WO2014142359A1

WO2014142359A1 - Method to passivate sintered anodes having a wire

Info

Publication number: WO2014142359A1
Application number: PCT/JP2014/057939
Authority: WO
Inventors: Osamu Kubota
Original assignee: Global Advanced Metals Japan K.K.
Priority date: 2013-03-15
Filing date: 2014-03-14
Publication date: 2014-09-18

Abstract

A method to passivate a sintered anode having at least one wire at least partially embedded in the sintered anode (or otherwise attached) is described. The method includes passivating the sintered anode in an atmosphere that contains oxygen and at least one inert gas and further has a relative humidity of less than 5%. A method to prevent embrittlement of a metal wire embedded in the sintered anode is further described.

Description

DESCRIPTION

METHOD TO PASSIVATE SINTERED ANODES HAVING A WIRE

BACKGROUND OF THE INVENTION

[0001] This application claims the benefit under 35 U.S.C. § 119(e) of prior U.S. Provisional Patent Application No. 61/794,626, filed March 15, 2013, which is incorporated in its entirety by reference herein.

[0002] The present invention relates to anodes and anodes having a wire at least partially embedded in the anode. The present invention further relates to methods to passivate sintered anodes having a wire at least partially embedded in the sintered anode, or welded onto the sintered anode.

[0003] Valve metals, e.g., tantalum and niobium, are generally extracted from their ores in the form of powders. Tantalum powders, for example, that are suitable for use in high performance capacitors, can be produced by chemical reduction, such as sodium reduction, of potassium fluorotantalate. In this process, the potassium fluorotantalate is recovered from processed ore in the form of a dry crystalline powder. The potassium fluorotantalate is melted and reduced to tantalum metal powder by sodium reduction. The tantalum powder formed is then water washed and acid leached, as described, for example, in U.S. Pat. Nos. 6,312,642, and 5,993,513, which are incorporated in their entireties herein by reference. The tantalum is then dried, resulting in what is known as a basic lot powder.

[0004] Typically, the basic lot powder is subjected to a heat treatment or thermal agglomeration step and then passivated and stabilized to obtain a powder cake that is subsequently ground up into a powder. A deoxidation step using an oxygen getter is then performed. After the deoxidation step, the tantalum powder can again be passivated to form a passive oxide coating on its surface to form stabilized powder particles. Other techniques used in the processing of tantalum powder to improve the performance characteristics of the finished products made from the metal powder include reacting small quantities of modifying agents to the tantalum powder. A range of additives or "dopants" have been used, including nitrogen, silicon, phosphorous, boron, carbon, and sulfur. Nitriding, for example, can occur between or during any of the aforementioned processing steps. The processed tantalum powder can then be pressed into a pellet and sintered for subsequent processing by capacitor anode manufacturers for example.

[0005] As mentioned, performance characteristics of the products made from the metal powder can be related to microstructure characteristics of the metal powder. Of particular interest, capacitance and DC leakage of metallic capacitors can be related to the specific surface area of the metal powder used to form the sintered metal body. Greater net surface area can be achieved, of course, by increasing the quantity (grams) of metal per pellet; but, cost and size considerations have dictated that development be focused on means to increase the specific area of the metal powder, that is, to increase volumetric efficiency. Due to the very fine particle size and high surface area, electrolytic capacitor grade metal powders such as tantalum and niobium need to be passivated to prevent a violent reaction upon exposure to atmospheric oxygen that can possibly result in combustion.

[0006] Conventional techniques to passivate tantalum and niobium particles involve controlled exposure to atmospheric air in a gradual or a step-wise increase in pressure. Depending on the surface area of the powder, multiple vent/evacuation cycles may be needed at each pressure step to equilibrate the surface with oxygen partial pressure. A primary reason for vent/evacuation cycles is that atmospheric air consists of approximately only 20 wt. % oxygen, with the balance being predominantly nitrogen. Residual nitrogen that is trapped between the powder particles is preferably evacuated before further venting of fresh air. [0007] When dealing with high surface area metal powders such as tantalum powders having a capacitance capability of over 100,000 CV/g or from 150,000 CV/g to over 200,000 CV/g, these powders are as explained above subjected to passivation. However, when the powders are further processed and formed into a pressed and sintered anode, the sintered anode again needs to be passivated since the previous passivation layer was diffused away or into the metal by being subjected to various processing steps and temperatures to form the anode. In addition, the forming of a passivation layer on the sintered anode is not to be confused with a dielectric layer. A passivation layer is different from a dielectric layer. The passivation layer on a sintered anode is not formed through anodization but is simply formed by subjecting the anode surface to oxygen or an oxygen containing atmosphere at generally room to elevated temperatures that do not exceed 100° C. The passivation layer on the sintered anode can ultimately be transformed at least partly into a dielectric layer by anodizing or anodization techniques.

[0008] Various anodes are made from metal powders or metal suboxide powders. The powder is typically pressed and then formed into a shape of an anode to form a green pellet or anode, wherein a wire or multiple wires are embedded in the anode prior to sintering. The wire is used for electrical connections. This wire, which is attached to the anode, can be considered or known as axial leads or anode lead wire, and can be one wire which extends along the axis of the anode such that it is exposed out of one or both ends of the anode, or it can be multiple leads.

[0009] However, when the wire or wires embedded in the anode are then subjected to sintering and other processing, along with the anode to form the sintered anode or a passivation layer on the anode, at times, the wire has become brittle which then leads to the breaking of the wire or to poor electrical conductivity. In the past, it was unclear what the cause was to this embrittlement of the wire. [0010] Accordingly, there is a need in the industry to create methods to prevent this brittle-wire problem on sintered anodes.

SUMMARY OF THE PRESENT INVENTION

[0011] A feature of the present invention is to provide a method to passivate a sintered anode having at least one wire at least partially embedded in the sintered anode, wherein the wire avoids brittlement afterwards.

[0012] A further feature of the present invention is to provide a method to prevent embrittlement of a metal wire(s) embedded in a sintered anode.

[0013] Additional features and advantages of the present invention will be set forth in part in the description that follows, and in part will be apparent from the description; or may be learned by practice of the present invention. The features and other advantages of the present invention will be realized and attained by means of the elements and combinations particularly pointed out in the description and appended claims.

[0014] To achieve these and other advantages, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention, in part, relates to a method to passivate at least one sintered anode having at least one wire at least partially embedded in the sintered anode. The wire can be welded or otherwise attached to the anode. The passivation occurs in a chamber. The method includes passivating the anode in an atmosphere that contains or comprises oxygen and at least one inert gas. The oxygen is present in an amount of from about 0.5 wt% to about 25 wt%, based on the weight of the atmosphere in the chamber or about 0.5% to about 25% by volume. The atmosphere in the chamber has a relative humidity below 5%. Preferably, the sintered anode is a tantalum anode, niobium anode, or a niobium suboxide anode. The anode can be a valve metal anode or valve metal suboxide anode.

[0015] The present invention further relates to a method to prevent embrittlement of a metal wire(s) embedded in a sintered anode. The method comprises passivating at least one sintered anode, such as a metal anode, or a metal suboxide anode, in a chamber with an atmosphere that comprises oxygen and at least one inert gas. The oxygen is present in an amount of from about 0.5 wt% to about 25 wt%, based on the weight of the atmosphere or from about 0.5% to 25% by volume. The atmosphere has a relative humidity below 5%. This sintered anode can be a tantalum anode, niobium anode, a niobium suboxide anode, or a valve metal or valve metal suboxide anode. The wire has low hydrogen embrittlement such that the wire can be flexed back and forth over 10 times without breaking and/or the sintered anode has an 0[ppm]/H[ppm] ratio of 50 or greater, which reflects low hydrogen present.

[0016] Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. The objectives and other advantages of the present invention will be realized and obtained by means of the elements and combinations particularly pointed out in the written description and appended claims.

[0017] It is to be understood that both the foregoing general description and the following detailed description are exemplary only and are not restrictive of the present invention as claimed. All patents, patent application, and publications mentioned above and throughout the present application are incorporated in their entirety by reference herein. DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0018] The present invention provides methods to passivate at least one sintered anode having one or more wires at least partially embedded in the sintered anode. The present invention also relates to a method to prevent embrittlement of a metal wire(s) embedded in a sintered anode.

[0019] In more detail, the method involves passivating at least one sintered anode that has one or more wires at least partially embedded in (or otherwise attached to) the sintered anode. As stated, these one or more wires can be considered leads, such as axial leads. The method comprises, consists essentially of, or consists of, or includes passivating at least one sintered anode in a chamber, such as a sealed chamber, like a furnace, wherein the atmosphere in the chamber comprises, consists essentially of, consists of, or includes oxygen and at least one inert gas. The oxygen present in the atmosphere can be present in an amount of from about 0.5 wt% to about 25 wt%, based on the weight of the atmosphere. The atmosphere in the chamber also has a relative humidity that is below 5%. The sintered anode can be a tantalum anode, niobium anode, or a niobium suboxide anode, or a valve metal or valve metal suboxide anode.

[0020] In further detail, as stated, the atmosphere within the chamber or environment within the chamber during the actual step of passivation comprises, consists essentially of, or consists of oxygen and at least one inert gas. The oxygen is present in an amount of from about 0.5 wt% to about 25 wt%, such as 0.5 wt% to about 20 wt%, 0.5 wt% to about 15 wt%, 0.5 wt% to about 10 wt%, or other amounts within these ranges based on the weight of the atmosphere. The term "oxygen" is a reference to oxygen gas (0₂). Further, for purposes of the present invention, as an option, the amounts for the oxygen gas present in the atmosphere within the chamber can be by volume with the same numerical ranges as for wt% 0 . Thus, the oxygen can be present by volume in an amount from 0.5% to about 25% by volume, such as 0.5% to about 20% by volume, 0.5% to about 15% by volume, 0.5% to about 10% by volume.

[0021] As part of the atmosphere, at least one inert gas is present. The inert gas can comprise, consist essentially of, or consist of nitrogen gas (N₂) or argon (Ar), or both. Other inert gases can be used entirely or in addition.

[0022] The at least one inert gas can form the rest of the atmosphere by weight or by volume. Put another way, if the oxygen gas is present in an amount of 25 wt% or 25% by volume, the inert gas(es) can be present in an amount of 75 wt% or 75% by volume. The inert gas can be present in an amount of 99.5 wt% to about 75 wt% or from 99.5 wt% to 80 wt%, 99.95 wt % to 85 wt%, 99.5 wt% to 90 wt% and the like. These numerical ranges can be by volume instead of by weight.

[0023] Other gases can be present in the atmosphere within the chamber and typically can form less than 10 wt% or less than 10% by volume, such as neon, carbon dioxide, methane, krypton, hydrogen, nitrous oxide, carbon monoxide, xenon, ozone, nitrogen dioxide, iodine, and/or ammonia. Each is in gas form.

[0024] As stated, in the atmosphere in the chamber, the relative humidity of the atmosphere for passivation is below 5%, such as from 0% to 4.9%, from 0.001% to 4.9%, from 0.01% to 4.9%, from 0.1% to 4.9%, from 1% to 4.9%, from 1.5% to 4.5%, from 2% to 4.5%, from 0.1% to 4%, from 0.1% to 3.5%, from 0.1% to 3%, from 0.1% to 2.5%, or below 4%, or below 3%, or below

2% or below 1%, or below 0.5%. Relative humidity is the ratio of the partial pressure of water

t

vapor in the atmosphere to the saturated vapor pressure of water at the temperature of the atmosphere in the chamber for each passivation cycle. The low relative humidity of the atmosphere used for each passivation cycle can be achieved by using dry air or bottled air which is dry and has a relative humidity of below 5%. [0025] In the method of the present invention, the passivating can occur at standard passivation temperatures and, generally, these temperatures occur at below 100° C, such as from about 15° C to about 95% C, or 15° C to about 80° C, or from about 25° C to about 80° C. Generally, the passivation temperature occurs at a temperature which prevents the migration of the oxygen from the surface of the sintered anode into the interior of the sintered anode.

[0026] The passivation can occur for any length of time, generally to achieve full passivation. Full passivation is wherein an oxide layer, typically, a metal oxide layer, (e.g., Ta₂0₅ or Nb₂0₅) is formed on the exposed surfaces of the sintered anode, such that the oxygen levels that form the passivated layer are in equilibrium with the oxygen levels found in the atmosphere of earth, namely, the atmosphere present at sea level to 1,000 feet above sea level. The passivation layer can be from 1 to 4 mm in thickness or thicker. Other thicknesses are possible. When a sintered anode that is fully passivated is present in such an atmosphere, then the sintered anode will be stable and not absorb any further significant amounts of oxygen (less than 5 wt% or less than 1 wt% by weight of anode).

[0027] The passivating can occur for at least 1 minute, at least 5 minutes, at least 10 minutes, at least 30 minutes, at least one hour, at least 2 hours, such as from about 1 minute to about 20 hours, or from about 5 minutes to about 15 hours, or from about 30 minutes to about 10 hours, or other times.

[0028] The step of passivating can comprise, consist essentially of, or consist of several steps. The passivating can include feeding in one or more gases into the chamber to create the atmosphere mentioned above. This passivating can comprise, consist essentially of, or consist of feeding in one or more gases to create the atmosphere in the chamber and then permitting the surface or surfaces of the sintered anode to reach equilibrium with that particular atmosphere in the chamber. Then, that atmosphere can be evacuated, such as by creating a vacuum or other techniques in the chamber. As an option, these steps of "feeding," "permitting the surfaces to reach equilibrium," and "evacuating the atmosphere" can be repeated any number of times until the sintered anode is fully passivated. This repeating of steps can occur once, twice, three times, or four or more times.

[0029] Typically, the "feeding" of the one or more gases to create the atmosphere occurs until the chamber reaches at least about 1 Torr of atmosphere within the chamber. This pressure can be at least 1 Torr, at least 2 Torr, at least 3 Torr, at least 4 Torr, at least 5 Torr, or more.

[0030] As an option, the passivating step can comprise, consist essentially of, or consist of:

(a) feeding in one or more gases to create the atmosphere in the chamber, wherein the feeding occurs until the chamber reaches at least about 1 Torr of said atmosphere; then

(b) permitting a surface or surfaces of the sintered anode to reach equilibrium with the atmosphere, and then

(c) evacuating the atmosphere by creating a vacuum in the chamber and repeating steps (a), (b), and (c), wherein for each repeating of the steps, the Torr of the atmosphere is increased to a higher Torr than the most previous Torr used, and the repeating then continues until the sintered anode is fully passivated.

[0031] Generally, the steps which involve repeating with feeding in new gas and permitting further passivation of the sintered anode to reach equilibrium with the current atmosphere in the

f

chamber and then evacuating that atmosphere can be repeated any number of times and, typically, occurs with an increase in the Torr of the atmosphere. This repetition can occur two to three to four times until full passivation of obtained.

[0032] Passivation can be achieved by a step-wise or cyclic increase in operating pressure in the chamber or container, a gradual increase in operating pressure, or a combination thereof (venting). Cyclic passivation can include venting and evacuation of the container. For purposes of the present invention, a cycle of passivation can include increasing the operating pressure in the container in which the sintered anode is contained by a predetermined amount, and maintaining or holding the increased container pressure for a predetermined amount of time, a complete cycle comprising venting/holding. Optionally, another cycle can then be initiated by a further increase in operating pressure. For purposes of the present invention, a cycle of passivation can also include increasing the operating pressure of the passivation container by a predetermined amount, and maintaining the increased container pressure for a predetermined time, followed by an evacuation of the passivation container or decreasing the operating pressure by a predetermined amount, a complete cycle comprising venting/holding/evacuation. Optionally, a subsequent passivation cycle can then be initiated by a further venting of the passivation container.

[0033] Preferably, passivation is achieved in an environment in which the sintered anode is stabilized by at least partially surface passivating of the anode in the fewest number of passivation cycles and/or in the least amount of passivation time as possible. The passivation container can have any starting pressure prior to passivation, and preferably, the passivation container is under vacuum, for example, from about 0.1 to about 1 torr. Passivation of the anode is initiated by cyclic e posure to progressively higher partial pressures of the atmosphere as described, that contains oxygen, at least one inert gas, and low relative humidity, for example, the pressure in the passivation container can be increased by an amount of from about 5 to about 100 torr, and preferably, from about 10 to about 25 torr by backfilling the passivation container with the atmosphere gas described herein. The pressure in the passivation container can then be maintained for a hold time of from about 1 to about 10 minutes or more. Preferably, the hold time is sufficient to allow at least some of the oxygen present in the gas to react with the anode so as to at least partially surface passivate the anode surface. This can constitute a passivation cycle.

[0034] The passivation cycle can include at least one evacuation step. The step of evacuating the passivation container preferably is sufficient to remove some, most, or all of any residual inert gas(es) present in the chamber or anode. Evacuating the passivation container can be achieved by reducing the pressure to a value of 0.1 to about 50 torr. The container can be evacuated to a pressure that is less than the initial pressure in the container, or is preferably evacuated to a pressure that is equal to or greater than the initial operating pressure. Upon achieving the desired vacuum pressure in the passivation container, the container can then be pressurized to a predetermined operating pressure by backfilling the container with further predetermined amount of gas, for example, from about 5 to about 100 torr, of the atmosphere gas that includes the oxygen and inert gas(es) and low relative humidity as described earlier. The operating pressure in the passivation container can then be maintained for a predetermined hold time. The hold time is sufficient to allow the oxygen present in the gas to react with the anode(s) so as to at least partially further surface passivate the anode. Following venting and holding, a further passivation cycle can be initiated by again evacuating the container to about 0.1 to about 50 torr. Evacuating the container can be to any operating pressure, and is preferably to a pressure that is greater than the operating pressure achieved by evacuation of the container in the immediate prior passivation cycle. Evacuation preferably is sufficient to at least partially remove any residual inert gas(es) that may be present. Upon achieving the desired operating pressure, the pressure within the container can then be increased to a predetermined operating pressure by backfilling the container with further atmosphere gas that includes oxygen and inert gas(es) and low relative humidity as described herein.

[0035] Passivation can include a fewer or a greater number of cycles than described above, sufficient to form a passivated sintered anode. The number of cycles needed to form a passivated sintered anode can relate to the specific surface area of the powder sintered, form, shape, type, amount, and the like of the powder, as well as to passivation pressures, temperatures, hold times, equipment, and passivating gas concentrations and the like. The number of passivation cycles can be, for example, from 1 to about 50 or more. A passivation cycle can be any amount of time, for example, from about 1 to about 30 minutes or more. Total passivation time can depend on any or all of the aforementioned parameters, and can be for a time of from about 30 to about 600 minutes or more, for instance. Any combination of vent/hold or vent/hold/evacuation cycles of passivation as described above can be used to form a passivated sintered anode.

[0036] The present invention further relates to a method to prevent embrittlement of a metal wire embedded in a sintered anode following the steps described above. By following these method steps, a sintered anode having at least one metal wire embedded in (or otherwise attached to) the sintered anode can avoid embrittlement after passivation, such that the wire has low hydrogen embrittlement such that the wire can be flexed back and forth over 10 times without breaking, and/or the sintered anode has an 0[ppm]/H[ppm] ratio of 50 or greater, which reflects low hydrogen present.

[0037] As a result of the present invention, the formation or migration of hydrogen is controlled or minimized. As a result the sintered anode (the passivated sintered anode) has a high 0[ppm] H[ppm] ratio of 50 or higher, such as 50 to 250 or higher, 50 to 225, 50 to 200, 50 to 175, 50 to 100, 60 to 250, 70 to 250, 80 to 250, 90 to 250, 100 to 250, over 60, over 75, over 80, over 90, over 100, over 150. This ratio reflects the low hydrogen content present after anode passivation.

[0038] The wire flex test can be described as where an anode lead wire (that is exposed on one side) is at the 12 o'clock position and then to flex, the wire is brought over to the 9 o'clock position and then over to the 3 o'clock position and this would considered one flex. The second flex would then take the wire at the 3 o'clock position and flex it back to the 9 o'clock position. The third flex would then take the wire at the 9 o'clock position and flex it back to the 3 o'clock position, and so on. And, this procedure is continued to determine the number of flexes that the anode lead wire can flex before breaking. In the case of the present invention, the anode lead wire can flex over 10 times, such as 11 times or more, 12 times or more, 15 times or more, 20 times or more and the like, without breaking.

[0039] As stated earlier, the passivation layer that is formed in the method of the present invention is not to be confused with a dielectric layer. A passivation layer is not a dielectric layer. The passivation layer is not formed through any anodization. The passivation layer is formed by oxygen forming a metal oxide layer on the anode surface. The dielectric layer can be formed by transforming (afterwards) at least part of the passivation layer at a later time. In addition, the passivation layer would not be sufficient as a dielectric layer in view of the defects that exist in the layer. It is to be understood that the passivation layer is for purposes of protecting and stabilizing the sintered anode especially when the sintered anode is formed by high surface area or high capacitance grade metal powders. For instance, the present invention is very effective when the sintered anode is formed from capacitor grade metal powder such as tantalum, niobium, or niobium suboxide powders, where the capacitance capability of the metal powder exceeds 100,000 CV/g or exceeds 150,000 CV/g or is 200,000 CV/g or more in capacitance capability. Any capacitance grade powder, however, can benefit with the present invention, such as below 100,000 CV/g or above 100,000 CV/g grade powder.

[0040] The capacitance (CV) value of the powder (e.g. tantalum powder) that can be used to form the sintered anodes of the present invention can be from 200,000 CV/g to 800,000 CV/g, such as from 450,000 to 800,000 CV/g. With respect to capacitance, the tantalum powders, when formed into an anode, can have a capacitance of from 200,000 to 800,000 CV/g, from about 500,000 to 800,000 CV/g, from about 550,000 to 800,000 CV/g, from about 600,000 to about 800,000 CV/g, from about 650,000 to about 800,000 CV/g, from about 700,000 to about 800,000 CV/g, from about 500,000 to about 750,000 CV/g, or from about 500,000 to 700,000 CV/g, and the like. With respect to leakage, the leakage can be 50 nA/CV or less, such as 30 nA/CV or less, such as 25 nA/CV or less, 20 nA/CV or less, 10 nA/CV or less, such as from 1.0 nA/CV to 30 nA/CV.

[0041] Regarding the measuring method of CV values for purposes of the present invention, first, tantalum pellets are produced. The pellets have tantalum lead wires present. The tantalum powder is formed into pellets using a press density of 4.5 g/cm³. In order to obtain this density, only the mass and pellet shape of the tantalum powder need to be defined, it is preferable to select the sintering temperature of the pellets arbitrarily such that the shrinkage ratio of the tantalum powder remains in a range of 5 to 10%. The sintering temperature is preferably in a range of from 900 to 1,000°C. Furthermore, the greater the CV value of the tantalum powder, the more preferable it is to select a lower temperature. Next, chemically converted substances are produced by chemically converting the pellets in a phosphoric acid aqueous solution of concentration 0.1 vol. % at a voltage of 6V. For the chemical conversion, in order to form a uniform (or substantially uniform) oxide film on the surface of tantalum powder, it is preferable to make an adjustment within a range when necessary: 30 to 60°C for temperature, 4 to 6V for voltage, and 90 to 120 minutes for the treatment time. Then, the CV values of the chemically converted substances are measured in a sulfuric acid aqueous solution of concentration 30.5 vol. % under the conditions: temperature 25°C, frequency 120 Hz, and voltage 1.5V. With respect to sintering, the sintering time can be from 5 minutes to 1 hour or more, such as from 10 minutes to 30 minutes, 10 minutes to 20 minutes, or 10 minutes to 15 minutes. Any desirable sintering time can be used. With respect to sintering temperature, any desirable sintering temperature can be used. For instance, the sintering temperatures can be from 800°C to 1 ,500°C, from 900°C to 1,450°C, from 900°C to 1,400°C, from 900°C to 1,350°C, from 900°C to 1 ,300°C, from 900°C to 1,250°C, from 900°C to 1,200°C, from 900°C to 1 , 150°C, and any sintering temperatures within these ranges. With respect to press density, other press densities can be used either in the test method or in use of the tantalum powders in general. The press densities can be from about 3.0 to about 6.0 g/cm³, such as 5.0 g/cm³, or 5.5 g/cm³, or 4.0 g/cm³. With respect to the present invention, it is to be understood that the test method used to determine capacitance is simply a test for determining capacitance. The tantalum powders of the present invention can be used under a variety of electrical conditions, various formation voltages, various working voltages, various formation temperatures, and the like. With respect to formation voltage, other formation voltages can be used, such as 5 volts, 4 volts, 3 volts, and the like, for instance, 5 to 10 volts, 5 to 16 volts, or 5 to 20 volts, can be used as a formation voltage.

[0042] For purposes of the present invention, valve metals generally include tantalum, niobium, and alloys thereof, and also may include metals of Groups IVB, VB, and VIB, and aluminum and copper, and alloys thereof. Valve metals are described, for example, by Diggle, in "Oxides and Oxide Films," Vol. 1, pp. 94 95, 1972, Marcel Dekker, Inc., New York, incorporated in its entirety by reference herein. Valve metals are generally extracted from their ores and formed into powders by processes that include chemical reduction, as described for example, in U.S. Pat. No. 6,348,113, by a primary metal processor. Further metal refining techniques typically performed by a primary metal processor include thermally agglomerating the metal powder, deoxidizing the agglomerated metal powder in the presence of a getter material, and then leaching the deoxidized metal powder in an acid leached solution, as disclosed, for example, in U.S. Pat. No. 6,312,642.

[0043] Examples of tantalum powders, including flakes, are described in U.S. Pat. Nos. 6,348,113 Bl; 5,580,367; 5,580,516; 5,448,447; 5,261,942; 5,242,481; 5,21 1,741 ; 4,940,490; and 4,441,927, which are incorporated herein in their entireties by reference. Examples of niobium powders are described in U.S. Pat. Nos. 6,420,043 Bl; 6,402,066 B l ; 6,375,704 Bl ; and 6,165,623, which are incorporated herein in their entireties by reference. Other examples of suitable powders and processes that can be used are described in U.S. Pat. Nos. 8, 1 10, 172; 7,323,017; 7,220,397; 7, 149,074; 7,749,297; 7,679,885; and 7,142,408.

[0044] The powder used can have a BET surface area of at least 1.5 m²/g, or preferably at least 1.7 m²/g, and more preferably, at least about 5 m²/g, and even more preferably from about 5 to about 8 m²/g, and most preferably at least 7.5 m²/g. The BET ranges are preferably based on pre-agglomerated powder. The powder can be hydrided or non-hydrided. Also, the powder can be agglomerated or non-agglomerated.

[0045] For purposes of the present invention, the one or more wires or metal wires embedded in the sintered anode occurs while the anode is in a green state or in a pre-sintered state. The wire can be any conventional diameter or metal material typically used for anodes, such as tantalum wire, niobium wire, aluminum wire, and the like. A typical diameter of the wire is from about 0.05 mm to about 0.5 mm. The anode lead wire can have a circular or non-circular cross- section. The non-circular can be oval or an ellipse, substantially flat, or flat. The anode lead wire(s) can be Al, Ta, Nb, Ti, Zr, Hf, W or mixtures, alloys or suboxides thereof.

[0046] The sintered anode can be used in a capacitor body. The capacitor body comprises an anode. Extending from the anode is at least one anode lead wire. The anode lead wire is preferably integral to the anode and extends into the anode a distance. It is preferable that the distance is as long as possible to insure as much surface area contact between the anode material and the anode lead wire embedded therein. The anode lead wire extends beyond the anode body by a distance which represents a sufficient amount for attachment of the anode lead wire to a lead frame or circuit trace. The anode can be a wet electrolyte anode or a solid electrolyte anode.

[0047] A capacitor can comprise the anode of the present invention. For example, the capacitor can have one anode terminal and one cathode terminal comprising: a) an anode compact formed from a sintered valve metal powder; b) at least one or two solid conductor anode lead wires embedded into the anode compact; c) a dielectric formed upon or with the passivated surface of the anode compact; d) a conductive material in contact with the dielectic to form a cathode; e) terminal connected to the cathode to form a cathode terminal; and f) a capsule formed around the anode and cathode exposing only the respective anode and cathode terminals. The valve metal can be Al, Ta, Nb, Ti, Zr, Hf, W or mixtures, alloys or suboxides thereof.

[0048] Prior to the passivation step described herein, the sintering of the anode can occur in any conventional manner and there are no limitations or restrictions on the methods for sintering or the sintering conditions. The material that forms the anode can be any conventional material as stated above, such as tantalum metal, niobium metal, niobium suboxide, or other valve metal or valve metals suboxide materials. The following patents and applications provide various materials that can be used to form the anode and, ultimately, the sintered anode that is passivated by the present invention. These are no limitations as to the capacitance, BET of the powder used to form the anode, amounts, or size of anodes with the present invention. The following patents and published applications describe various materials that can form the anode and, ultimately, the sintered anode, as well as further describe various sintering conditions that can be used and other techniques that can be used to form the anodes of the present invention which can be passivated by the present invention: U.S. Pat. Nos. 8,110,172; 7,323,017; 7,220,397; 7,149,074; 7,749,297; 7,679,885; and 7,142,408.

[0049] As an option, when the anode or pellet (e.g., pressed unsintered anode or green anode) is sintered to form the sintered anode, this can occur in a furnace. For instance, the sintering can occur in a furnace under vacuum. The sintering temperature (e.g., furnace temperatures) for sintering of a tantalum anode for instance can be from about 800° C to about 1500° C. The sintering time can be 5 minutes to 1 or 2 hours or more. The sintering temperature for a niobium anode can be from about 1000° C to about 1750° C. The sintering time can be 5 minutes to 1 or 2 hours or more. The sintering time for a niobium suboxide anode can be from about 1000° C to about 1750° C. The sintering time can be 5 minutes to 1 or 2 hours or more. As an option, the passivating step of the present invention (the method to passivate as disclosed herein) can occur in the same furnace as used for sintering. If this is done, preferably, once sintering is completed to form the sintered anode in a chamber that is the furnace used for sintering, the temperature (i.e., the elevated temperature) in the furnace needs to be reduced (e.g., cooling) to a temperature of 100° C or lower, for instance reduced to a temperature of from about 10° C to about 95° C or from about 15° C to about 80° C. While the furnace temperature is being reduced to this temperature range, as an option, and preferably, the furnace remains under dynamic vacuum. In other words, preferably the vacuum is maintained during cooling and the vacuum pump is not turned off. In other words, the vacuum pump (e.g., the diffusion pump) is kept running during the cooling process. Preferably, during the entire temperature reduction (e.g., cooling process), the vacuum is not only maintained but the vacuum pump is kept running. It has been found that parts of the furnace can absorb hydrogen and by keeping the furnace under vacuum and the vacuum pump running during the cooling process (after sintering), this reduces or controls or prevents hydrogen from migrating from one or more parts of the furnace to the sintered anodes and/or to the at least one wire at least partially embedded in the sintered anode, prior to the passivating step. Keeping the furnace under vacuum and constantly running the vacuum pump is considered 'under dynamic vacuum' conditions. By doing so, this further controls or prevents embrittlement of the metal wire(s) embedded in the sintered anode. Thus, preferably, and as option, prior to passivating by following the method of the present invention, the sintered anode with the wire at least partially embedded therein can be formed by: sintering, under vacuum and under elevated temperature, a pressed anode in a furnace to form the sintered anode, and while the sintered anode is present in the furnace, cooling (reducing) the furnace to a temperature under 100° C, and wherein the cooling occurs under dynamic vacuum, and then once the reduced temperature of 100° C or lower is reached, passivating occurs in accordance with the present invention.

ft

[0050] As a further aspect of the present invention, even if the atmosphere used for passivating has a relative humidity of 5% of higher, by cooling under dynamic vacuum conditions prior to passivating, and then passivating, one can reduce or control the hydrogen pick up levels in sintered anodes. For instance, even using air with a relative humidity of 25% to 30%, if dynamic vacuum conditions are used, the hydrogen levels in the sintered anodes can be reduced by over 60% (by ppm levels) compared to turning the vacuum pump off and sealing the furnace (not dynamic vacuum conditions). Thus, the present invention further relates to: prior to passivating the sintered anode with the wire at least partially embedded therein, a) sintering, under vacuum and under elevated temperature, a pressed anode in a furnace to form the sintered anode, and while the sintered anode is present in the furnace, b) cooling (reducing) the furnace to a temperature under 100° C, and wherein the cooling occurs under dynamic vacuum, and then once the reduced temperature of 100° C or lower is reached, c) passivating the sintered anodes (in an atmosphere having a relative humidity of below 5% or in an atmosphere having a relative humidity of 5% or higher).

[0051] The present invention will be further clarified by the following examples, which are intended to be exemplary of the present invention.

EXAMPLES

Example 1:

[0052] The effects of relative humidity in the atmosphere gas used during passivation was studied. In this experiment, a commercially-available 200 K (CV/g) tantalum powder and a 250 K (CV/g) tantalum powder were used and formed into sintered pellets. A tantalum wire having a diameter of 0.20 mm was embedded in the green body of the 250 K powder and a tantalum wire having a diameter of 0.28 mm was embedded into the green body of the 200 K powder. The green bodies were then sintered to form a sintered pellet or anode. The sintered pellets or anodes were then divided into several groups as shown in the tables below. One group was subjected to a passivation atmosphere, wherein the relative humidity was 25-30% at 20-25° C (this experiment was repeated at times as shown in the tables), and then another set of sintered pellets or anodes was subjected to an atmosphere which used dry air or bottled air where the relative humidity of the dry air was below 5% and more on the order of below 1-4% (this experiment was repeated at times as shown in the tables). The passivation times were kept the same per each experiment, and the only variation in the experiments was with regard to the relative humidity of the passivation atmosphere used. This same experiment was also repeated for the 200 commercially-available tantalum powder as shown in Table 2. The results are set forth below.

Table 1

[0053] The results show the O/H ratio in ppm and, further, the tables show the number of flexes that were made of the anode lead wire prior to it breaking. As can be seen, when the passivation atmosphere used a relative humidity below 5%, the O/H ratio was well over 50, and the anode lead wire could be flexed over ten times without breaking. Quite the opposite was the case when the passivation atmosphere was used in which the relative humidity was over 5%, and where the O/H ratio was below 50, and the anode lead wire broke after flexing 1-3 times.

Example 2:

[0054] In this experiment, a commercially-available 200 K (CV/g) tantalum powder and a 250 K (CV/g) tantalum powder were used and formed into sintered pellets. A tantalum wire having a diameter of 0.20 mm was embedded in the green body of the 250 K powder and a tantalum wire having a diameter of 0.28 mm was embedded into the green body of the 200 powder. The green bodies were divided into two groups before sintering. The first group was sintered at a temperature of 1225° C in a conventional furnace under vacuum to form a sintered pellets or anodes, and after sintering, the furnace was cooled down to a temperature of 50° C. In this first group, the cooling down occurred with the vacuum pump off. The sintered anodes were kept under vacuum by keeping the valves and seals closed in the furnace but the diffusion pump was turned off and therefore this was not considered under dynamic vacuum conditions. Once 50° C was reached, passivation was conducted as in Example 1 by using an atmosphere which used dry air or bottled air where the relative humidity of the dry air was below 5% and more on the order of below 1-4%. The second group of sintered anodes were subjected to the same sintering conditions in the same furnace, but in the second group, after sintering, during the cooling down period, the diffusion pump was left on (maintaining dynamic vacuum conditions).

[0055] For the first group, the average amount of hydrogen in the anodes, after two hours, was about 300 ppm H, and in the second group, the average amount of hydrogen in the anodes was about 160 ppm H. Thus, by keeping dynamic vacuum conditions during the cooling period after sintering, the amount of hydrogen pick up on the anodes was almost 50 % lower in H ppm levels. [0056] Accordingly, it can be seen that by using the methods of the present invention, hydrogen embrittlement of the anode lead wire can be controlled or minimized or even avoided.

[0057] The present invention includes the following aspects/embodiments/features in any order and/or in any combination:

1. A method to passivate at least one sintered anode having at least one wire at least partially embedded in said sintered anode in a chamber, said method comprising passivating said at least one sintered anode in an atmosphere that comprises oxygen and at least one inert gas, wherein said oxygen is present in an amount of from about 0.5 wt% to about 25 wt%, based on weight of said atmosphere (or from about 0.5% to about 25% by volume), and wherein said atmosphere has a relative humidity of below 5%, and wherein said sintered anode is a tantalum anode, niobium anode, or a niobium suboxide anode.

2. The method of any preceding or following embodiment/feature/aspect, wherein said inert gas comprises nitrogen or argon or both.

3. The method of any preceding or following embodiment/feature/aspect, wherein said passivating occurs at a temperature of from about 15° C to about 95° C.

4. The method of any preceding or following embodiment/feature/aspect, wherein said passivating occurs at a temperature of from about 15° C to about 80° C.

5. The method of any preceding or following embodiment/feature/aspect, wherein said passivating occurs for at least about 1 minute.

6. The method of any preceding or following embodiment/feature/aspect, wherein said passivating occurs for from about 2 minutes to about 20 hours. 7. The method of any preceding or following embodiment/feature/aspect, wherein said passivating comprises feeding in one or more gases in said chamber to create said atmosphere.

8. The method of any preceding or following embodiment/feature/aspect, wherein said passivating comprises

(a) feeding in one or more gases to create said atmosphere in said chamber;

(b) permitting a surface of the at least one anode to reach equilibrium with said atmosphere; and then

(c) evacuating said atmosphere by creating a vacuum in said chamber, and optionally repeating said (a), (b), and (c) until said at least one anode is fully passivated.

9. The method of any preceding or following embodiment/feature/aspect, wherein said feeding occurs until the chamber reaches a pressure of at least about one Torr of said atmosphere.

10. The method of any preceding or following embodiment/feature/aspect, wherein said feeding occurs until the chamber reaches at least about five Torr of said atmosphere.

11. The method, of any preceding or following embodiment/feature/aspect, wherein said passivating comprises

(a) feeding in one or more gases to create said atmosphere in said chamber, wherein said feeding occurs until the chamber reaches pressure of at least about one Torr of said atmosphere;

(b) permitting a surface of the at least one anode to reach equilibrium with said atmosphere; and then (c) evacuating said atmosphere by creating a vacuum in said chamber, and repeating said (a), (b), and (c), wherein for each repeating of (a), (b), and (c), the pressure in Torr of said atmosphere is increased to a higher pressure in Torr than the most previous pressure used and said repeating continues until said at least one anode is fully passivated.

12. A method to prevent embrittlement of at least one metal wire embedded in a sintered anode, comprising passivating said at least one sintered anode in a chamber with an atmosphere that comprises oxygen and at least one inert gas to form a passivated sintered anode, wherein said oxygen is present in an amount of from about 0.5 wt% to about 25 wt%, based on weight of said atmosphere (or from about 0.5% to about 25% by volume), and wherein said atmosphere has a relative humidity of below 5%, and wherein said sintered anode is a tantalum anode, niobium anode, or a niobium suboxide anode, and wherein said passivated sintered anode has an 0[ppm]/H[ppm] ratio of 50 or more and/or said metal wire has a hydrogen content, after passivating, such that the metal wire is capable of being flexed back and forth over 10 times without breaking.

[0058] The present invention can include any combination of these various features or embodiments above and/or below as set forth in sentences and/or paragraphs. Any combination of disclosed features herein is considered part of the present invention and no limitation is intended with respect to combinable features.

[0059] Applicants specifically incorporate the entire contents of all cited references in this disclosure. Further, when an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range.

[0060] Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the present specification and practice of the present invention disclosed herein. It is intended that the present specification and examples be considered as exemplary only with a true scope and spirit of the invention being indicated by the following claims and equivalents thereof.

Claims

2. The method of claim 1, wherein said inert gas comprises nitrogen or argon or both.

3. The method of claim 1, wherein said passivating occurs at a temperature of from about 15° C to about 95° C.

4. The method of claim 1, wherein said passivating occurs at a temperature of from about 15° C to about 80° C.

5. The method of claim 1, wherein said passivating occurs for at least about 1 minute.

6. The method of claim 1, wherein said passivating occurs for from about 2 minutes to about 20 hours.

7. The method of claim 1, wherein said passivating comprises feeding in one or more gases in said chamber to create said atmosphere.

8. The method of claim 1, wherein said passivating comprises

(a) feeding in one or more gases to create said atmosphere in said chamber; (b) permitting a surface of the at least one anode to reach equilibrium with said atmosphere; and then

9. The method of claim 8, wherein said feeding occurs until the chamber reaches a pressure of at least about one Torr of said atmosphere.

10. The method of claim 8, wherein said feeding occurs until the chamber reaches at least about five Torr of said atmosphere.

1 1. The method of claim 1, wherein said passivating comprises

(c) evacuating said atmosphere by creating a vacuum in said chamber, and repeating said (a), (b), and (c), wherein for each repeating of (a), (b), and (c), the pressure in Torr of said atmosphere is increased to a higher pressure in Torr than the most previous pressure used and said repeating continues until said at least one anode is fully passivated.

H

12. The method of claim 1, wherein said chamber is a furnace.

13. The method of claim 1, wherein said chamber is a furnace also used to sinter the anode to said sintered anode.

14. The method of claim 1, wherein said sintered anode is formed by sintering under vacuum and under elevated temperature, a pressed anode in a furnace to form said sintered anode, and while said sintered anode is present in said furnace, cooling the furnace to a temperature under 100° C, wherein said cooling occurs under dynamic vacuum, and then conducting said passivating.

15. A method to prevent embrittlement of at least one metal wire embedded in a sintered anode, comprising passivating said at least one sintered anode in a chamber with an atmosphere that comprises oxygen and at least one inert gas to form a passivated sintered anode, wherein said oxygen is present in an amount of from about 0.5 wt% to about 25 wt%, based on weight of said atmosphere (or from about 0.5% to about 25% by volume), and wherein said atmosphere has a relative humidity of below 5%, and wherein said sintered anode is a tantalum anode, niobium anode, or a niobium suboxide anode, and wherein said passivated sintered anode has an 0[ppm]/H[ppm] ratio of 50 or more and/or said metal wire has a hydrogen content, after passivating, such that the metal wire is capable of being flexed back and forth over 10 times without breaking.