CA1050361A - Method for preparing the container of an electrolytic capacitor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A slurry of valve metal particles is dispensed into a valve metal can. The can is spun and simultaneously heated, thereby forming a compacted coating of the valve metal particles adhered to the inner wall of the can. These steps may be repeated as often as desired to build the thickness of the coating. The coated can is subsequently sinter fired at from 1600°C to 2100°C.

Description

ilD36~L
This ir~vention relates to a method ~or preparilg the con~
tainer of an slec~roly~ic capacitor, and more particularly to a method for providing a porous val~e~ metal coat~g on the inslde of a valve metal can~
It is known in the mallufacture of electrolytic capacitors haviDg a wet electrolyte to provide a m~tal container with a porous metal liner æenTing as the cathode O Such arl electrolytic capacitor has a reverse voltage capability9 or wherl ~he porous metal is a valve metal the capacitor may perorm as a b~polar capacitor4 Various means have been e~ployed to prepare such a lin~d containerO In one method9 m~tal powder is pre~sed against the irl~
s~de walls of a metal call, the powder a~d the can beillg of th~ same film~ormiDg metal; the unit is then sinter ired, forming ~ porous metal linerO In another method, the i~side surface of a can is of noble m~tal and a paint containing colloidal carbon is pain~ed o~Ter the noble metal surface and heated to dri~e off the vehicle, l~aving a porous carbox~ linerO AlternativelyD a porous pla~inum coating may be applied by electrodeposition as described by Robinsorl and MilLard in U~,S0 3,082,3600 ~ feature of the present irlvention is ~he pro~rision of a method for applyi~g orlto a valve~metal inside surace of a container a~ adherent cor~pactQd and porous coating of valve metalO Anot~r feature is the provision of a contairler for a wet electroly~ic capa~
ci~s)r of the bipolar type4 In accordance w:ith this inven~ion a can ha~7iDg a valve~
metal irmer surface and conta~ng a valv~metal slurry i8 SpU~I and heated ~o provide a coating of valve~metal particles on the inner sur~ace.
In a drawing which illustrates ambodiments of the i~vention, F~gure 1 is a sectional view o~ a cylindrical can contain~
ing a small quantity of a slurry and having a removable stop therein~
and

- 2 ~ ~S ~ 3 6 Figure 2 is a sectional view of a wet electrolytic capa~
citor employing the container prepared by the me~hod of the pre~
ferred embodimentO
In general~ the method o:E this invantion includes dispen~
sing in~o a valve metal can having a~ open end9 a slurry comprising particles of said va~ve matal and a vehicla~ The can is spun and simNltaneously heated, thereby forming a compacted ~oating o~ ~he valve metal par~icles adhered to the inner walls of the canO The can is subse~uently sinter :Eired3 ther~by provldlng a porous val~re 10 m~tal co~ting inside ~he s:a~.
Prior to spinning and heating9 there is placed just inside the ope~ end of the can a stop such as a resilie~t ~leeve to stop the spun slurry from escaping the can and to keep the mouth portion of the ca~ clea~ for subsequent attachment of a cover and header.
Also, prior to spinni~g and heating, a mask~ng shee~ being conformal to the inside bottom of the can may be placed there~ ~o keep the can bo~tom free of slurry~
The vehicle consists of a solvent a~d a binder~ Tha sol~
v~nt evapora~es during the hea~ing step~ and the binder drys and holds the valve-me~al coating in place. A~ si~ter ~iring the bi~der is disintegratedO
~ he dispensing" spinning and heating steps may be repeated prior to sinteriDgb Altarnatively ~he dispensing~ spinning~ hea~ing and sintering steps may be repea~ed as required ~o achieve a coat~
25 iDg thickness as great as desiredO
A cylindrical can 10 made of tantalum and having an open e~d is sh~wn in Figure lp A plastic dise 12 of any mai:erial that can withstand heating to about 125Ca such a~ polypropylene or B Teflon (Trade ~aa of E~I,tDuPont), is placRd in the bottom of the 30 can lOo Inserted inside the can 10 ju~t below the open end is a Te10n sleeve 14 ha~ing a resilient interfer~nce fit therein~ Any suitable removable stop may be sub~tituted for the sleeve; for ~ ~5~D3 ~ 1 exa~ple, several layers of an adhe~ive tape or a plastic plug would be suitableO
A slurry 16 is prepared by mixing flnely divided tantalum powder of 1 to 25 microns average cliameter in a liqllid vehicle com-S posad of a binder such as methylmethacrylate and a solvent such asacetone9 toluene or a subs~i~utcd glycol ather~ A preerred ~ix~ure consists of 15 grams of tantalum pawder of average particle size about 4 microns~ 13 grams s:~f methas:rylata~ a~d 75 mulliliters of ethy~ne glycol mono~utyl eth~r~
A predetermined quantity of the slurry 16 is dispensed into the bot~om of can 109 The can 10 is th~n ~pun about its axis 11 by being moullted ~n a motor dr~ve~ spinni}lg apparatusO Th~
slurry 16 ~s ce~trifugally forced against the imler sur~ce of the cylindrical walls of the can 10~, The non~ettiDg surface charac-1~ teristics of the Te:Elon disc 12 encourages all of the slurry to be sw~p~ away from the bottom of the canO The ~ubber sleeve 14 ~tops or masks the spu~ slurry from running further up the can wallD and adva~tageously pravents t~e upper por~ion of the cylindrical wall from being coateda The quantity o:E ~he slurry th2lt is dispensed 20 will i~ a casl o given geo~etry dete~nine the liner ~hickness,. T~e sle~ve stop mat~rial must haye a ~hickr2ass equal to or greater than the ~pilming liner coating to efect~ely dam nd contain i~c"
Simultalleously with the spinDing~ heat is applied to the oan by any convenient means such as forced hot air3 radian~ heating, 25 flam~ heati~g alld the like~ While the spil~ning uniformly spreads the ~lurry over the cylindrical wall o the can9 the heating drys the vehicle by driving off the volatile solvent~, This simultaneous spinning and hPating step ~ypicall~ requir~s only a raction of a minute to spread and dry ~he m~xture in~o a non~flowing adherent 30 coatir~ 50 that the can may b~ handled without changing the position or shape o the coating1>

~ 4 -- .

~OS[)369L

The plastic disc 12 and t;he stop sleeve 14 are then rea m~ved~ alld the coated can is fired in a vacuum furnace at a tempera-ture b~tween 1600C to 2100C7 and preferably at 1750~Cc DuriDg ~iri~g the orgallic binder is burnecl of and the tantalum particles 5 are sin~ered ~o each other a~d to t:he tantalum canO Th~s" when the binder disintegrates~ a hard porous cover:LDg is sinter bound to Jche inside wall of th~ can.
If a~ e~en thicker coverillg ls desirad, the whola process may be repeatedO However~, a thiek ~ ltiple layer may a:lso be 10 achieved by the alterrlat~ve stepQ o~E placing the masking disc and stop sleeve in the can9 dispensing ~he slurry5, simultaneously Spil~-DiDg a~d heatingS and then repeating the dispansing alld spin~heat steps as often as necessary to build the desir~d thlckness, a~d finally removing the masks and firing., It should be noted tha~ it 15 is not n~cessary to use the maSkiDg disc si~ce the æpinning will ramcrva most of ~he slurry from ~he bottom of the canO Howe~era its us~ ~s preferred as will be bet~er understood ~rom ~he follow~g descrip~ion of a capacitor asse~blyO
With referencQ to Figure 2~ the tantalum ca~ 10 has a 20 porous layer of tantalum 16a that is sin~er bonded to the inside surface of the cyli~drical wallO A porous tantalum pelle~ 20~
having been formed by pressing tan~alum powder in a mold a~d fir~Dg at abo~t 1750~C~ ha~ welded at one end a tantalum lead wire 21~
The porous layer 16a of the tantalum ca~ and the porous pellet 20 are each oxidized in an electroly~ic solution with the tantalum being connacted to the positi~e side of the D~C~ forming voltage suppl~ O
me lead wire 21 is h~ld coax~ally in an orificc of a tantalum header plug ~S~ and glass 27 iæ ~used in the orifice seal~
ing a~d providing st~uctural support for the wire 21 in ~he header plug 250 An i~sulative cup 23 is ~it~ed into the bottom of the can lOo An electrolyt~ solution 24 such as white sulphuric acid -- 5 ~

~ 3 6 ~
gel is dispensed into the can, and the pelle~ and header assembly is inserted into the open s:lde of the can so ~hat the pellet 20 is snugly bottomed and held by the cup 23 at one e~d and supported at the other end by the l~ad wire~ 210 A bead weld 29 is formed~ pre-ferably by TIG (tungsten inert gas) welding~ between the can 10 andthe haader plug 250 From the foregoing it can be seen that the proper assembly of the pellet and header into the electrolyte containing ca~ is only mad~ possible by co~trolling a~d predeterminin~ the fit of the cup 23 wi~hin the coated ca~ lO and a~out ~he pellet 20u FurtherD
the amount of electrolyta 24 dispensed ~nto the can 10 mus~ be care-fully predeermined~ Also~ it is ~ecessary to accura~ely fastQn the pellet 20 by the lead wire 21 to the header 25 so that whe~ the pellet bottom~ in the cup 23, the header lies flush with the open end of the canO It is easier to control the bottoming of the cup in the can when a di~c has been used to prevent any coating o tan$alum powder on the can bottomO
It should be noted ~hat the fit of the cup 23 within the coated can 10 depends o~ k~owing bef~rehand the inner diameter of ~he coating 16a~ The spinni~g/heating step of ~his invention eom~
bined with a controlled amount o~ dispensed slurry provides a highly u~iform and repeatable m~ans or controlling in a predetermi~ed manner thi~ ~nner diameter r Prototype capacitors were made in accordance with the principles of thi~ iuvention~ employing a tantalum can 10~ a prassed and fired porous tantalum pellet 20 having a tantal~m wire 21 weld~
ed thereto~ The plug 25 was made ~f tantalum~ and the sleeve 14 as well as the disc 12 were of ~eflo~ The can 10 has a diameter o about 3/8 inch ~0~95 cm) a~d is 314 inch ~1~90 cm) longv The above me~tioned preferred slurry mixture lS was accu~
rately dispersed into the can 103 using a programmed pneumatic dispenser ~Model 101 made by Laurier Associates)0 The can 10 was ~ 3 ~ ~

spun about its axis at about 30 revolutions per minute for about one minute while a blast of hot air was dlrected at the spinning cana The temperature of the can re,ached about 125C~ although this temperature is not criticalO
After the hot spin molding s~ep9 ~he can with coating ~16a) was sintered in vacuum at 1750C and the capacitors were assembled as descr~bed aboveO Elec~rlcal measurements made on the prototype capacitors indicate that the spin molding method of this i~vention prov~des a more con~pact and efficient porous layer than could be aehievPd by casting a~d dryi~gO The telling figure of marit of the tar~talum liner is represent~d by the capacitor formed in the liner being characteri~ed by 100~000 micro~arad~volts per cubic inch~ Th~ spinDi~g and heating advantageously compacted the powder to provide ~his high figure of m~rit . Greater spinnlng 15 speeds would effect a greater compactionq In à typical po~sous tantal~m pelle~ formed under m~ny ton~ of pr~ssure per square inch~
the capacitor made therefrom would ~ormally have a figure of merit o:E about 600,000 microarad~volts per cubic iDIcho Tantalum metal is relatively dense making it par~icularly subject to compac~ion 20 by this spinning method co~pared with other commonly used valve metals 0 Although a rou~d can is prsferred~, any sh~pe of can will be suitable for use ~n the method of this i~ventioll~ For example a rectangular or more particularly a sq~are can may be used5, ~he 25 liner i~orming more thickly in the corners tharl the walls thareof~
In fact the liner may be discontlnuous about the per~phery o:E such a can.,

Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for preparing the container of an electrolytic capacitor comprising:
(a) placing a stop on the inside surface of a metal can near an open end thereof, and fitting a mask to the inside bottom of said call;
(b) dispensing into said can a slurry comprising parti-cles of a valve metal and a vehicle, the inside surface of said can being made of said valve metal;
(c) spinning said can causing said particles to be com-pacted by centrifugal force against said inside surface;
(d) heating said can simultaneously with said spinning;
(e) removing said stop and said mask; and (f) sinter firing said can and said compacted valve metal particles at 1600°C to 2100°C.

2. The method of claim 1 wherein said stop is a resilient sleeve whose normal outer perimeter is slightly larger than the inside perimeter of said can at said open end.

3. The method of claim 2 wherein said mask is a fluorocarbon plastic sheet conforming to said can bottom.

4. The method of claim 1 wherein said vehicle consists in a binder and a solvent, said solvent evaporating during said heating and said binder disintegrating during said sinter firing.

5. The method of claim 1 wherein said steps of dispensing, spinning and heating are repeated two or more times prior to said sintering.

6. The method of claim 1 wherein said steps of dispensing, spinning, heating and sintering are repeated two or more times.

7. The method of claim 1 wherein said valve metal is tantalum.