Patent Document

BACKGROUND OF THE INVENTION 
     The present invention concerns electrolytic capacitors, particularly those for use in medical devices, such as implantable defibrillators. 
     Every year more than half a million people in the United States suffer from heart attacks, more precisely cardiac arrests. Many of these cardiac arrests stem from the heart chaotically twitching, or fibrillating, and thus failing to rhythmically expand and contract as necessary to pump blood. Fibrillation can cause complete loss of cardiac function and death within minutes. To restore normal heart contraction and expansion, paramedics and other medical workers use a device, called a defibrillator, to electrically shock a fibrillating heart. 
     Since the early 1980s, thousands of patients prone to fibrillation episodes have had miniature defibrillators implanted in their bodies, typically in the left breast region above the heart. These implantable defibrillators detect onset of fibrillation and automatically shock the heart, restoring normal heart function without human intervention. A typical implantable defibrillator includes a set of electrical leads, which extend from a sealed housing into the heart of a patient after implantation. Within the housing are a battery for supplying power, heart-monitoring circuitry for detecting fibrillation, and a capacitor for storing and delivering a burst of electric charge through the leads to the heart. 
     The capacitor is typically an aluminum electrolytic capacitor, which usually includes a sandwich-like assembly of two strips of aluminum foil with two strips of paper, known as separators, between them. One of the aluminum foils serves as a cathode (negative) foil, and the other serves as an anode (positive) foil. Sometimes, two foils are stacked one on the other to form a dual anode. Attached to each foil is an aluminum tab which electrically connects the foil to other parts of the capacitor. 
     The foil-and-paper assembly, known as an active element, is then placed in a case, usually made of aluminum, and the paper is soaked, or impregnated, with a liquid electrolyte—a very electrically conductive solution containing free positive or negative ions. After the paper is impregnated, the case is sealed shut with a lid called a header. Extending from the header are two terminals connected respectively to the anode foil and cathode foil via the aluminum tabs. 
     In recent years, manufacturers of aluminum electrolytic capacitors have improved capacitor performance through the development of aluminum foils with increased surface areas. Increasing surface area of a foil, particularly the anode foil, increases capacitance and thus the charge-storage capacity of a capacitor. 
     One approach to increasing surface area of a foil is to chemically etch microscopic hills and valleys into both sides of the foil. The etching depth is controlled to leave a solid core layer between the sides of the foil. Thus, foils with this type of etching are called “core etched.” Although core-etched foils have more surface area, they don&#39;t work as well as expected in capacitors with two stacked anode foils, because the solid core layer of one anode foil shields the other anode foil from electrolyte flow. 
     Another approach, known as tunnel etching, entails etching both sides of a foil to form millions of tiny holes, or tunnels, completely through the foil, from one side to the other. The tunnels, which typically have an approximately circular cross-section about one-micron in diameter, allows electrolyte to flow through the foil. Thus, tunnel-etched foils overcome the electrolyte-flow problem of core-etched foils. 
     However, tunnel-etched foils not only have less surface area than core-etched foils but are also quite brittle and tend to break easily, particularly when rolling or winding the foils to form cylindrical capacitors. Accordingly, there remains a need to develop more durable foil structures. 
     SUMMARY OF THE INVENTION 
     To address these and other needs, the present inventors devised a new foil structure which combines the durability of core-etched foils with the electrolyte-flow advantages of tunnel-etched foils. In addition to devising methods for making the new foil structure, the inventors applied the new foil structure in novel ways to build new capacitor foil assemblies and new capacitors in cylindrical and flat configurations, for example. Ultimately, these advances allow construction of smaller medical devices, such as implantable defibrillators. 
     Specifically, one embodiment of the new foil structure is a foil having one or more holes or perforations and one or more cavities with a depth less than the foil thickness. In an exemplary embodiment, each perforation and cavity has a cross-sectional area, with the perforations having a larger, for example, 2 to 100 times larger, average cross-sectional area than the cavities. One method of making the new foil structure includes perforating a foil and forming cavities into one or both of its surfaces. Other methods form the cavities before perforating the foil. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an enlarged perspective view of an exemplary foil structure 8 that embodies the invention; 
     FIG. 2 is a perspective view of an exemplary cylindrical electrolytic capacitor  10  which incorporates the foil structure of FIG. 1; 
     FIG. 3 is a cross-sectional view of an exemplary electrolytic capacitor  10  which incorporates the foil structure of FIG. 1; 
     FIG. 4 is a cross-sectional view of a layered capacitive assembly  21  which forms an active element  20  of capacitor  10  and which incorporates the FIG. 1 foil structure; 
     FIGS. 5A-5C are perspective views of other capacitor configurations that incorporate the FIG. 1 foil structure; 
     FIG. 6 is a cross-sectional view of a symmetric capacitive assembly  60  which incorporates the FIG. 1 foil structure and which is particularly suited for flat capacitor configurations; and 
     FIG. 7 is a block diagram of generic implantable defibrillator  70  including a capacitor that incorporates the FIG. 1 foil structure. 
    
    
     DETAILED DESCRIPTION 
     The following detailed description, which references and incorporates FIGS. 1-7, describes and illustrates one or more exemplary embodiments of the invention, specifically a new foil structure and method of manufacture, several new foil assemblies, new capacitors incorporating the foil structure and foil assemblies, and an implantable defibrillator incorporating one or more of the new capacitors. These embodiments, offered not to limit but only to exemplify and teach, are shown and described in sufficient detail to enable those skilled in the art to implement or practice the invention. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art. 
     Exemplary Foil Structure and Methods of Manufacture 
     FIG. 1 shows an enlarged perspective view of a foil structure 8 which the inventors call a “perforated-core-etched” foil. Foil structure 8 can be made of aluminum, tantalum, hafniium, niobium, titanium, zirconium, and combinations of these metals. However, the invention is not limited to any particular foil composition or class of foil compositions. 
     Foil structure 8 includes opposing surfaces  8   a  and  8   b  which define an average foil thickness  8   t  and a set of perforations  8   p  which extend through foil structure 8 from surface  8   a  to surface  8   b . Surfaces  8   a  and  8   b  include respective sets of surface cavities (or depressions)  9   a  and  9   b , which have generally cylindrical, conical, or hemispherical shapes. However, the invention is not limited to any particular cavity form, class of cavity forms, or combination of cavity forms. Surface cavities  9   a  have an average maximum depth Da which is less than thickness  8   t , and surface cavities  9   b  having an average maximum depth Db which is also less than thickness  8   t . As FIG. 1 shows, depths Da and Db are measured along dimensions generally perpendicular to respective surfaces  8   a  and  8   b . Cavities  9   a  and  9   b  also have respective average maximum cross-sectional areas Sa and Sb (which are not shown in the figure.) Cross-sectional area is measured in a plane substantially parallel to one of surfaces  8   a  and  8   b.    
     In the exemplary embodiment, average maximum depths Da and depths Db are approximately equal to one third or one quarter of thickness  8   t , and cross-sectional areas Sa and Sb are substantially equal and range inclusively between about 0.16 and 0.36 square-microns. However, other embodiments use different equal and unequal depths Da and Db and different and unequal cross-sectional areas Sa and Sb. 
     More generally, the exemplary embodiment adheres to the constraint that the sum of average maximum depths Da and Db is less than thickness  8   t . Adherence to this constraint ensures that a significant number of cavities  9   a  are separated from a significant number of cavities  9   b  by a solid region of foil material. These regions of solid material not only provide foil structure 8 with greater structural integrity but also greater surface area than conventional tunnel-etched foils. However, in some embodiments of the invention one or more of cavities  9   a  intersect one or more of cavities  9   b , thereby forming openings through the foil. The number of these intersections and resultant openings can be regulated through selection of appropriate cavity formation techniques and cavity depths. 
     In addition to surface cavities  9   a  and  9   b , FIG. 1 shows that foil structure 8 includes a set of one or more perforations (or holes)  8   p . Perforations  8   p  have an average maximum cross-sectional area Sp measured in a plane substantially parallel to one of surfaces  8   a  and  8   b . Although perforations  8   p  have a generally circular cross-section in the exemplary embodiment, other embodiments use perforations with elliptical, triangular, square, or rectangular cross-sections. Thus, the invention is not limited to any particular shape or class of shapes. The layout or arrangement of perforations  8   p  takes any number of forms, including for example, a random distribution and a specific pattern with each perforation having a predetermined position relative to other perforations. The number of perforations per unit area is chosen to optimize relevant criteria, such as capacitor electrical performance or foil structural properties. 
     In the exemplary embodiment, average maximum cross-sectional area Sp of perforations  8   p  is larger than average maximum cross-sectional areas Sa and Sb of cavities  9   a  and  9   b . More precisely, area Sp in the exemplary embodiment ranges between about 500 square-microns and 32 square-millimeters. In other embodiments, area Sp ranges between 2-50, 10-75, 25-100, or 2-100 times larger than surface areas Sa and Sb. Additionally, the exemplary embodiment provides a total perforation area (number of perforations times average maximum cross-sectional area Sp) which is no more than about 20 percent of the foil surface area. 
     The inventors have devised a number of ways of making foil structure 8. For example, one method initially core-etches a foil using conventional etching techniques to form cavities  9   a  and  9   b  and then perforates the core-etched foil. Another method entails initially perforating a foil to form perforations  8   p  and then etching the perforated foil to form cavities  9   a  and  9   b . (For more details on a conventional etching technology, see, for example, U.S. Pat. No. 4,395,305 to Whitman, which is entitled Chemical Etching of Aluminum Capacitor Foil and incorporated herein by reference.) Perforations  8   p  can be formed using lasers, chemical etchants, or mechanical dies, for example. Conceptually, cavities  9   a  and  9   b  could also be formed using lasers. Thus, the invention is not limited to any particular technique or combination of techniques for forming perforations  8   p  and cavities  9   a  and  9   b.    
     In one embodiment of the invention, further processing of the foils, particularly those intended for electrolytic capacitors, entails applying an insulative, or dielectric, coating to one or both sides of the foils. Examples of suitable coatings include metallic oxides such as aluminum or tantalum oxide. 
     Exemplary Foil Assemblies Incorporating the New Foil Structure 
     Foil structure 8 can be combined with other foils structures to form various electrically and/or mechanically advantageous foil assemblies. Many of these assemblies are particularly useful as multiple anodes structures in flat, semi-cylindrical, and cylindrical capacitors. 
     In particular, the inventors devised several foil assemblies that combine foil structure 8 with core-etched and tunnel-etched foils. For example, one foil assembly stacks two or three foils incorporating foil structure 8 to form a dual- or triple-foil assembly which can serve as a dual or triple anode. Another foil assembly stacks a core-etched foil between two foils incorporating foil structure 8. Table 1 describes these and several other foil assemblies. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Foil Assembly No. 
                 Structure 
               
               
                   
               
             
             
               
                 1 
                 PP 
               
               
                 2 
                 PPP 
               
               
                 3 
                 PCP 
               
               
                 4 
                 PPCPP 
               
               
                 5 
                 PTP 
               
               
                 6 
                 PTPT 
               
               
                 7 
                 TPT 
               
               
                 8 
                 PTCTP 
               
               
                   
               
             
          
         
       
     
     In the table, P denotes a perforated foil similar to foil structure 8; C denotes a core-etched foil; and T denotes a tunnel-etched foil. Thus, for example, foil assembly 7 comprises a foil similar to foil structure 8 between two tunnel-etched foils. Other novel assemblies result from combining two or more of these assemblies. For instance, combining two assembly 1s yields a PPPP structure, and combining assemblies 2 and 3 yields a PPPPCP structure. Additionally, still other novel assemblies result from inserting insulators and electrolyte-impregnated substrates, such as paper, between adjacent foils of an assembly. 
     Exemplary Capacitor Incorporating the New Foil Structure 
     FIG. 2 shows a perspective view of an exemplary electrolytic capacitor  10  which incorporates one or more foils incorporating foil structure 8 or one or more of the foil assemblies described above. In addition to incorporating these novel foils and foil assemblies, capacitor  10  embodies many novel space-saving features. These features and their advantages are addressed in a co-pending U.S. Pat. application (attorney docket number 279.086US1) which is entitled Smaller Electrolytic Capacitors for Implantable Defibrillator, and which was filed on the same day as the present application. This application is incorporated herein by reference. 
     More specifically, FIG. 2 shows that capacitor  10  has a diameter  10   d  of about 14.5 millimeters and a total height  10   h  of about 30 millimeters, thereby occupying a total volume of about five cubic-centimeters. Capacitor  10  also includes a cylindrical aluminum case  12 , a header (or lid)  14 , and two aluminum terminals  16  and  18 . Two rivets  15  and  17  fasten terminals  16  and  18  to header  14 . Case  12 , which houses an active element  20  (not visible in this view), includes a circumferential seating groove  12   a  and a rolled lip  12   b , both of which secure header  14  to case  12 . 
     FIG. 3, a cross-sectional view taken along line  3 — 3  in FIG. 2, shows that case  12  has a thickness  12   t  and groove  12   a  is spaced a distance  12   d  from lip  12   b . Thickness  12   t  is about 0.010 inches, and distance  12   d  is about 0.145 inches. Additionally, groove  12   a  has a radius of about 0.035 inches, and lip  12   b , which is formed by rolling over the top edge of case  12 , has a radius of about 0.015 inches. (Some embodiments compress or flatten groove  12   a  to reduce capacitor height and volume.) Header  14 , which comprises a rubber layer  14   a  and a phenolic-resin layer  14   b , has a total thickness  14   t  of about two millimeters. 
     FIG. 3 also shows that capacitor  10  includes an active element  20  wound around mandrel region  28  and two pairs of insulative inserts  30   a - 30   b  and  32   a  - 32   b  respectively positioned adjacent the top and bottom of active element  20 . Mandrel region  28  has an exemplary width or diameter  28   w  of about 2.5 millimeters. And, insulative inserts  30   a - 30   b  and  32   a - 32   b  comprise respective pairs of paper disks, with each disk having a thickness of one one-thousandth of an inch and a diameter of about 14 millimeters The insulative inserts ensure electrical isolation of conductive portions of active element  20  from anode tab  25  and rivets  15  and  17  and from the bottom interior surface of case  12 . (As an alternative to insulative inserts, other embodiments enclose substantially all of active element  20  within an insulative bag.) For clarity, FIG. 3 omits a 1.125inch-wide plastic insulative sheath that surrounds the vertical surfaces of active element  20 . 
     Active element  20  comprises about  19  turns of a layered capacitive assembly  21 . As the cross-section in FIG. 4 shows, capacitive assembly  21  includes a cathode  22 , an anode structure  24 , and four electrolyte-impregnated separators  26   a ,  26   b ,  26   c , and  26   d . Cathode  22  and anode  24  each have a width (or height)  22   w . In this exemplary embodiment, cathode  22  and the one or more constituents of anode structure  24  are about 24 millimeters wide and 100 microns thick. Cathode  22  is about 422 millimeters long, and anode structure  24  is about 410 millimeters long. 
     Anode structure  24  can assume a variety of novel forms, the simplest being a single foil member incorporating foil structure 8 of FIG.  1 . Some embodiments provide anode structure  24  with one or more of the novel foil assemblies described using Table 1. 
     Although not shown in FIG. 4, the exemplary embodiment connects anode structure  24  to one anode tab regardless of the number of foils constituting the anode structure. (FIG. 3 shows an exemplary aluminum anode tab  25 .) Other embodiments, however, provide individual anode tabs for each anode members, with the tabs connected together to form a joint or composite anode tab. For more details on these or other types of tabs incorporated in other embodiments of the invention, see co-pending U.S. patent applications Ser. No. 09/063692 and 09/076023 (attorney docket numbers 279.094US1 and 279.101US1) which are respectively entitled Electrolytic Capacitor and Multi-Anodic Attachment and Wound Multi-Anode Electrolytic Capacitor with Offset Anodes and which are incorporated herein by reference. 
     Anode tab  25 , shown in FIG. 3, is ultrasonically welded to rivet  15  and thus electrically connected to terminal  16 . The exemplary embodiment folds anode tab  25  over itself; however, other embodiments omit this fold to reduce the space between header  14  and the top of active element  20 . Though not visible in FIG. 3 or  4 , cathode  22  includes a cathode tab which is similarly connected via rivet  17  to terminal  18 . 
     In addition to cathode  22  and anode  24 , FIG. 4 shows that capacitive assembly  21  includes thin electrolyte-impregnated separators  26 , specifically  26   a ,  26   b ,  26   c , and  26   d . In the exemplary embodiment, separators  26   a - 26   d  each consists of kraft paper impregnated with an electrolyte, such as an ethylene-glycol base combined with polyphosphates or ammonium pentaborate, and each has a thickness less than 0.001 inches. More specifically, the exemplary embodiment uses one or more papers of the following thicknesses: 0.000787, 0.0005 inches, and 0.00025 inches, with thicker papers preferably placed nearer the center of the active element to withstand the greater tensile stress that interior separators experience during winding. 
     Additionally, each of separators  26   a - 26   d  has a width  26   w  which is less than four millimeters wider than cathode  22  and anode  24  to provide end margins  27   a  and  27   b . In the exemplary embodiment, width  26   w  is about  27  millimeters, or three millimeters wider than cathode  22  and anode  24 , to provide end margins  27   a  and  27   b  of about 1.5 millimeters. Other embodiments of the invention provide at least one end margins of about 1.75, 1.25, 1, 0.75, 0.5, 0.25, and even 0.0 millimeters. 
     Although the exemplary capacitor  10  has a wound or cylindrical configuration, the invention is not limited to any particular type or category of configurations. For example, FIGS. 5A,  5 B, and  5 C show other capacitor configurations encompassed by the invention. More specifically, FIG. 5A shows a semi-cylindrical (or “D”) capacitor; FIG. SB shows an asymmetric semi-cylindrical capacitor; and FIG. 5C shows a flat (or more precisely a rectangular parallelepiped) capacitor. FIG. 6 show a cross-sectional view of a capacitive assembly  60  particularly useful for flat capacitors such as the one shown in FIG.  5 C. 
     In particular, capacitive assembly  60  includes an anode structure  62  between two cathode foils  64   a  and  64   b . Electrolyte-impregnated separators  63   a  and  63   b  lie respectively between anode structure  62  and cathode foils  64   a  and  64   b . In the exemplary embodiment separators  63   a  and  63   b  each comprise two or more layers of kraft paper of thicknesses similar to separators  26  of FIG.  4 . Anode structure  62  comprises one or more of the foil assemblies identified in Table 1. 
     Exemplary Embodiment of Implantable Defibrillator 
     FIG. 7 shows one of the many applications for exemplary capacitor  10 : a generic implantable defibrillator  70 . More specifically, defibrillator  70  includes a lead system  72 , which after implantation electrically contacts strategic portions of a patient&#39;s heart, a monitoring circuit  74  for monitoring heart activity through one or more of the leads of lead system  72 , and a therapy circuit  76  which delivers electrical energy through lead system  72  to the patient&#39;s heart. Therapy circuit  76  includes an energy storage component  76   a  which incorporates at least one capacitor having one or more of the novel features of capacitor  10 . Defibrillator  70  operates according to well known and understood principles. 
     In addition to implantable defibrillators, the innovations of capacitor  10  can be incorporated into other cardiac rhythm management systems, such as heart pacers, combination pacer-defibrillators, and drug-delivery devices for diagnosing or treating cardiac arrhythmias. They can be incorporated also into non-medical applications, for example, photographic flash equipment. Indeed, the innovations of capacitor  10  are pertinent to any application where small, high energy, low equivalent-series-resistance (ERS) capacitors are desirable. 
     Conclusion 
     In furtherance of the art, the inventors devised a new foil structure which combines the durability of core-etched foils with the electrolyte flow advantages of tunnel-etched foils. In addition to devising methods for making the new foil structure, the inventors applied the new foil structure to build new capacitors and implantable defibrillators. 
     The embodiments described above are intended only to illustrate and teach one or more ways of practicing or implementing the present invention, not to restrict its breadth or scope. The actual scope of the invention, which embraces all ways of practicing or implementing the concepts and principles of the invention, is defined only by the following claims and their equivalents.

Technology Category: 1