JP2020169817A - Gas sensor - Google Patents

Abstract

To provide a hybrid potential type sensor element having improved measurement accuracy of ammonia, and a gas sensor provided with the same.SOLUTION: Provided are: a first detection electrode 42bx and a second detection electrode 42by each of which are active for ammonia and NO2; and at least one respective reference electrodes 42ax and 42ay. The first detection electrode and the reference electrode form a first detection cell 42x, and the second detection electrode and the reference electrode form a second detection cell 42y. In a sensor element 100A that detects ammonia and in the first detection electrode and the second detection electrode, a porous layer 23g is provided on a surface of only the first detection electrode and an outer protective layer 23i having a larger porosity than the porous layer is provided on a surface of the porous layer and the second detection electrode.SELECTED DRAWING: Figure 4

Description

The present invention relates to a sensor element that detects ammonia contained in a gas to be measured, and a gas sensor including the same.

In recent years, a urea SCR (selective catalytic reduction) system has been attracting attention as a technique 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 ) with nitrogen oxides (NOx) to reduce nitrogen oxides to nitrogen (N ₂ ). It is a system.

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

As a sensor element for measuring the concentration of ammonia, a mixed potential type sensor element is used in which two detection electrodes having different activities with respect to ammonia are provided, and these electrodes form a reference electrode and a detection cell, respectively. Since this element detects not only ammonia but also NO ₂ , if the gas to be measured contains NO ₂ gas other than ammonia, the detection accuracy of ammonia is lowered.
Therefore, if _two detection cells having different sensitivities to ammonia and NO ₂ are provided, values with different sensitivities are detected from the two detection cells for two unknown concentrations of ammonia gas and NO ₂ gas. The concentration of ammonia gas and NO ₂ can be calculated.
Further, the surfaces of the two detection electrodes are coated with a porous protective layer.

JP-A-2018-81037

However, in the hybrid potential type sensor element that detects ammonia, there is a problem that the sensitivity difference of NO ₂ gas between the _two detection cells is small and the measurement accuracy is lowered.
The present invention has been made to solve the above problems, and an object of the present invention is to provide a hybrid potential type sensor element having improved ammonia measurement accuracy and a gas sensor provided with the same.

In order to solve the above problems, the sensor element of the present invention includes a first detection electrode and a second detection electrode, each of which is active against ammonia and NO ₂ , and at least one reference electrode, and the first detection In the sensor element in which the electrode and the reference electrode form a first detection cell, the second detection electrode and the reference electrode form a second detection cell, and ammonia is detected, the first detection electrode and the second detection electrode Of these, a porous layer is provided on the surface of only the first detection electrode, and an outer protective layer having a larger pore ratio than the porous layer is provided on the surface of the porous layer and the second detection electrode. It is characterized by being

According to this sensor element, in the outer protective layer having a larger porosity than the porous layer, not only ammonia gas having a small molecular weight but also NO ₂ gas having a large molecular weight passes through the outer protective layer and the surface of the second detection electrode. To reach quickly. On the other hand, in the porous layer having a smaller porosity than the outer protective layer, ammonia gas having a small molecular weight can quickly reach the surface of the first detection electrode through the porous layer, but NO ₂ gas having a large molecular weight is porous. It takes time to pass through the stratum corneum.
In the region of 500 ° C. or higher, which is the operating temperature of a general gas sensor, NO is chemically more stable than NO ₂ . Therefore, the NO ₂ gas reaches thermal equilibrium while staying in the porous layer, NO ₂ changes to NO, and the proportion of NO ₂ gas that reaches the surface of the first detection electrode increases. Since the first detection electrode and the second detection electrode do not detect NO, the sensitivity of the first detection electrode to NO ₂ can be reduced as compared with the second detection electrode. As a result, the difference in the sensitivity ratio of NH ₃ to NO ₂ can be increased between the first detection cell and the second detection cell, and the measurement accuracy of the concentrations of ammonia gas and NO ₂ is improved.

In the sensor element of the present invention, the porosity of the porous layer may be 40% or less.
According to this sensor element, it takes longer for the NO ₂ gas to pass through the gas sensor porous layer, so that the difference in the sensitivity ratio of NH ₃ to NO ₂ is further increased between the first detection cell and the second detection cell. can do.

In the sensor element of the present invention, the thickness of the porous layer may be 40 μm or less.
According to this sensor element, since the ammonia gas appropriately passes through the porous layer, the measurement accuracy of the concentrations of the ammonia gas and NO ₂ is further improved.

In the sensor element of the present invention, the sensor element includes a heater for heating the first detection cell and the second detection cell, and the maximum temperature of the first detection electrode and the second detection electrode when the heater is energized is set. It may be 500 to 950 ° C.
According to this sensor element, the first detection cell and the second detection cell can be held at a temperature of 500 ° C. or higher, where NO is chemically more stable than NO ₂ , and stays in the porous layer. Since the NO ₂ gas present is changed more than NO, the difference in the sensitivity ratio of NH ₃ to NO ₂ between the first detection cell and the second detection cell can be further increased.

The gas sensor of the present invention is a gas sensor including a sensor element and a main metal fitting for holding the sensor element, and uses the sensor element according to any one of claims 1 to 4 as the sensor element.

According to the present invention, a hybrid potential type sensor element having improved ammonia measurement accuracy and a gas sensor provided with the same can be obtained.

It is sectional drawing which follows the axial direction of the multi-gas sensor which concerns on embodiment of this invention. It is a block diagram which shows the structure of a multi-gas sensor and a gas sensor control device. It is sectional drawing which follows the width direction of the 1st detection cell and the 2nd detection cell. It is a partial cross-sectional view of FIG. 3 which shows the action by a porous layer.

Hereinafter, the multi-gas sensor according to the embodiment of the present invention will be described with reference to FIGS. 1 to 4. FIG. 1 is a cross-sectional view taken along the axis O direction (longitudinal direction) of the multi-gas sensor, FIG. 2 is a block diagram illustrating the configuration of the multi-gas sensor device, and FIG. 3 is a width showing the configurations of the first detection cell and the second detection cell. A cross-sectional view taken along the direction, FIG. 4 is a cross-sectional view taken along the width direction of the first detection cell and the second detection cell.

The multi-gas sensor 200A of the present embodiment is used in a urea SCR system that purifies nitrogen oxides (NOx) contained in exhaust gas (measured gas) discharged from a diesel engine. More specifically, the concentrations of nitric oxide (NO), nitrogen dioxide (NO ₂ ) and ammonia contained in the exhaust gas after reacting NOx contained in the exhaust gas with ammonia (urea) are measured. It is a thing.
The engine to which the multi-gas sensor 200A of the present embodiment is applied may be the diesel engine described above, or may be applied to a gasoline engine, and the type of the engine is not particularly limited.

As shown in FIG. 1, the multi-gas sensor 200A is an assembly in which a multi-sensor element unit 100A for detecting an ammonia concentration and a NOx concentration is assembled. The multi-gas sensor 200A includes a plate-shaped multi-sensor element 100A extending in the axial direction, a tubular main metal fitting 138 having a threaded portion 139 for fixing to an exhaust pipe formed on the outer surface, and a multi-sensor element 100A. The tubular ceramic sleeve 106 arranged so as to surround the radial circumference of the sensor and the inner wall surface of the contact insertion hole 168 penetrating in the axial direction are arranged so as to surround the rear end portion of the multi-sensor element portion 100A. It includes an insulating contact member 166 and a plurality of terminal fittings 110 (only two are shown in FIG. 1) arranged between the multi-sensor element portion 100A and the insulating contact portion 166.
The multi-gas sensor 200A and the multi-sensor element unit 100A correspond to the "gas sensor" and the "sensor element" in the claims, respectively.

The main metal fitting 138 has a through hole 154 penetrating in the axis O direction, and is configured in a substantially tubular shape having a shelf portion 152 protruding inward in the radial direction of the through hole 154. Further, in the main metal fitting 138, the tip side of the multi-sensor element portion 100A is arranged outside the tip side of the through hole 154, and a plurality of electrode pads 80 (see FIG. 4) are arranged outside the rear end side of the through hole 154. The multi-sensor element unit 100A 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.
For the sake of simplification, the electrode pads on the front surface and the back surface of the multi-sensor element unit 100A are represented by reference numerals 80 in FIG. 1, but as shown in FIG. 4, the electrode pads 80 are the NOx sensor unit described later. A plurality of electrodes are formed according to the number of electrodes and the like of 30A and the first and second detection cells 42x and 42y.

Inside the through hole 154 of the main metal fitting 138, an annular ceramic holder 151, a powder filling layer 153 (hereinafter, also referred to as a talc ring 153), and the above-mentioned are provided so as to surround the radial circumference of the multi-sensor element portion 100A. Ceramic sleeves 106 are laminated in this order from the front end side to the rear end side. Further, a crimping packing 157 is arranged between the ceramic sleeve 106 and the rear end portion 140 of the main metal fitting 138, and a talc ring 153 is provided between the ceramic holder 151 and the shelf portion 152 of the main metal fitting 138. And a metal holder (not shown) for holding the ceramic holder 151 is arranged. The rear end portion 140 of the main metal fitting 138 is crimped so as to press the ceramic sleeve 106 toward the tip side via the crimp packing 157.

On the other hand, a metal (for example, stainless steel) double external protector 142 that covers the protruding portion of the multi-sensor element portion 100A and has a plurality of holes on the outer periphery of the tip side (lower side in FIG. 1) of the main metal fitting 138. And the internal protector 143 is attached by welding or the like.

An outer cylinder 144 is fixed to the outer periphery of the main metal fitting 138 on the rear end side. Further, in the opening on the rear end side (upper side in FIG. 1) of the outer cylinder 144, a plurality of lead wires 146 (4 in FIG. 1) are electrically connected to the electrode pads 80 of the multi-sensor element unit 100A, respectively. The grommet 150 in which the lead wire insertion hole 161 through which the lead wire insertion hole 161 is formed and the second insulating contact portion 170 are arranged in this order from the rear end side.

Further, an insulating contact member 166 is arranged on the rear end side (upper side in FIG. 1) of the multi-sensor element portion 100A protruding from the rear end portion 140 of the main metal fitting 138. The insulating contact member 166 is arranged around the electrode pads 80 formed on the front and back surfaces of the multi-sensor element unit 100A on the rear end side. The insulating contact member 166 is formed in a tubular 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 arranged inside the outer cylinder 144 when the collar portion 167 comes into contact with the outer cylinder 144 via the holding member 169. Then, the terminal fitting 110 on the insulating contact member 166 side and the electrode pad 80 of the multi-sensor element unit 100A are electrically connected so as to be electrically connected to the outside by the lead wire 146.

FIG. 2 is a block diagram showing a configuration of a multi-gas sensor device 400 including a multi-gas sensor 200A according to an embodiment of the present invention. For convenience of explanation, FIG. 2 shows only a cross section of the multi-sensor element unit 100A housed in the multi-gas sensor 200A along the axis O direction.
The multi-gas sensor device 400 includes a control device (controller) 300 and a multi-gas sensor 200A (multi-sensor element unit 100A) connected thereto. The control device 300 is mounted on a vehicle equipped with an internal combustion engine (engine) (not shown), and the control device 300 is electrically connected to the ECU 220. The end of the lead wire 146 extending from the multi-gas sensor 200A is connected to a connector, and this connector is electrically connected to the connector on the control device 300 side.

Next, the configuration of the multi-sensor element unit 100A will be described. The multi-sensor element unit 100A includes a NO _x sensor unit 30A having the same configuration as a known NO _x sensor, and two ammonia sensor units, a first detection cell 42x and a second detection cell 42y, which will be described in detail later. The first detection cell 42x and the second detection cell 42y are formed on the outer surface of the NO _x sensor unit 30A.

First, the NO _x sensor unit 30A includes the insulating layer 23e, the first solid electrolyte body 2a, the insulating layer 23d, the third solid electrolyte body 6a, the insulating layer 23c, the second solid electrolyte body 4a, and the insulating layers 23b and 23a. It has a structure in which they are laminated in order. The first measurement chamber S1 is defined between the layers of the first solid electrolyte body 2a and the third solid electrolyte body 6a, and the first diffusion resistor 8a arranged at the left end (entrance) of the first measurement chamber S1 is used. Exhaust gas is introduced from the outside. A protective layer 9 made of porous material is arranged on the outside of the first diffusion resistor 8a.
A second diffusion resistor 8b is arranged at the end of the first measurement chamber S1 opposite to the inlet, and communicates with the first measurement chamber S1 on the right side of the first measurement chamber S1 via the second diffusion resistor 8b. The second measurement chamber S2 is defined. The second measurement chamber S2 penetrates the third solid electrolyte body 6a and is formed between the layers of the first solid electrolyte body 2a and the second solid electrolyte body 4a.

A long plate-shaped heat generating resistor (heater) 21 extending along the axis O direction of the multi-sensor element unit 100A is embedded between the insulating layers 23b and 23a. The heat generating resistor 21 is provided with a heat generating portion on the tip end side in the axis O direction, and is provided with a pair of lead portions from the heat generating portion toward the rear end side in the axis direction. This heater is used to raise the temperature of the gas sensor to the active temperature, increase the conductivity of oxygen ions in the solid electrolyte, and stabilize the operation.

The first pumping cell 2 includes a first solid electrolyte body 2a mainly composed of zirconia having oxygen ion conductivity, an inner first pumping electrode 2b arranged so as to sandwich the first solid electrolyte body 2a, and an outer first pumping electrode serving as a counter electrode. 2c is provided, and the inner first pumping electrode 2b faces the first measurement chamber S1. Both the inner first pumping electrode 2b and the outer first pumping electrode 2c are mainly made of platinum, and the surface of the inner first pumping electrode 2b is covered with a protective layer 11 made of a porous body.
Further, the insulating layer 23e corresponding to the upper surface of the outer first pumping electrode 2c is hollowed out and filled with the porous body 13, and the outer first pumping electrode 2c and the outside are communicated with each other to allow gas (oxygen) to flow in and out. There is.

The oxygen concentration detection cell 6 includes a third solid electrolyte body 6a mainly composed of zirconia, a detection electrode 6b and a reference electrode 6c arranged so as to sandwich the third solid electrolyte body 6a, and the detection electrode 6b is an inner first pumping electrode 2b. It faces the first measurement chamber S1 on the downstream side. Both the detection electrode 6b and the reference electrode 6c are mainly composed of platinum.
The insulating layer 23c is cut out so that the reference electrode 6c in contact with the third solid electrolyte body 6a is arranged inside, and the cut-out portion is filled with the porous body to form the reference oxygen chamber 15. .. Then, by passing a weak constant current through the oxygen concentration detection cell 6 in advance using the Icp supply circuit 54, oxygen is sent from the first measurement chamber S1 into the reference oxygen chamber 15 and used as the oxygen reference.

The second pumping cell 4 includes a second solid electrolyte body 4a mainly composed of zirconia, an inner second pumping electrode 4b and a counter electrode arranged on the surface of the second solid electrolyte body 4a facing the second measurement chamber S2. It is provided with a second pumping counter electrode 4c. Both the inner second pumping electrode 4b and the second pumping pair electrode 4c are mainly made of platinum.
The second pumping counter electrode 4c is arranged in the cutout portion of the insulating layer 23c on the second solid electrolyte body 4a, faces the reference electrode 6c, and faces the reference oxygen chamber 15.
The inner first pumping electrode 2b, the detection electrode 6b, and the inner second pumping electrode 4b are each connected to a reference potential.

Next, the first detection cell 42x and the second detection cell 42y, which are two ammonia sensor units (cells), will be described.
As shown in FIG. 3, the multi-sensor element unit 100A has a first detection cell 42x and a second detection cell 42y, which are separated from each other in the width direction.

The first detection cell 42x and the second detection cell 42y are formed on the insulating layer 23a forming the outer surface (lower surface) of the NO _x sensor unit 30A. More specifically, in the first detection cell 42x, the first reference electrode 42ax is formed on the insulating layer 23a, and the first solid electrolyte body 42dx is formed on the surface of the first reference electrode 42ax. Further, the first detection electrode 42bx is formed on the surface of the first solid electrolyte body 42dx. Then, the ammonia concentration in the gas to be measured is detected by the change in electromotive force between the first reference electrode 42ax and the first detection electrode 42bx.
Similarly, in the second detection cell 42y, the second reference electrode 42ay is formed on the insulating layer 23a, and the second solid electrolyte body 42dy is formed on the surface of the second reference electrode 42ay. Further, a second detection electrode 42by is formed on the surface of the second solid electrolyte body 42dy.

The first detection electrode 42bx and the second detection electrode 42by can be formed from a material containing Au as a main component (more than 50% by mass, for example, 70% by mass or more) and containing Pt or Pd. The first reference electrode 42ax and the second reference electrode 42ay can be formed of Pt alone or a material containing Pt as a main component (more than 50% by mass, for example, 70% by mass or more). The first detection electrode 42bx and the second detection electrode 42by are electrodes in which ammonia gas is difficult to burn on the electrode surface. Ammonia reacts with oxygen ions (electrode reaction) at the three-phase interface to detect the concentration of ammonia.
The first solid electrolyte body 42dx and the second solid electrolyte body 42dy are composed of, for example, partially stabilized zirconia (YSZ).

Further, of the first detection electrode 42bx and the second detection electrode 42by, a porous layer 23g is provided on the surface of only the first detection electrode 42bx. Here, the "surface of only the first detection electrode 42bx" means that the porous layer 23g is not provided on the surface of the second detection electrode 42by, and if it is a surface other than the second detection electrode 42by, the surface is the first. 1 The porous layer 23g may be provided on another member of the detection electrode 42bx (for example, the first solid electrolyte body 42dx in FIG. 3).
For example, in the example of FIG. 3, not only the upper surface of the first reference electrode 42ax but also the side surface is covered with the first solid electrolyte 42dx, and the first solid electrolyte 42dx is outside the first reference electrode 42ax when viewed in the stacking direction. Is sticking out. The porous layer 23g covers not only the upper surface but also the side surface of the first detection electrode 42bx, and the outer edge of the porous layer 23g is the surface of the first solid electrolyte body 42dx protruding to the outside of the first reference electrode 42ax. Will be provided in.

Further, an outer protective layer 23i having a porosity larger than that of the porous layer 23g is provided on the surfaces of the porous layer 23g and the second detection electrode 42by.
The outer protective layer 23i prevents the toxic substance from adhering to the first detection electrode 42bx and the second detection electrode 42by, and diffuses the gas to be measured that flows into the first detection cell 42x and the second detection cell 42y from the outside. It adjusts the speed.

The measurement of ammonia gas by the two ammonia sensor units of the first detection cell 42x and the second detection cell 42y is as follows. That is, since the ammonia sensor unit detects not only ammonia but also NO ₂ , if the gas to be measured contains NO ₂ gas other than ammonia, the detection accuracy of ammonia is lowered. Therefore, if two ammonia sensor units having different ratios of sensitivity to ammonia and sensitivity to NOx are provided, the values of the two unknown concentrations of ammonia gas and NO ₂ gas are obtained from the two ammonia sensor units with different sensitivities. Since it is detected, the concentrations of ammonia gas and NO ₂ can be calculated by the relationship (simultaneous equations) prepared for the known gas concentration in advance.
That is, the two ammonia sensor units measure the concentrations of ammonia gas and NO _{2 by} a hybrid potential type, but if the difference in the detection output of NO ₂ between the two ammonia sensor units is small, the detection accuracy of ammonia decreases. To do. Further, if the gas to be measured contains ammonia and NO ₂ , the ammonia and NO ₂ react on the surfaces of the detection electrodes 42bx and 42by, and the detection accuracy of ammonia is lowered.

Therefore, if the porous layer 23 g is provided on the surface of only the first detection electrode 42bx, the measurement accuracy of ammonia and NO ₂ can be improved as shown in FIG.
That is, as shown in FIG. 4, since the outer protective layer 23i has a higher porosity than the porous layer 23g, not only ammonia gas having a small molecular weight but also NO ₂ gas having a large molecular weight passes through the outer protective layer 23i. It quickly reaches the surface of the second detection electrode 42by.
On the other hand, since the porous layer 23g has a smaller porosity than the outer protective layer 23i, ammonia gas having a small molecular weight can quickly reach the surface of the first detection electrode 42bx through the porous layer 23g, but has a large molecular weight. It takes time for the NO ₂ gas to pass through the porous layer 23 g.

Here, NO is chemically more stable than NO _{2 in} the region of 500 ° C. or higher, which is the operating temperature of a gas sensor such as a general multi-gas sensor 200A. Therefore, the NO ₂ gas reaches thermal equilibrium while staying in the porous layer 23 g, and NO ₂ changes to NO, and the proportion of NO ₂ gas that reaches the surface of the first detection electrode 42bx increases. Since the first detection electrode 42bx and the second detection electrode 42by do not detect NO, the sensitivity of the first detection electrode 42bx to NO ₂ can be reduced as compared with the second detection electrode 42by. As a result, the ratio of the sensitivity to NOx can be increased in the first detection cell 42x and the second detection cell 42y, and the measurement accuracy of the concentrations of ammonia gas and NO ₂ is improved.

The outer protective layer 23i can be formed by, for example, a paste of ceramic particles such as alumina mixed with a pore-forming agent (burnable particles) such as carbon and fired after coating and printing. The average particle size of the pore-forming agent can be, for example, about 0.8 to 5 μm.
The porous layer 23g can be formed by applying, for example, a paste of ceramic particles such as alumina and firing it after coating and printing without using a pore-forming agent. In this case, it can be assumed that the porous layer 23 g does not have pores derived from the pore-forming agent and that voids (for example, pore diameter of about 0.2 μm) are formed between the ceramic particles. The maximum pore diameter of the porous layer 23 g is preferably 0.2 μm or less.

When the porosity of the porous layer 23g is 40% or less, it takes longer for the NO ₂ gas to pass through the gas sensor porous layer 23g, so that the above-mentioned effect can be further exhibited. When the porosity of the porous layer 23g exceeds 40%, NO ₂ gas easily passes through the porous layer 23g, and it may be difficult to change NO ₂ to NO due to thermal equilibrium. The lower limit of the porosity of the porous layer 23 g can be, for example, 15%.
The porosity of the outer protective layer 23i is preferably 60% or less.
The porosity can be measured by the mercury intrusion method. Further, a cross-sectional SEM image of the outer protective layer 23i and the porous layer 23g (the outer protective layer 23i has a magnification of 300 to 500 times, for example, a field of view of 0.24 × 0.18 mm, and the porous layer 23g has a magnification of 10000 times, for example. The porosity may be obtained by image analysis from a field of view of 12 × 9 μm).

When the thickness of the porous layer 23 g is 40 μm or less, the above-mentioned effect can be further exhibited. If the thickness of the porous layer 23 g exceeds 40 μm, it becomes difficult for ammonia gas to pass through the porous layer 23 g, and the measurement accuracy may rather decrease. If the thickness of the porous layer 23g is too thin, the effect of NO ₂ gas staying in the porous layer 23g becomes small, so that the thickness of the porous layer 23g is more preferably 10 to 40 μm.
The thickness of the porous layer 23g is obtained by image analysis from a cross-sectional SEM image (magnification of about 300 times) of the porous layer 23g to obtain an average thickness.

Further, in the case of a configuration in which the first detection cell 42x and the second detection cell 42y are heated by energizing the heat generation resistor (heater) 21 included in the multi-sensor element unit 100A, the first heat generation resistor 21 is energized. When the maximum temperature of the detection electrode 42bx and the second detection electrode 42by is 500 to 950 ° C., NO ₂ is within the temperature range of thermal equilibrium where NO changes to NO, so that the above-mentioned effect can be further exhibited.
If the maximum temperature is less than 500 ° C., NO ₂ may be lower than the temperature range of thermal equilibrium where NO changes to NO, and the above effect may be reduced.
If the maximum temperature exceeds 950 ° C., the heat resistance of the sensor may decrease.

Next, returning to FIG. 2, an example of the configuration of the control device 300 will be described. The control device 300 includes a (analog) control circuit 59 and a microcomputer (microcomputer) 60 on a circuit board. The microcomputer 60 controls the entire control device 300, includes a CPU (central processing unit) 61, a RAM 62, a ROM 63, a signal input / output unit 64, an A / D converter 65, and a clock (not shown), and is stored in advance in a ROM or the like. The program is executed by the CPU.
The control circuit 59 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, and a heater drive circuit 57, which will be described in detail later. The first electromotive force detection circuit 58a and the second electromotive force detection circuit 58b for detecting the electromotive force of the 42x and the second detection cell 42y are provided.
The control circuit 59 controls the NO _x sensor unit 30A, NO _x sensor unit first pumping current flowing through 30A Ip1, and outputs to the microcomputer 60 detects the second pumping current Ip2.
The first electromotive force detection circuit 58a and the second electromotive force detection circuit 58b detect the ammonia concentration output (electromotive force) between the electrodes of the first detection cell 42x and the second detection cell 42y and output the electromotive force to the microcomputer 60. ..

Specifically, the outer first pumping electrode 2c of the NO _x sensor unit 30A is connected to the Ip1 drive circuit 52, and the reference electrode 6c is connected in parallel to the Vs detection circuit 53 and the Icp supply circuit 54. Further, the second pumping counter electrode 4c is connected in parallel to the Ip2 detection circuit 55 and the Vp2 application circuit 56. The heater circuit 57 is connected to a heater (specifically, a heat generating resistor 21).
Further, the pair of electrodes 42ax and 42bx of the first detection cell 42x are connected to the first electromotive force detection circuit 58a, respectively. Similarly, the pair of electrodes 42ay and 42by of the second detection cell 42y are connected to the second electromotive force detection circuit 58b, respectively.

Each circuit 51 to 57 has the following functions.
The Ip1 drive circuit 52 supplies the first pumping current Ip1 between the inner first pumping electrode 2b and the outer first pumping electrode 2c, and detects the first pumping current Ip1 at that time.
The Vs detection circuit 53 detects the voltage Vs between the detection electrode 6b and the reference electrode 6c, and outputs the detection 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. Then, the Ip1 drive circuit 52 controls the direction and magnitude of the flow of the Ip1 current so that the voltage Vs becomes equal to the reference voltage, and a predetermined value such that NO _x does not decompose the oxygen concentration in the first measurement chamber S1. Adjust to.
The Icp supply circuit 54 allows a weak current Icp to flow between the detection electrode 6b and the reference electrode 6c, sends oxygen from the first measurement chamber S1 into the reference oxygen chamber 15, and uses the reference electrode 6c as a reference for a predetermined oxygen concentration. Expose to.
The Vp2 application circuit 56 has a constant voltage Vp2 (for example, 450 mV) between the inner second pumping electrode 4b and the second pumping counter electrode 4c so that the NO _x gas in the gas to be measured decomposes into oxygen and N ₂ gas. Is applied to decompose NO _x into nitrogen and oxygen.

The Ip2 detection circuit 55 enters the second pumping cell 4 when oxygen generated by the decomposition of NO _x is pumped from the second measurement chamber S2 to the second pumping counter electrode 4c side via the second solid electrolyte body 4a. The flowing second pumping current Ip2 is detected.
At this time, since there is a linear relationship between the second pumping current Ip2 and the NO _x concentration, the Ip2 detection circuit 55 detects the second pumping current Ip2 to detect the NO _x concentration in the gas to be measured. Can be done.

Further, the first electromotive force detection circuit 58a detects the ammonia concentration output (electromotive force) between the pair of electrodes 42ax and 42bx, and the second electromotive force detection circuit 58b detects the ammonia concentration output (electromotive force) between the pair of electrodes 42ay and 42by. ) Is detected by the mixed potential formula. Then, the concentrations of ammonia gas and NO _{2 in} the gas to be measured can be calculated as described above based on the relational equations (continuity equations) set in advance for the ammonia concentration and the NO ₂ concentration.

It goes without saying that the present invention is not limited to the above embodiments and extends to various modifications and equivalents included in the idea and scope of the present invention.
It may be a common reference electrode for the first detection cell and the first detection cell, or it may be a common solid electrolyte.
Further, the sensor element of the present invention is not limited to the multi-sensor element, and may be an ammonia sensor element that detects only ammonia. In the case of an ammonia sensor element that detects only ammonia, the present invention is effective for a gas in which NH ₃ and NO ₂ coexist.

21 Heat-generating resistor (heater)
23g Porous layer 23i Outer protective layer 42ax 1st reference electrode 42ay 2nd reference electrode 42bx 1st detection electrode 42by 2nd detection electrode 42x 1st detection cell 42y 2nd detection cell 100A Sensor element (multi-sensor element part)
138 Main metal fitting 200A gas sensor (multi-gas sensor)

Claims (5)

The first detection electrode and the second detection electrode, which are active against ammonia and NO ₂ , respectively,
A sensor element comprising at least one reference electrode, the first detection electrode and the reference electrode forming a first detection cell, the second detection electrode and the reference electrode forming a second detection cell, and detecting ammonia. In
Of the first detection electrode and the second detection electrode, a porous layer is provided on the surface of only the first detection electrode.
A sensor element characterized in that an outer protective layer having a porosity larger than that of the porous layer is provided on the surface of the porous layer and the second detection electrode. The sensor element according to claim 1, wherein the porosity of the porous layer is 40% or less. The sensor element according to claim 1 or 2, wherein the thickness of the porous layer is 40 μm or less. The sensor element includes a heater for heating the first detection cell and the second detection cell, and the maximum temperature of the first detection electrode and the second detection electrode when the heater is energized is 500 to 950 ° C. Item 2. The sensor element according to any one of Items 1 to 3. A gas sensor including a sensor element and a main metal fitting for holding the sensor element, wherein the sensor element according to any one of claims 1 to 4 is used as the sensor element.

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