Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
2001-10-18
2003-09-16
Wood, Elizabeth D. (Department: 1755)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C501S134000, C501S135000, C501S136000, C501S138000, C501S139000, C361S321200, C361S321400
Reexamination Certificate
active
06620753
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dielectric ceramic, a process for producing the same and a multilayer ceramic capacitor using the dielectric ceramic. Specifically, it relates to improvement for advantageously reducing the thickness of dielectric ceramic layers of multilayer ceramic capacitors.
2. Description of the Related Art
Multilayer ceramic capacitors are generally produced in the following manner.
Initially, ceramic green sheets comprising a dielectric ceramic material are prepared. Each of the ceramic green sheets carries a patterned conductive material to constitute an inner electrode on a surface thereof. A dielectric ceramic mainly containing, for example, BaTiO
3
is used in this procedure.
Plural plies of the ceramic green sheets including the ceramic green sheets carrying the conductive material are laminated and thermo-compressed to thereby yield a monolithic multilayer green compact.
Next, the multilayer green compact is fired to thereby yield a multilayer sintered compact (hereinafter briefly referred to as “laminate”). The laminate includes inner electrodes comprising the conductive material.
Outer electrodes are then formed on outer surfaces of the laminate so as to be electrically connected to specific ones of the inner electrodes. The outer electrodes are formed, for example, by applying a conductive paste containing a conductive metal powder and a glass frit to outer surfaces of the laminate and baking the applied conductive paste.
To minimize the production cost of a multilayer ceramic capacitor, nickel, copper and other base metals that are available at relatively low cost have been frequently used as the conductive material for the inner electrodes. However, upon the production of a multilayer ceramic capacitor including inner electrodes composed of a base metal, a multilayer green compact must be fired in a neutral or reducing atmosphere in order to avoid oxidation of the base metal during the firing operation. Accordingly, a dielectric ceramic used in the multilayer ceramic capacitor must be resistant to reduction.
Miniaturization of electronic parts has rapidly advanced in recent years accompanying the expansion of electronics, and multilayer ceramic capacitors must be miniaturized and must have larger capacities.
Accordingly, strong demands have been made for dielectric ceramics that have a high dielectric constant, low rates of temperature-dependent and time-dependent changes in dielectric constant even in such a firing atmosphere that a base metal for use in inner electrodes is not oxidized. These dielectric ceramics must have a high electrical insulating property and satisfactory reliability even when the thickness of constitutional dielectric ceramic layers is reduced. However, conventional dielectric ceramics do not necessarily satisfy these requirements.
For example, Japanese Unexamined Patent Application Publications No. 5-9066, No. 5-9067 and No. 5-9068 propose, as the dielectric ceramics being resistant to reduction, compositions each including BaTiO
3
, a rare earth metal oxide and Co
2
O
3
. However, these dielectric ceramics do not sufficiently satisfy the demands of the market in reliability when the thickness of the constitutional dielectric ceramic layers is reduced to, for example, 5 &mgr;m or less and specifically 3 &mgr;m or less, although these dielectric ceramics satisfy X7R characteristic specified by EIA specifications and exhibit high electrical insulation property.
Japanese Unexamined Patent Application Publications No. 6-5460 and No. 9-270366 each propose a dielectric ceramic having a high dielectric constant, a low rate of temperature-dependent change in dielectric constant and a long high-temperature load life. The dielectric ceramic described in the former publication exhibits decreased reliability with decreasing thickness of the dielectric ceramic layers and has a high rate of time-dependent change in electrostatic capacity, since the BaTiO
3
powder used in the dielectric ceramic has a large particle size. Likewise, the dielectric ceramic described in the latter publication exhibits decreased reliability with decreasing thickness of the dielectric ceramic layers and has a high rate of time-dependent change in electrostatic capacity under the application of a direct-current voltage.
When the thickness of dielectric ceramic layers is reduced in order to decrease the size of and to increase the capacity of the resulting multilayer ceramic capacitor, and the same rated voltage as that before reduction of the thickness is applied thereto, the field intensity applied per constitutional layer of the dielectric ceramic layers increases and insulation resistance at room temperature and at high temperatures decreases to thereby markedly decrease reliability of the resulting dielectric ceramic capacitor. Accordingly, the rated voltage must be decreased when the thickness of multilayer dielectric ceramic layers comprising a conventional dielectric ceramic is decreased.
Demands have therefore been made on a multilayer ceramic capacitor which does not require a reduced rated voltage even when the thickness of dielectric ceramic layers is reduced, has a high insulation resistance at a high field intensity and is satisfactory in reliability.
Multilayer ceramic capacitors are generally used under the application of a direct-current voltage and vary in electrostatic capacity with time under this condition. However, as the thickness of each of the dielectric ceramic layers of the multilayer ceramic capacitor is reduced in order to reduce the size of and to increase the capacity of the multilayer ceramic capacitor, the direct-current field intensity per a constitutional layer of the dielectric ceramic layers increases, and time-dependent change in electrostatic capacity further increases.
Demands have therefore been made on a multilayer ceramic capacitor which exhibits less time-dependent change in electrostatic capacity under the application of a direct-current voltage.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a dielectric ceramic and a process for producing the same, which dielectric ceramic has a high dielectric constant, exhibits less temperature-dependent change and time-dependent change under the application of a direct-current voltage, has a low dielectric loss, has a high product of insulation resistance (R) and electrostatic capacity (C) (CR product) and has a long life in insulation resistance at high temperatures and at high voltages, as well as to provide a multilayer ceramic capacitor using the dielectric ceramic.
Specifically, the present invention provides, in an aspect, a dielectric ceramic including ABO
3
as a major component, and R and M as accessory components, where A is at least one of Ba, Sr and Ca; B is at least one of Ti, Zr and Hf; R is at least one of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y; and M is at least one of Ni, Co, Fe, Cr and Mn. The dielectric ceramic has crystal grains including ABO
3
as a major component, and grain boundaries constituting interfaces between the crystal grains. In the dielectric ceramic, when the grain boundaries are analyzed at respective four points which equally divide a periphery of each of plural crystal grains selected arbitrarily form the crystal grains, the R and M exist at 70% or more of the sum of the respective four points.
The invented dielectric ceramic may further include Si as an accessory component. In this case, when the grain boundaries are analyzed at respective four points which equally divide a periphery of each of plural crystal grains selected arbitrarily form the crystal grains, the R, M and Si exist at about 70% or more of the sum of the respective four points.
The grain boundaries are preferably analyzed by an energy dispersion X-ray analysis method.
By these configurations, the resulting dielectric ceramic has a high dielectric constant, exhibits less temperature-dependent change and time-dependent change in dielectric constant und
Nakamura Tomoyuki
Sakai Kentaro
Sano Harunobu
Dickstein , Shapiro, Morin & Oshinsky, LLP
Murata Manufacturing Co. Ltd.
Wood Elizabeth D.
LandOfFree
Dielectric ceramic, process for producing the same and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Dielectric ceramic, process for producing the same and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dielectric ceramic, process for producing the same and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3093828