Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
2002-08-23
2004-07-20
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C501S138000
Reexamination Certificate
active
06764976
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a dielectric ceramic composition having a resistance to reduction and to a multilayer ceramic capacitor or other electronic device using the dielectric ceramic composition.
BACKGROUND ART
A multilayer ceramic capacitor, a kind of electronic devices, is broadly used as a compact, large-capacity, high reliability electronic device. The number of capacitors used in each piece of electronic equipments has also increased. In recent years, along with increasing miniaturization and improving performance of equipments, there has been increasingly stronger demand for further reductions in size, increases in capacity, reductions in price and improvements in reliability in multilayer ceramic capacitors.
Multilayer ceramic capacitors are normally produced by stacking a paste for the internal electrode layers and a paste for the dielectric layers using the sheet method or printing method, etc. and then co-firing the internal electrode layers and dielectric layers in the stack together.
As the electroconductive material for the internal electrode layers, generally Pd or Pd alloys are used, but since Pd is high in price, relatively inexpensive Ni, Ni alloys, and other base metals have come into use. When using a base metal as the electroconductive material of the internal electrode layers, firing in the air oxidizes the internal electrode layers, therefore the co-firing of the dielectric layers and internal electrode layers has to be done in a reducing atmosphere. When being fired in a reducing atmosphere, however, the dielectric layers end up being reduced and becoming lower in resistivity. Therefore, non-reducing type of dielectric materials is being developed.
In multilayer ceramic capacitors using a nonreducing dielectric ceramic composition, insulation resistence (IR) remarkably deteriorates when an electric field is applied, more specifically, there is a disadvantage of short IR lifetime, or low reliabity.
There also arises a disadvantage that, when the dielectric composition is exposed to the direct-current electric field, a specific permittivity &egr;r declines over time. Also, a direct-current voltage can be superimposed on a capacitor and there is a disadvantage that when a direct-current voltage is applied to a capacitor having a dielectric composition wherein a strong dielectric composition is a main composition, a capacitance generally declines (DC bias characteristics). When a dielectric composition layer is made thinner in order to make a chip capacitor more compact and larger in capacitance, an electric field affecting the dielectric composition layer at the time of applying a direct-current voltage becomes strong, so the change of permittivity &egr;r over time, that is, the capacitance change over time becomes remarkably large and DC bias characteristics decline.
Further, a capacitor is also required to have excellent temperature characteristics. In particular, in some applications, it is desired that the temperature characteristics be smooth under harsh conditions. In recent years, multilayer ceramic capacitors have come into use for various types of electronic equipments such as the engine electronic control units (ECU), crank angle sensors, antilock brake system (ABS) modules, etc., mounted in engine compartments of automobiles. These types of electronic equipment are used for stabilizing engine control, drive control, and brake control, therefore they are required to have excellent circuit temperature stability.
These types of electronic equipment are used in the environment in which the temperature falls to as low as −20° C. in the winter in cold areas or the temperature rises to as high as+130° C. in the summer while an engine is working. Recently, there has been a trend toward reduction of the number of wire harnesses used for connecting electronic apparatuses and the controlled equipment. Electronic apparatuses are also being mounted outside of the vehicles in some cases. Therefore, the environment is becoming increasingly severe for electronic apparatuses. Accordingly, capacitors used for these electronic apparatuses have to have smooth temperature characteristics over a broad temperature range.
Temperature-compensating capacitor materials superior in temperature characteristics such as, (Sr, Ca)(Ti, Zr)O
3
based, Ca(Ti, Zr) O
3
based, Nd
2
O
3
-2TiO
2
based, La
2
O
3
-2TiO
2
based, and other materials are generally known, but these compositions have extremely low specific permittivities (generally 100 or less), so it is substantially impossible to produce a capacitor having large capacity.
To create dielectric ceramic compositions having the high permittivity and smooth capacitance-temperature characteristics, compositions comprised of BaTiO
3
as a main component plus Nb
2
O
5
—Co
3
O
4
, MgO—Y, rare earth elements (Dy, Ho, etc.), Bi
2
O
3
—TiO
2
, etc. are known. A mechanism of smoothing the capacitance-temperature characteristic is not completely disclosed, but the Japanese Examined Patent Publication (Kokoku) No. 7-118431 proposes the way of smoothing the capacitance-temperature characteristic by dissolving Mg and rare earth elements inside a core-shell structure. However, in an article “Key Engineering Materials Vols. 17 to 24, 157 to 158 (1999); A study on Capacitance Aging in Ni-Electroded, BaTiO
2
—Based MLCCs with X7R Characteristics”, it is reported that the core-shell structure is not essential to satisfy the X7R characteristic of the EIA Standards (−55 to 125° C., &Dgr;C/C=±15% or less).
Also, when looking at the temperature characteristics of a dielectric ceramic composition comprising BaTiO
3
as a main component, because of the Curie temperature of pure BaTiO
3
close to 130° C., it is extremely difficult to satisfy the R characteristic of the capacitance-temperature characteristic (&Dgr;C/C=±15% or less) in the region higher than 130° C. Therefore, a BaTiO
3
based high permittivity material can only satisfy the X7R characteristic of the EIA standard (−55 to 125° C., AC/C=±15% or less). Satisfaction of the X7R characteristic is not good enough to be used in an electronic apparatus of an automobile which is used in the above-mentioned harsh environments. The above electronic apparatus requires a dielectric ceramic composition satisfying the X8R characteristic of the EIA standard (−55 to 150° C., &Dgr;C/C=+15% or less).
To satisfy the X8R characteristic in a dielectric ceramic composition comprised of BaTiO
3
as a main component, it has been proposed to have the Curie temperature of the composition shift to the high temperature side by replacing the Ba in the BaTiO
3
with Bi, Pb, etc. (Japanese Unexamined Patent Publication (Kokai) No. 10-25157 and No. 9-40465). Further, it has also been proposed to satisfy the X8R characteristic by selecting a BaTiO
3
+CaZrO
3
+ZnO+Nb
2
O
5
based composition (Japanese Unexamined Patent Publication (Kokai) No. 4-295048, No. 4-292458, No. 4-292459, No. 5-109319, and No. 6-243721).
In each of these compositions, however, Pb, Bi, and Zn which are easily vaporized and scattered are used, so firing in the air or another oxidizing atmosphere is a prerequisite. Therefore, there are the problems that it is not possible to use an inexpensive base metal such as Ni for the internal electrodes of the capacitor and it is necessary to use Pd, Au, Ag, or other high priced precious metals.
On the other hand, to enable to attain the high permittivity, to satisfy the X8R characteristic and to be fired in a reducing atmosphere, the present inventors have already proposed a dielectric ceramic composition described below (The Japanese Unexamined Patent Publication (Kokai) No. 2000-154057). The dielectric ceramic composition at least comprises BaTiO3 as a main component, a first subcomponent including at least one compound selected from MgO, CaO, BaO, SrO and Cr
2
O
3
, a second component expressed by (Ba, Ca)
x
Sio
2+x
(note that x=0.8 to 1.2), a third subcomponent includin
Kobayashi Hisashi
Nomura Takeshi
Sato Shigeki
Uchida Tomoko
Group Karl
TDK Corporation
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