JPH0246531B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for synthesizing barium titanate or strontium titanate powder having a fine and uniform particle size.

In recent years, as electronic products have become smaller and have higher performance, ceramic capacitors are also desired to be smaller and have higher capacity, and multilayer capacitors are one of them that is attracting attention. Barium titanate (BaTiO ₃ ) and strontium titanate (SrTiO ₃ ) are mainly used as ferroconductors in this multilayer ceramic capacitor, and there is a demand for finer powders of these materials in order to make the thickness thinner and more uniform. has been done.

Currently, the minimum distance between the electrodes of multilayer capacitors is about 20 μm, and further reductions are being considered in the future. Therefore, it is necessary that the particle size of the sintered body is 10 μm or less, and for this purpose, the particle size of the raw material powder is preferably 1 μm or less.

As a method for producing BaTiO ₃ or SrTiO ₃ with a particle size of 1 μm or less, barium carbonate or strontium carbonate is reacted with titanium dioxide at a high temperature of 1000°C or higher to contain BaTiO ₃ or SrTiO ₃ , and then mechanically pulverized. , there is a method of classification. However, this method has the disadvantage that it is essentially difficult to obtain powder with fine and uniform particle size because sintering progresses during the reaction and contains a large amount of coarse particles.

Recently, synthetic methods using metal alkoxides have been attempted, but the raw materials are expensive and there are problems with industrialization. In addition, the oxalic acid method uses oxalate at 600%
This method synthesizes titanate by firing at temperatures above ℃, but it is essentially a reaction between barium carbonate or strontium carbonate and titanium oxide, which is close to a solid phase reaction and has a uniform particle size. It is difficult to obtain granules.

On the other hand, the raw materials for the hydroxide method are relatively cheap;
The resulting powder is also attracting attention because of its high sinterability. For example, in JP-A-60-90825, a method is proposed in which titanic acid and barium hydroxide are heated at a temperature below the boiling point in the presence of a large amount of cedar. , an adjustment process is required.

When titanic acid is precipitated by neutralizing an aqueous solution of a titanium compound, there is a problem in that it is industrially difficult to wash and filter because it is a colloid. In addition, JP-A-50-86024 proposes a method of purifying and using titanic acid produced in the sulfuric acid method titanium dioxide production process, but the niobium and sulfate groups contained in titanic acid are , it is difficult to remove it sufficiently.

JP-A-59-9726 proposes a method for synthesizing barium titanate without preparing titanic acid in advance. The method of the invention includes dissolving barium chloride and barium nitrate in an aqueous solution of titanium salt,
This method involves adding alkali to adjust the pH to 13 or higher, and then heating below the boiling point. However, as a result of a detailed study of the invention, the reaction products TiO ₂ and BaO
It is difficult to adjust the molar ratio of
There is a problem of large shrinkage during sintering, which may be undesirable depending on the application.

However, as a result of intensive research into the hydroxide method, the present inventors found that an aqueous solution of titanium tetrachloride,
By dissolving any one type of barium or strontium carbonate, chloride, or nitrate compound, adding sodium hydroxide or potassium hydroxide, and heating in an autoclave, the average particle size is 0.05. It has been discovered that BaTiO ₃ or SrTiO ₃ having a uniform particle size of ~0.5 μm can be obtained, and the present invention has been completed based on this finding.

In the method of the present invention, when heating at the boiling point in the atmosphere without heating in an autoclave, for example, even if the molar ratio of BaO/TiO ₂ charged is 1.0, the reaction rate is as low as about 70%. of the product
Only BaO/TiO ₂ molar ratios as low as 0.7 can be obtained. If the molar ratio of the feed during the reaction is increased to 2.0, the molar ratio of the product will be close to 1.0, but
Economically unfavorable. Also, the primary particle size of the powder is
Although it is fine with a size of 100 Å or less, it forms agglomerated particles with a non-uniform particle size of 0.02 to 0.3 μm, and the particle density is also small, which is not preferable. However, it has been found that when a hydrothermal reaction is carried out using an autoclave, the reaction rate is 97% or more, and particles with very uniform particle size and good dispersibility can be obtained.

Furthermore, in the present invention, the synthesis reaction is carried out in the presence of large amounts of salts such as sodium chloride and potassium chloride, but if the reaction product is thoroughly washed, the alkali metal content can be reduced to about 0.01% by weight. is possible. Yamamura et al. reported in the Journal of the Chemical Society of Japan, No. 7, 1974, that BaTiO ₃ obtained by reacting titanic acid and barium hydroxide in the presence of Na ions contains 0.14% by weight of Na. The effects of the present invention were unexpected from these findings.

The present invention will be explained in detail below. First, a titanium chloride solution is prepared by gradually dissolving titanium tetrachloride in water. As the barium salt or strontium salt dissolved in this solution, carbonate can be used in addition to water-soluble chloride and nitrate. Since a large amount of hydrochloric acid is present in the solution of titanium chloride, carbonate can also be easily dissolved.

Next, sodium hydroxide or potassium hydroxide is added to this solution to make it alkaline. The amount of alkali added depends on the chloride ions and/or ions present in the titanium and barium or strontium solution.
Or in an equivalent amount or slightly in excess of the acid radical such as nitrate ion.

In the method of the present invention, before carrying out the hydrothermal reaction,
If barium or strontium is present in the form of carbonate, it will not react with titanium and will be mixed into the reaction product as it is, resulting in a heterogeneous composition, so the mixing and formation of carbonate must be avoided as much as possible.

The synthesis of BaTiO ₃ or SrTiO ₃ is performed using an autoclave equipped with a stirrer, but the heating temperature is
A temperature of about 120 to 200℃ is sufficient. As the temperature increases,
Although the density of the product particles increases, heating above 200°C has no significant effect and is not desirable from a cost perspective. If the temperature is around 120 to 200°C, there is no need to use special equipment materials, and the cost burden is not so large. After the reaction is completed, the product is separated, washed, and dried using a conventional method.

_BaTiO3 obtained by the method of the present invention and
_SrTiO3 powder has small particle size and uniform particle size, so it has high reactivity with various doping agents.
It is suitable not only as a raw material for multilayer capacitors, but also for various types of capacitors, PTC semiconductors, etc. In addition, the small particle size allows for lower sintering temperatures, making it possible to significantly reduce electrode costs in multilayer capacitors.

In the method of the present invention, part of the titanium chloride can be replaced with a metal salt such as zirconium, niobium, tin, etc., and part of the barium or strontium can be replaced within the range where a perovskite-type composite compound is formed. Strontium, barium, calcium, magnesium, lead, cerium,
It can be carried out by replacing it with a metal salt such as lanthanum.

Next, the present invention will be explained in more detail with reference to Examples.

Example 1 TiCl ₄ was added to ice water with stirring to prepare a solution containing 100 g/TiO ₂ . this solution
Dissolve _BaCO3 in an amount equivalent to Ti in 300 ml, add NaOH in an amount equivalent to the chlorine ion in the solution while blowing nitrogen gas and shutting off air, place in a stainless steel autoclave, and incubate at 130-140℃ for 6 hours. Stir and heat for an hour. The product from this reaction was filtered and washed with water, then diluted again, filtered and washed at 105°C.
After drying for 10 hours, a white powder was obtained. As a result of chemical analysis of this powder, the molar ratio of BaO/TiO ₂ was 0.99 and the content of Na ₂ O was 0.008% by weight.
Further, as a result of X-ray diffraction analysis, it was found to be cubic barium titanate with good crystallinity, and when observed under an electron microscope, it was found to be particles of 0.1 to 0.2 μm with very good dispersibility and uniform particle size.

Example 2 In the same manner as in Example 1, 0.99 equivalent of BaCl ₂ .
2H ₂ O was dissolved, and KOH in an amount equivalent to the chloride ion in the solution was added while blocking air, and the mixture was placed in an autoclave and stirred and heated at 170 to 180° C. for 3 hours. The product resulting from this reaction was filtered, washed and dried in the same manner as in Example 1 to obtain white powder. powder
The BaO/TiO ₂ molar ratio was 0.99 and the K ₂ O content was 0.012%. It is cubic barium titanate with good crystallinity, and the particle size is uniform at 0.2 to 0.3 μm.
It had good dispersibility.

Comparative Example In Example 2, 1.5 equivalents of BaCl ₂ 2H ₂ O were added and the amount of KOH added was increased accordingly,
Except for stirring and heating at 100℃ for 7 hours under normal pressure.
A white powder was obtained in the same manner as in Example 2. powder
The BaO/TiO ₂ molar ratio was 0.77, indicating a low reaction rate, and the K ₂ O content was 0.77%. Further, fine particles with a particle size of about 0.01 μm and particles with a particle size of about 0.3 μm are mixed, which is not preferable.

Example 3 A white powder was obtained in the same manner as in Example 1 except that SrCO ₃ was used instead of BaCO ₃ . As a result of X-ray diffraction analysis, it was found to be cubic strontium titanate, and when observed with an electron microscope, the particle size was 0.05 to 0.1 μm, and the particle size was uniform and had good dispersibility.

Claims (1)

[Claims] 1 Add one of barium or strontium carbonate, chloride, or nitrate to an aqueous solution of titanium tetrachloride.
A method for producing barium titanate or strontium titanate, which comprises dissolving seeds, adding sodium hydroxide or potassium hydroxide, and carrying out a hydrothermal reaction under stirring.