Preparation method of niobium-tantalum composite capacitor anode

Abstract

The invention relates to a preparation method of a niobium-tantalum composite capacitor anode, which comprises the following steps: (S1) carrying out ball milling on niobium powder with D50 of 25-35 mu m and span of less than or equal to 0.8, tantalum powder with D50 of 4-7 mu m and span of less than or equal to 0.8 and an adhesive to obtain niobium-tantalum mixed powder, the adhesive being a compound of polyvinyl alcohol and polyethylene glycol; (S2) the niobium-tantalum mixed powder is pressed into a blank block with a tantalum wire outgoing line, vacuum sintering is conducted, and a sintered block is obtained; and (S3) energizing the sintered block in an acid-containing electrolyte to obtain the niobium-tantalum composite capacitor anode. According to the invention, the tantalum powder and niobium powder with different particle sizes, a specific ball milling process and a specific adhesive are matched for compression molding, and then the composite material obtained after vacuum sintering and enabling processes is used as the anode of the capacitor, so that excellent comprehensive performance is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of capacitors, and in particular to a method for preparing an anode of a niobium-tantalum composite capacitor. Background Art

[0002] Niobium (Nb) and tantalum (Ta), both valve metals, are widely used in the manufacture of electrolytic capacitors due to their excellent dielectric properties and chemical stability. Tantalum capacitors use a solid electrolyte, offering greater stability and a longer service life. Known for their compact size, high capacitance, low ESR, high withstand voltage, and excellent temperature characteristics, they maintain stable performance over a wide temperature range of -50°C to 100°C, offering high capacitance within a compact size, making them suitable for circuit designs with limited space. Furthermore, tantalum capacitors offer excellent filtering and bypass performance in high-frequency applications. However, tantalum is expensive. Currently, the world's proven reserves of tantalum resources are only approximately 308,000 tons, while reserves of niobium, also a valve metal, are as high as 48.81 million tons. Tantalum is over 20 times more expensive than niobium. Due to the price and cost of tantalum, tantalum-based capacitors have long been used in high-end markets, such as the military and aerospace industries. Reducing capacitor manufacturing costs by partially replacing tantalum with niobium is a pressing issue.

[0003] There are reports on niobium-tantalum composite capacitors in the prior art. For example, CN106409510A discloses a niobium-tantalum composite capacitor, which is prepared by mixing niobium powder and tantalum powder, and then energizing them in sections, and combining them with a graphene oxide-multi-walled carbon nanotube-ruthenium trichloride composite material to produce a niobium-tantalum composite capacitor with large capacitance and good stability. However, the preparation process of this patent is complicated, requiring multiple steps to prepare a graphene oxide-highly hydrophilic multi-walled carbon nanotube-ruthenium trichloride composite material as a cathode material. On the one hand, the process is complicated, and on the other hand, expensive graphene, carbon nanotubes, and precious metal ruthenium are used, making it unsuitable for industrial production.

[0004] CN101859649A discloses a method for preparing a solid electrolyte niobium-tantalum composite capacitor. The capacitor is made by mixing niobium and tantalum. First, tantalum powder and niobium powder, or tantalum powder and pure niobium monoxide, are uniformly mixed in a certain proportion, wherein the tantalum content is 5% to 95% and the remainder is niobium. This is then pressed into shape to obtain a composite metal anode block, which is then vacuum sintered to obtain the anode of the composite capacitor. Electrochemical oxidation then forms a dielectric film of composite tantalum oxide and niobium oxide, which becomes the anode of the niobium-tantalum composite capacitor. The cathode is solid manganese dioxide. The product has an appearance similar to that of a chip-type solid electrolyte niobium-tantalum composite capacitor. The preparation method includes the steps of mixing tantalum powder and niobium powder or niobium monoxide, forming, sintering, energizing, and coating.

[0005] However, the above-mentioned patents all involve physical mixing of tantalum and niobium, which is prone to segregation during the pressing process, resulting in different ratios of niobium and tantalum in different parts of the resulting composite capacitor, causing a heterogeneous interface. It can be considered that in conventional composite processes, niobium and tantalum are prone to poor stability of the composite material due to problems with dielectric properties and interface compatibility.

[0006] CN104379792A discloses a capacitor-grade tantalum-niobium alloy wire powder and a preparation method thereof, wherein the niobium powder is divided into two parts, the entire tantalum powder and the first part of the niobium powder are first mixed at a gradually increasing speed, and then the second part of the niobium powder is added and the speed is increased to obtain a capacitor-grade tantalum-niobium alloy powder. This patented process avoids the unevenness caused by excessive addition of niobium powder at one time by partially adding niobium powder. However, the inventors found that the reason for the poor dispersion of niobium and tantalum in niobium-carbon composite capacitors is not only the unevenness during powder mixing, but more importantly, the difference in shrinkage behavior of the two during the subsequent vacuum sintering stage; and in the subsequent energizing process, due to the difference in oxidation potential, different regions of the composite material may have different oxidation degrees, resulting in oxygen vacancy defects, making it difficult to form a uniform and complete dielectric film on the surface of the niobium-tantalum composite anode. This leads to defects such as large leakage current, high loss, and poor high-temperature stability. Summary of the Invention

[0007] In order to solve the problem in the prior art that niobium-tantalum composite capacitors are difficult to give full play to the advantages of the two materials due to the poor dispersion of niobium and tantalum, resulting in poor performance, especially the defect of large leakage current. The present invention proposes a niobium-tantalum composite capacitor and a preparation method thereof. The present invention proposes a composite material obtained by pressing tantalum powder and niobium powder of different particle sizes, a specific ball milling process and a specific adhesive, and then performing vacuum sintering and energizing processes to form the composite material as the anode of the capacitor, thereby achieving excellent comprehensive performance. Specifically, the present invention provides the following technical solutions to solve the above technical problems:

[0008] A method for preparing a niobium-tantalum composite capacitor anode comprises the following steps:

[0009] (S1) ball milling niobium powder having a D50 of 25-35 μm and a span of ≤0.8, tantalum powder having a D50 of 4-7 μm and a span of ≤0.8, and a binder to obtain a niobium-tantalum mixed powder, wherein the binder is a compound of polyvinyl alcohol and polyethylene glycol;

[0010] (S2) pressing the niobium-tantalum mixed powder into a compact with a tantalum wire lead, and vacuum sintering to obtain a sintered block;

[0011] (S3) The sintered block is energized in an acid-containing electrolyte to obtain a niobium-tantalum composite capacitor anode.

[0012] Furthermore, in step (S1), the span is defined as span (Span) = (D90-D10) / D50. The smaller the span, the more concentrated the particle size distribution. The number average molecular weight of polyvinyl alcohol is 20,000-30,000, and the number average molecular weight of polyethylene glycol is 1600-3000. The use of two adhesives with different molecular weights can better improve the various performances of the product capacitor, especially reduce leakage current and dielectric loss at the same time. This shows that the two adhesives may have a certain synergistic effect. The high molecular weight polyvinyl alcohol can form a coating film on the surface of the particles, reducing the unevenness caused by transition cold welding. The low molecular weight polyethylene glycol has both lubrication and dispersing functions. Polyvinyl alcohol requires a higher molecular weight, not only to act as an adhesive, but also to suppress the gravity stratification caused by the different densities of niobium and tantalum through the steric hindrance effect of the polymer chain. However, the molecular weight of polyvinyl alcohol should not be too high, otherwise there will be residues in the vacuum sintering stage, affecting the performance of the capacitor. Low molecular weight polyethylene glycol, during the ball milling stage, the relatively short chain segments of PEG can play a role similar to a "lubricant", inserted between the long chains of polyvinyl alcohol and the metal powder, preventing excessive cold welding and improving the fluidity of ball milling.

[0013] Furthermore, in step (S1), the mass ratio of niobium powder, tantalum powder, and adhesive is 70-80:10-15:3.2-4.7. More preferably, the mass ratio of niobium powder to tantalum powder is less than 6:1. The adhesive is a compound of polyvinyl alcohol and polyethylene glycol in a mass ratio of 5-7:1. The ratio between niobium and tantalum must be appropriate to maximize the respective advantages of the two materials as capacitor raw materials. The amount of adhesive used should not be too high, otherwise excessive porosity will be caused during subsequent sintering to remove the adhesive, which is not conducive to the performance of the capacitor. After ball milling, the initial particle size of niobium powder is relatively large, and niobium has good ductility, and mainly breaks under the impact of ball milling energy. Tantalum powder has high hardness and a small initial particle size, which makes it more prone to cold welding and agglomeration during the ball milling process. After the final ball milling, the particle sizes of the two materials are close and they are evenly mixed.

[0014] Furthermore, in step (S1), the ball-to-material ratio of ball milling is 5-8:1, the rotation speed is 150-400 rpm, the ball milling time is 3-4 hours, and the ball medium is zirconia with a diameter of 4-6 mm.

[0015] Furthermore, in step (S1), the ball milling is segmented, and niobium powder and polyvinyl alcohol are first added and subjected to high-speed ball milling at a speed of 300-400 rpm for 1-2 hours. Then, tantalum powder and polyethylene glycol are added, the speed is reduced to 150-200 rpm, and ball milling is continued for 2-3 hours. The inventors unexpectedly discovered that segmented ball milling, combined with the addition of different adhesives at different stages, achieves performance improvements beyond conventional mixed ball milling by precisely controlling the ball milling conditions of different powders and the order of adding adhesives. The possible reason is that large-particle niobium powder is crushed under high-speed ball milling conditions, PVA is added for coating, and then small-particle tantalum powder and low-molecular-weight polyethylene glycol are added. This ball milling condition can significantly improve the dispersibility of niobium and tantalum in the resulting mixed powder.

[0016] Furthermore, in step (S2), the density of the briquette is controlled at 6.8-7.5 g / cm 3 Vacuum sintering is between 1×10 -3 to 1×10 -2 Under vacuum conditions of 0.5 Pa, pre-sintering is carried out at 1300-1500° C. for 0.5-1 h, then the temperature is raised to 1800-2000° C. and sintering is continued for 1-2 h, then the temperature is lowered to room temperature and returned to normal pressure to obtain a sintered block.

[0017] Furthermore, in step (S3), the acid-containing electrolyte has an acid concentration of 0.01-1 wt%, and the acid is selected from at least one of phosphoric acid, nitric acid, hydrochloric acid, boric acid, citric acid, and malic acid. The electrolyte also contains 20-30 wt% ethylene glycol, with the balance being water. The energizing step is performed at 60-90°C, a current density of 50-100 mA / g, and a voltage of 100-150 V for 1-2 hours. During the energizing step, a dense oxide film forms on the surface of the composite material block, thereby increasing the dielectric constant.

[0018] The niobium-tantalum composite capacitor anode prepared according to the preparation process of the present invention has uniform niobium and tantalum distribution, and the capacitor made of the anode material has excellent performance, fully utilizing the advantages of tantalum capacitors and niobium capacitors, while having improved stability.

[0019] The second object of the present invention is to provide a niobium-tantalum composite capacitor, wherein the anode of the niobium-tantalum composite capacitor is the anode of the niobium-tantalum composite capacitor prepared by the above method.

[0020] The niobium-tantalum composite capacitor is prepared by a preparation method comprising the following steps:

[0021] (S4) Immersing the anode of the niobium-tantalum composite capacitor prepared by the above preparation method in a manganese nitrate solution, removing the film, and repeating this process 3-5 times to form a manganese dioxide cathode. Graphite and silver paste are coated on the surface of the manganese dioxide cathode, connected to a lead frame, encapsulated with a polymer resin, and aged to obtain a niobium-tantalum composite capacitor.

[0022] Furthermore, in step (S4), the concentration of the manganese nitrate solution is 1-3wt%, and the film temperature is 180-230°C. The processes of coating graphite and silver paste, connecting leads, encapsulating, and aging are well known in the art. Furthermore, epoxy resin is used for encapsulation, and aging is performed at 1-5V for 1-2 hours, then increasing the voltage to 50-60V and maintaining it for 1-2 hours, then increasing the temperature to 50-70°C and maintaining it for 1-2 hours, then increasing the temperature to 100-110°C and maintaining it for 1-2 hours, then cooling and reducing the voltage to 50-70°C, and maintaining it at 30-40V for 30-40 hours. DETAILED DESCRIPTION

[0023] In order to make the technical means, creative features, objectives and effects of the present invention easier to understand, the present invention is further described below in conjunction with specific embodiments. The embodiments described below are exemplary and are only used to explain the present invention, and should not be understood as limiting the present invention.

[0024] Example 1

[0025] (S1) 7.3 kg of niobium powder having a D50 of 31.2 μm and a span of 0.57, 1.3 kg of tantalum powder having a D50 of 5.3 μm and a span of 0.71, and 0.35 kg of a binder were ball-milled together. The binder was a mixture of polyvinyl alcohol with a number average molecular weight of 26,000 and polyethylene glycol with a number average molecular weight of 2200 in a mass ratio of 6:1. The ball-milling process was a ball-to-material ratio of 8:1, a rotation speed of 250 rpm, a ball-milling time of 3 h, and a ball-milling medium of zirconia with a diameter of 5 mm. After ball milling, a niobium-tantalum mixed powder having a D50 of 17.3 μm and a span of 0.93 was obtained.

[0026] (S2) The niobium-tantalum mixed powder was pressed into a compact (3 mm × 2.5 mm × 1.5 mm) with a tantalum wire lead. The compact density was 7.1 g / cm 3 , in 1×10 -3 Pa vacuum conditions, pre-sintering at 1350℃ for 0.5h, then heating to 1900℃ and continuing sintering for 1.5h,

[0027] Cooling to room temperature and returning to normal pressure to obtain a sintered block;

[0028] (S3) The sintered block is energized in an electrolyte containing 0.1 wt% phosphoric acid, 0.01 wt% hydrochloric acid, 22 wt% ethylene glycol, and the remainder water. The energization is performed at 80° C., a current density of 50 mA / g, and a voltage of 130 V for 2 h, so that a dense oxide film is formed on the surface of the sintered block to obtain a niobium-tantalum composite capacitor anode.

[0029] (S4) The anode of the niobium-tantalum composite capacitor is immersed in a 2wt% manganese nitrate solution, taken out and placed in a coating device, and coated at 200°C, which is repeated 3 times to form a manganese oxide film on the surface of the anode of the niobium-tantalum composite capacitor. Then, graphite and silver paste are coated to form a cathode, the cathode lead is drawn out, the lead frame is connected, and epoxy resin is encapsulated. Aging is carried out according to the following conditions: 5V condition is maintained for 1 hour, the voltage is increased to 60V, maintained for 1 hour, the temperature is increased to 60°C and maintained for 1 hour, the temperature is increased to 110°C and maintained for 1 hour, and then the temperature is reduced to 40°C, the voltage is reduced to 40V and maintained for 40 hours.

[0030] Example 2

[0031] (S1) 8 kg of niobium powder having a D50 of 31.2 μm and a span of 0.57, 1 kg of tantalum powder having a D50 of 5.3 μm and a span of 0.71, and 0.40 kg of a binder were ball-milled together. The binder was a mixture of polyvinyl alcohol with a number average molecular weight of 26,000 and polyethylene glycol with a number average molecular weight of 2200 in a mass ratio of 5:1. The ball-milling process was a ball-to-material ratio of 8:1, a rotation speed of 250 rpm, a ball-milling time of 3 h, and a ball-milling medium of zirconia with a diameter of 5 mm. After ball milling, a niobium-tantalum mixed powder having a D50 of 17.1 μm and a span of 0.96 was obtained.

[0032] Steps (S2), (S3) and (S4) are the same as those in Example 1.

[0033] Example 3

[0034] (S1) 6 kg of niobium powder having a D50 of 31.2 μm and a span of 0.57, 1.5 kg of tantalum powder having a D50 of 5.3 μm and a span of 0.71, and 0.32 kg of a binder were ball-milled together. The binder was a mixture of polyvinyl alcohol with a number average molecular weight of 26,000 and polyethylene glycol with a number average molecular weight of 2200 in a mass ratio of 7:1. The ball-milling process was a ball-to-material ratio of 8:1, a rotation speed of 250 rpm, a ball-milling time of 3 h, and a ball-milling medium of zirconia with a diameter of 5 mm. After ball milling, a niobium-tantalum mixed powder having a D50 of 16.4 μm and a span of 1.05 was obtained.

[0035] Steps (S2), (S3) and (S4) are the same as those in Example 1.

[0036] Example 4

[0037] (S1) 7.3 kg of niobium powder with a D50 of 31.2 μm and a span of 0.57 and 0.3 kg of polyvinyl alcohol with a number average molecular weight of 26,000 were ball-milled at a ball-to-material ratio of 8:1 using zirconium oxide with a diameter of 5 mm as the milling medium at 350 rpm for 1 h. The milling was then stopped, and 1.3 kg of tantalum powder with a D50 of 5.3 μm and a span of 0.71 and 0.5 kg of polyethylene glycol with a number average molecular weight of 2200 were added and ball-milled at 200 rpm for 2 h to obtain a niobium-tantalum mixed powder with a D50 of 17.2 μm and a span of 0.86.

[0038] Steps (S2), (S3) and (S4) are the same as those in Example 1.

[0039] Example 5

[0040] Other conditions are the same as those in Example 1, except that in step (S1), the adhesive is a mixture of polyvinyl alcohol with a number average molecular weight of 30,000 and polyethylene glycol with a number average molecular weight of 1,600 in a mass ratio of 5:1.

[0041] Example 6

[0042] Other conditions are the same as those in Example 1, except that in step (S1), the adhesive is a mixture of polyvinyl alcohol with a number average molecular weight of 20,000 and polyethylene glycol with a number average molecular weight of 3,000 in a mass ratio of 5:1.

[0043] Example 7

[0044] Other conditions are the same as those in Example 1, except that in step (S1), the adhesive is a mixture of polyvinyl alcohol with a number average molecular weight of 10,000 and polyethylene glycol with a number average molecular weight of 2,200 in a mass ratio of 5:1.

[0045] Example 8

[0046] Other conditions are the same as those in Example 1, except that in step (S1), the adhesive is a mixture of polyvinyl alcohol with a number average molecular weight of 50,000 and polyethylene glycol with a number average molecular weight of 2,200 in a mass ratio of 5:1.

[0047] Comparative Example 1

[0048] Other conditions are the same as those in Example 1, except that in step (S1), the adhesive is all polyvinyl alcohol with a number average molecular weight of 26,000.

[0049] Comparative Example 2

[0050] Other conditions were the same as those in Example 1, except that in step (S1), the adhesive was all polyethylene glycol with a number average molecular weight of 2200.

[0051] Comparative Example 3

[0052] Other conditions are the same as those in Example 1, except that in step (S1), the D50 of the tantalum powder is 25.8 μm and the span is 0.64.

[0053] Comparative Example 4

[0054] Other conditions are the same as those in Example 1, except that in step (S1), the D50 of the niobium powder is 8.4 μm and the span is 0.75.

[0055] Application Examples

[0056] The performance of the capacitors prepared in the above examples was tested, and the results are shown in Table 1 below.

[0057] Table 1 Capacitor performance test results

[0058]

[0059]

[0060] From the data in Table 1, it can be seen that the niobium-tantalum composite capacitor prepared by the process of the present invention has the advantages of both niobium capacitors and tantalum capacitors. The reason why the composite capacitor obtained by the present invention has excellent performance is that the niobium-tantalum dispersion is more uniform through a specific process. Specifically, by compounding tantalum powder and niobium powder of different particle sizes, high molecular weight polyvinyl alcohol and low molecular weight polyethylene glycol, a niobium-tantalum composite capacitor with excellent comprehensive performance is obtained. In a preferred embodiment of the present invention, segmented ball milling is also used. First, a high-speed ball milling of large-particle niobium powder and polyvinyl alcohol is carried out. Then, small-particle tantalum powder and polyethylene glycol are added and a two-stage low-speed ball milling is carried out. Finally, a capacitor with the best performance can be obtained.

Claims

1. A method for preparing a niobium-tantalum composite capacitor anode, characterized in that: The following steps are involved: (S1) ball milling niobium powder having a D50 of 25-35 μm and a span of ≤0.8, tantalum powder having a D50 of 4-7 μm and a span of ≤0.8, and a binder to obtain a niobium-tantalum mixed powder, wherein the binder is a compound of polyvinyl alcohol and polyethylene glycol; (S2) pressing the niobium-tantalum mixed powder into a compact with a tantalum wire lead, and vacuum sintering to obtain a sintered block; (S3) The sintered block is energized in an acid-containing electrolyte to obtain a niobium-tantalum composite capacitor anode.

2. The preparation method according to claim 1, characterized in that In step (S1), the number average molecular weight of polyvinyl alcohol is 20,000-30,000, and the number average molecular weight of polyethylene glycol is 1,600-3,000.

3. The preparation method according to claim 1, characterized in that In step (S1), the mass ratio of niobium powder, tantalum powder, and adhesive is 70-80:10-15:3.2-4.7; the adhesive is a compound of polyvinyl alcohol and polyethylene glycol in a mass ratio of 5-7:1; Furthermore, the mass ratio of niobium powder to tantalum powder is less than 6:

1.

4. The preparation method according to claim 1, characterized in that In step (S1), the ball-to-material ratio of the ball milling is 5-8:1, the rotation speed is 150-400 rpm, the ball milling time is 3-4 hours, and the ball medium is zirconium oxide with a diameter of 4-6 mm.

5. The preparation method according to claim 1, characterized in that In step (S1), the ball milling is segmented ball milling. First, niobium powder and polyvinyl alcohol are added and high-speed ball milling is performed at a speed of 300-400 rpm for 1-2 hours. Then, tantalum powder and polyethylene glycol are added, the speed is reduced to 150-200 rpm, and ball milling is continued for 2-3 hours.

6. The preparation method according to claim 1, characterized in that In step (S2), the density of the briquette is controlled at 6.8-7.5 g / cm 3 Vacuum sintering is between 1×10 -3 to 1×10 -2 Under vacuum conditions of 0.5 Pa, pre-sintering is carried out at 1300-1500° C. for 0.5-1 h, then the temperature is raised to 1800-2000° C. and sintering is continued for 1-2 h, then the temperature is lowered to room temperature and returned to normal pressure to obtain a sintered block.

7. The preparation method according to claim 1, characterized in that In step (S3), the acid concentration in the acid-containing electrolyte is 0.01-1wt%, and the acid is selected from at least one of phosphoric acid, nitric acid, hydrochloric acid, boric acid, citric acid, and malic acid; the electrolyte also contains 20-30wt% ethylene glycol, and the balance is water.

8. The preparation method according to claim 1, characterized in that In step (S3), the energy is provided at 60-90°C, a current density of 50-100 mA / g, and a voltage of 100-150 V for 1-2 hours.

9. A niobium-tantalum composite capacitor, wherein the anode of the niobium-tantalum composite capacitor is prepared by the preparation method according to any one of claims 1 to 8.

10. The method for preparing the niobium-tantalum composite capacitor according to claim 9, characterized in that: The following steps are involved: (S4) immersing the niobium-tantalum composite capacitor anode obtained by the preparation method according to any one of claims 1 to 8 in a manganese nitrate solution, removing the coating, repeating this process 3 to 5 times to form a manganese dioxide cathode, coating the surface of the manganese dioxide cathode with graphite and silver paste, connecting a lead frame, encapsulating the capacitor with a polymer resin, and aging the capacitor to obtain a niobium-tantalum composite capacitor; Furthermore, in step (S4), the concentration of the manganese nitrate solution is 1-3 wt %, and the film temperature is 180-230°C.

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