JPH05133923A - Manufacture of gas sensor - Google Patents

Abstract

PURPOSE:To improve gas sensitivity with respect to carbon monoxide gas by soaking a palladium substituted precursor formed by soaking tin oxide in ammonium chloride solution successively in palladium chloride solution and then baking it. CONSTITUTION:An electrode 4 and a gas sensing body 2 are laminated on an alumina substrate 1, and a platinum heater 3 is formed on the other face of the substrate 1 to constitute a sensor. To apply palladium on the sensing body 2 comprising tin oxide, the substrate 1 with tin oxide formed is soaked in ammonium chloride solution of 2wt.% concentration at room temperature for approximately 60 minutes, and it is successively soaked, without rinsing, into palladium chloride water solution of 0.3wt.% concentration at room temperature for approximately 60 minutes, then to be taken out to be dried. Then the substrate is subjected to thermal treatment in flowing air at approximately 500 to 650 deg.C for approximately 3 hours, thereby providing a gas sensor excellent in sensitivity to carbon monoxide gas carrying palladium in tin oxide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a carbon monoxide gas sensor, and more particularly to a method for supporting palladium on tin oxide.

[0002]

2. Description of the Related Art A gas sensor using a tin oxide oxide semiconductor is known. This utilizes the fact that the electrical resistance of an oxide semiconductor changes in a test gas that is a reducing gas. The change in electric resistance in the reducing gas is further increased by supporting palladium on tin oxide. For example, JP-A-63-238456 discloses that when tin oxide is loaded with palladium, sensitivity to methane gas is improved. As a method of supporting palladium, a method of immersing tin oxide powder in an aqueous palladium chloride solution, followed by drying and firing, and then sintering the tin oxide powder is used. A method of heat-treating after drying is known.

[0003]

However, the conventional gas sensor as described above has a problem that it is not practical as a carbon monoxide gas sensor because it is not sufficiently sensitive to carbon monoxide gas harmful to the human body. The present invention has been made in view of the above points, and an object thereof is to provide a method of manufacturing a gas sensor that can be used for detecting carbon monoxide gas by improving a method of supporting palladium on tin oxide.

[0004]

According to the present invention, the above-mentioned object has a first step and a second step, wherein the first step involves immersing tin oxide in a solution of ammonium chloride to replace palladium. And the second step is a step of immersing the precursor in a palladium chloride solution and then calcining it.

[0005]

It is considered that when tin oxide is dipped in a solution of ammonium chloride, ammonium ion NH ₄ ⁺ is adsorbed by tin oxide and palladium ion Pd ²⁺ is ion-exchanged to form a precursor. When the precursor is dipped in a palladium chloride solution, NH ₄ ⁺ ions and palladium ions Pd ²⁺ are ion-exchanged and palladium is uniformly attached to the tin oxide surface. When the tin oxide to which palladium is attached is heat-treated, finely dispersed palladium fine particles are supported on the tin oxide surface.

[0006]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a sectional view showing a gas sensor according to an embodiment of the present invention. Electrode 4 on substrate 1 made of alumina
And the gas sensitive body 2 are laminated. A platinum heater 3 is formed on the other main surface of the substrate 1. The platinum heater 3 and the electrode 4 are formed by applying a platinum paste by a screen printing method and firing it. The gas sensitive body 2 is formed by adding a binder and a solvent to tin oxide powder to prepare a paste, coating the paste on an alumina substrate, and firing the paste at 650 ° C.

Palladium can be attached to the gas sensitive element 2 made of tin oxide as follows. First, an alumina substrate on which tin oxide is formed is immersed in an ammonium chloride aqueous solution having a concentration of 2% by weight at room temperature for 60 minutes. Subsequently, it is immersed in an aqueous palladium chloride solution having a concentration of 0.3% by weight for 60 minutes at room temperature without washing with water. The concentration of the ammonium chloride aqueous solution is preferably in the range of 0.1% by weight to 10% by weight, and the gas sensitivity to carbon monoxide decreases when the concentration is low or high. The concentration of palladium aqueous solution is 0.05
To 2.00% by weight is preferable, and the gas sensitivity to carbon monoxide is lowered when the concentration is low or high. Although an aqueous solution is used in the above system, it is also possible to use a non-aqueous solvent. The sample was taken out from the aqueous solution of palladium chloride, dried, and then placed in flowing air to obtain 5
Heat treatment is performed at a temperature of 50 to 650 ° C. for 3 hours. In this way, a gas sensor for carbon monoxide gas in which palladium is supported on tin oxide is obtained.

The gas sensitivity of the obtained gas sensor to carbon monoxide gas was measured according to a usual method. The temperature of the gas sensor at the time of measurement is 200 ° C. Letting R _{0 be} the electric resistance of the gas sensitive body in clean air and R _{g be} the electric resistance of the test gas containing carbon monoxide gas, R ₀ / R _g was taken as the gas sensitivity. FIG. 2 is a diagram showing the carbon monoxide gas concentration dependency (characteristic a) of the gas sensitivity of the gas sensor according to the embodiment of the present invention in comparison with the characteristic (characteristic b) of the conventional gas sensor. It can be seen that the gas sensor of the present invention has a gas sensitivity that is more than twice that of the conventional gas sensor.

[0009]

According to the present invention, there are a first step and a second step, and the first step is a step of immersing tin oxide in a solution of ammonium chloride to form a palladium-supported precursor. Since the second step is a step of immersing the precursor in a palladium chloride solution and then firing, tin oxide carries finely dispersed fine palladium particles, and as a result, the sensitivity to carbon monoxide gas increases. An excellent gas sensor can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view showing a gas sensor according to an embodiment of the present invention.

FIG. 2 is a graph showing the carbon monoxide gas concentration dependence (characteristic a) of the gas sensitivity of the gas sensor according to the embodiment of the present invention in comparison with the characteristic (characteristic b) of the conventional gas sensor.

[Explanation of symbols]

 1 substrate 2 gas sensor 3 platinum heater 4 electrode

Claims (4)

[Claims] 1. A first step and a second step, wherein the first step is a step of immersing tin oxide in a solution of ammonium chloride to form a palladium-substituted precursor, and a second step. Is a step of immersing the precursor in a palladium chloride solution and then calcining the precursor, and manufacturing the gas sensor. 2. The method according to claim 1, wherein the concentration of the ammonium chloride solution is in the range of 0.1 to 10% by weight. 3. The method of manufacturing a gas sensor according to claim 1, wherein the concentration of the palladium chloride solution is in the range of 0.05 to 2.00% by weight. 4. The method of manufacturing a gas sensor according to claim 1, wherein the firing is performed at a temperature in the range of 550 to 650 ° C.