DE19719722C2 - Process for producing an elongated high-Tc superconductor - Google Patents

Description

The invention relates to a method for producing an elongated superconductor with at least one superconducting conductor core, which has a superconductor material with a metal oxide high T _c phase, which is surrounded by a normally conductive, Ag-containing material, with which method one of which Conductor pre-product surrounded by normally conductive material is created with a primary material of the superconductor material which is subjected to several deformation steps and at least one heat treatment, with at least one alloy component increasing the hardness of the normally conductive material from the outside in the normally conductive material consisting of Ag or an Ag alloy, at least externally Parts of this normal conductive material is diffused. A corresponding manufacturing process is known from DD 296 179 A5. A similar manufacturing process can be found in "IEEE Transactions on Applied Superconducitivity", Vol. 5, No. 2, June 1995, pages 1145 to 1149.

Furthermore, DE 41 13 220 A1 are superconducting Composite body made of oxide ceramic superconductor material with known an outer shell in which the filler at least partially hardened from an oxide dipers or hardenable silver alloy, which is used to increase the electrical resistance of at least one of the Contains precious metal elements from the platinum group or gold. A similar procedure is known from EP 0 769 819 A1.

Superconducting metal oxide compounds with high transition temperatures T _c of over 77 K are known, which are therefore also referred to as high-T _c superconductor materials or HTS materials and in particular allow an LN ₂ cooling technology. Such metal oxide compounds include in particular cuprates of special material systems such as B. the types Y-BA-Cu-O or Bi-Sr-Ca-Cu-O or (Bi, Pb) -Sr-Ca-Cu-O. Several superconducting high-T _c phases can occur within individual material systems, which differ in the number of copper-oxygen network planes or layers within the crystalline unit cell and which have different transition temperatures T _c .

The known HTS materials attempt to produce elongated superconductors in wire or ribbon form. A method considered suitable for this is the so-called "powder-in-tube technology", which is known in principle from the production of superconductors using the classic metallic superconductor material Nb ₃ Sn. Corresponding to this technology, for the production of conductors made of HTS material, powder from a primary material of the HTS material is introduced into a tubular sheath or into a matrix made of a normally conductive material, in particular of Ag or an Ag alloy, which generally does not yet contain the desired superconducting high T _c phase or only to a small extent. The structure to be obtained in this way is then brought to the desired final dimension by means of deformation treatments, which may optionally be interrupted by at least one heat treatment at elevated temperature. Thereafter, the conductor intermediate product thus obtained is subjected to at least one annealing treatment in order to adjust or optimize its superconducting properties or to form the desired high T _c phase. This annealing treatment is carried out at least partially in an oxygen-containing atmosphere at an elevated temperature, which is the case for materials from the material system (Bi, Pb) -Sr-Ca-Cu-O when annealed in air in general between 835 ° C and 840 ° C and at reduced oxygen partial pressure is about 815 ° C (see, for example, also "Supercond. Sci. Technol.", Vol. 4, 1991, pages 165 to 171).

If one bundles several corresponding high-T _c superconductors or their conductor precursors in a manner known per se, then conductors with several superconducting conductor cores, so-called multi-core or multifilament conductors, can also be obtained (cf. also "IEEE Transactions on Applied Superconducitivity", Vol 5, No. 2, June 1995, pages 1259 to 1261). In particular for AC applications, the bundle of individual conductor wires can be twisted around the common conductor axis.

This well-known multifilament superconductor with HTS material ha ben prefers a band form. To this form of a corresponding  Obtaining the final conductor product must be according to the above cited at least one rolling step will be. It is generally from an initially zy linden-shaped structure based on an evenly distributed Arrangement of conductor cores seen across the cross section. The This structure is then in the flat by means of the rolling process Conveyed from the form of the band so that it can withstand high current necessary texture, d. H. a largely parallel out direction of the crystal planes of the superconducting phase pass. The result is a flat conductor with one Width-to-thickness ratio of, for example, 10 or higher forth.

However, it turns out that with such a production using a ribbon-shaped multifilament superconductor Further processing of Ag as a normal conductive material of the strip conductor in the annealed condition due to the low Hardness of the Ag material is extremely problematic. By already The lowest load on the strip conductor tends to be that superconducting material surrounding Ag matrix to a plasti deformation. The brittle conductor veins can do not follow the ceramic superconductor material of this expansion and break. In addition, a particularly good th conductivity of an Ag matrix lust.

Such AC losses are attempted to be counteracted by twisting the superconducting conductor wires and by using Ag alloys, which increase the electrical resistance, as a matrix material. The use of Ag alloys also increases the strength of the finished strip conductor, which can therefore be processed better. However, it can be seen that the use of tubular shells made of Ag alloys for the production of the conductor intermediate products according to the powder-in-tube technique mentioned affects the formation of the high T _c phase in such a way that a significant drop in the (critical ) Current carrying capacity can be observed. In addition, the entire manufacturing process must be tailored to the use of the alloy material. This leads to an undesirable increase in the time and equipment required.

The object of the present invention is therefore to Ver go out with the characteristics mentioned above design that this effort is reduced. In particular the process aims to make a conductor end product highly critical Current density can be obtained without the risk of loading damage to his at least one superconducting conductor to superconducting devices such as cables or magnetic coils conditions can be further processed. If necessary, too the AC losses of the conductor end product are relative be extremely small.

This object is achieved by the characterizing features of claim 1, becomes.

The invention is based on the consideration that, until the alloying step of the normally conductive material according to the invention, a simplified deformation of the conductor structure with normally conductive parts made of Ag or an Ag master alloy is possible, in particular a good bond between the normal conductive material and the material of the pre Superconductor material is to be guaranteed. Since the alloying step only takes place after annealing to form the high T _c phase, there is also no fear of impairment of the superconducting properties of the conductor. With the measures according to the invention, it is consequently advantageous to obtain an elongated superconductor, in particular with a band shape, which has an increased critical current density compared to the known conductors.

Advantageous embodiments of the method according to the invention emerge from the dependent claims.

The invention is described below with reference to the Drawing explained further in the single figure schematically shows a cross section through an inventive alloy structure of an HTS conductor is illustrated.

A conductor produced according to the invention represents an elongated composite body in tape or wire form which contains a high-T _c (HTS) material embedded in a specially alloyed matrix material, at least largely phase-pure. Practically all known high-T _c superconductor materials, in particular rare earth-free cupra te, are suitable as HTS material, with phases whose transition temperature T _{c is} above the evaporation temperature of liquid nitrogen (LN ₂ ) of 77 K. A corresponding example is the HTS material of the basic type (Bi, Pb) ₂ Sr ₂ Ca ₂ Cu ₃ O _x , which is assumed below as an exemplary embodiment. A powder-in-tube technique known per se can advantageously be used as the basis for producing a corresponding HTS conductor (cf., for example, DE 44 44 937 A). For this purpose, a powdery Vorproduktma material, which enables the formation of the desired superconducting phase, or the superconducting material is introduced into a cladding tube, the material of which serves as a matrix material for the finished end product of the superconductor. For the cladding tube, one preferably chooses a base material that does not undergo an undesirable reaction with the components of the HTS material such as with oxygen during the manufacture of the conductor and that can be easily deformed. An Ag material which contains either Ag in pure form or in the form of a master alloy with Ag as the main component (ie contains more than 50% by weight) is therefore particularly suitable as the base material. So z. B. pure Ag, for example in the form of work hardened silver or recrystallized silver. Powder-metal silver can also be used. Dispersion-hardened silver is also suitable.

The structure of the cladding tube and the one it encloses Core z. B. from the intermediate material of the HTS material then a sequence of several, especially across cutting-reducing deformation steps and at least one ner heat (glow) treatment to form the desired undergo superconducting phase. For the deformation steps come all known methods such. B. Extruded sen, rolling, rolling, hammering and pulling into question, which too can be combined with each other. This mechanical loading actions can take place both at room temperature and at he at a higher temperature. After the least two deformation steps then a raw conductor is in shape a composite body with a target end product at least largely corresponding shape, preferably in Band form, before. However, this composite body does not yet point the desired superconducting properties. That's why he the at least one heat or annealing treatment follows, as preferably in an oxygen-containing atmosphere such as e.g. B. is carried out in air, so the intermediate material for the formation of the desired superconducting phase required oxygen via the Ag-containing coating or To provide matrix.

The method described above can not only be used for manufacturing position of single core conductors, but is also how for adopted the figure of the drawing, for the production of multi-core conductors  suitable. Such multi-core or multifilament lines ter are generally by means of a known Bundling technology trained. Accordingly, e.g. B. a frets development of several starting products, each consisting of one Cladding tube and a core from the pre-pro duct material exist in a larger cladding tube from the Ma trix material possible. Of course, you can also shaped or, if necessary, preheated tube in a sol ches cladding tube are introduced.

A to get after a sequence of pressing and rolling operations, hereinafter referred to as a strip conductor raw conductor or another intermediate conductor of a band-shaped multi-core conductor to be treated according to the invention is shown in the figure of the drawing as a cross section. The band conductor is generally designated 2. Its 19 conductors, for example, 3 i cores (with 1 ≦ i ≦ 19) from the pre-product or finished superconductor material are embedded in a matrix 4 made of Ag. The strip conductor 2 has z. B. the following typical dimensions for strip shapes:
Band thickness D: 100 µm to 500 µm, preferably 200 µm to 350 µm
Band width B: 2 mm to 6 mm, preferably 3.5 mm to 4.5 mm;
Conductor core thickness d: 10 µm to 50 µm, preferably 20 µm to 35 µm;
Conductor core width b: 30 µm to 200 µm, preferably 150 µm to 180 µm.

The corresponding typical dimensions for a band-shaped single core conductor are:
50 µm ≦ D ≦ 400 µm, preferably 100 µm ≦ D ≦ 200 µm;
1.5 mm ≦ B ≦ 5 mm, preferably 2 mm ≦ B ≦ 3 mm;
10 µm ≦ d ≦ 40 µm, preferably 20 µm ≦ d ≦ 30 µm;
500 µm ≦ b ≦ 4.5 mm, preferably 1.5 mm ≦ b ≦ 2.5 mm.

In order to maintain the current carrying capacity of the superconducting strip conductor 2 shown in the figure with an Ag matrix or, if appropriate, a matrix made of an Ag pre-alloy, and nevertheless to ensure significantly improved strength when handling the conductor and lower AC losses during conductor operation, the Darge presented embodiment provided that an optimization of the Ag matrix 4 is only carried out when the strip conductor 2 has gone through the entire conventional manufacturing process. For this purpose, the strip conductor 2 is coated with a layer 5 of a thickness δ made of special elements or alloys. This coating can e.g. B. by an electroplating technique or by means of a PVD or CVD technique. The strip conductor can also be used in a different manner, e.g. B. be acted upon by a corre sponding atmosphere. The coating material is then, as is illustrated in the figure by arrowed lines 6 , diffused into the Ag matrix 4 during a specific annealing under protective gas or vacuum. During this annealing, oxygen and nitrogen should not come into contact with the conductor if non-noble metals are to be provided as coating materials. If you want to use noble metals as coating materials, the atmosphere to be selected for the annealing is not critical. The annealing should be carried out at a temperature T _g , which is significantly below the heat treatment temperature for the formation of the high T _c phase, so as not to damage the superconducting parts of the conductor 2 . Temperatures T _g below 700 ° C. are advantageously chosen, with annealing also being possible at different annealing temperatures. Under these conditions, a corresponding Ag alloy is then formed in at least outer parts of the Ag matrix in a zone of depth t _e , whereby the superconducting parts remain at least largely unaffected. Because with the present conditions, diffusion primarily takes place in the Ag material of the matrix, since the diffusion on the genetic material in the ceramic of the superconductor material is strongly suppressed. In addition, since the formation of the superconducting material has already taken place, damage to this material is not to be expected.

In order to ensure the required matrix properties, alloy elements or components must be alloyed, which have a sufficiently high strength with sufficient ductility lity and possibly also an increased electrical Wi guarantee the state. For this reason, Li never consider the following elements X for the Ag material: X = Cd, Hg, In, Sn, Zn, Al, Ga, Mn, Pb, Sb, Au, Pt, Cu, Ge, Pd and alloys containing these elements.

Through the formation of a corresponding Ag-X mixed crystal the resistance of the matrix material is dependent on Element X increased very significantly. Change at the same time the mechanical properties such as strength also advantageous ductility and ductility.

Of course, a diffusion of several elements is also possible, which are brought to the diffusion either simultaneously or in succession. A diffusion in limited areas of the surface of the conductor is also possible. The diffusion need not extend over the entire conductor cross section, but can also only take place to a predetermined depth t _e .

Furthermore, there is the additional possibility of alloying elements in the outer region of the conductor 2 by forming a second phase z. B. by means of oxidation (so-called "dispersoid formation") or excretion to increase hardness. It is also possible to nitrate in an N ₂ atmosphere or carburize in a carbon atmosphere, which leads to hardening dispersoids. In the outer edge layer, the alloying elements can be used for hardening, while they generate increased matrix resistance in the area of the individual superconducting conductor wires.

In the following, some specific exemplary embodiments are outlined, in which a superconducting strip conductor with a cross section according to the figure is used. The individual superconducting conductor wires consisted of the HTS material (Bi, Pb) ₂ Sr ₂ Ca ₂ Cu ₃ O _x with the so-called 2223 phase, the transition temperature T _{c of which} is about 110 K.

Embodiment 1

The superconducting Ag-Bi2223 multicore ribbon conductor 2 is coated, for example by means of electrodeposition, with a 5 μm thick layer 5 made of AgSn10 and then annealed under argon at 270 ° C. for 20 hours in order to thus transfer the Sn component of the layer material into the Ag Diffuse the matrix of the ribbon conductor 2 .

Embodiment 2

The strip conductor according to exemplary embodiment 1 is coated, for example by means of hot dipping, with a 5 μm thick layer 5 of AuGe12 and then annealed under argon for 20 hours at 300 ° C. in order to diffuse the Au and Ge into the Ag matrix 4 of the strip conductor 2 .

Embodiment 3

The strip conductor according to exemplary embodiment 1 is acted upon with Bi at 300 ° C. in a special gas atmosphere which contains gaseous Bi chloride, so as to diffuse the Bi into the Ag matrix 4 of the strip conductor 2 . For the diffusion z. B. provided a period of 10 hours.

Embodiment 4

In each of the embodiments 1 to 3 can additionally by an internal oxidation, for. B. for 10 hours at 600 ° C in an air atmosphere, with a 10 to 25 microns thick, ge hardened surface layer containing the oxides of the elements X Te (see also "Journal of Alloys and Compounds", Vol. 188, 1992 , Pages 268 to 271). So z. B. the X-element Sn diffused in accordance with embodiment 1 can be converted into SnO particles in a dispersoid formation. The inner area with the conductor cores 3 i and the Ag-Sn matrix surrounding them remains completely unaffected by this.

Embodiment 5

The exemplary embodiments 1 and 2 can also be in succession be carried out.

In the above exemplary embodiments, it was assumed that the diffusion of the at least one alloy component into the Ag material of the matrix takes place after the last deformation step to form the final shape of the superconductor. The diffusion of additional elements according to the invention into the Ag material of the matrix to increase the hardness of the normally conductive material and, if appropriate, to an associated increase in resistance is, however, not limited to aftertreatment of the manufactured super conductor, but can also be carried out during the entire manufacturing process . This means that hardening can be carried out at any point during the production of the conductor without mechanically stressing the conductor. The increase in resistance due to the diffusion according to the invention can also take place at any time. So z. B. diffuse the additional alloy component during a so-called thermo-mechanical treatment. Such a treatment is composed of annealing and further deformation steps in order to form the desired superconducting high-T _c phase and to produce a texture that is sufficient for a high critical current density and the superconductor material. If necessary, a special heat treatment can be provided between the further deformation steps of the thermo-mechanical treatment. The deformation steps can also be carried out at elevated temperature and / or combined with the annealing treatment to form the desired superconducting phase. The advantage of a diffusion, for example within the thermomechanical treatment before the last annealing, the hardening during this annealing lies in the increased strength of the finished strip conductor after the last annealing. If the strength requirements are already placed on the annealed conductor in order to exclude any damage to the finished annealed conductor, a corresponding procedure is particularly advantageous.

For example, the current carrying capacity of a pure silver leader as good as possible and still get one significantly improved strength and lower alternating current To ensure losses, especially the invention appropriate procedures are carried out such that the optimization Part of the matrix during the thermomechanical treatment  and partly only after the thermomechanical loading action is carried out.

As part of such a thermomechanical treatment generally an annealing in an atmosphere with reduced If there is partial oxygen pressure, an internal oxidation can occur on base elements that are no longer for an increase in the matrix resistance are available. It is therefore advantageous that during or before the thermo mechanical treatment only a change or increase the strength is achieved. An increase in resistance with Base elements should therefore only be used after the thermome chanic treatment through element loading and diff fusion under protective gas or vacuum. It follows thus a two-part process: hardening by coating, Diffusion and curing during thermomechanical loading action or before its last glow and an increase the electrical resistance according to the thermomechanical loading action.

Resistance and strength increases through noble alloy approximately elements such. B. Au, Pd, which under the usual procedure conditions or thermomechanical treatment can get mixed reaction, even during the band conductor production can be carried out.

Embodiment 6

Before a third annealing treatment during a thermomechanical treatment, an AgIn10 coating 5 is applied to the strip conductor 2 by means of galvanic deposition. This is followed by a modified annealing, since at about 800 ° C and in an oxygen atmosphere the In would oxidize immediately and thus cannot diffuse in and hinder oxygen diffusion. For this reason, the diffusion is first carried out at 600 ° C. for 50 hours under a protective gas atmosphere before the usual reaction parameters of the annealing treatment are set at the usual temperatures and in the usual oxygen atmosphere. After the thermomechanical treatment has been completed, Sb is introduced into the silver matrix, for example, in accordance with the method described above, in order to significantly increase the resistance.

Claims (11)

1. A method for producing an elongated Supralei ters with at least one superconducting conductor wire, which has a superconductor material with a metal oxide high-T _c - phase, which is surrounded by a normal conductive Ag-containing material, with which method one of the normal conductive material Surrounded preliminary conductor product is created with a pre material of the superconductor material, which is subjected to several deformation steps and at least one heat treatment, wherein in the normally conductive material consisting of Ag or an Ag pre-alloy from the outside at least one alloy component increasing the hardness of the normally conductive material in at least external parts of this normallei tend material is diffused, characterized in that after the final deformation step and / or after at least one of the preceding deformation steps and after the heat treatment to form the high T _c phase, the diff fusion of the at least one alloy component of the normal conductive material is made before. 2. The method according to claim 1, characterized in that the diffusion of the at least one alloy component is carried out at a temperature which is below a heat treatment temperature to form the high-T _c phase. 3. The method according to claim 2, characterized records that the diffusion at a temperature of a maximum of 700 ° C. 4. The method according to any one of claims 1 to 3, because characterized in that the diffusion sion in a vacuum or under a protective gas atmosphere men will.   5. The method according to claim 4, characterized records that a protective gas atmosphere is provided that is at least largely free of oxygen or Is nitrogen. 6. The method according to any one of claims 1 to 5, there characterized in that as diffusion founding alloy component of at least one of the elements te Cd, Hg, In, Sn, Zn, Al, Ga, Mn, Pb, Sb, Au, Pt, Cu, Ge, Pd or an alloy is provided which is at least one of the contains elements. 7. The method according to any one of claims 1 to 6, that the least least an alloy component in the form of a layer ( 5 ) is applied to the normally conductive material ( 4 ). 8. The method according to claim 7, characterized in that the layer ( 5 ) is brought up by means of a galvanic technology or a PVD technology or CVD technology. 9. The method according to any one of claims 1 to 6, there characterized by that the little least an alloy component in one of these components containing atmosphere in the normal conductive material is diffused. 10. The method according to any one of claims 1 to 9, there characterized in that after the one diffusion of the at least one alloy component in one outer area of the alloyed normal conducting material diffused alloy component in a predetermined Gas atmosphere to contain the element of the gas atmosphere material is converted.   11. The method according to claim 10, characterized by a nitration in an N ₂ atmosphere or a carbo ration in a carbon atmosphere or an oxidation in an oxygen-containing atmosphere.