JPH09159640A - Carbon dioxide gas sensor and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a carbon dioxide gas sensor in which detection sensitivity at low concentration is enhanced while suppressing deterioration after long time use by adding a mixture of CeO2 and BaCO3 to a carbon dioxide gas sensor element. SOLUTION: CeO2 powder, BaCO3 powder, and CuO powder are mixed uniformly using a dispersant of ethanol or the like and fired at 500-950 deg.C. It is then admixed again with ethanol and crushed and the grain size is arranged after ethanol is dried and removed. The powder is then pressed into disk-like, square or rectangular shape thus forming a carbon dioxide gas sensor element 1. The element 1 is coated, on the upper and lower surfaces thereof, with Pt paste to form an electrode 2 which is then fixed with a lead wire 3 and fired thus constituting a carbon dioxide gas sensor A. Since the element 1 of sensor has high carbon dioxide gas detection sensitivity even at low concentration, concentration of carbon dioxide gas can be detected accurately. Furthermore, deterioration is suppressed after long time use and durability is enhanced.

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 gas sensor used for a monitor system and the like and a method for manufacturing the same.

[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 that emits a laser and a light receiving element sensor that 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-Ti.
A laminated structure of electrodes / carbonate / solid electrolyte was formed using —O oxide or the like. 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 concentration of carbon dioxide gas by the change in electrostatic capacity, had a chemically stable oxide as a main component, and had a simple capacitor structure. As an example of this mixture B
mixture of the unconjugated based oxide ATiO ₃ and perovskite oxides such as SrTiO ₃ and the like CuO and NiO were disclosed in Japanese Patent Laid-Open No. 4-24548. Further, as another example, there is one that uses a carbonate such as CuO, CaO, or MgO.

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 ₃ 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.

[0004]

However, in the above-mentioned conventional carbon dioxide gas sensor, when detecting low-concentration carbon dioxide gas, the detection sensitivity is low and an accurate change amount of carbon dioxide concentration cannot be detected, and the measurement result is inaccurate. It had a problem of poor reliability. Further, if the carbon dioxide concentration is continuously detected, the amount of change in the capacitance of the carbon dioxide sensor decreases, the detection capability of the carbon dioxide concentration decreases, and deterioration over time occurs. It was

The present invention solves the above-mentioned conventional problems. The detection sensitivity in the low concentration range of carbon dioxide is extremely high, and the detection sensitivity of carbon dioxide does not decrease even when used for a long time, and the high detection sensitivity is maintained. An object of the present invention is to provide a carbon dioxide gas sensor which is less deteriorated with time and has excellent durability, and a manufacturing method of a carbon dioxide gas sensor which has high detection sensitivity of low-concentration carbon dioxide gas and is less deteriorated with time and excellent in durability.

[0006]

In order to achieve this object, the present invention comprises a carbon dioxide gas sensor element for detecting a carbon dioxide gas concentration, and a pair of electrodes laminated on the surface of the carbon dioxide gas sensor element. In the gas sensor, the carbon dioxide sensor element has a structure containing a mixture of CeO ₂ and BaCO ₃ .

As a result, it is possible to obtain a carbon dioxide sensor which has a significantly high detection sensitivity when the carbon dioxide concentration is low and has little deterioration with time even when used for a long time.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The invention according to claim 1 of the present invention comprises a carbon dioxide gas sensor element for detecting a 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 configured to contain a mixture of CeO ₂ and BaCO ₃ , and even when the carbon dioxide concentration is low, the capacitance changes greatly due to the variation of the carbon dioxide concentration, and Even if it is kept at the measurement temperature, the fluctuation rate of the capacitance does not change and a large fluctuation rate can be maintained.

According to a second aspect of the present invention, in the first aspect, the carbon dioxide sensor element contains CuO, and the capacitance value is large and the fluctuation is large even at a low carbon dioxide concentration. And, this fluctuation rate can be maintained for a long time.

[0010] The invention described in claim 3 is the invention according to claim 1 or 2.
In either case, the phase angle θ at the operating temperature is -5.
With the constitution of 0 ° ≦ θ ≦ −3 °, the carbon dioxide sensitivity and the stability of the fluctuation rate of the capacitance are compatible with each other, and an accurate carbon dioxide concentration measurement value can be obtained.

[0011] The invention according to claim 4 is the invention according to claims 1 to 3.
In any one of the above, CeO ₂ , BaCO ₃ , and
The mixing molar ratio of CuO is CeO ₂ / BaCO ₃ / CuO =
In X / Y / Z (X + Y + Z = 100), 30 ≦ X
The range of ≦ 80, 3 ≦ Y ≦ 50, and 0 ≦ Z ≦ 45 is adopted, and the fluctuation range of the capacitance is large even in the low concentration range of the carbon dioxide gas concentration of several hundred ppm, and the fluctuation range thereof is also large. Is highly reproducible, so that the CO ₂ concentration can be accurately measured. Further, even if the measurement temperature is kept for a long time, the capacitance and its variation rate are not deteriorated, and high sensitivity can be maintained.

[0012] The invention according to claim 5 provides the invention according to claims 1 to 4.
In any one of the above, the carbon dioxide sensor element is
The composition contains 6 mol% of Ag, and the sensitivity to carbon dioxide concentration can be improved.

[0013] The invention according to claim 6 is the invention according to claims 1 to 5.
In any one of the above, the carbon dioxide sensor element is 500
It has a structure formed by baking at ˜950 ° C., and it is possible to measure a stable and reproducible electrostatic capacity and to enhance the detection sensitivity to carbon dioxide gas.

The invention according to claim 7 is 500 to 950.
A carbon dioxide gas sensor having a high detection sensitivity to carbon dioxide gas, excellent reproducibility, and stable capacitance can be obtained by using a configuration including a carbon dioxide gas sensor element firing step of heating and molding in a temperature range of ° C. .

The embodiment of the present invention will be described below with reference to FIG.
This will be described with reference to FIGS. (Embodiment 1) FIG. 1 is a perspective view of a main part of a carbon dioxide sensor according to a first embodiment. In FIG. 1, A is the first
3 is a carbon dioxide sensor according to the embodiment. The carbon dioxide gas sensor element 1 acts to detect the carbon dioxide gas concentration based on the variation of the electrostatic capacitance value, and a mixture of CeO ₂ , BaCO ₃ and, if necessary, CuO is formed into a disk shape. The electrode 2 acts to apply a voltage to the carbon dioxide sensor element 1 during use, and is made of Pt, Ag, Au or the like laminated on the upper and lower surfaces of the carbon dioxide sensor element 1, respectively. Lead wire 3 is electrode 2
And a power source (not shown) are connected, and Cu wire, P
It consists of t-line etc.

The carbon dioxide sensor element 1 is made of CeO ₂ , Ba.
The mixing molar ratio of CO ₃ and CuO is CeO ₂ / BaC.
In O ₃ / CuO = X / Y / Z (X + Y + Z = 100), the ranges of 30 ≦ X ≦ 80, 3 ≦ Y ≦ 50, and 0 ≦ Z ≦ 45 are preferable. When CeO ₂ is less than 30 mol%, the conductivity is increased and the sensitivity of capacitance to carbon dioxide tends to be lowered, which is not preferable. CeO ₂
Is more than 80 mol%, the insulating property increases and the capacity sensitivity to carbon dioxide gas tends to decrease, which is not preferable. When BaCO ₃ is less than 3 mol%, the sensitivity to carbon dioxide gas tends to disappear, which is not preferable. BaC
When O ₃ exceeds 50 mol%, the characteristics tend to become unstable due to swelling with respect to water, which is not preferable. C
When uO exceeds 45 mol%, the electroconductivity increases and the sensitivity tends to decrease, which is not preferable.

It is preferable that the carbon dioxide sensor element 1 has a phase angle θ of −50 ° ≦ θ ≦ -3 ° at an operating temperature.
As the phase angle becomes less than -50 ° C, the sensitivity of the capacitance to carbon dioxide tends to decrease, which is not preferable. Since the conductivity becomes stronger as the phase angle exceeds -3 °, the variation of the capacitance with respect to carbon dioxide gas becomes large and the apparent sensitivity is large, but stable characteristics cannot be obtained, which is not preferable.

The carbon dioxide sensor element 1 contains 1 to 6 mol% of A
It is preferred to contain g. When Ag is less than 1 mol% or more than 6 mol%, the sensitivity of the capacitance to the carbon dioxide concentration tends not to be improved, which is not preferable.

Carbon dioxide sensor element 1 is 500 to 950 ° C.
Preferably, it is baked at. When the firing temperature is lower than 500 ° C., the measured value of the electrostatic capacity becomes unstable and the reproducibility tends to be poor, which is not preferable. When the firing temperature exceeds 950 °, blackening occurs due to evaporation or melting of CuO and sensitivity to carbon dioxide concentration tends to decrease, which is not preferable.

A carbon dioxide sensor A having the above structure
The manufacturing method will be described below. First, CeO
₂ powder, BaCO ₃ powder and CuO powder are uniformly mixed using a dispersant such as ethanol. Add this mixture to 500-9
After baking at 50 ° C. (calcination process for carbon dioxide gas sensor element), ethanol is added again for grinding, and ethanol is dried and removed for sizing. 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. A 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, CeO
_Since the carbon dioxide gas sensor element 1, which is a mixture of ₂ , BaCO ₃ , and CuO, is provided, the detection sensitivity of the carbon dioxide gas concentration can be improved even at a low carbon dioxide gas concentration, and the deterioration over time is small and the durability is excellent.

[0022]

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

(Examples 1 to 22, Comparative Examples 1 and 2) First,
CeO ₂ powder, BaCO ₃ powder and CuO manufactured by Wako Pure Chemical Industries
Powders were prepared in proportions (Table 1) and (Table 2).

[0024]

[Table 1]

[0025]

[Table 2]

Ethanol was added to this powder as a dispersant, and the mixture was uniformly mixed by a ball mill for 45 hours. The mixture was calcined in the air at 800 ° C. and 950 ° C. for 5 hours. Add ethanol to this baked product and use a ball mill for 45
After pulverizing for a time and pulverizing, ethanol was dried and sized using a mesh. This powder is pressed using a press machine 3
Pressurized with a pressure of 0 kg / cm ² , diameter 10 mm, thickness 0.4
A disk-shaped carbon dioxide sensor element 1 having a size of mm was formed. Pt paste was applied to the upper and lower surfaces of the carbon dioxide sensor element 1, the lead wire 3 was inserted, and heating was performed at 800 ° C. for 40 minutes to form the electrode 2 and the lead wire 3 on the carbon dioxide gas sensor element 1. For carbon dioxide sensor element 1, X-ray diffraction, FT-
Carbon dioxide sensor element 1 after IR and TG analysis
Was found to be a mixture of CeO ₂ , BaCO ₃ , CuO, Cu ₂ O.

Next, the changes in capacitance with respect to changes in carbon dioxide concentration in Examples 1 to 22 and Comparative Examples 1 and 2 were measured.
First, the carbon dioxide sensor A was held at the temperature (550 ° C., 650 ° C.) at which the electrostatic capacity was maximum. Carbon dioxide concentration is 350ppm by mixing dry air and carbon dioxide
(Air) and 2% were prepared. The capacitance value was 10 kHz using an LCR meter (manufactured by Hewlett-Packard),
An alternating voltage of 5 V was applied to the carbon dioxide sensor and measured. If the capacitance value in air is C _AIR and the capacitance value in a 2% carbon dioxide gas atmosphere is C _{2% CO2} , the sensitivity is | 10 × LOG.
(C _{2% CO 2} / C _AIR ) | (unit is dB). The sensitivity at the start of capacitance measurement was R ₀ , and the sensitivity 300 days after the start of capacitance measurement was R ₃₀₀ . Examples 1-22,
The results of the phase angles R ₀ and R ₃₀₀ of Comparative Examples 1 and 2 are shown in (Table 1).

As is clear from (Table 1) and (Table 2), it was found that the sensitivity was 5 dB or more in the present embodiment, and particularly, the sensitivity was 8.7 dB in Example 16. At this time, it was found that the phase angle was in the range of −48 ° to −3 °. Comparative Examples 1 and 2 have sensitivities of 0 and 1.2.
Very low as dB, phase angle at this time is -65 °, -76 °
And turned out to be low. Further, it was found that the present invention showed a very small sensitivity decrease of 0 to 0.5 dB after 300 days, and little deterioration with time.

Comparative Example 3 BaCO ₃ manufactured by Wako Pure Chemical Industries, Ltd.
The powder and TiO ₃ powder made by Nippon Aerosil are mixed at an equimolar ratio and baked in the air at 1200 ° C. for 12 hours to obtain BaT.
An iO ₃ powder was obtained. CuO powder and BaCO ₃ powder manufactured by Wako Pure Chemical Industries, Ltd. were mixed with the obtained BaTiO ₃ at a molar ratio of 49: 50: 1 and baked at 800 ° C. for 5 hours. The subsequent steps were the same as in Example 1 to produce a carbon dioxide sensor A in Comparative Example 3.

The sensitivity changes with time and carbon dioxide concentration in Example 3 and Comparative Example 3 were measured. The measurement method was the same as in Example 1. FIG. 2 is a diagram showing the relationship between the elapsed time and the sensitivity of Example 3 and Comparative Example 3, and FIG. 3 is Example 3 and Comparative Example 3.
It is a figure which shows the relationship between the carbon dioxide concentration and sensitivity of.

As is apparent from FIGS. 2 and 3, in Example 3 of the present invention, the sensitivity was 0.5 when kept for 60 days.
%, When it was kept for 150 days, it was found that only 1% deteriorated and the change with time was extremely small. Also, 500 to 5000
It was found that the sensitivity gradually increased to 2 to 6 dB in the carbon dioxide concentration range of ppm, the carbon dioxide detection sensitivity was high, and the rate of change was large.

(Examples 23 to 34) CeO_TwoPowder, Ba
CO_ThreePowder and CuO powder were blended in the same manner as in Example 3 to obtain Ag
NO _ThreeWas added so as to have an Ag ratio of (Table 3).

[0033]

[Table 3]

Otherwise, the same steps as in Example 3 were carried out and Example 2 was followed.
Carbon dioxide sensors 3 to 34 were manufactured. Examples 23-3
For No. 4, the phase angle and the sensitivity R ₀ were measured in the same manner as in Example 3, and the results are shown in (Table 3).

As is clear from this (Table 3), especially A
When 1 to 6 mol% of g is added, the sensitivity is 13.2 to 23.6.
It was found that it was improved by 1.8 to 3.3 times as much as dB.

[0036]

As described above, according to the carbon dioxide sensor of the present invention, the capacitance value is large in the low carbon dioxide concentration range of hundreds to thousands of ppm, and the static capacitance against the change in the carbon dioxide concentration at this low carbon dioxide concentration is high. Since the rate of change of the capacitance value is large, the carbon dioxide concentration can be accurately detected and the reliability is improved. Further, the capacitance value does not fluctuate even after long-term use and the sensitivity to the carbon dioxide concentration does not deteriorate, so that the reproducibility is excellent, the deterioration with time is small, and the durability is significantly improved.

Further, according to the method for producing a carbon dioxide gas sensor of the present invention, a carbon dioxide gas sensor having a remarkably high detection sensitivity of carbon dioxide in a low carbon dioxide concentration range and a small change over time and excellent durability is produced with high productivity and high production yield. Can be mass-produced in.

[Brief description of the drawings]

FIG. 1 is a perspective view of a main part of a carbon dioxide sensor according to a first embodiment.

FIG. 2 is a diagram showing the relationship between the passage of time and the sensitivity of Example 3 and Comparative Example 3.

FIG. 3 is a graph showing the relationship between carbon dioxide concentration and sensitivity in Example 3 and Comparative Example 3.

[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 (7)

[Claims] 1. A carbon dioxide gas sensor comprising a carbon dioxide gas sensor element for detecting a carbon dioxide gas concentration and a set of electrodes laminated on the surface of the carbon dioxide gas sensor element, wherein the carbon dioxide gas sensor element is CeO ₂ . A carbon dioxide gas sensor characterized by containing a mixture of BaCO ₃ . 2. The carbon dioxide sensor according to claim 1, wherein the carbon dioxide sensor element contains CuO. 3. The phase angle θ at the operating temperature is −50 ° ≦ θ
The carbon dioxide gas sensor according to claim 1 or 2, wherein ≤-3 °. 4. A mixed molar ratio of CeO ₂ , BaCO ₃ and CuO is CeO ₂ / BaCO ₃ / CuO = X / Y / Z.
In (X + Y + Z = 100), 30 ≦ X ≦ 80, 3
The carbon dioxide sensor according to any one of claims 1 to 3, wherein the ranges are ≤Y≤50 and 0≤Z≤45. 5. The carbon dioxide sensor element comprises 1 to 6 mol%.
5. The carbon dioxide sensor according to claim 1, wherein the carbon dioxide sensor contains Ag. 6. The carbon dioxide sensor element is 500 to 950.
The carbon dioxide sensor according to any one of claims 1 to 5, wherein the carbon dioxide sensor is formed by firing at ° C. 7. A method of manufacturing a carbon dioxide gas sensor, comprising a carbon dioxide gas sensor element firing step of heating and molding in a temperature range of 500 to 950 ° C.