JP5728426B2 - Manufacturing method of multi-gas sensor - Google Patents

Description

  The present invention relates to a method for manufacturing a multi-gas sensor suitable for detecting a nitrogen oxide concentration and an ammonia concentration contained in a gas to be measured.

In recent years, a urea SCR (Selective Catalytic Reduction) system has attracted attention as a technology for purifying nitrogen oxides (NOx) contained in exhaust gas discharged from an internal combustion engine such as a diesel engine. The urea SCR system purifies nitrogen oxides contained in exhaust gas by chemically reacting ammonia (NH ₃ ) and nitrogen oxides (NOx) to reduce nitrogen oxides to nitrogen (N ₂ ). System.

  In this urea SCR system, if the amount of ammonia supplied to the nitrogen oxides becomes excessive, unreacted ammonia may be discharged to the outside while being contained in the exhaust gas. In order to suppress such ammonia release, a multi-gas sensor capable of measuring a plurality of types of gas concentrations has been proposed, including a sensor element that measures the concentration of ammonia contained in exhaust gas (for example, Patent Document 1). reference).

  On the other hand, a process of detecting gas concentration stably (hereinafter referred to as “treatment”) for a detection element that detects the concentration of a specific gas component contained in the gas to be measured, for example, the concentration of nitrogen oxides. Has been proposed (see, for example, Patent Document 2).

  The treatment is performed in two treatments. The first treatment is generally performed because the element is not sufficiently activated and sufficient sensor characteristics cannot be obtained simply by providing an electrode on the solid electrolyte body in the detection element. In the first treatment, the detection element is heated in a state exposed to a predetermined atmosphere, and an alternating voltage having a preset magnitude is applied between the electrodes of the detection element to promote activation of the electrode. . In the second treatment, the electrode activity can be improved by the first treatment, but the initial activity is improved too much, and the initial fluctuation occurs in the sensor characteristics. As the second treatment, after the first treatment, heating is performed in a state where the moisture is exposed to a lean atmosphere with a substantially constant state, and the electrodes of the detection elements are energized.

JP 2011-075546 A JP 2004-294079 A

  The treatment described in Patent Document 2 can increase the detection sensitivity for a detection element that detects the concentration of a specific gas component, for example, the concentration of nitrogen oxide, but the concentration of another specific gas component, for example, ammonia. For other detection elements that detect the concentration of, the detection sensitivity may be reduced.

  In other words, for a multi-gas sensor having a sensor element portion having a nitrogen oxide concentration detection element and an ammonia concentration detection element, a treatment that promotes activation of the electrode of the detection element that detects the nitrogen oxide concentration in the first treatment. If this is done, the detection sensitivity of the detection element that detects the ammonia concentration will decrease, and even after the second treatment, the detection sensitivity of the detection element that detects the ammonia concentration will not recover, and the multi-gas sensor will be good overall There was a problem that stopped working. This is considered to be because the first treatment is performed in a rich atmosphere.

  The present invention has been made in order to solve the above-described problems, and provides a method for manufacturing a multi-gas sensor that can improve the overall detection sensitivity of each specific gas component concentration to be detected. Objective.

In order to achieve the above object, the present invention provides the following means.
Method for producing a multi-gas sensor of the present invention, NOx detector element for detecting the concentration of nitrogen oxides contained in the measurement gas, and wherein a ammonia detector element for detecting the ammonia concentration of the gas to be measured, zirconia A manufacturing method of a multi-gas sensor having a sensor element portion integrally provided with an ammonia detection element having a solid electrolyte body for ammonia mainly, a reference electrode mainly containing platinum, and an ammonia electrode mainly containing gold. Then, while heating the sensor element portion to the first temperature region in a rich atmosphere, an alternating voltage having a preset magnitude is applied between the electrodes of the NOx detection element, and the electrode of the NOx detection element is applied. The first treatment process to be activated, and after the first treatment process, the water content is set to more than 15 volume% and 30 volume% or less. At a lean atmosphere, while heating the sensor element to a second temperature region, and having a second treatment step of performing energizing electrode of the NOx detection element.

  According to the method for manufacturing a multi-gas sensor of the present invention, activation of the electrode of the NOx detection element can be promoted by performing the first treatment process. In addition, after the first treatment process performed in a rich atmosphere, by performing the second treatment process in a lean atmosphere where the water content is set to more than 15% by volume and not more than 30% by volume, The detection sensitivity of the lowered ammonia detection element can be recovered.

  Specifically, by performing the first treatment process, the electrode surface of the NOx detection element is cleaned, and the activity in the electrode is improved. On the other hand, in the ammonia detection element, the catalytic activity of the electrode is improved, so that the detection sensitivity is decreased.

  On the other hand, when the second treatment process is performed under the above-described conditions after the first treatment process, the catalytic activity at the electrode of the ammonia detection element is suppressed. As a result, the detection sensitivity of the ammonia detection element, which has been reduced by the first treatment process, is restored. In the second treatment process, the initial activity of the electrode of the NOx detection element is improved too much in the first treatment process, and the initial fluctuation of the sensor characteristics is generated. Gas concentration can be detected.

  According to the method for manufacturing a multi-gas sensor of the present invention, the activation of the electrode of the NOx detecting element can be promoted by performing the first treatment process, and the first treatment process is performed by performing the second treatment process. Since the lowered detection sensitivity of the ammonia detection element can be recovered, there is an effect that the detection sensitivity of each specific gas component concentration that is a detection target can be increased as a whole.

It is sectional drawing which follows the longitudinal direction explaining the structure of the multi-gas sensor which concerns on one Embodiment of this invention. It is a block diagram explaining the structure of the multi-gas sensor apparatus which concerns on one Embodiment of this invention. It is an expanded view explaining the structure of an ammonia sensor part. It is a bar graph showing the recovery rate from a comparative example and Example 1 to Example 14.

A multi-gas sensor device 1 according to an embodiment of the present invention will be described with reference to FIGS. The multi-gas sensor device 1 of this embodiment is used for a urea SCR system that purifies nitrogen oxides (NOx) contained in exhaust gas (gas to be measured) discharged from a diesel engine. More specifically, the concentration of nitrogen monoxide (NO), nitrogen dioxide (NO ₂ ), and ammonia contained in the exhaust gas after reacting NOx contained in the exhaust gas with ammonia (urea) is measured. Is.

  Note that the engine to which the multi-gas sensor device 1 of the present embodiment is applied may be the above-described diesel engine or can be applied to a gasoline engine, and the type of the engine is not particularly limited.

As shown in FIGS. 1 and 2, the multi-gas sensor device 1 controls the multi-gas sensor 2 which is a sensor main body, and controls the multi-gas sensor 2 and computes the sensor output to obtain concentrations of NO, NO ₂ and ammonia. And a control unit 3 for calculating.

  As shown in FIG. 1, the multi-gas sensor 2 is mainly provided with a sensor element unit 10, a metal shell 110, a separator 134, and a connection terminal 138. In the following description, the side (lower side in FIG. 1) where the sensor element unit 10 of the multi-gas sensor 2 is arranged is the front end side, and the side where the connection terminal 138 is arranged (upper side in FIG. 1) is the rear end. It is written as side.

  The sensor element unit 10 has a plate shape extending in the axis O direction. Electrode terminal portions 10 </ b> A and 10 </ b> B are disposed at the rear end of the sensor element portion 10. In FIG. 1, for ease of illustration, the electrode terminal portions formed on the sensor element portion 10 are only the electrode terminal portion 10A and the electrode terminal portion 10B. A plurality of electrode terminal portions are formed according to the number of electrodes and the like that the ammonia sensor portion 21 has. A more detailed description of the sensor element unit 10 will be described later.

  The metal shell 110 is a cylindrical member in which a screw portion 111 that fixes the multi-gas sensor 2 to an exhaust pipe of a diesel engine is formed on the outer surface. The metal shell 110 is mainly provided with a through hole 112 penetrating in the axial direction and a shelf 113 projecting radially inward of the through hole 112. The shelf 113 is formed as an inwardly tapered surface having an inclination that approaches the front end side from the radially outer side of the through hole 112 toward the center.

  The metal shell 110 holds the sensor element unit 10 in a state where the front end side of the sensor element unit 10 protrudes from the through hole 112 to the front end side and the rear end side of the sensor element unit 10 protrudes to the rear end side of the through hole 112. Is.

  Inside the through-hole 112 of the metal shell 110, in order from the front end side to the rear end side, a ceramic holder 114 that is a cylindrical member surrounding the radial periphery of the sensor element unit 10, and a talc ring that is a powder-filled layer 115 and 116 and a ceramic sleeve 117 are laminated.

  A caulking packing 118 is disposed between the ceramic sleeve 117 and the end portion on the rear end side of the metal shell 110. A metal holder 119 is disposed between the ceramic holder 114 and the shelf 113 of the metal shell 110. The metal holder 119 holds the talc ring 115 and the ceramic holder 114. The end portion on the rear end side of the metal shell 110 is a portion that is crimped so as to press the ceramic sleeve 117 toward the distal end side via the crimping packing 118.

  An external protector 121 and an internal protector 122 are provided at the end of the metal shell 110 on the front end side. The external protector 121 and the internal protector 122 are cylindrical members formed of a metal material such as stainless steel whose end on the distal end side is closed. The internal protector 122 is welded to the metal shell 110 in a state where the end of the sensor element unit 10 on the front end side is covered, and the external protector 121 is welded to the metal shell 110 in a state where the internal protector 122 is covered.

  At the end on the rear end side of the metal shell 110, the end portion on the front end side of the outer cylinder 131 formed in a cylindrical shape is fixed. Furthermore, a grommet 132 that closes the opening is disposed in an opening that is an end portion on the rear end side of the outer cylinder 131.

  The grommet 132 is formed with a lead wire insertion hole 133 through which the lead wire 141 is inserted. The lead wire 141 is electrically connected to the electrode terminal portion 10A of the sensor element portion 10 and the electrode terminal portion 10B.

  The separator 134 is a cylindrical member disposed on the rear end side of the sensor element unit 10. A space formed inside the separator 134 is an insertion hole 135 penetrating in the axial direction. On the outer surface of the separator 134, a flange 136 that protrudes radially outward is formed.

  The rear end portion of the sensor element portion 10 is inserted into the insertion hole 135 of the separator 134, and the electrode terminal portions 10 </ b> A and 10 </ b> B are disposed inside the separator 134.

  Between the separator 134 and the outer cylinder 131, a holding member 137 formed in a cylindrical shape is disposed. The holding member 137 contacts the flange 136 of the separator 134 and also contacts the inner surface of the outer cylinder 131, thereby fixing and holding the separator 134 with respect to the outer cylinder 131.

  The connection terminal 138 is a member disposed in the insertion hole 135 of the separator 134, and electrically connects the electrode terminal portion 10A and the electrode terminal portion 10B of the sensor element portion 10 and the lead wire 141 independently of each other. It is a conductive member. In FIG. 1, only two connection terminals 138 are shown for ease of illustration.

As shown in FIG. 2, the control unit 3 of the multigas sensor device 1 is electrically connected to an ECU 200 that is a vehicle-side control device of a vehicle on which the multigas sensor device 1 is mounted. The ECU 200 receives data indicating the NO concentration, NO ₂ concentration, and ammonia concentration in the exhaust gas calculated by the control unit 3, and executes control processing of the operating state of the diesel engine based on the received data, or the catalyst The accumulated NOx purification process is executed.

  Here, the detail of a structure of the sensor element part 10 is demonstrated, referring FIG. For convenience of explanation, FIG. 2 shows only a cross-sectional view along the longitudinal direction of the sensor element unit 10.

  The sensor element unit 10 is mainly provided with a NOx sensor unit (NOx detection element) 11 and an ammonia sensor unit (ammonia detection element) 21. The NOx sensor unit 11 and the ammonia sensor unit 21 in the present embodiment have the same configuration as a known NOx sensor and the same configuration as a known ammonia sensor, respectively.

  The NOx sensor unit 11 mainly includes an insulating layer 10e, a first solid electrolyte body 12a, an insulating layer 10d, a third solid electrolyte body 16a, an insulating layer 10c, a second solid electrolyte body 18a, and insulating layers 10b and 10a. The structure is laminated in this order. Each of the insulating layers 10a, 10b, 10c, 10d, and 10e described above is formed mainly of alumina.

  Further, in the NOx sensor unit 11, the first measurement chamber S1 is provided between the first solid electrolyte body 12a and the third solid electrolyte body 16a, and the second measurement chamber S2 corresponding to the NOx measurement chamber is provided in the first solid state. A third solid electrolyte body 16a is provided between the electrolyte body 12a and the second solid electrolyte body 18a.

  A first diffusion resistor 14 is disposed at the inlet end (left end in FIG. 2) of the first measurement chamber S1 into which the gas to be measured is introduced. A second diffusion resistor 15 that partitions the first measurement chamber S1 and the second measurement chamber S2 is disposed at the end opposite to the inlet end in the first measurement chamber S1 (the right end in FIG. 2). . The first diffusion resistor 14 and the second diffusion resistor 15 described above are made of a porous material such as alumina, and have permeability to the gas to be measured.

  The NOx sensor unit 11 is further provided with a heater 19 that raises the temperature of the NOx sensor unit 11 and the ammonia sensor unit 21 to the activation temperature and increases the conductivity of oxygen ions in the solid electrolyte body constituting each sensor. ing. The heater 19 is formed of platinum or an alloy containing platinum in the shape of a long plate extending along the longitudinal direction of the sensor element unit 10, and is embedded between the insulating layer 10b and the insulating layer 10a. .

  In addition, the NOx sensor unit 11 is provided with a first pumping cell 12, an oxygen concentration detection cell 16, and a second pumping cell 18.

  The first pumping cell 12 is mainly composed of a first solid electrolyte body 12a mainly composed of zirconia having oxygen ion conductivity, and an inner first pumping electrode 12b and outer first pumping electrode 12c mainly composed of platinum. Has been.

  The inner first pumping electrode 12b is provided on the surface of the first solid electrolyte body 12a exposed to the first measurement chamber S1. Further, the inner first pumping electrode 12b has a surface on the first measurement chamber S1 side covered with a protective layer 12d made of a porous body.

  The outer first pumping electrode 12c is an electrode serving as a counter electrode for the inner first pumping electrode 12b, and is disposed with the first solid electrolyte body 12a sandwiched between the inner first pumping electrode 12b. A portion corresponding to a region where the outer first pumping electrode 12c is disposed in the insulating layer 10e is hollowed out and filled with the porous body 12e. The porous body 12e allows gas (oxygen) to enter and exit between the outer first pumping electrode 12c and the outside.

  The oxygen concentration detection cell 16 is mainly composed of a third solid electrolyte body 16a mainly composed of zirconia, and a detection electrode 16b and a reference electrode 16c mainly composed of platinum and arranged with the third solid electrolyte body 16a interposed therebetween. It is configured.

  The detection electrode 16b is a surface exposed to the first measurement chamber S1 of the third solid electrolyte body 16a, and is provided downstream of the inner first pumping electrode 12b, in other words, in a region on the second diffusion resistor 15 side. It has been.

  A reference electrode 16c, which is a counter electrode of the detection electrode 16b, is disposed inside a reference oxygen chamber 17 formed by cutting out the insulating layer 10c. The reference oxygen chamber 17 is filled with a porous body. In the reference oxygen chamber 17, there is oxygen sent from the first measurement chamber S1, and oxygen in the reference oxygen chamber 17 is used as an oxygen reference.

  The second pumping cell 18 mainly includes a second solid electrolyte body 18a mainly composed of zirconia, an inner second pumping electrode (electrode) 18b mainly composed of platinum, and a second pumping counter electrode (electrode) 18c. Has been.

  The inner second pumping electrode 18b is provided in a region exposed to the second measurement chamber S2 in the second solid electrolyte body 18a. The second pumping counter electrode 18c is a region exposed to the reference oxygen chamber 17 in the second solid electrolyte body 18a, and is provided in a portion facing the reference electrode 16c.

  Further, the inner first pumping electrode 12b, the detection electrode 16b, and the inner second pumping electrode 18b are each connected to a reference potential.

  On the other hand, the ammonia sensor unit 21 is formed on the outer surface of the NOx sensor unit 11, more specifically, on the insulating layer 10e. The ammonia sensor unit 21 is disposed at substantially the same position (for example, the upper side in FIG. 2) in the direction of the axis O with the reference electrode 16c in the NOx sensor unit 11.

  In the present embodiment, when the control temperature of the second solid electrolyte body 18a of the NOx sensor unit 11 is 600 ° C., the ammonia sensor unit 21 is disposed at a position where the temperature of the outer surface of the NOx sensor unit 11 is 650 ° C. Has been.

  As shown in FIGS. 2 and 3, the ammonia sensor unit 21 is mainly composed of a reference electrode (electrode) 21a mainly composed of platinum, an ammonia solid electrolyte body 23 mainly composed of zirconia, and cobalt oxide and zirconia. It is mainly composed of an intermediate layer (electrode) 21b and an ammonia electrode (electrode) 21c mainly composed of gold. Furthermore, the ammonia sensor unit 21 is integrally covered with a porous protective layer 24.

  The reference electrode 21a is an electrode in which a combustible gas burns on the electrode surface, and is made of, for example, Pt alone or a material containing Pt as a main component. Furthermore, the reference electrode 21a is a rectangular electrode disposed on the outer surface (the upper surface in FIG. 3) of the insulating layer 10e, and platinum that extends in the longitudinal direction (the left-right direction in FIG. 3) of the insulating layer 10e. A main reference electrode lead 25 is provided. The end portion on the rear end side (right side in FIG. 3) of the reference electrode lead 25 forms an electrode terminal portion.

  The ammonia solid electrolyte body 23 is made of an oxygen ion conductive material such as yttria-stabilized zirconia (YSZ), and is disposed with the reference electrode 21a sandwiched between the insulating layer 10e.

The intermediate layer 21b is an outer surface of the ammonia solid electrolyte body 23 and is disposed at a position facing the reference electrode 21a.
In the present embodiment, the intermediate layer 21b is mainly composed of cobalt oxide and zirconia. However, the intermediate layer 21b contains 50% by weight or more of an oxygen ion conductive solid electrolyte component (that is, zirconia), and Mn, Cu, Ni. And a layer containing a first metal oxide which is an oxide of one or more metals selected from the group consisting of Ce and Ce. Further, for example, when the first metal oxide is cobalt oxide (Co ₃ O ₄ ), fluctuations in the sensitivity of the ammonia concentration in the ammonia sensor unit 21 are suppressed by H ₂ O contained in the gas to be detected. The The first metal oxide described above takes the form of a metal oxide or a composite oxide. The solid electrolyte component contained in the intermediate layer 21b may have the same composition as the solid electrolyte body constituting the gas sensor of the present invention, or may be a different component.

  The ratio of the first metal oxide contained in the intermediate layer 21b is preferably 1% by mass to 50% by mass. When the content ratio of the first metal oxide is less than 1% by mass, the intermediate layer 21b may not have sufficient selectivity for ammonia gas. Moreover, when the content rate of a 1st metal oxide exceeds 50 mass%, there exists a possibility that the ratio of the solid electrolyte component of the intermediate | middle layer 21b may decrease, and the oxygen ion conductivity of the intermediate | middle layer 21b may fall.

  Furthermore, the intermediate layer 21b is preferably porous. By forming the intermediate layer 21b from a porous body, the selectivity of the intermediate layer 21b with respect to ammonia gas is improved, and the ammonia gas detection sensitivity in the ammonia sensor unit 21 is improved.

  Whether or not the first metal oxide is contained in the intermediate layer 21b can be confirmed by analyzing the cross section of the ammonia sensor unit 21 with an EPMA (electron beam microanalyzer). In general, three portions of the cut surface are analyzed by EPMA, and the average value of the analysis results can be confirmed.

  The ammonia electrode 21c is disposed on the outer surface of the intermediate layer 21b. In other words, the ammonia electrode 21c is disposed with the solid electrolyte for ammonia 23 and the intermediate layer 21b sandwiched between the reference electrode 21a.

  In the ammonia electrode 21c, an ammonia electrode lead 21d is formed extending from the ammonia electrode 21c toward the rear end side. The ammonia electrode lead 21d is formed of a material whose main component is platinum. Further, the end portion on the rear end side of the ammonia electrode lead 21d forms an electrode terminal portion.

  The ammonia electrode 21c is an electrode formed from a material containing 70% by weight or more of Au and serves as a detection electrode. The ammonia electrode 21c may or may not contain the first metal oxide described above, but if it is not included, combustion of ammonia gas at the ammonia electrode 21c is suppressed. The ammonia gas that reaches the interface between the ammonia electrode 21c and the intermediate layer 21b is difficult to decrease. In other words, the detection accuracy of the ammonia sensor unit 21 is improved, and in particular, low concentration ammonia near 10 ppm can be detected with high accuracy.

  In addition, by forming the ammonia electrode 21c from a material containing 70% by weight or more of Au, the ability of the ammonia electrode 21c as a current collector is secured. Note that when the ammonia electrode 21c is formed using a material having an Au content of less than 70% by weight, it is difficult to ensure the ability of the ammonia electrode 21c as a current collector. That is, it becomes difficult to detect ammonia gas by the ammonia sensor unit 21.

  By forming the ammonia electrode 21c as a porous electrode containing a second metal oxide which is an oxide of one or more metals selected from the group consisting of Zr, Y, Al and Si, the ammonia electrode 21c has gas permeability. Can be granted. Therefore, ammonia gas can permeate the ammonia electrode 21c and can easily reach the interface between the ammonia electrode 21c and the intermediate layer 21b. In addition, it is preferable that the ammonia electrode 21c contains the 2nd metal oxide in the ratio from 5 mass% to 30 mass%.

  The ammonia gas that reaches the interface between the ammonia electrode 21c and the intermediate layer 21b reacts with oxygen ions (electrode reaction) at the interface. Therefore, the ammonia electrode 21c and the intermediate layer 21b can function as an ammonia gas detector. Here, by allowing the first metal oxide to exist at the interface between the ammonia electrode 21c and the intermediate layer 21b, the sensitivity of the ammonia electrode 21c and the intermediate layer 21b to gases other than ammonia gas, such as HC gas, is reduced. The selectivity of ammonia gas can be improved.

The reason why the selectivity of ammonia gas is improved is not clear, but it is considered that the first metal oxide interposed in the above-described interface modifies the electrode reaction field. Furthermore, since the first metal oxide has an acidic property, it is considered that the first metal oxide interacts strongly with the basic molecule NH ₃ and the electrode reaction with respect to NH 3 proceeds more favorably than other gases. From these facts, it is considered that the selectivity of ammonia gas in the ammonia electrode 21c and the intermediate layer 21b is improved.

  In the present embodiment, the ammonia electrode 21c and the intermediate layer 21b are provided separately. However, the intermediate layer 21b may be omitted by making the ammonia electrode 21c contain cobalt oxide contained in the intermediate layer 21b. .

The protective layer 24 adjusts the diffusion rate of the gas to be measured flowing from the outside into the ammonia sensor unit 21 while preventing the poisonous substance from adhering to the ammonia electrode 21c. Examples of the material for forming the protective layer 24 include at least one material selected from the group consisting of alumina (aluminum oxide), spinel (MgAl ₂ O ₄ ), silica alumina, and mullite. The diffusion rate of the gas to be measured by the protective layer 24 is adjusted by adjusting the thickness, particle size, particle size distribution, porosity, blending ratio, etc. of the protective layer 24.

  The protective layer 24 may be provided as in the above-described embodiment, or the ammonia electrode 21c and the like may be exposed without providing the protective layer 24, and is not particularly limited.

  As shown in FIG. 2, the control unit 3 includes a control circuit 50 that is an analog circuit disposed on a circuit board, and a microcomputer 60.

  The microcomputer 60 controls the entire control unit 3. The microcomputer 60 mainly includes a CPU 61 as a central processing unit, a RAM 62 and a ROM 63 as storage means, a signal input / output unit 64, an A / D converter 65, and a clock (not shown). Is provided. The microcomputer 60 performs various processes when the CPU 61 executes a program stored in advance in the ROM 63 or the like.

  The control circuit 50 includes a reference voltage comparison circuit 51, an Ip1 drive circuit 52, a Vs detection circuit 53, an Icp supply circuit 54, an Ip2 detection circuit 55, a Vp2 application circuit 56, a heater drive circuit 57, and an electromotive force. The detection circuit 58 is mainly configured.

  The Ip1 drive circuit 52 is electrically connected to the first outer pumping electrode 12c of the NOx sensor unit 11, and the Vs detection circuit 53 and the Icp supply circuit 54 are electrically connected in parallel to the reference electrode 16c. The Ip2 detection circuit 55 and the Vp2 application circuit 56 are electrically connected in parallel to the second pumping counter electrode 18 c, and the heater drive circuit 57 is electrically connected to the heater 19.

  The electromotive force detection circuit 58 is electrically connected to the reference electrode 21a and the ammonia electrode 21c in the ammonia sensor unit 21. Further, the electromotive force detection circuit 58 detects an ammonia electromotive force EMF, which is an electromotive force between the reference electrode 21a and the ammonia electrode 21c, and outputs it to the microcomputer 60.

  The Ip1 drive circuit 52 supplies the first pumping current Ip1 between the inner first pumping electrode 12b and the outer first pumping electrode 12c, and detects the supplied first pumping current Ip1.

  The Vs detection circuit 53 detects the voltage Vs between the detection electrode 16b and the reference electrode 16c, and outputs the detected result to the reference voltage comparison circuit 51. The reference voltage comparison circuit 51 compares the reference voltage (for example, 425 mV) with the output (voltage Vs) of the Vs detection circuit 53, and outputs the comparison result to the Ip1 drive circuit 52.

  The Ip1 drive circuit 52 controls the flow direction and magnitude of the Ip1 current so that the voltage Vs becomes equal to the above-described reference voltage, and has a predetermined value that does not decompose the oxygen concentration in the first measurement chamber S1. To adjust.

  The Icp supply circuit 54 allows a weak current Icp to flow between the detection electrode 16b and the reference electrode 16c. By supplying the current Icp, oxygen is sent into the reference oxygen chamber 17 from the first measurement chamber S1. The reference electrode 16c is exposed to a predetermined oxygen concentration as a reference.

The Vp2 application circuit 56 applies a constant voltage Vp2 (for example, 450 mV) between the inner second pumping electrode 18b and the second pumping counter electrode 18c, and decomposes NOx into nitrogen and oxygen. The constant voltage Vp2 is such a voltage that the NOx gas in the gas to be measured is decomposed into oxygen and N ₂ gas.

  The Ip2 detection circuit 55 detects the second pumping current Ip2 flowing through the second pumping cell 18. The second pumping current Ip2 is a current that flows when oxygen generated by the decomposition of NOx is pumped from the second measurement chamber S2 to the second pumping counter electrode 18c side through the second solid electrolyte body 18a.

  The Ip1 drive circuit 52 outputs the detected value of the first pumping current Ip1 to the A / D converter 65, and the Ip2 detection circuit 55 supplies the detected value of the second pumping current Ip2 to the A / D converter 65. Output. The A / D converter 65 digitally converts the values of the first pumping current Ip1 and the second pumping current Ip2 and outputs them to the CPU 61 via the signal input / output unit 64.

Next, control by the control circuit 50 will be described below.
First, when the engine is started and electric power is supplied to the control circuit 50 from the outside, electric power is supplied from the heater drive circuit 57 to the heater 19. The heater 19 supplied with electric power generates heat and heats the first pumping cell 12, the oxygen concentration detection cell 16, and the second pumping cell 18 to the activation temperature.

  When the NOx sensor unit 11 is heated to a target temperature by the heater 19, the temperature is raised to a desired temperature of the ammonia sensor unit 21 disposed on the NOx sensor unit 11.

  Further, the current Icp is supplied from the Icp supply circuit 54 between the detection electrode 16b and the reference electrode 16c. Then, oxygen is sent into the reference oxygen chamber 17 from the first measurement chamber S1, and the sent oxygen becomes the oxygen reference.

  When the first pumping cell 12, the oxygen concentration detection cell 16, and the second pumping cell 18 are heated to the activation temperature, the first pumping cell 12 pumps out oxygen in the first measurement chamber S1. Is called. That is, oxygen in the gas to be measured (exhaust gas) flowing into the first measurement chamber S1 is pumped from the inner first pumping electrode 12b of the first pumping cell 12 toward the outer first pumping electrode 12c.

  The oxygen concentration in the first measurement chamber S1 is a concentration corresponding to the interelectrode voltage Vs of the oxygen concentration detection cell 16. The Ip1 drive circuit 52 controls the first pumping current Ip1 flowing through the first pumping cell 12 so that the interelectrode voltage Vs becomes the above-described reference voltage. By doing so, the oxygen concentration in the first measurement chamber S1 is adjusted to such an extent that NOx is not decomposed.

  The gas to be measured whose oxygen concentration is adjusted in the first measurement chamber S1 then flows into the second measurement chamber S2. In the second measurement chamber S2, NOx contained in the gas to be measured is decomposed into nitrogen and oxygen. That is, when a constant voltage Vp2 (for example, 450 mV) is applied from the Vp2 application circuit 56 as a voltage between the electrodes of the second pumping cell 18, NOx is decomposed into nitrogen and oxygen. The constant voltage Vp2 is such a voltage that the NOx gas in the measurement gas is decomposed into oxygen and N2 gas, and is higher than the control voltage value of the oxygen concentration detection cell 16.

  Oxygen generated by the decomposition of NOx is pumped out of the second measurement chamber S2 by the second pumping cell 18. At this time, the second pumping cell 18 is supplied with a second pumping current Ip2 to pump out oxygen. Since there is a linear proportional relationship between the second pumping current Ip2 and the NOx concentration, the second pumping current Ip2 detected by the Ip2 detection circuit 55 is a value that is linearly proportional to the NOx concentration.

  On the other hand, an electromotive force is generated between the reference electrode 21a and the ammonia electrode 21c of the ammonia sensor unit 21 according to the ammonia concentration contained in the gas to be measured. The electromotive force detection circuit 58 detects an electromotive force between the reference electrode 21a and the ammonia electrode 21c as an ammonia electromotive force.

The CPU 61 of the microcomputer 60 performs a process of removing the influence of the oxygen concentration from the value of the second pumping current Ip2 and the ammonia electromotive force EMF based on various data stored in the ROM 63, and further, the NO concentration and the NO ₂ Processing for calculating the NOx concentration such as the concentration and the ammonia concentration is performed. In addition, as these processes, the process described in the above-mentioned patent document 1 etc. can be used, and it does not specifically limit.

  Next, a manufacturing method of the sensor element unit 10 in which the NOx sensor unit 11 and the ammonia sensor unit 21 which are features of the present embodiment are formed will be described. Here, the manufacturing method of the ammonia sensor unit 21 and the process for increasing the detection sensitivity of the sensor element unit 10 will be described. In addition, since the manufacturing method of the NOx sensor part 11 is a well-known manufacturing method, the detailed description is abbreviate | omitted in this embodiment.

First, the manufacturing method of the ammonia sensor unit 21 will be described below.
First, an electrode paste (hereinafter referred to as “Pt-based paste”) containing Pt, an alumina (inorganic oxide used as a common substrate) binder, and an organic solvent for forming the reference electrode 21a on the insulating layer 10e is screened. It arrange | positions by printing and it bakes at predetermined temperature (about 1400 degreeC or more).

  On the baked reference electrode 21a, a paste containing oxide powder, binder, and organic solvent, which are components of the solid electrolyte body forming the solid electrolyte body 23 for ammonia, is disposed by screen printing, and a predetermined temperature (for example, 1500). ℃). Thus, the reference electrode 21a and the ammonia solid electrolyte body 23 are formed.

  Next, a paste containing the first metal oxide forming the intermediate layer 21b and the solid electrolyte component is disposed on the solid electrolyte body 23 for ammonia by screen printing and fired at a predetermined temperature (for example, 1000 ° C.). Thus, the intermediate layer 21b is formed.

  Further, an Au-based paste for forming the ammonia electrode 21c is disposed on the intermediate layer 21b by screen printing, and is baked at a predetermined temperature (for example, 1000 ° C.), whereby the ammonia electrode 21c is formed. Finally, a paste containing alumina or the like is disposed by screen printing so as to cover the reference electrode 21a, the solid electrolyte for ammonia 23, the intermediate layer 21b, and the ammonia electrode 21c, and fired at a predetermined temperature (for example, 1000 ° C.). Thus, the protective layer 24 is formed. Thus, the ammonia sensor unit 21 is formed.

  Subsequently, a rich treatment (first treatment process) and a lean treatment (second treatment process) are performed on the sensor element unit 10. The rich treatment is performed for the purpose of promoting activation of the electrode of the NOx sensor unit 11, and the lean treatment recovers the detection sensitivity of the ammonia sensor unit 21 that has been lowered by the rich treatment, and the initial electrode of the NOx sensor unit 11. It is performed for the purpose of suppressing the occurrence of fluctuations.

  In the rich treatment, the sensor element unit 10 is disposed in a rich atmosphere and heated so that the temperature of the element is 500 ° C. to 800 ° C. In this state, a stepped alternating voltage having a voltage of 0.8 V and a half cycle of 60 s is applied to each electrode of the NOx sensor unit 11 for three cycles.

  Here, the rich atmosphere is an atmosphere in which the proportion of oxygen is small with respect to the theoretical air-fuel ratio (λ = 1). In other words, when the gas atmosphere burned at the stoichiometric air-fuel ratio, which is an ideal mixture ratio of air and fuel capable of complete combustion, is used as a reference, the oxygen ratio is less than the gas atmosphere (the oxygen partial pressure is low) ) Gas atmosphere.

As the above-mentioned rich atmosphere, for example, a gas whose moisture content is more than 0% by volume and not more than 5% by volume with respect to the gas whose H ₂ is several volume% and the balance is N 2 can be cited. Examples of the rich atmosphere include those containing water in a proportion of more than 0% by volume and 5% by volume or less with respect to a gas having 1% by volume of CO, 10% by volume of CO ₂ and the balance of N 2.

Subsequently, a lean treatment is performed on the sensor element unit 10 on which the above-described rich treatment has been performed.
In the lean treatment, the sensor element unit 10 is disposed in a lean atmosphere and heated so that the temperature of the element is 650 ° C. to 850 ° C., more preferably 750 ° C. In this state, the energization for 90 seconds and the non-energization for 10 seconds are alternately repeated with respect to each electrode of the NOx sensor unit 11 and the heater 19.

The lean atmosphere can be exemplified by a gas in which O ₂ is 1 to 20% by volume, moisture is more than 15% by volume and not more than 30% by volume, and the balance is N ₂ . More preferably, a gas in which O ₂ is 10% by volume, moisture is 20% by volume, and the balance is N ₂ can be exemplified.

  Next, an experimental result of measuring the recovery rate of the detection sensitivity of the ammonia sensor unit 21 when the lean treatment condition described in the manufacturing method of the sensor element unit 10 is changed will be described. The recovery rate is measured by measuring the ammonia electromotive force (hereinafter referred to as “initial sensitivity”), which is the detection sensitivity of the ammonia sensor unit 21 before the rich treatment and the lean treatment, and then applying the ammonia sensor unit 21 to the ammonia sensor unit 21. This is performed by performing a rich treatment and a lean treatment, and measuring an ammonia electromotive force (hereinafter referred to as “post-treatment sensitivity”) that is a detection sensitivity of the ammonia sensor unit 21 thereafter. The recovery rate is obtained by a calculation formula of post-processing sensitivity (mV) / initial sensitivity (mV) × 100.

Further, the gas to be measured used for measuring the initial sensitivity and the post-treatment sensitivity is a gas having 10% by volume of O ₂ , 5% by volume of CO ₂ , 5% by volume of H ₂ O, and the balance being N ₂ . , NH ₃ containing 50 ppm is used. Further, at the time of measuring the initial sensitivity and the post-processing sensitivity, the temperature of the gas to be measured is 150 ° C., the flow rate is 7.5 m / s, and the temperature of the ammonia sensor unit 21 is 650 ° C.

Table 1 below shows the fluctuation of the recovery rate when the ratio of moisture (H ₂ O) contained in the lean atmosphere during the lean treatment is changed in the range of 1% by volume to 30% by volume. . That is, Example 1 is an example in which the water content is 1% by volume, Example 2 is 10% by volume, Example 3 is 15% by volume, Example 4 is 20% by volume, and Example 5 is 25% by volume. 6 is an example of 30% by volume.

In addition, the lean atmosphere of Examples 1 to 5 is an atmosphere in which 10% by volume of O ₂ is contained in addition to the moisture that varies as described above, and the balance is N ₂ . The temperature of the ammonia sensor unit 21 during the lean treatment is 750 ° C., and the treatment time for the lean treatment is 20 minutes.

  According to Table 1 above, the recovery rate increases from 64% to 95% as the volume% of water increases from 1 volume% to 20 volume%. Thereafter, the recovery rate gradually decreases when the volume percentage of water exceeds 20 volume% (see FIG. 4). If it is determined that the recovery rate of the ammonia sensor unit 21 is 85% or more, the acceptable range is a range in which the volume percentage of water exceeds 15 volume% and is 30 volume% or less.

  In Table 1, as a comparative example, an example in which only the rich treatment is performed on the ammonia sensor unit 21 and the lean treatment is not performed is described. The recovery rate in the comparative example is 51%.

  Further, Table 2 below shows fluctuations in the recovery rate when the temperature of the ammonia sensor unit 21 during the lean treatment is changed in a range from 650 ° C. to 850 ° C. That is, Example 7 is an example in which the temperature of the ammonia sensor unit 21 is 650 ° C., Example 8 is 700 ° C., Example 9 is 750 ° C., Example 10 is 800 ° C., and Example 11 is 850 ° C. is there.

In addition, the lean atmosphere of Examples 7 to 11 is an atmosphere containing 10% by volume of O ₂ , 10% by volume of H ₂ O, and the balance being N ₂ . The treatment time for the lean treatment is 20 minutes.

  According to Table 2 above, it can be seen that even when the temperature of the ammonia sensor unit 21 is changed in the range from 650 ° C. to 850 ° C., the recovery rate value is 85% or more. Among them, it can be seen that the highest recovery rate (95%) is obtained when the temperature of the ammonia sensor section 21 is 750 ° C. (see FIG. 4).

Table 3 below shows fluctuations in the recovery rate when the concentration of O ₂ contained in the lean atmosphere during the lean treatment is changed from 1% by volume to 20% by volume. That is, Example 12 is an example in which O ₂ is 1% by volume, Example 13 is an example in which 10% by volume, and Example 14 is an example in which 20% by volume.

The lean atmosphere of Examples 12 to 14 is an atmosphere containing 20% by volume of moisture in addition to O ₂ changing as described above, with the balance being N ₂ . The temperature of the ammonia sensor unit 21 during the lean treatment is 750 ° C., and the treatment time for the lean treatment is 20 minutes.

According to Table 3 above, it can be seen that even when the concentration of O ₂ contained in the lean atmosphere is changed from 1% by volume to 20% by volume, the value of the recovery rate is 85% or more. Among them, it can be seen that the highest recovery rate (95%) is obtained when O ₂ is 10% by volume (see FIG. 4).

According to the manufacturing method of the multi-gas sensor 2 described above, the activation of the electrode of the NOx sensor unit 11 can be promoted by performing the rich treatment, and the ammonia sensor unit decreased by the rich treatment by performing the lean treatment. The detection sensitivity of 21 can be recovered. In other words, the detection sensitivity of the NO concentration, the NO ₂ concentration, and the NH ₃ concentration that are detection targets in the multi-gas sensor 2 can be increased as a whole.

  2 ... multi-gas sensor, 10 ... sensor element section, 11 ... NOx sensor section (NOx detection element), 18b ... inner second pumping electrode (electrode), 18c ... second pumping counter electrode (electrode), 21 ... ammonia sensor section ( Ammonia detection element), 21a ... reference electrode (electrode), 21b ... intermediate layer (electrode), 21c ... ammonia electrode (electrode)

Claims (1)

A NOx detecting element for detecting a nitrogen oxide concentration contained in the gas to be measured, and an ammonia detecting element for detecting the ammonia concentration contained in the gas to be measured , the solid electrolyte body for ammonia mainly comprising zirconia; A method of manufacturing a multi-gas sensor having a sensor element part integrally provided with an ammonia detection element having a reference electrode mainly composed of platinum and an ammonia electrode mainly composed of gold ,
While the sensor element portion is heated to a first temperature range in a rich atmosphere, an alternating voltage having a preset magnitude is applied between the electrodes of the NOx detection element to activate the electrodes of the NOx detection element A first treatment step to be performed;
After the first treatment step, the electrode of the NOx detection element is heated while the sensor element portion is heated to the second temperature region in a lean atmosphere in which moisture is set to more than 15 volume% and not more than 30 volume%. A second treatment process for energizing the
A method for manufacturing a multi-gas sensor, comprising:

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