JP2000097905A - APPARATUS AND METHOD FOR MEASURING NOx GAS CONCENTRATION - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make a NOx gas concentration measuring apparatus have a simple structure, a high in responsivity and low in dependency of a sensor output on an oxygen concentration by detecting an oxygen pump current flowing between a pair of electrodes corresponding to the NOx gas concentration when NOx is decomposed and dissociated within a void part. SOLUTION: A switch 20 is controlled by a microcomputer to be switched periodically, selectively connecting terminals 20a and 20b during an oxygen concentration detection term and terminals 20a and 20c during an NOx concentration detection term. A reference voltage is input to a reference input terminal of a differential amplifier 22. An output terminal of the differential amplifier is connected electrically to an outside electrode 6a of a first oxygen pump cell 6. An output of the differential amplifier 22 controls the first oxygen ion pump cell 6 to pump out, in the oxygen to the void part 2 so that an oxygen concentration in the void part 2 is a constant low concentration. After the oxygen concentration in the void part 2 becomes a predetermined value, the terminals 20a and 20c are connected and an oxygen pump current flowing between an inside common electrode 78a and an outside common electrode 78b is detected, thereby, an NOx gas concentration is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring the concentration of NOx gas in the exhaust gas of a moving or industrial internal combustion engine of an automobile, a ship, an airplane, or the like, or in the combustion gas of a boiler or the like, and the NOx gas concentration measurement. About the method.

[0002]

2. Description of the Related Art In recent years, with the tightening of exhaust gas regulations, studies have been conducted to directly measure the concentrations of NOx, HC, and CO in exhaust gas from engines and the like to control engines and catalysts. As a NOx gas sensor for this purpose, for example, SAE paper No. 960334 p137
No. 142-1996 or Japanese Patent Application Laid-Open No. 8-247959 discloses a NOx gas sensor having two internal voids,
It has two diffusion resistance parts and two oxygen ion pump cells. Specifically, a first oxygen ion pump cell and an oxygen concentration measurement cell are provided facing the first void. The oxygen concentration measuring cell is for detecting the oxygen concentration in the first void portion, and pumping oxygen from the first void portion by the first oxygen ion pump cell based on the oxygen concentration detection output of the oxygen concentration measuring cell. Is controlled. However, the first oxygen ion pump cell is controlled such that the oxygen partial pressure in the first gap is maintained at a level at which NOx is not decomposed in the first gap. Then, a second oxygen ion pump cell is provided facing the second gap, and when a voltage is applied to the second oxygen ion pump cell, based on an oxygen pump current corresponding to the concentration of NOx gas flowing through the cell. NO
The x gas concentration is measured.

[0003]

The above-mentioned conventional NOx gas sensor has at least three cells and has a complicated structure. Furthermore, since there are two voids and the diffusion path of the gas to be measured is long, there is a problem in responsiveness. Also, the first
Since the oxygen concentration measurement cell (especially, the oxygen concentration detection electrode) facing the void is separated from the second oxygen ion pump cell facing the second void, the first oxygen ion measurement cell based on the output of the oxygen concentration measurement cell is separated. In spite of controlling the oxygen concentration in the gap by the oxygen ion pump cell, the oxygen concentration near the second oxygen ion pump cell is unstable, and the sensor output largely depends on the oxygen concentration. In addition, two diffusion resistance portions, a first diffusion resistance portion communicating between the atmosphere and the first void portion, and a second diffusion resistance portion communicating between the first void portion and the second void portion,
Since two oxygen ion pump cells and an oxygen concentration measurement cell are required, the manufacturing process becomes complicated, and
There is a problem that variation between lots increases.

[0004] It is an object of the present invention to provide a simple NO
An object of the present invention is to provide an x gas sensor and a concentration measuring device using the same. Another object is to provide a NOx gas sensor having excellent responsiveness and low oxygen concentration dependence of sensor output, a concentration measuring device using the same, and a NOx gas concentration measuring method.

[0005]

Means for Solving the Problems In a sensor for measuring a NOx gas concentration based on an oxygen pump current flowing by oxygen ions dissociated from NOx, the present inventors face a sensor gap into which a gas to be measured is introduced. By applying a constant voltage to the oxygen concentration detection electrode for detecting the oxygen partial pressure in the gap for a certain period, the oxygen concentration detection electrode also functions as an oxygen pump electrode, and the oxygen concentration detection electrode and the oxygen It has been found that the NOx gas concentration can be measured by measuring the oxygen pump current flowing through the solid electrolyte layer between the concentration reference electrodes, and as a result of intensive research, the present invention has been completed.

The present invention includes the following items from the following viewpoints. First viewpoint: a void into which a gas to be measured is introduced via a diffusion resistor. A first oxygen ion pump cell that controls the pumping of oxygen out and / or into the void. A second cell comprising a solid electrolyte and a pair of electrodes formed on the solid electrolyte, wherein at least one electrode faces the void. Switching means for switching between application and non-application of a predetermined voltage between the pair of electrodes of the second cell; When a predetermined voltage is applied between the pair of electrodes of the second cell, NOx is decomposed in the gap and dissociated oxygen is pumped out through the second cell, so that NOx
Means for detecting an oxygen pump current flowing between a pair of electrodes according to a gas concentration. Means for controlling pumping of oxygen by the first oxygen ion pump cell based on an electromotive force generated between the pair of electrodes when a predetermined voltage is not applied between the pair of electrodes of the second cell. Note that the predetermined voltage is a voltage at least that dissociates NO.

Second viewpoint: The switching means includes a state in which the second cell functions as an oxygen concentration battery cell for detecting the oxygen concentration in the gap, and a second oxygen ion pump in which an oxygen pump current according to the NOx gas concentration flows. The function that functions as a cell must be periodically switched. Third perspective:
In the second cell, when the electrode facing the gap is defined as an oxygen concentration detection electrode and the other electrode is defined as an oxygen concentration reference electrode, the oxygen concentration reference electrode is a self-generated reference electrode. Oxygen present in the vicinity of the oxygen concentration reference electrode for forming the self-generated reference electrode is one in which NOx is decomposed and dissociated when a predetermined voltage is applied.

[0008] According to a fourth aspect, the first portion facing the gap is provided.
One electrode of the oxygen ion pump cell is composed of Pt, Rh, P
Including one or more metals selected from d, Ir, and Ru. Further, Ag, Au, Ni, Co, Cr, Fe, Mn, Cu, T
It contains at least one metal selected from i, Al, Zn, Sn, and In. One electrode of the second cell facing the void is P
At least one metal selected from the group consisting of t, Rh, Pd, Ir, and Ru, and containing Ag, Au, Ni, Co, Cr, Fe, Mn, Cu, T
and at least one metal selected from i, Al, Zn, Sn, and In.

Fifth viewpoint: A NOx gas sensor having first and second oxygen ion pump cells facing a gap into which a gas to be measured is introduced via a diffusion resistor is used. When no voltage is applied to the second oxygen ion pump cell, the first oxygen ion pump cell controls the pumping of oxygen into and / or out of the gap based on the electromotive force of the oxygen concentration cell generated in the cell. When a voltage is applied to the second oxygen ion pump cell, NOx in the gap is decomposed and dissociated oxygen is pumped out to detect an oxygen pump current corresponding to the concentration of the NOx gas flowing through the cell. The NOx gas concentration is measured based on the detected oxygen pump current. Sixth perspective:
Periodically switching between application and non-application of a voltage to the second oxygen ion pump cell. Seventh viewpoint: A cell including a pair of electrodes and a solid electrolyte layer is provided facing a gap into which a gas to be measured is introduced via a diffusion resistor. The cell acts as an oxygen ion pump cell for decomposing NOx and pumping out dissociated oxygen when a predetermined voltage is applied to the pair of electrodes. The cell functions as an oxygen concentration battery cell that generates an electromotive force (potential) according to the oxygen concentration in the gap (in the gas to be measured) when a predetermined voltage is not applied to the pair of electrodes.

In the NOx gas concentration measuring apparatus to which the NOx gas sensor according to the present invention is applied, since the NOx concentration is detected based on the oxygen pump current flowing by oxygen ions, the oxygen concentration in the internal space of the sensor element is kept constant. You need to control. Therefore, according to the present invention, one set of cells is arranged facing one void, and during one period, one cell (first oxygen ion) is detected based on the oxygen concentration detection output of the other cell (second cell). Pump cell) pumps oxygen into or out of the gap to control the oxygen concentration in the inner gap to be as constant and low as possible, and to apply a predetermined constant voltage to the other cell during other periods. This decomposes NOx in the gap and detects the NOx gas concentration from the oxygen pump current flowing through the cell. Therefore, according to the present invention, there is provided a gas sensor and a measuring device with good responsiveness since the configuration of the gas sensor element is simple. Further, in order to reduce the oxygen dependency of the sensor output, in particular, the oxygen concentration near the electrode where the NOx is actually dissociated (the electrode facing the gap of the other cell) is detected, and the oxygen concentration near this is detected. It is preferable to control the concentration to be constant and low. According to the present invention, since the electrode for dissociating NOx and the electrode for detecting oxygen concentration are common,
It is possible to provide a NOx gas concentration measuring device having extremely low oxygen dependency. Further, the manufacturing process of the NOx gas sensor element applied to the NOx gas concentration measuring device of the present invention is simplified as compared with the conventional element, and the variation between lots is reduced.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described. In a preferred embodiment of the NOx gas sensor applied to the NOx gas concentration measuring device of the present invention, the oxygen concentration detection electrode provided inside the gap and the inner electrode of the first oxygen ion pump cell are made of Pt, Rh. , P
It shall include at least one metal selected from d, Ir, and Ru. More preferably, the oxygen concentration detection electrode provided inside the gap and the inner electrode of the first oxygen ion pump cell are one or more metals selected from Pt, Rh, Pd, Ir, and Ru, and Ag, Au, Ni, Co, Cr, Fe, M
It contains at least one metal selected from n, Cu, Ti, Al, Zn, Sn and In.

The oxygen ion conductive solid electrolyte layer constituting the cell is made of ZrO in which Y ₂ O ₃ or CaO is dissolved.
_{Although 2} is a typical example, a solid solution of ZrO ₂ and an oxide of another alkaline earth metal element or a rare earth metal element may be used. Further, ZfO _{2 serving as} a base may contain HfO ₂ . In addition, partial stabilization, stabilization, or a mixture of ZrO ₂ and Ce
O ₂ , HfO ₂ , and ThO ₂ can be used. As the stabilizer, for example, one or more of CaO, MgO, and rare earth oxides (for example, Y ₂ O ₃ , La ₂ O ₃ , Gd ₂ O _3, etc.) are used. Preferably, yttria partially stabilized zirconia powder (YSZ) is used. Other stabilizers or other solid electrolytes can be used.

The oxygen concentration reference electrode is preferably a self-generated reference electrode capable of generating a stable potential. More preferably, the oxygen existing near the oxygen concentration reference electrode for forming the self-generated reference electrode is NOx. Is decomposed and dissociated to conduct the solid electrolyte layer. This eliminates the need for further providing a lead wire, and eliminates the need to newly provide a constant oxygen concentration chamber surrounding the oxygen concentration reference electrode.

Since the first oxygen ion pump cell and the second cell are formed on different solid electrolyte layers, leak current flowing between electrodes of different cells is reduced, and accurate concentration measurement can be performed. Preferably, an insulating layer is provided between each solid electrolyte layer. Further, in order to sufficiently dissociate NOx, it is preferable to provide a heater for heating the gap of the gas sensor element. For example, a heater layer containing a ceramic heater element is provided from above and below in the stacking direction of the solid electrolyte layers by bonding using cement or the like. The diffusion resistance part communicating the external measurement atmosphere and the void part is preferably made of alumina porous material, but may be made of another material.
If a predetermined diffusion resistance is obtained, a gap may be used. For example, alumina is used as a material of the insulating layer that defines a part of the gap, but another insulating material may be used.

The means for detecting the electromotive force of the oxygen concentration cell generated between the pair of electrodes of the second cell may not directly detect this electromotive force. For example, the oxygen concentration detecting electrode facing the void may be used. May be grounded so that a concentration cell electromotive force corresponding to the oxygen concentration in the void appears at the oxygen concentration reference electrode. The switching means for switching the application or non-application of the predetermined voltage between the pair of electrodes of the second cell selectively (1) connects the oxygen concentration detection electrode and the oxygen concentration reference electrode via a power supply, or ( 2) It is preferable that both the oxygen concentration detecting electrode and the electrode (inner electrode) facing the gap of the first oxygen ion pump cell are grounded. This switching means preferably switches periodically. It is preferable that each of the voltage application period and the voltage non-application period be 10 ms or more. In addition, the means for controlling the pumping of oxygen by the first oxygen ion pump cell includes an input terminal connected to the oxygen concentration reference electrode, and a voltage set according to a target oxygen concentration in the gap. The input terminal and the first
It is preferable to configure the differential amplifier having an output terminal electrically connected to the outer electrode of the oxygen ion pump cell. In this case, the oxygen concentration in the gap can be determined from the current IP1 flowing between the output terminal of the differential amplifier and the outer electrode of the first oxygen ion pump cell.

When controlling the oxygen concentration in the gap by the first oxygen ion pump cell, NOx (for example,
NO) may be partially decomposed, and it is possible to compensate for the decomposition amount of NO or the like based on the current IP1 flowing through the first oxygen ion pump cell. Further, NO ₂ may be decomposed in the void by the first oxygen ion pump cell.

[0017]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a NOx gas sensor element applied to a NOx gas concentration measuring device according to an embodiment of the present invention, and FIG. 8 is a NOx applied to a NOx gas concentration measuring device according to a comparative example.
It is a figure for explaining the structure of a gas sensor element.

First, a sensor element of a comparative example will be described. The sensor element of the comparative example shown in FIG. 8 has four solid electrolyte layers, two voids 2, 4, two diffusion resistance parts 1, 3
And two oxygen ion pump cells 6 and 8 and an oxygen concentration measurement cell 7 (three cells in total). The oxygen concentration (partial pressure) detection electrode 7a is dissociated from NOx and a current according to the NOx gas concentration flows. Inner electrode 8a of 2 oxygen ion pump cell 8
Are provided at remote locations. More specifically, an oxygen concentration detection electrode 7a and a first oxygen ion pump cell 6 (inner electrode 6b) are provided in the first void portion 2 communicating with the measurement atmosphere through the first diffusion resistance portion 1, and the oxygen concentration reference is provided. The electrode 7b is disposed between the second and third solid electrolyte layers,
A second oxygen ion pump cell 8 is provided facing the inside of the second gap 4 communicating with the first gap 2 through the porous second diffusion resistor 3 provided through the solid electrolyte layer. The inner electrode 8a of the oxygen ion pump cell 8 faces the second gap 4, and the outer electrode 8b is provided on the fourth solid electrolyte layer surrounded by an insulating layer.

A measuring device to which the sensor element of the comparative example (see FIG. 8) is applied is configured as follows. That is, the potential difference generated between the pair of electrodes 7a and 7b of the oxygen concentration measurement cell 7 is taken out, and the pair of electrodes 6a and 6 of the first oxygen ion pump cell 6 is set so that the potential difference becomes a predetermined constant potential.
A differential amplifier and a power supply for applying a voltage between the electrodes b and an ammeter for detecting a current flowing between the pair of electrodes 6a and 6b and measuring an oxygen concentration are provided. A power supply is connected between the pair of electrodes 8a and 8b of the second oxygen ion pump cell 8, and an ammeter for measuring the NOx gas concentration is connected in series with the power supply. Therefore, it is necessary to provide at least six lead wires inside the sensor element in order to extract the output of each of the six electrodes inside the sensor element.

Next, a method of measuring the concentration of NOx gas using the measuring apparatus of the comparative example will be described. Oxygen concentration measurement cell 7
Detects the oxygen concentration in the first gap 2, and controls the voltage applied to the first oxygen ion pump cell 6 by the differential amplifier so that the oxygen concentration becomes constant. As a result, oxygen is pumped and / or pumped from the first gap 2, and the gas having an approximately constant oxygen concentration diffuses and flows into the second gap 4, and the second voltage is applied to the second gap 4. The oxygen ion pump cell 8 dissociates NOx on the inner electrode 8a, and the dissociated oxygen ions conduct through the fourth solid electrolyte layer, thereby forming a pair of electrodes 8a and 8b of the second oxygen ion pump cell 8. During that time, an oxygen pump current corresponding to the NOx gas concentration flows, and by detecting this, the NOx gas concentration is measured.

On the other hand, the NOx gas sensor element applied to the NOx gas concentration measuring apparatus according to one embodiment of the present invention shown in FIG. 2 has three solid electrolyte layers, one void,
One diffusion resistance part, one oxygen ion pump cell and one
It has two common cells (two cells in total). Specifically, referring to FIG. 1, a first oxygen ion pump cell 6 and a common cell 78 are stacked facing one void 2,
Inner electrode 6b of oxygen ion pump cell 6 and common cell 7
Eight inner common electrodes 78 a are located in the gaps 2.
The outer electrode 6a of the first oxygen ion pump cell 6 is located in the measurement atmosphere, and the outer common electrode 78b of the common cell 78 is located between the solid electrolyte layers. Inner common electrode 78
a functions as an oxygen concentration detection electrode or an inner electrode of the second oxygen ion pump cell. The outer common electrode 78b functions as an oxygen concentration reference electrode or an outer electrode of the second oxygen ion pump cell. This sensor element has a structure in which a solid electrolyte layer having oxygen ion conductivity is laminated similarly to the sensor element of the comparative example, and electrodes are formed on the solid electrolyte layer. The void 2 is surrounded by the solid electrolyte layer in the laminating direction and the insulating layer and the diffusion resistor 1 in a direction along the plane of the solid electrolyte layer. The diffusion resistance portion 1 is formed on a part of both side surfaces in the lateral direction of the sensor element.

Referring to FIG. 2, a NOx gas concentration measuring apparatus according to an embodiment of the present invention to which the NOx gas sensor element shown in FIG. 1 is applied is configured as follows. That is,
In the switch 20, the terminal 20a is connected to the inner common electrode 78.
a, the terminal 20b is electrically connected to the inner electrode 6b and the ground of the first oxygen ion pump cell 6 via the node, and the terminal 20c is sequentially connected to the power supply 25, the ammeter 23, and the outer common electrode 78b. The switch 20 is periodically switched and controlled by a microcomputer (not shown),
The terminals 20a and 20b are selectively connected during the oxygen concentration detection period, and the terminals 20a and 20c are selectively connected during the NOx concentration detection period. Further, the outer common electrode 78b is connected to the terminals 20a, 20
It is electrically connected to the input terminal of the differential amplifier 22 via the sample and hold circuit 21 for maintaining the previous voltage during the connection period b. A reference voltage is input to the reference input terminal of the differential amplifier 22. This reference voltage corresponds to the control target oxygen concentration in the gap 2. The output terminal of the differential amplifier 22 is electrically connected to the outer electrode 6a of the first oxygen ion pump cell 6, and the output of the differential amplifier 22 causes the oxygen concentration in the gap 2 to be a constant low concentration. First,
The pumping of oxygen from the gap 2 and / or the pumping of oxygen into the gap 2 by the oxygen ion pump cell 6 is controlled.

Next, the operation of the measuring apparatus of this embodiment will be described with reference to FIG. (1) Oxygen concentration (partial pressure) control period (oxygen concentration detection period);
0b is electrically connected, and the inner common electrode 78a and the outer common electrode 78b are operated as a pair of electrodes of the oxygen concentration detection cell to detect the oxygen concentration in the gap 2 so that the oxygen concentration becomes constant. The voltage applied to the first oxygen ion pump cell 6 is controlled by the differential amplifier 22 (the oxygen pump current IP1 also changes). The ammeter 23 can also measure the oxygen concentration.

(2) Oxygen pump control period (NOx concentration detection period) Thus, after the oxygen concentration in the gap 2 has reached a predetermined stable low concentration, the terminals 20a and 20c of the switch 20 are now connected. Electrically connected to the inner common electrode 78
a, A predetermined voltage (a voltage at a level at which most of NO is dissociated) is applied to the outer common electrode 78b, and these are operated as a pair of electrodes of the second oxygen ion pump cell. As a result, NOx is dissociated on the inner common electrode 78a, and the dissociated oxygen ions are conducted through the solid electrolyte layer, so that an oxygen pump corresponding to the NOx gas concentration is provided between the pair of inner common electrode 78a and the outer common electrode 78b. Current IP2 flows. By detecting the oxygen pump current IP2, N
Measure the Ox gas concentration.

When the operations of (1) and (2) are repeated, as shown in FIG. 3, the oxygen pump current IP2 is constant regardless of the NOx gas concentration in the period of (1), and the period of (2) Then NOx
The oxygen pump current IP2 changes according to the gas concentration.
Therefore, it can be seen that the oxygen concentration and the NOx gas concentration can be measured by the above-described measuring device while keeping the oxygen concentration in the void portion constant.

According to the measuring apparatus of this embodiment,
Since the gas sensor element has one diffusion resistor, N
The response of Ox gas concentration detection is improved. Further, since the oxygen concentration detecting electrode also functions as the NOx concentration detecting section, it is possible to detect the NOx gas concentration independent of the oxygen concentration in the gas to be measured. Further, by sharing the oxygen concentration detection electrode and the oxygen pump electrode for NOx concentration detection, various advantages such as simplification of the manufacturing process and suppression of manufacturing variations can be obtained.

[Production Example] NOx applied to the NOx gas concentration measuring apparatus according to the embodiment of the present invention shown in FIG.
A production example of the gas sensor element will be described. FIG. 4 is a diagram for explaining the layout of the sensor element. This sensor element is manufactured by laminating and firing the ZrO ₂ green sheet and the electrode paste shown in FIG. The insulating coat and the paste material for the electrode are screen-printed on a predetermined ZrO ₂ green sheet, so that the insulating layer and the electrode are laminated at a predetermined position.
Hereinafter, an example of manufacturing each component such as a ZrO ₂ green sheet will be described.

[ZrO ₂ green sheet molding] The ZrO ₂ powder is calcined in an atmospheric furnace. A calcined ZrO ₂ powder, a dispersant, and an organic solvent are mixed together with cobblestone and dispersed, and a solution obtained by dissolving an organic binder in an organic solvent is added thereto.
Mix to obtain a slurry. A ZrO ₂ green sheet having a thickness of about 0.4 mm is prepared from the slurry by a doctor blade method and dried.

[Printing Paste] (1) Outer electrode 6a of first oxygen ion pump cell, oxygen concentration reference electrode (comparative example) 7b, outer common electrode 78b, second electrode
For inner electrode (comparative example) 8a and outer electrode (comparative example) 8b of oxygen ion pump cell: platinum powder, ZrO ₂ powder,
An appropriate amount of an organic solvent is mixed and dispersed, a solution obtained by dissolving an organic binder in the organic solvent is added, and a viscosity modifier is further added and mixed to prepare a paste.

(2) For the inner electrode 6b, oxygen concentration detecting electrode (comparative example) 7b and inner common electrode 78a of the first oxygen ion pump cell: Platinum powder, ZrO ₂ powder, gold powder, and an appropriate amount of an organic solvent are mixed. Then, a solution obtained by dissolving an organic binder in an organic solvent is added thereto, and a viscosity modifier is further added and mixed to prepare a paste. Or Zr
An Au solution (for example, chloroauric acid solution or the like) is impregnated in the O ₂ powder, dried and fired to deposit Au on the powder, thereby obtaining an Au-supported powder. The Au-supported powder and the Pt powder are mixed, and a binder, an organic solvent, and a viscosity modifier are appropriately added to obtain a paste.

(3) For insulating coat and protective coat: A mixture of alumina powder and an appropriate amount of an organic solvent is dissolved, a viscosity modifier is further added and mixed to prepare a paste.

(4) Pt-containing porous material (for lead wire): A powder is prepared by mixing and dispersing alumina powder, platinum powder, an organic binder, and an organic solvent, further adding a viscosity modifier, and mixing.

(5) For diffusion resistance section: Alumina powder, an organic binder, and an organic solvent are mixed and dispersed, and a viscosity modifier is added and mixed to prepare a paste.

(6) For carbon coating: carbon powder,
An organic binder and an organic solvent are mixed and dispersed, and a viscosity modifier is further added and mixed to prepare a paste. Note that by forming a carbon coat by printing, for example, electrical contact between the electrodes is prevented. Further, the carbon coat is used to form a void. Since carbon is burned off during firing, the carbon coat layer does not exist in the fired body.

[ZrO ₂ Laminating Method, Binder Removal and Firing] In the case of Example: One to three layers of ZrO ₂ green sheets are pressurized at a predetermined pressure for a predetermined time and pressed. The pressed compact is debindered and fired. In the case of the comparative example:
After crimping the second and third layers, the part where the second diffusion hole penetrates (diameter 1.
3mm). After the punching, a green columnar molded body serving as the second diffusion hole is embedded, and the first to third layers of the ZrO ₂ green sheet are pressed under a predetermined pressure for a predetermined time and pressed. The pressed compact is debindered and fired.

According to an embodiment of the present invention having the NOx gas sensor element of the embodiment thus obtained (see FIG. 1).
Various tests were performed using an Ox gas concentration measurement device (see FIG. 2). A similar test was performed using the NOx gas concentration measuring device according to the comparative example having the NOx gas sensor of the comparative example shown in FIG. The dimensions of the NOx gas sensor element of the embodiment are 1 mm in the solid electrolyte laminating direction, 45 mm in the longitudinal direction, and 3.5 mm in the lateral direction. NO of comparative example
x The dimensions of the gas sensor element are 1.3 m in the solid electrolyte stacking direction
m, 45 mm in the longitudinal direction and 3.5 mm in the lateral direction. The first diffusion resistance portion is made of porous alumina having an average particle diameter of φ2 μm. The size of the void is 50 in the solid electrolyte lamination direction.
μm, 7 mm in the longitudinal direction, and 2.2 mm in the lateral direction.

[Test Example 1] Using the NOx gas concentration measuring devices of the embodiment (see FIG. 2) and the comparative example (see FIG. 8), the NOx gas concentration and the distance between the inner common electrode 78a and the outer common electrode 78b ( In the case of the comparative example, the relationship between the currents (IP2) flowing between the electrodes 8a and 8b) was examined. In the case of the embodiment, a fixed concentration of NO or oxygen is supplied to the sensor element for a predetermined time, and the first half period is (1) the oxygen concentration (partial pressure) control period (oxygen concentration detection period) and the second half period is (2) The control period (NOx concentration detection period) was set (the same applies to other test examples). Other test conditions are shown below, and the measurement results are shown in FIG.

In the following test conditions, the cell electromotive force refers to an electromotive force generated between the inner common electrode 78a and the outer common electrode 78b (see FIGS. 1 and 2) in the embodiment. 3), and in the case of the comparative example, the electrodes 7a, 7
This is the electromotive force between b (see FIG. 8). IP2 is a current flowing between the inner common electrode 78a and the outer common electrode 78b in the embodiment, and a current between the electrodes 8a and 8b in the comparative example. In the NOx concentration detection, a cell to which a voltage is applied is the common cell 78 (same cell) in the embodiment, and the second oxygen ion pump cell 8 in the comparative example.

* Test conditions * Composition of gas to be measured NO: 0, 500, 1000, 1500 ppm O ₂ : 7% CO ₂ : 10% H ₂ O: 10% N ₂ : bal Temperature of gas to be measured 300 ° C. Detection part temperature 800 ℃ Oxygen concentration control: Control so that the cell electromotive force becomes 350 mV NOx concentration detection: Apply 700 mV to the cell and measure the oxygen pump current IP2

In FIG. 5, a continuous solid line indicates the sensor output IP2 of the measuring device of the embodiment, a diamond-shaped plot indicates the sensor output IP2 of the measuring device of the comparative example, and a broken line indicates the result of interpolation of the result of the comparative example. . Referring to FIG. 5, in the case of the embodiment, (1) the sensor output is constant during the period in which the oxygen concentration in the gap is controlled, while (2) the period during which the NOx concentration is detected is affected. It can be seen that the sensor output changes according to the NOx concentration in the measurement gas. In the period (2), the sensor output IP2 of the example and the sensor output IP2 of the comparative example substantially match. Therefore, it can be seen that the NOx gas concentration can be measured using the NOx gas concentration measurement device of the example, similarly to the device of the comparative example.

Test Example 2 The oxygen concentration dependency of the sensor output IP2 was examined using the NOx gas concentration measuring devices of the example and the comparative example. The test conditions are as follows. FIG. 6 shows the relationship between the oxygen concentration in the gas to be measured and IP2 when the test result is NO: 0 ppm.

* Test conditions * Gas composition to be measured NO: 0 ppm, O_Two: 0, 0.5, 1, 7, 16%,
CO_Two: 10%, N _TwoMeasured gas temperature 300 ° C; detector temperature 800 ° C; Oxygen concentration control: Control so that cell electromotive force becomes 350mV.
NOx gas concentration detection: Applying 700 mV to the cell and applying oxygen
Current IP2 measurement.

Referring to FIG. 6, according to the embodiment, the output of IP2 is stable even when the oxygen concentration changes from 0 to 16%, and the influence of the oxygen concentration in the gas to be measured is smaller than that of the comparative example. It is understood that the number is extremely small. Therefore, the inner common electrode 78a
It can be seen that the upper oxygen partial pressure is detected more accurately, and that the oxygen concentration in the gap 2 is stable.

Test Example 3 The responsiveness of the sensor output was examined using the NOx gas concentration measuring devices of the example and the comparative example.
The test conditions are as follows, and FIG.
The change of the sensor output when the input amount of NO is increased from 0 ppm to 1500 ppm is shown.

Test conditions Gas composition to be measured NO: 0 → 1500 ppm, O ₂ : 7%, CO ₂ : 10
%, H ₂ O: 10%, N ₂ : bal; Gas to be measured: 300 ° C .; Detection temperature: 800 ° C .; Oxygen partial pressure control: Control so that cell electromotive force becomes 350 mV; NOx gas concentration detection: To cell 700mV is applied and oxygen pump current IP2 is measured; oxygen concentration control and NOx gas concentration detection control are switched every 10msec.

Referring to FIG. 7, the response time of the embodiment is about half that of the conventional example, and it can be seen that the NO concentration can be measured with good responsiveness.

[0047]

As described above, according to the present invention,
An NOx gas concentration measurement device with a simplified sensor structure is provided. Further, in the NOx gas concentration measuring device according to the present invention, the dependence of the sensor output on the oxygen concentration is reduced, so that more accurate measurement of the NOx gas concentration can be realized and the responsiveness of NOx concentration detection is excellent. Further, according to the present invention, since the sensor structure is simplified, the manufacturing process is simplified, and variation between lots is reduced.

[Brief description of the drawings]

FIG. 1 is a view for explaining a structure of a NOx gas sensor element applied to a NOx gas concentration measuring device according to one embodiment of the present invention, and shows a surface of the sensor element cut in a longitudinal direction.

FIG. 2 is a view for explaining a NOx gas concentration measuring apparatus according to an embodiment of the present invention, to which the NOx gas sensor element shown in FIG. 1 is applied.

FIG. 3 is a diagram for explaining the operation of the device shown in FIG. 2;

FIG. 4 is a diagram for explaining a layout of the NOx gas sensor element shown in FIG.

5 is a diagram showing an example in which the NOx gas concentration measuring device of the embodiment shown in FIG. 2 and the NOx gas concentration measuring device of the comparative example to which the NOx gas sensor element of the comparative example shown in FIG. 8 is applied, respectively. FIG. 7 is a diagram for explaining a result of examining a relationship between a gas concentration and a sensor output IP2.

6 shows a sensor using the NOx gas concentration measuring device of the embodiment shown in FIG. 2 and a NOx gas concentration measuring device according to a comparative example to which the NOx gas sensor element of the comparative example shown in FIG. 8 is applied, respectively. FIG. 9 is a diagram for explaining the result of examining the oxygen concentration dependence of the output.

FIG. 7 shows a sensor using the NOx gas concentration measuring device of the embodiment shown in FIG. 2 and a NOx gas concentration measuring device according to a comparative example to which the NOx gas sensor element of the comparative example shown in FIG. 8 is applied. FIG. 9 is a diagram for describing a result of examining output responsiveness.

FIG. 8 is a diagram for explaining a structure of a NOx gas sensor element applied to a NOx gas concentration measuring device according to a comparative example.

[Explanation of symbols]

1: Diffusion resistance part 2: Void part 6: First oxygen ion pump cell 6a: Outer electrode 6b: Inner electrode 7: Oxygen concentration detecting cell 20: Switching means 22: Ammeter (oxygen pump current IP2 detecting means) 23: Difference Dynamic amplifier (means for controlling pumping of oxygen by the first oxygen ion pump cell) 78: common cell (second cell) 78a: inner common electrode (oxygen concentration measuring electrode) 78b: outer common electrode (oxygen concentration reference electrode)

Continued on the front page (72) Inventor Noboru Ishida 14-18, Takatsuji-cho, Mizuho-ku, Nagoya-shi Inside Japan Specialty Ceramics Co., Ltd.

Claims (7)

[Claims] 1. A gap into which a gas to be measured is introduced via a diffusion resistor; a first oxygen ion pump cell for pumping oxygen out and / or into the gap; a solid electrolyte; A second cell having a pair of electrodes formed, at least one of the electrodes facing the gap, and switching means for switching between application and non-application of a predetermined voltage between the pair of electrodes of the second cell. When a predetermined voltage is applied between the pair of electrodes of the second cell, NOx is decomposed in the gap and dissociated oxygen is pumped out through the second cell, so that the NOx gas concentration is reduced. Means for detecting an oxygen pump current flowing between the pair of electrodes in response to the electromotive force generated between the pair of electrodes when a predetermined voltage is not applied between the pair of electrodes of the second cell. , The first oxygen ion pump cell NOx gas concentration measuring device comprising means for controlling the pumping of oxygen by, that it has. 2. The switching means according to claim 1, wherein said second cell functions as an oxygen concentration battery cell for detecting an oxygen concentration in said gap, and a second oxygen ion through which an oxygen pump current flows according to a NOx gas concentration. 2. The NOx gas concentration measurement device according to claim 1, wherein a state of functioning as a pump cell is periodically switched. 3. In the second cell, if the electrode facing the gap is an oxygen concentration detection electrode and the other electrode is an oxygen concentration reference electrode, the oxygen concentration reference electrode is a self-generated reference electrode. Oxygen present near the oxygen concentration reference electrode to form a self-generated reference electrode becomes NOx when the predetermined voltage is applied.
3. The NOx gas concentration measuring device according to claim 1, wherein the NOx gas is decomposed and dissociated. 4. One or more electrodes of the first oxygen ion pump cell and one electrode of the second cell facing the void are selected from the group consisting of Pt, Rh, Pd, Ir, and Ru. Ag, Au, Ni, Co, Cr, Fe, Mn, Cu,
The NOx gas concentration measuring apparatus according to any one of claims 1 to 3, further comprising at least one metal selected from Ti, Al, Zn, Sn, and In. 5. A voltage is applied to said second oxygen ion pump cell by using a NOx gas sensor having first and second oxygen ion pump cells facing a gap into which a gas to be measured is introduced via a diffusion resistor. Is not applied, based on the electromotive force of the oxygen concentration cell generated in the cell, the outside of the gap and / or
Or, controlling the pumping of oxygen into the inside, applying a voltage to the second oxygen ion pump cell, the NOx in the void is decomposed, and the dissociated oxygen is pumped out to reduce the NOx gas concentration flowing through the cell. Detecting a corresponding oxygen pump current and measuring a NOx gas concentration based on the detected oxygen pump current.
x Gas concentration measurement method. 6. The NOx gas concentration measuring method according to claim 5, wherein the application and non-application of a voltage to said second oxygen ion pump cell are periodically switched. 7. A cell comprising a pair of electrodes and a solid electrolyte layer is provided facing a space into which a gas to be measured is introduced via a diffusion resistor. When a voltage is applied, it acts as an oxygen ion pump cell for decomposing NOx and pumping out the dissociated oxygen, and when the predetermined voltage is not applied to the pair of electrodes, the oxygen concentration in the gap is reduced. A NOx gas sensor that functions as an oxygen concentration battery cell that generates a corresponding electromotive force.