Specialized metallurgical processes – compositions for use therei – Processes – Producing or purifying free metal powder or producing or...
Reexamination Certificate
1999-08-19
2001-02-27
Jenkins, Daniel J. (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or purifying free metal powder or producing or...
C075S369000, C075S245000
Reexamination Certificate
active
06193779
ABSTRACT:
The present invention relates to a tantalum powder, to pressed, sintered anodes obtainable from the tantalum powder, and to a process for producing tantalum powders.
Metallic tantalum powder is usually produced by the reduction of K
2
TaF
7
by means of sodium. The physical properties of tantalum powders, such as the grain size or specific surface for example, are controlled by the addition of inert salts such as KCl, NaCl, KF or NaF. As the proportion of inert salts is increased, the resulting tantalum powder becomes finer, i.e. the resulting metal surface increases. However, the throughput of tantalum metal in the reduction decreases, corresponding to the increasing inert salt concentration.
After the salts have been washed out, the tantalum powder is dried and is subjected to further purification by high-temperature treatment in a vacuum or in an inert gas atmosphere. In this agglomeration step, the specific surface is significantly reduced and the oxygen content of the powder is considerably increased. The oxygen content is reduced again by thermal treatments, by means of metals having a reducing action, particularly magnesium. Another consequence of this deoxidation step or steps is a slight reduction in the specific surface. In order to optimise the electrical properties of capacitors produced from these tantalum powders, the tantalum powders are treated with dopants containing phosphorus and/or boron.
The electrical properties of tantalum powders, such as their specific charge or their leakage current, are tested on a pressed, sintered anode which has subsequently been anodically oxidised, i.e. formed. The specific charge, expressed in &mgr;FV/g, is a measure of the capacitance of the capacitor, and is directly proportional to the metal surface. The leakage current, expressed in nA/&mgr;FV, is an indicator of how well a capacitor holds its charge.
In the sodium reduction of K
2
TaF
7
in the molten salt which is usually carried out industrially, capacitor powders with specific charges of 18,000 to 70,000 &mgr;FV/g are economically produced. To obtain tantalum powders of small primary particle size which are required for capacitors which have a high capacitance it is necessary to carry out the sodium reduction of K
2
TaF
7
at a very high level of dilution (diluent salts: KCl, KF, NaCl) which results in smaller agglomerates (secondary particle size 1 to 5 &mgr;m with a primary particle size of about 0.3 elm). The small dimensions of the agglomerates make it necessary to agglomerate this tantalum powder thermally (by pre-sintering), whereupon firstly unwanted impurities are removed and secondly the specific surface is reduced again. However, the favourable pore structure of the agglomerates for capacitor production, due to which capacitors with low leakage currents are obtained by forming, is only obtained by repeated pre-sintering with comminution of the agglomerates by removing them between sintering operations. Tantalum capacitor powders with very high capacitances are described in DE 195 36 013 A1. Specific charges up to 91,810 &mgr;FV/g are obtained in sintered anodes produced therefrom, with the thermal agglomeration step which is otherwise customary being omitted. These tantalum powders contain troublesome impurities, such as fluoride for example, in concentrations >100 ppm. Part of the high fluoride content is decomposed when the anodes are sintered. The fluorides which are thereby evolved result in thermal corrosion in sintering furnaces. The fluoride which remains in the tantalum anode is probably responsible for the considerably increased leakage currents. Thus a tantalum powder which has been produced according to Example 6 of DE 195 36 013 A1 has an F content of 460 ppm and an Mg content of 200 ppm. Moreover, these powders have the disadvantage that their powder density is too low and the fracture strength of anodes pressed from them is too low compared with conventional powders which are available. Therefore, these powders have still not attained industrial importance. Furthermore, these tantalum powders are burdened with the disadvantage that they also contain residual contents of alkali, which even if they are only within the ppm range, make the leakage currents considerably worse.
Very finely divided powders are produced by the gas phase reduction of TaCl
5
with hydrogen. What are essentially discrete powders which are no longer free-flowing are obtained in this process. Due to the difficulty of processing them industrially, these powders have not been used in capacitor technology.
The object of the present invention is to provide a tantalum powder which does not have the aforementioned disadvantages. A further object of the present invention is to provide an economic process for producing tantalum powders which exhibit very high capacitances.
It has now been found that very finely divided tantalum powders which are already pre-sintered (agglomerates) are obtained by igniting a mixture of tantalum pentachloride and magnesium hydride, and that these tantalum powders are outstandingly suitable for the production of high capacitance capacitors. Due to the rapidly proceeding reaction, which is complete after only a few minutes, very small primary particles are produced, even without the use of diluent salts, which have typical dimensions of 50 to 300 nm, which come into contact with each other on account of the high content of tantalum in the reaction mixture and which sinter within the time of reaction due to their high sintering activity. This results in a foam-like, partially sintered, open-pored cake from which outstandingly free-flowing agglomerates with average secondary particle sizes of 10 to 50 &mgr;m or more are obtained by crushing and sieving. After washing and drying, these agglomerates have very high BET specific surfaces of 1.5 to 10 m
2
/g.
Accordingly, the present invention relates to a process for producing tantalum powder by igniting a mixture of tantalum pentachloride and magnesium hydride in an inert gas atmosphere and subsequently washing the reaction product with mineral acids.
The reaction of a homogenised mixture of tantalum pentachloride and magnesium hydride is preferably conducted in a tantalum reduction vessel. A thin tantalum wire is preferably embedded as an igniter in the top layer of this homogenised mixture, and can be particularly elegantly connected by an externally entering lead wire to a source of low voltage, whereupon it can be brought to bright red heat. This ignitable arrangement is accommodated in a protective chamber flushed with argon. After this reaction product, which has been heated to white heat due to the strongly exothermic reaction and which exists in the form of a partially sintered porous cake, has cooled, it is coarsely broken up, washed with mineral acids and dried. A sulphuric acid/hydrogen peroxide solution is preferably used as the washing liquid. Washing is continued until the wash water is free from chloride.
The quantitative ratio of the starting materials for the reaction mixture is not critical, since both magnesium and hydrogen are available for reduction after they have been released from the magnesium hydride. Accordingly, 1.25 to 1.3 moles of magnesium hydride can be used per mole of pentachloride. However, due to the low boiling temperature of tantalum pentachloride and due to the low decomposition temperature of magnesium hydride, losses in yield are observed at low molar ratios of the starting materials. A molar ratio of tantalum pentachloride to magnesium hydride of 2 to 3 is therefore preferred. Yields of 80 to 90% with respect to the tantalum used are then obtained. It is anticipated that the yield can be increased further if the reaction is conducted under pressure, in an autoclave for example.
Magnesium hydride of formula MgH, is preferably used as the reducing agent, where x is greater than 1.2 and is most preferably greater than 1.6.
By means of the invention, tantalum powders which consist of sintered primary particles are produced for the first time, wherein the primary parti
Rawohl Christine
Reichert Karlheinz
Wolf Rudiger
Eyl Diderico van
Gil Joseph C.
H. C. Starck GmbH & Co. KG
Jenkins Daniel J.
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