AU617357B2

AU617357B2 - Process for preparing superconducting materials and materials thereby obtained

Info

Publication number: AU617357B2
Application number: AU22233/88A
Authority: AU
Inventors: Marie-Odile Lafon; Claude Magnier
Original assignee: Rhone Poulenc Chimie SA
Current assignee: Rhodia Chimie SAS
Priority date: 1987-09-18
Filing date: 1988-09-15
Publication date: 1991-11-28
Anticipated expiration: 2008-09-15
Also published as: ZA886908B; NO884120L; IL87743A0; NO884120D0; CN1032261A; DK517988D0; FR2620866A1; JPH01122924A; AU2223388A; FI884270A0; PT88535A; BR8804803A; EP0308338A1; PT88535B; KR890005908A; FI884270A; FR2620866B1; DK517988A

Description

517 COMdMONWETALTH OF AUSTRALIA PATETACE 952 CMLT flSIFIAlQN NAME ADDRESS OF APPLICANT: RHONE-POULENC CHIMIE 25 Quai Paul Dourner 92408 Courbevoie France NAME(S) OF INVENTOR(S): Marie-Odile LAFON 9 Claude MAGNIER ADDRESS FOR SERVICE: 0; DAVIES COLLIISON Patent Attorneys I1 Little Collins Street, Melbourne, 3000, COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED: V Process for preparing superconducting materials and materials thereby obtained The following statement is a full description of this invention, including the best method of performing it known to me/us:la The present invention relates to the field of superconducting materials.

Superconductivity is known to be characterized, inter alia, by the disappearance of all electrical resistance in a body brought to very low temperature.

The superconducting state in materials appears only below a threshold temperature, known as the "critical temperature", which until recently was absolute zero. Such 9 a constraint obviously caused a serious holdup in the 10 large-scale development of all the potential practical *o applications that could derive frjm superconductivity.

Recent investigations have resulted in the production of new materials which possess superconducting properties at higher temperatures, i.e. temperatures ranging 15 from 70 to 90K, or even up to 100K.

There materials are, for the most part, based on rare earth metals, alkaline-earth metals, transition metals and oxygen. More specifically, the most promising systems studied appear to be those based on yttrium and/or lanthanum, on barium and/or strontium and/or calcium, on i i 2 copper, and/or nickel and/or cobalt and/or manganese, and on oxygen.

The method of synthesis generally described in the literature for obtaining these materials is based on a solid phase reaction, at high temperatures (1,000 0 between oxides and/or salts containing a volatile anion (for example carbonate) of the corresponding elements, the latter compounds taking the form of powders.

However, this process has the drawback of being a. 10 awkward to control, and leads to powders which are not sufficiently sinterable. For certain practical applications, only the use of superconducting sintered products is of genuine interest.

It is hence essential to provide dense and So. 15 homogeneous sintered materials which have preserved the intrinsic properties of superconductivity of the starting powders. It follows from this that homogeneous superconducting powders possessing good capacity for sintering are required.

20 The present invention seeks to provide such materials and to propose simple and effective means that are easy to carry out for reproducibly obtaining both homogeneous superconducting powders possessing an excellent capacity for sintering, and high-density superconducting sintered materials.

I i YI~C--*L- 3 The present invention provides a process for preparing a superconducting powder essentially based on at least one rare earth metal, at least one alkaline-earth metal, copper and oxygen, which comprises: a) mixing in the presence of alcohol an aqueous solution of a nitrate and/or an acetate of at least one rare earth metal, of at least one alkaline-earth metal and of copper, with an oxalic acid solution, ensuring that the pH of 10 the mixture is between 2 and 4; b) separating off the precipitate obtained; c) drying the precipitate; d) calcining the dried product; and e) optionally grinding the calcined powder, 15 In the description of the present invention which follows, rare earth metal is understood to include any element of the Periodic Table whose atomic number is between 57 and 71 inclusive, and also to include yttrium which, by convention, is classified here as a rare earth metal.

The especially preferred rare earth metals for carrying out the process according to the invention are yttrium and lanthanum.

Furthermore, the alkaline-earth metals used in the present process are preferably calcium, barium ylsi L C" 4 or strontium.

The choice of the elements within the above faTio lies will naturally be made in accordance with the nature and the composition of the superconducting powder which it is desired to obtain.

Similarly, the ratios between the various elements in the starting solution are adjusted in a conventional S ianner in accordance with the stoichiometry desired in the final product, the latter naturally having to cor- 10 respond to the production of superconducting properties.

The process according to the invention thus applies especially well to the manufacture of superconduct- S* ing systems of the Y-Ba-Cu-O type. These systems have been studied, in particular, in a paper which appeared in the Journal of the American Chemical Society, 1987, 109, 5 2528-2530.

According to the invention, the rare earth metal, alkaline-earth metal and copper are present in the initial aqueous solution in the form of acetates and/or nitrates.

However, it is preferable to work with a solution of acetates.

According to another important feature of the process according to the invention, the mixing between the solution of saLts (nitrates and/or acetates) and the oxalic acid solution is carried out in the presence of alcohol, the purpose of this being, in particular, to promote the subsequent precipitation operation.

i ~1 ;li 5 This alcohol may be introduced into the system either independently and separately, or by being mixed beforehand with at least one of the solutions of salts and oxalic acid. According to a particular embodiment of the process according to the invention, the solution of salts is first treated with alcohol, and the solution thus prepared is then mixed with the oxalic acid solution.

S The quantity of alcohol introduced is not critical; S in practice, approximately one volume of alcohol is added 10 per volume of aqueous salt soLution.

Any alcohol, provided it is substantially soluble in water, can be perfectly suitable for carrying out the present process. By way of example, short-chain aliphatic alcohols such as methanol, ethanol and propanol are very 15 suitable.

The quantity of oxalic acid introduced must be sufficient to cause a final pH of between 2 and 4 in the mixture. Thus, in the case of a system Y-Ba-Cu-0, this pH will preferably lie in the region of Naturally, the final pH of the mixture could, if appropriate, be adjusted by means of an addition of a supplementary acid such as nitric or acetic acid, or a soplementary base such as ammonia solution.

On completion of this mixing, a precipitate then appears, consisting essentially of a mixture of rare earth metal, alkaline-earth metal and copper oxalates.

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The precipitate obtained is then separated off by any means known per se, in particular by filtration.

It may be preferable, before carrying out the drying, to perform a washing of the precipitate, in particular with water, alcohol or a water/alcohol mixture.

The washing is preferably carried out using alcohol, such as ethanol.

The product is then dried, I This drying may be carried out by any known method, S 10 in particular by drying in an oven, or by atomization, S. that is to say by spraying the suspension into a hot atmosphere. The drying is preferably carried out in a conventional oven, and at a temperature of the order of 100 0

C.

The dried product is then calcined.

The calcination is performed at a temperature of between 850 0 C and 1,000 0 C, and preferably between 900 0 C and 950 0 C. The calcination time can vary between 30 min. and 24 hours, for example, and preferably between 5 and 15 hours.

This calcination is performed under an atmosphere of air or any air/oxygen mixture, jut preferably under air.

Furthermore, according to a particular embodiment of the invention, the calcined products are cooled very rapidly to room temperature, an operation equivalent to a quenching, for example by means of flushing with cold air. The cooling time is generally of the order of a I i r 7 few seconds.

After this calcination, a superconducting powder possessing a macroscopic particle size of 1 to 10 p is obtained, the 1 to 10 p particles consisting of elemeno 0 tary crystallites between 100 A and 500 A approximately in size.

The powders obtained should generally be ground, i preferably in the dry state, so that good sintering can j be obtained. The average particle size distribution of 10 the powders then Lies between approximately 0.5 and 2 pm, and preferably between 1 and 2 pm.

These superconducting powders possess the exceptional property of being able to give, after sintering, excellently homogeneous superconducting sintered materialz 15 whose density is greater than 95% of the theoretical density of the material in question.

The temperature of sintering the powders is between 900 0 C and 1,000 0 C, and the sintering time gene- |j rally between 2 and 10 hours. This sintering is preferj 20 ably carried out under an oxygen atmosphere.

More specifically, the characteristics of the superconducting powders according to the invention, in respect of their capacity for sintering, are determined in the following manner: the powder is first pelletized with or without a binder, under a uniaxial pressure of 1.5 T/cm 2 and then sintered for 2 hours at a temperature of 950 0 C and cooled I I _I 8-

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in the course of 7 hours to room temperature; the density of the product obtained is then measured. In all cases, a final density of greater than 95% of the theoretical density of the material in question is observed.

Furthermore, it is observed that the sintered product obtained possesses very good superconducting properties.

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*5 15 0O S Other advantages and aspects of the invention will become apparent on reading the example which follows.

Example This example illustrates the invention in the context of the preparation of a superconducting material of formula Y-Ba2-Cu3-Ox (6.5 x 7).

A Preparation of the powder 30 g of yttrium acetate, 43.5 g of barium acetate and 53.1 g of copper acetate are dissolved in 2.25 L of water.

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2.25 L of ethanol are added to this solution.

34.88 ml/min. of this solution are introduced in continuous fashion into a reactor with 9.3 ml/min. of a M oxalic acid solution, using pumps.

A precipitate forms, which is subsequently filtered off, then washed with alcohol (ethanol) and then dried in an oven at 100 0

C.

The powder thereby obtained is calcined at 900 0

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for 10-15 hours under air.

The product is then brought back very rapidly to a r ~I 9

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S.r .9 0 4 9 .9 9 9 15 room temperature.

A grinding is then performed in the dry state until a powder is obtained whose mean particle size is less than 2 pm.

The characteristics of the powder obtained are then as follows: specific surface area (BET) 2 m2/g total pore volume 0.32 cm 3 /g including an actual volume 0.25 cm 3 /g and an intergranular volume 0.07 cm 3 /g porosity range 0.02-5 pm mean pore diameter 0.9 pm mean particle size 1.7 pm mean size of crystallites approx.

0 300 A S. V D of a S.r *r S *9I 0 B Sintering of the powder The powder is pelletized by uniaxial pressing in the dry state under a pressure of 1.5 T/cm 2 then sintered at 9500C for 2 hours under oxygen and cooled in the course of 7 hours to room temperature.

A material possessing a density equal to 97% of its theoretical density is then obtained.

This material is superconducting; its "onset" temperature, at which transition begins, is 92K, and it corresponds to the average formula Y-Ba 2 -Cu 3 -0 6 9

Claims

1. Process for preparing a superconducting powder essentially based on at least one rare earth metal, at least one alkaline-earth metal, copper and oxygen, which comprises; a) mixing in the presence of alcohol an aqueous solution of a nitrate and/or an acetate of at least one rare earth metal, of at least one alkaline-earth metal and of copper, with an oxalic acid solution, ensuring that the pH or the mixture is between 2 and 4; b) separating off the precipitate obtained; c) drying the precipitate; d) calcining the dried product; and e) optionally grinding the calcined powder. 15 .2 Process according to claim 1 in which a solution of acetates is used.

3. Process according to any one of the preceding claims in which the alcohol is an aliphatic alcohol which is substantially soluble in water.

4. Process according to claim 3 in which the alcohol is methanol, ethanol or propanol. i i--m~ :0 11 Process acco-ding to claim 3 or 4 in which one volume of alcohol is used per volume of aqueous solution.

6. Process according to any one of the preceding claims in which the precipitate obtained is separ Po off by filtration.

7. Process according to any one of the preceding claims in which, before drying, the precipitate obtained is w. ashed using water, alcohol or a water/alcohol mixture, I C 8. Process according to claim 7 in which the I 10 washing substance i alcohol. e S. 9. Process according to any one of the preceding S. claims in which the calcination is performed at between 850°C and 1,000 0 C, 0 10. Process according to claim 9 in which the 15 calcination is performed at between 900 C and 950'C, t I1. Process according to any one of the preceding claims in which the calcination is performed under air.

12. Process according to any one of the preceding claims in which the calcined products are cooled very S 20 rapidly to room temperature.

13. Process according to any one of the preceding Sclaims in which the grinding is performed in the dry state,

14. Process according to claim 13 in which after grinding the calcined product has a mean particle size of less than 2 microns. -12- Process according to any one of the preceding claims In which the rare earth metal is yttrium or lanthanum.

16. Proces* according to any one of the preceding claims in which the alkaline-earth metal is calcium, barium or strontium.

17. Process according to any one of the preceding claims in which an aqueous solution comprising yttrium, barium and copper acetates is used. 10 18, Process according to claim 1 for preparing a superconducting powder substantially as described in the foregoing Examples.

19. superconducting ceramic powder essentially based on a rare earth metal, at least one Alkaline-earth K 15 metal, copper and oxygen, when obtained by carrying out a process as defined in any one of claims I. to 18, Supercondiucting ceramic powder according to :claim 19 oapable of being sintered to a density of at least of its theoretical density.

21. Super',*nduictinWT sintered ceramic material essentially based ona a rare earth metal, at least one J alkaline-earth metal, copper and oxygen, obtained by sintering a powder according to claim 19 or Dated this 18th day of September, 1991 RHONE-POULENC CHIMIE By its Patent Attorneys DAVIES COLLISON Mt&