Electrochemical etching method for anode foil of high-voltage tantalum capacitor

Abstract

According to the electrochemical etching method for the anode foil of the high-voltage tantalum capacitor, the tantalum foil is pre-oxidized before etching processing, isotropic etching of the tantalum foil can be achieved by controlling the thickness of an oxidation film on the surface of the tantalum foil, and therefore circular corrosion holes are formed; the corrosion hole with the morphology can eliminate the limitation of adverse crystal face orientation on the depth of the corrosion hole to the greatest extent; then, pulse current is used for hole forming, so that the density of initial corrosion holes can be improved; on the basis, direct current and constant current are used for reaming, so that the diameter and depth of a corrosion hole can be further increased; and finally, enabling the etched tantalum foil to form the high-voltage tantalum capacitor anode foil with high specific volume. The capacitance density of the tantalum anode foil formed by the method can reach 92.6 nF / mm < 2 > under the voltage of 100V, which is about 6 times higher than that before the process is improved. Therefore, the performance of the anode foil of the high-voltage tantalum capacitor is effectively improved.

Description

Technical Field

[0001] The present invention relates to an electrochemical etching method for an anode foil of a high-voltage tantalum capacitor, and belongs to the technical field of preparation of tantalum electrolytic capacitors for electronic components. Background Art

[0002] With the rapid development of artificial intelligence (AI) technology, tantalum electrolytic capacitors (tantalum capacitors) have become indispensable key electronic components in high-performance AI server power distribution / management systems due to their advantages of high volume efficiency, large capacitance, and high stability. However, traditional tantalum capacitors use high-temperature sintered porous tantalum blocks as anodes, and their special preparation processes result in the following two technical bottlenecks for tantalum capacitors: on the one hand, it is difficult to achieve an ultra-thin structure (thickness < 100 μm), and on the other hand, there are also obvious deficiencies in scenarios with large capacitance requirements (> 1000 μF). These technical limitations directly restrict the application expansion of tantalum capacitors in miniaturized electronic devices and high-voltage high-power systems. In contrast, a new type of tantalum capacitor based on etched tantalum foil anodes shows significant advantages. Through innovative anode structure design, this technology can achieve: 1) diverse product forms (including PCB-embedded, surface-mounted, stacked, and wound types); 2) the device thickness breaks through to the micron level; 3) the capacitance coverage range is extended from several microfarads to several thousand microfarads. This multi-dimensional technical breakthrough opens up a broad space for the application of tantalum capacitors in emerging fields such as flexible electronics and high-density integrated circuits.

[0003] Although etched tantalum foil has shown a relatively high capacitance density at low formation voltages (< 10V), its capacitance density is severely low at high voltages (> 100V) and cannot meet the actual requirements. The main reasons are as follows: Firstly, commercial tantalum foil has multiple crystal plane orientations, and the (211) and (111) crystal planes will generate inverted pyramid or inverted triangular prism-shaped corrosion holes, which hinder their longitudinal development; Secondly, tantalum electrode foils prepared by pulsed electrochemical etching have a relatively high pore density, but their diameters are generally small. Therefore, the performance of high-voltage tantalum capacitor electrode foils urgently needs to be further improved. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the limitations of existing etching technologies on the size and depth ratio of corrosion holes, and thus provide an electrochemical etching method for an anode foil of a high-voltage tantalum capacitor. By isotropic etching of tantalum foil, circular corrosion holes are prepared, the limitation of crystal plane orientation on the depth of corrosion holes is inhibited, and the size of corrosion holes is further increased by using the process of pulsed pore formation plus DC pore expansion, so as to significantly improve the capacitance density of high-voltage tantalum electrode foils and provide a new technology for the development of tantalum capacitors with high working voltage and high specific capacitance.

[0005] The present invention provides an electrochemical etching method for an anode foil of a high-voltage tantalum capacitor, comprising the following steps: Step 1, surface pretreatment of tantalum foil. The pretreatment method is one or a combination of chemical pickling, chemical alkali washing, electrochemical polishing, and mechanical polishing; Step 2, pre-oxidation. An oxide film is grown on the pretreated tantalum foil by electrochemical anodic oxidation. The pre-oxidation voltage is 1 - 5V, and the constant voltage duration is 5 - 30 min; Step 3, pore formation by pulsed current. The pre-oxidized tantalum foil is subjected to pore formation by pulsed electrochemical etching; Step 4, direct current constant current pore expansion. The size and depth of the corrosion pores are further increased by constant current electrochemical etching; Step 5, energization. After the electrochemically etched tantalum foil is thoroughly cleaned and boiled, a constant current is applied in the electrolyte to form an amorphous Ta 2 O 5 dielectric film.

[0006] The key of the present invention lies in that the tantalum foil should be pre-oxidized first before electrochemical etching. In this process, an oxide film with a thickness of 2 - 10 nm is grown on the surface of the tantalum foil, which can effectively slow down the mass transfer rate during the electrochemical corrosion process, making the mass transfer polarization become the control step of the tantalum anode dissolution rate, thereby realizing the isotropic etching of the tantalum foil. The circular corrosion pores generated by isotropic etching are more conducive to deepening than the inverted pyramid or inverted triangular prism-shaped corrosion pores, which is crucial for improving the specific capacitance of the tantalum electrode.

[0007] It should be noted that in the traditional manufacturing process of electrolytic capacitors, high-texture foils (such as cubic-textured aluminum foils) are the key to improving the specific capacitance of electrode foils. The cubic texture of aluminum foils can be greatly improved through annealing processes, but the preparation of cubic-textured tantalum foils is extremely difficult. Therefore, the present invention proposes to use isotropic etching to eliminate the influence of unfavorable crystal plane orientations, providing a low-cost feasible solution for further improving the specific capacitance of tantalum electrode foils.

[0008] As a further optimized technical solution of the present invention is as follows: Preferably, in step 2, the voltage range of pre-oxidation is 1 - 5V, the applied current density is 0.1 - 1 mA / cm 2 , and after reaching the target voltage, the constant voltage is maintained for 5 - 20 min.

[0009] Preferably, in step 3, the etchant is a methanol sulfuric acid solution, the concentration of the methanol sulfuric acid solution is 0.5M - 5M, the pulse frequency is 0.1Hz - 50Hz, the current density is 1 - 20 mA / cm 2 , the duty cycle is 10% - 50%, and the etching duration is 1 - 5 min.

[0010] Preferably, in the step 4, the etching solution used is an ethylene glycol solution of sodium chloride, the concentration of the ethylene glycol solution of sodium chloride is 0.01M - 0.1M, the current density is 1 - 10 mA / cm 2 , and the etching duration is 5 - 30 min.

[0011] Preferably, in the step 5, the boiling and washing temperature is 100°C, the electrolyte is a dilute phosphoric acid solution with a concentration of 0.01 wt.%, the electrolyte temperature is 70 - 95°C, the energizing voltage is 50 - 200V, and the applied current density is 0.1 - 0.5 mA / cm 2 , and the constant voltage duration is 30 - 60 min.

[0012] The present invention also provides a tantalum capacitor electrode foil prepared by the above method.

[0013] In the process of the present invention, a relatively thin oxide film is first grown on the surface of the tantalum foil by the pre - oxidation method, aiming to achieve isotropic etching of the tantalum foil and obtain circular corrosion holes; the reason for using pulsed electrochemical etching for pore formation is that denser initial corrosion holes can be obtained; while direct - current electrochemical etching for pore expansion is to further increase the diameter of the corrosion holes to meet the thickness of the oxide film prepared under high voltage. The specific capacitance of the high - voltage tantalum electrode foil prepared by the above method is greatly improved. After energization at 100V voltage, the specific capacitance can reach 92.6 nF / mm 2 , which is about 6 times higher than the original process.

[0014] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention realizes isotropic etching by pre - oxidizing the tantalum foil before electrochemical etching. The obtained circular corrosion holes are more conducive to obtaining a high aspect ratio than the inverted pyramid - shaped corrosion holes on the (211) crystal plane and the inverted triangular prism - shaped corrosion holes on the (111) crystal plane. Importantly, this process does not require the preparation of high - texture cubic tantalum foil, which is beneficial to saving manufacturing costs.

[0015] (2) The high - voltage tantalum capacitor electrode foil prepared by the present invention has a high specific capacitance. If packaged in a single - chip manner, it is expected that its total thickness is less than 100 microns, and it can meet the usage requirements of working voltages above 50V, providing a new type of electronic component for high - density packaging in high - power application scenarios; if packaged in a multi - chip laminated or wound manner, it can provide a larger capacitance than traditional bulk tantalum capacitors, thus meeting the applications in high - power power electronics. Description of the Drawings

[0016] Figure 1 It is an optical microscope image of a tantalum electrode foil prepared by the process in the comparative example.

[0017] Figure 2Optical microscope image of tantalum electrode foil prepared according to the process in the embodiment.

[0018] Figure 3 3D imaging effect of surface corrosion holes of tantalum electrode foil prepared according to the process in the embodiment.

[0019] Figure 4 Process flow chart of the present invention. Detailed implementation manners

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings: This embodiment is implemented on the premise of the technical solution of the present invention, and detailed implementation manners and specific operation processes are given, but the protection scope of the present invention is not limited to the following embodiments.

[0021] As Figure 4 shown, an electrochemical etching method for an anode foil of a high-voltage tantalum capacitor includes the following steps: Step 1, surface pretreatment of tantalum foil: The surface of the tantalum foil is pretreated by one or a combination of chemical pickling, chemical alkali washing, electrochemical polishing, and mechanical polishing, aiming to remove surface scratches, defects, and incomplete passivation films.

[0022] Step 2, pre-oxidation: An oxide film with a thickness of 2 - 10 nm is grown on the pretreated tantalum foil by electrochemical anodic oxidation. The pre-oxidation voltage is 1 - 5 V, and the constant voltage duration is 5 - 20 min; more preferably, the oxidation voltage is 3 V, and the constant voltage duration is 15 min.

[0023] Step 3, pore formation by pulsed current: The pretreated tantalum foil is subjected to pore formation using a forward pulsed current. The etching agent is a methanol sulfuric acid solution with a concentration of 0.5 M - 5 M, the pulse frequency is 0.1 Hz - 50 Hz, the current density is 1 - 20 mA / cm 2 , and the duty cycle is 10% - 50%. The etching duration is 1 - 5 min; more preferably, the concentration is 1 M, the pulse frequency is 10 Hz, the current density is 10 mA / cm 2 , and the duty cycle is 30%, and the etching duration is 2 min.

[0024] Step 4, direct current constant current hole expansion: The purpose is to further increase the size and depth of the corrosion holes by constant current electrochemical etching. The etching solution used is an ethylene glycol solution of sodium chloride with a concentration of 0.01 M - 0.1 M, and the current density is 1 - 10 mA / cm 2 , and the duration is 5 - 30 min; more preferably, the concentration is 0.04 M, the current density is 10 mA / cm 2 , and the duration is 20 min.

[0025] Step 5, Empowering: Thoroughly clean and boil the tantalum foil after electrochemical etching, and then apply a constant current in the electrolyte to form amorphous Ta 2 O 5 dielectric film. The boiling temperature is 100 °C, the electrolyte is a dilute phosphoric acid solution with a concentration of 0.01 wt.%, the electrolyte temperature is 70 - 95 °C, the empowering voltage is 50 - 200 V, and the applied current density is 0.1 - 0.5 mA / cm 2 , and the constant voltage duration is 30 - 60 min; more preferably, the electrolyte temperature is 85 °C, the boosting current is 0.5 mA / cm 2 , the empowering voltage is set to 100 V, and the constant voltage duration is maintained for 30 min.

[0026] In summary, in the present invention, the tantalum foil is pre-oxidized before etching. By controlling the thickness of the surface oxide film, isotropic etching of the tantalum foil can be achieved, thereby generating circular corrosion holes. The corrosion holes with this morphology can eliminate the limitation of the adverse crystal plane orientation on the depth of the corrosion holes to the greatest extent; then pulsed current is used to initiate holes, which can increase the density of the initial corrosion holes; on this basis, the holes are enlarged with direct current constant current, which can further increase the diameter and depth of the corrosion holes; finally, the etched tantalum foil is empowered to form a high specific capacitance high-voltage tantalum capacitor anode foil. The capacitance density of the tantalum anode foil formed in this way can reach 92.6 nF / mm 2 under a voltage of 100 V, which is about 6 times higher than that before process improvement. Therefore, the present invention effectively improves the performance of the high-voltage tantalum capacitor anode foil. Examples

[0027] (1) Using the tantalum foil as the anode and the platinum metal sheet as the cathode, a two-electrode system is constructed and placed in a mixed acid of hydrofluoric acid and concentrated sulfuric acid (where the volume ratio of hydrofluoric acid to concentrated sulfuric acid is 1:9, and the hydrofluoric acid used is 38% hydrofluoric acid) for surface pretreatment; (2) Thoroughly clean the pretreated tantalum foil, use it as the anode, and construct a two-electrode system with the platinum metal sheet as the cathode for pre-oxidation. The anodic oxidation voltage is set to 3 V, the boosting current density is 0.1 mA / cm 2 , and the electrolyte is a 0.01 wt.% aqueous phosphoric acid solution. After reaching a voltage of 3 V, the constant voltage is maintained for 15 min.

[0028] (3) Using the pre-oxidized tantalum foil as the anode and the platinum metal sheet as the cathode, a two-electrode system is constructed. Using a pulsed power supply as the current source and a 1 M sulfuric acid methanol solution as the etching solution, the pulse frequency is set to 10 Hz, the current density is 10 mA / cm 2 , the duty cycle is 30%, and the etching duration is 2 min.

[0029] (4) Using tantalum foil with pulsed electrochemical pore formation as the anode and a platinum metal sheet as the cathode, a two-electrode system was constructed. Using a DC power supply as the current source and an ethylene glycol solution of sodium chloride with a concentration of 0.04 M as the etching solution, the current density was 10 mA / cm 2 , and the etching duration was 20 min.

[0030] (5) The tantalum foil after electrochemical etching was thoroughly cleaned and boiled, and then energized in a phosphoric acid solution with a concentration of 0.01 wt.%. The boiling temperature was 100 °C, the solution temperature was 85 °C, and the boosting current was 0.5 mA / cm 2 , the energizing voltage was set at 100 V, and the constant voltage duration was maintained for 30 min. The specific capacitance of the obtained tantalum electrode foil was 92.6 nF / mm 2 , and the optical microscope image of the tantalum electrode foil is shown in Figure 2 , and the 3D imaging effect of the corrosion pores on its surface is shown in Figure 3 . Comparative Example

[0031] (1) Using tantalum foil as the anode and a platinum metal sheet as the cathode, a two-electrode system was constructed and placed in a mixed acid of hydrofluoric acid and concentrated sulfuric acid (where the volume ratio of hydrofluoric acid to concentrated sulfuric acid is 1:9, and the hydrofluoric acid used is 38% hydrofluoric acid) for surface pretreatment; (2) Using the pretreated tantalum foil as the anode and a platinum metal sheet as the cathode, a two-electrode system was constructed. Using a DC power supply as the current source and an ethylene glycol solution of sodium chloride with a concentration of 0.04 M as the etching solution, the current density was 10 mA / cm 2 , and the etching duration was 20 min.

[0032] (3) The tantalum foil after electrochemical etching was thoroughly cleaned and boiled, and then energized in a phosphoric acid solution with a concentration of 0.01 wt.%. The boiling temperature was 100 °C, the solution temperature was 85 °C, and the boosting current was 0.5 mA / cm 2 , the energizing voltage was set at 100 V, and the constant voltage duration was maintained for 30 min. The specific capacitance of the obtained tantalum electrode foil was 15.4 nF / mm 2 , and the optical microscope image of the tantalum electrode foil is shown in Figure 1 .

[0033] As described above, it is only the specific implementation manner in the present invention, but the protection scope of the present invention is not limited thereto. Any transformation or replacement that can be understood and conceived by those familiar with the technology within the technical scope disclosed by the present invention should be covered within the scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. An electrochemical etching method for anode foil of a high voltage tantalum capacitor, characterized in that: The following steps are involved: Step 1, pretreatment of the tantalum foil surface, the pretreatment method is one or a combination of chemical pickling, chemical alkali washing, electrochemical polishing, mechanical polishing; Step 2, pre-oxidation, using electrochemical anodization to grow an oxide film on the pre-treated tantalum foil, the pre-oxidation voltage is 1-5V, and the constant voltage time is 5-30min; Step 3, pulse current pore formation, using pulse electrochemical etching to form pores in the pre-oxidized tantalum foil; Step 4: direct current constant current hole expansion, using constant current electrochemical etching method to further increase the size and depth of the corrosion hole; Step 5, energizing, after the tantalum foil after electrochemical etching is fully cleaned and boiled, a constant current is applied in the electrolyte to form an amorphous Ta2O5 dielectric film.

2. The electrochemical etching method for high voltage tantalum capacitor anode foil according to claim 1, characterized in that: In step 2, the pre-oxidation voltage range is 1-5V, and the applied current density is 0.1-1 mA / cm 2 After reaching the target voltage, maintain constant voltage for 5-20 minutes.

3. The electrochemical etching method for anode foil of a high voltage tantalum capacitor according to claim 1, characterized in that: In step 3, the etchant is a methanol-sulfuric acid solution, the concentration of the methanol-sulfuric acid solution is 0.5M-5M, the pulse frequency is 0.1Hz-50Hz, and the current density is 1-20mA / cm 2 , the duty cycle is 10%-50%, and the etching time is 1-5min.

4. The electrochemical etching method for anode foil of a high voltage tantalum capacitor according to claim 1, characterized in that: In step 4, the etching solution used is a sodium chloride ethylene glycol solution with a concentration of 0.01M-0.1M and a current density of 1-10mA / cm 2 The etching time is 5-30min.

5. The electrochemical etching method for anode foil of a high voltage tantalum capacitor according to claim 1, characterized in that: In step 5, the boiling and washing temperature is 100°C, the electrolyte is a dilute phosphoric acid solution with a concentration of 0.01wt.%, the electrolyte temperature is 70-95°C, the energizing voltage is 50-200V, and the applied current density is 0.1-0.5mA / cm 2 , constant pressure time is 30-60min.

6. Tantalum capacitor anode foil prepared by the method according to any one of claims 1 to 5.

Images