WO1991006510A1

WO1991006510A1 - Process for the preparation of oxide type superconductors and precursors obtained therefor

Info

Publication number: WO1991006510A1
Application number: PCT/EP1990/001823
Authority: WO
Inventors: Pierluigi Villa
Original assignee: Pierluigi Villa
Priority date: 1989-11-07
Filing date: 1990-10-31
Publication date: 1991-05-16
Also published as: AU6541390A

Abstract

Precursors for superconductors containing yttrium or rare earth metals, barium and copper oxides, are obtained by a particular method comprising the concentration from a solution and a particularly slow drying process of the residue and an oxidative decomposition of the dried residue characterized by a careful graduality in the ozone enriched oxygen feeding and in the heating rate.

Description

PROCESS FOR THE PREPARATION OF OXIDE TYPE

SUPERCONDUCTORS AND PRECURSORS OBTAINED THEREFORE

The present invention concerns a process for the preparation of precursors of superconductors , particularly of the kind RBa₂ Cu₃O _{7 -} δ (with R = yttrium or rare earth metal ) , or (Bi , Tl ) Z₂ ( Sr ,Ca ,Ba )_n+1 Cu_n O_2(n+2)-δ wherein δ ranges from 0 to 1 or RBa₂ CU₃O_9-ε wherein δ ranges from 0 to 2, starting from amorphous citrates. The invention also concerns the obtained precursors.

Superconductors are defined as systems consisting of metals, alloys or metal compositions which usually have no resistivity at very low temperatures. The temperature at which the resistivity of a superconductor becomes zero or reaches at least a minimal value, is defined as critical superconductivity temperature (T_c). The T_c of known superconductors, consisting of special alloys or of intermetal compounds, ranges from about 10°K to about 25K; their applications (superconductive wires for the manufacture of magnets; magnetic suspensions; special electronic apparatuses) are possible only using liquid helium as cooling liquid, with all the technological and economic problems connected with said use.

More recently, superconductors with higher T_c were obtained so as to allow the substitution of liquid helium with remarkbly less expensive fluids, much easier to handle, such as liquid nitrogen (b.p. 77K instead of 4K of helium). These are systems consisting of complex oxides, for instance yttrium, barium, copper oxides or the like, simply abbrevated to "L-M-Cu-O", wherein

L = yttrium, lanthanum and lanthanides

M = barium, strontium, calcium.

Said Y-Ba-Cu-O system, which contains in particular the various elements in atomic ratios according to the formula YBa₂Cu₃O_7-δ, turned out to be most interesting. In said formula δ ranges from 0 to 1. For instance, the systems such as Bi₂Sr₃CaCuO_{10 -} δ (bismuth may also be partially substituted by lead), Tl₂Sr₂Ca₂Cu₃O_10- δ and Tl₂Ba₂CaCu₂O_8-δ (wherein δ has the above stated meaning) are highly interesting.

These superconductors, with T_c higher than 90K, are prepared by finely grinding predetermined amounts of metal compounds, particularly oxides and/or carbonates , and heating the homogeneous mixture of the compounds to temperatures of 800-1000°C in air or oxygen stream.

The so obtained "calcined" product is then ground very finely, pressed and formed; it is then reheated to the above temperature, always in the presence of oxygen, and his causes the sintering of the superconductive compound.

As already stated, this process requires particularly careful grinding steps in order to give a very high homogeneity to the superconductor: however the homogeiity is never perfect since it depends not only on the granule size but also on the surface characteristics thereof. Moreover, the prolonged heating at high temperature involves an increase in the particle size, with the double effect of impairing the homogeneity, and making the sintening difficult.

In order to overcome said drawbacks and the technological complications connected therewith, several trials were carried out, mainly based on the initial use of solutions of soluble compounds of the desired metals (nitrates, citrates, tartrates and the like) instead of powder mixtures.

These solutions are then concentrated to a more or less marked level of anhydrification. The so obtained residue is then subjected to thermal decomposition in oxidizing atmosphere, so as to remove (besides the residual water) the organic nitrogen and/or carbon; the last process step consists of a calcination of the precursor which is transformed thereby in the actual superconductor.

Is is evident that in the initial step, the use of solutions provides homogeneity characteristics, which cannot otherwise be achieved even by the most careful grinding of solid starting compounds. However, the concentration and decomposition steps involve drawbacks including uncontrolled oxidation phenomena (even actual combustions) or other types of drawbacks impairing sometimes irreparably the necessary homogeneity of the desired precursor and limiting in any way the preparation scale because of the difficulty of controlling the phenomenon with amounts of a certain entity.

Process of this kind are reported, for instance, in Powder Technology 7 (1973), 21-38; FR 1.604.707; IT 1.205.041; J. Am. Ceram. Soc. 70 [12] C-375-C-377 (1987); Proc. European Workshop of High T_c Superconductors and Potential Applications, Genoa, Italy (1987), Paper 18; Physica 145B (1987) 222-226.

A serious problem in the precursors preparation is the difficulty of obtaining actual solutions, i.e. clear and homogeneous solutions containing simultaneously the different metal ions.

The use of nitrates as starting materials assures this homogeneity but leads in the decomposition step (or even in the concentration step) to the previously cited phenomena of uncontrolled oxidation, especially when copper is present (Powder Technology, above cited). The use of other salts usually involves the above problems of partial insolubility which may be overcome by the contemporaneous use of hydroxypolyacids or glycols producing condensed polymers. The drawback of this process consists in the increase in the carbon content in the solution and in the consequent complication of the oxidative decomposition.

A process has nowbeen found for the preparation of precursors of superconductors having high critical temperature of the already cited type, characterized a) by a special preparation technique of the starting solution and b) by a procedure for the concentration of the solution and for the oxidative decomposition of the product of said concentration to give the precursor, carried out under critical conditions of gradualness and temperatures.

According to the invention, a suspension is prepared from an aqueous solution of yttrium (or rare earth metal, bismuth or thallium) acetate and from an aqueous solution of citric acid, at a temperature lower than 60°C and preferably lower than 10°C. At the same temperatures, said suspension is added to an ammonium hydroxide solution giving rise to a clear solution (A). The solution (A) is then added to a second solution (C), prepared by dissolving in water copper acetate and citric acid and adding thereto, at the same temperatures reported above, aqueous ammonium hydroxide; a third solution (B), prepared by dissolving barium peroxide (or calcium or strontium) in a citric acid aqueous solution, and adding the so obtained solution to aqueous ammonium hydroxide is then added to the mixture (A) and (C). The final pH must be higher than 7, preferably about 10.

Of course, the concentrations and the relative amounts of the various solutions are chosen so as to obtain, in the final solution (A+C+B), the desired ratios between the various metals.

According to the invention, the amount of citric acid with respect to the metals of the precursors, is usually higher than in the known methods. The ratio of moles of citric acid to g-atoms of metal ion is 0.5-1.5 for solution A; 0.5-3 for solution B; 0.5-1 for solution C. Ammonium hydroxide is also used in relatively high amounts, the ratio of moles of NH₄OH to g-atoms of metal ions being 2-3 in solution A, 2-4 in solution B, 1-2 in solution C.

The so prepared final solution (A+C+B) is evaporated under vacuum, e.g. in a revolving flask heated by means of a bath which has a temperature from 80 to 100ºC; the pressure is about 15-30 mmHg and heating is continued until the viscosity of the residue, measured at 20ºC, exceeds 200 cpoise. It is important to carry out the concentration step immediately after the preparation of the final solution, and before barium precipitation, starts which occurs within 12-24 hours.

At this point, it may be advantageous for the homogeneity of the precursor to add a volatile acid, e.g. aqueous HCl, to the viscous residue, so as to lower pH and bring about the formation of [H₂ citrate]- instead of [citrate]^3- anion. (This step can also be carried out later on, as will be specified).

Another critical step is then started, namely the thermal decomposition of said residue to give the precursor.

In a first step, said residue, is brought from the room temperature to about 60°C within about 1 hour in a vacuum oven, (< 0.1 bar) then, within another hour, to 80-95°C, preferably to 90°C and kept at this temperature for at least 18 hours, but preferably for 24 hours. The so obtained solid is subjected to a coarse grinding (particle size lower than 15-35 mesh), placed again in said oven under vacuum at 90°C and kept at this temperature for at least one day, but preferably for 4-6 days. The residue is then heated, at the rate of 25°C/hour, to 160-200ºC, preferably to 170°C, and this temperature is kept for 5-20 hours. The treatment with the volatile acid may be carried out also during or after the drying step, after adding to the solid the minimum amount of water sufficient to obtain a viscous solution which can be stirred. In this instance, drying under vacuum, is of course repeated after acidification.

The product from this first step, in form of a grey-black powdery solid, is ground to a particle size of about 50-70 mesh and charged into a gas-flow reactor consisting of a glass or quartz cylindrical body with a bottom porous septum on which the product is charged and through which gas can flow, said cylindrical body being surrounded by a tubular oven whose temperature as well as the product temperature is controlled by means of one or more thermocouples.

The second step is also characterized by a predetermined heating rate (°C/hour) and duration at various, very accurate temperature levels, which are of critical importance for the precursor characteristics. Equally important is the kind of flow gas, its amount relative to the amount of product charged into the reactor and the feeding rate of the gas itself. The gas flow, constant over all the thermal treatment, is 1/10 SY - 1/2 SY ml /minutes (measured under normal conditions), wherein S is the internal section of the reactor measured in cm² and Y is the amount in grams of the solid charged into the reactor and it is anyhow adjusted so as to minimize the entrainment of the powdered precursor. The gas flow rate can be further increased if the gas flows downwards so as to avoid any entrainment of the fowder and to shorten the duration of the decompositions. The gas is first nitrogen, at least for about 12 hours at about 200°C.

After this initial conditioning period, the slow decomposition herein claimed is started, controlled according to two criteria limiting the increase of both the pre-determined temperature and of the oxygen percent in the flow-gas. Said criteria are:

a) the difference between the reactor internal temperature and that in the oven, difference (due to reaction heat) which must be preferably lower than 5-10°C; and

b) the oxygen consumption by the precursor, which is preferably lower than 1% of the total oxygen and nitrogen flow.

By meeting these limitations the temperature is first increased from about 200°C to about 360°C constantly feeding about 1% of ozone-enriched oxygen, then the fed oxygen percent is increased keeping the temperature constant at about 360ºC. At least 24 hours at 360°C in ozone-enriched pure oxygen flow are eventually needed.

At the end of this process the temperature may be further increased to about 420°C to further decrease the carbon, hydrogen and nitrogen content.

As can be understood from what is reported, above, a particularly relevant feature of the process of the invention consists the graduality of temperature Increase. For instance, the oven of the flow reactor is brought from the room temperature to 180-222°C, preferably 200°C, only within 2-4 hours; however, this temperature is maintained for at least 5 hours, after which the determined temperature is increased according to the previously mentioned criteria. The complete operation of this second step takes several days, usually about 200-400 hours according to the linear velocity of the gas or of the amount of the charged product.

However if the gas flows downwards the duration of the decomposition can be significanthy shortened. The various critical factors just mentioned (overall duration of treatment; increase rate of the determined temperature; maximum product temperature; increase rate of oxygen percentage) may be reciprocally adjusted within certain limits, provided that the maximum temperature of the precursor does not exceed 380-420°C; the overall duration of the treatment in an upwards gas-flow reactor should not be shorter than 40-50 hours; the oxygen percent should be kept to very low values (i.e. lower than 3% by volume) for at least three quarters of the overall heating time.

As has already been mentioned, the oxygen should contain (for all the duration of treatment and for temperatures lower than 260-360°C) a certain amount of ozone. This amount should range from 0.5 to 5% (v/v) with respect to oxygen.

The precursor obtained at the end of this treatment has low cristallinity, is relatively homogeneous with carbon content preferably lower than 4%, nitrogen preferably lower than 2% and hydrogen preferably lower than 0.2% (always by weight) and it can be transformed in the above specified superconductors at remarkably lower temperatures than those usually needed for precursors prepared by the known process. For the latter precursors, in fact, a calcination at about 900-950°C is necessary whereas a calcination of 750-850ºC proved to be sufficient for the precursors obtained according to the invention. It has also been found that a single thermal cycle, such as the heating of the precursor in pure oxygen at 950ºC during 30-60 minutes, maintaining this temperature for about 12 hours and cooling to room temperature within 10 hours, allows to obtain orthorhombic monophasic samples, whereas the same results were obtainable with the known precursors only after two or more thermal cycles.

A further surprising result of the invention consists in the preparation of new superconductors having stoichiometry

Y Ba₂Cu₃O_9-ε wherein ε ≤ 2

therefore with oxygen stachionometry higher than 7.

The invention enables the preparation of superconductors much more satisfying from the crystallografie point of view, characterized by a remarkably higher density of critical current J_c.

According to the invention, it also possible to pre-form the desired manufact working directly the precursor powder instead of the already reacted powder, which should be necessarily subjected to a second treatment, thus allowing a much more convenient process.

The process of the invention is illustrated by the following Example.

EXAMPLE

Reagent

Yttrium Acetate,

Y(CH₃COO)₃.nH₂O MW=335.7 99.9% Ventron Copper Acetate,

Cu(CH₃COO)₂.H₂O MW=199.65 99% CARLO ERBA Citric Acid,

C₆H₈O₇.H₂O MW=210.14 99.8% CARLO ERBA Barium Peroxide,

BaO₂ MW=169.34 95% Fluka

Ammonium Hydroxide,

NH₄OH MW=35.04 30±2% CARLO ERBA a) Preparation of the solutions

(Y) 34.12 g of yttrium acetate are poured in 200 ml of H₂O and the mixture is heated to 90°C obtaining a colourless and transparent solution. In an other vessel 16.05 g of citric acid are dissolved in 200 ml of H₂O. This solution is added to the previous ice-cooled solution, obtaining a milky emulsion. 31.4 ml of ammonium hydroxide are cooled in an ice-bath and the previous emulsion is added thereto. A colourless and transparent solution (A) is obtained.

(Cu) 61.0 g of copper acetate are dissolved in 100 ml of H₂O at room temperature together with 32.0 g of citric acid. The solution is cooled in an ice-bath and 57.6 ml of NH₄OH are added to give a transparent, dark blue solution (C).

(Ba) 34.498 g of barium peroxide are added under strong sterring and at room temperature to a citric acid solution obtained by dissolving 88.0 g of the latter in 360 ml of H₂O. 80 ml of ammonium hydroxide are cooled in ice-bath and the barium solution is added thereto. A colourless, solution is obtained, opalescent because of a slight gas bubble development,

b) Preparation of the dried precursor

The solution (A) containing yttrium is added in ice-bath to the copper solution (C) and then coutiously the barium solution (B). The washing waters of the various containers are also added. The total volume is about 1600 ml. The final solution is then concentrated under vacuum in about 30 minutes by means of revolving flask with bath temperature from 80 to 100°C, until an homogeneous product with viscosity higher than 200 cpoise is obtained. The colour during this operation turns from dark blue to dark green,

c) Thermal decomposition of the amorphous dried precursor

At this point, the pH of the viscous solution can be adjusted by HCl addition, if desired.

The first part of the decomposition is carried out under vacuum in an oven. After a treatment at 90°C for 24 h a coarse grinding is carried out. Heating under vacuum is then continued, bringing the product at 90°C for 4 days, heating to 170°C with a 25°C/h increase and keeping the product to this temperature for 10 hours.

When this treatment (first step) is over, the product in the form of a pulverulent grey-black product is transferred to a glass or quartz flow reactor provided with porous septum. The temperature program and the composition of the fed gaseous mixture is reported in the following table, taking into account the various discussed parameters.

By means of a descending gas flow reactor it is possible to increase the gas flow without entrainment of powders. The decomposition time of the organic part is therefore significantly shorter as the following tables hows.

Claims

1. A process for the preparation of precursors of superconductors, particularly of the kind RBa₂ Cu₃O₇_δ (with R = yttrium or rare earth metal), or (Bi, T1)₂(Sr,Ca,Ba)_n+1 Cu_n O_2(n+2)-δ wherein δ ranges from 0 to 1 or RBa₂ Cu₃O_9-ε wherein ε ranges from 0 to 2, characterized in that:

a) A suspension is prepared from an aqueous solution of yttrium (or rare earth metal, bismuth or thallium) acetate and from an aqueous solution of citric acid, at a temperature lower than 60°C; at the same temperatures, said suspension is added to an ammonium hydroxide solution giving rise to a clear solution (A) which is then added to a second solution (C), prepared by dissolving in water copper acetate and citric acid and adding thereto, at the same temperatures reported above, aqueous ammonium hydroxide; a third solution (B), prepared by dissolving barium peroxide (or calcium or strontium) in a citric acid aqueous solution and adding the so obtained solution to aqueous ammonium hydroxide, is then added to the mixture (A) and (C); the concentrations and the relative amounts of the various solutions are being chosen so as to obtain, in the final solution (A+C+B), the desired ratios between the various metals; the so prepared final solution (A+C+B) is evaporated under vacuum, at a temperature not higher than 100°C until the viscosity of the residue, measured at 20ºC, exceeds 200 cpoise;

b) the residue, is heated to about 60°C within about 1 hour in an oven, under vacuum (0.1 - 0.01 mmHg) then, within another hour, to 80-95°C, and kept at this temperature for at least 18 hours; the so obtained solid is ground to a particle size lower than 15-35 mesh and heated again under vacuum to 90°C for at least one day, for 5-20 hours grinding to a particle size of 50-70 mesh;

c) said grinding product is heated in a gas-flow reactor for at least 12 hours at a temperature of about 180-200°C, under a nitrogen stream; 0.5-3% (by volume) of ozone enriched oxygen is added and the temperature is slowly increased to 360°C, keeping the temperature difference between the internal reactor and the oven less than 10°C and the percent of consumed oxygen less than 1% of the total gas flow; and when the oxygen percent in the effluent approaches values similar to those fed, the ozone enriched O ₂ feeding is increased up to 100% within about 10 hours, heating to about 360- 420°C; the gas flow, constant over all the thermal treatment, being 1/10 SY - 1/2 SY ml /minutes (measured under normal conditions), wherein S is the internal section of the reactor measured in cm² and Y is the amount in grams of the solid charged into the flow reactor.

2. A process according to claim 1 wherein, the ratio of moles of citric acid to g-atoms of metal ion is 0.5-1.5 for solution A; 0.5-3 for solution B; 0.5-1 for solution C.

3. A process according to claim 1 wherein, the ratio of moles of NH₄OH to g-atoms of metal ions are 2-3 in solution A, 2-4 in solution B, 1-2 in solution C.

4. A process according to claims 1-3, characterized in that in the preparation of the solutions A, C and B the mixing phase of the different solutions are carried out at temperatures lower than 10°C.

5. A process according to any one of previous claims wherein in the step b) the solid ground to 15-35 mesh is heated under vacuum for 4-6 days.

6. A process according to any one of previous claims characterized in that in the step c) the heating, starting from the beginning of the oxygen feeding in the gas flow, is continued for 15 hours before of increasing the oxygen percent.

7. A process according to any one of previous claims wherein in the step c) the oxygen is first added to nitrogen in an amount of 1% by volume.

8. A process according to any one of previous claims wherein the oxygen added to the gas flow contains, for all the duration of its use, from 0.5 to 10% by volume of ozone.

9. A process according to any one of previous claims wherein the gas flow charged into the flow reactor is about 3Y ml/minute, measured under normal conditions, wherein Y is the amount of grams of solid introduced in the reactor.

10. A process according to any one of previous claims characterized in that the viscous solution obtained from step a) is acidified with volatile acids.

11. A process according to any one of claims 1-9, wherein the product ground to 15-30 mesh from step b) is dissolved in the minimum amount of water necessary to obtain a viscous but stirrable solution, which is acidified with volatile acids and subjected again to drying.

12. Precursors for superconductors obtained according to the processes of claims 1-11.

13. Superconductors having substantially the following stoichiometry

Y Ba₂ Cu₃ O₉