WO2007109612A2 - Valve metal ribbon type fibers for solid electrolytic capacitors - Google Patents
Valve metal ribbon type fibers for solid electrolytic capacitors Download PDFInfo
- Publication number
- WO2007109612A2 WO2007109612A2 PCT/US2007/064305 US2007064305W WO2007109612A2 WO 2007109612 A2 WO2007109612 A2 WO 2007109612A2 US 2007064305 W US2007064305 W US 2007064305W WO 2007109612 A2 WO2007109612 A2 WO 2007109612A2
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- WIPO (PCT)
- Prior art keywords
- valve metal
- fibers
- capacitor
- sintering
- metal comprises
- Prior art date
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- 239000000835 fiber Substances 0.000 title claims abstract description 79
- 229910052751 metal Inorganic materials 0.000 title claims abstract description 37
- 239000002184 metal Substances 0.000 title claims abstract description 37
- 239000003990 capacitor Substances 0.000 title claims description 27
- 239000007787 solid Substances 0.000 title description 7
- 238000000034 method Methods 0.000 claims abstract description 26
- 238000005245 sintering Methods 0.000 claims abstract description 23
- 238000005096 rolling process Methods 0.000 claims abstract description 12
- 239000000463 material Substances 0.000 claims abstract description 10
- 230000009467 reduction Effects 0.000 claims abstract description 6
- GUVRBAGPIYLISA-UHFFFAOYSA-N tantalum atom Chemical compound [Ta] GUVRBAGPIYLISA-UHFFFAOYSA-N 0.000 claims description 36
- 229910052715 tantalum Inorganic materials 0.000 claims description 26
- 229910052802 copper Inorganic materials 0.000 claims description 18
- 239000010949 copper Substances 0.000 claims description 18
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 claims description 17
- 229910052758 niobium Inorganic materials 0.000 claims description 6
- 239000010955 niobium Substances 0.000 claims description 6
- 229910052782 aluminium Inorganic materials 0.000 claims description 5
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims description 5
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 claims description 5
- 229910052720 vanadium Inorganic materials 0.000 claims description 4
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 claims description 4
- 238000007743 anodising Methods 0.000 claims 1
- 230000015572 biosynthetic process Effects 0.000 description 20
- 238000005755 formation reaction Methods 0.000 description 20
- 238000011282 treatment Methods 0.000 description 15
- 239000000843 powder Substances 0.000 description 10
- 239000011159 matrix material Substances 0.000 description 8
- 239000002131 composite material Substances 0.000 description 7
- 230000008569 process Effects 0.000 description 7
- 239000002245 particle Substances 0.000 description 6
- 239000010407 anodic oxide Substances 0.000 description 4
- 238000013459 approach Methods 0.000 description 4
- 230000008901 benefit Effects 0.000 description 4
- 238000001125 extrusion Methods 0.000 description 4
- 230000006872 improvement Effects 0.000 description 4
- 150000002739 metals Chemical class 0.000 description 4
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 3
- 238000005516 engineering process Methods 0.000 description 3
- 239000000758 substrate Substances 0.000 description 3
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 description 2
- NBIIXXVUZAFLBC-UHFFFAOYSA-N Phosphoric acid Chemical compound OP(O)(O)=O NBIIXXVUZAFLBC-UHFFFAOYSA-N 0.000 description 2
- 239000002253 acid Substances 0.000 description 2
- 230000015556 catabolic process Effects 0.000 description 2
- 238000005530 etching Methods 0.000 description 2
- 239000012535 impurity Substances 0.000 description 2
- 238000002386 leaching Methods 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 210000003739 neck Anatomy 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 229910017604 nitric acid Inorganic materials 0.000 description 2
- 238000004806 packaging method and process Methods 0.000 description 2
- 238000003825 pressing Methods 0.000 description 2
- 238000005491 wire drawing Methods 0.000 description 2
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 1
- ROSDCCJGGBNDNL-UHFFFAOYSA-N [Ta].[Pb] Chemical compound [Ta].[Pb] ROSDCCJGGBNDNL-UHFFFAOYSA-N 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 229910000147 aluminium phosphate Inorganic materials 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
- 239000011651 chromium Substances 0.000 description 1
- 238000005253 cladding Methods 0.000 description 1
- 238000009694 cold isostatic pressing Methods 0.000 description 1
- 238000011109 contamination Methods 0.000 description 1
- CIYRLONPFMPRLH-UHFFFAOYSA-N copper tantalum Chemical compound [Cu].[Ta] CIYRLONPFMPRLH-UHFFFAOYSA-N 0.000 description 1
- 238000002425 crystallisation Methods 0.000 description 1
- 230000008025 crystallization Effects 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 230000001627 detrimental effect Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000001513 hot isostatic pressing Methods 0.000 description 1
- 238000005470 impregnation Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 1
- 230000001788 irregular Effects 0.000 description 1
- 238000000462 isostatic pressing Methods 0.000 description 1
- 238000000626 liquid-phase infiltration Methods 0.000 description 1
- 229910044991 metal oxide Inorganic materials 0.000 description 1
- 150000004706 metal oxides Chemical class 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 238000005457 optimization Methods 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 230000035699 permeability Effects 0.000 description 1
- 239000011148 porous material Substances 0.000 description 1
- 238000011946 reduction process Methods 0.000 description 1
- 229910052710 silicon Inorganic materials 0.000 description 1
- 239000007858 starting material Substances 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 238000007669 thermal treatment Methods 0.000 description 1
- 229910052719 titanium Inorganic materials 0.000 description 1
- 229910052721 tungsten Inorganic materials 0.000 description 1
- 239000011800 void material Substances 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
- 238000005303 weighing Methods 0.000 description 1
- 238000003466 welding Methods 0.000 description 1
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
- B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
- B21C37/04—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of bars or wire
- B21C37/047—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of bars or wire of fine wires
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
- B21C33/00—Feeding extrusion presses with metal to be extruded ; Loading the dummy block
- B21C33/004—Composite billet
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
- B22F3/002—Manufacture of articles essentially made from metallic fibres
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
- C25D11/02—Anodisation
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
- C25D11/02—Anodisation
- C25D11/04—Anodisation of aluminium or alloys based thereon
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
- C25D11/02—Anodisation
- C25D11/26—Anodisation of refractory metals or alloys based thereon
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
- H01G9/004—Details
- H01G9/04—Electrodes or formation of dielectric layers thereon
- H01G9/048—Electrodes or formation of dielectric layers thereon characterised by their structure
- H01G9/052—Sintered electrodes
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
- H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
- H01G9/004—Details
- H01G9/04—Electrodes or formation of dielectric layers thereon
- H01G9/048—Electrodes or formation of dielectric layers thereon characterised by their structure
- H01G9/052—Sintered electrodes
- H01G9/0525—Powder therefor
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
- B22F2998/10—Processes characterised by the sequence of their steps
Definitions
- valve metals which are metals such as tantalum, aluminum, niobium, vanadium and the like.
- tantalum is the preferred metal, and efforts to improve the performance of capacitors made of tantalum are highly desired.
- Miniaturization is one of the main technology drivers in the electronics industry. For capacitors, miniaturization is achieved by increasing volumetric efficiency, which is the normalized capacitance per volume or CV/cm 3 or normalized capacitance per gram or CV/g.
- the only feasible means to improve volumetric efficiency is to increase the available surface area by increasing the specific surface area of the valve metal substrate on which the anodic oxide layer is formed.
- the specific surface area depends on the morphology of the substrate on which the dielectric film is produced.
- tantalum powder considerable development has pushed the technology to exceptionally high CV/g levels.
- these high CV powders suffer from extremely rapid fall-off in CV/g with increasing formation voltages.
- Fibers particularly flat fibers which are essentially two- dimensional structures, should have similar properties to flakes.
- the potential advantages of fibers have been known for many years, and several approaches were proposed for making fibers suitable for capacitors. As far back as 1972, Douglas patented a method for making fibers (U.S. Patent No. 3,681 ,063), and capacitors from these fibers (U.S. Patent Nos.
- tantalum powder was mixed with a second metal powder using sufficient powder so that the second metal forms the matrix surrounding the tantalum particles.
- the blend was compacted into a billet, and the billet subsequently drawn to elongate the tantalum powder particles.
- the matrix material was removed by leaching in acid to release the tantalum fibers.
- Fife also described a method of making anodes by forming the fibers into a felt or mat structure (see U.S. Patent 5,306,462). Fife emphasized the need to have short fibers approximately 400 ⁇ m in length and to randomly orient the fibers in order to preserve maximum surface area on sintering.
- My earlier patented processes involved assembling a composite billet of solid tantalum rods in a soft metal matrix, and then drawing the rod to wire to reduce the size of the tantalum.
- Copper is the preferred matrix material since it is very ductile, has virtually no solubility in tantalum, and has deformation characteristics that are compatible with tantalum.
- the wire was cut into short lengths and bundled together in a secondary billet making a multifilament composite.
- the composite billet was further reduced by extrusion, drawing, or rolling, or a combination of these methods. The process was repeated a number of times to achieve very high reductions, and produce very fine tantalum fibers.
- a variation of the above processes is to draw the tantalum-copper composite until the fibers are a few microns in diameter, then flatten the fibers by rolling to produce a highly aspected, high surface area fiber that is a micron or less in thickness.
- the flattened fibers thus formed are thin ribbons that have many of the dimensional attributes of flakes and provide higher surface area per weight of metal than round fibers.
- a further advantage of making continuous fibers is that it avoids the inherent complexities of handling and pressing short fibers, The continuous lengths of fiber can be twisted or braided to form a fiber strip that holds the loose filaments together. Anodes can be stamped directly from the strip, thus eliminating the need to press powders.
- the present invention provides an improvement over prior art methods for making electrolytic capacitors. More particularly, the present invention provides an improved method for making capacitor anodes by producing filamentary valve metal fibers by a co-reduction of valve metal filaments within a copper matrix by a combination of drawing and rolling. The copper matrix is then removed leaving valve metal fibers in the form of continuous flat, ribbon-like fibers that have a relatively high aspect ratio of width to thickness, typically of at least about 10 to 1, and as a result relatively high surface area. By producing the fibers in a bundled continuous strip form, they can be made into thin anodes without pressing, thus maintaining the high surface area through subsequent anode sintering and formation.
- Fig. 1 is a plot of specific surface area to diameter or width of flattened and drawn ribbons or wires
- Fig. 2 is a flow-chart describing the steps followed in a preferred embodiment of the process of the present invention
- Fig. 3 is a plot comparing the affect of formation voltage on CV/g under different sintering treatments
- Fig. 4 is a graph showing CV/g versus formation voltage under a single sintering treatment
- Fig. 5 is a plot showing DC leakage after different sintering treatments.
- Tantalum fibers were produced as fine filamentary ribbons, which were made by a process of extrusion, drawing, and rolling of a multifilament composite following the teachings of my prior U.S. Patent No. 5,034,857.
- the overall process is as follows: In a preferred embodiment of the present invention, the process begins with pure tantalum rod or high purity tantalum rod having a small amount of impurities, e.g. Fe, Ni, Cr, Cu, Nb, Mo, Si, Ti, W, C or O.
- a plurality of tantalum rods 12 are assembled substantially parallel to one another, in a copper can.
- a copper nose and tail are welded onto the can to form a primary billet, and the billet is then evacuated and sealed.
- the can is then hot extruded to bond the copper to the tantalum and cold drawn to make a copper clad tantalum wire bundle following the teachings of my prior U.S. Patent No. 5,034,857. Bonding of the copper cladding to the tantalum is essential to prevent oxidation or other contamination of the tantalum during subsequent processing.
- the resulting copper clad tantalum wire bundle was cut to length, and bundled and restacked into a second copper container, a nose and tail are welded in place, and the secondary billet is evacuated and sealed as before.
- the secondary sealed billet is optionally prepared for extrusion by hot or cold isostatic pressing in order to collapse any void space within the billet and to promote filament uniformity.
- the secondary billet is machined to fit the extrusion liner, and the billet is then extruded and drawn followed by rolling to a final preferred thickness of less than 1 ⁇ m and preferably less than 0.5 ⁇ m.
- the tantalum fibers are immersed in an etching solution such as nitric acid and water to leach the copper.
- CV/g and DC leakage are characteristics that depend on the quality and morphology of the fibers. Capacitance is largely a function of surface area, but also depends on the packaging of the filaments in the anode body.
- the present invention results in part from the realization that for a fixed volume of material, ribbon type fibers have more surface area than round fibers when the ribbon type fibers have a thickness equivalent to the diameter of the round fibers. Thus it is possible to produce higher surface area fibers by flattening round fibers. This greatly facilitates the production of high surface area fibers, since it is difficult to make very fine, submicron fibers by wire drawing without producing fibers that have highly irregular cross-sections.
- valve metal fibers having non-uniform cross sections lead to lower CV/g performance.
- the starting material was a rod 12 of high purity tantalum.
- the rod was vacuum encapsulated in a copper can 14, extruded and cold drawn to make a copper clad Ta wire.
- the wire was cut to length, bundled and restacked into a second copper container, and further reduced by drawing followed by rolling to final thickness. After rolling, the resulting Ta fibers were removed from the matrix by leaching the copper with nitric acid.
- the drawing and rolling parameters can be varied to produce a wide range of fiber sizes and shapes, the particular deformation sequence and reduction scheduled used in this example resulted in fibers that were approximately 0.5 to 1 ⁇ m thick and 35-50 ⁇ m wide, and had a B.E.T. surface area greater than 0.300 m Ig.
- the fibers were twisted and cut into pieces weighing approximately 50 mg and a tantalum lead wire attached by spot welding.
- the dimensions of the anodes were approximately 0.3 x 4 x 8 mm. Since the fibers are continuous, the length of the fibers forming the anode is equivalent to one of the planar dimensions of the anode.
- the anodes were sintered under vacuum of greater than 10 "3 Pa (7.5 x 10 "6 torr) for 10 minutes or 50 minutes at temperatures of either 1300 0 C or 1500 0 C.
- the fibers received no other chemical or thermal treatment.
- the sintered anodes were anodized in a solution of 0.10 V/V% phosphoric acid at 80 0 C and a current of 100 mA per gram.
- Samples were anodized to formation voltages of 100 V, 140 V and 180 V. Capacitance and leakage current were measured in a wet cell of 15 W/W% H 2 SO 4 . DC leakage current was measured at a potential of 70% of the formation voltage.
- Capacitance values for 100 V, 140 V and 180 V formations are given in Table 1 and Figure 3 which reports on the effect of formation voltage on CV/g for each sintering treatment.
- the CV/g is highest for the 1500 0 C 10 minute sintering treatment and lowest for the 1500 0 C 50 minute sinter.
- the CV/g values are similar for all sintering treatments.
- the highest CV/g value was obtained with the 1500 0 C 50 minute sintering treatment, while the values for the 1300 0 C and 1500 0 C 10 minute sintering treatments were nearly identical.
- the CV/g values for the 1500 0 C 50 minute sinter treatment exhibits a linear fall-off with formations voltages as shown in Figure 4 which reports CV/g versus formation voltage for a 1500 0 C 50 minute sintering treatment showing the linear fall-off of capacitance between 100 V to 200 V formations and how the fall-off rate is less severe than for the 10 minute sintering treatment at both 1300 0 C and 1500 0 C.
- DC leakage values are given in Table 2. As can be seen leakage decreases with increasing sintering temperature and sintering time. The values for the two 1500 0 C sintering treatments are shown in Figure 5 which reports DC leakage for 1500 0 C treatments. As can be seen, above 140 V formation, leakage increases dramatically. The data also show that at 100 V formation, leakage below 0.5 nA/ ⁇ F-V can be obtained without a deoxidation treatment. Table 1. Specific Capacitance
- tantalum fibers produced by a composite co-reduction process in accordance with the present invention have properties suitable for use in forming capacitor anodes particularly those used for higher voltage ratings.
- the fibers can be produced in different sizes depending on the intended application voltage.
- the fibers are organized in a continuous filament structure which improves handling and packaging of the fibers into anodes. Further improvements in CV/g can be realized by producing a more uniform fiber structure, while improvements in DC leakage may be achieved by performing a deoxidation treatment or through optimization of the sintering cycle.
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- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Power Engineering (AREA)
- Organic Chemistry (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Electrochemistry (AREA)
- Mechanical Engineering (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Manufacturing & Machinery (AREA)
- Powder Metallurgy (AREA)
- Fixed Capacitors And Capacitor Manufacturing Machines (AREA)
- Superconductors And Manufacturing Methods Therefor (AREA)
Abstract
A method for making superconducting material useful for forming electrolytic devices comprising the steps of establishing multiple valve metal rods in a primary billet of a ductile material; working the primary billet to a series of reduction steps to form said valve metal rods into a plurality of elongated elements surrounded at least in part by the ductile material; cutting the elongated elements from step (b) and bundling the cut elements to form a secondary billet; working the secondary billet through a series of reduction steps followed by rolling to final thickness; removing the ductile material, whereby to leave valve metal elongated fibers; and sintering the elongated fibers from step (e) under vacuum.
Description
Valve Metal Ribbon Type Fibers for Solid Electrolytic Capacitors Solid electrolytic capacitors are made of valve metals, which are metals such as tantalum, aluminum, niobium, vanadium and the like. For high reliability devices, tantalum is the preferred metal, and efforts to improve the performance of capacitors made of tantalum are highly desired. Miniaturization is one of the main technology drivers in the electronics industry. For capacitors, miniaturization is achieved by increasing volumetric efficiency, which is the normalized capacitance per volume or CV/cm3 or normalized capacitance per gram or CV/g. The capacitance (C) of a dielectric is given by: C = εo-ε-AJd where ε0 is the permeability in vacuum, ε is the dielectric constant of the anodic oxide layer, and A and d are the surface area and thickness of the oxide respectively. Since ε0, is a physical constant and ε is a material property which is fixed by the dielectric constant of the valve metal, the only parameters that can be manipulated to enhance volumetric efficiency are area (A) and thickness (d). For practical purposes, the thickness of the anodic oxide film is set by reliability considerations. For a given voltage rating, a thinner anodic oxide layer will provide less resistance to dielectric breakdown leading to lower reliability. Thus, the only feasible means to improve volumetric efficiency is to increase the available surface area by increasing the specific surface area of the valve metal substrate on which the anodic oxide layer is formed. The specific surface area depends on the morphology of the substrate on which the dielectric film is produced. For tantalum powder, considerable development has pushed the technology to exceptionally high CV/g levels. However, as reported in Y. Pozdeev-Freeman, "How Far Can We Go With High CV Capacitors", T.I.C. Bulletin, No. 122, June 2005, pp 4-8, these high CV powders suffer from extremely rapid fall-off in CV/g with increasing formation voltages. As a consequence these powders are generally useful for only low voltage applications, and there is still a need for the development of higher CV/g tantalum substrates for solid capacitors rated in range of 35 to 50V range. A significant achievement in tantalum powder technology has been the development of powder having flake morphology. See J. Koenitzer, S. Krause, L. Mann, S. Yuan, T. Izumi and Y. Noguchi, "Tantalum Flakes - Powders for High Reliability Electrolytic Capacitor Applications", presented at the International Symposium Tantalum and Niobium World, October 2006; J. A. Fife, "Improvements to Volumetric
Efficiency", T.I.C. Bulletin, No. 81 , March 1995, pp. 5-8. Because of their structure, flakes have a higher surface to volume ratio than nodular powders. The flat surfaces can provide more contact area between particles resulting in better inter-particle bonding. Also the reduced curvature of flakes lowers the stresses in the oxide layer particularly at higher formation voltages where the oxide is thicker. These last two characteristics help achieve lower DC leakage. Fibers, particularly flat fibers which are essentially two- dimensional structures, should have similar properties to flakes. The potential advantages of fibers have been known for many years, and several approaches were proposed for making fibers suitable for capacitors. As far back as 1972, Douglas patented a method for making fibers (U.S. Patent No. 3,681 ,063), and capacitors from these fibers (U.S. Patent Nos. 3,742.369 and 3,827.865). The basic approach involved sintering tantalum powder into a porous compact and impregnating the compact with a softer metal, such as copper, nickel, or aluminum that does not react with tantalum. Impregnation was accomplished b> melt infiltration of the second metal. The solidified composite structure was drawn or rolled to elongate the tantalum particles to produce fibers. The matrix was removed by etching in a suitable acid resulting in a porous structure of elongated fibers. Fife in U.S. Patent No. 4,502,884 describes a method for making loose fibers from tantalum powder and capacitor anodes from these fibers. In this approach, tantalum powder was mixed with a second metal powder using sufficient powder so that the second metal forms the matrix surrounding the tantalum particles. The blend was compacted into a billet, and the billet subsequently drawn to elongate the tantalum powder particles. The matrix material was removed by leaching in acid to release the tantalum fibers. Fife also described a method of making anodes by forming the fibers into a felt or mat structure (see U.S. Patent 5,306,462). Fife emphasized the need to have short fibers approximately 400 μm in length and to randomly orient the fibers in order to preserve maximum surface area on sintering. While the benefits of using fibers are known in the art, what has not been fully appreciated is that since flat fibers or ribbon type fiber have greater surface area than round fibers of equivalent cross-sectional area, the thinner the fiber thickness the greater is the surface area as is shown in Fig. 1 , which leads to the possibility of increasing the efficiency of the capacitor by producing the flat fibers with a ribbon-like morphology. Additionally flat fibers with high surface area are easier to produce than high surface area round fibers because it is very difficult to
produce uniformly round fine filaments by conventional wire drawing techniques. Thus, large round filaments that are easy to produce can be rolled to thin cross-section to make high surface area fibers. In my previous U.S. Patents No. 5,034,857 and 5,869,196 I describe approaches intended for making continuous fibers. My earlier patented processes involved assembling a composite billet of solid tantalum rods in a soft metal matrix, and then drawing the rod to wire to reduce the size of the tantalum. Copper is the preferred matrix material since it is very ductile, has virtually no solubility in tantalum, and has deformation characteristics that are compatible with tantalum. At a suitable size, the wire was cut into short lengths and bundled together in a secondary billet making a multifilament composite. The composite billet was further reduced by extrusion, drawing, or rolling, or a combination of these methods. The process was repeated a number of times to achieve very high reductions, and produce very fine tantalum fibers. A variation of the above processes is to draw the tantalum-copper composite until the fibers are a few microns in diameter, then flatten the fibers by rolling to produce a highly aspected, high surface area fiber that is a micron or less in thickness. The flattened fibers thus formed are thin ribbons that have many of the dimensional attributes of flakes and provide higher surface area per weight of metal than round fibers. A further advantage of making continuous fibers is that it avoids the inherent complexities of handling and pressing short fibers, The continuous lengths of fiber can be twisted or braided to form a fiber strip that holds the loose filaments together. Anodes can be stamped directly from the strip, thus eliminating the need to press powders. Since the fibers can be readily processed into strip, relatively thin sections can be made from which to stamp anodes. The present invention provides an improvement over prior art methods for making electrolytic capacitors. More particularly, the present invention provides an improved method for making capacitor anodes by producing filamentary valve metal fibers by a co-reduction of valve metal filaments within a copper matrix by a combination of drawing and rolling. The copper matrix is then removed leaving valve metal fibers in the form of continuous flat, ribbon-like fibers that have a relatively high aspect ratio of width to thickness, typically of at least about 10 to 1, and as a result relatively high surface area. By producing the fibers in a bundled continuous strip form, they can be made into thin anodes without pressing, thus maintaining the high surface area through subsequent anode sintering and formation.
Further features and advantages of the present invention will be seen from the following detailed description of the invention, taken into conjunction with the accompanying drawings, wherein: Fig. 1 is a plot of specific surface area to diameter or width of flattened and drawn ribbons or wires; Fig. 2 is a flow-chart describing the steps followed in a preferred embodiment of the process of the present invention; Fig. 3 is a plot comparing the affect of formation voltage on CV/g under different sintering treatments; Fig. 4 is a graph showing CV/g versus formation voltage under a single sintering treatment; and Fig. 5 is a plot showing DC leakage after different sintering treatments. Tantalum fibers were produced as fine filamentary ribbons, which were made by a process of extrusion, drawing, and rolling of a multifilament composite following the teachings of my prior U.S. Patent No. 5,034,857. The overall process is as follows: In a preferred embodiment of the present invention, the process begins with pure tantalum rod or high purity tantalum rod having a small amount of impurities, e.g. Fe, Ni, Cr, Cu, Nb, Mo, Si, Ti, W, C or O. A plurality of tantalum rods 12 are assembled substantially parallel to one another, in a copper can. A copper nose and tail are welded onto the can to form a primary billet, and the billet is then evacuated and sealed. The can is then hot extruded to bond the copper to the tantalum and cold drawn to make a copper clad tantalum wire bundle following the teachings of my prior U.S. Patent No. 5,034,857. Bonding of the copper cladding to the tantalum is essential to prevent oxidation or other contamination of the tantalum during subsequent processing. The resulting copper clad tantalum wire bundle was cut to length, and bundled and restacked into a second copper container, a nose and tail are welded in place, and the secondary billet is evacuated and sealed as before. The secondary sealed billet is optionally prepared for extrusion by hot or cold isostatic pressing in order to collapse any void space within the billet and to promote filament uniformity. After isostatic pressing, the secondary billet is machined to fit the extrusion liner, and the billet is then extruded and drawn followed by rolling to a final preferred thickness of less than 1 μm and preferably less than 0.5 μm. After rolling to final thickness, the tantalum fibers are immersed in an etching solution such as nitric acid and water to leach the copper.
CV/g and DC leakage are characteristics that depend on the quality and morphology of the fibers. Capacitance is largely a function of surface area, but also depends on the packaging of the filaments in the anode body. To achieve high CV/g it is necessary to create a high amount of useful surface area, eliminate the very thin fiber segments that would be consumed during anodizatiυn, and package the fibers to maintain an open pore structure that does not close-off surface area during formation. DC leakage is largely related to surface chemistry, but is also affected by the regularity and uniformity of the oxide structure. To achieve low leakage, it is necessary to have a uniform amorphous oxide without irregularities or discontinuities caused by inclusion protruding through the oxide film or crystallization promoted by impurities at the metal oxide interface. An additional factor that can have a detrimental effect on leakage is inadequately formed neck structures which bond the particles of fibers together in a single network structure. Poorly formed necks will result in local hot spots due to highly resistive junctions causing a breakdown in the oxide particularly at higher formation voltages. The present invention results in part from the realization that for a fixed volume of material, ribbon type fibers have more surface area than round fibers when the ribbon type fibers have a thickness equivalent to the diameter of the round fibers. Thus it is possible to produce higher surface area fibers by flattening round fibers. This greatly facilitates the production of high surface area fibers, since it is difficult to make very fine, submicron fibers by wire drawing without producing fibers that have highly irregular cross-sections. When used to make anodes for electrolytic capacitors, valve metal fibers having non-uniform cross sections lead to lower CV/g performance. We have discovered that flattening fibers by rolling produces a more uniform surface structure that results in more useable surface area and thus produces a capacitor that has higher volumetric efficiency. Examples: The starting material was a rod 12 of high purity tantalum. The rod was vacuum encapsulated in a copper can 14, extruded and cold drawn to make a copper clad Ta wire. The wire was cut to length, bundled and restacked into a second copper container, and further reduced by drawing followed by rolling to final thickness. After rolling, the resulting Ta fibers were removed from the matrix by leaching the copper with nitric acid. While the drawing and rolling parameters can be varied to produce a wide range of fiber sizes and shapes, the particular deformation sequence and reduction scheduled used in
this example resulted in fibers that were approximately 0.5 to 1 μm thick and 35-50 μm wide, and had a B.E.T. surface area greater than 0.300 m Ig. To make anodes, the fibers were twisted and cut into pieces weighing approximately 50 mg and a tantalum lead wire attached by spot welding. The dimensions of the anodes were approximately 0.3 x 4 x 8 mm. Since the fibers are continuous, the length of the fibers forming the anode is equivalent to one of the planar dimensions of the anode. The anodes were sintered under vacuum of greater than 10"3 Pa (7.5 x 10"6 torr) for 10 minutes or 50 minutes at temperatures of either 1300 0C or 1500 0C. The fibers received no other chemical or thermal treatment. The sintered anodes were anodized in a solution of 0.10 V/V% phosphoric acid at 80 0C and a current of 100 mA per gram. Samples were anodized to formation voltages of 100 V, 140 V and 180 V. Capacitance and leakage current were measured in a wet cell of 15 W/W% H2SO4. DC leakage current was measured at a potential of 70% of the formation voltage. Capacitance values for 100 V, 140 V and 180 V formations are given in Table 1 and Figure 3 which reports on the effect of formation voltage on CV/g for each sintering treatment. At 100 V formation, the CV/g is highest for the 1500 0C 10 minute sintering treatment and lowest for the 1500 0C 50 minute sinter. At 140 V formation, the CV/g values are similar for all sintering treatments. At 180 V formation, the highest CV/g value was obtained with the 1500 0C 50 minute sintering treatment, while the values for the 1300 0C and 1500 0C 10 minute sintering treatments were nearly identical. The CV/g values for the 1500 0C 50 minute sinter treatment exhibits a linear fall-off with formations voltages as shown in Figure 4 which reports CV/g versus formation voltage for a 15000C 50 minute sintering treatment showing the linear fall-off of capacitance between 100 V to 200 V formations and how the fall-off rate is less severe than for the 10 minute sintering treatment at both 1300 0C and 1500 0C. DC leakage values are given in Table 2. As can be seen leakage decreases with increasing sintering temperature and sintering time. The values for the two 1500 0C sintering treatments are shown in Figure 5 which reports DC leakage for 15000C treatments. As can be seen, above 140 V formation, leakage increases dramatically. The data also show that at 100 V formation, leakage below 0.5 nA/μF-V can be obtained without a deoxidation treatment.
Table 1. Specific Capacitance
Table 2. DC Leakage
It is thus seen that tantalum fibers produced by a composite co-reduction process in accordance with the present invention have properties suitable for use in forming capacitor anodes particularly those used for higher voltage ratings. Like powders, the fibers can be produced in different sizes depending on the intended application voltage. However, unlike powders, the fibers are organized in a continuous filament structure which improves handling and packaging of the fibers into anodes. Further improvements in CV/g can be realized by producing a more uniform fiber structure, while improvements in DC leakage may be achieved by performing a deoxidation treatment or through optimization of the sintering cycle. While the invention has been described in detail in connection with the formation of tantalum fibers for solid electrolytic capacitors, the invention also advantageously may be employed with other valve metals commonly used for forming solid electrolytic capacitors. in particular niobium, aluminum, vanadium and like metals and alloys thereof. Yet other changes may be made without departing from the spirit and scope of the invention.
Claims
Claims What is claimed is: 1. A method for making valve metal fibers useful for forming electrolytic capacitors comprising the steps of (a) establishing multiple valve metal rods in a primary billet of a ductile material; (b) working the primary billet to a series of reduction steps to form said valve metal rods into a plurality of elongated elements surrounded at least in part by the ductile material; (c) cutting the elongated elements from step (b) and bundling the cut elements to form a secondary billet; (d) working the secondary billet through a series of reduction steps followed by rolling to the elongated elements into flattened fibers having a width to thickness aspect ratio of at least about 10 to 1 ; (e) removing the ductile material, whereby to leave valve metal elongated flattened 1 -s; and
(0 sintering the elongated flattened fibers from step (e) under vacuum.
2. The method of claim 1, wherein the ductile material comprises a ductile metal.
3. The method of claim 2, wherein the ductile metal comprises copper.
4. The method of claim 1 , wherein the sintering is conducted at a temperature in the range of 13000C to 18000C for a time period of 10 to 60 minutes.
5. The method of claim 4, wherein the sintering is conducted at a temperature in the range of 13000C to 15000C for a period of 10 to 50 minutes.
6. The method of claim 5, wherein the sintering is conducted at a temperature of about 15000C for about 50 minutes.
7. The method of claim 1 , including the step of anodizing the sintered flattened fibers from step (g).
8. The method of claim 1 , including the step of twisting the flattened filaments before sintering.
9. The method of claim 1 , wherein the valve metal comprises tantalum.
10. The method of claim 1, wherein the valve metal comprises niobium.
11. The method of claim 1, wherein the valve metal comprises aluminum.
12. The method of claim 1 , wherein the valve metal comprises vanadium.
13. An electrolytic capacitor comprising an anode formed of valve metal ffiillaammeennttss o c f substantially uniform thickness within a range of 0.3-1.0 microns, and having a ssppeecciiffiicc ccaapacitance in excess of about 10,000 CV/g.
14. The capacitor of claim 13, wherein the valve metal comprises tantalum.
15. The capacitor of claim 13 , wherein the valve metal comprises niobium.
16. The capacitor of claim 13, wherein the valve metal comprises aluminum.
17. The capacitor of claim 13, wherein the valve metal comprises vanadium.
18. The capacitor of claim 13, wherein the filaments comprise ribbon-like fibers.
19. The capacitor of claim 18, wherein the ribbon-like fibers have a width to thickness aspect ratio of at least about 10 to 1.
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US20110137419A1 (en) * | 2009-12-04 | 2011-06-09 | James Wong | Biocompatible tantalum fiber scaffolding for bone and soft tissue prosthesis |
US9031671B2 (en) | 2012-09-21 | 2015-05-12 | Composite Materials Technology, Inc. | Medical implantable lead and manufacture thereof |
US9312075B1 (en) | 2013-09-06 | 2016-04-12 | Greatbatch Ltd. | High voltage tantalum anode and method of manufacture |
USRE48439E1 (en) | 2013-09-06 | 2021-02-16 | Greatbatch Ltd. | High voltage tantalum anode and method of manufacture |
US9633796B2 (en) | 2013-09-06 | 2017-04-25 | Greatbatch Ltd. | High voltage tantalum anode and method of manufacture |
US9498316B1 (en) | 2014-07-10 | 2016-11-22 | Composite Materials Technology, Inc. | Biocompatible extremely fine tantalum filament scaffolding for bone and soft tissue prosthesis |
US9155605B1 (en) | 2014-07-10 | 2015-10-13 | Composite Materials Technology, Inc. | Biocompatible extremely fine tantalum filament scaffolding for bone and soft tissue prosthesis |
US10062519B2 (en) * | 2014-09-15 | 2018-08-28 | Kemet Electronics Corporation | Tantalum capacitor with polymer cathode |
EP3895832B1 (en) | 2016-08-12 | 2022-12-28 | COMPOSITE MATERIALS TECHNOLOGY, Inc. | Electrolytic capacitor and method for improved electrolytic capacitor anodes |
WO2018045339A1 (en) | 2016-09-01 | 2018-03-08 | Composite Materials Technology, Inc. | Nano-scale/nanostructured si coating on valve metal substrate for lib anodes |
CN107719188B (en) * | 2017-11-06 | 2023-07-07 | 成都金和工贸有限公司 | Copper-aluminum composite contact wire and manufacturing method thereof |
WO2023063185A1 (en) * | 2021-10-11 | 2023-04-20 | 国立大学法人大阪大学 | Composite metal material and production method therefor |
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US6819544B1 (en) * | 2003-05-30 | 2004-11-16 | Medtronic, Inc. | Dual-anode electrolytic capacitor for use in an implantable medical device |
US20040244185A1 (en) * | 2000-03-21 | 2004-12-09 | Composite Materials Technology, Inc. | Production of electrolytic capacitors and superconductors |
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