JPH0471177B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Fields The present invention is applicable to combustion equipment such as gas and kerosene stoves, boilers, automobile engines, etc., to reduce overheating, oxygen deficiency, and air/fuel ratio (A/F) (including regions other than equivalent composition). It relates to a multifunctional sensor that can perform detection with a single sensor. Conventional Structure and Problems Conventionally, in order to detect a power out or overheating, a thermistor was provided for each object to be detected, and the state was detected from the change in resistance of the thermistor. The need to provide individual sensors was due to the lack of a stable material that could measure with high sensitivity over a wide temperature range, from detecting a state near room temperature, such as turning off, to around 1,000 degrees Celsius, indicating overheating. In addition, even if the same operation is performed, such as detecting such a state and closing the gas valve, the output format from the sensor is different, so it is necessary to provide an individual electrical circuit for each sensor. It was hot. To detect A/F, which corresponds to an oxygen-deficient state or the equivalent composition of combustion, a stabilized or partially stabilized zirconia solid electrolyte and Pt electrodes are attached on both sides, and one electrode is used to maintain a constant oxygen partial pressure like air. (Po ₂ =
0.21 atm) and the other to exhaust gas to form an oxygen concentration cell, and utilize the fact that the generated superpower changes greatly depending on _the equivalent composition of _combustion . _{, MgCo2O4}
Sensors are used that take advantage of the fact that the electrical resistance of the fuel changes greatly depending on the combustion equivalent composition. However, of course, these cannot simultaneously detect turning off or overheating, and detection of A/Fs that cause sudden changes in superpower or resistance is limited to equivalent compositions.
In order to sharpen the degree of change in the equivalent composition of these sensors, the catalytic action of noble metals such as Pd and Pt is required, and for this reason, sensors that utilize the resistance change of oxides contain these noble metals in the sensor substrate. was added as a catalyst. Therefore, it had the disadvantage of being expensive. The inventors first investigated the oxygen deficiency state and equivalent composition of A/F.
When an electron-oxygen-ion mixed conductor consisting of Sr _1+X/2 La _1-X/2 Co _1-x Fe _x O ₃ is used to detect Not only is there no oxygen, but in a state of excess oxygen.
It has a low resistance of 10 ^-4 S/cm ² , and the resistance increases when there is an excess of reducing gas (contrary to the behavior of SnO ₂ and TiO ₂ sensors), so it is fail-safe against disconnection, and the sensor itself It was revealed that it is possible to provide a sensor that has the advantage of allowing direct control without a circuit by passing a current through the sensor (Japanese Patent Application Laid-open No. 103041/1983).
In addition, by hybridizing an oxygen pump to this sensor, it is possible to move from the situation where only A/Fs with equivalent composition had sudden changes in output to any A/F by changing the current flowing through the pump. Become. In addition, by adding SrTiO ₃ to the sensor base material, it is possible to match the thermal expansion with the electrode metal material, zirconia electrolyte material, and thin film substrate material, achieving a long life _. La _1-X/2 Co _1-x Fe _x O ₃ grain boundaries are formed and
By increasing the O ^2- ion conductivity and increasing the sensitivity and responsiveness of Cean, we are able to exhibit a high semiconductor-like temperature dependence around room temperature and a metallic temperature dependence around 1000℃.
We have succeeded in obtaining a sensor that can detect both turning off and overheating. However, the sensor substrate material Sr _1+X/2
The grain boundaries formed by mixing SrTiO ₃ with La _1-X/2 Co _1-x Fe _x O ₃ were naturally generated during firing of the sensor substrate. Therefore, the grain boundaries formed in the sensor substrate material are very irregular;
There is also a limit to the improvement in sensor sensitivity and response due to an increase in ionic conductivity, and in particular, the difference in sensitivity and response at temperatures below 300°C caused by the addition of SrTiO ₃ was not noticeable. OBJECTS OF THE INVENTION It is an object of the present invention to provide a sensor that has a long life, has high sensor sensitivity and responsiveness even in a low temperature range, and can detect turning off and overheating. Structure of the Invention The present invention uses sensor base material Sr _1+X/2 La _1-X/2 Co _1-x
By layering Fe _x O ₃ and SrTiO ₃ by forming thin films on each other using a sputtering method, the grain boundaries inside the base material can be controlled and the sensitivity of the sensor can be improved over a wide temperature range from around 200°C to around 1000°C. This sensor is capable of detecting a power outage, overheating, oxygen deficiency state, and air/fuel ratio with high responsiveness. DESCRIPTION OF EMBODIMENTS FIG. 1 is a diagram showing the basic structure of the sputtered film lamination method of the present invention, where A is a top view and B is a partially cutaway side view taken along line X-X'. , same figure C
is a partially cutaway side view taken along line Y-Y'. In the figure, 1 is a substrate made of alumina salamic, and 2 is a base material Sr _1+X/2 La _1-X/2 Co _1-x Fe _x O _3.
A sputtered laminated film with SrTiO ₃ , 3 is an electrode plate forming the base material 2, 3' is an electrode printed and baked on the base material 2, and 4 is a stabilized zirconia solid sprayed thereon. Electrolytes 5 and 5' are printed and baked onto the stabilized zirconia 4 and are of the same material as electrodes 3 and 3', and electrodes 6 and 6' are the leads to electrodes 3 and 3', respectively. Made of the same material. 7 is a solenoid for attracting and keeping the gas safety valve open, and 8 is a current for flowing current through the sensor 2 and solenoid 7, which may be direct current or alternating current. 9 is a constant current DC power supply for supplying current to the oxygen pump. These are circuit elements when the sensor 2 is used in a gas safety valve. In Figure 1, if only the A/F and detection of the equivalent composition of combustion are required, the zirconia electrolyte and the electrodes 5, 5'
And the power source 9 is of course unnecessary. Typical aspects of the sensor of the present invention have been described above, and next, specific examples based on these aspects will be described. Example 1 Electrode 3 with Sr _0.65 La _0.35 Co _0.7 Fe _0.3 O ₃ shown in Fig. 1
to form a layer of approximately 100 microns. A thin film layer of SrTiO ₃ of about 100 angstroms is then formed on top of this layer by sputtering. Then again
A thin film layer of about 100 angstroms of Sr _0.65 La _0.35 Co _0.7 Fe _0.3 O ₃ is formed by sputtering. This process is repeated 10 times, and then Sr _0.65 La _0.35 Co _0.7 Fe _0.3 O ₃ is sprayed to form a layer of approximately 100 microns. Finally, the electrodes shown in FIG. 1, 3' were printed on this upper surface to produce a sensor base having the laminated structure shown in FIG. 2. 4 in Figure 2
and 14 are approximately 100 micron sprayed films of Sr _0.65 La _0.35 Co _0.7 Fe _0.3 O ₃ , and 6, 8, 10, and 12 are approximately 100 micron sprayed films of the same material.
It is a 100 angstrom sputtered film. Further, numerals 5, 7, 9, 11, and 13 are sputtered films of SrTiO ₃ with a thickness of about 100 angstroms. The electrodes 3 and 3' are made of an alloy of the same material, 30% Pt-70% Pd.
Thereafter, Pd paste was applied to the lead having a thickness of 0.2 mm as shown in Fig. 1 and burned onto the electrodes 3 and 3'. The amount of Pd paste applied was limited to the amount necessary to bond the lead parts. Figure 3 shows the resistance change of this sensor against CO gas. The resistance change is 3 in Figure 1 C.
and 3'. In Figure 3, the solid line a
is the resistance at each temperature in air,
The solid line b shows the resistance at each temperature 5 seconds after starting to flow 10 ppm CO gas (remaining N ₂ ).
Similarly, the solid line c shows the resistance 10 seconds after the gas started flowing, and the solid line d shows the resistance 20 seconds after starting to flow the gas. Table 1 also shows the time required for this sensor to reach 90% of the steady resistance after the CO gas is introduced, and the patent application for this sensor
-120529 for comparison with sample number 1-5 sensor of Example 1. In this table, the diagonal line from 150 degrees to 300 degrees for the front sensor means that the front sensor is not capable of detecting CO gas at that temperature.

[Table] As is clear from Figure 3 and Table 1, when laminated by the sputtering method, the time required to detect CO gas is reduced and the detection ability is achieved even at lower temperatures. Example 2 Although Sr _0.75 La _0.25 Co _0.5 Fe _0.5 O ₃ was used as the substrate material and the structure was otherwise exactly the same as in Example 1, the resistance change against CO gas is shown in FIG. 4. As in Figure 3, the solid line a' shows the resistance at each temperature in air, and the solid line b' shows the resistance at 10 ppm.
This shows the resistance at each temperature 5 seconds after starting to flow CO gas (remaining N ₂ ), solid line c' shows the resistance 10 seconds after starting to flow the above gas, and solid line d' shows the resistance 20 seconds after starting to flow the above gas. It is. Table 2 also shows the above CO as in Table 1.
This shows the time it takes to reach 90% of the steady resistance after gas is introduced.

[Table] Effects of the Invention As described above, the present invention increases the sensor response by laminating the sensor base using the sputtering method, making it possible to detect oxygen deficient conditions and A/F even in low temperature regions. produce an effect.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a gas sensor structure according to an embodiment of the present invention, FIG. 2 is a partially cutaway top view of the sensor base,
3 and 4 are diagrams showing the gas sensitivity of the resistance of the sensor base. 1...Sensor holding board, 2...Sensor base, 3
...Electrode plate, 4,5,5'...Stabilized zirconia electrolyte, 6,6'...Lead, 7...Solenoid,
8, 9...Power supply.

Claims (1)

[Claims] 1 Chemical formula Sr _1+X/2 La _1-X/2 Co _1-x Fe _x O ₃ (0<x
0.3) A substrate made by alternately laminating sputtered layers of a sintered body of a substance represented by 0.3) and a sintered body of SrTiO ₃ , and at least two electrodes are provided on the substrate,
A gas sensor characterized in that the change in electrical resistance between the electrodes is measured. 2. The gas sensor according to claim 1, wherein the electrode is an alloy of at least two of Pt, Pd, and Ag. 3. The gas sensor according to claim 1, wherein the substrate has stabilized zirconia and a third electrode sequentially applied to its surface.