JPH0799358B2 - Gas detection method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for detecting a gas by utilizing a temperature change of a gas sensor using a change in resistance value of a metal oxide semiconductor.

[Prior Art] Japanese Examined Patent Publication No. 53-43,320 discloses that the temperature of a gas sensor using a change in the resistance value of a metal oxide semiconductor is alternately changed between a high temperature region and a low temperature region, and a specific gas is output from an output in a low temperature region. Is disclosed.

The inventor here finds that the apparent resistance of the sensor is the detection voltage,
That is, it was found that it depends on the voltage applied to detect the resistance value of the sensor. The voltage dependency of the resistance value becomes more significant as the distance between the two electrodes of the sensor is reduced and the contact area between the sensor and the semiconductor is reduced. Further, this voltage dependency becomes more remarkable as the sensor temperature is lowered, and increases as the atmospheric humidity is increased. Therefore, this voltage dependence is a phenomenon related to temperature, and it may be due to the adsorbed water on the sensor, possibly the interaction between the adsorbed water and impurities such as chlorine ions in the sensor.

Such voltage dependence deteriorates the humidity characteristics of the sensor and increases the power supply voltage dependence.

[Problem of the Invention] An object of the present invention is to suppress the voltage dependence of the resistance value of the gas sensor and improve the detection accuracy for gas.

[Configuration of the Invention] In the present invention, a load resistance is connected in series to a gas sensor that utilizes a change in the resistance value of a metal oxide semiconductor, and the temperature of the gas sensor is alternately changed between a high temperature range and a resistance range. Then, gases such as CO, ammonia and NO _x are detected from the gas sensor output in the low temperature range. Here, the detection voltage applied to the series piece of the gas sensor and the load resistance is 0.1 to 1.5 V, more preferably
Set to 0.1 to 1.2V to suppress the dependency of the sensor resistance on the detection voltage.

As shown in FIG. 1, FIG. 2 and FIG. 2, the resistance value of the gas sensor in the low temperature range depends on the detection voltage. As is clear from Table 2, when the detection voltage is set to 1.5 V or less, the dependency of the resistance value of the gas sensor on the detection voltage becomes small, and particularly 1.2 V or less becomes extremely small. On the other hand, if the detection voltage is less than 0.1 V, the output is too small to handle, so the detection voltage is 0.1 to 1.5 V, and more preferably 0.1 to 1.2 V.

It is a phenomenon associated with the surface conduction by the adsorbed water that the resistance value depends on the detection voltage. The original resistance value of the metal oxide semiconductor does not depend on the detection voltage, and it depends on the surface conduction by the adsorbed water. When the detection voltage is increased, surface conduction occurs and the resistance value of the gas sensor apparently decreases. This is because the apparent electrical conductivity of the sensor increases when the detection voltage is set to 2 V or higher in Table 2, and the apparent resistance value (Vc) is set to 2 V or 5 V in Figs. 1 and 2. It is clear from the decrease in Rs).

When the detection voltage dependency of the resistance value occurs, the humidity dependency of the sensor resistance increases. For example, FIG. 1 (in clean air) and second
In the figure (in CO 100 ppm), the chain line shows the resistance value when the detection voltage is 1 V, the broken line shows the resistance value when the detection voltage is 5 V, and the solid line shows the resistance value when the detection voltage is 2 V. Keep the ambient temperature constant,
The relative humidity was changed to three types, and the resistance value was measured in FIGS. 1 and 2. The interval between the three lines at the same detection voltage represents the humidity dependence of the resistance value of the sensor. As is clear from the figure, the humidity dependency increases as the detection voltage increases. This is because when the detection voltage is increased, surface conduction occurs due to the adsorbed water, and the degree of surface conduction is determined by humidity. Therefore, if the detection voltage is set to 1.5 V or less, not only the error when the detection voltage changes due to the power supply voltage change can be eliminated, but also the humidity dependency of the gas sensor can be reduced and the detection accuracy can be improved.

[Example] FIG. 4 shows a gas sensor (2). In the figure,
(4) and (6) are a pair of electrodes that also serve as heaters. Here, an Ir-Pd alloy coil having a wire diameter of about 60 μ was used, and the inner diameter of the coil was 0.3 mm (preferably 0.2 to 0.5 mm). (8) is a metal oxide semiconductor whose resistance value changes by adsorbing gas,
Here, SnO ₂ is used, and (10) is a NO _x reducing contact for reacting and removing NO _x with the gas to be detected. NO _x reducing contact (1
0) is not necessary when NO _x is the detection target. NO _x
The reduction in contact (10), the NO _x is reacted with a detector gas such as NO _x and CO or ammonia as long as it is reduced to N _2, it is possible to use any contact. The reduction contact (10) is, for example, a noble metal-based material such as RuO ₂ , Rh, Pd, or Pt, or Mn ₂ O ₃ , Cr.
₂ O _{3 and} other transition metal oxides, or RuO ₂
It is also possible to use those carrying a noble metal such as However, here, the same SnO ₂ as the semiconductor (8) was used, and its thickness F was increased (0.3 mm) and used as a contact.

In addition, SnO ₂ was used here, which was obtained by hydrolyzing SnCl ₄ with ammonia and calcining at 600 ° C., and 0.3% by weight of palladium was added to SnO ₂ as a material for semiconductor (8) and reduction contact (10). And

The detection voltage dependency of the sensor resistance decreases as the contact area between the semiconductor and the electrode increases. Therefore, both electrodes (4) and (6) are coiled here. Further, the dependency of the resistance on the detection voltage decreases as the electrode interval H increases. A preferable electrode interval is 0.4 to 1 mm, below which the detection voltage dependency is large, and above this, the heat capacity of the sensor increases,
This increases the time required for temperature changes and delays gas detection.

The dependence of the sensor resistance on the detection voltage depends on the electrode spacing and the detection voltage itself means that the strength of the electric field inside the sensor is related to the detection voltage dependence. Also, decreasing the contact area between the electrode and the semiconductor increases the detection voltage dependency.
This suggests that there is a problem with the contact resistance between the semiconductor and the electrode, and that the concentration of the electric field near the electrode increases the detection voltage dependence.

FIG. 5 shows another gas sensor (12). (14) is a linear noble metal electrode, (16) is a coil-shaped noble metal electrode that also serves as a heater, (18) is a semiconductor, and (20) is a NO _x reduction catalyst. Here, the inner diameter of the coil (16) was set to 0.6 mm, and the NO _x reduction catalyst (20) used was γ-alumina carrying 3 wt% of RuO ₂ in terms of metal. The thickness of the reduction catalyst (20) is 0.2m.
It was m. The sensor (12) shown in FIG. 5 has a greater dependency of the resistance value on the detected voltage than the sensor (2) shown in FIG. This is because the center electrode (14) is linear, the electric field is concentrated in the vicinity thereof, and the contact resistance is increased due to the small electrode area. The shape of the sensor may be any other shape.

FIG. 6 shows an example of the detection circuit. In the figure, (30) is a detection power source, for example, a power source of about 0.1 to 1.5 V, and its output voltage is the detection voltage (Vc). (32) is the heater power supply,
(34) is a load resistance of about 3 KΩ, (36) is a negative characteristic thermistor for compensating the temperature dependence of the sensor, and (38) is an operational amplifier. (40) is a control circuit containing a timer circuit, which is a publicly known one. The control circuit (40) operates, for example, in a cycle of 20 seconds, and switches (42) for the first 10 seconds.
Close and heat the sensor to a high temperature range. For the next 10 seconds, the switch (42) is opened to cool the sensor and keep it in the low temperature range. In the low sound range, the temperature of the sensor gradually decreases, the switch (44) is closed immediately before the elapse of 10 seconds, and the output of the differential amplifier (38) is input to the AD converter (46) also serving as a memory. The AD converter stores the output of the differential amplifier (38) and controls the display device (48). A light emitting diode, a meter, a buzzer, or the like is used for the display device (48). A large number of such detection circuits are known, and the detection circuit is not limited to that shown in FIG.

The dependence on the detection voltage (Vc) is shown in FIGS. 1 and 2.
The sensor is the sensor (2) in Fig. 4, and the electrode interval H is 0.5 mm.
Is. It is assumed that the sensor (2) is heated to 300 ° C. until time 0, and the heater is turned off at time 0 to shift to the low temperature range. Horizontal axis is time, vertical axis is sensor resistance, atmosphere is 45 ° C
The relative humidity is 75%, 15%, and 3%. Figure 1 shows the characteristics in clean air, and Figure 2 shows the characteristics in 100ppm CO.

The resistance value of the sensor (2) is affected by humidity. If the detected voltage is changed here, the measured resistance value also changes. Such changes are significant in clean air and at high humidity. The dependency of the resistance value on the detected voltage increases with the passage of time on the low temperature side. This phenomenon reduces the gas detection accuracy. Further, this phenomenon becomes a further problem when a low concentration gas, for example, CO of about 10 to 30 ppm is detected. Note that here
An example of using SnO ₂ as a semiconductor has been described, but this phenomenon is common to other semiconductors such as In ₂ O ₃ and ZnO. The resistance value when the detection voltage was 2 V at 45 ° C. and 3% relative humidity in FIG. 1 appeared between the results when the detection voltage was 1 V and 5 V. Also, in the following, relative humidity is omitted and is simply expressed by%. In FIG. 2, the result at the detection voltage of 2V appears in the middle of the results at the detection voltage of 1V and 5V.

The characteristics shown in FIGS. 1 and 2 are obtained by devising the sensor shape to reduce the detection voltage dependency. Table 1 shows the results shown in FIG. 5 and the results when the electrode interval H was changed in the sensor (2) shown in FIG. The atmosphere is 20 ° C and 65%,
It has the same absolute humidity as 45 ° C and 15%, and shows the electric conductivity at 5V based on the electric conductivity of the sensor at the detection voltage of 1V. In the sensor of FIG. 4, there is a large change between the electrode intervals of 0.2 mm and 0.5 mm, and in the sensor of FIG. 5, the contact area between the center electrode (14) and the semiconductor is small, so that the detection voltage dependency is large.

Table 2 shows the characteristics of the sensor (2) using two coil-shaped electrodes (4) and (6) when the electrode interval is fixed to 0.5 mm and the detection voltage is changed.

It can be seen from Table 2 that when the detection voltage is reduced, the dependency of the resistance value on the detection voltage is reduced, and particularly at 1.5 V, and more preferably 1.2 V or less, the dependency of the resistance value on the detection voltage hardly poses a problem.

Returning to FIGS. 1 and 2, it can be seen that the detection voltage dependency of the sensor resistance increases on the high humidity side. This exacerbates the humidity dependence of the sensor.

Next, the sensor resistance in CO should be 10 to 20 after the transition to the low temperature range.
The bottom centered on the second depends. On the other hand, the sensor resistance in air has a peak around 10 to 20 seconds after the transition to low temperature. During this period, the sensor corresponds to the temperature (80 to 100 ° C.) at which the sensor has the maximum sensitivity to CO, and it is preferable to detect CO during this period.

FIG. 3 shows temperature characteristics when the heating of the sensor (2) is turned on and off every 10 seconds. The sensor (2) is that of Fig. 4 and the room temperature is 20 ° C. The thermal time constant changes depending on the electrode interval H, and the time required for changing the temperature to the low temperature range also changes. CO
The temperature of the sensor needs to be changed to detect ammonia, ammonia, etc.
Increasing the thermal time constant delays detection. Therefore, the electrode spacing H is
It is preferably 1 mm or less, more preferably 0.8 mm or less.

Next, the effect of the NO _x reduction catalyst (10) will be shown. At 20 ℃, 65% atmosphere, sensor resistance in 100ppm CO and 50ppm
Calculate the ratio with the sensor resistance in the atmosphere with NO added. The measured value is 10 seconds after the transition to the low temperature range. The results are shown in Table 3. _The NO _x reduction catalyst (10), by (20), NO _x
The effect of can be mitigated.

Table 3 Ratio of resistance values of NO _x reduction catalyst sensor Fig. 4 F 0.3mm 0.7 〃 F to 0 0.4 Fig. 0.8 * In the sensor of Fig. 4, 0.3 wt% palladium catalyst was added to SnO ₂ in terms of metal. As a NO _x reduction catalyst,
In the case of the one shown in Fig. 5, 3w is converted to metal outside the coil (16).
γ-alumina containing t% RuO ₂ was coated to a thickness of 0.2 mm and NO
_x reduction catalyst.

[Advantages of the Invention] According to the present invention, it is possible to suppress the dependency of the resistance value of the gas sensor on the detection voltage and improve the gas detection accuracy.

[Brief description of drawings]

1 to 3 are characteristic diagrams of the embodiment, FIG. 4 is a sectional view of a gas sensor used in the embodiment, FIG. 5 is a sectional view of a gas sensor of a modified example, and FIG. 6 is used in the embodiment. It is a circuit diagram of a detection circuit. In the figure, (4), (6), (14), (16) electrodes, (8), (18) semiconductors, (10), (20) NO _x reduction catalysts, (40) control circuit.

Claims (2)

[Claims] 1. A gas sensor that utilizes a change in resistance of a metal oxide semiconductor is connected in series with a load resistor, and the temperature of the gas sensor is alternately changed between a high temperature range and a low temperature range, and a gas sensor output in a low temperature range is provided. The method for detecting a gas to be detected according to claim 1, wherein the detection voltage applied to the series piece of the gas sensor and the load resistor is 0.1 to 1.5V. 2. The gas detection method according to claim 1, wherein the detection voltage is 0.1 to 1.2V.