Sensor element and gas sensor

A sensor element includes an element body and a porous protective layer arranged to cover a part of a surface of the element body. The protective layer includes an inlet protective layer arranged to cover a gas inlet formed in the surface of the element body, and at least a part of a face included in the surface of the element body, the face on which the gas inlet is opens, and an arithmetic average roughness Rap of an inner peripheral surface of an internal space of the inlet protective layer satisfies at least one of conditions below: the arithmetic average roughness Rap is 8 μm or more, and the arithmetic average roughness Rap is higher than an arithmetic average roughness Rac of a bonding surface of the protective layer, the bonding surface at which the protective layer is bonded to the element body.

BACKGROUND OF THE INVENTION

1. Field of the Invention

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

2. Description of the Related Art

Gas sensors including a sensor element that detects the concentration of a specific gas, such as NOx, in a measurement-object gas, such as an automobile exhaust gas, are known. It is also known that some of these gas sensors include a protective layer that covers the surface of the sensor element, the protective layer having a space formed therein (e.g., PTL 1). In PTL 1, the protective layer has an exposure space at which the surface of the element body is exposed. The exposure space limits a reduction in the temperature of the element body which may occur when water is adhered onto the surface of the protective layer and thereby enhances the waterproofing performance of the element body.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

When the sensor element detects a specific gas concentration in a measurement-object gas, the detected specific gas concentration may vary although the specific gas concentration does not fluctuate in reality.

The present invention was made in order to address the above issue. A main object is to reduce variations in the specific gas concentration detected by the sensor element.

In the present invention, the following measures were adopted so as to achieve the main object.

A sensor element of the present invention detects a specific gas concentration in a measurement-object gas, the sensor element includes:an element body including an oxygen-ion-conductive solid electrolyte body, the element body having a measurement-object gas flow section formed therein, the measurement-object gas flow section through which a measurement-object gas is introduced and flows;a measurement electrode disposed on an inner peripheral surface of the measurement-object gas flow section;a reference electrode disposed in the element body, the reference electrode being exposed to a reference gas used as a reference for detecting the specific gas concentration; anda porous protective layer arranged to cover a part of a surface of the element body,wherein the protective layer includes an inlet protective layer arranged to cover a gas inlet formed in the surface of the element body, the gas inlet being an inlet of the measurement-object gas flow section, and at least a part of a face included in the surface of the element body, the face on which the gas inlet is opens,the inlet protective layer has an internal space formed therein, andan arithmetic average roughness Rap of an inner peripheral surface of the internal space of the inlet protective layer satisfies at least one of conditions below: the arithmetic average roughness Rap is 8 μm or more, and the arithmetic average roughness Rap is higher than an arithmetic average roughness Rac of a bonding surface of the protective layer, the bonding surface at which the protective layer is bonded to the element body.

The above-described sensor element includes a measurement electrode disposed on the inner peripheral surface of the measurement-object gas flow section and a reference electrode exposed to a reference gas used as a reference for detecting a specific gas concentration. This sensor element is capable of detecting the specific gas concentration in a measurement-object gas on the basis of the voltage between the measurement electrode and the reference electrode. The sensor element also includes an inlet protective layer covering a gas inlet formed in the surface of the element body, the gas inlet being an inlet of the measurement-object gas flow section, and at least a part of a face included in the surface of the element body, the face on which the gas inlet opens. The arithmetic average roughness Rap of the inner peripheral surface of an internal space formed in the inlet protective layer satisfies at least one of the conditions below: the arithmetic average roughness Rap is 8 μm or more, and the arithmetic average roughness Rap is higher than the arithmetic average roughness Rac of the bonding surface of the protective layer at which the protective layer is bonded to the element body. That is, the inner peripheral surface of the internal space of the introduction protective layer has a relatively high arithmetic average roughness Rap, that is, relatively large irregularities formed therein. Consequently, when a measurement-object gas is passed from the outside of the protective layer to the gas inlet through the internal space of the inlet protective layer, the irregularities present in the inner peripheral surface of the internal space cause the flow of the measurement-object gas in the internal space to be converted into a turbulent flow. The turbulent flow stirs the measurement-object gas and thereby increases the uniformity in the specific gas concentration in the measurement-object gas. As a result, variations in the specific gas concentration in the measurement-object gas introduced into the measurement-object gas flow section are reduced and, accordingly, fluctuations in the voltage between the measurement electrode and the reference electrode which are caused due to the variations in the specific gas concentration are reduced. Thus, variations in the specific gas concentration detected by the sensor element can be reduced.

In the above case, the arithmetic average roughness Rap may be 100 μm or less. If the arithmetic average roughness Rap is higher than 100 the irregularities present in the inner peripheral surface of the internal space of the inlet protective layer increase resistance to the flow of a measurement-object gas and reduce the likelihood of the measurement-object gas reaching the gas inlet. This may reduce the responsivity of the sensor element. When the arithmetic average roughness Rap is 100 or less, the reduction in responsivity can be prevented. The gas inlet may be formed in the internal space of the inlet protective layer. The internal space of the inlet protective layer may be an exposure space to which the surface of the element body is exposed. The element body may have an elongate shape having a longitudinal direction. The element body may have an elongate, rectangular parallelepiped shape.

In the sensor element according to the present invention, the arithmetic average roughness Rap may be 10 μm or more. When the arithmetic average roughness Rap is 10 μm or more, variations in the specific gas concentration detected by the sensor element may be further reduced. The arithmetic average roughness Rap may be 20 μm or more or may be 30 μm or more.

In the sensor element according to the present invention, the arithmetic average roughness Rac may be 0.1 or more and 1.0 μm or less. When the arithmetic average roughness Rac is 0.1 μm or more, the adhesion strength between the element body and the protective layer can be maintained at a certain level. When the arithmetic average roughness Rac is 1.0 μm or less, the strength of the protective layer can be maintained at a certain level.

In the sensor element according to the present invention, the surface of the element body may include the face on which the gas inlet opens and one or more adjacent faces that each meet the above face along a side of the above face. Moreover, the protective layer may include an adjacent-face protective layer that covers at least a part of the one or more adjacent faces. Furthermore, the adjacent-face protective layer may have an internal space formed therein, the internal space being directly communicated with the internal space of the inlet protective layer, an arithmetic average roughness Ras of an inner peripheral surface of the internal space of the adjacent-face protective layer satisfying at least one of conditions below: the arithmetic average roughness Ras is 8 μm or more, and the arithmetic average roughness Ras is higher than the arithmetic average roughness Rac. In such a case, the presence of the adjacent-face protective layer enhances the waterproofing performance of the element body. In addition, since the adjacent-face protective layer has an internal space, the conduction of heat from the outside of the adjacent-face protective layer toward the element body in the thickness direction of the adjacent-face protective layer can be suppressed by the internal space. This further enhances the waterproofing performance of the element body. Moreover, since the internal space of the adjacent-face protective layer and the internal space of the inlet protective layer are directly communicated with each other, the adjacent-face protective layer has a relatively wide internal space formed therein. This further enhances the waterproofing performance of the element body. Furthermore, the arithmetic average roughness Ras of the inner peripheral surface of the internal space of the adjacent-face protective layer satisfies at least one of the conditions below: the arithmetic average roughness Ras is 8 μm or more, and the arithmetic average roughness Ras is higher than the arithmetic average roughness Rac. In other words, the adjacent-face protective layer has an internal space having an inner peripheral surface having a relatively high arithmetic average roughness Ras. Thus, the irregularities of the internal space of the adjacent-face protective layer cause the flow of the measurement-object gas in the internal space to be converted into a turbulent flow. This reduces the likelihood of a measurement-object gas moving from the internal space of the inlet protective layer to the internal space of the adjacent-face protective layer. This enables the measurement-object gas present in the internal space of the adjacent-face protective layer to readily enter the measurement-object gas flow section from the gas inlet and consequently increases the responsivity of the sensor element. That is, while the internal space of the adjacent-face protective layer and the internal space of the inlet protective layer are directly communicated with each other in order to enhance the waterproofing performance of the element body, a reduction in responsivity which may occur when the above internal spaces are directly communicated with each other can be prevented by setting the arithmetic average roughness Ras to be relatively high. Note that the expression “directly communicated” used herein means that the above internal spaces are communicated with each other not through the pores present in the protective layer.

In the sensor element according to the present invention, the element body may have an elongate shape having a longitudinal direction, and the face on which the gas inlet opens may be an end surface of the element body in the longitudinal direction.

In the above case, the element body may be a layered body including a plurality of layers composed of the solid electrolyte body, the layers being stacked on top of one another in a stacking direction perpendicular to the longitudinal direction. Furthermore, the surface of the element body may include the end surface and a plurality of adjacent faces that each meet the end surface along a side of the end surface. Moreover, the protective layer may include an adjacent-face protective layer that covers the plurality of adjacent faces. In addition, parts of the adjacent-face protective layer which each cover a specific one of top and bottom surfaces included in the adjacent faces, the top and bottom surfaces being located at respective ends of the element body in the stacking direction, may each have an internal space formed therein and may include an outer protective layer arranged closer to the outside of the sensor element than the internal space and an inner protective layer arranged closer to the inside of the sensor element than the internal space, the inner protective layer being bonded on the surface of the element body. In such a case, the presence of the inner protective layer arranged in contact with the top and bottom surfaces increases the thermal capacity of the element body (to be exact, the element body and the inner protective layer). Therefore, even if a thermal shock transmits from the outside to the element body, a sudden change in the temperature of the element body can be avoided. This enhances the waterproofing performance of the element body.

A gas sensor according to the present invention includes the sensor element according to any one of the above-described aspects. Therefore, the above gas sensor may have the same advantageous effects as the above-described sensor element according to the present invention. That is, for example, variations in the specific gas concentration detected by the sensor element may be reduced.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention is described below with reference to the attached drawings.FIG.1is a perspective view of a sensor element101included in a gas sensor100according to an embodiment of the present invention.FIG.2is a cross-sectional view of the gas sensor100, schematically illustrating the structure of the gas sensor100. The cross section of the sensor element101illustrated inFIG.2is the section A-A ofFIG.1.FIG.3is a magnified view of the periphery of the measurement-object gas flow section9illustrated inFIG.2.FIG.4is a cross-sectional view taken along the section B-B ofFIG.1. Note that, in the cross-section illustrated inFIG.4, the parts of the inside of the element body102which are other than the measurement-object gas flow section9are omitted. The sensor element101has an elongate, rectangular parallelepiped shape. Hereinafter, the longitudinal direction of the sensor element101(the horizontal direction inFIG.2) is referred to as “front-rear direction”, the thickness direction of the sensor element101(the vertical direction inFIG.2) is referred to as “top-bottom direction”, and the width direction of the sensor element101(the direction perpendicular to the front-rear and top-bottom directions) is referred to as “left-right direction”.

The gas sensor100is attached to a piping, such as an automobile exhaust gas pipe, and used for measuring the concentration of a specific gas, such as NOx or O2, in the exhaust gas, which is a measurement-object gas. In this embodiment, the specific gas concentration measured by the gas sensor100is NOx concentration. The gas sensor100includes a sensor element101. The sensor element101includes an element body102and a porous protective layer84arranged to cover the element body102. Note that the element body102is a part of the sensor element101which is other than the protective layer84.

As illustrated inFIG.2, the sensor element101is an element having a structure in which six layers composed of a first substrate layer1, a second substrate layer2, a third substrate layer3, a first solid electrolyte layer4, a spacer layer5, and a second solid electrolyte layer6, each being formed from an oxygen ion-conductive solid electrolyte layer of zirconia (ZrO2) or the like, are stacked in that order from the bottom side inFIG.2. Also, the solid electrolyte constituting these six layers is dense and airtight. The above-described sensor element101is produced by, for example, subjecting ceramic green sheets corresponding to the individual layers to predetermined processing, printing of circuit patterns, and the like, stacking them thereafter, and further performing firing so as to integrate the ceramic green sheets.

In one front end portion (frontward end portion) of the sensor element101and between the lower surface of the second solid electrolyte layer6and the upper surface of the first solid electrolyte layer4, a gas inlet10, a first diffusion-controlled portion11, a buffer space12, a second diffusion-controlled portion13, a first internal space20, a third diffusion-controlled portion30, and a second internal space40are formed in that order so as to adjoin and communicate.

The gas inlet10, the buffer space12, the first internal space20, and the second internal space40are spaces in the inside of the sensor element101by hollowing the spacer layer5, where the upper portion is defined by the lower surface of the second solid electrolyte layer6, the lower portion is defined by the upper surface of the first solid electrolyte layer4, and the side portions are defined by the side surfaces of the spacer layer5.

The first diffusion-controlled portion11, the second diffusion-controlled portion13, and the third diffusion-controlled portion30are each formed as a pair of horizontal slits (the longitudinal direction of the openings is perpendicular to the drawing). Hereinafter, the space extending from the gas inlet10to the second internal space40is referred to as “measurement-object gas flow section9”. The measurement-object gas flow section9is formed in a substantially rectangular parallelepiped shape. The longitudinal direction of the measurement-object gas flow section9is parallel to the front-rear direction.

Meanwhile, at the position farther from the front end side than the measurement-object gas flow section9, a reference gas introduction space43is provided at the location between the upper surface of the third substrate layer3and the lower surface of the spacer layer5, where the side portions are defined by the side surfaces of the first solid electrolyte layer4. For example, the air serving as the reference gas at the time of measurement of the NOx concentration is introduced into the reference gas introduction space43.

An air introduction layer48is a layer composed of porous ceramics. The reference gas is introduced into the air introduction layer48through the gas introduction space43. Also, the air introduction layer48is formed so as to cover a reference electrode42.

The reference electrode42is an electrode formed so as to be sandwiched between the upper surface of the third substrate layer3and the first solid electrolyte layer4and, as described above, the air introduction layer48connected to the reference gas introduction space43is provided around the reference electrode42. In addition, as described later, it is possible to measure the oxygen concentrations (oxygen partial pressures) in the first internal space20and the second internal space40by using the reference electrode42.

In the measurement-object gas flow section9, the gas inlet10is a part made open to the outside space, and the gas to be measured is taken from the outside space into the sensor element101through the gas inlet10. The first diffusion-controlled portion11is a part for giving predetermined diffusion resistance to the gas to be measured, where the gas is taken from the gas inlet10. The buffer space12is a space provided so as to lead the gas to be measured, where the gas is introduced from the first diffusion-controlled portion11, to the second diffusion-controlled portion13. The second diffusion-controlled portion13is a part for giving predetermined diffusion resistance to the gas to be measured, where the gas is introduced from the buffer space12to the first internal space20. When the gas to be measured is introduced from the outside of the sensor element101into the first internal space20, the gas to be measured, which is taken into the sensor element101through the gas inlet10rapidly because of the pressure fluctuation of the gas to be measured in the outside space (pulsation of an exhaust pressure in the case where the gas to be measured is an automotive exhaust gas), is not directly introduced into the first internal space20but introduced into the first internal space20after pressure fluctuation of the gas to be measured are canceled through the first diffusion-controlled portion11, the buffer space12, and the second diffusion-controlled portion13. Consequently, pressure fluctuation of the gas to be measured, which is introduced into the first internal space20, are made to be at an almost negligible level. The first internal space20is provided as a space for adjusting the oxygen partial pressure in the gas to be measured which is introduced through the second diffusion-controlled portion13. The above-described oxygen partial pressure is adjusted by actuation of a main pump cell21.

The main pump cell21is an electrochemical pump cell composed of an inside pump electrode22having a ceiling electrode portion22aprovided on an almost entire surface of the lower surface of the second solid electrolyte layer6facing the first internal space20, an outside pump electrode23provided in a region, which corresponds to the ceiling electrode portion22aon the upper surface of the second solid electrolyte layer6, and the second solid electrolyte layer6sandwiched between these electrodes.

The inside pump electrode22is formed so as to extend over the upper and lower solid electrolyte layers (second solid electrolyte layer6and first solid electrolyte layer4) defining the first internal space20and the spacer layer5providing the side walls. Specifically, the ceiling electrode portion22ais formed on the lower surface of the second solid electrolyte layer6providing the ceiling surface of the first internal space20and a bottom electrode portion22bis formed on the upper surface of the first solid electrolyte layer4providing the bottom surface. Then, side electrode portions (not shown in the drawing) are formed on the side wall surfaces (inner surfaces) of the spacer layer5constituting both side wall portions of the first internal space20so as to connect the ceiling electrode portion22ato the bottom electrode portion22b. Thus, the inside pump electrode22is disposed in the form of a tunnel-like structure in a zone where the side electrode portions are disposed.

The inside pump electrode22and the outside pump electrode23are formed as porous cermet electrodes (for example, a cermet electrode of Pt containing 1% of Au and ZrO2). In this regard, the inside pump electrode22to contact with the gas to be measured is formed by using a material having weakened ability to reduce NOx components in the gas to be measured.

In the main pump cell21, oxygen in the first internal space20can be pumped out to the outside space or oxygen in the outside space can be pumped into the first internal space20by applying a predetermined pump voltage Vp0between the inside pump electrode22and the outside pump electrode23and passing a pump current Ip0between the inside pump electrode22and the outside pump electrode23in the positive direction or negative direction.

In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space20, an electrochemical sensor cell, that is, a main pump controlling oxygen partial pressure detection sensor cell80is constructed by the inside pump electrode22, the second solid electrolyte layer6, the spacer layer5, the first solid electrolyte layer4, the third substrate layer3, and the reference electrode42.

The oxygen concentration (oxygen partial pressure) in the first internal space20is determined by measuring the electromotive force V0of the main pump controlling oxygen partial pressure detection sensor cell80. Further, the pump current Ip0is controlled by feedback-controlling the pump voltage Vp0of a variable power supply25such that the electromotive force V0becomes a target value. Consequently, the oxygen concentration in the first internal space20can be maintained at a predetermined constant value.

The third diffusion-controlled portion30is a part which gives predetermined diffusion resistance to the gas to be measured, the oxygen concentration (oxygen partial pressure) of the gas having been controlled by the operation of the main pump cell21in the first internal space20, and leads the gas to be measured into the second internal space40.

The second internal space40is provided as a space for performing a treatment related to the measurement of the nitrogen oxide (NOx) concentration in the gas to be measured that is introduced through the third diffusion-controlled portion30. The NOx concentration is measured mainly in the second internal space40in which the oxygen concentration is adjusted by an auxiliary pump cell50and further the NOx concentration is measured by the operation of a measurement pump cell41.

In the second internal space40, the gas to be measured is further subjected to adjustment of the oxygen partial pressure by the auxiliary pump cell50, the gas to be measured having been subjected to adjustment of the oxygen concentration (oxygen partial pressure) in the first internal space20in advance and, thereafter, having been introduced through the third diffusion-controlled portion30. Consequently, the oxygen concentration in the second internal space40can be maintained constant with high accuracy and, therefore, the gas sensor100can measure the NOx concentration with high accuracy.

The auxiliary pump cell50is an auxiliary electrochemical pump cell constructed by an auxiliary pump electrode51having a ceiling electrode portion51aprovided on an almost entire surface of the lower surface of the second solid electrolyte layer6facing the second internal space40, an outside pump electrode23(not limited to the outside pump electrode23, and the sensor element101and an appropriate outside electrode will suffice), and the second solid electrolyte layer6.

The above-described auxiliary pump electrode51is arranged in the second internal space40so as to have a similar tunnel-like structure to the above-described inside pump electrode22disposed in the first internal space20. That is, a tunnel-like structure is constructed, in which the ceiling electrode portion51ais formed on the second solid electrolyte layer6providing the ceiling surface of the second internal space40, a bottom electrode portion51bis formed on the first solid electrolyte layer4providing the bottom surface of the second internal space40, and then, side electrode portions (not shown in the drawing) for connecting the ceiling electrode portion51ato the bottom electrode portion51bare formed on both side wall surfaces of the spacer layer5providing side walls of the second internal space40. In this regard, the auxiliary pump electrode51is formed by using a material having weakened ability to reduce NOx components in the gas to be measured in the same manner as the inside pump electrode22.

In the auxiliary pump cell50, oxygen in the atmosphere in the second internal space40can be pumped out to the outside space or oxygen in the outside space can be pumped into the second internal space40by applying a predetermined pump voltage Vp1between the auxiliary pump electrode51and the outside pump electrode23.

In addition, in order to control the oxygen partial pressure in the atmosphere in the second internal space40, an electrochemical sensor cell, that is, an auxiliary pump controlling oxygen partial pressure detection sensor cell81is constructed by the auxiliary pump electrode51, the reference electrode42, the second solid electrolyte layer6, the spacer layer5, the first solid electrolyte layer4, and the third substrate layer3.

In this regard, the auxiliary pump cell50performs pumping by a variable power supply52which is voltage-controlled on the basis of the electromotive force V1detected by the auxiliary pump controlling oxygen partial pressure detection sensor cell81. Consequently, the oxygen partial pressure in the atmosphere in the second internal space40is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

In addition to this, the pump current Ip1thereof is used for controlling the electromotive force of the main pump controlling oxygen partial pressure detection sensor cell80. Specifically, the pump current Ip1serving as a control signal is input into the main pump controlling oxygen partial pressure detection sensor cell80, and by controlling the above-described target value of the electromotive force V0thereof the gradient of the oxygen partial pressure in the gas to be measured, which is introduced from the third diffusion-controlled portion30into the second internal space40, is controlled so as to be always constant. In the case of application as a NOx sensor, the oxygen concentration in the second internal space40is maintained at a constant value of about 0.001 ppm by the functions of the main pump cell21and the auxiliary pump cell50.

The measurement pump cell41measures the NOx concentration in the gas to be measured in the second internal space40. The measurement pump cell41is an electrochemical pump cell constructed by a measurement electrode44disposed on the upper surface of the first solid electrolyte layer4facing the second internal space40and at the position apart from the third diffusion-controlled portion30, the outside pump electrode23, the second solid electrolyte layer6, the spacer layer5, and the first solid electrolyte layer4.

The measurement electrode44is a porous cermet electrode. The measurement electrode44also functions as a NOx reduction catalyst for reducing NOx present in the atmosphere in the second internal space40. Further, the measurement electrode44is covered with a fourth diffusion-controlled portion45.

The fourth diffusion-controlled portion45is a film composed of a ceramic porous body. The fourth diffusion-controlled portion45has a function of restricting the amount of NOx flowing into the measurement electrode44and, in addition, a function as a protective film for the measurement electrode44. In the measurement pump cell41, oxygen generated by decomposition of nitrogen oxides in the atmosphere around the measurement electrode44is pumped out and the amount of generation thereof can be detected as a pump current Ip2.

Also, in order to detect the oxygen partial pressure around the measurement electrode44, an electrochemical sensor cell, that is, a measurement pump controlling oxygen partial pressure detection sensor cell82is constructed by the first solid electrolyte layer4, the third substrate layer3, the measurement electrode44, and the reference electrode42. A variable power supply46is controlled on the basis of the electromotive force V2detected by the measurement pump controlling oxygen partial pressure detection sensor cell82.

The gas to be measured, which is introduced into the second internal space40, reaches the measurement electrode44through the fourth diffusion-controlled portion45under circumstances where the oxygen partial pressure is controlled. Nitrogen oxides in the gas to be measured around the measurement electrode44are reduced (2NO□N2+O2) and oxygen is generated. Then, the resulting oxygen is pumped by the measurement pump cell41. At that time, the voltage Vp2of the variable power supply46is controlled such that the control voltage V2that is detected by the measurement pump controlling oxygen partial pressure detection sensor cell82is constant (target value). The amount of oxygen generated around the measurement electrode44is proportional to the concentration of the nitrogen oxides in the gas to be measured and, therefore, the nitrogen oxide concentration in the gas to be measured is calculated by using the pump current Ip2in the measurement pump cell41.

In addition, in the case where the measurement electrode44, the first solid electrolyte layer4, the third substrate layer3, and the reference electrode42are combined so as to constitute an oxygen partial pressure detection device as an electrochemical sensor cell, the electromotive force in accordance with the difference between the amount of oxygen generated by reduction of NOx components in the atmosphere around the measurement electrode44and the amount of oxygen contained in the reference air can be detected and, thereby, the concentration of NOx components in the gas to be measured can be determined.

Further, an electrochemical sensor cell83is constructed by the second solid electrolyte layer6, the spacer layer5, the first solid electrolyte layer4, the third substrate layer3, the outside pump electrode23, and the reference electrode42. The oxygen partial pressure in the gas to be measured in the outside of the sensor can be detected by the electromotive force Vref obtained by the sensor cell83.

In the gas sensor100having the above-described configuration, the gas to be measured, which has an oxygen partial pressure always maintained at a low constant value (value that does not substantially affect the measurement of NOx) by actuation of the main pump cell21and the auxiliary pump cell50, is fed to the measurement pump cell41. Therefore, the NOx concentration in the gas to be measured can be determined on the basis of the pump current Ip2that flows because oxygen, which is generated by reduction of NOx nearly in proportion to the NOx concentration in the gas to be measured, is pumped out of the measurement pump cell41.

Further, in order to enhance the oxygen ion conductivity of the solid electrolyte, the sensor element101includes a heater portion70having a function of adjusting the temperature including heating the sensor element101and keeping the temperature. The heater portion70includes a heater connector electrode71, a heater72, a through hole73, a heater insulating layer74, and a pressure release hole75.

The heater connector electrode71is an electrode formed so as to contact with the lower surface of the first substrate layer1. The electric power can be supplied from the outside to the heater portion70by connecting the heater connector electrode71to an external power supply.

The heater72is an electric resistor formed to be sandwiched between the second substrate layer2and the third substrate layer3in the vertical direction. The heater72is connected to the heater connector electrode71through the through hole73and generates heat by being supplied with an electric power from the outside through the heater connector electrode71so as to heat the solid electrolyte constituting the sensor element101and keep the temperature.

Also, the heater72is embedded over an entire range from the first internal space20to the second internal space40and the entirety of the sensor element101can be adjusted to have a temperature at which the above-described solid electrolyte is activated.

The heater insulating layer74is an insulating layer formed on the upper and lower surfaces of the heater72by using an insulator, e.g., alumina. The heater insulating layer74is formed for the purpose of establishing electrical insulation between the second substrate layer2and the heater72and electrical insulation between the third substrate layer3and the heater72.

The pressure release hole75is a part provided so as to penetrate the third substrate layer3and communicate with the reference gas introduction space43and is formed for the purpose of reducing an internal pressure increase associated with a temperature increase in the heater insulating layer74.

As illustrated inFIGS.1to4, the element body102is partially covered with the porous protective layer84. Since the sensor element101is rectangular parallelepiped, the element body102(specifically, the layers1to6) has the following six external surfaces as illustrated inFIGS.1to4: a first surface102a(top surface), a second surface102b(bottom surface), a third surface102c(left-side surface), a fourth surface102d(right-side surface), a fifth surface102e(front-end surface), and a sixth surface102f(rear-end surface). The protective layer84includes first to fifth protective layers84ato84e, which are each disposed on a corresponding one of the five surfaces (the first to fifth surfaces102ato102e) out of the six surfaces of the element body102(the first to sixth surfaces102ato102f). The fifth protective layer84e(an example of the inlet protective layer) covers the fifth surface102e, which is one of the end surfaces of the element body102in the longitudinal direction (i.e., the front-rear direction), and the gas inlet10formed in the fifth surface102e(seeFIG.3). The first to fourth protective layers84ato84d(examples of the adjacent-face protective layer) each cover a corresponding one of the four surfaces (the first to fourth surfaces102ato102d, which are examples of the adjacent faces) of the element body102which touch the fifth surface102ealong a side of the fifth surface102e(seeFIGS.3and4). Hereinafter, the first to fifth protective layers84ato84eare referred to collectively as “protective layer84”. The protective layer84is arranged to cover and protect a part of the element body102. For example, the protective layer84reduces the cracking of the element body102which may be caused due to the adhesion of moisture or the like included in the measurement-object gas.

As illustrated inFIGS.3and4, the first protective layer84aincludes a first internal space90a, a first outer protective layer85aarranged closer to the outside than the first internal space90a, and a first inner protective layer86aarranged closer to the inside than the first internal space90a. The first inner protective layer86ais arranged in contact with the first surface102a. The first inner protective layer86acovers the outside pump electrode23. Similarly, the second protective layer84bincludes a second internal space90b, a second outer protective layer85b, and a second inner protective layer86b. The second inner protective layer86bis arranged in contact with the second surface102b. The third protective layer84cincludes a third internal space90cand a third outer protective layer85carranged closer to the outside than the third internal space90c. Since the third protective layer84cdoes not include a protective layer arranged closer to the inside than the third internal space90c, the third surface102cis exposed to the third internal space90c(seeFIG.4). Similarly, the fourth and fifth protective layers84dand84einclude fourth and fifth internal spaces90dand90eand fourth and fifth outer protective layers85dand85e, respectively. The fourth and fifth surfaces102dand102eare exposed to the fourth and fifth internal spaces90dand90e, respectively (seeFIGS.3and4). The gas inlet10is exposed to the fifth internal space90e. Hereinafter, the first to fifth outer protective layers85ato85eare referred to collectively as “outer protective layer85”, the first and second inner protective layers86aand86bare referred to collectively as “inner protective layer86”, and the first to fifth internal spaces90ato90eare referred to collectively as “internal space90”. The inner peripheral surfaces of the first to fifth internal spaces90ato90eare referred to as “first to fifth inner peripheral surfaces94ato94e”, respectively, which are referred to collectively as “inner peripheral surface94”.

The first to fifth outer protective layers85ato85eare each joined to adjacent outer protective layers. The outer protective layer85covers the front end of the element body102as a whole. The first and second inner protective layers86aand86bdirectly cover parts of the first and second surfaces102aand102bwhich are covered with the first and second outer protective layers85aand85b, respectively. Thus, the first and second surfaces102aand102bare not exposed to the first and second internal spaces90aand90b, respectively. The first to fifth internal spaces90ato90eare each directly communicated with adjacent internal spaces. The internal space90forms one space as a whole. Note that the expression “directly communicated” means that the above internal spaces are communicated with one another not through the pores present in the protective layer84(i.e., the outer protective layer85and the inner protective layer86). The outer protective layer85and the inner protective layer86are arranged in contact with each other at only the rear end of the protective layer84(seeFIG.3). Specifically, the first outer protective layer85aand the first inner protective layer86aare arranged in contact with each other at the rear end. Similarly, the second outer protective layer85band the second inner protective layer86bare arranged in contact with each other at the rear end. Among the parts of the outer protective layer85, the third and fourth outer protective layers85cand85dare arranged in contact with the third and fourth surfaces102cand102d, respectively, at only the rear end. The fifth outer protective layer85eis not arranged in contact with the element body102.

When viewed in a direction perpendicular to the first surface102a, the first protective layer84aoverlaps the entirety of a region of the first surface102awhich extends from the front end of the element body102a distance L rearward of the element body102(seeFIG.3). The same applies to the second to fourth protective layers84bto84d. The same also applies to the first to fourth outer protective layers85ato85dand the first and second inner protective layers86aand86b. When viewed in a direction perpendicular to the fifth surface102e(i.e., when viewed in the direction from front to rear), the fifth protective layer84eoverlaps the entirety of the fifth surface102e. That is, the fifth protective layer84ecovers the entirety of the fifth surface102eincluding the gas inlet10. Since the protective layer84is a porous body, the measurement-object gas can flow inside the protective layer84and reach the gas inlet10and the inside of the measurement-object gas flow section9.

The distance L illustrated inFIG.3is determined in the range of (0<Distance L<Longitudinal length of element body102) on the basis of the region of the gas sensor100in which the element body102is exposed to the measurement-object gas, the position of the measurement-object gas flow section9, and the like. The distance L is preferably determined so as to be larger than the length of the measurement-object gas flow section9, which is formed inside the element body102, in the front-rear direction. Since the longitudinal direction of the measurement-object gas flow section9is the same as the longitudinal direction (i.e., the front-rear direction) of the element body102as illustrated inFIGS.2to4, the distance L is larger than the longitudinal length of the measurement-object gas flow section9. In this embodiment, the length of the element body102in the front-rear direction, the width of the element body102in the left-right direction, and the thickness of the element body102in the top-bottom direction are different from one another as illustrated inFIG.1such that Length>Width>Thickness. The distance L is larger than either the width or thickness of the element body102.

The protective layer84is composed of a porous body, such as an alumina porous body, a zirconia porous body, a spinel porous body, a cordierite porous body, a titania porous body, or a magnesia porous body. In this embodiment, the protective layer84is composed of an alumina porous body. The thickness of the protective layer84is, for example, but not limited to, 100 to 1000 μm. The porosity of the protective layer84is, for example, but not limited to, 5% to 85%. The thickness of the outer protective layer85may be, for example, 50 to 800 μm. The thickness of the inner protective layer86may be, for example, 5 to 50 μm. The thickness (height) of the internal space90may be, for example, 5 to 800 μm. The porosities, materials, etc. of the outer protective layer85and the inner protective layer86may be different from each other. At least one of the outer protective layer85and the inner protective layer86may include a plurality of sublayers.

The arithmetic average roughness Rap of the fifth inner peripheral surface94eof the fifth internal space90eis relatively high. Specifically, the above arithmetic average roughness Rap satisfies at least one of the following conditions: the arithmetic average roughness Rap is 8 μm or more, and the arithmetic average roughness Rap is higher than the arithmetic average roughness Rac of a bonding surface97of the protective layer84at which the protective layer84is bonded to the element body102. This creates a turbulent flow in the fifth internal space90eand consequently reduces variations in the NOx concentration detected by the sensor element101. Details are described below. In this embodiment, since the fifth inner peripheral surface94eincludes only a fifth outside inner peripheral surface95ethat is an inner (element body102-side) surface of the fifth outer protective layer85e(seeFIG.3), the arithmetic average roughness Ra of the fifth outside inner peripheral surface95eis used as an arithmetic average roughness Rap. In this embodiment, since the bonding surface97includes a first bonding surface97athat is a bonding surface at which the first inner protective layer86ais bonded on the first surface102aand a second bonding surface97bthat is a bonding surface at which the second inner protective layer86bis bonded on the second surface102b, the average of the arithmetic average roughness values Ra of the first and second bonding surfaces97aand97bis used as an arithmetic average roughness Rac. In this embodiment, the arithmetic average roughness Rap satisfies both of the two conditions described above.

The arithmetic average roughness Rap is preferably 10 μm or more. When the arithmetic average roughness Rap is 10 μm or more, variations in the NOx concentration detected by the sensor element101may be further reduced. The arithmetic average roughness Rap may be 20 μm or more or may be 30 μm or more. The arithmetic average roughness Rap may be 100 μm or less. The arithmetic average roughness Rac may be 0.1 μm or more and 1.0 μm or less. When the arithmetic average roughness Rac is 0.1 μm or more, the adhesion strength between the element body102and the protective layer84can be maintained at a certain level. When the arithmetic average roughness Rac is 1.0 μm or less, the strength of the protective layer84can be maintained at a certain level.

The arithmetic average roughness Rap is determined by cutting the protective layer84such that the inner peripheral surface that is to be measured (i.e., the fifth outside inner peripheral surface95e) is exposed and subsequently conducting measurement in accordance with a method adhering to JIS B 0601:2013 using a spectrointerferometer. The arithmetic average roughness Rac is determined by the following method. First, a observation sample is prepared by cutting the sensor element101such that a cross section perpendicular to the bonding surface97serves as an observation surface, then embedding the cross section in a resin, and polishing the cross section. Subsequently, an image of the observation surface of the observation sample is taken with a scanning electron microscope (SEM) at a 300-fold magnification and a field of view of about 350 μm×250 μm. On the basis of the brightness data of pixels of the obtained image, a histogram of brightness values of all the pixels is prepared. The brightness values corresponding to the portions (valleys) between the three peaks of the histogram are used as thresholds. The brightness value of each pixel is converted into a ternary representation by comparing the brightness of the pixel with the thresholds. Thereby, for each of the pixels, whether the pixel corresponds to the particles constituting the protective layer84, the pores present in the protective layer84, or the element body102is determined. Subsequently, the boundary line between the particles constituting the protective layer84and the element body102is drawn. This boundary line is used as a “surface profile of real surface” of the bonding surface97defined in JIS B 0601:2013. An arithmetic average roughness Ra determined by performing an image processing of the surface profile of real surface in accordance with a method adhering to JIS B 0601:2013 is used as an arithmetic average roughness Rac.

The arithmetic average roughness of the inner peripheral surfaces of the first to fourth internal spaces90ato90d, which are directly communicated with the fifth internal space90e, is preferably relatively high. Specifically, it is preferable that the arithmetic average roughness Ras of the first to fourth inner peripheral surfaces94ato94dof the first to fourth internal spaces90ato90dsatisfy at least one of the following conditions: the arithmetic average roughness Ras is 8 μm or more, and the arithmetic average roughness Ras is higher than the arithmetic average roughness Rac. Hereinafter, the arithmetic average roughness values Ras of the first to fourth inner peripheral surfaces94ato94dare referred to as “arithmetic average roughness values Ra1sto Ra4s”, respectively. In this case, it is preferable that one or more of the arithmetic average roughness values Ra1sto Ra4ssatisfy at least one of the two conditions described above.

The arithmetic average roughness Ra1sis described below. In this embodiment, the first inner peripheral surface94aincludes a first outside inner peripheral surface95athat is an inner (element body102-side) surface of the first outer protective layer85aand a first inside inner peripheral surface96athat is an outer (first internal space90a-side) surface of the first inner protective layer86a(seeFIG.3). In this case, when the arithmetic average roughness Ra of at least one of the first outside inner peripheral surface95aand the first inside inner peripheral surface96ais defined as arithmetic average roughness Ra1s, the arithmetic average roughness Ra1spreferably satisfies at least one of the two conditions described above. In other words, it is preferable that the arithmetic average roughness Ra of at least one of the first outside inner peripheral surface95aand the first inside inner peripheral surface96asatisfy at least one of the two conditions described above. In this embodiment, the arithmetic average roughness Ra (=Ra1s) of the first outside inner peripheral surface95asatisfies both of the two conditions.

The arithmetic average roughness Ra2sis described below. In this embodiment, the second inner peripheral surface94bincludes a second outside inner peripheral surface95bthat is an inner (element body102-side) surface of the second outer protective layer85band a second inside inner peripheral surface96bthat is an outer (second internal space90b-side) surface of the second inner protective layer86b(seeFIG.3). In this case, similarly to the arithmetic average roughness Ra1s, when the arithmetic average roughness Ra of at least one of the second outside inner peripheral surface95band the second inside inner peripheral surface96bis defined as an arithmetic average roughness Ra2s, the arithmetic average roughness Ra2spreferably satisfies at least one of the two conditions described above. In this embodiment, the arithmetic average roughness Ra (=Ra2s) of the second outside inner peripheral surface95bsatisfies both of the two conditions.

The arithmetic average roughness Ra3sis described below. In this embodiment, the third inner peripheral surface94cincludes only a third outside inner peripheral surface95cthat is an inner (element body102-side) surface of the third outer protective layer85c(seeFIG.4). Therefore, the arithmetic average roughness Ra of the third outside inner peripheral surface95cis defined as an arithmetic average roughness Ra3s. Similarly, the fourth inner peripheral surface94dincludes only a fourth outside inner peripheral surface95dthat is an inner (element body102-side) surface of the fourth outer protective layer85d(seeFIG.4). Therefore, the arithmetic average roughness Ra of the fourth outside inner peripheral surface95dis defined as an arithmetic average roughness Ra4s. In this embodiment, each of the arithmetic average roughness values Ra3sand Ra4ssatisfies both of the two conditions.

Similarly to the arithmetic average roughness Rap, the arithmetic average roughness values Ra1sto Ra4sare each determined by cutting the protective layer84such that the inner peripheral surface that is to be measured (i.e., a corresponding one of the first to fourth outside inner peripheral surfaces95ato95d) is exposed and subsequently conducting measurement in accordance with a method adhering to JIS B 0601:2013 using a spectrointerferometer.

As illustrated inFIGS.3and4, the first to fifth inner peripheral surfaces94ato94eare all the inner (element body102-side) surface of the outer protective layer85. Therefore, in this embodiment, the arithmetic average roughness values Ra1sto Ra4sare set to be equal to one another and the arithmetic average roughness Rap is set to be equal to the arithmetic average roughness values Ra1sto Ra4s. However, the arithmetic average roughness values Rap and Ra1sto Ra4smay be different from one another. The arithmetic average roughness Ras (specifically, one or more of the arithmetic average roughness values Ra1sto Ra4s) may be 10 μm or more, 20 μm or more, or 30 μm or more. The arithmetic average roughness Ras may be 100 μm or less.

A method for producing the above-described gas sensor100is described below. In the method for producing the gas sensor100, first, an element body102is produced. Subsequently, a protective layer84is formed on the element body102to produce a sensor element101.

A method for producing the element body102is described below. First, six unbaked ceramic green sheets are prepared. A plurality of sheet holes used for performing positioning when printing is performed or the green sheets are stacked on top of one another, necessary through-holes, and the like are formed in each of the green sheets in accordance with a corresponding one of the layers1to6. A space that is to serve as a measurement-object gas flow section9is formed, by punching or the like, in the green sheet that is to be formed into a spacer layer5. Then, patterns such as electrodes and heaters are printed on each of the ceramic green sheets. Subsequent to the formation of the above patterns, the green sheets are dried. Subsequently, the green sheets are stacked on top of one another to form a layered body. A vanishing body (organic material, such as carbon or theobromine) capable of vanishing during baking may be charged into a part of the layered body which is to serve as a space such as a measurement-object gas flow section9. The above layered body includes a plurality of element bodies102. The layered body is cut into pieces having a size of the element body102, which are then baked at a predetermined baking temperature. Hereby, an element body102is produced.

A method for forming a protective layer84on the element body102is described below. First, an inner protective layer86is formed on the surface of the element body102. For forming the inner protective layer86, various methods such as mold casting, screen printing, dipping, and plasma spraying may be used. In the case where the inner protective layer86is formed by screen printing or plasma spraying, the first to fifth inner protective layers86ato86emay be formed one by one. Subsequently, a vanishing body is applied to the inner protective layer86, and the resulting coating film is dried to form a vanishing body having a shape of the internal space90. The application of the vanishing body may be performed using screen printing, gravure printing, ink-jet printing, or the like. The vanishing body may be formed by repeatedly performing the above application and drying steps. Examples of the material of the vanishing body include the above-described organic materials, such as carbon and theobromine, and thermally degradable polymers, such as a vinyl resin. Subsequently, an outer protective layer85is formed on the outer surfaces of the inner protective layer86and the vanishing body. The outer protective layer85can be formed as in the formation of the inner protective layer86. Hereby, a protective layer84including a vanishing body having a shape of the internal space90is formed. Then, the vanishing body is vanished by performing combustion. As a result, the part in which the vanishing body was present serves as an internal space90. That is, a protective layer84having an internal space90formed therein is formed. In the above-described manner, a protective layer84is formed on the element body102and, hereby, a sensor element101is produced. In the case where the protective layer84is formed by mold casting, screen printing, or dipping, a slurry that is to form the outer protective layer85and the inner protective layer86is solidified or dried and then baked to form a protective layer84. In such a case, the baking of the protective layer84and the combustion of the vanishing body may be performed simultaneously. In the case where the outer protective layer85and the inner protective layer86are formed by plasma spraying, the vanishing body may be vanished by performing combustion subsequent to the formation of the above two protective layers.

For setting the arithmetic average roughness values Rap and Ras to be relatively high, for example, the following methods may be used. First, a method in which the arithmetic average roughness Ra of the first to fifth outside inner peripheral surfaces95ato95eis increased in order to set the arithmetic average roughness values Rap and Ras to be relatively high is described below. In this case, for example, the outer protective layer85is formed by plasma spraying and the velocity at which the particles constituting the outer protective layer85are impinged onto the vanishing body is set to be relatively low by reducing the amount of gas used for generating plasma in plasma spraying or by increasing the distance between a plasma gun and the element body102. This reduces the likelihood of the particles constituting the outer protective layer85being crushed and flattened upon collision with the vanishing body and consequently increases the arithmetic average roughness Ra of the first to fifth outside inner peripheral surfaces95ato95e. Alternatively, using a vanishing body composed of a soft material also reduces the likelihood of the particles constituting the outer protective layer85being crushed upon collision with the vanishing body and consequently increases the arithmetic average roughness Ra of the first to fifth outside inner peripheral surfaces95ato95e. Increasing the size of particles of a powder spray material (raw-material powder particles that are the particles constituting the outer protective layer85) used in plasma spraying also increases the arithmetic average roughness Ra of the first to fifth outside inner peripheral surfaces95ato95e. For increasing the arithmetic average roughness Ra of the first and second inside inner peripheral surfaces96aand96b, for example, the inner protective layer86is formed so as to include a plurality of sublayers stacked on top of one another in the thickness direction and the size of the particles constituting a sublayer exposed to the internal space90is set to be larger than the size of the particles constituting a sublayer bonded to the element body102. Alternatively, subsequent to the formation of the inner protective layer86, the first and second inside inner peripheral surfaces96aand96bmay be roughened in order to increase the arithmetic average roughness Ra.

In the case where the protective layer84includes a plurality of sublayers in the thickness direction (i.e., the outer protective layer85and the inner protective layer86), the innermost later (i.e., the inner protective layer86) is preferably formed by forming the slurry on the surface of the element body102by mold casting, screen printing, dipping, or the like and then baking the slurry together with the element body102in an integrated manner in order to form an inner protective layer86. Since the surface of the element body102has a relatively small arithmetic average roughness Ra in many cases, the adhesion between the element body102and the inner protective layer86, which is directly bonded to the element body102, is likely small. Baking the slurry and the element body102in an integrated manner increases the adhesion between the element body102and the inner protective layer86. A surface of the inner protective layer86which is to come into contact with the outer protective layer85(i.e., a rear-end part of the surface of the inner protective layer86) preferably has a larger arithmetic average roughness Ra than the surface of the element body102. In such a case, the adhesion between the inner protective layer86and the outer protective layer85can be increased. The arithmetic average roughness Ra of the surface of the inner protective layer86which is to come into contact with the outer protective layer85may be 1 μm or more and 10 μm or less and may be 1 μm or more and 5 μm or less. In addition to the surface of the inner protective layer86which is to come into contact with the outer protective layer85, a first inside inner peripheral surface96aand a second inside inner peripheral surface96bwhich are to be exposed to the internal space90may have an arithmetic average roughness Ra of 1 μm or more and 10 μm or less or an arithmetic average roughness Ra of 1 μm or more and 5 μm or less.

When the outer protective layer85is prepared, the entirety of the outer protective layer85(the first to fifth outer protective layers85ato85e) may be integrally formed as a protective layer having a cap-like shape (also referred to as “bottomed cylindrical shape” or “shape of a box with one side open”). For example, the outer protective layer85may be prepared by forming a cap-like unbaked body having a shape of the outer protective layer85by mold casting, inserting the front end-side part of the element body102(in the case where the inner protective layer86is present, the element body102and the inner protective layer86) into the inside of the cap-like unbaked body, and subsequently baking the unbaked body. In the above case, forming the unbaked body in a shape having a space holding portion, such as a columnar or step-like portion, formed therein (therefore, the outer protective layer85, which is produced by baking the unbaked body, has a space holding portion) enables the internal space90to be formed between the outer protective layer85and the element body102without using the vanishing body having a shape of the internal space90but using the space holding portion. In the case where the outer protective layer85is prepared by the method in which the element body102is inserted into the cap-like unbaked body, the internal space90formed between the outer protective layer85and the element body102may have an opening directed toward the rear end of the element body102. In such a case, a sealing portion may be formed by plasma spraying or the like so as to block the opening. The sealing portion is preferably a porous body the principal constituent of which is the same as that of the outer protective layer85. It is possible to adjust the arithmetic average roughness values Rap and Ras by changing the shape of irregularities present in the surface (surface roughness) of a die used for preparing the unbaked body that is to be formed into the outer protective layer85.

After the sensor element101has been produced in the above-described manner, it is placed in a predetermined housing, which is then attached to a main body (not illustrated in the drawing) of a gas sensor100and connected to power supplies, etc. Hereby, a gas sensor100is produced.

While the above-described gas sensor100is used, a measurement-object gas present inside a piping reaches the sensor element101and enters the gas inlet10through the protective layer84. The sensor element101detects the NOx concentration in the measurement-object gas passed into the measurement-object gas flow section9through the gas inlet10on the basis of the voltage (i.e., electromotive force V2) between the measurement electrode44and the reference electrode42. For example, the value representing the specific gas concentration is obtained by the sensor element101outputting (measuring) the electromotive force V2or the pump current Ip2that flows when the voltage Vp2is controlled such that the electromotive force V2is constant.

In the sensor element101according to this embodiment, the arithmetic average roughness Rap of the fifth inner peripheral surface94e(i.e., the fifth outside inner peripheral surface95e) of the inlet protective layer (i.e., the fifth protective layer84e) satisfies at least one of the conditions below: the arithmetic average roughness Rap is 8 μm or more, and the arithmetic average roughness Rap is higher than the arithmetic average roughness Rac of the bonding surface97of the protective layer84at which the protective layer84is bonded to the element body102. That is, the fifth outside inner peripheral surface95ehas a relatively high arithmetic average roughness Rap, that is, relatively large irregularities formed therein. Consequently, when a measurement-object gas is passed from the outside of the protective layer84to the gas inlet10through the fifth internal space90e, the irregularities present in the fifth outside inner peripheral surface95ecauses the flow of the measurement-object gas in the fifth internal space90eto be converted into a turbulent flow. The turbulent flow stirs the measurement-object gas and thereby increases the uniformity in the NOx concentration in the measurement-object gas. As a result, variations in the NOx concentration in the measurement-object gas introduced into the measurement-object gas flow section9are reduced and, accordingly, fluctuations in the electromotive force V2between the measurement electrode44and the reference electrode42which are caused due to the variations in the NOx concentration are reduced. Thus, variations in the NOx concentration detected by the sensor element101can be reduced.

In the gas sensor100according to this embodiment described above in detail, the sensor element101includes a measurement electrode44disposed on the inner peripheral surface of the measurement-object gas flow section9and a reference electrode42exposed to a reference gas (i.e., air) used as a reference for detecting a specific gas concentration (i.e., NOx concentration). The sensor element101includes an inlet protective layer (i.e., the fifth protective layer84e) covering a gas inlet10formed in the surface of the element body102, which is an inlet of the measurement-object gas flow section9, and at least a part of the fifth surface102ein which the gas inlet10is formed. Furthermore, the arithmetic average roughness Rap of the fifth inner peripheral surface94e(i.e., the fifth outside inner peripheral surface95e) of the fifth internal space90eof the fifth protective layer84esatisfies at least one of the conditions below: the arithmetic average roughness Rap is 8 μm or more, and the arithmetic average roughness Rap is higher than the arithmetic average roughness Rac of the bonding surface97of the protective layer84at which the protective layer84is bonded to the element body102. That is, the fifth outside inner peripheral surface95ehas a relatively high arithmetic average roughness Rap. This reduces variations in the NOx concentration detected by the sensor element101.

If the arithmetic average roughness Rap is higher than 100 the irregularities present in the fifth inner peripheral surface94e(i.e., the fifth outside inner peripheral surface95e) of the fifth internal space90eof the inlet protective layer (i.e., the fifth protective layer84e) increase resistance to the flow of a measurement-object gas and reduce the likelihood of the measurement-object gas reaching the gas inlet10. This may reduce the responsivity of the sensor element101. When the arithmetic average roughness Rap is 100 μm or less, the reduction in responsivity can be prevented.

When the arithmetic average roughness Rap is 10 or more, variations in the NOx concentration detected by the sensor element101may be further reduced. When the arithmetic average roughness Rac is 0.1 μm or more, the adhesion strength between the element body102and the protective layer84can be maintained at a certain level. When the arithmetic average roughness Rac is 1.0 μm or less, the strength of the protective layer84can be maintained at a certain level.

Furthermore, the surface of the element body102includes a fifth surface102ein which the gas inlet10is formed and a plurality of adjacent faces (i.e., the first to fourth surfaces102ato102d) that each meet the fifth surface102ealong a side of the fifth surface102e. The protective layer84includes adjacent-face protective layers (i.e., the first to fourth protective layers84ato84d) that cover the first to fourth surfaces102ato102d, respectively. The first to fourth protective layer84ato84dhave first to fourth internal spaces90ato90dformed therein, respectively, the first to fourth internal spaces90ato90dbeing directly communicated with the fifth internal space90eof the fifth protective layer84e. The arithmetic average roughness Ras (i.e., each of the arithmetic average roughness values Ra1sto Ra4s) of the inner peripheral surfaces (i.e., the first to fourth inner peripheral surfaces94ato94d) of the first to fourth internal spaces90ato90dsatisfies at least one of the conditions below: the arithmetic average roughness Ras is 8 or more, and the arithmetic average roughness Ras is higher than the arithmetic average roughness Rac. While the sensor element101is used, moisture included in the measurement-object gas may adhere onto the surface of the sensor element101. Since the temperature of the element body102is adjusted to be the temperature (e.g., 800° C.) at which the solid electrolyte is activated by the heater72as described above, a quick reduction in the temperature of the element body102caused by the moisture adhered may result in cracking of the element body102due to thermal shock. In the sensor element101according to this embodiment, the presence of the first to fourth protective layers84ato84davoids a sudden reduction in the temperature of the element body102. This enhances the waterproofing performance of the element body102. In addition, since the first to fourth protective layers84ato84dhave the first to fourth internal spaces90ato90d, respectively, the conduction of heat from the outside of the first to fourth protective layers84ato84dtoward the element body102in the thickness directions of the first to fourth protective layers84ato84dcan be suppressed by the first to fourth internal spaces90ato90d, respectively. This further enhances the waterproofing performance of the element body102. Moreover, since the first to fourth internal spaces90ato90dand the fifth internal space90eare directly communicated with one another, the first to fourth internal spaces90ato90dare relatively wide. This further enhances the waterproofing performance of the element body102. Furthermore, the arithmetic average roughness Ras of the first to fourth inner peripheral surfaces94ato94dof the first to fourth internal spaces90ato90dsatisfies at least one of the conditions below: the arithmetic average roughness Ras is 8 μm or more, and the arithmetic average roughness Ras is higher than the arithmetic average roughness Rac. In other words, the first to fourth protective layers84ato84dhave first to fourth internal spaces90ato90dhaving first to fourth inner peripheral surfaces94ato94dhaving a relatively high arithmetic average roughness Ras, respectively. Thus, the irregularities of the first to fourth internal spaces90ato90dcause the flow of the measurement-object gas in the first to fourth internal spaces90ato90dto become turbulent. This reduces the likelihood of a measurement-object gas moving from the fifth internal space90eto the first to fourth internal spaces90ato90d. This enables the measurement-object gas present in the fifth internal space90eto readily enter the measurement-object gas flow section9from the gas inlet10. This increases the responsivity of the sensor element101. That is, while the first to fourth internal spaces90ato90dand the fifth internal space90eare directly communicated with one another in order to enhance the waterproofing performance of the element body102, a reduction in responsivity which may occur when the above internal spaces are directly communicated with one another can be limited by setting the arithmetic average roughness Ras to be relatively high.

The element body102is a layered body including a plurality of layers composed of a solid electrolyte body, the layers being stacked on top of one another in a stacking direction (the top-bottom direction) perpendicular to the longitudinal direction. Furthermore, the surface of the element body102includes the fifth surface102ethat is an end surface of the element body102in the longitudinal direction and a plurality of adjacent faces (i.e., the first to fourth surfaces102ato102d) that each meet the fifth surface102ealong a side of the fifth surface102e. The protective layer84includes adjacent-face protective layers (i.e., the first to fourth protective layers84ato84d) that cover the first to fourth surfaces102ato102d, respectively. The parts (i.e., the first and second protective layers84aand84b) of the first to fourth protective layers84ato84dwhich each cover a specific one of the top surface (i.e., the first surface102a) and the bottom surface (i.e., the second surface102b) included in the first to fourth surfaces102ato102d, the top and bottom surfaces being located at respective ends of the element body102in the stacking direction, have first and second internal spaces90ato90bformed therein, respectively, and include first and second outer protective layers85aand85barranged closer to the outside than the first and second internal spaces90aand90b, respectively, and first and second inner protective layers86aand86barranged closer to the inside than the first and second internal spaces90aand90b, respectively, the first and second inner protective layers86aand86bbeing bonded on the surface of the element body102. The presence of the first and second inner protective layers86aand86barranged in contact with the first surface102aand the second surface102bincreases the thermal capacity of the element body102(to be exact, the element body102and the first and second inner protective layers86aand86b). Therefore, even if a thermal shock transmits from the outside to the element body102, a sudden change in the temperature of the element body102can be avoided. This enhances the waterproofing performance of the element body102.

It is needless to say that the present invention is not limited to the foregoing embodiment and may be implemented in various aspects within the technical scope of the present invention.

For example, although the protective layer84includes the inner protective layer86in the above-described embodiment, the protective layer84does not necessarily include the inner protective layer86.FIG.5is a cross-sectional view of a protective layer184according to a modification example corresponding to the above case. The protective layer184includes an outer protective layer85and an internal space90. The surfaces of the element body102, that is, the first to fifth surfaces102ato102e, are exposed to the internal space90. In this case, the bonding surface97of the protective layer184is a bonding surface of the outer protective layer85at which the outer protective layer85is bonded to the element body102(e.g., the first and second bonding surfaces97aand97billustrated inFIG.5), and the arithmetic average roughness Rac is determined on the basis of the bonding surface97.

Although the inner protective layer86includes the first and second inner protective layers86aand86bin the above-described embodiment, the present invention is not limited to this. The inner protective layer86is arranged to cover at least one of the first to fifth surfaces102ato102e. For example, similarly to the protective layer284illustrated inFIGS.6and7according to an modification example, the inner protective layer86may include first to fifth inner protective layers86ato86ethat cover the first to fifth surfaces102ato102e, respectively. In the protective layer284, the third to fifth inner peripheral surfaces94cto94einclude third to fifth outside inner peripheral surfaces95cto95eand third to fifth inside inner peripheral surfaces96cto96e, respectively. In this case, since the bonding surface97of the protective layer284is a bonding surface of the inner protective layer86at which the inner protective layer86is bonded to the element body102(the first to fifth bonding surfaces97ato97eillustrated inFIGS.6and7), the arithmetic average roughness Rac is determined on the basis of the bonding surface97. Specifically, in the example illustrated inFIGS.6and7, the average of the arithmetic average roughness values Ra of the first to fifth bonding surfaces97ato97eis used as an arithmetic average roughness Rac. In the example illustrated inFIGS.6and7, since the fifth surface102eis covered with the fifth inner protective layer86e, either the fifth surface102eor the gas inlet10is not exposed to the fifth internal space90e.

Although the first to fifth internal spaces90ato90eare directly communicated with one another in the above-described embodiment, the present invention is not limited to this. For example, the fifth internal space90emay be directly communicated with at least one of the first to fourth internal spaces90ato90dand is not necessarily directly communicated with any of the first to fourth internal spaces90ato90d.

Although the first to fifth protective layers84ato84eeach have one internal space in the above-described embodiment, the present invention is not limited to this; each of the first to fifth protective layers84ato84emay have two or more internal spaces. In the case where a plurality of fifth internal spaces90eare present, the arithmetic average roughness Ra of the inner peripheral surface of one of the plurality of fifth internal spaces90ewhich is closest to the gas inlet10is used as an arithmetic average roughness Rap.

Although the protective layer84includes the first to fifth protective layers84ato84ein the above-described embodiment, the protective layer84includes at least the inlet protective layer (in the above-described embodiment, the fifth protective layer84e). The protective layer84does not necessarily include the adjacent-face protective layer (in the above-described embodiment, the first to fourth protective layers84ato84d) and may include at least one adjacent-face protective layer.

Although the longitudinal direction of the measurement-object gas flow section9is parallel to that of the element body102in the above-described embodiment, the present invention is not limited to this. Although the gas inlet10of the measurement-object gas flow section9is formed in the fifth surface102ein the above-described embodiment, the gas inlet10may be formed in the other surfaces, such as the first surface102a. In other words, the inlet protective layer is not limited to the fifth protective layer84e.

Although the element body102is rectangular parallelepiped in the above-described embodiment, the present invention is not limited to this. For example, the element body102may have an elongate shape having a longitudinal direction. For example, the element body102may have a shape of a polygonal column or a cylinder.

Although not mentioned in the above-described embodiment, each of the first to fifth internal spaces90ato90eformed in the protective layer84are distinguishable in size from the pores present in the components (e.g., the outer protective layer85and the inner protective layer86) of the protective layer84. That is, the pores present in the outer protective layer85and the inner protective layer86are not included in the internal space90. The internal space90(each of the first to fifth internal spaces90ato90e) is a space that is different from and larger than the pores present in the protective layer84. For example, the volume of a part of the first internal space90awhich is present in a region immediately above the first surface102amay be 0.03 mm3or more, 0.04 mm3or more, 0.07 mm3or more, 0.5 mm3or more, or 1.5 mm3or more. The volume of a part of the second internal space90bwhich is present in a region immediately below the second surface102bmay be 0.03 mm3or more, 0.04 mm3or more, 0.07 mm3or more, 0.5 mm3or more, or 1.5 mm3or more. The volume of a part of the third internal space90cwhich is present in a region left of the third surface102cmay be 0.015 mm3or more, 0.2 mm3or more, or 0.4 mm3or more. The volume of a part of the fourth internal space90dwhich is present in a region right of the fourth surface102dmay be 0.015 mm3or more, 0.2 mm3or more, or 0.4 mm3or more. The volume of a part of the fifth internal space90ewhich is present in a region forward of the fifth surface102emay be 0.010 mm3or more, 0.1 mm3or more, 0.2 mm3or more, or 0.3 mm3or more. Note that the expression “region immediately above the first surface102a” means a region that extends from the first surface102ain a direction perpendicular to the first surface102a, which does not include a region above and left of the first surface102a, a region above and right of the first surface102a, and the like. The same applies to the expressions “region immediately below the second surface102b”, “region left of the third surface102c”, “region right of the fourth surface102d”, and “region forward of the fifth surface102e”. In the case where the first internal space90aincludes a plurality of spaces, the volume of a part of at least one of the spaces which is present in the region immediately above the first surface102amay be 0.03 mm3or more, 0.04 mm3or more, 0.07 mm3or more, 0.5 mm3or more, or 1.5 mm3or more. Alternatively, the total of the volumes of parts of the spaces which are present in the region immediately above the first surface102amay be 0.03 mm3or more, 0.04 mm3or more, 0.07 mm3or more, 0.5 mm3or more, or 1.5 mm3or more. Similarly, for each of the second to fifth internal spaces90bto90e, in the case where the internal space includes a plurality of spaces, at least one of the spaces may satisfy the above volume range. Alternatively, the spaces may satisfy the above volume range in terms of the total of the spaces. The height of the first internal space90amay be 40% or more and 70% or less of the distance from the first surface102ato the top surface of the first outer protective layer85a. Similarly, the height of the second internal space90bmay be 40% or more and 70% or less of the distance from the second surface102bto the bottom surface of the second outer protective layer85b. The height of the third internal space90cmay be 40% or more and 70% or less of the distance from the third surface102cto the left surface of the third outer protective layer85c. The height of the fourth internal space90dmay be 40% or more and 70% or less of the distance from the fourth surface102dto the right surface of the fourth outer protective layer85d. The height of the fifth internal space90emay be 40% or more and 70% or less of the distance from the fifth surface102eto the front surface of the fifth outer protective layer85e. The height of the first internal space90amay be 5 times or more or 10 times or more the average pore size (by mercury intrusion porosimetry) of the protective layer84. Similarly, the heights of the second to fifth internal spaces90bto90emay be 5 times or more or 10 times or more the average pore size of the protective layer84.

Although the element body102is a layered body including a plurality of solid electrolyte layers (the layers1to6) in the above-described embodiment, the present invention is not limited to this. The element body102may be any layered body including at least one oxygen-ion-conductive solid electrolyte layer. For example, the layers1to5, other than the second solid electrolyte layer6, inFIG.2may be structure layers composed of a material other than a solid electrolyte (e.g., layers composed of alumina). In such a case, the electrodes included in the element body102are disposed in the second solid electrolyte layer6. For example, the measurement electrode44illustrated inFIG.2is disposed on the bottom surface of the second solid electrolyte layer6. Furthermore, the reference-gas introduction space43is formed in the spacer layer5, but not in the first solid electrolyte layer4, the air introduction layer48is interposed between the second solid electrolyte layer6and the spacer layer5, but not between the first solid electrolyte layer4and the third substrate layer3, and the reference electrode42is disposed on the bottom surface of the second solid electrolyte layer6at a position rearward of the second internal space40.

Although a gas sensor100that detects NOx concertation is described as an example in the above-described embodiment, the present invention may be applied to a gas sensor that detects oxygen concentration and a gas sensor that detects ammonia concentration.

EXAMPLES

Examples where the above-described sensor element was specifically prepared are described below as Examples. Test Examples 2 to 11 correspond to Examples of the present invention, while Test Example 1 corresponds to Comparative Example. Note that the present invention is not limited by Examples below.

Test Example 1

In Test Example 1, a sensor element101having the structure illustrated inFIGS.1to4was prepared by the method described below. First, an element body102as described inFIGS.1to4, which had a length of 67.5 mm, a width of 4.25 mm, and a thickness of 1.45 mm, was prepared. In the preparation of the element body102, ceramic green sheets corresponding to the layers1to6were prepared by mixing zirconia particles including 4 mol % yttria serving as a stabilizer with an organic binder and an organic solvent and subsequently performing tape casting. Patterns of electrodes, etc. were printed on each of the six green sheets. A slurry that was to form an inner protective layer86(first and second inner protective layers86aand86b) after baking was formed, by screen printing, on a surface (surface that is to serve as a first surface102a) of one of the six green sheets which was to serve as a second solid electrolyte layer6and on a surface (surface that is to serve as a second surface102b) of one of the six green sheets which was to serve as a first substrate layer1. The slurry used for forming the inner protective layer86was prepared by the following method. With 10 vol % of a raw-material powder (an alumina powder) having a particle size of D50=5 μm, 40 vol % of a binder solution (polyvinyl acetal and butyl carbitol), 45 vol % of a cosolvent (acetone), and 5 vol % of a dispersant (polyoxyethylene styrenated phenyl ether) were mixed. The resulting mixture was stirred with a pot mill mixer at a rotation speed of 200 rpm for 3 hours to form a paste. Subsequent to the printing of the patterns of electrodes, etc. and the slurry that was to form an inner protective layer86, the six green sheets were stacked on top of one another and then baked. Hereby, an element body102including an inner protective layer86was prepared.

Subsequently, an internal space90and an outer protective layer85were formed on the element body102including the inner protective layer86. Specifically, first, a vanishing body composed of a vinyl resin was formed on the first inner protective layer86a, the second inner protective layer86b, and the third to fifth surfaces102cto102eof the element body102by screen printing. The vanishing body was formed in a shape of the internal space90(first to fifth internal spaces90ato90e). Then, an outer protective layer85(first to fifth outer protective layers85ato85e) was formed on the surface of the vanishing body by plasma spraying with a plasma spray gun (“SinplexPro-90” produced by Oerlikon Metco). In the formation of the first outer protective layer85a, plasma spraying was performed under the following conditions. A mixture of an argon gas (flow rate: 50 L/min) and hydrogen (flow rate: 2 L/min) was used as a gas for plasma generation. The voltage applied for plasma generation was a direct-current voltage of 100 V. The current was 200 A. The raw material particles (powder spray material) used to form the first outer protective layer85awere alumina powder particles having an average particle size of 30 μm. The carrier gas used for feeding of the raw material particles was an argon gas (flow rate: 5 L/min). Plasma gun spraying was performed on the first surface102ain a direction perpendicular to the first surface102a. The distance between the plasma gun and the first surface102awas 120 mm. Plasma spraying was performed in air atmosphere at normal temperature. The second to fifth outer protective layers85bto85ewere formed by plasma spraying as in the formation of the first outer protective layer85a. In the formation of the first to fifth outer protective layers85ato85e, plasma spraying was performed under the same conditions. After the first to fifth outer protective layers85ato85ehad been formed in the above-described manner, the vanishing body was removed by combustion to form an internal space90. Hereby, a sensor element101of Test Example 1 was prepared.

In the sensor element101prepared in Test Example 1, the first inner protective layer86aand the second inner protective layer86bhad a thickness of 50 μm and a porosity of 50%. The arithmetic average roughness Rac of the bonding surface97of the first inner protective layer86aand the second inner protective layer86bwhich was measured by the above-described method was 1 The first to fifth outer protective layers85ato85ehad a thickness of 200 μm and a porosity of 20%. The arithmetic average roughness values Ras (=Ra1sto Ra4s) of the first to fourth outside inner peripheral surfaces95ato95dwhich were measured by the above-described method with a spectrointerferometer (optical measuring device, Zygo) were all 1 The arithmetic average roughness Rap of the fifth outside inner peripheral surface95ewhich was measured by the above-described method in the same manner was 1 The arithmetic average roughness Ras (=Ra1sto Ra4s) was calculated as the average of arithmetic average roughness values measured at the following three positions: the center of a corresponding one of the first to fourth outside inner peripheral surfaces95ato95d; and two positions that were 1 mm away from the above center in the longitudinal direction (the front-rear direction) of the sensor element101. The arithmetic average roughness Rap was calculated as the average of arithmetic average roughness values measured at the following three positions: the center of the fifth outside inner peripheral surface95e; and two positions that were 1 mm away from the above center. The thickness of the first and second internal spaces90aand90b(the distance between the outer protective layer85and the inner protective layer86in the thickness direction) was 200 The third to fifth internal spaces90cto90ehad a thickness of 200 μm.

Test Examples 2 to 7

In Test Examples 2 to 7, a sensor element101was prepared as in Test Example 1, except that the arithmetic average roughness values Rap and Ras were set to be higher than those in Test Example 1 by changing the conditions under which plasma spraying was performed in the formation of the outer protective layer85. The conditions for plasma spraying were changed as follows: in Test Example 2, the above-described distance was changed to 150 mm; in Test Example 3, the above distance was changed to 180 mm; in Test Example 4, the above distance was changed to 200 mm; in Test Example 5, the above distance was changed to 200 mm and the average particle size of the alumina powder was changed to 35 μm; in Test Example 6, the above distance was changed to 200 mm and the average particle size of the alumina powder was changed to 40 μm, and in Test Example 7, the above distance was changed to 200 mm and the average particle size of the alumina powder was changed to 50 In each of Test Examples 2 to 7, the arithmetic average roughness Rap was equal to the arithmetic average roughness Ras.

[Test for Evaluating Variations in Detected Value]

A gas sensor including the sensor element101prepared in Test Example 1 was attached to a piping constituting an automobile exhaust gas pipe. Subsequently, the temperature of the heater72was increased to 800° C. by energizing the heater72, in order to heat the sensor element101. Then, an automobile gasoline engine (1.8 L) was operated under predetermined operating conditions (engine rotation speed: 4500 rpm, air fuel ratio A/F: value 11.0, load torque: 130 N·m, gage pressure of automobile exhaust gas: 60 kPa, and temperature of automobile exhaust gas: 800° C.). Subsequently, the above-described pump cells21,41, and50were actuated in order to start measuring the NOx concentration with the sensor element101. After a lapse of 10 seconds since the operation of the pump cells was started, the measurement of the pump current Ip2(value corresponding to the NOx concentration in the automobile exhaust gas) was started. The above measurement was continued for 10 seconds. The difference between the maximum and minimum pump current values Ip2measured during the above measurement period was derived as a value representing the degree of variations in the NOx concentration detected by the sensor element101(value detected by the sensor element101). In Test Examples 2 to 7, the above value was derived in the same manner as described above. The values derived in Test Examples 2 to 7 were expressed on a percentage basis with the value derived in Test Example 1 being 100%. These percentages were used as the ratio of variations in the value detected by the sensor element101.

Table 1 lists the arithmetic average roughness values Rap, Rac, and Ras and the ratio of variations in the value detected by the sensor element101that were measured in each of Test Examples 1 to 7.FIG.8is a graph illustrating the relationship between the arithmetic average roughness values Rap and the ratios of variations in the value detected by the sensor element101which were measured in Test Examples 1 to 7.

The results listed in Table 1 and the results illustrated inFIG.8confirm that the ratio of variations in the value detected by the sensor element101was low in Test Examples 2 to 7, where Rap>Rac, compared with Test Example 1, where the arithmetic average roughness Rap was less than 8 μm and Rap=Rac. In Test Examples 1 to 7, the higher the arithmetic average roughness Rap, the lower the ratio of variations in the value detected by the sensor element101. In the case where the arithmetic average roughness Rap was less than 8 μm (Test Examples 1 to 4), the ratio of variations in the value detected by the sensor element101sharply decreased with an increase in the arithmetic average roughness Rap, while the ratio of variations in the detected value substantially did not change with an increase in the arithmetic average roughness Rap in the case where the arithmetic average roughness Rap was 8 μm or more (Test Examples 5 to 7). Therefore, setting the arithmetic average roughness Rap to be 8 μm or more may reduce variations in the NOx concentration detected by the sensor element101by a sufficient degree.

Test Examples 8 to 11

In Test Examples 8 to 11, a sensor element101was prepared in which the conditions under which plasma spraying was performed in the formation of the first to fourth outer protective layers85ato85dwere changed from the conditions under which plasma spraying was performed in the formation of the fifth outer protective layer85esuch that the arithmetic average roughness Rap and the arithmetic average roughness Ras were different from each other. Test Examples 8 to 11 were the same as Test Example 5, except the conditions under which plasma spraying was performed in the formation of the first to fourth outer protective layers85ato85d. In Test Examples 8, 9, 10, and 11, the conditions under which plasma spraying was performed in the formation of the first to fourth outer protective layers85ato85dwere the same as those of Test Examples 2, 4, 5, and 6, respectively. Thus, in Test Example 10, a sensor element101was prepared under the same production conditions as in Test Example 5, including the conditions under which plasma spraying was performed in the formation of the first to fourth outer protective layers85ato85d.

A gas sensor including the sensor element101prepared in Test Example 8 was attached to a piping constituting an automobile exhaust gas pipe. Subsequently, the temperature of the heater72was increased to 800° C. by energizing the heater72, in order to heat the sensor element101. A model gas prepared by mixing a base gas, which was nitrogen, with a predetermined concentration of oxygen and 70 ppm of NO was used as a measurement-object gas. The measurement-object gas was passed through the piping at a flow rate of 9 m/s. Subsequently, the above-described pump cells21,41, and50were actuated in order to start measuring the NOx concentration with the sensor element101. After the pump current Ip2(value corresponding to the NOx concentration in the measurement-object gas) had become stable, a change in the pump current Ip2with time which occurred when the NO concentration in the measurement-object gas passed through the piping was changed from 70 ppm to 500 ppm was examined. With the pump current Ip2measured immediately before the NO concentration was changed being 0% and the pump current Ip2measured after the pump current Ip2had been changed and stabilized subsequent to the change in NO concentration being 100%, the amount of time that elapsed from when the pump current Ip2exceeded 10% to when the pump current Ip2exceeded 90% was defined as a response time (sec) in the detection of NOx concentration. The smaller the response time, the higher the responsivity of the sensor element101. The response time was also measured in Test Examples 8 to 11 in the same manner as described above. In each test example, the measurement of the response time was conducted a plurality of times and the average thereof was used as a response time.

Table 2 lists the arithmetic average roughness values Rap, Rac, and Ras and the response time of the sensor element101that were measured in each of Test Examples 8 to 11.

The results listed in Table 2 confirm that the response time of the sensor element101was small in Test Examples 9 to 11, where Ras>Rac, compared with Test Example 8, where the arithmetic average roughness Ras was less than 8 μm and Ras=Rac. In Test Examples 8 to 11, the higher the arithmetic average roughness Ras, the smaller the response time of the sensor element101. A comparison between the results obtained in Test Examples 8 to 11 confirms that the response time of the sensor element101sharply decreased in Test Examples 10 and 11, where the arithmetic average roughness Ras was 8 μm or more, compared with Test Examples 8 and 9, where the arithmetic average roughness Ras was less than 8 Therefore, setting the arithmetic average roughness Ras to be 8 μm or more may increase the responsivity of the sensor element101by a sufficient degree.

The present application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2019-033351 filed on Feb. 26, 2019, and the prior Japanese Patent Application No. 2019-211703 filed on Nov. 22, 2019, which are incorporated herein by reference in their entirety.