JPH0754314B2 - Oxygen concentration detector - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an oxygen concentration detector, and is particularly used for control of an internal combustion engine and the like, and is used from a low concentration air-fuel ratio (lean) to a high concentration air-fuel ratio (litch). The present invention relates to a wide range detector of this type that can be measured over a wide range up to.

[Conventional technology]

Conventionally, for example, in an automobile combustion system,
A system has been put into practical use in which the excess air ratio λ (λ = actual air-fuel ratio / theoretical air-fuel ratio) of the engine is measured from the oxygen concentration in the exhaust gas and the air-fuel ratio is fed back controlled from this detected value.
An oxygen concentration detector used as this type of air-fuel ratio sensor is, for example, platinum on the inner and outer surfaces of a solid electrolyte element formed by mixing a small amount of yttrium oxide (Y ₂ O ₃ ) with zirconium oxide (ZrO ₂ ) and firing the mixture. In order to attach an electrode and enable stable detection, a gas diffusion resistance layer that limits the rate of oxygen and the like is provided on the side of the electrode (reaction electrode) exposed to exhaust gas.

By the way, it is generally difficult for the conventional oxygen concentration detector to measure the excess air ratio in the latch region of the air-fuel ratio (the reason will be described later). Therefore, it is used as a stoichiometric sensor (which detects the theoretical air-fuel ratio λ = 1) and a lean sensor (which detects only the low-concentration air-fuel ratio region), but ideally it is ideal for increasing the combustion efficiency of automobiles. ,
It is necessary to control the air-fuel ratio in a wide range from the rich side with a high air-fuel ratio concentration to the lean side with lean combustion, and the development of a wider range of oxygen concentration detectors has been under development from the past. Has the following points to be improved.

Here, the measurement principle from the lean air-fuel ratio region of the oxygen concentration detector to the rich air-fuel ratio region and points to be improved are 7, 8 and 9
A description will be given based on the figure.

FIG. 7 is a principle explanatory view for explaining the principle of wide-lensity measurement. In the detection part, the reaction electrodes 2 and 3 are formed by platinum plating on the front and back surfaces of the ZrO ₂ —Y ₂ O ₃ solid electrolyte element 1. A gas diffusion resistance layer 4 is provided on the reaction electrode 2. The gas diffusion resistance layer 4 is formed to have a thickness of 50 to 450 μm by plasma spraying using magnesia spinel powder, for example, and has a porosity of about 5 to 10%, and an average pore diameter measured by a mercury porosimeter of about 5 to 10%. It has the property of 200-500Å. The solid electrolyte element 1 is kept at a constant temperature by a heater (not shown), the reaction electrode 2 side is arranged in an exhaust gas atmosphere, and the electrode 3 side is arranged in the atmosphere.

An automobile combustion system that uses an air-fuel ratio sensor (oxygen concentration detector) can detect the combustion state by measuring the oxygen concentration in the exhaust gas, and inform the circuit that controls the gasoline supply amount and air amount. To control the mixture ratio of gasoline and air amount (air-fuel ratio A / F). (1) A region larger than the theoretical air-fuel ratio λ = 1 (A / F = 14.7 / 1), that is, lean On the side, most of the components in the exhaust gas are O ₂ , and the amount of CO, HC, and H _{2 in the} gas is extremely small (No. 8).
Figure). When a voltage is applied between the electrodes 2 and 3 in this state, O ₂ in the exhaust gas passes through the diffusion layer 4 and is ionized by the catalytic reaction at the outer reaction electrode 2. Since the solid electrolyte 1 has ion conductivity, oxygen ions pass through the solid electrolyte element 1 to reach the electrode 3 and emit electrons. In this process, oxygen ions become carriers, and the current I _P (I _P > 0) flows in the arrow direction (forward direction). In this case, in order to obtain a stable oxygen concentration detection value (limit current characteristic), it is necessary to rate-control the oxygen passing through the exhaust gas by the diffusion resistance layer 4, and the diffusion resistance layer 4 is required to have a certain degree of fineness. To be done. The oxygen reaching the reaction electrode 2 is ionized as described above, but the oxygen concentration in the exhaust gas differs depending on the air-fuel ratio, so that the output shows the limiting current characteristic as shown in FIG.

Here, the following theoretical formula (1) showing the limiting current will be described.

F: Faraday constant R: Gas constant Do ₂ : Diffusion constant of oxygen molecule T: Absolute temperature E: Diffusivity of gas (oxygen) diffusion resistance layer l: Effective diffusion distance of gas (oxygen) diffusion resistance layer S: Electrode area Po ₂ : Oxygen partial pressure This equation (1) is publicly known and the limiting current in FIG. 9 is determined by the value of each term.
The formula becomes the following formula (2).

That is, the limiting current I _P is determined by 1 corresponding to the denseness of the gas diffusion resistance layer and the electrode area S. If the electrode area S is small, the limiting current also becomes low, but if it is too small, the reaction rate and accuracy are affected, so a certain area must be secured. Therefore, the limiting current I _P depends on the effective diffusion distance 1 of the gas diffusion resistance layer.

(2) Next, at the complete combustion point (λ = 1), as a result of safe combustion, O ₂ does not move, no current flows, and the limiting current value becomes I _P = 0 as shown in FIG.

(3) Also, on the latch side (λ <1), the exhaust gas contains little O _{2 and a} large amount of unburned gases CO, HC, and H ₂ , so these gases pass through the diffusion layer 4, and In contrast to the lean case, the oxygen ions pass through the solid electrolyte 1 from the atmosphere side, and undergo an oxidation reaction with the unburned gas on the outer electrode 2. That is, in this case, when the oxygen concentration detector itself acts like a concentration battery based on the oxygen concentrations on the atmosphere side and the exhaust side to generate super power and the applied voltage is controlled to a certain value, the current I _P becomes (1 ) Flows in the opposite direction (I _P <0) to the case of the lean region. If the unburned component reaching the reaction electrode 2 is also appropriately rate-controlled by the diffusion resistance layer 4, as shown in FIG. 9, the limiting current characteristic I _{P is} similar to that in the lean region (however, the limiting current characteristic I _P in the lean region is _{If P} is a positive value, the limiting current characteristic I _{P in the} latch region is represented by a negative value). Since these limiting current characteristics I _P are proportional to the excess air ratio λ, it is possible to perform a linear air-fuel ratio control. Become. However, in the conventional detection element of this type, it is very difficult to measure the excess air ratio particularly in the latch region, and it is difficult to obtain the limiting current I _P as shown in FIG. The reason is that the particle size of the unburned gas component is much smaller than that of the O ₂ particle, so that the amount of the gas passing through the diffusion layer cannot be rate-controlled in the diffusion layer for O ₂ as in the past, and the latch is Side control becomes impossible. That is, in order to perform the control on the latch side, a dense diffusion layer is required so as to control the diffusion of the unburned gas particles.

However, due to the densification, the diffusion layer is clogged with impurities in the exhaust gas, which shortens the useful life of the detector,
The detector must not include the factor that the arrival time to the reaction electrode becomes long and the response speed of the detector is deteriorated. It was not possible to satisfy these conditions by changing only the porosity and thickness of a single layer.

This is a fundamental cause that a detector for detecting and controlling a so-called wide range from the rich region to the lean region does not appear until now.

In view of this point, it has recently been proposed that the diffusion layer has a two-layer structure with different densities, which is disclosed in Japanese Patent Laid-Open Nos. 53-13980 and 53-116.
It is proposed in Japanese Patent Publication No. 896.

In the former case, the first layer close to the electrode is densely formed by the plasma spraying alumina layer and has a thickness of 30 μm, and the second layer outside thereof is formed as a rough layer with a thickness of about 80 μm by the same method. ing. In the latter case, the first layer and the second layer are similarly formed by magnesia spinel of the plasma spraying method. The first layer is a rough layer, the thickness is 300 μm, the second layer is a dense layer, and the thickness is about 2 mm. Has been formed.

[Problems to be Solved by the Invention]

As described above, when either the first layer or the second layer of the diffusion resistance layer is made to be a relatively rough layer and the other is made to be a dense layer, in addition to O ₂ , the particles are smaller than this. Unburned gases such as CO, HC, and H ₂ can be rate-controlled appropriately, and further, clogging by impurities in the exhaust gas can be reduced as compared with a single dense layer, which can be improved.

However, even in such a conventional technique, sufficient consideration has not been given to the relationship between the thickness or denseness of the diffusion layer and the heat resistance, productivity, or responsiveness. For example, in the former case, a rough outer thick layer is used. However, there is a point to be improved in terms of heat resistance because a large crack is likely to occur due to the cold heat cycle.
In the latter case, it is difficult to form a dense and thick layer on the outside by the plasma spraying method, and it is desired to improve the productivity. Also, since the dense layer is thick, the diffusion resistance becomes too large and the responsiveness tends to decrease. There was a tendency.

The present invention has been made in view of the above points, and an object thereof is to have an optimum gas diffusion function (rate-controlling function) and to detect a wide range of air-fuel ratio states from the lean to the rich region. Moreover, it is to provide an oxygen concentration detector having excellent heat resistance and durability.

[Means for Solving the Problems]

The above object is achieved by configuring a gas diffusion resistance layer of this type as follows.

That is, as a first solution, at least one of the gas diffusion resistance layers is a plasma sprayed layer using an electrically insulating metal oxide having an appropriate grain size, and the other layers are PVD (Physical) layers.
Vapor Deposition) is used to form a thin film layer formed on the surface of the plasma sprayed layer, and the thin film layer of this PVD method is a denser layer than the plasma sprayed layer.

Here, as the PVD method, as described in the embodiments, for example, there are an ion mixing method, a dynamic ion mixing method, an ion plating method, a spattering method, and the like.

A second solution is to reverse the arrangement of the plasma sprayed layer and the PVD thin film layer described in the first solution to that of the first layer.
That is, of the gas diffusion resistance layer, the first layer closer to the electrode is a dense thin film layer formed by the PVD method, and the second layer formed on the surface of the first layer has an appropriate grain size. And a plasma sprayed layer of the electrically insulating metal oxide, which is a rougher layer than the thin film layer of the PVD method.

A third solution is to form the following surface modification layer (thin film layer) on the surface of the plasma spray layer instead of the PVD thin film layer. That is, as this type of gas diffusion resistance layer, at least one layer is formed by a plasma sprayed layer using an electrically insulating metal oxide having an appropriate particle size, and in addition, fine cracks generated on the surface of the plasma sprayed layer and the sprayed layer. By implanting appropriate elemental ions over the surface of the plasma sprayed layer and the surface of the fine cracks, a surface modification layer made of a compound of the plasma sprayed layer and an ionic element is formed. As the elemental ions, for example, O ions, N ions, Ar ions and the like are suitable.

[Action]

In the first and second solving means described above, the gas diffusion resistance layer for rate-controlling O ₂ and unburned gas (CO, H ₂ , HC, etc.) is
It is composed of a plasma sprayed layer and a thin film layer formed by the PVD method.
Since the thin film layer is formed by the PVD method,
A highly dense layer that can form an extremely thin layer and has a small porosity despite its thinness and that is capable of sufficiently controlling the diffusion rate of not only O ₂ but also gas with small particles such as CO, H ₂ , and HC. Can be formed. Since this layer is thin, the thickness of the diffusion resistance layer as a whole becomes thin, the occurrence of thermal strain due to the difference in thermal expansion coefficient from the solid electrolyte is reduced, and cracks based on this are less likely to occur.

And, in the first solution, the surface of the plasma sprayed layer is
A thin film layer is formed by the PVD method, and this thin film also extends to the surface of fine cracks generated in the plasma sprayed layer. This fine crack is an unavoidable one that occurs during the plasma sprayed layer formation process, and is extremely fine, unlike the crack that occurs due to the cooling / heating cycle when the oxygen concentration detector is used,
There is no problem in its existence itself, on the contrary, innumerable fine cracks are generated, and these fine cracks play an important role in controlling the passing gas together with the fine holes in the plasma sprayed layer. Then, by forming a thin film layer of the PVD method on the micropore surface and the microcrack surface in the plasma sprayed layer, the opening area of these micropores and microcracks is appropriately reduced, and other than O ₂ , The rate of unburned gas having particles smaller than this is also well controlled.

Therefore, the oxygen concentration detector outputs a good limiting current corresponding to the gas to be measured in the lean region as well as in the lean region of the air-fuel ratio, and can accurately detect the air-fuel ratio from the lean to the rich region. . Here, the plasma sprayed layer is rough and has a function of improving the catalytic reaction of the gas on the electrode and a role of accelerating the detection response. Since the measurement mechanism of the oxygen concentration detector has been described in the section of [Prior Art], the description thereof is omitted here.

According to the second solving means, a thin film layer of PVD method is formed on the electrode side and a plasma sprayed layer is formed on the outer side. In this case, however, the plasma sprayed layer is a relatively rough layer, so the plasma sprayed layer is formed. From the surface of PVD to the thin film layer of PVD method, the passing gas (for example, O ₂ or unburned gas such as CO, H ₂ and HC) enters relatively smoothly, and then fine PVD thin film layer is good. In addition to O _2, the rate is controlled satisfactorily for unburned gas with smaller particles. Therefore, also in this case, it is possible to accurately detect the air-fuel ratio from the lean region to the latch region.

The plasma sprayed layer has a function of rate-controlling to some extent, and also has a function of thermally and mechanically protecting the electrodes, the PVD thin film layer and the like.

The thickness of the plasma sprayed layer is preferably 10 to 500 μm, and the thickness of the PVD thin film layer is preferably set to 20 μm or less.

Next, in the case of the third solution, appropriate elemental ions are implanted into the surface of the plasma sprayed layer and the surface of the fine cracks generated on the surface of the sprayed layer, and the surface of the sprayed layer and the surface of the fine cracks are implanted. A surface modification layer made of a compound of a plasma sprayed layer and an ionic element is formed. The surface modification layer in this case, the surface of the fine holes and the fine cracks of the plasma spray layer, a part of the ionic element is attached, or the structure of the element of the plasma spray layer ejected by implantation is attached, The opening area of fine holes and fine cracks in the plasma sprayed layer is reduced. Therefore, similarly to the first solution, not only O ₂ but also unburned gas having smaller particles can be rate-controlled,
The air-fuel ratio detection from lean to rich can be performed well.

〔Example〕

Embodiments of the present invention are shown in FIGS. 1 to 6, 10 and 11.
A description will be given based on the figure.

FIG. 1 is a sectional view of a limiting current type oxygen concentration detector for air-fuel ratio control of an automobile to which the present embodiment is applied.

Numeral 1 is a solid electrolyte element of zirconia oxide (hereinafter abbreviated as ZrO ₂ ) partially stabilized by yttrium oxide (hereinafter abbreviated as Y ₂ O ₃ ), on which platinum (hereinafter P
Abbreviated as t. ) Is plated on the inner and outer surfaces of the element. Since the outer electrode 2 is related to the electrode area that affects the characteristics in the theoretical formula (1), it is accurately formed by masking during Pt plating.

A gas diffusion resistance layer 4 is composed of a first layer 4a and a second layer 4b. These layers 4a and 4b are essential parts of this embodiment and will be described in detail later.

Reference numeral 5 is a lead electrode connected to the outer electrode 2,
Is formed by Pt plating masked at the same time, but it is covered with a thin glass insulating layer to completely block the reaction with exhaust gas.

6 is a plug, 7 is an internal heater, 8a to 8c are lead wires, and 9 is a protective cap.

Now, the gas diffusion resistance layer 4a, 4b important to the present invention,
As shown in FIG. 4, first layer 4a is formed of a plasma sprayed layer formed by spraying magnesia spinel by a plasma spraying method. The first layer 4a is a layer of the second layer 4b described later.
It is important to be rougher than the thin film layer formed by the VD method, especially because it is closely related to the catalytic reaction on the electrode, and an appropriate density is required to improve the responsivity as a detector. Is. Although it is not the most suitable measure for this,
The porosity is about 5 to 10%, and the average pore diameter measured by the mercury porosimeter is 300 to 400Å. The thin thickness of the first layer is 10-50.
The thickness is 0 μm, and if the thickness is carelessly thick, cracks are likely to occur due to the difference in thermal expansion coefficient from the solid electrolyte, so 20 to 200 μm is preferable.

A scanning electron micrograph (hereinafter referred to as SEM photograph) showing the surface of the first layer (plasma sprayed layer) 4a is shown in FIG.
In FIG. 10, it can be seen that the semi-molten powder is attached. What is important here is not only that the gas diffuses only from the gaps (fine pores) where one powder is deposited,
Fine surface cracks (0.1 to 0.2)
It is also diffused from (μm or less). The fine cracks on this surface are characteristic of ceramic spraying. Since many fine cracks are present, they play an important role especially in gas diffusion. Further, this fine crack is suitable as a diffusion resistance for controlling the rate of O ₂ , but as it is, it is insufficient as a function for controlling the rate of particles HC, H ₂ , and CO below it.

Here, a specific forming process of the first layer 4a will be described.

In this embodiment, the average particle diameter of the sprayed powder of the first layer 4a is about 15
Using magnesia spinel (MgO.Al ₂ O ₃ ) of μm, 1
It was sprayed at a feed rate of about 30 g per minute. By the way, the powder is very fine and because of its nature, it has a very high hygroscopicity, is easily influenced by the humidity in the room, and it is difficult to stably supply the powder. When the supply amount changes, the film formation rate of the film deposited on the reaction electrode also changes, which greatly affects the denseness of the coating. Therefore, the limiting current characteristics fluctuate, and it becomes impossible to provide a stable oxygen concentration measuring detector. Therefore, it is necessary to always supply the thermal spraying powder in a constant dry state. In the production equipment of this embodiment, in order to solve this problem, the powder feeding device is provided with a preheating device for drying the powder to 80 to 100 ° C.
The plasma gas is a mixed gas of argon (Ar) and titanium (N ₂ ), and the spray output is 800A, 50V. The thermal spraying state rotates the solid electrolytic element at about 600 rpm, the plasma spraying gun that the semi-molten magnesia spinel powder injects under the above conditions, is sprayed at a relative speed of 1000 m / min with respect to the rotating element, The first layer was formed to a thickness of about 80 μm. The denseness of this coating is 5-10 when porosity is measured.
%, The average pore size is around 300Å according to the mercury porosimeter.

Next, the second layer 4b will be described.

The second layer 4b is a highly dense layer (thin film layer or surface modification layer) formed by the PVD method or ion implantation. This layer
In addition to O ₂ , in particular, in order to enable detection of the latch region as well, it is for limiting the rate of diffusion of fine particles of CO, HC, and H ₂ which are unburned gases. If the film thickness is too thick, gas diffusion will be difficult to occur, so in the PVD method, it is 0.01 to 20 μm, preferably 0.01 to 5 μm. In the case of the ion implantation method,
It is several Å to 1 μm, preferably 10 to 1000 Å.

Here, a specific example of forming the second layer 4b on the surface of the first layer (plasma sprayed layer) 4a will be described.

First, the case where the second layer 4b is formed by the PVD method will be described.

As the PVD method, there are a dynamic ion mixing method, an ion mixing method, an ion plating method (reactive ion plating method represented by HCD method, etc.), a spattering method (magnetron spattering method, etc.) and the like.

The dynamic ion mixing method is a method in which vapor deposition and ion implantation are simultaneously performed when forming a thin film layer.

As the thin film layer 4b, TiN film formation, Al ₂ O ₃ film formation and the like are suitable. For example, when forming a TiN film by a dynamic ion mixing method, Ti vapor deposition is performed on the surface of the plasma sprayed layer 4a in the vacuum or reduced pressure atmosphere, and N ions are implanted on the Ti vapor deposition surface at the same time. Then, at the boundary between the thin film layer 4b and the plasma sprayed layer 4a, a mixing zone (a mixed layer of a compound to be deposited and a base atom) is formed by an ion implantation effect.
Is formed. With this ion mixing zone,
It is characterized in that high adhesion can be obtained between the first and second layers 4a and 4b and that the compound layer (thin film layer) 4b can be formed at a relatively high speed. In the dynamic ion mixing method of the present embodiment, TiN film was formed after the magnesia spinel was already formed on the electrode 2 as the first layer 4a by plasma spraying. That is, the fixed electrolyte element 1 was set in a sample holder as in the case of ion implantation, the chamber was evacuated to 5 × 10 ^-6 Torr or less, and then nitrogen gas was introduced to make 1.5 × 10 ^-4 Torr and an acceleration voltage of 20 k.
V, output current 0.2A, Ti deposition rate 7Å / Sec, ion incident angle 65
The film formation time was 7 minutes. When the cross section after the thin film layer treatment was analyzed by SEM and EPMA, it was found that the TiN layer was formed to be 0.06 μm and the mixing zone due to the ion implantation was present at about 500Å. In the case of the TiN film formation this time, it is necessary to enable the diffusion of O ₂ as an oxygen concentration detector, and the fine crack-shaped open holes on the surface of the plasma sprayed layer described above are narrowed without being completely closed. Therefore, it is necessary to select the film forming conditions for this. In the surface observation by SEM in Fig. 11, the TiN film is not a perfectly uniform smooth film, and it is found that the TiN film is significantly uneven due to the influence of the surface condition of the plasma sprayed layer of the underlying layer, and it has entered the fine crack-shaped open holes. It became clear.

In this way, TiN by the dynamic ion mixing method is
The film formation is an effective means for narrowing the fine crack-shaped open holes on the surface of the magnesia spinel sprayed surface of the underlying layer to make it dense and limit (rate-control) the diffusion of the gas to be detected.

The ion mixing method is one in which ion implantation is performed after vapor deposition, and a thin film layer similar to the dynamic ion mixing method is used.
4b can be obtained, and in addition, a good thin film layer 4b can be obtained by the ion plating method and the sputtering method.

FIG. 5 is a schematic view in which the thin film layer 4b of the PVD method is formed on the surface of the plasma sprayed layer 4a, and the thin film layer 4b extends to the surface of the fine crack 10. Therefore, according to such a structure, as described above, the thin film layer 4b is formed in the fine crack 10 in addition to the fine holes (not shown) of the plasma spray layer 4a. ， The opening area of the fine crack is appropriately reduced, and a good gas diffusion rate controlling function can be achieved.

The plasma spray layer 4a and the thin film layer 4b of the PVD method,
The order may be reversed, and the thin film layer 4b may be the first layer (inner layer) and the plasma sprayed layer 4a may be the second layer (outer layer). In this case, the gas that has passed through the plasma spray layer 4a is diffusion-controlled due to the denseness of the thin film layer.

Next, a case where an ion implantation (ion implantation) thin film layer (surface modification layer) is formed instead of the PVD method will be described.

The ion implantation method is a method of ionizing a target element and accelerating it to implant it on the surface of a substrate to modify or alloy the surface layer. The implanted atoms have a Gaussian distribution with a peak inside and reach the inside as the acceleration voltage increases. Further, when the accelerating voltage is high, the implanted atoms do not remain on the outermost surface, but the crystal structure is disturbed in the portion where the ions pass through, and sometimes amorphous. This time, N ions are implanted on the surface of the plasma sprayed layer of the magnesia spinel to perform surface modification on the sprayed fine pores and fine cracks peculiar to the ceramics, thereby densifying the surface portion and limiting the diffusion of gas to be detected ( Rate-determining).

That is, by implanting ions into the plasma sprayed layer, a compound of the sprayed layer element and an ionic element (surface modification layer) is formed on the surface of the plasma sprayed layer. In this case, fine holes and fine cracks in the plasma sprayed layer are formed. A part of the ionic element adheres to the surface of the plasma spraying element or the element of the plasma sprayed layer ejected by implantation adheres to reduce the opening area of the fine holes and the fine cracks of the plasma sprayed layer. Therefore, similar to the thin film layer of the PVD method, not only O ₂ but also unburned gas having smaller particles can be rate-controlled. As an ion implantation method, a plasma sprayed detector element was clamped by a sample holder, and oxygen ions were implanted while rotating at 60 rpm. Injection conditions are acceleration voltage 35kV, output current 0.25
A, ion incidence angle 65 ° Implantation time is 10 min. In addition,
As a result of analyzing the concentration distribution of 0 from the surface of the injected sample by RBS (Razaford backscattering analysis), an O of about 500Å
It was found that the surface modification layer formed by ion implantation was formed.

FIG. 6 shows an apparatus used for thin film formation (surface modification) by the ion implantation method and the dynamic mixing method of this example. When the surface modification of the ion implantation method is performed by using this apparatus, the vapor deposition unlike the dynamic mixing method is not performed. This device was manufactured by the same company, and the irradiation diameter of the conventional ion implantation device was as narrow as several φ to several tens of φ, which was unsuitable for practical use for surface modification of structural members. In the developed device, the bucket type ion source was used, and the beam size was 150 mm × 150 mm, the acceleration voltage: continuous _max 40 kV, the output current: continuous _max 0.4 A, a large capacity ion beam mixing device was used.

Through the above steps, a detection element having an effective gas diffusion resistance layer is completed, in which one layer is a highly dense layer capable of controlling gas diffusion and the other layer is capable of gas diffusion to accelerate the reaction rate with the Pt electrode. . FIG. 1 shows an oxygen concentration measuring detector manufactured by using the detecting element 1. The detection element 1 is fixed to the plug body 5. An outer cylinder 7 for protecting the detection element is provided at the tip of the stopper, and the element is placed inside the element.
A heater 6 is built in to heat the element to zirconia as an electrolyte by heating to 0 to 700 ° C. Further, the outer reaction electrode 2b, the inner electrode 2a, and the heater 6 are provided with lead wires 8a to 8c for taking out electric signals and applying voltage. The oxygen concentration measuring detector manufactured in this way was attached to the exhaust pipe of an automobile, the heater was energized to heat the solid electrolyte of the element body to about 700 ° C, and a voltage was applied to the element to obtain the oxygen concentration. Output characteristics of the detector for measurement It was confirmed that the air-fuel ratio can be detected as a linear output up to λ = 0.6 especially on the latch side as shown by the solid line in FIG. The characteristic of the conventional diffusion film is λ on the latch side as shown by the broken line.
= 0.8, the output was saturated in the higher density latch region, and the problem was greatly improved by the present invention. Also, conventionally, λ =
Even if it was possible to detect up to 0.8, the diffusion resistance layer was not dense, so there were large variations, and the detection accuracy on the latch side was poor. As a result, if the output characteristics of the oxygen concentration detector of the present embodiment are replaced with drivability, normal running on flat ground (40-60 km
In / h), lean area control results in economical driving, and in uphill driving on mountain roads, etc., output in the Rich area control improves output, improving overall drivability. In addition, the current engine, which uses the oxygen sensor (stock sensor) to perform three-way feedback control (CO, NC, NO _X control in exhaust gas), has a λ at cold start or during rapid acceleration.
May reach up to about 0.6, so the detector according to the present invention can be used not only for lean-burn engines (engines for lean burn control) but also for wide-range air-fuel ratio control in current engines, improving fuel efficiency. ， There is a ripple effect that is effective in improving drivability and safety.

It should be noted that although there was not much difference in the initial detector characteristics between the case where O ions were injected onto the plasma sprayed layer and the TiN coating by the PVD method such as dynamic ion mixing, there was little difference in the characteristics after the durability test (3000 hours). In terms of accuracy, the O ion-implanted detector gave a slightly better result.

Further, as the plasma sprayed layer, an example using magnesia spinel powder was shown in the example, but the kind of powder is not particularly limited, and the coating film after thermal spraying has a porosity of, for example, 2 to 20.
%, The average pore diameter measured by mercury porosimeter may be 200 to 500 Å.

That is, the plasma sprayed layer 4a is effective even if the powder is a single substance of ceramics such as alumina, magnesia, silica, titania, zirconia, and calcia, or a composite powder, and the particle size of the powder does not matter.

In this example, the TiN coating film by the PVD method such as the O ₂ ion implantation method and the dynamic mixing method was shown, but in the case of the ion implantation method, N ₂ and N ₂ other than O ₂ are not affected regardless of the type of gas. Ar, He,
Ne is also effective, and the same effect can be obtained by injecting metal. Needless to say, the same effect can be obtained in the PVD method such as dynamic mixing with oxides such as Al ₂ O ₃ , TiC, and BN, nitrides, and carbides in addition to TiN.

〔The invention's effect〕

As described above, according to the present invention, it is possible to enhance the diffusion-controlling function of the gas to be measured and widen the measurement range of oxygen concentration detection, and further, at least one of the gas diffusion resistance layers is PVD.
Since it is formed of a thin film layer by the ion diffusion method or the ion implantation method, thermal strain due to the difference in thermal expansion coefficient between the gas diffusion resistance layer and the solid electrolyte element is reduced, and in turn cracking based on this is effectively prevented. It is possible to obtain an oxygen concentration detector which is excellent in durability and durability and has good responsiveness.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of an oxygen concentration detector to which the present invention is applied, and FIG. 2 is a diagram comparing output characteristics of an oxygen concentration detector as an embodiment of the present invention and a conventional oxygen concentration detector. , Third
FIG. 4 is a sectional view of an essential part of an oxygen concentration detector to which the present invention is applied. FIG. 4 is a sectional view of an essential part showing an embodiment of the present invention.
FIG. 7 is a perspective view of an essential part showing an embodiment of the present invention, FIG. 6 is a system diagram of an ion implantation and dynamic ion mixing apparatus used in an embodiment of the present invention, and FIG. 7 is a measurement principle of an oxygen concentration detector. Fig. 8 is an explanatory diagram showing the relationship between the air-fuel ratio and the gas components in the exhaust gas, Fig. 9 is an output characteristic diagram of the oxygen concentration detector, and Fig. 10 is a SEM photograph showing the surface of the magnesia spinel sprayed layer. , Fig. 11 is an SEM photograph showing the surface of a high-density layer in which a TiN film is formed on the surface of the magnesia spinel sprayed layer by the dynamic ion mixing method. 1 ... Solid electrolyte element, 2, 3 ... Electrode, 4 ... Gas diffusion resistance layer, 4a ... Plasma sprayed layer, 4b ... Highly dense thin film layer (surface modified layer), 5 ... Lead electrode, 7 ...... Internal heater.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mitsuo Haginotani 4026 Kuji Town, Hitachi City, Ibaraki Prefecture Hitachi Research Laboratory, Hitachi, Ltd. (56) References JP-A-59-109854 (JP, A) JP-A-53 -116896 (JP, A)

Claims (10)

[Claims] 1. Porous and catalytic electrodes are arranged on the front and back surfaces of a solid electrolyte element having oxygen ion conductivity, and a passing gas is formed on the surface of an electrode of the electrodes arranged on the side of the atmosphere to be measured gas. In the oxygen concentration detector formed by coating the gas diffusion resistance layer for rate limiting, the gas diffusion resistance layer is a plasma spray layer using at least one layer of an electrically insulating metal oxide having an appropriate particle size, and another layer. Consists of a thin film layer formed on the surface of the plasma sprayed layer by PVD method,
An oxygen concentration detector characterized in that the thin film layer of the PVD method is a denser layer than the plasma sprayed layer. 2. A porous and catalytic electrode is arranged on the front and back surfaces of a solid electrolyte element having oxygen ion conductivity, and a passing gas is formed on the surface of the electrode which is arranged on the measured gas atmosphere side of the electrodes. In an oxygen concentration detector formed by coating a gas diffusion resistance layer for rate limiting, the gas diffusion resistance layer is a plasma sprayed layer using at least one layer of an electrically insulating metal oxide having an appropriate particle size, and, On the surface of this plasma sprayed layer, a thin film layer denser than the plasma sprayed layer is formed.
It is formed by the PVD method, and the thin film layer of the PVD method is spread over the surface of the fine cracks generated in the plasma sprayed layer to reduce the opening area of the fine cracks of the plasma sprayed layer. Oxygen concentration detector. 3. The plasma sprayed layer is first formed on the surface of the electrode arranged on the side of the measured gas atmosphere, and then the thin film layer of the PVD method is formed on the surface of the electrode to be measured gas atmosphere side. Oxygen concentration detector. 4. A porous and catalytic electrode is arranged on the front and back surfaces of a solid electrolyte element having oxygen ion conductivity, and a passing gas is formed on the surface of the electrode, which is arranged on the measured gas atmosphere side, of the electrodes. In an oxygen concentration detector formed by coating a gas diffusion resistance layer for rate limiting, the gas diffusion resistance layer is a dense thin film layer in which the first layer closer to the electrode is formed by the PVD method, The second layer formed on the surface of the first layer is a plasma sprayed layer of an electrically insulating metal oxide having an appropriate grain size, and the plasma sprayed layer is a layer rougher than the thin film layer of the PVD method. An oxygen concentration detector characterized by. 5. Any one of claims 1 to 4
In the paragraph, the thickness of the plasma sprayed layer is 10 ~ 500μ
The oxygen concentration detector is characterized in that the thickness of the thin film layer of the PVD method is 20 μm or less. 6. Any one of claims 1 to 5
In the paragraph above, the thin film layer of the PVD method is an oxygen concentration detector formed by using a dynamic ion mixing method. 7. The invention according to any one of claims 1 to 5.
In the paragraph above, the thin film layer of the PVD method is an oxygen concentration detector formed by using an ion mixing method. 8. The invention according to any one of claims 1 to 5.
In the item above, the thin film layer of the PVD method is an oxygen concentration detector formed by using an ion plating method. 9. Any one of claims 1 to 5
In the item above, the thin film layer of the PVD method is an oxygen concentration detector formed by using a sputtering method. 10. A porous and catalytic electrode is arranged on the front and back surfaces of a solid electrolyte element having oxygen ion conductivity, and a passing gas is formed on the surface of the electrode, which is arranged on the side of the atmosphere to be measured gas, of the electrodes. In an oxygen concentration detector formed by coating a gas diffusion resistance layer for rate limiting, the gas diffusion resistance layer is a plasma sprayed layer using at least one layer of an electrically insulating metal oxide having an appropriate grain size, and By implanting appropriate elemental ions on the surface of the plasma sprayed layer and the surface of the fine cracks generated in the sprayed layer, a surface modification consisting of a compound of the plasma sprayed layer and an ionic element is formed on the surface of the plasma sprayed layer and the fine cracks. An oxygen concentration detector characterized by forming a quality layer.