JPH0477866B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a limiting current type oxygen concentration detector (limiting current type oxygen sensor), and in particular removes restrictions on the operating temperature range and oxygen concentration measurable range and improves durability. Regarding limiting current type oxygen sensor. Here, a limiting current type oxygen sensor is a plate or cylindrical object made of an oxygen ion conductor (solid electrolyte), with an anode provided on one side and a cathode on the other surface facing the anode. A positive voltage is applied to the gas to be measured, and the oxygen gas in the gas to be measured is reduced to oxygen ions mainly at the point where the cathode, the solid electrolyte, and the gas coexist. It is a sensor that moves through an ion conductor to reach the interface between the solid electrolyte and anode, is oxidized at the point where the solid electrolyte, anode, and gas coexist, becomes oxygen gas again, and discharges it to the outside of the element. The cathode itself, a box with small holes (perforated box) covering the cathode, and a porous layer allow diffusion, etc. The amount of oxygen gas per unit time that is reached by the oxygen gas is limited, and the amount of oxygen ions that are generated per unit time by reduction at the point where the cathode, solid electrolyte, and gas coexist is limited. The amount of charge (current) per unit time is limited and a certain voltage range (overvoltage control region)
This refers to an oxygen sensor that allows a constant current to flow regardless of the voltage. (Prior Art) It goes without saying that in today's society, many combustion devices such as thermal power plants and internal combustion engines for automobiles are in practical use and contribute to our lives in various ways. These devices can generate large amounts of harmful gases if operating conditions are not appropriate. There is also a strong demand for lower fuel consumption. Combustion in a lean fuel region (abbreviated as "lean") is a promising method for purifying exhaust gas and improving fuel efficiency. For example, diesel engines and the like are normally operated in a lean region, but gasoline engines are also expected to be operated in a lean region. Even in these engines that operate in the lean region, if the air-fuel ratio is improperly adjusted, inconvenient problems such as soot generation, unburned fuel discharge due to misfire, and reduced output occur, making it difficult to operate in the lean region. Not only does it not suit the purpose, it may even have the opposite effect. Therefore, adjusting the air-fuel ratio is extremely important. By the way, as with any control, the control target (in this case, the air-fuel ratio in the lean region)
must be able to be detected precisely and quickly. Conventionally, there has not always been a suitable air-fuel ratio sensor in this field. For example, magnetic oxygen concentration detectors have a slow transient response and are unsuitable for use in vehicles, and gas density or thermal conductivity type gas sensors have measurement accuracy that is significantly affected by trace amounts of hydrogen (H ₂ ). This problem made it unsuitable for engine combustion control. In response to this, we first proposed a sensor that measures the limiting current and analyzes the oxygen gas concentration (hereinafter referred to as the limiting current oxygen sensor) (Japanese Patent Application Laid-open No. 72286/1986), and also added a porous cathode. To solve this problem, we developed an oxygen concentration sensor coated with a carbonaceous layer (Japanese Unexamined Patent Publication No. 1983-48648). FIG. 19 shows an example of the structure of the limiting current type oxygen sensor 1. 2 is a plate or cylinder made of an oxygen ion conductor (solid electrolyte). This material includes zirconia, Y ₂ O ₃ , Yb ₂ O ₃ , Gd ₂ O ₃ ,
MgO, CaO, Sc ₂ O ₃ , etc. as a solid solution as a stabilizer, or Bi ₂ O ₃ as a stabilizer with Y ₂ O ₃ , Er ₂ O ₃ , WO _3, etc. as a solid solution, or HfO ₂ ,
It is a dense sintered body made by dissolving CaO, MgO, Y ₂ O ₃ , Yb ₂ O ₃ , etc. in ThO ₂ etc. as a stabilizer. An anode 3 is provided on one surface of the ion conductor, and a cathode 4 is provided on the other surface facing the anode. The yin and yang poles are
It is a heat-resistant electron conductor made of Pt, Ag, Rh, Ir, Pd, etc. or a mixture of these materials, and if these materials are used, the interfacial resistance between the oxygen ion conductor and the electrode can be practically reduced. Is possible. The cathode 4 is covered with a box having small holes (perforated box) or a porous layer. As an example, FIG. 19 shows an example of a structure covered with a porous layer 5. This has the function of limiting the flow rate of oxygen flowing into the cathode 4 by diffusion or the like. Further, in order to prevent the anode 3 from deteriorating due to deposits or the like, the anode is covered with a porous protective layer 6. Porous layers 5 and 6 are alumina,
Consists of heat-resistant inorganic substances such as magnesia, siliceous, spinel, and mullite. The gas permeability of the porous layer 6 is often equal to or greater than that of the porous layer 5. The reason for this is that during operation, the porous layer 5 functions to control the rate of oxygen permeation sucked into the oxygen ion conductor 2 from the outside via the cathode 4, whereas the porous layer 6 acts as an oxygen ion conductor. This is to discharge oxygen from the body 2 to the outside world via the anode 3 with less resistance. Lead wires 7 and 8 are taken out from the negative and positive poles, respectively. Similar to the electrodes, the lead wire materials include Pt, Ag, Rh, Ir,
It is a heat-resistant electron conductor made of Pd, etc., or a mixture thereof. When a negative voltage is applied to the cathode and a positive voltage is applied to the anode of the limiting current type oxygen sensor configured as described above, and the entire device is brought into contact with the gas to be measured, the oxygen gas in the gas to be measured mainly flows between the cathode and the solid electrolyte. At the point where gas and gas coexist, it is reduced to oxygen ions,
The oxygen ions move through the oxygen ion conductor, reach the interface between the solid electrolyte and the anode, and are oxidized mainly at the point where the anode, solid electrolyte, and gas coexist, becoming oxygen gas again, and flowing outside the device. is discharged to. If we limit the amount of oxygen gas per unit time that reaches the point where the cathode, solid electrolyte, and gas coexist using some method, oxygen gas will be generated by reduction at the point where the cathode, solid electrolyte, and gas coexist. Since the amount of oxygen ions per unit time is limited and the amount of charge (current) carried by oxygen ions per unit time is limited, only a constant current can flow regardless of the voltage, and as shown in Figure 20. As shown, a limiting current characteristic occurs. Therefore, in the limiting current characteristics of an oxygen sensor, when the voltage applied to both the negative and negative electrodes is gradually increased from zero, as shown in Figure 20, the current flowing between the negative and negative electrodes while the voltage is low. increases approximately proportionally to the voltage (this voltage region is referred to as the resistance dominated region), but within a certain voltage range, the current remains approximately constant regardless of the voltage (this voltage region is referred to as the overvoltage dominated region). The current in the overvoltage control region is called the limiting current, and the oxygen concentration inside the limiting body approaches zero, and the difference in oxygen concentration between the inside and outside becomes almost equal to the oxygen concentration outside the limiting body. The limiting current is proportional to the oxygen concentration. Furthermore, in a region where the voltage and current are higher than the overvoltage control region, there is a portion where the current suddenly increases in response to a small voltage increase (this region is referred to as an excessive current region). This is because carbon dioxide (CO ₂ ) and water vapor (H ₂ O), which are contained in large quantities in the exhaust gas, appear to be partially decomposed and the oxygen concentration increases due to the applied voltage exceeding a certain limit value. be. As mentioned above, when the applied voltage is low, the region becomes a resistance-dominated region, and conversely, when the applied voltage is high, the region becomes an excessive current region, so the detection of the limiting current must be performed at a portion sandwiched between the two regions. This range is about 1 [V]. As mentioned above, in this example, the limiter (i.e., means for controlling the rate of oxygen permeation into the oxygen ion conductor)
This paper describes a limiting current type oxygen sensor using a porous layer. In addition to this, there is also a system in which the cathode itself is used as the limiting body, but the characteristics tend to change at high temperatures and it is difficult to obtain a durable one. (Problems with the Prior Art) This limiting current type oxygen sensor solves various difficulties associated with conventional oxygen sensors. Although this method is very effective,
It is undeniable that there are some problems. That is,
In combustion devices such as automobile engines, the temperature of the exhaust gas usually fluctuates depending on the operating conditions. Therefore,
Limiting current type oxygen (concentration) sensors, which are exhaust sensors, are also required to operate in a wide temperature range from low to high temperatures. By the way, in a limiting current type oxygen sensor, the internal resistance increases as the temperature decreases, and the proportional coefficient of oxygen concentration versus current changes slightly. This increase in internal resistance causes the following two problems. In other words, in order to obtain the output current of a limiting current type oxygen sensor, the maximum value of the voltage drop due to internal resistance must be
The applied voltage is limited to about 0.5 [V], and the applied voltage is 1
It is necessary to set it to about 0.75 [V], which is between [V] and 0.5 [V]. However, this condition is subject to various restrictions because the internal resistance increases as the temperature decreases. The first problem is that the linearity of the oxygen concentration versus the limiting current deteriorates, and the second problem is that the upper limit of the oxygen concentration measurement range is suppressed, making the measurement range narrower. Furthermore, the fact that the proportionality coefficient of oxygen concentration versus limiting current changes with temperature causes a decrease in accuracy. As countermeasures to these problems, we have shown the following so far. First, to solve the problem caused by an increase in internal resistance, one possible method is to maintain the temperature at a high enough constant temperature that the internal resistance is sufficiently low to solve the problem with the straight line and measurement range. For this purpose, temperature detection and heating control are necessary. There are various methods for temperature detection.
It is conceivable to install a temperature-sensitive sensor such as a thermocouple or a temperature-sensitive resistor (thermistor) near the limiting current sensor. However, in such a method,
The drawback is that the sensor configuration is complex and expensive. Furthermore, errors caused by the temperature difference between the temperature sensor and the limit current sensor also pose a problem. Another method is to measure the internal resistance of the oxygen ion conductor to detect temperature, but this requires a complicated circuit. Another method is to measure the internal resistance, estimate the voltage drop, and apply an appropriate limit current detection voltage, but this requires a complicated circuit. Next, the above-mentioned method of maintaining a constant high temperature can be applied to the problem that the proportionality coefficient changes depending on the temperature and the accuracy decreases. Another possible solution to the problem of output changes due to temperature changes is to also detect the temperature and estimate and compensate for changes in the proportionality coefficient. However, conventional products had to be operated at high temperatures, which did not work at low temperatures, and had the following problems. (a) Durability is limited because it is maintained at high temperatures. (b) A large amount of electricity is required for heating. (c) It takes a long time to reach the set temperature. (d) High temperature dependence. (e) Measurable oxygen concentration range is narrow. (f) Poor transient response. (Objective of the Invention) The present invention solves the above-mentioned problems of the prior art in limiting current type oxygen sensors, has a wide oxygen concentration measurement range, has good output linearity, and has good characteristics even at relatively low temperatures. It is an object of the present invention to provide a limiting current type oxygen sensor that has excellent transient response, good stability, small variation in characteristics, is highly durable, easy to manufacture, and inexpensive. (Structure of the Invention) In order to achieve the above object, the present invention is characterized by a cathode coating layer. That is, in the limiting current type oxygen sensor of the present invention, an anode is provided on one surface of a plate or cylindrical object made of an oxygen ion conductor (solid electrolyte), and a cathode is provided on the other surface facing the anode.
Porosity 8-16%, thickness 3-3 as the first layer on the cathode
It is characterized in that a porous layer of 30 μm is provided, and a nearly dense layer with a porosity of 4 to 7% and a thickness of 2 to 50 μm is provided as the second layer. This will be explained in more detail below. The present invention can be applied to any sensor having a different shape, such as a cylindrical sensor with one end closed, a cylindrical sensor with both ends open, or a pellet-like sensor. Further, the present invention can be applied to a sensor in which a gas to be measured is introduced to the cathode side and a reference gas to the anode side, or a sensor in which a gas to be measured is introduced to both the cathode and anode sides.
Further, the present invention can be applied to a sensor based on a solid electrolyte or a sensor in which a thin film solid electrolyte is formed on another base. Therefore, here we will take as an example a sensor in which a solid electrolyte is used as a base to form a cylindrical shape with one end sealed, a gas to be measured is introduced to the cathode side, and air as a reference gas is introduced to the anode side. Give an explanation. FIG. 1 is an enlarged sectional view of the main part (operating area) of the limiting current type oxygen sensor of the present invention, and FIG. 2 is a diagram showing the entire sensor. Figures 1 and 2
In the figure, this sensor has a solid electrolyte 2 formed into a cylindrical shape with a sealed end, and an operating area 20 in the middle of the solid electrolyte 2.
is set. Within the working area 20, a porous anode 3 is placed on both sides of the solid electrolyte 2, and inside the solid electrolyte 2.
are in close contact with each other, and a porous cathode 4 is in close contact with the outside. Solid electrolyte 2 outside the operating region 20
An electrically insulating layer 13 is adhered to the outside of the electrode, and a portion serving as a continuous lead wire for the cathode is provided outside of the electrically insulating layer 13. Now, the cathode 4 in the working area 20
On top, the first layer has a porosity of 8-16% and a thickness of 3-3.
A porous layer 10 of 30 μm is provided, and a nearly dense layer with a porosity of 4 to 7% and a thickness of 2 to 50 μm is further provided as a second layer. A layer 1 for protecting the cathode and lead portion extends from outside the working area to the end of the working area.
2 is provided. Reason and Basis for Providing the First Layer It was previously stated that the oxygen gas in the gas to be measured is converted to oxygen ions at the point (triple point) where the cathode, the solid electrolyte, and the gas coexist. The cathode has countless points (triple points) where the cathode, solid electrolyte, and gas coexist. The smaller the gradient of current with respect to voltage in the overvoltage control region, the better the limiting current characteristics. In order to obtain good limiting current characteristics, the variation in oxygen gas concentration between the numerous points (triple points) where the cathode, solid electrolyte, and gas coexist must be small. To achieve this, gas must actively diffuse between points (triple points) where the cathode, solid electrolyte, and gas coexist. In the case of the above-mentioned perforated box type, there is a small room (abbreviated as cathode room) covered by a box with small holes above the cathode, so gas can diffuse freely within it. , a uniform gas concentration can be obtained within the cathode chamber. Since the cathode itself is porous, gas can freely diffuse from the cathode chamber to the point (triple point) where the cathode, solid electrolyte, and gas coexist. The gas concentrations at the point where the electrolyte and gas coexist (triple point) are close to each other. Therefore, the gas concentration at the point (triple point) where the cathode, solid electrolyte, and gas coexist also becomes nearly uniform. As shown in the figure in the above-mentioned patent publication, this method provides very good limiting current characteristics. However, this method had the problem of slow transient response. On the other hand, a method without a cathode chamber has the advantage of excellent transient response. However, on the other hand,
Since it does not have the function of making the gas concentration uniform within the cathode chamber, it has the problem that the oxygen concentration at the point (triple point) where the cathode, solid electrolyte, and gas coexist tends to become uneven. As a countermeasure, it is necessary to facilitate gas diffusion in the plane direction within the cathode layer and the porous structure provided thereon. Among these, the cathode layer is usually thin, about 0.5 to 2 μm, and there is a limit to how porous it can be, so it cannot be expected to make a large contribution. On the other hand, it is thought that the higher the porosity and the thicker the porous layer, the more effective it is to make it easier for gas diffusion to occur in the planar direction within the porous layer, but doing so will reduce the transient response. The problem is that it gets worse. As mentioned above, a sensor is desired to have a high-speed response, and it is difficult to obtain good control results with a sensor that responds slowly. The time constant is preferably within 1 second at the latest, usually about 100 to 300 ms. The first step for that purpose is
The highest critical porosity and thickness of the layer were found to be 16% and 30 μm. On the other hand, when the porosity and thickness are small, it becomes difficult to sufficiently alleviate the variation in gas concentration between the triple points, the limiting current characteristics deteriorate, the linearity is poor, and the temperature dependence becomes large. Become. Therefore, it has been found that in order to obtain good limiting current characteristics, the minimum porosity and thickness of the first layer are 8% and 2 μm. Reason for providing the second layer and its rationale The purpose of providing the second layer is to limit the amount of oxygen reaching the triple point from the outside to an appropriate value. The higher the porosity and the thinner the second layer, the more oxygen will reach the triple point. In this case, the current per oxygen concentration increases. The larger the current, the less susceptible it is to electrical noise.
In that sense, it is convenient. However, as the temperature decreases and the resistance of the solid electrolyte and the resistance at the interface between the solid electrolyte and the electrode increase, the voltage drop in the resistance-dominant region becomes an obstacle to measurement. In that case,
The greater the current, the greater the disturbance. Therefore, in order to obtain excellent low-temperature operability, a wide measurement range, and good linearity, it is necessary to suppress the current density to an appropriate level. As a result of various studies, we have found that good characteristics can be obtained by setting the current to about 0.005 to 0.5 mA/mm ² . For this purpose, the porosity is 4-7% and the thickness is 2-50μ.
It has been found that providing a second layer of m is suitable. Examination of pore diameter of porous layer 11 (second layer) As mentioned in the section on problems with the prior art, there is a problem that the proportional coefficient of oxygen concentration versus current changes slightly due to temperature change (decrease). This was causing a decrease in accuracy. After examining various causes and countermeasures for this problem, we found that the cause was the temperature dependence of the gas diffusion coefficient. As a countermeasure, we have come to the conclusion that by using an appropriate value for the pore diameter of the diffusion barrier, it is possible to minimize the temperature dependence of the diffusion coefficient within the pores. Based on the above prediction, a limiting current type oxygen concentration sensor having only one porous layer was prototyped, and the pore diameter was varied in the range of 200 Å to 2 μm. We measured the relationship between temperature and limiting current at a constant oxygen concentration (10%) for these limiting current type oxygen sensors.
Figure 3 shows the ratio to the current at 700°C. As is clear from the figure, it can be seen that there are various temperature dependencies depending on the pore diameter. Among them, the temperature coefficient of 2000 Å to 1 μm is low. In addition, 3000 Å to 4000 Å is 500 Å to 4000 Å.
It was found that the temperature coefficient was low between 700°C and 1000°C. [Example 1] Example 1 has the structure shown in FIGS. 1 and 2, in which 85 to 95 mol% of ZrO ₂ is used as the oxygen ion conductor 2 and 5% of Y ₂ O ₃ or Yb ₂ O ₃ is added as a stabilizer. ~
It uses a solid electrolyte made of a dense sintered body of stabilized zirconia in which 15 mol% is dissolved. Examination of stabilizer concentration Figure 4 shows the relationship between the concentration of stabilizer (Y ₂ O ₃ ) and resistivity for solid electrolyte (ZrO ₂ ). The test conditions are 800℃. As mentioned above, it is desirable for this sensor to have as low a resistivity as possible. As is clear from the figure, the lowest resistivity was obtained at about 8% stabilizer concentration within the range of about 6 to 10 mol%. The cathode 4 and the anode 3 are porous platinum electrodes with a thickness of 0.5 to 20 μm. Suitable methods for preparing the porous platinum electrode include vacuum deposition, sputtering, ion plating, chemical plating, and paste printing. Examination of electrode thickness Figure 5 shows the relationship between platinum electrode thickness adjusted by the plating method and durability time. The durability test was carried out by holding it at 700°C in 10% O ₂ in N ₂ and measuring the time until the output current decreased to 90% of its initial value. As is clear from the figure, when the thickness is less than 0.5 μm, the durability time becomes rapidly shorter. FIG. 6 shows the relationship between the transient response time and the thickness of the platinum electrode. The test method was to examine the time required for the output current to change by 50% when the oxygen concentration suddenly changed from 1% to 5%. The test conditions were 700°C and a flow rate of 5/min.
As is clear from the figure, it can be seen that the response time increases rapidly when the thickness is 20 μm or more. From the above two data, it was found that good characteristics in terms of both durability and response time can be obtained in the range of 0.5 to 20 μm. Consideration of material for porous layer 10 (first layer) In addition to the above-mentioned functions, this layer also ensures adhesion to the platinum electrode, and prevents peeling, cracking, and sintering due to durability. It is also necessary that the process does not progress and become denser. Further, it is also necessary that difficult conditions are not required during manufacturing. Furthermore, since the atmosphere in which the oxygen sensor is used is oxidizing, it is necessary to have oxidation resistance. Furthermore, since automotive sensors and the like are often exposed to a reducing atmosphere, it is also necessary to have resistance to reduction. Furthermore, those that are radioactive or highly toxic are not very desirable. In addition, materials whose coefficient of thermal expansion is too different from those of solid electrolytes and electrodes are unsuitable, and materials whose vapor pressure at the operating temperature is too high are also unsuitable. We conducted experimental studies on various heat-resistant materials. The prototype was manufactured using a plasma spray method. First, initial adhesion was tested. Adhesion evaluation method Regarding adhesion, after conducting a thermal shock test, the presence or absence of cracks and peeling of the porous layer was observed using a microscope. No cracks at all: ○ Some cracks: △ Many cracks: × No peeling at all: 0 Some peeling: △ Many peels: × Thermal shock test method Ceramic coating The evaluation method is as follows:
The sample was heated in a furnace at 900°C, then suddenly removed from the furnace and rapidly cooled by blowing room temperature air at 50/min. This cycle was added for 10 cycles. Second, the durability is evaluated at room temperature and at 900°C.
A temperature cycle test (see FIG. 7) of 100 degrees Celsius was conducted to observe the presence or absence of peeling and cracking. Note that the temperature cycle test was conducted only for those that were initially good. Table 1 shows the results when stabilized zirconia was used as the solid electrolyte. As is clear from the table, the materials with excellent overall evaluation are stabilized zirconia, alumina, spinel, steatite, siyamoto, lanthania,
Ittria, calcium zirconate, _SiO2・
Al ₂ O ₃ , Gadolinia, Samaria, Ittrivia,
These are scandia, erbia, hafnia, barium zirconate, barium aluminate, magnesium silicate, magnesium zirconate, magnesium silicate zirconium, followed by forsterite, calcia, and HfO ₂ ·CaO. Consideration of the material of the porous layer 11 (second layer) Since this layer mainly functions to control the rate of gas movement due to diffusion, alteration of this layer immediately leads to changes in characteristics. Therefore, among the materials for which good results were obtained in the tests conducted on the porous layer 10, a high temperature durability test was further conducted to examine suitable materials. High-temperature durability test method At high temperatures, the porous layer progresses to sintering and porosity decreases, which may make it difficult for gas to diffuse. The existence and extent of this tendency was evaluated using the output of the limiting current type oxygen sensor. This test was conducted by measuring the initial value of the limiting current and the value after 200 hours of durability, and examining the rate of change. The atmosphere of the durability test was air, and the holding temperature was
It is 900℃. The results are shown in Table 2. As is clear from Table 2, materials with excellent durability include stabilized zirconia, alumina, spinel, hafnia, lanthania, ittria, barium zirconate, calcium zirconate, gadolinia, samaria, ittrivia, scandia, erbia,
barium aluminate, magnesium zirconate, followed by calcia, _HfO2 .
CaO, magnesium silicate, magnesium zirconium silicate. Consideration of the material of the electrical insulating layer 13 This layer is provided to prevent the solid electrolyte from coming into contact with the cathode outside the operating area.
Although good results were obtained in the tests conducted on the porous layer 10, the amount of current leakage at a higher temperature (900° C.) was investigated. Electrical insulation layer leakage test method
mm width electrode (Pt 1μm), electrical insulation layer (100μm)
m), a sample with an electrode (Pt 1 μm) and a three-layer structure was prepared. Note that the electrical insulating layer was deposited by plasma spraying. Then, we investigated the resistance when 10V was applied at 900°C in air. If the resistance is less than 2kΩ, there will be a slight negative effect, and 200
It becomes noticeable below Ω. Therefore, leakage current of 2kΩ or more was evaluated as large (200Ω to 2kΩ), medium (200Ω or less), or small (small). The results are shown in Table 3. Third
As is clear from the table, alumina, spinel, forsterite, steatite, hafnia, calcia, lanthania, ittria, HfO ₂ · CaO, gadolinia, samaria, and eruvia are suitable for the electrical insulating layer, followed by siyamoto, SiO ₂・Al ₂ O ₃ , Ittrivia, Scandia,
There is magnesium silicate. Like the porous layer 10, the protective layer 12 must also have good adhesion to the platinum electrode and must not peel off or crack during durability. Therefore, the material suitable as the material for the porous layer 10 is also suitable as the material for the protective layer 12. In order to protect the platinum electrode, a dense protective layer has better protective power, but it is also possible to make it porous. Moreover, instead of providing the protective layer 12 independently, the porous layer 10 and the porous layer 11
Either or both of them may be extended outside the operating area to also serve as a protection function. [Example 2] Example 2 has a structure in which a porous layer 14 is further provided as a third layer on the porous layer (second layer) 11 in FIGS. 9 and 10. . Appropriate range of porosity and thickness for the third layer If the porosity of the third layer is less than 10%, fine particles of compounds such as lead (Pb) and phosphorus (P) will adhere to the surface, resulting in a relatively short durability. Over time, clogging occurs and the current value decreases. Furthermore, when the porosity of the third layer is 18% or more, clogging on the surface of the third layer becomes less likely to occur, but compounds such as Pb, P, and S pass through the third layer and reach the second layer. easier to reach. Then, clogging of the surface of the second layer occurs within a relatively short durability time. Therefore, a suitable porosity for the third layer is 10 to 18%. In addition, if the thickness of the third layer is 5 μm or less, even if the above-mentioned appropriate porosity is used, such compounds will reach the second layer within a relatively short durability time, causing a decrease in current. . Considering only from the viewpoint of durability, the thicker the third layer, the better, but on the other hand, the transient response becomes slower. Since it is also necessary to emphasize responsiveness, the upper limit is limited to 50 μm. Therefore, the thickness of the third layer is suitably 5 to 50 μm. The porosity of the third layer is 10-18% and the thickness is 5-50μm
A range of is suitable. As the material for the third layer, a material suitable as the material for the second layer is suitable. Also in this embodiment, instead of providing the protective layer 12 independently, the porous layers 10, 11 and 1
Any or all of 4 may be extended outside the operating area 20 to also serve as a protection function. By providing a third layer, it can be used in an atmosphere where there are many fine particles of compounds such as Pb, P, sulfur (S), calcium (Ca), zinc (Zn), barium (Ba), etc., such as in combustion exhaust. In this case, clogging of the second layer is also reduced. The third layer has a high porosity, so it is difficult to get clogged. Therefore, over a long period of time,
Changes in characteristics are reduced. [Example 3] In this example 3, FIGS. 1 and 2 or 9
In FIG. 10, palladium, rhodium, platinum, or a mixture thereof was supported as a catalyst on any or all of the porous layers 10, 11, and 14. Examination of the amount of catalyst supported Figure 8 shows the relationship between the amount of catalyst supported and the durability time. The durability test was conducted using an actual engine, with the exhaust temperature adjusted to 500 to 600 degrees Celsius, where phosphorus and lead particles are most likely to adhere. The sensor used was one in which the porous layer 14 supported the amount of catalyst shown in FIG. As is clear from the results in Figure 8, the amount of supported
When the amount is less than 0.1 wt%, there is only a small effect of improving durability compared to a catalyst not supported. On the other hand, at 0.1wt% or more, a remarkable durability improvement effect was observed. On the other hand, when the amount of catalyst supported is 50wt% or more, phosphorus and lead compounds are difficult to adhere to, but at 900℃
At such high temperatures, sintering of the catalyst itself progresses, resulting in poor gas flow within the catalyst support layer and slow transfer response. Therefore, the supported amount of catalyst is 0.1wt%~
50wt% is appropriate. By supporting the catalyst, oxygen, H ₂ , CO,
Even in an atmosphere where HC etc. coexist, the oxidation of HC etc. is promoted in the catalyst section, making it possible to measure accurately with less error. In addition, phosphorus and lead contained in exhaust gas are less likely to adhere. (Effects of the Invention) FIG. 11a shows the voltage [Va] vs. current (I) characteristics of a limiting current type oxygen concentration sensor manufactured by the prior art, using oxygen concentration as a parameter. The test conditions were 10% CO ₂ , 8% H ₂ O, and the rest N ₂ at a temperature of 750°C. Figure b shows the relationship between the current and oxygen concentration (O ₂ ) when 0.7V is applied, which is the characteristic of figure a. Figures 12a and 12b show the characteristics of the product of the present invention measured under the same conditions as in Figure 11. The one in FIG. 12 has a smaller voltage drop in the resistance dominated region and has better characteristics. Figure 13 shows the characteristics of the conventional product at 575°C.
Since the temperature has become lower than in the case of FIG. 11, the internal resistance has increased and the voltage drop in the resistance dominated region has become large, making it impossible to obtain the limiting current characteristics. Therefore, even if the oxygen concentration increases, the current does not increase proportionally and tends to saturate. On the other hand, the characteristics of the product of the present invention are shown in Figure 14. Although the voltage drop in the resistance dominated region is larger than that in Figure 12, it is still
The voltage drop in % is about 0.6V, which is relatively small and does not interfere with the purpose of oxygen concentration measurement. Further, the boundary between the resistance dominated region and the overvoltage dominated region is clear, resulting in good limiting current characteristics. Therefore, a current proportional to the oxygen concentration is obtained. Figure 15 shows the relationship between temperature and current at an applied voltage of 0.7V for the conventional product. As is clear from the figure
Temperature dependence is large below 700°C, and if accurate measurements are desired, it must be used at temperatures above 700°C. On the other hand, in the case of the product of the present invention shown in Fig. 16, the temperature dependence is small up to 575°C at 10% oxygen concentration and up to 550°C at 8%, and it has excellent low-temperature operability compared to conventional products. It can be used even at high temperatures, and it can be seen that it has good properties over a wide range of operating temperatures. Note that when the oxygen concentration is low, the temperature dependence tends to increase above 675°C, but this is because an applied voltage of 0.7V enters the excessive current region; ), this problem can be solved because the current does not fall into the excessive current region. FIG. 17 shows the characteristics of the conventional product in the high oxygen concentration region. The test conditions were a temperature of 700°C, 10% CO ₂ and the rest N ₂ . As is clear from the figure, oxygen concentration
At 10% or less, the voltage drop in the resistance dominated region is 0.6V
It is possible to correctly detect the limiting current with an applied voltage of 0.7 V; however, if the oxygen concentration is higher than that, the voltage drop becomes large and the limiting current cannot be detected. Therefore, the 17th
As shown in FIG. b, even if the oxygen concentration increases, a current proportional to the oxygen concentration cannot be obtained and the device tends to saturate, so it cannot be used in a high oxygen concentration region. On the other hand, in the case of the product of the present invention shown in Fig. 18, under the same conditions, the voltage drop in the resistance dominated region at 50% oxygen concentration was as small as 0.4 V, and the boundary between the resistance dominated region and the overvoltage dominated region was clear. It has good limiting current characteristics. Therefore, a current proportional to the oxygen concentration is obtained. As is clear from a comparison between FIG. 17 and FIG. 18, the measurement range of the product of the present invention is expanded to five times higher oxygen concentrations than the conventional product. In addition, "limit type oxygen concentration sensor manufactured using conventional technology" or "conventional product" refers to the
The "limiting current type oxygen concentration sensor" described in Publication No. 48648, especially the one with the specifications shown in Appendix Table 4,
The "product of the present invention" refers to the "limiting current type oxygen concentration sensor" described in [Example 1] of this specification, particularly the product with the specifications shown in Table 5 of the appendix. As explained above, according to the present invention, the oxygen concentration measurement range is wide, the linearity of the output is good, good characteristics can be obtained even at relatively low temperatures, the transient response is excellent, the stability is good, and the characteristics are It has the effect of having small variations, being highly durable, and being easy to manufacture and inexpensive.

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[Table] ○ Good △ Partially defective × Not possible

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【table】 [Brief explanation of drawings]

FIG. 1 is a sectional view showing a main part of an embodiment of the present invention, FIG. 2 is an enlarged partial sectional view, and FIG. 2 is a structural diagram of an embodiment of a limiting current type oxygen sensor of the present invention. Figure 3 shows the relationship between average pore diameter and temperature dependence, Figure 4 shows the resistivity of solid electrolyte, Figure 5 shows the relationship between platinum electrode thickness and durability, and Figure 6 shows the relationship between platinum electrode thickness and durability. Figure 7 shows the relationship between platinum electrode thickness and transient response time, Figure 7 shows the temperature cycle pattern, Figure 8 shows the relationship between catalyst loading and durability, and Figure 9 shows the relationship between the amount of catalyst supported and durability.
10 is a partial cross-sectional enlarged view of FIG. 9, FIG. 11a is a voltage vs. current characteristic diagram of a conventional product, and FIG. 11b is a diagram of FIG. 11a.
Diagram showing the relationship between current and oxygen concentration when 0.7V is applied,
Figure 12a is a voltage vs. current characteristic diagram of the product of the present invention;
Figure 2b shows the relationship between the current and oxygen concentration when 0.5V is applied in Figure 12a, and Figures 13a and b show the relationship between the current and oxygen concentration when 0.5V is applied in Figure 12a.
Characteristic diagrams at 575°C, Figure 14 a and b are for the product of the present invention.
Characteristic diagram at 575°C. Figure 15 shows the temperature dependence of the current of the conventional product. Figure 16 shows the temperature dependence of the current of the product of the present invention. Figures 17a and b show the high temperature dependence of the conventional product. Diagram showing characteristics in oxygen concentration region, Figure 18a, b
is the characteristic of the product of the present invention in the high oxygen concentration region. Figure 19 is a structural diagram of a conventional limiting current type oxygen sensor.
FIG. 20 is a characteristic diagram of a conventional limiting current type oxygen sensor. DESCRIPTION OF SYMBOLS 1... Limiting current type oxygen concentration sensor, 2... Oxygen ion conductor, 3... Anode, 4... Cathode, 5, 6... Porous layer, 7, 8... Lead wire, 10... First layer, 11... Second layer , 12...protective layer, 13...insulating layer, 14...third
layer.

Claims (1)

[Claims] 1. An anode is provided on one surface of a plate or cylinder made of an oxygen ion conductor, a cathode is provided on the other surface facing the anode, and a first layer with a porosity of 8 is provided on the cathode.
~16%, a porous layer with a thickness of 3 to 30 μm is provided, and the second
A limiting current type oxygen sensor characterized by having a nearly dense layer with a porosity of 4 to 7% and a thickness of 2 to 50 μm. 2. The limiting current type oxygen sensor according to claim 1, wherein the second layer has a pore diameter of 2000 Å to 1 μm. 3 The first layer is stabilized zirconia, alumina, spinel, steatite, siyamoto, hafnia, barium zirconate, lanthania, ittria, calcium zirconate, SiO ₂ Al ₂ O ₃ , gadolinia, samaria, ittrivia, scandia, eruvia, barium aluminate, magnesium silicate,
The limiting current type oxygen sensor according to claim 1, characterized in that it is made of any one of magnesium zirconium silicate, forsterite, calcia, HfO ₂ ·CaO, or a mixture thereof. 4 The second layer is stabilized with zirconia, alumina, spinel, hafnia, lanthania, yttria, barium zirconate, calcium zirconate, gadolinia, samaria, yttrivia, scandia, erbia, barium aluminate, magnesium zirconate, or calcia. , _HfO2・
The limiting current type oxygen sensor according to claim 1 or 2, characterized in that it is made of CaO, magnesium silicate, magnesium zirconium silicate, or a mixture thereof. 5 As the third layer on the second layer, porosity 10-18%,
The limiting current type oxygen sensor according to claim 1 or 4, characterized in that a porous layer having a thickness of 5 to 50 μm is provided. 6 The third layer is stabilized by any one of zirconia, alumina, spinel, hafnia, lanthania, yttoria, barium zirconate, calcium zirconate, gadolinia, samaria, yttrivia, scandia, erbia, barium aluminate, magnesium zirconate, or calcia. , _HfO2・
The limiting current type oxygen sensor according to claim 5, characterized in that it is made of CaO, magnesium silicate, magnesium zirconium silicate, or a mixture thereof. 7. The limiting current type oxygen sensor according to any one of claims 1 to 5, characterized in that a catalyst is supported on any or all of the first to third layers. 8 0.1wt% or more of palladium, rhodium, platinum, or a mixture thereof as a catalyst.
The limiting current type oxygen sensor according to claim 7, characterized in that the oxygen sensor carries 50 wt%.