CA2318113A1

CA2318113A1 - Methods for producing precursor material for the production of high-temperature superconducting wires

Info

Publication number: CA2318113A1
Application number: CA002318113A
Authority: CA
Inventors: Michael Baecker; Joachim Bock
Original assignee: Individual
Current assignee: Aventis Research and Technologies GmbH and Co KG
Priority date: 1998-01-30
Filing date: 1999-01-28
Publication date: 1999-08-05
Also published as: WO1999039392A1; DE19803447A1; EP1051760A1; KR20010040471A; JP2002502099A; CN1289457A; AU2830599A

Abstract

The invention relates to methods for producing a high-temperature superconductor precursor material for use in the production of strips or wire. According to said methods a mixture of oxides and/or their precursors is prepared, the mixture is heated to a temperature at which it presents a readily mixable and pourable melt in which individual phases can possibly still be present in the solid state, and the melt is introduced into an envelope where it solidifies as it cools. The melt solidifies at such a speed that substantially no reaction layer is formed from the material constituting the envelope and that of the melt. The solidified molten body is heated in an annealing furnace to a temperature at which the molten body material is converted into a precursor material containing at least 10 weight % of at least one high-temperature superconductive and/or high-temperature superconducting phase. No grinding takes place after melting of the mixture.

Description

METHODS FOR PRODUCING P ECURSOR MATERIAL FOR THE PRODUCTION.
OF HIGH-TEMPERATURE SUPERCONDUCTING WIRES
Description For industrial use in wires and solid components, high temperature superconductors studied have been especially .those from the family of bismuth-calcium-strontium cuprates. The phases which have a transition temperature T~ above the boiling point of nitrogen (77 K) are worthy of io particular note. These are the three-layer compound having the nominal composition (Pb,Bi)2Sr2Ca2Cu30x with a transition temperature of about 110 K and the two-layer compound having the nominal composition Bi2Sr2Ca~Cu20x with a transition temperature of about 92 K. The corresponding single-layer compound Bi2(Sr,Ca)2CuOX also exists, but this is of little interest for industrial use because of the low transition .temperature of about 10 K.
The most widespread method of producing superconducting wires is the OPIT (Oxide Powder In Tube) technology. Here, a precursor material in the form of powder or rods pressed from powder is placed in a metal tube 2 o which usually has a high proportion of silver. This is hammered and drawn out to form a monofilament wire and then bundled together to give a multifilament, drawn out again and usually subsequently rolled. The actual reaction to form the high temperature superconducting material occurs during the final process steps of wire production as a solid-state reaction at temperatures of preferably from 650°C to 850°C in a defined atmosphere.
The quality of the precursor material is critically dependent on the degree of reaction and thus on the reaction time in the wire for conversion into the desired target phase. Furthermore, the quality of the precursor material is important in determining the properties of the superconducting wire, e.g.
current carrying capacity, transition temperature and homogeneity. In most cases, the objective of the preparation of the precursor material is to prereact and optimize the high temperature superconductive material so that the actual striplwire production is, particularly in respect of the thermal treatment, particularly brief, effective and leads to the best superconducting properties.

In the following, no distinction will be made- betweenwire and strip production or wire and strip and reference will be made, in summary, only to wires and their production: As target phase of wire production, which is to be present in the finished wire, a distinction will be made between the two-layer and the three-layer compound, while the precursor material for wire production, i.e. the precursor for the wire, preferably has approximately the chemical composition of the target phase but not necessarily a high proportion of the target phase.
The precursor powder is ~ prepared in a variety ways from sometimes precalcined metal oxides, e.g. by a solid-state reaction, wet chemically, e.g.
by coprecipitation, or by spray pyrolysis. Another method is milling pieces of solidified melt. However, a feature common to all processes is that they always have to go through a plurality of reaction steps (e.g. calcination, heat treatment) and a plurality of processing steps (e.g. drying, milling).
This therefore leads to very complicated and expensive processes.
Nevertheless, the precursor material is used virtually exclusively as powder or as a rod pressed from powder. This is for two reasons in particular.
firstly, extremely fine milling to mean particle sizes of dip _< 5 ~m gives good mechanical deformability of the wires in the process steps of drawing and rolling and, secondly, the milling achieves very good homogeneity which is a particular challenge for the majority of components.
However, these advantages are associated with a series of disadvantages, one of the most serious being the large number of process steps and thus the high costs of powder production. In addition, the many process steps incur the risk of contamination, e.g. by abraded material from the milling processes and by atmospheric water and carbon dioxide which leads to formation of carbonates and hydroxides. Carbonates and hydroxides, particularly those of the alkaline earth metals, are, in contrast to the compounds of the other elements Bi, Pb and Cu, very thermally stable compounds. Such impurities impair, on the one hand, the quality of the superconducting wires by grain-grain contacts in the microstructure of the superconducting wire being adversely affected and, on the other hand, carbonates and hydroxides lead to uncontrolled decomposition reactions during the final solid-state reactions to form the superconducting material in the wire (decomposition temperatures: CaC03: 898°C, Ca(OH)2:
580°C, i Sr(OH)Z: 710°C) and thus to undesirable bubble formation-and constriction of the conductive cross section in the wire, so that current flow through the conductor may be restricted or even interrupted (W. Hellstrom et al., Supercond. Sci. Technol. 8, 1995, 317). In conventional processes, therefore, attention always has to be paid to very high purity and sometimes exclusion of air, which makes the known processes even more costly.
Particularly finely divided powders are desired for wire production by the OPIT method since the development of ever smaller filaments will be _ required. In particular, milling of the precursor to powders having particle sizes in the range d5p = 1 - 5 wm necessitates, as a result of the large reactive surface area of the hygroscopic material, expensive equipment to avoid contamination with carbon dioxide and water (C. Mao et al., Pysica C 281 (1997) 149).
In addition, the use of pressed powder achieves only a low degree of fill in the wire. While the theoretical density of the two- and three-layer compound is somewhat more than 6.5 glcm3, depending on compositio 3 the OPIT method only achieves bulk densities of from 3 to 4.5 g/cm depending on whether loose powder is stuffed into the silver tubes or pressed rods are used. Although this low density aids forming to produce the superconducting wire, it is associated with relatively large, undesired gas inclusions, again with the danger of bubble formation. Furthermore, when pressing powder to produce precursor rods, the dimensional accuracy required for introduction into tubes cannot be ensured because of variable pressing factors. The compacts therefore have to be, especially in the case of non-circular cross sections, machined afterwards, e.g. by sawing, turning or grinding, which results in a loss of up to 30% by weight of the precursor material.
The problems in the shaping of precursor material apply particularly to wires which are not made up of bundled monofilaments (Figure 1 a) but have a more complex structure, for example concentrically arranged layers (Figure 1~b), e.g. in the order silver-superconductor-silver-superconductor-silver-etc. (Supercond. Week, Vol. 11, 24 (1997) 2). This structure enables a finished wire having a multilayer structure to be produced in one step if the precursor material is already present in the spaces between a t concentrically arranged- silver -tubes. For this purpose; according to the pressing technology employed hitherto, tubes have to be pressed from the precursor powder. This is particularly problematical since precise adherence to dimensions of the individual components, the homogeneity of the precursor material and the later shaping of the pressed individual components to fit the tubes accurately is extremely difficult.
However, there is also another reason why tubes placed concentrically within one another constitute an interesting starting geometry for producing superconducting wires. Superconduction is primarily characterized by the complete loss of electrical resistance. However, this applies only to DC
applications, but AC applications are attracting increasing interest.
Alternating electric fields accelerate non-superconducting electrons in the superconductor (there is always only a small proportion of conduction electrons in the superconducting state in the superconductor) and in the metal matrix of the wire so as to generate eddy currents and thus an electrical resistance. To reduce these AC losses, the cross section of the wire in which such an eddy current is generated has to be reduced. This can be achieved by incorporation of resistive barriers, i.e. electrically insulating regions, in the matrix of the superconducting wire.
An electrically insulating layer between the individual superconducting monofilaments (Figure 2a) accordingly reduces such. AC losses. However, in the subsequent thermal treatment of the wires, diffusion of oxygen into the filaments of superconductive material has to be possible while at the same time the insulating material must neither destroy the usually predominantly silver-containing sheath at the high heat-treatment, sintering and annealing temperatures nor diffuse through the usually silver-containing layer.

These demanding requirements can be met only by very few compounds.
In addition, owing to the possibility of diffusion through the intermediate silver sheath into the superconductive material, high concentrations of extraneous elements which impair or destroy the superconducting properties of the superconductive or superconducting material are undesirable in the insulation. For this reason, various attempts have been made in the past to produce an insulating material comprising a greatly contaminated two- or three-layer compound. At concentrations of extraneous ions such as AI, Ti, B, etc. above 1000 ppm by weight, s w complete loss of the superconducting properties is observed. To obtain such an insulating material it is again necessary, as in the case of the conductor concept comprising concentric tubes, firstly to fill an inner silver tube with superconductor precursor material and then to fill the space 5 between the inner and outer tube with insulating material. Here too, it is necessary either to press tubes from powder or to press the powder directly into the hollow spaces, which gives extremely inhomogeneous pressing and is associated with great difficulties.
DE-A-196 13 163 describes a process in which powder is firstly introduced into a sheathing tube and this powder is then, however, heated and melted in the sheathing tube, as a result of which a high, homogeneous degree of fill is supposed to be achieved. When this is carried out in practice, diffiiculties can occur from both chemical and physical points of view:
If the sheathing tube, which usually consists mainly of silver, is heated to temperatures above the melting point of the molten filling material in the range from about 860-900°C depending on the desired superconductor material together with the superconductive material to be melted, the previously - fine-grained, homogeneous sheathing tube material recrystallizes and as a result of the formation of large grains offers starting points for rupture of the sheathing tube during cold forming to produce a wire or strip. Silver has a relatively low melting , point of only 962°C
(Handbook of Chemistry and Physics, CRC Press, Boca Raton, Florida, 70th Ed. 1900), while many of the silver-rich alloys used in industry have a melting point somewhat above 962°C. A significant softening of the silver-rich sheath material occurs far below the melting point, so that severe distortion of the sheathing tube can occur during melting. The rupture of the sheath during production of a wire or strip is also promoted by reaction of the filling melt with the sheathing tube, because the wall thickness is reduced. It is known that, of all metals, only platinum is sufficiently resistant to the aggressive oxide melts based on BiSrCaCuO.
A further problem with the process described is caused by the chemical properties of the superconductors. The phase diagram for superconductors based on bismuth (Phase diagrams for High Tc Superconductors II, The American Ceramic Society, 1997, p. 241 ) shows that all possible compositions have a miscibility gap in the melt below 1000°C, i.e. the melt separates completely into two components in a manner comparable to an -- - oiUwater mixture, so - thaf--- there is a risk that a homogeneous superconductor material cannot be produced.
It is an object of the invention to develop a more economical way of preparing precursor material with a smaller number of process steps, lower material losses, with significantly reduced impurity contents and with a reduced risk of bubble formation occurring during wire production. It is an additional object to propose a simple shaping method and various geometries for individual components for wire production which, in particular, should avoid the abovementioned problems of a melt route such as reaction of the melt with the sheathing tube, demixing of the melt and a temperature which is too high for the sheathing tube materials.
This object is achieved by a process for preparing a high temperature superconductor precursor material for use in strip or wire production, which comprises preparing a mixture of oxides orland their precursors, heating this mixture to a temperature at which a readily mixable and pourable melt is present but individual phases may still be present in the solid state, introducing this melt into a sheath where it solidifies on cooling and wherein the melt solidifies so rapidly that essentially no reaction layer is fom~ed from the material of the sheath and that of the melt, and subjecting the solidified melt to a heat treatment at a temperature at which the solidified melt is converted into a precursor material having a content of at least one high temperature superconductive orland high temperature superconducting phase of at least 10% by weight, wherein no milling is carried out after melting the mixture.
The object is also achieved by a process for preparing a high temperature superconductor precursor material for use in strip or wire production, which comprises preparing a mixture of oxides orland their precursors, heating this mixture to a temperature at which a readily mixable and pourable melt is present but individual phases may still be present in the solid state, introducing this melt into a sheath where it solidifies on cooling, wherein the melt is heated to a temperature at which no demixing of the melt for the superconductive material occurs, the solidified melt is heated in a heat treatment to a temperature at which the solidified melt is converted into a precursor material having a content of at least one high temperature supe~conductive or/and high temperature superconducting phase of at least 10% by weight and no milling is carried out after melting the mixture.
Demixing of the two superconductor components in the melt- can be remedied by further heating of the melt to above 1000°C or rapid quenching of the melt to freeze-in the microstructure before demixing.
In a process as described in DE-A-196 13 163, further heating is prohibited by the excessively low melting point of the sheath material, since the sheathing'tube material would otherwise melt and the sheathing tube would be destroyed.
Quenching of the melt in the context of a process as described in DE-A-196 13 163 is not possible because of the good thermal conductivity of silver which has the highest thermal conductivity of all metals at 3.82 W/(cm2K) (Handbook of Chemistry and Physics, CRC Press, Boca Raton, Florida, 70th Ed. 1900), since the silver-containing sheathing tube would level out the temperature differences required, so that continuous quenching in the longitudinal direction of the sheathing tube should not be practicable.
As a result of the rapid solidification on the contact surface with the sheathing tube, preferably less than 1 second, particularly preferably less than 0.5 seconds, no reaction can occur between the liquid melt and the preferably silver-containing sheath material. In the case of slow solidification, the entire sheathing tube comprising a silver-rich alloy can be reacted away by the melt; here, the silver can be dissolved in the melt and precipitated as silver oxide. In the case of incomplete dissolution of the sheathing tube, which usually has a wall thickness of about 1 mm, a reaction layer comprising mostly silver-containing mixed oxides can be observed; however, a sheathing tube which has been attacked in this way can no longer be used for wire or strip production. However, the sheathing tube together with the superconductive or superconducting material can usually still be used if the reaction layer is thinner than 10 Nm, preferably thinner than 5 Nm.
The process of the invention is notable for, in particular, the complete avoidance of milling processes after melting and thus the avoidance of extremely air-sensitive powders as intermediate, since the material is not - introduced into the usually silver-containing sheaths in the forma of powder or pressed powder, but in the form of a melt.
In the process of the invention, a mixture of metal oxides such as Bi203;
PbO, SrO, CaO, Cu0 or their precursor materials corresponding to the desired atomic ratio for the superconducting bismuth-{lead~strontium-calcium-copper oxides is melted at temperatures above 1000°C, preferably above 1050°C. Preferably, all rations are present in the mixture in the form of oxides. Preference is given to setting a stoichiometric or approximately stoichiometric ratio of the oxides in the mixture, based on the nominal composition of the two- or three-layer compound. If desired, up to 50 mol%
of the bismuth can be replaced by lead, antimony or/and yttrium; in addition, to improve the crystallinity, up to 30% by weight of a high-melting compound, in particular alkaline earth metal sulfates, preferably SrS04, BaS04 oNand (Ba,Sr)S04, can be added. These high-melting compounds should neither melt themselves nor decompose at temperatures up to the temperature selected for melting the mixture.
Melting is preferably carried out in platinum crucibles, in platinum-lined crucibles or in crucibles made of another suitable material such as barium zirconate in furnaces such as muffle furnaces. The melt can be agitated during the melting procedure by mechanical, preferably platinum-coated, stirrers, blowing in gas and/or swirling the crucible.
During the melting procedure, the oxide melt should, as a result of the choice of crucible materials, come into contact only with inert materials such as platinum, related noble metals or their alloys. Contamination of the melt or premature aging of the crucibles is avoided in this way. The high melt temperatures above 1000°C and possibly the motion of the melt suppress separation into two liquid phases and produce a homogeneous melt.
After melting, which does not have to be complete especially when high-melting compounds as described above, e.g. alkaline earth metal sulfates, have been added, the crucible is taken from the furnace and the pourable melt is, preferably without delay, introduced, poured or sucked directly into tubes or tube-like sheaths or via communicating systems. For the purposes of the present invention, the term "sheath" includes all tubes, tube systems and similar constructions which are suitable -together with the precursor material and possibly an insulating material, to give an essentially rod-shaped body which is referred to as a rod in the present patent application.
The sheaths used are here either at room temperature or preheated to temperatures of preferably up to 750°C, in particular up to 700°C, particularly preferably to from 200°C to 500°C. The longer the transport paths of the melt, the higher the preheating temperature should be. The greater the internal diameter of the sheath, the lower the preheating temperature should be in order to avoid an excessively large temperature increase in the sheath. The temperature difference between the melt and the preheated sheathing tube should preferably be at least 300°C. In this way, pouring the melt into the sheath results in immediate solidification on the inner surtace of the sheath in the form of a glass-like thin layer. The thickness of this protective layer is generally from 50 to 250 Nm, depending in particular on the temperature difference between melt and sheathing tube. This layer then protects the sheath against the further, liquid and aggressive oxide melt poured into the sheath. In addition, this layer insulates the sheath against the still liquid melt in the interior and thus reduces the maximum temperature of the sheath. As a result of this phenomenon, the sheath is not heated above its softening point but rather the maximum temperatures are < 800°C. For this reason, problems such as rupture of the silver sheath during the actual wire production process do not occur if the melt reacts with the silver-containing alloy or if the sheathing tube is heated to excessively high temperatures, in particular temperatures of more than 850°C.
The cooling rate of the melt can be varied within a wide range by casting the melt in the furnace, by casting outside the furnace and placing the filled sheath back in the furnace and subsequent slow cooling in the furnace or by means of specially applied heating elements. When cooling is carried out outside the furnace, the cooling rate is usually in the order of from 20 to 100 IVs. When filling the sheath, care should be taken to ensure that the gases present in the melt and in the sheaths can escape as completely as possible and that very few and only very small voids or pores are formed.
During cooling, however, care must always be taken to ensure that the protective layer produced on the inner surface of the sheath is not melted again. When the process is carried out well, there are no pores > 50 pm or even no pores > 20 pm present in the-solid~ed melt after cooling, if the uppermost parts of the solidified melt are disregarded.
The melting orland casting of the melt is preferably carried out in air, but 5 can also be carried out under protective gas, particularly preferably under nitrogen, argon or synthetic air, to reduce impurities further. The degassing of the melt can, if desired, be aided by melting under reduced pressure or/and under a protective gas.

10 The sheaths preferably have a diameter of from 5 to 20 mm and a wall thickness of from 0.5 to 5 mm and preferably consist of pure silver (> 99%
by weight) or a silver alloy which can have, in particular, contents of Mg, Au, Pd orland Cu of <_ 10% by weight and can have any cross section, in particular a round or angular cross section, and be welded shut on one side. The sheaths can also have, in their interior, lamellae and, if desired, tubular elements which are preferably an-anged concentrically or in a regular way next to one another; there are no limits to the variety of shapes for multichannel sheaths or for single-channel sheaths having fin-like lamellae. The individual channels (= chambers) can be filled with superconductive or electrically insulating materials. During casting, the sheaths serve primarily to delineate the external shape and to separate the superconductive and, if present, insulating material regions. During and after wire production, the tubular elements and lamellae serve, in particular, to align the superconductive or superconducting crystallites in preferred directions and as a ductile encapsulation of the brittle ceramic material during hammering, drawing, rolling, etc. for . manufacturing the superconducting wire. The sheaths can also assume cross sections and shapes which are atypical for conventional tubes and be, for example, bent into U shapes, where, in particular, the one side of the sheaths can be made oblique, for example, so as to be particularly matched to the casting process and the escape of the gases. The wall thickness of the tubular elements and lamellae is preferably made small but still sufficient to maintain an adequate separating layer between the adjacent material layers during subsequent wire production.
Such multihole sheaths are preferably provided with thin chamber walls;
they can have been produced by extrusion methods or by joining a plurality of individual components. The channels of the widely differing types of t sheaths are preferably accessible at both-ends so that the melt can enter at one end and the gases can escape at the other end. The sheaths can also have branches in the individual channels, for example to allow gases to escape even better. The sheaths preferably have essentially the same cross section and, if applicable, also the same type of dividing walls (_ lamellae) over their entire length. The lamellae do not necessarily have to separate the chambers completely from one another but can also project like fins into a free space. In place of a sheath closed at the side, it is also possible to use tubes having a more complicated construction in which individual chambers can be open to the outside or, particularly advantageously, wound foils, in particular spirally wound foils, which in the longitudinal direction on the outer edge can be open, partly contacted or closed in the form of a seam; these wound foils have the advantage of an improved distribution of the melt and, provided that they are not closed in a gastight manner on the outer edge in the longitudinal direction, the greater variety of paths for introducing the melt or for removing gas. The wound foils, too, can be combined with lamellae orland tubular elements. The sheaths having a more complicated construction can also comprise at least one tubular element and at least one set of lamellae, a plurality of nested tubular elements with or without lamellae or, in addition, further channels.
The number of nested tubular elements is preferably from 2 to 25, particularly preferably from 3 to 10, and that of the lamellae is preferably from 1 to 12, particularly preferably from 2 to 8, without a lamella running through a plurality of tubular elements or a tubular element running through a plurality of lamellae being counted more than once. The sheaths according to the invention having a more complicated construction comprising a plurality of tubular elements or/and lamellae have the great advantage that, particularly in the case of a relatively large number of tubular elements or lamellae, bundling together to form multifilaments is not necessary and process steps in wire production can thus be saved, since the sheath filled with superconductive material and, if desired, electrically insulating material in individual chambers can be regarded from the beginning as a "multifilament°. Such precursor rods preferably have a rectangular or circular cross section. In the present application, the wound foils are also covered by the generic term "sheath°.
The solidified melt has, depending on composition and additives content, a bulk density of >_ 5.5 g/cm3, corresponding to a porosity of <_ 25%. The precursor material-thus has a bulk density which is at least 20%, often at feast 50%, higher than that of conventional precursor material prepared via a powder route. The main crystalline constituent of the precursor material of the invention is preferably the single-layer compound having the nominal composition Bi2(Sr,Ca)2CuOx; in addition, alkaline earth metal cuprates, copper(I) oxide and, in the case of lead oxide in the starting mixture, alkaline earth metal plumbates often occur. Based on microscopic examination, the proportion of amorphous material is, depending on the cooling rate of the melt, preferably from 10 to 90% by weight, in particular from 20 to 80% by weight. As a general rule, the lower the cooling rate, the higher the proportion of crystalline material.
Since the melt solidifies at temperatures above the existence region of all hydroxides and all carbonates of the rations present with the exception of SrC03 (decomposition point: 1268°C), contamination with such materials can largely be avoided. The impurities are less than 500 ppm by weight of carbon, preferably less than 300 ppm by weight of carbon, and less than 300 ppm by weight of hydrogen, preferably less than 150 ppm by weight of hydrogen, in the solidified melts. To rule out contamination with strontium carbonate as well, the strontium oxide to be used can be ignited for a short time at temperatures of >_ 1250°C before use in the mixture, which reduces the carbon content to below 100 ppm by weight.
The intermediate, namely the solidified melt, is significantly less air sensitive than the fine powders which are obtained in the customary precursor material preparation, since only a thin surface layer at the openings of the sheaths can be attacked by air. In contrast to the precursor powders or the very open-pored, pressed rods very little gas, if any, can under normal circumstances penetrate into the interior of the compact solidified melt. The intermediates therefore do not have to be handled in the absence of air. The processing of the intermediates can, however, advantageously be carried out under protective gas or/and under reduced pressure.
The solidified melt usually has, in contrast to the oxide mixture used, a significant proportion of copper(I), equivalent to a deficiency of oxygen. If the rations used are taken to be in the stable oxidation states Bi(III), Sr(II), Ca(II), Cu(Il), Pb(II), the oxygen content of the two-layer compound should s be about S peg formula unit and that of the lead-free three-layer compound should be about 10 per formula unit. In the solidified melt, this oxygen content is usually reduced by more than 5 mol%. In the final heat-treatment step, oxygen has to be taken up and, in particular, diffuse into the wire from the outside in order to produce superconducting materials from superconductive materials and to set the oxygen content giving the optimum superconducting properties for the compound. In the case of an excess of oxygen, as is frequently encountered in precursor powders or in rods pressed from powders, this has to be removed through the sheath during heat treatment, which can lead to bubble formation when oxygen is evolved spontaneously.
If the final wire is made up of, for example, concentrically arranged, alternate layers of silver-superconductor-silver-etc., the casting mold for the precursor material melt can comprise a plurality of, for example, concentrically nested or worked tubular elements. The melt can then be introduced either in one pour or in a plurality of separate casting steps into the hollow spaces of the sheaths, in particular between the tubular elements or, if present, lamellae.
At least two casting steps, which can be carried out separately but also simultaneously, are necessary if different melts are, to be introduced into the various chambers. This is necessary to achieve electrical insulation of the superconductive "monofilaments" from one another in the "multifilament conductor'. As insulation, it is here possible to use, for example, the single-layer compound, but only for applications significantly above the transition temperature of the single-layer compound of about 10 K, the relatively strongly contaminated two- or three-layer compound without superconducting properties or a material based on BiSr-(Ca-)Cu-O of completely different stoichiometry without occurrence of high temperature superconducting phases. For all materials, the casting process remains unchanged from that for superconductivelsuperconducting materials. The composite materials obtained in this way can be treated further like the above-described simple precursor materials.
-The solidified melt in the sheath displays, owing to the high density, a significantly worse forming behavior in wire production than comparable OPIT silver tubes of the prior art. For this reason, a further thermal 14 ...
treatment- is advaritageously carried out before forming; during this heat __ treatment step, the precursor is partly converted into a precursor having a phase composition with a higher proportion of the significantly more ductile two-layer compound so that this heat-treated precursor can then be readily deformed during wire production.
The heat treatment is preferably carried out in muffle furnaces or tube furnaces in a stream of nitrogen, oxygen, air or a nitrogenloxygen gas mixture and if desired, under reduced gas pressure. The temperatures are preferably from 600°C to 900°C, particularly preferably from 700°C to 840°C; the reaction time is, in particular, from 0.2 to 50 hours, preferably from 0.5 to 20 hours. The reaction can be carried out to from 10 to 100% by weight of the two-layer compound, preferably to from 50 to 90% by weight.
Further phases which occur in the case of the two-layer compound as target phase are especially the single-layer compound, alkaline earth metal cuprates and copper(I) oxide. In the case of the three-layer compound as target phase, there may, in addition, be formation of initial amounts of the three-layer compound and formation of alkaline earth metal plumbates; with the exception of the single- and two-layer compound, the proportions of all other phases formed are usually less than 20% by weight, preferably less than 10% by weight.
A further variant of the process of the invention is first to pour the melt into divided dies, i.e. dies to be opened, or into molds, dishes or crucibles made of a heat-resistant material, preferably of copper or a copper-rich alloy, which are, in particular, at room temperature or at temperatures up to 500°C. Melting is here carried out in a similar way to when the melt is poured into sheaths. After taking the solidified melt from the die or from the mold, the dish or the crucible, the bars of solidified melt are first heat treated, preferably under the same conditions as described above. This can be followed by mechanical shaping, e.g. by milling, turning or sawing, so that the rods of solidified melt obtained can be fitted accurately into sheaths before further processing to produce wire.
In all heat treatment processes, both for the solidified melt in sheaths and for bars of solidified melt, the reaction rate is found to be only very slightly dependent on the atmosphere selected, which results from the compact structure of the solidified melt without relatively large open pores.
Likewise, there are virtually no differences in the reaction rates for solidified melts in different sheaths which have a different oxygen diffusion rate.
Compared to precursor material in the form of powder or pressed rods, the 5 solidified melts have a significantly increased reactivity. The reaction times for producing a defined proportion of two-layer compound are, surprisingly, often reduced to about one third of the otherwise customary heat treatment times. The cause of this behavior is that oxygen is given off on melting and is not taken up again on cooling, leading to an oxygen deficiency. This is 10 also shown by the occur-ence of Cu20 which solidifies to form a eutectic together with Bi-Sr-Ca-containing phases. The eutectic usually undergoes partial melting during heat treatment. The partial melting significantly reduces the reaction times compared to usual solid-state reactions in solid mixtures. Moreover, the atmosphere in the heat treatment of powders or 15 pressed, highly porous rods has to be monitored precisely using complicated measurement techniques while, in contrast, small fluctuations in the gas compositions have no influence on the solidified melts during heat treatment.
Drawings:
The invention is illustrated by way of example with the aid of drawings.
Figures 1 a and 1 b schematically show a multifitament wire (1 ) and a concentrically constructed wire (2) according to the prior art. These wires comprise a plurality of chambers (3) whose walls (4) and (5) comprise a silver-rich matrix or a silver-rich sheath material. The chambers are filled with a high temperature superconductor material (6).
Figure 2a shows a multifilament wire (7) having a plurality of chambers (8) according to the prior art, whose walls (9) comprise a silver-rich matrix.
Within the chambers there is a high temperature superconductor material (10) which is surrounded first by a silver-rich sheath (9a) and then by an insulating intermediate layer (11 ).
Figure 2b schematically shows a wire (12) constructed according to the invention and having a plurality of chambers (13) formed by concentric layers of silver-rich chamber walls (14) and a high temperature superconductor material (15) alternating with electrically insulating layers (16) in accordance with the invention.
Examples:
Example 1 (according to the invention):
200 g of a mixture of individual oxides (Bi203 99.9% by weight; Cu0 99.999% by weight; Ca0 99.99% by weight; Sr0 99.32% by weight) having the overall composition Bi2,p1Sr1,g2Ca1.05Cu2~x (Bi: 47.0% by weight; Sr:
18.8% by weight; Ca: 4.7% by weight; Cu: 14.2% by weight) was melted in a platinum crucible at a temperature of 1100°C for 30 minutes in a muffle furnace and poured into an AgIPd tube having a Pd content of 3% by weight and a square cross section with internal dimensions of 9 x 9 x 300 mm and a wall thickness of 1 mm. The tube together with the solidified melt was subsequently heat treated in air at 710°C for 20 hours in a muffle furnace. The precursor obtained in this way was characterized as follows:
a. Composition: ICP-AES analysis (ICP plasma 400 and AAS

11 OOB; Perkin-Elmer) Bi: 46.7% by weight;

Sr: 19.0% by weight; Ca: 4.8% by weight;
Cu:

14.1 % by weight; corresponding to:

Bi2.01 Sr1.96Ca1.08Cu2~x b. Impurities: C: 400 ppm by weight (Coulomat 702;
from StrtShlein) H: 80 ppm by weight (CHNS 932; from Leco) Pt: 450 pp3 by weight (1CP-AES) c. Bulk density: 6.02 g/cm d. X-ray powder Crystalline material (X-pert; from Philips):

patterns: 65 ( 5)% by weight of two-layer compound, 31 (t5)% by weight of single-layer compound e. Reflected-light micrographs of polished sections:

Amorphous content less than 10% by volume;

- homogeneous distribution across the entire cross section;

no reaction of the precursor material with the tube wall.

a The thickness of the glass-like protective layer on the inside of the sheathing tube was about 100 pm.
Example 2 (according to the invention):
As described in Example 1, but the melt was poured into a round Ag tube having an internal diameter of 10 mm and a wall thickness of 1 mm.
X-ray powder patterns: Crystalline material (X-pert, from Philips):
62 (t 5)% by weight of two-layer compound, 35 (t 5)% by weight of single-layer compound.
The thickness of the glass-like protective layer on the inside of the sheathing tube was about 100 pm.
Other analytical data as in Example 1.
Example 3 (according to the invention) As described in Example 2, but Sr0 was ignited for 1 hour at a temperature of 1300°C before preparation of the oxide mixture.
Impurities: C: 180 ppm by weight (Coulomat 702; from Strbhlein) Other analytical data as in Example 2. .
Example 4 (according to the invention):
As described in Example 2, but an additional 20% by weight of strontium sulfate was added to the starting oxide mixture. The precursor obtained in this way was characterized as follows:
a. Composition: ICP-AES analysis (ICP plasma 400 and AAS
11 OOB; Perkin-Elmer) Bi: 38.0% by weight;
Sr: 24.6% by weight; Ca: 3.6% by weight; Cu:
11.4% by weight; corresponding to, with subtraction of 20% by weight of SrS04:
Bi2.03Sr1.92Ca1.00Cu20x s b. Impurities: ~ - -- C: 350 ppm by weight (Coulorriat 702; from Strt~hlein) H: 90 ppm by weight (CHNS 932; from Leco) Pt: 420 ppm by weight (1CP-AES) c. Bulk density: 5.90 glcm3 d. X-ray powder Crystalline material (X-pert; from Philips):
patterns: 60 (t 5)% by weight of two-layer compound, 22 (t 5)% by weight of single-layer compound, 14 (t 5)% by weight of SrS04.
The thickness of the glass-like protective layer on the inside of the sheathing tube was about 150 pm.
Example 5 (according to the invention):
As described in Example 1, but the melt was poured into a copper die having the ~ internal dimensions 20 x 100 x 100 mm3 which had been preheated to 350°C. The solidified melt was taken from the die and, by means of a diamond-plated band saw, sawn into cuboids having dimensions of 9 x 20 x 100 mm3. These bars were subsequently heat treated for 10 hours at 750°C under a stream of nitrogen in muffle furnaces.
The precursor obtained in this way was characterized as follows:
a. Composition: ICP-AES analysis (ICP plasma 400 and AAS
11 OOB, Perkin-Elmer) Bi: 47.1 % by weight;
Sr: 18.7% by weight; Ca: 4.5% by weight; Cu:
14.4% by weight; corresponding to:
Bi1.99Sr1.88Ca0.99Cu20x b. Impurities: C: 380 ppm by weight (Coulomat 702; from Strohlein) H: 100 ppm by weight (CHNS 932; from Leco) - Pt: 390 pp3 by weight (ICP-AES) c. Density: 6.10 g/cm -~d:- -X=ray powder Crystalline material (X-pert; from Philips):
patterns: 81 (t 5)% by weight of two-layer compound, 16 (t5)% by weight of single-layer compound e. Reflected-light micrographs of polished sections:
Amorphous content less than 10 (t5)% by volume;
homogeneous distribution across the entire cross section.
Example 6 (according to the invention):
200 g of a mixture of individual oxides (Bi203 99.9% by weight; Pb0 99.9%
by weight, Cu0 99.999% by weight; Ca0 99.99% by weight; Sr0 99.32%
by weight) having the composition Bil,7pPbp.33Sr1.85Ca1,ggCu30x (Bi:
34.6% by weight; Pb: 6.7% by weight; Sr: 15.8% by weight; Ca: 7.8% by weight; Cu: 18.6% by weight) was melted in a platinum crucible at a temperature of 1050°C for 45 minutes in a muffle furnace and poured into an Ag tube having a round cross section with an internal diameter of 10 mm and wall thickness of 1 mm. The tube was subsequently heat treated in air at 710°C for 20 hours in a muffle furnace. The precursor obtained in this way was characterized as follows:
a. Composition: ICP-AES analysis (ICP plasma 400 and AAS
11008; Perkin-Elmer) Bi: 35.4% by weight; Pb:
6.9% by weight; Sr: 16.2% by weight; Ca: 7.0%
by weight; Cu: 19.2% by weight;
corresponding to:
Bi1.68Pb0.33Sr1.84Ca 1.73Cu3~x b. Impurities: C: 300 ppm by weight (Coulomat 702; from Strohlein) H: 100 ppm by weight (CHNS 932; from Leco) Pt: 380 ppm by weight (ICP-AES) c. Bulk density: 6.12 g/cm3 d. X-ray powder Crystalline material (X-pert; from Philips):
patterns: 52 (t 5) % by weight of two-layer compound, . .
43 (t 5) % by weight of single-layer compound.
e. Reflected-light micrographs of polished sections:
amorphous content less than 10% by volume;
homogeneous distribution across the entire cross section;
no reaction of the precursor material with the tube wall.
The thickness of the glass-like protective layer on the inside of the sheathing tube was about 200 pm.
5 Example 7 (according to the invention):
As described in Example 6, but the melt was poured into a copper die having internal dimensions of 20 x 100 x 100 mm3 which has been preheated to 350°C. The solidified melt was taken from the die and, by 10 means of a diamond-plated band saw, sawn into cuboids having dimensions of 9 x 20 x 100 mm3. These bars were subsequently heat treated for 20 hours at 750°C under a stream of nitrogen in muffle furnaces.
The precursor obtained in this way was characterized as follows:
a. Composition: ICP-AES analysis (ICP plasma 400 and AAS
1100B; Perkin-Elmer) Bi: 35.2% by weight; Pb:
6.8% by weight; Sr: 16.4% by weight; Ca: 7.3%
by weight; Cu: 19.1 % by weight;
corresponding to:
Bi1.68Pb0.33Sr1.87Ca1.82Cu3~x b. Impurities: C: 400 ppm by weight (Coulomat 702; from Strohlein) H: 70 ppm by weight (CHNS 932; from Leco) Pt: 370 ppm by weight (ICP-AES) c. Bulk density: 6.12 g/cm3 d. X-ray powder Crystalline material (X-pert; from Philips):
patterns: 93 (~ 5) % by weight of two-layer compound, 4 (~ 5) % by weight of single-layer compound.

y , i Exaniple-8:
In this example, the reactivities of various precursor materials for the two-layer compound are compared. All precursor materials were obtained from the same oxide mixture as in Example 1 either by pouring the melt into a silver tube (precursor material A; according to the invention) or into a die as described in Example 2 and subsequently sawing the solidified melt into bars (precursor material B; according to the invention). In addition, powder was obtained by m_ filling the bars of precursor material B in an air jet mill immediately after casting ~ (precursor material C; comparative example).
This powder was isostatically pressed to produce rods (precursor material D; comparative example) having a diameter of 10 mm and a length of 100 mm.
Table 1:
Heat treatment conditions and resulting content of the two-layer compound Precursor Temperature Time Atmosphere % by weight of material - [°C] [h] two-layer compound after heat treatment A 750 20 air 87 B 750 20 air . ' 85 C 750 20 air 0 D 750 20 air 25 Table 1 shows the surprisingly high, compared to the pulverulent precursor materials C and D, proportions of two-layer compound in solidified melts of precursor materials A and B. The precursor materials A and B have a high proportion of two-layer compound even after these short heat-treatment times.
Example 9 (according to the invention):
As described in Example 6, but three tubular Ag elements having round cross sections with internal diameters of 8, 13 and 18 mm and a wall thickness of 1 mm in each case were first placed inside one another and soldered at the lower end to centering rings so as to give uniform spacings 22 _ between the individual tubes. 400 g of a mixture as described in Example 6 were then melted, poured into the intermediate spaces and then treated further as described in Example 6.
Reflected-light micrographs of polished sections:
No difference between inner and outer precursor material layers in respect of crystallinity and phase composition;
amorphous content less than 10% by volume;
no reaction of the precursor material with the tube walls.
The thickness of the glass-like protective layer on the inside of the sheathing tube was about 80 ~m for the inner tube, about 120pm for the middle tube and about 220 pm for the outer tube.
Other analytical data as in Example 6.
Example 10 (according to the invention):
Two tubular silver elements having circular cross sections with internal diameters of 13 and 18 mm and a wall thickness of 1 mm in each case were first placed inside one another and soldered at the lower end to centering rings so as to give a uniform spacing between the tubular elements. 200 g of a mixture of individual oxides (Bi2O3 99.9% by weight;
Cu0 99.999% by weight; Ca0 99.99% by weight; Sr0 99.32% by weight) having the composition Bi2.01Sr1.92Ca1.05Cu20x (~i: 47.0% by weight; Sr:
18.8% by weight; Ca: 4.7% by weight; Cu: 14.2% by weight) was melted in a platinum crucible at a temperature of 1110°C for 30 minutes in a muffle furnace and poured into the inner tubular Ag element. Subsequently, 200 g of a mixture having the composition Bi~.99Sr1.95CuOX were melted in a platinum crucible at a temperature of 1000°C for 30 minutes in a muffle furnace and poured into the hollow space between inner and outer tubular Ag elements. The tube was subsequently heat treated in air at 710°C
for 20 hours in a muffle furnace. The precursor obtained in this way was characterized as follows:
a. Composition: ICP-AES analysis (ICP plasma 400 and AAS
11008; Perkin-Elmer) inner tubular element:
Bi: 47.3% by weight; Sr: 18.6% by weight;
Ca: 4.7% by weight; Cu: 14.3% by weight;

.~ v ~ . ....' .~.-.~..~.:-.. . -_-.._. ~ ....
v t :....-....L.-'.....:._.._ ...:-..
corresponding:-to~Bi2.01~~1.89~a~.o4Cu20x _ -_.__ outer tubular element:--r Bi: 55.7% by weight;-Sr:-22.8% by weight;
Cu: 8.5% by weight; -corresponding to: Bi~.99Sr1.95CuOx b. Impurities: C: 400 ppm by weight..(Coulomat 702; from Strdhlein) H: 80 ppm by weight (CHNS 932; from Leco) Pt: 450 p~m by weight (ICP-AES) c. Bulk density: 6.11 glcm X-ray powder patterns: Crystalline material~(X-pert; from Philips):
inner tubular element:
72 (t 5) % by weight of two-layer compound, 25 (t 5) % by weight of single-layer compound;
external tubular element virtually 100% single-layer compound.
Reflected-light micrographs of polished sections:
no difference between inner and outer precursor - material layers in respect of crystallinity and phase composition;
amorphous content less than 10% by volume;
no reaction of the precursor material with the tube walls.
The thickness of the glass-like protective layer on the inside of the sheathing tube was about 80 Nm for the inner tube and about 150 Nm for the outer tube.
Example 11 (according to the invention):
The procedure of Example 6 was repeated, but a tube furnace with Inconel steel tube which sealed very well was used in place of a muffle furnace (not gastight). Furthermore, synthetic, C02-free air was used as atmosphere.
The composition was identical to that in Example 6. The following were determined:
b. Impurities: C: 120 ppm by weight (Coulomat 702; from Strtihlein) H: 60 ppm by weight (CHNS 932; from Leco) ,. Pt: 390 ppm by viieight (ICP-AES) . . _..._... .
c. Bulk density: 6.11 g/cm3 d. X-ray powder Crystalline material (X-pert; from Phiiips):
patterns: 60 (t 5) % by weight of two-layer compound, 36 (t 5) % by weight of single-layer compound.

Claims

Claims:

1. A process for preparing a high temperature superconductor precursor material for use in strip or wire production, which comprises preparing a mixture of oxides or/and their precursors, heating this mixture to a temperature at which a readily mixable and pourable melt is present but individual phases may still be present in the solid state, introducing this melt into a sheath where it solidifies on cooling and where a glass-like thin protective layer forms on the inner surface of the sheath, with the melt solidifying so rapidly that essentially no reaction layer is formed from the material of the sheath and that of the melt, and, if desired, subjecting the solidified melt to a heat treatment at a temperature at which the solidified melt is converted into a precursor material having a content of at least one high temperature superconductive phase of at least 10% by weight, wherein milling processes are completely avoided after melting the mixture.

2. A process for preparing a high temperature superconductor precursor material for use in strip or wire production, which comprises preparing a mixture of oxides or/and their precursors, heating this mixture to a temperature at which a readily mixable and pourable melt is present but individual phase's may still be present in the solid state, introducing this melt into a sheath where it solidifies on cooling and where a glass-like thin protective layer forms on the inner surface of the sheath, wherein the melt is heated to a temperature at which no demixing of the melt for the superconductive material occurs, the solidified melt is, if desired, heated in a heat treatment to a temperature at which the solidified melt is converted into a precursor material having a content of at least one high temperature superconductive phase of at least 10%
by weight and milling processes are completely avoided after melting the mixture.

3. The process as claimed in claim 1 or 2, wherein an oxide mixture based on Bi-Sr-Ca-Cu-O and further comprising, if desired, additional chemical elements such as Pb, Sb, Y and/or further phases such as high-melting compounds is prepared.

4. The process as claimed in claim 3, wherein up to 50 mol% of the bismuth is replaced by lead, antimony or/and yttrium.

5. The process as claimed in any of claims 1 to 4, wherein a high-melting compound having a melting point above 1000°C, in particular an alkaline earth metal sulfate, preferably a barium or/and strontium sulfate, is added to the oxide mixture.

6. The process as claimed in any of claims 1 to 5, wherein the oxide precursors of the mixture are calcined prior to melting or/and the sheath is preheated at a temperature up to 750°C before introducing the melt.

7. The process as claimed in any of claims 1 to 6, wherein melting is carried out in a sheath, in a dish, in a crucible or on a base made of a heat-resistant material.

8. The process as claimed in any of claims 1 to 7, wherein the heat-resistant material is a noble metal or a noble metal alloy, in particular one having a high content of platinum, copper or/and silver.

9. The process as claimed in any of claims 1 to 8, wherein the melt is poured onto or into a silver-rich material which may remain permanently in contact with the resulting precursor material.

10. The process as claimed in any of claims 1 to 9, wherein the mixture based on Bi-Sr-Ca-Cu-O and further comprising, if desired, additional chemical elements and phases is heated to temperatures of from 950 to 1250°C, in particular to at least 1000°C, preferably to about 1050°C, preferably to not more than 1150°C, in particular for hold times of from 15 minutes to 8 hours, preferably at least 30 minutes, preferably not more than 2 hours.

11. The process as claimed in any of claims 1 to 10, wherein the solidified melt is subjected to a heat treatment at temperatures of from 600 to 950°C, preferably at least 650°C and not more than 850°C, in particular for hold times of from 15 minutes to 100 hours, preferably at least 45 minutes, preferably not more than 30 hours.

27a

12. The process as claimed in any of claims 1 to 11, wherein a precursor material having a content of high temperature super-conductive phases of from 15 to 95% by weight, in particular at least 40% by weight, preferably not more than 80% by weight, is formed in the heat treatment.

13. The process as claimed in claim 12, wherein a precursor material comprising predominantly a high temperature superconductive phase having a two-layer structure is formed in the heat treatment.

14. The process as claimed in any of claims 1 to 13, wherein a precursor material having a content of only low temperature super-conductive phases of up to 90% by weight, in particular at least 20%
by weight, preferably not more than 60% by weight, is formed in the heat treatment.

15. The process as claimed in any of claims 1 to 14, wherein a precursor material having a composition in the range Bi1.8-2.3 Sr1.7-2.1Ca0.7-1.2Cu2Ox, Bi1.6-1.9Pb0.2-0.5Sr1.7-2.1Ca0.7-1.2Cu2Ox, Bi1.8-2.3Sr1.7-2.1Ca1.7-2.2Cu3Ox or Bi1.6-1.9Pb0.2-0.5Sr1.7-2.1 Ca1.7-2.2Cu3Ox, if desired having an additional content of up to 30%
by weight of a high-melting compound, in particular an alkaline earth metal sulfate, particularly preferably a barium or/and strontium sulfate, is formed in the heat treatment.

16. The process as claimed in any of claims 1 to 15, wherein the sum of the carbon and hydrogen impurities in the precursor material is reduced to ~ 800 ppm by weight, preferably to ~ 400 ppm by weight, in particular to ~ 200 ppm by weight.

17. The process as claimed in any of claims 1 to 16, wherein the relative density of the precursor material is from 75 to 99%.

18. The process as claimed in any of claims 1 to 17, wherein the melt is introduced into a tube, into a tube having a plurality of chambers, into a wound and, if desired, edge-contacted foil or into a communicating tube system, where in each case a plurality of tubular elements or/and lamellae can be attached, of a 27b heat-resistant material, in particular by casting, sucking or by means of capillary forces.

19. The process as claimed in claim 18, wherein, in a tube having a plurality of chambers, a melt for high temperature superconductive recursor materials is introduced into part of the chambers and a material for electrical insulation is introduced into another part of the chambers.

20. The process as claimed in claim 19, wherein the material for electrical insulation is a strongly contaminated material based on Bi-Sr-Ca-Cu-O, if desired further comprising additional chemical elements or/and phases, a material based on an only low temperature superconductive phase or a material based on phases which are not high temperature superconductive based on Bi-Sr-Ca-Cu-O, if desired further comprising additional chemical elements or/and phases.

21. The process as claimed in any of claims 1 to 20, wherein a precursor rod is produced as starting material for strip or wire production.

22. The process as claimed in claim 21, wherein the precursor rod is processed by drawing, hammering, bundling, rolling or/and sintering to produce a high temperature superconducting strip or wire.

23. The process as claimed in any of claims 1 to 22, wherein the melt is introduced into a tube having a plurality of chambers or/and lamellae or into a wound foil which makes possible such a great layer sequence of sheath layer(s) to high temperature superconductive layers that the precursor rod can be used like a multifilament without bundling of a plurality of precursor rods.

24. The process as claimed in claim 1, wherein the melt is poured into divided molds, i.e. molds to be opened, or into molds, dishes or crucibles made of heat-resistant material, the solidified melt is taken from the divided mold, mold, dish or crucible and is heat-treated and, if desired, is treated by subsequent mechanical working so that the solidified melt rods obtained can be introduced into sheaths in an accurately fitting manner prior to further processing to produce wires.

25. A precursor rod produced as claimed in any of claims 1 to 24 which has a eutectic microstructure comprising crystalline single- and two-layer compounds whose grains have grain sizes essentially in the range from 20 to 400 µm, preferably from 50 to 300 µm, and in which the normal to the plane of the layer structure in the grains of the single- and two-layer compounds is preferably aligned essentially perpendicular to the temperature gradient during cooling, i.e. preferably essentially perpendicular to the outer surface of the solidified melt.

26. A precursor rod as claimed in claim 25 whose porosity is ~ 30% by volume, preferably ~ 20% by volume, and whose closed pores essentially have diameters of ~ 50 µm, preferably ~ 20 µm.

27. A precursor rod as claimed in either of claims 25 and 26 whose high temperature superconductor material has hydrogen impurities of ~ 350 ppm by weight, preferably ~ 200 ppm by weight, particularly preferably ~ 100 ppm by weight, and carbon impurities of ~ 400 ppm by weight, preferably ~ 250 ppm by weight, particularly preferably ~ 150 ppm by weight.

28. A precursor rod as claimed in any of claims 25 to 27 whose precursor material comprises a metallic sheath, preferably of silver or a silver-rich alloy.

29. The use of a precursor rod produced as claimed in claim 22, for example a multifilament for strip or wire production.