JPH01102355A - Sensor for controlling combustion - Google Patents

Abstract

PURPOSE:To measure the concn. of oxygen in combustion exhaust gas with good accuracy over a long period of time, by interposing an intermediate layer between a solid electrolyte and an electrode. CONSTITUTION:A pair of electrodes 2, 4, at least an electrode 4 working as a cathode, provided on a substrate composed of an oxygen ion conductive solid electrolyte 1 is composed of perovskite type composite oxide represented by predetermined formula (wherein Ln is at least one element selected from La, Ce, Pr and Nd, A is at least one element selected from Sr, Ca and Ba, Me is at least one element selected from Ni, Fe, Mn, Cr and V, 0<=x<=1, 0<=y<=1 and delta is an oxygen deficient amount). Electrode lead-out terminals 5, 6 are provided to the electrodes 2, 4 and a gas diffusion layer 7 is provided on the cathode 4 and, further, the outer peripheral side surface of the structure consisting of the electrodes 2, 4, the electrolyte 1 and the diffusion layer 7 is brought to a gas impervious state. Furthermore, an intermediate layer 3 composed of an oxygen ion conductor non-reactive with both of the electrolyte 1 and the electrodes 2, 4 is interposed. By this method, the concn. of oxygen in combustion exhaust gas can be measured with good accuracy over a long period of time.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention detects the air/fuel ratio based on the residual oxygen concentration in a gas to be measured such as combustion exhaust gas, and is used to maintain a proper combustion state. This invention relates to a combustion control sensor to be used.

BACKGROUND OF THE INVENTION Conventionally, this type of sensor has used stabilized zirconia as an oxygen ion conductive solid electrolyte, platinum as an anode and a cathode, and a gas diffusion layer provided on the cathode. In this sensor, oxygen ions move in a solid electrolyte by a voltage applied between two electrodes,
This can be extracted as current. Since this movement of oxygen ions is ultimately rate-limited by the gas diffusion layer provided on the cathode, the output current increases to a certain value and then becomes saturated. This saturation current value indicates a value that corresponds to the oxygen concentration in the atmosphere, so by measuring the current value, it is possible to know the oxygen concentration in the exhaust gas, and therefore combustion can be controlled to achieve an appropriate air-fuel ratio. becomes possible.

On the other hand, the inventors used the general formula L n 1-, A x Co 1-y M e y instead of platinum as the electrode material.
We have proposed a combustion control sensor using a perovskite complex oxide represented by O3-δ. In the case of platinum electrodes, polarization is large because the electrode reaction rate is low, and the potential of the electrode itself becomes unstable, making it difficult to apply a constant potential to the other electrode. To improve this point, it is necessary to increase the surface area, but platinum is prone to sintering at high temperatures, so it is extremely difficult to create a porous electrode that is homogeneous and has long-term stability. . On the other hand, when the perovskite type composite oxide is used as an electrode material, it has a high catalytic activity for the oxygen redox reaction, and thus provides a stable electrode potential with extremely low polarization during the electrode reaction. As a result, a constant potential is constantly applied to the other pole,
Excellent sensor characteristics with small variations can be obtained. Also,
It is thermally stable and can maintain excellent properties over a long period of time.

Problems to be Solved by the Invention When measuring the oxygen concentration in combustion exhaust gas, it is common to install a sensor directly in the exhaust gas passage near the combustion section. Therefore, depending on combustion conditions or due to abnormal combustion, the sensor element may be exposed to extremely high temperatures.

A major factor that influences the characteristics of the sensor is the characteristics of the contact interface between the electrode and the solid electrolyte. As mentioned above, perovskite-type composite oxide electrodes exhibit excellent electrode properties and are thermally stable, but at temperatures above 950-100°C, they react with the solid electrolyte and become substances with different crystal structures. It may be deposited at the interface between the electrode and solid electrolyte.

As a result, ionic conductivity decreases, polarization increases during electrode reactions, and stability of electrode potential is lost. Therefore, there is a risk that sensor characteristics may deteriorate or variations may become large, resulting in poor reliability.

Means to Solve the Problem This book was developed by focusing on the above-mentioned problem.In order to prevent the reaction between the solid electrolyte and the electrode, a solid electrolyte and an electrode are placed between the solid electrolyte and the electrode. An intermediate layer made of a thermally stable oxygen ion conductor that is non-reactive with respect to both of the two is interposed.

Function: By interposing an intermediate layer between the solid electrolyte and the electrode, which is made of a thermally stable oxygen ion conductor that is non-reactive with respect to both the solid electrolyte and the electrode, the solid electrolyte and the electrode are interposed. Can prevent electrode reactions. Further, since the intermediate layer itself has an oxygen ion conductor and does not react with both the solid electrolyte and the electrode, it does not interfere with oxygen ion conduction. As a result, the excellent catalytic action of the perovskite-type composite oxide against oxygen redox and the polarization characteristics during electrode reactions based on this are maintained stably, resulting in uniform and stable sensor characteristics.

Embodiment FIG. 1 is a schematic sectional view showing an embodiment of a sensor element according to the present invention. 1 is Bmo1%Y2o3-92mo1
% Z r 02 oxygen ion conductive solid electrolyte plate (5,6φX1tm), 2 is an anode (3μmt) formed by attaching platinum paste by screen printing, 3
is an intermediate layer formed by flame spraying an oxide represented by the chemical formula La0.9Ba0.1AIO3,...
1o/jmt), 4 is a cathode (20 μmt) formed by attaching perovskite type composite oxide by flame spraying, 6 is an anode lead terminal, 6 is a cathode lead terminal, 7 is an inorganic porous Gas diffusion layer (100 μmt), 8 is a glassy gas impermeable seal. For comparison, a sensor element without an intermediate layer as in No. 3 and a sensor element without an intermediate layer and having a cathode with a platinum paste deposited by screen printing were fabricated.

The sensor fabricated as described above was subjected to an operating characteristic test. FIG. 2 shows the measurement results of the output characteristics of the sensor.

The measurements were carried out as follows.

A sensor element is installed in an electric furnace, the temperature is controlled to a predetermined element temperature, and an oxygen-nitrogen mixed gas of a predetermined concentration is supplied at approximately 1ocIR/! Flow contact was carried out at a flow rate of ieC.

At this time, the output current with respect to the applied voltage was measured, and the output current when a constant voltage was applied was determined for each oxygen concentration. In Figure 2, as an example, the temperature is 800°C and the voltage is 1.
The case of v is shown.

In addition, measurements were performed on 10 elements each in both the example and the conventional example. Sensors using perovskite-type composite oxide cathodes show uniform output characteristics with small variations in characteristics regardless of the presence or absence of an intermediate layer, but sensors using conventional platinum cathodes show uniform output characteristics. The larger the size, and the higher the oxygen concentration, the more conspicuous it was. Perovskite-type composite oxides have high catalytic activity for acid reduction and a high reaction rate in electrode reactions, resulting in extremely small polarization and an electrode that exhibits a nearly constant potential. Therefore, in constant voltage driving, a constant potential is applied to the other electrode, and as a result, the flowing current accurately corresponds to the oxygen concentration. Because the sensor has such excellent electrode characteristics, differences in the fine structure of the electrodes of individual sensors have almost no effect on the output characteristics, and therefore characteristic variations are small and oxygen concentration can be detected with high precision and responsiveness. Even in the case of a sensor with an intervening intermediate layer, the intermediate layer itself functions as an oxygen ion conductor, so there is no loss in electrode properties; therefore, the sensor using a perovskite composite oxide cathode has excellent It shows its characteristics. On the other hand, with a platinum cathode, the reaction rate is low and polarization is large during the electrode reaction, so the potential applied to the other electrode is unstable and the flowing current does not accurately indicate the oxygen concentration. Therefore, slight differences in electrode porosity, surface area, etc. appear as variations in characteristics. Uniform control of microstructure is extremely difficult;
This is a major obstacle to manufacturing yield and ensuring constant quality. In addition, the measurements were carried out at different temperatures in the range of 600 to 900°C, and in each case the same results as in the case of 700°C were obtained.

Next, the thermal chamber constancy of sensor characteristics will be explained. The evaluation was performed as follows. After the sensor element was subjected to a cycle of holding at 800° C. for 1 hour and holding at 1000° C. for 10 minutes in the air, 60 times in succession, the same measurements as above were carried out using soo'c, and the output characteristics were compared. The results are shown in Figures 3a, b and c. The output characteristics of the sensor using the intermediate layer remain almost unchanged as shown in FIG. 3a.

In the case of a sensor that does not use an intermediate layer, as shown in FIG. 3, the output characteristics change compared to the initial state, and the variation becomes large. In the case of the sensor using platinum, a change in the characteristics as shown in FIG. 3C was observed, but it was milder than the characteristics of the element shown in FIG. 3S. As a result of analyzing the interface between the solid electrolyte and the cathode of a sensor element that does not use an intermediate layer,
The presence of a substance represented by the chemical formula S r 2 Z r O4 was observed.

This is an insulator, and it is thought that the output characteristics of the sensor changed as a result of the polarization characteristics becoming worse due to the production of this substance. Furthermore, when platinum is used, sintering progresses gradually in a high-temperature atmosphere, and as the microstructure of the electrode changes and the surface area decreases, the catalytic activity decreases and the output characteristics change. On the other hand, in the case of a sensor with an intermediate layer, no substances with different crystal structures were observed at the interface between the solid electrolyte and the intermediate layer and between the cathode and the intermediate layer, and the initial structure remained unchanged. Since it is well maintained, there is almost no change in the electrode characteristics. As a result, it is thought that the output characteristics of the sensor were maintained stably.

As is clear from the above examples, the combustion control sensor according to the present invention exhibits extremely excellent characteristics. In the example, the case where only the cathode was formed of a perovskite type composite oxide was described.

In the case of a sensor in which both the anode and the anode are formed of a perovskite-type composite oxide and an intermediate layer is also interposed between the solid electrolyte and the anode, the perovskite-type composite oxide exhibits excellent activity in redox catalytic reactions, In addition, because the intermediate layer is an excellent oxygen ion conductor, the variation in characteristics between individual sensors is smaller than when only the cathode is made of perovskite-type composite oxide, and output with excellent linearity is achieved. Show characteristics. In addition, in the example, Ln is La and A is Sr.
, using Fe as Ms, and x = 0.65.7=
The case where Ln becomes 0.3 was shown, but when Ln is Ce,
In the case of Pr, Nd, or in the case of La, Go, Pr, NdO being an element of two or more inner plate types, in the case of A being Ca, Ba, or in the case of A being an element of more than the inner plate type of Sr, Ca, Ba,
When Me is Ni, Mn, Cr, V or Ni, Fe
Similar results were obtained when two or more of , Mn, Ct, and V were zero elements, or when other composition ratios were used. Furthermore, S r M e ' 03 (M e ' is Ti.

When at least one element selected from Zr and Hf is mixed, when a platinum group element is added, the effect of increasing the oxygen redox catalytic activity is exhibited without impairing the uniformity of electrode characteristics. In addition, 8 m
Although o1%Y2o3-92mo1%Zr02 was used, it is not limited to this as long as it has a similar function. The intermediate layer material is not limited to the examples as long as it has the same function. Further, the gas diffusion layer is not limited to a porous material, and may be provided with diffusion holes, and the material may be any material as long as it is non-reactive with the cathode material, lead material, etc. On the other hand, the form of the sensor is not limited to the layered flat plate type, and may take any form as long as it does not go against the spirit of the invention. Furthermore, the methods for producing electrodes, gas diffusion layers, and the like are not limited to those in the examples, but may include sintering, sputtering, printing, coating pyrolysis, and other methods, as well as combinations thereof.

Effects of the Invention As described above, the combustion control sensor of the present invention exhibits extremely stable characteristics, so it is possible to accurately measure the oxygen concentration in the combustion exhaust gas over a long period of time, and to control the combustion to an appropriate state. It is something.

[Brief explanation of the drawing]

FIG. 1 is a schematic sectional view showing one embodiment of the combustion control sensor according to the present invention, FIG. 2 is an output characteristic diagram of the sensor element, and FIG. FIG. 3 is a diagram showing the thermal stability of each sensor characteristic of a conventional example (without intermediate layer) and a conventional example (platinum cathode). 1...Oxygen ion conductive solid electrolyte, 2...
... Anode, 3... Intermediate layer, 4... Cathode, 6... Anode lead terminal, 6... Cathode lead terminal, 7... ...Porous gas diffusion layer, 8...
...Gas impermeable seal. Name of agent: Patent attorney Toshio Nakao and 1 other person
21! 1 Ken He Me-Nong & (Zo J Ward -) 1 Break Shushin 1 c'1) ε Bowl Kai - Yin
in;

Claims (3)

[Claims] (1) Among a pair of electrodes provided on a substrate made of an oxygen ion conductive solid electrolyte (hereinafter referred to as solid electrolyte), at least the electrode serving as a cathode has the general formula Ln_1_-_xA_xCo_1_-_yMe_yO_
3_-_δ (Ln is at least one element selected from La, Ce, Pr, and Nd; A is at least one element selected from SrCa and Ba; Me is Ni, Fe, Mn, Cr, and V
At least one element selected from 0≦x≦1, 0≦y≦
1, δ is the amount of oxygen vacancies], each of the pair of electrodes is provided with an electrode lead terminal, a gas diffusion layer is provided on the cathode, and the electrode, solid electrolyte substrate and gas The outer peripheral side surface of the structure consisting of a diffusion layer is made gas-impermeable, and a structure is formed between the solid electrolyte and the composite oxide electrode that is thermally stable and non-permeable to both the solid electrolyte and the oxide electrode. A combustion control sensor characterized by interposing an intermediate layer made of a reactive oxygen ion conductor. (2) SrMe'O_3 (Me' is Ti, Z
0 to 80 mol%, preferably 40 to 70 mol% of at least one element selected from r, Hf) is added to the perovskite type composite oxide. sensor. (3) The combustion control sensor according to claim 1 or 2, wherein at least one platinum group element is added to the electrode material.