JPH04344450A - Gas sensor - Google Patents

Abstract

PURPOSE:To provide a gas sensor of high sensitivity and excellent stability by providing a gas sensitive layer and a covering layer on a substrate, and forming the gas sensitive layer as a sintered film of an oxide semiconductor, and the covering layer as a sintered film of an oxide semiconductor containing a catalyst. CONSTITUTION:Electrodes 11, 12, a gas sensitive layer 2 made of tin oxide, a covering layer 4, lead wires 51, 52 are provided on the respective portions of one main face of an alumina substrate 3 and a heater 6 and lead wires 71, 72 are provided on the respective portions of the other main face of the substrate 3. The gas sensitive layer 2 is a sintered film of an oxide semiconductor and the covering layer 4 is a sintered film of an oxide semiconductor containing a catalyst. The oxide semiconductor of the layer 2 has its electric resistance varied on contact with combustible gas. The catalyst-containing oxide semiconductor of the layer 4 generates activated oxygen and supplies it to the oxide semiconductor of the layer 2 which has no catalyst.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the structure of a gas sensor, and particularly to a gas sensor with excellent stability.

0002

2. Description of the Related Art As a gas sensor using an oxide semiconductor, for example, a thick film type gas sensor is known. FIG. 5 is a cross-sectional view showing a conventional thick film type gas sensor. In the thick film type gas sensor, a gas sensitive layer 2 such as tin oxide is formed on one main surface of a substrate 3, and a heater 6 for heating the gas sensitive layer 2 is formed on the other main surface. The gas-sensitive layer 2 is generally printed in a predetermined shape by thick film printing technology and then baked onto the substrate. 11 and 12 are electrodes, and 51, 52, 71, and 72 are lead wires.

When a gas-sensitive layer made of an n-type metal oxide semiconductor such as tin oxide or zinc oxide is heated in the atmosphere to a temperature of 300 to 500°C, oxygen in the atmosphere is activated and adsorbed on the particle surface, resulting in a high temperature. It becomes resistive, but when combustible gas comes into contact with it, the adsorbed oxygen and combustible gas react, the adsorbed oxygen is removed, and the resistance value decreases. Taking advantage of these properties, gas sensors using tin oxide are widely used in gas leak alarms for LP gas, city gas, and the like. A noble metal catalyst such as Pt or Pd is supported on the gas-sensitive layer 2 in order to increase the reaction rate between the oxygen adsorbed on the n-type metal oxide semiconductor and the combustible gas.

0004

However, although such conventional sensors have high sensitivity to combustible gases, there is a problem in that the sensitivity changes over time. The reason for this is that if a catalyst is involved in the desorption of oxygen, changes in the catalyst will affect the stability of the sensor.

The present invention has been made in view of the above-mentioned points, and its object is to provide a gas sensor that does not involve a catalyst in the desorption of oxygen and has high sensitivity to combustible gases and excellent stability. .

[0006]

[Means for Solving the Problems] According to a first embodiment of the present invention, the above-mentioned object has a gas-sensitive layer and a coating layer on a substrate, the gas-sensitive layer being a sintered film of an oxide semiconductor, This is achieved by configuring the coating layer to be a sintered film of an oxide semiconductor supporting a catalyst. According to a second embodiment of the present invention, the substrate has a gas-sensitive layer and a coating layer, the gas-sensitive layer is a sintered film of an oxide semiconductor supporting an oxide semiconductor and a catalyst, and the coating layer is a sintered film of an oxide semiconductor supporting an oxide semiconductor and a catalyst. It is characterized by being a sintered film of an oxide semiconductor supporting. Furthermore, according to a third aspect of the present invention, a gas-sensitive layer is provided on the substrate, and the gas-sensitive layer is a sintered film of an oxide semiconductor supporting an oxide semiconductor and a catalyst.

[0007]

[Operation] The oxide semiconductor changes its electrical resistance when it comes into contact with flammable gas. The oxide semiconductor supporting the catalyst produces activated oxygen and supplies it to the oxide semiconductor not having the catalyst. The oxide semiconductor supporting this catalyst also functions to remove alcohol interference.

[0008]

Embodiments Next, embodiments of the present invention will be described based on the drawings. FIG. 1 is a sectional view showing a gas sensor according to an embodiment of the invention. On one main surface of the alumina substrate 3, electrodes 11 and 12, a gas-sensitive layer 2 made of tin oxide, a coating layer 4,
Lead wires 51 and 52 are provided, and a heater 6 and lead wires 71 and 72 are provided on the other main surface. The gas-sensitive layer 2 is formed as follows.

That is, water and silica sol were added to tin oxide powder with a center particle diameter of 2 μm to form a paste, and then the gas-sensitive layer 2 was applied to a thickness of 60 μm on the electrodes 11 and 12 provided on the alumina substrate 3 (750). It was heated at ℃ for 30 minutes and baked onto an alumina substrate.

Next, the same tin oxide powder as above was impregnated with chloroplatinic acid to a concentration of 1 to 10% by weight of platinum, and heated at 600° C. for 2 hours to decompose the platinum. Water and silica sol were added to this platinum-supported tin oxide powder to form a paste.
A coating layer 4 was applied to a thickness of m. This was dried at room temperature and then heated at 730° C. for 30 minutes to form a coating layer 4.

FIG. 2 is a diagram showing the current conduction characteristics of the sensor resistance in combustible gas. Keep the sensor at a predetermined temperature using the heater 6, and set the sensor characteristics in 0.2% isobutane gas to Rg.
, R with the initial characteristic of the sensor as Rg0 and Rg0 as the reference
The time change of the value of the ratio Rg/Rg0 of g was investigated. Characteristic line 201 is the gas sensor of the present invention, characteristic line 202 is P
These are the characteristics of a conventional gas sensor using a gas-sensitive layer carrying d. The gas sensitivity Ra/Rg (Ra is the resistance of the sensor in air, Rg is the resistance of the sensor in 0.2% isobutane gas) is 10 to 30, which is sufficient sensitivity. If the sensor is constructed without supporting a catalyst, the gas sensitivity will be 2.
It is between 10 and 10. Zinc oxide can also be used instead of tin oxide.

FIG. 3 is an enlarged sectional view showing a gas sensor according to another embodiment of the invention. The gas sensitive layer is different from the above-mentioned gas sensor. The gas-sensitive layer is formed as follows. Water and silica sol are added to tin oxide powder with a center particle size of 2 μm to form a paste (paste A). Tin oxide powder was impregnated with chloroplatinic acid to a concentration of 1 to 10% by weight of platinum, and heated at 600° C. for 2 hours to decompose the platinum. Water and silica sol were added to this platinum-supported tin oxide powder to form a paste (paste B). Paste A and paste B are mixed in a weight ratio of 10:1. A gas-sensitive layer is formed using this mixed paste. This gas sensor is characterized by excellent stability and short initial response characteristics. Initial response time is 1.0 to 1.5 minutes. This initial response time depends on the mixing ratio of paste A and paste B, and is 0.5 to 1.0 minutes in the case of 10:2 and 0.5 minutes or less in the case of 10:5. This is because the catalyst-supported tin oxide produces activated oxygen and the efficiency of supplying it to the catalyst-free tin oxide is improved.

FIG. 4 is a sectional view showing a gas sensor according to still another embodiment of the present invention. It differs from the above-mentioned gas sensor in that it does not have a coating layer. This gas sensor is effective for alcohol, CO gas, etc. Excellent stability and short initial response time.

[0014]

According to the present invention, a substrate has a gas-sensitive layer and a covering layer, the gas-sensitive layer is a sintered film of an oxide semiconductor, and the covering layer is a sintered film of an oxide semiconductor supporting a catalyst. Forming as a sintered film, or having a gas-sensitive layer and a covering layer on a substrate, the gas-sensitive layer being a sintered film of an oxide semiconductor supporting an oxide semiconductor and a catalyst, and the covering layer supporting a catalyst. The oxide semiconductor can be formed as a sintered film of an oxide semiconductor, which has a gas-sensitive layer on the substrate, and the gas-sensitive layer is formed as a sintered film of an oxide semiconductor supporting an oxide semiconductor and a catalyst. When it comes into contact with a flammable gas, the electrical resistance changes, and the catalyst-supported oxide semiconductor creates activated oxygen, which is supplied to the catalyst-free oxide semiconductor.
A gas sensor with excellent sensitivity and stability can be obtained.

[Brief explanation of drawings]

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

FIG. 2 is a diagram showing the current conduction characteristics of the sensor resistance in flammable gas of the gas sensor according to the embodiment of the present invention.

[Figure 3
] An enlarged sectional view showing a gas sensor according to a different embodiment of the present invention.

FIG. 4 is a sectional view showing a gas sensor according to still another embodiment of the present invention.

[Figure 5] Cross-sectional view showing a conventional thick-film gas sensor

[Explanation of symbols]

2 Gas-sensitive layer 3　　　　Substrate 4 Coating layer 6 Heater 11 Electrode 12 Electrode 51 Lead wire 52 Lead wire 71 Lead wire 72 Lead wire

Claims (6)

[Claims] Claim 1: A gas-sensitive layer and a covering layer are provided on a substrate, the gas-sensitive layer is a sintered film of an oxide semiconductor, and the covering layer is a sintered film of an oxide semiconductor supporting a catalyst. Characteristic gas sensor. 2. A gas-sensitive layer and a covering layer are provided on the substrate, the gas-sensitive layer is a sintered film of an oxide semiconductor supporting an oxide semiconductor and a catalyst, and the covering layer is a sintered film of an oxide semiconductor supporting an oxide semiconductor and a catalyst. A gas sensor characterized by being a sintered semiconductor film. 3. A gas sensor comprising a gas-sensitive layer on a substrate, the gas-sensitive layer being a sintered film of an oxide semiconductor supporting an oxide semiconductor and a catalyst. 4. The gas sensor according to claim 1, wherein the oxide semiconductor is tin oxide. 5. A gas sensor according to claim 1, wherein the catalyst supports 1.0 to 10.0% by weight of platinum. 6. The gas sensor according to claim 2 or 3,
A gas sensor characterized in that the proportion of the oxide semiconductor supporting the catalyst in the sintered film is in the range of 5 to 50% by weight.