JPH03162656A - Gas sensor - Google Patents

Abstract

PURPOSE:To obtain the gas sensor having a high sensitivity with combustible gases and having excellent stability as well by providing a gas sensitive layer and a coating layer on a substrate, consisting the gas sensitive layer of a tin oxide semiconductor and forming the coating layer of a tin oxide semiconductor on which a noble metal is deposited at 2 to 10 wt.%. CONSTITUTION:Electrodes 11, 12, the gas sensitive layer 2 consisting of tin oxide, the coating layer 4, and lead wires 51, 52 are provided on one main surface of the alumina substrate 3 and a heater 6 and lead wires 71, 72 are provided on the other main surface. The gas sensitive layer 2 is formed by adding water and silica sol to tin oxide powder and forming the powder to a pasty state, then applying the paste on the electrodes 11, 12 provided on the substrate 3 and heating the coating, thereby baking the paste onto the substrate 3. For example, chloroplatinic acid is then impreg nated in the tin oxide powder so as to attain 2 to 10 wt.% as platinum and the powder is heated to decompose the platinum. The water and silica sol are added to the tin oxide powder deposited with the platinum and after the mixture is made into the pasty state, the paste is applied to cover the gas sensitive layer 2 to form the coating layer 4. The coating is heated after drying at ordinary temp., by which the coating layer 4 is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gas sensor for LP gas, city gas, etc., and particularly to a gas sensor with high gas sensitivity and excellent stability. [Prior art] Tin oxide. When n-type metal oxide semiconductors such as 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. Upon contact, the adsorbed oxygen and combustible gas react, removing the adsorbed oxygen and decreasing the resistance value. Utilizing these properties, gas sensors using tin oxide are widely used in gas leak alarms for LP gas, city gas, etc.
In conventional gas sensors, in order to increase the sensitivity to flammable gases, platinum. It supports noble metals such as palladium and is used as a gas-sensitive layer. [Invention tries to solve it! III) However, although such conventional sensors have high sensitivity to flammable gases, there is a problem in that the sensitivity changes over time. This invention has been made in view of the above points, and its purpose is to provide a gas sensor that is highly sensitive to combustible gases and has excellent stability by improving the sensor structure in which precious metals are supported. [Means for Solving the Problems] According to the present invention, the above-mentioned object has a gas-sensitive layer and a covering layer on a substrate, the gas-sensitive layer is made of a tin oxide semiconductor, and the covering layer is made of a tin oxide semiconductor. This is achieved by assuming that the metal supports 2 to IO weight percent of precious metals. [Operation] Some oxidative combustion of combustible gas occurs inside the gas-sensitive layer where the catalyst is supported, but this oxidative combustion decreases as the catalytic activity of the gas-sensitive layer deteriorates, and the effective flammability inside the gas-sensitive layer decreases. It is estimated that the gas concentration increases over time. Since the coating layer has a high catalyst concentration, it is estimated that there is no change in oxidative combustion. [Example] Next, an example of this invention will be explained based on the drawings. 1 is a schematic cross-sectional view showing a gas sensor according to an embodiment of the present invention.An electrode 11 is placed on one main surface of the alumina base Fi3.
, 12, gas-sensitive layer 2 made of tin oxide, coating layer 4, lead wire 51. 52 also has a heater 6 and a lead AI71.52 on the other main surface. 72 will be provided. The mysterious gas layer 2 takes shape as follows. That is, water and silica sol were added to tin oxide powder with an average particle size of 2n to form a paste, and then the first
Electrode 11 provided on the alumina substrate 3 shown in the figure. Gas-sensitive layer 2 is coated on layer 12 to a thickness of about 50 mm, and
It was baked on the alumina substrate 3 by heating at 0°C for 30 minutes.
Next, chloroplatinic acid was added to the same tin oxide powder as above as platinum, and 2 to xo! Impregnated to a concentration of 1% and heated at 600℃ for 2
The platinum was decomposed by heating for a period of time. Water and silica sol were added to this platinum-supported tin oxide powder to form a paste, and then a paste of approximately 50 ina was applied to cover the sensitive gas 12 shown in FIG.
Coating layer 4 was applied to a thickness of . After drying this at room temperature 73
The coating layer 4 was formed by heating at 0°C for 30 minutes. Note that palladium or the like can also be used as a catalyst. For comparison, a sensor without the coating layer 4 is referred to as a comparative sensor A, which also does not have the coating layer 4, but the gas-sensitive layer 2 is coated with 0.1 to 1% platinum.
The sensor carrying % by weight was designated as comparative sensor B. Figure 2 is a diagram showing the energization time dependence of sensor resistance in combustible gas. The sensor is kept at a predetermined temperature using the heater 6, and the sensor resistance in 0.2% isoptane gas is Rg.
The initial resistance of the sensor is Rgo, and R is based on Rgo.
Changes over time were investigated in the ratio of E, that is, the value of Rg/Rgo.
Characteristics &l21 and 22 are the characteristics of the gas sensor according to the embodiment of the present invention and comparative sensor A, respectively, and characteristic line 23 is the characteristic of comparative sensor B. It can be seen that when the gas-sensitive layer contains a catalyst, the sensor resistance in the combustible gas changes over time.

FIG. 3 is a diagram showing the dependence of the gas sensitivity on the energization time of the gas sensor according to the embodiment of the present invention and comparative sensors A and H. Gas sensitivity is defined as Ra/Rg, where Ra is the sensor resistance in air and Rg is the sensor resistance in <0.2% isobutane gas as described above. Since Ra does not change over time for each sensor, gas sensitivity (Ra/Rg) is determined by R. It is influenced by changes over time. A characteristic line 31 is the characteristic of the gas sensor according to the embodiment of the present invention. Sensitivity is 10
It is said that a sensitivity of 5 or higher is sufficient for the 5 gases, which are large and do not change over time. On the other hand, comparative sensor B has high gas sensitivity as shown by characteristic line 33, but changes over time are large. Comparative sensor A, which does not have a catalyst in the gas-sensitive layer, has the characteristic vA3.
As shown in 2, the gas sensitivity is low. However, there is no change in gas sensitivity over time. The gas sensor according to the present invention has high gas sensitivity and excellent stability as described above, but it also has the advantage of not being affected by interfering gases such as ethyl alcohol. This is a coating layer 4 made of tin oxide supporting a catalyst.
This is because interfering gases are removed by combustion. Flammable gases such as isobutane do not burn in the coating layer 4 over a wide temperature range. In this way, an excellent gas sensor that has high gas sensitivity, does not change over time, and is not interfered with by ethyl alcohol can be obtained. [Effects of the Invention] According to the present invention, a gas-sensitive layer and a coating layer are provided on the substrate, the gas-sensitive layer is made of a tin oxide semiconductor, and the coating layer is a tin oxide semiconductor containing 2 to 10% by weight of a noble metal. Since it is supported, there is no problem of combustion of flammable gas inside the gas-sensitive layer, and a gas sensor with excellent gas sensitivity and stability can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a gas sensor according to an embodiment of the present invention, and FIG. 2 is a comparison of the current supply time dependence of sensor resistance in flammable gas for the gas sensor according to an embodiment of the present invention. Characteristics of sensors A and B FIG. 3 is a diagram showing the dependence of the gas sensitivity on the energization time of the gas sensor according to the embodiment of the present invention in comparison with the characteristics of comparative sensors A and H. Tame》“Miryu No. 1 Closed”

Claims (1)

[Claims] 1) A gas-sensitive layer and a covering layer are provided on the substrate, the gas-sensitive layer is made of a tin oxide semiconductor, and the covering layer is a tin oxide semiconductor carrying 2 to 10% by weight of a noble metal. A gas sensor characterized by: