Anodizing electrolytes for high voltage capacitor anodes

Abstract

The present invention is a low temperature (below about 60° C.) anodizing electrolyte composition for valve metals including, and not limited to, aluminum, niobium, titanium, tantalum, zirconium, and alloys thereof. The low temperature anodizing electrolyte composition contains at least: (1) a protic solvent selected from the group consisting of alkylene glycols, polyalkylene glycols, and their mono ethers, and (2) a weak inorganic or organic acid or its salt. The present invention is capable of anodizing valve metals for high voltage capacitors of greater than 300 Volts with: (1) little to no gray-out at high formation voltage, (2) high formation breakdown voltage, and (3) high quality of oxide with low DC leakage and stable long term performance of the anode.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The invention relates to a low temperature-anodizing electrolyte and to an electrochemical process for anodizing valve metals with that electrolyte. [0003] 2. Prior Art [0004] Electrolytic capacitors contain anodes that are valve metal coated with its corresponding oxide (the dielectric) by anodizing. The anodizing electrolyte composition and the process (the protocol) of forming the anodes are crucial for the electrical properties and performance of the anodes or the capacitors, especially for high voltage capacitors. [0005] Anodizing electrolyte compositions have long been used to anodize valve metals for use in electrolytic capacitors. Melody et al. disclose, in U.S. Pat. No. 5,716,511 at col. 1, lines 49-60, one such 1950 brand of anodizing electrolyte composition. In particular, Melody et al. wrote, “After post-sintering separation and attachment to carrier strips or bars, the anodes are suspended in an electro...

AI Technical Summary

Benefits of technology

The present invention is a low temperature electrolyte composition for anodizing valve metals. It contains a protic solvent and a weak acid or its salt. The electrolyte is capable of anodizing valve metals for high voltage capacitors with low DC leakage and stable long term performance of the anode. It has high formation breakdown voltage and quality of oxide. The technical effect of the invention is to provide a reliable and efficient method for anodizing valve metals for high voltage capacitors.

Problems solved by technology

[Some of these disadvantages include and are not limited to] ethylene glycol is toxic, .

Melody et al., however, report that only using organic solvents that have a low viscosity can accomplish anodizing at such low temperatures.

Melody et al. asserted that the viscosity values of glycols, polyglycols, and higher alkyl ethers of the polyethylene glycols, such as diethyl, dipropyl or dibutyl ethers were unacceptable for use with anodizing electrolyte solutions at temperatures below 50° C. In particular, Melody et al. stated that (1) glycols and polyglycols become too viscous at such low temperatures, and (2) polyethylene glycol higher dialkyl ethers, such as diethyl, dipropyl or dibutyl ethers do not provide the requisite solubility and low viscosity.

It is a result of the formation of crystalline tantalum oxide having high DC leakage, and is undesirable.

On the other hand, too low an electrolyte conductivity results in excessive heating of the electrolyte and the anode pellet that would cause “gray-out”, breakdown, or poor quality of the oxide.

The low conductivity electrolyte also causes high voltage drop and renders the anodizing impractical.

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