JPH0915179A - Carbon dioxide gas sensor - Google Patents

Abstract

PURPOSE: To enhance the power for sensing CO2 and to improve long-term reliability by making a burned body contain one or more kinds of oxides selected from alkaline-earth metals, Fe, lanthanoide-series elements, Pb, etc., and one or more kinds of carbonates of the alkaline-earth metals. CONSTITUTION: A CO2 sensor is constructed by a method wherein a cylindrical burned disk 1 having a diameter of about 10mm and a thickness of about 0.2-0.6mm, for instance, is made to contain a composition of BaCO3 and CuO, a metal electrode 2 of Ag or the like is fitted to the disk 1, Pt is fixed on the surface of the electrode 2 and thereby a lead wire 3 is formed. According to this construction, the BaCO3 and the CuO being noncomposite-series metal oxides cause a reversible carbonate producing reaction, the electrical characteristics of the surface of the BaCO3 and the CuO and the interfaces thereof with other materials are changed thereby and the electrical characteristics of the whole are changed in conformity therewith. Therefore CO2 concentration can be measured with excellent accuracy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention measures the concentration of carbon dioxide gas,
The present invention relates to a carbon dioxide gas sensor used in facilities requiring control such as environmental hygiene, horticulture, disaster prevention, and industrial facilities, and particularly relates to a carbon dioxide gas sensor that detects a carbon dioxide concentration by changing electrical resistance, capacitance, etc. .

[0002]

2. Description of the Related Art In recent years, carbon dioxide gas sensors for detecting carbon dioxide concentration have been used in environmental hygiene, horticulture, production facilities and the like. As the carbon dioxide sensor, a detection method using infrared rays, a method using an electrolytic solution, a method utilizing heat conduction, N
a method using an alkali ion conductive solid electrolyte such as NASICON with aCO ₃ as an electrode, CO ₃ ² such as K ₂ CO ₃
A method using an ionic conductor, a method using a Li ionic conductor, a method using a resistance value of hydroxyapatite, and the like are used. In particular, a carbon dioxide gas sensor that detects the carbon dioxide concentration by changing the electrical characteristics by a reversible carbonate formation reaction between a non-composite oxide and carbon dioxide has been attracting attention because the device can be downsized. For example, JP-A-4-
Japanese Patent No. 24548 discloses "a carbon dioxide gas sensor using a change in capacitance of a mixture of a perovskite type metal oxide and a non-composite type metal oxide which is an oxide composed of one kind of metal." Japanese Patent Application No. 6-182261 discloses "a carbon dioxide gas sensor using a mixture of non-composite metal oxides." Further, in order to improve the detection ability of these carbon dioxide sensors, it is effective to add a noble metal element such as Ag and a transition metal element in JP-A-5-142180 and BaO in JP-A-6-313410. It is disclosed that there is.

[0003]

However, the above-mentioned conventional carbon dioxide gas sensor has a problem that it cannot be used for highly accurate measurement of carbon dioxide gas concentration because of its insufficient detection sensitivity. Further, the additive such as Ag or the additive including BaO has a problem that it cannot be practically used as a sensor because it has sufficient sensitivity but deteriorates in characteristics rapidly and lacks in durability.

The present invention solves the above-mentioned conventional problems, and has a high ability to detect carbon dioxide in practical use, and
It is an object of the present invention to provide a carbon dioxide sensor having excellent long-term reliability.

[0005]

To achieve this object, a carbon dioxide gas sensor according to claim 1 of the present invention comprises a fired body, and one or more pairs of electrodes on the surface or inside of the fired body.
A carbon dioxide gas sensor having a terminal capable of sending and receiving a signal to and from an electrode, wherein the burning disk is an alkaline earth metal, Fe, Co, Ni, Cu, Zn, Y, Zr, and a lanthanoid element having an atomic number of 57 to 71. , Pb, Bi, one or more oxides, and one or more alkaline earth metal carbonates.

A carbon dioxide gas sensor according to a second aspect of the present invention has the structure according to the first aspect, wherein the fired body is a composition containing a perovskite type metal oxide.

A carbon dioxide sensor according to a third aspect of the present invention is the carbon dioxide sensor according to the first and second aspects, wherein the fired body is Al ₂ O ₃ or SiO.
_It has a composition that is a composition containing ₂ .

A carbon dioxide gas sensor according to a fourth aspect of the present invention is the carbon dioxide gas sensor according to any one of the first to third aspects, wherein the fired body is copper oxide as an oxide and any one of selenium oxide, zinc oxide, magnesium oxide and nickel oxide. The above is a composition containing the above.

A carbon dioxide gas sensor according to a fifth aspect of the present invention has the structure according to the first to fourth aspects, wherein the fired body is a composition containing barium carbonate as a carbonate.

[0010]

With this structure, the non-composite metal oxide can react with carbon dioxide to cause a reversible carbonate formation reaction. Due to this generation reaction, the electrical characteristics of the surface of the metal oxide and the interface with other materials change, and the overall electrical characteristics change accordingly, so that the carbon dioxide concentration can be measured with high accuracy.

Oxygen is required to detect carbon dioxide,
By mixing the carbonate with the fired body, the amount of oxygen vacancies in the metal oxide is changed and the sensitivity to carbon dioxide can be increased. Therefore, the electrical characteristics of the fired body largely change, and the detection sensitivity to carbon dioxide can be increased.

Al ₂ which does not participate in the detection of carbon dioxide
The impedance value can be optimized by adding the O ₃ and SiO ₂ materials, the rate of change of the electrical value due to the change of the carbon dioxide concentration can be improved, and the detection capability of carbon dioxide can be improved.

By these means, it is possible to obtain a small-sized, highly sensitive carbon dioxide gas sensor having a long life.

[0014]

【Example】

(Embodiment 1) An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a perspective view of a carbon dioxide sensor according to the first embodiment of the present invention. Reference numeral 1 is a cylindrical firing disk (diameter 10 mm, thickness 0.2 to 0.6 mm) made of a composition of BaCO ₃ and CuO, 2 is an electrode made of Ag laminated on both sides of the firing disk 1, and 3 is an electrode 2 is a lead wire made of Pt bonded to 2. The electrode 2 is Au, P
It may be formed of t, RuO, or the like. Since these noble metals and noble metal oxides have various catalytic actions, they may be contained in the disc in a trace amount as a sensitizer. The lead wire 3 is a Pt wire, but may be any wire having a low resistance that can withstand an operating temperature, and may be an Au wire, a nickel wire, a stainless wire, or the like.

The method of manufacturing the carbon dioxide sensor having the above structure will be described below. BaCO ₃ with an average particle size of 1 μm and CuO with an average particle size of 3 μm were mixed at a molar ratio of 70:30, mixed and crushed in a mortar, and then molded into a disk shape, and 400 ° C. or higher in an electric furnace 800
It was fired at a temperature of ℃ or less for 5 hours. When the firing temperature is high, the raw materials react with each other to cause excessive sintering, which lowers the sensitivity. Therefore, it is preferable to perform the sintering at a low temperature at which the sample can be carried.

The blending amount of BaCO ₃ is preferably 50 to 80 mol%, particularly preferably 65 to 75 mol% based on the total number of mols. When the BaCO ₃ content is less than 50 mol%, the impedance tends to be small and the sensitivity to carbon dioxide gas tends not to appear, which is not preferable. When BaCO ₃ exceeds 80 mol%, the impedance tends to increase and the sensitivity tends not to appear, which is not preferable.

An Ag electrode 2 was attached to this firing disk 1,
The lead wire 3 is formed by fixing the Pt wire on the surface of the electrode 2.
A carbon dioxide sensor was completed.

Here, as the copper oxide, Cu ₂ O and C are used.
Although uO, Cu ₂ O _{3 and the} like are known, CuO is preferable because it tends to become CuO when heated in air.

Next, the sensitivity test of the carbon dioxide gas sensor in the first embodiment will be described. The carbon dioxide sensor in the first embodiment is installed in the ventilation pipe, heated by a heater, and YH
Sensitivity was measured by a two-terminal method using an impedance analyzer 4192A manufactured by P. As the sample gas, dry air and carbon dioxide gas (concentration 2 vol%) diluted with dry air were used. The measurement frequency is 50 KHz, and the signal is 0.5 mV.
It was found that the maximum value was obtained when the resistance value was 0 ° or more and the resistance value was about 50 kΩ to 0.1 kΩ. It was also found that the sensitivity can be adjusted by the sample thickness, the firing temperature and the composition ratio.

The state of detection of carbon dioxide gas is based on the capacity value (C ₀ ) of the sample heated to the measurement temperature, which was measured in a state in which air was introduced, and the air adjusted to a carbon dioxide concentration of 2% was introduced. It was grasped by the change of the capacitance value (C _2% ) at the time of measurement. The sensitivity was defined by the following formula.

Sensitivity = 10 * Log (C _2% / C ₀ ) [dB] When the sensitivity is 0, it means that the capacitance value does not change with the increase of the carbon dioxide concentration, and the carbon dioxide is detected. There will be no. When the capacity value after the inflow of carbon dioxide decreases, the sensitivity becomes a negative value. Sensitivity is also represented by a change in impedance value, but the sign of sensitivity is opposite to that in the case of capacitance value.

Similarly, the sensitivity of the mixture prepared from carbonate and various metal oxides to carbon dioxide was measured by the capacitance value. The results are shown in (Table 1) as "sensitivity A (dB)".

[0024]

[Table 1]

Further, the impedance value is changed in the same manner as the change of the capacitance value, and the impedance value may be used for detecting carbon dioxide gas.

As is clear from Nos. 4 to 17 in Table 1 above, the addition of carbonate increases the absolute value of sensitivity and improves the sensitivity. Further, as the alkaline earth metal _{carbonate, BaCO 3, CaCO 3, MgCO} 3, SrC
Although O ₃ is used, addition of BaCO ₃ is particularly effective, as is clear from Nos. 18 to 21 in (Table 1). Addition of BaCO ₃ does not show sensitivity by itself C
uO became sensitive to carbon dioxide.

Example 2 BaC having an average particle size of 1 μm
O ₃ , CuO with an average particle size of 3 μm and an average particle size of 0.4 μm
Mixing BaTiO ₃ in a molar ratio to the ratio of 2:50:50, and calcined 5 hours at 800 ° C. in an electric furnace after molded into a disk. An Ag electrode 2 was attached to this element to form the carbon dioxide gas sensor shown in FIG. Air adjusted to have a carbon dioxide gas concentration of 2% was periodically passed through this element, and the carbon dioxide detection sensitivity was measured in the same manner as in Example 1. Further, here, the sensitivity after 30 days was similarly measured in order to see the change with time of the carbon dioxide detection sensitivity. The results are shown in (Table 1) as "sensitivity B (dB)".

As is clear from this (Table 1), No1
From a system containing 8 to 28 perovskite-type metal oxide,
The sensitivity of the samples of the non-composite oxides containing no perovskite type metal oxides of Nos. 1 to 17 is smaller. Further, it is more effective to obtain a gas sensor having a long life and a small change in sensitivity of a sample containing CuO, CeO ₂ , ZnO, MgO, and NiO as components.

Example 3 BaC having an average particle size of 0.7 μm
O ₃ and CuO having an average particle size of 0.7 μm were mixed in equimolar amounts, formed into a disk shape, and then baked in an electric furnace at 800 ° C. for 5 hours. When an Ag electrode 2 was attached to this element and the carbon dioxide sensor shown in FIG. 1 was used for measurement, the impedance was small and no sensitivity to carbon dioxide was observed.
Therefore, in order to increase the impedance, BaCO ₃ and CuO are added with SiO ₂ having an average particle size of 100 μm to form BaC 3.
The mixed sample was molded so that the molar ratio of O ₃ : CuO: SiO ₂ was 40:40:20, and the temperature was 500 ° C. or higher in an electric furnace.
Baking was performed at 00 ° C. or lower for 5 hours. The addition of SiO ₂ is 1 to 30 mol%, particularly 5 to 25 mol% based on the total number of mols.
Mol% is preferably used. As SiO ₂ is less than 1, unfavorably tend to effect the SiO ₂ is eliminated. As SiO ₂ exceeds 30 mol%, the impedance tends to increase and the sensitivity tends not to appear, which is not preferable. The same applies to Al ₂ O ₃ .

An Ag electrode was attached to the obtained sample, and the carbon dioxide sensor shown in FIG. 1 was measured for sensitivity to carbon dioxide in the same manner as in Example 1.

The sensitivity of various materials prepared in the same manner is shown in (Table 2).

[0032]

[Table 2]

As is clear from this (Table 2), Cu
O, SiO ₂ alone, or CuO mixed with SiO ₂ does not provide sensitivity to carbon dioxide gas, but compositions with low resistance values of BaCO ₃ and CuO that cannot be subjected to sensitivity measurement have SiO ₂ or Al ₂ O ₃ When is mixed and the impedance is adjusted, sensitivity to carbon dioxide appears. From this, it is understood that the impedance value has an optimum value, and the sensitivity to carbon dioxide can be obtained by adjusting the resistance and the phase angle. That is, by mixing SiO ₂ or Al ₂ O ₃ to optimize the impedance value, good sensitivity can be obtained even for a composition having low sensitivity to carbon dioxide gas.

Further, SiO ₂ is an indispensable material when forming a paste, and this makes it possible to form a film on an alumina substrate and use it as a carbon dioxide sensor. Al ₂ O ₃ is a material that is often used for ceramics because it has a large strength and is unlikely to deteriorate. By mixing Al ₂ O ₃ , the range of applications is widened and the stability of the characteristics is very effective.

[0035]

As described above, according to the present invention, a carbonate is added to a conventional non-composite metal oxide-based carbon dioxide gas sensor, and in some cases, Al ₂ O ₃ , S is supplementarily added for impedance adjustment.
Carbon dioxide is detected by adding iO ₂ , and an excellent carbon dioxide sensor with higher sensitivity and high reliability for a long period of time is realized.

Further, carbon dioxide gas can be detected by utilizing changes in impedance and capacitance value, and can be handled easily without requiring a standard atmosphere. The structure is simple and can be miniaturized, and it realizes an excellent carbon dioxide sensor with high practicality that can be used for indoor air conditioning management by ventilation fans and other ventilation equipment and facilities, plant growth, storage, transportation, etc.

[Brief description of the drawings]

FIG. 1 is a perspective view of a carbon dioxide sensor according to a first embodiment of the present invention.

[Explanation of symbols]

 1 Firing disc 2 Electrode 3 Lead wire

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinji Morimoto 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Shinichiro Kaneko 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (5)

[Claims] 1. A carbon dioxide gas sensor having a calcined body and one or more pairs of electrodes on or in the surface of the calcined body, the electrodes being connected to terminals capable of sending and receiving signals, wherein the calcined body is an alkali. Earth metal, Fe, Co, Ni, Cu, Z
n, Y, Zr, a lanthanoid element having an atomic number of 57 to 71, one or more kinds of oxides selected from Pb and Bi, and one or more kinds of carbonates of alkaline earth metals are contained. Carbon dioxide sensor. 2. The carbon dioxide sensor according to claim 1, wherein the fired body contains a perovskite type metal oxide. 3. The carbon dioxide gas sensor according to claim 1, wherein the fired body contains at least one of Al ₂ O ₃ and SiO ₂ . 4. The oxide is copper oxide and any one of selenium oxide, zinc oxide, magnesium oxide, and nickel oxide.
And at least one species.
The carbon dioxide sensor according to any one of 1. 5. The carbon dioxide gas sensor according to claim 1, wherein the alkaline earth metal carbonate is barium carbonate.