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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 ph/~
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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 ph/l' ~05~678 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 .. ..
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 ' ~; "'': ' ph/,' ~
~- 1(359~78 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.
. . . -~, 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 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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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 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 ;~
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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~59f~7l~3 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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