WO1999038819A1

WO1999038819A1 - Method for producing oxidic powders with a low carbon and hydrogen content and their use as well as mixed oxides for high-temperature superconductors and high-temperature superconductors

Info

Publication number: WO1999038819A1
Application number: PCT/EP1999/000565
Authority: WO
Inventors: Joachim Bock; Michael BÄCKER; Dieter Sachse; Horst Hahn; Günther RIEDEL; Thomas Diehl
Original assignee: Aventis Research & Technologies Gmbh & Co. Kg
Priority date: 1998-01-30
Filing date: 1999-01-28
Publication date: 1999-08-05
Also published as: AU2425099A

Abstract

The invention relates to methods for producing oxidic powders having a carbon content of < 200 ppmw, a hydrogen content of < 150 ppmw and a composition which can yield a high-temperature superconductor after further processing. Said oxidic powders are made from powder-like oxide precursors which can be processed into formed pieces and possibly also to high-temperature superconductive sintered bodies. The powder particles of the oxide precursor are largely isolated, fed to a substantially vertical tubular reactor, passed through the reactor from top to bottom against the flow of a gas and thermally decomposed in the process. Alternatively the oxidic powder or the formed pieces made therefrom are treated with an acid which is added as an acid, especially to the furnace atmosphere, or as a thermally decomposable salt. The added acid and/or the acid released during thermal decomposition of the salt contributes to the removal of carbon and/or hydrogen. The invention further relates to the corresponding mixed oxides and high-temperature superconductors.

Description

1

description

Process for producing oxidic powders with low carbon and hydrogen contents, their use and mixed oxides for high-temperature superconductors and high-temperature superconductors

The invention relates to a method for producing oxidic powders, in particular for caicinizing oxide precursors to form oxidic powders, and the use of these powders for the production of moldings and high-temperature superconductors. The invention further relates to mixed oxides in the form of

Powders or moldings with a very low carbon content or with a very low hydrogen content, which are suitable for the production of high-temperature superconductors, and corresponding high-temperature superconductors, in particular in the form of strips, wires or solid parts.

For the production of high-temperature superconductors with high current carrying capacity, it is necessary that inorganic powdery material with high chemical purity, homogeneity, phase purity, defined phase composition and grain size is synthesized. This material contains mixed oxides, which besides

Oxygen predominantly contain one or more alkaline earth metals, copper and one or more elements from the group bismuth, lead, yttrium, lanthanum, lanthanides and thallium and optionally further oxides.

This oxidic powder can either be solid with superconducting parts

Properties above 77 K can be processed into superconducting wires or strips by pressing and subsequent annealing or by the powder-in-tube method (PIT method). In the PIT process, either the powdery material or a molded body produced therefrom by pressing is introduced into sleeves made of silver or silver alloys and with the usual

Technique formed into wires or ribbons. Here and with the aforementioned 2

Annealing to solid parts takes place in further thermal processes, in which the properties of the powder particles are also modified. In these processes, no volatile constituents may be released from the material that lead to bubble production. For this reason, for example, the residual carbon content of the powders and moldings should be as low as possible.

The low carbon content and, if possible, the low hydrogen content should be achieved during the production of the oxidic powder. At the same time, the oxidic powder for further processing into the end products should have a small grain size and high reactivity for the conversion into the desired superconducting phase, which takes place during the thermal treatment. For example, the oxidic powder used based on Bi, Pb, Sr, Ca cuprates with the so-called (Bi, Pb) -2212 phase should easily convert to the (Bi, Pb) -2223 phase during the thermal treatment.

If a calcination process of oxide precursors is used to produce the oxidic powder, these requirements contradict each other in principle, since high calcination temperatures and long treatment times at these temperatures promote the degradation of residual carbon, but reduce the reactivity for phase change and increase the grain sizes.

Various processes have been used to produce the oxide powders.

A conventional method is that oxide precursors in chamber or

Tube furnaces can be caicinised as fillings in several steps. This method requires long treatments at high temperatures.

In the unpublished DE 19 742 304 a calcination of oxide precursors in a rotary kiln is described, which is already shorter 3

Response times required. If the temperature control is not correct, there is a risk of deposits on the inner pipe wall.

From Uno et al. (Jap. J. Appl. Physics 27, No. 6, 1988, L1013-L1014) it is known that the solid-state reaction for mixed oxide formation and the degradation of residual carbon in a powder mixture of Bi ₂ O ₃ , SrCO ₃ , CaCO ₃ and CuO ₂ with a calcination in a vacuum at lower temperatures. Sintering of the powder particles, which can only be removed by an intensive grinding process, can thereby be avoided.

EP-A-0 369 117 describes a spray pyrolytic process in which a metal nitrate solution is sprayed into a calcination zone where it is brought into contact with a hot gas stream. In EP-B-0 473 621 oxidic ceramic powders are also obtained by a spray pyrolytic process of metal nitrate solutions by pyrolyzing the nitrates in the presence of organic substances serving as fuel. Both methods are quite complex in terms of equipment. In addition, the resulting nitrous gases require special measures for reasons of occupational safety and environmental protection.

From DE-A-195 14 202 and DE-A-195 05 133 processes for the production of highly disperse oxidic powders are known, in which the conversion to the oxides takes place in an oxyhydrogen burner by burning dissolved oxide precursors. These processes are disadvantageous due to the use of expensive raw materials and high process costs.

EP-A-0 573 804 describes oxide-ceramic superconductor materials with a carbon content in the range from 200 to 800 ppmw. This carbon content should be able to be set by prematurely stopping sales fires or in an additional annealing or melting treatment with a higher CO ₂ content than air. Above all, there will be an additional multi-level sales fire recommended, the material should even be shredded between the individual stages, which is a very complex process.

The object of the present invention was to create a cost-effective method for caicinizing powdery, homogenized oxide precursors, in which oxidic powders with a low residual carbon content and / or a low residual hydrogen content are obtained at the lowest possible temperatures and short reaction times.

It was also an object of the present invention to provide a process for the production of powders which is as generally applicable as possible and which is suitable for the production of high-temperature superconductors by means of which the content of carbon and / or hydrogen in the powder or, if appropriate, in the molded article or the product produced therefrom can possibly be further reduced in the high-temperature superconductor material made therefrom.

The object is achieved in that the powder particles of the oxide precursor are largely separated, are fed to an essentially vertical, tubular reactor, pass the reactor from top to bottom in countercurrent to a gas and are thermally decomposed in the process.

Furthermore, the object is achieved with a process for producing oxidic powders with a carbon content of <200 ppmw and with a hydrogen content of <150 ppmw with a composition which can result in a high-temperature superconductor from the powdery treatment

Oxide precursors, which can be further processed into shaped bodies and possibly also into high-temperature sintered bodies, in which the powder particles of the oxide precursor are largely separated, are fed to an essentially vertical, tubular reactor, pass the reactor from top to bottom in countercurrent to a gas and be thermally decomposed in the process. 5

Furthermore, the object is achieved with a method for producing oxidic powders with a carbon content of <200 ppmw and with a hydrogen content of <150 ppmw with a composition which can result in a high-temperature superconductor during further treatment, and optionally to give moldings and possibly also high-temperature superconductors Sintered bodies can be processed further, the oxidic powders or the moldings produced therefrom being treated with an acid which is added as an acid, in particular is added to the furnace atmosphere, or which is added as a thermally decomposable salt, the added or / and that in the thermal decomposition of the salt released acid to remove carbon or / and

Hydrogen contributes.

The superconductor material with these carbon and hydrogen contents is preferably in the equilibrium state.

The oxide precursor is a mixture of compounds of several elements, selected from the group Mg, Ca, Sr, Ba, Sc, Y, La, lanthanides, Zr, Pt, Ag, Cu, Hg, Al, Tl, Pb, Bi and Nd, which can be present as acetates, carbonates, citrates, hydroxides, lactates, nitrates, oxalates, tatrates or mixtures thereof. If nitrates are used, they are only present in small proportions in mixtures.

A mixture of compounds of the elements Bi, EA and Cu is particularly preferred; (Bi, Pb), EA and Cu; Y, EA and Cu; (Y, SE), EA and Cu; Tl, EA and Cu; (Tl.Pb), EA and Cu; Ti, (Y, EA) and Cu; or Hg, EA and Cu, where EA means alkaline earth metal, in particular Ba, Ca, and / or Sr and SE rare earth metals. Oxalates, hydroxides and acetates or mixtures thereof are particularly preferred

Element groups used as an oxide precursor.

The powdery, homogenized oxide precursor is preferably a coprecipitate powder. This powder can be obtained, for example, by starting from a nitrate solution of the metals and by adding oxalic acid or a corresponding other acid to precipitate the metal salts which 6 then be spray dried into the coprecipitate powder. However, the powdery, homogenized oxide precursor can also be produced in the dry state by mixing the corresponding salts and / or hydroxides. The powdery oxide precursor preferably has a residual moisture content of <1.5%.

The powder particles are largely separated so that they are fed to the reactor in fine distribution. This separation can take place, for example, in that the powder particles pass through a vibrating sieve. The separation can also be carried out by swirling in the gas stream.

If you choose a vibrating sieve, it can be particularly useful to provide a stirring blade in the sieve at a small distance (for example 1 to 5 mm) from the sieve bottom to produce a mixing movement which ensures uniform metering into the reactor and prevents clogging of the sieve . The

Mesh size of the sieve can be in the range from 20 to 400 μm, preferably in the range from 40 to 200 μm and in particular in the range from 63 to 125 μm. The choice of the mesh size of the sieve depends on the desired particle size that is to be fed to the reactor, which will be discussed in more detail below.

Our own studies have shown that when using oxalate mixtures with contents of the elements Bi, Pb, Sr, Ca, Cu, the use of an oven atmosphere containing air and / or nitrogen below atmospheric pressure with relatively short holding times of 1-5 h, preferably 3 h, and at low temperatures in the range from 700 ° C to 780 ° C, preferably 730 - 750 ° C, the oxalate decomposition and the mixed oxide formation compared to standard conditions with a multi-stage process - e.g. 4 h 790 ° C in air and subsequently 15 h 800 ° C in O ₂ - can be carried out in a shortened process step to the same state of about 0.1% residual carbon. 7

According to the invention, this is achieved with the relatively short holding times and low temperatures just mentioned by using a negative pressure of 0.01-10 mbar, preferably 0.1-1 mbar.

The reactor is preferably tubular and arranged essentially vertically. Essentially vertical means that even small slopes, i.e. small angles to the normal are possible. However, it is particularly preferred to be vertical. The reactor is preferably heated indirectly by means of an electric heater in the form of a plurality of fiber-insulated heating modules which surround the outside of the tube and each have separate temperature control devices. The reactor is preferably made of a heat-resistant material, for example quartz glass, ceramic or metal.

In the tubular reactor, the ratio of the heated length to the inside diameter of the reactor is generally in the range from 10 to 50: 1, preferably in the range from 15: 1 to 30: 1 and in particular in the range from 20: 1 to 30: 1. The heated one The length of the reactor can be between 0.2 to about 10 m, preferably between 1 to 5 m. The inside diameter of the reactor should not be too large. In the case of a heated length of 1.50 m, an inner diameter is chosen to be not substantially more than 100 mm, preferably not more than 80 mm. If the internal diameter is increased, the throughput cannot be increased significantly while the quality of the calcined product remains the same, since, as a result of a laminar flow with the somewhat slower flowing edge regions, powder accumulation takes place in the edge regions. If you ensure turbulent flow conditions, an increase in the

Inner diameter possible.

An increase in throughput is preferably achieved by using several tubular reactors which are arranged in the form of a bundle within the same heating system. The number of reactors is limited by the fact that, based on their cross section, 8th

the same temperature should be present. The temperature difference across the cross section should not be greater than + 1-2 K if possible, because otherwise the powder will not be homogeneously caicinated and different carbon contents can occur in the reaction product.

After being separated, the powder particles of the oxide precursor are fed to the tubular reactor from above, in which they sink downward in countercurrent to a gas. The gases used can be nitrogen, oxygen, argon, their mixtures, air or a more oxygen-rich mixture with an N ₂ / O ₂ ratio of up to 10:90. The gas preferably has an N ₂ / O ₂ -

Ratio of 50:50. Oxygen can also be used preferably.

In order to have the lowest possible carbon content in the thermally decomposed oxidic powder leaving the reactor and to prevent the formation of new carbonates with alkaline earth metals contained in the reaction mixture, it is advantageous if the gas contains a sufficiently high proportion of oxygen towards the end of the calcination. The further the thermal decomposition in the reactor has progressed, the higher the oxygen partial pressure should be in this area. An oxygen partial pressure of the gas conducted in countercurrent to the reaction mixture, increasing from top to bottom in the tubular reactor, is therefore particularly preferred. For reasons of cost, it is favorable to supply an oxygen-rich nitrogen-oxygen mixture or oxygen at the lower end of the tubular reactor and a mixture with a lower oxygen content, in particular air or nitrogen, in the central region of the reactor.

In order to lower the carbon content and to prevent the formation of carbonates, it is also important that the gaseous decomposition products, in particular CO ₂ , are removed sufficiently quickly in countercurrent. The amount of gas supplied should be selected in relation to the reactor diameter so that the resulting gas velocity, on the one hand, allows the gases which are produced in the decomposition reaction to be rapidly removed upward from the reactor 9

guaranteed, but on the other hand not to be too large, so that no or only minimal amounts of the powder are discharged from the reactor with the gas stream.

The production of oxidic superconductor materials with particularly low carbon contents is also possible after a single pass through the preferably tubular reactor, if the subsequent post-calcination in pure oxygen e.g. in the range of about 12 h at 800 ° C, the phase setting e.g. in the range of about 3 h at 780 ° C and the final annealing of the compacts made from this powder e.g. in the range of about 5 h at 740 ° C. in a nitrogen atmosphere with an oxygen content of 0.1-3%, preferably 1%. The repeated annealing of the moldings has the advantage that carbon accumulation that occurs through handling or mechanical reworking of the molded parts is compensated for again. This means that the carbon content can be reduced to values <100 ppm (50-80 ppm), that if properly packaged in synthetic air or another dry air

Gas with the exception of CO ₂ filled glass or metal sleeves can also be retained until it is inserted into the silver tubes during wire production.

In principle, it is also possible to generate the gas stream in the tubular reactor not by a regulated supply of the gas, but by one

Let chimney effect and self-suction arise.

The thermal decomposition of the oxide precursor takes place in the reactor at normal pressure and temperatures in the range from 400 ° C. to 900 ° C., preferably in the range from 550 ° C. to 900 ° C. and in particular in the range from 650 ° C. to 900 ° C.

The heated area of the reactor can be a zone with a uniform temperature, especially if the calcined powder particles are subjected to a post-calcination. In this case, the calcination is generally carried out at a temperature in the range from 550 ° C to 900 ° C, preferably 550 ° C to 830 ° C. The

Post-calcination can be carried out continuously in a rotary kiln, in a fluidized bed 10 or in batch operation as a bed in open containers in a gas stream in order to form the superconducting mixed oxide phases at temperatures above 77 K.

If a post-calcination is carried out, the heated zone in the reactor can be shorter and air or oxygen-enriched air can be supplied to the reactor as gas in order to achieve economical operation. For post-calcination in the rotary kiln or in the fluidized bed, agglomeration is desirable in order to achieve continuous transport or fluidizability. The agglomeration is advantageously achieved in that the

Powder particles of the oxide precursor are separated through a coarser sieve before they are fed to the reactor. In this case, mesh sizes of the sieve are selected which are in the range from 200 to 400 μm. In all other cases, a sieve with a small mesh size is preferably used in the method according to the invention.

It is also possible to feed the calcined powder particles to the same reactor or to a second reactor for a second pass, it being possible for the temperature and gas composition to be the same or different in the two passes in order to form the superconducting phases at temperatures above 77K. In a preferred embodiment, the heated area of the reactor has two or more zones with different temperatures. A lower temperature is selected for the upper or uppermost zone of the reactor, which the powder particles pass first, than for the lower or the lowest zone. The temperature in the upper part of the tubular reactor can range from 400 ° C to 700 ° C and the temperature in the lower part of the reactor can range from 750 ° C to 900 ° C. This procedure is particularly advantageous if oxide precursors are used which decompose in an exothermic reaction even at relatively low temperatures. For example, the decomposition reaction of bismuth oxalate begins at 236 ° C. The first heating zone through which the powder particles pass is therefore comparatively low at around 650 ° C 11 set. Overheating due to the heat released during oxalate decomposition is avoided, which would lead to undesired sintering of the powder particles into large and hard agglomerates.

The lower or lowest heating zone is set to a higher temperature in order to lower the carbon content in the powder particles and to initiate the formation of superconducting mixed oxide phases, which, however, usually only takes place to a sufficient extent in the subsequent thermal treatment. In the embodiment of the method with two or more different temperature zones, it is preferred to separate the powder particles of the oxide precursor by

Separate sieve with a small mesh size in the range of 40 to 125 μm. The powder particles of the oxide precursor fed to the reactor preferably have a size of <20 μm, in particular <15 μm and particularly preferably <10 μm (measured with laser light diffraction, Mastersizer from Malvern Instruments GmbH). This also applies if the heated area of the reactor is a zone with a uniform temperature, unless, as stated above, post-calcination in the rotary kiln or in the fluidized bed is provided. The small particle size and the use of oxygen as a gas greatly reduce the carbon content without significantly extending the heated reactor area. This allows the conventional

Calculation process necessary first two annealing steps are omitted.

In the embodiment just described with zones of different temperature, it is possible for the upper zone with a lower temperature to be a lower-oxygen gas or nitrogen and the lower zone for an oxygen-rich one

Add gas or oxygen. This procedure is less expensive and still provides a low carbon oxide powder.

The inventive method works continuously and has the advantage that with short treatment times of a few seconds, a decomposition of the

Oxide intermediates to form mixed oxides and oxides with low 12

Carbon content is achieved. This is achieved with little technical effort and little work. Furthermore, the agglomerates of the oxidic powder have a small average grain size, and the proportion of agglomerates> 40 μm, which is 20 to 30% in conventional processes, and the proportion of hard agglomerates are very small. Another advantage of the method according to the invention is the higher reactivity of the oxidic powder produced for the conversion into the superconducting target phase, for example (Bi, Pb) -2223.

In the preferred embodiments of the method according to the invention, the carbon content of the oxidic powder is reduced to <0.5% or even <0.1% in one pass. As a result, the thermal phase adjustment process can follow immediately, which is preferably carried out on moldings to which the oxidic powder is pressed. Compared to the calcination process in a rotary tube or fluidized bed reactor, the process according to the invention also has the advantage that metallic impurities on Fe, Cr and Ni, which negatively influence the superconducting properties, are avoided.

A shaped body can be produced from the oxidic powder, optionally without an additional grinding process, for example by isostatic pressing, and can be further processed in various ways by thermal treatment. Subsequent sintering can be used to produce a solid superconducting part, or the material for the production of superconducting wires and tapes can be obtained through a special annealing process with a defined oxygen partial pressure. can be used.

In a preferred embodiment, a homogeneous mixture of oxalates, for example of the elements Bi, Pb, Sr, Ca and

Cu. In the reactor, these become mixed oxides in the form of the non-superconducting 13

Phase Bi-2201 and (Sr, Ca) cuprates, plumbates and / or bismuthates and decomposed in the form of CuO. The formation of the superconducting phase (Bi, Pb) -2212 with a critical temperature of 92 K can then already begin. However, this phase is mainly obtained only in subsequent thermal steps, for example those described above

Post-calcination processes and thermal treatments such as sintering and special tempering processes. As already mentioned, the recalcination process can be omitted if the process is preferred. The formation of the superconducting phase (Bi, Pb) -2223 with a critical temperature of 1 10 K represents the target phase in the solid part and wire production. It is only formed during the sintering to the solid part or the thermomechanical treatment of the wire production.

In order to further reduce the carbon and hydrogen contents, it is recommended that the oxide powders or those produced therefrom

Moldings are treated with an acid which is added as an acid, in particular is added to the furnace atmosphere, or which is added as a thermally decomposable salt, the acid added or / and released during the thermal decomposition of the salt to remove carbon and / or hydrogen contributes.

The oxidic powders or the moldings produced therefrom can be treated with organic acids or / and mineral acids which have a greater acid strength than carbonic acid, in particular with formic acid, acetic acid, sulfuric acid and / or nitric acid. This procedure can be used

Room temperature, but also at elevated temperatures. It is advantageous to use gaseous acids, especially in the furnace atmosphere.

In addition, in this process according to the invention or in general in processes for the production of powdery oxide precursors to form oxidic powders with a carbon content <200 ppmw and with a hydrogen content <150 ppmw 14

With a composition which can result in a high-temperature superconductor during the further treatment, and optionally to give shaped bodies and possibly also high-temperature superconducting sintered bodies, the oxidic powders or the shaped bodies produced therefrom can be treated with an acid, which is a thermally decomposable salts of acids which are relatively large Have acid strength as carbonic acid, are added, in particular ammonium, ammonium hydrogen or metal hydrogen salts or in particular salts of formic acid, acetic acid, sulfuric acid or nitric acid, with the metallic or semimetallic elements present in the superconducting material being preferred as the metal. Due to the thermal decomposition, this process requires a minimum temperature of usually 150 ° C and is particularly suitable for moldings.

Due to the stronger acid, the residues of the weaker acids, especially water and carbonic acid, are made from their salts, especially the hydroxides and

Carbonates, displaced. If salts of this type are added, the decomposition temperature of these salts is reached first during heating and the reaction of the liberated acid with the hydroxides and carbonates only when heating continues; finally, at even higher temperatures, the salts previously formed with the acid are decomposed into oxides or multiple oxides.

In principle, the acid can be added in solid, liquid, dissolved or gaseous form or the salt in solid, dissolved or liquid form. The chemical reaction with the acid can in principle take place at room temperature or at low to very high temperatures, in particular when heating to a higher temperature, it being possible for the chemical reaction to be essentially completed before the end temperature is reached. Preferred is the addition in the form of a gaseous acid when acting on a powder, in particular in the form of a powder bed, preferably at 50 to 200 ° C, or the addition in the form of a thermally decomposable salt in solid form when acting on one

Shaped body, preferably at 150 to 500 ° C. 15

In both process variants, an alkaline earth metal sulfate can be formed in the powder or shaped body if sulfate is present, the alkaline earth metal sulfate being homogeneously finely dispersed in the powder or in the shaped body. The particles of this sulfate, in particular sulfates of Ba, Ca or / and Sr, are not decomposed even during the later sintering and / or thermal aftertreatment and act as

Crystallization nuclei, as structure-modifying components that have a positive influence on microstructure formation, and possibly as pinning components.

The present invention also relates to a mixed oxide, in particular in the form of a powder or a shaped body, with a carbon content <200 ppmw, in particular <150 ppmw, particularly preferably <100 ppmw or with a hydrogen content <150 ppmw, in particular <100 ppmw preferably <60 ppmw or <50 ppmw. Such mixed oxides are either after two passes through the reactor with two or more temperature zones and oxygen as a gas at least in the lower region of the

Reactor available or after a single run under these preferred process conditions with subsequent thermal treatment. Alternatively or additionally, the aforementioned acid treatment can be used to reduce the levels of these impurities. Mixed oxide means both the powder and the powder formed into a shaped body. The thermal treatment comprises annealing the powder or the shaped body at, for example, 770 ° C. in a stream of nitrogen, optionally mixed with oxygen, for three hours.

The present invention furthermore relates to a high-temperature superconductor with a carbon content of <100 ppmw, in particular <80 ppmw, particularly preferably <50 ppmw or else a high-temperature superconductor with a hydrogen content <100 ppmw, in particular <60 ppmw, particularly preferably <40 ppmw. Such low levels of carbon or hydrogen have been sought after for many years in high-temperature superconductor materials and have been

Knowledge of the applicant has never been achieved. The levels of carbon and 16

Hydrogen is lower than with the mixed oxides used, since any thermal treatment leads to a reduction as long as these traces are in the form of easily decomposable compounds. The compounds of these elements which are difficult to decompose in the usual temperature range up to about 800 ° C or up to about 850 ° C under the selected atmospheric conditions, e.g. However, strontium carbonate can only be removed with acids using a chemical process.

This is available after a single pass through the reactor, annealing in an oxygen stream and sintering to form a solid part or thermomechanical processing to form a wire or - also in other processes for the production of superconducting powders or moldings made therefrom - via the treatment according to the invention with acids. With a single pass through the reactor with different temperature zones and oxygen as a gas, at least in the lower region of the reactor, the high-temperature superconductor with the low carbon content is already available without an annealing step in the oxygen stream.

The oxidic powders or molded articles produced therefrom by the process according to the invention are for the manufacture of high-temperature superconducting wires, strips, rods, tubes, hollow and solid bodies, in particular for the manufacture of high-voltage cables, power lines, transformers, windings, magnets, power supplies, electric motors , Energy storage, current limiters or magnetic bearings can be used.

The invention is explained in more detail below with reference to the figure and the examples.

The figure shows a schematic illustration of the tubular reactor and the associated device parts for the process according to the invention. The oxide precursor is from the storage container 1 via a powder flap 2 and

Dosing device 3 with screw conveyor 4 transported into a sieve 5. The sieve 5 is with 17

a stirrer 6, for which a motor 7 is provided. The impeller of the stirrer 6 is located a short distance above the sieve bottom 8. The sieve 5 is set in vibration by a pneumatic vibrator 9.

The powder particles of the oxide precursor are separated as they pass through the sieve 5. They fall evenly distributed over the reactor cross section into the open tubular reactor 10. In its simplest embodiment, this has a zone heated by an electric heater 11 for which control electronics 12 are provided. Gas is supplied to the reactor 10 from below via lines 13 and 14. The line 13 is provided for the oxygen supply and the line 14 for the nitrogen supply. Any ratio of oxygen to nitrogen in the gas supplied to the reactor 10 can be set via control electronics 15, which act on the valves 16 and 17.

The powder particles sink in the reactor in countercurrent to the gas introduced from below and are thermally decomposed in the heated zone of the reactor. The oxidic powder falls into the collecting container 18 arranged under the reactor 10.

The speed of the gas flow can also be regulated via the valves 16 and 17. It is preferably set so that no powder or a maximum of 1% of the powder is discharged upwards. In order to minimize the discharge of the powder, the top part of the reactor 10 can be funnel-shaped (not shown in the figure), preferably by a factor of 1.5 to 3. This reduces the gas velocity in this area. The exhaust gas flow, which is predominantly passed to the side of the screen, can be passed through a Memtic particle filter, not shown, in order to separate out emerging powder particles.

Alternatively, this filter can be omitted if the exhaust gas is cleaned in a wet scrubber. This is particularly recommended if the

Reactor is operated in combination with a spray dryer in which a 18th

Suspension with wet homogenized oxide precursors is spray dried and this is already equipped with a wet washer. The exhaust gas stream from the reactor can then be fed to the latter. A wet washer is particularly recommended for lead-containing oxide precursors.

If a gas with a different composition than the lower zone is to be fed into the upper zone of the reactor 10, a further gas feed is provided in the central region of the reactor.

Several examples and a comparative example are given below, which are intended to illustrate the invention by way of example.

Example 1 :

The following raw materials were weighed out for a 1 kg batch: 304 g Bi ₂ (C ₂ O ₄ ) ₃ ; 30 g Bi (OH) ₃ ; 51 g Pb (C ₂ O ₄ ); 220 g of Sr (C ₂ O ₄ ); 145 g Ca (C ₂ O ₄ ); 250 g Cu (C ₂ O ₄ ).

These raw materials were dispersed in water, homogenized in the suspension and subsequently spray-dried. The residual moisture of the

Drying product was 0.9%.

Via a dosing device with screw conveyor, the product was placed on a sieve with a mesh size of 150 μm vibrating by means of a pneumatic vibrator and introduced into the reactor in an amount of 300 g / h by additional stirring of the feed material.

Reactor material: quartz glass Reactor geometry: heated length: 1, 50 m,

Pipe diameter: 70 mm

Temperature of the first heating zone: 650 ° C

Second heating zone temperature: 650 ° C

Temperature of the third heating zone: 650 ° C Gas composition: N ₂ : O ₂ = 50:50

Gas volume: 120 l / h 19

Residual carbon content of the calcine after one reactor run: 2.8% (measured by means of thermal oxidation and infrared detector: device from Elementar High TOC). Contaminant content of the calcine: 80 ppmw Fe, 20 ppmw Ni, 30 ppmw Cr (measured by ICP / OES). This did not represent an increase in the content of Fe, Ni and Cr, taking into account the loss of ignition during the calcination, compared to the starting products. The sum of the metallic impurities also remained constant, taking into account the loss of ignition during the calcination. The mean agglomerate grain size was 20 μm (dispersion in silicone oil with 60 s ultrasound treatment; laser granulometer, device type CILAS 715). Specific surface area: 1.1 m ² / g (measured by means of low-temperature nitrogen adsorption according to BET). These agglomerates were easily destroyed, for example when sieving on a 40 μm sieve using sieve aids in the form of ZrO ₂ spheres with 3-5 mm

Diameter or in the manufacture of a shaped body by a pressing process. Hard agglomerates were not contained in this powder.

The carbon content was further reduced by annealing in the powder bed for 12 hours at 800 ° C. in an oxygen stream. The

Carbon content reduced to 0.08%.

Production of the sintered body: The powder was sieved <40 μm and pressed at a pressure of 2000 bar. The compacts were annealed for 120 hours at 825 ° C. in a tube furnace in a gas stream (250 l / h) consisting of nitrogen and 1% oxygen, the conversion to the (Bi, Pb) -2223 phase being carried out.

Properties of the sintered body produced therefrom: Sintered bulk density: 95% of the theoretical density (6.34 g / cm ³ ); Carbon content: 0.008%; Step temperature Tc = 1 10 ° C;

Critical current density each = 1800 A / cm ² . 20th

Manufacture of shaped bodies for wires: cylinders with a diameter of 15 mm and a length of 100 mm were produced from the powder sieved <40 μm using isostatic pressing at 2000 bar using the dry die process. These were used to produce a phase composition of approximately 85% orthorhombic (Bi, Pb) -2212 phase as well as (Sr, Ca) cuprates and

CuO annealed for three hours at 770 ° C in nitrogen with a share of 1% oxygen. The carbon content was reduced to 0.01%.

These moldings could then be manufactured by a wire manufacturer after the powder-in-tube

Method for producing a mono- and / or multifilament strip conductor with a silver sheath and powder core can be processed further.

Example 2: The procedure was as in Example 1, but with the following differences:

The following raw materials were weighed out for a 1 kg batch: 342 g Bi ₂ (C ₂ O ₄ ) ₃ ; 50 g Pb (C ₂ O ₄ ); 217 g of Sr (C ₂ O ₄ ); 143 g Ca (C ₂ O ₄ ); 248 g Cu (C ₂ O ₄ ).

These oxide precursors were dispersed in water, homogenized in the suspension and subsequently spray-dried. The residual moisture was 0.8%.

Using a dosing device with a screw conveyor, the product was placed on a sieve with a mesh size of 125 μm, which vibrated by means of a pneumatic oscillator, and was introduced into the reactor in an amount of 200 g / h with additional stirring of the feed material.

Reactor geometry and material as in Example 1. Temperature of the first heating zone: 650 ° C Temperature of the second heating zone: 850 ° C Temperature of the third heating zone: 870 ° C

Gas: 100% oxygen 21

Gas volume: 100 l / h

The residual carbon content after one reactor run was 0.6%. The metallic impurities corresponded to those of Example 1. The average agglomerate grain size was 3 μm (dispersion in silicone oil at 60 seconds

Ultrasound treatment; Laser granulometer, device type CIL-AS 715). These agglomerates, like the powders prepared according to Example 1, were easily destructible. Hard agglomerates were not contained in this powder.

The further processing into sintered bodies and shaped bodies for wires was carried out as in

Example 1, wherein the carbon content after the oxygen annealing step was 0.05%. The carbon content of the sintered bodies was 0.004% and that of the shaped bodies for the wire production was 0.008%.

Example 3:

The procedure was as in Example 2, but with the following differences:

The powder of Example 2 was placed on a sieve vibrating by means of a pneumatic vibrator with a mesh size of 50 μm and introduced into the reactor in an amount of 100 g / h with additional stirring of the feed material.

The reactor geometry, temperature setting and gas corresponded to those of Example 2.

The residual carbon content after a reactor run was 0.08%, the metallic impurities corresponded to those of Example 1. The average agglomerate grain size was 8 μm (dispersion in silicone oil with 60 s ultrasound treatment; laser granulometer, device type GLAS 715). These agglomerates, like the powders produced in Example 1, were easily destructible. Hard agglomerates were not contained in this powder. 22

Because of the low carbon content of 0.08% achieved after the calcination in the vertical reactor, the oxygen annealing step described in Examples 1 and 2 could be dispensed with, and the powder could be pressed to the shaped body after sieving on a 40 μm sieve.

The pourability of this powder was significantly improved compared to the material conventionally processed in the powder filling, so that a mold with a diameter of 15 mm and a length of 120 mm could be filled uniformly without additional aids such as pressing aids or vibration. As in the case of sample preparation according to Examples 1 and 2, this prevented the eaicinated powders from absorbing moisture from the ambient atmosphere before processing.

The further processing into sintered bodies and shaped bodies for wires was carried out according to Example 1.

The carbon content of the sintered bodies was 0.006% and that of the shaped bodies for the wire production was 0.012%.

Example 4: 400 g of an oxide powder according to Example 2 eaicinated in a vertical reactor were placed in a silver boat in the middle of a gas-tight horizontal tube furnace. In the front part of the tube furnace, for example where the flowing nitrogen atmosphere is introduced, 50 ml of conc. Nitric acid submitted. The tube furnace was then heated to 150 ° C. with a flowing nitrogen atmosphere (20 l / min). At this temperature the nitric acid slowly evaporated and was passed from the inert gas stream over the powder, where the reaction of the residual hydroxides and carbonates to the corresponding nitrates took place. After the nitric acid had completely evaporated, the tube furnace was slowly heated to the annealing temperature of 800 ° C. and held at this temperature for 60 h, during which the nitrates formed were decomposed to form metal oxides and nitrogen oxides. 23

After this treatment, the powder had a residual carbon content of 50 ppmw and a residual hydrogen content of 40 ppmw.

Example 5: The spray-dried powder mixture from Example 1 was used in Ag glow boats

Dimensions 250 mm x 250 mm filled with a bed height of approx. 20 mm and caicinated in a chamber furnace for 6 hours at 690 ° C.

The residual carbon content after calcination was 3.5%, the metallic impurities correspond to example 1. The average agglomerate grain size was 30 μm (dispersion in silicone oil with 60 s ultrasonic treatment:

Laser granulometer, device type GLAS 715). Hard agglomerates of 10 - 40 μm in size appeared that could only be destroyed by grinding. Despite the use of sieve aids according to Example 1, this was also shown in a sieve residue> 40 μm of 40% and> 63 μm of 10%.

400 g of such an oxide powder was placed in a silver boat in the middle of a gastight horizontal tube furnace. In the front part of the tube furnace, for example where the flowing nitrogen atmosphere is introduced, 100 ml of conc. Nitric acid submitted. The tube furnace was then heated to 150 ° C. with a flowing nitrogen atmosphere (20 l / min). At this

Temperature slowly evaporated the nitric acid and was passed from the inert gas stream over the powder, where the reaction of the residual hydroxides and carbonates to the corresponding nitrates took place. After the nitric acid had completely evaporated, the tube furnace was slowly heated to the annealing temperature of 800 ° C. and held at this temperature for 100 h, during which the nitrates formed were decomposed to form metal oxides and nitrogen oxides. After this treatment, the powder had a residual carbon content of 85 ppmw and a residual hydrogen content of 55 ppmw. A measurement with the laser granulometer as in Example 1 showed that the agglomerates had been partially destroyed due to the acid treatment and only a medium one

Agglomerate grain size of about 8 microns had. 24

Example 6:

500 g of an eaicinated oxide powder according to Example 2 were mixed with 50 g of ammonium hydrogen sulfate (NH ₄ ) HSO ₄ in an Eirich intensive mixer for 1 hour. The batch was pressed into 10 bars (diameter 12 mm, length 10 cm, density 4.2 g / cm ³ ). These rods were in a tube furnace in one

Nitrogen atmosphere with 1000 ppmw oxygen slowly (50 ° C / h) heated to a final temperature of 800 ° C and held at this temperature for 60 h. After this treatment, the powder had a residual carbon content of 40 ppmw and a residual hydrogen content of 45 ppmw. The grinding of a compact embedded in synthetic resin showed, when viewed under the reflected light microscope, finely divided alkaline earth metal sulfate with average grain sizes of <3 μm. No agglomerates were noticed here.

Comparative Example 1: The spray-dried powder mixture from Example 1 was used in Ag glow boats

The residual carbon content after the calcination was 3.5%, the metallic impurities corresponded to example 1. The average agglomerate grain size was 30 μm (dispersion in silicone oil with 60 s ultrasound treatment:

The carbon content was further reduced by annealing in the powder bed for 18 hours at 800 ° C. in an oxygen stream. The carbon content was reduced to 0.1%.

Production of the sintered body: The powder was sieved to a particle size <40 μm and pressed at a pressure of 2000 bar. The compacts were at 120 h 25th

Annealed 825 ° C in a tube furnace in a gas stream (250 l / h) consisting of nitrogen and 1% oxygen, the conversion to the (Bi, Pb) -2223 phase was carried out.

Properties of the sintered body produced therefrom: Sintered bulk density: 93% of the theoretical density (6.34 g / cm ³ ); Carbon content: 0.03%;

Step temperature T _c = 1 10 ° C;

Critical current density J _c = 1300 A / cm ² .

Shaped body production for wires: Since the powders had to be ground before pressing because of the hard agglomerates that were too large for the wire production, the flowability was significantly poorer than that of the powders of Examples 1 to 3.

The powder, which was dry-ground using a vibrating mill and sieved again to a particle size of <40 μm, was used for isostatic pressing

Cylinder with a diameter of 15 mm and a length of 100 mm manufactured by the dry matrix process. These were annealed for three hours at 770 ° C. in nitrogen with a proportion of 1% oxygen to produce a phase composition of approximately 85% orthorhombic (Bi, Pb) -2212 phase and (Sr, Ca) cuprates and CuO. The carbon content was reduced to 0.035

% decreased.

Example 7:

500 g of a powder produced by co-precipitation of oxalates with contents of the elements Bi, Pb, Sr, Ca, Cu with a residual moisture content of 0.9% was placed in an Ag

Filled with dimensions of 250 x 250 mm ² with a maximum dumping height of 30 mm. The calcination was carried out in a gas-tight tube furnace at 730 ° C. for 3 hours using a vacuum of 0.01 mbar, starting from an air atmosphere. The residual carbon content was then 0.1%. 26

The further carbon depletion took place simultaneously with the setting of a phase composition in the form of superconducting orthorhombic (Bi, Pb) - 2212 phase, (Sr, Ca) cuprate and CuO with a subsequent heat treatment of 3 h at 780 ° C in a nitrogen atmosphere with a proportion of 1% oxygen. The residual carbon content was in the material produced in this way

120 ppmw.

Example 8:

500 g of a powder produced by co-precipitation of oxalates with contents of the elements Bi, Pb, Sr, Ca, Cu with a residual moisture of 0.9% was in

Downpipe reactor pre-caicinated according to Example 1. Then the post-calcination was carried out in pure oxygen for 12 h at 800 ° C, the phase adjustment for 3 h at 780 ° C in a nitrogen atmosphere with an oxygen content of 1% and the final glow of the compacts made from this powder for 5 h at 740 ° C in a nitrogen atmosphere with a Oxygen content of 0.1 - 3%.

The residual carbon content was then 75 ppmw.

Claims

27 Patent claims:

1. A process for the production of oxidic powders with a carbon content <200 ppmw and with a hydrogen content <150 ppmw with a composition which in the further treatment a

High-temperature superconductors can result from powdery oxide precursors, which can be processed into moldings and, if appropriate, also into high-temperature superconducting sintered bodies, characterized in that the powder particles of the oxide precursor are largely separated, are fed to an essentially vertical, tubular reactor, the reactor from top to bottom pass in countercurrent to a gas and are thermally decomposed.

2.Procedure for producing oxidic powders with a carbon content <200 ppmw and with a hydrogen content <150 ppmw with a composition which can result in a high-temperature superconductor in the further treatment and, if appropriate, can be further processed into moldings and possibly also into high-temperature superconducting sintered bodies, characterized in that the oxidic powders or the moldings produced therefrom are treated with an acid which is added as acid, in particular is added to the furnace atmosphere, or which is added as a thermally decomposable salt, the added or / and during thermal decomposition the acid released from the salt contributes to the removal of carbon and / or hydrogen.

3. The method according to claim 2 or according to claim 1 and 2, characterized in that the oxidic powder or the moldings produced therefrom are treated with organic acids and / or mineral acids which have a greater acid strength than carbonic acid, 28

especially with formic acid, acetic acid, sulfuric acid and / or nitric acid.

4. The method according to claim 2 or 3, characterized in that the oxidic powders or the moldings produced therefrom are treated with an acid, which are added as thermally decomposable salts of acids which have a greater acid strength than carbonic acid, in particular ammonium, Ammonium hydrogen or metal hydrogen salts or in particular salts of formic acid, acetic acid, sulfuric acid or nitric acid, the metal or semi-metallic elements present in the superconductor material being preferred as the metal.

5. The method according to any one of the preceding claims, characterized in that the powder particles of the oxide precursor are largely isolated, are fed to a substantially vertical, tubular reactor, pass the reactor from top to bottom in countercurrent to a gas and are thermally decomposed.

6. The method according to any one of the preceding claims, characterized in that the oxide precursor a mixture of compounds of several elements selected from the group Mg, Ca, Sr, Ba, Sc, Y, La, lanthanides, Zr, Pt, Ag, Cu, Hg, AI, Tl, Pb, Bi and Nd.

7. The method according to any one of the preceding claims, characterized in that the compounds acetates, carbonates, citrates,

Hydroxides, lactates, nitrates, oxalates, tartrates or mixtures thereof.

8. The method according to any one of the preceding claims, characterized in that the oxide precursor is a mixture of compounds of the elements Bi, EA and Cu; (Bi.Pb), EA and Cu; Y, EA and Cu; (Y, SE), EA and Cu; Tl, EA and Cu; (TI, Pb), EA and Cu; Ti, (Y, EA) and Cu; or Hg, EA 29 and Cu, where EA is alkaline earth metal, in particular Ba, Ca and / or Sr, and SE means rare earth metals.

9. The method according to any one of the preceding claims, characterized in that the powder particles of the oxide precursor are separated by a vibrating sieve.

10. The method according to claim 9, characterized in that the sieve has a mesh size in the range of 40 to 200 microns.

11. The method according to any one of the preceding claims, characterized in that the gas is nitrogen, oxygen, argon, their mixtures, air or an oxygen-rich mixture.

12. The method according to any one of the preceding claims, characterized in that the oxygen partial pressure of the gas in the tubular reactor increases from top to bottom.

13. The method according to any one of the preceding claims, characterized in that the thermal reaction at temperatures in

Range from 550 ° C to 900 ° C is carried out.

14. The method according to any one of the preceding claims, characterized in that the ratio of the heated length to the inner diameter of the tubular reactor is in the range from 15: 1 to 30: 1.

15. The method according to any one of the preceding claims, characterized in that a plurality of tubular reactors are arranged within a heating system in the form of a tube bundle. 30th

16. The method according to any one of the preceding claims, characterized in that the oxidic powders are fed to the same reactor or to a second reactor for a second pass, the temperature and gas composition in the two passes being the same or different.

17. The method according to any one of the preceding claims, characterized in that the heated area of the reactor is a zone with a uniform temperature.

18. The method according to claim 17, characterized in that the temperature is in the range from 550 ° C to 900 ° C.

19. The method according to any one of the preceding claims, characterized in that the eaicinated powder particles are subjected to a post-calcination in a rotary tube furnace, in a fluidized bed or as a bed in an oven in order to form the superconducting phases at temperatures above 77 K.

20. The method according to any one of claims 1 to 16 and 19, characterized in that the heated area of the reactor has two or more zones with different temperatures, the temperature of the upper zone or the uppermost zones in the range from 400 ° C to 700 ° C and the temperature of the lower zone or the lowest zones is in the range of 750 ° C to 900 ° C.

21. The method according to claim 20, characterized in that a lower oxygen or nitrogen gas is supplied to the upper zone or the uppermost zones and an oxygen-rich gas or oxygen is supplied to the lower zone or the lowest zones, so that the powder particles in countercurrent to an increasingly oxygen-rich gas pass the reactor. 31

22. The method according to any one of the preceding claims, characterized in that the powder particles of the oxide precursor supplied to the reactor have a size ≤ 20 microns.

23. The method according to any one of the preceding claims, characterized in that the eaicinated oxidic powder particles are optionally pressed into a shaped body after a grinding step.

24. The method according to claim 23, characterized in that the powder or the molded body is treated thermally.

25. The method according to claim 23, characterized in that the powder or the shaped body is thermally treated and processed in a sleeve made of silver or a silver alloy to form wires or strips.

26. The method according to any one of the preceding claims, characterized in that the oxidic powder or the moldings produced therefrom are treated with an acid which is added as an acid, in particular is added to the furnace atmosphere, or which is added as a thermally decomposable salt, the added or / and the acid released during the thermal decomposition of the salt contributes to the removal of carbon and / or hydrogen.

27. The method according to claim 26, characterized in that the oxidic

Powder or the moldings produced therefrom are treated with organic acids and / or mineral acids which have a greater acid strength than carbonic acid, in particular with formic acid, acetic acid, sulfuric acid and / or nitric acid. 32

28. The method according to claim 26, characterized in that the oxidic powders or the moldings produced therefrom are treated with an acid which is used as thermally decomposable ammonium, ammonium hydrogen, metal hydrogen salts or as salts of acids which have a greater acid strength than carbonic acid, be added, in particular

Salts of formic acid, acetic acid, sulfuric acid and / or nitric acid, the preferred metal being the metallic or semi-metallic elements present in the superconductor material.

29. The method according to any one of claims 26 to 28, characterized in that an alkaline earth metal sulfate is formed which is homogeneously finely dispersed in the powder or in the shaped body.

30. Mixed oxide in the form of a powder or a powder pressed into a shaped body for a high-temperature superconductor with a carbon content <

200 ppmw, in particular <150 ppmw, particularly preferably <100 ppmw, and with a hydrogen content <150 ppmw, in particular <100 ppmw, particularly preferably <60 ppmw or <50 ppmw.

31. High-temperature superconductor with a carbon content <100 ppmw, in particular <80 ppmw, particularly preferably <50 ppmw, and with a hydrogen content <100 ppmw, in particular <60 ppmw, particularly preferably <40 ppmw.

32. Use of the oxidic produced according to one of claims 1 to 29

Powder or shaped body for the production of high-temperature superconducting wires, strips, rods, tubes, hollow or solid bodies, in particular for the production of power cables, power lines, transformers, windings, magnets, power supplies, electric motors, energy storage devices, current limiters or magnetic bearings. 33

33. Use of a high-temperature superconductor according to claim 31 as a wire, ribbon, rod, tube, hollow or solid body, in particular for the production of power cables, power lines, transformers, windings, magnets, power supplies, electric motors, energy storage devices, current limiters or magnetic bearings.