JP5033017B2 - Ammonia gas sensor - Google Patents

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, urea was added to the SCR catalyst to generate ammonia, is intended to reduce NO _x with ammonia, ammonia gas sensor is used for the ammonia concentration to reduce the NO _x to determine whether an appropriate amount ing.

As such an ammonia gas sensor, an ammonia-selective detection electrode 21 is arranged on the surface of the solid electrolyte layer 20 made of zirconia, a reference electrode (reference electrode) 22 is arranged facing the detection electrode 21, and the reference electrode and the detection electrode An electromotive force sensor that detects an ammonia concentration based on an electromotive force between the two is disclosed (see Patent Document 1).
In this sensor, both the reference electrode and the detection electrode are exposed to the gas to be measured, and the influence of the difference in oxygen concentration and moisture concentration between the gas to be measured and the atmosphere is exposed, like a cylindrical sensor that exposes the reference electrode to the atmosphere. There is an advantage that the ammonia concentration detection accuracy is improved.
Further, in this sensor, soot in the gas to be measured is deposited on the detection electrode exposed outside the sensor, and the detection electrode is prevented from peeling off from the solid electrolyte layer, and accompanying the deposition of oil-derived components. In order to prevent poisoning of the detection electrode, the porous protective layer 23 covers the detection electrode.

US Patent Application Publication No. 2007/0045114 (FIG. 2, paragraphs 0033, 0037, 0039)

However, in the case of the sensor described in Patent Document 1, since not only the detection electrode but also the reference electrode is exposed to the gas to be measured, the durability of the reference electrode may be inferior.
That is, an object of the present invention is to provide an ammonia gas sensor in which not only the detection electrode but also the reference electrode has high durability and excellent long-term stability.

In order to solve the above-mentioned problem, the ammonia gas sensor of the present invention has a sensor element part, and detects the ammonia concentration in the gas to be measured. The sensor element part includes an oxygen ion conductive solid electrolyte layer, A detection electrode formed on one surface of the solid electrolyte layer and exposed to the gas to be measured, and located inside the sensor element unit and formed on the other surface of the solid electrolyte layer to A reference electrode which is a counter electrode and has a lower ammonia activity than the detection electrode and is exposed to the gas to be measured, a porous first protective layer covering the detection electrode, the reference electrode, and the sensor element before exposed on the outer surface of the part, and a second protective layer made of porous introducing said measurement gas from the outside, arriving into an interface between the reference electrode and the solid electrolyte layer When the gas diffusion time of the gas to be measured is VA and the gas diffusion time of the gas to be measured that reaches the interface between the solid electrolyte layer and the detection electrode from the outside is VB, | VA−VB | × 100 / ( The gas diffusion time difference ratio represented by (VA + VB / 2) is 50% or less.
With such a configuration, when the method of introducing the gas to be measured is also introduced to the reference electrode side, the reference electrode is covered with the second protective layer, so that the soot and poisonous substance in the gas to be measured are covered on the reference electrode. Can be prevented and defects such as separation of the reference electrode from the solid electrolyte layer can be suppressed, and the durability of the reference electrode can be improved. And, by introducing the gas to be measured also to the reference electrode side, it is not necessary to consider the influence of the difference in oxygen concentration and moisture concentration between the gas to be measured and the atmosphere compared to the case where the reference electrode is exposed to the atmosphere. The detection accuracy of ammonia concentration is improved.

  In addition, the gas diffusion time of the gas to be measured that reaches the interface between the solid electrolyte layer and the reference electrode from the outside is defined as VA, and the gas diffusion time of the gas to be measured that reaches the interface between the solid electrolyte layer and the detection electrode from the outside. When VB is VB, the gas diffusion time difference ratio represented by | VA−VB | × 100 / ((VA + VB) / 2) is 50% or less, so the diffusion rate of the gas reaching the detection electrode and the reference electrode, respectively. (Time) does not vary greatly, and even if the sensor output fluctuates due to a sudden change in the oxygen concentration in the gas to be measured, it does not take time to stabilize the output, and ammonia measurement accuracy is further improved.

  Note that the detection electrode has lower ammonia activity than the reference electrode. With such a configuration, ammonia in the gas to be measured reacts and decomposes on the reference electrode, and the gas to be measured whose ammonia concentration is reduced becomes the reference atmosphere, so that the ammonia concentration can be measured more accurately.

Moreover, it is preferable that a selective reaction layer is interposed between the detection electrode and the first protective layer.
In such a configuration, the selective reaction layer burns combustible gas components other than ammonia in the gas to be measured, so the ammonia in the gas to be measured is detected on the sensing electrode without being affected by the combustible gas components. Measurement accuracy can be improved.

The first protective layer and the second protective layer are made of a porous material containing at least one selected from the group consisting of Al ₂ O ₃ , MgAl ₂ O ₄ , SiO ₂ , SiO ₂ / Al ₂ O ₃ , zeolite, and SiC. May be.
With such a configuration, it is possible to easily obtain a porous material having excellent durability against the gas to be measured.

  According to the present invention, it is possible to obtain an ammonia gas sensor in which not only the detection electrode but also the reference electrode has high durability and excellent long-term stability.

Hereinafter, embodiments of the present invention will be described.
<First Embodiment>
FIG. 1 is a cross-sectional view taken along the longitudinal direction of an ammonia gas sensor 200A according to the first embodiment of the present invention. The ammonia gas sensor 200A is an assembly in which a sensor element unit 50A for detecting ammonia is assembled. The ammonia gas 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 portion 50A and the insulating contact portion 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 30A to 34A 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 filling layers 153 and 156 (hereinafter also referred to as talc rings 153 and 156) in a state of 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 30A to 34A 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 30A to 34A formed on the surface on the rear end side 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 FIG. The sensor element portion 50A has a long plate shape, and the first protective layer 9A is exposed on the upper surface on the front end side (left side in FIG. 2) of the sensor element portion 50A. The electrode terminal portions 40A to 44A are exposed.

3 is a cross-sectional view taken along line III-III in FIG. The sensor element unit 50A is an electromotive force type plate ammonia gas sensor.
The sensor element portion 50A has a structure in which insulating layers 24A and 26A made of alumina are stacked, and the solid electrolyte layer 6 is stacked outside the insulating layer 24A. The upper surface of the insulating layer 24A is cut out in a U-shape in plan view at a predetermined depth, and the U-shaped opening is arranged to face the left in FIG. A second protective layer 10A made of a porous material is formed in the U-shaped opening, whereby the gas to be measured is introduced into the second protective layer 10A from the left end of the sensor element portion 50A. .

On the lower surface of the solid electrolyte layer 6 facing the second protective layer 10A, a reference electrode 4A having a substantially rectangular shape in plan view is disposed, and the surface of the reference electrode 4A is covered with the second protective layer 10A. Further, the detection electrode 2A is disposed on the upper surface of the solid electrolyte layer 6 at a position facing the reference electrode 4A.
The lead 30A extends along the longitudinal direction of the upper surface of the solid electrolyte layer 6 connected to the detection electrode 2A, and the insulating layer 20A is covered on the lead 30A. However, the right end of the lead 30A is exposed without being covered with the insulating layer 20A, and forms an electrode terminal portion 40A (further, an electrode terminal portion 41A connected to a lead 37A described later). A lead 37 </ b> A extends along the longitudinal direction of the lower surface of the solid electrolyte layer 6 connected to the reference electrode 4 </ b> A, and is connected to the electrode terminal portion 41 </ b> A through a through hole formed in the solid electrolyte layer 6. An insulating layer 22A is formed between the lead 30A and the solid electrolyte layer 6, and an insulating layer 24A (an insulating layer 24A1 described later) is formed between the lead 37A and the solid electrolyte layer 6. .

A selective reaction layer 3A that completely covers the detection electrode 2A is formed on the detection electrode 2A, and the surface of the selective reaction layer 3A is covered with a porous first protective layer 9A.
The selective reaction layer 3A has a role of burning a combustible gas component other than ammonia in the gas to be measured. When the selective reaction layer exists, the ammonia in the gas to be measured is detected without being influenced by the combustible gas component. can do.
Then, the gas to be measured is introduced into the selective reaction layer 3A and the detection electrode 2A through the first protective layer 9A, and the potential difference between the detection electrode 2A and the reference electrode 4A is measured through the solid electrolyte layer 6, The ammonia concentration in the gas to be measured (exhaust gas) can be detected.

The detection electrode 2A has, for example, gold as a main component. The reference electrode 4A, the leads 30A and 37A, and the electrode terminal portions 40A and 41A can be mainly made of platinum, for example, and the solid electrolyte layer 6 can be made of an oxygen ion conductive material such as ZrO ₂ .

The selective reaction layer 3A usually has a metal oxide as a main component, but in particular, an oxide represented by A _x M _y O _z (A is one or more metals; M is vanadium, tungsten, or molybdenum. X, y and z are preferably an atomic ratio). Examples of A include one or more selected from the group of bismuth, lanthanum, strontium, calcium, copper, gadolinium, neodymium, yttrium, samarium, and magnesium.
Specific examples _{A x M y O z, V} 2 O 5, Cu 2 (VO 3) 2, WO 3, MoO 3, BiVO 4 may be exemplified. BiVO ₄ is obtained by mixing vanadium oxide (V ₂ O ₅ ) and bismuth oxide (Bi ₂ O ₃ ) at 1: 1 (molar ratio). The selective reaction layer 3A can be obtained by printing and baking a paste containing these materials or plasma spraying these powders.
When the selective reaction layer 3A is not provided, the detection electrode 2A may be formed using the above-described A _x M _y O _z .

The first protective layer 9A and the second protective layer 10A prevent poisoning of the base (detection electrode 2A, reference electrode 4A) and adhesion of dirt. For example, Al ₂ O ₃ , MgAl ₂ O ₄ , SiO ₂ , SiO ₂ / SiO ₂ / It consists of a porous material containing at least one selected from the group consisting of Al ₂ O ₃ , zeolite (aluminosilicate) and SiC. Each protective layer can be formed in advance by spraying a porous material as described above on the base surface or by paste printing, but the forming method is not limited to this.

On the other hand, on the outside (lower surface) of the insulating layer 26A, a temperature detecting means (temperature sensor) 14A, which is a resistance temperature detector, is arranged, and a resistance for heating the sensor element unit 50A between the insulating layer 24A and the insulating layer 26A. A body heater 16A is interposed. Furthermore, leads 32A and 33A extend from the temperature detecting means 14A along the longitudinal direction of the insulating layer 26A. Leads 35A and 36A extend from the heater 16A along the longitudinal direction of the insulating layer 26A, and are connected to the electrode terminal portions 42A and 44A via through holes formed in the insulating layer 26A.
The surface of the temperature detecting means 14A is covered with the insulating layer 12A, but the right ends of the leads 32A and 33A are exposed without being covered with the insulating layer 12A to form electrode terminal portions 42A and 43A, respectively.
Then, a temperature control device (circuit unit) (not shown) controls the voltage applied to the heater 16A based on the measurement value of the temperature detection means 14A, and the solid electrolyte layer of the sensor element unit 50A is heated to the optimum temperature (activation temperature). Be controlled.

The temperature detection means 14A, the heater 16A, the leads 32A, 33A, 35A, and 36A each have, for example, platinum as a main component.
As each of the insulating layers 12A, 20A, 22A, 24A, and 26A, an insulating ceramic sintered body can be used, and oxide ceramics such as alumina and mullite can be exemplified.

Next, an example of the operation of the ammonia gas sensor 200A will be described. First, after the solid electrolyte layer 6 is activated by the heating of the heater 16A, when the sensor is exposed to the gas to be measured, the gas to be measured is transmitted from the first protective layer 9A to the selective reaction layer 3A, and in the selective reaction layer 3A. The combustible gas (CO, HC, etc.) burns, and the gas from which the combustible gas is removed reaches the detection electrode 2A.
On the other hand, the gas to be measured passes through the second protective layer 10A at the front edge of the sensor to the reference electrode 4A, and an electromotive force generated between the detection electrode 2A and the reference electrode 4A according to the ammonia concentration in the gas to be measured. A sensor output is obtained from (potential difference), and the ammonia concentration can be detected.

Note that when the gas to be measured is introduced into both the detection electrode and the reference electrode, the gas composition is the same, so that no electromotive force is generated between the electrodes if the electrode composition is the same. Therefore, by making the ammonia activity of the detection electrode lower than that of the reference electrode, an electromotive force can be generated between the electrodes.
That is, when the ammonia activity of the reference electrode is increased, ammonia in the gas to be measured reacts and decomposes on the reference electrode. As described above, the gas to be measured having a lowered ammonia concentration becomes a reference atmosphere and stays in the second protective layer on the surface of the reference electrode, so that a difference occurs in the ammonia concentration in the gas with the detection electrode.
And a sensing electrode Au (or a material containing Au alloy and Au), an oxide such as _A x _M y _{O z} as described above, the use of Pt (or a material containing Pt alloy or Pt) as a reference electrode Thus, the ammonia activity of the detection electrode can be made lower than that of the reference electrode.

In this way, when the method of introducing the gas to be measured is also introduced to the reference electrode side, the oxygen concentration and moisture between the gas to be measured and the atmosphere, such as a cylindrical electromotive force sensor that exposes the reference electrode to the atmosphere, is used. There is no need to consider the influence of the concentration difference, and there is an advantage that the detection accuracy of the ammonia concentration is improved.
On the other hand, when soot or poisonous substance in the gas to be measured is deposited on the reference electrode, the reference electrode may be peeled off from the solid electrolyte layer. However, by covering the reference electrode with the second protective layer, Therefore, it is possible to improve the durability of the reference electrode.

By the way, the porous protective layer has a predetermined diffusion resistance. For this reason, when the detection electrode and the reference electrode are each covered with a protective layer, if the diffusion rate (time) of the gas that reaches the detection electrode and the reference electrode differs greatly, the oxygen concentration in the measured gas changes suddenly. Sensor output may fluctuate and stabilization (convergence) may take time (transient characteristics).
Therefore, the gas diffusion time of the measurement gas reaching from the outside to the interface between the solid electrolyte layer and the reference electrode is VA, and the gas diffusion time of the measurement gas reaching from the outside to the interface between the solid electrolyte layer and the detection electrode is VB. In this case, the gas diffusion time difference ratio represented by | VA−VB | × 100 / ((VA + VB) / 2) is set to 50% or less. When the gas diffusion time difference rate is 50% or less, there is no significant difference in the gas diffusion time of the gas to be measured between the detection electrode and the reference electrode, the above-mentioned transient characteristics are improved, and ammonia measurement accuracy is further improved. .
The method of adjusting the gas diffusion time difference ratio is as follows: the first protective layer, the second protective layer, the selective reaction layer, and the respective air permeability (diffusion resistance), film thickness, exposed area (opening area) ) And the like can be changed as appropriate.

The gas diffusion times VA and VB are measured as follows. First, as shown in FIG. 4, in the ammonia gas sensor 200 </ b> A, the space around the sensor element unit 50 </ b> A surrounded by the internal protector 143 is divided into two by the partition plate 300. For example, the measurement gas Ga introduced from the outside to the detection electrode 2 </ b> A side is provided as a reference electrode by arranging the partition plate 300 in parallel with the surface of the solid electrolyte layer 6 at a position passing through the center in the thickness direction of the solid electrolyte layer 6. The gas to be measured Gb introduced from the outside to the reference electrode 4A side does not flow to the detection electrode 2A side without flowing to the 4A side. The partition plate 300 has a dimension that penetrates the contour portion of the sensor element unit 50 </ b> A and can be accommodated in the internal protector 143.
In this way, it is possible to measure the electromotive force (sensor output) between the two electrodes by flowing gases having different compositions through the detection electrode 2A and the reference electrode 4A.

The gas diffusion time VA is such that when the oxygen concentration in the gas Ga flowing to the detection electrode 2A side is switched from 0.2% to 20% and the oxygen concentration in the gas Gb flowing to the reference electrode 4A side is constant at 20%, The time from when the oxygen concentration of the gas Ga is switched is set as the starting point, and the time from when the sensor output at the starting point starts to decrease to 0.3 seconds later is obtained as VA. The gas composition (components other than oxygen) used for the measurement is CO ₂ = 5%, H ₂ O = 5%, N ₂ = bal. And a gas temperature of 280 ° C., a sensor element control temperature of 650 ° C., and a gas flow rate of 4 m / s with a sensor attached to the model gas generator for measurement.
FIG. 5 shows an example of a temporal change in sensor output after the oxygen concentration of the gas Ga is switched. The sensor output when the oxygen concentrations in the gases Ga and Gb are both 20% is V ₀ . Assuming that the time during which the oxygen concentration in the gas Ga is switched from 0.2% to 20% is t ₀ , the sensor output becomes V ₁ at time t ₁ after a little time has elapsed from t ₀ . Further, when the time after 0.3 seconds from the time t ₁ is t ₂ (the sensor output at this time is V ₂ ), it is expressed as VA = t ₂ −t ₀ .

The reason for setting the time t ₁ from the time t ₂ after 0.3 seconds, in the region V _R from time t ₁ elapses sensor output considerable time decays, the electrode transport number (ammonia This is because the ionization rate due to decomposition of the gas becomes rate-determining and it becomes difficult to measure the gas diffusion time.
Note that measurement of τ90 response time and the like is not preferable because gas component adsorption or reaction may occur in the electrode, selective reaction layer, or the like, and the true gas diffusion time may not be measured.

  Similarly, the gas diffusion time VB is obtained when the oxygen concentration in the gas Gb flowing to the reference electrode 4A is switched from 0.2% to 20% and the oxygen concentration in the gas Ga flowing to the detection electrode 2A is constant at 20%. The time from when the oxygen concentration of the gas Gb is switched to the starting point and from when the sensor output at the starting point starts to decrease to 0.3 seconds later is obtained as VB.

Next, an example of a method for manufacturing the sensor element unit 50A will be briefly described with reference to a development view 6. FIG. 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, alumina (an inorganic oxide used as a common substrate) binder and an organic solvent are included on the insulating layer 26A. Electrode paste (hereinafter referred to as “Pt paste”) is screen-printed to form the heater 16A (and leads 35A and 36A extending therefrom).
Then, an insulating layer 24A5 is screen-printed on the heater 16A and the leads 35A and 36A, and an insulating layer 24A4 is screen-printed on the insulating layer 24A5.

  On the other hand, the temperature detecting means 14A (and leads 32A, 33A, 34A and electrode terminal portions 42A, 43A, 44A extending therefrom) are formed by screen printing a Pt paste on the lower surface of the insulating layer 26A, and the temperature detecting means 14A. The insulating layer 12A is formed by screen printing a paste containing alumina, a binder and an organic solvent on the surface.

Next, the green sheet used as the solid electrolyte layer 6 is manufactured from the slurry containing a zirconia-based powder, a binder, and an organic solvent by a doctor blade method, and an insulating layer 24A1 having a rectangular hole at the left end below the solid electrolyte layer 6 Are laminated. Then, a Pt-based paste is screen-printed in the rectangular hole of the insulating layer 24A1 to form the reference electrode 4A (and the lead 37A extending therefrom). Further, an insulating layer 24A2 having a rectangular hole at the left end is screen-printed so that the reference electrode 4A enters the rectangular hole. The reference electrode 4A is exposed to the lower surface from the rectangular hole of the insulating layer 24A2.
Next, a rectangular second protective layer 10A is screen-printed on the lower side of the reference electrode 4A so as to cover the exposed reference electrode 4A. 10 A of 2nd protective layers are accommodated in the interior space formed by the U-shape of insulating layer 24A3 mentioned later. The solid electrolyte layer 6 may be formed by screen printing a solid electrolyte paste.

  On the other hand, an insulating layer 22A having a rectangular hole at the left end is screen-printed on the solid electrolyte layer 6, and a Pt-based paste is screen-printed on the insulating layer 22A to form leads 30A and electrode terminal portions 40A and 41A. Further, an insulating layer 20A having a rectangular hole at the left end is screen-printed so as to cover the lead 30A.

  Then, an insulating layer 24A3 having a U-shape opening toward the left tip is screen-printed on the insulating layer 24A4. Then, the second protective layer 10A is laminated and pressure-bonded on the insulating layer 24A3 (so as to be accommodated in the U-shape of the insulating layer 24A3), further cut into a predetermined shape, and a predetermined temperature (eg, about 400 ° C.). ) And then baked at a predetermined temperature (for example, 1520 ° C.). The insulating layers 24A1 to 24A5 enhance the electrical insulation between the reference electrode 4A and the heater 16. In FIG. 3, the insulating layers 24A1 to 24A5 are collectively shown as the insulating layer 24A. The insulating layer 24A may be formed by alumina green sheet instead of screen printing.

Thereafter, an Au-based paste is screen printed in the rectangular holes of the insulating layers 20A and 22A to form the detection electrode 2A.
Then, the laminate is debindered at a predetermined temperature (for example, 250 ° C.) and then fired at a predetermined temperature (for example, 1000 ° C.). Here, the detection electrode 2A is connected to the lead 30A, and the reference electrode 4A and the detection electrode 2A are in contact with the upper and lower surfaces of the solid electrolyte layer 6.

  Further, the rectangular selection reaction layer 3A is formed by screen printing a paste mainly composed of a metal oxide so as to cover the detection electrode 2A, and finally the rectangular shape so as to cover the selection reaction layer 3A. The first protective layer 9A is screen-printed and baked at a predetermined temperature (for example, 750 ° C.).

<Second Embodiment>
Next, a sensor element portion 50B of an ammonia gas sensor according to a second embodiment of the present invention will be described with reference to a development view 7. FIG. In the ammonia gas sensor according to the second embodiment, since the laminated structure from the insulating layer 24A4 to the insulating layer 12A in the sensor element portion of FIG. 6 is used as it is, the same components are denoted by the same reference numerals as in FIG. Description is omitted.

In FIG. 7, a solid electrolyte layer 6 (same as in the first embodiment) is stacked on an insulating layer 24A4, and further, an insulating layer 22B in which two long side openings are arranged in the width direction at the left end is solidified. Screen printing is performed on the electrolyte layer 6, and Pt paste is screen printed on the insulating layer 22B to form the reference electrode 4B, the leads 30B and 31B, and the electrode terminal portions 40B and 41B. Further, the insulating layer 20B is screen-printed so as to cover the leads 30B and 31B. As a result, the leads 30B and 31B are completely covered with the insulating layer 20B. On the other hand, the reference electrode 4B is exposed from one opening of the insulating layer 20B and is in contact with the surface of the solid electrolyte layer 6 through the opening of the insulating layer 22B.
Further, the Au-based paste is screen-printed in the other opening of the insulating layer 20B (at the same position as the other opening of the insulating layer 22B) to form the detection electrode 2B. The detection electrode 2B is in contact with the surface of the solid electrolyte layer 6 through another opening of the insulating layer 22B and is connected to the lead 30B.

  Further, a long-sided selection reaction layer 3B is formed by screen printing a paste mainly composed of a metal oxide so as to cover the detection electrode 2B, and finally the selection reaction layer 3B and the reference electrode 4B are covered. Then, the same rectangular protective layer 11B is screen-printed and baked at a predetermined temperature (for example, 750 ° C.).

In the second embodiment, the detection electrode 2B and the reference electrode 4B are formed on the same surface side of the solid electrolyte layer 6, and the first protective layer and the second protective layer are the same layer 11B. With such a configuration, the detection electrode 2B and the reference electrode 4B are exposed to the gas flow to be measured under the same conditions, and the directionality of the gas flow can be relaxed with respect to the counter electrode.
Also in the second embodiment, since the ammonia activity of the detection electrode 2B is lower than that of the reference electrode 4B, ammonia in the measurement gas reacts and decomposes on the reference electrode 4B, and the measurement gas whose ammonia concentration has decreased is the reference. Since the atmosphere stays in the vicinity of the reference electrode 4B of the layer 11B, a difference occurs in the ammonia concentration in the gas between the detection electrode 2B and the ammonia concentration can be detected.

The present invention is not limited to the above embodiment. The present invention is applicable to any ammonia gas sensor in which ammonia concentration is detected from an electromotive force generated between a detection electrode and a reference electrode via a solid electrolyte layer, and both the detection electrode and the reference electrode are exposed to a gas to be measured. It is.
Moreover, in the said embodiment, although the selective reaction layer was provided on the detection electrode, it is not necessary to provide a selective reaction layer.
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.

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

The ammonia gas sensor according to the first embodiment was manufactured by the method described above. Each insulating layer 12A, 26A, 24A, 22A, 20A is a mixture of alumina powder, organic solvent and dispersant in a mortar, and after 4 hours of dispersion mixing with a rake machine, a predetermined amount of binder and viscosity modifier are added. Further, the paste prepared by wet mixing for 4 hours was printed and formed.
The green sheet to be the solid electrolyte layer 6 was formed by a doctor blade method from a slurry containing partially stabilized zirconia-based powder containing 5.4 mol% Y ₂ O ₃ , a binder and an organic solvent.
In the detection electrode 2A, Au powder, a small amount of zirconia powder, an organic solvent and a dispersing agent are put in a mortar, and after 4 hours of dispersion mixing with a rough machine, a predetermined amount of a binder and a viscosity modifier are added, and further 4 hours. It was formed by printing a paste prepared by wet mixing.
The reference electrode 4A, each lead, and each electrode terminal part were placed in a mortar with Pt powder, 14% by mass of partially stabilized zirconia powder, an organic solvent and a dispersant in a mortar, and dispersed and mixed for 4 hours with a rake machine. After that, a predetermined amount of a binder and a viscosity modifier were added, and the paste prepared by wet mixing for 4 hours was printed and formed.

The selective reaction layer 3A is prepared by adding vanadium oxide (V ₂ O ₅ ) powder and bismuth oxide (Bi ₂ O ₃ ) powder prepared in a 1: 1 (molar ratio) to the mortar, an organic solvent and a dispersant in the mortar. After mixing and mixing for 4 hours by a machine, a predetermined amount of a binder and a viscosity modifier were added, and a paste prepared by wet mixing for 4 hours was printed to form.
The first protective layer 9A and the second protective layer 10A are prepared by putting an alumina (Al ₂ O ₃ ) powder, an organic solvent and a dispersant in a mortar and dispersing and mixing them for 4 hours with a cracker, followed by a binder and a viscosity modifier. Was added, and a paste prepared by wet mixing for 4 hours was printed to form.
After laminating each paste to form insulating layers 12A to 20A, the binder was removed at 400 ° C. and fired at 1470 ° C.
Next, the detection electrode 2A is screen-printed and fired at 1000 ° C. for 1 hour, and then the selective reaction layer 3A and the first protective layer 9A are screen-printed and laminated, and fired at 750 ° C. Obtained. The obtained sensor element unit 50A was assembled as the ammonia gas sensor 200A as described above.

The sensor was the same as in Example 1 except that MgAl ₂ O ₄ powder was used instead of alumina (Al ₂ O ₃ ) powder as a paste for forming the first protective layer 9A and the second protective layer 10A. The element part was manufactured and assembled as an ammonia gas sensor.

The sensor element portion is the same as in Example 1 except that SiO ₂ powder is used instead of alumina (Al ₂ O ₃ ) powder as a paste for forming the first protective layer 9A and the second protective layer 10A. And assembled as an ammonia gas sensor.

As a paste for forming the first protective layer 9A and the second protective layer 10A, instead of alumina (Al ₂ O ₃ ) powder, SiO ₂ / Al ₂ O powder (SiO ₂ / Al ₂ O ₃ ratio = 1000) A sensor element part was manufactured in the same manner as in Example 1 except that was used, and assembled as an ammonia gas sensor.

As in Example 1, except that zeolite (ZSM-5) powder was used instead of alumina (Al ₂ O ₃ ) powder as a paste for forming the first protective layer 9A and the second protective layer 10A. The sensor element was manufactured and assembled as an ammonia gas sensor.

The sensor element portion was formed in the same manner as in Example 1 except that SiC powder was used instead of alumina (Al ₂ O ₃ ) powder as a paste for forming the first protective layer 9A and the second protective layer 10A. Manufactured and assembled as an ammonia gas sensor.

<Comparative example>
A sensor element portion was manufactured in the same manner as in Example 1 except that the second protective layer 10A was not formed and the reference electrode 4A was exposed in the U-shaped space of the insulating layer 24A3, and assembled as an ammonia gas sensor.

<Sensor characteristics (sensitivity) evaluation>
(1) Initial sensitivity of sensor The ammonia gas sensor of each Example and the comparative example was attached in the gas flow of the model gas generator, and the initial sensitivity of the sensor was evaluated. The gas temperature of the model gas was 280 ° C., the element control temperature (heater heating) was 650 ° C., and the gas composition was O ₂ = 10% CO ₂ = 5% H ₂ O = 5% N ₂ = bal. When the gas was flowed from the model gas generator and both the detection electrode and the reference electrode were exposed to the gas, the potential difference between the detection electrode and the reference electrode was measured and used as the base electromotive force. Next, NH ₃ = 100 ppm was added to the model gas, and the potential difference between the detection electrode and the reference electrode at this time was measured to obtain an electromotive force at the time of measurement. A value obtained by subtracting the base electromotive force from the electromotive force at the time of measurement was defined as the initial sensitivity of the sensor.

(2) Actual machine test The ammonia gas sensor of each Example and Comparative Example was attached to an actual engine, and the engine was actually operated to evaluate the sensor characteristics. The engine used was a 3.0-liter diesel engine, and an ammonia gas sensor was attached to the exhaust pipe (the wake of the DOC (Diesel Oxidation Catalyst) and DPF (black smoke removal device)).
The engine was idled for 10 minutes and then operated at 3000 rpm for 30 minutes as one cycle, and this cycle was repeated for 500 hours to conduct an actual machine test.
After the actual machine test was completed, the sensor was removed from the engine and attached to the model gas generator in the same manner as (1) to measure the sensitivity of the sensor. The sensitivity of the sensor was defined as electromotive force at measurement-base electromotive force.

  The obtained results are shown in Table 1 and FIG.

From Table 1 and FIG. 8, in the case of each example in which the porous second protective layer was provided on the reference electrode side, the rate of decrease in sensor sensitivity after the actual machine test was 5% or less (long-term) It became excellent in stability.
On the other hand, in the case of the comparative example in which the protective layer is not provided on the reference electrode side, the rate of decrease in sensor sensitivity (from the initial stage) after the actual machine test exceeds 20%, the long-term stability is inferior, and the shape change ( Partial peeling) was observed.
The ammonia gas sensor according to the second embodiment was manufactured by the above-described method, and the sensor sensitivity was evaluated in the same manner as the ammonia gas sensor according to the first embodiment. ) It was confirmed that the reduction rate was 5% or less. In addition, the paste used for each layer at the time of manufacturing the ammonia gas sensor according to the second embodiment was the same as that in the first embodiment.

<Evaluation of sensor transient characteristics (stability)>
In Example 1, a sensor element portion in which the diffusion resistance of the second protective layer 10A was changed was manufactured. The diffusion resistance was changed by adding an organic bead or carbon pore-forming agent in the paste, or using a protective layer material with a changed particle size and specific surface area. Moreover, the sensor element part of the said comparative example was prepared. These sensor element portions were assembled as an ammonia gas sensor.
Next, as shown in FIG. 4, in the ammonia gas sensor 200 </ b> A, the space around the sensor element unit 50 </ b> A surrounded by the internal protector 143 was partitioned into two by the partition plate 300.
The gas diffusion time VA is such that when the oxygen concentration in the gas Ga flowing to the detection electrode 2A side is switched from 0.2% to 20% and the oxygen concentration in the gas Gb flowing to the reference electrode 4A side is constant at 20%, Starting from the oxygen concentration switching of the gas Ga, the time from when the sensor output at the starting point starts to decrease to 0.3 seconds later was determined as VA. The gas composition (components other than oxygen) used for the measurement is CO ₂ = 5%, H ₂ O = 5%, N ₂ = bal. The gas temperature was 280 ° C., the sensor element control temperature was 650 ° C., and the measurement was performed with a sensor attached to the model gas generator at a gas flow rate of 4 m / s.
Similarly, the gas diffusion time VB is obtained when the oxygen concentration in the gas Gb flowing to the reference electrode 4A is switched from 0.2% to 20% and the oxygen concentration in the gas Ga flowing to the detection electrode 2A is constant at 20%. The time from when the oxygen concentration of the gas Gb was switched to the starting point and the sensor output at the starting point starting to decrease until 0.3 seconds later was determined as VB.
Then, the gas diffusion time difference ratio was calculated by | VA−VB | × 100 / ((VA + VB) / 2).

The partition plate was removed from the ammonia gas sensor and mounted in the gas flow of the model gas generator described above, and the sensitivity of the sensor was evaluated. The gas temperature of the model gas is 280 ° C, the element control temperature (heater heating) is 650 ° C, and the gas composition is O ₂ = 0.2% CO ₂ = 5% H ₂ O = 5% N ₂ = bal. The potential difference (electromotive force) between the reference electrodes was measured. Next, the gas was switched to a gas having an oxygen concentration of 20%, and the electromotive force between the detection electrode and the reference electrode was subsequently measured to determine the time variation of the electromotive force.
The obtained results are shown in Table 2 and FIG.

From Table 2 and FIG. 9, when the gas diffusion time difference rate is 0%, the change in the electromotive force of the sensor when the oxygen concentration in the gas suddenly changes is small (the change in electromotive force is about 0.2 mV). Returning, stable sensor output was obtained.
When the gas diffusion time difference rate was 20% and 50%, the change in electromotive force of the sensor when the oxygen concentration in the gas changed suddenly was about 6 mV or less. In this measurement, if the gas flow rate is 4 m / s, which is close to the actual use environment, and the sensor electromotive force change is about 5 to 6 mV, the sensor characteristics converge quickly even if the oxygen concentration in the gas changes suddenly, and the actual use It can be said that there is no problem.
On the other hand, when the gas diffusion time difference rate was 80%, if the oxygen concentration in the gas changed suddenly, the change in electromotive force of the sensor increased to 10 mV or more, and it took time until the output converged (stabilized).
From the above, it was found that the gas diffusion time difference rate is preferably 50% or less.

It is sectional drawing which follows the longitudinal direction of the gas sensor (ammonia gas sensor) which concerns on the 1st Embodiment of this invention. It is a perspective view which shows the structure of 50 A of sensor element parts. It is sectional drawing which follows the III-III line of FIG. It is a figure which shows the apparatus (partition plate) for measuring a gas diffusion time difference rate. It is a figure which shows an example of the time change of the sensor output after switching the oxygen concentration in gas as a method of calculating | requiring gas diffusion time. It is an expanded view of 50 A of sensor element parts. It is an expanded view of the sensor element part 50B in the ammonia gas sensor which concerns on 2nd Embodiment. It is a figure which shows the sensitivity change of the sensor after the long-term test by an actual machine. It is a partial expanded view of the sensor element part in the ammonia gas sensor which concerns on 3rd Embodiment. It is a figure which shows the electromotive force change of a sensor when the oxygen concentration in gas changes suddenly.

Explanation of symbols

2A, 2B Detection electrode 3A, 3B Selective reaction layer 4A, 4B Reference electrode 6 Solid electrolyte layer 9A First protective layer 10A Second protective layer 11B Same layer (first protective layer and second protective layer)
50A, 50B Sensor element part 200A Ammonia gas sensor

Claims (3)

In an ammonia gas sensor that has a sensor element portion and detects the ammonia concentration in the gas to be measured,
The sensor element unit is
An oxygen ion conductive solid electrolyte layer, a sensing electrode formed on one surface of the solid electrolyte layer and exposed to the gas to be measured;
A reference electrode that is located inside the sensor element portion and is formed on the other surface of the solid electrolyte layer to serve as a counter electrode of the detection electrode, has a lower ammonia activity than the detection electrode, and is exposed to the gas to be measured. When,
A first protective layer made of a porous material covering the detection electrode;
A second protective layer made of a porous material that covers the reference electrode, is exposed on the outer surface of the sensor element unit, and introduces the gas to be measured from the outside ;
The gas diffusion time of the measurement gas that reaches the interface between the solid electrolyte layer and the reference electrode from the outside is defined as VA, and the gas of the measurement gas that reaches the interface between the solid electrolyte layer and the detection electrode from the outside An ammonia gas sensor having a gas diffusion time difference ratio of 50% or less represented by | VA−VB | × 100 / ((VA + VB) / 2) where the diffusion time is VB.   The ammonia gas sensor according to claim 1, wherein a selective reaction layer is interposed between the detection electrode and the first protective layer. The first protective layer and the second protective layer are made of a porous material containing at least one selected from the group consisting of Al ₂ O ₃ , MgAl ₂ O ₄ , SiO ₂ , SiO ₂ / Al ₂ O ₃ , zeolite, and SiC. The ammonia gas sensor according to claim 1 or 2.

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