Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1996-05-21
2001-03-27
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C072S042000, C072S282000, C505S430000, C505S431000, C505S433000
Reexamination Certificate
active
06205645
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for converting multifilamentary superconducting ceramic precursors into textured and densified superconducting ceramic composite articles and to the resulting articles. More particularly, it relates to a method of tensioning the precursor during certain stages of the manufacturing process, which increases the efficiency of conventional deformation processing techniques used in texturing superconducting composite articles and improves the physical uniformity and performance of the resulting article.
BACKGROUND OF THE INVENTION
Since the discovery of oxide superconducting materials with transition temperatures above about 20 Kelvins, the possibility of using them to obtain greater efficiency in electrical and magnetic applications has attracted considerable interest.
Although superconducting oxides have complex, brittle, ceramic-like structures which cannot by themselves be drawn into wires or similar conductor forms using conventional metal-processing methods, progress has been made in manufacturing superconducting oxide conductors with high engineering current capacity as composite structures in which the superconducting oxides are supported by a matrix material, typically a noble metal, which adds mechanical robustness to the composite. In preferred manufacturing processes such as the “PIT” process, in both its oxide precursor and metallic precursor variations, and coated conductors, the desired superconducting oxide is formed within or upon the supporting matrix by a combination of phase transformation and oxidation reactions which occur during the manufacturing process. For example, the well-known powder-in-tube (“PIT”) process, which is used to prepare composites in elongated forms such as wires, tapes and cables, includes the steps of: (a) forming a powder of superconductor precursor material; (b) filling a metal container, such as a tube, billet, or grooved sheet, with precursor powder and deformation processing one or more filled containers to provide a composite of reduced cross-section including one or more filaments of superconductor precursor material in a surrounding metal matrix; and (c) further thermomechanically processing the composite to form and sinter filament material having the desired superconducting properties. Multifilamentary composites with the desired number of filaments may be obtained by successive rebundling or cabling iterations, with additional deformation and thermomechanical processing steps as needed.
A key requirement for improving the current-carrying capacity (J
c
) of polycrystalline superconducting ceramics is a high degree of densification and crystallographic alignment or texture of the superconducting grains. In conventional PIT processing, an initial deformation stage, commonly called the breakdown stage, is used to reduce a large diameter, low density precursor composite to a highly aspected, high density tape, cable or wire via one or more deformation drafts. Total reductions in excess of 40% during the breakdown stage are common. During the breakdown stage, the grains of the precursor phases are densified and physically aligned in relation to the direction of elongation, namely primarily such that the c-directions of the grains are aligned orthogonally to the desired current direction along the composite axis, which promotes good intergrain electrical connectivity after phase conversion. In fine multifilamentary composites, the breakdown stage also forms the basic shape and cross-section of the filaments in order to promote reaction induced texture during subsequent heat treatments. Flat, evenly shaped filaments in which one dimension of the filament is no greater than about the longest dimension of the desired superconducting grains have been found to provide improved J
c
performance. Additional intermediate deformation stages, typically at low reductions, may be used after the breakdown stage to reduce the severity of reaction induced defects in the textured superconductor phases and to modulate the mosaic spread of its grains in order to further improve its texture. Between deformation stages, reaction sintering heat treatments are used to convert the oxide particle fragments of the precursor to the desired superconductor or to an intermediate phase, typically also a superconductor, to heal cracks induced by deformation, and to promote texturing by enhancing the anisotropic growth of the superconducting grains. This type of uni-axial texturing has been particularly well developed for the PIT fabrication of the micaceous bismuth-strontium-calcium- copper-oxide (BSCCO) 2223 and 2212 superconducting phases (Bi
2
Sr
2
Ca
2
Cu
3
O
10-x
and Bi
2
Sr
2
Ca
1
Cu
2
O
8-x
respectively), because these oxides exhibit a modest amount of plastic deformation via the activation of the a-b plane slip systems, although it has also been applied with modest success to the rare earth-containing superconducting copper oxides, the thallium-containing superconducting copper oxides and the mercury-containing superconducting copper oxides. Typical prior art processes use a breakdown stage followed by one to four intermediate stages for a total of two to five iterations, each typically involving multiple deformation drafts, although processes employing a breakdown stage with a single draft and, in one embodiment, no further iterations, have also been disclosed. See, for example, co-pending application U.S. Ser. No. 08/468,089, (US '089) filed Jun. 6, 1995 and entitled “Improved Deformation Process for Superconducting Ceramic Composite Conductors”, which is herein incorporated in its entirety by reference. The deformation sequence may be designated by the term “nDS”, in which “D” refers to the deformation step, “S” refers to the sintering or heating step and “n” refers to the number of iterations. Common deformation techniques in nDS processes include extrusion, drawing, roll working, or pressing. The various forms of roll working, such as strip rolling, groove rolling, rod rolling, cover rolling, and turk's heading, are particularly well-suited for continuous processing of long lengths of superconducting material, such as wire, tape or cable. Like many “nDS” techniques, the approach described in US '089 also has an extremely sensitive process response surface, with small variations from optimum processing parameters creating high dimensional variations and large decreases in J
c
. This creates difficulties for large scale manufacturing, as extremely precise control over deformation conditions is hard to maintain over extremely long lengths of wire, cable or tape.
However, to be practical outside the laboratory, most electrical and magnetic applications require that the conductor be manufacturable to precise dimensions at reasonable cost, in addition to having high current-carrying capacity. Long, flexible conductors with specific geometries and low dimensional variations are needed in many applications, including coils, rotating machinery and long length cables. In comparison to conventional conductors of comparable cross-section, high performance superconducting ceramic composites are quite difficult to manufacture to precise specifications and tolerances because of the complexity of the manufacturing process, the number of in situ chemical reactions involved, and the difficulty in predicting the interactions between the components of the composite during the various deformation stages.
Deformation processing of any material is complex. Standard metal-working processes have both imposed stress and imposed displacement boundary conditions. In roll working, for example, standard process parameters that control these conditions include front and back tension, roll diameter, reduction, and friction coefficient. See, e.g. Avitzur, “Handbook of Metal-Forming Processes”, Ch. 13 for a discussion of the non-linear interactions of process parameters for roll working in the simplest case, that of deformation of a pure metal in a system where lateral spread of the metal cannot take place. T
Carter William L.
Christopherson Craig J.
Li Qi
Masur Lawrence J.
Michels William J.
American Superconductor Corporation
Arbes Carl J.
Choate Hall & Stewart
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