WO1992008672A1

WO1992008672A1 - Method for producing a powder of mixed metal oxides

Info

Publication number: WO1992008672A1
Application number: PCT/EP1991/002065
Authority: WO
Inventors: Henri Wautier; Franz Legrand; Joël Bourjot
Original assignee: Solvay
Priority date: 1990-11-08
Filing date: 1991-10-29
Publication date: 1992-05-29
Also published as: PT99449A; JPH06501445A; BE1004604A4; AU8752391A; KR930702222A; CN1062122A; EP0556224A1

Abstract

A method for producing a powder of mixed metal oxides, wherein a metal hydroxide, a metal alcoholate and water are mixed in a common organic solvent containing a dissolved organic salt. The method is suitable for the production of barium titanate powders doped with metal oxides and intended for the manufacture of electronic components.

Description

Method for manufacturing a powder of mixed metal oxides.

The invention relates to a process for the production of mixed metal oxide powders.

It is known that it is possible to manufacture mixed metal oxide powders by hydrolysis of a metal alcoholate in the presence of a metal hydroxide, under certain conditions. To this end, in patent US-A-4636378 (Hughes Aircraft Company), a process for the manufacture of a barium titanate powder having the tetragonal crystal structure of perovite is described, according to which an alcoholate is gradually introduced of titanium in an aqueous barium hydroxide solution, so as to form a hydrated titanium oxide gel, the reaction medium is then heated beyond 100 ° C. in an autoclave maintained under pressure, then, after having cooled down to room temperature, it is successively subjected to moderate reheating at atmospheric pressure and to dialysis. This known method has the drawback of great complexity and requires expensive equipment. Furthermore, the powder obtained has a heterogeneous morphology, being formed of irregular grains and having an extended particle size. In document EP-A-297646 (SOLVAY & Cie), there is a process which avoids the aforementioned drawbacks. According to this process, the hydrolysis of the metal alcoholate is carried out in the presence of a metal hydroxide and an acidic organic compound, under conditions regulated to precipitate the mixed metal oxides in the form of a powder, without formation through a gel.

In the known processes described above, the metal alcoholates constitute expensive raw materials.

According to document JP-A-64003019 (NIPPON CEMENT KK), an alcoholic solution of a metal alcoholate and a aqueous solution of metal acetates, the precipitate which forms is dried, then subjected to calcination under an oxidizing atmosphere. In this known process, the constituents of the powder are precipitated in several successive stages: a first precipitation is the result of the hydrolysis of the metal alcohol which takes place when the solutions are mixed, while a second precipitation is the result of a decomposition of metal acetates, which occurs during the subsequent stages of drying and calcination. This operating process is unfavorable for obtaining a homogeneous powder.

The invention remedies this drawback of the process disclosed in document JP-A-64003019, by providing a new process which makes it possible to produce powders of mixed metal oxides which are homogeneous, by using organic metallic salts such as metallic acetates, for example.

The invention therefore relates to a method for manufacturing a powder of mixed metal oxides, according to which a metal hydroxide, a metal alcoholate and water are mixed in a common organic solvent; according to the invention, before incorporating the water into the mixture, an organic salt is dissolved in the solvent.

In the context of the invention, the term “mixed metal oxide powder” is intended to denote a powder which contains oxides of different metals. Mixed metal oxides are, by definition, solid solutions, that is to say homogeneous mixtures at molecular or ionic level.

In the process according to the invention, the metal alcoholate designates any compound in which a metal is linked via an oxygen atom, to a hydro-carbon group such as an aromatic group or an alipha group. linear or cyclic tick, saturated or unsaturated, unsubstituted or partially or totally substituted. Metal alcoholates with aliphatic groups are especially recommended; those with saturated or substituted aliphatic groups are preferred, such as, for example, methyl, ethyl groups, n-propyl, isopropyl, n-butyl and isobutyl. It is possible to use either an isolated metal alcoholate or a mixture of metal alcoholates.

Organic salt is, by definition, a metal salt of an organic acid. It must be soluble in organic solvents. For this purpose, it is recommended to select it from the salts of carboxylic acids containing less than ten carbon atoms. Metal acetates are preferred.

In the process according to the invention, the water must be used in an amount sufficient to cause hydrolysis of the metal alcoholate. The organic salt and the metal alcoholate must also be used in respective amounts corresponding to more than one mole of the alcoholate radical per mole of the acid radical of the organic salt. For example, respective quantities are chosen for which the number of moles of the alcoholate radical is between 3 and 40 per mole of the acid radical of the organic salt, the values between 5 and 20 being preferred.

In the process according to the invention, the metal alcoholate undergoes hydrolysis on contact with water. The hydrolysis must be regulated, in a manner known per se, so that the mixed metal oxides precipitate in the form of a powder, without gelling in mass of the reaction medium resulting from the hydrolysis. To this end, it is recommended that the mixture of metal alcoholate, metal hydroxide, water and organic salt in the common organic solvent be made homogeneous as quickly as possible, before nucleation begins. Various procedures are available for making the mixture. According to a first operating mode, the metal hydroxide, the metal alcoholate, the organic salt and the water are dissolved separately in organic solvents, and the organic solutions thus obtained are introduced simultaneously but separately into a reaction chamber. According to a second operating mode, two separate premixes are produced, one of which contains the metal hydroxide and the water, while the other contains the alcoholate. metal and organic salt, and the premixes are combined in the reaction chamber. In these operating methods of the process according to the invention, it is advisable to avoid the presence of solid particles in the organic solutions, before mixing. The same or different organic solvents can be used for the metal alcoholate, metal hydroxide, organic salt and water, but the organic solvent in which the alcoholate is dissolved must be free of water. In the case of different organic solvents, it is necessary that these are miscible to form the common organic solvent together. Alcohols and their derivatives are well suited, in particular methanol, ethanol, n-propanol, isopropanol, n-butanol and isobutanol. The optimum dilution rates of the metal alcoholate, the metal hydroxide, the organic salt and the water in their respective solvents depend on various factors, in particular the alcoholate and the organic salt used, the temperature of work and of the quality sought for the powder of mixed metal oxides; they must be determined in each particular case by routine laboratory work. For example, it is advantageously possible to use alcoholic solutions, the respective contents of metal alcoholate, metal hydroxide and organic salt, before mixing, do not exceed 5 moles per liter and are preferably between 0, 02 and 0.5 mole per liter. Mixing can be done in ambient air. However, to avoid carbonation of the metal hydroxide, it is advisable to use an atmosphere free of Cθ2 "Furthermore, to avoid the risk of an uncontrolled decomposition of the metal alcoholate, it is preferable to work in an atmosphere free of humidity. Dry, dehydrated and decarbo-iated air, nitrogen and argon constitute examples of atmospheres which can be used in the process according to the invention. To ensure mixing, it is advantageously possible to operate in the manner described in patent application GB-A-2168334.

In a particular embodiment of the method according to the invention, a metal hydroxide is used which is hydrated, the water of hydration of the metal hydroxide then constituting at least part of the water necessary for the hydrolysis reaction of the metal alcoholate. It is preferred to select a hydrated metal hydroxide whose hydration rate is sufficient to provide all of the water necessary for the hydrolysis of the metal alcoholate. This embodiment of the method according to the invention advantageously applies to the case where the metal hydroxide is barium hydroxide and / or hydrated strontium.

In another preferred embodiment of the process according to the invention, an acidic organic compound containing more than six carbon atoms in its molecule is dissolved in the common organic solvent. In this embodiment of the process according to the invention, the term “acid organic compound” is intended to denote an organic acid or a derivative of an organic acid. The organic acid derivative may have an acidic character or be devoid of an acidic character, being then, for example, a neutral body. Saturated or unsaturated carboxylic acids and their derivatives are especially recommended. Acids or derivatives of acids containing more than six carbon atoms in their molecule should be selected. Carboxylic acids which have been found to be especially advantageous are those containing at least eight carbon atoms in their molecule, such as octanoic, lauric, palmitic, isopalmitic, oleic and stearic acids. Carboxylic acids having more than ten carbon atoms in their molecule are preferred. Examples of organic acid derivatives which can be used in the process according to the invention are the anhydrides, the esters and the salts of these acids.

In this embodiment of the method according to the invention, the dissolution of the acidic organic compound in the common organic solvent must be complete and homogeneous before nucleation begins. For this purpose, it is recommended to introduce it into the mixing chamber simultaneously with the organic solutions of the other constituents of the mixture, or dissolve it in one of these solutions before introducing them into the mixing chamber.

It has been observed that the acidic organic compound acts on the morphology of the powder of mixed metal oxides, by inhibiting the agglomeration of the grains and by giving them a spherical profile. As a general rule, it must be used in sufficient quantity so that its action is manifested on the morphology of the powder, however avoiding exceeding a threshold beyond which its action on the quality of the powder could be negative. In practice, the optimum amount of acidic organic compound to be used depends on many parameters, among which are in particular the selected acidic organic compound (mainly the length of its carbon chain), the metal alcoholate, the metal hydroxide. and the organic salt used, as well as the operating conditions and it must be determined in each particular case as a function of the quality sought for the morphology of the powder. In general, it is recommended to use a mass of acidic organic compound of between 20 and 200 g per mole of all the elemental metal oxides forming the mixed metal oxides to be produced. The amounts between 50 and 150 g are preferred, in the case where the acidic organic compound is selected from carboxylic acids.

At the end of the hydrolysis reaction which follows the mixing, a powder of fine particles containing a complex combination of metal oxides in the amorphous state, more or less hydrated, and organic residues is collected. The powder is formed, essentially, of generally spherical particles, having a diameter not exceeding 5 microns, usually between 0.05 and 2 microns. The powder can optionally undergo drying, followed by a heat treatment at a suitable temperature to remove the excess water, the organic solvent and, where appropriate, the acidic organic compound. Heat treatment can be set to control the porosity or eliminate it completely. It can also be adjusted to cause crystallization of metal oxides. The process according to the invention is in particular suitable for the use of hydroxides of metals selected from barium, strontium, boron, selenium, tellurium and those of group la of the periodic table of elements such as sodium, potassium, rubidium and cesium. The metal alcoholate can be any metal alcoholate capable of being converted, by hydrolysis, into the corresponding metal oxide or hydroxide; by way of nonlimiting examples, the method according to the invention applies to the use of alcoholates of metals selected from Li, Na, K, Be, Mg, Ca, Sr, Ba, Ti, Zr, Nb , Ta, Mn, Fe, Co, Cu, Zn, Cd, Al, Ga, In, Si, Ge, Sn, Pb, As, Sb, Bi, Te, Y, La and rare earths such as Nd, Sm, Eu and Gd (Ferroelectronics, Vol. 49, 1983, pages 285-296: "Ultrafine

Electroceramic Powder Preparation from Métal Alkoxides "). The organic salt can be selected from the metal salts mentioned above for the metal alcoholate.

The method according to the invention is well suited to the production of mixed metal oxide powders, intended for the use of ceramic materials which, by definition, are non-metallic inorganic materials, the use of which starts from a powder requires high temperature treatments, such as melting or sintering treatments (P. William Lee - "Ceramics" - 1961 - Reinhold Publishing Corp. - page 1; Kirk Othmer Encyclopedia of Chemical Technology - Third edition - Volume 5 - 1979, John Uiley & Sons, USA - pages 234 to 236: "Ceramics, scope").

The mixed metal oxide powders obtained by the process according to the invention consist of spherical grains. They are characterized by a very small particle size distribution, an almost absence of agglomerates and a remarkable chemical homogeneity, greater than that of the powders obtained with the process described in document JP-A-64003019 (NIPPON CEMENT KK) examined above.

The method according to the invention finds an application interesting for the production of mixed metal oxide powders intended for the manufacture, by sintering, of electronic components, especially semiconductors, PTC or NTC thermistors and dielectrics used in the construction of capacitors. The method according to the invention thus finds an application for obtaining powders of barium titanate doped with metal oxides selected from calcium, magnesium, zirconium and silicon, and intended for the manufacture of dielectrics by an operation metallurgical sintering. Another application of the process according to the invention relates to obtaining powders of barium titanate doped with metal oxides selected from antimony, manganese, calcium, silicon, lead and strontium, and intended for the production of PTC thermistors by a metallurgical sintering operation.

The few examples whose description follows serve to illustrate the invention. These examples are given with reference to the appended FIGS. 1 to 6, which are six photographic reproductions of mixed metal oxide powders according to the invention, observed under an electron transmission microscope, at 20,000 × magnification.

First series of examples

Examples 1 to 4, the description of which follows relate to the preparation of powders specially intended for the manufacture of dielectric ceramics. In the execution of these examples, the procedure was as follows. We prepared separately. a solution comprising, per liter, 0.08 mole of barium hydroxide monohydrate in an organic solvent formed from a mixture of equal volumes of methanol and isopropanol,

. a solution comprising per liter, 0.08 mole of titanium isopropoxide and 0.05 mole of oleic acid, in an organic solvent formed from a mixture of equal volumes of methanol and isopropanol, • a solution comprising, by liter, 0.08 mole of zirconium n-propoxide in isopropanol. . a solution comprising, per liter, 0.02 mole of calcium acetate hydrated in methanol,. a solution comprising, per liter, 0.01 mole of magnesium acetate hydrated in an organic solvent formed from a mixture of equal volumes of methanol and isopropanol,

. a solution comprising, per liter, 0.02 mole of tetraethylorthosilicate in isopropanol

In a reaction chamber maintained under an atmosphere of anhydrous nitrogen, the solutions of titanium isopropoxide, calcium acetate and, if necessary, magnesium acetate and tetraethylorthosilicate were first introduced. The solution of zirconium n-propoxide and the solution of hydrated barium hydroxide were then introduced therein separately. The mixture of solutions was subjected to intense stirring to produce a homogeneous reaction mixture before nucleation began. The reaction medium was then subjected to a two hour maturing at 60 ° C., with moderate stirring. After ripening, the reaction medium was subjected to evaporation and a dry powder of mixed metal oxides was collected. The dry powder was then subjected to a calcination treatment which included a treatment under a humid nitrogen atmosphere at 500 ° C, followed by a treatment in dry air for two hours, at 860 ° C. At the end of the calcination treatment, the powder was collected and a measurement of its particle size distribution was carried out by means of a MALVERN measurement device implementing a measurement method based on the scattering of laser radiation. . On the cumulative particle size distribution curve, the following characteristic quantities were noted:. the diameter D (0.5), corresponding to 50% of the amount (expressed by volume) of particles; . the diameter D (0.9), corresponding to 90 X of the quantity

(expressed by volume) of particles; . the diameter D (0.1), corresponding to 10% of the amount (expressed by volume) of particles;

. the deviation from the mean E, defined by the relation D (0, 9) - D (0, 1)

E =

D (0.5) Example 1 The solutions of barium hydroxide, titanium isopropoxide, zirconium propoxide and calcium acetate were mixed in controlled proportions to obtain the product of molar formula

^Ba l, ll ^Ca 0, l ^Zr 0.22 ^τi 03.65 Figure 1 shows the powder obtained after the calcination treatment. The following particle size characteristics were noted: D (0.5) = 0.79 μm D (0.9) = 2.03 μm D (0.1) = 0.34 μm E = 2.2

Example 2

The solutions of barium hydroxide, titanium isopropoxide, zirconium propoxide and calcium acetate were mixed in controlled proportions to obtain the product of molar formula

^Ba l, 13 ^Ca 0, l ^Zr 0.22 ^τ iθ3.67 Figure 2 shows the powder obtained after the calcination treatment. Example 3

The solutions of barium hydroxide, titanium isopropoxide, zirconium propoxide, magnesium acetate and calcium acetate were mixed in controlled proportions to obtain the product of molar formula ^Ba 1.077 ^Ca 0.075 ^M g0.008 ^Zr 0.16 ^τi0 3.48

FIG. 3 shows the powder obtained at the end of the calcination treatment. The following particle size characteristics were noted:

D (0.5) = 0.58 μm D (0.9) = 1.49 μm

D (0.1) = 0.26 μm

E = 2.1 Example 4

The solutions of barium hydroxide, titanium isopropoxide, zirconium propoxide, ethylorthosilicate, calcium acetate and magnesium acetate were mixed in controlled proportions to obtain the product of molar formula

^Ba l, 066 ^C 0.114 ^M ε0.005 ^Zr 0.185 ^τi Si0.005 ° 3.565 Figure 4 shows the powder obtained after the calcination treatment. The following particle size characteristics were noted: D (0.5) = 0.82 μm D (0.9) = 3.79 μm D (0.1) = 0.30 μm E * = 4.2

Second series of examples. Examples 5 and 6 relate to the production of powders intended for the manufacture of PTC type thermistors.

Example 5

This example relates to obtaining a powder of general molar formula B 0.9 ^Ca 0, l ^τi l, 01 ^sb 0.0034 ^Mn 0.0008 ° 3.026

In a reaction chamber, we introduced

. 259.9 ml of a solution comprising, in a mixture of equal volumes of methanol and isopropanol, per liter of solution,

. 0.0619 mole of titanium isopropoxide,. 0.00614 mole of calcium acetate,

. 0.0310 mole of oleic acid,

. 3.08 ml of a solution comprising, per liter, 0.02 mole of antimony acetate in methanol,

. 1.45 ml of a solution comprising, per liter, 0.01 mole of manganese acetate in methanol.

The mixture of these solutions was homogenized, then 200 ml of a solution comprising, per liter, 0.0816 mole of barium hydroxide monohydrate in a mixture of equal volumes of methanol and d 'isopropanol. The mixture of solutions was subjected to intense stirring to produce a homogeneous reaction medium before nucleation began. We then subjected the reaction medium to two hours maturing at 70 ° C., with moderate stirring. After ripening, the reaction medium was subjected to evaporation and a dry powder of mixed metal oxides was collected. The dry powder was then subjected to a calcination treatment which included a gradual heating up to 560 ° C., in an atmosphere of wet nitrogen, for 3.5 hours, followed by a treatment at 860 ° C. in dry air. , during two hours. The powder collected at the end of the calcination treatment is visible in FIG. 5. It was subjected to a particle size measurement using the “MasterSizer” measuring device (Malvern Instruments Limited) used in the first series of examples . We have deduced the characteristic quantities D (0.5), D (0.9), D (0.1) and E defined above, with reference to the first series of examples: D (0.5) = 0.48 μm D (0.9) = 0.93 μm D (0.1) = 0.25 μm E 1.4

Example 6 This example relates to the manufacture of a powder of molar formula

Ba ₀ , ^Q Ca ₀ , lTii _f 02 ^sb 0.0034 ^Mn 0.0008 °, 046 A reaction chamber was introduced. 259.9 ml of a solution comprising, in a mixture of equal volumes of methanol and isopropanol, per liter of solution,. 0.0619 mole of titanium isopropoxide,. 0.00614 mole of calcium acetate,. 0.0310 mole of oleic acid,. 0.054 ml of titanium isopropoxide,. 3.08 ml of a solution comprising, per liter, 0.02 mole of antimony acetate in methanol,. 1.45 ml of a solution comprising, per liter, 0.01 mole of manganese acetate in methanol.

The mixture of these solutions was homogenized, then 200 ml of a solution comprising, per liter, 0.0816 mole of barium hydroxide monohydrate in a mixture of equal volumes of methanol and isopropanol. The mixture of solutions was subjected to intense stirring to produce a homogeneous reaction medium before nucleation began. The reaction medium was then subjected to two hours maturing at 70 ° C., with moderate stirring. After ripening, the reaction medium was subjected to evaporation and a dry powder of mixed metal oxides was collected which was subjected to a calcination treatment under the conditions set out above with reference to Example 5. Figure 6 shows the powder collected at the end of the calcination treatment. A granulometric measurement was carried out under the same conditions as in Example 5 and the following granulometric characteristics were deduced therefrom: D (0.5) = 0.49 μm D (0.9) = 0.95 μm D (0.1) = 0.25 μm E ≈ 1.4

Claims

R E V E N D I CA T I O N S

1 - Method for manufacturing a powder of mixed metal oxides, according to which a metal hydroxide, a metal alcoholate and water are mixed in a common organic solvent, characterized in that before incorporating the water into the mixture, an organic salt is dissolved in the solvent.

2 - Process according to claim 1, characterized in that the organic salt is selected from the salts of carboxylic acids containing less than 10 carbon atoms.

3 - Process according to claim 2, characterized in that the organic salt is selected from metal acetates.

4 - Process according to any one of claims 1 to 3, characterized in that the organic salt and the metal alcoholate are used in amounts corresponding to more than one mole of the alcoholate radical per mole of the acid radical of the organic salt .

5 - Process according to any one of claims 1 to 4, characterized in that the metal hydroxide and at least part of the water are used in the state of hydrated metal hydroxide.

6 - Method according to any one of claims 1 to 5, characterized in that the common organic solvent is an alcohol or a mixture of alcohols.

7 - Process according to any one of claims 1 to 6, characterized in that dissolved in the organic solvent an acidic organic compound containing more than six carbon atoms in its molecule. 8 - Process according to claim 7, characterized in that the acidic organic compound is selected from carboxylic acids containing at least 8 carbon atoms in the molecule and the derivatives of these acids and is used in an amount between 20 and 200 g per mole of all the elemental metal oxides of the mixed metal oxides to be produced.

9 - Process according to any one of claims 1 to 8, characterized in that the mixing is carried out so as to make it homogeneous before nucleation begins.

10 - Method according to any one of claims 1 to 9, characterized in. that the metal hydroxide is selected from barium and strontium hydroxides, the metal alcoholate is selected from titanium and zirconium alcoholates, and the organic salt is selected from organic salts of calcium, magnesium, manganese and antimony.