Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
1999-01-20
2001-03-20
Kincaid, Kristine (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S321500, C361S320000, C361S313000, C501S137000, C423S598000
Reexamination Certificate
active
06205015
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dielectric ceramic which is advantageously used in a laminated ceramic electronic element such as a laminated ceramic capacitor having an internal conductor formed of a base metal such as nickel or nickel alloy, and to a method for producing the dielectric ceramic. The present invention also relates to a laminated ceramic electronic element which is formed of the dielectric ceramic and to a method for producing the same.
2. Description of the Related Art
Miniaturization and cost reduction of laminated ceramic electronic elements is in progress. For example, the ceramic layer has been thinned and a base metal has been employed as an internal conductor in such a ceramic electronic element. In the case of a laminated ceramic capacitor, which is one type of laminated ceramic electronic element, the dielectric ceramic layer has been formed as thin as about 3 &mgr;m and a base metal such as Cu or Ni has been employed as a material for producing an internal conductor, i.e., an internal electrode.
However, when the ceramic layer becomes thin, the strength of an electric field applied to the layer increases and causes a problem in that the ceramic layer dielectric exhibits a great change in dielectric constant induced by the electric field. Decrease of the size of ceramic grains in the thickness direction of the ceramic layer also causes a problem in reliability.
In order to cope with such situations, Japanese Patent Application Laid-Open (kokai) Nos. 9-241074, 9-241075, etc. have proposed ceramic materials which enable enhanced reliability by increasing the size of ceramic grains in the thickness direction of the dielectric ceramic layer. Thus, controlling the grain size of ceramic grains allows a reduction in change of dielectric constant induced by an electric field or temperature.
However, in the above-described conventional art, although reliability is maintained when the thickness of a dielectric ceramic layer is about 1 &mgr;m or less, variation in temperature-dielectric constant characteristics increases to thereby make it difficult to obtain products having stable temperature-dielectric constant characteristics with high reproducibility. In order to ensure stable temperature-dielectric constant characteristics, field intensity must be lowered and the rated voltage of the resultant laminated ceramic electronic elements has to be lowered. Therefore, realization of a thin layer having a thickness as thin as about 1 &mgr;m or less is difficult or impossible so long as the above-described conventional art is employed to solve the problem.
SUMMARY OF THE INVENTION
In view of the foregoing, the present invention is directed to a dielectric ceramic which is advantageously used in a laminated ceramic electronic element including a thin ceramic layer having a thickness as thin as about 1 &mgr;m or less and to a method for producing the dielectric ceramic. The present invention is also directed to a laminated ceramic electronic element which is formed of the dielectric ceramic and to a method for producing the same.
In one aspect of the present invention, there is provided a dielectric ceramic which is obtained by firing barium titanate powder having a perovskite structure in which the c-axis/a-axis ratio in the perovskite structure is in the range of about 1.003 to about 1.010 and the amount of OH groups in the crystal lattice is 1 wt. % or less.
In another aspect of the present invention, there is provided a method for producing the dielectric ceramic, which method comprises the steps of providing the above barium titanate powder in which the c-axis/a-axis ratio in the perovskite structure is in the range of about 1.003 to 1.010 and the amount of OH groups in the crystal lattice is 1 wt. % or less; and firing the barium titanate powder.
The amount of OH groups is determined based on the loss at 150° C. or more as measured during thermogravimetric analysis of specimens.
The barium titanate powder preferably has a maximum particle size of about 0.5 &mgr;m or less and an average particle size of about 0.1-0.3 &mgr;m.
Also, each particle of the above-described barium titanate powder preferably comprises a low-crystallinity portion and a high-crystallinity portion, the diameter of the low-crystallinity portion being less than about 0.65 times the particle size of the powder. As shown in
FIG. 1
, which is a transmission electron microscopic photograph of barium titanate powder, and
FIG. 2
, which is an explanatory sketch therefor, the term “low-crystallinity portion”
21
used herein refers to a domain containing a number of lattice defects such as a void
22
, whereas the term “high-crystallinity portion”
23
used herein refers to a domain containing no such lattice defects.
Also, when the ratio (average grain size of fired dielectric ceramic)/(average particle size of provided barium titanate powder) is represented by R, R preferably falls within the range of about 0.90-1.2.
Grains that constitute the dielectric ceramic of the present invention may have a core-shell structure in which the composition and crystal system differ between the core and the shell or a homogeneous structure having a uniform composition and crystal system.
The term “crystal system” used herein refers to the crystal system of perovskite crystals, i.e., to a cubic system having a c-axis/a-axis ratio in the perovskite structure of 1 or to a tetragonal system having a c-axis/a-axis ratio in the perovskite structure of 1 or more.
In yet another aspect of the present invention, there is provided a laminated ceramic electronic element including a laminate formed of a plurality of ceramic layers and an internal conductor formed along a specific interface between adjacent dielectric ceramic layers.
Specifically, in the present invention, the dielectric ceramic layer included in the laminated ceramic electronic element is constituted by a dielectric ceramic obtained by firing barium titanate powder having a perovskite structure in which the c-axis/a-axis ratio in the perovskite structure is in the range of about 1.003 to about 1.010 and an amount of OH groups in the crystal lattice is about 1 wt. % or less.
In the above-described laminated ceramic electronic element, the internal conductors preferably contain a base metal such as nickel or nickel alloy.
The laminated ceramic electronic element may further include a plurality of external electrodes at different positions on a side face or faces. In this case, the internal conductors are formed such that one end of each of the internal conductors is exposed to the side face so as to be electrically connected to one of the external electrodes. Such a structure is typically applied to laminated ceramic capacitors.
In a still further aspect of the present invention, there is provided a method for producing a laminated ceramic electronic element, which method comprises the steps of providing a barium titanate powder in which the c-axis/a-axis ratio in the perovskite structure is in the range of about 1.003 to 1.010 and the amount of OH groups in the crystal lattice is about 1 wt. % or less; fabricating a laminate in which a plurality of ceramic green sheets containing the barium titanate powder and internal electrodes are laminated so that the internal electrodes are present along specific interfaces of the ceramic green sheets; and firing the barium titanate powder to thereby provide a dielectric ceramic.
REFERENCES:
patent: 4701827 (1987-10-01), Fujikawa
patent: 4880758 (1989-11-01), Heistand, II et al.
patent: 4946810 (1990-08-01), Heistand, II et al.
patent: 5445806 (1995-08-01), Kinugasa et al.
Hamaji Yukio
Hiramatsu Takashi
Ikeda Jun
Wada Nobuyuki
Kincaid Kristine
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Thomas Eric W.
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