JPH04344457A - Carbon dioxide sensor - Google Patents

Abstract

PURPOSE:To improve the temperature dependency of the output of a solid electrolyte type carbon dioxide sensor and to enhance the durability of the sensor. CONSTITUTION:A catalyst material 2 is added to a reference electrode layer a1 and the temperature dependency of the output of a carbon dioxide sensor is improved by the catalytic action of the material 2 so as to prevent deterioration of the sensor due to corrosive gas: Or, a reference electrode layer 104 is covered with a gas shielding layer and thereby the reference electrode layer 104 is shielded from measurement atmosphere so as to eliminate the influence of water and gases in the atmosphere thus preventing the humidity dependency of the output of the sensor and preventing deterioration of the reference electrode layer 104: Or, an oxygen ion conductive material 5 is added to a reference electrode layer b4 so as to make oxygen partial pressure within the reference electrode layer be constant thereby providing outputs which are stable over a long period; the carbon dioxide sensor of high reliability can thus be obtained.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is applicable to greenhouse horticulture, environmental hygiene,
This invention relates to a solid electrolyte type carbon dioxide sensor used in disaster prevention, industrial, and other places where carbon dioxide concentration is measured and controlled.

0002

2. Description of the Related Art In recent years, the need for carbon dioxide gas sensors has increased mainly in the fields of air conditioning and agriculture and livestock farming, and various types of sensors have been developed and put into practical use. However, in practice, many problems remain regarding its reliability.

An example of a conventional carbon dioxide sensor will be described below with reference to FIG. As shown in the figure, a thin NAXS (sodium ion conductor) plate 101, which is a solid electrolyte, has a sensing electrode layer 102 and a reference electrode layer 104, which are a pair of electrode layers 109, at both ends thereof. 102
is formed by adding and dispersing sodium carbonate, which is a metal carbonate 103, into an electrode layer made of Au, and a reference electrode layer 104 is formed of an electrode layer made of Au, and constitutes a gas sensing section 106. Was. A heating unit 105 is fixed to the lower part of one side of the gas sensing unit 106 with an adhesive 108, and heats the gas sensing unit 106 to a measurement temperature. Further, the sensor output is taken out through lead wires 107a and 107b taken out from the detection electrode layer 102 and the reference electrode layer 104, respectively.

In the above configuration, the gas sensing section 106 constitutes the following battery. i.e.

[0005]

[Chemical formula 1]

Gas sensing section 10 constituting such a battery
6 is heated to the measurement temperature by the heating section 105, the following battery reaction occurs in the sensing electrode layer 102 and the reference electrode layer 104.

[0007]

[Chemical 2]

An electromotive force expressed by the following equation is generated between the two electrodes.

[0009]

[Math 1]

Here, E0 is a constant, R is a gas constant, T is an absolute temperature, F is a Faraday constant, αNa2O, αNa2
CO3 is the activity of Na2O and Na2CO3, respectively;
PCO2 indicates the partial pressure of CO2 in the air. αNa2CO3
can be set as 1 in a normal stable state, and αNa
Since 2O is determined by the composition of NAXS, it can be regarded as a constant. Therefore, if T, which is the operating temperature of the sensor, is constant, the electromotive force E (output) will be a function only of PCO2. Therefore, the CO2 concentration can be determined by extracting this electromotive force E (output) from the sensing electrode layer 102 and the reference electrode layer 104 via the lead wires 107a and 107b.

[0011]

[Problems to be Solved by the Invention] However, the above configuration has the following problems. In other words, if there is moisture in the measurement atmosphere, the detection electrode layer (chemical formula 2)
Water was involved in the electrode reaction shown in . Therefore, when the humidity of the measurement atmosphere changes, the electromotive force, that is, the sensor output changes in accordance with the change, which is the temperature dependence of the output. Therefore, accurate carbon dioxide concentration could not be measured.

Furthermore, the presence of corrosive gases such as hydrogen sulfide in the atmosphere contaminates the reference electrode layer and causes deterioration.

Furthermore, if dew condenses on the gas sensing part, the metal carbonate in the sensing electrode layer dissolves into the condensed water, and when the solution reaches the reference electrode, the gas sensing part no longer forms the battery shown in (Chemical formula 1). It was damaged.

The present invention solves the above-mentioned problems, and provides a highly reliable carbon dioxide sensor that reduces temperature dependence of output, prevents deterioration of the reference electrode.

[0015]

[Means for Solving the Problems] A first means for achieving the object of the present invention is to provide a pair of electrode layers at both ends of an ion conductive ceramic plate made of a solid electrolyte, and to A gas sensing section is formed by adding a metal carbonate that forms a dissociation equilibrium with carbon dioxide gas to one of the electrode layers to form a sensing layer, and adding a catalyst substance to the other reference electrode layer a. , and a heating section that heats the gas sensing section to a measurement temperature.

A second means is to provide a pair of electrode layers at both ends of an ion-conductive ceramic plate made of a solid electrolyte, and to dissociate carbon dioxide from one electrode layer of the ion-conductive ceramic plate. A detection layer containing a metal carbonate that forms an equilibrium is formed, and the other reference electrode layer or a part or all of the surface of the ion-conductive ceramic plate including the reference electrode layer is covered with a gas barrier layer. and a heating section that heats the gas sensing section to a measurement temperature.

A third means is one in which an oxygen ion conductive substance is added to the reference electrode layer b.

[0018]

[Operation] According to the first means described above, the present invention adds a catalytic substance to the reference electrode layer a, and its catalytic action causes the reference electrode layer a to react in the atmosphere in the electrode reaction shown in (Chemical formula 2). Since the influence of moisture is reduced and the corrosive gas is decomposed, the dependence of the output on atmospheric humidity and the deterioration of the reference electrode layer a due to the corrosive gas can be reduced.

Furthermore, with the configuration of the second means, by covering the reference electrode with a gas barrier layer, the reference electrode is isolated from the measurement atmosphere, thereby reducing atmospheric humidity dependence due to the influence of moisture in the atmosphere, and corrosiveness. This can prevent deterioration of the reference electrode layer due to gas, condensed water, etc.

Furthermore, according to the configuration of the third means, by adding an oxygen ion conductive substance to the reference electrode layer b, the oxygen partial pressure in the reference electrode layer b can be stabilized, and stable output can be obtained over a long period of time. be.

[0021]

[Embodiment] The first embodiment of the present invention will be described below with reference to FIGS.
This will be explained with reference to FIG.

[0022] Note that the same parts in the conventional example are given the same numbers and their explanations will be omitted. As shown in FIG. 1, a pair of electrode layers 109, a sensing electrode layer 102 and a reference electrode layer a1, are formed at both ends of a NAXS plate 101 made of sodium ion conductive ceramics. The reference electrode layer a1 is made of a perovskite oxide La0.5Sr0.5Co03 powder, which is a mixed conductive ceramic that is an oxidation catalyst and has conductivity for both oxygen ions and electrons, in an Au paste that is an electrode material, that is, a catalyst material. 2 was added in an amount of about 10% by weight, mixed to form a paste, which was screen printed or coated, dried and then fired.

Carbon dioxide sensor 1 configured as above
10, its operation will be explained below. In the above configuration, the influence of moisture in the atmosphere on (Chemical formula 2) shown in the conventional example is moderated by the catalytic action of the catalyst substance 2 added to the reference electrode layer a1. As a result, the dependence of the output on atmospheric humidity is reduced. Figure 2 shows the measurement results. The vertical axis shows the output change amount (ΔEmf), and the horizontal axis shows the humidity (dew point) of the measurement atmosphere. Further, since the catalyst substance 2 decomposes corrosive gas (for example, H2S, C12) that comes into contact with the reference electrode layer a1, it is possible to prevent the reference electrode layer a1 from deteriorating due to the corrosive gas.

As described above, according to the carbon dioxide sensor 110 of the first embodiment of the present invention, the influence of moisture on the reference electrode layer a1 can be reduced, and the dependence of the output on humidity can be reduced. Furthermore, deterioration of the reference electrode layer a1 due to corrosive gas can be prevented.

In this example, the catalyst material 2 is La0.
Although 5Sr0.5Co03 is used, any substance may be used as long as it is an oxidation catalyst and does not inhibit the reaction in the reference electrode layer a1 shown in (Chemical formula 2). Next, a second embodiment of the present invention will be described with reference to FIG. It should be noted that the same parts as in the conventional example are given the same numbers and the description thereof will be omitted.

As shown, the reference pole layer 104 and N
A gas barrier layer 3 is coated to cover a part of the AXS board 101. The gas barrier layer 3 is an amorphous glass layer formed by applying a paste-like amorphous glass sealing material, followed by drying and firing.

In the above structure, the reference electrode layer 104 is completely shielded from the atmosphere by the gas barrier layer 3. Therefore, the reference electrode layer 104 is not affected by moisture in the measurement atmosphere, and the dependence of the output on atmospheric humidity can be prevented. At the same time, corrosive gases and the like are also blocked, so that deterioration of the reference electrode layer 104 can be prevented. In addition, the detection electrode layer 10 may be damaged due to dew condensation water.
Even if the metal carbonate 103 of No. 2 is eluted, the solution does not reach the reference electrode layer 104 due to the gas barrier layer 4, so sensor deterioration due to metal carbonate 103 adhering to the reference electrode layer 104 can be prevented. .

As described above, the carbon dioxide sensor 110 of the second embodiment of the present invention can prevent the humidity dependence of the output and prevent the reference electrode layer 104 from deteriorating.

Although the gas barrier layer 3 is an amorphous glass layer in this embodiment, it may be a glass layer or a resin layer as long as it is stable at the operating temperature of the sensor and does not allow gas to pass therethrough.

Next, a third embodiment of the present invention will be described with reference to FIG. It should be noted that the same parts as in the conventional example are given the same numbers and the description thereof will be omitted.

As shown in the figure, the reference electrode layer b4 is doped with YSZ, which is stable zirconia doped with yttrium, as the oxygen ion conductive material 5, and the reference electrode layer b4 and N
A gas barrier layer 3 is coated to cover a part of the AXS board 101.

In the above structure, the reference electrode layer b4 is shielded from the atmosphere by the gas barrier layer 3, so that it is not affected by atmospheric humidity and is further protected from corrosive gases. Therefore, dependence of the output on atmospheric humidity and deterioration of the reference electrode layer b4 due to corrosive gas are prevented.

Furthermore, the oxygen ion conductive material 5 added to the reference electrode layer b4 stabilizes the oxygen partial pressure within the reference electrode layer b4 at a value inherent to the oxygen ion conductive material 5. Therefore, the potential of the reference electrode layer is kept constant and stable output can be obtained.

As described above, according to the carbon dioxide sensor 110 of the third embodiment of the present invention, it is possible not only to prevent humidity dependence of the output and prevent deterioration of the reference electrode layer 104, but also to maintain stability over a long period of time. I get the output.

In this example, the oxygen ion conductive material 5 was YSZ, but La0.5Sr0.5Co03 or CS, which is stabilized zirconia containing calcium, may also be used.
Z, platinum, or platinum black may be used.

[0036]

As described above, according to the present invention, by adding a catalyst substance to the reference electrode layer, the dependence of the output on atmospheric humidity can be reduced, and furthermore, deterioration due to corrosive gases can be prevented.

Furthermore, by coating part or all of the surface of the reference electrode layer or the ion conductive ceramic plate including the reference electrode layer with a gas barrier layer, it is possible to easily add a gas barrier layer to a conventional gas sensing section. Since the detection electrode is isolated from the atmosphere, it is possible to prevent humidity dependence of the output caused by the detection electrode.Furthermore, the gas barrier layer protects the reference electrode from corrosive gases in the atmosphere, preventing deterioration of the reference electrode. It can be prevented. Further, deterioration of the sensor due to dew condensation water can be prevented.

Furthermore, by adding an oxygen ion conductive substance to the reference electrode layer, the oxygen partial pressure in the sensing electrode layer becomes constant, making it possible to obtain stable output over a long period of time.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view showing the configuration of a carbon dioxide sensor according to a first embodiment of the present invention.

[Figure 2] Graph showing the atmospheric humidity dependence of the output of the carbon dioxide sensor and the conventional carbon dioxide sensor

FIG. 3 is a vertical cross-sectional view showing the configuration of a carbon dioxide sensor according to a second embodiment of the present invention.

FIG. 4 is a vertical cross-sectional view showing the configuration of a carbon dioxide sensor according to a third embodiment of the present invention.

[Figure 5] Vertical cross-sectional view showing the configuration of a conventional carbon dioxide sensor

[Explanation of symbols]

1 Reference electrode layer a 2 Catalyst material 3 Gas barrier layer 4 Reference electrode layer b 5 Oxygen ion conductive material 101 Ion conductive ceramic plate (NAXS
plate) 102 Sensing electrode layer 103 Metal carbonate 104 Reference electrode layer 105 Heating section 106 Gas sensing section 109 Pair of electrode layers 110 Carbon dioxide sensor

Claims (3)

[Claims] 1. A pair of electrode layers are provided at both ends of an ion-conducting ceramic plate made of a solid electrolyte, and one electrode layer of the ion-conducting ceramic plate is made of metal carbonate that forms a dissociative equilibrium with carbon dioxide gas. A gas sensing section configured by adding salt to form a sensing electrode layer and adding a catalyst substance to the other electrode layer to form a reference electrode layer a, and heating to heat the gas sensing section to a measurement temperature. A carbon dioxide gas sensor comprising: 2. A pair of electrode layers is provided at both ends of an ion-conductive ceramic plate made of a solid electrolyte, and one electrode layer of the ion-conductive ceramic plate is made of metal carbonate that forms a dissociative equilibrium with carbon dioxide gas. A gas sensing section formed by adding salt to form a sensing layer, and covering part or all of the surface of the other reference electrode layer or the ion conductive ceramic plate including the reference electrode layer with a gas barrier layer. and a heating section that heats the gas sensing section to a measurement temperature. 3. The carbon dioxide gas sensor according to claim 2, wherein the reference electrode layer b includes an oxygen ion conductive substance.