U.S. patent application number 11/574675 was filed with the patent office on 2008-01-17 for deoxidation of valve metal powders.
This patent application is currently assigned to H.C. Starck GmbH & Co. KG. Invention is credited to Ulrich Bartmann, Josua Loffelholz.
Application Number | 20080011124 11/574675 |
Document ID | / |
Family ID | 35445804 |
Filed Date | 2008-01-17 |
United States Patent
Application |
20080011124 |
Kind Code |
A1 |
Loffelholz; Josua ; et
al. |
January 17, 2008 |
Deoxidation of Valve Metal Powders
Abstract
Deoxidation of valve metal powders, in particular of niobium
powders, tantalum powders or their alloys, by treating the valve
metal powder with calcium, barium, lanthanum, yttrium or cerium as
deoxidising agent, and valve metal powders that are distinguished
by a ratio of the sum of the contents of sodium, potassium and
magnesium to the capacitance of less than 3 ppm/10,000
.mu.FV/g.
Inventors: |
Loffelholz; Josua;
(Langelsheim, DE) ; Bartmann; Ulrich; (Goslar,
DE) |
Correspondence
Address: |
CONNOLLY BOVE LODGE & HUTZ, LLP
P O BOX 2207
WILMINGTON
DE
19899
US
|
Assignee: |
H.C. Starck GmbH & Co.
KG
Im Schiecke 78-91
Goslar
DE
38642
|
Family ID: |
35445804 |
Appl. No.: |
11/574675 |
Filed: |
August 26, 2002 |
PCT Filed: |
August 26, 2002 |
PCT NO: |
PCT/EP05/09230 |
371 Date: |
June 4, 2007 |
Current U.S.
Class: |
75/245 ;
75/343 |
Current CPC
Class: |
B22F 9/22 20130101; C22B
5/12 20130101; C22B 5/04 20130101; C22B 34/24 20130101; H01G 9/0525
20130101 |
Class at
Publication: |
075/245 ;
075/343 |
International
Class: |
B22F 9/22 20060101
B22F009/22; B22F 1/00 20060101 B22F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 8, 2004 |
DE |
10 2004 043 343.7 |
Claims
1-10. (canceled)
11. A process for the deoxidation of valve metal powders which
comprises deoxidizing valve metal powders with a deoxidizing agent,
wherein the deoxidizing agent is calcium, barium, lanthanum,
yttrium or cerium.
12. The process according to claim 11, wherein the valve metal
powder is a niobium powder, a tantalum powder or a niobium-tantalum
alloy powder.
13. The process according to claim 11, wherein the deoxidizing
agent is calcium or lanthanum.
14. The process according to claim 11, wherein the deoxidizing
agent is calcium and the deoxidation is carried out at a
temperature of from 880 to 1050.degree. C.
15. The process according to claim 11, wherein the deoxidizing
agent is lanthanum and the deoxidation is carried out at a
temperature of from 940 to 1150.degree. C.
16. The process according to claim 12, wherein the deoxidizing
agent is calcium and the deoxidation is carried out at a
temperature of from 880 to 1050.degree. C.
17. The process according to claim 12, wherein the deoxidizing
agent is lanthanum and the deoxidation is carried out at a
temperature of from 940 to 1150.degree. C.
18. The process according to claim 11, wherein the deoxidation is
carried out in two steps.
19. The process according to claim 11, wherein the valve metal
powder obtained by reduction of a valve metal oxide with gaseous
calcium, barium, lanthanum, yttrium or cerium is deoxidised.
20. A valve metal powder which comprises the ratio of the sum of
the impurities sodium, potassium and magnesium to the capacitance
of the valve metal powder is less than 3 ppm/10,000 .mu.FV/g.
21. The valve metal powder according to claim 20, wherein the ratio
of the sum of the impurities sodium, potassium and magnesium to the
capacitance of the valve metal powder is less than 1 ppm/10,000
.mu.FV/g.
22. The valve metal powder according to claim 20, wherein the valve
metal powder is a niobium powder.
23. The valve metal powder according to claim 20, wherein the valve
metal powder is a tantalum powder.
24. The valve metal powder according to claim 21, wherein the valve
metal powder is a niobium powder.
25. The valve metal powder according to claim 21, wherein the valve
metal powder is a tantalum powder.
Description
[0001] The invention relates to a process for the deoxidation of
valve metal powders, in particular of niobium powders, tantalum
powders or their alloys, by treating the valve metal powder with a
deoxidising agent from the group calcium, barium, lanthanum,
yttrium and cerium, and to valve metal powders distinguished by a
low content of sodium, potassium and magnesium.
[0002] Valve metals, which are to be understood as being especially
niobium and its alloys, tantalum and its alloys, as well as the
further metals of groups IVb (Ti, Zr, Hf, Vb (V, Nb, Ta) and VIb
(Cr, Mo, W) of the periodic system of the elements, and their
alloys, are widely used in the manufacture of components.
[0003] Particular mention is to be made of the use of niobium or
tantalum in the manufacture of capacitors, especially of solid
electrolyte capacitors. In the manufacture of niobium or tantalum
capacitors there are conventionally used as starting material
corresponding metal powders, which are first compressed and then
sintered in order to obtain a porous body. This body is anodised in
a suitable electrolyte, whereby a dielectric oxide film forms on
the sintered body. The physical and chemical properties of the
metal powders used have a critical influence on the properties of
the capacitor. Critical characteristics are, for example, the
specific surface, the content of impurities and, as the most
important electrical parameters the specific capacitance at a given
forming voltage U.sub.f. The specific capacitance is generally
given in the unit microfarad*volt per gram (.mu.FV/g).
[0004] General trends in circuit design in the electronics industry
are towards ever higher clock frequencies at ever lower operating
voltages with minimal electric losses. For the solid electrolyte
capacitors used in such applications this means that ever lower
forming voltages are used and at the same time ever lower leakage
currents are required.
[0005] Valve metal powders which are to be used in the manufacture
of capacitors must therefore meet ever higher demands, with the
content of impurities being of great importance. This applies, for
example, to the content of oxygen in the valve metal powder, which
must not be too high, but also to metallic impurities, which have a
decisive influence on the leakage current properties of the
capacitor. Such impurities are especially Na, K, Mg, but also C,
Fe, Cr, Ni.
[0006] However, the impurities Na, K and Mg in particular are
introduced during preparation of the valve metal powders owing to
the process that is used. Thus, for example, the preparation of
tantalum powder is generally still carried out today according to
the reduction, known from U.S. Pat. No. 2,950,185, of
K.sub.2TaF.sub.7 with sodium or potassium, which results in high
contents of sodium and potassium in the product.
[0007] According to U.S. Pat. No. 4,141,720, tantalum powders
having a high oxygen and sodium content can be worked up by adding
K.sub.2TaF.sub.7 and alkali halides and heating the reaction
mixture. The contents of oxygen, sodium and potassium can be
reduced in that manner. However, even the powders so treated have a
sodium content of from 10 to 87 ppm and a potassium content of from
112 to 289 ppm.
[0008] For the preparation of tantalum powder having a high
specific surface and a minimal content of sodium and potassium,
U.S. Pat. No. 5,442,978 proposes reducing highly diluted
K.sub.2TaF.sub.7 by the stepwise addition of sodium, the addition
being carried out at a high rate. According to Example 1 it is
possible in this manner to obtain a tantalum powder having a sodium
content .ltoreq.3 ppm and a potassium content <10 ppm. However,
a deoxidation step is necessary to adjust the oxygen content. To
that end, the tantalum powder is mixed with magnesium and then
heated, as a result of which magnesium is introduced into the
tantalum powder.
[0009] In addition to the reduction of fluoride salts of the valve
metals with alkali metals, oxides of the valve metals are
increasingly being used as starting material recently, which
oxides, as described in U.S. Pat. No. 6,558,447 B1, are reduced
with gaseous magnesium to form the corresponding valve metal. The
content of alkali metal can be kept low in this manner. However,
there is an increased introduction of magnesium. In addition, this
procedure generally requires a deoxidation step to reduce the
oxygen content after the reduction, whereby the magnesium content
in the valve metal powder increases further.
[0010] Owing to their high ionic conductivity and the formation of
crystalline phases with the dielectric layer of amorphous valve
metal oxide produced during capacitor manufacture, the impurities
sodium, potassium and magnesium cause an increased leakage current
in the electric field or on thermal loading during the processing
process of the capacitor manufacturer. This is particularly
pronounced in the case of the ever thinner valve metal oxide layers
of <100 nm which capacitors have today. (1 V forming voltage
corresponds, for example, to about 2 nm tantalum oxide film
thickness).
[0011] The object of the present invention is accordingly to
provide an economical process for the preparation of valve metal
powders which makes available valve metal powders that are
distinguished by a low content of the elements sodium, potassium
and magnesium, which are critical for the residual current of a
capacitor. During capacitor manufacture, such valve metal powders
form very uniform amorphous oxide layers at a high specific charge
(>35,000 CV/g).
[0012] The object is achieved by subjecting the valve metal powder
to a deoxidation step in which a deoxidising agent having low ionic
mobility is used.
[0013] The invention accordingly provides a process for the
deoxidation of valve metal powders, wherein calcium, barium,
lanthanum, yttrium or cerium is used as the deoxidising agent.
[0014] The process according to the invention permits the
preparation of valve metal powders that have a very low content of
impurities having high ionic conductivity.
[0015] As a result, no crystalline phases form with the resulting
valve metal oxide during further processing of such valve metal
powders to capacitors, so that defects in the oxide lattice and
high residual currents are avoided.
[0016] The process according to the invention is suitable for the
deoxidation of a wide variety of valve metal powders. Preference is
given, however, to the deoxidation of niobium powder, tantalum
powder or niobium-tantalum alloy powder, particularly preferably
tantalum powder.
[0017] Accordingly, the valve metal is preferably tantalum.
[0018] According to the invention, calcium, barium, lanthanum,
yttrium or cerium is used as the deoxidising agent. Calcium or
lanthanum is preferably employed, particularly preferably calcium.
The valve metal powder to be deoxidised is mixed with the
deoxidising agent.
[0019] This mixture of the valve metal powder with the deoxidising
agent is heated to a temperature above the melting point of the
deoxidising agent. It is preferably heated to a temperature that is
at least 20.degree. C. above the melting point of the deoxidising
agent used.
[0020] If calcium is used as the deoxidising agent, the deoxidation
is preferably carried out at a temperature of from 880 to
1050.degree. C., particularly preferably at a temperature of from
920 to 1000.degree. C. When lanthanum is used, the preferred
deoxidation temperature is from 940 to 1150.degree. C.,
particularly preferably from 980 to 1100.degree. C.
[0021] The deoxidation is preferably carried out at normal
pressure. However, it is also possible to work at a lower pressure.
The presence of hydrogen is not necessary in the process according
to the invention. The process can be carried out, for example, in
vacuo or under inert gas, such as neon, argon or xenon. Nor does
the process require a solvent or agent for suspending the solids in
a liquid phase, such as, for example, a salt melt, as is
conventionally used in the reduction of valve metal compounds to
valve metals.
[0022] The amount of deoxidising agent added and the treatment time
may vary within wide limits and depend especially on the oxygen
content of the valve metal powder to be deoxidised and on the
deoxidation temperature.
[0023] A deoxidation time of from 2 to 6 hours is generally
sufficient. Preferably, deoxidation is carried out for from 2 to 4
hours.
[0024] There is preferably used a 1.1- to 3-fold stoichiometric
excess of deoxidising agent, based on the amount that is
theoretically required to reduce the oxygen content to 0. It has
been shown that it is generally sufficient to use the deoxidising
agent Ca in an amount of from 3 to 6 wt. % and the deoxidising
agent La in an amount of from 6 to 14 wt. %, based on the amount of
valve metal powder to be deoxidised, in order to achieve the
desired lowering of the oxygen content and of the elements sodium,
potassium and magnesium. There are preferably used from 3.5 to 5.9
wt. % of deoxidising agent Ca or from 9 to 11.5 wt. % La, based on
the amount of valve metal powder to be deoxidised, particularly
preferably from 4 to 4.7 wt. % Ca or from 10 to 115 wt. % La.
[0025] After the deoxidation, the oxides of the deoxidising agent
used that form during the deoxidation are preferably extracted with
an acid. The acid used is preferably nitric acid or hydrochloric
acid. It is to be noted that the use of sulfuric acid is to be
avoided when calcium is used as the deoxidising agent.
[0026] The deoxidation according to the invention is preferably
carried out in two steps. In this case, further deoxidising agent
is added to the valve metal powder after the above-described
deoxidation and acid extraction, and the mixture is subjected to
the described heat treatment again. The amount of deoxidising agent
is chosen to be lower in the second deoxidation step than in the
first deoxidation step and preferably corresponds to a
stoichiometric excess of from 1.3 to 2.0, based on the amount of
oxygen in the valve metal powder. The deoxidising agent is used in
the second deoxidation step preferably in an amount of from 1 to 3
wt. % when Ca is used as the deoxidising agent and in an amount of
from 1.5 to 7 wt. % when La is used, based on the amount of valve
metal powder to be deoxidised. Preferably, from 1 to 1.3 wt. % Ca
or from 3 to 6.1 wt. % La are used as the deoxidising agent, based
on the amount of valve metal powder to be deoxidised.
[0027] The process according to the invention is suitable for the
deoxidation of valve metal powders prepared by any method. For
example, it is possible to deoxidise niobium and tantalum powders
that are prepared by reduction of a fluoride salt of the valve
metal by means of sodium in the presence of a diluting salt. Such a
procedure is known from U.S. Pat. No. 5,442,978, for example.
[0028] In the deoxidation of tantalum powders, particularly
advantageous results are achieved when the tantalum powder used as
starting material is obtained by reaction of K.sub.2TaF.sub.7 with
sodium in the presence of potassium chloride and potassium fluoride
under the following reaction conditions: The salt mixture of
K.sub.2TaF.sub.7, potassium chloride and potassium fluoride is
placed in a test retort and heated preferably for 6 hours at
400.degree. C. in order to remove residual moisture from the salts.
The test retort is then heated to a temperature of from 850.degree.
C. to 950.degree. C., preferably from 850.degree. C. to 920.degree.
C., particularly preferably to a temperature of 900.degree. C.,
whereby the salt mixture liquefies. The liquid melt is stirred
under an argon atmosphere (1050 hPa) for the purpose of
homogenisation. When the reduction temperature is reached, liquid
sodium is added in portions. The total amount of sodium corresponds
to a 3 to 6 wt. % excess, based on the amount of potassium
heptafluorotantalate used. During the addition it must be ensured
that the temperature in the test retort always remains in the range
of the reduction temperature (T+/-20.degree. C.). In order to
adjust the surface of the precipitated tantalum powder, an additive
that influences the surface tension of the salt melt, for example
anhydrous sodium sulfate, is added to the mixture before the first
addition of sodium. When the reduction is complete, stirring is
continued for a further 0.5 to 3 hours in the range from
800.degree. C. to the reduction temperature. Preferably, stirring
is continued for about 3 hours while simultaneously cooling from
the reduction temperature to 800.degree. C. The reaction material
is cooled to room temperature and steam is passed through the test
retort in order to passivate excess sodium. The retort is then
opened and the reaction material is removed and pre-comminuted by
means of jaw breakers (<5 cm, preferably <2 cm). The inert
salts are then removed by washing, and the resulting tantalum
powder is dried. A step of doping with phosphorus can optionally be
inserted here, in which the tantalum metal powder is treated with a
(NH.sub.4)H.sub.2PO.sub.4 solution in order to adjust the P content
of the finished tantalum metal powder. The powder is then exposed
to a high temperature in vacuo. For example, heating is carried out
for 30 minutes at from 1250.degree. C. to 1500.degree. C.,
preferably from 1280.degree. C. to 1450.degree. C., particularly
preferably from 1280.degree. C. to 1360.degree. C. The tantalum
powder so prepared is then subjected to the deoxidation according
to the invention.
[0029] If is, of course, also possible to use as starting materials
valve metal powders which are obtained by reduction of the valve
metal oxides using gaseous magnesium, as described in U.S. Pat. No.
6,558,447 B1.
[0030] It has been shown that it is particularly advantageous to
use calcium, barium, lanthanum, yttrium or cerium as the reducing
agent in this case instead of magnesium.
[0031] In a particularly preferred embodiment of the process
according to the invention, therefore, there is used as the valve
metal powder to be deoxidised a valve metal powder that is obtained
by reduction of a valve metal oxide using gaseous calcium, barium,
lanthanum, yttrium or cerium.
[0032] The procedure for the preparation of the corresponding valve
metal powder is according to U.S. Pat. No. 6,558,447 B1, but
calcium, barium, lanthanum, yttrium or cerium is used as the
reducing agent.
[0033] For the preparation of a tantalum powder, which is
preferably used, tantalum oxide (Ta.sub.2O.sub.5) is, for example,
placed on a tantalum gauze in a tantalum dish. A 1.1-fold
stoichiometric amount, based on the oxygen content in the tantalum
oxide, of calcium, barium, lanthanum, yttrium or cerium is placed
beneath the tantalum gauze. The reduction is carried out at a
temperature that is sufficiently high to convert the reducing agent
to the gaseous state. In order to increase the vapour pressure of
the reducing agent at a given reduction temperature, it is possible
to work at a reduced overall pressure in the reactor. Accordingly,
the process is generally carried out at an overall pressure in the
reactor of less than or equal to 1000 mbar, preferably at an
overall pressure in the reactor of less than or equal to 500 mbar.
The reduction temperature is then preferably from 950 to
1100.degree. C., particularly preferably from 980 to 1050.degree.
C. In general, reducing times of up to 8 hours are sufficient. When
the reduction is complete, the reaction material is removed and the
resulting oxide of the reducing agent is extracted with nitric acid
or hydrochloric acid. Analogously to the above-described procedure,
a P-doping step may also optionally be inserted here. Finally, the
valve metal powder so obtained is subjected to a deoxidation
according to the invention.
[0034] Valve metal powders that are distinguished by a content of
Na, K and Mg of less than 3 ppm, based on a capacitance of 10,000
.mu.FV/g, are accessible for the first time by means of the
deoxidation process according to the invention.
[0035] The invention accordingly further provides valve metal
powders that have a ratio of the sum of the impurities sodium,
potassium and magnesium to the capacitance of the valve metal
powder of less than 3 ppm/10,000 .mu.FV/g.
[0036] The ratio of the sum of the impurities sodium, potassium and
magnesium to the capacitance of the valve metal powder is
preferably less than 2 ppm/10,000 .mu.FV/g, particularly preferably
less than 1 ppm/10,000 .mu.FV/g.
[0037] The content of the impurities K, Na, Mg is determined after
acid decomposition of the valve metal sample by means of
HNO.sub.3/HF. K and Na are determined by the method of flame atom
adsorption spectroscopy (FAAS) in an acetylene/air mixture, and
magnesium is determined by the ICP-OES method (inductive coupled
plasma-optical emission spectroscopy). For the acid decomposition,
2 ml of 65 wt. % HNO.sub.3 and 10 ml of 40 wt. % HF are added to
1.0 g of the valve metal sample to be tested, and stirring is
carried out for 10 hours at a temperature of 105.degree. C. under
normal pressure. After cooling, 5 ml of 30 wt. % HCl are added, and
the volume of the sample is made up to 100 ml with H.sub.2O. The
solution so obtained is then tested by means of FAAS or ICP-OES.
The contents that are determined are indicated in ppm (parts per
million).
[0038] The capacitance of the valve metal powder is determined by
the following procedure: Cylindrical compressed bodies having a
diameter of 4.1 mm and a length of 4.26 mm and having a compressed
density of 4.8 g/cm.sup.3 are each prepared from 0.296 g of a
deoxidised valve metal powder, a tantalum wire of 0.2 mm diameter
being inserted axially into the compression mould as contact wire
before the valve metal powders are introduced. The compressed
bodies are sintered at a sintering temperature of from 1330.degree.
C. to 1430.degree. C. for 10 minutes under a high vacuum
(<10.sup.-5 mbar) to form anodes. The anode bodies are immersed
in 0.1 wt. % phosphoric acid and formed at a current intensity
limited to 150 mA to a forming voltage of 30 V. After the current
intensity has diminished, the voltage is maintained for a further
100 minutes. In order to measure the capacitor properties, a
cathode of 18 wt. % sulfuric acid is used. Measurement is carried
out at a frequency of 120 Hz. The residual current is then measured
in phosphoric acid of conductivity 4300 .mu.S. The resulting values
of the capacitance of the individual anode and the residual current
of the individual anode are standardised to .mu.FV/g, where
.mu.F=capacitance, V=forming voltage, g=anode mass, or .mu.A/g,
where .mu.A=measured residual current and g=anode mass used, or
.mu.A/.mu.FV.
[0039] The valve metal powders according to the invention
preferably have a capacitance of at least 35,000 .mu.FV/g,
particularly preferably of at least 40,000 .mu.FV/g.
[0040] The valve metal powders according to the invention are
preferably niobium or tantalum powders, which are optionally doped
with one another and/or with one or more of the metals Ti, Mo, V,
W, Hf and Zr. Further doping elements, such as, for example,
phosphorus, are possible.
[0041] The valve metal powders according to the invention can be
used for a wide variety of applications and are suitable in
particular for the manufacture of solid electrolyte capacitors.
[0042] The examples which follow serve to illustrate the invention
in greater detail, the examples being intended to facilitate
comprehension of the principle according to the invention and not
to limit it.
EXAMPLES
[0043] Unless indicated otherwise, percentages are by weight (wt.
%).
Example 1
[0044] A tantalum primary powder was prepared at a reduction
temperature of 900.degree. C. starting from a mixture of 150 kg of
K.sub.2TaF.sub.7, 136 kg of KCl, 150 kg of KF, 4 kg of a superfine
tantalum powder and 300 g of Na.sub.2SO.sub.4 in a nickel-coated
INCONEL retort by the increment-wise addition of sodium,
analogously to U.S. Pat. No. 5,442,978. The tantalum powder was
isolated from the cooled and comminuted reaction mixture by washing
with weakly acidified water, a cleaning treatment with a washing
solution comprising sulfuric acid and hydrogen peroxide
subsequently also being carried out. The material was doped with 20
ppm of phosphorus using a sodium dihydrogen phosphate solution
containing 1 mg of P per ml of solution. After drying, heat
treatment was carried out under a high vacuum at 1430.degree. C.
Following this, the phosphorus content of the tantalum powder was
adjusted to 60 ppm by means of the sodium dihydrogen phosphate
solution (1 mg of P per ml). The powder exhibited the following
impurities (in ppm):
Mg: <1 ppm
Na: 0.7 ppm
K: 7 ppm
[0045] 2 kg of this powder (starting powder) were mixed with 90 g
(4.5 wt. %) of calcium powder and heated at 980.degree. C. for 3
hours in a covered tantalum crucible in a retort tinder an argon
atmosphere. After cooling and the controlled introduction of air
for passivation, the reaction material was removed and calcium
oxide that had formed was removed with a washing solution of dilute
nitric acid and hydrogen peroxide solution. The washing solution
was decanted oft and the powder on the suction filter was washed
with demineralised water until free of acid. The dried powder had
an oxygen content of 2831 ppm.
[0046] 1.8 kg of this powder were then subjected to a second
deoxidation step. To that end, 19.2 g of calcium powder (based on
the oxygen content, the 1.5-fold stoichiometric amount) were mixed
into the powder and the mixture was likewise heated at 980.degree.
C. for 3 hours. After cooling and passivation, the CaO that had
formed was again removed by acid washing, and the powder was washed
until free of acid.
[0047] The powder so prepared exhibited the following
impurities:
Mg: <1 ppm
Na: 1 ppm
K: 8 ppm
[0048] The electric test gave a capacitance of 37,419 .mu.FV/g at a
sintering temperature of 1400.degree. C.
Example 2
Comparison Example
[0049] 2 kg of the starting powder from Example 1 were mixed with
50 g of magnesium turnings (2.5 wt. %) and heated at 980.degree. C.
for 3 hours in a covered tantalum crucible in a retort under an
argon atmosphere. After cooling and the controlled introduction of
air for passivation, the reaction material was removed and
magnesium oxide that had formed was removed with a washing solution
of dilute sulfuric acid and hydrogen peroxide solution. The washing
solution was decanted off, and the powder on the suction filter was
washed with demineralised water until free of acid. The dried
powder had an oxygen content of 2781 ppm.
[0050] 1.8 kg of this powder were then subjected to a second
deoxidation step. To that end, 11.4 g of magnesium turnings (based
on the oxygen content, the 1.5-fold stoichiometric amount) were
mixed into the powder and the mixture was likewise heated at
980.degree. C. for 3 hours. After cooling and passivation, the MgO
that had formed was again removed by acid washing, and the powder
was washed until free of acid.
[0051] The powder so prepared exhibited the following
impurities:
Mg: 8 ppm
Na: 1 ppm
K: 6 ppm
[0052] The electric test gave a capacitance of 38,261 .mu.FV/g at a
sintering temperature of 1400.degree. C.
Example 3
200 g of the starting powder from Example 1 were mixed with 22 g of
lanthanum powder (11 wt. %) and heated at 980.degree. C. for 3
hours in a covered tantalum crucible in a retort under an argon
atmosphere. After cooling and the controlled introduction of air
for passivation, the reaction material was removed and lanthanum
oxide that had formed was removed with a washing solution of dilute
nitric acid and hydrogen peroxide solution. The washing solution
was decanted off, and the powder on the suction filter was washed
with demineralised water until free of acid. The dried powder had
an oxygen content of 3045 ppm.
[0053] 180 g of this powder were then subjected to a second
deoxidation step. To that end, 6.5 g of lanthanum powder (based on
the oxygen content, the 1.5-fold stoichiometric amount) were mixed
into the powder and the mixture was likewise heated at 980.degree.
C. for 3 hours. After cooling and passivation, the La.sub.2O.sub.3
that had formed was again removed by acid washing, and the powder
was washed until free of acid.
[0054] The powder so prepared exhibited the following
impurities:
Mg: <1 ppm
Na: 0.7 ppm
K: 8 ppm
[0055] The electric test gave a capacitance of 38,093 .mu.FV/g at a
sintering temperature of 1400.degree. C.
Example 4
[0056] A tantalum primary powder was prepared at a reduction
temperature of 920.degree. C. starting from a mixture of 75 kg of
K.sub.2TaF.sub.7, 125 kg of KCl, 225 kg of KF, 5 kg of a superfine
tantalum powder and 500 g of Na.sub.2SO.sub.4 in a nickel-coated
INCONEL retort by the increment-wise addition of sodium,
analogously to U.S. Pat. No. 5,442,978. The tantalum powder was
isolated from the cooled and comminuted reaction mixture by washing
with weakly acidified water, a cleaning treatment with a washing
solution comprising sulfuric acid and hydrogen peroxide
subsequently also being carried out. The material was doped with
100 ppm of phosphorus using a sodium dihydrogen phosphate solution
containing 1 mg of P per ml of solution. After drying, heat
treatment was carried out under a high vacuum at 1280.degree. C.
The powder exhibited the following impurities (in ppm):
Mg: <1 ppm
Na: 1 ppm
K: 49 ppm
[0057] 2 kg of this powder were mixed with 90 g (4.5 wt. %) of
calcium powder and heated at 960.degree. C. for 3 hours in a
covered tantalum crucible in a retort under an argon atmosphere.
After cooling and the controlled introduction of air for
passivation, the reaction material was removed and calcium oxide
that had formed was removed with a washing solution of dilute
nitric acid and hydrogen peroxide solution. The washing solution
was decanted off, and the powder on the suction filter was washed
with demineralised water until free of acid. The dried powder had
an oxygen content of 3700 ppm.
[0058] 1.8 kg of this powder were then subjected to a second
deoxidation step. To that end, 25 g of calcium powder (based on the
oxygen content, the 1.5-fold stoichiometric amount) were mixed into
the powder and the mixture was likewise heated at 960.degree. C.
for 3 hours. After cooling and passivation, the CaO that had formed
was again removed by acid washing, and the powder was washed until
free of acid.
[0059] The powder so prepared exhibited the following
impurities:
Mg: <1 ppm
Na: 1 ppm
K: 12 ppm
[0060] The electric test gave a capacitance of 59,764 .mu.FV/g at a
sintering temperature of 1400.degree. C.
Example 5
[0061] 500 g of tantalum pentoxide (Ta.sub.2O.sub.5) having a
particle size <400 .mu.m are placed on a tantalum gauze in a
tantalum crucible. The 1.1-fold stoichiometric amount, based on the
oxide content in the tantalum pentoxide, of calcium (249.4 g) is
placed beneath the tantalum gauze. The tantalum dish is introduced
into a scalable retort.
[0062] The reduction is carried out for 8 hours under an argon
atmosphere at 980.degree. C. and at a reaction pressure of 600
mbar. The reaction material is removed, and the resulting calcium
oxide is extracted with nitric acid. The tantalum powder, which has
been washed until free of acid, is doped with 100 ppm of P on the
suction filter using a sodium dihydrogen phosphate solution
containing 1 mg of P per ml of solution, and then dried. The
tantalum powder so prepared has an oxygen content of 7143 ppm.
[0063] 400 g of this powder are mixed with 18 g (4.5 wt. %) of
calcium powder and heated at 960.degree. C. for 3 hours in a
covered tantalum crucible in a retort under an argon atmosphere.
After cooling and the controlled introduction of air for
passivation, the reaction material is removed and calcium oxide
that has formed is removed with a washing solution of dilute nitric
acid and hydrogen peroxide solution. The washing solution is
decanted off, and the powder on the suction filter is washed with
demineralised water until free of acid. The dried powder has an
oxygen content of 4953 ppm.
[0064] 300 g of this powder are then subjected to a second
deoxidation step. To that end) 5.6 g of calcium powder (based on
the oxygen content, the 1.5-fold stoichiometric amount) are mixed
into the powder and the mixture is likewise heated at 960.degree.
C. for 3 hours. After cooling and passivation, the CaO that has
formed is again removed by acid washing, and the powder is washed
until free of acid.
[0065] The powder so prepared exhibits the following
impurities:
Mg: <1 ppm
Na: <1 ppm
K: 2 ppm
[0066] The electric test gave a capacitance of 70,391 CV/g at a
sintering temperature of 1400.degree. C.
* * * * *