JPH0197854A - Sensor for burning control - Google Patents

Abstract

PURPOSE:To obtain the uniform, stable characteristics of a sensor and to make it possible to display the stable characteristics for a long period, by using a specified perovskite type composite oxide as an electrode material. CONSTITUTION:An anode 2 and a cathode 3 are provided on a substrate of oxygen-ion conducive solid electrolyte. A porous gas diffused layer 6 is provided on the cathode 3. A seal 7, which does not transmit gas, is provided at the outer end surface. Thus, a burning control sensor, which detects a fuel ratio from the concentration of remaining oxygen in exhaust gas and the like, is formed. When the electrode, which is to become the cathode, is formed with a perovskite type composite oxide shown by the general formula I, uniform, stable sensor characteristics are obtained different from the case where platinum is used. The stable characteristics are displayed for a long time since the electrode is thermally stable. In the formula, Ln is La, Ce, Pr and Nd; Me is at least one element, which is selected among Ni, Fe, Mn, Cr and V; 0<=x<1; 0<=y<1; and delta is missing amount of oxygen.

Description

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

Conventional technology Conventionally, this type of sensor has used stabilized zirconia as the oxygen ion conductive solid electrolyte base, platinum as the anode and cathode, and a gas diffusion layer on the cathode. . In this sensor, oxygen ions move in the oxygen ion conductive solid electrolyte base by a voltage applied between the two electrodes, and this can be extracted as an electric current. This movement of oxygen ions is ultimately rate-limited by the gas diffusion layer provided on the cathode, so 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.

However, in the case of a platinum electrode, since the electrode reaction rate is low, 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, so it is extremely difficult to create a porous electrode that is homogeneous and has long-term stability. .

Problems to be Solved by the Invention Electrode characteristics are a major factor that influences the various characteristics of the sensor. At the cathode, oxygen must quickly diffuse to the interface between the electrode and electrolyte to cause an oxygen reduction reaction. On the other hand, at the anode, it is necessary to rapidly increase the degree of oxidation of oxygen ions. Therefore, it is common to use a porous platinum electrode that has high catalytic activity for oxygen redox and facilitates diffusion. However, in the case of platinum, the reaction rate during the electrode reaction is slow, so the polarization is large. As a result, a problem arises in the stability of the potential of the electrode itself, making it difficult to apply a constant potential to the other electrode.

In other words, it becomes difficult to keep the reaction rate constant.

Therefore, it is necessary to increase the surface area of the electrode, but it is extremely difficult to form a homogeneous porous electrode. In addition, platinum is prone to sintering at high temperatures, causing variations in catalyst activity and diffusion resistance, and changes over time, resulting in problems with the uniformity of sensor characteristics and long-term stability. Another problem is that platinum itself is expensive.

Means for Solving the Problems The present invention has been made in view of the above-mentioned problems, and uses a perovskite type composite oxide as an electrode material.

Function The perovskite type composite oxide used as the electrode material of the combustion control sensor according to the present invention is a material having both electronic conductivity and oxygen ion conductivity.

Therefore, it exhibits excellent catalytic activity for electrochemical redox reactions of oxygen on electrodes made of this material. In the case of platinum, the reaction occurs only at the three-phase interface consisting of platinum, electrolyte, and atmosphere, whereas in the case of this material, the reaction active sites are distributed over the entire electrode surface, so that the oxidation-reduction of oxygen occurs. The reaction progresses quickly and with high probability, and the polarization during the electrode reaction is extremely small. Therefore, the electrode potential is stabilized, and a constant potential is constantly applied to the other electrode, so that the current when a constant voltage is applied corresponds to changes in oxygen concentration. As a result, uniform and stable sensor characteristics can be obtained. Furthermore, since the material is thermally stable, it can exhibit stable characteristics over a long period of time.

Embodiment FIG. 1 is a schematic sectional view showing an embodiment of a sensor element according to the present invention. 1 is 8 mol% Y203-92 mol
%Z r O2 oxygen ion conductive solid electrolyte plate (5,5φx 1 t xw ), 2 is an anode (3t
μm), 3 has the chemical formula LaO, 35Sr0.65” 0.
Perovskite die cut 03 represented by 7FeO
3-δ A cathode (15 μm) formed by adhering the composite oxide by flame spraying, 4 is an anode lead terminal, 5 is a cathode lead terminal, 6 is an inorganic gas diffusion layer (70 t μm), 7
is a gas impermeable seal. For comparison, a sensor element equipped with a platinum cathode was 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 was installed in an electric furnace, the temperature was controlled to a predetermined element temperature, and an oxygen-nitrogen mixed gas of a predetermined concentration was brought into contact with the sensor element by flowing at a flow rate of about 10 CM/sH:. 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 70°C and the voltage is 1V.
The case of Note that measurements were performed on 10 elements each in the example, the conventional example, and the conventional example. As a result, conventional sensors using platinum cathodes had large variations in output, and this was more pronounced as the oxygen concentration increased. On the other hand, the sensor according to the present invention exhibited uniform output characteristics with less variation than the conventional sensor using a platinum cathode. Perovskite-type composite oxides have high catalytic activity for oxygen reduction and a high reaction rate in electrode reactions, resulting in electrodes with extremely low polarization and 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. On the other hand, with platinum cathodes, the reaction rate is slow, so 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, the stability of sensor characteristics over time will be described.

The evaluation was performed as follows. After holding the sensor element in air at 5oot: for 500 hours, it was heated to 700°C.
The same measurements as above were performed and the output characteristics were compared. The results are shown in Figures 3a and b.

The output characteristics of the conventional sensor using platinum (Fig. 3a) have changed significantly compared to the initial stage, but on the other hand, the output characteristics of the sensor according to the present invention (Fig. 3 b) have changed compared to the initial stage. It is clear that little has changed. In the case of platinum, sintering progresses gradually in a high-temperature atmosphere over a long period of time, resulting in a change in the fine structure of the electrode and a decrease in surface area, resulting in a decrease in catalytic activity and a change in output characteristics. On the other hand, in the case of perovskite-type composite oxides, the thermal stability is extremely excellent, so that almost no change in properties occurs.

Therefore, the electrode characteristics are stable and reliable, and oxygen concentration can be detected with high accuracy over a long period of time.

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, the case where only the cathode is formed of a perovskite-type composite oxide is described, but in the case of a sensor in which both the cathode and anode are formed of a perovskite-type composite oxide, compared to the case where only the cathode is formed of the perovskite-type composite oxide, The variation in characteristics between individual sensors is small, and the sensor exhibits output characteristics with excellent linearity. In addition, in the example, Ln is La and A is Sr.
, the case where Fe was used as Me was shown, but Ln
is Go, Pr, Nd or La, Ce, Pr.

When two or more elements of Nd are present, A is Ca.

In the case of Ba or in the case of two or more elements among Sr, Ca, and Ba, in the case of Me in the case of Ni, Mn, Cr, and V, or in the case of two or more elements among Ni, Fe, Mn, Cr, and V. Similar results were obtained in the case of Furthermore, S r
When M e' 03 (M e ' is at least one element selected from Ti, Zr, and Hf) is mixed, when a platinum group element is added, the catalytic activity of oxygen reduction increases without impairing the uniformity of electrode characteristics. Shows the effect of increasing In addition, 8 mol% was added to the oxygen ion conductive solid electrolyte used as the base.
Although Y2o3-92mol%Zr02 was used, it is not limited to this as long as it has a similar function. Further, the gas diffusion layer material may be any material as long as it is non-reactive with the cathode material, lead material, and the like. On the other hand, the shape 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 sputtering, printing, coating thermal decomposition, 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 an embodiment of the combustion control sensor according to the present invention, and FIG. 2 is an output characteristic diagram of the sensor element.
FIGS. 3a and 3b are diagrams showing the stability over time of the sensor characteristics of the conventional example and the above embodiment, respectively. 1...Oxygen ion conductive solid electrolyte, 2...
... Anode, 3... Cathode, 4... Anode lead-out terminal, 6... 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 level X (Cape Figure 3 (α) Oxygen concentration (%)

Claims (3)

[Claims] (1) Among a pair of electrodes provided on a substrate made of an oxygen ion conductive solid electrolyte, at least the 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, and Me is Ni, Fe, Mn, and Cr.
, at least one element selected from V, 0≦x≦1, 0≦
y≦1, δ is the amount of oxygen vacancies), a gas diffusion layer is provided on the cathode surface, electrode lead terminals are provided on the anode and the cathode, and an anode,
A combustion control sensor characterized in that an outer peripheral end face of a structure consisting of an oxygen ion conductive solid electrolyte base, a cathode, and a gas diffusion layer is rendered impermeable to gas. (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.