JPH09264867A - Carbon dioxide sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sensor which is highly reliable along with endurance. SOLUTION: This sensor A has a CO2 sensor element 1 for detecting CO2 and a pair of electrodes 2 laminated on the surface of the CO2 sensor element 1. In this case, the CO2 sensor element 1 is constituted of a mixture of CeO2 , BaCO3 and metal oxide thereby achieving very high detection sensitivity to a low concentration CO2 whereas the hourly degradation of detection sensitivity to the CO2 is remarkably limited. Thus, a highly reliable CO2 sensor A is obtained along with substantial endurance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air pollution monitor in a living space such as a room or a car, a monitor for an air conditioning system, a carbon dioxide concentration controller for transporting and storing fresh food, and a carbon dioxide concentration in a bio facility. The present invention relates to a carbon dioxide sensor used in a monitor system or the like.

[0002]

2. Description of the Related Art In recent years, various systems for measuring and controlling carbon dioxide concentration have been developed. Infrared absorption type, solid electrolyte type, etc. were mentioned as the carbon dioxide gas sensor used for it.

The infrared absorption type is composed of a light emitting element which emits a laser and a light receiving element sensor which receives the laser light emitted from the light emitting element and measures the intensity thereof. The infrared absorption type is capable of precise measurement, but its device is large and the structure of the carbon dioxide sensor is complicated, so that it is inferior in productivity and mass productivity, and was limited to special applications such as analytical instruments. The solid electrolyte type is NASICON or Li-
A laminated structure of electrodes / carbonate / solid electrolyte was formed using Ti—O oxide. The solid electrolyte type has a problem that if it contains water, the detection sensitivity of carbon dioxide gas is lowered and the water resistance is poor. In addition, since the structure is complicated and the material is easily changed and unstable, mass productivity is poor, reliability is low, and life is short. On the other hand, as a solution to the above problems, a mixture of ceramic powder and a non-composite metal oxide was developed as a carbon dioxide gas sensor. This mixture was able to detect the carbon dioxide concentration based on the change in electrostatic capacity, had a chemically stable oxide as a main component, and had a simple capacitor structure. Ba as an example of this mixture
Perovskite type oxides such as TiO ₃ and SrTiO ₃ and C
A mixture with a non-composite type oxide such as uO or NiO is disclosed in Japanese Patent Application Laid-Open No. Hei 4
No. 24548. Further, as another example, an oxide forming a carbonate such as CuO, CaO and MgO has been disclosed in Japanese Patent Application No. 6-313410.

Next, a method of manufacturing a carbon dioxide gas sensor made of a mixture of BaTiO ₃ and CuO will be described. First, Ba
An equimolar mixture of CO ₃ and TiO ₂ was calcined at 1200 ° C. for 12 hours to obtain BaTiO ₃ . CuTiO was mixed with BaTiO ₃ in an equimolar amount, compression-molded into a disk shape, and baked at 500 ° C. for 1 hour. A silver paste was applied to both sides of the burned disc to prepare a carbon dioxide sensor.

[0005]

However, the above-mentioned conventional carbon dioxide gas sensor has the problems that the heater has a large load because the measured temperature is as high as 550 ° C., the heater life is short, and the power consumption is large. It was

The present invention solves the above-mentioned conventional problems, and provides a carbon dioxide gas sensor which is durable and has high reliability while reducing the load on the heater to extend the life by lowering the measurement temperature. With the goal.

[0007]

In order to solve this problem, the present invention is a carbon dioxide gas sensor having a carbon dioxide gas sensor element for detecting the carbon dioxide gas concentration and a pair of electrodes laminated on the surface of the carbon dioxide gas sensor element. The carbon dioxide sensor element is made of a mixture of CeO ₂ , BaCO ₃ and a metal oxide.

According to the present invention, it is possible to provide a carbon dioxide gas sensor which is durable and has high reliability, as well as to reduce the load on the heater and prolong the life by lowering the measurement temperature.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The invention according to claim 1 of the present invention is a carbon dioxide gas sensor having a carbon dioxide gas sensor element for detecting a carbon dioxide gas concentration and at least a pair of electrodes on the carbon dioxide gas sensor element, The carbon dioxide sensor element is composed of a mixture of CeO ₂ , BaCO ₃ and a metal oxide, and even in a low concentration of carbon dioxide, the capacitance changes greatly due to the variation of carbon dioxide concentration and the carbon dioxide sensor is used for a long time. However, there is little deterioration with time, and 500
It has the effect that it can be used at a temperature of not higher than ° C.

According to a second aspect of the present invention, in the first aspect, the metal oxide is FeO, Fe ₂ O ₃ or Pb.
The configuration is such that it is at least one member of the group consisting of O, PbO ₂ , and Pb ₂ O ₃ , and the capacitance value is large at the same time even when the carbon dioxide concentration is low, and its fluctuation is large. Can be maintained across and 50
It has the effect that it can be used at 0 ° C or lower.

According to a third aspect of the present invention, in the first or second aspect, the carbon dioxide gas concentration measuring temperature of the carbon dioxide gas sensor is 400 to 500 ° C. It has an effect that both the stability of the fluctuation rate of the electrostatic capacity can be achieved and an accurate measured value of carbon dioxide concentration can be obtained.

An embodiment of the present invention will be described below with reference to FIGS. 1 to 3. (Embodiment) FIG. 1 is a perspective view of a main part of a carbon dioxide sensor according to an embodiment of the present invention. In FIG. 1, A is a carbon dioxide sensor. The carbon dioxide gas sensor element 1 acts to detect the carbon dioxide gas concentration based on the change in the capacitance value, and a mixture of CeO ₂ , BaCO ₃ , and a metal oxide is molded into a disc shape. The electrode 2 acts to apply a voltage to the carbon dioxide gas sensor element 1, and is made of Pt, RuO ₂ , Ag, Au or the like laminated on the upper and lower surfaces of the carbon dioxide gas sensor element 1, respectively. The lead wire 3 has a function of connecting the electrode 2 and a power source (not shown), and is made of a Cu wire, a Pt wire, or the like.

The carbon dioxide sensor A having the above structure
The manufacturing method will be described below.

First, CeO ₂ powder, BaCO ₃ powder and metal oxide powder are uniformly mixed using a dispersant such as ethanol. After this mixture is fired at 500 to 800 ° C. (carbon dioxide gas sensor element firing step), it is pulverized again to be sized. The carbon dioxide sensor element 1 is formed by press-molding this powder into a disk shape, a square shape, a rectangular shape, or the like. Pt paste is applied to the upper and lower surfaces of the carbon dioxide sensor element 1, and the lead wire 3 is attached to the paste and fired to form the electrode 2 and the lead wire 3 on the carbon dioxide sensor element 1 to form the carbon dioxide sensor A.

As described above, according to the present embodiment, Ce
Since the carbon dioxide gas sensor element 1 which is a mixture of O ₂ , BaCO ₃ and metal oxide is provided, the carbon dioxide gas detection sensitivity can be improved with respect to the low carbon dioxide concentration even at the operating temperature of 400 to 500 ° C. It has little deterioration over time, has excellent durability, and can reduce power consumption.

[0016]

Next, specific examples of the present invention will be described.

(Examples 1 to 20, Comparative Examples 1 to 3) First,
CeO ₂ powder, BaCO ₃ powder and Fe ₂ manufactured by Wako Pure Chemical Industries
O ₃ powder or PbO ₂ powder was prepared at the ratios shown in (Table 1) to (Table 2).

[0018]

[Table 1]

[0019]

[Table 2]

Ethanol as a dispersant was mixed with this powder, and the mixture was uniformly mixed with a ball mill for 12 hours. The mixture was calcined at 800 ° C. for 5 hours. After crushing and sizing this mixture, 30kg of this powder was pressed using a press machine.
Pressurized at a pressure of / cm ² , diameter 10 mm, thickness 0.4 mm
The disk-shaped carbon dioxide gas sensor element was molded. Pt paste was applied to the upper and lower surfaces of this carbon dioxide sensor, and lead wires were attached to the paste and heated at 800 ° C. for 10 minutes to form electrodes and lead wires on the carbon dioxide sensor element.

(Comparative Examples 4 and 5) BaC manufactured by Wako Pure Chemical Industries, Ltd.
O ₃ and TiO ₂ manufactured by Nippon Aerosil are mixed at an equimolar ratio,
BaTi was fired in the air at 1200 ° C for 12 hours.
O ₃ powder was obtained. 4 of the obtained BaTiO ₃ and CuO powder manufactured by Wako Pure Chemical Industries and BaCO ₃ powder manufactured by Wako Pure Chemical Industries
The mixture was mixed at a molar ratio of 9: 50: 1 and baked at 800 ° C. for 5 hours. The following steps were carried out in the same manner as in the example, and the carbon dioxide gas sensors of Comparative Examples 4 and 5 were manufactured as shown in (Table 3).

[0022]

[Table 3]

Next, the capacitances of Examples 1 to 20 and Comparative Examples 1 to 5 with respect to the carbon dioxide concentration were measured. First,
Set the carbon dioxide sensor to a constant temperature (400 ° C, 450 ° C, 5
The temperature was maintained at 00 ° C and 550 ° C). Carbon dioxide concentration is 350ppm by mixing dry air and carbon dioxide
(Air only) and 2% were prepared. The capacitance value is LCR
50KH using a meter (made by Hewlett-Packard)
An alternating voltage of z and 0.5 V was applied to the carbon dioxide sensor and measured. When the capacitance value in air Cair, a capacitance value of 2% carbon dioxide gas atmosphere and C2% CO _2, the sensitivity | 10
_{× LOG (C2% CO 2 /} Cair) | ( in dB) and defined. The sensitivity at the start of measurement was S ₀ , and the sensitivity 300 days after the start of measurement was S ₃₀₀ .

The phase angles of Examples 1 to 20 and Comparative Examples 1 to 5,
The results of S ₀ and S ₃₀₀ are shown in (Table 1) to (Table 3). As is clear from (Table 1) to (Table 3), in this example, it was found that the sensitivity was 4 dB or higher at the measurement temperature of 450 ° C., and was extremely high at 5.3 dB depending on the composition. It was found that Comparative Examples 1 to 3 had a very low sensitivity of 0 to 1.2 dB and a low phase angle of −64 to −76 °. Also, in Comparative Examples 4 and 5, the sensitivity was as high as 7.1 dB at 550 ° C., but 2.3 d at 450 ° C.
It turned out that it would be as small as B. Furthermore, in the present invention, the sensitivity decrease after 300 days from the start of measurement is 0 to 0.
It was as small as 2 dB, and it was found that the deterioration with time was extremely small as compared with Comparative Examples 4 and 5.

The sensitivity changes with time and carbon dioxide concentration in Example 2 and Comparative Example 5 were measured. The measuring method was the same as in Example 1. FIG. 2 shows Example 2 of the present invention and Comparative Example 5
FIG. 3 is a characteristic diagram showing the relationship between the elapsed time and sensitivity, and FIG. 3 is a characteristic diagram showing the relationship between carbon dioxide concentration and sensitivity in Example 2 of the present invention and Comparative Example 5.

As is clear from FIG. 2 and FIG. 3, in Example 2 of the present invention, the sensitivity did not change even after being kept for 50 days, and the amount of change was 1% or less even after being kept for 150 days. It turns out that it has hardly deteriorated. Also, from 350
Sensitivity is 1 in the carbon dioxide concentration range of 10000 ppm
It was found that the change rate was greatly changed to about 4 dB, the detection sensitivity of low-concentration carbon dioxide was high, and the rate of change was also large.

Also, in the long-term use, the measured temperature is 550 ° C. in Comparative Example 5 from the viewpoint of power consumption.
In Example 2, since it was as low as 450 ° C., it was found that the load on the heater can be reduced and power saving can be achieved.

[0028]

As described above, according to the carbon dioxide sensor of the present invention, the capacitance value is large at the measurement temperature of 400 to 500 ° C., and the capacitance value changes with the carbon dioxide concentration change at a low carbon dioxide concentration. Since the rate is high, the carbon dioxide concentration can be accurately detected, and the reliability is improved. In addition, since the capacitance value does not fluctuate and the sensitivity does not deteriorate even after long-term use, reproducibility is excellent, deterioration with time is small, and durability is significantly improved.

Further, even at 400 ° C., since the rate of change of the capacitance value with respect to the change of carbon dioxide concentration is large, the power consumption is reduced and the life of the heater is also improved.

Further, according to the method for producing a carbon dioxide gas sensor of the present invention, a carbon dioxide gas sensor having a significantly high carbon dioxide detection sensitivity in a low carbon dioxide concentration range, little change over time, excellent durability, and low power consumption is produced with good productivity. Further, it can be mass-produced with a high manufacturing yield.

[Brief description of drawings]

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

FIG. 2 is a characteristic diagram showing a relationship between elapsed time and sensitivity in Example 2 of the present invention and Comparative Example 5.

FIG. 3 is a characteristic diagram showing the relationship between carbon dioxide concentration and sensitivity in Example 2 of the present invention and Comparative Example 5.

[Explanation of symbols]

 A Carbon dioxide sensor 1 Carbon dioxide sensor element 2 Electrode 3 Lead wire

Front page continuation (72) Inventor Shinichiro Kaneko 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (3)

[Claims] 1. A carbon dioxide gas sensor having a carbon dioxide gas sensor element for detecting a carbon dioxide gas concentration and at least a pair of electrodes on the carbon dioxide gas sensor element, wherein the carbon dioxide gas sensor element is CeO ₂ , BaCO ₃ and a metal oxide. A carbon dioxide gas sensor characterized by comprising a mixture of 2. The metal oxide is FeO, Fe_TwoO_Three, Pb
O, PbO_Two, Pb_TwoO _ThreeAt least one member of the group
The carbon dioxide sensor according to claim 1, wherein the carbon dioxide sensor is provided. 3. The carbon dioxide gas sensor according to claim 1, wherein the carbon dioxide gas concentration measurement temperature of the carbon dioxide gas sensor is 400 to 500 ° C.