JP2001143970A - Tantalum powder, manufacturing method thereof, porous sintered body, and solid electrolytic capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide tantalum powder for a porous sintered body having a large surface area and hardly to cause a defect in formation of a solid state electrolytic coating film. SOLUTION: A raw material for tantalum ions is solved in molten salt 3. A reducing agent 8 in a crucible 9 is reacted with the molten salt 3 to reduce the tantalum ions in the molten salt 3 into a metallic state. Then, tantalum powder with radial aggregate particles, in which cylindrical particles are aggregated in a radial form, is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tantalum powder useful as a material for an anode constituting a solid electrolytic capacitor and a method for producing the same, and particularly to a low equivalent series resistance (hereinafter referred to as ES).
R) and an anode having a large capacity characteristic can be provided.

[0002]

2. Description of the Related Art The anode of a conventional solid electrolytic capacitor using tantalum is formed of, for example, a porous sintered body having a porosity of about 70% by volume. The tantalum powder as the material is sponge-like agglomerated particles having a large number of interconnected substantially spherical holes, and has a spherical shape with a particle size of several tens to several hundreds of μm. Conventional tantalum powder is produced, for example, as follows. First, a primary powder is produced by a known method such as sodium reduction of potassium fluorotantalate and hydrogen reduction of tantalum pentachloride. Then, the primary powder is subjected to an acid / water washing treatment as required, a degassing treatment, a high-temperature heat treatment at 1000 ° C. or higher, and a deoxidation treatment for removing excess oxygen to obtain a tantalum powder.

Then, by pressing this tantalum powder into a predetermined shape and sintering, an anode having a large number of pores caused by the pores of the aggregated particles (tantalum powder) can be obtained. For example, a solid electrolytic capacitor is formed by forming a solid electrolyte film (hereinafter, referred to as a solid electrolyte film) on the surface of the porous sintered body (anode), soldering a cathode made of Ni wire or the like thereon, Is integrally covered with an exterior resin such as a flame-retardant epoxy resin.

Conventionally, manganese oxide has been mainly used as a solid electrolyte. The solid electrolyte coating made of manganese oxide is formed, for example, by subjecting a porous sintered body to a chemical conversion treatment by a conventional method, impregnating the porous sintered body with a manganese nitrate solution, and thermally decomposing the porous sintered body. At this time, since the porous sintered body has a large number of substantially spherical holes interconnected as described above, when the porous sintered body is immersed in a manganese nitrate solution, the porous sintered body becomes porous. The manganese nitrate solution penetrates from the pores opened on the surface of the body, reaches the pores inside the porous sintered body connected to the manganese nitrate solution, and further opens in the open state on the surface of the porous sintered body. And the manganese nitrate solution spreads throughout the porous sintered body. As a result, a solid electrolyte film having a large area is formed, and the entire surface of the anode is effectively used. Recently, with the miniaturization and higher frequency of electronic devices and circuits, a solid electrolytic capacitor having a large capacity and a low ESR has been demanded. Since the capacity of the solid electrolytic capacitor is proportional to the surface area of the anode, it is preferable that the porosity of the porous sintered body be as high as possible in order to constitute a large-capacity solid electrolytic capacitor.

On the other hand, in a CPU and a power supply circuit of a personal computer in which a solid electrolytic capacitor is incorporated, an increase in the ESR of the anode causes a failure in signal processing accompanying an increase in the speed of an electronic circuit, and is an important characteristic. The main cause of the increase in ESR at the anode is poor formation of the solid electrolyte film.

[0006]

However, if the porosity of the tantalum powder is increased in order to increase the porosity of the porous sintered body, the strength of the tantalum powder decreases, and the porosity is reduced during the press working. The porous sintered body may be easily crushed, and conversely, the porosity of the porous sintered body may decrease. Therefore, it is necessary to adjust the porosity of the tantalum powder to such an extent that an appropriate strength can be obtained.

[0007] Further, the poor formation of the solid electrolyte film is mainly caused by the non-uniformity of the pores of the porous sintered body. That is,
If the pores inside the porous sintered body are not connected to the open pores opened on the surface of the porous sintered body but are closed and exist independently, In this case, the manganese nitrate solution does not penetrate, resulting in poor formation of the solid electrolyte film. Alternatively, even if the inner hole is connected to one of the open holes, if it is not connected to the other open hole, a so-called bottomed pot-shaped hole is formed and nitric acid is formed. The manganese solution does not penetrate sufficiently, resulting in poor formation of the solid electrolyte film. FIG. 6 is a micrograph of a conventional porous sintered body made of tantalum powder, taken by a scanning electron microscope (SEM), showing an example of a state in which holes are closed and a pot-like hole is formed. is there. The occurrence of closed holes and pot-shaped voids depends on the particle size distribution of the tantalum powder, the collapse of the voids in the tantalum powder during press working (cohesive strength), the state and ratio of the voids present inside the tantalum powder, etc. Change. Recently, a technique using a conductive polymer instead of manganese oxide has been put to practical use. Since conductive polymers have large molecules,
It is difficult to penetrate the pores of the porous sintered body, and it is necessary to precisely control the pores of the porous sintered body.

The present invention has been made in view of the above circumstances, and has as its object to obtain a tantalum powder capable of providing a porous sintered body having a large surface area and in which formation failure of a solid electrolyte film hardly occurs. Specifically, it is an object to provide a tantalum powder having uniform pores and appropriate strength. More specifically, it is an object of the present invention to provide a technique in which a closed hole or a pot-shaped hole is less likely to occur in the hole.

[0009]

Means for Solving the Problems The present inventors have found that tantalum powder can be obtained as columnar particles by a specific production method, and that the columnar particles can be obtained as radially aggregated particles which are radially aggregated. Thus, the present invention has been completed. That is, in order to solve the problem,
The present invention provides the following solutions. A first invention is a tantalum powder characterized in that columnar particles are aggregated. A second invention is a tantalum powder according to the first invention, characterized in that the tantalum powder contains radially aggregated particles in which a plurality of columnar particles are radially aggregated. A third invention is a tantalum powder obtained by reducing the tantalum salt without directly contacting the tantalum salt with a reducing agent in the tantalum powder according to the first or second invention. A fourth invention is the tantalum powder according to the third invention, wherein the reducing agent is a magnesium reducing agent or a sodium reducing agent. The fifth invention is
A method for producing a tantalum powder, characterized in that the tantalum salt is reduced to obtain a tantalum powder without directly contacting the tantalum salt with a reducing agent. A sixth invention is the method for producing a tantalum powder according to the fifth invention, wherein the reducing agent is a magnesium reducing agent or a sodium reducing agent. A seventh invention is a porous sintered body comprising the tantalum powder according to any one of the first to fourth inventions. An eighth invention is a solid electrolytic capacitor comprising an anode made of the porous sintered body of the seventh invention.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The tantalum powder of the present invention can be obtained, for example, by a production method having the following process. 1. A raw material serving as a tantalum ion source is dissolved in a molten salt (medium). 2. A reducing agent acts on the molten salt. Then
This reducing agent reduces the tantalum ions dissolved in the molten salt to a metal state, and as a result, the tantalum powder crystallizes and precipitates in the molten salt. At this time, the by-product of the reduction reaction dissolves in the molten salt, and the activity of the by-product decreases, so that the reactivity is good. Further, since a molten salt is present around the tantalum powder, the tantalum powder is unlikely to become coarse due to collision and bonding between the tantalum powders. Therefore, a relatively small powder can be obtained, which is preferable from the viewpoint of increasing the surface area. 3. The tantalum powder is recovered from the molten salt. 4. The reducing agent ions contained in the molten salt are collected and recycled as a reducing agent by a method such as electrolysis.

The reduction of the tantalum salt without directly contacting the tantalum salt and the reducing agent is different from the conventional operation of directly contacting the tantalum salt with the reducing agent by mixing the tantalum salt with the reducing agent. Is dissolved in a medium (molten salt), a liquefied or vaporized reducing agent is allowed to act on the medium, and the tantalum salt in the medium is reduced through the medium.

In the present invention, as a raw material, potassium fluoride tantalate, sodium fluoride tantalate and the like which have been conventionally used as raw materials in the tantalum metal refining industry can be used. In addition, tantalum pentachloride, tantalum pentafluoride, tantalum pentaiodide, and other tantalum halides can be used by adjusting the amount of use in consideration of solubility in the molten salt.

Examples of the reducing agent 13 include alkali metals such as sodium and potassium or alkaline earth metals such as calcium and magnesium which have the ability to thermodynamically reduce tantalum ions to a metallic state. Are preferably satisfied. (1) Dissolve in molten salt. (2) It does not dissolve mutually with tantalum. (3) chlorides and fluorides of reducing agents, which are by-products of the reaction,
Salts such as iodide dissolve in the molten salt. (4) The specific gravity difference from the molten salt is large. (5) After the reaction, only by-products can be recycled by electrolysis. (6) Inexpensive and resource-rich. Examples of those satisfying these conditions include sodium and magnesium. Magnesium is preferred because it is industrially easy to regenerate.

The molten salt preferably satisfies the following conditions. (1) It is thermochemically stable with respect to a reducing agent and tantalum powder, and serves as a medium for a chemical reaction. (2) Since it is used at a high temperature, it has a low vapor pressure and has a property of hardly scattering. (3) Inexpensive and resource-rich. (4) Melting at low temperature from the viewpoint of energy saving. Therefore, (5) in order to avoid mixing of impurities, for example, if the raw material is tantalum chloride, it is desirable that the molten salt is also chloride.

From the viewpoint of satisfying these conditions and stability against the reducing agent, an alkali metal or alkaline earth metal salt or the like is preferable.
It is preferable to select according to the raw material as shown in the above (5). Considering separation and recovery of reduced ions and tantalum after completion of the reduction reaction and recycling of the reducing agent, chlorides of alkali metals or alkaline earth metals are preferred.

Hereinafter, the method for producing tantalum powder of the present invention will be described with reference to first and second examples. (First Example) FIG. 1 is an explanatory view of the structure of an apparatus used in the method of manufacturing a tantalum powder of the first example. In the figure, reference numeral 1 denotes a container 1 made of dense mullite or stainless steel.
It is. The container 1 includes a main body 1a of a hollow cylindrical body having a bottom and an upper opening, and a lid 1b. A heater 1c is provided around the main body 1a. Thermometer 4 on lid 1b,
Pipes (lances) 7 are provided, and these are inserted into the container 1. Although the details of the pipe 7 will be described later, the pipe 7 is kept closed until the molten salt is produced.
Further, the lid 1b is provided with a gas flow pipe 6, from which the atmosphere inside the container 1 is replaced and the inside of the container 1 is evacuated.

First, a salt before melting is charged into a crucible 2 made of dense alumina (Al ₂ O ₃ ) or a magnesium oxide made of a hollow cylinder with a bottom and opened at the upper portion.
Install inside. In this example, the salt is KCl-NaCl
The crucible 2 has an inner diameter of 42 mm and a height of 155 mm.
Next, the heater 1c is operated, and the inside of the container 1 is evacuated through the gas flow pipe 6, so that the temperature from room temperature to
Vacuum dry at 600 ° C. for 1 to 24 hours. The specific condition of this example is 200 ° C. for 2 hours. By this vacuum drying, the stability of the molten salt can be enhanced, and the generated tantalum powder can be prevented from being oxidized by oxygen generated from water in the molten salt.

Then, after the inside of the container 1 is replaced with an inert gas such as argon gas or nitrogen gas through a gas flow pipe 6, the temperature is raised to 660 to 1000 ° C. and maintained for 0 to 2 hours. In a molten state (molten salt 3). In this example, the heating temperature is about 900 ° C., and the holding time is 0.5
Time. Then, the crucible 9 containing the reducing agent 8 is lowered into the molten salt 3, and at 660 to 1000 ° C., 0.5 to
Hold for about 12 hours. In this example, the holding temperature is about 900 ° C. and the holding time is 0.5 hour. A plurality of holes are formed in the wall surface of the crucible 9 so that gas and liquid can flow between the inside and the outside of the crucible 9 through the holes.

The material of the crucible 9 is selected from the viewpoint of thermodynamic stability to a reducing agent, chemical reactivity, and the like. In this example, since magnesium is used as the reducing agent, the material of the crucible 9 is selected from magnesium oxide which is thermodynamically stable with respect to magnesium. However, in this case, oxygen is dissolved in the molten salt 3 due to the thermodynamic oxygen potential of magnesium and magnesium oxide, and as a result, a tantalum powder containing oxygen is obtained. Therefore, when mixing of oxygen is a problem,
It is preferable to select a material that does not cause a chemical reaction with the reducing agent for the material of the crucible 9. When the reducing agent is magnesium,
For example, iron is suitable.

Next, with the pipe 7 inserted into the molten salt 3 from above the lid 1b, the temperature of the molten salt 3 is increased to 900-95.
While maintaining the temperature at 0 ° C., the raw material which is heated and gasified is supplied from the pipe 7 and is blown into the molten salt 3. Alternatively, since the temperature of the molten salt 3 usually exceeds the boiling point of the raw material, the reducing agent particles in a solid state at room temperature are put into a pipe 7 together with an inert gas such as an argon gas or a nitrogen gas and gasified. A method of blowing into the salt 3 can also be exemplified. In this example, tantalum pentoxide is used as the raw material, and its boiling point is about 240 ° C. Therefore, when it is put into the molten salt 3 maintained at about 900 ° C., it is quickly vaporized.

As a result, the chemical reaction between the reducing agent in the molten salt 3 and the raw material proceeds. In this example, magnesium oxide is selected as the material of the pipe 7 for the same reason as the material of the crucible 9. Then, crucible 9 and pipe 7
And let it cool naturally.

Next, the molten salt 3 is recovered by washing with running water and an acid such as acetic acid to obtain a mixture of water and tantalum powder.
The tantalum powder in the mixture is separated from water by an operation such as centrifugation or filtration, and dried to collect the tantalum powder. The shape and size of the particles constituting the tantalum powder can be specified by elemental analysis using energy dispersive X-ray spectroscopy (EDX), phase identification using a powder diffractometer, shape analysis using a scanning electron microscope (SEM), and the like. .

(Second Example) The method for producing tantalum powder of the second example is a first step of producing a mixed salt in which a tantalum ion is present in a molten salt, and a reducing agent is acted on this mixed salt. A second step of obtaining tantalum powder. Further, the same reducing agent, molten salt and raw material as in the first example are used.

FIG. 2 is a schematic configuration diagram of an apparatus used in the first step. The same components as those of the apparatus shown in FIG. In this example, the crucible 12 is made of dense magnesium oxide. Also, a tantalum foil 1 is provided at the tip of the pipe 17 for supplying the raw material.
7a, and a gaseous raw material is supplied through the tantalum foil 17a. By supplying the raw material through the tantalum foil 17a, an effect of disproportionation can be obtained.

First, the pipe 17 is set, and a temperature of 850 to 950 ° C. (900 in this example) is set in the vessel 1.
The raw material is blown under the condition of (° C.) to dissolve the raw material in the molten salt. Thereafter, the pipe 17 is removed and the mixture is naturally cooled to obtain a mixed salt composed of a molten salt containing tantalum ions in the raw material.

FIG. 3 is a schematic structural view of an apparatus used in the second step. The mixed salt 22 (30 g in this example) obtained as described above is put into a crucible 23 and
3 is placed in a sealed container 21 made of stainless steel or the like. The sealed container 21 is provided with a heater 21a.
In this example, the crucible 23 is made of alumina. On the other hand, the reducing agent 25 put in the crucible 24 is arranged in the sealed container 21. In this example, the material of the crucible 24 is made in consideration of thermal stability to magnesium as the reducing agent 25,
It is preferable to use magnesium oxide or iron,
In this example, magnesium oxide is used.

Next, after the inside of the sealed container 21 is evacuated using a decompression pump or the like, an inert gas such as argon gas or nitrogen gas is used to prevent the reducing agent from oxidizing and disappearing. 850 to 950
C. and maintain for 1-6 hours. In this example, the heating temperature is 900 ° C., and the holding time is 3 hours. Then, the reducing agent 25 in the crucible 24 evaporates and the sealed container 21
Is in a state in which a gaseous reducing agent is present in the atmosphere. The gaseous reducing agent is mixed with the mixed salt 22 in the crucible 23.
And a tantalum powder is generated in the mixed salt 22. Next, the mixed salt 22 is washed away with an acid such as water or acetic acid and collected, and a tantalum powder is obtained in the same manner as in the first example.

FIG. 4 is a photograph taken by an SEM of an example of the tantalum powder of the present invention. A plurality of square columnar particles are formed, and these columnar particles are 10 to 100.
Radially aggregated particles are formed in which approximately 0 radially aggregated particles are formed. In the tantalum powder of the present invention, the length of the columnar particles is 2 to 20 μm, and the length of one side of the square, which is a cross section orthogonal to the length direction, is 0.1 to 2 μm. The ratio of the length of one side of the square to the length of the particles is 1/2 to 1/100, preferably 1/5 to 1/30. If it exceeds １ ／, the difference from the case of the conventional spherical particles is small, and if it is less than 1/100, the strength of the columnar particles themselves tends to decrease. The average diameter of the radially aggregated particles is 0.2 to 10 μm, preferably 0.3 to 5 μm.
m. If it is less than 0.2 μm, handling is difficult, and if it exceeds 10 μm, the surface area of the tantalum powder becomes small, which is inconvenient.

When a porous sintered body is formed by a conventional method using the tantalum powder composed of columnar particles,
Since a plurality of columnar particles intersect to form pores in the porous sintered body, the pores are not easily crushed during press working, and have a large surface area and moderate strength. Is obtained. In addition, since a closed hole and a pot-shaped void are not easily generated, a solid electrolyte film that is hardly formed is poorly obtained.
When tantalum powder composed of radially aggregated particles is used, voids are surely formed between adjacent particles, so that these effects are further improved.

FIG. 5 is a chart showing the results of component identification by X-ray (EDX) of one example of the tantalum powder of the present invention. The horizontal axis is the energy of the X-ray, the unit is keV, the left end is 0.000 keV, and the right end is 1
0.110 keV. The symbol of Ta is automatically given to the position of the X-ray peak of tantalum which is known in advance. The vertical axis is the X-ray intensity, and the unit is the number of counts in 100 seconds. The bottom is zero and the top is 5000 counts. The strongest peak is over 5000 counts. This chart shows that tantalum is strongly detected. Further, aluminum caused by the material of the crucible was slightly detected as an impurity.

The solid electrolytic capacitor using the tantalum powder of the present invention is formed into a porous sintered body by the above-described ordinary method, and the surface of the porous sintered body (anode) is made of a solid material such as manganese oxide. It can be constituted by forming an electrolyte film, soldering a cathode made of a Ni wire or the like on the electrolyte film, and further integrally covering these with an exterior resin such as a flame-retardant epoxy resin.

[0032]

As described above, by using the tantalum powder of the present invention, a porous sintered body having appropriate strength and high porosity can be obtained. Therefore, a large-capacity solid electrolytic capacitor can be obtained. Further, in the porous sintered body, since closed holes and pot-shaped voids are hardly generated, poor formation of the solid electrolyte film does not easily occur, and an increase in ESR of the solid electrolytic capacitor can be prevented.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an apparatus configuration used for a method of manufacturing a tantalum powder according to a first example of the present invention.

FIG. 2 is an explanatory view of an apparatus configuration used in a first step of a method of manufacturing a tantalum powder according to a second example of the present invention.

FIG. 3 is an explanatory view of an apparatus configuration used in a second step of a method for producing a tantalum powder according to a second example of the present invention.

FIG. 4 shows an example of the tantalum powder of the present invention in SE
3 is a micrograph taken by M.

FIG. 5 is a chart showing the results of component identification by X-ray (EDX) for an example of the tantalum powder of the present invention.

FIG. 6 is a photomicrograph taken by SEM of an example of a conventional porous sintered body made of tantalum powder.

[Explanation of symbols]

 3 ... molten salt, 8 ... reducing agent, 22 ... mixed salt, 25 ... reducing agent.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22F 1/18 C22F 1/00 621 H01G 9/00 628 // C22F 1/00 621 661Z 628 H01G 9/05 K 661 9/24 C (72) Inventor Hitoshi Iijima Haseji, Higashi-Nagahara, Kato-cho, Kawanuma-gun, Fukushima Prefecture Showa Cabot Super Metal Co., Ltd. F-term (reference) 4K017 AA03 BA07 DA08 EH01 FB10 FB11 4K018 AA40 BA03 BB01 BB10 DA11 KA22 KA39

Claims (8)

[Claims] 1. A tantalum powder comprising columnar particles. 2. The tantalum powder according to claim 1, wherein said tantalum powder includes radially aggregated particles in which a plurality of columnar particles are radially aggregated. 3. The tantalum powder according to claim 1, wherein the tantalum salt and the reducing agent are not brought into direct contact with each other,
A tantalum powder obtained by reducing the tantalum salt. 4. The tantalum powder according to claim 3, wherein the reducing agent is a magnesium reducing agent or a sodium reducing agent. 5. A method for producing a tantalum powder, comprising reducing the tantalum salt without directly contacting the tantalum salt with a reducing agent to obtain a tantalum powder. 6. The method according to claim 5, wherein the reducing agent is a magnesium reducing agent or a sodium reducing agent. 7. A porous sintered body comprising the tantalum powder according to any one of claims 1 to 4. 8. A solid electrolytic capacitor comprising an anode made of the porous sintered body according to claim 7.