JPH087181B2 - 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 where carbon dioxide concentration is measured or controlled for facility horticulture, environmental hygiene, disaster prevention, industrial use, etc. 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, in practice, many problems remain in its reliability.

Hereinafter, an example of the conventional carbon dioxide gas sensor described above will be described with reference to FIGS. 5 and 6.

5 and 6 show the structure of a conventional solid electrolyte type carbon dioxide gas sensor. In Fig. 5, NASICON in the form of a thin plate as an ion conductor made of a solid electrolyte
The (sodium ion conductor ceramics) plate 11 has a pair of porous electrode layers 12a for extracting voltage signals at both ends thereof.
12b, sodium carbonate 13 forming a dissociation equilibrium with carbon dioxide in a shape covering part or all of one of the electrode layers 12a, 12b is attached, and the side to which sodium carbonate 13 is attached is the cathode. Layer 12a is referred to as the anode layer 1 on the non-side
The gas sensing unit 1 was constructed by calling 2b. The composition of NASICON is Na _{1 + X} Zr ₂ Si _X P _3-X O ₁₂ , (0 ≦ X ≦ 3). Further, the gas sensing portion 1 of FIG. 6 is provided with a heater 2 for heating on the lower surface of one side, and lead wires 3a, 3b drawn from a pair of negative / anode layers 12a, 12b and a heater 2 for drawing. Two
It is fixed to the lower pedestal 6 via lead wires 5a and 5b. The lead wires 3a and 3b and the lead wires 5a and 5b penetrate the pedestal 6 for signal output and voltage application, respectively.

The protector 4 includes a gas sensing unit 1, a heater 2,
Each of the lead wires 3a, 3b, 5a, 5b is made of a stainless wire mesh and is fixed to the pedestal 6 in order to protect it from mechanical damage and to improve the contact with the measurement atmosphere.

In the above structure, the gas sensing unit 1 can form the following battery. That is, When the gas sensing part 1 constituting such a battery is heated to the measurement temperature by the heater 2, the following battery reactions occur in the cathode layer 12a and the anode layer 12b, respectively. An electromotive force represented by the following equation is generated between both electrodes.

Where E: electromotive force P ^* : total atmospheric pressure R: gas constant ^P CO ₂ : partial pressure of carbon dioxide in the atmosphere F: Faraday constant ^a Na ₂ O: Na ₂ O activity in NASICON T: absolute temperature ^a Na ₂ CO ₃ : Na ₂ CO ₃ activity From equation (3), 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 12
The carbon dioxide concentration in the atmosphere was electrically detected by taking out from the a and the anode layer 12b through the lead wires 3a and 3b.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, the above configuration has the following problems. That is, the above battery reaction (1),
Sodium oxide Na ₂ O generated at the interface between the anode layer and NASICON due to (2) supplies oxygen ions involved in the cell reaction (2), so that water or carbon dioxide in the atmosphere is passed through the porous anode layer. Contact with gas causes the following reactions. That is, Na ₂ O (S) + H ₂ O (g) → 2NaOH …… (4) 2NaOH ＋ 2CO ₂ → 2NaHCO ₃ …… (5) 2NaHCO ₃ → Na ₂ CO ₃ ＋ H ₂ O ＋ CO ₂ …… (6) These reactions As a result, NaCO ₃ is formed at the interface between the anode layer and NASICON, and a pseudo cathode is formed on the anode layer, resulting in deterioration of characteristics and loss of detection sensitivity.

The present invention solves the above-mentioned problems, and an object of the present invention is to provide a carbon dioxide gas sensor having high reliability even when the gas sensing part is exposed to moisture and carbon dioxide in the measurement atmosphere.

Means for Solving the Problems The carbon dioxide sensor of the first means for achieving this object is provided with a pair of anion / anode layers at both ends and is electrically conductive with respect to metal ions capable of forming metal carbonate. An ion conductive ceramic and one of the cathode layers of the ion conductive ceramic are composed of an electron conductor, and are covered with a metal carbonate which forms a dissociation equilibrium with carbon dioxide in a shape covering part or all of the electron conductor, and the other. A positive electrode layer comprising a gas sensing part composed of a mixed conductive ceramic having conductivity for both oxygen ions and electrons, and a heating part for heating the gas sensing part to a measurement temperature. .

A second means for achieving this object is to mix the anode layer in the above means with a conductive ceramic La _0.5 Sr _0.5 CoO ₃ 30.
-40 mol% and 60-70 mol% of SrTiO ₃ are added.

A third means for achieving this object is, in addition to the above first and second means, a gas blocking layer on a part or all of the remaining portion of the ion conductive ceramic other than the surface for forming the negative and positive electrode layers. Is formed.

By this first means, the anode layer is formed of a dense mixed conductor that allows only oxygen ions and electrons to pass therethrough. Since H ₂ O (g) and CO ₂ (g) in the atmosphere are not involved in the above, it is possible to prevent the characteristic deterioration of the gas sensing part due to the following reaction.

Na ₂ O (S) + H ₂ O (g) → 2NaOH 2NaOH + 2CO ₂ → 2NaHCO ₃ 2NaHCO ₃ → Na ₂ CO ₃ + H ₂ O + CO ₂ Also, by the second means, the electronic conduction of the mixed conductive ceramics forming the anode layer. The temperature dependence of the temperature is reduced, the temperature characteristics of the gas sensor are improved, and stable operation is possible in a wide temperature range.

Furthermore, by the third means, the interface between NASICON and the anode layer is sealed from the atmosphere, and the characteristic deterioration of the gas sensing part due to the reaction of Na ₂ O of the anode product with moisture or CO ₂ in the atmosphere can be prevented. In addition, the adhesive strength between the electrode layer and NASICON is improved, and the thermal and mechanical strength of the device is improved.

Example Hereinafter, a first example of the present invention will be described with reference to FIG.

FIG. 1 shows the configuration of a gas sensing portion of a carbon dioxide gas sensor according to a first embodiment of the present invention. The same parts as those of the conventional example are designated by the same reference numerals and the description thereof will be omitted.

In FIG. 1, an anode layer 14 having an electrode formed at one end of a thin NASICON plate 11 is a mixed conductive ceramic having conductivity for both oxygen ions and electrons, and is a perovskite oxide La _0.5 Sr _0.5 CoO ₃ The cathode layer 12a, which is formed of a dense sintered body obtained by compression-molding the fine powder of, and sintered at about 1200 ° C. and formed on the other end of the NASICON plate 11, is composed of an electron conductor, and the surface of the cathode layer 12a is It is formed by covering with a metal carbonate that forms a dissociation equilibrium with carbon dioxide in a covering shape. The composition of NASICON is Na _{1 + X} Zr ₂ Si _X P _3-X O ₁₂ , (0 ≦ X ≦ 3).

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

The detection operation in the above configuration is the same as that of the conventional example, and the description thereof will be omitted. Gas detector 1a by heater 2 in FIG.
When is heated to the measurement temperature, the battery reaction (1) shown in the conventional example at the interface between NASICON 11 and the cathode layer 12a and anode layer 14 (1)
And (2) occur. At that time, the anode layer 14 and NASICON 11
Since the interface of is contacted only with oxygen in the atmosphere by the anode layer 14 and is shielded from water vapor and carbon dioxide gas, the hydration and carbonation reactions of the anode product Na ₂ O shown in the conventional example (4), ( 5) and (6) can be reduced.

As described above, by composing the anode layer with mixed conductive ceramics having conductivity for both oxygen ions and electrons, the anode product Na ₂ O prevents contact with moisture and carbon dioxide gas in the atmosphere. To be done. Therefore, the deterioration of the gas sensing portion 1a due to the oxidation and carbonation of the anode product Na ₂ O is reduced.

A second embodiment of the present invention will be described below with reference to FIGS. FIG. 2 is a sectional view showing the structure of the gas sensing portion of the carbon dioxide sensor according to the second embodiment of the present invention. The same parts of the cathode layer 12a shown in the conventional example and the first embodiment are designated by the same reference numerals and the description thereof will be omitted. As shown in FIG. 3, an anode layer 15 having an electrode formed on one end of a thin plate-shaped NASICON plate 11 has an electronic conductivity of the gas sensing unit 1b in the operating temperature range, that is, 400 ° C. to 600 ° C. It is formed of a dense sintered body obtained by compressing a mixture of 35 mol% of mixed conductive ceramic (that is, La _0.5 Sr _0.5 CoO ₃ ) and 65 mol% of SrTiO ₃ which has little temperature dependence and sintering at about 1200 ° C.

The operation of the carbon dioxide sensor configured as described above will be described below. Note that the detection operation of the carbon dioxide sensor is the same as that of the conventional example, and therefore its description is omitted.

In the above structure, when the gas sensing part 1b is heated to the operating temperature by the heater 2 shown in FIG. 6, both the negative / anode layers 12a,
An electromotive force proportional to the logarithm of the partial pressure of carbon dioxide in the atmosphere is generated between the 15 parts, but at that time, since the anode layer 15 has stable electron conductivity in the operating temperature range, the operating temperature fluctuates. A stable electromotive force can be obtained.

As described above, by composing the anode layer 15 with a mixed conductive ceramic having a low temperature dependence of electron conductivity, a stable output can be obtained against a change in operating temperature, and moisture or carbon dioxide gas in the atmosphere can be obtained. It is possible to reduce the deterioration of the gas sensing unit 1b caused by.

A third embodiment of the present invention will be described below with reference to FIG. FIG. 4 is a sectional view showing the structure of the gas sensing portion 1c of the carbon dioxide gas sensor according to the third embodiment of the present invention. The same parts as those of the present invention, the conventional example and the second embodiment are designated by the same reference numerals and the description thereof will be omitted.

In FIG. 4, the gas blocking layer 16 is formed by applying a low-melting-point glass-based insulating paste capable of preventing the permeation of gases such as water vapor and carbon dioxide to the surface of the NASICON 11 exposed to the atmosphere excluding the electrode layers 12a and 15 to It is baked and fixed at 600 ° C.

The operation of the carbon dioxide sensor configured as described above will be described below. Note that the detection operation of the carbon dioxide sensor is the same as that of the conventional example, and therefore its description is omitted.

In the above configuration, the gas sensing unit heated to the operating temperature
In 1c, the anode product Na ₂ O is produced at the interface 17 between the NASICON 11 and the anode layer 15 by the cell reactions (1) and (2). At that time, the interface 17 is insulated from the measurement atmosphere by the gas blocking layer 1b, and contact between sodium oxide Na ₂ O on the interface 17 and water vapor and carbon dioxide gas in the measurement atmosphere can be avoided. Also, the anode layer 15 and NASI
The adhesion strength of CON11 is improved by forming the gas barrier layer 16.

As described above, by providing the gas blocking layer in the gas sensing portion, which is made of the mixed conductive ceramic having the anode layer 15 having conductivity with respect to both oxygen ions and electrons, sodium oxide generated at the interface 17 is formed. It is possible to prevent hydroxylation or carbonation of the gas, improve the adhesive strength of the anode layer, and dramatically improve the durability of the gas sensing part.

EFFECTS OF THE INVENTION As is clear from the above description of the embodiments, according to the present invention, the anode layer is composed of the mixed conductive ceramic having conductivity for both oxygen ions and electrons. Only the gas is allowed to pass through, and moisture and carbon dioxide in the atmosphere do not participate in the cell reaction, and it is possible to prevent the deterioration of the gas sensing part due to the oxidation and carbonation of sodium oxide of the anode product.

In addition to the above-mentioned effect, the operating temperature fluctuation can be obtained by forming the anode layer from the mixed conductive ceramic having conductivity with respect to both oxygen ions and electrons whose temperature dependence of electron conductivity is small by the second means. In contrast, stable operation becomes possible.

In addition to the above structure, a gas blocking layer is formed on the gas sensing portion by the third means, so that the anode layer and the NASICO
Since the N interface is shielded from the external atmosphere, deterioration of the anode due to permeation of moisture or carbon dioxide into the interface can be prevented, and the adhesive strength of the anode layer with NASICON can be increased.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing the structure of a gas sensing portion of a carbon dioxide gas sensor according to a first embodiment of the present invention, and FIG. 2 is a structure of a gas sensing portion of a carbon dioxide gas sensor according to a second embodiment of the present invention. FIG. 3 is a longitudinal sectional view showing a temperature dependence of electron conductivity in a mixed conductive ceramic constituting an anode layer of a carbon dioxide sensor according to a second embodiment of the present invention, and FIG. 4 is a graph showing the present invention. FIG. 5 is a vertical sectional view showing the structure of a gas sensing portion of a carbon dioxide gas sensor in a third embodiment of the present invention, FIG. 5 is a vertical sectional view showing the structure of a gas sensing portion of a conventional carbon dioxide gas sensor, and FIG. 6 is a conventional carbon dioxide gas sensor. 3 is a partially cutaway vertical sectional view showing the configuration of FIG. 1 ... Gas sensing part, 2 ... Heater, 3a, 3b ... Lead wire, 4 ... Protector, 5a, 5b ... Lead wire, 6 ... Pedestal, 11 ... NASICON, 12a ... Cathode layer, 12b ...... Anode layer, 1
3 …… Sodium carbonate, 14 …… Anode layer, 15 …… Anode layer, 1
6 ... Gas barrier layer.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication location G01N 27/46 ZAB (72) Inventor Takeshi Takeda 1006 Kadoma, Kadoma-shi, Osaka Matsushita Electric Industrial Co., Ltd. (72) Inventor Satoshi Sekido 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (3)

[Claims] 1. An ion-conducting ceramic having a pair of anion / anode layers at both ends and having conductivity with respect to metal ions capable of forming a metal carbonate, and one cathode layer of the ion-conducting ceramic is made of an electron. Mixed conduction that is composed of a conductor and is covered with a metal carbonate that forms a dissociation equilibrium with carbon dioxide in a shape that covers part or all of it, and the other anode layer has conductivity for both oxygen ions and electrons. A carbon dioxide gas sensor, comprising: a gas sensing part made of a conductive ceramic; and a heating part heating the gas sensing part to a measurement temperature. 2. A mixed conductive ceramic La _0.5 Sr _0.5 for an anode layer.
The carbon dioxide gas sensor according to claim 1, wherein ₃₀ to 40 mol% of CoO _{3 and} 60 to 70 mol% of SrTiO ₃ are added. 3. The carbon dioxide gas sensor according to claim 1, wherein a gas blocking layer is formed on a part or all of the remaining portion of the ion conductive ceramic other than the surface on which the negative and positive electrode layers are formed.