U.S. patent application number 14/290017 was filed with the patent office on 2014-10-09 for process for producing electrolytic capacitors and capacitors made thereby.
This patent application is currently assigned to Kemet Electronics Corporation. The applicant listed for this patent is Keith R. Brenneman, Antony P. Chacko, Qingping Chen, Randolph S. Hahn, Philip M. Lessner, Yongjian Qiu, Hong Zhang. Invention is credited to Keith R. Brenneman, Antony P. Chacko, Qingping Chen, Randolph S. Hahn, Philip M. Lessner, Yongjian Qiu, Hong Zhang.
Application Number | 20140301022 14/290017 |
Document ID | / |
Family ID | 46636723 |
Filed Date | 2014-10-09 |
United States Patent
Application |
20140301022 |
Kind Code |
A1 |
Chen; Qingping ; et
al. |
October 9, 2014 |
Process for Producing Electrolytic Capacitors and Capacitors Made
Thereby
Abstract
A process for preparing a solid electrolytic capacitor
comprising application of a non-ionic polyol prior to application
of a conducting polymer layer.
Inventors: |
Chen; Qingping;
(Simpsonville, SC) ; Zhang; Hong; (Jiangsu,
CN) ; Chacko; Antony P.; (Simpsonville, SC) ;
Lessner; Philip M.; (Simpsonville, SC) ; Hahn;
Randolph S.; (Simpsonville, SC) ; Qiu; Yongjian;
(Simpsonville, SC) ; Brenneman; Keith R.;
(Simpsonville, SC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Chen; Qingping
Zhang; Hong
Chacko; Antony P.
Lessner; Philip M.
Hahn; Randolph S.
Qiu; Yongjian
Brenneman; Keith R. |
Simpsonville
Jiangsu
Simpsonville
Simpsonville
Simpsonville
Simpsonville
Simpsonville |
SC
SC
SC
SC
SC
SC |
US
CN
US
US
US
US
US |
|
|
Assignee: |
Kemet Electronics
Corporation
|
Family ID: |
46636723 |
Appl. No.: |
14/290017 |
Filed: |
May 29, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13396793 |
Feb 15, 2012 |
8771381 |
|
|
14290017 |
|
|
|
|
Current U.S.
Class: |
361/532 ;
977/948 |
Current CPC
Class: |
H01G 9/0036 20130101;
B82Y 99/00 20130101; H01G 9/028 20130101; H01G 9/052 20130101; B05D
1/185 20130101; Y10S 977/948 20130101; H01G 9/0032 20130101; Y10T
29/43 20150115; H01G 9/0425 20130101; H01G 9/15 20130101 |
Class at
Publication: |
361/532 ;
977/948 |
International
Class: |
H01G 9/042 20060101
H01G009/042; H01G 9/15 20060101 H01G009/15 |
Claims
1-21. (canceled)
22. A capacitor comprising: an anode a dielectric on said anode;
and a conductive polymeric cathode on said dielectric wherein said
conductive polymeric cathode comprises a non-ionic polyol.
23. The capacitor of claim 22 wherein non-ionic polyol has a
melting point of at least 50.degree. C. to no more than 250.degree.
C.
24. The capacitor of claim 23 wherein non-ionic polyol has a
melting point of at least 75.degree. C. to no more than 200.degree.
C.
25. The capacitor of claim 22 wherein said non-ionic polyol
comprises an alkyl of 3-20 carbons with at least two carbons each
comprising at least one hydroxyl group.
26. The capacitor of claim 25 wherein at least one carbon is
substituted with an alkyl group.
27. The capacitor of claim 26 wherein said alkyl group has 1 to 5
carbons.
28. The capacitor of claim 22 wherein said non-ionic polyol is
selected from the group consisting of
CH.sub.2OH(CHOH).sub.2CH.sub.2OH, CH.sub.2OH(CHOH).sub.3CH.sub.2OH,
CH.sub.2OHC(CH.sub.2OH).sub.2CH.sub.2OH,
CH.sub.2OHC(CH.sub.3).sub.2CH.sub.2OH,
CH.sub.2OH(CHOH).sub.4CH.sub.2OH, CH.sub.2OH(CHOH).sub.4CH.sub.2OH,
CH.sub.3C(CH.sub.2OH).sub.3,
O(CH.sub.2C(C.sub.2H.sub.5)CH.sub.2OH).sub.2).sub.2,
CH.sub.2OH(CHOH).sub.4COH, CH.sub.2O H(CHOH).sub.3COCH.sub.2OH,
sucrose and lactose.
29. The capacitor of claim 22 further comprising drying said
conductive polymer layer.
30. The capacitor of claim 22 wherein said anode comprises a valve
metal.
31. The capacitor of claim 30 wherein said anode is selected from a
group consisting of a valve metal and a conductive oxide of a valve
metal.
32. The capacitor of claim 30 wherein said anode comprises a
material selected from the group consisting of tantalum, aluminum,
niobium and niobium oxide.
33. The capacitor of claim 22 wherein said conductive polymer is
selected from the group consisting of polyaniline, polythiophene
and polypyrole and their derivatives.
34. The capacitor of claim 33 wherein said conductive polymer is
poly-3,4-ethylenedioxythiophene.
35. The capacitor of claim 22 further comprising forming a layer of
nanoparticles on said conductive polymer layer.
36. The capacitor of claim 35 wherein said nanoparticles have an
average particle size of no more than 100 nm.
37. The capacitor of claim 36 wherein said nanoparticles have an
average particle size of no more than 50 nm.
38. The capacitor of claim 35 wherein said nanoparticles are
selected from the group consisting of aluminum oxide, cerium oxide
silicon oxide and zinc oxide.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to pending U.S.
Provisional Patent Appl. No. 61/443,051 filed Feb. 15, 2011 and to
pending U.S. Provisional Patent Appl. No. 61/443,622 filed Feb. 16,
2011 both of which are incorporated herein by reference.
BACKGROUND
[0002] The present invention is related to an improved method of
forming a solid electrolyte capacitor and an improved capacitor
formed thereby. More specifically, the present invention is related
to materials and methods for improving corner and edge coverage of
solid electrolytic capacitors. The invention also discloses methods
for manufacturing the same.
[0003] The construction and manufacture of solid electrolyte
capacitors is well documented. In the construction of a solid
electrolytic capacitor a valve metal serves as the anode. The anode
body can be either a porous pellet, formed by pressing and
sintering a high purity powder, or a foil which is etched to
provide an increased anode surface area. An oxide of the valve
metal is electrolytically formed to cover all surfaces of the anode
and serves as the dielectric of the capacitor. The solid cathode
electrolyte is typically chosen from a very limited class of
materials that includes manganese dioxide or electrically
conductive organic materials such as polyaniline, polypyrrole,
polythiophene and their derivatives. Solid electrolytic capacitors
with intrinsically conductive polymers as the cathode material have
been widely used in the electronic industry due to their
advantageously low equivalent series resistance (ESR) and
"non-burning/non-ignition" failure mode. In the case of conductive
polymer cathodes the conductive polymer is typically applied by
either chemical oxidation polymerization, electrochemical oxidation
polymerization or spray techniques with other less desirable
techniques being reported.
[0004] The backbone of a conductive polymer consists of a
conjugated bonding structure. The polymer can exist in two general
states, an undoped, non-conductive state, and a doped, conductive
state. In the doped state, the polymer is conductive, due to a high
degree of conjugation along the polymer chain and the presence of
charges generated by doping, but has poor processibility. In its
undoped form, the same polymer loses its conductivity but can be
processed more easily because it is more soluble. When doped, the
polymer incorporates counter ionic moieties as constituents on its
charged backbone. In order to achieve high conductivity, the
conductive polymers used in the capacitor must be in doped form
after the completion of the process, although during the process,
the polymer can be undoped/doped to achieve certain process
advantages.
[0005] Various types of conductive polymers including polypyrrole,
polyaniline, and polythiophene are applied to the solid
electrolytic capacitors. The major drawback of conductive polymer
capacitors, regardless of the types of conductive polymers
employed, is their relatively low working voltage compared to their
wet electrolytic counterparts. For tantalum solid electrolytic
capacitors conductive polymer capacitors have lower working voltage
limits than those based on MnO.sub.2 as the solid cathode. The
polymer capacitors have reliability issues, to varying degrees,
when the voltage rating exceeds 25V. This is believed to be caused
by the relatively poor dielectric-polymer interface, which has poor
"self-healing" capability. The ability to withstand high voltage
can be best characterized by the breakdown voltage (BDV) of the
capacitors. Higher BDV corresponds with better reliability. For
reasons which were previously unknown the break-down voltage of
capacitors comprising conductive polymers has been limited to about
55V thereby leading to a capacitor which can only be rated for use
at about 25V. This limitation has thwarted efforts to use
conductive polymers more extensively.
[0006] U.S. Pat. No. 7,563,290, which is incorporated herein by
reference, describes the slurry/dispersion process. The resulting
capacitors show excellent high voltage performances, reduced DC
leakage (DCL) and improved long term reliability.
[0007] It is highly desirable that the capacitor devices are of
high reliability and that they can withstand stressful
environments. Therefore, the integrity of the anodes and the
robustness of conductive polymer cathodes are essential for high
quality capacitor products. However, it is a challenge to form a
conductive polymer coating on the anodes that is defect-free, and
which is capable of withstanding thermal mechanical stress during
anode resin encapsulation and surface-mounting. The improper
application of polymer slurry often leads to the exposure of the
dielectric and formation of cracks and delaminating of the polymer
coating thus formed.
[0008] A particular concern is the formation of adequate polymer
coatings on edges and corners. U.S. Pat. No. 7,658,986, which is
incorporated herein by reference, describes the difficulty in
coating the edges and corners of the anode with polymer slurry.
These materials tend to pull away from the corners and edges due to
surface energy effects. The resulting thin coverage at corners and
edges leads to poor reliability of the device.
[0009] One approach to mitigating poor coverage of the anode
corners and edges has been to alter the design of the anode as
disclosed in U.S. Pat. Nos. 7,658,986, D616,388, D599,309, and
D586,767 each of which is incorporated herein by reference. While
changes in the anode design are beneficial in some regards the
effect of poor coverage is still present even with anode designs
which facilitate corner and edge coverage by polymer slurry as the
primary cathode layer.
[0010] Another approach for improving coverage of the corners and
edges is provided in International Application WO2010089111A1,
which is incorporated herein by reference, which describes a group
of chemical compounds called crosslinkers, which are mostly
multi-cationic salts or multi-amines, such as an exemplary material
linear aliphatic .alpha.,.omega.-diamines. International
Application WO2010089111A1 teaches the application of a solution of
the crosslinker on the anodes prior to the application of polymer
slurry to achieve good polymer coverage on corners and edges of the
anodes. The effectiveness of the crosslinker is attributed to the
cross-linking ability of multi-cationic salts or multi-amines to
the slurry/dispersion particles. While crosslinkers are
advantageous for improving the coating coverage on corners and
edges of the anodes, the addition of these crosslinkers, which are
mostly ionic in nature, has the unintended consequences of
degrading the humidity performance of finished capacitors under
humid conditions.
[0011] Cross linkers, by definition, link one polymer chain to
another thus tending to be part of the polymer system. While
crosslinkers are advantageous in many applications, it is
undesirable to have an ionic crosslinker react with the polymer
chain and be part of the polymer chain. Ionic materials, especially
low molecular weight ionic compounds or mobile ionic compounds, can
diffuse through various cathode layers, especially under humid
conditions, and can cause higher leakage current. Unlike covalently
crosslinked molecules, ionically crosslinked molecules, have lower
bond strength and can be disassociated when exposed to high
temperature and high humidity conditions. Once disassociated, these
mobile ions can cause higher leakage current. So a need exists for
materials and methods which improves corner and edge coverage while
not crosslinking with the polymer system or increasing the ionic
content of the capacitor.
SUMMARY OF THE INVENTION
[0012] It is an object of the invention to provide an improved
solid electrolytic capacitor.
[0013] It is an object of the invention to provide an improved
method of preparing a solid electrolytic capacitor cathode with
good corner and edge coverage.
[0014] It has now been found that, surprisingly, corner and edge
coverage can be improved using non-ionic polyols as a precoat prior
to formation of the conductive polymeric cathode layer from
conductive polymer slurry.
[0015] These and other advantages, as will be realized, are
provided in a process for forming a solid electrolytic capacitor
and an electrolytic capacitor formed by a process which
includes:
providing an anode wherein the anode comprises a porous body and an
anode wire extending from the porous body; forming a dielectric on
the porous body to form an anodized anode; forming a solid cathode
layer inside the pores of the anode; applying a layer of non-ionic
polyol on the solid cathode layer; forming a conducting polymer
layer on the non-ionic polyol layer; and applying additional layers
of non-ionic polyol and conducting polymer until a desired
thickness of conducting polymer is obtained.
[0016] Yet another embodiment of the invention is provided in a
method of forming an electrolytic capacitor comprising:
providing an anode with an anode lead extending therefrom; forming
a dielectric on the anode; forming a conductive polymer layer on
the layer of dielectric; forming a layer of non-ionic polyol on the
conductive polymer layer; and forming a second conductive polymer
layer on the layer of non-ionic polyol.
[0017] Yet another embodiment is provided in a capacitor with an
anode and a dielectric on the anode. A conductive polymeric cathode
is on the dielectric wherein the conductive polymeric cathode
comprises a non-ionic polyol.
BRIEF DESCRIPTION OF FIGURES
[0018] FIG. 1 schematically illustrates a capacitor in schematic
cross-sectional view.
[0019] FIG. 2 schematically illustrates a capacitor in schematic
cross-sectional view.
[0020] FIG. 3 illustrates a method of the claimed invention in flow
chart representation.
[0021] FIGS. 4-7 illustrate the advantages of the invention.
DESCRIPTION
[0022] The present invention is related to an improved capacitor
and a method for making the improved capacitor. More particularly,
provided herein is a method that allows the production of
capacitors with improved corner and edge coverage.
[0023] The invention will be described with reference to the
figures forming an integral, non-limiting, component of the
disclosure. Throughout the various figures similar elements will be
numbered accordingly.
[0024] An embodiment of the invention is illustrated in schematic
cross-sectional view in FIG. 1. In FIG. 1, a capacitor, generally
represented at 10, comprises an anode, 12, with an anode wire, 14,
extending therefrom. A dielectric, 16, is on the anode at least
partially encasing the anode. A conductive polymeric cathode, 18,
is on the dielectric and separated from the anode by the
dielectric. Adhesion layers, 20, provide a layer which allows
adhesion to a cathode external termination, 22. An anode external
termination, 24, is in electrical contact with the anode wire. The
entire capacitor, except for the lower portion of the anode and
cathode external terminations, is preferably encased in a
non-conductive matrix, 26 or sealed in a metal can as known in the
art.
[0025] An embodiment of the invention is illustrated in FIG. 2
wherein a capacitor is generally represented at 110. A series of
anodes, 120, are arranged in parallel fashion. Each anode has a
dielectric, 116, thereon. A conductive polymer cathode, 118, is on
each dielectric. The anodes are fused at 123 and the cathodes are
commonly terminated.
[0026] The present invention provides a method for forming improved
coating on the edges and corners through the use of a precoat of a
non-ionic polyol prior to coating with a slurry of conductive
polymer. The precoat may be prior to the first application of
conductive polymer slurry or the first application of conductive
polymer slurry may occur initially with non-ionic polyol applied
between subsequent layers.
[0027] An embodiment of the invention is illustrated in flow-chart
form in FIG. 3. In FIG. 3, an anode is provided at 30. A dielectric
is formed on the anode at 32. An initial coating of conductive
polymer is formed at 34 preferably by in-situ chemical
polymerization. A particularly preferred conductive polymer is poly
3,4-ethylenedioxythiophene (PEDT). PEDT can be made by in situ
polymerization of ethylenedioxythiophene (EDT) monomer such as
Clevios M V2 which is commercially available from Hereaeus with an
oxidizer such as ferric tosylate solution available as Clevios.RTM.
C from Hereaeus. In one embodiment the oxidizer is applied first by
dipping following by dipping and drying in a monomer solution. An
optional layer of nonionic polyol can be applied on the initial
coating of conductive polymer at 36. A conductive polymeric coating
is applied at 38 by dipping in a slurry of conductive polymer. To
achieve the desired thickness of conductive polymer, sequential
repeated steps of forming a non-ionic polyol layer at 40 is
followed by forming a conductive polymeric layer at 42. The
conductive polymeric layer is preferably formed by dipping in a
slurry of conductive polymer. Once the desired thickness of
conductive polymer is achieved the capacitor is finished at 44 by
forming anodic and cathode external terminations and optionally
encapsulating or sealing the capacitor.
[0028] Improvements in conductive polymeric coatings can be
achieved, particularly on the edges and corners, by applying a
coating of non-ionic polyol prior to formation of conductive
polymeric coating. While not limited to any theory, it is
hypothesized that the hydrophilic interaction between the multiple
hydroxyl groups and the dispersed conductive particles will
increase the viscosity of the coating layer when the solvent is
evaporated during drying. The increased viscosity immobilizes the
particles thereby mitigating the migration of material away from
the edges and corners as typically occurs due to surface tension
effects.
[0029] For the purposes of the present invention a non-ionic polyol
is an alkyl alcohol with multiple hydroxyl groups or alkyl ethers
with multiple hydroxyl groups on the alkyl groups.
[0030] The non-ionic polyol preferably has a melting point which is
sufficiently high that the material remains in place as a solid
coating yet lower than the drying temperature of the conductive
polymer coating layer. During the drying of the conductive polymer
layer the non-ionic polyol will melt and diffuse into the
conductive polymer layer to eliminate the presence of a discrete
non-ionic layer which would function as an insulating layer between
the conductive polymer layers or conductive polymeric cathode and
the dielectric. The melting point of the non-ionic polyol is
preferable at least 50.degree. C. to no more than 250.degree. C.
More preferably the melting point of the non-ionic polyol is at
least 75.degree. C. to no more than 200.degree. C.
[0031] Preferred polyols comprise an alkyl or alkyl ether of 3-20
carbons, either linear, branched or in rings, with at least two
carbons each substituted with at least one hydroxyl group. Each
carbon may be otherwise unsubstituted or substituted with an alkyl
of 1 to 5 carbons. When the alkyl or alkyl ether has lower than 3
carbons the polyol tends to be a liquid which is unsuitable. When
the alkyl or alkyl ether has more than 20 carbons the solubility is
impaired and the material approaches functioning as a polymer
therefore becomes less effective.
[0032] Particularly preferred non-ionic polyols are
CH.sub.2OH(CHOH).sub.2CH.sub.2OH or erythritol,
CH.sub.2OH(CHOH).sub.3CH.sub.2OH as ribitol or xylitol,
CH.sub.2OHC(CH.sub.2OH).sub.2CH.sub.2OH or pentaerythritol,
CH.sub.2OHC(CH.sub.3).sub.2CH.sub.2OH or
2,2-dimethyl-1,3-propanediol; CH.sub.2OH(CHOH).sub.4CH.sub.2OH or
sorbitol, CH.sub.2OH(CHOH).sub.4CH.sub.2OH or manitol,
CH.sub.3C(CH.sub.2OH).sub.3 or trimethylolethane and
O(CH.sub.2C(C.sub.2H.sub.5)CH.sub.2OH).sub.2).sub.2 or
di-trimethylolpropane, CH.sub.2OH(CHOH).sub.4COH or glucose,
CH.sub.2OH(CHOH).sub.3COCH.sub.2OH or fructose,
C.sub.12H.sub.22O.sub.11 or sucrose or lactose.
[0033] Solid electrolytic capacitors generally comprise a porous
metal anode, an oxide layer on the anode, typically an oxide of the
anode metal, and an electrically conductive solid cathode, such as
manganese dioxide or an intrinsically conductive polymer,
incorporated into the pores and onto the dielectric. Additional
layers, such as silver and carbon layers, are then added to aid in
contact formation.
[0034] The solid electrolytic capacitors typically incorporate
valve metals or conductive oxides of valve metals with tantalum,
aluminum, niobium and niobium oxide being mentioned as particularly
preferred. An advantage of the high surface area is that a very
high capacitance can be achieved.
[0035] The dielectric is typically formed as an oxide of the anode
metal without limit thereto. Dielectric formation is well
documented in the art and the method of dielectric formation is not
limited herein.
[0036] Conductive polymers are particularly suitable for use as the
electrically conductive solid cathode with polyaniline,
polypyrroles and polythiophenes being most preferred. A
particularly preferred polymer for use as a cathode is
polythiophene. The polymer layer inside the pores is preferably
formed by chemical polymerization wherein the internal conductive
layer is formed by dipping the anodized substrate first in a
solution monomer of the conductive polymer. After a drying step,
the anode bodies are then immersed in a solution comprising
oxidizer and dopant. The chemical polymerization cycle can be
repeated multiple times to achieve the desired coverage of the
surface inside the pores. The polymer layer inside the pores can
also be formed by dip coating using a solution or dispersion of
conductive polymer. When a solution of conductive polymer such as
polyaniline is utilized a diluted solution is preferred so that the
solution viscosity would be sufficiently low to allow diffusion of
the solution into the porous structure. In case of a dispersion of
the conductive polymer the particle size must be sufficiently small
to allow impregnation of the porous structure.
[0037] After the application of the internal conductive polymer
layer, non-ionic polyol coating can be applied. The non-ionic
polyol is preferably applied to the conductive polymer layer as a
solution by dipping or by spraying. The non-ionic polyol can be
selectively applied to the corners or edges. It is preferable that,
at least, the edges and corners have polyol coated thereon. It is
more practical to apply polyol to the entire outer surface of the
capacitor body.
[0038] After the non-ionic polyol layer is formed a layer of
conductive polymer can be applied with a slurry or dispersion of
the conductive polymer. It is preferred to include a dopant in the
polymer as known in the art. A particularly preferred dopant is the
sodium salt of polystyrenesulfonate (PSS) or polestersulfonate
(PES).
[0039] The conducting polymer is preferably an intrinsically
conducting polymer comprising repeating units of a monomer of
Formula I:
##STR00001##
[0040] R.sup.1 and R.sup.2 of Formula I are preferably chosen to
prohibit polymerization at the .beta.-site of the ring. It is most
preferred that only .alpha.-site polymerization be allowed to
proceed. Therefore, it is preferred that R.sup.1 and R.sup.2 are
not hydrogen. More preferably R.sup.1 and R.sup.2 are
.alpha.-directors. Therefore, ether linkages are preferable over
alkyl linkages. It is most preferred that the groups are small to
avoid steric interferences. For these reasons R.sup.1 and R.sup.2
taken together as --O--(CH.sub.2).sub.2--O-- is most preferred.
[0041] In Formula I, X is S, Se or N. Most preferably X is S.
[0042] R.sup.1 and R.sup.2 independently represent linear or
branched C.sub.1-C.sub.16 alkyl or C.sub.1-C.sub.18 alkoxyalkyl; or
are C.sub.3-C.sub.8 cycloalkyl, phenyl or benzyl which are
unsubstituted or substituted by C.sub.1-C.sub.6 alkyl,
C.sub.1-C.sub.6 alkoxy, halogen or OR.sup.3; or R.sup.1 and R.sup.2
taken together, are linear C.sub.1-C.sub.6 alkylene which is
unsubstituted or substituted by C.sub.1-C.sub.6 alkyl,
C.sub.1-C.sub.6 alkoxy, halogen, C.sub.3-C.sub.8 cycloalkyl,
phenyl, benzyl, C.sub.1-C.sub.4 alkylphenyl, C.sub.1-C.sub.4
alkoxyphenyl, halophenyl, C.sub.1-C.sub.4 alkylbenzyl,
C.sub.1-C.sub.4 alkoxybenzyl or halobenzyl, 5-, 6-, or 7-membered
heterocyclic structure containing two oxygen elements. R.sup.3
preferably represents hydrogen, linear or branched
C.sub.1-C.sub.16alkyl or C.sub.1-C.sub.18 alkoxyalkyl; or are
C.sub.3-C.sub.8 cycloalkyl, phenyl or benzyl which are
unsubstituted or substituted by C.sub.1-C.sub.6 alkyl.
[0043] More preferably R.sup.1 and R.sup.2 independently represent
--CH.sub.3, --CH.sub.2CH.sub.3; --OCH.sub.3; --OCH.sub.2CH.sub.3 or
most preferably R.sup.1 and R.sup.2 are taken together to represent
--OCH.sub.2CH.sub.2O-- wherein the hydrogen can be replaced with a
solubilizing group, a halide or an alkyl.
[0044] A solvent is defined as a single solvent or a mixture of
solvents.
[0045] It is preferable to apply the dispersion comprising the
conductive polymer at a pH of no more than 10 and more preferably
no more than 8 with below 7 being more preferred and below 6 being
especially preferred.
[0046] The conductive polymer dispersion is applied onto the polyol
to form a layer that covers the edges and corners of the anodes.
The application of non-ionic polyol layer and the conductive
polymer layer can be repeated multiple times to achieve enough
thickness. Without limit thereto 1-10 cycles of non-ionic polyol
and conductive polymer layer application are suitable for
demonstration of the invention. Each application of conductive
polymer may use a unique composition and a unique solution or an
identical or similar material may be used for the various dipping
steps. A preferred thickness of the conductive polymer layer is at
least 2 micrometers to no more than 50 micrometers. A more
preferred thickness of the conductive polymer layer is from at
least 2 micrometers to no more than 40 micrometers. An even more
preferred thickness is from at least 3 micrometers to no more than
30 micrometers. If the layer of conductive polymer is below about 2
micrometers the dielectric is not adequately covered resulting in
defective capacitors. If the conductive polymer layer is over about
50 micrometers the equivalent series resistance of the resulting
capacitor is compromised.
[0047] In one embodiment a nanoparticle dispersion is applied after
formation of the initial conductive polymer layer and after
formation of subsequent conductive polymer layers. The
nanoparticles may be used in combination with the non-ionic polyols
or as a separate layer to enhance the formation of a layer that
adequately covers the edges and corners of the anodes. The sequence
of applying the nanoparticle dispersion material followed by
applying a conductive polymer layer is repeated until the desired
layer thickness is reached. Without limit thereto 2-10 cycles of
the nanoparticle dispersion and conductive polymer layer
application is suitable for demonstration of the invention.
Nanoparticle dispersions comprise nanoparticles with the particle
size of the nanoparticle of no more than 100 nm and more preferably
no more than 50 nm. Nanoparticles of the nanoparticle dispersion
are selected from aluminum oxide, zinc oxide, silicon oxide and
cerium oxide. These nanoparticle dispersions are available from Byk
Additives And Instruments under commercial name Nanobyk 3600 for
aluminum oxide, Nanobyk 3810 for cerium oxide and Nanobyk 3820 for
zinc oxide.
EXAMPLES
Inventive Example 1
[0048] A series of tantalum anodes were prepared with a diameter of
5.2 mm and a length of 10.7 mm comprising a tantalum lead wire.
Tantalum oxide dielectric was formed on the surface in accordance
with the teachings of U.S. Pat. No. 5,716,511. The anodes with
dielectric were immersed in a dilute PEDT dispersion with very
small particle sizes, available as Clevios.RTM. Knano from Haraeus,
for 1 minute and dried at 150.degree. C. for 20 minutes. They were
then dip-coated with 6% sorbitol solution in water and dried at
90.degree. C. for 20 minutes. They were then dip-coated using a
PEDT slurry, available as Clevios.RTM.K from Haraeus, to form an
external polymer layer. The dip-coating with sorbitol solution
followed by PEDT slurry dipping was repeated three more times. The
optical photograph of the coated anode is provided in FIG. 4
wherein external anode coating covers the edges adequately.
Control Example 1
[0049] Anodes which were identical to those of Inventive Example 1
where coated in analogous fashion with the exception that the
coating of sorbitol was deleted. The optical photograph of the
anode is provided in FIG. 5 wherein the coverage of the top and
bottom edges is poor with some exposed dielectric visible.
Inventive Example 2
[0050] Aluminum foil coupons with etched porous structure were
anodically formed to 11 volts for a rated working voltage of 6.3
volts. They were immersed in a solution of ethylenedioxythiophene
(EDT) in isopropanol. The isopropanol was evaporated at 40.degree.
C. for 2 minutes. The coupons were then dipped in an aqueous
solution of ammonium persulfate (APS) with dispersed particles of
sodium anthroquinone sulfonate (SAQS). EDT in contact with APS and
SAQS was allowed to polymerize for 6 minutes. This chemical
oxidative polymerization step was repeated multiple times to build
the conductive polymer on the surface inside the pores. The anodes
were dip-coated using a PEDT slurry (Clevios.RTM. K from Heraeus)
to form the first layer of an external polymer. They were then
immersed in an aqueous solution of 7% sorbitol for 1 minute and
dried at 90.degree. C. for 20 minutes. Next they were dip-coated
using a PEDT slurry (Clevios.RTM. K from Heraeus). Carbon and
silver containing conductive coatings were applied as part of the
procedure to build terminal links as is known to one skilled in the
art. The anodes after carbon and silver coating were
cross-sectioned. Multiple points along the conductive polymer layer
of the edges and bulk body locations were taken for measuring the
thickness of the conductive polymer layer. The averaged thickness
values of the bulk body and the edge were listed in Table 1.
Control Example 2
[0051] Aluminum anodes were prepared with the same procedure as
outlined in Inventive Example 2 except that the polyol precoat was
not used. The averaged thickness values of the bulk body and the
edge are listed in Table 1.
[0052] The experimental data in Table 1 shows that anodes with
sorbitol precoat had significantly improved edge coverage.
TABLE-US-00001 TABLE 1 Conductive polymer layer thickness on
aluminum anodes Body(.mu.m) Edge(um) Body/Edge Control Example 2
10.6 1.42 7.5 Inventive Example 2 6.77 2.23 3.0
Corrosion Test
Comparative Example 2
[0053] Aluminum anodes were prepared with the same procedure as
outlined in Inventive Example 2 except that the precoat was applied
using a commercial product, Clevios.RTM. K Primer from Heraeus.
[0054] The anodes prepared in the Inventive Example 2 and
Comparative Example 2 were placed in a chamber set at 121.degree.
C. and 85% RH under 1.76 atm pressure for 21 hours. The rated
working voltage of 6.3V was applied to the samples. After this
corrosion test samples prepared by the Inventive Example 2 showed
no sign of corrosion as shown in FIG. 6 while samples made by the
Comparative Example 2 showed severe corrosion as shown in FIG.
6.
[0055] The invention has been described with reference to the
preferred embodiments without limit thereto. One of skill in the
art would realize additional embodiments and alterations which are
not specifically stated but which are within the scope of the
invention as set forth in the claims appended hereto.
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