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Description  |
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TECHNICAL FIELD
The present invention relates to a ceramic capacitor and a method of
manufacturing it.
BACKGROUND ART
This kind of ceramic capacitor includes a dielectric layer and electrodes
disposed on obverse and reverse surfaces of the dielectric layer,
respectively.
Increase in capacity is required for the ceramic capacitor, and therefore
the dielectric layer must have a high dielectric constant. However, for
downsizing the ceramic capacitor and increasing the capacity of it, the
thickness of the dielectric layer must be 1 to 2 .mu.m or less. In the
ceramic capacitor employing the dielectric layer having the thickness of 1
to 2 .mu.m or less, the dielectric constant can be presently increased
only to about 3000. In other words, BaTiO.sub.3 powder having minimum
grain size must be used for forming the dielectric layer having thickness
of 1 to 2 .mu.m or less, but using the BaTiO.sub.3 having such a small
grain size decreases the dielectric constant rapidly. Therefore, at the
present time, the dielectric constant can be increased only to about 3000.
DISCLOSURE OF THE INVENTION
A ceramic capacitor includes a dielectric layer made of polycrystal mainly
composed of BaTiO.sub.3 having an average grain size of 0.5 .mu.m or less
and electrodes disposed on obverse and reverse surfaces of the dielectric
layer, respectively. The polycrystal has a tetragonal-perovskite-type
crystal structure and a c-axis/a-axis ratio of 1.005 through 1.009. In a
method of manufacturing the ceramic capacitor, an additive to BaTiO.sub.3
is selected so that the c-axis/a-axis ratio of the polycrystal is 1.005
through 1.009.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partially-broken perspective view showing a ceramic capacitor
in accordance with an exemplary embodiment of the present invention.
FIG. 2 is a pattern diagram showing internal electrodes and a dielectric
layer of the ceramic capacitor of FIG. 1.
FIG. 3 shows a crystal structure of crystal grains constituting the
dielectric layer of the ceramic capacitor of FIG. 1.
FIG. 4 is a characteristic diagram of the dielectric layer.
FIG. 5 shows a conventional crystal structure of crystal grains.
DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENT
The present invention provides a ceramic capacitor where the dielectric
constant of a dielectric layer can be 3500 or more even when thickness of
the dielectric layer is 1 to 2 .mu.m or less, and a method of
manufacturing the ceramic capacitor.
Embodiments of the present invention will be described hereinafter with
reference to the accompanying drawings.
FIG. 1 shows a ceramic capacitor in accordance with an exemplary embodiment
of the present invention. In FIG. 1, electrodes 12 are embedded at a
predetermined interval in dielectric layer 11. Electrodes 12 are
alternately pulled to respective ends and connected to respective external
electrodes 13.
Thickness of dielectric layer 11 sandwiched between electrodes 12 and the
crystal structure of them are important in the present invention. The
ceramic capacitor of FIG. 1 has a small size and a large capacity, and the
thickness of dielectric layer 11 between electrodes 12 is 1 to 2 .mu.m. In
other words, the thickness of dielectric layer 11 between electrodes 12 is
decreased to face electrodes 12 to each other as closely as possible,
thereby providing large electrostatic capacity.
For providing the large electrostatic capacity, dielectric constant of
dielectric layer 11 sandwiched between mutually close electrodes 12 must
be increased. Dielectric layer 11 is formed as follows in the present
embodiment. Various additives are added to BaTiO.sub.3 powder having the
average grain size of 0.2 .mu.m as a starting material The additives
specifically include MgO, MnO.sub.2, Dy.sub.2 O.sub.3, V.sub.2 O.sub.5,
and Ba--Al--Si--O based glass. These materials are mixed, dried, calcined,
and pulverized. The pulverized powder is mixed with various binders, and
molded to form a sheet. This sheet is dielectric layer 11 to be sandwiched
between electrodes 12.
Next, the sheets and electrodes 12 are alternately laminated, they are
burned at 1200 to 1300.degree. C. in that state, and then both end
surfaces of them are shaved, thereby exposing electrodes 12 from the both
end surfaces. External electrodes 13 are disposed on the exposing parts to
form the ceramic capacitor of FIG. 1.
Important items in this case are then described with reference to FIG. 2.
FIG. 2 is an enlarged pattern diagram showing electrodes 12 of FIG. 1 and
dielectric layer 11 sandwiched by electrodes 12. Electrodes 12 in FIG. 2
correspond to electrodes 12 of FIG. 1, and crystal grains 21 in FIG. 2
correspond to the crystal grains in dielectric layer 11 of FIG. 1. Space
22 exists between electrodes 12. After burning, space 22 is filled with
dielectric layer 11 having a width of scant about 1 to 2 .mu.m, as shown
in FIG. 2. Two, three, or four-tiers of crystal grains 21 having an
average grain size of 0.5 .mu.m or shorter are stacked in space 22 filled
with dielectric layer 11 having the width of about 1 to 2 .mu.m.
In the present embodiment, crystal grains 21 constitute a crystal structure
shown in FIG. 3 even when the average grain size is 0.5 .mu.m or less. In
FIG. 3, barium atoms (Ba) 31, titanium atoms (Ti) 32, and oxygen atoms (O)
33 constitute each crystal grain 21 of FIG. 2. Arrow 34 and arrow 35 show
two crystal axes, namely a-axis and c-axis, respectively. A c-axis/a-axis
ratio of crystal grain 21 is controlled to be 1.005 through 1.009 by
adjusting an amount of an additive, for example MgO, in FIG. 3.
A conclusion is described with reference to FIG. 4. In FIG. 4, horizontal
axis 41 shows the c-axis/a-axis ratio, and vertical axis 42 shows the
dielectric constant. Straight line 45 shows dielectric constant of 3500,
and polygonal line 43 indicates a relation between the c-axis/a-axis ratio
and the dielectric constant. Arrow 44 shows the range from 1.005 through
1.009 in c-axis/a-axis ratio. We found that the dielectric constant of
dielectric layer 11 between electrodes 12 can thus be 3500 or higher when
c-axis/a-axis ratio 41 of crystal grain 21 is set in the range from 1.005
through 1.009. We found that the dielectric constant of no lower than 3500
can be obtained only when c-axis/a-axis ratio 41 of crystal grain 21 is
set in range 44 from 1.005 through 1.009. The dielectric constant of 3500
cannot be achieved in the prior art. Based on the findings, we studied a
selecting method of materials for obtaining the findings, a specific
reason why the dielectric constant of no lower than 3500 can be obtained,
and the like.
FIG. 5 shows a conventional crystal structure when the average grain size
of crystal grains 21 is 0.5 .mu.m or less. In FIG. 5, atoms 51, atoms 52,
and atoms 53 indicate barium atoms (Ba), titanium atoms (Ti), and oxygen
atoms (O), respectively. Arrow 54 and arrow 55 show two crystal axes,
namely a-axis and c-axis, respectively. In the prior art, the
c-axis/a-axis ratio is about 1.000, as shown in FIG. 5, when the average
grain size of crystal grains 21 is 0.5 .mu.m or less. The smaller the
average grain size of crystal grains 21 is, the closer the c-axis/a-axis
ratio is to 1.000. When the c-axis/a-axis ratio is close to 1.000, the
dielectric constant is about 3000 at the highest, as shown in FIG. 4 and
as in the prior art.
We earnestly studied how to obtain high dielectric constant largely
exceeding 3000. As a result, it is found that controlling the amount of
MgO for BaTiO.sub.3 of 100 mol to be not more than 1 mol allows the
c-axis/a-axis ratio to be set in the range from 1.005 through 1.009. In
the prior art, for forming a ceramic capacitor, MgO of 2 mol or more is
added to BaTiO.sub.3 of 100 mol.
The reason why the c-axis/a-axis ratio can be set in the range from 1.005
through 1.009 cannot sufficiently be clarified presently, but the
following mechanism is estimated. Reducing the amount of MgO results in
generation of crystal grain 21 having no shell on the surface thereof in a
core shell structure, stress applied to crystal grain 21 having the core
shell structure enlarges c-axis value, and therefore the c-axis/a-axis
ratio lies in the range from 1.005 through 1.009.
Dielectric constant of 3500 or higher (this cannot be conventionally
achieved) can be achieved by setting the c-axis/a-axis ratio in the range
from 1.005 through 1.009 (our selection), as shown in FIG. 4. Thus, a
smaller ceramic capacitor with larger capacity can be obtained.
In the present invention, the c-axis/a-axis ratio is set in the range from
1.005 through 1.009 in the tetragonal-perovskite-type crystal structure of
BaTiO.sub.3, thereby providing the ceramic capacitor with a large capacity
which cannot be conventionally obtained. For example, an extremely high
dielectric constant of 3500 or higher can be obtained (it is
conventionally difficult).
INDUSTRIAL APPLICABILITY
In the present invention, the c-axis/a-axis ratio is set in the range from
1.005 through 1.009 in a tetragonal-perovskite-type crystal structure of
BaTiO.sub.3, thereby providing a ceramic capacitor with a large capacity
which cannot be conventionally obtained.
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