Electrolytic Capacitors With Pressure-Relief and Metal Electrode Structure Retaining Means

This invention relates to electrolytic capacitors, especially "miniature" capacitors having pressure-relief means.

Over the years many pressure-relief structures have been incorporated in the design of hermetically sealed electrolytic capacitors, often taking the form of valves. The can is sealed to preserve the electrolyte against drying out or leaking. Explosive pressure can thus develop inside the can due to malfunction or misapplication. Pressure-relief structures devised for large capacitors have been found impractical for incorporation into capacitors having small-size cans, e.g. about 1/2-inch diameter and less. In the absence of a distinct pressure-relief device, it has been known that the entire end-seal structure would be released suddenly upon development of internal pressures approaching explosive levels and even at much lower pressures, representing a safety hazard. The entire capacitor electrode assembly might then be projected violently and dangerously from the can.

As one solution to this problem for special application to small-diameter cans, it has been proposed that a weakening incision in the end or the side wall of the can would provide safe pressure-relief. That approach, while effective, has required the exercise of great care in achieving precisely the right amount of penetration of the incision into the wall of the can. Despite care in making the incisions, the pressure level at which venting occurs is variable. If the incision is too deep, the can vents too readily and the capacitor fails prematurely. If the incision is not deep enough, the pressure may well blow out the end seal before the incision functions. The incision cannot be tested to sort out the capacitors having the proper venting range since the test is destructive. Moreover, the incision represents a weak point that could develop into a leak. The incision can also be breached accidentally in handling, as by a fingernail, particularly where the incision is made on the outside surface of the can.

Another solution to the venting problem is disclosed and claimed in a companion application of Fred H. West filed concurrently herewith. That approach to the problem is based on accepting as non-hazardous the release of the end closure of a capacitor in response to excessive internal pressure, particularly where the end seal is a resilient plug. It is shown there that a suitably rigid barrier inside the end-seal and too large to pass by an inward-projecting formation of the can, will effectively prevent the electrode assembly from being expelled. The presently preferred embodiment of the broad concept in that application incorporates features of the present invention.

A metal barrier is interposed between the closure or end-seal and the wound-foil electrode section contained in the case of an electrolytic capacitor. A formation of the case prevents the barrier from leaving the case when internal pressure blows the end-seal free, and the barrier is firm enough to block the electrode assembly from being expelled powerfully when the end-seal is released by excessive internal pressure. The barrier is of a metal that is essentially inactive when exposed to the electrolyte, and thus an aluminum barrier can safely be used in a capacitor having an aluminum can and aluminum electrodes. In capacitors having tantalum electrodes and a silver can, the barrier may be tantalum or silver. Further, to avoid accidental short-circuiting across the upper edges of the foil electrodes, the upper edge of the foil that is connected to the riser is recessed in relation to the other foil electrode. The metal disc has a hole through which the riser extends. The hole is over-size to avoid contact with the riser and to assure exposure of the end seal to internal pressures. The periphery of the barrier is close to the wall of the case, thus centering the barrier about the riser.

The nature of the invention, and its various objects and novel features, will be recognized and better appreciated from the following detailed description of an illustrative embodiment shown in the accompanying drawing.

In the drawings, the single FIGURE is a longitudinal cross-section of a novel electrolytic capacitor as an embodiment of the invention.

The illustrative capacitor in the drawing includes a generally cylindrical can 10 as of aluminum containing a wound electrolytic capacitor section 12. The wound capacitor section here includes alternating convolutions of a cathode foil and an anode foil, separated from each other by porous separators as of paper, the wound section being impregnated with an electrolyte. In this capacitor, one end of the can is closed by end wall 10a that is an integral portion of can 10. The side and end walls are formed of a single metal part, as a deep-drawn can. The opposite end of the can is hermetically sealed by a resilient plug 14 as of butyl rubber. Riser 16 of aluminum is connected to the anode foil and extends through plug 14 to anode terminal wire 18, wire 18 being butt-welded to riser 16. The riser and the terminal wire constitute the anode terminal. Cathode terminal wire 20 is butt-welded to the exterior of end wall 10a of the can. The cathode foil 24 is wider than the anode foil 26. One edge of the cathode foil extends from the lower end of the wound section and freely and firmly contacts can 10. Both edges of the anode foil are recessed from both top and bottom edges of the cathode foil. A connecting strip of aluminum (not shown) extends from the cathode foil to the can, welded to the foil and to the inside of the can in a conventional manner. The connecting strip is long and the welds are done before the wound section is inserted.

Two formations 10b and 10c of the can squeeze resilient plug 14 in axial and radial directions and thereby form hermetic seals between the can and the plug and between riser 16 and plug 14. Bead 10c indents the lateral surface of plug 14. The lower or inner surface of plug 14 is supported by the assembly in the can. Pressure exerted by lip 10b on the top or outer face of plug 14 is firmly resisted by bead 10c.

The illustrative capacitor as thus far described is a well-known type of capacitor. In so-called miniature capacitors, the can has a maximum diameter (which varies with different capacitor ratings) of about 1/2-inch. Its length varies widely, also depending on the ratings.

Riser 16 has a smooth cylindrical surface where it passes through plug 14. Riser 16 has a diameter of 0.070 inch in an example. The outside diameter of cylindrical can 10 in this example is nominally 0.375 inch, and its wall thickness is about 0.020 inch. The can is of deep-drawn aluminum. Such capacitors are made by assembling the capacitor section into the can, slipping centrally pierced plug 14 into place along wire 18 and riser 16, and sealing the capacitor by rolling-over lip 10b and forming bead 10c in spinning operations. In this example, bead 10c has a minimum external diameter of about 0.312 inch. Accordingly, the internal diameter of bead 10c is 0.272 inch as compared with the internal can diameter of about 0.335 inch. Plug 14 in this example initially has a diameter of 0.320 inch and a thickness of 0.160 inch.

The capacitor section is impregnated with an electrolyte, either before the section is inserted into the can or after insertion but before the plug is sealed in place. The anode foil is treated to provide a so-called formed oxide, in a preparatory operation. The completed capacitor is electrically treated to reform any damaged oxide and to form anodically all other bare areas of the anode that are exposed to the electrolyte. The electrolyte, a liquid or a gel or the like, ordinarily is a formulation containing some water and in any case it tends to boil or to evolve gas when the capacitor malfunctions and when it is misapplied. Hydrogen is commonly evolved, or both hydrogen and oxygen may be evolved in case of decomposition of water in the electrolyte. The evolution of gas is rapid when the capacitor fails in use, because the absorbed electrical energy tends to cause rapid decomposition and/or boiling of the electrolyte. Evolving gas can increase the internal pressure to the point where plug 14 or other comparable end seal in prior-art forms of electrolytic capacitors is blown free. Where oxygen and hydrogen are both developed, there may even be an internal explosion caused by an internal spark that ignites the mixture of gases yielded by electrolytically decomposed electrolyte.

Blowing of the capacitor's end seal or other form of pressure-relief device must occur under these conditions. However, damage could result from violent expulsion of capacitor section 12. For this reason, some form of pressure-relief or venting has been built into the containers of electrolytic capacitors for many years. The illustrated and presently preferred capacitor meets the problem directly and effectively by including a metal barrier 22 inside plug 14 and having a diameter definitely too large to pass formation 10c of the can. In this example, this is the same formation that squeezes plug 14 to seal the can to the plug and to seal the plug to the riser. Barrier 22 may be a disc of aluminum having an outside diameter of 0.315 inch and a thickness of 0.060 inch, and a hole of 0.150 inch diameter through which riser 16 extends. The clearance between riser 16 and disc 22 provides space for insulation where the disc is of metal and touches--or can touch--the can or the cathode foil or both. The projecting margins of the inner convolutions of the cathode foil may extend into this clearance space; but suitable material such as the paper separators provides insulation in the space between the riser and the nearby portions of the cathode foil. In any case, the enlarged hole provides a means for unrestricted exposure of plug 14 to the gas pressure developed at the capacitor section 12. Thus disc 22 does not seal plug 14 from the space containing the electrolyte and consequently the end-seal, especially the plug type of end-seal, is subjected to any internal pressure that may develop at the section.

The upper edge of the anode foil 26 is recessed in relation to the upper edge of the cathode foil 24. This safeguards the assembly against a possibility of the anode foil becoming short-circuited to the cathode via the metal disc. When end pressure is applied to the assembled parts preparatory to sealing plug 14 in place, even if the paper separators project above the edges of both foils, short-circuiting might occur if the upper foil edges were approximately aligned. The foil edges could penetrate projecting paper separators and become short-circuited by the metal disc.

In the example given above, plug 14 is squeezed and severely deformed and retained in hermetic sealing contact with the can and the riser. The plug blows open and relieves internal pressure when the pressure rises to about 500 pounds per square inch or somewhat less, in the case of can diameters of about 1/2-inch or less. This is far below the rupturing strength of the can. The same pressure-relief occurs where disc 22 is present. However, disc 22 serves as a secure barrier against violent expulsion of wound section 12. Riser 16 and wire 18 are united securely to the anode foil, thus being a secure part of the electrode assembly. The riser and wire are not united to plug 14 and are free of obstruction that could block plug 14 from outward pressure-relieving shift. Otherwise such an obstruction could cause plug 14 to pull on the wound section and thus threaten forcible expulsion of the wound section. Of course, a limited enlargement of terminal 16, 18 immediately outside plug 14 can be present if it does not prevent pressure-responsive displacement of plug 14 apart from section 12. Where there is a readily rupturable conductor between the wound section and the anode terminal, the terminal could be united to the end-seal without interfering with safe pressure relief.

Barrier 22 in the above example has proved eminently successful where it was shaped as a centrally perforated disc. However, the barrier can have other shapes in capacitors having the illustrated form of end seal. At the loss of some strength, edge-notched discs or discs with plural holes can be used with the type of end plug illustrated. Where other forms of end seals are used, barrier shapes should be chosen such as to avoid impairment of the seal between the can and the end-seal, and between the riser and the end seal.

In the form of end seal illustrated, the plug is directly exposed, top and bottom, to lip 10b and to disc 22, respectively. Obviously, thin wafers could be interposed at these points without significant change since those wafers would function as part of the plug in responding to internal pressures.

In the described capacitor, disc 22 is cooperable with bead 10c in restraining the capacitor section against expulsion, bead 10c in this example serving dual functions. It is evident, however, that a separate formation of the can could be included (a needless complication) for arresting barrier 22. Bead 10c would then be relied on only to serve its seal-forming function.

Various other modifications will be obvious to those skilled in the art. Therefore, the invention should be construed broadly in accordance with its full spirit and scope.