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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
  • C04B35/495—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on vanadium, niobium, tantalum, molybdenum or tungsten oxides or solid solutions thereof with other oxides, e.g. vanadates, niobates, tantalates, molybdates or tungstates
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  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/622—Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/653—Processes involving a melting step
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  • 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
• • 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
  • C22B5/00—General methods of reducing to metals
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  • 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
  • C25C3/26—Electrolytic production, recovery or refining of metals by electrolysis of melts of titanium, zirconium, hafnium, tantalum or vanadium
• • H—ELECTRICITY
  • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
  • H10N60/00—Superconducting devices
  • H10N60/01—Manufacture or treatment
  • H10N60/0184—Manufacture or treatment of devices comprising intermetallic compounds of type A-15, e.g. Nb3Sn
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
  • C04B2235/32—Metal oxides, mixed metal oxides, or oxide-forming salts thereof, e.g. carbonates, nitrates, (oxy)hydroxides, chlorides
  • C04B2235/3231—Refractory metal oxides, their mixed metal oxides, or oxide-forming salts thereof
  • C04B2235/3251—Niobium oxides, niobates, tantalum oxides, tantalates, or oxide-forming salts thereof
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
  • C04B2235/40—Metallic constituents or additives not added as binding phase
  • C04B2235/402—Aluminium
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
  • C04B2235/30—Constituents and secondary phases not being of a fibrous nature
  • C04B2235/40—Metallic constituents or additives not added as binding phase
  • C04B2235/404—Refractory metals
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
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  • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
  • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
  • C04B2235/80—Phases present in the sintered or melt-cast ceramic products other than the main phase

Definitions

• the present invention relates to a method and an apparatus for fabricating materials, and in particular for fabricating superconducting materials.
• Nb 3 Al which forms an A15 superconducting phase and, by way of illustration, methods for fabricating this material include the following.
• the fabrication processes can be considered in three groups: low-temperature, high temperature and transformation processing.
• an A15 Nb 3 Al strand is processed by first making the final-size strand, with the constituents subdivided, and then heat-treating it to form the A15 phase.
• Low temperature ( ⁇ 1000C) processes ensure that the grain size of Nb 3 Al does not become too coarse because the Nb/Al constituents directly react with diffusion to suppress Nb 3 Al grain growth. But, at low temperatures, there is a deviation from A15 stoichiometry, thus affecting high-field properties, especial J c . Low temperature processes include the following.
• Jelly-roll (JR) Alternate foils of Nb and Al are wound onto a copper rod and inserted into holes drilled in a copper matrix before drawing to form the final-size strand.
• Rod-in-tuje, (RIT) - An alloy rod is inserted into a Nb tube and drawn down. A triple stacking operation gives the desired lOOnm core diameter to match that of the Al layer.
• Clad chip extrusion, (CCE) - A three-layered clad foil, of Al/Nb/Al, is cut into square chips, and then filled into a can in order to be extruded.
• Powder metallurgy, (PM) - A mixture of hydride- dehydride Nb powder and Al powder are put in a copper tube can so that they can be extruded and drawn into a monofilament wire. A bundle of these wires will give an Al layer thickness of lOOn .
• J c versus magnetic field curves are very similar for all of these processes. JR may have a slight advantage for producing long piece strands. High Temperature Processing
• the invention provides a method and an apparatus for fabricating materials as defined in the appended independent claims .
• Preferred or advantageous features of the invention are set out in dependent sub-claims .
• the invention uses a process for extracting metals and alloys from solid compounds by direct electrochemical reduction, or electrodecomposition, in molten salt, known as the Fray-Farthing-Chen Cambridge process (FFC) , as one of a series of steps to fabricate a material.
• FFC Fray-Farthing-Chen Cambridge process
• the FFC process is described in the present applicant's earlier International patent application PCT/GB99/01781 which is incorporated herein by reference.
• the FFC process allows the treatment of a solid material, which may be a compound between a metal (or semi-metal) and a substance (such as an anionic species) , or a solid solution of the substance in the metal, by electrodecomposition in a molten salt to remove the substance from the solid material .
• the solid material On completion of the process, the solid material has been converted to the metal.
• the solid material comprises more than one metal, being for example a mixture of metal compounds, or a mixture of a metal and a metal compound, or comprises a solid solution of metal compounds, then on completion of the process an alloy or interr ⁇ etallic compound of the metals is formed.
• the product of the FFC process is typically porous and, in the method of the present invention, is then infiltrated with an element, metal or alloy, typically as a liquid, to form a material which can be used or further processed to fabricate a product.
• the invention may be particularly efficacious for fabricating superconductors.
• a porous sample of NbTi alloy is produced. This can then be infiltrated with molten Al to form a material which can be further processed, for example by deformation and heat treatment, to form a high-performance superconductor, advantageously at lower cost than for conventional methods.
• the FFC process is performed on a preform comprising a mixture of powdered Nb and Sn oxides. A Nb 3 Sn superconductor can then be fabricated.
• the invention may advantageously provide a method having four steps as follows for fabricating Nb-based superconductors: 1) electrochemical reduction of the Nb-based compounds , 2) infiltration by Al-based alloys (or any other elements or alloys to form intermetallics or artificial pinning centres [APCs] ) , 3) deformation, and
• the invention is not limited to the field of superconductor fabrication but relates primarily to the technique of infiltrating a porous material formed by the FFC process.
• the FFC process is very flexible and can produce a wide range of metals, semi-metals, alloys and intermetallic compounds, including materials which are difficult to fabricate in other ways.
• the additional novel step of infiltrating a product of the FFC process, which is typically porous, with a metal or other material may advantageously allow the fabrication of a wide variety of novel and useful materials compositions and microstructures .
• the infiltration step may be carried out ex-situ or, preferably, in-situ.
• the FFC process can produce a porous alloy or intermetallic immersed in a molten salt.
• the molten salt is contained in a bath which also contains the molten material for infiltration.
• the infiltration material will usually be denser than the salt, in which case the salt will float on the infiltration material.
• the porous sample can then move directly from the salt into the infiltration material, which can displace the molten salt and infiltrate the porous sample.
• the porous sample can move directly from the salt to the infiltration material, advantageously avoiding contact with any other substances .
• the porous sample may be immersed in the infiltration material by moving the interface between the salt and the infiltration medium rather than by moving the porous sample.
• the bath containing the salt may be flooded with infiltration material to displace the salt, or where the bath contains both salt and infiltration material, the bath may be moved, rather than the porous sample.
• the molten infiltration material In ex-situ infiltration the molten infiltration material is held in a separate bath from the molten salt and the porous sample moves from one bath to the other for infiltration. If this is done in an oxidising atmosphere, disadvantageous oxidation of the porous sample may occur. An inert atmosphere may be used to alleviate this problem but nevertheless contamination of the porous sample may be more likely than with the in-situ method.
• the porous sample is withdrawn from the molten salt and allowed to cool, preferably in an inert atmosphere or in vacuum above the molten salt. As the sample is withdrawn, much or all of the salt within pores in the sample is retained and then solidifies. The sample is then transferred to a pool of molten infiltration material, where it is immersed and the salt melts and is displaced by the infiltration material to infiltrate the sample.
• This implementation has the advantage that the solidified salt in the pores of the sample during transfer to the immersion material helps to protect the sample surface from contamination or oxidation.
• the infiltration material wets the FFC product better than the molten salt, it may advantageously substantially entirely displace the salt from the porous FFC product.
• molten salts wet metals relatively poorly and so, where the FFC product is metallic and the infiltration material is also metallic, the infiltration material will tend to wet the FFC product more strongly than the molten salt.
• One method for this is to pump the molten salt out of the porous sample after or as it is immersed in the infiltration material.
• a second method which may be combined with the first, is to vibrate or agitate the porous sample or the infiltration material, for example by using an ultrasonic transducer.
• FIG. 1 illustrates an electrolytic cell for carrying out the FFC process
• Figure 2 illustrates the infiltration and subsequent steps in a first method embodying the invention
• Figure 3 is a micrograph of a sample of porous Nb alloy following the FFC process
• Figure 4 is a micrograph of a sample of Nb alloy following infiltration
• Figure 5 is a " micrograph of a Nb-Al-Ge (X) wire following mechanical reduction and diffusion treatment
• Figure 6 is a plot of AC susceptibility against temperature for Nb and NbTi rods embodying the invention
• Figure 7 is a plot of AC susceptibility against temperature for reduced Nb 2 0 5 -Sn0 2 rods embodying the invention
• Figure 8 illustrates a cell for in-situ infiltration according to an embodiment of the invention.
• Figure 9 illustrates a second stage in the in-situ infiltration method using the cell of figure 8; and Figure 10 is an element distribution plot for an infiltrated pellet of niobium oxide.
• the electrochemical reduction route of the FFC process may advantageously be a much easier, quicker and cheaper way to extract many metals and alloys than established metallurgical routes.
• a schematic of such a process is presented in figure 1.
• Figure 1 shows an apparatus for making the binary alloy NbTi. It comprises a cell 2 containing molten CaCl 2 4. A graphite anode 6 and a rod-shaped preform 8 of mixed Nb 2 0 5 and Ti0 2 are immersed in the salt. The preform is supported on and electronically connected to a Kanthal wire 10. The preform is made by mixing powdered Nb 2 0 5 and Ti0 2 in the desired proportion, slip casting and optionally partially sintering the mixture.
• a polymer binder may be added to improve the slip-casting process and the polymer then burned off.
• a prefabricated polymer matrix may be used to make the preform. In this case the polymer matrix is infiltrated with metal oxide powders and then the polymer is burned off.
• the preform of mixed Nb 2 0 5 ,Ti0 2 powders is made the cathode in the molten CaCl 2 , whose cation can form a more stable oxide, CaO, than Nb 2 0 5 and Ti0 2 .
• the oxygen in the Nb 2 0 5 and Ti0 2 mixture is thus ionised and dissolves in the salt, leaving Niobium- Titanium alloy metal of the desired composition behind.
• the extraction of Nb, NbTi, and Nb 3 Sn metals and alloys from oxides using this process has been carried out on a laboratory scale, as has the extraction of many other metals and alloys from their compounds.
• the final product of the FFC process in the embodiment is a porous, rod-shaped, metallic sponge of NbTi alloy, as shown in figure 3. Rapid oxidation of the Nb-based porous rod normally takes place after its removal from the chloride bath and may have a detrimental effect on its surface quality and any subsequent processing. Therefore a different approach is proposed to provide better infiltration conditions, using either in-situ or ex-situ infiltration as described below. Porosity
• the degree of porosity of the final percolative network of the Nb-based alloy sponge ' depends on the density of the oxide preform and on the initial preparation technique of the prefabricated oxide. For example, preforms which are sintered show a significant shrinkage (increased density) and greatly increased strength in comparison with those prepared by slip casting only. Materials Considerations in Superconductor Fabrication
• the metallic product of the electrochemical reduction (for NbTi or other materials) is soft and porous, without structural defects or secondary phases, it can not be regarded as a high critical current density, J c , superconducting material.
• Such semi-finished product has to be upgraded by introduction of, for example, Al or Sn alloys for the reactive diffusion formation of the intermetallic phase in the case of A15 intermetallic superconductors, and/or by introduction of Artificial Pinning Centres (APC) as in the case of NbTi.
• APC Artificial Pinning Centres
• the Al-Ge system forms a low melting point eutectic (424C) at the composition of 70% Al-30% Ge.
• the inherent brittleness of the Al-Ge eutectic when solidified requires particular attention to the temperature of the infiltrating bath, the temperature difference between the porous niobium or alloy rod and the bath, and the rate of infiltration.
• the -formability of the Al-Ge eutectic, entrapped and solidified within the pore volume of the rod, during form rolling or wire drawing can be significantly improved through the application of superplasticity principles.
• Superplastic behaviour requires a fine duplex microstructure that is stable at the deformation temperature .
• the invention may thus advantageously provide new techniques that allow manufacture of complex compositional superconducting alloy rods by ex-situ and in-situ infiltration processes. Ex-situ infiltration
• FIG. 2 A schematic representation of an embodiment of the process following the fabrication of the FFC process alloy sponge is shown in figure 2, which shows the steps of infiltration 50, here in a Sn/Ga/Al infiltrant bath 52, cladding 54, mechanical reduction 56 and diffusion processing 58 in a furnace 60.
• Figure 2 relates to (Nb, X) 3 (Sn, Al, Z) wire processing, using porous (Nb,X) rod fabricated using the FFC process, for example.
• a NbTi alloy was formed by direct electrochemical reduction to form an alloy sponge, or rod-shaped sample. Its microstructure is shown in cross-section in figure 3.
• the Nb-based alloy rod is immersed in a bath of molten Sn or Al-based alloy maintained at a temperature above melting. Lower temperatures are preferred in order to prevent the extensive, very often rapid, formation of brittle intermediate phases which could impair the ductility of the composite infiltrated material.
• the microstructure of the infiltrated alloy sponge is shown in figure 4.
• the final products of reducing Nb-oxide-based oxide mixtures in the FFC process typically have pore sizes in the range of 2-20/ ⁇ U
• special care should be taken to ensure that the Nb-based sponge ' surface is as clean and pure as possible to enable complete infiltration of the porous rod, efficient wetting by the infiltrating metal or alloy such as Sn, Al etc. and finally minimisation of superconductor filament damage caused by the formation of hard Nb 2 0 5 (or even of more complex insulating compounds) on the sponge surface during removal of the metallic Nb-based rod from the chloride bath.
• the oxygen content in the Nb can reach 2%-3at.%, which is about the solubility limit at the extrusion temperature used later in processing.
• Oxygen adsorbed at the surface of the particles in the sponge may also diffuse into the Nb, increasing its microhardness to 3500 MNirf 2 .
• the plastic deformation of the composite may then not be uniform because of severe solution hardening of the Nb, mainly due to interstitial oxygen.
• a successful co-deformation of Nb and Sn particles requires sufficient reduction of the oxygen content in the Nb. If the oxygen content in the Nb is reduced to 0.1at% the microhardness of the Nb matrix drops to ⁇ 1200MNm -2 , which is about the value of the surrounding Cu matrix in which the Nb material is typically subsequently encased and which is used for cryostability.
• a variant of ex-situ infiltration aims to address these concerns.
• the porous metal or alloy sponge is withdrawn from the molten salt using a control and positioning system until it is held in an inert atmosphere or in vacuo above or near the salt bath. If wetting of the metal by the salt is sufficient, the pores in the sponge remain filled with molten salt, and the sponge can be cooled to solidify the salt. The sponge surface is thus protected against contamination or oxidation by the presence of the solid salt and can be transferred to an infiltration bath without damage. On immersion in the infiltration bath, the salt melts and is displaced by the immersion material. Molten salt can also be more effectively retained in the porous metal by lowering the temperature of the salt bath, prior to removal of the metal, to close to the melting point of the salt.
• FIGS 8 and 9 illustrate the technique of in-situ infiltration.
• a cell 20 contains molten salt 22 floating above a molten metal alloy 24 held in an extended lower portion 26 of the cell.
• a Nb 2 0 5 /Ti0 2 preform 28 and a graphite anode 30 are immersed in the molten salt, which is CaCl 2 .
• the preform is supported on a tubular Kanthal support 32.
• in-situ infiltration of the porous rod is carried out by lowering it from the molten salt directly into the molten metal (in the embodiment, molten Al) beneath, as shown in figure 9.
• the rod is lowered by a control and positioning system coupled to the Kanthal support.
• This in-situ process is advantageous because there is no direct contact of the Nb with oxygen before infiltration, and so the metal surface on infiltration is oxide free.
• molten CaCl 2 is displaced from the sample, or sponge, by the molten metal. With these materials, effective infiltration may be expected due to the better wetting of the Nb by the Al than by the CaCl 2 .
• various methods may be used to encourage the full metal infiltration of the porous Nb-based rod.
• One of these is to pump out or suck out the molten CaCl 2 from the rod. This can be achieved by pumping the salt through the tubular Kanthal support shown in figures 8 and 9.
• sucking will be very effective and any excess of the molten metal within the core of the sample or in the Kanthal support can be easily removed after infiltration and replaced with internal cryogenically stabilising composite such as Cu with a protective Ta diffusion barrier.
• an ultrasonic device mechanically coupled to the rod or its support, or to the bath of infiltration material, may also be used to accelerate the infiltration process.
• In-situ infiltration of the Nb-based porous rod should advantageously minimise many of the negative effects related to the ex-situ process mentioned above, and in particular the risk of surface contamination of the sample before infiltration.
• Post-Infiltration Processing After the infiltration process, rods would be machined to the desired shape and inserted in subsequent tubes 62 to serve as a diffusion barrier and for electrical and thermal stabilisation. This is the cladding step 54 of figure 2. Although an elevated temperature during the infiltration stage may produce some A15 phase, it may be desired to subject the infiltrated rod or tape to a substantial reduction in thickness by cold rolling 56 prior to the final diffusion formation 58 of the intermetallic A15 layers in the conductor.
• microstructure control An important aspect of superconductor fabrication is microstructure control.
• Use of the FFC process as a step in fabrication enables an element of control through control of the particle size of the powder used to make the preform, densification of the preform through sintering, and the temperature and other electrolysis parameters at which the electrodecomposition is performed.
• ex-situ and in-situ infiltration processes can be applied to the niobium-titanium, niobium-tin and niobium-aluminium systems as described but in general these techniques can be used to infiltrate any metals or materials manufactured by the FFC process, whether for superconducting applications or other purposes. Additional Example of Infiltration for Materials Fabrication
• a cathode preform of Nb 2 0 5 was prepared by pressing oxide powder into a small cylindrical pellet (10mm diameter, 10mm height, approximately 1.5g mass) which was then sintered at 1000°C for 2 hours. After sintering the preform gained a reasonable strength and had a porosity of about 40-50%, depending on parameters including starting material parameters and pressing pressure. A hole (1.5mm diameter) was drilled through the sintered pellet which was then threaded onto Kanthal wire. This assembled cathode was placed in the molten eutectic mixture of CaCl 2 and NaCl at 950°C. FFC electrodecomposition was carried out with a graphite rod anode at 3.
• the pellet was broken into two halves, and the cross section examined by SEM (scanning electron microscopy) and EDX (energy-dispersive x-ray analysis). It was observed that the pellet contained two different phases.
• the outer layer of the pellet was about 400 micrometres in thickness and relatively dense, but the central part was porous.
• EDX analysis revealed that, as shown in Figure 10, the outer layer was composed mainly of niobium and aluminium with about 20at% oxygen, but the central part was of niobium and calcium with 58at% oxygen (Nb 2 0 5 contains 71at% oxygen) .
• the calcium content was also much lower in the outer layer than in the central part.

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• 2001
  • 2001-10-10 GB GBGB0124303.9A patent/GB0124303D0/en not_active Ceased
• 2002
  • 2002-10-10 CA CA002463396A patent/CA2463396A1/en not_active Abandoned
  • 2002-10-10 EP EP02767710A patent/EP1440175A2/en not_active Withdrawn
  • 2002-10-10 CN CN02822309.8A patent/CN1585828A/zh active Pending
  • 2002-10-10 JP JP2003534634A patent/JP2005505121A/ja active Pending
  • 2002-10-10 WO PCT/GB2002/004603 patent/WO2003031665A2/en not_active Application Discontinuation
  • 2002-10-10 US US10/492,180 patent/US20050016854A1/en not_active Abandoned
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