JP2000214130A - Method for measuring concentration of gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for measuring a gas concentration wherein the concentration of oxygen gas as well as the concentration of a specific gas in a gas can be measured, and a sensor element can be constructed more simply. SOLUTION: A gas sensor element 1 has a sensor cell 2 and a pump cell 3 while the sensor cell 2 is connected to a detecting circuit 25 including a first ammeter 251 and a power source 253 and, the pump cell 3 is connected to a pump circuit 35 including a second ammeter 351 and a variable power source 353. A concentration of oxygen gas in a gas is measured from a detected value of the second ammeter 351 with use of the gas sensor element 1. The variable power source 353 is controlled on the basis of the obtained concentration value of oxygen gas, and at the same time the concentration of a specific gas in the gas to be measured is measured from the detected value of the first ammeter 251.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a measuring method for measuring a specific gas concentration together with an oxygen gas concentration.

[0002]

2. Description of the Related Art Air pollution caused by exhaust gas from automobiles and the like has caused serious problems in modern society, and regulations on purification standards for pollutants in exhaust gas have become stricter year by year.
For this reason, more precise combustion control for gasoline engines, diesel engines, etc., and further reduction of pollutants in exhaust gas by catalytic converters installed in engine exhaust systems, etc. are being studied.

[0003] The OBD-II regulations in the United States require that the engine combustion control system and the like have a function of determining whether a catalyst for purifying exhaust gas in a catalytic converter is in an appropriate state. On the other hand, a monitor system using two oxygen sensors has been introduced.

However, the method using an oxygen sensor is not a method for directly measuring the amount of pollutants in exhaust gas accurately. In this method, the efficiency of purifying exhaust gas by a catalytic converter is increased by maintaining an appropriate air-fuel ratio in an engine. That is,
This was an indirect method of increasing the purification efficiency to reduce pollutants from the exhaust gas.

[0005] If, in the combustion control system or a catalyst monitor or the like of the engine, NOx (e.g. NO, nitrogen oxide such as NO ₂₎ if it is possible to directly detect the gas concentration, of reducing pollutants from the exhaust gas A more accurate and effective approach to the purpose can be expected. In view of the above background, in recent years, a gas sensor element as shown in FIG. 16 has been proposed as a sensor element for measuring the concentration of NOx gas in exhaust gas (Japanese Patent Application Laid-Open No. 8-271476).
issue).

The gas sensor element 9 has a first internal space 921 connected by a diffusion passage 920 functioning as a diffusion resistance.
And a second internal cavity 922. The gas to be measured is introduced into the first internal space 921 from the introduction path 910, where the amount of oxygen in the gas to be measured is controlled. Next, the gas to be measured in which the amount of oxygen is controlled is guided to the second internal space 922 through the diffusion passage 920, and the second internal space 922 is formed.
The concentration of NOx gas in the gas to be measured existing in the atmosphere is measured.

As a method for controlling the amount of oxygen in the gas to be measured in the first internal space 921, the following method is adopted for the gas sensor element 9. That is, the reference electrode 821 for detecting the oxygen partial pressure is provided in the first internal space 921.
And a pumping electrode 811, 812 for the oxygen pump cell 81.
The amount of electricity between the pumping electrodes 811 and 812 is controlled based on the oxygen partial pressure detected by the above. This gives
The oxygen partial pressure in the atmosphere in the first internal space 921 can be controlled. Note that reference numeral 925 in the figure is a reference gas chamber, and a reference electrode 820 provided here is a reference electrode of the reference cell 82 and a sensor cell 83 described below.
Is also a reference electrode. Reference numerals 91 and 93 are oxygen ion conductive solid electrolyte bodies.

In the second internal space 922, a sensor cell 83 having a sensor electrode 831 for measuring NOx gas is provided. This sensor electrode 831 allows N in the gas to be measured to be measured.
Measure the Ox gas concentration. As described above, the specific gas concentration such as NOx can be measured together with the oxygen partial pressure in the measured gas, that is, the oxygen gas concentration.

[0009]

However, the method for controlling the amount of oxygen in the gas to be measured by the conventional gas sensor element has the following problems. That is, it is necessary to have two types of electrodes having different functions, that is, a reference electrode and an oxygen pumping electrode, for controlling the amount of oxygen, which complicates the structure of the sensor element.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and it is possible to measure a specific gas concentration in a gas to be measured together with an oxygen gas concentration, and to further simplify the structure of a sensor element. It is an object of the present invention to provide a method for measuring a gas concentration which can be performed.

[0011]

According to a first aspect of the present invention, there is provided a sensor cell comprising a solid electrolyte body, a measurement electrode provided on a surface of the solid electrolyte body, and facing a gas chamber to be measured and a reference electrode facing a reference gas chamber. And a pump cell comprising a solid electrolyte body and a pair of pump electrodes provided on the surface of the solid electrolyte body and arranged facing the gas chamber to be measured,
The sensor cell is connected to a detection circuit having a first ammeter and a power supply, and the pump cell is a gas sensor element connected to a pump circuit having a second ammeter and a variable power supply. The oxygen gas concentration in the gas to be measured is measured from the detected value of the second ammeter, the variable power supply is controlled from the obtained oxygen gas concentration value, and the oxygen gas concentration in the gas to be measured is measured from the detected value of the first ammeter. A gas concentration measuring method characterized by measuring a specific gas concentration.

In the measurement according to the present invention, a voltage is applied to the pump cell by a variable power supply of a pump circuit, and the pump circuit is provided with a second ammeter. When a voltage is applied to the pump cell, oxygen gas in the measured gas chamber is ionized on the surface of the pump electrode, and is discharged outside the measured gas chamber through the solid electrolyte.

FIG. 7 shows the relationship between the pump cell current and the pump cell applied voltage. When the voltage is small, the pump current increases in proportion to the voltage.
The current does not change from a certain point (limit current value).
After this state continues for a while, when the voltage further increases, the current increases again in proportion to the voltage. As shown in FIG. 7, the voltage-current characteristic curve changes in accordance with the oxygen gas concentration, and moves to the lower left when the oxygen gas concentration decreases.

Therefore, a voltage is applied from the variable power supply to the pump cell so that the second ammeter indicates the limit current value, and oxygen gas is discharged outside the room so that the oxygen amount in the gas to be measured becomes the stoichiometric air-fuel ratio. This reduces the amount of oxygen in the room to about 10 ^-6
It can be easily and constantly reduced to about atm. Further, since the value of the limit current value is proportional to the oxygen gas concentration from FIG. 7, the measurement of the oxygen gas concentration in the gas to be measured introduced into the gas chamber to be measured can be performed simultaneously.

The application of a voltage to the sensor cell is performed by a power supply of a detection circuit, and the detection circuit is provided with a first ammeter. When a voltage is applied to the sensor cell, the specific gas in the gas to be measured is reduced on the measurement electrode to generate oxygen ions. Since a voltage is applied to the measurement electrode and the reference electrode, the concentration of the specific gas in the gas to be measured is increased. Current flows through the solid electrolyte in the sensor cell.

Since this ion current flows through the detection circuit connected to the sensor cell, it can be measured by the first ammeter in the detection circuit. Then, as shown in FIG. 8, the ion current changes according to the specific gas concentration, so that the specific gas concentration can be measured from the value of the first ammeter. Further, as described above, since oxygen gas has already been substantially extracted from the gas to be measured by the pump cell, only oxygen derived from the specific gas is ionized on the sensor cell. Therefore, the value of the first ammeter is proportional to the specific gas concentration.

As described above, according to the measuring method of the present invention, unlike the conventional method, the concentration of the specific gas and the oxygen gas concentration can be measured only by the pump cell and the sensor cell, so that the structure of the gas sensor element can be simplified. Can be.

As described above, according to the present invention, there is provided a gas concentration measurement method capable of measuring the oxygen gas concentration together with the specific gas concentration in the gas to be measured and simplifying the structure of the sensor element. be able to. Also,
Since the structure of the sensor element is simplified, manufacture of the gas sensor element can be facilitated. Furthermore, since the number of electrodes can be reduced, the amount of expensive noble metal materials used can be reduced, and cost reduction can be realized.

It should be noted that various types of specific gas concentrations can be measured by changing the type of the measurement electrode in the sensor cell of the gas sensor element used in the measurement method of the present invention. For example, NO
A NOx sensor element can be obtained by using an electrode active for x. In other words, N
It is preferable to use a substance having a function of decomposing NOx into nitrogen ions and oxygen ions by contact with Ox.

In this way, the decomposed oxygen ions become an ionic current flowing through the solid electrolyte. By measuring this current value, a value proportional to the NOx concentration can be obtained. Therefore, it functions as a NOx sensor element. In addition, since an element for measuring the concentration of CO, HC, H ₂ O, or the like is obtained, the measuring method according to the present invention can be applied to these gas concentration measurements.

In the gas sensor element used in the present invention, the pump electrode facing the gas to be measured needs to be made of a substance inert to a specific gas. As described above, the gas sensor element measures the specific gas concentration based on the amount of oxygen ions generated by decomposing the specific gas. Therefore, accurate measurement of the specific gas concentration cannot be performed unless the consumption of the specific gas in the pump cell is prevented.

Further, the pump cell can be arranged so as to face the gas chamber to be measured and also outside the gas sensor element (see FIG. 1). Further, the pump cell can be disposed so as to face the measured gas chamber and also to face the reference gas chamber (see FIG. 14).

According to a second aspect of the present invention, it is preferable that the gas chamber to be measured in the gas sensor element is formed of one space and is filled with a porous material having a porosity of 3 to 30%. The measured gas chamber of the gas sensor element used in the present invention has a simple shape consisting of only one space, and is solid, so that the shape of the measured gas chamber can be easily maintained even during manufacturing. And deformation and dimensional deviation are unlikely to occur. Therefore, according to the measuring method according to the present invention using such an element, highly accurate measurement can be performed.

In the gas sensor element used in the measuring method according to the present invention, as described above, since the gas chamber to be measured is filled with the porous body, diffusion of the gas to be measured occurs in the porous body. Will be done. Therefore, the diffusion of the gas to be measured is a mixture of Knudsen diffusion and molecular diffusion, and the temperature dependence is reduced. Therefore, measurement independent of temperature can be performed.

As described above, according to these, the measurement accuracy is high,
It is possible to provide a gas concentration measurement method having a small temperature dependency in measurement accuracy.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 A gas concentration measuring method according to an embodiment of the present invention and a structure of a gas sensor element used for the method will be described with reference to FIGS. Note that the gas concentration measuring method of the present embodiment is a method of measuring the NOx gas concentration in the gas to be measured.

As shown in FIG. 1, the gas sensor element 1 used in this embodiment includes a solid electrolyte 13 and the solid electrolyte 13.
And a sensor cell 2 comprising a measurement electrode 21 facing the measured gas chamber 101 and a reference electrode 22 facing the reference gas chamber 102, and a solid electrolyte body 11 and a surface of the solid electrolyte body 11. A pair of pump electrodes 3 provided
1 and 32 and a pump cell 3 disposed facing the gas chamber 101 to be measured. The sensor cell 2 is a detection circuit 25 having a first ammeter 251 and a power supply 253.
, And the pump cell 3 is connected to a pump circuit 35 having a second ammeter 351 and a variable power supply 353.

Using such a gas sensor element 1, the oxygen gas concentration in the gas to be measured is measured from the detection value of the second ammeter 351 and the variable power supply 353 is controlled based on the obtained oxygen gas concentration value. At the same time, the specific gas concentration in the gas to be measured is measured from the detection value of the first ammeter 251. The measured gas chamber 101 is formed of one space and is filled with a porous body.

The details will be described below. As shown in FIG. 1, the gas sensor element 1 according to the present embodiment is a stacked element, and includes solid electrolyte members 11, 12, 13, an insulating substrate 14, a heater 1
5 The solid electrolyte substrate 11 is provided with a pump cell 3 including a pair of pump electrodes 31 and 32, and the pump electrode 32 faces the gas chamber 101 to be measured. A gas chamber 101 to be measured is formed by filling a space surrounded by the solid electrolyte bodies 11, 12, and 13 with a porous body having a porosity of 12%. This porous body is provided for introducing the gas to be measured into the gas chamber 101 to be measured under a certain diffusion resistance and diffusing the gas to the vicinity of the measurement electrode 21 on the sensor cell 2.

Further, the solid electrolyte member 13 is provided with a measurement electrode 21 and a reference electrode 22, and these constitute a sensor cell 2. The measurement electrode 21 in the sensor cell 2 is an electrode made of platinum. When a voltage is applied to the sensor cell 2, the measurement electrode 21 decomposes NOx to generate nitrogen ions and oxygen ions, and converts the oxygen ions into a reference gas chamber. 102 can be pumped. That is, the measurement electrode 21 is an electrode active for NOx. The reference electrode 22 is also made of platinum, but may be made of a gold-platinum electrode.

The pair of pump electrodes 31, 32 in the pump cell 3 are composed of gold-platinum electrodes which do not decompose NOx. By applying an appropriate voltage, the pump cell 3 can pump oxygen ions from the measured gas chamber 101 to the outside or in the opposite direction.

Further, as shown in FIG. 2, the pump electrode 31 of the pump cell 3 is provided with a lead 311 for applying a voltage. Although not shown in FIGS. 1 and 2, the measurement electrode 21, the reference electrode 22, and the other pump electrode 32 of the pump cell 3 of the sensor cell 2 are similarly connected to a lead portion for applying voltage or extracting output. And terminal parts are provided.

Further, a heater 15 is laminated on the solid electrolyte body 13 via a spacer 14. And
A space surrounded by the solid electrolyte body 13, the spacer 14, and the heater 15 becomes the reference gas chamber 102. The heater 15 includes a heater substrate 151 and a heating element 150 containing platinum disposed on the heater substrate 151.
0 is covered by the covering substrate 152.

Also, for the heater substrate 151,
A lead portion (not shown) for supplying power to the heating element 150 is provided. The heating element 150 is preferably provided so as to cover the measurement electrode 21 and the reference electrode 22 in the sensor cell 2 and the pump electrodes 31 and 32 in the pump cell 3 when viewed in projection. By heating the electrodes 21, 22, 31, and 32 to the activation temperature, functions such as pumping and decomposition of NOx can be exhibited for the first time. For this reason, these electrodes 21, 22, 3
In order to heat 1, 32, such a configuration was adopted.

Next, a method for manufacturing the gas sensor element 1 of the present embodiment will be described. First, the solid electrolyte members 11, 12, 1
The preparation of the raw zirconia sheet for No. 3 will be described. 100 parts (parts by weight, hereinafter referred to as yttria) of 6 mol% yttria and 94 mol% zirconia and having an average particle size of 0.5 μm, and 1 part of α-alumina;
A ceramic mixture comprising 5 parts of PVB (polyvinyl butyral), 10 parts of DBP (dibutyl phthalate), 10 parts of ethanol and 10 parts of toluene was prepared. Then,
The above ceramic mixture was mixed in a ball mill, and the obtained slurry was dried to a thickness of 0.3 by a doctor blade method.
mm.

The sheet compact was cut into a rectangle of 5 × 70 mm, and a printing portion for the pump electrode 32 of the pump cell 3 was formed by printing using a paste containing 1 to 10 wt% of gold-added platinum and 10 wt% of zirconia. On the other hand, a pump electrode 31 provided at a position facing the pump electrode 32, and a printed portion serving as each lead portion connected to the pump electrodes 31, 32 were formed by screen printing using a platinum paste containing 10 wt% zirconia. . This becomes a raw sheet for the solid electrolyte body 11.

Further, the sheet compact is cut into a rectangle of 5 × 70 mm, and a 2 × 15 mm rectangle is formed so that the tip becomes a U-shape.
Is provided. This becomes a raw sheet for the solid electrolyte member 12.

Further, the above-mentioned sheet molded body was cut into a rectangle of 5 × 70 mm, and the printed portion for the measuring electrode 21 was cut by 10 wt%.
Printing was performed using a zirconia-added platinum paste. On the other hand, a printed portion serving as each lead portion connected to the measurement electrode 21 and the reference electrode 22 was formed by screen printing using a 10 wt% zirconia-added platinum paste. This becomes a raw sheet for the solid electrolyte body 13.

Next, the spacer 14, the heater substrate 15
1. The preparation of the raw alumina sheet for the coated substrate 152 will be described. 98 parts of α-alumina having an average particle size of 0.3 μm,
3 parts of 6 mol% yttria partially stabilized zirconia, PVB
10 parts, DBP 10 parts, ethanol 30 parts, toluene 3
A ceramic mixture consisting of 0 parts was prepared. Next, this ceramic mixture was mixed in a ball mill, and the resulting slurry was dried to a thickness of 0.3 by a doctor blade method.
mm.

The above sheet molded body was cut into a rectangle of 5 × 70 mm, and 2 × 65 m so that the tip became a closed U-shape.
m was provided. This becomes a raw sheet for the spacer 14. Further, the sheet molded body is cut into a rectangle having a size of 5 × 70 mm, and a paste containing 90 wt% of platinum and 10 wt% of alumina is used. Was formed by a screen printing method. This becomes a raw sheet for the heater substrate 151. Further, a raw sheet for the coated substrate 152 is manufactured by cutting the above sheet molded body at 5 × 70 mm.

Next, a method of manufacturing the porous body filled in the gas chamber 101 will be described. 10 parts of PVB as a binder, 5 parts of DBP as a plasticizer, 1 part of span as an antifoaming agent, 50 parts of terpineol as a solvent, and 100 parts of alumina powder were mixed, and the mixture was passed through three rolls 20 times to produce an alumina paste.

Then, the green sheets are laminated to form a laminate. This method will be described. First, a green sheet for the solid electrolyte body 12 and a green sheet for the solid electrolyte body 13 are laminated by thermocompression bonding at 80 degrees. Next, the above-mentioned alumina paste was applied to the solid electrolyte 1 by screen printing.
Then, a space to be the gas chamber 101 to be measured is prepared. Thereafter, as shown in FIG. 1, the remaining green sheets were sequentially laminated and pressed to obtain a laminated body. The laminate obtained by the above method was fired in the air at 1500 ° C. for 1 hour. Thus, the gas sensor element 1 according to this example was obtained.

Next, a method for measuring the specific gas concentration and the oxygen gas concentration by the gas sensor element 1 will be described with reference to the following block diagrams and the like. As shown in FIG. 3, in the gas sensor element 1, an oxygen gas concentration detecting means 6 is connected to the pump cell 3, and a specific gas concentration detecting means 5 is connected to the sensor cell 2. Both detection means 5, 6 are part of the control circuit 4. Then, the signals 1S and 2S emitted from the respective detecting means 5 and 6 become the output of the oxygen gas concentration and become the output of the NOx gas concentration.

Next, the control circuit 4 will be described in detail. As shown in FIG. 4, the pump electrode 32 and the measurement electrode 21 in the gas sensor element 1 are grounded,
It is kept at the common potential Va (GND). The oxygen gas concentration detecting means 6 includes a pump cell applied voltage command circuit 61 for controlling the applied voltage to the pump cell 3.
And the command voltage V from the pump cell applied voltage command circuit 61.
It comprises an amplifier circuit 62 in which b is input to the non-inverting input terminal, and a resistor 63 for detecting a pump cell current flowing according to the oxygen gas concentration.

The output from the amplifier circuit 62 is a resistor 63
Is input to one terminal 631. Further, a terminal 633 for detecting a current corresponding to the oxygen gas concentration is also connected to the same terminal 631. Also, terminal 6
The voltage of 33 is Vd.

The other terminal 632 of the resistor 63 is connected to the pump electrode 31 of the gas sensor element 1. The same terminal 632 is input to the inverting input terminal of the amplifier circuit 62 and is also connected to a terminal 634 for detecting a current corresponding to the oxygen gas concentration. The terminal 634 is
The same voltage as Vb which is a command voltage from the pump cell applied voltage command circuit 61 input to the non-inverting input terminal of the amplifier circuit 62.

Therefore, the pump cell applied voltage command circuit 61
Is applied as an applied voltage to the pump cell 3. The pump cell current is determined from the difference between the voltage Vd of the amplifier 63 on the amplifier circuit 62 side and the voltage Vb on the pump cell 3 side of the resistor 63 for detecting the pump cell current flowing according to the oxygen gas concentration and the resistance value of the resistor 63 for detecting the pump cell current. The following equation can be obtained. (Pump cell current Ip) = (Vd−Vb) / R63 where R63 is the resistance value of the resistor 63. This pump cell current corresponds to the oxygen gas concentration.

The specific gas concentration detecting means 5 includes a sensor cell applied voltage command circuit 51 for controlling the applied voltage of the sensor cell 2 and a command voltage Vc from the sensor cell applied voltage command circuit 51 inputted to the non-inverting input terminal. It comprises a circuit 52 and a resistor 53 for detecting a sensor cell current flowing according to the NOx gas concentration.

The output of the amplifier circuit 52 is connected to one terminal 531 of a resistor 53 for detecting a sensor cell current.
The terminal 531 is connected to a terminal 533 for detecting a current according to the NOx gas concentration. The voltage at the terminal 533 is Ve.

The other terminal 53 of the resistor 53
2 is connected to the reference electrode 22 of the gas sensor element 1 and is input to the inverting input terminal of the amplifier circuit 52 and is also connected to a terminal 534 for detecting a current corresponding to the NOx gas concentration. The terminal 534 has the same voltage as Vb which is a command voltage from the sensor cell applied voltage command circuit 51 input to the non-inverting input terminal of the amplifier circuit 52.

Therefore, the sensor cell applied voltage command circuit 51
Is applied as an applied voltage to the sensor cell 2, and the difference between the voltage Ve on the amplifier circuit 52 side of the resistor 53 for detecting the sensor cell current flowing according to the NOx gas concentration and the voltage Vc on the sensor cell 2 and the sensor cell current The sensor cell current can be obtained from the following equation from the resistance value of the resistor 53 for detecting. (Sensor cell current Is) = (Ve−Vc) / R53 where R53 is the resistance value of the resistor 53. This sensor cell current corresponds to the NOx gas concentration.

Next, the specific configurations of the pump cell applied voltage command circuit 61 and the sensor cell applied voltage command circuit 51 will be described. As shown in FIG. 6, both instruction circuits 51 and 61 are provided with a microcomputer 66 and an A / D converter 6.
5, it can be realized by the D / A converter 67. Further, a specific operation of the two command circuits 51 and 61 will be described with reference to FIG.

As shown in FIG. 5, steps 581 and 58
In step 2, the voltages Vd and Vb at both terminals 631 and 632 of the resistor 63 for detecting the current flowing through the pump cell 3 according to the oxygen gas concentration are read. Specifically, as shown in FIGS. 4 to 6, the voltage Vd of one terminal 631 of the resistor 63 for detecting the pump cell current is read into the microcomputer 66 at A / D1, and the voltage Vb of the other terminal 632 of the resistor 63 is read. In the A / D2 microcomputer 66
Read in.

Next, as shown in FIG.
At 3,584, both terminals 531 and 532 of the resistor 53 for detecting a current flowing through the sensor cell 2 according to the NOx gas concentration
Are read. Specifically, FIG.
6, a resistor 5 for detecting the sensor cell current.
3 reads the voltage Ve of one terminal 531 into the microcomputer 66 at A / D3, and
Voltage Vc at the A / D 4 by the microcomputer 6
Read in 6.

As shown in FIG. 5, in step 585, the current Ip flowing through the pump cell 3 is calculated from Vd and Vb and the resistance value R63 of the resistor 63. Subsequently, at step 586,
As shown in FIG. 7, the target applied voltage corresponding to the pump cell current lp is obtained (map calculation) using the applied voltage characteristic line LX1 for the pump cell 3.

FIG. 7 is a voltage-current characteristic line of the pump cell obtained by varying the oxygen gas concentration by taking the applied voltage to the pump cell on the horizontal axis and the current flowing in the pump cell on the vertical axis. The applied voltage characteristic line marked with the applied voltage LX1 has different oxygen gas concentrations at the center of the flat portion (limit current value) seen in each voltage-current characteristic line.
Are the straight lines obtained by connecting the respective curves. However, since the gas to be measured contains much less NOx to be measured than O ₂ in addition to oxygen,
Actually, as shown in FIG. 7, the target applied voltage must be set in a region where NOx is not decomposed. Further, in step 587, a new pump cell applied voltage Vb is obtained from the target applied voltage, and this is output from D / A1.

Next, at step 588, the current Is flowing through the sensor cell is calculated from Ve, Vc and R53.
In step 589, as shown in FIG. 8, a target applied voltage corresponding to the sensor cell current Is is obtained using the applied voltage characteristic line LX2 to the sensor cell (map calculation).

FIG. 8 is a voltage-current characteristic line of the sensor cell obtained by changing the NOx gas concentration by taking the applied voltage to the sensor cell on the horizontal axis and the current flowing in the sensor cell on the vertical axis. The applied voltage characteristic line marked with the applied voltage LX2 is a straight line obtained by connecting the center of the flat part seen in each voltage-current characteristic line with respect to each curve of different NOx gas concentrations. Further, in step 590, a new sensor cell applied voltage Vb is obtained from the target applied voltage, and this is output from the D / A2. As described above, the NOx gas concentration can be measured together with the oxygen gas concentration.

Next, the operation and effect according to this embodiment will be described. FIG. 7 shows the relationship between the pump cell current and the pump cell applied voltage. When the voltage of the pump current is small, the current increases in proportion to the voltage, but the current does not change from a certain point (limit current value). After this state continues for a while, when the voltage further increases, the current increases again in proportion to the voltage. As shown in FIG. 7, the voltage-current characteristic curve changes in accordance with the oxygen gas concentration, and moves to the lower left when the oxygen gas concentration decreases.

Therefore, a voltage is applied from the variable power supply to the pump cell so that the second ammeter indicates the limit current value, and oxygen gas is discharged outside the room so that the oxygen amount in the gas to be measured becomes the stoichiometric air-fuel ratio. This reduces the amount of oxygen in the room to about 10 ^-6
It can be easily and constantly reduced to about atm. Further, since the value of the limit current value is proportional to the oxygen gas concentration from FIG. 7, the measurement of the oxygen gas concentration in the gas to be measured introduced into the gas chamber to be measured can be performed simultaneously.

A voltage is applied to the sensor cell by a power supply of a detection circuit, and the detection circuit is provided with a first ammeter. When a voltage is applied to the sensor cell, the specific gas in the gas to be measured is reduced on the measurement electrode to generate oxygen ions. Since a voltage is applied to the measurement electrode and the reference electrode, the concentration of the specific gas in the gas to be measured is increased. Current flows through the solid electrolyte in the sensor cell.

Since this ion current flows through the detection circuit connected to the sensor cell, it can be measured by the first ammeter in the detection circuit. Then, as shown in FIG. 8, the ion current changes according to the specific gas concentration, so that the specific gas concentration can be measured from the value of the first ammeter. Further, as described above, since oxygen gas has already been substantially extracted from the gas to be measured by the pump cell, only oxygen derived from the specific gas is ionized on the sensor cell. Therefore, the value of the first ammeter is proportional to the specific gas concentration.

As described above, according to the measuring method of this embodiment, unlike the conventional method, the concentration of the specific gas and the oxygen gas concentration can be measured only by the pump cell and the sensor cell, so that the structure of the gas sensor element can be simplified. Can be.

As described above, according to the present embodiment, there is provided a gas concentration measuring method capable of measuring the specific gas concentration in the gas to be measured together with the oxygen gas concentration and simplifying the structure of the sensor element. be able to.

Further, the gas sensor element 1 used in this embodiment is
The gas chamber 101 to be measured is constituted by one space portion and is filled with a porous material having a porosity of 3 to 30%.
It is solid. Therefore, the shape of the gas chamber 101 to be measured can be easily maintained at the time of manufacture, use, or the like, and deformation, dimensional deviation, and the like hardly occur.

As a result, variations in the individual characteristics of the gas sensor element 1 can be reduced, and it is not necessary to adjust the variations in the individual characteristics by connecting the gas sensor element 1 to a circuit or the like. Therefore, the gas sensor element 1 according to the present embodiment
According to this, the manufacturing cost can be reduced, and the time and effort for manufacturing can be saved. Further, since there is little variation in the shape of the gas chamber 101 to be measured, a gas sensor element with high measurement accuracy can be obtained according to this example (see Embodiment 2 and FIG. 10).

In addition, the gas chamber 101 to be measured is filled with a porous body, and in the gas sensor element 1 of the present embodiment, diffusion of the gas to be measured occurs in the porous body. Therefore, the diffusion of the gas to be measured is a mixture of Knudsen diffusion and molecular diffusion, and the temperature dependence is reduced. Therefore, according to this example, it is possible to obtain the gas sensor element 1 in which the measurement accuracy hardly depends on the temperature (see Embodiment 2 and FIG. 11).

Further, in the gas sensor element 1 according to the present embodiment, the measurement electrode 21 of the sensor cell 2 and the pump electrode 32 of the pump cell 3 are arranged facing the gas chamber 101 to be measured. By the way, in the gas sensor element,
The measurement electrode must be active for NOx gas, while the pump electrode must be inactive.

According to the present embodiment, when the gas sensor element 1 is manufactured, since the gas chamber 101 to be measured is filled with the porous material, the components added to make the pump electrode 32 inactive volatilize. Even in this case, these components are scattered by the porous body and cannot reach the measurement electrode 21. Therefore, poisoning of the measurement electrode 21 can be prevented.
Therefore, according to the present embodiment, a gas sensor element capable of performing more accurate measurement can be obtained.

Embodiment 2 A gas sensor element according to Embodiment 1 and a gas sensor element having a configuration in which the gas chamber to be measured shown in FIG. 9 was not filled with a porous material were manufactured, and the performances of both were compared. The gas sensor element 1 of the sample 1 is shown in FIGS. 1 and 2, and the gas chamber to be measured is filled with a porous body. Further, the gas sensor element 99 of the comparative sample C1 has the same position, area, width and length of the gas chamber to be measured and the reference gas chamber as those of the gas sensor element 1 of the sample 1. However, as shown in FIG. 9, a pinhole 109 having a diameter of 0.2 mm is provided in the vicinity of the pump cell 3 of the solid electrolyte body 11, and the solid electrolyte body 12 has a size of 2 × 15 mm in order to form a gas chamber 101 to be measured. Square holes were provided. Other than that, it was manufactured using the same material and the same manufacturing procedure as Sample 1 described above.

Next, by using the sample 1 and the comparative sample C1, the characteristics with respect to NOx were evaluated by the following method, and the variation in the characteristics was examined. The output variation was used for evaluating the characteristic variation. This output variation was defined as the difference between the maximum value and the minimum value with respect to the average value, which was divided by the average value. That is, {(maximum value)-average value}
/ Average value or {(minimum value) −average value} / average value. This measurement was performed on an evaluation bench through which a specific gas can flow.

The conditions for this measurement are as follows: the temperature of the gas to be measured is 400 ° C., and the gas to be measured is 5% O ₂ containing 2000 ppm of NO and 95% N ₂ . Further, the temperature of the gas sensor element was maintained at 750 ° C. The above measurement was performed for Sample 1, Comparative Sample C1, and
This was performed on the gas sensor elements of the present invention. This measurement result is shown in FIG.

The temperature dependence of the output ratio was measured using the above-mentioned sample 1 and comparative sample C1. This output ratio was defined based on the value at 750 ° C, and the difference was divided by the value at 750 ° C to define the output ratio. This measurement was performed by the same method as described above. The measurement conditions were as follows: the temperature of the gas to be measured was 400 ° C., and the gas to be measured was 5% O ₂ containing 2000 ppm of NO and 95%.
% N ₂ . The flow rate of the gas to be measured is 1.2
Liter / minute, the temperature of the gas sensor element is 650 ° C, 7
The tests were performed at 00 ° C, 750 ° C, 800 ° C, and 850 ° C. The above measurement was performed for each of the sample 1, the comparative sample C1, and the five gas sensor elements. The result is shown in FIG.

As is clear from FIGS. 10 and 11, the gas sensor element according to the sample 1 is larger than the comparative sample C1.
Output variation was small and output temperature dependence was small. After the measurement in FIG. 10 was completed, the inside of the gas sensor element used was micro-observed. As a result, no change in the shape of the gas chamber to be measured was observed in the gas sensor element (sample 1) according to this example. In the sample C1, the shape of the gas chamber to be measured was not maintained, and the shape of each element varied. Also, a trace amount of gold was detected on the measurement electrode 21. Therefore, it is presumed that such deformation of the gas chamber to be measured and poisoning of the measurement electrode 21 by gold are the major factors causing the output variation.

The cause of the temperature dependence as shown in FIG. 11 is considered to be that the diffusion of the gas to be measured by the pinhole is dominated by molecular diffusion. This is because molecular diffusion has a strong temperature dependence of the diffusion rate.

Embodiment 3 Another measurement method using the gas sensor element 1 shown in Embodiment 1 and not using the microcomputer shown in Embodiment 1 will be described. This method is for the case of using hardware as shown in FIG. As shown in FIG. 12, the pump cell applied voltage command circuit 6 includes a reference voltage circuit 611, an amplifier circuit 612, an amplifier resistor 615 and an amplifier resistor 616 for determining the amplification factor of the amplifier circuit, a resistor 613 and a capacitor 614 constituting a low-pass filter. It comprises a circuit 617 for detecting the pump cell current.

The input of the circuit 617 for detecting the pump cell current is connected to a resistor R63 for detecting the pump cell current shown in FIG.
Terminals 633 and 634 are connected. Voltage (Vd-Vb) processed by the circuit 617 for detecting the pump cell current
Is connected to the non-inverting input terminal of the amplifier circuit 612 as an output. The inverting input terminal of the amplification circuit 612 is an amplification resistor 615 for determining the amplification factor, and a capacitor 61 of a low-pass filter.
4 is connected to one side.

Another terminal of the resistor 615 for determining the amplification factor is connected to the reference voltage circuit 611, and a reference voltage is applied. The output terminal of the amplifier circuit 612 is connected to another terminal of the amplification resistor 616 that determines the amplification factor of the amplifier circuit 612, and is connected to one side of the resistor 613 of the low-pass filter. Another terminal of the resistor 613 of the low-pass filter is connected to one side of the capacitor 614 of the low-pass filter, and is output as the pump command voltage Vb. The other side of the capacitor 614 is grounded to GND.

The circuit 617 for detecting the pump cell current outputs a voltage Vd-Vb corresponding to the pump cell current, and the amplifier circuit 612 compares the voltage of the reference voltage circuit 611 and the pump cell current value (Vd-Vb) with amplification resistors 615 and 616. For comparative amplification.

As a result, the applied voltage characteristic line LX shown in FIG.
The characteristic shown in FIG. Here, the reference voltage circuit 611 generates an offset voltage (applied voltage at 0 mA) of the applied voltage characteristic line LX1, and outputs the amplified circuit 612 and the amplified resistors 615,6.
16 generates the gradient of the applied voltage line LX1 shown in FIG. 7 (increase in applied voltage with increase in pump cell current).

The low-pass filter (resistor 613 and capacitor 614) provides positive feedback because the applied voltage characteristic line LX1 has a characteristic that the applied voltage increases with an increase in the pump cell current, and prevents the output voltage from oscillating. It is installed for the purpose.

The control circuit 4 can also be configured by hardware with the above configuration and operation. In addition,
Since the applied voltage command to the sensor cell can be realized by a similar circuit, the description is omitted here.

Embodiment 4 Another example of a gas sensor element usable in the measuring method according to the present invention is an element as shown in FIG.
As shown in the figure, the surface of the pump electrode 31 is covered with a protective layer 119 to protect the electrode from being directly exposed to the heat of the exhaust gas and causing agglomeration or the like. Also,
In the gas sensor element 1 according to the present embodiment, the gas to be measured is introduced from the side of the gas chamber 101 to be measured. Protected.

Further, in the gas sensor element 1 of this embodiment, the sensor cell 2 is provided between the reference gas chamber 102 and the gas chamber 101 to be measured, the pump cell 3 faces the gas chamber 101 to be measured, and the pump cell 3 The other surface was arranged so as to face the outside of the gas sensor element 1. As shown in FIG. 14, contrary to the gas sensor element 1, the pump cell 3
Is provided between the reference gas chamber 102 and the gas chamber 101 to be measured, and the sensor cell 2 faces the gas chamber 101 to be measured.
Further, the other surface of the sensor cell 2 can be arranged so as to face the outside of the gas sensor element 1.

Incidentally, the gas sensor element 1 according to this embodiment
Sets the NOx detection electrode 21 (see FIG. 1) to NOx
Although it was constituted by an active electrode, as will be described later, the measuring electrode 21 was used.
Can be constituted by an electrode inert to NOx. That is, the measuring electrode 21 is connected to the pump electrodes 31 and 3 of the pump cell 3.
2 and a NOx inert electrode containing the same gold-platinum. FIG. 15 shows N ₂ − when the measurement electrode 21 is composed of a gold-platinum electrode that is a NOx inert electrode.
FIG. 4 is a diagram showing Vi characteristics of an O ₂ —NOx-based gas to be measured.

As can be seen from the figure, when the voltage applied to the sensor cell 2 is small, NOx is not decomposed, and the current generated by oxygen pumping is output. The limit current at this time is c. When the applied voltage is further increased,
Even the NOx inactive electrode functions as an active electrode. That is, since the NOx can be decomposed and the oxygen in the NOx can be pumped, a limit current d is generated.

Therefore, the pump electrode 32 and the measurement electrode 21
Is composed of a NOx inert electrode made of gold-platinum,
By setting the applied voltages to a and b in the figure, a function as a gas sensor element can be achieved. in this case,
Since the limiting current value at the voltage a in the Vi characteristic of the sensor cell 2 indicates the amount of oxygen near this cell, the offset current based on the residual oxygen can be estimated from the current value at the voltage a, and the There is also an advantage that you can cancel.

[Brief description of the drawings]

FIG. 1 is a cross-sectional explanatory view showing a structure of a gas sensor element according to a first embodiment.

FIG. 2 is a plan view of a gas sensor element according to the first embodiment.

FIG. 3 is a block diagram showing a gas sensor element, an oxygen gas concentration detecting means, and a specific gas concentration detecting means in the first embodiment.

FIG. 4 is a block diagram showing details of an oxygen gas concentration detecting means and a specific gas concentration detecting means in the first embodiment.

FIG. 5 is an explanatory diagram showing a flow relating to oxygen gas concentration detection and specific gas concentration detection in the first embodiment.

FIG. 6 is a block diagram of a pump cell applied voltage command circuit and a sensor cell applied voltage command circuit by the microcomputer in the first embodiment.

FIG. 7 is a diagram showing a voltage-current characteristic line of the pump cell obtained by changing the oxygen gas concentration in the first embodiment.

FIG. 8 is a diagram showing a voltage-current characteristic line of the sensor cell obtained by changing the NOx gas concentration in the first embodiment.

FIG. 9 is an explanatory sectional view showing a gas sensor element according to Comparative Example C1 in Embodiment 2;

FIG. 10 is a diagram showing output variations of Sample 1 and Comparative Sample C1 in Embodiment 2.

FIG. 11 is a diagram showing the relationship between the output ratio of Sample 1 and Comparative Sample C1 and temperature in Embodiment 2;

FIG. 12 is a block diagram of a pump cell applied voltage command circuit using hardware according to a third embodiment.

FIG. 13 is an explanatory sectional view showing a gas sensor element provided with a protective layer and a poisonous substance trap layer in a fourth embodiment.

FIG. 14 is a cross-sectional explanatory view showing a gas sensor element according to a fourth embodiment in which a sensor cell is exposed to the outside and a pump cell is provided between a gas chamber to be measured and a reference gas chamber.

FIG. 15 shows an example in which NO is used as a measurement electrode in the fourth embodiment.
V in the sensor cell when an inert electrode is used for x
FIG. 3 is a diagram showing i characteristics.

FIG. 16 is an explanatory view showing a structure of a NOx sensor element according to a conventional technique.

[Explanation of symbols]

 1. . . Gas sensor element, 11, 12, 13. . . Solid electrolyte body, 101. . . Measured gas chamber, 102. . . 1. reference gas chamber; . . Sensor cell, 21. . . Measuring electrode, 22. . . Reference electrode, 25. . . Detection circuit, 251. . . First ammeter, 253. . . Power supply, 3. . . Pump cell, 31, 32. . . Pump electrode, 351. . . Second ammeter, 353. . . Variable power supply,

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akio Tanaka 1-1-1, Showa-cho, Kariya, Aichi Prefecture Inside Denso Corporation (72) Inventor Shinichiro Imamura 1-1-1, Showa-cho, Kariya City, Aichi Prefecture Denso Corporation (72) Inventor Satoshi Haneda 1-1-1, Showa-cho, Kariya-shi, Aichi Prefecture Inside Denso Co., Ltd.

Claims (2)

[Claims] 1. A solid electrolyte body, a sensor cell provided on a surface of the solid electrolyte body and comprising a measurement electrode facing a gas chamber to be measured and a reference electrode facing a reference gas chamber, and a solid electrolyte body. A pump cell comprising a pair of pump electrodes provided on the surface of the solid electrolyte body and disposed facing the gas chamber to be measured, wherein the sensor cell has a first ammeter and a power supply; Connected to
The pump cell is obtained by measuring the concentration of oxygen gas in the gas to be measured from the detected value of the second ammeter using a gas sensor element connected to a pump circuit having a second ammeter and a variable power supply. Controlling the variable power supply based on the oxygen gas concentration value and measuring a specific gas concentration in the gas to be measured based on a detection value of the first ammeter. 2. The gas sensor element according to claim 1, wherein the gas chamber to be measured in the gas sensor element is formed of one space and is filled with a porous material having a porosity of 3 to 30%. Gas concentration measurement method.