JP2000275200A - Nox sensor and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a high sensitivity NOx sensor. SOLUTION: The NOx sensor is equipped with an insulating substrate 1, a pair of the electrodes 2 provided on the substrate 1 and the gas sensing body 3 based on tin-added indium oxide (ITO) further provided on the substrate 1. The gas sensing body 3 is formed by using an org. soln. containing an inorg. indium salt, an org. tin salt, at least an org. compd. capable of being bonded to indium by a coordinate bond and an activator.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a low concentration NOx
Shows high sensitivity and selectivity to (nitrogen oxide) gas.
The present invention relates to a NOx detection sensor and a method for manufacturing the same.

[0002]

BACKGROUND ART NO NOx gas and (nitric oxide) mainly gas and NO ₂ (nitrogen dioxide) gas, automotive, discharged from such heating equipment of an internal combustion engine or home such as aircraft itself or other It is a highly dangerous gas that has a great effect on the human body, living organisms, living environment, etc. due to its combination, adsorption, and reaction with other gases. Conventionally, a chemiluminescence method, an infrared absorption method, an electrolysis method, and the like have been used for measurement of the NOx gas. However, these methods have a problem that a large-sized apparatus is required and maintenance is difficult.

In order to solve the above-mentioned problem, NO
Using an oxide semiconductor whose resistance value changes in contact with x gas
Compact, maintenance-free NOx sensor
Conductive gas sensors have been considered. Generally, semiconductor type
As shown in FIG. 2, the gas sensor has an insulated
On at least one pair of electrodes 2 on the edge substrate 1
It is composed of the formed gas sensitive body 3. Gas sensitive
The material used as the body 3 is obtained by gas adsorption.
As a method using electric conductivity change, SnO _Two, In_Two
O_ThreeAnd the like have been put to practical use. Gas sensitive body
As a forming method, a method using sintering, screen printing
Thick film method such as sputtering, thin film method such as sputtering method and vacuum evaporation method
Is used, and the film thickness is several tens nm to several tens μm.
Of the degree. Also, some semiconductor gas sensors
In order to improve gas selectivity,
SiO_TwoSome have a coating layer of metal oxide such as
You.

In the above-mentioned semiconductor type gas sensor, for example, Japanese Patent Application Laid-Open No. 60-155555-7 discloses a method in which a pair of electrodes are provided on the surface of an insulating substrate, and samarium oxide, gadolinium oxide and neodymium oxide are formed on indium oxide. JP-A-61-93945 discloses a gas-sensitive thin film mainly composed of a polycrystalline SnO2 containing Fe (iron) formed on a substrate. However, the non-stoichiometric parameter (δ) is 0.01 <δ <
Japanese Patent Application Laid-Open No. 6-207921, which has an oxygen vacancy of 0.5 and contains an oxide containing titanium atoms, discloses that indium oxide is a main component on an insulating substrate and 900 parts by weight of indium is used for 100 parts by weight of indium. There are known gas sensitive bodies containing gold parts by weight.

[0005]

As described above, the conventional method for measuring the NOx concentration generally requires a large-scale apparatus and complicated operations, and is expensive and cannot be easily used. Have.

On the other hand, a semiconductor type NOx sensor proposed as a simple NOx concentration measuring method has excellent sensitivity and responsiveness to NOx gas.

However, it is desired to develop a sensor that is excellent in sensitivity, selectivity, responsiveness, stability, reproducibility and the like for NOx gas. In addition, as a method of forming the gas sensitive body, a thin film method such as a sputtering method or a vacuum deposition method has a disadvantage that when the gas sensitive body is composed of multiple components, the composition tends to vary. Further, the thick film method such as screen printing has a disadvantage that the film thickness tends to vary.

An object of the present invention is to solve the above-mentioned problems of the conventional sensor and to provide a high-sensitivity NOx sensor and a manufacturing method capable of manufacturing the sensor with good reproducibility.

[0009]

SUMMARY OF THE INVENTION According to the present invention, it is possible to improve the long-term stability and life of a single phase of indium oxide by using a gas sensitizer containing tin-added indium oxide (ITO) as a main component. it can. At this time, tin forms a solid solution in indium oxide or forms a compound with indium.

In the present invention, an organic solution containing an inorganic indium salt, an organic tin salt, an organic compound capable of coordinating with indium, and an activator is used for forming a gas sensitizer. In general, inorganic indium salts often have deliquescence or are easily hydrolyzed, and it is difficult to stably produce a gas sensitive material having the same thickness and composition. For this purpose, an organic compound capable of coordinating with the indium is added to generate a coordination compound substituted with a part of the inorganic indium salt, thereby stabilizing the inorganic indium salt. Since the organic tin salt has compatibility with a compound having an indium coordination compound, the degree of dispersion of tin after film formation is good. The active agent is indium,
Metal salts other than tin, and the addition of these salts improves the sensitivity and selectivity of the gas sensitizer to NOx gas.

As a result, a composition for forming a gas sensitive body comprising an inorganic indium salt, an organic tin salt, an organic compound capable of coordinating with indium, and an organic solution containing an activator is applied and fired to form a composition. It is possible to stably produce a controlled gas sensing element. Then, as compared with a physical film forming method such as vacuum deposition or sputtering, the gas sensitive body causes mutual diffusion with the substrate at the time of firing, so that a highly reliable NOx sensor having good adhesion to the substrate can be obtained.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. .

FIG. 1 is a diagram showing a NO according to an embodiment of the present invention.
It is a schematic sectional drawing of an x sensor. In FIG. 1, 1 is an insulating substrate such as alumina or mullite, 2 is an electrode made of a metal such as gold, silver or platinum, and 3 is a gas sensitive body made of a metal oxide containing indium tin oxide as a main component.

As the substrate 1, any material can be used as long as it has an insulating surface and has a heating function, and the material and the configuration are not limited. However, the surface roughness of the substrate 1 is preferably between 0.01 and 1 μm.

The main purpose of the electrode 2 is to apply a voltage to the gas sensitive body 3 and measure the resistance value, and there is no limitation on the material, configuration, pattern, manufacturing method, and the like of the electrode.

The gas sensing element 3 can be formed as follows.

After a film of the composition for forming a gas sensitive body is formed on the substrate 1 on which the electrode 2 is attached , the coating is fired at a temperature of several hundred degrees Celsius or more to form the gas sensitive body 3. The gas sensitizer forming composition can be applied by a screen printing method, a roll coating method, a dip coating method, a spin coating method, or the like, but the dip coating method and the spin coating method are preferred. The firing temperature may be not lower than the temperature at which the composition for forming a gas sensitive body is decomposed and not higher than the deformation temperature of the substrate, and is preferably 400 to 800 ° C. Here, the composition for forming a gas sensitive body is synthesized as follows.

First, an inorganic indium salt is mixed with an organic compound capable of coordinating with indium. Here, the inorganic indium salt is an organic compound capable of coordinating with indium,
What is necessary is just to have the ligand which can be substituted. For example, indium nitrate and indium chloride are mentioned, and those having crystallization water are preferable. Organic tin salts
Those which are easily thermally decomposed are preferred, and carboxylate salts such as tin acetate, dicarboxylate salts such as tin oxalate, and tin acetylacetonate are preferred. In addition, an organic compound capable of coordinating with indium is necessary for coordinating one part with indium to stabilize an inorganic indium salt and to have solubility in an organic solvent, for example, β-diketones such as acetylacetone. And condensates of the above-mentioned polyhydric alcohols such as α- or β-ketone acids, polyhydric alcohols and diethylene glycol.

Next, an organic solvent and an activator are added to the solution,
The organic solutions are heat-treated. Here, the activator is a metal salt added for the purpose of improving the sensitivity and gas selectivity of the gas sensitizer, and an alkaline earth metal salt such as a metal magnesium salt, a metal calcium salt, a metal strontium salt, and a metal barium salt. Or metal titanium salt, metal zirconium salt,
Transition metal salts such as metal vanadium salts, metal chromium salts, metal manganese salts, metal iron salts, metal cobalt salts, metal nickel salts, metal copper salts, metal zinc salts, metal lead salts, metal cadmium salts, metal antimony salts, metals Bismuth salts and the like can be mentioned. The compound is relatively stable at room temperature, but may be easily decomposed by heat treatment, and may be an inorganic salt or an organic salt. For example, inorganic salts include nitrates, sulfates, and chloride salts, and organic salts include carboxylate salts, dicarboxylate salts, and acetylacetone complex salts. Also,
The organic solvent may be any as long as it can dissolve the organic compound and the inorganic compound used in the present invention. For example, ethanol, alcohols such as isopropanol, acetone,
Examples thereof include ketones such as diethyl ketone, ether alcohols such as methoxyethanol and ethoxyethanol, and tetrahydrofuran. Further, when the activator is hardly soluble at room temperature, an inorganic indium salt, an organic compound capable of coordinating with indium, and an organic solution containing the activator are subjected to heat treatment at or near the reflux temperature of the organic solution. You may.

Next, the gas sensitive body 3 is patterned as follows. First, after a resist ink is printed and cured on a portion of the substrate 1 where the gas sensitizer 3 is required, the substrate 1 is immersed in a hydrochloric acid solution of ferric chloride. 3 is melted to form a pattern of the gas sensitive body 3 on the substrate 1. Here, the resist ink only needs to be insoluble in an acidic solution after being cured by heat or ultraviolet rays, and is preferably composed mainly of epoxy acrylate, epoxy methacrylate, urethane acrylate, and urethane methacrylate. Next, the substrate 1
Is immersed in an alkaline solution such as sodium hydroxide to remove the resist.

The patterning of the gas sensing element 3 is performed by subjecting a gas sensing element formed by a thermal decomposition method such as dip coating, a thin film method such as a sputtering method or a vacuum evaporation method to photolithography or resist printing, curing, etching, or the like. Can also be produced.

Hereinafter, the present invention will be described with reference to more detailed examples, but the present invention is not limited to these embodiments.

[0023]

(Example 1) A gold organometallic compound paste was applied on a 0.4 mm-thick alumina substrate by screen printing and dried, and then fired at 800 ° C. to form an electrode layer. did.

In a 1 l Erlenmeyer flask, 18 g of indium nitrate (Chemical Formula 1) was weighed, and 20 g of acetylacetone was added and mixed and dissolved at room temperature. Tin oxalate and 130 g of acetone weighed so that (Equation 1) becomes 3 mol% were added to the solution, followed by stirring and mixing to obtain a desired composition for forming a gas sensitive body.

[0025]

Embedded image In (NO ₃ ) ₃ · 3H ₂ O

[0026]

## EQU1 ## Sn / (In + Sn) .times.100 The composition for forming a gas sensible body is applied on an alumina substrate having a thickness of 0.4 mm by dip coating.
By firing for a time, a gas-sensitive body having a thickness of 100 nm and made of a metal oxide containing indium as a main component was formed.

Next, a resist ink is printed on the gas sensitizer 3 and cured with ultraviolet rays. Then, the substrate is immersed in a hydrochloric acid solution of ferric chloride to remove unnecessary portions of the gas sensitizer on the electrode 2. Was dissolved, and the substrate was immersed in an alkaline solution such as sodium hydroxide to remove the resist, and a pattern of a gas sensitive material was formed on the substrate.

Next, NOx was prepared using the sensor element thus manufactured.
The response characteristic to was measured. The sensor element is fixed in a quartz glass measuring tube, and the element temperature is set to 300 ° C with a heater.
, And a change in resistance of the sensor element when air and air containing 100 ppm of NOx were alternately brought into contact with the sensor element was measured. R _A , N
Change the sensor element resistance one minute after changing to air containing Ox to R _G1
RG1 / _RA was determined as the sensor sensitivity. The sensitivity of the sensor of each composition thus obtained was 100.

Example 2 18 g was placed in a 1 l Erlenmeyer flask.
Of indium nitrate (chemical formula 1) was weighed and 20 g of acetyl acetate was weighed.
Seton was added and mixed and dissolved at room temperature. The solution
In addition, oxalic acid weighed so that (Equation 1) becomes 5 mol%
And oxalic acid weighed so that (Equation 2) becomes 5 mol%.
Add bismuth and 130 g of acetone, stir and mix
Thus, a desired composition for forming a gas sensitive body was obtained. Other examples
Same as 1. Note that the sensor sensitivity (R_G1/ R _A) Is 120
Was.

[0030]

As is apparent from the above description, the present invention is compact, lightweight, inexpensive, has excellent gas detection characteristics, and controls NOx concentration or NOx in combustion equipment.
An object of the present invention is to provide an ozone sensor suitable for applications such as detection.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing one embodiment of an ozone sensor of the present invention.

FIG. 2 is a schematic sectional view of a conventional semiconductor gas sensor.

[Explanation of symbols]

 1 substrate 2 electrode 3 gas sensitive body

Claims (5)

[Claims] At least an insulative substrate, a pair of electrodes provided on the substrate, and a gas sensitive body mainly composed of indium tin oxide (ITO) provided on the substrate are provided. A NOx sensor. 2. The gas responsiveness of the NOx sensor according to claim 1, wherein an inorganic solution containing an inorganic indium salt, an organic tin salt, an organic compound capable of coordinating at least indium, and an active agent is applied and fired. A method for manufacturing a NOx sensor, comprising a step of forming a body. 3. The method for manufacturing a NOx sensor according to claim 2, wherein said inorganic indium salt is selected from the group consisting of indium nitrate and indium chloride. 4. The method according to claim 2, wherein said organotin salt is selected from the group consisting of carboxylate, dicarboxylate and acetylacetone complex. 5. The organic compound capable of coordination is a β-diketone, an α- or β-ketone acid, a polyhydric alcohol,
The method for producing a NOx sensor according to claim 2, wherein the NOx sensor is selected from the group consisting of condensates of the polyhydric alcohol.