WO2008041007A1 - Procédé et appareil pour produire des poudres métalliques - Google Patents

Procédé et appareil pour produire des poudres métalliques Download PDF

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WO2008041007A1
WO2008041007A1 PCT/GB2007/003805 GB2007003805W WO2008041007A1 WO 2008041007 A1 WO2008041007 A1 WO 2008041007A1 GB 2007003805 W GB2007003805 W GB 2007003805W WO 2008041007 A1 WO2008041007 A1 WO 2008041007A1
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metal
temperature
aggregate
precursor material
alloy
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PCT/GB2007/003805
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English (en)
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Stewart Male
David Hodgson
David Jackson
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Metalysis Limited
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B34/00—Obtaining refractory metals
    • C22B34/20—Obtaining niobium, tantalum or vanadium
    • C22B34/24—Obtaining niobium or tantalum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F9/00—Making metallic powder or suspensions thereof
    • B22F9/16—Making metallic powder or suspensions thereof using chemical processes
    • B22F9/18—Making metallic powder or suspensions thereof using chemical processes with reduction of metal compounds
    • B22F9/20—Making metallic powder or suspensions thereof using chemical processes with reduction of metal compounds starting from solid metal compounds
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/34—Electrolytic production, recovery or refining of metals by electrolysis of melts of metals not provided for in groups C25C3/02 - C25C3/32
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C5/00—Electrolytic production, recovery or refining of metal powders or porous metal masses
    • C25C5/04—Electrolytic production, recovery or refining of metal powders or porous metal masses from melts
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10—Processes characterised by the sequence of their steps
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2999/00—Aspects linked to processes or compositions used in powder metallurgy
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B34/00—Obtaining refractory metals
    • C22B34/10—Obtaining titanium, zirconium or hafnium
    • C22B34/12—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08
    • C22B34/129—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08 obtaining metallic titanium from titanium compounds by dissociation, e.g. thermic dissociation of titanium tetraiodide, or by electrolysis or with the use of an electric arc
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B4/00—Electrothermal treatment of ores or metallurgical products for obtaining metals or alloys
    • C22B4/06—Alloys

Definitions

  • the present invention relates to a method and an apparatus for the production of powders, including aggregates, or aggregated powders, of tantalum, niobium and their alloys, and powders produced thereby.
  • a tantalum capacitor typically has small size and high capacitance, and has become a vital component in cellular phones and personal computers.
  • a tantalum capacitor typically has small size and high capacitance, and has become a vital component in cellular phones and personal computers.
  • Tantalum metal capacitor anodes are normally manufactured by compressing aggregated tantalum powder to a density value of less than half of the metal's true density (typically between 5 and 7.5g.cr ⁇ 3 compared with the true density of Ta, which is 16.6g.cm "3 ), with an embedded anode lead wire, to form a porous pellet.
  • the pellet is then sintered to form a porous body (i.e. an anode or anode body), and the porous body is anodised by impregnation with a suitable electrolyte to form a continuous dielectric oxide film on the tantalum surface.
  • the anodized porous body is then impregnated with a cathode material to form a uniform cathode coating, connected to a cathode lead wire and encapsulated with a resin casing.
  • the porous body must contain open, preferably uniform, pores in order to allow impregnation for the steps of anodizing and impregnation to form the dielectric film and the cathode.
  • Increasing the surface area (whilst maintaining an open, porous structure) of the aggregated tantalum powder used to form the anode may advantageously increase the capacitance of a tantalum capacitor.
  • the oxygen content of the powder and the body has to be maintained at or below 4000 ppm to ensure that a high-quality amorphous oxide film is produced during the anodization process.
  • High oxygen levels in the tantalum may lead to the production of crystalline defects in the anodic oxide film, which gives high leakage currents and poor component reliability.
  • the desirability of producing Ta metal powder of reduced primary particle size and lower oxygen content leads to significant technical difficulties and high costs in conventional production methods.
  • the standard preparation route for tantalum powder is by reducing potassium tantalum fluoride (K 2 TaF 7 ) with sodium ( e.g. Ref. [1-4]) and then water washing and acid leaching the product to remove the process salts.
  • the powder is then dried and is known at this stage as primary or raw tantalum powder.
  • This route usually produces powders with average particle size ranging from 0.2 to 3.0 ⁇ m and surface areas of the order 1 m 2 /g. At this stage the oxygen content of the powder can be in excess of 7000 ppm.
  • the raw tantalum powder is subjected to vacuum heat treatment at temperatures up to 1200C.
  • This process thermally agglomerates the fine powder, and to reduce its particle size the resulting agglomerate must then be crushed; this produces a granulated material with an average particle size in the region of 10-100 ⁇ m.
  • the steps of vacuum heat treatment and thermal agglomeration are necessary to reduce the impurity content and improve the handling and sintering characteristics of the Ta powder but has the significant disadvantage that the oxygen content is increased considerably; levels as high as 12000 ppm may be reached.
  • the aggregated tantalum powder must therefore undergo further processing in order to reduce the oxygen levels.
  • the powders are subjected to deoxidation with magnesium metal followed by acid leaching to remove the oxidation products (i.e. MgO) and further drying and classification stages.
  • Powders produced by the standard potassium tantalum fluoride method which are thermally-aggregated and deoxidized, usually have a BET (Brunauer-Emmett-Teller) specific surface area of approximately 1 m 2 /g (mean primary particle size on the basis of specific surface area of around 400nm) and a specific capacitance of approximately 50,000 CV/g.
  • BET Brunauer-Emmett-Teller
  • the invention provides an agglomerated metal or alloy powder, a capacitor made using the powder, methods for producing the agglomerated metal or alloy powder and the capacitor, and corresponding apparatus, as defined in the appended independent claims, to which reference should now be made. Preferred or advantageous features of the invention are set out in independent subclaims.
  • the invention may thus provide a process involving sintering followed by solid-state electro-decomposition or electro-reduction of suitable metal compounds under predetermined conditions in order to fabricate an agglomerated metal or alloy powder, and preferably such a powder suitable for use in the manufacture of electrolytic capacitors.
  • manufacturers of capacitors conventionally use as a raw material an agglomerated Ta powder in which Ta particles of approximately 0.2 to 3 ⁇ m have been thermally agglomerated into an agglomerate, or granular material, of particle size in the region of 10-100 ⁇ m.
  • This material provides the desired combination of high surface area (approximately 1 m 2 /g in conventional Ta agglomerates) and sufficiently large particle size for easy handling of the material.
  • conventional methods for fabricating such materials are complex and expensive.
  • the oxygen content of these materials needs to be reduced, after agglomeration, to a level low enough for the satisfactory formation of an anodized dielectric layer on the anode body surface.
  • the method of the invention may advantageously enable the direct formation of agglomerated powders, for example suitable for use by capacitor manufacturers, with the added advantage of low oxygen content.
  • the invention may allow the production of suitable agglomerates in a wide range of materials, specifically in Ta, Nb, or alloys comprising these metals, and not just of Ta. This may advantageously allow the fabrication of capacitors from these other materials, and possible improved capacitor performance.
  • a starting material in powdered form comprising a compound of the metal or a mixture of compounds of the component metals in the alloy, in the proportions required to form the alloy.
  • the starting material is formed into a precursor material suitable for electro- reduction by a method comprising the step of sintering the powder at a first temperature.
  • the metal compounds may be metal oxides, but other compounds suitable for sintering followed by electro-reduction may also be used.
  • the sintered powder is then cathodically connected in an electrochemical cell, in which a fused salt is contacted with a cathode comprising the sintered powder, and with an anode, such that the powder is electrochemically reduced to the metal, or to an alloy of the component metals.
  • the sintered powder during the electro-reduction is at a second temperature lower than the first temperature; this may be achieved by controlling the temperature of the fused salt, particularly in the vicinity of the cathode, to be at the second temperature.
  • the electrochemical reduction is carried out as described in the prior art, for example using the FFC method [7].
  • the metal compound or compounds are cathodically connected in a fused salt electrolyte such that the non-metal component(s) of the compound or compounds (e.g. oxygen in a metal oxide) dissolve in the melt and are transported to the anode.
  • the cathode potential is maintained below a potential for deposition of metal from the cations in the fused salt electrolyte.
  • a commonly-used electrolyte is calcium chloride, which may contain some calcium oxide in solution if a metal oxide is to be reduced at the cathode.
  • any suitable salt or mixture of salts may be used; for example an electrolyte providing a desired electro-reduction temperature may advantageously be selected in order to reduce or avoid sintering of the metal or alloy product.
  • an electrolyte providing an operating temperature of less than two thirds of the melting point of the metal or alloy may be selected.
  • the sintering conditions for the precursor material are preferably such that the sintered precursor material has a predetermined porosity preferably between 40% and 60%, or between 40% and 75%, porosity.
  • the second temperature is then such that any sintering of the metal or alloy during electro-reduction is negligible or is sufficiently limited that the metal or alloy product forms, or can be broken up (preferably by a low- energy process) to form, an aggregated powder which is substantially of a predetermined particle size or within a predetermined range of particle sizes.
  • the first temperature should thus be sufficient to enable sintering of the precursor material, optionally under pressure, and the second temperature may preferably be less than about two-thirds of the melting temperature of the metal or alloy product.
  • metal should be taken to refer to both metals and alloys
  • metal compound or precursor material should be taken to refer to both metal compounds and mixtures of metal compounds for use as precursor materials.
  • the metal compound before sintering is in powdered form, preferably of particle size less than 10 ⁇ m, or less than 3 ⁇ m, and particularly preferably about 1 ⁇ m or less.
  • the metal compound may then be sintered to form a precursor material of mechanical strength sufficient for the electro- reduction process but of sufficient porosity to allow ingress of the fused salt, or electrolyte, during electro-reduction.
  • sintering should be carried out with as little shrinkage of the precursor material as possible, to maximise porosity of the precursor material while maintaining adequate mechanical strength.
  • the sintering conditions may advantageously be chosen such that the volume shrinkage on sintering is less than 20%, preferably less than 15% and particularly preferably less than 10%. This may advantageously produce a precursor material having a porosity of between 40% and 60%, or between 40% and 75%.
  • the metal product may comprise a powder which is an agglomerate, or granular powder, of a smaller particle size.
  • the product may comprise an agglomerate of particle size 10 to 300 ⁇ m, or more preferably 20 to 200 ⁇ m.
  • Each such particle is advantageously a porous agglomerate of smaller metal particles, preferably of less than 5 ⁇ m, or 3 ⁇ m, and particularly preferably of 1 ⁇ m or less (this may be referred to as the primary particle size of the agglomerate).
  • any real material particle sizes vary within a range rather than being of one constant particle size and particle dimensions described herein should be interpreted accordingly. Where a single figure is given, this indicates a mean particle size of a particle size distribution, and where a range of particle sizes is given, the range indicates the size of the most significant portion of the particle size distribution; some particles may nevertheless fall outside the range specified.
  • the agglomerate advantageously flows, suitably for pouring into dies for fabrication of, for example, anode bodies, and the porosity of the agglomerate particles is preferably connected.
  • the level of porosity, and the surface area of the agglomerate particles, is preferably high.
  • the metal compound powder used to form the precursor material should be of small particle size, and advantageously of smaller particle size than the metal particle size in the product. For example it has been found that electro-reduction of a Ta oxide powder of about 300nm particle size forms a metal particle size of about 1 ⁇ m. Further, it has been found that sintering the Ta oxide powder to a volume shrinkage of only about 15% or 10% or less provides a precursor material with high connected porosity and adequate mechanical strength for the electro-reduction process, but from which the metal product is easily broken up into an agglomerate of 20 to 200 ⁇ m; the agglomerate particles are formed of a porous agglomeration of the 1 ⁇ m metal particles.
  • the metal compound particle size should be less than the desired metal particle size, for example by a factor of about one half or one third.
  • the metal compound particle size should therefore preferably be less than about 0.5 ⁇ m, preferably less than about 0.4 ⁇ m, and particularly preferably less than about 0.3 ⁇ m.
  • the cathode or the melt may advantageously be agitated.
  • the melt may be agitated by sparging.
  • the metal product should preferably be broken up by a low-energy process. Some breaking up may occur simply on washing the fused salt from the product after electro-reduction. A further suitable low- energy process would be mixing in a high-shear mixer.
  • the inventors have found that using low-energy processes tends to produce an agglomerate of more uniform particle size (agglomerate size) than using high- energy processes such as crushing, and may therefore reduce wastage if a particular particle size range is required for further fabrication.
  • the electro-reduction process used in the invention is preferably based on the Fray, Farthing and Chen (FFC) process [6, 7] where the reduction is performed electrochemically and therefore the rate can be controlled by the imposed voltage and the reduction can be completed in one step.
  • FFC Fray, Farthing and Chen
  • the inventors have found, particularly using Ta oxide precursors, that the oxygen levels can be reduced to 2000-3000 ppm in a single pass and surprisingly that the form of the starting oxide porous pellet determines the final tantalum metal particle structure. This is surprising as it is known [8] that the reduction pathways (for electro-reduction in electrolytes containing calcium, such as CaCI 2 -based electrolytes) always include mixed calcium metal oxides which have distinct morphological differences from that of tantalum metal.
  • the invention may allow production of a high- surface-area, low-oxygen Ta powder with higher efficiency than by the conventional chemical route. Specifically, it may produce an agglomerated tantalum powder which has a primary particle diameter of 0.1 to 2.0 ⁇ m, has a low oxygen content, and can be used in electrolytic capacitors.
  • the oxygen levels within the powder may be controlled by using a one-step deoxidation process to minimise the number and type of chemical and temperature excursions that the tantalum powder undergoes. In particular, this avoids the post-reduction processes that are essential in the prior art, such as thermal agglomeration, which increase the oxygen content and therefore require further deoxidation steps.
  • the invention may also avoid separate processes that, in the prior art, must be carefully controlled to prevent reduction in specific surface area.
  • the precursor material should be sintered sufficiently to achieve adequate mechanical strength and high, connected porosity. It is the inventors' understanding that this may advantageously be achieved as follows.
  • the material should be sintered to no further than approximately the boundary between the initial and intermediate stages.
  • this corresponds to a sintering temperature of between 1100 and 1300C 1 or 1000 and 1300C 1 a sintering time of no longer than 12 hours and a pre-sintering density of the tantalum oxide pellet of between 0.3 and 0.45, or between 0.2 and 0.45, of the theoretical density.
  • the final values can however be influenced by particle size and impurity level.
  • a preferred embodiment illustrates the object of simultaneously achieving an agglomerated and high surface area tantalum powder with low oxygen levels suitable for use as a feedstock for capacitor anodes. This is achieved by the direct electrochemical reduction of Ta oxide (Ta 2 O 5 ).
  • the tantalum oxide starting material is first made into the form of a porous solid.
  • Ultrafine particle size tantalum oxide is readily available, for example the inventors have used a powder with a primary particle size of 200-500nm or 100-500nm (particle nomenclature follows that of Ref [10]).
  • This powder is formed into a pellet with a density of between 3 and 5g cm “3 (40-60%, or 40-75%, porosity) and then sintered at temperature up to 1300C for twelve hours (at atmospheric pressure). Either longer sintering times or sintering under vacuum can adjust the primary particle size of the sintered oxide compact.
  • the compact has an open porosity of much the same structure as that required in the tantalum capacitor.
  • this may be achieved before reduction rather than after reduction. Aspects of this structure may advantageously be retained during electro-reduction and, consequently, the tantalum powder so produced does not need to be thermally agglomerated and so there may be no need for additional de-oxidation processes as used in the prior art.
  • FIG 1 is a schematic illustration of a suitable electrochemical cell for the electro-reduction, on a laboratory scale.
  • the cell comprises a crucible 2 containing the fused salt electrolyte 4 of CaCI 2 containing about 1 mol% CaO.
  • the anode 6 (which may be a carbon rod or plate) and the cathode, comprising three annular pellets 8 of sintered Ta oxide threaded onto a stainless steel wire or rod 10, are in contact with the fused salt.
  • a power source 12 applies the cell voltage between the anode and the cathode.
  • a Ta-coated stainless steel grid or mesh 14 is threaded onto the stainless steel wire between each annular pellet. These provide mechanical support for each pellet during electro-reduction to reduce the risk of pellet fragmentation in the electrochemical cell. If the mechanical strength of the pellets is sufficient to avoid fragmentation, however, these supports may be omitted.
  • the desired cell voltage can be calculated (11) using standard tables for Gibbs free energy changes and the Nernst equation.
  • thermodynamic reversible potentials for the oxide reduction and carbon/oxygen anodic reactions this is given by the thermodynamic reversible potentials for the oxide reduction and carbon/oxygen anodic reactions, the respective overpotentials and Ohmic drop (IR ce ii) contribution from the resistance of the electrolyte and electrodes.
  • IR ce ii Ohmic drop
  • the potential of an electrode through which current flows differs from the reversible potential established when no current flows through the electrode. This difference is the overpotential. It is determined by the kinetics of the electrode process and can be divided into charge transfer, diffusion, reaction and crystallisation overpotential according to the four possible types of rate control process.
  • the free energy is taken from Roine [9] for unit activities and 900C (the fused salt temperature in the cell).
  • the estimated minimum cell voltage components are given in Table 1. Both the overvoltages and the cell resistance have some degree of uncertainty, and consequently the minimum voltage which would lead to a reduction is between 2.1 and 2.6V.
  • the cells are usually operated at a constant voltage between 2.6 and 3.0V to ensure that the reduction process goes to completion.
  • the cell voltage is defined with reference to the components in Table 1 and measured at the cell terminals. So if, for example, the Ohmic losses are larger due to higher resistance current links, interface connections or a larger electrode gap, then the cell voltage would have to be adjusted accordingly, as would be within the capabilities of a person skilled in the art.
  • the electrolyte in this preferred embodiment consists of calcium chloride with up to 1 mol% CaO.
  • a potential is applied across the cell of a value determined as described above until there is little or no change in the current.
  • the reduced pellet is then withdrawn from the molten salt and cooled in an inert atmosphere in the space above the crucible. When the pellet has sufficiently cooled (for example to ⁇ 100C) it is removed from the furnace together with any adherent salt.
  • the salt can be removed by washing in deionised water and then acid leaching with acetic acid, followed by a final wash.
  • the electrochemical reduction process produces large (of the order of mm) granules which can be easily comminuted to an agglomerated powder of 50-1 OO ⁇ m agglomerate size with high surface area, for example by mixing in a high-shear mixer.
  • An EDAX scan of the powder showed that only Ta was present.
  • Figure 1 illustrates an apparatus for performing an electro-reduction process.
  • a pellet of Ta 2 O 5 was prepared by mixing 23g of Ta 2 O 5 with 2g Abryll 84 wax.
  • the tantalum oxide was Ta 2 O 5 nominal 2 ⁇ m powder (although the primary particle size was less than 1 ⁇ m) from PI-KEM Ltd., Yew Tree House, Tilley, Wem, Shropshire. SY4 5HE, UK. This was pressed into a pellet of approximately 35mm diameter and 10mm thickness and sintered in a two stage process involving a slow heat up to 600C and a dwell to burn out the wax, before a final heat to 1225C. The pellet was held at that temperature for 6 hours to allow sintering to occur, before furnace cooling. This process produced pellets with a typical mass of 21 g and a porosity of 55-60%.
  • the pellet was then placed in a cell, consisting of a crucible, carbon anode, cathode assembly and an electrolyte.
  • the cylindrical crucible was made from alumina with a zirconia wash on the inside; nominal size 110mm diameter by 170mm tall.
  • the anode consisted of a cylinder of carbon (SGL grade HLM-NOX), OD 95mm, ID 85mm, height 200mm. The salt depth would have submerged ⁇ 40mm of this anode cylinder.
  • a steel collar at the top of the anode allowed the attachment of a 6mm thick steel bar rising out of the cell to act as a current collector.
  • the oxide pellet had a 3mm diameter hole drilled into its centre before sintering.
  • the sintered pellet was attached to a 50mm length of 316 stainless steel M3 studding with a 316 stainless steel plate at the bottom to provide mechanical support.
  • the stainless steel studding and the attached tantalum oxide pellet formed the cathode assembly.
  • the cell was contained within a vertical furnace with a refractory cement top plate which allowed the furnace to be blanketed with argon gas.
  • the electrolyte consisted of 60Og of Briners Choice CaCI 2 , which had been vacuum dried at 190C, with an addition of 3g CaO.
  • the cell was held under an argon blanket, and the temperature was increased to 400C, and held there for 16 hours. The temperature was then further increased to 920C, and the cathode lowered into the molten salt. A voltage of 2.8V was applied across the cell terminals (measured at the exit from the furnace) for 56 hours to ensure the complete reduction of the tantalum oxide.
  • the cathode was withdrawn from the melt into the argon blanket, and the cell cooled to room temperature under argon.
  • the cathode was soaked in water to remove excess CaCI 2 .
  • the pellets were brittle and consequently gentle agitation during washing was all that was required to break up the pellet into powder.
  • the resulting agglomerated powder was washed firstly in hot (80C) deionised water and then in 20 v/v% acetic acid to remove the salt. Finally the powder was washed and dried; and characterised by SEM, BET, particle size and oxygen analysis.
  • the oxygen and nitrogen were analysed using an Eltra ON 900 automatic analyser. It is designed for the oxygen and nitrogen in steel, alloys, copper, zirconium, titanium, ceramics and other inorganic materials. Two duplicate measurements were always undertaken to ensure constancy.
  • the oxygen content of the product was found to be 2896 ppm, the primary particle size had increased to around 1 ⁇ m, and the mean (aggregate) particle size was 28 ⁇ m.
  • the BET surface area was 0.38m 2 /g.
  • a pellet was pressed from 25g of Ta 2 O 5 (as specified in Example 1) with 10% wax mix and sintered for 6 hours at 1225C. A 3mm hole was then drilled in the pellet, to allow it to be threaded onto the cathode.
  • the dimensions and arrangement of the crucible, anode and cathode were as described in Example 1.
  • the electrolyte 80Og Briners Choice CaCI 2 with an addition of 3.9Og CaO
  • the electrolyte 80Og Briners Choice CaCI 2 with an addition of 3.9Og CaO
  • An argon flow of 0.6l/min was started and the furnace heated to 400C. This temperature was maintained for 16 hours to dry the salt, before a further increase to the operating temperature of 920C was applied.
  • the cathode was lowered such that the pellet was submerged within the electrolyte, and a voltage of 3.0V was applied across the cell terminals at the furnace top plate exit. This voltage was maintained for 56 hours, and the cathode was periodically agitated by rotation. The cathode and anode were then withdrawn from the salt. The cathode was allowed to cool under argon and then removed from the cell.
  • the pellet was then processed by soaking in water until it could be removed from the cathode current collector rod. Washing then proceeded by rinsing in distilled water three times, acid washing in acetic acid three times and then further water washing to remove the acid (judged complete when the washing solution pH had risen to 5-6).
  • the BET surface area was 5.22m 2 /g, measured by the volumetric technique on a Micromeritics TriStar 3000 and the oxygen level (measured as in Example 1) was 8084 ppm.
  • a number of pellets of Ta 2 O 5 were prepared by pressing 28-3Og of Ta 2 O 5 into a mould of size approximately 35mm diameter and 10mm depth.
  • the tantalum oxide was Ta 2 O 5 nominal 2 ⁇ m powder (although the primary particle size was less than 1 ⁇ m) from PI-KEM Ltd., Yew Tree House, Tilley, Wem, Shropshire. SY4 5HE, UK.
  • the resulting compacts were then sintered at 1250 C for 12 hours in a vacuum furnace.
  • a typical dimension of a resultant pellet was 31 mm by 9.97mm thick with a porosity between 50 and 55%. However, the primary particle size had increased to around 1 ⁇ m.
  • the oxide pellets had a 4mm diameter hole drilled into their centre before sintering.
  • the crucible was rectangular and had dimensions 200mm (L) x 120mm (W) x 155mm (D) and was manufactured from alumina.
  • the crucible and all necessary connections, thermocouples and heat shields were placed in a vertical furnace with a sealed top plate which allowed the experiment to be run under an inert atmosphere of argon gas.
  • the electrolyte consisted of 240Og of Sigma-Aldrich anhydrous granules (C1016) CaCI 2 , which had been vacuum dried at 160C for 24 hours, with an addition of 1mol.% CaO.
  • the electrolyte was pre-melted in the crucible, and held at temperature of 820C for 24 hours under Ar.
  • the anode was a 1cm diameter Tokai graphite rod connected to a 4mm diameter stainless steel rod which formed the current collector.
  • the overall length was about 600mm (similar to the cathode assembly) after passing through the top plate.
  • a carbon anode was introduced such that it was submerged by about 10mm into the molten salt. It was maintained at this depth throughout the run.
  • the cathode assembly was placed about 60mm away at the other end of the crucible. The cell was then run for 16 hours at a terminal voltage of 2.8V, measured at the top of the cathode and anode rods after they passed through the top plate. A sparge of argon gas was injected under the anode to aid mixing and gas release from the anode.

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Abstract

Dans un procédé de fabrication d'un agrégat de Ta, Nb ou d'un alliage comprenant Ta ou Nb, un matériau précurseur est produit par frittage d'une poudre d'un composé du métal ou d'un mélange de composés des métaux pour former un alliage, le frittage étant réalisé à une première température. Le précurseur est ensuite amené à devenir la cathode dans un bain de sels fondus à une seconde température, inférieure à la première température, et réduit pour former un produit de métal ou d'alliage. La seconde température est prédéterminée de façon à ne pas diminuer sensiblement la porosité du matériau précurseur ou, après réduction, du métal ou de l'alliage.
PCT/GB2007/003805 2006-10-06 2007-10-05 Procédé et appareil pour produire des poudres métalliques WO2008041007A1 (fr)

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WO2020185166A1 (fr) * 2019-03-13 2020-09-17 Agency For Science, Technology And Research Procédé électrochimique de réduction d'oxyde métallique

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9393623B2 (en) 2009-02-13 2016-07-19 Metalysis Limited Method for producing metal powders
US9579725B2 (en) 2009-02-13 2017-02-28 Metalysis Limited Method for producing metal powders
WO2020185166A1 (fr) * 2019-03-13 2020-09-17 Agency For Science, Technology And Research Procédé électrochimique de réduction d'oxyde métallique

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