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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to dielectric ceramics, methods for making
and evaluating the same, and monolithic ceramic electronic components. In
particular, the present invention relates to thin monolithic ceramic
electronic components such as thin monolithic ceramic capacitors.
2. Description of the Related Art
Monolithic ceramic capacitors, as an example of monolithic ceramic
electronic components relating to the present invention, are typically
produced as follows.
Green ceramic sheets, each composed of a dielectric ceramic material and
provided with an internal electrode pattern of a conductive material, are
prepared. The dielectric ceramic material may comprise BaTiO.sub.3, for
example.
A plurality of green ceramic sheets, including the above sheets provided
with the internal electrode patterns, is stacked and is thermally
compressed to form a green composite.
The green composite is fired to prepare a sintered composite, which has
internal electrodes formed of the above-described conductive material.
External electrodes are formed on outer faces of the composite so that the
external electrodes are electrically connected to predetermined internal
electrodes. The external electrodes are formed, for example, by applying a
conductive paste containing a conductive metal powder and a glass frit on
the outer faces of the composite and baking the composite. A monolithic
capacitor is thereby formed.
In order to reduce production costs of the monolithic ceramic capacitors,
relatively inexpensive base metals such as nickel and copper are often
used nowadays as the conductive materials for the internal electrodes.
Unfortunately, the green composite must be fired in a neutral or reducing
atmosphere to prevent oxidation of the base metal in the production of
monolithic ceramic capacitors having such internal electrodes formed of
base metals. As a result, the dielectric ceramic used in the monolithic
ceramic capacitor must have resistance to reducing atmosphere.
BaTiO.sub.3 -rare earth oxide-Co.sub.2 O.sub.3 compositions for such
dielectric ceramics having resistance to reducing atmosphere are disclosed
in Japanese Unexamined Patent Application Publication Nos. 5-9066, 5-9067,
and 5-9068. Dielectric ceramics having a high dielectric constant, a small
change in dielectric constant with temperature and a long life at
high-temperature load are disclosed in Japanese Unexamined Patent
Application Publication Nos. 6-5460 and 9-270366.
Trends toward miniaturization and higher capacitance are noticeable in
monolithic ceramic capacitors with the rapid miniaturization of electronic
components as a result of recent great advances in electronics
technologies.
The requirements regarding reliability for dielectric ceramics which are
fired in an atmosphere which does not oxidize base metals used in internal
electrodes are a high dielectric constant, a small change in dielectric
constant with temperature and time, and high electrical insulation for
thinner dielectric ceramic layers. The above-described known dielectric
ceramics, however, do not completely satisfy these requirements.
For example, the dielectric ceramics disclosed in Japanese Unexamined
Patent Application Publication Nos. 5-9066, 5-9067, and 5-9068 above
satisfy the X7R characteristics in the EIA Standard and exhibit high
electrical insulation, but do not always satisfy the demands of the
market, namely, they may be sufficiently reliable, when the thicknesses of
the dielectric ceramics are reduced to about 5 .mu.m or less and
particularly 3 .mu.m or less.
In the dielectric ceramic disclosed in Japanese Unexamined Patent
Application Publication No. 6-5460, the particle size of the BaTiO.sub.3
powder used is large. Thus, its reliability decreases and the change in
electrostatic capacitance with time increases as the thickness of the
dielectric ceramic layer decreases.
Also, the reliability of the dielectric ceramic disclosed in Japanese
Unexamined Patent Application Publication No. 9-270366 decreases and the
change in electrostatic capacitance with time increases while applying a
DC voltage as the thickness of the dielectric ceramic layer decreases.
When the same rated voltage is applied to a dielectric ceramic layer having
a reduced thickness, which agrees with miniaturization and higher
capacitance requirements of the monolithic ceramic capacitor, a larger
electric field is applied to each layer of the dielectric ceramic. Thus,
the insulating resistance at room or high temperature decreases, resulting
in significantly decreased reliability. Accordingly, the rated voltage
must be reduced when the thickness of the dielectric ceramic layers in the
known dielectric ceramic is reduced.
There have been demands that monolithic ceramic capacitors have high
insulation resistance in high electric fields and have high reliability,
and that they can be used at high rated voltages even when the thicknesses
of the dielectric ceramic layers are reduced.
It is known that the electrostatic capacitance of a monolithic ceramic
capacitor varies over time because a DC voltage is applied in use. As the
thickness of the dielectric ceramic layers decreases, the DC electric
field per dielectric ceramic layer increases. As a result, the
electrostatic capacitance changes more significantly over time.
Thus, there have been demands that monolithic ceramic capacitors have a
small change in electrostatic capacitance when a DC voltage is applied in
use.
Also, monolithic ceramic electronic components other than the monolithic
ceramic capacitors have the above-described problems and demands.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a dielectric
ceramic exhibiting a high dielectric constant, small changes in dielectric
constant with temperature and over time when a DC voltage is applied in
use, a high product of insulation resistance and electrostatic capacitance
(CR product), and a prolonged lifetime, in terms of insulation resistance,
under accelerated testing at high temperature and high voltage.
Another object of the present invention is to provide a method for making
the dielectric ceramic.
Another object of the present invention is to provide a method for
evaluating the dielectric ceramic in which dielectric ceramics having the
above superior characteristics can be readily and efficiently selected,
for example, in a designing process.
Another object of the present invention is to provide a monolithic electric
component comprising the above dielectric ceramic.
The present invention is directed to a dielectric ceramic having a ceramic
structure comprising crystal grains and grain boundaries between the
crystal grains, the crystal grains comprising a main component represented
by the formula ABO.sub.3 and an additive containing a rare earth element
wherein A is at least one of barium, calcium and strontium, barium being
an essential element, and B is at least one of titanium, zirconium and
hafnium, titanium being an essential element.
This dielectric ceramic further satisfies the following conditions: (1) the
average rare earth element concentration in the interior of the crystal
grains is about 0.5 or less the average rare earth element concentration
at the grain boundaries, and (2) about 20% to 70% of the crystal grains
have a rare earth element concentration in the center of the crystal grain
of at least about 1/50 the maximum rare earth element concentration in a
region extending inward from the surface by a distance corresponding to
about 5% of the diameter of the crystal grain.
This dielectric ceramic exhibits a high dielectric constant, small changes
in dielectric constant with temperature and over time when a DC voltage is
applied in use, a high product of insulation resistance and electrostatic
capacitance (CR product), and a prolonged lifetime, in terms of insulation
resistance, under accelerated testing at high temperature and high
voltage.
A thin monolithic ceramic electronic component including dielectric ceramic
layers composed of this dielectric ceramic is highly reliable for a long
time.
The present invention is also directed to a method for making such a
dielectric ceramic. The method for making the dielectric ceramic comprises
the steps of mixing fractions of AO, BO.sub.2 and the rare earth element,
calcining the mixture in air, and pulverizing the mixture to prepare a
modified ABO3 powder in which the rare earth element is present in the
interiors of the particles; mixing the remaining fractions of the AO and
BO.sub.2, calcining the mixture in air, and pulverizing the mixture to
prepare an ABO.sub.3 powder in which the rare earth element is not present
in the interiors of the particles; and mixing the modified ABO.sub.3
powder, the ABO3 powder and the remaining fraction of the rare earth
element, and firing the mixture.
Since the ABO.sub.3 powder, the modified ABO.sub.3 powder and the rare
earth element are mixed at two stages, the above-described concentration
profile of the rare earth element is readily achieved in the crystal
grains. In conventional one-shot mixing, such a concentration profile is
barely achieved.
Furthermore, the present invention is directed to a method for evaluating a
dielectric ceramic that has a ceramic structure comprising crystal grains
and grain boundaries between the crystal grains, the crystal grains
comprising a main component represented by the formula ABO.sub.3 and an
additive containing a rare earth element wherein A is at least one of
barium, calcium and strontium, barium being an essential element, and B is
at least one of titanium, zirconium and hafnium, titanium being an
essential element.
This method comprises the steps of measuring the average rare earth element
concentration in the interiors of the crystal grains and the average rare
earth element concentration at the grain boundaries; determining whether
or not a first condition that the average rare earth element concentration
in the interior of the crystal grains is about 1/2 or less the average
rare earth element concentration at the grain boundaries is satisfied;
measuring the rare earth element concentration in the center of each
crystal grain and the maximum rare earth element concentration in a region
extending inward from the surface by a distance corresponding to about 5%
of the diameter of the crystal grain; determining whether or not a second
condition that about 20% to 70% of the crystal grains each have a rare
earth element concentration in the center of the crystal grain which is at
least about 1/50 of the maximum rare earth element concentration in the
region is satisfied; and assuming the dielectric ceramic to be
nondefective when the dielectric ceramic satisfies the first and second
conditions.
This method condenses a cycle of designing, making and evaluation of a
dielectric ceramic.
The present invention is also directed to a monolithic ceramic electronic
component comprising a composite comprising a plurality of stacked
dielectric ceramic layers; and internal electrodes formed along
predetermined interfaces between the dielectric ceramic layers, the
dielectric ceramic layers comprising the above-described dielectric
ceramic.
Preferably, the internal electrodes comprise a base metal.
Since the dielectric ceramic according to the present invention exhibits
high resistance to reducing environments, the base metal can be used as a
conductive component of the internal electrodes.
The monolithic ceramic electronic component is preferably a monolithic
ceramic capacitor. In such a case, the monolithic ceramic electronic
component further comprises a first external electrode and a second
external electrode formed on outer faces of the composite, wherein the
internal electrodes are arranged in the stacking direction of the
composite and are alternately and electrically connected to the first
external electrode and the second external electrode to define the
monolithic ceramic capacitor.
This monolithic ceramic capacitor has a large capacitance regardless of its
compactness and can be used at conventional rated voltages. Thus, the
thickness of the dielectric ceramic layers in the monolithic ceramic
capacitor can be reduced to, for example, about 1 .mu.m without problems.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross-sectional view of a monolithic ceramic
capacitor according to an embodiment of the present invention;
FIG. 2 is a schematic view illustrating analysis points for determining the
concentration profile of a rare earth element in a crystal grain; and
FIG. 3 is an EDX spectrum at ANALYSIS POINT 1 in FIG. 2 of the crystal
grain containing the rare earth element that is dissolved up to the
interior.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a schematic cross-sectional view of a monolithic ceramic
capacitor 21 according to an embodiment of the present invention.
The monolithic ceramic capacitor 21 includes a composite 22. The composite
22 includes a plurality of dielectric ceramic layers 23 and a plurality of
internal electrodes 24 and 25. Each of the internal electrodes 24 and 25
extends along a predetermined one among the interfaces between the
dielectric ceramic layers 23 and toward outer faces of the composite 22.
The internal electrodes 24 extending to an outer face 26 (first outer
face) and the internal electrodes 25 extending to the opposing outer face
27 (second outer face) are arranged alternately.
External electrodes 28 and 29 are formed on the first outer face 26 and the
second outer face 27, respectively. First plating layers 30 and 31 formed
of nickel, copper or the like, are formed on the external electrodes 28
and 29, respectively. Furthermore, second plating layers 32 and 33 of
solder, tin or the like, are formed on the first plating layers 30 and 31,
respectively.
Since the internal electrodes 24 and 25 are stacked in the monolithic
ceramic capacitor 21, each pair of adjacent internal electrodes 24 and 25
constitute an electrostatic capacitor. The internal electrodes 24 are
electrically connected with the external electrode 28 and the internal
electrodes 25 are electrically connected with the external electrode 29.
Thus, the electrostatic capacitance between the internal electrodes 24 and
25 is discharged through the external electrodes 28 and 29.
In this monolithic ceramic capacitor 21, the dielectric ceramic layer 23
has a ceramic structure that comprises crystal grains and grain boundaries
between the crystal grains. The crystal grains comprise the main component
represented by the formula ABO.sub.3 and an additive containing a rare
earth element where A is at least one of barium, calcium and strontium,
barium being an essential element, and B is at least one of titanium,
zirconium and hafnium, titanium being an essential element.
This dielectric ceramic satisfies the following conditions: (1) the average
rare earth element concentration in the interior of the crystal grains is
about 1/2 or less the average rare earth element concentration at the
grain boundaries (first condition); and (2) about 20% to 70% of numbers of
the crystal grains have a rare earth element concentration in the center
of the crystal grain of at least about 1/50 the maximum rare earth element
concentration in a region extending inward from the surface by a distance
corresponding to about 5% of the diameter of the crystal grain (second
condition).
The second condition is determined for the following reasons: If the
proportion of the crystal grains satisfying the concentration profile of
the second condition is less than about 20%, the lifetime in terms of
insulation resistance shortens under accelerated testing at high
temperature and high voltage, resulting in low reliability of thin
dielectric ceramic layers 23. If the proportion of the crystal grains
exceeds about 70%, the change in the dielectric constant with temperature
at high temperature increases due to the shifter effect of the rare earth
element.
The proportion of the crystal grains adequately containing the rare earth
element in the centers thereof is preferably at least about 30% in view of
prolonged high-temperature load lifetime.
The first condition is determined based on the following fact: If the
average rare earth element concentration in the interiors of the crystal
grains exceeds about 1/2 the average rare earth element concentration at
the grain boundaries, the change in electrostatic capacitance over time is
large when a DC voltage is applied.
In the present invention, the term "crystal grain boundary" represents both
a region formed between two crystal grains primarily containing ABO.sub.3
and a region formed at the boundary between three crystal grains
(so-called "triplet point"). Specifically, the grain boundary indicates a
definite layer that is crystallographically observed between crystal
grains in a cross-section of the dielectric ceramic. If a definite layer
is not observed between the crystal grains, the grain boundary includes
the connection point and/or connection line and a region extending
therefrom to a distance of 2 nm.
Such a grain boundary may further contain M, silicon, any one of A and B,
magnesium, vanadium, boron and aluminum, where M is at least one selected
from the group consisting of nickel, cobalt, iron, chromium and manganese,
A is at least one selected from the group consisting of barium, calcium
and strontium, and B is at least one selected from the group consisting of
titanium, zirconium and hafnium. These elements do not substantially have
adverse effects on the properties of the dielectric ceramic.
The overall rare earth element concentration in the dielectric ceramic is
not limited in the present invention. The overall rare earth element
concentration to 100 moles of the main component ABO.sub.3 is preferably
about 0.2 mole or more in order to achieve a prolonged high-temperature
load lifetime and is about 5 moles or less to achieve a high dielectric
constant.
The rare earth element is at least one selected from the group consisting
of lanthanum, cerium, promethium, neodymium, samarium, europium,
gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,
lutetium and yttrium. Desired characteristics are achieved by using one
type of rare earth element; however, a combination of at least two rare
earth elements facilitates the control of properties which satisfy the
demands of the market: a high dielectric constant; a high insulation
resistance, and a prolonged high-temperature load lifetime.
The average particle size of the ABO.sub.3 powder as the primary component
in the dielectric ceramic is not limited in the present invention. The
average particle size is preferably in the range of about 0.05 to 0.7
.mu.m to facilitate a reduction in thickness of the dielectric ceramic
layer 23. When the ABO.sub.3 has such an average particle size, the
thickness of dielectric ceramic layer 23 can be reduced to about 1 .mu.m
without problems.
A method for making the monolithic ceramic capacitor 21 will now be
described.
A base powder for the dielectric ceramic constituting the dielectric
ceramic layer 23 is prepared as follows.
A fraction of AO, a fraction of BO.sub.2 and a fraction of a rare earth
element are mixed according to a predetermined formula where A is at least
one element selected from the group consisting of barium, calcium and
strontium, barium being an essential element, B is at least one element of
titanium, zirconium and hafnium, titanium being an essential element, and
the rare earth element is at least one selected from the group consisting
of lanthanum, cerium, promethium, neodymium, samarium, europium,
gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,
lutetium and yttrium. The mixture is calcined in air and pulverized. A
modified ABO.sub.3 powder in which the rare earth element is present
inside the crystal grain is thereby prepared.
The remaining fractions of AO and BO.sub.2 are mixed and the mixture is
calcined in air and pulverized to prepare an ABO.sub.3 powder containing
no rare earth elements in the interior thereof.
Amounts of the modified ABO.sub.3 powder and unmodified ABO.sub.3 powder
are selected to achieve a desired result and are mixed with the remaining
fraction of the rare earth element. Furthermore, SiO.sub.2, MgO, MnO.sub.2
and B.sub.2 O.sub.3 may be added to the mixture, if necessary. The mixture
is used as a base powder for the dielectric ceramic.
A sintered dielectric ceramic layer 23 is readily prepared by sintering a
green compact containing this base powder, in which the crystal grains
have the above-described rare earth element concentration profile in the
interiors of crystal grains and at the grain boundaries.
When all the AO powder, the BO.sub.2 powder, the rare earth element and MO
and Si as optional components, where M is at least one of nickel, cobalt
and manganese, are mixed and allowed to react with each other at once, or
when an ABO.sub.3 powder, the rare earth element, and MO and SiO.sub.2 as
optional components are mixed and allowed to react with each other at
once, the crystal grains in the resulting dielectric ceramic barely have
the above-described rare earth element concentration profile in the
interiors of crystal grains and at the grain boundaries.
To the base powder, an organic binder and a solvent are added to prepare a
slurry. Using this slurry, green ceramic sheets for the dielectric ceramic
layers 23 are prepared.
A conductive paste film for forming the internal electrode 24 or 25 is
formed on each green ceramic sheet by, for example, screen-printing. The
conductive paste contains a base metal, for example, elemental nickel, a
nickel alloy, elemental copper or a copper alloy, as a conductive
component. The internal electrodes 24 and 25 may be formed by evaporation
or plating instead of printing.
A plurality of green ceramic sheets provided with the conductive paste
films are stacked and the composite is sandwiched by two green ceramic
sheets having no conductive paste films. The composite is compressed and
cut, if necessary to prepare a green composite for the composite 22. The
conductive paste films are exposed at the respective sides of the green
composite.
The green composite is fired in a reducing atmosphere to prepare the
sintered composite 22 shown in FIG. 1 in which the green ceramic sheets
constitute the dielectric ceramic layers 23 and the conductive paste films
constitute the internal electrodes 24 and 25.
The external electrodes 28 and 29, respectively, are formed on the first
and second outer faces 26 and 27 of the composite 22 and are thereby
connected to the internal electrodes 24 and 25.
The external electrodes 28 and 29 may be formed of the same material as
that of the internal electrodes 24 and 25. The external electrodes 28 and
29 may also be formed of a powder of elemental silver, elemental palladium
or a silver-palladium alloy. The metal powder may contain a glass frit of
B.sub.2 O.sub.3 --SiO.sub.2 --BaO glass, LiO.sub.2 --SiO.sub.2 --BaO
glass, B.sub.2 O.sub.3 --LiO.sub.2 --SiO.sub.2 --BaO glass, or the like.
These materials may be selected in view of the application and the
environment in the use of the monolithic ceramic capacitor 21.
The external electrodes 28 and 29 are generally formed by applying a paste
containing the above conductive metal powder on the surfaces of the
sintered composite 22 and baking the paste. Alternatively, the paste may
be applied to a green composite. In this case, sintering of the green
composite and baking of the paste can be simultaneously achieved in one
firing process.
The external electrodes 28 and 29 are plated with nickel, copper or the
like, to form first plating layers 30 and 31, respectively. The first
plating layers 30 and 31 are plated with solder, tin or the like, to form
second plating layers 32 and 33, respectively. The first plating layers 30
and 31 may be omitted according to the application.
The fabrication of the monolithic ceramic capacitor 21 is thereby
completed.
In any of the steps of preparing the base powder for the dielectric ceramic
and the step of fabricating the monolithic ceramic capacitor 21, the
monolithic ceramic capacitor 21 may incorporate impurities such as
aluminum, zirconium, iron, hafnium, sodium and nitrogen. Fortunately,
these impurities do not cause deterioration of electrical characteristics
of the monolithic ceramic capacitor 21.
Also in any of the steps for preparing the monolithic ceramic capacitor 21,
the internal electrodes 24 and 25 may incorporate impurities such as iron.
These impurities also do not cause deterioration of electrical
characteristics of the monolithic ceramic capacitor 21.
As described above, the dielectric ceramic constituting the dielectric
ceramic layers 23 in the resulting monolithic ceramic capacitor 21
satisfies the following conditions: (1) The average rare earth element
concentration in the interior of the crystal grains is about 1/2 or less
the average rare earth element concentration at the grain boundaries
(first condition); and (2) about 20% to 70% by number of the crystal
grains have a rare earth element concentration in the center of the
crystal grain of at least about 1/50 the maximum rare earth element
concentration in a region extending inward from the surface by a distance
corresponding to about 5% of the diameter of the crystal grain (second
condition). Thus, the dielectric ceramic has a high dielectric constant,
small change in dielectric constant with temperature and time when a DC
voltage is applied, a high product of the insulation resistance and the
electrostatic capacitance (CR product), and a prolonged lifetime, in terms
of insulation resistance, under accelerated testing at high temperature
and high voltage. Accordingly, the monolithic ceramic capacitor 21 is
highly reliable even the thickness of the dielectric ceramic layer 23 is
reduced.
As described above, the rare earth element concentration profile in the
dielectric ceramic significantly affects the electrical characteristics of
the dielectric ceramic. Thus, a method for evaluating the dielectric
ceramic can be provided based on the rare earth element concentration
profile. This method condenses a cycle of designing, making and evaluation
of a dielectric ceramic.
That is, the dielectric ceramic is evaluated based on the following steps.
The average rare earth element concentration in the interiors and the
average rare earth element concentration at the grain boundaries of the
crystal grains are measured to determine whether or not the first
condition that the average rare earth element concentration in the
interiors is about 1/2 or less the average rare earth element
concentration at the grain boundaries is satisfied.
Also, the rare earth element concentration in the center of the crystal
grain and the maximum rare earth element concentration in a region
extending inward from the surface by a distance corresponding to about 5%
of the diameter of the crystal grain are measured for all the crystal
grains to determine whether or not the second condition that about 20% to
70% of the crystal grains have a rare earth element concentration in the
center of the crystal grain of at least about 1/50 the maximum rare earth
element concentration in a region extending inward from the surface by a
distance corresponding to about 5% of the diameter of the crystal grain is
satisfied.
If the dielectric ceramic satisfies both the first and second conditions,
this is assumed to be satisfactory.
EXAMPLES
1. Preparation of Base Powders for Dielectric Ceramics
Example 1
BaTiO.sub.3 was used as ABO.sub.3, (Ba.sub.0.99 Dy.sub.0.01)TiO.sub.3 was
used as modified ABO.sub.3, and Dy.sub.2 O.sub.3 --NiO--MnO.sub.2
--SiO.sub.2 was used as an additive.
Barium carbonate (BaCO.sub.3) and titanium dioxide (TiO.sub.2) were weighed
in a molar ratio of Ba:Ti=1:1. These compounds were mixed together with
deionized water in a ball mill for 24 hours, and the water was evaporated
to prepare a powder mixture. The powder mixture was calcined at
1,000.degree. C. in air and was pulverized to form BaTiO.sub.3 powder.
Barium carbonate (BaCO.sub.3), titanium dioxide (TiO.sub.2), and dysprosium
oxide (Dy.sub.2 O.sub.3) were weighed in a molar ratio of
Ba:Dy:Ti=0.09:0.01:1, and these compounds were mixed together with
deionized water in a ball mill for 24 hours. Water was evaporated to
prepare a powder mixture. The powder mixture was calcined at 1,000.degree.
C. in air, and was pulverized to form (Ba.sub.0.99D Dy.sub.0.01)TiO.sub.3
powder.
The BaTiO.sub.3 powder and the (Ba.sub.0.99 Dy.sub.0.01)TiO.sub.3 powder
were compounded in a molar ratio of 50:50. With respect to a total of 100
moles of this mixture, 0.75 mole of powdered Dy.sub.2 O.sub.3, 0.5 mole of
powdered NiO, 0.2 mole of powdered MnO2 and 1.5 moles of powdered
SiO.sub.2 were added to prepare a base powder mixture for dielectric
ceramics.
Comparative Example 1
Powders of BaCO.sub.3, TiO.sub.2, Dy.sub.2 O.sub.3, NiO, MnO.sub.2 and
SiO.sub.2 were compounded in a molar ratio of 99.5:100:1.0:9.5:0.2:1.5 at
the same time. The mixture was calcined at 1,000.degree. C. and was
pulverized to prepare a base powder mixture for dielectric ceramics. The
mixture had the same composition as that in EXAMPLE 1.
Example 2
(Ba.sub.0.95 Sr.sub.0.05)TiO.sub.3 powder was used as ABO.sub.3,
(Ba.sub.0.93 Sr.sub.0.05 Gd.sub.0.02)TiO.sub.3 powder was used as modified
ABO.sub.3, and Ho.sub.2 O.sub.3 --Cr.sub.2 O.sub.3 --MgO--SiO.sub.2 powder
was used as an additive. The (Ba.sub.0.95 Sr.sub.0.05)TiO.sub.3 powder and
the (Ba.sub.0.93 Sr.sub.0.05 Gd.sub.0.02)TiO.sub.3 powder were prepared as
in EXAMPLE 1.
The (Ba.sub.0.95 Sr.sub.0.05)TiO.sub.3 powder and the(Ba.sub.0.93
Sr.sub.0.05 Gd.sub.0.02)TiO.sub.3 powder were compounded in a molar ratio
of 70:30. With respect to a total of 100 moles of this mixture, 0.8 mole
of powdered Ho.sub.2 O.sub.3, 0.5 mole of powdered Cr.sub.2 O.sub.3, 0.5
mole of powdered MgO and 2.0 moles of powdered SiO.sub.2 were added to
prepare a base powder mixture for dielectric ceramics.
Comparative Example 2
Powders of BaCO.sub.3, SrCO.sub.3, TiO.sub.2, Ho.sub.2 O.sub.3, Gd.sub.2
O.sub.3, Cr.sub.2 O.sub.3, MgO and SiO.sub.2 were compounded all at the
same time in a molar ratio of 99.4:5.0:100:0.8:0.3:0.5:0.5:2.0. The
mixture was calcined at 1,000.degree. C. and was pulverized to prepare a
base powder mixture for dielectric ceramics. The mixture had the same
composition as that in EXAMPLE 2.
Example 3
(Ba.sub.0.95 Ca.sub.0.05)(Ti.sub.0.95 Zr.sub.0.04 Hf.sub.0.01)O.sub.3
powder was used as ABO.sub.3, (Ba.sub.0.93 Ca.sub.0.05
Sm.sub.0.02)(Ti.sub.0.95 Zr.sub.0.04 Hf.sub.0.01)O.sub.3 powder was used
as modified ABO.sub.3, Ho.sub.2 O.sub.3 --MgO--MnO.sub.2 --B.sub.2 O.sub.3
--SiO.sub.2 powder was used as an additive. The (Ba.sub.0.95
Ca.sub.0.05)(Ti.sub.0.95 Zr.sub.0.04 Hf.sub.0.01)O.sub.3 powder and the
(Ba.sub.0.93 Ca.sub.0.05 Sm.sub.0.02)(Ti.sub.0.95 Zr.sub.0.04
Hf.sub.0.01)O.sub.3 powder were prepared as in EXAMPLE 1.
The (Ba.sub.0.95 Ca.sub.0.05)(Ti.sub.0.95 Zr.sub.0.04 Hf.sub.0.01)O.sub.3
powder and the (Ba.sub.0.93 Ca.sub.0.05 Sm.sub.0.02)(Ti.sub.0.95
Zr.sub.0.04 Hf.sub.0.01)O.sub.3 powder were compounded in a molar ratio
35:65. With respect to a total of 100 moles of this mixture, 0.5 mole of
powdered Ho.sub.2 O.sub.3, 1.0 mole of powdered MgO, 0.3 mole of powdered
MnO.sub.2, 0.5 mole of B.sub.2 O.sub.3 and 1.0 mole of powdered SiO.sub.2
were added to prepare a base powder mixture for dielectric ceramics.
Comparative Example 3
To 100 mol of powdered (Ba.sub.0.937 Ca.sub.0.05)(Ti.sub.0.95 Zr.sub.0.04
Hf.sub.0.01)O.sub.3, which was preliminarily prepared as ABO.sub.3,
powders of 0.5 mole of Ho.sub.2 O.sub.3, 0.65 mole of Sm.sub.2 O.sub.3,
and 1.0 mole of MgO were compounded. The mixture was calcined and was
pulverized. To this mixture, powders of 0.3 mole of MnO.sub.2, 0.5 mole of
B.sub.2 O.sub.3 and 1.0 mole of SiO.sub.2 were compounded as components
for an additive to prepare a base powder mixture for dielectric ceramics.
The mixture had the same composition as that in EXAMPLE 3.
Example 4
(Ba.sub.0.90 Ca.sub.0.10)TiO.sub.3 powder was used as ABO.sub.3,
(Ba.sub.0.89 Ca.sub.0.10 Y.sub.0.01)TiO.sub.3 powder was used as modified
ABO.sub.3, and Y.sub.2 O.sub.3 --MgO--SiO.sub.2 --B.sub.2 O.sub.3
--CoO--Fe.sub.2 O.sub.3 powder was used as an additive. The (Ba.sub.0.90
Ca.sub.0.10)TiO.sub.3 powder and the (Ba.sub.0.89 Ca.sub.0.10
Y.sub.0.01)TiO.sub.3 powder were prepared as in EXAMPLE 1.
The (Ba.sub.0.90 Ca.sub.0.10)TiO.sub.3 powder and the (Ba.sub.0.89
Ca.sub.0.10 Y.sub.0.01)Tio.sub.3 powder were com | | |
