JPH01206252A - Method of detecting gas and apparatus therefor - Google Patents

Abstract

PURPOSE:To reduce power consumption of a gas sensor by a method wherein a heater and a gas-pressure-sensitive element are laminated on a board, a power pulse of a width 1 second or below and a duty ratio 1/10 or below is given to the heater to heat the gas-pressure-sensitive element intermittently, and thereby a gas is detected. CONSTITUTION:A gas is detected by a gas sensor 2 fabricated by laminating a heater 12 and a gas-sensitive element 20 on a substrate 4. A power pulse of a width 1 second or less and a duty ratio 1/10 or below is given to the heater 12, so as to heat the gas-sensitive element 20 intermittently. By this heating the sensitive element 20 is activated intermittently and thereby the gas is detected. The width of the power pulse is set to be 50m/sec or below, and the interval thereof is set to be 500m/sec or above. It is preferable that the sensitive element 20 is made filmy with a film thickness set to be 50mu or below. By this constitution, power consumption of the gas sensor is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to gas detection, and particularly to reducing power consumption of a gas sensor used for detection.

[Prior Art] Reducing the power consumption of gas sensors is one of the fundamentals in the field of gas detection. Research to date has focused on miniaturizing sensors.

However, it is also true that there is a limit to the miniaturization of sensors.

The inventor considered applying a short heating pulse to the gas sensor to intermittently heat the gas sensor to operate the sensor. The inventor also considered reducing power consumption by using an existing sensor structure without complicating the structure of the gas sensor.

In addition, related prior art is shown here. Jitsukō 55-92
No. 0 proposes laminating a heater and a gas sensing section on a heat-resistant insulating substrate to form a gas sensor. That is, the gas sensor used in this invention is known per se. However, this publication does not mention intermittent heating of the gas sensitive section.

U.S. Patent No. 3906.473 discloses that a gas sensor is heated alternately to a high temperature range and a low temperature range, and CO is calculated from the output in the low temperature range.
proposed to detect.

The heating cycle in this case is, for example, 20 seconds on the high temperature side and 40 seconds on the low temperature side.

[Invention Section: jli] An object of the present invention is to reduce the power consumption of a gas sensor.

[Structure and function of the invention] The focus of this invention is on the following points.

(1) The sensor operates even if the gas-sensitive part of the sensor is heated for only a short period of time. For example, once per second 0°2
The sensor operates even if the gas sensitive part is heated by m5ec. Previous research has not determined a lower limit for the heating time required to operate or activate the sensor. The inventor has also discovered that the heating time required to activate the sensor is quite short. Therefore, power consumption can be reduced by intermittently heating the gas sensitive part of the sensor for short periods of time and substantially stopping heating the gas sensitive part during other periods.

(2) In order to activate the gas sensitive part by heating for a short time, it is necessary to increase the efficient conduction between the heater and the gas sensitive part.

If the conduction between the heater and the gas sensitive part is fast, the gas sensitive part can be heated in a short time. On the other hand, if heat conduction is slow during this time, the power pulse to the heater must be lengthened.

(3) Considering these requirements and not complicating the structure of the sensor, a sensor in which a heater and a gas sensing section are stacked on a substrate is suitable. In this structure, since the gas sensing section is stacked on the heater, heat conduction between the two is fast. On the other hand, the increase in power consumption due to the use of a substrate is not very large. Assuming that the heat conduction from the heater to the gas sensitive section is equal to the heat conduction to the substrate, the increase in power consumption by the substrate is only twice as large. This is extremely small compared to the effect of shortening the heating time of the gas sensitive section.

Furthermore, if a heat insulating layer such as a glass layer is provided between the heater and the substrate, heat loss to the substrate can be further reduced.

For example, the thermal conductivity of glass is only about 1/20 of that of alumina for the substrate.

In this invention, gas is detected based on these findings. That is, the heater and the gas sensitive part should be placed in a circle on the substrate to speed up heat conduction between the two. Then, a power pulse with a width of 1 second or less and a duty ratio of 1/10 or less is applied to the heater to intermittently heat the gas sensing part and detect gas.

The lower limit of the width of the power pulse is determined by the time required to activate the gas sensitive part. This time is 0.
It is less than 2w+sec. The lower limit of the pulse width is also limited by the time required for heat conduction from the heater to the gas sensitive section. Even if the width of the power pulse is made extremely short, the result will be the same as applying a longer power pulse because it takes time for heat to be conducted from the heater to the gas sensitive part. Therefore, the lower limit of the power pulse width has no particular meaning.

In this invention, short power pulses are applied intermittently;
The heating pulse ends before the substrate heats up. Therefore, the temperature of the substrate does not substantially increase, making it easier to handle the substrate. In traditional detection methods, the entire substrate is heated;
The board must be insulated from the housing. Therefore, the holding strength of the substrate is low, and the process of assembling the sensor becomes complicated.

On the other hand, if the width of the power pulse is shortened to prevent the temperature of the board from rising, the board can be directly fixed to the housing, increasing the holding strength of the board, and at the same time simplifying the assembly of the sensor.

Next, the gas sensitive part is preferably thin and has a small heat capacity. This is because if the heat capacity is large, it takes time to raise the temperature of the gas sensitive part, and the width of the power pulse cannot be shortened. For this purpose, it is preferable that the gas sensitive part is in the form of a film, especially m
It is preferable that W is 50 μm or less, more preferably 20 μm or less.

The material of the gas sensitive part is SnO, In1Os, BaS.
metal oxide semiconductors such as now, Pet's, Tilt,
An ion conductive solid electrolyte such as ZrOt or 5btOs or a catalytic oxidation catalyst is used. For catalytic oxidation catalysts,
Heat generated by the combustion of flammable gas is detected using a resistance temperature detector, etc.

If it is necessary to insulate the gas sensitive part and the heater, an insulating layer such as glass is provided between them. The insulating layer is preferably thinner than the heat insulating layer between the heater and the substrate so as not to impede heat conduction.

To carry out such a gas detection method, for example, a suitable pulse generating means is provided, a switch connected to the heater is turned on, and the heater generates heat in a short pulse manner. Then, gas is detected from the peak value of the sensor output or from the sensor output at a specific point in time.

[Example 1] Figs. 1 to 8 show an example in which a metal oxide semiconductor such as Snow is used for the gas sensitive part.

An aqueous solution of SnC14 was neutralized with ammonia to precipitate a stannic acid sol. After drying the sol, 700℃ in air
After firing for 1 hour, SnO was obtained. 5nO1 is irises i,
It may be changed to a metal oxide semiconductor such as 03 or ZnO) FewO+. Using this Sn0m, the gas sensor 2 shown in FIG. 1 was obtained.

In the figure, 4 is an alumina substrate with a thickness of 0.3+m, and a glass heat insulating layer 6 with a thickness of 50 μm was provided on the entire surface thereof. The heat insulating layer 6 is made of undercoat glass for IC manufacturing, and its thermal conductivity is about 1/20 that of the alumina substrate 4.

On the heat insulating layer 6, a gold Ti stock 8.10 and a Ru Ot heater (IA thickness 20 μm) were provided. Rub, over the heater 12, lay a layer of overcoat glass 14 for IC manufacturing (WX thickness 10 μm), and on top of that, gold electrodes 16.18 and S
A film 20 (film thickness lθμs) of nO was provided. All of these films were provided by printing. Snow0w has a total of 20 sensing parts.

Here, the board 4 is a Ru Ot heater! Since the substrate 4 is insulated from the substrate 2, a metal such as stainless steel may be used for the substrate 4.

When using an insulator for the substrate 4, especially when using glass with low thermal conductivity, the insulating film 6 may not be provided. The glass film 14 may be, for example, a 5ift film or an Altos film. These films have higher thermal conductivity than overcoating glass and can be made thinner. For example, when a silica film or an alumina film is provided by CVD or by ripe decomposition of silica sol or alumina sol, an insulation resistance of IMΩ or more can be obtained with a film thickness of 50 A to 1 μm. When one electrode of the SnO film 20 also serves as the heater 12, one of the electrodes 16 and 18 may be omitted. In this case, it is preferable that the heater 12 be made of a noble metal such as a PT film, and the insulating layer 14 becomes unnecessary. Furthermore, as a parallel resistance of the heater 12, Snow0w total 20
, Snown0w may be detected as a change in the parallel resistance of the heater 12. In this case, electrode 16.
18 is unnecessary.

FIG. 2 shows the product W5 structure of the gas sensor 2.

FIG. 3 shows the overall structure of the sensor 2. In the figure, 2
Reference numeral 2 denotes a housing, and a substrate 4 is fixed to a recess provided in the center with an adhesive. Further, 24 is a lead fixed to the housing 22, and is wire-bonded to each electrode 8.10, 16.18. Note that the glass layer 14 was not provided at the wire bonding portion of the electrode 8.10, making wire bonding possible. In this embodiment, the substrate 4 is used at almost room temperature. Therefore, it is not necessary to separate the housing 22 and the substrate 4, and it is possible to fix them with an adhesive. The gas sensor 2 is completed by combining the housing 22 with a cover (not shown).

FIG. 4 shows an example of a circuit using a peak hold circuit.

In the figure, Eb is an appropriate power source such as 5V, and 42 is an oscillation circuit as a pulse generating means, and here, the pulse width is 10 times per second.
Outputs esec pulse. Tr is a transistor switch, which may be any switch such as a relay, and connects the power source Eb to the heater 12 by the output pulse of the oscillation circuit 42.

R1 is a load resistance of the sensor 2, and the voltage across it is taken as the sensor output V.

A1. A2 is an operational amplifier, D! ~D3 is a diode, C
1 is a capacitor, R2-R4 are resistors, and these constitute a peak hold circuit 44.

That is, the peak of the output V is accumulated in the capacitor Ct, and is taken out from the operational amplifier A2 as the external output Vout.

FIG. 5 shows a modification in which the sampling timing of the sensor output V is specified. In this modification, a sampling signal is generated in the delay concave path 52 when a predetermined i suitable time has elapsed after the output pulse of the oscillation circuit 42 ends. This signal causes the A/D conversion circuit A/D with a built-in sample and hold circuit to
, trigger the edge. Then, the A/D converted output is displayed on the display circuit D1play, and the output of the A/D conversion circuit is taken out as a control output Control.

FIG. 6 shows operational waveforms of the embodiment. A power pulse with a width of 10 m5ec and a power of 800+aWatt was applied once per second to the sensor 2. Effective power consumption is 8 mWatt. In addition, when continuously heating sensor 2, turn off! 2 electric power is
When the sensor temperature is 350° C., it is about 200 steel Watt.

Along with this, the temperature Ts of the gas sensitive part (Sno, film 20) changes as shown in 2) in the figure. Between the power pulses, the temperature of the gas sensitive part is near room temperature, and since the heating time is short, the temperature of the substrate 4 is maintained near room temperature.

In response to this power pulse, the voltage V applied to the load resistance RN of 100 KΩ varied as shown in 3). 20 for the atmosphere
Results are shown for 100 PP in ethanol and 100 PP in CO using 65% relative humidity. Despite short intermittent heating, the sensor output is responsive to the atmosphere and can detect gas.

The peak output to ethanol is near the end of the power pulse, and there is a slight time delay between the peak output and the power pulse. Therefore, in the case of ethanol detection,
Preferably, the output is sampled near the end of the power pulse. Note that the delay in peeling is due to the Sn
This is probably due to the delay in heat conduction to the 0W film 20. The output to CO, on the other hand, remains after the end of the power pulse. Therefore, CO can be detected from the signal at any given point in time.

Note that as shown in 4) in the figure, i power pulses may be further divided into a plurality of power pulses. Also, although this is just a modification, a small amount of power may continue to be applied to the heater 12 between power pulses.

FIG. 7 shows the response waveform of IOOpprv to ethanol under the conditions of the example. The output was sampled using the circuit shown in FIG. 4 (as a general rule, the peak value of the sensor output V is sampled below). In the comparative example, the sensor 2 was continuously heated to 350°C.

Figure 8 shows the amount of ethanol in air and each t o ppm.
2 shows the resistance values of sensor 2 in water and isobutane.

In the comparative example, the sensor 2 was heated continuously. In the example, the power pulse conditions are 10 m5ec and 50 m/s.
There are three types: 5ec and 2m5ec, and the horizontal axis of the figure is electric power j.
The power during pulse application is shown. The effective power is multiplied by the duty ratio of the power pulse. 2 m5e
Gas was detected even at c/5ec (duty ratio 11500).

Table 1 shows the results in air and at 1100p for various heating conditions.
The resistance value of sensor 2 in ethanol of p is shown.

Table 1 Heating conditions Resistance value (Ω) Effective power 50m5e
c/sec IM 180K 40800
+IIWatt 10msec/sec 1.4M 280K
8800 mWatt 2msec/sec 2M 500K
1.6800 mWatt 1msec/see 3M 600K
1Watt 0.5m5ec/sec 2M 300K
1Watt 0.5m5ec/10sec 2M 40
0 K 0 , 1Watt 5msec/sin 400K 200K
O,”08Watt From the results in Table 1, the interval between power pulses is 2
It turns out that there is no problem up to a minute or so. And as the interval between power pulses increases, power consumption decreases.

Next, the narrower the width of the power pulse, the lower the power consumption. However, if the width of the pulse is made narrower, the power required to apply the pulse must be increased, so there is a limit to the reduction in power consumption.

According to the current results, even if the width of the power pulse is set to 2 m5ec or less, the effective power consumption does not decrease much. Note that the lower limit of the width of the power pulse has no particular meaning. For example, the width IOμse
This is because even if a power pulse of approximately c.

The results in Table 1 are gold li! This is not due to the manufacturing conditions of the oxide semiconductor. In the invention, a SnO film with a thickness of 1 μm was obtained by decomposing a compound of Sn (tin octylate). When we experimented with this as sensor 2 in Fig. 1, we found that in the case of continuous heating at 200 mWatt, (J
The t resistance value was 120Ω, and the resistance value in 100 ppI11 ethanol was 20Ω. When a power pulse of 800 mWatt is applied under heating conditions of 5 m5ec/see, the resistance value in air is 200Ω, and in ethanol it is 30Ω.
It was Ω.

[Example 2] Figures 9 and 10 show an example using a hydrogen ion conductive solid electrolyte such as Sb or O8. In the gas sensor 92 shown in FIG. 9, 94 is a printed film of Sb*Os (10μ
96 is a ventilation-limiting coating that covers only one of the electrodes. For example, a densely printed alumina or silica film may be used for the ventilation restriction [96].

Figure 10 shows an example of the auxiliary circuit, with Sb! as a gas sensing part! [Electrode of OS film 94] 6 and 18 are short-circuited, and R7
These gases are detected from the short circuit current caused by CO and CO. In this case as well, the Sb*Os film can be activated with a power pulse of about 10 m5ec/see. Note that Sb
*The Os film may be replaced with another solid electrolyte such as a Zr0t film.

[Example 3] Figures 11 to 13 show an example using a catalytic oxidation catalyst. In FIG. 1I, the structure of sensor 112 is similar to that of FIG. 1, 112 is a gold pole, 114.
6 is a heater using a PT film, which also serves as a temperature measuring resistor. 11B is an oxidation catalyst FJ (film thickness: lOμm) in which 2vt% PL catalyst is supported on a γ-alumina carrier; 120 is a p
t-catalyst-free γ-alumina (film thickness: lOμw+). All of these items were formed by printing. FIG. 12 shows a cross-sectional view of the sensor 112.

FIG. 13 shows an example of ancillary circuit. In the figure, 132
is a timer that outputs a pulse of, for example, 10 m5ec/see, which turns on the transistor Tr and causes the heaters 114 and 116 to generate heat. R5, R6
is a resistor, and constitutes a bridge circuit with heaters 114 and 116, and the bridge output is amplified by a differential amplifier 134, and the A/
Input to D conversion circuit A/D. Then, take out the sampling signal from the timer 132, for example, just before the transistor Tr is turned off, and operate the A/D conversion circuit A/D.

In this embodiment, the carrier layer 118 and the γ-alumina layer! 2
0 is intermittently heated by turning on the transistor Tr. The activation of the catalyst layer 118 by this heating is used to combust the combustible gas. This combustion heat is a heater! 14, and the temperature change of the heater 114 is extracted as a detection output. When continuously heating sensor II2 (operating temperature 380℃
), the power consumption was approximately 400 mWatt. In the example, the sensor 112 was operated by applying a power pulse of 1.6 Watt with a width of 10 m5 ec once every 10 seconds. The power consumption is I, 6 mWatt. The gain of the differential amplifier 134 was determined so that an output of 30 mV could be obtained for 1000 ppI11 of isobutane during continuous heating. In the example, 40 mV for isobutane of looOppm.
The output was obtained.

7 Effects of invention] This invention provides the following effects.

(1) Power consumption of the gas sensor can be reduced.

(2) A known structure of the gas sensor can be used, and the sensor structure does not become complicated due to reduction in power consumption.

(3) The temperature of the substrate itself used in the gas sensor does not rise, making it easier to assemble the sensor and improving the holding strength of the substrate.

(4) If a heat insulating layer is provided between the substrate and the heater, heat loss to the substrate can be suppressed and power consumption can be further reduced.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a gas sensor used in an example. FIG. 2 is an enlarged cross-sectional view of the main parts of the gas sensor used in the example. FIG. 3 is a plan view showing the overall structure of the gas sensor used in the example. FIG. 1 is a circuit diagram of an embodiment. FIG. 5 is a circuit diagram of a modified example. Figures 6 1) to 4) are waveform diagrams of the example, Figure 6 1) is a waveform diagram of power pulses, Figure 6 2) is a waveform diagram showing temperature changes in the gas sensing section, and Figure 6 3). 6 is a waveform diagram showing the output of the gas sensor, and FIG. 6 (4) is a waveform diagram of a modified power pulse. FIG. 7 is a characteristic diagram showing the response characteristics of the example to 1100 pp of ethanol. FIG. 8 is a characteristic diagram showing the relationship between sensor resistance and power pulse in the example. FIG. 9 is a sectional view of the gas sensor used in the second example. FIG. 1O is a circuit diagram of the second embodiment. 1st! The figure is a sectional view of the gas sensor used in the third example. FIG. 12 is a sectional view in the cutting direction of FIG. 11. FIG. 13 is a circuit diagram of the third embodiment. In the figure, 2.92,112 gas sensor, 4 substrate, 6 heat insulation layer, 12.114.116 heater, 14 insulation layer, 20,94,118 gas sensitive section. Figure 1 Figure 2, Figure 3 1f16 Figure 7 Figure T (min) Figure 9 Figure 10 m Figure 11 Melon 12 va Figure 13

Claims (16)

[Claims] (1) In a method of detecting gas using a gas sensor in which a heater and a gas sensing section are stacked on a substrate, a power pulse with a width of 1 second or less and a duty ratio of 1/10 or less is applied to the heater to intermittently activate the gas sensing section. 1. A gas detection method, characterized in that the gas is detected by heating the gas and intermittently activating the gas sensitive part by this heating. (2) The gas detection method according to claim 1, wherein the width of the power pulse is 50 msec or less, and the interval between the power pulses is 500 msec or more. (3) The width of the power pulse is 20 msec or less, the interval between power pulses is 0.8 seconds or more, and the duty ratio of the power pulse is 1
3. The gas detection method according to claim 2, wherein the gas detection method is 40% or less. (4) The width of the power pulse is shorter than the time required for the temperature of the gas sensitive part to reach a steady value, and the gas sensitive part is allowed to cool naturally to around room temperature between power pulses. The gas detection method according to claim 1, characterized in that: (5) The gas detection method according to claim 1, characterized in that no power is applied to the heater between power pulses. (6) The gas detection method according to claim 1, characterized in that the gas is detected based on the characteristics when the temperature of the gas sensitive section is increased by a power pulse. (7) The gas detection method according to claim 1, characterized in that the gas is detected from the characteristics of the gas sensitive section during cooling between the power pulses. (8) The gas detection method according to claim 1, wherein the gas sensing portion is a film-like metal oxide semiconductor. (9) The gas detection method according to claim 1, wherein the gas sensing portion is a film-like catalytic oxidation catalyst. (10) The gas detection method according to claim 1, wherein the gas sensing section is a membrane-like solid electrolyte. (11) A film-like heater is provided on the substrate, and a film-like gas sensitive part is laminated on this heater to enhance heat conduction from the heater to the gas sensitive part, and each power pulse to the heater Gas detection involves raising the temperature of the gas sensing part before the temperature rises, and intermittently applying this power pulse to intermittently heat the gas sensing part, and activating the gas sensing part by this heating to detect gas. Method. (12) The gas detection method according to claim 11, characterized in that a heat insulating layer is provided between the substrate and the heater. (13) The gas detection method according to claim 12, wherein the heat insulating layer is a glass layer. (14) In a gas detection device using a gas sensor in which a heater and a gas sensing part are laminated on a substrate, a pulse generating means for emitting an output pulse having a width of 1 second or less and a duty ratio of 1/10 or less; What is claimed is: 1. A gas detection device comprising: a switch operated by the switch; and a power source for applying power to a heater of a gas sensor when the switch is operated. (15) The gas detection device according to claim 14, further comprising a peak hold means for sampling the peak value of the output of the gas sensing section. (16) The gas detection device according to claim 14, further comprising sampling means for sampling the output of the gas sensing section at a specific point in time.