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Description  |
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Priority is claimed in Japanese Patent Applications No. 2003-48234 filed on
Feb. 25, 2003, and No. 2003-151139 filed on May 28, 2003, the disclosure
of which is incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multilayer ceramic capacitor and a
process for preparing the same, and particularly to a multilayer ceramic
capacitor that comprises an effective dielectric material section
consisting of thin dielectric ceramic layers and internal electrode layers
stacked alternately, and external cover dielectric layers that are stacked
on the upper and lower surfaces of the effective dielectric material
section for the protection thereof, and a process for preparing the same.
2. Description of Related Art
Recently, as electronic components become increasingly smaller in size and
higher in functionality, efforts have been made to manufacture multilayer
ceramic capacitors that are smaller in size and larger in capacity.
Specifically, such multilayer ceramic capacitors have been manufactured as
the dielectric ceramic layer thereof is made thinner to a thickness
(distance between internal electrodes) of 10 .mu.m or less and 100 or more
internal electrode layers are stacked. With such a trend toward thinner
dielectric ceramic layers, the mean grain size of main crystal phase that
constitute the dielectric ceramic layers has been reduced to about 1 .mu.m
while particle size of the dielectric material powder and glass powder
used to make it have been made smaller. Such technologies have been
disclosed in, for example, Japanese Unexamined Patent Publication No.
10-241987 and Japanese Unexamined Patent Publication No. 9-97733.
However, in such a multilayer ceramic capacitor made by using dielectric
material powder and glass powder of fine particles as described above,
high shrinkage ratio of the dielectric material powder after firing causes
the protective external cover dielectric layer 107 to shrink at a higher
rate than the effective dielectric material section 105 that includes the
dielectric ceramic layers 101 and the internal electrode layers 103. As a
result, the external cover dielectric layer 107 tends to become smaller in
size as shown in FIG. 3 (dimension of the external cover dielectric layer
107 before shrinkage is denoted as L1 and dimension thereof after
shrinkage is denoted as L2). In such a multilayer ceramic capacitor,
strain due to difference in shrinkage after firing has been causing cracks
and/or delamination between the external cover dielectric layer and the
effective dielectric material section and between the effective dielectric
material sections.
SUMMARY OF THE INVENTION
An advantage of the present invention is to provide a multilayer ceramic
capacitor that can suppress the occurrence of cracks and/or delamination
between the external cover dielectric layer and the effective dielectric
material section and between the effective dielectric material sections
due to the difference in shrinkage after firing, despite the use of
dielectric material power of finer particles, and a process for preparing
the same.
The multilayer ceramic capacitor of the present invention comprises an
effective dielectric material section where dielectric ceramic layers that
include a main crystal phase made mainly of at least BaTiO.sub.3 and a
secondary phase made mainly of SiO.sub.2 that forms grain boundary and
triple point boundary and internal electrode layers are stacked
alternately, external cover dielectric layers that are formed on the upper
and lower surfaces of the effective dielectric material section and
includes a main crystal phase and a secondary phase comprising at least
the same components as those of the dielectric ceramic layers, and
external electrodes that are connected with the internal electrode layers
led onto both end faces of the effective dielectric material section which
includes the external cover dielectric layers, wherein the external cover
dielectric layers comprises ceramics which has lower sinterability than
the dielectric ceramic layers of the effective dielectric material
section.
Specifically, the multilayer ceramic capacitor of the present invention has
such a constitution as the mean grain size of the main crystal phase in
the external cover dielectric layer is larger than the mean grain size of
the main crystal phase in the dielectric ceramic layer, and the amount of
the secondary phase in the external cover dielectric layer is more than
the amount of the secondary phase in the dielectric ceramic layer. This
constitution makes it possible to decrease the difference in final
shrinkage after firing between the external cover dielectric layer and the
effective dielectric material section, reduce the internal stress (strain)
generated between the external cover dielectric layer and the effective
dielectric material section by reducing the shift of the shrinkage
starting temperature toward higher temperature even when the mean particle
size of the dielectric material powder is made larger, thereby to suppress
cracks and delamination from occurring in the multilayer ceramic capacitor
made by stacking larger number of thinner layers.
The multilayer ceramic capacitor of the present invention may also be made
by setting the volume proportion of the secondary phase to the main
crystal phase in the external cover dielectric layer smaller than the
volume proportion of the secondary phase to the main crystal phase in the
dielectric ceramic layer. This constitution also produces the multilayer
ceramic capacitor free of delamination even when fine particles are used
in the main crystal phase that constitutes the dielectric ceramic layer.
A process for preparing the multilayer ceramic capacitor according to the
present invention comprises the steps of forming a laminate comprising an
effective laminate made by interposing internal electrode pattern between
a plurality of first dielectric material green sheets that include
dielectric material powder and glass powder and are stacked one on
another, and external cover layers that are placed on the upper and lower
surfaces of the effective laminate and are made from second dielectric
material green sheets which include the same dielectric material powder
and the glass powder as those of the first dielectric material green
sheets; and cutting and firing the laminate, wherein the dielectric
material powder and the glass powder are included in the green sheets in
such a proportion as the second dielectric material green sheets have
lower sinterability than the first dielectric material green sheets.
Specifically, the mean particle size of the dielectric material powder
included in the second dielectric material green sheet is made larger than
the mean particle size of the dielectric material powder included in the
first dielectric material green sheet, and the amount of glass powder
included in the second dielectric material green sheets is made larger
than the amount of the glass powder included in the first dielectric
material green sheet.
Such a manufacturing method enables it to make the mean grain size of the
main crystal phase of the external cover dielectric layer larger than the
mean grain size of the main crystal phase of the effective dielectric
material section, and set the amount of the secondary phase larger in the
external cover dielectric layer than in the effective dielectric material
section, so that the multilayer ceramic capacitor that enables it to
decrease the difference in final shrinkage after firing between the
external cover dielectric layer and the effective dielectric material
section can be easily fabricated. That is, shift of the shrinkage starting
temperature toward higher temperature can be made smaller even when the
mean particle size of the dielectric material powder is made larger,
thereby making it easier to manufacture the multilayer ceramic capacitor
that can suppress cracks and delamination from occurring, despite stacking
a larger number of thinner layers.
Alternatively, proportion of the glass component in the second dielectric
material green sheet may be made smaller than the proportion of the glass
component in the first dielectric material green sheet.
As a result, even in the multilayer ceramic capacitor made by stacking a
larger number of thinner layers made of finer powder, the volume
proportion of the secondary phase that forms the grain boundary and triple
point boundary in the ceramics structure of the external cover dielectric
layer may be made lower than the volume proportion of the secondary phase
in the effective dielectric material section. Specifically, when the
volume proportion of the secondary phase in the external cover dielectric
layer is set in a range from 60 to 95% of the volume proportion of the
secondary phase in the dielectric ceramic layers that constitute the
effective dielectric material section, it is made possible to delay the
start of shrinkage of the external cover dielectric layer thereby to
achieve a changing pattern near the shrinkage curve for the firing
temperature of the effective dielectric material section. Thus it is made
possible to suppress the stress generated in the interface between the
effective dielectric material section and the external cover dielectric
layer due to the difference in the shrinkage starting temperature during
firing, thereby to prevent delamination from occurring in the interface
and between the internal electrode layer and the dielectric ceramic layer
near the interface.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic sectional view showing an embodiment of the
multilayer ceramic capacitor according to the present invention.
FIG. 2 is an enlarged sectional view between the effective dielectric
material section and the external cover dielectric layer.
FIG. 3 is a schematic sectional view showing the multilayer ceramic
capacitor where the external cover dielectric layer has shrunk compared to
the effective dielectric material section.
DESCRIPTION OF PREFERRED EMBODIMENTS
(First aspect)
The multilayer ceramic capacitor of this embodiment comprises an effective
dielectric material section 1 that contributes to the development of
capacitance, external cover dielectric layers 3 that are placed on the
upper and lower surfaces of the effective dielectric material section 1
and do not contribute to the development of capacitance, and external
electrodes 5 formed at the ends of the effective dielectric material
section 1 and the external cover dielectric layers 3. The effective
dielectric material section 1 is constituted by stacking the external
cover dielectric layers 7 and the internal electrode layers 9 alternately.
It is preferable that the thickness (t1) of the effective dielectric
material section 1 and the thickness (t2) of the external cover dielectric
layer 3 satisfy the relation of t2/t1.gtoreq.0.05. It is particularly
preferable to apply the present invention to a case where the ratio t2/t1
is 0.1 or larger, and the external cover dielectric layer 3 has greater
influence on the effective dielectric material section 1.
FIG. 2 is an enlarged sectional view between the effective dielectric
material section 1 and the external cover dielectric layer 7.
Specifically, the dielectric ceramic layer 7 is constituted from main
crystal phase 11 made of ceramic grains, grain boundary 13 and triple
point grain boundary 15 formed in the interface of the main crystal phase
11. The main crystal phase 11 is made of at least BaTiO.sub.3 as the main
component.
The grain boundary 13 and the triple point grain boundary 15 are
constituted from the secondary phase 16 including as SiO.sub.2 the main
component. The external cover dielectric layers 3 are also constituted
from the main crystal phase 11 made of similar components as those of the
dielectric ceramic layer 7 that constitutes the effective dielectric
material section 1, and the secondary phase 16 consisting of the grain
boundary 13 and the triple point grain boundary 15.
According to the present invention, it is important that the mean grain
size (D2) of the main crystal phase 11 in the external cover dielectric
layer 3 is larger than the mean grain size (D1) of the main crystal phase
11 of the dielectric ceramic layer 7, and that the amount M2 of the
secondary phase in the external cover dielectric layer 3 is larger than
the amount M1 of the secondary phase in the dielectric ceramic layer 7.
Specifically, the mean grain size (D2) of the main crystal phase 11 in the
external cover dielectric layer 3 is preferably in range from 1.1 to 1.5
times and more preferably 1.2 to 1.4 times the mean grain size (D1) of the
main crystal phase 11 of the dielectric ceramic layer 7.
It is also preferable that the amount (M2) of the secondary phase in the
external cover dielectric layer 3 is from 1.01 to 1.5 times, more
preferably from 1.05 to 1.4 times the amount (M1) of the secondary phase
in the dielectric ceramic layer 7.
According to the present invention, as the mean particle size (DG2) of the
dielectric material powder on the external cover dielectric layer 3 side
before firing is made larger than the mean particle size (DG1) on the
dielectric ceramic layer 7 side, density before firing becomes higher and
less shrinkage occurs after firing, while the starting temperature of
sintering of the external cover dielectric layer 3 shifts toward higher
temperatures. On the other hand, shrinkage starting temperature can be
made lower so as to achieve a changing pattern near the shrinkage curve
for the firing temperature of the effective dielectric material section 1,
by setting the amount of glass powder (MG1) that makes the secondary phase
(M2) including silicon oxide as the main component larger than the amount
of glass powder (MG1) that makes the secondary phase (M1) on the
dielectric ceramic layer 7 side in correspondence to the increase in the
mean particle size (DG2) of the dielectric material powder on the external
cover dielectric layer 3 side.
As a result, it is made possible to suppress the strain generated in the
interface between the effective dielectric material section 1 and the
external cover dielectric layer 3 due to the difference in shrinkage
starting temperature during firing, thereby to easily manufacture the
multilayer ceramic capacitor, that is free of cracks and delamination that
would occur in the interface and between the internal electrode layer 9
and the dielectric ceramic layer 7 near the interface, with higher yield
of production.
The mean grain size (D1, D2) of the main crystal phase 11 can be determined
by the intercept method based on the observation of the cross section of
the ceramics with an electron microscope. Specifically, the mean grain
size is given as the length of the diagonal of a 30 .mu.m square area in
the photograph divided by the number of grains lying on the line.
Thickness of the dielectric ceramic layer 7 is 7 .mu.m or less, preferably
5 .mu.m or less, and more preferably 3 .mu.m or less. Number of stacks is
100 or more, preferably 150 or more, and more preferably 200. By reducing
the thickness of the dielectric ceramic layer 7 and stacking a larger
number of the layers, electrostatic capacitance of the multilayer ceramic
capacitor can be increased.
The mean grain size (D2, D1) of the main crystal phase 11 that constitutes
the dielectric ceramic layer 7 and the external cover dielectric layer 3
is 0.5 .mu.m or less and is preferably 0.3 .mu.m or less, so that the
present invention is best suited to the multilayer ceramic capacitor
wherein mean grain size (D2, D1) of the main crystal phase that
constitutes the dielectric ceramic layer 7 and the external cover
dielectric layer 3 is small.
Thickness of the internal electrode layer 9 is 5 .mu.m or less, preferably
3 .mu.m or less, and more preferably 2 .mu.m or less, in order to reduce
the influence of stress of the internal electrode layer 9 on the effective
dielectric material section 1.
The internal electrode layer 9 is preferably one kind of metal selected
from a group including Ni, Cu, Ag, Ag--Pd or an alloy thereof, in order to
cut down on the cost of the compact and high-capacitance multilayer
ceramic capacitor, and Ni is particularly preferable since it enables
simultaneous firing with BaTiO.sub.3 that is the main component.
Now the process for preparing the multilayer ceramic capacitor according to
the present invention will be described in detail below.
First, a dielectric material powder based on BaTiO.sub.3, a glass powder
that includes at least a predetermined quantity of SiO.sub.2 and various
additives of small quantities are dispersed in a dispersing medium that
includes a binder so as to obtain a ceramic slurry. The slurry is then
formed into a sheet using a known coater, for example a doctor blade, to
obtain a first dielectric material green sheet that would become the
dielectric ceramic layer 7 after firing.
A second dielectric material green sheet that makes an external cover layer
constituting the laminate before firing, namely that would become the
external cover dielectric layer 3 after firing is also made in a procedure
similar to that for the first dielectric material green sheet.
Here it is important that the mean particle size of the dielectric material
powder in the second dielectric material green sheet is larger than the
mean particle size of the dielectric material powder in the first
dielectric material green sheet, and that the amount of glass powder (MG2)
in the second dielectric material green sheet is larger than the amount of
glass powder (MG1) in the first dielectric material green sheet.
Specifically, the mean particle size (DG2) of the dielectric material
powder in the second dielectric material green sheet is preferably 1.1 to
1.5 times and more preferably 1.2 to 1.4 times the mean particle size
(DG1) of the dielectric material powder in the first dielectric material
green sheet.
The amount of glass powder (MG2) in the second dielectric material green
sheet is preferably 1.01 to 1.5 times and more preferably 1.05 to 1.4
times the amount of glass powder (MG1) in the first dielectric material
green sheet.
This constitution makes it possible to cancel the increase in the starting
temperature of shrinkage of the second dielectric material green sheet
that uses the dielectric material powder having larger mean particle size
by the increase in the amount of glass powder, and achieve a changing
pattern near the shrinkage curve for the firing temperature of the first
dielectric material green sheet that would make the effective dielectric
material section 1.
Thus it is made possible to suppress the stress generated in the interface
between the effective dielectric material section 1 and the external cover
dielectric layer 3 due to the difference in the shrinkage starting
temperature during firing, thereby to prevent delamination from occurring
in the interface and between the internal electrode layer 9 and the
dielectric ceramic layer 7 near the interface.
According to the manufacturing method of the present invention, mean
particle size (DG1, DG2) of the dielectric material powder that
constitutes the first dielectric material green sheet and the second
dielectric material green sheet is 0.5 .mu.m or less and preferably 0.4
.mu.m.
On the other hand, mean particle size of the glass powder is in a range
from 0.3 to 1.2 .mu.m, and is preferably in a range from 0.4 to 0.8 .mu.m.
The mean particle size of the dielectric material powder mentioned in the
present invention refers to the mean particle size in the prepared slurry.
The mean particle size of dielectric material powder mentioned in the
present invention is a value corresponding to cumulative relative
frequency of 50% (D50) in particle size distribution analysis.
Thickness of the first dielectric material green sheet of the present
invention is 8 .mu.m or less, preferably 6 .mu.m or less, and more
preferably 4 .mu.m or less. Number of stacks is 100 or more, preferably
150 or more, and more preferably 200.
The first dielectric material green sheet having the internal conductor
pattern formed thereon is made by printing an electrically electrode paste
that includes a powder of one kind of metal selected from a group
including Ni, Cu, Ag, Ag--Pd and drying it. Thickness of the internal
conductor pattern is 5 .mu.m or less and preferably 3 .mu.m or less. Mean
particle size of the metal powder for making such a thin internal
conductor pattern is preferably in a range from 0.2 to 0.5 .mu.m.
When a laminated electronic component is made by stacking a large number of
layers, step between the internal electrode pattern and a portion where
the internal electrode pattern is not formed, due to the thickness of the
internal electrode pattern, has a significant structural influence leading
to defect of the laminated electronic component. To avoid this problem, it
is preferable to form a ceramic pattern by printing a dielectric ceramic
paste of the same composition as that of the first dielectric material
green sheet in the portion of the first dielectric material green sheet
where the internal electrode pattern is not formed.
Then a plurality of the first dielectric material green sheets having the
internal electrode pattern formed thereon are stacked to form the
effective laminate that would develop electrostatic capacitance after
firing. Then a plurality of second dielectric material green sheets that
make the external cover layers are stacked on the upper and lower surfaces
of the effective laminate and is processed by thermo-compression bonding
so as to make the laminate. The laminate is cut into predetermined size to
obtain individual green compacts for the capacitor element which are not
yet fired. The green compacts for the capacitor element are fired under
predetermined conditions to make capacitor elements.
An external electrode paste is applied to the end faces of the capacitor
element where the internal electrode layers 9 are led out as shown in FIG.
1, and is baked to make laminated ceramic electronic components provided
with the external electrodes.
(Second aspect)
While the multilayer ceramic capacitor of this embodiment has basically the
same structure as that of the first aspect shown in FIG. 1 and FIG. 2, it
is important in this embodiment that volume proportion of the secondary
phase to the main crystal phase (main phase) in the external cover
dielectric layer 3 is lower than the volume proportion of the secondary
phase to the main crystal phase in the dielectric ceramic layer 7.
Specifically, the volume proportion of the secondary phase to the main
crystal phase in the external cover dielectric layer 3 is preferably set
in a range from 60 to 95%, particularly from 70 to 90% of the volume
proportion of the secondary phase to the main crystal phase in the
dielectric ceramic layer 7. This makes it possible to further suppress the
stress generated due to the shrinkage of the internal electrode layer 9
interposed by the dielectric ceramic layer 7 through firing, thereby to
suppress the occurrence of delamination.
The volume proportion can be determined, for example, by the following
equation from cross sectional areas of the main crystal phase and the
secondary phase measured through observation with an electron microscope.
Volume proportion (%)={(cross sectional area of the secondary phase)/(cross
sectional area of the main crystal phase)}.times.100
While the multilayer ceramic capacitor of this embodiment can be
manufactured basically in a process similar to that for the multilayer
ceramic capacitor of the first aspect shown in FIG. 1 and FIG. 2, it is
important in this embodiment that the amount of glass component consisting
of SiO2 as the main component included in the second dielectric material
green sheet is less than that of the first dielectric material green
sheet. Specifically, the amount of glass component in the second
dielectric material green sheet is preferably set in a range from 60 to
95%, particularly from 70 to 90% of the amount of glass component in the
first dielectric material green sheet. This makes it possible to delay the
start of shrinkage of the second dielectric material green sheet that
would become the external cover dielectric layer 3 thereby to achieve a
changing pattern near the shrinkage curve for the firing temperature of
the first dielectric material green sheet that would become the effective
dielectric material section 1.
Thus it is made possible to suppress the stress generated in the interface
between the effective dielectric material section 1 and the external cover
dielectric layer 3 due to the difference in the shrinkage starting
temperature during firing, thereby to prevent delamination from occurring
in the interface and between the internal electrode layer 9 and the
dielectric ceramic layer 7 near the interface. This embodiment is similar
to the first aspect with other respects.
Various improvements and modifications can be made to the present invention
within the scope of the appended claims.
The following examples further illustrate the manner in which the present
invention can be practiced. It is understood, however, that the examples
are for the purpose of illustration and the inventions are not to be
regarded as limited to any of the specific materials or condition therein.
Example I
First, BaTiO.sub.3 powder having a mean particle size of 0.3 .mu.m was used
as the ceramic powder, for making a ceramic slurry for the first
dielectric material green sheet, and a glass powder including SiO.sub.2 as
the main component having a mean particle size of 0.6 .mu.m was used as
the sintering assisting agent. A binder solution was prepared by
dissolving polyvinyl butyral and a plasticizer in a solvent for the
ceramic slurry made by mixing toluene and ethanol in proportions of 1:1 by
weight. BaTiO.sub.3 powder and glass powder were added to the binder
solution in predetermined proportions, and were dispersed by using a ball
mill so as to condition the ceramic slurry. The ceramic slurry was spread
over a carrier film such as polyethylene terephthalate (PET) by means of
doctor blade, so as to make the first dielectric material green sheets
having a thickness of 3 .mu.m, 6 .mu.m and 8 .mu.m.
Ceramic slurry for the second dielectric material green sheet was prepared
similarly to the process described above, except for using a dielectric
material powder having a mean particle size larger than that of the
ceramic slurry for the first dielectric material green sheet and
increasing the quantity of the glass powder, as shown in Table 1. The
ceramic slurry was spread over the carrier film by means of a doctor
blade, to make the second dielectric material green sheet for the external
cover having a thickness of 10 .mu.m. Same conditions for preparing he
slurry were employed for both sheets. Specifications of the slurry are
shown in Table 1.
Then an electrically conductive paste including Ni was applied to the first
dielectric material green sheet of various thickness to form the internal
electrode pattern. The first dielectric material green sheet having the
internal electrode pattern formed thereon was then peeled off the carrier
film. 300 pieces of the first dielectric material green sheet were stacked
one on another, and each of the upper and lower surfaces of this stack was
covered by stacking 20 external cover sheets having the specified quantity
of glass component thereon, thereby making the laminate of the present
invention. Thickness of the internal conductor was set at 0.5 times that
of each green sheet.
The laminate was cut into the shape of green compacts for the capacitor
element that were degreased and fired in a reducing atmosphere to obtain
the capacitor elements. Combinations of the first dielectric material
green sheet and the second dielectric material green sheet are shown in
Table 1.
An external electrode paste was applied to both end faces of the capacitor
element and was baked to form the external electrode and make the
multilayer ceramic capacitor measuring 3.2 mm in length and 2.5 mm in
width.
Ceramic structures of the dielectric ceramic layer and of the external
cover dielectric layer were observed with an electron microscope, to
determine the mean grain size of the crystal phase including BaTiO.sub.3
that was the main crystal phase and the amount of the secondary phase
consisting of boundary layer and triple point boundary layer. In the
present invention, proportions of the dielectric material powders and
proportions of the glass components used in the first dielectric material
green sheet and the second dielectric material green sheet were retained
after firing.
Number of occurrences of delamination in 100 pieces of the multilayer
ceramic capacitor was counted as the ratio of structural defects. Number
of occurrences of cracks in 100 pieces of the multilayer ceramic capacitor
was counted in soldering thermal shock resistance test at a temperature of
280.degree. C. as an indication of reliability of the multilayer ceramic
capacitor.
Similar multilayer ceramic capacitors were fabricated as comparative
examples by using green sheets of dielectric material having the same
glass content in the first dielectric material green sheet and the second
dielectric material green sheet. Results of similar evaluation tests of
the comparative examples are shown in Table 1.
TABLE 1
Ratio of Ratio of Occur-
Thickness of Mean amount of rence of
First Dielectric Particle Secondary Delami- Occurrence
Sample Material Green Diameter Phase nation of
No. Sheet (.mu.m) (*2) (*3) (%) Cracks
*I-1 3 1.3 1 85 5/100
I-2 3 1.05 1.01 10 3/100
I-3 3 1.1 1.01 5 0/100
I-4 3 1.2 1.05 0 0/100
I-5 3 1.3 1.1 0 0/100
I-6 3 1.3 1.2 0 0/100
I-7 3 1.3 1.3 0 0/100
I-8 3 1.3 1.4 0 0/100
I-9 3 1.4 1.4 0 0/100
I-10 3 1.4 1.4 0 0/100
I-11 6 1.5 1.35 0 0/100
I-12 6 1.5 1.4 0 0/100
I-13 6 1.5 1.45 0 0/100
I-14 8 1.5 1.5 0 1/100
*I-15 3 1.0 1.0 100 --
Sample numbers marked with * are not within the scope of the present
invention.
*2: Ratio of mean particle diameter of main crystal in relation of external
cover dielectric layer/dielectric ceramic layer.
*3: Ratio of amount of secondary phase in relation of external cover
dielectric layer/dielectric ceramic layer.
As will be apparent from Table 1, in the samples Nos. I-2 through I-14
where the mean grain size of the main crystal phase that constitutes the
external cover dielectric layer was larger than that of the dielectric
ceramic layer that constituted the effective dielectric material section
and the amount of the secondary phase was larger, rates of occurrences of
cracks and delamination between the effective dielectric material sections
due to the strain caused by the difference in the firing start temperature
between the external cover dielectric layer and the effective dielectric
material was 10% or less after firing and was 3% or less after the
soldering thermal shock resistance test.
In the samples Nos. I-3 through I-14 where the mean grain size of the main
crystal phase of the external cover dielectric layer was in a range from
1.1 to 1.5 times the mean grain size of the main crystal phase of the
dielectric ceramic layer, and the amount of the secondary phase was from
1.01 to 1.5 times, in particular, rates of occurrences of cracks and
delamination were 5% or less after firing and were 1% or less after the
soldering thermal shock resistance test.
In the comparative examples of the samples Nos. I-1 and I-15 where mean
particle size and the amount of glass were made equal in the external
cover dielectric layer and the dielectric ceramic layer, or either mean
particle size or the amount of glass was made equal in the external cover
dielectric layer or the dielectric ceramic layer, in contrast,
delamination occurred in the interfaces between the external cover
dielectric layer and the dielectric ceramic layer of all multilayer
ceramic capacitors after firing.
Example II
First dielectric material green sheets having a thickness of 3 .mu.m, 6
.mu.m and 8 .mu.m were made on carrier films similarly to Example I.
The second dielectric material green sheet for the external cover 10 .mu.m
in thickness was made similarly to the process described above, except for
using a ceramic slurry for the second dielectric material green sheet with
the glass content being controlled in a range from 60 to 95% by weight to
that of the first dielectric material green sheet.
Then an electrically conductive paste including Ni was applied to the first
dielectric material green sheets of various thickness to form the internal
electrode pattern. The first dielectric material green sheets having the
internal electrode pattern formed thereon were peeled off the carrier
films. 300 pieces of the first dielectric material green sheet were
stacked one on another, and each of the upper and lower surfaces of this
stack was covered by stacking 20 external cover sheets having the
specified quantity of glass component thereon, thereby making the laminate
of the present invention.
The laminate was cut into the shape of green compacts for the capacitor
element that were degreased and fired in a reducing atmosphere to obtain
the capacitor elements. Combinations of the first dielectric material
green sheet and the second dielectric material green sheet are shown in
Table 1.
An external electrode paste was applied to both end faces of the capacitor
element and was baked to form the external electrode and make the
multilayer ceramic capacitors measuring 3.2 mm in length and 2.5 mm in
width.
Ceramic structures of the dielectric ceramic layer and the external cover
dielectric layer were observed with an electron microscope, to determine
the difference in the volume proportion between the grains including
BaTiO.sub.3 that was the main crystal phase and the secondary phase
(boundary layer and triple point boundary layer). The mean grain size of
the main crystal phase that constituted the dielectric ceramic layer and
the external cover dielectric laye | | |
