U.S. patent application number 10/247518 was filed with the patent office on 2003-04-03 for conductive paste for terminal electrodes of monolithic ceramic electronic component, method for making monolithic ceramic electronic component, and monolithic ceramic electronic component.
Invention is credited to Hamada, Kunihiko, Miki, Takeshi, Noda, Satoru.
Application Number | 20030064873 10/247518 |
Document ID | / |
Family ID | 26622620 |
Filed Date | 2003-04-03 |
United States Patent
Application |
20030064873 |
Kind Code |
A1 |
Noda, Satoru ; et
al. |
April 3, 2003 |
Conductive paste for terminal electrodes of monolithic ceramic
electronic component, method for making monolithic ceramic
electronic component, and monolithic ceramic electronic
component
Abstract
A conductive paste for forming terminal electrodes of a
monolithic ceramic electronic component having nickel internal
electrodes is provided. The conductive paste includes at least one
of a silver powder and a silver alloy powder, a boron powder, an
inorganic binder and an organic vehicle. The amount of the boron
powder is at least about 0.5 percent by weight and less than about
7.0 percent by weight of the total weight of the conductive
paste.
Inventors: |
Noda, Satoru; (Hikone-shi,
JP) ; Miki, Takeshi; (Omihachiman-shi, JP) ;
Hamada, Kunihiko; (Kyoto-shi, JP) |
Correspondence
Address: |
Edward A. Meilman
DICKSTEIN SHAPIRO MORIN & OSHINSKY LLP
41st Floor
1177 Avenue of the Americas
New York
NY
10036-2714
US
|
Family ID: |
26622620 |
Appl. No.: |
10/247518 |
Filed: |
September 20, 2002 |
Current U.S.
Class: |
501/19 ;
252/500 |
Current CPC
Class: |
H01C 1/1406 20130101;
H05K 1/092 20130101; C03C 8/18 20130101; H01C 1/148 20130101; H01G
4/2325 20130101; C03C 8/14 20130101 |
Class at
Publication: |
501/19 ;
252/500 |
International
Class: |
C03C 008/18; H01B
001/00; H01C 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 20, 2001 |
JP |
2001-287487 |
Jun 13, 2002 |
JP |
2002-172870 |
Claims
What is claimed is:
1. A conductive paste for forming terminal electrodes of a
monolithic ceramic electronic component, comprising: at least one
of a silver powder and a silver alloy powder; a boron powder; an
inorganic binder; and an organic vehicle, wherein the amount of the
boron powder is at least about 0.5 percent by weight and less than
about 7.0 percent by weight of the total weight of the conductive
paste.
2. The conductive paste according to claim 1, wherein the inorganic
binder is at least one selected from the group consisting of
bismuth borate glass, bismuth borosilicate glass and zinc
borosilicate glass.
3. The conductive paste according to claim 2, wherein the inorganic
binder is lead free.
4. The conductive paste according to claim 3, wherein the average
particle diameter of the boron powder is about 60 .mu.m or
less.
5. The conductive paste according to claim 4, wherein the amount of
the inorganic binder is about 1 to 20 percent by volume of the
total volume of the inorganic binder, said at least one of the
silver powder and the silver alloy powder, and the boron
powder.
6. The conductive paste according to claim 1, wherein the amount of
the inorganic binder is about 1 to 20 percent by volume of the
total volume of the inorganic binder, said at least one of the
silver powder and the silver alloy powder, and the boron
powder.
7. The conductive paste according to claim 1, wherein the average
particle diameter of the boron powder is about 60 .mu.m or
less.
8. The conductive paste according to claim 1, wherein the paste is
lead free.
9. A method for making a monolithic ceramic electronic component,
comprising: providing a sintered compact having internal conductor
layers comprising nickel; and applying the conductive paste of
claim 1 to the sintered compact and baking the conductive paste to
form terminal electrodes electrically connected to the internal
conductor layers of the sintered compact.
10. A monolithic ceramic electronic component made by the method of
claim 9.
11. The monolithic ceramic electronic component according to claim
10, wherein the ceramic green sheets are dielectric, and the
monolithic ceramic electronic component is a monolithic ceramic
capacitor.
12. The monolithic ceramic electronic component according to claim
10, wherein the ceramic green sheets are semiconductive and the
monolithic ceramic electronic component is a monolithic positive
temperature coefficient thermistor.
13. The method according to claim 9, wherein the conductive paste
is baked at a temperature of about 400 to 900.degree. C.
14. The method of claim 13, further comprising: preparing ceramic
green sheets; forming a precursor of an internal conductor layer of
nickel or a material containing nickel as the primary component on
each of the ceramic green sheets; stacking the ceramic green sheets
to form a composite; and sintering the composite to form the
sintered compact.
15. The method of claim 9, further comprising: preparing ceramic
green sheets; forming a precursor of an internal conductor layer of
nickel or a material containing nickel as the primary component on
each of the ceramic green sheets; stacking the ceramic green sheets
to form a composite; and sintering the composite to form the
sintered compact.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a conductive paste for
forming terminal electrodes of a monolithic ceramic electronic
component such as a monolithic capacitor or a monolithic positive
temperature coefficient (PTC) thermistor. It also relates to a
monolithic ceramic electronic component using the conductive paste
and to a method for making the monolithic ceramic electronic
component.
[0003] 2. Description of the Related Art
[0004] Conventionally, internal electrodes of a monolithic ceramic
capacitor, i.e., an example of a monolithic ceramic electronic
component, have been made of silver, silver-palladium, or the like.
However, since these materials are expensive, nickel, which is a
relatively inexpensive base metal, has replaced these
materials.
[0005] On the other hand, terminal electrodes of a monolithic
ceramic capacitor are composed of silver, which exhibits superior
conductivity and can be baked at low temperatures. The silver
terminal electrodes are coated with a nickel layer and then a tin
layer or a solder layer to improve the solderability and to obtain
a monolithic ceramic capacitor.
[0006] The internal electrodes of a monolithic PTC thermistor,
which is another example of a monolithic ceramic electronic
component, are made of nickel which makes an ohmic contact to a
ceramic, i.e., an n-type impurity semiconductor. A PTC thermistor
having a desired positive resistance temperature characteristic is
made by simultaneously baking stacked ceramic green sheets and
internal electrodes in a reducing atmosphere and subsequently
forming terminal electrodes by baking in air while reoxidizing the
ceramic.
[0007] However, nickel and silver do not form a solid solution.
Thus, a monolithic ceramic capacitor having nickel internal
electrodes and silver terminal electrodes suffers from a problem in
that the bonding of the internal electrodes to the terminal
electrodes is difficult, thereby often failing to obtain a desired
capacitance.
[0008] In view of the above problem, copper which forms a complete
solid solution with nickel has drawn much attention as a material
of the terminal electrodes. However, since copper is easily
oxidized, baking of the copper-containing conductive paste to form
terminal electrodes must be performed in a reducing atmosphere.
Such a process increases the manufacturing cost. Moreover, since
the oxygen concentration in the reducing atmosphere is low, the
decomposition rate of the vehicle contained in the conductive paste
is also low. As a result, the characteristics of the product may be
affected by the residual carbon.
[0009] As with the monolithic ceramic capacitor, a monolithic PTC
thermistor having terminal electrodes composed of silver, which can
be baked in air, suffers from a problem in that silver does not
form a solid solution with nickel in the internal electrodes,
thereby inhibiting bonding of the internal electrode to the
terminal electrode. Other examples of the terminal electrode paste
that can be baked in air include an aluminum paste and a zinc
paste. However, electrodes made of aluminum or zinc have poor
solderability and suffer from the problem that aluminum or zinc in
the electrode elutes into a plating solution when electrolytic
plating is performed onto the electrodes.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to provide a silver
conductive paste for forming terminal electrodes of a monolithic
ceramic electronic component, such as a monolithic ceramic
capacitor and a monolithic PTC thermistor, that has nickel internal
conductors. With the silver conductive paste of the present
invention, the oxidation of the surface of the nickel internal
conductors can be prevented and a satisfactory connection between
the terminal electrodes and the internal conductors can be achieved
even when the terminal electrode are baked in air. Another object
of the present invention is to provide a method for making a
monolithic ceramic electronic component having terminal electrodes
made from this conductive paste, and to provide a monolithic
ceramic electronic component.
[0011] To attain these objects, a first aspect of the present
invention provides a conductive paste for forming terminal
electrodes of a monolithic ceramic electronic component, including:
at least one of a silver powder and a silver alloy powder; a boron
powder; an inorganic binder; and an organic vehicle. The amount of
the boron powder is at least about 0.5 percent by weight and less
than about 7.0 percent by weight of the total weight of the
conductive paste.
[0012] A second aspect of the present invention provides a method
for making a monolithic ceramic electronic component, the method
including the steps of: preparing ceramic green sheets; forming a
precursor of an internal conductor layer of nickel or a material
containing nickel as the primary component on each of the ceramic
green sheets; stacking the ceramic green sheets to form a
composite; sintering the composite to obtain a sintered compact
having the internal conductor layers; and applying the conductive
paste of the invention to the sintered compact and baking the
conductive paste to form terminal electrodes electrically connected
to the internal conductor layers of the sintered compact.
[0013] A third aspect of the present invention provides a
monolithic ceramic electronic component made by the method
described above.
[0014] In one preferred embodiment, the ceramic green sheets are
dielectric, and the monolithic ceramic electronic component is a
monolithic ceramic capacitor.
[0015] In another preferred embodiment, the ceramic green sheets
are semiconductive and the monolithic ceramic electronic component
is a monolithic positive temperature coefficient thermistor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a cross-sectional view of a monolithic ceramic
capacitor which is an example of a monolithic ceramic electronic
component of the present invention; and
[0017] FIG. 2 is a cross-sectional view of a monolithic positive
temperature coefficient thermistor which is another example of the
monolithic ceramic electronic component of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] A conductive paste for forming terminal electrodes of a
monolithic ceramic electronic component according to the present
invention contains: at least one of a silver powder and a silver
alloy powder; a boron powder; an inorganic binder; and an organic
vehicle. The amount of the boron powder in the total conductive
paste is at least about 0.5 percent by weight but less than about
7.0 percent by weight.
[0019] The conductive paste for terminal electrodes of the present
invention contains at least one of a silver powder and a silver
alloy powder as the conductive powder, and can be baked in air.
When the conductive paste is applied on the sintered monolithic
ceramic compact having nickel internal electrodes so that a
connection to the nickel internal electrodes are formed, the boron
powder contained in the conductive paste prevents the oxidation of
nickel in the internal electrodes. Moreover, although nickel in the
internal electrodes and silver in the terminal electrodes do not
form a solid solution by themselves, the presence of boron enables
nickel in the internal electrode and silver in the terminal
electrode to form a solid solution.
[0020] When boron oxide (B.sub.20.sub.3) or boric acid
(H.sub.3BO.sub.3) is used instead of semimetal boron (B), oxidation
of nickel in the internal electrode can still be prevented if they
are added in the same amount, on a boron basis, as that of boron.
However, it has been found that neither boron oxide nor boric acid
enables formation of a nickel-silver solid solution. Thus, a
semimetal boron powder must be used.
[0021] The amount of the boron powder is preferably at least about
0.5 percent by weight. At an amount of less than about 0.5 percent
by weight, an oxide film is formed on the nickel internal
electrode, thereby generating an electrical contact resistance. At
an amount of about 7.0 percent by weight or more, the conductivity
of the baked layer decreases.
[0022] Preferably, the average particle diameter of the boron
powder is about 60 .mu.m or less. At a diameter exceeding about 60
.mu.m, boron is likely to remain in the baked electrode film in an
uneven manner, thereby decreasing the conductivity.
[0023] The organic binder is preferably bismuth borate glass,
bismuth borosilicate glass, or zinc borosilicate glass in view of
environmental concerns since these glasses are lead free.
[0024] As for the viscosity of the glass at high temperatures, the
working point (log .eta.(Pa.multidot.s)=4) is preferably about
600.degree. C. or less, and more preferably, the baking temperature
is about 900.degree. C. or less. This is because the conductive
paste of the present invention is baked in air at a temperature of
about 400 to 900.degree. C. If the baking temperature is less than
about 400.degree. C., silver and nickel do not form a solid
solution, and thus the resulting terminal electrodes cannot be
satisfactorily bonded to the nickel internal electrodes. In
contrast, at a baking temperature exceeding about 900.degree. C.,
the sintering reaction of the silver powder or the silver alloy
powder becomes excessive, thereby failing to obtain uniform
electrode layers.
[0025] The amount of the inorganic binder is preferably about 1 to
20 percent by volume of the total volume (solid component volume)
of the silver powder and/or the silver alloy powder, the inorganic
binder, and the boron powder. At an amount of less than about 1
percent by volume, the adhesiveness of the baked electrode is
decreased. At an amount exceeding about 20 percent by volume, the
baked electrode does not exhibit conductivity.
[0026] A method for making a monolithic ceramic electronic
component of the present invention includes: preparing a ceramic
stack having internal conductors made of nickel or mainly made of
nickel; and applying the above-described conductive paste for
forming terminal electrodes on the ceramic stack and baking the
conductive paste to form terminal electrodes.
[0027] A method of the present invention will now be described with
reference to FIG. 1 using a monolithic ceramic capacitor 1 as an
example of the monolithic ceramic electronic component.
[0028] First, a dielectric ceramic powder, such as a BaTiO.sub.3
system powder, is prepared. After the dielectric ceramic powder is
made into a slurry, the slurry is formed into ceramic green sheets
which are precursors of dielectric ceramic layers 3.
[0029] Next, a precursor of an internal conductor layer for making
an internal electrode 4 is formed on each of the ceramic green
sheets by screen-printing or the like. The internal electrode 4 is
composed of nickel or a material containing nickel as the primary
component. A required number of ceramic green sheets with the
precursors of the internal conductor layers thereon are stacked,
and a ceramic green sheet not provided with the precursor of the
internal conductor layer is placed on the top of the stack. The
stacked ceramic green sheets are then press-bonded to prepare a
green composite.
[0030] The green composite is baked at a predetermined temperature
in a predetermined nonoxidative atmosphere to prepare a sintered
compact 2.
[0031] Terminal electrodes 5 are formed on the two side faces of
the sintered compact 2 to obtain a monolithic ceramic capacitor.
Each of the terminal electrodes 5 is electrically connected to
corresponding internal electrodes 4. The terminal electrodes 5 are
formed by applying the above-described conductive paste of the
present invention on the two side faces of the compact and baking
the applied conductive paste.
[0032] Subsequently, a plating layer 6 composed of nickel, copper
or the like, is formed on each of the terminal electrodes 5, and a
plating layer 7 composed of solder, tin or the like, is formed on
the plating layer 6, if necessary.
[0033] FIG. 2 shows a PTC thermistor, which is another example of
the monolithic ceramic electronic component of the present
invention.
[0034] A PTC thermistor 11 includes a sintered compact 12 including
stacked semiconductor ceramic layers 13 and nickel internal
electrodes 14 formed at corresponding interfaces between the
semiconductor ceramic layers 13. Terminal electrodes 17 are formed
at the side faces of the sintered compact 12 so that each of the
terminal electrodes 17 is connected to corresponding internal
electrodes 13.
[0035] Unlike the monolithic ceramic capacitor 1 described above,
the PTC thermistor 11 has surfaces covered with a glass layer 15.
Moreover, side electrodes 16 composed of nickel, which is the same
material as the internal electrodes, are provided so that the
internal electrodes 14 are securely connected to the side
electrodes 16. The conductive paste of the present invention is
applied on the side electrodes 16 and is baked to form the terminal
electrodes 17.
[0036] A plating layer 18 composed of nickel is formed on each of
the terminal electrodes 17, and a plating layer 19 composed of
solder, tin or the like, is formed on each of the terminal
electrodes 17, if necessary.
EXAMPLES
Example 1
[0037] Example 1 is explained below using a monolithic ceramic
capacitor as an example of the monolithic ceramic electronic
component.
[0038] Predetermined amounts of starting materials, i.e.,
TiCl.sub.4 and Ba(NO.sub.3).sub.2 were allowed to react with oxalic
acid to obtain a barium titanyl oxalate
(BaTiO(C.sub.2O.sub.4).4H.sub.2O) residue. The residue was
pyrolyzed at a temperature of 1,000.degree. C. or more so as to
synthesize the main component, i.e., BaTiO.sub.3.
[0039] Suitable oxides, carbonates or hydroxides of the components
described below were mixed and milled so as to obtain a powder
having a composition of 0.25 Li.sub.2O-0.65(0.30 TiO.sub.2.0.70
SiO.sub.2)-0.10 Al.sub.2O.sub.3 on a molar ratio basis. This powder
mixture was heated to 1,500.degree. C. in a platinum crucible, was
quenched, and was milled to obtain an oxide powder having an
average particle diameter of 1 .mu.m or less. This oxide powder was
a first auxiliary component.
[0040] Suitable oxides, carbonates or hydroxides of the components
described below were mixed and milled so as to obtain a powder
having a composition of 0.66 SiO.sub.2-0.17 TiO.sub.2-0.15 BaO-0.02
MnO on a molar ratio basis. This powder mixture was heated to
1,500.degree. C. in a platinum crucible, was quenched, and was
milled to obtain an oxide powder having an average particle
diameter of 1 .mu.m or less. This oxide powder was a second
auxiliary component.
[0041] Next, the main component, the first auxiliary component, and
the second auxiliary component were mixed at a weight ratio of main
component: first auxiliary component: second auxiliary
component=99:0.5:0.5. To the mixture, polyvinyl butyral, i.e., a
binder, and ethanol, i.e., a solvent, were added, and the resulting
mixture was mixed with a ball mill to obtain a ceramic slurry.
Subsequently, the ceramic slurry was formed into sheets by the
doctor blade process to obtain rectangular ceramic green sheets
having a thickness of 35 .mu.m.
[0042] A conductive paste containing nickel as the primary
component was applied by printing on each of the ceramic green
sheets to form a conductive paste layer, i.e., a precursor of an
internal conductor layer.
[0043] The ceramic green sheets provided with the conductive paste
layers were then alternately stacked in such a manner that the side
faces of the ceramic green sheets at which the conductive paste
layers were exposed alternately appeared in the side faces of the
stack. A ceramic green sheet not provided with a conductive paste
layer was placed on the top of the stack, and the resulting stack
was press-bonded to form a green composite.
[0044] The green composite was heated at a temperature of
350.degree. C. in an N.sub.2 atmosphere to decompose the binder.
The resulting composite was then baked at 1,300.degree. C. for 2
hours in a reducing atmosphere, i.e., an H.sub.2--N.sub.2--H.sub.2O
gas at an oxygen partial pressure of 10.sup.-9 to 10.sup.-12
MPa.
[0045] Conductive pastes of Samples 1 to 6 for terminal electrodes
were prepared as follows. A silver powder, a boron powder (the
average particle size D50 =16 .mu.m), and a zinc borosilicate glass
powder, i.e., an inorganic binder, were dispersed into an organic
vehicle containing ethyl cellulose as the resin component with a
kneader such as a triple roller to prepare the conductive pastes
for terminal electrodes. The paste samples contained 64.8 percent
by weight of the silver powder and 1.4 percent by weight of the
zinc borosilicate glass powder. The boron powder contents of the
paste samples are shown in Table 1.
[0046] Comparative Example 1 is a conductive paste for terminal
electrodes was prepared as in Samples 1 to 6 but using a boron
oxide (B.sub.2O.sub.3) powder (the average particle size D50=15
.mu.m) instead of the boron powder.
[0047] Next, each of the conductive pastes for terminal electrodes
was applied on the side faces of a sintered compact at which
internal electrodes are exposed. The applied pastes were baked in
air at 800.degree. C. for 1 hour to form terminal electrodes. Each
of the terminal electrodes was plated with nickel and then with tin
so as to obtain monolithic ceramic capacitors.
[0048] The capacitance and the dielectric loss (tan.delta.) of each
of the monolithic ceramic capacitors were measured at a temperature
of 20.degree. C., 1 kHz and 1 Vrms. The results are shown in Table
1. In Table 1, the samples marked with an asterisk and the
comparative example are outside the scope of the present invention.
The rest of the samples are within the scope of the present
invention.
1TABLE 1 Boron powder Capacitance Dielectric loss Sample No.
content (wt %) (nF) (%) *1 0 10.2 4.21 2 0.5 20.4 2.64 3 1.0 21.8
2.78 4 5.0 21.4 2.66 *5 7.0 16.8 3.67 *6 10.0 15.4 3.32 Comparative
5.0 (as B.sub.2O.sub.3) 10.6 3.86 Example 1
[0049] As is apparent from Table 1, the ceramic capacitors of
Samples 2 to 4 prepared by using the conductor paste for terminal
electrodes of the present invention exhibited superior
characteristics, i.e., a capacitance of 20 nF or more and a
dielectric loss of 3.0% or less, when compared with Samples 1, 5
and 6 and Comparative Example 1. This is because the boron powder
contained in the conductive pastes for terminal electrodes
prevented the oxidation of the surface of the nickel internal
electrodes. Thus, nickel and silver formed a solid solution, and
the nickel internal electrodes were satisfactorily connected to the
silver terminal electrodes.
Example 2
[0050] Example 2 is explained below using a PTC thermistor as an
example of the monolithic ceramic electronic component.
[0051] Suitable amounts of starting materials, i.e., BaCO.sub.3,
TiO.sub.2, and Sm.sub.2O.sub.3, were prepared so that
(Ba.sub.0.9998Sm.sub.0.0002)TiO.sub.3 can be formed. Deionized
water was added to the prepared powder mixture of the starting
material and was mixed and milled with a ball mill using zirconia
balls for 16 hours. The resulting mixture was then dried and
calcined at 1,200.degree. C. for 2 hours to obtain a calcined
powder.
[0052] Next, a polyvinyl butyral, i.e., a binder, and ethanol,
i.e., a solvent, were added to the calcined powder. The resulting
mixture was mixed with a ball mill to obtain a ceramic slurry. The
ceramic slurry was formed into sheets by the doctor blade method so
as to obtain rectangular ceramic green sheets having a thickness of
35 .mu.m.
[0053] On each of the ceramic green sheets, a conductive paste
mainly composed of nickel was applied by printing so as to form a
conductive paste layer for an internal conductor.
[0054] The ceramic green sheets provided with the conductive paste
layers were then alternately stacked in such a manner that the side
faces of the ceramic green sheets at which the conductive paste
layers were exposed alternately appeared in the side faces of the
stack. A ceramic green sheet not provided with a conductive paste
layer was placed on the top of the stack, and the resulting stack
was press-bonded to form a green composite. A nickel paste, which
has been prepared in advance, was applied on the side faces of the
green composite and was dried. Subsequently, the green compacts and
the nickel paste were baked at 1,200 .degree. C. in a reducing
atmosphere having a volume ratio H.sub.2/N.sub.2=0.03 so as to
obtain a sintered compact having nickel side electrodes.
[0055] Next, the sintered compact was dipped into an aqueous
solution containing glass having a softening point of 500 to
800.degree. C. lower than the baking temperature of the conductive
paste described below and a working temperature of 800 to
1,150.degree. C. higher than the baking temperature of the
conductive paste described below. The dipped sintered compact was
dried and heated at 500 to 600.degree. C. so as to form a glass
layer of approximately 0.5 to 5 .mu.m in thickness.
[0056] Meanwhile, conductive pastes for terminal electrodes were
prepared. A silver powder, a boron powder (the average particle
size D50=16 .mu.m) and a zinc borosilicate glass powder, i.e., an
inorganic binder, were dispersed into an organic vehicle containing
ethyl cellulose as the resin component with a kneader such as a
triple roller to prepare Samples 7 to 12 of the conductive pastes
for terminal electrodes. The paste samples contained 64.8 percent
by weight of the silver powder and 1.4 percent by weight of the
zinc borosilicate glass powder. The boron powder contents of the
paste samples are shown in Table 2.
[0057] Comparative Example 2 is a conductive paste for terminal
electrodes was prepared as with Samples 7 to 12 but using a boron
oxide (B.sub.2O.sub.3) powder (the average particle size D50=15
.mu.m) instead of the boron powder.
[0058] Each of the conductive pastes for the terminal electrodes
was applied on the nickel side terminals of the sintered compact
having glass layers on the surfaces. The resulting sintered
compacts were baked in air at 700.degree. C. for 1 hour so as to
form terminal electrodes. The terminal electrodes were plated with
nickel and then with tin so as to obtain PTC thermistors.
[0059] The initial resistance at 25.degree. C. and change in
resistance in the temperature range of 130.degree. C. to
150.degree. C., i.e., .alpha.=(resistance at 150.degree.
C.)/(resistance at 130.degree. C.) of the resulting PTC thermistors
were examined. The results are shown in Table 2. Note that in Table
2, the samples marked with an asterisk and the comparative example
are outside the scope of the present invention. The rest of the
samples are within the scope of the present invention.
2TABLE 2 Boron powder Initial resistance Change in Sample No.
content (wt %) (.OMEGA.) resistance (.alpha.) *7 0 2.80 9.5 8 0.5
0.12 10.2 9 1.0 0.10 10.4 10 5.0 0.08 10.2 *11 7.0 0.07 7.4 *12
10.0 0.05 6.2 Comparative 5.0 (as B.sub.2O.sub.3) 2.60 6.8 Example
2
[0060] As is apparent from Table 2, the PTC thermistors of Samples
8 to 10, which were prepared with the conductive pastes for
terminal electrodes of the present invention, exhibited an initial
resistance at 25.degree. C. of 0.1.OMEGA..+-.20%, and the
resistance did not increase due to the formation of the terminal
electrodes. Moreover, the change in resistance (.alpha.) in the
temperature range of 130 to 150.degree. C. was large, i.e., 10 or
more. The PTC thermistors of Samples 8 to 10 exhibited
characteristics superior to those of Samples 7, 11 and 12, and
Comparative Example 2. This is because the boron powder contained
in the conductive pastes prevented the oxidation of the surfaces of
the nickel internal electrodes and the nickel side electrodes. As a
result, nickel and silver formed a solid solution, and the nickel
internal electrodes were satisfactorily connected to the silver
terminal electrodes.
[0061] Note that Examples 1 and 2 above used a silver powder as a
conductive powder in the conductive paste for terminal electrodes.
The same effects can be obtained even with a silver alloy powder,
i.e., an alloy powder containing silver as the primary
component.
* * * * *