RU2081937C1 - Method of preparing high-temperature superconducting coating - Google Patents

Abstract

FIELD: superconducting materials. SUBSTANCE: method includes preparing initial solution by mixing aqueous solution of nitrate salts of metals composing high-temperature superconducting material with summary concentration of cations not higher than 1.5 mol/l with solution containing at least 2 mol of citric acid and at least 7 mol of ethylene glycol per 1 mol of above-mentioned metal cations followed by curing resultant solution at 343-353 K to form viscous polymer solution, which is applied onto a carrier and thermal decomposition of applied layer is effected through high-speed heating at 1113-1193 K. method may be used to manufacture high-temperature superconducting wires and other superconductors finding use in computer technics, radiotechnics, and power engineering. EFFECT: optimized parameters of procedure. 2 cl, 1 tbl

Description

 The invention relates to the field of superconductivity and can be used in the manufacture of high-temperature superconducting (HTSC) wires and other superconductors that can find application in computer engineering, radio engineering, energy.

Known methods for producing HTSC wires (ie, long conductors based on a superconducting compound) based on the mechanical processing (drawing, rolling) of metal tubes filled with HTSC or on the production of “sandwich” structures of HTSC metal ceramics followed by rolling and annealing [ 1]
The indicated methods for producing HTSC wires are laborious, poorly reproducible, and most importantly not technological, i.e. cannot be implemented in existing industrial facilities.

 There are a number of methods for the preparation of HTSC films and coatings on ceramic and single-crystal substrates [2] Suspensions of mixtures of HTSC oxides in various solvents and solutions of salts of the metal components of HTSC are used as precursors (initial solutions for coatings). The application of coatings from solutions to the substrate is carried out either by applying and pyrolyzing the layers of the solution, or using sol-gel technology.

A known method of producing citrate "sol" described in [3] which is as follows. Solutions of the metal components in nitric acid and separately a solution of citric acid in ethylene glycol are prepared. The solutions are drained and heated at a temperature of 343 363 ^o K for several hours until the formation of a viscous solution ("gel"). Volumetric ceramics are made from the obtained “gel”. A detailed description of the methodology presented allows us to identify both its positive and negative sides.

 The main advantage of the described method is the refusal to increase the pH of the solution. There are several options for citrate sol-gel method. Part of them consists in increasing the pH of the solution with the help of ammonia, triethanolamine and other bases [4–6], which leads to further dissociation of citric acid in the second and third stages. It is assumed that this leads to the formation of more durable complexes. But an increase in the pH of the solution often leads to the precipitation of bismuth oxosalts and insoluble alkaline earth metal citrates. The positive side is the use of dilute nitrate solutions, although the exact concentration is not indicated. At the first stage of complexation, water provides the dissolution of the complexes and prevents their salting out. When reproducing the well-known [3] methodology, its shortcomings were revealed. First of all, when the dosage of the components of citric acid 1 1 and ethylene glycol 2,3 1 per 1 mol of the total metal ion is difficult to obtain a "reversible gel", that is, a gel that easily forms a stable solution when mixed with ethylene glycol. An insufficient amount of citric acid 1 mol per mol of each metal ion of the HTSC component does not contribute to the formation of polynuclear citrate complexes. The amount of ethylene glycol solvent is clearly not enough to maintain the homogeneity of the solution during evaporation. The described disadvantages do not allow to obtain homogeneous products, which limits the applicability of the method.

 By its technical solution, the method described in the patent [4] which is chosen by us as a prototype is closest to the claimed method.

As precursors for the manufacture of coatings, a homogeneous solution containing the metal components of HTSC with a molar fraction of 5–30% is used. To prepare the solution, it is recommended to use chlorides, nitrates, citrates, oxalates, alcoholates, dissolved in various solvents. The solutions are applied to the substrate repeatedly until a thickness of 1,199 μm, preferably 1 5 μm, is reached. To achieve good adhesion, the decomposition temperature of the salts was chosen in the range of 1123–1123K. The coating is heated to 1223K, and then cooled at a rate of 1 3K / min in an oxygen atmosphere to 673 773K. At this temperature, the coating is kept for 4-16 hours and cooled to room temperature at a rate of 1 3K / min. According to this method, a YBa ₂ Cu ₃ O _{6 + x} coating with T _c 87K is obtained. The authors did not give the critical current density in the coatings obtained.

The described method has several significant disadvantages:
yttrium HTSC compounds used to obtain coatings have low air resistance;
the proposed method for producing solutions by mixing salts and solvent cannot be considered successful, since it is applicable only in the case of well soluble salts of chlorides and nitrates. Citrates and oxalates are less soluble, especially for alkaline earth salts, and salting out of poorly soluble salts may occur, which leads to a violation of the stoichiometry of the solution;
pyrolysis of nitrates of strong oxidizing agents on metal substrates can lead to severe corrosion of the substrate, especially in the presence of bases - alkaline earth metal oxides;
it is not clear how the authors managed to carry out the pyrolysis of chlorides.

It can be assumed that copper and yttrium chlorides react with oxygen to form the corresponding oxides, but this cannot be said for BaCl ₂ , which is stable in air even above 1233K;
the prototype essentially does not contain any specific example of the preparation of precursors, as well as quantitative characteristics of the obtained HTSC coatings. Reliable are only temperature ranges that do not have significant differences from the well-known.

The described disadvantages do not allow to obtain high-quality HTSC coatings with high critical currents of the order of 10 ³ 10 ⁴ A / cm ³ at a temperature of 77 K for use in high-current technology.

 The objective of the invention is to increase the stability, hemogenicity and wetting ability of the initial solutions of the complexes of metal components of HTSC with anions of polybasic organic hydroxy acids intended for the formation of HTSC coatings, as well as increase the density and uniformity of the coating made from this solution, increase its adhesion to the substrate and achieve higher critical currents.

 This problem is achieved by the fact that in the method of producing an HTSC coating on a substrate, comprising preparing an initial solution of salts of the metal components of HTSC and polybasic organic hydroxy acids in polyhydric alcohol, applying it to the substrate, followed by thermal arrangement of the deposited layer, anneals them to impart superconducting properties to the coating, according to the invention, the initial solution is obtained by mixing an aqueous solution of nitrates of the metal components of HTSC with a total concentration of cations of not more than 1.5 mol / l with thief citric acid in ethylene glycol, citric acid in respect of at least 2 1 and ethylene at least 7 1 moles per 1 mole of metal cations HTS components, then the solution was maintained at 353K 343 to form a viscous polymer solution.

 Thermal decomposition of the deposited layer is carried out in air by high-speed heating, and the temperature rises to 1113 1143 K within 20 30 seconds. As the substrate, either metal or ceramic material, resistant to temperature differences up to 900K, can be used.

Obtaining a citrate precursor according to the proposed method consists in preparing two solutions of a solution of metal nitrates of HTSC components in water and a solution of citric acid in ethylene glycol, when these two solutions are merged, metal cations interact with citric acid, and citrate complexes are formed
Me (NO ₃ ) _n + H ₃ Cit MeH _3-n Cit + nHNO ₃ (1)
the formation of complexes containing several cations and citric acid residues is also possible.

The mixture of the two solutions is heated for several (10 20) hours at a temperature not exceeding 353 K, which leads to the decomposition of nitric acid formed as a result of the exchange reaction
HNO ₃ + (CH ₂ OH) ₂ H ₂ O + CO ₂ + NO (or NO ₂ ) (2)
as well as water removal and polycondensation of citrate complexes and excess citric acid with ethylene glycol
HO-R ₁ COOH + HO (CH ₂ ) ₂ OH + HO-R ₂ -COOH HO-R ₁ -COO - (CH ₂ ) ₂ -OR ₂ -COOH + 2H ₂ O (3)
where R ₁ and R ₂ can relate to either the metal complex or citric acid. As the water is removed, the solution thickens and turns into a so-called “gel”, which is actually a viscous solution of polymers in ethylene glycol.

 An excess of citric acid was taken to create the necessary conditions for the formation of polynuclear citrate complexes. An excess of ethylene glycol of 7 mol for each mol of metal ion of the HTSC component contributes to the unhindered reaction (3). The upper limit of the concentration of the metal ion in the citrate solution is due to the poor solubility of oxynitrate, as well as the salting out of alkaline earth metal nitrates with an excess of nitric acid, which is necessary to prevent the deposition of basic bismuth salts.

 The upper limit of the solution thickening temperature of -353K is due to the danger of decomposition of the complexes and the precipitation of salts, and, as a result, loss of homogeneity. The lower limit of the thickening temperature of the 343K solution is due to the need to remove water and the decomposition of nitric acid released as a result of reaction (2). The thickening time was chosen so that it passed to the end of reaction (2) and (3).

 Thus obtained "gel" is stable for a long time (several months), easily forms a homogeneous solution when diluted with ethylene glycol. This makes it possible to control the concentration of components and the viscosity of the solution, and accordingly the thickness of one coating layer.

The method of obtaining consists of the following operations:
dissolution upon heating of copper and bismuth oxides in nitric acid;
dissolution of calcium carbonates and strontium in nitric acid;
merging the resulting solutions and diluting them with water;
preparation of a solution of citric acid in ethylene glycol and mixing with these solutions at 343 353K and stirring for 10 20 hours until a viscous solution forms;
applying the resulting viscous solution to the surface (wire). Depending on the required coating thickness, this operation is repeated several times;
thermal decomposition of the applied layer at 1113 1193K for 20-30 s;
annealing the deposited coating in the temperature range 1103–1133 K for 6–60 h to impart superconducting properties to the coating.

 Example 1

4.67 g of Bi ₂ O ₃ , 1.59 g of CuO are dissolved in 20 ml of HNO ₃ (solution 1) with heating. Separately, 2.95 g of SrCO ₃ and 1.00 g of CACO _{3 are} dissolved in a mixture of 10 ml of HNO ₃ and 100 ml of H ₂ O (solution 2). 45 g of citric acid are dissolved in 80 ml of ethylene glycol (solution 3). Solutions 1 and 2 are poured together and solution 3 is added. The resulting mixture is heated for 15 hours at a temperature of 343 K with continuous stirring until a viscous solution forms. If necessary, the resulting viscous solution is diluted with ethylene glycol to the desired consistency (usually 2 3 times). The cleaned surface (or wire) is immersed in the prepared precursor solution and slowly removed from it, then quickly introduced into the furnace heated to 1123 K and held there for 20 30 s. This operation is repeated 10 to 20 times, depending on how thick the coating needs to be obtained (thickness increase is approximately 1 μm per application operation).

To impart superconducting properties to the coating, annealing is carried out at 1108 K for 20 hours. The resulting coating has a composition of Bi ₂ Sr ₂ CaCu ₂ O _{8 + x} and has a superconducting transition temperature of 70 77 K, the critical current density measured by the non-contact method at 4.2 K is 10 ⁵ A / cm ² .

 Example 2

4.67 Bi ₂ O ₃ , 2.63 g CuO and 0.62 g PbO are dissolved in 40 ml of HNO ₃ (solution 1) with heating. Separately, 2.95 g of SrCO ₃ and 2.00 g of CaCO _{3 are} dissolved in a mixture of 30 ml of HNO ₃ and 100 ml of H ₂ O (solution 2). 60 g of citric acid are dissolved in 120 ml of ethylene glycol (solution 3). Solutions 1 and 2 are poured together and solution 3 is added. The resulting mixture is heated for 20 hours at a temperature of 343 K with continuous stirring until a viscous solution forms. If necessary, the resulting viscous solution is diluted with ethylene glycol to the desired consistency (usually 2 3 times). The cleaned surface (or wire) is immersed in the prepared precursor solution and slowly removed from it, then quickly introduced into the furnace, heated to 1153 ^o K and kept there for 20 -30 s. This operation is repeated 10 to 20 times, depending on how thick the coating needs to be obtained (thickness increase is approximately 1 μm per application operation).

To impart superconducting properties to the coating, annealing is carried out at 1113 K for 15 hours. The resulting coating has a composition of Bi _1.8 Pb _0.2 Sr ₂ Ca ₂ Cu ₃ O _{10 + x} and has a superconducting transition temperature of 96 105 K, the critical current density measured by the non-contact method at 4.2 K is 10 ⁴ A / cm ² .

 Example 3

4.67 g of Bi ₂ O ₃ , 2.63 g of CuO in 40 ml of HNO _{3 are} dissolved with heating (solution 1). Separately, 2.95 g of SrCO ₃ and 2.00 g of CaCo _{3 are} dissolved in a mixture of 30 ml of HNO ₃ and 100 ml of H ₂ O (solution 2). 60 g of citric acid are dissolved in 120 ml of ethylene glycol (solution 3). Solutions 1 and 2 are poured together and solution 3 is added. The resulting mixture is heated for 10 hours at a temperature of 343 K with continuous stirring until a viscous solution forms. If necessary, the resulting viscous solution is diluted with ethylene glycol to the desired consistency (usually 2-3 times). The cleaned surface (or wire) is immersed in the prepared precursor solution and slowly removed from it, then quickly introduced into the furnace heated to 1173K and held there for 20-30 s. This operation is repeated 10 to 20 times, depending on how thick the coating needs to be obtained (thickness increase is approximately 1 μm per application operation).

To impart superconducting properties to the coating, annealing is carried out at 1118 K for 20 hours. The resulting coating has the composition Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _{10 + x} and has a superconducting transition temperature of 100 107 K, the critical current density measured by the non-contact method at 4 , 2K, is 3.10 ⁴ A / cm ² .

Since the prototype does not contain any specific example indicating the quantitative characteristics of the obtained high-temperature superconducting coatings, we consider it possible to compare the products we obtained with those known from the literature [7 10] Examples [7 10] are selected in such a way that the material of high-temperature superconductors bismuth cuprates and the substrate material - silver is the same as in our products. Obviously, a high adhesion of the coating to the substrate while maintaining continuity will be characterized by a change in the critical current density upon application of mechanical stresses, such as tension, bending, etc. The table shows 3 intervals of mechanical stresses:
1 reversible, upon stress relieving, the initial superconducting characteristics of the coating are retained;
2 voltage interval within which the critical current decreases to a value equal to 50% of the initial value;
3 is the voltage range, upon application of which the critical current in the superconductor drops to zero.

Thus, the products obtained by the proposed method, which are bismuth-containing HTSC coatings on silver, retain superconducting properties (flow of macroscopic or local superconducting currents) up to very large degrees of deformation up to 30%
The high parameters characterizing bismuth-containing HTSC coatings on silver, namely the critical currents of 10 ⁴ 10 ⁵ A / cm ² at T 4.2 77 K, are due to the high adhesion of the coating to the substrate while maintaining its continuity and stoichiometry. In turn, it was possible to achieve high adhesion of the coating to the substrate while maintaining the continuity and stoichiometry of the coating due to the developed method for synthesizing the precursor and the pyrolysis mode. The homogeneity of the precursors and their high stability make it possible to obtain coatings with the cation ratio the same as in the precursor. The stoichiometry of cations determines the direction of the phase formation reaction, as well as the rate and complete conversion. Obtained according to the invention, the application solutions are stable for a long time, maintain homogeneity, while the viscosity and concentration of the components of HTSC can change in the right direction. The phase formation process using these precursors is completed within a few hours, after which the coating becomes superconducting.

The method allows to obtain coatings with higher critical currents
up to 10 ⁵ A / cm ² .

Sources of information
1. Seminozhenko V.P. Sobolev V.L. Khirny V.F. Reviews of high temperature superconductivity. 1991 - issue. 1 (5). p. 19 43.

 2. Grigoriev G.Yu. High Temperature Superconductivity Reviews 1990, no. 2 p. 25 69.

 3. Azaka T. Okazawa Y. Hirayama T, Tachikawa K. Jap. J. Appl. Phys. 1990. V.29, N 2 P. L280 L283.

 4. Mulav V.N. J. Mater. Sci.Lett. 1990. V. 9 P.1284 1287.

 5. The patent of Germany (DE) s. N 38382856, op. 07/06/89, C 04 B 35/00, 35/50, 41/87; H 01 B 12/00, H 01 L 39/24.

 6. The patent of Germany (DE) s. N 3815460, op. 11.16.89, C 04 B 35/00, C 23 C 28/04, H 01 L 39/24.

 7. Togano K.et al. J. Appl. Phvs. 1991. V.70. N 11. P6966 6969.

 8. Osamura K.et. Adv, Grvogen.Eng. (Materials). 1992. V38. P. 875 881.

 9. Kuroda T. et al. Adv, Gryogen. Eng. (Materials). 1992. V.38. P1045 1051.

 10. Ekin J. W. et al. Adv.Cryogen.End. (Materials). 1192. V.38. P1041-1043.

Claims (2)

 1. A method of obtaining a high-temperature superconducting coating on a substrate, comprising preparing an initial solution of metal salts - components of a high-temperature superconducting compound and polybasic organic acids in polyhydric alcohol, applying it to a substrate, followed by thermal decomposition of the applied layer and annealing to impart superconducting properties to the coating, characterized in that the initial solution is prepared by mixing an aqueous solution of nitrates of the metal components of the high-temperature super conductive compound with a total concentration of cations of not more than 1.5 mol / l with a solution of citric acid in ethylene glycol with a ratio of citric acid of at least 2 1 and ethylene glycol 7 1 moles per 1 mol of cations of the metal components of the high-temperature superconducting compound, then the prepared solution is kept at 343 353K before the formation of a polymer viscous solution, and the thermal decomposition of the applied layer is carried out in air by high-speed heating.  2. The method according to p. 1, characterized in that the thermal decomposition of the applied layer of the solution is carried out at a temperature of 1113 1193K.

Images