JP2006090898A - Nitrogen oxide sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitrogen oxide sensor capable of detecting the concentration of a nitrogen oxide gas of low concentration with high accuracy, hardly receiving the effect of the concentration of coexisting oxygen and showing sufficient detection precision at an operation temperature exceeding 700°C. <P>SOLUTION: The nitrogen oxide sensor is equipped with an oxygen ion conductive solid electrolyte 1, the detection electrode 2 formed on the solid electrolyte 1, a reference electrode 3, a means 8 for detecting the concentration of nitrogen oxide by the potential difference between the detection electrode 2 and the reference electrode 3 and a means for heating the solid electrolyte 1, the detection electrode 2 and the reference electrode 3. The main constituent substance of the detection electrode 2 is at least one kind of an oxide selected from among the group consisting of tin oxide, iron oxide, titanium oxide, gallium oxide, chromium oxide and zinc oxide and the ratio t/R<SP>2</SP>of the average thickness (t) of the detection electrode and the square of the average crystal particle size R of particles for constituting the detection electrode is 30 or below. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a sensor for detecting the concentration of nitrogen oxide gas in a measurement target gas, and more particularly to a sensor suitable for measurement of exhaust gas from an internal combustion engine such as a combustion furnace or an automobile.

  From the viewpoint of global environmental conservation, it is required to suppress nitrogen oxide gas discharged from internal combustion engines such as combustion furnaces and automobiles. In particular, as a measure for suppressing the concentration of nitrogen oxides emitted from automobiles, development of catalysts, combustion control, and development of control systems that integrate these technologies are actively performed. With the development of such a control system, a sensor that can directly detect the concentration of nitrogen oxides is required. In the automotive field, it is clear that exhaust emission regulations will become stricter globally, so the nitrogen oxide gas detection concentration range required for exhaust gas aftertreatment systems, etc. is expected to shift more and more to the low concentration side. ing.

  Furthermore, it is essential that a sensor for measuring the concentration of nitrogen oxide gas in the exhaust gas of an automobile can operate at a high temperature. Nitrogen oxide gas sensors are originally exposed to high-temperature exhaust gas, but in order to activate the catalyst in a short time in order to reduce the concentration of nitrogen oxide gas in the exhaust gas, the catalyst mounting position is placed on the engine side. There have been attempts to shift the temperature, and the temperature of the environment in which the sensor is used is considered to shift to the high temperature side, specifically, about 700 to 900 ° C. Therefore, the nitrogen oxide gas sensor is required to exhibit sufficient detection accuracy in such a high temperature environment.

  The sensors proposed so far are roughly classified into two types. One of them is a so-called limit current type sensor, and the other is a so-called electromotive force type sensor. The limiting current type sensor electrochemically electrolyzes nitrogen oxide gas and detects the nitrogen oxide concentration from the obtained electrolysis current value of oxygen, and is described in JP-A-8-271476. The limiting current type sensor is suitable for use in a relatively high temperature environment of about 700 ° C. However, when the concentration of nitrogen oxide is dilute, there is a problem of large interference with coexisting oxygen concentration, and since the signal current is weak, the signal / noise ratio (S / N) is used in an environment where large noise is generated. Ratio) is bad. Therefore, complicated control and circuit design are required to apply the limiting current sensor to an automobile or the like.

  On the other hand, an electromotive force type sensor detects a nitrogen oxide concentration by a potential difference generated between two electrodes having a difference in electrode potential with respect to a nitrogen oxide gas, and is described in Japanese Patent No. 3195758 (Patent Document 1). . In the case of an electromotive force type sensor, the nitrogen oxide concentration can be detected with high accuracy by using an electrode that generates a high electrode potential with respect to a low concentration of nitrogen oxide. However, since the electrode that generates the electrode potential with respect to nitrogen oxide generates the electrode potential with respect to oxygen, the influence of oxygen cannot be ignored. There is a problem that it is not suitable for the detection of the nitrogen oxide gas concentration therein.

The inventors of the present invention have previously proposed a hybrid electric potential type sensor that combines an electrochemical reaction between nitrogen oxides and oxygen among electromotive force sensors. Japanese Patent No. 3524980 (Patent Document 2) describes a mixed potential type nitrogen oxide sensor having a detection electrode mainly composed of an oxide of Mn, Fe, Co, or Ni. As typical gases of nitrogen oxides, there are NO and NO _2, but in the detection electrode of the mixed potential type sensor, when detecting NO, the reaction represented by the following formula (1) and the following formula (2) The reaction represented by
^{_{O 2 + 4e - → 2O 2-}} ········ (1)
_{^{2NO + 2O 2- → 2NO 2 +}} 4e - ···· (2)

On the other hand, when NO ₂ is detected, a reaction represented by the following formula (3) and a reaction represented by the following formula (4) occur simultaneously at the sensing electrode.
2O ^2- → O ₂ + 4e (3)
^{_{2NO 2 + 4e - → 2NO +}} 2O 2- ···· (4)

  The hybrid potential type nitrogen oxide sensor utilizes the reactions shown in the above formulas (1) to (4), and therefore has a feature that it is hardly affected by the coexisting oxygen concentration. However, it has been found that the output of the hybrid potential sensor exhibits a large temperature dependence. When the sensor operating temperature was increased, the output of the mixed potential sensor was decreased, and the sensor operating temperature was 500 to 700 ° C. in practical use.

Japanese Patent No. 3195758 Japanese Patent No. 3524980

  Therefore, an object of the present invention is to detect a low concentration of nitrogen oxide gas with high accuracy, to be hardly affected by the coexisting oxygen concentration, and to show sufficient detection accuracy even at an operating temperature exceeding 700 ° C. A nitrogen oxide sensor is provided.

As a result of earnest research in view of the above object, the present inventors are the first to investigate the relationship between the average thickness t of the detection electrode of the mixed potential type nitrogen oxide sensor and the average crystal grain size R of the particles constituting the detection electrode. Paying attention, if the ratio (t / R ² ) between the average thickness t of the detection electrode made of a metal oxide such as nickel oxide and the square of the average crystal grain size R of the particles constituting the detection electrode is 30 or less, It was discovered that a low concentration of nitrogen oxide gas can be detected with high accuracy even at an operating temperature exceeding 700 ° C., and the present invention has been conceived.

That is, the nitrogen oxide sensor according to the present invention includes a solid electrolyte body having oxygen ion conductivity, a detection electrode formed on the solid electrolyte body, a reference electrode, and a potential difference between the detection electrode and the reference electrode. A means for detecting an object concentration, and a means for heating the solid electrolyte body, the detection electrode, and the reference electrode, and the main constituents of the detection electrode are nickel oxide, tin oxide, iron oxide, titanium oxide, oxidation It is at least one oxide selected from the group consisting of gallium, chromium oxide and zinc oxide, and the ratio between the average thickness t of the detection electrode and the square of the average crystal grain size R of the particles constituting the detection electrode t / R ² is 30 or less.

  The sensing electrode of the nitrogen oxide sensor is preferably porous, and the porosity of the sensing electrode is preferably 5 to 30% by volume.

  The nitrogen oxide sensor of the present invention exhibits excellent sensitivity to nitrogen oxide gas even at operating temperatures in excess of 700 ° C. By using a nitrogen oxide sensor having such characteristics, it becomes possible to directly measure the nitrogen oxide gas concentration in the exhaust gas in the immediate vicinity of the engine, so that the combustion control accuracy of the engine can be improved. it can.

  FIG. 1 is a schematic view showing an example of the nitrogen oxide sensor of the present invention. The nitrogen oxide sensor shown in FIG. 1 includes a plate-shaped solid electrolyte body 1, a detection electrode 2 provided on one surface 1a of the solid electrolyte body 1, and a reference electrode 3 provided on the other surface 1b. Have. The detection electrode 2 and the reference electrode 3 are connected to the potential difference measuring means 8. Although not shown, the solid electrolyte body 1, the detection electrode 2 and the reference electrode 3 of the nitrogen oxide sensor are heated to a predetermined temperature (750 ° C., 800 ° C., 850 ° C., etc.) by an external heating means. It has become. For example, an external heater may be provided near the solid electrolyte body 1, the detection electrode 2, and the reference electrode 3, or the nitrogen oxide sensor may be operated in a heating furnace.

The solid electrolyte body 1 is made of a solid electrolyte having oxygen ion conductivity. The solid electrolyte body is preferably made of zirconia (ZrO ₂ ), and preferably contains a stabilizer. An example of a stabilizer is yttria (Y ₂ O ₃ ). The addition amount of a stabilizer such as yttria is preferably 3 to 8 mol%.

  The solid electrolyte body 1 is a fired body. As an example of the manufacturing method of the nitrogen oxide sensor, there is a method of sintering after forming the detection electrode 2 and the reference electrode 3 or their precursor (unfired body) on a green sheet made of zirconia or the like.

  The detection electrode 2 is mainly composed of at least one oxide selected from the group consisting of nickel oxide, tin oxide, iron oxide, titanium oxide, gallium oxide, chromium oxide and zinc oxide. The detection electrode 2 is a film formed by fusing particles made of metal oxide crystals. A preferable average crystal grain size R of particles (mainly metal oxide particles) constituting the detection electrode is 0.2 to 3 μm. The average crystal grain size R can be determined by observing the surface and / or cross section of the detection electrode 2 with an electron microscope according to the cutting method of JISH0501, and measuring the diameter by regarding the fused particles as spherical.

The ratio t / R ² between the average thickness t of the detection electrode ² and the square of the average crystal grain size R of the particles constituting the detection electrode is 30 or less. And the ratio t / R ² is greater than 30, due to the low sensitivity of the nitrogen oxides in operating temperatures above 700 ° C., sufficient detection accuracy is difficult to obtain. Considering the temperature dependence of sensor sensitivity, when the operating temperature is 750 to 850 ° C, it is preferable to set the t / R ² value to 10 or less, and when the operating temperature is 850 to 900 ° C, t / R ² The value is preferably 5 or less.

  As an example of a method for forming the detection electrode 2, (a) a paste made of oxide particles, a binder and a solvent is prepared, (b) a pattern is formed on the solid electrolyte body 1 by screen printing, and (c) degreasing and (d) The method of baking is mentioned.

When the detection electrode 2 is formed by the screen printing method, the average thickness t of the detection electrode 2 can be controlled by the printing plate conditions, the paste concentration and composition, the number of times of overprinting, and the like. The average thickness t of the detection electrode 2 is preferably about 1 to 20 μm. When the average thickness t is in this range, the t / R ² value is easily set to 30 or less. When using a vapor deposition method such as sputtering, the average thickness t of the detection electrode 2 can be easily controlled by the vapor deposition time. In any of the forming methods, the ratio t / R ² of the square of the average thickness t of the detection electrode 2 and the average crystal grain size R can be set to 30 or less by selecting an appropriate forming condition. . For example, a paste containing at least one oxide powder selected from the group consisting of nickel oxide, tin oxide, iron oxide, titanium oxide, gallium oxide, chromium oxide and zinc oxide, a binder and a solvent is applied to the solid electrolyte body 1. After that, when the sensing electrode 2 is formed by degreasing and firing, the average crystal grain size R of the particles constituting the sensing electrode 2 and the sensing electrode 2 are determined according to the metal oxide powder diameter, firing temperature and firing atmosphere. The average thickness t can be in a desired range.

  The detection electrode 2 is preferably porous. When the detection electrode 2 is porous, the porosity (porosity) is more preferably 5 to 30% by volume. The sensing electrode 2 having a porosity of 5 to 30% by volume exhibits a good response speed and excellent durability. If the porosity is less than 5% by volume, the ventilation resistance to the three-phase interface increases, and the response speed as a sensor may decrease. When the porosity is larger than 30% by volume, the bond strength of the crystal particles is lowered, and the durability may be deteriorated.

  The porous detection electrode 2 can be formed by, for example, a screen printing method. For example, when the screen printing method is used, the porosity of the detection electrode 2 can be controlled by the type and amount of binder (sublimation substance, burnout substance, etc.) and amount of solvent in the paste material for printing. . Examples of the sublimable binder include theobromine and polyethylene.

  The reference electrode 3 may be anything that can be formed in a form that acts as an electrode on the solid electrolyte body 1. However, when operating a nitrogen oxide sensor at 700-900 degreeC, it needs to consist of an electrode material which has heat resistance. An example of a preferred reference electrode 3 is a platinum electrode. In order to form the reference electrode 3 made of platinum, for example, a paste containing platinum may be patterned on the solid electrolyte body 1 by screen printing, and then degreased and fired. The reference electrode 3 may be formed via the solid electrolyte body 1 as shown in FIG. 1 or may be formed on the same surface as the detection electrode 2. It is obvious that the same effect can be obtained even if the reference electrode 3 and the detection electrode 2 are formed on the same surface of the solid electrolyte body 1.

  The potential difference measuring means 8 is connected to conductors (not shown) provided on the detection electrode 2 and the reference electrode 3, respectively. Each conductor may be any one that can extract an electrical signal, but preferably has heat resistance. An example of a heat-resistant conductor material is platinum. The potential difference measuring means 8 takes out the electrode potential of the detection electrode 2 and the electrode potential of the reference electrode 3, respectively, and measures the potential difference between the detection electrode 2 and the reference electrode 3.

  FIG. 2 schematically shows another example of the nitrogen oxide sensor of the present invention. The nitrogen oxide sensor shown in FIG. 2 is substantially the same as the example shown in FIG. 1 except that it has a cylindrical solid electrolyte body 1 sealed at one end, and only the differences will be described below. The solid electrolyte body 1 is a tube with one end sealed or a rectangular tube made of a plurality of flat plates. The detection electrode 2 is formed on the outer surface of the cylindrical solid electrolyte body 1 and is exposed to a test gas. Since the reference electrode 3 is formed on the inner surface of the cylindrical solid electrolyte body 1 and is exposed to the atmosphere, it is not in contact with the test gas. Thus, when the reference electrode 3 is arranged so as not to contact the test gas, the nitrogen oxide concentration can be detected with reference to the atmosphere.

  FIG. 3 shows yet another example of the nitrogen oxide sensor of the present invention. The nitrogen oxide sensor shown in FIG. 3 includes a plate-shaped solid electrolyte body 1, a detection electrode 2 and a reference electrode 3 provided on the surface 10 of the solid electrolyte body 1, and a thermocouple 6 embedded in the solid electrolyte body 1. And the resistor substrate 5 in close contact with the back surface 1b of the solid electrolyte body 1 and the resistor 4 sandwiched between the solid electrolyte body 1 and the resistor substrate 5.

  The solid electrolyte body 1 has a two-layer structure, and a thermocouple 6 is sandwiched between the layers. The detection electrode 2 and the reference electrode 3 are provided on the same surface (surface 1a) of the solid electrolyte body 1. Both the detection electrode 2 and the reference electrode 3 are exposed to the test gas. When not only the detection electrode 2 but also the reference electrode 3 is exposed to the test gas, if the electrode potential of the detection electrode 2 and the reference electrode 3 with respect to oxygen is set to the same level, the oxygen concentration coexisting in the detection target gas atmosphere can be reduced. The influence can be suppressed.

  The resistor 4 is in close contact with the solid electrolyte body 1 and heats the solid electrolyte body 1. A general resistor can be used as the resistor 4. The heat given to the solid electrolyte body 1 by the resistor 4 is transmitted to the detection electrode 2 and the reference electrode 3. By the heating by the resistor 4, the temperature of the detection electrode 2 and the reference electrode 3 is maintained at a predetermined temperature. An example of the material of the resistor 4 is platinum. In order to produce the resistor 4 made of platinum on the substrate 5, for example, a paste containing platinum may be patterned by screen printing or the like, and then degreased and fired.

  The resistor substrate 5 has a plate shape and is bonded to the back surface 1b of the solid electrolyte body 1. Since the resistor 4 is formed on a part of the joint surface of the resistor substrate 5 with the solid electrolyte body 1, the resistor 4 is in close contact with the solid electrolyte body 1. Since the resistor substrate 5 is in contact with the solid electrolyte body 1, it must have electrical insulation. Therefore, it is preferable that (a) be made of an insulating material, or (b) have an electrically insulating film on at least one surface of a non-insulating material plate. An example of the resistor substrate 5 made of an insulating material is an alumina plate. An example of the resistor substrate 5 having an electrically insulating film is a zirconia plate having an alumina film.

  In order to join the resistor substrate 5 to the solid electrolyte body 1, the resistor substrate 5 is fused to the solid electrolyte body 1 using, for example, glass. Examples of the material for fusing the resistor substrate 5 to the solid electrolyte body 1 include the same material as the solid electrolyte body 1 in addition to glass. When firing the resistor substrate 5 and the solid electrolyte body 1 in a lump, it is preferable to use the same material as the solid electrolyte body 1 as a fusing agent from the viewpoint of thermal characteristics.

  The thermocouple 6 includes a positive leg 61 and a negative leg 62 and extends in the longitudinal direction in the solid electrolyte body 1. The thermocouple positive leg 61 and the thermocouple negative leg 62 are made of different metals or alloys having different compositions. In the example shown in FIG. 3, the tip of the thermocouple 6 is at an approximately equal distance from the detection electrode 2 and the reference electrode 3, but may be at a position shifted toward the detection electrode 2 or the reference electrode 3. As long as it can be used in an environment of a predetermined operating temperature (for example, 700 to 900 ° C.) and the temperature of the solid electrolyte body 1 or the like can be accurately sensed, the materials and shapes of the thermocouple positive leg 61 and the thermocouple negative leg 62 Is not particularly limited, and may be the same as a general thermocouple.

  In order to provide the thermocouple 6 in the solid electrolyte body 1, the thermocouple positive leg 61 and the thermocouple negative leg 62 are formed on the first layer of the solid electrolyte body 1, and the second layer of the solid electrolyte body 1 is formed thereon. Are integrated. For example, after screen printing a paste containing a positive leg component (for example, a platinum-containing paste) and a paste containing a negative leg component (for example, a paste containing an alloy of platinum and gold) on a green sheet (first layer) Then, another green sheet (second layer) is pressure-bonded, degreased and fired.

  The thermocouple 6 is connected to a feedback mechanism (not shown), and the temperature of the solid electrolyte body 1 measured by the thermocouple 6 is fed back to the voltage applied to the resistor 4. By the feedback mechanism, the temperature of the solid electrolyte body 1, the detection electrode 2, and the reference electrode 3 can be precisely controlled to a preferable operating temperature, for example, 700 to 900 ° C. By controlling the temperature of the solid electrolyte body 1 or the like, it is possible to suppress variations in sensor signals resulting from temperature changes and to obtain high output accuracy.

  The nitrogen oxide sensor shown in FIGS. 4 (a) and 4 (b) is substantially the same as the example shown in FIG. 3 except that it has an air duct forming body 11 provided substantially in parallel to the solid electrolyte body 1, and therefore the difference is different. Only the point will be described below. For simplification of the figure, the potential difference measuring means 8 is omitted in FIG. The air duct forming body 11 is fixed to the upper surface of the spacer 10. As shown in FIGS. 4A and 4B, the spacer 10 and the air duct forming body 11 are provided so as to cover the reference electrode 3 and expose the detection electrode 2. Therefore, the space formed by the solid electrolyte body 1, the spacer 10 and the air duct forming body 11 becomes the air duct 12, and the reference electrode 3 is in contact with the air and exhibits an electrode potential corresponding to the air atmosphere and acts as a reference electrode. To do.

  The material of the spacer 10 and the air duct forming body 11 is not particularly limited as long as it has sufficient heat resistance and mechanical strength, but is preferably made of the same material as that of the solid electrolyte body 1 from the viewpoint of thermal characteristics.

  The nitrogen oxide sensor shown in FIGS. 5A and 5B shows still another example of the nitrogen oxide sensor. In FIG. 5B, the potential difference measuring means 8 is omitted. This nitrogen oxide sensor includes a solid electrolyte body 1 having a detection electrode 2 on the front surface 1a and a reference electrode 3 on the back surface 1b, and a resistor substrate 5 provided substantially parallel to the solid electrolyte body 1. As shown in FIG. 5, when the detection electrode 2 and the reference electrode 3 are formed on the front and back of the solid electrolyte body 1, the detection electrode 2 and the reference electrode 3 are close to each other.

  As shown in FIG. 5 (b), a U-shaped spacer 10 is sandwiched between the back surface 1b of the solid electrolyte body 1 and the resistor substrate 5, and the solid electrolyte body 1, the resistor substrate 5 and A space formed by the spacer 10 is an air duct 12. Since the reference electrode 3 is in the atmospheric duct 12, it is in contact with the atmosphere, and since the detection electrode 2 is outside the atmospheric duct 12, it is in contact with the test gas.

  The resistor substrate 5 includes a first substrate 51 and a second substrate 52. In the example shown in FIG. 5, the resistor 4 is sandwiched between the first substrate 51 and the second substrate 52, and the thermocouple 6 extends in the longitudinal direction in the first substrate 51. The structure of the resistor substrate 5 that supports the resistor 4 and the thermocouple 6 is not limited to this, and the resistor substrate 5 can be heated by the resistor 4 while sensing the temperature of the resistor substrate 5 by the thermocouple 6. If it is. From the viewpoint of thermal characteristics, the first substrate 51 and the second substrate 52 are preferably made of the same material as the solid electrolyte body 1.

When the nitrogen oxide sensor is applied to an internal combustion engine such as an automobile, the test gas includes a reducing gas such as a hydrocarbon gas or a carbon monoxide gas in addition to the nitrogen oxide gas. Therefore, the nitrogen oxide sensor may receive a considerable amount of interference from these reducing gases. In the case of a hybrid potential sensor, it is known that the output characteristics of NO and NO ₂ are opposite to each other and interfere with each other. In such a case, a function using oxygen pumping as described in Japanese Patent No. 3195758 may be adopted as appropriate. Since the function using the oxygen pumping is not related to the essential operation and effect of the present invention, detailed description thereof is omitted.

[2] Manufacturing Method of Nitrogen Oxide Sensor A nitrogen oxide sensor as shown in FIGS. 1 to 5 can be manufactured using a green sheet. Taking the nitrogen oxide sensor shown in FIG. 5 as an example, a manufacturing method using a green sheet will be described.

  First, the detection electrode 2, the reference electrode 3, and the lead conductor are screen-printed on a green sheet to obtain a sheet I. A paste containing a metal for the thermocouple 6 is printed on another green sheet (sheet II, for the upper layer of the first substrate 51). Furthermore, an insulating layer is formed on another green sheet at a portion in contact with the resistor 4 (sheet III, for the lower layer of the first substrate 51). Further, after applying a paste containing an insulating metal to another green sheet, a paste containing a metal for the resistor 4 is screen-printed (sheet VI, for the second substrate 52). The obtained sheet VI, sheet III, sheet II, spacer 10 and sheet I are fitted into a jig and are crimped. The resulting laminate is degreased at about 500 ° C. and then sintered usually at 1400 ° C. or higher. Finally, a lead wire such as Pt is welded to the lead conductor.

  The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

Example 1
A nitrogen oxide sensor having the structure shown in FIG. 1 was produced as follows.
(1) Green sheet for solid electrolyte body 1 Using a ball mill, zirconia powder to which 6 mol% of yttria was added, a binder and a solvent were kneaded to obtain a green sheet having a thickness of 0.25 mm by a doctor blade method. The obtained green sheet was cut into a size of 5 × 5 mm, and a Pt-containing paste was screen-printed thereon for the lead conductor.

(2) Printing detection electrode paste
The NiO reagent powder was pulverized by a ball mill and then classified to a diameter of 0.1 to 1 μm. The obtained powder was put into an agate mortar, and a binder and a solvent were added and kneaded to obtain a NiO-containing paste. NiO-containing paste was screen printed on a green sheet with a pattern size of 3 × 3 mm.

(3) Printing of Reference Electrode Paste Platinum powder, 8 mol% yttria-stabilized zirconia powder, binder and solvent with respect to the platinum powder were kneaded in an agate mortar to obtain a platinum-containing paste. This platinum-containing paste was screen-printed on a green sheet with a pattern size of 3 × 3 mm.

(4) Firing The green sheet on which the detection electrode 2, the reference electrode 3 and the lead conductor paste were printed was degreased at 500 ° C. for 2 hours in the air, and then fired at 1400 ° C. for 3 hours in the air. A Pt wire was welded to the lead conductor of the fired body to obtain an evaluation sample.

(5) Measurement of average crystal grain size of NiO particles In order to investigate the average crystal grain size of NiO particles constituting the detection electrode 2, scanning electron microscope (SEM) observation of the surface and cross section of the detection electrode 2 was performed. An SEM photograph of the sensing electrode 2 of the nitrogen oxide sensor and the average crystal grain size R are shown in FIG. The thickness of the detection electrode 2 was measured and found to be about 8 μm.

(6) Complex impedance measurement A nitrogen oxide sensor was set in a quartz tube. The quartz tube was placed in an electric furnace, and the inside of the electric furnace was maintained at 800 ° C. and an oxygen concentration of 5% by volume, and electrode interface impedance (complex impedance) was measured. Table 1 shows the measurement results.

Examples 2 and 3, Comparative Example 1
A nitrogen oxide sensor was prepared and complex impedance measurement was performed in the same manner as in Example 1 except that NiO particles were prepared so that the average particle diameter was as shown in Table 1. The measurement results are also shown in Table 1.

  The nitrogen oxide sensor having the sensing electrode 2 made of NiO particles having a smaller average crystal grain size has a smaller electrode interface impedance. This is probably because the smaller the average crystal grain size of NiO particles, the more electrode reaction sites.

Examples 4-8, Comparative Examples 2-4
As shown in Table 2, the average crystal grain size of NiO particles constituting the detection electrode is 0.2 μm, 0.5 μm, 1 μm, 1.5 μm and 2.5 μm, and the thickness of the detection electrode is 2 μm, 4 μm and 8 μm. A nitrogen oxide sensor was produced in the same manner as in (1) to (4) of Example 1 except that. Table 2 shows t / R ^binary values, Example No. or Comparative Example No. of each nitrogen oxide sensor.

* −: Not prepared.

Each sensor is set in a quartz tube held in an electric furnace, the inside of the electric furnace is made into an atmosphere of 5 vol% oxygen and 200 ppm NO ₂ concentration, and the sensor output is maintained at 700, 800 ° C or 900 ° C. It was measured. The sensitivity to 200 ppm NO _{2 at} each operating temperature is shown in Tables 3-5.

*: No sample.

*: No sample.

*: No sample.
**: Not measured.

Find t / R ² from the thickness (thickness t) of the sensing electrode and the average crystal grain size R of the NiO particles that make up the sensing electrode. The t / R ² value of each sensor and 200 ppm at each operating temperature The relationship of sensitivity to NO ₂ was examined. If it shows a sensitivity of 10 mV or more for 200 ppm NO ₂ , it can be said that it is a good sensitivity that can withstand practical use, whereas a sample with a t / R ² value of 30 or less is good at an operating temperature of 700 ° C. Showed high sensitivity. Samples with a t / R ² value of 10 or less show higher sensor sensitivity than samples with a t / R ² value greater than 10 at any operating temperature (700 ° C, 800 ° C and 900 ° C), and the operating temperature is 800 ° C. Also, the sensitivity was 10 mV or more. Samples with a t / R ² value of 5 or less showed higher sensor sensitivity than other samples at any operating temperature, and were considered to be sufficiently usable even at an operating temperature of 900 ° C.

Examples 9-11
A green sheet for a solid electrolyte body was produced in the same manner as in Example 1 (1), and a lead conductor paste was printed thereon.

  The NiO reagent powder was pulverized by a ball mill and then classified to an average diameter of about 0.5 μm. NiO powder and 1% by mass, 5% by mass and 10% by mass of theobromine were put in an agate mortar, kneaded with a binder and a solvent, and made into a paste. The obtained paste containing NiO and theobromine was screen-printed on a green sheet. The pattern size was 3 × 3 mm.

  In the same manner as in Examples 1 (3) to (5), a paste for a reference electrode was printed on a green sheet, and degreased and sintered.

The obtained nitrogen oxide sensor is set in a quartz tube held in an electric furnace, and the inside of the electric furnace is held at 800 ° C, 5 vol% oxygen, 200 ppm NO ₂ concentration atmosphere, and the sensor output is measured. did. Table 6 shows the measurement results. The thickness t of the detection electrode 2 of each sensor was about 8 μm, and the average crystal grain size R of NiO particles constituting the detection electrode was about 1 μm. For comparison, the sensitivity and the like of Example 1 using the detection electrode 2 produced without adding a sublimation material are also shown.

  It was found that a nitrogen oxide sensor having a sensing electrode 2 with a large porosity shows a larger sensor output in the range of porosity of 5 to 30% by volume.

Examples 12-15
A nitrogen oxide sensor was produced in the same manner as in (1) to (4) of Example 1 except for the average crystal grain size of NiO particles constituting the sensing electrode and the thickness of the sensing electrode (Example 12). Further, SnO ₂ particles, Fe ₂ O ₃ particles and TiO ₂ particles are blended in the paste for the sensing electrode 2 instead of NiO particles, and the t / R ² value is as shown in Table 7 below. Nitrogen oxide sensors were produced in the same manner as 1) to (4) (Examples 13 to 15).

Each sensor was set in a quartz tube held in an electric furnace, and the inside of the electric furnace was kept at 800 ° C., 5 vol% oxygen, and 200 ppm NO ₂ concentration atmosphere, and the sensor output was measured. Nitrogen oxidation having sensing electrode 2 composed of SnO ₂ particles, Fe ₂ O ₃ particles, and TiO ₂ particles, although not as good as the nitrogen oxide sensor of Example 12 (nitrogen oxide sensor having a sensing electrode composed of NiO particles). The object sensor also showed a sensitivity of 16 mV or higher, and a good output was obtained.

Examples 16-18
Instead of NiO particles, paste for the sensing electrode 2 was mixed with Ga ₂ O ₃ particles, Cr ₂ O ₃ particles and ZnO particles, and the t / R ² value was as shown in Table 8 below. 1) A nitrogen oxide sensor was fabricated in the same manner as in (4).

Each sensor was set in a quartz tube held in an electric furnace, and the inside of the electric furnace was held at 700 ° C., 5 vol% oxygen, and 200 ppm NO ₂ concentration atmosphere, and the sensor output was measured. Table 8 shows the measurement results. In addition, t / R ² value of each sensor was 3.4-3.8, respectively. Nitrogen having a sensing electrode 2 composed of Ga ₂ O ₃ particles, Cr ₂ O ₃ particles and ZnO particles, although not exceeding the nitrogen oxide sensor of Example 12 (nitrogen oxide sensor having a sensing electrode composed of NiO particles). The oxide sensor also showed a sensitivity of 15 mV or higher, and a good output was obtained.

Example 19
(1) Green sheet A nitrogen oxide sensor having the structure shown in FIG. 5 was produced as follows. In the same manner as in Example 1 (1), yttria-added zirconia powder was used as a starting material to obtain a green sheet having a thickness of 0.25 mm by the doctor blade method. Four green sheets obtained were cut to a size of 5 × 50 mm. A green sheet for the spacer was also cut off.

(2) Seat I
A lead conductor Pt paste was screen-printed on one of the green sheets obtained in Example 19 (1). Subsequently, a NiO-containing paste for the detection electrode 2 was printed on the green sheet I obtained in Example 19 (1) in the same manner as in Example 1 (2). Further, a platinum-containing paste for the reference electrode 3 was printed in the same manner as in Example 1 (3).

(3) Sheet II
After a paste containing alumina powder was printed on another green sheet, a platinum paste was overprinted on a portion where the resistor 4 was to be formed to obtain a sheet II.

(4) Sheet III
Furthermore, a paste containing 65% Au-35% Pd alloy for the thermocouple positive leg 61 and a paste containing Pt for the thermocouple negative leg 62 were printed on another green sheet. A paste containing alumina powder was printed on the portion of the back surface of the green sheet that was in contact with the resistor 4.

(5) Firing and assembly Sheet II, sheet III, an unprinted green sheet, a spacer, and sheet I were set in a jig, and placed in 80 ° C. warm water and pressurized. The obtained laminate was degreased in the atmosphere at 500 ° C. for 2 hours and then baked in the atmosphere at 1400 ° C. for 3 hours. A Pt wire was welded to the lead conductor of the obtained fired body and attached to a sensitivity evaluation jig to obtain an evaluation sample. At that time, a jig was erected so that the duct 12 was exposed to the atmosphere.

(6) Measurement of sensor sensitivity The thermocouple 6 was connected to the thermocouple output measuring means 9, and the output from the measuring means 9 was set to be fed back to the applied voltage of the resistor 4. A Pt wire welded to the lead conductor was connected to the potential difference measuring means 8.

The evaluation sample was placed in an atmosphere containing 5% by volume of oxygen and 200 ppm of NO ₂ , and the sensor operating temperature was maintained at 800 ° C. by feeding back the output of the thermocouple 6 to the applied voltage of the resistor 4. The sensor sensitivity of the evaluation sample was measured and found to be 27 mV. The thickness of the detection electrode of this nitrogen oxide sensor was 8 μm, the average crystal grain size was 1.5 μm, and t / R ² was 3.56. Even when the duct 12 was exposed to the atmosphere, the sensor sensitivity was comparable to that when the nitrogen oxide sensor was set in an electric furnace (see Table 4, Example 2).

It is sectional drawing which shows an example of the nitrogen oxide sensor of this invention. It is sectional drawing which shows another example of the nitrogen oxide sensor of this invention. It is sectional drawing which shows another example of the nitrogen oxide sensor of this invention. FIG. 3 shows still another example of the nitrogen oxide sensor of the present invention, where (a) is a cross-sectional view and (b) is a top view. FIG. 3 shows still another example of the nitrogen oxide sensor of the present invention, where (a) is a cross-sectional view and (b) is a top view. It is a figure which shows the average crystal grain diameter of the oxide particle | grains which are the photograph of the surface and cross section of a detection electrode, and the main component of a detection electrode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Solid electrolyte body 2 ... Detection electrode 3 ... Reference electrode 4 ... Resistor 5 ... Resistor board 6 ... Thermocouple
61 ... Thermocouple leg
62 ... Thermocouple negative leg 8 ... Means for measuring potential difference between detection electrode and reference electrode 9 ... Means for measuring thermocouple output
10 ... Spacer
11 ... Atmosphere duct formation
12 ... Air duct

Claims (2)

An oxygen ion conductive solid electrolyte body, a detection electrode formed on the solid electrolyte body, a reference electrode, means for detecting a nitrogen oxide concentration by a potential difference between the detection electrode and the reference electrode, and the solid A nitrogen oxide sensor comprising an electrolyte body, a means for heating the detection electrode and the reference electrode, wherein the main constituent material of the detection electrode is nickel oxide, tin oxide, iron oxide, titanium oxide, gallium oxide, oxide It is at least one oxide selected from the group consisting of chromium and zinc oxide, and the ratio t / R of the average thickness t of the detection electrode and the square of the average crystal grain size R of the particles constituting the detection electrode A nitrogen oxide sensor, wherein ² is 30 or less. The nitrogen oxide sensor according to claim 1, wherein the detection electrode has a porosity of 5 to 30% by volume.