U.S. patent application number 09/740973 was filed with the patent office on 2001-06-28 for process for producing powder metallurgy compacts free from binder contamination and compacts produced thereby.
This patent application is currently assigned to Kemet Electronics Corporation. Invention is credited to Kinard, John Tony, Melody, Brian John, Moore, Keith Lee, Wheeler, David Alexander.
Application Number | 20010004854 09/740973 |
Document ID | / |
Family ID | 23569605 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010004854 |
Kind Code |
A1 |
Moore, Keith Lee ; et
al. |
June 28, 2001 |
Process for producing powder metallurgy compacts free from binder
contamination and compacts produced thereby
Abstract
Metal powders are pressed into compacts more readily through the
addition of a minor percentage of dimethyl sulfone binder. Dimethyl
sulfone may be dry-blended with the metal powder by mixing it in
the form of a powder, or it may be wet-blended by first dissolving
it in a suitable solvent, then adding it to the metal powder and
evaporating the solvent. Dimethyl sulfone may be almost completely
removed from compacts pressed from tantalum, etc., either by vacuum
distillation or by water leaching, to leave compacts uncontaminated
by the binder and suitable for further processing into capacitor
anodes, etc.
Inventors: |
Moore, Keith Lee;
(Greenville, SC) ; Melody, Brian John; (Greer,
SC) ; Kinard, John Tony; (Simpsonville, SC) ;
Wheeler, David Alexander; (Williamston, SC) |
Correspondence
Address: |
BANNER & WITCOFF
1001 G STREET N W
SUITE 1100
WASHINGTON
DC
20001
US
|
Assignee: |
Kemet Electronics
Corporation
|
Family ID: |
23569605 |
Appl. No.: |
09/740973 |
Filed: |
December 21, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09740973 |
Dec 21, 2000 |
|
|
|
09397032 |
Sep 16, 1999 |
|
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Current U.S.
Class: |
75/252 |
Current CPC
Class: |
H01G 9/0525 20130101;
B22F 1/10 20220101 |
Class at
Publication: |
75/252 |
International
Class: |
B22F 001/02 |
Claims
What is claimed is:
1. A method of making an anode for an electrolytic capacitor,
comprising the steps of combining a metal powder and an effective
amount of dimethyl sulfone as a binder; pressing the powder and
dimethyl sulfone to form an anode body; and removing the dimethyl
sulfone.
2. The method of claim 1 further comprising removing the dimethyl
sulfone with vacuum distillation.
3. The method of claim 2 wherein the vacuum distillation occurs at
a temperature of between about 200.degree. C. and about 400.degree.
C.
4. The method of claim 3 wherein the vacuum distillation occurs at
a temperature of between about 250.degree. C. and about 350.degree.
C.
5. The method of claim 1 further comprising removing the dimethyl
sulfone by washing with an aqueous solution.
6. The method of claim 5 wherein the aqueous solution is water.
7. The method of claim 5 further comprising removing the dimethyl
sulfone by washing with an aqueous solution at a temperature of
between about 60.degree. C. and 95.degree. C.
8. The method of claim 1 wherein the amount of dimethyl sulfone is
between about 1 wt % and about 6 wt % based on total weight of
dimethyl sulfone and powder.
9. The method of claim 8 wherein the amount of dimethyl sulfone is
between about 2 wt % and about 4 wt % based on total weight of
dimethyl sulfone and powder.
10. The method of claim 9 wherein the amount of dimethyl sulfone is
about 2 wt %.
11. The method of claim 1 wherein the metal powder is tantalum.
12. A material for use in forming anodes for electrolytic
capacitors, said material comprising a metal powder and dimethyl
sulfone.
13. The material of claim 12 wherein the amount of dimethyl sulfone
is between about 1 wt % and about 6 wt % based on total weight of
dimethyl sulfone and powder.
14. The material of claim 13 wherein the amount of dimethyl sulfone
is between about 2 wt % and about 4 wt % based on total weight of
dimethyl sulfone and powder.
15. The material of claim 12 wherein the metal powder is
tantalum.
16. Anodes comprising a pressed metal powder prepared by combining
the metal powder and an effective amount of dimethyl sulfone as a
binder; pressing the metal powder and dimethyl sulfone to form an
anode body; and removing the dimethyl sulfone.
17. The anode of claim 16 wherein the amount of dimethyl sulfone is
between about 1 wt % and about 6 wt % based on total weight of
dimethyl sulfone and powder.
18. The anode of claim 17 wherein the amount of dimethyl sulfone is
between about 2 wt % and about 4 wt % based on total weight of
dimethyl sulfone and powder.
19. The anode of claim 16 wherein the metal powder is tantalum.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a method of producing powder
metallurgy compacts free of binder contamination by pressing metal
powder and a dimethyl sulfone binder.
BACKGROUND OF THE INVENTION
[0002] For many years powder metallurgy bodies fabricated from
metals have found use in industry. Powder metallurgy compacts
pressed from tantalum powder to approximately 1/4to 3/4of the
theoretical density have been sintered to produce the tantalum
capacitor anode bodies used to fabricate tantalum electrolytic
capacitors. In order to facilitate the mass production of tantalum
capacitor anodes, of which millions are fabricated daily, a binder
or lubricant is usually mixed with the tantalum powder prior to the
pressing step. This blending of tantalum powder and binder may be
accomplished by one of two basic means: the binder may be employed
in the form of a fine powder and mixed with the tantalum powder by
dry-blending, physically shaking or tumbling the powders together,
or the binder may be dissolved in a suitable solvent and the
solution sprayed on, or tumbled with, the tantalum powder (called
wet-blending), leaving binder-coated tantalum upon evaporation of
the solvent.
[0003] After the tantalum powder/binder combination is pressed to
form anode compacts, the binder has traditionally been removed via
a vacuum distillation step at 200.degree. C. to 600.degree. C.
prior to the high-temperature sintering step used to produce the
finished capacitor anode bodies. The vacuum distillation binder
removal step may also include the use of an inert "sweep" gas to
assist in removing the binder from proximity to the anodes as it is
volatilized.
[0004] The industry demand for ever smaller capacitors having
increasing CV density and decreasing cost has led to the
development of tantalum capacitor powders having an increasingly
higher surface area per gram and smaller average particle size. In
the 1960's, capacitor grade tantalum powder routinely had a surface
area of 0.05 square meter per gram and a CV product of 5,000
microfarad-volts (or microcoulombs) per gram. By the 1980's,
capacitor grade tantalum powders were available with a surface area
of 0.2 square meter per gram and a CV product of 20,000
microcoulombs per gram. Currently available capacitor grade
tantalum powders may have as high as 0.5 to 1.0 square meter per
gram surface area and 50,000 to 100,000 microcoulombs per gram. The
average particle size of capacitor grade tantalum powder in the
1960's was in excess of 5 microns. The average particle size of the
finest contemporary tantalum powders is below 0.2 micron and
sub-micron tantalum powders have been prepared.
[0005] The increase in surface area and reduction in the average
particle size of capacitor grade tantalum powders has made possible
both the size and cost reductions in tantalum capacitors sought by
industry. Unfortunately, the reduction in the pore size of tantalum
powder metallurgy anodes resulting from the use of these finer
tantalum capacitor powders makes the removal of the
binder/lubricant progressively more difficult with decreasing
particle size. Further complicating the binder removal is the
increasing surface energy and resulting reactivity of the tantalum
powder as the particle size is reduced. Thus, anodes pressed from
high surface area tantalum powders have very small pores through
which the binder vapor must diffuse and are composed of particles
that become very reactive at traditional binder removal
temperatures due to the high surface energy associated with the
small radii of curvature of these fine particles. The result is
that an increasingly high fraction of the binder reacts with the
tantalum anode material during the binder removal step and may be
detected by the standard carbon analysis tests used for reactive
metals.
[0006] Experience indicates that carbon residues formed by reaction
between carbonaceous binders and the tantalum powder comprising
tantalum anode compacts give rise to flaws in the anodic oxide film
dielectric formed via various anodizing processes. Flaws in the
anodic oxide give rise to elevated leakage current levels both in
liquid electrolyte solutions (used to test the anodes and to fill
"wet-slug" capacitors) and in finished "solid" tantalum
capacitors.
[0007] In order to help reduce the residual carbon content of
powder metallurgy capacitor anode bodies pressed from high surface
area tantalum powder (and the elevated leakage current and
short-circuit problems associated with this residual carbon),
capacitor manufacturers have employed various approaches to
enhancing the binder removal process. Anodes may be processed in
relatively small batches and spread out into relatively thin layers
in order to minimize the diffusion path length which the binder
vapor must transit to escape from the bulk of the anode bodies. As
mentioned above, an inert "sweep" gas may be employed to help
remove binder vapor from the vicinity of the anodes.
[0008] U.S. Pat. No. 4,664,883 describes a method of employing
mixed binders in which one component exhibits relatively good
binder properties, such as polyethylene oxide, and a second
component which, while not having particularly good binder
properties itself, decomposes at binder removal temperatures to
yield a large quantity of gases which serve to help sweep the first
binder component from the pores of the anode bodies. One
disadvantage of this method is that the volatile binders described,
ammonium carbonates, are hydroscopic and tend to absorb water
during processing in humid environments, resulting in problems with
powder flow.
[0009] Another relatively recently introduced binder material is
polypropylene carbonate, which is sold by PAC Polymers under the
name of "Q-PAC". This material thermally degrades in vacuum at
approximately 250.degree. C. to yield propylene carbonate,
propylene oxide, and carbon dioxide. This material has been found
to leave much less carbonaceous residue within vacuum sintered
powder metallurgy capacitor anodes pressed from high surface area
tantalum powder than is found with traditional binders, such as
stearic acid, CARBOWAX 8000, or ACRAWAX C. Unfortunately,
polypropylene carbonate is very difficult to mill for dry-blending
use due to the glass transition temperature of approximately
40.degree. C. (cryomilled powder tends to agglomerate into a solid
mass unless stored and shipped under refrigeration). Due to the
difficulties encountered in dry-blending polypropylene carbonate,
the material is usually wet-blended with tantalum powder. The
solubility of polypropylene carbonate is relatively high only in
chlorinated solvents and acetone. The wet-blending of polypropylene
carbonate on a manufacturing scale requires very thorough equipment
design and careful plant operation to prevent ignition of or worker
exposure to solvent fumes. Additionally, although polypropylene
carbonate represents a definite improvement in ease of removal
compared with traditional binder materials, such as stearic acid or
ACRAWAX C, it remains very difficult to remove the last traces of
polypropylene carbonate from powder metallurgy tantalum anode
compacts.
[0010] A simple and straightforward approach to removing
binders/lubricants from tantalum powder metallurgy anode compacts
is described in U.S. Pat. No. 5,470,525. The inventors employ
binders which are fairly water soluble and remove the binder
following the anode compacting step via warm water washing. This
method avoids reaction of the carbonaceous binder with the tantalum
at traditional binder removal temperatures by avoiding temperatures
sufficiently hot to decompose the binder. With this methodology,
virtually all of the binder may be removed with little or no damage
to the product. One disadvantage with this process is that
manufacturing plants already equipped for vacuum/thermal binder
removal must purchase and install equipment for the water washing
process and the binder must be soluble in water and have the
necessary lubriciousness and ability to agglomerate fine tantalum
powders into small tantalum-binder agglomerates having superior
flow and reduced dust formation compared to the tantalum powder,
alone.
[0011] Additionally, in a plant in which both processes,
vacuum/thermal binder removal and water wash binder removal, are
operated care must be taken that pressed tantalum anodes containing
binder suitable for one removal process are not inadvertently
subjected to the other process. For example, tantalum anode bodies
containing wet-blended polypropylene carbonate, which is removed
thermally, will contain grossly excessive amounts of carbon
following processing through a water wash process designed to
remove polyethylene glycol 8000, and capacitor anode bodies
compacted from high surface area tantalum powder containing
polyethylene glycol 8000 will be badly contaminated with carbon if
subjected to a vacuum/thermal binder removal process.
[0012] Ammonium carbonate or bicarbonate may be used as the sole
binder and may be completely removed from powder metallurgy
capacitor anode bodies compacted from high surface area tantalum
powder by either removal method. These materials have several
disadvantages, however. Due to their high vapor pressure, they must
be dry-blended. The ammonium carbonates are mechanically weak
substances and offer little or no lubrication value. Tantalum
powder blended with the ammonium carbonates as the sole binder is
subject to a great deal of dust generation and gives rise to
excessive wear of the anode compacting presses due to the settling
of fine, abrasive tantalum dust on the sliding and rotating parts
of the equipment. The ammonium carbonates also evaporate from open
containers of tantalum capacitor powder blended with them. The
resulting variable binder content due to evaporation complicates
control of the weight of the anodes produced.
SUMMARY OF THE INVENTION
[0013] The invention is directed to a method of making powder
metallurgy compacts with a dimethyl sulfone binder and the removal
of the binder from the compacts.
[0014] The invention provides a method of making a powder
metallurgy compact comprising the steps of: combining a metal
powder and dimethyl sulfone in an amount to be effective as a
binder; pressing the powder and dimethyl sulfone to form a powder
metallurgy compact, such as an anode body; and removing the
dimethyl sulfone.
[0015] The invention also provides a method of making powder
metallurgy compacts wherein the dimethyl sulfone is removed by
vacuum distillation or by washing with an aqueous solution.
[0016] Numerous other features, objects and advantages of the
invention will become apparent from the following detailed
description.
DETAILED DESCRIPTION OF THE INVENTION
[0017] It was discovered that dimethyl sulfone is an effective
binder for making compacts with a metal powder. Such metal powders
typically have a high surface area. The metal powder is pressed
into compacts more readily by the addition of a minor amount of
dimethyl sulfone binder.
[0018] The dimethyl sulfone is combined with the metal powder in
any suitable manner. For example, a powder form of dimethyl sulfone
may be dry-blended with the metal powder by mixing, or the dimethyl
sulfone may be wet-blended by first dissolving the dimethyl sulfone
a suitable solvent, then adding the solution to the fine reactive
metal powder, and finally evaporating the solvent. Suitable
solvents include, but are not limited to acetone and water.
[0019] The metal powder may be any suitable metal that is used to
prepare compacts. Examples of metal powders include tantalum,
niobium, titanium, zirconium, hafnium, nickel, copper, zinc,
aluminides, bronze, etc. for the manufacture of capacitor anodes.
Preferably the metal powder is tantalum powder.
[0020] The dimethyl sulfone is combined with metal powder in an
amount effective as a binder, typically about 1 wt % to about 6 wt
% based on total weight of binder and powder, preferably about 2 wt
% to about 4 wt %.
[0021] The powder-binder combination is pressed to form a compact,
for example, using conventional or traditional pressing
technology.
[0022] The binder is then removed by any suitable method such as
vacuum distillation or by washing the compact with water or other
aqueous solution (leaching.) The dimethyl sulfone is almost or
virtually completely removed from the compacts to leave compacts
uncontaminated by the binder and/or resulting carbon residues and
suitable for further processing into capacitor anodes, etc.
[0023] The vacuum distillation comprises heating the compact under
a vacuum. The compact is heated to at least 200.degree. C.,
preferably about 250.degree. C. to about 350.degree. C. for a time
sufficient to remove the dimethyl sulfone, typically, but not
limited to, about 0.5 to about 1.5 hours.
[0024] The water washing comprises rinsing the compact with water
at least one time, more preferably at least two times, and most
preferably about three times. The rinse preferably occurs at
elevated temperatures of about 50.degree. C. to about 95.degree.
C., preferably about 60.degree. C. to about 90.degree. C., most
preferably about 80.degree. C., for a time sufficient to remove the
dimethyl sulfone.
[0025] The compacts are preferably used as anodes for electrolytic
capacitors. Other uses include porous metal filters, gaseous
diffusion media, and catalytic reaction surfaces.
[0026] Dimethyl sulfone is a non-hydroscopic, white solid having
little or no odor, readily available and inexpensive. The material
is somewhat lubricious and is mechanically strong enough to form
tantalum powder/binder agglomerates for improved powder flow and
greatly reduced dust formation. Dimethyl sulfone is very soluble in
water at room temperature (up to approximately 40 wt. %) and is
infinitely miscible with water at 80.degree. C. Dimethyl sulfone
melts at approximately 110.degree. C. and boils at 238.degree. C.
to 240.degree. C. at atmospheric pressure. The ash content is
typically below 0.1%.
[0027] Literature from Gaylord Chemical, Bulletin No. 301 indicates
that dimethyl sulfone is characterized by uncommon inertness, even
at elevated temperatures. Heating dimethyl sulfone to a temperature
of 275.degree. C. for one hour results in autodecomposition of 0.3%
mole %. It is reported to be stable at 238.degree. C. in the
presence of aqueous sodium hydroxide and at 130.degree. C. in
nitrating acid (fuming nitric and sulfuric acids). The extreme
stability of the material allows for thermal/vacuum distillation
removal from anode compacts pressed from high surface area reactive
metals. The flash point of dimethyl sulfone is 290.degree. F.,
143.degree. C., open cup.
[0028] In addition, dimethyl sulfone has a very low level of
toxicity (the Gaylord Chemical Company's M.S.D.S. lists the Oral
Rat LD50 as 17,000 mg/kg). It is considered to be a non-irritant to
the skin and is readily biodegradable.
[0029] Dimethyl sulfone is compatible with tantalum and other
reactive metal powders. It is available in the powdered state to
facilitate dry-blending with tantalum powders. The vapor pressure
is sufficiently low so that it does not evaporate from open
containers of tantalum/binder blends nor does it evaporate during
removal the solvent during a wet-blending operation.
[0030] Although not wishing to be bound by any theory, it is
believed that the dimethyl sulfone binder can be removed completely
via thermal/vacuum distillation since it is extremely stable with
respect to autodecomposition and does not react with tantalum at
temperatures required to vaporize the material under vacuum. It is
also believed that the dimethyl sulfone binder can be also removed
completely via water washing since it is highly and rapidly water
soluble and solutions formed by the binder in hot water do not
attack tantalum. Dimethyl sulfone has a relatively low molecular
weight which appears to maximize diffusion during the removal
process. Thus, once the capacitor compacts have been pressed, the
dimethyl sulfone is capable of being removed either by
vacuum/thermal distillation or by water wash methods. A mistake in
the removal method used would have minimal negative consequences
for a plant employing both removal methods.
EXAMPLES
Example 1
[0031] In order to illustrate the low level of residual carbon
contamination which is obtainable with powder metallurgy compacts
pressed from tantalum powder containing dimethyl sulfone as a
binder, a quantity of Showa-Denko S705 tantalum powder was split
into three groups. The groups were blended with 2 wt. % binder and
were pressed into 0.26 gram tantalum anodes of the following
dimensions: 0.125 inch thick by 0.128 inch wide by 0.185 inch long.
The groups were then processed as indicated in Table 1.
[0032] It may readily be seen from the data in Table 1 that
dimethyl sulfone may be completely removed from the anode compacts
either by vacuum distillation or by water washing at 80.degree. C.
Values for carbon and oxygen were taken before sintering and after
sintering at 1375.degree. C. to 1385.degree. C./15 min.
1TABLE 1 Carbon Oxygen Group Binder Binder Removal ppm ppm Virgin
S705 none none 24 3079 1 Dimethyl Vacuum, 350.degree. C. 26 3662
pre sintering sulfone 1 1/2 Hours 1 Dimethyl 22 6485 post sintering
sulfone 2 Dimethyl 1 Rinse* 78 3733 pre sintering sulfone 2 Rinses*
34 3812 3 Rinses* 30 3787 2 Dimethyl 20 6168 post sintering sulfone
3 Polypropylene Vacuum, 350.degree. C. 124 6667 post sintering
carbonate 1 1/2 Hours *Each rinse consists of immersing the anodes
in 80.degree. C. deionized water for 1 hour; 2 liters of water per
1000 anodes.
[0033] It will be apparent to those skilled in the art that various
modifications and variations can be made in the compositions and
methods of the present invention without departing from the spirit
or scope of the invention. Thus, it is intended that the present
invention cover the modifications and variations of this invention
provided they come within the scope of the appended claims and
their equivalents.
* * * * *