Barium titanate powder

Details Barium titanate powder Barium titanate powder

2000-01-12

2001-09-04

Griffin, Steven P. (Department: 1754)

Chemistry of inorganic compounds

Oxygen or compound thereof

Metal containing

C423S610000, C423S611000, C423S612000, C423S635000, C502S340000, C502S350000, C501S137000

Reexamination Certificate

active

06284216

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a barium titanate powder having superior dispersion characteristics, which is suitable for dielectric materials for electrical parts such as PTC thermistors and multilayer ceramic capacitors.
BACKGROUND ART
Multilayer ceramic capacitors are manufactured by alternately laminating ceramic dielectric layers and internal electrode layers, and then firmly pressing and sintering them to form an integrated body. Barium titanate powder slurry is widely used for the ceramic dielectric layers. Recently, the ceramic dielectric layer is particularly made to be thinner, so as to meet requirements of compact design and increased capacity. In order to obtain a ceramic dielectric layer having satisfactory voltage tolerance characteristics, the particle size distribution of the barium titanate powder must be in a narrow range of 0.2 to 1.0 &mgr;m, and the dispersion characteristics in the slurrying of the barium titanate powder must be superior. If the dispersion characteristics of the barium titanate powder in the slurry are not good, the barium titanate particles partly agglomerate after sintering, so that the density distribution of layer is not uniform. As a result, an insulated barium titanate layer is formed in some cases, and this therefore poses a problem in that the capacity of the condenser may not be sufficient.
Heretofore, barium titanate powders have been produced by mixing titanium compounds and barium compounds, and then sintering them so as to cause a solid phase reaction. However, in such a production method, the above chemical compounds are made to react at high temperatures, causing the particle sizes of the obtained barium titanate powders to increase, the particle size distribution thereof to broaden, and the shapes thereof to become irregular. Therefore, the dispersion characteristics of the barium titanate powder in the slurry is not so good. Japanese Unexamined Patent Publication No. 330824/93 discloses a production process in which a cubic spherical barium titanate powder having good dispersion characteristics can be obtained. According to the disclosure, hydrogen peroxide is added to a titanium compound and a barium compound so as to wet react them and obtain a barium titanate powder. Furthermore, the barium titanate powder having particle sizes ranging from 0.2 to 5 &mgr;m, obtained by this production process, is presintered at temperatures ranging from 600 to 1100° C. Japanese Unexamined Patent Publication No. 155532/85 discloses a production process for a barium titanate powder by a wet method. According to the disclosure, the hydrolysis products of a titanium compound, a water-soluble barium salt, and a water-soluble strontium salt are reacted in a strong alkali, to obtain fine particles of barium titanate strontium.
In addition, Kyng-Hee Lee disclosed a wet synthetic method in the J. Korean Ceram. Soc., vol. 21, no. 4 (1984) pages 323-326. According to this synthetic method, a mixed solution of titanium chloride and barium chloride is reacted with KOH at 100° C. or less, so that a finely crystalline barium titanate powder of high purity is efficiently obtained. The wet synthetic method has advantages in that the control of particle size is easy, fine globular barium titanate powders can be obtained, and by-products other than barium titanate are not included in the fmal product.
For a barium titanate powder used for a ceramic dielectric layer of a multilayer ceramic capacitor, the dispersion characteristics in the slurry condition is the most important, as mentioned above. In the production processes disclosed in the above references, although improvements are made in the particle size, the particle size distribution, and the shape of the barium titanate powder, the dispersion characteristics of the barium titanate powder are still not satisfactory. Therefore, the agglomeration occurs when it is actually slurried, and satisfactory characteristics for dielectric layers have not yet been obtained.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a barium titanate powder having superior dispersion characteristics in slurried condition, in which partial agglomeration of the barium titanate particles after sintering can be inhibited, thereby providing suitable barium titanate powders for materials for dielectric layers in multilayer ceramic capacitors. In particular, an object of the present invention is to provide a barium titanate powder in which the density of the sintered compact is 95% or more of the theoretical density and the dielectric constant is 4000 or more.
The inventors have researched to achieve the above objects, and have discovered a barium titanate powder having superior dispersion characteristics in slurried conditions, which is suitable for materials for dielectric layers in electrical components such as multilayer ceramic capacitors and PTC thermistors, thereby completing the invention.
The present invention provides a barium titanate powder comprising an average particle size ranging from 0.1 to 1.0 &mgr;m, a CV value (standard deviation of the particle size/the average particle size) of the particle size distribution of 40% or less, and a zeta-potential ranging from −30 to −60 mV measured by a laser Doppler method using electrophoresis at pH 6.4.
The invention will be explained in more detail hereinafter. The barium titanate powder according to the invention has an average particle size ranging from 0.1 to 1.0 &mgr;m, enabling production of a dielectric layer as a thin laminate, preferably ranging from 0.1 to 0.5 &mgr;m, and more preferably ranging from 0.1 to 0.3 &mgr;m. In the invention, the CV value (standard deviation of the particle size/the average particle size), which is one index of the particle size distribution of the barium titanate powder, is 40% or less, preferably 35% or less, and more preferably 30% or less. The average particle size, the standard deviation of the particle size, and the CV value are measured by observing the barium titanate powder through an electron microscope and analyzing the observed image. The CV value is an index of the distribution of all particle sizes with respect to the average particle size. The particle size distribution narrows and the particles are more uniform, as the CV value decreases.
In the barium titanate powder according to the invention, the zeta-potential, measured by a laser Doppler method using electrophoresis at pH 6.4, is in the range of −30 to −60 mV, preferably in the range of −35 to −55 mV, and more preferably in the range of −40 to −50 mV. The zeta-potential refers to the difference between the potential of the outermost sliding surface of the ionic fixation layer which moves adhering to the solid, and the potential of the inside of the liquid, when solid and liquid electrolyte solutions move relatively. The zeta-potential varies according to the material. In a metal oxide such as barium titanate, the zeta-potential is an index showing acid and alkali characteristics of hydroxyl groups at the surfaces of particle.
In addition, the barium titanate powder of the invention is preferably presintered at temperatures ranging from 900 to 1200° C. By presintering prior to sintering, the dielectric constant, the dielectric loss and the insulation resistance may be improved.
Production processes for the barium titanate powder according to the invention are not limited. A method using the liquid-phase method, called the “sol-gel method” or the “direct synthesis method” is more desirable than method using reactions by heating to high temperatures, such as the solid phase method and the hydrothermal method. In particular, reaction methods, in which aqueous solutions of titanium compounds and aqueous solutions of barium compounds come into contact with each other in an alkali region may be preferable.
More particularly, an aqueous solution of titanium compounds and an alkali aqueous solution of barium compounds come into contact with each other while being stirred, and w

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