JP2911928B2 - Gas detection method - Google Patents

Description

The present invention relates to detection of a gas using a change in the resistance value of a SnO ₂ film, and is used, for example, for detecting a gas such as hydrogen sulfide, ethanol, CO, or isobutane.

[Prior Art] Japanese Patent Application Laid-Open No. 1-206,252 discloses that a gas sensor is driven by pulse heating to detect gas. In this technique, the sensor is generally kept at room temperature (RT) except during pulse heating, and the duty ratio of pulse heating is reduced to reduce power consumption.

The condition for the sensor in this technique is that the thermal time constant is short and the gas sensor can be heated in a short time. If this condition is satisfied, the structure of the sensor is arbitrary. The detection is performed from the output during pulse heating in principle, but can also be detected from the output during cooling. However, in this technique, the effect of the pulse heating temperature has not been sufficiently studied.

Next, the inventors of this application examined pulse heating driving of the gas sensor, and set the temperature during pulse heating to 100 to 300 ° C.
It was found that the sensor sensed only humidity and the gas sensitivity disappeared (Japanese Patent Application No. 1-128,468, Japanese Patent Application Laid-Open No. 2-307).
050).

In parallel with this, the inventor studied characteristics when the temperature during pulse heating was further increased. As a result, it has been found that when the pulse heating temperature is set to a certain temperature or higher, the following phenomena occur.

(1) The sensor resistance in air during cooling increases sharply. However, the sensor resistance during pulse heating does not particularly increase.

(2) The sensor resistance in the gas at the time of cooling does not particularly increase, and is not much different from the sensor resistance in the gas at the time of pulse heating.

(3) As a result, gas sensitivity during cooling and gas concentration dependency are improved.

[Purpose] An object of the present invention is to improve the sensitivity of a gas sensor and the gas concentration dependency of the gas sensor.

[Constitution of the Invention] In the present invention, a gas sensor using a SnO ₂ film is heated to 450 ° C. or more in a pulsed manner at a high temperature to form active oxygen ions on the SnO ₂ surface. Next, when the SnO ₂ film is rapidly cooled, the SnO ₂ film is cooled while maintaining the generated active oxygen ions, so that the resistance value in the air at the time of cooling sharply increases. The resistance in air at the time of cooling is several MΩ or more, usually 10 MΩ.
Above MΩ, virtually infinite. On the other hand, the resistance value in the gas during cooling does not increase so much. As a result, the sensitivity of the gas sensor increases. Further, since the ratio of the resistance value in air to the resistance value in gas is large, the gas concentration dependency of the sensor is also improved.

As a cause of the increase in the resistance value in the air at the time of cooling, it is considered that, besides the adsorption of active oxygen ions by pulse heating, the adsorbed water is removed by pulse heating. However, according to the experiment of the inventor, both the sensor output at the time of pulse heating and the sensor output at the time of cooling are affected by humidity. Therefore, it is difficult to imagine that the absorbed water is completely removed by the pulse heating. The only thing that remains as the cause of the increase in the resistance value in the air during cooling is that active oxygen ions are generated during pulse heating and remain even during cooling.

In order to significantly increase the resistance value of the SnO ₂ film in the air during cooling, the pulse heating temperature is set to 450 ° C. or higher. This temperature is the literature value of the temperature at which active oxygen ions such as O ⁻ and O ²⁻ are formed on the SnO ₂ surface, (Egashira ACS Symposium Serial N
o.309, p71, 1986). It is also known that the formation of these adsorbed oxygen ions significantly increases the low resistance of SnO ₂ . These are due to the fact that the resistance value in air during cooling increases due to pulse heating,
Supports the formation of active adsorbed oxygen ions.

Next, the present invention will be described individually. In order to shorten the thermal time constant of the gas sensor, SnO ₂ in the form of a film (thick film having a thickness of about 10 to 20 μm or thin film having a thickness of 1 μm or less) is used.

It is important for the structure of the gas sensor to reduce the thermal time constant of the SnO ₂ film. If the thermal time constant is large and cooling from pulse heating is slow, active oxygen ions are lost during cooling, and SnO ₂ during cooling does not have a very high resistance, and the gas sensitivity is low. For example, in a gas sensor in which a metal oxide semiconductor is molded into a chip shape, which is commonly used, the thermal time constant during cooling is extremely slow, about 10 seconds to several tens of seconds, and the resistance value at room temperature even when cooled from high temperature to room temperature. Does not become infinite and the gas sensitivity at room temperature is low. On the other hand, if the thermal time constant at the time of cooling is short, for example, 100 msec or less, and more preferably 10 msec or less, the resistance value at the time of cooling after pulse heating is infinite and the gas sensitivity is high. Reducing the thermal time constant has the effect of shortening the time required to reach the maximum heating temperature during pulse heating (hereinafter, this temperature is referred to as “pulse temperature”) and shortening the width of the heating pulse. This reduces the power consumption of the gas sensor.

In the case of a SnO ₂ film, the temperature of the pulse heating needs to be 450 ° C. or higher, and was 500 ° C. in the example. The temperature at the time of cooling is preferably room temperature in order to save power consumption.However, the temperature dependence of the resistance value in air or gas near room temperature is small, and it may be slightly higher than room temperature.
It should be below ° C.

The time width of the pulse heating is set to a width that can raise the temperature of the SnO ₂ film to a temperature at which active oxygen ions are formed. In the embodiment, a width of 20 mm is applied to a gas sensor having a thermal time constant of 20 msec when the temperature is raised.
A heating pulse of msec was applied. The time width during cooling is preferably equal to or longer than the time required for the characteristics of the SnO ₂ film to reach a steady value during cooling. This time is about 30 under the conditions of the embodiment.
It was 0 msec. Next, the upper limit of the time width during cooling is not important. When left at room temperature for several minutes after cooling, the resistance value of the SnO ₂ film decreases to a value that can be read practically, and the gas sensitivity also decreases. However, the output may be sampled before the resistance value of the SnO ₂ film decreases, and there is no problem even if the resistance value decreases after sampling.

[Example] Measuring method The measuring method used in the example will be described with reference to Figs. 7 to 10. FIG. 7 shows an example of the gas sensor 02. 2 is wire diameter 20
A linear heater using a μm Fe—Cr—Al alloy, 4 is an alumina insulating film having a film thickness of about 1 μm, and 6 and 8 are film-shaped gold electrodes. Reference numeral 10 denotes a SnO ₂ film having a thickness of about 0.5 μm, and the film thickness is
A CuO film having a thickness of 0.1 μm or less was laminated. The CuO film is a catalyst for promoting the adsorption and desorption of oxygen ions, and may be changed to a catalyst for the adsorption and desorption of arbitrary oxygen ions such as CoO, Mn ₂ O ₃ , and Cr ₂ O ₃ . The structure of the gas sensor O ₂ is the same as that of the applicant's gas sensor “TGS50
1 ". The gas sensor O ₂ was manufactured as follows. Alumina sol was adhered to the surface of the heater 2 at 800 ° C.
To form an alumina insulating film 4. Then gold electrode 6,
8 is vacuum-deposited, a solution of an organic compound of Sn is adhered to the surface of the alumina insulating film 4, and after drying, it is thermally decomposed at 550 ° C.
The nO ₂ film 10 was used.

This sensor 02 uses a thermal time constant of 20 ms during heating.
ec, because the thermal time constant during cooling is as short as 5 msec, other sensors may be used as long as the thermal time constant is small. For example, a heater and a SnO ₂ film may be laminated on an alumina substrate via a heat insulating glass film. In this case, the substrate having a large heat capacity does not rise by short-time pulse heating, and is kept at room temperature. Here, when the heater is turned on / off, the SnO ₂ film is stacked on the heater and the distance between the two is small, so that the temperature of the SnO ₂ film changes following the on / off of the heater. As a result, the thermal time constant at the time of heating can be set to about several milliseconds. In addition, a gas sensor in which a heater, an electrode, and a SnO ₂ film are formed on a thin film of SiO ₂ or the like cross-linked on the cavity may be used.

FIG. 8 shows an example of the circuit. In the figure, reference numeral 12 denotes a power supply, which is 5 V here. 14 is a power supply for pulse heating,
Here, the output 0.65V, 16 is a transistor switch, which may be an arbitrary switch such as a relay. R ₁ is the gas sensor O ₂
A load resistance of 30 KΩ to 300 KΩ was used for the measurement. Reference numeral 18 denotes a control timer that operates at a one-second cycle,
Turn on the switch 16 only for 20 msec and pulse temperature 500
Perform pulse heating at ° C. 20 is a sample and hold circuit,
Sampling the sensor output during cooling (output to the load resistor R ₁₎ immediately before the heating pulse. As a result, the sensor O2 is driven at a cycle of one _second, is heated to a pulse temperature of 500 ° C. for 20 msec, and is kept at room temperature during other periods. Then, the gas is detected from the output immediately before the pulse heating (immediately before the end of the cooling time to room temperature).

FIG. 9 shows the measurement conditions of the sensor characteristics. As described above, the SnO ₂ film 10 is heated at a heater voltage of 0.65 V once a second for 20 msec and heated up to 500 ° C. During other periods, the heater 2 is turned off, and the SnO ₂ film 10 is kept at room temperature. And a immediately before the end of the heating pulse, to the immediately preceding transition to the heating pulse from room temperature, from the output of the load resistor R _1, sampling the sensor characteristics.

FIG. 10 shows the temperature characteristics of the gas sensor O ₂ in N ₂ . In the vertical axis the output to the load resistor R _1, 100 msec from time 0
During this time, a heating pulse is applied to the heater 2 to heat the SnO ₂ film 10 from room temperature. The output of the sensor reaches a steady value in 20 to 25 msec, and it can be seen from this that the thermal time constant during heating is about 20 msec. On the other hand, when the heating pulse is terminated at time 0.1 second, the sensor output decreases in about 5 msec, which indicates that the thermal time constant during cooling is about several msec. Next, the SnO ₂ film 1 at a heater voltage of 0.65 V, determined using thermopaint, was used.
The steady-state value of the temperature of 0 is 500 ° C. Considering that the thermal time constant of the SnO ₂ film 10 is about 20 msec, the pulse temperature at a pulse voltage of 0.65 V is about 500 ° C.

The Figure 1-Figure 3 sensor characteristics, showing the gas sensitivity of the gas sensor O ₂ gas concentration dependency. The measurement conditions in Fig. 1 are as follows: operating cycle is 1 second, heating pulse width is 20msec, pulse temperature is 500
The temperature when no heating pulse is applied is room temperature. Note the load resistance R ₁ is 100 K.OMEGA, the gas hydrogen sulfide 2pp
m was used. FIG. 1 shows the characteristics at 500 ° C. and the characteristics at room temperature. The resistance in air at room temperature is infinite (more than 10 MΩ), and the sensitivity to 2 ppm hydrogen sulfide is extremely high.

FIG. 2 shows the sensor output Vout when a heating pulse is applied.
3 shows the waveforms of FIG. A heating pulse is applied for 20 msec from time 0, and then cooled to room temperature. The resistance value in air after cooling is extremely high, and the gas sensitivity is also large. The time at which the resistance value at room temperature becomes infinite was usually about 10 seconds to 1 minute after cooling. Therefore, if the heating cycle is set to 10 seconds to 1 minute or less, detection can be performed while keeping the resistance value in air at room temperature to infinity. However, this does not exclude extending the detection cycle to, for example, about 5 minutes. For example, if the output at about 1 second after cooling to room temperature is sampled and the subsequent output is not used, no problem occurs even if the resistance value at room temperature decreases from infinity.

FIG. 3 shows gas concentration characteristics with respect to hydrogen sulfide and ethanol. The figure shows the resistance value at room temperature and the resistance value at 500 ° C. based on the results of a cycle of 5 seconds between 500 ° C. (pulse width 20 msec) and room temperature. For comparison, the SnO ₂ film 10
The results when the temperature was kept at ℃ are also shown. The gas concentration dependence at room temperature is large.

FIG. 4 shows the effect of humidity. Pulse heating conditions are
A pulse voltage width in O.65V 20msec, 1 cycle 5 seconds, the load resistance R ₁ is 30 k.OMEGA. 1 ppm of hydrogen sulfide (humidity
50% or 100%) and 50% humidity during other periods
In the air. The dashed line at the top of the figure represents the sensor output at 500 ° C., the dashed line at the bottom represents the output at room temperature (immediately before the heating pulse), and the temperature of the atmosphere was 20 ° C. Comparing the second peak (1 ppm hydrogen sulfide, 50% humidity) with the third peak (1 ppm hydrogen sulfide, 100% humidity) in the figure, it was taken out of the gas and returned to the air at 20 ° C and 50% RH. Response is different. This indicates that the pulsed heating at 500 ° C. does not completely remove the water adsorbed on the SnO ₂ film 10 and the humidity remains on the sensor characteristics.

Possible causes of the rapid increase in resistance in air at room temperature due to pulse heating at 500 ° C are as follows: (1) Active oxygen ions are adsorbed on the SnO ₂ surface by pulse heating, and the resistance of the sensor increases. Or (2) that the water adsorbed on the SnO ₂ surface is completely removed. From the results shown in FIG. 4, it is unlikely that the absorbed water is completely removed by the pulse heating. Therefore, it is considered that the reason why the resistance value of the sensor becomes infinite is that active oxygen ions are formed on the SnO ₂ surface during pulse heating.

In FIG. 5, the pulse temperature was set to 30 under the same measurement conditions as in FIG.
The results when the temperature is changed from 0 ° C to 600 ° C are shown. As the pulse temperature increases, the resistance in air increases and the resistance in gas decreases. And this effect is remarkable above 450 ° C. This temperature also coincides with the adsorption temperature of active oxygen ions on SnO ₂ .

FIG. 6 shows the resistance value during cooling when the pulse temperature is set to 500 ° C. (pulse width: 20 msec, pulse cycle: 1 second) and the temperature during cooling is changed from room temperature to 300 ° C. There is no significant difference in the results when the cooling temperature is set to room temperature or 100 ° C. However, when the temperature at the time of cooling is set to 200 ° C., the resistance value in the air decreases, and the sensitivity decreases. For this reason, the cooling temperature is preferably 150 ° C. or lower, more preferably 100 ° C. or lower.

[Effect of the Invention] In the present invention, (1) the sensitivity of the gas sensor is increased, and (2) the gas concentration dependency of the sensor characteristics is increased.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a gas detection result in the embodiment. FIG. 2 is a characteristic diagram showing gas sensor characteristics near a heating pulse in the embodiment. FIG. 3 is a characteristic diagram showing gas concentration dependency in the embodiment. FIG. 4 is a characteristic diagram showing an influence of humidity on gas sensor characteristics in the embodiment. FIG. 5 is a characteristic diagram showing the influence of the pulse heating temperature on the characteristics at room temperature in the example. FIG. 6 is a characteristic diagram showing the effect of temperature during cooling in the embodiment. FIG. 7 is a plan view of a main part of the gas sensor used in the embodiment. FIG. 8 is a circuit diagram of the embodiment. FIG. 9 is a waveform diagram showing a heating pulse in the embodiment. FIG. 10 is a characteristic diagram showing a thermal response characteristic of the gas sensor used in the example.

Claims (4)

(57) [Claims] 1. A gas sensor using a SnO ₂ film is placed in a cycle of pulse heating and cooling at a temperature of 450 ° C. or more, so that the resistance value of the SnO ₂ film in air during cooling becomes a high resistance state. A gas detection method, wherein the gas is detected from the resistance value of the SnO ₂ film in the gas during cooling. 2. The gas detection method according to claim 1, wherein the temperature of the SnO ₂ film during cooling is room temperature. 3. The gas detection method according to claim 2, wherein the thermal time constant of the SnO ₂ film is shorter than the time during which the relaxation of oxygen ions to an inactive state in the cooling process occurs. 4. The gas detection method according to claim 3, wherein the surface of the SnO ₂ film is coated with a catalyst for promoting adsorption of oxygen ions.