Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Utilizing nonaqueous bath
Reexamination Certificate
2000-08-02
2002-08-20
Nguyen, Nam (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Utilizing nonaqueous bath
C205S322000, C205S332000
Reexamination Certificate
active
06436268
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to non-aqueous electrolytic solutions suitable for anodizing valve metal derivative anodes, to methods of anodizing using non-aqueous electrolytic solutions, and to capacitors prepared with non-aqueous electrolytic solutions.
BACKGROUND OF THE INVENTION
For many years, so-called solid tantalum capacitors have set the market standard for high capacitance per unit volume combined with high reliability. Since their introduction in the 1950's, solid tantalum capacitors have continued to shrink in size due to the introduction of tantalum powders having higher surface area per unit weight (i.e., smaller particle size). High surface area tantalum powders facilitate the use of smaller anodes having the same capacitance when anodized to equivalent anodic oxide thickness compared with older tantalum powders.
The utility of tantalum capacitors has been extended by the widespread introduction of surface mount solid tantalum capacitors in the 1980's. The heat-resistance properties inherent to solid tantalum capacitors due to the manganese dioxide counter electrode material used in the fabrication renders tantalum capacitors relatively immune to the destructive effects of heating during reflow-soldering compared with aluminum electrolytic capacitors which contain a liquid, organic-solvent-based electrolyte. Consequently, a large fraction of the applications which formerly utilized miniature aluminum electrolytic capacitors have been converted to solid tantalum capacitors of the surface mount configuration.
Further extending the utility of solid tantalum capacitors has been the introduction of inherently conductive polymers as counter electrode materials in place of the manganese dioxide traditionally present in these devices. The high electrical conductivity of inherently conductive polymers gives rise to a significant reduction in equivalent series resistance and loss of capacitance at higher frequencies in solid tantalum capacitors containing them. Solid tantalum capacitors with conductive polymer cathodes have the additional advantage of being resistant to ignition in the event of a short circuit occurring within the capacitor.
The recent introduction of surface mount, solid tantalum capacitors having multiple anode elements in parallel to reduce the equivalent series resistance of the devices to well under 10 milliohms further extends the use of solid tantalum capacitors to applications where previously only ceramic or metallized film capacitors could be used. The tantalum capacitors are generally much smaller than the ceramic or metallized film capacitors which they replace.
The improvements in tantalum capacitors, described above, combined with the explosive growth in the computer and mobile telephone industries have resulted in the growth in worldwide demand for tantalum capacitors from a few million pieces per year in the 1950's to well over a billion pieces per month today. In spite of the improvements in surface area per unit weight made by the suppliers of tantalum capacitor powder over the years, the demand for tantalum for capacitor purposes has grown steadily since the 1950's. Tantalum is a relatively rare element in nature, and this fact coupled with increasing demand, has resulted in a forty-fold or more increase in the price of tantalum powder over the past forty to fifty years.
It is widely recognized that the growth of the electronics industry is driven by greater device performance at lower cost, as time advances. Thus, while the capacitance per unit volume continues to increase, the price per unit of capacitance for solid tantalum capacitors continues to decrease with time, as it must in order for these devices to conform to the so-called learning curve of device manufacturing cost versus the logarithm of the cumulative number of devices sold worldwide. This learning curve of the cost requirements for components must be satisfied in order to maintain the growth rate of the electronics industry.
It is widely recognized that, in spite of the device manufacturing cost learning curve, surface area per unit weight of tantalum cannot be increased indefinitely. It is also recognized that the increasing demand for tantalum is forcing tantalum producers to process lower quality ores in order to meet demand for the metal. The extraction cost per pound of tantalum increases significantly with decreasing ore quality.
In an effort to reduce the cost of the valve metal component of solid capacitors, other valve metals in addition to tantalum have been tested for use in solid capacitor manufacture. The metal, niobium, is most closely related to tantalum in chemical and physical properties. For many years, attempts have been made to fabricate successful solid capacitors from niobium powder. Early niobium powders contained a relatively large amount of impurities and gave rise to highly flawed oxide during anodization at the temperatures normally used to anodize tantalum anodes (i.e., 80° C. to 90° C.) in dilute phosphoric acid. It was found that the production of blister-like flaws in the anodic oxide on niobium could be minimized through the use of anodizing temperatures below about 25° C. Unfortunately, solid niobium capacitors were found to give increasing leakage current and shorted devices upon testing under voltage at elevated temperatures (e.g., 85° C.).
Niobium powders prepared recently appear much improved with respect to impurity content and may be anodized at traditional anodizing temperatures (i.e., 80° C. to 90° C.) with the production of relatively flaw-free oxide at low anodizing voltages (60 volts or less). Particularly good dielectric properties, as indicated by wet-cell testing of anodized niobium anodes, are obtained through the use of the electrolytes and methods described in co-pending applications, Ser. No. 09/143,373 and Ser. No. 09/489,471.
Unfortunately, even solid capacitors manufactured from relatively pure niobium powder are subject to increasing leakage current and short circuit failures on life test at elevated temperature. The failures have been traced to the migration of oxygen from the anodic oxide into the niobium substrate. This failure mechanism is also known in tantalum capacitors, but the effect is much more pronounced with niobium.
Fortunately, a solution to the problem of oxygen migration from the anodic film to the valve metal substrate has been found and has been demonstrated for tantalum and niobium. On Mar. 9, 2000, at the 20th Capacitor And Resistor Technology Symposium, Dr. Terrance Tripp presented a paper, entitled: “Tantalum Nitride: A New Substrate for Solid Capacitors” (Authors: T. Tripp, R. Creasi, B. Cox; reprinted in the symposium proceedings, on pages 256-262). This paper describes the anodic oxide-to-valve metal substrate thermally-driven oxygen migration problem for the tantalum-tantalum oxide system. The authors also describe a method of overcoming this problem via the substitution of tantalum nitride or sub-nitride for tantalum powder in the fabrication of device anodes (the tantalum nitride, TaN, actually loses half of its nitrogen content during the vacuum sintering step used to consolidate the powder into an anode body, becoming tantalum sub-nitride, Ta
2
N, by the end of the sintering process).
The presence of nitrogen in the tantalum sub-nitride substrate material does not appear to interfere with the formation of the anodic oxide film dielectric. Anodes prepared by vacuum sintering tantalum nitride or sub-nitride may be anodized in the electrolytes traditionally used in the tantalum capacitor industry as well as the electrolytes described in co-pending applications, Ser. No.09/143,373 , abondoned, and Ser. No.09/489,471 now U.S. Pat. No. 6,162,345, as well as PCT No. WO 00/12783. The anodic oxide films produced on sintered tantalum nitride or sub-nitride have proven to be greatly improved with respect to resistance to thermally driven oxygen migration, oxide-to-substrate, compared with anodic oxide films grown upon tantalum metal. This enhanced th
Kinard John Tony
Lessner Philip Michael
Melody Brian John
Wheeler David Alexander
Banner & Witcoff , Ltd.
Kemet Electronics Corporation
Leader William T.
Nguyen Nam
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