Note: Descriptions are shown in the official language in which they were submitted.
2(~ :
The present invention re`lates to elec~rical devices,
and more particularly, to cathode electrodes adapted for use -
in electrolytic, dielectric oxide film-forming metal anode
capacitors and the like.
While the present invention will be discussed hereinafter
with reference to use in electrolytic capacitors, it is to be
understood that the invention may be utilized in a variety of other ~ ;
electrical devices where a cathode electrode is required.
Electrolytic capac;tors of the sintered dielectric oxide
film-forming metal anode type generally consist of a cathode
electrode, an electrically conductive electrolyte and a porous anode ~;
with a dielectric oxide film formed thereon. Typically, the cathode
electrode is composed of silver, an alloy of silver, copper, or an
alloy of copper to impart high electrical conductivity and corrosion
resistance, the electrolyte is an aqueous solution of an inorganic
acid, and the anode is composed of a film-forming metal such as tantalum,
aluminum, or niobium, preferably tantalum. Certain inorganic salts
may be added to the electrolyte, for example see U.S. Patent 2,616,953
issued to J. M. Booe on November 4, 1952, to act as a depolarizer and
to thereby help prevent the formation of potentially damaging gaseous
products and films on the cathode electrode surface.
In most situations, the cathode electrode also functions as
the case or housing for the electrolytic capacitor by surrounding and
retaining both the anode and the electrolyte. Present practice is to -
utilize silver as a main constituent of the cathode electrode because
of the desirable electrical and chemical properties of silver. Due to
the relatively high cost of silver, it would be advantageous to use
copper or
' " '' ,' , ''' ~,, :
.
~v~z~ :
copper alloys for tha cathode electrode while still retaining
the performance characteristics of silvex-based cathode elec-
trodes.
The anode in this type of electrolytic capacitor
is generally formed by pressing powders of the particular ~ilm-
forming metal into the desired shape and then sintering the press-
ed powder. The resultant sintered anode is characterized as
having a myriad of interconnecting void areas and therefore
has a very large surface area per unit of volume which contributes
greatly to the capacitance of the device in which it is utilized.
A dielectric oxide film of the metal is then formed over ;
the anode, typically by an electrolytic anodization process.
While the anode exhibits a relatively large capaci-
tance, the interface between the cathode electrode and the
electrolyte has an inherent charge separation due to polar-
ization and thus exhibits a cathodic capacitance. This cathodic
capacitance due to polarization may result from the formation
of an asymmetric conductive film on the cathode electrode
surface or from electrochemically developed insoluble insulat-
ing films or gas polarization films on the cathode electrodesurface.
Since both the anode and cathode electrode possess `~
inherent asymmetric conducting properties, the two electrodes
are series-opposed with respect to their arrangement in the
capacitor structure. When under the influence of an applied
pulsating voltage, the electrodes charge and discharge alter-
nately; that is, one electrode discharges as the other charges. ~ -
As a consequence, the electrolyte between the electrodes re-
mains at a negative potential toward the external electrodes
throughout an alternating cycle. This differs from the
charge-discharge function of two ordinary electrostatic capa-
-2-
,. .- . . .
:,. , ~.,., , ~ ,, .
~4~L~20
citors connected in series, however, the law governing the
admittance of the circuit remains the same: l/C (device) =
l/C (anode) + l/C (cathode) where C is capacitance. This
relationship results in the condition that the charge trans-
fer is limited by the smaller of the two capacitances in
either arrangement.
In the design of electrolytic capacitors, par-
ticularly relating to capacitor rating, the design is
established invariably from the design parameters of the
anode. Therefore the cathode capacitance should be made
several orders of magnitude higher than the anode capacitance
by suitable arrangement or treatment so as to be compatible
with the anode design. Thus, the term l/C (cathode) in -the
above relation would become small relative to the other terms
and the device capacitance would become essentially equal
to the anode capacitance. Ideally, the operating characteristics
of the capacitor approach optimum stability as the cathode
capacitance approaches infinity. This condition, of course,
can be attained only approximately in practical design of
capacitors.
Several methods are known to increase the capaci-
tance of the cathode electrode in electrolytic, film-forming
metal capacitors and they include;
(1) applying to the surface of the cathode elec-
trode a layer of finely divided, substantially inert conduc-
tive material such as carbon or certain of the platinum metals
or gold; when properly applied, these materials provide a
very high cathode surface area necessary for cathode capaci-
tance, or
(2) providing certain metal ions in the electro-
-3-
~L~4~
lyte which are capable of being electro-deposited on a cathode
electrode of a suitable metal and dissolved therefrom in sub-
stantially the exact proportion of the current flowing back
and forth across the cathode-electrolyte interface.
In system (1) it is thought that the current tra-
versing the electrolyte-cathode junction discharges hydrogen ~ -
or hydroxyl ions which are absorbed on the surface of the
metal to yield a dielectric film. In system (2), it is be-
lieved the electrochemical discharge and disso]ution of metal
ions creates an ionic double layer in the electrolyte at the
cathode electrode surface, and the space charge across this
layer exhibits a high capacitance.
Since film-forming metal anode type capacitors are
usually made as small as possible and the anode capacitance
is therefore very high per device volume, the usual practice
is to employ both methods of increasing the capacitance of the
cathode electrode. However, applying the layer of finely divid-
ed conductive material to the surface of the cathode electrode
is genexally tedious and costly, especially when gold or plati-
num are utilized. The method generally involves the steps ofcleaning the cathode, applying a coat of masking material
over a portion of the cathode electrode surface, etching the
surface with an acid, rinsing, filling with plating solution,
electroplating with a platinum anode, removing the anode and
plating solution, rinsing, drying and removing the mask material.
It is therefore a feature of the present invention
to provide a copper-based cathode electrode for an electrical
device with a layer or coating thereon to help increase the
capacitance of the cathode by increasing the effective surface
area of the cathode electrode. Another feature of the pres-
ent invention is that the layer applied to the cathode electrode
_4
~3L62C~
contains at least one compound selected from copper selenide,
copper telluride and copper sulfide. Yet another feature
of the present invention is that the method of applying the
layer includes relatively few process steps and is therefore
comparatively inexpensive. Another feature is that a copper-
based cathode electrode with a layer according to this invention
is substantially non-film forming, is non-gassing, offers mini-
mal internal series resistance, stabilizes electrical character-
istics of the capacitor and is substantially inert to corrosive
chemical attack by an operating electrolyte. Yet another fea-
ture of the present invention is that the electrolyte with
which the copper-based cathode electrode is in contact is de-
aerated so as to reduce the possibility of corrosion of the
cathode electrode by oxidizing substances in the electrolyte.
These and various other features of this invention as well as
many specific advantages will become more fully apparent from
a detailed consideration of the remainder of this disclosure
including the examples and the appended claims in conjunction
with the accompanying drawing, in which the figure is a cross-
sectional view of a typical electrolytic, film-forming metal
anode capacitor.
Generally, the present invention comprehends a ca-
thode electrode for an electrical device wherein the cathode
electrode is composed of a copper based material and has a
layer or coating over the cathode electrode surface of a com-
pound selected from the group consistin~ of copper telluride,
copper selenide, copper sulfide and mixtures thereof. The
invention also comprehends a method for applying the layer to
the cathode electrode surface. The invention is particularly
adapted for use in electrolytic, film-forming metal anode
-5-
- ;
.. ..
: ' ' .
0
capacitors where a high cathodic capacitance and stable electrical
characteristics are desired.
In particular, the invention pertains to a capacitor comprising
an anode electrode, a cathode electrode, and dielectric means between
the anode electrode and the cathode electrode, the cathode electrode in-
cluding a surface containing Cu contacted by at least semi-liquid elec-
trolyte, and a composition on the cathode electrode surface containing at
least one compound selected from the group of copper selenide, copper
telluride and copper sulfide.
As indicated previouslya the present invention can be utilized
in various electrical devices but will be discussed hereinafter with
reference to its application in electrolytic capacitors although not
limited thereto. The main advantages realized by the utilization of a
copper-based cathode electrode are a significant reduction in the final
cost of the completed capacitor without a reduction in performance capa-
bility as compared to capacitors having platinized silver-based cathode
electrodes, and the ease of forming the layer over the cathode electrode
surface. ~-
The invention can be more clearly understood with reference to
the drawing. The figure ~s a cross-sectional view of an electrolytic
capacitor 10 containing a dieleotri~ oxide film-~orm;ng metal anode 11
with anode riser 19. The anode 11 has a dielectric oxide film or layer
(not shown) over its surface. In combination with anode 11 is cathode
electrode 12 which also serves as the housing or case for capacitor 10.
The anode 11 ;s retained within the cathode housing 12 by means of the
insulative bottom spacer 13 and end seal assembly 14. End seal assembly
14 is shown as a glass-to-metal seal 15 in conjunction with a resiliant
washer 16, but the end seal assembly forms no part of the present in-
Yention and, as is known in the art9 a wide variety of different types
of structures may be employed to close the open end of the housing 12.
-'
--6-
- ., : : ~ . . . -
Z~
Electrolyte 17 contacts the anode 11 and the interior surface coating :~
18 of the cathode electrode 12. External electrical termination for ;
the cathode electrode is provided by cathode lead 20. ~ ~
: i.. '' ~: ~`
,. , ~
,~ .. ~.,;
,: : : :
C~
-6A-
~, . ,.,.. . ~, . .. ...
,' ,'' ''' '',' ' ~ '" ,; ~,''; : ` ' ', ' " ''''', '''''' ' ' ',.': ";'' ''i ' '. , , ~`
: , ,:,., , ; , ,. . :. . . . ... . .. .. ..
~L~34~6;~ i
The operating electrolyte 17 for the capacitor
is typically a dilute, non-oxidizing aqueous solution of an
inorganic acid such as sulfuric acid, phosphoric acid, hydro-
chloric acid, and the like. The presently preferred electro-
lyte is a 39 wt.% solution of sulfuric acid. To help prevent cor-
rosion of the interior surfaces of the cathode electrode 12
by the electrolyte 17, preferably the electrolyte is de-aerated
by means such as refluxing in a closed system before being
added to the capacitor 10.
According to the present invention, the cathode elec-
trode 12 is composed of a copper-based material, that is, a
material containing at least 50 wt.~ copper. Preferably, the
copper-based material contai~s at least 90% copper. One
particularly advantageous material is argentiferous copper,
CDA #116, which is a copper based alloy containing about 25
troy ounces of silver per ton of copper, or about 0.1 wt.
silverO This copper alloy has desirable mechanical proper-
ties such as ductility, retention of work-hardenad temper over
the operating temperature range of a typical capacitor, and
good corrosion resistance.
As mentioned previously, the surface area of the
case or housing acting as a cathode electrode 12 in the capaci~
tor 10 should be signi~icantly increased so as to take full
advantage of the capacitance of the anode 11. Conventional
methods of accomplishing this end include platinization by
electrochemical means and application of an artificial graphite
coating by means of a liquid dispersion or paint.
While conventional methods may be ade~uate for some
purposes, it has been found that treatment of the interior
of the cathode electrode 12 can be accomplished in fewer
, .. .. , : , . .. .....
: ,;, , , . . . -, . ,, , :
1~4~ 0
steps, and therefore generally at a lower cost, by forming
a coating 18 of copp~r selenide, copper telluride or copper
sulfide by reaction of a solution of the appropriate acid
or other reactive compound with the copper-based cathode
electrode 12. Thus, for example, a reaction of seleneous
acid with the copper-based cathode electrode 12 would produce
a coating 18 of copper selenide, tellurous acid would yield
copper telluride and a polysulfide would produce copper sulfide.
The presently pre~erred compound for the coating 18 over
the cathode electrode 12 is copper telluride. Generally,
the quantity of acid or compound required to form the coating
18 is in the range of about 0.2 to 1.0 milligrams per square
centimeter of cathode electrode surface.
The coating 18 formed by the reaction product of the
reactive acids or compounds with the cathode electrode 12 may
be applied at several different stages of the capacitor assem-
bly. The appropriate compound may be added to the fill electro-
lyte 17 and thus the reaction takes place in the completed
capacitor 10. Alternatively, the porous anode 11 may be dipped
in an aqueous solution of the appropriate compound and then
dried. The reaction will commence when the anode 11 is insert-
ed into the electrolyte 17 and any unreacted compound will
thereby remain in the capacitor 10. In another method, an
a~ueous solution of the compound is placed in a dry housing 12
and the reaction with the cathode surface 12 allowed to take
place. Then the remaining solution is removed and the housing
12 drained. The electrolyte 17 is added in a subsequent step
and the capacitor assembly is then completed.
One particular advantage in utilizing a coating 18
of copper selenide, copper telluride or copper sulfide as
oppo~ed to conventional methods of increasing the cathode
-8-
1~416~0
electrode surface area such as platinization ~nd graphite
coating is that many processing steps in the assembly of
an electrolytic capacitor can ba eliminated. For example,
platinization of the interior surface of the cathode electrode
requires the steps of masking, etching, rinsing, plating,
rinsing and drying as set forth previously. The layers or coat-
ing 18 of the present invention may be provided simply by add-
ing a small amount of an appropriate compound, for example,
tellurous acid, to the fill electrolyte. This particular acid
can be formed in acid electrolytes by adding the oxide of the
element, fox example, tellurium oxide (TeO2), to the acid -
electrolyte such as sulfuric acid.
In the presently preferre~ method, the coating or
layer over the cathode electrode is formed prior to the introduc-
tion of the fill electrolyte. In this method, the reactive
compound is formed in an aqueous media, usually an agueous
solution of an acid such as H2S04, and brought into contact
with the cathode electrode surface 12. The reaction between
the reactive compound and the copper component of the cathode -;
~0 electrode is allowed to take place to form the layer of copper
telluride, copper selenide, or copper sulfide. In most situa-
tions, the optimum reaction time is between about five and
twelve minutes. The presently preferred reaction time is eight
minutes plus or minus two minutes. After the reaction, the
solution is removed and the surface allowed to drain. Subse-
~uently, the fill electrolyte is dispensed, the anode inserted,
and the device sealed.
Thus, the method for providing the coating over
the cathode electrode surface according to this invention is
greatly simplified in comparison to the conventional methods
such as platinization of the cathode electrode surface.
.. . . ; ." ::
,
6ZC~
It is also advantageous to add certain soluble
inorganic salts as depolarizers to the operating electrolyte
of capacitors according to this invention as as to minimize
leakage current effects present during capacitor operation.
Such salts help to prevent the formation of gaseous products
which are liberated due to leakage current between the electrodes. -
One particular salt that can be used advantageously with a
capacitor according to this invention is CuSO4, prefera~ly
in its hydrated form and with sulfuric acid electrolytes.
The amount of salt needed is typically from about 0.1 wt.
to about 5 wt.% of the electrolyte.
Several capacitors made according to this invention
and performance data for these capacitors are presented in
the following examples. It should be understood that the
examples are given for the purpose of illustration only and
the examples do not limit the invention as has theretofore
been described.
EXAMPLE I
A 450 uf capacitor of 6 volt rating is prepared
according to the present invention.
A copper-based cathode housing composed of CDA #116
silver-copper, tough pitch, is fabricated with a length of
about 0.677 inches and a diameter of about 0.187 inches. The
interior of the housing is cleaned with a warm detergent solu-
tion and rinsed with de-ionized water and then the excess water
drained. An electrolyte of de-aerated 39 wt.% H2SO4 with the
addition of 0.4 wt.% SeO2 is then poured into the housing. A
sintered tantalum anode with a dielectric oxide coating and
a length of about 0.503 inches is placed in the electrolyte
and the open end of the housing sealed with an appropriate seal
--10--
.
" ~ ~ ' ! ', .
: :' ' ,,
~L~)41~;Z~
assembly .
The SeO2 forms an acid in the electrolyte and reacts
during and after filling with the copper of the interior sur~
face of the cathode housiny to produce an adherent layer of
copper selenide. The finished capacitor exhibits good per-
formance data on a foward life test at 85C and on shock-
vibration tests.
EXAMPLE II
A 78 uf capacitor of 50 volt rating is prepared
according to the present invention.
The housing is the same size as the housing utilized
in EXAMPLE I. Again the housing is cleansed by warm deter-
gent solution and then rinsed with de-ionized water and drained.
A treating solution of about one gram of TeO2 in about 100
grams of 39 wt.% H2SO~ solution is dispensed into the housing,
filling the housing to about 1/32 inch from the top of the
housing. Reaction between the copper of the housing and
the tellurous acid is allowed to take place to form the layer
of copper telluride and then the housing is flushed with de-
ionized water and dried at about 85C.
After the housing is dry, electrolyte consisting of
a de-aerated aqueous solution of about 39 w~.~ H2SO4 with about
3 wt.~ CuSO4 ~ 5H2O is dispensed into the housing. A sintered
tantalum anode with a dielectric oxide film thereon is in-
serted into the electrolyte and the open end of the housing
is closed by a seal assembly.
Two finished capacitors made by the above procedure
are tested and are found to have the following electrical proper- ~-
ties at various temperatures.
: ' , , , ,, "; '
~4~2C~
Capacitance Change (~C)
+25C -35C -55C ~85~ +25C
Unit 1 80~4 uf -12 -24 -0.37 -3.6
3.6%DF
Unit 2 81.2 uf -12 -20 -0.74 -3.8
3.7%DF
Acceptable ~40 tlO i5
Limits
,: ~
As can be noted from the above table, the performance
of both capacitors is well within acceptable limits for temper-
ature stability.
Forward life tests at 85C indicate that the abovetype capacitor is also within acceptable limits in terms of
performance even after 1000 hours of testing. Percentage
change in capacitance is less than 2~ and usually less than 1%,
D.C. leakage is well below the initial value and percentage
change in dissipation factor i5 less than 10~, usually less
than 5%, well within acceptable limits.
Thus the invention as has been described comprehends
the use of a copper-based~cathode electrode in an electrical
device wherein a surface of the cathode electrode has an ad-
herent layer or coating of a material selected from the group
of copper selenide, copper telluride, copper sulfide or mix-
tures thereof. The layer is relatively easy and inexpensive
to apply to the cathode electrode surface and permits the
efficient utilization of copper-based materials as cathode
electrodes for electrical devices, especially electrolytic
aapacitors.
While the present invention has been described with ~`
reference to particular embodiments thereof, it will be under-
stood that numerous modifications may be made by those skilled
in the art without actually departing from the spirit and
scope of the invention as defined in the appended claims.
-12-
:; . ' ~ ~ , -: ....................... , . '
:", ' '' , ~' , .
. . . . .. ..
