CS273935B1 - Method of high-temperature superconductor preparation - Google Patents

Abstract

The essence of the invention is the preparation of the chemical precursor of a high-temperature superconductor and its doping with ions of silver, gold or platinum by controlled crystallisation by precipitation. This method of preparation of high-temperature superconductors allows a precursor with mixing of components on the sub-micron level with a defined gradient of components with high ability to sinter. Doping the high-temperature allows the increase by a desirable method of electric, magnetic and mechanical characteristics of thin and thick layers or tablets.

Description

The present invention relates to a process for the preparation of AB-Cu-0 high temperature superconductors by solution precipitation crystallization, wherein A represents a cation in oxidation stage 3+ with an ionic radius in the range of 0.080 to 0.120 nm or any combination of such ions, and B represents a cation in oxidation stage 2+ with an ion radius in the range of 0.090 to 0.140 nm, or any combination of such ions.

High temperature superconductors with critical temperatures above the boiling point of liquid nitrogen offer a wide range of applications in engineering practice. For their practical use it is necessary to master a defined method of their preparation, when it is necessary to obtain superconducting materials with the following properties:

- the material should have a high current density and should not lose its properties even in the presence of strong magnetic fields,

- the material should have good mechanical properties, in particular sufficient fracture toughness and flexibility,

- the method of preparation should allow reproducible preparation of high temperature superconductor on a large scale.

A number of methods for preparing high temperature superconductors can be found in the published literature, which can in principle be divided into small-scale and large-scale preparation. Because the literature on the preparation of high temperature superconductors is very extensive, it will be useful to classify the methods of preparing high temperature superconductors according to their principle for ease of reference. Methods for preparing high temperature superconductors are shown in Table I.

For practical applications it is necessary to manage the preparation of various shapes of high temperature superconductors: tapes, wires, thin and thick layers and their application on various substrates, compact bodies and the like. A number of papers have been published in the literature, showing how difficult it is to optimize large-scale methods leading to obtaining high-temperature superconductors with the desired electrical, magnetic and mechanical properties that are additionally resistant to the undesirable effects of superconducting aging, e.g. atmosphere.

It is an object of the present invention to provide such a process for preparing high temperature superconductors which would allow the formation of thin or thick layers, their application to various substrates, the formation of tapes or compact superconductors while ensuring the required electrical, magnetic and mechanical properties.

It is an object of the present invention to provide a process for preparing a high temperature superconductor of the type A-B-Cu-Q.

A represents a cation in oxidation stage 3+ with an ionic radius size of 0.080 to 0.120 nm, or any combination of such ions, B is a cation in oxidation stage 2+ with an ionic radius of 0.090 to 0.140 nm, or any combination of such ions. The superconductor is doped with silver, gold or platinum atoms. Process for the preparation of the superconductor is characterized in that the chemical precursor of the high temperature superconductors and the dopant is prepared in one batch or separately controlled during crystallization by precipitation with such anion which forms with the cations of A, B, Cu, and dopant sparingly soluble compound on the value of the solubility product pK _a ≤ 6 , 0. Controlled precipitation is carried out in the presence of a hydrophilic colloid type of acrylic acid derivative, gelatin made from bones or skins of animals or fish, gelatin derivatives, gelatin grafts, graft polymers gelatin with other polymers, albumin, casein, cellulose derivatives, sugar derivatives, starch derivatives. Furthermore, synthetic hydrophilic high molecular weight substances such as polyvinyl alcohol and its derivatives 273 935 B1, poly-N-vinylpyrrolidone, polymethacrylic acid and derivatives thereof, polyacrylamide, polyvinyl imidazole, polyvinylpyrazole, polyvinylmethoxazolidone, or copolymers containing the repeating units of the above-mentioned polymers. The precipitate formed is stabilized by adding further compounds forming cations of the above compound málorozpustné value and solubility _PK)> Z 0, in an amount up to 0.1 mol / 1 cations and A, B, and Cu dopant. Controlled precipitation is carried out in the temperature range of ¹⁰ to 90 ° C, pH 0.5 to 12.0 at the concentration of precipitated anion in the crystallizer - log = 0.5 to (pK _s -2).

A high temperature superconductor and dopant chemical precursor suspension is prepared by controlled precipitation crystallization separately. They are mixed or applied in layers before being applied to the substrate.

The dopant is added to the crystallizer before, at the same time or after the cation of the cations A, B and Cu in one batch.

The function of the dopant added at some stage in the preparation of the high temperature superconductor is to:

1. Create an inert layer at the substrate-high temperature superconductor transition that prevents. diffusion and reaction of the high temperature superconductor with the substrate,

2. to provide an insulating layer that stabilizes the surface of the high temperature superconductor against environmental influences such as air humidity, carbon dioxide, and the like;

3. increase the reactivity of the superconductor grains to sintering and achieve high specific masses close to the theoretical value,

4. improve the mechanical properties of the high temperature superconductor.

Great emphasis is placed on selecting a suitable dopant to coat the superconductor surface. The following requirements should be met to select the appropriate dopant:

1. a comparable temperature coefficient with a high temperature superconductor;

2. sufficient mechanical strength,

3. the melting point of the dopant should be higher than the heat treatment temperature of the high temperature superconductor;

4. chemical inertness to high temperature superconductor;

5. oxygen permeability at temperatures at which the stoichiometry of the cations to oxygen is adjusted;

6. chemical inertness to oxygen;

7. relatively cheap material and available.

As a result of these strict requirements, the choice of dopant is very limited to silver and gold. From an economic point of view, it is preferable to use silver which melts in air at 961 ° C and in an oxygen atmosphere at 940 ° C. Metallic silver at high temperatures does not oxidize and permits oxygen. When silver oxide or carbonate is heated, it decomposes to metallic silver and other reaction residues at temperatures below 500 ° C.

Several methods of silver doping of high temperature superconductors have been published in the literature. Yurek (priority 26.1.1987) prepared metal alloy YBa ₂ Cu ₃ Ag, which was

CS 273 935 B1 oxidized in a clever manner and resulted in high-quality, high-temperature superconductors. A large group of works dealt with the conventional preparation of a high temperature superconductor: a mixture of oxides or carbonates was heat treated, the obtained high temperature superconductor was ground and mixed with silver oxide. However, this procedure cannot overcome the segregation of the components at the micrometer level. Another series of work dealt with the preparation of thin layers on a suitable support so that the individual cations were applied gradually including silver atoms. Thus, thin layers of silver-doped superconductor were prepared. The disadvantage of this process is its small scale and the use of complex apparatuses. The high temperature superconductor preparation method proposed in the present invention is a relatively simple, inexpensive and reproducible process. The chemical precursor of the high temperature superconductor and its doping with silver or gold are performed by controlled crystallization by precipitation in one or more steps. By performing the preparation of the high temperature superconductor according to the proposed method, the following is achieved:

1. The high temperature superconductor and dopant segregation is suppressed into the submicrometer range;

2. coating of all grains, high temperature superconductor with dopant of uniform thickness can be ensured,

3. for thin or thick layers, their thickness and composition can be assured at a submicrometer level.

The preparation of a high temperature superconductor by controlled crystallization by precipitation consists in the fact that in a two-stream precipitation the solutions of the mixture of cations and the precipitant are fed simultaneously but separately to the crystallizer, where the reaction conditions are kept at the required values during the precipitation process. In this way, the reactive steamers at the submicrometer level are intimately mixed and co-precipitated. If a suitable lyophilic colloid is used, after the unwanted byproducts of the precipitating reaction have been removed, a very defined coating can be carried out: uniform coating thickness and segregation of the components at the submicrometer level. In subsequent steps during drying, the volatiles are removed and during heat treatment the lyophilic colloid decomposes and the chemical precursors decompose. During generally known methods, the necessary stoichiometry of the cations to oxygen is formed, thereby creating a superconducting phase.

A feature of the present invention is a method of doping a high temperature superconductor with silver or gold atoms by controlled crystallization by precipitation, wherein doping can be performed in one step together with the chemical precursor preparation or in two or more steps. doping occurs when two suspensions are mixed or the layers are glazed onto a support.

The preparation process is illustrated by the accompanying drawings, in which Fig. 1 shows the course of drying a superconductor precursor in an air stream, Fig. 2 shows the thermal mode of superconductor preparation, and Fig. 3 schematically illustrates the conversion of the precursor to a high temperature superconductor and its densification.

Comparative example

Controlled crystallization precipitation were prepared by high-temperature superconductors YBazCUjO? _S ^ and Bij Pbg ^ Ca ^ SRJ GCU-j ₆ 0 _χ follows:

CS 273 935 B1 initial conditions in the crystallizer:

000 ml deionized water + ethanol (1 l) g inert gelatin pH = 5.0 Ρ ^ οΟλ = 2,0 t = 35 ° C.

Controlled crystallization by precipitation with the following solutions was added to the crystallizer:

(a) 1000 ml of a 0,7 M solution of Y ³⁺ , Ba ²⁺ and Cu ²⁺ cations in a molar ratio of 1,2: 3a

000 ml 0,7 M ethanolic oxalic acid solution,

(b) 1000 ml of a 1,0 M solution of cations Bi ^ ⁺ , Pb ²⁺ , Ca ²⁺ , Sr ²⁺ , Cu ²⁺ in a molar ratio (1,6: 0,4: 2: 1,8: 3; 6) and 1000 ml of 1.0 M ethanolic oxalic acid solution.

The solution flow method was chosen such that the volumetric flow rate of the crystallizer was linearly increased so that the end flow rate was 20 times higher than initially and the total flow time was 30 minutes. ^r

A constant pH = 5.0 was maintained during the reactant feed by a controlled flow of ammonia into the crystallizer and the concentration of oxalate ions at a pC of _{20 A} =

2.0 by controlling the inflow of oxalic acid solution.

After precipitation, the suspension was flocculated by addition of acetone. · After cooling to 10 ° C flocculate was washed three times with washing solution: water + ethanol (1: 1) pH 5.0 and PC ₂ O _A = 2.0. The washed flocculant was stored in the refrigerator and used to collect the fractions.

Tablet preparation:

- an amount of flocculate was withdrawn from which 5 g of a high temperature superconductor could be prepared. The flocculate was dried for 12 hours at 100 ° C to evaporate the gelatin-retained liquid. Calcination was then carried out at 250 ° C for 24 hours. Next, the powder was heated at 400 ° C for 24 hours in a stream of oxygen to remove carbon. The temperature was then raised to 930 ° C for 10 hours. After cooling, the tablets were compressed, sintered again at 930 ° C and annealed in an oxygen stream at 450 ° C. The standard method measured the dependence of R - T _c and the measured value

T = 92 K a = 450 A / cm ² . cc

Layer preparation:

- the flocculate was redispersed and after addition of formaldehyde was poured onto MgO, SrTiOj pads,

Zr0 ₂ (9 mol% Y ₂ 0.j), θϊθ2 ^{in an} amount that the layer thickness is in the range of 20 to

100 µm (after heat treatment). The critical steps of layer preparation are drying the slurry and thermal decomposition of the lyophilic colloid, which determines the quality of the resulting layer, since inadequate for the colloid drying and decomposition stages various defects occur, such as microcracks, uneven frosting. .

3 ₊ 2 4 *

Fig. 1 shows the drying conditions for the oxalate suspension layer Y, Ba and

2+

Cu, precipitated in the presence of inert gelatin, on the previously mentioned substrates. On

EN 273 935 Bl Fig. No. 2 contains the conditions of the heat treatment chemical precursor, in this manner was prepared YBa ₂ CUj0 ₂ _F value = 90 K and Bi-gPbg 4 ^ ^and 2 ^ ^r l 8 ^ ^u 3 6 ^ x ^to SrTiOj close to T _c = 102 K.

Example 1

By controlled crystallization by precipitation a silver oxalate suspension was prepared as follows: 1X · ·· ‘/ Wj o“ initial conditions in the crystallizer:

000 ml deionized water + ethanol (1: 1) g inert gelatin pH = 5.0 pC ₂ 0 ₄ = 2.0 t = 35 ° C.

Controlled crystallization was added to the crystallizer by precipitation with a solution of 1.4 M silver nitrate and 0.7 M ethanolic oxalic acid solution. The method of inflow of the solutions was chosen such that the volumetric rate to the crystallizer was linearly increased so that the inflow rate at the end was 20 times higher than initially and the total inflow time was 30 minutes and the volume of the solutions was 1000 ml.

During the reactant feed, a constant pH = 0.5 was maintained by controlled ammonia feed to the crystallizer and the oxalate ion concentration at pC ^O ^ = 2.0 by controlling the inflow of oxalic acid solution.

After precipitation, the suspension was flocculated by addition of acetone. After cooling to 10 ° C, the flocculate was washed three times with a wash solution: water + ethanol (1: 1), pH = 5.0 and pC ₂ 0 ₄ - 2.0. The washed flocculant was stored in a refrigerator and was used to remove the portions for doping the high temperature superconductor with silver.

Silver doping was performed according to the scheme shown in Figure 3a. A silver oxalate slurry layer was poured onto the support, and after drying and decomposition of gelatin and silver oxalate at 500 ° C, a high temperature superconductor chemical precursor chemical precursor was poured onto the support. According to the foregoing procedures, the suspension was dried and heat treated. Found value T _c and J _c are given in Table II for each type superconductors. The molar ratio of silver to high temperature superconductor was 1: 10.

Example2

In this example, the chemical precursors of the high temperature superconductor and dopant were prepared according to the above procedures. The suspensions were loaded onto a support according to the scheme shown in Figure 3b. Closer to the support was a suspension with silver oxalate. The layer was dried and heat treated as previously described. The molar ratio of silver to high temperature superconductor was 1: 10.

Example 3

In this example, a three-layer coating was made according to the scheme shown in Figure 3c. The superconductor doped in this way was tested for the effect of water on its stability. Diving

CS 273 935 B1 superconductors into water and after drying it was found that the structure of the high temperature euponductor was not destroyed.

Example 4

In this example, both precursor suspensions were mixed prior to coating onto the support in a 1: 10 dopant Ag: superconductor precursor precursor molar ratio. The suspensions were stirred for 35 min at 35 ° C and then the suspension was poured onto the support according to the scheme shown in FIG. 3d.

Example 5

Chemical precursors were prepared by controlled precipitation crystallization as in the comparative example. After the cation of Y ³⁺ , Ba ²⁺ , Cu ²⁺ or Bi ³⁺ , Pb ²⁺ , Ca ²⁺ , Sr ²⁺ , Cu ²⁺ cations was terminated, solutions of 1.4 M silver nitrate and 0.7 M oxalic acid at a rate of 10 ml / min for 10 minutes. The reaction conditions in the crystallizer were similar to those in the high-temperature superconductor chemical precursor particle growth phase.

Table I Properties of silver-doped high temperature superconductors

Example Method of icing YBa ₂ CUjO ₇ ^B 1, 6 ^Pb 0.4 ^Ca 2 ^Sr 1, B ^Cu 3.6 ° x ^T c (K) ³ C (A / cm ² ) ^T c (K) ^J c (A / cm ² ) Cf. VSV SrTiO-j 82 350 78 480 1 VSV Ag SrTiOj 90 750 101 850 2 VSV Ag SrTiOj 92 1050 105 1200 3 Ag VSV Ag SrTiOj 92 1300 105 1400 4 mixture of precursor suspensions SrTiO-j 92 3500 103 4500 5 layered Ag SrTiOj 93 6500. 105 8500

VSV = high temperature superconductor

CS 273 935 Bl

Claims (3)

SUBJECT OF THE INVENTION A process for preparing a high temperature superconductor of the type A-B-Cu-0, wherein A represents a cation in oxidation stage 3+ with an ionic radius in the range of 0.050 to 0.120 nm, or any combination of such ions and B represents a cation in oxidation stage 2+ s an ionic radius in the range of 0.090 to 0.140 nm, or any combination of such ions which is doped with silver, gold or platinum atoms, characterized in that the chemical precursor of the high temperature superconductor and dopant is prepared in a single batch or separately controlled crystallization by precipitation with such anion , which forms with the cations A, B, Cu and dopant a low solubility compound having a solubility product of pK 6.0, wherein the controlled precipitation is carried out in the presence of a hydrophilic colloid of the acrylic acid derivative type, gelatin made from animal or fish bones or skins; , gelatin grafts , graft polymers of gelatin with other polymers, albumin, casein, cellulose derivatives, sugar derivatives, starch derivatives, synthetic hydrophilic high molecular weight substances such as polyvinyl alcohol and its derivatives, poly-N-vinylpyrrolidone, polymethacrylic acid and derivatives thereof, polyacrylamide, polyvinylimidinyl, polyvinyl imidazole, or copolymers containing the repeating units of the above polymers and the precipitate formed is stabilized by the addition of a cationic compound having a solubility product pK X of 7.0, up to 0.1 mol / mol of cations A, B, Cu and dopant, wherein the controlled precipitation is carried out in the temperature range of 1 ° C and 90 ° C, pH = 0.5 to 12.0, with the concentration of precipitated anions in the crystallizer - log c = 0.5 to (pK - 2). with 2. The process of claim 1 wherein the high temperature superconductor and dopant precursor chemical precursors are prepared by controlled crystallization by precipitation separately and mixed or applied in layers sequentially prior to application to the substrate, 3. The process of claim 1 wherein the dopant is added to the crystallizer prior to, at the same time or after the cation of the cations A, B, and Cu in one batch.