FR2667586A1 - Lead titano-zirconate type cpds. with perovskite structure - can be sintered into ceramics with piezoelectric properties for e.g. voltage or standard frequency generation - Google Patents

Abstract

Pb-based compsns. with a Perovskite structure having a chemical homogenity measured by EDS (energy dispersion spectroscopy) analysis such that the regions of heterogenicity are less than or equal to 1 micrometre, pref. less than or equal to 0.5 micrometre. USE/ADVANTAGE - The cpds. have a Perovskite structure with very high chemical homogenity and can be made in the form of monodisperse particles which can be sintered to form ceramic materials with the same structure and high chemical homogenity having piezoelectric properties and useful, e.g., in voltage generators and standard frequency generators. Ceramics based on Pb titano-zirconate contg. La are transparent and therefore useful in electro-optical applications. (Dwg.0/0)

Description

LEAD-BASED PEROVSKITES AND THEIR METHOD OF PREPARATION
The present invention relates to lead-based perovskite structure compositions and their method of preparation.

 In recent years, much research has been carried out to develop processes for the preparation of such ceramics. These are indeed advantageous for many applications using in particular their piezoelectric properties. Thus we can cite their use in the manufacture of voltage generators, standard frequency generators.

 With regard more particularly to ceramics based on lead titano-zirconate comprising lanthanum, mention may be made of their use in the field of electro-optics because of their transparency.

 Furthermore, ceramics with a lead-based perovskite structure and more precisely ceramics of the lead titano-zirconate type and their derivatives are advantageous because of their relatively low sintering temperatures compared to barium titanate.

 Various methods are used for the synthesis of ceramics of this type. In particular, one of the most common consists in reacting in the solid phase at high temperature the various oxides and / or the various salts containing a volatile anion of the carbonate type for example, necessary for obtaining the desired phase.

Mention may likewise be made of the process for the precipitation of hydroxides from the salts of metal cations; most often the hydroxides are prepared separately and the three precipitates are combined, mixed and then calcines.

 However, all of these processes have drawbacks both in terms of chemical homogeneity and purity of the final products (presence of parasitic phases) as well as in their particle size distribution and the size of their particles. Indeed, such processes are delicate to control and most often require a grinding step prior to sintering in order to homogenize the powder and obtain a particle size suitable for this last step. However, such an operation increases the risks of pollution of the desired phase.

 Finally, no satisfactory solution has been provided to the problem of lead loss in terms of chemical homogeneity of the final product during the sintering of such compositions.

 Usually, this is done around 1300C while the lead departure is noted from 8500C. Thus, it is known, in order to compensate for the loss of this element, to carry out sintering under an atmosphere of lead oxide. However, such a process is detrimental to the homogeneity of composition of the final ceramic because the control, on the one hand of the lead loss and on the other hand of the introduction of this same element, is delicate if not impossible.

 Still with the aim of avoiding lead loss, sintering can also be carried out under hot pressing. However, the drawback of such a method is that it requires the implementation of heavy and costly industrial means and limits the size of the parts.

 The present invention therefore relates to a ceramic composition with a perovskite structure based on lead which does not have the drawbacks described above, concerning in particular the homogeneity of chemical composition, particle size distribution and size.

 In addition, the invention relates to a composition of perovskite structure based on lead which does not require extensive grinding but only a deagglomeration of the particles, in order to adjust the particle size before sintering.

 Furthermore, the invention describes a ceramic sintering at a lower temperature than known ceramics of the same type, without adding lead oxide or using heavy industrial means.

 Thus, the composition according to the invention of perovskite structure based on lead is characterized in that it has a chemical homogeneity measured by EDS analysis such that the domains of heterogeneity are less than or equal to 1ym.

 Another subject of the invention relates to a sintered material based on a composition with a perovskite structure comprising lead, characterized in that it exhibits chemical homogeneity measured by energy dispersive spectroscopy (EDAX) such as the domains of heterogeneity are less than or equal to sium.

 A third object of the invention consists of a sintered material of the ceramic type with a perovskite structure based on lead, characterized in that it is obtained by sintering a ceramic composition according to the invention.

 Finally, a last object of the invention relates to obtaining said composition.

The process for the preparation of a lead-based composition of perovskite structure is characterized in that it comprises the following steps
- An aqueous solution comprising salts of the required elements is prepared, then the said solution is emulsified in an organic solvent,
- the emulsion is reacted with a precipitating agent,
- the precipitate formed is separated,
- Said precipitate is calcined.

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

 One of the characteristics of the composition according to the invention lies in its very good chemical homogeneity (or homogeneity of composition). Indeed, it is such that the domains of heterogeneity are less than 1ym.

 More particularly, the homogeneity of the composition according to the invention is such that the areas of heterogeneity are less than or equal to 0.5ym.

 It should be noted that the homogeneity of composition was measured by the energy dispersive spectroscopy (EDS) mapping method using a transmission electron microscopy (TEM) microprobe.

Another advantageous characteristic of the composition of perovskite structure based on lead according to the invention is such that it is in the form of a powder whose average particle size is less than or equal to 1 μm, measured by the method
CILAS.

 Preferably, the average particle size is less than or equal to 0.9ym and more particularly between 0.1 and 0.6ym.

 Furthermore, the composition according to the invention can be in the form of monodisperse particles.

 The monodispersity 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
d84 - d16
ID = ~~~~~~~~~
2 d50 in which
- d84 represents the diameter for which 84% of the particles have a size smaller than the value of this diameter.

 - d16 represents the diameter for which 16% of the particles have a size smaller than the value of this diameter.

 - d50 represents the diameter for which 50% of the particles have a size smaller than the value of this diameter.

 Thus, according to a particular embodiment of the invention, the dispersion index ID of the composition according to the invention is less than or equal to 0.7 and preferably less than or equal to 0.5.

 The composition of the perovskite structure according to the invention and more particularly the composition of the lead titano-zirconate type (hereinafter called the PZT type composition) also has an advantageous characteristic in terms of densification.

 Indeed, it has little or no loss of lead when heated to a temperature of up to 11000C.

 This characteristic is particularly advantageous compared to known compositions of the same type since the latter exhibit a loss of lead from 8500C. Consequently, the sintering of such compositions could only be carried out in particular, as has been said previously, only under an atmosphere making it possible to compensate for the lead deficit, that is to say comprising lead oxide.

On the contrary, the composition of the PZT type according to the invention can be sintered under an atmosphere free of lead oxide due to the increase, relative to the sintering temperature, of the temperature at which the lead escapes from the composition
Thus, the chemical homogeneity of the composition according to the invention is preserved.

 The invention relates to various ceramics of lead-based perovskite structure.

 By perovskite structure is meant the compounds of general formula ABO3 in which A is an element coordinated with 12 oxygen atoms and B is an element coordinated with 6 oxygen atoms.

 The invention relates more precisely to various ABO3 type ceramics in which A is lead.

 Thus, according to a first variant of the invention, the compositions according to the invention, in addition to lead, are based on elements of classes 5A, optionally an alkaline-earth metal, optionally a transition element and oxygen.

 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, F.A. COTTON and G. WILKINSON (John Willey and Sons).

 Concerning the elements of class 5A, niobium and tantalum are perfectly suitable for the implementation of the invention.

 Among the alkaline earths, magnesium is generally used.

 As a transition element, mention may be made inter alia of scandium, iron, cobalt, nickel and zinc.

As an example of a composition of the type corresponding to this first embodiment of the invention, particular mention is made of
Pb (Mg1 / 3Nb2 / 3) O3, Pb (Fe1 / 2Nb1 / 2) O3, Pb (Sc1 / 2Nb1 / 2) O31
Pb (Sc1 / 2Ta1 / 2) O3r Pb (Mgl / 3Ta2 / 3) 03, Pb (Co1 / 3Nb2 / 3) o3t
Pb (Co1 / 3Ta2 / 303, Pb (Ni1 / 3, Nb2 / 3) O3, Pb (Zn1 / 3Nb2 / 3) O3.

 A second variant of the invention relates to compositions of the type comprising, in addition to lead, elements from classes 4A, 5A, 5B.

 Among the elements of class 4A, zirconium and / or titanium and / or hafnium are generally used.

 As element of class 5B, use is more particularly made of bismuth and / or arsenic.

 Corresponding to this second type of composition, there may be mentioned, among others, PbTiO3, PbNb206.

However, according to a preferred variant of the invention, the composition consists of ceramics of perovskite structure of the lead titano-zirconate type and its derivatives (composition of the type
PZT).

 According to a first embodiment of this preferred variant, the composition may also comprise a rare earth.

 By rare earth, we designate the elements whose atomic number varies from 57 to 71 as well as the yttrium.

 However, lanthanum is preferably used as the rare earth.

 Furthermore, according to a second particular mode of this same variant, the composition of the PZT type according to the invention can also comprise at least one dopant.

 This is generally chosen from alkalis, alkaline earths, transition elements and rare earths.

 As alkali, potassium is generally used.

 Regarding the alkaline earths, magnesium is suitable for implementing the process according to the invention.

 As transition elements, scandium, manganese and iron can be used as doping agent of the composition according to the invention.

Regarding rare earths, lanthanum, neodymium, cerium, gadolinium and samarium are generally used
According to another embodiment of the preferred variant of the invention, the composition of perovskite structure can be written according to the following formula
Pbx Al-x Zry Ti1-y Mz O3 in which
A represents at least one rare earth
M represents at least one dopant
x varies from 0.8 to 1.02
y varies from 0.2 to 0.9
z varies from O to 0.1.

 A second object of the invention resides in a sintered material having chemical homogeneity properties similar to those of the composition from which it comes. Indeed, this sintered material of perovskite structure based on lead has a homogeneity such that the domains of heterogeneity are less than or equal to 1 pm.

 It should be noted that the material homogeneity measurements are made by energy dispersion spectroscopy (EDAX).

 On the other hand, the sintered material according to the invention has a high density after sintering since it is greater than or equal to 90% of the theoretical density and preferably greater than 95% of the theoretical density.

 The invention likewise relates to a sintered material obtained by sintering the composition of perovskite structure based on lead described above.

 This operation is carried out according to all methods known to those skilled in the art.

 However, advantageously, the sintering is carried out in an atmosphere not containing lead oxide. It is especially possible to sinter under vacuum and / or under air.

 On the other hand, the heating temperature of the composition is relatively lower than that reached for the sintering of known compositions.

 Thus, the sintering of the composition according to the invention and more particularly in the case of the composition of the PZT type appears from 11000C and more particularly from 10000C.

 On the other hand, as far as the known compositions are concerned, sintering only appears around 13000C.

 Usually, the sintering time is between 2 and 10 hours.

 Finally, it is important to note that extensive grinding prior to sintering in order to adjust the particle size and to re-homogenize the powder is no longer necessary. In fact, the powder according to the invention has very good chemical homogeneity and a submicron and monodisperse particle size.

 Thus, unlike the known compositions where it is necessary to reduce the very size of the particles, one can be satisfied in the case of the compositions according to the invention with a simple deagglomeration of the primary particles.

 On the other hand, it was surprisingly noticed that the particles were not agglomerated after calcination if the calcination temperature is less than 9000C and more particularly less than 8000C.

 In this case, the particles obtained directly after calcination have the following characteristics, 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 obtaining the lead-based perovskite structure composition according to the invention will now be described.

This includes the following steps - an aqueous solution of the salts of the required elements is prepared, then
said solution is emulsified in an organic solvent, - the emulsion is reacted with a precipitating agent, - the precipitate formed is separated, - said precipitate is calcined.

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

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

 However, according to a particular embodiment, nitrates are chosen as inorganic acid salts.

 It should be noted that the inorganic salts of the elements constituting the composition according to the invention are marketed except the titanium salt which it is necessary to prepare beforehand.

 Thus titanyl nitrate is obtained by reaction of ammonia on TiOCl2. The precipitate is then washed by following several cycles of centrifugation - repulping in order to remove the chlorides.

 Titanyl nitrate is obtained by redissolving the precipitate washed in molar nitric acid in the proportions such that the molar ratio of acid to titanium is close to 6.

 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 salts of the elements are adjusted according to the stoichiometry of the desired final composition 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.

 Thus, 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, mention may be made of hexane, heptane, cyclohe xane but also solvents of the VARSOL z type, solvents of the isoparaffinic type including in particular ISOPAR and 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 the solvent, AROMATIC @ , ACTREL 400 QR said solvents can be used pure or in 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 using a ZEISS IM 35 microscope, equipped with Nomarski interference contrast and whose resolution limit is 0.15pu.

 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, alkyl sulfates, alkyl sulfonates, sulfosuccinates (AEROSOL OT (R)) or alkylbenzene sulfonic acids (SULFAMIN 1298) are used, among others.
With regard to cationic surfactants, the emulsion can be prepared with the alkyltrimethylammonium, dialkyldimethylammonium, alkylpyridinium salts, imidazolinium derivatives.

 As amphoteric surfactants, there may be mentioned alkylbetaines, amidopropylbétaines, sulfobétaines.

As non-ionic surfactants, it is possible to use pure or as a mixture the sorbitan ester derivatives of the SPAN type, the polyethoxylated sorbitan ester derivatives of the TWEEN z type, the polyethoxylated alkylphenols, the polyethoxylated fatty alcohols, the triglycerides. polyethoxylated, polyethoxylated fatty amines, these polyethoxylated derivatives belonging to the group of CEMULSOLS
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 [-O-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 preferably. For example, R can be a group derived from stearic acid such as the straight chain C17H35.

 R1 is a hydrogen atom or a monovalent C1-C24 hydrocarbon group, linear or branched. 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
HE -O-HCR3-CH2 Jg-O-HCR3-CH2OH (II) in which
R3 is a hydrogen atom or a C1-C3 alkyl group.

 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 (-! O-HCR3-CH2-Jr-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 marketed under the brand B 261 and B 246.

 According to another preferred embodiment, molecules used as surfactants resulting from the condensation of a poly (lower alkylene) imine on a polyester having a free acid group are used; 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.

Concerning 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 nonr comprising no 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 the patent
US No. 4,224,212 on behalf of Company IC 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.

 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 beaoniae,
Xanthomonas campestris, Xanthomonas carotae, Xanthomonas hederae,
Xanthomonas incanae, Xanthomonas malvacearum,
Xanthomonas papavericola, Xanthomonas phaseoli, Xanthomonas pisi, anthomonas vasculorum, Xanthomonas vesicatoria, Xanthomonas vitians, Xanthomonas pelarqonii; 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; to the genus Asrobacter and more particularly the species Asrobacterium radiobacter, Asrobacterium rhizoqènes, Asrobacter tumefaciens; in the genus Alcalisènes and more particularly the species Alcaliqènes faecalis; in the genus Rhizobium; in the genus Sclerotium and more particularly the species Sclerotium rolfsii and Sclerotium qlucanicum; 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. QR
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.

 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 350C.

The second step of the process according to the invention consists in reacting the emulsion with a precipitating agent
The precipitating agent can be in the form of a liquid, a solid or a gas. However, according to a preferred embodiment of the invention, the reaction is carried out with a precipitating agent in the form of a liquid.

 As such, aliphatic, cyclic, benzenic amines or quaternary ammoniums 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 mode, the precipitating agent is chosen from ammonia or a precursor such as urea, methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, dodecylamine, oleylamine, cyclohexylamine, aniline, methyltricaprylammonium hydroxide.

 In the case where methyltricaprylammonium hydroxide is chosen, it must be prepared from methyltricaprylammonium chloride, sold under the name ALIQUAT 336.

ALIQUAT hydroxide is obtained by liquid / liquid transfer of ions by successive impregnations of aqueous and organic phases. The first consists of 150 g of a sodium hydroxide solution (1 ON) for each impregnation and the second comprises 45 g of ALI
QUAT 336 (about 0.5 mole / kg) dissolved in 200 g of SOLVESSO 200.

 The two phases are stirred vigorously using a magnetic bar for 30 minutes and then separated after decantation.

 This operation is carried out until the chloride ions have disappeared from the aqueous phase.

 In the case where urea is used as precipitating agent, it is introduced into the aqueous solution of the salts of the elements constituting the composition according to the invention, then the emulsion is prepared as was said above.

 Said emulsion is then heated to a temperature between 60 and 650C to decompose the urea and thus release into the medium the actual precipitating agent.

 It should be noted that the emulsion is not destabilized by this heating.

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

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

 The molar ratio of the precipitating agent to the salts of the elements usually ranges from 1 to 10.

 The contacting of the emulsion and the precipitating agent can be done directly, that is to say without the use of an additional solvent. However, 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 the organic solvent of 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 comprising at most 10 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 polyhydric alcohols, ethylene glycol, diethylene glycol and its derivatives, for example ethylene glycol monobutyl ether or monethylene glycol acetate monobutyl ether, 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 the implementation of 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 emulsion is preferably introduced dropwise into the solution of the precipitating agent.

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

 The reaction is carried out at a temperature between and 1000C, 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 second 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.

 The precipitate thus obtained is then calcined.

 The calcination is carried out according to any usual method.

 More particularly, the precipitate is calcined in air and / or under oxygen in a static atmosphere or in a sweep at a temperature generally varying between 500 and 8500C and preferably between 700 and 8000C; the upper temperature being fixed by the nature of the final phase.

 The duration of the calcination is usually between 2 and 10 hours.

 It should be noted that a single calcination of a shorter duration compared to the known methods is sufficient to obtain a product of good phasic purity. Thus, it is not necessary to carry out a complex calcination cycle, that is to say comprising one or more intermediate grindings in order to obtain a pure and homogeneous product.

 Concrete examples will now be presented.

 Previously, two agglomeration measurement protocols using the CILAS method will be described.

Protocol 1
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 nO 1).

Protocol 2
150-200 mg of powder are taken and dispersed in 40 ml of a 1% aqueous hexametaphosphate solution. 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) containing water.

 The measurement is then carried out in said tank (measurement range n "1).

EXAMPLE 1 - synthesis of PZT
A mixed aqueous solution of titanyl nitrate (0.14M), zirconyl nitrate (0.14M) and lead nitrate (0.28M) is prepared.

The titanyl nitrate solution is prepared beforehand as follows
400 g of TiOCl2 are reacted at 250 ° C. with 237 g of ammonia. The precipitate obtained is washed with deionized water by a series of centrifugations - repulped so as to remove the chlorides.

The precipitate thus treated is redissolved in nitric acid in proportions such that the molar ratio of
HNO3 / Ti is around 6 and the titanium concentration is 0.6 mol / l.

 3.57 g of hypermerating LP8 surfactant are dissolved in 357 ml of SOLVESSO 200.

 143 ml of the aqueous nitrate solution are emulsified by sonication with the solution comprising the surfactant.

 A mixture is prepared comprising 200 ml of 20% ammonia and 200 ml of Normapur ethanol and the said mixture is transferred to a base stock thermostatically controlled at 250C.

 The emulsion is then introduced into the aforementioned mixture at a flow rate of 27 ml.mn-1 with mechanical stirring (mixed propeller 200 rpm. 1).

 The precipitate obtained is recovered by centrifugation, then washed with ethanol before drying in an oven at 300C.

 The dried precipitate is then calcined in air at 8000C for 3 hours.

 Two grams of the unmilled calcined powder are then pelletized in a mold with a diameter of 1.62 cm at 2 t / cm 2 in the presence of a binder of the RHODOVIOL 4/20 type.

 A descaling cycle is then carried out at 6000C, then the pellet thus obtained is sintered at 11000C for 3 hours.

PRODUCT ANALYSIS
X-ray diffraction analysis of the calcined powder indicates a pure PZT phase.

 The calcined powder is monodisperse and the particle size dispersion index is 0.42.

 The average particle size of the calcined powder is 0.5un (measured by the CILAS method).

 The domains of chemical heterogeneity measured by EDS are less than 0.5 film.

 The sintered powder has a density of 7.52 g / cm3, or 95% of the theoretical density.

 There is no evidence of lead departure during sintering.

COMPARATIVE EXAMPLE 2
This synthetic route consists in precipitating the aqueous lead solution and the aqueous solution of the Zr-Ti mixture separately.

 For this manipulation, an assembly is prepared consisting of two pumps (an acid and a basic) and a reactor where the reaction takes place. This takes place at constant pH, close to the precipitation pH of the elements in solution, and it is modulated by small pumps which regulate the pH. The precipitate is recovered by over-drawing and filtered on a glass of porosity 4.

 The "cake" obtained is dried and a sample is calcined to determine the amount of lead oxide present in the first precipitate and of titanium oxide and zirconia in the second.

 Subsequently, after weighing these oxides exactly, they are mixed and made into a suspension.

 A whole series of centrifugations and neutralizations is then carried out to remove the chlorides, while being careful not to dissolve the lead (the precipitation test is carried out with NH40H in the mother liquors after each centrifugation). After five identical operations, the cake is recovered, which is dried in an oven and then calcined at 8000C.

 The calcined powder requires grinding.

 The ground powder with a particle size centered around 0.65pu is sintered at 1100C after having been pelletized under the conditions described in Example 1.

PRODUCT ANALYSIS
The calcined powder has an X-ray diffraction spectrum where the position of the diffraction planes is intermediate between that of the PZT and PbTiO3 planes. It also identifies quadratic ZrO2.

 After sintering, the density is 6.7 g / cm3, or 84% of the theoretical density.

EXAMPLE 3 Synthesis of PLZT
The prepared phase corresponds to the following formula Pb0.91La0.09 (Zr0165Ti0.35) O3
An aqueous solution of 143 ml mixed with titanyl nitrate (0.155M), zirconyl nitrate (0.289M), lanthanum nitrate (0.040M) and lead nitrate (0.403M) is prepared.

The titanyl nitrate solution is prepared beforehand as follows
Reacted at 250C of TiOCl2 with 237 g of ammonia. The precipitate obtained is washed with deionized water by a series of centrifugations - repulped so as to remove the chlorides.

 The precipitate thus treated is redissolved in nitric acid in proportions such that the molar ratio of HNO3 / Ti is approximately 6 and that the concentration of titanium is 0.6 mol / l.

 3.57 g of hypermerating LP8 surfactant are dissolved in 357 ml of SOLVESSO 200.

 143 ml of the aqueous nitrate solution are emulsified by sonication with the solution comprising the surfactant.

 A mixture is prepared comprising 200 ml of 20% ammonia and 200 ml of Normapur ethanol and the said mixture is transferred to a base stock thermostatically controlled at 250C.

 The emulsion is then introduced into the aforementioned mixture at a flow rate of 27 ml.mn-1 with mechanical stirring (mixed propeller 200 rpm. 1).

 The precipitate obtained is recovered by centrifugation, then washed with ethanol before drying in an oven at 300C.

 The dried precipitate is then calcined in air at 72 ° C. for 3 hours.

 Two grams of the unmilled calcined powder are then pelletized in a mold with a diameter of 1.62 cm at 2 t / cm 2 in the presence of a binder of the RHODOVIOL 4/20 type.

 A descaling cycle is then carried out at 6000C, then the pellet thus obtained is sintered at 1100C for 3 hours.

PRODUCT ANALYSIS
X-ray diffraction analysis of the calcined powder indicates a pure PLZT phase.

 The calcined powder is monodisperse and the particle size dispersion index is 0.46.

 The average particle size of the calcined powder is 0.5pu.

 The domains of heterogeneity measured by EDS / MET have a size less than 0.5pu.

 The sintered powder has a density of 7.3 g / cm3 or 93% of the theoretical density.

 There is no loss of lead during sintering.

COMPARATIVE EXAMPLE 4
The prepared phase corresponds to the following formula Pb0.9iLa0.09 (Zr0.65Ti0.35) O3
A 850 ml base stock is prepared of an ethanol water mixture (50/50), the pH of which is brought to 9.3 by the addition of a few drops of NH40H.

 400 ml of an 0.26M aqueous solution consisting of lanthanum nitrate (0.022M), lead nitrate (0.23M), titanyl nitrate (0.088M) and zirconyl nitrate (0.16M) are prepared. ).

 This solution is added by a pump 1, at the bottom of the tank while a pump 2 introduces 275 ml of an ammonia mixture (20%) - ethanol (50/50). The total mixture is kept under stirring.

 The flow rates of the two pumps are adjusted so that the pH remains fixed at 9.6. The reagents are added in 35 minutes.

 The precipitate obtained is centrifuged and then washed with ethanol.

 It is dried under vacuum at 500C and then calcined at 8000C in air for 3 hours.

 The calcined powder is then pelletized (2 samples) without grinding as described in the previous examples, then sintered at 11000C and at 12000C.

PRODUCT ANALYSIS
The powder does not densify since 5.64 g / cm3 (72% of the theoretical density) and 5.57 g / cm3 (71% of the theoretical density) are obtained for the samples sintered at 1100C and 12000C respectively .

 In addition, the lead departure is noted from 8000C.

Claims (39)

1 - Composition of perovskite structure based on lead, characterized in that it has chemical homogeneity measured by analysis EDS such that the domains of heterogeneity are less than or equal to ium. 2 - Composition according to claim f, characterized in that the areas of heterogeneity are less than or equal to 0.5 pu. 3 - Composition according to one of the preceding claims, characterized in that it is in the form of a powder of average size of particles less than or equal to lun. 4 - Composition according to claim 3, characterized in that the average particle size is less than or equal to 0.7Fun and preferably between 0.1 and 0.6pu. 5 - Composition according to one of claims 3 or 4, characterized in that the particles are monodisperse. 6 - Composition according to claim 5, characterized in that the particle size dispersion index  d84 - d16  ID =  2 d50 is less than or equal to 0.7 and preferably less than or equal to 0.5. 7 - Composition according to one of the preceding claims, characterized in that it has no loss of lead when heated to a temperature up to 11000C. 8 - Composition according to one of the preceding claims, characterized in that it further comprises at least one element of class 4A and oxygen. 9 - Composition according to claim 8, characterized in that the elements of class 4A are titanium and zirconium. 10 - Composition according to one of the preceding claims, characterized in that it further comprises at least one rare earth. 11 - Composition according to claim 10, characterized in that the rare earth is lanthanum. 12 - Composition according to one of the preceding claims, characterized in that it further comprises at least one dopant. 13 - Composition according to claim 9, characterized in that the dopant is chosen from alkali metals, alkaline earth metals, transition elements and rare earths. 14 - Composition according to claim 12, characterized in that the dopant is chosen from potassium, magnesium, lanthanum, neodymium, gadolinium, cerium, samarium, scandium and iron. 15 - Composition according to one of the preceding claims, characterized in that it has the following formula  Pbx A1x Zry Til-y Mz O3 in which  A represents at least one rare earth  M represents at least one dopant  x varies from 0.8 to 1.02  y ranges from 0.2 to 0.9.  z varies from O to 0.1. 16 - Sintered material based on a composition with a perovskite structure comprising lead, characterized in that it has chemical homogeneity measured by EDAX, such that the domains of heterogeneity are less than or equal to 1 pm. 17 - Material according to claim 16, characterized in that it has a density greater than or equal to 90% of the theoretical density and preferably greater than or equal to 95% of the theoretical density. 18 - Sintered material based on a composition with a perovskite structure comprising lead, characterized in that it is obtained by sintering a composition according to one of claims 1 to 15. 19 - Process for preparing a composition of perovskite structure based on lead according to one of claims 1 to 15, characterized in that it comprises the following stages - a solution of the salts of the required elements is prepared and then said solution in emulsion in an organic solvent, - the emulsion is reacted with a precipitating agent, - the precipitate formed is separated, - said precipitate is calcined. 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 19, characterized in that a sorbitan monooleate is used as surfactant in combination with a biogum of the xanthan gum type. 22 - Process according to claim 20, characterized in that the surfactant has the following general formula  (A-COO) m-B in which m12  A is represented by the following general formula RCO [-O-HCR1- (R2) n-CO] p-O-HCR1- (R2) n-COOH  in which  R is a hydrogen atom or a substituted or unsubstituted monovalent hydrocarbon group  R1 is a hydrogen atom or a C1-C24 monovalent hydrocarbon group  R2 is a divalent C1-C24 hydrocarbon group  p is an integer varying between 0 and 100  - B is represented by the following general formula when m = 2  H [-o-HCR3-CHZ] q-O-HCR3-CHZOH  in which  R3 is a C1 - C3 alkyl group  q is an integer varying from 10 to 500  - B is represented by the following formula when m> 2 R4 (-CO-HCR3-CH2-] r-OH) m  in which  R3 and m have the same meaning as before  r is an integer varying from 1 to 500, knowing that the total number of -O-HCR3-CH2- units in the molecule is at least 10. 23 - 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 bound to each poly (lower alkylene) imine. 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 the salts of saturated aliphatic acids and the salts of hydroxycarboxylic acids. 26 - Process according to claim 25, characterized in that the salts of organic acids are chosen from formates, acetates, propionates, citrates, oxalates. 27 - Process according to claim 24, characterized in that the salts of inorganic acids are chosen from nitrates. 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 6 carbon atoms . 29 - Process according to claim 28, characterized in that the solvent for the above-mentioned emulsion is chosen from hexane, 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 a second 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 - Method 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 - Method according to one of claims 19 to 34, characterized in that the precipitating agent is a base. 36 - Process according to claim 35, characterized in that the precipitating agent is chosen from aliphatic, cyclic, benzene amines, diamines and quaternary ammoniums. 37 - Process according to claim 36, characterized in that the precipitating agent is chosen from ammonia, urea, methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, dodecylamine, l oleylamine, methyltricaprylammonium hydroxide. 38 - Method according to one of claims 32 and 34 to 37, characterized in that the second solvent miscible with the organic solvent of the emulsion is chosen from saturated alcohols, aromatic alcohols, polyhydric alcohols, said alcohols comprising at most 10 carbon atoms, esters of hydroxycarboxylic acids. 39 - A method according to claim 38, characterized in that the aforementioned second solvent is chosen from ethanol, benzyl alcohol, ethylene carbonate, propylene, ethylene glycol, diethylene glycol, monobutyl ether of diethylene glycol, monobutyl ether acetate, diethylene glycol. 40 - Method according to one of claims 19 to 39, characterized in that the emulsion is reacted by introducing it dropwise onto the precipitating agent.