JPH04218760A - Manufacture of gas sensor - Google Patents

Abstract

PURPOSE:To furnish a manufacturing method of a gas sensor which can monitor a carbonic acid gas continuously. CONSTITUTION:After a gas sensitive body constituted of an oxide semiconductor is formed on a substrate, it is impregnated with a noble metal salt solution and subjected to heat treatment, so that a noble metal is borne by the oxide semiconductor. Thereby the temperature whereat oxygen adsorbed on the oxide semiconductor reacts with a carbonic acid gas efficiently is shifted onto the high temperature side, adsorption of water in air is prevented and monitoring is conducted continuously.

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 gas sensor for detecting carbon monoxide gas using an n-type metal oxide semiconductor as a gas sensitive material.

0002

[Prior Art] When n-type metal oxide semiconductors such as tin oxide and zinc oxide are heated to a temperature of 200 to 500°C in the atmosphere, they activate and adsorb oxygen in the atmosphere on the surface of the oxide particles.
The surface becomes highly resistive. When activated and adsorbed oxygen comes into contact with flammable gas, the adsorbed oxygen reacts with the combustible gas, the adsorbed oxygen is removed, and the electrical resistance value decreases. Utilizing these properties, gas sensors using, for example, tin oxide semiconductors as gas sensing bodies are widely used in gas leak alarms for LP gas, city gas, and the like.

Another application of tin oxide based semiconductors to sensors is Sensor Technology Vol. 7, No. 13, '87
As described on page 44 of the December issue, there is a sensor for detecting carbon monoxide gas. This sensor is prepared by adding palladium (Pd) to a tin oxide semiconductor and sintering it.
Driven at sensor temperature below ℃. This is because the detection sensitivity of carbon monoxide gas is high at the above temperature.

0004

[Problems to be Solved by the Invention] However, although such a carbon monoxide gas sensor has high sensitivity, since it adsorbs moisture in the air, the response speed becomes slow as it is used, and the sensor temperature is periodically raised to 250°C or higher. It is necessary to raise the temperature and clean it, which makes it impossible to continuously detect carbon monoxide, and it also has problems such as a timer and two types of heater power supplies, which increases the cost. Intermittent measurements are insufficient to monitor incomplete combustion in water heaters, fan heaters, etc., and it is necessary to continuously detect carbon monoxide.

[0005] The present invention was made in view of the above points, and its purpose is to continuously monitor carbon monoxide gas by detecting carbon monoxide gas with high sensitivity at a high temperature that does not adsorb moisture. An object of the present invention is to provide a method of manufacturing a gas sensor that is possible.

[0006]

[Means for Solving the Problems] According to the present invention, the above-mentioned object has a first step and a second step, and the first step is to form a gas sensitive body made of an oxide semiconductor on a substrate. The second step is to impregnate the gas sensitive body with a noble metal salt solution,
This is achieved by drying and heat treating the gas sensitive material to support the noble metal.

[0007]

[Function] The supported noble metal accelerates the reaction between oxygen adsorbed on the surface of the oxide semiconductor and carbon monoxide gas at a certain temperature, but the noble metal catalyst produced by the production method of the present invention shifts this reaction temperature to a higher temperature side. let

[0008]

Embodiments Next, embodiments of the present invention will 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, in which a platinum electrode 5A is placed on a substrate 1 made of alumina.
, 5B, a heater 4, and a tin oxide gas sensitive body 2 supporting a noble metal.

[0010] Such a gas sensor is prepared as follows. Carbitol, ethyl cellulose, and colloidal silica were added to tin oxide powder with a particle size of 3 μm, and the mixture was kneaded and dispersed using a sieve machine to prepare a tin oxide paste. Next, using this paste, a gas sensitive member 2 was printed on a substrate 1 made of 96% alumina as shown in FIG. 1 by screen printing. Next, after drying at a temperature of 120° C. for 2 hours, heat treatment was performed in the air at a temperature of 700° C. for 15 minutes to sinter the gas sensitive body 2 on the substrate.

Next, palladium chloride dihydrate was dissolved in an acidic aqueous solution of hydrochloric acid, and the substrate 1 on which the gas sensitive member 2 was formed was immersed in this solution. After soaking, dry in the air at a temperature of 700℃.
The palladium chloride was decomposed by heat treatment for 30 minutes. The liquid concentration was adjusted so that the content of palladium in tin oxide was 0.5% by weight.

For comparison, a conventional sensor was fabricated as follows. The same tin oxide powder as above was mixed with a palladium chloride solution of a predetermined concentration and evaporated to dryness.
Palladium was precipitated on the tin oxide powder by heat treatment at 0° C. for 30 minutes. Here, the concentration and amount of palladium chloride added were adjusted to be 0.5% by weight of palladium based on the tin oxide powder. Next, carbitol, ethyl cellulose, and colloidal silica were added to this palladium-attached tin oxide powder to prepare a paste in the same manner as described above. Using this paste, a gas sensitive material 2 was printed on the alumina substrate 1 in the same manner as described above, and the gas sensitive material 2 was printed at a temperature of 120°C.
h, and was dried and heat-treated in the air at 700° C. for 15 minutes to sinter the gas sensitive body 2 on the substrate 1.

Gas sensitivity to carbon monoxide gas was measured for the sensor according to the manufacturing method of the present invention and the sensor according to the conventional method. For measurement, a current is applied to the heater 4 formed on the back side of the substrate 1 in Fig. 1, and the sensor temperature is set to 300°C.
was held at In addition, the gas sensitive body 2 has platinum electrodes 5A and 5B.
A DC voltage of 1 V was applied through the gas sensitive element 2, and the current flowing through the gas sensitive element 2 was determined from the terminal voltage of a load resistor connected in series with the gas sensitive element. The resistance of the gas sensitive body thus determined was defined as Rg in carbon monoxide gas and Ro in air, and (Ro/Rg) was defined as gas sensitivity.

Table 1 shows the gas sensitivity and 90% of the gas sensor according to the manufacturing method of the present invention and the gas sensor according to the conventional method.
Response times are shown in comparison. The measurement gas is carbon monoxide gas 2
The concentration was 00 ppm. Measurements were performed using 20 gas sensors, and the average value was calculated.

[0015]

[Table 1]

The gas sensor according to the manufacturing method of the present invention has about three times the gas sensitivity as compared to the gas sensor according to the conventional method. 9
The 0% response time for both is about 10 seconds, which is sufficient for practical use.

[0017]

According to the present invention, there are a first step and a second step, the first step is to form a gas sensitive body made of an oxide semiconductor on a substrate, and the second step is to The gas sensitive material is impregnated with a noble metal salt solution, then dried and heat-treated to support the precious metal on the gas sensitive material. Therefore, carbon monoxide is absorbed at a higher sensor temperature than conventional sensors without adsorbing moisture in the air. A gas sensor with high sensitivity to gas is obtained, and as a result, a gas sensor that can continuously monitor carbon monoxide gas at a high sensor temperature without the need to apply a temperature cycle is obtained.

[Brief explanation of the drawing]

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

[Explanation of symbols]

1　　　　Substrate 2 Gas sensitive body 4 Heater 5A Platinum electrode 5B Platinum electrode

Claims (4)

[Claims] Claim 1: A first step and a second step, wherein the first step is to form a gas sensitive body made of an oxide semiconductor on a substrate, and the second step is to form a noble metal on the gas sensitive body. 1. A method for producing a gas sensor, which comprises impregnating a salt solution, followed by drying and heat treatment to support a noble metal on a gas sensitive member. 2. The method of manufacturing a gas sensor according to claim 1, wherein the oxide semiconductor is tin oxide. 3. The method of manufacturing a gas sensor according to claim 1, wherein the gas sensitive body is formed by sintering. 4. The method of manufacturing a gas sensor according to claim 1, wherein the noble metal salt is palladium chloride.