JP3087982B2 - Gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor, and more particularly to a gas sensor having improved humidity dependency and aging characteristics.

[0002]

2. Description of the Related Art The inventors laminated a heater and a metal oxide semiconductor film on an insulating substrate via a heat insulating film such as glass, and caused the heater to generate heat instantaneously to heat the metal oxide semiconductor film instantaneously. (Japanese Patent Application Laid-Open No.
No. 313,751, No. 2-25,857). Through research into practical use of this sensor, the inventors have found that 1) the sensor's stability over time is insufficient and the metal oxide semiconductor for gas detection increases in resistance over long periods of use, and 2) the sensor is highly dependent on humidity. In particular, there was a problem that the humidity dependency in the case of detecting CO was large. One of the purposes of this sensor is to reduce costs through miniaturization. Downsizing the sensor has increased the cost of the housing, necessitating the development of a simpler housing.

[0003]

An object of the present invention is to provide a gas sensor having (1) a small dependence on humidity and (2) improved stability over time (Claim 1). The subject of claim 2 is:
In addition to these, an object is to provide a housing which is inexpensive and easy to assemble.

[0004]

According to the present invention, a sensor body including a heater and a metal oxide semiconductor is provided, and the heater is pulsed to generate heat instantaneously, thereby heating the metal oxide semiconductor instantaneously. In the gas sensor, a sorbent of water is held in a housing of the gas sensor, and outside air is introduced into the gas sensor via the sorbent.

[0005] Preferably, the metal oxide semiconductor is a film-shaped metal oxide semiconductor, and the sensor main body is provided with the heater and the metal oxide semiconductor on an insulating substrate. The gas sensor lead is pulled out from one end of the heat-shrinkable tube in the tube, and the heat-shrinkable tube is shrunk at this lead portion to close one end of the tube, and the water sorbent is attached to the other end of the heat-shrinkable tube. Is provided, and the heat-shrinkable tube is shrunk at this filter portion to hold the filter on the heat-shrinkable tube.

[0006]

8 to 10 show the structure of a gas sensor.
In FIG. 8, 2 is a gas sensor, 4 is a heat-shrinkable tube of synthetic resin, 6 is a sensor main body, and 8 is an activated carbon filter. 10 and 12 are wire meshes above and below the activated carbon filter 8;
16 and 18 are leads connected to the sensor body 6.

The activated carbon filter 8 is preferably used in an amount of 10 mg to 100 mg per sensor, and when it is 10 mg or more, the effect of suppressing the humidity dependency and stabilizing the change with time becomes remarkable.
The effect is particularly remarkable when the amount is 25 mg or more per sensor. Using an activated carbon filter 8 of 100 mg or more is not preferable because the size of the gas sensor 2 is increased.

[0008] The material of the filter 8 is not limited to activated carbon.
Silica gel, molecular sieve, kaolin, or those impregnated with polyethylene glycol, glycerin, or the like may be used. The activated carbon may be chemically modified, such as one having an increased adsorption ability to an alkaline substance by an acid treatment or one having an increased adsorption ability to an acidic substance by an alkali treatment. The shape of the activated carbon may be any of a granular shape, a fiber sheet shape, and a honeycomb shape. The basic function of the filter 8 is to absorb or absorb water in the air and prevent the sensor main body 6 from being exposed to a highly humid atmosphere. The filter 8 is a water sorbent, for example activated carbon is a weak water adsorbent. Molecular sieves and silica gel are strong water adsorbents, and kaolin is a weak adsorbent. In addition, polyethylene glycol and glycerin have a deliquescent effect and are thus sorbents that absorb water. These filters 8 absorb water when the surrounding humidity increases, and prevent the sensor main body 6 from touching a highly humid atmosphere. When the humidity decreases, water is released, and the water is regenerated so that the water can be absorbed when the humid atmosphere is next touched. When the sensor 2 is pulse-driven, the filter 8 is kept at room temperature without increasing the temperature, so that the water adsorbing ability and the absorbing ability of the filter 8 are maintained. The preferable amount of the filter 8 is
In the case of activated carbon, molecular sieve, or silica gel, each sensor is 10 mg to 100 mg and 10 m
At g or more, the effect of suppressing humidity dependency and stabilizing change with time can be obtained, and when the amount is 25 mg or more per sensor, the effect is particularly remarkable.

What is important here is that the sensor body 6 is pulse-driven. Since the sensor body 6 is driven in a pulsed manner, the temperature of the filter 8 does not rise, so that the water sorption capacity of the filter 8 is maintained. For example, in a gas sensor using a cycle in which a bulk metal oxide semiconductor is heated to 300 ° C. for 60 seconds and maintained at 80 ° C. for 90 seconds, the temperature of the filter 8 is about 60 ° C. Then, since the temperature is high, the water absorption capacity of the filter 8 decreases, and the filter 8 does not contribute to the suppression of the humidity dependency and the change with time.

Another important point is that in pulse driving, the metal oxide semiconductor of the sensor body 6 is kept near room temperature for most of the time, and water adsorption becomes particularly remarkable. It is considered that such water adsorption is mainly due to physical adsorption. Therefore, when the sensor 2 is used for a long time without the filter 8, the resistance of the metal oxide semiconductor is increased. Such a phenomenon is not observed in a sensor that continuously heats a metal oxide semiconductor, and when the sensor is continuously heated, the resistance does not increase but usually decreases. That is, in a sensor other than the pulse drive, the phenomenon of physical adsorption of the surrounding water does not become a problem, and the need to prevent the sensor main body 6 from being exposed to a high humidity atmosphere is low.

The inventor has confirmed this experimentally.
The bulk metal oxide semiconductor is heated at 300 ° C. and 80 ° C.
In the sensor heated alternately to ° C., even if 600 mg of activated carbon per sensor was used as a filter, there was no effect on the humidity dependency, and the filter did not affect the aging characteristics.

Here, the pulse driving means that the sensor body 6
The heating duty of the metal oxide semiconductor (the ratio of the heating time to the operation cycle, ignoring delays in temperature rise and temperature decrease due to the thermal time constant of the sensor) is 1/10 or less, particularly 1/20 or less, and is practical. More preferably, the heating time is 1/50 or less, and the heating time is 0.1 ms to 5 seconds, particularly 1 ms to 2 seconds, and particularly preferably 1 ms to 0.1 seconds.

FIG. 9 shows the structure of the sensor body 6. In the figure, reference numeral 20 denotes a substrate made of alumina or the like, and reference numeral 22 denotes a heat insulating film made of glass or the like having a thickness of, for example, about 20 to 200 μm. Reference numeral 24 denotes a thick film heater made of ruthenium oxide, and a thin film heater such as tantalum nitride or iridium oxide may be used. Reference numeral 26 denotes an insulating film made of a thin film of silica or alumina having a thickness of about 1 μm, and reference numeral 28 denotes a metal oxide semiconductor film of SnO2, In2O3, WO3 or the like. Reference numeral 30 denotes an electrode film of the heater 24, which is connected to the lead 14 for the heater 24. Leads 14, 16, 18
Are fixed by bumps such as solder provided on the front surface of the electrode 30 or the back surface of the substrate 20 using an F-shaped lead whose tip is cut into two or three.

Here, the substrate 20 and the heater 24 are formed by the heat insulating film 2.
2 shows the sensor body 6 insulated by 2 and capable of pulse heating. However, the present invention is not limited to this, and a heater 24, an insulating film 26, and a metal oxide semiconductor film 28 may be provided thereon, for example, using an SiO2 thin film provided on a cavity obtained by undercut etching a silicon substrate as an insulating substrate. In any case, it is important not to heat the metal oxide semiconductor film 28 continuously.

In assembling such a sensor 2,
The sensor main body 6 is manufactured by a known method, and then inserted into the synthetic resin heat-shrinkable tube 4. Next, the wire mesh 12 is set, and after the activated carbon of the filter 8 is filled, the wire mesh 10 is set. Leads 14, 16, 18 with local heaters or infrared rays
When the heat-shrinkable tube 4 is heated, the tube 4 shrinks and is closely attached to the leads 14, 16, and 18.
Of course, a slight air permeability may be left on the side of the leads 14, 16, 18. Similarly, when the heat-shrinkable tube 4 in the portion of the filter 8 is contracted by a local heater, infrared rays, or the like, the tube 4 is connected to the wire meshes 10 and 12, and the activated carbon can be confined as the filter 8. The heating condition of the heat-shrinkable tube 4 in this portion is determined so as to optimize the packing density and the air permeability of the filter 8. As a result, the sensor body 6 and the filter 8 can be easily held by the inexpensive heat-shrinkable tube 4.

Instead of the wire nets 10 and 12, for example, a sorbent such as activated carbon may be wrapped with a gas-selective permeable membrane such as Teflon, and the gas-selective permeable membrane may be connected to the tube 4 and held.

1 to 7 show characteristics of the gas sensor 2. FIG. In the sensor 2 used, the metal oxide semiconductor film 28
It is assumed that 1.5 wt% of Pd (in terms of metal) is added to O2 (film thickness: 5 [mu] m), the pulse heating condition is 8 msec heating (maximum temperature slightly over 400 [deg.] C.), and the heating cycle is 1 second. Further, the characteristics are characteristics 2 ms after the start of heating, and show the average values of several sensors. The filter 8 is granular activated carbon and is 30 mg per sensor. As a comparative example, the same filter was used except that the filter 8 was not provided.

FIG. 1 and FIG.
The resistance value when the temperature was fixed at 40 ° C. and the relative humidity was changed in the range of 35% to 100% in m. FIG. 1 shows the result of the example, and FIG. 2 shows the result of the comparative example. In the embodiment, the humidity dependency is small and the CO sensitivity is high.

FIG. 3 shows the resistance value of the sensor 2 of the embodiment when the relative humidity is fixed at 65% and the temperature is changed in the range of 10 ° C. to 40 ° C. Atmosphere is air ~ CO150p
pm. In the detection of CO using a metal oxide semiconductor, humidity dependency is generally a fatal problem, but here, as is clear from FIG. 3, the humidity dependency is small.

FIGS. 4 and 5 show the time-dependent characteristics of the pulse driving for 240 days. FIG. 4 shows the result of the example and FIG. 5 shows the result of the comparative example. The resistance value of the sensor of the example is stable, but the resistance of the sensor of the comparative example is increased for any gas. The gas concentration is LP
G is 3000 ppm, others are 1000 ppm. Further, the embodiment of FIG. 4 has higher CO sensitivity than the comparative example of FIG. This is the same tendency as shown in FIGS.

FIGS. 6 and 7 show the time-dependent characteristics of 240 days of pulse driving when Pd is changed to a Pt catalyst (addition amount: 1.0 wt%) as the metal oxide semiconductor film. FIG. 6 shows the result of the example, and FIG. 7 shows the result of the comparative example. In the example, the resistance value is stable, and in the comparative example, the resistance value gradually increases. Further, the embodiment of FIG.
High sensitivity.

When activated carbon is used for the filter 8, LPG and alcohol are adsorbed, so that it is not used for detecting LPG and alcohol. Therefore, when silica gel or molecular sieve was used instead of activated carbon, the humidity dependency was suppressed as in the results of FIGS. 1 to 3, and the high resistance due to long-term use was prevented as in FIGS. 4 and 5. . These filters do not adsorb LPG or alcohol,
It was effective in detecting G. Therefore, in the detection of LPG, methane, and alcohol, it is preferable to use silica gel or molecular sieve as the material of the filter 8.

[0023]

According to the present invention, a gas sensor having (1) a small dependence on humidity and (2) improved stability over time can be obtained. According to the second aspect of the present invention, in addition to the above, the cost of the sensor can be reduced by using an inexpensive housing that is easy to assemble.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram of a gas sensor of an embodiment showing an absolute humidity dependency at 40 ° C. and a relative humidity of 35 to 100%.

FIG. 2 is a characteristic diagram of a conventional gas sensor showing absolute humidity dependence at 40 ° C. and a relative humidity of 35 to 100%.

FIG. 3 is a characteristic diagram of the gas sensor of the embodiment showing the absolute humidity dependence of low concentration% in air at 10 ° C. relative humidity 65% to 40 ° C. 65%.

FIG. 4 is a characteristic diagram showing a time-dependent characteristic of the gas sensor of the example for 240 days.

FIG. 5 is a characteristic diagram showing 240-hour aging characteristics of a conventional gas sensor.

FIG. 6 is a characteristic diagram showing a temporal characteristic of a gas sensor according to another embodiment for 240 days.

FIG. 7 is a characteristic diagram showing a time-dependent characteristic of another conventional gas sensor for 240 days.

FIG. 8 is a perspective view of the gas sensor of the embodiment.

FIG. 9 is a sectional view of a main part of the gas sensor according to the embodiment.

FIG. 10 is a cross-sectional view showing a heat-shrinkable tube of the gas sensor according to the embodiment.

[Explanation of symbols]

 2 Gas Sensor 4 Heat Shrink Tube 6 Sensor Body 8 Activated Carbon Filter 10, 12 Wire Mesh 14, 16, 18 Lead 20 Substrate 22 Heat Insulating Film 24 Heater 26 Insulating Film 28 Metal Oxide Semiconductor Film 30 Electrode Film

Claims (2)

(57) [Claims] 1. A gas sensor comprising a sensor main body including a heater and a metal oxide semiconductor, wherein the heater is pulsed to generate heat instantaneously, thereby heating the metal oxide semiconductor instantaneously. 2. The gas sensor according to claim 1, wherein a sorbent for water is held in a housing of the gas sensor, and outside air is introduced into the gas sensor via the sorbent. 2. The heat-shrinkable tube according to claim 2, wherein the metal oxide semiconductor is a film-shaped metal oxide semiconductor, and the sensor main body is provided with the heater and the metal oxide semiconductor on an insulating substrate. The gas sensor lead is pulled out from one end of the heat-shrinkable tube, and the heat-shrinkable tube is contracted at this lead portion to close one end of the tube. 2. The gas sensor according to claim 1, wherein a sorption filter is provided, and the heat-shrinkable tube is shrunk at a portion of the filter so that the filter is held by the heat-shrinkable tube.