IE910806A1

IE910806A1 - Process for the preparation of a titanate of a divalent or¹trivalent cation

Info

Publication number: IE910806A1
Application number: IE80691A
Authority: IE
Original assignee: Rhone Poulenc Chimie
Priority date: 1990-03-14
Filing date: 1991-03-11
Publication date: 1991-09-25
Also published as: FR2659641B1; CA2038200A1; EP0448441A1; FR2659641A1; JPH085667B2; JPH04238814A; PT97024A

Abstract

This process is characterised in that at least one salt or hydroxide of the said cation is reacted in basic medium with a sol of titanium oxide of anatase structure obtained by a process comprising a thermohydrolysis. The product obtained is in the form of monodisperse particles with a mean size not exceeding 0.5 mu m.

Description

PROCESS FOR THE PREPARATION OF A TITANATE OF A DIVALENT OR TRIVALENT CATION The present invention relates to a process for the preparation of a titanate of a divalent of trivalent cation.

It relates more particularly to the preparation of alkaline earth metal titanates such as, for example, barium titanate.

These titanates are used in electronic applications, especially for the manufacture of capacitors or resistors .

For this use, these products must have in par15 ticular a high purity, a good sinterability and a small and monodisperse particle size. If these conditions are not satisfied simultaneously, the products do not have good dielectric properties, in particular, and are consequently unsuitable for the afore-mentioned appli20 cations.

Several processes for the preparation of titanates have already been proposed.

The conventional procedure is that of chamotte burning, i.e. reacting powders of TiO2 and, for example, a barium salt with one another at high temperature.

However, such a process is unsatisfactory because the products obtained have a large size and a very wide particle size distribution due to the formation of aggregates which are difficult to grind down.

Moreover, high-efficiency grinding is also a source of impurities which can detract from the dielectric properties of the material.

Furthermore, because of the poor initial chemical homogeneity of the starting materials, it is difficult to have precise control over the stoichiometry, resulting in the presence of spurious phases. This leads especially to high sintering temperatures and poor control over the size of the titanate particles.

Other methods have been proposed, in particular the hydrothermal method. In this case, a hydrated oxide gel is used as the titanium precursor. However, the product obtained again has an irregular morphology and a wide particle size distribution.

An improvement in the particle size distribution and better control over the final size of the product can nevertheless be achieved provided that the synthesis is carried out with very precise control over the rate of introduction of the cation, for example the barium, into the titanium precursor. However, this is a process which is difficult to carry out and which is restricted to relatively low concentrations.

A first subject of the invention is consequently a process for the preparation of a titanate which makes it possible to obtain a product containing par3 tides whose size is small, especially submicron, and monodisperse.

A second subject of the invention is a process which is simple to carry out and which, in particular, makes it possible to work with a high concentration of reactants.

With this in mind, the process according to the invention for the preparation of a titanate of at least one divalent or trivalent cation is characterized in that at least one salt or hydroxide of said cation is reacted in a basic medium with a sol of titanium oxide of anatase structure, obtained by a process comprising thermohydrolysis.

The process of the invention makes it possible especially to obtain very fine powders, i.e. powders with a size of at most 0.5 μτα. These powders consist of generally spherical particles with a narrow size distribution, for example φ 75 / φ 25 < 1.5, which are non-porous and readily dispersible.

Other characteristics, details and advantages of the invention will become clearer from the description and non-limiting Examples which follow.

It will be noted first of all that the invention applies to the preparation of a titanate of at least one divalent or trivalent cation, i.e. a titanate which can contain one or more divalent or trivalent cations in its formula, such as, for example, a mixed barium and strontium titanate. Consequently, through4 out the remainder of the description, all statements relating to a divalent or trivalent cation are to be understood as being applicable to several cations.

As far as divalent cations are concerned, there 5 may be mentioned lead and those of the alkaline earth group. Among the latter, barium and strontium may be mentioned more particularly.

Examples of trivalent cations which may be considered are those of the rare earth group, such as yttrium, lanthanum and the elements of the lanthanum series, such as praseodymium and neodymium. Bismuth may also be mentioned as a trivalent cation.

The divalent or trivalent cations can firstly be used in the form of salts.

These can be inorganic salts such as chlorides and nitrates.

It is also possible to use organic salts such as acetates, citrates, oxalates and tartrates.

The divalent or trivalent cations are also employed in the form of hydroxides.

According to one of the main characteristics of the invention, the divalent or trivalent cation reacts with a sol of titanium oxide of anatase structure, obtained by a process comprising thermohydrolysis.

A sol is understood here as meaning a colloidal dispersion of titanium oxide particles of 10 to 200 nm in diameter, it being possible for these particles themselves to consist of crystallites of about 5 to 7 nm.

Furthermore, the titanium oxide must be in essentially anatase form.

In addition, the titanium sol must have been 5 obtained by a process which comprises thermohydrolysis.

Thermohydrolysis is understood as meaning the operation which consists in heating a titanium salt in solution. It will be noted here that it is quite possible to use a sol obtained by a more complex process, i.e. a process comprising other steps, for example post-treatments of the sol obtained, in addition to thermohydrolysis.

Moreover, it is preferable to use a sol of titanium oxide which has not been calcined.

It is also advantageous to use a titanium oxide which is free of sulphur.

Finally, it is preferred to use an aqueous sol of titanium oxide.

According to a particular variant of the in20 vention, a sol of the above type can be obtained by the thermohydrolysis of a titanium compound in a specific medium. This compound will hereafter be called compound A.

Generally, compound A is selected from titanium halides, oxyhalides, nitrates and alkoxides.

The reaction medium in which the thermohydrolysis takes place is characterized in that it contains at least one compound (compound B) selected from: (i) acids which contain: - either one carboxyl group and at least two hydroxyl and/or amine groups, - or at least two carboxyl groups and at least one 5 hydroxyl and/or amine group; and (ii) salts of the acids mentioned under (i). This process for the preparation of the titanium sol will be described in greater detail below.

The first step of this process therefore involves firstly the preparation of a solution containing at least one compound A and at least one compound B, as defined above.

This initial solution, which is to be hydrolyzed, is preferably totally aqueous; if appropriate, another solvent, for example an alcohol, may be added provided, of course, that compounds A and B used are then substantially soluble in this mixture.

Also, it is preferable for titanium compound A to be free of sulphur, which in this case excludes the use of salts of the titanium sulphate or oxysulphate type.

It will be preferred to use titanium compounds of the titanium halide or oxyhalide type. The titanium halides or oxyhalides which can be used more particu25 larly are titanium fluorides, chlorides, bromides and iodides and, respectively, oxyfluorides, oxychlorides, oxybromides and oxyiodides.

It is very particularly possible to use titaIE 91806 nium oxychloride, TiOCl2.

According to the process, the initial solution must also contain at least one compound B as defined above, i.e. a compound B suitably selected from the general class of the hydroxy- and/or amino-carboxylic acids .

The following may be mentioned especially as non-limiting examples of compounds B falling within the framework of the present inventions - hydroxypolycarboxylic acids and more particularly hydroxydicarboxylic or hydroxytricarboxylic acids such as, for example, malic acid, citric acid and tartronic acid, - (polyhydroxy)monocarboxylic acids such as, for example, glucoheptonic acid and gluconic acid, - poly(hydroxycarboxylic) acids such as, for example, tartaric acid, - aminodicarboxylic acids and their corresponding amides, such as, for example, aspartic acid, asparagine and glutamic acid, and - hydroxylated or non-hydroxylated aminomonocarboxylic acids such as, for example, lysine, serine and threonine.

As already indicated, it is also possible to use any salts of the afore-mentioned acids as compounds B.

Preferably, these salts will be either alkali metal salts, more particularly sodium salts, or ammoilium salts.

Preferably, compounds B, especially as defined above, will be hydrocarbon compounds of the aliphatic type.

Finally, the length of the main hydrocarbon chain will not exceed preferably 15 carbon atoms and more preferably 10 carbon atoms.

The amount of titanium compound present in the solution to be hydrolyzed is generally such that the molar titanium concentration of said solution is between about 0.1 mol/litre and 1.5 mol/litre.

Titanium concentrations of less than 0.1 mol/ litre simply detract from the economy and viability of the process.

Titanium concentrations of more than 1.5 mol/ litre can detract from the yield of the hydrolysis reaction.

For titanium concentrations of about 1.5 mol/ litre or above, it can be advantageous to add aqueous ammonia, NH4OH, to the solution in a molar ratio [NH3]/[Ti] which preferably does not exceed 1.5, for the purpose of increasing the yield and/or the kinetics of the hydrolysis reaction.

The concentration of compound B in the initial solution can be between 0.002 mol/litre and 0.5 mol/ litre. In general, it is found that relatively low concentrations of compound B, i.e. concentrations of between 0.002 mol/litre and 0.1 mol/litre, are suf9 ficient to achieve the desired result.

More particularly, the conditions can be such that the molar ratio B/Ti is at least 1.5% and preferably at least 2%.

Finally, it will be noted that, according to a particular embodiment of the invention, the initial solution can also contain seeds of anatase , if appropriate. The amount of seeds can vary between 0.1 and 2% by weight, relative to the total TiO2. The presence of these seeds makes it possible to accelerate the hydrolysis rate and to have better control over the size of the sol particles.

The initial solution obtained in this way is then hydrolyzed.

This hydrolysis is preferably carried out at a temperature greater than or equal to 60eC. Lower temperatures can of course be used, but in this case the hydrolysis reaction is much longer, which obviously detracts from the economy of the process.

When the reaction is complete, the solid formed is recovered, especially by filtration.

The solid recovered in this way can then be washed, for example with water, in order to remove any remaining impurities, and subsequently dried. Σ-ray diffraction analysis shows that the product thus obtained is titanium oxide, TiO2, present essentially or solely in its anatase crystalline form.

ItZ As seen above, the process of the Invention comprises a reaction of a divalent or trivalent cation with a sol of the type described previously or of the type obtained by the process which has just been studied above.

This reaction generally takes place at a temperature of at least 50eC and more particularly of at least 70’C.

A temperature of 200’C is not normally excee10 ded.

Thus it is possible to carry out the reaction at between 70 and 180eC, for example, and more particularly at between 70 and 12O’C.

For temperatures above 100C, it may be neces15 sary to carry out the reaction in an autoclave.

It will be noted here that one of the advantages of the process of the invention is that the operating temperatures are relatively low.

Furthermore, the reaction is carried out in a 20 basic or even very basic medium.

Thus the reaction medium must be at a pH of at least 10, preferably of at least 12 and more particularly of at least 13.5.

If necessary, to obtain the desired pH values, 25 a mineral or organic base, such as alkali metal or alkaline earth metal hydroxides like NaOH or KOH, quaternary ammonium salts or hydrates, or amines, can be added to the reaction medium.

Furthermore, the reaction can be carried out in a medium having a titanium concentration of between 0.1 and 1 mol of titanium per kg of reaction medium.

The concentration of divalent or trivalent 5 cation in this same medium can be between 0.2 and 2 mol per kg of reaction medium.

It will be noted that these concentrations of reactant are high compared with those used in the prior art.

Finally, it is preferably to work in an inert atmosphere free of C02, for example under nitrogen or argon.

When the process is carried out, the reactants are introduced in any order and in any manner. The mixture thus obtained is then heated to the necessary temperature. The mixture is generally agitated.

The mixture is then kept at said temperature for a period of between about 10 and 240 min.

When the reaction is complete, a powder is obtained which is filtered off and, if necessary, washed, for example with a buffered solution at pH 4.8, and then finally dried at a moderate temperature, for example at a temperature between room temperature and 50°C, or by a process of the atomization type.

In general, the process of the invention makes it possible to obtain a product consisting of generally spherical particles with a size of at most 0.5 μτα, in particular equal to or less than 0.15 μία and possibly as small as 0.025 pm.

These particles have a very narrow size distribution, for example φ 75 / φ 25 < 1.5 and more particularly < 1.3.

Their specific surface area measured by the BET method varies between 1 and 25 m2/g.

These particles have little or no porosity.

They are very readily dispersible. Their degree of crystallization is greater than 95%.

The titanate powders obtained by the process of the invention are highly suitable for sintering (at least 95% of the theoretical density above 1200°C) without grinding or prior calcination. This of course enables substantial savings to be made in the overall process for the preparation of the material.

Particular embodiments of the invention can be envisaged.

A first variant will be described below. This variant makes it possible to obtain products whose size is in the upper part of the range given above, namely between 0.15 and 0.5 pm.

According to this variant, the reaction is carried out in two steps.

The first step is carried out under conditions t such that the molar ratio divalent or trivalent cation/ Ti is less than 1 and preferably at most 0.5, for example between 0.2 and 0.5. Then, in a second step, the necessary complementary amount of divalent or trivalent cation is added.

In the case of this variant, it is preferable to carry out the two steps at different temperatures.

The operating temperature is low in the first step and higher in the second.

The powder is then recovered in the same manner as that described above.

A second variant can also be used more particularly in the case where it is desired to obtain par10 tides having no porosity.

This variant consists in carrying out the reaction in at least two steps at different temperatures, that of the first step being lower than that of the last. For example, the first step can take place at a temperature of between 50 and 100°C and the last step can take place at between 150 and 200°C.

It is also possible to make provision for an intermediate step between the first and last steps so that the temperature rise is more gradual, in which case this intermediate step takes place at a temperature between those of the other two steps.

Concrete Examples will now be given.

Examples 1 to 3 are Comparative Examples. They make it possible to show the influence of the nature of the precursor on the final titanate obtained. Examples 4 to 11 are Examples according to the invention.

COMPARATIVE EXAMPLE 1 509 g of a suspension of hydrated titanium oxide (1.9% of TiO2), obtained by neutralizing a solution of titanium oxychloride with aqueous ammonia and amorphous to X-rays, 77.6 g of Ba(OH)2.8H2O and 142 g of decarbonated water were introduced into a 1 litre autoclave in an inert atmosphere. The mixture was kept at 100“C for 4 hours, with agitation (350 rpm). After washing with a buffered solution at pH 4.8, a powder consisting of BaTiO3 (cubic phase) was obtained with a yield of 97%. The particles are aggregated and polydisperse with a size distribution of between 0.07 and 0.2 pin.

COMPARATIVE EXAMPLE 2 Under the same conditions as those of Example 1, especially in respect of titanium and barium concentrations and temperature, a suspension of hydrated titanium oxide, prepared by the hydrolysis of titanium isopropanolate, amorphous to X-rays and consisting of spherical particles of 1.2 pm in diameter, was reacted. This gave highly polydisperse BaTiO3 with a particle size varying between 0.07 and 0.5 pm. The average diameter measured by a Brookhaven DCP-1000 granulometer is 0.3 pm with φ 75 / φ 25 equal to 1.86.

COMPARATIVE EXAMPLE 3 Under the same conditions as those of Example 1, especially in respect of titanium and barium concentrations and temperature, a suspension of titanium oxide of rutile crystallographic structure, prepared by the thermohydrolysis of TiOCl2 and taking the form of aggregated needles of 0.20 μπι by 0.007 μία, was reacted. After a reaction time of 4 hours, the yield of BaTiO3 formed was only 25% and the particles obtained were polydisperse with a size varying between 0.5 and 1.5 μτα.

EXAMPLE 4 A sol of titanium oxide of anatase structure (containing 16% of TiO2) is prepared by the following process: 0.02 mol of citric acid is added to one litre 15 of a solution of titanium oxychloride containing one mol of Ti. Seeds of anatase are also added to this solution at a rate of 2% by weight, relative to the total TiO2. The whole is brought to the boil and kept at the boil for 6 hours. When the reaction is com20 plete, this sol takes the form of particles of 30 nm in diameter, consisting of crystallites of 7 nm. It is then reacted under the same conditions as those of Example 1, especially in respect of titanium and barium concentrations and temperature. This gave BaTiO3 with a yield of at least 95%. The particles are spherical and non-aggregated, have an average size equal to 0.07 μτα and are monodisperse (φ 75 / φ 25 - 1.33).

EXAMPLE 5 Autoclaving of barium hydroxide with an anatase sol such as that described in Example 4, for 4 hours at 120eC, with a titanium concentration of 0.17 5 mol/kg and a molar ratio Ba/Ti equal to 1.25, resulted in a monodisperse BaTiO3 powder whose particles of spherical appearance had an average diameter equal to 0.125 pm (φ 75 / φ 25 = 1.30), with a yield of 93%.

EXAMPLE 6 Autoclaving of barium hydroxide with the anatase sol described in Example 4, for 4 hours at 120eC, with a titanium concentration of 0.60 mol/kg and a ratio Ba/Ti equal to 1.5, made it possible to obtain a monodisperse BaTiO3 powder whose particles of spheri15 cal appearance had an average diameter of 0.025 pm (Φ 75 / φ 25 = 1.25).

EXAMPLE 7 Autoclaving carried out under the concentration conditions of Example 5, in the presence of tetraethyl20 ammonium hydroxide (0.42 mol/kg) and at a temperature of 70°C, made it possible to obtain BaTiO3 with a yield of 95% after a reaction time of 4 hours. The particles formed are spherical and monodisperse and have a diameter of 0.060 pm (φ 75 / φ 25 = 1.24).

EXAMPLE 8 A mixture consisting of the anatase sol described in Example 4 and barium hydroxide, such that the titanium concentration was 0.4 mol/kg and the molar ratio Ba/Ti was equal to 1.25, was autoclaved at 70eC (1 hour), then at 100eC (1 hour) and finally at 180°C (2 hours). This gave non-porous BaTiO3 particles of spherical morphology. The average diameter measured by a BROOKHAVEN DCP-1000 granulometer is 0.075 pm and φ 75 / φ 25 = 1.26.

Their specific surface area measured with nitrogen is 13 m2/g. The ratio divalent cation/Ti is equal to 0.990. The powder, neither calcined nor ground, had a density of 96.5% of the theoretical density after sintering at 1300°C (2 hours).

EXAMPLE 9 364 g of a mixture of the anarsss sol described in Example 4 and barium hydroxide, such that the titanium concentration was 0.6 mol/kg and the molar ratio Ba/Ti was equal to 0.33, were heated at 80 °C for 30 min, with agitation. A solution of barium hydroxide (2 mol/kg, 364 g) was then added, after which the mixture was heated at 100’C (30 min) and then at 180°C for 2 hours. This resulted in monodisperse spherical BaTiO3 particles with an average size of 0.150 pm (φ 75 / φ 25 = 1.35).

EXAMPLE 10 A suspension consisting of anatase titanium oxide, such as that described in Example 4, at a concentration of 0.17 mol/kg and strontium hydroxide at a concentration of 0.21 mol/kg was heated at 80°C for 4 hours. After washing, a monodisperse SrTiO3 powder was obtained with an average particle diameter equal to 0.035 pm.

EXAMPLE 11 A suspension consisting of anstase titanium oxide, such as that described in Example 4, at a concentration of 0.17 mol/kg and calcium hydroxide at a concentration of 0.5 mol/kg, the pH of which was adjusted to 13.5 with NaOH, was heated at 150°C for 4 hours.

After washing, a CaTiO3 powder was obtained which consisted of parallelepipedal particles of 0.45 x 0.1 x 0.1 pm.

EXAMPLE 12 The autoclaving of baryum hydroxyde, of strontium hydroxide with the anatase sol described in example 4, is carried out, with a titanium concentration of 0,17M/kg and a ratio Ba/Sr equal to 1, a ration (Ba+Sr)/Ti equal to 1, for 4 hours at 120*C, in the presence of sodium hydroxide (0,66 M/kg).

The resulting product, washed with a buffered solution at pH 4,8, is made of Bao#5Srg ,5^03.

The particles have a spherical morphology, are monodispersed (075/025 K 1/35) and the mean size thereof ls 0,035pm.

EXAMPLE 13 The autoclaving of baryum hydroxyde, of strontium hydroxide with the anatase sol described in example 4, is carried out, with a titanium concentration of 0,17M/kg and a ratio Ba/Sr equal to 1, a ration (Ba+Sr)/Ti equal to 0,25, for 4 hours at 120*C, in the presence of sodium hydroxide (0,66 M/kg).

The resulting product, washed with a buffered solution at pH 4,6, is made of Bag,2Sro,8Tio3· The particles have a spherical morphology, are monodispersed (075/025 « 1/50) and the mean size thereof is 0,030pm.

EXAMPLE 14 The autoclaving of baryum hydroxyde, of strontium hydroxide with the anatase sol described in example 4, is carried out, with a titanium concentration of 0,l7M/kg and a ratio Ba/Sr equal to 1, a ration (Ba+Sr)/Ti equal to 4, for 4 hours at 120*C, in the presence of sodium hydroxide (0,66 M/kg).

The resulting product, washed with a buffered solution at pH 4,6, is made of Bao,eSrO,2Ti03· The particles have a spherical morphology, are monodispersed (075/025 B 1/25) and the mean size thereof is 0,027μπ».

Claims

1. A process for the preparation of a titanate of at least one divalent or trivalent cation, characterized in that at least one salt or hydroxide of 5 said cation is reacted in a basic medium with a sol of titanium oxide of anatase structure, obtained by a process comprising thermohydrolysis.

2. A process according to Claim 1, characterized in that the divalent cation is selected from the group 10 comprising alkaline earth metal cations.

3. A process according to Claim 2, characterized in that the cation is barium or strontium.

4. A process according to any one of the preceding claims, characterized in that the trivalent 15 cation is selected from the group comprising bismuth, yttrium and the rare earths.

5. A process according to any one of the preceding claims, characterized in that the above-mentioned sol is the product of the 20 thermohydrolysis of a titanium compound A in a medium comprising at least one compound B selected from: (i) acids which contain: - either one carboxyl group and at least two hydroxyl and/or amine groups, 25 - or at least two carboxyl groups and at least one hydroxyl and/or amine group; and (ii) salts of the acids mentioned under (i).

6. A process according to Claim 5, characterized in that compound A is a titanium halide, oxyhalide, nitrate or alkoxide. 57. A process according to any one of the preceding claims, characterized in that the above-mentioned reaction is carried out at a temperature of at least 50 e C and more particularly of between 70 and 180°C. 10 8. A process according to Claim 7, characterized in that the above-mentioned reaction is carried out at a temperature of between 70 and 120 °C.

7. 9. A process according to any one of the preceding claims, characterized in that the 15 above-mentioned reaction is carried out in a medium with a pH of at least 10, preferably of at least 12 and more particularly of at least 13.5.

8. 10. A process according to any one of the preceding claims, characterized in that the 20 above-mentioned reaction is carried out in a medium comprising a base.

9. 11. A process according to any one of the preceding claims, characterized in that the above-mentioned reaction is carried out in a medium 25 having a Ti concentration of between 0.1 and 1 mol/kg.

10. 12. A process according to any one of the preceding claims, characterized in that the above-mentioned reaction is carried out in a medium having a concentration of divalent cation of between 0.2 and 2 mol/kg.

11. 13. A process according to any one of the preceding claims, characterized in that the above-mentioned reaction is carried out in at least two steps at different temperatures, the temperature being higher in the last step.

12. 14. A process according to any one of the preceding claims, characterized in that the above-mentioned reaction is carried out in two steps, the first step being carried out under conditions such that the molar ratio divalent cation/Ti is less than 1, the complementary amount of divalent cation then being introduced in the second step.

13. 15. A process according to Claim 14, characterized in that the above-mentioned two steps are carried out at different temperatures, the temperature being higher in the second step.

14. 16· A process according to Claim 1 for the preparation of a titanate of at least one divalent or trivalent cation, substantially as hereinbefore described and exemplified.

15. 17. A titanate of at least one divalent or trivalent cation, whenever prepared by a process claimed in a preceding claim.