WO2003107446A2

WO2003107446A2 - Elongate superconductor structure comprising a high-tc superconductor material, an oxidic intermediate layer system, and a nickel support, and method for the production of said structure

Info

Publication number: WO2003107446A2
Application number: PCT/DE2003/001755
Authority: WO
Inventors: Sascha Danninger
Original assignee: Siemens Aktiengesellschaft
Priority date: 2002-06-13
Filing date: 2003-05-28
Publication date: 2003-12-24
Also published as: WO2003107446A3; DE10226392C1

Abstract

Disclosed is a superconductor structure (2) comprising a biaxially textured nickel support (3), an intermediate layer system (6) which is deposited on the support and is provided with at least two intermediate cerium oxide layers (4, 5) that are applied one after another, and a superimposed high-T_C superconducting layer (7) of type (RE)M₂Cu₃O_x (in which RE represents a rare earth and M represents an alkaline earth metal). The first intermediate layer (4) is subjected to an oxygen treatment before the second intermediate layer (5) is deposited.

Description

description

Elongated superconductor structure with high-T _c superconductor material, oxidic interlayer system and nickel carrier as well as processes for the production of this structure

The invention relates to an elongated superconductor structure for carrying an electrical current in a predetermined direction. This structure should have at least the following parts, namely a biaxially textured carrier made of nickel material, an intermediate layer system deposited on the carrier with at least two intermediate layers made of oxidic material and a superconducting layer made of a high-T _c superconducting material of the type deposited on the intermediate layer system (RE) M ₂ Cu3θ _x , wherein the component RE contains at least one rare earth metal (including yttrium) and the component M contains at least one alkaline earth metal. The invention further relates to a method for producing such a superconductor structure. A corresponding superconductor structure and a method for its production are described, for example, in “Applied Superconductivity *, Vol. 4, Nos. 10-11, 1996, pages 403 to 427.

Superconducting metal oxide compounds with high transition temperatures T _c of over 77 K at normal pressure are known, which are therefore also referred to as high-T _c superconductor materials or HTS materials and in particular allow liquid nitrogen (LN ₂ ) cooling technology. Such metal oxide compounds include, in particular, cuprates from the (RE) -M-Cu-O family of materials, component RE containing at least one rare earth metal (including Y) and component M containing at least one alkaline earth metal. The main representative of this family is the material YBa ₂ C 3θ _x , so-called YBCO.

These known HTS materials are tried to be used on different substrates for different applications. deposit, with a view to a high current carrying capacity for phase-pure, textured superconductor material.

In the case of a corresponding elongate superconductor structure for conductor applications, the HTS material mentioned is generally not deposited directly on a metallic carrier tape serving as a substrate; rather, this carrier tape is first covered with at least one thin intermediate layer, which is also referred to as a buffer layer (“buffer * layer). This intermediate layer with a thickness of the order of up to approximately 1 .mu.m is intended to prevent metal atoms from diffusing out of the carrier material into the HTS material in order to avoid an associated deterioration in the superconducting properties of the HTS material. At the same time, such an intermediate layer serving as a diffusion barrier can also smooth the surface and improve the adhesion of the HTS material. Corresponding intermediate layers consist in particular of metal oxides and are therefore generally electrically insulating.

In addition to the property as a diffusion barrier, this at least one intermediate layer should also fulfill the requirement that it enables textured growth of the HTS material to be applied to it. As a result, the intermediate layer itself must have an appropriate texture. The transfer of the crystallographic orientation during the growth of a layer on a chemically different type of substrate is known under the term “heteroepitaxy. In this case, the intermediate layer must have dimensions of its unit cells that are matched as closely as possible to the lattice constants of the HTS material. In addition, it should have a coefficient of thermal expansion that at least approximately matches that of the HTS material, in order to avoid undesirable mechanical stresses in the temperature that can be avoided for superconducting technology applications and layer preparation. avoid cycles or differences and, if necessary, damage such as spalling.

Similar requirements must also be placed on the selection of the “carrier intermediate layer *” system. Here are good ones too

To strive for adhesive properties, while at the same time the desired heteroepitaxy between the intermediate layer and the HTS layer growing thereon must not be impaired.

For the reasons mentioned above, a metal strip made of nickel material and biaxially textured on its surface by means of a rolling process is provided as the carrier strip in accordance with the literature reference mentioned at the beginning. Corresponding metal strips are known under the name “RABiTS * (“ Rolling-Assisted-Biaxially-Textured-Substrates *). In the known superconductor structure, an intermediate layer system in the form of a Ce0 ₂ layer (as a first buffer layer) and a thicker layer of Y-stabilized Zr0 ₂ , so-called YSZ (as a second buffer layer), are deposited on such a metal strip. It is assumed that a good texture transfer can be achieved with a Ce0 ₂ layer if this layer itself has a good in-plane and out-of-plane (001) texture. However, since the first buffer layer has to be grown in a reducing atmosphere due to the use of a Ni-containing tape and because of the high vacuum required it is not possible to offer enough oxygen during the further manufacturing process, the Ce0 ₂ always grows slightly deficient in oxygen. As a result, the Ce0 ₂ material contracts as soon as it is exposed to an oxygen-containing atmosphere (> 1 mbar). This contraction of the Ce0 ₂ buffer layer then creates cracks in well-oriented layers from a layer thickness of 50 nm. The consequence of this is that such a buffer layer is no longer effective as a diffusion barrier. Thus, (111) -oriented grains neutralize undesirable layer tensions; however, such grains grow in a columnar manner. This means that when layer stress occurs Gap the columns apart and there is no longer a dense surface or layer. At such defects, ie (111) -oriented grains, there is a risk of undesired diffusion. For this reason, in the prior art mentioned, the first buffer layer is covered with a second, sputtered buffer layer made of YSZ or Y ₂ 0 ₃ . However, non-sputtered layers are not dense enough, since YSZ and Y2O3 grow in columnar form and the Ni diffuses between the columns into the YBCO material. In such a case, cracks could be covered with a porous buffer layer with a relatively large thickness between 500 nm and 2000 nm, which does not cause any layer stress problems. However, since zirconate complexes can form during the deposition of YBCO on YSZ, which also lower the critical current density J _{c of} the HTS material, this in turn must be covered with a third buffer layer, for example made of Ce0 ₂ . However, such a 3-layer system as a buffer is technically very complex, therefore expensive and also time-consuming because of the sputtered intermediate layer. A YSZ layer directly on Ni is very difficult to produce because the (001) orientation is difficult to achieve during sputtering.

The object of the present invention is therefore to provide a layer system for the layer structure of the type mentioned at the outset which fulfills the requirements mentioned, so that even longer support pieces, in particular of more than one meter, preferably more than 100 m, can be coated with constant quality. Diffusion of metal atoms from the carrier material, in particular into a YBCO layer, as well as diffusion of oxygen, which is required for the deposition and formation of the YBCO, to the metal surface are to be prevented. Such an oxygen diffusion would namely lead to metal oxidation at the usual deposition and annealing temperatures of the YBCO material in the range of about 600 to 800 ° C. and thus reduce the adhesive strength of the interlayer system. For this Basically, the thickness of the interlayer system is generally between about 0.05 and 2 .mu.m, this thickness depending on the chosen interlayer material. Furthermore, a method is to be specified with which a corresponding superconductor structure can be produced in a relatively simple manner.

The stated object is achieved with respect to the conductor structure with the measures specified in claim 1. Accordingly, the intermediate layer system should have a first intermediate layer made of a cerium oxide facing the carrier and a second intermediate layer facing a superconducting layer made of a cerium oxide. In this context, a layer system is understood to mean a successive deposition of discrete layers, which, as in the present case, can consist of identical material.

It was recognized that it was not absolutely necessary to distribute the sub-tasks mentioned with regard to the layer system, namely with regard to texture transfer and with regard to a diffusion barrier, to layers made of different materials. By loading the oxygen after evaporation of the first buffer layer, unstoichiometric, but (200) -oriented Ce0 _{2 can be converted} into stoichiometric Ce0 ₂ . As a result, the Ce0 ₂ contracts and cracks appear in the first buffer layer. These deliberately created cracks mechanically relax the first buffer layer so that it is possible to seal these cracks again with a second buffer layer without the cracks reaching from the second buffer layer to the substrate. It is therefore now possible to completely cover the cracks in the first Ce0 ₂ layer underneath with a second, preferably at most equally thick, CeO ₂ layer. The second layer can of course have cracks for the same reasons as the first layer. However, these cracks occur at least largely not in the same places as in the first layer, because the layer tension is minimal at these points.

Another advantage of the construction according to the invention is the possibility of achieving high current densities even on supports whose buffer layer shows only a Ce0 ₂ (200) portion in the order of magnitude of the Ce0 ₂ (111) portion due to poor epitaxy. Such a buffer layer is normally leaky since mechanical stresses are reduced at grain boundaries of (111) and (200) oriented grains, as mentioned above. The fractures between the grains thus generated are filled with the second buffer layer from Ce0 ₂ . Although this is still (111) -oriented at the affected areas, it advantageously represents a dense barrier. It is advantageous here that YBCO can easily grow over smaller (111) -oriented areas.

Advantageous refinements of the superconductor structure according to the invention emerge from the dependent claims.

The superconductor material can preferably be of the YBa ₂ Cu ₃₂ θ _x (YBCO) type. Instead, a material can also be provided in which, based on YBCO, the Y-

Component and / or component Ba are / are at least partially replaced by an element from the corresponding group.

The first intermediate layer made of cerium oxide advantageously has a thickness of less than 200 nm, preferably 100 nm, and of at least 10 nm. With this layer thickness, good hetero-epitaxy must be ensured.

Furthermore, it is to be regarded as advantageous if the second intermediate layer made of cerium oxide has a comparatively smaller thickness than the first intermediate layer, in particular between 50 nm and 200 nm. Such a relatively thin second interlayer guarantees the tightness of the entire interlayer system even over great lengths of the superconductor structure and nevertheless represents a good epitaxial base for the HTS material to be deposited thereon. In addition, this thickness dimensioning must ensure that the layer stresses in the second Intermediate layer are sufficiently small that it is unable to tear open the first intermediate layer.

In order to obtain a thicker intermediate layer system, it is of course possible to produce a plurality of such double layers one above the other, as long as each individual one of the intermediate layers is loaded with oxygen after vapor deposition and is thus relaxed.

Further advantageous refinements of the superconductor structure according to the invention emerge from the material claims which are not addressed above and which are dependent on claim 1.

A preferred method for producing a corresponding superconductor structure according to the invention is characterized in that after the first intermediate layer has been deposited under a condition which is low or free from oxygen or reduced, this layer is exposed to an oxygen-containing atmosphere before the second intermediate layer is deposited. Such a method is particularly easy to carry out. For example, by ventilating the first intermediate layer with oxygen or an oxygen-containing atmosphere, unstoichiometric cerium oxide contracts with oxygen after flooding. An oxygen pressure or oxygen partial pressure higher than that of the coating, in particular of at least 50 mbar, preferably 100 mbar, is sufficient for this to ensure the desired formation of cracks in this intermediate layer. To further explain the invention, reference is made below to the drawing. In each case schematically their Figure 1 show a basic training possibility of a

Superconductor structure according to the invention and its Figure 2 a section of the layer system of this

Superconductor structure. In the figures, corresponding parts are each provided with the same reference symbols.

When designing the superconductor structure according to the invention, it is assumed that the embodiments are known per se (cf. the above-mentioned literature reference “Appl. Supercond. *” Or WO 00/46863). The superconductor structure, which is denoted generally by 2 in FIG. 1, therefore comprises at least one elongate carrier 3, also known as a substrate, with a thickness d1. At least two intermediate layers 4 and 5, which are also to be coated as buffer layers, are deposited on the carrier with thicknesses d2 and d3. A layer 7 with a thickness d4 of a special HTS material is applied to the intermediate layer system 6 thus formed.

For the carrier 3, a plate or a band or some other elongated structure made of a special metallic material is used with the thickness d 1 which is arbitrary per se and the dimensions of its area required for the respective application. The carrier can also be part of a composite body with at least one further element not shown in the figure. This element can be provided for mechanical reinforcement and / or for electrical stabilization and is located, for example, on the side of the carrier facing away from the interlayer system 6. Such a further element can consist of Cu, for example. For the construction according to the invention, 3 nickel (Ni) or a Ni alloy should be selected as the metallic material for the carrier. Its surface facing the interlayer system 6 should have a biaxial texture (so-called “cube-layer texture *). Corresponding carriers are also referred to as “RABiTS * (cf. WO 00/46863 mentioned).

All known metal oxide high-T _c superconductor materials which can be derived from the (RE) ιM ₂ Cu ₃ O _x type are suitable as HTS materials. A YBCO material is preferably provided. The thickness d4 of the corresponding HTS layer 7 is not critical per se and is generally less than 5 μm. In view of a high current carrying capacity, even larger layer thicknesses can be selected. If necessary, at least one further layer, such as a protective layer or a stabilizing layer, in particular made of Ag or an Ag alloy, can be applied to this layer.

In a departure from the known embodiment of a superconductor structure according to WO 00/46863, the two intermediate layers 4 and 5 of the layer system 6 of the layer structure 2 according to the invention consist of a Ce oxide, in particular CeO ₂ . The thickness d2 of the first layer 5 facing the carrier 3 is preferably above 10 nm and can generally reach up to 200 nm, optionally up to 2 μm. The second intermediate layer 5 facing the YBCO layer 7 can have the same thickness d3; in general, however, this layer is comparatively thinner or at most the same thickness. For example, the thickness d3 is between 50 nm and 200 nm.

All layers of the superconductor structure are applied with the aid of known deposition processes while heating the carrier. The deposition is generally carried out under an oxygen-poor or -free or -reducing condition, for example at an oxygen partial pressure p (0 ₂ ) with 10 ^{~ 6} mbar <p (0 ₂ ) <10 ^"2 mbar. Figure 2 shows a detail of the superconductor structure 2 according to Figure 1, the two intermediate layers 4 and 5 of the layer system 6 in an enlarged view. An embodiment was assumed in which the layer thicknesses d2 and d3 are approximately the same size and, for example, each amount to 100 nm. As can be seen from the highly schematic figure, both layers of the Ce0 ₂ material show cracks 9 and 10 which inevitably occur. However, these cracks advantageously lie in different areas of the layer system, so that this has a practically uninterrupted diffusion barrier between the Ni material of the carrier 3 and a YBCO layer 7 to be applied forms. This can be ensured by making sure that the cracks 9 already form in the first intermediate layer 4 before the second one is produced during the production of the layer system 6

Intermediate layer 5 is applied. To this end, the first intermediate layer can advantageously be aerated with oxygen, in particular immediately after its deposition, since unstochiometric.es Ce0 _{2 contracts} after flooding with oxygen. A vacuum system to separate the

Intermediate layer, for example under the aforementioned low oxygen partial pressure p (0 ₂ ), can simply be ventilated and / or opened once before the second layer is applied. In this case, such an oxygen pressure or an oxygen partial pressure is to be selected that complete oxidation of the first CeO ₂ intermediate layer 4 is ensured. A 0 ₂ atmosphere of 10 ^"3 mbar would only lead to an unstoichiometric Ce0 ₂ intermediate layer. In general, however, a 0 ₂ pressure of 50 mbar, preferably of at least 100 mbar, is sufficient. Experience has shown that this pressure should, however For example, a pressure of 200 mbar p (0 ₂ ) is selected. A cooling of the structure consisting of carrier 3 and first intermediate layer 4 during production is not necessary for the formation of the cracks in the first layer the second intermediate layer 5 of Ce0 ₂ and then the YBCO layer 7 are produced. In the exemplary embodiment described above, it was assumed that the interlayer system 6 is only formed from two discrete CeO ₂ intermediate layers 4 and 5. Of course, the intermediate layer system can also have a larger number of individual intermediate layers, especially when it comes to better diffusion tightness. For example, any number of 100 nm thick intermediate layers can be applied as long as oxygen is loaded after each individual layer, for example by ventilation. The maximum thickness of the individual intermediate layers essentially depends only on the maximum possible partial pressure of 0 ₂ during the deposition process. The total thickness of the intermediate layer system and thus the number of individual intermediate layers also depends on the coefficient of expansion of the intermediate layer system, which should generally be matched to the coefficients of expansion of the carrier material and / or superconductor material.

Claims

claims

1. Elongated superconductor structure (2) for carrying an electrical current in a predetermined direction with at least the following parts, namely with

- A biaxially textured carrier (3) made of nickel material, an intermediate layer system (6) deposited on the carrier (3), which has at least one first intermediate layer (4) facing the carrier (3) made of a cerium oxide and a applied to the second intermediate layer (5) made of a cerium oxide and

- A superconducting layer (7) deposited on the interlayer system (6) made of a high-T _c - superconducting material of the type (RE) M ₂ Cu3θ _x , wherein the component RE at least one rare earth metal (including yttrium) and the component M at least one alkaline earth metal contain.

2. Superconductor structure according to claim 1, characterized in that the superconductor material

3. Superconductor structure according to claim 2, so that the Y component and / or the Ba component are / are at least partially replaced by an element from the corresponding group.

4. Superconductor structure according to one of the preceding claims, that the first intermediate layer (4) has a thickness (d2) of less than 2 μm, in particular less than 200 nm, preferably less than 100 nm, and is at least 10 nm thick.

5. Superconductor structure according to one of the preceding claims, characterized in that the second intermediate layer (5) at most the same thickness (d3) as the first intermediate layer (4), preferably has a comparatively smaller thickness (d3).

6. superconductor structure according to claim 5, g e k e n n - z e i c h n e t by a thickness (d3) of the second intermediate layer (5) between 50 nm and 200 nm.

7. Superconductor structure according to one of the preceding claims, g e k e n n z e i c h n e t by a superconducting layer (7) with a thickness (d4) of at most 5 microns.

8. Superconductor structure according to one of the preceding claims, g e k e n n z e i c h n e t by a carrier (3) with a band shape.

9. A method for producing a superconductor structure according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that after a deposition of the first intermediate layer (4) under low or oxygen-free or reducing conditions, this layer is exposed to an oxygen-containing atmosphere before the second intermediate layer is deposited.

10. The method of claim 9, d a d u r c h g e k e n n - z e i c h n e t that an oxygen loading of the first

Intermediate layer (4) at an oxygen pressure or oxygen partial pressure of at least 50 mbar, preferably of at least 100 mbar.