CS270392B1 - Process for preparing a high temperature superconductor - Google Patents

Abstract

The solution concerns the preparation of a chemical precursor of a high-temperature superconductor by controlled crystallization by precipitation in the presence of a hydrophilic colloid. This preparation method allows for the deposition of thin and thick layers of the superconductor and the creation of compact bodies or superconductor powder.

Description

The invention relates to a method for preparing high-temperature superconductors of the A-B-0u-0 type by controlled crystallization by precipitation from a seed, where A represents a cation in the 3* oxidation state with an ionic radius in the range of 0.080 to 0.120 nm, or any combination of such ions, and B represents a calcium, strontium or barium ion, or any combination of these ions.

High-temperature superconductors with critical temperatures higher than the boiling point of liquid nitrogen offer a wide range of applications in technical practice. For their practical use, it is necessary to master a defined method of their preparation, when it is necessary to obtain superconducting materials not only with the required critical temperature T _o , but also with the required critical current density J _Q .

In the published literature, one can find a number of methods for preparing high-temperature superconductors, which can be divided in principle into small-scale preparation and large-scale preparation. Since the literature on the preparation of high-temperature superconductors is very extensive, it will be useful for easier orientation to classify the methods for preparing high-temperature superconductors according to their principle. Such a division of the methods for preparing high-temperature superconductors is given in Table I. For practical applications, it is necessary to master the preparation of various shapes of high-temperature superconductors: tapes, wires, thin and thick layers and their application on various substrates, compact bodies, etc. Although this problem is addressed by a huge number of published works, published in a number of professional journals, in proceedings of symposia and congresses or in patent applications, it turns out to be very difficult to optimize the previously known methods of preparation so that high-temperature superconductors with a high critical current density are prepared. ^χ ο is particularly relevant to methods for preparing high-temperature superconductors on a large scale (see Table I).

Table I

Methods for preparing high-temperature superconductors

Small-scale methods

Applying thin layers

Sputtering

Swagger

Organometallic layer

Precursors

Creating patterns with effective ions and reactive electrons

double ion beam nagnetron reactive in very high vacuum thermal laser evaporation electron beam discharge epitaxy molecular beam epitaxy solid phase/vapor/liquid deposition from organometallic vapor from a polymer precursor layer photolithography ion beam or laser etching ion beam machining ion implantation ion plasma screen printing electron beam lithography

CS 270 392 Bl

Single crystals from melting

Large-scale methods

Coatings

Powder production

Compaction of sleepers

chemical vapor deposition thermal spraying electrochemical deposition solid phase reactions sol - gel chemical precursors

hot pressing hot isostatic pressing clinker forging rapid compaction dynamic tamping extrusion, slurry casting fiber drawing, cladding

The aim of the invention is to find a method for preparing high-temperature superconductors that would allow for the creation of thin and thick layers, their deposition on various substrates, the creation of tapes or compact superconductors, and at the same time ensure a relatively high critical current density of such a superconductor.

The subject of the invention is a method for preparing a chemical precursor of high-temperature superconductors by controlled cryo-crystallization by precipitation from a solution in the presence of a protective colloidal environment. The function of the protective colloidal environment is:

1. contribute to the creation of a chemical precursor of a high-temperature*superconductor with a relatively narrow particle size distribution function,

2. ensure the separation of the suspension of the chemical precursor of the high-temperature superconductor from the solution,

3. ensure the creation of the desired shape of the high-temperature superconductor: coating a thin or thick layer, shaping into the desired compact shape.

The possibilities and methods of using the present invention will be described in detail in the following text and in the examples.

Controlled crystallization by precipitation allows the preparation of sets of colloidal particles with the following properties:

1. narrow particle size distribution,

2. the same concentration profile in the volume of each Particle,

3. the same state on the surface of all particles (particle morphology, epitaxy).

The preparation of sets of monodisperse particles with the above properties plays an important role in the production of catalysts, progressive ceramics, pigments, pharmaceuticals, photographic emulsions, magnetic recording media, etc. In the preparation of progressive ceramics, obtaining monodisperse particles allows the creation of ceramic materials with some more advantageous properties: higher specific density, hardness, ductility, etc.

To obtain particle sets with a narrow size distribution function controlled by precipitation crystallization, it is necessary to conduct the process so that the following conditions are met:

1. separation of the nucleation phase from the growth phase,

2. prevention of coagulation,

3. selection of a growth model leading to a monodisperse set of particles,

4. suppression of Ostwald ripening,

5. method of monomer inflow into the system,

6. homogeneous mixing.

OS 270 392 Bl

The above requirements for the preparation of nanodispersed particles can be met by the method of controlled precipitation crystallization. This method consists in that reactants flow into the crystallizer in two or more streams under controlled conditions, such as: the rate and composition of the reactants flowing into the crystallizer, the concentration of precipitated ions in the crystallizer, temperature, pH, concentration and type of solvents, concentration and type of growth modifiers, concentration and type of protective colloidal medium, etc.

The selection of reactants, by the reaction of which a chemical precursor of a high-temperature superconductor is prepared, is important. The composition of cations is determined by the desired type of high-temperature superconductor, the chemical precursor of which is to be prepared. The selection of a suitable anion is subject to the condition that it forms a sparingly soluble compound with the cations flowing into the crystallizer. A sparingly soluble compound in this case can be considered a compound whose solubility product has a value higher than pK_> 6.0. A number of anions 0 that can be advantageously used for a given purpose meet the above condition. The following anions can be mentioned among the important anions for the above method: carbonate, itavelate and 8-hydroxyquinoline.

The reactant flow into the crystallizer and the reaction conditions in the crystallizer are chosen so that after a certain time the nucleation (i.e. the formation of stably growing nuclei) is completed and that in the next phase all reactant flow serves only to grow a constant number of stable nuclei. In this way the nucleation phase is separated from the growth phase.

To prevent particle coagulation, it is necessary to stabilize lyophobic colloidal particles by precipitation in the presence of a protective colloidal environment, such as: lyophilic polymers, surfactants or complexing agents that adsorb on the particle surface. The stabilizing effect of these substances lies in the repulsive force due to Coulomb repulsive forces, in the ionic pressure and in the steric hindrance of the overlapping areas of the adsorbed layers on the individual particles.

Some of the following hydrophilic colloids may be used as suitable protective agents: derivatives of polyacrylic acid, gelatins made from bones or skins of animals or fish, gelatin derivatives, grafted polymers of gelatin and other polymers, albumin, casein, cellulose derivatives, e.g. hydroxyethylcellulose, carboxymethylcellulose, cellulose sulfate, sugar derivatives, starch derivatives, various synthetic hydrophilic high molecular substances such as polyvinyl alcohol and its derivatives, poly-N-vinylpyrrolidone, polymethacrylic acid and its derivatives, polyacrylamide, polyvinylimidazole, polyvinylpyrazole, polyvinylmethoxMOlidone, copolymers containing repeating units of the above polymers. Other hydrophilic colloids may also be used. In the case of gelatin, products resulting from hydrolysis or cleavage by enzymes may also be used, thereby obtaining gelatins with a low average molecular weight.

Ostwald ripening of growing crystals is prevented by reducing the solubility of the system in the crystallizer. The solubility can be reduced in several ways: during precipitation, a certain excess of the precipitated anion is set in the crystallizer, a suitable pH value is set, the crystal surface is stabilized by the addition of suitable organic substances, the temperature of the system is reduced, etc.

The concentration of precipitated ions in the crystallizer can be monitored using ion-selective electrodes in combination with a reference electrode and regulation of the ion inflow into the crystallizer to maintain this monitored concentration at the required value. Similarly, the pH of hydrogen ions is monitored during the precipitation process and the required value is set by the inflow of acid or base. In this way, the solubility of the system is very effectively maintained at the required value.

Reducing the solubility of the system not only suppresses Ostwald ripening, but in the nucleation phase the solubility value significantly affects the number of nuclei formed, which then remain in the system, which fundamentally affects the average crystal size. It is very well known from the production of progressive ceramic materials how the average particle size of the chemical precursor affects the course and result of the heat treatment of the chemical precursor to the prepared ceramic material. In the preparation of ceramic oxides, e.g. alumina,

OS 270 392 B1 zirconium oxide, yttrium oxide, etc., has shown that a mean particle size in the range of 0.1 µm to 0.5 µm is suitable for heat treatment of the chemical precursor.

The average particle size and stability to Ostwald ripening will very effectively affect the addition of such substances that adsorb onto the particle surface, or form sparingly soluble compounds with precipitated ions. The condition for selecting and using the necessary substances for a given purpose is their ability to form sparingly soluble compounds with precipitated ions. The value of the solubility product of these compounds should be higher than pK*>7.0, where the solubility product pK' · - log K*. Of the number of compounds satisfying this condition, many of them are known from analytical chemistry, where they are used for the determination of the mentioned cations, such as 8-hydroxy-quinoline and kupferon.

Another possibility of suppressing the effects of Ostwald ripening during particle growth is to increase the molar inflow of reactants into the crystallizer. In this way, the entire precipitation process is accelerated, and a set of crystals with a narrow size distribution function is obtained. The rate of inflow of reactants into the crystallizer cannot be increased arbitrarily, but only up to the so-called critical inflow rate, when all added growth material serves to grow already existing crystals in the crystallizer. The condition for increasing inflow is therefore not to exceed a certain supersaturation value, below which new nuclei could form in the crystallizer and continue to grow. ¹

The temperature of the system has a great influence on the solubility. The precipitation temperature can be varied in a wide range from 1 to 90 °C. The choice of the precipitation temperature depends on the required properties of the chemical precursor of the high-temperature superconductor being prepared. If it is necessary to reduce the average particle size, it is necessary to reduce the temperature of the system. The working temperature range depends on the type of protective colloidal environment, which does not lose its protective colloidal effects in a given temperature interval. For example, gelatin made from animal bones or skins has a sol-gel transition at a temperature of 31 to 33 °C, while gelatin made from fish bones or skins has a sol-gel transition temperature of 8 to 10 °C. This is the reason why precipitation cannot be carried out at lower temperatures, when the hydrophilic colloid turns into a gel. A further reduction in the precipitation temperature can be achieved by a suitable combination of the above-mentioned polymers, or by reducing the concentration of the hydrophilic colloid. This concentration cannot be reduced arbitrarily, as dilute solutions of hydrophilic colloids lose their protective colloidal effects. During controlled crystallization by precipitation, the concentration of hydrophilic colloid should be in the range of 0.05 to 10% by weight.

In addition to the cations that create the structure of the high-temperature superconductor, dopants that can modify some properties of high-temperature superconductors or stabilize high-temperature superconductor phases can be added to the crystallizer or to the incoming reactants during the entire precipitation process or only in some of its phases, e.g. before nucleation or after the precipitation is complete. Thus, any of the above dopants can be incorporated into the bulk of the chemical precursor or onto its surface, although in principle the choice of dopants can be much wider: silver, gold, alkali metals, magnesium, aluminum, gallium, indium, germanium, tin, lead, zinc, cadmium, chromium, manganese, iron, cobalt, nickel, platinum, osmium, iridium, etc.

After the precipitation process is completed, it is necessary to separate the obtained chemical precursor from unwanted impurities, such as soluble salts, solvents in the system, etc. Several methods can be used to separate the solid phase from the liquid phase, e.g. flocculation, decantation, filtration, noodleing with washing, diafiltration with semi-permeable membranes. If controlled crystallization is performed by precipitation in the presence of gelatin, it is advantageous to perform flocculation by changing the pH after the precipitation in the case of grafted gelatins, or to noodle the gel after cooling and wash it with washing solutions at low temperature.

After washing the flocculate, a concentrate of the chemical precursor is obtained, which is coated with a hydrophilic colloid. This concentrate can be redispersed in another portion of the hydrophilic colloid and the obtained suspension can be applied to various substrates such as: MgO, SrTiOj, ZrO ₂ , silicon, silver, etc. Similarly, washed noodles can be processed, which after dissolution

270 392 Bl tnings are applied to the substrates. The chemical precursor prepared in this way can be applied in thin or thick layers, tapes or compact bodies can be prepared: cylinders, tablets, etc. These wide possibilities are given by the properties of the hydrophilic colloid, whose adhesion to the substrates allows for a thin-layer or multi-layer coating, or removal of the hydrophilic colloid by heat treatment allows for obtaining a powder while simultaneously shaping it into the desired shape.

Since the hydrophilic colloid itself usually does not maintain the desired shape, layer thickness, etc., it is advisable to harden the shallow layer. The choice of hardener depends on the type of protective colloid used. For example, for gelatin, the following hardeners can be used, or any combination thereof: formaldehyde, glyoxal, glutardialdehyde, divinyl ketone, divinyl sulfone, 4,6-dichloro-s-triazine, 2,3-dihydroxydioxane, bis-2,3-epoxypropyl-ethylene glycol ether, diacetyl, 1,2-cyclopentanedione, Ν,Ν-dipropylcarbodiimide, mucochloric acid and its halogen- aldehyde derivatives, etc. Since the action of some hardeners is slow, auxiliary substances called hardening accelerators can be added to accelerate the hardening. In the case of hardening gelatin with formaldehyde, its effect is accelerated by the addition of resorcinol.

After coating the layer or forming the compact shape, drying is performed, the aim of which is to remove volatile substances. By increasing the temperature, the protective colloid decomposes in the range of 100 to 500 °C. Since part of the carbon remains in the material, which could form undesirable metal carbides, it is appropriate to heat the material in a stream of oxygen in the temperature range of 400 to 500 °C. This removes carbon from the system in the form of oxides. Further heat treatment of the chemical precursor is carried out according to a generally known procedure. Calcination is performed in the temperature range of 800 to 950 °C, when the superconducting phase begins to form. After the decomposition of the chemical precursor, pressure can be applied to thick or thin films, a similar process can also be used for compact bodies. After cooling to 300 to 550 °C, heating is performed in a stream of oxygen, which preferentially creates a superconducting phase in the material with a higher transition temperature.

The possibilities of using the present invention for the preparation of high-temperature superconductors will be documented in the following examples.

Example 1

The high-temperature superconductor YBa ₂ CUjO^_g was prepared by induced crystallization by precipitation in the following way:

initial conditions in the crystallizer 700 ml deionized water, 300 ml 10% inert gelatin, pH * 3.0 adjusted with oxalic acid, t 35 °C.

The following solutions were added to the crystallizer by controlled crystallization by precipitation: 1,000 ml of a 1.0 M solution of cations Y-$ ⁺ , Ba^* and Ou^* in a molal ratio of 1 : 2 : 3 and 1,000 ml of a 1.2 M solution of oxalic acid in ethanol, in such a way that the volumetric flow rate of the reactants into the crystallizer was linearly increased so that the flow rate at the end was 20 times higher than at the beginning and the flow time was 30 minutes.

During the inflow of the reactants, a constant pH value of * 3.0 was maintained by a controlled inflow of ammonia into the crystallizer and the concentration of oxalate ions at a value of pCgO^* = 2.0 by regulating the inflow of a 1.2 M oxalic acid solution.

After the precipitation was completed, the suspension was flocculated by adding acetone. After cooling to 10 °C, the flocculate was washed three times with washing water of pH 3.0 (the pH value was adjusted with oxalic acid). The washed flocculate was stored in a refrigerator and used for taking aliquots.

a) preparation of tablets:

A quantity of flocculate was taken from which 5 g could be prepared. The flocculate was dried for 12 hours at 100 °C to evaporate the liquid retained by the gelatin. Then calcination was carried out at 250 °C for 24 hours. In a further step at 400 °C, the powder was heated for 24 hours in a stream of oxygen to remove carbon. Then the temperature was increased to 930 °C for 10 hours. After cooling, tablets were pressed

OS 270 392 Bl ty, re-calcined at 930 °C and annealed in a stream of oxygen at 450 °C. The B - T _Q dependence was measured using the standard method and the T _e - 92 K value was measured.

b) preparation of a thick layer:

The flocculate was redispersed and after the addition of formaldehyde it was poured onto the MgO, SrTiO2, _ZrO2 (9mol% _Y2 °3^ ^T substrates in an amount such that the layer thickness was 100 µm (calculated * for a high-temperature superconductor). After curing, the layers were dried in a stream of air at a temperature of 35 °C. Then, the chemical precursor was thermally treated as in the previous case, with the difference that tablets were not pressed, but in some cases heating under pressure was performed. The following critical temperature values were measured:

washer T _c (K)

MgO76

SrTiO ₃ 85

ZrO ₂ 82.

c) preparation of a compact body: the flocculate was redispersed and after the addition of formaldehyde a cylinder was cast with dimensions: length 100 mm and diameter 10 mm. The thermal treatment was carried out as in the preparation of the tablet. The value T _c was measured to be - 85 K. Example 2

Controlled crystallization by precipitation of the chemical precursor TBs^Cu^O^^ was performed in a similar manner as in the first case, with the difference that the following conditions were set in the crystallizer:

000 ml of 3% gelatin solution made from fish bones, pH 3.0 (adjusted with oxalic acid, t 15 °C, 1 mmol of 8-hydroxyquinoline in 10 ml of ethanol.

After the precipitation was completed, the gelatin concentration was adjusted to 4 wt%, then the suspension was cooled to 1°C. The obtained gel was noodled and washed with washing water (pB = 3.0 adjusted with oxalic acid). The washed noodles were stored in the refrigerator.

The noodles were redispersed by heating to 15 °C and, after adding formaldehyde, were sprayed onto the substrates in such an amount as to obtain layers of superconducting material 1 µm thick. After heat treatment, the following values were measured:

washer T„ (K)

In MgO 88

SrTlOj 90.

Example 3

Controlled crystallization by precipitation of the chemical precursor TBa^u^O?^ was performed in a similar manner as in the first case, with the difference that a 3% solution of polyvinyl alcohol was used as the hydrophilic colloid and after the precipitation was complete, 1 mmol of 8-hydroxyquinoline in 10 ml of ethanol was added.

After flocculation, the sediment was washed and redispersed in another portion of polyvinyl alcohol. The suspension was then extruded as a fiber into a precipitation medium of aqueous solutions of NaOH and Na^SO^ at 60 °O and the fiber was wound onto a rotating drum at a speed of 1 m/s. Then the fiber was neutralized, washed and dried. After heat treatment, the critical temperature T was measured _{to be} 85 K. Example 4

Controlled crystallization by precipitation of the chemical precursor of the high-temperature superconductor BigCagSXgCu^C^ was performed similarly to the first example, with the difference that the 1.0 M cation solution was composed of cations Bp ⁺ - 0a ²⁺ - Sr ²⁺ - 0u ²⁺ in a molecular ratio of 1:1:1:2.

EN 270 392 B1

The heat treatment was modified for this type of high-temperature superconductor, i.e. calcination temperature 850 to 870 °C. The critical temperature T~80 K was found for the samples. c

Example 5

Controlled crystallization by precipitation of the chemical precursor of the high-temperature superconductor Bi.0a _o Sr _o 0u.0„ was performed similarly to the first example with the difference that 1.0 U

CCJ Λ ρψ Αψ ηψ * cation solution was composed of Bi ^J - Pb - Ca - Sr - Cu cations in the molar ratio 0.7 : 0.3 : 1.0 : 1.0 : 1.8.

After heat treatment, the critical temperature T _c » 103 K was measured.

Claims (1)

SUBJECT OF THE INVENTION A method for preparing a high-temperature superconductor of the type A - B - Cu - O, where A is a cation in the oxidation state 3+ with an ionic radius in the range of 0.080 to 0.120 nm, or any combination of such ions, and B is a calcium, strontium or barium ion, or any combination of these ions, characterized in that the chemical precursor of the high-temperature superconductor is prepared by controlled crystallization by precipitation with such an anion, 04. which forms with cations A, B and Cu poorly soluble compounds with a solubility product value of pK. >6.0, while controlled precipitation is carried out in the presence of a hydrophilic colloid with a concentration of 0.05 to 10% by weight, where the hydrophilic colloid may be, for example, derivatives of polyacrylic acid, gelatins made from bones or skins of animals or fish, gelatin derivatives, grafted polymers of gelatin with other polymers, albumin, casein, cellulose derivatives, ochre derivatives, starch derivatives, synthetic hydrophilic high-molecular substances such as polyvinyl alcohol and its derivatives, poly-N-vinylpyrrolidone, methacrylic acid and its derivatives, polyacrylamide, polyvinylimidazole, polyvinylpyrazole, polyvinylmethoxazolidone, or copolymers containing repeating units of the above polymers, and in individual phases of the precipitation process or during the entire precipitation process, a compound may be advantageously present which forms with cations A, B and Cu ²⁺ poorly soluble compounds with a solubility product value ρίζ>7.0, in an amount of up to 0.1 mol/mol of cations A, B and Cu ²⁺ , while controlled precipitation is carried out in the temperature range of 1 to 90 °C, pH « 0.5 to 12.0, at a concentration of the precipitated anion in the crystallizer - log o = 0.5 to (pK _g - 2).