Methods of anodizing valve metal anodes

Details Methods of anodizing valve metal anodes Methods of anodizing valve metal anodes

2002-01-28

2004-10-12

King, Roy (Department: 1742)

Electrolysis: processes, compositions used therein, and methods

Electrolytic coating

Forming nonmetal coating using specified waveform other than...

C205S148000, C205S322000

Reexamination Certificate

active

06802951

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to methods and process for anodizing sintered valve metal anodes for use in wet electrolytic capacitors. The methods are useful specifically for forming high voltage anodes of comparatively large volume. This type of anode may be chosen in high voltage capacitors incorporated into implantable medical devices (IMDs).
BACKGROUND OF THE INVENTION
The term “valve metal” stands for a group of metals including aluminium, tantalum, niobium, titanium, zirconium, etc., all of which form adherent, electrically insulating, metal-oxide films upon anodic polarization in electrically conductive solutions.
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 consist of stacked sheets of anodized aluminium or of tantalum sintered structures separated from the cathode by at least one separator layer as described further below. Such electrolytic capacitors find wide application in industry including in IMDs.
As described in commonly assigned U.S. Pat. No. 6,006,133, a wide variety of IMDs are known in the art. Of particular interest are implantable cardioverter-defibrillators (ICDs) that deliver relatively high-energy cardioversion and/or defibrillation shocks to a patient's heart when a malignant tachyarrhythmia, e.g., atrial or ventricular fibrillation, is detected. Electrical transformer circuitry charges one or more high voltage electrolytic capacitors to a high voltage a low voltage battery using a low voltage battery as a charge source, and the capacitor(s) are discharged into the patient's heart as a cardioversion/defibrillation shock. Current ICDs also typically possess single or dual chamber pacing capabilities for treating specified chronic or episodic atrial and/or ventricular bradycardia and tachycardia and were referred to previously as pacemaker/cardioverter/defibrillators (PCDs). Earlier automatic implantable defibrillators (AIDs) did not have cardioversion or pacing capabilities. For purposes of the present invention, ICDs are understood to encompass all such IMDs having at least high voltage cardioversion and/or defibrillation capabilities.
Energy, volume, thickness and mass are critical features in the design of ICD implantable pulse generators (IPGs) that are coupled to the ICD leads. The battery(s) and high voltage capacitor(s) used to provide and accumulate the energy required for the cardioversion/defibrillation shocks have historically been relatively bulky and expensive. Presently, ICD IPGs typically have a volume of about 40 to about 60 cc, a thickness of about 13 mm to about 16 mm and a mass of approximately 100 grams.
It is beneficial to patient comfort and minimizes complications due to erosion of tissue around the ICD IPG to reduce the volume, thickness and mass of such capacitors and ICD IPGs without reducing deliverable energy. Reductions in size of the capacitors may also allow for the balanced addition of volume to the battery, thereby increasing longevity of the ICD IPG, or balanced addition of new components, thereby adding functionality and additional features to the ICD IPG. It is also desirable to provide such ICD IPGs at low cost while retaining the highest level of performance. At the same time, reliability of the high-voltage capacitors cannot be compromised. Aluminium and Tantalum based electrolytic capacitors have usually been employed as high-voltage ICD capacitors. An aluminium electrolytic capacitor that is incorporated into an ICD IPG is disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 09/607,830 filed Jun. 30, 2000, for IMPLANTABLE MEDICAL DEVICE HAVING FLAT ELECTROLYTIC CAPACITOR FORMED WITH PARTIALLY THROUGH-ETCHED AND THROUGH-HOLE PUNCTURED ANODE SHEETS filed in the names of Yan et al.
The performance of electrolytic capacitors is dependent upon several factors, e.g., the effective surface area of the anodes and cathodes that can be contacted by electrolyte, the dielectric constant of the oxide formed on the metal surface, the thickness of the oxide layer on top of the metal surface, the conductivity of the electrolyte etc. In all electrolytic capacitors, the thickness of the anodic oxide layer is approximately proportional to the potential applied to the anode during the formation of the anode, i.e., at the time when the anode is immersed into the formation electrolyte. For aluminum, the oxide grows approximately by 1.2 nm per Volt, for Tantalum this “rate” is somewhat higher, approximately 1.7 nm per Volt
Niobium and tantalum anodes are typically made in the form of a ho pressed powder pellet or “slug” when used in an electrolytic capacitor. The density of the anode slugs is typically significantly less than the density of the metals themselves, i.e., up to ⅔ of the volume of a given slug may be open or pore space. The final density of the anode slug is largely determined at the time of pressing, when a known amount of powder is pressed into a known volume. For the proper formation of the anode slug it is critical to achieve a fairly homogeneous distribution of pores throughout the anode slug since the forming electrolyte needs to wet even the most “remote” cavities in the karst-like internal structure of the anode. This is specifically important for comparatively large anodes with volumes of the order 1 cm
3
or above. 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. During formation, local potential differences of several hundred volts together with local current densities of several tens of milliamperes may be encountered, i.e., electrical energy as high a 20 to 30 Watts may be dissipated as heat. Various methods are used to achieve a homogeneous distribution of pores throughout the anode, as is well known to those skilled in the art. Traditional methods of forming the oxide layers are described in the prior art, e.g., in U.S. Pat. Nos. 6,231,993, 5,837,121, 6,267,861 and in the patents and articles referenced therein. Typically, a power source capable of delivering a constant current and/or a constant potential is connected to the anode slug that is immersed in the electrolyte. The potential is then ramped up to a desired final potential while a constant current flows through the anode-electrolyte system.
Regardless of the process by which the valve metal powder was processed, pressed and sintered valve metal powder structures, and specifically Tantalum and Niobium pellets, are typically anodized by the controlled application of formation potential and current while the anode is immersed in formation electrolytes. A typical formation electrolyte consists of ethylene glycol or polyethylene glycol, de-ionized water and H
3
PO
4
and has a conductivity anywhere between 50 &mgr;S/cm (read: micro-Siemens per cm) to about 20,000 &mgr;S/cm at 40° C.
Conventional practice has been to form the anodically polarized valve metal to a target formation potential with a constant current flowing through the anode-electrolyte system. Typically, stainless steel cathodes are used with the glycol-containing electrolytes. The magnitude of the current depends on the electrolyte, the valve metal powder type and the size of the valve metal structure. Most of the electronic current flowing through the anode-electrolyte system is used in the process of the anodic oxidation for the electrolysis of water as outlined below:
Anodic process:
10OH
−
+2Ta→Ta
2
O
5
+5H
2
O+10e−
Cathodic process:
10H
+
+10e−→5H
2
Therefore, the current setting directly influences the speed of the anodization reaction: using Faraday's laws, it can be readily shown that very low form

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