Carbon monoxide gas sensor

Abstract

This carbon monoxide gas sensor 10 is a single chamber sensor for measuring the concentration of gaseous-phase carbon monoxide gas. The carbon monoxide gas sensor 10 has electrodes 12, 13 respectively arranged on the two surfaces of a solid electrolyte layer 11. One electrode among the electrodes is active for oxidization of carbon monoxide gas and the other electrode among the electrodes is less active for oxidization of carbon monoxide gas than the one electrode. The carbon monoxide gas sensor 10 is configured to measure a short circuit current between the electrodes. In the carbon monoxide gas sensor 10, the solid electrolyte layer has oxide ion conductivity.

Description

[Technical Field]

[0001] The present invention relates to a short-circuit current detection type carbon monoxide gas sensor. [Background technology]

[0002] Known sensors capable of measuring the concentration of carbon monoxide gas in the gas phase include potentiostatic electrolytic gas sensors and semiconductor gas sensors. However, potentiostatic electrolytic gas sensors have the disadvantage of having a short lifespan in high-temperature environments due to the use of an electrolyte. Semiconductor gas sensors have the disadvantage of being easily affected by combustible gases other than carbon monoxide.

[0003] In addition to the above-mentioned types of sensors, Patent Document 1 proposes a carbon monoxide gas sensor that includes a pair of electrodes and a solid electrolyte made of a BaCeO3-based oxide or CeO2-based oxide, which is an ion-conductive ceramic. This sensor is available in single-chamber and dual-chamber types. This sensor is configured to measure the concentration of carbon monoxide gas by measuring the short-circuit current, open-circuit potential difference, or voltage when current is flowing between the electrodes. Patent Document 2 also proposes a carbon monoxide gas sensor using a solid electrolyte. The solid electrolyte described in this document is an oxide ion conductor called LSGM8282. The sensor described in this document is configured to measure the concentration of carbon monoxide gas by measuring the voltage value when a current flows between the electrodes. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-207482 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-42222 Summary of the Invention

[0005] When using the two-chamber sensor described in Patent Document 1, it is necessary to separate the atmosphere to which the sensing electrode is exposed from the atmosphere to which the counter electrode is exposed, and therefore it is necessary to provide the sensor with an airtight structure, which makes the sensor structure complicated. In the type of sensor described in Patent Document 2 that measures voltage values ​​when current is flowing between electrodes, it may not be possible to obtain a sufficient voltage value, which may result in insufficient measurement accuracy. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a carbon monoxide gas sensor that does not have a complicated structure and has high measurement accuracy.

[0006] [1] The present invention provides a single-chamber carbon monoxide gas sensor for measuring the concentration of carbon monoxide gas in a gas phase, comprising: a solid electrolyte layer having anion conductivity; an electrode disposed on each side of the solid electrolyte layer; one of the electrodes is active in the oxidation of carbon monoxide gas; the other of the electrodes is more inactive with respect to the oxidation of carbon monoxide gas than the one of the electrodes; The above-mentioned problems are solved by providing a carbon monoxide gas sensor configured to measure a short circuit current between the electrodes.

[0007] [2] The present invention also provides a carbon monoxide gas sensor according to [1], wherein the solid electrolyte layer has oxide ion conductivity.

[0008] [3] The present invention provides a carbon monoxide gas sensor according to [1] or [2], wherein the solid electrolyte layer contains an oxide of a rare earth element other than cerium.

[0009] [4] The present invention provides a carbon monoxide gas sensor according to any one of [1] to [3], wherein the solid electrolyte layer contains a compound having an apatite-type crystal structure.

[0010] [5] The present invention relates to a solid electrolyte layer comprising a compound represented by formula (1): A 9.3+x [T 6.0-y M y ]O 26.0+z (wherein A is one or more elements selected from the group consisting of La, Ce, Y, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba; T is an element containing Si or Ge, or both; M is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Ge, Zn, Sn, W, and Mo; x is a number that is equal to or greater than -1.4 and equal to or less than 1.5; y is a number that is equal to or greater than 0.0 and equal to or less than 3.0; and z is a number that is equal to or greater than -5.0 and equal to or less than 5.2. The ratio of the number of moles of A to the number of moles of T is 1.3 or greater and equal to or less than 3.7.)

[0011] [6] The present invention achieves an absolute value of 0.01 μA / cm at a temperature of 350°C to 600°C in an atmosphere containing 10 ppm or more of carbon monoxide gas. 2 The present invention provides a carbon monoxide gas sensor according to any one of [1] to [5], which detects a short-circuit current density equal to or higher than this value.

[0012] [7] The present invention provides a carbon monoxide gas sensor according to any one of [1] to [6], wherein the electrode inert to the oxidation of carbon monoxide gas contains particles of elemental gold or a gold alloy.

[0013] [8] The present invention provides a carbon monoxide gas sensor according to any one of [1] to [7], wherein the electrode active in oxidizing carbon monoxide gas contains particles of a platinum group element or an alloy of a platinum group element. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic cross-sectional view taken along the thickness direction of one embodiment of a carbon monoxide gas sensor according to the present invention. [Figure 2] FIG. 2 is a schematic diagram illustrating the mechanism by which a short-circuit current occurs in the carbon monoxide gas sensor having the structure shown in FIG. [Figure 3] FIG. 3 is a graph showing the results of short-circuit current measured using the carbon monoxide gas sensor produced in Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will now be described based on preferred embodiments with reference to the drawings. Fig. 1 shows one embodiment of a carbon monoxide gas sensor according to the present invention. The carbon monoxide gas sensor 10 shown in the figure includes a layered solid electrolyte layer 11. The carbon monoxide gas sensor 10 includes a sensing electrode 12 on one side of the solid electrolyte layer 11 and a counter electrode 13 on the other side. 1 , a counter electrode-side intermediate layer 15 may be disposed between the counter electrode 13 and the solid electrolyte layer 11. Furthermore, a sensing electrode-side intermediate layer 14 may be disposed between the sensing electrode 12 and the solid electrolyte layer 11.

[0016] The carbon monoxide gas sensor 10 is configured to measure the short-circuit current between the sensing electrode 12 and the counter electrode 13. For this purpose, the sensing electrode 12 and the counter electrode 13 are connected by a conductor 16. An ammeter 17 is disposed midway along the conductor 16. The ammeter 17 is used to measure the current that flows when the sensing electrode 12 and the counter electrode 13 are short-circuited while measuring the concentration of carbon monoxide gas.

[0017] 1 , a counter electrode-side intermediate layer 15 and a sensing electrode-side intermediate layer 14 are disposed directly on either side of a solid electrolyte layer 11. In this embodiment, no member is interposed between the solid electrolyte layer 11 and the counter electrode-side intermediate layer 15, or between the solid electrolyte layer 11 and the sensing electrode-side intermediate layer 14. Similarly, in this embodiment, no member is interposed between the counter electrode 13 and the counter electrode-side intermediate layer 15, or between the sensing electrode 12 and the sensing electrode-side intermediate layer 14.

[0018] 1, the counter electrode 13 and the counter electrode-side intermediate layer 15 are shown to have different sizes, but the size relationship between them is not limited to this, and for example, the counter electrode 13 and the counter electrode-side intermediate layer 15 may be the same size. The same applies to the sensing electrode 12 and the sensing electrode-side intermediate layer 14; they may be the same size, or, for example, the sensing electrode 12 may be larger in size than the sensing electrode 12. 1, the size of the counter electrode-side intermediate layer 15 and the size of the solid electrolyte layer 11 are shown to be the same, but the size relationship between the two is not limited to this, and for example, the solid electrolyte layer 11 and the counter electrode-side intermediate layer 15 may be different sizes. The same applies to the sensing electrode 12 side.

[0019] The solid electrolyte layer 11 generally has a certain thickness and is made of a material having anion conductivity. A material having oxide ion conductivity is typically used as the solid electrolyte layer 11. A single crystal or polycrystalline material is used as the solid electrolyte that constitutes the solid electrolyte layer 11. In particular, it is preferable to use an oxide of a rare earth element other than cerium as the material that constitutes the solid electrolyte layer 11, since this further increases the oxide ion conductivity.

[0020] From the viewpoint of further increasing oxide ion conductivity, it is preferable to use lanthanum oxide as the oxide of a rare earth element (excluding cerium) contained in solid electrolyte layer 11. Examples of lanthanum oxide include a composite oxide containing lanthanum and gallium, a composite oxide obtained by adding strontium, magnesium, or cobalt to the composite oxide, and a composite oxide containing lanthanum and molybdenum. In particular, it is preferable to use an oxide ion conductor made of a composite oxide of lanthanum and silicon because of its high oxide ion conductivity.

[0021] The composite oxide of lanthanum and silicon includes, for example, an apatite-type composite oxide containing lanthanum and silicon. The apatite-type composite oxide contains lanthanum, which is a trivalent element, silicon, which is a tetravalent element, and O, and has a composition of La. x SiO 1.5x+12 (X represents a number of 8 or more and 10 or less) is preferred from the viewpoint of high oxide ion conductivity. When this apatite-type composite oxide is used as the solid electrolyte layer 11, it is preferred that the c-axis coincides with the thickness direction of the solid electrolyte layer 11. The most preferred composition of this apatite-type composite oxide is La 9.33 SiO 26 This composite oxide can be produced, for example, according to the method described in JP-A-2013-51101.

[0022] Another example of the material for forming the solid electrolyte layer 11 is a material represented by the formula (1): A 9.3+x [T 6.0-y M y ]O 26.0+zExamples of suitable composite oxides include those represented by the formula: These composite oxides also have an apatite-type crystal structure. In the formula, A represents one or more elements selected from the group consisting of La, Ce, Y, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba. In the formula, T represents an element containing Si or Ge, or both. In the formula, M represents one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Ge, Zn, Sn, W, and Mo. From the viewpoint of enhancing c-axis orientation, M is preferably one or more elements selected from the group consisting of B, Ge, and Zn.

[0023] In the above formula, x is preferably a number of -1.4 or more and 1.5 or less, more preferably 0.0 or more and 0.7 or less, and even more preferably 0.4 or more and 0.6 or less, from the viewpoint of increasing the degree of orientation and oxide ion conductivity. In the formula, y is preferably a number of 0.0 or more and 3.0 or less, more preferably 0.4 or more and 2.0 or less, and even more preferably 0.4 or more and 1.0 or less, from the viewpoint of filling the T element positions in the apatite-type crystal lattice. In order to maintain electrical neutrality within the apatite-type crystal lattice, z in the formula is preferably a number between −5.0 and 5.2, more preferably between −2.0 and 1.5, and even more preferably between −1.0 and 1.0.

[0024] In the above formula, the ratio of the number of moles of A to the number of moles of T, in other words, (9.3+x) / (6.0-y) in the above formula, is preferably 1.3 or more and 3.7 or less, more preferably 1.4 or more and 3.0 or less, and even more preferably 1.5 or more and 2.0 or less, from the viewpoint of maintaining the spatial occupancy rate in the apatite-type crystal lattice. 9.3+x [T 6.0-y M y ]O 26.0+z In the formula (9.3+x) / (6.0-y), when both T and M contain Ge, y=0.

[0025] Among the composite oxides represented by the above formula, composite oxides in which A is lanthanum, i.e., La 9.3+x [T 6.0-y M y ]O 26.0+z It is preferable to use a composite oxide represented by the formula: La 9.3+x [T 6.0-y M y ]O 26.0+z Specific examples of composite oxides represented by the formula include La 9.3+x (Si 4.7 B 1.3 )O 26.0+z , La 9.3+x (Si 4.7 Ge 1.3 )O 26.0+z , La 9.3+x (Si 4.7 Zn 1.3 )O 26.0+z , La 9.3+x (Si 4.7 W 1.3 )O 26.0+z , La 9.3+x (Si 4.7 Sn 1.3 )O 26.0+x , La 9.3+x (Ge 4.7 B 1.3 )O 26.0+z The composite oxide represented by the above formula can be produced, for example, according to the method described in International Publication WO2016 / 111110.

[0026] The thickness of solid electrolyte layer 11 is preferably 10 nm to 1000 μm, more preferably 50 nm to 700 μm, and even more preferably 100 nm to 500 μm, from the viewpoint of effectively reducing the electrical resistance of carbon monoxide gas sensor 10. The thickness of solid electrolyte layer 11 can be measured using, for example, a stylus step gauge or an electron microscope.

[0027] Next, the detection electrode 12 and the counter electrode 13 will be described. The counter electrode 13 functions as a counter electrode for the sensing electrode 12, which will be described later, in the carbon monoxide gas sensor 10 of this embodiment. The counter electrode 13 is preferably active in the oxidation of carbon monoxide gas. "Active in the oxidation of carbon monoxide gas" means that the counter electrode 13 has catalytic activity that causes carbon monoxide gas to react with oxygen molecules on the electrode surface and convert it into carbon dioxide gas in the environment in which the carbon monoxide gas sensor 10 is used. From the above viewpoints, it is preferable that the counter electrode 13 is made of a platinum group element or an alloy of platinum group elements. These materials may be in the form of particles.

[0028] Examples of platinum group elements include platinum, ruthenium, rhodium, palladium, osmium, and iridium. Examples of alloys of platinum group elements include those containing 50 mol% or more of a platinum group element and containing, as alloy components, Pt—Pd, Pt—Rh, Pt—Ni, Pt—Au, Pt—W, and Pt—Cu. When the platinum group element or the platinum group element alloy is in the form of particles, the particle size is determined by the volume cumulative particle size D at 50% cumulative volume measured by a laser diffraction / scattering particle size distribution measurement method from the viewpoint of ensuring the surface area of ​​the particles at the interface between the particles and the gas phase and from the viewpoint of suppressing deterioration due to sintering of the particles during the manufacture of the counter electrode. 50 It is preferable that the thickness is 0.01 μm or more and 100 μm or less.

[0029] On the other hand, the sensing electrode 12 is an electrode in the carbon monoxide gas sensor 10 of this embodiment that is exposed to the atmosphere to measure the concentration of carbon monoxide gas contained in the atmosphere. The sensing electrode 12 is preferably more inert to the oxidation of carbon monoxide than the counter electrode 13. In particular, the sensing electrode 12 is preferably inert to the oxidation of carbon monoxide. "Inert to the oxidation of carbon monoxide" means that the catalytic activity of the electrode surface for converting carbon monoxide gas into carbon dioxide gas is lower than that of the counter electrode 13 in the environment in which the carbon monoxide gas sensor 10 is used. From the above viewpoints, it is preferable that the sensing electrode 12 is made of gold or a gold alloy. These materials may be in the form of particles. Examples of gold alloys include those containing 50 mol % or more of gold and containing alloy components such as Au-Ag, Au-Pt, Au-Pd, Au-In, Au-Sn, and Au-Fe.

[0030] When the gold or gold alloy is in the form of particles, the particle diameter is set to a volume cumulative particle diameter D 50 It is preferable that the thickness is 0.01 μm or more and 100 μm or less.

[0031] From the viewpoint of effectively reducing the electrical resistance of the carbon monoxide gas sensor 10, the thickness of the sensing electrode 12 and the counter electrode 13 is preferably 10 nm or more and 1000 μm or less, and more preferably 50 nm or more and 700 μm or less.

[0032] In the present invention, as in the embodiment shown in Fig. 1 , a counter electrode-side intermediate layer 15 may be disposed between the solid electrolyte layer 11 and the counter electrode 13. In addition to or instead of this, a sensing electrode-side intermediate layer 14 may be disposed between the solid electrolyte layer 11 and the sensing electrode 12. The counter electrode-side intermediate layer 15 and the sensing electrode-side intermediate layer 14 (hereinafter, for convenience, they may be collectively referred to simply as "intermediate layer") are not essential parts of the carbon monoxide gas sensor of the present invention. However, when the carbon monoxide gas sensor of the present invention includes the counter electrode-side intermediate layer 15 and / or the sensing electrode-side intermediate layer 14, the performance of the sensor is improved.

[0033] The intermediate layer is preferably composed of cerium oxide containing one or more rare earth elements (hereinafter also referred to as "LnDC"). However, "rare earth elements" does not include cerium. In LnDC, rare earth elements other than cerium are contained in the matrix cerium oxide (CeO2) in the form of a solid solution (doped). The doped rare earth elements usually exist in the crystal lattice of cerium oxide by substituting for the sites where cerium is located.

[0034] From the viewpoint of further enhancing the oxide ion conductivity of the carbon monoxide gas sensor 10, the intermediate layer is preferably made of cerium oxide containing lanthanum and a rare earth element (excluding lanthanum and cerium) (hereinafter also referred to as "La-LnDC") In La-LnDC, lanthanum may exist in the crystal lattice of cerium oxide by substituting for the site where cerium is located, or it may exist at the grain boundary of the crystal grains of cerium oxide doped with the rare earth element.

[0035] From the viewpoint of further enhancing the oxide ion conductivity of the carbon monoxide gas sensor 10, it is preferable that the intermediate layer be composed of cerium oxide containing lanthanum and one or more elements selected from the group consisting of samarium, gadolinium, yttrium, erbium, ytterbium, and dysprosium. In particular, it is preferable that the intermediate layer contains cerium oxide containing lanthanum and either samarium or gadolinium, since this can further enhance the oxide ion conductivity of the entire carbon monoxide gas sensor 10. The La-LnDC constituting both intermediate layers 14, 15 may be the same or different. Alternatively, one of the counter electrode side intermediate layer 15 and the sensing electrode side intermediate layer 14 may be composed of La-LnDC, and the other may be composed of another material.

[0036] In La-LnDC, the ratio of rare earth element (excluding lanthanum and cerium) doped into cerium oxide is preferably 0.05 at% to 0.5 at%, more preferably 0.1 at% to 0.4 at%, and even more preferably 0.2 at% to 0.3 at%, expressed as Ln / Ce, which is the atomic ratio of rare earth element (Ln) to cerium. By setting the degree of rare earth element doping within this range, the oxide ion conductivity between the solid electrolyte layer 11 and the sensing electrode 12 and / or counter electrode 13 is improved. The presence of the rare earth element in the cerium oxide as a solid solution can be confirmed by X-ray diffraction.

[0037] Lanthanum is contained in the La-LnDC constituting the intermediate layer for the purpose of improving the oxide ion conductivity of the carbon monoxide gas sensor 10. For this purpose, the atomic ratio of lanthanum to cerium, La / Ce (at%), in La-LnDC is preferably 0.3 or more. Furthermore, since an excessive amount of lanthanum actually reduces the oxide ion conductivity, the La / Ce (at%) is preferably 1.2 or less. The La / Ce (at%) value is more preferably 0.4 to 1.1, and even more preferably 0.5 to 1.0.

[0038] The total amount of rare earth elements doped into the cerium oxide constituting the intermediate layer, i.e., the sum of the amount of lanthanum and the amount of rare earth elements other than lanthanum, Ln T is the atomic ratio to cerium, i.e., Ln T In terms of improving the oxide ion conductivity of the carbon monoxide gas sensor 10, it is preferable that the ratio of Ln to Ce (at %) is 0.3 or more and 1.5 or less. T The value of Ln / Ce (at%) is more preferably 0.4 or more and 1.4 or less, and even more preferably 0.5 or more and 1.3 or less. T A detailed method for measuring / Ce (at %) will be explained in the examples below.

[0039] As long as the intermediate layer has a certain thickness or greater, it can effectively improve the oxide ion conductivity between the solid electrolyte layer 11 and the sensing electrode 12 and / or the counter electrode 13. The thickness of the intermediate layer on each of the sensing electrode 12 side and the counter electrode 13 side is preferably 1 nm or more and 1000 nm or less, and more preferably 10 nm or more and 700 nm or less. The thickness of this intermediate layer can be measured using a stylus step gauge or an electron microscope. The thickness of the counter electrode-side intermediate layer 15 and the thickness of the sensing electrode-side intermediate layer 14 may be the same or different.

[0040] The carbon monoxide gas sensor 10 of the embodiment shown in Fig. 1 can be suitably manufactured, for example, by the method described below. First, the solid electrolyte layer 11 is manufactured by a known method. For manufacturing, for example, the methods described in JP 2013-51101 A and WO 2016 / 111110 can be adopted.

[0041] Next, the counter electrode-side intermediate layer 15 and the sensing electrode-side intermediate layer 14 are formed on the two main surfaces of the solid electrolyte layer 11, respectively. Sputtering, for example, can be used to form each of the intermediate layers 14, 15. The target used for sputtering can be manufactured, for example, by the following method: A powder of an oxide of a rare earth element (excluding cerium) and a powder of cerium oxide are mixed using a mixer such as a mortar or a ball mill, and then fired in an oxygen-containing atmosphere to obtain raw material powder. This raw material powder is molded into a target shape and hot-press sintered. The sintering conditions can be a temperature of 1000°C to 1400°C, a pressure of 20 MPa to 35 MPa, and a time of 60 minutes to 180 minutes. The atmosphere can be an inert gas atmosphere such as nitrogen gas or a rare gas. The sputtering target obtained in this manner is composed of LnDC. The manufacturing method for the sputtering target is not limited to this manufacturing method; for example, a target-shaped molded body can be fired in air or an oxygen-containing atmosphere.

[0042] Using the target thus obtained, a sputtering layer is formed on each surface of the solid electrolyte layer 11 by, for example, high-frequency sputtering. The substrate temperature may be raised to a range of 300 to 500°C in advance, and sputtering may be performed while maintaining this temperature. The sputtering layer is preferably made of LnDC.

[0043] After sputtering is completed, the sputtered layer is annealed. The annealing is performed to thermally diffuse the lanthanum contained in the solid electrolyte layer 11 into the sputtered layer, thereby incorporating lanthanum into the LnDC that constitutes the sputtered layer. For this purpose, the annealing conditions can be a temperature of 1300°C to 1600°C for a time of 10 to 120 minutes, more preferably a temperature of 1400°C to 1600°C for a time of 10 to 90 minutes. The atmosphere can be an oxygen-containing atmosphere such as air. Other deposition methods that can be used include atomic layer deposition, ion plating, pulsed laser deposition, plating, and chemical vapor deposition.

[0044] The above-mentioned annealing process yields intermediate layers made of lanthanum-containing LnDC (La-LnDC). Then, a sensing electrode 12 and a counter electrode 13 are formed on the surface of each intermediate layer. For the counter electrode 13, a paste containing particles of a platinum group element or an alloy of platinum group elements can be used. The paste is applied to the surface of the counter electrode-side intermediate layer 15 to form a coating film, and the coating film is fired to form the counter electrode 13 made of a porous body. The firing conditions can be a temperature of 600°C to 900°C for a time of 30 minutes to 120 minutes. The atmosphere can be an oxygen-containing atmosphere such as air. The sensing electrode 12 can be formed in the same manner as the counter electrode 13, and the sensing electrode 12 made of a porous material can be formed using a paste containing particles of gold or a gold alloy.

[0045] The above method yields the desired carbon monoxide gas sensor. When the carbon monoxide gas sensor of the present invention thus obtained is placed in a measurement target atmosphere and the counter electrode and sensing electrode are short-circuited by a conductor, oxygen pumping occurs according to the chemical potential difference between the two electrodes, generating a short-circuit current. When carbon monoxide gas, a flammable gas, is present in the measurement atmosphere, carbon monoxide burns on the surface of the counter electrode 13, which is an electrode with high oxidative activity, as shown in FIG. 2 (2CO + O → 2CO). In contrast, carbon monoxide is adsorbed onto the sensing electrode 12, which is an electrode with low oxidative activity, and reaches the interface between the sensing electrode 12 and the solid electrolyte layer 11 (or the interface between the sensing electrode 12 and the sensing electrode-side intermediate layer 14, if present). As a result, a chemical potential difference occurs between the electrodes 12 and 13, and an oxygen reduction reaction (O + 4e - →2O 2- ) occurs, and the carbon monoxide gas oxidation reaction (CO+O 2- →CO2+2e - ) occurs. As a result, a short-circuit current is generated between the two electrodes 12, 13. By preparing a calibration curve in advance for the value of the short-circuit current and the concentration of carbon monoxide gas contained in the atmosphere to be measured, the concentration of carbon monoxide gas contained in any atmosphere to be measured can be determined from the measured value of the short-circuit current. The carbon monoxide gas sensor of the present invention has the above-mentioned configuration, particularly the counter electrode and sensing electrode which have different oxidation activities for carbon monoxide gas, and therefore has a large value of short-circuit current, which allows it to measure the concentration of carbon monoxide gas in the gas phase over a wide concentration range. Moreover, accurate measurement is possible even at low concentrations of carbon monoxide gas.

[0046] Because the carbon monoxide gas sensor of the present invention generates a short-circuit current through the above mechanism, it can be used as a single-chamber sensor. A single-chamber sensor is a sensor in which both the counter electrode and the sensing electrode are exposed to the same atmosphere to be measured. When using a single-chamber sensor, there is no need to airtightly separate the atmosphere on the counter electrode side from the atmosphere on the sensing electrode side. Therefore, a single-chamber sensor has the advantage of being simple in structure.

[0047] From the viewpoint of being able to measure the concentration of carbon monoxide gas with higher accuracy, the carbon monoxide gas sensor of the present invention has an absolute value of preferably 0.01 μA / cm at a temperature of 350° C. or higher and 600° C. or lower in an atmosphere containing 10 ppm or more of carbon monoxide gas. 2 It is preferable that the device is configured to detect a short circuit current density of 0.02 μA / cm or more. 2 There is no particular upper limit to the short-circuit current density, and the higher the upper limit, the more accurately the concentration of carbon monoxide gas can be measured. 2 If the short circuit current density is high to a certain extent, the intended object of the present invention can be fully achieved. In the present invention, the current flowing from the sensing electrode 12 to the counter electrode 13 is defined as a positive current.

[0048] Although the present invention has been described above based on preferred embodiments thereof, the present invention is not limited to these embodiments. For example, in the carbon monoxide gas sensor 10 of the embodiment shown in Figures 1 and 2, the counter electrode-side intermediate layer 15 is disposed between the counter electrode 13 and the solid electrolyte layer 11, and the sensing electrode-side intermediate layer 14 is disposed between the sensing electrode 12 and the solid electrolyte layer 11. Alternatively, the counter electrode-side intermediate layer 15 and / or the sensing electrode-side intermediate layer 14 may not be disposed. [Example]

[0049] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to these examples.

[0050] Example 1 In this example, a carbon monoxide gas sensor 10 having the structure shown in FIG. 1 was manufactured according to the following steps (1) to (3). (1) Manufacturing of the solid electrolyte layer 11 La2O3 powder and SiO2 powder were blended in a molar ratio of 1:1, and ethanol was added and mixed in a ball mill. This mixture was dried, crushed in a mortar, and fired in a platinum crucible at 1650°C for 3 hours in an air atmosphere. Ethanol was added to this fired material, and it was then crushed in a planetary ball mill to obtain fired powder. This fired powder was placed in a 20 mm diameter molding machine and pressed from one direction to perform uniaxial molding. Further, cold isostatic pressing (CIP) at 600 MPa for 1 minute was performed to form pellets. This pellet-shaped compact was then heated in air at 1600°C for 3 hours to obtain a sintered pellet. X-ray diffraction measurement and chemical analysis of this sintered compact confirmed that it had a La2SiO5 structure.

[0051] 800 mg of the obtained pellets and 140 mg of B2O3 powder were placed in a sagger with a lid and heated in the air at 1550°C (furnace atmosphere temperature) for 50 hours using an electric furnace. This heating generated B2O3 vapor in the sagger and caused the B2O3 vapor to react with the pellets, resulting in the desired solid electrolyte layer 11. This solid electrolyte layer 11 contained La 9.3+x [Si 6.0-y B y ]O 26.0+z In this compound, x = 0.50, y = 1.17, z = 0.16, and the molar ratio of La to B was 8.38 (hereinafter, this compound will be abbreviated as "LSBO"). The oxide ion conductivity of LSBO at 500°C was 3.0 × 10 -2 The solid electrolyte layer 11 had a thickness of 350 μm.

[0052] (2) Manufacturing the Counter Electrode Side Intermediate Layer 15 and the Sensing Electrode Side Intermediate Layer 14 Sm 0.2 Ce 1.8The O2 powder was placed in a 50 mm diameter molding machine and pressed from one direction to form a uniaxial compact, followed by hot press sintering. The sintering conditions were a nitrogen gas atmosphere, a pressure of 30 MPa, a temperature of 1200°C, and 3 hours. In this way, a target for sputtering was obtained. Using this target, sputtering was performed on each side of the solid electrolyte layer 11 by radio frequency sputtering to form a sputtering layer of samarium-doped cerium oxide (hereinafter also referred to as "SDC"). The sputtering conditions were an RF output of 30 W and an argon gas pressure of 0.8 Pa. After sputtering, the SDC was annealed in air at 1500°C for 1 hour to thermally diffuse the lanthanum contained in the LSBO into the sputtered layer, thereby incorporating lanthanum into the SDC. In this manner, a counter electrode-side intermediate layer 15 and a sensing electrode-side intermediate layer 14 were formed, each composed of lanthanum-containing SDC (hereinafter also referred to as "La-SDC"). Each intermediate layer 14, 15 had a thickness of 300 nm. Quantitative analysis using energy dispersive X-ray spectroscopy (EDS) revealed that the atomic ratio of La / Ce (at%) in the counter electrode-side intermediate layer 15 and the sensing electrode-side intermediate layer 14 was 0.98.

[0053] (3) Manufacturing the sensing electrode 12 and the counter electrode 13 A paste containing platinum powder was applied to the surface of the counter electrode-side intermediate layer 15 to form a coating film. This coating film was dried in the air at 120°C for 3 hours and then fired at 750°C for 1 hour to obtain a porous counter electrode 13. The counter electrode 13 had a thickness of 10.0 μm. A paste containing gold powder was applied to the surface of the sensing electrode-side intermediate layer 14 to form a coating. This coating was dried in air at 120°C for 3 hours and then fired at 750°C for 1 hour to form the porous sensing electrode 12. The thickness of the sensing electrode 12 was 10.0 μm.

[0054] Example 2 The firing temperature and firing time of the sensing electrode 12 were changed as shown in Table 1. A carbon monoxide gas sensor 10 was obtained in the same manner as in Example 1 except for this.

[0055] Example 3 In this example, a carbon monoxide gas sensor 10 was manufactured according to the following steps (1) and (2). This sensor has a three-layer structure without the counter electrode side intermediate layer 15 and the sensing electrode side intermediate layer 14. (1) Manufacturing of the solid electrolyte layer 11 The same as in Example 1.

[0056] (2) Manufacturing the sensing electrode 12 and the counter electrode 13 A paste containing platinum powder was applied to one surface of the solid electrolyte layer 11 to form a coating. This coating was dried in air at 120°C for 3 hours and then fired at 850°C for 1 hour to obtain a porous counter electrode. The thickness of the counter electrode was 5.0 μm. A paste containing gold powder was applied to the other surface of the solid electrolyte layer 11 to form a coating. This coating was dried in air at 120°C for 3 hours and then fired at 850°C for 1 hour to form a porous sensing electrode. The sensing electrode had a thickness of 5.0 μm.

[0057] [Rating 1] The carbon monoxide gas sensors 10 obtained in Examples 1 to 3 were placed in a measurement atmosphere (a CO and O2-containing N2 atmosphere) with oxygen and carbon monoxide gas concentrations shown in Table 2, and the short-circuit current density was measured. The temperature of the measurement atmosphere was set as shown in Table 2. The measurement results are shown in Table 2.

[0058] [Table 1]

[0059] [Table 2]

[0060] As is clear from the results shown in Table 2, the carbon monoxide gas sensor 10 of each example can obtain a high short-circuit current density. The relationship between the CO concentration in the measurement atmosphere and the short-circuit current density of the carbon monoxide gas sensor 10 manufactured in Example 2 is shown in Figure 3. As is clear from the results shown in Figure 3, the short-circuit current density value changes linearly with changes in CO concentration. Therefore, it was confirmed that the carbon monoxide gas sensor of the present invention, which utilizes the difference in CO oxidation activity between the counter electrode and the sensing electrode, is useful as a single-chamber short-circuit current sensor. [Industrial Applicability]

[0061] As described above in detail, the present invention provides a carbon monoxide gas sensor that is not complicated in structure and has high measurement accuracy.

Claims

1. A single-chamber carbon monoxide gas sensor for measuring the concentration of carbon monoxide gas in a gas phase, a solid electrolyte layer having anion conductivity; an electrode disposed on each side of the solid electrolyte layer; one of the electrodes is active in the oxidation of carbon monoxide gas; the other of the electrodes is more inactive with respect to the oxidation of carbon monoxide gas than the one of the electrodes; A carbon monoxide gas sensor configured to measure a short circuit current between the electrodes.

2. 2. The carbon monoxide gas sensor according to claim 1, wherein the solid electrolyte layer has oxide ion conductivity.

3. 2. The carbon monoxide gas sensor according to claim 1, wherein the solid electrolyte layer contains an oxide of a rare earth element other than cerium.

4. 2. The carbon monoxide gas sensor according to claim 1, wherein the solid electrolyte layer contains a compound having an apatite-type crystal structure.

5. The solid electrolyte layer is a compound represented by the formula (1): A ９．３＋ｘ [T ６．０－ｙ M ｙ ]O ２６．０＋ｚ (In the formula, A is one or more elements selected from the group consisting of La, Ce, Y, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba. T is an element containing Si or Ge, or both. M is Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B , Ge, Zn, Sn, W, and Mo. x is a number between -1.4 and 1.

5. y is a number between 0.0 and 3.

0. z is a number between -5.0 and 5.

2. The ratio of the number of moles of A to the number of moles of T is between 1.3 and 3.

7.

6. In an atmosphere containing 10 ppm or more of carbon monoxide gas, at a temperature of 350°C to 600°C, the absolute value is 0.01 μA / cm ２ 2. The carbon monoxide gas sensor according to claim 1, wherein a short-circuit current density equal to or greater than 100 kJ / s is detected.

7. 2. The carbon monoxide gas sensor of claim 1, wherein the electrode inert to the oxidation of carbon monoxide gas comprises particles of elemental gold or an alloy of elemental gold.

8. 2. The carbon monoxide gas sensor according to claim 1, wherein the electrode active in oxidizing carbon monoxide gas comprises particles of a platinum group element or an alloy of platinum group elements.

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