U.S. patent application number 09/791295 was filed with the patent office on 2001-09-13 for dieletric ceramics, multilayer ceramic electric parts and method for the manufacture thereof.
This patent application is currently assigned to Taiyo Yuden Co. Ltd.. Invention is credited to Kohzu, Noriyuki, Mizuno, Youichi, Okino, Yoshikazu, Saito, Kenji.
Application Number | 20010021095 09/791295 |
Document ID | / |
Family ID | 18572737 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010021095 |
Kind Code |
A1 |
Mizuno, Youichi ; et
al. |
September 13, 2001 |
Dieletric ceramics, multilayer ceramic electric parts and method
for the manufacture thereof
Abstract
Dielectric ceramic includes 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. The shell portions
can be radially disposed on one another or be in direct contact
with the surface of the core.
Inventors: |
Mizuno, Youichi; (Tokyo,
JP) ; Okino, Yoshikazu; (Tokyo, JP) ; Saito,
Kenji; (Tokyo, JP) ; Kohzu, Noriyuki; (Tokyo,
JP) |
Correspondence
Address: |
SHAHAN ISLAM, ESQ.
ROSENMAN & COLIN LLP
575 Madison Avenue
New York
NY
10022-2585
US
|
Assignee: |
Taiyo Yuden Co. Ltd.
16-20 Ueno 6-chome, Taito-ku
Tokyo
JP
110-0005
|
Family ID: |
18572737 |
Appl. No.: |
09/791295 |
Filed: |
February 23, 2001 |
Current U.S.
Class: |
361/311 ;
361/321.4; 361/321.5 |
Current CPC
Class: |
C01G 23/006 20130101;
H01G 4/1227 20130101; H01G 4/20 20130101; C04B 35/4682
20130101 |
Class at
Publication: |
361/311 ;
361/321.4; 361/321.5 |
International
Class: |
H01G 004/06 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 28, 2000 |
JP |
2000-051029 |
Claims
What is claimed is:
1. 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.
2. The dielectric ceramic of claim 1, wherein the shell portions
are radially disposed on one another.
3. The dielectric ceramic of claim 1, wherein the shell portions
are unevenly distributed on the surface of the core.
4. The dielectric ceramic of claim 3, wherein parts of the shell
portions overlap with each other.
5. A multilayer ceramic electric part comprising the dielectric
ceramic of claim 1.
6. The multilayer ceramic electric part of claim 5, wherein the
shell portions are radially disposed on one another.
7. The multilayer ceramic electric part of claim 5, wherein the
shell portions are unevenly distributed on the surface of the
core.
8. The multilayer ceramic electric part of claim 7, wherein parts
of the shell portions overlap with each other.
9. A method for manufacturing the multilayer ceramic electric part
of claim 5, 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.
Description
FIELD OF THE INVENTION
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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
[0005] 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.
[0006] In accordance with one aspect of the present invention,
there is provided a dielectric ceramic comprising:
[0007] 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,
[0008] wherein the shell includes at least two shell portions
having different components diffused thereinto, respectively.
[0009] In accordance with another aspect of the present invention,
there is provided a multilayer ceramic electric part comprising the
dielectric ceramic.
[0010] 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:
[0011] 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
[0012] removing portions of surfaces of the ceramic particles
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] 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:
[0014] FIG. 1 schematically shows dielectric ceramic grains in
accordance with a first preferred embodiment;
[0015] FIG. 2 schematically illustrates dielectric ceramic grains
in accordance with a second preferred embodiment;
[0016] FIG. 3 schematically describes dielectric ceramic grains in
accordance with a third preferred embodiment;
[0017] FIG. 4 schematically depicts dielectric ceramic grains in
accordance with a third preferred embodiment;
[0018] FIG. 5 schematically shows an exemplary core-shell grain
structure in accordance with still another preferred
embodiment;
[0019] FIG. 6 is a partial cutaway view of an exemplary multilayer
ceramic capacitor; and
[0020] FIG. 7 is an exploded perspective view of a sintered body in
accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.2O.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.
[0026] 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.
[0027] 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.
[0028] 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.2O.sub.3 powder to
the BaTiO.sub.3 powder. To be more specific, the rare earth
Ho.sub.2O.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.2O.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.2O.sub.3 are diffused and an inner shell portion located
inside the outer shell portion and including BaTiO.sub.3 into which
Ho.sub.2O.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
[0029] 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.
[0030] 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 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.
[0031] 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.2O.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.
[0032] 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.
[0033] 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.
[0034] First, as described above, one of the MgO powder and
Ho.sub.2O.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.
manufacturing method thereof will be explained thereafter.
[0035] First, as described above, one of the MgO powder and
Ho.sub.2O.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.2O.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.2O.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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] The preferred embodiments of the invention will now be
described in further detail by way of illustration based on
Examples.
EXAMPLE 1
[0042] 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.2O.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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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
[0047] 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.2O.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.2O.sub.3 in Example 1 to the heat-treated mixture of
Ho.sub.2O.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.
[0048] 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
[0049] In this Example, 1.3 wt % of MgO and 1.3 wt % of
Ho.sub.2O.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.
[0050] 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
[0051] 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.
[0052] 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.
[0053] Table shows the test results of the Examples 1-3 and the
comparative Example.
1TABLE 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.2O.sub.3 first Example 3240 3.8 68360 sec. simultaneous
heat 3 treatment of MgO and Ho.sub.2O.sub.3 with breaking shells
Compara- 2780 4.2 1200 sec. simultaneous heat tive treatment of MgO
Example and Ho.sub.2O.sub.3 without breaking shells
[0054] 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.
[0055] 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.
* * * * *