FR2665896A1 - Metal carboxylates, their preparation and superconducting compositions obtained from them - Google Patents

Abstract

The invention relates to metal carboxylates in which the heterogeneity domains are less than 1 mu m. It likewise relates to superconducting compositions obtained by calcinations of the said carboxylates and in which the heterogeneity domains are less than 1 mu m. Finally, the invention relates to a process for the preparation of the abovementioned metal carboxylates consisting in preparing an aqueous solution of metal salts which is emulsified in an organic solvent, in then reacting the emulsion with a precipitating agent and finally in separating the precipitate thus obtained.

Description

METAL CARBOXYLATES, THEIR PREPARATION AND COMPOSITIONS
SUPERCONDUCTORS OBTAINED FROM THOSE
The present invention relates to metal carboxylates, their preparation and their application to the field of superconductivity.

 The phenomenon of superconductivity corresponds to an absence of electrical resistance of a material when this one is brought to a very low temperature, called critical temperature.

 Recently, superconductive materials with critical temperatures of the order of 80K, even 100K, have been developed.

 However, such results are all the more conceivable as the materials are the most homogeneous in composition.

 In addition, these same materials must be very dense for some of their uses. In other words, the powders from which they come must have a very fine and tight particle size.

 However, the methods known up to now do not make it possible to obtain materials having all of these characteristics.

 We know for example the process consisting in reacting in solid phase at high temperature the oxides and / or the salts containing a volatile anion, carbonate type in particular, of the elements constituting the superconductive product. This method has the disadvantage of being a delicate control and of not giving dense superconductive compositions, free of parasitic phases.

 A process is also known for preparing a superconductive phase by coprecipitation of the salts of the constituent elements of said phase and then by calcination of the precipitate obtained.

 In addition to the problems of homogeneity generally resulting from an uncoordinated precipitation of the salts, it is often necessary on the one hand to carry out several stages of calcination separate from a grinding in order to rehomogenize the powder. On the other hand, a thorough grinding prior to sintering is carried out to adjust the particle size, thereby increasing the risk of pollution of the final phase.

 The present invention therefore relates to the preparation of monophasic superconductive compositions which are perfectly homogeneous in composition, obtained from metal carboxylates having the same characteristics and with a process which does not require multiple calcinations, nor of thorough grinding before sintering.

 For this purpose, the metal carboxylate of the invention is characterized in that it exhibits chemical homogeneity measured by energy dispersion spectroscopy (EDS), such that the domains of heterogeneity are less than or equal to 1ym.

 In addition, the invention relates to a superconductive composition characterized in that it is obtained by calcination of a metal carboxylate of the aforementioned type.

 The invention also relates to a superconductive composition of the type comprising at least one rare earth, at least one transition element and at least one alkaline earth, and characterized in that it exhibits chemical homogeneity measured by EDS analysis, such as the domains of heterogeneity are less than or equal to lum.

 Finally, the process for the preparation of a metal carboxylate according to the invention is characterized in that it comprises the following stages - an aqueous solution of metal salts is prepared and then this solution is emulsified in an organic solvent, - one makes reacting the emulsion with a precipitating agent, - the precipitate obtained is separated.

 However, other advantages and characteristics will appear on reading the description and the examples which follow.

 The essential characteristic of the metal carboxylate according to the invention resides in its chemical homogeneity (or homogeneity of composition) such that the domains of heterogeneity of said metal carboxylate are less than or equal to 1 #. This homogeneity is measured by EDS mapping using a transmission electron microscopy (TEM) microprobe.

 According to an interesting variant of the metal carboxylate of the invention, it is in the form of a powder whose average particle size is at most 1Fun or submicron, measured by transmission electron microscopy.

 More particularly, the average particle size, measured by transmission electron microscopy, is between 0.1 and 1 µm and preferably between 0.2 and 0.6 ion.

 The metal carboxylate can also be in the form of a powder, the particle size distribution of which is monodisperse.

 The monodisperse nature of the powder can be assessed by scanning microscopy.

 Concerning the metallic part of the carboxylate according to the invention, this can be made up indifferently of one or more metallic ions, knowing that all the species can enter into the composition of the carboxylate.

 However, according to a first advantageous variant of the invention, the carboxylate comprises at least one rare earth, at least one transition element and optionally at least one alkaline earth.

 By rare earth is meant the lanthanides whose atomic number varies from 57 to 71 and also comprising yttrium.

 However, lanthanum, neodymium, cerium and yttrium are particularly suitable.

 As a transition element, mention may in particular be made of iron, cobalt, nickel, manganese, copper.

 Under the term alkaline earth, we mean among other things barium, strontium, calcium.

 However, according to a second preferred variant of the invention, the metal carboxylate comprises at least one rare earth, at least one transition element and at least one alkaline earth.

 According to a third variant of the invention, the metal carboxylate comprises at least one rare earth, at least one transition element, at least one alkaline earth and at least one element of class 4A.

 The carboxylate, according to a last advantageous variant, can comprise at least one transition element, at least one alkaline earth metal, at least one element from classes 3A, 4A, 5A.

It will be noted here and for the whole description that the classes of the elements cited correspond to the periodic classification of the elements published in the work "Advanced Inorganic
Chemistry "4th edition, FA.COTTON and G. WILKINSON (John Willey and
Sounds).

 According to particular embodiments of this latter variant, thallium enters the composition of the carboxylate as an element of class 3A.

 Usually, lead and tin are chosen from elements of class 4A to enter into the composition of the carboxylate according to the invention.

 Concerning the elements of class 5A, we generally use bismuth and antimony.

 The organic part of the metal carboxylate comes from carboxylic, hydroxycarboxylic, dicarboxylic, saturated, unsaturated or aromatic acids, so that the metal salts of these acids give precipitates which are insoluble in water or in a mixture of water and organic solvent.

 According to a particular embodiment of the invention, the abovementioned acids have at most 8 carbon atoms.

 Thus, mention may be made, among saturated dicarboxylic acids, of oxalic, succinic and malonic acids in particular.

 Among the saturated hydroxycarboxylic acids, tartaric acid is used, for example.

 As regards the unsaturated dicarboxylic acids, mention may be made of maleic and fumaric acids.

 Finally, as aromatic acids, the phthalic acid derivatives are suitable for implementing the invention.

 As mentioned above, one of the applications of the metal carboxylate according to the invention relates to the field of superconductivity.

 Thus, it is possible to prepare, by calcination of the abovementioned carboxylate, superconductive compositions of the type comprising at least one rare earth, at least one transition element and optionally at least one alkaline earth.

As an example, we can cite the phases of the following formulas
La2 # xAxCuO4 where A represents strontium or barium
and O sx <2
(NdCeSr) 2CuO4
According to a second preferred variant, the superconductive compositions correspond to the following formula
LnBa2Cu307 where Ln represents yttrium or lanthanum.

 It is thus possible to prepare superconductive phases of the YBa2Cu4O8 type.

 According to a third interesting variant, the superconductive compositions comprise at least one rare earth, at least one alkaline earth, at least one transition element and possibly at least one element of class 4A.

 By way of example, mention may be made of the phases of the Pb2Sr2NdCu3O8 type.

 The process according to the invention is also suitable for the preparation of superconductive compositions of the type comprising at least one transition element, at least one alkaline earth metal, at least one element of classes 3A, 4A, 5A.

The compositions according to this last variant can correspond to the following formula: TlmBa2Can # 1CunOm + 2n + 2 in which. thallium can be substituted with bismuth
partially or totally,
barium can be substituted by strontium,
partially or totally,
. m = 1 or 2
For example, the invention makes it possible to obtain the following phases
TlBa2cacu207
T1Ba2Ca2Cu309
TlBa2CuO6
Tl2Ba2CuO6
T12BaZCa2Cu3010
We can also cite the phases of the following formula, always within the framework of this same last variant:
Bix My Srz Cat Cuv Ow in which M can be lead, antimony or tin where
1.5 sx + ys 2.5 and y can be zero
2 svs 4
3 sz + ts5
Similarly, we can cite the phases of the type
Ca1 # x Sbx Co 3-6
Srl-x Sbx Co 3 ~ ô
Bal-x Bix Co 03-6 where O <xs 0.6 and O <ô s 0.3
The calcination of the metal carboxylate of the aforementioned type is carried out according to any usual method. More particularly, the above-mentioned carboxylate is calcined in air and / or under oxygen scavenging at a temperature varying between 600 and 9500C and preferably between 700 and 9000C.

 The calcination time is between 2 and 10 hours and preferably between 2 and 6 hours.

 It should be noted on the one hand that a single calcination of a shorter duration compared to known methods is sufficient to obtain a pure product and on the other hand that it is not necessary to carry out a complex calcination, that is to say comprising an intermediate grinding in order to obtain a pure and homogeneous phase.

 The invention also relates to the superconductive compositions of the aforementioned types as such.

 The superconductive compositions according to the invention are pure, this purity being determined by X-ray diffraction.

According to a first variant of the invention, said compositions are characterized in that they have a homogeneity of composition, measured by EDS analysis (microprobe
MET) such that the domains of heterogeneity are less than or equal to 1pm. More particularly, said domains are less than or equal to 0.5Fun.

According to a second variant of the invention, the analysis
Composition ESCA (Electron Spectroscopy for Chemical
Analysis, based on the photoelectric effect) indicates a surface stoichiometry relatively close to the theoretical stoichiometry; the differences being of the order of 10% compared to the theoretical stoichiometry.

 According to another interesting variant, the superconductive phases can be in the form of a powder, the average particle size of which is at most 1 μm or submicron and preferably between 0.2 and 0.6 Fun.

 In addition, the particle size distribution is monodisperse.

 The monodisperse nature of the powder is determined by measuring the dispersion index ID according to the CILAS method for example.

The dispersion index corresponds to the following formula
ID = d8p-di6
2d50 in which .dg4 represents the diameter for which 84% of
particles have a size smaller than the
value of this diameter
.d16 represents the diameter for which 16% of
particles are smaller than this
diameter
.dso represents the diameter for which 50% of
particles are smaller than this
diameter.

 Thus, according to a particular embodiment of the invention, the dispersion index of the above-mentioned superconductive phases is less than or equal to 0.7 and preferably less than or equal to 0.5.

 Another characteristic of the superconductive phases according to the invention is that they are in the form of fine particles which can be easily disaggregated.

 This advantage is particularly advantageous compared to known superconductive compositions.

 In fact, after calcination, these are generally in the form of elementary particles the size of which is 5 to 7 uni agglomerated or not.

 Thus, unlike the phases according to the invention, the grinding necessary to adjust the particle size before sintering is longer and difficult since it is not only necessary to dissociate agglomerates but to reduce the very size of the particles.

 In general, for the products of the invention, one can be satisfied with a disaggregation obtained by a grinder of the ball mill type or of the air jet type.

 On the other hand, it was surprisingly noticed that the particles were not agglomerated after calcination if the solvent used, during the preparation of the carboxylate of the invention, is of the paraffinic type, and more particularly isoparaffinic such as ISOPAR Q in particular.

 In this case, the particles obtained directly after calcination have the characteristics stated above, namely monodispersity, fine particle size and non-agglomeration of the particles.

 Thus, the grinding step prior to sintering is no longer necessary, but obviously it would not be departing from the scope of the present invention to carry out such an operation.

 The process for preparing the league metal carboxylate according to the invention will now be described.

 This essentially comprises the following stages - an aqueous solution of the metal salts is prepared, then this solution is put in an organic solvent, - the emulsion is reacted with a precipitating agent, - the precipitate obtained is separated.

 The process according to the invention can be used for the preparation of any type of carboxylate, whether it consists of one or more metallic species.

 Everything that has been said above about the nature of metal ions remains of course valid.

 The aqueous solution of the metal salts of the first stage of the process comprises salts of inorganic acids or of organic acids.

 All the inorganic or organic acids are suitable for carrying out the process insofar as they form salts soluble in water with the metal ions described above.

 However, according to a particular embodiment, the salts of inorganic acids are chosen from chlorides and nitrates.

 Concerning the salts of organic acids and according to a second particular embodiment of the invention, these are chosen from the salts of saturated aliphatic carboxylic acids or from the salts of hydroxycarboxylic acids.

 Thus, as saturated aliphatic carboxylic acids, mention may be made of formates, acetates, propionates.

 As for hydroxycarboxylic acids, citrates are usually used.

 The concentrations of the aqueous solution of the various metal salts are adjusted according to the final composition of the carboxylate and are generally between 0.1 and 5 M.

 The above-mentioned aqueous solution is then emulsified in an organic solvent.

 The continuous phase of the emulsion, that is to say the organic solvent, is chosen according to its density and its degree of miscibility in water.

 In general, in order to obtain a stable emulsion long enough to allow the compounds to react, the organic solvent chosen must be little or immiscible with water and must have a density close to that of the aqueous solution of the salts.

 Consequently, all organic solvents having such characteristics are suitable for carrying out the process according to the invention.

 However, according to a particular embodiment of the invention, the above-mentioned aqueous solution is emulsified in a solvent of the linear, cyclic or branched alkane type having at least 5 carbon atoms.

 In particular, there may be mentioned hexane, heptane, cyclohexane but also solvents of the VARSOL @ type, paraffinic and in particular isoparaffinic solvents of the ISOPAR type, as well as their mixtures.

According to another particular mode, the emulsion is prepared in an aromatic solvent, in particular of the benzene, naphthenic, anthracene type. We can use among others benzene, toluene, xylene, SOLVESSO @ corresponding to the petroleum cut from C10 to C13 or even the solvent, AROMATIC
ACTREL 400 @ said solvents can be used pure or as a mixture.

 According to a third particular mode, the alcohols having at least 6 carbon atoms are used as the continuous phase of the emulsion. By way of example, mention may be made of hexanol, cyclohexanol, ethyl-2-hexanol, benzyl alcohol and their mixtures.

 According to a fourth mode, the abovementioned aqueous solution is emulsified with a linear, branched or cyclic ketone comprising at least 6 carbon atoms.

 Mention may in particular be made of methyl-2-pentanone-4, heptanone-2, cyclohexanone.

 According to another particular mode, the hydroxycarboxylic acid esters are used as the continuous phase of the emulsion.

 Thus, propylene carbonate and ethylene carbonate are suitable for the implementation of the invention.

 Finally, according to a last embodiment of the invention, the emulsion is prepared using nitriles.

 Mention may in particular be made of acetonitrile, n-butyronitrile, lactonitrile.

 In general, when the aqueous solution is emulsified, the volume ratio of the aqueous solution to the organic solvent varies between 5 and 50%. Preferably, this is between 10 and 30%.

 According to a preferred embodiment of the method, the aqueous solution of the abovementioned salts is emulsified in the presence of a surfactant.

 The choice of this is based on several criteria relating to the emulsion obtained. First of all, this must be stable at least during the period of introduction of the reagents which is usually between a few minutes and an hour.

Then, the droplets of the emulsion must be submicron and preferably monodisperse, such characteristics can be observed by means of a microscope
ZEISS IM 35, equipped with Nomarski interference contrast and whose resolution limit is 0.15un.

 Generally, nonionic surfactants are used, but the anionic, cationic or amphoteric surfactants are suitable for carrying out the process insofar as they satisfy the above conditions.

 In addition, these surfactants can be used alone or as a mixture and be present in the organic phase or in the organic and aqueous phases.

As anionic surfactants, use is made, inter alia, of alkyl sulfates, alkyl sulfonates, sulfosuccinates (AEROSOL OT
Regarding cationic surfactants, the emulsion can be prepared with alkylbenzene sulfonic acids (SULFAMIN 1298
As amphoteric surfactants, mention may be made of alkyltrimethylammonium and alkylpyridinium salts, imidazolinium derivatives, alkylbetaines, amidopropylbétaines.

As non-ionic surfactants, sorbitan ester derivatives of the type can be used pure or as a mixture
ARLACEL @ 581-582, ARLATONE T @ but also of the SPAN type derivatives of polyethoxylated sorbitan esters of the TWEEN type, polyethoxylated alkylphenols, polyethoxylated fatty alcohols, polyethoxylated triglycerides, polyethoxylated fatty amines, these polyethoxylated derivatives CEMULSOLS
According to an interesting variant of the present invention, a biogum is used in combination with the active surfactant.

 Biogums are polysaccharides of high molecular weight, generally greater than a million, obtained by fermentation of a carbohydrate under the action of a microorganism.

As biogums which can be used in the preparation of the emulsion, mention may be made of those obtained by fermentation under the action of bacteria or fungi belonging to the genus Xanthomonas such as Xanthomonas beqoniae,
Xanthomonas campestris, Xanthomonas carotae, Xanthomonas hederae,
Xanthomonas incanae, Xanthomonas malvacearum,
Xanthomonas oaoavericola, Xanthomonas Phaseoli, Xanthomonas Disi, anthomonas vasculorum, Xanthomonas vesicatoria, Xanthomonas vitians, Xanthomonas Pelaraonii; to the genus Arthrobacter and more particularly the stabilizing Arthrobacter species, Arthrobacter viscosus; to the genus Erwinia; to the genus Azotobacter and more particularly the species Azotobacter indicus; in the genus Aarobacter and more particularly the species Asrobacterium radiobacter, Agrobacterium rhizoqènes, Asrobacter tumefaciens; in the genus Alcalictènes and more particularly the species Alcalictènes faecalis; in the genus Rhizobium; in the genus Sclerotium and more particularly the species Sclerotium rolfsii and Sclerotium ctlucanicum; to the genus Corticium; the genus Sclerotinia; like
Stromatinia.

 As biogum which can be obtained by fermentation by the microorganisms described above, use is particularly made of the gum obtained by fermentation of bacteria of the genus Xanthomonas and in particular xanthan gum.

 Of course, it would not be departing from the scope of the present invention to use a mixture of biogums as stabilizer.

Preferably, the biogum is used in combination with a surfactant of the SPAN @ type and in particular SPAN 80
The biogum is used in sparingly concentrated solutions, preferably in an aqueous solution of 0.01 to 0.1%.

 The advantage of using a biogum with a suitably chosen surfactant is to allow the recovery of metal carboxylate without having to destabilize the emulsion by a means such as the addition of a compound or by heating.

 According to a first preferred embodiment of the invention, block or graft copolymers are used as surfactant.

These correspond to the following general formula
(A-COO) m - B in which m is an integer greater than or equal to 2.

Polymer A has a molecular weight of at least 500 and is the residue of a complex, fat-soluble monocarboxylic acid of the following general formula
RCO tO-HCR1- (R2) n-CO] pO-HCR1- (R2) n-COOH (I) in which
R is a hydrogen atom or a monovalent hydrocarbon group or a substituted, linear or branched hydrocarbon group, preferably comprising up to 25 carbon atoms.

For example, R can be a group derived from stearic acid such as the straight chain C17H35.

 R1 is a hydrogen atom or a linear or branched C1-C24 monovalent hydrocarbon group. Preferably, R1 is a straight chain alkyl group.

 R2 is a linear or branched divalent C1-C24 hydrocarbon group. Preferably R2 is a straight chain alkylene group.

 n is O or 1.

 p is an integer varying from 0 to 20.

 The units -O-HCR1- (R2) n-CO can be identical or not.

 The monocarboxylic acid from which A derives has the structure of the product of the trans esterification of one or more monohydroxy-monocarboxylic acids with a monocarboxylic acid devoid of hydroxyl groups, the latter acid acting as a chain-terminating agent.

 Polymer B has a molecular weight of at least 500.

In the case where m = 2, said polymer is the divalent residue of a water-soluble polyalkylene glycol, of the following general formula
H IO-HCR3-CH2] qO-HCR3-CH20H (II) in which
R3 is a hydrogen atom or an alkyl group
C1-C3.

 q is an integer varying from 10 to 500.

 The polyalkylene from which B is derived in this first case, can in particular be polyethylene glycol.

In the case where m is greater than 2, the polymer B is the residue of valence m of a water-soluble polyether glycol, of the following general formula
R4 f - (O-HCR3-CH2-] r-OH3m (III) in which
R3 and m have the same meaning as before.

 r is O or is an integer varying from 1 to 500, knowing that the total number of groups: -O-HCR3-CH2- is at least 10.

 R4 is the residue of an organic compound comprising m hydrogen atoms reactive with an alkylene oxide.

 The units -O-HCR3-CH2- of formulas (II) and (III) may or may not be identical in each polymer.

 The polyether polyol from which B is derived in the latter case is the product of the condensation of an alkylene oxide such as ethylene oxide and / or propylene oxide with a compound having several active hydrogen atoms. The latter may be glycerol, or else the internal anhydride of a polyhydroxylated compound such as sorbitan, or else an amine such as in particular ethylene diamine, or else an amide, or alternatively a polycarboxylic acid.

 Such surfactants are described in particular in European patent No. 424 in the name of the Company I C I.

 In addition, some of these surfactants are sold under the brand names B 261 and B 246.

 According to a second preferred mode, surfactants are used with molecules resulting from the condensation of a poly (lower alkylene) imine on a polyester having a free acid group; said molecules having at least two polyester chains attached to a poly (lower alkylene) imine chain.

 The condensate is present either in the predominant form of an amide, or in the predominant form of a salt, depending on the operating conditions.

 As regards the poly (lower alkylene) imine group, the term lower alkylene is understood to mean a preferably branched chain containing 2 to 4 carbon atoms. Indeed, according to a preferred embodiment, at least 20% of the nitrogen atoms are in the form of tertiary amine. Mention may in particular be made, as chain of this type, of the branched formulas of polyethylene imines.

 The molecular weight of the poly (lower alkylene) imine group is generally greater than 500, preferably greater than 5000 and more especially between 104 and 105.

Regarding the polyester group, this comes, according to an interesting variant, from a hydroxycarboxylic acid of general formula
HO-X-COOH
On the one hand, X represents a divalent radical, saturated or unsaturated, containing at least 8 carbon atoms, and preferably between 12 and 20 carbon atoms.

 On the other hand, at least 4 carbon atoms separate the hydroxyl and carboxylic functions, preferably between 8 and 14 carbon atoms.

 By way of example, there may be mentioned in particular ricinoleic acid, hydroxystearic acid or fatty acid originating from hydrogenated beaver oil.

 The polyester group can also, according to another variant, come from the mixture of a hydroxycarboxylic acid of the aforementioned type with a carboxylic acid, saturated or not, not comprising a hydroxyl function. Thus, particularly suitable are carboxylic acids, on the one hand comprising an alkyl or alkenyl group, and on the other hand comprising 8 to 10 carbon atoms. Mention may in particular be made of lauric, palmitic, stearic and oleic acids.

 Regarding the molecular weight of polyester, it is between 560 and 5600.

 Such surfactants are described in particular in US Patent No. 4,224,212 in the name of the Company I C I.

In addition, some of these surfactants are marketed under the brand HYPERMER @ LP4, LP5, LP6, LP7 and
LP8.

 In general, the quantity of surfactants required during the emulsification of the aqueous solution of the abovementioned salts is between 0.1 and 3% by weight relative to the organic solvent.

 The emulsion is prepared by any means known to those skilled in the art and in particular, electroemulsification, ultrasound, the action of colloid mills of the ULTRATURRAX type, the action of an electric field, the use can be used. MENTON-GAULIN, CAVITRON, SUPRATON homogenizers, or the use of static mixers.

 The preparation of the emulsion is made between 0 and 1000C, the upper limit being imposed by the destabilization of the emulsion and / or the aqueous stability of the salts of the above-mentioned aqueous solution.

 However, the emulsification is preferably carried out at ambient temperature or in the vicinity thereof, for example between 20 and 35 C.

 The second step of the process according to the invention consists in reacting the emulsion with a precipitating agent.

As such, saturated, unsaturated or aromatic carboxylic, hydroxycarboxylic dicarboxylic acids can be used in so far as such agents give, by reacting with the metal salts of the emulsion, a precipitate insoluble in water or in a water mixture - organic solvent
According to a particular embodiment of the invention, the abovementioned acids have at most 8 carbon atoms.

 Thus, mention may be made, among saturated dicarboxylic acids, of oxalic, succinic and malonic acids in particular.

 Among the saturated hydroxycarboxylic acids, tartaric acid is used, for example.

 As regards the unsaturated dicarboxylic acids, mention may be made of maleic and fumaric acids.

 Finally, as aromatic acids, the phthalic acid derivatives are suitable for implementing the invention.

 The normality of the solution of the precipitating agent is calculated as a function of the normality of the aqueous solution of the metal salts.

 Generally, the concentration of precipitating agent is between 0.1 and 5M.

 The molar ratio of the precipitating agent to the metal salts usually varies from I to 10.

 The emulsion and the precipitating agent can be brought into contact directly, but according to an advantageous variant of the process, the agent is reacted in solution in a second organic solvent.

 Any organic solvent which may or may not be miscible in the solvent of the emulsion can be used insofar as it dissolves the precipitating agent.

 According to a first variant of the invention, the precipitating agent is dissolved in a second organic solvent which is the organic solvent for the emulsion.

 Therefore, all that has been said above about the nature of this solvent remains valid.

 According to a second variant and in cases where the precipitating agent is not soluble in the organic solvent of the emulsion, it is then possible to dissolve said agent in a second solvent which may or may not be miscible with the organic solvent of the emulsion.

 Saturated, aromatic or polyhydric alcohols are suitable in this case for the implementation of the invention.

 However, alcohols generally comprising at most 8 carbon atoms are generally used.

 As regards saturated alcohols, mention may in particular be made of ethanol, pentanol, 2-methyl butanol and their mixtures.

 In the case of aromatic alcohols, benzyl alcohol is used, among others.

 Finally, among the polyhydroxylated alcohols, ethylene glycol, diethylene glycol and its derivatives, including diethylene glycol monobutyl ether or diethylene glycol monobutyl ether acetate, can be named for example.

 Other types of solvents can be envisaged, for example the esters of hydroxycarboxylic acids.

 Thus, for example, ethylene carbonate or propylene carbonate are suitable for implementing the invention.

 The emulsion is brought into contact with the precipitating agent, pure or diluted, either by introducing the emulsion into the precipitating agent or vice versa.

 The case of the simultaneous introduction of the reagents is likewise possible.

 However, this is usually done by introducing the emulsion into the precipitating agent, pure or diluted.

 The two reactants are preferably brought into contact, that is to say the emulsion and the precipitating agent drop by drop.

 During the introduction, it is preferable to keep the medium stirred.

 The reaction is carried out at a temperature between 0 and 100 ° C., the upper limit always being the destabilization of the emulsion and / or the stability of the salts of the above-mentioned aqueous solution.

 Preferably, one works at a temperature between 20 and 350C.

 After the addition of the reagents, the stirring is generally maintained for a period of 1 to 3 hours.

 Optionally, the reaction medium can then be aged, that is to say leave it to stand for a period of between 6 and 15 hours.

 The separation of the precipitate resulting from the second step can be carried out in two stages.

 The first, optional, consists in destabilizing the emulsion either by heating or cooling, or by the introduction of a destabilizer, such as ethanol for example.

 The second consists in separating the precipitate by any known means, including filtration or centrifugation.

 Subsequently, the precipitate can optionally be washed, for example with the solvent in which the precipitating agent is dissolved.

 Finally, according to a variant of the invention, the precipitate thus obtained is dried by any known means. It is thus possible to carry out the drying under vacuum at 500C, or under air at room temperature.

 The drying time is between 3 and 12 hours.

 Concrete examples will now be presented.

 Previously, the agglomeration measurement protocol by the CILAS method will be described.

 400 mg of powder are taken which is dispersed in 40 ml of absolute ethanol. The mixture is then ultra-sonicated for 15 minutes (sonicator: "SONICS MATERIALS").

 The dispersion is then transferred to the measuring tank (granulometer HR 850 B) also containing absolute ethanol.

 The measurement is then carried out with ultrasound in said tank (measurement range n0 1).

EXAMPLE 1
2.67 g of HYPERMER LP8 are dissolved in 267 g of SOLVESSO 200.

An aqueous solution is prepared comprising
- 0.081 mole / l of yttrium acetate
- 0.162 mole / l of barium acetate
- 0.243 mole / l of copper acetate.

 150 ml of the aqueous solution and the organic solution containing the surfactant are emulsified with an apparatus of the ULTRATURRAX T 50 type (colloid mills) and then by sonication.

 14.18 g of oxalic acid (C2H204.2H20) are dissolved in 300 ml of ethanol 95 and the solution is then transferred to a reactor controlled at 30 C.

 The emulsion is introduced into the oxalic solution with a flow rate of 12 ml / min with stirring.

 Stirring is continued one hour after the introduction of the reactants and then the reaction medium is allowed to age for 12 hours.

 300 ml of ethanol are added to the reaction medium and the metal carboxylate thus obtained is separated by centrifugation.

This is then washed several times with ethanol before being dried for 12 hours at room temperature.

 The metal carboxylate is calcined at 8500C for 5 hours.

PRODUCT ANALYSIS
Metallic Carboxvlate
The domains of heterogeneity are less than 1ym (EDS).

 The elementary particles are monodisperse and have an average diameter less than Monday (SEM).

Superconductive composition
X-ray diffraction analysis indicates a pure phase
orthorhombic YBa2Cu307.

 The domains of heterogeneity are less than 0.5 dna (EDS).

 The elementary particles have an average size of less than 0.5 μm.

EXAMPLE 2
2.16 g of HYPERMER LP8 are dissolved in 267 g of hexanol 1.

An aqueous solution is prepared comprising
- 0.081 mole / l of yttrium acetate
- 0.162 mole / l of barium acetate
- 0.243 mole / l of copper acetate.

 150 ml of this aqueous solution and the organic solution containing the surfactant are emulsified with an apparatus of the ULTRATURRAX T 50 type (colloid mills) at maximum power for 2 minutes and then by ultrasound for 6 minutes.

 10.023 g of oxalic acid (C2H204.2H2O) are dissolved in 300 ml of hexanol 1, then the solution is transferred to a reactor controlled at 300C.

 The emulsion is introduced into the oxalic solution with a flow rate of 10 ml / min with stirring.

 Stirring is continued 3 hours after the introduction of the reagents.

 300 ml of ethanol are added in the reaction medium and then the metal carboxylate thus obtained is separated by centrifugation.

This is then washed several times with absolute ethanol before being dried under vacuum for 12 hours.

 The metal carboxylate is calcined at 8500C for 5 hours.

PRODUCT ANALYSIS
Metallic Carboxvlate
The areas of heterogeneity are less than Monday (EDS).

 The particles are monodisperse and the average size is less than 0.7pu (SEM).

Superconductive composition
X-ray diffraction analysis indicates a pure orthorhombic phase YBa2Cu307.

 The areas of heterogeneity are less than 0.5pu (EDS).

 The particles have an average size of between 0.4 and 0.5unm.

EXAMPLE 3
2.67 g of the surfactant polymer B 261 are dissolved in 267 g of SOLVESSO 200.

An aqueous solution is prepared comprising
- 0.081 mole / l of yttrium acetate
- 0.162 mole of barium acetate
- 0.243 mol of copper acetate.

 150 ml of this aqueous solution and the organic solution containing the surfactant are emulsified with an apparatus of the ULTRATURRAX T 50 type (colloid mills) at maximum power for 2 minutes and then by ultrasound for 6 minutes.

 10.023 g of oxalic acid (C2H204.2H2O) are dissolved in 300 ml of ethanol 95 and the solution is then transferred to a reactor controlled at 300C.

 The emulsion is introduced into the oxalic solution with a flow rate of 10 ml / min with stirring.

Stirring is continued 3 hours after the introduction of the reagents
300 ml of ethanol are added to the reaction medium and the metal carboxylate thus obtained is separated by centrifugation.

This is then washed several times with absolute ethanol before being dried under vacuum for 12 hours.

 The metal carboxylate is calcined at 8500C for 5 hours.

PRODUCT ANALYSIS
Metallic Carboxvlate
The particles are monodisperse and the average particle size is submicron (SEM).

 The areas of heterogeneity are less than lpm (EDS).

Composition on #conductive
X-ray diffraction analysis indicates a pure orthorhombic phase YBa2Cu307.

 The domains of heterogeneity are less than 0.5Fun (EDS).

 The average particle size is less than 0.5ym.

EXAMPLE 4
2.1 g of the surfactant polymer LP 8 are dissolved in 267 ml of ISOPAR M.

An aqueous solution is prepared comprising
- 0.081 mole / l of yttrium acetate
- 0.162 mole of barium acetate
- 0.243 mol of copper acetate.

 150 ml of this aqueous solution and the organic solution containing the surfactant are emulsified with an apparatus of the ULTRATURRAX T 50 type (colloid mills) at maximum power for 2 minutes and then by ultrasound for 6 minutes.

 10.023 g of oxalic acid (C2H204.2H2O) are dissolved in 300 ml of ethanol 95 and the solution is then transferred to a reactor controlled at 300C.

 The emulsion is introduced into the oxalic solution with a flow rate of 10 ml / min with stirring.

 Stirring is continued 3 hours after the introduction of the reagents.

 300 ml of ethanol are added to the reaction medium and the metal carboxylate thus obtained is separated by centrifugation.

This is then washed several times with absolute ethanol before being dried under vacuum for 12 hours.

 The metal carboxylate is calcined at 8500C for 5 hours under oxygen scavenging.

PRODUCT ANALYSIS
Metallic Carboxylate
The areas of heterogeneity are less than 1pm (EDS).

 The particles are monodisperse and the average size is less than one.

Superconductive composition
X-ray diffraction analysis indicates a pure YBa2Cu3O7 phase.

 Heterogeneity domains are less than 0.5pm (EDS).

 The particle size is less than 0.5pm.

 ESCA analysis gives the following atomic relationships between the elements Ba / Y: 1.82 and Cu / Y: 2.88 representative of surface stoichiometry.

EXAMPLE 5
3 g of SPAN 80 surfactant are dissolved in 350 g of cyclohexane.

An aqueous solution is prepared comprising
- 0.081 mole / l of yttrium acetate
- 0.162 mole / l of barium acetate
- 0.243 mole / l of copper acetate
150 g of the above-mentioned aqueous solution are mixed with 50 g of an 0.1% aqueous xanthan gum solution.

 The solution previously obtained and the organic solution containing the surfactant are emulsified with an apparatus of the ULTRATURRAX T 50 type (colloid mills) at maximum power for 2 minutes and then by ultrasound for 6 minutes.

 10 g of oxalic acid (C2H204, 2H20) are dissolved in 350 ml of ethanol 95 and then the solution is transferred to a reactor controlled at 300C.

 The emulsion is introduced into the oxalic solution with a flow rate of 10 ml / min with stirring.

 Stirring is continued 3 hours after the introduction of the reagents.

 The metal carboxylate thus obtained is separated by natural decantation. This is then washed several times with absolute ethanol before being dried under vacuum for 12 hours.

 The metal carboxylate is calcined at 8500C for 5 hours.

PRODUCT ANALYSIS
X-ray diffraction analysis of the superconductive composition shows a YBa2Cu307 phase comprising traces of BaCuO2.

 Analysis of this same composition by the CILAS method indicates an average diameter of 1ym.

COMPARATIVE EXAMPLE 6
An oxalic acid solution (C2H204, 2H20) is prepared at a concentration of 0.9M in water.

An aqueous solution is prepared comprising
- 0.081 mole / l of yttrium acetate
- 0.162 mole / l of barium acetate
- 0.243 mole / l of copper acetate
The two solutions are introduced continuously into a 1 liter reactor while respecting the following flow rates.
- oxalic acid: 8.6 ml / min
- aqueous solution: 14.3 ml / min
The precipitated solid is recovered by filtration and then washed with ethanol.

 The mixture is then calcined for 5 hours at 9000C.

PRODUCT ANALYSIS
SEM analysis of the precipitate before calcination shows the presence of fine grains of size less than 10 mm as well as the presence of large barium sticks of around 100 Fun. Hence the domains of heterogeneity on the order of 100un.

 X-ray diffraction analysis of the calcined product indicates the presence of a YBa2Cu3O7 phase comprising parasitic phases of the BaCuO2 and CuO type in a proportion of 5%.

Claims (35)

1 - Metallic carboxylate characterized in that it has a homogeneity measured by EDS mapping such that the domains of heterogeneity are less than or equal to luni. 2 - Metallic carboxylate according to claim 1, characterized in that it has a homogeneity measured by EDS mapping such that the domains of heterogeneity are less than or equal to 0.5 pu. 3 - Carboxylate according to claim 2, characterized in that it is in the form of particles of average size is at most 1pm or submicron. 4 - Carboxylate according to claim 3, characterized in that the average particle size is between 0.1 and 1pm and preferably between 0.2 and 0.6 well. 5 - Carboxylate according to one of the preceding claims, characterized in that the carboxylate comes from a carboxylic, hydroxycarboxylic, dicarboxylic acid saturated, unsaturated or aromatic and comprises at most 8 carbon atoms. 6 - Carboxylate according to claim 5, characterized in that the carboxylate comes from oxalic, malonic, succinic, tartaric, maleic, phthalic, fumaric acids. 7 - Carboxylate according to one of the preceding claims, characterized in that it comprises at least one rare earth, at least one alkaline earth and at least one transition metal. 8 - Carboxylate according to claim 7, characterized in that the rare earth is chosen from yttrium, lanthanum. 9 - Carboxylate according to claim 7, characterized in that the alkaline earth is chosen from strontium, barium, calcium. 10 - Carboxylate according to claim 7, characterized in that the transition metal is chosen from copper, cobalt, iron, manganese, nickel.  II - Superconductive composition obtained by calcination of a metal carboxylate according to claims 1 to 10. 12 - Superconductive composition of the type comprising at least one rare earth, at least one transition element, at least one alkaline-earth, characterized in that it has a homogeneity of composition measured by EDS mapping, such as the domains of heterogeneity are less than or equal to lun. 13 - Composition according to claim 12, characterized in that it has a homogeneity measured by EDS mapping such that the areas of heterogeneity are less than or equal to 0.5un. 14 - Superconductive composition according to one of claims 11 and 12, characterized in that the average particle size is at most luni or submicron. 15 - Superconductive composition according to claim 14, characterized in that the average size of the particles is between 0.1 and luni and preferably between 0.2 and 0.6un. 16 - Superconductive composition according to one of claims 11 to 15, characterized in that the particles are monodispersed. 17 - superconductive composition according to claim 16, characterized in that the dispersion index is less than or equal to 0.5 with  ID = d # A-d16  2d50 18 - Superconductive composition according to one of claims Il à 17, characterized in that it is in the form of a non-agglomerated powder. 19 - Process for the preparation of a metal carboxylate according to claims 1 to 10, characterized in that it comprises the following steps - an aqueous solution of metal salts is prepared and then put  this solution in emulsion in an organic solvent, - the emulsion is reacted with a precipitating agent, - the precipitate obtained is separated. 20 - Process according to claim 19, characterized in that the above-mentioned aqueous solution is emulsified in the presence of a surfactant. 21 - Process according to claim 20, characterized in that the surfactant has the following general formula: (A-COO) m-B in which  .m # 2  A A is represented by the following general formula RCO in which  R is a hydrogen atom or a monovalent group  substituted or unsubstituted hydrocarbon  R1 is a hydrogen atom or a hydrocarbon group  monovalent in C1 - C24  R2 is a divalent hydrocarbon group in Cî - C24  n is 0 or 1  P is an integer varying between 0 and 200 For m = 2 B is represented by the following general formula  H [-O-HCR3-CH2) q # O-HCR3-CH2OH in which:  R3 is a C1 - C3 alkyl group  q is an integer varying from 10 to 500.  For m> 2 B is represented by the following formula  R4 (-EO-HCR3-CH2-jr-OH # m in which  R3 and m have the same meaning as before  R4 is the residue of an organic compound comprising m hydrogen atoms reactive with an alkylene oxide.  R4 is the residue of an organic compound comprising hydrogen atoms reactive with an alkylene oxide.  r is an integer varying from 1 to 500, knowing that the total number of -0-HCR3-CH2- units in the molecule is at least 10. 22 - Process according to claim 20, characterized in that the surfactant corresponds to the condensation product of a poly (lower alkylene) imine with a polyester comprising a free carboxylic acid group, said condensation product having at least two polyester groups linked to each poly (lower alkylene) imine. 23 - Method according to claim 20 characterized in that one uses as surfactant a sorbitan monooleate in combination with a biogum type xanthan gum. 24 - Method according to one of claims 19 to 23, characterized in that the aqueous solution of metal salts is formed of salts of organic or inorganic acids. 25 - Process according to claim 24, characterized in that the salts of organic acids are chosen from salts of saturated aliphatic carboxylic acids or from salts of hydroxycarboxylic acids. 26 - Process according to claim 24, characterized in that the organic acid salts are chosen from citrates, formates, propionates or acetates. 27 - Process according to claim 24, characterized in that the salts of inorganic acids are chosen from nitrates or chlorides. 28 - Method according to one of claims 19 to 27, characterized in that the organic solvent of the emulsion is chosen from alkanes, having at least 5 carbon atoms, aromatics, alcohols having at least 5 carbon atoms .  29 - Process according to claim 28, characterized in that the solvent for the above-mentioned emulsion is chosen from hexane, heptane, cyclohexane, pentanol, hexanol, cyclohexanol, ethyl 2-hexanol. 30 - Method according to one of claims 19 to 29, characterized in that the volume ratio of water to the organic solvent of the above emulsion is between 5 and 50%, and preferably between 10 and 30% . 31 - Method according to one of claims 19 to 30, characterized in that the precipitating agent is used in solution in an organic solvent. 32 - Process according to claim 31, characterized in that the precipitating agent is used in solution in the organic solvent of the emulsion. 33 - Method according to claim 31, characterized in that the precipitating agent is used in solution in a second solvent miscible with the organic solvent of the emulsion. 34 - Process according to claim 31, characterized in that the precipitating agent is used in solution in a second organic solvent immiscible with the organic solvent of the emulsion 35 - Process according to claims 19 to 34, characterized in that precipitating agent is chosen from saturated, unsaturated or aromatic carboxylic, hydrocarboxylic, dicarboxylic acids, said acids having at most 8 carbon atoms. 36 - Process according to claim 35, characterized in that the precipitating agent is chosen from malonic, tartaric, succinic, oxalic, maleic, fumaric, phthalic acids.  37 - Process according to claim 33, characterized in that the solvent miscible with the organic solvent of the emulsion is chosen from saturated alcohols, aromatic alcohols, polyhydroxylated alcohols, said alcohols comprising at most 8 carbon atoms, esters of 'hydroxycarboxylic acids. 38 - Process according to claim 33, characterized in that the abovementioned solvent is chosen from ethanol, benzyl alcohol, ethylene carbonate, propylene carbonate, ethylene glycol, diethylene glycol, diethylene monobutyl ether glycol, monobutyl ether diethylene acetate glycol. 39 - Method according to one of claims 19 to 38, characterized in that the emulsion is reacted by introducing it dropwise on the precipitating agent.