JP2918394B2 - Nitrogen oxide detection sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the detection of nitrogen oxides used in fields such as nitrogen oxide reduction technology and nitrogen oxide decomposition technology.

[0002]

2. Description of the Related Art Known methods for measuring the concentration of nitrogen oxides in exhaust gas include a chemiluminescence system, infrared absorption system, ultraviolet absorption system, constant potential electrolysis system, and constant potential coulomb system. On the other hand,

[0003]

Embedded image It is known to employ a composition having an oxide semiconductor sensor.

[0004]

Here, the chemiluminescence type has the highest accuracy and reliability and is considered to be the most effective method at present. However, an ozone generator, a photomultiplier tube,
It requires a high voltage source, etc., and has limitations in miniaturization, price reduction, and maintenance conditions. On the other hand, the absorption method has a high detection threshold and requires correction. When strict accuracy is not required, the constant potential electrolysis method is simple, but there are difficulties in changing the electrode performance with time and maintaining the electrolyte solution. The devices used in these methods have the problem of costing hundreds of thousands of yen to several million yen, and have high measurement accuracy, but require correction due to drift, It has the disadvantage of lacking properties. On the other hand, the above-described oxide semiconductor sensor has a disadvantage that the sensitivity characteristic changes when water is present in the detection environment.

Accordingly, an object of the present invention is to provide an inexpensive and durable nitrogen oxide detection sensor having a relatively simple structure.

[0006]

In order to achieve the above-mentioned object, a characteristic feature of a nitrogen oxide detection sensor is that it has electrical conductivity or semi-conductivity.

[0007]

Embedded image

(A ′ is one or more elements selected from alkaline earth elements, B ′ is one or more elements selected from Group IIIa elements, x is 0 to 3, y is 0 to 1, n Is 1 or more,
δ is characterized by being composed of a gas detecting section mainly composed of a complex oxide represented by 0 to 1) and an electrode electrically connected to the gas detecting section.
The effects are as follows.

[0009]

The composite oxide employed in the gas detection portion of this sensor has a layered oxygen lattice defect structure in the crystal structure, and nitrogen oxide molecules are reversibly coordinated at the oxygen defect site. To detach. This reaction depends on the concentration of nitrogen oxides in the gas phase and is a reversible coordination / elimination reaction. Here, when this reaction occurs, the electric resistance value of the gas detector changes in accordance with the concentration of nitrogen oxide in the gas phase. This change is detected using an electrode provided in the sensor, and the sensor functions as a nitrogen oxide sensor.

[0010]

That is, in the sensor of the present invention, the change in the electric resistance value of the gas detecting portion is measured through the electrode, and the concentration of only the nitrogen oxide gas contained in the atmosphere or the combustion exhaust gas is measured. Can be detected. Furthermore, since it is only provided with a composite oxide electrode, it is configured as a simple and inexpensive device.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings in the order of sensor structure, sensor manufacturing method, and sensor characteristics.

1. Sensor structure FIG. 1 shows the configuration of the nitrogen oxide detection sensor 1 of the present application. The sensor 1 is provided with a thin-film gas detector 3 on the upper side of the MgO substrate 2, and a pair of Pt current flowing electrodes 4 and these current A Pt voltage detection electrode 5 for the electrode 4 is provided. Further, a ceramic heater plate 6 as a heating means is provided below the substrate 2. By the way, the main component of the above-mentioned gas detection unit 3 is

[0013]

Embedded image

That is, with this sensor configuration, a change in the electric resistance value of the gas detecting section 3 is detected. Here, when the gas detection unit 3 is configured as a thin film, when the nitrogen oxide contained in the gas to be inspected in the entire gas detection unit reacts with this portion, the resistance change is caused by a gas-solid reaction. The rate of the nitrogen oxide diffusion in the gas detection part can be reduced by increasing the ratio of the part that performs nitrogen oxide absorption, so the nitrogen oxide absorption / release speed (sensor response speed) can be improved. Becomes As a result, the sensitivity to low-concentration nitrogen oxides and the responsiveness to sensitivity to changes in nitrogen oxide concentration are improved. Further, with respect to the electrode configuration, by employing the above configuration, the current also flows to the resistance change portion in the gas detection unit, and it is possible to effectively detect the electric resistance change of the gas detection unit due to the nitrogen oxide gas. .

2. Manufacturing method of sensor The manufacturing method of the composite oxide thin film constituting the gas detection unit 3 will be described below. The oxides were mixed at a molar ratio and then fired to prepare a target, and then a thin film was formed by a laser ablation method. Table 1 shows the film forming conditions for laser ablation.

[0016]

[Table 1]

A gas detecting section 3 is formed on a substrate 2 on which a thin film has been formed, and an electrode 4.5 is provided for the detecting section 3.

3. Sensor Characteristics 3-1 Nitrogen oxide sensitivity characteristics Sensitivity characteristics were measured as follows. A constant voltage is applied to the ceramic heater plate 6, the gas detecting section is maintained at 200 ° C., a current of 10 mA is supplied to the current supplying electrode 4, and a gas containing various components in a predetermined concentration is brought into contact with the air. Was measured to determine the electric resistance value. FIG.
Each gas _{(NO, NO 2, C 2} H 5 OH, CH 3 OH,
H ₂ ). In the drawing, the resistance value in air is represented by R ₀ (Ω), and the resistance value in each gas is represented by R (Ω). In the figure, the horizontal axis is the concentration of each gas (ppm unit).
And the vertical axis indicates the response rate (log ( R / R ₀ )).

As a result, this sensor showed sufficient sensitivity characteristics to NO and NO ₂ .

3-2 Nitrogen Oxide Sensitivity of Pt Carrying the sensor 1 described above, the interference component (C ₂ H ₅ O
H, CH ₃ OH, H ₂ ) by oxidation reaction
An oxidation catalyst layer 7 having a platinum group element such as t, Pd or the like formed thereon (1% of Pt mixed and dispersed in activated alumina on a film constituting a gas detection portion) by screen printing to a thickness of 500 μm. Printing by a method and baking at 600 ° C. for 24 hours) is also effective for detecting nitrogen oxides. The configuration of this sensor 10 is shown in FIG. FIG. 4 shows a measurement result of the sensor 10 corresponding to FIG. As a result, the selectivity for the interference component is improved, and the sensitivity for the NO and NO ₂ components is improved. That is, it can be seen that by providing the catalyst layer 7 containing a platinum group element such as Pt and Pd on the composite oxide, it is possible to suppress the influence of a high concentration of interference components.

4. EXPERIMENTAL EXAMPLES As a study of a material suitable as a composite oxide constituting the gas detection unit 3, experimental results on various materials performed by the inventor are shown below. The sensor configuration was the same as that of FIG. 1, and the results of the sensitivity characteristics were arranged as in FIG. FIG. 5 shows the results for NO ₂ and FIG. 6 shows the results for NO. In these figures, R ₀ is the resistance value when NO ₂ or NO is not present, R is the resistance value when NO ₂ or NO is present at a certain concentration , and the response rate log (R
/ R ₀ ) indicates the sensitivity. Table 2 shows the composition of the composite oxide sample studied. 5 and 6, each number corresponds to the number of each sample.

[0022]

[Table 2]

As a result, it was found that any of the samples could detect nitrogen oxides. Therefore, as a composite oxide that can be employed in the gas detection section 3 of the present application, the crystal structure

[0024]

Embedded image

It can be seen that the composite oxide represented by In the formula, as the element A ′, Ca, Sr, Ba
And at least one element selected from the group IIa elements such as a mixture or a solid solution of two or more elements. The element B 'is selected from the group IIIa elements such as La and Y. More than one kind of element (may be a mixture or solid solution of two or more kinds of elements), and the valence of Cu can change to some extent. Therefore, the amount of oxygen is in the range of 2n + 6 ± δ (δ is 0 to 1). Can vary. A ', B', x,
Regarding the combination of y, n, and δ, this material should be electrically conductive or semiconductive depending on the sensor configuration and the detection principle configuration.

4-2 Durability Characteristics A sensor according to the present invention employing a composite oxide of No. 1 and No. 3 shown in Table 1 for a gas detecting portion;

[0027]

Embedded image FIG. 7 is a graph showing the durability of a conventional sensor adopting the above method under an atmosphere coexisting with moisture. As a result,

[0028]

Embedded image Of the composition of the present invention, while maintaining the initial performance as it is. Therefore, it can be seen that the sensor of the present application can maintain good durability.

[Another embodiment] Another embodiment will be described below. (1) Configuration of Oxidation Catalyst Layer In the above embodiment, an example in which the oxidation catalyst layer 7 in which Pt is supported on activated alumina is provided, but this may be configured as follows. a. A platinum group element such as Pt, Pd, etc.
% And mixed. b. A platinum group element such as Pt and Pd is supported on the inner surface of the composite oxide.

(2) Electrode Configuration In the above embodiment, an example was shown in which a pair of current-carrying electrodes 4 and a pair of voltage detection electrodes 5 were provided to detect a change in resistance. In the meantime, a constant voltage may be applied to detect the current between the pair of current detection electrodes. Further, as a method of forming the electrodes, it is also possible to use a normal two-terminal electrode (in which the current electrode and the voltage electrode are common). Since a portion that causes a partial resistance change due to absorption is generated, the current preferentially flows through only the low-resistance phase. Therefore, the change in the resistance value of the gas detection unit that can be detected from the electrode may be very small, and the configuration disclosed in the embodiment is preferable. However, as the function of the electrode, any configuration may be used as long as it can detect a change in resistance value occurring in the gas detection unit.

(3) Heating means In order to improve the response and recovery of the electrical resistance value of the nitrogen oxide detection sensor to the change in the concentration of nitrogen oxide gas, the sensor is heated to 150 ° C. or higher as in the embodiment. For this purpose, it is preferable to provide an arbitrary heating means in addition to the ceramic heater plate 6 of the embodiment. By providing a heating means, nitrogen oxide absorption and
The release rate becomes sufficiently large, and the responsiveness and recovery of the electrical resistance value of the nitrogen oxide detection sensor to changes in the concentration of the nitrogen oxide gas can be improved. However, if a change in the resistance value can be ensured, detection can be performed even without this heating means. A self-heating type is also conceivable.

(4) Film Forming Method In addition to the above-described methods, a film forming method for forming a gas detecting portion is a physical vapor deposition method such as a vacuum vapor deposition method and a sputtering method, and a chemical vapor deposition method such as MO-CVD and chloride CVD. There is also a vapor deposition method.

(5) Material of Substrate In addition to MgO, SrTiO ₃ can be used as the material of the substrate.

(6) Sensor Configuration The gas detector 3 can also exhibit the function of detecting nitrogen oxides when an arbitrary structure such as a lump or the like is employed in addition to a thin film.

Incidentally, reference numerals are written in the claims for convenience of comparison with the drawings, but the present invention is not limited to the configuration of the attached drawings by the entry.

[Brief description of the drawings]

FIG. 1 is a diagram showing a sensor configuration.

FIG. 2 is a diagram showing sensitivity characteristics to gas.

FIG. 3 is a diagram showing a configuration of a sensor including a catalyst layer carrying Pt.

FIG. 4 is a view showing gas sensing characteristics of a sensor having a catalyst layer.

FIG. 5 is a diagram showing the gas sensitivity characteristics of each composite oxide to NO _{2 in} an experimental example.

FIG. 6 is a graph showing gas sensitivity characteristics of each composite oxide of the experimental example to NO.

FIG. 7 is a diagram showing durability of a sensor.

[Explanation of symbols]

 2 Substrate 3 Gas detector 4 Electrode 5 Electrode 7 Oxidation catalyst layer

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Masamichi Ipponmatsu Osaka Gas Co., Ltd. 4-1-2, Hirano-cho, Chuo-ku, Osaka-shi, Osaka (56) Reference JP-A-2-126146 (JP, A) JP-A-62-192643 (JP, A) JP-A-3-200057 (JP, A) JP-A-3-200058 (JP, A) JP-A-50-144391 (JP, A) JP-A-51-15492 (JP, A) JP, A) Sensors and Actutors B, 9, (1992), p17-23 (58) Fields investigated (Int. Cl. ⁶ , DB name) G01N 27/12 JICST file (JOIS)

Claims (4)

(57) [Claims] (1) having electrical conductivity or semi-conductivity; (A ′ is one or more elements selected from alkaline earth elements, B ′ is one or more elements selected from Group IIIa elements, x is 0 to 3, y is 0 to 1, and n is 1 or more. , Δ is 0
It is characterized by comprising a gas detecting section (3) containing a composite oxide represented by 1) as a main component, and electrodes (4, 5) electrically connected to the gas detecting section (3). Nitrogen oxide detection sensor. 2. The nitrogen oxide detection sensor according to claim 1, wherein the alkaline earth element is Sr. 3. The nitrogen oxide detection sensor according to claim 1, wherein an oxidation catalyst layer supporting a platinum group element is provided on at least a surface side of the gas detection section. 4. The nitrogen oxide detection sensor according to claim 1, wherein the gas detection section is a thin film provided on a flat substrate.