WO1997017733A1 - Processing of oxide superconductor cables - Google Patents

Abstract

A method for preparing an oxide superconductor cable includes transposing a plurality of oxide superconductor strands along a longitudinal axis so as to form a cable and exposing the cable to a two step heat treatment after cabling of the oxide strands, the heat treatment comprising: (a) heating the cable to and maintaining the cable at a first temperature sufficient to partially melt the article, such that a liquid phase co-exists with the desired oxide superconductor phase; and (b) cooling the cable to and maintaining the cable at a second temperature sufficient to substantially transform the liquid phase into the desired oxide superconductor. The oxide superconductor multistrand cable includes a plurality of oxide superconductor strands, each of the strands including an oxide superconductor having an irreversible melt characteristic, wherein the plurality of oxide strands are transposed about a longitudinal axis, such that each of the strands are substantially electrically and substantially mechanically isolated; and wherein the cable exhibits critical transport properties (Jc) of at least about 10,000 A/cm2 at 77K, self field.

Description

Processing of Oxide Superconductor Cables

This application is a continuation-in-part application of co-pending application U.S.S.N. 08/041,822, filed April 3, 1993 and entitled "Improved Processing for Oxide Superconductors" and of co-pending application U.S.S.N. 08/198,912, filed February 17, 1994 and also entitled "Improved Processing for Oxide Superconductors".

Field of the Invention This invention relates to cabled superconducting oxide conductors and to a method for their manufacturing. The present invention further relates to a method for healing defects introduced into die oxide superconductor composite during cabling and thereby improving superconducting properties.

Background of the Invention

Since the discovery of oxide superconducting materials with transition temperatures above about 20 Kelvin the possibility of using them to obtain greater efficiency in electrical and magnetic applications has attracted considerable interest. However, to be practical outside the laboratory, most electrical and magnetic applications require flexible cabled lengths of conductor manufacturable with high packing factors which can be manufactured at reasonable cost and with high engineering current-carrying capacity. High packing factor forms are needed because limited space constraints and high overall current requirements are major design issues. Conductors which are flexibly cabled, that is, composed of twisted, helically wound, braided or odierwise transposed bundles of electrically, and sometimes mechanically, isolated conductor strands, are desired in many applications, including coils, rotating machinery and long length cables. In comparison to monolithic conductors of comparable composition and cross-section, cabled forms which are made from a number of isolated conductors strands will have much higher flexibility. Substantially mechanically isolated cable strands have some ability to move within the cable, although some degree of mechanical locking of the strands is desired for stability and robustness of the conductor to stay together during handling and winding. Electrical isolation of the cable strands is preferred but not required. In low temperature superconducting conductors, cables which are made from a number of substantially electrically isolated and transposed conductor strands have been shown to have greatly reduced AC losses in comparison to monolithic conductors. See "Superconducting Magnets" by Martin Wilson (1983.1990), pp 197, 307-309. It has been proposed that the same relation will hold for high temperature superconductors. Flexibility increases in proportion to the ratio between me cable cross-section and the strand cross-section. AC losses are believed to decrease in relation to cable cross-section, strand cross- section and twist pitch. Thus, me greater the number of strands in a cable of given dimension, the more pronounced mese advantages will be.

However, it has not been considered feasible to form oxide superconductors in high winding density, tightly transposed configurations because of me physical limitations of d e material. Superconducting oxides have complex, brittle, ceramic-like structures which cannot by tiiemselves be drawn into wires or similar forms using conventional metal-processing mediods and which do not possess me necessary mechanical properties to withstand cabling in continuous long lengths. Consequently, me more useful forms of high temperamre superconducting conductors usually are composite structures in which me superconducting oxides are supported by a matrix material, typically a noble metal, which adds mechanical robustness to the composite.

Even in composite forms, the geometries in which high-performance superconducting oxide articles may be successfully fabricated are constrained by die relative brittleness of me composite, by the electrical anisotropy characteristic of me oxide superconductor, and by the necessity of "texturing" the oxide material to achieve adequate critical current density. Unlike other known conductors, me current-carrying capacity of a superconducting oxide composite depends significantly on me degree of crystallographic alignment and intergrain bonding of the oxide grains, toge er known as "texturing", induced during me composite manufacturing operation. Known processing methods for obtaining textured oxide superconductor composite articles include an iterative process of alternating anneal and deformation steps. The anneal is used to promote reaction- induced texture (RIT) of me oxide superconductor in which me anisotropic growth of the superconducting grains is enhanced. Each deformation provides an incremental improvement in the orientation of me oxide grains (deformation-induced texturing or DIT). The texture derived from a particular deformation technique will depend on how closely die applied strain vectors correspond to me slip planes in the superconducting oxide. Thus, superconducting oxides such as the BSCCO family, which have a micaceous structure characterized by highly anisotropic preferred cleavage planes and slip systems, possess a highly anisotropic current-carrying capacity. Such superconducting oxides are known to be most effectively DIT textured by non-axisymmetric techniques such as pressing and rolling, which create highly aspected (greater than about 5:1) forms. Other methods of texturing BSCCO 2223 have been described in U.S.S.N. 08/302,601, filed 9/8/94 entiUed "Torsional Texturing of Superconducting Oxide Composite Articles", which describes a torsional texturing technique; U.S.S.N. 08/041,822 filed April 1, 1993, entitled "Improved Processing for Oxide Superconductors"; and U.S.S.N. 08/198,912 filed February 17, 1994, entitled "Improved Processing of Oxide Superconductors" which describes an RIT technique based on partial melting. These techniques have been observed to provide me greatest improvement in me Jc's of BSCCO 2223 samples when used in combination with a highly non- axisymmetric DIT technique, such as rolling.

Although superconducting oxide composite articles may be textured by various methods, including magnetic alignment, longitudinal deformation (DIT) or heat treatment (RIT), not all texturing methods are equally applicable to, or effective for, all superconducting oxides. For example, known techniques for texturing the two-layer and diree-layer phases of the bismuth-strontium-calcium- copper-oxide family of superconductors (Bi₂Sr₂CaιCu₂O. and Bi₂Sr₂Ca₂Cu₃O., also known as BSCCO 2212 and BSCCO 2223, respectively) are described in Tenbrink et al. , "Development of Technical High-T_c Superconductor Wires and Tapes", Paper MF-1, Applied Superconductivity Conference, Chicago(August 23-28,

1992), H. B. Liu and J. B. Vander Sande, submitted to Physica C, (1995), and Motowidlo et al. , "Mechanical and Electrical Properties of BSCCO Multifilament Tape Conductors", paper presented at Materials research Society Meeting, April 12-15, 1993. Micaceous oxides such as the BSCCO family which demonstrate high current carrying capacity in die absence of biaxial texture have been considered especially promising for electrical applications because they can be textured by techniques which are readily scalable to long-lengm manufacturing. Liquid phases in co-existence wim solid oxide phases have been used in processing of oxide superconductors. One type of partial melting, known as peritectic decomposition, takes advantage of liquid phases which form at peritectic points of me phase diagram containing me oxide superconductor. During peritectic decomposition, the oxide superconductor remains a solid until the peritectic temperamre is reached, at which point the oxide superconductor decomposes into a liquid phase and a new solid phase. The peritectic decompositions of Bi₂Sr₂CaCu₂O_g+x, (BSCCO 2212, where 0≤x ≤ 1.5), into an alkaline earth oxide and a liquid phase and of YBa₂Cu₃O_M (YBCO 123, where O≤δ≤ l.O) into Y₂BaCuO_s and a liquid phase are well known. Kase et al. in IEEE Trans. Mag. 27(2), 1254 (1991) report obtaining highly textured BSCCO 2212 by slowly cooling dirough the peritectic point, a RIT technique because BSCCO 2212 totally melts and reforms during melt textured growth, any texturing induced by deformation prior to the melting will not influence the final strucmre. However, BSCCO 2223 cannot be effectively textured by the melt-textured growth technique. Instead of peritectic melting, BSCCO 2223 exhibits irreversible melting in mat solid 2223 does not form directly from a liquid of 2223 composition. RIT techniques applicable to BSCCO 2223 rely on some type of partial melting, such as eutectic melting, solid solution melting or formation of non-equilibrium liquids, in which the oxide superconductor coexists wim a liquid phase rather than being totally decomposed.

Partial melting of (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀₊- ((Bi,Pb)SCCO 2223, where 0<x≤ 1.5) and (Bi)₂Sr₂Ca_ICu₂Oι_0+x ((Bi)SCCO 2223, where 0<x< 1.5) at temperatures above 870 C in air has been reported; see, for example, Kobayashi et al. Jap. J. Appl. Phys. 28, L722-L744 (1989), Hatano et al. Ibid. 27(11), L2055 (Nov. 1988), Luo et al. Appl. Super. 1, 101-107, (1993), Aota et al. Jap. J. Appl. Phys. 28, L2196-L2199 (1989) and Luo et al. J. Appl. Phys. 72, 2385-2389 (1992). The exact mechanism of partial melting of BSCCO-2223 has not been definitively established.

Such partial melting techniques are inherendy more dependent on me geometry of me initial phase than melt-textured growth, and texturing induced by deformation prior to the partial melting will have a significant impact on me texturing of the final product. In short, for superconducting oxides wim irreversible melting characteristics, such as BSCCO 2223, superior texturing and current-carrying capacity are most obtainable in highly aspected forms such as tapes. Unfortunately, highly aspected superconducting oxide tapes are particularly difficult to cable. All superconducting oxide composites are brittle by the standards of conventional conductors. It is well known that exerting any bend strain in excess of a critical strain which is determined by d e composition and geometry of the composite (and which is typically on me order of 0.1 - 1 %) will severely degrade its electrical and mechanical properties. Strands which are round in cross-section can be bent in any plane and me bend strain will be the same, but the strain on a highly aspected strand will depend on me bend direction, wim highest strains when die bend is in the plane of the longer cross-sectional dimension. The effect on strand performance can be considerable, since me bend strain increases proportionally to the thickness of me bent material and d e critical current drops asymptotically at bend strains in excess of me critical strain.

Since me lowest coupling losses are predicted to come from fully transposed cables, limitations on the direction in which the strands can be cabled also limits the potential usefulness of the cabled conductor. Unfortunately, some forms of transposition makes it inevitable that some portion of the cabled conductor will not be oriented in me preferred direction. Thus, an important consideration in fabricating high performance oxide superconducting conductors is maximizing the portions which do have the desired orientations. It is mus desirable to main a common orientation for all strands in the cable. In rigid cabling techniques the oxide superconducting strands rotate around cable axis resulting in strands of various orientations. In planetary cabling, the oxide superconductor strands do not rotate and transposition only results in slight misorientation.

The difficulties of handling superconducting oxide strands appear even more pronounced when me need for a low cost, scalable cable manufacturing process is considered. There are a number of well-known cabling techniques, such as Rutherford cabling, braiding, and other forms of Litz cabling, for transposing low aspect ratio strands of conventional conductor material on automated machinery, which rely on gradual radial bending of me conductor strands, but to make high packing factor cables on diese machines requires bending strains in excess of those tolerated by conventional oxide superconductor strands. The problem is even worse for aspected forms. The best-known automatic technique for cabling conventional highly aspected conductors requires sharp bends in me strand at regular intervals and so, not surprisingly, has never been demonstrated to be practicable for oxide superconducting composites.

Summary of the Invention These and omer objects of the invention are obtained by subjecting an oxide superconductor cable to one or more two step heat treatments, after cabling or deformation, or both, of the article. The two step heat treatment includes (a) heating the cabled article at a temperature sufficient to partially melt the cabled article, such mat a liquid phase co-exists with the desired oxide superconductor phase; and (b) cooling the cabled article to a temperamre sufficient to transform me liquid phase into the desired oxide superconductor, wim no deformation or cabling occurring after the final heat treatment. The deformation or cabling operations are those which introduce strains of about at least 5-10% and which introduce defects peφendicular to me direction of current flow resulting in significant loss of superconducting performance as measured by critical current. Strain is defined wim respect to the oxide superconductor cable itself, as opposed to strain of the individual oxide strands or oxide filaments. In another aspect of the invention, an oxide superconductor cable is prepared by exposing an oxide superconductor cable to a two step heat treatment and ereafter texturing the oxide superconductor cable. The heat treatment comprises d e steps of (a) heating die cable to and maintaining the cable at a first temperamre sufficient to partially melt the cable, such mat a liquid phase co-exists wim the desired superconducting oxide phase; and (b) cooling the cable to and maintaining the cable at a second temperamre sufficient to substantially transform the liquid phase into the desired oxide superconductor. The oxide superconductor cable is men textured. The texturing process may be selected such mat no further deformations are introduced into the cable. Suitable texturing processes include reaction induced texturing (RIT) and magnetic field induced grain alignment, discussed above. Alternatively, me texture operation may cause defects, i.e., deformation induced texturing (DIT), in whihc case, it may be desirable to perform a subsequent two step heat treatment, as described in steps (a) and (b), above.

In anodier aspect of me present invention, an oxide superconductor cable may be prepared by texturing an oxide superconductor cable and thereafter exposing the textured oxide superconductor cable to a two step heat treatment of the invention. Th texturing process may be selected such d at no fiirther deformations are introduced into the cable. Alternatively, the texture operation may cause defoects, i.e., deformation induced texturing (DIT).

In another aspect of the invention, an oxide superconductor cable containing a desired oxide superconductor phase is exposed to a one or more two step heat treatments after deformation or cabling, or bo , of the oxide superconducting cable which includes (a) forming a liquid phase in the oxide superconducting cable such mat the liquid phase co-exists wid the desired oxide superconductor solid phase; and men (b) transforming the liquid phase into me desired oxide superconductor, widi no deformation or cabling occurring after d e final heat treatment. Cabling and deformation after final heat treatment referred to are diose which introduce strains of about at least 5-10% and which introduce defects perpendicular to me direction of current flow resulting in significant loss of superconducting performance as measured by critical current. Strain is defined wim respect to the oxide superconductor cable itself, as opposed to stain of die individual oxide strands or oxide filaments. In preferred embodiments, the liquid phase wets surfaces of defects contained widiin the oxide superconductor cable strands. The defects are healed upon transformation of the liquid to die desired oxide superconductor. The partial melting of step (a) and me transformation of step (b) are effected by selection of appropriate thermodynamic state variables, for example, temperamre, P₀₂, P^ and total composition. In principle, deformation or cabling may occur during me final heat treatment up to immediately prior to the completion of step (a), providing mat die liquid phase is available for a period of time sufficient to wet defect surfaces. By "cable", as diat term is used herein, it is meant an assemblage of a number of individual strands in close proximity along their length in a periodic arrrangement by techniques including transposing, interweaving, twisting, braiding, helically winding, and me like, of the strands. Each strand of die cable may be substantially electrically isolated, and the cable may be flexible. By "strand", as d at term is used herein, it is meant the individual lengths of oxide superconductor which are used to weave, to transpose or odierwise form me oxide superconductor cable. The strands may be rounded or may have a flattened, aspected cross-sectional geometry because of the deformation processes used to DIT texture. The strands themselves may be composed of one or multiple filaments of oxide superconductor supported widiin or on a malleable, conductive matrix, preferably a noble metal, or may themselves be cables.

By "two step heat treatment", as that term is used herein, is meant a heat treatment for healing defects and forming an oxide superconductor. By "final two- step heat treatment", as that term is used herein, it is meant a heat treatment for forming an oxide superconductor after which no further deformation or cabling occurs. However, heat treatments for purposes other than diose stated herein, such as, for example, oxygenation of the oxide superconductor, are possible.

By "partial melt", as that term is used herein, it is meant me oxide superconductor article is only partially melted, and mat the desired oxide superconductor is present during melting.

By "deformation" as that term is used herein, it is meant a process which causes a change in the cross-sectional shape of the article, without loss of mass. By "oxide superconductor precursor", as diat term is used herein, it is meant any material that can be converted to an oxide superconductor upon application of a suitable heat treatment. Suitable precursor materials include but are not limited to metal salts, simple metal oxides, complex mixed metal oxides and intermediate oxide superconductors to the desired oxide superconductor.

By "desired oxide superconductor", as diat term is used herein, it is meant the oxide superconductor which it is desired to ultimately prepare. An oxide superconductor is typically me "desired" oxide superconductor because of superior electrical properties, such as high T_c and/or J_c. The desired oxide superconductor is typically a high T_c member of a particular oxide superconductor family, i.e., BSCCO 2223, YBCO 123, TBCCO 1212 and TBCCO 1223.

By "intermediate oxide superconductor", as mat term is used herein, it is meant an oxide superconductor which is capable of being converted into a desired oxide superconductor. However, an intermediate oxide superconductor may have desirable processing properties, which warrants its formation initially before final conversion into the desired oxide superconductor. The formation of an "intermediate oxide superconductor" may be desired, particularly during anneal/deformation iterations, where the intermediate oxides are more amenable to texturing than the desired oxide superconductor. In preferred embodiments, me intermediate oxide superconductor is

BSCCO 2212 or (Bi,Pb)SCCO 2212 because it is readily textured by the deformation/anneal iterations. The intermediate oxide superconductor is men converted to a desired oxide superconducting phase, typically BSCCO 2223 or (Bi,Pb)SCCO 2223. The partial melting of step (a) may be carried out at a temperamre in the range of 820-835 °C at 0.075 atm O₂. The transformation of the liquid in step (b) may be carried out at a temperature in the range of 790- 820°C at 0.075 atm O₂. In other preferred embodiments, the desired oxide superconductor, may be YBCO 123, Y₂Ba₄Cu₇O₁₄__δ (YBCO 247), (Tl,Pb),Ba₂Ca,Cu₂O_{60 ± y} (TBCCO 1212) or (Tl,Pb),Ba₂Ca₂Cu₃O₈.o _{± y} (TBCCO 1223), where O≤δ≤ 1.0 and y ranges up to 0.5. The stated stoichiometries are all approximate and intentional or unintentional variations in composition are contemplated wi iin the scope of the invention. In other preferred embodiments, die liquid phase is formed in me range of 0.1-30 vol%. In yet other preferred embodiments, me anneal of the first and second anneal/deformation iterations partially melts the oxide superconductor cable. In yet anomer aspect of the invention, an oxide superconductor cable is exposed to one or more two step heat treatments after a deformation or cabling step, which includes (a) heating the cable at a temperature substantially in the range of 810-860^*C for a period of time substantially in the range of 0.1 to 300 hours at a P₀₂ substantially in the range of 0.001-1.0 atm; and (b) cooling d e cable to a temperature substantially in the range of 780-845 'C for a period of time substantially in die range of 1 to 300 hours at a P₀₂ substantially in the range of 0.001-1.0 atm, with no deformation or cabling occurring after the final heat treatment.

In yet another aspect of the present invention, an oxide superconductor cable containing a desired oxide superconductor phase is exposed to a final heat treatment after a deformation or cabling step, which includes (a) subjecting me cable to an oxygen partial pressure sufficient to partially melt the oxide superconducting article, such that a liquid phase co-exists wim the desired oxide superconductor; and (b) raising to an oxygen partial pressure sufficient to transform the liquid phase into die desired oxide superconductor.

Thus, a highly textured cabled conductor wim improved AC loss characteristics containing a superconducting oxide wim irreversible melting characteristics such as BSCCO 2223, and a process for manufacturing it is provided. A transposed cabled conductor containing superconducting oxide strands in highly aspected forms and a method for manufacturing it is also provided. The novel cabled conductor manufacturing process of the invention allows a superconducting oxide composite to be used wim conventional high-speed cabling equipment. The method improves superconducting performance of oxide superconductor cables by healing cracks and defects formed during cabling of oxide superconductors strands. Cables having a critical current density of about 10,000 A/cm² at 77K, self field, have been prepared in accordance with me method of the invention. A feature of me invention is a two-step heat treatment which introduces a small amount of a liquid phase co-existing widi me oxide superconductor phase, and men transforms the liquid back into die oxide superconductor phase.

An advantage of me invention is the production of highly defect-free oxide superconductor cables which exhibit superior critical current densities.

Brief Description of the Drawing The invention is described wim reference to the Drawing, which is provided for die purpose of illustration only and is in no way limiting of the invention, and in which:

Figure 1 is a processing profile of me fmal heat treatment of the invention; Figure 2 is a processing profile used to obtain a textured oxide superconductor cable according to the method of me invention;

Figure 3 is a schematic diagram illustrating processes of the present invention;

Figure 4 is a schematic illustration of a cabling operation according to the invention;

Figure 5 is a power vs. current for a cable of the present invention determined at a variety of magnetic field strengms; Figure 6 is a V-I plot (electric field vs. current) for a cable of die present invention determined at a variety of magnetic field strengms; and

Figure 7 is a V-I plot (electric field vs. current) at 77K, self field, for a cable prepared widiout d e method of the invention.

Description of the Preferred Embodiment

The present invention is directed to a highly textured oxide superconductor cable having improved AC loss characteristics, as compared to a monolith conductor. The oxide superconductor used in die cable possesses irreversible melting characteristics which lends itself to me improved texmring and critical current density observed in die invention.

The present invention also is a method for improving me critical current density of oxide superconductor cables by healing defects, such as micro- and macrocracks and bending strain defects, incurred upon DIT deformation, cabling, or bo , of the individual oxide superconductor composite strands. The present invention calls for a one or more two-step treatments after deformation, cabling, or bo of die oxide superconductor cable, in which (a) a liquid phase is formed such that the liquid phase co-exists with me desired oxide superconductor; and (b) die liquid phase is men transformed into d e desired oxide superconductor widiout any intermediate deformation. The methods of me invention can be used to heal defects in any oxide superconductor or superconducting composite cable which result from DIT processing and/or cabling operations. The two step heat treatment operates in the following manner to heal defects. The liquid phase is formed upon partial melting of the oxide superconductor cable. During partial melting of the cable, non-superconducting materials and intermediate oxide phases may be present wim the desired oxide superconductor phase. During me partial melting step of the invention the desired oxide superconductor, d e non-superconducting materials, oxide superconducting precursors, the desired oxide superconductor or a mixture of these components may melt to form the liquid phase. The above process, which required diat liquid co-exist with die desired oxide superconductor phase, is distinguished from those which involve the peritecitic decomposition of the oxide superconductor, such as described by Kase et al. , in which me desired oxide superconductor decomposes during me melting process.

The type of cabling styles which are contemplated for use in d is process include, but are in no way limited to, Roebel cabling, Rutherford cabling, braiding and odier forms of Litz cabling. Rigid or planetary forms of any of tiiese may be used. Litz cable has complete transposition of strands. Roebel, Rutherford and braids are special types of Litz cables. Some cable types, such as a six around one configuration do not have complete transposition, but may also be satisfactory. Suitable strand texmring and cabling techniques are set forth in United States application entitled "Cabled Conductors Containing Anisotropic Superconducting Compounds and Mediod for Making Them", filed on even date herewith, in

United States application entitled "Low Resistance Cabled Conductors Comprising Superconducting Ceramics" filed on even date herewim, and in United States application entitled "Low Aspect Ratio Superconductor Wire" filed on even date herewidi.

The method of me invention is particularly useful for oxide superconductor articles which possess defects peφendicular to d e direction of current flow. In such instance, d e defects disrupt the percolative pamway for current flow. It is expected dierefore, that healing of such defects will have a marked effect on current carrying ability.

Fig. 1 shows a processing profile of me two-step heat treatment of the invention. A dashed line 10 indicates a processing point at which a liquid phase is foimed for a given set of processing conditions, e.g., T, P₀₂, P_t^ and/or oxide composition.

In me oxide superconductors and superconducting composites disclosed herein, processing conditions for obtaining me requisite liquid and solid oxide phases are well established and die relationship between temperamre, oxygen partial pressure and total pressure is reasonably well understood. For further information on me phase diagrams for YBCO, BSCCO and me thallium-based systems, the interested reader is directed to "Phase Diagrams for High T_c Superconductors", John D. Whitler and Robert S. Roth, Ed.; American Ceramic Society, Westerville, OH. Presence of a liquid phase can also be determined experimentally by use of such conventional techniques as differential diermal analysis (DTA). In DTA, exothermic and endomermic reactions as a function of temperature can be identified and attributed to various thermodynamic and chemical processes. It is possible to identify endodiermic processes corresponding to partial melting, i.e., liquid phase formation.

It is desired d at only a small amount of liquid be formed during partial melting. The reason for this is that, in preferred embodiments of the invention, at the time mat die two-step heat treatment is applied, the article may already possess substantial texmre.. Complete or significant liquid formation at this point would result in loss of texmre. Volume percent of the liquid phase is typically in die range of 0.1 to 30. The oxide superconductor strands are cabled at a point lla before a two- step heat treadnent, at which time bending strain may introduce defects, such as microcracks, into the cabled article. The oxide superconductor strands are typically deformed at a point 11 before a two-step heat treatment, at which time defects such as microcracks may be introduced into die article. Suitable deformation can include swaging, extruding, drawing, pressing, hot and cold isostatic pressing, rolling, and forging of wires, tapes and a variety of shaped articles. However, in some embodiments of the invention, the strands may be textured before or after cabling by an alternative texturing method which does not independently introduce defects into die cabled article. As will be seen in a discussion below, cabling and deformation or odier texturing steps may be performed at different stages in the process wid respect to each odier and widi respect to one or more of the two-step heat treatments, and such variations are widiin die scope of me invention. Conventional cabling machines used to cable conventional current carrying wires may be used. By way of example only, these may include Rudierford cabling, braiding, Roebel cabling, and odier forms of Litz cabling machines. A Litz cable is any cable with transposed, insulated strands; however, an uninsulated strand may also be cabled. These will not retain mechanical or electrical isolation but may be useful for DC applications. Referring again to Fig. 1, the processing conditions are adjusted to bring the cable to point 12 where die article is partially melted and a liquid phase co¬ exists widi d e desired oxide superconductor phase. The cable is held at point 12 for a period of time during which me defect surfaces contained widiin d e oxide superconductor are wet by me newly-formed liquid. In die case of BSCCO-2223, a temperamre of 820-835 ^*C at 0.075 atm O₂ for 0.1-300 hours and preferably 12- 300, and more preferably 50-200 hours is sufficient.

The processing parameters are then adjusted to bring d e oxide superconductor cable to point 13 where me liquid phase is consumed and me desired oxide superconductor phase is formed from the melt. In me case of BSCCO-2223, a temperature of 820-790' C at 0.075 atm O₂ for 1 to 300 hours is sufficient. The processing temperamre will vary dependent upon die oxygen pressure. Additionally, variations in me chemical composition of the article will also affect selection of temperamre and pressure. In particular, it has been noted diat addition of silver to the oxide composition lowers the temperamre range for partial melting, particularly at higher P₀₂ (0.1-1.0 atm).

Hence, me two-step heat treatment heals cracks and odier defects. The partial melting during the final part of die process can perform two tasks. Firstly, the final conversion of the oxide phases to d e desired oxide superconductor phase is kinetically enhanced by die presence of die liquid phase, in part, due to die enhanced diffusivity of die oxide superconductor constiments. The conversion rate of BSCCO 2212 to BSCCO 2223, for example, is greatly accelerated, allowing the formation of a microscopically crack-free, interconnected BSCCO 2223 phase. Secondly, d e cracks formed during die prior deformation or cabling steps are healed by rapid growdi of die oxide superconductor grains at die crack site.

Various processing parameters can be controlled to obtain die necessary partial melt and oxide reforming steps. For example, P₀₂ can be held constant and temperature can be raised to promote melting and formation of the liquid phase and lowered to regenerate die desired oxide superconductor. Alternatively, temperamre can be held constant, and P₀₂ can be lowered to promote the partial melting of me oxide superconductor article and raised to reform the oxide superconductor. For constant P_o2 conditions, temperamre should increase and for constant temperature conditions, P₀₂ should decrease sequentially through me two- step process. Thus, conditions are selected which give a two-step process, in which die d eπnodynamic state is changed from the first to the second condition. Ideally d e diermodynamic state is altered so as to destabilize die liquid in die second step witii respect to die desired oxide phase superconductor. This is in contrast to systems in which conditions are varied between a first and second step, but in which such adjustments to not change me thermodynamic state of the system with respect to d e stability of me liquid phase.

The processing conditions can be changed rapidly from point 12 to point 13 of me process (fast ramp rate). Alternatively, me oxide superconductor can be subjected to gradually changing conditions (of temperamre or pressure) between point 12 and point 13 of die process designated by d e curve 14 in Fig. 1 (slow ramp rate). In anomer alternative embodiment, there need be no "hold" at 13. The processing conditions can be slowly ramped from the processing conditions defined at point 12 to die processing conditions defined for point 13. This process is illustrated by curve 15 in Fig. 1.

The method of forming textured oxide superconducting cables is described with reference to oxides of me BSCCO family; however, this is in no way meant to limit the scope of me invention. The present invention can be practiced widi any oxide superconductor system in which a liquid phase co-exists wim an oxide superconductor phase such diat an irreversible melt occurs and which is amenable to deformation- induced texmre processing. Mediods of obtaining highly textured oxide superconducting strands using die two-step heat treatment of d e invention is described in detail in U.S.S.N. 08/041,822 and is incoφorated herein by reference. Fig. 2 shows a processing profile for a method of die invention used to obtain highly textured oxide superconductor cable using diis two-step heat treatment. The cabling operation may be performed at various stages in die process, as is discussed hereinbelow.

In a preferred embodiment, an oxide superconductor precursor is subjected to one or more first anneal/deformation iterations, denoted by step 20 and step 21, respectively, of Fig. 2. The oxide superconductor precursor can be any combination of materials which will yield die desired oxide superconductor upon reaction. In particular, it may be a metallic alloy containing me metallic constiments of me desired oxide superconductor and optionally containing silver. Alternatively, die constituent simple metal oxides, mixed metal oxides, metal salts and even intermediate oxide superconductors of the desired oxide superconductor may be used as a precursor. The precursor may optionally be mixed widi a matrix metal, such as silver, and/or may be sheathed in a matrix material in a powder-in-tube configuration.

The anneal 20 of die anneal/deformation iteration serves two puφoses in the process. Firstly, die anneal is sufficient to form an oxide superconductor and results typically in a mixture of superconducting and secondary phases. "Secondary phases" include sub-oxide or non-superconducting oxide species which require further processing to form an oxide superconductor phase. BSCCO-2212 is often die intermediate oxide superconductor because it is readily textured during mechanical deformation. BSCCO-2223 is the typical desired oxide superconducting phase because of its high critical temperamre. Secondly, the anneal promotes reaction-induced texmre.

The deformation 21 of the article promotes deformation-induced texture. One or more iterations can be performed. Fig. 2 shows two first anneal/deformation iterations, by way of example only. If more than one iteration is performed, bodi conversion to die superconducting phase and development of texmre can be done in incremental stages.

If me desired oxide superconductor is not formed in the first anneal/deformation iterations, die second step of d e process may consist of one or more second anneal/deformation iterations to form the desired oxide superconductor and to further texmre the oxide superconductor phase. The article is annealed in a step indicated by 22 whereby die desired oxide superconductor is formed and reaction-induced texmre can occur. Secondary phases react wim BSCCO-2212 to form me desired oxide superconductor, BSCCO-2223. The article is deformed in a subsequent step indicated by 23, whereby deformation- induced texmre can occur. One or more second anneal/deformation iterations can be performed. Fig. 2 shows two iterations, by way of example only. If more than one iteration is used, only a portion of me intermediate oxide superconductor, need be converted into d e desired oxide superconductor with each iteration.

Conditions known to form intermediate and desired oxide superconductors are well known in the art. Suitable conditions are described in Sandhage, et al. JOM, 21 (Mar. 1991), hereby incoφorated by reference.

Practically, d e incremental improvement in alignment for both anneal/deformation cycles will decrease markedly after several iterations, however, diere is no tiieoretical limit to the number of iterations mat can be used. The strain introduced in die deformation step can range up to 99%. The strains applied in each deformation/anneal iteration may be constant or they may be changed for each subsequent iteration. It is particularly desirable in some embodiments, to use decreasing strains with each subsequent iteration.

It is also possible to adjust die processing conditions to promote partial melting during the anneal 20 or 22 of die anneal/deformation iterations, indicated by step 24, to assist in grain growdi and enhance reaction kinetics (reaction- induced texture). An anneal in the range of 820-835 ^*C in 0.075 atm O₂ and 1 atm total pressure for 0.1 to 100 hours is typical for partial melting to occur.

The above description is directed to d e formation of a textured oxide superconductor widiin the individual strands. A cabling step 30 may be carried out at a number of stages during die processing of the oxide superconductor cable, designated 30a, 30b, 30c, 30d in Figs. 2 and 3. The choice of when in d e process to cable die oxide strands depends upon die namre of the oxide superconductor and the type of cabling operation to be performed. Generally, cabling early in the manufacturing process is preferred to get uniform good direction texture independent of cabling style; also earlier cabling may be preferred to minimize strain. It also may be desirable to coat each individual oxide superconductor strand with an insulating layer, such as for example, MgO prior to cabling. Alternatively, me final cabled article may be coated widi an insulating layer. Coordination of oxide superconductor formation and cabling operation is shown in Fig. 3.

In one embodiment, die oxide superconductor strand is processed by one or more suitable texmring methods in order to convert die precursor into die desired oxide superconductor and to substantially completely texmre the oxide superconductor. In die most preferred embodiment illustrated in Fig. 3(a), the texturing is accomplished by successive anneal/deformation iterations. Thereafter, die textured oxide strands are cabled in a step 30a (see, Figs 2 and 3). The cable is d en subjected to die final two-step heat treatment of me invention in order to heal die defects introduced in die cabling step, and if deformation was performed, in die deformation steps.

In anodier embodiment illustrated in Fig. 3(b), the oxide superconductor strand is processed by successive anneal/deformation iterations in order to convert the precursor into d e desired oxide superconductor and to substantially completely texmre the oxide superconductor. A two-step heat treatment of the invention is performed in order to heal defects (microcracks and die like) introduced in d e deformation processing steps. Thereafter, the individual oxide superconductor strands are cabled in a step 30b (see, Figs. 2 and 3). A final two-step heat treatment of the invention is performed in order to heal defects introduced in die cabling process. It is recognized diat odier wire processing operations may benefit from the two-step heat treatment of the invention. For example, coil formation and spooling at small radii of curvature may introduce bending strains similar to those introduced in cabling operations. It is expected that performing a two-step heat treatment of me invention will benefit current carrying properties after these operations as well. The processes illustrated in Figs. 3(a) and 3(b) benefit from the full texmring of me individual oxide strands before die cabling process. While optimal texmring in the individual strands is beneficial to current carrying capacity, the oriented strands may have a lower tolerance to bending strains.

Anodier embodiment is illustrated in Fig. 3(c). The oxide superconductor strand is processed as above by successive texmring operations, preferably including successive anneal/deformation iterations, in order to convert die precursor into d e desired oxide superconductor and to texture the oxide superconductor. At some point during diis iterative process, die not-yet-fully- reacted- and-textured oxide superconductor strands are cabled in a step 30c (see, Figs. 2 and 3). Further texmring is performed on die cable to complete the reaction to die oxide superconductor and to fully texture die oxide superconductor. Thereafter, a final two-step heat treatment is performed in order to heal defects introduced by bodi die deformation and cabling processes. A typical processing sequence may include a first anneal and deformation (20, 21), a cabling operation (30c), and a second anneal and deformation (22, 23), followed by die final two- step heat treatment of the invention. In yet anodier embodiment of the invention, die cabling operation is performed on a precursor strand before a significant texmre is developed in strands and while density is low. Thereafter, a two-step heat treatment is performed in order to heal defects introduced by the cabling processes, and texmring is completed. Fig. 3(d) illustrates one such approach, in which d e cabling operation 30d is performed on a precursor strand prior to a series of successive texture-inducing deformation and reaction iterations (see, Figs. 2 and 3) where the number of iterations, n, can vary from zero to five, and a final two-step heat treatment heals defects induced by bodi the cabling and deformation steps. However, a plurality of two step heat treatments might equally be performed. Because die cabling operation illustrated in Figs. 3(c) and 3(d) is performed on me oxide superconductor strands diat are not fully textured, d at is, me aspect ratio of d e oxide grains is less than optimal and me density is relatively low, it may be expected diat d e deleterious effect of cabling on die critical current density is reduced; however, subsequent texturing operations may be less efficient due to d e varied strand orientations after cabling.

The oxide superconductors which make up me oxide superconductor cables of the present invention are brittle and typically would not survive a mechanical deformation process, such as rolling or pressing. For mis reason, the oxide superconductors of me present invention are typically processed as a composite material including a malleable matrix material. In particular, silver is preferred as die matrix material because of its cost, nobility and malleability; however, odier noble metals may be used. A metal is considered noble when it is inert to oxidation and chemical reaction under die processing conditions of die oxide superconductor. The oxide superconductor strands may be processed in any shape, however, the form of wires, tapes, rings or coils are particularly preferred. The oxide superconductor strand may be encased in a silver sheatii, in a version of die powder- in-tube technology. The oxide superconductor strand can take the form of multiple filaments embedded widiin a silver matrix. For further information on superconducting tapes and wires; see, Sandhage et al.

Example 1 The following example describes d e manufacture of an oxide superconductor strand for use in die cabling operations of me present invention which is described in U.S.S.N. 08/041,822 and is hereby incoφorated by reference, and compares the transport critical current characteristics of a samples treated widi die two-step heat treatment of me present invention to those of conventionally processed samples. Precursor powders were prepared from the solid state reaction of freeze- dried precursor of die appropriate metal nitrates having me nominal composition of 1.7:0.3: 1.9:2.0:3.1 (Bi:Pb:Sr:Ca:Cu). Bi₂O₃, CaCO₃, SrCO₃, Pb,O₄ and CuO powders could be equally used. After thoroughly mixing the powders in the appropriate ratio, a multistep treatment (typically, 3-4 steps) of calcination (800 °C ± 10°C, for a total of 15 h) and intermediate grinding was performed in order to remove residual carbon, homogenize the material and to generate d e low T_c BSCCO-2212 oxide superconductor phase. The powders were packed into silver shead s having an inner diameter of 0.625" (1.5875 cm) and a length of 5.5" (13.97 cm) and a wall diickness of 0.150" (0.38 cm) to form a billet.

The billets were extruded to a diameter of 1/4" (0.63 cm). The billet diameter was narrowed wim multiple die passes, widi a final pass drawn dirough a 0.070" (0.178 cm) hexagonally shaped die into silver/oxide superconductor hexagonal wires. Nineteen of die wires were bundled togemer and drawn dirough a 0.070" (0.178 cm) round die to form a multifilamentary round wire. The round wire was rolled to form a 0.009" x 0.100" (0.023 cm x 0.24 cm) multifilamentary strand. A lengdi of me multifilamentary strand was dien subjected to a heat treatment according to die invention. The composite strand was heated in a furnace in a first anneal at 820 ^*C in 0.075 atm O₂ for 48 h. The first anneal formed significant amounts of the desired oxide superconductor phase, BSCCO- 2223. The composite strand was d en rolled to reduce diickness by 11 % (0.009" to 0.008"). Lastly, the rolled composite strand was subjected to a final two-step heat treatment, namely, heating from room temperature at a rate of l 'C/min to 820'C in 0.075 atm O₂ and holding for 54 h, cooling to 810'C in 0.075 atm O₂ and holding for 30 h. The sample was furnace cooled to room temperamre in 1 atm P₀₂. A length of multifilamentary strand was also subjected to a conventional heat treatment. The composite strand was heated in a furnace in a first anneal at 820 'C in 0.075 atm O₂ for 48 h. The first anneal caused significant amounts of me desired oxide superconductor phase, BSCCO-2223 to form. The multifilamentary strand was then rolled to reduce diickness by 11 % (0.009" to 0.008"). The control samples were then subjected to a second anneal at 810^*C in 0.075 atm O₂ for 84 h. This was a single step heat treatment in which no melting of the sample occurs. The microstructure of the samples were evaluated under an optical microscope. The samples prepared according to d e mediod of die invention had a higher density and far less cracks than d e control samples.

The critical currents of the samples using a criterion of lμV/cm, 77 K and zero applied field were determined. A single critical current was determined end- to-end over a long lengdi of strand (7-10 m). Critical current for a number of 10 cm lengdis of composite strands were determined and an average value was determined. The results are reported in Table 1 and show diat samples processed according to the method of die invention exhibited a factor of at least two improvement in critical transport properties.

Table 1. A comparative study of the method of the invention with a conventional process. sample no. length (m) UA) % a J_c (A/cm²)

Example 1-1 10 6.05 7563

Example 1-2 0.1 9.52 13 11,900

Control 1-1 7 2.23 2788

Control 1-2 0.1 4.08 16 5100

Further demonstrations of e superior performance of oxide superconductor strands prepared using a final two-step heat treatment of me invention are found in U.S.S.N. 08041,822, incoφorated herein by reference.

Example 2

This example demonstrates the manufacture of a multistrand power cable using oxide superconductor strands.

Precursor powders were prepared from the solid state reaction of freeze- dried precursor of die appropriate metal nitrates having the nominal composition of 1.7:0.3: 1.9:2.0:3.1 (Bi:Pb:Sr:Ca:Cu). Bi₂O₃, CaCO₃, SrCO₃, Pb₃O₄ and CuO powders could be equally used. After thoroughly mixing the powders in die appropriate ratio, a multistep treatment (typically, 3-4 steps) of calcination (800 °C ± 10°C, for a total of 15 h) and intermediate grinding was performed in order to remove residual carbon, homogenize the material and to generate d e low T_c BSCCO-2212 oxide superconductor phase. The formulated powder was packed into the open end of a silver billet under hydraulic pressure. The silver billet was open at one end and closed at d e odier and had a length of 8.00" ± 0.15" (20.32 cm), an outer diameter of 1.25" ± 0.005" (3.18 cm) and a inner diameter of 0.85" ± 0.005" (2.16 cm).

The billets were extruded to a diameter of 0.5" (1.27 cm), then were placed into a furnace at 450 'C for one hour to anneal. The billets were drawn through progressively smaller round dies until tiiey reach a diameter of 0.0785" (0.199 cm). Each pass dirough the die reduced the diameter by 5% to 11 %. The next step was to draw the wires dirough hexagonal shaped dies to tiieir final hexagonal wire dimension of 0.070 " (0.178 cm).

The hexagonal shaped wires were cleaned wim suitable cleaning agents; men cut into 85 equal lengtiis; and dien grouped togetiier to form a hexagonal shaped bundle. The bundle was inserted into a pure silver tube having an outer diameter of 0.840" ± 05.015" (2.133 cm) and an inner diameter of 0.760" ± 0.0015" (1.93 cm). After bundling, die multifilament bundle was placed in a furnace at 450 ^*C for four hours to anneal. The annealed multifilamentary bundle was allowed to cool before it is drawn dirough a round die of progressively smaller dimension until it reached die final wire diameter of 0.072" (0.183 cm). The wire was place into a furnace at 600 ^*C for 2 hours to tiiermally bond d e assembly.

The round strand was dien rolled in tiiree reduction passes to a final dimension of 0.010 inch (0.025 cm) x 0.0100 inch (0.254 cm) with intermediate heat treatments in which the strand is ramped to 815' C at 1 ^* C/min, held at 815 ' C for 16 hours and cooled to room temperamre, all at 7.5% oxygen. After a final anneal at 450 ^*C for one our, twelve lengths of the strand were cut to about 8 inches for cabling.

Figure 4 is a side view of a cabling operation used to form die cable of the present invention. It is understood that the hand assembly described in this example could be readily substimted by commercially available processes, such are currently used in the cable industry. Using a protractor, a silver tube 40 is mounted in a vise 42. The mbe 40 may be mounted at an angle θ in die range of 0' to 40" , and preferably at a an angle of 25 ' ± 5 ' . With a shaφ instrument, several guide lines were lightly scored into die silver mbe at die angle θ set by d e tube 40. Twelve lengtiis of oxide superconducting strands 44 were cut to a length of about two inches longer than the mbe 40 length. Six of the oxide strands 44a are secured to an uppermost edge 45 of die mbe 40 and were positioned so as to be aligned with the scored guidelines and/or to hang substantially peφendicular to die ground. Once aligned, the oxide strands 44a are spiral wrapped around die tube 40. The strands may be wrapped by rotation of die strands around a stationary tube or, in a preferred embodiment, by rotation of me tube, while maintaining the strands at die selected angle θ with respect to die mbe. Once spiral wrapped, die strands are secured at die lowermost edge of die tube. The strands may be wrapped one at a time or simultaneously. The above process is repeated at the complementary angle (-Θ), so that the remaining six oxide superconducting strands 44b are spiral wrapped in die opposite direction, such that a second layer of spiral wrapped strands is formed. The strands 44a, 44b are secured top and bottom with a silver wire 46.

When the second layer is completed, d e assembly is removed from the vise and lightly compressed in a hydraulic press at 10 Kpsig. The flattened cable assembly is subjected to a two-step heat treatment involving a 40 hour bake at 830^* C followed by a 40 hour bake at 811 ^* C followed by a 30 hour bake at 787' C in 7.5% oxygen at one atmosphere total pressure. Fig. 5 shows the power consumption of the cabled conductor in watts/m of die cable per amp of applied current in magnetic fields of 0, 100, 326 and 1070 Gauss. Fig. 6 shows die VI characteristics of diat same cabled conductor in die same applied magnetic fields and demonstrates that the cable has substantially linear VI characteristics.

Comparison Example 2 As a comparison, a cable was processed in substantially the same manner as described in Example 2, but without the final two-step heat treatment of the invention. the cable was evaluated for current carrying ability. The plot of electric field vs current plot at 77K shown in Fig. 7 demonstrates the VI characteristics of the cable. It can be seen that the cable was resistive and had no significant current carrying capability.

Example 3 Square cross-sectioned (0.070" x 0.070") multifilamentary (1254 filaments) precursor alloy/silver composite wires having me appropriate stoichiometry for BSCCO-2223 were fabricated, and oxidized at 405 ' C for 600 hours in 100 atm oxygen. After oxidation, diey were reacted in 7.5% oxygen gas at one atmosphere total pressure for 6 hours to foπn BSCCO 2212 + BSCCO 0011 reactant. They were en square bar rolled forward and in die reverse direction in 10% and 20% area reduction increments with anneals every third pass consisting of a 10 minute bake at 200" C in air until they were 0.033" x 0.033" in cross- section (78% area reduction). They were dien sheet rolled forward and in reverse at ambient temperature with a 2-high rolling mill with 4" diameter rolls until their dimensions were 0.023" x 0.045" in cross-section (4 passes). A series of experiments were men completed to determine a suitable final heat treatment sequence for forming sintered and textured BSCCO 2223 from d e textured BSCCO 2212 + BSCCO 0011 as well as forming me wires into a cable via a planetary winding scheme such that they are separated by a fairly resistive layer in the cable. In the best method, die wires were baked at 829" C for about 10 hours in

7.5% oxygen at a total pressure of one atmosphere followed by sheet rolling to a 15% thickness reduction in one pass, and a second bake at 829" C for about 10 hours in 7.5% oxygen gas at one atmosphere total pressure. The wires thus processed were tiien manually cabled to form 5-strand cable samples such that the same surface of each wire was parallel to the external cable surface regardless of position in the cable. The cable pitch was about 1.3". The wires were cabled by bending diem sequentially onto a θ.1 " x 0.01 " copper tape former without rotating the wire about its own or ti e cable axis, thereby preserving alignment of each wire surface with a corresponding surface of the cable. After cabling, the copper strip was removed.

Some wires were coated widi MgO prior to and after cabling by dipping into a fine MgO powder/alcohol suspension and drying witii forced hot air. The cables were tiien 2-high rolled wid 4" diameter rolls to a wire diickness reduction of 10%, tiiereby consolidating the cable into a well defined, structurally integral form. The cabled wires were tiien subjected to the two step heat treatment of tiie invention to sinter the BSCCO 2223. This heat treatment consisted of a 30 hour bake at 829" C followed by a 60 hour bake at 811 ' C followed by a 20 hour bake at 787" C in 7.5% oxygen at one atmosphere total pressure.

The transport properties of the cables and co-processed un-cabled wires were measured at 77K in self field. The results are presented in Table 2.

Table 2. Results of Electrical Measurements

Sample History Cable Tvpe Cable Ic at 77K IΔ)

1) Cable, no MgO slurry 5-wire, 10% HTS oxide 6.0 fill factor

2) Cable dipped in MgO 5-wire, 19% HTS oxide fill 10.1 slurry before the final factor heat treatment

3) Cable dipped in MgO 5-wire, 19% HTS oxide fill 10.8 slurry after cabling factor

Example 4 Oxide superconducting strands were prepared according to Example 2.

The oxide superconductor multistrand cable was assembled as follows.

Construction of die cable assembly is similar to that described in Example 2, except that two pieces of oxide superconductor wire having a diameter of 0.015" and a composition of substantially BSCCO 2212 are used in place of die silver core tube. The two wires are secured togetiier to form a core of 0.017" thick and 0.300" wide. Six strands of oxide superconductor strands prepared according to Example 2 above are spiral wrapped in one direction at an angle of 25 ' and six strands are wrapped in die otiier direction, also at an angle of 25 ^* . When die second layer is completed, the assembly is removed from the vise and lightly compressed in a hydraulic press at 10 Kpsig. The flattened cable C followed by a 40 hour bake at 811 " C followed by a 30 hour bake at 787" C in 7.5% oxygen at one atmosphere total pressure.

Example 5 Oxide superconducting strands were prepared according to Example 2. The oxide superconductor multistrand cable was assembled as follows.

A 0.003" quartz sheet was laid our on a flat metal surface and saturated widi polyvinyl alcohol (PVA). The PVA-saturated sheet was heated witii an iron to thermoset the plastic. After heating, tiie quartz sheet was cut into one half inch strips. A six inch silver tube, such as that described in Example 2, was wrapped witii the quartz strips. Construction of the cable assembly is similar to that described in Example 2. Six strands of oxide superconductor strands prepared according to Example 2 above are spiral wrapped in one direction at an angle of 25 ^* and six strands are wrapped in the other direction, also at an angle of 25 ' . When the second layer is completed, the assembly is removed from the vise and lightly compressed in a hydraulic press at 10 Kpsig. The flattened cable assembly is subjected to a final two-step heat treatment of to a final two-step heat treatment of a 40 hour bake at 830" C followed by a 40 hour bake at 811 ' C followed by a 30 hour bake at 787' C in 7.5% oxygen at one atmosphere total pressure.

Example 6 A 91 filament composite was made by die PIT process widi an approximately a hexagonal array filament pattern using standard monofilament 2223 precursor in a fine Ag sheath. Precursor powders were prepared from the solid state reaction of freeze-dried precursors of die appropriate metal nitrates having the nominal composition of 1.8:0.3:1.9:2.0:3.1 (Bi:Pb:Sr:Ca:Cu)?]. Bi₂O₃, CaCO₃ SrCO₃, Pb^ and CuO powders could equally be used. After thoroughly mixing the powders in the appropriate ratio, a multistep treatment (typically 3-4 steps) of calcination (800°C_±_10^PC, for a total of 15 h) and intermediate grinding was performed in order to remove residual carbon, homogenize the material and generate a BSCCO 2212 oxide superconductor phase. The powders were packed into silver sheaths to form a billet. The billets were extruded to a diameter of about 1/2 inch (1.27 cm) and annealed at 450 C for 1 hour. The billet diameter was narrowed witii multiple die steps, widi a fmal step drawn through a hexagonally shaped die into a silver/precursor hexagonal monofilament wires. Eighty-nine wires .049x.090", one .1318 round and one .055 round wires were assembled and inserted into a .840" outer diameter by .740" inner diameter silver mbe to form a bundle. The assembly was baked for four hours at 450 degrees d e bundle was allowed to cool and then drawn dirough to .072 via successive 20% and 10 % pass reductions to for a multi-filamentary round strand. At .072" it was annealed at 450 degrees for one hour, allowed to cool and drawn to .0354" It was again annealed at 450 degrees C. for one hour, allowed to cool and then drawn to .0245" diameter. The composite was annealed in air at 300C for nominally 10 minutes. The material was divided approximately equally into 8 parts and each was layer wound onto a cabling spool. An 8 strand Rutherford cable was made from 91 filament composite strand.

A rigid cabling configuration was used, where die spools' orientation are fixed relative to die rotating support diat holds diem. The tension on each strand was controlled by magnetic breaks and set to nominally 0.5 inch-pounds. The widtii and thickness of die cable were set by a non-powered turks-head to be 0.096 and 0.048 inch, respectively. The cable lay pitch was set by a capstan take-up speed relative to the rotations speed to be nominally 1.03 inch. After cabling, the material was heat treated at 760 C for 2 hr. in 0.1 atm of oxygen. The cable was then rolled to at diickness of 0.0157 inch and heat treated for 6 hr. at 827 C in 7.5 % oxygen in nitrogen atmosphere. The cable was finally turks head rolled to 0.0126 inch in thickness. A final heat treatment of 40 hr. at 827 C, 30 hr. at 808 C, and 30 hr. at 748 C, all in 0.075 atm of oxygen in nitrogen was employed. The Je at 77K (B=0) was 2996 A/cm² at a fill factor of nominally 25 % superconductor cross section. The voltage/current characteristics of the sample in 0 magnetic field are shown in Fig 1.

As can be seen by the above examples, the method of die invention is highly versatile and can be successfully used widi a variety of deformation processes, oxide superconductor compositions, silver alloy compositions and processing conditions.

Odier embodiments of me invention will be apparent to those skilled in d e art from a consideration of me specification or practice of the invention disclosed herein. It is intended diat the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.

What is claimed is:

Claims

1. A method for preparing an oxide superconductor cable, comprising: exposing an oxide superconductor cable to a one or more two step heat treatments, the cable comprising strands of at least a desired oxide superconductor phase, and each two step heat treatment comprising, (a) heating the cable to and maintaining d e cable at a first temperamre sufficient to partially melt the cable, such diat a liquid phase co-exists widi die desired oxide superconductor phase; and

(b) cooling the cable to and maintaining the cable at a second temperamre sufficient to substantially transform die liquid phase into the desired oxide superconductor, witii no transposition of the oxide strands or deformation occurring after the final heat treatment.

2. A method for preparing an oxide superconductor cable, comprising: exposing an oxide superconductor cable to one or more two step heat treatments, the cable comprising strands of at least a desired oxide superconductor phase, and each heat treatment comprising,

(a) forming a liquid phase in die oxide superconducting cable, such mat the liquid phase co-exists witii the desired oxide superconductor phase; and

(b) substantially transforming the liquid phase into the desired oxide superconductor phase, with no transposition of the oxide strands or deformation occurring after die final heat treatment.

3. A method for preparing an oxide superconductor cable, comprising: subjecting a plurality of oxide superconductor strands to an anneal/deformation iteration, the anneal effective to form at least a desired oxide superconductor phase and the deformation effective to induce alignment of the c- axes of the oxide superconductor; transposing the plurality of oxide superconducting strands along a longitudinal axis so as to form a cable; and exposing the cable to a final heat treatment after transposition and deformation of the oxide strands, die heat treatment comprising, (a) heating the cable to and maintaining me cable at a first temperamre sufficient to partially melt the article, such diat a liquid phase co-exists with the desired oxide superconductor phase; and

(b) cooling die cable to and maintaining me cable at a second temperamre sufficient to substantially transform the liquid phase into die desired oxide superconductor, witii no transposition or deformation of oxide strands occurring after d e final heat treatment.

4. A method for preparing an oxide superconductor cable, comprising: transposing a plurality of oxide superconductor strands along a longitudinal axis so as to form a cable, such mat each of said strands are substantially electrically isolated; subjecting d e plurality of oxide superconductor strands to an anneal/deformation iteration, the anneal effective to form an oxide superconductor and die deformation effective to induce alignment of the c-axes of die oxide superconductor; and exposing the cable to a final heat treatment after transposition and deformation of the oxide strands, the cable comprised of at least a desired oxide superconductor phase and die heat treatment comprising, (a) forming a liquid phase in die oxide superconducting article, such that the liquid phase co-exists witii the desired oxide superconductor phase; and

(b) substantially transforming the liquid phase into the desired oxide superconductor phase, with no transposition or deformation of oxide strands occurring after the final heat treatment.

5. The method of claim 3 or 4, wherein the anneal/deformation iteration comprises: subjecting the oxide superconductor strands to a first anneal/deformation iteration, the anneal effective to form an intermediate oxide superconductor, whereby an intermediate textured oxide superconductor phase is formed; and subjecting the intermediate oxide superconductor strands to a second anneal/deformation iteration, die anneal effective to form the oxide superconductor, whereby a textured oxide superconductor is formed.

6. A method for preparing a BSCCO 2223 oxide superconductor cable, comprising: exposing a BSCCO 2223 oxide superconductor cable to a final two-step heat treatment, me heat treatment comprising,

(a) heating die cable to and maintaining die cable at a temperature substantially in me range of 815-860°C for a period of time substantially in the range of 0.1 to 300 hours at a P₀₂ substantially in the range of 0.001-1.0 atm; and

(b) cooling d e cable to and maintaining the cable at a temperamre substantially in tiie range of 700-845 °C for a period of time substantially in the range of 1 to 300 hours at a P₀₂ substantially in the range of 0.001-1.0 atai, with no transposition of the oxide strands occurring after die final two-step heat treatment.

7. The method of claim 1, 2 or 6, wherein me oxide superconductor cable is prepared by transposing a plurality of oxide superconductor strands along a longitudinal axis.

8. The method of claim 1, 2, 3, 4 or 6, wherein me strands of the oxide superconductor cable are substantially electrically isolated.

9. The method of 3 or 4, wherein the oxide superconductor cables are fully transposed.

10. The method of claim 3 or 4, wherein the oxide superconductor cables are partially transposed.

11. The method of claim 7, wherein me oxide superconductor cables are fully transposed.

12. The method of claim 7, wherein the oxide superconductor cables are partially transposed.

13. The method of claim 1, 2, 3 or 4 wherein the liquid phase of step (a) wets surfaces of a defect contained widiin the oxide superconductor, whereby upon transfoπnation of me liquid in step (b) to die oxide superconductor, die defect is healed.

14. The method of claim 3 or 4, wherein me deformation is selected from the group consisting of rolling, pressing, isostatic pressing, drawing, swaging, extrusion and forging.

15. The method of claim 3 or 4, wherein die deformation comprises rolling.

16. The method of claim 1, 2, 3 or 4, wherein the transposition of oxide strands is selected from the group of cabling techniques consisting of winding, twisting, Rutherford cabling, Roebel cabling, braiding, and other forms of Litz cabling.

17. The method of claim 3 or 4, wherein me step of transposing die oxide strands of the cable is carried out before an anneal/deformation iteration.

18. The method of claim 3 or 4, wherein the step of transposing die oxide strands of d e cable is carried out after completion of all anneal/deformation iterations.

19. The method of claim 3 or 4, wherein the step of transposing the oxide strands of die cable is carried out between a first anneal/deformation iteration and a second anneal/deformation iteration.

20. A mediod for preparing an oxide superconductor multistrand cable, comprising: subjecting a plurality of oxide superconductor strands to an anneal/deformation iteration, die anneal effective to form an oxide superconductor and die deformation effective to induce alignment of e c-axes of the oxide superconductor; and exposing the plurality of oxide superconductor strands to a two-step heat treatment after deformation, d e strands comprised of the oxide superconductor phase, and the heat treatment comprising, (a) heating the oxide strands to and maintaining the oxide strands at a first temperamre sufficient to partially melt the article, such that a liquid phase co¬ exists with die desired oxide superconductor phase; and

(b) cooling die oxide strands to and maintaining the oxide strands at a second temperamre sufficient to substantially transform the liquid phase into the desired oxide superconductor, with no deformation of oxide strands occurring after die final heat treatment; transposing die heat treated oxide superconductor strands along a longitudinal axis so as to form a cable, such that each of said strands are substantially electrically isolated; and exposing die cable to a final two-step heat treatment after transposition of the oxide strands, die strands comprised of die oxide superconductor phase and d e heat treatment comprising,

(c) heating the cable to and maintaining the cable at a first temperamre sufficient to partially melt the article, such that a liquid phase co-exists with the desired oxide superconductor phase; and

(d) cooling the cable to and maintaining the cable at a second temperature sufficient to substantially transform die liquid phase into d e desired oxide superconductor, witii no deformation of or transposition of the oxide strands occurring after die final two-step heat treatment.

21. The method of claim 1, 2, 3, 4 or 21, wherein partial melting of step (a) is carried out at a temperature substantially in die range of 820 to 835 °C and in an oxygen atmosphere substantially in the range of 0.05 atm to .01 atm oxygen.

22. The method of claim 1, 2, 3, 4 or 21, wherein the transformation of step (b) of d e liquid phase into me oxide superconductor is carried out at a temperamre substantially in the range of 700 to 820 ^*C and in an oxygen atmosphere substantially in the range of 0.05 atm to 0.01 atmoxygen.

23. The method of claim 1, 2, 3, 4 or 21, wherein the liquid phase of step (a) is in the range of 0.1 to 30 vol%.

24. The method of claim 5, wherein the anneal step of tiie first anneal/deformation iteration partially melts the oxide superconductor article.

25. The method of claim 5, wherein the anneal step of die second anneal/deformation iteration partially melts the oxide superconductor article.

26. The method of claim 5, wherein the intermediate oxide superconductor precursor is selected from the group consisting of BSCCO 2212 and (Bi,Pb)SCCO 2212 and the desired oxide superconducting phase is selected from the group consisting of BSCCO 2223 and (Bi,Pb)SCCO 2223.

27. The method of claim 1, 2, 3 or 4, wherein the final heat treatment is performed two to twelve times.

28. The method of claim 1, 2, 3, 4 or 14, wherein me oxide superconductor strand is a multifilamentary composite, where the filaments are comprised of oxide superconductor and die filaments are substantially axially aligned within a matrix material.

29. An oxide superconductor multistrand cable, comprising: a plurality of oxide superconductor strands, each of said strands comprised of an oxide superconductor having an irreversible melt characteristic, wherein said plurality of oxide strands are transposed about a longitudinal axis, such that each of said strands are substantially electrically and substantially mechanically isolated; and wherein said cable exhibits critical transport properties (J_c) of at least about 10,000 A/cm² at 77K, self field.

30. The cable of claim 29, wherein the oxide superconductor comprises BSCCO 2223.

31. The cable of claim 29, wherein each of said strands comprises a multifilamentary composite, where the filaments are comprised of oxide superconductor and die filaments are substantially axially aligned widiin a matrix material.

32. The cable of claim 31, where in me matrix material is a noble metal selected from the group consisting of gold, silver, platinum, palladium and alloys diereof.

33. A method for preparing an oxide superconducting cable, comprising: exposing an oxide superconductor cable to a two step heat treatment, die cable comprising strands of at least a desired oxide superconductor phase, and d e heat treatment comprising, (a) heating die cable to and maintaining the cable at a first temperamre sufficient to partially melt the cable, such tiiat a liquid phase co-exists with the desired oxide superconductor phase; and

(b) cooling the cable to and maintaining me cable at a second temperamre sufficient to substantially transform the liquid phase into d e desired oxide superconductor; and texmring die oxide superconductor cable.

34. The method of claim 33, further comprising: exposing the textured oxide superconductor to a final two step heat treatment, the cable comprising strands of at least a desired oxide superconductor phase, and die heat treatment comprising, (a) heating the cable to and maintaining the cable at a first temperature suficient to partially melt the cable, such that a liquid phase co-exists witii the desired oxide superconductor phase; and

(b) cooling the cable to and maintaining me cable at a second temperature sufficient to substantially transform the liquid phase into the desired oxide superconductor, witii no transposition of the oxide strands or deformation occurring after the final heat treatment.

35. A method for preparing an oxide superconductor cable, comprising: texmring an oxide superconductor cable; and exposing the textured oxide superconductor cable to a two step heat treatment, me cable comprising strands of at least a desired oxide superconductor phase, and the heat treatment comprising,

(a) heating the cable to and maintaining the cable at a first temperamre sufficient to partially melt the cable, such that a liquid phase co-exisits with the desired oxide superconductor phase; and

(b) cooling the cable to and maintaining the cable at a second temperamre sufficient to substantially transform the liquid phase into the desired oxide superconductor, witii no transposition of the oxide strands or deformation occurring after the final heat treatment.