JP2006184252A - Gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas sensor which is equipped with a detection electrode having ample strength, and has superior output stability. <P>SOLUTION: In this gas sensor 10, a detection electrode 14 and a counter electrode 16 are provided on a substrate 12, and the detection electrode 14 is constituted of a current collecting layer 14a and a gas-detecting layer 14b, while the counter electrode 16 is constituted of a current collecting layer 16a and a protective layer 16b. In a preferred embodiment, the gas-detecting layer 14b contains a first metal oxide having a resistance value of 1×10<SP>10</SP>Ωcm or smaller, a second metal oxide, having a resistance value of 1×10<SP>10</SP>Ωcm or larger and a metal carbonate. The absolute value of the difference between the resistance value of the first metal oxide and the resistance value of the second metal oxide is 1×10<SP>3</SP>Ωcm or larger. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a gas sensor, and more particularly to a solid electrolyte type gas sensor.

  As a gas sensor that chemically detects a specific gas in the environment, a solid electrolyte gas sensor, a semiconductor gas sensor, and the like are known. In particular, since solid oxide gas sensors have excellent features such as easy handling and miniaturization, various studies have been conducted in recent years.

  A solid electrolyte type gas sensor has a structure in which a detection electrode and a counter electrode are provided on a substrate containing a solid electrolyte, and a change in electromotive force between the detection electrode and the counter electrode before and after contact between the gas to be detected and the detection electrode. The gas is detected based on the above.

  The solid electrolyte type gas sensor having the above structure is often used under a high temperature condition exceeding 400 ° C. in order to reduce the influence of humidity. However, under such high temperature conditions, the sensitivity is likely to vary due to the occurrence of convection due to heating. However, if a low temperature operation is attempted to avoid such inconvenience, the time until the gas detection operation can be stably performed after startup (hereinafter referred to as “rise time”) becomes long. There was a tendency to cause inconvenience.

Therefore, as a gas sensor (carbon dioxide sensor) that enables operation at a lower temperature than that of the prior art, more preferably near normal temperature, and can sufficiently shorten the rise time described above, a detection electrode containing a predetermined metal oxide is provided. What is provided is disclosed (for example, refer to Patent Document 1).
Japanese Patent Laid-Open No. 11-271270

  The gas sensor according to the prior art described above can operate at a low temperature and has a sufficiently short rise time, but there is still room for improvement in terms of the strength of the detection electrode containing a metal oxide. Specifically, the detection electrode may be damaged when a force greater than usual is applied to the detection electrode, for example, when static electricity is generated or the flow rate of the test gas is large. In this case, the detection of the gas to be detected by the detection electrode tends to be insufficient.

  Patent Document 1 describes that the detection electrode further includes a metal carbonate in addition to the metal oxide. However, in this case, the metal oxide and the metal carbonate may form a composite salt depending on the operating conditions (temperature, test gas flow rate, etc.). If it becomes like this, problems will arise in terms of output stability, such as it becomes difficult to stably obtain the electromotive force (output) of the gas sensor over a long period of time.

  Therefore, the present invention has been made in view of such circumstances, and an object thereof is to provide a gas sensor having a detection electrode having sufficient strength and having excellent output stability.

In order to achieve the above object, a gas sensor of the present invention includes a substrate including a solid electrolyte, a detection electrode and a counter electrode provided on the substrate, and the detection electrode has a resistance value of less than 1 × 10 ¹⁰ Ωcm. A first metal oxide, a second metal oxide having a resistance value of 1 × 10 ¹⁰ Ωcm or more, and a metal carbonate, and a resistance value of the first metal oxide And the absolute value of the difference between the second metal oxide and the resistance value of the second metal oxide is 1 × 10 ³ Ωcm or more.

  The gas sensor of the present invention contains a combination of the first metal oxide having a relatively low resistance and the second metal oxide having a relatively high resistance in the detection electrode. Among these, the former first metal oxide includes the metal oxide used in the detection electrode of the gas sensor described in Patent Document 1, and similarly to these, the strength when an electrode or the like is formed is low. Often it will be sufficient. On the other hand, the second metal oxide having a resistance value higher than a certain value than the first metal oxide is composed of the first metal oxide when used in combination with the first metal oxide. It is considered that a function of binding particles and the like can be expressed. For this reason, the sensing electrode including these two kinds of metal oxides has sufficient strength as a whole, and as a result, the above-described defect occurs due to generation of static electricity or increase in gas flow rate. It will be difficult. In addition, the effect | action of this invention is not necessarily limited to these.

  The second metal oxide has a characteristic that it is difficult to form a composite salt with a metal carbonate. According to the study by the present inventors, when the second metal oxide is combined with the first metal oxide, the formation of a composite salt by the first metal oxide and the metal carbonate is also sufficient. It was found that it can be reduced. Therefore, in the gas sensor having the above configuration, formation of a composite salt of the first metal oxide and the metal carbonate is very unlikely to occur at the detection electrode. As a result, the gas sensor of the present invention has excellent output stability.

In the gas sensor of the present invention, it is more preferable that the first metal oxide contained in the detection electrode has a resistance value of 10 ⁷ or less. Such a gas sensor having a detection electrode containing the first metal oxide can perform low-temperature driving better and can have a shorter rise time, but it is further insufficient in terms of strength. Tend to be. However, in the present invention, since the second metal oxide is contained in combination as described above, even when the first metal oxide having such a lower resistance value is used, detection is performed. The pole will have sufficient strength.

Furthermore, in the gas sensor of the present invention, the second metal oxide is at least one selected from the group consisting of Al ₂ O ₃ , ZrO ₂ , Y ₂ O ₃ , HfO ₂ , B ₂ O ₃ and SiO ₂ . A metal oxide is more preferable. When these metal oxides are combined with the first metal oxide, the effect of improving the strength of the detection electrode and the effect of suppressing the formation of a composite salt with the metal carbonate can be expressed more favorably. .

  Furthermore, in the gas sensor of the present invention, the content of the second metal oxide in the first metal oxide, the second metal oxide, and the metal carbonate is 0.01 to 2% by mass. More preferably. When the content ratio of the second metal oxide is adjusted in such a range, the above-described effect can be obtained more satisfactorily.

The gas sensor of the present invention includes a substrate containing a solid electrolyte, and a detection electrode and a counter electrode provided on the substrate, and the detection electrode is a metal oxide having a resistance value of 1 × 10 ¹⁰ Ωcm or more. And a metal carbonate may be contained. As described above, the metal oxide having such a resistance value can form a sensing electrode having a sufficient strength as compared with those having a lower resistance value. Moreover, such a metal oxide is difficult to cause a reaction with a metal carbonate. Therefore, the gas sensor having the above configuration can have sufficient detection pole strength and output stability.

From the viewpoint of obtaining better detection pole strength and output stability, in the gas sensor, the metal oxides contained in the detection pole include Al ₂ O ₃ , ZrO ₂ , Y ₂ O ₃ , HfO ₂ , B ₂ O. _At least one metal oxide selected from the group consisting of ₃ and SiO ₂ is preferred.

  ADVANTAGE OF THE INVENTION According to this invention, while providing the detection pole which has the outstanding intensity | strength, it becomes possible to provide the gas sensor which has sufficient output stability.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected to the same element through all figures, and the overlapping description is abbreviate | omitted.

  FIG. 1 is a diagram schematically showing a cross-sectional structure of a gas sensor according to an embodiment of the present invention. As shown in FIG. 1, the gas sensor 10 includes a detection electrode 14 and a counter electrode 16 on a substrate 12 so as to be in contact with the substrate 12. The detection electrode 14 has a configuration in which a current collecting layer 14a and a gas detection layer 14b are stacked in this order from the substrate 12 side. The counter electrode 16 has a configuration in which a current collecting layer 16a and a protective layer 16b are laminated in this order from the substrate 12 side.

The gas sensor 10 having such a configuration is a so-called solid electrolyte gas sensor, and can be applied as a carbon dioxide gas sensor, a hydrogen gas sensor, a nitrogen oxide gas sensor, a carbon monoxide gas sensor, or the like by appropriately changing the material of each component. Here, a carbon dioxide (CO ₂ ) gas sensor will be described as an example of the gas sensor.

(Substrate 12)
The substrate 12 is a so-called solid electrolyte substrate containing a solid electrolyte. The substrate 12 is a solid ionic conductor. As this ion conductor, for example, NASICON represented by Na _{1 + x} Zr ₂ Si _x P _3-x O ₁₂ (x = 0 to 3), zirconia (ZrO ₂ ) is stabilized with yttria (Y ₂ O ₃ ). And metal ion conductors such as yttria-stabilized zirconia (YSZ) and barium-cerium-based oxides containing rare earth elements. Of these, NASICON is preferable as the ion conductor.

The substrate 12 preferably has a thickness of about 1 μm to 1 mm, and more preferably the area of the main surface (the surface on which the detection electrode 14 and the counter electrode 16 are provided) is about 1 μm ^{2 to} 200 mm ² . Such a substrate 12 can be formed by a solid phase method, a sol-gel method, a coprecipitation method, or the like, and among them, the coprecipitation method is preferable.

(Detection pole 14)
As described above, the detection electrode 14 includes the current collecting layer 14a and the gas detection layer 14b. In the detection electrode 14, the gas detection layer 14 b covers the current collection layer 14 a and is in direct contact with the substrate 12 at a portion other than the region covering the current collection layer 14 a.

  In the detection electrode 14, the current collecting layer 14a has a function of transferring electrons to and from the substrate 12 (solid electrolyte). The current collecting layer 14a is made of a material having conductivity that can function as a current collector. Examples of the constituent material of the current collecting layer 14a include a metal material. Specifically, gold (Au), platinum (Pt), silver (Ag), rubidium (Rb), rhodium (Rh), palladium (Pd), iridium (Ir), nickel (Ni), copper (Cu), Examples thereof include chromium (Cr) and alloys thereof.

The thickness of the current collecting layer 14a is preferably about 0.01 to ¹⁰ μm, and the area of the main surface (the surface in contact with the substrate 12) can be about 0.1 μm ^{2 to} 200 mm ² . The current collection layer 14a is more preferably porous in order to allow uniform distribution of the test gas within the detection electrode 14. The current collecting layer 14a having the above-described configuration can be formed by, for example, a method of applying a powder of a metal material in a paste form by screen printing or the like, a sputtering method, or the like.

The gas detection layer 14b in the detection electrode 14 generates a dissociation equilibrium by contact with the test gas, thereby detecting the test gas. The gas detection layer 14b contains at least a metal oxide having a resistance value of 1 × 10 ¹⁰ Ωcm or more and a metal carbonate, and has a metal oxide (first metal having a resistance value of less than 1 × 10 ¹⁰ Ωcm). Oxide) It is particularly preferable to contain a metal oxide (second metal oxide) having a resistance value of 1 × 10 ¹⁰ Ωcm or more and a metal carbonate. Thus, when the gas detection layer 14b includes both the first and second metal oxides, the absolute value of the difference in resistance value between the first metal oxide and the second metal oxide is 1 ×. It is preferably 10 ³ or more, and more preferably 1 × 10 ⁶ or more. When the value of this difference is less than 1 × 10 ³ , it is possible to improve the strength by adding the second metal oxide having a higher resistance compared to the case where it is more than that, and the effects of suppressing the formation of the composite salt, etc. Tends to be difficult to obtain.

The first metal oxide contained in the gas detection layer 14b has a resistance value of less than 1 × 10 ¹⁰ Ωcm, and preferably has a resistance value of 1 × 10 ⁷ or less, and generally exhibits electronic conductivity. It is more preferable that the resistance value is such that the resistance value is about 1 × 10 ⁶ or less. In addition, the resistance value as used in this specification shall mean the value measured by shape | molding a metal oxide in the element shape, for example by the voltage drop method. Here, the voltage drop method specifically refers to forming a metal oxide into a pellet shape, providing electrodes at both ends, connecting the pellet and a reference resistor in series, and then applying a constant voltage with a constant voltage power source. In other words, it refers to a method of measuring the resistance value by measuring the voltage drop of the reference resistance.

Examples of the first metal oxide include indium oxide (In ₂ O ₃ ), tin oxide (SnO ₂ ), cobalt oxide (Co ₃ O ₄ ), tungsten oxide (WO ₃ ), zinc oxide (ZnO), Lead oxide (PbO), copper oxide (CuO), iron oxide (Fe ₂ O ₃ , FeO), nickel oxide (NiO), chromium oxide (Cr ₂ O ₃ ), cadmium oxide (CdO), bismuth oxide (Bi ₂ O) ₃ ), manganese oxide (MnO ₂ , Mn ₂ O ₃ ), antimony oxide (Sb ₂ O ₃ ), lanthanum oxide (La ₂ O ₃ ), cerium oxide (CeO ₂ ), praseodymium oxide (Pr ₆ O ₁₁ ), oxidation neodymium _{(Nd 2 O} 3), silver _(Ag 2 O) oxide, lithium oxide _(Li 2 O), sodium oxide _(Na 2 O), potassium oxide _(K 2 O), oxidation Rubijiu _(Rb 2 O), cesium oxide _(Cs 2 O), magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO) may be exemplified. As the first metal oxide, one or more of these can be used.

  Among them, indium oxide, tin oxide, cobalt oxide, tungsten oxide, zinc oxide, lead oxide, copper oxide, iron oxide, nickel oxide, chromium oxide, cadmium oxide or Bismuth oxide is preferable, indium oxide, tin oxide, copper oxide or zinc oxide is particularly preferable, and indium oxide is most preferable. These may be slightly deviated from the stoichiometric composition.

The second metal oxide contained in the gas detection layer 14b has a resistance value of 1 × 10 ¹⁰ Ωcm or more, and generally has a resistance value capable of expressing insulation, for example, 1 × 10 ^12. It is more preferable that it has a resistance value of Ωcm or more.

Examples of the second metal oxide include aluminum oxide (Al ₂ O ₃ ), zirconium oxide (ZrO ₂ ), yttrium oxide (Y ₂ O ₃ ), hafnium oxide (HfO ₂ ), and boron oxide (B ₂ O ₃ ), silicon oxide (SiO ₂ ) and the like. Among these, at least one metal oxide selected from the group consisting of Al ₂ O ₃ , ZrO ₂ and SiO ₂ is preferable. These may be slightly deviated from the stoichiometric composition.

The gas detection layer 14b further contains a metal carbonate in addition to the first and second metal oxides described above. By including the metal carbonate in this manner, the generation of hydrogen carbonate ions necessary for detecting CO ₂ in the gas detection layer 14b is promoted, and as a result, the sensitivity, response speed, and the like of the gas sensor 10 are further improved.

Examples of the metal carbonate include lithium carbonate (Li ₂ CO ₃ ), sodium carbonate (Na ₂ CO ₃ ), potassium carbonate (K ₂ CO ₃ ), rubidium carbonate (Rb ₂ CO ₃ ), and cesium carbonate (Cs ₂ CO). ₃ ), magnesium carbonate (MgCO ₃ ), calcium carbonate (CaCO ₃ ), strontium carbonate (SrCO ₃ ), barium carbonate (BaCO ₃ ), manganese carbonate (Mn (CO ₃ ) ₂ , Mn ₂ (CO ₃ ) ₃ ), Iron carbonate (Fe ₂ (CO ₃ ) ₃ , FeCO ₃ ), nickel carbonate (NiCO ₃ ), copper carbonate (CuCO ₃ ), cobalt carbonate (Co ₂ (CO ₃ ) ₃ ), chromium carbonate (Cr ₂ (CO ₃ ) _3), zinc carbonate (ZnCO _3), silver carbonate _(Ag 2 _CO 3), cadmium carbonate (CdCO _3), carbonate indium (In _{(CO 3) 3),} yttrium carbonate _{(Y 2 (CO 3) 3} ), lead carbonate (PbCO _3), bismuth subcarbonate _{(Bi 2 (CO 3) 3} ), lanthanum carbonate _{(La 2 (CO 3) 3} ), Examples include cerium carbonate (Ce (CO ₃ ) ₃ ), praseodymium carbonate (Pr ₆ (CO ₃ ) ₁₁ ), neodymium carbonate (Nd ₂ (CO ₃ ) ₃ ), dysprosium carbonate (Dy ₂ (CO ₃ ) ₃ ), and the like. . Such metal carbonates may be used alone or in combination of two or more. Of these, lithium carbonate, sodium carbonate or potassium carbonate is preferred. In the gas detection layer 14b, such a metal carbonate is preferably contained in an amount of 1 to 99% by mass, and 5 to 50% by mass with respect to the first and second metal oxides described above. More preferred.

  The content ratio of the second metal oxide in the first metal oxide, the second metal oxide and the metal carbonate contained in the gas detection layer 14b is preferably 0.01 to 2% by mass, More preferably, it is 0.01-0.3 mass%. By adjusting the content ratio of the second metal oxide to such a range, the strength of the gas detection layer 14b is further improved, and the formation of a composite salt by the metal carbonate and the first metal oxide is further improved. It becomes possible to suppress. When the content ratio of the second metal oxide is less than 0.01% by mass, the strength of the gas detection layer 14b is not sufficiently improved and the formation of a complex salt of the first metal oxide and the metal carbonate is insufficient. Tend to be. On the other hand, if it exceeds 2% by mass, the conductivity of the gas detection layer 14b is excessively lowered, and the output of the gas sensor 10 tends to be hardly obtained.

  The gas detection layer 14b having such a configuration can be formed as follows, for example. That is, first, the first metal oxide, the second metal oxide, the metal carbonate and the like described above are dispersed in a predetermined solvent to obtain a paste. Next, this paste is applied so as to cover the current collecting layer 14a provided on the substrate 12 and its peripheral portion is in contact with the substrate. Thereafter, the paste layer is heated to remove the solvent, thereby forming the gas detection layer 14b.

  As the solvent used in such a method, a solvent that does not dissolve or react with the first metal oxide, the second metal oxide, the metal carbonate, or the like is preferable. As such a solvent, for example, aliphatic alcohol, aliphatic alcohol acetate, aliphatic alcohol propionate, terpene oil and the like are suitable. Further, from the viewpoint of simplifying the production of the gas sensor 10, those that can be easily removed at a relatively low temperature are preferred. Specifically, a solvent having a boiling point of less than 300 ° C is preferred, and a solvent having a boiling point of less than 250 ° C is more preferred. preferable.

  Thus, in the gas sensor 10 of the present embodiment, the gas detection layer 14b contains the first metal oxide and the second metal oxide described above in combination. Since the second metal oxide has a high resistance value, conventionally, when used as a constituent material of the detection electrode of the gas sensor, the conductivity of the detection electrode is lowered and the output of the gas sensor is easily lowered. There was a trend. On the other hand, in the gas sensor 10 of the present embodiment, since the second metal oxide is used in combination with the first metal oxide having a lower resistance, the conductivity of the gas detection layer 14b is practically used. It can be maintained in a sufficient range.

  Conventionally, low resistance metal oxides such as the first metal oxide tend to be brittle when fired as a paste. For this reason, the detection electrode (gas detection layer) made only of the first metal oxide may cause a defect when a stronger impact than that assumed in normal use is applied, which causes a decrease in the output of the gas sensor. It was the cause of causing. On the other hand, the gas sensor 10 of the present embodiment further includes the second metal oxide in the gas detection layer 14b as described above. Such a second metal oxide is considered to have a function of binding particles made of the first metal oxide. Therefore, the gas detection layer 14b including the first and second metal oxides in combination has an extremely excellent strength as compared with that formed only from the first metal oxide. As a result, the durability of the gas detection layer 14b and the element life of the gas sensor 10 are greatly improved.

  Furthermore, when applied to the sensing electrode of a gas sensor, a low-resistance metal oxide such as the first metal oxide reacts with a metal carbonate added simultaneously to form a composite salt depending on the usage environment. There was a case. As a result, the content of the metal carbonate in the gas detection layer 14b gradually decreases, and as a result, the output of the gas sensor decreases over time. On the other hand, the above-mentioned second metal oxide is extremely difficult to form a composite salt with a metal carbonate, and can also reduce the formation of a composite salt by the first metal oxide contained together. have. Therefore, in the gas detection layer 14b of the present embodiment including the first metal oxide, the second metal oxide, and the metal carbonate, formation of the composite salt as described above is extremely difficult to occur. The gas sensor 10 provided can obtain a stable output even after long-term use.

(Counter electrode 16)
The counter electrode 16 is provided on the substrate 12 so as to be separated from the detection electrode 14 and is constituted by the current collecting layer 16a and the protective layer 16b as described above. Specifically, the protective layer 16b is provided so as to cover the current collecting layer 16a. The protective layer 16b does not necessarily need to cover the entire surface of the current collecting layer 16a. The thickness of the counter electrode 16 as described above can be about 0.01 μm to 1 mm, including the current collecting layer 16a and the protective layer 16b.

  The current collecting layer 16a in the counter electrode 16 is not particularly limited as long as it is composed of a conductive material, and examples thereof include those composed of a metal material. Specifically, a material composed of the same material as that of the current collecting layer 14a in the detection electrode 14 described above is preferable. The thickness of the current collection layer 16a is preferably equal to the thickness of the current collection layer 14a described above.

  The protective layer 16b is provided to protect the current collecting layer 16a from moisture in the atmosphere. Examples of the protective layer 16b include those formed of fluorine-based resin, ceramics, cobaltate, and the like. The counter electrode 16 does not necessarily have such a protective layer 16b.

  The gas sensor according to one embodiment of the present invention has been described above, but the gas sensor of the present invention is not necessarily limited to the above-described configuration, and can be changed as appropriate.

  For example, in the above embodiment, a gas sensor having a structure having a detection electrode and a counter electrode on the same side with respect to the substrate is illustrated, but the present invention is not limited thereto, and as shown in FIG. 2, a detection electrode is provided on one side of the substrate, and The structure may have a counter electrode on the opposite side.

  FIG. 2 is a view showing a cross-sectional structure of a gas sensor according to another embodiment of the present invention. As shown in FIG. 2, the gas sensor 20 includes a substrate 22, a detection electrode 24 provided on one surface of the substrate 22, and a counter electrode 26 provided on the surface of the substrate 22 opposite to the detection electrode 24. I have.

  Similarly to the detection electrode 14 described above, the detection electrode 24 is provided so as to cover the current collector 24 a provided in contact with the substrate 22 and the current collection layer 24 a, and does not cover the current collector 24. The region is composed of a gas detection layer 24 b in contact with the substrate 22. The counter electrode 26 includes a current collecting layer 26a provided in contact with the substrate 22 and a protective layer 26b provided on the opposite side of the current collecting layer 26a from the substrate. Each of these components can be formed of the same material as each component of the gas sensor 10 described above.

  The gas sensor of the present invention may further include a heater unit that heats the detection electrode to a temperature at which stable operation is possible. For example, in the gas sensor 10, such a heater unit can be provided on the surface of the substrate 12 opposite to the side where the detection electrode 14 and the counter electrode 16 are formed. In the gas sensor 20, a heater portion can be provided on a side portion of the substrate 22 where the detection electrode 24 and the counter electrode 26 are not formed.

  Furthermore, the gas sensor of the present invention may further include an external circuit for measuring an electromotive force between the detection electrode and the counter electrode, an external circuit for energizing the heater unit described above, and the like.

  EXAMPLES Hereinafter, although an Example demonstrates this invention still in detail, this invention is not limited to these Examples.

Example 1
First, a NASICON substrate having a rectangular main surface (4 mm × 4 mm) and a thickness of 0.5 mm was prepared as a solid electrolyte substrate. On the NASICON substrate, a current collecting layer made of Au and having a thickness of 0.1 μm and a counter electrode were formed by a sputtering method. Next, 0.8 g of particulate lithium carbonate (Li ₂ CO ₃ ) and 4 g of particulate barium carbonate (BaCO ₃ ) were mixed and kneaded while heating to 750 ° C. to obtain these composite salts. . Subsequently, 6 g of particulate indium oxide (In ₂ O ₃ ) is added to the composite salt as a first metal oxide, and aluminum oxide (Al ₂ O ₃ , resistance is added as a second metal oxide. Value: 1 × 10 ¹² Ωcm or more) was added to the total amount of the above components so as to be 0.3% by mass and stirred. The obtained mixture was dispersed in α-terpineol having the same mass as this to prepare a paste-like gas detection material. The obtained gas detection material was applied and dried so as to cover the current collection layer, and then fired at 600 ° C. to form a gas detection layer having a shape covering the current collection layer.

  Thereafter, a thin film of Pt was formed as a heating part on the surface opposite to the detection electrode (current collection layer and gas detection layer) and the counter electrode on the NASICON substrate by sputtering to obtain a gas sensor element.

(Example 2)
A CO ₂ gas sensor was prepared in the same manner as in Example 1 except that zirconium oxide (ZrO ₂ , resistance value: 1 × 10 ¹² Ωcm or more) was used as the second metal oxide instead of Al ₂ O _3. Obtained.

(Example 3)
A CO ₂ gas sensor was prepared in the same manner as in Example 1 except that silicon oxide (SiO ₂ , resistance value: 1 × 10 ¹¹ Ωcm or more) was used as the second metal oxide instead of Al ₂ O _3. Obtained.

Example 4
As the second metal oxide, instead _{Al 2 O 3, Al 2 O} 3 and SiO _{2 composite;} except for using, (weight ratio _{Al 2 O 3: 2: SiO} 2 = 1) A CO ₂ gas sensor was obtained in the same manner as in Example 1.

(Example 5)
A CO ₂ gas sensor was obtained in the same manner as in Example 1 except that the content of SiO ₂ was 2% by mass.

(Example 6)
A CO ₂ gas sensor was obtained in the same manner as in Example 1 except that the content of SiO ₂ was 0.01% by mass.

(Example 7)
A CO ₂ gas sensor was obtained in the same manner as in Example 1 except that In ₂ O ₃ and tin oxide (SnO ₂ ) were used as the first metal oxide.

(Example 8)
As the first metal oxide, along with use of _In 2 _{O 3} and SnO _2, as the second metal oxide, in place of the _{Al 2 O 3, Al 2 O} 3 and SiO ₂ of the composite (weight ratio; A CO ₂ gas sensor was obtained in the same manner as in Example 1 except that Al ₂ O ₃ : SiO ₂ = 1: 2) was used.

Example 9
A CO ₂ gas sensor was obtained in the same manner as in Example 1 except that the content of SiO ₂ was 3% by mass.

(Example 10)
CO ₂ was used in the same manner as in Example 1 except that boron oxide (B ₂ O ₃ , resistance value: 1 × 10 ¹² Ωcm or more) was used as the second metal oxide instead of Al ₂ O _3. A gas sensor was obtained.

(Example 11)
CO ₂ was used in the same manner as in Example 1 except that yttrium oxide (Y ₂ O ₃ , resistance value: 1 × 10 ¹² Ωcm or more) was used as the second metal oxide instead of Al ₂ O _3. A gas sensor was obtained.

(Comparative Example 1)
A CO ₂ gas sensor was obtained in the same manner as in Example 1 except that Al ₂ O ₃ as the second metal oxide was not added.

(Comparative Example 2)
A CO ₂ gas sensor was obtained in the same manner as in Comparative Example 1 except that In ₂ O ₃ and SnO ₂ were used as the first metal oxide.
[Characteristic evaluation]

Using the CO ₂ gas sensors of Examples 1 to 11 and Comparative Examples 1 and 2, the output stability and peeling resistance of each gas sensor were evaluated as follows. The results obtained are summarized in Table 1.

(Output stability)
First, for each CO ₂ gas sensor immediately after production, the electromotive force (output) was measured under the condition that the CO ₂ concentration was 500 ppm and the humidity was 30% RH. And the electromotive force after operating each gas sensor on the same conditions and having passed 1000 hours was measured similarly. And the difference of the electromotive force before and after 1000-hour progress was calculated, and the obtained value was made into the value of output fluctuation. In Table 1, the output stability value is the value of each CO ₂ gas sensor when the value of the electromotive force change obtained by the CO ₂ gas sensor of Comparative Example 1 which is a general CO ₂ gas sensor is 100%. The relative value (%) of the obtained value was shown. In other words, it can be said that the smaller this value is, the better the output stability is than the conventional gas sensor.

(Peeling resistance)
Air of 0.2 MPa was blown on each CO ₂ gas sensor, and whether or not the gas detection layer was peeled was visually observed. The evaluation results in Table 1 are obtained based on the evaluation criteria shown below.
×: Most of the gas detection layer is peeled off and cannot be used as a sensor. ○: A part of the gas detection layer is peeled off or cracked. ◎: The gas detection layer has almost no peeling, and it can be used satisfactorily as a sensor.

From Table 1, according to the CO ₂ gas sensor of Example 1 to 11 using a combination of first and second metal oxides, Comparative Examples 1 and 2 using no component corresponding to the second metal oxide It was found that excellent output stability and peeling resistance can be obtained as compared with the CO ₂ gas sensor.

It is a figure showing typically the section structure of the gas sensor concerning one embodiment of the present invention. It is a figure which shows typically the cross-section of the gas sensor which concerns on other embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Gas sensor, 12 ... Board | substrate, 14 ... Detection electrode, 14a ... Current collection layer, 14b ... Gas detection layer, 16 ... Counter electrode, 16a ... Current collection layer, 16b ... Protection layer, 20 ... Gas sensor, 22 ... Substrate, 24 ... Detection electrode, 24a ... current collection layer, 24b ... gas detection layer, 26 ... counter electrode, 26a ... current collection layer, 26b ... protective layer.

Claims (6)

A substrate including a solid electrolyte, and a detection electrode and a counter electrode provided on the substrate,
The detection electrode is
A first metal oxide having a resistance value of less than 1 × 10 ¹⁰ Ωcm;
A second metal oxide having a resistance value of 1 × 10 ¹⁰ Ωcm or more;
And the absolute value of the difference between the resistance value of the first metal oxide and the resistance value of the second metal oxide is 1 × 10 ³ Ωcm or more. is there,
Gas sensor. The gas sensor according to claim 1, wherein the first metal oxide has a resistance value of 1 × 10 ⁷ or less. The second metal oxide is at least one metal oxide selected from the group consisting of Al ₂ O ₃ , ZrO ₂ , Y ₂ O ₃ , HfO ₂ , B ₂ O ₃ and SiO _2. The gas sensor according to 1 or 2.   The content of the second metal oxide in the first metal oxide, the second metal oxide, and the metal carbonate is 0.01 to 2% by mass. The gas sensor according to any one of the above. A substrate including a solid electrolyte, and a detection electrode and a counter electrode provided on the substrate,
The detection electrode is
A metal oxide having a resistance value of 1 × 10 ¹⁰ Ωcm or more;
A metal carbonate,
Gas sensor. The gas sensor according to claim 5, wherein the metal oxide is at least one metal oxide selected from the group consisting of Al ₂ O ₃ , ZrO ₂ , Y ₂ O ₃ , HfO ₂ , B ₂ O ₃ and SiO _2. .

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