Ceramic capacitor and method for making the same

Details Ceramic capacitor and method for making the same Ceramic capacitor and method for making the same

2000-06-09

2001-10-09

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Fixed capacitor

C361S321200, C361S306300, C361S321400, C361S321500

Reexamination Certificate

active

06301092

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ceramic capacitors, particularly to monolithic ceramic capacitors having thin dielectric ceramic layers, and relates to methods for making the same.
2. Description of the Related Art
Ceramic capacitors, particularly monolithic ceramic capacitors, are typically produced as follows. Dielectric sheets, each primarily composed of, for example, BaTiO
3
, are prepared. An electrode material for an internal electrode is applied to a surface of each dielectric sheet. The dielectric sheets provided with electrode materials are laminated, are thermally compressed, and are sintered at 1,250 to 1,350° C. in an ambient atmosphere to form a dielectric ceramic having the internal electrodes. External electrodes connecting to the internal electrodes are provided by baking on both sides of the dielectric ceramic to form a monolithic ceramic capacitor.
Thus, the internal electrode material must satisfy the following conditions.
(A) Since the dielectric sheets and the internal electrode material are simultaneously sintered, the internal electrode material must have a melting point which is higher than the temperature at which the dielectric sheet material is sintered.
(B) The internal electrode material must not be oxidized and must not react with the dielectric sheet material in an oxidative high-temperature atmosphere.
As internal electrode materials satisfying such conditions, noble metals, such as platinum, gold, palladium and silver-palladium alloys, have been used since these noble metals exhibit superior properties when used as internal electrodes. These noble metals, however, are expensive, and this is a major factor increasing production costs of monolithic ceramic capacitors.
Other than noble metals, base metals having high melting points are, for example, Ni, Fe, Co, W, and Mo. These base metals, however, are readily oxidized in high-temperature oxidizing atmospheres and thus are not suitable for electrodes. When these base metals are used as internal electrodes of monolithic ceramic capacitors, the composite must be sintered in neutral or reducing atmospheres. Conventional dielectric ceramic materials, however, are significantly reduced during sintering in such neutral or reducing atmospheres and are thus converted to semiconductor materials.
In order to solve this problem, Japanese Examined Patent Application Publication No. 57-42588 discloses a dielectric material composed of a barium titanate solid solution in which the ratio of the barium site to the titanium site is stoichiometrically excessive. In addition, Japanese Unexamined Patent Application Publication No. 61-101459 discloses a dielectric material composed of a barium titanate solid solution containing an oxide of a rare earth element, such as La, Nd, Sm, Dy or Y.
Known dielectric materials exhibiting a small change in dielectric constant with temperature include, for example, a BaTiO
3
—CaZrO
3
—MnO—MgO based composition disclosed in Japanese Unexamined Patent Application Publication No. 62-25422, and a BaTiO
3
—(Mg,Zn,Sr,Ca)O—B
2
O
3
—SiO
2
based composition disclosed in Japanese Examined Patent Application Publication No. 61-14611. Since these dielectric materials are not converted into semiconductor materials during sintering in reducing atmospheres, base metals such as nickel can be used as internal electrodes in the production of monolithic ceramic capacitors.
In recent years, miniaturization of electronic components is making rapid progress as electronic technologies develop, and further miniaturization and larger capacitances are required in monolithic ceramic capacitors. Thus, development of higher dielectric constant dielectric materials and reduced thickness dielectric ceramic layers is rapidly progressing. Current dielectric materials are, therefore, required to have high dielectric constants, small changes in dielectric constant with temperature and high reliability.
Although the dielectric materials disclosed in Japanese Examined Patent Application Publication No. 57-42588 and Japanese Unexamined Patent Application Publication No. 61-101459 have large dielectric constants, segregation of rare earth elements readily occurs in dielectric ceramic matrices. Such segregation of rare earth elements causes significantly reduced reliability of thin dielectric ceramic layers having a thickness of 10 &mgr;m or less.
The dielectric material disclosed in Japanese Unexamined Patent Application Publication No. 62-256422 has a relatively large dielectric constant and changes in dielectric constant with temperature are small. In this material, however, CaZrO
3
, and CaTiO
3
are formed during sintering and readily form secondary phases together with Mn. Thus, this dielectric material exhibits less reliability at high temperatures.
When secondary phases are formed by segregation of one or more elements in the primary phase of a dielectric ceramic layer, reliability is significantly reduced in a monolithic ceramic capacitor comprising such dielectric ceramic layers having a thickness of 10 &mgr;m or less. When the secondary phases exhibit low resistance to reduction, sintering in a reducing atmosphere causes further decrease in the reliability of the monolithic ceramic capacitor.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a ceramic capacitor having high insulation resistance, superior load characteristics at high temperatures or high humidity, and high weather resistance, even when sintered in a reducing atmosphere.
It is another object of the present invention to provide a monolithic ceramic capacitor having large capacitance.
It is still another object of the present invention to provide a method for making the ceramic capacitor.
A ceramic capacitor of the present invention comprises a dielectric ceramic layer and at least one pair of electrodes, one member of the pair provided on each side of the dielectric, wherein the dielectric ceramic layer comprises a primary phase and secondary phases, and the size of the secondary phases in the thickness direction of the dielectric ceramic layer is not more than about one-third of the thickness of the entire dielectric ceramic layer.
Preferably, the dielectric ceramic layer comprises a nonreduced dielectric ceramic which is formed by sintering in a neutral or reducing atmosphere and which comprises a primary component represented by the formula ABO
3
, wherein A is at least one selected from Ba, Sr, Ca and Mg, and B is at least one selected from Ti, Zr and Hf; at least one rare earth oxide; and a glass component primarily composed of SiO
2
or B
2
O
3
.
Preferably, the secondary phases comprise the rare earth oxide.
A method for making a ceramic capacitor of the present invention comprises the steps of mixing compounds constituting secondary phases in a primary phase of a dielectric ceramic, annealing and then pulverizing the mixture to prepare a raw material for the secondary phases, mixing the raw material for the secondary phases with other materials for the dielectric ceramic which are not completely or sufficiently contained in the raw material for the secondary phases, shaping and sintering the mixture to form a dielectric ceramic, and forming electrodes on the dielectric ceramic.
Preferably, the ceramic capacitor is a monolithic ceramic capacitor comprising a plurality of dielectric ceramic layers, internal electrodes provided between the dielectric ceramic layers, and external electrodes electrically connected to the internal electrodes.
The secondary phases in the present invention indicate portions at which sub-components such as the rare earth oxide and glass are segregated and are present in high concentrations compared to the primary phase, and at least one sub-component is present in a concentration which is at least about ten times the concentration in the primary phase.
Since the secondary phases in the dielectric ceramic layer of the ceramic capacitor of the present invention are composed of fine particles, an electric field is not c

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