WO1993016012A1

WO1993016012A1 - Method for preparing a mixed metal oxide powder suitable for producing electrical capacitors

Info

Publication number: WO1993016012A1
Application number: PCT/EP1993/000271
Authority: WO
Inventors: Henri Wautier; Marianne Van Cauwenbergh
Original assignee: Solvay S.A.
Priority date: 1992-02-14
Filing date: 1993-02-04
Publication date: 1993-08-19

Abstract

A method for preparing a mixed metal oxide powder suitable for producing electrical capacitors, wherein a submicronic barium titanate powder is suspended in a solution containing one or more dissolved metals from groups IIb, IVa, IVb, Va, Vb, VIIb and VIIIb, the previously calcined barium titanate powder is impregnated with the solution, the solution is evaporated off, and the resulting powder is pressed then sintered. The method may be used to prepare mixed metal oxide powders for producing capacitors which comply with standard X7R.

Description

Process for the manufacture of a powder of mixed metal oxides suitable for the production of electric capacitors.

The invention relates to a process for the manufacture of mixed metal oxide powders suitable for the production of electrical capacitors of high capacity and its use for obtaining a powder intended for the manufacture of capacitors in accordance with standard X7R (ANSI / EIA-198-C-1983 standard), Mixed metal oxide compositions suitable for producing high-capacity electric capacitors are usually prepared by mixing powders of previously ground metal oxides such as, for example, BaTiOβ , Nb2 <-5, Bi2Û3, TJO2, Snθ2, Mnθ2- PbO, ZnO and CoO. (patent US-4499521 in the name of North American PHILIPS Corp.)

The manufacture of high-capacity multilayer capacitors, however, requires increasingly fine powders in order to be able to produce increasingly thin dielectric layers necessary for obtaining a high capacity.

The powders obtained by known conventional methods generally have morphologies typical of ground powders, with average diameters greater than μm and, therefore, no longer allow the thickness of the dielectric layers to be reduced.

The invention aims to overcome the drawbacks of known methods by providing a method capable of producing submicron powders suitable for the manufacture of ultra-thin dielectric layers for high-capacity multilayer capacitors. To this end, the invention relates to a process for the fabrication of a powder of mixed metal oxides suitable for the production of high capacity electric capacitors according to which a submicron powder of barium titanate and one or several metal oxides of groups Ilb, IVa, IVb, Va, Vb, Vllb and VlIIb. According to the invention, the previously calcined barium titanate powder is suspended in a solution containing the metals in the dissolved state, the barium titanate powder is impregnated with the solution, the solution is evaporated and the powder obtained is subjected to pressing followed by sintering. By mixed metal oxide powder is meant a powder comprising oxides of at least two different metals.

The high capacitance electric capacitors which can be manufactured according to the process of the invention are condens

10 multilayer sators each of which layers is made of a dielectric material whose dielectric constant ε at 25 ° C is greater than approximately 1800 and, preferably, greater than approximately 2000. This dielectric constant at 25 ° C can be very high. In practice, it usually does not exceed one

¹⁵ value of about 5000.

The submicron barium titanate powder which is used according to the invention can be obtained by any suitable method making it possible to obtain barium titanate grains of regular particle size whose average diameter is below

~~ μm.

In a preferred embodiment of the invention, the submicron barium titanate powder is prepared by coprecipitation resulting from the mixture of an alcoholic solution of oxalic acid and titanium alcoholate with a solution

~ - barium hydroxide or acetate as described in Belgian patent application No. 9100836 in the name of SOLVAY & Cie (Société Anonyme).

Advantageously, choose methanol as solvent for oxalic acid and titanium alcoholate.

In another embodiment of the invention which is also preferred, the submicron barium titanate powder is prepared by hydrolysis of a titanium alcoholate in the presence of a hydrated barium hydroxide and a carboxylic acid containing more of 6 carbon atoms in its molecule as it was

35 described in French patent application No. 87.09116 in the name of SOLVAY & Cie (Société Anonyme). According to this other embodiment of the invention, the carboxylic acid will preferably be selected from aliphatic acids with a linear carbon chain. Oleic acid has given excellent results. The metals of groups Ilb, IVa, IVb, Va, Vb, Vllb and VlIIb of the periodic table of the elements will advantageously be chosen from silicon, niobium, bismuth, manganese, and cobalt. It is imperative to use at least one and preferably at least two of these metals. Excellent results have been obtained using three different metals chosen from the five preferred metals mentioned above. The best results have been obtained with a mixture of Nb, Bi and Mn.

According to the invention, the metals are used in the dissolved state in a solution. Preferably, this solution is an alcoholic solution of one or more alcoholates and / or acetates of these metals. The preferred solvent is an optionally substituted aliphatic or alicyclic alcohol or a mixture of two or more of these alcohols which are miscible with each other in large proportions. Saturated linear aliphatic alcohols called "lower", that is to say those having 1 to 6 carbon atoms in their hydrocarbon chain are well suited. Alcohols having from 1 to 4 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl and isobutyl alcohols are most suitable. Among these, methyl alcohol has given excellent results.

According to the invention, by metallic alcoholate is intended to denote a compound of formula R-0-Me, in which Me is a metal atom and in which R denotes a linear or branched aromatic, alicyclic or aliphatic hydrocarbon group, saturated or comprising a or several unsaturations, unsubstituted or carrying one or more hydrocarbon substituents optionally comprising one or more heteroatoms in their main chain. Metal alcoholates derived from an optionally substituted saturated linear aliphatic alcohol are preferred such as, for example, alcoholates derived from a lower aliphatic alcohol having less than 5 carbon atoms, such as those where R is a methyl, ethyl, n-propyl, isopropyl, n-butyl or iso-butyl group. One can, indifferently, use a single metal alcoholate or a mixture of several alcoholates,

In a preferred embodiment of the process according to the invention, the alcoholic solution of the metals is a solution of a mixture of alcoholates or acetates of niobium, bismuth and manganese in a lower aliphatic alcohol. A solution of manganese acetate, bismuth acetate and a mixture of niobium ethylate and propylate in methyl alcohol gave the best results.

In the process of the invention, the quantities of metals to be used in the dissolved state are such that after impregnation and evaporation of the solution, the total content of metals other than Ti and Ba in the powder is greater than 0.5 X by weight and preferably greater than 1 X. This content should also be less than 10 and preferably less than 8 X. According to an advantageous variant of the process according to the invention, the alcoholic solution of the metals can advantageously contain one or more carboxylic acids. Aliphatic carboxylic acids such as formic, acetic and propionic acids are well suited. Acetic acid has given good results, especially in methanolic solutions of metals.

According to the invention, the sintering of the powder is carried out at a temperature sufficient for the particles of barium titanate powder impregnated with the acetates or the alcoholates of metals to give rise to the formation of oxides of these metals at the periphery. particles. This temperature will not however be too high so as to slow the diffusion of the oxides of these metals towards the core of the particles. In practice, the sintering temperature will be between 1050 ° C and 1350 ° C. The invention also relates to the use of the process of the invention for obtaining a powder intended for the manufacture cation of capacitors conforming to standard X7R.

The X7R standard designates a standard characterizing an electric capacitor whose dielectric constant does not vary by more than 15 X on either side of its value at 25 ° C in an operating temperature range from -55 ° C to + 125 ° C.

The examples which follow are given for the purpose of illustrating the invention, without however limiting its scope. Example 1R is given for comparison. Examples 2 to 7 were carried out in accordance with the invention. Example 1R: (not in accordance with the invention)

In a reactor containing a solvent formed from an equivolumic mixture of methanol and isopropanol, a solution of Ti isopropylate and oleic acid was introduced into the same solvent mixture so as to achieve a molar ratio of oleic acid / Ti 0.7 and a 0.0847 M solution of Ti isopropylate. A solution of Nb alcoholate in methanol previously prepared as described below was then added and a solution of manganese acetate in methanol in an amount adjusted so that the Nb / Ti molar ratio of the solution is equal to 0 , 02 and the Mn / Ti molar ratio at 0.002.

A solution of hydrated barium hydroxide in the methanol / isopropanol equivolumic mixture was then added with stirring in an amount adjusted so that the Ba / Ti molar ratio was equal to 1.02. The precipitate formed was then dried and then calcined to 860 ° C.

The particles obtained were submicron (90 Y. of these had a diameter of less than 1 μm) and had an average diameter of 0.5 μm.

The variation of the dielectric properties of the powder obtained as a function of the temperature has been plotted in graph in FIG. 1 showing on the ordinates the evolution of the dielectric constant ε as a function of the temperature (in degrees C) plotted on the abscissa, the measurement being carried out with alternating current at a frequency of 1.5 kHz. Figure 2 illustrates the relative variation, expressed in Z, of the capacity as a function of the temperature (dC / C25 °). The ordinates of Figure 2 represent the relative variation of the capacity (dC / C25 °) and are graduated in% and the abscissas represent the temperature and are graduated in degrees Centigrade. It can be seen that the variations in the dielectric constant as a function of the temperature are too great for the powder to be usable in the manufacture of electrical capacitors conforming to the X7R standard.

Preparation of the Nb alcoholate solution: 1 l of methanol and 34 g of bCls were introduced into a glass flask previously put under dry nitrogen pressure. 68.5 ml of propylene oxide were then added. so as to produce a propylene oxide / Nb molar ratio equal to 7.7 and by controlling the temperature so that it does not exceed 32 ^β C, after which 1 ml of triethylamine has been added. It was allowed to react at room temperature for 2 days, then the solution was heated to 40 ° C for 2 hours. Example 2: (according to the invention)

In a glass flask, there were successively introduced: 2.33 g of BaTiOβ obtained by cohydrolysis in the presence of oleic acid and previously calcined at 850 ° C for 2 hours; 30 ml of methanol;

2.05 ml of Nb alcoholate in 0.122 M solution in methanol prepared as in Example 1R; 25.0 ml of Bi (CH3COO) 3 in 0.01 M solution in a mixture of methanol and acetic acid and 2.0 ml of Mn (CH3C00) 2 in 0.01M solution in methanol.

The solution was then evaporated under reduced pressure, and the powder obtained was sintered at 1175 ° C. for 1 hour. The powder obtained had an Nb / BaTiθ3 molar ratio of 0.025, a Bi / BaTiθ3 molar ratio of 0.025 and an Mn / BaTiθ3 molar ratio of 0.002.

The dielectric properties of the powder obtained as a function of the temperature have been plotted in graphs in FIGS. 3 and 4 showing the evolution of the dielectric constant ε as a function of the temperature (in degrees C) and the variation relative capacity (in X) as a function of temperature, the measurement being carried out using alternating current at frequencies of 1.5 and 10 kHz. The abscissas and the ordinates of FIG. 3 represent the same quantities as in FIG. 1 and those of FIG. 4, the same as in FIG. 2. The curves in solid lines represent the properties measured at 1.5 kHz and the dashed lines, the properties measured at 10 kHz. The maximum dielectric constant measured at 1.5 kHz is 2043 and corresponds to a temperature of 19 ° C, that measured at 10 kHz is 2020 and corresponds to 102 ° C. At 25 ° C, these dielectric constants are worth 2042 (at 1.5 kHz) and 2003 (at 10 kHz).

It can be seen that the variations in capacity as a function of the temperature are small and less than 10 X over the entire range of temperatures envisaged. The powder obtained can therefore be used in the manufacture of electrical capacitors in accordance with the X7R standard.

The solution of Bi acetate in the methanol and acetic acid mixture was prepared as follows:

1.42 g of Bi (CH3C00) 3, 17.5 ml of acetic acid and 50 ml of methanol were introduced into an Erlenmeyer flask. After heating the mixture to reflux, 300 ml of methanol was then added with stirring.

Examples 3 to 6: (in accordance with the invention)

Example 2 was reproduced by modifying the contents of Nb, Bi and Mn so as to produce powders the contents of which follow:

The results obtained have been plotted in graphs Figures 5 to 12 which give the evolution of the dielectric constant ε as a function of the temperature (in degrees C) (Figures 5, 7, 9 and 11) and the relative variation of the capacity as a function of the temperature (Figures 6, 8, 10 and 12), the measurement being carried out with alternating current at frequencies of 1.5 and 10 kHz. The abscissa and ordinate of Figures 5, 7, 9 and 11 have the same meanings as those of Figure 1, those of Figures 6, 8, 10 and 12, the same as those of Figure 2. The curves in solid lines represent the properties measured at 1.5 kHz and the dashed lines, the properties measured at 10 kHz, except in FIGS. 11 and 12 where the measurement was carried out at 100 kHz rather than at 10 kHz. The maximum values and the values at 25 ° C. of the dielectric constants measured were as follows:

(*): values measured at 100 kHz We see that the variations in capacity as a function of temperature are in all cases between -15 and +15 X over the whole range of temperatures envisaged. The powders obtained can therefore all be used in the manufacture of electrical capacitors in accordance with the X7R standard.

Claims

R E V E N D I C A T I O N S

1 - Process for the manufacture of a powder of mixed metal oxides suitable for the production of high capacity electric capacitors according to which a submicron powder of barium titanate and one or more metal oxides of groups Ilb, IVa are used , IVb, Va, Vb, Vllb and VlIIb, characterized in that the barium titanate powder, previously calcined, is suspended in a solution containing the metals in the dissolved state, which the titanate powder is impregnated with. barium with the solution, the solution is evaporated and the powder obtained is subjected to pressing followed by sintering.

2 - Process according to claim 1, characterized in that the barium titanate powder is obtained by coprecipitation resulting from the mixture of an alcoholic solution of oxalic acid and titanium alcoholate with a hydroxide or acetate solution barium.

3 - Process according to claim 1, characterized in that the barium titanate powder is obtained by hydrolysis of a titanium alcoholate in the presence of a hydrated barium hydroxide and a carboxylic acid containing more than 6 carbon atoms in its molecule.

4 - Process according to claim 3, characterized in that the carboxylic acid is oleic acid.

5 _ Process according to any one of claims 1 to 4, characterized in that the metals are selected from niobium, bismuth, manganese and cobalt.

6 - Process according to any one of claims 1 to 5, characterized in that the solution containing the metals in the dissolved state is an alcoholic solution of one or more alcoholates and / or acetates of these metals.

7 - Method according to claims 5 and 6, characterized in that the alcoholic solution contains a mixture of alcoholates or acetates of niobium, bismuth and manganese.

8 - Process according to any one of claims 1 to 7, characterized in that the alcoholic solution is a methanolic solution.

9 - Process according to claim 8, characterized in that the methanolic solution contains a carboxylic acid selected from formic acid, acetic acid and propionic acid.

10 - Use of the method according to any one of claims 1 to 9 for the manufacture of mixed metal oxide powders intended for the production of capacitors in accordance with standard X7R.