JPH0763726A - Solid reference-electrode type co2 gas sensor - Google Patents

Abstract

PURPOSE:To provide a solid reference-electrode type CO2 gas sensor which is not affected by a coexisting gas (for example, steam) in an atmosphere, can operate at a low temperature, does not require any reference gas, and is reduced in size, weight, and power consumption. CONSTITUTION:An oxide is formed at the boundary between a solid electrolyte 1 composed of an alkali-ion conductor forming dissociation equilibrium with a CO2 gas and a solid reference electrode 2 which is an electron-ion mixture conductor by directly press-contacting the electrolyte 1 and electrode 2 with each other and lead wires 5 and 8 are respectively connected to the electrolyte 1 and electrode 2. Therefore, the CO2 gas can be detected from an electromotive force generated by a chemical reaction between the oxide and CO2 gas at the boundary.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor suitable for measuring the concentration of CO ₂ gas, and more particularly to a solid electrolyte of an alkali ion conductor and an electronic sensor.
Using a solid reference electrode is an ion mixing conductor, chemical reaction by utilizing the accurate measurement of CO ₂ gas concentration and CO ₂ gas, to a solid reference electrode type CO ₂ gas sensors capable of detecting.

[0002]

2. Description of the Related Art Recently, a technique for measuring various gas concentrations has been developed. Among them, in particular, measurement of CO ₂ concentration in the atmosphere, control of indoor environment such as living space, agricultural and industrial processes, and biological surface The use of CO ₂ gas sensors is increasing in various fields such as measurement of metabolic functions and medical fields. Conventional CO ₂ gas sensors include those using infrared absorption (optical measurement method), wet method (electrolysis method), semiconductors, and ceramics (alkali ion conductors, perovskite type oxides). However, there are merits and demerits, and there is still no satisfactory CO ₂ gas sensor.

[0003]

For example, since the optical measuring method utilizes infrared absorption, if dust, dirt, oil or the like adheres to the sensor portion for detecting gas and is contaminated, the accurate gas concentration will not be obtained. I can't measure. Therefore, the sensor unit needs a device for removing dust, moisture, etc. in the measurement gas atmosphere, and it is difficult to reduce the size and weight. And the price is expensive.

There is also a method of forming a concentration cell type sensor using a solid electrolyte that directly forms dissociation equilibrium with CO ₂ gas, and obtaining the CO ₂ concentration in the test gas from the electromotive force thereof (Japanese Patent Laid-Open No. 24548/1992). (See Japanese Patent Publication). The configuration of this concentration cell type sensor is shown in equation (1). _{CO 2 ', O 2',} Au / Li 2 CO 3 / AuCO 2 ", O 2" ··· (1) the electromotive force E of the battery having the structure represented by the (1) is expressed by the following equation. E = -RT / 4Fln [(PCO ₂ ') ² · PO ₂ ' / (PCO ₂ ") ² · PO ₂ ”] ··· (2) where PCO ₂ 'and PO ₂ ' are in the test gas, respectively. CO ₂ ,
O ₂ partial pressure, PCO ₂ ", PO ₂ " is C in the reference gas
O ₂ and O _{2 are} partial pressures, R is a gas constant, F is a Faraday constant, and T is an absolute temperature. Therefore, the CO ₂ concentration in the test gas can be obtained by using the above equation (2) by determining the temperature, PO ₂ in the test gas, and PCO ₂ , PO ₂ of the reference gas.

By the way, the concentration battery type sensor is
_The principle of detection of ₂ gases is simple, but since the reference gas is required for CO ₂ gas measurement as is clear from equation (2), the system for supplying the reference gas to the sensor and the containment of the reference gas are required. Gas seal of is required. Further, since the metal carbonate is used for the solid electrolyte, the sensor shows a response not only to CO ₂ gas but also to O ₂ gas. Therefore, by advance likewise the O ₂ partial pressure of the test gas and reference gas, or constitute a concentration cell of CO ₂ gas only, O ₂
A combination with a gas sensor is required. As described above, since the system of this sensor becomes complicated and there is a limit to miniaturization, it may be difficult to mount the sensor on a small device. When a carbonate equilibrium system such as CaO + CaCO ₃ system that gives a CO ₂ (O ₂ ) equilibrium partial pressure is used instead of the reference gas, the equilibrium CO ₂ and CO ₂ → CO + 1/2 O
_Since the oxygen pressure of the ₂ system depends on the temperature, the temperature change of the sensor element remarkably affects the electromotive force of the sensor, resulting in C
It has a drawback that the detection accuracy of O ₂ is deteriorated.

Further, there is a sensor of the type which does not require a reference gas, in addition to the CO ₂ gas sensor of the concentration battery type. This sensor constitutes a battery with a solid electrolyte having sodium ion conductivity and sodium carbonate (Japanese Patent Application Laid-Open No. Hei 3).
-134553). The configuration of this sensor is shown in equation (3). CO ₂ , O ₂ , Au / NaCO ₃ / NASICON / Au, O ₂ (3) The electromotive force E of the sensor shown in the above (3) is expressed by the following equation. E = ΔG ° (Na ₂ CO ₃ ) −RT / 2Fln (aNa ₂ O · PCO ₂ ) ... (4) where ΔG ° (Na ₂ CO ₃ ) is the standard free energy of formation, a
Na ₂ O is the activity of Na ₂ O, and PCO ₂ is the partial pressure of CO ₂ . R
Is a gas constant, F is a Faraday constant, and T is an absolute temperature.

[0007] This type of sensor, although activity of Na ₂ O is a prerequisite be constant, it is difficult to make constant the activity of Na ₂ O. As a result, when the Na ₂ O activity changes, the electromotive force of the sensor that depends on the CO ₂ gas concentration fluctuates, and the CO ₂ gas concentration may not be accurately measured. Further, Na ₂ CO ₃ absorbs water in the atmosphere and deliquesces, so that the storage management of the sensor element is troublesome.

In order to solve the above problems, the present invention is not affected by a coexisting gas (for example, water vapor) in the atmosphere, can operate at low temperature, is small and lightweight, and consumes little power for the device. A solid reference electrode type CO ₂ gas sensor is provided. In addition, a sensor having a simpler structure that does not require a reference gas is provided, thereby providing a smaller CO ₂ gas sensor. Further, the present invention provides a solid reference electrode type CO ₂ gas sensor capable of improving the stability of the electromotive force of the sensor.

[0009]

Therefore, in the solid reference electrode type CO ₂ gas sensor of the present invention, a solid electrolyte of an alkali ion conductor and an electron / electron electrode which directly forms a dissociation equilibrium with CO ₂ gas.
A solid reference electrode, which is an ionic mixed conductor, is pressure-bonded to form an oxide at the interface between the two, and a lead wire is attached to each of the solid electrolyte and the solid reference electrode so that the oxide and CO ₂ gas at the interface CO ₂ gas can be detected by an electromotive force generated by a chemical reaction between the _two . Further, a metal of the same conductive species as the alkali ion conductor is added to the mixed electron / ion conductor to obtain a solid standard. It is characterized by being a pole.

[0010]

In the present invention, a solid electrolyte of an alkali ion conductor and a solid reference electrode of an electron / oxygen ion conductor are combined to form an oxide at the interface between the two, and the chemical reaction at this interface is utilized. The CO ₂ concentration is calculated based on the activity of the oxide. This sensor has a sensor element of 350 to 600.
Operate by heating to ℃. The electromotive force of the sensor hardly changes after the oxide formation reaction at the interface stabilizes. The response speed of the sensor differs depending on the combination of the solid electrolyte and the solid reference electrode material. Further, by adding a metal of the same conductive species as the alkali ion conductor to the mixed electron / ion conductor, the activity of the oxide can be kept constant, and a more stable CO ₂ gas concentration can be detected. .

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure and operation of a solid reference electrode type CO ₂ gas sensor according to the present invention will be described below with reference to the drawings. [Structure of Sensor] In FIG. 1, 1 is a solid electrolyte as a disk-shaped alkali ion conductor (details will be described later), 2 is a solid reference electrode which is a mixed conductor of electrons and oxygen ions, and 3 is a porous Au electrode. 4 is Au as a current collector
A mesh, 5 is an Au lead wire, 6 is a high temperature adhesive, 7 is a Pt mesh as a current collector, 8 is a Pt lead wire, and 10 is a potentiometer, which constitute a solid reference electrode type CO ₂ gas sensor. There is.

This sensor uses a lithium ion conductor as a solid electrolyte, and specifically, a lithium ion conductor (Li ₂ CO ₃ -5 mol% Li ₃ PO ₄ -10 mol% Al ₂ O ₃ ) 1
Mixed conductor of solid electrolyte and electron / oxygen ion (La _2-X
A solid reference electrode 2 which is Sr _x CuO ₄ ) is adhered to each other, and both are fixed with a high temperature adhesive 6 and gas-sealed. A porous Au electrode 3 and an Au net 4 as a current collector are provided on one surface of the solid electrolyte side 1 and an Au lead wire 5 is provided. On the other hand, a Pt net 7 as a current collector is provided on the surface of the composite oxide 2 side as a solid reference electrode, and a Pt lead wire 8 is taken. The lead wires are connected to the potentiometer 10. When the sensor element formed as described above is heated by a heater and installed in a gas atmosphere to be measured, an electromotive force proportional to the CO ₂ gas concentration is obtained, and from this electromotive force, the CO ₂ gas concentration is determined by the following detection principle. Desired.

The configuration of the sensor is expressed by equation (5). CO ₂ , O ₂ , Au / lithium ion conductor / solid reference electrode / Au, O ₂ ··· (5) And the reaction at each interface (a), (b) and (c) of this sensor is (6),
Shown by (7) and (8). Interface a

[Chemical 1] Interface

[Chemical 2] Interface

[Chemical 3] So the whole reaction is

[Chemical 4] Becomes From this, the electromotive force E of this sensor is
It is indicated by 0). E = ΔG ° (Li ₂ CO ₃ ) −RT / 2Fln (aLi ₂ O · PCO ₂ ) (10) where ΔG ° (Li ₂ CO ₃ ) is the standard free energy of formation, a
Li ₂ O is the activity of Li ₂ O, and PCO ₂ is the CO ₂ partial pressure. R
Is a gas constant, F is a Faraday constant, and T is an absolute temperature. In this sensor, as is clear from the above reaction formula, if the activity of Li ₂ O is constant, all values other than PCO ₂ are known. Therefore, PCO ₂ (that is, CO 2 ₂ concentration) can be obtained. By the way, in the present invention, it is a necessary condition that the activity (aLi ₂ O) of Li ₂ O generated at the interface between the solid electrolyte and the solid reference electrode is constant as described above. Therefore, it is considered that the activity of Li ₂ O can be made constant. That is,
In the solid electrolyte used here, lithium ions can move, and in the mixed conductor of the solid reference electrode, electrons and oxygen ions can move. When the sensor configured as shown in FIG. 1 is heated by both of these, lithium ions in the solid electrolyte and oxygen ions in the solid reference electrode diffuse to the interface.
There, Li ₂ O is formed. However, as it is, L
The activity of i ₂ O is not constant. When lithium is not added to the solid reference electrode, lithium ions try to diffuse to the solid reference electrode side until the chemical potentials of lithium in the solid electrolyte and the solid reference electrode become equal. While this diffusion is occurring, the activity of Li ₂ O is not determined, and it is considered that the electromotive force of the sensor continues to fluctuate. However, when the diffusion spreads and the Li ₂ O activity (aLi ₂ O) becomes constant, the electromotive force of the sensor becomes stable, and as a result, the CO ₂ concentration can be accurately measured by this sensor. Further, by adding a certain amount of lithium to the solid reference electrode in advance, the electromotive force of the sensor can be stabilized quickly. This is by mixing Li ₂ O in La _2-X Sr _x CuO ₄ solid reference electrode, solid solution _{(La 2-XY Sr X Li} Y CuO 4) is formed. The activity of Li ₂ O in this solid solution is determined by the composition ratio of La _2-X Sr _x CuO ₄ and Li ₂ O, and by attaching this to the solid electrolyte, the activity of Li ₂ O formed at the interface is unique. Decided. Therefore, it is not necessary to wait until the activity of Li ₂ O generated at the interface becomes constant as described above. Therefore, the addition of lithium to the solid reference electrode has the effect of quickly stabilizing the electromotive force of the sensor.

In the experiments, it was confirmed that the CO ₂ gas concentration can be accurately measured by the sensor having the above-mentioned structure. From this, it is clear that the Li ₂ O activity becomes constant in the sensor having the above structure. It was FIG. 2 shows the result of measuring the CO ₂ concentration by the sensor having the above structure. Although the manufactured sensor showed fluctuations in electromotive force for several days, a stable electromotive force proportional to the CO ₂ concentration was obtained after the oxide formation reaction at the interface stabilized thereafter (see FIG. 2). Further, as compared with the conventional sensor, the electromotive force has a linear relationship with log (PCO ₂ ) even at a low CO ₂ gas partial pressure. Furthermore, when the same element as the conductive species of the alkali ion conductor is added to the solid reference electrode which is an electron / oxygen ion conductor (Li is added to the solid reference electrode) (La _{2- XY} Sr _X Li
It was confirmed that the fluctuation of _Y CuO ₄ ) and electromotive force was even smaller. The response speed of the sensor differs depending on the combination of the solid electrolyte and the solid reference electrode material.

A method for producing the solid electrolyte, the solid reference electrode and the sensor cell used in the above structure will be described. [Production of Solid Electrolyte] As a specific example, as a solid electrolyte of an alkali ion conductor, a lithium ion conductor (Li ₂ C
_{O 3 -5mol% Li 3 PO 4} - 10mol% Al 2 O 3) is used. Li ₂ CO ₃ , Li ₃ PO ₄ , and Al ₂ O ₃ powders were mixed in a mortar so as to have the above composition, and then melted at 750 ° C. in an alumina crucible. Pour this into a container of φ10 × 10 mm and solidify,
Allow to cool. Then, it is cut into a disk shape of φ8 × 2 mm, and both surfaces thereof are polished and smoothed to obtain a solid electrolyte.

[Preparation of Solid Reference Electrode] The solid reference electrode, which is an electron / ion mixed conductor, is a La _2-XY Sr _X Li _Y CuO _{4 layer obtained} by substituting a part of La _{2 -X} Sr _X CuO ₄ with Li. Use ties. Prepare powders of LaCO _3, SrCO _3, Li ₂ CO _3, CuO as starting materials,
Using a powder mixing method or freeze drying method,
An oxide powder adjusted to a predetermined ratio is obtained. This powder was formed using a press machine and fired at 1100 ° C. to obtain φ7 ×
A dense sintered body of 0.5 mm is obtained. Then, the disk-shaped both surfaces are polished and smoothed to form a solid reference electrode.

[Production of Cell] The solid electrolyte and the solid reference electrode produced by the above method are pressure-bonded so that no gaps are formed, and a high-temperature adhesive is applied to the outer peripheral surface of the joint,
dry. After that, it is heated at 650 ° C. and the high temperature adhesive is baked and integrated to fix and gas seal. Next, Au paste is applied to the surface of the solid electrolyte side and dried. After the above operation, the sensor is constructed as shown in FIG. A porous Au electrode 3 and an Au net 4 are provided on the solid electrolyte 1 side.
The reason why the Pt mesh 7 is used on the side of the solid reference electrode 2 is to facilitate the contact of the O ₂ gas in the test gas with the solid reference electrode and the solid electrolyte through the holes and meshes. Other than that, it is possible to use a structure that allows gas to pass therethrough and can fix the lead wire.

[CO₂Example of gas measurement)
Related solid reference pole type CO₂Use a gas sensor with appropriate means
CO when heated to 400-500 degrees and activated₂concentration
Solid reference electrode type CO₂Indicates the electromotive force of the gas sensor
You As is clear from this graph, this sensor is CO
₂It was found that a stable electromotive force proportional to the concentration was obtained.
It was

Next, another embodiment of the present invention will be described.
The configuration of the sensor is shown. 3 and 4 use the sensor more
To heat the sensor to operating temperature to facilitate
In this embodiment, a CO₂Gas concentration detection principle
Is the same as in the first embodiment. FIG. 3 is the same as that of the first embodiment.
A heater 9 is provided outside the Pt net 7. Figure 4
Wraps the solid reference electrode 2 around the heater 9 and
The solid electrolyte 1 is wound on the top, and the solid electrolyte 1 side is porous.
Connect the Au electrode 3 and the Au net 4 to the heater 9 on the end side of the heater 9.
A Pt mesh 7 is provided on the body reference electrode 2. 2nd and 3rd real
All of the examples have a heater built into the sensor.
Therefore, using this heater, the sensor operating temperature (4 degrees Celsius
(0 to 500 degrees) can be easily heated. the above
Like the solid reference electrode type CO of the present invention.₂The gas sensor is
Li as solid electrolyte ₂A solid reference electrode with a constant O activity
Stabilized the electromotive force of the sensor by mounting
The feature is the point. Solid reference electrode is La_2-XSr_XCuO_FourTo Li₂
By mixing O, a solid solution (La_2-XYSr_XLi_YCuO_Four)
Was formed. Li in this solid solution ₂O activity is La_2-X
Sr_XCuO_FourAnd Li₂Since the composition ratio of O can be determined, this is a solid
It was used as a reference electrode. Therefore, La containing lithium_2-X
Sr_XCuO_FourIs a stable CO₂The effect that enables concentration detection
is there.

[0020]

As described in detail above, according to the present invention, a sensor is constructed by combining a solid electrolyte of an alkali ion conductor and a solid reference electrode of an electron / ion mixed conductor to form a concentration cell. It is no longer necessary to use a reference gas such as a CO ₂ gas sensor of the formula. Therefore, the structure of the sensor is simplified. Further, since the sensor element has a simple structure, it is possible to further reduce the size. Furthermore, by adding the same element as the alkali ion conductive species to the oxide of the mixed electron / ion conductor, it was possible to stabilize the electromotive force of the sensor depending on the CO ₂ concentration. It is possible to obtain excellent effects such as.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a solid reference electrode type CO ₂ gas sensor of a first embodiment.

FIG. 2 is an electromotive force and CO of a solid reference electrode type CO ₂ gas sensor.
It is a figure which shows the relationship with ₂ density.

FIG. 3 is a diagram showing a configuration of a solid reference electrode type CO ₂ gas sensor according to a second embodiment.

FIG. 4 is a diagram showing a configuration of a solid reference electrode type CO ₂ gas sensor according to a third embodiment.

[Explanation of symbols]

 1 Solid Electrolyte 2 Solid Reference Electrode 3 Porous Au Electrode 4 Au Net 5 Au Lead Wire 6 High Temperature Adhesive 7 Pt Net 8 Pt Lead Wire 9 Heater

(72) Inventor Junichiro Mizusaki 2-20 Nishifune, Funabashi-shi, Chiba Nishifune Green Heights 7-204 (72) Inventor Hiroaki Tagawa 3-14-5 Higashi-Arima, Miyamae-ku, Kanagawa Prefecture

Claims (2)

[Claims] 1. Direct CO₂Forming a dissociation equilibrium with the gas
Solid electrolyte and electron / ion mixed conduction of Lucari ion conductor
Oxidize at the interface between the solid reference electrode, which is an electric body, by pressure bonding
Of the solid electrolyte and that of the solid reference electrode.
Attach a lead wire to each of the
CO₂The electromotive force generated by the chemical reaction with the gas
Ri CO₂Solid reference electrode characterized by being able to detect gas
Type CO ₂Gas sensor. 2. The solid reference electrode type CO ₂ gas sensor according to claim 1, wherein a metal of the same conductive species as the alkali ion conductor is added to the mixed electron / ion conductor to form a solid reference electrode. .