JP2002107329A - Carbon dioxide sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon dioxide sensor whose sensitivity is enhanced and in which irregularities in characteristics are small. SOLUTION: In the carbon dioxide sensor, a detection electrode and a counter electrode are installed respectively to come into contact with a solid electrolyte, the solid electrolyte contains alkaline metal ions and/or an alkaline-earth metal ion electrical conductor, and the detection electrode is provided with a metal oxide layer and a collector. The metal oxide layer forms a porous metal oxide layer, using sol-gel method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon dioxide sensor for use in bio-related fields such as indoor and outdoor environment control, agricultural and industrial processes such as facility horticulture, disaster prevention, and measurement of metabolic functions on living bodies, and medical fields. About.

[0002]

2. Description of the Related Art In recent years, there is a need for a carbon dioxide sensor mainly for detection of indoor air pollution accompanying the spread of air conditioning, detection of air pollution in facilities in livestock farming, plant growth control in horticultural facilities, various industrial processes, and the like. And various types of carbon dioxide sensors have been proposed.

Specifically, for example, an infrared absorption type carbon dioxide sensor has been put to practical use. However, this type of sensor has not been widely used due to its large size and high cost. A sensor using a semiconductor has also been proposed, but since this sensor has poor selectivity for carbon dioxide, it is difficult to measure the concentration of only carbon dioxide.

[0004] On the other hand, some sensors using a solid electrolyte have been proposed as small and inexpensive sensors.

[0005] Maruyama et al. The 10th Solid Ionics Symposium
Abstracts 69 (1983)｝ show dissociation equilibrium with carbon dioxide
Forming a pair of electrodes on a solid electrolyte such as potassium carbonate
Is formed, and one of them is brought into contact with a reference gas having a known concentration to form an atmosphere.
A density-polarized cell for measuring the electromotive force due to the concentration difference of ambient gas.
A sensor. In addition, Maruyama et al.
In addition to polarization type sensors, NASICON (Sodium Super)
-Ion conductor: Na _ThreeZr_TwoSi_TwoPO₁₂Al)
Forming a pair of electrodes on a potassium metal ion conductive solid electrolyte
And dissociation equilibrium with carbon dioxide such as sodium carbonate
A metal carbonate layer that forms
A so-called electromotive force detection type cell
Has also proposed.

In Japanese Patent Publication No. 4-79542, a pair of electrodes is formed on a solid electrolyte having metal ion conductivity of a metal salt forming a dissociation equilibrium with carbon dioxide, and one of the electrodes is coated with the metal salt. A carbon dioxide sensor has been proposed in which the other electrode and the remaining solid electrolyte surface are coated with a gas barrier layer.

Japanese Patent Application Laid-Open No. 7-63726 proposes a solid reference electrode type carbon dioxide sensor in which a solid reference electrode, which is a conductor of electrons and oxygen ions, is pressed against a solid electrolyte made of an alkali ion conductor. .

A problem with a small and inexpensive carbon dioxide sensor using a solid electrolyte is that, first, the metal carbonate used as a material is easily affected by humidity. In order to solve this problem, it is necessary to seal the portion other than the gas detecting portion of the sensor element, or to heat the sensor with a heater to increase the operating temperature to reduce the influence of humidity. The operating temperature of the above sensor is as high as 400 to 700 ° C. When the operating temperature is high, problems such as a large power consumption of the whole sensor and a thermal degradation of the material occur. Further, there is also a problem that heat of several hundred degrees Celsius has a subtle effect on the measurement environment, such as heating around the sensor even if it is from a small heater and generating convection of air.

Further, since the metal carbonate is easily affected by humidity, it is necessary to store the element in a dry atmosphere when the use of the sensor is stopped. Furthermore, during use, baking is necessary to remove moisture that has entered the element during stoppage, and it takes a long time for the output voltage of the sensor to stabilize, and there is a problem of workability and energy. These are inevitable problems when operating the sensor at a high temperature, and a sensor operating at a lower temperature is required.

In response to this problem, S. Breikhin et al. [Applied
PhysicsA 57, 37-43 (1993)] reports a carbon dioxide sensor in which a solid electrolyte and a semiconductor are bonded. This carbon dioxide sensor uses a SnO ₂ semiconductor doped with Sb or V as a detection electrode, and a sodium ion conductor, NASICON, is joined as a solid electrolyte to this. A Na _x CoO ₂ as a reference electrode is provided on the opposite side of the detection electrode. Has been arranged. This carbon dioxide sensor can be operated at low temperature (-35 ° C. to room temperature), but has a problem that it takes more than 4 minutes until a response is observed. In addition, the moisture resistance is poor, and the response time and sensitivity change depending on the relative humidity (humidity), and it is difficult to obtain stable performance.

In view of this situation, the inventors of the present invention have previously provided a detection electrode and a counter electrode in contact with a solid electrolyte such as NASICON, respectively, and the detection electrode is formed of a current collector and a predetermined metal oxide. Various carbon dioxide sensors that include a metal oxide layer containing carbon dioxide and that detect carbon dioxide using a detection electrode in the presence of hydrogen carbonate ions have been proposed (Japanese Patent Application No. 10-96604).
No., Japanese Patent Application No. 10-272608, Japanese Patent Application No. 10-272609, Japanese Patent Application No. 10
-272610, Japanese Patent Application No. 2000-98036).

The metal oxide layer provided in the detection electrode of such a carbon dioxide sensor is a porous layer. Generally, a paste containing a metal oxide, a binder and a coating solvent is applied on a solid electrolyte and baked. Obtained.

Since the paste is used as described above, there is a problem that the variation between lots is large, and the variation in sensor characteristics becomes large. Further, the sensitivity was not sufficient.

[0014]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a carbon dioxide sensor having a uniform porous metal oxide layer, which has less variation in characteristics and has improved sensitivity.

[0015]

This and other objects are achieved by the present invention described below. (1) A detection electrode and a counter electrode are respectively provided in contact with a solid electrolyte, the solid electrolyte contains an alkali metal ion and / or an alkaline earth metal ion conductor, and the detection electrode is formed of a metal oxide layer and a current collector. A carbon dioxide sensor having a body, wherein the metal oxide layer is a porous metal oxide layer formed by a sol-gel method. (2) The carbon dioxide sensor according to (1), wherein the metal oxide layer is formed from a metal alkoxide. (3) The carbon dioxide sensor according to (1) or (2), wherein the metal oxide layer contains indium oxide as a metal oxide.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The carbon dioxide sensor of the present invention is of a type in which a detection electrode and a counter electrode are respectively provided in contact with a solid electrolyte, and the solid electrolyte is an alkali metal ion and / or an alkaline earth metal ion conductor, preferably sodium. An ion conductor is contained, and the detection electrode has a metal oxide layer and a current collector.

This metal oxide layer is provided in contact with the solid electrolyte. Specifically, the current collector may be provided so as to face the solid electrolyte with the metal oxide layer interposed therebetween so that the metal oxide layer is located on the solid electrolyte side, for example, to obtain a quick response. Good. In addition, the current collector is provided on the solid electrolyte side. In such an installation method, since the current collector is usually a porous body such as a metal mesh, a metal oxide layer is formed. The sol-gel method composition invades the current collector, and as a result, the metal oxide layer and the solid electrolyte come into contact with each other (see FIG. 1 described later).

In the present invention, such a metal oxide layer is
It is formed as a porous layer by a sol-gel method.

Since it is formed by the sol-gel method, a uniform porous layer can be obtained without occurrence of variation due to the lot of the paste, as compared with the case where a paste containing a conventional metal oxide is used. Therefore, variations in the characteristics (for example, sensitivity) of the sensor are reduced, the yield is improved, and the productivity is improved. At the same time, the surface area of the porous layer is increased, and the sensitivity is improved as compared with the conventional product.

The fact that the metal oxide layer formed by the sol-gel method is a uniform porous layer is confirmed by a microscope (optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM), etc.). And that the surface area of the porous metal oxide layer formed by the sol-gel method is larger than that of the conventional product formed by the paste,
It can be confirmed by measuring the ET value. The formation by the sol-gel method is 10 to 10 times more than the formation by the paste.
The surface area is increased by about 80%.

The metal oxide layer has a pore diameter of 0.01 to 100.
It is about μm.

The porous metal oxide layer is formed by a sol-gel method. In this case, the sol-gel method employs a metal organic compound such as a metal alkoxide, a carboxylate, or an acetylacetonate salt, or a metal water. By hydrolyzing, polymerizing and condensing a metal-inorganic compound such as an oxide, a nitrate, or a chloride, the sol is converted into a sol in which fine particles are dispersed, and the reaction further proceeds to gel.

More specifically, a solution of the above compound is obtained and applied on a solid electrolyte, and then hydrolyzed to form a sol, or a sol obtained by hydrolyzing the above solution is formed on a solid electrolyte. It is applied, dried, gelled, and fired.

The solvent for obtaining the solution depends on the compound used, but is generally water, alcohols such as methanol, ethanol, propanol and terpineol, ketones such as methyl ethyl ketone, acetone and acetylacetone, and cellosolves. . Compound concentration is 1-50
% (Mass percentage).

The coating may be performed by any ordinary method.

Drying after solification is carried out at a temperature of room temperature to 100 ° C. for 30 minutes to 6 hours, and baking for forming the sol into a metal oxide is carried out at a temperature of 300 to 750 ° C. for 10 minutes to 3 hours. . Thereby, 99% (mole) or more is converted into a metal oxide.

As the metal alkoxide of the above compound,
Examples include propoxide, isopropoxide, butoxide, t-butoxide, and the like, and examples of the metal carboxylate include acetate, oxalate, and the like.

Of these, use of metal alkoxides is preferred. Indium oxide preferably used in the present invention is exemplified by indium (III) isopropoxide.

Other compounds for indium oxide include indium hydroxide, indium chloride, indium nitrate, indium (III) acetate, and indium (III) acetylacetonate. Furthermore, dimethylindium chloride, indium hexafluoropentanedionate, indium methoxyethoxide, indium methyl (trimethyl) acetyl acetate, indium 2,4-pentanedionate, indium (III) trifluoroacetate, indium trifluoropentanedionate And tris (2,2,6,6-tetramethyl-3,5-heptanedionato) indium (III).

These compounds are usually used alone, but may be used in combination of two or more.

The details of the sol-gel method follow the usual method.

The thickness of the metal oxide layer thus formed is about 1 nm to 10 μm.

The metal oxide layer is formed by a sol-gel method, but preferably contains a metal oxide having electronic conductivity, such as indium oxide (In ₂ O ₃ ), Tin (SnO ₂ ), cobalt oxide (Co ₃ O ₄ ), tungsten oxide (WO ₃ ), zinc oxide (ZnO), lead oxide (PbO), copper oxide (CuO), iron oxide (Fe ₂ O ₃ , FeO) , Nickel oxide (NiO),
Chromium oxide (Cr ₂ O ₃ ), cadmium oxide (CdO),
Bismuth oxide (Bi ₂ O ₃ ), manganese oxide (MnO ₂ )
Mn ₂ O ₃ ), yttrium oxide (Y ₂ O ₃ ), zirconium oxide (ZrO ₂ ), antimony oxide (Sb ₂ O ₃ ), aluminum oxide (Al ₂ O ₃ ), lanthanum oxide (La
₂ O ₃ ), cerium oxide (CeO ₂ ), praseodymium oxide (Pr ₆ O ₁₁ ), neodymium oxide (Nd ₂ O ₃ ), silver oxide (Ag ₂ O), lithium oxide (Li ₂ O), sodium oxide (Na ₂ O), potassium oxide (K ₂ O), rubidium oxide (Rb ₂ O), cesium oxide (Cs ₂ O), magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO) and barium oxide (BaO) )
It is preferable to contain any one or more of these. Among them, indium oxide, tin oxide, cobalt oxide, tungsten oxide, zinc oxide, lead oxide, copper oxide, iron oxide, nickel oxide, chromium oxide, cadmium oxide, bismuth oxide, zirconium oxide, and aluminum oxide are preferable.
In particular, indium oxide, tin oxide, copper oxide, and zinc oxide are preferable, and in order to obtain the effects of the present invention, indium oxide is most preferable, and the metal oxide in the layer is preferably made of indium oxide. However, when lithium oxide, sodium oxide, potassium oxide, rubidium oxide, cesium oxide, magnesium oxide, calcium oxide, strontium oxide, and barium oxide are used, the response is good, but the sensitivity is easily affected by humidity. It is preferable to use them in combination. Note that these may slightly deviate from the stoichiometric composition.

The metal oxide layer may contain a metal bicarbonate and / or a metal carbonate in addition to the metal oxide.

Although the carbon dioxide sensor of the present invention has the above-described metal oxide layer and current collector as the detection electrode, the configuration and the like other than the metal oxide layer described in detail above will be described.

<Current Collector of Detection Electrode> The carbon dioxide sensor of the present invention uses a current collector for the detection electrode.

The metal used for the current collector may be any one or more of gold, platinum, silver, rubidium, rhodium, palladium, iridium, nickel, copper, chromium and the like.

The current collector is preferably a porous metal. If the current collector is porous, the detection electrode itself functions as a gas diffusion layer, so that a quicker response can be obtained.

As the porous metal, a metal mesh or a powder electrode formed by pressing or screen printing a metal powder paste is preferable. In particular, powder electrodes are preferred. The mesh size of the metal mesh is not particularly limited as long as it has a holding force.

Screen printing is a method in which a paste of metal powder is applied to a substrate through a mesh screen. In this case, a porous electrode in which metal particles are connected to each other is formed. The average particle size of the metal powder used at this time is 10 nm to 100 μm, particularly 10 nm to 10 μm.
m is preferable because good printing can be performed. In addition, the solvent for the paste is an organic solvent in which the metal used does not dissolve or react, and has a relatively low vapor pressure at room temperature.
Anything that has good workability may be used. Particularly, α-terpineol, ethylene glycol, glycerin and the like are preferable.
The viscosity of the slurry is preferably 0.1 to 100,000P.

It is also preferable to apply a current collector metal powder paste on the upper surface of the metal oxide layer and take a lead.

The pore diameter of the porous electrode is 0.5 to 500 μm
Is preferred.

It is also possible to form an electrode having a porous surface by sputtering these metal materials. Ar, He, O ₂ , N ₂
And the like, and the pressure during film formation is preferably in the range of 0.1 to 66.5 Pa (0.1 to 500 mTorr). The electrode surface can also be made porous by resistance heating evaporation.

Further, in order to make the sensing electrode itself a gas diffusion layer and obtain a quicker response, the current collector may be provided to face the solid electrolyte with the metal oxide layer interposed therebetween.

<Solid Electrolyte> In the carbon dioxide sensor of the present invention, an alkali metal ion and / or alkaline earth metal ion conductor, preferably a sodium ion conductor is used as the metal ion conductor in the solid electrolyte. When the counter electrode is exposed to the same environment as the detection electrode, the solid electrolyte cannot take an electromotive force. Further, those having no electronic conductivity are preferable.

As such ionic conductors, for example, Na-β ″ alumina, Na-β alumina, Na _{1 + x}
NAS represented by _{Zr 2 Si x P 3-x} O 12 (x = 0~3)
ICON (for example, Na ₃ Zr ₂ Si ₂ PO ₁₂ ), Na-βGa ₂ O ₃ , Na-Fe ₂ O ₃ , Na ₃ Zr ₂
PSi ₂ P ₂ O ₁₂ , Li-β alumina, Li ₁₄ Zn (Ce
_{O 4), Li 5 AlO 4} , Li 1.4 Ti 1.6 In 0.4 P
₃ O ₁₂ , K-β alumina, K _1.6 Al _0.8 Ti _7.2 O ₁₆ , K
₂ MgTi ₇ O ₁₆ , CaS and the like. These may deviate somewhat from the stoichiometric composition. As the solid electrolyte, a sodium ion conductor is preferable, and Na, Z
A composite oxide of r, Si and P is preferable, and among them,
ASICON is preferred, and Na ₃ Zr ₂ is particularly preferred.
Although it is Si ₂ PO ₁₂ , it may slightly deviate from the stoichiometric composition.

The method for producing the solid electrolyte may be any of a commonly used solid phase method, a sol-gel method, a coprecipitation method and the like, and the solid phase method is preferably used.

In the solid electrolyte, in addition to the metal ion conductor, aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ),
Zirconium oxide (ZrO ₂ ), silicon carbide (Si
C), silicon nitride (Si ₃ N ₄ ), iron oxide (Fe ₂ O ₃ ), and the like may be contained in a mass percentage of 50% or less. These may deviate somewhat from the stoichiometric composition.

<Metal Bicarbonate and / or Metal Carbonate> It is preferable to support metal bicarbonate and / or metal carbonate on the surface of the solid electrolyte. As a result, the generation of bicarbonate ions essential for the detection of carbon dioxide is further promoted, and responsiveness such as sensitivity, response speed, and selectivity is improved. It is considered that the metal carbonate reacts with carbon dioxide and water to form a metal bicarbonate, which promotes the generation of hydrogen carbonate ions derived from carbon dioxide. In any case, the existence of hydrogen carbonate ions is important for the function as a carbon dioxide sensor.

Examples of the metal bicarbonate include alkali metal bicarbonate and the like. Sodium bicarbonate (NaHCO ₃ ), potassium bicarbonate (KHCO ₃ ), rubidium bicarbonate (RbHCO ₃ ), cesium bicarbonate ( CsHCO ₃ ). These may be used alone or in combination of two or more. Among them, it is preferable to use sodium hydrogencarbonate and potassium hydrogencarbonate, particularly sodium hydrogencarbonate.

As the metal carbonate, for example,
Lithium (Li_TwoCO_Three), Sodium carbonate (Na_TwoC
O_Three), Potassium carbonate (K_TwoCO_Three), Rubidium carbonate
(Rb_TwoCO_Three), Cesium carbonate (Cs_TwoCO_Three), Carbonic acid
Gnesium (MgCO_Three), Calcium carbonate (CaC
O_Three), Strontium carbonate (SrCO_Three), Barium carbonate
(BaCO_Three), Manganese carbonate (Mn (CO_Three)_Two, M
n_Two(CO_Three)_Three), Iron carbonate (Fe _Two(CO_Three)_Three, FeCO
_Three), Nickel carbonate (NiCO_Three), Copper carbonate (CuC
O_Three), Cobalt carbonate (Co_Two(CO_Three)_Three), Chromium carbonate
(Cr_Two(CO_Three)_Three), Zinc carbonate (ZnCO_Three), Silver carbonate
(Ag_TwoCO_Three), Cadmium carbonate (CdCO_Three), Carbonated
Indium (In)_Two(CO_Three)_Three), Yttrium carbonate
(Y_Two(CO_Three)_Three), Lead carbonate (PbCO_Three), Bismuth carbonate
(Bi_Two(CO_Three)_Three), Lanthanum carbonate (La_Two(C
O_Three)_Three), Cerium carbonate (Ce (CO_Three)_Three), Carbonated plastic
Theodymium (Pr₆(CO_Three)₁₁), Neodymium carbonate (Nd_Two
(CO_Three)_Three) And the like. Use one kind of metal carbonate
Or two or more of them may be used in combination. Among them, Lithium carbonate
It is preferable to use sodium carbonate, potassium carbonate,
New When used in combination with metal bicarbonate,
It is preferable to use a carbonate of the same metal as the bicarbonate of the genus
No.

In particular, it is preferable to use a bicarbonate or carbonate of the same metal as the mobile ion species of the solid electrolyte.

The metal bicarbonate and / or metal carbonate may be used in combination of two or more kinds. In this case, the metal bicarbonate may be used between metal carbonates or between two metal carbonates, or the metal bicarbonate may be used. And a metal carbonate can be used in combination.

The amount of the metal oxide, metal bicarbonate and / or metal carbonate to be added may be appropriately determined according to the element configuration and size.

In order for the metal bicarbonate and / or metal carbonate to be interposed between the solid electrolyte and the sensing electrode (preferably a metal oxide layer), that is, to be supported on the surface of the solid electrolyte, the metal bicarbonate and / or the metal bicarbonate are used. A solid electrolyte surface may be impregnated with a predetermined amount of a compound using a saturated to supersaturated aqueous solution of a metal carbonate to form a layer of these compounds, or may be formed of a metal bicarbonate and / or a metal carbonate. A layer may be formed by applying a predetermined amount to the surface of the solid electrolyte using a salt paste, baking it, and further dripping water (ion-exchanged water or the like). The amount of water dropped is the solid electrolyte 1
It is about 0.001 ml to 1 ml per cm ² . Cellulose or a cellulose derivative (eg, ethyl cellulose) or the like is used as a binder when the paste is used,
Α-Terpineol or the like is used as the solvent. The average particle size of each of the above compounds when forming a paste is 10
nm-100 μm is preferred, and the viscosity of the slurry is 0.1-10
0,000P is preferred.

When the above-described production method is employed, since the metal oxide layer is usually formed by firing, the application of water (an aqueous solution or a paste using water) may be applied to fix the detection electrode. Due to the need to be performed later, the current collector and the metal oxide layer of the detection electrode are porous, and the current collector is generally porous, but when it is porous,
After forming the sensing electrode, the aqueous solution or paste is
It may be carried on the solid electrolyte by passing through the detection electrode. In such a case, a part of the compound remains on the detection electrode (particularly, the metal oxide layer),
It may be left even if it remains, and it is rather preferable in terms of characteristics.

When a layer of the above compound is formed, it is considered that the compound diffuses at the solid electrolyte interface and the sensing electrode (particularly, metal oxide layer) interface. .

<Counter electrode> In the carbon dioxide sensor of the present invention,
A metal or metal oxide is used for the counter electrode. The metal or metal oxide used may be any one or more of the same metals or oxides thereof as the above-described current collector of the sensing electrode, and the same metal oxide as the metal oxide layer. By using a metal oxide for the counter electrode, the influence of the coexisting gas is reduced, and high carbon dioxide selectivity is obtained. Further, the moisture resistance is improved, and the influence of humidity, particularly at the time of measurement at a low temperature, is reduced.

The counter electrode is preferably made of a porous metal or a porous metal oxide, like the current collector. In particular, a powder electrode formed by pressing or screen printing a metal oxide powder paste is preferable. The mesh size of the metal mesh is not particularly limited as long as it has a holding force. The average particle size of the metal powder and the metal oxide powder used for the paste for forming the powder electrode is preferably from 10 nm to 100 μm, particularly preferably from 10 nm to 10 μm. The paste solvent may be any organic solvent that does not dissolve or react with the metal or metal oxide used, and has a relatively low vapor pressure at room temperature and good workability.
Particularly, α-terpineol, ethylene glycol, glycerin and the like are preferable. Slurry viscosity 0.1 ~ 100,000P
It is preferable that

The pore diameter of the porous electrode is 0.5 to 500 μm
Is preferred.

<Method of Measuring Carbon Dioxide Concentration>
In the presence of carbon dioxide, the sensor
Bicarbonate ion (HCO_Three ^-) Is the sensing electrode metal
Supposedly formed on oxide surface or solid electrolyte surface
It is. Usually, it is thought that all of them are formed.
And the dissociation equilibrium of bicarbonate ion
Convert to changes in carbon dioxide concentration by converting to ion activity
Detect as force. Using NASICON for solid electrolyte
HCO_Three ^-Is the solid electrolyte surface, the solid electrolyte of the sensing electrode
Whether formed on a porous surface or a metal oxide surface,
Na, a mobile ion in the electrolyte⁺But HCO_Three ^-Pull to
And move to the vicinity of the detection pole. Also, ionic conductivity
Excellent HCO on metal oxide surface_Three ^-Is formed,
Na which is a mobile ion during the decomposition⁺Penetrate into the metal oxide layer
Enter. At this time, NaHCO_ThreeMay be generated
You. In addition, the solid electrolyte component in the sensing electrode also supplies mobile ions.
Acts as a feeder and changes in carbon dioxide concentration
It is quickly converted by a change in the activity of the metal ion.

When bicarbonate ions are formed on the surface of a metal oxide having excellent electron conductivity, the metal oxide acts as a conductor and generates an electromotive force. Examples of metal oxides having excellent electron conductivity include indium oxide, tin oxide,
Cobalt oxide, tungsten oxide, zinc oxide, lead oxide,
Examples include copper oxide, iron oxide, nickel oxide, and chromium oxide.

When bicarbonate ions are formed on the surface of a metal oxide having excellent ion conductivity, as described above, an electromotive force is generated when mobile ion species in the solid electrolyte penetrate into the metal oxide. Examples of the metal oxide having excellent ion conductivity include bismuth oxide and cerium oxide.

Even if the formed hydrogen carbonate ion is directly adsorbed or bonded to the metal oxide surface or the solid electrolyte surface of the sensing electrode, the surface of the solid electrolyte component of the sensing electrode,
It may be adsorbed or bonded directly to —OH groups on the surface of the metal oxide or the surface of the solid electrolyte or via water (H ₂ O).

The fact that HCO ₃ ⁻ is involved in the measurement of carbon dioxide in the sensor of the present invention can be confirmed from the results of infrared absorption spectrum (IR) and mass spectrometry.

In addition, the influence of humidity upon measurement is reduced,
In addition, in order to reduce drift, it is preferable to use indium oxide as the metal oxide, but the mechanism is not clear.

<Sensor Structure> FIG. 1 shows a configuration example of the carbon dioxide sensor of the present invention. FIG. 1 shows that a detection electrode 3 and a counter electrode 6 composed of a metal oxide layer 4 and a current collector 5 are connected to a solid electrolyte 2.
Is a non-separable carbon dioxide sensor 1 provided on one surface of the sensor. Metal bicarbonate and / or metal carbonate is interposed between the solid electrolyte 2 and the current collector 5. The non-separable type is preferable because the formation of the current collector and the removal of the lead can be simplified in the process and the manufacturing process is simplified, so that the production efficiency is increased. Further, the size of the element can be reduced. Lead wires are drawn out from the detection electrode 3 and the counter electrode 6, respectively, and connected to a potentiometer.

In the illustrated example, the detection electrode and the counter electrode are provided on the same surface of the solid electrolyte. However, the present invention is not limited to this.
A separation type carbon dioxide sensor in which a detection electrode and a counter electrode are provided on opposite sides of a solid electrolyte may be used.

Although the size of the carbon dioxide sensor of the present invention is not particularly limited, when the surface on which the detection electrode is formed is the upper surface of the solid electrolyte, the thickness of the solid electrolyte is usually 1 μm to 5 m.
m and the area of the upper surface of the solid electrolyte is about 1 μm ^{2 to} 200 mm ² . The thickness of the sensing electrode is about 0.1 to 100 μm, and the area of the sensing electrode is about 0.5 μm ^{2 to} 200 mm ² . The thickness of the counter electrode is about 0.1 to 100 μm, and the area of the counter electrode is about 0.5 μm ^{2 to} 200 mm ² .

The optimum operating temperature of the carbon dioxide sensor of the present invention varies depending on the material constituting the sensor element, the type of coexisting gas, and the like, but is -70 ° C to 200 ° C, preferably -7 ° C.
The range is from 0 ° C to 150 ° C, more preferably from -50 ° C to 120 ° C. At a temperature higher than this, the change in electromotive force with respect to the carbon dioxide concentration becomes small, probably because the amount of adsorbed water decreases and bicarbonate ions are hardly formed, so that the carbon dioxide concentration cannot be measured. The performance of a sensor that cannot be measured at high temperature is restored when the temperature is lowered to about room temperature, and a response can be seen. The carbon dioxide sensor of the present invention can operate at a lower temperature than a conventional carbon dioxide sensor using a solid electrolyte, and can reduce power consumption. Since the temperature does not need to be high, the change in the measurement environment due to the heat of the heater can be sufficiently reduced.

The carbon dioxide sensor of the present invention has good responsiveness, and a response can be obtained in one second to three minutes.

In the element configuration, a heater is not necessary for a sensor that can be operated at room temperature, but it is preferable to provide a heater in consideration of a seasonal temperature difference.

[0073]

EXAMPLES Hereinafter, the present invention will be described specifically with reference to examples.
I do. 1. Fabrication of Carbon Dioxide Sensor As shown in FIG.
Typically, Na_ThreeZr_TwoSi _TwoPO₁₂) Pellets (10mm diameter,
Au film on the top surface (1 mm thick)
Brush (100 mesh) so that it does not touch the sensing electrode.
Provided as a counter electrode.

Next, a current collector Au mesh (100 mesh) was provided in a region smaller than about half of the upper surface of the NASICON pellet.

Further, an indium oxide layer was formed by using a sol-gel method using indium (III) isopropoxide so as to cover about half the upper surface of the NASICON pellet as follows. Indium (III) isopropoxide was dissolved in ethanol so as to form a 20% (mass percentage) solution, and an equimolar amount of water was added to the reactive group of the indium (III) isopropoxide, followed by hydrolysis. In addition, 40% of the amount of indium oxide generated by firing
An amount of polyethylene glycol (PEG: molecular weight: 20,000) corresponding to (mass percentage) was added and dissolved to prepare a coating solution. This coating solution is applied to the above-mentioned area, and then 80 ° C.
For 1 hour. Baking at 500 ° C for 2 hours, 5
An indium oxide (In ₂ O ₃ ) layer having a thickness of μm was formed.

NaHCO ₃ powder (average particle size: 0.1 to 1)
A supersaturated aqueous solution (00 μm) was impregnated into the above In ₂ O ₃ layer, supported on the solid electrolyte, and then dried. Thus, a detection electrode was produced.

Finally, a lead wire was connected from each electrode to obtain a non-separable carbon dioxide sensor as shown in FIG. This is designated as sensor No. 1.

A carbon dioxide sensor was obtained in the same manner as in sensor No. 1, except that an In ₂ O ₃ layer was formed using the following paste instead of the sol-gel method. This is the sensor
No.2. The paste was prepared by mixing 50 mg of indium oxide (average particle diameter: 50 nm) with 5% (mass percentage) of ethylcellulose and α-terpineol in a mixture of 50% and preparing a paste. The viscosity is 10,000-100,000P).

The In ₂ O ₃ layers of the sensors No. 1 and No. 2 are SE
M observation revealed that both were porous layers, but it was confirmed that sensor No. 1 formed a more uniform porous layer. In addition, when the BET value was measured, it was found that the specific surface area of the In ₂ O ₃ layer of the sensor No. 1 was increased by about 60% as compared with the sensor No. 2.

The sensors thus manufactured (each 100
Were evaluated for sensitivity.

Sensitivity At room temperature (25 ° C.), a CO ₂ concentration of 350 to 100
The prepared carbon dioxide sensor was inserted into a measurement cell in which a gas changed in the range of 00 ppm was passed, and the sensitivity was examined.

As a result, as an average value of 100 sensors, the sensitivity of sensor No. 1 was 45 mV / decade, the sensitivity of sensor No. 2 was 30 mV / decade, and the sensitivity of sensor No. 1 increased. I understand. The variation in sensitivity is ± 3 mV for sensor No. 1 and sensor No. 2 for 100 sensors.
Is ± 10 mV, and it was found that sensor No. 1 had less variation.

[0083]

According to the present invention, it is possible to obtain a carbon dioxide sensor having improved sensitivity and less variation in characteristics.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing one configuration example of a non-separable carbon dioxide sensor of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Carbon dioxide sensor 2 Solid electrolyte 3 Detecting electrode 4 Metal oxide layer 5 Current collector 6 Counter electrode

Claims (3)

[Claims] 1. A detection electrode and a counter electrode are provided in contact with a solid electrolyte, respectively, wherein the solid electrolyte contains an alkali metal ion and / or an alkaline earth metal ion conductor, and the detection electrode is formed of a metal oxide layer. A carbon dioxide sensor having a current collector, wherein the metal oxide layer is a porous metal oxide layer formed by a sol-gel method. 2. The carbon dioxide sensor according to claim 1, wherein said metal oxide layer is formed from a metal alkoxide. 3. The carbon dioxide sensor according to claim 1, wherein said metal oxide layer contains indium oxide as a metal oxide.