Anodizing Electrolytes Using A Dual Acid System For High Voltage Electrolytic Capacitor Anodes

Abstract

An improved formation electrolyte and method for anodizing valve metal anodes used in electrolytic capacitors, particularly for high voltage sintered tantalum powder anode, is described. The anodizing electrolyte composition is comprised of 1) a phosphorus oxyacid and / or its salt, such as phosphoric acid and ammonium phosphate; 2) a weak inorganic acid / salt (such as boric acid, ammonium borate) or a weak carboxylic acid / salt; 3) water; and 4) a protic solvent or a mixture of two or more protic solvents. The weak mono-carboxylic acid / salt has 2 to 7 carbon atoms and the weak di- or poly-carboxylic acid / salt has 3 to 13 carbon atoms. The present electrolytes have high anodizing breakdown voltage capability and the formed dielectric oxides have improved oxide quality including good oxide hydration resistant ability, and result in more stable capacitor performance. These properties are particularly important for critical applications such as implantable cardioverter defibrillators (ICDs). Significantly, this means that fewer capacitors are needed to meet an ICD's operating voltage.

Description

CROSS-REFERENCE TO RELATED APPLICATION [0001] This application claims priority from U.S. Application Ser. No. 60 / 776,168, filed Feb. 23, 2006.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] This invention relates to anodizing electrolytes and methods for anodizing valve metal anodes, particularly for high voltage sintered tantalum powder anodes used in electrolytic capacitors. [0004] 2. Prior Art [0005] Electrolytic capacitors are well known for use in a variety of electronic equipment such as consumer audio and video equipment, home appliances, power supplies, industrial electronics, military electronics, computers, telecommunication equipment, entertainment equipment, automotive devices, lighting ballasts, and implantable medical devices. In general, electrolytic capacitors comprise an anode and a cathode segregated from each other by at least one layer of separator material impregnated with a working electrolyte. The anode is a valve metal body coated with a l...

AI Technical Summary

Benefits of technology

[0014] The electrolytes have relatively high anodizing breakdown voltages, which allows for formation of high voltage dielectric oxides, and results in improved dielectric oxide quality. This means that the resulting capacitor has higher operating voltage, lower DC leakage, good hydration resistance and more stable lifetime performance. These properties are particularly important for critical applications such as required by implantable cardioverter defibrillators (ICDs). When used in ICDs, capacitors made with anodes formed according to the present invention, which can be formed to higher formation voltages, allow for fewer capacitors per device, provide shorter first charging time, higher energy efficiency, and more stable lifetime performance.

Problems solved by technology

However, development of higher voltage capacitors is, in part, limited by the ability of the anodizing electrolyte to form valve metal anodes to a higher voltage.

Unfortunately, electrolytic capacitors degrade when not in operation.

Nonetheless, degradation is mainly due to hydration of the dielectric oxide by water present in the capacitor working electrolyte.

Oxide degradation increases the time of the first charging cycle after an extended non-operation period and reduces the capacitor charge / discharge energy efficiency.

This is undesirable for critical applications such as ICDs, in which dielectric oxide degradation increases capacitor (device) charging time and decreases the useful life of the battery or increases battery and device volume.

Although lowering the electrolyte conductivity by reducing the phosphoric / phosphate concentration or increasing the solvent content can enhance the anodizing breakdown voltage, the electrolyte eventually becomes impractical when its IR drop becomes too high due to increased electrolyte resistivity.

But, the anodic oxides formed in these electrolyte compositions do not have the oxide hydration resistance and long-term performance stability characteristic of phosphoric / phosphate-based electrolytes.