Method for passivating sintered tantalum block of tantalum capacitor

Abstract

The invention belongs to the field of tantalum capacitor manufacturing, and particularly relates to a tantalum capacitor sintered tantalum block passivation method which comprises the following steps: 1) cooling: after the temperature of a sintering furnace is reduced to be less than or equal to 500 DEG C, vacuumizing, filling protective gas and maintaining pressure in sequence; the operation is repeated until the temperature in the furnace is-10 DEG C to 100 DEG C; 2) oxidation: sequentially vacuumizing, filling oxygen-containing gas and maintaining pressure; the operation is repeated until the pressure in the furnace is 0 Pa; (3) preparing for discharging: vacuumizing, introducing air and maintaining pressure; according to the method, parameter control such as passivation temperature control, passivation gas control, furnace pressure control, passivation time control and passivation frequency control is combined, high cooling efficiency, low oxidation initial temperature, high oxidation film purity and controllable passivation temperature are achieved, and therefore the leakage current value of the tantalum capacitor is greatly reduced.

Description

Technical Field

[0001] The invention belongs to the field of tantalum capacitor manufacturing, and in particular relates to a passivation method for sintered tantalum blocks of tantalum capacitors. Background Art

[0002] Tantalum capacitor anode blocks are typically made from tantalum powder pressed into shape and then sintered at high temperature and high vacuum. Tantalum has a strong affinity for oxygen, and through oxidation, a tantalum oxide film forms on the tantalum surface. This film protects the tantalum matrix from further oxidation. However, during the sintering process, oxygen from the tantalum oxide film migrates into the tantalum matrix or escapes at high temperatures, causing the oxide film to disappear after sintering. Due to the fine particle size of tantalum powder, its high specific surface area and surface activity make sintered tantalum powder highly susceptible to intense oxidation reactions upon contact with air. As the specific volume of tantalum powder increases, its specific surface area increases, its surface activity further increases, and the oxidation reaction becomes more intense. Tantalum oxidation is exothermic, and the heat released exacerbates the reaction, creating a vicious cycle that causes the temperature to rise continuously and may eventually cause the tantalum powder to combust, rendering it useless for use.

[0003] In order to avoid the above phenomenon, the existing technology usually performs a passivation treatment after sintering the tantalum block and before taking it out of the furnace. The passivation process is to introduce a small amount of air before taking it out of the furnace to cause a slight oxidation reaction on the surface of the tantalum powder. Then the amount of air introduced is gradually increased until the pressure in the furnace is balanced with the atmospheric pressure, and then the product is taken out. The extremely thin oxide film formed during the passivation process can isolate the internal tantalum matrix from external oxygen, thereby reducing the oxidation reaction rate and playing a protective role.

[0004] However, the existing passivation technology has the following shortcomings:

[0005] 1. High passivation temperature: The passivation starting temperature has a significant impact on the passivation effect. The higher the temperature, the more intense the oxidation reaction, the thicker the generated oxide film, and a crystalline oxide film may be formed. This crystalline oxide film will reduce the quality of the dielectric oxide film formed subsequently, resulting in increased leakage current. In the prior art, passivation usually begins after naturally cooling to 50°C or even below 35°C after sintering. When the sintering furnace naturally cools down to below 500°C, the cooling rate is slow, time-consuming, and the efficiency is low. In order to improve efficiency, passivation may begin at around 50°C, but the temperature is relatively high at this time. For high-specific capacitance tantalum powder (specific capacitance ≥ 50K), even if passivated at 35°C, a relatively intense oxidation reaction will still occur. Although it will not burn, the generated oxide film is thicker and the oxygen content of the tantalum block increases, resulting in a decrease in the quality of the dielectric oxide film formed subsequently.

[0006] 2. Low cooling efficiency: In the existing technology, the cooling speed of the sintering furnace below 500°C is slow, resulting in a long cooling process and low production efficiency.

[0007] 3. Low purity of passivation gas: Existing passivation processes usually use air as the oxygen source. However, in addition to oxygen and nitrogen, air also contains impurities such as hydrogen, carbon dioxide, water vapor, and dust. These impurities will be mixed into the oxide film during the passivation process and become inclusions in the subsequently formed dielectric oxide film, thereby reducing the quality of the dielectric oxide film and increasing the leakage current.

[0008] 4. Heat accumulation during the passivation process: Existing passivation processes typically introduce air multiple times in small increments until the pressure inside the furnace reaches the same level as the outside pressure. However, the heat released during the passivation process accumulates, causing the temperature inside the furnace to rise, further intensifying the oxidation reaction.

[0009] In summary, the existing passivation technology has obvious deficiencies in terms of passivation temperature control, cooling efficiency, gas purity, and heat management. Improvements are urgently needed to enhance the passivation effect of tantalum powder and the quality of subsequent dielectric oxide films, thereby ensuring low leakage current values of tantalum capacitors. Summary of the Invention

[0010] Aiming at the deficiencies of the prior art, the present invention proposes a passivation method for sintered tantalum blocks of tantalum capacitors.

[0011] This is achieved specifically through the following technical solutions:

[0012] A passivation method for sintered tantalum blocks of tantalum capacitors comprises the following steps:

[0013] 1) Cooling: After the tantalum block is sintered, the temperature of the furnace is lowered. When the temperature of the sintering furnace drops to ≤500°C, the furnace is first vacuumed to a pressure of -100kPa to 0Pa, and then filled with protective gas to a pressure of -100kPa to 0Pa, which is maintained for 1min to 72h; the "vacuuming - passing protective gas - maintaining pressure" step is repeated 1 to 100 times until the temperature in the furnace is -10°C to 100°C;

[0014] 2) Oxidation: After the sintering furnace temperature drops to -10°C to 100°C, first evacuate the furnace to a pressure of -100kPa to 0Pa, then fill it with oxygen-containing gas until the pressure in the furnace is -100kPa to 0Pa and maintain it for 1 minute to 72 hours; repeat the "evacuate - pass oxygen-containing gas - maintain pressure" operation 1 to 100 times until the pressure in the furnace after passing oxygen-containing gas is 0Pa;

[0015] 3) Preparation for furnace removal: After the pressure in the furnace is 0Pa, first evacuate the furnace to a pressure of -100kPa~0Pa, then introduce air to the furnace to a pressure of -5kPa~5kPa, and keep it constant for 1min~72h.

[0016] Preferably, the specific capacitance of the tantalum block is ≥1000 μF·V / g; further preferably, the specific capacitance of the tantalum block is ≥20000 μF·V / g.

[0017] The temperature of the protective gas is -10°C to 100°C, and the purity is ≥99.9%.

[0018] The protective gas is argon or nitrogen.

[0019] The temperature of the oxygen-containing gas is -10°C to 100°C.

[0020] The oxygen-containing gas is a mixed gas of oxygen and argon or nitrogen.

[0021] The oxygen content of the oxygen-containing gas is 1% to 99%, and the impurity content is less than 0.1%.

[0022] More preferably, the step 1) is specifically as follows:

[0023] S1.1 After the tantalum block is sintered, the furnace temperature is lowered. When the sintering furnace temperature drops to ≤500°C, the furnace is first vacuumed to a pressure of -100kPa to 0Pa, and then filled with protective gas to a pressure of -100kPa to 0Pa, which is maintained constant for 1 minute to 72 hours. Repeat this step several times until the furnace temperature reaches 30°C to 60°C.

[0024] S1.2 After the temperature in the furnace reaches 30℃~60℃, evacuate the furnace until the pressure is -100kPa~0Pa, then fill it with protective gas at a temperature of -10℃~30℃ until the pressure in the furnace is -100kPa~0Pa, and keep it constant for 1min~72h; repeat this step several times until the temperature in the furnace is -10℃~30℃.

[0025] Beneficial effects:

[0026] 1. High cooling efficiency. The present invention adopts a cycle of "vacuuming - passing protective gas - maintaining pressure", which greatly improves the cooling rate in the sintering furnace and achieves rapid reduction of the furnace temperature. It only takes 2 hours to reduce the furnace temperature from 500°C to 50°C, while the existing technology method takes 4 hours.

[0027] 2. Low initial oxidation temperature. The initial oxidation temperature of the prior art is 35-50°C, while the initial oxidation temperature of the present invention can be as low as below 30°C, or even -10°C. Lowering the initial oxidation reaction temperature can avoid the formation of a crystalline oxide film and reduce the thickness of the oxide film.

[0028] 3. The oxide film has high purity. The invention uses pure argon or pure nitrogen containing a certain amount of oxygen as the oxygen source. Only oxygen reacts with tantalum, and no other types of impurities are introduced. The oxide film generated after passivation has low impurity content, high purity, and controllable quality.

[0029] 4. Controllable passivation temperature. The existing technology uses a method of slowly increasing the amount of air introduced to control the oxidation reaction rate. However, the heat released during the reaction accumulates, causing the passivation temperature to rise. The temperature rise after passivation can reach 5°C to 10°C, resulting in a thicker oxide film with a higher oxygen content. The present invention removes the released heat by vacuuming after pressure maintenance. Combined with the temperature control of the oxygen-containing gas introduced each time, this achieves controllable passivation temperature and avoids the passivation temperature from rising. DETAILED DESCRIPTION

[0030] The specific embodiments of the present invention are further described in detail below, but the present invention is not limited to these embodiments. Any improvement or replacement based on the basic spirit of the present embodiment still falls within the scope of protection required by the claims of the present invention.

[0031] The hydrogen and oxygen contents of the present invention are detected using an oxygen, nitrogen, and hydrogen analyzer. Carbon dioxide is passed through an infrared detection cell and its concentration is measured using non-dispersive infrared technology (NDIR), thereby calculating the oxygen content. Hydrogen is determined by oxidizing hydrogen gas to water (H2O) using an oxidant (such as copper oxide), and its concentration is then measured using a non-dispersive infrared detector. Silicon content is detected using inductively coupled plasma mass spectrometry (ICP-MS). The principle is to use a plasma generated by a high-frequency electromagnetic field to ionize silicon atoms in the sample into ions. The mass-to-charge ratio of the ions is then measured using a mass spectrometer to infer the silicon content. The leakage current test method involves immersing the formed tantalum core in a 0.01% phosphoric acid solution (covering 2 / 3 of the height of the tantalum block) and testing using a TH2686C leakage current tester. The negative electrode is connected to the phosphoric acid solution, and the positive electrode is connected to the positive electrode of the tantalum core. The test voltage is 70% of the forming voltage, and the constant voltage is maintained for 1 minute. After the constant voltage is reached, the leakage current value is read.

[0032] Example 1

[0033] An anode tantalum block prepared from tantalum powder with a nominal specific capacitance of 50,000 μF·V / g was selected and sintered at 1,350°C for 30 minutes. The sintered tantalum block was then passivated using the following method, including:

[0034] 1) Cooling: After the sintering furnace is naturally cooled to 500°C, vacuum the furnace to -90kPa, introduce argon gas with a purity of ≥99.99% and room temperature to -10kPa, maintain for 15 minutes, repeat the "vacuum-pass argon-maintain" action 4 times, vacuum the furnace to -90kPa, introduce argon gas with a purity of ≥99.99% and about 25°C to -10kPa, maintain for 15 minutes, repeat the "vacuum-pass argon-maintain" action 4 times, until the furnace temperature is about 25°C;

[0035] 2) Oxidation: After the furnace temperature is about 25°C, evacuate the furnace until the pressure reaches -90kPa, introduce oxygen-containing gas until the pressure reaches -60kPa, maintain for 15 minutes, and then evacuate the furnace until the pressure reaches -90kPa; then introduce oxygen-containing gas until the pressure reaches -30kPa, maintain for 15 minutes, and then evacuate the furnace until the pressure reaches -90kPa; introduce oxygen-containing gas again until the pressure reaches 0Pa, and maintain for 15 minutes; the oxygen-containing gas comprises oxygen and argon, the oxygen content in the oxygen-containing gas is 25%, and the temperature of the oxygen-containing gas is about 25°C;

[0036] 3) Preparation for furnace removal: Evacuate the furnace until the pressure reaches -90kPa, then introduce air until the pressure reaches 0Pa, maintain for 15 minutes, and the passivation is completed.

[0037] Comparative Example 1

[0038] Anode tantalum blocks prepared from tantalum powder with a nominal specific capacity of 50,000 μF·V / g were selected and sintered at 1,350°C for 30 min. Passivation began after the sintering furnace was naturally cooled to 45°C. During passivation, the furnace was first vacuumed to a pressure of -90 kPa, and then air was introduced to a pressure of -60 kPa, maintained for 30 min, followed by air introduction to a pressure of -30 kPa, maintained for 30 min, and then air introduction to a pressure of 0 Pa, maintained for 30 min.

[0039] Table 1 Passivation results of Example 1 and Comparative Example 1

[0040]

[0041]

[0042] Note: The passivation time in Table 1-3 refers to the total time of the two stages of oxidation and furnace preparation, including vacuuming, filling, and holding, of which vacuuming and filling are calculated as 1 minute per time.

[0043] Example 2

[0044] An anode tantalum block prepared from tantalum powder with a nominal specific capacitance of 70,000 μF·V / g was selected and sintered at 1,300°C for 30 minutes. The sintered tantalum block was then passivated using the following method, including:

[0045] 1) Cooling: After the sintering furnace is naturally cooled to 400°C, vacuum the furnace until the pressure reaches -90kPa, and introduce argon gas with a purity of ≥99.99% and room temperature to the furnace pressure of -50kPa, maintain for 10 minutes, and repeat the "vacuum-pass argon-maintain pressure" action 5 times; then vacuum the furnace until the pressure reaches -90kPa, and introduce argon gas with a purity of ≥99.99% and about 20°C to the furnace pressure of -10kPa, maintain for 10 minutes, and repeat the "vacuum-pass argon-maintain pressure" action 5 times until the furnace temperature is about 20°C;

[0046] 2) Oxidation: After the furnace temperature is about 20°C, evacuate the furnace until the pressure reaches -90kPa, introduce oxygen-containing gas until the pressure reaches -60kPa, and maintain for 15 minutes; evacuate the furnace again until the pressure reaches -90kPa, introduce oxygen-containing gas until the pressure reaches -30kPa, and maintain for 15 minutes; evacuate the furnace again until the pressure reaches -90kPa, introduce oxygen-containing gas again until the pressure reaches 0Pa, and maintain for 15 minutes; the oxygen-containing gas comprises oxygen and argon, the oxygen content in the oxygen-containing gas is 20%, and the temperature of the oxygen-containing gas is 20°C;

[0047] 3) Preparation for furnace removal: Evacuate the furnace until the pressure reaches -90kPa, then introduce air until the pressure reaches 0Pa, maintain for 15 minutes, and the passivation is completed.

[0048] Comparative Example 1

[0049] Anode tantalum blocks prepared from tantalum powder with a nominal specific capacity of 70,000 μF·V / g were selected and sintered at 1,300°C for 30 minutes. Passivation began after the sintering furnace naturally cooled to 35°C. During passivation, the furnace was first vacuumed to a pressure of -90 kPa, and then air was introduced to a pressure of -60 kPa, maintained for 30 minutes, followed by air introduction to a pressure of -30 kPa, maintained for 30 minutes, and then air introduction to a pressure of 0 Pa, maintained for 30 minutes.

[0050] Table 2 Passivation results of Example 2 and Comparative Example 2

[0051]

[0052] Example 3

[0053] An anode tantalum block prepared from tantalum powder with a nominal specific capacitance of 100,000 μF·V / g was selected and sintered at 1250°C for 30 minutes. The sintered tantalum block was then passivated using the following method, including:

[0054] 1) Cooling: After the sintering furnace is naturally cooled to 300°C, vacuum the furnace until the pressure reaches -90kPa, introduce argon gas with a purity of ≥99.99% and room temperature until the pressure in the furnace reaches -50kPa, maintain for 10 minutes, and repeat the "vacuum-pass argon-maintain pressure" action 6 times; then vacuum the furnace until the pressure reaches -90kPa, introduce argon gas with a purity of ≥99.99% and a temperature of about 10°C until the pressure in the furnace reaches -10kPa, maintain for 10 minutes, and repeat the "vacuum-pass argon-maintain pressure" action 6 times until the temperature in the furnace is about 10°C;

[0055] 2) Oxidation: After the temperature in the furnace is about 10°C, evacuate the furnace until the pressure in the furnace reaches -90kPa, introduce oxygen-containing gas until the pressure in the furnace reaches -70kPa, and maintain for 10 minutes; evacuate the furnace again until the pressure in the furnace reaches -90kPa, introduce oxygen-containing gas until the pressure in the furnace reaches -50kPa, and maintain for 10 minutes; evacuate the furnace again until the pressure in the furnace reaches -90kPa, introduce oxygen-containing gas again until the pressure in the furnace reaches -30kPa, and maintain for 10 minutes; introduce oxygen-containing gas until the pressure in the furnace reaches 0Pa, and maintain for 10 minutes; the oxygen-containing gas is composed of oxygen and argon, the oxygen content in the oxygen-containing gas is 10%, and the temperature of the oxygen-containing gas is about 10°C;

[0056] 3) Preparation for furnace removal: vacuum the furnace until the pressure reaches -90kPa, then introduce air to the furnace until the pressure reaches -50kPa, maintain for 15 minutes, vacuum the furnace until the pressure reaches -90kPa, then introduce air to the furnace until the pressure reaches 0Pa, maintain for 15 minutes, and the passivation is completed.

[0057] Comparative Example 1

[0058] Anode tantalum blocks prepared from tantalum powder with a nominal specific capacity of 100,000 μF·V / g were selected and sintered at 1250°C for 30 min. Passivation began after the sintering furnace was naturally cooled to 30°C. During passivation, the furnace was first vacuumed to a pressure of -90 kPa, and then air was introduced to a pressure of -70 kPa, which was maintained for 30 min. Then, air was introduced to a pressure of -50 kPa, which was maintained for 30 min. Then, air was introduced to a pressure of -30 kPa, which was maintained for 30 min. Then, air was introduced to a pressure of 0 Pa, which was maintained for 30 min.

[0059] Table 3 Passivation results of Example 3 and Comparative Example 3

[0060]

[0061] By comparing Tables 1, 2, and 3, it can be seen that the cooling time of the embodiment is reduced by about 50% compared with the comparative example, the passivation temperature is reduced by 10°C to 20°C, and the passivation time is reduced by 24min to 43min. The high efficiency and low passivation temperature are conducive to controlling the oxidation rate during passivation, resulting in a thin and dense passivation film. From the comparison of impurity content, it can be seen that the oxygen content of the sintered tantalum block of the embodiment is reduced by 7.1% to 13% compared with the comparative example, the hydrogen content is reduced by 62.8% to 70.3%, and the silicon content is reduced by 56.2% to 75%. The leakage current after formation is reduced by 31.8% to 44.7%. Leakage current is a macroscopic manifestation of the quality of the formed dielectric oxide film. The passivation film is part of the structure of the formed dielectric oxide film. An increase in the impurity content of the passivation film will lead to an increase in the impurity content of the formed dielectric oxide film. The increase in the impurity content in the formed dielectric oxide film will cause the dielectric oxide film to be incomplete, thereby reducing insulation and increasing leakage current. In this embodiment, passivation is performed at a lower temperature, resulting in a thin, dense oxide film with a low oxygen content. Furthermore, the oxygen source used in this embodiment is of higher purity, resulting in a low impurity content in the oxide film after passivation. The resulting dielectric oxide film is more structurally sound, has a high density, a low electron throughput rate, high insulation properties, and reduced leakage current. As can be seen from the foregoing, the present invention provides a highly efficient, simple-to-operate passivation method for sintered tantalum ingots for tantalum capacitors, producing a high-purity passivation film and subsequently forming a higher-quality dielectric oxide film, offering significant advantages over existing technologies.

[0062] The above describes in detail the passivation method for sintered tantalum ingots for tantalum capacitors provided by the present invention. Specific examples are used herein to illustrate the present invention. The description of the above embodiments is intended only to facilitate understanding of the method and core concepts of the present invention. It should be noted that those skilled in the art will be able to make various improvements and modifications to the present invention without departing from the principles of the present invention, and such improvements and modifications fall within the scope of protection of the claims.

Claims

1. A passivation method for sintered tantalum blocks of tantalum capacitors, characterized in that: The steps include: 1) Cooling: The tantalum block is sintered and the furnace temperature is lowered. When the sintering furnace temperature drops to ≤500℃, the furnace is first vacuumed to a pressure of -100kPa~0Pa, and then filled with protective gas to a pressure of -100kPa~0Pa, which is maintained for 1min~72h; the "vacuuming-passing protective gas-maintaining pressure" steps are repeated 1~100 times until the furnace temperature is -10℃~100℃; 2) Oxidation: After the sintering furnace temperature drops to -10°C to 100°C, first evacuate the furnace to a pressure of -100kPa to 0kPa, then inject oxygen-containing gas to a pressure of -100kPa to 0Pa, and maintain this pressure for 1 minute to 72 hours; repeat the "evacuate - inject oxygen-containing gas - maintain pressure" operation 1 to 100 times until the pressure in the furnace after injecting oxygen-containing gas is 0Pa; 3) Preparation for furnace removal: After the pressure in the furnace is 0Pa, first evacuate the furnace to a pressure of -100kPa~0Pa, then introduce air to the furnace to a pressure of -5kPa~5kPa, and keep it constant for 1min~72h.

2. The passivation method for sintered tantalum blocks of tantalum capacitors according to claim 1, characterized in that: The specific capacitance of the tantalum block is ≥1000 μF·V / g.

3. The passivation method for sintered tantalum blocks of tantalum capacitors according to claim 1, characterized in that: The temperature of the protective gas is -10°C to 100°C, and the purity is ≥99.9%.

4. The passivation method for sintered tantalum blocks of tantalum capacitors according to claim 1, characterized in that: The protective gas is argon or nitrogen.

5. The passivation method for sintered tantalum blocks of tantalum capacitors according to claim 1, characterized in that: The temperature of the oxygen-containing gas is -10°C to 100°C.

6. The passivation method for sintered tantalum blocks of tantalum capacitors according to claim 1, characterized in that: The oxygen-containing gas is a mixed gas of oxygen and argon or nitrogen.

7. The passivation method for sintered tantalum blocks of tantalum capacitors according to claim 1, characterized in that: The oxygen content of the oxygen-containing gas is 1% to 99%, and the impurity content is less than 0.1%.

8. The passivation method for sintered tantalum blocks of tantalum capacitors according to claim 1, wherein: The step 1) is specifically as follows: S1.1 After sintering, the tantalum block is cooled with the furnace. When the temperature of the sintering furnace drops to ≤500℃, first evacuate the furnace to a pressure of -100kPa~0Pa, then fill it with protective gas to a pressure of -100kPa~0Pa, and keep it constant for 1min~72h; repeat this step several times until the temperature of the furnace reaches 30℃~60℃. S1.2 After the temperature in the furnace reaches 30℃~60℃, evacuate the furnace until the pressure is -100kPa~0Pa, then fill it with protective gas at a temperature of -10℃~30℃ until the pressure in the furnace is -100kPa~0Pa, and keep it constant for 1min~72h; repeat this step several times until the temperature in the furnace is -10℃~30℃.

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