JP3440135B2 - Manufacturing method of catalytic combustion type CO 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 method of manufacturing a catalytic combustion type CO gas sensor, and more specifically, a Pd-Fe alloy wire is used for each coil wire of a sensing element part and a compensating element part. The present invention relates to a contact combustion type CO gas sensor having a two-layer structure of a sintered body layer and a combustion catalyst layer, and particularly to a method for producing a contact combustion type CO gas sensor which can be stably used for a long period of time.

[0002]

2. Description of the Related Art In recent years, with the spread of gas leak alarms, gas explosion accidents have been drastically reduced, while CO gas poisoning accidents due to incomplete combustion of various combustion equipment are gradually increasing. is there. For this reason, the gas alarm maker uses CO
The development of alarm devices is progressing, and the alarm devices are starting to appear on the market, though only a few.

The sensors used in gas leak alarms are generally of two types: semiconductor type and catalytic combustion type. Here, although the semiconductor gas sensor has excellent gas sensitivity, the sensor characteristics are inferior to those of the catalytic combustion type gas sensor, and therefore, a highly reliable catalytic combustion type is generally desired as an alarm device application. Note that the gas sensitivity of the catalytic combustion type gas sensor is inferior as described above, because the gas sensitivity of the catalytic combustion type sensor is determined by the magnitude of the combustion energy of the gas, and therefore the CO gas sensitivity of LEL 12.5% is gas leakage. It can be understood from the fact that only about 1/10 of the target isobutane gas sensitivity can be obtained with the alarm device.

From this point of view, C which is currently on the market
The O-alarm is a semiconductor-type sensor only because the O-alarm needs to have a very low alarm concentration due to the toxicity of CO gas. Therefore, false alarms often occur, the reliability is poor, and the sensor is too expensive. Therefore, it has not been widely used.

Therefore, the development of an inexpensive and highly reliable catalytic combustion type CO gas sensor has been awaited in the market. Recently, as a sensor that responds to such needs, there is an invention described in, for example, Japanese Patent Application Laid-Open No. 4-140656 as a highly sensitive catalytic combustion sensor using a coil made of the same material as that of the present invention.

Specifically, the present invention is a catalytic combustion type CO
The coil wire used for the gas sensor is a Pd-Fe alloy wire.
It was formed. Specifically, Pd-Fe 20 μm
Using a wire, a coil with a diameter of 0.8 φ and 24 turns is used.
After being welded to the pipe, it is cleaned and then the surface of the coil is
Al in₂ O ₃ Fine powder of N is electrodeposited, N₂ Resin in airflow
Eliminate minutes and sinter.

Next, in the case of the sensing element, a resin solution of PdO and Al ₂ O ₃ obtained by strongly heating metallic palladium to oxidize it is electrodeposited, air-dried and then dried in an N ₂ gas furnace after sintering. , A protective film is applied, and after aging, a required cap is covered. In the case of a compensating element, Ni ₂ O
A resin solution of ₃ and Al ₂ O ₃ can be electrodeposited, and after natural drying and vacuum drying in a N ₂ gas furnace, sintering is performed, a protective film is applied, and after aging, a required cap is covered.

The catalytic combustion type gas sensor according to the present invention has a very high sensitivity, a good gas selectivity and a certain degree of durability against a drop impact. Among such Pd-Fe alloy wires,
The characteristics of the alloy wire of Fe25 wt% are shown in Table 1 in comparison with the pure Pt wire and the Pt alloy wire of Ni10 wt% which have been conventionally used.

[0009]

[Table 1]

As can be seen from Table 1, the Pd-Fe alloy wire has a resistivity of about 4 times that of a pure Pt wire and a P content of Ni of 10 wt%.
Since it is about twice as large as that of the t-alloy wire, a higher voltage than that of the pure Pt wire or the Pt alloy wire of 10 wt% Ni can be applied to the sensor, and as a result, a larger output signal can be obtained. In addition, the temperature resistance coefficient is about 2.
2 times, about 4.2 times that of Pt alloy wire with 10 wt% Ni,
A considerably larger output signal can be expected as compared with the conventional sensor using Pt.

That is, it can be seen that the catalytic combustion type CO gas sensor using such a Pd-Fe alloy wire has a very high sensitivity. Specifically, although it is very difficult for other catalytic combustion type sensors to detect CO of about 500 ppm, CO gas detection of about 500 ppm is possible by using this highly sensitive Pd-Fe alloy wire. It has become.

However, the catalytic combustion type gas sensor as disclosed in the present invention has poor stability over time,
In addition, it was greatly affected by changes in ambient temperature and humidity, and there were large variations during mass production. It is considered that this is mainly due to problems in the element manufacturing process and the catalyst material used.

Specifically, since this wire is poor in high temperature stability in air, there is no problem in the temperature range of 130 to 170 ° C., which is the operating temperature of the element for CO measurement.
The atmosphere in the process of sintering the catalyst material on the upper surface of the coil has been a problem. That is, this Pd-Fe alloy wire was oxidized when it was attempted to be sintered in the air, the material was changed, and desired characteristics could not be obtained. Therefore, in the present invention, the step of sintering the catalyst material on the upper surface of the coil is carried out by heat treatment in an N ₂ gas flow using an electric furnace to prevent oxidation.

However, in order to sinter the resin liquid and expel the resin component, it is necessary to burn the resin component. For this reason, the resin component cannot be completely removed by sintering in a complete N ₂ stream. Furthermore, when considering sintering in an electric furnace, there is probably oxygen back diffusion, which does not necessarily result in complete non-oxygen sintering, and it cannot be said that oxidation is completely prevented. It was.

It is considered that such oxidation of the wire material affects the variations in the characteristics of each coil during mass production. Further, as a particularly large point, in such an element, the oxidation of the wire is unavoidable, and the deterioration of the characteristics over time has been a problem due to the oxidation of the wire. For example, the one shown in FIG. 7 shows a change with time regarding the sensor output in the air after the zero point adjustment is performed by the sensor using the sensing element and the compensating element which are simultaneously manufactured by the same manufacturing method. It is a graph.

From these measurement results, it was found that the output variation at the zero point became too large, which was not practical. Furthermore, what is shown in FIG. 8 is CO500pp
It is a graph which showed the time-dependent change of the sensor output in m atmosphere. From this graph as well, even in a CO atmosphere of the same concentration, the output becomes 1 after about 2 weeks.
It was measured that it would be about / 2, and it was found that it was not in practical use at all.

[0017]

Therefore, according to the invention described in claims 1 to 3 of the present invention, as described above, while using a highly sensitive Pd-Fe alloy wire, variations in the characteristics of each coil during mass production are caused. It is an object of the present invention to provide a catalytic combustion type sensor which is eliminated and improved in stability especially for long-term use.

[0018]

In order to achieve the above-mentioned object, in the invention according to claim 1 of the present invention, as the wire material of the coil used in the sensing element section and the compensating element section,
Fe15~60wt%, have a composition of Pd40~85wt%, and a wire diameter with Fe-Pd alloy wire 20 to 30 [mu] m, the alloy wire inner diameter 0.3 to 0.4 mm, turn
After processing into a coil with a number of 10 to 20, it is fixed to a stem, and then a sintered material containing Al2O3 as a main component is applied and dried on the surface of the coil together with a binder to form the first layer. After that, it is sintered by internal heating by energizing a coil in a nitrogen stream containing a trace amount of hydrogen, and further , in the outer peripheral portion of the first layer, SnO2 is added to Pt, Pd, Rh, Ru or Ir as a sensing element. A catalyst material impregnated with one or more metal elements is coated with a binder, dried and then aged, and the compensating element is a material in which SnO2 is impregnated with Cu at the outer periphery of the first layer. It is characterized in that after aging is carried out after applying and drying the one to which the binder is added.

The invention according to claim 2 is characterized in that, in addition to the constitution of the invention according to claim 1, the hydrogen concentration is in the range of 0.1 to 4.0%. The invention according to claim 3 is characterized in that, in addition to the constitution of the invention according to claim 1 or 2, the sintering temperature of the first layer is set in a range of 500 to 1000 ° C. Here, the manufacturing method of the catalytic combustion type CO gas sensor will be described in more detail.

As the wire material of the coil used in the sensing element section and the compensating element section, Fe 15 to 60 wt% and Pd 40 to 8 are used.
An Fe-Pd alloy wire having a composition of 5 wt% is used. With regard to the composition ratio, it is desirable that Fe is 20 to 40 wt.
%, Pd of 60 to 80 wt% is excellent in that a larger temperature resistance coefficient can be obtained. Further here,
The wire diameter of the Fe-Pd alloy wire is preferably about 20 to 30 μm. The reason is that by thinning the wire to this extent, a high resistance coil can be obtained, that is, a larger voltage can be applied, and thus a larger output voltage can be obtained. Next, this Fe-Pd alloy wire is processed into a coil and then fixed to the stem. Here, the coiling process preferably has an inner diameter of about 0.3 to 0.4 mm. The reason is that if it is smaller than this, coil processing is difficult, and if it is too large, a wire rod having a wire diameter of about 20 to 30 μm cannot maintain the coil state due to its own weight. Further, the number of turns of the coil is 10 to 2
0 is desirable. The reason is that if the amount is too small, the resistance does not increase, and conversely, if the amount is too large, the self-weight problem occurs. Further, for fixing to the stem, means such as welding can be used.

Then, a sintered material containing Al ₂ O ₃ as a main component is applied onto the surface of the coil together with a binder and dried to form a first layer. Where Al ₂ O
_The sintered material containing ₃ as a main component is Al ₂ O ₃ of 60 to
It is the one used about 90 wt%. The reason why only Al ₂ O ₃ is not used here is that Al ₂ O ₃ alone cannot provide a sintered pair for fixing the coil. Further Al
_As components other than ₂ O ₃ , ₁₀ to 40 wt% of TiO ₂
%, 5 to 10 wt% of Mg ₂ Si ₃ O ₈ and 0 to 5 wt% of low melting point glass are desirable. By such addition, there is an effect that a relatively porous sintered body can be obtained. In addition to these, addition of a low-melting point oxide or the like can be considered.

Further, the binder is required to be a material having an appropriate viscosity, and specific examples thereof include alumina sol and ethylene glycol. In addition to these, glycerin and the like can be considered.
The sintering material and the binder are mixed at a ratio of about 10: 1. As a mixing means, specifically, mixing using a mortar is conceivable, but other means such as mixing using a ball mill is also conceivable.

It should be noted that the coil may be applied with a brush, but other means such as dipping may be used. The coating thickness is preferably about 0.1 to 0.2 mm. The reason is that the heat of the surface is transferred to the coil faster and the coil is firmly fixed.
Further, as a drying means, a means is used in which a voltage is applied to the coil and the stem itself is rotated for about 5 minutes while the coil itself is heated to about 150 ° C. by internal heating.

Next, the coil coated with the sintering material and the binder is sintered by internal heating by energizing the coil in a nitrogen stream containing a slight amount of hydrogen. here,
Sintering in a nitrogen stream containing a small amount of hydrogen is performed in an aging box. In addition, the reason why the flow of nitrogen containing a small amount of hydrogen is used here is that it is difficult to use only nitrogen in reality, and in order to avoid combustion due to the mixing of a small amount of oxygen, hydrogen is forcibly forced. Is mixed in so that oxygen is not mixed in. By doing so, oxidation caused by combustion during sintering is prevented.

If the hydrogen gas concentration in the nitrogen gas stream containing a trace amount of hydrogen is 0.1% or less, the reducing power is insufficient, so 0.1% or more is sufficient. However, if the concentration is increased too much and the concentration of LEL exceeds 4.0%, there is a risk of explosion, and it should be avoided in actual use. Further, the sintering is performed by applying a voltage to the coil fixed to the stem, and gradually increasing the voltage to heat the sintering material and the binder around the coil at a temperature of about 800 ° C. at 10 ° C. for 10 minutes.
Sinter for minutes.

Then, due to this sintering, the sintered body itself is fixed around the coil and does not flow down with a solution such as water. Further, by this sintering, the particles can be connected to each other with various additives. Here, the binder functions to improve the particle density.

Further, here, the sintering temperature is 500 to 1
It is carried out in the range of 000 ° C. Here, if the sintering temperature is 500 ° C. or less, sintering is insufficient. That is, in this case, the sintered body itself may not be fixed around the coil, and may fall off with a solution such as water. When the sintering temperature exceeds 1,000 ° C, the alloy wire as a wire material separates inside the wire for each composition element, causing characteristic changes, resulting in deterioration of the wire material and deterioration of durability. Therefore, long-time sintering at 1,000 ° C or higher should be avoided. As a result of EPMA analysis, 1,000
It has been confirmed that Fe is phase-separated on the surface of the wire when heated at ℃ or higher.

Then, in the case of a sensing element, SnO ₂ is further added to Pt, Pd, Rh, Ru or Ir on the outer periphery of the first layer.
In the case of a compensating element, Sn is added to the outer peripheral portion of the first layer after applying and drying a catalyst material impregnated with one or more of the above metal elements and a binder added and dried.
A material obtained by impregnating O ₂ with Cu and adding a binder is applied and dried, and then aging is performed.

Here, SnO ₂ 60 used in the sensing element is used.
For 95 wt%, the content of one or more metal elements of other Pt, Pd, Rh, Ru or Ir is 5 to 40 w.
Impregnation is performed in the range of t%. The impregnation means is a wet method. Here, as the binder, there are those such as alumina sol and ethylene glycol. Furthermore, the mixing of SnO ₂ impregnated with a metal element and the binder is a mixing ratio of about 10: 1. As a mixing means, specifically, mixing using a mortar is conceivable, but other means such as mixing using a ball mill is also conceivable.

Further, SnO ₂ 60 to 9 used in the compensating element
Cu is impregnated in the range of 5 to 40 wt% with respect to 5 wt%. The impregnation means is a wet method. Here, as the binder, there are those such as alumina sol and ethylene glycol. Furthermore,
The mixing of SnO ₂ impregnated with a metal element and the binder is a mixing ratio of about 10: 1. As a mixing means, specifically, mixing using a mortar is conceivable, but other means such as mixing using a ball mill is also conceivable.

Further, as the coating of the sensing element or the compensating element to which a binder is added, coating with a brush is considered, but other means such as dipping is also available. The coating thickness is preferably about 0.1 to 0.2 mm. The reason for this is that this level of curtain pressure is the minimum necessary for producing the combustion reaction on the surface, but if it exceeds this level, the blockage itself becomes large and the efficiency drops.

The drying is performed by leaving it at room temperature for about one day. Further, aging may be performed by energizing the coil in the air at about 240 to 250 ° C. for about 60 minutes, but the binder may be completely blown out. Further, the temperature range is set to 240 to 250 ° C. here because the binder flies at this temperature.

Thereafter, a wire mesh is attached to the stem to complete the catalytic combustion type CO gas sensor according to the present invention. As described above, this wire is easily oxidized in the air even in terms of material, and is limited to use at about 400 ° C. or lower in the air. However, in the present invention, in order to completely prevent the wire rod oxidation at the time of manufacturing the element, the chamber evacuated of air in advance is filled with nitrogen gas containing a slight amount of hydrogen gas, and the heat treatment of the element is internally heated by the coil energization there. It was done in.

As a result, when confirmed by a micrograph, it was confirmed that the temperature resistance coefficient after the heat treatment maintained the characteristics before the treatment, and the element distribution state equivalent to that before the heat treatment was maintained by the cross-sectional elemental analysis of the coil. It was That is, according to the present invention, it is possible to expect good temporal stability of the wire. As for the catalytic material covering the coil, a catalytic material based on SnO ₂ which has a long-established track record is used as the catalytic combustion gas sensor, and the first layer is Al ₂ O in order to further improve the stability of the coil. _It has a two-layer structure covered with a sintered body containing ₃ as a main component. Therefore, it can be said that the catalytic combustion gas sensor has excellent reliability.

Further, since this wire has a very large temperature resistance coefficient as described above, it is greatly affected by ambient temperature changes in normal use, but in the present invention, the constitution of the sensing element and the compensating element is used. Make the materials very similar,
In addition, the structure in which both elements are integrated without being separated corresponds to changes in ambient temperature and humidity. Conventionally, a method in which the sensing element and the compensating element are the same and only the compensating element is a closed system has been introduced.However, since the temperature resistance coefficient of the wire is too large, the compensating element should be a closed system. It was accompanied by the fact that the balance between the sensing element and the compensating element did not correspond to the ambient temperature change, but instead of adopting a structure in which both elements are integrated, it is also possible to cope with the ambient temperature change.

[0036]

EXAMPLES An example of a specific method for producing a catalytic combustion gas sensor according to the present invention will be described below. First, the coils used in the sensing element section and the compensating element section, which are the core of this sensor, will be described.

A Pd-Fe alloy wire is used for this coil. Here, in this embodiment, Pd 74 wt%, Fe
An alloy blended with 19 wt%, Mn 1.6 wt% and other 5.4 wt% is used. Furthermore, this Pd-Fe
The alloy wire has a wire diameter of 30 μm (−1.0, +0.0). Next, this Pd-Fe alloy wire was processed into a coil having an inner diameter of 0.4 mm (-0.0, +0.02) and 16 terns (-0.0, +0.0), and then the stem. Two pieces are welded as a sensing element section and a compensating element section.

Then, a sintered body material containing Al ₂ O ₃ as a main component is applied to the surface of the coil together with a binder and dried. The sintered material used here is Al ₂
Although O ₃ is the main component, TiO ₂ 25 wt
%, Mg ₂ Si ₃ O ₈ 4% by weight, low melting point glass 2% by weight
Is added. Further, the binder used is a mixture of alumina sol and ethylene glycol. Further, the sintered body and the binder are mixed using a mortar or the like. The coil is applied using a brush. Further, the drying is performed by applying a voltage to the coil and rotating the stem for about 5 minutes while keeping the coil itself at about 150 ° C. by internal heating.

This state is inserted into a predetermined jig and placed in an aging box, and the air in the box is completely replaced with nitrogen containing 3% hydrogen. The completion of this replacement is confirmed by an oximeter. Then, a voltage is applied to the coil here, and the material around the coil is sintered at about 800 ° C. for 10 minutes by internal heating while gradually increasing the voltage. Then, due to this sintering, the sintered body itself is fixed around the coil and does not flow down with a solution such as water.

By such sintering, the first layer is completed. The next sensing element, a combustion catalyst material impregnated respectively about 9 wt% of Pd and Pt on SnO _2, also the compensating element, binder material impregnated about 10 wt% of Cu on SnO _2, respectively - with After coating and drying, energization aging is performed in air at about 300 ° C. for 10 minutes to completely remove the binder. In addition, here, the application is performed with a brush so that the thickness is about 0.1 to 0.2 mm. Further, the drying is performed by leaving it at room temperature for 30 minutes or more.

After that, a wire mesh is attached to the stem to complete a catalytic combustion type CO gas sensor. Next, the sensitivity characteristics of the catalytic combustion type CO gas sensor manufactured in this way will be described. To measure the sensitivity characteristics,
First, a bridge circuit as shown in FIG. 1 is formed by the completed catalytic combustion type CO gas sensor and two fixed resistors of 510Ω. At this time, the voltage applied to the circuit is 2.4 V (element surface temperature is 140 to 150 ° C.), and the current is 30 to 32.
mA. Further, the output signal will be obtained in the mV order from between A and B.

FIG. 2 shows the effect of temperature on the sensor output of the catalytic combustion type CO gas sensor manufactured as described above. From this figure, the catalytic combustion type CO gas sensor according to the present invention is 10 mV at 500 ppm CO.
That is, it can be seen that the output is sufficient for practical use. Also, for C ₂ H ₅ OH measured at the same time, 1,
The output value at 000 ppm is 8 mV or less, and CO
It was confirmed that the selectivity between gas and C ₂ H ₅ OH was also excellent.

FIG. 3 shows the effect of temperature on the air level fluctuation value of the catalytic combustion type CO gas sensor manufactured as described above. From this figure, the catalytic combustion type CO gas sensor according to the present invention is stable within a range of −10 to + 50 ° C. with a fluctuation within ± 3 mV, and is sufficiently durable for practical use.

FIG. 4 shows measured response characteristics of the catalytic combustion type CO gas sensor manufactured as described above.
From this figure, it is confirmed that the catalytic combustion type CO gas sensor according to the present invention responds in about 30 seconds after introduction of CO 500 ppm and immediately after exhaustion, and has a practically sufficient high-speed response performance. confirmed. .

FIG. 5 shows changes in the sensor output of the catalytic combustion CO gas sensor manufactured as described above with time. As is clear from this figure, even after 50 days from the start of the measurement, an extremely stable output is shown with respect to CO 500 ppm or C ₂ H ₅ OH 1,000 ppm. Furthermore, it was confirmed that stable characteristics were maintained for 3 years or more over time.

FIG. 6 is a graph showing the change with time of the air level fluctuation value of the catalytic combustion type CO gas sensor manufactured as described above. As is clear from this figure, even after 50 days from the start of measurement, CO500ppm
Alternatively, with respect to C ₂ H ₅ OH of 1,000 ppm, the change in the air level is within the range where there is no practical problem. Furthermore, it was confirmed that stable characteristics were maintained for 3 years or more over time.

Further, in the above-mentioned embodiment, since the shape of the coil is smaller than that of the conventional product, it is expected that the variation in mass production is small and the mass productivity is excellent. Furthermore, since this wire rod has a very large temperature resistance coefficient as described above, it is greatly affected by ambient temperature changes in normal use.However, in the present invention, the constituent materials of the sensing element and the compensating element are extremely small. It was made to be similar, and the structure in which both elements were integrated without being separated was compatible with changes in ambient temperature and humidity. Originally, a method was adopted in which the sensing element and the compensating element were the same, and only the compensating element was a closed system.However, since the temperature resistance coefficient of the wire was too large, the sensing element by using the compensating element as a closed system was adopted. Since the balance of the compensation element and the compensating element does not correspond to the ambient temperature change, an integrated structure was adopted.

[0048]

As described above, the catalytic combustion type CO sensor obtained by the present invention has a practical level of gas sensitivity by using the Pd-Fe alloy for the coil of the sensing element and the compensating element. Further, the catalyst material on the coil is Al ₂
Sintered layer composed of O ₃ as a main component for coil fixing and SnO
_By adopting a two-layer structure with a _2- system catalyst layer, various characteristics including time-dependent characteristics have been dramatically improved.

[Brief description of drawings]

FIG. 1 is a circuit diagram for measuring sensitivity characteristics.

FIG. 2 is a graph showing the effect of temperature on the sensor output.

FIG. 3 is a graph showing the influence of temperature on the air level fluctuation value.

FIG. 4 is a graph showing measured response characteristics.

FIG. 5 is a graph showing a change with time with respect to a sensor output.

FIG. 6 is a graph showing changes over time with respect to air level fluctuation values.

FIG. 7 is a graph showing a change with time in terms of sensor output in air after performing zero point adjustment with a sensor using a conventional sensing element and a compensating element simultaneously manufactured by the same manufacturing method. .

FIG. 8 is a graph showing a change with time of a conventional sensor output in an atmosphere of CO 500 ppm.

─────────────────────────────────────────────────── --Continued front page (56) References JP-A-4-140656 (JP, A) JP-A-5-87759 (JP, A) JP-A-59-137849 (JP, A) JP-A-2- 161345 (JP, A) JP-A-61-246659 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/00-27/24

Claims (3)

(57) [Claims] 1. A wire material of a coil used for a sensing element part and a compensating element part, wherein Fe is 15 to 60 wt% and Pd is 40 to
Have a composition of 85 wt%, and a wire diameter with a Fe-Pd alloy wire 20 to 30 [mu] m, the alloy wire inner diameter 0.3 to 0.4 mm, the number of turns 1
After processed into a coil shape of 0 to 20, it was fixed to a stem, and then a sintered material containing Al2 O3 as a main component was applied to the surface of the coil together with a binder and dried to form a first layer. After that, it was sintered by internal heating by energizing a coil in a nitrogen stream containing a slight amount of hydrogen, and the sensing element was made to have SnO2 and P on the outer periphery of the first layer.
t, Pd, Rh, subjected to aging After coating and drying a plus binder catalyst material impregnated with one or more metal element selected from Ru or Ir, the compensating element, of the first layer A catalytic combustion type C characterized in that a material obtained by impregnating SnO2 with Cu and a binder is applied and dried on the outer peripheral portion, and then aging is performed.
Manufacturing method of O gas sensor. 2. The method for producing a catalytic combustion CO gas sensor according to claim 1, wherein the hydrogen concentration is in the range of 0.1 to 4.0%. 3. The sintering temperature of the first layer is 500 to 1000 ° C.
The method for producing a catalytic combustion CO gas sensor according to claim 1 or 2, characterized in that