JP4865572B2 - Gas sensor element, gas sensor and NOx sensor - Google Patents

Description

  The present invention relates to a gas sensor element, a gas sensor, and a NOx sensor, and more particularly to a gas sensor element, a gas sensor, and a NOx sensor that are suitable for detecting a gas concentration of a specific gas in an exhaust gas of an automobile.

  2. Description of the Related Art Conventionally, various gas sensor elements for detecting the concentration of a predetermined gas in a gas to be measured such as oxygen concentration and NOx concentration in an automobile exhaust gas are known.

For example, the following gas sensor elements are known as gas sensor elements for measuring the NOx concentration. That is, this gas sensor element has a first measurement chamber into which a gas to be measured is introduced through a first diffusion resistor, and a second measurement chamber that communicates with the first measurement chamber through a second diffusion resistor. Yes. Further, a first oxygen pump cell and an oxygen partial pressure detection cell are formed so as to face the first measurement chamber, and a second oxygen pump cell is formed so as to face the second measurement chamber. Each of the measurement chambers is formed so as to be sandwiched between two plate-shaped solid electrolyte bodies that are arranged with a gap therebetween. Then, the solid electrolyte body is formed with pump electrodes constituting the first and second oxygen pump cells, detection electrodes constituting the oxygen partial pressure detection cell, and the like so as to be located in the first and second measurement chambers. (For example, refer to Patent Document 1).
Japanese Patent Laid-Open No. 2004-151018

  However, the conventional gas sensor element described above has the following problems. In other words, it is preferable that the time (light-off time) required from the start of energization to startup until characteristics can be stably measured (light-off time) is as short as possible, but it is difficult to shorten the light-off time. There is a problem that there is. For example, in a NOx sensor that measures a small amount of NOx in a gas to be measured, the light-off time is about 300 seconds, and it is desired to shorten the light-off time. In addition, in the case of a gas sensor element that needs to measure a very small current even after being in a measurable state, there is a problem that high-precision measurement is hindered by noise such as variations in offset values.

  The present invention has been made to solve the above problems. The present invention can reduce the time (light-off time) required from the start to the state where measurement can be started compared to the conventional gas sensor element, and can improve the measurement accuracy compared to the conventional gas sensor element, An object is to provide a gas sensor and a NOx sensor.

In order to achieve the above object, a gas sensor element of the present invention includes a first solid electrolyte body, a pump cell having a pair of pump electrodes disposed on the first solid electrolyte body, and a second solid electrolyte body. And a detection cell having a pair of detection electrodes disposed on the second solid electrolyte body, and disposed between the first solid electrolyte body and the second solid electrolyte body, and the solid electrolyte. A first measurement chamber that introduces a gas to be measured through a diffusion rate-determining portion provided on the distal end side in the longitudinal direction of the body, and one of the pump electrodes and one of the detection electrodes are A gas sensor element disposed so as to face the first measurement chamber, and disposed such that the other of the pump electrodes and the other of the detection electrodes face the outside of the first measurement chamber ; And at least the one The rear end of the detection electrode is formed on the rear end or rear end side of the pump electrode of the one, and the tip or the rear end side of the pump electrode tip of the one of the one detection electrode are formed, between the rear end of at least the first measurement chamber to the rear end of the pump electrode of the one, the entire surface facing the first measurement chamber, said first and second An insulating layer is arranged to suppress oxygen adsorbed and desorbed from the solid electrolyte body from being mixed into the gas to be measured .

  As a result of detailed investigations by the present inventors, as one of the causes of prolonged light-off time and noise generation in the gas sensor element, the oxygen adsorbed on the solid electrolyte body is gradually separated into the measured gas. It turned out that there was a phenomenon of mixing. This phenomenon becomes prominent as a cause of prolonged light-off time and noise, especially when adsorbed and desorbed oxygen is mixed in the gas to be measured after pumping oxygen by the pump cell. is there. Specifically, the rear end of one detection electrode disposed in the measurement chamber of the detection cell is disposed on the rear end side of the rear end of one pump electrode disposed in the measurement chamber of the pump cell. Therefore, when oxygen is mixed in the gas to be measured after oxygen is pumped by the pump cell, the detection accuracy of the detection cell is lowered. Therefore, in the gas sensor element of the present invention, at least the rear end portion of the first measurement chamber and the rear end of one of the pump electrodes arranged in the first measurement chamber face the first measurement chamber. Insulating the entire surface to reduce the influence of adsorbed and desorbed oxygen. As a result, the light-off time can be shortened and the measurement accuracy can be improved as compared with the prior art. In addition, “insulating the entire surface facing the measurement chamber” not only covers the surface of the first solid electrolyte body and the surface of the second solid electrolyte body facing the measurement chamber with an insulating layer, When the side wall (when the first solid electrolyte body is the top wall and the second solid electrolyte body is the bottom wall) is also formed of a solid electrolyte body, the surface of the solid electrolyte body on the side wall must be covered with an insulating layer. is there. The insulating layer is preferably formed of an insulating sheet such as alumina, mullite, spinel, or an insulating paste.

  In the gas sensor element of the present invention, at least the surface (excluding the electrode portion) facing the first measurement chamber can be insulated. In the case of a gas sensor element in which the second measurement chamber is provided on the rear end side of the first measurement chamber via the second diffusion rate-limiting part, at least the entire surface facing the second measurement chamber is An insulated configuration can also be used. Furthermore, it can also be set as the structure which insulated the whole surface which faces a 2nd diffusion rate-limiting part at least. Note that “insulating the entire surface facing the second diffusion rate controlling portion” not only covers the surface of the first solid electrolyte body and the surface of the second solid electrolyte body facing the diffusion rate controlling portion with an insulating layer. For example, if the side wall of the measurement chamber (when the first solid electrolyte body is the top wall and the second solid electrolyte body is the bottom wall) is formed of a solid electrolyte body, the surface of the solid electrolyte body on the side wall is also an insulating layer. It is necessary to cover with. By adopting such a configuration, it is possible to further reduce the influence of adsorbed and desorbed oxygen, further shorten the light-off time, and improve the measurement accuracy.

  A gas sensor can be configured using the gas sensor element having the above configuration. Moreover, the NOx sensor which detects NOx density | concentration can be comprised using the gas sensor element of the said structure. According to such a gas sensor and NOx sensor, it is possible to shorten the light-off time and improve the measurement accuracy as compared with the conventional case.

  According to the present invention, it is possible to reduce the time (light-off time) required from the start to the state where measurement can be started compared to the conventional gas sensor, and to improve the measurement accuracy compared to the conventional gas sensor. Elements, gas sensors and NOx sensors can be provided.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, the configuration and operation principle of the gas sensor element for measuring the NOx concentration will be described with reference to FIG. FIG. 8 is a cross-sectional view showing a schematic configuration of a main part of the gas sensor element. As shown in FIG. 8, the gas sensor element includes a first oxygen pump cell 1, a second oxygen pump cell 2, and an oxygen partial pressure detection cell 3 that are stacked on each other. Further, a heater 4 for heating the gas sensor element to a predetermined operating temperature is provided adjacent to the gas sensor element.

  A first measurement chamber 5 is formed between the first oxygen pump cell 1 and the oxygen partial pressure detection cell 3. A gas to be measured is introduced into the first measurement chamber 5 via a first diffusion rate-determining portion 7 made of a porous body or pores. The first measurement chamber 5 communicates with the second measurement chamber 6 through a second diffusion rate-determining portion 8 made of a porous body or pores. The side wall portion of the first measurement chamber 5 is formed of an insulating ceramic made of alumina or the like.

  The first oxygen pump cell 1 includes a plate-like first solid electrolyte body 17 having oxygen ion conductivity such as zirconia, and a pair of pump electrodes 9 and 10 formed on the first solid electrolyte body 17. Yes. One pump electrode 10 is arranged so as to be located in the first measurement chamber 5, and the other pump electrode 9 is arranged so as to be located outside the first measurement chamber 5. In the first oxygen pump cell 1, oxygen ions generated by dissociating oxygen in the first measurement chamber 5 on the pump electrode 10 are pumped out to the pump electrode 9 side through the first solid electrolyte body 17. At this time, the current flowing through the first solid electrolyte body 17 is the first oxygen pump current Ip1.

  The second oxygen pump cell 2 includes a plate-like third solid electrolyte body 18 having oxygen ion conductivity such as zirconia, and a pair of pump electrodes 13 and 14 formed on the third solid electrolyte body 18. ing. The pump electrode 13 is disposed so as to be located in the second measurement chamber 6. The pump electrode 14 is disposed outside the second measurement chamber 6 and is exposed to an atmosphere having a stable oxygen concentration. Oxygen ions generated by dissociating NOx and the like in the second measurement chamber 6 on the pump electrode 13 are pumped to the pump electrode 14 side through the solid electrolyte body 18. At this time, the current flowing through the third solid electrolyte body 18 is the second oxygen pump current Ip2.

  The oxygen partial pressure detection cell 3 includes a plate-like second solid electrolyte body 19 having oxygen ion conductivity such as zirconia, and a pair of detection electrodes 11 and 12 formed on the second solid electrolyte body 19. Has been. The detection electrode 11 is disposed so as to be located in the first measurement chamber 5. By passing a constant controlled current Icp between the detection electrodes 11 and 12 through the oxygen partial pressure detection cell 3, the amount of oxygen ions pumped to the detection electrode 12 side through the oxygen partial pressure detection cell 3 becomes constant. . As a result, the detection electrode 12 is exposed to an atmosphere having a stable oxygen concentration. Therefore, the oxygen concentration in the first measurement chamber 5 can be detected based on the potential difference generated between the detection electrode 11 and the detection electrode 12.

  In FIG. 8, 20 is a first oxygen pump cell control means, 21 is an oxygen partial pressure detection cell control means, and 22 is a second oxygen pump cell control means. The oxygen partial pressure cell control means 21 is a concentration difference between the oxygen concentration in the first measurement chamber 5 appearing in the oxygen partial pressure detection cell 3 and the oxygen concentration pumped into the detection electrode 12 through the oxygen partial pressure detection cell 3. (Potential difference) is detected. The first oxygen pump cell control means 20 controls the voltage of the first oxygen pump cell so that the potential difference detected by the oxygen partial pressure cell control means 21 becomes a constant predetermined potential difference. The oxygen concentration is controlled as constant as possible. Further, the oxygen concentration of the gas to be measured can be known by measuring the first oxygen pump current Ip1.

  The second oxygen pump cell control means 22 detects the second oxygen pump current Ip2 flowing through the second oxygen pump cell 2 by applying a predetermined voltage as constant as possible to the second oxygen pump cell 2. The first oxygen pump current Ip1 can be extracted from the resistor 120, and the second oxygen pump current Ip2 can be extracted from the resistor 122.

  Next, the configuration of the gas sensor element 100 according to the embodiment of the present invention will be described with reference to FIG. In the gas sensor element 100 of FIG. 1, portions corresponding to those of the gas sensor element of FIG. 8 described above are denoted by the same reference numerals, and redundant description is omitted. As shown in FIG. 1, in the gas sensor element 100 of the present embodiment, at least the surfaces of the first solid electrolyte body 17 and the second solid electrolyte body 19 facing the inside of the first measurement chamber 5 (the pump electrode 10 and the detection electrode). The electrode layer 11 is excluded). As this insulating layer 60, for example, mullite, spinel, alumina or the like can be used.

  In the present embodiment, the surfaces (opposite surfaces) of the first solid electrolyte body 17 and the second solid electrolyte body 19 on the tip side of the first measurement chamber 5 (left side of the first solid electrolyte body 7 in FIG. 1), Further, the surface (inner side surface) of the first solid electrolyte body 17 on the rear end side (the right side of the second solid electrolyte body 8 in FIG. 1) from the first measurement chamber 5 is also covered with the insulating layer 60.

  FIG. 2 shows a gas sensor in which the gas sensor element 100 described above is incorporated. Specifically, an example of a NOx sensor 600 attached to an exhaust pipe of an internal combustion engine and used for measuring oxygen concentration and NOx concentration in exhaust gas. It is the whole sectional view shown.

  The metal shell 30 shown in FIG. 2 has a male thread portion 31 for attaching the NOx sensor to the exhaust pipe, and a hexagonal portion 32 to which an attachment tool is applied at the time of attachment. Further, the metal shell 30 is provided with a metal side step portion 33 protruding radially inward, and this metal side step portion 33 supports a metal holder 34 for holding the gas sensor element 100. . Inside the metal holder 34, a ceramic holder 35 and a talc 36 for arranging the gas sensor element 100 at a predetermined position are arranged in this order from the tip side.

  The talc 36 includes a first talc 37 disposed in the metal holder 34 and a second talc 38 disposed over the rear end of the metal holder 34. An alumina sleeve 39 is disposed on the rear end side of the second talc 38. The sleeve 39 is formed in a multi-stage cylindrical shape, and is provided with a shaft hole 391 along the axis, and the gas sensor element 100 is inserted through the shaft hole 391. The caulking portion 301 on the rear end side of the metal shell 30 is bent inward, and the sleeve 39 is pressed to the front end side of the metal shell 30 via the stainless steel ring member 40.

  Further, a metal protector 24 having a plurality of gas intake holes 241 is attached to the outer periphery of the metal shell 30 at the front end side thereof by covering the front end portion of the gas sensor element 100 protruding from the tip of the metal shell 30 by welding. . The protector 24 has a double structure, and has a bottomed cylindrical outer protector 41 having a uniform outer diameter on the outer side and an outer diameter of the rear end 421 on the inner side than the outer diameter of the front end 422. An inner protector 42 having a bottomed cylindrical shape that is formed to be larger is also arranged.

  On the other hand, the front end side of the outer cylinder 25 is inserted into the rear end side of the metal shell 30. The outer cylinder 25 has a distal end portion 251 whose diameter is enlarged on the distal end side fixed to the metal shell 30 by laser welding or the like. A separator 50 is disposed inside the rear end side of the outer cylinder 25, and a holding member 51 is interposed in a gap between the separator 50 and the outer cylinder 25. The holding member 51 is fixed to the outer cylinder 25 and the separator 50 by engaging a protruding portion 501 of the separator 50 described later and crimping the outer cylinder 25.

  The separator 50 is provided with through holes 502 for inserting the lead wires 111 to 116 of the gas sensor element 100 from the front end side to the rear end side (note that the lead wires 114 to 116 are not shown). ). A connection terminal 118 that connects the lead wires 111 to 116 and an external terminal of the gas sensor element 100 is accommodated in the through hole 502. Each of the lead wires 111 to 116 is connected to a connector (not shown) outside. Electric signals are input and output between the external devices such as the ECU and the lead wires 111 to 116 via this connector. Although not shown in detail, each lead wire 111 to 116 has a structure in which a conductive wire is covered with an insulating film made of resin.

  Further, a substantially cylindrical rubber cap 52 for closing the opening 252 on the rear end side of the outer cylinder 25 is disposed on the rear end side of the separator 50. The rubber cap 52 is fixed to the outer cylinder 25 by caulking the outer periphery of the outer cylinder 25 toward the radially inner side in a state where the rubber cap 52 is mounted in the rear end of the outer cylinder 25. A through hole 521 for inserting the lead wires 111 to 116 is also provided in the rubber cap 52 from the front end side to the rear end side.

  According to the gas sensor element 100 and the NOx sensor 600 configured as described above, at least the surfaces of the first solid electrolyte body 17 and the second solid electrolyte body 19 facing the first measurement chamber 5 of the gas sensor element 100 are: Since it is covered with the insulating layer 60, it is possible to reduce the time (light-off time) required from the start to the state where measurement can be started compared to the conventional case, and to improve the measurement accuracy compared to the conventional case. be able to.

  That is, as shown in FIG. 8, in the case of a gas sensor element that does not have the insulating layer 60 that covers the surfaces of the first solid electrolyte body 17 and the second solid electrolyte body 19 facing the first measurement chamber 5, Oxygen is adsorbed on the surfaces of the first solid electrolyte body 17 and the second solid electrolyte body 19 facing the measurement chamber 5, and the adsorbed oxygen is gradually released. As a result, the light-off time is prolonged. The graph of FIG. 3 shows the result of actually measuring the change in Ip2 current at start-up with the Ip2 current on the vertical axis and the time on the horizontal axis. As shown by the curve B in this graph, the insulating layer 60 is not provided. In the case of a gas sensor element, the time (light-off time) required until the characteristic (Ip2) can be stably measured after the start of energization is about 300 seconds. On the other hand, as shown by a curve A in FIG. 3, the gas sensor element 100 of this embodiment having the insulating layer 60 can reduce the influence of the above-described adsorption and desorption of oxygen, and the light-off time. Was reduced to about 100 seconds.

  FIG. 4 shows the result of measuring the variation in the offset value of the Ip2 current in the gas sensor element 100 of the present embodiment and in the gas sensor element that does not have the insulating layer 60. As shown in FIG. 4, in the gas sensor element 100 of the present embodiment having the insulating layer 60, the influence due to the adsorption and desorption of oxygen as described above can be reduced, and in the case of the gas sensor element not having the insulating layer 60. Compared to the above, variation in the offset value of the Ip2 current could be reduced. This makes it possible to perform highly accurate measurement with less noise and improve measurement accuracy. The Ip2 current offset value indicates the value of the Ip2 current that flows even when the NOx concentration is zero, as shown in the graph of FIG. 5 where the vertical axis represents the Ip2 current and the horizontal axis represents the NOx concentration. . The measurement of the offset value of the Ip2 current was performed 30 minutes after the gas sensor element was started.

  Next, another embodiment will be described. In the gas sensor element 100 shown in FIG. 1, the example in which at least the entire surfaces of the first solid electrolyte body 17 and the second solid electrolyte body 19 facing the inside of the first measurement chamber 5 are covered with the insulating layer 60 has been described. It is not always necessary to cover all of them, and some of them may be used. In this case, as in the gas sensor element 101 shown in FIG. 6, at least a portion in the first measurement chamber 5 from the rear end of the first measurement chamber 5 to the rear end of the electrode 10 constituting the first oxygen pump cell 1. As for, the surfaces of the first solid electrolyte body 17 and the second solid electrolyte body 19 facing the first measurement chamber 5 are preferably covered with the insulating layer 60. With such a configuration, the first solid electrolyte body 17 and the second solid electrolyte body 19 are adsorbed to the gas to be measured after the oxygen is pumped in the first oxygen pump cell 1 (the gas to be measured having a low oxygen concentration). It is possible to reduce the mixing of the separated oxygen. Therefore, it can prevent that the detection accuracy of a detection cell falls.

  Further, as in the gas sensor element 102 shown in FIG. 7, the first solid faced not only inside the first measurement chamber 5 but also inside the second measurement chamber 6 provided on the rear end side of the first measurement chamber 5. The surfaces of the electrolyte body 17 and the second solid electrolyte body 19 may be covered with an insulating layer 60. In the gas sensor element 102 of FIG. 7, the surface of the third solid electrolyte body 18 facing the inside of the second measurement chamber 6 is also covered with the insulating layer 60. With such a configuration, the influence of oxygen adsorbed and desorbed further than the gas sensor element 100 described above can be reduced, and the light-off time can be shortened and the measurement accuracy can be improved.

  In the above-described embodiment, the case where the present invention is applied to the gas sensor element and the NOx sensor for measuring the NOx concentration has been described. However, the present invention is not limited to such an embodiment, and other gases are measured. The same applies to gas sensor elements and gas sensors, for example, gas sensor elements and gas sensors that measure oxygen concentration. Moreover, in the said embodiment. The side wall portion of the first measurement chamber 5 is formed of an insulating ceramic made of alumina or the like. However, the first measurement chamber 5 is formed of a solid electrolyte body made of the same material as the first solid electrolyte body 17 and the second solid electrolyte body 19. You may do it. In this case, the side wall of the first measurement chamber 5 needs to be covered with the insulating layer 60.

The figure which shows the principal part cross-section schematic structure of the gas sensor element which concerns on one Embodiment of this invention. The figure which shows the cross-sectional schematic structure of the NOx sensor using the gas sensor element of FIG. The figure which shows the measurement result of the time change at the time of starting of Ip2 electric current. The figure which shows the measurement result of the dispersion | variation in the offset value of Ip2 electric current. The figure for demonstrating the offset value of Ip2 electric current. The figure which shows the principal part cross-section schematic structure of the gas sensor element which concerns on other embodiment. The figure which shows the principal part cross-sectional schematic structure of the gas sensor element which concerns on other embodiment. The figure for demonstrating the structure of a gas sensor element which measures NOx concentration, and an operation principle.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... 1st oxygen pump cell, 2 ... 2nd oxygen pump cell, 3 ... Oxygen partial pressure detection cell, 4 ... Heater, 5 ... 1st measurement chamber, 6 ... 2nd measurement chamber, 7 ... 1st 1 diffusion limiting part, 8 ... second diffusion limiting part, 9, 10 ... pump electrode, 11, 12 ... detection electrode, 13, 14 ... pump electrode, 17 ... first solid electrolyte body, 19: Second solid electrolyte body.

Claims (6)

A pump cell having a first solid electrolyte body and a pair of pump electrodes disposed on the first solid electrolyte body;
A detection cell having a second solid electrolyte body and a pair of detection electrodes disposed on the second solid electrolyte body;
A first gas to be measured is introduced via a diffusion rate controlling portion disposed between the first solid electrolyte body and the second solid electrolyte body and provided at the front end side in the longitudinal direction of the solid electrolyte body . And one of the pump electrodes and one of the detection electrodes are arranged so as to face the first measurement chamber , and the other of the pump electrodes and the A gas sensor element arranged so that the other of the detection electrodes faces the outside of the first measurement chamber ,
At least the rear end of the one detection electrode is formed on the rear end side of the rear end of the one pump electrode, and the front end of the first detection electrode is higher than the front end of the one pump electrode. It is formed on the rear end side,
At least the entire surface facing the first measurement chamber is adsorbed by the first and second solid electrolyte bodies between the rear end of the first measurement chamber and the rear end of the one pump electrode. A gas sensor element, wherein an insulating layer for suppressing the separated oxygen from being mixed into the gas to be measured is disposed . The gas sensor element according to claim 1,
An insulating layer for suppressing oxygen adsorbed and desorbed from the first and second solid electrolyte bodies from being mixed into the gas to be measured is disposed on at least the entire surface facing the first measurement chamber. A characteristic gas sensor element. The gas sensor element according to claim 1 or 2,
A second measurement chamber is provided on the rear end side of the first measurement chamber via a second diffusion rate-determining unit, and the first and second surfaces are at least on the entire surface facing the second measurement chamber . A gas sensor element characterized in that an insulating layer is arranged to suppress oxygen adsorbed and desorbed from the solid electrolyte body from being mixed into the gas to be measured . The gas sensor element according to claim 3,
An insulating layer is disposed on at least the entire surface facing the second diffusion rate-determining portion to suppress oxygen adsorbed and desorbed from the first and second solid electrolyte bodies from being mixed into the gas to be measured. A characteristic gas sensor element.   A gas sensor comprising the gas sensor element according to claim 1.   A NOx sensor comprising the gas sensor element according to any one of claims 1 to 4 and configured to detect a NOx concentration.

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