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Title:
HIGH-TEMPERATURE SUPERCONDUCTOR TAPE WITH BUFFER HAVING CONTROLLED CARBON CONTENT
Document Type and Number:
WIPO Patent Application WO/2021/063723
Kind Code:
A1
Abstract:
Disclosed is an at least 10 m long superconductor tape comprising a substrate, a buffer layer, and a high-temperature superconductor layer, wherein the buffer layer contains 5 to 30 at% carbon. Processes for producing such a tape comprise two or three heating steps of a carbon-containing buffer layer precursor solution in different reducing and/or oxygen-containing atmospheres.

Inventors:
RIKEL MARK (DE)
BRUNKAHL OLIVER (DE)
FALTER MARTINA (DE)
BAECKER MICHAEL (DE)
Application Number:
PCT/EP2020/076288
Publication Date:
April 08, 2021
Filing Date:
September 21, 2020
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
BASF SE (DE)
International Classes:
H01L39/24; H01L39/14
Domestic Patent References:
WO2001008170A22001-02-01
WO2006015819A12006-02-16
WO2016150781A12016-09-29
Foreign References:
US20110312500A12011-12-22
US20090298698A12009-12-03
EP0830218A11998-03-25
EP1208244A12002-05-29
EP1198846A22002-04-24
EP2137330A22009-12-30
Other References:
NARAYANAN V ET AL: "X-ray Photoelectron Spectroscopy (XPS) Depth Profiling for Evaluation of La2Zr2O7 Buffer Layer Capacity", MATERIALS, vol. 5, 27 February 2012 (2012-02-27), pages 364 - 376, XP055756417, ISSN: 1996-1944, DOI: 10.3390/ma5030364
SELBMANN D ET AL: "Structural properties of epitaxial YSZ and doped CeO2 films on different substrates prepared by liquid sources MOCVD (LSMOCVD)", JOURNAL DE PHYSIQUE IV, vol. 10, no. 2, 2000, pages Pr2-27 - Pr2-33, XP000991256, ISSN: 1155-4339, DOI: 10.1051/jp4:2000204
DAWLEY J T ET AL: "Chemical solution deposition of <100>-oriented SrTiO3 buffer layers on Ni substrates", JOURNAL OF MATERIALS RESEARCH, vol. 17, no. 7, July 2002 (2002-07-01), pages 1678 - 1685, XP002602136, ISSN: 0884-2914
"PhD thesis at the technical university of Dresden", 2005, CUVILLIER VERLAG GOTTINGEN, article "Chemisch abgeschiedene Lanthanzirkonatpufferschichten auf technischen Substraten zur Realisierung von Yttriumbariumkuperoxidbandleitern"
Attorney, Agent or Firm:
BASF IP ASSOCIATION (DE)
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Claims:
Claims:

1. A superconductor tape comprising a substrate, a buffer layer and a superconductor layer, wherein the buffer layer contains 5 to 30 at.-% carbon and wherein the superconductor tape has a length of at least 10 m.

2. The superconductor tape according to claim 1 , wherein the buffer layer contains lantha num and zirconium.

3. The superconductor tape according to claim 1 or 2, wherein the buffer layer has a thick ness of 50 to 400 nm.

4. The superconductor tape according to any of the claims 1 to 3, wherein the superconduc tor tape contains at least two buffer layers.

5. The superconductor tape according to any of the claims 1 to 4, wherein the buffer has a degree of cube texture of at least 90 %.

6. The superconductor tape according to any of the claims 1 to 5, wherein the buffer layer has a lower carbon content on side facing the substrate than on the side facing the super conductor layer.

7. The superconductor tape according to any of the claims 1 to 6, wherein the superconduc tor tape has a critical current density of at least 200 A/cm width at 77 K.

8. The superconductor tape according to any of the claims 1 to 7, wherein the superconduc tor layer has a fluorine content of 0.001 to 5 at.-%.

9. An intermediate product comprising a substrate and a buffer layer, wherein the buffer lay er contains 5 to 30 at.-% carbon, wherein it does not contain more than 15 at.-% ele mental carbon.

10. A process for producing a superconductor tape comprising depositing a carbon-containing precursor solution for a buffer layer on a substrate, subjecting this deposited precursor so lution to a first heating at a first temperature of 200 to 650 °C for 0.5 to 30 min in a first atmosphere containing 1 atm or less oxygen and to a second heating to a second tem perature of 750 to 1200 °C for 5 to 90 min in a second atmosphere which is reducing, and depositing a superconductor layer on the buffer layer.

11. A process for producing a superconductor tape comprising depositing a carbon-containing precursor solution for a buffer layer on a substrate, subjecting this deposited precursor so lution to a first heating at a first temperature of 200 to 650 °C for 0.5 to 30 min in a first atmosphere, to a second heating to a second temperature of 750 to 1200 °C for 5 to 90 min in a second atmosphere, to a third heating to a temperature of 400 to 800 °C for 15 min to 25 h in an atmosphere containing an oxygen-containing compound, and depositing a superconductor layer on the buffer layer.

12. The process according to claim 10 or 11 , wherein depositing the superconductor layer is performed by chemical solution deposition.

13. The process according to any of the claims 10 to 12, wherein the process is performed on a reel-to-reel apparatus.

14. The process according to any of the claims 10 to 13, wherein the precursor solution con- tains a carboxylic acid and/or a metal carboxylate.

15. The process according to any of the claims 10 to 14, wherein the sequence comprising depositing the precursor solution and exposing it to the first and the second heating is per formed at least twice.

Description:
HIGH-TEMPERATURE SUPERCONDUCTOR TAPE WITH BUFFER HAVING CONTROLLED CARBON CONTENT

Description

The present invention is in the field of buffers for high-temperature superconductor tapes. High- temperature superconductors have been discovered in the 1980ies, but their commercial use is still limited. One reason is that it remains a challenge to produce superconductors of high quality and performance on a large scale at low price. For high-temperature superconductors of the second generation the superconducting material is deposited on tape-shaped substrates, wherein the substrate and the superconductor are separated by a buffer layer. A promising path towards low-cost production is the use of chemical solution deposition (CSD) not only for the superconducting material, but also for the buffer layer.

WO 2006 / 015819 A1 discloses a process to produce buffer layers for superconductor tapes via chemical solution deposition. Kerstin Knoth has analyzed this process in her PhD thesis at the technical university of Dresden in 2005 which is published by Cuvillier Verlag Gottingen with the title “Chemisch abgeschiedene Lanthanzirkonatpufferschichten auf technischen Substraten zur Realisierung von Yttriumbariumkuperoxidbandleitern” (ISBN 3-86537-888-9). For small samples under highly controlled laboratory conditions, superconductor tapes with high perfor mance can be obtained. However, on larger production facilities, superconductive tape of suffi cient and sufficiently uniform quality can hardly be obtained with this process. This is usually because higher carbon contents lead to peeling off of the layers on top during subsequent heat ing.

It was therefore an object of the present invention to provide a superconductor tape on large scale at high production speed. The superconductor tape was aimed to have a high critical cur rent density with low variation along the tape length. The superconductor tape should be ob tained efficiently in terms of time, space, apparatus requirements, and energy consumption in order to enable low-cost production.

These objects were achieved by a superconductor tape comprising a substrate, a buffer layer and a superconductor layer, wherein the buffer layer contains 5 to 30 at.-% carbon and wherein the superconductor tape has a length of at least 10 m.

The invention further relates to an intermediate product comprising a substrate and a buffer lay er, wherein the buffer layer contains 5 to 30 at.-% carbon, wherein it does not contain more than 15 at.-% elemental carbon.

The invention further relates to a process for producing a superconductor tape comprising de positing a carbon-containing precursor solution for a buffer layer on a substrate, subjecting this deposited precursor solution to a first heating at a first temperature of 200 to 650 °C for 0.5 to 30 min in a first atmosphere containing 1 atm or less oxygen and to a second heating to a sec ond temperature of 750 to 1200 °C for 5 to 90 min in a second atmosphere which is reducing, and depositing a superconductor layer on the buffer layer. The invention further relates to a process for producing a superconductor tape comprising de positing a carbon-containing precursor solution for a buffer layer on a substrate, subjecting this deposited precursor solution to a first heating at a first temperature of 200 to 650 °C for 0.5 to 30 min in a first atmosphere, to a second heating to a second temperature of 750 to 1200 °C for 5 to 90 min in a second atmosphere, to a third heating to a temperature of 400 to 800 °C for 15 min to 25 h in an atmosphere containing an oxygen-containing compound, and depositing a superconductor layer on the buffer layer.

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.

According to the present invention, the superconductor tape contains a substrate. The substrate may be any material capable of supporting buffer and/or superconducting layers. For example, suitable substrates are disclosed in EP 830218, EP 1 208 244, EP 1 198 846, EP 2 137 330. Often, the substrate is a metal and/or alloy strip/tape, whereby the metal and/or alloy may be nickel, silver, copper, zinc, aluminum, iron, chromium, vanadium, palladium, molybdenum, tung sten and/or their alloys. Preferably the substrate contains nickel. More preferably, the substrate contains nickel and 1 to 10 at-%, in particular 3 to 9 at-%, tungsten. Laminated metal tapes, tapes coated with a second metal like galvanic coating or any other multi-material tape with a suitable surface can also be used as substrate.

The substrate can be textured or non-textured, preferably it is textured, i.e. it has a textured sur face, in particular a cube textured surface with a degree of cube texture of at least 90 %, such as at least 95 %. 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 °. The substrates are typically 20 to 200 pm thick, preferably 40 to 100 pm. The length of the substrate usually substantially resembles the length of the superconductor tape.

The surface of the substrate can have a roughness in a relatively wide range, for example with root-mean-squared roughness (rms) according to DIN EN ISO 4287 and 4288 of 5 to 50 nm, such as 10 to 30 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. However, it is also possible to use rougher and thus less expensive substrates. In this case, the buffer layer can serve as planarization lay er.

According to the present invention the superconductor tape contains a buffer layer. In the con text of the present invention, a buffer layer is any layer between the substrate and the super conductor layer which serves to planarize the substrate and/or separates the substrate from the superconductor layer against diffusion of atoms from the one to the other and/or transfers tex ture from the substrate to the superconductor layer.

It is possible that the superconductor tape contains one buffer layer or more than one buffer layer, preferably, the superconductor tape contains more than one buffer layer, for example two, three or four buffer layers, in particular two buffer layers. If the superconductor tape contains more than one buffer layer, at least one of the buffer layers has the characteristics described herein, preferably the buffer layer in closest proximity to the substrate has the characteristics described herein, in particular all buffer layers have the characteristics described herein.

Preferably, the buffer layer contains a metal oxide, such as TbO x , GaO x , CeC>2, yttria-stabilized zirconia (YSZ), Y 2 0 3 , LaAIOs, La 2 Zr 2 0 7 , SrTiOs, CaTiOs, Gd 2 0 3 , LaNi0 3 , l_aCu0 3 , SrRu0 3 , NdGa0 3 , NdAI0 3 , CeGdO and/or some nitrides as known to those skilled in the art. Preferably, the buffer layer contains at least one rare earth metal, i.e. Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, preferably the buffer layer contains an oxide containing at least one rare earth metal. Preferred buffer layer materials are yttrium-stabilized zirconium oxide (YSZ); zirconates, such as gadolinium zirconate, lanthanum zirconate; titanates, such as strontium ti- tanate; 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. In the context of the present invention, a region of substantially the same composition is regarded as one buffer layer, even though it may be made by stacking several layers of about the same composition.

The buffer layer according to the present invention contains 5 to 30 at.-% carbon, preferably 8 to 25 at.-%, in particular 10 to 20 at.-%, such as 12 to 16 at.-%. The carbon content relates to all atoms of that particular buffer layer. The carbon content can be measured for example by secondary neutral mass spectrometry, wherein an area of at least 0.5 mm 2 is subject to analysis in order to obtain a sufficiently averaged value. The carbon can be concentrated in carbon-rich pores within the buffer layer. The carbon-rich pores can have a weight-average diameter of 10 to 50 nm. The carbon-rich pores can have a facetted form.

Before the superconductor layer is deposited, i.e. in the intermediate product according to the present invention, the buffer layer contains not more than 15 at.-% elemental carbon, more preferably 0.1 to 12 at.-%, in particular 0.5 to 8 at.-%, such as 1 to 5 at.-%. It has surprisingly been found out that the final tape has a lower tendency to peel off even at high total carbon contents if the intermediate product had such a low content of elemental carbon. The carbon content relates to all atoms of that particular buffer layer. In the context of the present invention, elemental carbon is any carbon in the oxidation state 0, for example carbon black, graphite, graphene, carbon nanotubes or fullerenes. The content of carbon and of elemental carbon can be measured by combining Raman spectroscopy and secondary neutrals mass spectrometry (SNMS) or Rutherford back-scattering (RBS) measurements.

Preferably, the buffer layer has a thickness of 50 to 400 nm, more preferably 100 to 350 nm, in particular 150 to 300 nm. 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. Preferably, the surface of the buffer layer facing towards the superconductor layer has a low roughness such as a rms of less than 50 nm, more preferably less than 30 nm, in particular less than 20 nm. The surface of the buffer layer is preferably textured, in particular cube-textured. The degree of texture is preferably at least 90 %, more preferably at least 96 %, in particular at least 98 %.

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 °.

The superconductor tape according to the present invention further contains a superconductor layer. Preferably, the superconductor layer contains a compound of the formula RE x Ba y Cu3C>7-6. RE stands for one or more than one rare earth metal, preferably yttrium, dysprosium, 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 .9 Gdo .i . 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. The superconductor layer can cover the complete buffer layer or only parts of it, for example the superconductor layer has a lower dimension in the width direction of the tape, for example 100 mm, 40 mm, 12 mm, 10 mm or 4 mm.

Preferably, the superconductor layer contains residual amounts of fluorine arising from the precursor composition. The fluorine content of the superconductor layer can be 0.001 to 5 at.-%, preferably 0.01 to 2 at.-%, in particular 0.05 to 1 at.-%.

The superconductor layer preferably further contains non-conductive particles which act as pinning centers and can minimize the critical current density loss upon application of magnetic fields. Typical pinning centers contain ZrC>2, stabilized ZrC>2, HfC>2, Ta20s, SrTiCh, BaZrCh, BaHfCh, La2Zr2C>7, CeC>2, BaCeCh, Y2O3 or RE2O3, in which RE stand for one or more rare earth metals. Usually, the particles 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 a root-mean- squared roughness (rms) according to DIN EN ISO 4287 and 4288 of less than 150 nm or even less than 100 nm, such as less than 50 nm. The superconducting 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 1.0 MA/cm 2 , more preferably at least 1.5 MA/cm 2 . Preferably, the critical current density decreases by less than 30 % if a magnetic 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 superconductor layer, more preferably it decreases by less than 10 %. Preferably, the superconductor layer has a critical current of at least 200 A/cm width, more preferably at least 300 A/cm width, in particular at least 500 A/cm width.

According to the present invention, the superconductor tape has a length of at least 10 m, preferably at least 50 m, more preferably at least 100 m, in particular at least 200 m. Usually, the superconductor tape has a length of not more than 1 km. The superconductor tape can have various widths, for example 4 mm, 10 mm, or 12 mm. During production, however, the tapes are usually wider, preferably 10 to 200 mm, more preferably 15 to 150 mm, in particular 39 to 110 mm. The final widths is obtained by slitting these tapes.

Preferably, the superconductor tape further contains a stabilizer layer. The stabilizer layer typi cally has a low electrical resistance, preferably lower than 1 pQm at room temperature, more preferably lower than 0.2 pQm at room temperature, in particular lower than 0.05 pQm at room temperature. Often, the stabilizer layer comprises a metal, preferably copper, silver, tin, zinc, stainless steel or an alloy containing one of these such as brass, in particular copper. Alterna tively, conductive polymers can also be used as stabilizor layer. Preferably, the stabilizer layer contains at least 50 at-% copper, tin or zinc, more preferably at least 65 at-%, in particular at least 85 at-%. It is possible that the stabilizer layer has a uniform composition or that the stabi lizer layer has locally different compositions, for example on one side of the tape the composi tion is different to the other side of the tape. 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 just overlie the superconducting layer. Preferably, the stabilizer layer covers the whole circumference of the tape, i.e. it overlies the superconducting layer, the sub strate and at least two of the side surfaces. It is possible that the stabilizer layer has a different thickness on the different sides of the tape or the same. If the thickness is different, the thick ness ranges above refer to the average thickness. In particular, if the stabilizer layer is a galva nized layer, the so called “dog-bone” effect often leads to higher thicknesses at the edges com pared to flat areas. In this case, the thickness at the edges is not included into the calculation of the average thickness.

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 superconductor layer when the stabilizer layer is deposited. It also increases the conductivity of the tape for the deposition of the stabilizer layer, which is useful if electrodeposition is used. Typically, the noble metal comprising layer contains silver. Silver has the additional advantage that it can act as catalyst for increasing the oxygen content of the superconductor layer.

The superconducting tape according to the present invention can be made in various ways. Two processes are particularly useful and are thus described hereinafter. The first process is aimed to reduce the carbon content of the buffer layer by removing a large amount of carbon while the second process is aimed at converting elemental carbon into oxidized forms of carbon thereby reducing the amount of elemental carbon before the superconductor layer is deposited.

The first process comprises depositing a carbon-containing precursor solution for a buffer layer on a substrate. The carbon-containing precursor solution preferably contains at least one or- ganometallic salt, for example a metal carboxylate such as a metal acetate or propionate, or a metal acetylacetonate. Preferably the carbon-containing precursor solution contains a solvent, more preferably a carboxylic acid, in particular propionic acid. Depending on the type of organ- ometallic salt, the concentration of the organometallic salt in the carbon-containing precursor solution varies from 1 mmol/l to 1 mol/l, preferably 10 mmol/l to 500 mmol/l, for example 20 to 50 mmol/l or 200 to 300 mmol/l. The deposition of the carbon-containing precursor solution can be achieved in various ways, for example by inkjet printing, roller coating or slot die coating.

The substrate on which the carbon-containing precursor solution is deposited is preferably wider than the substrate in the final superconductor tape. This allows faster and thus more cost- effective production. Preferably the substrate on which the carbon-containing precursor solution is deposited has a width of 20 to 100 mm, more preferably 30 to 50 mm, for example 40 mm. The desired width of the superconductor tape is obtained later by slitting.

According to the present invention the deposited precursor solution is subjected to a first heat ing at a first temperature of 200 to 650 °C, preferably 220 to 450 °C, in particular 250 to 400 °C. The temperature of the first heating can be constant over time or it can be varied. Typically, it is varied by slowly heating up and keeping the temperature a certain time at the maximum fol lowed by quick cooling down. The first temperature thus is the maximum temperature which is reached during the first heating.

The first heating is performed for 0.5 to 30 min, preferably 1 to 20 min, in particular 2 to 10 min. This time period refers to the time at which the deposited precursor solution is exposed to a temperature of at least 140 °C. The first heating is performed in a first atmosphere containing 10 9 to 1 atm oxygen, preferably 1 to 1000 ppm, in particular 5 to 50 ppm. Without being bound by any theory it is believed that within the temperature range described above this oxygen con centration is sufficiently high to suppress or reduce the formation of elemental carbon and at the same time sufficiently low to avoid oxidation of the substrate and suppress formation of unde sired phases in the buffer layer. Previously, the first heating was done in an atmosphere of ni trogen and hydrogen which has a significantly lower oxygen content than 0.1 ppm. An atmos phere according to the present invention can for example be made by evaporating liquid nitro gen which usually contains about 10 ppm oxygen.

According to the present invention a second heating to a second temperature of 750 to 1200 °C, preferably 900 to 1100 °C is performed. The temperature of the second heating can be constant over time or it can be varied. Typically, it is varied by slowly heating up and keeping the temper ature a certain time at the maximum followed by quick cooling down. The second temperature thus is the maximum temperature which is reached during the second heating.

The second heating is performed for 5 to 90 min, preferably 15 to 75 min, in particular 30 to 60 min. This time period refers to the time at which the deposited buffer precursor solution is exposed to a temperature of at least 600 °C. The second heating is performed in a second at mosphere which is reducing. Usually, a mixture of an inert gas, such as nitrogen or argon, with hydrogen is used. The hydrogen content is preferably 1 to 10 vol.-%, in particular 3 to 8 vol.-%. Preferably, the during the second heating, the gas atmosphere moves parallel to the supercon ductor tape in the opposite direction with regard to the movement of the superconductor tape. More preferably, the during the second heating, the gas atmosphere moves parallel to the su perconductor tape at a speed sufficient to exchange the whole atmosphere in the heating zone at least twice during the time period of the second heating, even more preferably at least three times, in particular 3.5 times. For example, if the heating zone has an area perpendicular to the movement of the superconducting tape of 500 cm 2 and the second heating takes 60 min, the flow of the second atmosphere through the heating zone can be 10 to 500 l/(h dm).

Depending on the desired thickness of the buffer layer, the sequence comprising depositing the precursor solution and exposing it to the first and the second heating can be performed more than once, for example at least twice or at least three times. Each sequence can be performed using the same set of parameters or with different ones. If different ones are chosen, the chemi cal composition typically varies between the parts of the buffer layer made by performing each sequence. For example, the first time the sequence is performed, the first heating is done in an atmosphere with a lower oxygen content then the subsequent times the sequence is performed. In this way, the buffer layer may have a lower carbon content on the side facing the substrate than on the side facing the superconductor layer.

If the superconductor tape comprises more than one buffer layer, at least one of these buffer layers is made as described above, preferably the buffer layer in closest proximity to the sub strate is made as described above, in particular all buffer layers are made as described above.

In some cases, for example if the buffer layer on the substrate is not textured, one buffer layer having cubic texture is added by a different process, for example by ion-beam assisted deposi tion (I BAD) or inclined substrate deposition (ISD).

According to the present invention a superconductor layer is deposited on the buffer layer. Pref erably, the superconductor layer is deposited in a chemical solution deposition process. Prefer ably, an ink containing a rare earth-containing compound, a barium-containing compound, a copper-containing compound and an alcohol are deposited and heated as for example describe in WO 2016 / 150 781 A1.

The second process differs from the first process in that it does not require a certain atmos phere during the first heating, thus for example a reducing atmosphere such as nitrogen con taining 5 vol.-% hydrogen can be used as well. The second process comprises a third heating to a temperature of 400 to 800 °C, preferably 500 to 750 °C, in particular 600 to 700 °C. The tem perature of the third heating can be constant over time or it can be varied. Typically, it is varied by slowly heating up and keeping the temperature a certain time at the maximum followed by quick cooling down. The first temperature thus is the maximum temperature which is reached during the first heating.

The third heating is performed for 15 min to 25 h, preferably 20 min to 5 h, in particular 20 min to 2 h. This time period refers to the time at which the deposited precursor solution is exposed to a temperature of at least 140 °C. The third heating is performed in an atmosphere containing an oxygen-containing compound. Generally, any oxygen-containing compound which can oxi dized elemental carbon under the given conditions is suitable. Preferred examples are CO, CO2, or mixtures of water with nitrogen and/or hydrogen.

The processes according to the present invention are preferably performed on a reel-to-reel apparatus. As the process of the present invention is very robust, the superconducting tape can be made at high speeds, such as 50 to 200 m/s depending on the size of the ovens. Examples

Example 1

A 40 mm wide nickel substrate containing 5 at.-% Wwas coated with 15.25 ml/m 2 of a propionic acid solution containing 0.33 mol/l La(OOCCH2CH3)3 and 0.38 mol/l Zr(OOCCH2CH3)4. The coated substrate was heated to 300 °C for 3 min in nitrogen containing 5 vol.-% hydrogen, wherein the oxygen content is less than 10 19 ppm. Afterwards, it was heated for 30 min in a temperature gradient from 750 °C to 1000 °C in nitrogen containing 5 vol-% hydrogen. The thus obtained lanthanum zirconate buffer layer had a carbon content of 5 at-%. The gas exchange rate in the furnace was about 2 h -1 . The preceding procedure was repeated twice resulting in a lanthanum zirconate buffer layer of 180 nm thickness. The carbon content was analyzed by SNMS and Raman. The LZO film contained 14 at-% of elemental carbon. No oxidized carbon species could be detected.

On the lanthanum zirconate buffer layer a cerium oxide buffer layer and thereon a layer of YBa2Cu307- x is deposited by chemical solution deposition including a heating step to a tempera ture of up to 800 °C in an atmosphere containing about 0.4 mbar oxygen. The inductively measured critical current density at 78 K was 2.3 MA/cm 2 .

Example 2

Example 1 was repeated by replacing the Nitrogen and 5 vol.-% H2 mixture by nitrogen which contained naturally 10 ppm of oxygen. The lanthanum zirconate buffer layer as obtained after the second heating had content of elemental carbon of 3 at.-%. The final superconductor tape also did not delaminate after the YBCO process and the current density was 1.1 MA/cm 2 .

Example 3

Example 2 was repeated by reducing the time for the second heating from 30 min to 15 min. The lanthanum zirconate buffer layer had a carbon content of 18 at-%, all of which was in the form of elemental carbon. Delamination of YBa2Cu3C>7 and buffer layers was observed after YBCO processing, so the critical current could not be measured in a meaningful way.

Example 4

The tape obtained in example 3 before deposition of a superconductor layer was subject to a third heating for 60 min at 800 °C in an atmosphere of nitrogen containing 5 vol.-% hydrogen and water. The water content was 5 10 -9 bar. After this third heating, the elemental carbon content was below 15 at.-%.