Precursor solutions suitable for the preparation of high-performance epitaxial REBa2Cu3O7-x superconductors

Abstract

The present invention relates to a REBCO precursor solution comprising propionates of Re, Ba, and Cu; an amine; and a solvent system, wherein the metals RE, Ba, and Cu are present only as propionates, which are the only salts present in the solution; the total metal concentration is 1-2M in the solution; the solvent system is a mixture of alcohol (C1-C4):propionic acid in a ratio of 20:80-60:40, the amine is dissolved in the solvent system, and is present at a concentration of 1-8% by volume of the total volume of the solution; the amine:copper molar ratio is 0.3:1-2:1; and the solution is fluorine- and acetate-free. These solutions can contain nanoparticles and are useful for preparing superconducting REBCO (or YBCO) layers.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of European Patent Application No. EP22382741, filed July 29, 2022.

[0002] FIELD OF THE INVENTION The present invention relates to the field of high temperature superconducting materials, and in particular to the synthesis of REBa2Cu3O for the preparation of such superconducting materials. 7-x (RE=rare earth or Y) precursor solutions. [Background technology]

[0003] The development of alternative energy sources has made great strides over the past few decades, driven by the need to reduce the environmental impact of the long-standing use of fossil fuel energy sources. Superconducting materials offer new opportunities to solve the problem of efficient electrical transport due to their unique property of transporting large currents without any losses. The discovery of high-temperature superconductors (HTS) has taken this technology even further, although there have been many challenges to overcome.

[0004] Currently, HTS is a promising candidate for a variety of applications, including clean power energy devices such as power cables, fault current limiters, and transformers, as well as devices operating under very high magnetic fields (accelerators, NMR, fusion reactors) and transportation (ships, levitated trains, electric aircraft).

[0005] After the discovery of HTS, REBa2Cu3O 7-xThe widespread adoption of superconductivity was delayed by the need to develop manufacturing methods for REBCO materials (REBCO materials, where RE is yttrium or a rare earth element, x<0.1). The introduction of high thermal conductivity superconductors (HTS) in flexible tape structures, particularly coated conductors (CC), enabled industrial applications, and production scaled up, with hundreds of meters of length successfully produced at current capacities of 200–700 A per cm-wide tape. CC requires the adoption of thin-film technology for applications spanning kilometers in length, requiring the deposition of epitaxial multilayers on long, flexible metal substrates. However, the manufacturing costs of available technologies remain prohibitive. Common techniques for growing HTS layers include pulsed laser deposition (PLD), metalorganic chemical vapor deposition (MOCVD), evaporation, and trifluoroacetic acid metalorganic decomposition-chemical solution deposition (TFA-CSD), which often imply expensive vacuum equipment and / or slow diffusion rates.

[0006] Recently, a chemical-based non-equilibrium process called transient liquid-assisted growth (TLAG) has been gaining attention as a low-cost method for the fabrication of YBCO superconductors. This method enables growth rates two orders of magnitude higher than existing techniques (see L. Soler et al.; Nat. Commun., 2020, vol. 11, p. 344). It is based on the formation of a transient Ba-Cu-O liquid containing dissolved Y ions, where the epitaxial growth of the YBCO layer is kinetically dominated by a fast atomic diffusion process toward the substrate growth surface. For this purpose, it was necessary to overcome the removal of BaCO3 (an intermediate compound in the formation of YBCO, the removal of which is the rate-limiting step of the reaction) at low temperatures (see P. Vermeir et al.; "Elucidation of the Mechanism in Fluorine-Free Prepared YBa2Cu3O7-δ Coatings" Inorg. Chem., 2010, vol. 49, pp. 4471-4477). However, environmentally friendly non-fluorinated precursors were developed for the preparation of YBCO solutions.

[0007] The use of yttrium, barium, and copper acetates dissolved in HProp-based media as precursors for preparing fluorine-free YBCO precursor solutions is known in the art (see P. Vermeir et al., “Influence of sintering conditions in the preparation of acetate-based fluorine-free CSD YBCO films using a direct sintering method,” Mater. Res. Bull., 2012, vol. 47, pp. 4376-4382; and Yue Zhao et al., “Growth of Highly Epitaxial YBa2Cu3O7-δ Films from a Simple Propionate-Based Solution,” Inorg. Chem. 2015, vol. 54, pp. 10232-10238). However, in the case of barium acetate in particular, the acetate salt may not be completely converted to propionate salt (see S. Rasi et al., "Relevance of the Formation of Intermediate Non-Equilibrium Phases in YBaCuO during Growth by Transient Liquid Assisted Growth," The Journal of Physical Chemistry, 2020, Vol. 124, pp. 15574-15584). The presence of a mixture of products in the solution, especially acetate salts, may lead to different decomposition pathways, preventing optimal and reproducible results for the final pyrolyzed film.

[0008] M. Nasui et al. have disclosed that F-free REBCO solutions can be obtained by mixing previously prepared solutions of Y, Ba, and Cu propionates and adding a chelating agent (glycerol). Because the corresponding metal acetates are used as reagents to prepare such propionate solutions of Y, Ba, and Cu, the final REBCO solution may still contain traces of barium acetate (M. Nasui et al.; "Fluorine-free propionate route for the chemical solution deposition of YBa2Cu3O 7-x "superconducting films"; Ceramics International 2015, vol. 41, pp. 4416-4421). In this case too, the removal of acetic acid present in the propionic acid solution is not guaranteed.

[0009] As disclosed by L. Soler et al., a fluorine-free YBCO solution can be used in combination with a transient liquid-assisted growth process and chemical solution deposition to achieve growth rates of over 100 nm / s. The YBCO solution is prepared from the corresponding acetate salt at a concentration of 1.5 M using triethylamine (see L. Soler et al., "Ultrafast transient liquid-assisted growth of high current density superconducting films," Nat. Commun., 2020, vol. 11, p. 344).

[0010] Finally, CN106242553A discloses the preparation of a fluorine-free precursor solution by dissolving rare earth (RE) propionate, barium propionate, and copper propionate in an atomic ratio of RE:Ba:Cu=1:1.75:3 in a mixed solvent of propionic acid and ethanol at a temperature of 40°C to 100°C to obtain a fluorine-free precursor solution with a total cation molar concentration of 1.5 mol / L. This document teaches that the produced BaCO3 can be removed by introducing some fluorine precursor, thus converting this route into a fluorine-based route, but this method is outside the scope of this patent.

[0011] However, despite what is disclosed in the art, there remains a need to provide a stable solution of yttrium, barium, and copper propionates for TLAG-CSD growth of thick epitaxial YBCO superconducting films and that is free of residual acetate as a result of inefficient conversion of the original acetate to propionate salts in the YBCO precursor solution. Summary of the Invention

[0012] The present inventors have developed a novel type of fluorine-free solution based on fluorine-free pure metal propionates of yttrium, barium, and copper by introducing an amine, such as monoethanolamine (MEA), as an additive. This solution is suitable for the transient liquid-assisted growth-chemical solution deposition (TLAG-CSD) growth of thick REBCO superconducting epitaxial thin films to obtain reproducible results. The fluorine-free solution meets the requirements of green chemistry. The use of amines not only promotes the dissolution of Cu(Prop)2 through the formation of Cu-amine complexes, thereby increasing solubility, but also advantageously increases the thickness of the pyrolyzed layer compared to solutions without amines. The selection of amines and amine content was useful for obtaining nanoscale homogeneous pyrolyzed layers with the properties required for TLAG. Furthermore, a stable solution was obtained, achieving nanoscale chemical and microstructural homogeneity in the pyrolyzed film, which was useful for further growth of TLAG.

[0013] Therefore, a first aspect of the present invention relates to a REBCO precursor solution comprising RE propionate, Ba propionate, Cu propionate, an amine, and a solvent system, wherein RE means either Y or an earth metal, and the metals RE, Ba, and Cu are present only in the form of propionate; the propionate is the only salt present in the solution; the metals, in total, are at a molar concentration in the range of 1 to 2 M of the solution; the solvent system is a mixed solvent system of alcohol (C1-C4):propionic (Hprop) acid in a volume ratio of 20:80 to 60:40 by volume; the amine is miscible in the solvent system at room temperature and is present at a volume concentration of 1 to 8% of the total volume of the solution; the molar ratio of amine:copper is 0.3:1 to 2:1, and the solution does not contain F and acetic acid.

[0014] The method employed for the synthesis of yttrium, barium, and copper propionates successfully eliminates the possibility of product mixtures resulting from incomplete conversion of acetate precursors in solvent mixtures through a robust and scalable process performed with high efficiency. Furthermore, the synthetic method is easy to perform, provides high-purity products in high yield, and most importantly, is extremely cost-effective compared to commercially available acetate precursors.

[0015] Therefore, a second aspect of the present invention relates to a method for preparing the above-mentioned REBCO precursor solution, comprising the steps of: a) preparing a mixture of (C1-C4) alcohol:propionic acid in a volume ratio ranging from 20:80 to 60:40; b) successively adding barium propionate, copper propionate, and RE propionate to the solvent mixture, each propionate being added after the previous one has dissolved, and the amount of propionate is such that the total metal concentration of the propionates is in the range of 1 to 2 M; and c) adding an amine in a concentration of 1 to 8% relative to the total volume of the solution, with the amine:copper molar ratio ranging from 0.3:1 to 2:1.

[0016] REBCO solutions not only allow for the acquisition of homogeneous layers but also the preparation of multilayer superconducting films on substrates. During the epitaxial growth of superconducting layers in the TLAG framework, ultrafast growth rates of 100 nm / s to 2500 nm / s can be achieved using transient liquids. The TLAG-CSD process used is based on a stable solution of RE(Y), Ba, and Cu propionates in the appropriate stoichiometric ratio. This solution's stability allows the use of high-performance printing deposition methods such as inkjet printing and slot-die coating. Furthermore, the use of amine additives in the precursor solution allows for high concentrations, resulting in multilayer depositions with thicknesses of up to 2.5 μm. This facilitates epitaxial growth via the TLAG process without interfering with the highly homogeneous results of thermal decomposition methods. The thickness is related to the solution's concentration and viscosity.

[0017] Therefore, a third aspect of the present invention is the use of a REBCO precursor solution for preparing a superconducting REBCO layer having a thickness of 0.1 to 2.5 μm.

[0018] Another aspect of the present invention relates to a method for preparing a superconductor by growing a superconducting REBCO layer with a thickness of 0.1 to 2.5 μm, the method comprising the following steps: a) depositing the precursor solution defined above on the surface of a suitable substrate, for example, a metal or single crystal having a deposited biaxially oriented oxide layer (buffer layer), by any method that can uniformly control the film thickness to form a precursor film; b) subjecting the precursor film to a pyrolysis process by heat treatment in a controlled atmosphere; c) repeating steps a) and b) as necessary to obtain a thicker pyrolyzed film in a multilayer deposition process; d) subjecting the deposited REBCO layer of step b) or the REBCO multilayer of step c) to a TLAG process to grow a final epitaxial REBCO layer; and e) subjecting the superconductor thus obtained to an oxygenation process. Thus, the superconductor according to the present invention is a superconducting layer deposited in multiple layers on a metal substrate having a biaxially oriented oxide layer.

[0019] Finally, another aspect of the invention relates to a superconducting layer deposited in a multilayer on a suitable substrate, for example a biaxially textured substrate, obtained by the process described above. This multi-conductor multilayer substrate is an epitaxial copper oxide superconducting film, more precisely with a critical current density of 2-4 MA / cm at 77 K. 2 , especially the critical current density at 77K is 2.5MA / cm 2 The thickness is over 500 nm, and therefore it is a high performance REBCO superconducting layer. [Brief explanation of the drawings]

[0020] [Figure 1] ATR FT-IR of Cu(Prop)2. [Figure 2] High-resolution XRD of Cu(Prop)2. [Figure 3] ATR FT-IR of Ba(Prop)2. [Figure 4] High-resolution XRD of Ba(Prop)2. [Figure 5] ATR FT-IR of Y(Prop)3. [Figure 6] High-resolution XRD of Y(Prop)3. [Figure 7] ATR FT-IR of Gd(Prop)3. [Figure 8] High-resolution XRD of Gd(Prop)3. [Figure 9] (a) Viscosity measurements of 1 M and 1.75 M (3:7) solutions with varying amounts of MEA. (b) HO wt% release over time for (3:7) solutions with and without MEA measured using Karl Fischer titration. [Figure 10] Optical microscopy (OM) images and XRD patterns of pyrolyzed samples are shown, including two layers of (a, b) a (2:3) composition (a YBCO-stoichiometric mixture with a Y-Ba-Cu ratio of 1:2:3) using a 1.75M + 4.3% v / v MEA solution; (c, d) a (3:7) composition (a Cu-rich mixture with a Y-Ba-Cu ratio of 1:2:4.66) using a 1.75M + 4% v / v MEA solution; (e, f) a (4:11) composition (a Cu-rich mixture with an excess of Y-Ba-Cu ratio of 1:2:5.5) using a 1.75M + 4% v / v MEA solution; and one layer of (g, h) a (3:7) composition using an excess of MEA, in this case a 1.75M + 8% v / v MEA solution. [Figure 11] Low-magnification STEM-HAADF cross-sectional images of as-pyrolyzed thin films deposited using a 1.75M + 4% v / v MEA solution (3:7) are shown. (a) 2-layer, (b) 4-layer, and (c) 8-layer. Zoomed-in STEM-HAADF images show similar thicknesses of the individual layers in (d) 2-layer, (e) 4-layer, and (f) 8-layer. (g-i): Pore density analysis performed using Image J from the red rectangular areas is shown in (d-f), respectively, with pores colored red for quantification. Histograms of pore area are used to calculate the average pore diameter. [Figure 12](a) STEM-HAADF image of a bilayer (3:7) pyrolyzed film deposited using a 1.75M+4% v / v MEA solution. (b) Elemental analysis EELS maps of the Cu-L edge, (c) Ba-M edge, and (d) OK edge. (e) Composite EELS map of Cu (blue) and Ba (yellow) from the red rectangular area in (a). (f-h) High-resolution TEM images collected from different regions of the pyrolyzed film, showing the presence of individual size and precursor phases in the same area, indicating a uniform distribution. [Figure 13] STEM-EDX cross-sectional elemental maps of Cu, Y, and Ba. [Figure 14] a) TEM images of BaZrO3 nanoparticles redispersed in ethanol, b) TEM images of BaZrO3 nanoparticles in YBCO nanocomposite precursor solution, c) Solution XRD of BaZrO3 nanoparticles redispersed in YBCO precursor solvents of MEA, ethanol, methanol, and propionic acid. DETAILED DESCRIPTION OF THE INVENTION

[0021] All terms used herein are to be understood in their ordinary meaning as known in the art unless otherwise specified. Other more specific definitions of terms used herein are as defined below and are intended to be applied consistently throughout the specification and claims unless a broader definition is provided where otherwise expressly defined.

[0022] The term "room temperature" refers to a temperature of about 20°C to about 25°C.

[0023] The terms "pyrolysis" and "carbonization" are used interchangeably herein.

[0024] The term "REBCO" includes "YBCO" and whenever RE is used it refers to both the rare earth elements and Y. REBa2Cu3O, also known as REBCO material 7-x is a material in which RE is yttrium (Y) or a rare earth element and x is less than 0.1.

[0025] The preparation of REBCO, and especially YBCO, precursor solutions follows the same procedure, regardless of the solution stoichiometry. Solutions of various stoichiometries have different RE-Ba-Cu molar ratios and are named after the Ba-Cu molar ratio of the transient liquid formed during the transient liquid-assisted growth (TLAG) process: a REBCO stoichiometric mixture with an RE-Ba-Cu molar ratio of 1:2:3 ((2:3) composition); a Cu-rich mixture with an RE-Ba-Cu molar ratio of 1:2:4.66 ((3:7) composition); and an excessively Cu-rich composition with an RE-Ba-Cu molar ratio of 1:2:5.5 ((4:11) composition).

[0026] Please note that as used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0027] Specifically, the YBCO precursor solution of the present invention, REBCO, comprises RE(Y) propionate, Ba propionate, Cu propionate, an amine, and a solvent system, in which the metal RE(Y), Ba, and Cu are present only in the propionate form; the metals are present in a total metal concentration of 1-2 M in the solution; the solvent system is a mixture of alcohols (C1-C4):propionic (Hprop) acid in a volume ratio of 20:80 to 60:40; the amine is miscible in the solvent system and present at a concentration of 1-8% of the total solution volume; the amine:copper molar ratio is 0.3:1 to 2:1; and the solution is fluorine-free and acetate-free. In these precursor solutions, propionate is the only salt present in the solution. As confirmed by infrared (IR) and electron paramagnetic resonance (EPR) analysis, the copper and amine are partially coordinated.

[0028] In a particular embodiment, the metals are at a total metal concentration in the solution ranging from 1 to 1.5 M. In another particular embodiment, the metals are at a concentration of 1.5 M total metal concentration in the solution.

[0029] In another particular embodiment of the present invention, the REBCO precursor solution is one in which the RE is selected from gadolinium or yttrium.

[0030] Suitable rare earth metals for use in the present invention are neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Preferably, the RE metal is yttrium or gadolinium.

[0031] In another particular embodiment of the present invention, the REBCO precursor solution is selected from the following group: a) REBCO-stoichiometric mixture with a molar ratio of Y:Ba:Cu of 1:2:3; b) Cu-rich mixture with RE:Ba:Cu molar ratio of 1:2:4.66; c) an excessively Cu-rich mixture with a molar ratio of RE:Ba:Cu of 1:2:5.5; and d) A Y-excess, Cu-rich mixture with a RE:Ba:Cu molar ratio of 1.35:2:5.5, preferably RE is yttrium (Y).

[0032] In a particular embodiment of the present invention, the REBCO precursor solution is a Cu-rich mixture of RE:Ba:Cu in a molar ratio of 1:2:4.66, which is particularly good at promoting epitaxial growth of REBCO on TLAG.

[0033] Any of the above embodiments where RE=Y is a specific embodiment of the invention.

[0034] The amine forms a complex with Cu(Prop)2, helping to increase the solubility of Cu(Prop)2 in the medium used. It also increases the stability of the solution and the final film thickness. During the pyrolysis process, the propionate decomposes and forms part of a complex metal-organic framework in a manner that properly releases strain while avoiding cracking during the pyrolysis process. In certain embodiments, the REBCO precursor solution according to the present invention has an amine selected from the group consisting of (C1-C4)-alcoholamines, secondary and tertiary amines as catecholamines, and pyridine. Examples of catecholamines include epinephrine (adrenaline), norepinephrine (noradrenaline), and dopamine. Examples of (C1-C4)-alcoholamines include methanolamine and ethanolamine. In another specific embodiment, the amine contains at least an -OH functional group along with an -NH2 functional group that, apart from the possibility of complexation, can be stabilized by H-bonding. In another specific embodiment, the amine is a (C1-C4)-alcoholamine. In another specific embodiment of the present invention, the REBCO precursor solution is one in which the amine is selected from the group consisting of methanolamine, ethanolamine, catecholamine, and pyridine. In another specific embodiment of the present invention, the REBCO precursor solution is one in which the amine is selected from the group consisting of (C1-C4)-alcoholamine, catecholamine, triethylamine, lutidine, 4-formylpyridine, 2-acetylpyridine, and pyridine.

[0035] In another specific embodiment of the present invention, the REBCO precursor solution is one in which the amine is selected from the group consisting of (C1-C4)-alcoholamine, triethylamine, pyridine, lutidine, 4-formylpyridine, and 2-acetylpyridine. In a preferred embodiment, the amine is monoethanolamine (MEA).

[0036] In another specific embodiment, the REBCO precursor solution has an amine concentration of 1-5% by volume of the total solution volume. In another specific embodiment, the REBCO precursor solution has an amine concentration of 1-4% by volume of the total solution volume. In another specific embodiment, the REBCO precursor solution has an amine concentration of 2-4% by volume of the total solution volume. In another specific embodiment, the REBCO precursor solution has an amine concentration of 4% by volume of the total solution volume. This percentage, combined with a metal concentration in solution of, for example, 1.75 M, can result in a particularly stable solution and a high-quality pyrolyzed film having a thickness of 450 nm when the deposition technique is spin coating.

[0037] In another specific embodiment of the present invention, the REBCO precursor solution, particularly the REBCO precursor solution, has an amine:copper molar ratio of 0.5:1 to 2:1. In another specific embodiment of the present invention, the REBCO precursor solution has an amine:copper weight ratio of 0.6:1.

[0038] In certain embodiments of the present invention, the amine is methanolamine and the viscosity of the REBCO solution is in the range of 3 to 25 mPa·s. In another particular embodiment, the viscosity of the REBCO solution is in the range of 4 to 15 mPa·s. In another particular embodiment, the viscosity of the REBCO solution is in the range of 4 to 5 mPa·s.

[0039] In another specific embodiment of the present invention, the REBCO precursor solution is one in which the alcohol (C1-C4) is selected from the group consisting of methanol, ethanol, butanol, and mixtures thereof. In another specific embodiment, the solvent system is a mixture of alcohol (C1-C4):propionic (Hprop) acid in a volume ratio ranging from 20:80 to 50:50. In another specific embodiment, the solvent system is a mixture of alcohol (C1-C4):propionic (Hprop) acid in a volume ratio of 50:50. In another specific embodiment, the solvent system is a mixture of methanol:Hprop in a volume ratio of 50:50. In another specific embodiment, the solvent system is a mixture of butanol:Hprop in a volume ratio of 50:50. In another specific embodiment, the solvent system is a mixture of ethanol:methanol:Hprop in a volume ratio of 25:25:50. In another specific embodiment, the solvent system is a mixture of ethanol:propionic (Hprop) acid in a volume ratio of 20:80 to 50:50.

[0040] In another particular embodiment, the amount of water in the solution is in the range of 0.5 to 10% by weight, in another particular embodiment, the amount of water in the solution is less than 2% by weight, in another particular embodiment, the amount of water in the solution is 0.6% by weight.

[0041] In a specific embodiment, the REBCO solution has a concentration of 1.75 M (total metal concentration) in a 50:50 solvent mixture of HProp:MeOH, a Cu-rich mixture with a molar ratio of RE:Ba:Cu of 1:2:4.66, and contains 4% v / v MEA.

[0042] As noted above, any of the above embodiments where RE=Y is a specific embodiment of the present invention.

[0043] In another specific embodiment of the present invention, the REBCO precursor solution is combined with nanoparticles. In another specific embodiment, the REBCO precursor solution of the present invention is one in which the nanoparticles are BaMO (M=Zr, Hf, Ti) nanoparticles, BaMO (M=Ta, Nb) nanoparticles, or metal oxide nanoparticles. Thus, preformed nanoparticles can be added to the initial precursor solution to form nanocomposites that improve the performance of superconductors in high magnetic fields.

[0044] All features of the above specific embodiments that define the precursor solutions as products in themselves are also features of the corresponding specific embodiments according to the invention, relating to the processes for their preparation or their uses.

[0045] The REBCO solution, particularly the YBCO solution, of the present invention can be prepared by a process comprising the following steps: a) preparing a mixture of (C1-C4) alcohol:propionic acid in a volume ratio ranging from 20:80 to 60:40; b) sequentially adding barium propionate, copper propionate, and RE propionate to the solvent mixture while stirring, with each propionate added after the previous one is dissolved, in an amount appropriate to achieve a total metal concentration of 1 to 2 M; and c) adding the barium propionate at 1 to 8% by volume relative to the total solution volume, with an amine:copper molar ratio ranging from 0.3:1 to 2:1. This is a simple and rapid method that maintains stability and allows for precise control of the final film thickness.

[0046] In certain embodiments, the mixture of (C1-C4) alcohol:propionic acid is in a volume ratio ranging from 20:80 to 50:50.

[0047] The REBCO precursor solution may contain additional additives other than triethanolamine, i.e., may be free of triethanolamine.

[0048] In a specific embodiment, the barium carbonate, copper oxide, and rare earth oxides used as starting materials have a purity of 99.9% by weight or greater. In another specific embodiment, the barium carbonate, copper oxide, and rare earth oxides used as starting materials have a purity of 99.99% by weight or greater. In another specific embodiment, the propionic acid has a purity of 99.5% by weight or greater.

[0049] The three metal propionates used in this invention can be prepared by a facile, high-purity, one-pot synthesis without the use of hazardous media that may interfere with the continuous preparation of highly concentrated YBCO precursor solutions.

[0050] To avoid the precipitation of Ba(NO3)2 during the synthesis of the REBCO precursor solution, no nitrogen medium is used for the preparation of RE-propionates, especially Y-propionates.

[0051] The precursors chosen for the metal propionates were CuO, RE2O3, and BaCO3, given their commercially available low cost and high purity. For the metal Gd, the precursor of choice was Gd2O3. Indeed, significant cost differences were observed compared to the respective acetates, most notably for the copper precursor, where a cost reduction of at least 10x was achieved.

[0052] The process for obtaining the propionate salt raw materials of the present invention is cost-effective, robust, reproducible, and allows for the preparation of high purity three metal propionates without the use of hazardous media.

[0053] In certain embodiments, the process for preparing the REBCO precursor solution defined above further comprises reacting barium carbonate, copper(II) oxide, and yttrium(III) oxide with an excess of propionic acid at a suitable temperature, respectively, to pre-prepare the starting materials barium propionate, copper propionate, and RE propionic acid, respectively, and isolating the products thus obtained, respectively.

[0054] In another particular embodiment, the temperature for preparing the starting materials barium propionate, copper propionate, and RE propionate ranges from RT to 150°C. In another particular embodiment, this temperature ranges from 120 to 140°C. In another particular embodiment, the isolation of each of barium propionate, copper propionate, and RE propionic acid includes a washing step to remove residual propionic acid with an organic solvent. In another particular embodiment, the solvent is a polar solvent such as diethyl ether or acetone. Advantageously, the compounds thus obtained are free of residual propionic acid.

[0055] As an example, copper propionate (CuProp2) can be prepared by a process comprising the following steps: a) adding cupric oxide having a purity of 99.9% by weight or greater to excess propionic acid (99.5% or greater) at a concentration of approximately 0.3-1 M, particularly 0.5 M; b) carrying out the reaction under reflux; c) cooling and removing the excess solvent, for example, by filtration or rotary evaporation, to obtain a solid. A washing procedure can be performed to remove residual solvent. The solid can be washed several times with a solvent, for example, diethyl ether. The solid can be ground, for example, by ball milling using an agate mortar (Fritsch, Pulverisette 6, 250 5 mm diameter balls) to obtain a fine powder. The copper propionate obtained by the above process can be characterized by ATR-FTIR spectroscopy, XRD, and SEM under the conditions described in the Examples. Thermogravimetric analysis (TGA) of the final product, when heated in air from 50 to 200 °C at 10 K / min, shows no mass loss in the temperature range where hydration water molecules are lost (up to 140 °C), indicating that the product is anhydrous. After the grinding process, the copper propionate particles have a size range of 1 μm to 20 μm as measured by scanning electron microscopy (SEM).

[0056] Barium propionate (BaProp2) can be prepared by a process that includes the following steps: a) adding barium carbonate (BaCO3) with a purity of 99.9% or higher to a mixture of propionic acid (≥99.5%) and distilled water in a ratio of, for example, 1.1:1, and mixing at an appropriate temperature, such as room temperature (T 25°C); b) removing excess solvent and promoting crystallization of the solid product by cooling to below 0°C, for example, using an ice and acetone bath. The solid can be washed several times to remove residual solvent, for example, using acetone and / or diethyl ether. A fine powder can be obtained by ball milling the solid, for example, using the agate mortar described above. The product can be characterized by ATR-FTIR spectroscopy, XRD, and SEM under the conditions described in the Examples. Furthermore, nuclear magnetic resonance (NMR) spectroscopy was performed using a Bruker Avance DPX 250 MHz (5.8 T) spectrometer. 13 C(CD3OD) reveals only peaks due to barium propionate species, indicating the absence of unwanted products, whereas the TG heating process from 50 to 200 °C at 10 K / min in air produces a TG curve revealing the presence of 2 wt% H2O and 4 wt% propionic acid in the products.

[0057] Yttrium propionate (YProp3) can be prepared by adding yttrium(III) oxide (99.99%) at a concentration of 0.3-1 M, e.g., 0.5 M, to excess propionic acid (≥99.5%), reacting under reflux, cooling, and removing excess solvent to obtain a white solid. A washing procedure may be performed to remove residual solvent. The solid can be washed several times with solvents such as acetone and / or diethyl ether. Finally, the solid can be crushed. The product can be characterized by ATR-FTIR spectroscopy, XRD, and SEM under the conditions described in the Examples. 13A C NMR spectrum was obtained using a Bruker Avance DPX 250 MHz (5.8 T) spectrometer with CD3OD as the solvent. It showed only peaks due to yttrium propionate species, indicating the absence of undesired products. The TG heating process used was from 50 °C to 200 °C at 10 K / min in air. The TG curve showed that the product contained 6% HO by mass, with the onset of dehydration occurring at 100 °C, indicating that the HO was coordinately bonded to the structure. Further TG analysis showed that this amount did not change over time.

[0058] Gadolinium propionate (GdProp3) can be prepared in the same manner as yttrium propionate. The resulting solid can be washed several times with diethyl ether. The product can be characterized by ATR-FTIR spectroscopy, XRD, and SEM under the conditions described in the Examples. The TG heating process was from 50 to 200 °C at 10 K / min in air. The TG curve shows that the product contains 6.2 wt% HO, and most of the mass loss due to dehydration occurs above 100 °C, indicating that HO is coordinated to the structure.

[0059] Simple addition of three metal propionates to a 50:50 mixture of propionic acid and methanol can produce a solution with a maximum total salt concentration of 1 M to 2 M. In certain embodiments, the solution is 1 M.

[0060] A REBCO precursor colloidal solution containing nanoparticles can be prepared by a process comprising the following steps: a) providing a solution of nanoparticles, such as preformed metal oxide nanoparticles, in an alcohol, such as methanol or ethanol; b) adding ethanolamine to the solution; c) adding methanol and / or ethanol followed by propionic acid; and d) adding salts, preferably barium propionate, copper propionate, and rare earth metal propionate, such as yttrium propionate, in this order. In certain embodiments, the nanoparticles are first dispersed in ethanol. Typically, the nanoparticle dispersion has a molar concentration of nanoparticles appropriate to achieve a molar concentration of 6-30 mol% in the final REBCO precursor solution containing nanoparticles. In certain embodiments, the molar concentration of nanoparticles is 8-20 mol%, more particularly 8-12 mol%, or even more particularly 12 mol%. All ranges contained herein are inclusive of their endpoints.

[0061] BaMO3 (M = Zr, Hf, Ti) nanoparticles can be synthesized using thermal activation methods such as autoclave and microwave, respectively, as follows. Both methods allow the synthesis of dispersed nanoparticles of controlled size. The nanoparticle size can be adjusted in the range of 4-5 nm, 7-8 nm, and 10-12 nm.

[0062] As an example, the desired n-butoxide precursor (M=Ti 4+ , Zr 4+ , or Hf 4+) can be added to room-temperature ethanol under a nitrogen atmosphere. Depending on the desired nanoparticle (NP) size, barium hydroxide octahydrate can then be added, followed by triethylene glycol and HO or ammonia. The reagents can be added at room temperature with continuous stirring. The final suspension is heated to a high temperature (100–180 °C) in an autoclave, maintained at this temperature for an appropriate time (e.g., 1 h), and then slowly cooled to room temperature. The resulting suspension can be washed first with an excess of ethyl acetate, then with EtOH, MeOH, or BuOH, especially 1-butanol, and sonicated until the pellets are completely dispersed. Additional centrifugation can be repeated until a uniform particle size distribution is observed by TEM or DLS.

[0063] When using microwave activation, the process can be carried out in the same manner as the solvothermal reaction using the desired amount of HO. After mixing, the resulting solution is heated at a rate (5-40 °C / min) to form BaMO (M = Zr 4+ , Hf 4+ The solvo-thermal method can be used to heat the powder to 180°C, and the BaTiO3 to 100°C, and then hold the powder at the corresponding final temperature for an appropriate time, such as 5-10 minutes. The powder is then washed in the same manner as the solvothermal method, and finally dispersed in absolute ethanol or methanol.

[0064] BaM2O6 (M = Ta, Nb) nanoparticles can be prepared by a surfactant-free solvothermal reaction using an autoclave system capable of reaching high temperatures (up to 300 °C) and pressures (3-14 bar) to avoid aggregation through post-synthesis surface functionalization. This reaction can be carried out in an autoclave system. Stoichiometric mixed-metal precursors, Ta(OCH2CH3)5 and Ba(OH)2·8H2O, can be mixed in EtOH under a nitrogen atmosphere at room temperature (20-25 °C). The solution can be heated in an autoclave for a period of time (24-70 hours) at high temperatures (220-260 °C). The resulting solution containing the precipitate can be washed with deionized water using centrifugation. Finally, the nanoparticles can be redispersed in ethanol. Post-synthesis surface functionalization can be carried out by a process involving redispersing the precipitated surfactant-free nanoparticles in acetonitrile (MeCN) solution into DMF, followed by the addition of Me3OBF4 in MeCN. The resulting nanoparticle solution can be characterized by DLS, TEM, XRD, and finally TGA, NMR, and IR.

[0065] In certain embodiments, the solution of nanoparticles in alcohol has a molar concentration such that the final REBCO precursor colloidal solution is 8-30% molar with respect to the REBCO concentration. In other specific embodiments, such a ratio is 8-20 mol%, and in other specific embodiments, such a ratio is 6-18 mol%, and in other specific embodiments, such a ratio is 6%, 12%, or 18 mol%.

[0066] In another particular embodiment of the present invention, the process for preparing the REBCO precursor solution is a 50:50 mixture of propionic acid and methanol by volume.

[0067] In another specific embodiment of the present invention, the process for preparing the REBCO precursor solution is one in which the amine is present in the range of 4-6% v / v. The use of the additive helps to obtain a homogeneous pyrolyzed layer with low porosity, and is also suitable for multilayer deposition.

[0068] Also part of the present invention is a process for preparing a superconductor by growing a superconducting REBCO (or YBCO) layer with a thickness of 0.1 μm to 2.5 μm, which comprises the following steps: a) depositing the precursor solution defined above on a substrate surface by any method that allows uniform control of the film thickness to form a precursor film; b) subjecting the precursor film to a pyrolysis process by heat treatment under a controlled atmosphere; c) repeating steps a) to b) as needed to obtain a thicker pyrolyzed film by a multilayer deposition process; d) subjecting the deposited REBCO layer of step b) or the REBCO multilayer of step c) to a TLAG process for the growth of a final epitaxial REBCO layer; and e) subjecting the superconductor thus obtained to an oxygenation process. This process therefore combines the deposition of a chemical solution with a TLAG growth process based on the decomposition and crystal growth of the deposited product.

[0069] In a particular embodiment, the process is for the preparation of a superconductor based on growing a superconducting REBCO (or YBCO) layer having a thickness of 0.1 μm to 2.5 μm, and in another particular embodiment, the process is for the preparation of a superconductor based on growing a superconducting REBCO (or YBCO) layer having a thickness of 0.2 μm to 1.5 μm.

[0070] The substrate is preferably SrTiO or a metal substrate comprising a biaxially textured oxide layer on a metal substrate. Other substrates, such as LaAlO, SrTiO, MgO, etc., may also be suitable for the purposes of the present invention. Although a buffered metal substrate using manganite is the preferred choice, other buffer materials compatible with the REBCO cell parameters are also possible, such as cerium oxide, RE-doped cerium oxide, MgO, GdCuO, or other combinations.

[0071] By biaxially textured substrate is understood a metal substrate which has already been textured by a thermomechanical process and which has one or more oxide buffer layers thereon, or a polycrystalline metal substrate on which a biaxially textured oxide layer has been grown by any method.

[0072] In a specific embodiment, a process for preparing a superconductor by growing a superconducting REBCO (including YBCO) layer with a thickness of 0.1 to 2.5 μm comprises the following steps: a) optionally subjecting a single-crystalline SrTiO substrate to an annealing treatment at a temperature of 800 to 950 °C for an appropriate time to obtain a flat terraced surface, followed by sequential washing with acetone and methanol; b) applying the precursor solution defined above to the substrate surface by spin-coating to form a precursor film; c) subjecting the precursor film to a pyrolysis treatment by heating it to 240 °C in a humidified oxygen stream at an appropriate heating rate and then to 500 °C at an appropriate heating rate; d) subsequently cooling to room temperature; e) optionally repeating steps b) to d) to obtain a thick pyrolyzed film by a multilayer deposition process; f) subjecting the deposited REBCO layer of step d) or the REBCO multilayer of step e) to a TLAG process; and g) subjecting the superconductor thus obtained to an oxygenation process. Advantageously, the TLAG-CSD technique provides a cheaper, higher throughput process compared to currently available methods for producing high critical current density superconducting films, thereby enabling the production of high temperature superconductors from the REBCO precursor solutions of the present invention at commercially attractive prices.

[0073] In a specific embodiment of this process, the single crystal SrTiO3 substrate is subjected to an annealing treatment at a temperature in the range of 850-900°C for a time period of approximately 5 hours.

[0074] In another specific embodiment, the precursor film is subjected to a pyrolysis treatment by heating in a humidified oxygen flow (0.01-0.24 L / min) at a heating rate of 2-20°C / min to 500°C. In another specific embodiment, the precursor film is subjected to a pyrolysis process in a humidified oxygen flow (0.12 L / min) by heating to 240°C at a heating rate of 5°C / min, followed by heating to 500°C at a heating rate of 3°C / min.

[0075] In a particular embodiment of the process, the pyrolysis layer has a thickness in the range of 400 to 2700 nm.

[0076] Pyrolyzed samples can reach thicknesses of 0.2-5 μm. For example, a two-layer pyrolyzed sample will result in an 800 nm thick film, a four-layer sample will be 1.5 μm thick, and most notably, an eight-layer sample will have a final thickness of 2.7 μm.

[0077] In certain embodiments, multilayer deposition of eight layers prepares a uniform pyrolytic film having a thickness of 0.2 to 5 μm. Thus, the REBCO solution of the present invention produces a thick monolayer and is suitable for multilayer deposition.

[0078] The precursor solution can be used in scalable deposition techniques such as inkjet printing and slot-die coating. Inkjet printing is a chemical deposition (CSD) method that deposits chemical solutions in the form of droplets in the picoliter (pL) range. Its advantage is that thick films can be produced in a single deposition run by adjusting the printing parameters, allowing large volumes of solution to be uniformly deposited on the substrate. As an example, we used the Autodrop Professional System MD-P802 from MICRODROP Technologies, an inkjet printer equipped with four nozzles with independent electronic circuits. Slot-die coating is a pre-metered coating deposition technique that allows for uniform layers with lengths ranging from a few nanometers to mm on different substrates. As an example, we used a metal tape with the following structure: LSMO / epi-MgO / IBAD-MgO / YO / AlO / Hastelloy C276. This protocol involves three steps: a) tape cleaning, b) deposition by slot-die coating and drying, and c) pyrolysis.

[0079] Layers prepared using this solution exhibit the desired precursor phases of TLAG, namely BaCO (predominantly orthorhombic phase), CuO, and YO, by XRD analysis (see Figure 10). The REBCO solution of the present invention provides smooth films without cracks.

[0080] Preferably, the superconducting material has the formula ReBa2Cu3O 7-x It has a critical temperature of 90K and a specific resistance of 2-4 MA / cm at 77K. 2 It is characterized by having a current density of

[0081] Finally, a superconducting multilayer on a suitable substrate, i.e., a coated conductor, obtained by the above-described process, is part of the present invention. The substrate may be as defined above, in particular a biaxially textured metal substrate. The superconducting multilayer is a multilayer stack.

[0082] The production of KM-long coated conductors with high-temperature superconducting layers deposited on buffer metal substrates can be used in many applications, particularly in nuclear magnetic resonance, magnetic resonance imaging, wind turbines, power cables, fault current limiting devices, rotating machinery for electric aircraft, high-field magnets for nuclear fusion, accelerators, or superconducting magnetic energy storage devices.

[0083] Throughout the specification and claims, the use of "comprises" and variations thereof is not intended to exclude other technical features, additives, components, or steps. Furthermore, the word "comprises" also encompasses the term "consisting of." Additional objects, advantages, and features of the present invention will become apparent to those skilled in the art upon examination of the specification or may be learned by practice of the present invention. The following examples and drawings are provided for illustrative purposes and are not intended to limit the invention. Reference signs in connection with the drawings and within parentheses in the claims are intended to enhance understanding of the claims and should not be construed as limiting the scope of the claims. Furthermore, the present invention covers all possible combinations of the specific and preferred embodiments described herein.

[0084] Example Example 1: Synthesis of metal propionate precursors Example 1A: Preparation of (Cu(Prop)2): CuO (cupric oxide, Puratronic®, 99.7% (metal basis), purchased from Alfa Aesar) was added to excess propionic acid (HProp) (≥99.5%, purchased from Sigma Aldrich) to a total concentration of 0.5 M. Reagents and solvents were used without further purification. The reaction was carried out overnight at 140 °C under reflux with vigorous stirring until a clear blue solution was obtained. Excess solvent was removed using a rotary evaporator (Buchi) to yield a dry, dark blue copper propionate (Cu(Prop)2) solid.

[0085] Example 1B: Preparation of Y(Prop): Y2O3 (yttrium(III) oxide, REacton®, 99.99% (REO), purchased from Alfa Aesar) is added to excess HProp to a concentration of 0.5 M. The reagents and solvents are used without further purification. The reaction is carried out under reflux at 140 °C overnight with vigorous stirring until a clear, transparent solution is obtained. The excess solvent is removed using a rotary evaporator (Buchi) to obtain a white solid of dried yttrium propionate (Y(Prop)3).

[0086] Example 1C: Preparation of Ba(Prop)2: The synthesis of barium propionate (Ba(Prop)2) differs from previous syntheses. BaCO3 (barium(II) carbonate, 99.95% (metal basis), purchased from Alfa Aesar) is added to a mixture of HProp and distilled water in a ratio of 1.125:1. The reagents and solvents are used without further purification. The reaction begins as a very foamy, white solution and is stirred vigorously for 24 hours. The excess solvent is removed using a rotary evaporator (Buchi) to yield a clear gel. To induce crystallization of the solid product from the gel, an ice and acetone bath is applied for 4 hours. Finally, as a necessary step to remove residual HProp, all three solid products are washed with diethyl ether (stabilized with Et2O, AGR, ACS, ISO, and BHT, purchased from Labbox) using a Buchner funnel to obtain a high-purity powder product. This synthesis method ensured a yield of over 90%.

[0087] For Ba(Prop)2 and Y(Prop)3, a two-step washing procedure was performed starting with acetone (Acetone, Multisolvent® HPLC grade ACS ISO UV-VIS), which was necessary to improve the gel removal process, followed by subsequent washing with Et2O. The yields increased from 20% to 90% for Ba-prop and from 60% to 90% for Y-prop, respectively.

[0088] However, in the case of Cu(Prop)2, the final powder product synthesized exhibited large particle sizes, with the majority of particles exceeding 100 μm. Furthermore, the crystal-like morphology prevented complete dissolution in the REBCO precursor solution for subsequent high-concentration applications. Therefore, the product was milled using a Fritsch Pulverisette 6 monoplanetary ball mill. Two 80 mL agate bowls were filled with 14 g of the product and 250 5 mm diameter agate balls. Because this process is high-energy, milling at 650 rpm for 1 minute was sufficient to significantly reduce the particle size, with the majority of the milled particles measuring approximately 30 μm in diameter.

[0089] The final product was analyzed by ATR FT-IR spectroscopy (spectrophotometer Jasco 4700, energy range: 300–7800 cm). -1 , equipped with an attenuated total reflection accessory) (Figures 1, 3, 5, 7). -1 The three peaks in the range are characteristic of the aliphatic chain of the propionic acid group present in the three spectra. They can also be characterized by XRD patterns, which show characteristic peaks of propionate (Figures 2, 4, 6, and 8). SEM (QUANTA FEI 200 FEG-ESEM) was used to evaluate particle size. The particle size of copper propionate after the milling process, as measured by scanning electron microscopy (SEM), ranges from 1 μm to 20 μm in diameter. The particle size of BaProp2 after milling ranges from 2 μm to 6 μm. The particle size of YProp3 powder after milling ranges from 5 μm to 50 μm. The particle size of GdProp3 powder after milling ranges from 5 μm to 80 μm. Characterization by NMR spectroscopy (Bruker Advance DPX, 250 MHz (5.8 T)) was limited to Ba(Prop)2 and Y(Prop)3 due to the paramagnetic nature of Cu(II) in Cu(Prop)2.

[0090] Example 2: Preparation of YBCO precursor solution The preparation of YBCO precursor solutions follows the same procedure regardless of the solution stoichiometry. The various solution stoichiometries differ in the Y-Ba-Cu ratio and are named after the Ba-Cu molar ratio of the transient liquid formed during the transient liquid-assisted growth (TLAG) process: a YBCO stoichiometric mixture with a Y-Ba-Cu molar ratio of 1:2:3 ((2:3) composition); a Cu-rich mixture with a Y-Ba-Cu molar ratio of 1:2:4.66 ((3:7) composition); an excessively Cu-rich mixture with a Y-Ba-Cu molar ratio of 1:2:5.5 ((4:11) composition); and a Y-rich mixture with a Y-Ba-Cu molar ratio of 1.35:2:5.5 (Y-rich 4:11) composition. These solutions were prepared using a 50:50 mixture of hydroxypropyl propyl paraben and methanol (MeOH) (methanol, 99.9%, anhydrous (maximum 0.003% HO), purchased from Scharlabs). Ethanolamine (MEA) (ethanolamine, purified by double distillation, ≥99.5%, purchased from Sigma-Aldrich) was also added at various volumetric ratios. The concentration of the YBCO solution for spin-coating deposition was 1.75 M total metal salts when MEA was used, regardless of the solution composition. MEA promotes complete precursor dissolution, enhances solution stability, and increases the uniformity and thickness of the final film. Without the amine additive, only a 1 M total salt concentration was achieved. The metal propionate precursor salts were added sequentially to the solvent mixture, ensuring complete dissolution of each precursor before adding the next. Ba(Prop)2 was added first to the solvent mixture, and dissolution was rapid. The solution became clear upon completion. Cu(Prop)2 was then added and heated to 30 °C. Y(Prop)3 was then added. The solution was maintained at 30 °C and 450 rpm for an additional 30 minutes. All precursors were completely dissolved, resulting in a deep blue solution. The solvent mixture was adjusted to a 50:50 volume ratio (as described above) and filtered. The final solution was stored in a sealed vial under argon.

[0091] The rheological properties of the solutions, such as viscosity and contact angle, were measured using a HAAKE RheoStress RS600 from Thermo Electron Corp. and a Kruss Drop Shape Analyzer DSA 100, respectively (Table S2†). The water content of the solutions is crucial, as it can affect the final properties of the REBCO layer. Therefore, it was monitored by Karl Fischer titration (reference) (Nittoseiko Analytech, Model CA-310 equipped with a VA-200 vaporizer), and each solution was used only while the HO content was above 2 wt%. Electron paramagnetic resonance (Bruker ELEXYS E500 X-band EPR spectrometer) measurements were performed on selected solutions to investigate and explain the role of the MEA (see the Results section).

[0092] viscosity Figure 9a shows a series of viscosity measurements performed on two solutions with total metal concentrations of 1 M and 1.75 M. The addition of MEA is very beneficial for the purpose of increasing the final thickness.

[0093] water content This type of solution, using a mixture of HProp and MeOH (50:50) as the solvent, is subject to Fischer esterification, producing HO as a by-product of the reaction. Figure 9b shows the time course of the HO release weight percent for the two (3:7) composition solutions. A complex with MEA is formed. Because the amine (MEA) is not present in stoichiometric amounts with Cu(Prop)2, the two species coexist in solution. The Cu-MEA complex is more stable in solution than Cu(Prop)2 alone. The formation of this complex is confirmed by EPR.

[0094] Example 3: Thin film deposition, pyrolysis and growth Thin films of different compositions were deposited by spin-coating (SMA 6000 Pro, purchased from Suministro de Materiales y Asistencia, SL) using the as-prepared YBCO precursor solution.

[0095] Single-crystal (001) SrTiO (STO) substrates (CrysTech GmbH) were used. Prior to deposition, they were annealed at 900 °C for 5 h to obtain a flat terraced surface. They were then washed sequentially with acetone (acetone, Multisolvent® HPLC grade ACS ISO UV-VIS, purchased from Scharlab) and methanol (methanol, Multisolvent® HPLC grade ACS ISO UV-VIS KF, purchased from Scharlab) to remove any possible residues. The solution was deposited at a rotation speed of 6000 rpm for 2 min in an ISO7 cleanroom with 10% humidity. The pyrolysis process consisted of heating to 240 °C at a heating rate of 5 °C / min in a humidified oxygen flow (0.12 L / min), then to 500 °C at a heating rate of 3 °C / min, followed by cooling to room temperature. To obtain thicker pyrolyzed films, a multilayer deposition process was performed, repeating the above procedure. Finally, the growth of the final epitaxial YBCO layer was improved by an innovative process called TLAG, using a tube furnace connected to a vacuum system.

[0096] The transient liquid-assisted growth (TLAG) process via PO2 route experiments was carried out in a tube furnace equipped with a vacuum system that can be switched from low to high vacuum quickly (~a few seconds). PO2 was introduced into the vacuum system through a gas line and controlled by a control valve. Samples were grown at low PO2 (10 -5 ~10 -6 The samples were heated at 1 bar (10 bar) with an average heating rate of 1°C / s to the desired temperature. The PO2 was then ramped up to reach the desired final PO2 in a time span of 1 second. The samples were cooled at the same heating rate. Following the TLAG process, an oxygenation process was performed. The samples were heated in a tube furnace at 1 bar with a heating rate of 10°C / min to 450°C, held for 210 minutes, and then cooled to room temperature at the same rate. This process was performed under continuous O2 flow.

[0097] Sample characterization Characterization of the pyrolyzed samples was performed using various techniques, starting with optical microscopy (OM) (Leica DM1750M) analysis to check the film homogeneity and further reflectometry (Filmetrics F50) to obtain the film thickness in a fast and non-destructive manner. The results for the different samples are shown in Figure 10(a, c, e, g).

[0098] Structural characterization The structure and phase composition of the as-prepared pyrolyzed layers were characterized using XRD on a Bruker-AXS D8 Advance diffractometer (Cu-Kα equipped with a general area detector diffraction system (GADDS)). XRD characterization of the grown YBCO thin films was performed using both a Bruker-AXS D8 Advance diffractometer (Cu-Kα, equipped with a GADDS (general area detector diffraction system)) and a Bruker D8 Discover system (Cu-Kα, X-ray energy = 8.049 keV, equipped with a Lynxeye XE-T energy-dispersive one-dimensional (1D) detector) in two configurations: a θ-2θ geometry to characterize the epitaxy, and a grazing incidence (GI) geometry to amplify the signal from secondary phases and facilitate their identification.

[0099] XRD analysis of layers prepared using this solution, shown in Figure 10(b, d, f, h), showed the desired precursor phases of TLAG: BaCO3 (predominantly orthorhombic phase), CuO, and YO3, confirming the suitability of this solution class for growing thick, robust YBCO films. It also confirmed that the precursor solution according to the present invention can provide smooth, crack-free films for all three compositions: (2:3), (3:7), and (4:11).

[0100] Microstructural characterization The surface morphology was evaluated using scanning electron microscopy (SEM) with the use of a QUANTA FEI 200 FEG-ESEM in combination with energy dispersive X-ray (EDX) spectroscopy.

[0101] For thickness evaluation, nanocrystalline phase distribution, and size, the microstructure of the pyrolyzed thin films was characterized using the following techniques: a) High-resolution transmission electron microscopy (HRTEM), b) High-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF), c) Energy-dispersive X-ray spectroscopy (EDX), and d) Electron Energy-Loss Spectroscopy (EELS). For this purpose, an FEI Tecnai F20(S)TEM operated at 200 kV in TEM and STEM mode and equipped with a Gatan quantum electron energy-loss spectrometer for EELS analysis was used.

[0102] Pore ​​density was assessed from cross-sectional STEM-HAADF micrographs. Images were processed with the image analysis software ImageJ41, and clear dark contrast areas were defined as pores using the threshold tool and analyzed with the "Particle Analysis" function. Average pore size was calculated from the histogram of measured pore area, taking into account circular pores.

[0103] EELS data processing involved principal component analysis (PCA) to reduce statistical noise in EELS spectral images. Reconstruction was performed using the first 10 principal components using the weighted PCA multivariate statistical analysis (MSA) plugin 42 in Gatan Digital Micrograph software.

[0104] Furthermore, the microstructure, atomic defect structure, and phase composition of the grown YBCO thin films were investigated using an FEI Tecnai F20(S)TEM operating at 200 kV and an FEI Titan operating in STEM mode at 300 kV equipped with an X-FEG gun, a CESCOR Cs probe corrector, and a Gatan TRIDIEM 866 ERS energy filter with monochromator.

[0105] Cross-sectional specimens for transmission electron microscopy (TEM) were prepared by conventional methods: cutting, gluing the slices together, thinning by tripod mechanical polishing, and Ar+ ion milling by Gatan PIPS until electron transparency was achieved.

[0106] Transverse high-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF) images show that the two-layer pyrolyzed sample reaches a thickness of 800 nm, the four-layer sample reaches a thickness of 1.5 μm, and most notably, the eight-layer sample reaches a final thickness of 2.7 μm, suggesting no loss of nanocrystalline matrix homogeneity or variation (Figure 10). Upon closer inspection, low-magnification STEM-HAADF images reveal smooth surfaces along with the thickness of all three (3:7) pyrolyzed thin films (Figure 11(a-c)). Furthermore, Figure 11(d-f) shows that the CuO nanocrystals are uniformly distributed within the BaCO3 matrix, with no segregation or interfaces, an ideal situation for successful YBCO epitaxial growth. In fact, segregation of the CuO interlayer at the interface between different layers in multiple depositions with intermediate pyrolysis is a common drawback, particularly prevalent in the TFA route. In Figure 11 (g–i), STEM-HAADF image analysis using the software ImageJ identified low estimated pore densities of 1 ± 0.2%, 1.15 ± 0.1%, and 1.08 ± 0.2% for the pyrolyzed membranes with 2, 4, and 8 layers, respectively.

[0107] Further elemental analysis by electron energy loss spectroscopy (EELS) was performed on these pyrolyzed films, and CuL 2,3 , BaM 4,5 , and the elemental maps of OK revealed that these precursor phases were highly homogeneous at the nanoscale (Fig. 12a–e).

[0108] Complementary compositional analysis was performed using energy dispersive X-ray spectroscopy (EDX) (Figure 13), and cross-sectional elemental mapping by STEM-EDX of Y, Ba, and Cu also confirmed the uniform distribution of nanocrystalline phase in the pyrolyzed thin films of (3:7) composition deposited using a 1.75M+4 vol / vol% MEA solution.

[0109] Analysis of high-resolution transmission electron microscopy (HR-TEM) images allowed the determination of the spatial distribution, crystalline state, and size of the precursor phase.

[0110] Specifically, the three nanocrystalline phases of BaCO3, CuO, and YO3 were clearly identified in multiple HR-TEM images, supporting a stable solution composed of these precursor intermediates. Their typical diameters remained small: 10–30 nm for orthorhombic BaCO3, 5–7 nm for monoclinic BaCO3, 10–25 nm for CuO, and 5–6 nm for YO3 (Figure 12, f–h). Because TLAG is an ultrafast liquid-assisted process, the small size and uniform distribution of the nanocrystalline precursors greatly facilitates homogeneous and rapid liquid formation, facilitating atomic mobility and promoting high epitaxial layer growth at ultrafast growth rates. Similarly, the majority of BaCO3 exists in the orthorhombic phase, favoring its direct reaction with nanocrystalline CuO in the subsequent TLAG process.

[0111] Electrical characterization The self-field critical current density (Jc) values ​​were obtained from out-of-plane induction measurements using a commercial Quantum Design MPMS XL SQUID DC magnetometer equipped with a 7 T magnet. The Jc self-field (at 5 K and 77 K) was estimated from the remanent magnetization width of the hysteresis loop using the Bean critical state model for thin disks. A Quantum Design Physical Property Measurement System (PPMS) instrument was used for resistivity measurements and Tc determination using the Van der Pauw configuration. The same PPMS was also used to determine the I(V) characteristics and transport critical current density at a field reference of 1 μV / cm.

[0112] The values ​​obtained for YBCO superconducting layers with thicknesses greater than 500 nm are 2-4 MA / cm at 77 K in the self-field. 2 , 20-30MA / cm at 5K 2 and a critical temperature of 90K.

[0113] Example 4: Preparation of REBCO precursor colloidal solution using metal oxide nanoparticles REBCO precursor solutions were synthesized using preformed metal oxide nanoparticles (5–10 nm in size) and different REBCO stoichiometries: Y:Ba:Cu 1:2:4.66 (3–7), Y:Ba:Cu 1:2:3 (2–3), and other compositions. Two methods were used for this synthesis, consisting of the following steps:

[0114] For high REBCO total metal concentrations (1-1.5M): To a round-bottom flask, 1-4% v / v of monoethanolamine (MEA) was added, depending on the desired total metal concentration (1-1.5M). The desired NP mol% (6-24 mol%) was added to the flask under an inert atmosphere (Ar) with stirring at room temperature for 5 minutes. The final desired volume of 50% ethanol and / or methanol was added (e.g., 50% ethanol, 50% methanol, or 25% ethanol and 25% methanol). 50% of the final total volume of propionic acid was added. The solution was stirred until complete NP redispersion occurred. After the NPs were redispersed, barium propionate and copper propionate were added. The mixture was stirred under argon gas until the salts were completely dissolved, and the temperature was maintained at 30–50 °C depending on the total metal concentration (30 min–2 h).

[0115] Again, yttrium was added under argon flow, after which the argon flow was stopped and the mixture was stirred until complete dissolution (30 min to 1 h). After the salt was completely dissolved (a clear blue solution), the reaction mixture was cooled to room temperature with stirring. The solution was transferred to a volumetric flask and made up to the appropriate volume with the same solvent mixture as before. Finally, the solution was filtered through a special filter for viscous solutions, transferred to a vial, sealed under argon, and stored in a desiccator.

[0116] For low REBCO total metal concentrations (0.5-0.875M): Following the same procedure as described in section 1.2, a REBCO precursor solution was prepared without the addition of nanoparticles at the desired total metal concentration (1-1.75 M). 50% of the nanoparticle-free REBCO precursor solution prepared in step 1 was mixed with 50% of a nanoparticle solution with the desired NP mol % (NPs redispersed in 100% methanol, 100% ethanol, or a methanol / ethanol mixture). Finally, the viscosity and water content of the REBCO nanocomposite precursor solution were characterized. Furthermore, the stability of the NPs in the REBCO nanocomposite precursor solution was confirmed by TEM and solution XRD. Viscosity: 11.8MPa·s. H2O content: 1.1% (first day of synthesis)

[0117] TEM images confirm the stability of the nanoparticles, as they have similar size and distribution in the YBCO nanocomposite precursor solution compared to nanoparticles redispersed in ethanol alone. Solution XRD confirmed the crystallinity and stability of the nanoparticles in the YBCO precursor solution and in the presence of the additive MEA.

[0118] References list Patent documents -CN106242553A Non-patent literature -L.Soler et al.;Nat. Commun.,2020,vol.11,p.344 -P.Vermeir et al.;“Elucidation of the Mechanism in Fluorine-Free prepared YBa2Cu3O 7-δ Coatings” Inorg.Chem.,2010,vol.49,pp.4471-4477 -P.Vermeir et aL.,“Influence of sintering conditions in the preparation of acetate-based fluorine-free CSD YBCO films using a direct sintering method”;Mater.Res.Bull.,2012,vol.47,pp.4376-4382 -Yue Zhao et al;“Growth of Highly Epitaxial YBa2Cu3O 7-δ Films from a Simple Propionate-Based Solution”,Inorg.Chem.2015,vol.54,pp.10232-10238 -S.Rasi et al.;”Relevance of the Formation of Intermediate Non-Equilibrium Phases in YBaCuO im Growth by Transient Liquid Assisted Growth”;The Journal of Physical Chemistry 2020 vol.124;pp.15574-15584 -M.Nasui et al;“Fluorine-fee propionate route for the chemical solution deposition of YBa2Cu3O 7-x superconducting films”;Ceramics International 2015,vol.41,pp.4416-4421

Claims

1. A REBCO precursor solution comprising: -Re propionate; -Ba propionate; -Cu propionate; -amines; and - solvent systems; Including, RE means Y or a rare earth metal; the metals are RE, Ba, and Cu, which are present only in the form of propionate, and the propionate is the only salt present in the solution; the metals, in total, are at a metal molarity of the solution in the range of 1-2 M; The solvent system is a mixture of alcohol (C １ -C ４ ): a mixture of propionic acid, the amine is miscible in the solvent system at room temperature and is present at a concentration of 1 to 8% by volume of the total volume of the solution; the molar ratio of amine:copper is from 0.3:1 to 2:1, and The solution is fluorine and acetic acid free, REBCO precursor solution.

2. The solvent system is a mixture of alcohol (C １ -C ４ 2. The REBCO precursor solution of claim 1, wherein the REBCO precursor solution is a mixture of hydroxybenzoates, ...

3. The amine is １ -C ４ 3. The REBCO precursor solution of claim 1, wherein the amine is selected from the group consisting of 4-alcoholamine, catecholamine, triethylamine, lutidine, 4-formylpyridine, 2-acetylpyridine, and pyridine.

4. 4. The REBCO precursor solution according to claim 1, wherein the concentration of the amine in the solution is 1 to 4% by volume, based on the total volume of the solution.

5. 5. The REBCO precursor solution of claim 1, wherein the molar ratio of amine to copper is 0.6:

1.

6. 6. The REBCO precursor solution of claim 1, wherein the solvent system is selected from the group consisting of a 50:50 volume ratio of methanol:Hprop, a 50:50 volume ratio of butanol:Hprop, a 25:25:50 volume ratio of ethanol:methanol:Hprop, and a volume ratio ranging from 20:80 to 50:50 of ethanol:Hprop.

7. The REBCO precursor solution of any one of claims 1 to 6 in combination with nanoparticles.

8. The REBCO precursor solution according to any one of claims 1 to 7, selected from the group consisting of: a) REBCO-stoichiometric mixture with a molar ratio of RE-Ba-Cu of 1:2:3; b) Cu-rich mixture with RE-Ba-Cu molar ratio of 1:2:4.66; c) an excessively Cu-rich mixture with a RE-Ba-Cu molar ratio of 1:2:5.5; and d) RE-rich and excessively Cu-rich mixture with RE-Ba-Cu molar ratio of 1.35:2:5.

5.

9. If the REBCO precursor solution does not contain nanoparticles, a) The volume ratio is in the range of 20:80 to 60:40 (C １ -C ４ 2.) Providing an alcohol:propionic acid mixture; b) sequentially adding barium propionate, copper propionate, and RE propionate to said mixture of solvents, each propionate being added after the previous propionate has dissolved, and the amount of each propionate being such that the sum of the metal concentrations of said propionates is in the range of 1-2 M; and c) adding an amine in the range of 1 to 8% v / v, wherein the molar ratio of amine to copper is in the range of 0.3:1 to 2:1; Including, When the REBCO precursor solution contains nanoparticles, a) providing a solution of preformed nanoparticles, such as metal oxide nanoparticles, in an alcohol, such as methanol or ethanol; b) adding ethanolamine to the solution; c) adding methanol and / or ethanol, followed by adding propionic acid; and d) adding the salts in the following order: barium propionate, copper propionate, and RE propionate; 9. A method for preparing the REBCO precursor solution of claim 1, comprising:

10. 10. The method for preparing a REBCO precursor solution of claim 9, wherein the amine is present in the range of 4 to 6% v / v.

11. 11. The method for preparing a REBCO precursor solution according to claim 10, wherein the barium propionate, copper propionate, and RE propionate used as starting materials have a purity of 99.9% by weight or greater.

12. 12. The method for preparing a REBCO precursor solution according to any one of claims 9 to 11, further comprising the steps of reacting barium carbonate, copper (II) oxide, and yttrium (III) oxide with an excess of propionic acid at an appropriate temperature, respectively, to preliminarily prepare the barium propionate, copper propionate, and RE propionate starting materials, and isolating each of the products thus obtained.

13. Use of a REBCO precursor solution as defined in any one of claims 1 to 8 for preparing a superconducting REBCO layer having a thickness of 0.1 μm to 2.5 μm.

14. 1. A method for preparing a superconductor by growing a superconducting REBCO layer having a thickness of 0.1 μm to 2.5 μm, comprising: a) depositing a precursor solution as defined in any one of claims 1 to 11 on a surface of a substrate by any method that allows for uniform control of the thickness of the film to form a precursor film; b) subjecting said precursor film to a pyrolysis process by heat treatment in a controlled atmosphere; c) repeating steps a)-b) as necessary to obtain a thicker pyrolytic film by a multi-layer deposition process; d) subjecting the deposited REBCO layer of step b) or the REBCO multilayer of step c) to a TLAG process to grow a final epitaxial REBCO layer; and e) subjecting the superconductor thus obtained to an oxygenation process; A method comprising:

15. A multi-layer deposited superconducting layer on a suitable substrate obtained by the method of claim 14.

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