JP2600482B2 - Carbon dioxide sensor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a carbon dioxide gas sensor used in places where carbon dioxide concentration is measured or controlled, such as for horticulture, environmental hygiene, disaster prevention, and industrial use.

[Prior Art] FIGS. 6 (a) and 6 (b) show Japanese Patent Application Laid-Open No. 1-2690 / 1990.
FIG. 6 is a side sectional view showing a sensing part of a conventional carbon dioxide sensor disclosed in Japanese Patent Publication No. 48 and a view showing an entire configuration of the carbon dioxide sensor. In FIG. 6 (a), (1) shows an alkali metal ion conductor made of a solid electrolyte. Ceramics (NASICON),
(2) and (3) are a pair of porous electrodes for extracting voltage signals on both sides thereof, and (4) and (5) are these electrodes (2).
(3) is a lead wire, (6) is a sodium carbonate adhered to the electrode (2), and ( 7 ) is a gas sensing part constituted by these.

In FIG. 6 (b), (8) is a heater provided with the gas sensing part ( 7 ) on the upper surface, (9) and (10) are lead wires drawn out from the heater, (11) and (12). Are metal pins for voltage application connected to these leads (9) and (10), (13) and (14) are metal pins for signal output connected to the leads (4) and (5), and (15) ) Is a metal pin (11)
(12) (13) The pedestal on which (14) is fixed, (16) is the built-in gas sensor ( 7 ), heater (8), and lead wire (4)
(5) (9) (10), metal pin (11) (12) (13) (1
4) Protector made of stainless steel wire mesh to protect

 Next, the operation will be described.

The gas detection unit ( 7 ) having the above configuration is based on the detection principle proposed by Saito and Maruyama of Tokyo Institute of Technology, and the gas detection unit ( 7 ) includes the following batteries.

When the gas sensing part ( 7 ) constituting such a battery is heated to the measurement temperature by the heater (8), the electrodes (2) and (3) serve as a cathode and an anode, respectively, at the boundary, as follows. A battery reaction occurs.

Boundary ^{_{Na 2 CO 3 = 2Na + +}} CO 2 + 1 / 2O 2 + 2e - ...... [I] Boundary ^{2Na + = 2Na + (in NASICON} ) ...... [II] Boundary _{^{2Na + + 1 / 2O 2 +}} 2e - = Na 2 O ... ... [III] A voltage is generated between both electrode layers by an electromotive force represented by the following Nernst equation.

Where E: generated electromotive force, F: Faraday constant R: gas constant, T: absolute temperature △ ▲ G ^゜ _i ▼: standard generation energy of chemical species i P ^* : total atmospheric pressure EMF generated from equation [IV] E is the partial pressure of carbon dioxide in the atmosphere
Proportional to P _CO2 of the logarithm. Therefore, the generated electromotive force E
Is extracted from the electrode layers (2) and (3), the carbon dioxide concentration can be detected electrically.

[Problems to be Solved by the Invention] The conventional carbon dioxide sensor is configured as described above, and has the following problems. First, sodium oxide (Na ₂ O) generated at the interface between the electrode (3) and the NASICON by the above-described battery reactions [I] to [III] is converted to a battery reaction [II].
Contact with moisture or carbon dioxide gas in the surrounding atmosphere through the porous electrode (3) for supplying oxygen ions involved in [I], causing the following reaction.

Na ₂ O (solid) + H ₂ O (gas) → 2NaOH... [V] 2NaOH + 2CO ₂ → 2NaHCO ₃ ... [VI] 2NaHCO ₃ → Na ₂ CO ₃ + H ₂ O + CO _2. Electrode (3) and NASICON
Sodium carbonate (Na ₂ CO ₃ ) is formed at the interface, and a pseudo-cathode is formed on the anode, the detection sensitivity gradually decreases, and eventually disappears. Second, when the measurement atmosphere is in a high-humidity environment or in a condition where dew condensation is likely to occur, when the carbon dioxide gas sensor is in a non-heated state, sodium carbonate (4), which is a component of the gas sensing unit ( 7 ), has its own hygroscopic action. Due to the solubility in water, hydrates and hydrides are formed respectively, and when heated again, the absorbed water evaporates. As a result, expansion and contraction occur. The gas sensing part itself could not be regenerated.

As a third problem, when the gas sensor (10) and the heater (11) are at a high temperature of 500 ° C. or more, if an organic solvent such as alcohol is present in the atmosphere to be measured, the following reaction occurs on these surfaces. Get offended.

C ₂ H ₅ OH + 3O ₂ → 2CO ₂ + 3H ₂ O… [VIII] At this time, the generated electromotive force of the carbon dioxide gas sensor was changed by the generated carbon dioxide gas, so that there was a problem that the carbon dioxide gas concentration could not be measured accurately.

The present invention solves the above-described problems, and has high reliability even when the gas sensing unit is exposed to moisture or carbon dioxide gas in the measurement atmosphere or repeatedly placed in a heated or non-heated state in a high humidity environment. It is another object of the present invention to provide a gas sensor and a carbon dioxide gas sensor capable of obtaining high detection accuracy and high reliability data even when an organic solvent such as alcohol is present in an atmosphere to be measured.

[Means for Solving the Problems] A first invention of the present invention is directed to an alkali metal ion conductive ceramic comprising a solid electrolyte having a pair of porous electrode layers at both ends, and at least a part of any one of the electrodes. And a closed space provided to cover the electrode on the opposite side of the electrode, a mixture of dry nitrogen gas and a metal oxide filled in the closed space, the dry nitrogen gas and the A gas barrier layer for shielding the mixture of metal oxides from an external measurement space.

According to a second aspect of the present invention, there is provided an alkali metal ion conductive ceramic comprising a solid electrolyte having a pair of porous electrode layers at both ends, a metal carbonate covering at least a part of one of the electrodes, and an opposite side of the electrode. A sealed space provided so as to cover the electrodes, a mixture of dry nitrogen gas and a metal oxide filled in the closed space, and a mixture of the dry nitrogen gas and the metal oxide from an external measurement space. It was provided with a gas blocking layer for blocking and a porous moisture absorbent provided in contact with the metal carbonate.

According to a third aspect of the present invention, there is provided a carbon dioxide sensing unit comprising an alkali metal ion conductive ceramic comprising a solid electrolyte having a pair of porous electrode layers at both ends, and a metal carbonate covering at least a part of one of the electrodes. And a gas absorber that absorbs gas excluding water vapor that forms part or all of a protector that includes a heating unit that heats the carbon dioxide gas sensing unit to a measurement temperature, and the atmosphere to be measured passes through the gas absorber. It reached the above-mentioned carbon dioxide gas detection part.

[Operation] electrode serving as an anode in the carbon dioxide sensor of the present invention is exposed to an oxygen atmosphere having a certain partial pressure, the following cell reaction 2Na at the anode _{^{+ + 1 / 2O 2 + 2e}} - = Na 2 O in the ambient atmosphere H ₂ O (gas) and CO ₂ (gas) are not involved,
Therefore, it is possible to prevent the characteristics of the gas sensing unit from deteriorating due to the following reactions.

Na ₂ O (solid) + H ₂ O (gas) → 2NaOH 2NaOH + 2CO ₂ → 2NaHCO ₃ 2NaHCO ₃ → Na ₂ CO ₃ + H ₂ O + CO _{2 In} the second invention, the metal carbonate covering one electrode is porous moisture absorbing Because it is covered with the agent, no moisture adheres to the metal carbonate even if it is placed in a humid state without heating. Therefore, peeling and falling off of the metal carbonate from the electrode are prevented.

Further, in the third invention, the atmosphere to be measured passes through a gas absorber that absorbs gas except for water vapor, and at this time, an organic solvent such as alcohol is adsorbed by the gas absorber, so that carbon dioxide due to a decomposition reaction of the organic solvent is generated. A correct measurement atmosphere is provided without a change in gas concentration.

[Example] Hereinafter, an example of the present invention will be described.

FIG. 1 (a) is a configuration diagram of an embodiment of the first invention, and in the figure, (21) is an alkali metal ion conductive ceramic such as a sodium ion conductive ceramic made of a solid electrolyte, and has a bottomed cylindrical shape. Is formed. (22) was filled in this internal space with, for example, dry nitrogen gas.
The mixture (23) of a metal oxide such as Cu ₂ O / CuO is a glass seal for closing the other end opening of the alkali metal ion conductive ceramic (21). FIG. 1 (b) shows the first
In this figure, (24) is a low-melting glass-based insulating pace covering the electrode (2), the alkali metal ion conductive ceramics (21) and the glass seal (23). 50 for gas barrier layer made of sintered body
Sintered at 0-600 ° C.

In each of the above figures, the porous electrode (2) in contact with the outer bottom of the alkali metal ion conductive ceramic (21) has sodium carbonate (6), which forms dissociation equilibrium with carbon dioxide, on its end face. Has been stripped. A porous electrode (3) is brought into contact with the interior space of the alkali metal ion conductive ceramics (21), and the mixture of metal oxides (22) is filled, and the open end thereof is a glass seal (23). A sealed space ( A ) is formed.

 Next, the operation will be described.

Gas sensing unit by heater not shown ( 7 )
Conductive ceramics that heats the sample to the measurement temperature (21)
The following dissociation equilibrium reaction occurs in the mixture (22) of the metal oxide filled in the internal space.

Therefore, alkali metal ion conductive ceramics (2
The battery reactions [I] to [III] shown in the conventional example occur at the interface between 1) and the electrodes (2) and (3). At this time, oxygen and nitrogen having a constant partial pressure come into contact with the electrode (3), and are shielded from water vapor and carbon dioxide. Therefore, the oxidation and carbonation reaction of the anode product Na ₂ O shown in the conventional example [ V]
[VI] and [VII] do not occur, and the deterioration of the gas sensing portion ( 7 ) is prevented.

On the other hand, in the embodiment of FIG. 1 (b), in addition to the configuration of FIG. 1 (a), an electrode having a low melting point glass-based insulating paste capable of preventing the penetration of gas such as water vapor or carbon dioxide gas is exposed to the surrounding atmosphere. It is characterized in that it is applied to the entire surface of (2) and the alkali metal ion conductive ceramics (21) and is fired at 500 to 600 ° C. to form a gas barrier layer (24). The detection operation as a carbon dioxide sensor is the same as that of FIG. 1A, and therefore the description thereof is omitted. However, in the configuration of FIG. 1B, the sensor is generated at the boundary by the battery reactions [I] to [III]. The contact between Na ₂ O and water vapor and carbon dioxide in the surrounding atmosphere is avoided. Further, the adhesive strength between the electrodes (2) and (3) and the alkali metal ion conductive ceramics (21) is improved by covering the gas barrier layer (24). In the configuration shown in FIG. 1 (c), the electrodes (2) and (3) and the rod-like alkali metal ion conductive ceramics (21) between them are placed in a glass tube (25) such as Virex so as to house them. The end is welded and fixed, the metal oxide mixture (22) is put in the extension of the glass tube, and the glass seal (23) is formed to form a closed space ( B ).

An embodiment of the second invention will be described with reference to FIG. Second
FIG. 1A is a configuration diagram of one embodiment, in which the same reference numerals are given to the same configurations as before. (26) is a granular silica gel, (30) is a protector, (31) is a tubular insulator, and an alkali metal ion conductive ceramic (21) is sealed with a gas barrier adhesive layer (20).

This operation is the same as that of FIG. 1, and the gas sensor ( 7 ) is heated to the measurement temperature by a heater (not shown).
Battery reactions [I] to [III] occur.

Incidentally, only the oxygen gas and nitrogen gas constant partial pressure in the electrode (3) serving as the anode in this arrangement is in contact, since the moisture and carbon dioxide in the atmosphere to be measured is interrupted, hydroxide Na ₂ O, And no carbonation reaction occurs. Therefore, a decrease in detection sensitivity can be prevented.

Further, the atmosphere to be measured reaching the sodium carbonate (6) passes through the silica gel, so that moisture is adsorbed. The sodium carbonate (6) is kept in a low humidity state, and the peeling and falling off described in the conventional example can be prevented. The water adsorbed on the silica gel separates from the silica gel when heated to the measurement temperature, so that it can be used repeatedly.

FIG. 2 (b) shows a hygroscopic gas permeable membrane (29) composed of a mixture of a powdery silica gel (27) and an amorphous glass seal (28) as a hygroscopic agent. The operation is the same as that of FIG.

The third invention will be described with reference to FIG. Fig. 3 (a)
Is a configuration diagram of an embodiment. In the figure, portions described in the above description are given the same reference numerals and description thereof is omitted. (Five
Reference numeral 0) denotes a cylindrical heating coil, in which a gas sensing portion ( 7 ) is located, and heats to a measurement temperature during measurement. The gas sensor ( 7 ) and the heating coil (50) are shut off by a heat-resistant gas shut-off protector (51) and a pedestal (15), and the atmosphere to be measured is a metal mesh (53) and a miscellaneous gas absorber (50). Only through 52) is the gas sensor ( 7 ) reached.

The miscellaneous gas absorber (52) is a gas absorber that absorbs gas excluding water vapor.

In an example in which activated carbon is used as a miscellaneous gas absorber, even in a humid atmosphere, if there is an organic solvent such as alcohol, it is adsorbed, and the organic solvent component does not reach the gas sensing portion.

For example, the effect of activated carbon when an alcohol is present in the desiccator of FIG. 3 is shown in FIG. 4, compared with a conventional product without a miscellaneous gas absorber, according to the present invention provided with 0.8 g of activated carbon in FIG. In the improved product, the generated electromotive force does not change even if alcohol is present in the atmosphere to be measured.

Miscellaneous gas components removed by activated carbon include fatty acids, mercaptans, phenols, hydrocarbons (aliphatic and aromatic), organic chlorine compounds, alcohols other than methanol, ketones, aldehydes other than formaldehyde, and esters. There is kind.

FIGS. 3 (b) and 3 (c) show another embodiment. In FIG. 3 (b), the gas sensing part ( 7 ) is a metal mesh (53).
The atmosphere to be measured is the miscellaneous gas absorber (52) because the surface of the container and the miscellaneous gas absorber (52) are insulated by the closed storage box (51).
Only reaches the gas sensing part.

Further, for example, the storage container itself of the gas sensor may be constituted by a porous cap-shaped filter obtained by processing a miscellaneous gas absorber.

FIG. 3 (c) shows an example in which a separation heater (54) is inserted in a miscellaneous gas absorber (52). Even if the miscellaneous gas absorber adsorbs miscellaneous gas to a saturated state, the adsorbed miscellaneous gas is released by reheating to several hundred degrees C, and the capacity is regenerated after a certain time. For example, in the operation flow shown in FIG. 5, it is possible to perform highly reliable measurement for a long time without being affected by miscellaneous gases.

In FIG. 3 (c), a dedicated separation heater is provided. However, when the miscellaneous gas absorber has conductivity such as activated carbon, electrodes are provided at both ends of the activated carbon, and between these electrodes. The same effect can be obtained even when Joule heat is generated by applying a voltage.

[Effect of the Invention] As described above, the carbon dioxide gas sensor of the present invention comprises: an electrode serving as an anode; a metal oxide mixture for providing a constant oxygen partial pressure by dissociation equilibrium; First, the battery reaction does not involve moisture or carbon dioxide gas in the surrounding atmosphere, and the gas sensing part is formed by the oxidation and carbonation of sodium oxide as the anode product. Deterioration can be prevented.

Second, since the metal carbonate is covered with the porous moisture absorbing agent, the metal carbonate can be prevented from peeling and falling due to repeated heating and non-heating even in a humid environment, and the detection sensitivity can be prevented from decreasing and deteriorating.

Third, since the gas sensing unit is covered with the miscellaneous gas absorber or the gas barrier protector, it is possible to prevent the detection accuracy of the gas sensing unit from deteriorating even in an atmosphere where an organic solvent is present.

[Brief description of the drawings]

FIGS. 1 (a), 1 (b), and 1 (c) are configuration diagrams showing an embodiment of the first invention, FIGS. 2 (a) and 2 (b).
FIGS. 3 (a), 3 (b) and 3 (c) are configuration diagrams showing an embodiment of the second invention, and FIGS. 4 (a) and 4 (b) are configuration diagrams showing an embodiment of the third invention. FIG. 3 is a characteristic diagram showing a comparison between the improved product according to the invention of FIG. 3 and a conventional product, FIG. 5 is an operation flow diagram to which the invention of FIG. 3 (c) is applied, FIG. 6 (a), FIG. 2) is a configuration diagram of a gas sensing unit of the conventional carbon dioxide gas sensor and the entire sensor. In the figure, (2) and (3) are electrodes, (6) is a metal carbonate, (7) is a gas sensing part, (20) is a gas barrier adhesive layer, (21) is an alkali metal ion conductive ceramic, ( 22) is a mixture of metal oxides, (24) is a gas barrier layer, (25) is a glass tube, (26) is granular silica gel,
(27) is a powdery silica gel, (28) is an amorphous glass seal, (29) is a hygroscopic gas permeable membrane, (30) is a protector, (31) is a tubular insulator, (50) is a heating coil, ( 51)
Is a gas shut-off protector, (52) is a miscellaneous gas absorber (53) is a metal mesh, and (54) is a separation heater. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (3)

(57) [Claims] 1. An alkali metal ion conductive ceramic comprising a solid electrolyte having a pair of porous electrode layers at both ends,
A metal carbonate covering at least a part of any one of the electrodes, a closed space provided to cover the electrode on the opposite side of the electrode, and a dry nitrogen gas and a metal oxide filled in the closed space. A carbon dioxide sensor comprising: a mixture; and a gas blocking layer that blocks a mixture of the dry nitrogen gas and the metal oxide from an external measurement space. 2. An alkali metal ion conductive ceramic comprising a solid electrolyte having a pair of porous electrode layers at both ends,
A metal carbonate covering at least a part of any one of the electrodes, a closed space provided to cover the electrode on the opposite side of the electrode, and a dry nitrogen gas and a metal oxide filled in the closed space. A carbon dioxide sensor comprising a mixture, a gas blocking layer for blocking a mixture of the dry nitrogen gas and the metal oxide from an external measurement space, and a porous moisture absorbent provided in contact with the metal carbonate. . 3. An alkali metal ion conductive ceramic comprising a solid electrolyte having a pair of porous electrode layers at both ends;
Water vapor constituting a part or all of a protector including a carbon dioxide gas sensing part comprising a metal carbonate covering at least a part of any one of the electrodes, and a heating part for heating the carbon dioxide sensing part to a measurement temperature. A gas absorber that absorbs gas except for the above, wherein the measured atmosphere reaches the carbon dioxide gas detection unit through the gas absorber.