U.S. patent application number 14/361041 was filed with the patent office on 2014-10-02 for method for manufacturing fine tungsten powder.
This patent application is currently assigned to SHOWA DENKO K.K.. The applicant listed for this patent is Kazumi Naito, Shoji Yabe. Invention is credited to Kazumi Naito, Shoji Yabe.
Application Number | 20140294663 14/361041 |
Document ID | / |
Family ID | 48535094 |
Filed Date | 2014-10-02 |
United States Patent
Application |
20140294663 |
Kind Code |
A1 |
Naito; Kazumi ; et
al. |
October 2, 2014 |
METHOD FOR MANUFACTURING FINE TUNGSTEN POWDER
Abstract
A method for finely powdering tungsten powder, which includes
electrolytically oxidizing tungsten powder while stirring in an
aqueous mineral-acid solution to form an oxide film in the surface
of the tungsten powder and removing the oxide film with an alkaline
aqueous solution; a method for producing tungsten powder to obtain
fine tungsten powder by a process including the above method for
finely powdering; and a tungsten powder having an average particle
size of 0.04 to 0.4 .mu.m, in which the dMS value (product of an
average particle size d (.mu.m), true density M (g/cm.sup.3) and
BET specific surface area S (m.sup.2/g)) is within the range of
6.+-.0.4.
Inventors: |
Naito; Kazumi; (Tokyo,
JP) ; Yabe; Shoji; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Naito; Kazumi
Yabe; Shoji |
Tokyo
Tokyo |
|
JP
JP |
|
|
Assignee: |
SHOWA DENKO K.K.
Minato-ku, Tokyo
JP
|
Family ID: |
48535094 |
Appl. No.: |
14/361041 |
Filed: |
August 29, 2012 |
PCT Filed: |
August 29, 2012 |
PCT NO: |
PCT/JP2012/071761 |
371 Date: |
May 28, 2014 |
Current U.S.
Class: |
420/430 ;
75/345 |
Current CPC
Class: |
B22F 2998/10 20130101;
H01G 9/0525 20130101; B22F 1/0081 20130101; C22C 27/04 20130101;
B22F 1/0018 20130101; B22F 2998/10 20130101; H01G 9/052 20130101;
B22F 9/04 20130101; B22F 9/04 20130101; B22F 2003/244 20130101;
B22F 1/02 20130101 |
Class at
Publication: |
420/430 ;
75/345 |
International
Class: |
B22F 1/00 20060101
B22F001/00; C22C 27/04 20060101 C22C027/04 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2011 |
JP |
2011-259926 |
Claims
1. A method for finely powdering tungsten powder, comprising
electrolytically oxidizing tungsten powder while stirring in an
electrolytic solution to form an oxide film in the particle surface
of the tungsten powder and removing the oxide film with an alkaline
aqueous solution.
2. The method for finely powdering tungsten powder as claimed in
claim 1, wherein the removal of the oxide film with an alkaline
aqueous solution includes mechanically removing the reaction
product in the particle surface of the tungsten powder.
3. The method for finely powdering tungsten powder as claimed in
claim 1, wherein the electrolytic solution is an aqueous solution
of mineral acid.
4. The method for finely powdering tungsten powder as claimed in
claim 3, wherein the mineral acid is selected from phosphoric acid,
nitric acid, hydrochloric acid, boric acid and sulfuric acid.
5. The method for finely powdering tungsten powder as claimed in
claim 4, wherein the mineral acid is phosphoric acid or boric
acid.
6. A method for finely powdering tungsten powder, comprising
dispersing tungsten powder in an aqueous solution containing an
oxidizing agent to form an oxide film in the surface of the
tungsten powder, removing the oxide film with an alkaline aqueous
solution, followed by the method claimed in claim 1.
7. A method for producing fine tungsten powder, comprising
obtaining tungsten powder having an average particle size of 0.04
to 0.4 .mu.m by a process including the method claimed in claim
1.
8. A method for producing fine tungsten powder, comprising
obtaining tungsten powder, in which the product of an average
particle size (.mu.m), true density (g/cm.sup.3) and BET specific
surface area (m.sup.2/g) is within the range of 6.+-.0.4, by a
process including the method claimed in claim 1.
9. A tungsten powder having an average particle size of 0.04 to 0.4
.mu.m, in which the dMS value (product of an average particle size
d (.mu.m), true density M (g/cm.sup.3) and BET specific surface
area S (m.sup.2/g)) is within the range of 6.+-.0.4.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing fine
tungsten powder. Specifically, the present invention relates to a
method for processing a tungsten powder into the one having a
smaller particle size which is useful for use in an electrolytic
capacitor; and a method for producing fine tungsten powder using
the above method.
BACKGROUND ART
[0002] With the progress of small-size, high-speed and lightweight
electronic devices such as cellular phones and personal computers,
the capacitor used for these electronic devices is demanded to have
a smaller size, a larger capacitance and a lower ESR (Equivalent
Series Resistance).
[0003] As an example of such a capacitor, the electrolytic
capacitor has been proposed, which capacitor is produced by
anodically oxidizing an anode body for capacitors comprising a
sintered body made of a valve-acting metal powder which can be
anodized such as tantalum to form a dielectric layer made of the
oxide of the metal in the surface of the anode body.
[0004] The electrolytic capacitor using tungsten as a valve-acting
metal and employing the sintered body of the tungsten powder as an
anode body can attain a larger capacitance compared to the
electrolytic capacitor obtained with the same formation voltage by
employing the anode body of the same volume using the tantalum
powder having the same particle size. However, the electrolytic
capacitor having the sintered body of the tungsten powder has been
unpracticed as an electrolytic capacitor due to the large leakage
current (LC). In order to solve this issue, a capacitor using the
alloy of tungsten and other metals has been studied and has
achieved some improvement in the leakage current, but it was not
enough (JP-A-2004-349658 (U.S. Pat. No. 6,876,083 B2); Patent
Document 1).
[0005] Patent Document 2 (JP-A-2003-272959) discloses a capacitor
using an electrode of a tungsten foil having formed thereon a
dielectric layer selected from WO.sub.3, W.sub.2N and WN.sub.2, but
the capacitor is not to solve the above-mentioned leakage current
problem.
[0006] Also, Patent Document 3 (WO 2004/055843 publication (U.S.
Pat. No. 7,154,743 B2)) discloses an electrolytic capacitor using
an anode body selected from tantalum, niobium, titanium and
tungsten, but it does not describe a specific example using
tungsten in the specification.
[0007] In an anode body for an electrolytic capacitor which is
obtained by molding tungsten powder and sintering it, the smaller
particle size enables the production of an anode body having a
larger capacitance, if the volume of the anode body is the same.
Therefore the smaller size of the raw material tungsten powder is
more preferable but the average particle size of a
commercially-available tungsten powder is 0.5 to 20 .mu.m.
[0008] Tungsten powder can be manufactured by treating oxide,
halide or ammonium salt of tungsten as a raw material with a
reducing agent such as hydrogen. However, increase in the rate of
reduction may give rise to a problem of generating a composite
oxide and the like.
[0009] Therefore, it is necessary to decrease the rate of reduction
in order to produce finer powder, and it leads to low the
production efficiency and high cost. Also, it is necessary to
produce the fine powder by a complicated process equipped with an
expensive controlling device. Furthermore, there has been a problem
of handling a material having a wide explosibility range such as
hydrogen gas.
PRIOR ART
Patent Document
[0010] Patent Document 1: JP-A-2004-349658
[0011] Patent Document 2: JP-A-2003-272959
[0012] Patent Document 3: WO 2004/055843
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0013] An object of the present invention is to provide a method
for processing tungsten powder to obtain tungsten powder having a
smaller particle size as a material of a capacitor comprising
tungsten as an anode (hereinafter referred to as a tungsten
capacitor), and a method for producing fine tungsten powder using
the method.
Means to Solve the Problem
[0014] As a result of intensive study to solve the above-mentioned
problem, the present inventors have found that a fine tungsten
powder which is more suitable for a capacitor can be obtained by
electrolytic oxidization of the surface of a tungsten powder which
is currently available and have accomplished the present
invention.
[0015] That is, the present invention relates to a method for
finely powdering tungsten powder and a method for producing
tungsten powder as below.
[0016] [1] A method for finely powdering tungsten powder,
comprising electrolytically oxidizing tungsten powder while
stirring in an electrolytic solution to form an oxide film in the
particle surface of the tungsten powder and removing the oxide film
with an alkaline aqueous solution.
[0017] [2] The method for finely powdering tungsten powder as
described in [1] above, wherein the removal of the oxide film with
an alkaline aqueous solution includes mechanically removing the
reaction product in the particle surface of the tungsten
powder.
[0018] [3] The method for finely powdering tungsten powder as
described in [1] or [2] above, wherein the electrolytic solution is
an aqueous solution of mineral acid.
[0019] [4] The method for finely powdering tungsten powder as
described in [3] above, wherein the mineral acid is selected from
phosphoric acid, nitric acid, hydrochloric acid, boric acid and
sulfuric acid.
[0020] [5] The method for finely powdering tungsten powder as
described in [4] above, wherein the mineral acid is phosphoric acid
or boric acid.
[0021] [6] A method for finely powdering tungsten powder,
comprising dispersing tungsten powder in an aqueous solution
containing an oxidizing agent to form an oxide film in the surface
of the tungsten powder, removing the oxide film with an alkaline
aqueous solution, followed by the method described in any one of
[1] to [5] above.
[0022] [7] A method for producing fine tungsten powder, comprising
obtaining tungsten powder having an average particle size of 0.04
to 0.4 .mu.m by a process including the method described in any one
of [1] to [6] above.
[0023] [8] A method for producing fine tungsten powder, comprising
obtaining tungsten powder, in which the product of an average
particle size (.mu.m), true density (g/cm.sup.3) and BET specific
surface area (m.sup.2/g) is within the range of 6.+-.0.4, by a
process including the method described in any one of [1] to [6]
above.
[0024] [9] A tungsten powder having an average particle size of
0.04 to 0.4 .mu.m, in which the dMS value (product of an average
particle size d (.mu.m), true density M (g/cm.sup.3) and BET
specific surface area S (m.sup.2/g)) is within the range of
6.+-.0.4.
EFFECTS OF THE INVENTION
[0025] According to the present invention, using a currently
available tungsten powder or a tungsten powder which can be
produced by a known method, a tungsten powder having a small
particle size and a substantially spherical shape which is suitable
for an electrolytic capacitor can be obtained.
[0026] Since the tungsten powder obtained by the present invention
have a small particle size, a capacitor obtained thereof has a
large capacitance. Also, the tungsten powder has a high flowability
due to the more spherical particle shape. Accordingly, the powder
can be handled more easily in the process of producing granulate
powder and the like.
MODE FOR CARRYING OUT THE INVENTION
[Raw Material Tungsten Powder]
[0027] The average particle size of the raw material tungsten
powder to be finely powdered in the present invention is preferably
within the range of 0.1 to 10 .mu.m.
[0028] A raw material tungsten powder can be obtained by , in
addition to using a commercially-available product, manufacturing
by a known method. For example, it can be obtained by manufacturing
by appropriately selecting from a method of crushing tungsten
trioxide powder under hydrogen atmosphere; a method of reducing
tungsten acid or tungsten halide with hydrogen or sodium and the
like. Also, a tungsten powder may be obtained by reducing the
tungsten-containing mineral directly or through several steps and
by selecting reducing conditions.
[0029] However, since it is difficult to obtain a raw material
tungsten powder having a small particle size by these methods, a
tungsten power treated by chemical oxidization in advance as
mentioned below or a fine particle tungsten powder obtained
according to the method of the present invention may be used as a
raw material tungsten powder. Using these tungsten powers having
been finely-powdered as a raw material, a tungsten powder having an
even smaller particle size can be obtained. Thus, a tungsten powder
having an average particle size of, for example, 0.04 .mu.m or less
can be obtained by repeatedly applying the method of the present
invention.
[0030] However, in the case of forming a dielectric layer by anode
oxidation, there is a lower limit of the particle size of the
powder which can be suitably used for a capacitor. The lower limit
of the particle size of the tungsten powder used for a capacitor is
twice the thickness of the dielectric layer to be formed. For
example, when the rated voltage is 1.6 V, the lower limit of the
particle size is to be 0.04 .mu.m. If the particle size is smaller
than the lower limit, a conductive portion of the tungsten is not
left sufficiently when performing anodic oxidation and it becomes
difficult to construct an anode of an electrolytic capacitor.
[0031] Specifically, when used for a high capacitance capacitor
having low rated voltage, the particle size of the tungsten powder
is preferably 0.04 to 0.4 .mu.m, more preferably 0.08 to 0.2
.mu.m.
[0032] The raw material tungsten powder used in the method of the
present invention may contain impurities within a range which does
not affect the capacitor properties or may be processed to contain
elements such as silicon, nitrogen, carbon, boron, phosphorus and
oxygen in order to improve the capacitor properties. However, it is
preferable that the particle surface treatment such as
silicidation, nitridation, carbonization or boronization to be
described later is conducted in a process later than applying the
present invention.
[0033] In the present invention, a finely powdered tungsten powder
is obtained by oxidizing the surface of particles of a raw material
tungsten powder and removing the oxide film in the surface.
[0034] Oxidization of the particle surface of a tungsten powder can
be performed either by chemical oxidization or electrolytic
oxidization, and a method of fine powdering using electrolytic
oxidization (referred to as electrolytic oxidization method
hereinafter) can be operated more easily than a method of fine
powdering using chemical oxidization, because generation amount of
the oxide film can be controlled only by adjusting an applied
voltage during the electrolytic oxidization. Accordingly, the
electorlytic oxidization method can be preferably applied to
production of a finer tungsten powder in which generation amount of
the oxide film needs to be controlled more precisely.
[0035] Fine powdering of tungsten powder may be performed only by
the electrolytic oxidization method. In the case of fine powdering
of relatively large particles (for example, average particle size
of 1 .mu.m or more), tungsten powder may be pre-treated by chemical
oxidization for fine powdering to some extent (for example, to
average particle size of 0.5 .mu.m or less) before applying the
electrolytic oxidization method, and the applied voltage at the
time of electrolytic oxidization can be reduced, which makes the
operation easier.
[0036] (1) Electrolytic Oxidization Method
[0037] Oxidization of surface of tungsten powder particle:
[0038] As an electrolyte, an electrolyte solution such as an
aqueous solution of mineral acid and salt thereof can be used, and
aquueous solution of mineral acid is preferable because washing
after oxidization is easy. Examples of mineral acid include
phosphoric acid, nitric acid, hydrochloric acid, sulfuric acid and
boric acid, and phosphoric acid or boric acid are preferable from
the viewpoint that an oxide film having defects is obtained
relatively easily, and the oxide film is easy to be removed with
alkali aqueous solution later. Preferable concentration of the
mineral acid aqueous solution is 0.1 to 5 mass %. If the
concentration increases, cleaning of tungsten powder in the
subsequent step becomes complicated.
[0039] The electrolytic oxidization is, for example, conducted as
below. A material tungsten powder is put in a metal container
containing an electrolyte while stirring, a predetermined voltage
is applied between a metal stirring stick as an anode and the
container as a cathode, and the tungsten powder is oxidized by
applying a current at a temperature preferably from room
temperature to the boiling point of the aqueous solution, more
preferably 30.degree. C. to 80.degree. C. for preferably 10 minutes
to 100 hours and more preferably 1 to 10 hours. A solvent component
is supplied in the amount corresponding to the amount lost by
evaporation, as needed.
[0040] The applied voltage may be set depending on the desired
degree of fine powdering. Higher applied voltage leads to an
increased amount of oxide film which results in smaller particle
size. Specific voltage can be determined by a preliminary
experiment. However, it takes time for electrolytic oxidization
under high voltage, and preferable applied voltage may be set at
100V or less, and more preferably 50V or less, and the operation of
fine powdering may be repeated, if necessary.
[0041] After the electrolytic oxidization is finished, the
operation of removing the liquid by decantation and the like is
repeated, and the tungsten powder is washed with a solvent such as
water. The color of the tungsten in this state changes from black
to yellowish blue.
[0042] Removal of the oxide film:
[0043] The oxide film of the tungsten powder obtained as mentioned
above, in which the surface is oxidized, is subjected to treatment
with an alkaline aqueous solution and removed at least
chemically.
[0044] Preferably, using a device such as a homogenizer which is
capable of vigorous stirring, the above-mentioned stirring is
conducted while mechanically removing the product generated in the
surface of tungsten particles as well.
[0045] As an alkaline solution, for example, sodium hydroxide
aqueous solution, potassium hydroxide aqueous solution, ammonia
water and the like can be used, and sodium hydroxide aqueous
solution and potassium hydroxide aqueous solution are
preferable.
[0046] Specifically, an alkaline aqueous solution is added to a
tungsten powder in which the surface is oxidized. The solution is
allowed to stand after stirring. After removing the solution by
decantation, a series of operations of feeding a solvent such as
water into the tungsten powder, stirring the resultant solution,
allowing it to stand and subjecting it to decantation is repeated
several times. By these operations, the color of the tungsten
powder becomes black, and the oxide formed in the surface of
tungsten particles is removed. Subsequently, the solution is dried
in a vacuum dryer under reduced pressure (e.g. with the reduced
pressure reduced of 10.sup.4 to 10.sup.2 Pa at the temperature of
50 to 180.degree. C.) and cooled to room temperature. Then, by
gradually introducing air into the dryer so that ignition may not
occur and taking out the powder into the air, a tungsten powder
having a smaller particle size than that of the raw material
tungsten powder can be obtained.
[0047] (2) Chemical Oxidization Method
[0048] In the chemical oxidization carried out as a pre-treatment,
if desired, a raw tungsten powder is dispersed in an aqueous
solution of oxidant by stirring and the like and retained for a
predetermined time to oxidize the surface of the tungsten powder. A
device such as a homogenizer which is capable of vigorous stirring
is preferably used in order to keep a good dispersion state and
make the surface of the tungsten powder oxidized rapidly. Further,
oxidization proceeds at a faster pace at a high temperature.
[0049] Examples of the oxidant include a manganese (VII) compound
such as permanganate; a chrome (VI) compound such as chromium
trioxide, chromate and dichromate; a halogen acid compound such as
perchloric acid, chlorous acid, hypochlorous acid and salt thereof;
peroxide such as hydrogen peroxide, diethyl peroxide, sodium
peroxide and lithium peroxide; peroxoacid such as peracetic acid
and persulfate and salt thereof. Particularly preferable are
hydrogen peroxide and ammonium persulfate, due to its
handleability, stability as an oxidant and high solubility to
water.
[0050] Concentration of the oxidant in an aqueous solution is
within a range of about 1% to a saturated solubility of the
oxidant. Concentration of the oxidant can be appropriately
determined by a preliminary experiment.
[0051] The time period for oxidization is one hour to 1,000 hours
and preferably one hour to 100 hours. Oxidization temperature is
from room temperature to the boiling point of the solvent and
preferably 50.degree. C. to the boiling point of the solution.
[0052] After oxidization reaction, tungsten powder is taken out
from the oxidization reaction solution by decantation and the like,
and a series of operations of feeding the tungsten powder into a
solvent, stirring the resultant solution, allowing it to stand and
subjecting it to decantation is repeated for washing the tungsten
powder. The color of the tungsten at this point changes from black
of the raw material to yellowish blue, and it can be visually
confirmed that the surface of the tungsten powder is oxidized.
[0053] Regarding a solvent used in each step of the present
invention, not only water but also a mixed aqueous solution of
water and water-soluble organic solvent (e.g. ethanol and methanol)
can be selected from the viewpoint of dispersibility of the powder
and time required for decatation.
[0054] Removal of oxide film of the tungsten powder in which the
surface is oxidized is conducted in a similar manner to the removal
of the oxide film in the electrolytic oxidization method.
[0055] According to the method of the present invention, almost
spherical tungsten particles can be obtained unless the raw
material tungsten powder particles have a shape having especially
high anisotropy. The fact that the particle shape is spherical can
be confirmed by that the average particle diameter (d) (.mu.m),
true density (M) (g/cm.sup.3) and BET specific surface area (S)
(m.sup.2/g) of the obtained tungsten powder satisfy the following
formula.
d=6/(M.times.S) (1)
[0056] That is, it can be said that if the product
(d.times.S.times.M; abbreviated as dSM) of the average particle
diameter (d) (.mu.m), true density (M) (g/cm.sup.3) and BET
specific surface area (S) (m.sup.2/g) of the obtained tungsten
powder is near to 6, the obtained tungsten powder particles have a
almost-spherical shape. The dMS value of the tungsten powder
obtained by the present invention is generally within the range of
6.+-.0.4. Furthermore, by using the tungsten powder obtained by
applying the method of the present invention as a raw material
powder, it is also possible to obtain a tungsten powder composed of
particles with higher degree of sphericity.
[0057] Since the dielectric layer formed in the surface of an
almost-sphere particle has a substantially constant curvature and
has no highly-curved portion with a small curvature in which stress
tends to be concentrated, it undergoes little degradation. As a
result, a capacitor having better LC characteristics can be
obtained.
[0058] The tungsten powder produced by the method of the present
invention may be directly sintered to be made into a sintered body,
or may be granulated into granules about the size of 10 to 300
.mu.m to be sintered and made into a sintered body. The granulated
tungsten powder is easier to handle and to keep the ESR as low as
possible.
[0059] Furthermore, the tungsten powder produced by the method of
the present invention may be subjected to silicidation,
nitridation, carbonization or boronization treatment to be made
into a tungsten powder containing at least one of tungsten
silicide, tungsten nitride, tungsten carbide and tungsten boride in
a part of the surface of the tungsten particles. These treatments
may be conducted when the tungsten powder has become a granulated
powder or a sintered body. An electrolytic capacitor is fabricated
comprising the sintered body as one electrode (anode), a counter
electrode (cathode) and a dielectric body interposed
therebetween.
EXAMPLES
[0060] The present invention is described below by referring to
Examples and Comparative Examples, but the present invention is not
limited thereto.
[0061] In the present invention, the particle diameter, specific
surface area and true density were measured by the methods
described below.
[0062] The particle diameter was measured by using HRA9320-X100
manufactured by Microtrack Inc. and the particle size distribution
was measured by the laser diffraction scattering method. A particle
diameter value (D.sub.50; .mu.m) corresponding to cumulative volume
% of 50 volume % was designated as the average particle size (d).
The diameter of the secondary particles is to be measured by this
method. However, since the tungsten powder generally has good
dispersibility, the measurement results near to the primary powder
particle diameter can be obtained. Therefore, the measurement
results can be substantially regarded as a primary particle
diameter and applied to the above-described formula (1) to judge
the particle shape.
[0063] The specific surface area (S; m.sup.2/g) was measured by the
BET method by using NOVA2000E (manufactured by SYSMEX Corp.)
[0064] The true density (M; g/cm.sup.3) was measured by a
picnometer method (20.degree. C.).
Example 1
[0065] 200 g of tungsten powder having an average particle diameter
of 1 .mu.m obtained by reducing ammonium tungstate with hydrogen
was put in 500 ml of distilled water in which 5 mass % of ammonium
persulfate was dissolved and stirred at 50.degree. C. for 24 hours
using homogenizer NS-51 manufactured by MICROTEC Co., Ltd. An
amount of water lost through evaporation was successively added all
the period. After allowing the liquid to stand at room temperature
for 17 hours to sedimentate the powder, the liquid was removed by
decantation. After adding another 200 ml of distilled water, the
liquid was stirred by a homogenizer for five minutes and allowed to
stand for several hours, and the liquid was removed by decantation.
The series of operations of feeding distilled water, stirring,
allowing the water to stand, and decantation was repeated four
times. The tungsten powder at this point underwent a change in
color to yellowish blue, which revealed that the surface of the
tungsten powder was oxidized. Subsequently, 100 ml of a 5 mass %
sodium hydroxide aqueous solution was added to the powder and
stirred by a homogenizer for one hour. As described above, after
allowing the liquid to stand and removing the liquid by
decantation, the series of operations of feeding distilled water,
stirring, allowing the liquid to stand, and decantation was
repeated four times. The tungsten powder at this point was black
and the oxide formed in the particle surface was removed. The
produced powder had an average particle diameter of 0.5 .mu.m.
[0066] Next, a tungsten powder containing water after decantation
(100 g for the powder itself) was moved to a separately prepared
container made of stainless steel, and 300 ml of 1 mass %
phosphoric acid aqueous solution was fed as an electrolytic
solution. A stirring stick made of stainless steel (four 4 cm-long
blades made of stainless steel are set in the lower part of the
stick, at an angle of 90.degree. to each other) is placed in the
electrolytic solution from the upper part of the container, and
electrolytic oxidization was conducted by applying a voltage of 20
V between the stirring stick as an anode and the container as a
cathode at 50.degree. C. for five hours while stirring the solution
at a rotation speed of 100 rpm. An amount of water lost through
evaporation was sucessively added all the period. After allowing
the liquid to stand at room temperature for 40 hours to sedimentate
the powder, the liquid was removed by decantation. After adding
another 200 ml of distilled water, the liquid was stirred by the
stirring stick for 20 minutes and allowed to stand for 20 hours,
and the liquid was removed by decantation. The series of operations
of feeding distilled water, stirring, allowing the water to stand,
and decantation was repeated four times. The tungsten powder at
this point underwent a change in color to yellowish blue, which
revealed that the surface of the tungsten powder was oxidized.
[0067] Subsequently, 100 ml of a 5 mass % sodium hydroxide aqueous
solution was added to the powder and stirred by the stirring stick
for one hour. As described above, after allowing the liquid to
stand and removing the liquid by decantation, the series of
operations of feeding distilled water, stirring, allowing the
liquid to stand, and decantation was repeated four times. The
tungsten powder at this point was black and the oxide formed on the
particle surface was removed. Subsequently, a part of the tungsten
powders is moved to a vacuum dryer and dried under reduced pressure
at the temperature of 50.degree. C., and cooled to room
temperature. Then, air was introduced gradually into the dryer so
that ignition may not occur, and the powder was taken out into the
air. The produced powder had an average particle diameter (d) of
0.2 .mu.m, specific surface area (S) of 1.5 m.sup.2/g and true
density (M) of 19.3. The product of the average particle diameter,
specific surface area and true density of the obtained tungsten
powder (dMS) was 5.8.
Example 2
[0068] The electrolytic solution used in Example 1 was changed from
300 ml of 1 mass % phosphoric acid aqueous solution to a mixed
solution of 100 ml of 1.5 mass % boric acid methanol and 350 ml of
water. Decantation liquid was changed to a mixed solution of
methanol and water in the same proportion as above. Also, an amount
of methanol lost through evaporation was successively added all the
period. A tungsten powder was obtained in a similar manner as in
Example 1 except the above. The produced powder had an average
particle diameter (d) of 0.16 .mu.m, specific surface area (S) of
2.0 m.sup.2/g and true density (M) of 19.3. The product of the
average particle diameter, specific surface area and true density
of the obtained tungsten powder (dMS) was 6.2. It was confirmed
that the particles of the powder obtained both in Examples 1 and 2
were almost spherical because the dMS value was within the range of
6.+-.0.2.
* * * * *