 2;

[0091] Among them, the porosity is measured by the liquid penetration method according to "DIN 30911-3 1990 Sintered Metals - Sintering Inspection Standards (SPN) - Sintered Density".

[0092] The ring tensile strength is detected according to "YS / T 1010-2014 Determination of Ring Tensile Strength of Sintered Metal Porous Materials".

[0093] Fix the two ends of the sample for support according to "GB / T232-1999 Metallic Materials - Bend Test Method", and gradually increase the pressure with a stress detector to detect the deformation amount and pressure value.

[0094] The clean gas permeability of the metal membrane was tested according to "EN 779:2002 Filters for general ventilation - Determination of filtration performance" and "VDI 3926 Standard test method for evaluating cleanable filter media".

[0095] After testing, on the premise that the metal membranes prepared in Example 1 and Comparative Examples 1-2 of the present invention have the same precision, the porosity of the metal membrane prepared in Example 1 is 56.05%, the porosity of the metal membrane prepared in Comparative Example 1 is 18.76%, and the porosity of the metal membrane prepared in Comparative Example 2 is 35.45%;

[0096] The circumferential tensile strength of the metal membrane prepared in Example 1 is 12,610 N, and the circumferential tensile strength of the metal membrane prepared in Comparative Example 1 is 11,090 N;

[0097] The flexural strength of the metal membrane prepared in Example 1 is 2,460 N, and the flexural strength of the metal membrane prepared in Comparative Example 1 is 2,000 N;

[0098] Under the same pressure difference, as Figure 1 shown, the gas permeability of the metal membrane prepared in Example 1 is 3 times that of the metal membrane prepared in Comparative Example 1, and the gas permeability of the metal membrane prepared in Example 1 is 2 times that of the metal membrane prepared in Comparative Example 1.

[0099] In summary, under the condition that the precision, thickness and detection conditions of the metal membranes are completely the same, the asymmetric metal membrane prepared in the embodiment of the present invention has excellent strength, porosity and gas permeability compared with the existing metal membranes, is suitable for large-flux filtration, and has excellent filtration performance.

[0100] Examples 1-3 of the present invention can all achieve comparable technical effects.

[0101] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements or improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An asymmetric metal film, characterized in that, It includes a film layer, a support layer, and a transition mesh layer disposed between the film layer and the support layer; Among them, 0.05μm ≤ film layer precision < transition mesh layer precision < support layer precision ≤ 150μm, and the pore size ratio of the support layer to the film layer is (300 - 2000):1; The thickness of the film layer is 20μm - 150μm, and the precision is 0.05μm - 0.5μm.

2. The asymmetric metal film according to claim 1, wherein The thickness of the transition mesh layer is 50μm - 150μm, and the precision is 10μm - 30μm.

3. The asymmetric metal film according to claim 1, wherein, The thickness of the support layer is 800μm - 3500μm, and the precision is 30μm - 150μm.

4. A method for preparing the asymmetric metal film according to any one of claims 1 to 3, characterized in that, It includes the following steps: Step 1: Spread the first metal powder on one side of the wire mesh, and press the first metal powder at 0.5MPa - 5MPa to obtain a film layer precursor; Step 2: Press the second metal powder at 2MPa - 13MPa on the other side of the wire mesh to obtain an asymmetric metal film precursor; Step 3: Keep the asymmetric metal film precursor at 315°C - 325°C for the first heat preservation, and then keep it at 1300°C - 1310°C for the second heat preservation to obtain an asymmetric metal film; Among them, the mesh number of the second metal powder < the mesh number of the first metal powder < the mesh number of the wire mesh.

5. The method for preparing an asymmetric metal film according to claim 4, characterized in that, In Step 1, the mesh number of the first metal powder is 325 - 550 mesh; and / or In Step 1, the mesh number of the wire mesh is 500 - 1000 mesh.

6. The method for preparing an asymmetric metal film according to claim 4, wherein In Step 2, the mesh number of the second metal powder is 100 - 200 mesh.

7. The method for preparing an asymmetric metal film according to claim 4, characterized in that, In Step 3, it further includes: heating the asymmetric metal film precursor to 315°C - 325°C at a rate of 5°C / min - 10°C / min.

8. The method for preparing an asymmetric metal film according to claim 4, wherein In Step 3, the time for the first heat preservation is 1h - 2h.

9. The method for preparing the asymmetric metal film according to claim 4, characterized in that, In Step 3, the temperature is raised from 315°C - 325°C to 1300°C - 1310°C at a heating rate of 2°C / min - 4°C / min.

10. The method for preparing an asymmetric metal film according to claim 4, characterized in that, In Step 3, the time for the second heat preservation is 8h - 12h.

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