Sensor element and gas sensor

Abstract

To further suppress a change in oxygen concentration around a standard electrode.SOLUTION: A sensor element 101 includes: an element body 101a that has solid electrolyte layers (layers 1 to 6) having oxygen ion conductivity and has a measured gas circulation part for introducing and circulating a measured gas inside; a standard electrode 42 arranged in the element body 101a; and a reference gas introduction part 62 that is arranged in the element body 101a and circulates a reference gas as a reference of detection of a specific gas concentration within a measured gas to the standard electrode 42. The reference gas introduction part 62 includes: a reference gas flow path 64; and at least one preliminary chamber 65 that is arranged in the middle of the reference gas flow path 64 and has smaller diffusion resistance than the reference gas flow path 54. The reference gas flow path 64 is constituted of a porous body at least partially such that any of at least one preliminary chamber 65 does not directly communicate with the outside of the element body 101a.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a sensor element and a gas sensor. [Background technology]

[0002] Conventionally, gas sensors that detect the concentration of a specific gas such as NOx in a measurement gas such as an exhaust gas from an automobile are known. For example, Patent Document 1 describes a gas sensor including a long plate-shaped sensor element formed by stacking a plurality of oxygen ion conductive solid electrolyte layers.

[0003] FIG. 10 is a schematic cross-sectional view showing an example of the configuration of such a conventional gas sensor 900. As shown in the figure, the gas sensor 900 includes a sensor element 907. The sensor element 907 is an element having a structure in which dense oxygen ion conductive solid electrolyte layers 901 to 906 are laminated. In the sensor element 907, a measurement gas flow section for introducing a measurement gas is formed between the lower surface of the solid electrolyte layer 906 and the upper surface of the solid electrolyte layer 904, and a measurement electrode 944 is formed in the measurement gas flow section. An outer pump electrode 923 is formed on the upper surface of the solid electrolyte layer 906. Meanwhile, a reference gas introduction space 943 is formed between the upper surface of the solid electrolyte layer 903 and the lower surface of the solid electrolyte layer 905, into which a reference gas (e.g., air) serving as a reference for detecting a specific gas concentration in the measurement gas is introduced. A reference electrode 942 is formed on the upper surface of the solid electrolyte layer 903 facing the reference gas introduction space 943. The reference electrode 942 is covered with a porous reference gas introduction layer 948. A reference gas is introduced into the reference electrode 942 from the outside of the sensor element 907 through the reference gas introduction space 943 and the reference gas introduction layer 948. In this gas sensor 900, when a measured gas is introduced into the measured gas flow portion, an electromotive force Va is generated between the measurement electrode 944 and the reference electrode 942. Based on this electromotive force Va, oxygen is pumped out or pumped in via the outer pump electrode 923 and the measurement electrode 944. Then, the concentration of a specific gas in the measured gas is detected based on a current Ip2 during the pumping out or pumping in. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2015-200643 A Summary of the Invention [Problem to be solved by the invention]

[0005] In the conventional gas sensor 900 shown in Fig. 10, the oxygen concentration of the reference gas around the reference electrode 942 may change, affecting the detection accuracy of the specific gas concentration. For example, a small amount of the measured gas may enter the reference gas introduction space 943, causing a decrease in the oxygen concentration around the reference electrode 942. As a countermeasure against this, a pump current may be applied between the reference electrode 942 and the outer pump electrode 923 to pump oxygen around the reference electrode 942. However, even in this case, the oxygen concentration of the reference gas around the reference electrode 942 may change if the amount of oxygen pumped in is too large or too small.

[0006] The present invention has been made to solve such problems, and has as its main object to further suppress changes in the oxygen concentration around the reference electrode. [Means for solving the problem]

[0007] In order to achieve the above-mentioned main object, the present invention adopts the following means.

[0008] The sensor element of the present invention comprises: A sensor element for detecting a specific gas concentration in a measurement gas, comprising: an element body having an oxygen ion conductive solid electrolyte layer and a measurement gas flow section for introducing and flowing the measurement gas therein; a reference electrode disposed inside the element body; a reference gas inlet portion disposed inside the element body for passing a reference gas serving as a reference for detecting the concentration of the specific gas in the measurement gas to the reference electrode; Equipped with the reference gas inlet portion has a reference gas flow path and one or more spare chambers disposed midway along the reference gas flow path and having a smaller diffusion resistance than the reference gas flow path; At least a portion of the reference gas flow path is made of a porous material so that none of the one or more spare chambers is directly connected to the outside of the element body. It is something.

[0009] In this sensor element, one or more spare chambers having a smaller diffusion resistance than the reference gas flow path are provided in the reference gas inlet part in the middle of the reference gas flow path. In addition, at least a part of the reference gas flow path is made of a porous material, so that none of the one or more spare chambers are in direct communication with the outside of the element body. As a result, the spare chambers act as a buffer for changes in the oxygen concentration in the reference gas, suppressing changes in the oxygen concentration around the reference electrode.

[0010] Here, "arranged in the middle of the reference gas flow path" includes an embodiment in which the spare chamber is arranged so as to be inserted between the reference gas flow paths, and an embodiment in which the spare chamber is arranged so as to be in contact with the reference gas flow path in the middle of the reference gas flow path. Also, "not in direct communication" means that there is no communication with the outside only through the space that does not pass through the porous body that constitutes the reference gas flow path. In other words, for each of one or more spare chambers, if the spare chamber and the porous body are arranged so that the reference gas always passes through the porous body in the reference gas flow path when moving from the spare chamber to the outside of the element body, it can be said that the spare chamber "does not directly communicate" with the outside of the element body.

[0011] In the sensor element of the present invention, the reference gas inlet portion is provided with the reference electrode. standard A reference gas chamber having a smaller diffusion resistance than the gas flow passage may be provided, in which case the reference gas chamber also functions as a buffer to suppress changes in the oxygen concentration around the reference electrode, similar to the spare chamber.

[0012] In the sensor element of the present invention, the volume ratio, which is the ratio of the total volume of the one or more spare chambers to the volume of the reference gas chamber, may be 1.5 times or more and 30 times or less. If the volume ratio is 1.5 times or more, the effect of suppressing the change in oxygen concentration around the reference electrode due to the spare chambers can be sufficiently obtained. If the volume ratio is 30 times or less, the decrease in strength of the element body due to the presence of the spare chambers can be suppressed. The volume ratio may be 8 times or less, or 4 times or less. The volume of each of the one or more spare chambers may be 8 times or less, or 4 times or less, the volume of the reference gas chamber.

[0013] In the sensor element of the present invention, the reference gas introduction part may have a plurality of the pre-chambers. Here, for example, when a single large pre-chamber is provided and a plurality of small pre-chambers are provided, if the total volume of the pre-chambers is the same, the buffer function described above is about the same in both cases, while the strength of the element body can be suppressed more effectively when a plurality of small pre-chambers are provided. Therefore, by having a reference gas introduction part have a plurality of pre-chambers, it is possible to suppress the change in the oxygen concentration around the reference electrode while suppressing the strength of the element body. In this case, two or more of the plurality of pre-chambers may be different in size from each other.

[0014] In the sensor element of the present invention, each of the one or more preliminary chambers may be a space or may be filled with a porous body.

[0015] The gas sensor of the present invention includes the sensor element according to any one of the above-described aspects, and therefore, the gas sensor can obtain the same effects as those of the sensor element of the present invention described above, for example, the effect of suppressing changes in the oxygen concentration around the reference electrode. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a vertical cross-sectional view of a gas sensor 100. [Diagram 2] FIG. 2 is a schematic cross-sectional view illustrating an example of the configuration of a sensor element 101. [Diagram 3] FIG. 4 is a partial cross-sectional view of the sensor element 101 around a spare chamber 65. [Figure 4] Cross section AA of Figure 2. [Diagram 5] FIG. 4 is a block diagram showing the electrical connection between a control device 95 and each cell. [Figure 6] FIG. 11 is a partial cross-sectional view showing a spare chamber 65 of a modified example. [Figure 7] FIG. 11 is a partial cross-sectional view showing a reference gas introduction part 62 of a modified example. [Figure 8] FIG. 11 is a partial cross-sectional view showing a reference gas introduction part 62 of a modified example. [Figure 9] FIG. 11 is a partial cross-sectional view showing a reference gas introduction part 62 of a modified example. [Figure 10] FIG. 1 is a schematic cross-sectional view illustrating an example of the configuration of a conventional gas sensor 900. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Next, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a longitudinal sectional view of a gas sensor 100 according to an embodiment of the present invention. Fig. 2 is a schematic sectional view showing an example of the configuration of a sensor element 101 included in the gas sensor 100. Fig. 3 is a partial sectional view of the sensor element 101 around a spare chamber 65. Fig. 4 is a sectional view taken along line AA in Fig. 2. Fig. 5 is a block diagram showing the electrical connection between a control device 95 and each cell.

[0018] As shown in FIG. 1, the gas sensor 100 includes a sensor element 101, a protective cover 130 for protecting the front end side of the sensor element 101, and a sensor assembly 140 including a connector 150 that is electrically connected to the sensor element 101. The gas sensor 100 is attached to a pipe 190, such as an exhaust gas pipe of a vehicle, as shown in the figure, and is used to measure the concentration of a specific gas, such as NOx or O2, contained in exhaust gas as a measurement gas. In this embodiment, the gas sensor 100 measures the NOx concentration as the specific gas concentration. The sensor element 101 includes an element body 101a and a porous protective layer 85 that covers the element body 101a. The element body 101a has a long rectangular parallelepiped shape, and the longitudinal direction of the element body 101a (the left-right direction in FIG. 2) is the front-rear direction, and the thickness direction of the element body 101a (the up-down direction in FIG. 2) is the up-down direction. The width direction of the element body 101a (the direction perpendicular to the front-rear direction and the up-down direction) is the left-right direction.

[0019] The protective cover 130 includes a cylindrical inner protective cover 131 with a bottom that covers the front end of the sensor element 101. 1 The protective cover 130 is provided with a cylindrical outer protective cover 132 having a bottom that covers the protective cover 131. The inner protective cover 131 and the outer protective cover 132 are formed with a plurality of holes for allowing the measurement gas to flow inside the protective cover 130. A sensor element chamber 133 is formed as a space surrounded by the inner protective cover 131, and the front end of the sensor element 101 is disposed inside the sensor element chamber 133.

[0020] The sensor assembly 140 includes an element sealing body 141 which seals and fixes the sensor element 101, a bolt 147 attached to the element sealing body 141, an outer cylinder 148, and a connector 150 which contacts and is electrically connected to connector electrodes (only connector electrode 69 and heater connector electrode 71, which will be described later, are shown in Figures 1 and 2) formed on the surfaces (top and bottom surfaces) of the rear end of the element body 101a of the sensor element 101.

[0021] The element sealing body 141 includes a cylindrical metal shell 142, a cylindrical inner cylinder 143 welded and fixed coaxially to the metal shell 142, and ceramic supporters 144a to 144c, powder compacts 145a and 145b, and a metal ring 146, which are filled in through holes inside the metal shell 142 and the inner cylinder 143. The sensor element 101 is located on the central axis of the element sealing body 141 and penetrates the element sealing body 141 in the front-rear direction. The inner cylinder 143 is formed with a reduced diameter portion 143a for pressing the powder compact 145b in the central axial direction of the inner cylinder 143, and a reduced diameter portion 143b for pressing the ceramic supporters 144a to 144c and the powder compacts 145a and 145b forward via the metal ring 146. Due to the pressing force from the reduced diameter portions 143a, 143b, the powder compacts 145a, 145b are compressed between the main metal fitting 142 and the inner tube 143 and the sensor element 101, so that the powder compacts 145a, 145b seal the space between the sensor element chamber 133 in the protective cover 130 and the space 149 in the outer tube 148 and fix the sensor element 101 in place.

[0022] The bolt 147 is fixed coaxially to the metallic shell 142, and has a male thread formed on its outer circumferential surface. The male thread of the bolt 147 is inserted into a fixing member 191 which is welded to the pipe 190 and has a female thread formed on its inner circumferential surface. In this way, the gas sensor 100 is fixed to the pipe 190 with the front end of the sensor element 101 and a portion of the protective cover 130 of the gas sensor 100 protruding into the pipe 190.

[0023] The outer cylinder 148 covers the inner cylinder 143, the sensor element 101, and the connector 150, and a plurality of lead wires 155 connected to the connector 150 are drawn out from the rear end. The lead wires 155 are electrically connected to the electrodes (described later) of the sensor element 101 via the connector 150. The gap between the outer cylinder 148 and the lead wires 155 is sealed by a rubber plug 157. A space 149 within the outer cylinder 148 is filled with a reference gas (atmosphere in this embodiment). The rear end of the sensor element 101 is disposed within this space 149.

[0024] As shown in FIG. 2, the element body 101a of the sensor element 101 is an element having a laminate in which six layers, namely, a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each of which is made of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO2), are laminated in this order from the bottom as viewed in the drawing. The solid electrolyte forming these six layers is dense and airtight. The element body 101a is manufactured, for example, by performing a predetermined processing and printing a circuit pattern on ceramic green sheets corresponding to each layer, laminating them, and further firing them to integrate them.

[0025] At one end of the element body 101a (the left side in FIG. 2), between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, a gas inlet 10, a first diffusion rate-controlling section 11, a buffer space 12, a second diffusion rate-controlling section 13, a first internal void 20, a third diffusion rate-controlling section 30, a second internal void 40, a fourth diffusion rate-controlling section 60, and a third internal void 61 are formed adjacent to each other and communicated with each other in this order.

[0026] Gas inlet 10, buffer space 12, first internal void 20, second internal void 40, and third internal void 61 are spaces within element body 101a that are defined by hollowing out spacer layer 5, with an upper portion defined by the underside of second solid electrolyte layer 6, a lower portion defined by the upper surface of first solid electrolyte layer 4, and sides defined by the side surfaces of spacer layer 5.

[0027] The first diffusion rate-controlling section 11, the second diffusion rate-controlling section 13, and the third diffusion rate-controlling section 30 are each provided as two horizontally elongated slits (with the opening extending in the direction perpendicular to the drawing). The fourth diffusion rate-controlling section 60 is provided as one horizontally elongated slit (with the opening extending in the direction perpendicular to the drawing) formed as a gap with the lower surface of the second solid electrolyte layer 6. The portion from the gas inlet 10 to the third internal space 61 is also referred to as a measurement gas flow section.

[0028] The element body 101a includes a reference gas inlet 62 for passing a reference gas from the outside of the element body 101a to the reference electrode 42 when measuring the NOx concentration. The reference gas inlet 62 includes a reference gas chamber 63, a reference gas flow path 64, and a spare chamber 65. A part of the reference gas inlet 62 is exposed to the rear end surface of the element body 101a, and this part functions as an inlet 67 of the reference gas inlet 62. The inlet 67 is exposed in the space 149 (see FIG. 1). The reference gas is introduced from the inlet 67 into the reference gas inlet 62. The reference gas inlet 62 introduces the reference gas introduced from the inlet 67 into the reference electrode 42 while providing a predetermined diffusion resistance to the reference gas. In this embodiment, the reference gas is the atmosphere (the atmosphere in the space 149 in FIG. 1). The reference gas inlet 62 will be described in detail later.

[0029] The reference electrode 42 is an electrode formed in a manner to be sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As will be described later, the reference electrode 42 can be used to measure the oxygen concentrations (oxygen partial pressures) in the first inner space 20, the second inner space 40, and the third inner space 61. The reference electrode 42 is formed as a porous cermet electrode (for example, a cermet electrode of Pt and ZrO2).

[0030] In the measurement gas flow section, the gas inlet 10 is a section that opens to the external space, and the measurement gas is introduced from the external space into the element body 101a through the gas inlet 10. The first diffusion rate-controlling section 11 is a section that imparts a predetermined diffusion resistance to the measurement gas introduced from the gas inlet 10. The buffer space 12 is a space provided for guiding the measurement gas introduced from the first diffusion rate-controlling section 11 to the second diffusion rate-controlling section 13. The second diffusion rate-controlling section 13 is a section that imparts a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20. When the measurement gas is introduced from the outside of the element body 101a into the first internal space 20, the measurement gas is suddenly taken into the element body 101a from the gas inlet 10 due to pressure fluctuations of the measurement gas in the external space (exhaust pressure pulsations if the measurement gas is automobile exhaust gas), but is not introduced directly into the first internal space 20, but is introduced into the first internal space 20 after the pressure fluctuations of the measurement gas are canceled through the first diffusion rate-controlling section 11, the buffer space 12, and the second diffusion rate-controlling section 13. As a result, the pressure fluctuations of the measurement gas introduced into the first internal space 20 are almost negligible. The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the measurement gas introduced through the second diffusion rate-controlling section 13. The oxygen partial pressure is adjusted by the operation of the main pump cell 21.

[0031] The main pump cell 21 is an electrochemical pump cell including an inner pump electrode 22 having a ceiling electrode portion 22a provided on almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, an outer pump electrode 23 provided in a region corresponding to the ceiling electrode portion 22a on the upper surface of the second solid electrolyte layer 6 so as to be exposed to the external space (the sensor element chamber 133 in FIG. 1), and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0032] The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal space 20, and the spacer layer 5 that provides the side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the spacer layer 5 that constitute both side wall portions of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are arranged in a tunnel-shaped structure at the positions where the side electrode portions are arranged.

[0033] The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (for example, a cermet electrode of Pt containing 1% Au and ZrO2). The inner pump electrode 22, which comes into contact with the measurement gas, is formed using a material with a weakened ability to reduce the NOx component in the measurement gas.

[0034] In the main pump cell 21, by applying a desired pump voltage Vp0 between the inner pump electrode 22 and the outer pump electrode 23 and flowing a pump current Ip0 in a positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23, it is possible to pump oxygen in the first internal space 20 out to the external space, or to pump oxygen in the external space into the first internal space 20.

[0035] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 80 for controlling the main pump, is configured by 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.

[0036] The oxygen concentration (oxygen partial pressure) in the first internal space 20 can be determined by measuring the electromotive force (voltage V0) in the main pump control oxygen partial pressure detection sensor cell 80. Furthermore, the pump current Ip0 is controlled by feedback controlling the pump voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes a target value. This allows the oxygen concentration in the first internal space 20 to be maintained at a predetermined constant value.

[0037] The third diffusion rate-controlling section 30 is a section that applies a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been controlled by the operation of the main pump cell 21 in the first internal space 20, and guides the measurement gas to the second internal space 40.

[0038] The second internal space 40 is provided as a space for further adjusting the oxygen partial pressure by the auxiliary pump cell 50 for the measurement gas introduced through the third diffusion rate-controlling section 30 after the oxygen concentration (oxygen partial pressure) has been adjusted in advance in the first internal space 20. This makes it possible to keep the oxygen concentration in the second internal space 40 constant with high accuracy, thereby enabling the gas sensor 100 to measure the NOx concentration with high accuracy.

[0039] The auxiliary pump cell 50 is an auxiliary electrochemical pump cell including an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode on the outside of the element body 101a will suffice), and the second solid electrolyte layer 6.

[0040] The auxiliary pump electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to that of the inner pump electrode 22 provided in the first internal space 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 providing the ceiling surface of the second internal space 40, a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 providing the bottom surface of the second internal space 40, and side electrode portions (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5 providing the side walls of the second internal space 40, respectively, forming a tunnel-shaped structure. Note that, like the inner pump electrode 22, the auxiliary pump electrode 51 is also formed using a material with a weakened ability to reduce NOx components in the measurement gas.

[0041] In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, it is possible to pump oxygen in the atmosphere within the second internal space 40 out to the external space or pump oxygen from the external space into the second internal space 40.

[0042] In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal space 40, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump, is constituted by 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.

[0043] The auxiliary pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. This allows the oxygen partial pressure in the atmosphere in the second internal space 40 to be controlled to a low partial pressure that does not substantially affect the measurement of NOx.

[0044] In addition, the pump current Ip1 is used to control 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-mentioned target value of the voltage V0 is controlled to always keep a constant gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion rate-controlling part 30 into the second internal space 40. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is kept constant at about 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

[0045] The fourth diffusion rate-controlling part 60 is a part that applies a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been controlled by the operation of the auxiliary pump cell 50 in the second internal space 40, and guides the measurement gas to the third internal space 61. The fourth diffusion rate-controlling part 60 serves to limit the amount of NOx flowing into the third internal space 61.

[0046] The third internal space 61 is provided as a space for carrying out a process for measuring the nitrogen oxide (NOx) concentration in the measurement gas, which has been introduced through the fourth diffusion-controlling part 60 after the oxygen concentration (oxygen partial pressure) has been adjusted in advance in the second internal space 40. The measurement of the NOx concentration is mainly carried out in the third internal space 61 by the operation of the measurement pump cell 41.

[0047] The measurement pump cell 41 measures the NOx concentration in the measurement gas in the third internal space 61. The measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61, 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 is a porous cermet electrode made of a material having a higher reduction ability for the NOx component in the measurement gas than the inner pump electrode 22. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere in the third internal space 61.

[0048] In the measurement pump cell 41, oxygen produced by decomposition of nitrogen oxides in the atmosphere surrounding the measurement electrode 44 is pumped out, and the amount of oxygen produced can be detected as a pump current Ip2.

[0049] In order to detect the oxygen partial pressure around the measurement electrode 44, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 82 for controlling the measurement pump, is configured by the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump.

[0050] The measurement gas introduced into the second internal space 40 reaches the measurement electrode 44 in the third internal space 61 through the fourth diffusion rate-controlling part 60 under the condition that the oxygen partial pressure is controlled. Nitrogen oxides in the measurement gas around the measurement electrode 44 are reduced (2NO→N2+O2) to generate oxygen. The generated oxygen is then pumped by the measurement pump cell 41, and the voltage Vp2 of the variable power supply 46 is controlled so that the voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 becomes constant (target value). The amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxides in the measurement gas, so that the nitrogen oxide concentration in the measurement gas is calculated using the pump current Ip2 in the measurement pump cell 41.

[0051] In addition, 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, and the electromotive force (voltage Vref) obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measured gas outside the sensor.

[0052] Furthermore, an electrochemical reference gas regulating pump cell 90 is configured by 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. This reference gas regulating pump cell 90 pumps oxygen by causing a control current (oxygen pumping current) Ip3 to flow due to a control voltage Vp3 applied by a power supply circuit 92 connected between the outer pump electrode 23 and the reference electrode 42. As a result, the reference gas regulating pump cell 90 pumps oxygen from the space around the outer pump electrode 23 (the sensor element chamber 133 in FIG. 1) to the periphery of the reference electrode 42.

[0053] In the gas sensor 100 having such a configuration, the main pump cell 21 and the auxiliary pump cell 50 are operated to supply a measurement gas in which the oxygen partial pressure is always kept at a constant low value (a value that has substantially no effect on the measurement of NOx) to the measurement pump cell 41. Therefore, the NOx concentration in the measurement gas can be known based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out of the measurement pump cell 41, which is approximately proportional to the NOx concentration in the measurement gas.

[0054] Furthermore, the element body 101a includes a heater section 70 that adjusts the temperature of the element body 101a by heating and keeping it warm in order to increase the oxygen ion conductivity of the solid electrolyte. The heater section 70 includes a heater connector electrode 71, a heater 72, a through hole 73, a heater insulating layer 74, and a lead wire 76.

[0055] The heater connector electrode 71 is an electrode formed in a manner to contact the lower surface of the first substrate layer 1. By connecting the heater connector electrode 71 to an external power source, it is possible to supply power to the heater section 70 from the outside.

[0056] The heater 72 is an electrical resistor sandwiched between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to a heater connector electrode 71 via a lead wire 76 and a through hole 73, and generates heat when power is supplied from the outside through the heater connector electrode 71, thereby heating and keeping warm the solid electrolyte that forms the element body 101a.

[0057] The heater 72 is embedded throughout the entire area from the first inner space 20 to the third inner space 61, making it possible to adjust the temperature of the entire element body 101a to a temperature at which the solid electrolyte is activated.

[0058] The heater insulating layer 74 is an insulating layer made of porous alumina and formed of an insulator such as alumina on the upper and lower surfaces of the heater 72. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72, and between the third substrate layer 3 and the heater 72.

[0059] The porous protective layer 85 covers the front end surface and the top, bottom, left and right surfaces of the element body 101a. The porous protective layer 85 covers only the front end and the area near the front end of the element body 101a. As a result, the porous protective layer 85 covers the peripheral parts of the electrodes 22, 23, 51, 44, 42 used to detect the specific gas concentration of the element body 101a, in other words, the parts of the element body 101a that are arranged in the sensor element chamber 133 and exposed to the measured gas. The porous protective layer 85 also exists behind the rear end of the measured gas flow part and the rear end of the reference gas chamber 63 of the element body 101a. In other words, when the element body 101a is viewed perpendicularly to the longitudinal direction (for example, when the element body 101a is viewed from above), the measured gas flow part and the reference gas chamber 63 are located so as to be included in the porous protective layer 85. The porous protective layer 85 serves to prevent the element body 101a from cracking due to adhesion of moisture or the like in the gas to be measured.

[0060] The porous protective layer 85 is made of a ceramic porous body such as a porous alumina body, a porous zirconia body, a porous spinel body, a porous cordierite body, a porous titania body, a porous magnesia body, etc. In this embodiment, the porous protective layer 85 is made of a porous alumina body.

[0061] 5, the control device 95 includes the variable power supplies 24, 46, and 52, the heater power supply 78, the power supply circuit 92, and a control unit 96. The control unit 96 is a microprocessor including a CPU 97, a RAM (not shown), and a storage unit 98. The storage unit 98 is a non-volatile memory such as a ROM, and is a device for storing various data. The control unit 96 receives the voltages V0 to V2 and the voltage Vref of the sensor cells 80 to 83. The control unit 96 receives the pump currents Ip0 to Ip2 and Ip3 flowing through the pump cells 21, 50, 41, and 90. The control unit 96 outputs control signals to the variable power supplies 24, 46, and 52 and the power supply circuit 92 to control the voltages Vp0 to Vp3 output by the variable power supplies 24, 46, and 52 and the power supply circuit 92, thereby controlling the pump cells 21, 41, 50, and 90. The control unit 96 outputs a control signal to the heater power supply 78 to control the power supplied from the heater power supply 78 to the heater 72, thereby adjusting the temperature of the sensor element 101. The memory unit 98 stores target values ​​V0*, V1*, V2*, Ip1*, etc., which will be described later.

[0062] The control unit 96 feedback controls the pump voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes the target value V0* (that is, so that the oxygen concentration in the first internal space 20 becomes the target concentration).

[0063] The control unit 96 feedback-controls the voltage Vp1 of the variable power supply 52 so that the voltage V1 becomes a constant value (referred to as the target value V1*) (i.e., so that the oxygen concentration in the second internal space 40 becomes a predetermined low oxygen concentration that does not substantially affect the measurement of NOx). At the same time, the control unit 96 sets (feedback-controls) a target value V0* of the voltage V0 based on the pump current Ip1 so that the pump current Ip1 flowing by the voltage Vp1 becomes a constant value (referred to as the target value Ip1*). As a result, the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion rate-controlling unit 30 into the second internal space 40 becomes constant at all times. In addition, the oxygen partial pressure in the atmosphere in the second internal space 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx. The target value V0* is set to a value such that the oxygen concentration in the first internal space 20 is higher than 0% and becomes a low oxygen concentration.

[0064] The control unit 96 feedback-controls the voltage Vp2 of the variable power supply 46 so that the voltage V2 becomes a constant value (referred to as a target value V2*) (i.e., so that the oxygen concentration in the third internal space 61 becomes a predetermined low concentration). As a result, oxygen is pumped out from the third internal space 61 so that the oxygen generated by the reduction of a specific gas (here, NOx) in the measurement gas in the third internal space 61 becomes substantially zero. The control unit 96 then obtains the pump current Ip2 as a detection value corresponding to the oxygen generated in the third internal space 61 derived from NOx, and calculates the NOx concentration in the measurement gas based on this pump current Ip2. The target value V2* is determined in advance as a value such that the pump current Ip2 flowing due to the feedback-controlled voltage Vp2 becomes a limiting current. The storage unit 98 stores a relational expression (for example, a linear function expression) or a map as a correspondence relationship between the pump current Ip2 and the NOx concentration. Such a relational expression or map can be obtained in advance by experiment. Then, the control unit 96 detects the NOx concentration in the measurement gas based on the acquired pump current Ip2 and the above-mentioned correspondence relationship stored in the storage unit 98.

[0065] The control unit 96 controls the power supply circuit 92 so that the voltage Vp3 is applied to the reference gas adjustment pump cell 90, thereby causing the pump current Ip3 to flow. In this embodiment, the voltage Vp3 is a DC voltage that causes the pump current Ip3 to have a predetermined value (a constant DC current). Therefore, when the pump current Ip3 flows, the reference gas adjustment pump cell 90 pumps a constant amount of oxygen from the periphery of the outer pump electrode 23 to the periphery of the reference electrode 42.

[0066] Incidentally, the control device 95, including the variable power sources 24, 46, 52 and the power supply circuit 92 shown in FIG. 2, is actually connected to each electrode inside the element body 101a via lead wires (not shown in the figure) formed within the element body 101a (only the reference electrode lead 47 described later is shown in FIG. 4), the connector 150 in FIG. 1, and lead wires 155.

[0067] Here, the reference gas introduction part 62 and its surrounding structure will be described in detail with reference to Figs. 2 to 4. As described above, the reference gas introduction part 62 includes a reference gas chamber 63, a reference gas flow path 64, and a spare chamber 65. The reference gas flow path 64 includes a reference gas flow path 64a which is a reference gas flow path between the reference gas chamber 63 and the spare chamber 65, and a reference gas flow path 64b which is a reference gas flow path between the spare chamber 65 and the outside of the element body 101a. The reference gas chamber 63 and the reference gas flow path 64 are provided between the upper surface of the third substrate layer 3 and the lower surface of the first solid electrolyte layer 4. The reference electrode 42 is disposed in the reference gas chamber 63. In this embodiment, the reference gas chamber 63 is filled with a porous body, and this porous body covers the reference electrode 42. In this embodiment, the reference gas flow path 64a and the reference gas flow path 64b are also made of a porous body. The porous body which constitutes the reference gas chamber 63 and the reference gas flow path 64 is made of a ceramic such as alumina. A part (here, the rear end) of the reference gas flow passage 64b is exposed on the surface (here, the rear end face) of the element body 101a, and this part functions as the inlet portion 67 described above.

[0068] The preliminary chamber 65 is a space formed in the element body 101a. As shown in Figs. 2 to 4, in this embodiment, the preliminary chamber 65 is a cylindrical space formed as a hole penetrating the first solid electrolyte layer 4 from top to bottom. The preliminary chamber 65 is disposed in the middle of the reference gas flow passage 64. In this embodiment, the preliminary chamber 65 is disposed so as to be inserted between the reference gas flow passages 64, specifically, between the reference gas flow passages 64a and 64b. Therefore, the reference gas flow passage 64 is divided into the reference gas flow passage 64a and the reference gas flow passage 64b before and after the preliminary chamber 65. Therefore, when the reference gas flows between the inlet portion 67 and the reference electrode 42, the reference gas always passes through the preliminary chamber 65 between the reference gas flow passage 64a and the reference gas flow passage 64b. The reference gas introduced into the reference gas introduction part 62 from the inlet part 67 passes through the reference gas flow passage 64b, the preliminary chamber 65, the reference gas flow passage 64a, and the reference gas chamber 63 in this order, and reaches the reference electrode .

[0069] At least a part of the reference gas flow passage 64 is made of a porous material so that the preliminary chamber 65 does not directly communicate with the outside of the element body 101a. In this embodiment, the reference gas flow passage 64b, which is a part of the reference gas flow passage 64 between the preliminary chamber 65 and the outside of the element body 101a, is made of a porous material as a whole, so that the preliminary chamber 65 does not directly communicate with the outside of the element body 101a. In addition, since the preliminary chamber 65 is surrounded by the lower surface of the spacer layer 5, the upper surface of the third substrate layer 3, and the side surface of the first solid electrolyte layer 4 (see Figures 2 and 3), there is no flow passage of the reference gas that communicates with the outside without passing through the reference gas flow passage 64b.

[0070] The reference gas chamber 63 and the spare chamber 65 each have a smaller diffusion resistance than the reference gas flow path 64. In this embodiment, the reference gas chamber 63 and the reference gas flow path 64 are layers of the same predetermined thickness (here, the length in the vertical direction), and are layers whose cross section cut along a plane perpendicular to the longitudinal direction of the element body 101a is rectangular. The reference gas chamber 63 and the reference gas flow path 64 (64a, 64b) are both rectangular in shape when viewed in plan, that is, from above. As shown in FIG. 4, the width (here, the length in the horizontal direction) of the reference gas chamber 63 is wider than the width of the reference gas flow path 64. In this way, the reference gas chamber 63 and the reference gas flow path 64 have the same thickness, but the reference gas chamber 63 is wider, so that the reference gas chamber 63 has a larger flow path cross-sectional area (the area of ​​the cross section perpendicular to the flow direction of the reference gas). In addition, the reference gas chamber 63 and the reference gas flow path 64 are formed of the same porous material throughout, and therefore the porosity of both is the same. For these reasons, the reference gas chamber 63 has a smaller diffusion resistance than the reference gas flow path 64. Also, as shown in FIG. 4, the preliminary chamber 65 has the same width as the reference gas flow path 64 but a larger thickness (height) than the reference gas flow path 64, and therefore the flow path cross-sectional area of ​​the preliminary chamber 65 is larger than the flow path cross-sectional area of ​​the reference gas flow path 64. Moreover, the preliminary chamber 65 is not filled with a porous body but is empty space. For these reasons, the preliminary chamber 65 has a smaller diffusion resistance than the reference gas flow path 64.

[0071] The diffusion resistance of each of the reference gas chamber 63, the reference gas flow passage 64, and the preliminary chamber 65 is the diffusion resistance per unit length along the reference gas flow direction (front-to-back direction here). For example, the diffusion resistance of the preliminary chamber 65 is calculated by multiplying the length (unit: cm, for example) of the preliminary chamber 65 in the reference gas flow direction (front-to-back direction here) by the volume (unit: cm, for example) of the preliminary chamber 65. 3) to calculate the diffusion resistance. In other words, the diffusion resistance of the preliminary chamber 65 is the reciprocal (1 / average cross-sectional area) of the average cross-sectional area of ​​the cross section perpendicular to the reference gas flow direction. In addition, when a porous body is filled, such as the reference gas chamber 63 and the reference gas flow path 64, the diffusion resistance can be calculated by further dividing the value obtained by dividing the above length by the volume by the porosity of the porous body. In other words, the diffusion resistance of the reference gas chamber 63 and the reference gas flow path 64 is the reciprocal (=1 / (average cross-sectional area x porosity)). Note that even if the value of the diffusion resistance itself cannot be calculated accurately, it is sufficient to compare the magnitude relationship of the diffusion resistance between the reference gas chamber 63 and the preliminary chamber 65 and the reference gas flow path 64. For example, when the reference gas chamber 63 and the reference gas flow path 64 are formed of the same porous material as described above, it can be seen that the diffusion resistance is smaller when the average cross-sectional area is larger, even without considering the porosity. Furthermore, since the preliminary chamber 65 is a space and not a porous body, it can be seen that the preliminary chamber 65 has a smaller diffusion resistance without taking into account the porosity of the reference gas flow path 64, so long as the average cross-sectional area of ​​the preliminary chamber 65 is at least equal to or greater than the average cross-sectional area of ​​the reference gas flow path 64.

[0072] The thickness of the reference gas chamber 63 and the reference gas flow passage 64 is not particularly limited, but may be set appropriately within a range of, for example, 10 to 30 μm. The porosity of the porous body forming the reference gas chamber 63 and the reference gas flow passage 64 is not particularly limited, but may be set appropriately within a range of, for example, 10 to 50%. The reference gas chamber 63 and the reference gas flow passage 64 may differ from each other in one or more of the material, porosity, and thickness.

[0073] The volume ratio (=volume of the preliminary chamber 65 / volume of the reference gas chamber 63), which is the ratio of the volume of the preliminary chamber 65 to the volume of the reference gas chamber 63, is preferably 1.5 times or more and 30 times or less. The "volume" refers to the volume of the space. Therefore, for the reference gas chamber 63 formed of a porous body, the volume of the reference gas chamber 63 is the product of the volume based on the shape of the reference gas chamber 63 and the porosity of the reference gas chamber 63. In this embodiment, the preliminary chamber 65 is not filled with a porous body and is entirely empty, so the volume based on the shape of the preliminary chamber 65 is the volume. The volume ratio may be 8 times or less, or 4 times or less. The volume of the reference gas chamber 63 is 0.01 mm 3 It may be more than 0.02 mm. 3 The volume of the reference gas chamber 63 is 0.05 mm 3 It may be less than 0.04 mm 3 The following may also be used.

[0074] The porosity of the reference gas chamber 63 is a value derived as follows using an image (SEM image) obtained by observation using a scanning electron microscope (SEM). First, the sensor element 101 is cut so that the cross section of the reference gas chamber 63 is the observation surface, and the cut surface is filled with resin and polished to obtain an observation sample. Next, an SEM image of the reference gas chamber 63 is obtained by photographing the observation surface of the observation sample using an SEM photograph (secondary electron image, acceleration voltage 15 kV, magnification 1000 times, but if a magnification of 1000 times is inappropriate, a magnification greater than 1000 times and less than or equal to 5000 times is used). Next, the obtained image is analyzed to determine a threshold value by a discriminant analysis method (Otsu's binarization) from the brightness distribution of the brightness data of the pixels in the image. After that, based on the determined threshold value, each pixel in the image is binarized into an object part and a pore part, and the area of ​​the object part and the area of ​​the pore part are calculated. Then, the ratio of the area of ​​the pores to the total area (the total area of ​​the object and pores) is calculated as the porosity [%]. The porosity of the reference gas flow passage 64 can be calculated in a similar manner. In addition, when the preliminary chamber 65 is filled with a porous body, the porosity of the preliminary chamber 65 can be calculated in a similar manner.

[0075] The reference electrode lead 47 is electrically connected to the reference electrode 42. The reference electrode lead 47 is not shown in FIG. 2, but is shown by a broken line in FIG. 4. The reference electrode lead 47 extends leftward from the right side surface of the element body 101a, goes under the reference gas flow passage 64b, and is bent forward along the longitudinal direction of the reference gas flow passage 64 to reach the reference electrode 42. However, the reference electrode lead 47 is wired so as to bypass the preliminary chamber 65 midway as shown in FIG. 4. The reference electrode lead 47 is connected to a connector electrode 69 disposed on the upper surface of the element body 101a (or a connector electrode (not shown) disposed on the lower surface of the element body 101a). Through the reference electrode lead 47 and the connector electrode 69, it is possible to apply electricity to the reference electrode 42 from the outside, and to measure the voltage and current of the reference electrode 42 from the outside. The reference electrode lead 47 is made of a material having electrical conductivity, and may be mainly composed of a precious metal, for example. In this embodiment, the noble metal contained in the reference electrode lead 47 is Pt.

[0076] Next, an example of a method for manufacturing such a gas sensor 100 is described below. First, six unfired ceramic green sheets containing an oxygen ion conductive solid electrolyte such as zirconia as a ceramic component are prepared. A plurality of sheet holes and necessary through holes used for positioning during printing and lamination are formed in advance in these green sheets. A space to be a measurement gas flow section is provided in advance in the green sheet to be the spacer layer 5 by a punching process or the like. A space (hole) to be a preliminary chamber 65 is provided in advance in the green sheet to be the first solid electrolyte layer 4 by a punching process or the like. Then, a pattern printing process and a drying process are performed to form various patterns on each ceramic green sheet corresponding to each of the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6. Specifically, the patterns to be formed are, for example, the patterns of the above-mentioned electrodes, the lead wires connected to each electrode, the reference gas chamber 63, the reference gas flow path 64, and the heater section 70. The pattern printing is performed by applying a pattern forming paste prepared according to the characteristics required for each formation target on the green sheet using a known screen printing technique. The drying process is also performed using a known drying means. After the pattern printing and drying are completed, a printing and drying process of an adhesive paste for laminating and bonding the green sheets corresponding to each layer is performed. Then, the green sheets on which the adhesive paste is formed are laminated in a predetermined order while being positioned by the sheet holes, and are pressed by applying a predetermined temperature and pressure condition to form one laminate. The laminate thus obtained contains a plurality of element bodies 101a. The laminate is cut and cut into pieces of the size of the element body 101a. Then, the cut laminate is fired at a predetermined firing temperature to obtain the element body 101a. When stacking a plurality of green sheets, it is preferable to fill the part that becomes the measurement gas flow part and the part that becomes the space of the reference gas introduction part 62 (here, the preliminary chamber 65) with a paste made of a vanishing material (e.g., theobromine) that disappears during firing. Next, the porous protective layer 85 is formed on the element body 101a by plasma spraying, thereby obtaining the sensor element 101.As a method for producing the porous protective layer 85, in addition to plasma spraying, screen printing, mold casting, dipping, etc. can also be used.

[0077] Once the sensor element 101 is obtained in this manner, a sensor assembly 140 (see FIG. 1) incorporating the sensor element 101 is manufactured, and a protective cover 130, a rubber plug 157, etc. are attached. Then, the control device 95 and the sensor element 101 are connected via lead wires 155, thereby obtaining the gas sensor 100.

[0078] The diffusion resistance of the reference gas chamber 63 and the reference gas flow passage 64 can be adjusted by adjusting the shape of each of the reference gas chamber 63 and the reference gas flow passage 64 or by adjusting the porosity. For example, the shape can be adjusted by adjusting the pattern when forming the porous body constituting each of the reference gas chamber 63 and the reference gas flow passage 64. The porosity can be adjusted by adjusting the particle size of the ceramic particles contained in the paste for forming the pattern of the porous body of each of the reference gas chamber 63 and the reference gas flow passage 64 or by adjusting the particle size or the blending ratio of the pore former. For example, organic polymers and theobromine can be used as the pore former. The diffusion resistance of the preliminary chamber 65 can be adjusted by adjusting the shape of the preliminary chamber 65. For example, the shape of the preliminary chamber 65 can be adjusted by adjusting the shape (e.g., the diameter of the hole) of the punched hole formed in the green sheet to be the first solid electrolyte layer 4.

[0079] The process performed by the control unit 96 when the gas sensor 100 detects the NOx concentration in the measurement gas will be described. First, the CPU 97 of the control unit 96 starts driving the sensor element 101. Specifically, the CPU 97 sends a control signal to the heater power supply 78 to heat the sensor element 101 with the heater 72. Then, the CPU 97 heats the sensor element 101 to a predetermined driving temperature (for example, 800°C). Next, the CPU 97 starts controlling the pump cells 21, 41, 50, 90 and acquiring the voltages V0, V1, V2, Vref from the sensor cells 80 to 83. In this state, when the measurement gas is introduced from the gas inlet 10, the measurement gas passes through the first diffusion rate-controlling section 11, the buffer space 12, and the second diffusion rate-controlling section 13 and reaches the first internal space 20. Next, the oxygen concentrations of the measurement gas in the first internal space 20 and the second internal space 40 are adjusted by the main pump cell 21 and the auxiliary pump cell 50, and the adjusted measurement gas reaches the third internal space 61. Then, the CPU 97 detects the NOx concentration in the measurement gas based on the acquired pump current Ip2 and the correspondence relationship stored in the memory unit 98.

[0080] Here, the measurement gas is introduced from the sensor element chamber 133 shown in FIG. 1 into the measurement gas flow section such as the gas inlet 10 of the sensor element 101. On the other hand, the reference gas (air) in the space 149 shown in FIG. 1 is introduced into the reference gas inlet 62 of the sensor element 101. The sensor element chamber 133 and the space 149 are partitioned by the sensor assembly 140 (particularly the powder compacts 145a, 145b) and are sealed so that gas does not flow between them. However, when the pressure on the measurement gas side is high, the measurement gas may slightly enter the space 149, and the oxygen concentration in the space 149 may decrease. At this time, if the oxygen concentration around the reference electrode 42 decreases, the reference potential, which is the potential of the reference electrode 42, changes. In that case, for example, the voltage based on the reference electrode 42, such as the voltage V2 of the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump during operation of the sensor element 101, changes, and the detection accuracy of the NOx concentration in the measurement gas decreases. In contrast to this, in the sensor element 101 of this embodiment, a spare chamber 65 having a smaller diffusion resistance than the reference gas flow path 64 is provided between the reference gas flow path 64a and the reference gas flow path 64b in the reference gas inlet portion 62. In addition, the reference gas flow path 64b is made of a porous material, so that the spare chamber 65 does not directly communicate with the outside of the element body 101a. As a result, the spare chamber 65 serves as a buffer for changes in the oxygen concentration in the reference gas, and a decrease in the oxygen concentration around the reference electrode 42 is suppressed.

[0081] In addition, in this embodiment, while the sensor element 101 is being driven, the control unit 96 pumps oxygen from the periphery of the outer pump electrode 23 to the periphery of the reference electrode 42 using the reference gas adjustment pump cell 90 as described above. This makes it possible to compensate for the decreased oxygen when the oxygen concentration around the reference electrode 42 decreases, and to suppress a decrease in the detection accuracy of the NOx concentration. However, depending on the degree of decrease in the oxygen concentration of the reference gas around the sensor element 101, the oxygen concentration around the reference electrode 42 may decrease even when the reference gas adjustment pump cell 90 pumps oxygen around the reference electrode 42. In addition, the oxygen concentration around the reference electrode 42 may increase due to a large amount of oxygen pumped around the reference electrode 42 by the reference gas adjustment pump cell 90. Therefore, even when the reference gas adjustment pump cell 90 pumps oxygen, the oxygen concentration around the reference electrode 42 may change (increase or decrease). In response to this, in the sensor element 101 of this embodiment, the spare chamber 65 serves as a buffer for changes in the oxygen concentration in the reference gas, so that it is possible to suppress changes in the oxygen concentration around the reference electrode 42. Therefore, regardless of whether oxygen is pumped in by the reference gas adjustment pump cell 90, the presence of the spare chamber 65 makes it possible to suppress changes in the oxygen concentration around the reference electrode 42. As a result, it is possible to suppress changes in the reference potential, and to suppress a decrease in the detection accuracy of the NOx concentration in the measured gas.

[0082] Here, the correspondence between the components of this embodiment and the components of the present invention will be clarified. The element body 101a of this embodiment corresponds to the element body of the present invention, the reference electrode 42 corresponds to the reference electrode, the reference gas inlet 62 corresponds to the reference gas inlet, the reference gas flow path 64 corresponds to the reference gas flow path, and the spare chamber 65 corresponds to the spare chamber. Also, the reference gas chamber 63 corresponds to the reference gas chamber.

[0083] In the gas sensor 100 of the present embodiment described above in detail, the sensor element 101 has a spare chamber 65 having a smaller diffusion resistance than the reference gas flow path 64, which is provided in the reference gas inlet portion 62 of the sensor element 101 in the middle of the reference gas flow path 64 (here, between the reference gas flow path 64a and the reference gas flow path 64b). At least a part of the reference gas flow path 64 is made of a porous material, so that none of the spare chambers is in direct communication with the outside of the element body. As a result, the spare chamber 65 acts as a buffer for changes in the oxygen concentration in the reference gas, and changes in the oxygen concentration around the reference electrode 42 are suppressed.

[0084] The reference gas inlet 62 is provided with a reference electrode 42. standard It has a reference gas chamber 63 which has a smaller diffusion resistance than the gas flow passage 64. As a result, the reference gas chamber 63, like the spare chamber 65, can also function as a buffer that suppresses changes in the oxygen concentration around the reference electrode .

[0085] Furthermore, the volume ratio, which is the ratio of the total volume of the preliminary chamber 65 to the volume of the reference gas chamber 63, is 1.5 times or more and 30 times or less. When the volume ratio is 1.5 times or more, the effect of suppressing the change in oxygen concentration around the reference electrode 42 by the preliminary chamber 65 can be sufficiently obtained. When the volume ratio is 30 times or less, a decrease in strength of the element body 101a due to the presence of the preliminary chamber 65 can be suppressed.

[0086] It goes without saying that the present invention is not limited to the above-described embodiment, and can be embodied in various forms within the technical scope of the present invention.

[0087] For example, in the above-described embodiment, the preliminary chamber 65 is a space, but is not limited thereto. As shown in FIG. 6, the preliminary chamber 65 may be filled with a porous body. In the example of FIG. 6, the preliminary chamber 65 is filled with the same porous material as the reference gas flow passage 64. In this case, the diffusion resistance of the preliminary chamber 65 is expressed by the reciprocal of (average cross-sectional area x porosity). The volume of the preliminary chamber 65 is the product of the volume based on the shape of the preliminary chamber 65 and the porosity of the preliminary chamber 65. The porosity of the porous body filled in the preliminary chamber 65 may be 10 to 50%. Note that the space 149 may contain a poisoning substance that poisons the reference electrode 42, but the preliminary chamber 65 filled with the porous body serves to trap the poisoning substance. This makes it possible to prevent the poisoning substance from reaching the reference electrode 42.

[0088] In the above-described embodiment, the reference gas introduction part 62 has only one preliminary chamber 65, but the number of the preliminary chambers may be one or more. FIG. 7 is a partial cross-sectional view showing a reference gas introduction part 62 of a modified example. The reference gas introduction part 62 of FIG. 7 has a reference gas chamber 63, a preliminary chamber 65 having a plurality of preliminary chambers 65a to 65c, a reference gas flow path 64 having reference gas flow paths 64a to 64e, and a reference gas introduction space 66 having reference gas introduction spaces 66a and 66b. The reference gas flow paths 64a and 64b are provided between the first solid electrolyte layer 4 and the third substrate layer 3, the reference gas flow path 64c is provided between the first solid electrolyte layer 4 and the spacer layer 5, the reference gas flow path 64d is provided between the spacer layer 5 and the second solid electrolyte layer 6, and the reference gas flow path 64e is provided between the second substrate layer 2 and the third substrate layer 3. The preliminary chamber 65a is a space provided in the third substrate layer 3, the preliminary chamber 65b is a space provided in the first solid electrolyte layer 4, and the preliminary chamber 65c is a space provided in the spacer layer 5. The reference gas introduction space 66a is a hole provided in the second solid electrolyte layer 6, and the reference gas introduction space 66b is a hole provided in the first substrate layer 1 and the second substrate layer 2. The reference gas introduction spaces 66a and 66b are each open to the surface of the element body 101a and each function as an inlet 67 of the reference gas introduction part 62. That is, the reference gas introduction part 62 in FIG. 7 has a plurality of inlets 67. Also, in FIG. 7, the inlet 67 of the reference gas introduction part 62 exists on a surface (here, the upper surface and the lower surface) other than the rear end surface of the element body 101a. Thus, the inlet 67 is not limited to one, and may be multiple, and the inlet 67 may be located on any surface of the element body 101a as long as it is exposed to the space 149. 7, the reference gas introduced from the reference gas introduction space 66a into the reference gas introduction part 62 passes through the reference gas flow path 64d, the preliminary chamber 65c, the reference gas flow path 64c, the preliminary chamber 65b, the reference gas flow path 64b, the preliminary chamber 65a, the reference gas flow path 64a, and the reference gas chamber 63 in this order, before reaching the reference electrode 42. The reference gas introduced from the reference gas introduction space 66b into the reference gas introduction part 62 passes through the reference gas flow path 64e, the preliminary chamber 65a, the reference gas flow path 64a, and the reference gas chamber 63 in this order, before reaching the reference electrode 42. The preliminary chambers 65a to 65c are shown to be equal in size in FIG. 7, but may be different in size.When the pre-chamber 65 includes a plurality of pre-chambers 65a to 65c as shown in FIG. 7, the above-mentioned volume ratio is the ratio of the total volume of the pre-chambers 65a to 65c to the volume of the reference gas chamber 63 (=total volume of the pre-chambers 65 / volume of the reference gas chamber 63). In this case, the volume ratio is preferably 1.5 times or more and 30 times or less as described above. The volume ratio may be 8 times or less, or 4 times or less. When the pre-chamber 65 includes a plurality of pre-chambers such as the pre-chambers 65a to 65c, the volume of each of the plurality of pre-chambers may be 8 times or less than the volume of the reference gas chamber, or 4 times or less. In FIG. 7, the plurality of pre-chambers 65a to 65c are all spaces, but one or more of the pre-chambers 65a to 65c may be filled with a porous body as shown in FIG. 6. Here, for example, when a case where one large pre-chamber 65 is provided is compared with a case where a plurality of small pre-chambers 65 are provided, if the total volume of the pre-chambers 65 is the same, the above-mentioned buffer function will be about the same in both cases. On the other hand, a decrease in strength of the element body 101a can be suppressed more effectively when multiple small auxiliary chambers 65 are provided. Therefore, by having the multiple auxiliary chambers 65 in the reference gas introduction part 62, a decrease in strength of the element body 101a can be suppressed while a change in oxygen concentration around the reference electrode 42 is suppressed.

[0089] Since the reference gas flow passage 64 made of a porous material exists between each of the preliminary chambers 65a to 65c in FIG. 7 and the outside of the element body 101a, none of the preliminary chambers 65a to 65c are directly connected to the outside of the sensor element 101. In contrast, the reference gas introduction spaces 66a and 66b in FIG. 7 are both open to the surface of the element body 101a, and are directly connected to the outside of the element body. Therefore, the reference gas introduction spaces 66a and 66b are not a type of the preliminary chamber 65, and are distinguished from the preliminary chamber 65. Similarly, the reference gas introduction space 943 in the sensor element 907 of the conventional example shown in FIG. 10 is also directly connected to the outside of the sensor element 907, and is therefore different from the preliminary chamber 65. In addition, the sensor element 907 in FIG. 10 has a pressure release hole 975 that penetrates the reference gas introduction layer 948 and opens into the reference gas introduction space 943. The pressure release hole 975 is formed for the purpose of mitigating the internal pressure increase caused by the temperature increase of the heater. This pressure relief hole 975 also communicates directly with the outside of the element body via the reference gas introduction space 943 , and is therefore different from the preliminary chamber 65 .

[0090] In the above-mentioned embodiment, the reference gas chamber 63 is filled with a porous body, and the reference gas flow passage 64 is made of a porous body, but this is not limited thereto. For example, the reference gas chamber 63 may be configured as a space, similar to the preliminary chamber 65. In addition, the reference gas flow passage 64 may be configured as at least a part of a porous body so that the preliminary chamber 65 does not directly communicate with the outside of the element body 101a. For example, even if a part of the reference gas flow passage 64b in FIG. 2 is configured as a porous body and the rest is configured as a space, it can be said that the preliminary chamber 65 does not "directly communicate" with the outside of the element body 101a as long as the preliminary chamber 65 and the porous body are arranged so that the reference gas always passes through the porous body in the reference gas flow passage 64b when moving from the preliminary chamber 65 to the outside of the element body 101a. In addition, since the reference gas flow passage 64a is not a flow passage of the reference gas between the preliminary chamber 65 and the outside of the element body 101a, the entire reference gas flow passage 64a may be configured as a space. 7, when the reference gas introduction part 62 has a plurality of spare chambers 65, at least a part of the reference gas flow passage 64 may be made of a porous material so that none of the plurality of spare chambers 65 is in direct communication with the outside of the element body 101a. When a part of the reference gas flow passage 64 or the reference gas chamber 63 is formed as a space, a pattern of the shape of the space may be formed during the manufacture of the sensor element 101 using a paste made of, for example, a dissipative material (e.g., theobromine).

[0091] In the above-described embodiment and the example shown in FIG. 7, the multiple pre-chambers 65 are all located at the rear end side of the element body 101a, but this is not limited thereto. For example, the reference gas introduction section 62 may be configured as shown in FIG. 8. In FIG. 8, the pre-chamber 65a is located at a position close to the reference electrode 42 in the element body 101a, more specifically, at the front end side of the center in the front-rear direction of the element body 101a. In this way, one or more of the multiple pre-chambers 65 may be located at the front end side of the center in the front-rear direction of the element body 101a. Also, the multiple pre-chambers 65 may be arranged at equal intervals from each other in the front-rear direction.

[0092] In the above-mentioned embodiment, the reference electrode lead 47 is arranged to bypass the preliminary chamber 65, but may be arranged to pass through the preliminary chamber 65 (exposed in the preliminary chamber 65) without bypassing. The reference electrode lead 47 may also serve as at least a part of the reference gas flow path 64. FIG. 9 is a cross-sectional view showing a reference gas inlet 62 of a modified example. Unlike FIG. 4, FIG. 9 shows a cross-section including the reference electrode 42 and the reference electrode lead 47 of the sensor element 101. The reference gas inlet 62 of FIG. 9 does not include the reference gas flow paths 64a and 64b, and instead the reference electrode lead 47 also serves as the reference gas flow path 64. If the reference electrode lead 47 is configured as a porous body, the reference gas can be circulated, so that it can also serve as the function of the reference gas flow path 64. When the reference electrode lead 47 also serves as the reference gas flow path 64, if the preliminary chamber 65 divides the reference gas flow path 64 as in the above-mentioned embodiment, the conduction between the reference electrode 42 and the connector electrode 69 cannot be ensured. Therefore, in the example of FIG. 9, the reference gas flow path 64 (reference electrode lead 47) is arranged so as to pass through the preliminary chamber 65 (exposed in the preliminary chamber 65) without being divided. In other words, the preliminary chamber 65 is arranged so as to contact the reference gas flow path 64 midway through the reference gas flow path 64. In this case as well, the reference gas flowing through the reference gas flow path 64 diffuses into the preliminary chamber 65, which has a smaller diffusion resistance than the reference gas flow path 64, so that the preliminary chamber 65 can serve as a buffer for changes in the oxygen concentration in the reference gas, as in the above-mentioned embodiment. Also, in the example of FIG. 9, the part of the reference electrode lead 47 exposed on the right side surface of the element body 101a functions as the inlet portion 67 of the reference gas introduction portion 62.

[0093] In the above-described embodiment, the reference gas chamber 63 and the reference gas flow passage 64 (64a, 64b) each have a constant width in the left-right direction, but this is not limited thereto. For example, the width of the reference gas flow passage 64a may gradually change along the front-rear direction, or a part of the reference gas flow passage 64a may have a width different from that of other parts. As described above, the diffusion resistance of the reference gas chamber 63 and the reference gas flow passage 64 is expressed by the reciprocal of the average cross-sectional area, so that the diffusion resistance can be calculated even if the width is not constant.

[0094] In the above-described embodiment, the reference gas inlet 62 has the reference gas chamber 63 having a smaller diffusion resistance than the reference gas flow path 64, but this is not limited thereto. For example, in the above-described embodiment, the width of the reference gas chamber 63 may be set to be the same as that of the reference gas flow path 64, so that the diffusion resistance of the reference gas chamber 63 is the same as that of the reference gas flow path 64, the reference gas chamber 63 is made to be a part of the reference gas flow path 64 (i.e., the reference gas chamber 63 is omitted), and the reference electrode 42 is covered by the reference gas flow path 64.

[0095] In the above-mentioned embodiment, the element body 101a is a laminate having a plurality of solid electrolyte layers (layers 1 to 6), but is not limited thereto. The element body of the sensor element 101 may include at least one solid electrolyte layer having oxygen ion conductivity. For example, one or more of the layers 1 to 6 may be a structure layer made of a material other than the solid electrolyte layer (for example, a layer made of alumina). By changing one or more of the layers 1 to 6 to a structure layer made of alumina, the element body 101a can be heated by the heater 72 in a short time because alumina has a higher thermal conductivity than zirconia constituting the solid electrolyte layer. Therefore, the solid electrolyte in the element body 101a can be activated early when the sensor element 101 is used. When a structure layer is provided on the element body 101a, the positions of the electrodes 22, 23, 51, 44, and 42 may be appropriately changed from the above-mentioned embodiment so that they are disposed on the surface of the solid electrolyte layer rather than on the structure layer. From the viewpoint of ensuring the strength of the element body 101a, it is preferable to provide the spare chamber 65 in the solid electrolyte layer rather than in the structure layer. Therefore, the layer adjacent to the layer in which the preliminary chamber 65 is provided in the element body 101a may be a structure layer. For example, in Figs. 7 and 8, the first substrate layer 1, the second substrate layer 2, and the second solid electrolyte layer 6 in which the preliminary chamber 65 is not provided may be changed to a structure layer made of alumina. Also, in Figs. 7 and 8, a structure layer may be added above the second solid electrolyte layer 6 and below the first substrate layer 1, so that the layers 1 to 6 are sandwiched between the structure layers. In this case, the reference gas introduction space 66 may penetrate the structure layer and open to the surface (upper and lower surfaces) of the element body 101a, or may open to the left and right side surfaces of the element body 101a. Also, when a structure layer is added on the second solid electrolyte layer 6, it is preferable to adopt a structure that allows the flow of the measurement gas (particularly oxygen) between the outer pump electrode 23 and the sensor element chamber 133. For example, the part of the structure layer that covers the outer pump electrode 23 may be made porous. A hole serving as a flow path for oxygen between the outer pump electrode 23 and the sensor element chamber 133 may be provided in a part of the structural layer.

[0096] In the above-described embodiment, the pump current Ip3 is a constant-value DC current, but is not limited to this. For example, the pump current Ip3 may be a pulsed, intermittent current. Also, in this embodiment, the pump current Ip3 is a constant-value DC current that always flows in a direction that pumps oxygen into the periphery of the reference electrode 42, but is not limited to this. For example, there may be a period during which the pump current Ip3 flows in a direction that pumps oxygen out from the periphery of the reference electrode 42. Even in this case, as long as the overall direction of oxygen movement over a sufficiently long predetermined period is the direction in which oxygen is pumped into the periphery of the reference electrode 42, Ba good.

[0097] In the above-described sensor element 101, the circuit of the reference gas adjusting pump cell 90 may be omitted, and the gas sensor 100 may not include the power supply circuit 92. Moreover, the gas sensor 100 may not include the control device 95. For example, the gas sensor 100 may include, instead of the control device 95, an external connection connector that is attached to the lead wire 155 and that connects the control device 95 and the lead wire 155.

[0098] In the above embodiment, the reference gas is the atmosphere, but it is not limited to this as long as it is a gas that serves as a reference for detecting the concentration of a specific gas in the measured gas. For example, a gas adjusted to a predetermined oxygen concentration (> the oxygen concentration of the measured gas) may be filled in the space 149 as the reference gas.

[0099] In the above-mentioned embodiment, the sensor element 101 detects the NOx concentration in the measured gas, but is not limited to this, as long as it detects the concentration of the specific gas in the measured gas. For example, the specific gas concentration may be other oxide concentrations, not limited to NOx. When the specific gas is an oxide, oxygen is generated when the specific gas itself is reduced in the third internal space 61 as in the above-mentioned embodiment, so that the measurement pump cell 41 can obtain a detection value (e.g., pump current Ip2) corresponding to this oxygen to detect the specific gas concentration. In addition, the specific gas may be a non-oxide such as ammonia. When the specific gas is a non-oxide, the specific gas is converted to an oxide (e.g., ammonia is converted to NO) and oxygen is generated when the converted gas is reduced in the third internal space 61, so that the measurement pump cell 41 can obtain a detection value (e.g., pump current Ip2) corresponding to this oxygen to detect the specific gas concentration. For example, the inner pump electrode 22 of the first internal space 20 functions as a catalyst, so that ammonia can be converted to NO in the first internal space 20. [Explanation of symbols]

[0100] 1 first substrate layer, 2 second substrate layer, 3 third substrate layer, 4 first solid electrolyte layer, 5 spacer layer, 6 second solid electrolyte layer, 10 gas inlet, 11 first diffusion rate limiting section, 12 buffer space, 13 second diffusion rate limiting section, 20 first internal space, 21 main pump cell, 22 inner pump electrode, 22a ceiling electrode section, 22b bottom electrode section, 23 outer pump electrode, 24 variable power supply, 30 third diffusion rate limiting section, 40 second internal space, 41 measurement pump cell, 42 reference electrode, 44 measurement electrode, 46 variable power supply, 47 reference electrode lead, 50 auxiliary pump cell, 51 auxiliary pump electrode, 51a ceiling electrode section, 51b bottom electrode section, 52 variable power supply, 60 fourth diffusion rate limiting section, 61 third internal space, 62 reference gas inlet, 63 Reference gas chamber, 64, 64a to 64e reference gas flow path, 65, 65a to 65c spare chamber, 66, 66a, 66b reference gas introduction space, 67 inlet portion, 69 connector electrode, 70 heater portion, 71 heater connector electrode, 72 heater, 73 through hole, 74 heater insulating layer, 76 lead wire, 78 heater power supply, 80 oxygen partial pressure detection sensor cell for controlling main pump, 81 oxygen partial pressure detection sensor cell for controlling auxiliary pump, 82 oxygen partial pressure detection sensor cell for controlling measurement pump, 83 sensor cell, 85 porous protective layer, 90 reference gas adjustment pump cell, 92 power supply circuit, 95 control device, 96 control unit, 97 CPU, 98 memory unit, 100 gas sensor, 101 sensor element, 101a element body, 130 protective cover, 131 inner protective cover, 132 Outer protective cover, 133 sensor element chamber, 140 sensor assembly, 141 element sealing body, 142 metal shell, 143 inner cylinder, 143a, 143b reduced diameter portion, 144a to 144c ceramic supporter, 145a, 145b powder compact, 146 metal ring, 147 bolt, 148 outer cylinder, 149 space, 150 connector, 155 lead wire, 157 rubber plug, 190 piping, 191 fixing member, 900 gas sensor, 901 to 906 solid electrolyte layer, 907 sensor element, 923 outer pump electrode, 942 reference electrode, 943 reference gas introduction space, 944 measurement electrode, 948 reference gas introduction layer, 975 pressure relief hole.

Claims

1. A sensor element for detecting a specific gas concentration in a measurement gas, comprising: an element body having an oxygen ion conductive solid electrolyte layer and a measurement gas flow section for introducing and flowing the measurement gas therein; a reference electrode disposed inside the element body; a reference gas inlet portion disposed inside the element body for passing a reference gas serving as a reference for detecting the concentration of the specific gas in the measurement gas to the reference electrode; Equipped with the reference gas inlet portion has a reference gas flow path and one or more spare chambers disposed midway along the reference gas flow path and having a smaller diffusion resistance than the reference gas flow path; the reference gas flow path is at least partially made of a porous material so that none of the one or more spare chambers is in direct communication with the outside of the element body; the reference gas inlet portion has a reference gas chamber in which the reference electrode is disposed and which has a smaller diffusion resistance than the reference gas flow path; a volume ratio, which is a ratio of the total volume of the one or more spare chambers to the volume of the reference gas chamber, is 1.5 times or more and 30 times or less; Sensor element.

2. A sensor element for detecting a specific gas concentration in a measurement gas, comprising: an element body having an oxygen ion conductive solid electrolyte layer and a measurement gas flow section for introducing and flowing the measurement gas therein; a reference electrode disposed inside the element body; a reference gas inlet portion disposed inside the element body for passing a reference gas serving as a reference for detecting the concentration of the specific gas in the measurement gas to the reference electrode; Equipped with the reference gas inlet portion has a reference gas flow path and one or more spare chambers disposed midway along the reference gas flow path and having a smaller diffusion resistance than the reference gas flow path; the reference gas flow path is at least partially made of a porous material so that none of the one or more spare chambers is in direct communication with the outside of the element body; The reference gas introduction unit has a plurality of the preliminary chambers. Sensor element.

3. the reference gas inlet portion has a reference gas chamber in which the reference electrode is disposed and which has a smaller diffusion resistance than the reference gas flow path; The sensor element according to claim 2 .

4. a volume ratio, which is a ratio of the total volume of the one or more spare chambers to the volume of the reference gas chamber, is 1.5 times or more and 30 times or less; The sensor element according to claim 3.

5. Each of the one or more preliminary chambers is a space or is filled with a porous body. The sensor element according to any one of claims 1 to 4.

6. A gas sensor comprising the sensor element according to any one of claims 1 to 5.

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