A sintered foil containing a core-shell structure composite fiber and a method for manufacturing the same

Abstract

This invention discloses a sintered foil containing core-shell composite fibers and its preparation method, belonging to the technical field of aluminum electrolytic capacitors. The sintered foil containing core-shell composite fibers uses aluminum foil as a matrix, on which core-shell composite fibers are loaded. The core layer of the core-shell composite fibers is Al2O3 fiber, and the shell layer is a barium strontium titanate nanocrystalline layer. The preparation method is as follows: Al2O3 fibers are prepared by electrospinning combined with a sol-gel method; a barium strontium titanate nanocrystalline layer is formed on the surface of the Al2O3 fibers by a hydrothermal method or a sol-gel method to obtain composite fibers; then, the composite fibers are mixed with aluminum powder to form a slurry, which is coated onto the aluminum foil, and then dried and sintered at a programmed temperature to obtain the final product. This invention, by introducing a core-shell composite fiber reinforcing phase, significantly enhances the mechanical strength, interfacial bonding, and bending resistance of the sintered foil while improving the dielectric constant and specific capacitance, and reduces dielectric loss, making it suitable for the preparation of high-performance aluminum electrolytic capacitors.

Description

Technical Field

[0001] This invention belongs to the field of aluminum electrolytic capacitor technology, specifically relating to a sintered foil containing core-shell structured composite fibers and its preparation method. Background Technology

[0002] The performance of aluminum electrolytic capacitors largely depends on the dielectric properties and microstructure of their anode materials. Traditional high-voltage anode foils primarily achieve their specific surface area through electrochemical corrosion to form a porous structure, followed by formation to create a dielectric oxide layer. However, the dielectric constant of aluminum oxide (Al₂O₃) alone is limited, restricting further improvements in the capacitor's specific capacitance. In recent years, researchers have attempted to introduce high-dielectric-constant ceramic materials, such as barium titanate (BaTiO₃), into the anode foil to construct composite dielectric layers. However, directly introducing ceramic powder presents problems such as weak adhesion to the matrix, uneven dispersion, and increased losses due to pore blockage. How to integrate high-dielectric-constant materials into the three-dimensional structure of the anode foil in a more efficient and stable manner is key to improving the overall performance of the capacitor. Summary of the Invention

[0003] In view of the above-mentioned prior art, the present invention provides a sintered foil containing core-shell structured composite fibers and its preparation method, which solves the problems of weak bonding force with the matrix, uneven dispersion, and increased loss due to pore blockage when ceramic powder is directly introduced into the prior art. The present invention provides an anode sintered foil for aluminum electrolytic capacitors with higher specific capacitance, lower loss, and better reliability.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a sintered foil containing core-shell structured composite fibers is provided. The sintered foil uses aluminum foil as a substrate, and core-shell structured composite fibers are loaded on the aluminum foil. The core layer of the core-shell structured composite fibers is Al2O3 fiber, and the shell layer is barium strontium titanate nanocrystalline layer ((Ba,Sr)TiO3, abbreviated as BST).

[0005] Based on the above technical solution, the present invention can be further improved as follows.

[0006] Furthermore, the core-shell structured composite fibers have a diameter of 50~500 nm and a length of 1~20 μm.

[0007] Furthermore, the molar ratio of barium, strontium, and titanium in the barium strontium titanate nanocrystalline layer is 1~9:1~9:10.

[0008] Furthermore, the preparation method of the above-mentioned sintered foil containing core-shell structured composite fibers includes the following steps: S1. Preparation of core-shell composite fibers: Al2O3 fibers are prepared by electrospinning combined with calcination. Strontium barium titanate precursor is coated on the surface of Al2O3 fibers by sol-gel method or hydrothermal method, and core-shell composite fibers are formed by heat treatment. S2. Slurry preparation: The core-shell composite fiber, aluminum powder, dispersant and binder are added to a solvent to prepare a slurry; S3. Coating and drying: The slurry is coated onto aluminum foil and dried to obtain a green blank; S4. Sintering: The green blank is sintered in a protective atmosphere with programmed temperature rise to obtain a sintered foil containing core-shell structured composite fibers.

[0009] Furthermore, the preparation of the core-shell structured composite fiber in step S1 includes the following steps: (1) Preparation of Al2O3 fiber: Prepare a mixed aqueous solution of polyethylene oxide and aluminum nitrate, and spin it by electrospinning process to obtain precursor fiber felt; calcine the precursor fiber felt and cool it to obtain Al2O3 fiber. (2) Coating of barium strontium titanate shell: Barium strontium titanate precursor sol was prepared by sol-gel coating method. The Al2O3 fiber obtained in step (1) was dispersed in ethanol. Under stirring conditions, the barium strontium titanate precursor sol was dropped into the fiber dispersion. After coating, the fiber coated with precursor was collected by centrifugation, washed with ethanol and dried. (3) Heat treatment crystallization: The fiber obtained in step (2) is placed in a protective atmosphere and calcined to obtain the product.

[0010] Furthermore, in step S2, the mass ratio of core-shell composite fiber to aluminum powder is 0.5~5:95~99.5, the aluminum powder is spherical aluminum powder with a particle size of 3~5 μm and a purity greater than 99.99%; the dispersant is phenolic resin, the binder is polyvinylidene fluoride, polyvinyl butyral or ammonium polyacrylate, and the solvent is ethanol.

[0011] Furthermore, the slurry preparation was carried out in a ball mill with a ball-to-material ratio of 5:1, a rotation speed of 200 rpm, and a ball milling time of 8 hours.

[0012] Furthermore, the aluminum foil is 30 μm thick, the slurry is coated on both sides with a thickness of 180 μm on each side, and the green body is 130 μm thick after drying.

[0013] Furthermore, the programmed temperature rise sintering process is as follows: first, the temperature is raised to 300~500 ℃ at a rate of 1 ℃ / min and held for 60~70 min; then, the temperature is raised to 610~650 ℃ at a rate of 5 ℃ / min for sintering and held for 480~720 min; finally, the temperature is cooled to below 100 ℃ in the furnace to obtain the final product.

[0014] Furthermore, the temperature is first increased to 400 °C at a rate of 1 °C / min and held for 60 min, then increased to 625 °C at a rate of 5 °C / min for sintering and held for 600 min; finally, the temperature is cooled to below 100 °C in the furnace.

[0015] The beneficial effects of the above-mentioned further technical solutions of the present invention are as follows: the green blank is subjected to programmed heating treatment in a protective atmosphere (such as argon, nitrogen or vacuum). First, it is held at a lower temperature to remove any possible organic additives (degreasing); then it is heated to the sintering temperature (which needs to be higher than the recrystallization temperature of aluminum but lower than its melting point) and held for a period of time, so that the aluminum powder particles diffuse and combine to form a strong aluminum skeleton porous structure. At the same time, the core-shell structure composite fiber is firmly embedded in the aluminum skeleton and pores, and finally an integrated sintered foil containing core-shell structure composite fiber is obtained.

[0016] The beneficial effects of this invention are: 1) Advantages of integrated sintering structure: This invention abandons the traditional composite method of "corrosion before loading" and adopts powder metallurgy method for integrated sintering; the aluminum skeleton formed by aluminum powder sintering itself has an excellent three-dimensional interconnected porous structure, the composite fiber is uniformly dispersed in the mixing stage, and after sintering it is firmly bonded to the aluminum skeleton, with extremely high structural stability, avoiding the problem of medium shedding.

[0017] 2) Enhanced Mechanical and Structural Reliability: Core-shell composite fibers, acting as a reinforcing phase, effectively improve the mechanical properties of the sintered foil. The one-dimensional fibrous dielectric in the aluminum matrix functions similarly to "fiber reinforcement," helping to refine the sintered structure, inhibit crack propagation, and thus improve the toughness, bending resistance, and overall structural strength of the sintered foil. This enhanced mechanical strength makes the fabrication and application of ultra-thin, high-capacitance sintered foils possible and significantly improves the reliability of capacitors under harsh conditions such as mechanical vibration and impact.

[0018] 3) Superior dielectric recombination effect: Core-shell fibers construct a stable three-dimensional dielectric network within the pores of the sintered body. The Al2O3 core provides excellent insulation and good physicochemical compatibility with the aluminum matrix; the high-dielectric BST shell greatly enhances the effective dielectric constant under an electric field. Together, they significantly improve specific capacitance while maintaining a high breakdown field strength.

[0019] 4) Optimized interface and low loss: The sintering process creates a tighter interface between the aluminum matrix and the Al2O3 core. The one-dimensional fiber morphology reduces the density of interface defects caused by randomly distributed powder, and reduces charge traps, thereby significantly reducing dielectric loss and leakage current.

[0020] 5) Process flexibility and universality: By adjusting the aluminum powder particle size, fiber content, molding pressure and sintering process, the porosity, specific surface area and composite dielectric distribution of the sintered foil can be flexibly controlled to meet the needs of capacitors of different voltage levels. This process is highly compatible with existing sintered foil production lines. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a core-shell composite fiber structure. Figure 2 The images shown are SEM images of the sintered foil containing core-shell structured composite fibers prepared in Example 1, where (a) is an image magnified 750 times and (b) is an image magnified 4000 times. Detailed Implementation

[0022] The specific embodiments of the present invention will be described in detail below with reference to examples.

[0023] Example 1: A sintered foil containing core-shell composite fibers (low fiber content, high strontium ratio BST shell), the preparation method of which is as follows: Step S1: Preparation of core-shell structured composite fibers (Al2O3@BST) (1) Preparation of Al2O3 fibers: Prepare a mixed aqueous solution of polyethylene oxide (PEO, molecular weight 900,000) and aluminum nitrate (Al(NO3)3·9H2O) with a concentration of 8 wt%, wherein Al 3+ The concentration was 0.5 mol / L. Electrospinning was performed at a voltage of 18 kV, a feed rate of 0.8 mL / h, and a receiving distance of 15 cm to obtain precursor fiber mats. The precursor fiber mats were heated to 600 °C in air at a rate of 2 °C / min and held for 2 h, then heated to 900 °C at a rate of 5 °C / min and held for 3 h. After natural cooling, γ-Al₂O₃ fibers with a diameter of 150–300 nm and a length of 5–15 μm were obtained.

[0024] (2) Coating of the BST shell: The sol-gel coating method was adopted. First, barium acetate, strontium acetate, and tetrabutyl titanate were weighed in a molar ratio of Ba:Sr:Ti = 1:9:10. Barium acetate and strontium acetate were dissolved in a mixed solvent of glacial acetic acid and ethylene glycol methyl ether and stirred at 70°C until completely dissolved. Tetrabutyl titanate was dissolved separately in another part of ethylene glycol methyl ether. Under vigorous stirring, the tetrabutyl titanate solution was slowly added dropwise to the barium acetate and strontium acetate mixture to obtain a clear BST precursor sol (total metal ion concentration of 0.1 mol / L). 0.5 g of Al2O3 fiber obtained in step (1) was dispersed in 200 mL of ethanol and sonicated for 30 min. Under stirring conditions, 100 mL of BST precursor sol was slowly added dropwise to the fiber dispersion, and stirring was continued for 12 h after the addition was completed. The fiber coated with the precursor was collected by centrifugation, washed twice with ethanol, and dried at 80°C.

[0025] (3) Heat treatment crystallization: The dried fibers from step (2) are placed in a tube furnace and heated to 750 ℃ ​​at a rate of 3 ℃ / min under the protection of flowing argon gas (purity 99.99%). The temperature is held for 2 h to allow the BST precursor to completely decompose and crystallize into the perovskite phase, finally obtaining the core-shell structured composite fiber (Al2O3@BST), the structural schematic diagram of which is shown below. Figure 1 As shown.

[0026] Step S2: Preparation of composite slurry Weigh 1.0 g of the above-mentioned core-shell structured composite fiber (accounting for 1.0% of the final solid component by mass), 99.0 g of high-purity spherical aluminum powder with an average particle size of 4 μm (purity >99.99%), 1.5 g of phenolic resin (dispersant), 4.0 g of PVDF (binder, soluble in ethanol), and 200 mL of anhydrous ethanol (solvent). Place the above raw materials in a zirconia ball mill jar, add zirconia grinding balls, and ball mill and mix for 8 h at a ball-to-material ratio of 5:1 and a rotation speed of 200 rpm to form a uniform and stable composite slurry.

[0027] Step S3: Coating and Drying The composite slurry prepared in step S2 was uniformly coated on both sides of a 30 μm thick high-purity bright aluminum foil (purity >99.99%) substrate using a coating machine. The wet film thickness was precisely controlled to be 180 μm on each side by adjusting the gap of the coating machine. After coating, the foil was initially dried in a forced-air drying oven at 80 ℃ for 10 min, and then transferred to a vacuum drying oven at 120 ℃ for 2 h to completely remove organic solvents. After drying, a green aluminum foil preform with a thickness of approximately 130 μm was obtained.

[0028] Step S4: Sintering and Shaping The dried aluminum foil blank was placed in a high-temperature sintering furnace. First, under the protection of flowing high-purity nitrogen (>99.999%), the temperature was raised to 400 ℃ at a rate of 1 ℃ / min and held for 60 min to fully decompose and remove organic binders and dispersants (degreasing process). Then, the temperature was raised to 625 ℃ at a rate of 5 ℃ / min and held at this temperature for 600 min for sintering. During the sintering process, the aluminum powder particles achieved densification and connection through diffusion neck growth and atomic migration, forming a strong three-dimensional porous aluminum skeleton, in which the core-shell composite fibers were firmly embedded. After sintering, the foil was cooled to below 100 ℃ in the furnace and removed to obtain a sintered foil containing core-shell composite fibers (denoted as sample A1).

[0029] SEM image of sample A1 as shown Figure 2As shown, the aluminum powder particles on the surface of the sintered foil diffuse and combine to form a strong porous aluminum skeleton structure. The core-shell composite fiber is firmly embedded in the aluminum skeleton and pores, ultimately resulting in an integrated sintered foil containing core-shell composite fiber.

[0030] Example 2: A sintered foil containing core-shell composite fibers (low fiber content, equimolar barium-strontium ratio BST shell). The difference between this example and Example 1 is the composition of BST in the core-shell composite fibers: In step S1 (2), the ratio of BST precursor sol is adjusted to Ba:Sr:Ti = 5:5:10 (molar ratio). Other preparation conditions are the same as in Example 1. Finally, a sintered foil containing core-shell composite fibers (denoted as sample A2) is obtained.

[0031] Example 3: A sintered foil containing core-shell composite fibers (low fiber content, high barium ratio BST shell). The difference between this example and Example 1 is the composition of BST in the core-shell composite fibers: In step S1 (2), the ratio of BST precursor sol is adjusted to Ba:Sr:Ti = 9:1:10 (molar ratio). Other preparation conditions are the same as in Example 1. Finally, a sintered foil containing core-shell composite fibers (denoted as sample A3) is obtained.

[0032] Example 4: A sintered foil containing core-shell composite fibers (high fiber content, equimolar barium-strontium ratio BST shell). The difference between this example and Example 1 is the BST composition and fiber addition amount of the core-shell composite fibers: In step S1 (2), the ratio of BST precursor sol is adjusted so that Ba:Sr:Ti = 5:5:10 (molar ratio); In step S2, the amount of core-shell composite fibers added is increased to 3.0 g (accounting for 3.0% of the final solid component mass fraction), and the mass of high-purity spherical aluminum powder (purity >99.99%) with an average particle size of 4 μm is reduced to 97.0 g. Other slurry components and ball milling processes are prepared in the same way as in Example 1; Finally, a sintered foil containing core-shell composite fibers is obtained (denoted as sample A4).

[0033] Comparative Example 1: A pure aluminum powder sintered foil, prepared according to the method in Example 1, but without the addition of any functional media. That is, without the addition of core-shell structured composite fibers, only 100.0 g of high-purity spherical aluminum powder with an average particle size of 4 μm (purity >99.99%) was used; other preparation conditions were the same as in Example 1, and the pure aluminum-based porous sintered foil (denoted as sample D1) without any composite media was obtained through the slurry preparation, coating, drying and sintering processes in steps S2 to S4.

[0034] Comparative Example 2: A composite sintered foil containing BST powder. The difference between this example and Example 1 is that the same mass fraction (1.0%) of commercially available barium strontium titanate (BST) nanoparticles (average particle size of approximately 100 nm) is used instead of the core-shell structured composite fibers. Specifically, 1.0 g of BST nanoparticles (equivalent to the mass of the core-shell structured composite fibers in step S2 of Example 1) is mixed with 99.0 g of high-purity spherical aluminum powder (purity >99.99%) with an average particle size of 4 μm; other preparation conditions are the same as in Example 1, and the sintered foil (denoted as sample D2) is obtained through the slurry preparation, coating, drying, and sintering processes in steps S2 to S4.

[0035] Experimental Example The sintered foils obtained in Examples 1-4 and Comparative Examples 1-2 were formed at 520 V in a boric acid system, and their performance was tested. The results are shown in Table 1.

[0036] Table 1 Performance tests of sintered foils prepared in Examples 1-4 and Comparative Examples 1-2

[0037] In terms of capacitance performance, all samples containing core-shell composite fibers (A1~A4) showed higher areal specific capacitance than pure aluminum sintered foil (D1). Among them, sample A3 (Ba:Sr=9:1) had the highest specific capacitance, reaching 1.105 μF / cm. 2 The specific capacitance was approximately 46.9% higher than that of pure aluminum foil. This significant improvement is directly attributed to the high dielectric properties of the BST shell and the excellent interfacial bonding between the core-shell composite fibers and the aluminum matrix. Even the A1 and A2 samples, with lower fiber content, showed increases in specific capacitance of 33.8% and 38.6%, respectively, demonstrating the effective enhancement of dielectric properties by the core-shell structure design. Notably, the D2 sample, which used conventional BST powder mixing, also showed an increase in specific capacitance (0.935 μF / cm). 2 (This represents a 24.3% improvement over pure aluminum foil), but is still significantly lower than the core-shell composite fiber sample. More importantly, the leakage current of the D2 sample is as high as 27 μA / cm. 2 It is much higher than that of the core-shell fiber sample (approximately 18 μA / cm). 2 ) and pure aluminum foil (17 μA / cm 2 This indicates that simple mechanical mixing of BST powder introduces numerous interfacial defects, leading to a decrease in insulation resistance. In contrast, the Al2O3 core layer in the core-shell composite fiber effectively isolates the direct contact between BST and the aluminum matrix, providing an additional insulation barrier and thus maintaining a low leakage current level.

[0038] Regarding withstand voltage, although all samples met the basic requirement (>477 V), the BST powder sample (D2) had the lowest withstand voltage (477.6 V), while the A2 sample (Ba:Sr=5:5) with a moderate barium-strontium ratio had the highest withstand voltage (513.0 V). This indicates that simple powder addition may introduce local electric field concentration points, reducing the overall withstand voltage capability; while the core-shell composite fiber samples (especially A1 and A2) achieved a good balance between withstand voltage performance and high specific capacitance through optimized interface design.

[0039] In terms of mechanical properties, the reinforcing effect of core-shell composite fibers is particularly significant. Bending test results show that all samples containing core-shell composite fibers (A1~A4) have a higher number of bending cycles than pure aluminum foil (D1) and BST powder samples (D2). In particular, sample A4 (high fiber content) achieved a bending cycle of 215 cycles, approximately 11.4% higher than pure aluminum foil and approximately 20.8% higher than BST powder samples. This improvement directly demonstrates the reinforcing effect of fibers in the aluminum matrix: the one-dimensional fibrous medium acts as a "fiber reinforcement" in the aluminum matrix, effectively refining the sintered structure, inhibiting crack propagation, and thus improving the toughness, bending resistance, and overall structural strength of the sintered foil.

[0040] Although specific embodiments of the present invention have been described in detail with reference to examples, they should not be construed as limiting the scope of protection of this patent. Various modifications and variations that can be made by those skilled in the art without inventive effort within the scope described in the claims are still within the scope of protection of this patent.

Claims

1. A sintered foil containing core-shell structure composite fibers, characterized by: The sintered foil uses aluminum foil as a substrate, and core-shell structured composite fibers are loaded on the aluminum foil. The core layer of the core-shell structured composite fibers is Al2O3 fiber, and the shell layer is strontium barium titanate nanocrystalline layer.

2. The sintered foil of core-shell structure composite fiber according to claim 1, characterized by: The core-shell composite fiber has a diameter of 50~500 nm and a length of 1~20 μm.

3. The sintered foil of core-shell structure composite fiber according to claim 1 or 2, characterized in that: The molar ratio of barium, strontium, and titanium in the barium strontium titanate nanocrystalline layer is 1~9:1~9:

10.

4. The method of producing a sintered foil of core-shell structure composite fiber according to any one of claims 1 to 3, characterized by, Includes the following steps: S1. Preparation of core-shell composite fibers: Al2O3 fibers are prepared by electrospinning combined with calcination. Strontium barium titanate precursor is coated on the surface of Al2O3 fibers by sol-gel method or hydrothermal method, and core-shell composite fibers are formed by heat treatment. S2. Slurry preparation: The core-shell composite fiber, aluminum powder, dispersant and binder are added to a solvent to prepare a slurry; S3. Coating and drying: The slurry is coated onto aluminum foil and dried to obtain a green blank; S4. Sintering: The green blank is sintered in a protective atmosphere with programmed temperature rise to obtain a sintered foil containing core-shell structured composite fibers.

5. The method of producing a sintered foil of core-shell structure composite fiber according to claim 4, characterized by, The preparation of the core-shell structured composite fiber in step S1 includes the following steps: (1) Preparation of Al2O3 fiber: Prepare a mixed aqueous solution of polyethylene oxide and aluminum nitrate, and spin it by electrospinning process to obtain precursor fiber felt; calcine the precursor fiber felt and cool it to obtain Al2O3 fiber. (2) Coating with barium strontium titanate shell: Barium strontium titanate precursor sol was prepared by sol-gel method. The Al2O3 fiber obtained in step (1) was dispersed in ethanol. Under stirring conditions, the barium strontium titanate precursor sol was dropped into the fiber dispersion. After coating, the fiber coated with precursor was collected by centrifugation, washed with ethanol and dried. (3) Heat treatment crystallization: The fiber obtained in step (2) is placed in a protective atmosphere and calcined to obtain the product.

6. The method for preparing sintered foil containing core-shell structured composite fibers according to claim 4, characterized in that: In step S2, the mass ratio of the core-shell composite fiber to aluminum powder is 0.5~5:95~99.5, the aluminum powder is spherical aluminum powder with a particle size of 3~5 μm and a purity greater than 99.99%; the dispersant is phenolic resin, the binder is polyvinylidene fluoride, polyvinyl butyral or ammonium polyacrylate, and the solvent is ethanol.

7. The method for preparing sintered foil containing core-shell structured composite fibers according to claim 6, characterized in that: The slurry was prepared in a ball mill with a ball-to-material ratio of 5:1, a rotation speed of 200 rpm, and a mixing time of 8 h.

8. The method for preparing sintered foil containing core-shell structured composite fibers according to claim 4, characterized in that: The aluminum foil has a thickness of 30 μm, the slurry is coated on both sides with a thickness of 180 μm on each side, and the green blank has a thickness of 130 μm after drying.

9. The method for preparing sintered foil containing core-shell structured composite fibers according to claim 4, characterized in that, The programmed temperature rise sintering process is as follows: first, the temperature is raised to 300~500 ℃ at a rate of 1 ℃ / min and held for 60~70 min; then, the temperature is raised to 610~650 ℃ at a rate of 5 ℃ / min for sintering and held for 480~720 min; finally, the temperature is cooled to below 100 ℃ in the furnace to obtain the final product.

10. The method for preparing sintered foil containing core-shell structured composite fibers according to claim 9, characterized in that, First, the temperature is increased to 400 ℃ at a rate of 1 ℃ / min and held for 60 min. Then, the temperature is increased to 625 ℃ at a rate of 5 ℃ / min for sintering and held for 600 min. Finally, the temperature is cooled to below 100 ℃ in the furnace.

Images