Method for producing metal powders

Abstract

A method of producing metallic powder for use in the manufacture of a capacitor comprises the step of reducing a non-metallic compound to metal in contact with a molten salt. The salt comprises, for at least a portion of the process, a dopant element that acts as a sinter retardant in the metal. In preferred examples, the metallic powder is Ta or Nb powder produced by the reduction of a Ta or Nb oxide and the dopant is boron, nitrogen, or phosphorous.

Claims

1. A method of producing metallic powder or sponge for use in the manufacture of a capacitor comprising the steps of: selecting a non-metallic compound capable of reduction to the metallic powder or sponge, and reducing the non-metallic compound to metal in contact with a molten salt, wherein the non-metallic compound is arranged to be in contact with a cathode in an electrolysis cell, the cell further comprising an anode and the molten salt, wherein a sufficient potential is applied between the anode and the cathode to reduce the non-metallic compound to metal; and wherein the non-metallic compound comprises oxygen and the reaction occurs by electro-deoxidation, wherein, before reduction of the selected non-metallic compound, the selected non-metallic compound comprises between 50 ppm and 10000 ppm of a dopant that acts as a sinter retardant in the metal.

2. The method according to claim 1, wherein the selected non-metallic compound is doped with between 50 ppm and 5000 ppm of the sinter retardant dopant prior to reduction to the metallic powder or sponge.

3. The method according to claim 1, wherein the selected non-metallic compound is doped by reaction with a solid, liquid or gas comprising the dopant, such that the dopant diffuses into the compound.

4. The method according to claim 1, wherein the dopant is boron, phosphorous, or nitrogen.

5. A method of producing metallic niobium or tantalum powder for use in the manufacture of a capacitor comprising the steps of: selecting a non-metallic niobium or tantalum compound capable of reduction to metallic niobium or tantalum, and reducing the non-metallic niobium or tantalum compound to metallic niobium or tantalum powder in contact with a molten salt, wherein the non-metallic niobium or tantalum compound is arranged to be in contact with a cathode in an electrolysis cell, the electrolysis cell further comprising an anode and the molten salt, wherein a sufficient potential is applied between the anode and the cathode to reduce the non-metallic niobium or tantalum compound to metallic niobium or tantalum powder, wherein, prior to reduction of the selected non-metallic niobium or tantalum compound, the selected non-metallic niobium or tantalum compound is doped with between 50 ppm and 5000 ppm of a dopant that acts as a sinter retardant in the metallic niobium or tantalum powder.

6. A method of producing metallic niobium or tantalum powder for use in the manufacture of a capacitor comprising the steps of: selecting a niobium or tantalum oxide capable of reduction to metallic niobium or tantalum, and reducing the niobium or tantalum oxide to metallic niobium or tantalum powder in contact with a molten salt, wherein the niobium or tantalum oxide is arranged to be in contact with a cathode in an electrolysis cell, the electrolysis cell further comprising an anode and the molten salt, wherein a sufficient potential is applied between the anode and the cathode to reduce the niobium or tantalum oxide to metallic niobium or tantalum powder, wherein, prior to reduction of the selected niobium or tantalum oxide, the selected niobium or tantalum oxide is doped with between 50 ppm and 5000 ppm of boron, phosphorous, or nitrogen, the boron, phosphorous, or nitrogen being a dopant that acts as a sinter retardant in the metallic niobium or tantalum powder.

Description

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

(1) Specific embodiments of the invention will now be described by way of examples, with reference to the accompanying drawing, in which;

(2) FIG. 1 illustrates an electro-reduction apparatus for performing a method according to an aspect of the invention.

EXAMPLE 1

(3) A first example describes the production of boron-doped Ta powder by direct electro-deoxidation of Ta.sub.2O.sub.5. A schematic of the apparatus used in the method is illustrated in FIG. 1.

(4) The apparatus used comprised a container or crucible 10 filled with a salt 20. The salt was a calcium chloride salt. The salt further comprised about 50 ppm boron.

(5) Heating elements (not shown) acted to raise the temperature of the salt to above its melting point. A cathode 30 and a carbon anode 40 were then arranged in the molten salt and connected via a power supply 50.

(6) Ta.sub.2O.sub.5 powder was formed into precursor agglomerates by a process of pressing and sintering. These precursor agglomerates may alternatively be described as preforms, particles or pellets.

(7) The agglomerates of Ta.sub.2O.sub.5 (the precursor material) 35 were arranged in contact with the cathode 30. This contact was achieved by placing the agglomerates within a basket 31 that forms part of the cathode 30. Contact between agglomerates and cathode could be achieved by any other suitable means including, for example, placing the agglomerates on the surface of a plate that is part of, or in contact with, the cathode, or by threading agglomerates onto a rod or wire that is electrically coupled to the cathode.

(8) As part of a pre-processing protocol, the temperature of the salt was first raised to 400 C. for 16 hours to dry the salt and then further raised to 920 C. It is understood, however, that any suitable pre-processing protocol may be used to prepare a salt for use in a method according to the invention.

(9) Electro-deoxidation was performed on the Ta.sub.2O.sub.5 agglomerates 35 such that they were reduced to metallic Ta. This electro-deoxidation was performed according to the Cambridge FFC process (described in WO 99/64638). A voltage of 2.8 V was applied across the cell terminals (30, 40) for 56 hours to ensure the complete reduction of the oxide to Ta metal. The cathode 30 with its associated basket 31 containing reduced metal was then withdrawn from the molten salt under an argon blanket and allowed to cool.

(10) The cathode and reduced metal were rinsed in water to remove excess salt and then the metal was thoroughly washed first in hot deionised water (80 C.) and then in 1M HCL acid to remove the salt and calcium oxide. After drying, capacitor grade Ta powder was obtained. The resulting powder had a boron content of 250 ppm, a specific surface area of about 1 m.sup.2/g and a specific charge of about 5010.sup.3 F.Math.V/g (CV/g)

(11) The resulting powder could then be processed via a known route to produce a capacitor. A typical example of a capacitor production route is described above in the background to the invention.

EXAMPLE 2

(12) A second example describes the production of phosphorous-doped Ta powder by direct electro-deoxidation of Ta.sub.2O.sub.5.

(13) Ta.sub.2O.sub.5 pellets were produced by pressing Ta.sub.2O.sub.5 powder and then sintering, such that the pellets had sufficient strength to be handled and a porosity of about 60 to 65%. The sintering temperature and time required to achieve these criteria will vary, depending on the particular sinter characteristics of the batch of Ta.sub.2O.sub.5 powder being used.

(14) An apparatus substantially as described above in relation to Example 1 (and illustrated in FIG. 1) was used to reduce the pellets. The electro-deoxidation reaction was performed in a phosphorous-doped molten salt, however, rather than the boron-doped salt used for Example 1.

(15) A phosphorous-doped calcium chloride salt was produced by adding 1000 ppm by weight of calcium phosphate (Ca.sub.3(PO.sub.4).sub.2) to LP grade CaCl.sub.2. The level of calcium oxide (CaO) in the salt was adjusted to be about 4000 ppm by weight. The ratio of the mass of salt in the cell to mass of oxide being reduced was approximately 36:1.

(16) Before use, the doped calcium chloride salt underwent a pre-processing protocol to dry the salt as described above in relation to Example 1. Any suitable salt pre-processing protocol may be used to prepare the salt for use.

(17) An electro-deoxidation of the Ta.sub.2O.sub.5 pellets was performed by applying a voltage of about 3 V between the anode and cathode for a period of 24 hours, such that the Ta.sub.2O.sub.5 pellets were reduced to metallic Ta. The temperature of the salt was maintained at about 830 C. for the duration of the electro-deoxidation reaction.

(18) The cell was then cooled and the reduced metal removed and washed to remove excess salt. The resulting Ta metal was analysed and found to contain 484 ppm phosphorous (ICP-MS).

EXAMPLE 3

(19) A third example describes the production of phosphorous-doped Ta powder by direct electro-deoxidation of Ta.sub.2O.sub.5.

(20) The apparatus and electro-deoxidation method used in Example 3 were substantially the same as described for Example 2, with the one difference that the salt was doped with higher levels of phosphorous.

(21) The salt for use in this third example was produced by adding about 3000 ppm by weight of calcium phosphate to LP grade calcium chloride salt. The total CaO content of the salt was adjusted to be about 4000 ppm by weight. The ratio of the mass of salt in the cell to mass of oxide being reduced was approximately 36:1.

(22) Ta.sub.2O.sub.5 pellets were reduced at a potential of about 3V for a period of 24 hours. The resulting Ta metal was analysed and found to have a phosphorous content of 1400 ppm.

EXAMPLE 4

(23) A fourth example describes the production of nitrogen-doped Ta powder by direct electro-deoxidation of nitrogen-doped Ta.sub.2O.sub.5. The apparatus used was substantially as described above in relation to Example 1. The molten salt used was a CaCl.sub.2 salt having 0.4 wt % CaO.

(24) Ta.sub.2O.sub.5 powder having a nitrogen content of 783 ppm (as measured by an Eltra ON900 nitrogen analyser) was selected, pressed into pellets and sintered. The pellets were placed in the apparatus and electro-deoxidation of the pellets was performed by applying a voltage of about 3 V between the anode and cathode for a period of 24 hours, such that the pellets were reduced to metallic Ta. The temperature of the salt was maintained at about 830 C. for the duration of the electro-deoxidation reaction. The resulting metal powder was analysed and found to contain 252 ppm of nitrogen.