Patent Application
20250164436
The present application claims priority from Japanese application JP2023-198186, filed on Nov. 22, 2023, the content of which is hereby incorporated by reference into this application.
The present invention relates to a sensor element and a gas sensor.
A known gas sensor in the related art includes a sensor element configured to detect the concentration of a specific gas, such as NOx, in a measurement-object gas, such as exhaust gas from an internal combustion engine (see PTL 1, for example). The sensor element disclosed in PTL 1 includes an element body, an inner electrode, a terminal, and a lead. The element body includes an oxygen-ion-conductive solid electrolyte layer and has a columnar shape elongate in a longitudinal direction. The element body has a front end and a rear end that are opposite ends in the longitudinal direction, and a peripheral face that is a surface extending in the longitudinal direction. A part of the element body that is at the front end is allowed to be exposed to a measurement-object gas. The inner electrode is disposed inside the element body. The terminal is disposed at a part of the peripheral face of the element body that is near the rear end. The lead is disposed in such a manner as to connect the inner electrode and the terminal to each other. The lead includes an inner portion disposed inside the element body, and a peripheral-face conducting section disposed on the peripheral face and exposed from the element body.
In such a sensor element, some kind of gas outside the sensor element (element body) may enter the inside of the element body through the end of the peripheral-face conducting section side of the inner portion of the lead or through a gap between the inner portion and the element body, move toward the inner electrode along the inner portion or the gap between the inner portion and the element body, and reach the inner electrode. If such a gas reaches the inner electrode, the accuracy in the detection of the concentration of the specific gas may be deteriorated.
The main object of the sensor element and gas sensor according to the present invention is to reduce the likelihood of accuracy deterioration in the detection of the concentration of the specific gas.
In order to achieve the above main object, the sensor element and gas sensor according to the present invention employs the following configuration.
[1]A sensor element according to the present invention is a sensor element configured to detect a concentration of a specific gas in a measurement-object gas, the sensor element comprising: an element body including an oxygen-ion-conductive solid electrolyte layer and having a columnar shape elongate in a longitudinal direction, the element body having a front end and a rear end that are opposite ends in the longitudinal direction and further having a peripheral face that is a surface extending in the longitudinal direction, a part of the element body that is at the front end being allowed to be exposed to the measurement-object gas; an inner electrode disposed inside the element body; and a conducting section including an inner conducting section and an outer conducting section, the inner conducting section being disposed inside the element body and being electrically continuous with the inner electrode, the outer conducting section including a connector electrode disposed at a part of the peripheral face that is near the rear end, the outer conducting section further including a segment disposed on the peripheral face and/or a segment disposed on the peripheral face and exposed to an outside of the sensor element, the outer conducting section being electrically continuous with the inner conducting section, wherein letting a porosity be Rp [%] and a thickness be Dc [mm], a covering portion of the outer conducting section that covers the inner conducting section satisfies Rp/Dc≤145%/mm.
In the sensor element according to the present invention, letting porosity be Rp [%] and thickness be Dc [mm], a covering portion of the outer conducting section that covers the inner conducting section satisfies Rp/Dc≤145%/mm. Such a configuration reduces the passage of some kind of gas outside the sensor element (element body) through a part of the outer conducting section that covers the inner conducting section, reduces the entry of gas into the element body through the inner conducting section or the gap between the inner conducting section and the element body, and reduces the likelihood of the gas reaching the inner electrode. Consequently, the likelihood of accuracy deterioration in the detection of the concentration of the specific gas is reduced. The present inventors have demonstrated these facts through experiments, analyses, and the like.
[2] In the sensor element according to the present invention (the sensor element according to [1] above), the covering portion may satisfy Rp/Dc≤120%/mm. Such a configuration further reduces the passage of some kind of gas outside the sensor element (element body) through the part of the outer conducting section that covers the inner conducting section.
[3] In the sensor element according to the present invention (the sensor element according to [1] or [2] above), the covering portion may satisfy Rp/Dc≥1.5%/mm. Such a configuration increases adhesiveness between the covering portion and the element body. The present inventors have demonstrated these facts through experiments, analyses, and the like.
[4] In the sensor element according to the present invention (the sensor element according to [3] above), the covering portion may satisfy Rp/Dc≥4.5%/mm. Such a configuration further increases the adhesiveness between the covering portion and the element body.
[5] In the sensor element according to the present invention (the sensor element according to any one of [1] to [4] above), the porosity Rp may be 0.01% or higher and 9.0% or lower.
[6] In the sensor element according to the present invention (the sensor element according to any one of [1] to [5] above), the thickness Dc may be 0.001 mm or greater and 0.080 mm or less.
[7] In the sensor element according to the present invention (the sensor element according to any one of [1] to [6] above), the inner electrode may be a measurement electrode to be used in detecting the concentration of the specific gas.
[8] In the sensor element according to the present invention (the sensor element according to any one of [1] to [7] above), the element body may be a layered body obtained by stacking a plurality of layers including the solid electrolyte layer in a layered direction that is orthogonal to the longitudinal direction, the layered body may have a first face and a second face that are opposite end faces in the layered direction; and a third face and a fourth face that are opposite end faces in a direction orthogonal to the longitudinal direction and to the layered direction, the first to fourth faces each being the peripheral face, the inner conducting section may include an inner lead that is routed to the third face or the fourth face, and the covering portion may be a peripheral-face lead disposed on the third face or the fourth face and covering the inner lead.
[9] In the sensor element according to the present invention (the sensor element according to any one of [1] to [7] above), the element body may be a layered body obtained by stacking a plurality of layers including the solid electrolyte layer in a layered direction that is orthogonal to the longitudinal direction, the layered body may have a first face and a second face that are opposite end faces in the layered direction and are each the peripheral face, the element body may have a through-hole with an opening provided in a part of the peripheral face that is near the rear end, the peripheral face being the first face or the second face, the through-hole extending through one or more of the plurality of layers in the layered direction, the inner conducting section may include a through-hole conductor disposed in the through-hole, and the covering portion may be the connector electrode and covers the through-hole conductor.
[10] In the sensor element according to the present invention (the sensor element according to any one of [1], [2] and [5] to [7] above), the element body may be a layered body obtained by stacking a plurality of layers including the solid electrolyte layer in a layered direction that is orthogonal to the longitudinal direction, the layered body may have a first face and a second face that are opposite end faces in the layered direction and are each the peripheral face, the element body may have a through-hole with an opening provided in a part of the peripheral face that is near the rear end, the peripheral face being the first face or the second face, the through-hole extending through one or more of the plurality of layers in the layered direction, and the covering portion may include a part of a through-hole conductor that is exposed to the outside at the opening, the through-hole conductor is disposed in the through-hole, includes the part exposed to the outside and is connected to the connector electrode. In this case, the covering portion may be a part of the through-hole conductor or the entire through-hole conductor.
[11] The sensor element according to the present invention (the sensor element according to any one of [1] to [10] above) may be a sensor element to be included in a gas sensor, the gas sensor including the sensor element; a case having a cylindrical shape elongate in the longitudinal direction of the sensor element, the case having a second front end and a second rear end that are opposite ends in the longitudinal direction, the sensor element being disposed inside the case; and a sealing member sealing the second rear end of the case.
[12] A gas sensor according to the present invention includes the sensor element according to any one of [1] to [10]; a case having a cylindrical shape elongate in the longitudinal direction of the sensor element, the case having a second front end and a second rear end that are opposite ends in the longitudinal direction, the sensor element being disposed inside the case; and a sealing member sealing the second rear end of the case.
Since this gas sensor according to the present invention includes the above sensor element, the gas sensor produces the same advantageous effects as those produced by the above sensor element. For example, an advantageous effect is the reduction of the likelihood of some kind of gas outside the sensor element (element body) reaching the inner electrode, which in turn reduces the likelihood of accuracy deterioration in the detection of the concentration of the specific gas. Here, examples of some kind of gas outside the sensor element includes a gas that is generated from the sealing member (for example, a volatile organic gas generated from the rubber stopper serving as the sealing member).
Embodiments of the present invention will now be described with reference to the drawings.
As illustrated in
The protection cover 130 includes a bottomed cylindrical inner protection cover 131 covering the front end part of the element body 102, and a bottomed cylindrical outer protection cover 132 covering the inner protection cover 131. The inner protection cover 131 and the outer protection cover 132 each have a plurality of holes for allowing the measurement-object gas to flow into the protection cover 130. The space enclosed by the inner protection cover 131 serves as a sensor element chamber 133. The front end part of the element body 102 is positioned in the sensor element chamber 133.
The sensor assembly 140 includes an element sealing unit 141 that seals and secures the sensor element 101, a bolt 147 and an outer cylinder 148 that are attached to the element sealing unit 141, and the connector 150 that is in contact with and electrically connected to connector electrodes 75 disposed on surfaces (upper and lower surfaces) of the element body 102 of the sensor element 101 at a rear end part.
The element sealing unit 141 includes a cylindrical main metal fitting 142, and a cylindrical inner cylinder 143 welded coaxially to the main metal fitting 142. The element sealing unit 141 further includes ceramic supporters 144a to 144c, compacts 145a and 145b, and a metal ring 146 that are sealed in a through-hole provided on the inner side of the main metal fitting 142 and the inner cylinder 143. The sensor element 101 is located on the center axis of the element sealing unit 141 and extends through the element sealing unit 141 in the front-rear direction. The inner cylinder 143 has a diameter reduction section 143a for pressing the compact 145b toward the center axis of the inner cylinder 143, and a diameter reduction section 143b for pressing the ceramic supporters 144a to 144c and the compacts 145a and 145b frontward via the metal ring 146. The pressing forces generated by the diameter reduction sections 143a and 143b compress the compacts 145a and 145b between the sensor element 101 and the combination of the main metal fitting 142 and the inner cylinder 143. Thus, the compacts 145a and 145b seal between the sensor element chamber 133 in the protection cover 130 and a space 149 in the outer cylinder 148, and also secure the sensor element 101.
The bolt 147 is secured coaxially to the main metal fitting 142 and has a male threaded section around the outer peripheral surface thereof. The male threaded section of the bolt 147 is screwed into a securing member 191 welded to the pipe 190 and having a female threaded section around the inner peripheral surface thereof. Thus, the gas sensor 100 is secured to the pipe 190, with a part of the gas sensor 100 inclusive of the front end part of the element body 102 of the sensor element 101 and the protection cover 130 protruding into the pipe 190.
The outer cylinder 148 encloses the inner cylinder 143, the sensor element 101, and the connector 150. A plurality of lead wires 155 are connected to the connector 150 and are routed to the outside from the rear end of the outer cylinder 148. The lead wires 155 are electrically continuous with electrodes (to be described below) of the sensor element 101 via the connector 150. At the rear end of the outer cylinder 148, the gap between the outer cylinder 148 and the lead wires 155 is sealed by a rubber stopper 157. The outer cylinder 148 has swaged sections 148a and 148b that restrict the front-rear movement of the rubber stopper 157 relative to the outer cylinder 148. The space 149 in the outer cylinder 148 is filled with a reference gas. The rear end part of the element body 102 of the sensor element 101 is positioned in the space 149.
As illustrated in
At the front end part of the element body 102 and between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4 are provided a gas inlet 10, a first diffusion control section 11, a buffer space 12, a second diffusion control section 13, a first internal cavity (oxygen-concentration adjustment chamber) 20, a third diffusion control section 30, a second internal cavity (oxygen-concentration adjustment chamber) 40, a fourth diffusion control section 60, and a third internal cavity (measurement chamber) 61 that are next to one another and communicate with one another in that order.
The gas inlet 10, the buffer space 12, the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 are spaces provided inside the sensor element 101 by hollowing out the spacer layer 5, and each have an upper side defined by the lower surface of the second solid electrolyte layer 6, a lower side defined by the upper surface of the first solid electrolyte layer 4, and lateral sides defined by the side surfaces of the spacer layer 5.
The first diffusion control section 11, the second diffusion control section 13, and the third diffusion control section 30 are each provided as two horizontally elongate slits (i.e., openings that are elongate in the direction perpendicular to the plane of the drawing). The fourth diffusion control section 60 is provided as a single horizontally elongate slit (i.e., an opening that is elongate in the direction perpendicular to the plane of the drawing) providing a gap with respect to the lower surface of the second solid electrolyte layer 6. A section extending from the gas inlet 10 to the third internal cavity 61 inclusive is also referred to as a measurement-object gas flow section.
The element body 102 includes a reference-gas introduction portion 49 that allows the reference gas to flow from outside the element body 102 to a reference electrode 42 in the measurement of NOx concentration. The reference-gas introduction portion 49 has a reference-gas introduction space 43 and a reference-gas introduction layer 48. The reference-gas introduction space 43 is a space extending from the sixth face 102f (rear end face) of the element body 102 toward the fifth face 102e (front end face). The reference-gas introduction space 43 is provided at a position between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5 and has lateral sides defined by the side surfaces of the first solid electrolyte layer 4. The reference-gas introduction space 43 has an opening at the rear end face of the element body 102. This opening functions as an entrance 49a of the reference-gas introduction portion 49. The reference gas is to be introduced into the reference-gas introduction space 43 through the entrance 49a. The reference-gas introduction portion 49 introduces the reference gas to the reference electrode 42 while applying a predetermined diffusion resistance to the reference gas received through the entrance 49a. In the present embodiment, the reference gas is ambient air.
The reference-gas introduction layer 48 is disposed between the upper surface of the third substrate layer 3 and the lower surface of the first solid electrolyte layer 4. The reference-gas introduction layer 48 is a porous body composed of a ceramic material such as alumina. A part of the upper surface of the reference-gas introduction layer 48 is exposed in the reference-gas introduction space 43. The reference-gas introduction layer 48 is provided over the reference electrode 42. The reference-gas introduction layer 48 allows the reference gas to flow from the reference-gas introduction space 43 to the reference electrode 42.
The reference electrode 42 is interposed between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4 and is surrounded by the reference-gas introduction layer 48 connected to the reference-gas introduction space 43, as described above. Furthermore, as will be described below, the reference electrode 42 is to be used in measuring the oxygen concentration (oxygen partial pressure) in the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61.
In the measurement-object gas flow section, the gas inlet 10 is open to an external space, so that the measurement-object gas is to be taken into the sensor element 101 from the external space through the gas inlet 10. The first diffusion control section 11 is provided for applying a predetermined diffusion resistance to the measurement-object gas taken in through the gas inlet 10. The buffer space 12 is provided for guiding the measurement-object gas introduced through the first diffusion control section 11 to the second diffusion control section 13. The second diffusion control section 13 is provided for applying a predetermined diffusion resistance to the measurement-object gas to be introduced to the first internal cavity 20 from the buffer space 12. When the measurement-object gas is introduced from outside the sensor element 101 to the first internal cavity 20, the measurement-object gas quickly taken into the sensor element 101 through the gas inlet 10 because of pressure fluctuation of the measurement-object gas (i.e., pulsation of exhaust pressure, if the measurement-object gas is exhaust gas from an internal combustion engine) in the external space is not directly introduced to the first internal cavity 20 but is introduced to the first internal cavity 20 after the pressure fluctuation of the measurement-object gas is negated by traveling through the first diffusion control section 11, the buffer space 12, and the second diffusion control section 13. Accordingly, the pressure fluctuation of the measurement-object gas to be introduced to the first internal cavity 20 becomes substantially negligible. The first internal cavity 20 is provided as a space for adjusting the oxygen partial pressure in the measurement-object gas introduced thereto via the second diffusion control section 13. The oxygen partial pressure is adjusted by actuating a main pump cell 21.
The main pump cell 21 is an electrochemical pump cell constituted of an inner pump electrode 22 including a ceiling electrode portion 22a disposed on the lower surface of the second solid electrolyte layer 6 over substantially the entirety of an area that faces the first internal cavity 20; an outer pump electrode 23 disposed on the upper surface of the second solid electrolyte layer 6 over an area that corresponds to the ceiling electrode portion 22a in such a manner as to be exposed to the outside of the element body 102; and the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4 that form a current path between the electrodes 22 and 23.
The inner pump electrode 22 is provided astride the upper and lower solid electrolyte layers (i.e., the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal cavity 20, and the spacer layer 5 that provides sidewalls of the first internal cavity 20. Specifically, the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal cavity 20 is provided with the ceiling electrode portion 22a, the upper surface of the first solid electrolyte layer 4 that provides the bottom surface of the first internal cavity 20 is provided with a bottom electrode portion 22b, and the ceiling electrode portion 22a and the bottom electrode portion 22b are connected to each other by lateral electrode portions (not illustrated) provided on respective sidewalls (inner surfaces) of the spacer layer 5 that form opposite sidewalls of the first internal cavity 20. Thus, the inner pump electrode 22 has a tunnel-like structure in a region where the lateral electrode portions are provided.
In the main pump cell 21, a desired voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23, whereby a pump current Ip0 is caused to flow in the positive direction or the negative direction between the inner pump electrode 22 and the outer pump electrode 23. Thus, the oxygen in the first internal cavity 20 is pumped out to the external space, or the oxygen in the external space is pumped into the first internal cavity 20.
Furthermore, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere within the first internal cavity 20, the inner pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 constitute an electrochemical sensor cell, that is, a main-pump-control oxygen-partial-pressure detection sensor cell 80.
The oxygen concentration (oxygen partial pressure) in the first internal cavity 20 is to be determined by measuring an electromotive force (voltage V0) in the main-pump-control oxygen-partial-pressure detection sensor cell 80. Furthermore, feedback control is performed on the voltage Vp0 of a variable power source 24 such that the voltage V0 becomes a target value, whereby the pump current Ip0 is controlled. Thus, the oxygen concentration in the first internal cavity 20 is maintained at a predetermined fixed value.
The third diffusion control section 30 is provided for applying a predetermined diffusion resistance to the measurement-object gas having an oxygen concentration (oxygen partial pressure) controlled in the first internal cavity 20 under the operation of the main pump cell 21, and then guiding the measurement-object gas to the second internal cavity 40.
The second internal cavity 40 is provided as a space for an auxiliary pump cell 50 to further adjust the oxygen partial pressure in the measurement-object gas having an oxygen concentration (oxygen partial pressure) preliminarily adjusted in the first internal cavity 20 and introduced to the second internal cavity 40 via the third diffusion control section 30. Thus, the oxygen concentration in the second internal cavity 40 is maintained at a fixed level with high accuracy, which enables the gas sensor 100 to achieve highly accurate NOx concentration measurement.
The auxiliary pump cell 50 is an auxiliary electrochemical pump cell constituted of an auxiliary pump electrode 51 including a ceiling electrode portion 51a disposed on the lower surface of the second solid electrolyte layer 6 over substantially the entirety of an area that faces the second internal cavity 40; the outer pump electrode 23 (but not limited to the outer pump electrode 23 and may possibly be an appropriate electrode disposed on the outer peripheral surface of the element body 102); and the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.
The auxiliary pump electrode 51 is disposed in the second internal cavity 40 and has a tunnel-like structure similar to the inner pump electrode 22 disposed in the first internal cavity 20. Specifically, the second solid electrolyte layer 6 that provides the ceiling surface of the second internal cavity 40 is provided with the ceiling electrode portion 51a, the first solid electrolyte layer 4 that provides the bottom surface for the second internal cavity 40 is provided with a bottom electrode portion 51b, and the ceiling electrode portion 51a and the bottom electrode portion 51b are connected to each other by lateral electrode portions (not illustrated) provided on opposite wall surfaces of the spacer layer 5 that provide respective sidewalls of the second internal cavity 40. Thus, the auxiliary pump electrode 51 has a tunnel-like structure.
In the auxiliary pump cell 50, a desired voltage Vp1 is applied between the auxiliary pump electrode 51 and the outer pump electrode 23 so that the oxygen in the atmosphere within the second internal cavity 40 can be pumped out to the external space or the oxygen can be pumped into the second internal cavity 40 from the external space.
Furthermore, in order to control the oxygen partial pressure in the atmosphere within the second internal cavity 40, the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3 constitute an electrochemical sensor cell, that is, an auxiliary-pump-control oxygen-partial-pressure detection sensor cell 81.
A variable power source 52 whose voltage is controlled based on an electromotive force (voltage V1) detected by the auxiliary-pump-control oxygen-partial-pressure detection sensor cell 81 causes the auxiliary pump cell 50 to perform pumping. Thus, the oxygen partial pressure in the atmosphere in the second internal cavity 40 is controlled to be a low partial pressure that has substantially no effect on NOx measurement.
Meanwhile, a pump current Ip1 generated by the auxiliary pump cell 50 is used in controlling the electromotive force of the main-pump-control oxygen-partial-pressure detection sensor cell 80. Specifically, the pump current Ip1 is input as a control signal to the main-pump-control oxygen-partial-pressure detection sensor cell 80, and the above target value for the voltage V0 of the main-pump-control oxygen-partial-pressure detection sensor cell 80 is controlled, whereby the gradient of the oxygen partial pressure in the measurement-object gas to be introduced to the second internal cavity 40 from the third diffusion control section 30 is controlled to be constantly fixed. To use the gas sensor 100 as a NOx sensor, the oxygen concentration in the second internal cavity 40 is maintained at a fixed value of about 0.001 ppm under the cooperation of the main pump cell 21 and the auxiliary pump cell 50.
The fourth diffusion control section 60 is provided for applying a predetermined diffusion resistance to the measurement-object gas having an oxygen concentration (oxygen partial pressure) controlled in the second internal cavity 40 under the operation of the auxiliary pump cell 50, and then guiding the measurement-object gas to the third internal cavity 61. The fourth diffusion control section 60 has a role of limiting the amount of NOx flowing into the third internal cavity 61.
The third internal cavity 61 is provided as a space where a process for measuring the concentration of nitrogen oxide (NOx) in the measurement-object gas is performed on the measurement-object gas having an oxygen concentration (oxygen partial pressure) preliminarily adjusted in the second internal cavity 40 and introduced to the third internal cavity 61 via the fourth diffusion control section 60. The NOx concentration is measured mainly in the third internal cavity 61 under the operation of a measurement pump cell 41.
The measurement pump cell 41 is configured to measure the NOx concentration in the measurement-object gas in the third internal cavity 61. The measurement pump cell 41 is an electrochemical pump cell constituted of the measurement electrode 44 disposed on the upper surface of the first solid electrolyte layer 4 in an area that faces the third internal cavity 61; and the outer pump electrode 23, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces the NOx existing in the atmosphere in the third internal cavity 61.
In the measurement pump cell 41, oxygen resulting from the decomposition of the nitrogen oxide (NOx) in the atmosphere surrounding the measurement electrode 44 is pumped out, and the amount of oxygen thus produced is detected as a pump current Ip2.
Furthermore, in order to detect the oxygen partial pressure around the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42 constitute an electrochemical sensor cell, that is, a measurement-pump-control oxygen-partial-pressure detection sensor cell 82. A variable power source 46 is controlled based on an electromotive force (voltage V2) detected by the measurement-pump-control oxygen-partial-pressure detection sensor cell 82.
The measurement-object gas introduced to the second internal cavity 40 reaches the measurement electrode 44 in the third internal cavity 61 via the fourth diffusion control section 60 under a condition where the oxygen partial pressure is controlled. The nitrogen oxide (NOx) in the measurement-object gas surrounding the measurement electrode 44 is reduced (2NO→N2+O2) to produce oxygen. Then, the oxygen thus produced undergoes pumping by the measurement pump cell 41. During the pumping of the oxygen, a voltage Vp2 of the variable power source 46 is controlled such that the voltage V2 detected by the measurement-pump-control oxygen-partial-pressure detection sensor cell 82 becomes a fixed value (i.e., a target value). Because the amount of oxygen produced around the measurement electrode 44 is proportional to the concentration of the nitrogen oxide in the measurement-object gas, the nitrogen oxide concentration in the measurement-object gas is calculated by using the pump current Ip2 of the measurement pump cell 41.
Alternatively, if the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined into an electrochemical sensor cell serving as an oxygen-partial-pressure detection device, an electromotive force corresponding to the difference between the amount of oxygen resulting from the reduction of the NOx component in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference air is detectable. Thus, the concentration of the NOx component in the measurement-object gas may be obtained.
Furthermore, 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 constitute an electrochemical sensor cell 83. Using an electromotive force (voltage Vref) generated by the sensor cell 83, the oxygen partial pressure in the measurement-object gas outside the element body 102 of the sensor element 101, specifically, around the outer pump electrode 23, is detectable.
In the gas sensor 100 having the above configuration, the measurement pump cell 41 receives the measurement-object gas whose oxygen partial pressure is constantly maintained at a fixed low value (i.e., a value that substantially has no effect on NOx measurement) as a result of actuation of the main pump cell 21 and the auxiliary pump cell 50. Thus, the NOx concentration in the measurement-object gas can be ascertained based on the pump current Ip2 flowing as a result of oxygen produced by NOx reduction being pumped out by the measurement pump cell 41 substantially in proportion to the NOx concentration in the measurement-object gas.
Here, the electrodes 22, 23, 42, 44, and 51 will be described. The inner pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44 each contain Class 1 noble metal with catalytic activity. The Class 1 noble metal may be at least one of Pt, Rh, Ir, Ru, and Pd, for example. The outer pump electrode 23 and the reference electrode 42 each also contain Class 1 noble metal. The inner pump electrode 22 and the auxiliary pump electrode 51 each further contain Class 2 noble metal that reduces the catalytic activity of the Class 1 noble metal on the specific gas (NOx). Thus, the reducing ability of each of the inner pump electrode 22 and the auxiliary pump electrode 51 with respect to the NOx component in the measurement-object gas is weakened. The Class 2 noble metal may be Au, for example. The measurement electrode 44 does not contain any Class 2 noble metal. Thus, the reducing ability of the measurement electrode 44 with respect to the NOx component in the measurement-object gas is made higher than that of the inner pump electrode 22 and the auxiliary pump electrode 51. The measurement electrode 44 preferably contains at least one of Pt and Rh among Class 1 noble metals, or may contain both Pt and Rh. The outer pump electrode 23 and the reference electrode 42 each preferably contain none of Class 2 noble metals. The electrodes 22, 23, 42, 44, and 51 are each preferably a cermet that contains noble metal and oxygen-ion-conductive oxide (such as ZrO2). Preferably, the electrodes 22, 23, 42, 44, and 51 are each a porous body. In the present embodiment, the inner pump electrode 22 and the auxiliary pump electrode 51 are each a porous cermet electrode composed of Pt and ZrO2 with 1% of Au. The outer pump electrode 23 and the reference electrode 42 are each a porous cermet electrode composed of Pt and ZrO2. The measurement electrode 44 is a porous cermet electrode composed of Pt, Rh, and ZrO2.
In order to enhance oxygen ion conductivity of the solid electrolyte in the element body 102, the heater section 70 has a role of temperature adjustment for keeping the sensor element 101 warm by heating the sensor element 101. The heater section 70 includes a heater 71a, a heater insulation layer 71b, and a pressure release hole 71c.
The heater 71a is an electrical resistor interposed between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 71a generates heat when supplied with electricity from a heater power source 72, thereby heating and keeping warm the solid electrolyte in the element body 102 of the sensor element 101. Furthermore, the heater 71a is embedded in such a manner as to overlap the entire region from the first internal cavity 20 to the third internal cavity 61, and is capable of adjusting the entirety of the sensor element 101 to a temperature at which the solid electrolyte is activated.
The heater insulation layer 71b is provided on the upper and lower surfaces of the heater 71a and is an insulator composed of alumina or the like. The heater insulation layer 71b is provided for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 71a and electrical insulation between the third substrate layer 3 and the heater 71a.
The pressure release hole 71c extends through the third substrate layer 3 and the reference-gas introduction layer 48 in such a manner as to communicate with the reference-gas introduction space 43. The pressure release hole 71c is provided for the purpose of alleviating an increase in internal pressure occurring with a temperature increase in the heater insulation layer 71b.
The gas sensor 100 further includes the control device 95. As illustrated in
The controller 96 is configured to perform an auxiliary-pump control process in which the auxiliary pump cell 50 is controlled such that the oxygen concentration in the second internal cavity 40 becomes a target concentration. Specifically, the controller 96 performs feedback control on the voltage Vp1 of the variable power source 52 in such a manner as to set the voltage V1 to a fixed value (referred to as “target value V1*”), thereby controlling the auxiliary pump cell 50. The target value V1* is defined as a value that makes the oxygen concentration in the second internal cavity 40 a predetermined low concentration that has substantially no effect on NOx measurement.
The controller 96 is configured to perform a main-pump control process in which the main pump cell 21 is controlled such that the pump current Ip1 generated when the auxiliary pump cell 50 adjusts the oxygen concentration in the second internal cavity 40 in the auxiliary-pump control process becomes a target current (referred to as “target value Ip1*”). Specifically, the controller 96 sets (i.e., performs feedback control on), based on the pump current Ip1, the target value for the voltage V0 (referred to as “target value V0*”) in such a manner as to set the pump current Ip1 flowing at the voltage Vp1 to a fixed target value Ip1*. Furthermore, the controller 96 performs feedback control on the voltage Vp0 of the variable power source 24 in such a manner as to set the voltage V0 to the target value V0* (i.e., to set the oxygen concentration in the first internal cavity 20 to a target concentration). Through the main-pump control process, the gradient of the oxygen partial pressure in the measurement-object gas to be introduced from the third diffusion control section 30 to the second internal cavity 40 is constantly fixed. The target value V0* is set to a value at which the oxygen concentration in the first internal cavity 20 is higher than 0% and is a low concentration. The pump current Ip0 that flows during the main-pump control process changes with the oxygen concentration in the measurement-object gas that flows from the gas inlet 10 into the measurement-object gas flow section (i.e., the measurement-object gas around the sensor element 101). Therefore, the controller 96 is also capable of detecting the oxygen concentration in the measurement-object gas based on the pump current Ip0.
The above main-pump control process and auxiliary-pump control process are also collectively referred to as “adjustment-pump control process”. Furthermore, the first internal cavity 20 and the second internal cavity 40 are also collectively referred to as “oxygen-concentration adjustment chamber”. The main pump cell 21 and the auxiliary pump cell 50 are also collectively referred to as “adjustment pump cell”. In the adjustment-pump control process performed by the controller 96, the adjustment pump cell adjusts the oxygen concentration in the oxygen-concentration adjustment chamber.
The controller 96 is configured to perform a measurement-pump control process in which the measurement pump cell 41 is controlled such that the voltage V2 becomes a fixed value (referred to as “target value V2*”) (i.e., such that the oxygen concentration in the third internal cavity 61 becomes a predetermined low concentration). Specifically, the controller 96 performs feedback control on the voltage Vp2 of the variable power source 46 in such a manner as to set the voltage V2 to the target value V2*, thereby controlling the measurement pump cell 41. Through the measurement-pump control process, oxygen is pumped out from the third internal cavity 61.
Through the measurement-pump control process, oxygen produced as a result of the NOx in the measurement-object gas being reduced in the third internal cavity 61 is pumped out from the third internal cavity 61 such that the oxygen becomes substantially zero. Then, the controller 96 acquires the pump current Ip2 as a value detected in correspondence with the oxygen produced in the third internal cavity 61 from the specific gas (in this case, NOx), and calculates the NOx concentration in the measurement-object gas based on the pump current Ip2.
The storage unit 98 stores, for example, a relational expression (e.g., a linear function or quadratic function expression) or a map that represents the correspondence relationship between the pump current Ip2 and the NOx concentration. Such a relational expression or map can be preliminarily obtained through experiments.
The controller 96 is configured to perform a heater control process in which a control signal is output to the heater power source 72 so that the temperature of the heater 71a is controlled to be a target temperature (800° C., for example). Here, the temperature of the heater 71a can be expressed as a linear function of the resistance value of the heater 71a. In the heater control process, the controller 96 calculates the resistance value of the heater 71a as a value equivalent to the temperature of the heater 71a (as a value convertible into temperature), and performs feedback control on the heater power source 72 such that the resistance value thus calculated becomes a target resistance value (a resistance value corresponding to the target temperature). The controller 96 is capable of acquiring, for example, the voltage of the heater 71a and the current flowing through the heater 71a and calculating the resistance value of the heater 71a based on the voltage and current thus acquired. Alternatively, the controller 96 may calculate the resistance value of the heater 71a through, for example, a three-terminal method or a four-terminal method. In powering the heater 71a, the heater power source 72 changes, based on the control signal from the controller 96 for example, the value of the voltage to be applied to the heater 71a, thereby adjusting the electric power to be supplied to the heater 71a.
The control device 95 inclusive of the variable power sources 24, 46, and 52, the heater power source 72, and so forth illustrated in
The plurality of connector electrodes 75 each function as a terminal that allows the sensor element 101 and an external element to be electrically continuous with each other. As illustrated in
Here, details of the measurement electrode 44 and the corresponding conducting section 74 (the connector electrode 75a and the lead 76) will be described with reference to
The measurement electrode 44 is disposed on the upper surface of the first solid electrolyte layer 4 in an area that faces the third internal cavity 61. The lead 76 provided for the measurement electrode 44 includes an inner lead 77 and a peripheral-face lead 78. The inner lead 77 is disposed inside the element body 102 and is connected to (electrically continuous with) the measurement electrode 44 and the peripheral-face lead 78. Specifically, the inner lead 77 includes first to fourth segments 77a to 77d. The first segment 77a is disposed on the upper surface of the first solid electrolyte layer 4 in an area that faces the third internal cavity 61. The right end of the first segment 77a is connected to the measurement electrode 44. The first segment 77a extends linearly in the left-right direction up to the left end of the third internal cavity 61. The right end of the second segment 77b is connected to the left end of the first segment 77a. The second segment 77b extends linearly in the left-right direction but does not reach the left end of the element body 102. The front end of the third segment 77c is connected to the left end of the second segment 77b. The third segment 77c extends linearly in the front-rear direction up to a position near the rear end of the element body 102. The right end of the fourth segment 77d is connected to the rear end of the third segment 77c. The fourth segment 77d extends linearly in the left-right direction up to the left end of the element body 102. The left end of the fourth segment 77d reaches (is routed to) the third face 102c (left end face) of the element body 102 and is covered by and connected to the peripheral-face lead 78. The inner lead 77 is a conductor mainly composed of a noble metal such as platinum (Pt) or a high-melting-point metal such as tungsten (W) or molybdenum (Mo). The inner lead 77 is preferably a cermet conductor containing a noble metal or a high-melting-point metal and zirconia that is the same as the main component of the first solid electrolyte layer 4.
In the inner lead 77, the entirety of the second and third segments 77b and 77c and the outer periphery of a part of the fourth segment 77d excluding a left end part thereof are enclosed by a lead insulation layer 79. The lead insulation layer 79 insulates the portion of the inner lead 77 whose outer circumference is surrounded by the lead insulation layer 79 from the first solid electrolyte layer 4 and the spacer layer 5. Note that the outer periphery of the left end part of the fourth segment 77d is not enclosed by the lead insulation layer 79. That is, during the manufacture of the sensor element 101, the outer periphery of the left end part of the fourth segment 77d is prevented from being enclosed by the lead insulation layer 79. Therefore, the connection (electrical continuity) between the left end of the fourth segment 77d and the peripheral-face lead 78 is less likely to be hindered. The lead insulation layer 79 is an insulator composed of ceramic such as alumina.
The peripheral-face lead 78 has a substantially rectangular parallelepiped shape and is disposed near the rear end of the third face 102c of the element body 102 and is connected to the inner lead 77 and the connector electrode 75a. Specifically, the peripheral-face lead 78 is disposed on the third face 102c in such a manner as to cover the entirety of the left end of the fourth segment 77d of the inner lead 77, thereby preventing the left end of the fourth segment 77d from being exposed to the outside of the sensor element 101. A central part of the right end face of the peripheral-face lead 78 is connected to (electrically continuous with) the left end of the fourth segment 77d. An upper end part of the right end face of the peripheral-face lead 78 is connected to the left end face of the front end part of the connector electrode 75a.
The peripheral-face lead 78 is a conductor mainly composed of noble metal such as platinum (Pt) or the like. The peripheral-face lead 78 is preferably a conductor containing noble metal and alumina (Al2O3). The peripheral-face lead 78 is further preferably a conductor containing noble metal, alumina, and silica (SiO2). In a case where the peripheral-face lead 78 is the conductor containing noble metal, alumina, and silica, for example, letting the volumes of noble metal, alumina, and silica be Vp, Va, and Vs [vol %], respectively, the peripheral-face lead 78 may be as follows. The peripheral-face lead 78 may satisfy Va/Vs≤1.5. The peripheral-face lead 78 may also satisfy Vp/(Va+Vs)≥1.5. In this case, the peripheral-face lead 78 may satisfy Vp/(Va+Vs)≥2.6. The peripheral-face lead 78 may satisfy Vp/(Va+Vs)≤19.0. The peripheral-face lead 78 may satisfy 1.0≤Va/Vs≤1.5. The ratio of the total mass of alkali metal and alkaline-earth metal contained in the peripheral-face lead 78 on an oxide basis may satisfy 0.1 wt % or lower.
Letting a porosity of the peripheral-face lead 78 be Rp [%] and a thickness of the peripheral-face lead 78 be Dc [mm], the peripheral-face lead 78 is configured to satisfy Rp/Dc≤145%/mm. If some kind of gas outside the sensor element 101 (element body 102) enters the inside of the element body 102 through the fourth segment 77d of the inner lead 77 or through a gap between the fourth segment 77d and the element body 102, the gas may move toward the measurement electrode 44 along the inner lead 77 or the gap between the inner lead 77 and the element body 102 (a gap between the inner lead 77 and the lead insulation layer 79 or a gap between the lead insulation layer 79 and the element body 102) and reach the measurement electrode 44. Examples of some kind of gas outside the sensor element 101 includes a volatile organic gas generated from the rubber stopper 157 if, for example, the sensor element 101 is exposed to a high-temperature environment. If such a gas reaches the measurement electrode 44, the oxygen concentration around the measurement electrode 44 may decrease. This decrease may lead to an increase in the voltage V2 to be detected by the measurement-pump-control oxygen-partial-pressure detection sensor cell 82. Consequently, less oxygen may be pumped out from the third internal cavity 61 during the measurement-pump control process, leading to a reduction in the pump current Ip2. This reduction in the pump current Ip2 may cause the accuracy deterioration of detection of the concentration of the specific gas (NOx). In view of the above, the peripheral-face lead 78 according to the present embodiment is configured to satisfy Rp/Dc≤145%/mm. Such a configuration makes the porosity Rp of the peripheral-face lead 78 small and/or the thickness Dc of the peripheral-face lead 78 large, thereby reducing the passage of some kind of gas outside the element body 102 through the peripheral-face lead 78. Consequently, the entry of such a gas into the inside of the element body 102 through the fourth segment 77d of the inner lead 77 or through a gap between the fourth segment 77d and the element body 102 is reduced, which reduces the likelihood of such a gas reaching the measurement electrode 44. As a result, the likelihood of a reduction in the pump current Ip2 is reduced, which in turn reduces the likelihood of accuracy deterioration in the detection of the concentration of the specific gas (NOx). The present inventors have demonstrated these facts through experiments, analyses, and the like. Furthermore, reducing the likelihood of such a gas reaching the measurement electrode 44 reduces the deterioration of the measurement electrode 44. The peripheral-face lead 78 is preferably configured to satisfy Rp/Dc≤120%/mm.
The peripheral-face lead 78 is preferably configured to satisfy Rp/Dc≥1.5%/mm. Such a configuration makes the porosity Rp of the peripheral-face lead 78 large and/or the thickness Dc of the peripheral-face lead 78 small, thereby increasing adhesiveness between the peripheral-face lead 78 and the third face 102c of the element body 102. Consequently, the likelihood of the peripheral-face lead 78 peeling from the third face 102c of the element body 102 is reduced when the sensor element 101 contracts during the calcinating process in its manufacture. Furthermore, the likelihood of the peripheral-face lead 78 peeling from the third face 102c of the element body 102 due to thermal expansion and contraction of the sensor element 101 is also reduced. The present inventors have demonstrated these facts through experiments, analyses, and the like. The peripheral-face lead 78 is further preferably configured to satisfy Rp/Dc≥4.5%/mm.
The porosity Rp of the peripheral-face lead 78 may be 0.01% or higher. The porosity Rp of the peripheral-face lead 78 may be 9.0% or lower. The thickness Dc of the peripheral-face lead 78 may be 0.001 mm or greater. The thickness Dc of the peripheral-face lead 78 may be 0.080 mm or less.
The porosity, such as the porosity Rp of the peripheral-face lead 78, is defined as a value derived as follows by using an image (SEM image) obtained in an observation through a scanning electron microscope (SEM). First, a measurement object is cut to have a section (if the measurement object is the peripheral-face lead 78, a section taken in the thickness direction of the peripheral-face lead 78). The section, regarded as an observation surface, is embedded with resin and is polished, whereby an observation sample is obtained. Subsequently, the observation surface of the observation sample is photographed by SEM photography (with a secondary electron image, an acceleration voltage of 15 kV, and a magnifying power of 1000, but a magnifying power higher than 1000 and lower than or equal to 5000 if the magnifying power of 1000 is inappropriate), whereby a SEM image of the measurement object is obtained. Subsequently, the obtained image is analyzed, so that a threshold value is determined using the discriminant analysis method (Otsu binarization method) from a brightness distribution of brightness data on the pixels in the image. Then, each pixel in the image is binarized into an object section and a pore section based on the determined threshold value, and the area of the object section and the area of the pore section are calculated. Furthermore, the percentage of the area of the pore section relative to the overall area (i.e., the total area of the object section and the pore section) is derived as the porosity (in %).
Now, an exemplary method of manufacturing the sensor element 101 included in the gas sensor 100 will be described. First, six non-calcinated ceramic green sheets each containing an oxygen-ion-conductive solid electrolyte, such as zirconia, as a ceramic component are prepared. In each of these green sheets, a plurality of sheet holes to be used for positioning during printing or stacking as well as necessary through-holes and the like are provided in advance. Furthermore, the green sheet that is to become the spacer layer 5 is preliminarily subjected to a punching process or the like in which a space that is to become the measurement-object gas flow section is provided. Likewise, the green sheet that is to become the first solid electrolyte layer 4 is subjected to a process for providing a space that is to become the reference-gas introduction space 43. Then, a pattern-printing process and a drying process for forming various patterns in the ceramic green sheets are performed in correspondence with 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. Specific patterns to be formed include, for example, patterns that serve as electrodes such as the measurement electrode 44 described above, inner leads such as the inner leads 77 connected to the electrodes, lead insulation layers such as the lead insulation layers 79, the connector electrodes 75, the reference-gas introduction layer 48, and the heater section 70. The pattern-printing process is performed in which a pattern-forming paste prepared in accordance with the properties required for an object to be formed is applied onto a green sheet by using a known screen printing technique. The drying process is performed by using a known drying technique. When the pattern-printing process and the drying process are completed, a printing process and a drying process are performed in which a bonding paste that is to become the bonding layers (including the above bonding layer 7) for stacking and bonding together the green sheets corresponding to the respective layers are printed and dried. Subsequently, the green sheets provided with the bonding paste are stacked in a predetermined order while being positioned at the sheet holes, and are then subjected to a pressure bonding process in which the green sheets are put under predetermined temperature and pressure conditions to be pressure bonded into a single layered body. The layered body thus obtained contains a plurality of sensor elements 101 therein. The layered body is cut into pieces each having the size of the sensor element 101. Subsequently, a pattern that is to become the peripheral-face lead 78 is formed by screen printing on a section obtained by cutting the layered body, i.e., on a part that serves as the third face 102c of the element body 102 of the sensor element 101. Furthermore, the pattern is dried. Then, the layered body is calcinated at a predetermined calcination temperature, whereby a sensor element 101 is obtained.
To form patterns that are to become the inner lead 77 and the lead insulation layer 79 on the green sheet that is to become the first solid electrolyte layer 4, the following steps may alternatively be taken, for example. First, a pattern that is to become a part of the lead insulation layer 79 that covers the lower side of the inner lead 77 is formed on the green sheet. Subsequently, a pattern that is to become the inner lead 77 is formed. Then, parts of the lead insulation layer 79 that cover the lateral sides and upper side of the inner lead 77 are formed.
The pattern-forming paste for forming the peripheral-face lead 78 is a paste containing noble metal such as platinum or the like. The pattern-forming paste for forming the peripheral-face lead 78 is preferably a paste containing noble metal and alumina. The pattern-forming paste for forming the peripheral-face lead 78 is further preferably a paste containing noble metal, alumina, and silica. The porosity Rp of the peripheral-face lead 78 is adjustable by, for example, adjusting particle diameters of particles contained in the pattern-forming paste for forming the peripheral-face lead 78, adjusting a particle diameter and content ratio of porosity material, or adjusting a calcination temperature and calcination time when the layered body is calcinated. The thickness Dc of the peripheral-face lead 78 is adjustable by, for example, adjusting the viscosity of the pattern-forming paste for forming the peripheral-face lead 78 or changing the number of times of printing to be performed in the pattern-forming process.
Subsequently, a gas sensor 100 having the sensor element 101 incorporated therein is manufactured. For example, an element sealing unit 141 is attached to the sensor element 101 to seal and secure the sensor element 101, and a protection cover 130 is attached to a part of the element sealing unit 141 that is near the front end of the element body 102 of the sensor element 101. Furthermore, a connector 150 and lead wires 155 are attached to the rear end of the element body 102 of the sensor element 101 in such a manner as to be electrically continuous with the connector electrodes 75. Furthermore, an external cylinder 148 is attached to a part of the element sealing unit 141 that is near the rear end of the element body 102, the lead wires 155 are routed from the external cylinder 148 to the outside, and the external cylinder 148 is welded to the main metal fitting 142. Subsequently, the lead wires 155 are passed through the through-holes provided in the rubber stopper 157, the rubber stopper 157 is fitted into the external cylinder 148, and the external cylinder 148 is swaged such that the diameter thereof is reduced to form swaged sections 148a and 148b. Thus, the rubber stopper 157 and the external cylinder 148 are secured to each other. Then, a control device 95 and the sensor element 101 are connected to each other via the lead wires 155. Thus, a gas sensor 100 is obtained.
Now, an exemplary usage of the gas sensor 100 will be described. The CPU 97 in the controller 96 first controls the heater power source 72 to supply electric power to the heater 71a such that the temperature of the heater 71a becomes a target temperature (800° C., for example). The CPU 97 controls the temperature of the heater 71a by, for example, acquiring a value (for example, the resistance value or current value of the heater 71a) that is convertible into the temperature of the heater 71a and performing feedback control on the heater power source 72 based on the thus acquired value. When the temperature of the heater 71a reaches the target temperature (or near the target temperature), the CPU 97 starts controlling the above pump cells 21, 41, and 50 (the adjustment-pump control process and the measurement-pump control process) and acquiring the voltages V0, V1, V2, and Vref from the above sensor cells 80 to 83. When a measurement-object gas is introduced through the gas inlet 10 in this state, the measurement-object gas travels through the first diffusion control section 11, the buffer space 12, and the second diffusion control section 13 and reaches the first internal cavity 20. Then, the oxygen concentration in the measurement-object gas is adjusted in the first internal cavity 20 and the second internal cavity 40 by the main pump cell 21 and the auxiliary pump cell 50, and the measurement-object gas thus adjusted reaches the third internal cavity 61. Subsequently, the CPU 97 detects the NOx concentration in the measurement-object gas based on the acquired pump current Ip2 and the correspondence relationship stored in the storage unit 98.
Here, the correspondence between the elements according to the present embodiment and the elements according to the present invention is clarified. 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 according to the present embodiment each correspond to the solid electrolyte layer according to the present invention. The element body 102 corresponds to the element body. The first to fourth faces 102a to 102d each correspond to the peripheral face. The measurement electrode 44 corresponds to the inner electrode. The conducting section 74 corresponds to the conducting section. The inner lead 77 corresponds to the inner conducting section. The connector electrode 75a and the peripheral-face lead 78 each correspond to the outer conducting section. The peripheral-face lead 78 corresponds to the covering portion. A combination of the protection cover 130 and the sensor assembly 140 corresponds to the case. The rubber stopper 157 corresponds to the sealing member.
In the sensor element 101 included in the gas sensor 100 according to the present embodiment described in detail above, the peripheral-face lead 78 covers the left end of the fourth segment 77d of the inner lead 77. Letting the porosity of the peripheral-face lead 78 be Rp [%] and the thickness of the peripheral-face lead 78 be Dc [mm], the peripheral-face lead 78 is configured to satisfy Rp/Dc≤145%/mm. Such a configuration reduces the passage of some kind of gas outside the sensor element 101 (element body 102), such as the volatile organic gas generated from the rubber stopper 157, through the peripheral-face lead 78. Consequently, the entry of such a gas into the inside of the element body 102 through the fourth segment 77d of the inner lead 77 or through a gap between the fourth segment 77d and the element body 102 is reduced, which reduces the likelihood of such a gas reaching the measurement electrode 44. As a result, the likelihood of a reduction in the pump current Ip2 is reduced, which in turn reduces the likelihood of accuracy deterioration in the detection of the concentration of the specific gas.
In the sensor element 101, the peripheral-face lead 78 is configured to satisfy Rp/Dc≤120%/mm. Such a configuration reduces the passage of some kind of gas outside the element body 102 through the peripheral-face lead 78.
In the sensor element 101, the peripheral-face lead 78 is configured to satisfy Rp/Dc≥1.5%/mm. Such a configuration increases adhesiveness between the peripheral-face lead 78 and the third face 102c of the element body 102. Consequently, the likelihood of the peripheral-face lead 78 peeling from the third face 102c of the element body 102 is reduced when the sensor element 101 contracts during the calcinating process in its manufacture. Furthermore, the likelihood of the peripheral-face lead 78 peeling from the third face 102c of the element body 102 due to thermal expansion and contraction of the sensor element 101 is also reduced.
In the sensor element 101, the peripheral-face lead 78 is configured to satisfy Rp/Dc≥4.5%/mm. Such a configuration further increases adhesiveness between the peripheral-face lead 78 and the third face 102c of the element body 102.
The present invention is not limited whatsoever to the above embodiment, and various embodiments are conceivable so long as they belong to the technical scope of the present invention.
For example, while the peripheral-face lead 78 according to the above embodiment is constituted of a single layer, such an embodiment is not limiting. For example, as in a sensor element 201 according to a modification illustrated in
Illustrated in
In the sensor element 301 illustrated in
While the above embodiment relates to a case where the connector electrode 75a is connected to the measurement electrode 44 via the inner lead 77 and the peripheral-face lead 78, such an embodiment is not limiting. For example, as in a sensor element 401 according to a modification illustrated in
While the connector electrode 75a of the sensor element 401 is constituted of a single layer, the connector electrode 75a may be constituted of two layers or three or more layers. In this case, the connector electrode 75a can be regarded as either of the peripheral-face leads 278 and 378 of the sensor elements 201 and 301 according to the modifications illustrated in
While the sensor element 401 includes the through-hole conductor 478 whose upper end is covered by the connector electrode 75a, such an embodiment is not limiting. For example, as in a sensor element 501 according to a modification illustrated in
In the above embodiment, the conducting section 74 provided for the measurement electrode 44 has been described. Another conducting sections 74 provided for any of the inner pump electrode 22, the auxiliary pump electrode 51, and the reference electrode 42 may be configured the same as above. For example, as with the case where the conducting section 74 provided for the measurement electrode 44 includes the connector electrode 75a and the lead 76 including the inner lead 77 and the peripheral-face lead 78, if the conducting section 74 provided for the inner pump electrode 22 includes a connector electrode 75h and a lead including an inner lead and a peripheral-face lead, the peripheral-face lead may be configured similar to the peripheral-face lead 78 of the sensor element 101, for example, configured to satisfy Rp/Dc≤145%/mm.
While the oxygen-concentration adjustment chamber according to the above embodiment includes the first internal cavity 20 and the second internal cavity 40, such an embodiment is not limiting. For example, the oxygen-concentration adjustment chamber may further include another internal cavity, or one of the first internal cavity 20 and the second internal cavity 40 may be omitted. Likewise, while the adjustment pump cell according to the above embodiment includes the main pump cell 21 and the auxiliary pump cell 50, such an embodiment is not limiting. For example, the adjustment pump cell may further include another pump cell, or one of the main pump cell 21 and the auxiliary pump cell 50 may be omitted. For example, if the oxygen concentration in the measurement-object gas can be satisfactorily reduced by using the main pump cell 21 alone, the auxiliary pump cell 50 may be omitted. If the auxiliary pump cell 50 is omitted, the controller 96 only needs to perform the main-pump control process as the adjustment-pump control process. Furthermore, in the main-pump control process, the setting of the target value V0* based on the pump current Ip1 described above may be omitted. Specifically, if a predetermined target value V0* is preliminarily stored in the storage unit 98, the controller 96 only needs to control the main pump cell 21 by performing feedback control on the voltage Vp0 of the variable power source 24 such that the voltage V0 becomes the target value V0*.
While the sensor element 101 of the gas sensor 100 according to the above embodiment has the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61, such an embodiment is not limiting. For example, the third internal cavity 61 may be omitted, as in a sensor element 601 according to a modification illustrated in
The outer pump electrode 23 according to the above embodiment has a role as an electrode (also referred to as “outer main pump electrode”) paired with the inner pump electrode 22 in the main pump cell 21, a role as an electrode (also referred to as “outer auxiliary pump electrode”) paired with the auxiliary pump electrode 51 in the auxiliary pump cell 50, and a role as an electrode (also referred to as “outer measurement electrode”) paired with the measurement electrode 44 in the measurement pump cell 41. However, such an embodiment is not limiting. Any one or more of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be disposed separately from the outer pump electrode 23 and outside the element body 102 in such a manner as to come into contact with the measurement-object gas.
While the sensor element 101 according to the above embodiment is configured to detect the concentration of NOx in the measurement-object gas, such an embodiment is not limiting as long as the sensor element is configured to detect the concentration of any specific gas in the measurement-object gas. For example, the concentration of any oxide other than NOx may be defined as the specific gas concentration. If the specific gas is an oxide, oxygen is produced when the specific gas itself is reduced in the third internal cavity 61, as with the case of the above embodiment. Therefore, the measurement pump cell 41 can detect the specific gas concentration by acquiring a value (for example, the pump current Ip2) detected in correspondence with the oxygen. Alternatively, the specific gas may be a non-oxide, such as ammonia. If the specific gas is a non-oxide, the specific gas is converted into an oxide (e.g., into NO in the case of ammonia), so that oxygen is produced when the converted gas is reduced in the third internal cavity 61. Thus, the measurement pump cell 41 can acquire a value (e.g., the pump current Ip2) detected in correspondence with this oxygen and thus detect the specific gas concentration. For example, since the inner pump electrode 22 in the first internal cavity 20 functions as a catalyst, the ammonia is converted into NO in the first internal cavity 20.
While the element body 102 of the sensor element 101 according to the above embodiment is a layered body including a plurality of solid electrolyte layers (i.e., the layers 1 to 6), such an embodiment is not limiting. The element body 102 only needs to include at least one oxygen-ion-conductive solid electrolyte layer. For example, the layers 1 to 5 other than the second solid electrolyte layer 6 in
In the above embodiment, the controller 96 sets (i.e., performs feedback control on) the target value V0* of the voltage V0 based on the pump current Ip1 so as to set the pump current Ip1 to the target value Ip1*, and performs feedback control on the pump voltage Vp0 so as to set the voltage V0 to the target value V0*, but may perform another type of control. For example, the controller 96 may perform feedback control on the pump voltage Vp0 based on the pump current Ip1 so as to set the pump current Ip1 to the target value Ip1*. Specifically, the controller 96 may omit the acquisition of the voltage V0 from the main-pump-control oxygen-partial-pressure detection sensor cell 80 and the setting of the target value V0*, and may directly control the pump voltage Vp0 (and by extension the pump current Ip0) based on the pump current Ip1.
While the above embodiment has been described as an embodiment of the gas sensor 100 including any of the sensor elements 101, 201, and so forth, the above embodiment may also be applied to any of the sensor elements 101, 201, and so forth to be included in a gas sensor 100, of course.
Specifically fabricated examples of the sensor element will now be described. Note that the present invention is not limited to the following examples.
Each of Experimental Examples was obtained by fabricating the sensor element 101 illustrated in
The reference gas sensor and the gas sensors 100 according to Experimental Examples were each subjected to an evaluation test for testing the accuracy in the detection of the pump current Ip2. First, each of the reference gas sensor and the gas sensors 100 according to Experimental Examples was attached to a pipe such that the front end part of the element body 102 of the sensor element 101 protruded into the pipe. Further temperature sensors were attached to the rubber stopper 157 of each gas sensor. Subsequently, the heater control process was started to heat the heater 71a to 800° C. Furthermore, an air-fuel mixture that is a mixture of liquefied petroleum gas (LPG) and air was burned with a burner to obtain a gas, which was taken as a measurement-object gas and was caused to flow into the pipe. In this step, the flow rates of the LPG and the air were adjusted to set the air-fuel ratio λ to 1.05 and the temperature of the measurement-object gas to 740° C. Note that the present inventors demonstrated in advance through other experiments and analyses that the NOx concentration of the measurement-object gas under the above conditions became substantially 100 ppm. Thus, while the measurement-object gas was caused to flow through the pipe, the adjustment-pump control process and the measurement-pump control process were executed, in which the pump current Ip2 was detected for two hours. A value obtained by converting the pump current Ip2 of the reference gas sensor into NOx concentration [ppm] was defined as the reference value. Furthermore, a value obtained by converting the pump current Ip2 of each of the gas sensors 100 according to Experimental Examples into NOx concentration [ppm] was defined as an evaluation-object value. For each of the gas sensors 100 according to Experimental Examples, concentration difference was calculated at each of different points of time by subtracting the evaluation-object value from the reference value. If the maximum value of the concentration difference was 7 ppm or less, the gas sensor was evaluated as very good (A). If the maximum value of the concentration difference was greater than 7 ppm and less than 15 ppm, the gas sensor was evaluated to be good (B). If the maximum value of the concentration difference was 15 ppm or greater, the gas sensor was evaluated as bad (F). As the reference gas sensor has been processed to prevent the generation of the volatile organic gas, as described above, it is assumed that the reference value of the reference gas sensor at each time is not sufficiently sensitive to Rp/Dc [%/mm].
The gas sensors 100 according to Experimental Examples were each subjected to a thermal expansion and contraction test, to evaluate the durability of the peripheral-face lead 78 against peeling from the third face 102c of the element body 102. Specifically, each of the gas sensors 100 was placed inside a temperature cycling furnace and subjected to a process of being held at −40° C. for 15 minutes, followed by 200° C. for 15 minutes. This sequence constituted one cycle, and the cycle was repeated 600 times. Next, each of the gas sensors 100 was removed from the temperature cycling furnace. Continuity between the measurement electrode 44 and the connector electrode 75a via the peripheral-face lead 78 was examined. If the continuity was present, whether there was any peeling of the peripheral-face lead 78 from the third face 102c of the element body 102 was further examined.
As can be seen from the results of Evaluation Test 1, among Experimental Examples, as illustrated in
As can be seen from the results of Evaluation Test 2, among Experimental Examples, as illustrated in
The present application claims priority from Japanese Patent Application No. 2023-198186 filed Nov. 22, 2023, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2023-198186 | Nov 2023 | JP | national |
