JPH0382945A - Semiconductor gas sensor - Google Patents

Abstract

PURPOSE:To increase selective oxidization of an oxide layer in a sensor with respect to alcohol by providing a gas sensitive layer having a catalyst of noble metal added to an n-type oxide semiconductor with a specific ratio. CONSTITUTION:Powder of tin oxide having the central particle size of 2mum is impregnated with palladium chloride so that the palladium becomes 0.5wts.%. Then, the powder is dried and heated at 600 deg.C for two hours, thereby dissolving the palladium chloride. After the powder is turned into paste, the tin oxide is screen printed on electrodes 1A, 1B provided on an alumina substrate 5, heated at 120 deg.C for two hours, thereby forming a gas sensitive layer 2 on the substrate 5. Thereafter, a paste including gamma alumina particles is printed on the gas sensitive layer 2, dried at 120 deg.C for two hours, thereby forming an insulating layer 3. Subsequently, the same powder of the tin oxide as above is impregnated with chloroplatinate so that the platinum becomes 0.5wts.%, which is then heated at 600 deg.C for two hours to be dissolved. The tin oxide powder with platinum is turned into paste, applied on an oxide layer 4, dried at room temperatures, and heated at 600 deg.C for three hours. As a result, an oxide layer 4 is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gas sensor using a tin oxide semiconductor, and particularly to a semiconductor type gas sensor that is free from interference by alcohol and has high selectivity to a target gas.

[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 high resistance. Upon contact, the adsorbed oxygen and combustible gas react, the adsorbed oxygen is removed, and its electrical resistance value decreases.

Utilizing these properties, sensors using tin oxide are
Widely used as a gas leak alarm for P gas, city gas, etc. To the tin oxide, platinum or palladium is added in an amount of 0.1 to 1% by weight to increase gas sensitivity.

It is known that this type of sensor exhibits a large change in resistance value even when exposed to alcohol in the atmosphere.In order to prevent malfunctions due to alcohol generated from cooking, warming, etc., the sensor has been designed to reduce its sensitivity to alcohol as much as possible. A sensor is required that can selectively detect LP gas, city gas, etc.

As one method for solving this problem, a method is known in which an oxide layer having the function of oxidizing and removing alcohol is formed on the outer surface of a gas-sensitive layer made of tin oxide. The oxide layer used is one in which noble metals such as platinum and palladium are supported on activated alumina.

(Problems to be Solved by the Invention) However, in the semiconductor type gas sensor using activated alumina as an oxide layer as described above, there is a problem in that interference due to alcohol cannot be sufficiently eliminated.

For example, Figure 4 shows isobutane characteristics 9 and alcohol characteristics 1 of a conventional semiconductor gas sensor at a sensor temperature of 375°C.
0 is shown. A semiconductor type gas sensor issues an alarm when its resistance value falls below a predetermined value, but when the isobutane characteristic 9 and the alcohol characteristic 10 are close to each other as described above, the gas sensor is likely to issue a false alarm due to alcohol. Note that the above-mentioned sensor temperature of 375° C. is the set temperature at which the isobutane characteristics and the alcohol characteristics are the most distant from each other.

The present invention has been made in view of the above-mentioned points, and its object is to provide a semiconductor gas sensor that is free from alcohol interference by improving the oxide layer.

Means for Solving Problem C1] According to the present invention, the above-mentioned object
A gas sensor comprising W2, an insulating layer 3, and N4 oxide, wherein the gas sensitive layer is an n-type oxide semiconductor coated with a noble metal catalyst.
．． The insulating layer is a porous layer laminated on the gas-sensitive layer, and the oxide layer is an n-type oxide semiconductor laminated on L of the insulating layer. However, this can be achieved by adding 0.1 to 1% by weight of a noble metal catalyst.

[Effect]

An oxide layer in which 0.1 to 1% by weight of a noble metal is supported on tin oxide has increased selective oxidation property for alcohol compared to a conventional oxide layer.

〔Example〕

Next, embodiments of the present invention will be described based on the drawings.

FIG. 1 is a cross-sectional view showing a semiconductor gas sensor according to an embodiment of the present invention, in which electrodes 1^ and IB, gas-sensitive layers 2. Insulating layer 3. Oxide layers 4 are sequentially laminated, and lead wires 6A, 6B are connected to electrodes IA, IB. A heater 7 is provided on the other main surface of the substrate 5. Lead M8A
, 8B supplies current for the heater.

Such a gas sensor is prepared as follows.

That is, tin oxide powder having a center particle size of 2irm was impregnated with palladium chloride to give a concentration of 0.5% by weight as palladium, dried and 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 tin oxide was screen printed on the electrodes IA and 1B provided on the alumina substrate 5'' to a thickness of about 50 mm, and heated at 120℃ for 2 hours. Then, a gas-sensitive layer 2 was formed on the alumina substrate 5. Next, a paste containing γ-alumina particles was screen printed on the gas-sensitive layer 2 and dried at 120° C. for 2 hours to form an insulating layer 3 with a thickness of 50 μm. Next, the same tin oxide powder as above was impregnated with chloroplatinic acid at a concentration of 0.5% as platinum, and 6
Platinum was decomposed by heating at 00°C for 2 hours.

Water and silica sol were added to this platinized tin oxide powder to form a paste, and then an oxide layer 4 was applied to a thickness of about 50 mm.

This was dried at room temperature and then heated at 600°C for 3 hours.

For comparison, a conventional gas sensor was fabricated in which the gas-sensitive layer was coated and baked using a paste of γ alumina carrying 3% by weight of platinum in place of the insulating layer 3 and oxide layer 4 described above.

FIG. 2 is a diagram showing the relationship between sensor temperature and sensor resistance of the gas sensor according to the embodiment of the present invention.

Song j6i13 is the sensor resistance RO in air (kΩ)
.

Curve M14 is the sensor resistance R in air containing 0.2% isobutane, (B) (kΩ) 1 curve 15 is the sensor resistance R in air containing 0.2% alcohol, (A) (k
Ω). As the temperature rises, the sensor resistance in air containing isobutane increases and the sensor resistance decreases, but eventually the isobutane begins to burn in the oxidized layer and the amount of isobutane that reaches the sensitive gas layer decreases, causing the sensor resistance to increase. The monotonically decreasing trend of resistance weakens. On the other hand, the sensor resistance in air containing alcohol is lower than that in the case of isobutane because the oxidation layer does not function while the sensor temperature is low, so the alcohol directly reaches the sensitive gas layer, and the sensor resistance is smaller than in the case of isobutane. As the temperature rises, the oxidized layer becomes functional and alcohol burns in the oxidized layer, reducing the amount of alcohol that reaches the gas-sensitive layer and increasing the sensor resistance. In this way, the curve 14 showing the sensor resistance in air containing isobutane and the curve 15 showing the sensor resistance in air containing alcohol intersect, and the sensor resistance in air containing alcohol intersects at a temperature higher than the temperature at the intersection. The sensor resistance is greater than that in air containing isobutane. Curve 14 and curve 15 are 450℃
The difference in sensor resistance becomes maximum at a temperature of . The relationship between sensor resistance and gas temperature at 450° C. is shown in FIG. Characteristic 4111 is the sensor characteristic in air containing isobutane, and characteristic line 12 is the sensor characteristic in air containing alcohol. The difference in sensor resistance between characteristic line 11 and characteristic line 112 is large. It can be seen that alcohol interference is less than with conventional sensors.

FIG. 3 is a diagram showing the relationship between sensor resistance and sensor temperature for a conventional gas sensor. Curve 16 shows sensor characteristics in air.Curve 17 shows sensor characteristics in air containing 0.2% isobutane.Curve 18 shows sensor characteristics in air containing 0.2% alcohol.
These are sensor characteristics in air containing 2%. The difference in sensor resistance value becomes largest at a temperature of 375° C. on the higher temperature side than the temperature where curves 17 and 18 intersect. The relationship between sensor resistance and gas concentration at 375° C. is shown by the characteristic line 9110 in FIG. 4, as described above.

FIG. 5 is a diagram showing the sensor temperature dependence of gas sensitivity γ, γ. Here, gas sensitivity γA1 and γrecommendation are alcohol sensitivity and isobutane sensitivity, respectively, and are defined as follows.

rA=Ro/Re (A) rs = R1)/Re (B) Curve 19.20 is the sensor temperature dependence of γ,,γ, regarding the gas sensor according to the embodiment of this invention, and curve 21.22 is γ with respect to conventional gas sensors.

If we define the sensor temperature range that satisfies the sensor temperature dependence of γ, 1 rm'; Range 23 is the operating temperature range 24 of conventional sensors.
You can see that it is wider.

〔Effect of the invention〕

According to this invention, there is provided a gas sensor having a gas-sensitive layer, an insulating layer, and an oxide layer on a substrate, wherein the gas-sensitive layer is formed by adding 0.1 to 1% by weight of a noble metal catalyst to an n-type oxide semiconductor. The insulating layer is a porous layer laminated on the gas-sensitive layer, and the oxide layer is a porous layer laminated on the insulating layer.
type oxide semiconductor is laminated, and a noble metal catalyst is added to this layer by 0.1~
Since it is added in an amount of 1% by weight, the selective oxidation property of the oxidation layer toward alcohol increases, and as a result, the ratio of alcohol reaching the gas-sensitive layer to the target gas is significantly different from that of conventional gas sensors, reducing the interference of alcohol. It is possible to obtain a semiconductor gas sensor with a small number of semiconductors. Furthermore, the gas sensor according to the present invention has the advantage that its operating temperature range is wider than that of conventional sensors.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a semiconductor gas sensor according to an embodiment of the present invention, FIG. 2 is a diagram showing the temperature dependence of sensor resistance for the sensor according to an embodiment of the invention, and FIG. 3 is a diagram showing a conventional sensor. FIG. 4 is a diagram showing the temperature dependence of the sensor resistance of the sensor according to the embodiment of the present invention, in comparison with the gas concentration dependence of the sensor resistance of the conventional sensor; FIG. 5 is a diagram showing the temperature dependence of gas sensitivity of the sensor according to the embodiment of the present invention in comparison with the temperature dependence of a conventional sensor. 2: Gas sensitive layer, 3: Insulating layer, 4 Dioxide layer. 2503003504004505005501=-/
→ Horn-1 change'tL ('c) Fig. 2 250300350400450500t Excess (0
C) Fig. 3 0.1 0.2 0.5 1.0 Force'°S temperature (X) Fig. 4 300350400450500550tSother temperature (0
C) Figure 5

Claims (1)

[Claims] 1) A gas sensor having a gas-sensitive layer, an insulating layer, and an oxide layer on a substrate, the gas-sensitive layer comprising an n-type oxide semiconductor coated with a noble metal catalyst of 0.1%.
~1% by weight is added, the insulating layer is a porous layer laminated on the gas-sensitive layer, the oxide layer is a n-type oxide semiconductor laminated on the insulating layer, A semiconductor gas sensor characterized in that 0.1 to 1% by weight of a noble metal catalyst is added thereto.