JP2748809B2 - Gas detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detector used for controlling an air-fuel ratio of an automobile internal combustion engine.

[0002]

2. Description of the Related Art As a gas detector for detecting an oxygen concentration in exhaust gas of an automobile internal combustion engine, for example, an oxygen concentration electromotive force type using a ZrO ₂ solid electrolyte is well known and put into practical use. ing. An example of such an electromotive force gas detector is an oxygen concentration detector 9 shown in FIG. 17 (Japanese Patent Publication No. 15017/1990).

The oxygen concentration detector 9 has a sensor element 90 at its tip. As shown in FIG. 18, the sensor element 90 is a bottomed cylindrical body in which an inner electrode 32, a ZrO ₂ solid electrolyte sintered body 4, an outer electrode 31, and an outermost layer 2 are sequentially formed in a laminated shape. In the lumen 901 of the sensor element 90,
The heater 5 is inserted. The outermost layer 2 is formed of a ceramic coating layer for protecting electrodes and controlling diffusion, or a layer provided with a γ 例 え ば Al ₂ O ₃ layer on the ceramic coating layer.

The outer electrode 31 and the inner electrode 32 are shown in FIG.
As shown in FIG. 7, it is connected to a connector 98 above the sensor element 90 via lead wires 91 and 92. The heater 5 is connected to the connector 98 via a lead wire 95. The sensor element 90 is covered by a housing 96 and a case 961 with a window. The oxygen concentration detector is fixed to an exhaust pipe or the like by a flange 97 attached to a housing 96.

The response speed and output of the oxygen concentration detector are determined by the diffusion speed of the exhaust gas between the exhaust gas atmosphere and the outer electrode 31 in the sensor element 90. Therefore,
To obtain a stable sensor output over a long period of time, it is necessary to maintain a stable exhaust gas diffusion rate.

[0006]

However, under certain practical conditions, as shown in FIG. 19, the surface of the outermost layer 2 of the sensor element is covered with a deposit 7 derived from exhaust gas. The deposit 7 is made of a film-like vitreous material composed of oil-mixed components such as P, Ca, Zn, and Si, and gasoline-mixed components such as K, Na, and Pb.

[0007] The deposit 7 covers the surface of the outermost layer 2 and may prevent the exhaust gas from diffusing into the outer electrode 31. For this reason, deterioration such as a decrease in sensor output and a decrease in responsiveness may occur. Therefore, when the deposits are attached in this manner, the gas detector cannot obtain a stable sensor output for a long time.

In order to solve the above problems, for example, a relatively porous sprayed film or γ─Al having a particle size of several μm is used.
An improvement has been proposed in which an outermost layer made of ₂ O ₃ particles or the like is provided on the element surface so that clogging does not occur. The first
Is to cover the surface of the sensor element exposed to the exhaust gas with an insulating coating made of a metal oxide that is heat-resistant and porous and is permeable to the detection gas, and a catalyst is supported on the insulating coating. (Japanese Patent Laid-Open No. 52-7 / 1982)
No. 3089).

As the insulating film, γ─Al ₂ O ₃ ,
There is a coating made of ZrO ₂ , MgO or the like. Examples of the catalyst include Pt, Pd, and Rh. The insulating coating captures the poisoning component and prevents the poisoning component from directly adhering to the sensor element, thereby improving the durability of the gas detector.

Second, an oxygen sensor in which a protective layer made of alumina spinel is provided on the surface of the detection gas side electrode of the sensor element, and an outermost layer of a poisoning substance made of gamma-alumina having a high adsorption ability is provided on the outer layer surface. (Japanese Patent Laid-Open No. 61-153)
561). The alumina spinel has a function of limiting the flow rate of the detection gas, improving the reactivity at the electrode, and protecting the electrode.

This gas detector is particularly effective for detecting the exhaust gas component of an internal combustion engine, and includes Pb, P, S, Si and Zn.
It is very effective for capturing poisonous components such as. Further, there is an effect that the poisoning deterioration of the internal catalyst component is prevented and the durability is greatly improved. However, in recent years, with the improvement of fuel efficiency and performance of internal combustion engines, the usage environment of gas detectors has become more severe. For this reason, the operating temperature of the gas detector has increased and the amount of poisoning components has increased.

Therefore, the captured poisoning components cause a chemical reaction or melting at a high temperature. Therefore, by subsequent cooling,
A dense glass layer or a vitreous adherent film which does not have air permeability by itself is formed, causing clogging.

As described above, by forming the outermost layer on the surface of the sensor element, the internal poisoning deterioration can be prevented.
Deterioration of the gas detector due to clogging of the surface by the glassy adhered coating has become a major problem. In view of the above problems, the present invention provides a gas detector capable of preventing clogging of the outermost layer, having an excellent poisoning component capturing effect, and maintaining a stable sensor output for a long period of time. It is something to offer.

[0014]

According to the present invention, there is provided a sensor element having a pair of electrodes, one of which is exposed to an atmosphere of a gas to be measured, and a surface layer formed on the outermost periphery of the one electrode. A gas detector comprising :
It is made of heat-resistant particles, and between the heat-resistant particles.
The formed pores are non-linear in the thickness direction of the surface layer.
It extends through the predetermined thickness of the one electrode direction the T, an average pore diameter of the predetermined hole of the surface layer d, the surface
In the layer, the opening formed by the heat-resistant particles
And drop it at right angles to one surface of the electrode.
When the predetermined depth until it contacts the refractory particles with a, T of the surface layer, d, a, along with satisfying the d ≦ a ≦ T, the average pore diameter d of the surface layer is
0.5 μm or more, the thickness T of the surface layer is 10 to 500 μm
m, wherein the porosity of the surface layer is 50 to 90% .

What is most notable in the present invention is that the surface layer is a porous body formed by a large number of particles. The particles are thermally stable and are continuously bonded to form the surface layer. The pores formed between the particles penetrate nonlinearly in the thickness direction of the surface layer to the surface of the outermost layer.

The average pore diameter d of the surface layer is larger than the average pore diameter of the porous outermost layer. If the average pore diameter is less than the average pore diameter of the outermost layer, there is a possibility that deposits made of poisoning components may become clogged in the surface layer, cover the surface, and impede gas passage. The average pore diameter d of the surface layer, Ru der least 0.5 [mu] m. If it is less than 0.5 μm, the amount of the deposit is small when the use time is short, and the opening of the opening near the surface of the surface layer is maintained, but the amount of the deposit increases when the use time is long. For this reason, a continuous deposit layer may be formed near the surface of the surface layer, which may cause clogging. Furthermore,
Preferably, the average pore diameter d is between 5 and 50 μm.

The thickness T of the surface layer is 10 to 500 μm.
You. If it is less than 10 μm, the surface layer may be too thin to exhibit its effect as a surface layer. Conversely, if the thickness is 500 μm, the surface layer is too thick, and the adhesion strength between the outermost layer and the surface layer may be reduced. Further, the diffusion of the exhaust gas from the surface layer itself may be hindered, which may adversely affect the initial characteristics of the sensor. Furthermore, preferably 5
0 to 200 μm. The porosity of the surface layer is 50-90%
Der Ru. If it is less than 50%, the surface layer may be too dense to cause clogging. If it exceeds 90%, the strength of the surface layer may decrease.

[0018] The depth a pore in the vertical direction in the opening portion of the surface layer, Ru d ≦ a <T der. In the case of a <d, the attached matter attached to the lower particles and the attached matter attached to the side surface of the upper particles may come into contact with each other, and the opening may be closed. In addition, it is natural that the upper limit of T ≦ a is smaller than the thickness T of the surface layer.

The surface layer is made of α─ alumina, γ─ alumina,
It is preferable to use one or more heat-resistant particles of mullite and MgO.Al ₂ O ₃ spinel. Further, the particle shape can be selected from spherical, massive, plate-like, fiber-like, foam-like, column-like, needle-like, and the like. The surface layer is 1μ
Secondary particles formed by a collection of dense primary particles of m or less can also be used.

In forming the surface layer, alumina sol as an inorganic binder and aluminum nitrate (Al) as a dispersant are used.
(NO ₃ ) ₃ ), and water and the particles for the surface layer are mixed to form a slurry. Next, the slurry is applied to the surface of the outermost layer by dipping or spraying. Alternatively, the particles are sprayed to form a surface layer.

Further, particles made of a resin material or an organic material which burn off at 500 to 900 ° C. and disappear by sublimation are mixed in the slurry, and these are adhered to the surface by a dipping method or the like, and then baked. You can do it. Thereby, the porosity of the surface layer can be easily controlled. The gas detector of the present invention can be applied to gas concentration electromotive force sensors, limit current sensors, stacked sensors, and the like.

[0022]

In the present invention, the surface layer is a porous body, and the average pore diameter d is larger than the average pore diameter of the outermost porous layer. Therefore, even in the case of use in a harsh use environment, even if the above-mentioned deposits adhere to the surface layer, the opening portion of the pore near the surface of the surface layer is reliably secured.
There is no clogging.

That is, even if deposits derived from the poisoning components adhere to the surface of the surface layer and a dense glass-like layer is formed,
The adhering substance layer does not adhere widely in the form of a film between adjacent particles in the surface layer. Therefore, the surface layer always maintains a large number of apertures, and does not prevent the gas to be detected from reaching the electrodes. Further, the deposit does not reach the outermost layer formed inside the surface layer.

As a result, the gas to be detected can easily reach the electrode from the measurement atmosphere through the surface layer and the outermost layer, and the diffusion speed does not change significantly. Therefore, a stable sensor output can be obtained for a long time. As described above, according to the present invention, there is no clogging of the outermost layer,
It is possible to provide a gas detector that has an excellent poisoning component capturing effect and can maintain a stable sensor output for a long period of time.

[0025]

【Example】

Embodiment 1 An embodiment according to the present invention will be described with reference to FIGS. The gas detector 9 of this embodiment is an oxygen concentration electromotive force type sensor, and its sensor element 90 has a pair of outer electrode 31 and inner electrode 32 and a surface layer 1 covering the outermost layer 2 of the outer electrode 31. The surface layer 1 is formed of a porous body for capturing poisoning components.

As shown in FIG. 1, the sensor element 90 has a heater 5 at the center thereof, and the inner electrode 32, the ZrO ₂ solid electrolyte sintered body 4, the outer electrode 31, The outer layer 2 is arranged in order. The outermost layer 2
It is formed of a thermal spray protective film such as MgO.Al ₂ O ₃ spinel for protecting the electrodes and controlling diffusion. This oxygen concentration electromotive force type sensor has, for example, an air-fuel ratio (A
/ F) is measured.

The surface layer 1 is a porous body formed by a large number of particles 10 as shown in FIGS. The particles 10 are thermally stable and continuously bond to form the surface layer 1. The pores 6 formed between the particles 10 penetrate non-linearly up to the surface of the outermost layer 2 in the thickness direction of the surface layer 1. The outermost layer 2 is porous. The average pore diameter d of the surface layer 1 is larger than the average pore diameter of the outermost layer 2.

The average pore diameter d of the surface layer 1 is 10 to 50 μm.
It is. The thickness T of the surface layer 1 is 30 to 200 μm.
The porosity of the surface layer 1 is 50 to 70%. Further, the depth a of the pores 6 in the vertical direction in the opening 60 of the surface layer 1 is:
d ≦ a <T. The particles 10 of the surface layer 1 have α─
Alumina, mullite, γ─ alumina, MgO · Al ₂ O
₃ Thermally stable ceramic particles such as spinel.

In forming the surface layer 1, alumina sol as an inorganic binder and aluminum nitrate (Al) as a dispersant are used.
(NO ₃ ) ₃ ), and water and particles 10 are mixed to form a slurry. Next, this slurry is adhered to the surface of the outermost layer 2 by dipping. Other structures of the gas detector are the same as those of the conventional example (see FIG. 17).

Next, the operation and effect of this embodiment will be described.
In this example, as shown in FIG. 5 and FIG. 6, the surface layer 1 is a porous body, and the average pore diameter d is the outermost layer 2 of the porous layer.
Is larger than the average pore size. Therefore, even when the vitreous deposit layer 7 derived from the poisoning component adheres to the surface layer 1 when used in a harsh use environment, the opening portions 60 of the pores 6 near the surface of the surface layer 1 are surely formed. And clogging does not occur.

That is, even if the adhered substance layer 7 adheres to the surface of the surface layer 1 and a dense glass-like layer is formed, the adhered substance layer 7 is formed between adjacent particles 10 on the surface layer 1. Does not adhere widely. Therefore, the surface layer 1
A large number of apertures 60 are maintained at all times, and do not prevent the gas to be detected from reaching the outer electrode 31 and the inner electrode 32.

The particles 10 have low adsorptivity of poisonous components,
Also, the wettability with glass is small. Therefore, it is possible to prevent the glass component derived from the poisoning component from adhering to the surface of the particle 10. Further, the attachment layer 7 is prevented from reaching the outermost layer 2 formed inside the surface layer 1.

Further, since the outermost layer 2 is inside, the poisoning component that has entered through the pores 6 of some surface layers 1 is also captured by the outermost layer 2 and does not reach the solid electrolyte sintered body 4. Therefore, the response of the solid electrolyte sintered body 4 due to the catalyst deterioration does not deteriorate. According to the gas detector 9 of this embodiment, the outermost layer 2
Without clogging, has excellent poisoning component capturing effect,
In addition, a stable sensor output can be maintained for a long period of time.

Embodiment 2 In this embodiment, in the gas detector shown in Embodiment 1, the average pore diameter d (μm), porosity (%), and thickness T (μm) of the surface layer are shown in Table 1. Various changes were made as shown, and the poisoning durability and the initial (pre-test) response were measured. Table 1 shows the results. The average pore diameter d of the surface layer is 3 to 70 μm, the porosity is 20 to 80%, and the thickness T is 30 to
(Sample Nos. 1 to 1)
3).

The poisoning durability was determined by the rate of change in sensor response before and after the accelerated poisoning durability test. And, when the change rate is less than 5%, ◎, when the change rate is 5% or more and less than 10%, ，,
A case of 10% or more and less than 20% was judged as Δ, and a case of 20% or more was judged as ×.

The initial response is indicated by ◎ when the response time is less than 100 ms, ○ when the response time is 100 ms or more and less than 150 ms, and 1
The case of 50 ms or more and less than 200 ms was judged as Δ, and the case of 200 ms or more was judged as ×.

As can be seen from the table, in the electromotive force type sensor of this embodiment, the surface pore diameter d is 10 to 50 μm,
The porosity is 50 to 70%, and the thickness T of the surface layer 1 is 30 to 2
It can be seen that, in the case of the range of 00 μm, good poisoning durability can be obtained without lowering of gas diffusivity and deterioration of initial response.

[0038]

[Table 1]

Embodiment 3 The gas detector of this embodiment is a limiting current type oxygen concentration detector as shown in FIG.
Is formed in a cylindrical shape on the side surface. The inner electrode 32 is formed so as to cover the inside of the sensor element 90. As shown in FIG. 8, the limiting current type oxygen concentration detector measures a limiting current (I) with respect to, for example, an air-fuel ratio (A / F). Others are the same as the first embodiment. In this embodiment, the same effect as in the first embodiment can be obtained.

Embodiment 4 In this embodiment, in the gas detector shown in the above Embodiment 3, the average pore diameter d (μm) of the surface layer and the pore depth a
(Μm) and the thickness T (μm) of the surface layer were variously changed as shown in Table 2, and the change rate (%) of the output current with respect to the use time in the exhaust gas of the engine was measured.

That is, as shown in Table 2, the average pore diameter d of the surface layer is 2 to 10 μm, the depth a of the pore is 5 to 10 μm,
The thickness T was changed in the range of 30 to 100 μm (sample N
o. 21-24). For comparison, a gas detector (sample No. C1) similar to that of Example 3 was prepared except that no surface layer was provided, and the same measurement was performed as described above.

FIG. 9 shows the results, in which the horizontal axis shows the endurance time of the gas detector in the exhaust gas, and the vertical axis shows the change rate (%) of the output current. In Table 2, the judgment indicates the evaluation of the rate of change of the output current at the endurance time of 100 hours. The case of 0% is ○, the case of more than 0% and less than 2% is Δ, and the case of 2% or more is ○. X.

As can be seen from FIG.
o. In No. 21, the initial output current was still maintained even after use for 200 hours or more. In addition, the sample No. of the present invention. In Nos. 22 to 24, an output current close to the initial state was maintained even after 100 hours of use. On the other hand, the sample No. C1 can be used for 50 hours
The output current decreased by about 5%.

The following can be understood from this. That is,
The sample No. according to the present example. In Nos. 21 to 24, even after use for 100 hours or more, the pores are not blocked by deposits derived from poisoning components. Therefore, the exhaust gas diffuses near the outer electrode and the inner electrode at a diffusion rate similar to that of the measurement atmosphere.
Therefore, a stable and stable sensor output can be maintained over a long period of time.

[0045]

[Table 2]

Embodiment 5 As shown in FIG. 10, the gas detector 9 of this embodiment is a stacked oxygen concentration detector having a sensor element 90. The sensor element 90 has electrodes 33 and 34 disposed on an alumina (Al ₂ O ₃ ) substrate 50. The surfaces of the electrodes 33 and 34 are sequentially covered with the transition metal oxide sintered body 40, the outermost layer 2, and the surface layer 1.

As shown in FIG. 11, the gas detector of this embodiment measures, for example, the resistance value (R) with respect to the air-fuel ratio (A / F). Inside the alumina substrate 50, Pt
The heating element is embedded (not shown). Electrode 3
For Pt 3, 34, Pt or the like is used.

The transition metal oxide sintered body 40 is made of titanium oxide (TiO ₂ ), cobalt oxide (CoO), zinc oxide (Z
A catalyst such as platinum (Pt) is supported on a metal oxide such as nO), tin oxide (SnO ₂ ), or niobium oxide (NbO). The thickness of the transition metal oxide sintered body 40 is 100 to 2
00 μm. The outermost layer 2 is made of a porous metal oxide such as γ─ alumina, activated alumina, perovskite, etc.
A catalyst for gas equilibrium is supported. The outermost layer 2
Is an electrical insulator that has a high gas adsorption capacity and is permeable to gas.

As a method of forming the outermost layer 2, first, a powder of alumina or the like having a large specific surface area of 50 m ² / g or more and having an average particle size of 2 to 5 μm and having an excellent adsorption capacity is formed into a slurry. This slurry is coated on the surface of the transition metal oxide sintered body 40, dried and baked. Next, a catalyst is supported. Thereby, the outermost layer 2 having a thickness of 20 to 100 μm is formed. The outermost layer 2 has a large surface area of alumina and a large effective diffusion length of the pores, despite the relatively small average pore diameter of 0.15 to 0.25 μm, so that the capturing ability of the poisoning component is high. Is high.

In the method of forming the surface layer 1, first, a coarse crystalline powder made of a heat-resistant metal oxide such as alumina, magnesia, calcia, titania, etc. is slurried. Next, this slurry is coated on the surface of the outermost layer 2, dried and fired. Thereby, the thickness is 10 to 50 μm
Is formed. In this embodiment, the same effect as in the first embodiment can be obtained.

Embodiment 6 Next, the material and average pore diameter of the surface layer and the outermost layer in the stacked oxygen concentration detector according to Embodiment 5 were variously changed, and these were mounted on an actual engine. , Durability test. The gas response time before and after that (1
/ 1000 seconds).

The surface layer is made of γ─ alumina or α
─Using alumina, the average pore size is 0.2-20μm
Was changed within the range. The thickness of the surface layer is 20 μm. The outermost layer is made of γ─ alumina or α─
Alumina is used, and the average pore diameter is 0.2 μm or 0.5 μm. The thickness of the outermost layer is 50 μm.

In the above durability test, the gas response time, that is, λ
When switching from 0.9 to λ1.0, the output is 0.6
This is a study on the time required to change from V to 0.3V.
The measurement was performed on a 2000 cc in-line 6-cylinder engine equipped with a fuel injection device using unleaded gasoline at a rotational speed of 1100 rpm. p.
m. The operation was performed at a detector temperature of 700 ± 10 ° C. The above measurements are made before and after the durability test.

The test conditions in the endurance test were as follows: a 2000 cc in-line four-cylinder engine with a fuel injection device was rotated at 4000 rpm after 30 minutes of idling. p. m. The condition of rotating for 30 minutes at was repeated continuously. The detection temperature is 500 to 700 ° C. The gasoline used was unleaded gasoline obtained by adding 5 wt% of a detergent to engine oil. The durability time is 100 hours. Table 3 shows the results of the durability test.

[0055]

[Table 3]

In the table, the initial stage is the case before the endurance test, and the after endurance is the case after the endurance test. And
Each response time was measured. The judgment in the table is
9 shows the results of a deposit microsurvey. In this determination, the case where the response time change rate was less than 30% was evaluated as ○, and the case where the response time change rate was 30% or more was evaluated as ×. As can be seen from Table 3, when the average pore diameter d is 0.5 μm or more, the response change rate is 54%.
Excellent durability was shown below. The response time was 100 ms or less even after the poisoning durability test.

In the microscopic examination of the deposit, the sample No. 31,
The opening of the surface layer corresponding to No. 32 was closed by the glassy deposit layer. The sample No. In Nos. 33 to 40, the opening was considerably open and hardly closed. Sample No. Sample Nos. 38 and 39 are sample Nos. 31-
Compared with No. 37, the amount of the adhering substance layer was small because particles having a small adsorption capacity were used. Sample No. In 40,
Although the opening was not closed, the poisoning substance had penetrated to the outermost layer, and the catalyst carried on the outermost layer was poisoned.
As a result, it is considered that the trapping ability of the outermost layer was reduced and the response was deteriorated.

When the thickness of the surface layer was separately tested, it was found that 10 μm could sufficiently capture the poisoning components. In addition, it was found that when the average pore diameter of the surface layer was larger than the thickness of the surface layer, the trapping ability was reduced. From the above test results, the gas detector according to the present invention has an average pore diameter of the surface layer of 0.5 μm or more,
It is preferably at least 5 μm and has a porosity of 40 μm.
% Or more, and when the outermost layer covered by the surface layer has a high trapping ability, it can be seen that the overall poisoning resistance is excellent.

The sample No. shown in Table 3 was used. 31-37
FIG. 12 shows the relationship between the average pore diameter (μm) of the surface layer and the rate of change [%] of the response time. As can be seen from the figure, when the average pore diameter of the surface layer was 0.5 μm or more, the response time change rate was 30% or less. 0.5
If the thickness is less than μm, the response change rate is significantly increased due to clogging of the surface layer.

Embodiment 7 As shown in FIG. 13, the gas detector 9 of this embodiment is a stacked gas detector having a sensor element 90. The sensor element 90 includes electrodes 33 and 34 disposed at the center, a transition metal oxide sintered body 40 surrounding the electrodes 33 and 34, an outermost layer 2 covering the transition metal oxide sintered body 40, and a surface. And layer 1. The transition metal oxide sintered body 40 uses a titania sintered body. The electrodes 33 and 34 use Pt. Electrodes 33, 3
4 are lead wires 9 outside, respectively, as shown in FIG.
3,94 are extended. Others are the same as the fifth embodiment. In this embodiment, the same effect as that of the fifth embodiment can be obtained.

Embodiment 8 In this embodiment, as shown in FIGS. 15 and 16, two kinds of shapes of the particles 10 of the surface layer 1 are shown. FIG. 15 shows a case where the particles are relatively spherical. In this case, the unevenness and porosity of the surface of the surface layer 1 become large, and even when the poisoning component, particularly the glassy deposit layer 7, is covered, the opening 60 through which the exhaust gas flows in is blocked. There is no structure. Therefore, no clogging occurs.

FIG. 16 shows an example in which the particles are relatively plate-shaped and are arranged substantially perpendicular to the surface of the surface layer 1. In this case, since the particles 10 of the surface layer 1 are long, the depth of the unevenness of the surface of the surface layer 1 is large. The structure is such that the part 60 is not closed. for that reason,
No clogging occurs.

[Brief description of the drawings]

FIG. 1 is a sectional view of a main part of a gas detector according to a first embodiment.

FIG. 2 is a diagram illustrating a relationship between an electromotive force and an air-fuel ratio of the gas detector according to the first embodiment.

FIG. 3 is an enlarged sectional view of a main part of the gas detector according to the first embodiment.

FIG. 4 is a plan view of a surface layer according to the first embodiment.

FIG. 5 is an enlarged cross-sectional view of a main part of the gas detector according to the first embodiment when a deposit adheres to the surface of the surface layer.

FIG. 6 is a plan view of the surface layer to which the deposit is attached according to the first embodiment.

FIG. 7 is a sectional view of a main part of a gas detector according to a third embodiment.

FIG. 8 is a diagram showing a relationship between a limiting current and an air-fuel ratio of the gas detector according to the third embodiment.

FIG. 9 is a diagram illustrating a change rate of an output current according to a durability time according to the fourth embodiment.

FIG. 10 is a sectional view of a main part of a gas detector according to a fifth embodiment.

FIG. 11 is a diagram showing a relationship between a resistance value and an air-fuel ratio of a gas detector according to a fifth embodiment.

FIG. 12 is a diagram showing the relationship between the average pore diameter of the surface layer and the response time change rate according to Example 6.

FIG. 13 is a sectional view taken along line AA of FIG. 14;

FIG. 14 is a perspective view of a gas detector according to a seventh embodiment.

FIG. 15 is a sectional view of a surface layer according to an eighth embodiment.

FIG. 16 is a sectional view of a surface layer according to an eighth embodiment.

FIG. 17 is a partially cut-away side view of a conventional gas detector.

FIG. 18 is a sectional view of a main part of a sensor element in a conventional gas detector.

FIG. 19 is an enlarged cross-sectional view of a main part of a conventional sensor element when an adhering substance adheres to the surface of the outermost layer.

[Explanation of symbols]

 1. . . Surface layer, 10. . . Particles, 2. . . Outermost layer, 31, 310, 32, 33, 34. . . Electrodes, 4. . . 40. solid electrolyte sintered body, . . 5. transition metal oxide sintered body; . . Pores, 60. . . 6. Opening, . . 8. deposit layer, . . Gas detector, 90. . . A sensor element, a. . . Pore depth, d. . . Average pore size, T. . . Thickness of surface layer,

Continuation of the front page (72) Inventor Masaya Fujimoto 1-1-1, Showa-cho, Kariya, Aichi Japan Inside Denso Co., Ltd. (56) References JP-A-4-303754 (JP, A) JP-A-52-50290 (JP) JP-A-4-204046 (JP, A) JP-A-2-276956 (JP, A) JP-A-55-20423 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB G01N 27/409 G01N 27/12 G01N 27/41

Claims (2)

(57) [Claims] 1. A gas detection device comprising: a sensor element having a pair of electrodes, one of which is exposed in a gas atmosphere to be measured; and a surface layer formed on the outermost periphery of said one electrode. Vessel, wherein the surface layer is formed of a large number of heat-resistant particles.
In addition, the pores formed between the heat-resistant particles are formed in the surface layer.
It penetrates in a non-linear manner in the thickness direction, and the predetermined thickness in the direction of the one electrode is T, the average pore diameter of predetermined pores in the surface layer is d, and the surface layer is formed by the heat-resistant particles.
From the opening formed, a direction perpendicular to one surface of the electrode
Lower countercurrent, when a predetermined depth to contact the other refractory particles and a, T of the surface layer, d, a, along with satisfying the d ≦ a ≦ T, the average of the surface layer A gas detector having a pore diameter d of 0.5 μm or more, a thickness T of the surface layer of 10 to 500 μm, and a porosity of the surface layer of 50 to 90% . 2. The method according to claim 1, wherein the surface layer is formed of α-alumina, γ-alumina, mullite, MgO.Al ₂ O _3.
A gas detector comprising one or more heat-resistant particles of spinel.