JPH01197646A - gas sensor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gas sensor used in an LP gas leak alarm.

[Conventional technology]

It is generally widely known that the electrical resistance value of tin oxide semiconductors changes depending on the gas (the electrical conductivity improves), and gas sensors that take advantage of this property are often used.

FIG. 5 is a sectional view showing the configuration of a gas sensor for detecting LP gas, in which a pair of platinum electrodes 2.3 are provided on the surface of a sensor substrate (alumina substrate) l, and both platinum electrodes 2.3 are connected to a tin oxide based semiconductor. The gas sensitive body 4 is connected. This gas sensitive body 4
The surface of the oxidation catalyst N is coated with alumina powder supporting platinum to prevent false alarms caused by ethyl alcohol.
5 is formed. This oxidation catalyst layer 5 has the function of oxidizing ethyl alcohol to carbon dioxide and not acting on the gas sensitive body 4 of the tin oxide based semiconductor inside the oxidation catalyst layer 5. 6.7 is a lead wire for leading both platinum electrodes 2.3 to the outside. An electric heater 8 for heating the gas sensitive body 4 made of a tin oxide semiconductor is provided on the back surface of the sensor substrate 4, and is connected to a power source through lead wires 9.about.10. The reason why the gas sensitive body 4 made of a tin oxide semiconductor is heated with the electric heater 8 is that by heating the gas sensitive body 4, the oxidation catalyst layer is also heated, its catalytic activity is increased, and the oxidation of ethyl alcohol is promoted. The temperature is 400°
A value around C is considered appropriate.

[Problem to be solved by the invention]

When using the gas sensor for detecting LP gas described above, the lead wires 9.10 of the electric heater 8 are connected to a heater power source (not shown) to energize the electric heater 8, and the platinum electrodes 2.3
Connect the lead wires 6 and 7 in series with a detection power supply and a load resistor (not shown) to form a detection circuit, or connect the lead wires 6 and 7 directly to the alarm (inside the power supply) not shown. Connecting. When the gas to be detected comes into contact with the gas sensing body 4 made of a tin oxide semiconductor of the gas sensor, the electrical resistance value of the semiconductor changes and the current flowing through the detection circuit changes, resulting in a change in the voltage between the terminals of the load resistor. Gas can be detected by capturing it. In the case of an alarm, the alarm is directly activated by a change (increase) in the current flowing through the semiconductor due to contact with gas, and the gas can be detected.

In this case, if the sensor temperature rises due to ambient conditions or other reasons, the gas sensor will oxidize isobutane gas in the same way as ethyl alcohol, and the amount of gas that should be detected will become oxidized.
This method has the disadvantage of lowering the detection sensitivity of isobutane, which is the main component of P gas. To explain this using a curve diagram, first, in order to determine gas-sensitive characteristics, a gas sensor was assembled as shown in the perspective view of FIG. 6. That is, the electrode stem 12.1 has the lead wire 6.7 of the platinum electrode 2.3 and the lead wire 9.10 of the electric heater 8 implanted in the base 11, respectively.
3 and heater stems 14 and 15 to determine gas sensitivity characteristics. At this time, the electrical resistance value of the gas sensor (gas sensitive body 4 made of a tin oxide semiconductor) in the air is Ro, 0
．． The electrical resistance value in 2% isobutane gas or 0.2% ethyl alcohol gas was defined as Rg, and Ro/Rg was defined as gas sensitivity. The temperature of the gas sensor is measured with an infrared radiation thermometer, and the voltage applied to the electric heater 8 is adjusted to 350.
The temperature was changed from °C to 400 °C to 450 °C. FIG. 7 shows the gas sensitivity of the gas sensor at each temperature at that time. Curve P is the gas sensitivity of 0.2% ethyl alcohol gas (
R.

/Rg) and the temperature of the gas sensor is 400°C to 45°C.
At 0'C, the gas sensitivity (Ro/Rg) decreases and there is an effect that it does not affect the gas sensitive body of the tin oxide semiconductor. Although Ro/Rg) is good when the gas sensor temperature is low, it rapidly decreases when the temperature exceeds 400°C, and is therefore the main component of LP gas. Detection of isobutane gas becomes difficult.

In view of the above-mentioned reasons, this invention aims to improve the structure of the gas sensor, especially the oxidation catalyst layer, so as to reduce the alcohol sensitivity over a wide temperature range while not reducing the gas sensitivity to isobutane gas, which is the main component of LP gas. purpose.

[Means to solve the problem]

In order to solve the above-mentioned problems in this invention, the oxidation catalyst layer was constructed as follows based on the experimental results.

That is, the oxidation catalyst layer on the surface of the gas sensitive member was formed by mixing platinum and cobalt and supporting it on alumina. In addition, the platinum and cobalt supported in this oxidation catalyst layer are 0.2 to 4 wt% of platinum and 1 to 10 wt% of cobalt, and the supported weight ratio of platinum and cobalt is 0.25 to 50. It was also confirmed that it is favorable.

[Effect]

When an oxidation catalyst layer made of a mixture of platinum and cobalt supported on alumina is provided on the surface of a gas sensitive material made of a tin oxide semiconductor,
It oxidizes and removes ethyl alcohol gas over a wide temperature range and reduces its sensitivity, yet the isobutane gas sensitivity remains unchanged.

〔Example〕

FIG. 1 is a front view of a gas sensor according to an embodiment of the present invention, FIG. 2 is a back view of the same gas sensor, and FIG. 3 is a sectional view taken along the line A--A in FIG. In the figure, 101 is a sensor substrate (alumina substrate), 102 and 103 are a pair of platinum electrodes, 104 is a gas sensitive body made of a tin oxide semiconductor, 105 is an oxidation catalyst layer, 106 and 107 are platinum electrode lead wires, 10
8 is a platinum heater, and 109° and 110 are heater lead wires.

This gas sensor is manufactured as follows.

First, platinum electrodes 102 and 103 are formed on the front surface of a sensor substrate (alumina substrate) 101, and a platinum heater 108 is formed on the back surface of the sensor substrate (alumina substrate) 101, respectively, by baking, and this pair of platinum electrodes 102.
A tin oxide layer is formed on the surface of 103 by reacting tin vapor and oxygen by arc discharge. This tin oxide layer becomes the gas sensitive body 104.

The tin oxide layer may be formed by other methods, such as mixing tin oxide powder with a binder to form a paste, applying the paste, and sintering the paste.

Next, the oxidation catalyst layer 1 covers the surface of the gas sensitive body 104.
05 is formed as follows. That is, the surface of the gas sensitive body 104 is coated with an alumina paste made by mixing alumina powder and colloidal alumina, and then dried. After drying, it is fired at 550° C. for 3 hours to form an alumina layer with a thickness of 0.3 mm. This was then immersed for 30 minutes in a mixed aqueous solution prepared by mixing a chloroplatinic acid aqueous solution and a cobalt nitrate aqueous solution to a predetermined concentration, and then dried in air for 60 minutes.
When thermally decomposed at 0°C for 3 hours, platinum and cobalt are mixed to obtain an oxidation catalyst layer supported on alumina. In this example, an alumina layer was formed on the surface of the gas sensitive body 104 and then platinum and cobalt were supported thereon. Mix and apply this to the gas sensitive body 10
Similar effects can be obtained by coating the surface of 4 to form an oxidation catalyst layer 105. In this way, the oxidation catalyst layer 105
When the gas sensitivity of the gas sensor according to the embodiment of the present invention in which the gas sensor is formed is determined by the same test method and test conditions as the conventional gas sensor described above, the characteristic curve diagram shown in FIG. 4 is obtained. is the gas sensitivity (Ro/Rg) of 0.2% ethyl alcohol gas, and there is no significant difference in the effect of reducing the ethyl alcohol gas sensitivity compared to the conventional curve P shown in FIG. However, regarding the isobutane gas sensitivity, as shown by the conventional curve Q in FIG. 7, when the temperature of the gas sensor becomes high (the isobutane gas sensitivity decreases dangerously), the curve S of the gas sensor according to the present invention shown in FIG.
In this case, even if the temperature of the gas sensor increases, the isobutane gas sensitivity hardly changes and remains constant.

According to experiments, such an effect is observed when the weight ratio of platinum to cobalt supported in the oxidation catalyst layer is in the range of 0.25 to 50, and the amount of platinum supported is 0.2 to 4 wt%.

The effect is recognized when cobalt is in the range of 1 to 10 wt%.

This is because when the amount of platinum and cobalt supported is small, the sensitivity to ethyl alcohol becomes high, and when the amount of supported platinum and cobalt is large, the sensitivity to isobutane becomes low, which is not preferable for gas detection.

Furthermore, it is not clear why the isobutane gas sensitivity of the gas sensor that has the above-mentioned oxidation catalyst layer made of a mixture of platinum and cobalt supported on alumina is constant regardless of temperature, but cobalt has oxidation activity against ethyl alcohol. Since it is inert to isobutane, it can be inferred that this is to suppress the oxidation activity of platinum at high temperatures.

〔Effect of the invention〕

According to this invention, in a gas sensor using a tin oxide semiconductor as a gas sensitive material, an oxidation catalyst layer made of a mixture of platinum and cobalt supported on alumina is formed on the surface of the gas sensitive material, thereby reducing sensitivity to ethyl alcohol gas. , and the isobutane gas sensitivity remains constant regardless of temperature.
It is possible to obtain a gas sensor that prevents false alarms due to ethyl alcohol and does not reduce sensitivity to isobutane gas, which is the main component of LP gas.

[Brief explanation of the drawing]

FIG. 1 is a front view of a gas sensor that is an embodiment of the present invention, FIG. 2 is a back view of the same gas sensor, FIG. 3 is a sectional view taken along the line A-A in FIG. A characteristic curve diagram showing the temperature dependence of gas sensitivity of isobutane and ethyl alcohol. Figure 5 is a cross-sectional view of a conventional gas sensor.
FIG. 6 is an assembled configuration diagram of the gas sensor, and FIG. 7 is a characteristic curve diagram showing the temperature dependence of the gas sensitivity of isobutane and ethyl alcohol in a conventional gas sensor. 101... Sensor board, 102, 103... Electrode,
104... Gas sensitive body, 105... Oxidation catalyst layer, 1
0B...Electric heater. lol 1 Figure 5 m Figure 6 D Figure 7

Claims (1)

[Claims] 1) A gas sensitive body made of a tin oxide semiconductor is electrically connected to a pair of electrodes mounted on the surface of the sensor substrate, an oxidation catalyst layer is formed on the surface of this gas sensitive body, and the gas sensitive body is formed on the back side of the sensor substrate. A gas sensor equipped with an electric heater for heating a sensing element, wherein the oxidation catalyst layer on the surface of the gas sensing element is formed by mixing platinum and cobalt and supporting it on alumina.