WO2009082631A1

WO2009082631A1 - Methods for fabrication of improved electrolytic capacitor anode

Info

Publication number: WO2009082631A1
Application number: PCT/US2008/086460
Authority: WO
Inventors: James Wong
Original assignee: Composite Materials Technology, Inc.
Priority date: 2007-12-26
Filing date: 2008-12-11
Publication date: 2009-07-02

Abstract

A process for making superconducting material useful for forming electrolytic devices comprising the steps of: establishing multiple tantalum or niobium components in a billet of a ductile material; working the billet to a series of reduction steps to form said tantalum or niobium components into elongated elements; cutting the elongated elements from step b) and leaching the ductile metal from the elements; washing or mixing the cut elements from step c); and forming the cut elements from step d) into a sheet.

Description

METHODS FOR FABRICATION QF IMPROVED ELECTROLYTIC

CAPACITOR ANODE

The present invention relates to the production of electrolytic materials. The invention has particular applicability to production of tantalum and niobium-based materials useful as electrolytic capacitor anodes and will be described in connection with such utility, although other utilities are contemplated.

Tantalum-based electrolytic capacitors have found increasing use in microelectronics. The combination of small package size, insensitivity to operating temperature, and excellent reliability have made them the choice over ceramic multilayer and aluminum foil-based capacitors for many applications. As the state of the art in microelectronics continues to progress, demand has grown for smaller, more cost- effective tantalum electrolytic capacitors. This demand entails improvements in the quality and performance of the tantalum compacts that are at the heart of such capacitors.

STANDARD INDUSTRY PRACTICE Heretofore, electrolytic tantalum capacitors have primarily been fabricated using fine particle tantalum powder. The powder is pressed into a green compact (20 to 50 percent dense) and is sintered under vacuum at a temperature of 1500°-2000° C. for 15- 30 minutes to form a porous, mechanically robust body in which the tantalum is electrically continuous. The sintering process is. in some cases, relied upon to attach a lead wire to the compact. In these cases, the lead is inserted into the green compact prior to sintering. If the lead is not attached in this manner, it usually will be welded into place immediately following sintering of the compact. An important ancillary benefit of the sintering operation is purification of the tantalum particle surfaces; impurities, such as oxygen, are driven off. After sintering, the compact is anodized to form the dielectric tantalum pentoxide

(Taa O₅) on the exposed surfaces. The porous regions of the anodized compact are then infiltrated with a conductive electrolyte. Typically, conductive polymers or manganese dioxide (MnO₂) or sulfuric acid are used as the electrolyte. In the case Of MnO₂, a manganese nitrate solution is infiltrated and then thermally decomposed into MnO₂. The final capacitor manufacturing stages are assembly of a current collector and packaging of the capacitor. POWDER MANUFACTURING

State of the art tantalum powder is produced by the sodium reduction process of K₂TaF?. Improvements in the process have resulted in commercially available powders capable of yielding a specific capacitance of over 50,000 CV/g. Better control of input tantalum particle size, reaction temperature, and other variables has led to the improvements in specific capacitance. Λ key advance was the introduction of doping agents that enabled the production of very high specific capacitance powders. The doping agents serve to prevent surface loss during sintering. Typical additives are nitrogen, oxygen, sulfur, and phosphorus compounds in the range from 50 to 500 ppm. While select dopants are beneficial, it is important to limit other contamination, which can weaken the dielectric film or even prevent the formation of a continuous Ta₂O₅ layer that could lead to premature breakdown of the dielectric film and loss of capacitance. Higher capacitance tantalum particles have been obtained by ball milling powders. Ball milling turns the roughly spherical powder particles into flakes. The benefit is that the flakes can have a higher specific capacitance at higher formation voltage than powder particles. This translates into greater volumetric efficiency for the flakes when they are formed into anodes, Aspecting tantalum particles by ball milling and other processes aimed at improving powder performance, while effective, have practical drawbacks, including increased manufacturing costs and a marked decrease in product yield. Currently, a premium of 2-3 times is charged for the very highest capacitance flakes over standard powder product.

The very fine tantalum powders commercially available today have several serious problems with respect to anode fabrication. One significant problem in particular is a sensitivity to surface area loss during sintering. Ideal sintering conditions arc high temperatures and short times. A higher temperature serves to purify the tantalum surface and provide a mechanically strong compact. Capacitors having lower equivalent series resistance (ESR) and equivalent series inductance (ESL) can be fabricated if higher sintering temperatures are employed. Unfortunately, the fine particles of high capacitance powders and flakes lose surface area at temperatures over 1500° C. A loss of surface area results in lower capacitance, reducing the benefit of using the higher specific capacitance powder. The capacitor manufacturer must balance sintering temperature, mechanical properties, and ESR and ESL levels in order to maximize capacitor performance.

Fine powders and flakes are also sensitive to forming voltage during anodization. The anodization process consumes some of the metallic tantalum to form the dielectric layer. As the forming voltage increases, more of the tantalum is consumed, resulting in a loss of capacitance. As the powder becomes finer, this problem becomes increasingly serious.

In practice today, high surface area powders are sintered at low temperatures (below 1500⁰C.) and are anodized at voltages below 50 volts. Most of these capacitors are restricted to operating voltages below 16 volts.

Another drawback to fine powders is "tortuousity". which can lead to poor electrolyte fill factor. As particle size is reduced, the pathways that the electrolyte must follow to infiltrate the compact grow increasingly narrow and more convoluted, or tortuous. Eventually, it becomes extremely difficult, if not impossible, to completely infiltrate the compact. Incomplete infiltration results in lower capacitance.

A final difficulty in using high specific capacitance tantalum powders is their poor flowability. In state of the art capacitor manufacturing processes, tantalum powder is metered into a die for pressing into the green compact. It is very important that the metering process is accurate and reliable in order that each capacitor manufactured contains the same quantity of powder. Flakes and highly aspectcd powders tend not to flow uniformly, which can lead to large variability in production runs.

FIBER PRODUCTION

In my prior U.S. Pat. No. 5,034,857, 1 disclose an approach to the production of very fine valve metal filaments, preferably tantalum, for capacitor use. The benefits of fine filaments relative to fine powders are higher purity, lower cost, uniformity of cross section, and ease of dielectric infiltration, while still maintaining high surface area for anodization. The uniformity of cross section results in capacitors with high specific capacitance, lower ESR and ESL, and less sensitivity to forming voltage and sintering temperature as compared to fine powder compacts. As disclosed in my aforesaid '857 U.S. Patent, valve metal filaments, preferably tantalum, are fabricated through the combination of the filaments with a ductile metal so as to form a billet. The second, ductile metal is different from the metal that forms the filaments. The filaments are substantially parallel, and are separated from each other and from the billet surface by the second, ductile metal. The billet is reduced by conventional means--e.g., extrusion and wire drawing~to the point where the filament diameter is in the range of 0.2 to 5.0 microns in diameter. At that point, the second, ductile metal is removed, preferably by leaching in mineral acids, leaving the valve metal filaments intact. The filaments are suitable for use in tantalum capacitor fabrication.

Other patents involving valve metal filaments and fibers, their fabrication, or articles made therefrom include U.S. Pat. Nos. 3,277,564, (Webber), 3.379,000 (Webber), 3,394,213, (Roberts), 3,567.407 (Yoblin), 3.698,863 (Roberts), 3,742,369 (Douglass), 4,502,884 (Fife), 5,217,526 (Fife), 5,306,462 (Fife), 5,284,531 (Fife), and 5,245,514 (Fife).

See also my earlier U.S. Patent 5,869,196 in which I describe a process for fabrication of fine-valve metal filaments for use as porous metal compacts used in the manufacture of electrolytic capacitors. According to my ' 196 U.S. Patent, a metal billet consisting of multiple filaments of a valve metal, preferably tantalum, is contained within and spaced apart by a ductile metal, preferably copper. The billet is reduced by conventional means, such as extrusion and wire drawing, the resulting composite product is cut into lengths, and the ductile metal separating the valve metal components is removed by leaching in acid. A similar compaction technique has been proposed to fabricate composites by providing continuous layers of tantalum and copper sheets layered together in a jellyroll. The jellyroll is then reduced to a small diameter size by extrusion and drawing. Starting with sheets of tantalum and copper offer advantages over working with filaments. However, at reduced sizes, the copper cannot readily be leached out due to the presence of the continuous tantalum layers.

The foregoing discussion of the prior art derives, in part, from my co-pending U.S. Application Serial No. 1 1 /859.687 in which I describe improvements over the prior art much as described in my ' 196 U.S. patent by creating one or more open slots in the starting billet stage and filling the slots with ductile metal prior to extrusion and drawing. After extrusion and drawing to small size, the slots remain. As a result, the ductile metal readily may be leached and removed from between the tantalum layers. The resulting product is a series of compacted tantalum layers each progressively of smaller width. In one embodiment of the invention, continuous layers of tantalum and copper are layered together in a jellyroll and formed into a billet which is circular in cross-section, and the slots are concentrically evenly spaced radially around the billet. The resulting product is a series of concentric split tubes each progressively of smaller diameter towards the center.

As described in my '687 application employing a foil or sheet of tantalum as opposed to filaments greatly simplifies assembly of the billet. Employing sheet tantalum also ensures greater uniformity since the thickness of the starting sheet can be controlled more readily that using a multiple of separate filaments. This in turn produces substantially more uniform capacitor material resulting in substantially higher values of CV/g.

The present invention provides improvements over the prior art such as discussed above by cutting the conventional valve metal filaments or wires in a ductile metal matrix into short segments, leaching the ductile metal leaving the short valve metal filaments intact, and forming or casting the valve metal filaments into a thin sheet from a slurry. In a preferred embodiment of my invention, the valve metal comprises tantalum while the ductile metal comprises copper, and the tantalum fibers are cast in the form of a thin sheet from an aqueous slurry using a continuous Doctor Blade casting method.

Further features and advantages of the present invention will be seen from the following detailed description taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a schematic block diagram of the process of the present invention; and Fig. 2 is a simplified side elevational view showing casting of a sheet in accordance with the present invention.

Referring to Figs. 1 and 2, the process starts with the fabrication of valve metal filaments, preferably tantalum, by combining filaments or wires of tantalum with a ductile material, such as copper to form a billet at step 10. The billet is then sealed in an extrusion can in step 12, and extruded and drawn in step 14 following the teachings of my ' 196 U.S. patent. The extruded and drawn filaments are then cut or chopped into short segments, typically l/16^th-l/4^lh inch long at a chopping station 16. Preferably the cut filaments all have approximately the same length. Actually, the more uniform the filament, the better. The chopped filaments arc then passed to an etching station 18 where the ductile metal is leached away using a suitable acid. For example, where copper is the ductile metal, the etchant may comprise nitric acid.

Etching in acid removes the copper from between the tantalum filaments. After etching, one is left with a plurality of short filaments of tantalum. The tantalum filaments arc then washed in water in a washing station 20, and the wash water is partially decanted to leave a slurry of tantalum filaments in water. The slurry of tantalum filaments in water is then cast as a thin sheet using, for example, a Doctor Blade at casting station 22. Excess water is removed, for example, by rolling at a rolling station 24. The resulting mat is then further compressed and dried at a drying station 26.

As an alternative to "Doctor Blade formation", the thin sheet may be formed by spray casting the slurry onto to a substrate, excess water removed and the resulting mat pressed and dried as before.

There results a thin sheet of substantially uniform thickness comparted tantalum fibers.

A feature and advantage of the present invention is the discovery that an aqueous slurry of chopped filaments will adhere together sufficiently so that the fibers may be cast as a sheet which can be pressed and dried into a stable mat. This is surprising since the metal filaments do not in itself absorb water. Notwithstanding, as long as the filaments are not substantially thicker than about 10 microns, they tend to adhere together. On the other hand, if the filaments are much larger than about 10 microns, they will not form a stable mat. Thus, it is preferred that the filaments have a thickness of less than about 10 microns, and preferably below 1 micron thick. To ensure even distribution of the chopped filaments, and thus ensure production of a uniform mat, the slurry preferably is subjected to vigorous mixing by mechanical stirrer or vibration.

The density of the resulting tantalum sheet may be varied simply by changing the final thickness of the sheet.

Also, if desired, multiple layers may be stacked to form thicker anodes that may be desired, for example, for high capacitance applications. ADVANTAGES OF THE INVENTION

The present invention provides several advantages over the prior art including:

(1 ) The invention allows for large-scale manufacturing using conventional sheet forming equipment.

(2) The resulting sheets are flexible and have sufficient strength to permit further handling.

(3) consistent and reliable CV/g values also can be achieved due to uniformity dispersion of the tantalum elements. (4) The process ensures cross-sectional uniformity and high surface-to- volume ratio. Both factors are important for reliable fabrication of high capacitance capacitors on an industrial scale.

(5) The process is flexible. By modifying the thickness of the mat, we can vary the amount of tantalum/unit volume of the resulting product.

(6) The process is economical as compared to existing processes employing powders.

(7) The process ensures uniform density. As a result, multiple layers may be stacked for high capacitance applications. (8) The process provides high yield, low cost per unit and consistent results.

While the invention has been described in connection with the use of tantalum and copper, other valve metals, such as niobium or an alloy of tantalum or niobium, and other ductile valve metals such as titanium or aluminum can be used. Still other changes may be made without departing from the spirit and scope of the invention.

Claims

The invention claimed is:

1. A process for making superconducting material useful for forming electrolytic devices comprising the steps of:

(a) establishing multiple tantalum or niobium components in a billet of a ductile material;

(b) working the billet to a series of reduction steps to form said tantalum or niobium components into elongated elements;

(c) cutting the elongated elements from step b) and leaching the ductile metal from the elements; (d) washing and mixing the cut elements from step c); and

(e) forming the cut elements from step d) into a sheet.

2. The process of claim 1, wherein said leaching is in an acid etchant.

3. The process of claim 2, wherein the components are tantalum, and the acid etchant is nitric acid.

4. The process of claim 1 , wherein the cut elements are washed with water.

5. The process of claim 1, wherein the sheet is formed by casting from a slurry containing the cut elements.

6. The process of claim 5, wherein the sheet is formed by Doctor Blading.

7. The process of claim 4, including the step of rolling the sheet after Doctor Blading to remove excess water.

8. The process of claim 7, including the step of pressing and drying the sheet after rolling.

9. Thin anodes made less than 500μ thick.