JPH0827253B2 - Carbon dioxide detection sensor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid electrolyte type carbon dioxide gas sensor.

[0002]

2. Description of the Related Art Currently developed solid electrolyte type sensors are constructed by providing electrodes on both sides of a solid electrolyte plate body which is usually made of an ion conductor. As such a solid electrolyte type carbon dioxide gas sensor, for example, a gold ion electrode is provided on both sides of a pellet-shaped ion conductor obtained by press-molding potassium carbonate and then firing, which has a structure as shown in FIG. There is a carbon dioxide sensor X that uses the above-mentioned element (Proceedings of the 1st International Conference on Chemical Sensors, pp. 353, 1
September 983).

In this sensor, the carbon dioxide concentration in the gas can be detected by measuring the potential difference between the detection electrode and the reference electrode which are in contact with the gas to be measured. The electromotive force characteristic with respect to concentration is shown. However, the electromotive force characteristics change depending on the moisture contained in the gas to be measured, and it is necessary to correct the error due to humidity, which is not practical.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems in the prior art and to provide a highly reliable carbon dioxide gas detection sensor with improved humidity characteristics.

[0005]

In order to achieve the above object, a carbon dioxide gas detection sensor of the present invention has two electrodes arranged opposite to each other with an ionic conductor interposed therebetween, and detects the concentration of gas contacting each electrode. In a gas sensor that outputs an electric signal corresponding to a difference, the ionic conductor is a solid solution of an alkaline earth metal carbonate and an alkali metal carbonate, and does not include a crystal of the alkali metal carbonate. To do.

The alkaline earth metal carbonate used for the ion conductor of the carbon dioxide sensor of the present invention is preferably selected from calcium carbonate, strontium carbonate and barium carbonate, and the alkali metal carbonate is It is preferably one or more carbonates selected from lithium carbonate, sodium carbonate and potassium carbonate.

The alkaline earth metal carbonate and the alkali metal carbonate selected from these are uniformly mixed in powder form and then pressed to form pellets.
Further heat treatment is performed to obtain a solid solution. The mixing ratio at this time varies depending on the combination of materials, but it is generally preferable to use a large amount of alkaline earth metal carbonate and a small amount of alkali metal carbonate. The solid solution thus obtained essentially retains the crystal structure of the alkaline earth metal carbonate,
Does not contain single crystals of alkali metal carbonates. This can be proved by analyzing the crystal structure of the solid solution using an X-ray diffraction method.

FIG. 1 shows the structure of a carbon dioxide gas detection sensor of the present invention assembled using an ionic conductor made of such a solid solution. In the figure, 1 is a sensing element,
Reference numeral 2 is an ion conductor pellet constituting a detection element, 3 is a detection electrode, and 4 is a reference electrode. Although (a), (b), and (c) in the figure have different forms, they basically have a common structure.

Here, for the detection electrode and the reference electrode, for example, a noble metal such as platinum, palladium, rhodium, ruthenium, iridium, gold and silver is vacuum-deposited or sputtered, or a conductive paste having a noble metal as a conductive material is printed. It is preferable to provide it so as to face the surface of the pellet 2 or the substrate 6 on which the pellet 2 is placed by a suitable method such as the above.

The sensing element 1 thus constructed is fused to the other surface of the ceramic substrate 6 having the heater 5 provided on one surface and then sealed around the reference electrode 4 with the inorganic sealing material 7. The terminals of the heater 5, the detection electrode 3, and the reference electrode 4 are connected to the respective pins 9 that are formed through the pedestal 8 by lead wires 10 and assembled. For the heater 5, an electric resistance film of a noble metal such as platinum, palladium, rhodium, ruthenium, iridium, gold, silver, etc. is formed on the surface of the ceramic substrate 6 by a method such as vacuum deposition, sputtering, or plating.
Alternatively, it can be provided by forming an electric resistance film on the surface of the ceramic substrate 6 by a method such as printing a conductive paint containing these noble metals as a conductive material.

[0011]

In the carbon dioxide detection sensor of the present invention, when the gas to be measured is brought into contact with the heater in a heated state, an electromotive force corresponding to the concentration of carbon dioxide in the gas is output between the electrodes. Since the carbon dioxide sensor according to the present invention uses the ionic conductor formed of a solid solution of an alkaline earth metal carbonate and an alkali metal carbonate, which does not contain a crystal of the alkali metal carbonate, Even if the temperature is not particularly high, it has a good detection output characteristic, and a measurement error due to humidity hardly occurs.

[0012]

EXAMPLE (First Example) Barium carbonate and sodium carbonate were mixed at a molar ratio of 1.7: 1 and press-molded at a pressure of 10 kg / cm ² to form 3 mm × 3 mm × 0.5 mm ions. Conductor pellets were made. The results of crystal structure analysis by X-ray diffraction of the pellets are shown in FIG. 15, and comparing the X-ray diffraction chart of barium carbonate of FIG. 25 and the X-ray diffraction chart of sodium carbonate of FIG. No peak due to sodium crystals was observed, and it can be seen that the solid solution basically has a structure close to that of barium carbonate.

On the other hand, platinum is sputtered on the back surface of an alumina-based ceramic substrate having a size of 4 mm × 3 mm × 0.3 mm to form a heating resistance wire to form a heater, and the heater has a width of 0 on both sides along the long side of the upper surface. A 0.5 mm electrode provided by sputtering platinum was prepared.

Next, by connecting the heater terminal and the electrode of the substrate to the heater pin and the electrode terminal pin, which are provided through the pedestal of tetrafluoroethylene resin, respectively with lead wires, the substrate is mounted on the pedestal. Fixed Next, after placing the ion conductor pellets on the upper surface of the substrate, energizing the heater to fuse and fix the pellets, and further apply an alumina-based inorganic heat-resistant paint around the pellet portion where one of the platinum electrodes is provided and atmosphere. Gas was sealed and used as a reference electrode.

The carbon dioxide detecting sensor A thus assembled as shown in FIG. 1 (a) is installed in the chamber 15 of the measuring device shown in FIG. 2, heated to a predetermined temperature by the heater 5, and then the flow rate is increased. Air, oxygen, and carbon dioxide gas were mixed so as to have a predetermined gas concentration through a total of 11, 12, 13 and passed through a water tank 14 to be adjusted to a predetermined humidity and then supplied to the chamber 15. Thus, the electromotive force between the detection electrode 3 and the reference electrode 4 was measured by the voltmeter 16. In addition, 17 is a water tank for shutting off outside air, and 18 is an exhaust port.

Thus, the element temperature is set to 550 ° C. and the concentration of carbon dioxide gas in humid air and dry air is set to 200 to 500.
The detection output characteristics for 0PPM were measured and the results are shown in Fig. 5.
It was shown to. From this result, it is understood that the carbon dioxide detecting sensor A of the present invention has the same detection output characteristic as that in the dry air even in the air having the humidity of 50% and is hardly affected by the humidity.

(First Comparative Example) Ionic conductor pellets were prepared in the same manner as in the first example except that the molar ratio of barium carbonate and sodium carbonate was set to 1: 1 instead of 1.7: 1. did. FIG. 16 shows the result of crystal structure analysis by X-ray diffraction of this pellet in the same manner as in Example 1. The X-ray diffraction chart shows peaks due to sodium carbonate crystals.

A carbon dioxide gas detection sensor a assembled using this ionic conductor pellet in exactly the same manner as in the first embodiment.
The measurement output characteristics of carbon dioxide gas were measured in humid air and dry air in the same manner as in Example 1. The results are shown in FIG. 6, and it can be seen that the detection output characteristics change depending on the humidity in the air.

(Second Example) Barium carbonate and lithium carbonate were mixed at a molar ratio of 2.5: 1 to prepare an ion conductor pellet having the same shape as in the first example. The results of the crystal structure analysis of this pellet by X-ray diffraction are shown in FIG.
When compared with the X-ray diffraction chart of barium carbonate shown in FIG. 25 and the X-ray diffraction chart of lithium carbonate shown in FIG. 29, no peak due to the crystal of lithium carbonate is observed and the structure is basically close to that of barium carbonate. It can be seen that the solid solution has.

This ionic conductor pellet was used, and a platinum-based conductive heat-resistant paint was applied to the periphery of the sensing electrode portion.
In the same manner as in Example 1 except that it is configured as in (b) of
A carbon dioxide detection sensor B was assembled.

The detection output characteristics of this carbon dioxide detection sensor B were measured in the same manner as in Example 1, and the results are shown in FIG. Looking at this result, the carbon dioxide detection sensor B of the present invention
It can be seen that has the same detection output characteristic as that in dry air even when the air has a humidity of 50% and is hardly affected by humidity.

Second Comparative Example An ionic conductor pellet was prepared in the same manner as in the second example except that the molar ratio of barium carbonate and lithium carbonate was 2.5: 1 instead of 2.5: 1. did. FIG. 18 shows the result of analyzing the crystal structure of this pellet by X-ray diffraction in the same manner as in Example 1. The X-ray diffraction chart shows peaks due to lithium carbonate crystals.

A carbon dioxide detecting sensor b assembled using this ionic conductor pellet in exactly the same manner as in the second embodiment.
The measurement output characteristics of carbon dioxide gas were measured in humid air and dry air in the same manner as in Example 1. The results are shown in FIG. 8, and it can be seen that the detection output characteristic changes depending on the humidity in the air.

(Third Example) Barium carbonate and potassium carbonate were mixed at a molar ratio of 3: 1 to prepare an ion conductor pellet having the same shape as in the first example. The results of the crystal structure analysis of this pellet by X-ray diffraction are shown in FIG.
When compared with the X-ray diffraction chart of barium carbonate of 5 and the X-ray diffraction chart of potassium carbonate of FIG. 30, a peak due to crystals of potassium carbonate is not observed, and a structure basically close to the crystal structure of barium carbonate is obtained. It can be seen that the solid solution has.

This ionic conductor pellet was used, and a platinum-based conductive heat-resistant paint was applied to the periphery of the sensing electrode portion.
In the same manner as in Example 1 except that the configuration (c) of FIG.
A carbon dioxide detection sensor C was assembled.

The detection output characteristics of this carbon dioxide detection sensor C were measured in the same manner as in Example 1, and the results are shown in FIG. Looking at these results, the carbon dioxide gas detection sensor C of the present invention
It can be seen that has the same detection output characteristic as that in dry air even when the air has a humidity of 50% and is hardly affected by humidity.

(Third Comparative Example) Except for using barium carbonate and potassium carbonate in a molar ratio of 1: 1 instead of 3: 1, the same procedure as in the third example was carried out.
An ionic conductor pellet was created. The results of crystal structure analysis of this pellet by X-ray diffraction in the same manner as in Example 1 are shown in FIG. 20. In this X-ray diffraction chart, peaks due to crystals of potassium carbonate are seen.

A carbon dioxide gas detection sensor c assembled using this ionic conductor pellet in exactly the same manner as in the third embodiment.
The measurement output characteristics of carbon dioxide gas were measured in humid air and dry air in the same manner as in Example 1. The results are shown in FIG. 10, and it can be seen that the detection output characteristics change depending on the humidity in the air.

Fourth Example Strontium carbonate and sodium carbonate in a molar ratio of 2: 1
Were mixed so as to obtain an ionic conductor pellet having a shape similar to that of the first embodiment. The results of the crystal structure analysis by X-ray diffraction of the pellets are shown in FIG. 21. As a result of comparison with the X-ray diffraction chart of strontium carbonate in FIG. 26 and the X-ray diffraction chart of sodium carbonate in FIG. No peak due to sodium crystals was observed, and it can be seen that the solid solution basically has a structure close to that of strontium carbonate.

A carbon dioxide detection sensor D having the same structure as in Example 1 except that this ionic conductor pellet was used was assembled, and its detection output characteristics were measured in the same manner as in Example 1, and the results are shown in FIG. It was shown to. From this result, it can be seen that the carbon dioxide detection sensor D of the present invention has the same detection output characteristic as that in dry air even in the air having a humidity of 50% and is hardly affected by the humidity.

Fourth Comparative Example Strontium carbonate and sodium carbonate in a molar ratio of 2: 1
Ion conductor pellets were prepared in exactly the same manner as in the fourth example except that the ratio was changed to 1.5: 1. FIG. 22 shows the result of crystal structure analysis by X-ray diffraction of this pellet in the same manner as in Example 1. The X-ray diffraction chart shows peaks due to sodium carbonate crystals.

A carbon dioxide detecting sensor d assembled using this ionic conductor pellet in exactly the same manner as in the fourth embodiment.
The measurement output characteristics of carbon dioxide gas were measured in humid air and dry air in the same manner as in Example 1. The results are shown in FIG. 12, and it can be seen that the detection output characteristics change depending on the humidity in the air.

Fifth Example Calcium carbonate and lithium carbonate were mixed at a molar ratio of 3: 1 to prepare an ion conductor pellet having the same shape as that of the first example. The results of crystal structure analysis by X-ray diffraction of the pellets are shown in FIG. 23. When comparing the X-ray diffraction chart of calcium carbonate in FIG. 27 and the X-ray diffraction chart of lithium carbonate in FIG. No peak due to the crystal of lithium was observed, and it is understood that the solid solution has a structure basically similar to that of calcium carbonate.

A carbon dioxide gas detection sensor E having the same structure as in Example 1 except that this ionic conductor pellet was used was assembled, and its detection output characteristics were measured in the same manner as in Example 1, and the results are shown in FIG. It was shown to. From this result, it is understood that the carbon dioxide detection sensor E of the present invention has the same detection output characteristic as that in dry air even in the air having the humidity of 50% and is hardly affected by the humidity.

Fifth Comparative Example Ionic conductor pellets were prepared in the same manner as in the fifth example except that the molar ratio of calcium carbonate to lithium carbonate was 2: 1 instead of 3: 1. FIG. 24 shows the result of analyzing the crystal structure of this pellet by X-ray diffraction in the same manner as in Example 1. The X-ray diffraction chart shows peaks due to lithium carbonate crystals.

A carbon dioxide gas detection sensor e assembled using the ion conductor pellets in exactly the same manner as in the fifth embodiment.
The measurement output characteristics of carbon dioxide gas were measured in humid air and dry air in the same manner as in Example 1. The results are shown in FIG. 14, and it can be seen that the detection output characteristics change depending on the humidity in the air.

[0036]

As described above, the sensor for detecting carbon dioxide of the present invention is a solid solution of an alkaline earth metal carbonate and an alkali metal carbonate as an ionic conductor, and does not contain a crystal of the alkali metal carbonate. In addition to the feature that the detection output characteristics are not affected by humidity, it can detect with high accuracy at a relatively low temperature, and it can efficiently heat the element, which reduces power consumption. It has the advantages of being less and therefore having a longer service life, and of being simple in construction and easy to manufacture.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a carbon dioxide detection sensor of the present invention, in which (a), (b), and (c) show different configurations.

FIG. 2 is a configuration diagram of a detection output characteristic measuring device of a carbon dioxide detection sensor.

FIG. 3 is a configuration diagram of an example X of a conventional carbon dioxide gas sensor.

FIG. 4 is an electromotive force characteristic diagram of Example X of a conventional carbon dioxide gas sensor.

FIG. 5 is a detection output characteristic diagram of the carbon dioxide detection sensor A of the present invention.

FIG. 6 is a detection output characteristic diagram of a carbon dioxide detection sensor a of a comparative example.

FIG. 7 is a detection output characteristic diagram of carbon dioxide detection sensor B of the present invention.

FIG. 8 is a detection output characteristic diagram of a carbon dioxide detection sensor b of a comparative example.

FIG. 9 is a detection output characteristic diagram of the carbon dioxide detection sensor C of the present invention.

FIG. 10 is a detection output characteristic diagram of a carbon dioxide detection sensor c of a comparative example.

FIG. 11 is a detection output characteristic diagram of the carbon dioxide detection sensor D of the present invention.

FIG. 12 is a detection output characteristic diagram of a carbon dioxide detection sensor d of a comparative example.

FIG. 13 is a detection output characteristic diagram of the carbon dioxide detection sensor E of the present invention.

FIG. 14 is a detection output characteristic diagram of a carbon dioxide detection sensor e of a comparative example.

FIG. 15 is an X-ray diffraction chart of an ion conductor used in the carbon dioxide detection sensor A of the present invention.

FIG. 16 is an X-ray diffraction chart of an ion conductor used in the carbon dioxide detection sensor a of Comparative Example.

FIG. 17 is an X-ray diffraction chart of an ionic conductor used in the carbon dioxide sensor B of the present invention.

FIG. 18 is an X-ray diffraction chart of an ionic conductor used in a carbon dioxide sensor b of a comparative example.

FIG. 19 is an X-ray diffraction chart of an ionic conductor used in the carbon dioxide detection sensor C of the present invention.

FIG. 20 is an X-ray diffraction chart of an ion conductor used in a carbon dioxide detection sensor c of a comparative example.

FIG. 21 is an X-ray diffraction chart of the ion conductor used in the carbon dioxide detection sensor D of the present invention.

FIG. 22 is an X-ray diffraction chart of an ionic conductor used in a carbon dioxide detection sensor d of a comparative example.

FIG. 23 is an X-ray diffraction chart of an ion conductor used in the carbon dioxide detection sensor E of the present invention.

FIG. 24 is an X-ray diffraction chart of an ionic conductor used in a carbon dioxide detection sensor e of a comparative example.

FIG. 25 is an X-ray diffraction chart of barium carbonate.

FIG. 26 is an X-ray diffraction chart of strontium carbonate.

FIG. 27 is an X-ray diffraction chart of calcium carbonate.

FIG. 28 is an X-ray diffraction chart of sodium carbonate.

FIG. 29 is an X-ray diffraction chart of lithium carbonate.

FIG. 30 is an X-ray diffraction chart of potassium carbonate.

[Explanation of symbols]

 1 Detecting Element 2 Ion Conductor Pellet 3 Detecting Electrode 4 Reference Electrode 5 Heater 6 Ceramic Substrate 7 Inorganic Sealing Material 8 Pedestal 9 Pin 10 Leader Wire 11, 12, 13 Flowmeter 14 Water Tank 15 Chamber 16 Voltmeter 17 Water Tank 18 Exhaust Port

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Norio Miura 3-17-5-301 Hirao, Chuo-ku, Fukuoka-shi, Fukuoka

Claims (3)

[Claims] 1. A gas sensor which has two electrodes arranged opposite to each other with an ionic conductor sandwiched therebetween and which outputs an electric signal corresponding to a difference in concentration of gas contacting the respective electrodes, wherein the ionic conductor is an alkaline earth metal. A carbon dioxide gas detection sensor, which is a solid solution of a carbonate and an alkali metal carbonate and does not contain crystals of the alkali metal carbonate. 2. The method according to claim 1, wherein the alkaline earth metal carbonate is calcium,
The carbon dioxide detection sensor according to claim 1, wherein the carbon dioxide detection sensor is one of strontium and barium carbonate. 3. The carbon dioxide detection sensor according to claim 1, wherein the alkali metal carbonate is at least one of lithium, sodium and potassium carbonates.