Sensor element, gas sensor, and method for producing element body

Abstract

To provide a sensor element enabling higher strength of the element body.SOLUTION: A sensor element 101 for detecting a specific gas concentration in a gas to be measured, the sensor element 101 comprising an element body 102 having a solid electrolyte body 103 containing, as the main component, a solid electrolyte with oxygen ion conductivity. The solid electrolyte body 103 contains an additive that is at least one selected from silicon oxide, sodium oxide, and iron oxide, the content of the additive being 0.004 wt.% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a sensor element, a gas sensor, and a method for manufacturing an element body. [Background technology]

[0002] Conventionally, sensor elements for detecting the concentration of a specific gas such as NOx in a measurement gas such as exhaust gas from an internal combustion engine are known (see, for example, Patent Document 1). The sensor element of Patent Document 1 includes an element body having multiple solid electrolyte layers mainly composed of an oxygen ion conductive solid electrolyte. The element body includes a gas flow section through which the measurement gas is introduced and circulated, multiple electrodes provided in the gas flow section and on the outer surface of the element body, and a heater embedded in the element body. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2019 / 188613 Brochure Summary of the Invention [Problem to be solved by the invention]

[0004] However, in such sensor elements, if minute cracks occur due to the low strength of the element body, the characteristics of the sensor element may be affected, so it is desirable to increase the strength of the element body.

[0005] The present invention has been made to solve such problems, and a main object of the present invention is to increase the strength of the element body in the sensor element. [Means for solving the problem]

[0006] In order to achieve the above-mentioned main object, the present invention employs the following means.

[0007] [1] The sensor element of the present invention comprises: A sensor element for detecting a specific gas concentration in a measurement gas, an element body having a solid electrolyte body mainly composed of an oxygen ion conductive solid electrolyte; Equipped with the solid electrolyte body contains an additive selected from the group consisting of silicon oxide, sodium oxide, and iron oxide, and the content of the additive is 0.004 wt% or more; It is something.

[0008] In this sensor element, the additive content in the solid electrolyte body is 0.004 wt% or more, which increases the strength of the solid electrolyte body and therefore increases the strength of the element body. The inventors confirmed this through experiments and analysis. Here, the "main component" refers to the component with the highest content (wt%).

[0009] [2] In the sensor element described in [1] above, the solid electrolyte body may have an additive content of 0.055 wt% or less, and the sensor element may further include a heater disposed inside the element body and generating heat when energized. This prevents the additive from making the solid electrolyte body too conductive, thereby suppressing leakage current from the heater when energized. Therefore, by setting the additive content to 0.004 wt% or more and 0.055 wt% or less, it is possible to achieve both improved strength of the element body and suppression of leakage current.

[0010] [3] In the sensor element described above (the sensor element described in [2] above), the solid electrolyte body may contain the additive in an amount of 0.040 wt % or less. This can further suppress leakage current.

[0011] [4] In the sensor element described above (the sensor element described in any one of [1] to [3] above), the element body may have a measurement gas flow section for introducing and circulating the measurement gas, an inner electrode provided in the measurement gas flow section, and an outer electrode provided on the outer surface of the element body. In particular, in the sensor element described in [2] or [3] above, when the inner electrode and the outer electrode are provided, leakage current between the heater and the inner electrode and / or the outer electrode is suppressed.

[0012] [5] In the above-described sensor element (the sensor element according to any one of [1] to [4]), the solid electrolyte body may have a plurality of solid electrolyte layers containing the solid electrolyte as a main component, and at least one of the plurality of solid electrolyte layers may have a content of the additive of 0.004 wt % or more.

[0013] [6] A gas sensor of the present invention includes the sensor element according to any one of [1] to [5] above. This gas sensor has the same effects as those of the sensor element described above, for example, the effect of increasing the strength of the element body.

[0014] [7] The method for manufacturing the element body of the present invention includes: A method for manufacturing an element body of a sensor element for detecting the concentration of a specific gas in a measurement gas, the element body having a solid electrolyte body mainly composed of an oxygen ion conductive solid electrolyte, comprising: A first step of preparing a slurry containing the solid electrolyte as a main component; a second step of forming a pre-fired element body having a pre-fired solid electrolyte body using the slurry; a third step of firing the pre-fired element body to obtain the element body; Including, the solid electrolyte body contains an additive selected from the group consisting of silicon oxide, sodium oxide, and iron oxide, and the content of the additive is 0.004 wt % or more; The slurry contains at least an additive raw material that will become the additive after firing in the third step. It is something.

[0015] This method for manufacturing an element body can produce an element body in which the additive content in the solid electrolyte is 0.004 wt% or more. Therefore, an element body with high strength can be produced. Note that "the slurry contains at least an additive raw material that will become the additive after firing in the third step" includes cases in which the slurry contains a substance that will become the additive after firing, and cases in which the slurry contains the additive itself before firing. In other words, the additive raw material may be a substance that will become the additive after firing (a precursor of the additive), or it may be the additive itself, or it may contain both. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a cross-sectional view schematically illustrating an example of the configuration of a gas sensor 100. FIG. [Figure 2] FIG. 3 is a block diagram showing the electrical connection relationship between the control device 95 and each cell, etc. [Figure 3] 3 is a process diagram of a method for manufacturing the sensor element 101. [Figure 4] FIG. 10 is a cross-sectional view of a sensor element 201 according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0017] Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view schematically illustrating an example of the configuration of a gas sensor 100 according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating the electrical connections between a control device 95, each cell, and a heater 72. The gas sensor 100 is attached to a pipe such as an exhaust gas pipe of an internal combustion engine. The gas sensor 100 detects the concentration of a specific gas, such as NOx or ammonia, in the measured gas, which is exhaust gas from the internal combustion engine. In this embodiment, the gas sensor 100 measures the NOx concentration as the specific gas concentration. The gas sensor 100 includes a sensor element 101 having an element body 102 shaped like a long rectangular parallelepiped; cells 21, 41, 50, 80 to 83 included in the sensor element 101; a heater unit 70 provided inside the sensor element 101; and a control device 95 having variable power supplies 24, 46, 52, and a heater power supply 76, and controlling the entire gas sensor 100. The longitudinal direction of the sensor element 101 (left-right direction in Figure 1) is defined as the front-to-back direction, the thickness direction of the sensor element 101 (up-to-down direction in Figure 1) is defined as the up-to-down direction, and the width direction of the sensor element 101 (direction perpendicular to the front-to-back direction and up-to-down direction) is defined as the left-to-right direction.

[0018] The element body 102 includes a solid electrolyte body 103 whose main component is an oxygen-ion conductive solid electrolyte such as zirconia (ZrO). The solid electrolyte body 103 includes multiple solid electrolyte layers whose main component is a solid electrolyte. In this embodiment, the solid electrolyte body 103 includes six solid electrolyte layers: 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. The solid electrolyte body 103 is a laminate in which the six solid electrolyte layers are stacked in this order from bottom to top as viewed in the drawing. The solid electrolyte forming these six layers is dense and airtight. The element body 102 is manufactured, for example, by performing predetermined processing and printing circuit patterns on ceramic green sheets corresponding to each layer, stacking them, and then firing them to integrate them.

[0019] On the tip end side (left end side in FIG. 1 ) of the sensor element 101 (element body 102), 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 space (oxygen concentration adjusting chamber) 20, a third diffusion rate-controlling section 30, a second internal space (oxygen concentration adjusting chamber) 40, a fourth diffusion rate-controlling section 60, and a third internal space (measurement chamber) 61 are formed adjacent to each other and communicate with each other in this order.

[0020] The gas inlet 10, the buffer space 12, the first internal space 20, the second internal space 40, and the third internal space 61 are spaces inside the sensor element 101, which are defined by hollowing out the spacer layer 5, with an upper portion defined by the underside of the second solid electrolyte layer 6, a lower portion defined by the upper surface of the first solid electrolyte layer 4, and sides defined by the side surfaces of the spacer layer 5.

[0021] The first diffusion-controlling section 11, the second diffusion-controlling section 13, and the third diffusion-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-controlling section 60 is provided as a single 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 region from the gas inlet 10 to the third internal space 61 is also referred to as a measurement gas flow section.

[0022] The sensor element 101 (element body 102) includes a reference gas inlet 49 that passes a reference gas from the outside of the sensor element 101 to the reference electrode 42 when measuring the NOx concentration. The reference gas inlet 49 includes a reference gas inlet space 43 and a reference gas inlet layer 48. The reference gas inlet space 43 is a space extending inward from the rear end surface of the sensor element 101. The reference gas inlet space 43 is located between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and is defined laterally by the side surface of the first solid electrolyte layer 4. The reference gas inlet space 43 opens at the rear end surface of the sensor element 101, and this opening functions as an inlet 49a of the reference gas inlet 49. The reference gas is introduced into the reference gas inlet space 43 from the inlet 49a. The reference gas inlet 49 applies a predetermined diffusion resistance to the reference gas introduced from the inlet 49a and introduces the reference gas into the reference electrode 42. In this embodiment, the reference gas is air.

[0023] The reference gas introduction layer 48 is provided 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 made of ceramics such as alumina. A portion of the upper surface of the reference gas introduction layer 48 is exposed within the reference gas introduction space 43. The reference gas introduction layer 48 is formed to cover 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.

[0024] Reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of third substrate layer 3 and first solid electrolyte layer 4, and as described above, is surrounded by reference gas introduction layer 48 connected to reference gas introduction space 43. As will be described later, reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) in first internal space 20, second internal space 40, and third internal space 61. Reference electrode 42 is formed as a porous cermet electrode (for example, a cermet electrode of Pt and ZrO2).

[0025] 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 into the sensor element 101 from the external space through the gas inlet 10. The first diffusion-controlling section 11 is a section that imparts a predetermined diffusion resistance to the measurement gas introduced through the gas inlet 10. The buffer space 12 is a space provided to guide the measurement gas introduced through the first diffusion-controlling section 11 to the second diffusion-controlling section 13. The second diffusion-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 sensor element 101 into the first internal space 20, the measurement gas is suddenly taken into the sensor element 101 through the gas inlet 10 due to pressure fluctuations of the measurement gas in the external space (exhaust pressure pulsations in the case where the measurement gas is automobile exhaust gas). The measurement gas 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 out through the first diffusion-controlling section 11, buffer space 12, and second diffusion-controlling section 13. This makes the pressure fluctuations of the measurement gas introduced into the first internal space 20 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-controlling section 13. The oxygen partial pressure is adjusted by operating the main pump cell 21.

[0026] 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 on the upper surface of the second solid electrolyte layer 6 in a region corresponding to the ceiling electrode portion 22a so as to be exposed to the outside of the sensor element 101, and the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4 which form a current path between these electrodes.

[0027] 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 configure 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-like structure at the locations where the side electrode portions are provided.

[0028] The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (for example, a cermet electrode of Pt and ZrO containing 1% Au). 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.

[0029] In the main pump cell 21, by applying a desired 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 from the first internal space 20 out to the external space, or to pump oxygen from the external space into the first internal space 20.

[0030] 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.

[0031] 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 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.

[0032] The third diffusion control section 30 is a section that imparts 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.

[0033] 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-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 precision, thereby enabling the gas sensor 100 to measure the NOx concentration with high precision.

[0034] The auxiliary pump cell 50 is an auxiliary electrochemical pump cell that is configured by 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 appropriate electrode on the outside of the sensor element 101 will suffice), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0035] The auxiliary pump electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to the inner pump electrode 22 disposed in the first internal space 20. That is, a ceiling electrode portion 51a is formed on the lower surface of the second solid electrolyte layer 6, which provides the ceiling surface of the second internal space 40, and a bottom electrode portion 51b is formed on the upper surface of the first solid electrolyte layer 4, which provides the bottom surface of the second internal space 40. Side electrodes (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, which provide the side walls of the second internal space 40. 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.

[0036] 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.

[0037] 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 configured 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.

[0038] 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 within the second internal space 40 to be controlled to a low level that does not substantially affect the measurement of NOx.

[0039] 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 target value of the voltage V0 is controlled to always maintain a constant gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion-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 maintained at a constant value of approximately 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

[0040] The fourth diffusion rate-controlling part 60 is a part that applies a predetermined diffusion resistance to the measurement gas, the oxygen concentration (oxygen partial pressure) of which 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 that flows into the third internal space 61.

[0041] The third internal space 61 is provided as a space for performing processing related to measurement of the nitrogen oxide (NOx) concentration in the measurement gas, which is 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 NOx concentration is measured mainly in the third internal space 61 by the operation of the measurement pump cell 41.

[0042] 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 that has 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.

[0043] In the measuring pump cell 41, oxygen produced by decomposition of nitrogen oxides in the atmosphere surrounding the measuring electrode 44 is pumped out, and the amount of oxygen produced can be detected as a pump current Ip2.

[0044] 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, i.e., an oxygen partial pressure detection sensor cell 82 for controlling the measurement pump. 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.

[0045] 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-controlling part 60 under conditions where 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. This 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 remains constant (target value). Because the amount of oxygen generated around the measurement electrode 44 is proportional to the nitrogen oxide concentration in the measurement gas, the pump current Ip2 in the measurement pump cell 41 is used to calculate the nitrogen oxide concentration in the measurement gas.

[0046] Furthermore, by combining the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 to form an oxygen partial pressure detection means as an electrochemical sensor cell, it is possible to detect an electromotive force corresponding to the difference between the amount of oxygen generated by reduction of the NOx components in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference gas, thereby making it possible to determine the concentration of the NOx components in the measured gas.

[0047] 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, and the electromotive force (voltage Vref) obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measurement gas outside the sensor.

[0048] In the gas sensor 100 having such a configuration, the measurement gas, in which the oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx) by operating the main pump cell 21 and the auxiliary pump cell 50, is supplied to the measurement pump cell 41. Therefore, the NOx concentration in the measurement gas can be determined 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.

[0049] In order to enhance the oxygen ion conductivity of the solid electrolyte, the sensor element 101 includes a heater unit 70 that adjusts the temperature by heating and maintaining the temperature of the sensor element 101. The heater unit 70 includes a heater connector electrode 71, a heater 72, a through-hole 73, a heater insulating layer 74, and a pressure release hole 75.

[0050] 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 a heater power supply 76 (see FIG. 2), it is possible to supply power from the heater power supply 76 to the heater section 70.

[0051] 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 through hole 73, and generates heat when power is supplied from a heater power supply 76 through the heater connector electrode 71, thereby heating and keeping warm the solid electrolyte that forms the sensor element 101.

[0052] The heater 72 is embedded throughout the entire area from the first internal space 20 to the third internal space 61, and is capable of adjusting the temperature of the entire sensor element 101 to a temperature at which the solid electrolyte is activated.

[0053] The heater insulating layer 74 is an insulating layer made of an insulator such as alumina and formed 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.

[0054] The pressure release hole 75 is a portion that penetrates the third substrate layer 3 and the reference gas introduction layer 48 and is provided so as to communicate with the reference gas introduction space 43, and is formed for the purpose of alleviating the increase in internal pressure that accompanies a rise in temperature within the heater insulation layer 74.

[0055] 2, the control device 95 includes the variable power supplies 24, 46, and 52, the heater power supply 76, and a control unit 96. The control unit 96 is a microprocessor including a CPU 97 and a storage unit 98. The storage unit 98 is a rewritable nonvolatile memory capable of storing, for example, various programs and data. The control unit 96 receives the voltage V0 of the oxygen partial pressure detection sensor cell 80 for controlling the main pump, the voltage V1 of the oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump, the voltage V2 of the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump, the voltage Vref of the sensor cell 83, the pump current Ip0 flowing through the main pump cell 21, the pump current Ip1 flowing through the auxiliary pump cell 50, and the pump current Ip2 flowing through the measurement pump cell 41. The control unit 96 also outputs control signals to the variable power supplies 24, 46, 52 to control the voltages Vp0, Vp1, Vp2 output by the variable power supplies 24, 46, 52, thereby controlling the main pump cell 21, the measurement pump cell 41, and the auxiliary pump cell 50. The control unit 96 also outputs control signals to the heater power supply 76 to control the power supplied from the heater power supply 76 to the heater 72. The memory unit 98 also stores target values ​​V0*, V1*, V2*, etc., which will be described later. The CPU 97 of the control unit 96 controls each of the cells 21, 41, 50 by referring to these target values ​​V0*, V1*, V2*.

[0056] The control unit 96 performs an auxiliary pump control process to control the auxiliary pump cell 50 so that the oxygen concentration in the second internal space 40 becomes a target concentration. Specifically, the control unit 96 controls the auxiliary pump cell 50 by feedback-controlling the voltage Vp1 of the variable power supply 52 so that the voltage V1 becomes a constant value (referred to as a target value V1*). The target value V1* is set as a value that makes the oxygen concentration in the second internal space 40 a predetermined low concentration that does not substantially affect the measurement of NOx.

[0057] The control unit 96 performs a main pump control process to control the main pump cell 21 so that the pump current Ip1 flowing when the auxiliary pump cell 50 adjusts the oxygen concentration in the second internal space 40 by the auxiliary pump control process becomes a target current (referred to as a target value Ip1*). Specifically, the control unit 96 sets (feedback controls) a target value (referred to as a target value V0*) of the voltage V0 based on the pump current Ip1 so that the pump current Ip1 flowing due to the voltage Vp1 becomes a constant target value Ip1*. The control unit 96 then feedback controls the voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes the target value V0* (i.e., so that the oxygen concentration in the first internal space 20 becomes the target concentration). This main pump control process ensures that 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 is always constant. The target value V0* is set to a value such that the oxygen concentration in the first internal space 20 is higher but lower than 0%. Furthermore, the pump current Ip0 flowing during this main pump control process varies depending on the oxygen concentration of the measurement gas (i.e., the measurement gas around the sensor element 101) flowing into the measurement gas flow section from the gas inlet 10. Therefore, the control section 96 can also detect the oxygen concentration in the measurement gas based on the pump current Ip0.

[0058] The above-described main pump control process and auxiliary pump control process are also collectively referred to as adjustment pump control process. The first internal space 20 and the second internal space 40 are also collectively referred to as the oxygen concentration adjustment chamber. The main pump cell 21 and the auxiliary pump cell 50 are also collectively referred to as the adjustment pump cell. The control unit 96 performs the adjustment pump control process, causing the adjustment pump cell to adjust the oxygen concentration in the oxygen concentration adjustment chamber.

[0059] Furthermore, the control unit 96 performs a measurement pump control process to control the measurement pump cell 41 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). Specifically, the control unit 96 controls the measurement pump cell 41 by feedback-controlling the voltage Vp2 of the variable power supply 46 so that the voltage V2 becomes the target value V2*. Oxygen is pumped out of the third internal space 61 by this measurement pump control process.

[0060] By performing the measurement pump control process, oxygen is pumped out of the third internal space 61 so that the amount of oxygen generated by the reduction of NOx in the measurement gas in the third internal space 61 becomes substantially zero. Then, the control unit 96 acquires the pump current Ip2 as a detection value corresponding to the oxygen generated in the third internal space 61 due to the specific gas (here, NOx), and calculates the NOx concentration in the measurement gas based on this pump current Ip2.

[0061] The storage unit 98 stores a relational expression (for example, a linear or quadratic function) 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.

[0062] The control unit 96 performs heater control processing, which outputs a control signal to the heater power supply 76 to control the temperature of the heater 72 to a target temperature (e.g., 800°C). Here, the temperature of the heater 72 can be expressed as a linear function of the resistance of the heater 72. Therefore, in the heater control processing, the control unit 96 calculates the resistance of the heater 72 as a value that can be regarded as the temperature of the heater 72 (a value that can be converted to temperature), and feedback-controls the heater power supply 76 so that the calculated resistance becomes the target resistance (the resistance corresponding to the target temperature). The control unit 96 can, for example, acquire the voltage of the heater 72 and the current flowing through the heater 72 and calculate the resistance of the heater 72 based on the acquired voltage and current. The control unit 96 may calculate the resistance of the heater 72 using, for example, a three-terminal method or a four-terminal method. When energizing the heater 72, the heater power supply 76 adjusts the power supplied to the heater 72 by, for example, changing the value of the voltage applied to the heater 72 based on a control signal from the control unit 96.

[0063] The control device 95, including the variable power supplies 24, 46, 52 and heater power supply 76 shown in FIG. 2, is actually connected to each electrode inside the sensor element 101 via lead wires (not shown) formed within the sensor element 101 and connector electrodes (not shown) formed on the rear end side of the sensor element 101 (only the heater connector electrode 71 is shown in FIG. 1).

[0064] Here, the solid electrolyte substrate 103 will be described in detail. As described above, the solid electrolyte substrate 103 is mainly composed of an oxygen ion conductive solid electrolyte, and in this embodiment, the solid electrolyte is zirconia. Here, the main component refers to the component with the highest content (wt%). The solid electrolyte is preferably stabilized zirconia to which a stabilizer has been added. Examples of stabilizers include yttria (YO), calcia (CaO), magnesia (MgO), and ceria (CeO), and the stabilized zirconia may contain one or more of these stabilizers. The zirconia in this embodiment is yttria-stabilized zirconia (YSZ) to which yttria has been added as a stabilizer.

[0065] The solid electrolyte substrate 103 also contains an additive selected from the group consisting of silicon oxide (SiO and / or SiO), sodium oxide (NaO), and iron oxide (FeO and / or FeO), with the additive content being 0.004 wt% or more. In this embodiment, the solid electrolyte substrate 103 contains silicon dioxide (SiO), sodium oxide, and iron (III) oxide (FeO), with the total content being 0.004 wt% or more. By ensuring that the additive content in the solid electrolyte substrate 103 is 0.004 wt% or more, the strength of the solid electrolyte substrate 103 is increased, thereby increasing the strength of the element body 102. This is thought to be because, as grain growth of the solid electrolyte (here, zirconia) progresses during firing (step S120, described below) during the manufacture of the element body 102, the strength of the solid electrolyte body 103 of the element body 102 after firing decreases. However, this grain growth is suppressed by the additive content of 0.004 wt% or more after firing. As described above, the solid electrolyte body 103 has multiple solid electrolyte layers (layers 1 to 6). In this embodiment, the additive content is the same for all of the multiple solid electrolyte layers (layers 1 to 6). The solid electrolyte body 103 may contain at least sodium oxide (NaO) and iron (III) oxide (FeO) as additives. The solid electrolyte body 103 may not contain alumina (AlO). The solid electrolyte body 103 may not contain any of alumina (AlO), zinc oxide, calcium carbonate, barium carbonate, magnesium carbonate, silicon nitride, aluminum nitride, titania, or spinel. The term "not containing" includes cases where the content is below the detection limit.

[0066] The solid electrolyte body 103 preferably contains an additive at a content of 0.055 wt% or less. The higher the additive content, the higher the conductivity of the solid electrolyte body 103. Therefore, by setting the additive content to 0.055 wt% or less, the conductivity of the solid electrolyte body 103 can be prevented from becoming too high due to the additive, thereby suppressing leakage current from the heater 72 when power is applied to the heater 72. Therefore, by setting the additive content to 0.004 wt% or more and 0.055 wt% or less, both the strength of the element body 102 and leakage current suppression can be achieved. When leakage current from the heater 72 occurs, the leakage current flows between the heater 72 and at least one of the outer pump electrode 23, the inner pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44. This leakage current causes noise in at least one of the pump currents Ip0, Ip1, and Ip2, which may reduce the accuracy of detecting the concentration of a specific gas. In particular, if the leakage current affects the pump current Ip2, the accuracy of detecting the specific gas concentration is likely to decrease. By setting the additive content of the solid electrolyte body 103 to 0.055 wt% or less, the decrease in the accuracy of detecting the specific gas concentration caused by the leakage current can be suppressed. The additive content of the solid electrolyte body 103 is more preferably 0.040 wt% or less. This can further suppress the leakage current, and therefore the decrease in the accuracy of detecting the specific gas concentration. The additive content of the solid electrolyte body 103 is a value measured using time-of-flight secondary ion mass spectrometry (TOF-SIMS).

[0067] Next, a method for manufacturing the sensor element 101 of the gas sensor 100 will be described. FIG. 3 is a process chart showing an example of the steps of the method for manufacturing the sensor element 101. As shown in FIG. 3, in the method for manufacturing the sensor element 101, first, a slurry containing an oxygen ion conductive solid electrolyte as a main component is prepared (step S100). Next, the prepared slurry is used to form a pre-fired element body 102 having a pre-fired solid electrolyte body 103 (step S110). Then, the pre-fired element body 102 is fired to obtain the element body 102 (step S120).

[0068] Step S100 will be described. In step S100, a slurry is prepared by mixing slurry raw materials containing an oxygen-ion conductive solid electrolyte. In addition to the oxygen-ion conductive solid electrolyte, the slurry raw materials also contain the stabilizer, additive raw materials, and a solvent. Examples of the solvent include organic solvents such as acetone and monohydric alcohols, and inorganic solvents such as water. The additive raw materials refer to substances that will become the additives described above after firing in step S120. That is, the additive raw materials may be substances that will become the additives after firing (additive precursors), the additives themselves, or both. Among the additive raw materials, examples of substances that will become silicon oxide after firing (silicon sources) include organic silicon compounds such as tetraethyl orthosilicate (TEOS) and tetramethylsilane (TMS), silicic acids such as orthosilicic acid, or silicon oxide. Among the additive raw materials, examples of substances that will become iron oxide after firing (iron sources) include organic iron compounds such as ferrocene, or iron oxide. Among the additive raw materials, examples of the substance (sodium source) that becomes sodium oxide after firing include organic sodium compounds such as sodium phenoxide, sodium carbonate, or sodium oxide. The additive raw material may also include sodium orthosilicate, which is a silicon and sodium source, or iron silicate, which is a silicon and iron source. The content of the additive raw material in the slurry is determined in advance based on experiments and analyses so that the additive content of the solid electrolyte layer 103 in the element body 102 after firing in step S120 is a desired value of 0.004 wt% or more (preferably 0.055 wt% or less, and more preferably 0.040 wt% or less). In this embodiment, the slurry raw materials are zirconia as a solid electrolyte, yttria as a stabilizer, orthosilicate methyl ester, sodium carbonate, and ferrocene as additive raw materials, and toluene and isopropanol as solvents, and a slurry is prepared by mixing these. In step S100, instead of preparing the slurry by mixing the slurry raw materials, a pre-prepared slurry may be prepared.

[0069] Step S110 will be described. In step S110, first, the slurry prepared in step S100 is used to form a plurality of (six in this embodiment) unsintered ceramic green sheets corresponding to the layers 1 to 6 of the solid electrolyte body 103. Examples of the forming method include tape casting. Subsequently, as needed, notches, through-holes, grooves, etc. are formed in each green sheet by punching, or patterns for the electrodes 22, 23, 42, 44, 51 and the heater section 70 are formed by screen printing. Each green sheet is then dried, and the plurality of green sheets are then stacked to form a laminate. The resulting laminate includes a plurality of pre-sintered element bodies 102. The laminate is then cut to the size of the element bodies 102. As a result, pre-sintered element bodies 102 having pre-sintered solid electrolyte bodies 103 are obtained.

[0070] Step S120 will be described. In step S120, the element body 102 before firing is fired at a predetermined firing temperature to obtain the element body 102 (sensor element 101). After step S120, a porous protective layer that covers the outer surface of the front end side of the element body 102 may be formed by, for example, plasma spraying or dipping, to manufacture the sensor element 101 including the element body 102 and the porous protective layer.

[0071] Here, the correspondence between the components of this embodiment and the components of the present invention will be clarified. The solid electrolyte body 103 of this embodiment corresponds to the solid electrolyte body of the present invention, and the element body 102 corresponds to the element body. Furthermore, the heater 72 corresponds to the heater, the inner pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44 each correspond to an inner electrode, and the outer pump electrode 23 corresponds to an outer electrode. Furthermore, step S100 corresponds to the first step, step S110 corresponds to the second step, and step S120 corresponds to the third step.

[0072] The sensor element 101 provided in the gas sensor 100 of this embodiment described above has an additive content of 0.004 wt% or more in the solid electrolyte body 103, which increases the strength of the solid electrolyte body 103 and therefore increases the strength of the element body 102.

[0073] Furthermore, by setting the additive content of the solid electrolyte body 103 to 0.055 wt% or less, the conductivity of the solid electrolyte body 103 can be prevented from becoming too high due to the additive, and leakage current from the heater 72 when current is applied to the heater 72 can be suppressed. Therefore, by setting the additive content to 0.004 wt% or more and 0.055 wt% or less, it is possible to improve the strength of the element body 102 and suppress leakage current at the same time. Furthermore, by setting the additive content of the solid electrolyte body 103 to 0.040 wt% or less, leakage current can be further suppressed.

[0074] It goes without saying that the present invention is not limited to the above-described embodiment, and can be embodied in various forms as long as they fall within the technical scope of the present invention.

[0075] For example, in the above-described embodiment, the additive content of each of the multiple solid electrolyte layers (layers 1 to 6) of the solid electrolyte substrate 103 is the same. However, this is not limiting and solid electrolyte layers with different additive content percentages may exist. In this case, it is sufficient that at least one of the multiple solid electrolyte layers of the solid electrolyte substrate 103 has an additive content of 0.004 wt% or more. This increases the strength of the element body 102 compared to when all of the multiple solid electrolyte layers have an additive content of less than 0.004 wt%. Furthermore, it is preferable that at least one of the multiple solid electrolyte layers of the solid electrolyte substrate 103 has an additive content of 0.055 wt% or less, and more preferably 0.040 wt% or less. It is also preferable that all of the multiple solid electrolyte layers of the solid electrolyte substrate 103 have an additive content of 0.004 wt% or more. It is preferable that the additive content of each of the plurality of solid electrolyte layers of the solid electrolyte body 103 is 0.055 wt% or less, and it is more preferable that the additive content of each of the plurality of solid electrolyte layers is 0.040 wt% or less.

[0076] In the above-described embodiment, the outer pump electrode 23 serves as an electrode paired with the inner pump electrode 22 in the main pump cell 21 (also referred to as an outer main pump electrode), an electrode paired with the auxiliary pump electrode 51 in the auxiliary pump cell 50 (also referred to as an outer auxiliary pump electrode), and an electrode paired with the measurement electrode 44 in the measurement pump cell 41 (also referred to as an outer measurement electrode), but is not limited to this. Any one or more of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be provided separately from the outer pump electrode 23 on the outer surface of the element body 102 so as to come into contact with the measurement gas.

[0077] In the above-described embodiment, the outer pump electrode 23 is exposed to the outside of the sensor element 101, but this is not limiting and the outer pump electrode 23 may be provided on the outer surface of the element body 102 so as to be in contact with the measurement gas. For example, the sensor element 101 may be provided with a porous protective layer that covers the element body 102 and allows the measurement gas to pass through, and the outer pump electrode 23 may also be covered with the porous protective layer.

[0078] In the above-described embodiment, the sensor element 101 of the gas sensor 100 includes the first internal space 20, the second internal space 40, and the third internal space 61. However, this is not limiting. For example, a sensor element 201 shown in FIG. 4 may not include the third internal space 61. In the modified sensor element 201 shown in FIG. 4, a gas inlet 10, a first diffusion-controlling section 11, a buffer space 12, a second diffusion-controlling section 13, the first internal space 20, the third diffusion-controlling section 30, and the second internal space 40 are adjacently formed and communicated in this order between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The measurement electrode 44 is disposed on the upper surface of the first solid electrolyte layer 4 within the second internal space 40. The measurement electrode 44 is covered with a fourth diffusion-controlling section 45. The fourth diffusion rate-controlling portion 45 is a film made of a porous ceramic material such as alumina (Al2O3). Similar to the fourth diffusion rate-controlling portion 60 of the above-described embodiment, the fourth diffusion rate-controlling portion 45 serves to limit the amount of NOx flowing into the measurement electrode 44. The fourth diffusion rate-controlling portion 45 also functions as a protective film for the measurement electrode 44. The ceiling electrode portion 51a of the auxiliary pump electrode 51 is formed up to directly above the measurement electrode 44. Even with the sensor element 201 configured in this manner, the NOx concentration can be detected based on, for example, the pump current Ip2, as in the above-described embodiment. In this case, the area around the measurement electrode 44 functions as a measurement chamber.

[0079] In the above-described embodiment, the sensor element 101 detects the NOx concentration in the measurement gas. However, this is not limiting, as long as it detects the concentration of a specific gas in the measurement gas. For example, the specific gas concentration may be other oxide concentrations, not just 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-described embodiment. The measurement pump cell 41 can detect the specific gas concentration by obtaining a detection value corresponding to this oxygen (e.g., pump current Ip2). Alternatively, 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). When the converted gas is reduced in the third internal space 61, oxygen is generated. The measurement pump cell 41 can detect the specific gas concentration by obtaining a detection value corresponding to this oxygen (e.g., pump current Ip2). For example, the inner pump electrode 22 in the first internal space 20 functions as a catalyst, thereby converting ammonia to NO in the first internal space 20.

[0080] In the above-described embodiment, the solid electrolyte body 103 of the sensor element 101 is a laminate having multiple solid electrolyte layers (layers 1 to 6), but this is not limiting. The solid electrolyte body 103 may be composed of a single solid electrolyte layer. In other words, the element body 102 of the sensor element 101 only needs to include at least one oxygen-ion conductive solid electrolyte layer. For example, in FIG. 1, layers 1 to 5 other than the second solid electrolyte layer 6 may be layers made of a material other than a solid electrolyte (e.g., a layer made of alumina). In this case, each electrode of the sensor element 101 may be disposed on the second solid electrolyte layer 6. For example, the measurement electrode 44 in FIG. 1 may be disposed on the underside of the second solid electrolyte layer 6. Alternatively, the reference gas introduction space 43 may be provided in the spacer layer 5 instead of in the first solid electrolyte layer 4, the reference gas introduction layer 48 may be provided between the second solid electrolyte layer 6 and the spacer layer 5 instead of between the first solid electrolyte layer 4 and the third substrate layer 3, and the reference electrode 42 may be provided behind the third internal space 61 and on the underside of the second solid electrolyte layer 6. In this case, the second solid electrolyte layer 6 corresponds to the solid electrolyte body 103.

[0081] In the above-described embodiment, the control unit 96 sets (feedback-controls) the target value V0* of the voltage V0 based on the pump current Ip1 so that the pump current Ip1 becomes the target value Ip1*, and then feedback-controls the voltage Vp0 so that the voltage V0 becomes the target value V0*. However, other control may be performed. For example, the control unit 96 may feedback-control the voltage Vp0 based on the pump current Ip1 so that the pump current Ip1 becomes the target value Ip1*. That is, the control unit 96 may omit obtaining the voltage V0 from the main pump control oxygen partial pressure detection sensor cell 80 and setting the target value V0*, and may directly control the voltage Vp0 (and thus the pump current Ip0) based on the pump current Ip1. [Example]

[0082] Hereinafter, specific examples of fabricating a sensor element will be described as examples, but the present invention is not limited to the following examples.

[0083] [Examples 1 to 5, Comparative Example 1] The sensor elements 101 were fabricated by the same manufacturing method as in the above-described embodiment, and were designated as Examples 1 to 5 and Comparative Example 1. The sensor elements 101 of Examples 1 to 5 and Comparative Example 1 were fabricated by the same method, except that the content ratio of the additive raw material in the slurry for the solid electrolyte body 103 prepared in step S100 was changed in various ways.

[0084] [Measurement of additive content] For each of the sensor elements 101 of Examples 1 to 5 and Comparative Example 1, the composition of the solid electrolyte body 103 was analyzed using the above-mentioned TOF-SIMS to measure the content ratio of the additive in the solid electrolyte body 103. The measurement results are shown in Table 1. As shown in Table 1, in all of Examples 1 to 5 and Comparative Example 1, the components of the solid electrolyte body 103 other than the additive were all yttria-stabilized zirconia (YSZ). The TOF-SIMS measurement conditions were as follows:

[0085] Measurement equipment: ION-TOF TOF-SIMS 5 Etching ion species: O2+ Etching ion acceleration energy: 1 keV Primary ion species: Bi + Primary ion energy: 25 keV Secondary ion polarity: positive Analysis mode: high mass resolution Charge compensation: E-gun, Pt-coat

[0086] [Table 1]

[0087] [Evaluation of leakage current] For each of the sensor elements 101 of Examples 1 to 5 and Comparative Example 1, the leakage current from the heater 72 in the sensor element 101 was measured using a method conforming to the insulation test described in Japanese Patent No. 4881334. Specifically, an insulation test device for applying a voltage and measuring a current between the outer pump electrode 23 and the heater 72 of the sensor element 101 was first connected, and the sensor element 101 was placed in a high-temperature furnace at 700°C to activate the solid electrolyte in the solid electrolyte body 103. Next, the current flowing due to the thermoelectric power between the outer pump electrode 23 and the heater 72 in this state was measured using the insulation test device. It was confirmed that a current equal to or greater than a predetermined threshold value (e.g., a value less than 1 μA) flowed, confirming that there was no continuity defect in the insulation test device. Next, a voltage of approximately 30 V was applied between the outer pump electrode 23 and the heater 72 using a variable power supply provided in the insulation test device, and the leakage current flowing between the outer pump electrode 23 and the heater 72 was measured. If the measured leakage current was greater than 500 μA, the leakage current was evaluated as "poor (F)." If the measured leakage current was less than 500 μA but greater than 100 μA, the leakage current was evaluated as "good (B)." If the measured leakage current was less than 100 μA, the leakage current was evaluated as "excellent (A)." The leakage current evaluation results are shown in Table 1. The smaller the leakage current value, the better the insulation between the heater 72 and the outer pump electrode 23 is maintained by the solid electrolyte body 103.

[0088] [Evaluation of the strength of the element body] For each of the sensor elements 101 of Examples 1 to 5 and Comparative Example 1, the strength of the element body 102 of the sensor element 101 was evaluated by a method conforming to the crack inspection described in Japanese Patent No. 5139833. Specifically, first, a test temperature rise profile (a profile of voltage applied to the heater 72) for the heater 72 was prepared, which would result in a faster temperature rise rate than the temperature rise of the heater 72 when the control unit 96 executes the heater control process in the above-described embodiment (i.e., the temperature rise of the heater 72 during actual use of the sensor element 101). Then, the control device 95 was connected to the sensor element 101, and the control unit 96 controlled the heater power supply 76 in accordance with this test temperature rise profile to raise the temperature of the heater 72. By applying such a voltage, minute cracks occurring in the element body 102 of the sensor element 101 were made apparent by thermal stress. In this state, the front end side of the sensor element 101 was exposed to a model gas with a known NOx concentration, and the pump current Ip2 that flowed when the control device 95 performed the above-mentioned adjustment pump control process and measurement pump control process was measured. If the measured value of the pump current Ip2 was within a predetermined allowable range that could be considered to correspond to the known NOx concentration of the model gas, it was determined that there were not many microcracks in the element body 102 and that the strength of the element body 102 was "high (A)." If the measured value of the pump current Ip2 was outside the predetermined allowable range, it was determined that there were few microcracks in the element body 102 and that the strength of the element body 102 was "low (F)."

[0089] As can be seen from Table 1, Comparative Example 1, in which the additive content in the solid electrolyte substrate 103 was less than 0.004 wt%, gave the element body 102 a strength evaluation of "low (F)," whereas Examples 1 to 5, in which the additive content was 0.004 wt% or more, gave the element body 102 a strength evaluation of "high (A)." These results confirmed that the strength of the element body 102 was improved when the additive content in the solid electrolyte substrate 103 was 0.004 wt% or more.

[0090] Furthermore, none of Examples 1 to 5 and Comparative Example 1 were rated as "poor (F)" for leakage current, but Example 1, in which the additive content in the solid electrolyte body 103 was 0.055 wt%, was rated as "good (B)" for leakage current, whereas Examples 2 to 5 and Comparative Example 1, in which the additive content was 0.040 wt% or less, were rated as "excellent (A)" for leakage current. These results confirmed that the additive content in the solid electrolyte body 103 of 0.055 wt% or less can suppress leakage current from the heater 72, and that the additive content of 0.040 wt% or less can further suppress leakage current.

[0091] From the results of Examples 1 to 5 and Comparative Example 1, it was confirmed that, unlike Comparative Example 1, Examples 1 to 5, in which the additive content of the solid electrolyte body 103 is 0.004 wt% or more and 0.055 wt% or less, can achieve both improved strength of the element body 102 and suppression of leakage current. [Industrial Applicability]

[0092] The present invention can be used in a gas sensor that detects the concentration of a specific gas such as NOx in a measurement gas such as an automobile exhaust gas. [Explanation of symbols]

[0093] 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-controlling section, 12 buffer space, 13 second diffusion rate-controlling 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-controlling section, 40 second internal space, 41 measurement pump cell, 42 reference electrode, 43 reference gas introduction space, 44 measurement electrode, 46 variable power supply, 48 reference gas introduction layer, 49 reference gas introduction section, 49a inlet section, 50 auxiliary pump cell, 51 auxiliary pump electrode, 51a ceiling electrode section, 51b bottom electrode section, 52 variable power supply, 60 fourth diffusion rate-controlling section, 61 Third internal cavity, 70 heater portion, 71 heater connector electrode, 72 heater, 73 through hole, 74 heater insulating layer, 75 pressure release hole, 76 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, 95 control device, 96 control unit, 97 CPU, 98 memory unit, 100 gas sensor, 101, 201 sensor element, 102 element body, 103 solid electrolyte body.

Claims

1. A sensor element for detecting a specific gas concentration in a measurement gas, an element body having a solid electrolyte body mainly composed of an oxygen ion conductive solid electrolyte; Equipped with the solid electrolyte body contains an additive selected from the group consisting of silicon oxide, sodium oxide, and iron oxide, and the content of the additive is 0.004 wt % or more; Sensor element.

2. 2. The sensor element according to claim 1, the solid electrolyte body has a content of the additive of 0.055 wt % or less, The sensor element further includes a heater provided inside the element body and configured to generate heat when energized. Sensor element.

3. 3. The sensor element according to claim 2, The solid electrolyte body has a content of the additive of 0.040 wt % or less. Sensor element.

4. The sensor element according to any one of claims 1 to 3, The element body includes a measurement gas flow portion through which the measurement gas is introduced and flows, an inner electrode provided in the measurement gas flow portion, and an outer electrode provided on an outer surface of the element body. Sensor element.

5. The sensor element according to any one of claims 1 to 3, the solid electrolyte body has a plurality of solid electrolyte layers containing the solid electrolyte as a main component, and at least one of the plurality of solid electrolyte layers has a content of the additive of 0.004 wt % or more; Sensor element.

6. A gas sensor comprising the sensor element according to any one of claims 1 to 3.

7. A method for manufacturing an element body of a sensor element for detecting the concentration of a specific gas in a measurement gas, the element body having a solid electrolyte body mainly composed of an oxygen ion conductive solid electrolyte, comprising: a first step of preparing a slurry containing the solid electrolyte as a main component; a second step of forming a pre-fired element body having a pre-fired solid electrolyte body using the slurry; a third step of firing the pre-fired element body to obtain the element body; Including, the solid electrolyte body contains an additive selected from the group consisting of silicon oxide, sodium oxide, and iron oxide, and the content of the additive is 0.004 wt % or more; The slurry contains at least an additive raw material that will become the additive after firing in the third step. Manufacturing method of the element body.

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