A method for preparing low-oxygen high-specific-capacity tantalum powder for tantalum capacitors

Abstract

This invention discloses a method for preparing low-oxygen, high-specific-capacitance tantalum powder for tantalum capacitors, addressing the problems commonly found in existing magnesothermic reduction methods for tantalum powder preparation, such as high oxygen content, difficulty in suppressing the byproduct magnesium tantalate, and poor powder particle uniformity leading to low specific capacitance, high leakage current, and poor electrical performance. This method involves uniformly coating a completely removable soluble additive onto the surface of porous spherical tantalum oxide particles. After drying, these particles are mixed with alkali metal halides and a single-phase addition of metallic magnesium powder. A gradient-temperature reduction process is employed: in the intermediate temperature stage, the additive decomposes to construct a porous framework and initiates initial reduction; in the high-temperature stage, deep reduction is completed. A molten salt medium synergistically regulates the heat of reaction and mass transfer, completely suppressing the formation of magnesium tantalate. The product, after post-treatment, yields tantalum powder with an oxygen content ≤2500 ppm, a specific capacitance ≥110000 μFV / g, and a uniform coral-like porous structure. This method offers a clean and highly controllable process, resulting in high-purity tantalum powder with excellent electrical properties, making it suitable for high-end tantalum electrolytic capacitors.

Description

Technical Field

[0001] This invention belongs to the field of rare metal powder metallurgy and advanced functional material preparation technology, specifically relating to a method for preparing low-oxygen, high-specific-capacitance tantalum powder for manufacturing high-performance tantalum electrolytic capacitors. Background Technology

[0002] Solid tantalum electrolytic capacitors are widely used in aerospace, military electronics, high-end industrial control, and consumer electronics due to their advantages such as small size, large capacitance, high reliability, and wide operating temperature range. As their core anode material, the performance of capacitor-grade tantalum powder directly determines the final performance of the capacitor, including key indicators such as rated voltage, capacitance efficiency, equivalent series resistance, and leakage current. With the continuous miniaturization and high performance development of electronic devices, the market demand for tantalum powder with higher specific capacitance and higher reliability is becoming increasingly urgent.

[0003] Magnesium reduction is one of the mainstream technologies for the industrial production of capacitor-grade tantalum powder. Traditional magnesium reduction processes typically involve mixing tantalum oxide (Ta₂O₅) with excess metallic magnesium and alkali or alkaline earth metal halides, followed by a one-time reduction under a high-temperature, inert atmosphere. However, this reaction is violently exothermic, with highly concentrated heat, easily leading to abnormal particle growth and sintering of tantalum powder. Furthermore, the reaction byproduct magnesium oxide (MgO) readily reacts with unreacted Ta₂O₅ at high temperatures to form complex oxides such as magnesium tantalate (Mg₄Ta₂O₉), which are difficult to remove. These complex oxides are extremely difficult to remove in post-processing, becoming the main source of oxygen impurities in tantalum powder and severely limiting the improvement of specific capacitance and the reduction of oxygen content. Although increasing the amount of magnesium and extending the reaction time can improve the reduction degree to some extent, this often results in coarser tantalum powder particles, excessively high bulk density, and the inability to effectively eliminate the formation of tantalates, making it difficult to stably control the oxygen content at a low level.

[0004] Existing technologies have explored various approaches to improve reaction uniformity and control heat.

[0005] Patent CN120421499A proposes using CaCl2 molten salt to encapsulate Ta2O5 particles, utilizing its endothermic and fluxing effects to suppress the formation of magnesium tantalate to a certain extent, reducing the oxygen content to below 3000 ppm. However, the fluxing and lattice activation effects of CaCl2 are limited, resulting in a bottleneck in promoting deep reduction, and the single, one-time reduction process does not provide precise control over the reaction process.

[0006] Patent CN202510904947.3 uses gaseous magnesium as a reducing agent, improves mass transfer uniformity through a vertical multi-layer tray structure, and introduces fluxes such as NaCl to form a molten salt medium to promote oxygen ion migration. Although this method improves reaction uniformity, the gaseous magnesium preparation and control system is complex and costly, and the final product still has an oxygen content in the range of 3000-6000 ppm, which is difficult to meet the current demand for ultra-low oxygen (<2500 ppm) ultra-high specific volume tantalum powder.

[0007] In existing technologies, a large amount of inert diluent is typically added to control the heat of reaction and improve powder morphology. However, excessive diluent not only increases the difficulty and cost of post-processing but may also become a new source of impurities. Other technologies attempt to introduce a carbon source and use carbothermal reduction to help reduce oxygen content, but a single carbothermal reaction often requires high temperatures and is prone to leaving carbon impurities, affecting electrical properties.

[0008] Therefore, developing a new process for preparing tantalum powder that can fundamentally suppress side reactions, achieve deep reduction, and obtain low oxygen content and ideal particle morphology while ensuring high reduction degree is of great significance for promoting the technological advancement of high-performance tantalum capacitors. Summary of the Invention

[0009] The purpose of this invention is to overcome the problems of weak pore structure control and incomplete suppression of side reactions in existing magnesia reduction processes, and to provide a novel multi-stage magnesium reduction method based on the synergistic effect of a residue-free soluble additive and a molten salt medium. This method introduces a thermally decomposable additive onto the surface of Ta2O5, utilizing its decomposition process to create pores in situ and activate the surface. Combined with the heat control and mass transfer promotion of the molten salt medium, this method achieves efficient and deep reduction of Ta2O5 during a gradient-heating reduction process, ultimately obtaining high-quality tantalum powder with extremely low oxygen content, extremely high specific volume, and an ideal structure.

[0010] This invention provides a method for preparing low-oxygen, high-specific-capacitance tantalum powder for high-performance capacitors, comprising the following steps: (1) Preparation of composite precursor: High specific surface area porous spherical Ta2O5 powder was selected as raw material. Soluble additives were weighed at a mass ratio of Ta2O5 to soluble additives of 1:(0.01-0.05). Soluble additives, such as sodium bicarbonate (chemical formula NaHCO3), were dissolved or dispersed in deionized water to prepare a homogeneous solution with a mass concentration of 5%-20%. The Ta2O5 powder was poured into the solution and subjected to high-intensity ultrasonic dispersion (power 200-400W, time 20-40 minutes) and mechanical stirring (speed 200-400 rpm, time 0.5-2 hours) to ensure that the Ta2O5 particles were fully wetted and coated by the solution. Subsequently, vacuum drying was carried out at a temperature of 80-120℃ to obtain a composite powder with the soluble additive uniformly coated on the surface of the Ta2O5 particles. The particle size distribution D50 was controlled at 10-50μm. The core of this step is to construct a uniform porous reaction framework for subsequent reactions.

[0011] (2) Raw material mixing: The composite powder obtained in step 1) is thoroughly mechanically mixed with KCl powder under an inert atmosphere. KCl is used as the molten salt medium for subsequent reactions, and its molar ratio with Ta2O5 is (0.5-2.0):1.

[0012] (3) Magnesium reduction reaction: After uniformly mixing the mixture from step 2) with metallic magnesium powder (Mg to Ta2O5 molar ratio of 10:1), place it in a closed reactor. Introduce high-purity argon gas with a purity ≥99.99% and perform 3-5 purging cycles to completely remove air. Under inert atmosphere protection, program the temperature to 700-800℃ at a heating rate of 5-10℃ / min and hold at this temperature for 1-2 hours. During this stage, maintain the system pressure at 0.10-0.15 MPa and maintain an inert gas flow rate of 100-300 mL / min. During this stage, sodium bicarbonate decomposes upon heating, and the resulting gas escapes from the surface and interior of the Ta₂O₅ particles. This process etches abundant initial micropores and interconnected channels on the particle surface and between aggregates, constructing a highly active porous structure layer. Simultaneously, KCl, with a melting point of 773℃, begins to melt, forming a liquid molten salt medium. This medium immediately absorbs and disperses the initial heat of reaction, effectively buffering the temperature rise of the system and acting as an ionic conductor to initially initiate the reduction reaction between Ta₂O₅ and magnesium. The in-situ formed porous structure greatly increases the effective reaction interface, with the molten salt permeating within it. Together, these elements create a reaction environment with superior mass and heat transfer conditions for subsequent deep reduction.

[0013] (4) Deep Reduction: Continue heating at a rate of 5-15℃ / min to 900-1000℃, and hold at this temperature for 2-4 hours to allow tantalum oxide to be deeply reduced to metallic tantalum in the molten salt medium. During this high-temperature stage, the previously constructed porous framework is consolidated and expanded under the support and protection of the molten salt medium. Completely molten KCl thoroughly wets the entire porous system, acting as an efficient "thermal buffer" to ensure uniform distribution of reaction heat and prevent local sintering. Simultaneously, as an excellent ion transfer medium, it greatly promotes the diffusion of Mg vapor to the reaction interface and O... 2- Plasma migration. The porous structure formed by the decomposition of the additives, in synergy with the depth of the molten salt, ensures that the reduction reaction proceeds fully, uniformly, and stably on the huge internal surface, and effectively isolates the generated MgO and residual Ta2O5. This dual suppression of the formation of composite oxides such as magnesium tantalate from both physical space and reaction kinetics perspectives is the fundamental reason for obtaining tantalum powder with ultra-low oxygen content.

[0014] (5) Cooling and separation: After the reaction is complete, stop the inert gas supply and immediately turn on the vacuum pump to evacuate for 40-80 minutes to remove excess magnesium vapor from the reaction chamber and separate it from the solid product. Then turn off the power, restart the inert gas supply, control the pressure at 0.14-0.20 MPa, and cool down to room temperature at a rate of 10-20℃ / min. The product is then removed from the furnace and is in block form.

[0015] (6) Water washing: After cooling, the blocky product is mechanically crushed and passed through a 100-mesh sieve. Taking advantage of the fact that KCl is easily soluble in water, it is stirred, dissolved, and filtered with hot deionized water at 60-90℃ to completely remove it.

[0016] (7) Acid washing: The product after water washing is acid washed with 5-15% hydrochloric acid and stirred at 60-80℃ for 2-4 hours to remove impurities such as MgO. Then, it is washed with hot pure water until the conductivity of the washing solution is ≤10μS / cm. This washing method can more thoroughly remove impurities from the product and improve the purity of the obtained tantalum powder. Since sodium bicarbonate has completely decomposed into gas and escaped (reaction equation 2NaHCO3 → Na2CO3 + CO2↑ + H2O(g)↑), the sodium salt produced by its decomposition is easily soluble in water or acid, and is therefore completely removed in water washing and acid washing.

[0017] (8) Vacuum drying: Place the washed tantalum powder in a vacuum drying oven and dry it at 70-90℃ for 6-12 hours to obtain the low oxygen high specific volume tantalum powder.

[0018] The beneficial effects of this invention are as follows: 1. This method abandons the traditional physical mixing or difficult-to-remove pore-forming agents and innovatively adopts soluble additives that can be completely decomposed and removed by thermal decomposition. The gas escape effect generated by its thermal decomposition in the early stage of reduction can construct abundant and interconnected micro- and nano-scale pores in situ on the surface of Ta2O5 particles and within the aggregates, directly constructing a high specific surface area and highly active reaction precursor. This is the key basis for obtaining the coral-like porous structure and high specific volume of the final product.

[0019] 2. This method not only optimizes the main reaction through the synergistic effect of soluble additives and molten salt medium, but also effectively isolates the contact between MgO and Ta2O5 in space, inhibits the formation of magnesium tantalate, and significantly reduces the oxygen content of the final product.

[0020] 3. The tantalum powder prepared by this method exhibits excellent comprehensive performance indicators, especially achieving a perfect balance between low oxygen and high specific capacitance. The soluble additives and their decomposition products used do not introduce any difficult-to-remove impurities. The process is clean, and the oxygen content is significantly lower than that of the comparative example and products prepared by existing technologies. The specific capacitance (≥110000 μFV / g) is much higher than that of traditional methods, and the powder morphology is ideal, fully meeting the manufacturing requirements of next-generation ultra-high specific capacitance capacitors.

[0021] 4. Thanks to its ultra-low oxygen content and ideal coral-like porous structure, the capacitor anode block prepared by the tantalum powder of this invention exhibits high specific capacitance, low leakage current and good loss characteristics, and its overall electrical performance is significantly better than that of existing technology products. Attached Figure Description

[0022] Figure 1 This is a scanning electron microscope (SEM) image of tantalum powder prepared by the magnesian reduction process without the addition of soluble additives.

[0023] Figure 2 This is an SEM image of the low-oxygen, high-specific-capacity tantalum powder prepared in Example 1 of this invention.

[0024] Figure 3 The image shows the X-ray diffraction (XRD) pattern of the tantalum powder prepared in Example 1 of this invention, which is a pure tantalum phase. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0026] The technical details are further illustrated below with reference to the embodiments. Example

[0027] Weigh 200g of Ta₂O₅ powder. Dissolve 6g of sodium bicarbonate in 114g of deionized water at a mass ratio of Ta₂O₅:sodium bicarbonate = 1:0.03 to prepare a solution with a mass fraction of approximately 5%. Add the Ta₂O₅ powder, ultrasonically disperse for 30 min, and then mechanically stir for 2 h. Subsequently, vacuum dry at 80-120℃ for 6 h to obtain a composite powder with sodium bicarbonate uniformly coated on the surface of the Ta₂O₅ particles.

[0028] Weigh and mix the above composite powder and KCl powder in a molar ratio of Ta2O5:KCl = 1:1.2.

[0029] All the mixtures from the above steps were uniformly mixed with magnesium powder in one go, with a Mg to Ta₂O₅ molar ratio of 10:1. The mixture was then placed in a reactor, and high-purity argon gas was introduced to purge the air four times. The temperature was increased to 720℃ at 8℃ / min and held for 1.5 hours, with an argon gas flow rate of 200 mL / min and a pressure of 0.12 MPa. The temperature was then increased to 960℃ at 10℃ / min and held for 3 hours.

[0030] After the reaction was complete, the argon gas supply was stopped, and a vacuum was applied for 60 minutes. Then, the power was turned off, and argon gas was reintroduced, with the pressure controlled at 0.14-0.20 MPa. The mixture was then cooled to room temperature at a rate of 10-20 °C / min, and the container was opened to remove the product.

[0031] The product was mechanically crushed and passed through a 100-mesh sieve, then washed with deionized water at 80℃ to remove KCl. It was then acid-washed with 10% hydrochloric acid at 70℃ for 3 hours to remove impurities such as MgO. After acid washing, it was washed with hot pure water until the conductivity of the washing solution was ≤10μS / cm. Finally, the washed tantalum powder was placed in a vacuum drying oven and dried at 75℃ for 6 hours to obtain metallic tantalum powder.

[0032] The SEM morphology of the obtained tantalum powder is as follows Figure 2 It has a uniform porous coral-like appearance. Figure 3 This is the X-ray diffraction (XRD) pattern of the tantalum powder, showing that it is a pure tantalum phase.

[0033] The oxygen, nitrogen, and hydrogen content of the prepared tantalum powder was determined using a LECO ONH836 oxygen, nitrogen, and hydrogen analyzer (USA). The results showed: O = 1820 ppm, N = 50 ppm, H = 15 ppm. The loose bulk density of the tantalum powder was determined to be 1.38 g / cm³ according to GB / T 1479.1-2011.

[0034] The tantalum powder was pressed into a standard cylindrical anode block with a diameter of φ3.0 mm and a weight of approximately 0.15 g under a pressure of 150 MPa, and tantalum wire leads were embedded. Electrochemical anodizing (enabling) was performed in a phosphoric acid electrolyte at 200 °C, with a formation voltage of 25 V and a formation current density of 50 mA / g.

[0035] At a formation voltage of 25V, the actual specific capacitance of the anode block was measured to be 125000μFV / g, and the measured leakage current (DCL), loss (DF), and equivalent series resistance (ESR) are listed in Table 1 below. Example

[0036] Soluble additives were replaced with sodium citrate (chemical formula C6H5Na3O7). 200g of Ta2O5 powder was weighed, and 4g of sodium citrate was dissolved in 36g of deionized water at a mass ratio of Ta2O5:sodium citrate = 1:0.02 to prepare a solution with a mass fraction of approximately 10%. The Ta2O5 powder was added, and the mixture was first ultrasonically dispersed for 30 min, followed by mechanical stirring for 2 h. Subsequently, it was vacuum dried at 80-120℃ for 6 h to obtain a composite powder with sodium citrate uniformly coated on the surface of the Ta2O5 particles.

[0037] Alkali metal halides were replaced with NaCl. The above composite powder and NaCl powder were weighed and mixed in a molar ratio of Ta2O5:NaCl = 1:1.5.

[0038] All the mixture from the above steps was uniformly mixed with magnesium powder in one go, with a Mg to Ta₂O₅ molar ratio of 10:1. The mixture was then placed in a reactor, and high-purity argon gas was introduced to purge the air four times. The temperature was increased to 750℃ at 10℃ / min and held for 2 hours, with an argon gas flow rate of 180 mL / min and a pressure of 0.12 MPa. The temperature was then increased to 980℃ at 10℃ / min and held for 4 hours.

[0039] After the reaction was complete, the argon gas supply was stopped, and a vacuum was applied for 60 minutes. Then, the power was turned off, and argon gas was reintroduced, with the pressure controlled at 0.14-0.20 MPa. The mixture was then cooled to room temperature at a rate of 10-20 °C / min, and the container was opened to remove the product.

[0040] The product was mechanically crushed and passed through a 100-mesh sieve, then washed with deionized water at 80°C to remove NaCl. It was then acid-washed with 10% hydrochloric acid at 70°C for 3 hours to remove impurities such as MgO. After acid washing, it was washed with hot pure water until the conductivity of the washing solution was ≤10 μS / cm. Finally, the washed tantalum powder was placed in a vacuum drying oven and dried at 75°C for 6 hours to obtain tantalum powder.

[0041] The oxygen content of the prepared tantalum powder was determined to be 1580 ppm using a LECO ONH836 oxygen, nitrogen, and hydrogen analyzer (USA). The loose density of the tantalum powder was determined to be 1.35 g / cm³ according to GB / T 1479.1-2011.

[0042] The tantalum powder was pressed into a standard cylindrical anode block with a diameter of φ3.0 mm and a weight of approximately 0.15 g under a pressure of 150 MPa, and tantalum wire leads were embedded. Electrochemical anodizing (enabling) was performed in a phosphoric acid electrolyte at 200 °C, with a formation voltage of 25 V and a formation current density of 50 mA / g.

[0043] At a formation voltage of 25V, the actual specific capacitance of the anode block was measured to be 132000μFV / g. The measured leakage current (DCL), loss (DF), and equivalent series resistance (ESR) are listed in Table 1 below.

[0044] Comparative Example 1 Without adding any reagents, 200g of the same Ta₂O₅ powder was mixed with CaCl₂ at a molar ratio of 1:1, and then mixed with magnesium powder at a ratio of Mg:Ta₂O₅ of 12:1. Under the protection of high-purity argon, the mixture was directly heated to 1050℃ at a rate of 10℃ / min and held at that temperature for 5 hours. The cooling and post-processing steps were the same as in Example 1.

[0045] The oxygen content was measured to be as high as 5100 ppm using a LECO ONH836 oxygen, nitrogen, and hydrogen analyzer from the United States. The loose density of the tantalum powder was measured to be 2.05 g / cm³ according to GB / T1479.1-2011. The tantalum powder was pressed into a standard cylindrical anode block with a diameter of φ3.0 mm and a weight of approximately 0.1 g under a pressure of 150 MPa, and tantalum wire leads were embedded. Electrochemical anodizing (enabling) was performed in a phosphoric acid electrolyte at 200 °C, with a formation voltage of 25 V and a formation current density of 50 mA / g.

[0046] At a formation voltage of 25V, the powder specific capacitance was measured to be only 68000μFV / g. The measured leakage current (DCL), loss (DF), and equivalent series resistance (ESR) are listed in Table 1 below.

[0047] Comparative Example 2 A gaseous magnesium reduction apparatus was used, but without adding any reagents. Ta₂O₅ and KCl were mixed in a 1:1 molar ratio and spread on the reactor, with magnesium blocks (Mg:Ta₂O₅ = 8:1) placed at the bottom. The reactor was directly heated to 1100°C under a high-purity argon flow and held at that temperature for 4 hours. After the holding period, the post-treatment was the same as in Example 1.

[0048] The oxygen content was measured to be as high as 4800 ppm using a LECO ONH836 oxygen, nitrogen, and hydrogen analyzer from the United States. The loose density of the tantalum powder was measured to be 1.70 g / cm³ according to GB / T1479.1-2011. The tantalum powder was pressed into a standard cylindrical anode block with a diameter of φ3.0 mm and a weight of approximately 0.15 g under a pressure of 150 MPa, and tantalum wire leads were embedded. Electrochemical anodizing (enabling) was performed in a phosphoric acid electrolyte at 200 °C, with a formation voltage of 25 V and a formation current density of 50 mA / g.

[0049] At a formation voltage of 25V, the specific capacitance of the powder was measured to be 88000μFV / g, and the measured leakage current (DCL), loss (DF), and equivalent series resistance (ESR) are listed in Table 1 below.

[0050] Table 1: Comparison of main performance test results between Examples 1 and 2 and Comparative Examples 1 and 2 sample Oxygen content (O / ppm) Specific volume (μFV / g) Leakage current (nA / µFV) loss(%) ESR (Ω) Core process features Example 1 1820 125000 0.45 34 0.42 Sodium bicarbonate + KCl molten salt + two-stage reduction Example 2 1580 132000 0.38 30 0.40 Sodium citrate + NaCl molten salt + two-stage reduction Comparative Example 1 5100 68000 1.2 45 0.85 <![CDATA[Only CaCl2 molten salt, one-time reduction]]> Comparative Example 2 4800 88000 0.9 40 0.70 KCl molten salt alone, simulating a gaseous magnesium environment The following conclusions can be drawn from the comparison of the data in Table 1 and the attached figures: This invention employs soluble additives for in-situ pore formation and surface activation, combined with multi-stage magnesium reduction and synergistic regulation using molten salt media. Compared to traditional processes using only molten salt (Comparative Example 1) or improved gaseous magnesium processes (Comparative Example 2), it achieves significant breakthroughs in reducing oxygen content, increasing specific capacitance, and improving powder morphology. More importantly, the tantalum powder prepared by this invention exhibits superior performance in key electrical indicators: higher specific capacitance means higher material utilization, allowing for the manufacture of capacitors with the same capacitance using less tantalum powder; lower leakage current (DCL) and loss (DF) directly indicate better energy storage efficiency and long-term reliability of the capacitor; and lower ESR is beneficial for the capacitor's performance at high frequencies. These excellent electrical properties stem from the high purity and ideal microstructure of the tantalum powder prepared by the method of this invention.

[0051] This invention discloses an innovative method for preparing tantalum powder. By introducing a completely removable soluble additive, its thermal decomposition process constructs ideal pores in the reaction precursor. Combined with the synergistic effect of molten salt medium and multi-stage reduction process, this method successfully solves the problems of severe side reactions and high oxygen content in traditional methods, producing high-quality tantalum powder for capacitors with ultra-low oxygen content, ultra-high specific capacitance, and ideal morphology. The tantalum powder prepared by this method has low oxygen impurity content and an ideal microstructure, exhibiting excellent electrical properties, providing key material support for the development of next-generation high-performance tantalum electrolytic capacitors.

[0052] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing low-oxygen, high-specific-capacitance tantalum powder for tantalum capacitors, characterized in that, Includes the following steps: 1) Preparation of composite precursor: Prepare porous spherical tantalum oxide (Ta2O5) powder and soluble additive. Weigh the soluble additive according to the mass ratio of Ta2O5 powder to soluble additive 1:(0.01-0.05), dissolve or disperse it in deionized water to prepare a solution with a mass concentration of 5%-20%. Then pour the porous spherical tantalum oxide (Ta2O5) powder into the solution, disperse it by ultrasonication and mechanical stirring, and then dry it to obtain a composite powder in which the soluble additive is uniformly coated on the surface of Ta2O5 particles. The soluble additive is a non-toxic thermally decomposable sodium salt that can decompose into gas and escape in the subsequent high-temperature reduction step, and whose decomposition products can be completely removed during acid washing or water washing. 2) Raw material mixing: The composite powder described in step 1) is thoroughly mixed with the alkali metal halide powder to obtain a mixture. 3) Magnesium reduction reaction: The mixture obtained in step 2) is mixed with magnesium powder in one step (Mg to Ta2O5 molar ratio is 10:1), placed in a closed reactor, and inert gas is introduced for multiple replacements. Under the protection of an inert atmosphere, the temperature is raised to 700-800℃ at 5-10℃ / min and held for 1-2 hours. During the holding period, the inert atmosphere is kept circulating at a flow rate of 100-300 mL / min and the pressure is maintained at 0.10-0.15 MPa. This promotes the decomposition of soluble additives and the formation of a porous activation layer on the surface of Ta2O5 particles. At the same time, alkali metal halides melt to form a molten salt medium, which absorbs the heat of reaction and initiates the initial magnesium thermal reduction of tantalum oxide to obtain low-valence tantalum oxide. 4) Deep reduction: Continue to increase the temperature to 900-1000℃ at 5-15℃ / min and hold at this temperature for 2-4 hours to allow tantalum oxide to be deeply reduced to metallic tantalum in the molten salt medium. The alkali metal halide continues to act as an endothermic agent and mass transfer medium during the high-temperature stage to suppress local overheating and side reactions. 5) Cooling and separation: After the reaction is complete, stop the inert gas supply, turn on the vacuum pump to separate the excess magnesium vapor, tantalum powder and alkali metal halide molten salt, turn off the power and cool down, continue to supply inert gas, control the pressure at 0.14-0.20 MPa, and cool to room temperature at a rate of 10-20℃ / min before removing from the furnace. 6) Post-processing: The cooled product obtained in step 4) is crushed and sieved, then dissolved in deionized water and filtered to remove the alkali metal halide molten salt. It is then subjected to acid washing, water washing and vacuum drying in sequence to obtain the low oxygen high specific volume tantalum powder.

2. The method according to claim 1, characterized in that, The soluble additive mentioned in step 1) is selected from sodium bicarbonate and sodium citrate. The average particle size of the Ta2O5 powder is 0.5-3.0 μm and the BET specific surface area is not less than 10 m² / g.

3. The method according to claim 1, characterized in that, The drying method described in step 1) is vacuum drying, and the particle size distribution D50 of the composite powder is 10-50 μm.

4. The method according to claim 1, characterized in that, The alkali metal halide in step 2) is at least one of potassium chloride and sodium chloride, and its molar ratio with tantalum oxide (Ta2O5) is (0.5-2.0):

1.

5. The method according to claim 1, characterized in that, The magnesium powder has a particle size of 50-300 μm and a purity of not less than 99.5%.

6. The method according to claim 1, characterized in that, The pickling in step 6) uses a 5-15% hydrochloric acid solution, is carried out by stirring at 60-80℃, and the pickling time is 2-4 hours.

7. The method according to claim 1, characterized in that, The water washing described in step 6) uses hot pure water at 60-90℃, and the washing is continued until the conductivity of the washing liquid is no greater than 10 μS / cm.

8. A low-oxygen, high-specific-capacitance tantalum powder for use in tantalum capacitors, characterized in that, Prepared by the method described in any one of claims 1-7, the oxygen content is not higher than 2000 ppm, the specific volume is not lower than 110000 μFV / g, the loose density is 1.3-1.6 g / cm³, and the powder particles have a porous coral-like structure and uniform particle size distribution.

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