FR2628664A1 - Ultrafine oxide type mineral powder prepn. - by thermal treatment polymer matrix contg. dispersed (in)organic species - Google Patents

Abstract

Prepn. of an oxide type mineral powder consists in thermally treating a polymeric matrix (I) in which are homogeneously dispersed, as opt. different (in)organic species (II), all the chemical elements (III), except optionally oxygen, which are required for the desired inorganic compsn. (I) is pref. at least partly crosslinked and is pref. a poly(meth)acrylamide, esp. a partly crosslinked polyacrylamide. (II) are pref. acid salts (pref. satd. aliphatic acid salts) or alkoxides for org. species and hydroxides or salts for inorganic species. USE/ADVANTAGE - For prepn. of a large number of (mixed) oxides in the fields of ceramics, glass, catalysts, electrochromes pigments and others, e.g. silica, silicates, alumina, aluminate, TiO2, titanates, zirconates, cuprates etc.

Description

PROCESS FOR THE PREPARATION OF ULTRAFINE MINERAL POWDERS
OF OXIDE TYPE AND POWDERS THUS OBTAINED
The present invention relates to a novel process for the preparation of ultrafine and homogeneous powders of mineral oxides, mixed or simple.

 More specifically, it relates to the preparation of such materials by means of a new general method based essentially on the use of polymeric precursor compositions as defined below.

 It will be noted already, as will become clear from reading the following description, that the method according to the invention is not limited to obtaining a particular mineral composition; it must be to the cortrare considered to be of general scope, that is to say as being applicable to the preparation of a large number of oxides and sixth oxides in fields as varied as ceramics, glasses, catalysts, electrochromics, pigments and others.

 By way of nonlimiting examples of mineral compositions above ~ e can be obtained directly in the form of ultrafine powders and homogeneous by the process according to the invention, we can already quote taken only OL in meiange, silica, silicates, alumina, alumina, titania, titanates (for example barium or strontium), zirconia, stabilized zirconia (for example with yttrium, calcium or magnesium +, zirconates (for example example of barium or stontium), cuprates, as well as any other mixed oxides such as those encountered in the field of superconducting ceramics (mixture of rare earth oxide, alkaline earth and transition metals).

 We have now found, and it is one of the first objects of the present invention, that one can obtain ultrafine and homogeneous powders of mineral oxides, mixed or simple, with the. by means of the novel general method of preparation according to the invention, said method being characterized in that it consists in preparing a composition of matter comprising a polymeric matrix in which are homogeneously dispersed in the form of organic and / or inorganic species , identical or different all the chemical elements, possibly excluding oxygen, necessary for obtaining the desired mineral composition, and subjecting said composition of material to a heat treatment.

 But the invention, and the advantages it provides, will be better understood on reading the description which follows and concrete examples, but not limiting, relating to its implementation.

 The first step of the general process according to the invention therefore involves the preparation of a composition of matter as defined above.

 This composition of matter is in a way the precursor of the ultrafine powders according to the invention which will be characterized later.

 The preparation of this composition of matter by simple means, effective and easy to implement, is one of the second objects of the present invention.

 To this end, the particular process developed by the Applicant consists, in principle, in "gelling" an initial stable solution or sol containing, in the stoichiometric ratios and in the form of organic and / or inorganic species, identical or different, all the chemical elements, with the possible exception of oxygen, necessary to obtain the desired mineral composition, and this by formation "in situ" of an organic macromolecular matrix resulting from the polymerization and / or copolymerization and / or polycondensation of one or more organic monomers previously introduced into said solution or said sol.

 In simple terms, this process amounts to "freezing" in space a solution or a soil, stable and homogeneous, containing all the precursors required to obtain the desired mineral composition.

 This gelation "in situ" thus makes it possible to obtain a polymeric matrix in which the precursor species of the desired mineral composition are found in a diluted and perfectly homogeneous distribution in space.

 This particular method of gelation "in situ" will now be developed further.

 For reasons of chemical homogeneity, it is preferred to operate on an initial mixture in the form of a solution.

 This initial solution, intended to be gelled, is generally aqueous; of course, it would be possible to use another solvent than water, an alcohol for example, provided that the precursor species defined below are then substantially soluble in this solvent.

 The chemical elements intended to constitute the final mineral powder, apart from oxygen, can then be introduced into the solvent phase in the form of an organic species or an inorganic species, hereinafter called precursor species.

 Among the organic species, mention may be made more particularly of the salts of acids, in particular of saturated aliphatic acids, for instance formates, acetates, propionates and citrates, as well as alkoxides, for example methoxides, ethoxides, propoxides or butoxides.

 Among the inorganic species, mention may be made of hydroxides and salts such as nitrates, carbonates, sulphates and halides (chlorides, bromides, iodides, fluorides).

 Preferably; salts are used.

 Nevertheless, care should be taken to avoid the use of inorganic salts which could leave, after subsequent thermal decomposition of the gel, undesirable compounds which could be difficult to eliminate later; Therefore, as preferred inorganic salts for carrying out the process according to the invention, mention may be made more particularly of nitrates and carbonates.

 Of course, the choice in the nature of the precursors used, as defined above or otherwise, will also be based on the actual possibilities of effectively resulting in a stable solution or a chemically homogeneous mixture.

 Thus, to increase the solubility and / or stability of certain precursors in the initial solution, it may eventually be necessary to add chemical compounds such as coupling or chelating agents well known to those skilled in the art.

 S-hydroxy acids, such as citric acid, glycolic acid or lactic acid, or else ethylenediaminetetraacetic acid (ED, TA) and the like can be cited as examples. acetylacetone.

 Of course, when the use of such compounds is necessary, it will be ensured here again that the selected compound on the one hand does not promote the formation of undesirable stable products with one or more of the precursors after the heat treatment, and on the other hand does not interfere with the polymerization "in situ" of the organic monomer or monomers, defined below, in the solution.

 Similarly, it may be necessary to adjust the pH of the initial solution to a value such that it is compatible with the practical conditions of polymerization of said organic monomers.

 It is finally recalled that, of course, the precursors defined above must be introduced into the initial solution in proportions such that they correspond to the stoichiometry of the desired mineral composition.

 It is generally carried out with solutions comprising between 1 and 10% by weight of dry extract.

 The initial stable solution of precursors being thus obtained, one adds one or more organic monomers soluble in said solution, and capable, by conventional polymerization and / or copolymerization and / or polycondensation mechanisms, of generating organic macromolecules which will constitute the gelatinous matrix trapping the various precursors of the desired mineral composition.

 This or these monomers can be introduced into the solution in solid form or in the form of a solution.

 According to a preferred embodiment of the process according to the invention, the starting monomer or monomers are chosen in such a way that, after reaction, they confer, at least partially, a three-dimensional (cross-linked) macromolecular structure on the polymeric matrix. obtained.

 For this purpose, it is possible to use either polymerizing agents which, taken alone, make it possible to lead directly to a three-dimensional macromolecular structure, or to polymerizing agents which, in combination with conventional crosslinking agents, make it possible to obtain this type of structure.

 The amount of gelling agents to be introduced into the solution is not critical; it must nevertheless be sufficient to allow.

obtaining a sufficiently strong gel.

 Of course, it is possible to add, with the polymerization and / or copolymerization and / or polycondensation and / or crosslinking agents intended to give rise to the gel according to the invention, other compounds which are well known in the art. skilled in this type of chemistry, such as initiators and reaction accelerators.

 Depending on the nature and quantity of the gelling agents introduced, and possibly the various additives, the gelation may be spread over a period ranging from a few minutes to several hours.

 According to a particularly preferred embodiment of the invention, the gel used for the preparation of the powders is a polyacrylamide gel or polymethacrylamide, but preferably polyacrylamide, partially crosslinked.

 In this case, and taking into account the above, this gel can then be obtained by polymerizing "in situ" acrylamide or methacrylamide, and in the presence of a crosslinking agent such as N, N ' ethylene diacrylamide, in an initial aqueous solution containing all the required precursors.

 More precisely, it is possible to polymerize acrylamide (respectively methacrylamide) by initiating the polymerization using a free radical generating substance, such as ammonium persulfate, and adding a transfer agent. from the initiator to the monomer (polymerization accelerator), such as, for example, N, N, N ', N'-tetramethylethylenediamine (TEMED).

 Amounts of acrylamide of between 3 and 10% by weight, based on the initial solution, and amounts of crosslinking agents of 3 to 10% by weight, based on acrylamide, are generally sufficient to obtain a suitable gel.

 Note that in the particular case tool exists in the initial solution one or more elements capable of forming one or more stable compounds with ammonium persulfate (case of an initial solution containing the barium element, the latter can be found after heat treatment in the form of very stable barium sulfate), it is preferable to replace said persulfate with another initiator which does not have this disadvantage, such as hydrogen peroxide.

 At the end of this polymerization / crosslinking of acrylamide, one.

obtain a composition in the form of a gel, and which consists essentially of a partially crosslinked polyacrylamide matrix in which are held homogeneously dispersed in space, the various precursors initially present in the starting solution.

Since the composition of matter, intended to constitute the precursor of the desired mineral powder, is thus obtained, it undergoes a heat treatment.
This heat treatment, which therefore constitutes the second step of the general process according to the invention, aims not only to dissolve the polymer matrix containing the previously defined organic and / or inorganic precursors, but also to decompose said precursors into their oxides. respective so that they react with each other to give the desired mineral composition.

 This heat treatment is carried out until the desired inorganic oxide compound is obtained.

 The temperature at which the thermal decomposition treatment is conducted is therefore a function of the nature of the mineral composition to be obtained.

 However, it is observed that in most cases, this temperature is between 650 and 9500 C, and even more generally between 700 and 800 C.

 The duration of this heat treatment may vary from a few minutes to several hours, depending on the progressivity of the rise in temperature, and in general, the higher the treatment temperature, the shorter this duration may be.

 It is conducted under an air atmosphere, or under an air / oxygen mixture, but preferably under air.

 During this heat treatment, the various precursors retain the diluted and homogeneous distribution in the space they had in the initial material composition.

 This may explain to a large extent the reason why, after the heat treatment, a powder is obtained which is both very fine and very homogeneous. It will be noted on the other hand, which may seem surprising, that the volume occupied by the powder at the end of treatment is substantially the same as the volume initially occupied by the composition of material before treatment.

 The powders obtained are characterized first of all by their extreme finesse; the average size of the particles determined by scanning electron microscopy is indeed between 0.1 and 0.5 microns.

 They are characterized by the fact that they are perfectly homogeneous and exactly adjusted to the stoichiometry of the desired mineral composition.

 The fine and homogeneous powders resulting from the process according to the invention can then be used for the preparation of dense bodies by using conventional sintering processes.

 In addition to the preparation of the powders as defined above, the polymeric dobby compositions according to the invention can potentially find other useful applications.

 By adjusting their viscosity and their precursor content to suitable values, it is indeed conceivable to shape them at will to obtain for example fibers (extrusion) or coating films.

 The pyrolysis of these may then lead respectively to mineral fibers or to pieces (ceramics, metals, composites, etc.) coated with a mineral film.

 Concrete examples illustrating the invention will now be given.

EXAMPLE i
This example is intended to illustrate the invention in the context of the preparation of a fine and homogeneous powder of alpha alumina (OC-A12 03)
63 g (0.3 mole) of citric acid monohydrate are dissolved in 0.5 liter of distilled water. To this solution is added 20.4 g (0.1 mole) of aluminum isopropoxide Al (OC 3 H 7) 3.

 The mixture is heated with stirring until the mixture becomes substantially clear. The mixture is filtered and the filtered solution is brought to 7.3 by addition of concentrated ammonia, and the solution is then added with distilled water to finally have 1 liter of precursor solution.

 In 100 ml of this precursor solution, 6.2 g of acrylamide, 0.25 g of N, N'-methylenediacrylamide (crosslinking silver), 0.1 ml of N, N, N ', N' tetramethylethylenedia are dissolved. ine (TEMED) (polymerization accelerator) and 20 mg of ammonium persulfate (polymerization initiator).

 The solution obtained is slightly degassed and then left to stand for a few hours, usually sufficient time to proceed with the gelation thereof.

 After gelation, the gel is transferred to an alumina crucible and heated under air at 8800 ° C. in 12 hours.

 0.5 g of very fine and chemically homogeneous powder of amorphous alumina is then obtained.

 The average particle size of this powder is 0.1 p.

 This amorphous powder is crystallized directly and completely in alumina by heat treatment at 10500 C for 24 hours.

EXAMPLE 2
This example is intended to illustrate the invention in the context of the preparation of a fine and homogeneous silica SiO2 powder.

 114 g (0.54 moles) of citric acid monohydrate are dissolved in 0.8 liters of distilled water.

 To this solution is added 28.4 g (0.136 mole) of Si (OC 2 H 5) 4 tetraethylorthosilicate, which is then heated until it becomes substantially clear. The mixture is filtered and the pH of the filtered solution is brought to 7 by addition of concentrated ammonia. Hydrolysis of the complex results in a sol at about 0.17 mole / liter of silica.

 100 ml of this precursor sol are taken, which is gelled with 5 g of acrylamide, 0.165 g of N, N'-methylenediacrylamide, 0.1 ml of TEMED and 0.02 g of ammonium persulfate.

 The gel obtained is transferred into an alumina crucible and then heated in air to 7000 C in 10 hours.

 A very fine and chemically homogeneous powder of silica SiO 2 is then obtained.

 The average particle size of this powder is 0.1 μm.

EXAMPLE 3
As in Example 2, this example is intended to illustrate the invention in the context of the preparation of a fine and homogeneous Six2 silica powder.

 42 g (0.14 mol) of ethylenediaminetetraacetic acid (EDTA) are suspended in 0.5 liter of distilled water. To this suspension is added, with stirring, concentrated ammonia dropwise until a clear solution is obtained.

14.5 g (0.07 mol) of tetraethylorthosilicate are then added
Si (oC2H5) 4, and the mixture is heated with stirring until a substantially clear solution is obtained.

 The mixture is filtered and the pH of the filtered solution is brought to 7.1 by addition of concentrated ammonia.

 A clear precursor solution is then obtained.

100 ml of this precursor solution are gelled by adding 6 g of acrylamide, 0.48 g of N, N'-methylene diacrylamide, 0.1 ml of
TEMED and 0.02 g of ammonium persulfate.

 The gel obtained is transferred into an alumina crucible and is then heated under air at 8000 C in 10 hours.

 Then a very fine and chemically homogeneous powder of SiO2 silica is obtained.

 The average particle size of this powder is 0.2 μm.

EXAMPLE 4
This example is intended to illustrate the invention in the context of the preparation of a fine and homogeneous powder of SiO 2 silica, but from an acid solution.

 126 g (0.6 mole) of citric acid monohydrate are dissolved in 0.8 liter of distilled water.

To this solution is added 28.7 g (0.137 mole) of tetraethylorthosilicate Si (OC 2 H 5) 4. The mixture is slowly heated with stirring until it becomes substantially clear. We filter and thus obtain a perfectly clear precursor solution
100 ml of this solution are gelled by adding 5 g of acrylamide, 0.25 g of N, N'-methylenediacrylamide, 0.1 ml of TEMED, 1 mg of ammonium persulfate and keeping in a water bath around 750 C for one hour.

 The gel obtained is transferred into an alumina crucible and then carried under air at 7000 C in 10 hours.

 A very fine and chemically homogeneous powder of silica SiO 2 is then obtained.

 The average particle size of this powder is 0.1 μm.

EXAMPLE 5
This example is intended to illustrate the invention in the context of the preparation of a fine and homogeneous powder of TiO 2 titanium oxide.

 23.9 g (0.084 mol) of titanium tetraisopropoxide Ti (oe3H7) 4 are added in 0.2 liter of a solution of distilled water containing 57.7 g (0.27 mol) of citric acid monohydrate. This mixture is then heated until a substantially clear solution is obtained.

The mixture is filtered and the pH of the filtered solution is brought to 7 by addition of concentrated ammonia.

 A stable and clear TiO 2 precursor solution is then obtained at approximately 0.3 mole / liter of TiO 2.

 100 ml of this precursor solution are gelled by adding 7 g of acrylamide, 0.4 g of N, N'-methylenediacrylamide, 0.1 ml of TEMED and 0.03 g of ammonium persulfate.

 The gel thus obtained is transferred into an alumina crucible and then, under air, at 7000 C in 10 hours.

A very fine and chemically homogeneous powder of crystallized titanium TiO 2 oxide, present in its rutile and anatase forms, is then obtained.
This powder has an average particle size of 0.2 μm.

EXAMPLE 6
This example is intended to illustrate the invention in the context of the preparation of a fine and homogeneous powder of yttria zirconia.

 32 g (0.15 mole) of citric acid monohydrate are dissolved in 0.5 liter of distilled water. 13.16 g of zirconium propoxide Zr (OC 3 H 7) 4 at 95 t, ie 0.038 mol, are added to this solution.

 The mixture is heated until a substantially clear solution is obtained.

 0.837 g (0.0022 mol) of yttrium nitrate Y (NO 3) 3-5.5 H 2 O and concentrated ammonia are filtered and filtered to bring the pH to 7.

 Thus, a precursor solution of zirconia at 3 mole of Y2O3, limpid and stable, is obtained at about 0.07 mole / liter.

 80 ml of this precursor solution are gelled by adding 4 g of acrylamide, 0.12 g of N, N'-methylenediacrylamide, 0.1 ml of TEMED and 0.04 g of ammonium persulfate.

 The gel obtained is transferred into an alumina crucible and then carried under air at 7000C in 10 hours.

 0.8 g of a very fine and chemically homogeneous powder of yttriated zirconia crystallized in its tetragonal form (stabilized zirconia) is then obtained.

 The average particle size of this powder is 0.1 μm.

EXAMPLE 7
This example is intended to illustrate the invention in the context of the preparation of a fine and homogeneous powder of barium titanate BaTiO3.

 25.2 g (0.12 mol) of citric acid monohydrate are dissolved in 0.4 liter of distilled water.

 To this solution is added 5.75 g (0.02 mole) Ti (OC 3 H 7) 4 titanium isopropoxide, and the mixture is then heated until a substantially clear solution is obtained. Filtered 3.98 g (0.02 mol) of barium carbonate are added to the filtered solution, followed by concentrated ammonia to bring its pH to 7, then complete with distilled water to finally obtain 0 , 5 liter of a precursor solution BaTiO3, stable and clear, about 0.04 mol / liter.

 In 100 ml of this precursor solution, 6 g of acrylamide, 0.3 g of N, N'-methylenediacrylamide, 0.1 ml of TEMED and 5 ml of 10-volume hydrogen peroxide are dissolved and the solution is maintained. obtained in a water bath at about 700 C for 1 to 2 hours, time generally sufficient to proceed to the gellation thereof.

 The gel thus obtained is transferred into an alumina crucible and then, under air, at 7000 C in 10 hours.

 0.9 g of a very fine and chemically homogeneous powder of barium titanate BaTiO 3 crystallized in a pseudo-cubic form are then obtained.

 The average particle size of this powder is 0.2 μm.

EXAMPLE 8
As Example 7, this example is intended to illustrate the invention in the context of the preparation of a fine and homogeneous powder of batik3 barium titanate.

 10.36 g (0.012 mol) of mixed citrate of barium and titanium, obtained according to the method described by GA Hutchins, GH Maher and SD Ross (AIL, Ceram Soc., Bull, 66 (4), 681-684, ( 1987)) are suspended in 0.25 liter of distilled water. Ammonia concentrate is slowly added to bring the pH of the mixture to 7.6.

 A precursor solution of BaTiO3, perfectly stable and clear, is thus obtained at about 0.04 mole / liter and in which the Ba / Ti ratio is exactly adjusted to 1.

 100 ml of this solution are treated under the same gelling and heat treatment conditions as Example 6.

 0.9 g of a very fine and chemically homogeneous powder of Batik3 crystallized in a pseudo-cubic form are thus obtained.

 The average particle size of this powder is 0.2 μm.

EXAMPLE 9
This example is intended to illustrate the invention in the context of the preparation of a fine and homogeneous powder of a mixed oxide of barium and copper of formula BaCuO 2.

 42 g (0.2 mol) of citric acid monohydrate are dissolved in 0.5 liter of distilled water. 9.868 g (0.05 mol) of barium carbonate and 9.983 g (0.05 mol) of copper acetate monohydrate Cu (CH 3 CO 2) 2, H 2 O are dissolved in this solution, and concentrated ammonia is finally added to the solution. bring its pH to 7.4.

 This gives a precursor solution of BaCuO2, stable and clear, 0.1 mol / liter.

 100 ml of this solution are gelled with 6 g of acrylamide, 0.5 g of N, N'-methylenediacrylamide, 0.1 ml of TEMED and 2 ml of 10 volumes hydrogen peroxide.

 The gel thus obtained is transferred into an alumina crucible and then heated, in air, to 8250 ° C. at 700 ° C. per hour.

 2.3 g of a very fine and chemically homogeneous powder of mixed copper and barium oxide of formula BaCuO 2 are obtained.

 The average particle size of this powder is 0.2 μm.

Claims (23)

1 / Process for the preparation of an oxide-type mineral powder  characterized in that it consists in thermally treating a  composition of matter comprising a polymeric matrix in  which are homogeneously dispersed, in the form of  organic and / or inorganic, identical or different, all  chemical elements, with the possible exception of oxygen,  necessary to obtain the desired mineral composition. 2 / A method of preparation according to claim 1 characterized in that  that said polymeric matrix is, at least partially,  abutment. 3 / A method of preparation according to one of claims 1 and 2 carac  characterized in that said polymeric matrix is a polyacrylamide or  polymethacrylamide. 4 / A method of preparation according to claim 3 characterized in that  that the polymeric matrix is a partially polyaorylamide  reticle. 5 / A method of preparation according to any one of the claims  previous ones characterize itself. what said organic species are  selected from o'old salts, in particular aliphatic acids  stues, and lcoxyfe ~ -. 6 / A method of preparation according to claim 5 characterized in that  that the acid salts are chosen from formates,  acetates, propionates and citrates, and selected alkoxides  among the methoxides, ethylene derivatives, propoxides and butovides. 7 / A method of preparation according to any one of the claims  characterized in that said inorganic species are  selected from hydroxides and salts. 8 / A method of preparation according to claim 7 characterized in that  inorganic species are salts chosen from  nitrates and carbonates. 9 / Preparation process according to any one of the claims  characterized in that said composition of matter is  obtained by a method of polymerizing and / or  copolymerizing and / or polycondensing one or more organic monomers  in a solution or a stable soil containing, in the  stoichiometric ratios and in the form of organic species  and / or inorganic, identical or different, all elements  chemicals, with the possible exception of oxygen, necessary for  obtaining the desired mineral composition. 10 / A method of preparation according to claim 9 characterized in that  that we use a solution. 11 / A method of preparation according to claim 10. characterized in that  that said solution is an aqueous solution. 12 / A method of preparation according to any one of claims 9 to  Characterized in that said organic species are selected  among the salts of acids, in particular of saturated aliphatic acids, and  alkoxides. 13 / A method of preparation according to claim 12 characterized in that  that the acid salts are chosen from formates,  acetates, propionates and citrates, and selected alkoxides  methoxides, ethoxides, propoxides and  butoxide. 14 / A method of preparation according to any one of claims 9 å  Characterized in that said inorganic species are selected  among hydroxides and salts.  1E / preparation process according to claim 14 characterized in that  that inorganic species are salts. 16 / A method of preparation according to claim 15 characterized in that  said salts are selected from nitrates and carbonates. 17 / A method of preparation according to any one of claims 9 &  Characterized in that, as a monomer,  acrylamide or methacrylamide.  18 / Preparation process according to claim 17 characterized in that  in addition, a crosslinking agent is used.  -19 / Preparation process according to claim 18 characterized in that  said crosslinking agent is N, N'-methylenediacrylamide. 20 / A method of preparation according to any one of claims 17  at 19 characterized in that in addition an initiator of  polymerization. 21 / A method of preparation according to claim 20 characterized in that  that ammonium persulfate or hydrogen peroxide is used.  22 / A method of preparation according to any one of claims 17  characterized in that an accelerator of  polymerization. 23 / A method of preparation according to claim 22 characterized in that  that N, N, N ', N'-tetramethylethylenediamine is used. 24 / A method of preparation according to any one of the claims  characterized in that said heat treatment is  conducted between 6500 C and 9500 C. 25 / A method of preparation according to claim 24 characterized in that  that said temperature is between 700 and 8000 C. 26 / Preparation process according to any one of the claims  characterized in that the heat treatment is  performed under air. 27 / oxide-type mineral powder characterized in that it is obtained  according to a method as defined in any one of  Claims 1 to 2.