Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
2000-07-18
2003-05-06
Koslow, C. Melissa (Department: 1755)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C501S138000, C361S311000, C361S321500
Reexamination Certificate
active
06559084
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dielectric ceramic composition having a resistance to reduction and to a multi-layer ceramic capacitor or other electronic device using the same, more particularly relates to a dielectric ceramic composition having a capacity-temperature characteristic satisfying the X8R characteristic of the Electronic Industries Association (EIA) standard (−55 to 150° C., &Dgr;C=±15% or less) and capable of improving mechanical strength of an electronic device.
2. Description of the Related Art
A multi-layer ceramic capacitor, one type of electronic device, is being broadly used as a compact, large capacity, high reliability electronic device. The number used in each piece of electronic equipment has also become larger. In recent years, along with the increasing miniaturization and improved performance of equipment, there have been increasingly stronger demands for further reductions in size, increases in capacity, reductions in price, and improvements in reliability in multi-layer ceramic capacitors.
Multi-layer ceramic capacitors are normally produced by stacking a paste for forming the internal electrode layers and a paste for forming the dielectric layers using the sheet method or printing method etc. and then cofiring the internal electrode layers and dielectric layers in the stack together.
As the electroconductive material for the internal electrode layers, generally Pd or a Pd alloy is 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 atmosphere ends up oxidizing the internal electrode layers and therefore the cofiring 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 specific resistance. Therefore, nonreducing type dielectric materials are being developed.
Further, a capacitor is also required to be excellent in temperature characteristics. In particular, in some applications, it is desired that the temperature characteristics be smooth under harsh conditions. In recent years, multi-layer ceramic capacitors have come into use for various types of electronic equipments such as the engine electronic control units (ECU) mounted in engine compartments of automobiles, crank angle sensors, antilock brake system (ABS) modules, etc. These electronic equipments are used for stabilizing engine control, drive control, and brake control, and therefore are required to have excellent circuit temperature stability.
The environment in which these electronic equipments are used is envisioned to be one in which the temperature falls to as low as −20° C. or so in the winter in cold areas or the temperature rises to as high as +130° C. or so in the summer right after engine startup. Recently, there has been a trend toward reduction of the number of wire harnesses used for connecting electronic apparatuses and the equipment they control. 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 in a broad temperature range.
As temperature-compensating capacitor materials superior in temperature characteristics, (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 dielectric constants (generally less than 100), so it is substantially impossible to produce a capacitor having a large capacity.
As a dielectric ceramic composition having a high dielectric constant and a smooth capacity-temperature characteristic, a composition 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. is known. Looking at the temperature characteristic of a dielectric ceramic composition comprising BaTiO
3
as a main component, since the Curie temperature of pure BaTiO
3
is close to about 130° C., it is extremely difficult to satisfy the R characteristic of the capacity-temperature characteristic (&Dgr;C=±15% or less) in the region higher in temperature than that. Therefore, a BaTiO
3
based high dielectric constant material can only satisfy the X7R characteristic of the EIA standard (−55 to 125° C., &Dgr;C=±15% or less). If only satisfying the X7R characteristic, the material is not good enough for 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=±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 shift the Curie temperature 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 as well, however, since Pb, Bi, and Zn which are easily vaporised and scattered are used, firing in air or another oxidizing atmosphere becomes 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 another high priced precious metal.
Furthermore, in a dielectric ceramic composition of the related art, there are problems of not having resistance to repeated heat impulses from a low temperature to a high temperature, etc. so improvement in mechanical strength is required.
On the other hand, a method of improving the strength by forming acicula crystalline on outer surface regions of the both ends in the stack direction of a ceramic layered body has been proposed (Japanese Unexamined Patent Publication (kokai) No. 9-312234). In the method described in the publication, an oxide paste composed of Tio
2
as a main component is applied on the surface of the ceramic layered body, which is dried and then subjected to heat processing, consequently, the acicula crystalline is formed on the outer surface region of the ceramic. It is considered that the acicula crystalline is deposited substance of Ba
4
Ti
13
O
30
, Ba
6
Ti
17
O
40
, etc.
In the method described in the above publication, however, since the ceramic base is reduced when firing the ceramic layered body in the reducing atmosphere, acicula crystalline cannot be formed inside or close to the center of the ceramic layered body. Accordingly, forming of the acicula crystalline is limited only near the outer surface of the ceramic layered body and the mechanical strength is not sufficiently improved. Thus, when a thickness of a capacitor cover for protecting the ceramic layered body cannot be sufficiently secured, an electronic device having sufficient strength able to be used in practice cannot be obtained.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a dielectric ceramic composition having a high specific dielectric constant and a capacity-temperature characteristic satisfying the X8R characteristic of the EIA standard (−55 to 150° C., &Dgr;C=±15% or less), and able to be fired in a reducing atmosphere, and further, to provide a multi-layer ceramic capacitor or other electronic de
Fujikawa Yoshinori
Sato Shigeki
Terada Yoshihiro
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