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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dielectric ceramic composition which is
advantageously used in a laminated ceramic capacitor having an internal
electrode formed of a base metal such as nickel or nickel alloy. The
present invention also relates to a laminated ceramic capacitor which is
formed from the dielectric ceramic composition and to a method for
producing the capacitor.
2. Description of the Related Art
A laminated ceramic capacitor includes a laminate formed of a plurality of
laminated dielectric ceramic layers and an internal electrode laminated
therein. Recently, the internal electrode has been formed of an
inexpensive base metal such as Ni rather than an expensive noble metal
such as Ag or Pd in order to reduce cost.
When the internal electrode is formed of a base metal such as Ni, the
electrode must be fired in a reducing atmosphere so as to avoid oxidizing
the base metal. However, when fired in a reducing atmosphere, a ceramic
formed of barium titanate is disadvantageously reduced to become
semiconductive.
In order to solve this problem, there has been developed a technique for
preventing reduction of dielectric materials by modifying the ratio of the
barium sites/titanium sites in the barium titanate solid solution such
that it exceeds the stoichiometric ratio (Japanese Patent Publication
(kokoku) No. 57-42588). Through this technique, a laminated ceramic
capacitor having an internal electrode formed of a base metal such as Ni
can be put into practical use, and production of such capacitors has
increased.
With recent advances in development of electronics, miniaturization of
laminated ceramic electronic elements has progressed rapidly. In the field
of laminated ceramic capacitors, trends towards miniaturization and
increased capacitance are also noticeable. In addition, laminated
capacitors must have an electrostatic capacity that is higher and have a
lower dependence on temperature. Thus, a variety of materials having high
dielectric constant and excellent temperature-related characteristics have
been proposed and put into practical use.
Thus far, all the proposed materials comprise BaTiO.sub.3 as a primary
component and a rare earth element, which is diffused into BaTiO.sub.3
grains during sintering, as an additive. Grains that constitute the
obtained sintered compacts are known to have a core-shell structure
comprising a core portion containing no diffused additive component and a
shell portion containing the diffused additive component. Therefore, the
combination of the core portion and the shell portion--which differ
according to the temperature dependence of the dielectric
constant--provides a composition whose dielectric constant has a low
dependence on temperature.
These materials realize laminated ceramic capacitors having high
electrostatic capacity and low dependence on temperature, and thus have
greatly contributed toward broadening of the market.
However, the core-shell structure, which is attained through sintering of
ceramics and control of diffusion of the additive component, also involves
a disadvantage. Specifically, as sintering progresses the additive
component diffuses excessively to fail to provide low dependence on
temperature, whereas insufficient sintering results in poor reliability.
Achieving control of sintering and diffusion is relatively difficult with
the above-described materials, causing undesirable variation in the
temperature dependence of dielectric constant.
Furthermore, in order to satisfy demand for miniaturization and high
electrostatic capacity, dielectric ceramic layers formed in a laminated
compact must be made thinner and the laminates must comprise a greater
number of layers. However, when the ceramic layers become thin, a smaller
number of ceramic grains are included between internal electrodes and this
remarkably deteriorates the reliability of the capacitor. Thus, the
decrease in the thickness must be limited. Therefore, development of
materials having high reliability and exhibiting low variation in
dielectric constant with temperature and electric field must be achieved
through a decrease in the size of ceramic grains.
Meanwhile, many electronic elements such as those used in automobiles are
used in a high-temperature environment, and therefore those whose
characteristics remain stable at high temperature are desired.
Specifically, there is desired a laminated ceramic capacitor of high
reliability and having a dielectric constant having a low temperature
dependence at higher temperature (e.g., 150.degree. C.).
However, the sinterability of conventional materials having a core-shell
structure and diffusion of an additive component increases as the
BaTiO.sub.3 grains become smaller, which causes difficulty in maintaining
low temperature dependence characteristics. Since BaTiO.sub.3 exhibits a
large variation in dielectric constant at high temperature (e.g.,
150.degree. C.), maintaining a dielectric constant having low temperature
dependence up to high temperature is relatively difficult.
As described hereinabove, according to the state of the art, realization of
a sufficiently thin laminated ceramic capacitor and a dielectric constant
of sufficiently low temperature dependence by use of a material having a
core-shell structure is difficult.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide
a dielectric ceramic composition to solve the above-described problems.
Another object of the invention is to provide a laminated ceramic
capacitor produced from the composition. Still another object of the
invention is to provide a method for producing the ceramic laminated
capacitor.
In short, the dielectric ceramic composition according to the present
invention is a material that does not have a core-shell structure formed
through diffusion of an additive component, i.e., a material whose
temperature-dependent characteristics and reliability do not depend on
diffusion of an additive component. A laminated ceramic capacitor produced
from the dielectric ceramic according to the present invention satisfies
the B characteristics specified by JIS specifications and satisfies the
X7R and X8R characteristics specified by EIA specifications.
In one aspect of the present invention, there is provided a dielectric
ceramic composition comprising a complex oxide containing Ba, Ca, Ti, Mg
and Mn as metal elements.
In another aspect of the present invention, there is provided a dielectric
ceramic composition represented by the following formula: {Ba.sub.1-x
Ca.sub.x O}.sub.m TiO.sub.2 +.alpha.MgO+.beta.MnO wherein
0.001.ltoreq..alpha..ltoreq.0.05; 0.001.ltoreq..beta..ltoreq.0.025;
1.000<m.ltoreq.1.035; and 0.02.ltoreq.x.ltoreq.0.15.
Preferably, the dielectric ceramic composition according to the present
invention further contains a sintering aid in an amount of about 0.2-5.0
parts by weight based on 100 parts by weight of the remaining components
of the dielectric ceramic composition. Preferably, the sintering aid
comprises SiO.sub.2 as its primary component.
In another aspect of the present invention, there is provided a laminated
ceramic capacitor which is formed of a dielectric ceramic composition
comprising a complex oxide containing Ba, Ca, Ti, Mg and Mn as metal
elements.
More specifically, the laminated ceramic capacitor includes a laminate
formed of a plurality of dielectric ceramic layers and further includes a
plurality of external electrodes provided at different positions on side
faces of the laminate, wherein each of a plurality of internal electrodes
are formed along an interface between two adjacent dielectric ceramic
layers such that each of the internal electrodes has one end exposed to
one of the side faces so as to establish electric contact with one of the
external electrodes. The dielectric ceramic layers are formed of the
above-described dielectric ceramic composition. The internal electrodes of
the laminate ceramic capacitor preferably contain Ni or an Ni alloy.
In yet another aspect of the present invention, there is provided a method
for producing a laminated ceramic capacitor comprising the following
steps:
a step for preparing a mixture comprising a compound represented by
{Ba.sub.1-x Ca.sub.x O}TiO.sub.2, an Mg compound, and an Mn compound;
a step for fabricating a laminate by laminating a plurality of ceramic
green sheets containing the mixture and a plurality of internal electrodes
each formed along an interface between two adjacent ceramic green sheets
such that each of the internal electrodes has one end exposed to one of
the side faces;
a step for firing the laminate; and
a step for forming a plurality of external electrodes on each side face of
the laminate such that the one end of each of the internal electrodes
exposed to the side face is electrically contacted with one of the
external electrodes.
Preferably, in the method for producing a laminated ceramic capacitor, the
content of an alkali metal oxide present as an impurity in the compound
represented by {Ba.sub.1-x Ca.sub.x O}TiO.sub.2 is about 0.03 wt. % or
less.
Preferably, the compound represented by {Ba.sub.1-x Ca.sub.x O}TiO.sub.2
has an average particle size of about 0.1-0.8 .mu.m.
The average particle size of the compound represented by {Ba.sub.1-x
Ca.sub.x O}TiO.sub.2 may be about 0.1 .mu.m -0.3 .mu.m, or may be more
than 0.3 .mu.m but not more than about 0.8 .mu.m. More preferably, the
maximum particle size of the compound is about 0.5 .mu.m or less for the
former case and is about 1.0 .mu.m or less for the latter case.
In the method for producing a ceramic capacitor according to the present
invention, the ratio of (average grain size of dielectric ceramic
product)/(average particle size of provided starting material powder),
which is represented by R, is preferably about 0.90-1.2.
In the above-described aspects of the present invention which relate to the
compositions and method for producing a laminated ceramic capacitor, the
dielectric ceramic compositions may further contain a rare earth element,
which is represented by RE. RE is preferably selected from the group
consisting of Y, Gd, Tb, Dy, Ho, Er and Yb.
BRIEF DESCRIPTION OF THE DRAWING
Various other objects, features, and many of the attendant advantages of
the present invention will be readily appreciated as the same becomes
better understood with reference to the following detailed description of
the preferred embodiments when considered in connection with an
accompanying drawing in which
FIG. 1 is a cross-sectional view of a laminated ceramic capacitor 1
according to one mode for carrying out the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
As mentioned above, the dielectric ceramic composition according to the
present invention comprises a complex oxide containing Ba, Ca, Ti, Mg and
Mn as metal elements. More specifically, the dielectric ceramic
composition according to the present invention is represented by the
following formula:
{Ba.sub.1-x Ca.sub.x O}.sub.m TiO.sub.2 +.alpha.MgO+.beta.MnO
wherein 0.001.ltoreq..alpha..ltoreq.0.05; 0.001.ltoreq..beta..ltoreq.0.025;
1.000<m.ltoreq.1.035; and 0.02.ltoreq.x.ltoreq.0.15.
Such a ceramic can be fired without becoming semiconductive, even when
fired in a reducing atmosphere. From the dielectric ceramic, there can be
obtained a laminated ceramic capacitor that satisfies the B
characteristics specified by JIS specifications within the range of
-25.degree. C. to +85.degree. C. and variation in electrostatic capacity
of -10% to +10% that satisfies the X7R characteristics within the range of
-55.degree. C. to +125.degree. C. and variation in electrostatic capacity
of .+-.15%, and that satisfies the X8R characteristics specified by EIA
specifications within the range of -55.degree. C. to +155.degree. C. and
variation in electrostatic capacity of .+-.15%. Moreover, the capacitors
have high reliability and high breakdown voltage at both room temperature
and high temperature.
The dielectric ceramic composition according to the present invention
typically contains a sintering aid, which is incorporated in an amount of
about 0.2-5.0 parts by weight based on 100 parts by weight of the
remaining components of the dielectric ceramic composition. Preferably,
the sintering aid predominantly comprises SiO.sub.2.
The above-described dielectric ceramic composition is used, for example, to
producing a laminated ceramic capacitor 1 illustrated in FIG. 1.
As shown in FIG. 1, the laminated ceramic capacitor 1 comprises a laminate
3 containing a plurality of laminated dielectric layers 2 and a first
external electrode 6 and a second external electrode 7 which are provided
on a first side face 4 and a second side face 5 of the laminate 3,
respectively. In its entirety, the laminated ceramic capacitor 1
constitutes a rectangular parallelepiped-shaped chip-type electronic
element.
In the laminate 3, first internal electrodes 8 and second electrodes 9 are
disposed alternately. The first internal electrodes 8 are formed along
specific interfaces between dielectric ceramic layers 2 such that each of
the internal electrodes 8 has one end exposed to the first side face 4 so
as to be electrically connected to the first external electrode 6, whereas
second internal electrodes 9 are formed along specific interfaces between
dielectric ceramic layers 2 such that each of the internal electrodes 9
has one end exposed to the second side face 5 so as to be electrically
connected to the second external electrode 7.
In the laminated ceramic capacitor 1, the dielectric ceramic layers 2
included in the laminate 3 are produced from the above-mentioned
dielectric ceramic composition.
The laminated ceramic capacitor 1 is produced according to the following
steps:
In a first step, a mixture comprising a compound represented by {Ba.sub.1-x
Ca.sub.x O}TiO.sub.2, an Mg compound and an Mn compound is prepared
through mixing, such as wet-mixing. Mixing ratios of the compounds are
preferably selected so as to obtain a dielectric ceramic composition
represented by {Ba.sub.1-x Ca.sub.x O}.sub.m TiO.sub.2
+.alpha.MgO+.beta.MnO wherein 0.001.ltoreq..alpha..ltoreq.0.05;
0.001.ltoreq..beta..ltoreq.0.025; 1.000<m.ltoreq.1.035; and
0.02.ltoreq.x.ltoreq.0.15.
The average particle size of the compound represented by {Ba.sub.1-x
Ca.sub.x O}TiO.sub.2 is preferably about 0.1 .mu.m-0.8 .mu.m. When the
average particle size is 0.1 .mu.m-0.3 .mu.m (in this case, the maximum
particle size is preferably 0.5 .mu.m or less), the laminated ceramic
capacitor 1 can possess a dielectric constant having low temperature
dependence up to 125.degree. C. and high reliability, even when the
dielectric ceramic layer 2 has a thickness as thin as 3 .mu.m or less.
When the average particle size is at least 0.3 .mu.m and not more than 0.8
.mu.m (in this case, the maximum particle size is preferably 1.0 .mu.m or
less), the laminated ceramic capacitor 1, having a dielectric ceramic
layer 2 with a thickness of more than 3 .mu.m, can possess a dielectric
constant having low temperature dependence up to 150.degree. C.
Usually, the above-described compound represented by {Ba.sub.1-x Ca.sub.x
O}TiO.sub.2 contains alkali metal oxide as an impurity. The present
inventors have confirmed that the content of the alkali metal oxide
greatly influences electric characteristics of the dielectric ceramic
composition. Specifically, they have confirmed that the content of the
alkali metal oxide must be regulated to about 0.03 wt. % or less,
preferably about 0.02 wt. % or less, in order to obtain a dielectric
ceramic having high reliability.
To the above-described mixture, a sintering aid, e.g., one predominantly
comprising SiO.sub.2, is incorporated in an amount of about 0.2-5.0 parts
by weight based on 100 parts by weight of the raw starting composition for
forming the dielectric ceramic composition. By addition of such a
sintering aid, the dielectric ceramic can be sintered at a relatively low
temperature, such as 1250.degree. C. or less, during the below-described
firing step.
Then, an organic binder and a solvent are added to the mixed powder, to
thereby obtain a slurry, and a ceramic green sheet forming the dielectric
ceramic layer 2 is produced from the slurry.
Subsequently, electrically conductive paste films forming internal
electrodes 8 and 9 are formed on the predetermined ceramic green sheets.
The conductive paste film contains a base metal such as nickel or copper,
or an alloy thereof, and is formed through a method such as screen
printing, vapor deposition, or plating.
A plurality of ceramic green sheets, including those on which conductive
paste film has been formed as described above, are laminated, pressed, and
then cut, if necessary. Thus, there is produced a green laminate 3 in
which ceramic green sheets and the internal electrodes 8 and 9 formed
along specific interfaces between ceramic green sheets are laminated, such
that each of the internal electrodes 8 has one end exposed to the side
face 4 and each of the internal electrodes 9 has one end exposed to the
side face 5.
The laminate 3 is then fired in a reducing atmosphere. As described above,
since a sintering aid comprising SiO.sub.2 is added to the mixture, the
dielectric ceramic can be sintered at a relatively low temperature, such
as 1250.degree. C. or less, to thereby minimize shrinkage of internal
electrodes 8 and 9 during the firing step. Therefore, the reliability of
the laminated ceramic capacitor 1 having a thin dielectric ceramic layer 2
can be enhanced. As described above, materials containing a base metal
such as nickel, copper, or an alloy thereof there can be employed as the
internal electrodes 8 and 9 without any problem.
During sintering to obtain the dielectric ceramic, the ratio of (average
grain size of dielectric ceramic product)/(average particle size of
provided starting material powder), which is represented by R, is
preferably about 0.90-1.2. The range of the ratio is such that
considerable grain growth does not occur during sintering of the ceramic.
When the ratio falls within the above-described range, there can be
obtained a dielectric ceramic having a dielectric constant of low
temperature dependence.
The first external electrode 6 is formed on the first side face 4 of the
laminate 3 so as to be in contact with the exposed ends of the first
internal electrodes 8, and the second external electrode 7 is formed on
the second side face 5 of the laminate 3 so as to be in contact with the
exposed ends of the second internal electrodes 9 in the fired laminate 3.
No particular limitation is imposed on the materials for producing the
external electrodes 6 and 7. Specifically, the same materials used for
producing the internal electrodes 8 and 9 may be used. The external
electrodes may also be constructed of a sintered layer comprising
electrically conductive metal powder such as powder of Ag, Pd, Ag--Pd, Cu
or a Cu alloy; or a sintered layer comprising the above conductive metal
powder blended with glass frit such as B.sub.2 O.sub.3 --Li.sub.2
O--SiO.sub.2 --BaO, B.sub.2 O.sub.3 --SiO.sub.2 --BaO, Li.sub.2
O--SiO.sub.2 --BaO or B.sub.2 O.sub.3 --SiO.sub.2 --ZnO. The materials for
producing the external electrodes 6 and 7 are appropriately determined in
consideration of factors relating to the laminated ceramic capacitor 1
such as use or environment of use.
As described above, the external electrodes 6 and 7 may be formed by
applying the metal powder paste forming them on the fired laminate 3 and
burning. Alternatively, the electrodes may be formed by applying the paste
on the unfired laminate 3 and burning simultaneous with firing of the
laminate 3.
The external electrodes 6 and 7 may be coated with plating layers 10 and
11, respectively, formed of Ni, Cu, Ni--Cu alloy, etc., in accordance with
need. The plating layers 10 and 11 may further be coated with second
plating layers 12 and 13, respectively, formed of solder, tin, etc.
In the above-described dielectric ceramic compositions and method for
producing a laminated ceramic capacitor, the dielectric ceramic
compositions may further contain a rare earth element, which is
represented by RE. RE is preferably selected from the group consisting of
Y, Gd, Tb, Dy, Ho, Er and Yb.
EXAMPLES
The present invention will next be described in detail by way of examples,
which should not be construed as limiting the invention.
Example 1
The laminated ceramic capacitor produced in the present Example is a
laminated ceramic capacitor 1 having a structure shown in FIG. 1.
High-purity TiO.sub.2, BaCO.sub.3, and CaCO.sub.3 serving as starting
materials were weighed in amounts such that the produced mixtures had
respective Ca contents shown in the following Table 1, then were mixed and
crushed. Each of the resultant powders was dried and heated to
1000.degree. C. or more, to thereby synthesize (Ba, Ca)TiO.sub.3 having
respective average particle sizes shown in Table 1.
TABLE 1
Average
Type of Alkali metal CaO (mol particle
BaTiO.sub.3 oxide (wt. %) fraction) size (.mu.m)
A 0.003 0.004 0.25
B 0.010 0.100 0.25
C 0.012 0.150 0.25
D 0.015 0.170 0.25
E 0.062 0.100 0.25
F 0.003 0.100 0.15
G 0.020 0.050 0.25
H 0.010 0.100 0.40
I 0.010 0.100 0.09
J 0.010 0.020 0.25
In order to obtain an oxide powder which serves as a sintering aid
predominantly comprising SiO2, component oxides or carbonates and
hydroxides thereof were weighed such that the produced mixtures attained
respective molar compositional ratios shown in the following Table 2, and
were then mixed and crushed. Each sample of the resultant powders was
heated to 1500.degree. C. in a platinum crucible, quenched, and crushed,
to thereby obtain average particle sizes of 1 .mu.m or less.
TABLE 2
Type of
sintering Composition of sintering aid (wt. %)
aid SiO.sub.2 TiO.sub.2 BaO CaO
a 100 0 0 0
b 80 15 5 0
c 50 30 0 20
BaCO.sub.3, MgO, and MnO were employed so as to adjust the molar ratio of
(Ba, Ca)/Ti in (Ba, Ca)TiO.sub.3 represented by m.
Subsequently, respective sintering aids were added to each of the starting
material powders, to thereby obtain mixtures having compositions shown in
Table 3. To each mixture, a polyvinyl butyral binder and organic solvent
such as ethanol were added, and the ingredients were wet-milled in a ball
mill so as to prepare a ceramic slurry. The resultant slurry was molded
into a sheet by the doctor blade method to thereby obtain a rectangular
green sheet having a thickness of 2.7 .mu.m. Then, a conductive paste
containing Ni as a primary component was applied on the resultant ceramic
green sheet by way of printing to form conductive paste film for forming
internal electrodes.
TABLE 3
Sintering aid
{Ba.sub.1-x Ca.sub.x O}.sub.m TiO.sub.2 + .alpha.MgO +
.beta.MnO Parts
Sample Type of by
No. BaTiO.sub.3 x m .alpha. .beta. Type weight
1* A 0.004 1.01 0.02 0.005 a 1
2* D 0.17 1.01 0.02 0.005 a 1
3* B 0.1 1.01 0.0008 0.005 a 1
4* B 0.1 1.01 0.06 0.005 a 1
5* B 0.1 1.01 0.02 0.0008 a 1
6* B 0.1 1.01 0.02 0.028 a 1
7* B 0.1 0.995 0.02 0.005 a 1
8* B 0.1 1 0.02 0.005 a 1
9* B 0.1 1.036 0.02 0.005 a 1
10* B 0.1 1.01 0.02 0.005 a 0
11* B 0.1 1.01 0.02 0.005 a 5.5
12* E 0.1 1.01 0.02 0.005 a 1
13 B 0.1 1.01 0.005 0.005 a 1
14 B 0.1 1.01 0.02 0.005 a 1
15 H 0.1 1.01 0.02 0.005 a 2
16 I 0.1 1.01 0.02 0.005 a 1
17 B 0.1 1.01 0.02 0.005 b 1
18 B 0.1 1.005 0.02 0.002 a 1
19 B 0.1 1.025 0.02 0.005 b 4
20 C 0.15 1.01 0.01 0.002 a 1
21 J 0.02 1.01 0.01 0.005 a 1
22 F 0.1 1.015 0.05 0.02 a 3
23 G 0.05 1.01 0.02 0.005 a 1
Subsequently, a plurality of the thus-obtained ceramic green sheets were
laminated such that leading ends of the above-mentioned conductive paste
films on the sheets were arranged alternately, to thereby obtain a
laminate. The resultant laminate was heated at 350.degree. C. in an
atmosphere of N.sub.2, so as to burn the binder, and then fired for two
hours at a temperature shown in Table 4 in a reducing atmosphere of
H.sub.2 --N.sub.2 --H.sub.2 O gas containing oxygen at a partial pressure
of 10.sup.-9 to 10.sup.-12 MPa.
A silver paste containing B.sub.2 O.sub.3 --Li.sub.2 O--SiO.sub.2 --BaO
glass frit was applied to the opposite side faces of the fired laminate,
followed by burning in a nitrogen atmosphere at 600.degree. C. to obtain
external electrodes electrically connected with the internal electrodes.
The outer dimensions of the resultant laminated ceramic capacitor were 5.0
mm width, 5.7 mm length, and 2 mm thickness, and the thickness of the
dielectric ceramic layer disposed between internal electrodes was 2.4
.mu.m. The number of effective dielectric ceramic layers was five, and in
each layer the opposing electrodes had an area of 16.3.times.10.sup.-6
m.sup.2.
The electrical properties of the resultant samples were measured as
follows.
Electrostatic capacity (C) and dielectric loss (tan .delta.) were measured
by use of an automatic bridge instrument and according to JIS 5102, and
dielectric constant (.epsilon.) was calculated from the measured
electrostatic capacity.
Resistance (R) was measured by use of an insulation tester; 10 V DC was
applied for two minutes to obtain resistance (R) at 25.degree. C., and
specific resistivity was calculated from resistance.
With regard to temperature dependence of electrostatic capacity, the rate
of variation (.DELTA.C/C.sub.20) with respect to electrostatic capacity at
20.degree. C. is shown for the temperature range of -25.degree. C. to
+85.degree. C. and the rate of variation (.DELTA.C/C.sub.25) with respect
to electrostatic capacity at 25.degree. C. is shown for the temperature
range of -55.degree. C. to +125.degree. C.
The rate of variation of electrostatic capacity under electric field of 5
kV/mm (.DELTA.C %) was also obtained.
In a high temperature loading test, time-course change of resistance upon
application of 20 V DC at 150.degree. C. was measured. In this test,
sample life was considered to be equal to the time until breakdown when
the resistance (R) of the sample dropped to 10.sup.5 .OMEGA. or less, and
average life was calculated for several samples.
Breakdown voltage was measured by applying DC voltage at a voltage
elevation rate of 100 V/sec.
Average particle size of the starting material was obtained through
observation under a scanning microscope, and average grain size of the
dielectric ceramic contained in the resultant laminated ceramic capacitor
was obtained by chemically etching polished cross-sectional surfaces of
the laminate and observing of the surfaces under a scanning microscope.
From the results, ratio R, i.e., (average grain size of the dielectric
ceramic product)/(average particle size of the starting material) was
calculated.
The results are shown in Table 4.
TABLE 4
Rate of
variation in
Rate of variation
electrostatic capacity Specific
Firing Dielectric in electrostatic with
temperature resistance Breakdown
Sample temperature Grain size Dielectric loss tan .delta. capacity DC
.DELTA.C/C.sub.20 .DELTA.C/C.sub.25 .rho. log.rho. voltage DC
No. (.degree. C.) ratio R constant (%) 5 kV/mm (.DELTA.C %)
(%) (%) (.OMEGA. .multidot. cm) (kV/mm) Average life (h)
1* 1200 1.01 2800 2.1 -55
-9.9 -15.3 13.2 15 22
2* 1150 1.01 1120 3.4 -35 -4.6
-6.6 13.1 14 20
3* 1150 3.55 2590 2.5 -64 -15.6
-25.0 11.4 12 12
4* 1300 1.01 1830 2.7 -49 -8.0
-15.2 13.2 14 1
5* 1200 1.01 1920 2.3 -55 -9.1
-15.2 11.0 15 3
6* 1200 1.01 1710 2.4 -53 -14.2
-20.2 11.3 14 8
7* 1200 1.01 2060 4.6 -60 -12.4
-19.0 11.1 8 Not measurable
8* 1200 1.01 1990 2.7 -62 -12.4
-17.3 11.5 9 Not measurable
9* 1300 1.02 2080 3.0 -50 -8.9
-14.1 12.3 8 2
10* 1300 1.02 1520 3.9 -48 -8.7
-14.0 11.4 8 Not measurable
11* 1150 1.02 1730 2.8 -47 -15.1
-29.9 13.5 14 4
12* 1200 1.11 1700 2.3 -59 -15.2
-16.7 13.2 13 11
13 1250 1.25 2540 2.2 -52 -5.3
-14.9 13.0 13 41
14 1200 0.85 1180 2.5 -51 -8.2
-14.9 13.2 14 115
15 1200 1.01 2890 1.6 -49 -4.9
-6.6 13.1 14 53
16 1100 1.01 1040 2.2 -31 -9.8
-14.9 13.2 15 214
17 1175 1.02 2200 2.2 -35 -9.4
-14.0 13.3 15 118
18 1150 1.02 2120 2.2 -40 -8.7
-13.7 13.1 14 146
19 1200 1.02 2150 2.1 -42 -9.1
-14.0 13.2 14 125
20 1175 1.05 1680 2.3 -39 -5.2
-8.2 13.1 15 175
21 1175 1.01 2330 2.1 -41 -9.6
-14.6 13.1 14 85
22 1150 1.02 1280 1.9 -33 -8.2
-12.6 13.2 15 191
23 1200 1.11 2510 2.2 -43 -9.4
-14.3 13.1 14 101
The preferable composition of the dielectric ceramic according to the
present invention is represented by {Ba.sub.1-x Ca.sub.x O}.sub.m
TiO.sub.2 +.alpha.MgO+.beta.MnO, wherein 0.001.ltoreq..alpha..ltoreq.0.05;
0.001.ltoreq..beta..ltoreq.0.025; 1.000<m.ltoreq.1.035; and
0.02.ltoreq.x.ltoreq.0.15.
The above-described characteristics preferably fall within the following
ranges: dielectric constant of 1000 or more; dielectric loss of 2.5% or
less, and decrease in electrostatic capacity of 55% or less. With regard
to the rate of variation in electrostatic capacity with temperature within
the range of -25.degree. C. to +85.degree. C., the rate of variation with
reference to electrostatic capacity at 20.degree. C. is .+-.10% or less,
and within the range of -55.degree. C. to +125.degree. C., the rate of
variation (.DELTA.C/C.sub.25) with reference to electrostatic capacity at
25.degree. C. is .+-.15% or less. The specific resistivity is 13.0
.OMEGA./cm or more and the breakdown voltage is 10 kV/mm or more.
In Tables 3 and 4, samples marked with * fall outside the above-described
preferable compositional range.
As is clear from Table 4, each of Sample Nos. 13 to 23, whose composition
falls within the preferable range, exhibits a rate of variation in
electrostatic capacity with temperature that satisfies the B
characteristics specified by JIS specifications within the range of
-25.degree. C. to +85.degree. C. and that satisfies the X7R
characteristics specified by EIA specifications within the range of
-55.degree. C. to +125.degree. C. In addition, the average life of most of
the samples in the high-temperature loading test are in excess of 100
hours, to yield high reliability. The samples can be fired at a firing
temperature of 1250.degree. C. or less, and most can be fired at
1200.degree. C. or less. The reasons why the above-described preferable
compositional range is limited to the above values will next be described.
When the content of added Ca represented by x is less than about 0.02, as
in Sample No. 1, the variation in dielectric constant with voltage, i.e.,
electric field, may be significant, and the average life in the high
temperature loading test may be short. When x is in excess of about 0.15,
as in Sample No. 2, the relative dielectric constant may be low and tan
.delta. may be high.
More preferably, the content of Ca, represented by x, is about 0.05 or
more. This is more advantageous than the case where x falls within the
range of at least about 0.02 and less than about 0.05.
When the content of added MgO represented by .alpha. is less than about
0.001, as in Sample No. 2, the specific resistivity may be low and
temperature dependence of dielectric constant may fail to satisfy the B
and X7R characteristics; whereas when .alpha. is in excess of about 0.05,
as in Sample No. 4, the sintering temperature may increase and life in a
high temperature loading test may be short.
When the content of added MnO represented by .beta. is less than about
0.001, as in Sample No. 5, specific resistivity may be low; whereas when
.beta. is in excess of about 0.025, as in Sample No. 6, specific
resistivity may be low and temperature dependence of dielectric constant
may fail to satisfy the B and X7R characteristics.
When the ratio of (Ba, Ca)/Ti represented by m is less than about 1.000, as
in Sample No. 7, specific resistivity may be low. When m is about 1.000,
as in Sample No. 8, specific resistivity may also be low. Sample Nos. 7
and 8 may have shorter life in the high temperature loading test, and some
samples were broken immediately upon application of voltage at high
temperature. When ratio of (Ba, Ca)/Ti represented by m is in excess of
about 1.035, as in Sample No. 9, sinterability may be poor and the sample
may have a short life in the high temperature loading test.
When no sintering aid is added, as in sample No. 10, sintering may be poor
to cause a large loss of specific resistivity due to plating and the
sample may have a short life in the high temperature loading test; whereas
when a sintering aid is added in an amount in excess of about 5.0 parts by
weight, as in Sample No. 11, a secondary phase attributed to a glass
component contained in the sintering aid may increase to thereby shorten
life in the high temperature loading test.
When the content of an alkali metal oxide serving as an impurity contained
in (Ba, Ca)TiO.sub.3 is in excess of about 0.03 wt. %, as in Sample No.
12, life in the high temperature loading test may be short.
In Sample No. 14, the crushing conditions employed during the preparation
of ceramic slurry was reinforced such that the particle size of powder in
the slurry was smaller than that of starting raw material powder. The
ratio of (average grain size of dielectric ceramic product)/(average
particle size of provided starting material powder), which is represented
by R, was as low as about 0.85, to cause a drop in dielectric constant.
When the ratio R is as high as 1.25, as in Sample No. 13, grain growth is
induced during firing, the ceramic grains between internal electrodes
decrease in number, and the dielectric ceramic layer is thinned, to
possibly shorten life in the high temperature loading test.
When the average grain size is as large as about 0.40 .mu.m, as in Sample
No. 15, the dielectric constant is high. In contrast, thinning a
dielectric ceramic layer as in the case of Example 1 results in a short
life in the high temperature loading test and large variation in
dielectric constant under a high electric field. When the average grain
size is as small as about 0.09 .mu.m, as in Sample No. 16, the dielectric
constant may be low and may have high dependence on temperature.
Particularly, in Sample Nos. 17 to 23, the dielectric constant falls within
the range of 1280 to 2510 and variation of electrostatic capacity under
high voltage is as low as less than 42%. No deterioration due to plating
is observed. The samples have a long life in the high temperature loading
test, as well as excellent reliability.
In the above Example, a powder was produced from (Ba, Ca)TiO.sub.3 as a
starting material through a solid phase synthesis method. However, the
powder may be produced through a wet synthesis method such as an alkoxide
method or a hydrothermal synthesis method.
Mg oxide and Mn oxide serving as additive components and a sintering aid
containing SiO.sub.2 were added in the form of oxide powders. No
particular limitation is imposed on the form of addition so long as the
dielectric ceramic phase falls within the scope of the present invention,
and there may used a solution of a precursor thereof such as alkoxide or
an organometallic compound. The yielded characteristics do not depend on
the form of addition.
The surfaces of Ni grains contained in internal electrodes of a laminated
ceramic capacitor intrinsically contain NiO. When firing conditions are
favorable for oxidation, NiO forms in a large amount. The NiO may diffuse
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