JPH0252247A - Gas sensor - Google Patents

Abstract

PURPOSE:To selectively oxidize only the alcohol vapor which is a disturbing gas by coating the outside surface of a gas sensitive layer with an n type oxide semiconductor added with a noble metal and increasing the concn. of the noble metal in this coating layer to the concn. higher than the concn. of the catalyst in the gas sensitive layer. CONSTITUTION:A tin oxide layer 2 which is the gas sensitive layer and is added with about 0.1-1.0% platinum, palladium, etc., is provided on electrodes 11, 12 provided to an alumina substrate 3 and the coating layer 4 consisting of tin oxide powder added with the platinum, palladium, etc., at the ratio higher by about 2-10% than the concn. in the tin oxide layer 2 is formed by firing on the outside surface thereof. The alcohol vapor is oxidized away by this coating layer and the detecting gas is hardly oxidized. The sensitivity to the alcohol vapor is thus suppressed. The alcohol oxidation activity of the coating layer 4 decreases if the concn. of the platinum, etc., in the coating layer 4 is about <=2% and the alcohol sensitivity exceeds isobutane sensitivity at about <=500 deg.C. The isobutane oxidation activity of the coating layer 4 is significant at a high temp. if the concn. is about >=10% and therefore, the usable temp. region narrows.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention uses an n-type metal oxide semiconductor such as tin oxide or zinc oxide as a gas-sensitive layer, and is used as a sensor for interfering gases other than the target gas, such as LP gas. The present invention relates to a gas sensor that suppresses sensitivity to alcohol vapor and increases gas selectivity.

[Conventional technology]

When n-type metal oxide semiconductors such as tin oxide and zinc oxide are heated to a temperature of 300 to 500°C in the atmosphere, atmospheric oxygen is activated and adsorbed onto the particle surface, resulting in a high resistance, but they are not flammable. When a flammable gas comes into contact with the adsorbed oxygen, the adsorbed oxygen reacts with the combustible gas, the adsorbed oxygen is removed, and the resistance value decreases. Utilizing these properties, gas sensors using tin oxide can detect LP gas,
Widely used in gas leak alarms such as city gas.

[Problem to be solved by the invention]

It is known that this type of gas sensor exhibits a large change in resistance value even when exposed to alcohol vapor in the atmosphere. Resistance value/
By minimizing the resistance value (when in contact with alcohol vapor), LP
A gas sensor that can selectively detect gas, city gas, etc. is needed.

One method for solving the above problem is to coat the outer surface of the gas-sensitive layer with an oxidation catalyst layer that has the function of oxidizing and removing alcohol. In general, alcohol vapor has the property of being catalytically oxidized at a lower temperature on an oxidation catalyst supporting a noble metal than LP gas, and the above method utilizes this property to selectively remove alcohol vapor to the oxidation catalyst layer. It also has the role of a filter. Usually, as this oxidation catalyst, an activated alumina-supported noble metal catalyst in which a noble metal such as platinum or palladium is supported on activated alumina is said to exhibit high activity.

However, as a result of various studies conducted by the present inventors on oxidation catalysts for alcohol removal, it has been found that catalysts in which noble metals are supported on activated alumina have the following problems.

(b) Although it shows sufficient oxidizing activity for alcohol vapor, it also has some oxidizing activity for LP gas, so when the sensor temperature rises, the oxidation reaction of LP gas in the catalyst layer tends to proceed, and LP gas Sensitivity tends to decrease. As a result, the usable temperature range of the gas sensor becomes narrower.

(b) The adhesion strength of the oxidation catalyst layer covering this outer peripheral surface is lower than the adhesion strength of the tin oxide layer. For example, a method for producing a late-night type sensor that forms a tin oxide layer on an insulating substrate is to knead tin oxide powder containing precious metals with a solvent and a colloidal inorganic binder to form a paste, and then print this on an insulating substrate. Then bake at high temperature. Next, oxidation catalyst powder in which precious metals are supported on activated alumina is mixed with a solvent and a solvent in the same manner as above.
There is a method of kneading it with a binder to form a paste, applying it to the outer circumference of the tin oxide layer, and baking it by heating.

However, in the gas sensor manufactured by the above method, the adhesion strength of the oxidation catalyst layer is lower than that of the tin oxide layer. If this is incorporated into an alarm, the oxidation catalyst layer may peel off due to falling, vibration, etc., which is undesirable from the viewpoint of ensuring reliability. The details of the reason why the adhesion strength of the oxidation catalyst layer is lower than that of the tin oxide layer are unknown, but it is thought to be related to the fact that activated alumina has lower sinterability in the presence of a binder than tin oxide.

Therefore, the purpose of the present invention is to eliminate the above-mentioned conventional drawbacks, and to provide a coating layer coated on the outer peripheral surface of the gas-sensitive layer that can suppress the oxidation of the detection gas over a wide temperature range and selectively remove only alcohol vapor, which is an interfering gas. To obtain a gas sensor that can be oxidized and has high adhesive strength.

[Solve the problem. means for

According to the present invention, the present invention provides a gas sensor having a gas-sensitive layer made of an n-type oxide semiconductor doped with a noble metal, in which the outer surface of the gas-sensitive layer is coated with an n-type oxide semiconductor doped with a noble metal; This is achieved by making the noble metal concentration in the layer higher than the catalyst concentration in the gas sensitive layer.

It is generally believed that tin oxide itself has extremely low oxidation activity against alcohol vapor compared to an oxidation catalyst made of active alumina supporting noble metals.

On the other hand, in order to increase the gas sensitivity of tin oxide used in combustible gas sensors, approximately 0.1 to 1 trich of a platinum group element such as platinum or palladium is added as a sensitizer. However, with only a tin oxide sensitive gas layer to which an increase/decrease agent is added in an amount of 0.1 to 1 as described above, the sensitivity to alcohol vapor is 2 to 5 times higher than the sensitivity of LP gas, which is a detection gas. , alcohol vapor interference is too thick. As a result of repeated studies by the present inventors on methods for reducing the interference of alcohol vapor, we found that by increasing the amount of noble metal added to the tin oxide layer, which is made by adding platinum, palladium, etc. to tin oxide, to 2% or more, alcohol vapor interference can be reduced. It was found that it has steam oxidation activity. That is, a tin oxide layer containing 0.1-1% of platinum, palladium, etc. is formed on the gas-sensitive layer, and a tin oxide layer containing 2-10% of platinum, palladium, etc. is coated on the outer periphery of the layer. Alcohol vapor is oxidized and removed in the layer, and oxidation of LP gas, which is the detection gas, is suppressed.

[Effect]

In the present invention, the amount of platinum and palladium in the gas-sensitive layer is 0.1 to 1.
．． A coating layer containing 2 to 10% of platinum and palladium is formed on the outer surface of the 0% tin oxide layer, so alcohol vapor is oxidized and removed by this coating layer, and the detection gas is hardly oxidized. Sensitivity to alcohol vapor can be suppressed.

Furthermore, by using tin oxide as the material for the coating layer, the adhesion strength is greatly increased compared to a coating layer made of activated alumina.

〔Example〕

An embodiment of the present invention will be described below based on the drawings.

FIG. 1 is a sectional view of a gas sensor showing one embodiment of the present invention. The tin oxide layer 2 as a gas-sensitive layer is formed as follows. That is, tin oxide powder having a center particle size of 2 μm was impregnated with palladium chloride to a weight of 0.5 parts as palladium, and after drying, it was heated at 600° C. for 2 hours to decompose the palladium chloride. Next, water and silica sol were added to this powder to form a paste, and a tin oxide layer 2 was applied to the electrode 11.12 provided on the alumina substrate 1 shown in FIG. 1 to a thickness of about 70 μm. The tin oxide layer 2 on the alumina substrate 3 was baked by heating with C for 30 minutes. Next, the same tin oxide powder as above was impregnated with chloroplatinic acid to a concentration of 3% by weight of platinum, and heated at 60.0°C for 2 hours to decompose the platinum. After adding water and silica sol to this platinized tin oxide powder and making it into a paste,
A coating layer 4 of approximately 50 μm was applied to cover the tin oxide layer 2 shown in the figure. After drying at room temperature,
was heated for 30 minutes to form a coating layer 4 (this is used as a sensing element).

On the other hand, for comparison, 3% by weight as a coating layer in the above manufacturing method.
A device was prepared in which the platinum-supported γ-alumina powder was used to form a paste in the same manner as above, and the outer peripheral surface of the tin oxide layer 2 was coated with the paste (this was used as a comparative device). Next, a resistance value detection lead 51.5 is connected to the detection element and comparison element.
2 were connected to electrodes 11.degree. 12, and leads 71.degree. 72 were welded to the heater 6 for heating the element provided on the lower surface of the alumina substrate 1, and the gas sensitivity of the element was measured.

To measure gas sensitivity, the element is kept at a predetermined temperature using a heater 6, and the resistance value in the atmosphere (Ro) and the resistance value in 0.2% isobutane gas or 0.2% ethyl alcohol vapor are determined by By ( B) +"f (A), and the gas sensitivity was determined using the following formula. Here, isobutane gas is generally used as an index gas when evaluating the characteristics of an element with respect to LP gas.

FIG. 2 compares the dependence of gas sensitivity on sensor temperature between the sensing element of the present invention and a comparative element. Solid line A
, B are the temperature dependence of γB and γ of the sensing element, respectively,
Further, broken lines C and D indicate the temperature dependence of γBr of the comparative element, respectively. From FIG. 2, it can be seen that the reduction in isobutane sensitivity at high temperatures is significantly suppressed in the smart element whose coating layer is made of trithiplatinum and tin oxide compared to the comparative element made of 3% platinum and activated alumina. The temperature region 14 where the number of people is 5 or more and where one person is lower than γB is wider than the temperature regions 1 and 4' of the comparison element.

Next, in order to examine the characteristics of the sensing elements, the temperature dependence of gas sensitivity was measured for several types of elements obtained by the same manufacturing method as in the previous example but with different catalyst concentrations in the coating layer.

The results are shown in Table 1. For comparison, Table 1 shows the temperature range (ΔT) that satisfies rB to <rB and the temperature (T) at which γB=-.

Table 1 Relationship between catalyst concentration and properties of the coating layer As shown in Table 1, when the platinum concentration in the coating layer is less than 2%, the alcohol oxidation activity of the coating layer decreases, and below 500'C, the alcohol sensitivity is lower than the isobutane sensitivity. It exceeds. Moreover, if it is 10% or more, the isobutane oxidation activity in the coating layer becomes significant at high temperatures, so the usable temperature range (corresponding to ΔT) becomes narrow.

Therefore, it can be seen that the preferable concentration for a gas sensor is 2 to 10%.

Next, the detection element created in the above example was incorporated into a gas leak alarm device for normal use, and this was placed on a concrete floor at a height of 50 CTL.
After repeating the dropping operation from a height of 5 times, the sensing element was taken out and visually inspected for peeling of the laminated layer. The results are shown in Table 2. This test was conducted using 10 detection elements and 10 comparison elements, and the proportion of elements that achieved lfIJM1 was determined.

Table 2 Peeling frequency of coating layer in drop test As shown in Table 2, compared to the oxidation catalyst layer made of activated alumina, the coating layer made of the noble metal-tin oxide of the present invention has a higher adhesion strength and is more reliable in practice. I found out that I can get it.

〔Effect of the invention〕

According to this invention, the surface of the gas-sensitive layer made of a metal oxide is covered with a coating layer made of the same material as the gas-sensing layer and containing a noble metal at a higher concentration than that of the gas-sensitive layer, for example, 2 to 10%. This suppresses the decrease in sensitivity due to oxidation of the detection target gas at high temperatures, makes it possible to selectively oxidize and remove only alcohol vapor, and widens the warm summer range in which the sensor can be used. As a result, a very large effect can be obtained, such as narrowing the range of fluctuations in the scum detection sensitivity due to fluctuations in the power supply voltage, for example.

Furthermore, in the present invention, by using the same material as the gas-sensitive layer for the coating layer, it is possible to increase the adhesion strength of the coating layer, and high reliability against drop impact, vibration, etc. can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a gas sensor showing an embodiment of the present invention;
FIG. 2 is a characteristic diagram showing the temperature dependence of gas sensitivity of the gas sensor of the present invention. 2. Tin oxide layer, 4 Noble metal/tin oxide coating layer. 4θD UO, 9σθ Sanzu 1h seat ('C) No. 21 ¥1

Claims (1)

[Claims] 1) In a gas sensor whose gas-sensitive layer is an n-type oxide semiconductor doped with a noble metal, the outer periphery of the gas-sensitive layer is coated with an n-type oxide semiconductor doped with a noble metal, and the noble metal concentration of this coating layer is determined as the gas-sensitive layer. A gas sensor characterized by having a catalyst concentration higher than that of the layer.