Nitrided valve metals and processes for making the same

This application is a divisional of U.S. patent application Ser. No. 09/795,947 filed Feb. 28, 2001 now U.S. Pat. No. 6,679,934, which in turn claims the benefit under 35 U.S.C. § 119(e) of prior U.S. Provisional Patent Application No. 60/185,979 filed Mar. 1, 2000, which is incorporated in its entirety by reference herein.

The present invention relates to valve metals and processes of preparing valve metals of acceptable quality for use in such applications as capacitors and the like.

There is always a desire in the industry to improve on the properties of valve metals, such as tantalum, particularly in the areas of capacitance and DC leakage achieved by valve metals when formed into capacitor anodes. The manner in which the various valve metal properties are improved upon include modifying various steps of the processes involved in making the valve metal, including purifying the valve metal.

There are general processes known to those skilled in the art for preparing valve metals. For instance, tantalum is obtained from ore and subsequently crushed into a powder. The tantalum is then separated from the crushed ore through the use of an acid solution and density separation of the acid solution containing the tantalum from the acid solution containing niobium and other impurities. The acid solution containing the tantalum is then crystallized into a salt and this tantalum containing salt is then reacted with pure sodium in order to reduce the tantalum containing salts to tantalum and to form salts with the non-tantalum containing elements. Water washing is then used to remove the salts and recover the tantalum which is subsequently subjected to one or more acid Teachings to remove chemical impurities. The tantalum is then dried resulting in what is known as a basic lot powder. Typically, this basic lot(s) is subjected to a heat treatment or thermal agglomeration step and then passivated to obtain a powder cake that is subsequently ground up into a powder. A deoxidation step using oxygen getters, such as magnesium, is then performed. After the deoxidation step, the tantalum powder is generally subjected to an acid wash and dried. The powder is then pressed into a pellet and sintered for subsequent processing by capacitor anode manufacturers.

There is an interest by those skilled in the art to nitride valve metals, such as tantalum, with the belief that such nitriding can decrease the DC leakage in capacitor anodes formed from such nitrided tantalum. The current nitriding techniques primarily involve the use of nitrogen containing compounds or nitrogen gas during the deoxidation stage. This method has several disadvantages including the lack of uniform distribution of the nitrogen in the valve metal.

Accordingly, there is a desire to improve on the methods of nitriding valve metals as well as improving on the resulting product.

A feature of the present invention is to provide processes to nitride valve metals, such as tantalum.

Another feature of the present invention is to provide a process which provides a more uniform distribution of the nitrogen throughout the valve metal.

A further feature of the present invention is to provide nitrided valve metals, such as tantalum.

An additional feature of the present invention is to provide a nitrided valve metal, such as tantalum, having high capacitance capability along with excellent flow properties and/or Scott density.

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.

To achieve these and other advantages, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention relates to a process of preparing a nitrided valve metal which involves the step of nitriding a valve metal powder at a sufficient temperature and pressure prior to a deoxidation step. Preferably, the nitriding of the valve metal occurs during a heat treatment step but always prior to the deoxidation step.

The present invention further relates to nitrided valve metals wherein the nitrogen content is from about 1,500 ppm to about 4,000 ppm and is substantially and uniformly distributed throughout the valve metal. All ppm referenced herein are by weight.

The present invention also relates to a nitrided valve metal, such as tantalum, wherein at least a portion of the valve metal has pore sizes equal to or greater than 2 microns in size.

The present invention in addition relates to a nitrided tantalum having a capacitance of from about 40,000 CV/g to about 80,000 CV/g when formed into a capacitor anode and having a Scott Density of from about 25 to about 40 g/in 3 and/or a flow of from about 70 to about 300 mg/s.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide a further explanation of the present invention, as claimed.

The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate several embodiments of the present invention and together with the description serve to explain the principles of the present invention.

FIGS. 1 and 2 are graphs depicting the temperature and pressure conditions during a heat treatment, wherein tantalum powder was nitrided.

FIGS. 3 and 4 are graphs depicting pore size diameter of nitrided tantalum vs. Log Differential Intrusion (mL/g) and pore size diameter vs. Cumulative Pore Volume (mL/g).

The present invention relates to nitrided valve metals and processes for making nitrided valve metals.

For purposes of the present invention, valve metals include, but are not limited to, tantalum and niobium. Preferably, the valve metal is tantalum or niobium, and more preferably is tantalum.

The valve metal that is nitrided can be in any form and is preferably in the form of a powder. The powder can be any shape, such as flaked, nodular, or combinations thereof, and the like. Further, the powder can have any surface area and particle size and/or agglomerate size. For instance, the powder can have a BET surface area of from about 0.5 to about 10 m 2 /g and more preferably from about 1 to about 3 m 2 /g.

Furthermore, when the powder, such as tantalum, is formed into a capacitor anode, the capacitor preferably has a capacitance of from about 20,000 to about 80,000 CV/g, and more preferably from about 26,000 to about 64,000 CV/g when formed at 30 volts and a press density of 5.5 g/cc.

Preferably, the valve metal powder, especially tantalum, has a flow of from about 70 to about 300, and more preferably from about 80 to about 200 mg/s, as determined by the flow test recited in WO 99/61184, incorporated in its entirety by reference herein. Also, or alternatively, preferably the valve metal powder, especially tantalum, has a Scott Density of from about 25 to about 40, and more preferably, from about 26 to about 32 g/in 3 .

The amount of nitrogen present in the valve metal powder can be any amount suitable for such applications as capacitor anodes. Preferably, the nitrogen content is from about 1,500 ppm to about 4,000 ppm. The inventor has found that amounts generally greater than about 4,000 ppm do not lead to any additional improvement in properties compared to lower amounts of nitrogen. Similarly, amounts lower than about 1,500 ppm nitrogen generally do not accomplish the desired improvements sought with nitrogen doping. Accordingly, the range of from about 1,500 ppm to about 4,000 ppm is beneficial, and more preferably the range of from about 2,000 ppm to about 3,000 ppm nitrogen.

The valve metal can also have a phosphorus content, such as from about 50 ppm to about 100 ppm. The valve metal can also have an oxygen content, such as from about 1,500 ppm to about 3,500 ppm.

In addition, the valve metal powder, preferably tantalum, has at least a portion of the pores with a pore size of 2 microns or greater leads to beneficial properties once a capacitor anode is made and impregnated, since such pore sizes permit the more uniform and complete impregnation with the counter electrode liquid.

As stated earlier, the process of nitriding the valve metal preferably begins with the basic lot powder of the valve metal. The following discussion uses tantalum as an example but it must be realized that the following discussion is applicable to any valve metal and the process conditions, such as temperature and time of heat treatment, varies depending upon the type of valve metal involved.

In the present process, the basic lot powder can be obtained by any processing techniques known to those skilled in the art. Such general processing techniques have been discussed above in the background of the invention and typically involve recovering the ore, crushing the ore, conducting solvent extraction to recover the tantalum in a salt form, and reducing the tantalum salt with sodium in order to form tantalum and other salts. These other salts are generally removed by water washing, and acid leaching can then be conducted to remove chemical impurities. The tantalum is then dried to obtain the basic lot. These above steps can be modified in any manner or alternative steps can be used in order to achieve what is known to those skilled in the art as a basic lot powder.

Once the basic lot is obtained, it is preferred (though, optional) for purposes of the present invention to conduct further processing of the tantalum powder or other valve metal prior to heat treatment. One preferred processing step before heat treatment is conducting water agglomeration as described in PCT Publication No. WO 99/61184 published Dec. 2, 1999, incorporated in its entirety herein by reference. The tantalum powder in the form of a powder or powder cake, with or without the prior water agglomeration or other agglomeration procedures, can then be subjected to a heat treatment wherein the nitriding of the tantalum powder occurs.

For purposes of the present invention it is understood that heat treatment and thermal agglomeration refer to the same processing step which involves subjecting the tantalum powder or other valve metal powder to elevated temperatures generally in a furnace and generally under vacuum.

Preferably, in the heat treatment step and with reference to tantalum powder, if significant levels of hydrogen are present in the tantalum powder, such as on the order of greater than about 200 or 300 ppm hydrogen, then a degassing step is preferably conducted in order to substantially remove the hydrogen present. Typically, the degassing step will remove enough hydrogen from the tantalum powder so that after the degassing step, the amount of hydrogen present in the tantalum powder is below 50 ppm hydrogen and preferably below 40 ppm hydrogen gas.

Once the tantalum powder has been degassed (if needed), the heat treatment or thermal agglomeration of the tantalum powder can occur. With respect to tantalum, typically the sintering phase of the heat treatment occurs at a temperature of from about 1100° C. to about 1500° C., which is the approximate average temperature reached on the surface of the powder in the furnace. Generally, the heat treatment at this temperature range occurs for about 10 minutes to about 2 hours, although other times can be used depending upon the desired properties. For purposes of the present invention, the heat treatment includes the time necessary to reach the sintering phase which generally causes some loss of surface area of the powder. Furthermore, the heat treatment typically occurs under vacuum and preferably at about 0.01 Torr or less. The nitriding of the tantalum powder occurs during heat treatment and prior to the deoxidation step.

The nitriding of the tantalum powder or other valve metal preferably occurs at a temperature of about 600° C. or lower, and more preferably from about 250° C. to about 600° C., and more preferably from about 300° C. to about 400° C. Since the nitriding is preferably occurring at temperatures not above 600° C., it is preferred to conduct the nitriding where these temperatures are achievable, meaning, nitriding right before the sintering phase of heat treatment occurring at 1250° C.–1500° C. or after this high temperature phase has occurred. In the present invention, the nitriding of the metal powder preferably occurs once the temperature of the metal powder has reached some stability and is not fluctuating significantly (e.g., not fluctuating by more than 50° C. or so). Further, preferably the nitriding begins at a temperature of about 200°–300° C. to avoid the absorption of nitrogen in one location of the powder. Preferably, nitriding is started at a low temperature, like 200° C.–300° C., and then the temperature is ramped up at approximately 1° C. to about 10° C. per minute. Thus, a steady increase in temperature ensures that nitrogen absorption is uniform throughout the powder and further ensures that the temperature is substantially uniform for absorption, which can vary depending on powder and surface area, is reached for a sufficient time to permit absorption.

The nitriding of the tantalum powder can be accomplished with the use of a nitriding agent, such as nitrogen gas or a nitrogen containing gas or with nitrogen containing or generating compounds (e.g, TaN). Preferably, a nitrogen gas is used in this process. Generally, once the tantalum powder is at the preferred temperature range of from about 250° C. to about 600° C., the nitriding process is preferably started. In the preferred embodiment, the nitrogen gas is introduced into the furnace under vacuum and the amount of nitrogen gas introduced is dependent upon the amount of nitrogen desired in the resulting nitrided powder and the amount of basic lot powder present in the furnace. In the Examples, it can be seen that various amounts of nitrogen were introduced in order to achieve various nitrogen contents in the powder. In view of the present invention, one skilled in the art can readily determine the amount of nitrogen to be introduced into the furnace for a desired amount of nitrogen in the final metal powder.

It is preferred that the nitrogen gas or other nitrogen generating techniques do not occur at temperatures above 600° C. because the combination of nitrogen with tantalum is an exothermic reaction which generates heat and leads to an autocatalytic process which can be uncontrollable. This reaction leads to a non-uniformed distribution of the nitrogen in the basic lot powder.

Generally, during the nitriding process, the nitrogen is introduced into the furnace and is readily absorbed by the basic lot powder present in the furnace. Once the nitrogen gas has been absorbed, or prior to this step, the basic lot powder is subjected to the higher heat treatment phase or sintering phase of heat treatment described earlier. Once the desired heat treatment at high temperatures and the desired nitriding of the basic lot powder has occurred, the temperature is preferably substantially reduced in order to passivate the powder. Generally, the powder is in the form of a powder cake which is subsequently subjected to grinding. The powder can then be subjected to a deoxidation process. Any conventional deoxidation process can be used, such as with magnesium or any other oxygen getters. Once the deoxidation step and subsequent acid leaching is complete, the powder can be further processed in conventional manners, such as by pressing into pellets and sintering at desired temperatures which is dependent upon the desired capacitance and the type of valve metal being sintered. The sintered pellets can then be used as capacitor anodes using standard techniques known in the industry such as those set forth in U.S. Pat. Nos. 4,805,074; 5,412,533; 5,211,741; and 5,245,514, and European Patent Application Nos. 0 634 762 A1, incorporated in their entirety herein by reference.

The nitriding of the valve metal during the heat treatment step is beneficial compared to other nitriding methods which typically occur during the deoxidation step. By nitriding during the heat treatment stage, a more uniform distribution of the nitrogen throughout the entire valve metal powder is achieved. One reason this occurs may be due to the fact that the nitriding occurs at an early stage of the metal processing and thereafter there are many other stages which involve subjecting the valve metal to high temperatures. These additional stages assist in uniformly distributing the nitrogen. Thus, the earlier the nitriding can occur, the more uniform distribution of the nitrogen throughout the valve metal can be achieved. In the examples, as can be seen, uniform nitrogen distribution was accomplished.

It is certainly possible and within the bounds of the present invention to conduct more than one nitriding step in order to achieve any amount of nitrogen content desired in the final product.

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