Compositions – Barrier layer device compositions – Organic
Reexamination Certificate
2001-08-09
2003-12-16
Gupta, Yogendra N. (Department: 1751)
Compositions
Barrier layer device compositions
Organic
C252S518100, C252S520200, C252S520500, C338S0220SD, C338S0220SD, C029S610100, C029S612000, C264S614000, C264S618000, C423S605000, C423S606000
Reexamination Certificate
active
06663794
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a reducing-atmosphere-resistant thermistor element capable of accurately detecting a temperature in a broad temperature range and having stable characteristics even in a reducing atmosphere, and a production method of such a thermistor element. The present invention is particularly suitable for use in temperature sensors for automobile exhaust gases.
2. Description of the Related Art
Thermistor elements for temperature sensors have been used for measuring temperatures in medium and high temperature ranges of about 400 to 1,300° C. such as the temperatures of automobile exhaust gases, the gas flame temperatures of gas hot water supply systems, the temperatures of heating furnaces, and so forth. The characteristics of the thermistor elements of this kind are generally represented by a resistance value and a resistance-temperature coefficient (temperature dependence of the resistance value). To provide a practical resistance value to a temperature detection circuit constituting the temperature sensor, the resistance value of the thermistor element must remain within a predetermined range (100&OHgr; to 100 k&OHgr; within an ordinary use temperature range, for example). Perovskite type composite oxide materials have been used mainly as the materials having resistance value characteristics suitable for the thermistor element.
Japanese Unexamined Patent Publication (Kokai) No. 6-325907, for example, discloses a thermistor element using peroviskite type materials. To provide a thermistor element that can be used over a broad temperature range, this reference teaches to mix oxides of Y, Sr, Cr, Fe, Ti, etc, in a predetermined composition ratio and to bake the mixture to obtain a thermistor element that is in a completely solid-solution state. The reference describes that this thermistor exhibits stable characteristics in a high temperature range.
In temperature sensors for automobile exhaust gases, a metal case covers a thermistor element, as a sensing element, at the distal end of the temperature sensor to prevent deposition of dust and soot from the exhaust gases. When the exhaust gas reaches a high temperature of around 900° C., however, the metal case is thermally oxidized by the heat of the high temperature exhaust gas, and the internal atmosphere of the metal case is likely to change to a reducing atmosphere. In consequence, a problem develops in that oxides constituting the thermistor element are reduced and the resistance value changes.
To cope with this problem, the temperature sensor is generally placed into an electric furnace and is subjected to thermal aging treatment at 900 to 1,000° C. for about 100 hours to stabilize the resistance value. However, if any hole appears in the metal case or the exhaust gas enters the case due to loosening of the case, during use, of the temperature and the thermistor element is exposed to the reducing atmosphere, the resistance value is likely to change as described above. In recent engine control systems, the temperature sensor is often mounted at a position closer to the engine that generates the high-temperature exhaust gas. Therefore, an exhaust gas having a high temperature (e.g. 1,100 to 1,200° C.) reaches the temperature sensor. During the thermal aging treatment at 900 to 1,000° C., there is the possibility that the metal case is re-oxidized depending on the operation mode of the engine, the thermistor element is again reduced, and the resistance value changes. In other words, a thermal aging treatment cannot completely solve the problem. In addition, the number of production steps increases and invites an increase in the cost of production of in temperature sensor.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 9-69417 describes a technology that shapes a metal case by machining a specific metal material such as an alloy consisting of Ni—Cr—Fe as a principal component, suppresses the change of the atmosphere inside the case and minimizes the resistance value change of a thermistor element. However, when the metal case is made of the specific metal material, the material cost and the machining cost increase. Further, the problem of the resistance change when the thermistor element itself is exposed to the reducing atmosphere remains to be solved.
As described above, a thermistor element that exhibits stable resistance value characteristics even under the condition where the internal atmosphere of the metal case of the temperature sensor becomes a reducing atmosphere is not available.
SUMMARY OF THE INVENTION
In view of the problems described above, the present invention aims at economically providing a reduction-resistant thermistor element the resistance value of which does not greatly change even when the thermistor element is exposed to a reducing atmosphere, and which has high accuracy and excellent resistance value stability.
To solve the problems, the inventors of the present invention have conducted intensive studies and have discovered that when an oxygen occlusion-release composition is added to, and dispersed in, a composition constituting a thermistor element so as to provide the thermistor element with a function of occluding and releasing oxygen in accordance with an environment, migration of oxygen from the thermistor element can be limited and the resistance change can be suppressed in the reducing atmosphere.
The invention according to claim 1 is completed on basis of the finding described above. The thermistor element of this invention is characterized by a construction which consists of a metal oxide sintered body as a principal component, and in which an oxygen occlusion-release composition having oxygen occlusion-release characteristics is dispersed in the metal oxide sintered body.
As the thermistor element according to the present invention has a construction in which the oxygen occlusion-release composition is dispersed in the metal oxide sintered body having thermistor characteristics. Therefore, when the internal atmosphere of the metal case becomes a reducing atmosphere, the oxygen occlusion-release composition emits absorbed oxygen and prevents migration of oxygen from the metal oxide sintered body. In consequence, the thermistor element can prevent fluctuation of the composition resulting from the reduction of the metal oxide sintered body and the resistance change resulting from the former, and can improve resistance value stability.
As described above, the thermistor element according to the present invention has reducing-atmosphere-resistance and its resistance value does not greatly change even when it is exposed to the reducing atmosphere. Therefore, the thermistor element can accurately detect the temperature for a long time, and can provide a temperature sensor having high reliability. Since the metal case need not be made of a specific metal material and a thermal aging treatment is not necessary, either, the production cost can be reduced.
According to a second aspect of the present invention, there is provided a reducing-atmosphere-resistant thermistor element wherein the oxygen occlusion-release composition is an oxide containing at least one metal oxide selected from the group consisting of Ce, Pr and Tb.
According to a third aspect of the present invention, there is provided a occlusion-release thermistor element, as described above, wherein the oxygen occlusion-release composition is at least one oxide selected from the group consisting of CeO
2
, Pr
6
O
11
, Tb
4
O
7
, 2CeO
2
.Y
2
O
3
and CeO
2
.ZrO
2
. Since these oxides have the function of occluding and releasing oxygen in accordance with the environment, the reducing-atmosphere-resistance of the thermistor element can be drastically improved when these oxides are added and dispersed.
According to a fourth aspect of the present invention, there is provided a reducing-atmosphere-resistant thermistor element, wherein a starting material of the oxygen occlusion-release composition is ultra-fine particles having a mean particle diameter
Kurano Atsushi
Kuzuoka Kaoru
Makino Daisuke
Ogata Itsuhei
Gupta Yogendra N.
Harness Dickey & Pierce PLC
Nippon Soken Inc.
Vijayakumar Kallambella
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