JP2001221774A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To supply sufficient oxygen into a measuring chamber without enlarging the sectional area of an air introduction duct. SOLUTION: An anode electrode 8 of an oxygen pump cell is arranged in an internal cavity 3 enclosed by zirconia solid electrolyte, so that a cathode electrode 11 is led to a test gas atmosphere outside the internal cavity 3, and a prescribed voltage Vp is applied on the oxygen pump cell.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor, and more particularly to a gas sensor for detecting the concentration of nitrogen oxides in combustion gas.

[0002]

2. Description of the Related Art In recent years, attention has been paid to all-solid-state gas sensors capable of continuous measurement by directly inserting them into the exhaust gas of an engine of an automobile or the like. The present inventors have already found that total NOx in exhaust gas
A hybrid potential type NOx sensor capable of measuring the concentration in real time has been proposed. For example, JP
In Japanese Patent Application Laid-Open No. 23526/1995, a mixed potential type N is provided in a measurement chamber including an internal space formed of a zirconia solid electrolyte.
An Ox detection electrode is installed, and an oxygen pump (NOx conversion electrode) is installed in the same measurement room so as to face the Ox detection electrode. That is, NOx (NO and NO ₂ ) in the exhaust gas is electrochemically converted into a NO ₂ single gas, and the NO ₂ detection electrode detects the NO ₂ concentration as the total NO x concentration. In the example of Japanese Patent Application Laid-Open No. 11-23526, the NO
An oxygen pump (conversion pump) for converting x to NO ₂ and an oxygen pump for adjusting the oxygen concentration in the canister are provided. Further, the above-mentioned reference electrode of the NOx detection electrode and the oxygen detection electrode for measuring the oxygen concentration in the can chamber are provided in the measurement chamber. By setting the potential of the oxygen detection electrode caused by the oxygen concentration in the same atmosphere as the reference potential of the NOx detection electrode, the sensor configuration is such that the output of the NOx sensor is hardly affected even if the oxygen concentration in the measurement chamber fluctuates. .

[0003] However, for example, in an exhaust gas from an automobile, an interference gas such as a hydrocarbon gas (HC) or CO which is likely to affect the detection accuracy is simultaneously present in NOx to be measured. Therefore, it is necessary to maintain a high oxygen concentration in the measurement chamber of the sensor element. That is, the catalytic effect of the zirconia solid electrolyte or the like forming the measurement chamber oxidizes the interfering gas such as HC and CO entering the measurement chamber and converts it into H ₂ O or CO ₂ which does not affect NOx detection. .

[0004] Therefore, in a conventional method as disclosed in Japanese Patent Application Laid-Open No. 11-23526, oxygen in the atmosphere is supplied into a measurement chamber by driving an oxygen pump through an air introduction duct. In the conventional structure, in order to maintain a high oxygen concentration in the measurement chamber, it is necessary to reduce the diffusion resistance of oxygen, and for that purpose, it is necessary to increase the cross-sectional area of the air introduction duct. However, when the cross-sectional area of the air introduction duct is increased, the distortion during sintering in the production of the sensor element, particularly in the sintering step, becomes large, causing a problem that the element is liable to be cracked or peeled off.

[0005]

As described above, in the conventional structure, in order to maintain a high oxygen concentration in the measurement chamber, if the cross-sectional area of the air introduction duct is increased, the firing is performed during the manufacturing of the sensor element, particularly in the firing step. There was a problem that distortion at the time of knotting was increased and cracks, peeling, and the like were likely to occur in the element. Therefore, a means for supplying sufficient oxygen into the measurement chamber without having such a large cross-sectional area has been desired.

[0006]

SUMMARY OF THE INVENTION In view of the above problems, the present invention proposes to solve the problem by the following means. That is, in the internal space having a gas introduction hole surrounded by a zirconia solid electrolyte having oxygen ion conductivity, a detection electrode active on the test gas and oxygen installed at least on the zirconia solid electrolyte in the internal space. A gas sensor comprising a reference electrode paired with the detection electrode and active on oxygen and not active on the test gas, and a pair of oxygen pump cells provided on the solid electrolyte in the internal space. The anode electrode of the oxygen pump cell is installed in the internal space, and the cathode electrode is installed so as to communicate with the test gas atmosphere outside the internal space. By applying a predetermined voltage to the oxygen pump cell, the oxygen in the test gas atmosphere is Alternatively, the oxygen compound gas is electrochemically decomposed to supply oxygen into the internal space, or the detection electric current is supplied while oxidizing a part of the gas. Suggest gas sensor for detecting the test gas concentration by measuring the potential difference between the reference electrode. In the above-described sensor configuration, the reference electrode paired with the detection electrode is provided in the internal space, so that the sensor configuration is hardly affected even if the oxygen concentration in the test gas atmosphere fluctuates. it can.

On the other hand, in an internal space having a gas introduction hole surrounded by a zirconia solid electrolyte having oxygen ion conductivity, at least a test gas and oxygen active on a zirconia solid electrolyte in the internal space are provided. A detection electrode, a counter electrode that is active on oxygen and is not active on the test gas, a reference electrode that is active only on oxygen provided in a reference space where a predetermined oxygen partial pressure is maintained, and a solid electrolyte in the internal space. A gas sensor comprising a pair of oxygen pump cells provided in the gas pump, wherein an anode electrode of the oxygen pump cell is provided in the internal space, and a cathode electrode is provided so as to communicate with a test gas atmosphere outside the internal space. By applying a predetermined voltage to the oxygen pump cell, oxygen or oxygen compound gas in the test gas atmosphere is electrochemically decomposed and Simultaneously measure the potential difference between the detection electrode and the reference electrode and the potential difference between the counter electrode and the reference electrode while supplying oxygen or oxidizing a part of the gas, and detect the concentration of the test gas from the two potential differences. The proposed gas sensor is also proposed.

Further, in the above-mentioned sensor configuration,
A cathode electrode of an oxygen pump cell is installed in a second internal space having a second gas introduction hole communicating with a test gas atmosphere provided separately, and a cathode electrode of the oxygen pump and a second electrode are provided in the second internal space. A second anode electrode which is paired with the first and second anode electrodes, and a voltage is simultaneously applied between the cathode electrode and the first and second anode electrodes, so that oxygen is introduced into the first and second internal spaces. Is detected while simultaneously supplying oxygen, thereby enabling stable oxygen supply from the atmosphere of the test gas.

The present invention is particularly applicable to a gas sensor for detecting the concentration of nitrogen oxides in exhaust gas. That is, the first anode electrode is made of an electrode material active on oxygen and nitrogen oxide, and supplies oxygen from the test gas atmosphere into the first internal space by the oxygen pump cell, while converting one NO to NO ₂ electrochemically, it is possible to obtain a gas sensor for detecting the total NOx concentration by potentiometric measuring the between the sensing electrode and the reference electrode.

Further, a third oxygen pump cell is provided in the second internal space, wherein an anode electrode of the first oxygen pump cell is active at least for oxygen,
The anode electrode of the third oxygen pump cell has oxygen and NO
Active in x, the controllability of the oxygen concentration in the first internal space is greatly improved by arranging the detection electrode opposite to the third anode electrode. On the other hand, a configuration is provided in which the cathode electrode of the third oxygen pump cell is provided in a duct that communicates with the atmosphere, which is isolated from the second internal space,
The anode electrode of the first oxygen pump cell is active only on oxygen, the anode electrode of the third oxygen pump cell is active on oxygen and NOx, and a detection electrode is provided opposite to the third anode electrode. , The ability to supply oxygen to the first internal space and the ability to convert NOx can be controlled without interfering with each other.

Further, according to the present invention, a predetermined voltage is applied to the above-mentioned oxygen pump cell, and even if the oxygen concentration in the test gas atmosphere is at least zero, the oxygen concentration in the internal space becomes 0.1 vol% or more. It is practically possible to set the gas diffusion resistance of the first gas introduction hole so as to be maintained. Further, based on the potential difference between the aforementioned counter electrode and the reference electrode,
By controlling the driving voltage of the first oxygen pump cell and maintaining the oxygen concentration in the first internal space at 0.1 vol% or more and within a predetermined range, more accurate detection becomes possible.

[0012] By setting the gas diffusion resistance of the atmospheric duct so that the third oxygen pump cell is in a diffusion-controlled state in which the pump current does not change above a predetermined applied voltage, the duct cross-sectional area is reduced. And a constant oxygen supply rate from the third oxygen pump.

[0013]

1 to 6 show a cross-sectional structure of a sensor according to a preferred embodiment of the present invention. FIGS. 1, 2, and 3 show examples of the most basic sensor configuration. In the drawings, the same reference numerals indicate the same configurations. A description of parts common to the drawings is partially omitted. In FIG. 1, a gas detection electrode 5 is provided in an internal space (gas measurement chamber) 3 which is surrounded by a zirconia solid electrolyte 13 which is an oxygen ion conductor and communicates with a test gas atmosphere via a gas introduction hole 1. I have. In addition, the anode electrode 8 of the oxygen pump is
Are arranged in the internal space 3, and the cathode electrodes 11 forming a pair as an oxygen pump have a structure in which the cathode electrodes 11 are exposed to the atmosphere of a test gas. The reference electrode 6, which is paired with the detection electrode 5, is a reference duct 1 that communicates with the atmosphere to make the atmospheric oxygen concentration a reference.
5. In this structure example, at least the substrate on which the sensing electrode (reference electrode) and the oxygen pump are formed is required to be a solid electrolyte of an oxygen ion conductor, but the element sintering distortion in the firing step during manufacturing is reduced. For this purpose, it is desirable that the other substrates be the same solid electrolyte.
Reference numeral 16 denotes a heater for maintaining the sensor at a measurement temperature (for example, about 650 ° C.).

In the sensor structure shown in FIG. 1, a predetermined DC voltage Vp is applied between the cathode electrode 11 and the anode electrode 8 of the oxygen pump. At this time, oxygen in the test gas atmosphere and oxygen compound gas such as H ₂ O and CO ₂ are electrochemically decomposed by an oxygen pump, and the generated oxygen is ionized and passes through the solid electrolyte, and Supplied to Therefore, even in an atmosphere of a test gas in which the oxygen concentration fluctuates greatly from zero to several tens of percent, such as in automobile exhaust gas, the presence of an oxygen compound gas such as H ₂ O or CO ₂ stably results. Oxygen can be supplied to the internal space.

The detection electrode 5 needs to be installed in the internal space 3. As the detection electrode 5 in the present invention, a mixed potential type detection electrode is used. A hybrid potential sensing electrode is an electrode potential obtained due to two or more electrode reactions occurring on the electrode. For example, in the case of NO detection of nitrogen oxide, an electrode reaction represented by the formulas (1) and (2) occurs, and in the case of NO ₂ detection, an electrode reaction represented by the formulas (3) and (4) occurs.

[0016] _{^{O 2 + 4e - → 2O 2-}} .............................. (1) 2NO + 2O 2- → 2NO 2 + 4e - ............ (2) 2O 2- → O 2 + 4e - ......... ............ (3) 2NO ₂ + 4e ⁻ → 2NO + 2O ^2- ……… (4)

Therefore, when NO and NO ₂ coexist, the output is canceled and the total NOx concentration cannot be detected. The sensor output measures the electrode potential difference between the sensing electrode and the reference electrode. The reference electrode 6 is NO for the aforementioned reason.
If no electrode reaction occurs for x, a potential difference between the electrodes occurs. Therefore, the reference electrode 6 only needs to satisfy, for example, either an electrode material having no activity on NOx or a structure not exposed to NOx. In the case of FIG. 1, the structure is not exposed to NOx. When the reference electrode 6 has no activity on NOx and has activity on oxygen, the counter electrode 7 and the detection electrode 5 can be simultaneously installed in the internal space 3 as shown in FIG. In such a structure, even if the oxygen concentration in the internal space 3 fluctuates, if the oxygen activities of the detection electrode 5 and the counter electrode 7 are made equal, the oxygen concentration dependence of the sensor output can be extremely reduced. It is needless to say that this sensor structure is completely impossible with other density electromotive force type detection methods. Further, as shown in FIG. 3, a counter electrode 7 is provided in the internal space 3 and a detection electrode 5 similarly obtained by using a potential difference between the reference electrode 6 and the counter electrode 7 provided in the atmospheric reference duct. When used in combination with the potential difference between the reference electrode 6 and the reference electrode 6, the detection accuracy of the sensor can be improved. That is, even if the oxygen partial pressure dependency of the sensing electrode 5 and the oxygen partial pressure dependency of the counter electrode 7 are different,
By setting an appropriate coefficient in the detection circuit, even if the oxygen concentration in the test gas atmosphere fluctuates greatly, its dependence can be almost eliminated. Furthermore, as shown in FIGS. 6 and 7, the oxygen partial pressure in the internal space 3 can be directly measured to control the oxygen pump cell voltage of the present invention.

A test gas diffuses and flows into the internal space 3 through the gas inlet 1. For example, when detecting the NOx concentration in the combustion exhaust gas of an automobile or the like, the NOx includes NO and NO ₂
And hydrocarbon gas (HC) and C
Other gases such as O, CO ₂ and H ₂ O coexist. In this,
To detect the total concentration of NOx, it is necessary to convert the exclusion of other gases that affect the sensor output and to NO or NO ₂ in a single gas. In the case of FIG. 1, it is necessary to set the oxygen concentration in the internal space 3 to 0.1 vol% or more in order to oxidize HC and the like so as not to affect the detection electrode potential. In the case of automobile exhaust gas, it is desirable to keep the oxygen concentration at 1 vol% or more.

The oxygen pump in FIG 1 converts a supply of oxygen in the internal space, NOx to NO ₂ electrochemically 2
It has two functions. Therefore, the anode electrode 8
Must have activity on both oxygen and NOx. On the other hand, the cathode electrode 11 only needs to have at least oxygen activity. However, in a case where a large amount of HC coexists in an atmosphere in which oxygen hardly exists, such as an automobile exhaust gas during a rich operation, HC may be preferentially adsorbed to the cathode 11 and may hinder oxygen pumping. . This is a phenomenon called electrode blocking. To prevent this blocking, FIG.
The use of such a sensor structure is also included in the present invention.

In FIG. 4, the oxygen pump cathode 11 is surrounded by a second internal space 4 and a second gas inlet 2 communicating with the gas to be detected is provided. For example, trace amounts of oxygen and H
_{Even when 2} O and HC are present at the same time, only HC is oxidized and made harmless in the internal space 4, and a blocking phenomenon does not occur. That is, the second anode 9 is installed in the internal space 4 and oxygen is supplied to the internal space 4 so that the oxygen concentration can be kept high. The oxygen concentration in the internal space 4 needs to be 0.1 vol% or more, preferably 1 vol% or more. Therefore, the second gas inlet 2 is set to have an appropriate gas diffusion resistance.

In the above-described structural example of the present invention, the oxygen pump and the NOx conversion pump are used in combination.
However, in order to freely control the oxygen concentration in the internal space 3, it is preferable to separate the oxygen pump for simply supplying oxygen and the oxygen pump for converting NOx or the like in order to improve the sensor accuracy. 5 and 6 show a sensor structure provided with a third anode electrode. In FIG. 5, the inner space 4
FIG. 6 shows a case where the cathode electrodes 11 and 12 are installed in the inside, and FIG. However, in FIG. 5, the cathode electrodes 11 and 12 can be shared by one electrode. In the case of FIG. 5, there is an advantage that the oxygen supply source for the conversion pump is also a test gas atmosphere. In the case of FIG. 6, only the oxygen supply source for the conversion pump is introduced from the atmosphere. However, since the amount of oxygen ions for the conversion pump may be extremely small, the cross-sectional area of the duct 14 can be reduced. At this time, there is an advantage that the potential setting of the conversion pump can be set very wide. That is, when electrolyzing H ₂ O in a test gas atmosphere, a cell voltage of about 0.4 V or more is required. Further, in FIG. 6, if the diffusion resistance of the duct 14 is set so as to generate a limiting current, the supply amount of oxygen from the conversion pump is always constant, and the adjustment of the oxygen concentration in the internal space 3 tends to be more stable. .

In order to obtain the sensor of the present invention as described above,
Usually, it is provided by the following production method. The sensor structure having the internal space and the air duct as described above is usually formed by laminating and pressing green sheets of zirconia and degreased.
It is obtained by firing at around 00 ° C. Zirconia (Z
The rO ₂ ) green sheet is usually added with _{3 to} 8 mol of yttria (Y ₂ O ₃ ) at the same time in order to impart oxygen ion conductivity. In a general method for producing this green sheet, a predetermined amount of zirconia powder to which yttria is added is subjected to ball mill mixing with an organic binder such as PVA, a plasticizer and an organic solvent. At this time, a dispersant may be added in order to improve the dispersion of the powder particles. The zirconia slurry thus obtained was coated on a PET film by a sheet forming machine using a doctor blade for several tens of minutes.
Formed to a thickness of 0 μm. When this is dried to evaporate the solvent, the sheet is very flexible and can be strongly adhered to each other when the sheets are pressed together while heating.
In order to form an internal space in this green laminate,
It can be formed by filling the green sheet with theobromine or the like which sublimates at a temperature lower than the degreasing temperature at which the organic binder is oxidized and removed.

The electrode material used in the present invention is not particularly limited, except that it needs to satisfy the above-mentioned activity for the test gas or oxygen. However, chemical stability is necessary for firing at a high temperature. NiCr ₂ O ₄ , MnCr ₂ O ₄ , FeCr ₂
O ₄ , Cr ₂ O ₃ , NiO or the like can be used. In the noble metal system, Rh, Ir, Pt-Rh alloy, Pt
-Ir-Rh alloy or the like can be used. They are,
The electrode material powder, an organic binder and an organic solvent are kneaded to form a paste, which is applied on a zirconia green sheet by a screen printing method or the like.

The material of the oxygen pump electrode used in the present invention is not particularly limited as long as it is active at least for oxygen. However, the anode electrode for the conversion pump needs to have activity for both the test gas and oxygen. Pt is usually used as the oxygen pump electrode material. Total N
The anode electrode of the conversion pump for Ox detection is Pt-Rh
The use of a noble metal such as an alloy, a Pt—Ru alloy, and Rh is suitable for obtaining a high ion current.

FIG. 7 shows the IV characteristics of the pump cell in which both the cathode electrode and the anode electrode are made of Pt with respect to water vapor or CO ₂ . This data shows the relationship between the cell voltage and the pump current when the temperature of the pump cell is 650 ° C. A large pumping current is obtained despite the zero oxygen in the measurement atmosphere. However, it can be seen that the activity of the electrode is slightly lower for CO ₂ than for water (steam). FIG. 8 shows a Pt cathode electrode and Pt-Rh
(5 wt%) IV of pump cell composed of anode electrode
Show characteristics. The measurement atmosphere does not contain oxygen at all,
10 vol% of steam is added. The relationship between the pump cell voltage and the pumping current when the temperature of the pump cell is 600 ° C., 650 ° C., and 700 ° C. is shown. It can be seen that the pumping amount increases as the pump cell temperature increases. It is necessary to suppress the setting of the pump cell voltage to be equal to or lower than the cell voltage at which electrolysis (blackening phenomenon) of the zirconia solid electrolyte itself occurs. Therefore, it is desirable to set the pump cell voltage at 1.2 V or less.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.
It goes without saying that the invention includes all those having the same idea.

(Example 1) NO of the sensor structure shown in FIG.
An x sensor sample was prepared as follows. First, a green sheet of zirconia to which 6 mol% of yttria was added was prepared by the above-described method. The thickness of the zirconia green sheet was about 200 μm. This was cut to the size of the sensor substrate before firing, and windows were simultaneously formed in the internal space and the part where the air duct was formed. Electrodes, lead conductors, heaters, and the like respectively formed on the cut green sheets were printed by a screen printing machine. Pt paste was used as a printing material for the cathode electrode 11, counter electrode 7, reference electrode 6, heater, and current collector of the oxygen pump. The amount of Rh added to the anode electrode 8 of the oxygen pump was 3
A wt% Pt-Rh alloy paste was used. However, the porosity of the electrode was adjusted by adding about 10 wt% of a zirconia solid electrolyte powder to the Pt paste and the Pt-Rh alloy paste. For the detection electrode 5, an oxide paste of NiCr ₂ O ₄ was used.

The green sheets thus formed by printing were superimposed sequentially while inserting a Pt wire into the lead portion so as to obtain the lamination state shown in FIG. 3, and lamination was performed by isostatic pressing in warm water. This laminate was degreased at about 600 ° C., and subsequently fired in a high-temperature atmosphere at 1400 ° C. The sensor sample thus produced was set on a holder for measurement and connected to a measurement circuit and a control circuit shown in FIG. 3 to evaluate sensor characteristics. That is, a DC constant voltage source Vp was connected between the electrode 8 and the electrode 11, and a constant voltage of 0.7 V was applied using the electrode 11 as a negative electrode (cathode) and the electrode 8 as a positive electrode (anode). At the same time, the input impedance 1 between the detection electrode 5 and the reference electrode 6
A potentiometer of 0 MΩ or more was connected. Similarly, the potential difference between the counter electrode 7 and the reference electrode 6 was measured. Although not shown in FIG. 3, a circuit for measuring the bulk impedance of the zirconia substrate between the electrode 8 and the electrode 11 is connected in parallel, a sensor temperature signal is taken out, and connected between two lead wires from the heater substrate. Connected to the temperature control circuit
Was used to control the sensor temperature to be 650 ° C. As a measurement gas, a sensor output for NO was measured by using a mixed gas of 100 ppm of NO, oxygen and H ₂ O (the rest being nitrogen). FIG. 9 shows the oxygen concentration dependency of the sensor output with respect to 100 ppm of NO at this time. As is apparent from the graph, even when the oxygen concentration in the measurement gas is zero, a substantially constant sensor output can be obtained. It is considered that the reason why the output slightly increases near the oxygen concentration of zero is that the oxygen concentration in the internal space of the sensor element slightly decreases and the hybrid potential output increases accordingly.

[0029] (Example 2) under the same conditions the sensor samples used in Example 1, NO100p measurement gas containing CO ₂
The sensor output for pm was measured. Example 1
Is the same as However, the measurement gas was N 2 using a mixed gas of 100 ppm of NO, oxygen and CO ₂ (the rest was nitrogen).
The sensor output for O was measured. Oxygen concentration in the measurement gas, and 0～5Vol%, and the CO ₂ concentration and 10 vol% constant. FIG. 10 shows the oxygen concentration dependency of the sensor output with respect to 100 ppm of NO at this time. As is clear from FIG. 10, even when the oxygen concentration in the measurement gas is zero, an almost constant sensor output can be obtained. The reason why the sensor output level is slightly lower than in the first embodiment and the oxygen concentration dependency is slightly higher is that the oxygen pumping amount is different due to a difference in electrochemical activity between H ₂ O and CO ₂ .

(Example 3) A sensor sample having the sensor structure shown in FIG. However, in this embodiment, the second anode electrode in the second internal space 4 was not provided. Example 1
Operated under the same conditions as. The sensor element was heated to 650 ° C. At this time, as a measurement gas, 100 ppm of NO and oxygen,
The sensor output for NO was measured using a mixed gas of steam and C ₃ H ₆ (propylene) (the rest being nitrogen).
The oxygen concentration of the measurement gas was 0 to 5 vol%, and the water vapor concentration was constant at 7 vol%. C ₃ H ₆ concentration 0ppm or 2000pp
m was fixed. FIG. 11 shows the oxygen concentration dependency of the sensor output with respect to 100 ppm of NO at this time. When the C ₃ H ₆ concentration was 0 ppm (black circle data in the figure), almost the same oxygen concentration dependence as in Example 1 was exhibited. However, when the C ₃ H ₆ concentration is 2000 ppm (open circle data in the figure),
It can be seen that the sensor output sharply decreases when the oxygen concentration in the measurement gas decreases. This is presumably because C ₃ H ₆ was preferentially adsorbed on the cathode of the oxygen pump, thereby reducing the oxygen pumping ability.

Example 4 A sensor sample having the structure shown in FIG. 4 was manufactured in the same manner as in Example 3. In the sample of this example, a second anode electrode made of Pt was provided in the second internal space 4. A predetermined voltage was applied between the second oxygen pump cells, and the second internal space was set to a concentration at which 2000 ppm of C ₃ H ₆ could be sufficiently oxidized. At this time, the second gas inlet does not have a diffusion resistance that controls oxygen pumping. The sample was examined under the same measurement conditions as in Example 3 for the oxygen concentration dependency in the measurement gas.
FIG. 12 shows the result. It can be seen that even if C ₃ H ₆ is present, the oxygen concentration dependency is almost eliminated. That is, it is considered that C ₃ H ₆ is oxidized and removed in the second internal space. Of course, it is obvious that the effect of suppressing HC blocking can be further improved by providing a substance having a high oxidation catalytic property also in the internal space 4.

Example 5 A sample having the sensor structure shown in FIG. 5 was manufactured as in Example 1. In the second internal space of the sample, the same operation as in the fourth embodiment is performed.
Of the oxygen pump cell. The driving of the second oxygen pump cell controlled the oxygen concentration in the internal space 3 to 4% by feeding back the electromotive force between the Pt counter electrode 7 and the reference electrode 6 in the first internal space 3. A constant cell voltage was applied to the third oxygen pump cell. At this time, the sensor output for NO was measured by using a mixed gas of 100 ppm of NO, oxygen, water vapor, and C ₃ H ₆ (propylene) (the remainder being nitrogen) as a measurement gas. Oxygen concentration of measurement gas is 0-5vo
l%, and the water vapor concentration was kept constant at 7 vol%. The C ₃ H ₆ concentration was fixed at 2000 ppm. A method in which the oxygen concentration in the internal space 3 was not performed was also implemented for comparison. At this time, N
FIG. 13 shows the oxygen concentration dependency of the sensor output with respect to 100 ppm of O. When the oxygen concentration control in the internal space 3 in FIG. 13 is not performed (black circle data), the output slightly increases when the oxygen concentration is near zero, but when the oxygen concentration control is performed (open data). It can be seen from FIG. 4 that an extremely constant sensor output can be obtained even when the oxygen concentration in the measurement gas is zero.

[0033]

According to the present invention, a detection electrode which is installed in an internal space having a gas introduction hole surrounded by a zirconia solid electrolyte and which is active on oxygen, and a detection electrode which is active on oxygen and which has a A gas sensor comprising an inactive reference electrode and a pair of oxygen pump cells provided on a solid electrolyte in the internal space, wherein an anode electrode of the oxygen pump cell is provided in the internal space, and a cathode electrode is provided in the internal space. It is installed outside the internal space, and by applying a voltage to the oxygen pump cell, the oxygen or oxygen compound gas in the test gas atmosphere is electrochemically decomposed to supply oxygen into the internal space, or By measuring the potential difference between the detection electrode and the reference electrode while oxidizing the portion, the concentration of the test gas can be detected. That is, the present invention eliminates the necessity of forming an air duct for introducing a large amount of oxygen from the air atmosphere, so that a great advantage is obtained from the viewpoint of sensor element performance and sensor element manufacture.

[Brief description of the drawings]

FIG. 1 is a sectional structural view showing an example of the sensor of the present invention.

FIG. 2 is a sectional view showing an example of the sensor of the present invention.

FIG. 3 is a sectional structural view showing an example of the sensor of the present invention.

FIG. 4 is a sectional structural view showing an example of the sensor of the present invention.

FIG. 5 is a sectional structural view showing an example of the sensor of the present invention.

FIG. 6 is a sectional structural view showing an example of the sensor of the present invention.

FIG. 7 is a graph showing IV characteristics of a Pt: Pt pump cell.

FIG. 8 is a graph showing IV characteristics of a Pt: Pt-Rh pump cell.

FIG. 9 is a graph showing NOx sensor output characteristics having the sensor structure shown in FIG. 3;

FIG. 10 is a graph showing NOx sensor output characteristics having the sensor structure shown in FIG. 3;

FIG. 11 is a graph showing NOx sensor output characteristics having the sensor structure shown in FIG. 4;

FIG. 12 is a graph showing NOx sensor output characteristics having the sensor structure shown in FIG. 4;

FIG. 13 is a graph showing NOx sensor output characteristics having the sensor structure shown in FIG. 5;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st gas introduction port 2 2nd gas introduction port 3 1st internal space (gas measurement chamber) 4 2nd internal space 5 detection electrode 6 reference electrode 7 counter electrode (oxygen detection electrode) 8 1st anode electrode 9 2nd anode electrode 10 3rd anode electrode 11 1st cathode electrode 12 2nd cathode electrode 13 Solid electrolyte 14 Air introduction duct 15 Air reference duct 16 Heater

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takashi Unchi, 4-14-1 Suehiro, Kumagaya-shi, Saitama Pref. Riken Kumagaya Plant (72) Inventor Politics Hasei 4-4-1, Suehiro, Kumagaya-shi, Saitama RIKEN Kumagaya Office

Claims (11)

[Claims] In an internal space surrounded by a zirconia solid electrolyte having oxygen ion conductivity and having a gas introduction hole, active detection of a test gas and oxygen installed on the zirconia solid electrolyte in the internal space is performed. An electrode, a reference electrode paired with the detection electrode and active on oxygen but not on the test gas, and a reference electrode provided on the solid electrolyte in the internal space.
A gas sensor comprising a pair of oxygen pump cells, wherein an anode electrode of the oxygen pump cell is installed in the internal space, and a cathode electrode is installed so as to communicate with a test gas atmosphere outside the internal space. By applying oxygen to the internal space by electrochemically decomposing oxygen or oxygen compound gas in the test gas atmosphere, or oxidizing a part of the gas, A gas sensor for detecting a concentration of a test gas by measuring a potential difference between the gas and a reference electrode. 2. The gas sensor according to claim 1, wherein in the sensor configuration, a reference electrode paired with a detection electrode is provided in the internal space. 3. An internal space surrounded by a zirconia solid electrolyte having oxygen ion conductivity and having a gas introduction hole, and at least a test gas and oxygen active on a zirconia solid electrolyte provided in the internal space. A detection electrode, a counter electrode that is active on oxygen and is not active on the test gas, a reference electrode that is active only on oxygen provided in a reference space where a predetermined oxygen partial pressure is maintained, and a solid electrolyte in the internal space. A gas sensor comprising a pair of oxygen pump cells provided in the gas pump, wherein an anode electrode of the oxygen pump cell is provided in the internal space, and a cathode electrode is provided so as to communicate with a test gas atmosphere outside the internal space. By applying a predetermined voltage to the oxygen pump cell, the oxygen or oxygen compound gas in the test gas atmosphere is electrochemically decomposed and oxygen is introduced into the internal space. While supplying the element or oxidizing a part of the gas, the potential difference between the detection electrode and the reference electrode and the potential difference between the counter electrode and the reference electrode are simultaneously measured, and the concentration of the test gas is determined from the two potential differences. A gas sensor characterized by detecting. 4. In the above sensor configuration, a cathode electrode of an oxygen pump cell is provided in a second internal space having a second gas introduction hole which communicates with a test gas atmosphere provided separately, and the second internal space is provided. A cathode electrode of the oxygen pump and a second anode electrode forming a second pair are provided therein, and a voltage is simultaneously applied between the cathode electrode, the first anode electrode, and the second anode electrode. First
The gas sensor according to claim 1, wherein the detection is performed while simultaneously supplying oxygen to the second internal space. 5. The gas sensor according to claim 1, wherein the test gas is a nitrogen oxide. 6. The first anode electrode is made of an electrode material active on oxygen and nitrogen oxides. The oxygen pump cell supplies oxygen from a test gas atmosphere into a first internal space and nitrogen gas. NO, which is an oxide, is converted to NO ₂
The gas sensor according to claim 5, wherein the total NOx concentration is detected by measuring a potential difference between the detection electrode and the reference electrode while electrochemically converting the NOx concentration into a total value. 7. A structure in which a third oxygen pump cell is provided in the second internal space, wherein an anode electrode of the first oxygen pump cell is active at least in oxygen, and The anode electrode is oxygen and NOx
6. The gas sensor according to claim 5, wherein a detection electrode is provided so as to face the third anode electrode. 8. A structure in which a cathode electrode of a third oxygen pump cell is provided in a duct communicating with an atmospheric atmosphere independent of the second internal space, wherein an anode electrode of the first oxygen pump cell is made of oxygen only. 6. The gas sensor according to claim 5, wherein an anode electrode of the third oxygen pump cell is active on oxygen and NOx, and a detection electrode is provided so as to face the third anode electrode. . 9. A predetermined voltage is applied to the oxygen pump cell, and even if the oxygen concentration in the test gas atmosphere is at least zero, the oxygen concentration in the internal space is maintained at 0.1 vol% or more. 6. The gas sensor according to claim 1, wherein the gas diffusion resistance of the first gas introduction hole is set as described above. 10. A driving voltage of the first oxygen pump cell is controlled based on a potential difference between the counter electrode and the reference electrode so that the oxygen concentration in the first internal space is 0.1 vol% or more and a predetermined value. The gas sensor according to claim 3, wherein the gas sensor is maintained in a range. 11. The gas diffusion resistance of an atmospheric duct is set so that the third oxygen pump cell is in a diffusion-controlled state in which the pump current does not change at a predetermined applied voltage or higher. 9. The gas sensor according to 8.