JP5773419B2 - Gas sensor and gas detector - Google Patents

Description

  The present invention relates to gas detection by a MEMS gas sensor, and particularly to prevention of poisoning by an organic solvent or the like.

The inventors are developing a gas sensor using a microfabrication on a silicon substrate (sometimes referred to as a MEMS sensor), and in particular developing a sensor that detects methane using a SnO2 thick film. The MEMS sensor is equipped with an activated carbon fill to remove miscellaneous gases such as organic solvents, and is heated to around 500 ° C. for 0.1 seconds in a cycle of 30 seconds, for example, to detect methane, and the others are left at room temperature. The inventor has found that this MEMS sensor is poisoned when exposed to a high concentration of ethanol for a long time. And from experiments on poisoning,
・ Ethanol passes through the activated carbon filter and reaches the SnO2 membrane.
・ The resistance value of SnO2 film in methane, resistance in hydrogen, and resistance in air increased.
・ Methane sensitivity (ratio of resistance in air to resistance in methane) decreases,
・ It is not enough to completely burn the adsorbed ethanol by heating to the detection temperature of methane (470 ℃) every 30 seconds for 0.1 second.
It has been found.

  As a measure against poisoning with ethanol, it is difficult in terms of power consumption to increase the heating time to the methane detection temperature. Covering the SnO2 film with a transition metal oxide film having high oxidation activity is not preferable because the transition metal may diffuse into SnO2 and change the characteristics. Increasing the amount of activated carbon is not preferable in that the ventilation resistance greatly increases while the effect of improving the poisoning resistance is small. The inventor studied the prevention of poisoning by ethanol or the like while keeping the configuration of the existing MEMS sensor basically unchanged and making the increase in ventilation resistance small and the increase in cost small. Invented.

  Related prior art is shown. Patent Document 1 (JP2825149) proposes that a catalytic combustion gas sensor is provided with a siloxane gas filter in which activated carbon and Pt are mixed with cellulose. However, Patent Document 1 does not mention poisoning with ethanol or the like, a MEMS type gas sensor, or a sensor using a SnO2 film.

JP2825149

  An object of the present invention is to prevent poisoning of a gas sensor by an organic solvent or the like with a simple configuration.

In the present invention, the insulating film on the surface of the silicon substrate is provided with a SnO2 film, a gas detection unit including an electrode connected to the SnO2 film and a heater, and a cavity is provided around the gas detection unit at the bottom of the insulating film In the gas sensor in which the silicon substrate is accommodated in a housing,
An adsorbent of an organic solvent that is activated carbon and does not contain a noble metal and an oxidation catalyst of an organic solvent that is an activated carbon supporting a noble metal are provided in the housing in order of the adsorbent and the oxidation catalyst. The detection atmosphere is guided to the gas detection unit through the adsorbent and the oxidation catalyst.

Oxidation catalysts such as activated carbon-Pt, activated carbon-Pd, activated carbon-RuO2, carbon black-Pt, Fe2O3-Au, LaCoO3-Pt, LaCrO3-Pt, MnO2-CuO-Pt have ethanol oxidation activity even at room temperature. . Therefore, by providing the housing with an oxidation catalyst that is activated carbon supporting noble metals such as activated carbon-Pt, activated carbon-Pd, activated carbon-RuO2, organic solvents such as ethanol can be removed, and poisoning of the gas detection unit can be prevented.
If an atmosphere to be detected is introduced in the order of adsorbent, oxidation catalyst, and gas detector by providing an adsorbent before the oxidation catalyst, a small amount of organic solvent that could not be treated with the adsorbent is treated with the oxidation catalyst. This reduces the burden on the oxidation catalyst. Therefore, a noble metal-supported oxidation catalyst such as Pt can be used efficiently.
In particular, the combination of activated carbon and precious metal-supported activated carbon makes it possible to unify the basic filter material into activated carbon.
In addition, in the case of a gas detector that heats the SnO2 film and the electrode with a heater, SnO2 is a mild oxidation catalyst compared to a catalyst in which Pt or the like is supported on γ-alumina or the like, so the influence of poisoning by ethanol or the like is particularly significant . For this reason, it is significant to prevent poisoning with an oxidation catalyst.

Further, according to the present invention, the insulating film on the surface of the silicon substrate is provided with a SnO2 film, a gas detection unit including an electrode connected to the SnO2 film and a heater, and a cavity is formed around the gas detection unit at the bottom of the insulating film. In a gas detection apparatus comprising: a gas sensor in which the silicon substrate is housed in a housing; a power source; and a drive circuit for the gas sensor.
An adsorbent of an organic solvent that is activated carbon and does not contain a noble metal and an oxidation catalyst of an organic solvent that is an activated carbon supporting a noble metal are provided in the housing in order of the adsorbent and the oxidation catalyst. The detection atmosphere is guided to the gas detection unit via the adsorbent and the oxidation catalyst,
The drive circuit changes the power to the heater between a low level suitable for evaporating or oxidizing the organic solvent from the gas detection unit, a high level suitable for detection of the detection target gas, and a zero level. It is comprised so that it may make it.
In this specification, the description relating to the gas sensor also applies to the gas detection device as it is.

In this way, poisoning can be significantly reduced by a combination of oxidizing the organic solvent with the oxidation catalyst and evaporating or oxidizing the organic solvent from the gas detector at a low level. The low level temperature is, for example, 60 to 200 ° C., preferably 60 to 120 ° C., particularly preferably 70 to 110 ° C., so that the increase in power consumption is within an allowable range. In this specification, when a range is indicated by ~, it means that it is not less than the lower limit and not more than the upper limit.

Cross section of gas sensor Cross section of sensor body Block diagram of gas detector It is a wave form diagram which shows the heating pattern of the gas sensor in an Example, 1) shows heating power and 2) shows the temperature of a gas detection part. The flowchart which shows the algorithm of an Example Characteristic chart showing the gas concentration in the cap of the gas sensor when exposed to 3000ppm ethanol for 2 days a day for 2 days A characteristic diagram showing changes in sensor characteristics when exposed to 3000 ppm ethanol for 20 days a day for 20 minutes. The sensor is equipped with 120 mg activated carbon and 30 mg activated carbon-Pt catalyst, 100 ° C x 0.4 seconds-470 ° C x Drive in order of 0.1 seconds-room temperature 29.5 seconds A characteristic diagram showing changes in sensor characteristics when exposed to 3000 ppm ethanol for 20 days a day for 20 minutes. The sensor is equipped with 120 mg activated carbon and 30 mg activated carbon-Pt catalyst, 470 ° C x 0.1 seconds, room temperature 29.9 seconds Drive alternately with A characteristic diagram showing changes in sensor characteristics when exposed to 3000 ppm of ethanol for 20 minutes a day for 21 days. The sensor is equipped with 150 mg of activated carbon and is driven alternately at 470 ° C x 0.1 seconds and at room temperature 29.9 seconds.

  BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an optimum embodiment for carrying out the present invention will be shown, but the present invention is not limited to the embodiment, and can be modified by adding matters known to those skilled in the art to the description of the specification and drawings.

  1 to 9 show examples and their characteristics. In the figure, reference numeral 2 denotes a gas sensor, in which a MEMS type sensor body 4 is fixed to a base, and a pad (not shown) of the sensor body 4 is connected to a pin 13 by a lead wire 11. The sensor body 4 is disposed in a space surrounded by the cap 12 and the base 10. 14 is an adsorbent made of granular or sheet-like activated carbon, 16 is an oxidation catalyst made of, for example, granular or sheet-like activated carbon-Pt, 18 is a nonwoven fabric for fixing the adsorbent 14 and the oxidation catalyst 16, and is made of paper or porous If the adsorbent 14 and the oxidation catalyst 16 are in the form of a sheet, the nonwoven fabric 18 is not necessary. Reference numeral 19 denotes a pressing ring for pressing the lower nonwoven fabric 18 in FIG. The structure, shape, and material of the gas sensor 2 is a MEMS type gas sensor. An adsorbent 14 is placed outside the air passage from the outside to the sensor body 4, an oxidation catalyst 16 is placed in the middle, and the sensor body 4 is placed inside. Points are important and other points are optional.

Here, the adsorbent 14 is granular activated carbon , and the shape may be granular or sheet-like. The oxidation catalyst 16 is granular activated carbon-Pt here, but Pt may be supported on sheet-like activated carbon . The activated carbon -Pd, may be in an oxidation catalyst such as activated carbon -RuO2, which are oxidation catalyst with ethanol oxidizing activity at room temperature. In order to oxidize an organic solvent such as ethanol at room temperature, the oxidation catalyst 16 is a catalyst in which a noble metal such as Pt, Au, Pd, Rh, RuO2 is supported on a support made of activated carbon . By arranging the adsorbent 14 in the preceding stage, the burden on the oxidation catalyst can be reduced and the required amount can be reduced. Moreover, the material can be unified by using the material of the adsorbent 14 as the carrier of the oxidation catalyst 16.

  FIG. 2 shows the structure of the sensor body 4. An insulating film 22 such as silica or tantalum oxide is provided on one surface of the silicon substrate 20, and a cavity 21 is provided at the bottom of the insulating film 22. A film-like heater 6 such as a Pt film is provided on the insulating film 22 above the cavity 21, and is covered with the second insulating film 23. On the insulating film 23, for example, electrodes 24 and 24 made of a pair of Pt films and an SnO2 film 8 are provided. The heater 6 may also be used as one electrode. In the embodiment, the thickness of the SnO 2 film 8 is 30 μm, and contains 1.5 mass% of Pd with respect to 100 mass% of SnO 2. The structure and material of the sensor 2 are arbitrary, and other metal oxide semiconductors such as WO3 and In2O3 may be used instead of SnO2. Further, a catalytic combustion type gas sensor for detecting a combustion heat of a combustible gas by supporting a catalyst such as Pt on a carrier such as γ-alumina may be used. However, according to the inventor's experience, poisoning with ethanol or the like was serious in a metal oxide semiconductor gas sensor such as SnO2, and was minor in a catalytic combustion type gas sensor. This is presumably because the catalytic combustion gas sensor has a sufficiently high ability to oxidize to ethanol or the like, so that poisoning is weakened.

  FIG. 3 shows a circuit configuration of the gas detection device 30. A load resistor 31 is connected to the SnO2 film 8, and a battery 32 is a power source. Reference numeral 34 denotes a microcomputer as a drive circuit for the gas sensor 2, which acts as a heater drive 36, a detection circuit 38, and a start circuit 40. The heater drive 36 controls the power to the heater 6 by, for example, PWM (pulse width modulation control), and drives the heater 6 in a cycle of, for example, 30 seconds in the order of low level (0.4 seconds), high level (0.1 seconds), and 0 level. To do. The detection circuit 38 detects methane, which is a detection target gas, from the resistance value of the SnO2 film 8 when the heater power is at a high level, or an amount corresponding thereto, in the embodiment, the voltage applied to the SnO2 film 8. The resistance value of the SnO2 film 8 or an amount corresponding thereto is called a sensor output, and the detection circuit 38 is provided with an AD converter to determine the resistance value of the SnO2 film and compare it with an appropriate reference value to obtain the methane concentration. Ask for. The detection circuit 38 may obtain the concentration of an organic solvent such as ethanol from the sensor output at a low level. When starting the gas detection device 30 that has been stopped, the start circuit 40, for example, when the battery 32 and the microcomputer 34 are connected by a switch (not shown) or the like, is 1 second to 20 seconds longer than the usual 0.4 seconds. A signal is sent to the heater drive 36 to heat the sensor 2 to a low level for a second, in the example, 4 seconds.

  4 and 5 show the driving algorithm of the gas sensor 2. Here, the temperature of the SnO2 film 8 at the low level is 100 ° C., and the temperature of the SnO2 film 8 at the high level is 470 ° C. The temperature at the high level is, for example, 300 ° C. to 550 ° C. Also, heating the sensor to a predetermined level or temperature means heating the SnO2 film 8 with a predetermined power or to a predetermined temperature. When the power is turned on, the sensor is heated to a low level, for example, for 4 seconds, preferably 1 to 20 seconds, and the organic solvent accumulated in the SnO2 film 8 during standing is evaporated or oxidized (step 1). Then, for example, in a 30 second cycle, the gas sensor 2 is driven, and the low level is set for the first 0.4 seconds, preferably the first 0.1 to 2 seconds (step 2), and then the high level for the next 0.1 seconds, preferably the next 0.02 seconds to 0.5 seconds. (Step 5), and in the remaining section, the heater is turned off (step 6). The period of 30 seconds is to make the detection delay of methane about 30 seconds, and the period is, for example, 5 seconds to 10 minutes.

  The adsorbent 14 adsorbs organic solvents such as ethanol and other poisoning gases such as silicone vapor. It is conceivable that a poison gas having a higher concentration as it passes through the adsorbent 14 exists for a longer time in the case of an organic solvent such as ethanol. When the organic solvent passes through the adsorbent 14, the low concentration organic solvent reaches the oxidation catalyst 16 for a long time. The oxidation catalyst 16 oxidizes the organic solvent and reduces the amount of the organic solvent that reaches the sensor body. The sensor is heated to, for example, 100 ° C. by heating to a low level. Since an organic solvent such as ethanol generally has a boiling point of slightly less than 100 ° C., the organic solvent evaporates from the sensor body 4 at 100 ° C. or is oxidized by SnO 2 supporting a noble metal. For this reason, it is possible to prevent the organic solvent from changing from the sensor body to a substance that is difficult to desorb due to condensation or the like.

  Since the gas sensor 2 has sensitivity to an organic solvent at a low level of heating temperature, the concentration of the organic solvent is obtained at a low level or an intermediate temperature between the low level and the high level. Also, heating to a low level is performed, and if it is less than a predetermined value, heating to a low level may be omitted from the next time (steps 2 and 3). This determination is performed, for example, every 100 cycles, preferably every 10 to 1000 cycles. The meaning of steps 2 and 3 is to save power consumption, and the problem is that there is a possibility that the organic solvent is accumulated in the sensor, so steps 2 and 3 may be omitted.

The characteristics of the gas sensor 2 are shown in FIGS. The total amount of activated carbon as adsorbent and activated carbon-Pt as the oxidation catalyst is 150 mg / sensor,
・ Conventional example in which all 150mg is simply activated carbon and no oxidation catalyst is provided,
An example in which 20 mg is activated carbon-Pt and 130 mg is simply activated carbon,
Examples where 30 mg is activated carbon-Pt and 120 mg is simply activated carbon,
Three types of gas sensors were manufactured. The Pt concentration in activated carbon-Pt is 5% by mass. The amount of noble metal of the oxidation catalyst 16 is preferably 0.5 to 5 mg per sensor, for example. In order to obtain the gas concentration in the cap, instead of the methane sensor, a MEMS type ethanol sensor (with a noble metal content in the SnO2 film reduced for ethanol detection and an operating temperature of 300 ° C at all times) was placed. Daily exposure to 3000 ppm ethanol for 2 hours a day. FIG. 6 shows the gas concentration in the cap (in terms of ethanol). With activated carbon alone, ethanol of several tens of ppm exists in the cap, but by adding activated carbon-Pt, the ethanol concentration in the cap becomes less than 10 ppm and the time lag until the ethanol concentration increases is lengthened. As described above, the activated carbon-Pt decreases the maximum value of the organic solvent concentration in the cap and lengthens the time until the organic solvent concentration increases.

  FIG. 7 shows a sensor having 120 mg of granular activated carbon and 30 mg of activated carbon-Pt (Pt 5 mass%) driven at a cycle of 30 seconds in which the sensor is left at 100 ° C. for 0.4 seconds, 470 ° C. for 0.1 seconds, and room temperature for 29.5 seconds. The characteristics when exposed to 3000 ppm ethanol for 20 minutes every day are shown. FIG. 8 shows the characteristics when the same sensor as FIG. 7 is exposed to 3000 ppm of ethanol for 20 minutes every day while being driven at a period of 30 seconds of 470 ° C. for 0.1 second and room temperature of 29.5 seconds. Fig. 9 shows the characteristics when a sensor having only 150 mg of activated carbon (without oxidation catalyst) is exposed to 3000 ppm of ethanol for 20 minutes every day while driving at 470 ° C for 0.1 seconds and at room temperature for 2 seconds for 30 seconds. . Two sensors were used for each measurement, and FIGS. 7 to 9 show average results.

  In FIG. 9, methane sensitivity was almost lost in one week, and the resistance value in methane, the resistance value in hydrogen, and the resistance value in air all increased. The poisoned sensor recovers by driving for about a month in a normal atmosphere. In FIG. 8, the influence of poisoning is small in the first three weeks, but it is greatly influenced in the fourth week. In FIG. 7, the influence of poisoning can be ignored even after 4 weeks. Since the boiling point of ethanol is a little less than 100 ° C, it can be estimated that the ethanol is evaporated from the SnO2 film by heating to 100 ° C. Moreover, since the ignition point of ethanol in SnO2 carrying a precious metal catalyst is less than 100 ° C, it is considered that the oxidation of ethanol in SnO2 also contributed. As described above, the influence of poisoning can be reduced by providing an oxidation catalyst, and the influence of poisoning can be ignored by combining this with heating to around 100 ° C.

  The inventor examined the optimum temperature at a low level. As a result, the same performance was obtained at 80 ° C and 100 ° C, and the same performance was obtained at 60 ° C and 120 ° C. However, it was found that the anti-poisoning performance is improved compared to the case where low level heating is not performed, but the effect is small. Therefore, the low-level heating temperature is most preferably 80 to 100 ° C, more widely 70 to 110 ° C, more widely 60 to 120 ° C, and most widely 60 to 200 ° C. The heating time per cycle at a low level is, for example, 0.1 to 2 seconds because it is effective at 0.1 seconds or more, and the effect is large at 0.2 seconds or more, and if it exceeds 1 second, the effect on power consumption is large, which is preferable. Is 0.2 to 1 second.

In the examples, poisoning with ethanol was shown, but poisoning with organic solvents such as methanol and isopropanol can also be removed with an oxidation catalyst, and by adding heating to a lower level, the influence of poisoning can be ignored. Can be small. Alternatively, the silicon substrate 20 may be fixed to a printed circuit board provided with a through hole or another silicon substrate, and the oxidation catalyst may be supported in the through hole.

2 Gas sensor 4 Sensor body 6 Heater 8 SnO2 film 10 Base 12 Cap 13 Pin 14 Adsorbent 16 Oxidation catalyst 18 Non-woven fabric 19 Holding ring 20 Silicon substrate 21 Cavity 22 and 23 Insulating film 24 Electrode 30 Gas detection device 31 Load resistance 32 Battery 34 Micro Computer 36 Heater drive 38 Detection circuit 40 Start circuit

Claims (2)

An insulating film on the surface of the silicon substrate is provided with a SnO2 film, a gas detection unit including an electrode connected to the SnO2 film and a heater, and a cavity is provided at the bottom of the insulating film around the gas detection unit. In the gas sensor in which the substrate is accommodated in the housing,
An adsorbent of an organic solvent that is activated carbon and does not contain a noble metal and an oxidation catalyst of an organic solvent that is an activated carbon supporting a noble metal are provided in the housing in order of the adsorbent and the oxidation catalyst. A gas sensor characterized in that a detection atmosphere is guided to a gas detection unit via the adsorbent and the oxidation catalyst. An insulating film on the surface of the silicon substrate is provided with a SnO2 film, a gas detection unit including an electrode connected to the SnO2 film and a heater, and a cavity is provided at the bottom of the insulating film around the gas detection unit. In a gas detection device comprising a gas sensor in which a substrate is housed in a housing, a power source, and a drive circuit for the gas sensor,
An adsorbent of an organic solvent that is activated carbon and does not contain a noble metal and an oxidation catalyst of an organic solvent that is an activated carbon supporting a noble metal are provided in the housing in order of the adsorbent and the oxidation catalyst. The detection atmosphere is guided to the gas detection unit via the adsorbent and the oxidation catalyst,
The drive circuit changes the power to the heater between a low level suitable for evaporating or oxidizing the organic solvent from the gas detection unit, a high level suitable for detection of the detection target gas, and a zero level. It is comprised so that it may cause. The gas detection apparatus characterized by the above-mentioned.

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