WO2001010781A1

WO2001010781A1 - METHOD FOR MANUFACTURING BaTiO3 BASED POWDERS

Info

Publication number: WO2001010781A1
Application number: PCT/KR1999/000434
Authority: WO
Inventors: Kyoung Ran Han; Hee Jin Koo
Original assignee: Korea Institute Of Science And Technology
Priority date: 1999-08-05
Filing date: 1999-08-05
Publication date: 2001-02-15

Abstract

A method for manufacturing BaTiO3 based powders such as BaTiO3 or Ba0.7Sr0.3TiO3 is disclosed. The BaTiO3 based powders is manufactured by doping a precursor of TiO2, presumably TiO(OH)2 acid-containing Ti solution or TiCl4 on the surface of BaCO3 or a mixture of BaCO3 and SrCO3, forming a fine amorphous type or anatase type TiO2 on the surface of a BaCO3 powder during a heat treatment, and accelerating a solid solution reaction by a diffusion between BaCO3 and TiO2. Thereby, it is possible to decrease the manufacturing cost of a BaTiO3 group powder by a simple process and to manufacture powders having an excellent characteristic such as a controlled stoichiometry and small particle size of (<1νm).

Description

METHOD FOR MANUFACTURING BaTiQ₃ BASED POWDERS

TECHNICAL FIELD

The present invention relates to a method for manufacturing a BaTi0₃ group powder, and in particular to an easier method for manufacturing BaTi0₃ based powders having high density and controlled stoichiometry than by known method.

BACKGROUND ART Since BaTi0₃ based powders such as BaTi0₃ or Ba₀₇Sr₀₃TiO₃

(hereinafter, referred to as "BST") are becoming widely used as a material for PTC, monolith and MLCC (MultiLayer Ceramic Capacitor), and thus the need for the same is increased.

Conventionally, in order to manufacture BaTi0₃ powder, a solid oxide manufacturing method in which BaC0₃ and Ti0₂ are mixed is generally used. However, it is difficult to implement such method because a calcination temperature of 1000°C ~ 1200°C is needed in such method. In addition, a milling process is additionally required for grinding an agglomerate formed during the calcination process, impurities may be introduced during the process, and only an irregular-shaped BaTi0₃ powder having an average particle size of larger than 1 μm can be obtained. Therefore, the MLCC which is manufactured using such powder has a poor performance as a capacitor.

The BaTi0₃ powder having an average particle size of smaller than 1 μm is generally manufactured by solution processes. In a chemical method in which an organic ligand is used, it is possible to obtain a high purity BaTi0₃ powder. However, in this case, an agglomeration occurs due to a heat treatment which is performed at a temperature of about 1000°C which necessitates an additional grinding process. The density of the BaTi0₃ powder manufactured by such method is about 5.5g/cm³. In another method, an ultra-fine BaTi0₃ powder with high purity may be manufactured by hydrolyzing a Ba-Ti alkocide under a C0₂free environment , or by hydrolyzing a Ti(OR)₄ in a Ba(OH)₂ solution. In this method, an expensive reagent is needed, and the hydrolysis speed, density, etc. may affect the degree of precipitation and it takes a long time.

Generally, in order to manufacture an ultra fine BaTi0₃ powder with high purity, a hydrothermal reaction is used. German Patent publication DE 3526674 discloses that a hydroxide, a chloride and an alkoxides of Ba and Ti are 5 hydrothermal ly reacted at 5~15atm at a temperature of 150~200°C for thereby manufacturing a BaTi0₃. European patent publication No. EP-A141551 discloses that an orthotitanium acid obtained by a titanium salt (chloride, sulfate) with alkali is hydrothermally reacted with Ba(OH)₂ at a temperature of 60~100°C for thereby manufacturing a BaTi0₃ powder. However, in the above-described methods, o impurities such as chloride, sulfate, or alkali ions may be contained in the crystal structure, so that it is difficult to manufacture a powder with high purity.

In addition, U.S. Patent No. 4,764,493 discloses that an excessive Ba(OH)₂ and ultra-fine Ti0₂ powder (Anatase>75%) is dispersed into distilled water and is hydrothermally reacted at a temperature of 100~125°C for thereby 5 manufacturing BaTi0₃ powder having a density of 5.7g/cm³, and that a Ba(OH)₂ and Ti(0-iPr)₄ or Ti(OEt)₄ are hydrothermally reacted at a boiling point of alcohol for thereby manufacturing a crystalline BaTi0₃ powder having a density higher than 5.7g/cm³. According to U.S. Patent No. 4,859,448, an excessive Ba(OH)₂ and amorphous Ti0₂ powder are dispersed into distilled water which does not o contain C0₂ and then are heated at a temperature of 60 to 95°C for 90 hours for thereby manufacturing a BaTi0₃ powder with a single cubic structure.

Also, U.S. Patent No. 5,009,876 discloses that BaTi(H₂C₂0₄)₃ obtained by mixing a mixture of TiOCI₂ and oxalic acid with a BaCI₂ aqueous solution is separated and washed and then is calcined at a temperature of 900 to 1200°C 5 for thereby manufacturing a BaTi0₃ powder.

Also, U.S. Patent No. 4,670,243 discloses that Ti(Ac)₄ is obtained by adding acetic acid to Ti(0-iPr)₄ and then distilled water is added thereto for thereby manufacturing clear solution and then BaC0₃, SrC0₃, CaC0₃ or a mixture of the same is dissolved into the resultant solution, and the resultant solution is o precipitated in a state of pH>13 using a NaOH solution for thereby obtaining Ba₍₁. _x)Sr_xTi0₃ powder.

In the conventional method in which BaC0₃ and Ti0₂ are mixed and simply heat-treated, it is difficult to obtain a single phase BaTi0₃ powder. Thus, Ba²⁺ and Ti⁴⁺ under the state of a mixed solution are precipitated with a strong alkaline solution for thereby obtaining a Ba-Ti hydroxide, and then a BaTi0₃ crystal is obtained by a hydrothermal reaction thereof. In such a reaction, in order to prevent the formation of BaC0₃ which is a byproduct, the process is performed under conditions preventing the infiltration of C0₂.

In addition, as another method for obtaining a BaTi0₃ powder, an organic ligand is added to Ba²⁺ and Ti⁴⁺ under the state of a mixed solution for thereby manufacturing a Ba-Ti complex compound. In the above-described method, BaC0₃ , an intermediate, is formed, and BaC0₃ is reacted with Ti0₂ on the surface, so that BaTi0₃ is formed.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to provide a method for simply and economically manufacturing an ultra-fine BaTi0₃ powder with high purity by doping a Ti0₂ precursor having a nano size on the surface of a BaC0₃ particle and performing a heat treatment with respect to the resultant structure. It is another object of the present invention to provide a method for manufacturing an ultra-fine BST(Barium-Strontium-Titanate) powder with high purity at a low cost. To achieve the above objects, there is provided a method for manufacturing BaTi0₃ based powders which is formed by doping a precursor of Ti0₂ on the surfaces of a mixture of BaC0₃ and SrC0₃, forming a fine Ti0₂ in amorphous or anatase form on a BaC0₃ powder surface during a heat treatment process and accelerating a solid solution reaction by diffusion. Additional advantages, objects and features of the invention will become more apparent from the following description.

MODES FOR CARRYING OUT THE INVENTION

The method for manufacturing a BaTi0₃ group powder according to the present invention will be explained in detail.

The method includes adding an ammonia solution diluted with distilled water of 40 vol% into an aqueous carbonate slurry comprising BaC0₃ or a mixed powder of BaC0₃ and SrC0₃ of below 10wt%, preferably 1 to 5wt% and distilled water of more than 90 wt% and maintaining the pH of the carbonate slurry in a range of 9.5 to 12.0, preferably, 9.5 to 10.3, obtaining a precipitate by adding a Ti solution into the carbonate slurry containing the ammonia water, filtering and washing the precipitate formed in the carbonate slurry to form a first powder, dispersing the first powder into an alcohol, adding a certain amount of a hydroxypropyl cellulose(HCP), and dehydrating the resultant compound to form a second powder, and heat-treating the second powder at a temperature of 750 to 1150°C, preferably, 900 to 1050°C to form a BaTi0₃ powder or a Ba₀₇Sr₀₃TiO₃ powder. As the Ti solution, a TiCI₄ aqueous solution diluted with distilled water or a Ti solution (Ti(OR)₅) containing formic acid is used wherein R represents iPr, iBu, nBu, etc.

As the dilution ratio of the TiCI₄ aqueous solution which is added into the carbonate slurry to form a precipitate, the density of Ti⁺ (TiO(OH)₂) should be below 1 mmol/ml, preferably, below 0.5 mmol/ml.

The alcohol which disperses the first powder is selected from the group consisting of ethanol, isopropanol, a compound of ethanol and isopropanol. However, the alcohol is not limited thereto.

The amount of hydroxypropyl cellulose (HCP) is 0.1 to 5% of the weight amount of the first powder, preferably, 0.2 to 3%. The heat treatment is performed until the phase transformation is completed.

A wet milling process may be further included to increase the fineness of the BaTi0₃ group powder formed by the heat treatment, and the washing is performed using ammonia water until the chloride ions are fully removed from the precipitate. The filtering is performed by dispersing the precipitate into the alcohol until the distilled water is fully removed.

In the BST manufactured according to the present invention, the average fineness of the powder was 0.2μm after the calcination process was performed In the case of BaTi0₃, the average fineness was 0.3 to 0.5μm which was below 1 μm, and thus a uniform particle fineness was implemented. By X-Ray

Diffraction(XRD), it was determined that the powder had a cubic structure. In the case of BaTi0₃, the density measured using a helium picometer was 5.75g/cm³, and in the case of Ba₀₇Sr₀₃TiO₃, the density was 5.73/cm³. Next, the method of the present invention will be described with the following examples, which are illustrative of one embodiment of the present invention.

Example 1

BaCO₃(0.08578mol, Merck Company, German, purity 99.0%) was placed in a polypropylene container together with zirconia balls having a diameter of 5mm and distilled water and was ball-milled for about 4 hours, and the resultant solution was diluted with distilled water so that the concentration of the solution became 2.3wt%. Then 331 ml of distilled water was added to TiCI₄ (0.08571 mol, 7.2025x10^'4mol/ml, Alfa Product Company, USA, purity 99.0%), the resultant solution was filled into one burette. In another burette, ammonia water(Junsei Company, Japan, purity 28 to 30%) was diluted with distilled water of 40vol%. The pH of the aqueous BaC0₃ slurry was maintained at 10.0, and the diluted aqueous TiCI₄ solution was slowly added into the BaC0₃ slurry, and ammonia water was added thereto for thereby forming precipitate of Ti⁺⁴ while maintaining the pH at 10. The resultant slurry was agitated for 30 minutes, and settled, and then the supernatant was decanted. This precipitate was filterde and washed using ammonia water of pH 10.3 until the chloride ions were fully removed. This precipitate was dispersed in isopropanol and then was filtered, so that H₂0 content was minimized. When the H₂0 was nearly removed, the resultant powder was dispersed into the isopropanol, and a hydroxypropyl cellulose(the average amount of molecules was about 80,000, Aldrich Chemical Company, USA) was added to the resultant powder by 0.2wt%. Thereafter, the resultant solution was filtered and dried. The temperature of the resultant powder was increased by 10°C per minute and then the resultant powder was heat-treated at a temperature of 850°C for 4 hours and at a temperature of 950°C for 2 hours. By XRD, it was determined that the powder had a cubic structure. In order to measure the size of the particles, the resultant powder was dispersed in the distilled water, and the concentration was properly adjusted to 0.1g/300ml, and then the resultant solution was ultrasonicated for about 15 minutes and was settled for about 3 minutes. The sizes of the particles are shown in Table 1. [Table 1] The average particle size of BaTi0₃ powder versus heat treatment temperature

In addition, after heat treatment of 2 hours, the helium pycnometer examination of the BaTi0₃ powder indicated the density of 5.75g/cm³.

Example 2

BaCO₃(0.07275mol, purity 99.0%, Merck Company, German,) was placed in a polypropylene container together with zirconia balls having a diameter of 5mm and distilled water and was ball-milled for about 4 hours. The resultant slurry was diluted with distilled water so that the concentration of the slurry became 4.1wt%. Then 200ml of distilled water was added to TiCI₄(0.0720mol, 7.2025x10^" ⁴mol/ml, Alfa Product Company, USA, purity 99.0%), and the resultant solution was filled into one burette. In another burette, ammonia water(Junsei Company, Japan, purity 28 to 30%) was diluted to 60vol% with distilled water. The pH of the BaC0₃ slurry was maintained at 10.0, and the diluted aqueous TiCI₄ solution was added to with the aqueous BaC0₃ slurry, and ammonia water was added thereto for thereby forming precipitate of Ti⁺⁴ while maintaining the pH at 10. The resultant slurry was stirred for 30 minutes, and then settled. This precipitate was filtered and washed using the ammonia water of pH 10.3 until the chloride ions were fully removed. When the chloride ions were removed, the precipitate was dispersed into isopropanol, and precipitate was filtered. In this state, H₂0 content was minimized. When the H₂0 was nearly removed, the resultant powder was dispersed in isopropanol. A slurry A in which hydroxypropyl cellulose (the average amount of molecules was about 80,000, Aldrich Chemical Company, USA) was contained by 0.2% of the powder weight was prepared, and a slurry B in which hydroxypropyl cellulose was contained by 2.0% of the powder weight was prepared. The slurry A and slurry B were dehydrated using a rotary type evaporator for thereby obtaining powders A and B. The thusly obtained powders A and B were heat-treated at a temperature of 950°C for 2 hours and were heat- treated at a temperature of 1000°C for 2 hours. As a result of the XRD, it was confirmed that the powders A and B had a cubic structure. Powder A heat treated at 1000°C for 2 h showed mole ratio of 1.0079 of Ba /Ti by XRF. And mole ratio of powder A heat treated at 950°C for 2 h was found 1.0073 of Ba Ti by XRF. In order to measure the particle size of the powders, the powders A and

B were dispersed in distilled water and were controlled to have a proper concentration of 0.1g/300ml and then were ultrasonicated for about 15 minutes and were settled for about 3 minutes before measurement. Table 2 illustrates the result of the measurement. The densities of the powders were measured by a helium pycnometer.

[Table 2]

Example 3 Ti(O(CH₂)₂CH₃)₄ (0.10mol, purity 99.0%, Aldrich Chemical Company, USA) was slowly added to isopropanol. Formic acid (0.20mol, Junsei Co., Japan, purity 85.0%)was diluted with isopropanol for thereby obtaining a concentration of 16.7wt%. The formic acid solution was slowly added to the Ti solution. The Ti solution containing the formic acid was refluxed for 2 hours. BaCO₃ (0.10mol, purity 99.0%, Merck company, German) was placed in a polypropylene container together with zirconia balls having a diameter of 5mm and distilled water and was ball-milled for about 4 hours, and the resultant slurry was diluted using distilled water so that the concentration of the slurry became 4.0wt%. Hydroxypropyl cellulose (the average amount of molecules was about 80,000, Aldrich Chemical Company, USA) was added to the slurry by 0.2wt%. The pH of the BaC0₃ slurry was maintained at 9.5 using ammonia diluted to 20vol% with distilled water. The formic acid-added Ti solution was added to the slurry with the ammonia for thereby forming precipitates. The slurry was agitated for 30 minutes and then settled. Thereafter, the precipitate was filtered and washed, and then was dispersed in isopropanol and was dehydrated using rotary type evaporator for removing the residual water from the slurry. The temperature of the thusly- obtained powder was increased by 10°C per minute, and the powder was heat- treated at a temperature of 950°C for 2 hours and at a temperature of 1050°C for 2 hours, respectively. As a result of XRD with respect to the said powder, it was confirmed that the perovskite phase was only about 60% for both of them.

Example 4 BaCO₃ (0.0504mol, purity 99.0%, Merck Company, German) and SrC0₃

(0.0216mol, purity 99.0%, High Purity Chemical Company, Japan) were placed in a polypropylene container together with zirconia balls having a diameter of 5mm and distilled water and was ball-milled for about 4 hours. The resultant compound was diluted with distilled water so that the density of BaC0₃ became 1.7wt%. Aqueous TiCI₄ solution (0.0720mol, 7.2025x10-⁴mol/ml, Alfa Product Company) was diluted with distilled water of 80vol% in a burette. Ammonia water(purity 28-30%, Junsei Company, Japan) was diluted to 60vol% with distilled water, and the pH of the BaC0₃ and SrC0₃ slurry was maintained at 10.0, and the diluted TiCI₄ aqueous solution was slowly added to the slurry with the diluted ammonia for thereby forming precipitate. The thusly- obtained slurry was stirred for 30 minutes and then settled. The precipitate was washed using ammonia water of pH 10.3 until the chloride ions were removed. After the chloride ions were fully removed, the precipitate was filtered and washed for thereby obtaining a powder. The powder was dispersed into isopropanol, and the precipitate was filtered, and the H₂0 content was minimized. After the H₂0 was removed from the powder, the resultant powder was dispersed in the isopropanol. At this time, the powder was divided into two parts. The amount of the alcohol 5 contained in one part(4B) was three times the amount in the other part(4A) of the powder. Then hydroxypropyl cellulose (the average amount of molecules was about 80,000, Aldrich Chemical Company, USA) was added to the isopropanol slurries of the two powders by 0.2wt of the powder. The resultant slurry was dehydrated using a rotary type evaporator for thereby obtaining powders. The o powder was heat-treated at a temperature of 950°C and 1000°C for 2 hours, to yield a BST in which the perovskite structure was confirmed at 100% as a result of XRD. The powder had a cubic structure. In order to measure the sizes of the particles of the powder 4A which was heat-treated at a temperature of 950°C for 2 hours, the powder was dispersed in distilled water and had a proper 5 concentration of 0.1g/300ml, and the resultant powder was ultrasonicated for about 15 minutes and was set for about 3 minutes. At the first measurement, the average fineness was 0.18μm, and at the second measurement, the average fineness was 0.21 μm.

As described above, in the present invention, BaC0₃, SrC0₃, TiCI₄, etc. o which may be obtained at a lower cost are used, and it is possible to manufacture

BaTi0₃ based powders having a controlled stoichiometric composition having an average particle size of less than 1 μm by a simple process.

Namely, in the present invention, it is possible to decrease the manufacturing cost of BaTi0₃ based powders which is used for manufacturing a 5 multilayer ceramic capacitor (MLCC), and a mass production of an excellent

BaTi0₃ based powders is implemented by a simple process.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing 0 from the scope and spirit of the invention as recited in the accompanying claims.

Claims

1. A method for manufacturing a BaTi0₃ group powder, comprising the steps of: precipitating a Ti solution with a dilute ammonia over aqueous BaC0₃ slurry at pH 9.5 ~ 12; filtering and washing the slurry to form a first powder; diffusing the first powder into alcohol, and adding and dehydrating a small amount of HCP (Hydroxypropyl cellulose), to form a second powder; and forming a BaTi0₃ powder or a Ba₀₇Sr₀₃TϊO₃ powder by heat-treating the second powder formed by the dehydration process.

2. The method of claim 1 , wherein said Ti solution is a TiCI₄ aqueous solution diluted with distilled water.

3. The method of claim 2, wherein as a dilution ratio of the TiCI₄ aqueous solution, the density of Ti⁺⁴ is below 1 mmol/ml, preferably in the range of 0.1 to 0.5mmol/ml.

4. The method of claim 1 or 2, wherein the concentration of the carbonate slurry is in the range of 1 to 5wt%.

5. The method of claim 1 or 2, wherein in said Ti⁺⁴ precipitation step is the pH of the carbonate slurry containing ammonia water is maintained at 9.5 to 10.3 for thereby implementing the precipitation.

6. The method of claim 1 or 2, wherein said alcohol is selected from the group comprising ethanol, isopropanol and a mixture of ethanol and isopropanol.

7. The method of claim 1 or 2, wherein said addition amount of the hydroxypropyl celluose is in the range of 0.1 to 5% of the first powder weight, preferably in the range of 0.2 to 3%.

8. The method of claim 1 or 2, wherein said heat treatment temperature is in the range of 750 to 1150°C, preferably, in the range of 900 to 1050°C, and said heat treatment is performed until a structure variation is completed.