Process for treating impregnated electrolytic capacitor anodes

A process for treating an impregnated electrolytic capacitor anode whereby the anode body is immersed in a liquid electrolytic solution and a voltage is applied to the anode body, whereby a current flows through and repairs flaw sites in the anode body. The liquid electrolytic solution includes an organic solvent comprising at least one of polyethylene glycol, polyethylene glycol monomethyl ether, and polyethylene glycol dimethyl ether. Alternatively, the electrolytic solution includes an organic solvent and an alkali metal phosphate salt. Preferably, the electrolytic solution contains both an alkali metal phosphate salt and an organic solvent comprising at least one of polyethylene glycol, polyethylene glycol monomethyl ether, and polyethylene glycol dimethyl ether.

FIELD OF THE INVENTION
 The present invention is directed to contamination-resistant reformation
 electrolytes and more particularly to a process for treating impregnated
 electrolytic capacitor anodes.
 BACKGROUND OF THE INVENTION
 Electrolytic capacitors are generally the capacitors of choice for
 applications demanding high capacitance/size and high capacitance/cost
 ratios. These devices exploit the relatively high dielectric constants and
 very high withstanding voltages per unit thickness which may be obtained
 in the anodic oxide films grown upon valve metals with appropriate
 electrolytes and anodizing conditions.
 Aluminum is typically employed as the valve metal in electrolytic
 capacitors where low cost per unit capacitance is a primary concern, while
 tantalum is usually employed for applications in which high reliability
 and volumetric efficiency (i.e., high capacitance per unit volume) are the
 primary concerns. Other valve metals, including niobium, titanium, and
 titanium aluminides have also been used to construct capacitors, but
 aluminum and tantalum remain the main materials of choice. The valve metal
 may be utilized in the form of an etched foil or as a porous, sintered
 powder-metallurgy compact at significantly below theoretical density. In
 either form, the valve metal is anodized to produce the anodic dielectric
 film prior to application of the cathode material.
 Electrolytic capacitors may be classified by the type of cathode material
 employed in their construction. So-called "wet" capacitors contain
 aluminum or other valve metal foil immersed in a container of liquid
 electrolyte. Although these devices were once commonplace, their low
 volumetric efficiency has rendered them obsolete. "Wet slug" tantalum
 capacitors in which sintered, powder metallurgy, anodized tantalum anodes
 are immersed in a minimum volume of highly conductive liquid electrolyte
 (e.g. 40% sulfuric acid) find application where high volumetric efficiency
 and capacitance combined with high reliability are desired. These devices
 tend to have relatively high ESR due to the resistivity inherent in
 ionically conductive liquids. In the event of seal failure, the acid and
 concentrated salt electrolytes employed in their construction tend to be
 very corrosive to circuit boards and other components.
 So-called "dry" electrolytic capacitors were developed to increase the
 volumetric efficiency of "wet" aluminum capacitors while avoiding the
 highly corrosive electrolytes and additional expense associated with "Wet
 slug" tantalum capacitors. This type of capacitor has one or more anode
 and cathode foils separated by highly absorbent paper. The foil/paper
 combination is wound to form a cylinder having protruding tabs for
 electrical connection, and this cylinder assembly is then impregnated with
 a liquid electrolyte prior to assembly into a case which surrounds the
 impregnated assembly.
 The "wet," "wet slug," and "dry" capacitor constructions all have in common
 the presence of a liquid electrolyte in contact with the valve metal
 anode. Any cracks or defects in the anodic oxide (due to the capacitor
 assembly process, etc.) may be at least partially healed during use by the
 application of voltage, which results in the growth of fresh anodic oxide
 or the isolation of flaws by the presence of gas bubbles from electrolysis
 of the electrolyte.
 In the 1950's, a new type of electrolytic capacitor was introduced in which
 the cathode material is a true "solid" These devices usually contain a
 sintered, powder metallurgy anodized tantalum anode which has been
 impregnated with manganese dioxide via pyrolysis of manganese nitrate
 solution. More recently, "solid" electrolytic capacitors have been
 introduced which employ intrinsically conductive polymers, such as
 polypyrrole, polythiophenes, etc., as the cathode materials.
 The introduction of conductive polymer cathode materials has facilitated
 the use of aluminum and other valve metals in addition to tantalum in
 "solid" capacitors due to the elimination of the multiple pyrolysis steps
 at the relatively high temperatures (200-400.degree. C.) required to
 produce manganese dioxide cathode material within the pore structures of
 anodes after first impregnating the anode bodies with manganese nitrate
 solution.
 The construction of "solid" electrolytic capacitors eliminates the contact
 to the anodic oxide by a liquid electrolyte. The absence of a liquid
 electrolyte minimizes the amount of dielectric flaw "healing" or isolation
 which can be accomplished in the finished device due to extreme heating of
 the oxide at flaw sites brought about by the higher currents supported by
 manganese dioxide or conductive polymers compared with the more resistive
 liquid electrolytes. The elimination of liquid electrolyte also minimizes
 the heat sink action of the cathode material at flaw sites (localized
 boiling of liquid electrolyte tends to carry heat away from flaw sites).
 In order to overcome the difficulty of repairing flaws in the anodic oxide
 dielectric of assembled "solid" electrolytic capacitors, one or more
 electrolytic treatment steps (known as "reformation" or, simply, "reform"
 steps; the initial anodization which produces the anodic oxide is known as
 the "formation" step(s)) are carried out in which the anode bodies
 containing manganese dioxide or conductive polymeric material are immersed
 in a liquid electrolyte and a positive voltage is applied to the anode
 bodies while a negative voltage is applied to the electrolyte. The voltage
 applied to the anodes is generally lower than that used to produce the
 anodic oxide, so that the vast majority of any current flowing through the
 anode bodies flows through the flaw sites. This current flow is thought to
 repair the flaws by the growth of new oxide at the flaw or, especially, by
 thermally and electrochemically degrading the cathode material locally,
 thereby isolating the flaw sites electrically. Reformation electrolytes
 generally contain a small amount of phosphoric acid as the ionogen.
 Although many other ionogens have been employed in "reform" electrolytes
 including sulfuric acid, nitric acid, acetic acid, and sulfosalicylic
 acid, the presence of the orthophosphate ion has generally been found to
 give the best results with respect to the leakage current of the finished
 devices.
 As stated above, the liquid electrolyte used for reformation serves as a
 heat sink to prevent run-away heating at flaws and the resistivity of the
 electrolyte serves to act as a resistor in series with each capacitor
 anode, limiting the current and the resulting current-driven heating of
 the flaws during the reform process.
 Due to the current-limiting aspect of the reform electrolyte the
 resistivity of this electrolyte is usually carefully controlled. The
 optimal resistivity range for reform electrolytes depends upon the applied
 voltage, electrolyte temperature, and the chemical nature of the cathode
 material involved. What is generally desirable, however, is minimal
 resistivity change during use.
 Unfortunately, anodes which have been impregnated with solid cathode
 materials frequently contain ionic materials which leach into the
 reformation electrolyte during the reformation step(s). Manganese dioxide
 containing anodes tend to contain nitrogen oxides adsorbed on the high
 surface area manganese dioxide, as well as a small amount of unreacted
 manganese nitrate. Organic polymer containing anodes tend to contain a
 certain amount of uncombined dopant acid, such as toluene sulfonic acid.
 It has proven to be very difficult to reduce the level of these ionic
 contaminants to the degree that they do not result in resistivity
 depression of the reformation electrolyte; even when anodes are exposed to
 prolonged hot de-ionized water rinsing prior to the reform steps, some
 ionogens are released by the electrochemical action.
 In a manufacturing environment, it is highly desirable to reduce the
 resistivity depression of the reformation electrolyte so as to avoid the
 necessity of frequent changes of the electrolyte. Traditionally, this
 problem has been addressed by the use of aqueous phosphoric acid solutions
 containing a substantial percentage of ethylene glycol. The glycol acts to
 raise the resistivity of the electrolyte for a given ionogen content and
 temperature. As the electrolyte becomes contaminated by ionogens from the
 solid impregnated anodes, the ethylene glycol content has been
 progressively increased in order to maintain the resistivity within
 specified limits. Thus the reformation electrolyte may be used for a
 significantly larger number of anodes prior to replacement, thereby
 facilitating greater manufacturing throughput per tank of reformation
 electrolyte.
 SUMMARY OF THE INVENTION
 It was discovered that reformation electrolytic solutions containing
 polyethylene glycol, polyethylene glycol monomethyl ether, and/or
 polyethylene glycol dimethyl ether in place of ethylene glycol are more
 resistant to resistivity depression than reformation electrolytes
 containing the same percentage of ethylene glycol. It also was discovered
 that reformation electrolytic solutions containing alkali metal phosphates
 are more resistant to resistivity depression by ionic contaminants than
 reformation electrolytic solutions containing phosphoric acid. Further, it
 was discovered that reformation electrolytic solutions containing both
 polyethylene glycol, polyethylene glycol monomethyl ether, and/or
 polyethylene glycol dimethyl ether and alkali metal phosphates demonstrate
 greater resistance to resistivity depression by ionic contaminants from
 solid impregnated capacitor anodes than with either material in
 combination with prior art organic (ethylene glycol) or ionic (phosphoric
 acid) constituents.
 In accordance with the present invention, an impregnated electrolytic
 capacitor anode is treated by immersing an anode body in a liquid
 electrolytic solution and applying a voltage to the anode body, whereby a
 current flows through and repairs flaw sites in the anode body.
 In one embodiment of the present invention, the liquid electrolytic
 solution includes an organic solvent comprising at least one of
 polyethylene glycol, polyethylene glycol monomethyl ether, and
 polyethylene glycol dimethyl ether.
 According to another embodiment of the present invention, the liquid
 electrolytic solution includes an organic solvent and an alkali metal
 phosphate salt.
 In a preferred embodiment of the invention, the electrolytic solution
 contains both an alkali metal phosphate salt and an organic solvent
 comprising at least one of polyethylene glycol, polyethylene glycol
 monomethyl ether, and polyethylene glycol dimethyl ether.

DETAILED DESCRIPTION OF THE INVENTION
 In accordance with the present invention, an impregnated electrolytic
 capacitor anode is treated by immersing an anode body in a liquid
 electrolytic solution and applying a voltage to the anode body, whereby a
 current flows through and repairs flaw sites in the anode body. The
 voltage is usually applied in stepwise fashion, a few percent of the
 original anodizing voltage at a time, until an empirically determined
 optimum reform voltage is reached. The voltage is then held constant for a
 period of time while the current decays. The present invention provides
 greater resistance to change in resistivity with ionic (acidic)
 contamination during the reform process than prior art processes.
 The reformation process according to a first embodiment of the present
 invention employs an electrolytic solution having an organic solvent which
 includes at least one organic solvent selected from polyethylene glycols,
 polyethylene glycol monomethyl ethers, or polyethylene dimethyl ethers.
 The reformation process according to a second embodiment of the present
 invention employs an electrolytic solution having one or more alkali metal
 phosphate salts in combination with water and an organic solvent to give
 an alkaline or neutral electrolytic solution. The organic solvent may be
 any organic solvent that is typically used in this field such as ethylene
 glycol, glycerol, or propylene glycol. The alkali metal phosphate salt and
 organic solvent should be selected such that minimum resistivity change
 upon acid addition is achieved.
 The reformation process according to the preferred embodiment of the
 present invention employs an electrolytic solution having one or more
 alkali metal phosphate salts and an organic solvent which includes at
 least one solvent selected from polyethylene glycols, polyethylene glycol
 monomethyl ethers, or polyethylene dimethyl ethers.
 The amount of organic solvent should be less than the solubility limit of
 the organic solvent in water. Generally, the organic solvent is above
 about 1 vol % and less than about 95 vol % of the electrolytic solution.
 Preferably, the organic solvent is from about 5 vol % to about 75 vol % of
 the electrolytic solution. More preferably, the organic solvent is from
 about 5 vol % to about 55 vol % of the electrolytic solution.
 The alkali metal phosphate salt preferably is a water soluble dibasic or
 tribasic salt, such as dibasic potassium phosphate and dibasic sodium
 phosphate, most preferably tribasic potassium phosphate. The amount of
 alkali metal phosphate salt is generally from about 0.01 wt % to about 10
 wt % of the total weight of the electrolytic solution, preferably, from
 about 0.5 wt % to about 5 wt % of the total weight of the electrolytic
 solution, more preferably from about 0.5 wt % to about 2.5 wt % of the
 total weight of the electrolytic solution.
 EXAMPLES
 The following illustrative examples are provided for a better understanding
 of the invention. These examples are illustrative of preferred aspects of
 the invention and are not intended to limit the scope of the invention.
 Example 1
 In order to illustrate the superior resistivity depression resistance to
 contamination of "reformation" electrolytes containing alkali metal
 phosphates in place of phosphoric acid, a series "reformation"
 electrolytes was prepared employing solvents of the present invention as
 well as the traditional solvents, de-ionized water and aqueous ethylene
 glycol. For each solvent, two electrolytes were prepared: one containing
 phosphoric acid and the other containing dibasic potassium phosphate. All
 of these electrolytes were adjusted to a resistivity of approximately
 20,000 ohm-cm at 25.degree. C..+-.3.degree. C. by varying the ionogen
 content. The resistivity of each electrolyte was measured vs. incremental
 additions of 1 normal nitric acid to 1 liter of electrolyte.
 FIGS. 1-5 show that, in each case, the nitric acid addition-induced
 resistivity depression is lower for the dibasic potassium phosphate
 containing electrolyte than for its phosphoric acid-containing
 counterpart.
 In manufacturing practice this enhanced resistivity depression resistance
 of the alkali metal phosphate-containing "reformation" electrolytes
 translates into the ability to absorb several times as much contamination
 from anodes being "reformed" before a minimum resistivity limit (e.g.
 15,000 ohm-cm) is violated.
 Example 2
 The superior resistance to resistivity depression from contamination of the
 "reformation" electrolytes based upon the solvents of the present
 invention was determined by comparing the resistivity vs. nitric acid
 content of a series of electrolytes, each containing 55 volume % organic
 material in water. 1-liter samples of each electrolyte solvent were
 adjusted to a resistivity of approximately 20,000 ohm-cm at 25.degree.
 C..+-.3.degree. C. with phosphoric acid. Incremental amounts of 1N nitric
 acid were then added to each sample of "reformation" electrolyte while
 monitoring the resistivity.
 FIGS. 6-8 show that in each case, the resistivity depression of the 55
 volume % ethylene glycol electrolyte is greater at the same concentration
 of nitric acid than is found with the electrolytes containing the solvents
 of the present invention.
 Example 3
 The superior resistance to resistivity depression from contamination of
 "reformation" electrolytes based upon the solvents of the present
 invention was further determined with resistivity testing, as in Example
 2, except that the ionogen used to prepare the electrolytes was dibasic
 potassium phosphate in place of phosphoric acid.
 In each case, the resistivity depression vs. nitric acid content was less
 for the electrolyte of the present invention than for the 55 volume %
 ethylene glycol plus water electrolyte (see FIGS. 9-11), indicating the
 superior resistivity depression resistance of the electrolytes of the
 present invention.
 Example 4
 Sometimes it is desirable to conduct the "reformation" process at elevated
 temperature, e.g. at 80.degree. C. In order to illustrate the superior
 elevated temperature resistance to resistivity depression of "reformation"
 electrolytes containing alkali metal phosphates in place of phosphoric
 acid, resistivity vs. incremental additions of 1 normal nitric acid was
 compared for 1 liter "reformation" electrolyte samples containing 55
 volume % of organic plus water solvents and adjusted to approximately
 10,000 ohm-cm. with an ionogen consisting of phosphoric acid or dibasic
 potassium phosphate.
 In both cases the electrolytes containing the alkali metal phosphate,
 dibasic potassium phosphate, exhibited greater resistivity depression
 resistance then the electrolytes containing phosphoric acid. See FIGS. 12
 and 13.
 Example 5
 The superior elevated temperature resistance to resistivity depression by
 contamination of "reformation" electrolytes containing the solvents of the
 present invention was determined by preparing 1 liter samples of
 electrolyte solvents containing 55 volume % organic content plus water.
 The samples were then adjusted to a resistivity of approximately 10,000
 ohm-cm at 80.degree. C. with phosphoric acid or dibasic potassium
 phosphate. The resistivity of each "reformation" electrolyte was then
 measured vs. incremental additions of 1 normal nitric acid at 80.degree.
 C.
 For both phosphoric acid and dibasic potassium phosphate ionogens at
 80.degree. C., the electrolytes containing a solvent of the present
 invention (i.e., aqueous polyethylene glycol) proved to be more resistant
 to resistivity depression over a broad range of acid additions than
 electrolytes containing a conventional solvent (i.e., aqueous ethylene
 glycol) having the same volume % organic content. See FIGS. 14 and 15.
 Example 6
 It was found that alkali metal phosphates other than dibasic potassium
 phosphate are also useful in the formulation of "reformation" electrolytes
 having superior resistance to resistivity depression by contamination when
 compared with "reformation" electrolytes containing phosphoric acid as the
 ionogen.
 FIG. 16 shows the resistivity depression vs. incremental 1 normal nitric
 acid additions for two "reformation" electrolytes having a solvent
 consisting of 55 volume % polyethylene glycol 300 plus water. One
 electrolyte was adjusted to approximately 20,000 ohm-cm with phosphoric
 acid, while the other was adjusted to approximately 20,000 ohm-cm with
 tribasic potassium phosphate, both at 25.degree. C.+/-3.degree. C.
 The resistivity vs. acid addition curves indicate that many times as much
 nitric acid is required to depress the resistivity of the tribasic
 potassium phosphate containing electrolyte to an arbitrary minimum
 resistivity (e.g., 15,000 ohm-cm) as is required to depress the
 resistivity of the phosphoric acid containing electrolyte to the same
 resistivity.
 Example 7
 In order to illustrate the great improvement over prior art of the solvents
 and ionogens of the present invention in the preparation of "reformation"
 electrolytes which are resistant to resistivity depression by
 contamination, a 55 volume % ethylene glycol plus water electrolyte
 containing a sufficient quantity of phosphoric acid to yield a 25.degree.
 C.+/-3.degree. C. resistivity of 20,000 ohm-cm was compared with a 55
 volume % polyethylene glycol 300 electrolyte containing sufficient
 tribasic potassium phosphate to yield 25.degree. C.+/-3.degree. C.
 resistivity of 20,000 ohm-cm. See FIG. 17.
 The amount of nitric acid required to depress the electrolyte resistivity
 to an arbitrary minimum (e.g., 15,000 ohm-cm) is many times higher for the
 "reformation" electrolyte containing aqueous polyethylene glycol 300 plus
 tribasic potassium phosphate than for the prior art electrolyte containing
 aqueous ethylene glycol plus phosphoric acid.
 Example 8
 Due to the large resistivity depression vs. amount of contamination
 observed for prior art "reformation" electrolytes, capacitor manufacturers
 have been forced to disregard used "reformation" electrolyte frequently or
 to resort to a progressive increase in the organic content of the
 electrolyte in order to maintain the resistivity above a predetermined
 minimum value (usually determined empirically by part performance).
 The use of increased organic content to maintain a minimum resistivity is
 illustrated in Table 1, which depicts the resistivity versus ethylene
 glycol content for a 20,000 ohm-cm/25.degree. C. prior art "reformation"
 electrolyte containing 55 volume % aqueous ethylene glycol and phosphoric
 acid and to 1 liter of which has been added 1.0 ml of 1 normal nitric
 acid.
 TABLE 1
 Resistivity Versus Ethylene Glycol Content
 0.1N HNO.sub.3 Ethylene Glycol Ethylene Glycol 1 KHz Resistivity
 Added (ml) Added (ml) (% by volume) (Ohm-cm), 25 C
 0 0 55 20,000
 1 0 55 7,240
 1 100 59 9,670
 1 200 62.5 12,000
 1 300 65.3 14,600
 1 320 65.9 15,200
 Table 1 shows that 320 ml of ethylene glycol was added before the
 resistivity increased to an arbitrary minimum of approximately 15,000
 ohm-cm, giving a 65.9 volume % ethylene glycol containing "reformation"
 electrolyte. This is not only wasteful of the organic, but is difficult to
 carry-out with "reformation" electrolytes operated near room temperature
 due to the low evaporation rate of the water component of the electrolyte
 (without evaporation the change to a 65.9 volume % ethylene glycol
 solution, above, involves an increase in electrolyte volume of 32%).
 By contrast, a "reformation" electrolyte containing 55 volume % aqueous
 polyethylene glycol 300 and adjusted to a 25.degree. C. resistivity of
 approximately 20,000 ohm-cm with dibasic potassium phosphate exhibits a
 resistivity in excess of 15,000 ohm-cm/25.degree. C. after the addition of
 1 ml of 1 normal nitric acid to 1 liter of this electrolyte. Thus a large
 savings in time required to empty/adjust "reformation" tanks and a large
 savings in chemicals as well as a more uniform organic/water ratio may be
 realized through the use of the solvents and ionogens of the present
 invention.
 Example 9
 In order to demonstrate the efficacy of the electrolytes and process of the
 present invention, a batch of sintered, anodized tantalum anodes was split
 into 3 groups. All 3 groups were impregnated with manganese dioxide via
 wetting the anodes with manganese nitrate, followed by pyrolysis at
 approximately 260.degree. C. in an oven having live steam injection as is
 common to the industry. Multiple wetting and pyrolysis cycles were used to
 produce the desired manganese dioxide coating thickness.
 The batch of anodes was split into 3 groups for a "reformation" step after
 approximately half of the impregnation process and, then, at the end of
 the impregnation process (i.e., after the completion of the manganese
 oxide coating process). During the "reformation" process, the anodes were
 immersed in a "reformation" electrolyte and voltage was applied step-wise
 until approximately 55% of the anodizing voltage was reached.
 The anodes were then rinsed in de-ionized water, dipped in a graphite
 suspension, and coated with a silver paint to produce finished (but
 unencapsulated) capacitor anodes.
 The electrical performance for the group of anodes "reformed" in a
 traditional electrolyte containing aqueous ethylene glycol and phosphoric
 acid and for groups "reformed" at 30.degree. C. and 80.degree. C. in
 electrolytes of the present invention containing polyethylene glycol 300
 and dibasic potassium phosphate is presented in Table 2.
 TABLE 2
 Polyethylene Glycol and Dibasic Potassium Phosphate Performance
 Cap DF ESR Leakage
 Electrolyte Type, Temp (.mu.F) (%) (ohms) (.mu.A)
 Ethylene Glycol / H.sub.3 PO.sub.4, 80.degree. C. 22.13 2.24 0.320
 0.138
 PEG 300 / K.sub.2 HPO.sub.4, 30.degree. C. 21.90 2.23 0.298 0.109
 PEG 300 / K.sub.2 HPO.sub.4, 80.degree. C. 21.92 2.22 0.292 0.089
 At both 30.degree. C. and 80.degree. C., the electrolyte/process of the
 present invention, which was shown above to be more resistant to
 contamination with respect to resistivity depression, is seen to give
 results equivalent to conventional "reform" electrolyte/process.
 It will be apparent to those skilled in the art that various modifications
 and variations can be made in the compositions and methods of the present
 invention without departing from the spirit or scope of the invention.
 Thus, it is intended that the present invention cover the modifications
 and variations of this invention provided they come within the scope of
 the appended claims and their equivalents.