JPS63171356A - Detector for measuring concentration of oxygen - Google Patents

Abstract

PURPOSE:To enhance heat resistance and response, by a method wherein a diffusion layer is formed into a double structure and one layer is formed coarsely in a relatively thick state by a plasma spraying method while the other layer is formed as an extremely dense and relatively thin layer by a sol/gel method. CONSTITUTION:An element main body 1 is composed of a zirconia solid electrolyte partially stabilized by yttrium oxide and platinum is plated to the inner and outer surfaces of the element main body 1 as reactive electrodes 2a, 2b. Further, a magnesia spinnel is applied as the first layer 3a of a gas diffusion resistance layer by a plasma spraying method and the thickness of the layer 3a is set to 10-500mum, desirably, 20-200mum. Furthermore, the second layer 3b is formed on the layer 3a as a highly dense ceramic film by a sol/gel method in a thickness of 0.01-20mum, desirably 0.5-5mum. By reducing the thickness of the total layer as mentioned above, heat resistance and response can be enhanced.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to a detector for measuring oxygen concentration, and is particularly used for controlling an internal combustion engine, and is used to control oxygen concentration from a low concentration air-fuel ratio (lean) to a high concentration air-fuel ratio (lean). This type of detector has a wide range that can be used in a wide range of applications up to rich.

[Conventional technology]

In conventional stoic sensors (detects the theoretical air-fuel ratio λ = 1) and lean sensors (detects only the low concentration range), the oxygen diffusion layer is formed with a thickness of 50 to 450 μm by plasma spraying using magnesia spinel powder. Its porosity is approximately 5-10%.

The average pore diameter is approximately 20 when measured using a mercury porosimeter.
It has the characteristics of 0 to 500 people.

However, in order to improve the combustion efficiency of automobiles, it is necessary to control the air-fuel ratio over a wide range from the rich side with high concentration to the lean burn side.

However, in order to measure the oxygen concentration on the rich side, an automotive combustion system that uses a diffusion resistance air-fuel ratio sensor rather than the conventional oxygen diffusion resistance layer described above needs to grasp the combustion state by measuring the oxygen concentration in the exhaust gas. and feeds back information to the circuit that controls the amount of gasoline supplied and the amount of air.
It controls the mixture ratio of gasoline and air amount (excess air ratio A/F), and the stoichiometric air-fuel ratio λ=l (A/F=1
4.7/1) In the larger region, that is, on the lean side, the components in the exhaust gas are mostly 02, and the unburned gas G O, H
C, Hz is extremely small. (Figure 3) Here 02
．． passes through the diffusion layer and is ionized by a catalytic reaction at the outer reaction electrode, and the oxygen ions move from the exhaust gas side to the atmosphere. (Figure 4) At this time, oxygen passing through the diffusion resistance layer needs to be rate-controlled by the diffusion resistance layer, so the diffusion resistance layer requires a certain degree of density.The oxygen that reaches the reaction electrode is However, since the oxygen concentration in the exhaust gas varies depending on the air-fuel ratio, the output shows a limiting current characteristic as shown in FIG.

Here, the following theoretical formula (1) representing the limiting current will be explained.

F: Faraday constant R: Gas constant Dot: Diffusion constant of oxygen molecules T: Absolute temperature E: Diffusion rate of gas (oxygen) diffusion resistance layer Q: Effective diffusion distance of gas (oxygen) diffusion resistance layer s: ft! Pole area Pot: Oxygen partial pressure This equation (1) is well known and the limiting current shown in Fig. 5 is determined by the value of each term, but when each constant is shown collectively, (1)
The formula is the following formula (2).

T p ”K-1...(
2) Q That is, the limiting current rp is determined by a, which corresponds to the density of the gas diffusion resistance layer, and the electrode area S.

If the electrode area S is small, the limiting current will also be low, but if it is too small, it will affect the reaction speed and accuracy, so a certain area must be secured.Therefore, the limiting current Ip is the effective diffusion distance of the gas diffusion resistance layer. It depends on a, and Q
is larger, that is, the denser the gas diffusion resistance layer is, the IP
is small, which is effective for detection control in rich areas.

On the rich side, there is less O2 in the exhaust gas and unburned gas CO,
Since there are a lot of HC and Hz, these unburned gases pass through the diffusion layer, and contrary to the case of oxygen ions, they pass from the atmosphere side through the solid electrolyte and react with the unburnt gases on the outer electrode. Become. However, the particle size of the unburned gas component is 0.
Since it is much smaller than 2 particles, the amount passing through the diffusion layer cannot be controlled by the conventional diffusion layer, and only rich side control is possible. That is, in order to perform rich side control, a dense diffusion layer that can control the rate of diffusion of unburned gas particles is required.

However, due to densification, the diffusion layer may become clogged with impurities in the exhaust gas, reducing the useful life of the detector.
It must not contain any factors that degrade the response speed of the detector due to the long time it takes to reach the reaction electrode. It was not possible to titrate these conditions by changing only the porosity or thickness of a single layer.

In view of this point, it is proposed that the diffusion layer has a two-layer structure with different densities, as disclosed in JP-A-53-13980 and JP-A-53-1.
It has been proposed in Publication No. 16896, etc.

In the above, the first layer of alumina close to the electrode was coated with plasma spraying to a thickness of 30 μm, and the second layer on the outside was coated with a coarse layer of 8 μm using the same method.
It is formed with a thickness of 0 μm. In the latter case, the first layer of magnesium spinel was coated with a rough thickness of 300 μm, and the second layer was coated with magnesium spinel using the same plasma spraying method.
The layer is densely formed to a thickness of 2 m.

[Problem that the invention seeks to solve]

The above conventional technology does not consider the relationship between the thickness and density of the diffusion layer and heat resistance, productivity, or response.
In the former case, there is a problem in that the rough and thick outer layer cracks due to heating and cooling cycles. The latter is not practical because it is difficult to form a dense and thick outer layer and the resistance becomes too large, resulting in poor response.

An object of the present invention is to obtain a detector for measuring oxygen concentration that is equipped with a gas diffusion layer having an optimal gas diffusion function.

[Means for solving problems]

The above purpose is to make the diffusion layer a double structure, one layer is coarse and relatively thick, and the other layer is very dense and relatively thin, and this is formed by the sol-gel method. This can be achieved by

Optimally, the first layer on the electrode side should be plasma sprayed to a thickness of 10-50%.
0μm, and the second layer on the outside is 0.01~ by sol-gel method.
It is preferable to form it to a thickness of 20 μm.

[Effect]

In a detector configured in this manner, one layer is formed by the sol-gel method, so that although the layer is thin, it has a small porosity and exhibits a sufficient diffusion-limiting function. Since this layer is thin, the overall thickness of the diffusion resistance layer is reduced, and thermal strain due to the difference in thermal expansion coefficient with the solid electrolyte is reduced.
Cracks become less likely to generate magnetism.

- [Example] The basic configuration of an example of the detector according to the present invention and the action of the oxygen diffusion layer will be described below. The first layer on the outer electrode is a magnesia spinel layer formed by plasma spraying. It is important that this first layer is rougher than the second layer, a diffusion layer formed by the sol-gel method, as it has a close relationship with the catalytic reaction on the electrode, and also improves the responsiveness of the detector. Appropriate density is required for good performance. As a guideline! & Although it is not a suitable scale, the porosity is 5 to 1
Approximately 0%, average pore diameter measured by mercury porosimeter is 300
~400 people. The thickness of the first layer is 10 to 500 μm.
Therefore, if the thickness is carelessly increased, cracks are likely to occur due to the difference in thermal expansion coefficient between the solid electrolyte and the solid electrolyte.
0 to 200 μm is suitable.

The second M is a very dense ceramic layer formed by a sol-gel method, and this layer is unburned gas, especially for the detection of rich regions. Go, H.C.

This is to limit and rate the diffusion of H2 fine particles. If the film thickness is too thick, gas diffusion will be difficult, so set the film thickness to 0.
01 to 20 μm, preferably 0.5 to 5 μm. Examples will be described in more detail. FIG. 5 is a sectional view of a limiting current type oxygen concentration measuring detector used for controlling an automobile according to the present invention. That is, the element main body 1 is a solid electrolyte of zirconia oxide (hereinafter abbreviated as Zr0z) partially stabilized with yttrium oxide (hereinafter abbreviated as Y2O2), and platinum (hereinafter abbreviated as P) is added to this as reaction electrodes 2a and 2b. The inside and outside surfaces of the element are plated. Outside mold%2b
Since it is related to the electrode area which affects the characteristics in the above-mentioned theoretical formula (1), it is formed with high precision by masking during Pt plating. Also, outside? ! ! Lead electrodes 4 connected to the poles are formed by masking P and plating at the same time, but are covered with a thin glass insulating layer to completely block reaction with exhaust gas. Regarding the gas diffusion resistance layers 3a and 3b, which are important to the present invention, first, magnesia spinel was sprayed as the first layer 3a by a plasma spraying method.

A SEM image of the surface is shown in FIG. 9, and it can be seen that semi-molten powder is attached. What is important here is that gas not only diffuses from the gap where one powder is deposited, but also that the surface has a fine rack (0.1 to 0.2 - or less).
This means that it also spreads from This is a characteristic of ceramic spraying. Average particle size as thermal spray powder: approx. 15μm
Magnesia spinel (M g O・A Q zoa)
Thermal spraying was carried out at a feed rate of approximately 10 g per minute. By the way, powder has very high hygroscopicity due to its fine particle nature and is easily affected by indoor humidity, so stable powder supply is only possible through rotation. When the supply amount changes, the rate at which the film is deposited on the reaction electrode also changes, which greatly affects the density of the coating. Therefore, the limiting current characteristics will fluctuate, making it impossible to provide a stable oxygen concentration measurement detector. For this reason, thermal spray powder must always be supplied in a constant dry state. In order to solve this problem in the production equipment of this embodiment, a preheating device for drying the powder to 80 to 100° C. is attached to the powder supply device. In addition, the plasma gas is argon (Ar
) and nitrogen (NZ), the thermal spraying conditions were 800A and 50V.
．． A plasma spray gun that is rotated at 0 rpm and injects semi-molten magnesia spinel powder under the above conditions is set at a relative speed of 1000 m/w to the rotating element.
The first layer was thermally sprayed to a thickness of about 80 μm. The density of this film is 5 to 10% when measured in terms of porosity.
According to a mercury porosimeter, the average pore diameter is 300 Ajii.
Later. Reference FIG. 1 shows a surface SEM image of the first layer.

Next, the second layer [1, which was a highly dense layer, was formed by a sol-gel method to form a ceramic (hereinafter abbreviated as SiOz) film on the first layer. Silica sol was used for coating by the sol-gel method. The element subjected to plasma spraying is dipped in silica sol at room temperature and baked at 700°C for 30 minutes. By repeating this process twice, the film thickness is approximately 1 μm.
And so. This second layer is very dense, and an SEM image of its cross section taken with an electron microscope is shown in Reference FIG. 2.

Through the above steps, the outermost layer has a highly dense layer that can control the rate of gas diffusion, and below that, there is a gas diffusion resistance layer that allows appropriate gas diffusion and is effective in speeding up the reaction rate with the Pt electrode. The detection element is completed.

FIG. 1 shows a concentration measuring detector manufactured using this detection element 1. The detection element 1 is fixed to the stopper 5. The end of the stopper is equipped with an outer cylinder 7 for protecting the detection element, and the inside of the element contains 600 to 70
A heater 6 is built in to heat the device to 0° C. and turn the element material zirconia into an electrolyte. Furthermore, the outer reaction electrode 2b and the inner type Iji! A lead wire 8 is installed in each of the H.289 heaters 6 for extracting electrical signals and applying voltage. The oxygen concentration measuring detector manufactured in this way is attached to the exhaust pipe of a car, the heater is energized to heat the solid electrolyte in the device body to about 700°C, and voltage is applied to the device, and the oxygen concentration increases. Output characteristics of the measurement detector As shown by the solid line in Figure 2, especially on the rich side, λ = 0.6.
It was confirmed that the air-fuel ratio can be detected as a linear output. As shown by the broken line, the characteristics of conventional diffusion films were such that they could only detect up to λ = 0.8 on the rich side, and the output was saturated in the rich region where the concentration was higher. This is a huge improvement. If this is translated into drivability, normal driving on flat ground (4
In the range of 0 to 60 am/h), lean region control provides economical driving, while rich region control improves output and improves overall drivability when driving uphill, such as on mountain roads. In addition, 3-way feedback control (Go, HC, NOX control in exhaust gas) with oxygen sensor (stoic sensor)
In current engines that perform this, λ may become rich to about 0.6 during a cold start or sudden acceleration, so the detector according to the present invention It can be used not only for wide-range air-fuel ratio control in current engines (engines), but also for wide-range air-fuel ratio control in current engines, which has a ripple effect that is effective in improving fuel efficiency, drivability, and safety.

The present invention relates to a detector for measuring oxygen concentration used for air-fuel ratio control, and is particularly characterized by the gas diffusion resistance layer of the detection element. There are some important points. First, in plasma spraying, an example was shown in which magnesia spinel powder was used, but there is no particular control on the type of powder, and the coating after spraying has a porosity of, for example, 2 to 20%. Based on the average pore diameter measured by a mercury porosimeter, the effects of the present invention can be exerted on 200 to 500 people. In other words, the powder is alumina, magnesia, silica, or titania.

Single ceramics such as zirconia and calcia,
Alternatively, it is effective even if it is a composite powder, and the particle size of the powder does not matter.Also, the second highly dense layer is a silica coating made by the sol-gel method in this example, but this material Needless to say, the same effect can be achieved with alumina, zirconia, titania, calcia, magnesia, etc.

Furthermore, the same effect can be obtained by forming an Ht-dense layer on the electrode by the sol-gel method and then forming a rough layer on the outside by the plasma spraying method.

〔Effect of the invention〕

As explained above, according to the present invention, since the layer thickness can be reduced, it has excellent heat resistance and responsiveness.

[Brief explanation of the drawing]

Figure 1 is a sectional view of the oxygen concentration measuring detector of the present invention, Figure 2 is a cross-sectional view of the oxygen concentration measuring detector of the present invention;
The figure is an output characteristic diagram obtained by the present invention, Figure 3 is an explanatory diagram of the air-fuel ratio and gas components in exhaust gas, and Figure 4 is a limit 1'
A diagram explaining the principle of lf flow type air-fuel ratio control, Figure 5 is a diagram explaining the relationship between limiting current and air-fuel ratio, Figure 6 is a cross-sectional view of a detection element having a gas diffusion resistance layer, and Figure 7 is a conventional gas diffusion resistance. Layer explanatory diagram, Figure 8 is an explanatory diagram of the gas diffusion resistance layer of the present invention, reference diagram 1
is a surface SEM image of the magnesia spinel sprayed layer, reference figure 2
is a cross-sectional view of the S x Oz highly dense layer produced by the sol-gel method.
This is an EM image. DESCRIPTION OF SYMBOLS 1... Solid electrolyte, 2... Reaction electrode, 3a... Gas diffusion resistance layer, 3b... Highly dense gas diffusion resistance layer, 4...
...Lee's agent Patent attorney Katsuma Ogawa ゛-' Figure 1 Figure 2 0, g I / 5 Figure 4-Figure 60 Figure 7 Figure 80 Procedural amendment (method) 1 % formula % of the case Indication 1988 Patent Application No. 1663 Name of invention
Relationship with the case of a person who corrects a detector for measuring oxygen concentration Patent applicant

Claims (1)

[Scope of Claims] 1. Porous thin film electrodes are provided on the front and back surfaces of a solid electrolyte element made of an oxygen ion conductive metal oxide, and the element surface electrode is a gas diffusion layer made of a porous electrically insulating metal oxide. By covering with a resistive layer and applying a constant voltage between both electrodes, oxygen ions in the atmosphere in which the element is placed are diffused into the element, and the oxygen concentration is determined by determining the limiting current value for the oxygen ion concentration. In the limiting current type oxygen concentration measuring detector to be measured, in which the diffusion resistance layer is composed of at least two or more multilayers of a coarse layer and a very dense layer, the coarse layer has a moderate particle size. A detector for measuring oxygen concentration, which is formed by a plasma spraying method using an electrically insulating metal oxide, and the very dense layer is an electrically insulating metal oxide layer formed by a sol-gel method. . 2. In the product described in claim 1, the thickness of the rough layer formed by the plasma spraying method is 10 to 5.
00 μm, and the very dense layer formed by the sol-gel method has a thickness of 0.01 to 20 μm. 3. In the device described in claims 1 and 2, first the rough layer is formed on the outer electrode surface by plasma spraying, and then the dense layer is formed on the outside by sol-gel method. A detector for measuring oxygen concentration, characterized in that: