Note: Descriptions are shown in the official language in which they were submitted.
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B CKGROUND OF THE INVEN'rION
This invention broadly relates to an improved
capacitor construction. More par-ticularly this invention
relates to an improved capacitor, such as a tantalum
capacitor construc-tion, wherein a special thermally stable
coating composition is used in constructing the capacitor.
The state of the art is indicated by the following
references: U.S. Patents 3,573,230; 2,983,624; 3,025,185;
2,968,649; 3,132,12~; 3,051,677; DuPont Viton Bulletin
No. 16 "Solution Coatings of Viton" by J.M. Bowman; and,
DuPont's "The Engineering Properties of Viton Fluoroelastomer";
and, Acheson Industries, Inc. product data sheet for
"Electrodag ~05".
It is a main object of this invention to provide a
new and improved capaci-tor.construction which includes a
special thermally stable coating material as a part of its
. construction.
In one aspect the present invention provides in a
solid capacitor for providing electrical capacitance due to
a dielectric component in the capacitor, the improved con-
struction including an anode, a cathode, and at least one
coating composition applied in the construction between the
anode and the cathode, the coating composition being comprised
of a fluoroelastomer for providing the coating with thermal ~.
stability to temperatures of about ~00-700F., and a con- .
duc-tive pigment for providing conductivity to the coating,
wherein the conductive pigment is made substantially of a
finely particulated.ma-terial selected from at Ieast one of
the group consisting of noble metals, silver, copper, and
alloys thereof.
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Other objects, features, and advantages of the
present invention will become apparent from the subsequent
description and the appended claims taken in conjunction
with the accompanying dr~wing.
BRIEF DESCRIPTION OF THE DRAWING
The drawing ~igure illustrates in cross section
a preferred embodiment of a capacitor construction in ac- i
cordance with the invention.
SUMMARY OF THE INVENTION
Tantalum capacitors have been produced in the
past wherein a resin bonded silver coating composition
containing an acrylic resin binder was used. Such prior
capacitors have had a maximum stable long term operating
temperature limit of approximately 185F. Above this
temperature such capacitors were not reliable. One purpose
of this invention is to provide a new capacitor construction
which can operate at high temperatures, that is for example
at a temperature in the range of 400F-700F, as this has
generally not been possible with prior capacitors. The
invention herein at least in part comprises employing ther-
mally stable ~luoroelastomer containing coating composi-
tions, which coating compositions readily wet the sur~ace
to which they are applied in fabricating the capacitor as-
sembly, typically à graphite layer. This special coating
composition herein described dries rapidly to form a
coating which is notable for its ability to be wetted by
the solder which is required for attachment of the conduc-
tor o~ the capacitor construction; and for its low
dissipation factor at high frequency, for example at one
megahertz; and for its thermal stability during the life
of the impxoved capacitor construction.
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"DESCRIPTION OF PREFERRED EMBODIMENTS
The drawincJ illustrates a preEerred capaci-tor
construction in accordance with the invention comprising
a capacitor generally designated 200 which includes an
anode lead 202 passing through and electrically insulated
from a closure means 204 such that the anode lead com-
municates with the interior of the capacitor 200. The
anode lead 202 is in electrical contact with a sintered
tantalum powder anode 206 which has a conductive coating
208 thereon comprised for example of colloidal graphite
coating.
A special thermally stable dispersion coating ~ i
compositlon 216 overlies the coating 208. The exterior `
of the capacitor 200 is bonded with solder 218 to the
coating 216, this being the cathode connection 218 which
terminates through the can 219 in cathode lèad 220. The
special thermally stable dispersion coating 216 forming
- a part of the construction of capacitor 200 provides
this capacitor with the highly useful and advantageous
electrical properties as is explained herein.
The coating composition (for ~orming the layer
216) contains total solids within the broad range of
about 5% to about 80% by weight of the total weight of ,~
the coating composition and, preferably the percent solids
is maintained within the range of about 45% to about 70
by weight.
The fluoroelastomer material used in the coa~
ting should be present within the broad range of about
3% to about 20% by weight of the coating solids and pre-
ferably from about 5% to about 10% by weight. Thisfluoroelastomer material prov1des the special function
of endowing the coating with great thermal stability,
that is, resistance to temperatures of about 400F ~
700F. The fluoroelastomer material used in the invention
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~hould be one which provides this high temperature resis-
tance while at the same ti~e possessing the property of
being able to operate as an effective binder material for
the applied coating. Particularly suitable materials for
use as the fluoroelastomer herein m,ay be described as
vinyl/olefinic fluoroelastomeric polymers, vinyl-fluoro-
carbon elastomeric copolymers, vinylidene/fluoro-olefinic
elastomeric polymers, C2-C4 olefinic/fluorocarbon elasto-
meric polymers and fluorinated acrylic polymers. A pre-
ferred material is the vinylidene fluoride/hexaEluoropro-
pylene copolymer fluoroelastomer. Commercially availablefluoroelastomers which may be used are Viton A or B,
~DuPont trademark) and Fluorel FC-2170 ox KF-2140 (3M Com-
pany trademark). Still further fluoroelastomers which may
be used herein are described in U.S. Patents 2,968,649,
3,051,677 and 3/172,124.
The conductive particles or pigment for use in
the coating 216 should be present within the broad range
of about 80% to about 97~ by weight of the to-tal solids
of the coating and preferably this range should be from
about 90% to about 95% by weight. Suitable pigments for
use in the coating are various finely particulated pigments
such as silver particles, copper particles, noble metal
particles and alloys thereof, and silver coated particles.
Silver particles are preferred.
The silver pigment preferably employed has a mesh ;
size of about 90~ through 325 mesh; and an apparent density
of about 16-32 grams per cubic inch (Scott Volumeter).
The percentage of conductive pigment particles
required in this coating composltion should be above a min-
imum-level as generally described above in order to obtain
coatings to which solder will adhere readily. This is a
function to some extent of the particle size and shape of
the silver particles, and the particular fluoroelastomer
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material being used. When the conduc-tive particle content,
such as the silver particles, is over 97~ khe coatlngs ap-
pear to lose integrity and adhesion.
The flow control agent used in the coating may be `
present broadly within the range of zero up to about lO~i by -
weight of the coating solids and preferably no flow control
agent is used.- Particularly suitable materials for use as
the flow control agent are finely divided materials selected
from the group consisting of silicas and silicates. Specific
I0 materials for this case are Cab-o-sil~ and Bentone~. ;
The solvent carrier material used for forming solu-
tions or dispersions of the coating may satisfactorily be
selected from any number of different solvents or blends
thereof, such as methylethyl ketone, acetone, various other ,
ketone type solvents, esters, dimethylformamide, and numer-
ous other organic solvent materlals-. The solvent forms
the balance of the coating composition when the coating is
- ~ formulated in a solution having a total solids content be-
tween about 1% and 80% by weight solids with the solvent
subsequently being driven off or evaporated leaving the ap-
plied coating.
The coating composition herein is applied during
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fabrication of the capacitor and provides a surface to which i;
an electrical lead can be readily soldered.
Resistance character of coating compositions such
as 216 herein are evaluated by forming dried or cured coat-
ings under uniform-conditions, upon which the electrical re~
sistance measurements can be made. It is convenient to de-
posit the coatings by use of a blade coatin~ device which ~ ;
will form on a glass plate layers of predetermined thickness.
Measurements heréin have been made on a dry coating after
curing for ten minutes at 300F.
The electrical characteristics of the coating
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compositions herein have been discovered to make them highly
suitable for use in capacltor constructions.
The electrical resistance in the following examples
was measured using a special fixture which gave data in ohms
per square. The resistance measurement was made with a suit-
able precision mllliohmmeter. These values demonstrate in
a convenient manner the relative electrical conductivities
of these materials.
EXAMPLE 1
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Parts by Weight
Silver Pigment 90.91
Fluoroelastomer ~Viton B) 9.09
Methyl Ethyl Ketone 62.33
162.33
Resistance 0.19 ohms per square
at 1 mil thickness
Solderability good :
EXAMPLE 2 .
Silver Pigment 93.35 :.
Fluoroelastomer (3M FC-2170) 6.65 .
Methyl Ethyl Ketone 66.68 .. .
. . 166.68 ,
Resistance 0.15 ohms per square
. at 1 mil thickness
Solderability excellent
EXAMPLE 3
Silver Powder 93.35
Fluoroelastomer (3M KF-2140) 6.65
Methyl Ethyl Ketone 66.68 .
166.68
Resistance 0.3 ohms per square
at 1 mil thickness ;-:
Solderability good ~
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EXAMPLE 4
Parts by Wei~Jh-t
Silver Powder 95.0
Fluoroelastomer (Viton B) 5.0
Mekhyl Ethyl Ketone 135 7---
Resistance 0.12 ohms per square
at 1 mil thickness
Solderability very good
EXAMPLE 5
Silver Powder 93.35
Fluoroelastomer (Viton B) 6.65 .-~
Methyl Ethyl Ketone _ 34 i-
123.34
Resistance .152 ohms per square
at l mil thickness
Solderability Good ;~
EXAMPLE 6
Silver Powder 90.32
Fluoroelastomer (3M KF-2140) 9.68
Methyl Ethyl Ketone 77.42 ~ -
177.42
Resistance .23 ohms per square
at l mil thickness
Solderability excellent
All of the above coating examples were applied to
a glass plate by a 3 mil "Bird"~ blade and cured 15 minutes
at 300F. ~11 formulations were manufactured by appropriate
ball or pebble milling techniques. All of the above coatings
were applied in the conventional manner to tantalum slugs and
then checked for dissipation factor. The impedance value
from the material of Example 6 applied to tantalum slugs was
0.485 (at resonant frequency of tantalum capacitor and taken
at ambient room temperature).
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EXA~PLE_7
~ tantalum capacitor i~ manufac~:ured by firs-t
taking a tantalum wire and forming thereon a sintered tan-
talum slug which encapsulates one end of the tantalum wire.
The tantalum riser wire is then welded to a fixture bar for
further processing. Durlng the sintering operation under
oxidizing conditions a tantalum oxide layer forms coating the
surface of tantalum in the slugO Following this, a manganese
dioxide layer is formed on the slug by dipping the slug several
times into a manganese nitrate solution and pyrolyzing to
form an oxide layer. Next a graphite coating is applied by
dipping in a 10~ aqueous dispersion of Aquadag~ E colloidal
graphite; and, then airdrying. The special thermally stable
coating composition in accordance with Example 1 hereinahove
is applied as the next layer by dipping one or two times in
a coating dispersion formed in accordance with Example 1
above, and then air-drying. Following this, a solder coating
~- is applied by dipping in a molten solder bath. ~astly, a
"canning" operation is carried out, for example, as shown
in the drawing figure; or, "potting" of the final capacitor
construction may be carried out by dipping the capacitor in
an epoxy potting composition after first attaching a lead
wire to the solder.
While it wlll be apparent that the preferred
embodiments of the invention disclosed are well calculated
to fulfill the objects above stated, it will be appreciated ;
that the invention is susceptible to modification, variation,
and change without departing from the proper scope or fair
meaning of the subjoined claims. :~
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