JPS644146B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an oxygen concentration detection element for detecting oxygen concentration in a gas, particularly in the exhaust gas of a combustion engine such as an automobile engine, and an oxygen concentration detection device using the element. It is related to.

Conventionally, oxygen concentration sensors for the above applications have applied the principle of oxygen concentration batteries, that is, platinum electrodes are formed on both sides of a zirconia solid electrolyte body, and combustion engine exhaust gas is introduced and brought into contact with the surface of one electrode.
A standard oxygen concentration gas, such as air, is introduced to the other electrode surface, and the superpower E generated between the two electrodes is expressed as E=RI/4FlnP ₀₂ , ₁ /P ₀₂ , ₂ . Devices were used that detected the oxygen concentration of the gas being measured by measuring superpower. Here, R is a gas constant, F is a Faraday constant, T is an absolute temperature, and P ₀₂ , ₁ , P ₀₂ , ₂ are the reference and measured oxygen concentrations. The oxygen concentration in the exhaust gas of a car engine with a lean fuel mixture ratio is in the range of 1 to 20%, but this is close to the oxygen concentration of air, which is 20%, so the generated electromotive force is small, and the range of change in oxygen concentration is small. Due to the narrow area, conventional oxygen concentration sensors can only detect minute changes in electromotive force, so highly sensitive instruments are required.
It had the disadvantage of being undesirable for use in automobiles.

On the other hand, an oxygen gas concentration analyzer (see Japanese Patent Laid-Open No. 72286/1986) that utilizes the fact that the diffusion-limiting current of oxygen is proportional to the oxygen concentration does not require a reference oxygen concentration gas and the sensor output current is proportional to the oxygen concentration. Since it is proportional to the concentration, it is especially suitable for measuring high concentrations of oxygen, such as 1 to 20%, and is superior to conventional oxygen concentration detection methods as an oxygen detector for lean mixture engines, etc. . An example of the current-voltage characteristics of this type of oxygen detector is shown in FIG. If a voltage of about 1V is applied to an element with such characteristics, an output current proportional to the oxygen concentration can be obtained. The saturation current characteristics (referred to as limiting current characteristics) shown in Figure 1 are as follows:
This is caused by the following mechanism.

In an element with a cathode on one side of an oxygen ion conductor plate and an anode on the other side, if a positive voltage is applied to the anode and a negative voltage is applied to the cathode, and a current is passed, the oxygen gas near the cathode is converted to oxygen by the cathode. Ion (divalent negative ion)
It travels through the oxygen ion conductor and reaches the anode, where it is converted back into oxygen gas and released out of the device. In this case, in order for current to flow through the element, oxygen gas must be supplied to the cathode, but if this amount is limited, the current will be limited. When the voltage applied to the element is gradually increased from zero, the current can increase while the element current is smaller than the current corresponding to the maximum amount of oxygen supplied to the cathode, but the amount of oxygen supplied to the cathode Once the corresponding current is reached, the current cannot be increased even if the voltage is increased thereafter. As a result, the current-voltage characteristics as shown in FIG. 1 are obtained.

The present invention relates to an oxygen concentration detection element (hereinafter abbreviated as a limiting current type oxygen sensor or simply oxygen sensor) based on the above principle, which uses a porous layer which is one form of a perforated box. It is something.

As a means for obtaining a limiting current due to the rate-limiting diffusion of oxygen gas, a structure including a perforated box, a cathode, an oxygen ion conductor, and an anode may be used. Here, the perforated box means a gas diffusion rate limiting body that covers the cathode and has gas diffusion holes in a part thereof.

FIG. 2 shows an oxygen sensor having a structure in which the cathode is coated with a porous material. This acts as an oxygen detection element in which the relationship between the oxygen concentration and the magnitude of the limiting current is determined by the gas permeability of the multiphoton body 4. The porous layer 5 is for protecting the anode from contaminants such as soot and residual combustion residue contained in the gas to be measured. The oxygen sensor shown in FIG. 2 has a simple structure and is suitable for applications that require mass production at low cost, especially for automobiles.

FIG. 3 shows an example of the current-voltage characteristics of the oxygen sensor shown in FIG. 2. The characteristics shown in Figure 3 are the results of measurements made by keeping the oxygen sensor at 770°C in the exhaust gas after mixing C ₄ H ₁₀ gas and air to the excess air ratio λ shown in the diagram and burning the mixture. be.

As is clear from Fig. 3, clear limiting current characteristics can be seen in the range where the excess air ratio is greater than 1 (fuel lean region, hereinafter simply referred to as lean region).
By applying a constant voltage of about 0.7 to 0.8 V to the oxygen sensor, a limiting current corresponding to the excess air ratio λ, that is, an oxygen sensor output current can be obtained. however,
In Fig. 3, when the excess air ratio λ is less than 1 (fuel rich region, hereinafter simply referred to as rich region)
In this case, the current only increases monotonically with respect to the voltage applied to the device, and no limiting current characteristics are observed. Fourth
The figure shows the results of applying a constant voltage of 0.7V to the oxygen sensor and recording the current flowing while continuously changing the excess air ratio of the combustion system. In this way, current flows not only in the lean region but also in the rich region, so it cannot be determined whether the oxygen sensor is in the rich region or the lean region by simply observing the oxygen sensor current.
Limit current type oxygen sensors are extremely suitable for measuring the air-fuel ratio of lean fuel mixture ratio engines for automobiles, but depending on the engine load condition, the sensor may be in the rich region, so it is not suitable for applications where such cases may occur. If you try to apply a limiting current type oxygen sensor, you will need some kind of auxiliary means to determine whether the area is rich or lean, that is, a separate detector, which has the disadvantage of complicating the device. .

The present invention seeks to provide an oxygen sensor that eliminates the above-mentioned drawbacks.

FIG. 5 shows an embodiment of the oxygen sensor according to the present invention, in which 7 is an oxygen ion conductor, 8 is a cathode,
9 is an anode, 10 is a porous layer, 11 is an oxygen accumulation layer,
12 is a lead wire.

The oxygen ion conductor 7 is ZrO ₂ , H ₅ O ₂ , ThO ₂ ,
A dense sintered body made of Bi ₂ O ₃ etc. with CaO, MgO, Y ₂ O ₃ , Yb ₂ O ₃ , Sc ₂ O ₃ etc. as a solid solution as a stabilizer is molded into a disk shape. On both sides of this oxygen ion conductor, a cathode 8, which is a heat-resistant, electronically conductive layer made of Pt, Ph, Ir, Pd, Ag, etc. or an alloy thereof, and an anode 9 are formed by sputtering, plating, vacuum evaporation, etc. It is formed by a method such as paste baking. The porous layer 10 is made of a heat-resistant inorganic material such as silica, alumina, spinel, magnesia, or zircon, and covers the cathode 8 . Oxygen in the gas to be measured is supplied to the cathode 8 by gas diffusion, but the porous layer 10 has the effect of controlling the rate of this oxygen diffusion, and the current flowing through the oxygen sensor corresponds to this rate-limiting amount of oxygen. It becomes what it is. The oxygen storage layer 11 is a porous body that temporarily stores oxygen, and covers the anode 9 . Oxygen storage layer 1
1 is a porous body having fine pores in order to accumulate as much oxygen as possible than the porous layer 10. When the gas to be measured changes from a lean atmosphere to a rich atmosphere, the oxygen accumulation layer 11 functions to delay the arrival of the rich atmosphere gas at the anode compared with the arrival at the cathode. In other words, when the gas to be measured changes from lean to rich, rich gas is quickly supplied to the cathode, but since the oxygen accumulated in the oxygen accumulation layer 11 is present at the anode, the rich gas and the accumulated oxygen are replaced. Until then, the anode is kept in an oxygen-rich atmosphere for a while, and during this period, a superpower of approximately 1V is generated from the cathode to the anode according to the raw material of the oxygen concentration battery. This superpower has a direction that opposes the externally applied voltage, and reverses the current flowing through the oxygen sensor if the externally applied voltage is smaller than the back electromotive force generated between the cathode and anode. Therefore, by detecting the direction of the current or the back electromotive force, it can be seen that the atmosphere has changed from lean to rich.
Even if some time passes, if the anode also becomes a rich atmosphere, the counter electromotive force will disappear, so if a voltage is applied from the outside to the oxygen sensor, a current will flow due to it. This state is undesirable because the device is operated in an oxygen-free atmosphere, which may accelerate deterioration of the cathode, anode, or oxygen ion conductor. A simple way to prevent this is to detect the generation of the back electromotive force and cut off the element current. Contrary to the above, when the gas to be measured changes from a rich atmosphere to a lean atmosphere, the gas in the lean atmosphere quickly arrives at the cathode.
After a while, the lean atmosphere gas also arrives at the anode. Therefore, after the lean atmosphere gas arrives at the cathode, the rich atmosphere gas remains at the anode for a while, but during that time an electromotive force is generated in the direction from the anode to the cathode. If this electromotive force is detected, it can be seen that the state has changed from rich to lean.

In order for the above-mentioned operation to be performed stably and to ensure the electromotive force signal at this time, it is preferable that the time length in which the back electromotive force is generated is equal to or longer than the response time of the sensor to a change in oxygen concentration. .
For this purpose, the amount of oxygen accumulated at the anode must be at least twice as large as that at the cathode. That is, in a lean atmosphere, it operates as a limiting current type oxygen sensor, and a current of approximately several mA normally flows between the cathode and anode. When the gas to be measured changes from a lean atmosphere to a rich atmosphere, the oxygen sensor generates a back electromotive force as mentioned above, but in order to stably detect that the direction of the current has reversed, it operates as a limit current sensor. The current must continue to flow in the opposite direction for a certain period of time, which is approximately the sensor response time. In other words, until it is detected that the current has reversed, at least enough oxygen must be accumulated to cause the current to flow in the opposite direction. If only a porous layer is provided on the anode layer, current reversal will occur in the sensor, but the current reversal time in this case is extremely short, so the current reversal detection circuit must be highly sensitive. However, by doing so, it is unavoidable that the device becomes susceptible to the effects of noise, etc., and this is not suitable for stable operation.
The approximate reversal time of a sensor with only a porous layer provided on the anode layer is as follows.

At 700° C. and 1 atm, the number of moles of gas per cm ³ is approximately 1.25×10 ⁻⁵ mol. In the case of a simple porous layer with no significant oxygen accumulation effect, the amount of oxygen present in the space within the porous layer is the amount of accumulated oxygen.For example, if the porosity of the porous layer is 10% and the oxygen concentration in the gas is 5%. For example, oxygen exists in an amount of 1.25×10 ^-5 ×0.1×0.05=6.25×10 ^-8 mol per cm ³ of the porous layer. Taking the same size as the oxygen sensor in the example described later, the sensor diameter is 3.5 mm,
When the thickness of the porous layer on the anode side is 1 mm, the porous layer volume is
0.00962 cm ³ , and therefore the oxygen accumulated therein is calculated to be 6.25×10 ^-8 ×0.00962 = 6×10 ^-10 moles. If half of the above amount of oxygen becomes a reversal current (this is assumed because oxygen exists only in the porous layer and is also lost by diffusion to the outside of the sensor), the amount of electricity in this case is is obtained by multiplying by 4F with Faraday's constant as F, and is 6×10 ⁻¹⁰ ×0.5×4×964870.116×10 ⁻³ coulombs.If the reversal current is 1 mA, this is consumed in 0.116 seconds. This time is the maximum value, and in reality it will be much shorter. Note that since the gas accumulation effect is the main cause of the sensor response delay, this time is approximately the sensor response time. As described above, if the anode layer is simply coated with a porous layer, the current reversal time will be extremely short.

In order to lengthen the current reversal time, it is sufficient to increase the amount of oxygen accumulated on the anode side. The first possible way to do this is to thicken the porous layer on the anode side and increase the porosity, but the former is undesirable because it increases the size of the oxygen sensor, and the latter is not recommended if the porosity is increased too much. This is not preferable because the target strength decreases.

In the present invention, in order to obtain a porous layer that accumulates a large amount of oxygen, fine particles of a metal with a catalytic action such as platinum, palladium, or rhodium are dispersed in the porous layer with a large specific surface area, or fine particles of a metal with a catalytic action such as platinum, palladium, or rhodium are dispersed in the porous layer. Metal oxide such as CeO ₂ fine particles are dispersed therein. Note that if the sensor current is cut off when the oxygen sensor atmosphere is rich, the electromotive force generated will be large when the atmosphere changes from rich to lean because the oxygen in the porous layer on the cathode side will not be consumed to flow the current. Even if the porous layer on the cathode side is not specifically used as an oxygen accumulator, the electromotive force can be easily detected and the above-mentioned problem does not occur.

An example of a method for manufacturing the oxygen sensor according to the present invention shown in FIG. 5 will now be described.

The composition of the oxygen ion conductor 7 is (ZrO ₂ ) _0.92 (Y ₂ O ₃ )
It is a dense sintered disk with a diameter of _0.08 , and was manufactured using the following method. First, high purity (more than 99.9%) _ZrO2
The powder and Y ₂ O ₃ powder were mixed at a molar ratio of 92:8, ground in a wet ball mill for 10 hours, and dried. The obtained mixed powder is calcined at 1250°C for 10 hours, and then further ground into fine powder in an agate mortar. This powder is mixed with 0.5% by weight of polyvinyl alcohol in the form of an aqueous solution, granulated, and compressed using a mold with a press method at a load of 1000 kg/cm ² to form a disc with a thickness of approximately 0.4 mm and a diameter of 4 mm. Molded. this
Sintering was carried out in air at 2000°C for 2 hours to obtain a dense sintered body having a disc size of 3.5 mm in diameter and 0.3 to 0.35 mm in thickness after sintering. The cathode 8 and the anode 9 were formed into disk shapes with a thickness of 1 μm and a diameter of 1.9 mm by sputtering platinum at the center of the upper and lower surfaces of the oxygen ion conductor.
The lead wire 12 was a platinum wire with a diameter of 0.3 mm, and was connected to the cathode and anode by thermocompression bonding, respectively.
The porous layer 10 covers the cathode 8 and has a thickness of 600μ.
It was formed to a thickness of m by plasma spraying using spinel powder as a raw material. The average particle size of the spinel powder was 47 μm, and the volume porosity of the porous layer after formation was 9%. The oxygen storage layer 11 is made of a porous material mainly composed of alumina in which fine platinum particles are dispersed, and has a thickness of 1 μm. This layer has an average grain size of 0.02μ
A slurry mainly composed of fine powder of δ-alumina of m is applied to cover the anode 9, dried at 110°C for 5 hours, and then fired at 700°C for 3 hours to form a porous body made of alumina. This was further impregnated with an aqueous solution of chloroplatinic acid, dried at 120°C for 1 hour, and then heated to 700°C.
It was manufactured by dispersing platinum fine particles by heating at ℃ for 3 hours. In this example, the amount of platinum is 3wt% based on the weight of the oxygen storage layer.

In addition to the above manufacturing method, in order to uniformly disperse and add substances such as platinum and _CeO2 , which have a remarkable oxygen storage effect, into the oxygen storage layer, δ alumina is mainly used when forming the porous layer. It is also possible to mix platinum, CeO ₂ , etc. in the form of fine powders or chloroplatinic acid, cerium nitrate, etc. with the slurry that is the component, and then apply this to the anode, dry it, and bake it. good. In this case, an oxygen accumulation layer in which platinum, CeO ₂ and the like are extremely uniformly dispersed can be formed.

The characteristics of the oxygen sensor manufactured in this way are shown in FIG. Figure 6a shows the gasoline engine,
This figure shows how the excess air ratio is changed in a rectangular waveform at a 5-second period between 0.95 and 1.15. The oxygen sensor of this example was placed in this exhaust gas flow and heated to 700°C by a heater near the oxygen sensor. Figure 6b shows the results of recording the temporal changes in the voltage output between the lead wires of the oxygen sensor in this state. In the oxygen sensor of this example, the peak value is approximately
A back emf signal of 0.7V was generated. Also, when changing from a rich atmosphere to a lean atmosphere, the peak value is approximately
A forward voltage signal of 0.5V was generated. Figure 6c shows the oxygen sensor of this example in the atmosphere shown in Figure 6a.
This shows the results of recording the current waveform flowing through the oxygen sensor when a constant voltage of 0.4V was applied.In this way, the sensor applied voltage was made smaller than the peak value of the back electromotive force region seen in Figure 6b. The sensor current could be temporarily reversed when the atmosphere changed from lean to rich.

Here, the longer the current reversal time, the better. In the oxygen sensor of the present invention, the peak value and width of the back electromotive force due to a change in atmosphere from lean to rich, and the forward direction due to a change in atmosphere from rich to lean. If the electromotive force is not maintained in an appropriate relationship, malfunctions may occur.
For example, if the amount of oxygen accumulated in the oxygen accumulation layer coated on the anode side is excessive, a back electromotive force is generated for a long time, and the forward electromotive force generated by the sensor when returning from rich to lean during this period is If the counter electromotive force is smaller than the back electromotive force, the sensor terminal voltage does not become positive, and therefore no change in ambient air can be detected. In this case, if an oxygen storage effect is added to the porous layer on the cathode side within a range that does not exceed the oxygen storage amount on the anode side, the electromotive force generated when changing from rich to lean will also increase, so the above problem can be solved. Experimentally, it has been found that the above problem can be solved by setting the amount of oxygen accumulated on the cathode side to about 1/20 to 1/2 times that on the anode side. In other words, in the oxygen sensor of the present invention, the relationship between the amount of oxygen accumulated on the cathode side and the anode side is always larger on the latter side than on the former.
It is necessary that the latter be within about 2 to 20 times the former. In the example, a substance that exhibits an oxygen accumulation effect such as platinum is not added to the cathode side porous layer, but even without addition, it still has an accumulation effect of about 0.1 seconds, that is, a response delay, as described above. Therefore, if the amount of accumulation on the anode side corresponds to about 0.2 to 2 seconds, the above condition is satisfied. In the case of the example, oxygen was accumulated for only about 0.8 seconds as shown in FIG. 6b. Fine particles such as platinum have a significantly large oxygen accumulation effect, so when these are added to both the anode and cathode sides, the amount of oxygen accumulation is determined by the amount of these particles added. In order to make the amount of oxygen accumulated on the cathode side 1/20 to 1/2 times that of the anode as described above, the amount of platinum etc. added to the oxygen accumulation layer on the cathode side should be approximately 1/20 to 1/2 times that on the anode side. It should be 1/2 times. In other words, in the case of platinum, the addition amount that still shows a remarkable oxygen accumulation effect is 0.5~
The concentration was 10wt%.

Furthermore, if the back electromotive force value when changing from lean to rich is smaller than the back electromotive force value when changing from rich to lean, the electric circuit means detects that the sensor terminal voltage has suddenly changed. Although it is also possible to detect changes in the atmosphere by using this method, the circuit configuration becomes complicated and it becomes susceptible to the effects of electrical noise, etc., so this is not very preferable.

The oxygen sensor element of the present invention described above generates an electromotive force when the gas to be measured changes from rich to lean or vice versa, and does not generate a steady electromotive force. By temporarily storing the electromotive force of the oxygen sensor for a period when no electromotive force is generated, it is possible to always know whether the state of the gas to be measured is rich or lean.

FIG. 7 shows an example of the output processing circuit of the oxygen sensor element of the present invention. The power supply 14 is a power supply that applies a constant voltage to the oxygen sensor 15, and one terminal thereof is connected to one lead wire 12 of the oxygen sensor 15.
The other terminal is directly connected to the current detection circuit 1.
6, the other lead wire 1 via the switch circuit 17
Connected to 2. The current detection circuit 16 is a circuit that detects and outputs the current flowing to the oxygen sensor 15.
Its output is connected to an output terminal 18 and an input of a current reversal detection circuit 19. The current reversal detection circuit 19 is a circuit that detects when the current flowing through the oxygen sensor is reversed and outputs the result. The voltage comparison circuit 20 is a circuit that compares whether the terminal voltage of the oxygen sensor is higher or lower than a reference voltage and outputs the result. The bistable circuit 21 has two input terminals, the first input terminal of which is connected to the output terminal of the current reversal detection circuit 19, the second input terminal of which is connected to the output terminal of the voltage comparison circuit 20, When a signal is applied to the first input terminal, the first
state and maintain this state until a signal is applied to the second input terminal, maintain the second state from this point until the next signal is applied to the first input terminal, and maintain the second state until the next signal is applied to the first input terminal. This circuit outputs an output signal corresponding to the second state. The output signal limits the switch circuit 17 and opens and closes the current flowing to the oxygen sensor 15.

In the output processing circuit configured as described above, if the atmosphere to be measured by the oxygen sensor changes from lean to rich while the switch circuit 17 is closed, the current flowing through the oxygen sensor will change as shown in Figure 6c. is inverted, so this is detected by the current detection circuit 16 and the current reversal detection circuit 19, and this output is input to the first input terminal of the bistable circuit 21. As a result, the output of the bistable circuit 21 is reversed, thereby opening the switch circuit 17, thereby cutting off the current flowing to the oxygen sensor. This interruption prevents the oxygen sensor from detecting current during a period when the oxygen sensor is not operating normally, that is, during a period when the atmosphere to be measured is rich.
In this state, the oxygen sensor is disconnected from the power supply 14, so the oxygen sensor terminal voltage shows a temporal change as shown in FIG. 6b. In other words, the magnitude of the back electromotive force of the oxygen sensor gradually decreases, and when the atmosphere changes from rich to lean, the oxygen sensor terminal voltage generates a forward electromotive force, so the sensor terminal voltage is compared with the voltage. The voltage comparator circuit 20 inputs its output to the second input terminal of the bistable circuit 21 when the voltage is compared with a certain reference voltage (for example, +0.1V) by the circuit 20 and the former exceeds the reference voltage. The output of the bistable circuit is restored, thereby returning the switch circuit 17 to the closed state. After this point, a voltage is applied to the oxygen sensor from the power supply 14, and the current detection circuit 16 outputs oxygen concentration information of the gas to be measured.

As mentioned above, the circuit configuration shown in Figure 7 temporarily stops the operation of the oxygen sensor while the atmosphere is rich, and generates a signal to return to the original state from the electromotive force generated by the oxygen sensor itself. It is.
The bistable circuit 21 in FIG. 7 needs to be initially set to a state in which the switch circuit 17 is closed if the oxygen sensor atmosphere at startup is lean. However, even if the initial settings are not changed, if the atmosphere is forcibly changed from lean to rich for one cycle, the initial state is set by the sensor electromotive force signal generated at that time. In addition, the oxygen accumulation layer is suitably a porous material with a pore radius in the range of 50 Å to 1 μm, and the catalyst material such as platinum impregnated into it is 0.1 wt%.
10wt% is good. In addition, the thickness of the oxygen accumulation layer is
In order to allow the oxygen accumulated inside to reach the anode efficiently, it is preferably within the diameter of the anode.

In the above embodiment, platinum fine particles were dispersed in the oxygen accumulation layer 11, but the porous layer 10 covering the cathode
Similarly, if platinum fine particles are dispersed in the cathode, the amount of oxygen accumulated on the cathode side will increase, and the electromotive force signal of the oxygen sensor when the atmosphere changes from rich to lean can be increased and the time span can be lengthened. However, if the amount of oxygen accumulated in the cathode is too large, the response time of the oxygen sensor becomes longer, so the upper limit is limited by the allowable response time required of the oxygen sensor.

In addition to the device structure shown in FIG. 5, there is also a porous layer 10 and an oxygen storage layer 11 as shown in FIG.
A layer 13 made of a dense heat-resistant inorganic material is coated on the outer periphery of the device to prevent the gas to be measured from entering the porous layer 10 or the oxygen storage layer 11 from the lateral direction of the device so that the oxygen storage effect is not reduced. You can also do it.

In addition, in the circuit shown in FIG. 7, since the sensor terminal voltage is also inverted during the period when the oxygen sensor current is inverted, a sensor terminal voltage inversion detection circuit is used instead of the current reversal detection circuit 19. The same thing will happen if this output is input to a bistable circuit.

In addition, in the circuit shown in FIG. 7, in order to stably detect the reversal of the oxygen sensor current, a secondary circuit is added as a function of the current reversal detection circuit 19, which outputs the information after a certain period of time has elapsed since the current reversal. Good too. Furthermore, similar additional functions can be provided in the voltage comparison circuit 20 as well.

As described above, the oxygen sensor according to the present invention has a cathode and an anode on both sides of an oxygen ion conductor, coats the cathode with a porous layer, and controls the rate of oxygen supplied to the cathode. By obtaining an output current that corresponds to the concentration and further covering the anode layer with an oxygen accumulation layer, it is possible to reduce the amount of current generated between the cathode layer and the anode layer when the atmosphere changes from lean to rich or vice versa. It is designed to generate electricity. The output processing circuit is configured to detect this electromotive force, determine whether the gas to be measured is in a rich region or a lean region, and control the oxygen sensor output based on the determination result. As described above, the oxygen sensor of the present invention has an extremely simple configuration, has a large thermal intensity, can obtain a sufficient signal to determine whether the area is rich or lean, and has an output Since the configuration of the processing circuit is simple, the present invention is suitable as an oxygen concentration detection device that can be applied to applications where the gas to be measured is in a rich region, and is particularly suitable for use in automobiles.

[Brief explanation of drawings]

Figure 1 is a diagram showing the current-voltage characteristics of a limiting current type oxygen sensor, Figure 2 is a cross-sectional view of an oxygen sensor whose cathode is coated with a porous material, and Figure 3 is a diagram showing the current-voltage characteristics of the oxygen sensor shown in Figure 2. 4 is a diagram showing the excess air ratio output current characteristics of the oxygen sensor in FIG. 2, and FIG.
The figure is a cross-sectional view of an oxygen sensor according to an embodiment of the present invention, and FIG. 6 shows the sensor terminal voltage when the oxygen sensor of FIG. 5 is changed to lean, rich, and lean atmospheres.
FIG. 7 is a diagram showing a temporal change in sensor current, FIG. 7 is a diagram showing an example of an oxygen sensor output processing circuit of the present invention, and FIG. 8 is a cross-sectional view of an oxygen sensor according to another embodiment of the present invention. 1... Oxygen ion conductor, 2... Cathode, 3...
Anode, 4... Porous layer, 5... Porous protective layer, 6
... Lead wire, 7 ... Oxygen ion conductor, 8 ...
Cathode, 9... Anode, 10... Porous layer, 11...
Oxygen storage layer, 12... Lead wire, 13... Layer made of dense inorganic material, 14... Power source, 15... Oxygen sensor, 16... Current detection circuit, 17... Switch circuit, 18... Output terminal , 19... Current reversal detection circuit, 20... Voltage comparison circuit, 21... Bistable circuit.

Claims (1)

[Scope of Claims] 1. A cathode layer is provided on one surface of an oxygen ion conductor, and an anode layer is provided on the other surface facing the cathode layer, and the anode layer is made of metal fine powder such as platinum, palladium, rhodium, etc. that has a catalytic action or a comparative material. The cathode is coated with a porous layer in which fine particles of a metal oxide such as CeO ₂ are dispersed, and the amount of metal fine powder or metal oxide in the porous layer that coats the cathode is reduced to the porous layer that coats the anode. An oxygen concentration detection element characterized in that the amount of oxygen concentration is smaller than that of fine metal powder or metal oxide in the substance. 2 Cover the cathode with a porous layer, and reduce the amount of metal fine powder or metal oxide in the porous layer covering the cathode to 1/20 of the amount of metal fine powder or metal oxide in the porous layer covering the anode. The oxygen concentration detection element according to claim 1, which is 1/2 times as large. 3. An oxygen concentration detection element characterized in that the oxygen concentration detection element according to claim 1 or 2 is covered with a dense layer so as to have openings on the cathode side and the anode side. 4 A cathode layer is provided on one side of the oxygen ion conductor, and an anode layer is provided on the other side facing the cathode layer, and the anode layer is made of fine metal powder such as platinum, palladium, rhodium, etc. that has a catalytic action or is relatively easily reduced. The amount of metal fine powder or metal oxide in the porous layer covering the cathode is determined by the amount of metal fine powder or metal oxide in the porous layer covering the anode _. An oxygen concentration detection element whose amount is smaller than the amount of powder or metal oxide, a power source that applies voltage to it, a current detection circuit that detects the current flowing through the oxygen concentration detection element, and whether the measured gas changes from fuel lean to fuel rich or A rich/lean reversal detection circuit that detects a reverse change in the oxygen concentration detection element by the current or voltage of the oxygen concentration detection element, and a control circuit that controls the current flowing to the oxygen concentration detection element based on the output of the detection circuit. , the lean reversal detection circuit includes a current reversal detection circuit that detects a reversal of the current flowing through the oxygen concentration detection element, and a voltage comparison circuit that compares a voltage generated in the oxygen concentration detection element with a reference voltage, and the oxygen concentration detection element A control circuit for controlling the current flowing through the oxygen source is characterized in that it comprises a bistable circuit into which the outputs of the current reversal detection circuit and the voltage comparison circuit are input, and a switch circuit driven by this output. Concentration detection device. 5 The cathode is covered with a porous layer, and the amount of metal fine powder or metal oxide in the porous layer covering the cathode is 1/20 of the amount of metal fine powder or metal oxide in the porous layer covering the anode. 5. The oxygen concentration detection device according to claim 4, wherein the oxygen concentration is 1/2.