JPH1130603A - Concentration cell type carbon dioxide gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve responsiveness and prevent the exfoliation and fall of a layer by laminating an auxiliary electrode material -catalyst mixed layer supporting catalyst acting in such a way as to produce alkenes from alcohol, on an auxiliary electrode material layer laminated on a detecting electrode so as to keep dissociation equilibrium with CO2 . SOLUTION: A detecting electrode 2 is laminated on one side of a solid electrolyte layer 1, and a reference electrode 3 is laminated on the other side. An auxiliary electrode material layer 4a formed of carbonate, and a carbonate- catalyst mixed layer 4b formed of carbonate and a catalyst, are laminated as an auxiliary electrode layer on the detecting electrode 2. The whole periphery of the side face of the solid electrolyte layer 1 is covered with a seal 6 formed of an inorganic adhesive. A gas sensor element is accommodated in a recessed part of a ceramic heater body 7 including a heating body 7a, and the gas sensor element and the ceramic heater body 7 are jointed to each other by the seal 6. This constitution prevents the exfoliation and fall of the auxiliary electrode material - catalyst mixed layer formed in order to suppress the influence of miscellaneous gas such as alcohol.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a concentration cell type carbon dioxide sensor, and more particularly to a concentration cell type carbon dioxide sensor used for air conditioning, agriculture, industry, and medical use.

[0002]

2. Description of the Related Art Generally, a concentration cell type carbon dioxide sensor is
Since it is sensitive not only to CO ₂ but also to organic gases such as alcohol and water vapor, fluctuations in electromotive force caused by the influence of such miscellaneous gases are directly output as errors in CO ₂ concentration. In such a concentration cell type (solid electrolyte type) carbon dioxide gas sensor, providing a catalyst having a function of decomposing alcohol into CO ₂ and H ₂ O, that is, providing a combustion effect, requires an output of the sensor due to generated CO ₂ . It is not suitable because it fluctuates. Therefore, in the concentration cell type carbon dioxide gas sensor, a catalyst having an action of generating alkenes from the primary alcohol, specifically, an oxidation catalyst such as silica-alumina, γ-alumina, and zeolite is used.

[0003] Various gas sensors have been proposed to reduce the influence of miscellaneous gases using such a catalyst or the like. For example, Japanese Unexamined Patent Publication No. 6-222042 discloses a concentration cell type gas (CO ₂ , NOx, SOx) sensor using a solid electrolyte, in which gold or platinum is deposited on the solid electrolyte for the purpose of improving the stability of sensor output. There has been proposed a structure in which a detection electrode formed by mixing zeolite is laminated, and an auxiliary electrode layer (sub-electrode layer) made of lithium carbonate or barium nitrate is further laminated on the detection electrode.

Further, Japanese Utility Model Laid-Open No. 6-12954 discloses that
Concentration cell type gas (CO ₂ , NOx,
Regarding the SOx) sensor, a carbon dioxide sensor having the following structure has been proposed for the purpose of removing the influence of alcohol. That is, this carbon dioxide gas sensor is laminated in the order of a solid electrolyte layer, a detection electrode, a detection material layer (sub-electrode layer made of alkali metal carbonate), and an oxidation catalyst layer. Then, the gas sensor element including the outer peripheral portion of the upper surface of the detection material layer is sealed with a sealing material, and the oxidation catalyst layer covers the central portion of the detection material layer and also covers the gas sensor element from outside the sealing material. Wrapping. Note that the sealing material adheres the detection material layer and the oxidation catalyst layer. In addition, an alcohol adsorption filter is mounted at a predetermined position of the oxidation catalyst layer, and an atmosphere gas is introduced into the atmosphere gas contact surface (the contact surface with the oxidation catalyst layer) of the detection material layer via the alcohol adsorption filter. You.

[0005]

However, according to the gas sensor disclosed in JP-A-6-222042, the detection electrode is made of a sintered body in which a noble metal and an oxidation catalyst are mixed.
Since the alkali metal carbonate (sub-electrode substance) is laminated on the detection electrode, the three-phase interface between the detection electrode, the solid electrolyte, and the atmospheric gas is reduced, and as a result, the sensor characteristics are reduced. .

Further, according to the gas sensor disclosed in Japanese Utility Model Laid-Open No. 6-12954, alcohol is adsorbed using a filter. However, the adsorbability of this filter is limited, and the filter is saturated in a relatively short time. Therefore, this gas sensor cannot be expected to have such a large effect in practical use.
In addition, since the gas sensor element is wrapped by the sealing material, there is a problem that the test gas does not reach the element part, and the response speed of the sensor is reduced. The reason for using a sealing material to wrap the entire gas sensor element is that
The melting point of the alkali metal carbonate is generally low, and the sinterability of the catalyst material at a temperature lower than the melting point of the carbonate is poor.
It is difficult to firmly install these oxidation catalysts on the surface of the sensing material layer (alkali metal carbonate), and this is because the catalyst layer is often prevented from peeling and falling. But,
On the other hand, the use of a sealing material for sealing most of the gas sensor element causes the above-described problem of reduced responsiveness.

[0007] In view of the above circumstances, an object of the present invention is to provide a concentration-cell-type carbon dioxide sensor having good responsiveness and preventing peeling and falling off of a layer.

[0008]

According to a first aspect of the present invention, there is provided a concentration cell type carbon dioxide sensor according to the present invention, wherein a sub-electrode layer is laminated on a detection electrode and dissociates with CO _2. A sub-electrode material layer composed of a sub-electrode material, and a sub-electrode material-catalyst mixture in which a catalyst laminated on the sub-electrode material layer and acting to generate alkenes from alcohol is carried on the sub-electrode material. And a layer.

According to a second aspect, based on the first aspect, the sub-electrode material-catalyst mixed layer contains 20 to 50% by weight of the catalyst.

According to a third aspect, based on the second aspect, the sub-electrode material-catalyst mixed layer is formed by heating the mixture of the sub-electrode material and the catalyst at a temperature near the melting point of the sub-electrode material. In this case, it is baked on the sub-electrode material layer.

Hereinafter, the operation of the present invention will be described. In the concentration cell type carbon dioxide gas sensor, it is possible to avoid the influence of H ₂ O by appropriately selecting the solid electrolyte material, but since the influence of the organic gas cannot be avoided by such a method, another means is required. Need to take. The catalyst (preferably an oxidation catalyst) used in the present invention is one that generates an alkene by dehydrating a primary alcohol at least as shown in the following formula.

[0012] _{C n H 2n + 1 OH →} C n H 2n + H 2 O

Such a catalyst has some effect on some other kinds of organic vapors (for example, vapors of acetone, methyl ethyl ketone, etc.). When a mixture of a sub-electrode material (for example, carbonate) and a catalyst is baked on a sub-electrode material layer made of carbonate, the carbonate contained in the mixture serves as a sintering aid. The mixture itself is relatively firmly sintered, and the bonding strength between the underlying sub-electrode material layer and the sub-electrode material-catalyst mixed layer also becomes relatively high. Therefore, it is not necessary to bond the sub-electrode material layer and the sub-electrode material-catalyst mixed layer using a sealing material. Further, since no additive is contained in the sub-electrode substance in contact with the detection electrode, the three-phase interface between the solid electrolyte layer, the detection electrode, and the sub-electrode does not decrease as compared with the case where no catalyst is provided, and is equivalent to Sensor characteristics can be obtained.

Incidentally, the sub-electrode material forms a dissociation equilibrium with CO ₂ . In certain high temperature, solid electrolyte of the reference electrode side of the O _2, carbonates of the detection electrode side of the CO ₂ and O _2, for making the respective dissociation equilibrium, the partial pressure of CO ₂ and O ₂ in the gas phase Accordingly, a gradient of the chemical potential occurs in the solid electrolyte. To offset this, a concentration cell electromotive force according to the Nernst equation is generated. Under normal conditions, the O ₂ partial pressure near the reference electrode and the detection electrode is almost constant, so that the sensor output actually depends only on the CO ₂ concentration near the detection electrode.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred method for manufacturing a sub-electrode layer according to one embodiment of the present invention will be described. First, a substantially plain sub-electrode material layer made of carbonate, which is a sub-electrode material, is laminated on the sensing electrode laminated on the solid electrolyte layer. As for the laminating method, known methods such as printing, brush painting, and vapor deposition can be applied without any limitation. Next, a mixture of the oxidation catalyst and the sub-electrode substance is applied by printing, brush painting, or the like so as to cover the detection electrode, and is baked at a temperature near the melting point of the base carbonate, preferably lower than the melting point. In this case, since the carbonate contained in the mixture serves as a sintering aid, the mixture itself is relatively firmly sintered, and the adhesive strength between the underlying carbonate and the mixture is relatively high. The response is improved as the thickness of the solid electrolyte layer is reduced, but the strength is reduced. As for the sub-electrode material layer constituting one of the sub-electrode layers, a thinner one is advantageous in strength,
It is preferable to secure a thickness enough to cover the wire mesh. As the thickness of the sub-electrode material-catalyst mixed layer constituting the two sub-electrode layers increases, the catalytic effect increases, but the strength tends to decrease.

The mixing ratio between the carbonate and the oxidation catalyst is not particularly limited. However, if the amount of the oxidation catalyst is too small as compared with the carbonate, the effect as the oxidation catalyst layer is reduced, and the amount of the oxidation catalyst is large. If the amount is too high, the adhesion strength to the carbonate as a base is reduced. Therefore, the ratio of the catalyst to the whole mixture is preferably about 20 to 50% by weight.

The solid electrolyte layer, which is an ionic conductor layer, preferably has an alkali metal ion or an alkaline earth metal ion as a conductive species. More preferably, lithium or sodium ions are used as the conductive species. As a solid electrolyte using lithium ions as a conductive species, for example, LISICON (Lithium Super IonicCon
ductor), LiAlSiO ₄ , LiRSiO ₄ (R; lanthanide element such as Sm, La, Nd or Y),
Li _3.6 Si _0.6 P _0.4 O ₄ and LiTi ₂ (PO ₄ ) ₃ can be used. Among them, particularly preferred is Li
The main crystal phase is SmSiO ₄ . The lithium ion conductive solid electrolyte may contain Al ₂ O ₃ or ZrO _2, yttria-stabilized zirconia, calcia-stabilized zirconia, or the like, in addition to the main crystal phase. By containing these, the mechanical strength of the solid electrolyte itself is improved, and a sensor characteristic that is less affected by humidity is obtained.

As a solid electrolyte using sodium ions as a conductive species, for example, Na _{1 + X} referred to as NASICON
_{Zr 2 Si X P 3-x} O 12 (0 ≦ X ≦ 3), although beta-alumina _{(Na 2 O · nAl 2 O} 3, 5 ≦ n ≦ 11) or the like is used, NaAlSi ₃ O _10, NaAlSi ₄ O _{10 and the} like can also be used. Of these, NAS is particularly preferred.
ICON (particularly, X = 2 in the above general formula) is used as a main crystal phase. Incidentally, in the sodium ion conductive solid electrolyte, ZrO
₂ or yttria-stabilized zirconia, calcia-stabilized zirconia, or the like. This is because, by containing these, changes in sensor output due to humidity, that is, humidity dependency can be reduced.

The material of the reference electrode or the sensing electrode is preferably platinum, gold, silver, copper, or an alloy containing these, which are good electrical conductors. The electrode is made of the above-mentioned metal or its compound, or a paste obtained by blending an organic binder or the like with the above-mentioned metal or a compound thereof.
For example, baking for minutes, electroplating, electroless plating, hot-dip plating, thermal spraying, vapor deposition, ion plating, mechanical plating, or by forming a metal coating on a predetermined surface of the solid electrolyte layer by a known method such as a printing method It is preferable to make it. In particular, it is preferable to use gold or platinum or a paste containing them from the viewpoint of corrosion resistance. Further, the solid electrolyte layer, the reference electrode, and the detection electrode may be integrally fired. Alternatively, the metal electrode may be made porous by using a metal mesh (wire mesh) or adding a combustible substance such as an organic bead to a paste containing a metal and burning it off after application.

Examples of the material of the sub-electrode material include alkali metal or alkaline earth metal carbonates such as sodium carbonate, lithium carbonate, calcium carbonate and magnesium carbonate.
Various carbonates that dissociate with O ₂ can be used. Specifically, these carbonates are appropriately mixed with water, a solvent such as alcohol and an organic binder to form a paste or a solution, applied to the surface on which the detection electrode is formed or arranged, and dried at a predetermined temperature. It is formed by firing. It is preferable that the carbonate is laminated on the sensing electrode in a predetermined amount.

As the catalyst, a catalyst having an action of generating alkenes from the primary alcohol, for example, an oxidation catalyst such as silica-alumina, γ-alumina, zeolite or the like is used.
This is mixed with the sub-electrode material and sintered to form a sub-electrode material-catalyst mixed layer. Specifically, for example, a mixed paste of a carbonate and an oxidation catalyst is printed on the surface of the sub-electrode material layer,
The baking is performed at a temperature near the melting point of the carbonate, and more preferably at a temperature lower than that.

The obtained carbon dioxide gas sensor can measure the CO ₂ concentration by measuring the potential difference between the reference electrode and the detection electrode using a voltmeter or the like. The lead for connection to the voltmeter can be formed by superimposing one end of the lead wire on one end of each of the above-mentioned electrodes formed by a printing method or the like and simultaneously firing, but after forming each electrode, the lead wire is brazed. You may connect. Alternatively, a part of the electrode may be metallized, and the lead wire from the voltmeter may be connected by brazing. Further, the carbon dioxide sensor element may be attached to a ceramic heater or the like, or may be inserted therein. In this case, a lead portion connected to the electrode of the carbon dioxide sensor is provided on the ceramic heater side, and the ceramic heater is provided. And the carbon dioxide sensor can be electrically connected. When a ceramic heater is provided, the ceramic heater may be provided on the reference electrode side of the carbon dioxide gas sensor. Conversely, a ceramic heater or the like may be provided inside the carbon dioxide gas sensor.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS LiSmSiO as a solid electrolyte layer_Four, Vice
Li as an extreme substance (carbonate)_TwoCO _ThreeCarbon dioxide gas
A sensor was fabricated. That is, first, Li_TwoCO_Three, Sm_Two
O_Three, SiO_TwoOf each powder reagent in the molar ratio of metal elements
Li: Sm: Si = 1: 1: 1
After mixing them in a resin mill, Al_TwoO_Three10 in the crucible
Calcined at 00 ° C and pulverized in an alumina ball mill to obtain Li
SmSiO_FourA powder was obtained. A binder (hi) is added to the obtained powder.
(Droxypropyl cellulose) (LiSmS
iO_Four5 parts by weight for 100 parts by weight of powder)
After forming, cold isostatic pressing (CIP)
LiSmSiO fired in air at 1100 ° C for 3 hours_FourSystem
A sintered body of the body electrolyte was obtained.

Separately, a reference electrode was formed by thick-film printing using gold paste on a ceramic heater body sandwiching a heating element using alumina as a base. A solid electrolyte layer prepared as described above is laid thereon, and a detection electrode and a grid-like wire mesh (φ0.12Au wire plainly woven with a detection electrode) electrically connected to each other by thick film printing using gold paste on the upper surface of the solid electrolyte layer. (# 50 mesh) and wiring for connecting lead wires were formed. Then, in order to cut off the contact between the reference electrode and the test gas or the contact between the side surface of the solid electrolyte layer 1 and the test gas, the side of the solid electrolyte is coated with an inorganic adhesive (commercially available SiO ₂ -Al ₂ O). _(3- BaO-based dielectric paste), and the ceramic heater body and the solid electrolyte layer were bonded. Next, a thick film of a carbonate (Li ₂ CO ₃ ) paste is printed on the detection electrode, dried, and baked at 550 ° C. to form a substantially simple sub-electrode material layer. Carbonate (Li ₂ CO ₃ ) and oxidation catalyst (γ-Al ₂ O ₃ ) on top
Was mixed at a mass ratio of 1: 1 to form a thick film, and the paste was baked at a temperature (500 ° C.) having a melting point of 618 ° C. or less of carbonate to laminate a carbonate-catalyst mixed layer.

FIG. 1 schematically shows the structure of the carbon dioxide gas sensor thus manufactured. Referring to FIG. 1, a detection electrode (upper electrode, detection electrode) 2 is laminated on one side of solid electrolyte layer 1, and a reference electrode (reference electrode, lower electrode) 3 is laminated on the other side. As the layers, a sub-electrode material layer (carbonate layer) 4a made of carbonate and a carbonate-catalyst mixed layer 4b made of carbonate and catalyst are laminated so as to cover the sub-electrode material layer 4a. The entire periphery of the solid electrolyte layer 1 is covered with a seal 6 made of an inorganic adhesive. The gas sensor element is housed in a concave portion of the ceramic heater main body 7 including the heating element 7a, and the gas sensor element and the ceramic heater main body 7 are joined to each other by a seal 6. In addition, a wire mesh 8 is disposed in the sub-electrode material layer 4a so as to be electrically connected to the detection electrode 3. The thickness of each layer is as follows. Solid electrolyte layer 1: 0.2 mm, sensing electrode 2: 0.15 mm, reference electrode 3: 0.1 mm, sub-electrode material layer 4a: 0.35 mm, carbonate-catalyst mixed layer 4b:
0.1 mm or less, seal 6: 0.2 mm, ceramic heater body 7: 0.78 mm, heating element 7 a: 0.01 m
m, wire mesh 8: about 0.31 mm (apparent thickness).

Further, the following two types of sensors were manufactured as comparative examples. In the sensor of Comparative Example 1 shown in FIG. 2, a catalyst-only paste is baked on a sub-electrode material layer 4a made of a carbonate alone to laminate a catalyst layer 4c having a thickness of 0.1 mm or less. The sensor of Comparative Example 2 shown in FIG.
Only one layer is a mixed paste of carbonate and catalyst (1:
1) is obtained by laminating a sub-electrode substance-catalyst mixed layer 4d, which includes a wire mesh 8. Other structures and manufacturing methods of the sensors of Comparative Examples 1 and 2 are the same as those of the above-described example.

[Measurement Example 1] In order to examine the strength of the carbonate-catalyst mixed layer, the sensors of the above embodiment and comparative example 1 were respectively dropped on a concrete floor from a height of about 200 mm, and the sub-electrode layer was formed. 4a each laminated on carbonate-
The number of peelings of the catalyst mixed layer 4b (Example) and the catalyst layer 4c (Comparative Example 1) were examined. As shown in Table 1, according to the sensor of the example, peeling of the carbonate-catalyst mixed layer 4b laminated on the sub-electrode material layer 4a did not occur. On the other hand, according to the sensor of Comparative Example 1, peeling of the catalyst layer 4c laminated on the sub-electrode material layer 4a occurred every time.

[0028]

[Table 1]

[Measurement Example 2] The sensitivity of the sensors of Example and Comparative Examples 1 and 2 was measured. In the field of the concentration cell type carbon dioxide sensor, a linear relationship is established between the sensor electromotive force and the test gas concentration.
The change in electromotive force when the change is 0 times is defined as sensitivity. The same applies to this measurement example. A voltmeter is electrically connected to the reference electrode and the detection electrode, a test gas of a predetermined concentration is supplied, the heater power consumption is 1.3 W, and the heater temperature is 4
The temperature was set to 50 ° C. (set value), and the electromotive force generated between the reference electrode and the detection electrode in dry air was measured. As shown in Table 2 and FIG. 4, the sensor (catalyst supporting structure) of the example had sufficient sensitivity. Note that the sensor of Comparative Example 2 has lower sensitivity than the sensor of Example and Comparative Example 1 because the catalyst was added to the sub-electrode material (carbonate-catalyst mixed layer 4d), and the sensitivity of the sensor was lower than that of the solid electrolyte layer 6. The three-phase interface between the electrode 2 and the carbonate-catalyst mixed layer 4d as the sub-electrode layer is reduced,
As a result, the electrode reaction was inhibited.

[0030]

[Table 2]

[Measurement Example 3] Ethanol having a predetermined concentration was supplied to each of the sensors of Example and Comparative Examples 1 and 2, and fluctuations in sensor outputs (electromotive force) were measured.
As shown in Table 3, the results showed that the variation of the sensor output in the example was as small as +5 mV or less, which was within the general allowable range.

[0032]

[Table 3]

[0033]

According to the present invention, exfoliation and dropping of the sub-electrode material-catalyst mixed layer formed for suppressing the influence of miscellaneous gases such as alcohol are prevented, and the sensitivity and the response are excellent. A carbon dioxide sensor is provided. INDUSTRIAL APPLICABILITY The carbon dioxide sensor of the present invention is suitably applied to, for example, an automatic ventilation device for air conditioning, a promotion of crop growth in house cultivation and the like for agriculture, and a freshness control during transport and storage of crops. The content of the catalyst in the auxiliary electrode material-catalyst mixed layer is 20 to 50 watts.
The required catalytic effect can be obtained and peeling of the layer can be sufficiently prevented. In particular, even when the sub-electrode material-catalyst mixed layer is baked at a temperature lower than the melting point of the sub-electrode material layer (relatively low temperature) (the relatively low temperature), the sub-electrode material contained in the mixture may be sintering aid Therefore, the mixture itself is relatively firmly sintered, and the adhesive strength between the underlying sub-electrode material layer and the sub-electrode material-catalyst mixed layer also increases.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a carbon dioxide sensor according to one embodiment of the present invention.

FIG. 2 is a diagram for explaining a carbon dioxide sensor according to Comparative Example 1.

FIG. 3 is a diagram for explaining a carbon dioxide sensor according to Comparative Example 2.

FIG. 4 is a view showing gas concentration-electromotive force characteristics of carbon dioxide sensors according to an example and comparative examples 1 and 2.

[Explanation of symbols]

 Reference Signs List 1 solid electrolyte layer 2 detection electrode 3 reference electrode 4a sub-electrode material layer (one of sub-electrode layers) 4b carbonate-catalyst mixed layer (two sub-electrode layers) 6 seal 7 ceramic heater main body 7a heating element 8 wire mesh

Claims (3)

[Claims] 1. A carbonate comprising a solid electrolyte layer, a reference electrode and a detection electrode formed on the solid electrolyte layer, and a sub-electrode layer dissociating equilibrium with CO ₂ laminated on the detection electrode. In the gas sensor, the sub-electrode layer is stacked on the detection electrode and is formed of a sub-electrode material that dissociates with CO _2, and an alkene is generated from alcohol stacked on the sub-electrode material layer. A concentration cell type carbon dioxide sensor comprising: a sub-electrode substance-catalyst mixed layer in which a catalyst acting as described above is carried on the sub-electrode substance. 2. The concentration cell type carbon dioxide sensor according to claim 1, wherein the catalyst is contained in the sub-electrode substance-catalyst mixed layer in an amount of 20 to 50 wt%. 3. The sub-electrode material-catalyst mixed layer is baked on the sub-electrode material layer by heating the mixture of the sub-electrode material and the catalyst at a temperature near the melting point of the sub-electrode material. 3. The method according to claim 1, wherein
The concentration cell type carbon dioxide sensor as described in the above.