FR2628665A1 - Compsn. for prepn. of oxide type mineral powder - comprises polymeric matrix contg. dispersed (in)organic species - Google Patents

Abstract

Compsn. which can be used for prepg. an oxide type mineral powder comprises a polymeric matrix (I) in which are homogeneously dispersed, as opt. different (in)organic species (II), all the chemical elements (III), except opt. 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 organic species and hydroxides or salts for inorganic species. Prepn. of the compsn. is also claimed. USE/ADVANTAGE - For prepn. of a large number of (mixed) oxides in the fields of ceramics, glass, catalysts, electrochromes, superconductors, pigments and others, e.g. silica, silicates, alumina, aluminates, TiO2, titanates, zirconates, cuprates, etc.

Description

MATERIAL COMPOSITIONS LIKELY TO BE USED FOR THE
PREPARATION OF ULTRAFINE OXIDE-TYPE MINERAL POWDERS,
AND ONE OF THEIR PREPARATION PROCESS
The present invention relates to new compositions of matter capable of being used for the preparation of ultrafine and homogeneous powders of mineral oxides, mixed or simple, as well as to a process for their preparation.

 It also relates, by way of application, to the preparation of said mineral oxides by heat treatment of said matter compositions.

 It will already be noted, as will become clear from reading the following description, that the present invention is not limited to obtaining a particular mineral composition; on the contrary it must be considered as being of general scope, it is è. Dre as puvant applies to the preparation of a large number of oxides and mixed oxides in fields as varied as ceramics, glasses, catalysts, electrochromes, pigments and others.

 By way of nonlimiting examples of mineral compositions which can be obtained directly in the form of ultrafine and homogeneous powders according to the invention, mention may already be made, taken in salt or in admixture, of silica, silicates, alumina, alumina, titanium dioxide, titanates (for example barium or strontium), zirconia, zirconia stabilized (for example by yttrium, calcium or magnesium), zirconates (for example of barium or stontlum), cuprates, as well as all other mixed oxides such as, for example, those encountered in the field of spraziconductor ceramics reelarge of rare earth oxide, alkaline earth metal and transition metals).

 It has therefore now been found that it is possible to obtain ultrafine and homogeneous powders of mineral oxides, mixed or simple, by means of a preparation process consisting in first of all preparing a composition of matter in accordance with the invention which comprises a polymeric matrix in which are homogeneously dispersed in the form of organic and / or inorganic species, identical or different, all the chemical elements, except possibly oxygen, necessary for obtaining the desired mineral composition, then & submit said material composition has a heat treatment.

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

 The first step of the process therefore involves the preparation of a composition of matter as defined above and constituting a first object of the present invention.

 This composition of matter constitutes in a way the precursor of ultrafine powders which will be characterized later.

 The preparation of this composition of matter by simple, effective and easy to use means constitutes 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 stoichiometric ratios and in the form of organic and / or inorganic species; identical or different, all the chemical elements, with the exception of the possibility of oxygen, necessary for obtaining the desired mineral composition, and this by formation ";; situ" of an organic macromolecular matrix originating 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 "fIge ':" in space a stable or homogeneous solution or soil, containing all the precursors required to obtain the desired mineral composition.

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

 This particular "in situ" gelling process will now be further developed.

 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, a solvent other than water, an alcohol for example, could be used, 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, there may be mentioned more particularly the salts of acids, in particular of saturated aliphatic acids such as for example formates, acetates, propionates and citrates, as well as alkoxides, such as for example methoxides, ethoxides , propoxides or butoxides.

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

 Preferably, salts are used.

 Care should be taken to avoid the use of inorganic salts which would be liable to leave, after subsequent thermal decomposition of the gel, undesirable compounds which it might be difficult to remove thereafter; this is why, & As preferred inorganic salts for carrying out the process according to the invention, there may be mentioned more particularly nitrates and carbonates.

 Of course. the choice in the nature of the precursors used, as defined = 1 above or others, will also be made according to the possibilities actually existing to effectively end up with a stable solution or a chemically homogeneous mixture.

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

 examples of α-hydroxy acids, in particular citric acid, glycolic acid or lactic acid, or alternatively ethylenediaminetetraacetic acid (E.D.T.A.) and acetylacetone, may thus be cited as examples.

 Of course. when the use of such compounds proves necessary, it will also be taken care that the chosen compound on the one hand does not favor the formation of undesirable stable products with one or more of the precursors after the heat treatment, and on the other hand part does not interfere with the "in situ" polymerization of the organic monomer (s), defined below, in the solution.

 Likewise, it may be necessary to adjust the pH of the initial solution to a value such that it is compatible with the practical conditions for 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.

 We generally operate with solutions comprising between 1 and 10% by weight of dry extract.

 The initial stable solution of precursors being thus obtained, there is added one or more organic monomers soluble in said solution, and capable, by conventional mechanisms of polymerization and / or copolymerization and / or polycondensation, 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 (s) are chosen so that, after reaction, they confer, at least partially, a three-dimensional (crosslinked) macromolecular structure & the polymer 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 polymerizing agents which, in combination with conventional crosslinking agents, make it possible to achieve 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 a sufficiently solid gel to be obtained.

 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 well known to man. art in this type of chemistry, such as initiators and accelerators of reactions.

 Depending on the nature and quantity of the gelling agents introduced, and possibly of the various additives, the gelling can 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 or polymethacrylamide, but preferably polyacrylamide, partially crosslinked gel.

 In this case, and taking into account the above, this gel can then be obtained by polymerizing "in situ" 1 acrylamide, respectively methacrylamide, and in the presence of a crosslinking agent such as for example N, N ' methylenedediacrylamide, in an initial aqueous solution containing all the required precursors.

 Even more precisely, it is possible to polymerize acrylamide (respectively methacrylamide) by initiating polymerization using a substance which generates free radicals, such as ammonium persulfate, and by adding a transfer agent. from the initiator to the monomer (polymerization accelerator), such as, for example, N, N, N ', N'-tetratnethylethylénédiamine (TEMED).

 Amounts of acrylamide between 3% and 10% by weight relative to the initial solution and amounts of crosslinking agents between 3 and 1% by weight relative to the acrylamide are generally sufficient to obtain a suitable gel.

 It will be noted that in the particular case where there 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 may 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 drawback, such as for example hydrogen peroxide.

 At the end of this polymerization / crosslinking of acrylamide, a coFcsitisn is obtained which is in the form of a gel, and which essentially consists of a matrix of partially crosslinked polyacrylamide in which are maintained. homogeneously dispersed in space, the various precursors initially present in the starting solution.

The composite of material, intended to constitute the precursor of the desired mineral powder, being thus obtained, it then undergoes a heat treatment
The purpose of this heat treatment is not only to decompose the polymer matrix containing the organic and / or inorganic precursors defined above, but also to decompose said precursors in their respective oxides so that the latter react with one another to give the desired mineral composition .

 This heat treatment is carried out until the desired mineral oxide compound has been obtained.

 The temperature at which the thermal decomposition treatment is carried out therefore depends on the nature of the mineral composition to be obtained.

 However, we observe that in most cases, this temperature is between 650 and 9500 C, and even more generally between 7000 and 8000C.

 The duration of this heat treatment can vary from a few minutes to several hours, depending on the progressiveness of the temperature rise, and in general, the higher the treatment temperature and the more this duration can be reduced.

 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 distribution. dIluated and homogeneous in the space they had in the composition of initial matter.

 Ce-I can largely explain the reason why at the end of the heat treatment, a powder is obtained which is both very fine and very homogeneous. It will also be noted, which may seem surprising, that the volume occupied by the powder at the end of treatment is roughly the same as the volume initially occupied by the composition of matter before treatment.

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

 They are further 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 implementing conventional sintering methods.

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

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

coating films.

 The pyrolysis of the latter can then lead respectively to mineral fibers or to parts (ceramics, metals, composites, etc.) coated with a mineral film.

 Concrete examples illustrating the invention will now be given.

EXAMPLE 1
The purpose of this example is to illustrate the invention - in the context of the preparation of a fine and homogeneous powder of alpha alumina (<x-Al2 03 J.

 e ^ - g -'C, 3 ncle) of citric azide monohydrate are dissolved in 0.5 liters of distilled water. 20.4 g (0.1 mol) of aluminum isopropoxide Al (OC3H7) 3 are added to this solution.

 The mixture is heated with stirring until the mixture becomes practically lipid. Filtered and the pH of the filtered solution is brought to 7.3 by adding concentrated ammonia, then the solution is completed 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 agent), 0.1 ml of N, N, N ', N' tetramethylethylenediamine are dissolved. (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, a time generally sufficient to gel it.

 After gelation, the gel is transferred to an alumina crucible and brought, under air, to 880 ° C. in 12 h.

 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 jin.

 This amorphous powder is crystallized directly and completely from alumina - Far heat treatment at 10500 C for 24 hours.

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

 114 g (0.14 mole) of citric acid monohydrate are dissolved in 0.8 liters of distilled water.

 28.4 g (0; 136 mole) of tetraethylorthosilicate Si (oC2H5) 4 are added to this solution, then this mixture is heated until it becomes practically clear. The mixture is filtered and the pH of the filtered solution is brought to 7 by adding concentrated ammonia. There is hydrolysis of the complex and a sol at about 0.17 mole / liter of silica is then obtained.

 100 ml of this precursor sol are taken and 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 to an alumina crucible and then heated in air to 7000 C in 10 hours.

 This gives a very fine and uniformly homogeneous powder of silica Six2.

 The average particle size of this powder is 0.1 μm.

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

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

 Then added 14.5 g (0.07 mole) of tetraethylorthosilicate Si (oC2H5) 4, and the mixture is heated with stirring until a practically clear solution is obtained.

 It is filtered and then the pH of the filtered solution is brought to 7.1 by adding 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 '- methylenediacrylamide 0.1 ml of
TEMED and 0.02 g of ammonium persulfate.

 The gel obtained is transferred to an alumina crucible and then brought to 8000 C in air in 10 hours.

 A very fine and chemically homogeneous powder of silica SiO2 is then obtained.

The average particle size of this powder is 0.2 μm,
EXAMPLE 4
Like Examples 2 and 3, the purpose of this example is to illustrate the invention in the context of the preparation of a fine and homogeneous powder of silica SiO2, but starting from an acid solution.

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

 28.7 g (0.137 mol) of tetraethylorthosilicate Si (OC2H5) 4 are added to this solution. The mixture is heated slowly with stirring until it becomes practically clear. It is filtered and a perfectly clear precursor solution is thus obtained.

 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 maintaining in a water bath 750 C for one hour.

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

 This gives a very fine and chemically homogeneous powder of silica SiO2.

 The average particle size of this powder is 0.1 m.

EXAMPLE 5
The purpose of this example is to illustrate the invention in the context of the preparation of a fine and homogeneous powder of titanium oxide TiO.

 23.9 g (0.084 mole) of titanium tetraisopropoxide Ti (OC3H7) 4 are added to G, 2 liters of a solution of distilled water containing 57.7 g (0.27 mole) of -itric acid monohydrate. This mixture is then heated until a practically clear solution is obtained.

Filtered and o .. brings the pH of the filtered solution & 7 by adding concentrated ammonia.

 One then obtains a precursor solution of TiO2, stable and clear, at about 0.3 mole, 'liter of TiO2.

 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 to an alumina crucible and then brought, under r, to 7000 C in 10 hours.

 We get. then a very fine and chemically homogeneous powder of crystallized titanium oxide Ti02, present in its rutile and anatase forms.

 This powder has an average particle size of 0.2 μm.

EXAMPLE 6
The purpose of this example is 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 liters of distilled water. To this solution is added 13.6 g of zirconium propoxide Zr (CC3H7) 95%, scit C, 938 mole.

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

 Filtered and added to the filtered solution 0.837 g (0.0022 mole) of yttrium nitrate Y (N03) 3-5.5 H2O, and concentrated ammonia to bring its pH to 7.

 One thus obtains a zirconia precursor solution at 3 mol% of y2o3, limpid and stable, at about 0.07 mol / liter.

 80 ml of this precursor solution are gelled by addition of 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 to an alumina crucible and then brought to 7000C in air in 10 hours.

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

 The average particle size of this powder is 0.1 μm.

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

 25.9 g (0.12 mole) of citric acid monohydrate are dissolved in 0.4 liters of distilled water.

 To this solution is added 5.75 g (0.02 mole) of titanium isopropoxide Ti (OC3H7) 4, then the mixture is heated until a practically clear solution is obtained. It is filtered and the resulting solution is 3.98 g (0.02 mole) of barium carbonate, then concentrated ammonia to bring its pH to 7, then it is completed with distilled water to finally obtain 0 , 5 liters of a precursor solution of BatiO3, stable and clear, at 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 hydrogen peroxide at 10 volumes are dissolved, and the solution obtained is maintained. in a water bath at about 700 C for 1 to 2 hours, time generally sufficient to gel it.

 The gel obtained is transferred to an alumina crucible and then brought to 700 C in air in 10 hours.

 0.9 g of a very fine and chemically homogeneous powder of barium titanate BaTiC3 crystallized in a pseudo-cubic form is then obtained.

 The average particle size of this powder is 0.2 μm.

EXAMPLE 8
Sum of Example 7, this example aims to illustrate the invention in the context of the preparation of a fine and homogeneous powder of barium titanate BaTiC
10.36 g (0.012 mole) of mixed barium and titanium citrate, obtained according to the method described by GA Hutchins, G. Hlaher and SD Ross (Am. Ceram. Soc. Bulb, 66 (4), 681-684, (1987)), are suspended in 0.25 liter of distilled water. Concentrated ammonia is added slowly to bring the pH of the mixture to 7.6.

 This gives a precursor solution of BaTiO3, perfectly stable and clear, & about 0.04 mole / liter and in which the Ba / Ti ratio is exactly adjusted to 4.

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

 0.9 g of a very fine, chemically homogeneous powder of BaT 103, crystallized in a pseudo-cubic form, is thus obtained.

 The average particle size of this powder is 0.2 μm.

EXAMPLE 9
The purpose of this example is to illustrate the invention in the context of the preparation of a fine and homogeneous powder of a mixed barium and copper oxide of formula BacuO2.

 42 g (0.2 mole) of citric acid monohydrate are dissolved in 0.5 liters of distilled water. Was dissolved in this solution 9.868 g (0.05 mole) of barium carbonate then 9.983 g (0.05 mole) of copper acetate monohydrate Cu (CH3CO2) 2, H2O, and finally added concentrated ammonia for bring its pH to 7.4.

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

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

 The gel thus obtained is transferred to an alumina crucible and then brought, under alr, to 8250 C at the rate of 700 C per hour.

 2.3 g of a very fine and chemically homogeneous powder of mixed copper and barium oxide of formula BaCuO2 were obtained.

 The average particle size of this powder is 0.2 µm.

Claims (16)

1 / Composition of material capable of being used for the  preparation of an oxide mineral powder, characterized in  that it comprises a polymer matrix in which are  homogeneously dispersed, in the form of organic species and / or.  inorganic, identical or different, all elements  chemicals, and possibly oxygen, necessary for  obtaining the desired mineral composition. 2 / composition of matter according to claim 1 characterized in that  that said polymer matrix is, at least partially,  crosslinked. 3 / Composition of matter according to any one of claims 1  and 2 characterized in that said polymer matrix is a  polyacrylamide su a polymethacrylamide.  At ~ ompositic .. de * atLère according to claim 3 characterized in  that the polymer matrix is a partially polyacrylamid  rti-llé.  5 / Composition of matter according to any one of the claims  previous characterized in that said organic species are  chosen from acid salts, especially aliphatic acids  saturated, and alkoxides. 6 / Composition of the natter cake according to claim 5 characterized in that  that the acid salts are chosen from formates,  acetates, propionates and citrates, and selected alkoxides  among methoxides, ethoxides, propoxides and  butoxides. 7 / Composition of matter according to any one of claims  previous characterized in that said inorganic species  are chosen from hydroxides and salts.  8i composition of matter according to claim 7 characterized in that  that the inorganic species are salts chosen from  strata and carbonates. 9 / Process for preparing a composition of matter such as  in particular defined in any one of claims 1 to 8,  characterized in that it consists in polymerizing and / or  copolymerize and / or polycondense one or more monomers  in a stable solution or soil containing, in the  stoichiometric reports and in the form of organic species  and / or inorganic, identical or different, all the elements  except chemicals. possibly oxygen, necessary to  obtaining the desired mineral composition. 10 / A method according to claim 9 characterized in that one uses  a solution. 11 / A method according to claim 10 characterized in that said  solution is an aqueous solution. 12 / A method according to any one of claims 9 to he characterized  in that said organic species are chosen from salts  acids, especially saturated aliphatic acids, and alkoxides.  13 / A method according to claim 12 characterized in that the salts  the acids are chosen from formates, acetates,  propionates and citrates, and alkoxides selected from  methoxides, ethoxides, propoxides and butoxides. 14 / A method according to any one of claims-9 to 13 characterized  in that said inorganic species are chosen from  hydroxides and salts. 15 / A method according to claim 14 characterized in that the species  inorganic are salts.  16 / A method according to claim 15 characterized in that said salts  are chosen from nitrates and carbonates.  17 / A method according to any one of claims 9 & 16 characterized  what we use as a monomer, acrylamide or  methacrylanide. 18 / A method according to claim 17 characterized in that one uses  further a crosslinking agent.  19 / A method according to claim 1 characterized in that said agent  is N, N'-methylenediacrylamide. 20 / Method according to any one of claims 17 & 49  laughed in that in addition a polymerization initiator is used. 21 / A method according to claim 20 characterized in that one uses  ammonium persulfate or hydrogen peroxide.  22 / A method according to any one of claims 17 to 21  characterized in that an accelerator is also used  polymerization. 23 / A method according to claim 22 characterized in that one uses  N, N, N ', N'-tetramethylethylenediamine. 24 / Composition of matter characterized in that it is obtained by the  implementation of a method as defined in any one of  claims 9 to 23.