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FIELD OF THE INVENTION
The present invention relates to dielectric ceramics for use in dielectric
layers of multilayer ceramic capacitors; and more particularly, to a
dielectric ceramics with core-shell grain structures capable of providing
favorable B temperature characteristics, wherein various shell portions
having different functions can be adaptably arranged around a core in a
manner suitable for obtaining desired characteristics.
BACKGROUND OF THE INVENTION
When manufacturing multilayer ceramic capacitors having desired B
temperature characteristics by using dielectric ceramics principally
composed of barium titanate (BaTiO.sub.3) , it has been considered to be
essential that crystal grains constituting the dielectric ceramics have a
core-shell grain structure, wherein the core-shell grain structure
includes a ferroelectric core and a paraelectric shell encompassing the
core. The core-shell grain structure in the dielectric ceramics is
obtained by using such additives as Mg and rare earth elements.
In manufacturing conventional dielectric ceramics having the core-shell
grain structure, Mg and rare earth elements are added simultaneously to
the dielectric ceramic material containing therein, e.g., BaTiO.sub.3 as a
main component and diffused together into grains to form shells thereof.
Moreover, no measure has been taken to control the distribution of Mg and
the rare earth elements in the grains, resulting in Mg diffusion
substantially deep into the dielectric grains.
Therefore, the conventional dielectric ceramics including dielectric grains
having core-shell grain structures to improve B temperature
characteristics may not be adaptably controlled to have required
properties. The thickness of the dielectric layers has been continuously
reduced to obtain an ever-increasing capacitance of multilayer ceramic
capacitors, necessitating various quality requirements in such scaled down
dielectric layers. However, the conventional core-shell grain structure
cannot effectively meet such various quality requirements for the
multilayer ceramic capacitors.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide multilayer
ceramic capacitors with an improved performance and reliability by
adaptively tailoring the shell structure of ceramic grains according to
the required characteristics.
In accordance with one aspect of the present invention, there is provided a
dielectric ceramic comprising:
dielectric ceramic grains having BaTiO.sub.3 as a major component thereof,
a portion of the dielectric ceramic grains having a ferroelectric core and
a paraelectric shell into which Mg and a rare earth element are diffused,
the shell being located at least on a part of a surface of the core,
wherein the shell includes at least two shell portions having different
components diffused thereinto, respectively.
In accordance with another aspect of the present invention, there is
provided a multilayer ceramic electric part comprising the dielectric
ceramic.
In accordance with still another aspect of the present invention, there is
provided a method for manufacturing the multilayer ceramic electric part
comprising the steps of:
producing a ceramic powder mixture having ceramic particles, the producing
step including the step of mixing MgO and a substance containing a rare
earth element with a BaTiO.sub.3 based dielectric ceramic material; and
removing portions of surfaces of the ceramic particles
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects and features of the present invention will
become apparent from the following description of preferred embodiments
given in conjunction with the accompanying drawings, in which:
FIG. 1 schematically shows dielectric ceramic grains in accordance with a
first preferred embodiment;
FIG. 2 schematically illustrates dielectric ceramic grains in accordance
with a second preferred embodiment;
FIG. 3 schematically describes dielectric ceramic grains in accordance with
a third preferred embodiment;
FIG. 4 schematically depicts dielectric ceramic grains in accordance with a
third preferred embodiment;
FIG. 5 schematically shows an exemplary core-shell grain structure in
accordance with still another preferred embodiment;
FIG. 6 is a partial cutaway view of an exemplary multilayer ceramic
capacitor; and
FIG. 7 is an exploded perspective view of a sintered body in accordance
with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, there are schematically shown dielectric ceramic
grains 1 constituting dielectric ceramic layers of multilayer ceramic
capacitors, each dielectric ceramic grain generally having a core-shell
grain structure in accordance with a first preferred embodiment of the
present invention. The dielectric ceramic of the present invention is
mainly composed of BaTiO.sub.3 and each of the dielectric ceramic grains 1
generally includes a ferroelectric core 2, and a paraelectric shell 3
where Mg and one or more rare earth elements are diffused into
BaTiO.sub.3. The shell 3 encompassing the core 2 is basically formed of
two shell portions, i.e., an outer shell portion 3a and an inner shell
portion 3b into which different components are diffused. The outer shell
portion 3a is formed of BaTiO.sub.3 and Mg and one or more rare earth
elements, e.g., Ho, both diffused into BaTiO.sub.3. The inner shell
portion 3b is formed of BaTiO.sub.3 and Mg diffused thereinto.
The inner shell portion 3b formed by the diffusion of Mg into BaTiO.sub.3
has a paraelectric phase with a high insulation resistance. Mg therein
acts as an acceptor of the main component BaTiO.sub.3. The presence of Mg
in the inner shell portion 3b encompassing the core 2 provides BaTiO.sub.3
with a reductive resistance and therefore the reduction of the
ferroelectric core 2 formed of BaTiO.sub.3 is prevented during a sintering
process and operating life characteristics can be improved.
The outer shell portion 3a is formed of BaTiO.sub.3 into which Mg and rare
earth elements, e.g., Ho are diffused and has a paraelectric phase with a
high insulation resistance. The rare earth element therein acts as a donor
of the major component BaTiO.sub.3. Therefore, the oxygen deficiency in
the dielectric grains can be effectively compensated, thereby obtaining a
high dielectric constant.
Further, the shell 3 may include a shell portion into which Zr is diffused
in addition to Mg and/or one or more rare earth elements. Zr is effective
in improving temperature characteristics and can be advantageously
employed to obtain required temperature characteristics.
The shell structure with two separated portions 3a, 3b having distinct
diffusion components can be obtained as follows: First, MgO powder is
added to and mixed with the main component BaTiO.sub.3 powder by a wet
method and dispersed and heat-treated until the MgO powder is uniformly
distributed in the powder mixture. The powder mixture is then made to
obtain BaTiO.sub.3 particles having peripheral portion into which Mg is
diffused. Next, the powder of one or more rare earth elements, e.g.,
Ho.sub.2 O.sub.3 powder, is added to and mixed with the heat-treated
powder mixture thus obtained by a wet method until the rare earth powder
is uniformly distributed in the mixture. The powder mixture is then
heat-treated. Through the procedure described above, a heat-treated powder
is obtained, wherein each particle in the heat-treated powder generally
has a core-shell structure provided with a core portion located at the
center portion of the particle and essentially composed of BaTiO.sub.3, an
inner shell portion located outside the core potion and composed of
BaTiO.sub.3 and Mg diffused thereinto, and an outer shell portion located
outside the inner shell portion and composed of BaTiO.sub.3 into which Mg
and a rare earth element, e.g., Ho are diffused. Next, water and an
organic binder are added to the heat-treated powder to make slurry. The
slurry is used to produce ceramic green sheets and internal electrode
patterns are then printed thereon. The pattern printed sheets are stacked
against one another and the stack is diced into a multiplicity of chips.
The chips are then sintered to thereby produce dielectric ceramics having
the core-shell grain structure as shown in FIG. 1.
In the core-shell structured dielectric ceramics, there are also formed
glassy grain boundary portions 4 at portions of shells 3 adjoining
neighboring grains as shown in FIG. 1. The glassy grain boundary portions
4 have a large electrical resistance and include a glass component
precipitated therein. The grain boundaries 4 can be formed by adding as a
sintering additive a glass component, e.g., SiO.sub.2, to raw materials of
the dielectric ceramics and the thickness of the glassy grain boundaries 4
can be adjusted by varying the amount of the sintering additives. The
grain boundaries 4 have a large electrical resistance, but are in the
paraelectric phase having a lower dielectric constant than that of the
core 2.
Referring to FIG. 2, there is schematically shown dielectric ceramic grains
constituting a dielectric ceramic and each having a second core-shell
grain structure in accordance with another preferred embodiment of the
present invention. The first core-shell grain structure illustrated in
FIG. 1 includes the shell 3 having the inner shell portion 3b formed by
BaTiO.sub.3 into which Mg is diffused and the outer shell portion 3a
formed by BaTiO.sub.3 into which rare earth elements, e.g., Ho, and Mg are
diffused. On the other hand, the second core-shell structure of the
present invention includes the shell 3 having an outer shell portion 3a
into which Mg and Ho are diffused as in the first embodiment and an inner
shell portion 3c where one or more rare earth elements, e.g., Ho, are
diffused into BaTiO.sub.3. The outer and the inner shell portions 3a, 3c
respectively exhibit similar properties and therefore function similarly
as those of the first preferred embodiment.
The second core-shell grain structure of the present invention is obtained
by inverting the order of adding the MgO powder and the rare earth powder,
e.g., Ho.sub.2 O.sub.3 powder to the BaTiO.sub.3 powder. To be more
specific, the rare earth Ho.sub.2 O.sub.3 powder is first added to and
mixed with the main component BaTiO.sub.3 powder and then the mixture is
heat-treated, so that the heat-treated powder mixture having BaTiO.sub.3
particles around which rare earth oxide such as Ho.sub.2 O.sub.3 is
diffused can be obtained. Next, the MgO powder is added to and mixed with
the heat-treated powder mixture thus obtained and then the mixture is
heat-treated. Consequently, the heat-treated powder obtained includes
particles, wherein each particle generally has an outer shell portion
including BaTiO.sub.3 into which MgO and Ho.sub.2 O.sub.3 are diffused and
an inner shell portion located inside the outer shell portion and
including BaTiO.sub.3 into which Ho.sub.2 O.sub.3 is diffused. Next, the
heat-treated powder thus provided is mixed with water and an organic
binder to make slurry. The slurry is used to produce ceramic green sheets
and then internal electrode patterns are printed thereon. The pattern
printed sheets are stacked against one another and the stack is diced into
a plurality of chips. The chips are then sintered to thereby produce
dielectric ceramics having the second core-shell grain structure shown in
FIG. 2
The inner shell portion 3c containing BaTiO.sub.3 and rare earth element
such as Ho serves to improve a dielectric loss tangent "tan .delta." of a
capacitor. The outer shell 3a formed of BaTiO.sub.3 into which Mg and rare
earth such as Ho are diffused functions to increase a dielectric constant.
Referring to FIGS. 3 and 4, there are shown schematic views of two
exemplary core-shell grain structures in accordance with a third preferred
embodiment of the invention. In this embodiment, the respective shell
portions 3a, 3b, 3c are not radially separated but are unevenly
distributed on the surface of the core 2, exposing some portions of the
core 2. The effect of the improvements of the electrical characteristics
due to the fact that shell portions have different diffusion components
can be optimized by using the core-shell structures shown in FIGS. 3 and
4.
In the core-shell structures of the third preferred embodiment of the
invention, grain boundaries of neighboring grains can be formed between
shell portions of either a same type or different types as shown in FIG. 3
or between a core and a shell portion or between cores as shown in FIG. 4.
The exemplary core-shell grain structures shown in FIGS. 3 and 4 can be
obtained by using a slurry formed in a similar manner described above with
reference to the first and the second preferred embodiments excepting that
the MgO powder and the rare earth powder, e.g., Ho.sub.2 O.sub.3 powder
are simultaneously added to the main material BaTiO.sub.3 powder and
heat-treated. In addition, the slurry is ball milled by using large beads
so that some parts of the shells are removed, thereby partially exposing
some parts of cores.
Referring to FIG. 5, there is shown an exemplary core-shell grain structure
in accordance with still another preferred embodiment, the shell portions
3a, 3b, 3c are not radially separated completely but are partially
distributed on the surface of the core 2 with some parts of the shell
portions overlapping with each other.
Next, a multilayer ceramic capacitor will be described as an example of
multilayer ceramic electrical parts which can be made by using the
dielectric ceramics, and a manufacturing method thereof will be explained
thereafter.
First, as described above, one of the MgO powder and Ho.sub.2 O.sub.3
powder is added to and mixed with the main material BaTiO.sub.3 powder.
The mixture is sintered and then the other powder is added to and mixed
with the heat-treated mixture. The second mixture is also sintered as in
the first and the second embodiments. Or, as in the third embodiment of
the invention, the Mgo and the Ho.sub.2 O.sub.3 powder can be mixed with
the BaTiO.sub.3 powder and sintered at the same time. Other additives can
be used together with the MgO powder and/or the Ho.sub.2 O.sub.3 powder.
Then, the sintered mixture is dispersed uniformly in an organic binder
such as ethyl cellulose dissolved in a solvent to produce slurry. The
slurry is uniformly coated on a base film, e.g., terepthalate film, and
dried to produce thin film green sheets. Then, the green sheets are cut to
obtain ceramic green sheets of a proper size.
Next, a conductive paste is printed on the ceramic green sheets to produce
two types of internal electrode patterns. The conductive paste contains a
100 wt % conductive powder of Ni, Cu, Ag, Pd, Ag-Pd and the like; a 3-12
wt % binder of ethyl cellulose, acryl, polyester and etc, and a 80-120 wt
% solvent of butyl carbitol, butyl carbitol acetate, terpineol, ethyl
cellosolve, hydrocarbon and etc uniformly mixed and dispersed.
The ceramic green sheets having internal electrodes patterns printed
thereon are alternately stacked. Then, dummy sheets, on which the internal
electrode patterns are not printed, are stacked on the lower side and the
upper side of the stacked green sheets, and pressed together to produce a
laminated body. The laminated bodies are cut into separate laminated
elements. The internal electrodes are alternatingly exposed at opposite
end surfaces of the laminated elements.
Thereafter, another conductive paste for use in forming external electrodes
is applied on surfaces of both end portions of each laminated element. The
laminated elements having the conductive paste thus applied are dried to
produce multilayer ceramic elements. Then, the multilayer ceramic elements
are sintered. During the sintering process, the ceramic layers are
sintered and at the same time the internal electrode patterns and the
conductive paste applied on the surfaces of the end portions are
heat-treated. Thereafter, Sn or solder plating is performed on the
conductive layers on the surfaces of the end portions thereby completing a
manufacture of the multilayer ceramic capacitor. A partial cutaway view of
an exemplary multilayer ceramic capacitor thus produced is illustrated in
FIG. 6, wherein reference numeral 12 represents the external electrodes
and 13 represents a sintered body of internal electrodes 15 and 16 and
ceramic layers 17.
Referring to FIG. 7, there is illustrated an exemplary view of the sintered
body 13 shown in FIG. 6. As shown, the sintered body 13 is made by
stacking the dielectric ceramic layers 17 having the internal electrodes
15, 16 thereon and several ceramic dummy layers 17' having no internal
electrode, on the lower and the upper side of the stacked ceramic layers
17. The internal electrodes 15, 16 facing each other through a dielectric
ceramic layer therebetween are alternatingly exposed at the opposite end
surfaces of the sintered body 13. The dielectric ceramic layers 17, 17'
are formed of dielectric ceramic having a core-shell grain structure
described in detail with reference to FIGS. 1-5.
It should be noted that the core-shell grain structure of the present
invention could be applied in other types of electric parts than the
multilayer ceramic capacitor described by way of illustrating in the
present invention. For instance, the inventive core-shell grain structure
can be equally applied to a multilayer ceramic LC hybrid component having
a capacitor portion.
The preferred embodiments of the invention will now be described in further
detail by way of illustration based on Examples.
EXAMPLE 1
To obtain dielectric ceramics for multilayer ceramic capacitors, a ceramic
powder mixture was prepared by mixing 97.5 wt % of BaTiO.sub.3 powder with
a mean diameter of 0.4 .mu.m, 1.3 wt % of MgO powder and 1.2 wt % of
SiO.sub.2 powder as a sintering additive. The ceramic powder mixture was
ball milled with pure water for 15 hours and then heat-treated at
1200.degree. C. for 2 hours. Thereafter, 1.5 wt % of Ho.sub.2 O.sub.3
powder and 1.5 wt % of SiO.sub.2 powder were added to the 97.0 wt % of the
heat-treated ceramic powder mixture thus obtained and this mixture was
ball milled with pure water for 15 hours and then heat-treated at
1000.degree. C. for 2 hours. Water and an organic binder were added to the
final ceramic powder mixture thus produced to obtain slurry.
The slurry was formed into ceramic green sheets with a thickness of 10
.mu.m by a reverse coater. Then, a conductive paste was coated on the
ceramic green sheets to form internal electrode and 10 green sheets thus
provided were stacked to produce a laminated body. The laminated body was
cut into a plurality of separate laminated elements. Thereafter, external
electrodes were formed on two opposite end portions of the laminated
elements to produce multilayer ceramic elements.
The multilayer ceramic elements were sintered at 1200.degree. C. in a
reductive atmosphere for 1.5 hours, thereby obtaining multilayer ceramic
capacitors of 3.2 mm.times.1.6 mm.times.1.6 mm.
Dielectric ceramic layers included in the multilayer ceramic capacitors
thus fabricated were formed of a plurality of dielectric ceramic grains 1
as schematically shown in FIG. 1. Each of the dielectric ceramic grains
generally had the core-shell grain structure including ferroelectric core
2 mainly composed of BaTiO.sub.3 at the center of the grain 1 and the
paraelectric shell 3 encompassing the core 2. The shell 3 was divided into
two layers, i.e., an inner shell portion 3b where Mg was diffused in
BaTiO.sub.3 and an outer shell portion 3a where Mg and Ho were diffused in
BaTiO.sub.3. The core was encompassed by the inner shell portion 3b, which
in turn was surrounded by the outer shell portion 3a.
The dielectric ceramic obtained through the process described above had a
dielectric constant of 3350, greater than 3000. The tan .delta. of the
multilayer ceramic capacitors was about 3.9, less than 4.0. Endurance life
of the capacitors obtained by the accelerated life test performed under
the condition of 150.degree. C., 100 V was 39860 seconds.
EXAMPLE 2
While the 1.3 wt % of MgO was first added to the dielectric ceramic
material having BaTiO.sub.3 as a major component thereof in Example 1, the
1.3 wt % of Ho.sub.2 O.sub.3 was first added and the mixture of the both
was heat-treated at 1000.degree. C. for 2 hours in Example 2. Thereafter,
1.3 wt % of MgO was added in lieu of 1.3 wt % of Ho.sub.2 O.sub.3 in
Example 1 to the heat-treated mixture of Ho.sub.2 O.sub.3 and the
BaTiO.sub.3 based ceramic powder. Dielectric ceramic slurry was made by
employing the same method as in Example 1 except that the processes
described above and multilayer ceramic capacitors were manufactured by
using the slurry.
The dielectric ceramic thus obtained exhibited a dielectric constant of
3210, greater than 3000, and the tan .delta. of 3.4, not greater than 4.0.
The endurance life of the capacitors obtained by the accelerated life test
performed under the condition of 150.degree. C., 100 V was 52980 seconds.
EXAMPLE 3
In this Example, 1.3 wt % of MgO and 1.3 wt % of Ho.sub.2 O.sub.3 were
mixed together with the BaTiO.sub.3 dielectric ceramic material and the
mixture was heat-treated at 1000.degree. C. for 2 hours. When the mixture
was ball milled to make slurry, large beads were used to remove some parts
of shells. The slurry was made by the same method as in Example 1 except
that the processes described above and multilayer ceramic capacitors were
manufactured by using the slurry.
The dielectric ceramic thus obtained exhibited a dielectric constant of
3240, not less than 3000, and the tan .delta. of 3.8, not greater than
4.0. The endurance life of the capacitors obtained by means of the
accelerated life test performed under the same condition as in Example 1
was 68360 seconds.
COMPARATIVE EXAMPLE
The ceramic capacitors of the comparative example were fabricated in a
similar manner as in the Example 3, excepting that large beads were not
used in producing the slurry.
The dielectric ceramic thus obtained exhibited a dielectric constant of
2780, less than 3000, and the tan .delta. of 4.2, greater than 4.0. The
endurance life of the capacitors measured by means of accelerated life
test performed under the same condition as in Example 1 was 1200 seconds.
Table shows the test results of the Examples 1-3 and the comparative
Example.
TABLE
Endurance
Life
Delectric (150.degree. C.,
Example Constant tan .delta. 100 V) Remarks
Example 3350 3.9 39860 sec. heat treatment of
1 MgO first
Example 3210 3.4 52980 sec. heat treatment of
2 Ho.sub.2 O.sub.3 first
Example 3240 3.8 68360 sec. simultaneous heat
3 treatment of MgO
and Ho.sub.2 O.sub.3 with
breaking shells
Compara- 2780 4.2 1200 sec. simultaneous heat
tive treatment of MgO
Example and Ho.sub.2 O.sub.3 without
breaking shells
In accordance with the present invention as described above, the shell can
be constituted by at least two separate shell portions respectively having
different functions of improving, e.g., a reduction resistance
characteristic, a breakdown voltage and operating life characteristic, and
a temperature, especially B temperature characteristic. These shell
portions can be adaptively disposed on the surface of the core. For
instance, the shell portions can be disposed radially on the surface of
the core as in FIGS. 1 and 2, non-radially but in direct contact with the
surface of the core as in FIGS. 3-5. Further, some parts of the core can
be exposed as shown in FIGS. 3 and 4. Therefore, the properties of the
dielectric ceramics and electric parts employing therein such dielectric
ceramics can be optimized by adaptively forming shell portions of desired
characteristics to have a shell structure suitable for the purpose.
While the invention has been shown and described with respect to the
preferred embodiments, it will be understood by those skilled in the art
that various changes and modifications may be made without departing from
the spirit and scope of the invention as defined in the following claims.
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