JP2000206089A - Solid electrolyte type carbon dioxide gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve stable measurement for a long time by equipping a sensor element consisting of a detection electrode, a solid electrolyte layer, a counted electrode, and a reference element consisting of a reference electrode, the solid electrolyte layer, and a counter electrode. SOLUTION: A composite sensor element is provided with a detection electrode 3, with a metal carbonate layer 2 at its periphery directly in direct contact with a solid electrolyte layer 1, a counter electrode 4, and a reference electrode 5. In the sensor element consisting of the detection electrode (a carbonate layer) 3, the solid electrolyte 1, and the counter electrode 4, and the reference element consisting of the reference electrode 5, the solid electrolyte layer 1, and the counter electrode 4, the solid electrolyte layer 1 and the counter electrode 4 are common. The detection electrode 3 and the reference electrode 4 of the sensor element A are of the same size. Also, the gas sensor is separately provided with a heater through a substrate. As a result, by correcting a potential difference or current due to the sensor element by the potential difference or current according to the reference element, the deterioration and aging as a solid electrolyte type carbon dioxide gas sensor can be canceled out.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a solid electrolyte type carbon dioxide gas sensor technology.

[0002]

2. Description of the Related Art A solid electrolyte type carbon dioxide gas sensor is
Since it has good sensitivity and can be made compact, research and development are being conducted to apply it to various fields. FIG. 7 shows a model sectional view of a general solid electrolyte type carbon dioxide gas sensor element.

[0003] NASICO having Na ⁺ ion conductivity
A detection electrode γ and a counter electrode δ covered with a layer made of a metal carbonate (metal carbonate layer β) are arranged with a layer made of a solid electrolyte such as N (solid electrolyte layer α) interposed therebetween. Is connected to a lead wire. In use, the sensor element is heated by a heater (not shown) during use, and is maintained in a temperature region (400 ° C.) where the solid electrolyte layer conducts ions.

Using such a sensor element according to the prior art, the output (E) to air containing various concentrations of carbon dioxide
MF (mV)) is shown in FIG. It is understood that a good linear relationship is obtained between the sensor element output and the logarithmic value of the carbon dioxide concentration.

[0005] However, such a solid electrolyte type carbon dioxide gas sensor element has a drawback that the change with time is large and the sensitivity characteristic is significantly impaired when left unused and not heated.

FIG. 9 shows the results of examining the change over time due to the use of such a sensor element according to the prior art using the output value with respect to air containing 350 ppm of carbon dioxide, and FIG.
The results of examining the changes over time when left unused (without heating the heater) at 30 ° C. and 90% RH (relative humidity) using the output value for air containing 350 ppm of carbon dioxide are shown in FIG. Show.

[0007] From these results, it can be seen that the solid electrolyte type carbon dioxide gas sensor according to the prior art is greatly deteriorated due to aging, and it is necessary to make some correction in order to perform accurate measurement.

For this reason, there has been proposed a method of restoring characteristics by applying a voltage in a direction opposite to the direction of electromotive force.
Even with this method, it is difficult to generate a carbonate serving as a nucleus for generating a potential of the sensor element, which is not practical.

It has been proposed to take measures such as adding an oxygen ion conductive material to the reference electrode of the element in order to reduce the change with time. However, the carbonate of the detection electrode has a relatively high heat resistance. , The use of a high temperature near the temperature at which the oxygen ion conductive material of the reference electrode has sufficient conductivity causes deterioration of the carbonate, resulting in a short-lived sensor.

On the other hand, a microcomputer or the like
It is also considered to perform some kind of calculation on the sensor element output value to perform correction, but the cost of these correction circuits is high because the measurement range must be secured and the accuracy (including resolution) must be guaranteed. It is difficult to reduce the size, and furthermore, the power required for these correction circuits is large, so that the sensor cannot be driven by a battery, or the battery life is extremely shortened.

[0011]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems, that is, a solid electrolyte which enables stable measurement over a long period of time and which is less affected by standing at room temperature when not in use. It is an object of the present invention to provide a carbon dioxide gas sensor.

[0012]

According to a first aspect of the present invention, there is provided a solid electrolyte type carbon dioxide gas sensor according to the present invention.
A sensor element comprising a solid electrolyte layer and a counter electrode, a reference electrode,
It has a configuration having a solid electrolyte layer and a reference element comprising a counter electrode.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The solid electrolyte type carbon dioxide gas sensor of the present invention has the above-described structure, and corrects a potential difference or current caused by a sensor element by a potential difference or current caused by a reference element. Therefore, it is possible to cancel the change with the passage of time, so that the influence of deterioration due to standing at room temperature can be eliminated or accurate measurement can be performed over a long period of time.

In the above configuration, since no metal carbonate is in contact with the reference electrode of the reference element, no electromotive force should be generated between the reference electrode and the counter electrode of the reference element.
However, when the output (potential difference or current) of the sensor element is corrected by the output (potential difference or current) of the reference element, accurate and stable measurement can be performed as described above, although the detailed reason is unknown. .

The solid electrolyte type carbon dioxide gas sensor of the present invention is, for example, a so-called composite in which the solid electrolyte layer and the counter electrode of the sensor element and the reference element are common, or the counter electrode of the sensor element and the reference element are common. It may be composed of a sensor element. At this time, manufacture becomes easy, and at the same time, a compact sensor can be obtained.

In the present invention, it is desirable that the shape and material of the detection electrode and the reference electrode are the same. Further, as the metal carbonate layer disposed around the sensing electrode, usually, a carbonate mixture or a eutectic mixture in which lithium carbonate is mainly used and barium carbonate, calcium carbonate, and strontium carbonate are added to further improve water resistance. Is formed by, for example, melting, but all other metal carbonates used in the solid electrolyte type carbon dioxide gas sensor can be used.

In order to make the history and thermal characteristics of the reference electrode the same as those of the detection electrode, a layer made of a substance that does not cause a potential difference between the counter electrode and the reference electrode with respect to carbon dioxide gas and other gases contained in the environment. It may be provided around the reference electrode. The detection electrode, the reference electrode, and the counter electrode are preferably made of a noble metal such as gold or platinum, or a cermet made of these noble metals and various ceramics in terms of heat resistance and durability.

The sensor element needs to be maintained at a temperature suitable for sodium ion conduction of the solid electrolyte.
Therefore, the heater may be provided via a substrate made of, for example, alumina or mullite.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The solid electrolyte type carbon dioxide gas sensor of the present invention will be specifically described below. In addition, each sensor element used below was produced as follows. A predetermined amount of commercially available sodium phosphate, zirconium silicate and silicon dioxide was taken and pulverized by a ball mill for 24 hours. After applying pressure to this powder and forming it into a circular shape,
A NASICON chip obtained by sintering for a time, polishing to a thickness of 0.5 mm, and then cutting was used as a solid electrolyte.

The detection electrode, the reference electrode, and the counter electrode were formed by screen printing gold paste. Further, an alumina substrate having a platinum heater formed on the chip by sputtering was baked using a platinum paste. A platinum lead wire was connected to each electrode using a platinum paste. Note that a metal carbonate layer was formed around the sensing electrode using molten lithium carbonate to cover the sensing electrode.

(Embodiment 1) FIG. 1 shows a sensor element A of a solid electrolyte type carbon dioxide gas sensor according to the present invention (Embodiment 1).
FIG.

This is in direct contact with the solid electrolyte layer 1,
A composite sensor element having a sensing electrode 3, a counter electrode 4, and a reference electrode 5 having a metal carbonate layer 2 around the sensor element, the sensor element including a sensing electrode (carbonate layer), a solid electrolyte layer, and a counter electrode. The solid electrolyte layer and the counter electrode are common to the electrode, the solid electrolyte layer, and the reference element for the counter electrode, respectively. The detection electrode 3 and the reference electrode 4 of the sensor element A have the same size. Further, a heater is separately provided via a substrate, but is not shown in FIG.

FIG. 2 shows the results of examining changes in the output (EMF (mV)) with respect to air containing various concentrations of carbon dioxide using the composite sensor element. In the measurement, the sensor element was kept at 400 ° C. by a heater (the same applies hereinafter).

FIG. 2 shows that the measurable range of the sensor element A is the same as that of the sensor element according to the prior art, and that the output of the sensor element and the logarithmic value of the carbon dioxide gas concentration have a good linear relationship. .

Next, the change in the sensitivity characteristics when left unused and not used was examined. 30 ° C, RH
(Relative humidity) The change with time when left unused (without heating the heater) under the condition of 90% (assumed to be left in the hot and humid summer) is examined by the output value with respect to air containing 350 ppm of carbon dioxide. The results are shown in FIG.

FIG. 3 shows that the sensor element A according to the present invention can accurately measure even when left at room temperature. In addition, when a change with time in a use state was examined using a sensor element manufactured similarly to the sensor element A, the output of the sensor element was more stable than the sensor element according to the related art, and the drift was lower. It was confirmed that it was much smaller.

(Embodiment 2) FIG. 4 shows a sensor element B of a solid electrolyte type carbon dioxide gas sensor (Embodiment 2) according to the present invention.
FIG. This is a composite sensor element having a sensing electrode 3, a counter electrode 4, and a reference electrode 5 directly surrounding the solid electrolyte layers 1 and 1 'and having a metal carbonate layer 2 around the sensing electrode (carbonate). The sensor element comprising the layer, the solid electrolyte layer and the counter electrode and the reference element for the reference electrode, the solid electrolyte layer and the counter electrode have a common counter electrode. Further, a heater is provided via an alumina substrate. In addition,
The detection electrode 3, the reference electrode 4, and the solid electrolyte elements 1 and 1 'of the sensor element A have the same size.

FIG. 5 shows the results of examining changes in the output (EMF (mV)) with respect to air containing various concentrations of carbon dioxide using this composite sensor element. From FIG. 5, it can be seen that the measurable range of the sensor element A is the same as that of the sensor element according to the related art, and the output of the sensor element and the logarithmic value of the carbon dioxide gas concentration have a good linear relationship.

Next, the change in the sensor element output when not used and left unheated was examined. 30
The change with time when left unused (assuming high temperature and high humidity in the summer) when left unused (without heating the heater) under the conditions of ° C and RH (relative humidity) 90% is the output to air containing 350 ppm of carbon dioxide. FIG. 6 shows the results obtained by examining the values.

It can be seen from FIG. 6 that the sensor element A according to the present invention can perform accurate measurement even at room temperature. In addition, when a time-dependent change in use condition was examined using a sensor element separately manufactured in the same manner as the sensor element A, the output of this sensor element was more stable than the sensor element according to the related art, and the drift was lower. It was confirmed that it was much smaller.

[0031]

The solid electrolyte type carbon dioxide gas sensor of the present invention has a sensor element comprising a sensing electrode, a solid electrolyte layer and a counter electrode having a metal carbonate layer around the sensor element, and a reference element comprising a reference electrode, a solid electrolyte layer and a counter electrode. The element has little change over time during use, furthermore, there is little deterioration when not in use, and no expensive and complicated circuit for correction is required. It is an excellent solid electrolyte type carbon dioxide gas sensor that can be driven by a battery and can be made compact.

[Brief description of the drawings]

FIG. 1 is a model sectional view of a sensor element of a solid oxide carbon dioxide gas sensor A according to the present invention.

FIG. 2 is a diagram showing the results of examining changes in the output (EMF (mV)) of air containing various concentrations of carbon dioxide using the sensor element of FIG.

FIG. 3 is a diagram showing a result of examining a change in sensor element output when the sensor element of FIG. 1 is left in a non-heated state without being used.

FIG. 4 is a model sectional view of a sensor element of a solid oxide carbon dioxide gas sensor B according to the present invention.

5 is a diagram showing the results of examining changes in output (EMF (mV)) with respect to air containing various concentrations of carbon dioxide using the sensor element of FIG. 4;

FIG. 6 is a diagram showing a result of examining a change in sensor element output when the sensor element of FIG. 4 is left unheated without being used.

FIG. 7 is a model sectional view of a sensor element of a solid oxide carbon dioxide gas sensor according to the related art.

8 is a diagram showing the results of examining changes in output (EMF (mV)) with respect to air containing various concentrations of carbon dioxide using the sensor element of FIG. 7;

9 is a diagram showing a result of examining stability of a sensor output when the sensor element of FIG. 7 is used.

FIG. 10 is a diagram showing a result of examining a change in sensor element output when the sensor element of FIG. 7 is left in a non-heated state without being used.

[Explanation of symbols]

A, B Sensor element of solid electrolyte type carbon dioxide gas sensor according to the present invention 1, 1 'solid electrolyte layer 2 metal carbonate layer 3 detection electrode 4 counter electrode 5 reference electrode 6 alumina substrate 7 heater

Claims (3)

[Claims] 1. A solid electrolyte comprising a sensor element comprising a sensing electrode having a metal carbonate layer around it, a solid electrolyte layer and a counter electrode, and a reference element comprising a reference electrode, a solid electrolyte layer and a counter electrode. Type carbon dioxide gas sensor. 2. The solid electrolyte type carbon dioxide gas sensor according to claim 1, wherein a solid electrolyte layer and a counter electrode of the sensor element and the reference element are common. 3. The solid electrolyte type carbon dioxide gas sensor according to claim 1, wherein a counter electrode of the sensor element and a reference element is common.