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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dielectric ceramic which is
advantageously used in a laminated ceramic electronic element such as a
laminated ceramic capacitor having an internal conductor formed of a base
metal such as nickel or nickel alloy, and to a method for producing the
dielectric ceramic. The present invention also relates to a laminated
ceramic electronic element which is formed of the dielectric ceramic and
to a method for producing the same.
2. Description of the Related Art
Miniaturization and cost reduction of laminated ceramic electronic elements
is in progress. For example, the ceramic layer has been thinned and a base
metal has been employed as an internal conductor in such a ceramic
electronic element. In the case of a laminated ceramic capacitor, which is
one type of laminated ceramic electronic element, the dielectric ceramic
layer has been formed as thin as about 3 .mu.m and a base metal such as Cu
or Ni has been employed as a material for producing an internal conductor,
i.e., an internal electrode.
However, when the ceramic layer becomes thin, the strength of an electric
field applied to the layer increases and causes a problem in the ceramic
layer dielectric exhibits a great change in dielectric constant induced by
the electric field. Decrease of the size of ceramic grains in the
thickness direction of the ceramic layer also causes a problem in
reliability.
In order to cope with such situations, Japanese Patent Application
Laid-Open (kokai) Nos. 9-241074, 9-241075, etc. have proposed ceramic
materials which enable enhanced reliability by increasing the size of
ceramic grains in the thickness direction of the dielectric ceramic layer.
Thus, controlling the grain size of ceramic grains allows a reduction in
change of dielectric constant induced by an electric field or temperature.
However, when a barium titanate material exhibiting a strong dielectric
property is used as a material for a dielectric ceramic layer having a
thickness of about 1 .mu.m or less in the above-described conventional
art, the effect of electric field intensity on the dielectric ceramic
layer increases to thereby lower the dielectric constant considerably.
When a laminated ceramic electronic element is constructed thereof, the
rated voltage must be lowered. Therefore, realization of a thin layer
having a thickness as thin as 1 .mu.m or less is difficult or impossible
so long as the above-described conventional art is employed to solve the
problem.
SUMMARY OF THE INVENTION
In view of the foregoing, the present invention is directed to a dielectric
ceramic which is advantageously used in a laminated ceramic electronic
element including a thin ceramic layer having a thickness as thin as about
1 .mu.m or less and to a method for producing the dielectric ceramic. The
present invention is also directed to a laminated ceramic electronic
element which is formed of the dielectric ceramic and to a method for
producing the same.
In one aspect of the present invention, there is provided a dielectric
ceramic which is obtained by firing a barium titanate powder having a
perovskite structure in which the c-axis/a-axis ratio in the perovskite
structure is about 1.000 or more and less than about 1.003 and the amount
of OH groups in the crystal lattice is about 2.0 wt. % or less.
In another aspect of the present invention, there is provided a method for
producing the dielectric ceramic, which method comprises the steps of
providing a barium titanate powder in which the c-axis/a-axis ratio in the
perovskite structure is about 1.000 or more and less than about 1.003 and
the amount of OH groups in the crystal lattice is about 2.0 wt. % or less;
and firing the barium titanate powder.
The amount of OH groups is determined based on the loss at 150.degree. C.
or more as measured during thermogravimetric analysis of specimens.
The barium titanate powder preferably has a maximum particle size of about
0.3 .mu.m or less and an average particle size of about 0.05-0.15 .mu.m.
Also, each particle of the above-described barium titanate powder
preferably comprises a low-crystallinity portion and a high-crystallinity
portion, and the diameter of the low-crystallinity portion is preferably
about 0.5 times or more the particle size of the powder. As shown in FIG.
1, which is a transmission electron microscopic photograph of barium
titanate powder, and FIG. 2, which is an explanatory sketch therefor, the
term "low-crystallinity portion" 21 used herein refers to a domain
containing a number of lattice defects such as a void 22, whereas the term
"high-crystallinity portion" 23 used herein refers to a domain containing
no such lattice defects.
Also, when the ratio (average grain size of fired dielectric
ceramic)/(average particle size of provided barium titanate powder) is
represented by R, R preferably falls within the range of about 0.90-1.2.
Grains that constitute the dielectric ceramic of the present invention may
have a core-shell structure in which the composition and crystal system
differ between the core and the shell or a homogeneous structure having a
uniform composition and crystal system.
The term "crystal system" used herein refers to a crystal system of
perovskite crystals, i.e., to a cubic system having a c-axis/a-axis ratio
in the perovskite structure of about 1 or to a tetragonal system having a
c-axis/a-axis ratio in the perovskite structure of about 1 or more.
In yet another aspect of the present invention, there is provided a
laminated ceramic electronic element including a laminate formed of a
plurality of ceramic layers and an internal conductor formed along a
specific interface between adjacent dielectric ceramic layers.
Specifically, in the present invention, the dielectric ceramic layer
included in the laminated ceramic electronic element is constituted by a
dielectric ceramic obtained by firing a barium titanate powder having a
perovskite structure in which the c-axis/a-axis ratio in the perovskite
structure is about 1.000 or more and less than about 1.003 and the amount
of OH groups in the crystal lattice is about 2.0 wt. % or less.
In the above-described laminated ceramic electronic element, the internal
conductors preferably contain a base metal such as nickel or nickel alloy.
The laminated ceramic electronic element may further include a plurality of
external electrodes at different positions on a side face or faces. In
this case, the internal conductors are formed such that one end of each of
the internal conductors is exposed to the side face so as to be
electrically connected to one of the external electrodes. Such a structure
is typically applied to laminated ceramic capacitors.
In a still further aspect of the present invention, there is provided a
method for producing a laminated ceramic electronic element, which method
comprises the steps of providing a barium titanate powder in which the
c-axis/a-axis ratio in the perovskite structure is about 1.000 or more and
less than about 1.003 and the amount of OH groups in the crystal lattice
is about 2.0 wt. % or less; fabricating a laminate in which a plurality of
ceramic green sheets containing the barium titanate powder and internal
electrodes are laminated so that the internal electrodes are present along
specific interfaces of the ceramic green sheets; and firing the barium
titanate powder to thereby provide a dielectric ceramic.
BRIEF DESCRIPTION OF THE DRAWINGS
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 the
accompanying drawings, in which:
FIG. 1 is a photograph of barium titanate powder provided for producing a
dielectric ceramic according to the present invention obtained by
transmission electron microscopy;
FIG. 2 is an explanatory sketch of the electron microscopic photograph
shown in FIG. 2; and
FIG. 3 is a cross-sectional view showing a laminated ceramic capacitor 1
according to one embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The barium titanate powder used in the present invention has a composition
represented by formula: (Ba.sub.1-x X.sub.x)m(Ti.sub.1-y Y.sub.y)O.sub.3.
The composition is not further limited specifically. X may comprise Ca,
single species of rare earth elements, and a combination of two or more
thereof. Y may comprise single species such as Zr or Mn or a combination
of two or more species thereof. In general, m is preferably about
1.000-1.035, depending on the composition of the barium titanate powder,
in order to obtain a non-reducing dielectric ceramic.
The barium titanate powder which is advantageously used has a perovskite
structure in which the c-axis/a-axis ratio is about 1.000 or more and less
than about 1.003. Moreover, the amount of OH groups in the crystal lattice
is about 2.0 wt. % or less; the maximum particle size is about 0.3 .mu.m
or less; and the average particle size is about 0.05-0.15 .mu.m. Such a
barium titanate powder can be obtained by thermally treating barium
titanate powder which is produced through a wet synthesis method such as a
hydrothermal synthesis method, a hydrolysis method, or a sol-gel method.
There may also be employed a solid phase synthesis method in which a
carbonate, an oxide, etc. of elements constituting the barium titanate
powder are mixed and thermally treated.
In the above-described thermal treatment, the conditions for moderate grain
growth are selected so as to realize a c-axis/a-axis ratio of about 1.000
or more and less than about 1.003. For example, the treatment is performed
in air or performed in a nitrogen stream or H.sub.2 O stream by
controlling the temperature and time of the treatment. When a solid-phase
method is used, because the c-axis/a-axis ratio might decrease depending
on conditions for disintegration of the synthesized powder, the
disintegration conditions must be controlled.
The diameter ratio of the low-crystallinity portion 21, i.e., the ratio of
the diameter of the low-crystallinity portion 21 to the particle size of
the powder, after the above-described thermal treatment, is predetermined
to be about 0.5 or more in each particle of the barium titanate powder
shown in FIG. 1 and FIG. 2. Such a diameter ratio may be obtained through
thermal treatment at a temperature elevation rate of 5.degree. C./min or
more.
The relationship between the average particle size of the thus-provided
barium titanate powder and the average grain size of the fired dielectric
ceramic, i.e., the ratio of (average grain size of fired ceramic)/(average
particle size of provided barium titanate powder), which is represented by
R, is preferably about 0.90-1.2. Briefly, considerable grain growth is
preferably prevented during sintering for producing ceramics. For example,
for this purpose an Mn component and/or an Mg component, an Si-base
sintering aid, etc. are added to barium titanate powder. In general, these
additives may be incorporated into barium titanate powder in the form of
an oxide powder or carbonate powder. Alternatively, there may be used a
method in which barium titanate powder is coated with a solution
containing these additives and then thermally treated.
Such barium titanate powder is fired to thereby produce a dielectric
ceramic, which is used in a laminated ceramic electronic element, e.g., a
laminated ceramic capacitor 1 illustrated in FIG. 3.
As shown in FIG. 3, 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. The laminated ceramic capacitor 1 in its entirety
constitutes a rectangular parallelepiped-shaped chip-type electronic
element.
In the laminate 3, first internal electrodes 8 and second internal
electrodes 9 are alternately disposed as internal conductors. The first
internal electrodes 8 are formed along specific interfaces between
dielectric ceramic layers 2 such that one end of each of the internal
electrodes 8 is exposed to the first side face 4 of the laminate 3 so as
to be electrically connected to the first external electrode 6, while
second internal electrodes 9 are formed along specific interfaces between
dielectric ceramic layers 2 such that one end of each of the internal
electrodes 9 is exposed to the second side face 5 of the laminate 3 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 comprise the above-mentioned dielectric
ceramic.
In order to produce the laminated ceramic capacitor 1, there are provided
starting materials comprising a primary component such as barium titanate
and an additive to improve characteristics and sinterability. The
materials are weighed in predetermined amounts and wet-mixed to form a
mixed powder.
Then, an organic binder and a solvent are added into the mixed powder to
thereby obtain a slurry, and a ceramic green sheet forming the dielectric
ceramic layer 2 is produced by use of the slurry.
Subsequently, electrically conductive paste films forming internal
electrodes 8 and 9 are formed on the specific 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
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, one end of
each of the internal electrodes 8 and 9 being exposed to the side face 4
or 5.
The laminate 3 is then fired in a reducing atmosphere, to thereby transform
barium titanate powder into the dielectric ceramic. In this step, the
above-described grain size ratio R is controlled so as to fall within the
range of 0.90.ltoreq.R.ltoreq.1.2.
The first external electrode 6 and the second external electrode 7 are
formed on the first side face 4 and on the second side face 5 of the
laminate 3, respectively, so as to contact with the exposed ends of the
first internal electrodes 8 and second internal electrodes 9 in the fired
laminate 3.
No particular limitation is imposed on the composition of the materials for
producing the external electrode 6 and 7. Specifically, there may be used
the same materials as those of the internal electrodes 8 and 9. 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 composition
of the materials for producing the external electrode 6 and 7 is
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. The electrodes may also be formed by applying the paste on the
unfired laminate 3 and burning simultaneous with firing the laminate 3.
The external electrodes 6 and 7 may be coated with plating layers 10 and 11
formed of Ni, Cu, an Ni--Cu alloy, etc., respectively, in accordance with
need. The plating layers 10 and 11 may further be coated with second
plating layers 12 and 13 formed of solder, tin, etc., respectively.
The present invention will next be described in detail by way of examples,
which should not be construed as limiting the invention.
EXAMPLES
The laminated ceramic capacitor produced in this Example is a laminated
ceramic capacitor 1 having a structure shown in FIG. 3.
Different barium titanate materials having compositions shown in Table 1
were prepared by hydrolysis. Barium titanate materials having a calcium
content as high as 10 mol % were prepared by mixing barium titanate
prepared by hydrolysis and calcium titanate materials prepared by
hydrothermal synthesis (see H and I shown in Table 1). The resultant
material powders have a particle size of 50 to 70 nm and a cubic structure
containing many OH groups in lattices of a perovskite structure. Through
heat-treatment of these materials under a variety of conditions in an
atmosphere of air, barium titanate powders A to N having different "c/a"
values (c-axis/a-axis ratio), average particle sizes, maximum particle
sizes, amounts of OH groups and diameter ratios were prepared.
Aggregations produced during heat-treatment were disintegrated after
heat-treatment.
TABLE 1
Average Maximum Amount
particle particle of OH
BaTiO.sub.3 (Ba.sub.1-x Ca.sub.x).sub.m TiO.sub.3 size
size groups Diameter
powder x m c/a (.mu.m) (.mu.m) (%) ratio
A* 0.00 1.005 1.001 0.08 0.20 2.24 0.9
B 0.00 1.010 1.002 0.10 0.20 1.40 0.8
C 0.00 1.010 1.002 0.14 0.27 1.05 0.5
D 0.00 1.015 1.002 0.13 0.28 1.23 0.6
E* 0.00 1.015 1.002 0.14 0.38 0.82 0.6
F* 0.00 1.015 1.004 0.25 0.35 0.60 0.3
G 0.05 1.010 1.000 0.07 0.20 1.75 0.9
H 0.10 1.010 1.002 0.14 0.27 0.96 0.5
I* 0.10 1.010 1.004 0.25 0.38 0.65 0.3
J* 0.10 1.010 1.002 0.13 0.25 0.92 0.4
The "c/a" values shown in Table 1 were determined by X-ray diffraction of
barium titanate powders. That is, the results obtained from X-ray
diffraction were subjected to X-ray profile fitting using Rietveld
analysis to precisely determine lattice constants. The average particle
size and the maximum particle size were measured by observation of barium
titanate powders under a scanning electron microscope. The amount of OH
groups was measured by way of a loss of weight at a temperature of
150.degree. C. or higher as measured by thermogravimetric analysis of
barium titanate powders.
The diameter ratio shown in Table 1 is a ratio of the diameter of the
low-crystallinity portion to the particle size of the powder and was
determined by subjecting the powder to cut-processing so as to obtain a
thin film specimen and observation under a transmission electron
microscope. When the film-like specimen of powder is prepared by
cut-processing, the particle size of the powder and the diameter of a
low-crystallinity portion in the powder vary. In particular, since the
low-crystallinity portion is not always located in the center of a powder
particle, the size of the portion must be observed several times depending
on the cutting site upon preparation of thin film. Thus, for observation
there were selected particles having a particle size similar to the
particle size observed by scanning electron microscopy. The diameter ratio
was determined by observation of 10 or more such particles and calculating
the average diameter ratio.
In the "BaTiO.sub.3 powder" column in Table 1, "x"s of the materials
(Ba.sub.1-x Ca.sub.x).sub.m TiO.sub.3 powders A to F are 0.00. Thus,
powders A to F contain no Ca, but powders G to J, in which "x"s are 0.05
or 0.10, contain Ca.
As additives for the barium titanate powders shown in Table 1, those having
the compositions shown in Tables 2 and 3 were provided. Specifically, with
respect to Sample Nos. 1 to 10 shown in Table 2, RE (RE represents any one
of Gd, Dy, Ho, and Er), Mg and Mn were provided to be added to BaTiO.sub.3
in the form of one of the above-mentioned samples A to F. A sintering aid
containing Si as a primary component was also provided. With respect to
Sample Nos. 11 to 19 shown in Table 3, Mg and Mn were provided as
additives to the (Ba.sub.1-x Ca.sub.x)TiO.sub.3 in the form of any one of
the above-mentioned samples G to J. A sintering aid containing (Si,
Ti)--Ba as a primary component was also provided.
TABLE 2
Si-
BaTiO.sub.3 + .alpha.Mg + .gamma.Mn containing
.beta. (parts .gamma. (parts
sintering
Sample Type of .alpha. (parts by mole) by by aid (parts
No. BaTiO.sub.3 Gd Dy Ho Er mole) mole) by mole)
1 A 0.03 0.030 0.005 3
2 B 0.03 0.010 0.005 5
3 B 0.03 0.020 0.005 4
4 B 0.03 0.020 0.005 3
5 B 0.03 0.020 0.005 4
6 B 0.03 0.020 0.005 3
7 C 0.02 0.020 0.020 4
8 D 0.02 0.020 0.005 3
9 E 0.02 0.020 0.005 3
10 F 0.02 0.020 0.005 3
TABLE 3
(Si,Ti)--Ba-
(Ba.sub.1-x Ca.sub.x)TiO.sub.3 + .beta.Mg + .gamma.Nb
containing
.beta. (parts by .gamma. (parts by
sintering aid
Sample No. Type of BaTiO.sub.3 mole) mole) (parts by
mole)
11 G 0.01 0.005 6
12 G 0.02 0.005 4
13 H 0.02 0.005 6
14 H 0.02 0.005 4
15 H 0.02 0.005 4
16 H 0.02 0.005 3
17 I 0.02 0.003 4
18 I 0.02 0.003 4
19 J 0.02 0.005 4
Respective additives shown in Tables 2 and 3 were transformed into alkoxide
compounds, which are soluble in organic solvent, and then were added to
the barium titanate powders which had been dispersed in an organic
solvent. Specifically, with respect to Sample Nos. 1 to 10, respective
additives were added to the barium titanate powders such that ".alpha.,"
".beta.," ".gamma.," and "Si-containing sintering aid," based on parts by
mole, in "BaTiO.sub.3 +.alpha.RE+.beta.Mg+.gamma.Mn" were as shown in
Table 2. With respect to Sample Nos. 11 to 19, respective additives were
added to the barium titanate powders such that ".beta.," ".gamma.," and
"(Si,Ti)--Ba-containing sintering aid," all based on parts by mole, in
"(Ba.sub.1-x Ca.sub.x)TiO.sub.3 +.beta.Mg+.gamma.Mn" were as shown in
Table 3.
In order to dissolve the above-mentioned additives in an organic solvent,
they may be transformed into alkoxides as described above, or may be
transformed into acetylacetonates or metal soaps.
The resultant slurries were subjected to evaporation of the organic solvent
to dryness and further heat-treatment, to thereby remove the organic
components.
Subsequently, to each sample of the barium titanate powders to which
respective additives had been added, a polyvinyl butyral binder and
organic solvent such as ethanol were added, and the ingredients were
subjected to wet milling so as to prepare a ceramic slurry. The resultant
slurry was molded into a sheet by use of a doctor blade to thereby obtain
a rectangular green sheet having a thickness of 1.0 .mu.m. Then, on the
resultant ceramic green sheet, a conductive paste containing Ni as a
primary component was applied by way of printing to form a conductive
paste film for forming internal electrodes.
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 the temperature shown in Table 4 in a reducing atmosphere of
H.sub.2 --N.sub.2 --H.sub.2 O gas with a partial pressure of 10.sup.-9 to
10.sup.-12 MPa oxygen.
To the opposite side faces of the fired laminate, a silver paste containing
B.sub.2 O.sub.3 --Li.sub.2 O--SiO.sub.2 --BaO glass frit was applied,
followed by burning in a nitrogen atmosphere at 600.degree. C. to obtain
external electrodes electrically connected with the internal electrodes.
The outer size of the resultant laminated ceramic capacitor was 5.0 mm
width, 5.7 mm length and 2.4 mm thickness, and the thickness of the
dielectric ceramic layer existing between internal electrodes was 0.6
.mu.m. The total number of effective dielectric ceramic layers was five,
and the area of the opposing electrodes per layer was 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 according to JIS 5102, and
dielectric constant (.di-elect cons.) was determined by use of the
resultant electrostatic capacity.
In order to measure insulation resistance (R), an insulation tester was
used; by application of 6 V DC for two minutes, insulation resistance (R)
at 25.degree. C. was obtained, and resistivity was calculated.
Regarding the rate of change in electrostatic capacity with respect to
temperature change, the rate of change (.DELTA.C/C.sub.20) within a range
of -25.degree. C. to +85.degree. C. with reference to the electrostatic
capacity at 20.degree. C. and the rate of change (.DELTA.C/C.sub.25)
within a range of -55.degree. C. to +125.degree. C. with reference to the
electrostatic capacity at 25.degree. C. are shown.
In a high temperature loading test, time-course change of insulation
resistance upon application of 6 V DC at 150.degree. C. was measured. In
this test, the average life of the samples were evaluated, wherein the
life of a sample was considered to be equal to time until breakdown when
the insulation resistance (R) of each sample dropped to 10.sup.5 .OMEGA.
or less.
Breakdown voltage was measured by applying DC voltage at a voltage
elevation rate of 100 V/sec.
The average grain size of dielectric ceramic contained in the resultant
laminated ceramic capacitor was obtained by chemically etching polished
cross-sectional surfaces of the laminate and observation of the surfaces
under a scanning microscope. By use of the results and average particle
sizes of the starting raw materials shown in Table 1, a ratio R, i.e.,
(average grain size of the dielectric ceramic)/(average particle size of
the starting raw material) was measured.
The results are shown in Table 4.
TABLE 4
Rate of Rate of
capacitance
capacitance change with
respect
Dielectric change to
temperature Breakdown
Firing loss .DELTA.C % change
Resistivity voltage Average
Sample temperature Size Dielectric tan .delta. DC3kV/mm
.DELTA.C/C.sub.20 .DELTA.C/C.sub.25 log .rho. DC life
No. (.degree. C.) ratio R constant (%) (%) (%)
(%) (.OMEGA. .multidot. cm) (kV/mm) (h)
1* 1050 1.55 1260 2.3 -14.6 -10.6
-27.0 13.1 76 0.4
2* 1100 1.76 1280 3.4 -22.1 -14.6
-30.5 13.1 81 0.5
3 1100 1.04 960 3.6 -3.4 -8.8
-12.4 13.2 88 65
4 1150 1.08 860 2.8 -3.2 -9.8
-14.7 13.2 94 96
5 1050 1.05 910 2.9 -3.3 -9.7
-14.8 13.2 91 88
6* 1050 0.80 630 3.3 -2.4 -12.6
-26.7 13.2 88 91
7 1150 1.16 1130 3.6 -4.6 -9.7
-14.8 13.0 91 75
8 1100 1.10 1040 3.4 -3.8 -8.8
-12.8 13.2 88 63
9* 1150 1.08 1060 3.3 -3.6 -9.4
-14.5 13.1 81 4.5
10* 1175 1.04 1670 2.3 -10.9 -9.4
-14.4 13.2 64 2.1
11* 1100 1.84 1120 3.8 -10.7 2.4
8.7 13.2 68 1.6
12 1100 1.05 740 2.6 1.3 -1.4
-4.6 13.2 72 66
13* 1150 1.63 1210 5.7 -11.4 3.4
9.7 13.1 64 2.5
14 1150 0.97 760 2.3 -2.4 -4.5
-4.5 13.2 96 68
15 1175 1.05 870 2.1 -1.5 -4.2
-5.8 13.2 88 64
16* 1150 0.88 230 1.9 2.4 -11.4
-17.9 13.2 87 96
17* 1250 1.05 1080 4.6 -8.4 -4.2
-8.5 13.2 75 13
18* 1200 0.75 330 2.5 1.5 -11.3
-16.4 13.2 83 67
19* 1175 1.04 1030 2.8 -8.6 -2.2
-2.5 13.2 85 72
The dielectric ceramic of the present invention is characterized in that it
is obtained by firing barium titanate powder having a perovskite structure
in which the c-axis/a-axis ratio in the perovskite structure is about
1.000 or more and less than about 1.003 and the amount of OH groups in the
crystal lattice is about 2.0 wt% or less. Preferably, the barium titanate
powder serving as the raw material has a maximum particle size of about
0.3 .mu.m or less, and an average particle size of about 0.05 to about
0.15 .mu.m. Also, each particle of the above-described barium titanate
powder preferably comprises a low-crystallinity portion and a
high-crystallinity portion, the diameter of the low-crystallinity portion
being about 0.5 times or more the particle size of the powder, and a ratio
R, i.e., (average grain size of the dielectric ceramic)/(average particle
size of barium titanate powder), of about 0.90 to 1.2, so that there is no
occurrence of considerable grain growth during sintering ceramic.
The sample numbers marked with * in Table 4, and the powders marked with *
in Table 1 fall outside the scope of the present invention or the
above-stated preferable ranges.
Regarding Sample Nos. 1 to 10 shown in Table 4 obtained through use of one
of the raw material powders A to F shown in Table 1, transmission electron
microscopic analysis of fired ceramic showed that near the grain boundary
of a ceramic grain, the rare-earth element (RE) such as Gd, Dy, Ho, or Er
diffused and formed a shell portion; and, in the center of the ceramic
grain, a core portion was formed; namely, the fired ceramic assumes,
within each grain, a core-shell structure in which the core and shell have
different compositions and crystal systems.
As is apparent from the data of Sample No. 1 in Table 4, use of a powder
having an amount of OH groups of 2.0 wt% or more, such as the material
powder A shown in Table 1, is not preferable because the reactivity is too
high, making the sintered ceramic grain size large; temperature
characteristics of dielectric constant become excessive; and average life
becomes short.
In the case of use of material powder such as the material powder E shown
in Table 1 having a maximum particle size larger than 0.3 .mu.m, as is
apparent from Sample No. 9 in Table 4, the average life of the capacitor
may become short, and, in the case of the present Example wherein the
dielectric ceramic layer has a thickness of 1 .mu.m or less, the
reliability may become poor.
In the case of use of powder material such as the material powder F shown
in Table 1 having an average particle size larger than 0.15 .mu.m and a
maximum particle size larger than 0.3 .mu.m, as is apparent from Sample
No. 10 shown in Table 4, when the dielectric ceramic layer is thin, the
reliability may become poor, and electrostatic capacity change at 3 kV/mm
may become excessive.
As the material powder B shown in Table 1, even in the case in which the
c-axis/a-axis ratio of the material powder is 1.000 or more and less than
1.003, the amount of OH groups is 2.0 wt% or less, the maximum particle
size is 0.3 .mu.m or less, and the average particle size is within a range
of 0.05 to 0.15 .mu.m; as is apparent from Sample No. 2 shown in Table 4,
when ratio R is greater than 1.2, change in the dielectric constant with
temperature may become excessive and reliability may become poor.
As in the case of use of the material powder B shown in Table 1, as is
apparent from Sample No. 6 shown in Table 4, when the grain size of the
sintered body is made small relative to the material particle size by
intensive crushing during ingredient preparation so as to achieve a ratio
R of less than 0.90, the dielectric constant may be low, and temperature
characteristics of electrostatic capacity may become poor.
In contrast, although in Sample Nos. 3, 4, 5, 7, and 8 shown in Table 4 the
thickness of the dielectric ceramic layer is as thin as 0.6 .mu.m, the
rate of change in electrostatic capacity with temperature satisfies the B
characteristic specified by JIS specifications within the range of
-25.degree. C. to +85.degree. C., and satisfies the X7R characteristic
specified by EIA specifications within the range of -55.degree. C. to
+125.degree. C. Further, the samples can endure for 60 hours or longer
until breakdown occurs in a high temperature loading test, and they can be
fired at 1200.degree. C. or lower. The change in electrostatic capacity
upon application of DC voltage is as small as 5% or less and thus can
ensure a high voltage rating.
Fired ceramics of Sample Nos. 11 to 19 shown in Table 4 which were obtained
by use of the material powders G to J shown in Table 1 were subjected to
transmission electron microscopic analysis. It was confirmed that
compositional non-uniformity and non-uniform crystal system as found for
individual ceramic grains in connection with Sample Nos. 1 to 17 were not
observed.
The material powders G and H shown in Table 1--in which the c-axis/a-axis
ratio of the material powder is 1.000 or more and less than 1.003, the
amount of OH groups is 2.0 wt% or less, the maximum particle size is 0.3
.mu.m or less, and the average particle size is within a range of 0.05 to
0.15 .mu.m--shows that the grain size of the ceramic increases, and in the
case in which the particle size ratio R is more than 1.2, reliability
becomes poor and change in electrostatic capacity at 3 kV/mm may becomes
large, as is apparent from Sample Nos. 11 to 13 in Table 2.
In the case of use of the material powder H shown in Table 1, as is
apparent from Sample No. 16 in Table 4, when the grain size of the
sintered body is made small relative to the material particle size by
intensive crushing during ingredient preparation so as to achieve a ratio
R of less than 0.90, the rate of change in electrostatic capacity with
temperature may be large, and relative dielectric constant may become
poor.
As the material powder I shown in Table 1, even in the case in which
average particle size is in excess of 0.15 .mu.m and maximum particle size
is in excess of 0.3 .mu.m, as is apparent from Sample No. 17 in Table 4, a
ratio R falling within the range of 0.90-1.2 does not necessarily ensure
reliability.
As in the case of use of the material powder I shown in Table 1, as is
apparent from Sample No. 18 in Table 4, when the grain size of the
sintered body is made small relative to the material particle size by
intensive crushing during ingredient preparation so as to achieve a ratio
R of less than 0.90, the change in electrostatic capacity with temperature
may be large, and relative dielectric constant may become poor.
As is clear from Sample No. 19 in Table 4, when there is used the material
powder J shown in Table 1 wherein each particle of barium titanate powder
includes a low-crystallinity portion having a diameter 0.5 times or less
the particle size of the powder--which indicates that a high-crystallinity
portion occupies a large area of the powder--dielectricity might be
enhanced and electrostatic capacity at 3 kV/mm might increase.
In contrast, with reference to Sample Nos. 12, 14, and 15 shown in Table 4,
although the thickness of the dielectric ceramic layer is as thin as 0.6
.mu.m, the rate of change in electrostatic capacity with temperature
satisfies the B characteristic specified by JIS specifications within the
range of -25.degree. C. to +85.degree. C., and satisfies the X7R
characteristic specified by EIA specifications within the range of
-55.degree. C. to +125.degree. C. Further, the samples can endure for 60
hours or longer until breakdown occurs in a high temperature loading test,
and the samples can be fired at a temperature of not higher than
1200.degree. C. The electrostatic capacity change upon application of DC
voltage is 5% or less and can ensure a high voltage rating.
As stated above, even if grains of a dielectric have a homogeneous
composition and a uniform crystal system within each particle--namely,
even if the grains do not have a core-shell structure--controlling grain
growth during sintering can produce a dielectric ceramic having excellent
temperature-dielectric constant characteristic and high reliability, as in
the cases of Sample Nos. 12, 14, and 15.
The above-described Example is directed to a laminated ceramic electronic
element in the form of a laminated ceramic capacitor; however, in the case
of other laminated ceramic electronic elements such as a multilayered
ceramic substrate which is produced by almost the same method, the same
results have been confirmed to be obtained.
As described hereinabove, ferroelectricity of the barium titanate material
in the dielectric ceramic of the present invention, is suppressed by
controlling the c-axis/a-axis ratio in the perovskite structure and the
amount of OH groups in the crystal lattice, preferably by further
controlling the maximum particle size, average particle size, structure of
powder particles based on the crystallinity and the grain growth during
firing of barium titanate powder serving as a starting material.
Therefore, the dielectric ceramic has an excellent temperature-dielectric | | |
