JP2918990B2 - Gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is caused by a gas leak (that is, mixing of oxygen gas from the outside) of a sealed container filled with SF ₆ gas or an abnormality or deterioration of a gas insulating device. The present invention relates to a gas sensor for detecting a decomposition gas, and more particularly, to a gas sensor capable of improving accuracy, reproducibility, or simultaneously measuring a plurality of components.

[Prior Art] In general, as a gas sensor applied to a gas insulating device, an all-solid-type gas sensor utilizing characteristics of a solid electrolyte has been proposed. It is particularly suitable in that it can be converted and an output signal can be extracted as an electric signal. Such a gas sensor is described in, for example, “Electrochemistry 52, No. 6 (1984)”.
On page 376 or on page 1518 of "The Chemical Society of Japan, No. 8 (1987)".

FIG. 5 is a sectional view showing a conventional gas sensor described in the above document.

In the drawing, (1) is a detection electrode whose one surface is exposed to a gas to be detected, and is formed of an electronic conductive metal (for example, compression bonding of platinum black powder).

(2) is a solid electrolyte having one surface joined to the other surface of the detection electrode (1), and is formed on a hydrogen ion conductive H ⁺ type Nafion film (for example, 0.2 mm thick) or an antimony film. It is composed of a film (for example, 0.5 mm thick) made by mixing Teflon powder (for example, 10 wt%).

Reference numeral (3) denotes a reference electrode having one surface joined to the other surface of the fixed electrolyte (2), and is formed of a mesh-like or porous electron conductive metal (for example, a platinum or silver thin film). (4) is a constant activity layer bonded to the other surface of the reference electrode (3), and is composed of an antimonic acid film (KH ₂ PO ₄ ) or the like.

(5a) and (5b) are output terminals for extracting a potential difference V generated between the electrodes (1) and (3), and (9) is a reference electrode (3) and a part together with a part of the solid electrolyte (2). It is a resin such as epoxy which covers the constant activity layer (4).

Next, the operation of the conventional gas sensor shown in FIG. 5 will be described.

First, when the gas sensor is installed in an atmosphere of a detection target gas (for example, oxygen) under a constant humidity, only one upper surface of the detection electrode (1) is exposed to the detection target gas. At this time, since hydrogen ions H ⁺ and electrons e ⁻ are supplied from the solid electrolyte (2), a reaction of O ₂ + H ⁺ + e ⁻ → HO ₂ occurs on the detection electrode (1) side, and HO ₂ is generated. You.

Here, assuming that the entire gas sensor is a battery, the equilibrium voltage Es is represented by Es = Es ゜ − (RT / F) · 1n (α _HO2 / PO ₂ α _{H +} ) according to the Nernst equation. Where Es ゜ is the standard electrode voltage, R is the gas constant, T is the absolute temperature, F is the Faraday constant, α _HO2 is the activity of HO ₂ (corresponding to the concentration), α _{H +} is the activity of H ⁺ , P _O2 Is the partial pressure of oxygen (O ₂ ). In addition, each activity α _HO2 and α _{H +} )
Is assumed to be constant regardless of the oxygen partial pressure P _O2 .

On the other hand, the reference electrode (3) side, in Teikatsu weight layer _{(4), KH 2 PO 4} → KPO 4 + 2H + + 2e - because the reaction occurs, the equilibrium voltage E _R is, E _R = E _R ° - ( RT / F) · 1n (KPO ₄ / αH ⁺ ). Here, E _R゜ is a standard electrode voltage, and all variables are regarded as constant.

Therefore, the potential difference V between the detection electrode (1) and the reference electrode (3) is as follows: V = Es−E _R = A + B · log P _O2 Here, A and B are constants.

Thus, a potential difference V proportional to the logarithm of the oxygen concentration on the detection electrode (1) side is generated between the detection electrode (1) and the reference electrode (3), and this potential difference V is output from the output terminal (5a). And (5b). The potential difference V is the oxygen concentration [% vo
l], the oxygen concentration can be detected based on the potential difference V.

At this time, by using the all-solid-type gas sensor as described above, downsizing and weight reduction can be realized and liquid leakage does not occur. Further, by extracting the concentration of the detection target gas as an electric signal, the function as the oxygen sensor can be sufficiently exhibited.

By the way, since the reference electrode (3) is an electron conductive metal, a portion where the solid electrolyte (2), the constant activity layer (4) and the reference electrode are in contact with each other is provided to function as an electrode for causing the above reaction. It is necessary. Therefore, the reference electrode (3)
However, since the reference electrode (3) cannot completely separate the solid electrolyte (2) and the constant activity layer (4), the constant activity layer (4) The active material therein diffuses into the layer of the solid electrolyte (2). This diffusion of the active material occurs when there is a concentration difference (usually present) of the active material between the solid electrolyte (2) and the constant activity layer (4).

For this reason, the constant activity layer (4) does not show a constant activity, the potential of the reference electrode (3) is not constant, and the reference electrode (3)
Cannot perform its function. That is, the correspondence between the potential difference V and the concentration of the detection target gas is not clear.

Also, the detection electrode (1) and the reference electrode (3) are each 1
Therefore, one gas sensor detects only one type of gas, and the same gas sensor cannot measure the concentrations of two or more detection target gases.

[Problems to be Solved by the Invention] As described above, the conventional gas sensor is a solid electrolyte (2).
And the constant activity layer (4) cannot be completely separated, the activity in the constant activity layer (4) diffuses into the solid electrolyte (2) and the potential of the reference electrode (3) Cannot be kept constant, and the concentration of the gas to be detected cannot be detected with high accuracy.

In addition, since the same gas sensor cannot measure two or more detection target gases, there has been a problem that the applicable range is limited.

That is, two components are separately required for the two functions of a path for hydrogen ions (H ⁺ ) (for example, solid electrolyte fine particles) and a path for electrons (e ⁻ ) (for example, metal wire mesh). Therefore, there is a problem that the structure becomes complicated and the factors of the measurement error increase.

The present invention has been made to solve the above problems, and has as its object to obtain a gas sensor capable of maintaining a constant activity of a constant activity layer.

Another object of the present invention is to provide a gas sensor that can simultaneously measure a plurality of detection target gases.

[Means for Solving the Problems] In a first gas sensor according to the present invention, a reference electrode is formed of a mixed conductor of ions and electrons or holes.

Further, the second gas sensor according to the present invention has at least one of the detection electrode and the reference electrode constituted by a plurality.

Further, the third gas sensor according to the present invention is provided with activity adjusting means for adjusting the activity of the constant activity layer by selectively applying a voltage between the detection electrode and the reference electrode. It is.

[Operation] In the first gas sensor of the present invention, the active material in the constant activity layer is completely separated from the solid electrolyte by the reference electrode made of the mixed conductor, and the activity is kept constant.
The potential of the reference electrode is kept constant. Therefore, the potential difference between the detection electrode and the reference electrode accurately corresponds to the concentration of the gas to be detected and has a high reproducibility.

In the second gas sensor according to the present invention, a plurality of detection electrodes or reference electrodes simultaneously measure a plurality of components with the same gas sensor.

Further, in the third gas sensor according to the present invention, the activity adjusting means selectively applies a voltage between the detection electrode and the reference electrode to adjust the activity of the constant activity layer in accordance with the concentration of the gas to be detected. Is adjusted to an arbitrary value to output an appropriate potential difference.

Embodiment An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing one embodiment of the present invention, and (1)
(4), (5a), (5b) and (9) are the same as described above.

In this case, the detection electrode (1) is made of, for example, a porous plating layer of platinum (or a platinum material similar to the conventional one). The reference electrode (3) is a mixed conductor of hydrogen ions and electrons (or holes), for example, HxWO ₃ (0.012 ≦ x
≤ 0.071), and no pores through which the active material passes are formed. Further, the constant activity layer (4) is formed of a metal hydride obtained by storing a predetermined amount of hydrogen in LaNi ₅ , for example, LaNiHx (0
≤ x ≤ 6).

(6) is a DC power supply, (7) is a changeover switch for selectively connecting the DC power supply (6) to the detection electrode (1) and the reference electrode (3), and the detection electrode (1) and the reference electrode (3). And an activity adjusting means for adjusting the activity of the constant activity layer (4) by selectively applying a voltage between the two.

In general, a substance in which both ionic conductivity and electronic conductivity are remarkable, and which involves not only electrons but also ions (or holes) as carriers of charge in a solid, is called a mixed conductor. This can be referred to, for example, on page 201 of “Electrochemical Association Handbook, edited by Electrochemical Society (Maruzen, 1992)”.

Further, the H _x WO ₃ is a recognized material of hydrogen ions (protons) as a mixed conductor and electronic, for example,
"Solid Ionics: Tokuichi Kudo, Kazuo Fueki" (Kodansha,
1986), p. 112.

Similarly, Ce _1-X Gd _X O _2-X is a substance recognized as a mixed conductor of electrons and oxygen ions (O ⁻ ),
Reference can be made to page 95 of the above-mentioned document "Solid Ionics".

These substances are described in the above-mentioned document “Solid Ionics”.
As shown on page 99, the diffusion (chemical diffusion) of ions (H ⁺ or O ⁻ ) through ion vacancies is fast and the electronic conductivity is large, so that a kind of mixed conductivity phenomenon can be exhibited. I understand.

That is, the following reaction occurs in the reference electrode (3), and an electromotive force corresponding to the concentration of the detection gas (Pt, O ₂ ) is generated.

H ₂ ⇔ 2H ⁺ + 2e ^{− In the} above equation, hydrogen (H ₂ ) on the left side is stored in the constant activity layer (4) made of a hydrogen storage alloy (LaNi ₅ ).

The right side is a reaction on the reference electrode (3) made of a mixed conductor, in which hydrogen ions (H ₊ ) are introduced into the solid electrolyte (2), and electrons (e ⁻ ) are led to an external circuit.

As described above, by forming the reference electrode (3) with a mixed conductor of ions and electrons, an electromotive force corresponding to the concentration of the detection gas based on the above reaction formula can be obtained.

At this time, the reference electrode (3) made of the mixed conductor is H
⁺ Ions, e ^- has a both passages (electronic),
Since two functions (ionic conduction and electronic conduction) are achieved by one component, there is an advantage that a measurement error is reduced with a simple configuration.

Next, the operation of the embodiment of the present invention shown in FIG. 1 will be described.

Normally, the output terminal (5a) is operated by the changeover switch (7).
And (5b) are connected to the detection electrode (1) and the reference electrode (3), and the DC power supply (6) is disconnected.

Therefore, when the gas sensor is installed in the gas to be detected,
As described above, for example, a potential difference V proportional to the logarithm of the oxygen concentration is generated between the output terminals (5a) and (5b), and functions as an oxygen sensor.

At this time, for example, in the constant activity layer (4) made of metal hydride, the hydrogen activity does not pass through the reference electrode (3), so that the hydrogen activity is the same as the metal hydrogen in the constant activity layer (4). Retained in the compound and does not decrease. Therefore, for a certain concentration of the gas to be detected, the output terminals (5a) and (5
b) The potential difference V generated between them always has the same value, and high reproducibility is obtained.

Further, in order to always obtain an appropriate output voltage V regardless of the concentration range of the detection target gas, the activity of the constant activity layer (4) may be adjusted according to the concentration of the detection target gas. In this case, by operating the changeover switch (7), the output terminals (5a) and (5b)
Are short-circuited, and both terminals of the DC power supply (6) are connected to the detection electrode (1) and the reference electrode (3), and a DC voltage is applied between the two electrodes to supply a predetermined amount of current.

For example, when increasing the hydrogen activity in the constant activity layer (4), the detection target gas contains hydrogen gas so that the detection electrode (1) becomes a positive electrode and the reference electrode (3) becomes a negative electrode. What is necessary is just to apply a DC voltage and to energize. At this time, a reaction of H ₂ O → 2H ⁺ + (1/2) O ₂ + 2e ⁻ occurs on the detection electrode (1). Therefore, oxygen (O ₂ ) remains in the gas to be detected, and hydrogen ions (H ⁺ ) move in the solid electrolyte (2) toward the reference electrolysis (3). Then, a reaction of 2H ⁺ + 2e ⁻ → H ₂ ... Occurs on the reference electrode (3). Thereby, the hydrogen activity of the constant activity layer (4) can be increased, for example, a large potential difference V can be obtained for a low concentration of the gas to be detected, and the low concentration gas to be detected can be detected with high accuracy. Can be. Further, by controlling the amount of direct current applied, the activity of the constant activity layer (4) can be set to a predetermined value.

Similarly, when reducing the hydrogen activity in the constant activity layer (4), a DC voltage may be applied so that the detection electrode (1) is a negative electrode and the reference electrode (3) is a positive electrode. At this time, a reaction of H ₂ → 2H ⁺ + 2e ⁻ occurs on the reference electrode (3), and hydrogen ions (H ⁺ ) move in the solid electrolyte (2) toward the detection electrode (1). On the detection electrode (1), a reaction of 2H ⁺ + (1/2) O ₂ + 2e ⁻ → H ₂ O occurs. Thereby, the hydrogen activity of the constant activity layer (4) can be reduced, and the activity can be set to a predetermined value by controlling the amount of direct current applied.

Further, since the reference electrode (3) is formed of a mixed conductor of hydrogen ions and electrons, the hydrogen activity in the constant activity layer (4) set to a predetermined value passes through the reference electrode (3). There is no danger of diffusion (transmission dissipation), and it is kept constant as described above.

FIG. 2 is a characteristic diagram showing the relationship between oxygen concentration and potential difference (output voltage) V when the gas sensor of FIG. 1 is applied to an oxygen sensor. As apparent from FIG. 2, the value of the potential difference V changes in proportion to the logarithm of the oxygen concentration.

In the above embodiment, the case where the active material of the constant activity layer (4) is hydrogen is shown, but the active material may be oxygen. Also in this case, the solid electrolyte (2) is a hydrogen ion conductive solid,
The reference electrode (3) may be a mixed conductor of hydrogen ions and electrons.

First, at the time of manufacturing the gas sensor, moisture is contained in the constant activity layer (4), and when the manufacturing is completed, a DC power supply (6) is set so that the reference electrode (3) becomes a positive electrode and the detection electrode (1) becomes a negative electrode.
To apply a DC voltage. Thereby, the reference electrode (3)
Above, the reaction of the formula occurs, the hydrogen ions (H ⁺ ) move toward the detection electrode (1), and the reaction of the formula or any of the formulas occurs on the detection electrode (1). At this time, oxygen is generated from the formula in the reference electrode (3), so that the amount of oxygen in the oxygen constant activity layer (4) can be set to an arbitrary value by controlling the amount of energized electricity. Can be.

For example, when the measurement range of the oxygen concentration in the gas to be detected is a high concentration region, the oxygen activity in the constant activity layer (4) is set to be lower, and the measurement range of the oxygen concentration in the gas to be detected is low. In the case of the concentration region, the oxygen activity of the constant activity layer (4) may be set higher. As a result, the output voltage V for a predetermined oxygen concentration region can be made relatively high, and the oxygen concentration can be measured accurately.

Further, in the above embodiment, as the material of the solid electrolyte (2) and the resin (9), a material suitable for a low-temperature type oxygen sensor usable at a maximum of 200 ° C. or less was used. By using a solid solution based on ionic conductive perovskite type ceramics (SrCeO ₃ or BaCeO ₃ ) and using insulating ceramics such as alumina in place of resin (9), a high temperature range of around 900 ° C. Can also be applied.

Similarly, in the case of a gas sensor used in a high temperature region,
The resin (9) is made of alumina or the like, and an oxygen ion conductive solid such as (Bi
With ₂ O _{3) 0.78} (WO _{3) 0.22,} as the reference electrode (3), by using the mixed conductor of oxygen ions and electrons of _{Ce 1-X Gd X O 2} -X / 2 (X = 0.2) Good.

In the above embodiment, the oxygen sensor has been described as an example. However, if a salt such as a metal, a hydrochloride, a sulfite, a sulfate, a nitrite, and a carbonate is used as a component of the constant activity layer (4). , Respectively, can be applied to the detection of gas concentrations of SO ₃ , SO ₂ , NO, NO ₂ , CO, CO ₂ , etc.

For example, taking the case of a CO sensor as an example, the solid electrolyte (2) is a hydrogen ion conductive solid and the constant activity layer (4)
A case in which CaCO ₃ is contained in the water will be described. At this time, on the detection electrode (1), a reaction of CO + H ₂ O22H ⁺ + CO ₂ + 2e ⁻ occurs, and on the reference electrode (3), a reaction of 2H ⁺ + CaCO ₃ ⇔CaO + CO ₂ + H _2. Occur.

Here, at a constant temperature, the activity in the constant activity layer (4) is determined by the amount of CaCO ₃ contained in advance, and the potential difference V generated at the output terminals (5a) and (5b) is The value is proportional to the logarithm of the CO concentration.

Further, in the above embodiment, the case where one kind of gas concentration is detected by the pair of electrodes (1) and (3) has been described. However, the plurality of detection electrodes (1) made of different materials are connected to the solid electrolyte (2). If the bonding is performed at different positions above, or a plurality of reference electrodes (3) are bonded to the solid electrolyte (2), and a plurality of constant activity layers (4) having different components are bonded to each other, 1
One gas sensor can simultaneously measure the concentration of a plurality of components.

For example, as shown in FIG. 3, by using a plurality of reference electrodes (3) and a constant activity layer (4), a plurality of detection target gases (SO ₂ , C
When simultaneously detecting the concentrations of l ₂ and F ₂ ), the reference electrode (3) is physically divided and arranged, and a different constant activity layer (4) is provided for each reference electrode (3). What is necessary is just to join and detect the potential difference V of each output terminal (5a) and (5b). Here, Ag is used as the detection electrode (1), Ag ₃ SI is used as the solid electrolyte (2), and Ag ₂ S, AgCl, and AgF are used as the three divided constant activity layers (4).

When the vapor concentration (vapor pressure) and the oxygen concentration are measured simultaneously, a hydrogen ion conductive solid (for example, hexatrimethylenediamine) is used as the solid electrolyte (2), and a mixture of hydrogen ions and electrons is used as the reference electrode (3). Using a conductor (eg, HxWO ₃ , 0.012 ≦ x ≦ 0.071) and filling a plurality of constant activity layers (4) with a hydrogen storage alloy,
By using those containing manganese fluoride and water, these constant activity layers (4) are each independently bonded to a hydrogen ion conductive solid, and applied to a detection target gas in which water vapor and oxygen coexist. Good.

As shown in FIG. 4, when detecting the concentration of a plurality of detection target gases (SO ₂ , Cl ₂ , F ₂ ) using a plurality of detection electrodes (1), the detection electrode (1 ) May be made of a mixed conductor of electrons and ions, similarly to the reference electrode (3), and the composition of each Ag content may be changed so that the reaction to each gas to be detected is different. Accordingly, even in a mixed atmosphere of a plurality of detection target gases, each detection target gas can be similarly detected individually.

In the above embodiment, a hydrogen ion conductive solid or an oxygen ion conductive solid was used as the solid electrolyte (2).
For example, if a silver ion conductive solid, an alkali metal conductive solid, a copper ion conductive solid, a fluorine ion conductive solid, a mercury ion conductive solid, or the like is used as appropriate, the applicable detection target gas can be expanded and measured. The accuracy is improved.

The reference electrode (3) is made of a mixed conductor,
As shown in FIG. 3, by dividing into a plurality of parts and further providing the activity adjusting means (6) and (7), high accuracy and space saving can be realized at the same time.

Further, in the above embodiment, the reference electrode (3) or the detection electrode (1) and the reference electrode (3) are composed of a mixed conductor of ions and electrons, but a mixed conductor of ions and holes. It is needless to say that the same operation and effect can be obtained even if it is constituted by.

[Effects of the Invention] As described above, according to the first aspect of the present invention, the reference electrode is composed of a mixed conductor of ions and electrons or holes, and the active material of the constant activity layer is the reference electrode. As a result, the solid electrolyte and the constant activity layer are completely separated to keep the activity constant, so that the potential of the reference electrode is kept constant and a gas sensor with high reproducibility of the detected potential difference is obtained. Has the effect.

Further, according to the second aspect of the present invention, since at least one of the detection electrode and the reference electrode is constituted by a plurality, a gas sensor capable of simultaneously measuring a plurality of components with the same gas sensor is obtained. .

According to the third aspect of the present invention, an activity adjusting means for selectively applying a voltage between the detection electrode and the reference electrode to adjust the activity of the constant activity layer is provided. Since the activity of the constant activity layer is adjusted to an arbitrary value according to the gas concentration and an appropriate potential difference is output, there is an effect that a gas sensor with improved accuracy according to the concentration can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing one embodiment of the first and second inventions of the present invention, FIG. 2 is a characteristic diagram showing the relationship between the potential difference obtained by the embodiment of FIG. 1 and the oxygen concentration, and FIG. FIG. 4 is a sectional view showing another embodiment of the second invention of the present invention. FIG. 4 is a sectional view showing another embodiment of the second invention of the present invention.
FIG. 1 is a sectional view showing a conventional gas sensor. (1) detection electrode, (2) solid electrolyte (3) reference electrode, (4) constant activity layer (6) DC power supply, (7) changeover switch (6) , (7) ... activity adjusting means V ... potential difference In the drawings, the same reference numerals indicate the same or corresponding parts.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasushi Tanaki 8-1-1, Tsukaguchi-Honcho, Amagasaki-shi, Hyogo Inside Itami Works, Mitsubishi Electric Corporation (72) Inventor, Arinobu Nagahata 8-1-1, Tsukaguchi-Honcho, Amagasaki-shi, Hyogo No. 1 Mitsubishi Electric Corporation Itami Works (56) References JP-A-55-50149 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01N 27/409, 27/416 G01N 27/406

Claims (3)

(57) [Claims] 1. A detection electrode having one surface exposed to a gas to be detected, a solid electrolyte having one surface joined to the other surface of the detection electrode, and one surface having a surface connected to the other surface of the solid electrolyte. A joined reference electrode, and a constant activity layer joined to the other surface of the reference electrode, wherein the concentration of the detection target gas is determined based on a potential difference generated between the detection electrode and the reference electrode. Wherein the reference electrode comprises a mixed conductor of ions and electrons or holes. 2. A detection electrode having one surface exposed to a gas to be detected, a solid electrolyte having one surface joined to the other surface of the detection electrode, and one surface having a surface connected to the other surface of the solid electrolyte. A joined reference electrode, and a constant activity layer joined to the other surface of the reference electrode, wherein the concentration of the detection target gas is determined based on a potential difference generated between the detection electrode and the reference electrode. A gas sensor characterized in that at least one of the detection electrode and the reference electrode comprises a plurality. 3. A detection electrode having one surface exposed to a gas to be detected, a solid electrolyte having one surface joined to the other surface of the detection electrode, and a surface having one surface joined to the other surface of the solid electrolyte. A joined reference electrode, and a constant activity layer joined to the other surface of the reference electrode, wherein the concentration of the detection target gas is determined based on a potential difference generated between the detection electrode and the reference electrode. In the gas sensor for detecting the activity, an activity adjusting means for selectively applying a voltage between the detection electrode and the reference electrode to adjust the activity of the constant activity layer is provided. Gas sensor.