WO1992011660A1 - Aqueous solutions of metallic cations suitable for the formation of a high-tc superconductor and containing one or more noble-metal cations, precursors obtainable by decomposition of said solutions and composites obtainable from said precursors - Google Patents

Abstract

The present invention concerns homogeneous aqueous solutions which are obtained from cheap reagents and contain the metallic cations necessary for the formation of a high-Tc-superconductor and one or more noble-metal cations such as Ag, Au or Pd for example. The invention also concerns the process for the exsiccating of said solutions and for the decomposition of the organic part, the exsiccates and the precursors prepared in said way and the composites obtained from these.

Description

ACQtTEOϋS SOLUTIONS OF METALLIC CATIONS SUITABLE FOR T FORMATION OF A HIGH-T- SUPERCONDUCTOR AND CONTAINI ONE OR MORE NOBLE-METAL CATIONS- PRECURSORS OBTAINAB BY DECOMPOSITION OF SAID SOLUTIONS AND COMPOSIT OBTAINABLE FROM SAID PRECURSORS

The present invention concerns homogeneo acσueous solutions which are obtained from che reagents and contain the metallic cations necessary f the formation of a high-T -Superconductor and one more noble-metal cations such as Ag, Au or Pd fo example.

The invention also concerns the process for th exsiccating of said solutions and for the decompositio of the organic part, the exsiccates and the precursor prepared in said way and the composites obtained fro these.

Superconductors are defined as those system consiεting of metals, alloys or metal composition usually having no resistivity at very low temperatures The temperature at which the resistivity of superconductor becomes zero or reaches at least minimum value, is defined as critical superconductivit temperature (Tc). The Tc of known superconductors consisting of special alloys or of intermetalli compounds, ranges from about 10 K to about 25 K therefore their applications (superconductive wires fo the manufacture of magnets; magnetic suspensions special electronic apparatuses) are possible only usin liquid helium as cooling liquid, with all th technological and economic problems connected with sai use.

More recently, superconductors with higher T were obtained, so as to allow the substitution of liquid helium with remarkably less expensive and more easy-to- handle fluids such as liquid nitrogen (b.p. 77 K instead of 4 K of helium) . These are systems consisting of complex oxides, for instance yttrium, barium, copper oxides or the like, shortly referred to with the abbreviation "L-M-Cu-O", wherein L = yttrium, lanthanum and lanthanides M = barium, strontium, calcium.

Said Y-Ba-Cu-O system turned out to be particularly interesting from the practical point of view. These superconductors, with T higher than 90^βC, 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 subjected again to a very fine grinding, then to pressing and forming; a further heating to the above temperature, always in the presence of oxygen, causes the sintering of the superconductive compound.

As already stated, this process requires particularly careful grinding steps in order to impart a very high homogeneity to the superconductor: however the homogeneity 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 th particle size, with the double effect of impairing th homogeneity, and making the sintering difficult.

In order to overcome said drawbacks and th technological complications connected therewith several attempts have been carried out, mainly based o the initial use of solutions of soluble compounds o the desired metals, (nitrates, citrates, tartrates an the like) instead of powder mixtures. These solution are then concentrated to a more or less marked level o anhydrification. The so obtained residue is the subjected to thermal decomposition in oxidizin atmosphere, so as to remove (besides the residua water) the organic carbon; the last process ste consists of a calcination of the precursor which i transformed thereby in the actual superconductor.

It is evident that in the initial step the use o solutions provides homogeneity characteristics whic cannot otherwise be achieved even by the most carefu grinding of solid starting compounds. However, th concentration and decomposition steps involve drawback comprising uncontrolled oxidation phenomena (eve actual combustions) or of other nature, impairin sometimes irreparably the necessary homogeneity of th desired precursor and limiting in some way th preparation scale because of the difficulty o controlling the phenomenon with amounts of a certai entity.

Proceedings 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 Hig TcSuperconductors and Potential Applications, Genoa Italy (1987), Paper 18; Physica 145B (1987) 222-226.

A serious problem in the precursors preparatio consists in the difficulty of obtaining actua solutions, i.e. clear and homogeneous solution containing simultaneously the different metal ions.

The use of nitrates as starting materials assure this homogeneity but leads in the decomposition ste (or even in the concentration step) to the previousl mentioned phenomena of uncontrolled combustion, 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 and glycols producing condensed polymers. The drawback of this process consists in the increase of the carbon content of the solution and in the consequent complication of the oxidative decomposition. Moreover these superconductors as ceramic materials, are intrinsicly brittle and thus difficult to work mechanically. The realization of composites noble metal(s) - superconducting oxide is intended to improve the workability of the cited superconductors during the production and the mechanical resistance during the use ,due to the pronounced malleability and ductility of the noble metals such as Ag, Au and Pd which do not impair the superconducting phase even at the above mentioned calcination temperatures. Ag has the advantage of a good permeability for oxygen, which is necessary for the conferring of superconducting properties to the final product o YBCO. Namely, it is easier to obtain the right oxyge stoichiometry also for dense and compact material because of the elevated coefficient of diffusion o oxygen in Ag. The drawback of the low melting point o Ag (960.8°C in air and 939°C in oxygen at 1 atm) can b overcome by alloying Ag with Au or Pd. For every mixin ratio, Ag and Pd form a solid solution whose meltin temperature increases with the Pd content. Other purposes of the fabrication of composite are: a) the protection of the superconductor fro chemical agents, particularly those present in th atmosphere such as humidity and CO-, b) the establishment of a parallel path of hig electrical conductivity for the case of local return o the superconductor in the normal state.

According to the traditional techniques, the nobl metal is added to the superconductor by a mechanica mixing of powders. For example, powders of Y₂°V ^Cu0 _>

BaCO.- and Ag_0 are mixed together and calcined t obtain composites Ag-YBCO

[C.F.Shen, Mat.Res.Bull. , 2±, 1231-1239 (1989)]. Otherwise powders of already reacted YBCO and powders of metallic Ag can be mixed [T.Nishio, Y.Itohi et al., J.Mater.Sci. , 2 , 3228-3234 (1989)]. These techniques generally have the disadvantage of a non optimal interspersion between metal and superconductor and of repeated grindings. From this point of view a wet method which makes use of a solution of the cations of Y, Ba, Cu and the noble metal is certainly preferable because it carries out an atomic-scale mixing of the mentioned cations. This elevated level of interspersion is then mantained in the following steps of the treatment up to the final product. Where the primary object of the present invention is concerned, homogeneous acqueous solutions have now been found which contain not only the cations of the kind and in the ratios necessary for the formation of a high-T superconducting oxide, but also one or more noble-metal cations such as Ag, Au or Pd. In the following the process for the preparation of these solutions and the process for their concentration and exsiccation, and the oxidative decomposition of the exsiccated product to "precursor" are illustrated. According to the invention, the process includes the following steps: a) (1) an acqueous solution of an Yttrium salt (solution A), (2) an acqueous solution of a Copper salt (solution B) and (3) an acqueous solution of Barium peroxide or oxide (solution C) , all three solutions containing a hydroxypolyacid and ammonia, are mixed; b) by rapid evaporation the solution obtained in a) is concentrated up to a viscosity of more than 0.2 kg/m*s (200 cps) at 20°C; c) possible addition of an alkaline solution containing Ag (solution D) or other metals which are soluble at the employed pH, and successive re- concentration up to a viscosity of more than 0.2 kg/m*s (200 cps) at 20°C; d) exsiccation at a pressure of less than 0.1 bar up to a weight substantially constant at about 90^βC; e) possible addition of solutions of Gold salts o of other metals which are soluble at acid pH values eventually containing a hydroxypoly cid (solution E) followed by exsiccation according to point d) an coarse grinding until the desired granulometry i reached.

In particular, starting from a solution of yttriu acetate (or, respectively, the acetate of a rare-eart metal) and from an acqueous solution of citric acid o of another hydroxypolyacid (in the following we refe synthetically to citric acid; for the extension t other hydroxypolyacids it has to be considered tha citric acid is tricarbossilic) , a suspension is firs prepared at a temperature of less than 60°C an preferably less than 10°C and then added to an ammoni solution with the same temperature prescriptions. Th so obtained clear solution (A) is then added to second solution (C), prepared by dissolving coppe acetate and citric acid in water and by adding acqueou ammonium hydroxide with the above temperatur prescriptions; a third solution (B) is obtained b dissolving of barium peroxide in an acqueous solutio of citric acid and by adding this solution (at les than 6 ^βC and preferably less than 10^CC) to acqueou ammonium hydroxide and finally added to the mixture o (A) and (C). The mixing order of A, B and C isn' critical.

The final pH of the solution has to be greate than 7 and conveniently equal to about 10. Naturally the concentrations and the relative quantities of the various solutions are arranged to reach the desired ratios of the various metals in th final solution(A+B+C) .

According to the present invention, the amount o citric acid used in relation to the metals which ar present in the precursor is, as a rule, higher than that of already known techniques. Thus the molar ratio of citric acid to metal ion is 0.5-1.5 for solution A; 0.5-3.0 for solution B and 0.5-1.0 for solution C. Also the ammonium hydroxide is employed in rather high relative amounts, the molar ratio of NH.OH to metal ion being 2-3 in solution A, 2-4 in solution B and 1-2 in solution C.

The so prepared final solution (A+B+C) is evaporated under vacuum, e.g. in a rotary evaporator heated by means of a bath having a temperature from 80 to 100°C; the pressure is about 15-30 mm Hg and heating is continued until the viscosity of the residue, measured at 20°C, exceeds 0.2 kg/m*s (200 cps). It is important to carry out the concentration step immmediately after the preparation of the final solution and absolutely essential before the barium begins to precipitate, which may occur within 12-24 h.

It may be advantageous for the homogeneity of the precursor to add at this point a volatile acid, for example acqueous HCl, to the viscous residue, so as to lower the pH and to allow the formation of [H₂citrate]~ instead of [citrate] ~ -anions. (This step can also be carried out later on, as will be specified. )

The addition of the ammoniacal Ag-citrate solution (solution D) can also be conveniently made at this point. In comparison to the direct addition to the (A+B+C) solution, there is the advantage of handlin more concentrated solutions.

At this point may also be added the solutions o other metals which are soluble in ammoniacal solution (basic pH) as citrates.

Solution D can be prepared by dissolving AgNO corresponding to the Ag desired in the final produc

(except a slight excess to compensate the losses in th following steps of precipitation and filtration). By means of a strong base, e.g. tetramethy1ammonium hydroxide, in a slight excess i comparison to the stoichiometric amount, Ag^ precipitates which is filtered and washed directly o the filter with deionized water up to neutrality of the filtrate.

An acqueous solution of citric acid is prepared, with a molar ratio of citric acid to Ag included in the interval 0.2-2.0 and preferably 0.3-1.0. In this solution the fresh precipitate of Ag₂0 is dispersed. To the so obtained and ice-cooled suspension cold ammonium hydroxide is added drop by drop until the complete dissolution is obtained.

The so obtained solution (solution D) is added to the concentrated (A+B+C) product, obtaining a perfectly clear solution of a strong blue colour, which is again concentrated in a rotary evaporator.

Another critical step of the proceeding is then started, namely the thermal decomposition of the mentioned residue to give the precursor. In a first step this residue is heated from room temperature to about 60^CC within about 1 hour in a vacuum oven (< 0.1 bar) and then to 80-95°C, preferabl 90^CC, within a further hour and kept at this level fo at least 18 hours, preferably 24 hours. The so obtaine solid undergoes a coarse grinding (particle size lowe than 420-1190 μm) , placed again in the mentioned vacuu oven at 90°C and kept at this temperature for at leas one day, preferably for four-six days.

Au and other metals which precipitate in alcaline solutions can be added at this point. In fact it is possible to re-fluidize the exsiccate with a minimum amount of water. The pH of the so obtained product is acid because of the nearly total evaporation of ammonia during the exsiccation. An acqueous solution of auric chloride containing citric acid is added. The content of citric acid in the auric- chloride solution depends on the excess of citric acid in the exsiccate. Hence it can vary from zero to 1.5 moles of citric acid / g- toms of Au. The so obtained solution is placed in a rotary evaporator and exsiccated in about 40 minutes.

The residue is then heated, at a rate of about 25°C/hour, up to 160-200^βC, preferably 170°C, and this temperature is kept for about five-twenty hours. The treatment with the volatile acid may be carried out also during or after the drying step, after treatment of the solid with the minimum amount of water sufficient to obtain a stirrable viscous solution. In this case the drying under vacuum is, of course, repeated after the acidification. The product of this first step of treatment, a grey-black powdery solid, is ground to a particle size of about 50-70 mesh and placed in a gas flow react consisting of a glass or quartz cylindrical body with porous septum at the bottom on which the product i placed and through which gas can flow, said cylindric body being surrounded by a tubular oven whos temperature, as well as the product temperature, i controlled by means of one or more thermocouples.

The second step is also characterized by a ver accurate programmation of the heating rate (°C/hour and the duration of persistance at the variou temperature levels. This accuracy is of critica importance for the precursor's characteristics Similarly critical are the kind of flowing gas, it flow rate relative to the charged amount of product and the flowing regime of the gas itself. The flo rate, constant over all the thermal treatment, is abou 1/10 SY - 1/2 SY ml/minute (measured under norma conditions) where S is the internal section of th reactor, measured in cm 2, and Y is the amount of soli charged in the reactor, and it is adjusted anyway so a to minimize drag of the powdered precursor. The gas i initially nitrogen, at least for 12 hours at abou

200°C.

The gas flow rate may be enhanced significantly b using a downwards gas flow reactor so as to avoi powder drag and to reduce the duration of th decomposition.

After this initial conditioning period, the slo decomposition herein claimed is started, controlle according to two criteria limiting the increase of bot the predetermined temperature and of the oxygen percen in the flow gas. Said criteria are: a) the temperature difference between th reactor's internal and the oven, difference (due t reaction heat) which has to be preferably less than 5 10°C; and b) the oxygen consumption of the precursor, whic is preferably less than 1 % of the total flow of oxyge and nitrogen.

To meet these limitations, the temperature is first increased from about 200°C to the settled maximu temperature (of about 360°C) constantly feeding about 1% of ozone-enriched oxygen, then the fed oxygen percent is increased keeping the temperature constant at about 360^CC e.g.. Finally there follows at least 24 hours at e.g. 360^CC in ozone-enriched pure oxygen flow. At the end of this process the temperature may be further increased up to about 420°C to further decrease the carbon content.

As can be understood from what is above reported, a particularly important feature of the process according to this invention consists in the long heating time, combined with the graduality of the temperature increase. For instance, the oven of the flow reactor is brought from room temperature to 180- 220°C, preferably 200°C, within only 2-4 hours; but this temperature is mantained for at least 5 hours, after which the programmed temperature is increased according to the previously mentioned criteria. At any rate the complete operation of this second step takes several days, typically about 200-400 hours depending on the linear velocity of the gas and on the charged amount of product.

However, if the gas flow is descendent, th duration of the decomposition may be reduce significantly. The various critical factors just mentione (overall duration of treatment; increasing rate of the programmed temperature; maximum product temperature; increasing rate of oxygen content) 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 the gas-flow reactor should not be shorter than 40-50 hours; the oxygen percent should be kept at very low values (i.e. less than 3% by volume) for at least three quarters of the overall heating time.

As has already been mentioned, the oxygen can contain (for all the duration of the treatment and for temperatures lower than 260-360°C) a certain amount of ozone. This amount suitably ranges from 0.5 to 5.0 % (v/v) with respect to the oxygen.

The precursors obtained at the end of this treatment have low cristallinity, are relatively homogeneous with a carbon content preferably lower than 4 % (b.w.), and can be transformed in composited superconductors with the above specified advantages.

It is possible in this way to obtain superconductors which are much more satisfactory from the cristallographic point of view and which have a considerable higher critical current intensity. These superconductors can also be obtained as films by deposition of the solutions directly on substrates and following exsiccation, decomposition o the organic part and calcination at high temperatures A specific advantage of the precursors containing A and other noble metals is the possibility of obtainin superconductors containing alloys with a higher fusio point than Ag (>930°C).

The manufacturing process according to th invention is illustated by the following example, whic is, nevertheless, not limiting.

PREPARATION OF YBCO PRECURSOR WITH (a) 10% OF SILVER, (b) 1.95% OF GOLD Reagent Yttrium Acetate,

Y(CH₃COO)₃ nH₂0 M.W.=335.7 99.9% Ventron

Copper Acetate,

Cu(CH₃C00)₂ H₂0 M.W.=199.65 99% CARLO ERBA

Citric Acid,

C₆H₈0₇ H₂0 M.W.=210.14 99.8% CARLO ERBA

Barium Peroxide,

Ba0₂ M.W.=169.34 95% Fluka

Ammonium Hydroxide,

NH4.OH M.W.=35.04 30.04±2% CARLO ERBA

Silver Nitrate,

AgN0₃ M.W.=169.88 MP

Tetramethylammonium Hydroxide,

N(CH₃)₄OH M.W.=91.15 10% MERCK Auric Chloride,

AuCl. M.W.=303.33 K&K

Preparation of the solutions:

(Y); 34.12 g of yttrium acetate are poured into

200 ml of H~0 and the mixture is heated to 90°C obtaining a colourless and transparent solution (A) 23.52 g of citric acid are separately dissolved in 20 ml of H_0. This solution is added to the previous, ic cooled solution A, obtaining a milky emulsion (B). 39. ml of ammonium hydroxide are cooled in an ice-bath an emulsion B is added thereto. A colourless an transparent solution is obtained.

(Ba) : 34.498 g of barium peroxide are added unde strong stirring and at room temperature to a solutio obtained by dissolving 88.0 g of citric acid in 360 m of H₂0 (solution D). 80 ml of ammonium hydroxide ar cooled in an ice-bath and the barium solution (D) i added thereto. A colourless solution is obtained opalescent, because of a slight gas bubble development. (Cu) ; 61.0 g of copper acetate are dissolved i 150 ml of H₂0 at room temperature together with 46.7 of citric acid. The solution is ice-cooled afte dissolution and 72.6 ml of ammonium hydroxide are adde to give a transparent solution of intense blue colour. (Ag) : 11.873 g of Ag 0₃ are dissolved in 40 ml o H₂0. 70 ml of N(CH₃).0H (10%) are added. Ag₂ precipitates which must be washed on a filter unti neutral pH of the filtered water. 5.4 g of citric aci are dissolved in 200 ml of H₂0. Thereto Ag₂0 is added The resulting suspension is ice-cooled and 80 ml o previously cooled ammonium hydroxide are added.

(Au) : 2.084 g of auric chloride are added to 30 m of H₂0.

Preparation of the exsiccated YBCO-precursor. In an ice-bath the yttrium-containing solution an then, with caution, the barium-containing solution ar added to the solution containing copper. The waters used for washing of the beakers is added. The total volume is about 1600 ml.

The final solution is then placed into a rotary evaporator at a temperature between 80 and 100°C and exsiccated under vacuum in about 50 minutes, until a homogeneous product with viscosity higher than 0.2 kg/m*s (200cps) is obtained. During this operation the colour turns from dark blue to dark green. (a) (silver composite): To the exsiccated residue the silver solution is added under stirring until a solution of dark blue colour is obtained. Again the solution is placed into the rotary evaporator for the final exsiccation. The residue is heated from room temperature to 90°C within one hour under vacuum (P=13.3vl.33 Pa) and mantained at this level for one day. The so obtained solid is subjected to a coarse grinding (up to a grain size lower than 420-1190 μm) . (b) (gold composite) : The residue is heated from room temperature to 90°C within one hour under vacuum (P=13.3÷1.33 Pa) and mantained at this level for one day. The so obtained solid is subjected to a coarse grinding (up to a grain size lower than 420-1190 μm) . At this point, due to the reversibility of the exsiccation, the residue can be re-fluidized in about 120 ml of water, using a 500 ml glass balloon, heating to 50-60°C up to complete dissolution. After this the viscous solution is cooled to 40°C (better 25^CC) in an ice-bath. Finally a solution of 2.084 g of auric chloride in 30 ml of water is added. The so obtained solution is immediately placed in the rotary evaporat and exsiccated in about 40 minutes. The temperatu must not exceed 40÷60°C. Thermal decomposition of the amorphous dried precursor. The first step of the decomposition is carried ou in a vacuum oven. After a treatment at 90^CC for abou 48h a coarse grinding is carried out. The product i then heated first to 90°C with a rate of 45°C/h an then to 170°C with a rate of 15°C/h; the heatin duration at 170^βC is 7h.

When this treatment is over, the product, in for of a grey-black pulverulent solid, is transferred in glass- or quartz flow reactor provided with a porou septum. The temperature program and the composition o the gaseous feed are reported in the following table.

TABLE (a) Decomposition of Ag-composite total gas flow: 5000 ml/min ozonized oxygen

262 290 10 0₂ adjusted to 20%

265 290 20 0₂ adjusted to 50%

290 290 50 0₂ adjusted to 100%

320 290 100 end of activation

TABLE (b) Decomposition of Au-composite total gas flow: 800 ml/min ozonized oxygen Time (h) Tem erature (°C) %0 Operations start increase 90°C/h constant 200°C start increase 15°C/h 0« adjusted to 1% constant 280°C start increase 60°C/h constant 300°C start increase 60°C/h constant 305^CC start increase 60°C/h constant 310^βC start increase 30°C/h constant 320°C start increase 30°C/h constant 330°C start increase 60°C/h constant 360°C 0₂ adjusted to 2% 0₂ adjusted to 4% 0₂ adjusted to 8%

0₂ adjusted to 16% 16 0₂ adjusted to 32% 32 0₂ adjusted to 64%

64 0₂ adjusted to 100%

100 end of activation

Claims

1. Homogeneous acqueous solutions of the meta cations of the kind and in the proportions required fo the formation of a high-T superconducting oxide an containing one or more metallic cations like Ag, Au o Pd, and exsiccated products obtained from therefrom.

2. Method for the preparation of the above mentioned solutions and exsiccates, characterized in that: a) there are mixed: (1) an acqueous solution of an yttrium salt containing a hydroxypolyacid and ammonia (solution A); (2) an acqueous solution of a copper salt containing a hydroxypolyacid and ammonia (solution B) and (3) an acqueous solution of barium peroxide or oxide containing a hydroxypolyacid and ammonia (solution C) ; b) the solution obtained in a) is rapidly concentrated by evaporation up to a viscosity of more than 0.2 kg/m*s (200 cps) at 20^CC; c) optional addition of an alkaline solution containing Ag (solution D) or other metals which are soluble at the employed pH, and successive re- concentration up to a viscosity of more than 0.2 kg/m*s (200 cps) at 20°C; d) exsiccation at a pressure of less than 0.1 bar up to a substantially constant weight at about 90°C; e) optional addition of solutions of Gold salts or of other metals which are soluble at acid pH values, containing if necessary a hydroxypolyacid (solution E) , followed by exsiccation according to point d) and coarse grinding until the desired granulometry is reached.

3. A process according to the previous claim, where the exsiccation step preceeding the final grinding carried out at a temperature of about 170°C.

4. A process according to claims 2 or 3, wherein t ratio of moles of citric acid to g-atoms of metall ion is0.5-1.5 for solution A, 0.5-3.0 for solution 0.5-1.0 for solution C, 0.2-2.0 for solution D and 1.5 for solution E.

5. A process according to any one of claims 2- wherein the ratio of moles of ammonium hydroxide to atoms of metal ions is 2-3 in solution A, 2-4 solution B, 1-2 in solution C and the amount necessa for complete dissolution in solution D.

6. A process according to any one of claims 2- wherein the mixing of the various solutions during t preparation of solutions A, B and C is^" carried out at temperature not higher than 10°C.

7. A process according to any one of claims 2- wherein in step d) the solid after grinding (partic size less than 420-1190 μm)is heated under vacuum f 4-6 days.

8. Precursors of superconducting composit obtainable by exsiccation of the solutions of claim 1.

9. A process for the decomposition of the organi part of the above mentioned exsiccates, wherein th product of step e) , ground to a particle size of les than 250-177 μm, is heated in a gas-flow reactor for a least 12 hours while a nitrogen flow passes through th product itself; then 0.5-3.0% (b.v.) of oxygen eventually enriched with ozone, are added to th nitrogen, and the temperature is slowly inceased up to about 360°C, keeping the temperature difference between the reactor's internal and the oven lower than 10°C and the oxygen consumption lower than 1% of the total gas flow; when the oxygen content of the output stream returns to values close to those of the input stream the oxygen feed (containing eventually ozone) is gradually increased up to 100% of the total gas flow keeping the temperature constant at about 360°C; at the end there follows at least 24 hours at 360°C in that gas flow.

10. A process according to claim 9 wherein 1% (b.v.) of oxygen is added to the nitrogen at the beginning of the decomposition.

11. A process according to claim 9 wherein the oxygen introduced in the gas flow contains from 0.5 to 10% (b.v.) of ozone for at least a part of the treatment.

12. Films of various thicknesses obtained from the mentioned solutions by deposition on a suitable substrate and following decomposition of the organic part.

13. Superconducting composites and artif cts obtainable from the precursors of claim 7.

14. Superconducting composites and artifacts according to claim 13, wherein the metal alloy melts at T>930^CC.