U.S. patent application number 09/458584 was filed with the patent office on 2002-06-06 for single, very high volt capacitor for use in an implantable cardioverter defibrillator.
Invention is credited to MARSHALL, TIMOTHY R., STRANGE, THOMAS F..
Application Number | 20020067589 09/458584 |
Document ID | / |
Family ID | 23821350 |
Filed Date | 2002-06-06 |
United States Patent
Application |
20020067589 |
Kind Code |
A1 |
MARSHALL, TIMOTHY R. ; et
al. |
June 6, 2002 |
SINGLE, VERY HIGH VOLT CAPACITOR FOR USE IN AN IMPLANTABLE
CARDIOVERTER DEFIBRILLATOR
Abstract
The present invention is directed toward a very high volt
capacitor for use in an implantable cardioverter defibrillator. In
particular, by the inclusion of a polymer matrix of a hydrogel,
preferably of the family of polyhydroxyalkylmethacrylate) but also
including polyvinyl alcohol (PVA), polyacrylnitrile (PAN), into a
standard fill electrolyte, the breakdown voltage of the enhanced
very high volt electrolyte of the present invention is raised to as
much as 800 V. A very high volt electrolytic capacitor according to
the present invention, impregnated with the enhanced very high volt
electrolyte of the present invention, is able to support voltages
of 700 to 800 volts, while maintaining the described desired
properties, and is therefore superior to other known electrolytic
capacitors for use in implantable cardioverter defibrillators. The
production of a very high volt capacitor capable of operating at a
voltage of 700 to 800 volts allows a single high volt electrolytic
capacitor to replace the conventional two capacitors-in-series
arrangement of an Implantable Cardioverter Defibrillator (ICD).
Having a single high voltage capacitor results in savings in cost
and in space required, especially where internal volume is at a
premium, such as in an ICD and related medical implant devices.
Inventors: |
MARSHALL, TIMOTHY R.;
(PICKENS, SC) ; STRANGE, THOMAS F.; (EASLEY,
SC) |
Correspondence
Address: |
STEVEN M MITCHELL
PACESETTER INC
701 EAST EVELYN AVENUE
SUNNYVALE
CA
94086
US
|
Family ID: |
23821350 |
Appl. No.: |
09/458584 |
Filed: |
December 9, 1999 |
Current U.S.
Class: |
361/503 |
Current CPC
Class: |
H01G 9/022 20130101 |
Class at
Publication: |
361/503 |
International
Class: |
H01G 009/00; H01G
009/02; H01M 006/04 |
Claims
What is claimed is:
1. A very high volt electrolytic capacitor impregnated with a
polymer electrolyte mixture comprising a polymer matrix of a
hydrogel in a solvent-based liquid electrolyte.
2. The electrolytic capacitor of claim 1, wherein said polymer
matrix is a poly(hydroxyalkylmethacrylate) based polymer
matrix.
3. The electrolytic capacitor of claim 2, wherein said polymer
matrix comprises hydroxyethylmethacrylate.
4. The electrolytic capacitor of claim 3, wherein said polymer
electrolyte mixture has a ratio between 30% and 60% by weight
hydroxyethylmethacrylat- e.
5. The electrolytic capacitor of claim 1, wherein said polymer
matrix is a polyvinylalcohol based polymer matrix.
6. The electrolytic capacitor of claim 1, wherein said polymer
matrix is a polyacrylonitrile based polymer matrix.
7. The electrolytic capacitor of claim 1, wherein said
solvent-based electrolyte comprises ethylene glycol.
8. The electrolytic capacitor of claim 1, wherein said
solvent-based electrolyte comprises propylene glycol.
9. The electrolytic capacitor of claim 1, wherein said
solvent-based electrolyte comprises 1-methyl-2-pyrrolidone.
10. The electrolytic capacitor of claim 1, wherein said
solvent-based electrolyte comprises gamma-butyrolactone.
11. The electrolytic capacitor of claim 1, wherein said
solvent-based electrolyte comprises dimethyl formamide.
12. The electrolytic capacitor of claim 1, wherein said
solvent-based electrolyte comprises dimethyl acetamide.
13. The electrolytic capacitor of claim 1, wherein said
solvent-based electrolyte comprises boric acid and phosphoric acid
in an ethylene glycol solvent base.
14. The electrolytic capacitor of claim 13, wherein said
solvent-based electrolyte comprises an ethylene glycol solution of
2.0%.+-.0.5% Boric acid, 5.0%.+-.2% water, and 50 ppm phosphoric
acid.
15. The electrolytic capacitor of claim 1, wherein said polymer
electrolyte mixture further comprises a polymerization initiator
compound.
16. The electrolytic capacitor of claim 15, wherein said
polymerization initiator compound comprises a persulfate
(S.sub.2O.sub.8.sup.-2) salt.
17. The electrolytic capacitor of claim 1, wherein said polymer
electrolyte mixture further comprises a crosslinking compound.
18. The electrolytic capacitor of claim 17, wherein said
crosslinking compound is tetraethyleneglycoldiacrylate.
19. The electrolytic capacitor of claim 17, wherein said polymer
electrolyte mixture has 1%.+-.0.5% by weight of said crosslinking
compound.
20. The electrolytic capacitor of claim 1, wherein said capacitor
is of the flat capacitor design.
21. The electrolytic capacitor of claim 1, wherein said capacitor
is of the rolled capacitor design.
22. The electrolytic capacitor of claim 1, comprising an aluminum
foil anode.
23. The electrolytic capacitor of claim 1, wherein said capacitor
has a breakdown voltage of greater than 700V.
24. An electrolytic capacitor comprising an electrolytic capacitor
stack or wound roll prepared by a process for impregnating said
electrolytic capacitor stack or wound roll, said process
comprising: (a) impregnating said stack or wound roll with a
polymerization initiator (b) contacting said stack or wound roll
with a polymer electrolyte solution comprising a polymer matrix of
a hydrogel in a solvent-based liquid electrolyte; and (c) curing
said polymer.
25. A process according to claim 24, wherein said polymer matrix is
a poly(hydroxyalkylmethacrylate) based polymer matrix.
26. A process according to claim 25, wherein said polymer matrix
comprises hydroxyethylmethacrylate.
27. A process according to claim 24, wherein said polymer matrix is
a polyvinylalcohol based polymer matrix.
28. A process according to claim 24, wherein said polymer matrix is
a polyacrylonitrile based polymer matrix.
29. A process according to claim 24, wherein said solvent-based
electrolyte comprises boric acid and phosphoric acid in an ethylene
glycol solvent base.
30. A process according to claim 24, wherein said solvent-based
electrolyte comprises an ethylene glycol solution of 2.0%.+-.0.5%
Boric acid, 5.0%.+-.2% water, and 50 ppm phosphoric acid.
31. An electrolytic capacitor comprising an electrolytic capacitor
stack or wound roll impregnated with a polymer electrolyte solution
according to a process for impregnating said electrolytic capacitor
stack or wound roll, comprising: (a) impregnating said stack or
wound roll with a solution comprising a polymerization initiator
and a polymer matrix of a hydrogel in a solvent-based liquid
electrolyte; and (b) curing said polymer.
32. A process according to claim 31, wherein said polymer matrix is
a poly(hydroxyalkylmethacrylate) based polymer matrix.
33. A process according to claim 32, wherein said polymer matrix
comprises hydroxyethylmethacrylate.
34. A process according to claim 31, wherein said polymer matrix is
a polyvinyl alcohol based polymer matrix.
35. A process according to claim 31, wherein said polymer matrix is
a polyacrylonitrile based polymer matrix.
36. A process according to claim 31, wherein said solvent-based
electrolyte comprises boric acid and phosphoric acid in an ethylene
glycol solvent base.
37. A process according to claim 31, wherein said solvent-based
electrolyte comprises an ethylene glycol solution of 2.0%.+-.0.5%
Boric acid, 5.0%.+-.2% water, and 50 ppm phosphoric acid
38. An implantable cardioverter defibrillator (ICD) comprising a
very high volt electrolytic capacitor impregnated with an
electrolyte comprising a polymer matrix of a hydrogel in a
solvent-based liquid electrolyte.
39. An ICD according to claim 38, wherein said polymer matrix is a
poly(hydroxyalkylmethacrylate) based polymer matrix.
40. An ICD according to claim 39, wherein said polymer matrix
wherein said polymer matrix comprises hydroxyethylmethacrylate.
41. An ICD according to claim 38, wherein said polymer matrix is a
polyvinylalcohol based polymer matrix.
42. An ICD according to claim 38, wherein said polymer is a
polyacrylonitrile based polymer matrix.
43. An ICD according to claim 38, wherein said solvent-based
electrolyte comprises boric acid and phosphoric acid in an ethylene
glycol solvent base.
44. An ICD according to claim 38, wherein said solvent-based
electrolyte comprises an ethylene glycol solution of 2.0%.+-.0.5%
Boric acid, 5.0%.+-.2% water, and 50 ppm phosphoric acid.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to a very high volt
electrolyte for use in electrolytic capacitors and to an
electrolytic capacitor impregnated with the electrolyte of the
present invention for use in implantable cardioverter
defibrillators (ICD). More specifically, the invention relates to
the incorporation of a polymer matrix into a standard solvent-based
fill electrolyte to raise the breakdown voltage (limit) of the
electrolyte to as much as 800 V and, in turn, to a very high volt
aluminum electrolytic capacitor impregnated with the electrolyte of
the present invention, operating at a voltage of 700 to 800
volts.
[0003] 2. Related Art
[0004] Compact, high voltage capacitors are utilized as energy
storage reservoirs in many applications, including implantable
medical devices. These capacitors are required to have a high
energy density since it is desirable to minimize the overall size
of the implanted device. This is particularly true of an
Implantable Cardioverter Defibrillator (ICD), also referred to as
an implantable defibrillator, since the high voltage capacitors
used to deliver the defibrillation pulse can occupy as much as one
third of the ICD volume.
[0005] Implantable Cardioverter Defibrillators, such as those
disclosed in U.S. Pat. No. 5,131,388, incorporated herein by
reference, use two electrolytic capacitors in series to achieve the
desired high voltage for shock delivery. For example, an
implantable cardioverter defibrillator may utilize two 350 to 400
volt electrolytic capacitors in series to achieve a voltage of 700
to 800 volts.
[0006] To further reduce the size of the implanted device, there is
a need for a single capacitor arrangement for an ICD, capable of
operating at a voltage of 700 to 800 volts, which can replace the
current two capacitors in series arrangement. However, this has not
been possible since available electrolytic capacitor technology has
limited photo flash electrolytic capacitor voltages to 600V and
below.
[0007] Electrolytic capacitors are used in ICDs because they have
the most nearly ideal properties in terms of size, reliability and
ability to withstand relatively high voltage. Conventionally, such
electrolytic capacitors include an etched aluminum foil anode, an
aluminum foil or film cathode, and an interposed kraft paper or
fabric gauze separator impregnated with a solvent-based liquid
electrolyte. While aluminum is the preferred metal for the anode
plates, other metals such as tantalum, magnesium, titanium,
niobium, zirconium and zinc may be used. A typical solvent-based
liquid electrolyte may be a mixture of a weak acid and a salt of a
weak acid, preferably a salt of the weak acid employed, in a
polyhydroxy alcohol solvent. The electrolytic or ion-producing
component of the electrolyte is the salt that is dissolved in the
solvent. The entire laminate is rolled up into the form of a
substantially cylindrical body, or wound roll, that is held
together with adhesive tape and is encased, with the aid of
suitable insulation, in an aluminum tube or canister. Connections
to the anode and the cathode are made via tabs. Alternative flat
constructions for aluminum electrolytic capacitors are also known,
comprising a planar, layered, stack structure of electrode
materials with separators interposed therebetween, such as those
disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
[0008] Conventional electrolytic capacitors that employ a standard
solvent-based liquid electrolyte utilize a thick mechanical
separator, typically made of kraft paper, that is impregnated with
and acts as a reservoir for the electrolyte. However, it has been
suggested that by using a polymer based electrolyte, the thickness
of the separator material can be greatly reduced and, in some
cases, the separator material can be eliminated entirely.
[0009] U.S. Pat. No. 4,942,501 and its continuations, U.S. Pat.
Nos. 5,146,391 and 5,153,820, each of which is incorporated herein
by reference, suggest reducing the volume of electrolytic
capacitors by completely eliminating the need for a mechanical
separator. They provide an electrolytic capacitor that instead
employs, between its anode and cathode, a layer of solid
electrolyte comprising a solid solution of a metal salt in a
polymer matrix. The preferred method of constructing these
capacitors is to deposit a liquid prepolymer electrolyte mixture
onto the surface of the anode, and then to cause polymerization to
take place to cure the electrolyte. The cathode is thereafter
formed by deposition upon the surface of the cured electrolyte
layer.
[0010] U.S. Pat. No. 5,585,039, incorporated herein by reference,
suggests producing a solid polymer electrolyte consisting of the
containment of an electrolyte solution within a polymer matrix
having a multiphase structure, suitable for use in high energy
density batteries, such as lithium batteries. Also disclosed is a
method of manufacturing the solid polymer electrolyte comprising
the steps of preparing a polymer matrix having a multiphase
structure first, followed by impregnating the electrolyte solution
into the polymer matrix. Alternatively, the method may comprise the
steps of preparing a polymer matrix having a multiphase structure
and containing an electrolyte first, then impregnating a solvent
into the polymer matrix having the multiphase structure containing
the electrolyte.
[0011] In known processes for impregnating electrolytic capacitor
stacks or wound rolls with solid polymer electrolytes, a
polymerization initiator is typically mixed with the electrolyte
prior to impregnation. For example, U.S. Pat. No. 5,628,801
discloses an electrolytic capacitor where a solid electrolyte alone
or a separator impregnated with an elastomeric solid electrolyte is
utilized in the dual capacity of electrolyte and adhesive material
to hold together the anode and cathode plates of the capacitor. The
preferred electrolyte consists of: 17.5 parts of
hydroxyethylmethacrylate, 32.5 parts ethylene glycol, 7.0 parts
ammonium adipate, 6.7 parts ammonium glutarate, 0.45 parts
tetraethyleneglycoldiacrylate, and 2.2 parts of initiator solution.
The preferred initiator solution consists of a solution of 3.6 g of
Cu(No.sub.3).sub.2.3H.sub.2O and 42.4 g of K.sub.2S.sub.2O.sub.8
per liter of pure water. The capacitor assembly is impregnated with
this polymerizable liquid electrolyte/adhesive and then heated to
approximately 55.degree. C. for at least 2 hours, but preferably 24
hours to cure the electrolyte/adhesive.
[0012] Similarly, U.S. Pat. No. 5,748,439 discloses an electrolytic
capacitor having interposed between the electrically conductive
anode and cathode layers thereof a reduced thickness spacer
comprised of a mechanical separator means such as kraft paper
impregnated with a crosslinked elastomeric electrolyte. The
electrolyte is preferably made up as a liquid prepolymer
electrolyte mixture prior to impregnation into the capacitor
element and the polymer is preferably formed in situ thereafter
from the prepolymer mixture. The prepolymer electrolyte mixture is
preferably made up by first dissolving a salt into a liquid
plasticizer component by stirring at elevated temperatures, cooling
the mixture to room temperature, and then adding to the mixture a
monomer corresponding to the desired polymer and a crosslinking
agent, as well as a polymerization initiator. As a result, the
electrolyte acts to strengthen the separator material, allowing a
storage device to be constructed with separator materials of
reduced thickness.
[0013] The problem with the above polymer electrolytes and
processes for impregnating electrolytic capacitors with such
polymer electrolytes is incomplete filling of the macroscopic
tunnels in the etched aluminum anodes. The processes described
above suggest combining a polymerization initiator compound with
the polymer electrolyte mixture, prior to impregnation, to promote
the break down of the ionic salt of the electrolyte mixture.
However, when the polymerization initiator is mixed with the
polymer electrolyte, polymerization begins, increasing the
viscosity of the solution, which reduces the working pot life.
Heating the electrolyte mixture to reduce viscosity, a common
practice in the industry, only serves to hasten the curing of the
polymer and thus defeats the intended purposes. Because of the
increased viscosity and the reduced working time, the polymer
mixture has insufficient time to fully incorporate itself into the
microscopic features of the anode foil. Capacitance is lost due to
the incomplete use of the etched foil. Consequently, such
capacitors have a breakdown voltage of less than 700 volts. Thus,
there is a need for an improved electrolyte and impregnation
process which solves these problems.
SUMMARY OF THE INVENTION
[0014] The present invention is directed to an enhanced very high
volt electrolyte for use in electrolytic capacitors. By the
inclusion of a polymer matrix of a hydrogel, preferably of the
family of poly(hydroxyalkylmethacrylate) but also including
polyvinylalcohol (PVA) and polyacrylonitrile (PAN), into a standard
fill electrolyte mixture, the breakdown voltage of the electrolytic
capacitor is enhanced by 20 to 100 volts over an electrolytic
capacitor impregnated with a standard, straight, or neat fill,
electrolyte, raising the breakdown voltage of the capacitor to 700
to 800 V, making a single capacitor ICD more practical. In order to
achieve 800 V with the polymer electrolyte, the standard fill
electrolyte must be capable of over 650V by itself. Any standard
fill electrolyte will benefit from the addition of the HEMA polymer
by improving its breakdown voltage by 20-100V. The standard fill
electrolyte may be a mixture of a weak acid and a salt of a weak
acid, preferably a salt of the weak acid employed, in a polyhydroxy
alcohol solvent. The preferred solvent-based electrolyte consists
of an ethylene glycol solution of a long chain dicarboxylic acid or
acids, boric acid, a base such as an amine or ammonia, with a small
amount of water. Examples of long chain dicarboxylic acids include
dodecanedioic, undecanedioic, dimer and trimer acids. The
electrolyte may also contain other cosolvents such as DMSO, DMF,
NMF and acetonitrile and may also include small quantities of a
long chain monocarboxylic acid.
[0015] The breakdown voltage of the electrolytic capacitor can be
further enhanced by impregnating the electrolytic capacitor with a
polymerization initiator prior to the impregnation of the polymer
electrolyte mixture. This process improves the incorporation of the
polymer into the anode foil, which thereby increases the
capacitance. This is accomplished by separating the polymerization
initiator from the polymer electrolyte mixture and locating the
polymerization initiator in intimate contact with the areas where
polymerization is desired (as in the anode foil tunnels, paper, or
cathode structure). This allows the polymer electrolyte mixture of
the present invention to be heated to any desired temperature, up
to 90.degree. C., prior to impregnation, thereby reducing the
viscosity of the solution, and allowing full impregnation into the
initiator treated stack or wound roll. The reduced viscosity
lessens resistance when the solution is filling the voids of the
anode foil. Additionally, separating the polymerization initiator
from the polymer electrolyte mixture has the advantage of
increasing the working pot life of the polymer electrolyte mixture.
Polymerization does not begin to occur until after impregnation of
the capacitor.
[0016] The very high volt aluminum electrolytic capacitor of the
present invention is capable of operating at a voltage of 700 to
800 volts, 20 to 100 volts higher than prior electrolytic
capacitors impregnated with a standard electrolyte. The design of a
very high volt capacitor according to the present invention can
include an aluminum electrolytic capacitor of the flat capacitor
design with 1 to 4 anodes per layer or of the wound or rolled
capacitor design.
[0017] This capacitor is able to support voltages of 700 to 800
volts, while being of reduced size, and is therefore superior to
other known electrolytic capacitors for use in implantable
cardioverter defibrillators. The production of a very high volt
capacitor capable of operating at a voltage of 700 to 800 volts
allows a single high volt electrolytic capacitor to replace the
conventional two capacitors-in-series arrangement of an ICD.
Replacing two lower voltage electrolytic capacitors with a single
very high volt electrolytic capacitor results in space savings,
especially where internal volume is at a premium, such as in ICDs
and related medical implant devices, and results in a reduction in
capacitor cost and in the complexity of assembly, while increasing
reliability.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] The present invention is directed toward an enhanced very
high volt electrolyte and a very high volt capacitor impregnated
with the electrolyte of the present invention for use in an ICD. In
particular, by the inclusion of a polymer matrix of a hydrogel,
preferably of the family of poly(hydroxy alkyl methacrylate) but
also including polyvinylalcohol and polyacrylonitrile, into a
standard fill electrolyte, the breakdown voltage of the enhanced
very high volt electrolyte of the present invention is raised to as
much as 800 V, making a single capacitor ICD more practical. A very
high volt electrolytic capacitor according to the present
invention, impregnated with the enhanced very high volt electrolyte
of the present invention, is capable of operating at a voltage of
700 to 800 volts.
[0019] According to the present invention, prior to the
impregnation of the capacitor, the etched and formed anode foil is
preloaded with a polymerization initiator. The polymerization
initiator is preferably an aqueous solution of a persulfate
(S.sub.2O.sub.8.sup.-2) salt, typically an alkyl metal or ammonium
salt, such as potassium persulfate, ammonium persulfate. Other free
radical initiators are suitable as well, such as
azoxyisobutyronitrile (AIBN) or benzoyl peroxide. For example, the
initiator may be incorporated into the anode foil by means of
soaking the anode foil or capacitor stack in a dilute, 0.2+/-0.05%,
aqueous solution of a persulfate salt. Additionally, miscible
organic solvents of high vapor pressure, such as methanol, ethanol,
acetone, methylethyl ketone, toluene, and other low boiling organic
solvents may be added to speed drying. Copper nitrate may also be
added as a catalyst to the polymerization initiator. A preferred
polymerization initiator solution comprises 0.09 g of
Cu(No.sub.3).sub.2 and 25 g of K.sub.2S.sub.2O.sub.8 per liter of
pure water. The impregnation of the anode foil with the
polymerization initiator may be done prior to the construction of
the capacitor or as a precursor to the impregnation of the
capacitor with the polymer electrolyte mixture. Alternatively, the
polymerization initiator compound may be added directly to the
polymer electrolyte mixture, prior to impregnation into the
capacitor, however, upon warming the electrolyte mixture,
polymerization begins, decreasing the working time.
[0020] After the foils have been dried at room temperature (less
than 35.degree. C.) to prevent the premature break down of the
persulfate salt into the sulfate ion, anodes are cut and the
capacitor assembled. In the case of a capacitor stack, the stack is
vacuum dried at room temperature (less than 35.degree. C.).
[0021] The polymer based electrolyte is prepared by adding a
polymer matrix to a standard fill electrolyte, in an oxygen limited
atmosphere where the total oxygen concentration is less than 2%.
The polymeric matrix may be based on the family of acrylate
monomers, such as hydroxyethylmethacrylate (HEMA) or
hydroxyethylacrylate (HEA), or may be based on polyvinylalcohol or
polyacrylonitrile. The preferred polymer matrix is based on
2-hydroxyethylmethacrylate, with ratios of 30 to 60% HEMA, with a
preferred ratio of 40% HEMA. The solvent-based fill electrolyte may
be any of the various fill electrolytes known to those skilled in
the art as suitable for use in electrolytic capacitor manufacture.
A typical solvent-based electrolyte is a mixture of a weak acid and
a salt of a weak acid, preferably a salt of the weak acid employed,
in a polyhydroxy alcohol solvent. Examples of acids used in the
fill electrolyte include boric, undecanedioic, dodecanedioic, dimer
and trimer acids. The salts of these acids can be ammonium salts or
salts of various amines such as monomethylamine, dimethylamine,
trimethylamine, isopropylamine. The solvents for the fill
electrolyte can include ethylene glycol, propylene glycol,
1-methyl-2-pyrrolidone, gammabutyrolactone, dimethylformamide,
dimethyl acetamide, mixtures of these materials and the like, as
would be apparent to one or ordinary skill in the relevant art.
Ethylene and propylene glycol are the preferred solvents. For
example, the solvent-based liquid electrolyte may be boric acid and
phosphoric acid in an ethylene glycol solvent base. The preferred
fill electrolyte is an ethylene glycol solution containing 4% to 9%
dimer acid, up to 2.0% boric acid, 1.0% degassing agent such as
nitroanisole, and up to 5.0% of a 20% solution of colloidal silica
in ethylene glycol, with the pH adjusted by the addition of
ammonium hydroxide. This electrolyte may also include up to 20% by
weight a co-solvent of butyl carbitol, preferably 10%. This mixture
may also contain a compound for crosslinking such as, but not
limited to, tetraethyleneglycoldiacrylate (TEGDA). For example, the
addition of 1.0+/-0.5% TEGDA to the electrolyte may be used for
cross linking of the polymer. Alternative divinyl crosslinking
agents include ethylene glycol dimethacrylate (EGDMA) and
diethylene glycol dimethacrylate (DEGDMA).
[0022] The polymer matrix is mixed with the standard solvent-based
electrolyte and is warmed to a temperature of 50 to 90.degree. C.
with a preferred temperature of 70.degree. C., to decrease
viscosity and allow for increased penetration into the microscopic
features in the anode foil. The pre-loaded capacitor is then vacuum
impregnated with the warmed mixture, by placing the capacitor in
contact with the warmed electrolyte and reducing the pressure to
less than 50 cm Hg. The capacitor is held at this low pressure for
5 to 45 minutes with a preferred time of 15 minutes, and then
pressure is restored, using the pressure to force the electrolyte
mixture into the capacitor. The capacitor is then removed and
placed in a 65 to 90.degree. C. oven with a preferred temperature
of 90.degree. C. and a maximum oxygen atmospheric concentration of
2% for a period of 2 to 24 hours, with a preferred time of 4 hours,
to break down the persulfate salt and polymerize the HEMA. The
capacitor is then aged in a normal manner by applying the working
voltage to the capacitor, allowing the capacitor to reach this
voltage, and then allowing the current to decrease.
[0023] While the above discussion has been directed to a method of
impregnation using a hydroxyethylmethacrylate(HEMA) and ethylene
glycol based electrolyte mixture, it will be apparent to one of
ordinary skill in the relevant art that a similar method could be
employed using an polymer electrolyte mixture incorporating a
different polymer matrix of a hydrogel, such as polyvinylalcohol or
polyacrylonitrile and/or a propylene glycol,
1-methyl-2-pyrrolidone, gammabutyrolactone, dimethylformamide,
dimethyl acetamide or alternative solvent-based electrolyte.
[0024] According to the present invention, an aluminum electrolytic
capacitor can be produced of the flat capacitor design with 1 to 4
anodes per layer or of the rolled capacitor design, either of which
has for its anode, aluminum foil that has been etched for use at
very high voltages, and formed at voltages of 800 to 1000 volts,
with an effective formation voltage of 920 volts for a 800 V
capacitor.
[0025] A flat capacitor according to the present invention is
constructed of anode and cathode layers, stacked with a paper
insulator or spacer between each layer. The anode layer is composed
of one or more anode foils stacked together without any paper
spacer, to form a high energy density anode element. The layers are
then grouped together in a parallel connection to produce
sufficient capacitance for the intended function. This finished
stack is inserted into a case with a geometry closely following the
contour of the stack, and designed to minimize the space occupied
inside the finished defibrillator.
[0026] In one embodiment of the present invention, the design of
the very high volt capacitor is that of a flat capacitor with a
single, one to four anode per layer design with a highly etched
aluminum anode foil having an effective formation voltage of 800 to
1000 volts. The electrolyte utilized is the very high volt polymer
electrolyte of the present invention with a breakdown voltage of
700-800 V. According to the present invention, the polymer
electrolyte mixture comprises a standard photoflash electrolyte
combined with a polymer matrix of a hydrogel, preferably of, but
not exclusive of, the family of poly(hydroxyalkylmethacrylate),
polyvinylalcohol, or polyacrylonitrile. The preferred hydrogel is a
polymerized 2-hydroxyethylmethacrylate. The photoflash electrolyte
is a standard fill electrolyte capable of reaching 650 volts
without breakdown, without the addition of HEMA. The solvent used
in the fill electrolyte may be ethylene or propylene glycol,
1-methyl-2-pyrrolidone, gamma-butyrolactone, dimethyl formamide,
dimethyl acetamide, mixtures of these materials, and the like, as
would be apparent to one of ordinary skill in the relevant art. The
preferred solvent-based electrolyte comprises boric acid and
phosphoric acid in an ethylene glycol solvent base. Most preferred,
is an ethylene glycol solution containing 4% to 9% dimer acid, up
to 2.0% boric acid, 1.0% degassing agent such as nitroanisole, and
up to 5.0% of a 20% solution of colloidal silica in ethylene
glycol, with the pH adjusted by the addition of ammonium hydroxide.
This electrolyte may also include up to 20% by weight a co-solvent
of butyl carbitol, preferably 10%.
[0027] In a second embodiment of the present invention, the design
of the very high volt electrolytic capacitor may be a traditionally
designed rolled capacitor in either a cylindrical or flattened
cylindrical shape. The anode foil has an effective formation
voltage of 800 to 1000 volts but has a lower capacitance per square
centimeter of projected area than the first design due to the fact
that the anode foil must have enough strength to be rolled. The
electrolyte utilized is the very high volt polymer electrolyte of
the present invention with a breakdown voltage of greater than 700
V, as discussed above with respect to the first embodiment.
[0028] Electrolytic capacitors according to the present invention
can be incorporated into implantable medical devices, such as
implantable cardioverter defibrillators (ICDs), as would be
apparent to one skilled in the art, as described in U.S. Pat. No.
5,522,851 issued to Fayram.
EXAMPLES
[0029] An investigation was conducted examining the capacitance and
voltage characteristics of the capacitors produced according to the
present invention. The experiments provided below are exemplary of
the capacitor constructions described above and are not intended to
limit the scope of the present invention.
[0030] The voltage measurements show that the capacitors produced
in accordance with the present invention will support voltages in
excess of 700 Volts, and up to about 800 Volts.
Example 1
[0031] Three single anode sandwich capacitors were created using
900V foils and standard fill electrolyte in a polymer base. Double
thickness paper was used as the separator. The polymer electrolyte
solution consisted of 9.6 g of a solution consisting of 95 g
standard fill electrolyte, 0.9 g TEGDA, 6.0 g HEMA and 0.4 ml
K.sub.2S.sub.2O.sub.8(sat- .). Copper nitrate was added as a
catalyst to the polymerization initiator (0.09 g Cu(NO.sub.3).sub.2
to 25 g K.sub.2S.sub.2O.sub.8). The capacitors were heated in an
oven at 70.degree. C. for 2 hours.
[0032] Full polymerization was achieved. All three single anode
capacitors reached 800V. Some aging took place, so that the current
bled down to approximately 400 .mu.A. During aging, some hissing
and popping occurred, probably due to the heating giving off water
and other liquids. Bridge measurements showed capacitance values of
1.62 .mu.F, 1.62 .mu.F and 1.47 .mu.F and an effective series
resistance (ESR) of approximately 70 .OMEGA., possibly due to
design.
[0033] While various embodiments of the present invention have been
described above, it should be understood that they have been
presented by way of example only, and not limitation. Thus, the
breadth and scope of the present invention should not be limited by
any of the above-described exemplary embodiments, but should be
defined only in accordance with the following claims and their
equivalents. Additionally, all patents, patent applications and
publications mentioned above are incorporated by reference
herein.
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