JP2004205408A - Solid electrolyte type carbon dioxide sensor element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid electrolyte type carbon dioxide sensor element capable of highly accurate measurement even when an operation temperature is lowered to a low-temperature region of about 300°C, and having the unchanged electromotive force even when left for long hours in the non-heated state. <P>SOLUTION: In this solid electrolyte type carbon dioxide sensor element, a working electrode layer containing an electron conductive material and a metal carbonate and a reference electrode layer containing the electron conductive material are formed on the solid electrolyte layer surface. The sensor element has a characteristic wherein the working electrode layer contains a carbonate oxide of a rare earth element, preferably La<SB>2</SB>O<SB>2</SB>CO<SB>3</SB>, Nd<SB>2</SB>O<SB>2</SB>CO<SB>3</SB>or the like, and a zeolite. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００５２】
【表２】

【００５３】
【発明の効果】
本発明の固体電解質型炭酸ガスセンサ素子は、加熱されて一定の温度に保たれた条件下で被検出ガスである炭酸ガスを含み得る雰囲気下に放置することによって使用され、被測定ガス濃度に応じて発生する起電力に基づいて雰囲気中の炭酸ガス濃度を測定することができる。そして、作用電極層に希土類金属元素の炭酸酸化物とゼオライトとを含むことにより、非加熱時耐湿性が高いばかりでなく、動作温度を３００℃程度の低温域に下げても高精度での計測が可能である。
このため、省エネルギー運転が求められる炭酸ガスセンサ素子として特に好適に使用できる。
【図面の簡単な説明】
【図１】図１は、固体電解質型炭酸ガスセンサ素子の代表的な態様を示す断面図である。
【符号の説明】
１．作用電極層
２．固体電解質層
３．参照電極層
４．セラミックス板
５．接着剤
６．ヒータ
７．電源
８．電圧計[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a solid oxide carbon dioxide sensor element for measuring the concentration of carbon dioxide in an atmosphere.
[0002]
[Prior art]
In recent years, there has been an increasing interest in living environments, particularly indoor environments. For example, an air purifier equipped with a carbon dioxide sensor element that monitors the indoor carbon dioxide concentration and automatically ventilates when the carbon dioxide concentration becomes high. Equipment is being developed. In addition, from the viewpoint of pollution prevention or countermeasures, the importance of so-called environmental measurement, which constantly measures changes in the concentration of carbon dioxide in the environment, is increasing. As a carbon dioxide sensor element incorporated in a control device or an alarm device such as an air purifying device, or an environmental measurement device, it has features of high gas selectivity, easy downsizing, wide operating conditions, and excellent maintainability. A solid electrolyte type carbon dioxide sensor element is suitable.
[0003]
A solid electrolyte type carbon dioxide sensor element is generally composed of (a) a solid electrolyte layer, which is an ionic conductor, and (b) a substance capable of causing an equilibrium reaction with an electron conductive material and carbon dioxide, which is a gas to be measured. Has a basic structure in which a “working electrode” containing a metal carbonate and (c) a “reference electrode” containing an electron conductive material are joined. In such a carbon dioxide sensor element, when it is left in an atmosphere containing the gas to be measured while being heated to a constant temperature of 450 ° C. to 600 ° C. by an attached heater, the metal carbonate contained in the working electrode layer is removed. The dissociation equilibrium reaction with the gas to be measured proceeds until the equilibrium reaction reaches equilibrium, and the mobile ion concentration of the solid electrolyte layer changes near the working electrode layer, and as a result, the working electrode layer Since a certain electromotive force corresponding to the concentration of the gas to be measured is accurately generated between the electrode and the reference electrode layer, the concentration of the gas to be measured can be known by measuring the electromotive force.
[0004]
Conventionally, in a generally known solid electrolyte type carbon dioxide gas sensor element, NASICON (Na _{1 + A} Zr ₂ Si _A P _3-A O ₁₂ , where 0 ≦ A ≦ 3) is used as a solid electrolyte forming a solid electrolyte layer. A sodium ion conductive material such as β-Al ₂ O ₃ and a lithium ion conductive material such as Li ₂ TiSiO ₅ and LiTi ₂ (PO ₄ ) ₃ are used as an electron conductive material contained in the working electrode and the reference electrode. Is a substance necessary to detect electromotive force) as a noble metal material such as gold or platinum which has excellent heat resistance and oxidation resistance. Further, as a metal carbonate contained in the working electrode, an alkali metal carbonate or an alkali metal is used. Earth metal carbonates are commonly used.
[0005]
However, a solid electrolyte type carbon dioxide gas sensor using an alkali metal carbonate or an alkaline earth metal carbonate as the above-mentioned metal carbonate, such as a high-humidity atmosphere or a dew atmosphere, in a state where heating is interrupted and operation is stopped. If left in an atmosphere with a high moisture concentration, the electromotive force will be significantly lower than the value before the operation when the operation is started again afterwards. Low).
[0006]
As a method of solving such a problem, a method of adding a rare earth metal carbonate to the working electrode is known (see Patent Document 1), and La ₂ O ₂ CO ₃ and Nd ₂ O ₂ are added to the working electrode. By adding a carbonate of a rare earth metal such as CO _3, it is possible to make the electromotive force hardly change from that before leaving even when left for a long time in a high humidity atmosphere without heating. It is possible to set the time from the start of operation to the stabilization of the electromotive force within 2 hours.
[0007]
Further, a method of adding a zeolite such as USY or ZSM-5 to the working electrode (see Patent Document 2) and a method of adding a rare earth metal oxide such as La ₂ O ₃ or Nd ₂ O ₃ and a zeolite to the working electrode ( In Patent Document 3), the same effect as described above is obtained.
[0008]
[Patent Document 1]
JP-A-11-153576 [Patent Document 2]
Japanese Patent Application Laid-Open No. H11-174024 [Patent Document 3]
JP-A-11-271261
[Problems to be solved by the invention]
By the way, in the solid electrolyte type carbon dioxide sensor, it is desired to reduce the operating temperature from the viewpoint of energy saving, apart from the above-mentioned problem of moisture resistance during non-heating. By reducing the operating temperature, power consumption is reduced, so that the cost can be reduced by reducing the number of parts of the sensor, and the power source can be converted from a commercial power supply to a dry battery.
[0010]
However, in the solid electrolyte type carbon dioxide gas sensor disclosed in the above publication, although the problem of moisture resistance at the time of non-heating is well solved, the operating temperature is the same as the conventional solid electrolyte type carbon dioxide gas sensor, and is not necessarily. It was not satisfactory. For example, when the operating temperature is set in a low temperature range of 200 ° C. to 300 ° C., the sensitivity is lowered, and it is difficult to perform a reliable measurement one step at a time.
[0011]
Therefore, an object of the present invention is to provide a solid electrolyte type carbon dioxide sensor capable of accurately measuring a carbon dioxide concentration even at an operating temperature lower than before, particularly in a temperature range of 200 ° C. to 300 ° C.
[0012]
[Means for Solving the Problems]
As a result of repeated studies to solve the above problems, the present inventor has found that, when a specific compound is added to a working electrode formed on the surface of a solid electrolyte layer, solid electrolyte type carbonate satisfying all the above-mentioned required performances is obtained. They have found that a gas sensor can be obtained, and have proposed the present invention.
[0013]
That is, the present invention provides a solid electrolyte type carbon dioxide gas sensor element in which a working electrode layer containing an electron conductive material and a metal carbonate and a reference electrode layer containing an electron conductive material are formed on the surface of a solid electrolyte layer. A solid electrolyte type carbon dioxide sensor element, characterized in that the layer contains a rare earth carbonate and zeolite.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the configuration of the solid oxide carbon dioxide sensor element of the present invention will be described in detail.
[0015]
In the solid oxide carbon dioxide sensor element of the present invention, it is important to use a working electrode containing a carbonate of a rare earth element and zeolite. By using these two substances in combination and making them act synergistically, the solid electrolyte type carbon dioxide sensor element of the present invention can measure the carbon dioxide gas concentration with high sensitivity and high accuracy even in a low temperature range of 200 to 300 ° C. be able to. If the working temperature is set to 300 ° C. or lower when the working electrode layer does not contain any one of these substances, sufficient sensitivity cannot be obtained, and the accuracy of measuring the carbon dioxide concentration is greatly reduced. The same applies to the case where a rare earth metal oxide and zeolite are contained. Further, when neither substance is contained, the value of the electromotive force is significantly reduced when left unheated in a high-humidity environment, as compared to before.
[0016]
The carbonate of a rare earth element used in the present invention includes R ₂ O ₂ CO ₃ , R ₂ O (CO ₃ ) ₂ and a hydrate thereof (R ₂ O (CO ₃ ) ₂ .xH ₂ O (x is 0 any number greater _{than)) means} a _{R a O L (CO 3)} M (OH) T (a, L, M, carbonate oxide T is represented by the general formula any integer), and the like. Here, R is a rare earth metal element, and examples include Sc; Y; and lanthanoids having atomic numbers 57 to 71 such as La, Ce, Pr, Nd, and Pm.
[0017]
Of the carbonates of the rare earth element represented by the above general formula, the carbonate of the rare earth element in which the rare earth element is a lanthanoid reduces the time from the start of operation of the solid electrolyte type carbon dioxide sensor element until the electromotive force is stabilized. In particular, a carbonate of a rare earth element in which the rare earth element is La or Nd is preferable.
[0018]
Further, among the carbonates of the rare earth elements, the carbonates of the rare earth elements represented by R ₂ O ₂ CO ₃ and R ₂ O (CO ₃ ) ₂ generate electromotive force from the start of operation in a high humidity environment. Is preferred from the viewpoint of shortening the time required for stabilization. In particular, it is most preferable to use R ₂ O ₂ CO ₃ .
[0019]
The content of the rare earth element carbonate in the working electrode layer is not particularly limited, but is preferably 0.5 to 80% by weight based on the total weight of 100% by weight of the working electrode, and more preferably 1 to 60% by weight. % By weight.
[0020]
As the zeolite used in the present invention, known zeolites are used without any limitation. For example, in addition to A-type, X-type, Y-type, USY, ZSM-5 and the like, there can be mentioned those obtained by exchanging cations contained in these zeolites with other cations.
Of the above zeolites, USY containing Na ⁺ (hereinafter abbreviated as Na-USY).
) And Na ⁺ contained in Na-USY are preferably replaced by Li ⁺ or K ⁺ , or ZSM-5 is preferable because the effect of the present invention is more easily exerted. Particularly, Na ⁺ contained in Na-USY is preferably used. Those replaced with Li ⁺ (hereinafter abbreviated as Li-USY) and ZSM-5 are most preferred.
[0021]
The method of exchanging cations contained in zeolite with other cations is not particularly limited.A typical example is a method of exchanging zeolite powder before ion exchange with a cation to be exchanged. After immersion and stirring in a contained aqueous solution, drying and baking can be exemplified.
In the present invention, the rate at which the cations contained in the zeolite are exchanged for other cations is not particularly limited, but in order to achieve the effects of the present invention, the cations before exchange should be reduced to 100%. On the other hand, it is preferable that 70% or more be exchanged.
[0022]
The content of zeolite in the working electrode layer is not particularly limited, but is preferably 0.1 to 8% by weight, more preferably 0.5 to 5% by weight, based on 100% by weight of the total weight of the working electrode. Preferably, there is.
[0023]
In the present invention, the metal carbonate contained in the working electrode layer can cause an equilibrium reaction with carbon dioxide in an atmosphere containing carbon dioxide, and known materials are used without any limitation. For example, alkali metal carbonates such as sodium carbonate and lithium carbonate and mixtures thereof, or alkaline earth metal carbonates such as calcium carbonate and magnesium carbonate and mixtures thereof are employed. It is preferable to use an alkali metal carbonate, particularly sodium carbonate or lithium carbonate, because it easily occurs.
[0024]
The combination of the rare earth carbonate and zeolite with the metal carbonate is not particularly limited, but the combination of the rare earth carbonate and zeolite USY in which the rare earth element is a lanthanoid and the alkali metal carbonate is high. It is preferable in terms of shortening the time from the start of operation in a humid environment to the stabilization of the electromotive force. In particular, a combination of lanthanum carbonate or neodymium carbonate and zeolite Li-USY and sodium carbonate or lithium carbonate is preferable. .
[0025]
The content of the metal carbonate in the working electrode layer is not particularly limited, but is preferably from 1 to 70% by weight, more preferably from 5 to 50% by weight based on 100% by weight of the total weight of the working electrode. Is preferable because the fluctuation of the electromotive force of the sensor element during continuous use is reduced.
[0026]
In the present invention, the electron conductive material contained in the working electrode layer is a substance necessary for outputting an electromotive force of the sensor element, similarly to the electron conductive material contained in the reference electrode layer described later, and a known material is used. Used without restrictions. For example, noble metal elements such as platinum, gold, palladium, and silver and alloys thereof, or a mixture of two or more of the above noble metal elements are used. In particular, platinum, gold, and mixtures and alloys thereof are resistant. It is suitable because of its excellent corrosiveness.
[0027]
The content of the electron conductive material in the working electrode layer is not particularly limited, but is preferably 10 to 95% by weight, more preferably 25 to 90% by weight based on 100% by weight of the total weight of the working electrode. Is preferred.
[0028]
In the present invention, the structure of the working electrode layer including the electron conductive material, the metal carbonate, the carbonate of the rare earth element, and the zeolite is not particularly limited. A typical structure is, for example, a structure in which an electron conductive material, a metal carbonate, a carbonate of a rare earth element and zeolite are layered on the surface of a solid electrolyte layer, and a metal carbonate, a rare earth element in an electron conductive material of a working electrode layer. Structure in which carbon oxides and zeolites are present in a dispersed state, and a structure in which a part or all of a mixture layer of metal carbonate, rare earth carbonate and zeolite formed on the surface of the solid electrolyte layer is covered with an electron conductive material In particular, a structure in which a metal carbonate, a carbonate of a rare earth element and zeolite exist in a dispersed state in an electron conductive material is preferable because a working electrode layer can be easily formed.
[0029]
As a method for forming the working electrode layer, a known method is used without any particular limitation. For example, the above-mentioned electron conductive material, metal carbonate, carbonate of rare earth element and zeolite are used alone or mixed and then kneaded with a solvent and a binder to form a paste, and the paste is formed on a solid electrolyte surface by a screen printing method or the like. A method of baking, a method of forming an electron conductive material, a metal carbonate, a carbonate of a rare earth element and zeolite by a thin film forming technique such as sputtering or vapor deposition is suitably employed.
[0030]
The thickness of the working electrode layer is not particularly limited, but is generally adopted in the range of 0.001 to 0.03 mm.
[0031]
In the present invention, the electron conductive substance contained in the reference electrode layer is a substance necessary for outputting an electromotive force of the sensor element, similarly to the electron conductive substance contained in the above-mentioned working electrode layer. Used without restrictions. For example, noble metal elements such as platinum, gold, palladium, and silver and alloys thereof, or a mixture of two or more of the above noble metal elements are used. In particular, platinum, gold, and mixtures and alloys thereof are resistant. It is suitable because of its excellent corrosiveness.
[0032]
As a method for forming the above-mentioned reference electrode layer, a known method is used without any particular limitation. For example, the method as described in the method for manufacturing the working electrode layer described above can be used.
[0033]
The thickness of the reference electrode layer is not particularly limited, but is generally adopted in the range of 0.001 to 0.03 mm.
[0034]
In the present invention, the arrangement of the working electrode layer and the reference electrode layer is not particularly limited as long as the working electrode layer and the reference electrode layer are in contact with the solid electrolyte layer. For example, those having a structure in which a working electrode layer is formed on one surface of the solid electrolyte layer and a reference electrode layer is formed on the other surface, and both the working electrode layer and the reference electrode layer are fixed on one surface of the solid electrolyte. May have a structure formed at a distance of.
[0035]
In the present invention, a known solid electrolyte is used without limitation for the solid electrolyte layer. For example, the above-mentioned NASICON, β-Al ₂ O _{3 and the} like can be mentioned.
[0036]
As a method for forming the solid electrolyte layer, a known method is employed without particular limitation. Typical forming methods include: a method in which a solid electrolyte synthetic material is fired, molded and then heated, a method in which the solid electrolyte synthetic material is molded and then sintered, and a method in which the solid electrolyte synthetic material is mixed with a solvent and a binder. Kneading into a paste, and printing the paste on a ceramic or glass substrate by a screen printing method or the like, and baking the paste.
[0037]
The thickness of the solid electrolyte layer is not particularly limited, but is generally adopted in the range of 0.02 mm to 2.0 mm.
[0038]
The solid electrolyte type carbon dioxide sensor element is usually heated to a constant temperature of 400 ° C. to 600 ° C. in order to cause a dissociation equilibrium reaction between the metal carbonate and carbon dioxide. In the present invention, measurement can be performed with high sensitivity and high accuracy even when used at a lower constant temperature of 200 ° C. to less than 400 ° C. In particular, when used at a temperature of 200 ° C to 300 ° C, the effect is more remarkably exhibited. As a method for heating the sensor element, heating from a heat source outside the sensor element may be used, or a ceramic or glass substrate on which a heater is formed is joined to the sensor element, and a DC or AC voltage is applied to the heater. May be heated. The mounting position of the heater joined to the sensor element is not particularly limited as long as it does not hinder the operation of the sensor element, such as on the reference electrode layer.
[0039]
【Example】
The present invention will be described specifically with reference to the following examples, but the present invention is not limited to these examples.
(1) Test method for operating temperature dependence of electromotive force and sensitivity Immediately after fabricating a solid electrolyte type carbon dioxide sensor element, place it in a chamber where the concentration of carbon dioxide can be freely controlled, and apply a DC voltage to the heater from a power supply. Was heated to an operating temperature of 200 ° C., and while the sensor element was kept at the temperature, the concentration of carbon dioxide in the chamber was adjusted to 350 ppm, and the electromotive force was measured. Next, the concentration of carbon dioxide in the chamber was adjusted to 2000 ppm, and the electromotive force was measured at the same temperature. Thereafter, the device temperature (operating temperature) was increased to 300 ° C. and 450 ° C., and the electromotive force at 350 ppm and 2000 ppm was measured at each temperature in the same manner. Further, a difference between the electromotive force at 350 ppm and the electromotive force at 2000 ppm at each temperature was determined, and this value was defined as the sensitivity at each operating temperature.
(2) Moisture resistance test method without heating Immediately after the measurement in the above test (1), the solid electrolyte type carbon dioxide sensor element is placed in a chamber in which the carbon dioxide concentration can be freely controlled, and a DC voltage is applied to the heater from a power supply. By heating the element to 450 ° C., while maintaining the temperature of the sensor element at 450 ° C., the concentration of carbon dioxide in the chamber was adjusted to 350 ppm and 2000 ppm, and the electromotive force at each concentration was measured. The initial electromotive force was used. Further, the difference between the electromotive force value at 350 ppm and the electromotive force value at 2000 ppm was determined, and this was defined as the initial sensitivity at an element temperature of 450 ° C. After measuring the initial sensitivity at an element temperature of 450 ° C., the sensor element was taken out of the chamber, placed in a constant temperature bath maintained at a temperature of 60 ° C. and a humidity of 90%, and allowed to stand in a non-heated state for 7 days. It was taken out of the thermostat, and the electromotive force (electromotive force after the test) at 350 ppm and 2000 ppm when the element temperature was 450 ° C. was measured in the same manner to calculate the sensitivity (the sensitivity after the test).
[0040]
Examples 1 to 6
An element having a cross-sectional structure as shown in FIG. 1 was manufactured as a solid electrolyte type carbon dioxide sensor. In this solid electrolyte type gas sensor element, a working electrode layer 1 is formed on one surface of a solid electrolyte layer 2 and a reference electrode layer 3 is formed on the other surface, and a ceramics plate 4 is bonded on the reference electrode layer 3 with an adhesive 5. It was. Further, a heater 6 was formed on the surface of the ceramic plate 4 opposite to the surface to which the reference electrode layer 3 was joined, and was supplied with electricity from a power supply 7. Further, lead wires were drawn out from the working electrode layer 1 and the reference electrode layer 3 and connected to the voltmeter 8 to measure the electromotive force.
[0041]
The solid electrolyte powder for forming the solid electrolyte layer 2 is obtained by mixing zirconium silicate and sodium phosphate so as to have a composition of Na ₃ Zr ₂ SiPO ₁₂ and firing the mixture in an air atmosphere at 1100 ° C. for 6 hours. Obtained.
[0042]
The solid electrolyte layer 2 was formed into a disk-shaped pellet by uniaxially molding the solid electrolyte powder and then sintering at 1200 ° C. in an air atmosphere for 10 hours.
[0043]
The working electrode layer was prepared by kneading 5% by weight of ethylcellulose in terpineol with kneading gold powder, metal carbonate, rare earth carbonate and zeolite as an electron conductive material in the proportions shown in Table 1 to obtain a paste. One side of the solid electrolyte layer was formed by screen printing, drying and baking for 30 minutes in the air at 650 ° C. Thus, a working electrode layer having a thickness of 0.015 mm was obtained.
[0044]
The rare earth element carbonate was obtained by calcining La ₂ (CO ₃ ) ₃ or Nd ₂ (CO ₃ ) ₃ , which is a metal carbonate of the rare earth element, in the air at 700 ° C. for 30 minutes.
Among the zeolites, Li-USY was obtained by stirring Na-USY in a 25 ° C. aqueous solution of lithium nitrate for 12 hours, washing with water, and then calcining in a 500 ° C. atmosphere for 8 hours. By this treatment, it was confirmed by a fluorescent X-ray analyzer that Li-USY in which 90% of Na ⁺ in Na-USY was replaced with Li ⁺ was obtained.
[0045]
As other materials, commercially available materials were used as they were.
[0046]
The reference electrode layer is formed by kneading gold powder as an electron conductive substance and terpineol in which 5% by weight of ethylcellulose is dissolved to form a paste. The paste is formed on the surface of the solid electrolyte layer opposite to the surface on which the working electrode layer is formed. It was formed by screen printing, drying, and firing for 30 minutes in the air at 650 ° C. Thus, a reference electrode layer having a thickness of 0.015 mm was obtained.
[0047]
On the above-mentioned reference electrode layer, an alumina substrate mounted with a platinum heater formed by a screen printing method using a commercially available platinum paste is bonded with an adhesive made of glass so that the surface on which the heater is not formed becomes a bonding surface. Joined.
[0048]
An operating temperature dependency test and a non-heating moisture resistance test were performed on the solid electrolyte type carbon dioxide sensor element manufactured by the above method. The results are shown in Table 2.
[0049]
Comparative Examples 7 to 15
The solid oxide carbon dioxide sensor elements of Comparative Examples 7 to 15 were produced in the same manner as in Examples 1 to 6, except that the components of the working electrode layer were set to the ratios shown in Table 1.
[0050]
An operating temperature dependency test and a non-heating moisture resistance test were performed on the manufactured sensor element, and the results are shown in Table 2.
[0051]
[Table 1]

[0052]
[Table 2]

[0053]
【The invention's effect】
The solid electrolyte type carbon dioxide gas sensor element of the present invention is used by being left in an atmosphere that can contain carbon dioxide as a gas to be detected under the condition of being heated and maintained at a constant temperature, and depending on the concentration of the gas to be measured. The concentration of carbon dioxide in the atmosphere can be measured based on the generated electromotive force. In addition, since the working electrode layer contains carbonate and zeolite of a rare earth metal element, not only high humidity resistance during non-heating but also high accuracy measurement even when the operating temperature is lowered to a low temperature range of about 300 ° C. Is possible.
For this reason, it can be used particularly suitably as a carbon dioxide gas sensor element requiring energy-saving operation.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a typical embodiment of a solid oxide carbon dioxide sensor element.
[Explanation of symbols]
1. Working electrode layer2. 2. solid electrolyte layer 3. Reference electrode layer Ceramic plate5. Adhesive 6. Heater 7. Power supply 8. voltmeter

Claims (1)

In a solid electrolyte type carbon dioxide sensor element in which a working electrode layer containing an electron conductive material and a metal carbonate and a reference electrode layer containing an electron conductive material are formed on the surface of a solid electrolyte layer, the working electrode layer is made of a rare-earth carbon dioxide. A solid oxide carbon dioxide sensor element comprising an oxide and zeolite.

Images