Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating predominantly single metal or alloy substrate of...
Reexamination Certificate
2001-10-16
2003-04-01
Nguyen, Nam (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Coating predominantly single metal or alloy substrate of...
C205S175000, C205S224000, C205S229000, C205S332000
Reexamination Certificate
active
06540900
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of anodizing aluminum capacitor foil.
BACKGROUND OF THE INVENTION
So-called “solid” tantalum capacitors which employ pyrolytically deposited manganese dioxide as the cathode material readily lend themselves to surface mount applications and have become the device of choice for circuit designers desiring the maximum capacitance in a given case size, high reliability, and high resistance to the temperatures associated with the reflow soldering techniques used to attach surface mount components to circuit boards. Aluminum electrolytic capacitors containing a liquid electrolyte may be fabricated with a base which facilitates surface mounting under carefully controlled solder reflow conditions, but these devices lack the resistance to high solder reflow temperatures and the parametric stability over a broad temperature range characteristic of their tantalum surface mount counterparts.
In recent years, tantalum and aluminum capacitors containing intrinsically conductive polymer cathode materials, such as polypyrole, polyaniline, polythiophene, and derivatives thereof, have been introduced commercially. The high electrical conductivity and thermal stability of certain of these intrinsically conductive polymers makes them almost ideally suitable for use as cathode materials for surface mount electrolytic capacitors. The introduction of these materials has led to a reduction in device ESR and reduced ignition damage from shorted capacitors as well as to the introduction of truly surface mountable aluminum electrolytic capacitors.
Unfortunately, aluminum electrolytic capacitors which contain intrinsically conductive polymer cathode materials, and which are not hermetically sealed against contact with the atmosphere, tend to undergo degradation of their leakage current performance with time, particularly when exposed to a humid environment. The tendency for the leakage current of conductive polymer containing, surface mount aluminum capacitors to increase with time has been traced in large part to the tendency of the anodic aluminum oxide to react with the humidity in the atmosphere to form hydrated aluminum oxide which is known to have very poor properties as an insulating dielectric film. In “wet” aluminum capacitors, which contain organic liquid electrolyte solutions, hydration degradation of the aluminum oxide film is minimized by limiting the water content of the electrolyte solution and by including a small amount of orthophosphate ion in the electrolyte composition. Orthophosphate, adsorbed on the anodic oxide surfaces, acts as a barrier layer, which inhibits the hydration reaction.
In surface mount devices containing conductive polymer cathode materials, the moisture of the atmosphere has ready access to the anodic oxide film due to the absence of a hermetic seal and the relatively porous nature of the usual conductive polymer cathode materials. Destructive hydration of the anodic oxide in polymer cathode aluminum capacitors is inhibited by the presence of an adsorbed layer of phosphate applied after the anodizing step(s) and prior to the application polymer cathode material. Unfortunately, the relatively thin coating of phosphate adsorbed on the outer surface of the anodic oxide is easily damaged during assembly of the capacitors. Any break or crack in the outer phosphate layer allows atmospheric moisture access to the reactive anodic oxide film and generally results in hydration damage and increased device leakage current.
Provisional Patent Application Serial No. 60/296,725 and co-pending U.S. application Ser. No. 09/891,208, now Pat. No. 6,450,565, describe the large hydration stability advantage obtained through the use of aluminum capacitor anode foil which has been anodized in a phosphate solution rather than in the dicarboxylic acid salt solutions normally used to anodize low voltage, high surface area capacitor foil. As described in this application, aluminum foil, which has been anodized in a phosphate solution has been found to be extremely resistant to hydration degradation even if the anodic oxide coating is cracked or damaged during the assembly of the finished capacitors.
Unfortunately, aqueous solutions containing orthophosphate as the sole anionic species tend to have a solvent action upon the aluminum foil during anodizing; this aluminum dissolution occurs even at near-neutral pH. The dissolved aluminum tends to form an aluminum phosphate precipitate which rapidly clouds the anodizing solution and which precipitates upon the anodizing tank surfaces as well as upon the foil being anodized. With the aluminum foil used for “phase shifting” or “motor start” capacitors, which has relatively shallow, wide diameter etch tunnels, the above described precipitation of aluminum phosphate upon the surface of the foil being anodized in a phosphate solution is of little consequence and aqueous, phosphate-based anodizing electrolyte solutions were used to anodize this type of foil for many years. With modern, highly etched aluminum anode foils having deep, closely packed, narrow diameter etch tunnels, aqueous phosphate anodizing tends to be even more corrosive towards the foil; the fine etch tunnel structure of modern aluminum capacitor foils may be partially or completely destroyed or blocked by precipitates during aqueous phosphate anodizing. Thus, modern, high surface area anode foil is not anodized in phosphate-containing electrolyte solutions.
In co-pending application Ser. No. 09/709,742, now Pat. No. 6,409,905, the use of glycerine-containing phosphate solutions for anodizing aluminum capacitor foils are described. Aluminum dissolution during anodizing may be reduced or eliminated through the use of glycerine-containing phosphate solutions rather than the use of aqueous phosphate solutions containing no glycerine. The nearly complete absence of aluminum dissolution in phosphate solutions containing an appropriate amount of glycerine facilitates the use of these solutions for the anodizing of modern, highly-etched foils. The hydration resistance advantages obtained with surface mount aluminum electrolytic capacitors, containing conductive polymer cathode materials and phosphate-anodized anode foil, have already been described.
BRIEF SUMMARY OF THE INVENTION
The invention is directed to a process for anodizing aluminum foil comprising anodizing the foil in a first aqueous electrolyte solution, heating the foil in an oven, and anodizing the foil in a second anodizing solution, wherein the first aqueous electrolyte solution and second aqueous electrolyte solution each comprise about 5 wt % to about 50 wt % glycerine, about 0.01 wt % to about 0.2 wt % ammonium phosphate, and water, and wherein the foil is anodized in the first aqueous electrolyte solution for at least about 3.5 minutes. Preferably, the water is de-ionized.
In a preferred embodiment, the first and second aqueous electrolyte solutions each comprise about 5 wt % to about 25 wt % glycerine. In another preferred embodiment, about 0.1% ammonium phosphate is present in the first and/or second aqueous electrolytic solutions.
In a further preferred embodiment, the temperature of the first aqueous electrolyte solution and second aqueous electrolyte solution are each between about 80° C. and 90° C.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a process for the continuous anodizing of aluminum foil for use in aluminum electrolytic capacitors. Specifically, etched anode foil is anodized to relatively low voltage in a two-step reel-to-reel process. The present invention is particularly useful for anodizing highly-etched aluminum foil for use in surface mount aluminum capacitors containing conductive polymer cathode material. The process of the invention is economical and provides high foil quality.
The process of continuous anodizing of aluminum foil via passing the foil through tanks containing anodizing electrolyte solution in a “reel-to-reel” process has been known for many years and is described in Paul M Deeley,
Electrolytic Capa
Harrington Albert Kennedy
Kinard John Tony
Lessner Philip Michael
Melody Brian John
Reed Erik Karlsen
Kemet Electronics Corporation
Leader William T.
Nguyen Nam
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