Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2001-08-20
2004-10-05
Fiorilla, Christopher A. (Department: 1731)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C029S025030, C156S089160, C264S615000
Reexamination Certificate
active
06800270
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of production of a dielectric ceramic composition and a method of production of an electronic device containing dielectric layers such as a multilayer ceramic capacitor.
2. Description of the Related Art
A multilayer ceramic capacitor is broadly used as a compact, large capacity, high reliability electronic device. A large number are used in electrical equipment and electronics. In recent years, along with the reduction in size and improvement in performance of such equipment, increasingly tough demands are being made for further reduction of size, increase of capacity, lowering of price, and improvement of reliability of such multilayer ceramic capacitors.
A multilayer ceramic capacitor is normally produced by stacking and firing a paste of internal electrodes and a slurry of a dielectric (paste) by the sheet method or printing method. In general, Pd or Pd alloy had been used for such internal electrodes, but Pd is high in price, so relatively inexpensive Ni or Ni alloy is now being used. When forming the internal electrodes by Ni or an Ni alloy, however, if firing in the atmosphere, there is the problem that the electrodes end up oxidizing. Therefore, in general, after the binder is removed, firing is performed at an oxygen partial pressure lower than the equilibrium oxygen partial pressure of Ni and NiO, then the dielectric layers are reoxidized by heat treatment (Japanese Unexamined Patent Publication (Kokai) No. 3-113116 and Japanese Patent No. 2787746).
If firing in a reducing atmosphere, however, the dielectric layers are reduced and the specific resistance ends up becoming smaller. Therefore, a reduction resistant dielectric material which is not reduced even if fired in a reducing atmosphere has been proposed (I. Burn et al., “High Resistivity BaTiO
3
Ceramics Sintered in CO—CO
2
Atmospheres”,
J. Mater. Sci
., 10, 633 (1975); Y. Sakabe et al., “High-Dielectric Constant Ceramics for Base Metal Monolithic Capacitors”, pn
J. Appl. Phys
., 20 Supple. 20-4, 147 (1981)).
A multilayer ceramic capacitor using such reduction resistant dielectric materials, however, suffers from the problem of a short high temperature accelerated lifetime of the insulation resistance (IR) and a low reliability. Further, it suffers from the problem that the specific dielectric constant of the dielectric falls along with time. This is particularly remarkable under a DC electric field. If the thickness of the dielectric layers is reduced to make the multilayer ceramic capacitor smaller in size and larger in capacity, the strength of the electric field applied to the dielectric layers when applying a DC voltage becomes larger. Therefore, the change in the specific dielectric constant becomes remarkably larger.
In the standard known as the X7R characteristic set in the EIA standard, the rate of change of the capacity is set within ±15% between −55° C. to 125° C. (reference temperature of 25° C.). As a dielectric material satisfying the X7R characteristic, for example, the BaTiO
3
+SrTiO
3
+MnO-based composition disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-36170 is known. This composition, however, changes a large degree in capacity along with time under a DC electric field. For example, if a DC electric field of 50 V is applied at 40° C. for 1000 hours, the rate of change of the capacity ends up becoming about −10 to −30% or so and therefore the X7R characteristic can no longer be satisfied.
Further, in the standard called the “B characteristic”, that is, the temperature characteristic of the capacity (EIAJ standard), the rate of change is set to within ±10% between −25 to 85° C. (reference temperature of 20° C.).
Further, as other reduction resistant dielectric ceramic compositions, the BaTiO
3
+MnO+MgO disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-71866, the (Ba
1−x
Sr
x
O)
a
Ti
1−y
Zr
y
O
2
+&agr;((1-z)MnO+zCoO)+&bgr;(1-t)A
2
O
5
+tL
2
O
3
)+wSiO
2
(where A═Nb, Ta, V; L═Y or a rare earth element) disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-250905, the barium titanate adding Ba
a
Ca
1-a
SiO
3
disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-83256, etc. may be mentioned.
However, even with these dielectric ceramic compositions, if the thickness of the dielectric layers is a superthin one of for example less than 4 &mgr;m, it is extremely difficult to satisfy all of the properties of the temperature characteristic of the capacity, the change in capacity along with time under a DC electric field, the accelerated lifetime of the insulation resistance, and the drop in capacity under a DC bias. For example, in the compositions disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-250905 and Japanese Unexamined Patent Publication (Kokai) No. 2-83256, the problem arises of a short accelerated lifetime of the insulation resistance and a large drop in capacity under a DC bias.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of production for obtaining a multilayer ceramic capacitor or other electronic device containing dielectric layers able to satisfy all of the temperature characteristics of capacity, that is, the X7R characteristic (EIA standard) and B characteristic (EIAJ standard), even when the dielectric layers are superthin layers and having a small change in capacity along with time under a DC electric field, a long accelerated lifetime of the insulation resistance, and small drop in capacity under a DC bias. Another object of the present invention is to provide a method of production of a dielectric ceramic composition able to be suitably used as a dielectric layer of a multilayer ceramic capacitor or other electronic device containing dielectric layers having such superior properties.
To achieve the first object, according to a first aspect of the present invention, there is provided a method of production of a dielectric ceramic composition having at least
a main component expressed by a formula Ba
m
TiO
2+n
, wherein m is 0.995≦m≦1.010, n is 0.995≦n≦1.010, and the ratio of Ba and Ti is 0.995≦Ba/Ti≦1.010,
a first subcomponent containing at least one compound selected from MgO, CaO, BaO, SrO, and Cr
2
O
3
,
a second subcomponent containing at least one compound selected from SiO
2
, MO (where M is at least one element selected from Ba, Ca, Sr, and Mg), Li
2
O, and B
2
O
3
,
a third subcomponent containing at least one compound selected from V
2
O
5
, MoO
3
, and WO
3
, and
a fourth subcomponent containing an oxide of R (where R is at least one element selected from Y, Dy, Td, Gd, and Ho), wherein
the ratio of the subcomponents with respect to 100 moles of the main component is
first subcomponent: 0.1 to 3 moles,
second subcomponent: 2 to 12 moles,
third subcomponent: 0.01 to 3 moles,
fourth subcomponent: 0.1 to 10.0 moles (where, the number of moles of the fourth subcomponent is a ratio of R alone),
said method of producing the dielectric ceramic composition comprising the step of:
mixing in said main component at least part of other subcomponents except for said second subcomponent to prepare a pre-calcination powder,
calcining the pre-calcination powder to prepare a calcined powder, and
mixing at least said second subcomponent in said calcined powder to obtain the dielectric ceramic composition having molar ratios of the subcomponents to the main component of the above ratios.
In the method of the present invention, preferably a dielectric ceramic composition further containing a fifth subcomponent containing MnO and having a ratio of the fifth subcomponent to 100 moles of the main component of 0.05 to 1.0 mole is obtained.
Preferably, a dielectric ceramic composition having a molar ratio of the third subcomponent to 100 moles of the main component of 0.01 to 0.1 mole, more preferably 0.01 to less than 0.1 mole, is obtained.
In the present
Nomura Takeshi
Sato Shigeki
Fiorilla Christopher A.
Olif & Berridge PLC
TDK Corporation
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