Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
2003-01-15
2004-10-26
Brunsman, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C501S139000, C252S06230Q, C361S321300, C361S321400, C361S321500
Reexamination Certificate
active
06809052
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dielectric ceramic composition used, for example, as a dielectric layer of a multilayer ceramic capacitor and an electronic device using the dielectric ceramic composition as a dielectric layer.
2. Description of the Related Art
A multilayer ceramic capacitor as an example of electronic devices is produced by alternately stacking, for example, ceramic green sheets made by a predetermined dielectric ceramic composition and internal electrode layers having a predetermined pattern, then making the same one body to obtain a green chip, and simultaneously firing the green chip. The internal electrode layers of the multilayer ceramic capacitor are made to be one body with ceramic dielectrics by firing, so it has been necessary to select materials not reacting with ceramic dielectrics. Therefore, inevitably, platinum, palladium and other precious metals have been conventionally used as materials for composing the internal electrode layers.
While, in recent years, a dielectric ceramic composition wherein nickel and other inexpensive base metals can be used has been developed and a wide reduction of the costs has realized.
In recent years, demands for compact electronic devices have become strong as electronic circuits become higher in density, and multilayer ceramic capacitors have rapidly become more compact and gained a larger capacitance. Along therewith, a thickness per one dielectric layer in a multilayer ceramic capacitor has become thinner, so a dielectric ceramic composition capable of maintaining its reliability as a capacitor even with thin layers has been desired. Particularly, when making midvoltage capacitors used with a high rated voltage compact and high capacitance, very high reliability is required to a dielectric ceramic composition.
The present inventors have proposed a dielectric ceramic composition disclosed in the patent articles 1 and 2, etc. as techniques capable of using base metals as materials to compose internal electrodes, and by which a temperature dependence of a capacitance satisfies the X7R characteristic (−55 to 125° C., &Dgr;C=±15% or less) of the EIA Standards. All of the techniques were to improve an accelerated lifetime of insulation resistance (IR) by adding Y
2
O
3
. However, further improvement of reliability has been desired in the circumstance where capacitors rapidly become more compact and obtain a larger capacitance.
On the other hand, there is known a dielectric ceramic composition, for example, disclosed in the patent article 3 as another technique satisfying the X7R characteristic.
The above dielectric ceramic compositions are those obtained by adding an oxide of at least one kind of rare-earth elements, Sc and Y, and an oxide of at least one kind of rare-earth elements, Gd, Tb and Dy to barium titanate. Namely, the technique disclosed in the patent article 3 is to improve an accelerated lifetime of insulation resistance by satisfying the X7R characteristic of the EIA Standards by adding oxides of at least two kinds of rare-earth elements selected from each of freely divided two element groups to barium titanate.
In the technique disclosed in the patent article 3, however, since the accelerated lifetime of the insulation resistance becomes short after firing when trying to satisfy the X7R characteristic, so there was a problem of keeping balance of the X7R characteristic and the lifetime. Moreover, along with attaining a further compact size and a larger capacitance, a dielectric loss (tan &dgr;) becomes large and a DC bias and other reliability are liable to decline, so improvements have been desired.
Particularly, when trying to use as a material of a midvoltage multilayer ceramic capacitor having a high rated voltage, it was necessary to thicken at least 15 &mgr;m of thickness per one dielectric layer if considering the reliability.
Note that there is disclosed in the patent article 4 a dielectric material having a preferable temperature characteristic for an object of satisfying a range of the X8R characteristic of the EIA Standards. Here, a rare-earth element is added for maintaining the temperature characteristic preferable, thus, a kind of the rare-earth element is different and an ionic radius of the rare-earth element is not focused on.
The patent Article 1: The Japanese Unexamined Patent Publication No. 6-84692
The patent Article 2: The Japanese Unexamined Patent Publication No. 6-342735
The patent Article 3: The Japanese Unexamined Patent Publication No. 10-223471
The patent Article 4: The Japanese Unexamined Patent Publication No. 2000-154057
SUMMARY OF THE INVENTION
An object of the present invention is to provide a dielectric ceramic composition which has an excellent reducing resisting property, exhibits excellent temperature dependence of capacitance after firing and an improved accelerated lifetime of insulation resistance. Another object of the present invention is to provide an electronic device, such as a multilayer ceramic capacitor, having high reliability produced by using the dielectric ceramic composition, particularly to provide an electronic device, such as a midvoltage multilayer ceramic capacitor having a high rated voltage.
To attain the above objects, according to a first aspect of the present invention, there is provided a dielectric ceramic composition, comprising
a main component including barium titanate,
a fourth subcomponent including an oxide of R1 (note that R1 is at least one kind selected from a first element group composed of rare-earth elements having a effective ionic radius of less than 108 pm when having a coordination number of nine), and
a fifth subcomponent including an oxide of R2 (note that R2 is at least one kind selected from a second element group composed of rare-earth elements having a effective ionic radius of 108 pm to 113 pm when having a coordination number of nine).
Preferably, a effective ionic radius of rare-earth elements composing the first element group is over 106 pm.
Preferably, when assuming a effective ionic radius of rare-earth elements composing the first element group is r1 and a effective ionic radius of rare-earth elements composing the second element group is r2, the first element group and the second element group are composed so that a ratio of r1 and r2 (r2/r1) satisfies a relationship of 1.007<r2/r1<1.06.
According to a second aspect of the present invention, there is provided a dielectric ceramic composition, comprising
a main component including barium titanate,
a fourth subcomponent including an oxide of R1 (note that R1 is at least one kind selected from a first element group composed of rare-earth elements having a effective ionic radius of less than 108 pm when having a coordination number of nine, and at least includes Y), and
a fifth subcomponent including an oxide of R2 (note that R2 is at least one kind selected from a second element group composed of rare-earth elements having a effective ionic radius of 108 pm to 113 pm when having a coordination number of nine).
Preferably, when assuming a effective ionic radius of Y included in the first element group is ry and a effective ionic radius of rare-earth elements composing the second element group is r2, the second element group is composed so that a ratio of ry and r2 (r2/ry) satisfies a relationship of 1.007<(r2/ry)<1.05
Preferably, when assuming a effective ionic radius of Y included in the first element group is ry and a effective ionic radius of rare-earth elements composing the second element group is r2, the second element group is composed so that a ratio of ry and r2 (r2/ry) satisfies a relationship of 1.007<(r2/ry)<1.03
According to a third aspect of the present invention, there is provided a dielectric ceramic composition, comprising
a main component including barium titanate,
a fourth subcomponent including an oxide of R1 (note that R1 is at least one kind selected from a first element group composed of rare-earth elements having a effective ionic radius of less
Horie Yuusaku
Masumiya Kaori
Nakano Yukie
Brunsman David
TDK Corporation
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