JPH08189915A - Carbon dioxide gas sensor - Google Patents

Abstract

PURPOSE: To detect concentration of carbon dioxide gas accurately even under the usage condition of intermittent energizing of a heater by using a material, represented by a specific formula, as solid electrolyte, so that recovery time to steady electromotive farce is shortened. CONSTITUTION: On one surface of a solid electrolyte 1, a reference pole 2 and a detection pole 3 are provided, and the detection pale 3 is coated with carbonate 4. On the other surface of the solid electrolyte 1, a heater substrate 6 provided with a heater 5 is bonded and fixed with bonding agent 7. Then, the heater 5 is energized, and used in to-be-detected gas under the condition an element is kept at high temperature. Sensor output is taken out of the detection pole 3. The solid electrolyte 1 is a material which is represented by a chemical formula Li4-0.5 XTi5-x Al1.5x O12 , where in a value of X is in a range 0<=X<=0.30, or the one represented by a chemical formula Li4-2x Ti5-x Mg3x O12 , where a value of X in in a range 0<=X<=0.40.

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 sensor for measuring the concentration of carbon dioxide in the atmosphere.

[0002]

2. Description of the Related Art A solid electrolyte type gas sensor detects a gas concentration by an electromotive force generated when a gas comes into contact with a solid electrolyte as a sensing material. When the operating principle of the gas sensor of this solid electrolyte type gas sensor is briefly described,
Now, if electrodes are formed on both sides of the solid electrolyte and this solid electrolyte is kept at a high temperature of several hundred degrees Celsius to give a partial pressure difference of gas to both sides of the solid electrolyte, ionic conduction occurs in the solid electrolyte, depending on the difference in gas concentration. Generated electromotive force. The gas concentration can be detected by measuring the electromotive force. Conventionally, as a sensing material for a solid electrolyte type carbon dioxide gas sensor, reliability with respect to temperature and stability over time have been improved. Na ₃ Zr ₂ PSi ₂ O ₁₂ (NASICON; Na
-Super; IonicConducton), Li
₁₄ Zn (Ge ₃ ) 4 (LISICON; Lithium
Various substances such as Super Ionic Conductor) are used, but most of these solid electrolytes have a low electric conductivity σ at room temperature and cannot obtain sufficient gas detection characteristics. At this temperature, σ needs to be about 10 ⁻⁴ Ω ⁻¹ cm ⁻¹ or more. For this reason, in a normal solid electrolyte type carbon dioxide gas sensor, a heater for heating is provided and the solid electrolyte is used while being kept at a high temperature of several hundred degrees.

However, in the actual use of the sensor, when the sensing is not necessary, the energization of the heating heater is stopped, and the heating heater is operated only when the sensing is necessary. is there.
FIG. 1 shows a change in electromotive force when a carbon dioxide gas sensor using NASICON as a solid electrolyte is used in intermittent operation of a heater in one day cycle. When the operation of the heater that has been continuously energized is stopped at the end of the first day, the sensor starting force decreases from an output of approximately 250 mV to a point where measurement is impossible. On the second day, the heater operation is stopped and the heater is activated at the beginning of the third day. At this time, the sensor starting force rises rapidly to some extent, but after that, even if the heater is continuously energized all day long, the sensor starting force does not reach the steady electromotive force when the heater is continuously energized. It shows a value as low as 10 mV. Then, in order to recover the starting force of the sensor to the steady state, it is necessary to wait for two days or more. Therefore, the carbon dioxide gas sensor using the conventional solid electrolyte has a problem that the carbon dioxide gas concentration cannot be accurately detected when the heater is used in the intermittent operation state.

[0004]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, shortens the recovery time to a steady electromotive force, and allows accurate carbon dioxide emission even under the operating conditions of intermittent energization of a heater. An object is to provide a solid electrolyte type carbon dioxide gas sensor capable of detecting gas concentration.

[0005]

In order to achieve the above object, the solid electrolyte type carbon dioxide gas sensor of the present invention has a solid electrolyte of Li _4-0.5x Ti _5-x Al _1.5x O ₁₂ ( _{0≤x≤0.30).} ) or, it is characterized in that it comprises a substance represented by the chemical formula _{Li 4-2x Ti 5-x Mg 3x} O 12 (0 ≦ x ≦ 0.40).

[0006]

As a solid electrolyte of the solid electrolyte type carbon dioxide gas sensor of the present invention, Li _4-0.5x Ti _5-x Al _1.5x O ₁₂ ( _{0≤x ≤0.30} ) or Li _4-2x Ti _5-x Mg _3x O ₁₂ (0 ≦ x ≦ 0.4
Since the substance represented by the chemical formula (0) is used, it is possible to obtain sufficient sensitivity and stability of electromotive force over time, and to accurately detect the concentration of carbon dioxide gas even under the operating conditions of intermittent energization of the heater. can do.

[0007]

The details of the present invention will be described below with reference to FIGS. 2 to 6. The configuration of the carbon dioxide sensor of the present invention will be described at the most part. FIG. 2A is a sectional view showing an example of the element structure of the solid electrolyte type carbon dioxide gas sensor of the present invention, in which one side of the solid electrolyte 1 is provided with a reference electrode 2 and a detection electrode 3, and a detection electrode. 3 is coated with carbonate 4. A heater substrate 6 provided with a heater 5 is adhered and fixed to the other surface of the solid electrolyte 1 with an adhesive 7. 2B is a cross-sectional view showing another element structure of the solid electrolyte type carbon dioxide sensor of the present invention. The difference between this element structure and the element structure shown in FIG. Is only provided on the surface opposite to the surface of the solid electrolyte 1 on which the detection electrode 3 is provided, and both element structures show the same characteristics.

The solid electrolyte carbon dioxide gas sensor of the present invention having such an element structure is used by being placed in a gas to be detected while the heater 5 is energized and the element is kept at a high temperature of several hundreds of degrees. Then, the sensor output is taken out from the detection electrode. The solid electrolyte 1 is a substance represented by a chemical formula of Li _4-0.5x Ti _5-x Al _1.5x O ₁₂ , and the value of X is _0≤x≤0 .
A substance represented by the chemical formula of Li _4-2x Ti _5-x Mg _3x O ₁₂ in which the value of X is 0 ≦ x ≦≦
It is in the range of 0.40.

Next, the characteristics of the solid electrolyte type carbon dioxide gas sensor of the present invention will be described based on experimental results. (Experimental Result 1) (Temperature Dependence of Conductivity σ) FIG. 3A shows a solid electrolyte Li _4-0.5x Ti _5-x Al _1.5x.
It is an experimental result showing the temperature dependence of the conductivity σ of Li ⁺ ions of O ₁₂ , and the conductivity σ (Ω ⁻¹ · cm ⁻¹ ) and the absolute temperature (K) are expressed as the reciprocal of the absolute temperature T (K). Product σT (Ω ^-1
・ Cm ⁻¹ · K) is plotted. In this case, ●: x = 0 ○: x = 0.1 ◎:
x = 0.2 (4): x = 0.3 □: x = 0.4 ◆: x = 0.5. From this experimental result, the value of X satisfying the condition of conductivity σ> 10 ^-4 Ω ^-1 cm ^-1 (400 ° C) that can be practically used as a sensor is 0 ≤ x ≤ 0.30, and the value of X is It can be seen that if it is 0.3 or more, the internal resistance of the solid electrolyte becomes large and a sufficient electromotive force cannot be obtained. Therefore, the solid electrolyte 1 is _replaced with Li _4-0.5x Ti _5-x Al _1.5x O ₁₂
When the substance represented by the chemical formula is, the value of X is 0 ≤ x
≤0.30 is optimal.

FIG. 3B shows the solid electrolyte Li _4-2x Ti.
These are experimental results showing the temperature dependence of the electrical conductivity σ of Li ⁺ ions of _5-x Mg _3x O ₁₂ , and the electrical conductivity σ (Ω ⁻¹ · cm ⁻¹ ) and the absolute ^value of the reciprocal of the absolute temperature T (K) It is a plot of the product of temperature (K), σT (Ω ⁻¹ · cm ⁻¹ · K). In this case, ●: x = 0 ○: x = 0.1 ◎:
x = 0.2 ■: x = 0.3 □: x = 0.4 ◆: x = 0.5 ◇: x = 0.6 ▲: x = 0.7 From this experimental result, the value of X satisfying the condition of conductivity σ> 10 ^-4 Ω ^-1 cm ^-1 (400 ° C) that can be practically used as a sensor is 0 ≤ x ≤ 0.40, and the value of X is 0
_It can be seen that when it exceeds 0.4, the internal resistance of the solid electrolyte becomes large and a sufficient electromotive force cannot be obtained. Therefore, when the solid electrolyte 1 is a substance represented by the chemical formula of Li _4-2x Ti _5-x Mg _3x O ₁₂ , the value of X is 0 ≤ x ≤ 0.
40 is the best.

(Experimental Result 2) (Confirmation of Sensitivity as Carbon _{Dioxide Gas Sensor} ) FIG. 4 (a) shows the case where seven kinds of solid electrolytes Li _4-0.5x Ti _5-x Al _1.5x O ₁₂ having different X values are used. , Fig. 2 (a)
And a carbon dioxide gas sensor having the element structure shown in FIG. 2B is configured, and the CO ₂ sensitivity (m
V) is plotted. The CO ₂ sensitivity is the difference in electromotive force of the sensor when the carbon dioxide concentration is changed from the atmospheric concentration to 2000 ppm. From this experimental result, the sensor sensitivity sharply decreases when the value of X exceeds 0.3, but when the value of X is in the range of 0 ≤ x ≤ 0.30, the sensor sensitivity is about 40 mV, which is sufficient as a carbon dioxide sensor. It was confirmed that it has various sensitivity.

FIG. 4 (b) is obtained by using nine kinds of solid electrolytes Li _4-2x Ti _5-x Mg _3x O ₁₂ having different X values, and FIG.
And a carbon dioxide gas sensor having the element structure shown in FIG. 2B is configured, and the CO ₂ sensitivity (m
V) is plotted. The CO ₂ sensitivity is the difference in electromotive force of the sensor when the carbon dioxide concentration is changed from the atmospheric concentration to 2000 ppm. From this experimental result, when the value of X exceeds 0.4, the sensor sensitivity sharply decreases,
When the value of X is 0 ≦ × ≦ 0.40, the sensor sensitivity is 4
About 0 mV was obtained, and it was confirmed that the carbon dioxide sensor had sufficient sensitivity.

(Experimental result 3) (Stability of sensor electromotive force with time) FIG. 5 shows a change with time of the sensor electromotive force when the heater of the carbon dioxide sensor is continuously energized. A white circle indicates a conventional carbon dioxide gas sensor using NASICON as a solid electrolyte. A black circle indicates a solid electrolyte of Li ₄ Ti ₅ O ₁₂ , that is, a solid electrolyte L.
_{i 4-0.5x Ti 5-x Al 1.5x} O 12 or the value of X a in _{Li 4-2x Ti 5-x Mg 3x} O 12 shows the present invention carbon dioxide sensor which was used as a 0. Square mark □ is L as a solid electrolyte
_{i 3. 85 Ti 4. 7} Al 0. 45 O 12, namely the solid electrolyte Li
₄ shows the carbon dioxide gas sensor of the present invention using _4-0.5x Ti _5-x Al _1.5x O _{12 having} an X value of 0.3.
The triangles △ are solid electrolytes Li _3.4 Ti _4.7 Mg _0.9
O ₁₂ or solid electrolyte Li _4-2x Ti _5-x Mg _3x O ₁₂
2 shows the carbon dioxide gas sensor of the present invention in which the value of X is 0.3. In any of the above carbon dioxide sensors of the present invention, the electromotive force of the sensor did not change with time as compared with the conventional carbon dioxide sensor using NASICON, and no deterioration of the electromotive force was observed after 300 days had passed.

[0014] (Experiment Result 4) (recovery time of the sensor electromotive force) FIGS. 6 (a) and _{6 (b) Li 4 Ti 5} O 12 as the respective solid _{electrolyte, Li 3. 4 Ti 4.} 7 Mg 0. ₉ O ₁₂ , L
_{i 3. 85 Ti 4. 7} Al 0. is ₄₅ O ₁₂ of the present invention carbon dioxide gas sensor using a shows the sensor electromotive force of the recovery time when used in the heater intermittently energizing day cycle in the atmosphere . As is clear from these characteristics, in the carbon dioxide gas sensor of the present invention, the electromotive force reached a steady electromotive force in about 10 minutes from the start of energization of the heater, and compared with the conventional carbon dioxide gas sensor using NASICON as the solid electrolyte. It was possible to significantly reduce the recovery time. Further, when the intermittent energization cycle of the heater was made longer than one day, the recovery of electromotive force was further delayed in the conventional carbon dioxide gas sensor, but in the carbon dioxide gas sensor of the present invention, it took about the same time of about 10 minutes. The electromotive force was recovered.

[0015]

As described in detail above, the present invention ensures sufficient sensitivity as a carbon dioxide sensor and stability of the starting force with time, and recovers the starting force of the sensor even under the use condition of intermittent energization of the heater. Since the time can be remarkably shortened, high reliability and accurate detection of the carbon dioxide concentration can be realized, and restrictions on use conditions can be greatly reduced.

[Brief description of drawings]

FIG. 1 is a starting force characteristic diagram of a conventional carbon dioxide gas sensor in an intermittent heater state.

FIG. 2 is a block diagram of the carbon dioxide sensor of the present invention.

FIG. 3 is a characteristic diagram of temperature dependence of electric conductivity of the solid electrolyte of the present invention.

FIG. 4 is a sensitivity characteristic diagram of the carbon dioxide sensor of the present invention.

FIG. 5 is a time-dependent characteristic diagram of the starting force of the carbon dioxide sensor of the present invention.

FIG. 6 is a characteristic diagram of recovery of starting force of the carbon dioxide sensor of the present invention.

[Explanation of symbols]

 1 ... Solid electrolyte 2 ... Reference electrode 3 ... Detection electrode 4 ... Carbonate 5 ... Heater 6 ... Heater substrate 7 ... Adhesive

Claims (2)

[Claims] 1. A solid electrolyte type carbon dioxide gas sensor, wherein the solid electrolyte is Li _4-0.5x Ti _5-x Al _1.5x O ₁₂ ( _0≤x≤0.30 ). 2. A solid electrolyte type carbon dioxide gas sensor, wherein the solid electrolyte is Li _4-2x Ti _5-x Mg _3x O ₁₂ (0 ≦ x ≦ 0.40).