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Production of high-purity tantalum flake powder

Production of high-purity tantalum flake powder description/claims

The present invention relates generally to methods of manufacturing tantalum flake for high CV/g/high voltage capacitors, using a process of mechanically flattening a nodular powder. Such an alternative method results in the production of tantalum flake with the potential for higher capacitance.

Tantalum electrolytic capacitors date back to the late 1940's. Since that time, many improvements have been made to the manufacturing process, allowing for smaller, more reliable, and better performing components. Today, high CV/g/high voltage tantalum capacitors are dominated by flake technology. The particular flake morphology allows for high voltage applications, as the contacts between particles are line, rather than point with traditional powder metallurgy. As the dielectric is grown on the tantalum, it consumes a portion of the underlying metal. The thicker the dielectric, the more metal is consumed, resulting in thinner contacts between particles. The entire sinter neck eventually is consumed and the particles become electrically isolated. Flake morphology allows the dielectric to be formed to higher voltages before choking off the sinter necks between particles.

Currently, tantalum flakes are typically produced by mechanically flattening tantalum particles. The particles are either from tantalum EB ingots or from the reduction of K2TaF7 with Na metal in a molten salt reactor. The malleable metal particles are flattened in a high-energy ball mill before being hydrided and reduced in size by impact milling. In order to reduce contamination, the tantalum is usually ball milled in an organic solvent, and acid leached. A deoxidation step is needed to reduce the oxygen to suitable levels for capacitor use, and a heat treatment is then applied to produce the necessary physical properties such as flow, and Scott Density.

The mechanical flake process produces a distribution of particle thickness. This variation in flake thickness reduces the performance of the capacitor at a given formation voltage. Flakes thinner than a specific value will be completely choked off, while flakes thicker than that value will not, for a given forming voltage. Capacitor powder manufacturers are continually improving the manufacturing process, in an attempt to reduce distribution of flake thicknesses. The present invention relates to a process to produce tantalum flake suitable for use in high CV/g/high voltage capacitor applications that overcomes technical difficulties in existing processes.

The present invention relates to a high-purity tantalum flake powder, produced by a hydride-dehydride process comprising: (a) cold working tantalum metal into a thin sheet; (b) hydriding the thin sheet, forming a brittle tantalum foil; (c) adjusting the tantalum foil to a desired particle size; and (d) removing hydrogen from the tantalum foil by vacuum sintering, forming a tantalum flake (agglomerate) powder.

In accordance with the present invention, tantalum flake is produced by sizing ultra-thin tantalum foil via the hydride-dehydride (HDH) process. Tantalum is an extremely malleable metal and can be cold worked into extremely thin sheets less than 1 micron thick. Once hydrided, this foil is brittle, and can be easily sized by suitable milling processes. The hydrogen is removed by vacuum sintering, resulting in an extremely thin Ta metal flake. In embodiments, the hydriding occurs at a temperature of about 400-800° C.

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