JPH06186199A - Carbon dioxide sensor element of solid-electrolyte type - Google Patents

Abstract

PURPOSE:To provide an element whose working temperature is remarkably reduced in comparison with a conventional sensor element, and of superior sensitivity and responsiveness. CONSTITUTION:In this element, electrode layers 7 are disposed respectively on both surfaces of solid-electrolyte layer 9, an auxiliary electrode 8 is provided on at least one of electrode layers 7, and detecting electrodes are formed. The auxiliary electrode is made of a carbonate selected from Bi2(CO3)O2, Ag2CO3, La2(CO3)3 and CuCO3. In the sensor element, heaters 2 are provided to the electrode layer 7, and each electrode layer 7 is connected to a detector 6 and used as a sensor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel solid electrolyte type carbon dioxide sensor element (hereinafter also simply referred to as "sensor element") capable of detecting carbon dioxide in an atmosphere. More specifically, it is a solid electrolyte type carbon dioxide sensor element which has a significantly lower operating temperature than conventional sensor elements and which has excellent sensitivity and response performance.

[0002]

2. Description of the Related Art Conventionally, carbon dioxide in the atmosphere has been measured in various fields such as combustion control, plant growth control, fermentation control, environmental control or medical diagnosis. Infrared absorption type and glass electrode type have been mainly used for measuring carbon dioxide, but it has been pointed out that the device is large and expensive and requires maintenance. Therefore, a sensor that utilizes the change in electromotive force of the solid electrolyte has been proposed as a small-sized, simple, and inexpensive carbon dioxide sensor.
Solid electrolyte type carbon dioxide sensor (hereinafter, simply “sensor”)
(Also referred to as), generally, an electrode layer is formed on both sides of a solid electrolyte having ion conductivity, and one electrode layer is provided with a metal carbonate having dissociation equilibrium with carbon dioxide in the atmosphere as an auxiliary electrode. It is configured using a sensor element that constitutes a detection pole.

For example, according to Japanese Patent Laid-Open No. 60-256043, β-A which is a sodium ion conductor is used as a solid electrolyte.
l ₂ O ₃ or Na _{1 + x} ZrSi _x P _{3 -x} O ₁₂ (where x is 0
3 to 3), generally referred to as NASICON), and Na ₂ CO _3, which is a carbonate of the same element as the mobile ions of the solid electrolyte, was used as the auxiliary electrode. When the sensor element having the above structure is placed in an atmosphere containing carbon dioxide, an electromotive force according to the Nernst equation shown in (1) below is generated between the two electrodes, and the electromotive force of carbon dioxide is detected by detecting this electromotive force. The concentration can be measured.

EMF = C−RT / nF · ln [CO ₂ ] (1) where EMF is the electromotive force of the sensor element, C is a constant, R is a gas constant, T is the temperature of the sensor element, and n is the reaction order. F is the Faraday constant and [CO ₂ ] is the concentration of carbon dioxide in the atmosphere.

However, in order to detect carbon dioxide according to the equation (1), it is necessary that the dissociation equilibrium reaction between Na ₂ CO ₃ used for the auxiliary electrode and carbon dioxide should proceed rapidly. Therefore, in the carbon oxide sensor, it is necessary to heat the sensor element to a temperature close to the decomposition temperature of the auxiliary electrode. The above Na ₂ CO ₃ has a decomposition temperature of 851 ° C. into an oxide, and generally 500 ° C. in order to promptly proceed the dissociation equilibrium reaction between Na ₂ CO ₃ and carbon dioxide.
It was said that it was necessary to heat it to a certain degree before use.

Further, as an auxiliary electrode, a carbonate of an element different from the mobile ions of the solid electrolyte, for example, Li ₂ CO ₃
Attempts to use SrCO ₃ or BaCO ₃ etc.
Published by J Mater. Sci. Lett. 5
Vol. 285, see report.

However, any of the above metal carbonates, like Na ₂ CO ₃ , has a high decomposition temperature, and it was necessary to heat the sensor element to a high temperature in order to operate it as an auxiliary electrode. For example, Li having a relatively low decomposition temperature of 618 ° C.
_{Even with 2} CO ₃ , it was necessary to heat the sensor element to about 450 ° C. in order to obtain sufficient carbon dioxide sensitivity.

[0008]

As described above, when the sensor element is operated at a high temperature, the power consumption of the heater for heating the sensor becomes large, which makes it difficult to apply it to a portable type driven by a dry battery or the like. is there. Further, there is a problem that the members constituting the sensor are also required to have high heat resistance. Further, there was a problem that the CO ₂ sensitivity of the sensor and the deterioration of the heater proceeded rapidly during long-term use.

Therefore, it is desired to develop a sensor element that operates at a low temperature.

[0010]

As a result of repeated research to develop a sensor element having such characteristics, the present inventor has used a specific carbonate selected from carbonates having a low decomposition temperature as an auxiliary electrode. When used, it operates at low temperature, and
It has been found that a sensor element having excellent sensitivity and response performance can be obtained, and the present invention has been proposed. That is, the present invention is
In a solid electrolyte type carbon dioxide sensor element having electrode layers on both sides of a solid electrolyte layer and at least one of the electrode layers provided with an auxiliary electrode to form a detection electrode, the auxiliary electrode is Bi ₂ (CO ₃ ) O ₂ , Ag ₂ CO ₃ , La ₂ (CO ₃ ) ₃
And a carbonate selected from CuCO ₃ and a solid electrolyte type carbon dioxide sensor element.

A typical structure of a sensor element according to the present invention is shown in FIG.

In the present invention, as the material of the solid electrolyte layer 9, a known solid ion conductor applicable to the sensor element is used without particular limitation. For example, NASICON, β- having ion conductivity of sodium, lithium, etc.
Al ₂ O ₃ , β-Ga ₂ O ₃ , Li _16-2x Zn _x (GeO ₄ ) ₄
(Where x is 0 ≦ x <8 and is generally called LISON), Li ₄ GeO ₄ —Li ₃ VO _{4, and the} like. Among them, NASICON and β-Al with high ionic conductivity
₂ O ₃ and LISICON are particularly suitable.

The solid electrolyte layer is generally formed by a molding method that does not impair the ion conductivity of the ion conductive material, for example, by a means such as sintering. A known shape is also adopted without particular limitation, but it is generally formed by molding into a flat chip-like shape.

In the present invention, the electrodes 7 are provided on both surfaces of the solid electrolyte 9 so as to be independently present. It suffices that the electrodes are made of a conductive material, and the electrodes may be made of the same kind of conductive material or different kinds of conductive materials. Preferable conductive materials include, for example, noble metals such as platinum, gold, silver, palladium, rhodium and oxides thereof, and the general formula La _1-x Sr _x B
0 ₃ (wherein B represents an element such as Co, Cu, Fe, and Ni, and x is a number of 0.01 to 0.5) and the like, and the above noble metal and metal oxide are used. Examples include mixed composite compositions.

The formation of the electrode 7 is not particularly limited.
Generally, a method of forming by degreasing and sintering after printing a paste comprising the above substance and a binder by screen printing,
A method of forming by a sputtering method, a method of forming by an ion plating method, a method of forming by an evaporation method, or the like is adopted.

The thickness of the electrode layer 9 is not particularly limited, but generally 0.5 to 20 μm is preferable.

In the present invention, the material of the auxiliary electrode 8 is B
i ₂ (CO ₃ ) O ₂ , Ag ₂ CO ₃ , La ₂ (CO ₃ ) ₃ and C
Among the specific carbonates selected from uCO ₃ , one kind or a mixture of two or more kinds is formed.

That is, the inventors of the present invention conducted various experiments on the properties of the substance forming the auxiliary electrode and the sensor element, and as a result, when the above carbonate was used as the auxiliary electrode, the low temperature operability of the sensor element was found. It has been found that not only is excellent, but also the sensor element has high characteristics in sensitivity and responsiveness to carbon dioxide.

The form of providing the auxiliary electrode 8 is not particularly limited as long as it is formed so as to come into contact with the solid electrolyte layer 9 together with the electrode 7. For example, in FIG.
As shown in, when the electrode 7 is formed on the solid electrolyte layer 9, the auxiliary electrode layer 9 is formed so as to cover the surface of the electrode layer 7 and also contact the solid electrolyte layer 9.

The method of forming the electrode layer is not particularly limited. In general, a method of impregnating the surface of the electrode layer with a slurry solution of carbonate and then firing, forming a paste of carbonate with terpineol, etc., printing a part of this on the solid electrolyte layer on the electrode layer, and then firing Methods and the like.

As another mode of providing the auxiliary electrode 8, a mode in which the conductive material and the carbonate are mixed and sintered so that the auxiliary electrode is dispersed and present in the electrode layer can also be adopted. In this case, it is necessary to select and use, as the conductive material forming the electrode, a conductive material that can be sintered at a temperature lower than the carbon dioxide temperature that is the material of the auxiliary electrode. In such an embodiment, the carbonate serving as the auxiliary electrode preferably has a concentration of 91 to 97.5% by weight at least at the interface with the solid electrolyte layer, and the other portions have a concentration lower than the concentration. It is more preferable to control the amount present.

The structure of the sensor formed by using the sensor element of the present invention is not particularly limited as long as the structure of the sensor element described above is satisfied.

For example, as a carbon dioxide detection method,
As shown in FIG. 1, a mode in which an auxiliary electrode is provided only on one electrode to serve as a detection electrode and the other electrode serves as a reference electrode,
A mode in which an auxiliary electrode is provided on both electrodes can be mentioned. In the mode in which the auxiliary electrode is provided only on one electrode, it is not necessary to seal the other electrode against the contact atmosphere, and the sensor element can be used by placing it in a carbon dioxide atmosphere. In addition, in the mode in which the auxiliary electrode is provided on both electrodes, the electrode to be the detection electrode is brought into contact with a carbon dioxide atmosphere, and the other electrode is sealed with a sealing material such as glass, or a fixed carbon dioxide is used. It can be used by contacting it with a reference gas atmosphere having a concentration.

Therefore, it may be appropriately selected from the above-mentioned modes according to the measuring method, the application, and the like.

Further, the sensor element is provided with heating means for heating the sensor element to an operating temperature. As such heating means, heating may be performed by radiation from a heat source outside the gas sensor, or a heater may be directly attached to this for heating. The embodiment shown in FIG. 1 is the former embodiment, in which a DC voltage or an AC voltage is applied from a power source 1 to a heater 2 obtained by screen-printing and heating a heating element forming material such as platinum paste on an alumina substrate 3 as a support. The mode which does is shown. The position where the heater is attached to the gas sensor is not particularly limited as long as it does not hinder the operation of the gas sensor. In the embodiment of FIG. 1, the alumina substrate to which the heater is attached is used as a part of the seal portion.

The sensor element of the present invention generally works well by heating it in the range of 100 to 300 ° C., although there are some differences depending on the carbonate used.

Further, in the present invention, when an organic gas such as toluene, ethyl acetate or ethanol coexists in the atmosphere to be measured, a filter for removing the organic gas may be used. As the filter, known filters can be used without any limitation.

A sensor using the sensor element of the present invention is
Each electrode is electrically connected to a detector 6 such as a voltmeter by a conductive wire 5. In the sensor of FIG. 1, a change in the carbon dioxide concentration of the atmosphere causes a change in the cation concentration of the carbonate of the auxiliary electrode 8 having a dissociation equilibrium with carbon dioxide, which causes the mobile ions on both sides of the solid electrolyte 9 to change. The activity changes, and this change appears as a change in electromotive force between the electrodes 7. Then, the concentration of carbon dioxide gas in the atmosphere is measured by directly or indirectly detecting the change in the voltage with the detector 6 such as a voltmeter. The carbon dioxide sensor having the above-described configuration has a feature that it does not require a reference gas and has a small size and a simple structure.

[0029]

The carbon dioxide sensor of the present invention is 300 ° C.
It is characterized in that carbon dioxide in the atmosphere can be measured with high sensitivity and good responsiveness and accuracy even at a low temperature such as below. Further, since carbon dioxide can be detected at such a low temperature, it is possible to further reduce power consumption as compared with a sensor using a conventionally known alkali metal carbonate or alkaline earth metal carbonate as a reference electrode. In addition, the problem that high heat resistance is required for the members constituting the sensor is also solved, and it is possible to prevent the CO ₂ sensitivity of the sensor and the deterioration of the heater during long-term use.

[0030]

In the sensor element of the present invention, as auxiliary electrodes, Bi ₂ (CO ₃ ) O ₂ , Ag ₂ CO ₃ , La ₂ (C
The mechanism by which O ₃ ) ₃ and CuCO ₃ exert the above-mentioned effect is not clear, but the decomposition temperature of the above-mentioned carbonate is low, and the decomposition of the carbonate into an oxide is extremely high. It is presumed that the above-mentioned excellent characteristics are exhibited by some action due to the characteristics that they occur at a high speed.

[0031]

EXAMPLES The present invention will be described with reference to the following examples in order to specifically describe the present invention, but the present invention is not limited to these examples.

Example 1 A carbon dioxide sensor having a structure as shown in FIG. 1 was produced.
As the solid electrolyte 9, a disk-shaped sintered body having a diameter of about 5 mm and a thickness of about 0.7 mm obtained by molding and sintering NASICON powder was used. The electrodes 7 on both sides were formed by screen-printing a platinum paste, drying and baking at 800 ° C. A lead wire 5 made of a platinum wire is attached to the electrode 7. Bi is used as the auxiliary electrode 8 on one of the electrodes.
₂ (CO ₃ ) O ₂ was added to terpineol in an amount of 5 wt. (weight)
A sensor element was prepared by screen-printing, drying, and baking at 300 ° C. what was made into a paste with a vehicle in which 1% ethyl cellulose was dissolved. On the other electrode side, a heater 2 formed by screen printing with a platinum paste on an alumina substrate 3 was joined with a sealing material 4 made of glass. The sensor was used by applying a DC voltage from the power source 1 to the heater 2 and heating it to 300 ° C.

The sensitivity of the sensor to carbon dioxide gas is
Sensor is 350p which is almost equal to carbon dioxide concentration in the atmosphere
It was evaluated by measuring the change in the electromotive force of the sensor when the carbon dioxide concentration in the atmosphere was changed to 30350 ppm by measuring with a detector 6 including a voltmeter. The sensitivity is indicated by a value obtained by subtracting the electromotive force of the sensor in the measurement gas atmosphere from the electromotive force of the sensor in the carbon dioxide concentration of 350 ppm.

Table 1 shows the relationship between the electromotive force and the sensitivity of the sensor element with respect to various carbon dioxide concentrations. The electromotive force decreased with the increase of carbon dioxide concentration. FIG. 2 shows the relationship between the carbon dioxide concentration and the sensitivity. It was found that the sensitivity increased with an increase in the carbon dioxide concentration and was proportional to the logarithm of the carbon dioxide concentration. From this result, this carbon dioxide sensor can detect the carbon dioxide concentration in the atmosphere by measuring the electromotive force by using FIG. 2 as a calibration curve even when the operating temperature is a relatively low temperature of 300 ° C. It became clear that

Further, in the above sensor element, when the carbon dioxide concentration in the atmosphere was changed from 350 ppm to 1000 ppm, the time required to reach 90% of the sensitivity at 1000 ppm (hereinafter referred to as 90% response time) was measured. As a result, a good response of 38 seconds was shown.

Examples 2 and 3 The same sensor element as in Example 1 was manufactured to construct a sensor. In the above-described sensor, by applying a DC voltage from the power source 1 to the heater 2, the sensor is set to 25 in the second embodiment.
The heating was carried out at 0 ° C. and in Example 3 at 200 ° C., respectively.

In Examples 2 and 3, the relationships between various carbon dioxide concentrations and the electromotive force and sensitivity of the sensor element were measured in the same manner as in Example 1 and shown in Tables 2 and 3, respectively.

It was found that the electromotive force decreases with an increase in the carbon dioxide concentration, and that the sensitivity is proportional to the logarithm of the carbon dioxide concentration as in the case of Example 1. Also, the operating temperature is 25
By lowering the temperature to 0 and 200 ° C., the sensitivity was slightly higher than that when the operating temperature in Example 1 was 300 ° C.
From this result, it was clarified that the sensor element can detect the carbon dioxide concentration in the atmosphere by measuring the electromotive force even when the operating temperature is as low as 200 ° C.

The 90% response time of the sensor element was 42 seconds, which was a good response.

Example 4 In Example 1, as the auxiliary electrode 8, Ag ₂ CO ₃ was used.
A sensor element was manufactured and a sensor was formed in the same manner as in Example 1 except that the sensor element was formed by firing at 200 ° C.

A DC voltage was applied to the heater 2 from the power source 1 to the sensor 2 and the sensor was heated to 150 ° C. to measure electromotive force and sensitivity in a carbon dioxide atmosphere.

With respect to the above sensor element, the relationship between the carbon dioxide concentration in the atmosphere and the electromotive force and sensitivity of the sensor was measured in the same manner as in Example 1, and is shown in Table 4.

The electromotive force decreased with increasing carbon dioxide concentration. It was found that the relationship between the carbon dioxide concentration and the sensitivity was proportional to the logarithm of the carbon dioxide concentration, as in Example 1. From this result, it became clear that even if the operating temperature of the present carbon dioxide sensor is as low as 150 ° C., it is possible to know the carbon dioxide concentration of the atmosphere by measuring the electromotive force.

Further, the 90% response time of the above sensor element showed a good response of 45 seconds.

Example 5 In Example 1, as the auxiliary electrode 8, La ₂ (CO ₃ ) was used.
_A sensor element was manufactured and a sensor was formed in the same manner as in Example ₃ except that the same was formed and fired at 400 ° C.

A DC voltage was applied to the heater 2 from the power source 1 to the heater 2 and the sensor was heated to 300 ° C. to measure electromotive force and sensitivity in a carbon dioxide atmosphere.

With respect to the above sensor element, the relationship between the carbon dioxide concentration in the atmosphere and the electromotive force and sensitivity of the sensor were measured in the same manner as in Example 1 and shown in Table 5.

The electromotive force increased with increasing carbon dioxide concentration. Although the slope of the relationship between the carbon dioxide concentration and the sensitivity was opposite to that in the case of Example 1, it was found that the sensitivity was proportional to the logarithm of the carbon dioxide concentration. From this result, it became clear that even if the operating temperature of the present carbon dioxide sensor is as low as 300 ° C., it is possible to know the carbon dioxide concentration of the atmosphere by measuring the electromotive force.

The 90% response time of the above sensor element showed a good response of 32 seconds.

Example 6 In Example 1, electrodes 7 were formed by screen-printing a gold paste, drying and baking at 700 ° C., and auxiliary electrodes 8 were made of CuCO ₃ in the same manner as in Example 1. A sensor element was manufactured and a sensor was constructed in the same manner except that it was formed by firing at 250 ° C.

A DC voltage was applied to the heater 2 from the power source 1 to the sensor 2 and the sensor was heated to 200 ° C. to measure electromotive force and sensitivity in a carbon dioxide atmosphere.

With respect to the above sensor element, the relationship between the carbon dioxide concentration in the atmosphere and the electromotive force and sensitivity of the sensor were measured in the same manner as in Example 1 and shown in Table 6.

The 90% response time of the above sensor element showed a good response of 37 seconds.

Example 7 In Example 1, silver was used as the electrode on the side where the auxiliary electrode was provided, and a mixed powder of silver and Bi ₂ (CO ₃ ) O ₂ was added to terpineol in an amount of 5 wt. A paste prepared with a vehicle in which (wt)% ethyl cellulose is dissolved is screen-printed, dried, and then sintered at 300 ° C. to obtain a Bi ₂ (CO ₃ ) O ₂ concentration at the interface of the solid electrolytic layer 9. Is 92 vol. A sensor element was manufactured and a sensor was formed in the same manner except that an electrode provided with an auxiliary electrode having a (capacity)% was formed.

With respect to the above sensor element, the relationship between the carbon dioxide concentration in the atmosphere and the electromotive force and sensitivity of the sensor was measured in the same manner as in Example 1, and is shown in Table 7.

The electromotive force decreased with increasing carbon dioxide concentration. It was found that the relationship between the carbon dioxide concentration and the sensitivity was proportional to the logarithm of the carbon dioxide concentration, as in Example 1. From this result, it became clear that even if the operating temperature of the present carbon dioxide sensor is as low as 250 ° C., it is possible to know the carbon dioxide concentration of the atmosphere by measuring the electromotive force. Further, an improvement in sensitivity was recognized as compared with Example 1.

Further, the 90% response time of the above sensor element showed a good response of 25 seconds.

Comparative Example 1 In Example 1, as the auxiliary electrode 8, Y ₂ (CO ₃ ) _{3 was used.}
Was formed in the same manner as in Example 1 and was formed by baking at 200 ° C., and a sensor element was manufactured and a sensor was formed in the same manner.

A DC voltage was applied to the heater 2 from the power source 1 to the heater 2 and the sensor was heated to 200 ° C. to measure electromotive force and sensitivity in a carbon dioxide atmosphere.

With respect to the above sensor element, the relationship between the carbon dioxide concentration in the atmosphere and the electromotive force and sensitivity of the sensor were measured in the same manner as in Example 1 and shown in Table 8.

The above sensor element hardly operated at a low temperature of 300 ° C. and could not be used as a sensor at such a temperature.

Further, the responsiveness of the sensor element could not be measured because it had no sensitivity.

[0063]

[Table 1]

[0064]

[Table 2]

[0065]

[Table 3]

[Brief description of drawings]

FIG. 1 is a sectional structure of a carbon dioxide sensor using a representative solid electrolyte type carbon dioxide sensor element of the present invention.

FIG. 2 is a graph showing the relationship between carbon dioxide concentration and sensitivity in the carbon dioxide sensor obtained in Example 1.

[Explanation of symbols]

 1 Power Supply 2 Heater 3 Alumina Substrate 4 Sealing Material 5 Conductive Wire 6 Detector 7 Electrode Layer 8 Auxiliary Electrode 9 Solid Electrolyte Layer

Claims (1)

[Claims] 1. A solid electrolyte type carbon dioxide sensor element having electrode layers on both sides of a solid electrolyte layer, and an auxiliary electrode provided on at least one of the electrode layers to form a detection electrode, wherein the auxiliary electrode is Bi ₂ ( CO ₃ ) O ₂ , Ag ₂ CO ₃ , L
A solid electrolyte type carbon dioxide sensor element comprising a carbonate selected from a ₂ (CO ₃ ) ₃ and CuCO ₃ .