JPH03248054A - Gas sensor - Google Patents

Abstract

PURPOSE:To restrict the deterioration of sensitivity due to the oxidation of a gas to be detected at high temperatures by forming an oxidative catalyst layer of a mixture of active alumina and zinc oxide, and adding noble metal at least to the active alumina. CONSTITUTION:A pair of platinum electrodes 2, 3 are provided at one surface of an alumina sensor substrate 1. A gas sensitive body 4 is provided astride the electrodes 2, 3. The surface of the sensitive body 4 is covered with an oxidative catalyst layer 5 in order to prevent the erroneous operation from steam of alcohol. For obtaining the catalyst layer 5, gamma-alumina powders are soaked in a solution of platinous chloride containing 1wt.% platinum, and dried, thereby to prepare powders wherein platinum is supported by gamma-alumina. The obtained gamma-alumina powders to which platinum is added is mixed with zinc oxide powders, and then water is added to the mixture. A paste of the mixture is applied to cover the sensitive body 4. An electric heater 8 is provided at the other surface of the substrate 1 to heat the sensitive body 4. In this manner, the deterioration of sensitivity due to the oxidation of a gas to be detected at high temperatures can be restricted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gas sensor using an n-type metal oxide semiconductor as a gas sensitive body.

[Conventional technology]

When n-type metal oxide semiconductors such as tin oxide and zinc oxide are heated to a temperature of 300℃ to 500℃ in the atmosphere, oxygen in the atmosphere is activated and adsorbed onto the surface of the oxide particles, resulting in a high resistance surface. do. When a combustible gas comes into contact with and is adsorbed here, the adsorbed oxygen reacts with the combustible gas, the NR-absorbing element is removed, and the electrical resistance value decreases. Utilizing these properties, gas sensors using, for example, zinc oxide-based semiconductors as gas sensing bodies are widely used in gas leak alarms for LP gas, city gas, and the like.

The cross-sectional structure of such a gas sensor is shown in FIG. A pair of platinum electrodes 2 on one side of an alumina sensor substrate 1.
A gas sensitive body 4 made of a zinc oxide semiconductor is provided across the gold electrodes 2 and 3 on both sides. The surface of this gas sensitive body 4 is coated with an oxidation catalyst layer 5 to prevent false alarms due to alcohol vapor. This oxidation catalyst layer 5 has the function of oxidizing alcohol vapor to carbon dioxide and preventing it from acting on the gas sensitive body 4. 6.7 is both days gold electrode 2゜3
This is the lead edge for the external drawer. An electric heater 8 for heating the gas sensitive body 4 is provided on the other side of the sensor base &1, and a lead edge 9. , 10 to the power supply. The reason why the gas sensitive body 4 is heated by the electric heater 8 is that when the gas sensitive body 4 is heated, the oxidation catalyst layer 5 covering it is also heated, increasing its catalytic activity and promoting the oxidation of alcohol vapor. The appropriate temperature is around 400°C.

[Problem to be solved by the invention]

In the above-mentioned gas sensor, as the oxidation catalyst layer 5, an active alumina-supported noble metal catalyst, in which a noble metal such as platinum or palladium is supported on activated alumina, has been conventionally used because it exhibits a high degree of activity.

However, the inventor of the present invention has conducted extensive research on oxidation catalysts for alcohol removal and has found that the above-mentioned activated alumina-supported metal catalyst has the following problems.

That is, the first problem is that although the active alumina-supported noble metal catalyst exhibits sufficient oxidizing activity against alcohol vapor, it also has some oxidizing activity against LP gas, which is the test gas. Gas sensor @! If the temperature becomes too high, the oxidation reaction in the oxidation catalyst layer 5 tends to proceed, and the detection sensitivity of the gas sensitive body 4 to LP gas tends to decrease.

As a result, there is a practical problem that the usable temperature range of the gas sensor becomes narrow.

Next, the second problem is that the sensor substrate 1 and the picture electrode 2,
Compared to the adhesion strength of the metal oxide semiconductor gas sensitive member 4 to the gas sensitive member 3, the adhesion strength of the oxidation catalyst layer covering the gas sensitive member 4 to the gas sensitive member is low.

This was revealed by the following experience. To form a zinc oxide-based semiconductor as a gas sensitive material, it is necessary to knead zinc oxide powder with water to form a paste, apply this onto a substrate, and then apply an inorganic material such as colloidal alumina or silica. After being impregnated with the binder, the zinc oxide gas sensitive layer is formed by baking at a temperature of 300° C. to 800° C., and then the active alumina-supported noble metal catalyst powder is made into a paste in the same manner as above to cover the gas sensitive layer. A prototype was created by coating, heating, and baking to form an oxidation catalyst layer.

However, as a result of trial manufacturing using this method, it was found that the strength with which the oxidation catalyst layer adheres to the gas-sensitive layer is weaker than the strength with which the gas-sensitive layer adheres to the sensor substrate. Although the details of the reason why the adhesion strength of the oxidation catalyst layer (active alumina-supported precious metal layer catalyst layer) is lower than that of the gas-sensitive layer (#l zinc oxide layer) are unknown.

This is thought to be related to the fact that activated alumina has lower sinterability in the presence of a binder than zinc oxide.

The above is because when a gas sensor is incorporated into a gas leak alarm, there is a possibility that the oxidation catalyst layer may peel off due to vibrations, drops, etc. during transportation of the gas leak alarm, and it is important to ensure the reliability of the product. Undesirable.

The purpose of the present invention is to coat the surface of a gas reactant made of a metal oxide semiconductor with an oxidation catalyst layer, and selectively oxidize easily combustible interfering gases such as alcohol vapor with this catalyst layer. A gas sensor with a configuration that increases selectivity for the gas to be detected suppresses oxidation of the gas to be detected over a wider temperature range, selectively oxidizes only the interfering gas, and has an adhesion strength of the oxidation catalyst that is within the range of optical spectroscopy in practical use. The objective is to configure the oxidation catalyst layer so that it has sufficient strength.

[Means to solve the problem]

In this invention, a gas sensor is configured as follows in order to achieve the above-mentioned object. That is, the surface of a gas sensitive body made of an n-type metal oxide semiconductor provided on one side of the sensor substrate is coated with an oxidation catalyst layer, and this catalyst layer oxidizes and removes combustible gases other than the components to be detected. In a configuration in which the gas sensitive body is heated to 77f+ by an electric heater provided on the other surface of the sensor substrate, the catalyst layer is made of a mixture of activated alumina and zinc oxide, and at least the activated alumina supports a noble metal.

[Effect]

Activated alumina has excellent oxidizing activity against combustible gases, and therefore partially oxidizes the LP gas, which is the gas to be detected, reducing the detection sensitivity of the gas sensor for LP gas, but it still functions as an oxidation catalyst. By supporting activated alumina with zinc oxide, which has almost no oxidation activity, it has the effect of suppressing oxidation activity. By mixing zinc oxide with activated alumina, the activated alumina particles are bonded to each other by highly sinterable zinc oxide particles, and the adhesion strength to the zinc oxide layer, which is a gas sensitive material, is increased.

〔Example〕

FIG. 1 is a sectional view of a gas sensor, and the present invention will be explained in detail based on FIG. A pair of platinum electrodes 2 and 3 are provided on one side of the sensor substrate 1 made of alumina, and the gold electrodes 2 and 3 are provided on both sides.
, 3, a gas sensitive body 4 is provided across them. This gas sensitive body 4
is created as follows. That is, the specific surface area is 10 m
/g, zinc oxide powder with a center particle size of 3μ and palladium 0.
It is impregnated with a palladium chloride solution having a concentration of 5% by weight, and after drying is heated at 600° C. for 2 hours to decompose the palladium chloride.

Next, this powder was mixed with water to form a paste, which was applied across the gold electrodes 2 and 3 on both days, dried at room temperature, impregnated with alumina sol binder, dried, and then heated at 750°C for 31) minutes. A gas sensitive body 4 was formed by baking on the alumina sensor base #i.

The palladium added to the zinc oxide powder is used to increase the resistance of zinc oxide to combustible gases. The bottom surface of the gas sensitive body 4 is coated with an oxidation catalyst layer 5 to prevent a fat layer from forming due to alcohol vapor, and the manufacturing method thereof is as follows. That is, the specific surface area is 150 i
/g, γ-alumina powder with a center particle size of 3P and platinum 1
% by weight of a chloroplatinic acid aqueous solution, dried, and then heated at 600° C. for 2 hours to prepare a powder in which platinum is supported on r-alumina. After mixing this platinum-added γ-alumina powder and zinc oxide powder so that the weight ratio was 1=1, water was added to make a paste, and this was mixed into a gas sensitive material 4.
It was coated to a thickness of 100p so as to cover the entire area.

In this state, after drying at room temperature, an alumina sol binder was impregnated, and after drying, the oxidation catalyst layer 5 was formed by heating at 730° C. for 9 minutes.

Reference numeral 6.7 denotes lead wires for resistance measurement drawn out from the gold electrodes 2 and 3 on both sides. An electric heater 8 for heating the gas sensitive body 4 is provided on the other surface of the sensor substrate l, and a lead wire 9 is provided.
, 10 to connect to a power source (not shown).

As mentioned above, in order to introduce the effect of the oxidation catalyst layer of the gas sensor, the following experiment was conducted. First of all
In this experiment, a device-2 was prepared for comparison with the abrogated catalyst layer 5 (hereinafter referred to as device-1) formed in the above example. This element-2 has a single layer and one layer in the manufacturing method of the element-1.
Using only γ-alumina powder supporting % platinum (vR
The paste is made into a paste by adding water (without mixing the zinc oxide powder), and the gas sensitive member 4 is coated with the paste. To measure the gas sensitivity of this pixel element, the element is heated with an electric heater 8 to maintain it at a predetermined temperature, and the electric resistance value Ro of the element in the atmosphere is 0.2.
% electrical resistance value ILg (B) in isobutane gas,
Electrical resistance value Rg in 0.2% ethyl alcohol vapor
It was determined by the following formula based on (A).

That is, inbutane gas sensitivity ra = Ro/Rg (
B) -Ethyl alcohol sensitivity γA: Ro/Rg
(A).

Figure 2 compares the dependence of gas sensitivity on the sensor temperature ('C) of Element-1 according to the present invention and Comparative Element-2.
The dashed curve θ shows the temperature dependence of the temperature.

0 indicates the temperature dependence of γB and γ of element-2, respectively. From this characteristic curve diagram, it can be seen that element-1, in which the oxidation catalyst layer is made of a mixture of γ-alumina and zinc oxide, has no zinc oxide.
- Compared to Element-2 made only of alumina, the decrease in isobutane gas sensitivity rs at high temperatures is significantly suppressed; for example, in the temperature range 1 where isobutane gas sensitivity Krm is 5 or more and ethyl alcohol g
1 is wider than the 81 degree region 12 of element-2.

Next, the second experiment was conducted using a 7-layer element obtained by changing the mixing ratio of r-alumina and zinc oxide in the oxidized catalyst IX layer 5 in the above-mentioned example and stubble manufacturing method. The leak alarm was assembled into a leak alarm (IL amount: 260 g), and the operation of dropping it from a vortex on the floor of a concrete IJ was repeated 5 times. After this, the device was oxidized for a few days, and TIi was oxidized.
This test was carried out by visual inspection to check whether there was any peeling of the layer.
- 10 elements each with alumina/zinc oxide mixing ratio were tested and the percentage of peeled elements was determined.
Extremely high adhesion strength can be obtained compared to alumina.

〔Effect of the invention〕

According to this invention, by making the oxidation catalyst layer a mixture of activated alumina and zinc oxide, it is possible to suppress a decrease in sensitivity due to oxidation of the gas to be detected at high temperatures, thereby widening the usable temperature range of the gas sensor. was completed. This has the extremely large advantage that, for example, the range of fluctuation in gas detection sensitivity due to fluctuations in power supply voltage can be narrowed.

Furthermore, the adhesion strength of the oxidation catalyst layer is increased by mixing zinc oxide, and peeling of the oxidation catalyst layer due to drop impact and vibration can be prevented.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a gas sensor, and FIG. 2 is a characteristic curve diagram showing the dependence of gas sensitivity on temperature of the gas sensor according to the present invention. 1: Sensor board, 4: Gas sensitive body, 5: Oxidation catalyst 1

Claims (1)

[Claims] 1) The surface of a gas sensitive body made of an n-type metal oxide semiconductor provided on one side of the sensor substrate is coated with an oxidation catalyst layer, and this catalyst layer oxidizes and removes combustible gases other than the components to be detected. , in a configuration in which the gas sensitive body is heated by an electric heater provided on the other surface of the sensor substrate, the catalyst layer is made of a mixture of activated alumina and zinc oxide, and at least a noble metal is added to the activated alumina. A gas sensor characterized by: