U.S. patent application number 10/151922 was filed with the patent office on 2003-01-09 for dielectric ceramic composition and ceramic capacitor.
This patent application is currently assigned to Taiyo Yuden Co., Ltd.. Invention is credited to Hagiwara, Tomoya, Mizuno, Youichi, Morita, Koichiro.
Application Number | 20030007315 10/151922 |
Document ID | / |
Family ID | 19017894 |
Filed Date | 2003-01-09 |
United States Patent
Application |
20030007315 |
Kind Code |
A1 |
Morita, Koichiro ; et
al. |
January 9, 2003 |
DIELECTRIC CERAMIC COMPOSITION AND CERAMIC CAPACITOR
Abstract
A dielectric ceramic composition includes sintered ceramic
grains having a core-shell structure, wherein smaller than or equal
to 50% of the grains have a domain width of twin less than 20 nm;
30% to 70% of the grains have a domain width of twin in the range
from 20 nm to 50 nm; less than or equal to 50% of the grains have a
domain width of twin greater than 50 nm or have no twin. A ceramic
capacitor includes more than one internal electrode and one or more
dielectric layers composed of a dielectric ceramic composition,
each of the dielectric layers being sandwiched between two
neighboring internal electrodes.
Inventors: |
Morita, Koichiro; (Tokyo,
JP) ; Hagiwara, Tomoya; (Tokyo, JP) ; Mizuno,
Youichi; (Tokyo, JP) |
Correspondence
Address: |
BACON & THOMAS, PLLC
625 SLATERS LANE
FOURTH FLOOR
ALEXANDRIA
VA
22314
|
Assignee: |
Taiyo Yuden Co., Ltd.
Tokyo
JP
|
Family ID: |
19017894 |
Appl. No.: |
10/151922 |
Filed: |
May 22, 2002 |
Current U.S.
Class: |
361/321.4 |
Current CPC
Class: |
H01G 4/1227
20130101 |
Class at
Publication: |
361/321.4 |
International
Class: |
H01G 004/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 12, 2001 |
JP |
2001-176980 |
Claims
What is claimed is:
1. A dielectric ceramic composition comprising: sintered ceramic
grains having a core-shell structure, wherein smaller than or equal
to 50% of the grains have a domain width of twin less than 20 nm;
30% to 70% of the grains have a domain width of twin in the range
from 20 nm to 50 nm; less than or equal to 50% of the grains have a
domain width of twin greater than 50 nm or have no twin.
2. The dielectric ceramic composition of claim 1, wherein the
ceramic grains include oxides of Ba and Ti as main components.
3. The dielectric ceramic composition of claim 1, wherein the
sintered ceramic grains include an oxide of Re, an oxide of Mg, and
oxides of one or more elements selected from the group consisting
of Mn, V and Cr, Re representing one or two elements selected from
the group consisting of Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and
Y.
4. The dielectric ceramic composition of claim 1 further comprising
about 100 mol part of an oxide of Ba and Ti; about 0.25 to 1.5 mol
part of an oxide of Re; about 0.2 to 1.5 mol part of an oxide of
Mg; and 0.025 to 0.25 mol part of oxides of one or more elements
selected from the group consisting of Mn, V and Cr, wherein the
content of the oxide of Ba and Ti is calculated by assuming that
the oxide of Ba and Ti is BaTiO.sub.3; the content of the oxide of
Re is calculated by assuming that the oxide of Re is
Re.sub.2O.sub.3; the content of the oxide of Mg is calculated by
assuming that the oxide of Mg is MgO; and the content of oxides of
Mn, V and Cr is calculated by assuming that the oxides of Mn, V and
Cr are Mn.sub.2O.sub.3, V.sub.2O.sub.5 and Cr.sub.2O.sub.3,
respectively, Re representing one or two elements selected from the
group consisting of Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Y.
5. The dielectric ceramic composition of claim 1, wherein the
sintered ceramic grains include SiO.sub.2 or a glass component
having SiO.sub.2.
6. The dielectric ceramic composition of claim 5, wherein the glass
component ranges from 0.05 to 5 wt %.
7. The dielectric ceramic composition of claims 1, wherein a
dielectric constant is equal to or greater than 3000.
8. A ceramic capacitor comprising: more than one internal
electrode; and one or more dielectric layers composed of a
dielectric ceramic composition, each of the dielectric layers being
sandwiched between two neighboring internal electrodes, wherein the
dielectric ceramic composition includes sintered ceramic grains
having a core-shell structure, wherein smaller than or equal to 50%
of the grains have a domain width of twin less than 20 nm; 30% to
70% of the grains have a domain width of twin in the range from 20
nm to 50 nm; less than or equal to 50% of the grains have a domain
width of twin greater than 50 nm or have no twin.
9. The ceramic capacitor of claim 8, wherein the sintered ceramic
grains include oxides of Ba and Ti as main components.
10. The ceramic capacitor of claim 8, wherein the sintered ceramic
grains include an oxide of Re, and oxide of Mg, and oxides of one
or more elements selected from the group consisting of Mn, V and
Cr, Re representing one or two elements selected from the group
consisting of Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Y.
11. The ceramic capacitor of claim 8 further comprising about 100
mol part of an oxide of Ba and Ti; about 0.25 to 1.5 mol part of an
oxide of Re; about 0.2 to 1.5 mol part of an oxide of Mg; and 0.025
to 0.25 mol part of oxides of one or more elements selected from
the group consisting of Mn, V and Cr, wherein the content of the
oxide of Ba and Ti is calculated by assuming that the oxide of Ba
and Ti is BaTiO.sub.3; the content of the oxide of Re is calculated
by assuming that the oxide of Re is Re.sub.2O.sub.3; the content of
the oxide of Mg is calculated by assuming that the oxide of Mg is
MgO; and the content of oxides of Mn, V and Cr is calculated by
assuming that the oxides of Mn, V and Cr are Mn.sub.2O.sub.3,
V.sub.2.sub.5 and Cr.sub.2O.sub.3, respectively, Re representing
one or two elements selected from the group consisting of Sm, Eu,
Gd, Tb, Dy, Ho, Er, Tm, Yb and Y.
12. The ceramic capacitor of claim 8, wherein the sintered ceramic
grains include SiO.sub.2 or a glass component having SiO.sub.2.
13. The ceramic capacitor of claim 12, wherein the glass component
ranges from 0.05 to 5 wt %.
14. The ceramic capacitor of claim 8, wherein the internal
electrodes are formed of a base metal.
15. The ceramic capacitor of claim 8, wherein the ceramic capacitor
fulfills X7R characteristic (EIA standards) or B characteristic
(EIAJ standards).
16. The ceramic capacitor of claim 8, wherein a dielectric constant
is equal to or greater than 3000.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a reduction resistive
dielectric ceramic composition containing therein ceramic grains of
a core-cell structure and a ceramic capacitor including therein
dielectric layers made by using such a ceramic composition.
DESCRIPTION OF THE PRIOR ART
[0002] Recently, a base metal, e.g., Ni, has been widely used in
forming internal electrodes of a multilayer ceramic capacitor for
the purpose of reducing manufacturing costs and various reduction
resistive dielectric ceramic compositions capable of being sintered
simultaneously with the internal electrodes composed of the base
metal have been developed. One of the reduction resistive
dielectric ceramic compositions is a barium titanate-based
dielectric ceramic composition including ceramic grains of a
core-shell structure.
[0003] However, in case where a multilayer ceramic capacitor, which
fulfilled the X7R characteristic (EIA standards) or the B
characteristic (EIAJ standards), is manufactured by employing a
barium titanate-based dielectric ceramic composition having a
dielectric constant equal to or greater than 3000 and internal
electrodes composed of the base metal such as Ni, capacitance aging
become deteriorated.
SUMMARY OF THE INVENTION
[0004] It is, therefore, an object of the present invention to
provide a multilayer ceramic capacitor yielding a maximum temporal
capacitance variation not smaller than -30% with a voltage bias of
2 V/.mu.m, even when a dielectric ceramic composition having a
dielectric constant (.epsilon.) of greater than 3000 and a good
reduction resistance to is used.
[0005] In accordance with one aspect of the present invention,
there is provided a dielectric ceramic composition including:
sintered ceramic grains having a core-shell structure, wherein
smaller than or equal to 50% of the grains have a domain width of
twin less than 20 nm; 30% to 70% of the grains have a domain width
of twin in the range from 20 nm to 50 nm; less than or equal to 50%
of the grains have a domain width of twin greater than 50 nm or
have no twin.
[0006] In accordance with another aspect of the present invention,
there is provided a ceramic capacitor including more than one
internal electrodes; and one or more dielectric layers composed of
a dielectric ceramic composition, each of the dielectric layers
being sandwiched between two neighboring internal electrodes,
wherein the dielectric ceramic composition includes sintered
ceramic grains having a core-shell structure, wherein smaller than
or equal to 50% of the grains have a domain width of twin less than
20 nm; 30% to 70% of the grains have a domain width of twin in the
range from 20 nm to 50 nm; less than or equal to 50% of the grains
have a domain width of twin greater than 50 nm or have no twin.
BRIEF DESCRIPTION OF THE DRAWING
[0007] 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
drawing, which shows an optimum range of twin domain width
distribution of the grains constituting dielectric layers
incorporated in a ceramic capacitor in accordance with the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0008] The term "twin" used herein refers to that represented by a
group of parallel black and white line patterns formed in a barium
titanate grain having a core-cell structure when observed along the
(110) direction by a TEM (Transmission Electron Microscope); and
the term "twin domain width" is a period of the line patterns
measured in a perpendicular direction to the group of lines.
EXAMPLE 1
[0009] Compound powders of Ho.sub.2O.sub.3 of 1.0 mol part, MgO of
0.5 mol part, Mn.sub.2O.sub.3 of 0.25 mol part and oxide glass of
1.0 wt % having Li.sub.2O--B.sub.2O.sub.3--SiO.sub.2--BaO as a main
component were mixed with BaTiO.sub.3 of 100 mol part and ground
for 15 to 24 hours by employing a wet method in a ball mill
containing therein PSZ (Partially Stabilized Zirconia) balls, to
thereby obtain a mixture.
[0010] The main component, i.e., BaTiO.sub.3, was in a form of
powder manufactured by using a hydrothermal synthesis method
yielding high crystallinity. The glass serves as a sintering
additive, that makes a sintering be completed before a solid
solution of an excessive amount of additive elements is formed in
the core.
[0011] Next, the mixture was dehydrated, dried and calcined in air
at 800.degree. C. to obtain a calcined material. The calcined
material was ground in ethanol and dried. The dried material was
then mixed with an organic binder and a solvent to provide a
ceramic slurry, which was then used for preparing green sheets
having a thickness of 5 .mu.m by employing a doctor blade
method.
[0012] Subsequently, a conductive paste having Ni powder as a main
component was applied on the green sheets by using a print method
to form internal electrodes. Ten sheets of the ceramic green sheets
having the internal electrodes thereon were stacked and
thermocompressed to form a laminated body. The laminated body was
then diced into a multiplicity of 3216 type chip-shaped ceramic
bodies having a size of 3.2 mm .times.1.6 mm.
[0013] Thereafter, Ni external electrodes were formed on the
chip-shaped ceramic bodies by using a dipping method. The organic
binder contained in the chip-shaped ceramic bodies was removed in
an N.sub.2 atmosphere. The binder-removed ceramic bodies were then
heat treated under an atmosphere having oxygen partial pressure in
the order of 10.sup.-5 to 10.sup.-8 atm to obtain sintered bodies
of a chip shape.
[0014] The sintered bodies were reoxidized in an N.sub.2 atmosphere
in a temperature range of 600 to 1000.degree. C. to thereby obtain
multilayer ceramic capacitors, wherein a thickness of each layer
incorporated in the multilayer ceramic capacitor was about 3
.mu.m.
[0015] Next, multilayer ceramic capacitors were polished in a
direction perpendicular to the stacking direction of the ceramic
green sheets until a thickness thereof became about 30 .mu.m. Then,
they were further thinned by applying ion milling. 300 grains in
the dielectric layers disposed between the internal electrodes were
observed by a TEM (Transmission Electron Microscope) along the
(110) direction and the domain widths of the twins formed in each
grain were measured.
[0016] The ratio of grains having the domain width less than 20 nm,
from 20 nm to 50 nm, and greater than 50 nm are tabulated in Table.
Since the observed domain width of twin varies depending on the
observation direction, the domain width was measured in one
direction, i.e., the (110) direction.
[0017] Next, for each specimen, dielectric constant and the
temperature dependency of the temporal capacitance were measured
and an experiment for the capacitance variations was performed.
[0018] The dielectric constant (.epsilon.) and a saturation or
maximum value of temporal capacitance variation for each specimen
are represented in Table. The temperature dependency of capacitance
satisfied both X7R characteristic (EIA standards) and B
characteristic (EIAJ standards).
[0019] Electrical characteristics were measured as follows:
[0020] (A) The dielectric constant (.epsilon.) was computed based
on a facing area of a pair of neighboring internal electrodes, a
thickness of a dielectric layer positioned between the pair of
neighboring internal electrodes, and the capacitance of a
multilayer ceramic capacitor obtained under the condition of
applying at 20 .degree. C. a voltage of 1.0 V (root mean square
value) with a frequency of 1 kHz.
[0021] (B) The saturation value of the temporal capacitance
variation (%) was obtained by measuring a capacitance (C.sub.0) at
40.degree. C. in a thermostatic (or constant temperature) oven and
thereafter periodically measuring the capacitance under the
condition of applying bias of 2 V/.mu.m until 1000 hours lapsed.
The saturation value of the temporal capacitance was obtained by
the formula .DELTA.S=(.DELTA.C.sub.1000/C.sub- .0).times.100,
wherein .DELTA.C.sub.1000 is the difference between those measured
at t=0 and t=1000 hours.
1 TABLE Microscopic structure Ratio of grains as a function of of
Domain width [%] Electrical characteristics Greater of ceramic
capacitor than 50 Saturation value of Less nm or temporal
capacitance Sample than 20 to 50 without Dielectric variation No.
20 nm nm twin constant [%] 1 24 20 56 2630 -17 2 13 34 53 2870 -20
3 9 42 49 3090 -24 4 4 54 42 3140 -25 5 0 73 27 3310 -31 6 3 70 27
3550 -29 7 6 68 26 3620 -25 8 18 53 29 3460 -22 9 26 46 28 3630 -22
10 38 41 21 3680 -24 11 41 37 22 3710 -27 12 49 34 17 3240 -30 13
52 32 16 3180 -32 14 39 28 33 2950 -21 15 40 51 9 3660 -25 16 44 55
1 3720 -27 17 48 52 0 3730 -27 represents a comparative
specimens.
[0022] Referring to Table, the test specimens 3, 4, 6 to 12 and 15
to 17 show that if the portion of the grains with the domain width
less than 20 nm is smaller than or equal to 50%, that with the
domain width in the range from 20 nm to 50 nm is 30% to 70%, and
that with the domain width greater than 50 nm or without twin is
equal to or less than 50%, the dielectric constant (.epsilon.) is
equal to or greater than 3000 and the saturation values of the
temporal capacitance variation is not less than -30%.
[0023] However, if the portion of the grains with the domain width
less than 20 nm is greater than 50% as in the specimen 13, the
saturation value is more than -30%. The test specimens 1 and 14
shows that, in case where the portion of the grains with the domain
width in the range from 20 nm to 50 nm is smaller than 30%, a
dielectric constant becomes less than 3000. Further, the test
specimen 5 show that, in case where the portion of the grains
having a domain width in the range from 20 to 50 nm is greater than
70%, the saturation value becomes smaller than -30%. In addition,
the test specimens 1 and 2 show that in case where the portion of
the grains with the domain width greater than 50 nm or without the
twin is more than 50%, a dielectric constant becomes less than
3000.
EXAMPLE 2
[0024] Ho.sub.2O.sub.3 of Example 1 was partially or entirely
replaced by other rare earth oxide, e.g., Sm.sub.2O.sub.3,
E.sub.U2O.sub.3, Tb.sub.2O.sub.3, Dy.sub.2O.sub.3, Er.sub.2O.sub.3,
Tm.sub.2O.sub.3, Yb.sub.2O.sub.3, Y.sub.2O.sub.3. It was found that
the capacitors obtained through same manufacturing procedure as in
Example 1 exhibited same microscopic twin structure and electrical
characteristics as those of Example 1.
[0025] Further, when Mn.sub.2O.sub.3 was replaced partially or
entirely by Cr.sub.2O.sub.3 and V.sub.2O.sub.5, capacitors obtained
through a same manufacturing procedure as in Example 1 also
exhibited same microscopic twin structure and electrical
characteristics as those of Example 1.
[0026] Furthermore, when the glass of 1.0 wt % was replaced by
SiO.sub.2 of 1.0 wt %, capacitors obtained through a same
manufacturing procedure as in Example 1 also exhibited same
microscopic twin structure and electrical characteristics as those
of Example 1.
[0027] As the additives are diffused into BaTiO.sub.3 grains to
form solid solution and the crystallinity of the BaTiO.sub.3 grains
deteriorates, the period of the line patterns of the twin in the
forms of parallel black and white lines becomes greater. If the
crystallinity becomes further worsened, the line patterns of twin
cannot be observed. Thus, in the present invention, the grains
without twin were equally treated as those having the domain width
greater than 50 nm.
[0028] Further, it is preferable that the ceramic grains include
oxides of Ba and Ti as main components, oxides of Re and Mg, and
one or more oxides selected from the group consisting of Mn, V and
Cr, wherein Re represents one or two elements selected from the
group consisting of Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Y.
[0029] Furthermore, it is preferable that the dielectric ceramic
composition includes 100 mol part of an oxide of Ba and Ti; 0.25 to
1.5 mol part of an oxide of Re; 0.2 to 1.5 mol part of an oxide of
Mg; and 0.025 to 0.25 mol part of oxides of one or more elements
selected from the group consisting of Mn, V and Cr, wherein the
content of the oxide of Ba and Ti is calculated by assuming that
the oxide of Ba and Ti is BaTiO.sub.3; the content of the oxide of
Re is calculated by assuming that the oxide of Re is
Re.sub.2O.sub.3; the content of the oxide of Mg is calculated by
assuming that the oxide of Mg is MgO; and the content of oxides of
Mn, V and Cr is calculated by assuming that the oxides of Mn, V and
Cr are Mn.sub.2O.sub.3, V.sub.2O.sub.5 and Cr.sub.2O.sub.3,
respectively.
[0030] Further, it is preferable that the sintered ceramic body
includes SiO.sub.2 or a glass component having SiO.sub.2, wherein
the content of the glass component ranges from 0.05 to 5 wt %.
[0031] 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.
* * * * *