Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
1999-09-29
2001-05-15
Gorgos, Kathryn (Department: 1741)
Stock material or miscellaneous articles
Composite
Of metal
C205S189000, C205S105000, C205S106000, C205S108000, C205S171000, C205S175000, C205S325000, C205S332000, C205S917000, C205S229000
Reexamination Certificate
active
06231993
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an anode for a capacitor, and more particularly, to an electrolytic capacitor including an anode of a valve metal having an improved breakdown voltage.
2. Prior Art
Conventional construction of an electrolytic capacitor consists of anodizing a sintered porous valve metal structure, such as a tantalum pellet, in an electrolyte. In the case of tantalum, this forms a dielectric film of tantalum pentoxide on the exposed surfaces of the pellet. The electrolyte is typically composed of phosphoric acid, ethylene glycol or polyethylene glycol and water.
For a thorough discussion of the anodization process for a tantalum structure, reference is made to B. Melody et al., “An Improved Series of Electrolytes For Use In The Anodization Of Tantalum Capacitor Anodes”, CARTS, pp. 40-50, 1992. This article is incorporated herein by reference.
For a thorough discussion of a conventional anodized tantalum pellet, reference is made to D. M. Smyth et al., “Heat-Treatment of Anodic Oxide Films on Tantalum”, Journal of the Electrochemical Society, vol. 110, No. 12, pp 1264-1271, December 1963; D. M. Smyth et al., “Heat-Treatment of Anodic Oxide Films on Tantalum”, Journal of the Electrochemical Society, vol. 110, No. 12, pp. 1271-1276, December 1963; D. M. Smyth et al., “The Heat-Treatment of Anodic Oxide Films on Tantalum”, Journal of the Electrochemical Society, vol. 111, No. 12, pp. 1331-1336, December 1964; D. M. Smyth et al., “The Heat-Treatment of Anodic Oxide Films on Tantalum”, Journal of the Electrochemical Society, vol. 113, No. 2, pp. 100-104, February 1966; and D. M. Smyth, “The Heat-Treatment of Anodic Oxide Films on Tantalum”, Journal of the Electrochemical Society, vol. 113, No. 12, pp. 1271-1274, December 1966. These publications are incorporated herein by reference.
A pressed tantalum powder pellet is a porous structure. During the prior art anodization process, the tantalum pellet is continuously oxidized to a desired formation voltage by applying a current to the pellet. Because the tantalum pellet is porous, the electrolyte is able to flow into the pellet where it becomes heated during the anodization process. Since the anodization process is continuous, the heated electrolyte is unable to readily flow out of the pellet. Instead, the temperature of the electrolyte continues to increase through out the course of the process. It is believed that heated electrolyte is responsible for cracks, fissures and similar imperfections formed in the oxide coating and inside the tantalum pellet. These faults degrade the voltage at which the anode can be charged to.
What is needed is an improved method for manufacturing a valve metal anode such as of the kind typically used in an electrolytic capacitor. Particularly, it is desirable to provide tantalum anodes with oxide coatings devoid of cracks, fissures and similar imperfections and, consequently, having breakdown voltages that are superior to those known in the prior art.
SUMMARY OF THE INVENTION
Valve metals include vanadium, niobium and tantalum, and when used as an anode in an electrolytic capacitor are typically in the form of a pressed powder pellet. For tantalum, the powder material can be provided by either the beam melt process or the sodium reduction process, as is well known to those skilled in the art.
Regardless of the process by which the valve metal powder was processed, pressed valve metal powder structures, and particularly tantalum pellets, are typically anodized in formation electrolytes consisting of ethylene glycol or polyethylene glycol, de-ionized water and H
3
PO
4
. These formation electrolytes have conductivities of about 2,500 &mgr;S to about 2,600 &mgr;S at 40° C. The other main type of formation electrolyte is an aqueous solution of H
3
PO
4
. This type of electrolyte has conductivities up to about 20,000 &mgr;S at 40° C.
Conventional practice has been to form the valve metal to a target formation voltage at a constant current. The current used depends on the electrolyte, the valve metal powder type and the size of the valve metal structure. Adjusting these parameters according to conventional practice is well within the knowledge of those skilled in the art. A target formation voltage is achieved when a coating of tantalum pentoxide is provided of a thickness sufficient to hold a charge of a desired voltage.
There are, however, problems with conventional valve metal anodization processing. Importantly, electrolyte inside the porous tantalum pellet becomes heated during the anodization process. Since the anodization process is continuous, the heated electrolyte is unable to readily flow out of the pellet. Also, tantalum pentoxide is a relatively poor thermal conductor. This causes “hot spots” to form inside the pellet in the vicinity of the heated electrolyte. These “hot spots” are sites of potential cracks, fissures and where voltage is likely to breakdown. Secondly, as the electrolyte becomes heated, its constituent balance changes. This effects the conductivity of the electrolyte, especially inside the anode pellet. An electrolyte of a greater or lesser conductivity than desired is detrimental to the anodization process. An electrolyte of a conductivity lesser or greater than that intended can result in the oxide coating forming faster or slower than anticipated, which can effect the breakdown voltage of the final product.
Accordingly, the present invention is an improved process for providing an anodized dielectric coating on a valve metal structure by periodically relieving heated electrolyte from inside the structure. This is done to prevent the formation of internal “hot spots” by replacing heated electrolyte inside the anodized structure with fresh material from the anodization electrolyte bath. That way, the present invention ensures that electrolyte conductivity both inside and outside the structure remains relatively constant throughout the anodization process.
REFERENCES:
patent: 4436599 (1984-03-01), Bissot et al.
patent: 4504369 (1985-03-01), Keller
patent: 4571287 (1986-02-01), Okubo et al.
patent: 4921584 (1990-05-01), Koski et al.
patent: 5131987 (1992-07-01), Nitowski et al.
patent: 5716511 (1998-02-01), Melody et al.
B. Melody et al., “An Improved Series of Electrolytes For Use In The Anodization Of Tantalum Capacitor Anodes”, CARTS, pp. 40-50, 1992. No month available.
D.M. Smyth et al., “Heat-Treatment of Anodic Oxide Films on Tantalum”, Journal of the Electrochemical Society, vol. 110, No. 12, pp. 1264-1271, Dec. 1963.
D.M. Smyth et al., “Heat-Treatment of Anodic Oxide Films on Tantalum”, Journal of the Electrochemical Society, vol. 110, No. 12, pp. 1271-1276, Dec. 1963.
D.M. Smyth et al., “Heat-Treatment of Anodic Oxide Films on Tantalum”, Journal of the Electrochemical Society, vol. 111, No. 12, pp. 1331-1336, Dec. 1964.
D.M. Smyth et al., “Heat-Treatment of Anodic Oxide Films on Tantalum”, Journal of the Electrochemical Society, vol. 113, No. 2, pp. 100-104, Feb. 1966.
D.M. Smyth et al., “Heat-Treatment of Anodic Oxide Films on Tantalum”, Journal of the Electrochemical Society, vol. 113, No. 12, pp. 1271-1274, Dec. 1966.
Cymerman Martin D.
Muffoletto Barry C.
Stephenson Donald H.
Gorgos Kathryn
Hodgson & Russ LLP
Keehan Christopher
Wilson Greatbatch Ltd.
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