Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
2000-01-12
2002-01-01
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C501S138000, C501S139000, C361S321400, C361S321500
Reexamination Certificate
active
06335302
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dielectric composition and a ceramic capacitor using the same, more particularly relates to a dielectric composition superior in resistance to reduction at the time of sintering, exhibiting a high dielectric constant and low distortion factor, and superior in the temperature-dependence of the capacity and to a ceramic capacitor having a high insulation resistance, high capacity, and low distortion characteristic, having a flat temperature-dependence of the capacity, and using a base metal for the internal electrodes.
2. Description of the Related Art
As a dielectric composition used for a ceramic capacitor, one having a high dielectric constant comprised mainly of barium titanate (BaTiO
3
) is known, but since it is a strong dielectric, the nonlinear characteristic of the voltage is strong and the distortion factor originate from D-E hysterisys loop is a large −50 dB to −70 dB. Therefore, a BaTiO
3
based capacitor cannot be used for a capacitor for which a low distortion factor is required such as a coupling circuit capacity, audio circuit capacitor, or image processing circuit capacitor. Film capacitors, electrolytic capacitors, etc. are exclusively being used for these, but such film capacitors or electrolytic capacitors are hard to reduce in size and there are problems in surface mounting.
On the other hand, among the ceramic capacitors, ones comprised of ordinary dielectrics such as CaTiO
3
, SrTiO
3
, CaSrZrO
3
, and NdTiO
3
are low in distortion factor and therefore can be used for coupling circuits, audio circuits, etc., but since they are ordinary dielectrics, the dielectric constant er is a low 30 to 200 and obtaining a high capacity capacitor would be difficult.
Therefore, as a dielectric composition exhibiting both a high dielectric constant and low distortion factor, ones comprised mainly of SrTiO
3
, Bi
2
TiO
3
, CaTiO
3
, and PbTiO
3
have been proposed (for example, see Japanese Unexamined Patent Publication (Kokai) No. 3-97669).
Internal electrodes of a ceramic capacitor, however, use a precious metal such as platinum Pt, gold Au, Pd or silver Ag, and their alloy for example, Pt—Au, Ag—Pd but from the viewpoint of cost, nickel or another base metal is preferably used.
The above-mentioned dielectric composition contains bismuth Bi or lead Pb which have a low vapor pressure, so when sintered in a reducing atmosphere, these end up evaporating. Accordingly, while predicated on sintering in an oxidizing atmosphere, if sintered in an oxidizing atmosphere, when using a low cost base metal, for example, nickel, for the internal electrodes, the nickel ends up being oxidized. In the end, there is no choice but to use a precious metal such as platinum, gold, or silver for the internal electrodes.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a dielectric composition superior in resistance to reduction at the time of sintering, exhibiting a high dielectric constant and low distortion factor, and superior in temperature-dependence of the capacity and a capacitor having a high insulation resistance, high capacity, and low distortion characteristic, having a flat temperature-dependence of the capacity, and using a base metal for the internal electrodes.
(1) The present inventors et al. engaged in in-depth research in order to obtain a balanced dielectric composition having a large dielectric constant, a small distortion factor, and a small rate of change of the capacity with respect to temperature (hereinafter referred to as a flat temperature-dependence of the capacity) and as a result obtained the following discoveries.
First, the distortion factor depends on the field dependency and non-linearity of the dielectric constant (that is, a strong dielectric property). As measures for suppressing the factor, it may be considered effective to improve the linearity of the dielectric constant or reduce the crystalline anistrophy so as to reduce the strong dielectric property or to use an ordinary dielectric phase. If the strong dielectric property is reduced, however, the dielectric constant falls, so balancing these becomes important. Further, an ordinary dielectric exhibits a negative temperature-dependence of the capacity, while a strong dielectric exhibits a positive temperature-dependence of the capacity, so balancing the strong dielectric and the ordinary dielectric becomes important for the temperature-dependence of the capacity as well.
The balance of the dielectric constant, distortion factor, and temperature-dependence of the capacity becomes suitable by controlling the molar ratios of composition of the calcium titanate, strontium titanate, and barium titanate. In the present invention, by adding the strong dielectric barium titanate to the ordinary dielectric calcium titanate (or strontium titanate), the balance of the ordinary dielectric phase and strong dielectric phase is promoted and it is possible to obtain a dielectric composition having a large dielectric constant, a small distortion factor, and a flat temperature-dependence of the capacity.
That is, if the molar ratio of composition of barium titanate is increased, while the dielectric constant becomes larger, the distortion factor and the rate of change of the capacity with respect to the temperature also become larger. As opposed to this, if the molar ratio of composition of strontium titanate is increased, while the distortion factor becomes smaller, the dielectric constant also becomes smaller. Further, when the molar ratio of composition of calcium titanate is increased too, while the distortion factor becomes small, the dielectric constant also becomes small.
Further, comparing calcium titanate and strontium titanate, if the molar ratio of composition of calcium titanate is increased, the rate of change of capacity with respect to temperature becomes smaller, but the dielectric constant also becomes smaller. As opposed to this, if the molar ratio of composition of strontium titanate is increased, the dielectric constant becomes larger, but the rate of change of the capacity with respect to temperature also becomes larger.
(2) Based on this discovery, the first aspect of the invention provides a dielectric composition containing at least calcium titanate, strontium titanate, and barium titanate, wherein the molar ratios of composition of the three are such that at least a molar ratio of composition of the barium titanate is not more than 0.3 and wherein the composition comprised of the calcium titanate, strontium titanate, and barium titanate contains at least one crystal structure from among tetragonal and orthorhombic crystal structures.
The composition comprising calcium titanate, strontium titanate, and barium titanate exhibits an insoluble phase and cubic crystal structure as well due to these three ratios of composition, but ratios of composition exhibiting mainly an insoluble phase are not desirable since the distortion factor is large, while ratios of composition exhibiting mainly a cubic crystal structure is not desirable since the distortion factor is large and the rate of change of the capacity with respect to the temperature is also large.
Further, a cubic crystal structure is exhibited in the region where the barium titanate is rich, but in this region, the molar ratio of composition of the barium titanate is large, so the dielectric constant becomes large, but the distortion factor also becomes large and the balance between the dielectric constant and the distortion factor is poor. Therefore, a tetragonal crystal structure in the region where the molar ratio of composition of barium titanate is not more than 0.3 is more preferable.
In the above aspect of the invention, the crystal structure of the dielectric composition obtained is not one which is completely comprised of at least one of a tetragonal crystal and orthorhombic crystal structure. It means the main crystal structure is at least one of a tetragonal crystal and orthorhombic crystal structure. Therefore,
Fujikawa Yoshinori
Masumiya Kaori
Nagai Akiko
Satoh Shigeki
Group Karl
Oliff & Berridg,e PLC
TDK Corporation
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