Processes and systems for formation of high voltage, anodic oxide on a valve metal anode

Abstract

Processes and systems for formation of high voltage, anodic oxide on a valve metal anode. The processes generally includes immersing a valve metal anode in an electrolyte forming bath comprised of a formation electrolyte, performing an anodization step; and maintaining or regulating the temperature of the formation electrolyte accurately at a temperature at or below 40° C. during the anodization step. The anodization firstly under constant current until a target potential is reached and secondly under constant potential at the target potential until the current falls below a predetermined termination current level. The systems generally include a tank configured to receive one or more anodes in an electrolyte forming bath comprised of a formation electrolyte; and a subsystem for cooling and maintaining the formation electrolyte at the desire processing temperature. The systems may further include electronic controls for monitoring and adjusting system or process parameters.

Description

FIELD OF THE INVENTION [0001] This invention relates to processes and systems for forming high voltage, anodized valve metal anodes for use in wet electrolytic capacitors. This type of anode is suitable for use in high voltage capacitors particularly for use in implantable medical devices (IMDs). BACKGROUND OF THE INVENTION [0002] The term “valve metal” stands for a group of metals including aluminum, tantalum, niobium, titanium, zirconium, etc., all of which form adherent, electrically insulating, metal-oxide films upon anodic polarization in electrically conductive solution, e.g., formation electrolytes. [0003] Wet electrolytic capacitors generally consist of an anode, a cathode, a barrier or separator layer for separating the anode and cathode and an electrolyte. In tubular electrolytic capacitors, anodes are typically composed of wound anodized aluminum foil in which subsequent windings are separated by at least one separator layer. The anodes in flat electrolytic capacitors may...

AI Technical Summary

Benefits of technology

[0029] In an embodiment of a process for formation of oxide layers according to the invention, a process for forming a high-voltage, anodic oxide on a valve metal anode is provided, comprising: immersing a valve metal anode in electrolyte forming bath, comprising a formation electrolyte; performing an anodization step under a constant current until a target potential is reached and then at the target potential until the current falls below a predetermined termination current level; circulating a flow of formation electrolyte from the forming bath through a heat exchanger to provide a cooled flow of formation electrolyte, and accurately maintaining a relatively cool temperature of the formation electrolyte in the forming bath (e.g., at a temperature at or below 40° C.) during anodization accompanied by a flow of relatively cool formation electrolyte in and about the forming bath.

[0030] In another embodiment of a process according to the invention a process for forming a high-voltage, anodic oxide on a valve metal is provided, comprising: providing an electrolyte forming tank configured to receive one or more anodes, the tank containing an electrolyte forming bath, comprising a formation electrolyte; providing an electrolyte circulation subsystem for circulating and cooling the formation electrolyte; immersing one or more anodes in the formation electrolyte; circulating formation electrolyte from the forming bath through the circulation subsystem to provide a cooled flow of formation electrolyte; applying an electrical potential to the one or more anodes, the electrical potential being ramped up to a target voltage under constant current until a target potential is reached; continuing application of the electrical potential to the one or more anodes at the target potential until the current falls below a predetermined termination current level; and regulating electrolyte flow rate and temperature so that the temperature of the formation electrolyte in the forming tank is accurately maintained at a temperature at or below 40° C. during application of the electrical potential.

[0031] In an embodiment of a system according to the invention an electrolytic bath system is provided, comprising: a tank configured to receive one or more anodes, the tank containing an electrolyte forming bath comprising a formation electrolyte. The tank may be configured with an fluid inlet and a fluid outlet. An electrolyte circulation subsystem connected in flow-through communication with the tank is provided. The subsystem is configured to receive a flow of electrolyte from the outlet, to lower the temperature of the flow of electrolyte, and to return the flow of electrolyte to the fluid inlet.

[0037] The invention is specifically useful for forming high voltage, high capacitance anodes as it allows for managing the thermal energy dissipation during the formation process and provides for a high yield of fully formed anodes with improved energy density and low leakage currents at the operating voltage.

Problems solved by technology

Earlier automatic implantable defibrillators (AIDs) did not have cardioversion or pacing capabilities.

Given the rising popularity, efficacy, declining prices and recent over-the-counter status of certain AEDs, such devices are almost certainly becoming small and more portable.

Both ICDs and AEDs have historically utilized relatively bulky and expensive battery and high voltage capacitor units to provide the energy required for the therapies they provide.

At the same time, reliability of the high-voltage capacitors cannot be compromised.

Furthermore, it is critical that electrolyte may flow fairly readily through the structure because a significant amount of electrical power may be dissipated as heat during the formation process.

The above-referenced '993 patent reports that there are problems with conventional valve metal anodization processing due to heating of the electrolyte inside the interstitial pores of the porous tantalum pellet during the anodization process.

In the hot areas of the karst-like structure, which may be likened to an assembly of steam vessels, the electrolyte may decompose and / or the sinter-structure may crack because of the increased internal pressure.

As a consequence, instabilities may be introduced into the system, which adversely affect the performance of the capacitor.

Such instabilities are, of course, unacceptable.

This method clearly can become very time consuming, as can be readily estimated using Faraday's laws.

In addition, the application of the method suggested in '993 results in prolonged periods of anodization time during which low currents are used together with high potentials, specifically in the potential regime just below the target formation potential.

In the long term, the growth of crystalline oxide seeds beneath the previously grown amorphous layer may lead to the destruction of the anode.

In the short term, crystalline growth may result in unfavorably high leakage currents.

In summary, the prior art anodization processes for comparatively large, high voltage wet electrolytic valve metal anodes tend to take a considerable amount of valuable production time and they tend to produce low yields, either because deposits of electrolyte decomposition products may render the anode unusable or because field crystallization has caused unacceptably high leakage currents.

This overheating adversely affects oxide formation and may cause electrolyte residue (polymer-like deposits) to accumulate within the pores or interstices.

The presence of such residue can negatively impact crystalline structure of oxides during formation or the migration of ions in the forming electrolyte during formation or in the working electrolyte during operation.

The residue can also result in lower efficiency of a capacitor measured as a ratio of energy out to energy in (EOUT / / EIN) of the capacitor.

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