U.S. patent application number 11/688000 was filed with the patent office on 2007-09-20 for valve metal ribbon type fibers for solid electrolytic capacitors.
Invention is credited to William T. Nachtrab, James Wong, Terence Wong.
Application Number | 20070214857 11/688000 |
Document ID | / |
Family ID | 38523224 |
Filed Date | 2007-09-20 |
United States Patent
Application |
20070214857 |
Kind Code |
A1 |
Wong; James ; et
al. |
September 20, 2007 |
VALVE METAL RIBBON TYPE FIBERS FOR SOLID ELECTROLYTIC
CAPACITORS
Abstract
A method for making superconducting material useful for forming
electrolytic devices comprising the steps of establishing multiple
valve metal rods in a primary billet of a ductile material; working
the primary billet to a series of reduction steps to form said
valve metal rods into a plurality of elongated elements surrounded
at least in part by the ductile material; cutting the elongated
elements from step (b) and bundling the cut elements to form a
secondary billet; working the secondary billet through a series of
reduction steps followed by rolling to final thickness; removing
the ductile material, whereby to leave valve metal elongated
fibers; and sintering the elongated fibers from step (e) under
vacuum.
Inventors: |
Wong; James; (Shrewsbury,
MA) ; Wong; Terence; (Shrewsbury, MA) ;
Nachtrab; William T.; (Shrewsbury, MA) |
Correspondence
Address: |
HAYES SOLOWAY P.C.
3450 E. SUNRISE DRIVE, SUITE 140
TUCSON
AZ
85718
US
|
Family ID: |
38523224 |
Appl. No.: |
11/688000 |
Filed: |
March 19, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60783329 |
Mar 17, 2006 |
|
|
|
Current U.S.
Class: |
72/275 ;
204/291 |
Current CPC
Class: |
B22F 2998/10 20130101;
H01G 9/0525 20130101; C25D 11/04 20130101; B21C 33/004 20130101;
C25D 11/02 20130101; H01G 9/052 20130101; B22F 2998/00 20130101;
B22F 2998/10 20130101; B22F 3/002 20130101; C25D 11/26 20130101;
B21C 37/047 20130101; B22F 3/002 20130101; B22F 2998/00 20130101;
B22F 3/1007 20130101; B22F 1/004 20130101; B22F 2201/20 20130101;
B22F 3/1007 20130101 |
Class at
Publication: |
072/275 ;
204/291 |
International
Class: |
C25B 11/04 20060101
C25B011/04; B21C 1/00 20060101 B21C001/00 |
Claims
1. A method for making valve metal fibers useful for forming
electrolytic capacitors comprising the steps of (a) establishing
multiple valve metal rods in a primary billet of a ductile
material; (b) working the primary billet to a series of reduction
steps to form said valve metal rods into a plurality of elongated
elements surrounded at least in part by the ductile material; (c)
cutting the elongated elements from step (b) and bundling the cut
elements to form a secondary billet; (d) working the secondary
billet through a series of reduction steps followed by rolling to
the elongated elements into flattened fibers having a width to
thickness aspect ratio of at least about 10 to 1; (e) removing the
ductile material, whereby to leave valve metal elongated flattened
fibers; and (f) sintering the elongated flattened fibers from step
(e) under vacuum.
2. The method of claim 1, wherein the ductile material comprises a
ductile metal.
3. The method of claim 2, wherein the ductile metal comprises
copper.
4. The method of claim 1, wherein the sintering is conducted at a
temperature in the range of 1300.degree. C. to 1800.degree. C. for
a time period of 10 to 60 minutes.
5. The method of claim 4, wherein the sintering is conducted at a
temperature in the range of 1300.degree. C. to 1500.degree. C. for
a period of 10 to 50 minutes.
6. The method of claim 5, wherein the sintering is conducted at a
temperature of about 1500.degree. C. for about 50 minutes.
7. The method of claim 1, including the step of anodizing the
sintered flattened fibers from step (g).
8. The method of claim 1, including the step of twisting the
flattened filaments before sintering.
9. The method of claim 1, wherein the valve metal comprises
tantalum.
10. The method of claim 1, wherein the valve metal comprises
niobium.
11. The method of claim 1, wherein the valve metal comprises
aluminum.
12. The method of claim 1, wherein the valve metal comprises
vanadium.
13. An electrolytic capacitor comprising an anode formed of valve
metal filaments of substantially uniform thickness within a range
of 0.3-1.0 microns, and having a specific capacitance in excess of
about 10,000 CV/g.
14. The capacitor of claim 13, wherein the valve metal comprises
tantalum.
15. The capacitor of claim 13, wherein the valve metal comprises
niobium.
16. The capacitor of claim 13, wherein the valve metal comprises
aluminum.
17. The capacitor of claim 13, wherein the valve metal comprises
vanadium.
18. The capacitor of claim 13, wherein the filaments comprise
ribbon-like fibers.
19. The capacitor of claim 18, wherein the ribbon-like fibers have
a width to thickness aspect ratio of at least about 10 to 1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Application Ser. No. 60/783,329, filed Mar. 17, 2006.
BACKGROUND OF THE INVENTION
[0002] Solid electrolytic capacitors are made of valve metals,
which are metals such as tantalum, aluminum, niobium, vanadium and
the like. For high reliability devices, tantalum is the preferred
metal, and efforts to improve the performance of capacitors made of
tantalum are highly desired. Miniaturization is one of the main
technology drivers in the electronics industry. For capacitors,
miniaturization is achieved by increasing volumetric efficiency,
which is the normalized capacitance per volume or CV/cm.sup.3 or
normalized capacitance per gram or CV/g. The capacitance (C) of a
dielectric is given by: C=.epsilon..sub.0.epsilon.A/d where
.epsilon..sub.0 is the permeability in vacuum, .epsilon. is the
dielectric constant of the anodic oxide layer, and A and d are the
surface area and thickness of the oxide respectively. Since
.epsilon..sub.0, is a physical constant and .epsilon. is a material
property which is fixed by the dielectric constant of the valve
metal, the only parameters that can be manipulated to enhance
volumetric efficiency are area (A) and thickness (d). For practical
purposes, the thickness of the anodic oxide film is set by
reliability considerations. For a given voltage rating, a thinner
anodic oxide layer will provide less resistance to dielectric
breakdown leading to lower reliability. Thus, the only feasible
means to improve volumetric efficiency is to increase the available
surface area by increasing the specific surface area of the valve
metal substrate on which the anodic oxide layer is formed.
[0003] The specific surface area depends on the morphology of the
substrate on which the dielectric film is produced. For tantalum
powder, considerable development has pushed the technology to
exceptionally high CV/g levels. However, as reported in Y.
Pozdeev-Freeman, "How Far Can We Go With High CV Capacitors",
T.I.C. Bulletin, No. 122, June 2005, pp 4-8, these high CV powders
suffer from extremely rapid fall-off in CV/g with increasing
formation voltages. As a consequence these powders are generally
useful for only low voltage applications, and there is still a need
for the development of higher CV/g tantalum substrates for solid
capacitors rated in range of 35 to 50V range. A significant
achievement in tantalum powder technology has been the development
of powder having flake morphology. See J. Koenitzer, S. Krause, L.
Mann, S. Yuan, T. Izumi and Y. Noguchi, "Tantalum Flakes--Powders
for High Reliability Electrolytic Capacitor Applications",
presented at the International Symposium Tantalum and Niobium
World, October 2006; J. A. Fife, "Improvements to Volumetric
Efficiency", T.I.C. Bulletin, No. 81, March 1995, pp. 5-8. Because
of their structure, flakes have a higher surface to volume ratio
than nodular powders. The flat surfaces can provide more contact
area between particles resulting in better inter-particle bonding.
Also the reduced curvature of flakes lowers the stresses in the
oxide layer particularly at higher formation voltages where the
oxide is thicker. These last two characteristics help achieve lower
DC leakage. Fibers, particularly flat fibers which are essentially
two-dimensional structures, should have similar properties to
flakes.
[0004] The potential advantages of fibers have been known for many
years, and several approaches were proposed for making fibers
suitable for capacitors. As far back as 1972, Douglas patented a
method for making fibers (U.S. Pat. No. 3,681,063), and capacitors
from these fibers (U.S. Pat. Nos. 3,742,369 and 3,827,865). The
basic approach involved sintering tantalum powder into a porous
compact and impregnating the compact with a softer metal, such as
copper, nickel, or aluminum that does not react with tantalum.
Impregnation was accomplished by melt infiltration of the second
metal. The solidified composite structure was drawn or rolled to
elongate the tantalum particles to produce fibers. The matrix was
removed by etching in a suitable acid resulting in a porous
structure of elongated fibers.
[0005] Fife in U.S. Pat. No. 4,502,884 describes a method for
making loose fibers from tantalum powder and capacitor anodes from
these fibers. In this approach, tantalum powder was mixed with a
second metal powder using sufficient powder so that the second
metal forms the matrix surrounding the tantalum particles. The
blend was compacted into a billet and the billet subsequently drawn
to elongate the tantalum powder particles. The matrix material was
removed by leaching in acid to release the tantalum fibers. Fife
also described a method of making anodes by forming the fibers into
a felt or mat structure (see U.S. Pat. No. 5,306,462). Fife
emphasized the need to have short fibers approximately 400 .mu.m in
length and to randomly orient the fibers in order to preserve
maximum surface area on sintering. While the benefits of using
fibers are known in the art, what has not been fully appreciated is
that since flat fibers or ribbon type fiber have greater surface
area than round fibers of equivalent cross-sectional area, the
thinner the fiber thickness the greater is the surface area as is
shown in FIG. 1, which leads to the possibility of increasing the
efficiency of the capacitor by producing the flat fibers with a
ribbon-like morphology. Additionally flat fibers with high surface
area are easier to produce than high surface area round fibers
because it is very difficult to produce uniformly round fine
filaments by conventional wire drawing techniques. Thus, large
round filaments that are easy to produce can be rolled to thin
cross-section to make high surface area fibers.
[0006] In my previous U.S. Pat. Nos. 5,034,857 and 5,869,196 1
describe approaches intended for making continuous fibers. My
earlier patented processes involved assembling a composite billet
of solid tantalum rods in a soft metal matrix, and then drawing the
rod to wire to reduce the size of the tantalum. Copper is the
preferred matrix material since it is very ductile, has virtually
no solubility in tantalum, and has deformation characteristics that
are compatible with tantalum. At a suitable size, the wire was cut
into short lengths and bundled together in a secondary billet
making a multifilament composite. The composite billet was further
reduced by extrusion, drawing, or rolling, or a combination of
these methods. The process was repeated a number of times to
achieve very high reductions, and produce very fine tantalum
fibers.
[0007] A variation of the above processes is to draw the
tantalum-copper composite until the fibers are a few microns in
diameter, then flatten the fibers by rolling to produce a highly
aspected, high surface area fiber that is a micron or less in
thickness. The flattened fibers thus formed are thin ribbons that
have many of the dimensional attributes of flakes and provide
higher surface area per weight of metal than round fibers. A
further advantage of making continuous fibers is that it avoids the
inherent complexities of handling and pressing short fibers. The
continuous lengths of fiber can be twisted or braided to form a
fiber strip that holds the loose filaments together. Anodes can be
stamped directly from the strip, thus eliminating the need to press
powders. Since the fibers can be readily processed into strip,
relatively thin sections can be made from which to stamp
anodes.
BRIEF DESCRIPTION OF THE INVENTION
[0008] The present invention provides an improvement over prior art
methods for making electrolytic capacitors. More particularly, the
present invention provides an improved method for making capacitor
anodes by producing filamentary valve metal fibers by a
co-reduction of valve metal filaments within a copper matrix by a
combination of drawing and rolling. The copper matrix is then
removed leaving valve metal fibers in the form of continuous flat,
ribbon-like fibers that have a relatively high aspect ratio of
width to thickness, typically of at least about 10 to 1, and as a
result relatively high surface area. By producing the fibers in a
bundled continuous strip form, they can be made into thin anodes
without pressing, thus maintaining the high surface area through
subsequent anode sintering and formation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Further features and advantages of the present invention
will be seen from the following detailed description of the
invention, taken into conjunction with the accompanying drawings,
wherein:
[0010] FIG. 1 is a plot of specific surface area to diameter or
width of flattened and drawn ribbons or wires;
[0011] FIG. 2 is a flow-chart describing the steps followed in a
preferred embodiment of the process of the present invention;
[0012] FIG. 3 is a plot comparing the affect of formation voltage
on CV/g under different sintering treatments;
[0013] FIG. 4 is a graph showing CV/g versus formation voltage
under a single sintering treatment; and
[0014] FIG. 5 is a plot showing DC leakage after different
sintering treatments.
DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS
[0015] Tantalum fibers were produced as fine filamentary ribbons,
which were made by a process of extrusion, drawing, and rolling of
a multifilament composite following the teachings of my prior U.S.
Pat. No. 5,034,857. The overall process is as follows:
[0016] In a preferred embodiment of the present invention, the
process begins with pure tantalum rod or high purity tantalum rod
having a small amount of impurities, e.g. Fe, Ni, Cr, Cu, Nb, Mo,
Si, Ti, W. C or O. A plurality of tantalum rods 12 are assembled
substantially parallel to one another, in a copper can. A copper
nose and tail are welded onto the can to form a primary billet, and
the billet is then evacuated and sealed. The can is then hot
extruded to bond the copper to the tantalum and cold drawn to make
a copper clad tantalum wire bundle following the teachings of my
prior U.S. Pat. No. 5,034,857. Bonding of the copper cladding to
the tantalum is essential to prevent oxidation or other
contamination of the tantalum during subsequent processing. The
resulting copper clad tantalum wire bundle was cut to length, and
bundled and restacked into a second copper container, a nose and
tail are welded in place, and the secondary billet is evacuated and
sealed as before. The secondary sealed billet is optionally
prepared for extrusion by hot or cold isostatic pressing in order
to collapse any void space within the billet and to promote
filament uniformity. After isostatic pressing, the secondary billet
is machined to fit the extrusion liner, and the billet is then
extruded and drawn followed by rolling to a final preferred
thickness of less than 1 .mu.m and preferably less than 0.5
.mu.m.
[0017] After rolling to final thickness, the tantalum fibers are
immersed in an etching solution such as nitric acid and water to
leach the copper.
[0018] CV/g and DC leakage are characteristics that depend on the
quality and morphology of the fibers. Capacitance is largely a
function of surface area, but also depends on the packaging of the
filaments in the anode body. To achieve high CV/g it is necessary
to create a high amount of useful surface area, eliminate the very
thin fiber segments that would be consumed during anodization, and
package the fibers to maintain an open pore structure that does not
close-off surface area during formation. DC leakage is largely
related to surface chemistry, but is also affected by the
regularity and uniformity of the oxide structure. To achieve low
leakage, it is necessary to have a uniform amorphous oxide without
irregularities or discontinuities caused by inclusion protruding
through the oxide film or crystallization promoted by impurities at
the metal oxide interface. An additional factor that can have a
detrimental effect on leakage is inadequately formed neck
structures which bond the particles of fibers together in a single
network structure. Poorly formed necks will result in local hot
spots due to highly resistive junctions causing a breakdown in the
oxide particularly at higher formation voltages.
[0019] The present invention results in part from the realization
that for a fixed volume of material, ribbon type fibers have more
surface area than round fibers when the ribbon type fibers have a
thickness equivalent to the diameter of the round fibers. Thus it
is possible to produce higher surface area fibers by flattening
round fibers. This greatly facilitates the production of high
surface area fibers, since it is difficult to make very fine,
submicron fibers by wire drawing without producing fibers that have
highly irregular cross-sections. When used to make anodes for
electrolytic capacitors, valve metal fibers having non-uniform
cross sections lead to lower CV/g performance. We have discovered
that flattening fibers by rolling produces a more uniform surface
structure that results in more useable surface area and thus
produces a capacitor that has higher volumetric efficiency.
EXAMPLES
[0020] The starting material was a rod 12 of high purity tantalum.
The rod was vacuum encapsulated in a copper can 14, extruded and
cold drawn to make a copper clad Ta wire. The wire was cut to
length, bundled and restacked into a second copper container, and
further reduced by drawing followed by rolling to final thickness.
After rolling, the resulting Ta fibers were removed from the matrix
by leaching the copper with nitric acid. While the drawing and
rolling parameters can be varied to produce a wide range of fiber
sizes and shapes, the particular deformation sequence and reduction
scheduled used in this example resulted in fibers that were
approximately 0.5 to 1 .mu.m thick and 35-50 .mu.m wide, and had a
B.E.T. surface area greater than 0.300 m.sup.2/g.
[0021] To make anodes, the fibers were twisted and cut into pieces
weighing approximately 50 mg and a tantalum lead wire attached by
spot welding. The dimensions of the anodes were approximately
0.3.times.4.times.8 mm. Since the fibers are continuous, the length
of the fibers forming the anode is equivalent to one of the planar
dimensions of the anode. The anodes were sintered under vacuum of
greater than 10.sup.-3 Pa (7.5.times.10.sup.-6 torr) for 10 minutes
or 50 minutes at temperatures of either 1300.degree. C. or
1500.degree. C. The fibers received no other chemical or thermal
treatment. The sintered anodes were anodized in a solution of 0.10
V/V % phosphoric acid at 80.degree. C. and a current of 100 mA per
gram. Samples were anodized to formation voltages of 100 V. 140 V
and 180 V. Capacitance and leakage current were measured in a wet
cell of 15 W/W % H.sub.2SO.sub.4. DC leakage current was measured
at a potential of 70% of the formation voltage.
[0022] Capacitance values for 100 V. 140 V and 180 V formations are
given in Table 1 and FIG. 3 which reports on the effect of
formation voltage on CV/g for each sintering treatment. At 100 V
formation, the CV/g is highest for the 1500.degree. C. 10 minute
sintering treatment and lowest for the 1500.degree. C. 50 minute
sinter. At 140 V formation, the CV/g values are similar for all
sintering treatments. At 180 V formation, the highest CV/g value
was obtained with the 1500.degree. C. 50 minute sintering
treatment, while the values for the 1300.degree. C. and
1500.degree. C. 10 minute sintering treatments were nearly
identical. The CV/g values for the 1500.degree. C. 50 minute sinter
treatment exhibits a linear fall-off with formations voltages as
shown in FIG. 4 which reports CV/g versus formation voltage for a
1500.degree. C. 50 minute sintering treatment showing the linear
fall-off of capacitance between 100 V to 200 V formations and how
the fall-off rate is less severe than for the 10 minute sintering
treatment at both 1300.degree. C. and 1500.degree. C.
[0023] DC leakage values are given in Table 2. As can be seen
leakage decreases with increasing sintering temperature and
sintering time. The values for the two 1500.degree. C. sintering
treatments are shown in FIG. 5 which reports DC leakage for
1500.degree. C. treatments. As can be seen, above 140 V formation,
leakage increases dramatically. The data also show that at 100 V
formation, leakage below 0.5 nA/.mu.FV can be obtained without a
deoxidation treatment. TABLE-US-00001 TABLE 1 Specific Capacitance
Sintering CV/g - .mu.F V/g Treatment 100 V 140 V 180 V 1300.degree.
C. - 10 min. 20,300 18,200 16,700 1500.degree. C. - 10 min. 21,400
18,400 16,800 1500.degree. C. - 50 min. 19,400 18,300 17,000
[0024] TABLE-US-00002 TABLE 2 DC Leakage Sintering DCL.sub.2 min -
nA/.mu.F V Treatment 100 V 140 V 180 V 1300.degree. C. - 10 min.
9.3 9.4 31.3 1500.degree. C. - 10 min. 1.5 1.8 3.5 1500.degree. C.
- 50 min. 0.4 0.7 1.8
[0025] It is thus seen that tantalum fibers produced by a composite
co-reduction process in accordance with the present invention have
properties suitable for use in forming capacitor anodes
particularly those used for higher voltage ratings. Like powders,
the fibers can be produced in different sizes depending on the
intended application voltage. However, unlike powders, the fibers
are organized in a continuous filament structure which improves
handling and packaging of the fibers into anodes. Further
improvements in CV/g can be realized by producing a more uniform
fiber structure, while improvements in DC leakage may be achieved
by performing a deoxidation treatment or through optimization of
the sintering cycle.
[0026] While the invention has been described in detail in
connection with the formation of tantalum fibers for solid
electrolytic capacitors, the invention also advantageously may be
employed with other valve metals commonly used for forming solid
electrolytic capacitors, in particular niobium, aluminum, vanadium
and like metals and alloys thereof. Yet other changes may be made
without departing from the spirit and scope of the invention.
* * * * *