Description

The present invention is in the field of oriented metal substrates for high temperature supercon- ductor tapes.

High-temperature superconductors in tape form are typically produced by epitaxial deposition on flexible metal substrates. In most cases substrates are made of nickel due to its high ten- dency to form a highly oriented cubic texture. During the process of depositing the supercon- ductor material on the substrate, high temperature and tensile stress is exerted on the sub- strate. Pristine nickel is mechanically too weak. Therefore, nickel alloys are used, for example nickel alloys containing tungsten.

WO 2008 / 125 091 discloses alloys which are suitable for these purposes. Nickel containing about 5 at-% tungsten can be very well processed into tapes with high crystal orientation, for ex- ample as disclosed in WO 03 / 024 637 A1. However, this alloy is magnetic, so it cannot be used in applications with high alternating magnetic fields, such as electric generators or en- gines. Higher tungsten contents render non-magnetic tapes. However, the more tungsten is added to the nickel, the lower is the tendency to form highly oriented crystals. This is a conse- quence of the decreased stacking fault energy of the alloy.

J. Eickemeyer et al. disclose in Superconductor Science and Technology volume 23 (2010), 085012 a process to produce Ni-9.0 at. % W substrate with high texture. Also U. Gaitzsch et al. disclose in Superconductor Science and Technology volume 26 (2013), 085024 such a process. However, the initial deformation process is not disclosed making reproduction difficult as de- scribed by F. Peng et al. in Rare Metals volume 37 (2018) page 662.

It was therefore the objective of the present invention to provide a process for producing low stacking fault energy tapes with high crystal orientation. The process was aimed to be robust in order to allow industrial production on very large scale. At the same time, the process was aimed to be efficient in terms of energy and time consumption.

These objectives were achieved by a process for preparing metal tapes with a high degree of cube texture comprising

(a) reducing the thickness of the metal tape and

(b) heating the metal tape to a temperature above 40 % of its melting point in Kelvin, wherein the number density of cube-oriented grains nc _B after thickness reduction is at least 10 ¹² nr ³ .

The present invention further relates to a metal tape containing Ni and at least 8 at-% W having a cube texture component of at least 99 %. The present invention further relates to the use of the metal tape according to the present inven- tion as a substrate for a high temperature superconductor.

The present invention further relates to a superconducting tape containing the substrate accord- ing to the present invention.

Preferred embodiments of the present invention can be found in the description and the claims. Combinations of different embodiments fall within the scope of the present invention.

The process according to the present invention is suitable for preparing metal tapes with a high degree of cube texture. The metal tape can contain any face centered cubic (fee) metal which forms a cube texture including nickel alloys, for example Ni-W, Ni-Mo, Ni-Cr, Ni-Re; aluminum alloys, for example Al-Mg, Al-Si-Mg, Al-Mn; copper alloys, for example Cu-Mn, Cu-Ni, Cu-Zn; iron alloys, for example Fe-Ni. Preferably, the metal tape contains nickel, more preferably the metal tape contains nickel and tungsten. The process yields good results for most compositions of nickel and tungsten. However, it is particularly suitable for metal tapes containing Ni and at least 7 at-% W, more preferably at least 8 at-% W, in particular at least 9 at-% W to generate a high degree of cube texture. The degree of cube texture in a rectangular object, for example a sheet or tape, of a polycrystalline material generally means that the percentage of grains with a deviation of their crystallographic axes from the principal axes of the rectangular object is equal or less than 16 °. Preferably, the degree of cube texture is at least 97 %, more preferably at least 99 %, in particular at least 99.5 %.

The process according to the present invention starts with the metal alloy, for example in form of a metal sheet which is obtained by hot deformation of an ingot which is formed from cooling down a melt of the metal alloy. It is generally known how melting and hot deformation are done, for example as disclosed in WO 2014 / 194 881 A2. After hot deformation the metal sheet often has a thickness of several millimeters, for example 5 to 20 mm.

The process according to the present invention comprises in step (a) reducing the thickness of the metal tape. Preferably, the thickness reduction is accomplished by rolling. Usually, the tem- perature increases during thickness reduction as a result of the mechanical energy input. How- ever, it should not exceed 40 % of the melting temperature of the metal tape in Kelvin, prefera- bly it should not exceed 30 % of the melting temperature of the metal tape in Kelvin, in particular it should not exceed 25 % of the melting temperature of the metal tape in Kelvin. Therefore, preferably, the metal tape is not heated in addition to this mechanical energy input, or even cooled in order to remove some of the heat resulting from the mechanical input in order to not exceed the maximum temperature. Sometimes, the speed of thickness reduction needs to be adjusted in order to avoid excessive heating during thickness reduction. Preferably, for metal tapes containing nickel and tungsten the temperature of the metal tape during thickness reduc- tion is below 500 °C, more preferably below 300 °C, in particular below 175 °C. Usually, the temperature not below 0 °C. Preferably, the thickness of the metal tape is reduced by cold roll- ing.

The process according to the present invention comprises in step (b) heating the metal tape to a temperature above 40 % of its melting point in Kelvin. Obviously, the metal tape should not be heated above its melting point. Preferably, the temperature is 42 % to 60 % of its melting point in Kelvin, in particular 45 % to 55 % of its melting point in Kelvin. For a nickel alloy processed in batch mode, the temperature of heating the metal tape is preferably 550 °C to 725 °C, more preferably 550 °C to 620 °C, in particular 570 °C to 610 °C. For a nickel alloy processed in reel- to-reel mode, the temperature of heating the metal tape is preferably 600 °C to 900 °C, in partic- ular 610 °C to 725 °C. Preferably, the heating is done in a reducing atmosphere, i.e. an atmos- phere with low oxygen content, for example in forming gas, i.e. a mixture of 95 vol-% nitrogen and 5 vol-% hydrogen. A reducing atmosphere usually avoids oxidation of the major elements of the metal alloy tape as well as majority of impurity elements in the metal alloy tape. Preferably, the heating is done in an atmosphere with low humidity, in particular in an atmosphere with a dew point below -40 °C.

Without being bound by any theory, it is believed that due to the difference in the activation en- ergy for recovery of cubic nuclei and grains of other orientation, heating the metal tape to these relatively low temperatures allows to partly revert rotation around the longitudinal direction of the metal tape, such as the rolling direction, in the regions of cube nuclei without noticeable recov- ery of dislocations in grains of other orientation and increases the degree of cube texture for relatively long heating times, in particular, for materials with small nc _B in the state right after thickness reduction of the metal tape. The heating can be done for a time period of 1 to 10 h, preferably 2 to 8 h, in particular 3 to 5 h.

According to the present invention the number density of cube-oriented grains nc _B after thickness reduction is at least 10 ¹² nr ³ , preferably at least 10 ¹³ nr ³ , more preferably at least 5 10 ¹³ nr ³ , in particular at least 10 ¹⁴ nr ³ . The value nc _B can for example be determined by electron backscatter diffraction (EBSD). From a thus obtained orientational map one can determine the number N of cube grains in a certain area A. The area density N _A is obtained by the quotient N/A. Assuming simple convex shape of cube-oriented grains with the average size <dcB>, one can assess ncB using the standard stereological relation nc _B = N _A /<dc _B >. Details of this method are for example disclosed by A. M. Gokhale in“Quantitative Characterization and Representation of Global Mi- crostructural Geometry”, ASM Handbook, 2004, volume 9: Metallography and Microstructures, pages 428-447.

During reducing the thickness of the metal tape, the volume fraction of cube texture decreases as a consequence of mechanical deformation. Upon heating the metal tape to a temperature above 40 % of its melting point in Kelvin, the cube-oriented grains grow back in expense of non- recrystallized grains of different orientation. Without being bound by any theory, it is believed that increasing the density of cube-oriented grains nc _B for a given level of overall deformation increases the speed of formation of cube texture component disfavoring other undesired texture component. In this way, the degree of cube texture can be increased.

Generally, nc _B continuously decreases with increasing deformation during the thickness reduction of the metal tape. Typically, the lower the stacking fault energy of the material is, the stronger is the decrease of nc _B during the thickness reduction of the metal tape. Therefore, in order to keep nc _B high, the thickness reduction of the metal tape in one step should be chosen to be the lower the lower the stacking fault energy of the material is. Preferably, the thickness reduction in step (a) is less than or equal to 80 %, more preferably less than 70 %, even more preferably less than 60 %, in particular less than 50 %. The thickness reduction value indicates the difference in thick- ness before and after reduction divided by the thickness before reduction. Consequently, the lower the stacking fault energy of the material is, the more often the sequence comprising steps

(a) and (b) has to be performed, preferably at least twice, more preferably at least three times, in particular at least four times. Preferably, the thickness reduction is lowered each time step (a) is performed, preferably, the thickness reduction of the last time step (a) is performed is less than 40 %, more preferably less than 35 %, in particular less than 30 %.

If the sequence containing steps (a) and (b) are performed more than once, the duration of step

(b) when it is performed for the last time depends on the density of cube-oriented grains and the degree of overall deformation. Often it is at least 15 h, more preferably at least 20 h, in particular at least 25 h. In most cases, it is sufficient that the final step (b) is performed for 60 h. If steps (a) and (b) are performed more than once, the total thickness reduction of the metal tape is often more than 80 %, preferably more than 90 %, more preferably at least 95 %, in particular at least 97 %.

Preferably, after having performed step (b) for the last time, the metal tape is heated to a temper- ature of at least 800 °C, more preferably at least 900 °C, in particular 1000 °C, for example 1050 to 1090 °C. Obviously, the temperature should be below the melting point of the material. This heating is usually done for a short period of time, preferably for 5 min to 2 h, more preferably 10 min to 1 h, in particular 20 to 40 min, such as 30 min.

The metal tapes obtained in the process according to the present invention are typically 20 to 200 pm thick, preferably 30 to 100 pm. The length is typically 1 to 1000 m, for example 100 m, the width is typically 0.4 cm to 1 m. The ratio of length to width is typically at least 100, prefera- bly at least 200, in particular at least 500.

The metal tapes are well suited for the use as a substrate for a high temperature superconduc- tor, in particular for high temperature superconductor tapes. Usually, such a tape contains a substrate, a buffer, a high-temperature superconductor layer and a stabilizer layer.

Preferably the substrate has a surface of low roughness. For this reason, surface is preferably planarized, for example by electropolishing. It is often advantageous to subject the thus planarized substrate to a thermal treatment. This thermal treatment includes heating the sub- strate to 600 to 1000 °C for 2 to 15 minutes, wherein the time refers to the time during which the substrate is at the maximum temperature. Preferably, the thermal treatment is done under re- ducing atmosphere such as a hydrogen-containing atmosphere. The planarization and/or ther- mal treatment may be repeated.

Preferably, the surface of the substrate has a roughness with rms according to DIN EN ISO 4287 and 4288 of less than 15 nm. The roughness refers to an area of 10 x 10 pm within the boundaries of a crystallite grain of the substrate surface, so that the grain boundaries of the metal substrate do not influence the specified roughness measurement.

The metal tapes according to the present invention are particularly suitable for the use as sub- strate in superconducting tapes. Therefore, the present invention further relates to the use of the metal tape according to the present invention as a substrate for a high temperature super- conductor and to a superconducting tape containing the substrate according to the present in- vention.

The superconducting tape according to the present invention often further comprises a buffer layer. The buffer layer can contain any material capable of supporting the superconductor layer. Examples of buffer layer materials include metals and metal oxides, such as silver, nickel, TbO _x , GaO _x , Ce0 ₂ , yttria-stabilized zirconia (YSZ), Y2O3, LaAIOs, SrTiC>3, Gd ₂ 03, LaNiOs, LaCuOs, SrRuOs, NdGaOs, NdAIOs and/or some nitrides as known to those skilled in the art. Preferred buffer layer materials are yttrium-stabilized zirconium oxide (YSZ); zirconates, such as gadolin- ium zirconate, lanthanum zirconate; titanates, such as strontium titanate; and simple oxides, such as cerium oxide, or magnesium oxide. More preferably the buffer layer contains lanthanum zirconate, cerium oxide, yttrium oxide, magnesium oxide, strontium titanate and/or rare-earth- metal-doped cerium oxide such as gadolinium-doped cerium oxide. Even more preferably the buffer layer contains lanthanum zirconate and/or cerium oxide. The surface of the buffer layer is preferably textured. The lattice parameters of the textured part of the buffer layer resemble the lattice parameters of the superconductor layer showing only a small mismatch to the lattice con- stant.

To enhance the degree of texture transfer and/or the efficiency as diffusion barrier, the super- conducting tape preferably contains more than one buffer layer on top of each other. Preferably the superconducting tape comprises two or three buffer layers, for example a first buffer layer comprising lanthanum zirconate and a second buffer layer containing cerium oxide.

The buffer layer preferably covers the whole surface of the substrate on one side, which means at least 95 % of the surface, more preferably at least 99 % of the surface. The buffer layer typi- cally has a thickness of 5 to 500 nm, for example 10 to 30 nm or 150 to 300 nm. The buffer layer can be made in various ways including physical deposition, such as ion beam assisted deposition (IBAD) or laser deposition, or by chemical solution deposition. If the buffer layer is made by chemical solution deposition, the buffer layer is often made in several steps such that it contains several individual layers of the same chemical composition, for example three layers of each 100 nm. Such a process is for example described in

WO 2006 / 015 819 A1. The buffer layer preferably has a low surface roughness, for example a rms according to DIN EN ISO 4287 and 4288 of less than 50 nm or even less than 30 nm.

The superconducting tape according to the present invention further comprises a superconduc- tor layer. Preferably, the superconductor layer contains a compound of the formula

RE _x Ba _y Cu ₃ 0 _7-6 . RE stands for one or more than one rare earth metal, preferably yttrium, dys- prosium, holmium, erbium, gadolinium, europium, samarium, neodymium, praseodymium, or lanthanum, in particular yttrium. An example, in which RE stands for more than one rare earth metals is RE = Yo _. 9Gdo _. 1- The index x assumes a value of 0.9 to 1.8, preferably 1.2 to 1.5. The index y assumes a value of 1.4 to 2.2, preferably 1.5 to 1.9. The index d assumes a value of 0.1 to 1.0, preferably 0.2 to 0.5. The superconductor layer preferably has a thickness of 200 nm to 5 pm, more preferably 400 nm to 3.5 pm, for example 1 to 2 pm. Preferably, the superconductor layer has crystal grains with a high degree of orientation to each other. If the superconductor layer is made by chemical solution deposition, it is often made in several steps such that it con- tains several individual layers of the same chemical composition, for example three layers of each 100 nm. Such a process is for example described in WO 2016 / 150 781 A1.

The superconductor layer preferably further contains non-conductive particles which act as pin- ning centers and can minimize the critical current density loss upon application of magnetic fields. Typical pinning centers contain Zr0 ₂ , stabilized Zr0 ₂ , Hf0 ₂ , BaZrOs, l_n ₂ Zr ₂ 07, Ce0 ₂ , Ba- Ce0 ₃ , Y ₂ 0 ₃ or RE ₂ 0 ₃ , in which RE stand for one or more rare earth metals. Usually, the parti- cles have an average diameter of 1 to 100 nm, preferably 2 to 20 nm.

The superconducting layer preferably has a low surface roughness, for example an rms accord- ing to DIN EN ISO 4287 and 4288 of less than 100 nm or even less than 50 nm. The supercon- ducting layer typically has a resistance close to zero at low temperatures, preferably up to a temperature of at least 77 K. Preferably, the superconductor layer has a critical current density without externally applied magnetic field of at least 1 10 ⁶ A/cm ² , more preferably at least 1.5 - 10 ⁶ A/cm ² . Preferably, the critical current density decreases by less than 30 % if a mag- netic field of 0.1 T is applied perpendicular to the surface of the superconductor layer, more preferably it decreases by less than 20 %. Preferably, the critical current density decreases by less than 15 % if a magnetic field of 0.1 T is applied parallel to the surface of the superconduc- tor layer, more preferably it decreases by less than 10 %.

The superconducting layer can be made in various ways, including physical vapor deposition methods such as pulsed laser deposition (PLD), sputtering or coevaporation; or chemical solu- tion deposition (CSD). Often, in particular if the superconductor layer is made by CSD, fluorine- containing precursors, such as BaF ₂ or Ba(TFA)2, wherein TFA stands for trifluoroacetate, are used in these processes. In this case, the superconducting layer often contains trace amounts of residual fluorine, for example 10 ¹⁰ to 10 ^_5 at-%.

Preferably, the superconducting tape according to the present invention further comprises a sta- bilizer layer. The stabilizer layer typically has a low electrical resistance, preferably lower than 1 pQrn at room temperature, more preferably lower than 0.2 pQrn at room temperature, in particu- lar lower than 0.05 pQrn at room temperature. Often, the stabilizer layer comprises a metal, preferably copper, silver, tin, zinc or an alloy containing one of these, in particular copper. Pref- erably, the stabilizer layer contains at least 50 at-% copper, tin or zinc, more preferably at least 70 at-%, in particular at least 85 at-%. Preferably, the stabilizer layer has a thickness of 0.1 to 50 pm, more preferably 0.5 to 20 pm, in particular 1 to 10 pm.

The stabilizer layer can be made in various ways including physical vapor deposition, chemical solution deposition, sputtering, electrodeposition, or lamination. Electrodeposition is preferred which means that the stabilizer layer is preferably an electrodeposited layer, more preferably the stabilizer layer is an electrodeposited layer on a noble metal layer. Electrodeposition of a stabilizer layer is for example described in WO 2007 / 032 207 A1.

The stabilizer layer can just overlie the superconducting layer. Preferably, the stabilizer layer co- vers the whole circumference of the tape, i.e. it overlies the superconducting layer, the substrate and at least two of the side surfaces. It is possible that the stabilizer layer has a different thick ness on the different sides of the tape or the same. If the thickness is different, the thickness ranges above refer to the side with the highest thickness. In particular if the stabilizer layer is a galvanized layer, the so called“dog-bone” effect often leads to higher thicknesses at the edges compared to flat areas.

Preferably, the superconducting tape further contains a noble metal comprising layer in between the superconductor layer and the stabilizer layer. Such a layer avoids the degradation of the su- perconductor layer when the stabilizer layer is deposited. It also increases the conductivity of the tape for the deposition of the stabilizer layer, which is particularly relevant if electrodeposi- tion is used. Typically, the noble metal comprising layer contains silver. A method of making a noble metal comprising layer on a superconducting layer is disclosed for example in WO 2008 / 000 485 A1.

Examples

Example 1

A 10 t Ni9W ingot produced by melting Ni and W in an industrial mold, was homogenized by zone melting and annealing for about 60 h at 1 180°C. Then it was multiple time hot deformed by forging at about 900-1 100°C and then hot rolled into a sheet with a thickness of 6.0 ± 0.3 mm. This metal sheet containing 8.8 ± 0.1 at-% tungsten was subjected to repeated cold rolling and thermal treatment according to the following table.

After first deformation (a) and annealing (b) step, the number density of cube-oriented grains in 3 mm thick metal tape was estimated from EBSD measurements as nc _B = (2.5 ± 0.6) 10 ¹⁴ rrr ³ . The number density of cube-oriented grains in the final (0.06 mm thick) as rolled metal tape was assessed as nc _B = (1.9 ± 0.2) 10 ¹⁴ nr ³ , whereas after the last heating step (b) the degree of cube texture was found to be 60 ± 5 %.

After subsequent annealing the metal tape at 1080 °C for 30 min, the degree of cube texture achieved in the metal tape was 99.5 ± 0.5 %. The cube texture sharpness measured by XRD is characterized by out-of-plane FWHM = 5.24 ± 0.05° (around translational direction); 9.7 ± 0.2° (around the rolling direction) and in-plane FWHM = 4.0 ± 0.1 ° (around the normal direction).

Example 2

The metal tape prepared as in Example 1 , with the difference that the last heating step (b) was done at 575°C for 245 h

After subsequent annealing the metal tape at 1080 °C for 30 min, the almost ULTIMATE degree of cube texture 99.97 ± 0.01 % was achieved in the metal tape with % Cube(<10°) = 98.0 ±0.3 %, The cube texture sharpness measured by XRD is characterized by out-of-plane FWHM = 5.20 ± 0.05° (around translational direction); 9.40 ± 0.05° (around the rolling direction) and in- plane FWHM = 3.9 ± 0.1 ° (around the normal direction).

Example 3

The metal tape prepared as in Example 1 , with the difference that the cold rolling was stopped at 0.074 mm thickness and the last heating step (b) at 605°C was done for 41 h. After subsequent annealing the metal tape at 1080 °C for 30 min, the degree of cube texture achieved in the metal tape was 95.8 ± 0.7 %.

Example 4

The metal tape prepared as in Example 3, with the difference that the last heating step (b) at 605°C was done for 88 h

After subsequent annealing the metal tape at 1080 °C for 30 min, the degree of cube texture achieved in the metal tape was 98.1 ± 0.5 %.

Example 5

The metal tapes prepared as in Examples 3, with the difference that the cold rolling was stopped at 0.1 mm thickness and the last annealing was done at 605 °C for 41 h.

After subsequent annealing the metal tapes at 1080 °C for 30 min, the degree of cube texture achieved in the metal tape was 91.8 ± 0.7 %.

Example 6

The metal tape prepared using the same thickness reduction scheme as in Example 1 , but us- ing different conditions for heating steps (b). In particular, the first step (b), which results in re- crystallization of 3 mm thick sheet without texture formation was done at 960°C, and subse- quent heating steps were done at 560°C and 600°C. As the result, EBSD measurements gave the number density of cube-oriented grains in the final (0.06 mm thick) as rolled metal tape nc _B = (0.8 ± 0.3) 10 ¹⁴ rrr ³ , whereas after the last heating step (b) the degree of cube texture was found to be 35 ± 5 %.

After subsequent annealing the metal tape at 1080 °C for 30 min, the degree of cube texture achieved in the metal tape was 95.1 ± 0.5 %. The cube texture sharpness measured by XRD is characterized by out-of-plane FWHM = 5.40 ± 0.05° (around translational direction); 9.65 ±

0.05° (around the rolling direction) and in-plane FWHM = 4.55 ± 0.08° (around the normal direc- tion).