Production method for tungsten anode body

Abstract

A method for producing an anode body in a capacitor, which includes making a molded body by molding a tungsten powder and making an anode body by sintering the molded body, which includes a step of bringing the tungsten powder or the molded body thereof into contact with a solution of a silicon compound before sintering the molded body so as to adjust the silicon content in the anode body to 0.05 to 7 mass %.

Claims

1. A method for producing an anode body in a capacitor, comprising making a molded body by molding a tungsten powder and making an anode body by sintering the molded body, which comprises a step of bringing the tungsten powder or the molded body thereof into contact with a solution of a silicon compound before sintering the molded body so as to adjust the silicon content in the anode body to 0.05 to 7 mass %, and wherein the solution of a silicon compound is a solution comprising the silicon compound and a solvent in which the silicon compound is dissolved.

2. The method for producing an anode body as claimed in claim 1, wherein the silicon content in the anode body is adjusted to 0.05 to 7 mass % by controlling the ratio of the solvent and the silicon compound in the solution of a silicon compound.

3. The method for producing an anode body as claimed in claim 1, wherein bringing the tungsten powder into contact with the solution of a silicon compound is conducted by mixing the tungsten powder into the solution of a silicon compound.

4. The method for producing an anode body as claimed in claim 1, wherein bringing the molded body into contact with the solution of a silicon compound is conducted by impregnating the molded body with the solution of a silicon compound.

5. The method for producing an anode body as claimed in claim 1, wherein the process of bringing the tungsten powder or the molded body thereof into contact with the solution of a silicon compound is conducted as a process of making a molded body by molding the tungsten powder using a solution of a silicon compound containing a binder.

6. The method for producing an anode body as claimed in claim 1, wherein the solvent of the solution of a silicon compound is removed from the tungsten powder or the molded body thereof after the process of bringing the tungsten powder or molded body thereof into contact with the solution of a silicon compound.

7. The method for producing an anode body as claimed in claim 1, wherein the silicon compound is a compound which is decomposed at a sintering temperature or lower, and at least part of the decomposed product reacts with tungsten to thereby form tungsten silicide.

8. The method for producing an anode body as claimed in claim 1, wherein the silicon compound is a silane compound.

9. The method for producing an anode body as claimed in claim 1, wherein the solvent of the solution of a silicon compound is one member selected from alcohol, ester and ether, or a mixed solvent of two or more thereof.

10. The method for producing an anode body as claimed in claim 1, further comprising a process of incorporating at least one element selected from nitrogen, carbon, boron, phosphorus and oxygen on at least a part of the surface of the sintered body.

11. The method for producing an anode body as claimed in claim 1, wherein at least part of silicon constituting the silicon compound is combined with tungsten on the surface of the sintered body in the sintering step to thereby form tungsten silicide.

12. A method for producing an anode body in a capacitor, comprising making a molded body by molding a tungsten powder and making an anode body by sintering the molded body, which comprises bringing the tungsten powder into contact with a solution of a silicon compound before sintering the molded body so as to adjust the silicon content in the anode body to 0.05 to 7 mass %, isolating the tungsten powder from the solution of a silicon compound, calcining the isolated tungsten powder, pulverizing the calcined powder to obtain a granulated powder and molding the granulated powder, wherein the solution of a silicon compound is a solution comprising the silicon compound and a solvent in which the silicon compound is dissolved.

13. The method for producing an anode body as claimed in claim 12, wherein at least part of silicon constituting the silicon compound is combined with tungsten on the surface of the sintered body in the sintering step to thereby form tungsten silicide.

Description

EXAMPLES

(1) The present invention is described below by referring to Examples and Comparative Examples, but the present invention is not limited thereto.

(2) The measurement of the specific surface area and average particle diameter, the production of a sintered body, and the analysis of the elements other than tungsten (silicon, oxygen, nitrogen and other elements) of the granulated powder of Examples and Comparative Examples were conducted by the following methods.

(3) Specific Surface Area:

(4) The specific surface area was measured by the BET method by using Macsorb HM model-1208 (manufactured by Mountech Co., Ltd.).

(5) Average Particle Diameter:

(6) The particle diameter distribution was measured by the laser diffraction scattering method using HRA 9320-X100 manufactured by Microtrac Inc. A particle size value (D.sub.50; m) corresponding to cumulative volume % of 50 volume % was designated as the average particle diameter.

(7) Production of a Sintered Body:

(8) A molded body being 1.01.54.5 mm in size was produced by molding the granulated powder made in the examples. A tantalum wire of 0.29 mm in diameter stands upright in the 1.01.5 mm surface of the molded body, which is embedded 3.7 mm inside the molded body and protruded outside by 7 mm. The molded body was vacuum-sintered in a high-temperature vacuum furnace at a temperature described later for 20 minutes to obtain a sintered body of 60 mg in mass.

(9) Measurement of the Each Content of Elements of Silicon, Oxygen and Other Elements:

(10) Each content of the elements in the anode body was determined by ICP emission spectrometry. The nitrogen element content and oxygen element content were determined by the thermal conductivity detection and the infrared absorption method, respectively, using an oxygen/nitrogen elemental analyzer (TC600; manufactured by LECO Corporation).

(11) Capacitance and LC Value of the Electrolytic Capacitor:

(12) The anode body for a capacitor, which was composed of a sintered body of tungsten powder, was subjected to chemical conversion in an aqueous solution of 3 mass % of ammonium persulfate at 9 V for six hours, washed with alcohol and dried at 190 C. to form a dielectric layer on the surface of the anode body. The anode body having a dielectric layer formed thereon was immersed in an aqueous solution of 30% sulfuric acid to form an electrolytic capacitor, and the capacitance and LC value of the capacitor were measured. The capacitance was measured by using an LCR meter manufactured by Agilent Technologies, Inc. at room temperature, 120 Hz and bias voltage of 2.5 V. The LC value was measured 30 seconds after applying a voltage of 4 V at room temperature. Arbitrarily selected 40 pieces of the anode body in each example were measured and the average values were calculated.

Example 1

(13) A tungsten powder (ungranulated powder) having an average particle diameter of 0.6 m was obtained by reducing tungsten trioxide powder in the stream of hydrogen. After 100 g of the powder was put in 100 ml of ethanol solution of 0.5 mass % tetraethoxysilane and fully mixed, the mixture was put in a vacuum dryer to remove ethanol and be dried at 60 C.

(14) Next, the resultant powder was calcined under vacuum condition of 510.sup.3 Pa at 1,400 C. for 20 minutes and cooled to room temperature. Subsequently, the powder was pulverized with a hammer mill to obtain a granulated powder having an average particle diameter of 110 m (particle diameter distribution: 26 to 180 m), a specific surface area of 0.3 m.sup.2/g and a silicon content of 0.05 mass %.

Examples 2 to 5, Comparative Examples 1 to 3

(15) Granulated powders of Examples 2 to 5 and Comparative Examples 1 to 3 were produced in the same manner as in Example 1 except that the concentration of the above-mentioned tetraethoxysilane was changed as in Table 1 so as to make the silicon concentration (mass %) in the anode body fall within a range of 0 to 7.6%.

(16) A molded body was formed by molding the granulated powder produced in each of the examples, and a sintered body was obtained by sintering the molded body. The measurement results of the silicon content and the oxygen content in the obtained sintered bodies are also shown in Table 1.

(17) The granulated powder was analyzed by an X-ray diffractometer (X'pert PRO; manufactured by PANalytical B.V.), and tungsten silicide was detected as a reaction product. Most of the detected tungsten silicide was W.sub.5Si.sub.3. Sputtered surface of the granulated powder was also analyzed in a similar manner and it was found that tungsten silicide as a reaction product exists in a range within 30 nm in depth from the particle surface of the granulated powder. That is, it was confirmed that silicon exists as tungsten silicide in at least a part of the surface layer of the particles of the granulated powder. Although these values are the results of the analysis of the granulated powder, the granulated powders show similar results when they are processed into an anode body.

(18) A molded body was formed by molding the granulated powder produced in each of the examples, and a sintered body was obtained by sintering the molded body at 1,550 C. in a high-temperature vacuum furnace. The capacitance and LC value of the electrolytic capacitor using the obtained sintered body as an anode body were measured and the results are also shown in Table 1.

(19) TABLE-US-00001 TABLE 1 Sintered body Silicon Content of other Anode body content principal element Capacitance LC (mass %) (ppm by mass) (F) (A) Example 1 0.05 Oxygen 5,800 294 3.6 Example 2 0.2 Oxygen 6,700 310 0.5 Example 3 1.1 Oxygen 7,500 303 0.5 Example 4 4.4 Oxygen 8,400 308 0.7 Example 5 6.3 Oxygen 9,200 313 3.9 Comparative 0.03 Oxygen 6,100 288 46 Example 1 Comparative 7.6 Oxygen 9,600 311 22 Example 2 Comparative 0.0 Oxygen 5,200 282 91 Example 3

Example 6

(20) 200 g of commercially-available tungsten powder (ungranulated powder) having an average particle diameter of 0.5 m was put in 400 g of water, in which 10 mass % of ammonium persulfate was dissolved, and the water was fully stirred with a homogenizer to oxidize the surface layer of the tungsten powder. After washing with water, 500 ml of 2N aqueous sodium hydroxide solution was added thereto and the resultant solution was stirred to thereby remove the oxide on the surface layer. A series of operations of the oxidation and removal of the oxide was repeated three times to obtain a finely-powdered tungsten powder having an average particle diameter of 0.3 m was obtained. 100 g of the powder was put in 100 ml of ethanol solution of 0.5 mass % of dimethyldimethoxysilane and fully mixed. Then, the solution was placed in a vacuum dryer to remove ethanol and be dried at 60 C.

(21) Next, the resultant powder was calcined under vacuum condition of 510.sup.3 Pa at 1,370 C. for 20 minutes and cooled to room temperature. Subsequently, the powder was pulverized with a hammer mill to obtain a granulated powder having an average particle diameter of 105 m (particle diameter distribution: 20 to 180 m), a specific surface area of 2.1 m.sup.2/g and a silicon content of 0.1 mass %.

Examples 7 to 10, Comparative Examples 4 to 6

(22) By changing the concentration of the above-mentioned dimethyldimethoxysilane as in Table 2, granulated powders of Examples 7 to 10 and Comparative Examples 4 to 6 were produced so as to make the silicon concentration (mass %) in the anode body fall within a range of 0 to 8.2%.

(23) A sintered body was produced from the granulated powder of each of the examples. The measurement results of each content of silicon, oxygen and nitrogen in the obtained sintered bodies are also shown in Table 2.

(24) A molded body was formed by molding the granulated powder produced in each of the examples, and a sintered body was obtained by sintering the molded body at 1,500 C. in a high-temperature vacuum furnace. The capacitance and LC value of the electrolytic capacitor using the obtained sintered body as an anode body were measured and the results are also shown in Table 2.

(25) TABLE-US-00002 TABLE 2 Sintered body Anode body Silicon Capac- content Content of other principal itance LC (mass %) element (ppm by mass) (F) (A) Example 6 0.1 Oxygen 7,700 Nitrogen 710 767 4.4 Example 7 0.3 Oxygen 8,500 Nitrogen 680 772 0.7 Example 8 1.4 Oxygen 9,400 Nitrogen 620 774 0.8 Example 9 4.8 Oxygen 11,500 Nitrogen 550 785 0.8 Example 10 7.0 Oxygen 12,700 Nitrogen 800 789 4.1 Comparative 0.01 Oxygen 7,400 Nitrogen 720 772 63 Example 4 Comparative 8.2 Oxygen 13,400 Nitrogen 800 746 30 Example 5 Comparative 0.0 Oxygen 6,700 Nitrogen 840 741 126 Example 6

Example 11

(26) A finely-powdered tungsten powder having an average particle diameter of 0.1 m was obtained by repeating a series of operations of the oxidation and removal of the oxide six times in the same way as in Example 6. A granulated powder having a silicon content of 0.3 mass % was produced in the same way as in Example 6 except that triethoxyphenylsilane was used instead of dimethyldimethoxysilane and the calcination temperature was set to 1,320 C. After putting 80 g of the granulated powder in 200 ml of an aqueous phosphoric acid solution to be mixed, the solution was vacuum-dried at 100 C. and the water was removed. Subsequently, resintering at 1,320 C. for 20 minutes was conducted and the powder was cooled to room temperature and pulverized with a hammer mill to obtain a granulated powder having an average particle diameter of 93 m (particle diameter distribution: 20 to 160 m) and a specific surface area of 10.3 m.sup.2/g. The sintered body made of the granulated powder contained 0.3 mass % of silicon, 14,700 ppm by mass of oxygen, 890 ppm by mass of nitrogen and 70 ppm by mass of phosphorus as in Table 3.

Comparative Example 7

(27) A granulated powder was obtained in the same way as in Example 11 except that a silicon compound (triethoxyphenylsilane) was not added. Silicon was not detected in the sintered body made of the granulated powder and the sintered body contained 11,900 ppm by mass of oxygen, 850 ppm by mass of nitrogen and 70 ppm by mass of phosphorus as in Table 3.

Comparative Examples 8 to 10

(28) A dispersion of a silicon powder was prepared in the same way as in Example 3 and Example 8 except that instead of a silicon compound, a commercially-available silicon powder (average particle diameter of 1.5 m) was added to ethanol, and a tungsten powder and the dispersion were mixed. A granulated powder was prepared in the same way as in Example 3 (calcination temperature: 1,400 C.) and Example 8 (sintering temperature: 1,370 C.) except that a solution of a silicon compound was changed to a dispersion of silicon powder. The sintered body made of the granulated powder of Comparative Example 8 had a silicon concentration of 1.0 mass % and an oxygen concentration of 6,400 ppm by mass and the granulated powder of Comparative Example 9 had a silicon concentration of 1.5 mass %, an oxygen concentration of 9,300 ppm by mass and a nitrogen concentration of 710 ppm by mass as shown in Table 3.

(29) The granulated powder of Comparative Example 10 was produced under the conditions of Example 3 and a dispersion of silicon dioxide prepared by adding a silicon dioxide powder having an average particle diameter of 1 m to ethanol so as to adjust the concentration to 1 mass % was used instead of the ethanol solution of tetraethoxysilane, and the tungsten powder was mixed with the dispersion. A granulated powder was obtained in the same way as in Example 3 except that a solution of a silicon compound was changed to a dispersion of silicon dioxide powder. The granulated powder had a silicon concentration of 1.0 mass % and an oxygen concentration of 12,200 ppm by mass as shown in Table 3. Further, it was confirmed that silicon dioxide exists in isolation between the particles of the tungsten powder by the observation under a scanning electron microscope (SEM) and the energy dispersive spectrometry (EDS), and that the silicon dioxide is in a crystalline state by the X-ray diffractometry. In the granulated powder, a compound of tungsten and silicon was not observed.

(30) A molded body was formed by molding the granulated powder produced in each of Example 11 and Comparative Examples 7 to 10, and a sintered body was obtained by sintering the molded body in a high-temperature vacuum furnace at a temperature as shown in Table 3. The capacitance and LC value of the electrolytic capacitor using the obtained sintered body as an anode body were measured and the results are also shown in Table 3.

(31) TABLE-US-00003 TABLE 3 Production of a sintered Sintered body body Silicon Sintering Anode body content Content of other principal temperature Capacitance LC (mass %) element (ppm by mass) ( C.) (F) (A) Example 11 0.3 Oxygen Nitrogen Phosphorus 1,460 1,860 1.3 14,700 890 70 Comparative 0.0 Oxygen Nitrogen Phosphorus 1,460 1,948 307 Example 7 11,900 850 70 Comparative 1.0 Oxygen 1,550 305 5.8 Example 8 6,400 Comparative 1.5 Oxygen Nitrogen 1,500 768 7.9 Example 9 9,300 710 Comparative 1.0 Oxygen 1,550 296 103 Example 10 12,200

(32) The capacitance and LC values shown in Tables 1 to 3 are an average value of arbitrarily-selected 40 units of anode bodies in each of Examples and Comparative Examples, and the element contents are an average value of arbitrarily-selected two pieces of sintered bodies in each of Examples and Comparative Examples. In any of Examples and Comparative Examples, there was no granulated powder in which the total content of the elements other than tungsten, silicon, oxygen, nitrogen and phosphorus exceeded 1,000 ppm.