JPH0731153B2 - Flammable gas sensor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a combustible gas sensor, and more specifically, to oxidation and reduction reactions on the surfaces of a working electrode and a counter electrode, and transfer of protons between both electrodes. The present invention relates to a combustible gas sensor that detects the concentration of a combustible gas based on a generated current caused by.

<Prior Art> Conventionally, as a combustible gas sensor, as shown in FIG. 9, a DC power supply (22) is connected to an element (21) whose resistance value changes in response to a combustible gas such as SnO ₂ . And an ammeter (23) are connected in series, and the above element (21) is connected to about 10
Power supply (25) for the heater (24) that heats to 0 ℃ to 400 ℃
As shown in FIG. 10, the sulfuric acid aqueous solution (32) as an electrolyte is stored in the casing (31), and the casing (31) is opened at a predetermined position. (33) (3
4), and a gas-permeable Teflon (registered trademark) membrane (3
5) (36) is provided, and a working electrode (37) and a counter electrode formed by applying platinum black at predetermined positions on the inner surface side of the gas-permeable Teflon membranes (35) (36). (38) and reference electrode (39) are provided (However, the potential between the working electrode (37) and the reference electrode (39) is kept constant by the potentiostat circuit (40). Was provided).

<Problems to be Solved by the Invention> In the combustible gas sensor having the configuration shown in FIG. 9, since the current signal corresponding to the adsorption amount of the combustible gas particles is generated, the combustible gas particles are generated. Is adsorbed to some extent and becomes saturated, and there is a problem that it is not possible to continuously detect the concentration of the combustible gas, and an element (21) for detecting the presence of the combustible gas is used as a heater. It is indispensable to heat to about 100 ° C to 400 ° C by (24), which not only complicates the structure but also causes the heater (24) to cause gas explosion. Further, due to the manufacturing conditions of the element (21) and the like, quite large variations in the characteristics are likely to occur, and it becomes necessary to adjust the compensation circuit and the like in accordance with the variations in the characteristics of the elements, and the combustible gas sensor is manufactured as a whole. There is a problem that the adjustment becomes complicated. Furthermore, since it is necessary to energize the heater for heating, there is a problem that not only a long time is required until a measurable state is reached but also power consumption is significantly increased.

In the flammable gas sensor having the configuration shown in FIG. 10,
The characteristics of the sensor as a whole will be affected by the characteristics of the gas permeable Teflon membrane, the characteristics of the electrodes, the positional accuracy of the electrodes, etc., so these characteristics, the positional accuracy, etc. must be matched to the set values. However, there is a problem that the manufacturing is extremely troublesome, and it is almost impossible to obtain a product having uniform characteristics even if the manufacturing is time-consuming.

There is also a problem that the reaction rate is affected by the concentration of the gas that permeates the gas-permeable Teflon membrane and cannot be increased so much.

Further, since it is necessary to store the electrolytic solution inside the casing, in a long-term use, leakage of the electrolytic solution occurs at a fairly high probability, and reliability in detecting flammable gas is impaired. There's a problem. This point becomes particularly noticeable when the combustible gas sensor is a portable type, and it becomes almost impossible to reduce the overall size and cost.

<Objects of the Invention> The present invention has been made in view of the above problems,
An object of the present invention is to provide a combustible gas sensor that can achieve simplification of the configuration, can significantly reduce the variation in characteristics, and can achieve high measurement accuracy for a long period of time.

<Means for Solving Problems> In order to achieve the above object, the combustible gas sensor of the present invention has a working electrode, a reference electrode, and a counter electrode, which have a predetermined positional relationship with each other on the surface of an insulating base material. The gas permeable proton conductor film is provided in a state of being held and integrally covers the electrode.

However, it is preferable that the reference electrode and the counter electrode are concentrically formed around the working electrode.

And, the reference electrode may be provided between the working electrode and the counter electrode, the working electrode is a platinum group metal,
Alternatively, it is preferable that the reference electrode is formed of gold and the reference electrode is formed of a metal that does not react with the combustible gas.

Further, the counter electrode may be provided between the working electrode and the reference electrode. In this case, both the working electrode and the counter electrode are formed of platinum group metal or gold. Is preferred.

The flammable gas sensor of another invention has, on the surface of the insulating base material,
The working electrode and the counter electrode are provided in a state of maintaining a predetermined positional relationship with each other, and the area of the counter electrode is set to several tens of times the area of the working electrode or more, and the above electrode is integrally formed. And a gas permeable proton conductor film covering the above. However, the area of the counter electrode is preferably set to 100 times or more the area of the working electrode.

<Operation> In the case of the combustible gas sensor having the above configuration, the combustible gas is passed through the gas-permeable proton conductor film in the state where a bias is applied between the working electrode and the counter electrode provided on the surface of the insulating base material. When guided to the surface of the working electrode, an oxidation reaction occurs on the surface of the working electrode, and a current corresponding to the amount of the oxidation reaction flows into the working electrode, and the protons generated as a result of the oxidation reaction face each other. Move towards the electrode. Therefore, it is possible to prevent the protons generated as a result of the oxidation reaction from staying on the surface of the working electrode and always obtain a current signal corresponding to the amount of combustible gas.

Then, by controlling the potential between the working electrode and the reference electrode, it is possible to prevent fluctuations in the potential on the surface of the working electrode. In this case, if the reference electrode and the counter electrode are concentrically formed around the working electrode, the effect of the oxidation or reduction reaction may occur only locally on the working electrode. Also, the potential between the working electrode and the reference electrode can be quickly and reliably controlled.

Further, when the reference electrode is provided between the working electrode and the counter electrode, the potential between the working electrode and the reference electrode can be controlled more quickly and reliably. When the working electrode is formed of platinum group metal or gold and the counter electrode is formed of metal that does not react with combustible gas, expensive platinum group metal or gold is used. The amount of can be reduced, and the cost can be reduced as a whole.

Furthermore, when the counter electrode is provided between the working electrode and the reference electrode, the response of the potential control between the working electrode and the reference electrode to the change in the potential of the counter electrode may be slightly lowered. However, the measurement can be performed with extremely high accuracy. In particular, when the working electrode and the counter electrode are both formed of a platinum group metal or gold, the counter electrode is formed of a metal that does not react with a combustible gas such as silver or lead. As compared with the above, the measurement accuracy can be improved and the amount of expensive platinum group metal can be suppressed to the necessary minimum.

In the case of a flammable gas sensor of another invention, the working electrode and the counter electrode are provided on the surface of the insulating base material while maintaining a predetermined positional relationship with each other, and the area of the counter electrode is equal to that of the working electrode. It is set to several tens of times the area and moreover, since the gas permeable proton conductor film covering both electrodes is provided, the working electrode is caused by increasing the area ratio of both electrodes. The potential of the surface of can be stabilized in the same manner as when the reference electrode is provided.

<Example> Hereinafter, detailed description will be given with reference to the accompanying drawings illustrating an example.

FIG. 1 is a schematic perspective view showing an embodiment of the combustible gas sensor of the present invention, and FIG. 2 is a schematic vertical sectional view, in which the surfaces of the insulating substrate (1) are kept in a predetermined positional relationship with each other. In this state, the working electrode (2), the reference electrode (3), and the counter electrode (4) are provided, and the gas-permeable proton conductor film (5) that integrally covers these electrodes is provided. There is.

More specifically, the insulating substrate (1) is, for example, a ceramic substrate and the working electrode (2).
Is a thin film electrode formed of Pt which is a kind of platinum group metal, and the reference electrode (3) is a kind of metal that does not react with the combustible gas to be measured, that is, does not cause a potential change. The counter electrode (4) is a thin film electrode formed of Ag, and the counter electrode (4) is a thin film electrode formed of a commonly used electrode metal, and each of which is a lead terminal (7) via a wiring pattern (6). Connected with. As the gas-permeable proton conductor membrane (5), one obtained by copolymerizing tetrafluoroethylene and perfluorosulfonylfluoride vinyl ether (trade name: Nafion, manufactured by DuPont) is used. ing. Then, the inverting input terminal of the operational amplifier (8) whose output terminal is connected to the extraction terminal for the counter electrode is connected to the extraction terminal for the reference electrode, and the non-inversion input terminal is connected to the DC power supply (9) (electromotive force is v The potentiostat circuit is formed by connecting it to the ground (10) via (). Further, the inverting input terminal of the operational amplifier (11) whose non-inverting input terminal is connected to the ground (10) is connected to the extraction terminal for the working electrode, and is connected to the inverting input terminal via the feedback resistor (12). The output terminal is used as a density detection signal extraction terminal.

The operation of the combustible gas sensor having the above configuration is as follows.

When CO gas as the measurement gas is introduced as shown by arrow A in FIG. 2, Pt passes through the proton conductor membrane (5).
To the working electrode (2) consisting of Pt, and the reaction represented by CO + H ₂ O → CO ₂ + 2H ⁺ + 2e is performed on the surface of the working electrode (2) by the catalytic action of Pt, resulting in the proton H ⁺ and the electron e.
Is generated. Therefore, if the proton stays as it is, the reaction represented by 2H ⁺ + 1 / 2O ₂ + 2e → H ₂ O is performed, and it is considered that the current does not flow through the working electrode (2). Since the protons freely move in (5), it effectively prevents the reduction reaction from being performed on the surface of the working electrode (2), and corresponds to the amount of the oxidation reaction through the working electrode (2). An inflow of electric current can appear.

The protons that have moved in the proton conductor membrane (5) are
Contributes to the reduction reaction on the surface of the counter electrode (4),
It is possible to reliably prevent the excessive presence of protons in the proton conductor film (5), and it is possible to reliably prevent the reversible reaction from being performed on the surface of the working electrode (2).

The potential of the counter electrode (4) may change during the oxidation and reduction reactions described above, and the potential difference between the working electrode (2) and the working electrode (2) may change. As shown in FIG. , The potential of the reference electrode (3) can be automatically changed by the potentiostat circuit, and the potential difference between the working electrode (2) and the reference electrode (3) can be kept at v. A highly accurate CO gas concentration detection signal is obtained by obtaining a current signal corresponding to the CO gas concentration and converting it into a voltage signal by the operational amplifier (11) while keeping the potential on the surface of the electrode (2) constant. Can be generated.

Moreover, although the case where CO gas is introduced to the surface of the working electrode (2) has been described above, when H ₂ gas is introduced, H ₂ → 2H ⁺ + 2e 2H ⁺ + 1 / 2O ₂ + 2e → H ₂ O reaction is performed, and a voltage signal corresponding to the gas concentration can be generated as in the case of CO gas.

Therefore, the concentrations of CO gas and H ₂ gas in the measurement target atmosphere can be measured with high accuracy.

Further, the combustible gas sensor having the above-mentioned structure can be easily manufactured by the thin film forming technique, can significantly reduce the variation in characteristics between products, and can maintain stable characteristics for a long period of time. In addition to being able to achieve this, the overall size can be reduced, and further cost reduction can be achieved easily.

FIG. 4 is a schematic perspective view showing another embodiment of the combustible gas sensor. The difference from the above embodiment is that the working electrode (2) is formed linearly, the reference electrode (3), and the counter electrode ( 4) is a semi-elliptical shape.

Therefore, in the case of this embodiment, it is possible to perform the same operation as in the above embodiment.

More specifically, in the case of this embodiment, the reference electrode (3) is provided between the working electrode (2) and the counter electrode (4), and the majority of the working electrode (2) is provided. Since it is surrounded, the potential followability of the reference electrode (3) with respect to the potential fluctuation of the counter electrode (4) can be remarkably improved as compared with the case of the above embodiment, and the measurement accuracy of the combustible gas concentration can be remarkably improved. Can be improved. Therefore, even when the ionic conductivity of the proton conductor film (5) is low,
The stability of the potential difference between the reference electrode (3) and the working electrode (2) can be improved, and the measurement accuracy of the combustible gas concentration can be maintained high.

FIG. 5 is a schematic perspective view showing still another embodiment of the combustible gas sensor. The points different from the above embodiment are the reference electrode (3) and the counter electrode (4), and the working electrode (2). It is only the points formed in a concentric shape.

Therefore, in the case of this embodiment, the potential followability of the reference electrode (3) can be further improved, and the measurement accuracy of the combustible gas concentration can be further improved.

In any of the above embodiments, the working electrode (2) may be made of a platinum group metal other than Pt or Au, and the reference electrode (3) may be made of a material other than Ag. It is possible to use a metal that does not change in potential due to a combustible gas, for example, one formed of Pb, and a proton conductor membrane (5) formed of antimonic acid, zirconium phosphate, or the like is used. It is possible.

However, as the proton conductor membrane (5), it is preferable to obtain the copolymer of tetrafluoroethylene and perfluorosulfonyl fluoride vinyl ether shown in the above-mentioned examples by plasma polymerization, and obtain it by polymerization in vacuum. As a result, it has the advantage that the amount of impurities is small and the film quality is highly uniform. Moreover, by controlling the film thickness and the degree of polymerization, the permeability of flammable gas can be arbitrarily adjusted. It has the advantage that it can be set.

FIG. 6 is a schematic perspective view showing still another embodiment of the combustible gas sensor. The difference from the embodiment of FIG. 5 is that the positional relationship between the reference electrode (3) and the counter electrode (4) is reversed. Only the set points.

Therefore, also in the case of this embodiment, the potential of the reference electrode (3) changes following the potential change of the counter electrode (4), and the potential difference between the working electrode (2) and the reference electrode (3) changes. It is possible to output a voltage signal corresponding to the combustible gas concentration while being kept constant.

However, in the case of this embodiment, the potential followability of the reference electrode (3) is slightly lowered as compared with the case of the embodiment of FIG. By using the counter electrode as a counter electrode, it is possible to suppress a decrease in potential followability and to measure the combustible gas concentration with high accuracy.

FIG. 7 is a schematic perspective view showing still another embodiment of the combustible gas sensor. The difference from the above embodiment is that the temperature sensor (13) in which Pt is formed in a thin film on the insulating substrate (1). It is only the point that is provided.

Therefore, in the case of this embodiment, the combustible gas concentration can be detected in the same manner as in the above embodiment, and the temperature can also be detected by the temperature sensor (13). As a result, the combustible gas concentration can be detected. , And a sensor suitable for fire alarm and other applications based on temperature. Further, in this embodiment, not only the temperature sensor (13) is provided, but it is possible to further incorporate a signal processing circuit such as a bridge circuit,
Even if the number of incorporated circuits increases, the use of semiconductor integrated circuit technology can minimize the complexity of the manufacturing process.

FIG. 8 is a schematic perspective view showing still another embodiment. The difference from the embodiment of FIG. 1 is that the reference electrode (3) is omitted, and the area of the counter electrode (4) is the working electrode (2). ), And that the operational amplifier (8) is omitted and the DC power supply (9) is directly connected to the counter electrode (4).

Therefore, in the case of this embodiment, it is possible to simplify the electrode structure and to stabilize the surface potential of the working electrode due to the fact that the area of the counter electrode (4) is significantly increased. Therefore, it is possible to detect the combustible gas concentration with high accuracy.

The present invention is not limited to the above-described embodiment, and the positions of the reference electrode (3) and the counter electrode (4) are set to be opposite to each other in the combustible gas sensor shown in FIGS. 1 and 4, respectively. In addition to the above, each electrode can be embedded in the insulating substrate (1),
Various other design changes can be made without departing from the scope of the invention.

<Effects of the Invention> As described above, the first invention is that the working electrode is formed on the surface of the insulating substrate,
Since the reference electrode and the counter electrode are provided in a state where a predetermined positional relationship is maintained with each other, and a gas-permeable proton conductor film that integrally covers the electrode is provided,
By controlling the potential between the working electrode and the reference electrode, it is possible to prevent fluctuations in the potential on the surface of the working electrode, and it is possible to exhibit good characteristics without variations and to stabilize for a long period of time. It is possible to perform the above-described measurement, and further, it is possible to easily manufacture, and it is possible to achieve a unique effect that the overall size and cost can be reduced.

According to a second aspect of the present invention, the working electrode and the counter electrode are provided on the surface of the insulating base material while maintaining a predetermined positional relationship with each other, and the area of the counter electrode is several ten times or more the area of the working electrode. Since the gas permeable proton conductor film that is set and covers the above electrodes integrally is provided, it is possible to exhibit good characteristics without variation as in the case where the reference electrode is provided. In addition, it is possible to perform stable measurement over a long period of time, and moreover, it is possible to easily manufacture, and it is possible to achieve the downsizing and cost reduction as a whole.

[Brief description of drawings]

FIG. 1 is a schematic perspective view showing an embodiment of a combustible gas sensor of the present invention, FIG. 2 is a schematic vertical sectional view, and FIG. 3 illustrates an operation for holding a constant potential difference between a working electrode and a reference electrode. FIG. 4, FIG. 4 to FIG. 8 are schematic perspective views showing other embodiments, and FIG. 9 and FIG. 10 are schematic diagrams showing conventional flammable gas sensors. (1) ... Insulating substrate, (2) ... Working electrode, (3) ... Reference electrode, (4) ... Counter electrode, (5) ... Proton conductor membrane

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Claims (7)

[Claims] 1. A gas-permeable proton conductive material, which comprises a working electrode, a reference electrode, and a counter electrode provided on a surface of an insulating base material in a state of maintaining a predetermined positional relationship with each other and integrally covering the electrode. A combustible gas sensor having a body membrane. 2. The flammable gas sensor according to claim 1, wherein the reference electrode and the counter electrode are concentrically formed with the working electrode as a center. 3. A reference electrode according to claim 1, wherein the reference electrode is provided between the working electrode and the counter electrode.
The combustible gas sensor according to item. 4. A working electrode made of a platinum group element or gold, and a counter electrode made of a metal which does not react with a combustible gas. The combustible gas sensor according to any one of items. 5. The counter electrode according to claim 1, wherein the counter electrode is provided between the working electrode and the reference electrode.
The combustible gas sensor according to item. 6. The flammable gas sensor according to claim 5, wherein both the working electrode and the counter electrode are formed of a platinum group element or gold. 7. A working electrode and a counter electrode are provided on the surface of an insulating base material while maintaining a predetermined positional relationship with each other, and the area of the counter electrode is set to several ten times or more the area of the working electrode. A combustible gas sensor characterized in that a gas-permeable proton conductor film integrally covering the electrode is provided.