EP1051760A1

EP1051760A1 - Methods for producing percursor material for the production of high-temperature superconducting wires

Info

Publication number: EP1051760A1
Application number: EP99908831A
Authority: EP
Inventors: Michael Baecker; Joachim Bock
Original assignee: Aventis Research and Technologies GmbH and Co KG
Current assignee: Nexans Industrial Solutions GmbH
Priority date: 1998-01-30
Filing date: 1999-01-28
Publication date: 2000-11-15
Also published as: DE19803447A1; JP2002502099A; CN1289457A; CA2318113A1; WO1999039392A1; AU2830599A; KR20010040471A

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

Process for the production of precursor material for the production of high-temperature superconductor wires

description

For technical applications in wires and solid parts, high-temperature superconductors, especially from the bismuth-calcium-strontium-cuprate family, have been investigated for a long time. Particular attention is paid to the phases, the one

Jump temperature T _c above the boiling point of nitrogen (77 K). These are the three-layer connection with the nominal composition (Pb, Bi) ₂ Sr ₂ Ca ₂ Cu ₃ O _x with a transition temperature of approx. 1 10 K and the two-layer connection with the nominal composition Bi ₂ Sr ₂ Ca _t Cu ₂ O _x with a Jump temperature of approx. 92 K. Furthermore there is the corresponding one

Single-layer compound Bi ₂ (Sr, Ca) ₂ CuO _x , which, due to the low transition temperature of around 10 K, does not appear to be of interest for technical applications.

The most common method of making superconducting wires is

OPIT (Oxide Powder In Tube) technology. Here, a preliminary product, the so-called precursor material, is filled in the form of powder or bars pressed from powder into a metallic tube which usually has a high proportion of silver. This is hammered out and drawn into a monofilament wire, then bundled into a multifilament, pulled out again and usually rolled afterwards. The actual reaction to the high-temperature superconducting material takes place in the last process steps of wire production as a solid-state reaction at temperatures preferably between 650 ° C and 850 ° C under a defined atmosphere. The quality of the precursor material is largely determined by the degree of reaction and thus by the reaction time in the wire for conversion into the desired target phase. In addition, the quality of the precursor material determines the properties of the superconducting wire such. B. current carrying capacity, 2

Jump temperature and homogeneity essential. In most cases, the aim of the production of the precursor material is to pre-react and optimize the high-temperature superconductive material to such an extent that the actual strip / wire production takes place particularly shortly, cheaply and with the best superconducting properties, particularly with regard to the thermal treatment.

In the following, no distinction is made between wire and tape manufacture or wire and tape and in summary only spoken of wires and their manufacture. A distinction is made between the two- and three-layer connection as the target phase of the wire production, which should be present in the finished wire, while the precursor material for the wire production, i.e. the preliminary product for the wire, then preferably approximately has the chemical composition of the target phase, but not necessarily a high proportion of the target phase.

The precursor powder is made from partially precalcined in a variety of ways

Metal oxides, such as. B. by a solid-state reaction, wet chemical, for. B. by co-precipitation, or by spray pyrolysis. Another method is the grinding of melted parts. However, all processes have in common that several reaction (e.g. calcining, tempering) and several work-up steps (e.g. drying, grinding) must always be carried out. This therefore leads to very complex and expensive processes.

Nevertheless, the precursor material is used almost exclusively as a powder or as a rod pressed from powder. There are two main reasons for this: on the one hand, extremely fine grinding with average grain sizes of d ₅ o <5 μm results in good mechanical ductility of the wires in the process steps of drawing and rolling, and on the other hand, grinding results in very good homogeneity achieved, which is a particular challenge given the large number of components. 3

However, these advantages result in a number of disadvantages, one of the most serious of which is the large number of process steps and thus the high costs for powder production. The many process stages also involve the risk of contamination such. B. by abrasion from the grinding processes and by contamination with water and carbon dioxide from the air, which leads to the formation of

Carbonates and hydroxides leads. In contrast to the compounds of the other elements Bi, Pb and Cu, carbonates and hydroxides, in particular of the alkaline earth metals, are very thermally stable compounds. Such impurities on the one hand impair the quality of the superconducting wires by deteriorating grain-grain contacts in the structure of the superconducting wire, on the other hand carbonates and hydroxides lead to uncontrolled decomposition reactions during the final solid-state reactions to the superconducting material in the wire (decomposition temperatures: CaC0 ₃ : 898 ° C, Ca (OH) ₂ : 580 ° C, Sr (OH) ₂ : 710 ° C) and thus to undesirable blistering and conductor cross-section restrictions in the wire, so that the current flow through the conductor can be restricted or even interrupted (W. Hellstrom et al., Supercond. Sci. Technol. 8, 1995, 317). Therefore, in the case of conventional processes, great purity and, in part, air exclusion must always be ensured, which further increases the cost of the known processes.

Particularly fine powders are desirable for wire production using the OPIT method, since the development of ever smaller filaments is required. Above all, the grinding of the preliminary product into powders with grain sizes in the range from d ₅₀ = 1 to 5 μm requires a large amount of equipment due to the large reaction surface of the hygroscopic material in order to avoid contamination with carbon dioxide and water (C. Mao et al., Pysica C 281 (1997) 149).

In addition, only a low degree of filling in the wire is achieved through the use of compressed powder. While the theoretical density of the two- and three-layer connection is slightly more than 6.5 g / cm ³ depending on the composition, the OPIT method only uses bulk densities between 3 and 4.5 g / cm ³ 4

achieved depending on whether loose powder is stuffed into the silver tubes or pressed bars are used. Although this low density makes it easier to form the superconducting wire, it does cause larger undesirable gas inclusions - again with the risk of blistering. In addition, when pressing powder to form precursor bars, the dimensional stability required for insertion into tubes cannot be guaranteed due to varying pressing factors. The compacts must therefore be reworked mechanically, especially with non-round cross-sections, e.g. B. by sawing, turning or grinding, which causes a loss of up to 30 wt .-% of the precursor material.

The problem of the shaping of precursor material applies particularly to wires that are not made up of bundled monofilaments (FIG. 1 a), but a more complex structure, such as, for example, concentrically arranged layers (FIG. 1 b) z. B. the sequence silver-superconductor-silver-superconductor-Siiber-etc. (Supercond. Week, Vol. 1 1, 24 (1997) 2). This structure causes a finished

Wire with a multilayer structure can be produced in one work step if the precursor material is already present in the spaces between concentrically arranged silver tubes. For this purpose, tubes have to be pressed out of the precursor powder according to the pressing technique previously used. This is particularly problematic because the dimensional accuracy of the individual components, the homogeneity of the

Precursor materials or the later processing of the pressed individual components to fit the pipes is extremely difficult.

For another reason, concentrically nested tubes are an interesting starting geometry for producing superconducting wires.

Superconductivity is initially characterized by the complete loss of electrical resistance. However, this only applies to applications with direct current (DC).

However, AC applications are becoming increasingly interesting.

However, alternating electric fields induce the acceleration of non-superconducting electrons in the superconductor - there is always only a small proportion of the conduction electrons in the superconducting state - and in the 5

metallic matrix of the wire eddy currents and thus an electrical resistance. If one wants to reduce these alternating current losses, the cross section of the wire in which such an eddy current is generated must be reduced. This is achieved by installing resistive barriers, i.e. electrically insulating areas, in the matrix of the superconducting wire.

An electrically insulating layer between the individual superconducting monofilaments (FIG. 2a) accordingly reduces such AC losses. In the final thermal treatment of the wires, however, oxygen diffusion into the filaments made of superconducting material must be possible on the one hand, and on the other hand the insulating material may be used at the high tempering, sintering and

Annealing temperatures neither destroy the shell, which mostly contains silver, nor diffuse through the layer, which mostly contains silver.

Very few connections can meet these high requirements. Because of the possibility of diffusion through the delimiting

Silver shells in the superconducting material are also foreign elements in high concentrations, which deteriorate or destroy the superconducting properties in the superconducting or superconducting material, not desired in the insulation. For this reason, various attempts have been made to achieve an insulating material with a highly contaminated two or three-layer connection. At concentrations of foreign ions such as Al, Ti, B etc. above 1000 ppmw, a complete loss of the superconducting properties is observed. In order to achieve such an insulating material, again, as in the case of the conductor concept with concentric tubes, first an inner silver tube must be filled with superconductor precursor material and then the space between the inner and outer tube with insulating material. For this too, either pipes made of powder have to be pressed or the powder has to be pressed directly into the cavities, which causes extremely inhomogeneous pressing and is associated with great difficulties. 6

DE-A-196 13 163 describes a method in which powder is first poured into a cladding tube and this powder is then heated and melted in the cladding tube, whereby a high, homogeneous degree of filling is to be achieved. Difficulties can arise during practical implementation from both a chemical and a physical point of view:

If the cladding tube, which mostly consists of silver, is heated to temperatures above the melting temperature of the molten filler material in the range of approx. 860 - 900 ° C depending on the desired superconductor material together with the superconductive material to be melted, then the previously fine-grained, homogeneous cladding tube material recrystallizes and, due to the formation of large grains, offers starting points for tearing open the cladding tube during cold forming into wire or tape. Silver has a relatively low melting point of only 962 ° C (Handbook of Chemistry and Physics, CRC Press, Boca Raton, Florida, 70 ^th Ed. 1900), while many of the silver-rich alloys used in practice a somewhat above lying of 962 ° C

Have melting point. A significant softening of the silver-rich cladding material occurs well below the melting temperature, so that a strong deformation of the cladding tube can occur during the melting. The tearing of the sheath during wire or strip production is also promoted by the reaction of the fill melt with the sheath tube because the wall thickness is weakened. It is known that of all metals, only platinum is sufficiently stable against the aggressive oxide melts based on BiSrCaCuO. Another problem with the method described lies in the chemical properties of the superconductors. The bismuth-based phase diagram for High Tc Superconductors II, The American

Ceramic Society, 1997, p. 241) shows that all the compositions in question have a miscibility gap in the melt below 1000 ° C., ie the melt completely separates into two components, comparable to an oil-water mixture, so that there is a risk that a homogeneous superconductor material cannot be produced. 7

It was therefore the task of developing a more economical way of producing precursor material with a smaller number of process steps, less material losses, with significantly reduced impurity contents and with a reduced risk of blistering occurring during wire production. In addition, there was the task of proposing a simple shape and different geometries for individual components for wire production, the problems of a melting route, such as the reaction of the melt with the cladding tube, separation of the melt and an excessively high temperature for the cladding tube material, in particular being avoided.

The object is achieved with a method for producing a high-temperature superconductor precursor material for use in strip or wire production, in which a mixture of oxides or / and their precursors is prepared, in which this mixture is heated so high that one at temperature there is good mixing and pourable melt, in which individual phases may still be in the solid state, this melt being introduced into a shell and solidifying there on cooling, and which is characterized in that the melt solidifies so quickly that here essentially no reaction layer made of the material of the shell with which the melt is formed, that the solidified melting body is heated in a temper firing to a temperature at which a conversion of the melting body material into a precursor material containing at least one high-temperature superconducting and / or high-temperature superconducting P hare of at least 10 wt .-% occurs and that no grinding is carried out after melting the mixture.

The object is further achieved with a method for producing a high-temperature superconductor precursor material for use in strip or wire production, in which a mixture of oxides and / or their precursors is prepared, in which this mixture is heated to such an extent that one at

The temperature of the melt that mixes well and is pourable, at which 8th

individual phases may still be in the solid state, this melt being introduced into a shell and solidifying there on cooling, which is characterized in that the melt is heated to such an extent that no segregation of the melt is formed for the superconducting material that the solidified melting body is heated in a temper firing to a temperature at which a conversion of the melting body material into a precursor material with a content of at least one high-temperature superconducting and / or high-temperature superconducting phase of at least 10% by weight occurs and that no grinding occurs after the mixture has melted is made.

Remedy for segregation of the two superconductor components in the melt can be achieved by further heating the melt above 1000 ° C or by rapidly quenching the melt to freeze the structure before segregation.

Further heating is prohibited in a method according to DE-A-196 13 163 due to the melting temperature of the cladding material being too low, since otherwise the cladding tube material would melt and the cladding tube would be destroyed. Quenching the melt as part of a process according to DE-A-196 13

163 is not because of the good thermal conductivity of silver, which has the highest thermal conductivity of all metals with 3.82 W / (cm ² K) (Handbook of Chemistry and Physics, CRC Press, Boca Raton, Florida, 70 ^th Ed. 1900) possible, since the silver-containing casing tube would compensate for the required temperature differences, so that continuous quenching in the longitudinal direction of the casing tube should not be practical.

Due to the rapid solidification at the contact surface with the cladding tube, preferably in less than 1 second, particularly preferably in less than 0.5 seconds, no reaction between the liquid melt and the preferably silver-containing cladding material can occur. With slow solidification, the entire 9

React the cladding tube made of a silver-rich alloy with the melt; the silver can be dissolved in the melt and excreted as silver oxide. In the event of incomplete dissolution of the cladding tube, which usually has a wall thickness of around 1 mm thickness, a reaction layer which mostly contains mixed oxides containing silver can be recognized; a cladding tube attacked in this way can no longer be used for wire or strip production. However, the sleeve tube together with the superconductive or superconducting material can usually still be used if the reaction layer is thinner than 10 μm, preferably thinner than 5 μm.

The process according to the invention is characterized in particular by the complete avoidance of grinding processes after melting and thus by the avoidance of extremely air-sensitive powders as an intermediate product, since the material is not filled in in the form of powder or compressed powder, but rather in the form of a melt in the majority covers containing silver.

In the inventive method, a mixture of the metal oxides such. B. Bi ₂ 0 ₃ , PbO, SrO, CaO, CuO or their precursor materials according to the desired atomic ratio for the superconducting bismuth (lead) strontium calcium copper oxides at temperatures above 1000 ° C, preferably above 1050 ° C , melted. Preferably all cations are in the form of oxides in the batch. A stoichiometric or approximately stoichiometric ratio of the oxides with respect to the nominal composition of the two- or three-layer compound in the mixture is preferably set. If necessary, bismuth can be substituted up to 50 mol% by lead, antimony and / or yttrium; In addition, up to 30% by weight of a high-melting compound, in particular of alkaline earth metal sulfates, preferably SrS0, BaS0 or / and (Ba, Sr) S0 ₄ , can be added to improve the crystallinity. These high-melting compounds should neither melt themselves nor decompose up to the selected temperature of the melting process of the batch. 10

The melting preferably takes place in platinum crucibles, in platinum-lined crucibles or in crucibles made of another suitable material such as, for. B. Ba umzirkonat in ovens such. B. in muffle furnaces. The melt can during the melting process by mechanical, preferably platinum-coated

Agitators, blowing gas and / or swiveling the crucible are moved.

During the melting process, the oxide melt should only be used with inert materials such as e.g. Platinum, related precious metals or their alloys come into contact. So be

Contamination of the melt and premature aging of the crucibles are avoided. Due to the high melting temperatures above 1000 ° C and possibly due to the movement of the melt, the separation into two liquid phases is suppressed and a homogeneous melt is produced.

After melting, which does not have to take place completely, particularly when adding optional high-melting compounds such as alkaline earth metal sulfates, the crucible is removed from the furnace, and the pourable melt is preferably poured directly into pipes or pipe-like casings or via communicating systems, without delay, poured or sucked. The term “shell” is intended to mean all pipes, pipe systems and similar designs which are suitable, together with the precursor material and possibly with an insulating material, to give an essentially rod-shaped molded body which is referred to in this application as a rod. The sleeves used are either at room temperature or are on

Temperatures preferably up to 750 ° C, in particular up to 700 ° C, particularly preferably to 200 ° C to 500 ° C, preheated. The longer the transport routes of the melt, the higher the preheating should be. The larger the inside diameter of the casing, the lower the preheating temperature should be in order to avoid an excessive temperature rise in the casing. The temperature difference between the melt and the preheated cladding tube should preferably be used 11

be at least 300 ° C. As a result, when the melt is poured in, an immediate solidification takes place on the inner surface of the shell in the form of a glass-like thin layer. The thickness of this protective layer is usually 50 to 250 μm, especially depending on the temperature difference between the melt and the cladding tube. This layer now protects the shell from the further poured liquid and aggressive

Oxide melt. In addition, this layer insulates the casing against the still molten melt inside, thus reducing the maximum temperature of the casing. This phenomenon means that the casing is not heated above its softening point, but the maximum temperatures are <800 ° C. Therefore, there are no problems such as tearing open the silver sheath in the actual wire production process when the melt reacts with the silver-containing alloy or when the sheath tube is heated to excessively high temperatures, in particular at temperatures of more than 850 ° C.

The cooling rate of the melt can be achieved by pouring the melt into the

Oven, by pouring outside the oven and putting the filled casing back into the oven and then slowly cooling in the oven or by means of specially fitted heating elements over a wide range. Cooling outside the furnace usually results in a cooling rate of the order of 20 to 100 K / s. When filling the casing, care must be taken that the gases contained in the melt and in the casings can escape as completely as possible and that as few and only very small cavities or pores are formed. When cooling, however, care must always be taken to ensure that the protective layer formed on the inner surface of the casing is not melted again. If the process is good, after cooling,

There are no pores> 50 μm or even no pores> 20 μm if the top parts of the melting body are not used.

The melting or / and casting of the melt is preferably carried out in air, but can also be carried out under in order to further reduce impurities

Protective gas, particularly preferably under nitrogen, argon or synthetic air, 12 done. When melting in the negative pressure and / or in a protective gas, the degassing of the melt can be facilitated if necessary.

The sleeves preferably have a diameter of 5 to 20 mm or a wall thickness of 0.5 to 5 mm and can preferably be made of pure silver (> 99

Wt .-%) or a silver alloy, which may have in particular contents of Mg, Au, Pd or / and Cu <10 wt .-%, and have any cross-section, in particular a round or angular, and be welded on one side. The envelopes can also contain fins and possibly also tubular elements inside, which are preferably arranged concentrically or regularly next to one another; There are no limits to the variety of shapes for multi-channel casings or for single-channel casings with lamellar arranged lamellas, whereby the individual channels (= chambers) can be filled with superconductive or electrically insulating materials. During casting, the shells serve primarily to delineate the outer shape and to delineate the superconductive or, if appropriate, also the insulating material areas. During and after the wire production, the tube elements and fins serve in particular to align the superconductive or superconducting crystallites in preferred directions and as a ductile sheathing of the brittle ceramic material during hammering, drawing, rolling, etc. for producing the superconducting wire. The sleeves can also have atypical cross-sections and shapes for conventional pipes and, for example, have a U-shaped curve, in particular one side of the sleeves can preferably be developed obliquely, for example in order to be particularly adapted to the casting process and the escape of the gases. The wall thickness of the tube elements and fins should preferably be chosen to be small, but in such a way that a sufficient separating layer between the adjacent material layers is still retained during the further wire production.

Such multi-hole covers are preferably provided with thin chamber walls; they can be made by extrusion processes or by joining several

Individual components have been manufactured. The channels of the most varied 13

Shells are preferably accessible at both ends so that the melt can penetrate from one side and allow the gases to escape on the other side. The envelopes can also have branches of the individual channels, for example in order to allow gases to escape even better. The sleeves preferably have essentially the same length

Cross-section and, if necessary, similar partition walls (= slats). The slats do not necessarily have to separate the chambers from each other, but can also protrude into a free space like wings. Instead of a laterally closed envelope, it is also possible to use pipes of more complicated construction, in which individual chambers can be open to the outside, or particularly advantageously wound foils, in particular spirally wound foils, which can be open, partially contacted or closed in the longitudinal direction on the outer edge; these wound foils have the advantage of a better distribution of the melt and, insofar as they are not gas-tight in the longitudinal direction on the outer edge, the more diverse ways of filling the

Melt or for degassing. The wrapped foils can also be combined with lamellas and / or tubular elements. The more complex sleeves can also consist of at least one tube element and at least one set of fins, of several tube elements lying one inside the other without or with fins or additionally of further channels. The number of tube elements lying one inside the other is preferably 2 to 25, particularly preferably 3 to 10, that of the fins preferably 1 to 12, particularly preferably 2 to 8, without counting a lamella leading through several tube elements or a tube element leading through several fins. The sleeves of the invention with a more complicated structure of several

Pipe elements or / and lamellae have the great advantage that bundling into multifilaments can be dispensed with, in particular when there are a large number of tube elements or lamellae, and process steps in wire production can also be saved, since they are electrically conductive with superconductive and possibly also in individual chambers insulating material filled casing can be regarded from the start as a "multifilament". Such precursor rods 14

preferably have a cuboid or circular cross section. In this application, the wrapped foils are also addressed under the generic term "envelope".

The solidified melt shows depending on its composition and content

Aggregates have a bulk density of> 5.5 g / cm ³ , which corresponds to a porosity of <25%. The precursor material thus has a bulk density that is at least 20%, often at least 50% higher than conventional precursor material that was produced via a powder route. The main crystalline component of the precursor material according to the invention is preferably the single-layer compound with the nominal composition Bi ₂ (Sr, Ca) ₂ CuO _x ; In addition, alkaline earth metal cuprates, copper (l) oxide and, in the case of lead oxide, alkaline earth metal plumbates often occur in the starting mixture. According to microscopic examinations, the amorphous fraction, depending on the cooling rate of the melt, is preferably between 10 and 90% by weight, in particular between 20 and 80% by weight. The lower the cooling rate, the higher the crystalline content.

Due to the solidification of the melt at temperatures above the area of existence of all hydroxides and all carbonates of the cations involved

With the exception of SrC0 (decomposition point 1268 ° C), such contamination can be largely avoided. The impurities are less than 500 ppmw carbon, preferably less than 300 ppmw carbon, and less than 300 ppmw hydrogen, preferably less than 150 ppmw hydrogen in the solidified melting bodies. In order to exclude contamination with strontium carbonate, the strontium oxide to be used can be briefly annealed at temperatures> 1250 ° C before use in the batch, whereby the C content drops below 100 ppmw.

The intermediate product, the solidified melt, is significantly less sensitive to air than the fine powders used in the usual precursor material production 15

occur, since only a thin surface layer at the openings of the envelopes can be attacked by air. In contrast to the precursor powders or the very open-pored pressed rods, gas can mostly or hardly penetrate into the interior of the compact melting body. The intermediate products therefore do not have to be handled in the absence of air. Editing the

However, intermediate products can advantageously be carried out under protective gas and / or under a reduced pressure.

The solidified melt is usually characterized in that, in contrast to the oxide mixture used, there is a clear copper (I) content, which is synonymous with an oxygen deficit. If one considers the stable oxidation levels Bi (III), Sr (II), Ca (II), Cu (II), Pb (II) in the cations used, the oxygen content of the two-layer compound should be about 8 per formula unit, in In the case of the lead-free three-layer connection of about 10 per formula unit. In the solidified melt, this oxygen content is usually reduced by more than 5 mol%. In the final tempering step, oxygen must be taken up and diffuse into the wire, in particular from the outside, in order to produce superconducting materials from superconductive materials and to adjust the oxygen content with the optimal superconducting properties for the connection. In the case of an excess of oxygen, as is often the case with

Precursor powder or in the case of bars pressed from powders, this must be removed through the casing during tempering, which can lead to the formation of bubbles if oxygen is released spontaneously.

In the case of a construction of the final wire from, for example, concentrically arranged, alternating layers of silver-superconductor-silver-etc. the casting mold for the precursor material melt can contain several, for example, concentrically nested or machined tubular elements. The melt can now either in one casting or in several separate casting processes into the cavities of the shells, in particular between the pipe elements or, if appropriate,

Slats to be filled. 16

At least two casting processes, which can be carried out separately, but also at the same time, are necessary if different melts are to be filled into the different chambers. This is necessary in order to achieve electrical insulation of the super-conductive "monofiaments" from one another in the "multifilament conductor" here, for example, the single-layer connection - but only for

Applications well above the transition temperature of the single-layer connection of about 10 K, the heavily contaminated two- or three-layer connection without superconducting properties or a material based on BιSr- (Ca) Cu-0 from completely different stochiometry without the occurrence of high-temperature superconducting phases. The casting process remains unchanged for all materials compared to superconducting / superconducting materials. The composite materials obtained in this way can be further treated like the simple precursor materials described above

Due to the high density, the solidified melt has a significantly poorer forming behavior in wire production than comparable OPIT silver tubes according to the prior art.This is why another thermal treatment before forming, the so-called annealing, is advantageously carried out, during which part Conversion of the preliminary product to a preliminary product with a phase stock with a higher proportion of the significantly more ductile two-layer connection takes place, which can then be formed cheaply during wire production

The annealing reaction is preferably carried out in a muffle furnace or tube furnace under flowing nitrogen, oxygen, air or a nitrogen / oxygen gas mixture, if appropriate also under reduced gas pressure. The temperatures here are preferably between 600 ° C. and 900 ° C., particularly preferably between 700 ° C. and 840 ° C, the reaction time is in particular 0.2 to 50 hours, preferably 0.5 to 20 hours. The reaction can be 10 to 100% by weight of the

Two-layer connection is carried out, preferably 50 to 90% by weight 17

In the case of the two-layer connection, phases occur primarily as the single-layer connection, alkaline earth metal cuprates and copper (I) oxide. In the case of the three-layer connection as the target phase, portions of the three-layer connection and alkaline earth metal plumbate can additionally appear; with the exception of the one and two-layer connection, the proportions of all others occurring

Phases usually below 20% by weight, preferably below 10% by weight.

A further variant of the method according to the invention is to first pour the melt into split molds, i.e. molds to be opened, or into molds, dishes or crucibles made of a heat-resistant material, preferably of copper or a copper-rich alloy, which in particular are at room temperature or at temperatures up to 500 ° C. The melting takes place in the same way as for casting in casings. After the solidified melting body has been removed from the mold or from the mold, the dish or the crucible, the melting bars are first annealed, preferably under the same conditions as mentioned above. Then a mechanical post-processing such. B. done by milling, turning or sawing, so that the melt body bars obtained can be inserted with a precise fit before further processing into wire in sleeves.

With all tempering processes, both for the solidified melt in casings and for melt ingots, there is only a very small dependence of the conversion rate on the selected atmosphere, which is due to the compact structure of the melting body without large open pores. Consequently, there are almost no differences in the

Conversion speeds for melting bodies in different shells, which have a differently high oxygen diffusion rate.

Compared to precursor material in the form of powder or pressed rods, the melting bodies show a significantly increased reactivity. The reaction times for generating a defined proportion of two-layer connection are 18th

Surprisingly often reduced to about a third of the usual tempering times. The reason for this behavior is that when it melts, oxygen escapes, which is not taken up again when it cools down and thus leads to an oxygen deficit. This is also evident from the occurrence of Cu ₂ 0, which solidifies together with phases containing Bi-Sr-Ca to form a eutectic. The eutectic usually melts partially during tempering. The partial melting significantly reduces the reaction times compared to conventional solid-state reactions in solid mixtures. In addition, the atmosphere during the tempering of powder or pressed highly porous rods must be precisely controlled using complex measuring technology, whereas slight fluctuations in the

Gas composition have no influence on the melting body during tempering.

Drawings:

The invention will be explained by way of example with reference to drawings.

Figures 1 a and 1 b schematically represent a multifilament wire (1) and a concentrically constructed wire (2) according to the prior art. These wires contain several chambers (3), the walls (4) and (5) of which consist of a silver-rich matrix or a silver-rich casing material. The chambers are filled with a high-temperature superconductor material (6).

Figure 2a shows a multifilament wire (7) with several chambers (8) according to the prior art, the walls (9) of which consist of a silver-rich matrix. The inside of the chambers contains a high-temperature superconductor material (10), first of all of a silver-rich shell (9a) and then of an insulated one

Intermediate layer (1 1) is surrounded.

FIG. 2b shows a wire (12) constructed according to the invention with a plurality of chambers (13) made of concentrically constructed layers of silver-rich chamber walls (14) and a high-temperature superconductor material (15) 19

Alternating storage with electrically insulating layers (16) according to the invention again schematically.

EXAMPLES

Example 1

200 g of a mixture of individual oxides (Bi ₂ 0 ₃ 99.9% by weight; CuO 99.999% by weight; CaO 99.99% by weight, SrO 99.32% by weight) of the total composition Bi ₂ , oιSrι , ₉₂ Caι, o5Cu ₂ Oχ (Bi 47.0 wt .-%; Sr 18.8 wt .-%; Ca 4.7 wt .-%, Cu 14.2

% By weight) was melted in a platinum crucible at a temperature of 1100 ° C. within 30 minutes in a muffle furnace and into an Ag / Pd tube with a Pd content of 3% by weight and with a square cross section of the internal dimensions of 9 x 9 x 300 mm and the wall thickness 1 mm cast. The tube with the solidified melting body was then annealed in a muffle furnace in air at 710 ° C. for 20 hours. The preliminary product thus obtained was characterized as follows: a. Composition: ICP-AES analysis (ICP-Plasma 400 and AAS

1100B; Perkin-Elmer) Bi 46.7% by weight; Sr 19.0% by weight; Ca 4.8% by weight; Cu 14.1% by weight corresponds to: B .oiSrLgeCa-i.oβC ^ Ox b. Impurities: C 400 ppmw (Coulomat 702; Ströhlein)

H 80 ppmw (CHNS 932; Leco) Pt 450 ppmw (ICP-AES) c. Bulk density: 6.02 g / cm ³ d. X-ray powder images: crystalline fraction (X-pert; Philips):

65 (± 5)% by weight two-layer connection, 31 (± 5)% by weight single-layer connection e. Incident light microscopic micrographs: Amorphous fraction below 10% by volume; homogeneous distribution over the entire cross-section; 20th

no reaction of the precursor material with the pipe wall. The thickness of the glass-like protective layer on the inside of the cladding tube was approximately 100 μm.

Example 2 (According to the Invention)

Like example 1, but the melt was poured into a round Ag tube with an inner diameter of 10 mm and a wall thickness of 1 mm. X-ray powder images: crystalline fraction (X-pert, Philips): 62 (± 5)% by weight two-layer compound,

35 (± 5)% by weight single-layer connection The thickness of the glass-like protective layer on the inside of the cladding tube was approximately 100 μm. Other analysis data as in example 1.

Example 3

Like example 2, but SrO was annealed at temperatures of 1300 ° C for 1 hour before producing the oxide mixture.

Impurities: C 180 ppmw (Couiomat 702; Ströhlein)

Other analysis data as in example 2.

Example 4

As in Example 2, but an additional 20% by weight of strontium sulfate was added to the starting oxide mixture. The preliminary product thus obtained was characterized as follows:

a. Composition: ICP-AES analysis (ICP-Plasma 400 and AAS

1100B; Perkin-Elmer) Bi 38.0% by weight, 21

Sr 24.6% by weight; Ca 3.6% by weight; Cu corresponds to 11.4% by weight, minus 20% by weight SrS0 ₄ : Bi ₂ , o3Srι _ι 9 ₂ Caι _ι ooCu ₂ Oχ

b. Impurities: C 350 ppmw (Couiomat 702; Ströhlein) H 90 ppmw (CHNS 932; Leco) Pt 420 ppmw (ICP-AES)

Bulk density: 5.90 g / cm ³

d. X-ray powder images: crystalline fraction (X-pert; Philips):

60 (± 5)% by weight two-layer connection, 22 (± 5)% by weight single-layer connection, 14 (± 5)% by weight SrS0 ₄ .

The thickness of the glass-like protective layer on the inside of the cladding tube was approximately 150 μm.

Example 5

Like example 1, but the melt was preheated to 350 ° C.

Cast copper mold with the inner dimensions 20 x 100 x 100 mm ³ . The melting body was removed from the mold and sawed into cuboids measuring 9 x 20 x 100 mm ³ using a diamond-coated band saw. These bars were then annealed at 750 ° C for 10 hours under flowing nitrogen in muffle furnaces. The preliminary product thus obtained was characterized as follows:

a. Composition: ICP-AES analysis (ICP-Plasma 400 and AAS 1100B, from Perkin-Elmer) Bi 47, 1% by weight; Sr 18.7% by weight; Ca 4.5% by weight; Cu 14.4% by weight corresponds to: Bii.ggSn.ssCao. gC ^ Ox 22

b. Impurities: C 380 ppmw (Couiomat 702; Ströhlein) H 100 ppmw (CHNS 932, Leco) Pt 390 ppmw (ICP-AES)

Density: 6, 10 g / cm ³

X-ray powder images: crystalline fraction (X-pert; Philips):

81 (± 5) wt .-% two-layer connection, 16 (± 5) wt .-% single-layer connection reflected light microscopic micrographs:

Amorphous fraction below 10 (± 5)% by volume; homogeneous distribution over the entire cross-section.

Example 6

200 g of a mixture of individual oxides (Bi ₂ 0 ₃ 99.9% by weight, PbO 99.9% by weight,

CuO 99.999 wt%; CaO 99.99% by weight; SrO 99.32% by weight) of the composition BiiyoPbo.ssSr ^ ssCa _L ggCusOx (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. within 45 minutes in a muffle furnace and into an Ag tube with a round cross section of 10 mm

Cast inside diameter and 1 mm wall thickness. The tube was then annealed in a muffle furnace in air at 710 ° C for 20 hours. The preliminary product thus obtained was characterized as follows:

Composition: ICP-AES analysis (ICP-Plasma 400 and AAS

1100B; 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 corresponds to: Bi ₁ , 68Pbo, 33Sr ₁ , ₈₄ Ca ₁ , ₇ 3CU ₃ Oχ

b. Impurities: C 300 ppmw (Couiomat 702; Ströhlein) 23

H 100 ppmw (CHNS 932, Fa Leco) Pt 380 ppmw (ICP-AES)

Bulk density 6.12 g / cm ³

d X-ray powder images crystalline fraction (X-pert, Philips)

52 (± 5)% by weight two-layer connection, 43 (± 5)% by weight single-layer connection e incident light microscopic micrographs

10 Amorphous fraction below 10% by volume homogeneous distribution over 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 hollow tube was approx. 200 μm

15

Example 7

As in Example 6, but the melt was poured into a copper mold preheated to 350 ° C. with an internal dimension of 20 × 100 × 100 mm ³

20 enamel bodies were removed from the mold and, by means of a diamond-coated band saw, said to form cuboids with the dimensions 9 x 20 x 100 mm ^3. These rods were then annealed at 750 ° C. for 20 hours under flowing nitrogen in a muffle furnace. The preliminary product obtained in this way was characterized as follows

T a composition of ICP-AES analysis (ICP-Plasma 400 and AAS 1100B, from 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

30 corresponds to BH esPbo ∞Sr ^" _{! 87} Caι ₈ 2Cu ₃ O _x 24

Impurities: C 400 ppmw (Couiomat 702, Ströhlein) H 70 ppmw (CHNS 932; Leco) Pt 370 ppmw (ICP-AES)

Bulk density: 6, 12 g / cm ³

d. X-ray powder images: crystalline fraction (X-pert; Philips):

93 (± 5)% by weight two-layer connection, 4 (± 5)% by weight single-layer connection.

Example 8:

In this example, the reactivities of different precursor materials for the two-layer connection are compared. All precursor materials were made from the same oxide mixture as in Example 1 either by casting the

Melt in a silver tube (precursor material A; according to the invention) or in a mold according to Example 2 and subsequent sawing to melt rods (precursor material B; according to the invention). In addition, powder was obtained by grinding the fusible rods of precursor material B in an air jet mill directly after the casting (precursor material C; comparative example). Isostatic rods were pressed from this powder (Precursormateπal D; comparative example) with a diameter of 10 mm and a length of 100 mm.

Table n

Annealing conditions and thus the content of the two-layer connection

Precursor temperature time atmosphere% by weight material [° C] [h] two-layer compound after annealing

A 750 20 air 87 B 750 20 air 85 25th

C 750 20 air 0

D 750 20 air 25

Table 1 shows the surprisingly high proportions of two-layer compound in comparison with the powdery precursor materials C and D in the case of solidified ones

Melting bodies of precursor materials A and B. Precursor materials A and B already have a high proportion of two-layer compound at these short annealing times.

Example 9

As example 6, but first three Ag tube elements with a round cross-section of 8, 13 and 18 mm inside diameter and 1 mm wall thickness were put into each other and soldered at the lower end with centering rings so that there were even distances between the individual tubes. Then 400 g of a mixture as in Example 6 were melted and poured into the interstices and then further treated as in Example 6.

Incident light microscopic micrographs: no difference between inner and outer

Precursor material layers in terms of crystallinity and

Phase composition; amorphous fraction below 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 cladding tube was approx. 80 μm for the inner tube, approx. 120 μm for the middle tube and approx. 220 μm for the outer tube. Other analysis data as in example 6.

Example 10 26

First, two silver tube elements with a circular cross-section of 13 and 18 mm inside diameter and 1 mm wall thickness were put into each other and soldered to the lower end with centering rings so that there was a uniform distance between the tube elements. 200 g of a mixture of individual oxides (Bi ₂ 0 ₃ 99.9% by weight; CuO 99.999% by weight; CaO 99.99% by weight; SrO 99.32% by weight) of the composition B. iSr-i ^ Ca _L osC ^ Ox (Bi 47.0% by weight, Sr 18.8% by weight, Ca 4.7% by weight; Cu 14.2% by weight) was placed in a platinum crucible at a temperature of Melted 1100 ° C within 30 minutes in a muffle furnace and poured into the inner Ag tube element. Then 200 g of a mixture of the composition Bh.ggSn. sCuOx, melted in a platinum crucible at a temperature of 1000 ° C within 30 minutes in a muffle furnace and poured into the cavity between the inner and outer Ag tube element. The tube was then annealed in a muffle furnace in air at 710 ° C for 20 hours. The preliminary product thus obtained was characterized as follows:

Composition: ICP-AES analysis (ICP-Plasma 400 and AAS

1100B, Perkin-Elmer) inner tube element:

Bi 47.3% by weight; Sr 18.6% by weight; Ca 4.7% by weight;

. Cu 14.3 wt -% corresponds: Bi ₂ ₀ ιSr _1ι8 GCA _1, o ₄ Cu ₂ Oχ outer tubular member:

Bi 55.7% by weight; Sr 22.8% by weight; Cu 8.5% by weight corresponds to: Bi-i.g Sr-i.gsCuOx b. Impurities: C 400 ppmw (Couiomat 702; Ströhlein)

H 80 ppmw (CHNS 932; Leco)

Pt 450 ppmw (ICP-AES) c. Bulk density: 6.11 g / cm ³ X-ray powder exposures: crystalline fraction (X-pert; Philips): inner tube element:

72 (± 5)% by weight two-layer connection, 27

25 (± 5)% by weight o single-layer connection; outer tubular element: almost 100% single-layer connection. Incident light microscopic micrographs: no difference between inner and outer

Layers of precursor material in terms of crystallinity and phase composition; amorphous fraction below 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 cladding tubes was approximately 80 μm for the inner tube and approximately 150 μm for the outer tube.

Example 11

The procedure was as in Example 6, but instead of a (not gas-tight) muffle furnace, a very well sealed tube furnace with Inconel steel tube was used. In addition, synthetic, C0 ₂ -free air was used as the atmosphere. The composition was identical to that in Example 6. The following were determined: b. Impurities: C 120 ppmw (Couiomat 702; Ströhlein)

H 60 ppmw (CHNS 932; Leco)

Pt 390 ppmw (ICP-AES)

Bulk density: 6.11 g / cm ³

X-ray powder images: crystalline fraction (X-pert; Philips):

60 (± 5)% by weight two-layer connection, 36 (± 5)% by weight single-layer connection.

Claims

28 patent claims

1. A process for producing a high-temperature superconductor precursor material for use in ribbon or wire production, in which a mixture of oxides and / or their precursors is prepared, in which these

Mixture is heated so high that a melt which mixes well at the temperature is present, in which individual phases may still be in the solid state, this melt being introduced into a shell and solidifying there on cooling, characterized in that the melt solidifies so quickly that essentially none

Reaction layer made of the material of the shell with which the melt is formed, that the solidified melting body is heated in a temper firing to a temperature at which a conversion of the melting body material into a precursor material with a content of at least one high-temperature superconducting and / or high-temperature superconducting phase of at least 10 % By weight occurs and that no grinding is carried out after the mixture has melted.

2. A process for producing a high-temperature superconductor precursor material for use in strip or wire production, in which a mixture of oxides and / or their precursors is prepared, in which this mixture is heated to such an extent that a mixture which mixes well at the temperature and pourable melt is present, in which, if necessary, individual phases can still be in the solid state, this melt being introduced into a shell and solidifying there on cooling, characterized in that the melt is heated to such an extent that there is no separation of the melt for the superconducting material is formed such that the solidified melting body is heated in a temper firing to a temperature at which a conversion of the melting body material into a precursor material containing at least one high-temperature superconductor or / and 29 high-temperature superconducting phase of at least 10 wt .-% occurs and that no grinding is carried out after melting the mixture.

3. The method according to claim 1 or 2, characterized in that an oxide mixture based on Bi-Sr-Ca-Cu-0 is produced, which may contain further chemical elements such as Pb, Sb, Y and / or other phases such as high-melting compounds .

4. The method according to claim 3, characterized in that bismuth is replaced by lead, antimony and / or yttrium up to a proportion of 50 mol%.

5. The method according to any one of claims 1 to 4, characterized in that a high-melting compound having a melting point of over 1000 ° C, in particular an alkaline earth metal sulfate, preferably a barium and / or strontium sulfate, is added to the oxide mixture.

6. The method according to any one of claims 1 to 5, characterized in that the oxide precursors of the mixture are calcined before melting or / and that the casing is preheated at a temperature up to 750 ° C before the introduction of the melt.

7. The method according to any one of claims 1 to 6, characterized in that the melting takes place in a shell, in a bowl, in a crucible or on a base made of a temperature-resistant material.

8. The method according to any one of claims 1 to 7, characterized in that the temperature-resistant material is a noble metal or a noble metal alloy, in particular with a high content of platinum, copper or / and silver. 30th

9. The method according to any one of claims 1 to 8, characterized in that the melt is poured onto or into a silver-rich material which may remain in contact with the precursor material produced therefrom in the long run.

10. The method according to any one of claims 1 to 9, characterized in that the mixture based on Bi-Sr-Ca-Cu-0 and optionally with other chemical elements and phases is heated to temperatures between 950 and 1250 ° C, in particular to at least 1000 ° C, preferably to about 1050 ° C, preferably to at most 1150 ° C, especially with holding times between

15 minutes and 8 hours, preferably at least 30 minutes, preferably at most 2 hours.

1 1. Method according to one of Claims 1 to 10, characterized in that the melting body material is tempered to temperatures of 600 to

950 ° C is heated, preferably to at least 650 ° C, at most up to 850 ° C, in particular with holding times between 15 minutes and 100 hours, preferably at least 45 minutes, preferably at most 30 hours.

12. The method according to any one of claims 1 to 1 1, characterized in that a temper material with a content of high-temperature superconductive phases of 15 to 95 wt.%, In particular at least 40 wt.%, Preferably at most 80 wt becomes.

13. The method according to claim 12, characterized in that a precursor material is formed in the temper firing, which predominantly has a high-temperature superconductive phase with a two-layer structure.

14. The method according to any one of claims 1 to 13, characterized in that a precursor material is formed in the temper firing with a content of 31 only low-temperature superconductive phases of up to 90% by weight, in particular at least 20% by weight, preferably at most 60% by weight.

15. The method according to any one of claims 1 to 14, characterized in that in the temper firing a Precursormateπal with a composition in the range of Bii, 8-2.3Sri - _2ι ιCao, 7-ι, ₂ Cu ₂ 0χ, Biι, 6-ι , gPbo, ₂ -o, 5Srι _ι 7. _2ι ιCao _ι 7. _{1 2} Cu ₂ Oχ, Biι _ι8 _ _{2 3} Srι _ι7 . _2ι1 Caι, ₇ . _2ι2 Cu ₃ 0χ or Bh.ei.gPbo ^ -o.sSrL /.^- iCau- ^ CusOx is formed, optionally with an additional content of up to 30% by weight of a high-melting compound, in particular an alkaline earth metal sulfate, particularly preferably one Ba or / and Sr sulfates.

16. The method according to any one of claims 1 to 15, characterized in that the sum of the impurities of carbon and hydrogen in the precursor material is reduced to <800 ppmw, preferably to <400 ppmw, in particular to <200 ppmw.

17. The method according to any one of claims 1 to 16, characterized in that the relative density of the precursor material is 75 to 99%.

1 8. The method according to any one of claims 1 to 17, characterized in that the melt in a tube, in a tube with several chambers, in a wound and possibly contacted at the edge of the film or in a communicating pipe system, with several pipe elements or / and fins can be attached, is introduced from a temperature-resistant material, in particular by casting, suction or by means of

Capillary forces.

19. The method according to claim 18, characterized in that a melt for high-temperature superconductive precursor materials is introduced into a tube with several chambers in one part of the chambers, a material for electrical insulation in another part of the chambers. 32

20. The method according to claim 19, characterized in that the material for electrical insulation is a heavily contaminated material based on Bi-Sr-Ca-Cu 0 optionally with other chemical elements and / or phases, a material based on a low-temperature super-conductive only Phase or a material on

Basis of non-high-temperature superconductive phases Basis Bi-Sr-Ca-Cu-0 with other chemical elements and / or phases.

21. The method according to any one of claims 1 to 20, characterized in that a precursor rod is produced as the starting material for the strip or wire production.

22. The method according to claim 21, characterized in that the precursor rod is processed by drawing, hammering, bundling, rolling or / and sintering to form a high-temperature superconducting strip or wire.

23. The method according to any one of claims 1 to 22, characterized in that the melt is introduced into a tube with a plurality of chambers or / and with lamellas or in a wound film, which enables such a large layer sequence from the coating layer (s) to high-temperature superconducting layers that the precursor rod can be used like a multifilament without bundling several precursor rods.

24. Precursor rod, produced according to one of claims 1 to 23, characterized in that it has a eutectic melting structure with crystalline

Has proportions of the one- and two-layer connection, the grains of which have grain sizes essentially in the range from 20 to 400 μm, preferably from 50 to 300 μm, and that the surface normal to the layer structure in the grains of the one- and two-layer connection is preferably essentially perpendicular to the temperature gradient when cooling, ie 33 is preferably aligned substantially perpendicular to the outer surface of the melting body.

25. Precursor rod according to claim 24, characterized in that its porosity is <30 vol .-%, preferably <20 vol .-%, and that its closed pores have essentially a diameter <50 microns, preferably <20 microns.

26. Precursor rod according to claim 24 or 25, characterized in that its high-temperature superconductor material contains impurities with hydrogen <350 ppmw, preferably <200 ppmw, particularly preferably <100 ppmw, and impurities with carbon <400 ppmw, preferably <250 ppmw, particularly preferably <150 ppmw.

27. Precursor rod according to one of claims 24 to 26, characterized in that its precursor material contains a metallic shell, preferably made of silver or a silver-rich alloy.

28. Ribbon or wire, produced according to claim 22 or 23, characterized in that it is substantially free of gas bubbles inside and that it has a current density of at least 2000 A / mm ² at 4.2 K, preferably of at least 2500 A / mm ² at 4.2 K,

29. Ribbon or wire according to claim 28, characterized in that it has no or only slight AC losses due to the inclusion of at least one electrically insulating layer.

30. Use of a precursor rod, produced according to claim 23, such as a multifilament for tape or wire production.