JP2021148612A - Sensor element and gas sensor - Google Patents

Abstract

To measure correctly, density of a specific oxide gas, regardless of an atmosphere of a gas to be measured.SOLUTION: A sensor element 101 comprises: a main pump cell 21 comprising an external pump electrode 23 including Pt on a part exposed to an exhaust gas of an element body 101a, and adjusting oxygen density of a first internal dead space 20; and a porous protective layer 91 for covering a tip part including a gas introduction hole 10 and the external pump electrode 23 on an external plane of the electrode body 101a. A first gas circulation path is a path reaching the gas introduction hole 10 from a position facing the gas introduction hole 10 on a surface of the porous protective layer 91, at a shortest distance. A second gas circulation path is a path reaching a near side of the gas introduction hole 10 through the external pump electrode 23 from a position facing the external pump electrode 23 out of the surface of the porous protective layer 91 at a shortest distance. A gas circulation inhibition part 92 is provided at a region for blocking the second gas circulation path, and a ratio of the first gas circulation path to a porosity of the gas circulation inhibition part 92 is 1.03 or greater.SELECTED DRAWING: Figure 2

Description

The present invention relates to a sensor element and a gas sensor.

Conventionally, a gas sensor including a sensor element for detecting the concentration of a specific gas such as NOx in a gas to be measured such as an exhaust gas of an automobile has been known. For example, Patent Document 1 discloses a sensor element using a laminated body. The laminate is a laminate of a plurality of oxygen ion conductive solid electrolyte layers. The laminated body includes an internal measurement gas distribution unit that introduces and distributes the measurement gas. The laminate includes a gas-side electrode to be measured and a measurement electrode. Of the laminate, the inlet of the gas flow section to be measured and the tip including the electrode on the gas side to be measured are covered with a porous protective layer. The electrode to be measured is provided on the outer surface of the laminate exposed to the gas to be measured, and is used to pump oxygen from the gas flow section to be measured. Pt is used for the electrode to be measured on the gas side. The measurement electrode is provided on the inner peripheral surface of the measurement chamber in the gas flow section to be measured. NOx in the gas to be measured is reduced at the measurement electrode to generate oxygen. Here, since the amount of oxygen generated is proportional to the NOx concentration in the gas to be measured, the NOx concentration in the gas to be measured is calculated based on the amount of oxygen.

Japanese Unexamined Patent Publication No. 2016-65552

However, for example, when the gas to be measured is the exhaust gas of a gasoline engine, when the exhaust gas becomes a stoichiometric atmosphere, NOx is purified at the gas-side electrode to be measured containing Pt as in the case of the three-way catalyst, and the NOx concentration is increased. Sometimes it was not possible to measure accurately.

The present invention has been made to solve such a problem, and an object of the present invention is to accurately measure a specific oxide gas concentration regardless of the atmosphere of the gas to be measured.

The present invention has taken the following measures to achieve the above-mentioned main object.

The sensor element of the present invention is
A sensor element for detecting a specific oxide gas concentration in the gas to be measured.
An element body having an oxygen ion conductive solid electrolyte layer and having a gas measurement unit for introducing and circulating the gas to be measured inside.
An adjustment pump cell having a gas-measured gas-side electrode containing Pt in a portion exposed to the exhaust gas on the outside of the element body, and adjusting the oxygen concentration in the oxygen concentration adjustment chamber of the gas flow-in portion to be measured. When,
A measuring electrode arranged in a measuring chamber provided on the downstream side of the oxygen concentration adjusting chamber in the gas flow section to be measured, and a measuring electrode.
Of the outer surface of the element body, a porous protective layer that covers the inlet of the gas flow section to be measured and the tip including the electrode on the measurement gas side, and
The porous protective layer is a sensor element provided with the above.
A first gas flow path from a position facing the inlet on the surface of the porous protective layer to the inlet at the shortest distance.
A second gas flow path from a position on the surface of the porous protective layer facing the gas-side electrode to be measured, through the gas-side electrode to be measured, and to the front of the inlet at the shortest distance.
A gas flow obstruction portion provided in a region that blocks the second gas flow path and having a smaller porosity than the first gas flow path.
With
The ratio of the porosity of the first gas flow path to the porosity of the gas flow obstruction portion is 1.03 or more.

Using this sensor element, for example, the concentration of a specific oxide gas contained in the gas to be measured can be detected as follows. First, the adjusting pump cell is operated to adjust the oxygen concentration of the gas to be measured introduced into the gas distribution section to be measured. As a result, the adjusted gas to be measured reaches the measurement chamber. Then, a detection value corresponding to oxygen generated in the measurement chamber derived from the specific oxide gas (oxygen generated when the specific oxide gas itself is reduced in the measurement chamber) is acquired, and the acquired detection value is used. Based on this, a specific oxide gas concentration in the gas to be measured is detected. The porous protective layer of this sensor element is provided with a gas flow obstructing portion. Therefore, the concentration of a specific oxide gas in the gas to be measured can be accurately measured regardless of the atmosphere of the gas to be measured. The reason for this is considered as follows. The electrode to be measured gas containing Pt tends to reduce a specific oxide gas contained in the gas to be measured when the atmosphere of the gas to be measured becomes stoichiometric. When the gas to be measured, whose specific oxide gas concentration has decreased due to the reduction, reaches the measurement chamber, the oxygen generated from the specific oxide gas decreases, and the measurement accuracy of the specific oxide gas concentration decreases. It is thought that. On the other hand, in this sensor element, the second gas flow path from the position facing the gas side electrode to be measured on the surface of the porous protective layer, passing through the gas side electrode to be measured, and reaching just before the inlet is blocked. A gas flow obstruction portion having a small porosity is provided in the region, and the ratio of the porosity of the first gas flow path to the porosity of the gas flow obstruction portion is 1.03 or more. For example, when the gas flow obstruction portion is provided between the surface of the porous protective layer facing the gas side electrode to be measured and the gas flow obstruction portion to be measured, the gas flow obstruction portion is to be measured. Since it prevents the gas from reaching the electrode to be measured, it suppresses the reduction of a specific oxide gas in the gas to be measured at the electrode to be measured. Further, when the gas flow obstruction part is provided between the electrode to be measured and the shortest distance to the front of the inlet, the gas flow obstruction part is reduced by the electrode to be measured and specific oxidation. It prevents the gas to be measured having a reduced substance gas concentration from reaching the inlet of the gas flow section to be measured. Therefore, in any case, the concentration of a specific oxide gas in the gas to be measured can be accurately measured regardless of the atmosphere of the gas to be measured.

In the sensor element of the present invention, the region that blocks the second gas flow path is at least from the position of the surface of the porous protective layer facing the gas-measured gas-side electrode to the gas-measured gas-side electrode. It may be an area.

In the sensor element of the present invention, the ratio of the porosity of the first gas flow path to the porosity of the gas flow obstruction portion may be 1.03 or more and 3.09 or less. In this way, a specific oxide gas concentration can be measured more accurately regardless of the atmosphere of the gas to be measured. In this case, the porosity of the gas flow blocking portion may be 9.8% or more and 25.4% or less, and the porosity of the first gas flow path may be 20.0% or more and 30.3% or less.

In the sensor element of the present invention, the ratio of the porosity of the first gas flow path to the porosity of the gas flow obstruction portion may be 1.96 or more and 3.09 or less. In this way, the specific oxide gas concentration can be measured sufficiently accurately regardless of the atmosphere of the gas to be measured. In this case, the porosity of the gas flow blocking portion may be 9.8% or more and 15.0% or less, and the porosity of the first gas flow path may be 20.0% or more and 30.3% or less.

In the sensor element of the present invention, the gas to be measured may be exhaust gas from a spark-ignition internal combustion engine. Since the exhaust gas from the spark-ignition internal combustion engine tends to have a relatively stoichiometric atmosphere, it is highly significant to use the sensor element of the present invention.

In the sensor element of the present invention, the oxide gas concentration may be a NOx concentration. Since the electrode to be measured containing Pt easily reduces NOx in a stoichiometric atmosphere, it is highly significant to use the sensor element of the present invention for measuring the NOx concentration.

The gas sensor of the present invention
With the sensor element of the present invention in any of the above-described embodiments,
An adjustment pump cell control unit that operates the adjustment pump cell so that the oxygen concentration in the oxygen concentration adjustment chamber becomes a target concentration.
A measurement voltage detection unit that detects a measurement voltage between the reference electrode and the measurement electrode,
Based on the measurement voltage, a detection value corresponding to oxygen generated in the measurement chamber derived from the oxide gas is acquired, and the oxide gas concentration in the exhaust gas is detected based on the detection value. Specific gas concentration detector and
Is provided.

This gas sensor includes any of the sensor elements described above. Therefore, this gas sensor can accurately measure the same effect as the sensor element of the present invention described above, for example, a specific oxide gas concentration regardless of the atmosphere of the gas to be measured.

FIG. 5 is a perspective view schematically showing an example of the configuration of the sensor element 101. FIG. 2 is a cross-sectional view taken along the line AA of FIG. FIG. 5 is a cross-sectional view schematically showing a first gas flow path P1 and a second gas flow path P2. Explanatory drawing of plasma spraying using a plasma gun 170. The block diagram which shows the electrical connection relationship with each cell of a control device 90. FIG. 5 is a perspective view schematically showing an example of the configuration of the sensor element 201. The perspective view which showed the example of the structure of the sensor element 301 schematicly. The graph which shows the relationship between the A / F of the measured gas and the Ip2 relative sensitivity in each of Experimental Examples 1, 5 and 19.

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view schematically showing an example of the configuration of the sensor element 101 according to the embodiment of the present invention. FIG. 2 is a cross-sectional view taken along the line AA of FIG. FIG. 3 is a cross-sectional view schematically showing the first gas flow path P1 and the second gas flow path P2. FIG. 4 is an explanatory diagram of plasma spraying using the plasma gun 170. FIG. 5 is a block diagram showing an electrical connection relationship between the control device 90 and each cell. In the present embodiment, the vertical direction and the front-back direction are as shown in FIG. 1, and the back side with respect to the paper surface of FIG. 1 is referred to as the left direction, and the front side with respect to the paper surface is referred to as the right direction.

The gas sensor 100 is attached to a pipe such as an exhaust gas pipe of a gasoline engine which is a spark-ignition internal combustion engine, for example. The gas sensor 100 detects the concentration of a specific oxide gas (NOx in this case) in the exhaust gas of the internal combustion engine. The gas sensor 100 controls the sensor element 101 having a long rectangular parallelepiped shape, the cells 21, 41, 50, 80 to 83 included in the sensor element 101, the variable power supplies 24, 46, 52, and the entire gas sensor 100. The device 90 and the like are provided.

The sensor element 101 includes an element body 101a and a porous protective layer 91 that covers the tip end side of the element body 101a. The element body 101a refers to a portion of the sensor element 101 other than the porous protective layer 91.

The element body 101a has a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, and a first solid electrolyte layer 4 _{, each of which is composed of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO 2).} The element has a laminated body in which six layers, the spacer layer 5 and the second solid electrolyte layer 6, are laminated in this order from the lower side in the drawing. Further, the solid electrolyte forming these six layers is a dense and airtight one. The element main body 101a is manufactured, for example, by performing predetermined processing, printing of a circuit pattern, or the like on a ceramic green sheet corresponding to each layer, laminating them, and further firing and integrating them.

A gas introduction port 10 and a first gas inlet 10 are located between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4 on the tip end side (left end side in FIG. 1) of the element main body 101a. Diffusion rate-determining section 11, buffer space 12, second diffusion rate-determining section 13, first internal space 20, third diffusion rate-determining section 30, second internal space 40, and fourth diffusion rate-determining section 60. , The third internal space 61 is formed adjacent to each other in this order.

The gas introduction port 10, the buffer space 12, the first internal space 20, the second internal space 40, and the third internal space 61 have an upper portion provided in a manner in which the spacer layer 5 is hollowed out. The space inside the sensor element 101 is defined by the lower surface of the second solid electrolyte layer 6, the lower portion is the upper surface of the first solid electrolyte layer 4, and the side portion is partitioned by the side surface of the spacer layer 5.

The first diffusion rate-determining section 11, the second diffusion rate-determining section 13, and the third diffusion rate-determining section 30 are all provided as two horizontally long slits (the openings have a longitudinal direction in the direction perpendicular to the drawing). .. Further, the fourth diffusion rate-determining portion 60 is provided as one horizontally long slit (the opening has a longitudinal direction in the direction perpendicular to the drawing) formed as a gap with the lower surface of the second solid electrolyte layer 6. The portion from the gas introduction port 10 to the third internal space 61 is also referred to as a gas distribution section to be measured.

Further, at a position far from the tip side of the gas flow portion to be measured, between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, the side portion is on the side surface of the first solid electrolyte layer 4. A reference gas introduction space 43 is provided at a position to be partitioned. For example, the atmosphere is introduced into the reference gas introduction space 43 as a reference gas for measuring the NOx concentration.

The atmosphere introduction layer 48 is a layer made of porous ceramics, and the reference gas is introduced into the atmosphere introduction layer 48 through the reference gas introduction space 43. Further, the atmosphere introduction layer 48 is formed so as to cover the reference electrode 42.

The reference electrode 42 is an electrode formed so as to be sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and as described above, the reference electrode 42 is connected to the reference gas introduction space 43 around the reference electrode 42. An air introduction layer 48 is provided. Further, as will be described later, the oxygen concentration (oxygen partial pressure) in the first internal space 20, the second internal space 40, and the third internal space 61 can be measured using the reference electrode 42. It is possible. The reference electrode 42 is formed as a porous cermet electrode (for example, a cermet electrode of Pt and ZrO ₂ ).

In the gas flow section to be measured, the gas introduction port 10 is a portion that is open to the external space, and the gas to be measured is taken into the sensor element 101 from the external space through the gas introduction port 10. There is. The first diffusion rate-determining unit 11 is a portion that imparts a predetermined diffusion resistance to the gas to be measured taken in from the gas introduction port 10. The buffer space 12 is a space provided for guiding the gas to be measured introduced from the first diffusion rate-determining unit 11 to the second diffusion rate-determining unit 13. The second diffusion rate-determining unit 13 is a portion that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal space 20. When the gas to be measured is introduced from the outside of the sensor element 101 to the inside of the first internal space 20, the pressure fluctuation of the gas to be measured in the external space (if the gas to be measured is the exhaust gas of an automobile, the pulsation of the exhaust pressure). ) Suddenly taken into the inside of the sensor element 101 from the gas introduction port 10), the gas to be measured is not directly introduced into the first internal space 20, but the first diffusion rate-determining unit 11, the buffer space 12, and the second. After the pressure fluctuation of the gas to be measured is canceled through the diffusion rate-determining unit 13, the gas is introduced into the first internal space 20. As a result, the pressure fluctuation of the gas to be measured introduced into the first internal space 20 becomes almost negligible. The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the gas to be measured introduced through the second diffusion rate-determining unit 13. The oxygen partial pressure is adjusted by operating the main pump cell 21.

The main pump cell 21 has an inner pump electrode 22 having a ceiling electrode portion 22a provided on substantially the entire lower surface of the lower surface of the second solid electrolyte layer 6 facing the first internal space 20, and an upper surface of the second solid electrolyte layer 6. An electrochemical pump cell composed of an outer pump electrode 23 provided in a region corresponding to the ceiling electrode portion 22a so as to be exposed to an external space, and a second solid electrolyte layer 6 sandwiched between these electrodes. be.

The inner pump electrode 22 is formed so as to straddle the upper and lower solid electrolyte layers (second solid electrolyte layer 6 and first solid electrolyte layer 4) that partition the first internal space 20 and the spacer layer 5 that provides the side wall. There is. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom portion is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. A spacer layer in which electrode portions 22b are formed, and side electrode portions (not shown) form both side wall portions of the first internal space 20 so as to connect the ceiling electrode portions 22a and the bottom electrode portions 22b. It is formed on the side wall surface (inner surface) of No. 5 and is arranged in a structure in the form of a tunnel at the arrangement portion of the side electrode portion.

The inner pump electrode 22 and the outer pump electrode 23 are formed as a porous cermet electrode (for example, a cermet electrode of Pt and ZrO ₂ ). The inner pump electrode 22 and the outer pump electrode 23 that come into contact with the gas to be measured are formed by using a material having a weakened reducing ability for the NOx component in the gas to be measured.

In the main pump cell 21, a desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23, and a pump current is applied in the positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23. By flowing Ip0, the oxygen in the first internal space 20 can be pumped into the external space, or the oxygen in the external space can be pumped into the first internal space 20.

Further, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, the inner pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4 are used. The third substrate layer 3 and the reference electrode 42 constitute an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 80 for controlling a main pump.

By measuring the electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump, the oxygen concentration (oxygen partial pressure) in the first internal space 20 can be known. Further, the pump current Ip0 is controlled by feedback-controlling the pump voltage Vp0 of the variable power supply 24 so that the electromotive force V0 becomes the target value. As a result, the oxygen concentration in the first internal space 20 can be maintained at a predetermined constant value.

The third diffusion rate-determining unit 30 imparts a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal space 20, and applies the gas to be measured. It is a part leading to the second internal space 40.

In the second internal space 40, after the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space 20, the auxiliary pump cell 50 is further applied to the gas to be measured introduced through the third diffusion rate-determining unit 30. It is provided as a space for adjusting the oxygen partial pressure. As a result, the oxygen concentration in the second internal space 40 can be kept constant with high accuracy, so that the gas sensor 100 can measure the NOx concentration with high accuracy.

The auxiliary pump cell 50 includes an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, and an outer pump electrode 23 (outer pump electrode 23). It is an auxiliary electrochemical pump cell composed of a suitable electrode on the outside of the sensor element 101) and a second solid electrolyte layer 6.

The auxiliary pump electrode 51 is arranged in the second internal space 40 in a structure having a tunnel shape similar to that of the inner pump electrode 22 provided in the first internal space 20. That is, the ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40 is formed. , The bottom electrode portion 51b is formed, and the side electrode portion (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b provides a side wall of the second internal space 40 of the spacer layer 5. It has a tunnel-like structure formed on both walls. The auxiliary pump electrode 51 is also formed by using a material having a weakened reducing ability for the NOx component in the gas to be measured, similarly to the inner pump electrode 22.

In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, oxygen in the atmosphere in the second internal space 40 is pumped out to the external space or outside. It is possible to pump from the space into the second internal space 40.

Further, in order to control the oxygen partial pressure in the atmosphere in the second internal space 40, the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte are used. The layer 4 and the third substrate layer 3 constitute an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 81 for controlling an auxiliary pump.

The auxiliary pump cell 50 pumps with the variable power supply 52 whose voltage is controlled based on the electromotive force V1 detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. As a result, the partial pressure of oxygen in the atmosphere in the second internal space 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

At the same time, the pump current Ip1 is used to control the electromotive force of the oxygen partial pressure detection sensor cell 80 for controlling the main pump. Specifically, the pump current Ip1 is input to the oxygen partial pressure detection sensor cell 80 for controlling the main pump as a control signal, and the electromotive force V0 is controlled, so that the third diffusion rate-determining unit 30 to the second internal space are vacant. The gradient of the oxygen partial pressure in the gas to be measured introduced into the 40 is controlled to be always constant. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of about 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

The fourth diffusion rate-determining unit 60 imparts a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the auxiliary pump cell 50 in the second internal space 40, and transfers the gas to be measured. It is a part leading to the third internal space 61. The fourth diffusion rate-determining unit 60 plays a role of limiting the amount of NOx flowing into the third internal space 61.

The third internal space 61 is in the gas to be measured with respect to the gas to be measured introduced through the fourth diffusion rate-determining unit 60 after the oxygen concentration (oxygen partial pressure) is adjusted in advance in the second internal space 40. It is provided as a space for performing a process related to the measurement of the nitrogen oxide (NOx) concentration of the above. The measurement of the NOx concentration is mainly performed by the operation of the measurement pump cell 41 in the third internal space 61.

The measurement pump cell 41 measures the NOx concentration in the gas to be measured in the third internal space 61. The measurement pump cell 41 includes a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61, an outer pump electrode 23, a second solid electrolyte layer 6, and a spacer layer 5. , An electrochemical pump cell composed of the first solid electrolyte layer 4. The measurement electrode 44 is a porous cermet electrode made of a material having a reduction ability for NOx components in the gas to be measured higher than that of the inner pump electrode 22. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx existing in the atmosphere in the third internal space 61.

In the measurement pump cell 41, oxygen generated by the decomposition of nitrogen oxides in the atmosphere around the measurement electrode 44 can be pumped out, and the amount of oxygen generated can be detected as the pump current Ip2.

Further, in order to detect the oxygen partial pressure around the measurement electrode 44, an electrochemical sensor cell, that is, a reference electrode 42 is used by the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42. The oxygen partial pressure detection sensor cell 82 for controlling the measurement pump is configured. The variable power supply 46 is controlled based on the electromotive force V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump.

The gas to be measured guided into the second internal space 40 reaches the measurement electrode 44 in the third internal space 61 through the fourth diffusion rate-determining unit 60 under the condition that the oxygen partial pressure is controlled. .. Nitrogen oxides in the gas to be measured around the measurement electrode 44 are reduced (2NO → N ₂ + O ₂ ) to generate oxygen. Then, the generated oxygen is pumped by the measurement pump cell 41, and at that time, a variable power source is used so that the electromotive force V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 becomes constant. The voltage Vp2 of 46 is controlled. Since the amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxides in the gas to be measured, the nitrogen oxides in the gas to be measured are used by using the pump current Ip2 in the measurement pump cell 41. The concentration will be calculated.

Further, if the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined to form an oxygen partial pressure detecting means as an electrochemical sensor cell, the measurement electrode It is possible to detect the electromotive force according to the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around 44 and the amount of oxygen contained in the reference atmosphere, thereby detecting the NOx component in the gas to be measured. It is also possible to determine the concentration of.

Further, the electrochemical sensor cell 83 is composed of the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42. The electromotive force Vref obtained by the sensor cell 83 makes it possible to detect the partial pressure of oxygen in the gas to be measured outside the sensor.

In the gas sensor 100 having such a configuration, the oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx) by operating the main pump cell 21 and the auxiliary pump cell 50. The gas to be measured is supplied to the measurement pump cell 41. Therefore, the NOx concentration in the gas to be measured is determined based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out from the measurement pump cell 41 in substantially proportional to the concentration of NOx in the gas to be measured. You can know it.

Further, the sensor element 101 includes a heater unit 70 that plays a role of temperature adjustment for heating and keeping the sensor element 101 warm in order to increase the oxygen ion conductivity of the solid electrolyte. The heater unit 70 includes a heater connector electrode 71, a heater 72, a through hole 73, a heater insulating layer 74, and a pressure dissipation hole 75.

The heater connector electrode 71 is an electrode formed so as to be in contact with the lower surface of the first substrate layer 1. By connecting the heater connector electrode 71 to an external power source, power can be supplied to the heater unit 70 from the outside.

The heater 72 is an electric resistor formed in a manner of being sandwiched between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 72 is connected to the heater connector electrode 71 via a through hole 73, and generates heat when power is supplied from the outside through the heater connector electrode 71 to heat and retain heat of the solid electrolyte forming the sensor element 101. ..

Further, the heater 72 is embedded in the entire area from the first internal space 20 to the third internal space 61, and the entire sensor element 101 can be adjusted to a temperature at which the solid electrolyte is activated. ing.

The heater insulating layer 74 is an insulating layer formed on the upper and lower surfaces of the heater 72 by an insulator such as alumina. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72 and electrical insulation between the third substrate layer 3 and the heater 72.

The pressure dissipation hole 75 is a portion provided so as to penetrate the third substrate layer 3 and the atmosphere introduction layer 48 and communicate with the reference gas introduction space 43, and the internal pressure rises with the temperature rise in the heater insulating layer 74. It is formed for the purpose of alleviating.

Further, as shown in FIGS. 1 and 2, the element main body 101a is partially covered with the porous protective layer 91. The porous protective layer 91 includes porous protective layers 91a to 91e formed on five of the six surfaces of the element body 101a, respectively. The porous protective layer 91a covers a part of the upper surface of the element body 101a. The porous protective layer 91b covers a part of the lower surface of the element body 101a. The porous protective layer 91c covers a part of the left surface of the device body 101a. The porous protective layer 91d covers a part of the right surface of the device body 101a. The porous protective layer 91e covers the entire front end surface of the device body 101a. In the porous protective layers 91a to 91e, adjacent layers are connected to each other. Therefore, the porous protective layer 91 (porous protective layers 91a to 91e) is a region (front end surface) of the surface of the element body 101a from the front end surface of the element body 101a to a distance L (see FIG. 2) from the front end surface to the rear. Includes). The porous protective layer 91 plays a role of suppressing, for example, moisture in the gas to be measured from adhering to the element body 101a to cause cracks. The distance L is set to a length that exceeds the rear end portion of the outer pump electrode 23 from the front end surface of the element main body 101a. Further, in the following, when the porous protective layers 91a to 91e are not particularly distinguished, they may be referred to as the porous protective layer 91.

The porous protective layer 91 is made of, for example, an alumina porous body, a zirconia porous body, a spinel porous body, a cordierite porous body, a titania porous body, a magnesia porous body, or the like. In the present embodiment, the porous protective layer 91 is made of an alumina porous body. Further, although not particularly limited, the film thickness of the porous protective layer 91 is, for example, 100 to 700 μm.

The porous protective layer 91e covers the gas introduction port 10. However, the porous protective layer 91e is a porous body, and the gas to be measured can basically flow inside the porous protective layer 91e. Therefore, the gas to be measured can reach the gas introduction port 10 from the surface of the porous protective layer 91e. In the present embodiment, as shown in FIG. 3, from the position of the surface of the porous protective layer 91e facing the gas introduction port 10, it flows through the inside of the porous protective layer 91e and reaches the gas introduction port 10 in the shortest time. The distribution route of the gas to be measured is defined as the first gas distribution route P1.

The porous protective layer 91a covers the portion where the outer pump electrode 23 is formed. However, the porous protective layer 91a is a porous body, and the gas to be measured can basically flow inside the porous protective layer 91a. Therefore, if the gas flow obstructing portion 92 described later is not formed, the gas to be measured can reach from the surface of the porous protective layer 91a to the outer pump electrode 23. Further, since the porous protective layer 91a is connected to the porous protective layer 91e and the porous protective layer 91e is also a porous body, if the gas flow obstructing portion 92 is not formed, the porous protective layer 91a The gas to be measured that has reached the outer pump electrode 23 from the surface can reach the gas introduction port 10. In the present embodiment, as shown in FIG. 3, the gas is introduced from the surface of the porous protective layer 91a from a position facing the outer pump electrode 23 through the inside of the porous protective layer 91a and passed through the outer pump electrode 23. The second gas flow path P2 is the flow path of the gas to be measured that reaches the front of the mouth 10 in the shortest time. The sensor element 101 is formed with a gas flow obstruction portion 92 in a region that blocks the second gas flow path P2, which has a smaller porosity than the first gas flow path P1 and makes it difficult for the gas to be measured to flow. The gas flow blocking portion 92 may be formed in any region as long as it is a region that blocks the second gas flow path P2, but in the present embodiment, as shown in FIG. 2, the porous protective layer 91a It is formed in the region from the position facing the outer pump electrode 23 to the outer pump electrode 23 on the surface of the above. Further, the ratio of the porosity of the first gas flow path P1 to the porosity of the gas flow obstruction section 92 is preferably 1.03 or more, and the gas flow obstruction section 92 is preferably formed so as to be 1.03 or more. It is more preferable that the gas flow obstruction portion 92 is formed so as to be .09 or less, and it is further preferable that the gas flow obstruction portion 92 is formed so as to be 1.96 or more and 3.09 or less. In this case, the porosity of the gas flow obstructing portion 92 is preferably 25.4% or less, more preferably 9.8% or more and 25.4% or less, and 9.8% or more and 15.0% or less. The porosity of the region (including the first gas flow path P1) of the porous protective layer 91 excluding the gas flow obstruction portion 92 is 20.0% or more and 30.3% or less. preferable. The gas flow obstruction unit 92 plays a role of preventing the measured gas from reaching the outer pump electrode 23.

Next, a method of manufacturing such a sensor element 101 will be described. In the method for manufacturing the sensor element 101, the element main body 101a is first manufactured, and then the porous protective layer 91 is formed on the element main body 101a to manufacture the sensor element 101.

A method of manufacturing the element body 101a will be described. First, six unfired ceramic green sheets are prepared. Then, corresponding to each of the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6, respectively. Print patterns such as electrodes, insulating layers, and heaters on the ceramic green sheet. Next, six ceramic green sheets having various patterns formed in this way are laminated to form a laminated body. The laminate is cut into pieces of the size of the element body 101a and fired at a predetermined firing temperature to obtain the device body 101a.

Next, a method of forming the porous protective layer 91 on the element body 101a will be described. In the present embodiment, the porous protective layers 91a to 91e are formed one by one by plasma spraying. FIG. 4 is an explanatory diagram of plasma spraying using the plasma gun 170. Note that FIG. 4 shows the formation of the porous protective layer 91a as an example, and the plasma gun 170 is shown in cross section. The plasma gun 170 includes an anode 176 and a cathode 178 that serve as electrodes for generating plasma, and a substantially cylindrical outer peripheral portion 172 that covers them. The outer peripheral portion 172 includes an insulating portion (insulator) 173 for insulating the anode 176. At the lower end of the outer peripheral portion 172, a powder supply portion 182 for supplying the powder sprayed material 184, which is a material for forming the porous protective layer 91, is formed. A water-cooled jacket 174 is provided between the outer peripheral portion 172 and the anode 176, whereby the anode 176 can be cooled. The anode 176 is formed in a cylindrical shape and has a nozzle 176a that opens downward. A plasma generating gas 180 is supplied from above between the anode 176 and the cathode 178.

When forming the porous protective layer 91a, a voltage is applied between the anode 176 and the cathode 178 of the plasma gun 170, and arc discharge is performed in the presence of the supplied plasma generation gas 180 to generate plasma. The gas 180 is brought into a high temperature plasma state. The gas in the plasma state is ejected from the nozzle 176a as a high-temperature and high-speed plasma jet. On the other hand, the powder spraying material 184 is supplied together with the carrier gas from the powder supply unit 182. As a result, the powder sprayed material 184 is heated, melted and accelerated by plasma, collides with the surface (upper surface) of the device body 101a, and rapidly solidifies, thereby forming the porous protective layer 91a.

As the plasma generating gas 180, an inert gas such as argon gas can be used. The flow rate of argon gas is, for example, 40 to 50 L / min, and the supply pressure is, for example, 0.5 to 0.6 MPa. The voltage applied between the anode 176 and the cathode 178 is, for example, a DC voltage of 80 to 90 V, and the current is, for example, 300 to 400 A.

The powder sprayed material 184 is a powder that is a material for the porous protective layer 91 described above, and is an alumina powder in the present embodiment. The particle size of the powder sprayed material 184 is, for example, 1 μm to 50 μm, more preferably 20 μm to 30 μm. As the carrier gas used for supplying the powder spraying material 184, for example, the same argon gas as the plasma generating gas 180 can be used. The flow rate of the carrier gas is, for example, 3 to 5 L / min, and the supply pressure is, for example, 0.5 to 0.6 MPa.

When performing plasma spraying, the distance W between the nozzle 176a, which is the outlet of the plasma gas in the plasma gun 170, and the surface of the element body 101a, which forms the porous protective layer 91 (the upper surface of the element body 101a in FIG. 5), is 150 mm. It is preferably about 200 mm. In this embodiment, the distance W is 180 mm. Plasma spraying may be performed while appropriately moving the plasma gun 170 (moving in the left-right direction in FIG. 5) according to the area forming the porous protective layer 91, but even in that case, the distance W is kept within the above range. Is preferable. The time for plasma spraying may be appropriately determined according to the film thickness and area of the porous protective layer 91 to be formed. When the porous protective layer 91 is formed on a part of the surface of the element body 101a (the region from the front end to the rear to the distance L) as in the porous protective layer 91a to the porous protective layer 91d, the porous protective layer 91 is porous. The region that does not form the quality protection layer 91 may be covered with a mask.

The porous protective layers 91b to 91e are also formed one by one in the same manner except that the surfaces formed on the element main body 101a are different. Plasma spraying is performed, for example, in an atmosphere or an atmosphere at room temperature. Two or more of the porous protective layers 91a to 91e may be formed at the same time. As described above, the porous protective layers 91a to 91e are formed on the upper, lower, left and right surfaces and the front end surface of the element main body 101a, respectively, to form the porous protective layer 91.

Subsequently, the porosity of the region of the surface of the porous protective layer 91a facing the outer pump electrode 23 to the outer pump electrode 23 is adjusted to form the gas flow obstruction portion 92, and the sensor element 101 is formed. To get. In forming the gas flow blocking portion 92 on the porous protective layer 91a, in the present embodiment, the porosity is adjusted by impregnating the porous protective layer 91a with highly fluid alumina paste and then heat-treating it. , The gas flow obstruction section 92 is formed.

The pore ratios of the first gas flow path P1 and the gas flow obstruction section 92 are values derived as follows using an image (SEM image) obtained by observing with a scanning electron microscope (SEM). And. First, the porous protective layer 91e is cut along the thickness direction of the measurement target so that the cross section of the measurement target (for example, the first gas flow obstruction portion) is the observation surface, and the cut surface is filled with resin and polished. Use as an observation sample. Subsequently, an SEM image to be measured is obtained by photographing the observation surface of the observation sample with an SEM photograph (secondary electron image, acceleration voltage 5 kV, magnification 7500 times). Next, by performing image analysis on the obtained image, a threshold value is determined by a discriminant analysis method (binarization of Otsu) from the luminance distribution of the luminance data of the pixels in the image. Then, based on the determined threshold value, each pixel in the image is binarized into an object portion and a pore portion, and the area of the object portion and the area of the pore portion are calculated. Then, the ratio of the area of the pore portion to the total area (total area of the object portion and the pore portion) is derived as the porosity (unit:%).

The control device 90 is a microprocessor including a CPU 93, a memory 94, and the like. The control device 90 includes an electromotive force V0 detected by the main pump control oxygen partial pressure detection sensor cell 80, an electromotive force V1 detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81, and a measurement pump control oxygen content. Electromotive force V2 detected by the pressure detection sensor cell 82, electromotive force Vref detected by the sensor cell 83, pump current Ip0 detected by the main pump cell 21, pump current Ip1 detected by the auxiliary pump cell 50, and for measurement. The pump current Ip2 detected in the pump cell 41 is input. Further, the control device 90 outputs a control signal to the variable power supplies 24, 46, 52 to control the main pump cell 21, the measurement pump cell 41, and the auxiliary pump cell 50.

The control device 90 sets the pump voltage Vp0 of the variable power supply 24 so that the electromotive force V0 becomes the target value (referred to as the target value V0 *) (that is, the oxygen concentration in the first internal space 20 becomes the target concentration). Feedback control. Therefore, the pump current Ip0 is the oxygen concentration of the gas to be measured, and thus the air-fuel ratio (A / F) of the gas to be measured and the excess air ratio λ (= the amount of air supplied to the internal combustion engine / the theoretically minimum amount of air). It changes accordingly.

Further, the control device 90 has a predetermined low so that the electromotive force V1 becomes a constant value (referred to as a target value V1 *) (that is, the oxygen concentration in the second internal space 40 does not substantially affect the measurement of NOx). The voltage Vp1 of the variable power supply 52 is feedback-controlled (so that the oxygen concentration becomes). At the same time, the control device 90 sets the target value V0 * of the electromotive force V0 based on the pump current Ip1 so that the pump current Ip1 flowing by the voltage Vp1 becomes a constant value (referred to as the target value Ip1 *) (feedback control). )do. As a result, the gradient of the oxygen partial pressure in the gas to be measured introduced from the third diffusion rate-determining unit 30 into the second internal space 40 is always constant. Further, the partial pressure of oxygen in the atmosphere in the second internal space 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx. The target value V0 * is set to a value such that the oxygen concentration of the first internal space 20 is higher than 0% and the oxygen concentration is low.

Further, the control device 90 uses the variable power supply 46 so that the electromotive force V2 becomes a constant value (referred to as a target value V2 *) (that is, the oxygen concentration in the third internal space 61 becomes a predetermined low concentration). The voltage Vp2 of the above is feedback-controlled. As a result, oxygen is pumped out from the third internal space 61 so that the oxygen generated by the reduction of NOx in the gas to be measured in the third internal space 61 becomes substantially zero. Then, the control device 90 acquires the pump current Ip2 as a detected value according to the oxygen generated in the third internal space 61 derived from the specific oxide gas (NOx in this case), and is based on this pump current Ip2. The NOx concentration in the gas to be measured is calculated. In this way, oxygen derived from the specific gas in the gas to be measured introduced into the sensor element 101 is pumped out, and the specific gas concentration is based on the amount of oxygen pumped out (based on the pump current Ip2 in this embodiment). The method of detecting the above is called the limit current method.

In the memory 94, a relational expression (for example, an expression of a linear function), a map, or the like is stored as a correspondence relationship between the pump current Ip2 and the NOx concentration. Such a relational expression or map can be obtained experimentally in advance.

An example of using the gas sensor 100 configured in this way will be described below. The CPU 93 of the control device 90 is in a state of controlling the pump cells 21, 41, 50 described above and acquiring the voltages V0, V1, V2, Vref from the sensor cells 80 to 83 described above. In this state, when the gas to be measured is introduced from the gas introduction port 10, the gas to be measured passes through the first diffusion rate-determining section 11, the buffer space 12, and the second diffusion rate-determining section 13, and the first internal space 20 To reach. Next, the oxygen concentration of the gas to be measured in the first internal space 20 and the second internal space 40 is adjusted by the main pump cell 21 and the auxiliary pump cell 50, and the adjusted gas to be measured reaches the third internal space 61. do. Then, the CPU 93 detects the NOx concentration in the gas to be measured based on the acquired pump current Ip2 and the correspondence stored in the memory 94.

In this way, when the CPU 93 detects the NOx concentration using the sensor element 101, in the present embodiment, the porous protective layer 91a is provided with the gas flow blocking portion 92. Therefore, the NOx concentration in the gas to be measured can be accurately measured regardless of the atmosphere of the gas to be measured. The reason for this is considered as follows. The outer pump electrode 23 containing Pt tends to reduce NOx contained in the gas to be measured when the atmosphere of the gas to be measured becomes stoichiometric. When the gas to be measured whose NOx concentration has decreased due to the reduction reaches the third internal space 61, it is considered that the oxygen generated from NOx decreases and the measurement accuracy of the NOx concentration decreases. The gas flow obstruction portion 92 is formed on the surface of the porous protective layer 91a from a position facing the outer pump electrode 23 to the electrode to be measured gas. Since the gas flow obstruction unit 92 prevents the gas to be measured from reaching the outer pump electrode 23, NOx in the gas to be measured is suppressed from being reduced by the outer pump electrode 23. Therefore, the NOx concentration can be accurately measured regardless of the atmosphere of the gas to be measured.

Here, the correspondence between the constituent elements of the present embodiment and the constituent elements of the present invention will be clarified. Six layers of the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6 of the present embodiment are laminated in this order. The laminate corresponds to the element body of the present invention, the second solid electrolyte layer 6 corresponds to the solid electrolyte layer, the first internal space 20 corresponds to the oxygen concentration adjusting chamber, and the outer pump electrode 23 corresponds to the gas to be measured. Corresponds to the electrode, the main pump cell 21 corresponds to the adjustment pump cell, the third internal space 61 corresponds to the measurement chamber, the measurement electrode 44 corresponds to the measurement electrode, the reference electrode 42 corresponds to the reference electrode, and is porous. The quality protection layer 91 corresponds to the porous protection layer, and the gas flow obstruction section 92 corresponds to the gas flow obstruction section. Further, the CPU 93 and the variable power supply 24 correspond to the adjustment pump cell control unit, the CPU 93 corresponds to the oxide gas concentration detection unit, and the measurement pump control oxygen partial pressure detection sensor cell 82 corresponds to the measurement voltage detection unit. The pump current Ip2 corresponds to the detected value. The first gas distribution route P1 corresponds to the first gas distribution route, and the second gas distribution route P2 corresponds to the second gas distribution route.

According to the sensor element 101 of the present embodiment described above, the gas flow obstructing portion 92 is provided between the surface of the porous protective layer 91a and the position facing the outer pump electrode 23 and the electrode to be measured. Therefore, the gas flow obstructing unit 92 prevents the gas to be measured from reaching the outer pump electrode 23, and thus suppresses the reduction of NOx in the gas to be measured by the outer pump electrode 23. Therefore, the NOx concentration can be accurately measured regardless of the atmosphere of the gas to be measured.

Further, the ratio of the porosity of the first gas flow path P1 to the porosity of the gas flow obstruction unit 92 may be 1.03 or more and 3.09 or less. In this way, the NOx concentration can be measured more accurately regardless of the atmosphere of the gas to be measured. In this case, the porosity of the gas flow blocking portion 92 may be 9.8% or more and 25.4% or less, and the porosity of the first gas flow path P1 may be 20.0% or more and 30.3% or less.

Further, the ratio of the porosity of the first gas flow path P1 to the porosity of the gas flow obstruction unit 92 may be 1.96 or more and 3.09 or less. In this way, the NOx concentration can be measured sufficiently accurately regardless of the atmosphere of the gas to be measured. In this case, the porosity of the gas flow blocking portion 92 may be 9.8% or more and 15.0% or less, and the porosity of the first gas flow path P1 may be 20.0% or more and 30.3% or less.

Furthermore, since the exhaust gas from the spark-ignition internal combustion engine tends to have a relatively stoichiometric atmosphere, it is highly significant to use the sensor element 101.

Since the outer pump electrode 23 containing Pt easily reduces NOx in a stoichiometric atmosphere, it is highly significant to use the sensor element 101 for measuring the NOx concentration.

Further, the gas sensor 100 includes a sensor element 101. Therefore, the gas sensor 100 can accurately measure the same effect as the sensor element 101, for example, the NOx concentration regardless of the atmosphere of the gas to be measured.

It goes without saying that the present invention is not limited to the above-described embodiment, and can be implemented in various aspects as long as it belongs to the technical scope of the present invention.

For example, in the above-described embodiment, the gas sensor 100 detects the NOx concentration as a specific oxide gas concentration, but the present invention is not limited to this, and other oxide concentrations may be used as the specific oxide gas concentration. When measuring a specific oxide gas concentration, oxygen is generated when the specific oxide gas itself is reduced in the third internal space 61 as in the above-described embodiment, so that the CPU 93 responds to this oxygen. It is possible to detect a specific oxide gas concentration by acquiring the detected value.

In the above-described embodiment, the case where the present invention is applied to a gasoline-fueled internal combustion engine as a spark-ignition internal combustion engine is illustrated, but a natural gas-fueled internal combustion engine or an ethanol-added gasoline-fueled internal combustion engine is illustrated. The present invention may be applied to an engine.

In the above-described embodiment, the element body of the sensor element 101 is a laminate having a plurality of solid electrolyte layers (layers 1 to 6), but the present invention is not limited to this. The element main body of the sensor element 101 may include at least one oxygen ion conductive solid electrolyte layer, and may be provided with a gas flow section to be measured inside. For example, in FIG. 1, the layers 1 to 5 other than the second solid electrolyte layer 6 may be a structural layer made of a material other than the solid electrolyte layer (for example, a layer made of alumina). In this case, each electrode of the sensor element 101 may be arranged on the second solid electrolyte layer 6. For example, the measurement electrode 44 of FIG. 1 may be arranged on the lower surface of the second solid electrolyte layer 6. Further, the reference gas introduction space 43 is provided in the spacer layer 5 instead of the first solid electrolyte layer 4, and the atmosphere introduction layer 48 is provided between the first solid electrolyte layer 4 and the third substrate layer 3 instead of being provided in the second solid. It may be provided between the electrolyte layer 6 and the spacer layer 5, and the reference electrode 42 may be provided behind the third internal space 61 and on the lower surface of the second solid electrolyte layer 6.

In the above-described embodiment, the control device 90 sets (feedback control) the target value V0 * of the electromotive force V0 based on the pump current Ip1 so that the pump current Ip1 becomes the target value Ip1 *, and the electromotive force V0 is set. Although the pump voltage Vp0 is feedback-controlled so as to reach the target value V0 *, other control may be performed. For example, the control device 90 may feedback control the pump voltage Vp0 based on the pump current Ip1 so that the pump current Ip1 becomes the target value Ip1 *. That is, the control device 90 directly controls the pump voltage Vp0 based on the pump current Ip1 without acquiring the electromotive force V0 from the main pump control oxygen partial pressure detection sensor cell 80 and setting the target value V0 *. (By extension, the pump current Ip0 may be controlled).

In the above-described embodiment, the porous protective layer 91a is impregnated with the alumina paste and heat-treated to adjust the porosity and form the gas flow blocking portion 92, but the present invention is not limited to this. For example, the gas flow obstructing portion 92 may be formed in the porous protective layer 91a by changing the conditions of plasma spraying and adjusting the porosity of the porous protective layer 91e and the porous protective layer 91a.

In the above-described embodiment, the sensor element 101 of the gas sensor 100 forms the gas flow obstruction portion 92 in the region of the surface of the porous protective layer 91 from the position facing the outer pump electrode 23 to the outer pump electrode. However, it is not limited to this. For example, as in the sensor element 201 of FIG. 6, the gas flow obstructing portion 92 may be formed in a region of the surface of the porous protective layer 91e from a position facing the gas introduction port 10 to the outer pump electrode 23. In this way, the gas flow obstructing unit 92 prevents the gas to be measured, which has been reduced by the outer pump electrode 23 and whose NOx concentration has decreased, from reaching the gas introduction port 10. Therefore, the concentration of a specific oxide gas in the gas to be measured can be accurately measured regardless of the atmosphere of the gas to be measured.

In the above-described embodiment, the sensor element 101 of the gas sensor 100 includes a first internal space 20, a second internal space 40, and a third internal space 61, but is not limited thereto. For example, as in the sensor element 301 of FIG. 7, the third internal space 61 may not be provided. In the modified example sensor element 301 shown in FIG. 7, a gas introduction port 10 and a first diffusion rate-determining portion 11 are provided between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The buffer space 12, the second diffusion rate-determining section 13, the first internal space 20, the third diffusion rate-determining section 30, and the second internal space 40 are formed adjacent to each other in this order. .. Further, the measurement electrode 44 is arranged on the upper surface of the first solid electrolyte layer 4 in the second internal space 40. The measurement electrode 44 is covered with a fourth diffusion rate-determining portion 45. The fourth diffusion rate-determining unit 45 is a film made of a ceramic porous body such as _{alumina (Al 2} O _3). The fourth diffusion rate-determining unit 45 plays a role of limiting the amount of NOx flowing into the measurement electrode 44, similarly to the fourth diffusion rate-determining unit 60 of the above-described embodiment. The fourth diffusion rate-determining unit 45 also functions as a protective film for the measurement electrode 44. The ceiling electrode portion 51a of the auxiliary pump electrode 51 is formed up to directly above the measurement electrode 44. Even with the sensor element 301 having such a configuration, the NOx concentration can be detected based on, for example, the pump current Ip2, as in the above-described embodiment. In this case, the periphery of the measurement electrode 44 functions as a measurement chamber.

Hereinafter, an example in which the sensor element is specifically manufactured will be described as an example. Experimental Examples 1 to 4, 6 to 8, 11 and 12 correspond to Examples of the present invention, and Experimental Examples 5, 9, 10 and 13 to 19 correspond to Comparative Examples. The present invention is not limited to the following examples.

[Experimental Example 1]
-Manufacture of sensor element As Experimental Example 1, a plurality of sensor elements 101 were manufactured according to the method for manufacturing the sensor element 101 of the above-described embodiment. First, six ceramic green sheets were prepared by mixing zirconia particles to which 4 mol% of the stabilizer yttria was added, an organic binder, and an organic solvent and molding them by tape molding. A plurality of sheet holes and necessary through holes used for positioning during printing and laminating are formed in advance on this green sheet. In addition, each green sheet was printed with a pattern of conductive paste for forming each electrode. Then, six green sheets were laminated in a predetermined order and crimped by applying predetermined temperature and pressure conditions. An unfired laminate having the size of the element body 101a was cut out from the pressure-bonded body thus obtained. Then, the cut out unfired laminate was fired to obtain an element body 101a. Subsequently, the porous protective layers 91a, 91b, 91c, 91d, 91e were formed on the surface of the element body 101a in this order. The conditions for plasma spraying to form the porous protective layer 91 were as follows. As the plasma generating gas 180, a mixture of argon gas (flow rate 50 L / min, supply pressure 0.5 MPa) and hydrogen (flow rate 10 L / min, supply pressure 0.5 MPa) was used. The voltage applied between the anode 176 and the cathode 178 was a DC voltage of 70 V. The current was 500A. As the powder spraying material 184, alumina powder having an average particle size of 30 μm was used. The carrier gas used for supplying the powder sprayed material 184 was argon gas (flow rate 4 L / min, supply pressure 0.5 MPa). The distance W was set to 150 mm. The distance L was set to 10 mm. Moreover, plasma spraying was carried out in the atmosphere and the atmosphere at room temperature. The direction of thermal spraying of the plasma gun 170 (direction of the nozzle 176a) was perpendicular to the formation surface of the porous protective layer 91 in the sensor element 101. When the thicknesses of the formed porous protective layers 91a to 91e were measured with a micrometer, they were all 100 μm. The porosity of the formed porous protective layers 91a to 91e was 30.3%. Subsequently, the gas flow obstructing portion 92 was formed to obtain the sensor element 101. In the sensor element 101 of Experimental Example 1, the entire porous protective layer 91a is impregnated with alumina paste so that the porosity of the gas flow blocking portion 92 is 9.8%, and then heat treatment is performed at 200 ° C. for 30 minutes. The porosity was adjusted to form the gas flow obstruction portion 92.

-Measurement of Porosity The sensor element 101 of Experimental Example 1 was prepared, samples were prepared by mechanically polishing the outermost surfaces and cross sections of the porous protective layers 91a and 91e of the sensor element 101, and SEM observation was performed to determine the porosity. It was measured. An image with a magnification of 10,000 times was used for measuring the porosity. The results are shown in Table 1. The porosity of the first gas flow path P1 is referred to as "porosity 1", and the porosity of the region above the outer pump electrode 23 of the porous protective layer 91a is referred to as "porosity 2."

[Experimental Examples 2 to 19]
In Experimental Examples 2 to 19, the sensor element 101 was used in the same manner as in Experimental Example 1 except that the porosity 1, the porosity 2, and the porosity 1 / porosity 2 were variously changed as shown in Table 1. Created. In all of the sensor elements 101 of Experimental Examples 2 to 19, the thickness of the porous protective layers 91a to 91e was 100 μm. In the experimental example in which the porosity was 1 = the porosity was 2, the porous protective layers 91a to 91e were formed by setting the plasma spraying conditions so that the porosity was 1. For Experimental Examples 1 to 4, 6 to 8, 11 and 12, plasma spraying conditions were first set so that the porosity was 1, and the porous protective layers 91a to 91e were formed, and then the porous protective layer 91a was formed. By impregnating with alumina paste and heat-treating, the upper region of the outer pump electrode 23 in the porous protective layer 91a was made to have a porosity of 2. The porosity 2 at this time corresponds to the porosity of the gas flow blocking unit 92. In the experimental example of porosity 1 <porosity 2, the plasma spraying conditions were first set so that the porosity was 2, and the porous protective layers 91a to 91e were formed, and then alumina was formed on the porous protective layer 91e. The paste was impregnated and heat-treated so that the first gas flow path P1 had a porosity of 1.

[Evaluation of measurement accuracy]
The sensor element 101 of Experimental Example 1 was connected to the above-mentioned control device 90 and variable power supplies 24, 46, 52, and the sensor element 101 was driven by the control device 90 in the same manner as in the above-described embodiment. Then, the pump current Ip2 was measured when the A / F of the gas to be measured before being introduced into the gas introduction port 10 of the sensor element 101 was variously changed. A model gas was used as the gas to be measured. As the model gas, nitrogen was used as the base gas, and 500 ppm of NO was used as the specific oxide gas component, and the water concentration was set to 3% by volume. Then, by using ethylene gas (C ₂ H ₄ ) as the fuel gas and changing the ethylene gas concentration and the oxygen concentration in the model gas in various ways, the A / F of the model gas was changed in various ways. The temperature of the model gas was 250 ° C., and the gas was circulated in a pipe having a diameter of 20 mm at a flow rate of 200 L / min. The pump current Ip2 was measured after the pump current Ip2 became sufficiently stable after the model gas flow was started. In addition, 11 types of model gases having different A / Fs were used as the gas to be measured, and the pump current Ip2 corresponding to each A / F was measured. A / F was measured using MEXA-730λ manufactured by HORIBA. Then, the pump current Ip2 when the A / F of the gas to be measured is 15.435 (λ is 1.05) is set to a value of 100, and the measured pump current Ip2 is relativized (referred to as Ip2 relative sensitivity). Was derived. For Experimental Examples 2 to 19, the Ip2 relative sensitivity was derived in the same manner. Table 1 shows the Ip2 relative sensitivities when the Ip2 relative sensitivities were the lowest (A / F = 14.553, λ = 0.99) for each of Experimental Examples 1 to 19. Further, FIG. 5 is a graph showing the relationship between A / F and Ip2 relative sensitivity for Experimental Example 1, Experimental Example 5, and Experimental Example 19. In FIG. 8, the excess air ratio λ (= (A / F) /14.7) converted from A / F is also shown in parentheses. As the Ip2 relative sensitivity changes from 100, it means that the measurement accuracy of the NOx concentration decreases.

The sensor element 101 of Experimental Example 5 is a sensor element having a porosity of 1 and 2 of 29.6% and 30.3%, respectively, and a porosity of 1 / a porosity of 2 of 0.98. As can be seen from Tables 1 and 8, in Experimental Example 5, when the atmosphere of the gas to be measured was in the vicinity of stoichiometric, the Ip2 relative sensitivity decreased by 2.0%, and the measurement accuracy of the NOx concentration decreased. The sensor element 101 of Experimental Example 19 is a sensor element having a porosity of 1 and 2 of 14.9% and 29.8%, respectively, and a porosity of 1 / a porosity of 2 of 0.50. As can be seen from Table 1 and FIG. 8, in Experimental Example 19, when the atmosphere of the gas to be measured is in the vicinity of stoichiometric, the Ip2 relative sensitivity is reduced by 15.0%, and the measurement accuracy of NOx concentration is higher than that in Experimental Example 5. It was declining. The sensor element 101 of Experimental Example 1 is a sensor element having a porosity of 1 and 2 of 30.3% and 9.8%, respectively, and a porosity of 1 / a porosity of 2 of 3.09. As can be seen from Table 1 and FIG. 8, in Experimental Example 1, even when the atmosphere of the gas to be measured is in the vicinity of stoichiometric, the Ip2 relative sensitivity is reduced by only 1.0%, and the measurement accuracy of NOx concentration is reduced. Was able to be suppressed. Further, as can be seen from Table 1, the sensor element 101 of Experimental Examples 5, 9, 10, 13 to 19 in which the porosity 1 / porosity 2 is 1.01 or less is used when the atmosphere of the gas to be measured is in the vicinity of stoichiometric. The relative sensitivity of Ip2 was lowered by 2.0% to 15.0%, and the relative sensitivity of Ip2 was lowered to the same level as that of Experimental Example 5 or higher than that of Experimental Example 5, and the measurement accuracy of NOx concentration was lowered. Further, as can be seen from Table 1, in Experimental Examples 1 to 4, 6 to 8, 11 and 12 in which the porosity 1 / porosity 2 is 1.03 or more and 3.09 or less, the atmosphere of the gas to be measured is in the vicinity of stoichiometric. Even in a certain case, the decrease in the relative sensitivity of Ip2 was 1.0% to 1.9%, and the decrease in the measurement accuracy of the NOx concentration could be suppressed as compared with Experimental Example 5. Of these, Experimental Examples 1, 2, 6 and 11 having a porosity 1 / porosity 2 of 1.96 or more and 3.09 or less have Ip2 relative sensitivities even when the atmosphere of the gas to be measured is near stoichiometricity. The decrease in NOx concentration was 1.0% to 1.5%, and the decrease in measurement accuracy of NOx concentration could be further suppressed.

1 1st substrate layer, 2 2nd substrate layer, 3 3rd substrate layer, 4 1st solid electrolyte layer, 5 spacer layer, 6 2nd solid electrolyte layer, 10 gas inlet, 11 1st diffusion rate controlling part, 12 buffer Space, 13 2nd diffusion rate control section, 20 1st internal space, 21 main pump cell, 22 inner pump electrode, 22a ceiling electrode section, 22b bottom electrode section, 23 outer pump electrode, 24 variable power supply, 30 3rd diffusion rate control section , 40 2nd internal space, 41 measurement pump cell, 42 reference electrode, 43 reference gas introduction space, 44 measurement electrode, 45 4th diffusion rate control part, 46 variable power supply, 48 atmosphere introduction layer, 50 auxiliary pump cell, 51 auxiliary pump Electrode, 51a Ceiling electrode part, 51b Bottom electrode part, 52 Variable power supply, 60 4th diffusion rate control part, 70 Heater part, 71 Heater connector electrode, 72 Heater, 73 Through hole, 74 Heater insulation layer, 75 Pressure dissipation hole, 80 Oxygen partial pressure detection sensor cell for main pump control, 81 Oxygen partial pressure detection sensor cell for auxiliary pump control, 82 Oxygen partial pressure detection sensor cell for measurement pump control, 83 sensor cell, 90 control device, 91, 91a to 91e Porous protective layer, 92 Gas flow obstruction part, 93 CPU, 94 memory, 100, 200, 300 gas sensor, 101, 201, 301 sensor element, 101a, 201a, 301a element body, 170 plasma gun, 172 outer periphery, 173 insulation part, 174 water-cooled jacket 176 anode, 176a nozzle, 178 cathode, 180 plasma generating gas, 182 powder supply section, 184 powder spray material, P1 first gas flow path, P2 second gas flow path.

Claims (7)

A sensor element for detecting a specific oxide gas concentration in the gas to be measured.
An element body having an oxygen ion conductive solid electrolyte layer and having a gas measurement unit for introducing and circulating the gas to be measured inside.
An adjustment pump cell having a gas-measured gas-side electrode containing Pt in a portion exposed to the exhaust gas on the outside of the element body, and adjusting the oxygen concentration in the oxygen concentration adjustment chamber in the gas flow-in part to be measured. When,
A measuring electrode arranged in a measuring chamber provided on the downstream side of the oxygen concentration adjusting chamber in the gas flow section to be measured, and a measuring electrode.
Of the outer surface of the element body, a porous protective layer that covers the inlet of the gas flow section to be measured and the tip including the electrode on the measurement gas side, and
The porous protective layer is a sensor element provided with the above.
A first gas flow path from a position facing the inlet on the surface of the porous protective layer to the inlet at the shortest distance.
A second gas flow path from a position on the surface of the porous protective layer facing the gas-side electrode to be measured, through the gas-side electrode to be measured, and to the front of the inlet at the shortest distance.
A gas flow obstruction portion provided in a region that blocks the second gas flow path and having a smaller porosity than the first gas flow path.
With
The sensor element in which the ratio of the porosity of the first gas flow path to the porosity of the gas flow obstruction portion is 1.03 or more. The region that blocks the second gas flow path is at least a region on the surface of the porous protective layer from a position facing the gas-side electrode to be measured to the gas-side electrode to be measured.
The sensor element according to claim 1. The ratio of the porosity of the first gas flow path to the porosity of the gas flow obstruction portion is 1.03 or more and 3.09 or less.
The sensor element according to claim 1 or 2. The ratio of the porosity of the first gas flow path to the porosity of the gas flow obstruction portion is 1.96 or more and 3.09 or less.
The sensor element according to any one of claims 1 to 3. The gas to be measured is an exhaust gas from a spark-ignition internal combustion engine.
The sensor element according to any one of claims 1 to 4. The oxide concentration is the NOx concentration.
The sensor element according to any one of claims 1 to 5. The sensor element according to any one of claims 1 to 6, and the sensor element.
An adjustment pump cell control unit that operates the adjustment pump cell so that the oxygen concentration in the oxygen concentration adjustment chamber becomes a target concentration.
A measurement voltage detection unit that detects a measurement voltage between the reference electrode and the measurement electrode,
Based on the measurement voltage, a detection value corresponding to oxygen generated in the measurement chamber derived from the oxide gas is acquired, and the oxide gas concentration in the exhaust gas is detected based on the detection value. Specific gas concentration detector and
Gas sensor equipped with.

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