Compositions – Electrically conductive or emissive compositions – Metal compound containing
Reexamination Certificate
1999-08-25
2001-07-17
Gupta, Yogendra (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Metal compound containing
C252S500000, C252S519100, C252S519200, C252S518100, C252S520200, C252S06230Q, C252S521100, C338S0220SD, C428S416000
Reexamination Certificate
active
06261480
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thermistor element which can detect a temperature ranging from room temperature to high temperature of about 1000° C., i.e. so-called wide-range type thermistor element, and the thermistor element is particularly suitable for use in a temperature sensor for an automobile exhaust gas.
2. Description of the Related Art
A thermistor element for a temperature sensor is used in the measurement of a temperature ranging from moderate to high temperature (e.g. 400 to 1300° C., etc.) such as temperature of an automobile exhaust gas, gas flame temperature of gas hot-water supply device, temperature of a heating oven, etc.
Characteristics of this kind of a thermistor element are indicated by the resistivity and resistivity temperature coefficient (temperature dependence of the resistivity). In order to cope with a practical resistivity range of a temperature detecting circuit constituting the temperature sensor, it is desired that the resistivity of the thermistor element is within a predetermined range. Therefore, perovskite materials are exclusively used as those having resistivity characteristics suitable for a wide-range type thermistor element.
As the thermistor element using perovskite material, for example, those described in Japanese Patent Kokai Publication Nos. Hei 6-325907 and Hei 7-201528 are suggested. These thermistor elements are produced by mixing oxides of Y, Sr, Cr, Fe, Ti, etc. in a predetermined composition proportion and calcining the mixture to form a perfect solid solution in order to realize a thermistor which can used in a wide temperature range.
The resistivity characteristics of the wide-range type thermistor element are indicated by the resistivity and resistivity temperature coefficient. In a normal temperature sensor, it is necessary that the resistivity of the thermistor element is from 50 to 300 k&OHgr; within a working temperature range in view of the resistivity range of the temperature detecting circuit. In case of affording a heat history from room temperature to 1000° C. to the thermistor element, the smaller a change between the resistivity after heat history and the initial resistivity, the better.
In the above Japanese Patent Publications, various thermistor elements of a perfect solid solution are suggested, but only data of the thermistor element resistivity at 300° C. or more are disclosed. Therefore, the present inventors have examined the resistivity characteristics at about room temperature of various thermistor elements in the above Japanese Patent Publications.
As a result, regarding those having a resistivity stability in the heat history from room temperature to 1000° C., the resistivity becomes higher in the temperature range from room temperature to 300° C. Therefore, it is impossible to discriminate it from insulation and the temperature can not be detected. On the other hand, regarding those satisfying low resistivity of 50 to 300 k&OHgr; the resistivity changes by 10% or more relative to the initial resistivity in the heat history. It has been found that the stability is poor.
There has never been obtained a thermistor element which can satisfy two resistivity characteristics which are contrary to each other, i.e. low resistivity characteristics within a range from room temperature to high temperature of 1000° C. and resistivity stability in the heat history (so-called wide-range type thermistor element).
In the light of the above problems, an object of the present invention is to provide a thermistor element which has stable characteristics (i.e. small change in resistivity in the heat history from room temperature to 1000° C.) and has a resistivity of 50 to 300 k&OHgr; within the temperature range from room temperature to 1000° C.
SUMMARY OF THE INVENTION
First Aspect
(A) In the first aspect of the present invention for accomplishing the above object, the present inventors have considered that a conventional thermistor element is composed of a perfect solid solution having a perovskite type structure but it is difficult for a perfect solid solution as a single compound to satisfy the above resistivity characteristics which are liable to be contrary to each other.
Thus, the above object has been accomplished by using a novel thermistor material composed of a mixed sintered body prepared by mixing two compounds, i.e. a perovskite material (oxide) having a comparatively low resistivity and a material having a comparatively high resistivity in place of the perfect solid solution.
The present inventors have tested and studied various perovskite materials. As a result, it has been found that a composition M
1
M
2
O
3
(M
1
is at least one element selected from the elements of the groups II and IIIA excluding La in the Periodic Table, and M
2
is at least one element selected from the elements of the groups IIB, IIIB, IVA, VA, VIA, VIIA and VIII) is preferable as a material having resistivity characteristics which are suitable for accomplishing the above object.
Since La has high moisture absorption property, there is a problem that La reacts with water in the air to form an unstable hydroxide, which results in breakage of the thermistor element. Therefore, La is not used as M
2
.
On the other hand, it has been decided that Y
2
O
3
(yttrium oxide), which has a comparatively high resistivity and stabilizes resistivity of the thermistor material, is used as another material to be mixed, as a result of the study.
By preparing a mixed sintered body from M
1
M
2
O
3
and Y
2
O
3
, a thermistor element of a mixed sintered body M
1
M
2
O
3
.Y
2
O
3
. The term “mixed sintered body” used herein means a sintered body wherein grains constituting the sintered body comprise a mixture of grains of a first component M
1
M
2
O
3
and grains of a second component Y
2
O
3
.
1) That is, this mixed sintered body is a mixed sintered body M
1
M
2
O
3
.Y
2
O
3
of the above M
1
M
2
O
3
and Y
2
O
3
, wherein M
1
is at least one element selected from the elements of the groups IIA and IIIA excluding La in the Periodic Table, and M
2
is at least one element selected from the elements of the groups IIB, IIIB, IVA, VA, VIA, VIIA and VIII in the composition M
1
M
2
O
3
. More specifically, it can also be represented as aM
1
M
2
O
3
.bY
2
O
3
.
This thermistor element was incorporated into a temperature sensor and the resistivity characteristics of the element were examined. As a result, it could be confirmed that it is stable, that is, a change in resistivity is small (e.g. few %, etc.) even in the heat history from room temperature to 1000° C. and the resistivity is from 50 to 300 k&OHgr; within the temperature range from room temperature to 1000° C.
Therefore, according to this invention, it is possible to provide a thermistor element which can detect a temperature ranging from room temperature to high temperature of 1000° C. and has stable characteristics, that is, a change in resistivity is small even in the heat history from room temperature to 1000° C., so-called wide-range type thermistor element.
2) As a result of the study of the present inventors, regarding each element in the above perovskite compound M
1
M
2
O
3
, M
1
is preferably at least one element selected from Y, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, Mg, Ca, Sr, Ba and Sc, and M
2
is preferably at least one element selected from Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Al, Ga, Zr, Nb, Mo, Hf, Ta and W, in view of the practical use.
3) As a result of a further study about a mixing ratio of M
1
M
2
O
3
and Y
2
O
3
, it has been found that the effect of the present invention can be accomplished more certainly if the mixing ratio is within a predetermined range, that is, when a molar fraction of the above M
1
M
2
O
3
is a and b is a molar fraction of the above Y
2
O
3
, these molar fraction a and b satisfy the relations 0.05≦a<1.0, 0<b≦0.95 and a+b=1 in the composition formula aM
1
M
2
O
3
.bY
2
O
3
.
Since the molar fractions can be changed within a wide range in such way, the resistivity
Kataoka Takumi
Kuzuoka Kaoru
Ogata Itsuhei
Yamada Masanori
Yasuda Eturo
Denso Corporation
Gupta Yogendra
Hamlin Derrick
Pillsbury Madison & Sutro LLP
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