Process for the manufacture of powders for superconductive ceramic materials

Abstract: A process for the pyrolysis of YBCO powders, characterized by the preparation of a clear aqueous solution containing the ions yttrium, barium and copper, in the final proportions desired for the powder, in the form of complex compounds, preferably citric complex compounds, and in the presence of a detonating system, like for instance a combination of ammonium ions and of nitrate ions. The clear solution is then made to concentrate by means of evaporation, until triggering a violent combustion which is carried out at a high temperature, higher than 250°C and preferably higher than 850°C.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The instant invention refers to a process for the manufacture of powders for superconductive ceramic materials. In particular, the invention refers to the manufacture of powders of superconductive ceramic oxides of the type Y 1 Ba 2 Cu 3 O 7-x (YBCO), x being comprised between 0 and 1, bounds included, by means of the pyrolysis of a solution containing the ions of the metals forming the same oxides. THE PRIOR ART It is known that ceramic materials, like the ones belonging to the class of the complex oxides as the YBCO, can show superconductivity at a temperature higher than the one of liquid nitrogen. Such a feature is considerably interesting, from a practical/applicatory point of view, but the interest is tempered and restrained both by the poor availability of oxides showing suitable features and by the high manufacturing cost of the same oxides. Particularly important, and obtainable with difficulty, is the combination of homogeneity ad fine granulometry of the starting powders of such oxides, which have to subsequently undergo compacting (hot-pressing) and syntering. In the case of YBCO, the most commonly used methods, exploiting the solid-state reaction of a mixture of copper oxide, yttrium oxide and barium carbonate, are showing a drawback, residing in that very unlikely a single-phase compound can be obtained by means of only one thermal treatment (at high temperature) and huge amounts of barium carbonate remain thus unreacted. That's why further onerous annealing treatments are required, which treatments anyway cause an undesired agglomeration of the powders, which therefore shall undergo a grinding. In particular, in the case of YBCO, the grinding cannot be wet-ground in water, but only in the presence of an organic solvent, which obviously involves further charges and problems. Therefore, the preliminary techniques hereinabove are considered particularly unfavourable. The chemical methods for the manufacture of the powders are for instance the sol-gel method, the coprecipitation and the pyrolysis. Such methods are more reliable and give rise to powders having a homogeneous composition and a fine granulometry, but they are complex, critical and expensive in the peculiar case of YBCO. In particular, the pyrolysis method for the manufacture of YBCO, as described for instance on November 8, 1988 by F. Celani et al., in the report 88/62(P) of the Ist. Naz. di Fisica Nucleare (Lab.Naz.Frascati) entitled "PYROLYTIC CITRATE SYNTHESIS AND OZONE ANNEALING. TWO KEY STEPS TOWARD THE OPTIMIZATION OF SYNTERED YBCO", gives rise to such a low yield that an industrialization is not justified. The method hereinabove is based on the combustion of a solution of citrates of the metals building up the YBCO, said solution being made flammable by the presence of ammonium nitrate. Such a method grants the dry pyrolysis residue the same microscopic homogeneity which is typical of the starting solution, provided said solution be clear without showing any suspended body. The reaching of such a condition is however critical and can be reproduced only with difficulty. Moreover, the thus obtained powders are not single-phase and shall undergo long (tens of hours) critical (because of powder agglomeration) thermal post-treatments, bound to the reaching of single-phase features. The Applicant found out that by working in a specific way and by a strict control of the operative conditions, the drawbacks hereinabove can be overcome, thus obtaining a highly satisfactory reproducibility and a constant quality of the oxides, in terms of homogeneity of the composition and of granulometry. SUMMARY OF THE INVENTION It is therefore a purpose of the present invention to manufacture powders of superconductive oxides like YBCO in such a way that the solution, containing the metals forming the oxide, be clear. Another purpose of the invention is an improved process for the pyrolysis of said clear solution. Further purposes of the invention will be pointed out by the following disclosure of the invention. DISCLOSURE OF THE INVENTION The present invention involves a process for the production of YBCO powders wherein a clear aqueous solution containing the ions yttrium, barium and copper in the final proportions desired for the powders in the form of complex compounds, preferably complex citric compounds, and in the presence of a detonating system, like for instance a combination of ammonium ions and of nitrate ions, is continuously spread as a thin layer over a moving carrier and in sequence a pyrolysis step is initiated consisting of evaporating said thin layer and in triggering its combustion, the resulting combustion product being kept at a temperature higher than 850°C. A very satisfactory temperature range is between 870° and 1000°C. Particularly advantageous pyrolysis conditions are a temperature of 870°-980°C for a time from 60 to 360 s. Temperatures over 1000°C are useless, as the amount of formed YBCO phase doesn't appreciably increase, and are of course not advisable because they promote the agglomeration of the powder, thus reducing its specific surface. The temperatures below 850°C give rise to an intimate YBCO mixture, with the presence of spurious phases, which is excellent as to the powder homogeneity but requires successive long thermal annealing post-treatments. If the clear solution containing Y, Ba and Cu is then directly concentrated and pyrolysed at a temperature higher than 850°C, a nearly single-phase YBCO powder is obtained in an extremely short treatment time. 60 s, at 900°C, can be an already sufficient time for granting the powders a single-phase feature equal to 95% by volume. A particularly advantageous operative condition resides in that the clear solution is pyrolised in the form of a thin layer, having for instance a thickness of the order of 100 µm on a suitable carrier. The carrier on which the pyrolysis is occurring can be a metal conveyor belt, optionally surface treated with a coating resistant to the high temperatures and inert with respect to the solution of metal ions. The thermal treatment of the thin layer solution, consisting of the evaporation and of the triggering of the combustion, gives rise to a flame having a duration which is not particularly critical. However, a too fast combustion is causing undesired throws of materials, whereas a too slow combustion can lead to an undesired phase separation. Typically, the times concerning the combustion will be of the order of 1-10 s. After the combustion, the temperature is steadily kept at a level higher than 850°C. A very favourable temperature range is 870°-1000°C. Particularly advantageous conditions are represented by a temperature of 870°-980°C for a time of 60-360 s. The subsequent cooling step, of the formed YBCO powder, is not critical. In order to obtain a constant quality of the YBCO powders, as already mentioned, it is essential that the solution containing the metal ions be clear and free from any suspended body. Particularly preferred, for the purposes of the invention, is the preparation of a clear aqueous solution containing the yttrium, barium and copper ions, in the final proportions desired for the powder, in the form of complex citric compounds and in the presence of a detonating system containing ammonium nitrate. In order to obtain all this it is advisable to work as follows. The starting copper, yttrium and barium compounds, previously dried at approximately 110°C, up to constant weight and in a stoichiometrical amount, as to have the Y, Ba and Cu cations, when dissolved, in the final ratio of the YBCO, equal to 1:2:3, are brought into solution. A suitable operative way is residing in the use of nitric acid for the dissolving step. Anyway, the amount of the nitrate ions in the solution, anyway, hasn't to be too high, as the combination with ammonia, which is subsequently added for creating the detonating system, leads to the formation of ammonium nitrate, which is one of the elements for managing the combustion strength. It is therefore necessary to meter the nitrate ions and the ammonia in such a way that the operative conditions of the pyrolysis be the ones hereinabove. In order to prepare the solution, the temperatures can be in the range from 80° to 90°C. Optimum reaction levels are obtainable starting from a final solution containing about 100 potential grams of YBCO in the liquid volume of 1 litre. A complexing agent like citric acid, acting as a combustion moderator, is then added to the solution. It is desirable that the ratio citric acid/ammonium nitrate be kept in the range 1:2 + 1:5. At this point the solution is reasonably acid. Finally NH 4 OH is added and the solution takes on a typical dark blue colouring. In the operative practice such colouring can be used as an indicator for the reaching of the final pH, desired for the solution, which has to be higher than 6.5 and preferably higher than 6.8. A particularly preferred pH range is from 7.0 to 7.5. It was found out that the obtaining of a clear solution is bound to the combinations of two technical expedients: a careful control of the pH of the solution and contemporaneously an addition of ammonia to be carried out in a time as short as possible, consistently with the operative conditions bound to the preparation of the solution. In the case of a discontinuous manufacture of small amounts of YBCO, ammonia will be added as NH 4 OH at a rate of 0.3 + 1 l/s of 25% b.w. NH 4 OH solution when the volume of the solution of Y, Ba and Cu is approximately comprised between 6 and 10 l. In the case of a continuous manufacture of YBCO, the operative conditions, for the ammonia addition, will be optimized according to the gist of the invention. The subsequent pyrolysis thermal treating at a high temperature, performed as above, allows one to obtain a powder containing an overall amount of carbon very restrained and surely reduced with respect to the traditional technologies. The undeniable huge advantage of the disclosed method is residing in the possibility to obtain formed YBCO in a largely shorter time, with respect to the times of the known art, thus making the pyrolysis method industrializable in an easy way. By the process according to the invention, the manufactured YBCO powders are characterized by a purity level higher than 95%, with the presence of traces of minor phases which cannot be detected with the usual X ray diffraction techniques. The powders obtained following the process according to the invention, because of the extremely low density of the reaction product, can be morphologically compared to a very light sponge and are therefore showing a high reactive surface and contemporaneously a high homogeneity of the composition, with respect to the partition of the cations present in the starting solution, the solution stoichiometry of said cations being strictly observed. In view of the above, the advantages of the disclosed method may be hence summarized as: a high productivity; a low manufacturing cost; the possibility of obtaining a product which is no more a precursor based on oxides and carbonates, being directly YBCO, (single-phase for at least 95% b.w.) and containing an overall amount of carbon lower than 0.2% b.w.. The following example is supplied for illustrative purposes and shall not be construed as to limit the invention's scope. EXAMPLE An acid solution, containing nitrate ions and the ions Y, Ba and Cu in the ratio 1:2:3 respectively, is prepared in such a way that the ratio between the solution volume and the Y moles be approximately 6/1. 1400 g of citric acid monohydrate , per mole of Y, are added under stirring by heating at approximately 60°C in order to promote the dissolving. The ratio citric acid/ammonium nitrate is ca. 1:3. There are eventually added 2500 cm 3 of a 25% b.w. NH 4 OH solution per mole of formed Y, with an addition rate of about 0.3 + 1 l/s, making sure the final pH be alkaline: if the pH is below the recommended range, the same pH can be adjusted by addition of further ammonia. A layer of about 100 µm of the thus prepared solution is spread over a conveyor belt and subjected to pyrolysis in an oven in the following way. Within a time of 3 s the solution layer is brought to a temperature of 900°C and maintained at such a temperature for 150 s. The thus obtained powder is then made to cool down spontaneously, whereafter it results as very fine and not agglomerated. Such powder is consisting of practically single-phase YBCO (absence of second phases under X ray analysis).

Claims: A process for the production of YBCO powders, wherein a clear aqueous solution, containing the ions yttrium, barium and copper in the final proportions desired for the powders, in the form of complex compounds and in the presence of a detonating system, is continuously spread as a thin layer over a moving carrier, and in sequence a pyrolysis step is initiated consisting in evaporating said thin layer and in triggering its combustion, the resulting combustion product being kept at a temperature higher than 850 °C. Process according to claim 1, wherein the carrier on which the pyrolysis does occur is a metal conveyor belt, optionally coated with a high temperature resistant coating. Process according to claim 1, wherein the thickness of the thin layer is of the order of 100 µm. Process according to any of the preceding claims, wherein the complex compounds are obtained by means of an addition of citric acid, wherein the detonating system comprises ammonium nitrate and wherein the ratio citric acid:ammonium nitrate is from 1:2 to 1:5. Process according to any of the preceding claims, wherein the combustion product is kept, immediately after the combustion, at a temperature of between 870 and 1000 °C. Process according to claim 5, wherein the combustion product is kept, immediately after combustion, at a temperature of between 870 and 980 °C for a time of between 60 and 360 s. Process according to any of the preceding claims, wherein the duration of the combustion is of the order of 1-10 s. The process of claim 1, comprising preparing the clear aqueous solution by: (i) dissolving starting compounds comprising yttrium, barium and copper, previously dried at approximately 110 °C until reaching a constant weight and metered according to stoichiometrical amounts, as to get in the solution the cations of yttrium, barium and copper in a final YBCO ratio equal to 1:2:3, in such a way that nitrate ions are present in the solution and that the solution is acid; (ii) adding a complexing agent to the solution; (iii) further adding NH 4 OH, wherein said addition of NH 4 OH is executed in a time as short as possible, bringing the solution at a pH higher than 6,8. Process according to claim 8, wherein in order to prepare the solution, the working temperature is kept in the range of between 80 and 90 °C. Process according to claim 8, wherein the final pH of the solution is comprised between 7,0 a,d 7,5. Process according to claim 8, wherein the addition of ammonia is carried out utilizing a 25% b.w. NH 4 OH solution at a rate of between 0.3 and 1 l/s when the volume of the solution containing yttrium, barium and copper ions is of between approximately 6 and 10 l.

Inventors: FORTUNATI STEFANO, RISSO LUCA, TULUI MARIO, FORTUNATI, STEFANO, RISSO, LUCA, TULUI, MARIO
Applicants: SVILUPPO MATERIALI SPA, CENTRO SVILUPPO MATERIALI S.P.A.
Application year: 1992
Main classification: C01G3
IPC codes: C04B35, C01G3