JPH095282A - Contact-buring type gas sensor and its manufacture - Google Patents

Abstract

PURPOSE: To provide a contact-burning type gas sensor by which the concentration of a low-concentration combustible gas in a gas including incombustible gas elements such as carbon dioxide, etc., can be measured accurately. CONSTITUTION: A contact-burning type gas sensor is provided with a detection element and a comparison element, and the detection element is formed by arranging a γ-alumina depositing a catalyst around a metallic wire, while the comparison element is formed by arranging an α-alumina around the metallic wire. When assuming that the mass of the γ-alumina is the detection element is 100, that of the α-alumina in the comparison element is more than 70 and less than 130.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalytic combustible gas sensor for detecting carbon monoxide in the environment or combustible gas such as methane gas.

[0002]

2. Description of the Related Art A catalytic combustible gas sensor usually has two elements, a sensing element for measuring the concentration of combustible gas and a comparison element for performing temperature compensation. Here, an example of the sensing element will be described with reference to FIG. FIG. 1 is a diagram showing a state in which a part of the ceramic carrier 2 of the detection element 3 is removed.
In the figure, reference numeral 1 is a metal wire made of platinum or the like, and 2 is a ceramic carrier such as alumina carrying a catalyst such as rhodium, palladium or platinum for catalytically burning a combustible gas for measurement.

The comparison element has the same structure as the above-mentioned detection element except that the ceramic coating layer does not contain a catalyst. The detection element A and the comparison element B are shown in FIG.
As shown in (1), a bridge circuit is formed with the electric resistance R and the electric resistance R ′ having the same resistance value, and the contact combustion gas sensor is formed with the power source D. In addition, t in FIG.
Is a terminal and is connected to display means such as a meter.

In the conventional catalytic combustion type sensor, γ-alumina has been used as a ceramic carrier. However, due to the catalytic activity of γ-alumina itself, combustion heat of the combustible gas is generated even in the comparative element, and as a result, the sensitivity is lowered. In order to solve this drawback, the comparative element was subjected to heat treatment to convert γ-alumina into α-alumina having no catalytic activity, but it was found that this can prevent a decrease in sensitivity.

However, the catalytic combustion type gas sensor comprising the comparison element having the α-alumina layer and the detection element having the catalyst-supporting γ-alumina layer is suitable for a gas containing a non-combustible component such as carbon dioxide gas. Also has sensitivity. Therefore, when the concentration of such an incombustible component is high and the concentration of the target combustible gas is low, the measurement becomes difficult.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a catalytic combustion type gas sensor which can also measure the concentration of a low concentration combustible gas in a gas having a non-combustible component such as carbon dioxide.

[0007]

[Means for Solving the Problems] As described above, catalyst-supporting γ
A contact gas sensor consisting of a sensing element with an alumina layer and a comparative element with an α-alumina layer is sensitive to non-combustible components such as carbon dioxide. As a result of various investigations on the cause, the following was found.

The coating layers of the sensing element and the comparison element are both γ-
In the case of using alumina, these coating layers were formed in the same shape and size in order to equalize the thermal behaviors of both and eliminate the influence on the measured value of temperature. The relationship between the size and the shape of each of the detection element and the comparison element was inherited even when the coating layer of the comparison element was formed by α-alumina, but as a result of the investigation, it was found that this was the case. It was found to cause noise due to non-combustible gas

That is, it has been found that, when the sensing element having the α-alumina coating and the comparative element having the γ-alumina coating have the same size, their behaviors with respect to changes in the concentration of nonflammable gas such as carbon dioxide are different.

The present invention solves the above problems by making the behaviors of these elements even with respect to changes in the concentration of non-combustible gas such as carbon dioxide. That is, in the catalytic combustion gas sensor of the present invention, the detection element is a γ-
Alumina is arranged around the metal wire, and the comparison element is formed by arranging α-alumina around the metal wire, and the mass of γ-alumina in the detection element is 100. The mass of the α-alumina supporting the catalyst in the comparative element at that time is 70 or more and 130 or less.

Here, the sensing element having the α-alumina coating layer can be obtained by a known method. For example, it can be obtained by molding a coating layer of α-alumina obtained by firing γ-alumina at 1100 ° C. or higher around a metal wire.

In the present invention, it is necessary that the mass of α-alumina supporting the catalyst in the comparative element is 70 to 130 when the mass of γ-alumina in the detecting element is 100. If the mass of α-alumina is less than 70 or more than 130, noise due to the influence of non-combustible gas such as carbon dioxide in the gas to be measured becomes large, and the reliability of the measured value of the combustible gas as the measurement target gas is high. Inferior in sex.

The combustible gas which is the measuring gas is
In the case of toxic carbon monoxide, a strict measurement value at a low concentration is required, so that the mass of α-alumina is 85 or more and 115 or less when the mass of γ-alumina in the sensing element is 100. It is desirable to have such a strict measurement value because it is possible to sufficiently deal with such a measurement value.

Here, MCA of carbon monoxide (allowable concentration:
It is said that the maximum concentration at which poisoning does not occur even if exposed to poisonous substances for 8 hours in moderate labor) is 100 ppm, and strict measurement below this concentration is required. On the other hand, carbon dioxide is said to have no adverse effect on the human body when the concentration is about 5%.

[0015]

Since the comparative element in the catalytic combustion type gas sensor of the present invention has α-alumina arranged around the metal wire, it has no sensitivity to combustible gases such as carbon monoxide, so that the catalyst supported γ -It is possible to make full use of the sensitivity of the sensing element formed by arranging alumina around the metal wire to combustible gas.

Further, since the mass of α-alumina in the comparative element is 70 or more and 130 or less when the mass of γ-alumina in the detecting element is 100, the influence of noncombustible gas such as carbon dioxide in the gas to be measured. Less is.
The factor that the non-combustible gas such as carbon dioxide affects the element is considered to be due to the difference in heat conduction between these elements and the non-combustible gas. It is believed that the configuration of the present invention that adjusts the layer mass adjusts for the difference in heat capacities of these devices.

[0017]

[Example] First, a platinum wire having a diameter of 25 μm was used and a coil diameter was 1 m.
A coil with m and 16 turns was created. Next, γ-alumina obtained by firing boehmite at 700 ° C. has a particle size of 1 to
After crushing to 10 μm and supporting metal rhodium on this, 10% by weight of an alumina-based binder was added, and water was added to the extent that it had an appropriate viscosity, followed by kneading into a paste.

This alumina paste was formed into a spherical shape on the platinum coil and then fired at 700 ° C. to obtain a sensing element T. First, the sensitivity of the sensing element T to carbon monoxide was examined. That is, the relationship between the voltage (element output) and the concentration of carbon monoxide in the air when the means for converting the change in the resistance value of the detection element T into a voltage was used was investigated. The results are shown in FIG.

From FIG. 3, it can be seen that the output of the detection element T per 100 ppm of carbon dioxide corresponds to 0.3 mV. In the following, when the element output is expressed by voltage, the same resistance value-voltage conversion means as used here was used.

Next, a comparative element was produced. 1 boehmite
The particle size of α-alumina obtained by firing at 100 ° C. is 1 to 10
It is pulverized to have a particle size of
% By weight, and after adding water to an extent that it has an appropriate viscosity, kneading into a paste, forming the same platinum coil used in the sensing element T into a spherical shape, and then firing at 700 ° C.,
Comparative devices a, b, c, d, e and f were obtained. The results of examining the particle size and mass of these coating layers are shown in Table 1.

[0021]

[Table 1]

The sensing element T and the comparative elements a to f were brought into contact with 5% or 10% carbon dioxide in nitrogen, and the output was obtained as a voltage. Detection element T at this time
FIG. 4 shows the difference between the output of each of the detection elements a to f and the output of each of the detection elements a to f. From FIG. 3, as compared with the case where the comparative element a which is a conventional example is used, in the case where the comparative elements b, c, d, e and f which are Examples 1 to 5 according to the present invention are used, It can be seen that the influence of the change in carbon concentration is small.

Further, using the sensing element T and the comparison element d, the same circuit as shown in FIG. 2 is formed by using them, and a catalytic combustion type gas sensor I is manufactured. The sensitivity to the carbon oxide concentration was examined, and the influence of temperature at that time was examined in the range of 10 to 40 ° C. The result is a catalytic combustion type gas sensor according to a conventional technique, that is, a gas sensor formed by using a detection element T and a comparison element a.
Comparing with the results of II, it was found that the gas sensor II according to the related art has the same performance in terms of sensitivity to carbon monoxide in air and influence of temperature.

Further, when the concentration of carbon monoxide in air (20 ° C.) containing 5% carbon dioxide and 100 ppm of carbon monoxide was measured for these gas sensors I and II, the gas sensor I showed no carbon dioxide. The same output as in the case of 100 ppm of carbon monoxide was obtained, but with the gas sensor II, it was 3
The output corresponds to carbon monoxide at a concentration of 80 ppm,
The exact value was not obtained.

[0025]

As described above, the catalytic combustion gas sensor according to the present invention has a carbon monoxide concentration equivalent to that of the conventional gas sensor, but has a non-combustible gas concentration such as carbon dioxide in the gas to be measured. It is an excellent one with little effect.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure in an example of a detection element of a contact combustion gas sensor.

FIG. 2 is a diagram showing a circuit of a catalytic combustion gas sensor.

FIG. 3 is a diagram showing the sensitivity of the sensing element to the carbon monoxide concentration.

FIG. 4 is a diagram showing a difference in element output between a sensing element and each comparative element in Examples and Comparative Examples.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Metal wire 2 Catalyst carrying ceramic carrier 3 Detection element 4 Comparison element D Power supply R, R'Electrical resistance t Output terminal

Front page continuation (72) Inventor Kazutoshi Agata 23 Kashima, Minamata-cho, Tenryu City, Shizuoka Prefecture Yazaki Keiki Co., Ltd. (72) Inventor Masanori Enomoto 3-4 Fukamidai, Yamato-shi, Kanagawa Prefecture (72) Inventor Shingo Kimura 3-4, Fukamidai, Yamato-shi, Kanagawa Prefecture, inside Gaster Co., Ltd. (72) Takuji Shigeoka, 3-4, Fukamidai, Yamato-shi, Kanagawa Inside Gaster Co., Ltd.

Claims (5)

[Claims] 1. A catalytic combustion gas sensor comprising a detection element and a comparison element, wherein the detection element carries a catalyst γ
-Alumina is arranged around the metal wire, and the comparison element is formed by arranging α-alumina around the metal wire, and the mass of γ-alumina in the detection element is 100. The catalytic combustion gas sensor, wherein the mass of α-alumina in the comparative element is 70 or more and 130 or less. 2. The catalytic combustion type gas sensor according to claim 1, wherein the mass of α-alumina in the comparative element is 85 or more and 115 or less when the mass of γ-alumina in the detecting element is 100. 3. The catalytic combustion gas sensor according to claim 1, wherein the gas to be detected is carbon monoxide. 4. A method for manufacturing a catalytic combustion gas sensor, which comprises a detection element having a ceramic coating layer supporting a catalyst and a comparison element having a ceramic coating layer, and a mass ratio of the detection element, the comparison element and each coating layer. Is adjusted to a range in which the concentration of the noncombustible gas in the gas to be measured does not affect the sensor output, and a method for manufacturing a catalytic combustion gas sensor. 5. A method of manufacturing a catalytic combustion gas sensor comprising a detection element having a ceramic coating layer supporting a catalyst and a comparison element having a ceramic coating layer, wherein a heat capacity ratio between the detection element, the comparison element and each coating layer. Is adjusted to a range in which the concentration of the noncombustible gas in the gas to be measured does not affect the sensor output, and a method for manufacturing a catalytic combustion gas sensor.