JPH03185351A - Detection of gas - Google Patents

Abstract

PURPOSE:To enhance the sensitivity of a gas sensor and the concn. dependence of gas by detecting the gas from the reduction in the resistance value of a metal oxide semiconductor film enhanced in sensitivity at the time of cooling. CONSTITUTION:A power supply 14 for pulse heating turns a transistor switch 16 ON to perform the pulse heating of the metal oxide film of a gas sensor 02 and an active oxygen ion is formed to the surface of the metal oxide semiconductor film. At this time, a control timer 18 is operated in an one-sec unit to turn the switch 16 ON for a predetermined time to heat the film. Subsequently, when the metal oxide semiconducte film is rapidly cooled, the signal from the sensor is sampled by a sample hold circuit 20 while an oxygen ion is held. At the time of cooling, the load resistor RL of the sensor 02 increases in its resistance value in air but does not increase in its resistance value in gas. When the gas is detected from the reduction in the resistance value of the metal oxide semiconductor film RL enhanced in sensitivity at the time of cooling, the sensitivity of the sensor 02 is enhanced and the concn. dependence of the gas can be enhanced.

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention relates to the detection of gases using changes in the resistance value of metal oxide semiconductor films, such as hydrogen sulfide, ethanol, carbon dioxide, etc.
O1 Used to detect gases such as isobutane.

[Prior Art 1 Japanese Patent Application Laid-open No. 1-206.252 discloses detecting gas by driving a gas sensor by pulse heating.

In this technique, the sensor is basically placed at room temperature (R, T) except during pulse heating, and the duty ratio of pulse heating is reduced to reduce power consumption.

The sensor requirements for this technology are that the thermal time constant is short;
The gas sensor can be heated in a short time. As long as this condition is met, the structure of the sensor is arbitrary. In principle, detection is performed from the output during pulse heating, but it can also be detected from the output during cooling. However, in this technique, the effect of pulse heating temperature is not studied much.

Next, the inventors of this application studied the pulse heating drive of the gas sensor and set the temperature during pulse heating to 100 to 3
It was discovered that when the temperature is set to 00°C, the sensor senses only humidity and the gas sensitivity disappears (Patent Application No. 1-128.468)
issue).

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

(1) Sensor resistance in the air increases dramatically during cooling.

However, the sensor resistance during pulse heating does not particularly increase.

(2) Sensor resistance in gas during cooling does not particularly increase;
There is not much difference from the sensor resistance in gas during pulse heating.

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

[Objective 1 An object of the present invention is to improve the sensitivity of a gas sensor and to improve the gas concentration dependence of the gas sensor.

[Structure of the Invention] In the present invention, a metal oxide semiconductor film of a gas sensor is heated to a high temperature in a pulsed manner to form active oxygen ions on the surface of the metal oxide semiconductor. Next, when the metal oxide semiconductor film is rapidly cooled, the metal oxide semiconductor film is cooled while retaining the generated active oxygen ions, so that the resistance value in the air during cooling increases dramatically. The resistance value in air during cooling is several MΩ or more, usually IOMΩ or more, and is substantially infinite. On the other hand, the resistance value in gas during cooling does not increase much. As a result, the sensitivity of the gas sensor increases.

Furthermore, since the ratio of resistance values in air and gas is large, the dependence of the sensor on gas concentration is also improved.

The reason for the increase in the resistance value in air during cooling is thought to be not only the adsorption of active oxygen ions by pulse heating but also the removal of adsorbed water by pulse heating. However, according to the inventor's experiments, both the sensor output during pulse heating and the sensor output during cooling are affected by humidity. Therefore, it is difficult to imagine that adsorbed water will be completely removed by pulse heating. The remaining cause of the increase in resistance value in air during cooling is the production of active oxygen ions during pulse heating.
This is the only thing that remains upon cooling.

When the metal oxide semiconductor film is a 5n02 film, the increase in resistance value in air during cooling is due to the pulse heating temperature being 450°C.
By doing the above, the problem becomes significant. This temperature is Sn
Literature values for the temperature at which active oxygen ions such as o- and o”- are formed on the O1 surface, (Egashira ^, C, S, Symp
osium 5erial No. 309. p71
゜1986), which is almost the same. It is also known that the formation of these adsorbed oxygen ions significantly increases the resistance value of metal oxide semiconductors such as SnO.

These facts confirm that the increase in resistance value in air during cooling due to pulse heating is due to the formation of active adsorbed oxygen ions.

Next, this invention will be explained individually. In order to shorten the thermal time constant, the gas sensor has a film-like structure (II thickness lO ~ 20 μm).
A metal oxide semiconductor with a thickness of about 1 pm or a thin film of 1 pm or less is used. Metal oxide semiconductors include, for example, SnO
, In, 03, and ZnO are used.

Regarding the structure of the gas sensor, it is important to reduce the thermal time constant of the metal oxide semiconductor film.

If the thermal time constant is large and the cooling from pulse heating is slow, active oxygen ions are lost during cooling, and the resistance of the metal oxide semiconductor during cooling does not increase much and the gas sensitivity is low. For example, in a commonly used chip-shaped gas sensor with a metal oxide semiconductor bottom, the thermal time constant during cooling is extremely slow, ranging from 10 seconds to several tens of seconds, and even when cooled from high temperature to room temperature, the resistance at room temperature is The value is not infinite, and the gas sensitivity at room temperature is also low. On the other hand, if the thermal time constant during cooling is shortened, e.g.
oOmsec or less, more preferably lQ+++sec
If it is set below, the resistance value during cooling after pulse heating will be infinite and the gas sensitivity will also be high. Furthermore, reducing the thermal time constant has the effect of shortening the time required to reach the maximum heating temperature during pulse heating (hereinafter referred to as "pulse temperature") and shortening the width of the heating pulse. This means that
This will reduce the power consumption of the gas sensor.

The temperature of pulse heating needs to be 450°C or higher in the case of a SnO2 film, and was set at 500°C in the example. 5n
When using a metal oxide semiconductor other than 02, depending on the change in the formation temperature of active oxygen ions in the metal oxide semiconductor,
Change pulse temperature.

The temperature during cooling is preferably room temperature in order to save power consumption, but since the temperature dependence of the resistance value in air or gas near room temperature is small, the temperature may be slightly higher than room temperature. 5n02
In the case of a film, the temperature during cooling is 150°C or less.

The time width of the pulse heating is set to be enough to raise the temperature of the metal oxide semiconductor film to a temperature at which active oxygen ions are formed. In the example, a heating pulse with a width of 20 II sec was applied to a gas sensor with a thermal time constant of 2 O-sec during temperature rise.

It is preferable that the time width during cooling is equal to or longer than the time required for the characteristics of the metal oxide semiconductor film to reach a steady value during cooling. This time is about 3 under the conditions of the example using 5n01.
It was 00m5ec. Next, the upper limit of the cooling time is
not important. When the metal oxide semiconductor film is left at room temperature for several minutes after cooling, the resistance value of the metal oxide semiconductor film decreases to a value that can be practically read, and the gas sensitivity also decreases. However, it is sufficient to sample the output before the resistance value of the metal oxide semiconductor film decreases, and there is no problem even if the resistance value decreases after sampling.

[Example] Measurement method The measurement method used in the example will be explained with reference to FIGS. 7 to 1O. FIG. 7 shows an example of the gas sensor 02. 2 is a linear heater made of Fe-Cr-Al alloy with a wire diameter of 20 μm, 4 is an alumina insulating film with a film thickness of lpm, and 6.8 is a film-like gold electrode. 1O is a 5n02 film with a thickness of 0.5 μm, and the surface has a film thickness of 0.5 μm. CuO films of 1/Jm or less were laminated. The CuO film is a catalyst that promotes adsorption and desorption of oxygen ions, and
od.

Any oxygen ion adsorption/desorption catalyst such as Mn2O3 or Cr2O may be used instead. The structure of the gas sensor 02 is the same as the applicant's gas sensor "TGS501" except that a CuO film is provided. Gas sensor 02 was manufactured as follows.

Alumina sol was attached to the surface of the heater 2 and thermally decomposed at 800° C. to form an alumina insulating film 4. Next, gold electrode 6.
8 was vacuum-deposited, a solution of an organic compound of Sn was attached to the surface of the alumina insulating film 4, and after drying, it was thermally decomposed at 550° C. to form a SnO film 10.

This sensor 02 was used because the thermal time constant during heating is 29.
This is because the thermal time constant during cooling is as short as m5ec and 5 wesec, and other sensors may be used as long as the thermal time constant is small. For example, a heater and a metal oxide semiconductor film may be laminated on an alumina substrate with a heat insulating glass film interposed therebetween. In this case, short-time pulse heating causes the substrate with a large heat capacity to heat up, but is kept at room temperature. When the heater is turned on/off here, the metal oxide semiconductor film is laminated on the heater and the distance between the two is small, so the temperature of the metal oxide semiconductor film changes following the on/off of the heater. As a result, the thermal time constant during heating can be set to about several m5ec. In addition to this, a gas sensor may be used in which a heater, an electrode, and a metal oxide semiconductor film are formed on a thin film such as Sing cross-linked on a cavity.

FIG. 8 shows an example of the circuit. In the figure, 12 is a power supply,
Here, a voltage of 5V was used. 14 is a power supply for pulse heating, and here the output is 0.65V.

16 is a transistor switch, which may be any switch such as a relay. R1 is the load resistance of the gas sensor 02, and a resistance of 30KΩ to 300Ω was used for the measurement. 18 is a timer for control, which operates at a cycle of 1 second, once every second at 20II.
Only for ISeC, turn on switch 16 and set pulse temperature to 5.
Pulse heating is performed at 00°C. 20 is a sample hold circuit that samples the sensor output during cooling (output to the load resistor R1) immediately before the heating pulse. As a result, the sensor 02 is driven at a cycle of 1 second, heated to a pulse temperature of 500° C. for 29 m5 ec, and kept at room temperature for the rest of the period.

Then, gas is detected from the output immediately before pulse heating (just before the end of the cooling time to room temperature).

FIG. 9 shows measurement conditions for sensor characteristics. As mentioned above, the SnO, film 1O is heated once every second for 20 m5ec at 0.
It is heated with a heater voltage of 65V to a maximum of 500°C. During other periods, the heater 2 is turned off and the SnO
, the membrane IO is kept at room temperature. Immediately before the end of the heating pulse and immediately before transition from room temperature to the heating pulse, sensor characteristics are sampled from the output of the load resistor R1.

FIG. 10 shows the temperature characteristics in N of the gas sensor 02. The vertical axis is the output to the load resistor R1, from time 0 to 100
During m5ec, a heating pulse is applied to heater 2, and SnO
, the membrane 10 is heated from room temperature. The output of the sensor is 20-2
A steady-state value is reached at 511 SeC, and from this it can be seen that the thermal time constant during heating is about 20 m5ec. On the other hand, time 0.
If the heating pulse is interrupted at 1 second, the sensor output will be 5 +1
The thermal time constant during cooling will decrease to several meters from now on.
It can be seen that it is about 5ec. Next, SnO,
The steady-state value of the temperature of the membrane IO is 500°C. SnO, film
Considering that the thermal time constant of O is about 2011 ISeC, the pulse temperature at a pulse voltage of 0.65 V is about 500
It is ℃.

Sensor characteristics FIGS. 1 to 3 show the gas sensitivity and gas concentration dependence of the gas sensor 02. The measurement conditions in FIG. 1 are that the operating cycle is 1 second, the heating pulse width is 2011 SeC, the pulse temperature is 500° C., and the temperature when no heating pulse is applied is room temperature. Note that the load resistance R1 was 100 KΩ, and 2 ppm of hydrogen sulfide was used as the gas. FIG. 1 shows the characteristics at 500° C. and the characteristics at room temperature. The resistance value in air at room temperature is infinite (more than 10 MΩ), and the sensitivity to 2 ppm hydrogen sulfide is extremely high.

Figure 2 shows the sensor output Vou when a heating pulse is applied.
The waveform of L is shown. A heating pulse is applied for 20 m5 ec from time 0, then cooled to room temperature. The resistance value in the air after cooling is extremely high, and the gas sensitivity is also high. Further, the time required for the resistance value to reach infinity at room temperature was usually about 10 seconds to 1 minute after cooling. Therefore, if the heating period is set to 10 seconds to 1 minute or less, detection can be performed while always keeping the resistance value in air at room temperature infinite. However, this does not preclude increasing the detection period to, for example, about 5 minutes. For example, if the output is sampled after about 1 second has elapsed after cooling to room temperature and the subsequent outputs are not used, no problem will occur even if the resistance value at room temperature decreases from infinity.

FIG. 3 shows gas concentration characteristics for hydrogen sulfide and ethanol. The figure shows 500℃ (pulse width 20 m5ec)
From the results of 5 seconds period between and room temperature, the resistance value at room temperature and 5
The resistance value at 00°C is shown. For comparison, SnO,
The results are also shown when the membrane IO was constantly maintained at 350°C. The gas concentration dependence at room temperature is large.

Figure 4 shows the influence of humidity. The pulse heating conditions were a pulse voltage of 0.65V and a width of 20m5ec.

(2) The period is 5 seconds, and the load resistance R1 is 30Ω.

For measurement, hydrogen sulfide 1 ppm (humidity 50% or 100%)
), and was kept in air with a humidity of 50% for the rest of the time. The dashed line at the top of the figure represents the sensor output at 500°C, and the dashed line at the bottom represents the output at room temperature (just before the heating pulse), where the ambient temperature was 20°C. The second peak in the figure (hydrogen sulfide tl) pm.

humidity 50%) and the third peak (fL hydrogen hydride lppm.

Humidity lOO%)
The response when returned to air at 0°C, R, 8, and 50% is different. This is because 5n02 film 10
This shows that the adsorbed water on the sensor is not completely removed, and the influence of humidity remains on the sensor characteristics.

Possible causes of the dramatic increase in resistance value in air at room temperature due to pulse heating at 500'O are: (1) Active oxygen ions adsorb to the SnO2 surface due to pulse heating, increasing the resistance value of the sensor. or (2) water adsorbed on the surface of SnO is completely removed. From the results shown in FIG. 4, it is difficult to imagine that adsorbed water would be completely removed by pulse heating. Therefore, it is thought that the reason why the resistance value of the sensor becomes infinite is that active oxygen ions are formed on the surface of SnO during pulse heating.

Figure 5 shows the pulse temperature at 30°C under the same measurement conditions as Figure 1.
The results are shown when changing the temperature from 0°C to 600°C. With increasing pulse temperature, the resistance in air increases and the resistance in gas decreases. This effect is significant above 450°C. This temperature also coincides with the adsorption temperature of active oxygen ions to 5n02.

Figure 6 shows the pulse temperature at 500°C (pulse intensity 2Q m5
ec, pulse period 1 second), and the resistance value during cooling is shown when the temperature during cooling is varied in the range from room temperature to 300°C. There is no significant difference in the results whether the temperature during cooling is room temperature or 100°C. However, if the temperature during cooling is set to 200° C., the resistance value in air decreases, resulting in lower sensitivity. Therefore, the temperature during cooling should be 150°C or less, more preferably 150°C or less.
00°C or less is preferable.

[Effects of the Invention] The present invention (1) increases the sensitivity of the gas sensor, and (2) increases the dependence of sensor characteristics on gas concentration.

[Brief explanation of drawings]

FIG. 1 is a characteristic diagram showing gas detection results in an example. FIG. 2 is a characteristic diagram showing the gas sensor characteristics near the heating pulse in the example. FIG. 3 is a characteristic diagram showing gas concentration dependence in the example. FIG. 4 is a characteristic diagram showing the influence of humidity on the gas sensor characteristics in the example. 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 influence of temperature during cooling in the example. FIG. 7 is a plan view of the main parts of the gas sensor used in the example. FIG. 8 is a circuit diagram of the embodiment. FIG. 9 is a waveform diagram showing heating pulses in the example. FIG. 1O is a characteristic diagram showing the thermal response characteristics of the gas sensor used in the example. Fig. HzS (ppm) Fig. 5 RILSE TEMP, ('c) No. R,T. 00 00 00 TEMP, at PLILSE OFF ('c) Fig. 2 Fig. HEATERC0NTR0L 119 Maro No. 10 Fig. IME (sec)

Claims (6)

[Claims] (1) In a method of detecting gas by subjecting the metal oxide semiconductor film of a gas sensor to a cycle of pulse heating and cooling, active oxygen ions are adsorbed onto the metal oxide semiconductor surface by pulse heating, and the adsorbed oxygen ions are removed. By cooling the metal oxide semiconductor film while maintaining the temperature, the resistance of the metal oxide semiconductor film increases in the air during cooling, making the metal oxide semiconductor film highly sensitive. A gas detection method characterized by detecting gas from a decrease in resistance value of a metal oxide semiconductor film. (2) In a method of detecting gas in a metal oxide semiconductor film of a gas sensor through a cycle of pulse heating and cooling, pulse heating substantially reduces the resistance value in air when the metal oxide semiconductor film is cooled. It is assumed to be infinite, and the resistance value decreases during cooling due to contact with gas of the metal oxide semiconductor film.
A gas detection method characterized by detecting a gas. (3) By placing a gas sensor using a SnO_2 film in a cycle of pulse heating and cooling at a temperature of 450°C or higher, the resistance value of the SnO_2 film in the air during cooling is placed in a high resistance state, and when cooling A gas detection method in which gas is detected from the resistance value of the SnO_2 film in the gas. (4) The gas detection method according to claim 3, wherein the temperature of the SnO_2 film during cooling is set to room temperature. (5) The gas detection method according to claim 4, characterized in that the thermal time constant of the SnO_2 film is made shorter than the time during which oxygen ions relax to an inactive state during the cooling process. (6) The gas detection method according to claim 5, characterized in that the surface of the SnO_2 film is coated with an oxygen ion adsorption promoting catalyst.