The manufacture of superconductive metal/ceramic compos- ites is a relatively new discipline in the field of mate- rial deformation processes. High temperature superconductors, HTSC, may be manufactured by the so- called oxide-powder-in-tube process (OPIT). A ceramic powder, which contains lead, calcium, bismuth, strontium and copper oxides, is arranged in a silver tube whose cross-section is reduced subsequently by drawing in several steps. The individual filaments thus obtained are packaged in a new silver tube, thereby providing a multi- filament wire having e. g. 37 individual filaments also denoted"single filament"whose cross-section is subsequently reduced by drawing and rolling to tapes of a thickness of e. g. about 0.2 mm and a width of about e. g.

3 mm. Following the mechanical deformation, the tapes are heat treated at about 835 °C, whereby the powder cores are converted into superconductive ceramic individual filaments by phase diffusion and grain growth. The geometry, density and texture of the powder cores prior

to the heat treatment are essential to the quality and the electrically conductive properties of the finished superconductive wires.

In the finished product the amount of silver or silver alloy are normally above 50% by vol., which makes the wire very expensive, as it will be explained later on.

Drawing and rolling are mechanical deformation processes which are used in connection with the manufacture of su- perconductors for achieving a reduction in the area of the cross-section of the superconductor, an extension of the superconductor, a change in the cross-sectional ge- ometry of the superconductor and the wires, a change in the mechanical properties of the superconductor and a change in the texture of the superconductive material.

These processes involve great differences in the deforma- tion of the treated material, seen over its cross-sec- tion. Both the strain of the material and the inner state of stress vary greatly depending on the position in the cross-section of the deformed material. Thus, rolling may result in great compressive stresses which are considerably greater at the center of the wire seen in cross-section than at the cross-sectional periphery of the wire. The conditions of strain are frequently a little more complicated, but these, too, vary greatly in dependence on the position in the deformed cross-section.

If a sufficient compression/density increase is to be achieved in the entire cross-section of the superconductor, then such a strong thickness reduction is required in practice as will result in undesired dimensional conditions, i. e. the superconductive tape becomes too wide and too thin.

Another problem is that rolling of materials may cause shear bands, which are local shear planes that form angles of about 35° with the rolling plane. These shear bands are particularly unfortunate in connection with the rolling of multi-filament superconductors, as sausaging may occur, i. e. locally repeated constrictions of the individual superconductive filaments seen in longitudinal section, which reduces the electrically conductive properties. Another typical phenomenon is waving caused by shear bands in the width direction, which manifests itself in that the superconductive filaments together get a characteristic appearance wave-shaped in cross-section. Such wave-shaped filaments may result in formation of cracks, which is highly undesired.

A good deal of research has been devoted to the optimiza- tion of parameters such as multifilament wire geometry and size, number of filaments, roller size, number of rolling steps, heat treatment, etc. to achieve a critical current density as great as possible. The critical current density is the maximum current density which the superconductor can achieve with superconductive properties, i. e. an ohmic resistance of 0.

WO 00/38251 discloses a method as mentioned in the open- ing paragraph, wherein mounting of a round preform in a round copper tube prior to rolling makes it possible to improve the critical current density of the finished su- perconductive tape. It is explained in the WO document that a greater density of the superconductive material is achieved because of the copper tube, since the supercon- ductive cable may be subjected to greater deformation without breaking because of the great tensile strength and hardness of the copper.

An objective of the invention is to provide a new method of manufacturing superconductive wires, such as tapes by use of which method the risk of obtaining superconducting tape with undesired current-carrying capability, particularly uneven current density Jc (current density as it varies over the cross-sectional area of a superconducting wire) has been reduced.

Another objective of the invention is to provide a method of manufacturing superconductive wires, such as tapes having improved current-carrying capability.

Accordingly it is also an objective of the invention to provide a superconductive wires, such as tapes having improved current-carrying capability.

A further objective of the invention is to provide a method of manufacturing superconductive wires, such as tapes by use of which is possibly to reduce the cost.

And a final objective is to provide a superconductive wire, such as a tape, which is cheaper to produce than corresponding wires having corresponding current-carrying capabilities.

These and other objectives have been achieved by the invention as it is defined in the claims.

A method of the type included in the first aspect of the invention is characterized in that the dummy jacket has considerably greater external transverse dimensions than the primary metal jacket.

Since the external dimension of the dummy jacket is considerably greater than the external dimension of the preform, the following may be utilized by a mechanical deformation: In practice, two cross-sections of the same shape, but with different areas will have the same stress and strain variation across the cross-section with the same degree of deformation. When, according to the inven- tion, a tube or jacket having considerably larger cross- sectional dimensions than the superconductive preform itself is used, the stress and strain variations will thus be distributed across a considerably larger cross- section. This results in considerably smaller stress and strain variations within the cross-section of the superconductive tape itself, which just constitutes a small part of the total cross-section. Thus, during rolling a high homogeneous pressure as well as a uniform deformation may be achieved, and thereby it is possible to achieve a high uniform critical current density across the cross-section of the entire superconductive wire.

Further, according to the invention, the preform is manu- factured by arranging one or more filaments of supercon- ductive material each having its own surrounding filament metal jacket in the primary metal jacket, said preform being subsequently mechanically deformed to a smaller cross-section. The method is particularly suitable for multi-filament wires, since it is hereby possible to achieve an essentially uniform deformation of all filaments in the superconductive wire, such as a tape- irrespective of the position of the filaments in the cross-section.

Also, according to the invention, the cross-sectional area of the dummy jacket wall may be at least two, more

expediently at least three and most expediently at least four times as a large as the cross-sectional area of the preform. Hereby an almost completely homogeneous stress and strain in the superconductor may be achieved; i. e. the stress and strain gradients are close to 0.

In the first aspect of the invention the method of manufacturing a superconductive wire comprises a number of steps including a step A) of providing a preform, a step B) of arranging the preform in a dummy jacket, and a step C) of deforming the dummy jacket containing the preform The preform provided in the step A) comprises a core of at least one filament comprising a superconductive material or a precursor for a superconductive material and a primary metal jacket, The superconductive material or the precursor therefore may in practice be any type of superconductive material or precursor. Examples of useful superconductive material and precursor materials can be found in US 5,338,721, US 5,508,254, US 5,610,123 and PCT application WO 01/20690 which are hereby incorporated by reference.

Preferred the superconductive material or the precursor therefore comprises a copper oxide comprising two or more components of the groups consisting of Ba, Y, La, Sr, La, Ca, Bi, Ti, said superconductive material or a precursor therefore preferably being a BSCCO material, such as BSCCO-2212/2223 or a precursor therefore.

The primary jacket should be of a material which is substantially inert with respect to oxygen and the oxides

of the superconducting or precursor material, preferably the material is permeable to oxides. Preferred materials for the primary jacket includes Ag, Au, Pd and alloys thereof such as alloy comprising up to about 40 % by weight of other metals. The preferred material for the primary jacket are silver or silver alloy, such as silver alloys comprising up to 30 % by weight of one or more other alloying metals such as Ni, Cu, Mg, Sn and Mn. The primary jacket may comprise coatings in the form of diffusions layers such as described in US 4,952,554 wherein the primary jacket consist of a primary jacket with two inner diffusion coatings, however generally it is preferred that the metal jacket or all layers of the metal jacket if it contain several layers is/are of silver or silver alloy, e. g. an innermost layer of silver and an outer layer of silver alloy. Most preferred the primary metal jacket is in the form of one layer of silver or silver alloy.

The filament or each of the filaments preferably comprises a superconducting or a precursor powder in a tube formed filament jacket.

The filament jacket may be of the materials as described above for the primary metal jacket.

If the core comprise only one filament the primary metal jacket may constitute the filament jacket. If the core comprises two or more filaments each filament comprise a filament jacket, and the filaments being contained in the primary metal jacket.

The production of the filament may be made using any method e. g. such as described in US 4,906,609, but the

preferred method is the"powder-in-tube"method which includes the step of preparing a tube of the primary jacket, filling the tube with the superconducting material or the precursor and closing the ends of the tube. Thereafter the filament may preferably be deformed and/or heat-treated. Useful methods of preparing the filaments are described in US 5,338,721, US 5,508,254, US 5,610,123 and PCT application WO 01/20690.

The outer surface of the preform should preferably be free of cavities and grooves.

In step B) the preform are arranged in a dummy jacket. such as a metal dummy jacket having an internal cross- section which essentially corresponds to the external cross-section i. e. the outer surface of the preform.

The dummy jacket has considerably larger external cross- sectional dimensions than the primary metal jacket.

The volume of the dummy jacket should preferably be at least twice, more preferably at least three times, even more preferred at least four times the total volume of the other materials contained in the dummy jacket.

The dummy jacket may in general be made of any material which has a sufficient hardness and strength to withstand the deforming forces applied in the deforming step c), and which preferably differs sufficiently from the material of the primary metal jacket, so that the material of the dummy jacket may be removed from the primary metal jacket. The primary jacket may be of reinforced polymer material e. g. fiber reinforced thermoplastic e. g. co-block polymers such as SBS, the

fiber preferably being glass fibers or carbon black fibers. It is generally preferred that the dummy jacket being of a metal, more preferably of a metal selected from the group of copper, aluminum or alloys thereof, such as alloys comprising one or more of the components copper and aluminum and optionally up to about 40 % by weight by weight of other metals including Ni, Au, Ag e. g. a Cu alloy comprising up to 20 % by weight of Ag. In one embodiment the dummy jacket comprise less than 5 % by weight of silver more preferably it is substantially free of silver The metal tube is most preferably of Cu, because copper is inexpensive, ductile, compatible with silver and may be etched away by means of e. g. sulfuric acid, which does not etch silver.

The preform may preferably be arranged in the dummy jacket using a modified Powder-in-tube method wherein the dummy jacket is prepared in the form of a tube and the preform is inserted into the dummy jacket tube.

Alternatively the dummy jacket may be coated or wrapped onto the preform.

In order to provide a wire with even current-carrying capability among the length of the wire the cross- sectional dimension of the wire should be substantially identical along the length of the wire.

"Substantial identical"as used herein means that the variation of the cross-sectional dimensions, diameter, width and thickness', should be less than 10 %, preferably less than 2,5 %, and more preferably less than 1 % from one sectional cut to another sectional cut along

the length of the wire. Generally it is preferred that "substantial identical"should mean"as identical as possible using the normal technique for producing such wires".

Percentage variations as used in this application unless other is mentioned should mean percentage variation from the largest of the dimensions which is compared, i. e. the difference in percent between two sizes means the variation in percent calculated on the largest size.

Generally it is preferred that the dummy jacket should be substantially identical through out the length of the wire prior to and/or after the step of deforming.

The dummy jacket and the primary metal jacket each, has a wall extending around the primary metal jacket and the filaments, respectively. It is generally preferred that the dummy jacket wall is thicker, e. g. more than two times or more than three times thicker than the wall of the primary metal jacket which is covered with the dummy jacket wall. Thereby the effect of obtaining a uniform distribution of force during the step of deforming and thereby obtaining a wire having improved current-carrying capability, is even higher.

The thickness of the wall prior to and/or after the deforming step may vary along its extension around the primary metal jacket. The variation should preferably be systematic so that the outer surface do not have cavities or grooves. The thickness may e. g. vary between 5 and 95 %, such as between 10 and 50 % from the thickest part to the thinnest part, calculated as percentage of the thickest part.

In addition, according to the invention, the preform and the dummy jacket may have rectangular or square cross- sections, as better results may be achieved with these cross-sections than with e. g. round cross-sections.

In one embodiment the dummy jacket wall has an outer shape which is substantially identical along its length, and wherein the cross-sectional outer shape of the dummy jacket wall is angular, more preferably rectangular such as square. Thereby the pressure provided onto the dummy jacket during the deforming step may be even more homogenous distributed and add further to the improved current-carrying capability of the resulting wire.

The dummy jacket may further have an inner shape, which is substantially identical along its length, and wherein the cross-sectional inner shape of the dummy jacket wall preferably is angular, more preferably rectangular such as square.

This angular inner and/or outer shapes of the dummy jacket may be present prior to the deforming step.

Alternatively, the angular inner and/or outer shapes of the dummy jacket may be provided during the deforming step.

In one embodiment both the cross-sectional inner shape and the cross-sectional outer shape of the dummy jacket wall being rectangular, thereby dividing the wall into four wall-segments, which may differ in thickness. Preferably the pair wise opposite wall-segments have similar thickness, and the non-opposite wall-segments have different wall thickness."Similar thickness"means

that the thinnest wall-segment if any should not be more than 20 %, preferably less than 10 % and more preferably less than 5 % thinner than the thickest wall-segment, the calculation being based on the thickness on the thickest wall-segment.

In the step C) of deforming, the dummy jacket containing the preform is plastically deformed by e. g. by swaging and/or drawing and/or rolling such that the ceramic superconductive material is compressed, as it is generally known in the art, e. g. from the above described prior art patent publications.

It is preferred that the step of deforming includes a sub-step of pressing or rolling the metal dummy jacket with the preform into a tape.

Generally, it is preferred that the wire is a tape, and in this embodiment the invention in its first. aspect is even more advantageous, as the pressure applied in the step of deforming the metal dummy jacket with the preform into a tape is very high, and there therefore is a large tendency that the tape being highly wave shaped, and that the current density Jc, may be uneven distributed.

The tape should preferably have a thickness/width dimension which is substantially identical through out the length of the tape. The dimension of the tape may be as it is generally known in the art e. g. with a width which it at least 5 times its thickness.

The method according to the invention may preferably include one or more steps of heat treating the wire before intermediate and/or after the step of deforming,

as it is generally known in the art e. g. as described in the above mentioned prior art documents.

At least if the filaments includes precursor material for a superconducting material, the method should preferably include one or more steps of sintering the supreconducting material or the precursor therefore, to thereby optimize the superconducting properties.

The dummy jacket is preferably removed after termination of the deforming step. Generally it is preferred that the dummy jacket is removed prior to the final sintering.

If the dummy jacket is of a polymeric material the dummy jacket should be removed prior to any step of heating above the melting point of the dummy jacket unless this heating step is used for removing the dummy jacket.

The preform may comprise a plurality of filaments, such as at least 4, preferably at least 15 more preferably at least 20 filaments, each comprising superconductive material or a precursor for a superconductive material surrounded by a filament metal jacket.

The number of filaments is in principal unlimited but normally the number will not exceed 200, as the final wire may be too inflexible.

As mentioned above the material for the primary jacket and the filament jacket (s) is most often silver or silver alloy. Alternatively gold or platinum may be used, however du to cost these materials are seldom used in the production of superconducting wires. When the term "silver"is used in the following, it should be

understood to includes also silver alloys with up to 30 % by weight CU, and further it may also include gold and platinum.

In order to reduce cost, it is preferred to reduce the amount of silver used in the production. It has surprisingly been found that this may be done by reducing the amount of silver in some of the filaments. Therefore, in a preferred embodiment at least one, preferably at least about 20 %, more preferably at least about 50 % of the filaments, each has a filament jacket which is thinner than filament jackets of the filaments lying closest to the primary jacket.

It is furthermore preferred that filaments are arranged in a pattern, which in a cross-sectional view provides an outer circle of filaments, which are circumferencing the other filaments. In the following"the other filaments" is"the inner filaments". The filaments of the outer circle have each a filament metal jacket, which is thicker, preferably at least 10 % thicker, more preferably at least 40 % thicker than the average thickness of the filament metal jackets of the inner filaments. The filaments of the outer circle may even have each a filament metal jacket, which is thicker, preferably at least 10 % thicker, more preferably at least 40 % thicker than all of the filament metal jackets of the inner filaments.

In any event, it is preferred that the metal tube is re- moved mechanically or chemically before the heat treat- ment of the superconductive tape. Aluminum may be etched away in a NaOH solution, and copper may be etched away in an H2SO4/H202 solution.

The invention also relates to a dummy metal tube for use as dummy jacket in the manufacturing of a superconductive tape. The cross-sectional area of the dummy metal tube wall is preferably at least two, more expediently three and most expediently at least four times as large as the cross-sectional area of the bore of the dummy metal tube.

According to the invention, the thickness of the dummy metal tube wall may vary, seen in cross-section. With rolling, it is a well-known phenomenon that the thickness/width ratio of a preform/tape is decisive for how the strain is in the longitudinal and the transverse directions. Thus, a wide tape becomes relatively longer by rolling than a narrow tape. Suitable configuration of the cross-section of the dummy metal tube, i. e. varying the wall thickness across the cross-section, allows the flow of the material to be controlled during the drawing or rolling, and thereby the tendency of forming defects such as shear bands, sausaging and waving may be affected.

According to an embodiment, the external and internal cross-sections of the dummy metal tube may be essentially rectangular or square, as two opposed wall sections have a first thickness and the two other, opposed wall sections have a second thickness. Such a dummy metal tube is particularly simple to configure and particularly suitable for the method according to the invention.

The invention also relates to a superconductive wire manufactured by the method according to the method of the first aspect of the invention.

The superconductive wire of the first aspect comprises a core of at least one filament comprising a superconductive material as described above and a primary metal jacket. The primary metal jacket is covered by a dummy jacket, such as a metal dummy jacket.

The primary metal jacket, the filament jackets and the dummy jacket may preferably be of materials and dimensions as described above.

Preferably the cross-sectional dimensions of the dummy jacket is substantially identical through out the length of the wire, which is preferably in the form of a tape also as described above.

The wire preferably includes a plurality of filaments e. g. a number as described above, such as between 4 and 100.

The filament jackets may have different thickness, e. g. so that at least one, preferably at least about 20 %, more preferably at least about 50 % of the filaments, each has a filament jacket which is thinner than filament jackets of the filaments lying closest to the primary jacket.

It is preferred that the jacket of one filament be totally or partly fused with the jacket of a neighboring filament, which should mean that the filaments stick together. The filament jackets may be fused together by cold welding due to the deformation of the filaments or due to a sintering of the material of the filament jackets or preferably due to both. Even though the jackets stick together, it is generally possible to

determine the interface between the jackets, and thereby determine the thickness of the respective jackets.

The filament jackets, should preferably have even wall thickness, which means that one filament jacket preferably should have substantial identical wall thickness through out its length and circumference.

The filaments may be arranged in a pattern, which in a cross-sectional view provides an outer circle of filaments circumferencing the other filaments designated the inner filaments, the filaments of the outer circle having a filament metal jacket, which is thicker, preferably at least 10 % thicker, more preferably at least 40 % thicker than the average thickness of the filament metal jackets of the inner filaments. It is further preferred that the filaments of the outer circle have a filament metal jacket, which is thicker, preferably at least 10 % thicker, more preferably at least 40 % thicker than all of the filament metal jackets of the inner filaments.

In its second aspect of the invention it has particularly been achieved to provide a cost-effective method of manufacturing a multi-filament superconducting wire. Compared to known methods the amount of silver/silver alloy is reduced substantially.

Thus it has been found that the filament jackets do not need having equal thickness, and further it has been found that the primary metal jacket may dispensable.

The invention in it second aspect therefore relates to a method of manufacturing a multi-filament superconductive wire comprising the steps of i. providing a plurality of single filaments each comprising a superconductive material or a precursor for a superconductive material and filament metal jacket, ii. arranging the filaments in an inner jacket surrounding the filaments iii. deforming the inner jacket containing the filaments plastically by e. g. drawing or rolling such that the ceramic superconductive material is compressed, wherein at least one of the filaments has a filament jacket which is thinner than the filament jacket of at least one other of the filaments.

In manufacturing of superconductive wires such as tapes, a number of requirements may appear. High Je (the average current density i. e. the average Jc) and low price are two of the most important requirements, and these requirement are highly improved according to the invention in this second aspect. In prior art superconducting wires consist normally of 30-45 % by vol. of superconducting powder. The rest is silver. According to the present invention the amount of silver or silver alloy may be reduce to e. g. up to about 30 % by vol. and thereby the savings in silver consumption for a given amount of superconducting powder may be up to between 45 and 57 % by vol.

Though it is not preferred, the inner jacket may be a primary metal jacket, preferably of Ag or Ag alloy as described for primary jackets above. The method may

further comprise a step of coating the primary metal jacket with a dummy jacket.

The material for a dummy jacket in the second aspect may preferably be as described above for the first aspect of the invention.

It is generally preferred that the inner jacket is a dummy jacket, preferably a metal dummy jacket.

The filament jackets should preferably be of a material as described for the filament jacket in the first aspect of the invention.

In order to optimize the saving of filament jacket material, while still obtaining a wire with an acceptable tensile strength and cohesiveness (sticking together of the filament jackets), at least one, preferably at least about 20 %, more preferably at least about 50 % of the filaments, each has a filament jacket which is thinner than filament jackets of the filaments lying closest to the inner jacket.

By arranging the filaments so that the outermost filaments having thicker filament jackets than the filament jackets of the inner filaments, the strengths and the cohesiveness of the wire will be optimal.

The number of filaments should preferably be as least 4, and may include any number of filaments more than 1, describes for the invention in its first aspects.

The filaments should preferably be arranged in the inner jacket so that the filament jacket of one filament being

totally or partly fused with the filament jacket of a neighboring filament. The filament jackets may be fused together by cold welding due to the deformation of the filaments or due to a sintering of the material of the filament jackets or preferably due to both. The"sticking together"effect obtained during the step of deforming may therefore preferably be improved by applying heat to the filaments so that the filament jackets are sintered to each other. This may e. g. be done prior, to during or after the deforming step.

In a preferred embodiment the filaments are arranged in a pattern, which in a cross-sectional view provides an outer circle of filaments circumferencing the other filaments designated the inner filaments, the filaments of the outer circle having a filament metal jacket, which is thicker, preferably at least 10 % thicker, more preferably at least 40 % thicker than the average thickness of the filament metal jackets of the inner filaments The filaments of the outer circle may even have a filament metal jacket, which is thicker, preferably at least 10 % thicker, more preferably at least 40 % thicker than all of the filament metal jackets of the inner filaments.

Furthermore, the filaments may be arranged in a pattern, which in a cross-sectional view provides an outer circle of filaments and two or more inner circles of the inner filaments, wherein the thickness of the filament jackets decreases from the outer circle towards the center of the wire prior the step of deforming.

The inner jacket in the form of a dummy jacket may in principle have any cross-sectional dimension, provided

that it sufficient strong and thick to withstand the pressure applied to it during the deforming. The dummy jacket may e. g. be dimensioned as the dummy jacket described in the first aspect of the invention.

The step of deforming may preferably be as described for the first aspect of the invention. It is particularly preferred that the step of deforming comprise a step of deforming the wire to a tape, which may preferably have dimensions as described above for the tape in the first aspect of the invention.

The step of deforming may also include reinforcing of the inner jacket i. e. by applying a coating of yet another material, which may be removed prior to a subsequent heat-treatment The deforming step may preferably include one or more steps of heat treatment also as described for the first aspect of the invention.

The deforming and optional heating should preferably be so that the surfaces of the filament jackets are partly or totally fused together as described above. A person skilled in the art will by using his skill be able to optimize the deforming and heat treatment.

The superconductive material or the precursor therefore may preferably as described for the first aspect of the invention.

The dummy jacket may preferably be removed after termination of the deforming step, e. g. prior to a final

step of heat treating, e. g. sintering the superconductive material or its precursor.

The invention also relates to a multi-filament superconductive wire obtainable by the method of the second aspect of the invention.

The invention includes a multi-filament superconductive wire comprising a plurality of single filaments each comprising a superconductive material or a precursor for a superconductive material and filament metal jacket, said filaments being contained in an inner jacket surrounding the filaments wherein at least one of the filaments has a filament jacket which is thinner than the filament jacket of at least one other of the filaments.

The arrangement and number of filaments, the materials used and the dimensions may be as described for the method of the second aspects.

It is preferred that the jacket of one filament is totally or partly fused with the jacket of a neighboring filament, which should mean that the filaments stick together. The filament jackets may be fused together by cold welding due to the deformation of the filaments or due to a sintering of the material of the filament jackets or preferably due to both. Even though the jackets stick together, it is generally possible to determine the interface between the jackets, and thereby determine the thickness of the respective jackets.

The filament jackets, should preferably have even wall thickness, which means that one filament jacket

preferably should have substantial identical wall thickness through out its length and circumference.

The filaments may be arranged in a pattern as described above when describing the method of the second aspect of the invention.

The invention will be explained more fully below with reference to the examples and the drawings, which shows prior art and embodiments of the invention.

Example 1 A precursor powder BSCCO was cold isostatically pressed to a round bar with a diameter of 16 mm and a length of 400 mm. The bar was placed in an Ag tube with an inner diameter of 16 mm and an outer diameter of 20 mm. The tube was closed in both ends, and thereafter it was drawn to a single filament with a diameter of 1.54 mm, and it was cut into shorter filaments. 85 of the filaments were bundled together and inserted in a copper tube with an inner diameter of 17 mm and an outer diameter of 20 mm, This multi filament rod was drawn through a series of dies down to a round cross section of 1.71 mm or a square cut area of 1.5x1.5 mm. One part of the drawn multi filament wire, the Cu was stripped off. The remaining part of round and square wire with Cu-clad was flat rolled to different dimension and subsequently the Cu was stripped off.

Short pieces from 200 mm to 3000 mm in length of drawn as well as of subsequently flat rolled wire, were heat treated in 10 hours at a temperature above 800 °C. As a

result the single filaments of drawn as well as of flat rolled wire were sticking together.

Example 2 A precursor powder BSCCO is prepared as in example 1 but with a diameter of 18'mm. The bar is placed in an Ag tube with an inner diameter of 18 mm and an outer diameter of 20 mm. The tube is closed in both ends, and thereafter it is drawn to a single filament with a diameter of 1.54 mm, and it is cut into shorter filaments. 50 of the filaments are bundled together with 35 of the single filaments prepared in example 1 and inserted in a copper tube with an inner diameter of 19 mm and an outer diameter of 22 mm. The filaments are arranged so that the filaments with the thickest Ag walls are placed closes to the copper wall. This multi filament rod is treated as in example 1, and the Cu is stripped of prior to heat treatment as in example 1.

Short pieces from 200 mm to 3000 mm in length of drawn as well as of subsequently flat rolled wire, are heat treated in 10 hours at a temperature above 800 °C. As a result the single filaments of drawn as well as of flat rolled wire are sticking together.

In the drawing: figure 1 shows a preform with a relatively thin dummy jacket according to the prior art before rolling, figure 2 shows the same after a first rolling step, figure 3 shows the same after a last rolling step,

figure 4 shows a preform with a dummy jacket before rolling according to the method of the invention, figure 5 shows the same after a first rolling step, figure 6 shows the same after a second rolling step, figure 7 shows the same after a last rolling step, figure 8 shows a graphic illustration of a simulation of rolling of a superconductor without a dummy jacket, figures 9 and 10 show a graphic illustration of a simulation of a superconductor with a relatively thin dummy jacket before and after rolling according to the prior art, figure 11 shows a graphic illustration of the cross- section of a preform with a dummy jacket before rolling according to the method of the invention, figure 12 shows a graphic illustration of the same after rolling by the method according to the invention, figures 13-17 show various configurations of a dummy jacket for use in the method according to the invention, figure 18 is a process diagram of a method according to the second aspect of the invention, figure 19 show a cross-sectional cut of a plurality of filaments contained in an inner dummy jacket in a

production step according to the invention in its second aspect, figures 20a, 20b, 20c disclose single filaments, which may be used in the invention.

Figures 1-7 are photographs of thin sections, where the object has been cut, ground and polished so that the transitions between the individual components are visible.

Figure 1 shows a preform containing filaments 2, with filament jackets 3 and a primary jacket 4 which is arranged in a copper tube 5 according to the prior art after drawing, but before rolling. Before the rolling, the preform measured about 1.3 mm times 1.3 mm and the copper tube about 1.58 mm times 1.58 mm. The preform consists of a plurality of ceramic filaments, in this case 55, which are based on BSCCO-2212/2223. The ceramic filaments are separated from each other by means of walls 3 of Ag or an Ag alloy.

Figure 2 shows the same as figure 1, but after a first rolling step, and figure 3 shows the same after a last rolling step. It will be seen clearly from figure 3 how the superconductor has got a cross-sectional shape, which resembles a UFO or a"flying saucer"for reasons which will be explained later.

Figures 4,5,6 and 7 show cross-sections through a pre- form/superconductive tape comprising a preform containing filaments 22, with filament jackets 23 and a primary jacket 24 corresponding to preform 2,3,4 in figures 1, 2, and 3 with a dummy jacket in the form of a copper tube

25 whose cross-sectional dimensions are considerably greater than those of the preform/tape 22,23,24, according to the invention. Before the rolling, the preform measured about 1.05 mm times 1.05 mm and the copper tube about 2.06 times 2.06 mm. The preform used in this test has 37 filaments, but this is not essential to the effect of the invention. Because of the copper tube 25, the superconductor is subjected to considerably more uniform deformation forces, and therefore the final superconductor has a cross-section, as will be seen in figure 7, which has a more uniform filament thickness to width ratio than the cross-section shown in figure 3.

Figures 8,9,10,11 and 12 show the result of so-called finite element calculations of the shape of the cross- section before and after rolling. It should be noted that only about one-fourth of the cross-section is seen.

Figure 8 shows the cross-section through a superconductor 2,3,4 after it, from being a square preform and without being provided with a surrounding copper tube, has been rolled flat by a roller 9. It will be seen clearly how the lines, which extend horizontally and vertically in the preform, now wind greatly as an indication of a high inhomogeneous deformation and a displacement of the filaments. The reference numeral 20 refers to the point, which is the transition between the horizontal and vertical sides of the preform before the rolling. As will be seen, a rotation of the material around the point 20 has almost taken place after rolling. Thus, a portion of the originally vertical face is now in contact with the roller 9 and extends horizontally.

In figure 9, the white area represents the preform 2,3, 4 and the dark area a copper tube 5 corresponding to the prior art before rolling. After the rolling, the preform 2, 3,4 and the tube 5 have been deformed strongly, as shown in figure 10. After the rolling, the tube 5 is etched away, and the superconductive tape 2, 3,4 thus gets an external geometry corresponding to the white area in figure 10. Like in the example shown in figure 8, there is a strong deformation, a rotation of the material around the prior corner point 21. The shape of the preform thus obtained-the white area 2,3,4-is the one which is also found in figure 3.

Figures 11 and 12 illustrate the method according to the invention, where the preform 22,23,24 is surrounded by a thick dummy jacket 25. In figure 12, which shows the superconductor 22,23,24 and the dummy jacket 25 after the rolling by the method according to the invention, it will be seen clearly how the deformation of the tape 22, 23,24 is very uniform, seen across the cross-section, as the vertical and horizontal lines 7,8 do not differ significantly from their original orientation.

Furthermore, there is substantially no rotation of the material around the prior corner point 21'.

Figures 8-12 graphically show the deformation, and the associated calculations showed that a stress variation of 260% and a strain variation of 300% were achieved across the cross-section for the tape in figure 8 without a metal tube or jacket. For the embodiment shown in figures 9 and 10, in which the cross-sectional area ratio of the metal tube 5 to the conductor 2,3,4 is about 0.3, the calculations showed a stress variation across the cross- section of 250% and a strain variation of 195%. In the

method according to the invention, which is illustrated in figures 11 and 12 and where the area ratio of the dummy jacket 25 to the superconductor 22,23,24 is about 5, the calculations showed a stress variation of just 30% and a strain variation of just 25%.

Figures 13-17 show various configurations of the cross- section of a dummy jacket 25a, 25b, 25c, 25d and 25e. Figure 13 thus shows a square metal dummy jacket 25a with a square bore lla. Figure 14 shows an embodiment of a dummy jacket 25b with two vertical wall sections 10 and two horizontal wall sections 9, wherein the vertical wall sections 10 are thicker than two horizontal wall sections 9. The use of this dummy jacket in the production of a superconducting wire according to the invention thus results in a change in the relation between the strain in the longitudinal direction and the transverse direction, as a greater strain is achieved in the longitudinal direction than in the transverse direction, compared to the cross-section shown in figure 13. The dummy jacket 25b comprise a square bore as the dummy jacket shown in figure 13, and the two dummy jackets may thus be used for the same preform geometry.

In the dummy jackets shown in figures 15,16 and 17 the external geometry of the jacket 25c, 25d, 25e are square as the dummy jacket 25 a shown in figurel3. The jackets 25c, 25d, 25e, respectively has a bore llc, lld, lle which should correspond to the external geometry of the preform to be filled into it differs from each other. Thus, optimum rolling of various preform geometries may be achieved without changing the other rolling parameters.

In one embodiment according to the invention in its second aspect, the method is carried out according to the process diagram indicated in figure 18. According to this embodiment of the invention a first single filament is produced by filling precursor powder into a silver or silver alloy tube, and there after the tube is deformed e. g. by drawing. A second filament is produced using the same method, but the tube used for preparing the second single filament has a thicker wall than the tube used for the production of the first single filament. The first single filament is cut into a number of filaments A having substantial similar length. The second single filament is cut into a number of filaments B having substantial similar length as the filaments A. The filaments A and B are bundled together and inserted into a dummy tube. The dummy tube is drawn to a wire, which subsequently is flat rolled into a tape. The dummy tube is removed by etching or mechanical stripping. Finally the tape is heated e. g. to sintering the precursor powder to thereby convert into a superconducting material.

Figure 19 show a cross-sectional cut of a plurality of filaments contained in an inner dummy jacket in a production step according to the invention in its second aspect. In this embodiment the dummy jacket 33 contain two different filaments 31a, 31b having equal outer diameter but with a different wall thickness 32a, 32b of their filament jackets.

Figures 20a, 20b, 20c shows different types of filaments 34,35,36, with precursor powder 34b, 35b, 36b, and a filament jacket 34a, 35a, 36a, and 36aa, which may be used in the invention. The filament 36 has a doubled walled filament jacket with an inner filament jacket wall

36aa and an outer filament jacket wall, the inner and outer jacket wall may preferably be of different material selected between Ag and Ag alloys. The filaments shown in figures 20a, 20b, 20c may e. g. be used in the multifilament of the invention in any combinations.

The invention is not restricted to the examples and em- bodiments shown here.