JPH01102354A - 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 constituting an electrode of perovskite type composite oxide and oxygen ion conductive solid electrolyte. CONSTITUTION:A pair of electrodes 2, 3 at least an electrode 3 working as a cathode, provided on a substrate composed of an oxygen ion conductive solid electrolyte 1 consists of perovskite composite oxide 3 represented by formula I (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) and the electrolyte 1. Electrode lead-out terminals 4, 5 are provided to the electrodes 2, 3 and, further, the outer peripheral end surface of the structure consisting of the electrodes 2, 3, the substrate composed of the electrolyte 1 and a gas diffusion layer 6 is brought to a gas impervious state. 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 is used to detect the ratio of air to combustion based on the residual oxygen concentration in a gas to be measured such as combustion exhaust gas, and to maintain an appropriate combustion state. This invention relates to a combustion control sensor.

Conventional technology Conventionally, this type of sensor has used stabilized zirconia as the oxygen ion conductive solid electrolyte, platinum as the anode and cathode, and a force diffusion layer on the cathode. . In this sensor, oxygen ions are moved into the solid T solute by a voltage applied between two electrodes, and this can be extracted as an electric 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 control combustion to achieve an appropriate air-fuel ratio. It becomes possible to do so.

In contrast, the inventors used Lnl-! instead of platinum as the electrode material. We have proposed a combustion control sensor using perovskite-type composite oxides represented by A, Co1-, Me, 03-, and 5. In the case of platinum, the electrode reaction rate is low, so polarization is large, 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 tends to sinter at high temperatures, making it 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 high catalytic activity in the redox reaction of oxygen, so polarization during the electrode reaction is extremely small and a stable electrode potential is provided. As a result, a constant potential is constantly applied to the other electrode, resulting in excellent sensor characteristics with extremely small variations.

Problems to be Solved by the Invention In general, W composite oxides have a larger coefficient of thermal expansion than solid electrolytes, and when placed in an environment with thermal fluctuations for a long period of time, the adhesion between electrodes and solid electrolytes decreases. As a result, the interfacial resistance tends to increase, and the electrode characteristics may fluctuate and deteriorate.

Means for Solving the Problems The present invention has been made in view of the above-mentioned problems, and consists of an electrode made of a perovskite type composite oxide and an oxygen ion conductive solid electrolyte.

Function: In the combustion control sensor according to the present invention, the solid electrolyte contained in the electrode functions as a binding material and enhances the adhesion between the electrode and the solid electrolyte base.

Embodiment FIG. 1 is a schematic sectional view showing an embodiment of a sensor element according to the present invention. 1 is 8 mo 1% Y 20 a
- Oxygen ion conductive solid electrolyte plate (6,6 $XIWt) consisting of 92 mol % ZrO2, 2 is an anode (3
p m t ), 3 is the chemical formula “o, asSro, C6”
Peropskite type composite oxide represented by o, rFeo, frame 70wt%8 mol%Y O-92mol%Z
a cathode (16 μmt) formed by flame spraying of a mixture consisting of r O2s o vr t%;
4 is an anode lead terminal, 5 is a cathode lead terminal, 6 is an inorganic gas diffusion layer (70 μmt), and 7 is a gas impermeable seal. For comparison, “0.35SrO,65C00,7”0
．． A sensor element provided with a cathode made of only 303-8 and a sensor element provided with a platinum cathode were respectively produced.

The Senna element fabricated as described above was subjected to an operating characteristic test to be described later. On the other hand, in order to evaluate the adhesion between the electrode and the solid electrolyte substrate, amol%Y2o3-92mO1
%L a 0.3sS r□ on ZrO2 substrate, 6
5”O, yF @0.303- J 7C1vvt%.

8mol%Y O-92mol%Z r O230vv
Approximately 115 tons of the mixture consisting of
For comparison, L”0.3 with a sample attached to a thickness of μm.
Samples were prepared in which only 68"0.65"0.7"0.303-J was deposited to the same thickness.

First, the evaluation results of adhesion will be shown. Evaluation of device performance was performed using the method described below. The sprayed sample was placed under an electric light, and the cycle of heating and cooling at 300 and 2900°C (1000°C/h temperature rise and fall) was repeated 100 times. Adhesive tape was attached to the surface of the sprayed film and then peeled off. The presence or absence of peeling of the sprayed film was investigated. Table 1 shows that in each of the 20 sample examples, no peeling of the sprayed film occurred, but in the conventional example, peeling occurred at a rate of 26%. Further, as a result of magnifying observation of the surface of the sprayed film after the heat cycle test, it was found that many cracks had occurred in the conventional example, but there were almost no cracks in the example. In the examples, &no1%Y2o3-g2mol% in the sprayed film
This is because Z r O2 functions as a binding material and a buffering material, and even during to-to cycling, a decrease in adhesion to the substrate and an increase in distortion within the film can be suppressed.

It is considered that no cracking or peeling occurs.

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, and the temperature is controlled so that the temperature of the element reaches a predetermined value.
Flow contact was carried out at a flow rate of c.

At this time, the output current with respect to the printing voltage was measured, and the output current when a constant voltage was applied was determined for each oxygen concentration. Figure 2 shows an example where the temperature is 700°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. As a result, conventional sensors using platinum cathodes had large variations in output, and the variation was more pronounced as the oxygen concentration was higher. In contrast, the sensor of the present invention not only exhibits uniform output characteristics with less variation compared to conventional sensors using platinum cathodes, but also -LaO
, 35S x 0.65"0.7" 0.3o3-J.

Perovskite-type composite oxides have high catalytic activity for oxygen 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 opposite voltage, and as a result, the flowing current accurately corresponds to the oxygen concentration.

The excellent properties of such a perovskite type composite oxide are as shown in the examples.
It is not inhibited by mixing m, ol % Z r O2. That is, 8 mol%
The cathode containing Y263-92 mol % ZrO2 has good adhesion to the substrate and exhibits a good bonding state.

Since it exhibits excellent oxygen ion conductivity, oxygen ions reduced on the cathode quickly migrate to the solid electrolyte substrate. Because it has such excellent electrode characteristics, differences in the fine structure of the individual electrodes of Sunsa have almost no effect on the output characteristics, so variations in characteristics are small, and oxygen concentration can be detected with high accuracy and responsiveness. On the other hand, since the reaction rate of platinum cathodes is slow, slight differences in electrode porosity, surface area, etc. will result in variations in properties.

Uniform control of the microstructure is extremely difficult, which reduces manufacturing yield and
This is a major obstacle to ensuring a certain level of quality.

In addition, the measurements were carried out at different temperatures in the range of 600 to 900°C, but the same results as in the case of 70°C were obtained in each case.

Next, we will discuss the stability of sensor characteristics over time.

The evaluation was performed as follows. After holding the sensor element in air at 860° C. for 500 hours.

The same measurements as above were performed at 700° C. and the output characteristics were compared. The results are shown in Figure 3 (a), (b) and (C).
It was shown to. Output characteristics of the sensor according to the present invention (Fig. 3 (a)
)) showed almost the same characteristics as the initial one. Against this.

LaO,35SrO,65”0.7F00.3o3-J
The output characteristics of the sensor using a cathode with only a small amount of force (Fig. 2(b)) have changed compared to the initial stage, and the dispersion has become slightly larger.

In addition, the output characteristics of the conventional sensor using platinum (Figure (C)
)) has changed significantly compared to the beginning. In the sensor according to the present invention, the adhesion between the cathode and the substrate is excellent, and the bonding state is not changed due to thermal stability.

Therefore, the electrode characteristics are stable and reliable, and the oxygen concentration can be detected with high accuracy over a long period of time. - "o, 5sSro, e5C0o, 7"o, 3°3
In the sensor having a cathode made only of −δ, it is thought that the electrode characteristics changed due to the formation of cracks and the occurrence of partial peeling in the electrode. On the other hand, in the case of a platinum cathode, sintering progresses gradually in a high-temperature atmosphere over a long period of time, resulting in a change in the microstructure of the electrode and a decrease in surface area, resulting in a decrease in catalytic activity and a change in output characteristics. .

As is clear from the above examples, it can be seen that the combustion control sensor according to the present invention is extremely excellent. In the example, only the cathode was formed of a perovskite-type composite oxide and a solid electrolyte. However, in the case of a sensor in which both the cathode and anode are formed of a perovskite-type composite oxide and a solid electrolyte, the electrode has excellent redox catalytic ability. In addition to demonstrating
Because the adhesion between the electrode and the solid electrolyte substrate is excellent, the variation in characteristics between individual sensors is smaller than when only the cathode is made of perovskite composite oxide and solid electrolyte, and the linearity is excellent. The output characteristics are shown below. In addition, in the example, Ln is La, A is Sr, and Me is F.
The case where x = 0.65 and y = 0.3 using e is shown, but when Ln is Co, Pr, or Nd, or when it is two or more elements among La, Ce, Pr, or Nd. , when A is Ca, Ba or Sr.

When two or more elements are present in Ca and BaO, Me is Ni
, Mn, Cr, V or Ni, Fe, Mn, O
r.

Similar results were obtained when two or more of the elements (2) or other composition ratios were used. Furthermore, SrM
When e'03 (Me' is at least one element selected from Ti, Zr, and Hf) is mixed with light, or when a platinum group element is added, oxygen oxidation and reduction can be achieved without impairing the uniformity of electrode characteristics. shows the effect of increasing the catalytic activity of In addition, the solid electrolyte mixed and used as an electrode material also contains 8 mol% Y2
It is not limited to o3→2 mol% ZrO2, but any material having a similar function may be used.

On the other hand, the solid electrolyte used as a base also contains &no 1% Y.
Although 2O392mol%Zr02 was used, it is not limited to this as long as it has a similar function. Further, the gas diffusion layer is not limited to a porous material, and may be of a type provided with diffusion holes, and the material of the diffusion layer may be any material as long as it is non-reactive with the electrode 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. .

[Brief explanation of the drawing]

Fig. 1 is a schematic cross-sectional view of a combustion control sensor according to an embodiment of the present invention, Fig. 2 is an output characteristic diagram of the sensor element, and Fig. 3 a
, b and c are diagrams showing the stability over time of the sensor output characteristics of the embodiment and the conventional example, respectively. 1...Oxygen ion conductive solid electrolyte, 2...
... Anode, 3... Cathode, 4... Anode lead-out terminal, 5... Cathode lead-out terminal, 6...
- Porous gas diffusion layer, 7... Gas impermeable seal. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 Oxygen concentration X (%) Figure 83 Acid! Concentration (42 Figure 3 Oxygen immersion (illusion)

Claims (4)

[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 Sr, Ca, and Ba; Me is Ni, Fe, Mn, Cr,
At least one element selected from V, 0≦x≦1, 0≦y
≧1, δ is the amount of oxygen vacancies) and a solid electrolyte, an electrode lead terminal is provided on the pair of electrodes, a gas diffusion layer is provided on the cathode,
Furthermore, a combustion control sensor characterized in that an outer peripheral end face of the structure including the electrode, solid electrolyte base, and gas diffusion layer is made gas impermeable. (2) SrMe'O_3 (Me' is Ti, Z
The thermal sintering control according to claim 1, characterized in that at least one element selected from r and Hr is added in an amount of 0 to 80 mol%, preferably 40 to 70 mol%, 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. (4) Claims 1 and 2, characterized in that the diffusion layer is made of MgO or a material mainly composed of MgO.
Combustion control sensor according to item 1 or 3.