Formation method of high-voltage sintered foil for aluminum electrolytic capacitor

Abstract

The invention belongs to the technical field of sintered foil electronic materials, and particularly relates to a formation method of a high-pressure sintered foil for an aluminum electrolytic capacitor, which comprises the procedures of pretreatment, sectional power-up formation, high-temperature heat treatment, stroke compensation, phosphoric acid treatment and drying treatment. By constructing a three-in-one formation process system of porous structure permeation optimization, dynamic parameter accurate regulation and control and post-treatment enhanced stabilization, the prepared high-voltage sintered foil can be directly applied to the fields of new energy automobile inverters, industrial frequency converters, 5G base station power supplies and the like, and a key material support is provided for performance upgrading of high-voltage aluminum electrolytic capacitors; and the transformation of the electronic material industry from middle and low voltage to high voltage and high capacity is promoted.

Description

Technical Field

[0001] This invention belongs to the field of sintered foil electronic materials technology, and particularly relates to a formation method of high-voltage sintered foil for aluminum electrolytic capacitors. Background Technology

[0002] As a core passive component in electronic circuits that performs functions such as energy storage, filtering, and coupling, aluminum electrolytic capacitors directly affect the stability and reliability of electronic devices, holding an irreplaceable position in power supply, automotive electronics, industrial control, and consumer electronics. With the rapid development of industries such as new energy, 5G communication, and industrial automation, the performance requirements of electronic devices for aluminum electrolytic capacitors show a significant trend of "higher voltage, higher capacitance, and longer lifespan." For example, new energy vehicle inverters need to withstand operating voltages of over 680V to achieve efficient energy conversion, industrial frequency converters need to maintain stable capacity output under high-frequency conditions, and 5G base station power modules require capacitors with a lifespan of over 10 years.

[0003] In the structure of aluminum electrolytic capacitors, the anode electrode foil is a key material that determines its capacitance, withstand voltage, and lifespan. Traditional anode electrode foils are mainly etched foils, which form a porous structure on the surface of the aluminum foil through electrochemical etching to increase the specific surface area. However, due to the limitation of the etching process in controlling the pore size (the pore size is usually ≥5μm), the potential for increasing the specific surface area is limited (the maximum specific surface area is about 200cm² / cm²). Furthermore, under high voltage conditions above 600V, oxide film breakdown is prone to occur, making it difficult to meet the requirements of high-end equipment.

[0004] Sintered foil, as a new generation of anode electrode foil material, is prepared by powder metallurgy (pressing and sintering aluminum powder). It can form a three-dimensional porous structure with uniform pore size distribution (1-5μm) and high porosity (60%-80%). The specific surface area is increased by 30%-50% compared with traditional etched foil (up to 260-300cm² / cm²). The metallurgical bonding between aluminum powder particles ensures excellent conductivity and mechanical strength. Theoretically, it can achieve a performance combination of "high voltage (≥680V) + high capacitance (≥0.7μF / cm²)", making it an ideal material to solve the performance bottleneck of high voltage aluminum electrolytic capacitors. However, the porous structure of sintered foil also presents unique challenges to its formation process. The essence of the formation process is to generate a uniform and dense Al2O3 oxide film (dielectric layer) on the surface of aluminum substrate through electrochemical oxidation reaction. The thickness, uniformity, and density of the oxide film directly determine the voltage resistance (the oxide film thickness needs to reach 120-130nm to withstand 680V voltage, and the thickness deviation needs to be ≤5%), capacity (the oxide film needs to uniformly cover the inner wall of the porous structure to avoid capacity loss), and leakage current (the oxide film must be free of defects to reduce current leakage).

[0005] Currently, the formation process for electrode foil in the industry is mainly based on traditional etched foil or low-voltage (≤500V) sintered foil development. A mature technical solution adapted to high-voltage sintered foil above 680V has not yet been formed. The existing processes mainly have the following four types of technical defects: 1. The electrolyte system is incompatible with the porous structure, resulting in poor film uniformity. Traditional formation systems (such as single boric acid systems and simple mixed acid systems) have high surface tension (approximately 65 mN / m for boric acid systems), which cannot fully wet the three-dimensional porous structure of the sintered foil. During the formation process, the electrolyte can only penetrate to the surface layer of the porous structure (depth ≤ 50 μm), while the internal channels (depth 100-200 μm) suffer from insufficient electrolyte supply, resulting in slow oxide film growth and insufficient thickness (only 50%-70% of the surface layer), forming a non-uniform oxide film structure that is "thick on the outside and thin on the inside." During 680V high-voltage testing, the thin internal oxide film area is easily broken down, resulting in a product withstand voltage pass rate of only 30%-40% (industry requirement ≥ 95%). Furthermore, the breakdown locations are mainly concentrated inside the porous structure, making it impossible to remove unqualified products through conventional screening methods.

[0006] 2. Rigid process parameter control leads to severe capacity loss. Traditional processes employ a "fixed current density + fixed temperature" control mode, which does not consider the electrochemical reaction characteristics of sintered foils with high specific surface areas. Current density defects: Under a fixed current density (1-1.5A / dm²), the current distribution on the surface and inside the sintered foil is uneven. The surface current density is too high (up to 2-2.5A / dm²), which leads to excessive oxide film growth (thickness exceeding the requirement by 10%-15%), resulting in capacity redundancy loss. The current density inside the holes is too low (only 0.5-0.8A / dm²), which leads to insufficient oxide film growth, resulting in a lack of effective capacity area. Ultimately, the 680V capacity is only 0.45-0.55μF / cm², which is far lower than the market requirement of 0.7μF / cm². Temperature control defects: The traditional process temperature (60-80℃) is lower than the optimal temperature range (85-90℃) required for the densification of Al2O3 oxide film. In the high-voltage stage (≥600V), it cannot provide sufficient energy for the crystallization of oxide film, resulting in low crystallinity of oxide film (amorphous Al2O3 content ≥70%), which easily leads to microcracks (crack width 10-20nm), further aggravating capacity loss and increased leakage current.

[0007] 3. Lack of post-processing steps leads to insufficient oxide film stability. Traditional processes rely solely on simple water washing after formation (washing with deionized water at 70-80℃ for 5-10 minutes), without designing specific post-treatment for the characteristics of sintered foils. Impurity residue problem: Electrolyte impurities (such as borate and organic acid ions) are easily left inside the porous structure. These impurities will react chemically with the electrolyte during the subsequent capacitor assembly process to generate corrosive substances (such as organic acids), which will accelerate the degradation of the oxide film and cause the leakage current to increase by 3-5 times after 1000 hours of use. Microcrack repair failure: Microcracks generated during the formation process are not repaired and are prone to expand into through-hole defects under high voltage conditions (680V), resulting in an initial leakage current of up to 5-8μA / cm², and a high temperature (105℃) life of only 2000-3000 hours (high-end equipment requires ≥5000 hours).

[0008] In summary, existing formation processes cannot solve the synergistic optimization problem of "film uniformity-capacity-stability" for high-voltage sintered foils above 680V. This results in high-voltage sintered foils possessing theoretical performance advantages but failing to achieve industrial application, thus hindering the development of high-voltage aluminum electrolytic capacitors into high-end fields. Therefore, developing a formation method adapted to the porous structure of sintered foils, with dynamically controllable process parameters and efficient post-processing, has become a crucial technological bottleneck that urgently needs to be overcome in this field. Summary of the Invention

[0009] To address the shortcomings of existing technologies, the core technical concept of this invention is: based on the core requirements of sintered foil—"the porous structure needs to be fully wetted, the high specific surface area needs to be uniformly reacted, and the high-pressure film formation needs to be dense and stable"—a three-in-one formation process system is constructed, which integrates "porous structure penetration optimization, dynamic parameter precise control, and post-treatment to enhance stability." This achieves technological breakthroughs through the following three dimensions: Permeation optimization dimension: Abandoning the traditional single electrolyte system, a segmented electrolyte system of "organic acid system - transitional mixed acid system - high-pressure boric acid system" is designed by adjusting the composition of the electrolytic electrolyte. The mixed acid system is used for anodic oxidation, and the low surface tension characteristics (surface tension 45-50mN / m) of organic acids (fumaric acid, maleic acid) are used to improve the wettability of the electrolyte to the porous structure. Then, the transitional system is used to gradually adapt to the high-pressure film formation requirements, ensuring that the electrolyte is evenly distributed in the porous structure, so as to facilitate the formation of a dense and flexible alumina film. Parameter control dimension: Breaking the "fixed parameter" mode, a dynamic control strategy of "stepped current rise + constant high temperature" is adopted. The initial low current density ensures that the film is formed synchronously inside the hole and on the surface. The current density is gradually increased as the voltage increases to match the high voltage film formation requirements. The constant high temperature (85-90℃) provides energy for the densification of the oxide film, achieving "uniform film formation + low capacity loss". Stability Enhancement Dimension: A new post-treatment combination process of "high temperature heat treatment - stroke filling - phosphoric acid treatment" is added. Through high temperature heat treatment to repair microcracks, stroke filling to fill oxide film gaps, and phosphoric acid treatment to form a passivation layer, a full-chain guarantee mechanism of "oxide film repair - defect sealing - stability improvement" is constructed.

[0010] Based on the above technical concept, this application provides a method for forming high-voltage sintered foil for aluminum electrolytic capacitors, including pretreatment, segmented electroforming, high-temperature heat treatment, stroke compensation, phosphoric acid treatment, and drying processes. The specific steps are as follows: 1) Pretreatment: The sintered foil to be treated is stirred and boiled in water above 90°C for 5 minutes. The stirring operation can accelerate the water flow to wash away the porous structure, remove the oil stains (such as the lubricating oil left over from the rolling process) and dust impurities (aluminum powder particles left over from the sintering process) attached to the surface. At the same time, the thermal expansion of hot water can increase the pore size of the porous structure by 10%-15% (from 1-5μm to 1.1-5.8μm), creating channels for subsequent electrolyte penetration.

[0011] 2) Segmented electrification: Nine segments of electrification are performed sequentially, with parameters for each segment as follows: The first stage of electrochemical formation involves using a mixture of ammonium adipate, fumaric acid, and sodium hypophosphite as the electrolyte. The temperature is 85-90℃, and a 150V DC voltage is applied for 8-12 minutes. The core function is to achieve the initial wetting of the porous structure by the electrolyte and the pre-formation of the surface oxide film.

[0012] Two-stage electroforming: The electrolyte is a mixture of citric acid and maleic acid, the temperature is 85-90℃, and a DC voltage of 350V is applied for 8-12 minutes; the oxide film is gradually thickened (to a thickness of 50-60nm) by utilizing the weak oxidizing properties of organic acids.

[0013] Three-stage electroforming: The electrolyte is a mixture of citric acid and boric acid. The temperature is 85-90℃, and a 500V DC voltage is applied for 8-12 minutes to achieve the transition from the organic acid system to the boric acid system, forming a dense and flexible alumina film and avoiding oxide film peeling caused by electrolyte abrupt change.

[0014] Electroforming in stages four through seven: The electrolyte is a mixture of ammonium pentaborate and boric acid, the temperature is 85-90℃, and DC voltages of 550V, 600V, 620V and 640V are applied respectively. The processing time for each stage is 8-12 minutes. The oxide film thickness is gradually increased to 100-110nm.

[0015] Eight-stage electrochemical formation: The electrolyte is a mixture of ammonium pentaborate and boric acid. The temperature is 85-90℃, and a DC voltage of 660V is applied for 10-20 minutes. The time is extended to ensure uniform growth of the oxide film in the pores. Nine-segment electrochemical formation: The electrolyte is a mixture of ammonium pentaborate and boric acid. The temperature is 85-90℃, and a DC voltage of 680V is applied for 20-30 minutes to form an oxide film with a thickness of 120-130nm.

[0016] 3) High-temperature heat treatment: The sintered foil after segmented electroforming is treated at 500℃ for 1-3 minutes. This temperature can prevent recrystallization of the aluminum substrate (the recrystallization temperature of aluminum is about 300℃, but short-term treatment at 500℃ can control grain growth), effectively repair microcracks in the oxide film, and promote crystal phase transformation, that is, promote the formation of highly stable α-Al2O3 and improve the stability of the oxide film.

[0017] 4) First replenishment process: The electrolyte is boric acid system, the temperature is 85-90℃, and a 680V DC voltage is applied for 5-15 minutes to fill the oxide film gaps that may appear after heat treatment.

[0018] 5) Phosphoric acid treatment: The sintered foil after the first replenishment process is treated at 60°C for 5-15 minutes; this temperature ensures a moderate chemical reaction rate (approximately 0.5 nm / min) and avoids excessive corrosion of the oxide film caused by excessive phosphoric acid concentration or excessive temperature.

[0019] 6) Second treatment: The electrolyte is boric acid system, the temperature is 85-90℃, and a 680V DC voltage is applied for 20-30 minutes to further enhance the density of the oxide film and ensure that the thickness deviation is ≤5%.

[0020] 7) Drying treatment: After the second pass, the sintered foil is treated at 200℃ for 1-3 minutes. This step can quickly remove residual moisture from the surface and pores of the sintered foil (the moisture content is reduced to below 0.1%), avoiding the hydrolysis reaction between moisture and the oxide film (Al2O3+3H2O=2Al(OH)3) which would lead to oxide film failure.

[0021] Furthermore, in the mixed system of ammonium adipate, fumaric acid and sodium hypophosphite described in step 2), the concentration of ammonium adipate is 50-80 g / L, the concentration of fumaric acid is 20-30 g / L, and the concentration of sodium hypophosphite is 5-10 g / L.

[0022] Furthermore, in the mixed system of citric acid and maleic acid described in step 2), the concentration of citric acid is 40-60 g / L and the concentration of maleic acid is 15-25 g / L.

[0023] Furthermore, in the citric acid and boric acid mixture described in step 2), the concentration of citric acid is 30-50 g / L and the concentration of boric acid is 60-80 g / L.

[0024] Furthermore, in the mixed system of ammonium pentaborate and boric acid described in step 2), the concentration of ammonium pentaborate is 20-40 g / L, and the concentration of boric acid is 70-90 g / L.

[0025] Furthermore, the concentration of the boric acid system described in steps 4) and 6) is 80-100 g / L.

[0026] Furthermore, the phosphoric acid concentration used in step 5) is 10-20 g / L.

[0027] Furthermore, the sintered foil to be processed is a substrate for aluminum electrolytic capacitors, and its capacitance under 520V testing conditions is 1-1.05μF / cm².

[0028] Furthermore, in the segmented electroforming process, a stepped current boost control mode is adopted, with an initial current density of 0.5-1A / dm². During each segment of electroforming, the current density gradually increases by 0.2-0.3A / dm², until the current density reaches 1.8-2.2A / dm² during the ninth segment of electroforming.

[0029] Furthermore, the water temperature for pretreatment is 95-100℃, and a stirring operation with a stirring rate of 50-80 r / min is used during the cooking process.

[0030] The segmented electrolyte system in this invention operates on the following principle of synergistic permeation-film formation: Penetration mechanism: The first stage of formation adopts a mixed system of ammonium adipate-fumaric acid-sodium hypophosphite. The carboxyl group (-COOH) of fumaric acid can form hydrogen bonds with the hydroxyl group (-AlOH) on the aluminum surface, reducing the contact angle between the electrolyte and the aluminum substrate and improving wettability. Sodium hypophosphite, as a reducing agent, can remove oxide impurities on the aluminum surface and further enhance the contact between the electrolyte and the substrate.

[0031] Film formation transition mechanism: Boric acid is gradually introduced in the second and third stages of formation, utilizing its weak acidity to slowly adjust the oxidation capacity of the electrolyte and avoid oxide film detachment due to abrupt changes in the electrolyte system; the fourth to ninth stages employ an ammonium pentaborate-boric acid system, where ammonium pentaborate can provide a high concentration of borate ions (B4O7). 2- Under high pressure, it reacts with Al 3+ The formation of a dense Al(BO2)3 interlayer can inhibit excessive growth of the oxide film and reduce capacity loss.

[0032] Observation using scanning electron microscopy (SEM) showed that after adopting the segmented electrolyte system, the electrolyte penetration depth in the porous structure of the sintered foil can reach more than 200 μm (covering the entire thickness), and the oxide film thickness deviation is controlled within 3%, which is a significant improvement over the traditional system.

[0033] The dynamic film formation principle of stepped upflow and constant high temperature in the technical solution of this invention is as follows: Stepped current rise mechanism: The initial current density (0.5-1A / dm²) is low, which allows the current to be evenly distributed in the porous structure, ensuring that the oxide film starts growing synchronously; as the voltage increases, the current density is gradually increased, which can match the current requirements for the increase of oxide film thickness and avoid film formation stagnation due to insufficient current.

[0034] Constant high temperature mechanism: A constant temperature of 85-90℃ is within the optimal crystallization temperature range of Al2O3 oxide film—at this temperature, Al 3+ With O 2- Its high diffusion coefficient can promote the transformation of amorphous Al2O3 to crystalline γ-Al2O3. The density of γ-Al2O3 is higher than that of amorphous Al2O3, which can improve the compactness and insulation strength of the oxide film.

[0035] Based on Faraday's law, a film formation rate model was established. It was calculated that under the parameters of this invention, the oxide film growth rate is stable at 10-12 nm / min, and the thickness deviation of each segment is ≤2 nm. The total thickness can be precisely controlled at 125±5 nm (the thickness required for 680V), which is a significant improvement in control precision compared with the traditional process (thickness deviation ±15 nm).

[0036] The principle of oxide film stabilization in the post-treatment combination process of this invention is as follows: High-temperature heat treatment repair mechanism: High-temperature treatment at 500℃ can generate thermal stress, which can cause microcracks in the oxide film to undergo plastic deformation and close, while promoting the transformation of γ-Al2O3 to the more stable α-Al2O3. α-Al2O3 has excellent chemical stability and can withstand the corrosion of electrolyte at a high temperature of 105℃.

[0037] Phosphoric acid treatment passivation mechanism: 10-20 g / L phosphoric acid solution can react chemically with active Al atoms on the surface of the oxide film (2Al + 2H3PO4 = 2AlPO4 + 3H2↑).

[0038] Beneficial effects (1) Comprehensive breakthroughs in performance indicators, solving core pain points in the industry Significant capacity improvement: 680V capacity reaches 0.73-0.77μF / cm², which is 27%-71% higher than the traditional process (0.45-0.55μF / cm²), meeting the high capacity requirements of high-end equipment; Significantly improved withstand voltage: 750V (1.1 times rated voltage) breakdown rate ≤5%, which is more than 90% lower than the traditional process (58.3%), and the product qualification rate reaches more than 95%. Leakage current and lifespan optimization: initial leakage current ≤3μA / cm², lifespan at 105℃ ≥6000 hours, which is 54% lower and 88% longer than traditional processes (leakage current 6.5μA / cm², lifespan 3200 hours), meeting the requirements of long-life application scenarios.

[0039] (2) Strong process stability and high consistency in mass production Through segmented parameter control and multiple post-processing steps, the process fluctuation range is small: in 10 batches of mass production verification, the 680V capacity fluctuation is only 0.73-0.77μF / cm² (fluctuation amplitude 5.5%), which is far better than the traditional process (capacity fluctuation 0.45-0.55μF / cm², fluctuation amplitude 22%), ensuring the performance consistency of mass-produced products.

[0040] (3) Wide range of applications, driving industry upgrading The high-voltage sintered foil prepared can be directly applied to fields such as new energy vehicle inverters, industrial frequency converters, and 5G base station power supplies, providing key material support for the performance upgrade of high-voltage aluminum electrolytic capacitors and promoting the transformation of the electronic materials industry from "medium and low voltage" to "high voltage and high capacity". Attached Figure Description

[0041] Figure 1 This is a flowchart of the formation method of high-voltage sintered foil for aluminum electrolytic capacitors according to the present invention. Detailed Implementation

[0042] The following non-limiting embodiments are intended to enable those skilled in the art to gain a more comprehensive understanding of the invention, but do not limit the invention in any way.

[0043] Example 1 A method for forming high-voltage sintered foil for aluminum electrolytic capacitors, the specific steps of which are as follows: 1) Pretreatment: Select aluminum electrolytic capacitor substrate sintered foil (capacity of 1.03μF / cm² under 520V testing conditions), put it into 95℃ deionized water, and stir continuously at a stirring rate of 60r / min for 5 minutes.

[0044] 2) Segmented electrification: The first stage of electrochemical formation was carried out using a mixture of ammonium adipate (60 g / L), fumaric acid (25 g / L), and sodium hypophosphite (8 g / L) as the electrolyte. The temperature was controlled at 88°C, and a DC voltage of 150 V was applied for 10 minutes. The initial current density was set to 0.8 A / dm².

[0045] Two-stage electroforming: The electrolyte was switched to a mixed system of 50 g / L citric acid and 20 g / L maleic acid, the temperature was maintained at 88℃, and a 350V DC voltage was applied for 10 minutes, increasing the current density to 1.1 A / dm².

[0046] Three-stage electroforming: The electrolyte was adjusted to a mixed system of 40 g / L citric acid and 70 g / L boric acid, the temperature was 88℃, a DC voltage of 500V was applied for 10 minutes, and the current density was adjusted to 1.3A / dm².

[0047] Four-stage electroforming: The electrolyte was replaced with a mixed system of 30 g / L ammonium pentaborate and 80 g / L boric acid, the temperature was 88℃, and a 550V DC voltage was applied for 10 minutes, increasing the current density to 1.5 A / dm².

[0048] Five-stage electroforming: The electrolyte is the same as in the fourth stage, the temperature is 88℃, a 600V DC voltage is applied for 10 minutes, and the current density is adjusted to 1.7A / dm².

[0049] Six-stage electroforming: The electrolyte is the same as in the fourth stage, the temperature is 88℃, and a 620V DC voltage is applied for 10 minutes, increasing the current density to 1.9A / dm².

[0050] Seven-stage electroforming: The electrolyte is the same as in the fourth stage, the temperature is 88℃, a 640V DC voltage is applied for 10 minutes, and the current density is adjusted to 2.0A / dm².

[0051] Eight-stage electroforming: Electrolyte same as in stage four, temperature 88℃, apply 660V DC voltage for 15 minutes, and maintain current density of 2.0A / dm².

[0052] Nine-stage electroforming: The electrolyte is the same as in the fourth stage, the temperature is 88℃, a 680V DC voltage is applied for 25 minutes, and the current density is maintained at 2.0A / dm².

[0053] 3) High-temperature heat treatment: The sintered foil after segmented electroforming is placed in a muffle furnace and treated at a constant temperature of 500℃ for 2 minutes.

[0054] 4) First replenishment process: The electrolyte is a 90g / L boric acid system, the temperature is 88℃, and a 680V DC voltage is applied for 10 minutes with a current density of 2.0A / dm².

[0055] 5) Phosphoric acid treatment: Prepare a 15 g / L phosphoric acid solution, immerse the sintered foil after the first replenishment process in it, and treat it at a constant temperature of 60°C for 10 minutes.

[0056] 6) Second replenishment cycle: Electrolyte is the same as the first replenishment cycle (boric acid 90g / L), temperature 88℃, apply 680V DC voltage for 25 minutes, current density 2.0A / dm².

[0057] 7) Drying treatment: Place the sintered foil after the second supplementary stroke into a hot air drying oven and treat it at 200℃ for 1 minute.

[0058] Example 2 A method for forming high-voltage sintered foil for aluminum electrolytic capacitors, the specific steps of which are as follows: 1) Pretreatment: Select sintered foil of the same specifications as in Example 1, place it in deionized water at 100°C, and stir and cook for 5 minutes at a stirring rate of 80 r / min.

[0059] 2) Segmented electrification: Electrochemical formation: The electrolyte is a mixture of ammonium adipate 80g / L, fumaric acid 30g / L and sodium hypophosphite 10g / L. The mixture is treated at 88℃ and 150V DC voltage for 10 minutes with a current density of 1.0A / dm².

[0060] Two-stage electroforming: The electrolyte is a mixture of 60 g / L citric acid and 25 g / L maleic acid. The treatment is carried out at 88°C and 350 V DC voltage for 10 minutes, with a current density of 1.3 A / dm².

[0061] Three-stage electroforming: The electrolyte is a mixed system of 50g / L citric acid and 80g / L boric acid, treated at 88℃ and 500V DC voltage for 10 minutes, with a current density of 1.6A / dm².

[0062] Four-stage electroforming: The electrolyte is a mixture of 40 g / L ammonium pentaborate and 90 g / L boric acid. The treatment is carried out at 88°C and 550 V DC voltage for 10 minutes, with a current density of 1.8 A / dm².

[0063] Five-stage electroforming: Electrolyte same as in stage four, 88℃, 600V DC voltage treatment for 10 minutes, current density 2.0A / dm².

[0064] Six-stage electroforming: Electrolyte same as in stage four, 88℃, 620V DC voltage treatment for 10 minutes, current density 2.1A / dm².

[0065] Seven-stage electroforming: Electrolyte same as in stage four, 88℃, 640V DC voltage treatment for 10 minutes, current density 2.2A / dm².

[0066] Eight-stage electroforming: Electrolyte same as in stage four, 88℃, 660V DC voltage treatment for 15 minutes, current density 2.2A / dm².

[0067] Nine-stage electroforming: The electrolyte is the same as in the fourth stage. The process involves treating the oxide film at 88℃ and 680V DC voltage for 25 minutes with a current density of 2.2A / dm², resulting in a main body oxide film with a thickness of 128±3nm.

[0068] 3) High-temperature heat treatment: constant temperature treatment at 500℃ for 2 minutes.

[0069] 4) First replenishment process: The electrolyte is a 100g / L boric acid system, treated at 88℃ and 680V DC voltage for 10 minutes, with a current density of 2.2A / dm².

[0070] 5) Phosphoric acid treatment: 20 g / L phosphoric acid solution, constant temperature treatment at 60℃ for 10 minutes.

[0071] 6) Second replenishment cycle: Electrolyte same as the first replenishment cycle, 88℃, 680V DC voltage for 25 minutes, current density 2.2A / dm².

[0072] 7) Drying treatment: Treat at 200℃ for 1 minute.

[0073] Example 3 A method for forming high-voltage sintered foil for aluminum electrolytic capacitors, the specific steps of which are as follows: 1) Pretreatment: Select sintered foil of the same specifications as in Example 1, place it in deionized water at 90°C, and stir and cook for 5 minutes at a stirring rate of 50 r / min.

[0074] 2) Segmented electrification: Electrochemical formation: The electrolyte is a mixture of 50 g / L ammonium adipate, 20 g / L fumaric acid and 5 g / L sodium hypophosphite. The mixture is treated at 88°C and 150V DC voltage for 10 minutes with a current density of 0.5A / dm².

[0075] Two-stage electroforming: The electrolyte is a mixture of 40 g / L citric acid and 15 g / L maleic acid. The mixture is treated at 88°C and 350 V DC voltage for 10 minutes with a current density of 0.7 A / dm².

[0076] Three-stage electroforming: The electrolyte is a mixed system of 30g / L citric acid and 60g / L boric acid, treated at 88℃ and 500V DC voltage for 10 minutes, with a current density of 0.9A / dm².

[0077] Four-stage electroforming: The electrolyte is a mixture of 20 g / L ammonium pentaborate and 70 g / L boric acid. The treatment is carried out at 88°C and 550 V DC voltage for 10 minutes, with a current density of 1.1 A / dm².

[0078] Five-stage electroforming: electrolyte same as in the fourth stage, 88℃, 600V DC voltage treatment for 10 minutes, current density 1.3A / dm².

[0079] Six-stage electroforming: Electrolyte same as in stage four, 88℃, 620V DC voltage treatment for 10 minutes, current density 1.5A / dm².

[0080] Seven-stage electroforming: Electrolyte same as in stage four, 88℃, 640V DC voltage treatment for 10 minutes, current density 1.6A / dm².

[0081] Eight-stage electroforming: Electrolyte same as in stage four, 88℃, 660V DC voltage treatment for 15 minutes, current density 1.7A / dm².

[0082] Nine-stage electroforming: The electrolyte is the same as in the fourth stage. The process involves treating the oxide film at 88℃ and 680V DC voltage for 25 minutes with a current density of 1.8A / dm², resulting in a main body oxide film with a thickness of 122±3nm.

[0083] 3) High-temperature heat treatment: constant temperature treatment at 500℃ for 2 minutes.

[0084] 4) First replenishment process: The electrolyte is an 80g / L boric acid system, treated at 88℃ and 680V DC voltage for 10 minutes, with a current density of 1.8A / dm².

[0085] 5) Phosphoric acid treatment: 10 g / L phosphoric acid solution, constant temperature treatment at 60℃ for 10 minutes.

[0086] 6) Second replenishment cycle: Electrolyte same as the first replenishment cycle, 88℃, 680V DC voltage for 25 minutes, current density 1.8A / dm².

[0087] 7) Drying treatment: Treat at 200℃ for 1 minute.

[0088] Comparative Example 1 (lacking a segmented electrolyte system, using a single boric acid system) A method for forming high-voltage sintered foil for aluminum electrolytic capacitors, the specific steps of which are as follows: Pretreatment: exactly the same as in Example 1 (95℃ deionized water, stirred and cooked at 60r / min for 5 minutes).

[0089] Electrochemical formation: The entire process uses a mixed system of 30 g / L ammonium pentaborate and 80 g / L boric acid (without segmented electrolyte switching), at a temperature of 88°C. The process is carried out stepwise according to the voltage and current parameters of Example 1 (150V / 0.8A→350V / 1.1A→500V / 1.3A→550V / 1.5A→600V / 1.7A→620V / 1.9A→640V / 2.0A→660V / 2.0A→680V / 2.0A, with the same time as in Example 1).

[0090] Post-processing: Completely consistent with Example 1 (heat treatment at 500℃ → first replenishment of 90g / L boric acid → treatment with 15g / L phosphoric acid → second replenishment of 90g / L boric acid → drying at 200℃).

[0091] Comparative Example 2 (lacking stepped current ramp, using a fixed current density) A method for forming high-voltage sintered foil for aluminum electrolytic capacitors, the specific steps of which are as follows: Pretreatment: Completely consistent with Example 1.

[0092] Segmented electroforming: The electrolyte system is exactly the same as in Example 1 (the types and concentrations of electrolytes in segments one through nine are the same), the temperature is 88°C, and the voltage step parameters are the same as in Example 1, but a fixed current density of 1.5A / dm² is used throughout the process (without step-up current), and the processing time for each segment is the same as in Example 1.

[0093] Post-processing: Completely consistent with Example 1.

[0094] Comparative Example 3 (Missing high-temperature heat treatment process) A method for forming high-voltage sintered foil for aluminum electrolytic capacitors, the specific steps of which are as follows: Pretreatment: Completely consistent with Example 1.

[0095] Segmented electroforming: completely consistent with Example 1 (electrolyte system, voltage, current, temperature, and time are all the same).

[0096] Post-processing: Skip the high-temperature heat treatment process and proceed directly to the first supplementary stroke (subsequent supplementary strokes, phosphoric acid treatment, and drying steps are completely consistent with Example 1).

[0097] Comparative Example 4 (lacking phosphoric acid treatment process) A method for forming high-voltage sintered foil for aluminum electrolytic capacitors, the specific steps of which are as follows: Pretreatment: Completely consistent with Example 1.

[0098] Segmented electrification: completely consistent with Example 1.

[0099] Post-processing: After high-temperature heat treatment, the process proceeds directly to the first supplementary stroke, skipping the phosphoric acid treatment process (the supplementary stroke and drying steps are exactly the same as in Example 1).

[0100] Comparative Example 5 (Traditional single boric acid system process, without core technology modules) A method for forming high-voltage sintered foil for aluminum electrolytic capacitors, the specific steps of which are as follows: Pretreatment: Select sintered foil of the same specifications as in Example 1, and wash it in 70°C deionized water for 5 minutes without stirring.

[0101] Chemical formation treatment: a single boric acid system of 80 g / L, temperature of 70℃, fixed current density of 1.2 A / dm², and direct application of 680 V DC voltage for 30 minutes.

[0102] Post-treatment: Wash with 70℃ deionized water for 5 minutes, dry at 150℃ for 5 minutes, without high-temperature heat treatment, stroke replenishment, or phosphoric acid treatment.

[0103] Performance testing To ensure the objectivity and comparability of the test results, all samples were tested using the following methods, conforming to national standards such as T / CECA22—2017 "Electrode Foil for Aluminum Electrolytic Capacitors" and GB / T6346.11-2015 "Fixed Capacitors for Electronic Equipment Part 11: Specifications for Aluminum Electrolytic Capacitors": 1. Capacity test Equipment: Agilent E4980A Precision LCR Tester; Conditions: Apply 680V DC voltage (polarize for 5 minutes), test frequency 120Hz, ambient temperature 25±2℃, relative humidity 45%-65%; Calculation: Take the average value of 5 test points (different areas of the sample), in μF / cm², with a test error ≤ ±2%.

[0104] 2. Pressure resistance test Equipment: Chroma 19053 withstand voltage tester; Conditions: Constant voltage boost mode, voltage rise rate 100V / s, from 0V to 750V (1.1 times the rated voltage 680V), hold for 1 minute; test environment temperature 25±2℃, relative humidity 45%-65%, samples should be placed in the test environment for 2 hours in advance to balance temperature and humidity; Judgment: A sudden increase in current (exceeding 10mA / cm²) is considered a breakdown; 50 samples are tested in each group, and the breakdown rate is calculated as (number of broken samples / total number of samples × 100%). A breakdown rate ≤ 5% is considered acceptable.

[0105] 3. Leakage current test Equipment: Tonghui TH2686 leakage current tester; Conditions: Apply 680V DC voltage, let stand for 1 minute until the current stabilizes, then take the reading; test ambient temperature 25±2℃, relative humidity 45%-65%; wipe the sample surface with anhydrous ethanol and let it air dry before testing; Calculation: Take the average value of 5 test points, in μA / cm. 2 Test error ≤ ±0.1μA / cm 2 Leakage current ≤3μA / cm 2 It is acceptable.

[0106] 4. High-temperature life test Equipment: ESPECSH-641 high temperature test chamber, Agilent E4980A precision LCR meter; Conditions: High temperature load life mode, test temperature 105±2℃, continuous application of 680V DC voltage; samples were taken out at 0, 1000, 2000, 3000, 4000, 5000 and 6000 hours respectively, and placed at room temperature (25±2℃) for 2 hours before testing the 680V capacity. Judgment: Stop the test when the capacity decays to 80% of the initial capacity and record the lifetime; if the capacity is still ≥ 80% of the initial capacity after 6000 hours, the lifetime is judged to be ≥ 6000 hours.

[0107] 5. Oxide film thickness and crystal phase analysis Equipment: Fisher XDV-SDDX X-ray fluorescence thickness gauge, Bruker D8 Advance X-ray diffractometer; Oxide film thickness test: 10 uniform test points were selected on the sample surface, and the thickness was measured by X-ray fluorescence method. The average value was taken, and the test error was ≤ ±3nm. Crystal phase analysis: CuKα rays (λ=0.154nm), scanning range 2θ=20°-80°, scanning rate 2° / min, the relative contents of α-Al2O3, γ-Al2O3 and amorphous Al2O3 were calculated by the diffraction peak area ratio.

[0108] Test Data and Analysis Following the above testing methods, performance tests were conducted on the samples from Examples 1-3 and Comparative Examples 1-5, and the obtained test data are shown in Tables 1 and 2 below: Table 1 Basic performance test data (capacity, withstand voltage, leakage current, oxide film characteristics)

[0109] Table 2 High-temperature life test data (105℃, 680V, ​​capacity retention %)

[0110] The experimental results above show that Comparative Example 1, lacking a segmented electrolyte system (a single boric acid system throughout), exhibited insufficient wettability of the electrolyte in the porous structure. The 680V capacity decreased to 0.65 μF / cm² (13.3% lower than Example 1), while the 750V breakdown rate increased to 18.2% (810% higher than Example 1). This is because the single boric acid system has a high surface tension (65 mN / m), preventing it from fully penetrating into the porous structure. This results in insufficient oxide film growth within the pores, forming a "thick outside, thin inside" structure, leading to a decrease in both withstand voltage and capacity.

[0111] Comparative Example 2, lacking a stepped current ramp (fixed current 1.5 A / dm²), exhibited uneven current distribution on the surface and within the vias. Consequently, the 680V capacitance decreased to 0.61 μF / cm² (18.7% lower than Example 1), while the leakage current increased to 4.8 μA / cm² (60% exceeding the acceptable standard). This was because, under a fixed current, excessively high surface current led to excessive oxide film growth (capacity loss), while excessively low current within the vias resulted in increased oxide film defects (increased leakage current).

[0112] In Comparative Example 3, due to the absence of high-temperature heat treatment, the α-Al₂O₃ content decreased from 32% to 15%, the proportion of amorphous Al₂O₃ increased to 37%, the leakage current increased to 5.2 μA / cm² (exceeding the acceptable standard by 73.3%), and the 5000-hour lifespan decreased to less than 4000 hours (more than 20% shorter than in Example 1). This is because high-temperature heat treatment can repair microcracks and promote phase transformation; its absence leads to increased oxide film defects and decreased chemical stability, resulting in increased leakage current and shortened lifespan.

[0113] Comparative Example 4, lacking phosphoric acid treatment, showed a 750V breakdown rate of 9.5% (375% higher than Example 1), and a 6000-hour capacity retention rate of 86.2% (5.6 percentage points lower than Example 1). This is because the passivation layer formed by phosphoric acid treatment can seal defects on the oxide film surface; its absence exposes these defects, increasing the risk of breakdown, accelerating electrolyte corrosion at high temperatures, and reducing lifespan.

[0114] Therefore, it can be seen that Examples 1-3 achieve the following through the synergistic effect of "segmented electrolyte system + stepped upflow + high-temperature heat treatment + phosphoric acid treatment": Breakthrough in capacity: 680V capacity is 0.73-0.77μF / cm², an improvement of 46%-54% compared to traditional processes (Comparative Example 5); Withstand voltage guarantee: 750V breakdown rate 1.5%-4.5%, a reduction of 92.3%-95.7% compared to traditional processes; Low leakage current: leakage current 1.6-2.8μA / cm², meeting the requirements of high-end equipment; Long lifespan: ≥6000 hours at 105℃, which is more than 60% longer than traditional processes.

[0115] The formation method of this invention solves the core problems of poor film uniformity, large capacity loss, and insufficient stability in traditional processes by constructing a process system of "porous structure permeation optimization - dynamic parameter precise control - post-treatment to enhance stability". The synergistic effect of each core technology module ensures that the high-voltage sintered foil possesses performance advantages such as high capacity, high withstand voltage, low leakage current, and long lifespan. Furthermore, the process parameters are controllable, and the cost is comparable to traditional processes. This method can meet the needs of high-voltage aluminum electrolytic capacitors in fields such as new energy vehicles, industrial frequency conversion, and 5G communications, and has industrialization potential.

[0116] Those skilled in the art should understand that the above embodiments are merely exemplary embodiments, and various changes, substitutions, and modifications can be made without departing from the spirit and scope of this application.

Claims

1. A method for forming high-voltage sintered foil for aluminum electrolytic capacitors, characterized in that, The process includes pretreatment, segmented electrochemical formation, high-temperature heat treatment, stroke replenishment, phosphoric acid treatment, and drying. The specific steps are as follows: 1) Pretreatment: Stir and boil the sintered foil to be treated in water at above 90°C for 4-6 minutes; 2) Segmented electrification: Nine segments of electrification are performed sequentially, with parameters for each segment as follows: Electrochemical formation: The electrolyte is a mixture of ammonium adipate, fumaric acid and sodium hypophosphite. The temperature is 85-90℃, and a DC voltage of 150V is applied for 8-12 minutes. Two-stage electrochemical formation: The electrolyte is a mixture of citric acid and maleic acid, the temperature is 85-90℃, and a 350V DC voltage is applied for 8-12 minutes. Three-stage electrochemical formation: The electrolyte is a mixture of citric acid and boric acid, the temperature is 85-90℃, and a 500V DC voltage is applied for 8-12 minutes; Electrochemical formation in stages four through seven: The electrolyte is a mixture of ammonium pentaborate and boric acid, the temperature is 85-90℃, and DC voltages of 550V, 600V, 620V and 640V are applied respectively, with each stage taking 8-12 minutes. Eight-stage electrochemical formation: The electrolyte is a mixture of ammonium pentaborate and boric acid, the temperature is 85-90℃, and a DC voltage of 660V is applied for 10-20 minutes; Nine-stage electrochemical formation: The electrolyte is a mixture of ammonium pentaborate and boric acid. The temperature is 85-90℃, and a DC voltage of 680V is applied for 20-30 minutes. 3) High-temperature heat treatment: The sintered foil after segmented electroforming is treated at 500℃ for 1-3 minutes; 4) First stroke: The electrolyte is boric acid system, the temperature is 85-90℃, and a 680V DC voltage is applied for 5-15 minutes; 5) Phosphoric acid treatment: The sintered foil after the first replenishment process is treated at 60°C for 5-15 minutes; 6) Second treatment: The electrolyte is boric acid system, the temperature is 85-90℃, and a 680V DC voltage is applied for 20-30 minutes; 7) Drying treatment: The sintered foil after the second replenishment process is treated at 200℃ for 1-3 minutes.

2. The formation method according to claim 1, characterized in that, In step 2), the concentration of ammonium adipate, fumaric acid and sodium hypophosphite in the mixed system is 50-80 g / L, the concentration of fumaric acid is 20-30 g / L and the concentration of sodium hypophosphite is 5-10 g / L.

3. The formation method according to any one of claims 1-2, characterized in that, In the mixed system of citric acid and maleic acid described in step 2), the concentration of citric acid is 40-60 g / L and the concentration of maleic acid is 15-25 g / L.

4. The formation method according to any one of claims 1-2, characterized in that, In the mixed system of citric acid and boric acid described in step 2), the concentration of citric acid is 30-50 g / L and the concentration of boric acid is 60-80 g / L.

5. The formation method according to any one of claims 1-2, characterized in that, In the mixed system of ammonium pentaborate and boric acid described in step 2), the concentration of ammonium pentaborate is 20-40 g / L and the concentration of boric acid is 70-90 g / L.

6. The formation method according to any one of claims 1-2, characterized in that, The concentration of the boric acid system described in steps 4) and 6) is 80-100 g / L.

7. The formation method according to any one of claims 1-2, characterized in that, The phosphoric acid concentration used in step 5) is 10-20 g / L.

8. A formation method according to any one of claims 1-2, characterized in that, The sintered foil to be processed is a substrate for aluminum electrolytic capacitors, and its capacitance under 520V testing conditions is 1.03μF / cm².

9. The formation method according to any one of claims 1-2, characterized in that, In the segmented electroforming process, a stepped current boost control mode is adopted. The current density of the first electroforming stage is 0.5-1A / dm², and the current density gradually increases by 0.2-0.3A / dm² in each electroforming stage until the current density reaches 1.8-2.2A / dm² in the ninth electroforming stage.

10. The formation method according to any one of claims 1-2, characterized in that, The water temperature for pretreatment is 95-100℃, and the stirring operation during cooking is carried out at a stirring rate of 50-80r / min.

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