Method of making a multifilamentary super-conducting article

Details Method of making a multifilamentary super-conducting article Method of making a multifilamentary super-conducting article

1996-05-21

2002-04-16

Arbes, Carl J. (Department: 3729)

Metal working

Method of mechanical manufacture

Electrical device making

C505S100000, C505S231000

Reexamination Certificate

active

06370762

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 rolling at a reduced coefficient of friction during the breakdown stage, 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 their discovery less than a decade ago, the superconducting ceramics have attracted wide interest, due to their ability to carry currents (below critical temperature, field and current values which are characteristic of each material) almost without resistive losses at relatively high temperatures, above about 20 Kelvin.
Composites of superconducting materials and metals are often used to obtain better mechanical and electrical properties than superconducting materials alone provide. These composites may be prepared in elongated forms such as wires, tapes and cables by processes such as the well-known powder-in-tube (“PIT”) process, which 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 Jc 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 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 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 repair cracks induced by deformation, and to promote texturing by enhancing the anisotropic growth of the superconducting grains. 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. When the steps of deforming and sintering are carried out several times, the process may be both time-consuming and expensive.
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. It is important to note that in conventional PIT processing, the deformation is applied directly to the phases of the initial precursor in the breakdown stage and to the phases of the desired superconducting ceramic or an intermediate (which typically possess either a single set of predominant slip systems as in the case of the BSCCO superconducting oxides, or no active slip system at all, as is the case with all the rare earth-containing superconducting copper oxides, the thallium-containing superconducting copper oxides and the mercury-containing superconducting copper oxides) in the remaining iterations, often called the intermediate stages.
Deformation processing of any material is complex because 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. Some of these, such as tension, will most directly influence stress and others, such as reduction, will most directly influence strain. However, the influence of other process parameters, such as roll diameter and friction, is not easily predicted even in the simplest case, that of deformation of a pure metal in a system where lateral spread of the metal cannot take place. 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 such a case. Lateral spread, a significant issue in the real world, complicates matters by turning a two-dimensional system into a three-dimensional one.
The situation is even more complex in a composite material for which there are discontinuities in materials parameters at each internal interface between one material and another. Where there are significant differences in mechanical properties, such as hardness, between the two materials, geometry can be very important in determining the dominant effects. The greater the differences in the material properties, the more likely it is that localized distortions will be created at the interfaces. Moreover, material properties and processing parameters can interact in unpredictable ways. For example, deformation of a precursor powder may increase its structural integrity over time due to compaction, or decrease it due to breakup of the powder grains and/or macroscopic shear failure.
Common measures of the effectiveness of the deformation process for superconducting composites are expressed as degree of texture, core microhardness, core density, filament homogeneity and filament uniformity. High core microhardness has been associated with improved texturing and core density, but excessive microhardness has been associated with cracking. Core microhardness is a measurement of the hardness of the filament material and matrix microhardness is a measurement of the hard

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