U.S. patent application number 11/415314 was filed with the patent office on 2006-09-21 for stacked ceramic body and production method thereof.
This patent application is currently assigned to Denso Corporation. Invention is credited to Akira Fujii, Toshiatsu Nagaya, Hitoshi Shindo, Atsuhiro Sumiya, Takashi Yamamoto, Eturo Yasuda.
Application Number | 20060210781 11/415314 |
Document ID | / |
Family ID | 26625075 |
Filed Date | 2006-09-21 |
United States Patent
Application |
20060210781 |
Kind Code |
A1 |
Nagaya; Toshiatsu ; et
al. |
September 21, 2006 |
Stacked ceramic body and production method thereof
Abstract
This invention provides a stacked ceramic body that prevents
reaction between components of dielectric layers and components of
electrode layers of an unsintered stacked body during sintering and
in which both components do not easily form a liquid phase, and a
production method of such a stacked ceramic body. A print portion
13 is formed on a green sheet 1, 12 containing lead by use of an
electrode paste consisting of copper oxide as its main component. A
desired number of print sheets 10 are stacked to give an unsintered
stacked body 15. Degreasing is conducted in an atmosphere to
degrease organic components. The print portion 13 is subjected to
reducing treatment in a reducing atmosphere containing hydrogen and
is converted to a print portion 13 containing copper as its main
component. The unsintered stacked body 15 is sintered in a reducing
atmosphere. Dielectric layers containing lead and electrode layers
for applying a voltage to the dielectric layers are alternately
stacked, and an oxidation portion containing copper is formed in
the proximity of a surface of the electrode layer. A thickness of
the oxidation portion in a stacking direction is 0.5 to 2 .mu.m and
a copper content in the oxidation portion is 1 to 30 wt %.
Inventors: |
Nagaya; Toshiatsu;
(Kuwana-city, JP) ; Yamamoto; Takashi;
(Chiryu-city, JP) ; Fujii; Akira; (Yokkaichi-city,
JP) ; Sumiya; Atsuhiro; (Kekinan-city, JP) ;
Shindo; Hitoshi; (Okazaki-city, JP) ; Yasuda;
Eturo; (Okazaki-city, JP) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
Denso Corporation
Aichi-pref.
JP
|
Family ID: |
26625075 |
Appl. No.: |
11/415314 |
Filed: |
May 2, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10318359 |
Dec 13, 2002 |
7063813 |
|
|
11415314 |
May 2, 2006 |
|
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Current U.S.
Class: |
428/210 |
Current CPC
Class: |
Y10T 428/24926 20150115;
H01L 41/0471 20130101; H01L 41/273 20130101; H01L 41/083
20130101 |
Class at
Publication: |
428/210 |
International
Class: |
B32B 18/00 20060101
B32B018/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 2001 |
JP |
2001-381933 |
Nov 15, 2002 |
JP |
2002-332217 |
Dec 14, 2001 |
JP |
2001-38199 |
Claims
1.-11. (canceled)
12. A stacked ceramic body produced by a method comprising the
steps of: arranging a printed portion formed of an electrode paste
consisting of copper or a copper compound as its main component on
a green sheet formed of an oxide dielectric containing a lead oxide
as its constituent element to form a print sheet; stacking a
plurality of said print sheets to form an unsintered stacked body;
conducting degreasing treatment by heat-treating for removing
organic components contained in said unsintered stacked body; and
sintering said unsintered stacked body in a reducing atmosphere
while controlling an oxygen partial pressure from room temperature
to 400 to 600.degree. C. to an oxygen partial pressure at which
copper and lead oxide can coexist, or an oxygen partial pressure
higher than said oxygen partial pressure at which copper and lead
oxide can coexist, wherein dielectric layers containing lead and
electrode layers containing copper for applying a voltage to said
dielectric layers are alternately stacked, and exposed portions of
said electrode layers and portions in the proximity of said exposed
portions are oxidation portions.
13. A stacked ceramic body according to claim 12, wherein an
oxidation width of said oxidation portion measured in a direction
vertical to a stacking direction of said electrode layers is 0.05
to 2 mm.
14. A stacked ceramic body according to claim 12, wherein a
diffusion portion in which at least one kind of component
constituting said electrode layer is diffused exists in the
proximity of an interface with said electrode layer in said
dielectric layer.
15. A stacked ceramic body according to claim 14, wherein copper
originating from said electrode layer is in a diffused state in
said diffusion portion, a diffusion distance from the interface
between said dielectric layer and said electrode layer in said
diffusion portion is 0.5 to 2 .mu.m, and a copper content in said
diffusion portion is 0.1 to 30 wt %.
16. A stacked ceramic body according to claim 12, wherein said
stacked ceramic body is a piezoelectric device.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a stacked ceramic body that can be
utilized as a piezoelectric device for a piezoelectric actuator,
and a production method thereof.
[0003] 2. Description of the Related Art
[0004] Piezoelectric devices comprising stacked ceramic bodies,
fabricated by alternately stacking a plurality of dielectric layers
and electrode layers for applying a voltage to the dielectric
layers, are known. Among them, a stacked ceramic body the
dielectric layer of which is formed of lead zirconate titanate
(PZT) and the electrode of which is formed of copper has gained a
wide application as a piezoelectric device because it is economical
and is considerably free from the migration that has been observed
in a silver-palladium electrode.
[0005] In the stacked ceramic body including the dielectric layers
containing lead and the electrode layers containing copper,
however, metallic lead is isolated from the dielectric layer during
sintering of an unsintered stacked body in a production process of
the stacked ceramic body, and this metallic lead and metallic
copper of the electrode layer together form a liquid phase and flow
out in some cases. Further, the electrode layer often aggregates
and is interrupted due to the reaction between metallic lead
isolated from the dielectric layer and metallic copper of the
electrode layer (see later-appearing FIGS. 1(a) and 1(b)). Lead
oxide contained in the dielectric layer forms a liquid phase with
copper oxide, is diffused into the dielectric layer, and sometimes
denatures the dielectric layer.
SUMMARY OF THE INVENTION
[0006] In view of the problems of the prior art technologies
described above, this invention provides a stacked ceramic body
that prevents reaction between components of dielectric layers and
components of electrode layers of an unsintered stacked body during
sintering and in which both components do not easily form a liquid
phase, and a production method of such a stacked ceramic body.
[0007] According to a first aspect of the invention, there is
provided a method of producing a stacked ceramic body comprising
the steps of arranging a print portion formed of an electrode paste
consisting of copper or a copper compound as its main component on
a green sheet formed of an oxide dielectric containing a lead oxide
as its constituent element to form a print sheet; stacking a
plurality of the print sheets to form an unsintered stacked body;
conducting degreasing treatment by heat-treating and removing
organic components contained in the unsintered stacked body; and
sintering the unsintered stacked body in a reducing atmosphere from
room temperature to 400 to 600.degree. C. while controlling an
oxygen partial pressure to an oxygen partial pressure at which
copper and lead oxide can coexist, or an oxygen partial pressure
higher than the oxygen partial pressure at which copper and lead
oxide can coexist.
[0008] The operation and effect of the first invention will be
explained. The first invention conducts degreasing treatment of the
unsintered stacked body and sinters the unsintered stacked body in
a reducing atmosphere.
[0009] When lead is isolated from the green sheet to operate as the
dielectric layer during sintering in the reducing atmosphere, this
lead reacts with copper of the print portion, forms a liquid phase
and is pushed out in some cases while involving other components of
the green sheet and print portions. Aggregation is also likely to
occur in the print portion.
[0010] Such isolation of lead occurs when the dielectric material
in the green sheet is reduced during the reducing treatment of the
unsintered stacked body and also when the dielectric material in
the green sheet is reduced at a sintering temperature of less than
600.degree. C. in the reducing atmosphere.
[0011] According to the first invention, the reducing atmosphere
from the room temperature to 400 to 600.degree. C. is controlled to
an oxygen partial pressure at which copper and lead oxide can
coexist, or an oxygen partial pressure higher than the oxygen
partial pressure at which copper and lead oxide can coexist.
Furthermore, lead that has already been isolated is oxidized.
Because isolation of lead during sintering is thus prevented, lead
does not increase any more. Therefore, the reaction does not easily
occur between isolated lead and copper, and aggregation of the
print portion and flow-out of the material from inside the
unsintered stacked body do not easily occur.
[0012] As the oxygen partial pressure is controlled in reduction
and sintering, copper in the proximity of an exposed portion of the
print portion of the unsintered stacked body is oxidized to copper
oxide that does not react with lead. Therefore, even when lead and
copper react inside the unsintered stacked body and part of them is
liquefied, flow-out of the materials from inside the stacked body
does not easily occur.
[0013] According to a second aspect of the invention, there is
provided a stacked ceramic body produced by alternately stacking
dielectric layers containing lead and electrode layers containing
copper for applying a voltage to the dielectric layers, wherein
exposed portions of the electrode layers to outside and portions in
the proximity of the exposed portions are oxidation portions.
[0014] In the second invention, the oxidation portion exists at the
portion of the electrode layer exposed to the outside and in
proximity to the exposed portion. This oxidation portion can
prevent flow-out of the materials from inside the liquefied
unsintered stacked body. As the stacked ceramic body is produced by
the method according to the first invention, aggregation of the
print portion does not easily occur.
[0015] The first and second inventions described above provide a
stacked ceramic body in which the reaction does not easily occur
between the component of the dielectric layer and, in the
components of the electrode layer of the unsintered stacked body
during its sintering, both components are not easily liquefied and
the liquefied component does not easily flow out, and also provides
a production method thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIGS. 1(a) to 1(d) are explanatory views each showing a
production method of an unsintered stacked body in an embodiment of
the invention;
[0017] FIG. 2 is a diagram showing a temperature profile during
degreasing in the embodiment of the invention;
[0018] FIGS. 3(a) to 3(c) are explanatory views each showing an
arrangement state of an unsintered stacked body during degreasing
in the embodiment of the invention;
[0019] FIG. 4 is a diagram showing a temperature profile during
reducing treatment in the embodiment of the invention;
[0020] FIGS. 5(a) and 5(b) are explanatory views each showing an
arrangement state of an unsintered stacked body during sintering in
the embodiment of the invention;
[0021] FIG. 6 is an explanatory view showing a state of a saggar
during sintering in the embodiment of the invention;
[0022] FIG. 7 is an explanatory view of a sintering furnace in the
embodiment of the invention;
[0023] FIG. 8 is a diagram showing control of a temperature and an
oxygen partial pressure during sintering;
[0024] FIG. 9 is a diagram showing an internal temperature and
oxygen partial pressure inside a sintering furnace during
sintering;
[0025] FIG. 10 is a diagram showing a Cu+PbO coexisting range and
the range for the temperature and the oxygen partial pressure
during sintering in the embodiment of the invention;
[0026] FIG. 11(a) is an explanatory view of an electrode layer and
an oxidation portion (a sectional explanatory view taken along line
A-A in FIG. 11(b)), and FIG. 11(b) is an explanatory view of a
stacked ceramic body; and
[0027] FIG. 12(a) is a schematic view showing an electrode layer
and an oxidation portion in a comparative example and FIG. 12(b) is
a schematic view showing an electrode layer and an oxidation
portion in the embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] In the first and second inventions, the dielectric material
that constitutes the green sheet is a composite compound containing
a lead oxide such as lead zirconate titanate,
Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3 and so forth. The print portion
described above is formed of copper or a copper compound. It may be
copper not containing any impurity or a compound of copper with Ni
or Zn.
[0029] A print portion formed of an electrode paste that contains
copper oxide as its main component is preferably arranged to form a
print sheet.
[0030] The print portion consisting of copper oxide contained in
the unsintered stacked body is preferably subjected to reducing
treatment in a hydrogen-containing reducing atmosphere to form the
print portion consisting of copper as its main component. When
subjected to the reducing treatment in the hydrogen-containing
reducing atmosphere, copper oxide is converted to the electrode
layer consisting mainly of copper. A material capable of achieving
such conversion is selected as a material for the electrode paste.
The copper oxide includes both monovalent and divalent oxides.
[0031] The volume of the unsintered stacked body is preferably 8
mm.sup.3 or more. When this condition is satisfied, prevention of
the reaction between isolated lead and copper, aggregation of the
print portion and the flow-out of the materials from inside the
unsintered stacked body mentioned in the first invention can be
more reliably achieved.
[0032] In the first invention, sintering is carried out while the
oxygen partial pressure is controlled to an oxygen partial pressure
at which copper and lead oxide can coexist, or to a higher oxygen
partial pressure than the oxygen partial pressure oxygen partial
pressure at which copper and lead oxide can coexist, until the
temperature is elevated from the room temperature to 400 to
600.degree. C. When the oxygen partial pressure is controlled to
the pressures other than the range of the oxygen partial pressure
described above in this temperature range, metallic lead is
isolated, forms a liquid phase with metallic copper and is likely
to flow out.
[0033] When the volume of the unsintered stacked body is smaller
than 8 mm.sup.3 in the first invention, copper in the proximity of
the surface of the unsintered stacked body is likely to be oxidized
by oxygen that diffuses from an external atmosphere into the
unsintered stacked body during sintering. In this case,
conductivity of the electrode layer drops when the print portion
changes to the electrode layer, and the operation as the electrode
cannot be exhibited easily.
[0034] Even if machining such as grinding is applied after
sintering, oxidation of the electrode proceeds in most cases to a
portion deeper than a range that can be removed by grinding
(grinding margin), and it is difficult to allow the electrode layer
operate as the electrode by grinding. When the electrode layer is
partially oxidized, a sufficient voltage cannot be applied easily
to the dielectric layer due to conduction defects when a stacked
ceramic body obtained from the unsintered stacked body is used as a
piezoelectric device. Therefore, performance of the piezoelectric
device is likely to drop.
[0035] The shapes of the green sheet and the print portion (that
is, the dielectric layer and the insulating layer) are square in
the later-appearing example, but they can be formed into desired
shapes such as a rectangular shape, a polygonal shape, a round
shape, a barrel shape, an elliptic shape, and so forth.
[0036] Next, to sinter the unsintered stacked body in the reducing
atmosphere, the temperature elevation is started from the room
temperature. In this case, the oxygen partial pressure in the
reducing atmosphere is preferably controlled to 10.sup.-10 to
10.sup.-20 atm (1.013.times.10.sup.-5 (=10.sup.-4.994) to
1.013.times.10.sup.-15 (=10.sup.-14.99) Pa).
[0037] When the oxygen partial pressure is controlled to the
specific range described above in the temperature range from the
room temperature to 400 to 600.degree. C., sintering can be carried
out at the oxygen partial pressure at which copper and lead oxide
can coexist or at an oxygen partial pressure higher than the oxygen
partial pressure at which copper and lead oxide can coexist.
Therefore, the reaction between metallic lead and metallic copper
can be prevented. An oxidation portion can be formed more easily,
as will be later described.
[0038] Next, to sinter the unsintered stacked body in the reducing
atmosphere, the temperature elevation is continued from 400 to
600.degree. C. In this case, it is preferred to control the oxygen
partial pressure in the reducing atmosphere to the oxygen partial
pressure at which copper and lead oxide can coexist, or to a higher
oxygen pressure than the oxygen partial pressure at which copper
and lead oxide can coexist, until the temperature reaches 900 to
1,000.degree. C.
[0039] When the temperature elevation is continued from 400 to
600.degree. C. and the oxygen partial pressure is so controlled as
to satisfy the condition described above until the temperature
reaches 900 to 1,000.degree. C., reduction sintering can be carried
out while the state under which copper and lead coexist is kept as
such.
[0040] When the oxygen partial pressure is less than 10.sup.-20 atm
(1.013.times.10.sup.-5 (=10.sup.-4.994) Pa) while the temperature
reaches 400 to 600.degree. C., sintering is conducted in a range
other than the Cu+PbO coexisting range. Therefore, metallic lead is
isolated from the green sheet to operate as the dielectric layer,
and metallic copper of the print portion to operate as the
electrode layer and metallic lead together form a liquid phase and
are likely to flow out.
[0041] When the oxygen partial pressure is greater than 10.sup.-10
atm (1.013.times.10.sup.-5 (=10.sup.-4.994) Pa) till the
temperature reaches 400 to 600.degree. C., the portion of the print
portion that comes into contact with oxygen is broadly oxidized,
the electric resistance of the electrode layer obtained from this
print portion becomes great and the function of the electrode is
likely to drop. Lead oxide and copper oxide of the green sheet
together form the liquid phase and are likely to diffuse into the
green sheet, that is, into the dielectric layer.
[0042] When the oxygen partial pressure, until the temperature
reaches 900 to 1,000.degree. C., is within the range of the oxygen
partial pressure at which copper and lead oxide can coexist,
metallic lead is isolated from the green sheet to operate as the
dielectric layer, and metallic copper and metallic lead of the
print portion to operate as the electrode layer together form the
liquid phase and are likely to flow out. When the oxygen partial
pressure is lower than the oxygen partial pressure at which copper
and lead oxide can coexist, metallic lead is isolated, and metallic
copper and metallic lead of the print portion to operate as the
electrode layer together form the liquid phase and are likely to
flow out. Furthermore, formation of an oxidation portion to be
later described becomes difficult and peeling is likely to occur
between the dielectric layer and the electrode layer.
[0043] Next, the requirement "to control the oxygen partial
pressure to an oxygen partial pressure at which copper and lead
oxide can coexist, or to a higher oxygen partial pressure than the
oxygen partial pressure at which copper and lead oxide can coexist,
until the temperature reaches 900 to 1,000.degree. C. while the
temperature elevation is continued" will be explained. An oxygen
partial pressure of an atmosphere determines whether copper changes
to a copper oxide or lead changes to a lead oxide in a system
containing both copper and lead. A specific range of the oxygen
partial pressure exists at which copper is not oxidized but lead is
oxidized to lead oxide and both of them coexist. According to claim
5, the oxygen partial pressure is kept at the oxygen partial
pressure at which copper and lead oxide coexist or at a higher
oxygen partial pressure (towards the oxidation side) than this
Cu+PbO coexisting oxygen partial pressure range.
[0044] As concretely shown in the diagram of FIG. 10, the range
encompassed by solid line a and solid line b is the coexisting
range of copper and lead oxide, and claim 5 stipulates that
sintering is carried out in this range or in a range having a
higher oxygen partial pressure. Incidentally, 1 atm=1,013
hPa=1.013.times.10.sup.5 Pa.
[0045] When the unsintered stacked body is sintered in the reducing
atmosphere, the oxygen partial pressure is preferably controlled so
as to satisfy the following condition at each temperature:
1,000.degree. C.:
[0046] 10.sup.-4 to 10.sup.-7.9 atm (10.13 (=10.sup.1.0056) to
1.276.times.10.sup.-3 (=10.sup.-2.894)Pa) 900.degree. C.: [0047]
10.sup.-5 to 10.sup.-10.1 atm (1.013 (=10.sup.0.0056) to
8.049.times.10.sup.-6 (=10.sup.-5.094)Pa) 800.degree. C.: [0048]
10.sup.-6 to 10.sup.-12.2 atm (1.013.times.10.sup.-1
(=10.sup.-0.9944) to 6.393.times.10.sup.-8 (=10.sup.-7.194)Pa)
700.degree. C.: [0049] 10.sup.-7 to 10.sup.-14.5 atm
(1.013.times.10.sup.-2 (=10.sup.-1.994) to 3.204.times.10.sup.-10
(=10.sup.-9.494)Pa) 600.degree. C.: [0050] 10.sup.-8 to
10.sup.-16.6 atm (1.013.times.10.sup.-3 (=10.sup.-2.994) to
2.545.times.10.sup.-12 (=10.sup.-11.59)Pa) 500.degree. C.: [0051]
10.sup.-9 to 10.sup.-18.8 atm (1.013.times.10.sup.-4
(=10.sup.-3.994) to 1.606.times.10.sup.-14 (=10.sup.-13.79)Pa)
[0052] When sintering is carried out while the oxygen partial
pressure is kept at the value at each temperature range described
above, sintering can be conducted at the oxygen partial pressure at
which copper and lead oxide can coexist, or at a higher oxygen
partial pressure than the Cu+PbO coexisting oxygen partial
pressure, and it is possible to prevent metallic lead from being
isolated from the green sheet to operate as the dielectric layer,
and to prevent metallic copper and metallic lead of the print
portion to operate as the electrode layer from forming the liquid
phase and flowing out, as can be understood from later-appearing
FIG. 10.
[0053] When sintering is conducted within a range deviated towards
the reduction side from the oxygen partial pressure corresponding
to each temperature range, metallic lead is isolated, and metallic
copper and metallic lead of the print portion to operate as the
electrode layer together form the liquid phase and are likely to
flow out. Formation of the oxidation portion to be later described
becomes difficult and the dielectric layer and the electrode layer
are likely to peel. When sintering is conducted within a range
deviated towards the oxidation side from the oxygen partial
pressure corresponding to each temperature range, other dielectric
materials, non-reacted lead oxide and copper oxide slightly formed
at the print portion to operate as the electrode layer together
form the liquid phase and are likely to diffuse into the stacked
body.
[0054] Next, in the second invention, an oxidation width of the
oxidation portion measured in a direction vertical to the stacking
direction of the electrode layer is preferably from 0.05 to 2 mm.
When this condition is satisfied, it is possible to acquire an
oxidation portion sufficient to cut off the outflow of the
materials when the liquid phase is formed inside. When the
oxidation width of the oxidation portion is less than 0.05 mm, it
becomes impossible in some cases to prevent the outflow when
metallic copper and metallic lead isolated from inside the green
sheet react with each other and form the liquid phase. When the
oxidation width is greater than 2 mm, on the other hand, the
electrode layer is covered with a thick oxide film and conductivity
of the electrode layer is likely to drop.
[0055] Even when machining such as grinding is applied after
sintering, oxidation of the electrode layer practically proceeds in
many cases to a deeper portion than the range that can be removed
by grinding, and it is difficult to let the electrode layer
function as the electrode by grinding. When the stacked ceramic
body having a broad oxidation portion is used as the piezoelectric
device, performance of the piezoelectric device is likely to drop
due to conduction defect of the electrode layer.
[0056] Incidentally, the oxidation width employs a maximum width
that is measured from an end portion of the stacked ceramic body to
the end of the oxidation portion in a direction vertical to the
stacking direction of the stacked ceramic body.
[0057] In the second invention, at least one diffusion portion
formed by diffusion of at least one kind of components constituting
the electrode layer preferably exists in the proximity of the
interface with the electrode layer in the dielectric layer. This
oxidation portion provides sufficient adhesion strength between the
dielectric layer and the electrode layer. Incidentally, the
thickness of the oxidation portion in the invention uses a maximum
thickness measured from the surface of the electrode layer in the
stacking direction of the stacked ceramic body.
[0058] In the diffusion portion in the second invention, copper
originating from the electrode layer is under the diffused state, a
diffusion distance from the interface between the dielectric layer
and the electrode layer in the diffusion portion is 0.5 to 2 .mu.m,
and the content of copper is preferably 0.1 to 30 wt %. When this
condition is satisfied, the drop of the insulation resistance can
be suppressed.
[0059] When the diffusion distance is less than 0.5 .mu.m, the
adhesion strength is not sufficient between the dielectric layer
and the electrode layer, and peeling is likely to occur between
these layers. When the diffusion distance is greater than 2 .mu.m,
a dielectric portion or a low insulation resistance portion having
another composition that contains copper is formed in a laminar
form or in a sparsely scattered form inside the dielectric layer.
As the insulation resistance of the dielectric layer drops in this
case, dielectric breakdown is likely to occur in the dielectric
layer when this stacked ceramic body is used as the piezoelectric
device.
[0060] When the copper content is less than 0.1 wt %, the adhesion
strength of the oxidation portion becomes weak and peel is likely
to occur between the dielectric layer and the electrode layer. When
the copper content is greater than 30 wt %, the insulation
resistance of the dielectric layer is likely to drop.
[0061] In the second invention, the stacked ceramic body is
preferably the piezoelectric device. In the stacked ceramic body
according to the second invention, the dielectric layer and the
electrode layer do not easily peel. Therefore, when the stacked
ceramic body is caused to operate as the piezoelectric device, the
stacked ceramic body undergoes extension and contraction in the
stacking direction. However, the dielectric layer and the electrode
layer do not easily peel due to this extension and contraction, and
the piezoelectric device has excellent durability.
[0062] As the electrode layer keeps sufficient electric
conductivity, the voltage can be reliably applied to the dielectric
layer. Moreover, a voltage necessary for sufficiently extending and
contracting the dielectric layer can be reliably applied.
Therefore, an excellent piezoelectric device can be obtained. In
this way, the stacked ceramic body according to the second
invention can be utilized as an excellent piezoelectric device.
[0063] Hereinafter, an example of the invention will be explained
with reference to the accompanying drawings. A method of producing
a stacked ceramic body 1 (see FIGS. 1(a) to 1(d)) according to this
example first forms a print portion 13 formed of an electrode paste
consisting of copper oxide as a main component on green sheets 11
and 12 containing a lead oxide as its constituent element to obtain
a print sheet 10, and then stacks a plurality of print sheets 10 to
obtain an unsintered stacked body 15 as shown in FIGS. 1(a) to
1(d).
[0064] Next, a degreasing treatment for removing a binder contained
in the unsintered stacked body is carried out by heat-treating the
organic components contained in the unsintered stacked body 15 in
the atmosphere. Reducing treatment of the print portion 13
consisting of copper oxide contained in the unsintered stacked body
as the main component is carried out to convert the print portion
13 to the one that consists of copper as the main component. The
unsintered stacked body is then sintered in a reducing atmosphere
and at an oxygen partial pressure at which copper and lead oxide
can coexist, or at a higher oxygen partial pressure than the oxygen
partial pressure at which copper and lead oxide can coexist, until
the temperature is elevated from room temperature to 400 to
600.degree. C.
[0065] In the way described above, there is obtained the stacked
ceramic body 1 in which the dielectric layers 31 and 32 containing
lead and the electrode layers 33 containing copper for applying the
voltage to these dielectric layers 31 and 32 are alternately
stacked and the exposed portion of the electrode layers 33 to the
outside and the portions near the exposed portion are formed of the
oxidation portion 335 as shown in FIGS. 11(a) and 11(b).
[0066] Hereinafter, the explanation will be given in detail. The
stacked ceramic body in this example is a piezoelectric device that
can be utilized as a driving source of a piezoelectric actuator.
This stacked ceramic body 1 is fabricated by alternately stacking
the dielectric layers 31 and 32 formed of lead zirconate titanate
(hereinafter called "PZT"; its detailed composition will be
described in a later-appearing production method) and the electrode
layers 33 consisting of copper oxide as its main component.
Incidentally, FIGS. 1(a) to 1(d) show the stacking state of the
unsintered stacked body, but the same structure is essentially kept
after sintering (though shrinkage occurs to some extents due to
sintering).
[0067] In other words, the stacked ceramic body 1 includes the
dielectric layers 31 and 32, the electrode layer 33 formed on the
surface of each dielectric layer 31 and 32, and a non-formation
portion 330, where the electrode layer 33 is not formed, on one of
the side surfaces of each dielectric layer 31 and 32 (see FIGS.
11(a) and 11(b)). In the stacked ceramic body 1 of this example,
the dielectric layers 31 and 32 are stacked in regular order in
such a manner that the non-formation portions 330 of the electrode
layers 33 alternately appear on the different side surfaces 35 and
36.
[0068] Next, the production method of the stacked ceramic body 1
will be explained in detail. First, green sheets 11 and 12 for the
dielectric layers 31 and 32 are prepared. Lead oxide and tungsten
oxide are weighed to 83.5 wt % and 16.5 wt %, respectively, are dry
mixed and are then sintered at 500.degree. C. for 2 hours. There is
thus obtained calcined powder (chemical formula:
Pb.sub.0.835W.sub.0.165O.sub.1.33) in which lead oxide and a part
of tungsten oxide react with each other. This calcined powder is
finely granulated and dried in a medium-stirring mill to improve
reactivity and to obtain assistant oxide powder.
[0069] The dielectric layers 31 and 32 in the piezoelectric device
1 of this example is PZT which has a ternary solid solution of
Pb(Y.sub.0.5Nb.sub.0.5)O.sub.3--PbTiO.sub.3--PbZrO.sub.3 system as
its basic composition and in which Sr replaces a part of Pb. The
composition of the starting materials is selected so that the final
composition achieves the compound described above. The starting
materials are dry mixed and are sintered at 850.degree. C. for 7
hours. In this way is obtained dielectric calcined powder.
[0070] Next, 2.5 L of water and a dispersant (2.5 wt % on the basis
of the dielectric calcined powder) are mixed in advance, and 4.7 kg
of the dielectric calcined powder is gradually mixed to obtain
slurry of the dielectric calcined powder. This dielectric calcined
powder slurry is stirred in a medium-stirring mill, and the
particle diameter of the dielectric calcined powder in the slurry
is controlled to 0.2 .mu.m or below by use of a pearl mill.
[0071] To the dielectric calcined powder slurry are added 4 wt % of
a binder on the basis of the weight of the dielectric calcined
powder in the slurry, 1.9 wt % of a mold release agent on the basis
of the weight of the dielectric calcined powder in the slurry and
13.5 g of the assistant oxide powder described above on the basis
of 1,600 g of the dielectric calcined powder in the slurry (0.5
atom % in the chemical formula Pb.sub.0.835W.sub.0.165O.sub.1.33 of
the assistant oxide powder). The mixture is stirred for 3 hours and
is dried by use of a spray dryer to give a granulated powder.
[0072] The granulated powder is further granulated finely for a
night and a day and is mixed with water. A sheet is then shaped
with a blade interval of 125 .mu.m by a doctor blade method. After
being dried at 80.degree. C., the sheet is cut into a size of 100
mm by 150 mm by using a sheet cutter. There are thus obtained the
green sheets 11 and 12.
[0073] Next, the print portion 13 is formed on each green sheet 11
and 12. To 1,800 g of CuO paste (CuO content: 50 wt %, CuO specific
surface area: 10 m.sup.2/g with the balance of binder) are added
1.11 g of 1050YPCu powder (mixed powder of yttria, phosphorus and
copper), a product of Mitsui Metal Co. and 0.09 g of duplicate
powder (powder having the same components as the calcined powder
for the dielectric layer or containing a part of the components).
These materials are then mixed inside a centrifugal de-foaming
apparatus to give an electrode paste. The electrode paste is
printed to a thickness of 5 to 8 .mu.m on the green sheets by use
of a screen-printing apparatus and is dried at 130.degree. C. for 1
hour.
[0074] The green sheets 11 and 12 having the print portion 13 is
obtained in this way as shown in FIG. 1(a).
[0075] The non-formation portions 130 of the print portions 13 are
disposed in such a manner as to appear on the opposing side
surfaces between the adjacent dielectric layers 31 and 32 at the
time of stacking. Therefore, as shown in FIG. 1(a), two kinds of
green sheets having the non-formation portion 130 positioned in a
different direction are prepared. Since the periphery of each green
sheet 11 and 12 is finally cut, the print portion 13 is disposed in
consideration of the cut margin.
[0076] Next, twenty green sheets 11 and 12 are stacked as shown in
FIG. 1(b). The stacked body is fixed to a press jig and is
thermally press-bonded at 120.degree. C. and 80 kg/m.sup.2 for 10
minutes to give a mother block. This mother block is cut to a side
of 9 mm.times.9 mm by use of a sheet cutter. Each fragment cut from
the mother block is put to a laminator and is again thermally
press-bonded at 120.degree. C. and 160 kg/m.sup.2 for 10 minutes.
Consequently, there is obtained a unit device 145 shown in FIG.
1(c).
[0077] Twenty unit devices 145 are stacked and put to the laminator
and are further thermally press-bonded at 80.degree. C. and 500
kg/m.sup.2 for 10 minutes to give an unsintered stacked body 15 as
shown in FIG. 1(d). Each green sheet 11, 12 has a square shape
having a side of 9 mm and a thickness of 0.1 mm inside the
unsintered stacked body 15. Therefore, the volume of the unsintered
stacked body 15 is 3,240 mm.sup.3 (the thickness of the print
portion 13 can be neglected because it is extremely small).
[0078] Magnesium oxide sheets having a porosity of 20% (15.times.15
mm.times.1 mm) are placed on and below the unsintered stacked body
15. Degreasing is carried out inside a gas circulation type
degreasing furnace in an atmosphere in accordance with a
temperature profile (a diagram showing the relation between the
time from the start of the degreasing treatment and the
temperature) shown in FIG. 2.
[0079] Degreasing is carried out while a ventilation plate 211, the
unsintered stacked body 15 and an upper side ventilation plate 212
are placed on a bottom surface 219 of a saggar 21 (the same as the
one used for sintering; see FIG. 6) as shown in FIG. 3(a). The
ventilation plates 211 and 212 may be formed of ceramic. In this
case, a ceramic plate having a porosity of at least 10% is
preferably used to secure ventilation though the ceramic material
is not specifically limited. The upper and lower ventilation plate
may be formed of the same material. The size of the upper and lower
ventilation plates 211 and 212 may be different so long as the
ventilation property remains substantially equal.
[0080] A spacer 213 may be disposed between the bottom surface 219
of the saggar 21 and the ventilation plate 211 to secure
ventilation at the lower part as shown in FIG. 3(b). This example
uses a cordierite honeycomb for the spacer 213. Further, a metallic
mesh plate can be used for the ventilation plates 211 and 212 as
shown in FIG. 3(c).
[0081] Incidentally, the ventilation plate may have a honeycomb
shape, a porous shape, a mesh shape, etc, so long as it has high
ventilation and can withstand degreasing (particularly, heat). A
suitable metal plate such as alumina or titania can be used as the
material. The maximum degreasing temperature is 500.degree. C. in
this example, but the temperature is not limited if it is within
the range of 400 to 650.degree. C. Though degreasing is carried out
in the atmosphere in this example, it may be conducted in a pure
oxygen atmosphere.
[0082] After degreasing is finished, the unsintered stacked body is
subjected to reducing treatment in a hydrogen atmosphere and is
then sintered. This reducing treatment is conducted at
1.times.10.sup.-23.5 atm on the basis of the temperature profile (a
diagram showing the relation between the lapse of time from the
start of sintering and the temperature) in an atmosphere containing
5,000 mL of Ar--H.sub.2 (1%) and 6.5 to 6 mL of O.sub.2 (pure)
while the oxygen partial pressure during reduction is managed by an
outer-furnace oxygen partial pressure.
[0083] Though this example uses Ar--H.sub.2 (1%) and O.sub.2, the
gas concentration and the processing amount are not particularly
limited so long as the environment of 1.times.10.sup.-16 to
1.times.10.sup.-24 atm can be achieved by the outer-furnace oxygen
partial pressure. (At this time, the substantial ratio of H.sub.2
and O.sub.2 charged into the furnace is H.sub.2:O.sub.2=50:50 to
5.5). Though the temperature may be within the range of 250 to
600.degree. C., it is preferably from 300 to 400.degree. C.
[0084] In this example, a stainless metal having higher reactivity
with oxygen than the constituent material of the electrode layer 33
is used for the furnace wall material of the furnace chamber. In a
certain oxygen partial pressure atmosphere, the furnace wall reacts
with a trace amount of oxygen and forms an oxide film layer that is
capable of reversible reaction. This film emits oxygen when the
oxygen partial pressure shifts towards the reduction side, and
builds up oxygen when the oxygen partial pressure shifts towards
the oxidation side, thereby keeping the change of the oxygen
partial pressure within a predetermined range. In an atmosphere
where the constituent materials of the electrode layer 33 are
slightly oxidized, the furnace wall material is oxidized more
quickly than the constituent materials of the electrode layer 33,
and thus protects the electrode layer 33 (particularly,
copper).
[0085] When sintering is carried out at an outer-furnace oxygen
partial pressure that is outside the range described above, lead
oxide of the dielectric layers 31 and 32 is reduced and metallic
lead is isolated. This metallic lead reacts with copper of the
electrode layer 33 and undesirably forms the liquid phase at a
temperature of 327.degree. C. or above. The same phenomenon occurs
when the sintering time gets elongated. Therefore, the sintering
time is preferably from 0.25 to 16 hours.
[0086] A concrete sintering method will be explained. As shown in
FIGS. 5(a), 5(b) and 6, the unsintered stacked body 15 is put on
the bottom surface 219 of the saggar 21 formed of magnesium oxide
together with the cordierite honeycomb plates 221 and 224,
magnesium oxide plates 222 and 223 (15.times.15 mm.times.1 mm) and
a magnesium oxide weight 225 (1 to 10 g). Reference numeral 210 in
FIG. 6 denotes a cover of the saggar. To prevent lead oxide from
evaporating away from the unsintered stacked body 15 during
sintering, a suitable amount of masses 226 of PbZrO.sub.3 are
placed at corners of the saggar 21 as shown in FIG. 6. The saggar
21 having the unsintered stacked body 15 arranged therein is
subjected to reduction sintering inside the sintering furnace 3
capable of sintering in the reducing atmosphere by use of CO.sub.2
(pure), Ar--CO (10%) and O.sub.2 (pure) in accordance with the
temperature/atmosphere pattern shown in FIG. 8.
[0087] FIG. 7 shows the sintering furnace 3 used for this reduction
sintering. The sintering furnace 3 includes a furnace chamber 30 in
which the saggar is placed and in which sintering is conducted, an
inner-furnace oxygen partial pressure sensor 315 inserted into the
furnace chamber 30 and an outer-furnace oxygen partial pressure
gauge 316 for acquiring a detection value from the sensor 315. The
furnace 3 further includes mass flow controllers 311, 312 and 313
for respectively introducing Ar--CO, CO.sub.2 and O.sub.2 into the
furnace chamber 30 and a flow path 31 equipped with a solenoid
valve 314 for appropriately switching the flow paths from the mass
controllers 311, 312 and 313 to the furnace chamber 30. An
outer-furnace oxygen partial pressure sensor 317 and an
outer-furnace oxygen partial pressure gauge 318 for acquiring an
output value from the sensor 317 are interposed at intermediate
portions of an exhaust system 310 extending from the furnace
chamber 30 to the outside. The outer-furnace oxygen partial
pressure sensor 317 and its pressure gauge 318 and the
inner-furnace oxygen partial pressure sensor 315 and its pressure
gauge 316 control the oxygen partial pressure of the furnace
chamber 30.
[0088] The outer-furnace oxygen partial pressure sensor 317 is a
zirconia O.sub.2 sensor. A built-in heater always heats the sensor
to 600.degree. C. or above so that the oxygen partial pressure in
the gas introduced into the outer-furnace oxygen partial pressure
sensor 317 can be measured throughout the entire temperature range.
On the other hand, the inner-furnace oxygen partial pressure sensor
315 is a zirconia O.sub.2 sensor but does not have a built-in
heater. When the furnace chamber 30 of the sintering furnace 3 is
heated to about 400 to 500.degree. C. or above, the oxygen partial
pressure of the furnace chamber 30 can be measured. In this
sintering furnace, the outer-surface oxygen partial pressure sensor
317 is used when the inner-furnace temperature is outside the
measurement temperature range of the inner-furnace oxygen partial
pressure sensor 315.
[0089] When the temperature is elevated, the outer-furnace oxygen
partial pressure sensor 317 and the pressure gauge 318 control the
oxygen partial pressure from room temperature to 580.degree. C.,
and the inner-furnace oxygen partial pressure sensor 315 and the
pressure gauge 316 control the oxygen partial pressure from
580.degree. C. and above. When the temperature is lowered, the
inner-furnace oxygen partial pressure sensor 315 and the pressure
gauge 316 control the oxygen partial pressure from the maximum
temperature to 600.degree. C., and the outer-furnace oxygen partial
pressure sensor 317 and the pressure gauge 318, that are installed
outside the furnace, control the oxygen partial pressure from
600.degree. C. and below.
[0090] At the temperature of 600.degree. C. or below, the gas flow
rates are again controlled to CO.sub.2 (pure) to 5,000 mL+Ar--CO
(10%) to 150 mL+O.sub.2 (pure) to 2.8 to 5 mL, and the indication
value of the outer-surface oxygen partial pressure sensor 317 is
controlled to 10.sup.20 atm to 10.sup.-10 atm. The control ranges
of the temperature and the oxygen partial pressure under this state
are described in a range f of a black belt in later-appearing FIG.
10. FIG. 8 shows the mode of this control with reference to the
relation among the time, the temperature and the oxygen partial
pressure. The value of the inner-furnace oxygen partial pressure
sensor, the value of the outer-furnace oxygen partial pressure
sensor and the inner temperature of the furnace in the practical
sintering process are shown in the diagram of FIG. 9.
[0091] FIG. 10 shows the control ranges of the oxygen partial
pressure and the temperature from 580.degree. C. or above during
temperature elevation and from 600.degree. C. or below during
temperature lowering. The inner-furnace atmosphere is controlled in
such a manner as to keep the temperature and the oxygen partial
pressure inside the range e represented by a black belt in the
drawing. Incidentally, FIG. 10 represents the Cu+PbO coexisting
range by the range encompassed by solid lines a and b under the
condition of the production method of this example. The production
method of the stacked ceramic body according to this example can be
accomplished when the oxygen partial pressure is controlled to the
range encompassed by solid line a and solid line c during sintering
of the unsintered stacked body 15. Sintering is possible in some
cases at a low oxygen partial pressure encompassed by solid line b
and solid line d (sintering is achieved by oxygen built up inside).
In FIG. 10, the abscissa represents the temperature and the
ordinate does an x value when the oxygen partial pressure is
expressed by 10.sup.x atm. Incidentally, 10.sup.x atm is equal to
1.013.times.10.sup.5.times.10.sup.5.times.10.sup.x Pa.
[0092] The unsintered stacked body 15 is sintered as described
above and the stacked ceramic body of this example shown in FIG.
11(b) is obtained. In the electrode layer 33 of the stacked ceramic
body 1 so obtained, the peripheral portions near the end portions
exposed on the side surface are covered with the oxidation portion
335 formed of copper oxide as shown in FIG. 11(a). The width W of
this oxidation portion 335 is the length from the end portion
opposing the side surface of the stacked ceramic body 1 to the
deepest position at which the oxidation portion 335 is formed. It
is 0.4 mm in this example.
[0093] FIG. 12(b) schematically shows the section of the resulting
stacked ceramic body 1. The electrode layer 33 is formed between
the dielectric layers 31 and 32, and the diffusion portion 330
expands from the surface of the electrode layer 33 towards the
dielectric layers 31 and 32. The stacked ceramic body obtained by
the production method of this example has the electrode layer 33
the thickness of which is substantially uniform. The maximum
thickness of the electrode layer 33 is 8 .mu.m and that of the
diffusion portion is 1 .mu.m in this example.
[0094] A stacked ceramic body as a comparative example is
fabricated by similarly conducting sintering while the inside of
the furnace is kept on the reducing side, that is, on the side at
which the oxygen partial pressure is low, in the diagram of FIG.
10. FIG. 12(a) schematically shows the section of the resulting
stacked ceramic body 1. In this case, an electrode layer 33 that is
interrupted and discontinuous (see reference numeral 390) is
formed, and the diffusion portion 330 exists, too. However, the
electrode layer 33 is much thinner than the electrode layer 33 of
the stacked ceramic body 1 obtained by this example (see FIG.
12(b)). This is because metallic lead isolated from the dielectric
layers 31 and 32 reacts with copper of the electrode layer 33, is
partially fused, and is shaped into islands. When the stacked
ceramic body having such an interrupted electrode layer 33 is used
as the piezoelectric device, conduction defect to the dielectric
layers 31 and 32 occurs. Further, because the voltage application
area to the dielectric layers 31 and 32 becomes small, the
extension/contraction amount is small, and the piezoelectric device
has a poor performance.
[0095] The operation and effect of this example will be explained.
In this example, the unsintered stacked body 15 after degreasing is
subjected to reducing treatment in the hydrogen atmosphere and is
then sintered during its production process. Copper oxide contained
in the print portion 13 changes to metallic copper in this reducing
treatment in the hydrogen atmosphere. At this time, lead oxide and
oxide compounds of lead contained in the green sheets 11 and 12
(dielectric layers 31 and 32) are also reduced to lead as the side
reaction. As the atmosphere is kept at the reducing atmosphere
hardly containing oxygen during sintering from room temperature,
lead oxide and oxide compounds of lead contained in the green
sheets 11 and 12 (dielectric layers 31 and 32) in the unsintered
stacked body 15 are also reduced and metallic lead is isolated.
[0096] When lead exists in the unsintered stacked body 15 and when
sintering is carried out on the reducing side (the range where the
oxygen partial pressure is small) from the Cu+PbO coexisting range
shown in FIG. 10, lead and copper react with each other and change
to the liquid phase in the unsintered stacked body 15, and lead and
copper so liquefied are likely to aggregate in an island form,
giving interrupted electrode layers (see FIG. 12(a)). These lead
and copper in the liquid phase are also likely to be pushed out
from the unsintered stacked body 15. Since the reaction between
lead and copper occurs at a low temperature of about 320.degree.
C., it is difficult to prevent the reaction by merely adjusting the
temperature. To solve this problem, therefore, it is necessary to
suppress the generation of metallic lead and to return metallic
lead that has already been formed to the original lead oxide.
[0097] When the sintering condition is controlled to the oxidizing
side (the range where the oxygen partial pressure is high) from the
Cu+PbO coexisting range as shown in FIG. 10, lead oxide contained
in the green sheets 11 and 12 (dielectric layers 31 and 32) and
copper oxide contained in the print portion 13 react with each
other and are liquefied inside the unsintered stacked body 15.
Consequently, copper oxide diffuses in the unsintered stacked body
15 at a temperature of 680.degree. C. or above.
[0098] To return metallic lead to original lead oxide and oxide
compounds of lead, this example controls the gas flow rates of
CO.sub.2 (pure) 500 mL+Ar--CO (10%) 150 mL+O.sub.2 (pure) 2.8 to 5
mL from the room temperature to 580.degree. C. during temperature
elevation as described above, so that the outer-furnace oxygen
partial pressure sensor 317 has the value of 10.sup.-20 to
10.sup.-10 atm. Since this example conducts such a control, it can
suppress the generation of metallic lead and at the same time, can
oxidize metallic lead that has already been formed, and can return
it to lead oxide and the oxide compounds of lead. Therefore, the
reaction between copper and lead hardly occurs.
[0099] When lead formed during the reduction of the electrode
reacts with copper and forms the liquid phase inside the unsintered
stacked body 15, the liquefied electrode material is pushed
outside. Moreover, this liquefaction is likely to occur from low
temperatures.
[0100] When sintering is conducted while the temperature and the
oxygen partial pressure are controlled as described in this
example, copper exposed on the surface of the electrode layer 33 in
the unsintered stacked body 15 is oxidized to copper oxides
(monovalent and divalent: solid) from the room temperature to
580.degree. C., and the copper oxide operates as a barrier and
forms the oxidation portion 335 shown in FIGS. 1(a) and 1(b) even
when metallic lead and copper react with each other and are
liquefied inside the unsintered stacked body. As a consequence,
internal copper is prevented from being discharged outside the
unsintered stacked body 15, and the formation of the interrupted
electrode layer 33 is prevented, too.
[0101] As described above, this example can provide a stacked
ceramic body which prevents the reaction between the component of
the dielectric layer and the component of the electrode layer in
the unsintered stacked body during sintering, and in which both
components are not easily liquefied, and a production method of
such a stacked ceramic body.
* * * * *