Note: Descriptions are shown in the official language in which they were submitted.
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REDUCTION OF Nb OR Ta OXIDE POWDER BY
A GASEOUS LIGHT METAL OR A HYDRIDE THEREOF
Field and L'aclcoround of the Invention
This invention relates to (he production of tantalum, niobium and other metal
powders and their alloys by the reduction of the corresponding metal oxide
with
gaseous active metals such as Mg, Ca and ofher elemental and compound reducing
materials, in gaseous form.
Tantalum and niobium are members of a group of inetals that are difficult to
isolate
in the free state because of the stability of their compounds, especially some
of their
oxides. A review of the methods developed to produce tantalum will serve to
illustrate the history of a typical manufacturing process for these metals.
Tantalum
metal powder was first produced on a commercial scale in Germany at the
beginning
of the 20`h Century- by the reduction of the double salt, potassium
heptafluorotantalate (K-)TaF7) with sodium. Small pieces of sodium were mixed
with
the tantalum containing salt and sealed into a steel tube. The tube was heated
at the
top with a ring burner and, after ignition, the reduction proceeded quickly
down the
tube. The reaction mixture was allowed to cool and the solid mass, consisting
of
tantalum metal powder, unreacted K2TaF7 and sodium, and other products of the
reduction was removed by hand using a chisel. The mixture was crushed and then
leached with dilute acid to separate the tantalum from the components. The
process
was difficult to control, dan-erous, and produced a coarse, contaniinated
powder, but
neverthcless pointed the %vav to what beeamc- the principal mearu cif
produetion of
llit'lh purity tantaluni in later years.
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Comniercial production of tantalum metal in the United States began in the
1930's. A
molten mixture of K2TaF7 containing tantalum oxide (Ta205) was electrolyzed at
700 C in a steel retort. When the reduction was conlpleted, the system was
cooled
and the solid mass removed from the electrolysis cell, and then crushed and
leached
to separate the coarse tantalum powder froni the other reaction products. The
dendritic powder was not suitable for use directly in capacitor applications.
The modern method for manufacturing tantalum was developed in the late 1950's
by
Hellier and Martin (Hellier, E.G. and Martin, G.L., US Patent 2950185, 1960).
Fol-
lowing Hellier and Martin, and llundreds of subsequently described
implementations
or variatioiis, a molten mixture of K2TaF7 and a diluent salt, typically NaCI,
is re-
duced with molten sodium in a stirred reactor. Using this system, control of
the in1-
portant reaction variables, such as reduction temperature, reaction rate, and
reaction
composition, was feasible. Over the years, the process was reftned and
perfected to
the point wliere high quality powders with surface area exceeding 20,000
cm2/gm are
produced and materials with surface area in the 5000-8000 cm2/gm range being
typi-
cal. The manufacturing process still requires the renioval of the solid
reaction pro-
ducts from the retort, separation of the tantalum powder from the salts by
leaching,
and treatments like agglomeration to improve the physical properties. Most
capacitor
grade tantalum powders are also deoxidized with magnesium to minimize the
oxygen
content (Albrecht, W.W., Hoppe, H., Papp, V. and Wolf, R., US Patent 4537641,
1985). Artifacts of preagglomeration of primary particles to secondary
particle form
and doping with inaterials to enhance capacitance (e.g. P, N, Si, and C) are
also
known todav.
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While the reduction of K2TaF7 with sodium has allowed the industry to make
high
performance, high quality tantalum powders thus, according to Ullmann's
Encyclo-
pedia of Industrial Chemistry, 5"' Edition, Volume A 26, p. 80, 1993, the
consump-
tion of tantalum for capacitors had already reached a level of more than 50%
of the
world production of tantalum of about 1000 tons per annum, whereas there had
essentially been no use of niobium for capacitors, even though the raw
material base
for niobium is considerably broader than that for tantalum and most of the
publications on powder preparation and capacitor manufacturing methods mention
niobium as well as tantalum.
Some of the difficulties of applying that process to niobium are as follows:
While the manufacturing process of the type shown in Hellier and Martin (US
2,950,185) for the reduction of potassium heptaflorotantalate by means of
sodium in
a salt melt is available in principle for the production of high purity
niobium powders
via potassium heptafluoroniobate, it doesn't work well in practice. This is
due, in
part, to the difficulti of precipitating the corresponding heptafluoroniobate
salts and
is due, in part, to the aggressively reactive and corrosive nature of such
salts, such
that niobium produced by that process is very impure. Further, niobium oxide
is
usually unstable. See, e.g., N.F. Jackson et al, Electrocomponent Science &
Technology, Vol. 1, pp. 27-37 (1974).
Accordingly, niobium has only been used in the capacitor industry to a very
minor
extent, predominantly in areas with lower quality requirements.
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However, niobium oxide dielectric constant is about 1.5 times as high as that
of a
similar tantalum oxide layer, which should allow in principle, for higher
capacitance
of niobium capacitors, subject to considerations of stability and other
factors.
As for tantalum itself, despite the success of the K2TaF7/sodium reduction
process,
there are several drawbacks to this method.
It is a batch process subject to the inherent variability in the system; as a
result, batch
to batch consistency is difficult. Post reduction processing (mechanical and
hydro-
metallurgical separations, filtering) is complex, requiring considerable human
and
capital resources and it is time consuming. The disposal of large quantities
of
reaction products containing fluorides and chlorides can be a problem. Of
fundamental significance, the process has evolved to a state of maturity such
that the
prospects for significant advances in the performance of the tantalum powder
produced are limited.
Over the years, numerous attempts were made to develop alternate ways for
reducing
tantalum and similar metal compounds, including Nb-compounds, to the metallic
state (Miller, G.L. "Tantalum and Niobium," London, 1959, pp. 188-94; Marden,
J.W. and Rich, M.H., US Patent 1728941, 1927; and Gardner, D., US Patent
2516863 1946; Hurd, U.S. Patent 4687632). Among these were the use of active
metals other than sodium, such as calcium, magnesium and aluminum and raw mate-
rials such as tantalum pentoxide and tantalum chloride. As seen in Table I,
below,
the negative Gibbs free energy changes indicate that the reduction of the
oxides of
Ta, Nb and otlier metals with magnesiuin to the metallic state is favorable;
reaction
rate and method detertnine the feasibility of using this approach to produce
high
quality powders on a coniinercial scale. To date, none of these approaches
were
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commercialized significantly because they did not produce high quality
powders.
Apparently, the reason these approaches failed in the past was because the
reductions
were carried out by blending the reducing agents with the metal oxide. The
reaction
took place in contact with the niolten reducing agent and under conditions of
inability to control the temperature of highly exothermic reactions.
Tlierefore, one is
unable to control morpltology of the produets and residual reducing metal
content.
Table 1
Gibbs Free Energy Change for Reduction of Metal Oxides with Magnesiuni
MxOy(s) + yMg (g) ~ yMgO(s) + xM(s)
Temperature Gibbs Free Energy Change (KcaUmole oxide)
C
W02
Ta205 Nb205 Ti02 V2O3 Zr02
200 -219 -254 -58 -133 -22 --143
400 -215 -249 -56 -130 -21 -141
600 -210 -244 -55 -126 -20 -139
800 -202 -237 -52 -122 -18 -137
1000 -195 -229 -50 -116 -15 -134
1200 -186 -221 -47 -111 -13 -131
1400 -178 -212 -45 -106 -11 -128
The use of inagnesium to deoxidize or reduce the oxygen content of tantalum
metal
is well known. The process involves blending the metal powder with 1-3 percent
magnesiuni and lieating to achieve the reduction process. The magnesium is in
the
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molten state during a portion of the heating time. In this
case, the objective is to remove 1000-3000 ppm oxygen and
only a low concentration of MgO is produced. However, when
a much greater quantity of tantalum oxide is reduced a large
quantity of magnesium oxide is generated. The resulting
mixture of magnesium, tantalum oxide and magnesium oxide can
under conditions of poorly controlled temperature, form
tantalum-magnesium-oxygen complexes that are difficult to
separate from the tantalum metal.
Summary of the Invention
The invention provides a new approach to
production of high performance, capacitor grade tantalum and
niobium powders that provides a means of eliminating one or
more, preferably all, the problems of traditional double
salt reduction and follow on processing. The invention
enables a continuous production process. Further, the
invention provides improved metal forms. The invention also
provides niobium/tantalum alloy powders of capacitor grade
quality and morphology.
We have discovered that the prior art problems can
be eliminated or at least mitigated when metal oxides such
as Ta205 and Nbz05 and suboxides in massive amounts are
reduced with magnesium in gaseous form, substantially or
preferably entirely. The oxide source should be
substantially or preferably entirely in solid. The oxide is
provided in the
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form of a porous solid with high access throughout its mass by the gaseous
reducing
agent.
The metals that can be effectively produced singly or in multiples (co-
produced)
through the present invention are in the group of Ta, Nb, and Ta/Nb alloy, any
of
these alone or with further inclusion of added or co-produced Ti, Mo, V, W, Hf
and/or Zr. The metals can also be mixed or alloyed during or after production
and/or
formed into useful compounds of such ntetals. The respective stable and
unstable
oxide forms of these metals can be used as sources. Metal alloys may be
produced
from alloyed oxide precursors, e.g. resulting from coprecipitation of a
suitable
precursor for the oxide.
Vapor pressures of some of the reducing agents are given as follows:
Temperature ( C) Aluminum P (Atmospheres)
2,000 53 x 10-2
2,100 1.0 x 10-1
2,200 1.9 x 10- 1
2,300 3.3 x 10-1
2,400 5.6 x 10-1
2,500 9.0 x 10-1
2,600 1.4
Temperature ( C) Magnesium P (Atmospheres)
800 4.7 x 10-2
850 8.9 x 10-2
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900 1.6 x 10" 1
950 2.7 x 10' 1
1000 4.8 x 10- 1
1050 7.2 x 10-1
1100 1.1
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Temperature ( C) Calcium P (Atmospheres)
1,000 1.7 x 10-2
1,100 5.1 x 10-2
1,200 1.3 x 10- h
1,300 2.9 x 10-1
1,400 6.0 x 10-1
1,500 1.1
Temperature ( C) Lithium P (Atmospheres)
i 1,000 5.1 x 10-2
~
1,100 1.4x10-1
1,200 3.8 x 10-1
1,300 7.2 x 10-1
1,400 1.4
The temperature of reduction varies significantly depending on the reducing
agent
used. The temperature ranges for reduction of (Ta, Nb) oxide are:
with Mg(gas)-800-1,100 C, Al(gas)- 1,100-1,500 C, Li(gas)- 1,000-1,400 C,
13a(,,as)
1,300-1,900 C.
Different physical properties as well as morphology of the metal powder
produced
by reduction can be achieved by variations of temperature and other conditions
of
processing v;ithin the effective reduction range.
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One embodiment of the invention includes a first step of reducing an oxide
source of
selected metal(s) substantially to free 80-100% (by weight) of the metal
values
therein as priniary powder particles, then leaching or other steps of
hydrometallurgy
to separate the metal fronl residual reducing agent oxide and other byproducts
of the
reduction reaction and from residual condensed reducing agent (optionally),
followed
by one or nlore deoxidation steps under less concentrated reagent conditions
than in
the first gross reduction step (and with better tolerance of molten state of
the
reducing agent), then further separation as might be needed.
In accordance with this first embodiment the invention provides for a single
stage
reduction process for the production of metal powders as cited above,
coniprising the
steps of:
(a) providing an oxide or mixed oxides of the metal(s), the oxide itself being
in a
forni that is traversable by gas,
(b) generating a gaseous reducing agent at a site outside the oxide mass and
passing the gas through the mass at an elevated temperature,
(c) the reactants selection, porosity of the oxide, temperature and time of
the
reduction reaction beiiig selected for substantially coniplete reduction of
the
oxide(s) to free the metal portion thereof, the residual oxide of reducing
agent
formed in the reaction being easily removable,
whereby a high surface area, flowable metal powder is formed in a process that
es-
sentially avoids use of molten state reducing agent in production of metal or
alloy
powder.
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More particularly, the first embodiment of the
invention provides a process for producing (i) a Ta or Nb
metal powder, (ii) a Ta/Nb alloy powder or (iii) the powder
of (i) or (ii) further alloyed with one or more metals
selected from the group consisting of Ti, Mo, W, Hf, V and
Zr, comprising the steps of: (a) providing a powdered oxide
or mixed oxides of the metal or metals defined in (i), (ii)
or (iii), the powdered oxide or mixed oxides being in a
porous solid form with high access throughout the mass
thereof by a gaseous reducing agent as defined in (b);
(b) generating a gaseous reducing agent selected from the
group consisting of Mg, Ca, Al, Li, Ba, Sr and a hydride
thereof at a site outside the powdered oxide or mixed oxides
mass and passing the gaseous reducing agent through the mass
at an elevated temperature; and (c) selecting the gaseous
reducing agent and the powdered oxide or mixed oxides,
porosity of the powdered oxide or mixed oxides, temperature
and time of the reduction reaction for substantially
complete reduction of the powdered oxide or mixed oxides to
free the metal (i) or alloy (ii) or (iii), wherein the
residual oxide formed in the reaction is removed by
leaching; whereby a high surface area powder is formed in a
process that essentially avoids use of a molten state
reducing agent in the production of the metal (i) or alloy
(ii) or (iii) powder.
Preferred reducing agents used in this reduction
process of the first embodiment are Mg, Ca and/or their
hydrides. Particularly preferred is Mg.
Preferred is the production of Nb and/or Ta
metals, optionally alloyed with each other and/or with
alloying elements, selected from the group consisting of Ti,
Mo, W, Hf, V and Zr.
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A second embodiment of the invention provides for
a two-stage reduction process, comprising the steps of:
(a) providing an oxide or mixed oxide of the
metal(s), the oxide being in a form that is traversible by
gas,
(b) passing a hydrogen containing gas, alone or
with gaseous diluent, through the mass at an elevated
temperature in a manner for partial reduction of the
oxide (s) ,
(c) the porosity of the oxide, temperature and
time of reduction reaction being selected to remove at least
20% of the oxygen contained in the oxide to produce a
suboxide,
(d) reducing the suboxide with reducing metal(s)
and/or hydrides of one or more reducing metals, thereby
substantially completely reducing the oxide to free the
metal portion thereof.
More particularly, the second embodiment of the
invention provides a process for producing (i.) a Nb metal
powder or (ii) the powder of (i) alloyed with one or more
metals selected from the group consisting of Ti, Mo, W, Hf,
V and Zr, comprising the steps of: (a) providing a powdered
oxide or mixed oxides of the metal or metals defined in (i)
or (ii), the powdered oxide or mixed oxides being in a
porous solid form with high access throughout the mass
thereof by a hydrogen containing gas; (b) passing the
hydrogen containing gas through the mass at an elevated
temperature; (c) selecting the porosity of the powdered
oxide or mixed oxides, temperature and time of the reduction
reaction to remove at least 20% of the oxygen contained in
the powdered oxide or mixed oxides to produce a suboxide;
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and (d) further reducing the suboxide in a second stage with
a reducing agent as defined in step (b) above, thereby
substantially completely reducing the suboxide to free the
metal (i) or alloy (ii).
Preferably, the oxide mass provides a void volume
of 90%.
Preferably the reducing metals and/or metal
hydrides are brought into contact with the suboxide in
gaseous form.
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Preferred reducing metals in the second reduction step of this second
embodiment
are Mg and/or Ca and/or their hydrides. Particularly preferred is Mg.
Reduction temperature preferably (for Mg) is selected between 850 C up to
normal
boiling point (1150 C)
The process according to the present invention (both embodiments) specifically
has
been developed to provide capacitor grade tantalum and niobium and tantalum
nio-
bium alloy powders and Ta/Nb materials or application of equivalent purity
and/or
morphology needs. The greatest gap of the state of the art is filled in part
by the
availability of capacitor grade niobium enabled by this invention, but a
segment of
the tantalum art is also enhanced thereby. In all cases the tantaluni and/or
niobium
may be enhanced by alloying or compounding with other materials during the
reduction reaction production of the tantalum/niobium or thereafter. Among the
requirements for such powders is the need for a high specific surface
presintered
agglomerate structure of approximately spherical prin-ary particles which
after
pressing and sintering results in a coherent porous mass providing an
interconilected
system of pore channels with gradually narrowing diameter to allow easy
entrance of
the forming electrolyte for anodization and nlanganese nitrate solution
[Mn(NO3)2]
for manganization.
The reduction of oxides with gaseous reducing agents at least during the
initial re-
duction phase allows for easy control of temperature during reduction to avoid
excessive presintering. Furthermore, as compared to prior art proposals using
liquid
reducing metals, the controlled reduction with gaseous reducing nietals does
not lead
to contamination of the reduced nietal with the reducing metal by
incorporation into
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the reduced metal lattice. It has been found that such contamination mainly
occurs
during the initial reduction of (in case of Nb) Nb205 to NbO,. This at first
appeared
surprising because niobium suboxide (Nb02) contains only 20% less oxygen than
niobium pentoxide (Nb02.5). This effect was traced back to the fact that the
suboxide forms a considerably more dense crystal lattice than the pentoxide.
The
density of NbO2.5 is 4.47 g/cm3, whilst that of Nb02 is 7.28 g/cm3, i.e., the
density
is increased by 1.6 times by the removal of only 20% of the oxygen. Taking
into
account the different atomic weights of niobium and oxygen, a reduction in
volume
of 42% is associated with the reduction of Nb02.5 to Nb02. Accordingly,
Applicants state (without limiting the scope of the invention thereby or being
bound by any
theory) that the effect according to the invention can be explained in that
during the reduction
of the pentoxide magnesium in contact with the oxide is able to diffuse
relatively easily into
the lattice, where it has a high mobility, whereas the mobility of magnesium
in the
suboxide lattice is significantly reduced. Accordingly, during the reduction
of the
suboxide the niaanesium substantially remains on the surface and remains
accessible
to attack by washing acids.
This even applies in case of a controlled reduction with gaseous magnesium.
Obvi-
ously in this case reduction occurs also during the critical initial reduction
to. sub-
oxide only at the surface of the oxide, and magnesium oxide formed during
reduction
does not enter the oxide or suboxide powder. Preferred temperature during
reduction
with magnesium gas is between 900 and 1100 C, particularly preferred between
900
and 1000 C.
Temperature may be increased up to 1200 C after at least 20% of the oxygen is
removed to improve presintering.
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The reduction of the pentoxide with hydrogen produces a suboxide which is
already
sintered with the formation of agglomerates comprising stable sintered
bridges,
wliich llave a favorable structure for use as a capacitor material.
Lower temperatures necessitate longer times of reduction. Moreover, the degree
of
sintering of the metal powders to be produced can be adjusted in a
predeterminable
manner by the choice of reduction temperature and reduction time. The reactors
are
preferably lined with molybdenum slleet or by a ceramic which is not reduced
by H2,
in order to prevent contamination.
Furthermore, the reduction time and reduction temperature should be selected
so that
at least 20% of the oxygen is removed from the pentoxide. Higher degrees of
reduction are not harmful. However, it is generally not possible to reduce
more than
60% of the oxygen within practicable time scales and at tolerable temperature.
After a degree of reduction of 20% or more has been reached, the suboxide is
present. According to this process embodiment the reduction product is
preferably
still held (annealed) for some tinie, most preferably for about 60 to 360
minutes, at a
temperature above 1000 C. It appears that this enables that the new, dense,
crystal
structure can be formed and stabilized. Since the rate of reduction decreases
very
considerably with the degree of reduction, it is sufficient to heat the
suboxide at the
{ reduction temperature under hydrogen, optionally with a slight decrease in
temperature. Reduction and annealing times of 2 to 6 hours within the
temperature
range from I 100 to I500 C are typically sufficient. Moreover, reduction witll
hydrogen llas the advantage that impurities such as F, Cl and C, which are
critical for
capacitor applications, are reduced to less than 10 ppm, preferably less than
2 ppm.
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The suboxide is subsequently cooled to room temperature (<100 C) in the
reduction
apparatus, the suboxide powder is mixed with finely divided powders of the
reducing
metals or metal hydrides and the mixture is heated under an inert gas to the
reduction
temperature of the second stage. The reducing metals or metal hydrides are
preferably used in a stoichiometric amount with respect to residual oxygen of
the
acid earth metal suboxide, and are most preferably used in an amount which is
slightly in excess of the stoichiometric amount.
One particularly preferred procedure consists of using an agitated bed in the
first
stage and of carrying out the second stage, without intermediate cooling, in
the same
reactor by introducing the reducing metals or metal hydrides. If magnesium is
used
as the reducing metal, the magnesium is preferably introduced as magnesium
gas,
since in this manner the reaction to form metal powder can readily be
controlled.
After the reduction whether according to the one-stage or to the two-stage
reduction
process to metal is complete, the metal is cooled, and the inert gas is
subsequently
passed through the reactor with a gradually increasing content of oxygen in
order to
deactivate the metal powder. The oxides of the reducing metals are removed in
the
manner known in the art by washing with acids.
Tantalum and niobium pentoxides are preferably used in the form of finely
divided
powders. The primary grain size of the pentoxide powders should approximately
correspond to 2 to 3 times the desired primary grain size of the metal powders
to be
produced. The pentoxide particles preferably consist of free-flowing
agglomerates
with average particle sizes of 20 to 1000 m, including a specific preference
of a
narrower range of most preferably 50 to 300 pm particle size. The oxide may
also be
provided in the form of agglomerated primary oxide particles with diameters of
between
100 to 1000 nm and an average agglomerate size of 10 to 1000 m (Mastersizer
D50).
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~ Reduction of niobium oxide with gaseous reducing agents can be conducted in
an
agitated or static bed, such as a rotary kiln, a fluidized bed, a rack kiln,
or in a sliding
batt kiln. If a static bed is used, the bed deptli should not exceed 5 to 15
cm, so that
the reducing gas can penetrate the bed. Greater bed depths are possible if a
bed
packing is employed tlirougli which the gas flows froni below. For tantalunl,
preferred equipment choices are described in Exanlple 2 and the paragrapli
between
Examples 2 and 3, below, with reference to FIGS. 1-4.
Niobium powders which are particularly preferred according to the invention
are
obtained in the form of agglomerated primary particles with a primary particle
size of
100 to 1000 nn1, wherein ttle agglomerates have a particle size distribution
as
determined by Mastersizer (ASTM-B822) corresponding to D 10 = 3 to 80 m,
particularly preferred 3 to 7 m, D50 = 20 to 250 Ftm, particularly preferred
70 to
250 m, most preferably 130 to 180gm and D90 = 30 to 400, particularly
preferred
230 to 400 in, most preferably 280 to 350 m. The powders according to the
invention exhibit outstanding flow properties and pressed strengths, which
determine
their processability to produce capacitors. The agglomerates are characterized
by
stable sintered bridges, which ensure a favorable porosity after processing to
fornl
capacitors.
Preferably niobium powder according to the invention contains oxygen in
amounts of
2500 to 4500 ppm/m2 surface and is otherwise low in oxygen, up to 10,000 ppm
nitrogen and up to 150 ppm carbon, and without taking into account a content
of
alloying metals has a maximum content of 350 ppm of other metals, wherein the
metal content is mainly that of the reducing metal or of the hydrogenation
catalyst
metal. The total content of other metals amounts to not more than 100 ppm. The
total content of F, Cl, S is less than 10 ppm.
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Capacitors can be produced from the niobium powders which are preferred
according
to the invention, immediately after deactivation and sieving through a sieve
of mesh
size 400 m. After sintering at a pressed density of 3.5 g/cm3 at 1100 C and
forming
at 40 V these capacitors have a specific capacitance of 80,000 to 250,000
FV/g (as
measured in phosphoric acid) and a specific leakage current density of less
than 2
nA/ FV. After sintering at 1150 C and forming at 40 V, the specific capacitor
capacitance is 40,000 to 150,000 FV/g with a specific leakage current density
of
less than 1 nA,' FV. After sintering at 1250 C and forming at 40 V, capacitors
are
obtained which have a specific capacitor capacitance (as measured in
phosphoric
acid) of 30,000 to 80,000 FV/g and a specific leakage current density of less
than 1
nA/ FV.
The niobium powders which are preferred according to the invention have a BET
specific surface of 1.5 to 30 m2/g, preferably of 2 to 10 m2/g.
Surprisingly it has been found that capacitors can be made from Nb/Ta-alloy
powders in way that the capacitors have an appreciably higher specific
capacitance
obtained from capacitors made from pure Nb-and pure Ta-powers or anticipated
for
an alloy be simple linear interpolation. Capacitances ( FV) of capacitors with
sintered Nb-powder anodes and sintered Ta-powder anodes having the same
surface
area are approximately equal. The reason is that the higher dielectric
constant of the
insulating niobium oxide layer (41 as compared to 26 of tantalum oxide) is
compensated by the larger thickness of the oxide layer per volt (anodization
voltage)
formed durino anodization. The oxide layer thickness per volt of Nb is about
twice
as thick as that formed on Ta (about 1.8 nm/V in the case of Ta and about 3.75
nm/V
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in the case of Nb). The present invention can provide a surface related
capacitancc
( FV/m2) of alloy powder capacitors which is up to about 1.5 to 1.7 higher
than the
expected value from linear interpolation between Nb powder capacitors and Ta
powder capacitors. This seems to indicate that oxide layer thickness per volt
of
anodization voltage of alloy powders of the invention is closer to ttiat of
Ta, whereas
the dielectric constant of the oxide layer is closer to that of Nb. The
foregoing
surprisingly lligh capacitance of the alloy may be associated with a different
structural form of oxide of alloy components compared to structure of oxides
on
surfaces of pure Nb powders. Indeed, preliminary measurements have revealed
that
oxide layer growth of a 15 at. -%Ta -- 85 at.-% Nb alloy is almost 2.75
nm/volt.
The present invention accordingly further comprises an alloy powder for use in
the
manufacture of electrolyte capacitors consisting primarily of niobium and
containing
up to 40 at.-% of tantalum based on the total content of Nb and Ta. Alloy
powder in
accordance with the present invention shall mean that the minor Ta-component
shall
be present in an amount greater than the amount of ordinary impurity of
niobium
~ nietal, e.g. in an amount of more than 0.2% by weight (2000 ppm,
corresponding to 2
at.-% for Ta).
Preferrably, the content of Ta is at least 2 at.-% of tantalum, particularly
preferred at
least 5 at.-% of tantalum, most preferably at least 12 at.-% of tantalum,
based on the
total content of Nb and Ta.
Preferably the content of tantalum in the alloy powders in accordance with the
in-
vention is less than 34 at.-% of tantalum. The effect of capacitance increase
is
gradually increasing up to a ratio of Nb- to Ta-atoms of about 3. Higher than
25 at.-
CA 02331707 2000-11-03
, .
-19-
% Ta based on the total content of Nb and Ta does only slightly furtlier
increase the
effect.
The alloy powders according to the invention preferably have BET'-surfaces
multi-
plied with the alloy density of between 8 and 250 (m2/g) x(g/cm3),
particularly
preferred between 15 and 80 (m2/g) x (g/cm3). The density of the alloy
material may
be calculated from the respective atomic ratio of Nb and Ta multiplied by the
densities of Nb and Ta respectively.
The effect of capacitance increase of alloying is not limited to powders
having the
structure of agglomerated spherical grains. Accordingly the alloyed powders in
accordance of the invention may have a morphology in the form agglomerated
flakes
preferably haviiig liave a BET-surface times density of between 8 and 45
(m2/g) x
(g/cm3).
Particularly preferred alloy powders are agglomerates of substantially
spherical pri-
niary particles having a BET-surface times density of 15 to 60 (m2/g)
x(g/cm3). The
primary alloy powders (grains) may have mean diameters of between 100 to
1500 nm, preferrably 100 to 300 nm. Preferrably the deviation of dianleter of
primary particles from mean diameter is less than a factor 2 in both
directions.
The agglomerate powders may have a mean particle size as determined in
accordance with ASTM-B 822 (Mastersizer) as disclosed for niobium powders
above.
Particularly preferred alloy powders have a ratio of Scott density and alloy
density of
between 1.5 and 3 (g/inch3)!(g/cm3).
CA 02331707 2000-11-03
-20-
Any production method known in the art for the production of capacitor grade
tan-
taluni powder may be used, provided that a precursor is used which is an
alloyed
precursor containing niobium and tantalum approximately at the atomic ratio of
Nb
and Ta desired in the metal powder alloy instead of precursor containing
tantalurn
alone.
Useful alloy precursors may be obtained from coprecipitation of (Nb, Ta)-
compounds from aqueous solutions containing water soluble Nb- and Ta-compounds
e.g. coprecipitation of (Nb, Ta)-oxyhydrate from aqueous solution of
heptafluoro-
complexes by the addition of ammonia and subsequent calcination of the
oxhydrate
to oxide.
Flaked powders may be obtained by electron beani melting of a blend of high
purity
tantalum and niobium oxides, reducing the molten ingot, hydriding the ingot at
ele-
vated temperature, and comminuting the brittle alloy, dehydriding the alloy
powder
and forming it into flakes. The flakes are thereafter agglomerated by heating
to 1100
to 1400 C in the presence of a reducing metal such as Mg, optionally witli
doping
with P and/or N. This process for the manufacture of "ingot derived" powder is
generally knoxvn from US-A 4,740,238 for the production of tantalum flaked
powder
and from WO 98/19811 for niobium flaked powder.
Particularly preferred Nb-Ta-alloy powders having the morphology of
agglomerated
spherical grains are produced froni mixed (Nb, Ta)-oxides by reduction with
gaseous
reducing aoent as described herein.
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~ , .
-21 -
The metal powders produced are suitable for use in electronic capacitors and
other
applications including, e.g. the production of complex electro-optical,
superconductive and other metal and ceranlic compounds, such as PMN structures
and high temperatures form metals and oxide.
The invention comprises the said powders, the methods of producing such
powders,
certain derivative products made from such powders and methods for making such
derivative products.
The capacitor usage can be accompanied by other known artifacts of capacitor
pro-
duction such as doping with ageitts to retard sinter densification or
otherwise
enllance end product capacitance, leakage and voltage breakdown.
The invention enables several distinct breakthroughs in several of its various
fields of
application.
First, the well known high performance tantalum powders for making
computer/telecomnlunications grade solid electrolyte, small size capacitors
(higll
capacitance per unit volume and stable performance characteristics) can now be
made with substantial net savings of cost, complexity and time.
Second, other reactive metals - especially Nb and alloys, e.g. Ta-Nb, Ta-Ti,
Nb-Ti,
can be introduced as replacement for Ta in capacitors in certain applications
with a
cost saving or as replacement for the high eiid Al market with much better
performance, particularly enabling much smaller sizes for equivalent
capacitance and
use of solid electrolyte. Commercial aluminunl electrolytic capacitors use wet
electrolyte systems.
CA 02331707 2005-10-14
30877-28
-22-
Other aspects, features and advantages will be apparent from the following
detailed
description of preferred embodiments taken in conjunction with the
accompanying
drawing in which:
Brief Description of the Drawing
FIGS. 1-4 show sketch outlines of processing systems for practice of the
present
invention;
FIGS. 5A-12C are scanning electron micrographs (SEMs) of powders produced
according to the present invention, including some SEMs of state of the art or
comparison examples of metal powders made otherwise than in accordance with
the
present invention;
FIGS. 13 and 14 are flow charts illustrating diverse usages of the powder and
derivatives; and
FIG. 15 is a schematic representation of an end item according to usage as a
capacitor (one of several forms of capacitor usage).
FIG. 16 is a trace of capacitance and surface area of Ta-Nb alloy powders in
relation
to alloy composition.
Detailed Description of Preferred Embodiments
Example 1 (comparison)
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-23-
A mixture of Ta205 arid magnesium was loaded into a tantalum tray and covered
with tantalum foil. The magnesium stoichionietry was 109% of that required to
completely reduce the tantalum oxide. The mixture was heated at 1000 for six
hours in an argon atniosphere. The mixture was not agitated during the
reduction
process. After cooling, the products were passivated by programmed addition of
oxygen. The result of tiie reduction process was a black spongy material that
was
difficult to break up. The product was leached with dilute niineral acid to
rerr-ove the
magnesium oxide, dried and screened. The yield of the coarse (+40 mesh)
niaterial
was lligh at 25 percent. The impurity content of each (as % or ppm) and
surface
areas (SA, cm2/gm) of the +40 and -40 fractions are given in Table 1.1, below.
Both
the magnesium and oxygen contents were high. The large percentage of coarse
material and poor quality of the product made it unsuitable for use in
capacitor
applications.
Table 1.1
0 N C S Na K Mg Sa
% ppm PPm ppni PPm PPm ppm cm2/gm
+40 mesh 7.6 840 21 <5 <I <10 >7000 17,000
-40 mesh 4.7 413 57 <5 <5 <10 >7000 35,000
Example 2
Referring to FIG. 1, a bed (3) of 200 grams of tantalum pentoxide was placed
on a
porous tantalum plate 4 suspended above magnesium metal chips (5) contained in
a
tantalum boat. The container was covered with a tantalum lid and placed in a
sealed
retort with argon (Ar) passed through the sealed volume via nozzle (6). The
boat
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-24-
was heated to and maintained at 1000 C for six hours in an argon/magnesium gas
atmosphere utilizing a bed (5) of solid magnesium cliips maintained in a
region
wholly separate from the oxide bed. After cooling to rooni temperature, the
product
mixture was passivated by introdttcing argon-oxygen mixtures, containing 2, 4,
8, 15
inches (Hg, partial pressure) of 02 (g), respectively, into the furnace. Each
mixture
was in contact with powder for 30 mintttes. The liold time for the last
passivation
with air was 60 minutes.
The magnesium oxide was separated from the tantalum powder by leaching with
dilute sulfuric acid and then rinsed with high purity water to remove acid
residues.
The product was a free flowing, powder. Samples of the product (designated as
Ta
GR-2D) are shown in scanning electron micrographs (SEMs) at FIGS. 5A, 513, 5C
at
15,700, 30,900 and 60,300 magnifications, respectively, taken in an electron
microscope operated at 15 kilovolts. A comparison is given in FIGS. 5D and 5E
which are 70,000 magnification (x) SEMs of tantalum powder made by sodium
reduction. Properties of the tantalum powder of FIGS. 5A, 513, 5C are given in
Table
2.1, below.
Table 2.1
Surface
Content of Included Chemical Elements (ppm) area
(cm 2/gm
O N C Cr Fe Ni Na K Ca Si
12,900 126 75 <5 23 <5 <I <10 <2 <8 37,600
The oxygen concentration to surface area ratio was consistent with surface
oxygen
only, indicatino, that the tantalum oxide was completel), reduced.
CA 02331707 2005-10-14
30877-28
-25-
Alternate forms of reactor to the one shown in FIG. 1(and discussed in Example
2)
are shown in FIGS. 2-4. FIG. 2 shows a flash reactor 20 with a vertical tube
22 surroun-
ded by a heater 24, a feed source 25 of metal oxide and a source 26 of
reducing agent
(e.g. Mg) vapor (mixed in argon), an argon outlet 26' and a collector 28 for
metal and
oxide of the reducing agent. Valves V1, V2 are provided. Particles of the
oxide drop
through the tube and are flash reduced. FIG.3 shows a rotary kiln 30 with an
inclined rotating tube 32, heater 34, oxide hopper 35, gas source 36 (reducing
agent and
diluent, e.g. argon), outlet 36', and collector 38 for metal and reducing
agent oxide.
FIG. 4 shows a multiple hearth furnace 40 with a retort 42 containing rotary
trays 43
and splined paddles 43', heater 44, oxide source 45, gas source 46, exit 46'
and collector
48. Still other forms of reactors such as conventional per se fluid bed
furnace reactors
or Contop, KIVCET types can be used.
Example 3
Tantalum powder with surface area of 57,000 cm2/gm made according to the
procedure in Example 2 was deoxidized by blending the powder with 2 W/W% Mg
and heating at 850 C for two hours in an argon atmosphere. Separation of
reducing
agent source and oxide is not necessary in this follow up deoxidation step.
The
deoxidized powder was allowed to cool and then passivated, leached, and dried.
A
SEM (100,000 x) of the deoxidized (finished) powder appears at FIG. 7A and a
SEM
(70,000 x) of finished sodium reduced powders appears at FIG. 7B. the
morphology
differences are apparent. After doping with 100 ppm P by adding an appropriate
amount of NH4H2PO4, the powder was pressed into pellets weighing 0.14 grams at
a
press density of 5.0 g/cc. A SEM of the further deoxidized powder is given at
FIG.
6. The pellets were sintered in vacuum at 1200 C for 20 minutes. The pellets
were
CA 02331707 2000-11-03
-26-
anodized to 30 volts in 0.1 volume percent (V/V%) H3PO4 solution at 80 C. The
formation current density was 100 mA/gm and the hold time at the formation
voltage
was two hours. The average capacitance of the anodized pellets was105, 000 F
(V)/gm and the leakage current nleasured after five minutes application of 21
V was
0.1 nA/ F (V).
Exaniplc 4
Powder with surface area of 133,000 cm2/gm and bulk density of 27.3 g/m3 niade
as
described in Example 2 was treated as in Example 3. A SEM (56,600 x) of the
finished powder appears at FIG. 7C. Pellets made from the deoxidized powder
were
anodized to 16V using the conditions in Example 3. The average capacitance of
the
anodized pellets was 160,000 F (V)/gm.
Cxample 5
Nine hundred grams of Ta205 was reduced with gaseous magnesium at 900 C for
two hours. The magnesium oxide was removed from the reduction product by
leaching with dilute sulfuric acid. The resulting powder had a surface area of
70,000
cm2/gm and was deoxidized at 850 C for two hours using 8 W/W% magnesium.
One (1.0) W/W% NH4CI was added to the charge to nitride the tantalum. The
deoxidized powder was treated as described in Example 3. The P doping level
was
200 ppm. The powder was deoxidized again using the same time and temperature
profile with 2.0 W/W% Mg and no NHaCI. Residual magnesium and magnesium
oxide were removed by leaciiing with dilute mineral acid. The chemical
properties of
the poxvder are given in Table 5.1, below. The powder liad a surface area of
9,000
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=
-27-
cm2/gm and excellent flowability. Pressed pellets were sintered at 1,350 C for
twenty minutes and anodized to 16V in 0.1 VN% H3P04 at 80 C.
The capacitance of the anodized pellets was 27,500 F (V)/gnl and the leakage
was
0.43 nA/ F (V).
Table 5.1
Cliemical Element (ppm)
0 N C Cr Fe Ni Na K Ca Si
2610 2640 95 8 18 <5 1 <10 <2 41
Example 6
500 granis of Ta205 were reduced at 1,000 C for six hours witli gaseous
magnesium.
Properties of the primary powder so produced are given in Table 6.1, below:
Table 6.1
0, ppm N, ppm C, ppm Na, ppm K, ppln SA, cmZ/g
19,000 1693 49 <1 <10 60,600
The primary powder was deoxidized at 850 C for two hours. 4 W/W% Mg and I
W/W% NH
4CI were added. MgO was leached with mineral acid. Then the powder
was doped at 200 ppnl P by adding the equivalent amount of NH4H2PO4. The pow-
der was deoxidized for the second tinie at 850 C for two hours and then
nitrided at
CA 02331707 2000-11-03
-28-
325 C by adding a gaseous mixture containing 80% argon and 20% nitrogen. Some
properties of the finished powder are given in Table 6.2, below.
Table 6.2
Table 6.2
0-ppm N, ppm C, ppm Na, ppm K, ppm SA, cm2/g
6050 3430 54 <1 <10 24,300
Pellets were made from the powder at a press density of 5.0 gm/cc. The
sintered
pellets were anodized at 80 C to 16 volts in 0.1 W/W% H3POq solution.
Capacitances and leakages as a function of sintering temperature are given in
Table
6.3, below.
Table 6.3
Sintering Capacitance Leakage
Temperature ( C) F (V)/gm A/ F(V)
1,200 143,000 0.77
1,250 121,000 0.88
1,300 96,000 1.01
Example 7 (comparative)
Potassium heptatluoroniobate (K2NbF7) was reduced with sodiurn using a stirred
reactor molten salt process siniilar to the ones described by Hellier et al.
and 1-fildretli
CA 02331707 2000-11-03
-29-
et al., US patent 5,442,978. The diluent salt was sodium chloride and the
reactor was
nlade ftom Inconel alloy. The niobium nletal powder was separated from the
salt
matrix by leaching with dilute nitric acid (HNO3) and then rinsing witli
water.
Selected physical and chemical properties are given in Table 7. 1, below. The
very
high concentrations of the metallic elements, nickel, iron and chromium, make
the
powders unsuitable for use as capacitor grade material. The contamination
resulted
because of the inherent corrosive nature of the K2NbF7. This property nlakes
the
sodium reduction process unsuitable for making capacitor grade niobium powder.
Table 7.1
Sample SA SBD FAPD O(ppm) Ni Cr Fe
1 13820 8.7 1.76 6080 18000 2970 2660
2 11700 9.4 1.48 4930 11300 4790 2060
SBD=Scott Bulk Density (g/in'), FAPD=Fislier Average Particle Diameter ( )
Cxaniple 8
Two hundred grams of niobium pentoxide was reduced as described in Example 2.
The resulting product was a free flowing black powder and had a surface area
of
200,800 cm2/gm. The passivated product was leached with dilute nitric acid
solution
to remove maanesium oxide and residual magnesium and then with high purity
water
to remove residual acid. This material was blended with ten (10.0) W/W% Mg and
deoxidized at 850 C for two hours. Physical and chemical properties of the
powder
are listed in table 8.1, beloNti-. The powder was doped with 100 ppni P as
described in
Example 3.
CA 02331707 2000-11-03
-30-
Table 8.1
Physical and Chemical Properties of Niobium Powder
Surface
Chemical Element (ppm) Area
cm2/gm
0 N C Cr Fe Ni Na K Ca Si
13000 620 40 27 45 21 8 I 3 26 40,900
SEMs (70,000 x) appear at FIGS. 8A and 8B, respectively, for niobium powders
produced by liquid sodium (Ex. 7) and magnesium gas (Ex. 8) reduction. Note
the
clustering of small particles as barnacles on large ones is much more
pronounced in
FIG. 8B than in 8A. FIGS. 8C, 8D are SEMs (2,000 x) of, respectively niobium
powder as produced by sodium reduction and magnesium gas reduction.
The niobium powder produced by liquid sodium reduction has large (>700 nm)
joined (300 nm+) grains protruding and facets that give the product a blocky
shape
and fine grain material (order of 10 nm, but some up to 75 nm) as barnacles
while the
niobium powder produced by magnesiuni gas reduction has a base grain_size of
about
400 nm and many smaller grains of about 20 nm thereon many of which smaller
grains are themselves agglomerates of up to 100 nm in size.
Example 9
Pellets weighing 0.14 gm were prepared from the niobium powder produced in
Example 8. The pellets were anodized in 0.1 V/V% 143P04 solution at 80 C. The
current density was 100 mA/gni and the hold time at the formation voltage was
two
CA 02331707 2000-11-03
.
`
-31-
hours. Electrical results as a function of pellet press density, formation
voltage and
sintering temperature are given in Table 9.1, below.
Table 9.1
Sunimary of Electrical Properties (capacitance, leakage) of Niobium Powder at
3.0, 3.5 (g/cc) Press Densities
Sintering Capacitance Leakage
Temperature ( F (V)/gm) (nA/ F(V))
( C)
3.0 3.5 3.0 3.5
16 V Formation
1300 29,500 20,000 1.6 4.7
1350 21,000 16,000 0.7 1.5
40 V Formation
1250 53,200 44,500 2.1 4.0
1300 31,000 22,300 1.2 4.7
1350 26,500 20,000 0.7 1.0
Example 10
Niobium oxide was reduced witli gaseous magnesium as described in Example 8.
The resulting powder was deoxidized twice. During the first deoxidation, 2.0
W/W% NH4CI was added to the charge to nitride the powder. The deoxidation
conditions Nvere 850 C for two hours with 7.0 W/W% Mg. After leaching and
CA 02331707 2000-11-03
. .
-32-
drying, the powder was doped with 200 ppm P. The second deoxidation was
carried
out at 850 C for two hours using 2.5 W/W% Mg. The finished powder has a
surface
area of 22,000 cm2/gm and good flowability. The chemical properties are given
in
Table 10.1, below. Pellets were anodized to 16 volts in 0.1 V/V% H3PO4
solution at
80 C using a current density of 100 mA/g and a two-hour hold. The electrical
properties are given in Table 10.2, below.
Table 10.1
Chemical Clenient (ppm)
0 N C S Cr Fe Ni Si Ta
7490 8600 166 9 <20 114 <20 34 <200
CA 02331707 2000-11-03
-33-
Table 10.2
Electrical Properties
Sintering Capacitanee Leakage
Temperature ( C) ( F(V)/gm (nA/ F(V)
1250 68,000 0.24
1300 34,500 0.14
1350 11,300 0.32
Exatnple 11
~
a) The Nb205 used had a particle size of 1.7 m as determined by FSSS (Fisher
Sub Sieve Sizer) and comprised the following contents of impurities:
Total (Na, K, Ca, Mg) 11 ppm
Total (Al, Co, Cr, Cu, Fe, Ga,
Mn, Mo, Ni, Pb, Sb, Sn,
Ti, V, W, Zn, Zr) 19 ppm
Ta 8 ppm
Si 7 ppin
C <1 ppm
C1 <3 ppm
F 5 PPm
S <1 PPm
CA 02331707 2000-11-03
-34-
The Nb205 was passed in a molybdenum boat through a sliding batt kiln,
under a slowly flowing hydrogen atmosphere, and was maintained in the hot
zone of the kiln for 3.5 hours.
The suboxide obtained had a composition corresponding to Nb02.
b) The product was placed on a fine-mesli grid under which a crucible was
situated which contained magnesium in 1.1 times the stoichionletric amount
with respect to the oxygen content of the suboxide.
The arrangement comprising the grid and crucible was treated for 6 liours at
IO00 C under an argon protective gas. In the course of this procedure, the
magnesium evaporated and reacted with the overlying suboxide. The kiln
was subsequently cooled (<100 C) and air was gradually introduced in order
to passivate the surface of the metal powder.
The product was washed with sulfuric acid tintil magnesiunl could no longer
be detected in the filtrate, and thereafter was washed tintil neutral with
deionized water and dried.
Analysis of the niobium powder gave the following impurity contents:
0 20,000 ppm (3300 ppm/m2)
Mg 200 ppm
Fe 8 ppm
Cr 13 ppm
Ni 3 ppni
CA 02331707 2000-11-03
-35-
Ta 110 ppm
C 19ppm
N 4150 ppm
The particle size distribution, as determined by Mastersizer, corresponded to
D 10 4.27 gm
D50 160.90 m
D90 318.33 }Lm
The primary grain size was determined visually as about 500 nm. The Scott
bulk density was 15.5 g/inch3. The BET specific surface was 6.08 mz/g. The
flowability, determined as the Hall Flow, was 38 seconds.
c) Anodes with a diameter of 3 mm, a length of 5.66 nim, an anode mass of 0.14
g and a pressed density of 3.5 g/cm3 were produced from the niobiunl powder
by sintering on a niobium wire for the times and at the temperatures given in
Table 11.1.
The pressed strength of the anodes, as determined according to Chatillon, was
6.37 kg. The anodes were formed at 80 C in an electrolyte containing 0.1 %
by volume of H3PO4 at a current density of 100/150 mA at the voltage given
in Table 11.1 and the capacitor characteristics were determined; see Table
11.1
CA 02331707 2000-11-03
-36-
Table 11.1
Sample Sintering Sintered Wire draw- Forming Capacitance Leakage
Temp. density ing strength voltage FV/g current
/tinie g/cm' N V nA/ FV
oC
min
a 1250/20 5.1 16 41,126 0.47
b 5 40 41,725 0.7
c 5 70 23,721 2.13
d 1150/20 3.9 35.6 16 111,792 0.77
e 4 35.6 40 147,292 0.43
f 1100/20 3.75 36.6 16 194,087 0.4
g 3.7 36.1 40 194,469 0.36
Example 12
Example 11 Nvas repeated, with the difference that the tenlperature in the
first
reduction stage was 1300 C.
The metal powder had the following properties:
Mastersizer D 10 69.67 m
D50 183.57 gm
D90 294.5 m
Primary grain size (visual) 300 - 400 nm
BET specific surface 5 rn2/g
CA 02331707 2000-11-03
=
-37-
Free-flowing.
The pressed strength was extremely high:
13 kg at a pressed density of 3.5 g/cm3, and
8 kg at a pressed density of 3 g/cm3.
After sintering at 1100 C for 20 minutes (pressed density 3 g/m3), and after
forrning
at 40 V, a capacitance of 222,498 FV/g and a leakage current of 0.19 nA/ FV
were
measured.
Cxample13
This example shows the effect of the reduction tenlperature in the first stage
on the
properties of the niobium powder:
Three batches of niobiunl pentoxide were treated for 4 hours under hydrogen at
1100 C, 1300 C or 15000C, under conditions which were otherwise the same.
The batches were subsequently reduced to niobium metal witli Mg gas (6 hours,
1000 C). The MgO which was formed in the course of the reaction, together with
excess Mg, were washed out with sulfuric acid. The following powder properties
were obtained:
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=
-38-
Reduction temperature 1100 C 1300 C 1500 C
Suboxide:
BET m2/gO 1.03 0.49 0.16
Hall FlowZ) non- 25 g in 25 g in
flowing 48 sec. 20 sec.
Niobium metal:
BET m2/g 9.93 7.8 5.23
FSSS m3) 0.6 0.7 6.8
Hall Flow non- 25 g in 25 g in
flowing 85 sec. 19 sec.
SD g/inch4) 16.8 16.5 16.8
Mg ppm 240 144 210
O ppm 40,000 28,100 16,600
BET specific surface
2)
flowability
3) particle size as determined by Fisher Sub Sieve Sizer
4) bulk density
Example 14
A(Nbx, Tal-r)2O5 precursor is prepared by coprecipitation of (Nb, Ta)-
oxyhydrate
from mixed aqueous solution of niobium and tantalum heptafluorocomplexes by
the
addition of ammonia with stirring and subsequent calcination of the oxyhydrate
to
oxide.
CA 02331707 2000-11-03
-39-
A lot of the mixed oxide powder having a nominal contposition of Nb: Ta =
90:10
(weight ratio) was placed in a molybdenum boat and passed tlirough a sliding
batt
kiln under slowly flowing hydrogen atmosphere and was maintained in the hot
zone
of the kiln for 4 hours at 1300 C. After cooling down to rooni temperature the
composition was determined from weiglit loss to be approxiniately
(Nb0.944Ta0.054)0=
The suboxide was placed on a fine mesh grid under which a crucible was
situated
which contained magnesiuni in 1,2 times the stoichiometric amount with respect
to
the oxygen content of the suboxide. The arrangement comprising grid and
crucible
was treated for 6 hours at 1000 C under an argon protective gas. The kiln was
subsequently cooled to below 100 C and air was gradually introduced in order
to
passivate the surface of the metal powder.
The product was washed kvith sulfuric acid until nlagnesiuni could no longer
be
detected in the filtrate, and thereafter washed until neutral with deionized
water and
dried.
Analysis of the alloy powder gave a tantalum content of 9.73 wt.-% and the
following impurity contents (ppm):
0: 20500, Mg: 24, C: 39, Fe: 11, Cr: 19, Ni: 2, Mo: 100.
The primary grain size as determined visually was roughly 450 nm. BET specific
surface was 6.4 m2/g, Scott density 15.1 g/1n3, particle size (FSSS) was 0.87
m.
CA 02331707 2000-11-03
-40-
Anodes with a diameter of 2.94 mm, a length of 3.2 mm and a pressed density of
3.23 g/cm3 were produced from the alloy powder by sintering on a niobium wire
for
20 minutes at 1 150 C. Sintered density was 3.42 g/cm3. The electrodes were
anodized in an electrolyte containiiig 0.25% of H3PO4 until a final voltage of
40 V.
The capacitor characteristics were determined by using a 10% H3PO4 aqueous
solu-
tion as follows: Capacitance: 209117 FV/g, Leakage current: 0.55 nA/ Pg.
Exam le 1~
An alloy powder was prepared as in Example 14, using an oxide powder with
nominal composition of Nb:Ta = 75:25 (weiglit ratio).
Analysis of the nietal alloy powder gave a tantalunl content of 26,74 wt.-%
and the
following impurity contents (ppm):
0: 15000, Mg: 25, C: 43, Fe: 9, Cr: 20, Ni: 2, Mo: 7, N: 247.
The primary grain size as deterniined visually was roughly 400 nm. BET
specific
surface was 3.9 m2/g, Scott density 17.86 g/in3, particle size (FSSS) was 2.95
m,
Hall Flow 27.0 s.
Anodes with a diameter of 2.99 mm, a length of 3.23 mm and a pressed density
of
3.05 g/cm~ were produced from the alloy powder by sintering on a niobium wire
for
20 minutes at 1,150 C. Sintered density was 3.43 g/cm3. The electrodes were
anodized in an electrolyte containing 0.25% of H3PO4 until a final voltage of
40 V.
CA 02331707 2000-11-03
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The capacitor characteristics were determined by using a 10%H3PO4 aqueous
solution as follows: Capacitance: 290173 FV/g, Leakage current: 0.44 nA/ Fg.
Exaninle 16
Tantalum hydroxide was precipitated from an aqueous tantalum fluorocomplex
solution by addition of anlmonia. The precipitated hydroxide was calcined at
1100 C
for 4 hours to provide a Ta205 precursor with the following physical data:
average
particle diameter with Fisher Sub Sieve Sizer (FSSS): 7.3 m, bulk density
(Scott):
27.8 g/inch3, specific surface area (BET): 0.36 mz/g particle size
distribution with
laser diffraction on Master Sizer S, measured without ultrasound: D10 = 15.07
m,
D50 = 23.65 .m, D90 = 34.03 m.
The morphology of agglomerated spheres is sllown on Fig. 9A -9C (SEM-
pictures).
300 g of the precursor pentoxide was placed on the screen and 124 g Mg (1.5
times
the stoichio-netric amount necessar), to reduce the pentoxide to nietal) was
placed on
the bottom of a retort shown in Fig. 1.
The retort was evacuated, filled with argon and heated to 950 C for 12 hours.
After
cooling to below 100 C, and passivation the product was leached with an
aqueous
solution containing 23 wt.-% sulfuric acid and 5.5 wt.-% hydrogen peroxide and
thereafter washed with water until neutral. The product was dried over night
at 50 C
and screened < 400 m.
The tantalum powder showed the following analytical data:
Average particle size (FSSS): 1.21 m,
bulk density (Scott): 25.5 g/inch,
3
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BET surface: 2,20 m2/g,
good flowability,
MasterSizer D 10 = 12.38 m, D50 = 21.47 gm, D90 = 32.38 nl,
morphology: see Fig. 1 OA - 10C (SEM-pictures).
Chenlical analysis:
0: 7150 ppm
N: 488 ppm
H: 195 ppm
C: 50 ppm
Si: 30 ppm
F: 2 ppm
Mg: 6 ppm
Na: I ppm
Fe: 3 ppm
Cr: <2 ppm
Ni: < 3ppm.
The powder was soaked with gentle stirring with NH4H2PO4-solution containing
1 mg P per nil, dried over night at 50 C for doping with 150 ppin P and
screened
< 400 m.
Capacitor anodes were prepared from 0.047 g of Ta-powder each at pressed
density
of 5.0 g/cm3 by sintering at 1260 C with 10 minutes holding time.
Forming current density was 150 mA/g with 0,1 wt.-% H3PO4 solution as forming
electrolyte at 85 C until final voltage of 16 V which was held for 100
minutes.
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Test results:
Sintered density: 4.6 g/cm3,
capacitance: 100 577 FV/g
leakage current: 0.73 nA/gFV.
Example 17
High purity optical grade Ta205 was calcined first at 1700 C for 4 hours and
thereafter for 16 hours at 900 C to provide for more compact and coarser
precursor
particles. Pliysical properties of the pentoxide powder are:
Average particle size (FSSS): 20 m
bulk density (Scott): 39 g/inch3
Screen analysis: 400-500 m 8.7 %
200-400 m 63.6 %
125-200 m 15.0%
80-125 m 7.2 %
45- 80 m 3.8%
<45 m 1.7%
Morphology is shown in Fig. I IA-1 1C (SEM-pictures).
The oxide powder was reduced to metal as described in example 16, however at
1000 C for 6 liours.
Leaching and P-doping was as in example 16.
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The tantalum powder showed the following analytical data:
Average particle size (FSSS): 2.8 m,
bulk density (Scott): 28.9 g/inch3,
BET surface: 2.11 m2/g,
flowability through nonvibrated funnel with 60 -angle and 0,1 inch opening: 25
g in
35 seconds,
Master Sizer D 10 = 103.29 m, D50 = 294.63 m, D90 = 508.5 m,
morphology: see Fig. 12A - 12C (SEM-pictures).
Chemical analysis:
O: 7350 ppm
N: 207 ppm
H: 174 ppm
C: 62 ppm
Mg: 9 ppm
Fe: 5 ppm
Cr: <2 ppm
Ni: < 3ppm.
P: 150 ppm
Capacitor anodes were prepared and anodized as in example 16.
Test results:
Sintered density: 4.8 g/cm3
Capacitance: 89 201 FV/g
I_eakage current: 0.49 nA/pFV
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-45 -
A second series of capacitors were prepared the same way, however with
sintering
temperature raised to 1310 C.
Test results:
Sintered density: 5.1 g/cm3
Capacitance: 84 201 FV/g
Leakage current: 0.68 nA/ FV
Example 18
Several samples, each approximately 25 grams, of W03, Zr02, and V203 were
reduced individually with gaseous magnesiunl at 950 C for 6 hours. The
reduction
products were leaclled with dilute sulfuric acid to remove residual magnesium
oxide.
The product was a black metal powder in each case. The tungsten and zirconium
powders had oxygen contents of 5.9 and 9.6 W/W% respectively, indicating that
the
nletal oxides were reduced to the metallic state.
The present process appears to represent the only demonstrated way of making
high
quality chemically reduced niobium powder. The reduction of the metal oxide
with a
gaseous reacting agent, such as magnesium, as shown herein is thus
particularly
suitable for producing powders useable as metal-metal oxide capacitor
substrates.
Although the reduction process was carried out with the metal oxide in a bed
in con-
tact with a source of magnesium gas, the reduction can take place in a
fluidized bed,
rotary kiln, flash reactor, multiple hearth or similar systenls provided the
magnesium
or other reducing agent is in the gaseous state. The process will also work
xvitli other
nietal oxides or metal oxide mixtures for which the. reduction reaction with
gaseous
magnesium or otlier reducing agent has a negatix=e Gibbs free energy cllange.
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-46-
There are several advantages to the gaseous reduction processes described
herei .
Treatment of the reduction products is much less complicated and expensive
than
post reduction work-up of tantalum powder produced by liquid phase reactions
such
as the sodium reduction of K2TaF7 in a molten salt system. No fluoride or
cliloride
residues are produced in the present process. This eliminates a potentially
serious
disposal problem or the need to institute an expensive waste recovery system.
The
reduction of metal oxides with gaseous reducing agents gives powders witll
much
higher surface areas than powders produced by the molten salt/sodium reduction
process. The new process easily makes powders witli very higli surface area
compared to the traditional method; the potential for making very high
performance
capacitor grade powders is great with magnesium or other gaseous reducing
agent.
The present invention further for the first time demonstrates the superiority
of Ta-Nb
alloy powders for use in the production of capacitors.
Fig. 16 shows the ratio of maximum obtainable capacitance ( FV/g) a.nd BET-
surface of powder (m2/g) in relation to the alloy composition. A and C
represent pure
Ta-, Nb-powders, respectively, as measured in present Example 16. B represents
the
highest known values of pure Ta powder capacitors as disclosed in Examples 2,
5
and 7 of WO 98/37249. Line 1 represents expectable values for alloy powder
capacitors from linear interpolation from pure Ta, and Nb powder capacitors. E
represents a fictive Nb-powder capacitor wherein the insulating oxide layer
has the
same thickness per volt as in Ta powder capacitors, however, the dielectric
constant
of niobium oxide differs. Line 11 represents linear interpolation between B
and E. D
represents a nieastired value of 25 %vt.-% Ta/75 wt.-% Nb alloy powder
capacitor as
presented in present Exaniple 15. Curve III represents the estiniated
dependency of
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= ~
-47-
capacitance on alloy composition of alloy powder capacitors in accordance with
the
present inventioii.
FIG. 13 is a block diagram of steps for achieving an electrolytic capacitor
usage of
the invention. The steps comprise reduction of metal oxide with gaseous
reducing
agent; separation of reduction agent oxide from a mass of resultant metal;
breakdown
to powder forin and/or primary powder particle size; classification;
optionally,
presinter to establish agglomerated secondary particles (controlled mechanical
rnethods and control of original reduction or separation steps also being
affective to
establish agglomerates); deoxidation to reduce the oxygen concentration;
compaction
of primary or secondary particles to a porous coherent mass by cold isostatic
pressing
with or without use of compacting binders or lubricants; sintering to a porous
anode
form (which can be an elongated cylindrical, or slab or of a short length from
such as
a chip); anode lead attachment by embedding in the anode before sinteriilg or
welding to the sintered anode'compact; forming the exposed metal surfaces
within
the porous anode by electrolytic oxidation to establish a dielectric oxide
layer; solid
electrode impregnation by impregnating precursors into the porous mass and
pyrolysis in one or more stages or other methods of impregnation; cathode
completioil; and packaging. Various additional steps of cleaning and testing
are not
sllown. The end product is illustrated (in a cylindrical form) in FIG. 15 as a
Ta or Nb
(or Ta-Nb- alloy) capacitor 101 in partial cut-away form as a porous Ta or Nb
(or Ta-
Nb alloy) anode 102, impregnated with a solid electrolyte, surrounded by a
counter-
electrode (cathode) 104 and packaging sheath 105 with a dense lead wire 106 of
Ta
or Nb (generall), niatchin? the powder composition) that is joined to the
anode by a
weld joint 107. As stated above, other known per se capacitor fornis
(different stlape
factors, different nietals, different electrolyte systems anode lead joinder,
etc.) are
accessible through the present invention.
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-48-
PIG. 14 is a block diagram collectively illustrating production of some of the
other
derivative products and uses of the invention including use of the powders as
slips, in
molding and loose pack form for further reaction and/or consolidation by way
of
sintering, liot isostatic pressing (H.I.P.) or in sinter/H.I.P. methods. The
powders per
se and/or as consolidated can be used in making composites, in combustion, in
chemical synthesis (as reactants) or in catalysis, in alloying (e.g.
ferrometallurgy)
and in coatings. The consolidated powders can be used to make mill products
and
fabricated parts.
In some instances the end use products made using the gas reduction produced
powders will resemble state of the art powders made witll state of the art
(e.g.
reduced) powders and in other instances the products will be novel and have
unique
physical, chemical or electrical characteristics resulting from the unique
forms as
described lierein of the powders produced by reduction by gaseous reducing
agents.
The processes of going from powder production to end product or end use are
also
modified to the extent the powdets, and methods of producing the same, produce
inodified impurity profiles and morphology.
The mill products and fabricated parts manufacture can involve remelting,
casting,
annealing, dispersion strengthening and other well known per se artifacts. The
end
products made through further reaction of the metal powders can include high
purity
oxides, nitrides, silicides and still further derivatives such as complex
ceramics used
in ferroelectrics and in optical applications, e.g. perovskite structtire PMW
compounds.
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. t.
-49-
It will now be apparent to those skilled in the art that other etnbodiments,
improve-
ments, details, and uses can be made consistent with the letter and spirit of
the fore-
going disclosure and within the scope of this patent, wliich is liniited only
by the
following claims, construed in accordance with the patent law, including the
doctrine
of equivalents.
