JPH0514859B2 - - Google Patents

Description

[Detailed description of the invention]

FIELD OF THE INVENTION The present invention relates to a sensor that controls the ratio of air to fuel supplied to a combustor such as a stove or boiler, or an internal combustion machine such as an automobile engine, by changing the resistance of a thin film sensitive member. Conventional Technology Conventionally, there are sensors made of stabilized zirconia solid electrolyte as air-fuel ratio control sensors for combustion in the lean region (Japanese Patent Laid-Open No. 60253/1983,
Publication No. 83048). In the sensor made of the stabilized zirconia solid electrolyte described above, the pump current for causing oxygen ions to flow through the zirconia changes in proportion to the oxygen concentration. In response, the inventors proposed a sensor that can specifically change the resistance of the sensor sensitive body at a given oxygen concentration by changing the current flowing through the oxygen ion conductive solid electrolyte plate. . Problems to be Solved by the Invention The present invention solves the problem that in conventional sensors, a large amount of oxygen diffuses from the outside of the sensor to the sensing body, and in an atmosphere with a high oxygen concentration, the ability of the solid electrolyte as an oxygen pump becomes rate-limiting, and the sensor becomes sensitive. This solves the problem that the body's resistance does not change much and that it hardly operates at low temperatures. Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides a method in which a thin film sensitive material is directly attached to the center of the other surface of an oxygen ion conductive solid electrolyte having a cathode electrode attached to one surface. A ring-shaped anode electrode for oxygen extraction and an electrode for resistance measurement are attached to the outer periphery of the electrode, and another electrode for resistance measurement is attached to the center of the upper surface of the thin film sensitive body, and a dense ceramic plate is attached to them. The diffusion area of gas from outside the sensor is limited to the gap between the solid electrolyte substrate and the dense ceramic plate, thereby limiting the amount of oxygen gas diffusion. A noble metal catalyst was added to the susceptor so that it could operate at low temperatures. Function The lean burn sensor according to the present invention is one in which a thin film of a sensor sensitive material is formed directly on a solid electrolyte substrate that acts as an oxygen pump, and the diffusion path of oxygen from the outside of the sensor to the sensitive material is connected to the outside of the sensitive material. It is limited to only the peripheral electrode layer. In this way, we formed a sensor sensitive thin film directly on a solid electrolyte substrate that acts as an oxygen pump, and
By providing an electrode layer on the outer periphery that allows gas to diffuse more slowly than the sensor sensitive thin film and using it as a diffusion path, and by reducing the space inside the sensor, the solid electrolyte, which acts as an oxygen pump, enhances the oxygen extraction effect. This makes it possible to specifically change the electrical resistance of the sensor sensitive thin film even at high oxygen concentrations. In addition, in conventional sensors, the surface reaction of the sensitive material was very slow at low temperatures, so it did not work as a sensor.For this reason, we added noble metal catalysts of the Pt group and Pd group to accelerate the surface reaction. , it was possible to operate even at low temperatures. Example The structure of the lean burn sensor according to the present invention is explained as follows.
This will be explained using figures. FIG. 1 shows the configuration of this sensor, with a being a side sectional view and b being a top view. In Figure 1, 1 has the chemical formula Sr _1+x/2 La _1-x/2 Co _1-x Me _x O _3- (Me is Fe, Mn,
At least one element among Cr, V, O≦x≦
1, O≦δ≦0.5)
SrMe′O ₃ (Me′ is at least one element among Ti, Zr, and Hf) is mixed with an oxide represented by Ti, Zr, and Hf, and at least one element from the Pt group and Pd group is added to the fired material. It is a thin film-like sensitive material. The film was formed by thermal spraying and screen printing. The membrane had a diameter of approximately 3.5 mm and was formed in the center of a zirconia solid electrolyte plate, approximately 5.5 mm in diameter. A ring-shaped electrode 2 for extracting oxygen was attached to the remainder of the outer periphery of the zirconia solid electrolyte plate 5 at a width of about 1 mm, and a counter electrode was attached to the entire back surface of the zirconia solid electrolyte plate. A resistance measuring electrode 3 is attached to the center of the sensitive body, a dense ceramic plate 6 is bonded,
A lead wire 4 was attached to the ring-shaped electrode, and cement was applied to the side and center hole of the sensor to harden it. Lead wire 4
An extension lead wire 9 made of heat-resistant metal is joined to the
Pass it through the hole in the alumina support 8 and attach the cement 7 at the bottom.
and fixed it to create a sensor. The sensor prepared as described above was placed in a quartz glass tube passed through a tube furnace, maintained at 800°C, and exposed to O ₂ ,
Using a mixed gas of CO and N ₂ , O ₂ gas inflow concentration is 2%
Test the characteristics of the sensor by measuring the DC current flowing through the zirconia solid electrolyte and the rate of change in the resistance of the sensor when the resistance of the sensor sensor changes significantly by changing the inflow concentration of CO gas. I went there. _Sr0.65 La _0.35 Co _0,7 Fe _0.3 O _3- 〓 was mixed with SrTiO ₃ as shown in Table 1, and a sensor made of the fired material was prepared and tested.

[Table] The results are shown in Figures 2 and 3. Figure 2 shows
Sensitivity S (S = Rh (high resistance) / R _L (low resistance)) is the DC current value applied to the zirconia solid electrolyte and the amount of change in the resistance of the sensor sensitive body with respect to the O ₂ and CO gas concentration ratio.
It is shown as follows. FIG. 3 shows the results of a test in which a direct current of 9 μA was passed through the zirconia solid electrolyte, and the oxygen concentration was cycled from 5% for 5 minutes to 2% for 1 minute. R _LO is the resistance at an initial oxygen concentration of 5%, and ΔR is the difference between R _LO and the resistance after 5 minutes at an oxygen concentration of 5% after the cycle has elapsed. The resistance of the sensor sensitive body changes significantly even in an atmosphere with a high oxygen concentration by passing a direct current through the zirconia solid electrolyte, and the sensor sensitivity S is
The sensor (1-3) containing 60 mol % of SrTiO ₃ had the highest resistance value, and the results of the cycle test shown in FIG. 3 indicate that the sensor has a very stable resistance value. FIG. 4 shows the response raw of each of these sensors. From this result, it was confirmed that each sensor reached 90% or more within about 1 minute when the oxygen concentration was changed from 5% to 2%. In order to understand the effect of Sr _1+x/2 La _1-x/2 Co _{1 -x} Fe _x O _3- Fabricated and tested.

[Table] The results are shown in FIGS. 5 and 6. Fifth
The figure shows the current value and sensor sensitivity S of the zirconia solid electrolyte with respect to O ₂ and CO gas concentrations, and Figure 6 shows the change in resistance at an oxygen concentration of 5% in a cycle test. be. The value of current flowing through the zirconia solid electrolyte hardly changed with respect to the change in the value of x. sensor sensitivity s
The larger the value of x, the larger the value, and in the cycle test, it was found that the resistance was very stable when the value of x was 0.3 or less. 60 mol% of SrTiO ₃ in Sr _0.65 La _0.35 Co _0.7 Fe _0.3 O _3-
A sensor was fabricated using the mixed and calcined material with the addition of a precious metal catalyst as shown in Table 3, and the mixture was heated to 400°C.
Tested with.

[Table] The results are shown in Figure 7. Figure 7 shows the current value of the zirconia solid electrolyte and the sensitivity of each sensor with respect to the O ₂ and CO gas concentration ratio. The current value of the zirconia solid electrolyte hardly changed. The sensor sensitivity was highest for Pt, and the sensor with Pd added was also higher than the sensor without noble metal added. In the example, Sr _0.65 La _0.35 Co _0.7 Fe _0.3 O _3- 〓 and SrTiO ₃
Although the case was shown in which a mixture of the two was used and fired, almost the same results could be obtained when a mixture of these was used. In addition, in the example, Me is Fe, Me' is
I mentioned Ti, but as Me, Mn, Cr, V,
Almost similar results were obtained for Zr and Hf as Me′. The film was made by thermal spraying, but
Similar results can be obtained by screen printing methods. Effects of the Invention The lean burn sensor according to the present invention limits the diffusion path of oxygen from outside the sensor to only the diffusion from the electrode layer around the outer periphery of the sensor, thereby reducing the amount of diffusion. This increases the effectiveness and allows it to operate as a sensor even at higher oxygen concentrations. By adding a noble metal catalyst such as Pt group or Pd group, it can operate even at low temperatures.

[Brief explanation of the drawing]

Fig. 1 shows the structure of a sensor according to an embodiment of the present invention, and Fig. 2 shows the current value of the zirconia solid electrolyte and the sensor sensitivity with respect to O ₂ and CO gas concentrations.
Figure ₃ shows the effect of mixing SrTiO 3 in a cycle test. Figure 4 shows the effect of mixing SrTiO ₃ in a cycle test. Figure 4 shows the response characteristics at an oxygen concentration of 2%.
Figure 5 shows the effect of mixing SrTiO ₃ with O ₂ and CO
Figure 6 shows the effect of chemical formula x on zirconia solid electrolyte current value and sensor sensitivity on gas concentration. Figure 6 shows the effect of chemical formula x on cycle tests. Figure ₇ shows zirconia solid electrolyte on gas concentration ratio. FIG. 3 is a diagram showing the effect of adding a noble metal on the current value of the solid electrolyte and the sensor sensitivity. DESCRIPTION OF SYMBOLS 1...Sensor, 2...Ring-shaped electrode, 3...Resistance measuring electrode, 4...Lead wire, 5...Zirconia solid electrolyte, 6...Ceramics plate, 7...Cement, 8...Alumina support Body, 9...extension lead wire.

Claims (1)

[Claims] 1. An oxygen ion conductive solid electrolyte plate with electrodes attached on one side has the chemical formula Sr _1+x/2 La _1-x/2 Co _1-x Me _x O _3- on the other side. 〓(Me is Fe, Mn,
At least one element among Cr, V, O≦x≦
1, O≦δ≦0.5)
A mixture with an oxide represented by SrMe′O ₃ (Me′ is at least one element among Ti, Zr, and Hf) or a fired product is used as an additive in the Pt group and Pd.
forming a thin film-like sensitive body containing at least one element of the group at the center, attaching a ring-shaped electrode for oxygen extraction to the outer periphery of the thin-film sensitive body, and attaching an electrode for resistance measurement to the center of the upper surface of the thin film sensitive body; A lean burn exhaust gas sensor featuring a dense ceramic plate.