Classifications

• • 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
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B23/00—Single-crystal growth by condensing evaporated or sublimed materials
  • C30B23/02—Epitaxial-layer growth
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
  • C30B29/10—Inorganic compounds or compositions
  • C30B29/16—Oxides
• • 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
  • H10N60/0268—Manufacture or treatment of devices comprising copper oxide
  • H10N60/0296—Processes for depositing or forming copper oxide superconductor layers
  • H10N60/0324—Processes for depositing or forming copper oxide superconductor layers from a solution
• • 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
  • H10N60/0268—Manufacture or treatment of devices comprising copper oxide
  • H10N60/0296—Processes for depositing or forming copper oxide superconductor layers
  • H10N60/0548—Processes for depositing or forming copper oxide superconductor layers by deposition and subsequent treatment, e.g. oxidation of pre-deposited material

Definitions

• This invention relates to high temperature superconductors (HTS), and more particularly to superconducting layers and methods of depositing precursor compositions for such layers.
• Coated conductors comprising a single or multiple combinations of a biaxially textured high temperature superconductor ("HTS") layer on a thin buffer layer and a substrate tape, are a cost-performance-effective technology for manufacturing long length flexible HTS wire for magnet, coil and power applications.
• these conductors should be useful for power transmission cables, rotor coils of motors and generators, and windings of transformers, as well as for magnets for medical magnetic resonance imaging (MRI), magnetic separation, ion-beam steering and magnetic levitation.
• MRI medical magnetic resonance imaging
• Particularly of interest here are applications which use ac currents and fields, or fast ramps of current and field, for example ac power transmission cables, transformers, faultcurrent limiters, magnetic separation magnets and high energy physics magnets.
• coated conductors include at least, for example, a substrate and a superconducting layer (such as YBCO) deposited thereon.
• a superconducting layer such as YBCO
• One or more buffer layers may be included between the substrate and the superconductor material.
• YBCO YBa 2 Cu 3 O x , or Yttrium-Barium- Copper-Oxide
• REBa 2 Cu 3 O x where the Y has been partially or completely replaced by rare earth elements (RE).
• RE rare earth elements
• Certain challenges in this field include the need for cost effective methods for producing chemically compatible biaxially textured buffer layers, as well as the need to deposit sufficient thickness of the high critical current density superconducting layer.
• deformation textured substrates with epitaxial buffer layers can be made cost effective.
• ion beam assisted deposition of thin MgO layers with epitaxial top layers may prove to be economically viable.
• CVD chemical vapor deposition
• PVD physical vapor deposition
• Trifluoroacetate (TFA) solution processes offer low costs for precursor compositions, high deposition rate, and non-vacuum processing advantages. Such processes are described, for example, in U.S. Patent No. 5,231,074 to Cima et al., and PCT Publication No. WO 98/58415, published December 23, 1998 and require dissolution of the constituents of the precursor composition to form a solution phase.
• compositions serving as a precursor to superconducting films, which can be coated onto large area substrates in a single application using high-deposition rate, to produce a desired film thickness.
• the precursor composition is preferably convertible to the superconducting phase by way of simple thermal processes.
• the invention provides a low cost method for fabricating thick film precursor compositions of rare-earth superconductors on long lengths of substrate.
• the final thicknesses of such films are preferably between about 1 micron and about 5 microns.
• the specific superconductors of interest are high temperature superconductors of the class of rare-earth barium cuprate species (REBCO), including, for example, YBa 2 Cu 3 O 7 . x (YBCO), or systems based on thallium/barium/calcium/copper/oxide (ThBCCO) or bismuth/strontium/calcium/copper/oxide (BSCCO), and other known superconducting materials, including versions doped with other species.
• REBCO rare-earth barium cuprate species
• ThBCCO thallium/barium/calcium/copper/oxide
• BSCCO bismuth/strontium/calcium/copper/oxide
• the precursor compositions for the superconducting layer of such high temperature superconductors include solid-state, or semi solid-state precursors deposited in the form of a dispersion . These precursor compositions allow for example the substantial elimination of in the case of YBCO BaCO 3 formation in final YBCO superconducting layers, while also allowing control of film nucleation and growth.
• biaxially textured refers to a surface for which the crystal grains are in close alignment with a direction in the plane of the surface.
• One type of biaxially textured surface is a cube textured surface, in which the crystal grains are also in close alignment with a direction perpendicular to the surface.
• Examples of cube textured surfaces include the (100) [001] and (100) [011] surfaces, and an example of a biaxially textured surface is the (113)[211] surface.
• epitaxial layer refers to a layer of material, the crystallographic orientation of which is directly related to the crystallographic orientation of the surface of a layer of material onto which the epitaxial layer is deposited.
• a substrate for a multi-layer superconductor having an epitaxial layer of superconductor material deposited onto a substrate, the crystallographic orientation of the layer of superconductor material is directly related to the crystallographic orientation of the substrate.
• a substrate in addition to the above-discussed properties of a substrate, it can also be desirable for a substrate to have a biaxially textured surface or a cube textured surface.
• a "dispersion” is a two-phase system in which one phase includes finely divided particles distributed throughout a liquid second phase.
• "ultrafine particles” are those particles sufficiently small to allow a uniform distribution of cation elements within the precursor composition, and a chemically homogeneous superconducting film.
• the particle diameters are typically less than about 10% of the final film thickness.
• the particle sizes are small enough, and uniformly distributed enough, to allow rapid local diffusion of cationic constituents of the precursor compositions, for the efficient formation of substantially stoichiometric superconducting layers.
• substantially stoichiometric refers to the elemental ratios in mixtures of materials, in which the atomic ratios of cationic elements are within about 10% of whole number values. Such deviations from whole number stoichiometries can be deliberately introduced or can arise from the supply of materials, and can be desirable to aid in material processing. Experiments can show that excess elements are typically rejected by the material as a whole, resulting in nearly stoichiometric amounts of the elements.
• the invention allows the deposition of thick films in a single deposition step. Although multiple deposition steps can be carried out according to the particular application, each of said steps can give a film thicker than that available from previously known solution processes. Simplified binder removal or decomposition may sigmficantly improve the prospect of a low cost superconductor preparation method.
• Figs. 1-3 are alternate configurations for high temperature superconductor coated conductors.
• Figs. 4-6 are furnace profile diagrams used for decomposition of samples according to particular embodiments of the invention.
• Fig. 7 is a furnace profile diagram used for decomposition of a prior art sample.
• Fig. 8 is a ⁇ -2 ⁇ X-ray diffraction spectrum of a sample according to a particular embodiment of the invention.
• Fig. 9 is a ⁇ -2 ⁇ X-ray diffraction spectrum of a prior art sample.
• Fig. 10 is a furnace profile diagram used for reaction of samples according to particular embodiments of the invention.
• Fig. 11 is a ⁇ -2 ⁇ X-ray diffraction spectrum of a sample according to a particular embodiment of the invention.
• Fig. 12 is a ⁇ -2 ⁇ X-ray diffraction spectrum of a prior art sample.
• Fig. 13 is an expanded scale view of Fig. 11.
• Fig. 14 is an expanded scale view of Fig. 12.
• Figs. 15-16 are pole figures for samples according to particular embodiments of the invention.
• Fig. 17 is a pole figure for a prior art sample.
• Fig. 18 is a plot of elemental molar ratios taken at different locations on a sample tape according to a particular embodiment of the invention.
• Fig. 19 is a ⁇ -2 ⁇ X-ray diffraction spectrum of decomposed samples according to particular embodiments of the invention.
• Fig. 20 is a ⁇ -2 ⁇ X-ray diffraction spectrum of reacted samples according to particular embodiments of the invention.
• the invention results from the discovery that relatively high critical current densities can be achieved in high temperature superconductors (HTS) from precursor compositions containing solid-state constituents.
• HTS high temperature superconductors
• the invention includes precursor compositions, and methods for depositing precursor compositions of superconducting material on substrates, either directly onto the substrate, or onto a buffer- and/or intermediate-coated substrate, thereby forming biaxially textured superconducting oxide films from the precursor compositions.
• the precursor compositions comprise solid-state constituent-containing components.
• high temperature superconductor (HTS) articles such as 10, particularly in the form of wires or tapes, generally comprise a substrate 12, at least one buffer coating 14, a superconducting layer 16 (formed of, for example,
• Layer 14 can be formed of any material capable of supporting layer 16.
• layer 14 can be formed of a buffer layer material.
• buffer layer materials include metals and metal oxides, such as silver, nickel, TbO x , GaO x , CeO 2 , yttria-stabilized zirconia (YSZ), Y O 3 , LaAlO 3 , SrTiO 3 , LaNiO 3 , Gd 2 O 3 , LaCuO 3 , SrRuO 3 , NdGaO 3 , NdAlO 3 and nitrides as known in the art.
• a buffer material can be prepared using solution phase techniques, including metalorganic deposition, such as disclosed in, for example, S.S. Shoup et al., J. Am. Cer. Soc, vol. 81, 3019; D. Beach et al., Mat. Res. Soc. Symp. Proc, vol. 495, 263 (1988); M. Paranthaman et al., Superconductor Sci. Tech., vol. 12, 319 (1999); D.J. Lee et al., Japanese J. Appl. Phys., vol. 38, L178 (1999) and M.W. Rupich et al., I.E.E.E. Trans, on Appl. Supercon. vol. 9, 1527.
• Cap layer 18 can be formed of one or more layers, and preferably includes at least one noble metal layer.
• "Noble metal,” as used herein, is a metal, the reaction products of which are thermodynamically unstable under the reaction conditions employed to prepare the HTS tape.
• exemplary noble metals include, for example, silver, gold, palladium, and platinum.
• Noble metals provide a low interfacial resistance between the HTS layer and the cap layer.
• cap layer 18 can include a second layer of normal metal (for example, copper or aluminum or alloys of normal metals.
• the substrate can be formed of alloys having one or more surfaces that are biaxially textured (e.g., (113)[211]) or cube textured (e.g., (100)[001] or (100)[011]).
• the alloys can have a relatively low Curie temperature (e.g., at most about 80K, at most about 40K, or at most about 20K).
• the substrate is a binary alloy that contains two of the following metals: copper, nickel, chromium, vanadium, aluminum, silver, iron, palladium, molybdenum, gold and zinc.
• a binary alloy can be formed of nickel and chromium (e.g., nickel and at most 20 atomic percent chromium, nickel and from about five to about 18 atomic percent chromium, or nickel and from about 10 to about 15 atomic percent chromium).
• a binary alloy can be formed of nickel and copper (e.g., copper and from about five to about 45 atomic percent nickel, copper and from about 10 to about 40 atomic percent nickel, or copper and from about 25 to about 35 atomic percent nickel).
• a binary alloy can further include relatively small amounts of impurities (e.g., less than about 0.1 atomic percent of impurities, less than about 0.01 atomic percent of impurities, or less than about 0.005 atomic percent of impurities).
• the substrate contains more than two metals (e.g., a ternary alloy or a quarternary alloy).
• the alloy can contain one or more oxide formers (e.g., Mg, Al, Ti, Cr, Ga, Ge, Zr, Hf, Y, Si, Pr, Eu, Gd, Tb, Dy, Ho, Lu, Th, Er, Tm, Be, Ce, Nd, Sm, Yb and/or La, with Al being the preferred oxide former), as well as two of the following metals: copper, nickel, chromium, vanadium, aluminum, silver, iron, palladium, molybdenum, gold and zinc.
• oxide formers e.g., Mg, Al, Ti, Cr, Ga, Ge, Zr, Hf, Y, Si, Pr, Eu, Gd, Tb, Dy, Ho, Lu, Th, Er, Tm, Be, Ce, Nd, Sm, Yb and/or La, with Al being the preferred oxide former
• the alloys can contain at least about 0.5 atomic percent oxide former (e.g., at least about one atomic percent oxide former, or at least about two atomic percent oxide former) and at most about 25 atomic percent oxide former (e.g., at most about 10 atomic percent oxide former, or at most about four atomic percent oxide former).
• the alloy can include an oxide former (e.g., at least about 0.5 aluminum), from about 25 atomic percent to about 55 atomic percent nickel (e.g., from about 35 atomic percent to about 55 atomic percent nickel, or from about 40 atomic percent to about 55 atomic percent nickel) with the balance being copper.
• the alloy can include an oxide former (e.g., at least about 0.5 atomic aluminum), from about five atomic percent to about 20 atomic percent chromium (e.g., from about 10 atomic percent to about 18 atomic percent chromium, or from about 10 atomic percent to about 15 atomic percent chromium) with the balance being nickel.
• the alloys can include relatively small amounts of impurities (e.g., less than about 0.1 atomic percent of impurities, less than about 0.01 atomic percent of impurities, or less than about 0.005 atomic percent of impurities).
• An alloy can be produced by, for example, combining the constituents in powder form, melting and cooling or, for example, by diffusing the powder constituents together in solid state.
• the alloy can then be formed by deformation texturing (e.g, annealing and rolling, swaging, extrusion and or drawing) to form a textured surface (e.g., biaxially textured or cube textured).
• the alloy constituents can be stacked in a jelly roll configuration, and then deformation textured.
• a material with a relatively low coefficient of thermal expansion e.g, Nb, Mo, Ta, V, Cr, Zr, Pd, Sb, NbTi, an intermetallic such as NiAl or Ni 3 Al, or mixtures thereof
• Nb, Mo, Ta, V, Cr, Zr, Pd, Sb, NbTi, an intermetallic such as NiAl or Ni 3 Al, or mixtures thereof
• stable oxide formation can be mitigated until a first epitaxial (for example, buffer) layer is formed on the biaxially textured alloy surface, using an intermediate layer disposed on the surface of the substrate.
• Intermediate layers suitable for use in the present invention include those epitaxial metal or alloy layers that do not form surface oxides when exposed to conditions as established by Po and temperature required for the initial growth of epitaxial buffer layer films.
• the buffer layer acts as a barrier to prevent substrate element(s) from migrating to the surface of the intermediate layer and forming oxides during the initial growth of the epitaxial layer. Absent such an intermediate layer, one or more elements in the substrate would be expected to form thermodynamically stable oxide(s) at the substrate surface which could significantly impede the deposition of epitaxial layers due to, for example, lack of texture in this oxide layer.
• the intermediate layer is transient in nature.
• Transient refers to an intermediate layer that is wholly or partly incorporated into or with the biaxially textured substrate following the initial nucleation and growth of the epitaxial film. Even under these circumstances, the intermediate layer and biaxially textured substrate remain distinct until the epitaxial nature of the deposited film has been established.
• the use of transient intermediate layers may be preferred when the intermediate layer possesses some undesirable property, for example, the intermediate layer is magnetic, such as nickel.
• Exemplary intermediate metal layers include nickel, gold, silver, palladium, and alloys thereof. Impurities or alloys may include alloys of nickel and/or copper.
• Epitaxial films or layers deposited on an intermediate layer can include metal oxides, chalcogenides, halides, and nitrides. In preferred embodiments, the intermediate metal layer does not oxidize under epitaxial film deposition conditions.
• the deposited intermediate layer is not completely incorporated into or does not completely diffuse into the substrate before nucleation and growth of the initial buffer layer structure causes the epitaxial layer to be established.
• the thickness of the deposited metal layer has to be adapted to the epitaxial layer deposition conditions, in particular to temperature.
• Deposition of the intermediate metal layer can be done in a vacuum process such as evaporation or sputtering, or by electro-chemical means such as electroplating (with or without electrodes).
• deposited intermediate metal layers may or may not be epitaxial after deposition (depending on substrate temperature during deposition), but epitaxial orientation can subsequently be obtained during a post-deposition heat treatment.
• solution coating processes can be used for deposition of one or a combination of any of the oxide layers on textured substrates; however, they can be particularly applicable for deposition of the initial (seed) layer on a textured metal substrate.
• the role of the seed layer is to provide 1) protection of the substrate from oxidation during deposition of the next oxide layer when carried out in an oxidizing atmosphere relative to the substrate (for example, magnetron sputter deposition of yttria-stabilized zirconia from an oxide target); and 2) an epitaxial template for growth of subsequent oxide layers.
• the seed layer should grow epitaxially over the entire surface of the metal substrate and be free of any contaminants that may interfere with the deposition of subsequent epitaxial oxide layers.
• oxide buffer layers can be carried out so as to promote wetting of an underlying substrate layer.
• the formation of metal oxide layers can be carried out using metal alkoxide precursors (for example, "sol gel" precursors), in which the level of carbon contamination can be greatly reduced over other known processes using metal alkoxide precursors.
• This heating step can be carried out after, or concurrently with, the drying of excess solvent from the sol gel precursor film. It must be carried out prior to decomposition of the precursor film, however.
• the carbon contamination accompanying conventional oxide film preparation in a reducing environment is believed to be the result of an incomplete removal of the organic components of the precursor film.
• the presence of carbon-containing contaminants C x H y and C a H t ,O c in or near the oxide layer can be detrimental, since they can alter the epitaxial deposition of subsequent oxide layers.
• the trapped carbon-containing contaminants buried in the film can be oxidized during the processing steps for subsequent oxide layers, which can utilize oxidizing atmospheres. The oxidation of the carbon-containing contaminants can result in CO 2 formation, and the subsequent blistering of the film, and possible delamination of the film, or other defects in the composite structure.
• the carbon-containing contaminants are oxidized (and hence removed from the film structure as CO 2 ) as the decomposition occurs.
• the presence of carbon-containing species on or near film surfaces can inhibit the epitaxial growth of subsequent oxide layers.
• the precursor solution after coating a metal substrate or buffer layer, can be air dried, and then heated in an initial decomposition step.
• the precursor solution can be directly heated in an initial decomposition step, under an atmosphere that is reducing relative to the metal substrate.
• the oxygen level of the process gas is increased, for example, by adding water vapor or oxygen. The nucleation step requires from about 5 minutes to about 30 minutes to take place under typical conditions.
• an epitaxial buffer layer can be formed using a low vacuum vapor deposition process (e.g., a process performed at a pressure of at least about lxlO "3 Torr).
• the process can include forming the epitaxial layer using a relatively high velocity and/or focused gas beam of buffer layer material.
• the buffer layer material in the gas beam can have a velocity of greater than about one meter per second (e.g., greater than about 10 meters per second or greater than about 100 meters per second). At least about 50% of the buffer layer material in the beam can be incident on the target surface (e.g., at least about 75% of the buffer layer material in the beam can be incident on the target surface, or at least about 90% of the buffer layer material in the beam can be incident on the target surface).
• the method can include placing a target surface (e.g., a substrate surface or a buffer layer surface) in a low vacuum environment, and heating the target surface to a temperature which is greater than the threshold temperature for forming an epitaxial layer of the desired material on the target surface in a high vacuum environment (e.g., less than about lxlO "3 Torr, such as less than about lxlO "4 Torr) under otherwise identical conditions.
• a gas beam containing the buffer layer material and optionally an inert carrier gas is directed at the target surface at a velocity of at least about one meter per second.
• a conditioning gas is provided in the low vacuum environment.
• the conditioning gas can be contained in the gas beam, or the conditioning gas can be introduced into the low vacuum environment in a different manner (e.g., leaked into the environment).
• the conditioning gas can react with species (e.g., contaminants) present at the target surface to remove the species, which can promote the nucleation of the epitaxial buffer layer.
• the epitaxial buffer layer can be grown on a target surface using a low vacuum (e.g., at least about lxlO "3 Torr, at least about 0.1 Torr, or at least about 1 Torr) at a surface temperature below the temperature used to grow the epitaxial layer using physical vapor deposition at a high vacuum (e.g., at most about lxl 0 "4 Torr).
• the temperature of the target surface can be, for example, from about 25°C to about 800°C (e.g., from about 500°C to about 800°C, or from about 500°C to about 650°C).
• the epitaxial layer can be grown at a relatively fast rate, such as, for example, at least about 50 Angstroms per second.
• a buffer layer can be formed using ion beam assisted deposition (IBAD).
• IBAD ion beam assisted deposition
• a buffer layer material is evaporated using, for example, electron beam evaporation, sputtering deposition, or pulsed laser deposition while an ion beam (e.g., an argon ion beam) is directed at a smooth surface of a substrate onto which the evaporated buffer layer material is deposited.
• the buffer layer can be formed by ion beam assisted deposition by evaporating a buffer layer material having a rock-salt like structure (e.g., a material having a rock salt structure, such as an oxide, including MgO, or a nitride) onto a smooth, amorphous surface (e.g., a surface having a root mean square roughness of less than about 100 Angstroms) of a substrate so that the buffer layer material has a surface with substantial alignment (e.g., about 13° or less), both in-plane and out-of- plane.
• a buffer layer material having a rock-salt like structure e.g., a material having a rock salt structure, such as an oxide, including MgO, or a nitride
• a smooth, amorphous surface e.g., a surface having a root mean square roughness of less than about 100 Angstroms
• the conditions used during deposition of the buffer layer material can include, for example, a substrate temperature of from about 0°C to about 400°C (e.g., from about room temperature to about 400°C), a deposition rate of from about 1.0
• Angstrom per second to about 4.4 Angstroms per second an ion energy of from about 200 eV to about 1200 eV, and/or an ion flux of from about 110 microamperes per square centimeter to about 120 microamperes per square centimeter.
• the substrate is formed of a material having a polycrystalline, non-amorphous base structure (e.g., a metal alloy, such as a nickel alloy) with a smooth amorphous surface formed of a different material (e.g., Si N 4 ).
• a material having a polycrystalline, non-amorphous base structure e.g., a metal alloy, such as a nickel alloy
• a smooth amorphous surface formed of a different material e.g., Si N 4
• a plurality of buffer layers can be deposited by epitaxial growth on an original IBAD surface.
• Each buffer layer can have substantial alignment (e.g., about 13° or less), both in-plane and out-of-plane.
• an epitaxial buffer layer can be deposited by sputtering from a metal or metal oxide target at a high throughput. Heating of the substrate can be accomplished by resistive heating or bias and electric potential to obtain an epitaxial morphology. A deposition dwell may be used to form an oxide epitaxial film from a metal or metal oxide target.
• the oxide layer typically present on substrates can be removed by exposure of the substrate surface to energetic ions within a reducing environment, also known as Ion Beam etching.
• Ion Beam etching can be used to clean the substrate prior to film deposition, by removing residual oxide or impurities from the substrate, and producing an essentially oxide-free preferably biaxially textured substrate surface. This improves the contact between the substrate and subsequently deposited material.
• Energetic ions can be produced by various ion guns, for example, which accelerate ions such as Ar + toward a substrate surface.
• gridded ion sources with beam voltages greater than 150 ev are utilized.
• a plasma can be established in a region near the substrate surface. Within this region, ions chemically interact with a substrate surface to remove material from that surface, including metal oxides, to produce substantially oxide-free metal surface.
• Another method to remove oxide layers from a substrate is to electrically bias the substrate. If the substrate tape or wire is made negative with respect to the anode potential, it will be subjected to a steady bombardment by ions from the gas prior to the deposition (if the target is shuttered) or during the entire film deposition. This ion bombardment can clean the wire or tape surface of absorbed gases that might otherwise be incorporated in the film and also heat the substrate to elevated deposition temperatures. Such ion bombardment can be further advantageous by improving the density or smoothness of the epitaxial film.
• deposition of a buffer layer can begin.
• One or more buffer layers, each including a single metal or oxide layer, can be used.
• the substrate is allowed to pass through an apparatus adapted to carry out steps of the deposition method of these embodiments. For example, if the substrate is in the form of a wire or tape, the substrate can be passed linearly from a payout reel to a take-up reel, and steps can be performed on the substrate as it passes between the reels.
• substrate materials are heated to elevated temperatures which are less than about 90% of the melting point of the substrate material but greater than the threshold temperature for forming an epitaxial layer of the desired material on the substrate material in a vacuum environment at the predetermined deposition rate.
• high substrate temperatures are generally preferred.
• Typical lower limit temperatures for the growth of oxide layers on metal are approximately 200°C to 800°C, preferably 500°C to 800°C, and more preferably, 650°C to 800°C.
• Various well-known methods such as radiative heating, convection heating, and conduction heating are suitable for short (2 cm to 10 cm) lengths of substrate, but for longer (lm to 100 m) lengths, these techniques may not be well suited.
• the substrate wire or tape must be moving or transferring between deposition stations during the process.
• the substrates are heated by resistive heating, that is, by passing a current through the metal substrate, which is easily scaleable to long length manufacturing processes. This approach works well while instantaneously allowing for rapid travel between these zones. Temperature control can be accomplished by using optical pyrometers and closed loop feedback systems to control the power supplied to the substrate being heated. Current can be supplied to the substrate by electrodes which contact the substrate in at least two different segments of the substrate. For example, if the substrate, in the form of a tape or wire, is passed between reels, the reels themselves could act as electrodes.
• the guides could act as electrodes.
• the electrodes could also be completely independent of any guides or reels as well.
• current is applied to the tape between current wheels.
• the metal or oxide material that is deposited onto the tape is desirably deposited in a region between the current wheels. Because the current wheels can be efficient heat sinks and can thus cool the tape in regions proximate to the wheels, material is desirably not deposited in regions proximate to the wheels.
• the charged material deposited onto the tape is desirably not influenced by other charged surfaces or materials proximate to the sputter flux path.
• the sputter chamber is preferably configured to place components and surfaces which could influence or deflect the sputter flux, including chamber walls, and other deposition elements, in locations distant from the deposition zone so that they do not alter the desired linear flux path and deposition of metal or metal oxide in regions of the tape at the proper deposition temperature. More details are provided in commonly owned United States Patent Application Serial No. 09/500,701, filed on February 9, 2000, and entitled "Oxide Layer Method," and commonly owned United States Patent Application Serial No. , filed on even date herewith, and entitled "Oxide Layer Method.” In preferred embodiments, three buffer layers are used. A layer of Y 2 O 3 or
• CeO 2 (e.g., from about 20 nanometers to about 50 nanometers thick) is deposited (e.g., using electron beam evaporation) onto the substrate surface.
• a layer of YSZ (e.g., from about 0.2 micron to about 1 micron thick, such as about 0.5 micron thick) is deposited onto the surface of the Y 2 O 3 or CeO 2 layer using sputtering (e.g, using magnetron sputtering).
• a CeO 2 layer (e.g., about 20 nanometers thick) is deposited (e.g, using magnetron sputttering) onto the YSZ surface.
• the surface of one or more of these layers can be chemically and/or thermally conditioned as described herein.
• the underlying layer e.g., a buffer layer or a different superconductor layer
• the underlying layer can be conditioned (e.g., thermally conditioned and/or chemically conditioned) so that the superconductor layer is formed on a conditioned surface.
• the conditioned surface of the underlying layer can be biaxially textured (e.g., (113)[211]) or cube textured (e.g., (100)[011] or (100)[011]), have peaks in an X-ray diffraction pole figure that have a full width at half maximum of less than about 20° (e.g., less than about 15°, less than about 10°, or from about 5° to about 10°), be smoother than before conditioning as determined by high resolution scanning electron microscopy or atomic force microscopy, have a relatively high density, have a relatively low density of impurities, exhibit enhanced adhesion to other material layers (e.g., a superconductor layer or a buffer layer) and/or exhibit a relatively small rocking curve width as measured by x-ray diffraction.
• a superconductor layer or a buffer layer e.g., a superconductor layer or a buffer layer
• “Chemical conditioning” as used herein refers to a process which uses one or more chemical species (e.g., gas phase chemical species and/or solution phase chemical species) to affect changes in the surface of a material layer, such as a buffer layer or a superconductor material layer, so that the resulting surface exhibits one or more of the above noted properties.
• “Thermal conditioning” as used herein refers to a process which uses elevated temperature with or without chemical conditioning to affect changes in the surface of a material layer, such as a buffer layer or a superconductor material layer, so that the resulting surface exhibits one or more of the above noted properties.
• thermal conditioning occurs in a controlled environment (e.g., controlled gas pressure, controlled gas environment and/or controlled temperature).
• Thermal conditioning can include heating the surface of the underlying layer to a temperature at least about 5°C above the deposition temperature or the crystallization temperature of the underlying layer (e.g., from about 15°C to about 500°C above the deposition temperature or the crystallization temperature of the underlying layer, from about 75 °C to about 300°C above the deposition temperature or the crystallization temperature of the underlying layer, or from about 150°C to about 300°C above the deposition temperature or the crystallization temperature of the underlying layer). Examples of such temperatures are from about 500°C to about 1200°C (e.g., from about 800°C to about 1050°C).
• Thermal conditioning can be performed under a variety of pressure conditions, such as above atmospheric pressure, below atmospheric pressure, or at atmospheric pressure. Thermal conditioning can also be performed using a variety of gas environments (e.g., an oxidizing gas environment, a reducing gas environment, or an inert gas environment).
• Deposition temperature refers to the temperature at which the layer being conditioned was deposited.
• Crystalstallization temperature refers to the temperature at which a layer of material (e.g., the underlying layer) takes on a crystalline form.
• Chemical conditioning can include vacuum techniques (e.g., reactive ion etching, plasma etching and/or etching with fluorine compounds, such as BF and/or CF 4 ). Chemical conditioning techniques are disclosed, for example, in Silicon Processing for the VLSI Era, Vol. 1, eds. S. Wolf and R.N. Tanber, pp. 539-574, Lattice Press, Sunset Park, CA, 1986.
• chemical conditioning can involve solution phase techniques, such as disclosed in Metallurgy and Metallurgical Engineering Series, 3d ed., George L. Kehl, McGraw-Hill, 1949.
• Such techniques can include contacting the surface of the underlying layer with a relatively mild acid solution (e.g., an acid solution containing less about 10 percent acid, less than about two percent acid, or less than about one percent acid).
• mild acid solutions include perchloric acid, nitric acid, hydrofluoric acid, hydrochloric acid, acetic acid and buffered acid solutions.
• the mild acid solution is about one percent aqueous nitric acid.
• bromide-containing and/or bromine-containing compositions e.g., a liquid bromine solution
• This method can be used to form multiple buffer layers (e.g., two, three, four, or more buffer layers), with one or more of the buffer layers having a conditioned surface.
• the method can also be used to form multiple superconductor layers, with one or more of the superconductor layers having a conditioned surface.
• a superconductor layer can be formed and then thermally and/or chemically conditioned as described above.
• An additional superconductor layer can then be formed on the conditioned surface of the first superconductor layer. This process can be repeated as many times as desired.
• Superconducting articles formed from such precursor compositions can include more than one superconductor layer (e.g., two superconductor layers disposed on each other).
• the combined thickness of the superconductor layers can be at least about one micron (e.g., at least about two microns, at least about three microns, at least about four microns, at least about five microns, or at least about six microns).
• the combined critical current density of the superconductor layers can be at least about 5xl0 5 Amperes per square centimeter (e.g., at least about 1x10° Amperes per square centimeter, or at least about 2x10° Amperes per square centimeter).
• the metal oxyfluoride is converted into an oxide superconductor at a rate of conversion selected by adjusting temperature, vapor pressure of gaseous water or both.
• the metal oxyfluoride can be converted in a processing gas having a moisture content of less than 100% relative humidity (e.g., less than about 95% relative humidity, less than about 50% relative humidity, or less than about 3% relative humidity) at 25°C to form some oxide superconductor, then completing the conversion using a processing gas having a higher moisture content (e.g., from about 95% relative humidity to about 100% relative humidity at 25°C).
• the temperature for converting the metal oxyfluoride can be in the range of from about 700°C to about 900°C (e.g., from about 700°C to about 835°C).
• the processing gas preferably contains from about 1 volume percent oxygen gas to about 10 volume percent oxygen gas.
• methods can be employed to minimize the formation of undesirable a-axis oriented oxide layer grains, by inhibiting the formation of the oxide layer until the required reaction conditions are attained.
• the diffusion of H 2 O into the film and the diffusion of HF out of the film occur at rates such that the formation of the YBa 2 Cu 3 O 7-x phase does not begin at any significant rate until the sample reaches the processing temperature.
• the rates of gaseous diffusion into and out of the film decrease, all other parameters being equal. This results in longer reaction times and or incomplete formation of the YBa 2 Cu 3 O -x phase, resulting in reduced crystallographic texture, lower density, and reduced critical current density.
• the overall rate of YBa 2 Cu 3 O 7-x phase formation is determined, to a significant extent, by the diffusion of gases through the boundary layer at the film surface.
• One approach to eliminating these boundary layers is to produce a turbulent flow at the film surface. Under such conditions, the local gas composition at the interface is maintained essentially the same as in the bulk gas (that is, the pH 2 O is constant, and the pHF is approximately zero). Thus, the concentration of the gaseous products/reactants in the film is not controlled by the diffusion through the gas/film surface boundary layer condition, but rather by diffusion through the film.
• the formation of the YBa 2 Cu 3 O 7- ⁇ phase is inhibited until desired process conditions are reached. For example, the formation of the YBa 2 Cu 3 O 7- ⁇ phase can be inhibited until desired process temperature is reached.
• a combination of: 1) low (non-turbulent) process gas flow, so that a stable boundary layer is established at the film/gas interface, during the ramp to temperature, and 2) high (turbulent) process gas flow, so that the boundary layer is disrupted at the film/gas interface is employed.
• the flow in a three inch tube furnace, can be from about 0.5 to about 2.0 L/min during the temperature ramp from ambient temperature to the desired process temperature. Thereafter, the flow can be increased to a value of from about 4 to about 15 L/min during the time at which the film is being processed.
• a-axis nucleated grains are desirably present in an amount of less than about 1%, as determined by scanning electron microscopy.
• a superconducting layer is deposited on the substrate, intermediate-coated substrate, or buffer-coated substrate, in the form of a precursor composition.
• Two general approaches are presented for the formulation of precursor compositions. In both approaches, the advantages of the method will be redirected if at least a portion of the components is present in solid form.
• the cationic constituents of the precursor composition are provided in components taking on a solid form, either as elements, or preferably, compounded with other elements.
• the precursor composition is provided in the form of ultrafine particles which are dispersed so that they can be coated onto and adhere onto the surface of a suitable substrate, intermediate-coated substrate, or buffer-coated substrate.
• ultrafine particles can be created by aerosol spray, by evaporation or by similar techniques which can be controlled to provide the chemical compositions and sizes desired.
• the ultrafine particles are less than about 500 nm, preferably less than about 250 nm, more preferably less than about 100 nm and even more preferably less than about 50 nm. In general, the particles are less than about 50% the thickness of the desired final film thickness, preferably less than about 30% most preferably less than about 10% of the thickness of the desired final film thickness.
• Precursors for the preparation of ultrafine particles can be metalorganic solutions such as those disclosed in U.S. Patent No. 5,231,074 to Cima et al., PCT Publication No. WO 98/58415, published on December 23, 1998, entitled Controlled Conversion of Metal Oxyfluorides into Superconducting Oxides.”
• the precursor components can be prepared from elemental sources, or from a substantially stoichiometric compound comprising the desired constituents.
• a solid comprising a substantially stoichiometric compound of desired REBCO constituents for example, YBa 2 Cu 3 O 7-x
• a number of solids, each containing a particular constituent of the desired final superconducting layer for example, Y 2 O 3 , BaF 2 , CuO
• spray drying or aerosolization of a metalorganic solution comprising a substantially stoichiometric mixture of desired REBCO constituents could be used to produce the ultrafine particles used in the precursor compositions.
• the precursor composition comprises ultrafine particles of one or more of the constituents of the superconducting layer in a substantially stoichiometric mixture, present in a carrier.
• This carrier comprises a solvent, a plasticizer, a binder, a dispersant, or a similar system known in the art, to form a dispersion of such particles.
• the precursor composition comprises ultrafine particles of one or more of the constituents of the superconducting layer in a substantially stoichiometric mixture, present in a carrier, the carrier being substantially as described above.
• Each ultrafine particle contains a substantially compositionally uniform, homogeneous mixture of such constituents.
• each particle can contain BaF 2 , and rare-earth oxide, and copper oxide or rare earth/barium/copper oxyfluoride in a substantially stoichiometric mixture.
• one or more of the cationic constituents is provided in the precursor composition as a metalorganic salt or metalorganic compound, and is present in solution.
• the metalorganic solution acts as a solvent, or carrier, for the other solid-state elements or compounds.
• dispersants and/or binders can be substantially eliminated from the precursor composition.
• the precursor composition comprises ultrafine particles of rare-earth oxide and copper oxide in substantially a 1 :3 stoichiometric ratio, along with a solublized barium-containing salt, for example, barium-trifluoroacetate dissolved in an organic solvent, such as methanol.
• the substrate is desirably uniformly coated to yield a superconducting film of from about 1 to about 10 microns, preferably from about 1 to about 5 microns, more preferably from about 2 to about 4 microns.
• the final superconducting layer is desirably a film of the oxide complex containing rare earth:barium:copper (REBCO), in a stoichiometric ratio of 1 :2:3 and an oxygen stoichiometry slightly deficient in oxygen.
• REBCO rare earth:barium:copper
• These complexes have the chemical formula "rare-earth”Ba Cu 3 O 7- ⁇ , where "rare earth” includes yttrium, praesodium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, lanthanum and cerium.
• Such complexes include YBa 2 Cu O 7-x , also known as YBCO.
• the extent of oxygen deficiency (that is, the amount that the stoichiometry of oxygen in such complexes is less than 7) is somewhat variable, but can range from about 6.5 to about 6.9.
• the precursor composition can contain a rare earth element, barium, and copper in the form of their oxides; halides such as fluorides, chlorides, bromides and iodides; carboxylates and alcoholates, for example, acetates, including trihaloacetates such as trifluroracetates, formates, oxalates, lactates, oxyfluorides, propylates, citrates, and acetylacetonates, and, chlorates and nitrates.
• halides such as fluorides, chlorides, bromides and iodides
• carboxylates and alcoholates for example, acetates, including trihaloacetates such as trifluroracetates, formates, oxalates, lactates, oxyfluorides, propylates, citrates, and acetylacetonates, and, chlorates and nitrates.
• the precursor composition can include any combination of such elements (rare earth element, barium, and copper) in their various forms, which can convert to an intermediate containing a barium halide, plus rare earth oxyfluoride and copper(oxyfluoride) without a separate decomposition step or with a decomposition step that is substantially shorter than that which may be required for precursors in which all constituents are solubilized, and without substantial formation of BaCO 3 , and which can subsequently be treated using high temperature reaction processes to yield an epitaxial REBCO film with T c of no less than about 89K, and J c greater than about 500,000 A/cm 2 at a film thickness of 1 micron or greater.
• the precursor composition could contain barium halide (for example, barium fluoride), yttrium oxide (for example, Y 2 O ), and copper oxide; or yttrium oxide, barium trifluoroacetate in a trifluoroacetate/methanol solution, and a mixture of copper oxide and copper trifluoroacetate in trifluoroacetate/methanol.
• the precursor composition could contain Ba-trifluoroacetate, Y 2 O 3 , and CuO.
• the precursor composition could contain barium trifluoroacetate and yttrium trifluoroacetate in methanol, and CuO.
• the precursor composition could contain BaF 2 and yttrium acetate and CuO.
• barium-containing particles are present as BaF 2 particles, or barium acetate. It is believed to be undesirable to allow the formation of barium carbonate during processing of the superconductive layer.
• the precursor could be substantially a solublized metalorganic salt containing some or all of the cation constituents, provided at least a portion of one of the compounds containing cation constituents present in solid form.
• the precursor in a dispersion includes a binder.
• the binder functions to hold the fine particles together as an aid in deposition on the underlying layer.
• Suitable binders include cellulose derivatives such as nitrocellulose, methylcellulose, hydroxyethylcellulose, carboxymethylcellulose and other cellulose derivatives known in the art.
• polymeric binders and copolymeric binders such as polyvinyl binders and copolyvinyl binders including polyvinyl aldehydes and copolyvinyl aldehydes such as polyvinyl butyral polymers and copolymers.
• Other binders include various starches from various sources, and chemically modified starches.
• Dispersants can also be included in certain embodiments.
• a wide variety of dispersants can be utilized in particular embodiments.
• any organic solvents which do not solublize all precursor constituents can be used.
• sorbitan esters, sorbitan diesters and sorbitan triesters, including sorbitan trioleate are suitable.
• Further suitable dispersants can readily be chosen by those of skill in the art.
• Dispersions can be maintained mechanically by stirring, shaking, mixing or by ultrasonic agitation.
• Solvents can also be included in certain embodiments, particularly those in which a precursor composition includes a constituent salt or compound dissolved in a solvent, in addition to constituents present as ultrafine particles in a dispersion.
• Solvents can be any which serve to solublize a precursor constituent.
• common volatile organic solvents including ethers such as ethyl ether, dioxane and tetrahydrofuran; esters including alkyl acetates, alkyl propionates and alkyl formates, alcohols including straight chain and branched alkanols, and other solvents such as dimethylformamide, dimethylsulfoxide and acetonitrile can be used.
• ethers such as ethyl ether, dioxane and tetrahydrofuran
• esters including alkyl acetates, alkyl propionates and alkyl formates
• alcohols including straight chain and branched alkanols
• Other suitable solvents can readily be put to use by those of skill in the art.
• the precursor compositions can be applied to substrate or buffer-treated substrates by a number of methods, which are designed to produce coatings of substantially homogeneous thickness.
• the precursor compositions can be applied using spin coating, slot coating, gravure coating, dip coating, tape casting, or spraying.
• multi-layer high temperature superconductors including first and second high temperature superconductor coated elements.
• Each element includes a substrate, at least one buffer layer deposited on the substrate, a high temperature superconductor layer, and optionally a cap layer.
• the first and second high temperature superconductor coated elements can be joined at the first and second cap layers, or can be joined with an intervening, preferably metallic, layer. Exemplary joining techniques include soldering and diffusion bonding.
• Such a multi-layer architecture provides improved current sharing, lower hysteretic losses under alternating current conditions, enhanced electrical and thermal stability, and improved mechanical properties.
• Useful conductors can be made having multiple tapes stacked relative to one another and/or laminated to provide sufficient ampacity, dimensional stability, and mechanical strength.
• Such embodiments also provide a means for splicing coated tape segments and for termination of coated tape stackups or conductor elements.
• this architecture can provide significant benefits for alternating current applications.
• AC losses are shown to be inversely proportional to the effective critical current density within the conductor, more specifically, the cross-sectional area within which the current is carried. For a multifilimentary conductor, this would be the area of the "bundle" of superconducting filaments, excluding any sheath material around that bundle.
• the "bundle" critical current density would encompass only the high temperature superconductor films and the thickness of the cap layer structure.
• the cap layer can be formed of one or more layers, and preferably includes at least one noble metal layer.
• Noble metal is a metal, the reaction products of which are thermodynamically unstable under the reaction conditions employed to prepare the HTS tape.
• exemplary noble metals include, for example, silver, gold, palladium, and platinum.
• Noble metals provide a low interfacial resistance between the HTS layer and the cap layer.
• the cap layer can include a second layer of normal metal (for example, copper or aluminum or alloys of normal metals). In direct current applications, additional face-to-face wires would be bundled or stacked to provide for the required ampacity and geometry for a given application.
• the high temperature superconductor film on the surface of the tapes could be treated to produce local breaks, that is, non-superconducting regions or stripes in the film only along the length of the tape (in the current flow direction).
• the cap layer deposited on the high temperature superconductor film would then serve to bridge the nonsuperconducting zones with a ductile normal metal region.
• An offset in ihe edge justification of the narrow strips or filaments, similar to a running bond brick pattern, would allow current to transfer to several narrow superconducting filaments both across the cap layers and to adjacent filaments, further increasing the redundancy and improving stability.
• a normal metal layer could be included along the edge of the conductor to hermetically seal the high temperature superconductor films and to provide for current transfer into the film, and if necessary, from the film into the substrate.
• coated conductors can be fabricated in a way that minimizes losses incurred in alternating current applications.
• the conductors are fabricated with multiple conducting paths, each of which comprises path segments which extend across at least two conducting layers, and further extend between these layers.
• Each superconducting layer has a plurality of conductive path segments extending across the width of the layer, from one edge to another, and the path segments also have a component of direction along the length of the superconducting layer.
• the path segments in the superconducting layer surface are in electrically conductive communication with interlayer connections, which serve to allow current to flow from one superconducting layer to another.
• Paths, which are made up of path segments are periodically designed, so that current flow generally alternates between two superconducting layers in bilayered embodiments, and traverses the layers through interlayer connections.
• Superconducting layers can be constructed to contain a plurality of path segments which extend both across their widths and along their lengths.
• superconducting layers can be patterned so as to achieve a high resistivity or a fully insulating barrier between each of the plurality of path segments.
• a regular periodic array of diagonal path segments can be imposed on the layer along the full length of the tape. Patterning of superconducting layers to give such arrays can be accomplished by a variety of means known to those skilled in the art, including for example, laser scribing, mechanical cutting, implantation, localized chemical treatment through a mask, and other known methods.
• the superconducting layers are adapted to allow the conductive path segments in their surfaces to electrically communicate with conducting interlayer connections passing between the layers, at or near their edges.
• the interlayer connections will typically be normally conducting (not superconducting) but in special configurations could also be superconducting.
• Interlayer connections provide electrical communication between superconducting layers which are separated by non-conducting or highly resistive material which is positioned between the superconducting layers. Such nonconducting or highly resistive material can be deposited on one superconducting layer. Passages can be fabricated at the edges of the insulating material to allow the introduction of interlayer connections, followed by deposition of a further superconducting layer.
• One can achieve a transposed configuration with coated conductors by patterning a superconducting layer into filaments parallel to the axis of the tape and winding the tape in a helical fashion around a cylindrical form. More details are provided in commonly owned United States Patent
• Barium acetate was purchased from Alfa Aesar (Ward Hill, MA) as crystalline particles, 99.0 to 102.0% (assay), stock number 12198.
• Trifluoracetic acid was purchased from Alfa Aesar, stock number 31771.
• Semiconductor grade methanol was purchased from Alfa Aesar, stock number 19393. 1% and 10% solutions of nitrocellulose in amyl acetate were purchased from Ernest Fullham, Inc.
• Yttrium oxide was purchased from Nanophase Technologies Corp. (Burr Ridge, IL), as 99.5% purity (total rare earth oxide) particles, product code 1600.
• the manufacturer's stated average particle size was 17-30 nm, as determined by BET analysis of specific surface area.
• Barium fluoride was purchased from Alfa Aesar as 99% pure (assay) particles, with less than 40 mesh particle size, stock number 12338. To reduce particle size, the as-received barium fluoride was jet-milled to a best-effort particle size reduction by Jet Pulverizer Co. (Palmyra, New Jersey). Copper oxide particles were purchased from Nanophase Technologies, Inc. as 99.5+% pure particles, product code 0500. The manufacturer's stated average particle size was 16- 32 nm, as determined by BET analysis of the specific surface area.
• the Y 2 O 3 and CuO particles were investigated by transmission electron microscopy.
• the as-received particles were typically spherical, individually less than about 50 nm in diameter, and bridged into networks.
• Barium fluoride was investigated by scanning electron microscopy before and after jet-milling. Before milling, the particles appeared blocky and smaller than the 40 mesh size specified by the manufacturer. The jet-milled particles appeared more uniform in size and were less blocky. Purity of materials was investigated by inductively coupled plasma atomic emission spectroscopy (ICP/AES). The nanophase Y 2 O was assayed as 91% pure, BaF 2 as 88% pure and CuO as 99.4% pure. These materials were also examined for the presence of common impurities, which are shown in Table 1. These materials were not baked prior to analysis, and the major contaminant in Y 2 O 3 and BaF 2 was presumed to be moisture. Jet-milling did not appear to introduce additional contaminants.
• the ⁇ -2 ⁇ X-ray diffraction (XRD) pattern of the as-received Y 2 O 3 matched the monoclinic phase of this material.
• the XRD patterns for BaF 2 before and after milling matched the pattern for frankdicksonite, which is a cubic pattern.
• the XRD pattern of the as-received CuO matched the pattern for the monoclinic (tenorite) phase of this material.
• Three precursor slurries were prepared, as summarized in Table 3. Two of the precursors were prepared by dispersing Y 2 O 3 and CuO into Ba-trifluoroacetate (Ba- TFA) dissolved in methanol, with differences in the amount of methanol used.
• the Ba-TFA was made by dissolving barium acetate in water, then adding trifluoroacetic acid. The trifluoroacetic acid replaced the acetate group on the barium.
• the aqueous Ba-TFA solution was then vacuum dried, and the Ba-TFA obtained from the drying operation was dissolved in methanol.
• a third dispersion was made by dispersing Y 2 O , BaF , and CuO in a 3% solution of nitrocellulose in amyl acetate.
• the yttrium molarity was calculated as (moles Y)/(volume of solution + volume of solids in dispersion).
• the stoichiometry of these precursors, determined by ICP/AES, is presented in Table 4.
• the MOD process employed yttrium acetate, barium acetate, and copper acetate. Each salt was dissolved in water, fluorinated with trifluoroacetic acid, and vacuum dried. The dried salts were then dissolved in methanol to produce the yttrium/barium/copper (YBC) solution for coating. Control of the precursor chemistry, and hence the stoichiometry, is very precise.
• CeO 2 -capped yttria-stabilized zirconia (CeO/YSZ) single crystals except sample AP9 (described below), which was coated onto single crystal strontium titanate.
• the CeO YSZ system was chosen because these are the materials typically employed as the top two buffer layers in development of coated conductor articles. Table 5 provides a summary of the samples that were fully processed and measured electrically. Samples which were processed for development of coating and decomposition cycles but not reacted or measured are not included.
• the time from first introduction to the furnace to the start of cool down was recorded as the decomposition time.
• the furnace profile used to decompose sample AP6 is shown in Fig. 4, the profile used for samples AP9-AP20, and for samples AP29-AP30 is shown in Fig. 5, and the profile used for samples AP22-AP23 is shown in Fig. 6.
• the typical profile used for decomposing an MOD film is shown in Fig. 7.
• Multiple decomposition times indicated in Table 5 specify that samples were repeatedly coated and decomposed as often as necessary to achieve film thickness before reaction.
• the samples were patterned to form bridges to enable I c measurement, with the exception of sample API 8, which was patterned after reaction.
• the XRD pattern of AP6, which was typical of all scans run on decomposed solid-state precursor-coated CeO/YSZ is shown in Fig. 8.
• a typical scan of MOD-coated lanthanum aluminate is shown in Fig. 9.
• the BaF 2 peak at 24.8° in the pattern generated from the MOD-coated sample was shifted to a slightly higher angle, whereas this same peak was not shifted in the scan of the samples according to the invention.
• the isothermal hold time at peak temperature was recorded as the reaction time.
• the furnace profile used for reaction of all samples, except for sample AP9, which had a 735°C peak temperature isothermal hold, is shown in Fig. 10.
• the XRD pattern of the 1.5 micrometer thick sample AP23 after reaction to form YBa 2 Cu O 7- ⁇ is shown in Fig. 11.
• the pattern clearly indicates c-axis texture for the fully converted YBCO film.
• the XRD pattern of a baseline 1.2 ⁇ m thick MOD-coated CeO/YSZ substrate is shown in Fig. 12.
• Figs. 13 and 14, respectively, show expanded scales of the scans in Figs. 11 and 12, indicating that in both systems, a BaCeO 3 reaction product was formed.
• the phase content and c-axis texture of YBCO films formed by the two methods on CeO/YSZ were similar.
• Pole figure analysis was performed using a dedicated Siemens X-ray system with a 2-D detector system to increase analysis speed and sensitivity.
• Fig. 15 shows the (102) YBCO pole figure for sample AP21, which had a relatively weak biaxial texture.
• Fig. 16 shows the (102) YBCO pole figure for the 0.5 micrometer thick sample API 8, which had a stronger biaxial texture.
• the pole figure analysis of a baseline 0.8 ⁇ m thick MOD-coated and reacted film is shown in Fig. 17.
• the thickness of the films used in the measured bridge after reaction were measured by scanning the bridge depth using a white light interferometer. Some conservatism is built into the J c calculations due to such measurements. Since the film surface is relatively low density and rough, the average thickness taken from the rough surface is greater than the actual current-carrying dimension.
• the surfaces of coated and reacted films were examined optically at 8x, 62x, and 500x magnification. Some surfaces were also examined by scanning electron microscopy (SEM) to gain an understanding of the general morphology of the films.
• the general surface morphology of the inventive films is similar to that of the baseline MOD films, although the MOD films appear denser and perhaps smaller in "grain" size.
• the sizes of the surface features are likely functions of both processing parameters (time, temperature) and thickness. It is not clear whether this surface density difference is due to the relative thickness of the samples or if it represents an inherent difference in the YBCO structures produced by the different methods.
• Bridges were patterned on the samples after decomposition by physically scribing parallel lines to define the bridge. Silver pads were deposited onto the fully converted film surface using thermal evaporation to provide for current input and voltage taps. Samples were immersed in liquid nitrogen and critical current density (I c ) measured using the standard four-point measurement technique and a 1 ⁇ volt/cm criterion. This method is known to those of skill in the art. To calculate J c , the I c was divided by the average measured thickness and the width of each bridge as determined from optical analysis. Transport transition temperatures (T c ) were measured by cooling the samples in helium vapor across their transition temperature. J c and I c results are shown above in Table 5.
• I c critical current density
• the high level of impurities in the BaF component of this precursor might have been a likely cause contributing to the poor current carrying performance of these samples.
• T c° values for these inventive samples ranged from 87.3 to 90.1, with ⁇ T C values ranging from 1.1-1.8 K.
• typical T c° values in baseline MOD films are 90-92 K, with ⁇ T C values of 1-2°.
• precursor B was web coated onto a fine silver strip.
• the silver strip was purchased from Cimini and Associates, Inc. (Pawcatuck, CT) and was 3 meters long, 1 inch wide x 0.003 inches thick, and was in the half hard condition.
• Precursor B was applied to the silver as a 18.2 ⁇ m thick layer, which corresponds to a 1.5 ⁇ m final reacted YBCO layer thickness.
• a syringe pump delivered the precursor at 0.77 ml/min into a 14 mm wide stainless steel slot die applicator, which had a 0.004 inch slot height.
• the precursor was agitated in an ultrasonic bath before fluid was drawn into the syringe, and approximately four minutes elapsed before the beginning region of the 3 m length was coated.
• the gap between the applicator and the web was 0.005 inches, and line speed was 3 meter/min. Approximately 3 m of silver was coated and allowed to air-dry before being wrapped into a coil.
• the results presented herein indicate that the J c obtained for the inventive method of preparing superconducting layers are already on the order of 25-50% of the highest J c obtained for YBCO films of comparable thickness below about 1 micron derived from optimized metalorganic solution processes.
• the performance of films produced by the inventive processes is comparable to that of the films produced by the baseline MOD process under the same growth conditions, for films with thicknesses greater than about 1.5 micrometers.
• the inventive process also eliminates the complex decomposition thermal cycle without producing any of the known defects (i.e. cracks, blisters, dewetting) of the MOD processes.
• reaction conditions specifically disclosed herein can be further optimized according to routine experimentation. It is, for example, well known that YBCO reaction processes, and resulting film structure and properties, are strongly affected by such variables as temperature, water and oxygen content in the reaction gas, and gas dynamics within the furnace.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Manufacturing & Machinery (AREA)
• Crystallography & Structural Chemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Superconductors And Manufacturing Methods Therefor (AREA)
• Superconductor Devices And Manufacturing Methods Thereof (AREA)
• Inorganic Compounds Of Heavy Metals (AREA)

Applications Claiming Priority (11)

Publications (1)

Family

ID=27538244

Family Applications (1)

Country Status (6)

Families Citing this family (31)

Family Cites Families (2)

• 2000
  • 2000-07-14 KR KR1020027000994A patent/KR100683186B1/ko active IP Right Grant
  • 2000-07-14 CN CN00810764A patent/CN1364322A/zh active Pending
  • 2000-07-14 KR KR1020027000684A patent/KR20020035837A/ko not_active Application Discontinuation
  • 2000-07-14 KR KR1020027000685A patent/KR20020025957A/ko not_active Application Discontinuation
  • 2000-07-14 AU AU62166/00A patent/AU6216600A/en not_active Abandoned
  • 2000-07-14 JP JP2001512648A patent/JP2003517984A/ja active Pending
  • 2000-07-14 EP EP00948699A patent/EP1198848A1/en not_active Withdrawn
  • 2000-07-14 WO PCT/US2000/019344 patent/WO2001008236A1/en not_active Application Discontinuation

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events