JP2012021925A - Ammonia gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ammonia gas sensor which reduces the influence of oxygen in a gas to be measured, and is superior in ammonia selectivity.SOLUTION: The ammonia gas sensor 200A includes a solid electrolyte body 22A having oxygen ion conductivity, and a detection electrode 2A and a reference electrode 4A provided respectively on the surface of the solid electrolyte body. The detection electrode includes precious metal as a main component, contains 2 to 30 mass% of a Ce oxide, and the Ce oxide and the precious metal exist at least in an interface area between the detection electrode and the solid electrolyte body.

Description

  The present invention relates to an ammonia gas sensor suitably used for measuring ammonia gas concentration in combustion gas and exhaust gas of, for example, a combustor and an internal combustion engine.

A urea SCR (Selective Catalytic Reduction) system has been developed as a method for purifying nitrogen oxides (NO _x ) in exhaust gas of internal combustion engines such as automobiles. Urea SCR system, by adding urea to the SCR catalyst to generate ammonia, is intended to reduce NO _x with ammonia, the urea SCR system, since the ammonia concentration for reducing NO _x to determine whether an appropriate amount An ammonia gas sensor is used.

As such an ammonia gas sensor, a sensor in which a reference electrode and a detection electrode are formed on the surface of an oxygen ion conductive solid electrolyte body and an ammonia concentration is detected based on an electromotive force between the electrodes has been proposed. For example, a sensor having a two-layer structure of a first electrode layer (Au) in contact with a solid electrolyte body and a second electrode layer (a mixture of Au and CeO ₂ ) covering the surface of the first electrode layer is proposed as a detection electrode. (See Patent Document 1).
According to this sensor, the concentration of combustible gas (HC gas, CO gas, ammonia gas, etc.) can be measured without being affected by the oxygen concentration even in a lean burn engine with a large change in oxygen concentration (that is, the combustible gas Selectivity can be secured).

JP 2001-108649 A

However, in the case of the ammonia gas sensor described in Patent Document 1, HC gas and CO gas other than ammonia gas have the same sensitivity as ammonia gas, so there is a problem in the selectivity of ammonia gas, and the concentration of only ammonia gas is reduced. It is difficult to measure. In addition, the output of the ammonia sensor varies due to the influence of oxygen in the gas to be measured, and it is difficult to accurately measure the ammonia gas.
That is, an object of the present invention is to provide an ammonia gas sensor excellent in selectivity of only ammonia gas by reducing the influence of oxygen in the gas to be measured and reducing the influence of combustible gas other than ammonia gas.

In order to solve the above problems, an ammonia gas sensor of the present invention includes an oxygen ion conductive solid electrolyte body, and a detection electrode and a reference electrode provided on the surface of the solid electrolyte body, respectively. It contains 2 to 30% by mass of Ce oxide as a component, and Ce oxide and the noble metal exist at least in the interface region between the detection electrode and the solid electrolyte body.
Thus, when Ce oxide is contained in the detection electrode and Ce oxide is present in the interface region, the selectivity of only ammonia can be secured and the influence of oxygen in the gas to be measured is reduced. This is presumably because the Ce oxide intervening at the interface suppresses the electrode reaction with combustible gas species other than NH ₃ at the interface. Further, Ce oxide is an insulator, but the conductivity of the detection electrode can be ensured by coexistence of a noble metal at the interface.
In addition, the presence of the noble metal in the interface region ensures the ability of the detection electrode as a current collector. If no precious metal is present in the interface region, the detection electrode may not be able to secure the ability as a current collector.

  The interface region refers to a region where the components of the solid electrolyte body and the detection electrode are mixed. Specifically, the cross section of the ammonia gas sensor including the detection electrode and the solid electrolyte body is represented by EPMA (electron beam). By analyzing with a microanalyzer etc., it is possible to specify a region where both components are mixed.

  Further, the interface region may have a two-layer structure of a mixed layer of the solid electrolyte component and the noble metal and a mixed layer of the solid electrolyte component and the metal oxide. It is preferable that it is a mixed layer of three components of bismuth and metal oxide because the selectivity of only ammonia is further improved.

The content ratio of the noble metal in the detection electrode is preferably 70% by mass or more.
When the content ratio of the noble metal in the detection electrode is less than 70% by mass, the characteristics of the Ce oxide that is an insulator appear, and a conduction failure of the detection electrode may occur, resulting in a decrease in conductivity.

In addition, it is preferable that the Ce oxide contains at least CeO ₂ from the viewpoint of improving the selectivity of only ammonia and reducing the influence of oxygen.

  According to the present invention, it is possible to obtain an ammonia gas sensor excellent in selectivity only for ammonia gas by reducing the influence of oxygen in the gas to be measured and reducing the influence of combustible gas other than ammonia gas.

It is sectional drawing in alignment with the longitudinal direction of the ammonia gas sensor which concerns on embodiment of this invention. It is an expanded view which shows the structure of a sensor element part. It is sectional drawing which follows the III-III line of FIG. It is a schematic diagram which shows an interface area | region. It is sectional drawing which shows the modification of the detection electrode in an interface area | region. It is sectional drawing which shows the modification of a detection electrode. NH ₃ is a graph showing the concentration conversion equation representing the relationship between the output of the ammonia gas sensor for concentration. It is a figure which shows ammonia selectivity when the composition in a detection electrode is changed. Is a graph showing the relationship between the content of CeO ₂ and ammonia selectivity in sensing electrode. It is sectional drawing of the comparative examples 6 and 7 when a detection electrode is made into two layers. It is a figure which shows the selectivity of ammonia when a detection electrode is made into two layers. It is a figure which shows the selectivity of ammonia when the detection electrode in an interface area | region is made into two layers. It shows the effect of O ₂ to ammonia sensitivity of the ammonia gas sensor of Example 1. It shows the effect of O ₂ to ammonia sensitivity of the ammonia gas sensor of Comparative Example 1. It shows the effect of O ₂ to ammonia sensitivity of the ammonia gas sensor of Comparative Example 2. It shows the effect of O ₂ to ammonia sensitivity of the ammonia gas sensor of Comparative Example 3.

Hereinafter, embodiments of the present invention will be described.
FIG. 1 shows a cross-sectional view along the longitudinal direction of an ammonia gas sensor (ammonia sensor) 200A according to an embodiment of the present invention. The ammonia sensor 200A is an assembly in which a sensor element unit 50A for detecting ammonia is assembled. The ammonia sensor 200A includes a plate-shaped sensor element portion 50A extending in the axial direction, a cylindrical metal shell 138 formed on the outer surface with a screw portion 139 for fixing to the exhaust pipe, and the diameter of the sensor element portion 50A. Insulating contact member disposed in a state in which a cylindrical ceramic sleeve 106 disposed so as to surround the periphery of the direction and an inner wall surface of the contact insertion hole 168 penetrating in the axial direction surround the periphery of the rear end portion of the sensor element unit 50A 166, and a plurality (only two are shown in FIG. 1) of connection terminals 110 disposed between the sensor element unit 50A and the insulating contact member 166.

  The metal shell 138 has a substantially cylindrical shape having a through hole 154 that penetrates in the axial direction, and a shelf 152 that protrudes radially inward of the through hole 154. Further, the metal shell 138 is arranged in such a manner that the front end side of the sensor element portion 50A is arranged outside the front end side of the through hole 154 and the electrode terminal portions 40A to 44A are arranged outside the rear end side of the through hole 154. 50A is held in the through hole 154. Further, the shelf portion 152 is formed as an inwardly tapered surface having an inclination with respect to a plane perpendicular to the axial direction.

  In addition, inside the through hole 154 of the metal shell 138, an annular ceramic holder 151 and powder-filled layers 153 and 156 (hereinafter also referred to as talc rings 153 and 156) surrounding the sensor element portion 50A in the radial direction. The ceramic sleeve 106 is laminated in this order from the front end side to the rear end side. A caulking packing 157 is disposed between the ceramic sleeve 106 and the rear end portion 140 of the metal shell 138, and a talc ring 153 is disposed between the ceramic holder 151 and the shelf 152 of the metal shell 138. In addition, a metal holder 158 for holding the ceramic holder 151 and maintaining hermeticity is disposed. Note that the rear end portion 140 of the metal shell 138 is crimped so as to press the ceramic sleeve 106 toward the distal end side via the crimping packing 157.

  On the other hand, as shown in FIG. 1, a metal (for example, stainless steel) having a plurality of holes and covering the protruding portion of the sensor element portion 50 </ b> A on the outer periphery of the front end side (lower side in FIG. 1) of the metal shell 138. A double external protector 142 and an internal protector 143 are attached by welding or the like.

  An outer cylinder 144 is fixed to the outer periphery of the rear end side of the metal shell 138. In addition, five lead wires 146 (in FIG. 1) that are electrically connected to the electrode terminal portions 40A to 44A of the sensor element portion 50A are respectively provided in the opening on the rear end side (upper side in FIG. 1) of the outer cylinder 144. A grommet 150 in which lead wire insertion holes 161 into which only three are inserted is formed is disposed.

  Further, an insulating contact member 166 is disposed on the rear end side (upper side in FIG. 1) of the sensor element portion 50A protruding from the rear end portion 140 of the metal shell 138. The insulating contact member 166 is disposed around the electrode terminal portions 40A to 44A formed on the rear end surface of the sensor element portion 50A. The insulating contact member 166 is formed in a cylindrical shape having a contact insertion hole 168 penetrating in the axial direction, and is provided with a flange portion 167 protruding radially outward from the outer surface. The insulating contact member 166 is disposed inside the outer cylinder 144 by the flange portion 167 coming into contact with the outer cylinder 144 via the holding member 169. The connection terminal 110 on the insulating contact member 166 side and the electrode terminal portions 40A to 44A of the sensor element portion 50A are electrically connected, and are electrically connected to the outside by the lead wire 146.

Next, the configuration of the sensor element unit 50A will be described with reference to the development view 2. FIG. The sensor element portion 50A is a long plate, and a detection portion for detecting ammonia gas in the exhaust gas is exposed at the tip portion, and electrode terminal portions 40A to 44A are exposed at the rear end portion of the sensor element portion 50A. is doing.
In FIG. 2, a lead 31A extends along the longitudinal direction on the upper surface of the insulating layer 6A, and the terminal of the lead 31A forms an electrode terminal portion 41A. Furthermore, on the insulating layer 6A, the lead 30A extends in parallel with the lead 31A, and the terminal of the lead 30A (the right end portion of the insulating layer 6A) forms the electrode terminal portion 40A. The leads 30A and 31A extend in the longitudinal direction from the central portion to the end of the insulating layer 6A. Further, an insulating layer 20A is formed so as to cover the leads 30A and 31A. However, the tip side of the insulating layer 6A (the portion where the leads 30A and 31A are not formed), the tip side of the leads 30A and 31A, and the electrode terminal portions 40A and 41A are exposed without being covered with the insulating layer 20A.

On the other hand, a solid electrolyte body 22A is laminated on a portion of the insulating layer 6A that is not covered with the insulating layer 20A. Further, a reference electrode 4A is formed on the solid electrolyte body 22A, and a detection electrode 2A is formed in parallel with the reference electrode. The reference electrode 4A is connected to the lead 31A, and the detection electrode 2A is connected to the lead 30A.
Thus, the reference electrode 4A and the detection electrode 2A are exposed on the same surface side of the solid electrolyte body 22A and are exposed to the gas to be measured. The solid electrolyte body 22A, the reference electrode 4A, and the detection electrode 2A constitute a cell 70.

On the other hand, on the lower surface of the insulating layer 26A (the lower surface in FIG. 2), temperature detection means (temperature sensor) 14A and leads 32A and 34A, which are resistance temperature detectors, are formed. The end of the lead 34A forms the electrode terminal portion 44A. A lead 32A extends parallel to the lead 34A, and the end of the lead 32A forms an electrode terminal portion 42A. A heating resistor 16A and leads 35A and 36A extending from the heating resistor 16A are formed on the upper surface of the insulating layer 26A. The temperature detecting means 14A and the leads 32A and 34A are covered with an insulating layer 11A, and the heating resistor 16A and the leads 35A and 36A are covered with an insulating layer 6A. Further, through holes 26x and 26y are opened at the right end of the insulating layer 26A, respectively. The leads 35A and 36A are connected to electrode terminal portions 42A and 43A disposed on the lower surface of the insulating layer 26A through through holes 26x and 26y, respectively.
A gas-permeable protective layer may be provided on both or either of the detection electrode 2A and the reference electrode 4A.

The detection electrode 2A is an electrode in which combustible gas hardly burns on the electrode surface. And since ammonia reacts with oxygen at the interface with the solid electrolyte body through the detection electrode 2A and burns, it functions as an ammonia gas detection electrode.
The detection electrode 2A contains a noble metal as a main component and contains 2 to 30% by mass of a Ce oxide, and the Ce oxide and the noble metal exist at least in an interface region between the detection electrode 2A and the solid electrolyte body 22A. It is necessary to be.
Here, the precious metal as a main component means that the ratio of the precious metal in the detection electrode 2A exceeds 50% by mass. The noble metal is Au, Pt, Ag, Ir, Ru, and alloys thereof, and Au is particularly preferable. As the Ce oxides, for example, CeO ₂ and the like.

3 is a cross-sectional view taken along line III-III in FIG. In FIG. 3, the configuration other than the cell 70 is illustrated in a simplified manner. The detection electrode 2A contains, for example, 90% by mass of Au and 10% by mass of Ce oxide, and Au and CeO ₂ are present in the interface region between the detection electrode 2A and the solid electrolyte body 22A. .
The presence of Ce oxide (CeO ₂ ) and the above-mentioned noble metal (Au) is confirmed by EPMA (electron beam microanalyzer) analysis of the cross section of the ammonia gas sensor including the detection electrode 2A and the solid electrolyte body 22A. can do.
Here, as shown in FIG. 4, when the cross section is subjected to EPMA analysis in the thickness direction from the solid electrolyte body 22A toward the detection electrode 2A, components of the solid electrolyte body 22A (in this example, YSZ (yttria stabilized zirconia)) are obtained. ) Abruptly decreases from a substantially constant maximum value (Max), and eventually becomes a substantially constant minimum value (Min). At this time, the density at a point where the maximum value (Max) has decreased at a predetermined rate is P1, the density at the point where the minimum value (Min) has been increased at a predetermined rate is P2, and the thickness at each concentration P1, P2 The position in the direction is L1 and L2. At this time, the region between L1 and L2 is defined as “interface region R” in the claims. If the Ce oxide and the above-described noble metal are detected in the interface region R exceeding the EPMA detection limit, it is considered that these components are present in the interface region R.

The points where the deviation rate from the maximum value (Max) and the minimum value (Min) is 5% are defined as P2 and P1, respectively. This is because if the points having the maximum value (Max) and the minimum value (Min) are set to P2 and P1, respectively, some fluctuations and errors are included, and the accuracy may be lowered. For the maximum value (Max) and the minimum value (Min), for example, a plurality of component concentrations of the solid electrolyte body 22A are measured in the thickness direction in a region sufficiently separated from the L1 and L2 in the thickness direction, and an average value thereof is adopted. can do. The maximum value (Max) is not necessarily 100% by mass. For example, when the solid electrolyte body 22A is formed of 80% by mass of YSZ and 20% by mass of alumina (Al ₂ O ₃ ), the maximum value is obtained. (Max) may be regarded as 80% by mass. Further, the minimum value (Min) does not need to be 0% by mass. For example, when the detection electrode 2A contains 20% by mass of YSZ as a common substrate, the minimum value (Min) can be regarded as 20% by mass. That's fine.
Further, the EPMA analysis is more preferable because the measurement of a plurality of (for example, three) 15 μm square visual fields in the above-described cross section and the accuracy of specifying the interface region R are improved.

As shown in FIG. 5 (a), in the interface region R (L1 to L2), the detection electrode 2A is composed of ₂ layers of a layer 2AA containing Au and a layer 2AB containing CeO ₂ in this order from the solid electrolyte layer 22A side. It may be a layered structure. Further, as shown in FIG. 5 (b), contrary to FIG. 5 (a), in the interface region R, the detection electrode 2A includes the layers 2AB, Au containing CeO ₂ in order from the solid electrolyte layer 22A side. It may have a two-layer structure of the containing layer 2AA. Further, as shown in FIG. 5 (c), in the interface region R, the detection electrodes 2A, in order from the solid electrolyte layer 22A side, a layer 2AC comprising a mixture of Au and CeO _2, the layer 2AD containing CeO ₂ It may be a two-layer structure. Further, as shown in FIG. 5 (d), in contrast to FIG. 5 (c), in the interface region R, the detection electrode 2A is arranged in order from the solid electrolyte layer 22A side with the layers 2AD, Au containing CeO ₂ and Au. A two-layer structure of a layer 2AC made of a mixture with CeO ₂ may be used. Further, as shown in FIG. 5E, in the interface region R, the detection electrode 2A is composed of a layer 2AE made of a mixture of Au and CeO _{2 in} order from the solid electrolyte layer 22A side, and a layer 2AF containing Au 2AF. It may be a layered structure. Further, as shown in FIG. 5 (f), contrary to FIG. 5 (e), in the interface region R, the detection electrode 2A is composed of the Au-containing layer 2AF, Au and CeO in order from the solid electrolyte layer 22A side. ₂ may be a two-layer structure of a layer 2AE made of a mixture of the two. However, it goes without saying that the components of the solid electrolyte body 22A are contained in the layer 2AA, the layer 2AB, the layer 2AC, the layer 2AD, the layer 2AE, and the layer 2AF in FIGS. 5A to 5F.

FIG. 6 is a cross-sectional view showing a modification of the detection electrode 2A. FIG. 6A shows a cross-sectional structure in which a coating layer made of CeO ₂ is provided on the surface and side surfaces of the detection electrode 2A2 made of a mixture of Au and CeO ₂ . FIG. 6B shows a cross-sectional structure in which a coating layer made of CeO ₂ is provided on the surface of the detection electrode 2A3 made of a mixture of Au and CeO ₂ . Also in the detection electrodes 2A2 and 2A3, Ce oxide and Au (noble metal) are surely present in the interface region R.

As described above, when Ce oxide and the above-described noble metal are present in the interface region R, the sensitivity to a gas other than ammonia gas (HC gas or the like) is reduced, and the selectivity of ammonia gas is improved. Although the cause of this is not clear, it is considered that the Ce oxide intervening in the interface region R suppresses the electrode reaction with combustible gas species other than NH ₃ at the interface. Further, when Ce oxide and the above-described noble metal exist in the interface region R, the ammonia gas concentration can be measured without being affected by the oxygen concentration. The reason for this is not clear, but is presumed to be as follows. Normally, in this type of sensor, when the oxygen concentration in the atmosphere changes, the electrode reactivity of oxygen changes, so that the electromotive force changes. However, when Ce oxide is present at the interface, it is presumed that the oxygen concentration change in the vicinity of the interface is suppressed and the influence is suppressed by the oxygen storage / release ability of Ce oxide.
In addition, Ce oxide is an insulator, and when only Ce oxide exists in the interface region R, conduction failure of the detection electrode occurs. Therefore, in order to ensure conductivity, both the Ce oxide and the noble metal need to exist in the interface region R.

It is necessary that the content ratio of Ce oxide in the detection electrode 2A is 2 to 30% by mass. When the Ce oxide content in the detection electrode 2A is less than 2% by mass, the effect of improving the selectivity of the ammonia gas described above is not sufficient, and the influence of oxygen in the gas to be measured increases. When the content ratio of the metal oxide in the detection electrode 2A exceeds 30% by mass, the conductivity of the detection electrode 2A is lowered, resulting in poor conduction.
Further, it is preferable that the sensing electrode contains no less than 70% by mass of noble metal because of its excellent current collecting ability. When the precious metal in the detection electrode is less than 70% by mass, the ability as a current collector cannot be obtained, and poor conduction occurs, and ammonia gas cannot be detected.

The detection electrode 2A only needs to include a mixture of Ce oxide and the above-described noble metal, and may further include other subcomponents. Examples of the subcomponent include a component constituting the solid electrolyte body (for example, YSZ) and Al ₂ O ₃ . These subcomponents may be mixed and used. The components constituting the solid electrolyte body can improve adhesion of the detection electrode and reduce contact resistance as a common substrate.

On the other hand, the reference electrode 4A is an electrode on which combustible gas burns on the electrode surface, and is made of, for example, Pt alone or a material mainly containing Pt.
Each of the leads 30A, 31A, 32A, 34A, 35A, 36A, the electrode terminal portions 40A to 44A, the temperature detecting means 14A, and the heating resistor 16A are made of, for example, a material mainly composed of Pt, Pd, or an alloy thereof. Yes.
Each of the insulating layers 6A, 11A, 20A, and 26A is made of an insulating ceramic such as alumina.

  The solid electrolyte body 22A is made of, for example, partially stabilized zirconia (YSZ or the like). The solid electrolyte body 22A is controlled to the activation temperature by the heating resistor 16A. Examples of the solid electrolyte body 22A include YSZ (yttria partially stabilized zirconia), SDC (samaria doped ceria), and ScSZ (scandia stabilized zirconia).

  Next, an example of a manufacturing method of the sensor element unit 50A will be briefly described. First, an alumina insulating layer 26A of a relatively thick green sheet (for example, 300 μm) serving as a main body of the sensor element portion is prepared, and Pt, an alumina (inorganic oxide used as a common substrate) binder, and an organic solvent are formed on the upper surface of the insulating layer 26A. The electrode paste (hereinafter referred to as “Pt paste”) is screen-printed to generate the heating resistor 16A (and leads 35A and 36A extending therefrom), and the temperature detecting means 14A (and leads 32A and 34A extending therefrom) on the lower surface. The electrode terminal portions 42A, 43A, and 44A are formed. Further, the insulating layer 11A is formed on the lower surface of the temperature detecting means 14A by screen printing a paste containing an insulating material (such as alumina), a binder and an organic solvent. Note that through-hole conductors are appropriately filled in the inner surfaces of the through-holes 26x and 26y of the insulating layer 26A.

  Next, an alumina insulating layer 6A of a relatively thick (for example, 300 μm) green sheet serving as the main body of the sensor element portion is laminated on the heating resistor 16A. Then, leads 30A and 31A and electrode terminal portions 40A and 41A are formed on the insulating layer 6A. Further, the insulating layer 20A is screen-printed so as to cover the leads 30A and 31A. The insulating layer 6A may be formed by screen printing an insulating paste.

  Then, the laminate is debindered at a predetermined temperature (for example, 250 ° C.) and fired at a predetermined temperature (for example, 1400 ° C.).

  Next, a paste containing oxide powder, binder, and organic solvent, which are components of the solid electrolyte body, is screen-printed on the fired insulating layer 6A to form the solid electrolyte body 22A, and a predetermined temperature (for example, 750 ° C.) Bake with.

  Then, on the solid electrolyte body 22A, a Pt-based paste is screen-printed to form the reference electrode 4A, and an Au-based paste (including the Ce oxide) is screen-printed to form the detection electrode 2A at a predetermined temperature. Baking at (for example, 750 ° C.).

It goes without saying that the present invention is not limited to the above-described embodiments, and extends to various modifications and equivalents included in the spirit and scope of the present invention. For example, a two-chamber sensor structure may be employed in which a detection electrode and a reference electrode are provided on the front and back surfaces of the solid electrolyte body, respectively, so that the reference electrode is exposed to the atmosphere and the detection electrode is exposed to the gas to be measured.
Also, a sensor structure in which the solid electrolyte body has a cylindrical shape, and a detection electrode and a reference electrode are provided on the outer surface and the inner surface of the tube, respectively, the inner surface of the tube is exposed to the atmosphere, and the detection electrode on the outer surface of the tube is exposed to the gas to be measured. It is good.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated concretely, this invention is not limited to these examples of course.

An ammonia gas sensor according to the above embodiment shown in FIGS. 1 and 2 was produced. First, the Pt paste is screen-printed on the upper surface of the alumina substrate (insulating layer) 26A to form the heating resistor 16A (and leads 35A and 36A extending therefrom), and the Pt paste is screen-printed on the lower surface to temperature. The detection means 14A (and leads 32A, 34A extending therefrom) and electrode terminal portions 42A, 43A, 44A were formed. Further, the insulating layer 11A was formed by screen printing a paste containing an insulating material, a binder and an organic solvent on the heating resistor 16A and the temperature detecting means 14A.
Next, an alumina insulating layer 6A of a relatively thick (for example, 300 μm) green sheet serving as the main body of the sensor element portion was laminated on the heating resistor 16A. Then, Pt paste was screen printed on the insulating layer 6A to form the leads 30A and 31A and the electrode terminal portions 40A and 41A, and the insulating layer 20A was screen printed so as to cover the leads 30A and 31A. Then, it baked for 60 minutes at 14500 degreeC.
Next, a YSZ (Y-stabilized zirconia) paste as a material for the solid electrolyte body 22A was printed on the insulating layer 6A. This laminate was fired at 1480 ° C. for 60 minutes. The YSZ paste was prepared by putting YSZ, an organic solvent, and a dispersant in a mortar and dispersing and mixing them for 4 hours with a rough machine, then adding a predetermined amount of a binder and a viscosity modifier, and further performing wet mixing for 4 hours.

Then, a Pt-based paste is screen-printed on the solid electrolyte body 22A to form the reference electrode 4A and baked at 1420 ° C., and then the Au-based paste formed with the following components is screen-printed and detected. Electrode 2A was formed and fired at 1000 ° C. The obtained sensor element part was assembled to a metal shell or the like to produce an ammonia gas sensor.
The Au-based paste was prepared by mixing CeO ₂ powder in a proportion described in Table 1 with a commercially available Au paste and mixing the mixture for 10 minutes or more so as to be uniform.
The ammonia gas sensors of Examples 1 to 6 and Comparative Examples 1 to 5 were prepared by changing the type and content ratio of the metal oxide powder in the Au-based paste as shown in Table 1. Incidentally, with respect to the detection electrode, the amount of the case of adding subcomponent is a percentage of the total weight of (Au + CeO _2).
The Pt paste is mixed with Pt powder, 10 wt% of YSZ powder and organic solvent in a mortar, and dispersed and mixed for 4 hours with a roughing machine. Then, organic binder and viscosity modifier are added in predetermined amounts. The mixture was further wet mixed for 4 hours.

<Evaluation>
1. Evaluation of ammonia selectivity by Ce oxide The ammonia gas sensors of Examples 1 and 2 and Comparative Examples 1 to 3 were attached to the gas flow of the model gas generator, and the sensitivity of the sensor was evaluated. The gas temperature of the model gas was 280 ° C., the control temperature of the sensor element (heater heating) was 650 ° C., and the gas composition was O ₂ = 10% H ₂ O = 5% CO ₂ = 5% N ₂ = bal.
First, the relationship of the output of the ammonia gas sensor to the NH ₃ concentration was obtained, and a concentration conversion formula was created. First, when NH ₃ was set to 0 ppm and the above gas was flowed from the model gas generator, the potential difference between the reference electrode 4A and the detection electrode 2A was measured to obtain a base electromotive force (base EMF). Next, the potential difference between the reference electrode 4A and the detection electrode 2A when NH ₃ is mixed with a predetermined amount of model gas between 0 and 100 ppm and the gas flows is measured, and the electromotive force at the time of measurement (at the time of measurement) EMF). Then, the relationship of the EMF output of the ammonia gas sensor to the NH ₃ concentration was determined by (electromotive force)-(base electromotive force) (base electromotive force; electromotive force when not exposed to the gas to be measured) at the time of measurement. The concentration conversion formula obtained for the sensor of Example 1 is shown in FIG.
Then, the output of the ammonia gas sensor when a current of gas mixture of C ₃ H ₆ as an interference gas 100ppmC model gas (EMF values) are substituted into the concentration conversion equation 7, the NH ₃ concentration conversion value Calculated. NH ₃ converted concentration value indicates the degree to detect the C ₃ H ₆ as ammonia, higher NH ₃ equivalent concentration value, it is possible to have detected the C ₃ H ₆ as ammonia, when the low selectivity of ammonia I can judge. On the other hand, as the NH ₃ concentration conversion value is smaller, C ₃ H ₆ is not detected as ammonia, and it can be determined that the selectivity of ammonia is higher. In consideration of actual use, the effect of C ₃ H ₆ is preferably zero. However, if the NH ₃ concentration conversion value is 5 ppm or less (accuracy within ± 5 ppm), the selectivity of ammonia is good. It was judged.

The obtained result is shown in FIG. In the case of Example 1, the NH ₃ concentration conversion values are all 5 ppm or less (about 2 ppm), which shows that the influence of interfering gas is reduced and the selectivity of ammonia is good.
In addition, in Example 2 in which YSZ, which is a common substrate for the purpose of improving the adhesion of the sensing electrode and reducing the contact resistance, was added to the sensing electrode as an auxiliary component other than CeO ₂ , the selectivity of ammonia was good. It was confirmed that the selectivity of ammonia was improved if CeO ₂ was contained in the detection electrode. Similar results were obtained when Al ₂ O ₃ was added to the detection electrode instead of YSZ as a subcomponent.
On the other hand, in the case of Comparative Example 1 in which the detection electrode is made of Au alone, and in the case of Comparative Example ₂ in which In ₂ O ₃ is used instead of CeO ₂ , the NH ₃ concentration converted value greatly exceeds 5 ppm, and the interference gas As a result, the selectivity of ammonia decreased. In Patent Document 1 described above, it is described that the addition of In ₂ O ₃ has an effect of reducing the oxygen concentration. However, the sensor described in Patent Document 1 is large not only for ammonia but also for C ₃ H _6. Since sensitivity is shown, it turns out that application is difficult as an ammonia sensor.
In Comparative Example 3 in which YSZ was blended in the sensing electrode without using CeO ₂ , the NH ₃ concentration conversion value greatly exceeded 5 ppm, and the ammonia selectivity was lowered due to the influence of the interference gas.

2. Evaluation of Ce Oxide Content and Ammonia Selectivity Next, the ammonia selectivity of each of the ammonia gas sensors of Examples 1, 3 to 6, and Comparative Examples 1, 4, and 5 was evaluated in the same manner as Evaluation 1. It was. The obtained results are shown in FIG.
FIG. 9 shows the relationship between the CeO ₂ content ratio in the detection electrode and the NH ₃ concentration converted value for Examples 1, 3 to 6 and Comparative Examples 1, 4, and 5.
In Examples 1 and _{3 to} 6 in which the content ratio of CeO ₂ in the detection electrode is 2 to 30% by mass, the NH ₃ concentration conversion value is 5 ppm or less, and the ammonia selectivity is good. .
On the other hand, in the case of Comparative Examples 1 and 4 in which the CeO ₂ content in the sensing electrode was 0% by mass and 1% by mass, respectively, the NH ₃ concentration conversion value exceeded 5 ppm, and the ammonia selectivity was lowered. Moreover, in the case of the comparative example 5 in which the content ratio of CeO _{4 in} the detection electrode exceeds 30% by mass, the conduction failure of the detection electrode occurred and the evaluation was impossible. Here, when the impedance between the reference electrode and the detection electrode is measured with an impedance analyzer (Solartron 1260/1287 manufactured by Toyo Technica Co., Ltd.), if the impedance value at 1 Hz exceeds 1 MΩ, a conduction failure has occurred. I saw it.

3. Evaluation of Presence of Metal Oxide and Ammonia Selectivity in Interface Region R Next, the interface region R of the sensing electrode is shown in Example 1 and Comparative Examples 6 and 7 having the two-layer cross-sectional structure shown in FIG. Comparison was made with an ammonia gas sensor. In the ammonia gas sensor of Comparative Example 6, as shown in FIG. 10A, a single layer made of CeO _{2 with a} thickness exceeding the interface region R is provided on the solid electrolyte body, and an Au single layer is provided on the surface thereof. ing. That is, only CeO ₂ exists in the interface region R except for the components of the solid electrolyte body. In addition, as shown in FIG. 10B, the ammonia gas sensor of Comparative Example 7 is provided with a single layer of Au having a thickness exceeding the interface region R on the solid electrolyte body, and a single layer of CeO ₂ provided on the surface thereof. ing. In other words, only Au is present in the interface region R except for the components of the solid electrolyte body. In Comparative Examples 6 and 7, the content of Au contained in the detection electrode and the content of Ce oxide (CeO ₂ ) were the same as in Example 1 except that the Au content was 90% by mass, and CeO ₂ 10% by mass.

  Further, whether or not Au and a metal oxide exist in the interface region R between the detection electrode and the solid electrolyte body was determined by measuring three 15 μm square visual fields with EPMA in the cross section including both. That is, as described in FIG. 4, a plurality of component concentrations of the solid electrolyte body 22 </ b> A are measured in the thickness direction in the region of the 15 μm square field on both ends in the thickness direction, and the average value thereof is maximized. The value (Max) and the minimum value (Min) were used. Then, points where the deviation rate from the maximum value (Max) and the minimum value (Min) was 5% were P2 and P1, respectively, and the corresponding positions in the thickness direction were L1 and L2. The region L1 to L2 was defined as an interface region R, and if metal oxide and Au were detected in the interface region R exceeding the detection limit of EPMA, it was considered that Au and metal oxide were present in the interface region R.

The ammonia selectivity of each of the ammonia gas sensors of Example 1 and Comparative Examples 6 and 7 was evaluated in the same manner as in Evaluation 1.
The obtained results are shown in FIG. In the case of Example 1, the NH ₃ concentration conversion value is 5 ppm or less, which shows that the influence of the interfering gas is reduced and the selectivity of ammonia is good. In FIG. 11, the values of Example 1 are also shown.
On the other hand, in the case of Comparative Example 7 in which only Au exists in the interface region R, the NH ₃ concentration conversion value greatly exceeded 5 ppm, and the selectivity of ammonia was lowered. Further, in the case of Comparative Example 6 where only CeO ₂ exists in the interface region R, the conduction failure of the detection electrode occurred and the evaluation became impossible.
From these facts, it was found that even if the sensing electrode has a two-layer structure, the ammonia selectivity is improved if Au and Ce oxides are present in the interface region R.

Next, in the same manner as in Examples 1 to 6, except that the detection electrode interface region R has a two-layer cross-sectional structure shown in FIGS. Each ammonia gas sensor was produced. FIG. 5A shows the ammonia gas sensor of the seventh embodiment, FIG. 5B shows the ammonia gas sensor of the eighth embodiment, FIG. 5C shows the ammonia gas sensor of the ninth embodiment, and FIG.
About each ammonia gas sensor of Examples 7-10, it carried out similarly to the evaluation 1, and evaluated ammonia selectivity.
The obtained result is shown in FIG. In Examples 7 to 10, the NH ₃ concentration conversion values are all 5 ppm or less, which shows that the influence of the interfering gas is reduced and the ammonia selectivity is good.

3. Evaluation of influence of O _{2 on} ammonia sensitivity The ammonia gas sensors of Example 1 and Comparative Examples 1 to 3 were installed in the gas flow of the model gas generator, and the influence of O _{2 on} the ammonia sensitivity was evaluated. The gas composition of the model gas was H ₂ O = 5% CO ₂ = 5% N ₂ = bal., The gas temperature was 280 ° C., and the sensor element control temperature (heater heating) was 650 ° C. Then, when O ₂ was mixed with the model gas at 1, 5 and 10%, NH ₃ was mixed in the range of 0 to 100 ppm, and the above gas was allowed to flow from the model gas generator, Examples 1 to 6 and Similarly, the output (EMF value) of the ammonia gas sensor was substituted into the concentration conversion formula of FIG. 6, and the NH ₃ concentration conversion value was calculated.
When the NH ₃ concentration conversion value when NH ₃ = 60 ppm is in the range of 55 to 65 ppm (60 ± 5 ppm), it was considered that the influence of O _{2 on} the ammonia sensitivity was small. When there is no influence of O ₂ , the NH ₃ concentration conversion value is 60 ppm.

The obtained results are shown in FIGS. Figure 13 shows the effect of O ₂ to ammonia sensitivity in Example 1, Figure 14 shows the effect of O ₂ to ammonia sensitivity in Comparative Example 1, FIG. 15, the O ₂ to ammonia sensitivity in Comparative Example 2 FIG. 16 shows the influence of O _{2 on} the ammonia sensitivity in Comparative Example 3. In the case of Example 1, the NH ₃ concentration conversion value when NH ₃ = 60 ppm was in the range of 55 to 65 ppm, and the influence of O _{2 on} the ammonia sensitivity was small.
On the other hand, in Comparative Examples 1 and 3 in which the sensing electrode does not contain CeO ₂ , the NH ₃ concentration conversion value when NH ₃ = 60 ppm exceeds the range of 55 to 65 ppm, and the influence of O _{2 on} ammonia sensitivity increases. It was. Further, in Comparative Example 2 using In ₂ O ₃ instead of CeO ₂ , the influence of O _{2 on} the ammonia sensitivity was small, but the ammonia selectivity was originally low.

2A, 2A2-2A6 Detection electrode 4A Reference electrode 22A Solid electrolyte body 50A Gas sensor element 200A Ammonia gas sensor

Claims (3)

An oxygen ion conductive solid electrolyte body, and a detection electrode and a reference electrode provided on the surface of the solid electrolyte body,
The detection electrode contains noble metal as a main component, contains 2 to 30% by mass of Ce oxide, and Ce oxide and the noble metal are present at least in an interface region between the detection electrode and the solid electrolyte body. Gas sensor.   The ammonia gas sensor according to claim 1, wherein a content ratio of the noble metal in the detection electrode is 70 mass% or more. The ammonia gas sensor according to claim 1, wherein the Ce oxide includes at least CeO ₂ .

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