U.S. patent application number 09/360313 was filed with the patent office on 2001-07-19 for composite material for positive temperature coefficient thermistor, ceramic for positive temperature coefficient thermistor and method for manufacturing ceramics for positive temperature coefficient thermistor.
Invention is credited to HIROTA, TOSHIHARU, NABIKA, YASUHIRO, OKAMOTO, TETSUKAZU, YAMAMOTO, NORIYUKI.
Application Number | 20010008867 09/360313 |
Document ID | / |
Family ID | 26494598 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008867 |
Kind Code |
A1 |
NABIKA, YASUHIRO ; et
al. |
July 19, 2001 |
COMPOSITE MATERIAL FOR POSITIVE TEMPERATURE COEFFICIENT THERMISTOR,
CERAMIC FOR POSITIVE TEMPERATURE COEFFICIENT THERMISTOR AND METHOD
FOR MANUFACTURING CERAMICS FOR POSITIVE TEMPERATURE COEFFICIENT
THERMISTOR
Abstract
A ceramic for the PTC thermistor having a resistivity at room
temperature of 5 .OMEGA..multidot.cm or less, static withstanding
voltage of 60 V/mm or more and temperature resistance coefficient
of 9.0%/.degree. C., having small dispersion of the resistance, is
composed of principal components of about 30 to 97 mol % of
BaTiO.sub.3, about 1 to 50 mol % of PbTiO.sub.3, about 1 to 30 mol
% of SrTiO.sub.3 and about 1 to 25 mol % of CaTiO.sub.3 (the total
content of them being 100 mol %), as well as about 0.1 to 0.3 mole
of Sm, about 0.01 to 0.03 mole of Mn and 0 to about 2.0 mole of Si
relative to 100 moles of the principal components, the composite
material being preferably heat-treated in an oxidative atmosphere
after being fired in a reducing or neutral atmosphere for obtaining
the ceramic.
Inventors: |
NABIKA, YASUHIRO;
(SHIGA-KEN, JP) ; OKAMOTO, TETSUKAZU; (SHIGA-KEN,
JP) ; HIROTA, TOSHIHARU; (HIKONE-SHI, JP) ;
YAMAMOTO, NORIYUKI; (SHIGA-KEN, JP) |
Correspondence
Address: |
OSTROLENK FABER GERB & SOFFEN
1180 AVENUE OF THE AMERICAS
NEW YORK
NY
100368403
|
Family ID: |
26494598 |
Appl. No.: |
09/360313 |
Filed: |
July 23, 1999 |
Current U.S.
Class: |
501/137 ;
252/520.21; 252/520.22; 501/138; 501/139 |
Current CPC
Class: |
H01C 7/025 20130101;
H01C 17/30 20130101 |
Class at
Publication: |
501/137 ;
501/138; 501/139; 252/520.21; 252/520.22 |
International
Class: |
C04B 035/46; C04B
035/49; H01B 001/02; C04B 035/48 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 24, 1998 |
JP |
10-209026 |
Jun 18, 1999 |
JP |
11-172136 |
Claims
What is claimed is:
1. A composite material for a PTC thermistor comprising a principal
component containing about 30 to 97 mol % of BaTiO.sub.3; about 1
to 50 mol % of PbTiO.sub.3; about 1 to 30 mol % of SrTiO.sub.3; and
about 1 to 25 mol % of CaTiO.sub.3, wherein the total content of
BaTiO.sub.3 PbTiO.sub.3 SrTiO.sub.3 and CaTiO.sub.3 is 100 mol %,
and relative to 100 moles of the principal component, as additives:
about 0.1 to 0.3 moles of Sm as Sm in a compound containing Sm;
about 0.01 to 0.03 moles of Mn as Mn in a compound containing Mn;
and 0 to about 2.0 moles of Si as Si in a compound containing
Si.
2. The composite material for a PTC thermistor according to claim 1
containing about 45 to 97 mol % of BaTiO.sub.3; about 1 to 30 mol %
of PbTiO.sub.3; about 1 to 15 mol % of SrTiO.sub.3; about 1 to 15
mol % of CaTiO.sub.3, about 0.2 to 0.3 moles of Sm and about 0.02
to 0.03 moles of Mn.
3. The composite material for a PTC thermistor according to claim 2
containing about 1.0 to 2.0 moles of Si.
4. The composite material for a PTC thermistor according to claim 2
containing about 0 moles of Si.
5. A ceramic for use in a PTC thermistor comprising a fired and
oxidative atmosphere heat treated composite material of claim
1.
6. A ceramic for use in a PTC thermistor comprising neutral
atmosphere fired and oxidative atmosphere heat treated composite
material of claim 1.
7. A ceramic for use in a PTC thermistor comprising reducing
atmosphere fired and oxidative atmosphere heat treated composite
material of claim 1.
8. A ceramic for use in a PTC thermistor comprising a fired and
oxidative atmosphere heat treated composite material of claim
2.
9. A ceramic for use in a PTC thermistor comprising neutral
atmosphere fired and oxidative atmosphere heat treated composite
material of claim 2.
10. A ceramic for use in a PTC thermistor comprising reducing
atmosphere fired and oxidative atmosphere heat treated composite
material of claim 2.
11. A method for manufacturing the ceramic for use in a PTC
thermistor comprising providing a BaTiO.sub.3 containing composite
material containing a semiconducting additive, firing the composite
material in a neutral or reducing atmosphere and then heat-treating
the fired material in an oxidative atmosphere.
12. The method for manufacturing the ceramic for use in the PTC
thermistor according to claim 11, wherein the semiconducting
additive comprises Sm.
13. The method for manufacturing the ceramic for use in the PTC
thermistor according to claim 12, wherein the firing is in a
neutral atmosphere.
14. The method for manufacturing the ceramic for use in the PTC
thermistor according to claim 13, wherein the composite material
comprises a principal component containing about 30 to 97 mol % of
BaTiO.sub.3; about 1 to 50 mol % of PbTiO.sub.3; about 1 to 30 mol
% of SrTiO.sub.3; and about 1 to 25 mol % of CaTiO.sub.3, wherein
the total content of BaTiO.sub.3 PbTiO.sub.3 SrTiO.sub.3 and
CaTiO.sub.3 is 100 mol %; and relative to 100 moles of the
principal component, as additives, about 0.1 to 0.3 moles of Sm as
Sm in a compound containing Sm; about 0.01 to 0.03 moles of Mn as
Mn in a compound containing Mn; and 0 to about 2.0 moles of Si as
Si in a compound containing Si.
15. The method for manufacturing the ceramic for use in the PTC
thermistor according to claim 12, wherein the firing is in a
reducing atmosphere.
16. The method for manufacturing the ceramic for use in the PTC
thermistor according to claim 15, wherein the composite material
comprises a principal component containing about 30 to 97 mol % of
BaTiO.sub.3; about 1 to 50 mol % of PbTiO.sub.3; about 1 to 30 mol
% of SrTiO.sub.3; and about 1 to 25 mol % of CaTiO.sub.3, wherein
the total content of BaTiO.sub.3 PbTiO.sub.3 SrTiO.sub.3 and
CaTiO.sub.3 is 100 mol %; and relative to 100 moles of the
principal component, as additives, about 0.1 to 0.3 moles of Sm as
Sm in a compound containing Sm; about 0.01 to 0.03 moles of Mn as
Mn in a compound containing Mn; and 0 to about 2.0 moles of Si as
Si in a compound containing Si.
17. The method for manufacturing the ceramic for use in the PTC
thermistor according to claim 11 wherein the firing is in a neutral
atmosphere.
18. The method for manufacturing the ceramic for use in the PTC
thermistor according to claim 11, wherein the firing is in a
reducing atmosphere.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a composite material for a
positive temperature coefficient thermistor (referred to PTC
thermistor hereinafter), ceramics for use in the PTC thermistor and
a method for manufacturing the PTC thermistor.
[0003] 2. Description of the Related Art
[0004] Related art concerning the present invention are disclosed
in Japanese laid-open Patent Application Nos. 3-88770 and No.
3-54165.
[0005] The former application discloses a barium titanate based
semiconductor ceramic composition for use in the PTC thermistor
containing, as principal components, 45 to 85 molar percentage (mol
%) of BaTiO.sub.3, 1 to 20 mol % of PbTiO.sub.3, 1 to 20 mol % of
SrTiO.sub.3 and 5 to 20 mol % of CaTiO.sub.3 as well as 0.1 to 0.3
mol % of a semiconductor forming agent, 0.006 to 0.025 mol % of Mn
and 0.1 to 1 mol % of SiO.sub.2 as additives, wherein BaTiO.sub.3,
PbTiO.sub.3, SrTiO.sub.3 and CaTiO.sub.3 as principal components
are formed by a citric acid method.
[0006] The application describes that characteristic values such as
a resistivity at room temperature of 8 .OMEGA..multidot.cm or less
(4 to 8 .OMEGA..multidot.cm), a temperature resistance coefficient
a of 9%/.degree. C. or more and an static withstanding voltage of
60 V/mm or more can be obtained with respect to the ceramic
composition. The laid-open text also shows examples using La, Sb
and Nb as semiconductor forming agents.
[0007] The latter laid-open text discloses a barium titanate based
semiconductor ceramic composition for use in the PTC thermistor
containing, as principal components, 45 to 87 mol % of BaTiO.sub.3,
3 to 20 mol % of PbTiO.sub.3, 5 to 20 mol % of SrTiO.sub.3 and 5 to
15 mol % of CaTiO.sub.3 as well as 0.2 to 0.5 mol % of a
semiconductor forming agent, 0.02 to 0.08 mol % of Mn and 0 to 0.45
mol % of SiO.sub.2 as additives, wherein BaTiO.sub.3, PbTiO.sub.3,
SrTiO.sub.3 and CaTiO.sub.3 as principal components are formed by a
liquid phase method.
[0008] The publication shows that characteristic values such as a
resistivity at room temperature of 3 to 10 .OMEGA..multidot.cm and
an static withstanding voltage of 10 to 200 V/mm can be obtained
with respect to the ceramic composition. It also shows examples
using Sb, Y and La as semiconductor forming agents. La, Sb, Nb and
Y are used as a semiconductor forming agent in the examples
disclosed in the foregoing two patent publications. However, these
semiconductor forming agents involve problems, as shown in the
comparative examples hereinafter. It is a problem that the
resistance is largely dispersed when La, Sb or Nb are used for the
semiconductor forming agent, although the resistivity at room
temperature becomes low. The resistivity at room temperature can
not be lowered when Y is used for the semiconductor forming
agent.
SUMMARY OF THE INVENTION
[0009] The present invention provides a composite material for the
PTC thermistor as well as a ceramic for use in the PTC thermistor
obtained by firing the composite material, and a method for
manufacturing the same.
[0010] In a first aspect, the present invention provides, in short,
a composite material for the PTC thermistor using Sm as a
semiconductor forming agent or, in more detail, a composite
material for a PTC thermistor containing as principal components
about 30 to 97 molar percentage (mol %) of BaTiO.sub.3, about 1 to
50 mol % of PbTiO.sub.3, about 1 to 30 mol % of SrTiO.sub.3 and
about 1 to 25 mol % of CaTiO.sub.3 (wherein the total content of
them accounts for 100 mol %), and containing as additives about 0.1
to 0.3 mole of Sm in terms of elemental Sm in a compound containing
Sm, about 0.01 to 0.03 mole of Mn in terms of elemental Mn in a
compound containing Mn and 0 to about 2.0 mole of Si in terms of
elemental Si in a compound containing Si, relative to 100 mole of
the principal components.
[0011] The present invention also provides a ceramic for use in the
PTC thermistor obtained by firing the composite material as
described above. In a second aspect, the present invention provides
a ceramic for use in a PTC thermistor containing as principal
components about 30 to 97 molar percentage (mol %) of BaTiO.sub.3,
about 1 to 50 mol % of PbTiO.sub.3, about 1 to 30 mol % of
SrTiO.sub.3 and about 1 to 25 mol % of CaTiO.sub.3 (wherein the
total content of them accounts for 100 mol %), and containing as
additives about 0.1 to 0.3 mole of Sm calculated as Sm in samarium
oxide, about 0.01 to 0.03 mole of Mn in terms of Mn in manganese
oxide and 0 to about 2.0 mole of Si in terms of Si in silicon
oxide, relative to 100 mole of the principal components.
[0012] The present invention also provides a ceramic for use in the
PTC thermistor obtained by firing the composite material for the
PTC thermistor according to the first aspect in a neutral
atmosphere followed by heat-treatment in an oxidative
atmosphere.
[0013] The present invention also provides a ceramic for use in the
PTC thermistor obtained by firing the composite material for the
PTC thermistor according to the first aspect in a reducing
atmosphere followed by heat-treatment in an oxidative
atmosphere.
[0014] The present invention also provides a method for
manufacturing a ceramic for use in the PTC thermistor.
[0015] In one aspect, the present invention provides a method for
manufacturing the ceramic for use in the PTC thermistor with the
step of firing the composite material for the PTC thermistor in a
neutral atmosphere followed by heat-treating in an oxidative
atmosphere.
[0016] In another aspect, the present invention provides a method
for manufacturing the ceramic for use in the PTC thermistor with
the step of firing the composite material for the PTC thermistor in
a reducing atmosphere followed by heat-treatment in an oxidative
atmosphere.
[0017] A nitrogen atmosphere may be used for the neutral atmosphere
while air containing about 20% of oxygen or a high oxygen
containing atmosphere containing about 100% of oxygen may be used
for the oxidative atmosphere. An atmosphere containing 1% of
hydrogen and 99% of nitrogen may be used for the reducing
atmosphere.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 shows a firing profile used for executing Example 2
or 5 according to the present invention.
[0019] FIG. 2 is a firing profile used for executing Example 3 or 6
according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
EXAMPLE 1
[0020] BaCO.sub.3, TiO.sub.2, PbO, SrCO.sub.3, CaCO.sub.3,
Sm.sub.2O.sub.3, MnCO.sub.3 and SiO.sub.2 were used as starting
materials and they were mixed in the proportions shown in TABLE 1
with wet mixing. The term "Ba", "Pb", "Sr" and "Ca" in TABLE 1
denote the composition ratios of "BaTiO.sub.3", "PbTiO.sub.3",
"SrTiO.sub.3" and "CaTiO.sub.3", respectively. The composition
ratios of "BaTiO.sub.3", "PbTiO.sub.3", "SrTiO.sub.3" and
"CaTiO.sub.3" to serve as principal components are expressed by mol
% units (the combined amount of them accounts for 100%) while "Sm",
"Mn" and "SiO.sub.2" to serve as additives are expressed in mol %
relative to 100 moles of the principal component.
1TABLE 1 No. Ba Pb Sr Ca Sm Mn SiO.sub.2 1-1 97.0 1 1 1 0.2 0.02 1
1-2 60.0 10 15 15 0.2 0.02 1 1-3 40.0 30 15 15 0.2 0.02 1 1-4 20.0
50 15 15 0.2 0.02 1 * 1-5 30.0 50 10 10 0.2 0.02 1 1-6 30.0 60 5 5
0.2 0.02 1 * 1-7 74.0 10 1 15 0.2 0.02 1 1-8 45.0 10 30 15 0.2 0.02
1 1-9 35.0 10 40 15 0.2 0.02 1 * 1-10 74.0 10 15 1 0.2 0.02 1 1-11
50.0 10 15 25 0.2 0.02 1 1-12 45.0 10 15 30 0.2 0.02 1 * 1-13 60.0
10 15 15 0.05 0.02 1 * 1-14 60.0 10 15 15 0.1 0.02 1 1-15 60.0 10
15 15 0.3 0.02 1 1-16 60.0 10 15 15 0.4 0.02 1 * 1-17 60.0 10 15 15
0.2 0.005 1 * 1-18 60.0 10 15 15 0.2 0.01 1 1-19 60.0 10 15 15 0.2
0.03 1 1-20 60.0 10 15 15 0.2 0.04 1 * 1-21 60.0 10 15 15 0.2 0.02
2 1-22 60.0 10 15 15 0.2 0.02 3 * The mark "*" denotes that the
composition is out of the range of the present invention.
[0021] Each mixture shown in TABLE 1 was then dehydrated and dried
and, after calcinating at 1100 to 1200.degree. C., a binder was
added for granulation. The granulated particles were subjected to a
uniaxial press molding and the molded body obtained was fired at
1300 to 1400.degree. C. in the air to obtain a sintered disk with a
diameter of 11.0 mm and a thickness of 0.5 mm.
[0022] Respective sintered disks were used for ceramics for use in
the PTC thermistor. In--Ga electrodes were formed on each principal
face of the ceramic disk to obtain a PTC thermistor as a sample
piece.
[0023] The resistivity at room temperature (.rho..sub.25), static
withstanding voltage and temperature resistance coefficient
(.alpha.), as well as dispersion of the resistance (CV %), were
determined with respect to the PTC thermistor using each
sample.
[0024] In more detail, the temperature resistance coefficient
(.alpha.) was calculated by the following equation:
.alpha.=[ln(.rho..sub.2/.rho..sub.1) /(T.sub.2-T.sub.1)].times.100
(%/.degree. C.)
[0025] wherein .rho..sub.1 denotes the resistivity at 10 times
.rho..sub.25 and and T.sub.1 denotes the corresponding temperature
while .rho..sub.2 denotes the resistivity at 100 times .rho..sub.25
and and T.sub.2 denotes the corresponding temperature.
[0026] Scattering of the resistance (CV %) was evaluated from the
variation of the resistivity at room temperature among each lot
containing 10 pieces of the PTC thermistors manufactured under the
same conditions using respective compositions shown in TABLE 1. The
values were actually determined by the following equation:
CV %=(standard deviation of .rho..sub.25 among 10 lots)/(mean value
of .rho..sub.25 among 10 lots).times.100
[0027] TABLE 2 shows the resistivity at room temperature
(.rho..sub.25), static withstanding voltage, and the temperature
resistance coefficient (.alpha.) and its scattering (CV %).
2TABLE 2 STATIC WITH- SCATTER- STANDING ING OF VOLTAGE RESISTANCE
No. .rho..sub.25(.OMEGA. .multidot. cm) (V/mm) .alpha.(%/.degree.
C.) (CV %) 1-1 2.1 60 10.0 3.2 1-2 3.1 90 11.5 3.5 1-3 3.9 110 13.0
3.9 1-4 5.3 120 14.1 3.8 * 1-5 4.9 110 13.1 3.9 1-6 5.5 105 12.7
4.0 * 1-7 2.7 75 10.3 3.3 1-8 4.5 100 12.6 3.8 1-9 6.6 105 13.2 5.5
* 1-10 2.8 70 10.4 3.4 1-11 4.1 100 12.2 3.7 1-12 5.2 105 12.5 3.9
* 1-13 5.6 115 13.6 4.3 * 1-14 4.5 105 12.9 3.6 1-15 3.2 75 10.1
3.5 1-16 4.1 60 8.7 3.6 * 1-17 2.0 55 9.4 3.1 * 1-18 2.3 70 10.0
3.4 1-19 4.8 110 13.0 3.8 1-20 7.3 130 14.4 5.2 * 1-21 4.2 100 12.3
3.7 1-22 5.5 110 12.7 4.3 * The mark "*" denotes that the
composition is out of the range of the present invention.
[0028] PTC thermistors were manufactured as comparative examples
using respective compositions in TABLE 3 by the same method as used
in Examples. While Sm.sub.2O.sub.3 was used as a starting material
for the semiconductor forming agent in the foregoing examples,
La.sub.2O.sub.3, Y.sub.2O.sub.3, Sb.sub.2O.sub.3 and
Nb.sub.2O.sub.3 were used as a starting material for each
semiconductor forming agent in the comparative examples.
3TABLE 3 SEMICONDUCTOR No. Ba Pb Sr Ca FORMING AGENT Mn SiO.sub.2
1-23 60.0 10 15 15 La 0.2 0.02 1 1-24 60.0 10 15 15 Y 0.25 0.02 1
1-25 60.0 10 15 15 Sb 0.2 0.02 1 1-26 60.0 10 15 15 Nb 0.2 0.02
1
[0029] The term "Ba", "Pb", "Sr" and "Ca" in TABLE 3 denote, as in
TABLE 1, the composition ratios of "BaTiO.sub.3", "PbTiO.sub.3",
"SrTiO.sub.3" and "CaTiO.sub.3", respectively. The composition
ratios of "BaTiO.sub.3", "PbTiO.sub.3", "SrTiO.sub.3" and
"CaTiO.sub.3" to serve as principal components are expressed by mol
% (the combined amount of them accounts for 100%) while "the
semiconductor forming agents (La, Y, Sb and Nb)l", "Mn" and "S" to
serve as additives are expressed in mol % relative to 100 moles of
the principal components.
[0030] Respective characteristics of the samples in the comparative
examples were evaluated by the same method as used in the examples
described above. The results are listed in TABLE 4
4TABLE 4 SCATTER- STATIC ING OF WITHSTANDING RESISTANCE No.
.rho..sub.25(.OMEGA. .multidot. cm) VOLTAGE (V/mm)
.alpha.(%/.degree. C.) (CV %) 1-23 2.9 85 10.0 7.0 1-24 6.0 120
12.5 3.4 1-25 2.8 75 9.5 7.0 1-26 3.1 80 9.5 6.5
[0031] It is evident from TABLE 1 and TABLE 2 that characteristics
such as the resistivity at room temperature (.rho..sub.25) of 5
.OMEGA..multidot.cm or less, static withstanding voltage of 60 V/mm
or more and temperature resistance coefficient (.alpha.) of
10%/.degree. C. or more are obtained with less variation of the
temperature resistance coefficient among the lots with respect to
the PTC thermistors manufactured using the composite material for
the PTC thermistors having the compositions within the range of the
present invention.
[0032] Samples 1-4, 1-6, l-9, 1-12, 1-13, 1-20 and 1-22 having
compositions out of the range of the present invention have, on the
other hand, a resistivity at room temperature (.rho..sub.25) of
more than 5 .OMEGA..multidot.cm while samples 1-16 and 1-17 have a
temperature resistance coefficient (.alpha.) of less than
10%/.degree. C.
[0033] Although it is possible to lower the resistivity at room
temperature (.rho..sub.25) in the samples 1-23, 1-25 and 1-26 in
which La, Sb and Nb are used as semiconductor forming agents,
respectively, as shown in TABLE 3 and TABLE 4, scattering of the
values among the lots is increased as compared with the samples
shown in TABLE 1 and TABLE 2. The resistivity at room temperature
(.rho..sub.25) can not be sufficiently lowered in the sample 1-24
in which Y is used as the semiconductor forming agent.
[0034] Accordingly, Sm is used for the semiconductor forming agent
in the examples according to the present invention in order to
obtaining the effects hitherto described. It is believed this
result is because the ionic radius of Sm is appropriate for
obtaining the foregoing effect.
[0035] TABLE 5 below show the ionic radii of the semiconductor
forming agents used in the examples and comparative examples.
5 TABLE 5 IONIC SPECIES IONIC RADIUS (.ANG.) Ba SITE Ba.sup.2+ 1.61
12-COORDINATION La.sup.3+ 1.36 NUMBER Sm.sup.3+ 1.24 Y.sup.3+ 0.96
Ti SITE Ti.sup.4+ 0.61 6-COORDINATION Nb.sup.5+ 0.64 NUMBER
Sb.sup.5+ 0.60
[0036] While the Ba-site can be in principle substituted with the
trivalent ions La, Sm and Y judging from ionic radii of these ions,
a part of them are thought to occupy the Ti-site. The proportions
of substitution of the Ti-site with these ions La, Sm and Y are
supposed to be increased as the ionic radii of them become smaller
in the order of Y>Sm>La. An acceptor level is generated when
the Ti-site is substituted with these trivalent ions, trapping free
electrons generated by substitution of the Ba site with these
ions.
[0037] Accordingly, the trapping probability of the liberated free
ions with the acceptors becomes higher when Y having a small ionic
radius and higher probability of Ti-site substitution is used for
the semiconductor forming agent. Although the rate of change of the
resistivity against the amount of addition of Y is small (the
resistivity is stable to the amount of addition of Y), the
resistivity of the thermistor has not been sufficiently reduced by
the addition of Y as seen in the sample 1-24 in TABLE 4.
[0038] An acceptor level is hardly formed to reduce the resistivity
when La having a higher tendency for substituting the Ba-site is
used as the semiconductor forming agent. However, the rate of
change of the resistivity against the addition of La becomes so
large that dispersion of the resistivity among the lots becomes
larger, as shown in the sample 1-23 in TABLE 4.
[0039] Sm has an ability to lower the resistivity and reduce
dispersion of the resistivity among the lots since the ionic radius
of Sm is within an appropriate range.
[0040] While the Ti-site may be in principle substituted with Nb or
Sb judging from their ionic radii, the acceptor level is never
formed even when the Ti-site is substituted with Nb or Sb since the
ion serves as a pentavalent ion, the ion serving as ions similar to
La.
EXAMPLE 2
[0041] The firing profile shown in FIG. 1 is used in this Example 2
in the firing step of the molded body comprising the composite
material for the PTC thermistor. In short, after firing the molded
body in a neutral atmosphere such as N.sub.2 atmosphere, the molded
body is heat-treated in an oxidative atmosphere such as in a high
oxygen concentration atmosphere or in the air in order to obtain
the ceramic for use in the PTC thermistor.
[0042] Except for the firing profile as described above, the molded
bodies of the examples and comparative examples in Example 2 were
obtained by the same treatment as described in the examples and
comparative examples in Example 1 using the same compositions
listed in TABLE 1 and TABLE 3. Electrodes were formed on the
sintered bodies after firing by the same method as in Example 1 to
manufacture the PTC thermistors to evaluate their
characteristics.
[0043] The neutral atmospheres for firing under the firing profile
and oxidative atmospheres for the heat-treatment shown in FIG. 1
are applied for respective samples shown in TABLE 6 below.
6 TABLE 6 FIRING HEAT-TREATMENT No. ATMOSPHERE ATMOSPHERE 2-1
N.sub.2 100% O.sub.2 100% 2-2-1 N.sub.2 100% O.sub.2 100% 2-2-2
N.sub.2 100% O.sub.2 20% 2-3 N.sub.2 100% O.sub.2 100% 2-4 N.sub.2
100% O.sub.2 100% * 2-5 N.sub.2 100% O.sub.2 100% 2-6 N.sub.2 100%
O.sub.2 100% * 2-7 N.sub.2 100% O.sub.2 100% 2-8 N.sub.2 100%
O.sub.2 100% 2-9 N.sub.2 100% O.sub.2 100% * 2-10 N.sub.2 100%
O.sub.2 100% 2-11 N.sub.2 100% O.sub.2 100% 2-12 N.sub.2 100%
O.sub.2 100% * 2-13 N.sub.2 100% O.sub.2 100% * 1-14 N.sub.2 100%
O.sub.2 100% 2-15 N.sub.2 100% O.sub.2 100% 2-16 N.sub.2 100%
O.sub.2 100% * 2-17 N.sub.2 100% O.sub.2 100% * 2-18 N.sub.2 100%
O.sub.2 100% 2-19 N.sub.2 100% O.sub.2 100% 2-20 N.sub.2 100%
O.sub.2 100% * 2-21 N.sub.2 100% O.sub.2 100% 2-22 N.sub.2 100%
O.sub.2 100% * The mark (*) denotes that the composition is out of
the range of the present invention.
[0044] The resistivity at room temperature (.rho..sub.25), static
withstanding voltage, temperature resistance coefficient (.alpha.)
and scattering of the resistivity (CV %) of each sample in TABLE 6
are listed in TABLE 7 below.
7TABLE 7 STATIC SCATTER- WITH- ING OF STANDING RESISTIVITY No.
.rho..sub.25(.OMEGA. .multidot. cm) VOLTAGE .alpha.(%/.degree. C.)
(CV %) 2-1 1.8 60 11.0 3.0 2-2-1 2.3 70 12.5 3.2 2-2-2 2.2 70 12.0
3.1 2-3 2.7 85 14.0 3.5 2-4 3.6 85 15.1 3.9 * 2-5 3.3 90 14.1 3.1
2-6 3.6 75 13.7 3.9 * 2-7 2.1 60 11.3 3.2 2-8 3.2 75 13.6 3.4 2-9
4.4 70 14.2 3.8 * 2-10 2.1 65 11.4 3.1 2-11 2.8 75 13.2 3.4 2-12
3.6 80 13.5 3.8 * 2-13 3.9 90 14.6 3.9 * 2-14 3.1 85 13.9 3.2 2-15
2.4 65 11.1 3.1 2-16 2.8 45 9.7 3.3 * 2-17 1.7 55 10.4 3.2 * 2-18
1.8 60 11.0 3.2 2-19 3.2 85 14.0 3.6 2-20 4.8 90 15.4 3.9 * 2-21
3.0 85 13.3 3.3 2-22 3.7 80 13.7 3.5 * The mark (*) denotes that
the composition is out of the range of the present invention.
[0045] Sample numbers 2-1 to 2-22 in TABLE 6 and 7 correspond to
the sample numbers 1-1 to 1-22 in TABLE 1 and TABLE 2,
respectively, in which the samples same sample numbers have the
same compositions with each other.
[0046] Samples with the sample number 2-2 in TABLE 6 and TABLE 7
were thermally-treated under different two kinds of oxidative
atmospheres in the firing profile shown in FIG. 1. Accordingly, the
samples were distinguished with each other by attaching sub-numbers
as 2-2-1 and 2-2-2.
[0047] TABLE 7 shows that the samples with compositions within the
range of the present invention have the resistivities at room
temperature (.rho..sub.25) of 3.5 .OMEGA..multidot.cm or less,
static withstanding voltages of 50 V/mm or more and temperature
resistance coefficients of 11.0 %/.degree. C., enabling one to
obtain the PTC thermistors with small variation of resistance (CV
%).
[0048] When the resistivities at room temperature (.rho..sub.25) is
especially noticed, lower values could be obtained in the samples
in TABLE 6 and TABLE 7 having compositions within the range of the
present invention as compared with the samples in the foregoing
TABLE 2 whose compositions are within the range of the present
invention, because the effect of firing in a neutral atmosphere and
heat-treatment in an oxidative atmosphere had been expressed to
increase the grain size in the ceramic due to such firing and
heat-treatment.
[0049] It is also evident from the comparison between the samples
2-2-1 and 2-2-2 in TABLE 6 and TABLE 7 that the oxygen
concentration during the heat-treatment step in the firing profile
shown in FIG. 1 has substantially no effects on the characteristics
of the PTC thermistors at an oxygen concentration from 20 to
100%.
[0050] When comparing the samples having the compositions within
the range of the present invention with the samples having the
compositions out of the range of the present invention in TABLE 7,
then the same tendencies as the comparison of these samples in
TABLE 2 are recognizable.
[0051] The comparative examples in which semiconductor forming
agents other than Sm were used are listed in TABLE 8 and TABLE 9
below. TABLE 8 shows the neutral atmospheres for firing and
oxidative atmosphere for the heat-treatment to be applied to each
sample as in TABLE 6, while TABLE 9 shows the results of
evaluations as shown in TABLE 7.
8TABLE 8 FIRING HEAT-TREATMENT No. ATMOSPHERE ATMOSPHERE 2-23
N.sub.2 100% O.sub.2 100% 2-24 N.sub.2 100% O.sub.2 100% 2-25
N.sub.2 100% O.sub.2 100% 2-26 N.sub.2 100% O.sub.2 100%
[0052]
9TABLE 9 SCATTER- STATIC ING OF WITHSTANDING RESISTANCE No.
.rho..sub.25(.OMEGA. .multidot. cm) VOLTAGE (V/mm)
.alpha.(%/.degree. C.) (CV %) 2-23 2.1 65 10.0 6.3 2-24 4.0 80 12.0
3.1 2-25 2.0 60 9.5 6.5 2-26 2.2 60 9.5 6.2
[0053] Sample numbers 2-23 to 2-26 in TABLE 8 and 9 correspond to
the sample numbers 1-23 to 1-26 in TABLE 3 and TABLE 4,
respectively, in which the same compositions are used in the
samples with the same sample numbers with each other.
[0054] In comparing the samples in the comparative examples shown
in TABLE 9 with the samples with the compositions within the range
of the present invention shown in TABLE 7, or comparing the samples
in the comparative examples shown in TABLE 4 with the samples with
the compositions within the range of the present invention shown in
TABLE 2, the same tendencies in the former comparison are also seen
in the latter comparison. However, it can be understood that the
effect for decreasing the resistivity at room temperature
(.rho..sub.25) by firing in a neutral atmosphere and heat-treatment
in an oxidative atmosphere is recognizable in the comparative
examples when the comparative examples shown in TABLE 9 are
compared with the comparative examples shown in TABLE 4.
EXAMPLE 3
[0055] The firing profile shown in FIG. 2 is used in Example 3 for
firing the molded bodies comprising the composite materials for the
PTC thermistors. In short, after firing the molded bodies in a
reducing atmosphere such as the H.sub.2/N.sub.2 atmosphere, the
molded bodies were heat-treated in an oxidative atmosphere such as
in a high oxygen concentration atmosphere or in air.
[0056] Except for the firing profile as described above, the molded
bodies of the examples and comparative examples in Example 3 were
obtained by the same treatment as described in the examples and
comparative examples in Example 1 using the same compositions
listed in TABLE 1 and TABLE 3. Electrodes were formed on the
sintered bodies after firing by the same method as in Example 1 to
manufacture the PTC thermistors to evaluate their
characteristics.
[0057] The reducing atmospheres for firing under the firing profile
and oxidative atmospheres for the heat-treatment shown in FIG. 2
are applied for respective samples are shown in TABLE 10 below.
10 TABLE 10 FIRING HEAT-TREATMENT No. ATMOSPHERE ATMOSPHERE 3-1
H.sub.2 1%/N.sub.2 99% O.sub.2 100% 3-2-1 H.sub.2 1%/N.sub.2 99%
O.sub.2 100% 3-2-2 H.sub.2 1%/N.sub.2 99% O.sub.2 20% 3-3 H.sub.2
1%/N.sub.2 99% O.sub.2 100% 3-4 H.sub.2 1%/N.sub.2 99% O.sub.2 100%
* 3-5 H.sub.2 1%/N.sub.2 99% O.sub.2 100% 3-6 H.sub.2 1%/N.sub.2
99% O.sub.2 100% * 3-7 H.sub.2 1%/N.sub.2 99% O.sub.2 100% 3-8
H.sub.2 1%/N.sub.2 99% O.sub.2 100% 3-9 H.sub.2 1%/N.sub.2 99%
O.sub.2 100% * 3-10 H.sub.2 1%/N.sub.2 99% O.sub.2 100% 3-11
H.sub.2 1%/N.sub.2 99% O.sub.2 100% 3-12 H.sub.2 1%/N.sub.2 99%
O.sub.2 100% * 3-13 H.sub.2 1%/N.sub.2 99% O.sub.2 100% * 3-14
H.sub.2 1%/N.sub.2 99% O.sub.2 100% 3-15 H.sub.2 1%/N.sub.2 99%
O.sub.2 100% 3-16 H.sub.2 1%/N.sub.2 99% O.sub.2 100% * 3-17
H.sub.2 1%/N.sub.2 99% O.sub.2 100% * 3-18 H.sub.2 1%/N.sub.2 99%
O.sub.2 100% 3-19 H.sub.2 1%/N.sub.2 99% O.sub.2 100% 3-20 H.sub.2
1%/N.sub.2 99% O.sub.2 100% * 3-21 H.sub.2 1%/N.sub.2 99% O.sub.2
100% 3-22 H.sub.2 1%/N.sub.2 99% O.sub.2 100% * The mark (*)
denotes that the composition is out of the range of the present
invention.
[0058] The resistivities at room temperature (.rho.25), static
withstanding voltage, temperature resistance coefficient (.alpha.)
and scattering of resistance (CV %) of each sample in TABLE 10 are
shown in TABLE 11.
11TABLE 11 STATIC WITH- SCATTER- STANDING ING OF VOLTAGE
RESISTIVITY No. .rho..sub.25(.OMEGA. .multidot. cm) (V/mm)
.alpha.(%/.degree. C.) (CV %) 3-1 1.7 60 10.8 2.9 3-2-1 2.3 70 12.8
3.2 3-2-2 2.1 70 11.7 3.3 3-3 2.6 85 13.8 3.5 3-4 3.6 85 15.1 3.8 *
3-5 3.2 90 13.8 3.1 3-6 3.6 75 13.5 3.8 * 3-7 2.0 60 11.0 3.2 3-8
3.2 75 13.6 3.0 3-9 4.3 70 14.0 3.8 * 3-10 2.1 65 11.2 3.0 3-11 2.6
75 13.0 3.4 3-12 3.6 80 13.5 3.7 * 3-13 3.8 90 14.3 3.9 * 3-14 3.1
85 13.9 3.3 3-15 2.3 65 10.9 3.1 3-16 2.8 45 9.5 3.5 * 3-17 1.6 45
10.1 3.2 * 3-18 1.8 60 10.8 3.4 3-19 3.1 85 13.9 3.6 3-20 4.8 90
15.6 3.8 * 3-21 2.9 85 13.0 3.3 3-22 3.7 80 13.8 3.4 * The mark (*)
denotes that the composition is out of the range of the present
invention.
[0059] Sample numbers 3-1 to 3-22 in TABLE 10 and 11 correspond to
sample numbers 1-1 to 1-22 in TABLE 1 and TABLE 2, respectively, in
which the samples with the same sample numbers with each other have
the same compositions.
[0060] The samples with the sample number 3-2 in TABLE 10 and TABLE
11 were heat-treated under different two kinds of oxidative
atmospheres in the firing profile shown in FIG. 2. Accordingly, the
samples were distinguished with each other by attaching sub-numbers
as 3-2-1 and 3-2-2.
[0061] TABLE 11 shows that the samples with the compositions within
the range of the present invention have the resistivities at room
temperature (.rho..sub.25) of 3.5 .OMEGA..multidot.cm or less,
static withstanding voltages of 50 V/mm or more and temperature
resistance coefficients (.alpha.) of 10.8 %/.degree. C., enabling
one to obtain the PTC thermistors with small scattering of
resistance (CV %).
[0062] When the resistivities at room temperature (.rho..sub.25) is
especially noticed, lower values could be obtained as compared with
the samples in TABLE 2 or TABLE 7 whose compositions are within the
range of the present invention because the effect of firing in a
reducing atmosphere and heat-treatment in an oxidative atmosphere
had been expressed to increase the concentration of conductive
electrons in the grains due to such firing and heat-treatment.
[0063] It is also evident from the comparison between the samples
3-2-1 and 3-2-2 in TABLE 10 and TABLE 11 that the oxygen
concentration during the heat-treatment steps in the firing profile
shown in FIG. 2 has substantially no effect on the characteristics
of the PTC thermistors in an oxygen concentration range from 20 to
100%.
[0064] The same tendencies as the comparison of these samples in
TABLE 2 are recognizable when the samples with the concentrations
within the range of the present invention are compared with the
samples with the concentrations out of the range of the present
invention in TABLE 11.
[0065] The comparative examples in which semiconductor forming
agents other than Sm were used are listed in TABLE 12 and TABLE 13
below. TABLE 12 shows the reducing atmospheres for the sintering
step and oxidative atmosphere for the heat-treatment to be applied
to each sample as in TABLE 10, while TABLE 13 shows the results of
evaluations as shown in TABLE 11.
12 TABLE 12 SINTERING HEAT-TREATMENT No. ATMOSPHERE ATMOSPHERE 2-23
H.sub.2 1%/N.sub.2 99% O.sub.2 100% 2-24 H.sub.2 1%/N.sub.2 99%
O.sub.2 100% 2-25 H.sub.2 1%/N.sub.2 99% O.sub.2 100% 2-26 H.sub.2
1%/N.sub.2 99% O.sub.2 100%
[0066]
13TABLE 13 SCATTER- ING OF STATIC RESIST- WITHSTANDING ANCE No.
.rho..sub.25(.OMEGA. .multidot. cm) VOLTAGE (V/mm) .alpha.
(%/.degree. C.) (CV %) 2-23 1.9 65 10.1 6.8 2-24 3.9 80 12.2 3.5
2-25 2.0 60 9.5 7.0 2-26 2.1 60 9.4 6.8
[0067] The sample numbers 3-23 to 3-26 in TABLE 12 and 13
correspond to the sample numbers 1-23 to 1-26 in TABLE 3 and TABLE
4, respectively, in which the same compositions are used in the
samples with the same sample numbers with each other.
[0068] Comparing the samples in the comparative examples shown in
TABLE 13 with the samples with the compositions within the range of
the present invention shown in TABLE 11, or comparing the samples
in the comparative examples shown in TABLE 4 with the samples with
the compositions within the range of the present invention shown in
TABLE 2, then the same tendencies in the former case are also seen
in the latter case. However, it can be understood that the effect
for decreasing the resistivity at room temperature (.rho..sub.25)
by firing in a reducing atmosphere and heat-treatment in an
oxidative atmosphere is recognizable in the comparative examples
when the comparative examples shown in TABLE 13 are compared with
the comparative examples shown in TABLE 4.
[0069] In contrast to Examples 1 to 3 as hitherto described,
SiO.sub.2 is not used in the starting materials in the samples in
Example 4 and in Examples to be described hereinafter, or the
ceramics obtained do not contain SiO.sub.2. The atmospheres to be
used in the firing and sintering steps in Examples 4, 5 and 6
correspond to the atmospheres in Examples 1, 2 and 3, respectively,
as hitherto described.
EXAMPLE 4
[0070] BaCO.sub.3, TiO.sub.2, Pbo, SrCO.sub.3, CaCO.sub.3,
Sm.sub.2O.sub.3 and MnCO.sub.3 were used as starting materials and
they were blended with wet-mixing to be the compositions as shown
in TABLE 14. The terms "Ba", "Pb", "Sr" and "Ca" in TABLE 14 denote
the composition ratios of the principal components "BaTiO.sub.3"
"PbTiO.sub.3", "SrTiO.sub.3" and "CaTiO.sub.3", respectively,
represented by mol % (the combined amount of them accounts for 100
mol %) while the additives "Sm" and "Mn" are represented by molar
ratios relative to 100 moles of the principal components. It should
be noted that the content of "SiO.sub.2" is zero.
14TABLE 14 No. Ba Pb Sr Ca Sm Mn SiO.sub.2 4-1 97.0 1 1 1 0.2 0.02
0 4-2 60.0 10 15 15 0.2 0.02 0 4-3 40.0 30 15 15 0.2 0.02 0 4-4
20.0 50 15 15 0.2 0.02 0 * 4-5 30.0 50 10 10 0.2 0.02 0 4-6 30.0 60
5 5 0.2 0.02 0 * 4-7 74.0 10 1 15 0.2 0.02 0 4-8 45.0 10 30 15 0.2
0.02 0 4-9 35.0 10 40 15 0.2 0.02 0 * 4-10 74.0 10 15 1 0.2 0.02 0
4-11 50.0 10 15 25 0.2 0.02 0 4-12 45.0 10 15 30 0.2 0.02 0 * 4-13
60.0 10 15 15 0.05 0.02 0 * 4-14 60.0 10 15 15 0.1 0.02 0 4-15 60.0
10 15 15 0.3 0.02 0 4-16 60.0 10 15 15 0.4 0.02 0 * 4-17 60.0 10 15
15 0.2 0.005 0 * 4-18 60.0 10 15 15 0.2 0.01 0 4-19 60.0 10 15 15
0.2 0.03 0 4-20 60.0 10 15 15 0.2 0.04 0 * The mark (*) denotes
that the composition is out of the range of the present
invention.
[0071] The compositions shown in TABLE 14 were subsequently
subjected to dehydration, drying, calcination, molding and
sintering steps followed by forming external electrodes under
substantially the same conditions as used in Example 1 to obtain
PTC thermistors as samples. Then, the resistivities at room
temperature (.rho..sub.25), static withstanding voltages,
temperature resistance coefficients (.alpha.) and dispersions of
the resistance (CV %) were determined with respect to the PTC
thermistors.
[0072] TABLE 15 shows the resistivities at room temperature
(.rho..sub.25), static withstanding voltages, temperature
resistance coefficients (.alpha.) and dispersions of the resistance
(CV %) of the samples listed in TABLE 14.
15TABLE 15 Static DISPERSION withstanding OF voltage RESISTIVITY
No. .rho..sub.25(.OMEGA. .multidot. cm) (V/mm) .alpha.(%/.degree.
C.) (CV %) 4-1 2.0 60 9.0 2.9 4-2 2.7 75 10.5 3.3 4-3 3.4 95 12.0
3.6 4-4 5.1 105 13.1 3.4 * 4-5 4.7 95 12.1 3.6 4-6 5.3 90 11.7 3.6
* 4-7 2.4 60 9.3 3.0 4-8 4.0 85 11.6 3.6 4-9 6.1 90 12.2 5.2 * 4-10
2.5 60 9.4 3.0 4-11 3.6 85 11.2 3.4 4-12 5.1 90 11.5 3.5 * 4-13 5.1
100 12.6 4.0 * 4-14 4.0 90 11.9 3.3 4-15 2.9 60 9.1 3.1 4-16 3.6 50
7.7 3.4 * 4-17 1.5 50 8.4 2.8 * 4-18 2.0 60 9.0 3.2 4-19 4.3 100
12.0 3.5 4-20 6.8 120 13.4 4.8 * The mark (*) denotes that the
composition is out of the range of the present invention.
[0073] It is evident from TABLE 14 and TABLE 15 that the
resistivities at room temperature (.rho..sub.25) of 5
.OMEGA..multidot.cm or less, static withstanding voltages of 60
V/mm and temperature resistance coefficients (.alpha.) of
9%/.degree. C. can be obtained with respect to the PTC thermistors
manufactured by using the composite materials for the PTC
thermistors having the compositions within the range of the present
invention with less dispersion of the resistance at room
temperature.
[0074] On the contrary, the samples 4-4, 4-6, 4-9, 4-12, 4-13 and
4-20 having compositions out of the range of the present invention
have the resistivities at room temperature (.rho..sub.25) of 5
.OMEGA..multidot.cm or more and the samples 4-1 and 4-17 have the
temperature resistance coefficients (.alpha.) of less than
10%/.degree. C.
EXAMPLE 5
[0075] The firing profile as shown in FIG. 1 in which molded bodies
were heat-treated in an oxidative atmosphere after firing in a
neutral atmosphere was used as in Example 2 in firing the molded
bodies comprising the composite materials for the PTC
thermistors.
[0076] The same composite materials as shown in TABLE 14 and the
same methods as used in the examples and comparative examples in
Example 4 were used in the examples and comparative examples in
Example 5, except that the firing profile as described above was
used.
[0077] Electrodes were formed on the sintered body after firing by
the same method as used in Example 4 to manufacture the PTC
thermistors to be evaluated as described previously.
[0078] The neutral atmosphere for firing and oxidative atmosphere
for the heat-treatment shown in FIG. 1 were applied to each sample
and the conditions are listed in TABLE 16.
16 TABLE 16 FIRING HEAT-TREATMENT No. ATMOSPHERE ATMOSPHERE 5-1
N.sub.2 100% O.sub.2 100% 5-2-1 N.sub.2 100% O.sub.2 100% 5-2-2
N.sub.2 100% O.sub.2 20% 5-3 N.sub.2 100% O.sub.2 100% 5-4 N.sub.2
100% O.sub.2 100% * 5-5 N.sub.2 100% O.sub.2 100% 5-6 N.sub.2 100%
O.sub.2 100% * 5-7 N.sub.2 100% O.sub.2 100% 5-8 N.sub.2 100%
O.sub.2 100% 5-9 N.sub.2 100% O.sub.2 100% * 5-10 N.sub.2 100%
O.sub.2 100% 5-11 N.sub.2 100% O.sub.2 100% 5-12 N.sub.2 100%
O.sub.2 100% * 5-13 N.sub.2 100% O.sub.2 100% * 5-14 N.sub.2 100%
O.sub.2 100% 5-15 N.sub.2 100% O.sub.2 100% 5-16 N.sub.2 100%
O.sub.2 100% * 5-17 N.sub.2 100% O.sub.2 100% * 5-18 N.sub.2 100%
O.sub.2 100% 5-19 N.sub.2 100% O.sub.2 100% 5-20 N.sub.2 100%
O.sub.2 100% * The mark (*) denotes that the composition is out of
the range of the present invention.
[0079] The resistivities at room temperature (.rho..sub.25), static
withstanding voltages, temperature resistance coefficients
(.alpha.) and dispersions of the resistances (CV %) of the samples
listed in TABLE 16 are shown in TABLE 17 below.
17TABLE 17 STATIC WITH- DISPERSION STANDING OF VOLTAGE RESISTIVITY
No. .rho..sub.25(.OMEGA. .multidot. cm) (V/mm) .alpha.(%/.degree.
C.) (CV %) 5-1 1.5 50 10.0 3.0 5-2-1 2.0 60 11.5 3.0 5-2-2 1.9 60
11.0 3.1 5-3 2.4 75 13.0 3.0 5-4 3.6 85 14.3 3.9 * 5-5 3.2 80 13.1
2.9 5-6 3.6 70 13.2 3.9 * 5-7 1.8 50 10.3 3.0 5-8 2.9 65 12.6 3.3
5-9 4.1 60 13.2 3.8 * 5-10 1.8 55 10.4 3.1 5-11 2.5 65 12.2 3.3
5-12 3.6 75 13.0 3.8 * 5-13 3.6 80 13.6 3.7 * 5-14 2.8 75 12.9 3.2
5-15 2.1 50 10.1 3.0 5-16 2.5 35 8.7 3.3 * 5-17 1.4 40 9.4 3.0 *
5-18 1.5 50 10.0 3.2 5-19 2.9 75 13.0 3.5 5-20 4.5 80 14.4 3.9 *
The mark (*) denotes that the composition is out of the range of
the present invention.
[0080] The samples 5-1 to 5-20 in TABLE 16 and TABLE 17 correspond
to the samples 4-1 to 4-20 in TABLE 14 and TABLE 15, respectively,
in which the same composite materials are used for each sample
having the same sample numbers.
[0081] The samples with the sample number 5-2 in TABLE 16 and TABLE
17 were heat-treated under different two kinds of oxidative
atmospheres in the firing profile shown in FIG. 1. Accordingly, the
samples were distinguished with each other by attaching sub-numbers
as 5-2-1 and 5-2-2.
[0082] TABLE 17 shows that the samples having the compositions
within the range of the present invention have the resistivities at
room temperature (.rho..sub.25) of 3.5 .OMEGA..multidot.cm or less,
static withstanding voltage of 50 V/mm or more and temperature
resistance coefficients (.alpha.) of 10.0 %/.degree. C. or less,
enabling one to obtain the PTC resistors having small dispersion of
resistivities (CV %).
[0083] Looking at the resistivities at room temperature
(.rho..sub.25), it is clear that the samples having the
compositions within the range of the present invention in TABLE 17
have smaller values as compared with the samples having the
compositions within the range of the present invention in TABLE 15.
This is because the effects of firing in the neutral atmosphere and
heat-treatment in the oxidative atmosphere have been displayed to
increase the grain size in the ceramics due to such firing and
heat-treatment steps.
[0084] As shown by the comparison of the sample 5-2-1 with the
sample 5-2-2, are oxygen concentration within the range of 20 to
100% in the heat-treatment step in accordance with the firing
profile shown in FIG. 1 has substantially no effect on the
characteristics of the PTC thermistors.
[0085] Comparing the samples having the compositions within the
range of the present invention with the samples having compositions
out of the range of the present invention in TABLE 17, it can be
seen that the same tendency obtained by comparison among these two
kinds of samples in TABLE 15 are also displayed in TABLE 17.
EXAMPLE 6
[0086] The firing profile in which the molded bodies were
heat-treated in an oxidative atmosphere after firing in a reducing
atmosphere as shown in FIG. 2 was used in Example 6 for firing the
molded bodies comprising the composite materials for the PTC
thermistors.
[0087] The same composite materials and treatment conditions as
used in the examples and comparative examples in Example 4 were
also used in the examples and comparative examples in Example 6 to
obtain the molded bodies, except that the firing profile as
described above was used. Electrodes were formed on the sintered
bodies after firing by the same method as used in Example 4 to
manufacture the PTC thermistors as samples for the same
evaluation.
[0088] The reducing atmosphere for firing and oxidative atmosphere
for the heat-treatment for the firing profile shown in FIG. 2
applied to each sample are shown in TABLE 18.
18 TABLE 18 FIRING HEAT-TREATMENT No. ATMOSPHERE ATMOSPHERE 6-1
H.sub.2 1%/N.sub.2 99% O.sub.2 100% 6-2-1 H.sub.2 1%/N.sub.2 99%
O.sub.2 100% 6-2-2 H.sub.2 1%/N.sub.2 99% O.sub.2 20% 6-3 H.sub.2
1%/N.sub.2 99% O.sub.2 100% 6-4 H.sub.2 1%/N.sub.2 99% O.sub.2 100%
* 6-5 H.sub.2 1%/N.sub.2 99% O.sub.2 100% 6-6 H.sub.2 1%/N.sub.2
99% O.sub.2 100% * 6-7 H.sub.2 1%/N.sub.2 99% O.sub.2 100% 6-8
H.sub.2 1%/N.sub.2 99% O.sub.2 100% 6-9 H.sub.2 1%/N.sub.2 99%
O.sub.2 100% * 6-10 H.sub.2 1%/N.sub.2 99% O.sub.2 100% 6-11
H.sub.2 1%/N.sub.2 99% O.sub.2 100% 6-12 H.sub.2 1%/N.sub.2 99%
O.sub.2 100% * 6-13 H.sub.2 1%/N.sub.2 99% O.sub.2 100% * 6-14
H.sub.2 1%/N.sub.2 99% O.sub.2 100% 6-15 H.sub.2 1%/N.sub.2 99%
O.sub.2 100% 6-16 H.sub.2 1%/N.sub.2 99% O.sub.2 100% * 6-17
H.sub.2 1%/N.sub.2 99% O.sub.2 100% * 6-18 H.sub.2 1%/N.sub.2 99%
O.sub.2 100% 6-19 H.sub.2 1%/N.sub.2 99% O.sub.2 100% 6-20 H.sub.2
1%/N.sub.2 99% O.sub.2 100% * The mark (*) denotes that the
composition is out of the range of the present invention.
[0089] The resistivities at room temperature (.rho..sub.25) ,
static withstanding voltages, temperature resistance coefficients
(.alpha.) and dispersions of the resistances (CV %) of the samples
listed in TABLE 18 are shown in TABLE 19 below.
19TABLE 19 STATIC WITH- DISPERSION STANDING OF VOLTAGE RESISTIVITY
No. .rho..sub.25(.OMEGA. .multidot. cm) (V/mm) .alpha.(%/.degree.
C.) (CV %) 6-1 1.4 50 10.1 3.0 6-2-1 2.0 60 11.4 2.9 6-2-2 1.7 60
11.2 3.2 6-3 2.3 75 13.1 3.0 6-4 3.6 80 14.4 3.9 * 6-5 3.1 80 14.2
2.6 6-6 3.6 70 13.1 3.9 * 6-7 1.7 50 10.1 3.1 6-8 2.7 65 12.5 3.4
6-9 4.1 60 13.3 3.8 * 6-10 1.6 55 10.5 3.3 6-11 2.5 65 12.2 3.5
6-12 3.4 75 12.9 3.8 * 6-13 3.6 80 13.5 3.7 * 6-14 2.8 75 13.0 3.0
6-15 1.9 50 10.1 3.0 6-16 2.4 40 8.6 3.5 * 6-17 1.3 40 9.4 3.1 *
6-18 1.5 50 9.7 3.3 6-19 2.7 75 13.0 3.7 6-20 4.5 80 14.6 3.9 * The
mark (*) denotes that the composition is out of the range of the
present invention.
[0090] Samples 6-1 to 6-20 in TABLE 18 and TABLE 19 correspond to
the samples 4-1 to 4-20 in TABLE 14 and TABLE 15, respectively, in
which the same composite materials are used for each sample having
the same sample numbers.
[0091] Sample number 6-2 in TABLE 18 and TABLE 19 were heat-treated
under different two kinds of oxidative atmospheres in the firing
profile shown in FIG. 2.
[0092] Accordingly, the samples were distinguished with each other
by attaching sub-numbers as 6-2-1 and 6-2-2.
[0093] TABLE 19 shows that the samples having the compositions
within the range of the present invention have resistivities at
room temperature (.rho..sub.25) of 3.5 .OMEGA..multidot.cm or less,
static withstanding voltage of 50 V/mm or more and temperature
resistance coefficients (.alpha.) of 9.7%/.degree. C. or less,
enabling to obtain the PTC resistors having small dispersion of
resistivities (CV %).
[0094] Looking at the resistivities at room temperature
(.rho..sub.25), it is clear that the samples having the
compositions within the range of the present invention in TABLE 19
have smaller values as compared with the samples having the
compositions within the range of the present invention in TABLE 15,
or as compared with the samples having the compositions within the
range of the present invention in TABLE 17. This is because the
effects of firing in the reducing atmosphere and heat-treatment in
the oxidative atmosphere have been displayed to increase the
concentration of conductive electrons in the grains due to such
firing and heat-treatment steps.
[0095] As shown by the comparison of the sample 6-2-1 with the
sample 6-2-2, an oxygen concentration within the range of 20 to
100% in the heat-treatment step in accordance with the firing
profile shown in FIG. 2 has substantially no effect on the
characteristics of the PTC thermistors.
[0096] Comparing the samples having the compositions within the
range of the present invention with the samples having the
compositions out of the range of the present invention in TABLE 19,
it can be seen that the same tendency obtained by comparison among
these two kinds of samples in TABLE 15 are also displayed in TABLE
19.
[0097] According to the present invention as hitherto described, Sm
is used for the semiconductor forming agent contained in a
prescribed quantity in the composite material for use in the barium
titanate based PTC thermistors. Accordingly, resistivity at room
temperature of the ceramic for the PTC thermistor obtained by
firing the composite material described above can be lowered, for
example, to 5 .OMEGA..multidot.cm or less, the static withstanding
voltage can be increased, for example, to 50 to 60 V/mm or more,
and the temperature resistance coefficient can be increased, for
example, to 9%/.degree. C., along with reducing dispersion of the
resistance. Therefore, the PTC thermistor to be used for protecting
electric circuits can be manufactured with excellent
productivity.
[0098] Firing the composite material for the PTC thermistor
according to the present invention in a neutral atmosphere followed
by heat-treating in an oxidative atmosphere allows the ceramic for
the PTC thermistor to have a lower resistivity at room
temperature.
[0099] When a reducing atmosphere is used in the foregoing firing
step instead of the neutral atmosphere, the resistivity at room
temperature can be reduced.
* * * * *