Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
1999-10-25
2001-05-15
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C427S126100, C427S319000, C075S622000
Reexamination Certificate
active
06231689
ABSTRACT:
BACKGROUND OF THE INVENTION
Valve metals such as tantalum and niobium are characterized by a stable oxide coating which imparts useful properties such as corrosion resistance and electrical resistance which make such metals attractive for various applications such as an anode material in electrolytic capacitors and as a material of construction in certain corrosive environments. The oxide coating can be a hindrance in bonding valve metals, e.g. in sintering pressed valve metal powders into a pellet bonded to a valve metal lead wire or lead tab. As finer valve metal powders are used to achieve higher capacitance, lower sintering temperatures are used to prevent excessive loss of surface area from exposure to sintering temperatures. Moreover, as satisfactory particle to particle bonding is achieved in the pressed powder, the particle to lead connection bond can be deficient at lower sintering temperatures allowing the sintered pellet to be readily pulled off of the lead wire or lead tab during fabrication or use of the capacitor.
SUMMARY OF THE INVENTION
This invention provides valve metal including a second metal where the second metal is preferably at a greater concentration at or near the peripheral portion of the valve metal than within the valve metal. Such valve metal may be in a variety of forms, e.g. wire, sheet including foil, or particulate including flaked, nodular or angular powder. In such forms, the second metal is concentrated in the valve metal at the periphery of the valve metal form or valve metal article. Valve metal wire according to this invention is useful in the fabrication of anodes for electrolytic capacitors to provide enhanced bonding strength between such wire and sintered pellets of valve metal powder. Valve metal sheet according to this invention is similarly useful to form lead tabs on sintered valve metal powder anodes.
This invention also provides a method of adding a second metal to valve metal comprising:
(a) coating a valve metal surface with a solution comprising a second metal; and
(b) heating said valve metal and coating in the presence of an oxygen getter at a temperature sufficient to remove oxygen from said valve metal and to provide said valve metal with said second metal in a peripheral region of said valve metal. Preferably, the second metal is provided at or near to the surface of the valve metal, e.g. in a peripheral region, to improve a surface property of the valve metal.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
As used herein the term “valve metal” refers to tantalum, niobium, alloys of valve metals including alloys of tantalum and niobium, and valve metals including grain growth inhibitors such as tantalum with up to about 1000 ppm of silicon or yttrium oxide, niobium with zirconium and the like. Unlike the second metal which is preferably concentrated at a peripheral region, other additives to a valve metal such as grain growth inhibitors and alloying elements are more uniformly distributed throughout the valve metal.
As used herein the term “second metal” refers to a metal component in a valve metal which is not uniformly distributed throughout the valve metal. Useful second metals can be found in Groups IVB, VB, VIB, VIIB, and VIII of the periodic table and include tantalum, niobium, nickel, titanium, zirconium, tungsten and iron. Nickel is a preferred second metal for tantalum articles to be bonded to sintered tantalum powders.
In the method of this invention a second metal is added to a valve metal by coating the valve metal surface with a compound of the second metal and heating the coated valve metal in the presence of an oxygen getter at a temperature sufficient to remove oxygen from said valve metal and provide the metal in the peripheral region of the valve metal. The getter should also be effective in removing anions associated with the second metal in the coating. This method is especially useful in preparing valve metal wire, powder and sheet with a second metal.
When such second metal-containing wire is used as lead wire for preparing sintered anodes of pelletized valve metal powder, higher bond strength between the sintered powder and lead wire can be achieved. Such wire is expected to be useful with valve metal powder which is preferably sintered at low temperatures, e.g. less than 1500° C. for tantalum powders or lower such as in the range of 1200 to 1400° C.
It is expected that the improved bonding of an anode pellet to an anode lead wire in accordance with the present invention will reduce (and thereby improve) the susceptibility of the anode to inrush surge current. While not wishing to be bound by theory, it is understood that inrush surge current, as occurs when a discharged circuit is initially energized, can cause disruption of tantalum capacitors if the contacts between powder and wire are poorly established and therefore of small total cross-section area. The high instantaneous current forced to traverse poorly developed bonds results in high current density and ohmic heating in the area of the wire-anode interface. The high current density and heat generation can result in capacitor failure and possibly even ignition of the valve metal pellet. The improved wire to pellet bonding provided by this invention is expected to reduce the occurrence of disruptions caused by current surge.
Valve metal sheet according to this invention is also expected to be useful in making lead tabs for sintered anodes of high capacitance valve metal powder. Thus, preferred articles of this invention are valve metal wires and sheets having a second metal which improves a surface property, e.g. bonding to sintered bodies of valve metal powder. Such lead wires commonly have diameters in the range of about 100 to 1000 micrometers (&mgr;m); sheets may be thinner, e.g. on the order of 50 &mgr;m; and foil, even thinner. Second-metal-containing powders according to this invention may be useful as bonding agents for joining valve metal parts or for low temperature sintering valve metal powders or simply providing metal additives, e.g. dopants, to valve metals in peripheral regions.
Preferred wire and sheet provided by this invention have a second metal concentrated in the peripheral region of the valve metal article where the thickness of the peripheral region will depend on the diffusion rate of the second metal into the valve metal. Factors influencing diffusion are expected to include the concentration per unit surface area of the second metal in the coating and the time and temperature at which the valve metal is subjected to thermal treatment as with exposure to the oxygen getter. In many cases it is preferable that the peripheral region in which the second metal is concentrated be as thin as possible to provide enhanced surface properties without affecting bulk properties of the valve metal. Another benefit of a thin peripheral region of concentrated second metal is that the second metal concentration can be reduced to minimal levels after the enhanced surface properties have been utilized, e.g. by thermal treatment to facilitate diffusion of the second metal into the bulk of the valve metal which has a low concentration, if any, of the second metal, e.g. into the core region of a wire or center region of a sheet. In preferred aspects of this invention the peripheral region of concentrated second metal can be within 1 to 2 micrometers from the surface. For some applications the second metal concentration in the peripheral region can be up to about 1 weight percent or more. Based on the total weight of valve metal in an article, the bulk concentration of the second metal can be in the range of 2 to 2000 ppm. For some applications bulk concentration of the second metal can be 5 to 200 ppm. For other application the bulk concentration of the second metal can be 5 to 50 ppm.
A valve metal wire in accordance with the invention can be annealed (55,000-90,000 psi), unannealed—half hard (105,000-160,000), or unannealed—hard (130,000-215,000 psi).
A useful second metal for tantalum is nickel which is stable and resistant to corrosi
Cabot Corporation
Oltmans Andrew L.
Sheehan John
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