JP2678045B2 - Carbon dioxide sensor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a solid structure having an ionic conductor which is used in a place for measuring and controlling carbon dioxide concentration such as facility horticulture, environmental hygiene, disaster prevention, and industrial use. The present invention relates to an electrolyte type carbon dioxide gas sensor.

2. Description of the Related Art In recent years, there has been an increasing need for carbon dioxide sensors mainly in the fields of air conditioning and agriculture and livestock, and various types of sensors have been developed and put into practical use. However, many problems remain in the reliability in practical use.

Hereinafter, an example of the above-described conventional carbon dioxide sensor will be described with reference to FIGS. 3 and 4. FIG.

In FIG. 3, a thin plate-shaped solid electrolyte, NAXS (sodium ion conductor) Na20-Al203-XSiO2 (X = 2,
4, 6, 8) The plate 11 is provided with a pair of electrode layers, a cathode layer 12a and an anode layer 12b, at both ends thereof. The cathode layer 12a is formed by adding sodium carbonate dispersed in an electrode layer using Au, The anode layer 12b is formed of an electrode layer using Au, and the cathode layer 12a is covered with the gas permeable film 13 so that the gas sensing portion 1 is formed.
Was composed.

Next, in FIG. 4, the gas sensing portion 1 is provided with a heater 2 for heating on the lower side of one surface thereof, and a pair of negative / anode layers 12a, 12b is provided.
The two lead wires 3a, 3b drawn from and the lead wires 5a, 5b drawn from the heating heater 2 are fixed to the lower pedestal 6 via the connected pins 7a, 7b and pins 8a, 8b, respectively. The pins 7a, 7b, 8a, 8b penetrate the pedestal 6 for signal output and voltage application, respectively, so that they can be connected to the outside. The protector 4 includes a gas sensing unit 1, a heater 2, lead wires 3a, 3b, 5a, 5b, pins 7a, 7 included therein.
In order to protect b, 8a and 8b from mechanical damage and to improve contact with the measurement atmosphere, they are made of stainless wire mesh and fixed to the pedestal 6.

In the above structure, the gas sensing unit 1 can form the following battery. That is, (Au) O2, CO2 | Na2CO3 | NAXS | O2 (Au) When the gas sensing part 1 constituting such a battery is heated to the measurement temperature by the heater 2, the cathode layer 12a and the anode layer 12b are heated.
The following battery reactions occur (cathode) Na2CO3 = 2Na ⁺ ＋ 1 / 2O2 ＋ 2e ⁻ …… (1) (anode) 2Na ⁺ ＋ CO2 ＋ 1 / 2O2 ＋ 2e ⁻ ＝ Na2O …… (2) The following formula is used between both electrodes. Generated electromotive force E =-(RT / 2F) ・ ln ((aNa2O ・ PCO2) / (aNa2CO3 ・
P *)) (3) where E: electromotive force, R: gas constant, F: Faraday constant, T: absolute temperature P *: total atmospheric pressure, PCO2: carbon dioxide partial pressure in the atmosphere aNa2O: NAXS The activity of Na2O and the activity of aNa2CO3: Na2CO3 in formula (3) show that the generated electromotive force E is proportional to the logarithm of the partial pressure of carbon dioxide in the atmosphere. Therefore, this electromotive force is applied to the cathode layer.
The carbon dioxide concentration in the atmosphere was electrically detected from the anode layer 12a and the anode layer 12b.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, the above configuration has the following problems. That is, sodium oxide Na2O generated at the interface between the cathode layer and the NAXS plate by the battery reaction shown in the above formulas (1) and (2) supplies oxygen ions involved in the battery reaction shown in the above formula (2). Therefore, they are in contact with each other in an atmosphere through a porous anode layer. When Na2O3 comes into contact with moisture or carbon dioxide in the atmosphere, the following reactions occur. That is, Na2O (S) + H2O (g) → 2NaOH ...... (4) 2NaOH + 2CO2 → 2NaHCO3 ...... (5) 2NaHCO3 → Na2CO3 + H2O + CO2 ...... (6) These reactions form Na2CO3 at the interface between the anode layer and the NAXS, and the anode layer A pseudo-cathode is formed on the top, which deteriorates the characteristics and loses the detection sensitivity.

In addition, the presence of corrosive gas such as hydrogen sulfide H2S in the atmosphere causes the following reactions. That is, Na2O + H2S → Na2S + H2O (7) By this reaction, sodium sulfide is formed at the interface between the anode layer and the NAXS.
Na2S will be formed and the potential of the anode will change, and therefore the absolute value of the electromotive force will change.

The present invention solves the above problems, and a first object is to
An object of the present invention is to provide a carbon dioxide gas sensor that stabilizes the potential of the anode layer and has high reliability.

The second purpose is that even if the gas sensing part is placed in an atmosphere in which moisture, carbon dioxide gas, corrosive gas, etc. exist, Na2O in the anode layer
It is intended to provide a carbon dioxide sensor having high reliability by preventing the above reaction.

Means for Solving the Problem The first means of the present invention for achieving the first object is an ion which conducts an alkali metal ion which is composed of a solid electrolyte and which is capable of forming a metal carbonate which undergoes dissociation equilibrium with carbon dioxide gas. A pair of electrode layers are provided at both ends of the conductive ceramic plate, and one of the electrode layers of the ion conductive ceramic plate is an electrode material having conductivity without reacting with the alkali metal ions and the ion conductive ceramic plate. And a gas sensing part formed by dispersing and adding a metal carbonate composed of an alkali metal ion capable of conducting to form a cathode layer, and adding an oxygen ion conductive substance to the other anode layer, and the gas sensing part. And a heating unit for heating to a measurement temperature.

A second means for achieving the second object is a structure in which a part or all of the surface of the anode layer or the ion conductive ceramic plate including the anode layer is covered with a gas blocking layer.

Action With this configuration, the action by the first means is that the oxygen partial pressure that contributes to the battery reaction of the formula (2) shown in the above-mentioned conventional example is in the anode layer because the oxide ion conductor is added in the anode layer. Since the oxygen ion-conductive substance has a specific constant value, the potential of the anode is stable and a stable electromotive force can be obtained.

Further, by the second means, since the anode layer is isolated from the measurement atmosphere by the gas barrier layer, deterioration of the element due to the reaction of moisture, carbon dioxide gas, corrosive gas and Na2O in the atmosphere is prevented, and further, Since the oxygen ion conductive substance is added to the anode layer, the potential of the anode layer is stabilized and a stable electromotive force is obtained.

Example Hereinafter, a first example of the present invention will be described with reference to FIG. The same parts as those of the conventional example are designated by the same reference numerals and the description thereof will be omitted.

In the figure, the anode layer formed on one end of the thin NAXS plate 11
14 is a mixed conductive ceramic that has conductivity for both oxygen ions and electrons in the Au paste, which is an electrode material, and is a powder of the perovskite oxide La0.5Sr0.5CoO3, which is a so-called oxygen ion conductive substance 15 of 10 wt. %, Mixed and applied as a paste, dried at about 150 ° C, then 8
It was baked at 00 ° C.

The operation of the carbon dioxide sensor configured as described above will be described below.

In the above-mentioned configuration, the detecting operation is similar to that of the conventional example, and therefore its explanation is omitted. When the gas sensing part 1 is heated to the measurement temperature, the interfaces of the NAXS plate 11, the cathode layer 12a, and the anode layer 12b are
The battery reactions of the formulas (1) and (2) shown in the conventional example occur. At that time, the interface between the anode layer 14 and the NAXS plate 11 is kept at a constant oxygen partial pressure by the oxygen ion conductive material 15La0.5Sr0.5CoO3 powder added in the anode layer 14. Therefore, the potential of the anode layer 14 is stable and a stable electromotive force can be obtained. Further, since oxygen exists as O − ions in the anode layer 14, the oxygen moves faster to the reaction interface and the reaction speed increases. Therefore, the gas response of the device is also improved.

As described above, by adding the oxygen ion conductive substance 15 to the anode layer 14, a stable electromotive force can be obtained and the responsiveness can be improved. In this embodiment, the ion conductive ceramics plate 11 is the NAXS plate, but NASICON (Na1 + XZr2P3
A −XSiXO12 (0 ≦ X ≦ 3)) plate may be used. Although the oxygen ion conductive material 15 is La0.5Sr0.5CoO3, it may be YSZ which is a stabilized zirconia containing yttrium or CSZ which is a stabilized zirconia containing calcium.

Next, a second embodiment will be described with reference to FIG. The same parts as those of the conventional example and the first embodiment of the present invention are designated by the same reference numerals and the description thereof will be omitted.

In the figure, the anode layer 14 and the NAXS plate 11 formed at one end of a thin plate-shaped NAXS plate 11 except for the cathode layer 12a of the gas sensing unit 1.
The surface of is covered with a gas barrier layer 16. The gas barrier layer 16 is formed by applying a pasty amorphous glass sealing material,
An amorphous glass layer formed by drying at 0 ° C for 10 minutes and firing at about 800 ° C.

The operation of the carbon dioxide sensor configured as described above will be described below.

In the above-mentioned configuration, the detecting operation is similar to that of the conventional example, and therefore its explanation is omitted. When the gas sensing part 1 is heated to the measurement temperature, the cell reaction shown by the conventional formulas (1) and (2) occurs at the interfaces of the NAXS plate 11, the cathode layer 12a and the anode layer 14. At that time, the interface between the anode layer 14 and the NAXS plate 11 is a gas blocking layer.
The 16 shields the measurement atmosphere and avoids the oxidation of Na2O on the interface, carbonation and reaction with corrosive gases.

Further, since the oxygen ion conductive substance 15La0.5Sr0.5CoO3 powder that acts as an oxygen supply source is added to the anode layer 14, the potential of the anode layer 14 is stabilized and a stable electromotive force is obtained.

As described above, the anode layer 14 and the oxygen ion conductive substance
By adding 15 and covering a part or all of the surfaces of the anode layer 14 and the NAXS plate 11 with the gas blocking layer 16, the gas sensing unit 1 by moisture, carbon dioxide gas, corrosive gas, etc. in the atmosphere
Is prevented and stable electromotive force is obtained.

In this embodiment, the ion conductive ceramic plate 11
Was used as the NAXS plate, but NASICON (Na1 + XZr2P3-XSiXO12 (0
≦ X ≦ 3)) plate may be used. Also, oxygen ion conductive substances
Although 15 is La0.5Sr0.5CoO3, YSZ which is a stabilized zirconia containing yttrium or CSZ which is a stabilized zirconia containing calcium may be used.

EFFECTS OF THE INVENTION As described above, according to the present invention, the oxygen partial pressure of the anode layer becomes constant and stable electromotive force can be obtained by adding the oxygen ion conductive substance into the anode layer.

Furthermore, by covering part or all of the surface of the anodic and ion conductive ceramics plate with a gas barrier layer, the anode layer is shielded from moisture, carbon dioxide gas, and corrosive gas in the atmosphere, and the gas sensing part deteriorates. Can be prevented, stable electromotive force can be obtained, and a great effect can be obtained in practical use.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing the structure of a gas sensing portion of a carbon dioxide gas sensor according to an embodiment of the present invention, and FIG. 2 is a gas sensing portion of a carbon dioxide gas sensor according to an embodiment of the second invention. FIG. 3 is a vertical cross-sectional view showing the structure, FIG. 3 is a vertical cross-sectional view showing the structure of a gas sensing portion of a conventional carbon dioxide gas sensor, and FIG. 4 is a vertical cross-sectional view showing the structure of the carbon dioxide gas sensor. 1 ... Gas sensing part, 2 ... Heater, 3a, 3b ... Lead wire, 11 ... NAXS plate, 12a ... Cathode layer, 12b ... Anode layer, 14
…… Anode layer, 15 …… Oxygen ion conductive material, 16 …… Gas blocking layer.

 ─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-60-256043 (JP, A) JP-A 1-213565 (JP, A) Sensors and Actuators, Vol. 12 (1987) p. 449 ~ 453

Claims (2)

(57) [Claims] 1. A pair of electrode layers are provided at both ends of an ion conductive ceramics plate which is made of a solid electrolyte and conducts alkali metal ions capable of forming a metal carbonate which is in dissociation equilibrium with carbon dioxide gas. A cathode layer by dispersing and adding a metal carbonate composed of an alkali metal ion capable of conducting the ion conductive ceramics plate to an electrode material having electrical conductivity without reacting with the alkali metal ion to one of the electrode layers of the plate A carbon dioxide gas sensor comprising: a gas sensing part formed by adding an oxygen ion conductive substance to the other anode layer; and a heating part heating the gas sensing part to a measurement temperature. 2. The carbon dioxide gas sensor according to claim 1, wherein a part or all of the surface of the anode layer or the ion conductive ceramics plate including the anode layer is covered with a gas blocking layer.