Superconductor technology: apparatus – material – process – Processes of producing or treating high temperature... – Process of making wire – tape – cable – coil – or fiber
Reexamination Certificate
2000-09-20
2003-04-29
Kopec, Mark (Department: 1751)
Superconductor technology: apparatus, material, process
Processes of producing or treating high temperature...
Process of making wire, tape, cable, coil, or fiber
C505S433000, C505S501000, C505S822000
Reexamination Certificate
active
06555503
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to processing of oxide superconductor composites to obtain high density, textured oxide superconductor articles.
Polycrystalline, randomly oriented oxide superconductor materials are generally characterized by their low density and low critical current densities. High oxide density, good oxide grain alignment and grain interconnectivity, however, are associated with superior superconducting properties.
Composites of superconducting materials and metals are often used to obtain better mechanical properties than superconducting materials alone provide. These composites may be prepared in elongated forms such as wires and tapes by the well-known “powder-in-tube” or “PIT” method. When powders include metal oxides or other oxidized metal salts, the method is referred to as “oxide-powder-in-tube” or OPIT. For multifilamentary articles, the method generally includes the three stages of (a) forming a powder of superconducting precursor materials (precursor powder formation stage), (b) filling a noble metal billet with the precursor powder, longitudinally deforming and annealing it, forming a bundle of billets or of previously formed bundles, and longitudinally deforming and annealing the bundle to provide a composite of reduced cross-section including one or more filaments of superconductor precursor material surrounded by a noble metal matrix (composite forming stage); and (c) subjecting the composite to successive asymmetric deformation and annealing cycles and further thermally processing the composite to form and sinter a core material having the desired superconducting properties (thermomechanical processing stage). General information about the OPIT method described above and processing of the oxide superconductors is provided by Sandhage et al. in
JOM
, Vol. 43, No. 3 (1991), pp 21-25, and references cited therein; by Tenbrink et al., “Development of Technical High-Tc Superconductor Wires and Tapes”, Paper MF-1, Applied Superconductivity Conference, Chicago (Aug. 23-28, 1992); and by Motowidlo et al., “Properties of BSCCO Multifilament Tape Conductors”, Materials Research Society Meeting, Apr. 12-15, 1993, all of which are incorporated by reference.
The deformations of the thermomechanical processing state are asymmetric deformations, such as rolling and pressing, which create alignment of precursor grains in the core (“textured” grains) and facilitate the growth of well-aligned and sintered grains of the desired oxide superconducting material during the later thermal processing stages. A series of heat treatments is typically performed during the thermomechanical processing stage to promote powder reactions, including the final thermomechanical processing stages employed to fully convert the filaments to the desired highly textured superconducting phase.
In the practice of the above prior art approach, it has been found that when heating during the thermomechanical processing stage, the oxide grains experience dilation leading to reduced oxide core density and increased porosity of the oxide core. Dilation is the loss of core material density due to introduction of pore space and/or changes in grain size and structure. Dilation is thought to be caused by gas evolution and by the growth of non-aligned oxide grains during heating.
Achieving high density in ceramics and ceramic composites is not a new problem. For other ceramic systems, such as Al
2
O
3
for structural problems, high density is achieved by heating the final product under high pressure.
Current approaches to rectifying the de-densification arising from the annealing process include mechanical deformation to redensify the oxide material. For example, Dou et al., in “Improvements of Critical Current Density in the Bi—Pb—Sr—Ca—Cu—O System Through Hot Isostatic Pressing” (
Physica
C, 167:525 (1990)), report similar results by hot isostatically pressing (HIPing) BSCCO pellets and powders. Bourdillon et al., in “Hot Isostatically Pressed Bi2Sr2Ca2Cu3O10 Coils Made with Novel Precursors,” describe HIPing of a BSCCO 2223 coil. Nhien et al., in “Bulk Texturing of Prereacted Bi/Pb(2223) under Triaxial Stresses at Room Temperature” (
Physica
C 235-240:3404 (1994)), use isostatic confinement coupled with an overload in one direction to promote grain alignment of a fully formed (Bi,Pb)SCCO 2223 material. International Application Publication No. WO 94/00886, entitled “High Tc Superconductor and Method of Making” and published Jan. 6, 1994, also describes an isostatic pressing operation after a heat treatment to impart superconducting properties to the precursor and before a final heat treatment to complete the phase conversion.
These approaches represent attempts to modify the oxide grain structure after de-densification of the oxide core has occurred. Such deformation steps are carried out when phase conversion to the final desired oxide superconductor is complete or nearly complete. While deformation processing may result in increased core density, at this late stage in the process it introduces both intergranular and intragranular cracks in the oxide phase that are highly resistant to healing by conventional annealing processes.
Other examples in the prior art use HIPing to introduce texture into the oxide superconductor composite, in particular, in those instances where asymmetric deformation is not preferred. EP 0 503 525 discloses the preparation of a twisted, multifilamentary oxide superconductor composite. The method relies upon drawing to alter the cross-sectional size and shape of the filaments during assembly of the multifilamentary composite, a method that is known to be ineffective in producing a high density, highly textured oxide phase, i.e., such methods result in f<0.7 (as defined herein) and density within the filaments is less than 60% theoretical. In order to texture to a degree approaching acceptable levels, the composite is HIPed. Thus, HIPing is used to introduce density or texture into the composite and not to retain any previously introduced texture of the composite during subsequent processing steps. EP 0 503 525 does not address the problem of dilation, since the oxide phase was not significantly densified and textured in the first place.
Furthermore, not all deformation processes have the desired effect of texturing and/or densifying the oxide material. See Pachla et al., in “Thick textured films of Bi-type ceramics by hot pressing” (
Applied Superconductivity
, 1(1-3):745 (1993)), who report hot pressing of BSCCO 2212 phase. The process resulted in a significant crushing of the oxide superconductor phase, and did not show evidence of texturing or densification of the BSCCO 2212 phase.
Still other groups have used HIPing to densify powder compacts of the oxide superconductor. Tien et al., in “Densification of Oxide Superconductors by Hot Isostatic Pressing” (
Metallur. Trans. A
, 19A:1841 (July 1988)), report an increase in density of a YBa
2
Cu
3
O
x
powder compact from 65% theoretical density to 99% of theoretical. HIPing was performed on a fully formed oxide superconductor.
Thus, there remains a need to overcome the problem of dilation, i.e., de-densification, of the oxide core material in multifilamentary composites during heat treatments, without the drawback of introducing cracks or other defects in the process. What is needed is a process that prevents or substantially prevents dilation from occurring in the first instance. Such a process would clearly present great advantages over the prior art processes in that no remedial action is required.
SUMMARY OF THE INVENTION
The present invention overcomes the limitations of the prior art by converting a highly textured oxide superconducting precursor into an oxide superconductor, while simultaneously applying a force to the oxide superconductor precursor which at least matches the expansion force experienced by the precursor during phase conversion to the oxide superconductor, whereby the density and the degree of texture of the oxide superconductor precursor are retained or substantially
DeMoranville Kenneth L.
Hellstrom Eric E.
Larbalestier David C.
Li Qi
Parrell Jeffrey A.
American Superconductor Corporation
Hale and Dorr LLP
Kopec Mark
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