Sensor element

A sensor element includes: an element base including: a ceramic body made of an oxygen-ion conductive solid electrolyte, and having a gas inlet at one end portion thereof; at least one internal chamber located inside the ceramic body, and communicating with the gas inlet under predetermined diffusion resistance; an electrochemical pump cell including an electrode located on an outer surface of the ceramic body, an electrode facing the chamber, and a solid electrolyte located therebetween; and a heater buried in the ceramic body, and an leading-end protective layer being porous, and covering a leading end surface and four side surfaces in a predetermined range of the element base on the one end portion. The leading-end protective layer has an extension extending into the gas inlet, and fixed to an inner wall surface of the ceramic body demarcating the gas inlet.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese application JP2018-161589, filed on Aug. 30, 2018, the contents of which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a gas sensor detecting a predetermined gas component in a measurement gas, and, in particular, to a configuration of a leading end portion of a sensor element included in the gas sensor.

Description of the Background Art

As a gas sensor for determining concentration of a desired gas component in a measurement gas, a gas sensor that includes a sensor element made of an oxygen-ion conductive solid electrolyte, such as zirconia (ZrO2), and including some electrodes on the surface and the inside thereof has been widely known. As the sensor element, a sensor element including a protective layer formed of a porous body (porous protective layer) at an end portion at which a gas inlet for introducing the measurement gas is provided has been known (see Japanese Patent Application Laid-Open No. 2013-64605, Japanese Patent No. 5533767, and Japanese Patent No. 4583187, for example).

The above-mentioned gas sensor is mainly installed onto an exhaust pipe of an internal combustion engine, such as a vehicle engine, and is used to detect a predetermined gas component contained in an exhaust gas from the internal combustion engine and further to measure the concentration of the gas component. When the gas sensor is used for such an application, the sensor element is frequently subjected to thermal shock due to repeated heating up in use of the internal combustion engine and cooling down not in use of the internal combustion engine. To achieve long-term stable operation of the gas sensor, it is required to provide the porous protective layer so that delamination and, further, detachment thereof do not occur upon receipt of thermal shock caused repeatedly.

Such delamination and, further, detachment occurring during long-term use of the gas sensor is not preferable because an introduction path of the measurement gas increases to more than expected at product designing, diffusion resistance of the measurement gas decreases, and, as a result, an output from the sensor element increases to more than a predetermined value.

As for this point, Japanese Patent Application Laid-Open No. 2013-64605 discloses a configuration in which a side surface of a sensor element is covered with an inorganic fiber sheet, and a protective layer is provided over the sheet to prevent delamination of the protective layer occurring due to thermal shock and the like.

Japanese Patent No. 5533767 discloses a configuration, of a gas sensor element including a porous protective layer provided at a portion exposed to a measurement gas, in which an upper end surface of the porous protective layer and the surface of the sensor element form a contact angle of 80° or less, and the upper end surface of the porous protective layer is smoothly curved to have an upwardly convex substantially circular arc-like or substantially parabolic shape in a planar direction of the sensor element to make delamination of the porous protective layer less likely to occur when the gas sensor element is subjected to shock or vibration.

Japanese Patent No. 4583187 discloses a configuration in which two or more porous ceramic layers are provided outside a ceramic heater element including a sensor portion on the surface or the inside thereof, and an inclined portion meeting a predetermined shape condition is provided at an end portion of an outermost ceramic layer to prevent delamination of the porous ceramic layers from the ceramic heater element due to a slight difference in coefficient of thermal expansion between the ceramic heater element and the porous layers.

However, none of Japanese Patent Application Laid-Open No. 2013-64605, Japanese Patent No. 5533767, and Japanese Patent No. 4583187 discloses or suggests a configuration, of a sensor element having a gas inlet in a leading end surface thereof, to positively secure adhesion of a porous protective layer to the leading end surface.

SUMMARY

The present invention relates to a gas sensor detecting a predetermined gas component in a measurement gas, and is, in particular, directed to a configuration of a leading end portion of a sensor element included in the gas sensor.

According to the present invention, a sensor element included in a gas sensor detecting a predetermined gas component in a measurement gas includes: an element base including: an elongated planar ceramic body made of an oxygen-ion conductive solid electrolyte, and having a gas inlet at one end portion thereof; at least one internal chamber located inside the ceramic body, and communicating with the gas inlet under predetermined diffusion resistance; at least one electrochemical pump cell including an outer pump electrode located on an outer surface of the ceramic body, an inner pump electrode located to face the at least one internal chamber, and a solid electrolyte located between the outer pump electrode and the inner pump electrode, the at least one electrochemical pump cell pumping in and out oxygen between the at least one internal chamber and an outside; and a heater buried in a predetermined range on a side of the one end portion of the ceramic body, and a leading-end protective layer being porous, and covering a leading end surface and four side surfaces in a predetermined range of the element base on the one end portion, wherein the leading-end protective layer has an extension extending into the gas inlet, and fixed to an inner wall surface of the ceramic body demarcating the gas inlet.

Accordingly, the sensor element in which delamination and, further, detachment of the porous leading-end protective layer on a side of the leading end surface of the element caused by application of thermal shock is suitably suppressed, and adhesion of the leading-end protective layer to the element base is suitably secured can thereby be achieved.

Preferably, the sensor element according to the present invention further includes a buffer layer being porous, having a larger porosity than the leading-end protective layer, and located outside the four side surfaces of the element base, wherein the leading-end protective layer is located further outside the buffer layer.

In this case, a so-called anchoring effect thereby acts between the leading-end protective layer and the buffer layer, and delamination of the leading-end protective layer from the element base caused by a difference in coefficient of thermal expansion between the leading-end protective layer and the element base is thus more suitably suppressed when the sensor element is in use.

It is thus an object of the present invention to provide a sensor element for a gas sensor in which adhesion of a porous leading-end protective layer to an element base on a leading end surface side is suitably secured.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Overview of Sensor Element and Gas Sensor>

FIG. 1is a schematic external perspective view of a sensor element (gas sensor element)10according to an embodiment of the present invention.FIG. 2is a schematic diagram illustrating a configuration of a gas sensor100including a sectional view taken along a longitudinal direction of the sensor element10. The sensor element10is a main component of the gas sensor100detecting a predetermined gas component in a measurement gas, and measuring concentration thereof. The sensor element10is a so-called limiting current gas sensor element.

In addition to the sensor element10, the gas sensor100mainly includes a pump cell power supply30, a heater power supply40, and a controller50.

As illustrated inFIG. 1, the sensor element10has a configuration in which one end portion of an elongated planar element base1is covered with a porous leading-end protective layer2.

As illustrated inFIG. 2, the element base1includes an elongated planar ceramic body101as a main structure, main surface protective layers170are provided on two main surfaces of the ceramic body101, and, in the sensor element10, the leading-end protective layer2is further provided outside both an end surface (a leading end surface101eof the ceramic body101) and four side surfaces, on one leading end portion. The four side surfaces other than opposite end surfaces in the longitudinal direction of the sensor element10(or the element base1, or the ceramic body101) are hereinafter simply referred to as side surfaces of the sensor element10(or the element base1, or the ceramic body101).

The ceramic body101is made of ceramic containing, as a main component, zirconia (yttrium stabilized zirconia), which is an oxygen-ion conductive solid electrolyte. Various components of the sensor element10are provided outside and inside the ceramic body101. The ceramic body101having the configuration is dense and airtight. The configuration of the sensor element10illustrated inFIG. 2is just an example, and a specific configuration of the sensor element10is not limited to this configuration.

The sensor element10illustrated inFIG. 2is a so-called serial three-chamber structure type gas sensor element including a first internal chamber102, a second internal chamber103, and a third internal chamber104inside the ceramic body101. That is to say, in the sensor element10, the first internal chamber102communicates, through a first diffusion control part110and a second diffusion control part120, with a gas inlet105opening to the outside on a side of one end portion E1of the ceramic body101(to be precise, communicating with the outside through the leading-end protective layer2), the second internal chamber103communicates with the first internal chamber102through a third diffusion control part130, and the third internal chamber104communicates with the second internal chamber103through a fourth diffusion control part140. A path from the gas inlet105to the third internal chamber104is also referred to as a gas distribution part. In the sensor element10according to the present embodiment, the distribution part is provided straight along the longitudinal direction of the ceramic body101.

The first diffusion control part110, the second diffusion control part120, the third diffusion control part130, and the fourth diffusion control part140are each provided as two slits vertically arranged inFIG. 2. The first diffusion control part110, the second diffusion control part120, the third diffusion control part130, and the fourth diffusion control part140provide predetermined diffusion resistance to a measurement gas passing therethrough. A buffer space115having an effect of buffering pulsation of the measurement gas is provided between the first diffusion control part110and the second diffusion control part120.

An outer pump electrode141is provided on an outer surface of the ceramic body101, and an inner pump electrode142is provided in the first internal chamber102. Furthermore, an auxiliary pump electrode143is provided in the second internal chamber103, and a measurement electrode145is provided in the third internal chamber104. In addition, a reference gas inlet106which communicates with the outside and through which a reference gas is introduced is provided on a side of the other end portion E2of the ceramic body101, and a reference electrode147is provided in the reference gas inlet106.

In a case where a target of measurement of the sensor element10is NOx in the measurement gas, for example, concentration of a NOx gas in the measurement gas is calculated by a process as described below.

First, the measurement gas introduced into the first internal chamber102is adjusted to have a substantially constant oxygen concentration by a pumping action (pumping in or out of oxygen) of a main pump cell P1, and then introduced into the second internal chamber103. The main pump cell P1is an electrochemical pump cell including the outer pump electrode141, the inner pump electrode142, and a ceramic layer101athat is a portion of the ceramic body101existing between these electrodes. In the second internal chamber103, oxygen in the measurement gas is pumped out of the element by a pumping action of an auxiliary pump cell P2that is also an electrochemical pump cell, so that the measurement gas is in a sufficiently low oxygen partial pressure state. The auxiliary pump cell P2includes the outer pump electrode141, the auxiliary pump electrode143, and a ceramic layer101bthat is a portion of the ceramic body101existing between these electrodes.

The outer pump electrode141, the inner pump electrode142, and the auxiliary pump electrode143are each formed as a porous cermet electrode (e.g., a cermet electrode made of ZrO2and Pt that contains Au of 1%). The inner pump electrode142and the auxiliary pump electrode143to be in contact with the measurement gas are each formed using a material having weakened or no reducing ability with respect to a NOx component in the measurement gas.

NOx in the measurement gas caused by the auxiliary pump cell to be in the low oxygen partial pressure state is introduced into the third internal chamber104, and reduced or decomposed by the measurement electrode145provided in the third internal chamber104. The measurement electrode145is a porous cermet electrode also functioning as a NOx reduction catalyst that reduces NOx existing in the atmosphere in the third internal chamber104. During the reduction or decomposition, a potential difference between the measurement electrode145and the reference electrode147is maintained constant. Oxygen ions generated by the above-mentioned reduction or decomposition are pumped out of the element by a measurement pump cell P3. The measurement pump cell P3includes the outer pump electrode141, the measurement electrode145, and a ceramic layer101cthat is a portion of the ceramic body101existing between these electrodes. The measurement pump cell P3is an electrochemical pump cell pumping out oxygen generated by decomposition of NOx in the atmosphere around the measurement electrode145.

Pumping (pumping in or out of oxygen) of the main pump cell P1, the auxiliary pump cell P2, and the measurement pump cell P3is achieved, under control performed by the controller50, by the pump cell power supply (variable power supply)30applying voltage necessary for pumping across electrodes included in each of the pump cells. In a case of the measurement pump cell P3, voltage is applied across the outer pump electrode141and the measurement electrode145so that the potential difference between the measurement electrode145and the reference electrode147is maintained at a predetermined value. The pump cell power supply30is typically provided for each pump cell.

The controller50detects a pump current Ip2flowing between the measurement electrode145and the outer pump electrode141in accordance with the amount of oxygen pumped out by the measurement pump cell P3, and calculates a NOx concentration in the measurement gas based on a linear relationship between a current value (NOx signal) of the pump current Ip2and the concentration of decomposed NOx.

The gas sensor100preferably includes a plurality of electrochemical sensor cells, which are not illustrated, detecting the potential difference between each pump electrode and the reference electrode147, and each pump cell is controlled by the controller50based on a signal detected by each sensor cell.

In the sensor element10, a heater150is buried in the ceramic body101. The heater150is provided, below the gas distribution part inFIG. 2, over a range from the vicinity of the one end portion E1to at least a location of formation of the measurement electrode145and the reference electrode147. The heater150is provided mainly to heat the sensor element10to enhance oxygen-ion conductivity of the solid electrolyte forming the ceramic body101when the sensor element10is in use. More particularly, the heater150is provided to be surrounded by an insulating layer151.

The heater150is a resistance heating body made, for example, of platinum. The heater150generates heat by being powered from the heater power supply40under control performed by the controller50.

The sensor element10according to the present embodiment is heated by the heater150when being in use so that the temperature at least in a range from the first internal chamber102to the second internal chamber103becomes 500° C. or more. In some cases, the sensor element10is heated so that the temperature of the gas distribution part as a whole from the gas inlet105to the third internal chamber104becomes 500° C. or more. These are to enhance the oxygen-ion conductivity of the solid electrolyte forming each pump cell and to desirably demonstrate the ability of each pump cell. In this case, the temperature in the vicinity of the first internal chamber102, which becomes the highest temperature, becomes approximately 700° C. to 800° C.

In the following description, from among the two main surfaces of the ceramic body101, a main surface (or an outer surface of the sensor element10having the main surface) which is located on an upper side inFIG. 2and on a side where the main pump cell P1, the auxiliary pump cell P2, and the measurement pump cell P3are mainly provided is also referred to as a pump surface, and a main surface (or an outer surface of the sensor element10having the main surface) which is located on a lower side inFIG. 2and on a side where the heater150is provided is also referred to as a heater surface. In other words, the pump surface is a main surface closer to the gas inlet105, the three internal chambers, and the pump cells than to the heater150, and the heater surface is a main surface closer to the heater150than to the gas inlet105, the three internal chambers, and the pump cells.

A plurality of electrode terminals160are provided on the respective main surfaces of the ceramic body101on the side of the other end portion E2to establish electrical connection between the sensor element10and the outside. These electrode terminals160are electrically connected to the above-mentioned five electrodes, opposite ends of the heater150, and a lead for detecting heater resistance, which is not illustrated, through leads provided inside the ceramic body101, which are not illustrated, to have a predetermined correspondence relationship. Application of a voltage from the pump cell power supply30to each pump cell of the sensor element10and heating by the heater150by being powered from the heater power supply40are thus performed through the electrode terminals160.

The sensor element10further includes the above-mentioned main surface protective layers170(170a,170b) on the pump surface and the heater surface of the ceramic body101. The main surface protective layers170are layers made of alumina, having a thickness of approximately 5 μm to 30 μm, and including pores with a porosity of approximately 20% to 40%, and are provided to prevent adherence of any foreign matter and poisoning substances to the main surfaces (the pump surface and the heater surface) of the ceramic body101and the outer pump electrode141provided on the pump surface. The main surface protective layer170aon the pump surface thus functions as a pump electrode protective layer for protecting the outer pump electrode141.

In the present embodiment, the porosity is obtained by applying a known image processing method (e.g., binarization processing) to a scanning electron microscope (SEM) image of an evaluation target.

The main surface protective layers170are provided over substantially all of the pump surface and the heater surface except that the electrode terminals160are partially exposed inFIG. 2, but this is just an example. The main surface protective layers170may locally be provided in the vicinity of the outer pump electrode141on the side of the one end portion E1compared with the case illustrated inFIG. 2.

In the sensor element10, the leading-end protective layer2that is a porous layer made of alumina having a purity of 99.0% or more is provided around an outermost periphery in a predetermined range from the one end portion E1of the element base1having a configuration as described above.

In the following description, a portion of the leading-end protective layer2being in contact with the leading end surface101eof the ceramic body101is referred to as an end surface portion201, and a portion of the leading-end protective layer2being in contact with the four side surfaces including the two main surfaces (the pump surface and the heater surface) on which the main surface protective layers170are provided is referred to as a side surface portion202.

The leading-end protective layer2is provided to surround a portion of the element base1in which the temperature becomes high when the gas sensor100is in use to thereby obtain water resistance in the portion. The leading-end protective layer2suppresses the occurrence of cracking (water-induced cracking) of the element base1due to thermal shock caused by local temperature reduction upon direct exposure of the portion to water.

Since the leading-end protective layer2is a porous layer, gas flows in and out between the gas inlet105and the outside at all times regardless of the presence of the leading-end protective layer2. That is to say, introduction of the measurement gas into the element base1(ceramic body101) through the gas inlet105is performed without any problems.

The leading-end protective layer2is preferably formed to have a thickness of 150 μm or more and 600 μm or less. A thickness of the leading-end protective layer2of less than 150 μm is not preferable because, due to reduction in strength of the leading-end protective layer2itself, resistance to thermal shock is reduced and water resistance is reduced, and, further, resistance to shock acting due to vibration or other factors is reduced. On the other hand, a thickness of the leading-end protective layer2of more than 600 μm is not preferable because, due to an increase in heat capacity of the leading-end protective layer2, power consumption increases when the heater150performs heating, and, due to an increase in gas diffusion time, responsiveness of the sensor element10is degraded.

The leading-end protective layer2preferably has a porosity of 15% to 40%. In this case, adhesion to the element base1, in particular, to the main surface protective layers170, which are in contact with most of the leading-end protective layer2, is suitably secured. A porosity of the leading-end protective layer2of less than 15% is not preferable because diffusion resistance increases, and responsiveness of the sensor element10is degraded. On the other hand, a porosity of more than 40% is not preferable because adhesion to the element base1(specifically, adhesion to the leading end surface101eand the main surface protective layers170) is reduced, and the strength of the leading-end protective layer2is not secured.

FIGS. 3 and 4are diagrams for describing further details of the leading-end protective layer2in the vicinity of the gas inlet105.FIG. 3is an enlarged view in the vicinity of a portion Q on the one end portion E1of the sensor element10, which is shown by a broken line inFIG. 2, andFIG. 4is a diagram for describing the size of each portion of the gas inlet105on the leading end surface101e.

Although simplified for illustration inFIG. 2, in the sensor element10, the leading-end protective layer2has an extension201aextending into the gas inlet105from the end surface portion201adhering to the leading end surface101eof the ceramic body101, as illustrated inFIG. 3. The extension201ais fixed to an inner wall surface101fdemarcating the gas inlet105from four sides in the ceramic body101. However, fixing only to opposite two portions of the inner wall surface101fis shown inFIG. 3for illustrative purposes.

The above-mentioned thickness of the leading-end protective layer2refers to the thickness of a portion of the leading-end protective layer2excluding the extension201a.

More particularly, the extension201ais formed in a range according to the size of the gas inlet105of the sensor element10. The gas inlet105is provided so that a distance L0from the leading end surface101eof the ceramic body101to an innermost part of the gas inlet105(the beginning of the first diffusion control part110) satisfies an equation 100 μm≤L0≤500 μm.

A distance L0of less than 100 μm is not preferable because it is likely that formation particles of the leading-end protective layer2scattered during formation of the leading-end protective layer2(in particular, the extension201a) enter the first diffusion control part110to cause clogging to thereby increase diffusion resistance to more than expected at designing.

On the other hand, a distance L0of more than 500 μm is not preferable because, to maintain a predetermined element size, it is required to shorten the diffusion control part, and it becomes difficult to achieve desired diffusion resistance, or, to secure the size of the diffusion control part, the element size is elongated.

The extension201ais formed so that a formation range L1of the extension201afrom the leading end surface101ein the longitudinal direction of the element is 8% or more and 75% or less of the distance L0(0.08≤L1/L0≤0.75), in other words, an adherence ratio of the extension201afrom a leading end surface side in the gas inlet105is 8% or more and 75% or less.

In the sensor element10according to the present embodiment, the leading-end protective layer2, which surrounds the portion of the element base1in which the temperature becomes high when the gas sensor100is in use, has the extension201ain the above-mentioned manner, so that adhesion of the leading-end protective layer2to the leading end surface101eof the ceramic body101, which is the end surface of the element base1, is more sufficiently secured compared with that in a conventional sensor element.

A ratio L1/L0of less than 0.08 is not preferable because the effect of securing adhesion of the extension201ais not sufficiently obtained.

On the other hand, a ratio L1/L0of more than 0.75 is not preferable because the extension201ais similar to the diffusion control part having a slit-like shape, and operation of each pump cell of the sensor element10becomes different from operation expected at designing. A problem in that formation of the extension201ais difficult and requires higher cost arises.

The gas inlet105preferably has an aspect ratio t/w that is a ratio of an opening height (thickness) t to an opening width w of the gas inlet105of 0.015 to 0.15 and an opening area S=wt of 0.1 mm2to 0.9 mm2.

An aspect ratio t/w of less than 0.015 or an opening area S of less than 0.1 mm2is not preferable because formation of the extension201ais difficult.

On the other hand, an aspect ratio t/w of more than 0.15 or an opening area S of more than 0.9 mm2is possible by increasing at least one of the opening width w and the opening height t, but is not preferable because a discrepancy from the size and the shape of the internal space becomes prominent, and productivity decreases due to difficulty of simultaneous formation with the internal space in this case.

InFIG. 3, a groove g is formed between two portions of the extension201avertically arranged inFIG. 3. However, this is just an example, and is not necessarily required. That is to say, a portion that becomes the groove g may be buried by a material for forming the leading-end protective layer2.

The extension201aof the leading-end protective layer2suitably suppresses delamination and, further, detachment of the leading-end protective layer2on the leading end surface of the element base1even if the sensor element10is frequently subjected to thermal shock due to repeated heating up and cooling down during long-term use. That is to say, the sensor element10can be said to be less likely to undergo a sensitivity change caused by delamination and, further, detachment of the leading-end protective layer even during long-term use, and thus have high reliability.

As described above, according to the present embodiment, the leading-end protective layer as the porous layer is provided in the element base of the sensor element included in the gas sensor at least around the portion thereof in which the temperature becomes high when the gas sensor is in use, and a portion of the leading-end protective layer is extended into the gas inlet on the one end portion of the element base and is fixed to the inner surface of the gas inlet. The sensor element in which delamination and, further, detachment of the leading-end protective layer on the leading end surface side of the element base caused by application of thermal shock is suitably suppressed can thus be achieved.

<Process of Manufacturing Sensor Element>

One example of a process of manufacturing the sensor element10having a configuration and features as described above will be described next.FIG. 5is a flowchart of processing at the manufacture of the sensor element10.

At the manufacture of the element base1, a plurality of blank sheets (not illustrated) being green sheets containing the oxygen-ion conductive solid electrolyte, such as zirconia, as a ceramic component and having no pattern formed thereon are prepared first (step S1).

The blank sheets have a plurality of sheet holes used for positioning in printing and lamination. The sheet holes are formed to the blank sheets in advance prior to pattern formation through, for example, punching by a punching machine. Green sheets corresponding to a portion of the ceramic body101in which an internal space is formed also include penetrating portions corresponding to the internal space formed in advance through, for example, punching as described above. The blank sheets are not required to have the same thickness, and may have different thicknesses in accordance with corresponding portions of the element base1eventually formed.

After preparation of the blank sheets corresponding to the respective layers, pattern printing and drying are performed on the individual blank sheets (step S2). Specifically, a pattern of various electrodes, a pattern of the heater150and the insulating layer151, a pattern of the electrode terminals160, a pattern of the main surface protective layers170, a pattern of internal wiring, which is not illustrated, and the like are formed. Application or placement of a sublimable material (vanishing material) for forming the first diffusion control part110, the second diffusion control part120, the third diffusion control part130, and the fourth diffusion control part140is also performed at the time of pattern printing.

The patterns are printed by applying pastes for pattern formation prepared in accordance with the properties required for respective formation targets onto the blank sheets using known screen printing technology. A known drying means can be used for drying after printing.

After pattern printing on each of the blank sheets, printing and drying of a bonding paste are performed to laminate and bond the green sheets (step S3). The known screen printing technology can be used for printing of the bonding paste, and the known drying means can be used for drying after printing.

The green sheets to which an adhesive has been applied are then stacked in a predetermined order, and the stacked green sheets are crimped under predetermined temperature and pressure conditions to thereby form a laminated body (step S4). Specifically, crimping is performed by stacking and holding the green sheets as a target of lamination on a predetermined lamination jig, which is not illustrated, while positioning the green sheets at the sheet holes, and then heating and pressurizing the green sheets together with the lamination jig using a lamination machine, such as a known hydraulic pressing machine. The pressure, temperature, and time for heating and pressurizing depend on a lamination machine to be used, and these conditions may be determined appropriately to achieve good lamination.

After the laminated body is obtained as described above, the laminated body is cut out at a plurality of locations to obtain unit bodies eventually becoming the individual element bases1(step S5).

The unit bodies as obtained are then each fired at a firing temperature of approximately 1300° C. to 1500° C. (step S6). The element base1is thereby manufactured. That is to say, the element base1is generated by integrally firing the ceramic body101made of the solid electrolyte, the electrodes, and the main surface protective layers170. Integral firing is performed in this manner, so that the electrodes each have sufficient adhesion strength in the element base1.

After the element base1is manufactured in the above-mentioned manner, the leading-end protective layer2is formed with respect to the element base1. The leading-end protective layer2is formed by a method of plasma-spraying.FIG. 6schematically illustrates formation of the leading-end protective layer2by plasma-spraying.

The leading-end protective layer2is formed by plasma-spraying slurry containing alumina power as a material for forming the leading-end protective layer2at a predetermined formation target location (step S7).

Specifically, as illustrated inFIG. 6, after the element base1is inclined to have a predetermined inclination angle α with a side of the leading end surface101ebeing up, the element base1is continuously rotated about the longitudinal direction of the element as shown by an arrow AR1while changing the inclination angle α. During the rotation, the slurry is thermal sprayed from a thermal spray gun1000towards the side of the leading end surface101eas shown by an arrow AR2. The slurry thus adheres to the side surfaces of the element base1, the end surface of the element base1(the leading end surface101eof the ceramic body101), and a predetermined range in the gas inlet105.

As the alumina power, powder having a maximum particle diameter of 50 μm or less and D50of 23 μm or less is suitable.

The inclination angle α and a rotation speed of the element base1are adjusted as appropriate to enable the slurry to adhere to the inner wall surface101fdemarcating the gas inlet105so that an adherence ratio has a predetermined value within a range of 8% to 75% in the leading-end protective layer2eventually formed.

The sensor element10is completed by formation of the thermal sprayed film.

The sensor element10thus obtained is housed in a predetermined housing, and built into the body, which is not illustrated, of the gas sensor100.

The above-mentioned embodiments are targeted at a sensor element having three internal chambers, but the sensor element may not necessarily have a three-chamber configuration. That is to say, the configuration in which the extension extending into the gas inlet is provided to the leading-end protective layer that is the porous layer surrounding the end surface and the predetermined range of the side surfaces on the one end portion of the element base is applicable to a sensor element having one internal chamber or two internal chambers.

Although the leading-end protective layer2is provided directly to the element base1in the above-mentioned embodiment, the leading-end protective layer2may not necessarily be provided directly to the element base1.FIG. 7is a schematic block diagram of the gas sensor100in a case where the sensor element10includes a buffer layer180between the element base1and the leading-end protective layer2.

The sensor element10illustrated inFIG. 7includes the buffer layer180outside the four side surfaces (on an outer periphery other than the leading end surface101e) of the element base1on the one end portion E1. The leading-end protective layer2is provided further outside the buffer layer180. InFIG. 7, a pump surface-side portion180aand a heater surface-side portion180bof the buffer layer180are illustrated.

The buffer layer180is a porous layer made of alumina, having a relatively large porosity of 30% to 50%, and having a thickness of 20 μm to 50 μm.

In a case where the buffer layer180is provided, the leading-end protective layer2preferably has a smaller porosity than the buffer layer180. When the buffer layer180has a larger porosity, a so-called anchoring effect acts between the leading-end protective layer2and the buffer layer180as an underlying layer. Due to the action of the anchoring effect, in the sensor element10, delamination of the leading-end protective layer2from the element base1caused by a difference in coefficient of thermal expansion between the leading-end protective layer2and the element base1is more suitably suppressed when the sensor element10is in use.

The buffer layer180has a role of preventing poisoning and exposure to water of the sensor element10along with the leading-end protective layer2and the main surface protective layers170. In particular, the buffer layer180has higher heat insulating properties than the leading-end protective layer2and the main surface protective layers170when the buffer layer180has a larger porosity than the leading-end protective layer2. This contributes to improvement in water resistance of the sensor element10.

The buffer layer180also has a role as the underlying layer when the leading-end protective layer2is formed with respect to the element base1. From this viewpoint, the buffer layer180is only required to be formed, on the side surfaces of the element base1, at least in a range surrounded by the leading-end protective layer2.

Manufacture of the sensor element10including the buffer layer180as illustrated inFIG. 7is achieved by further performing, with respect to an individual element body obtained by the procedures shown inFIG. 5, a process of forming (applying and drying) a pattern eventually becoming the buffer layer180, and then firing. Formation of the pattern is performed using a paste prepared in advance so that the buffer layer180as desired is eventually formed. That is to say, the element base1of the sensor element10illustrated inFIG. 7is generated by integrally firing the ceramic body101made of the solid electrolyte, the electrodes, the main surface protective layers170, and the buffer layer180.

EXAMPLES

As the sensor element10, five types of sensor elements10(Examples 1 to 5) having adherence ratios of the extension201aof the leading-end protective layer2to the inner wall surface101fof 10%, 30%, 50%, 60%, and 75% were manufactured. The gas inlet105was set to have a distance L0of 300 μm, an aspect ratio t/w of an opening of 0.08, and an area S of the opening of 0.5 mm2.

As comparative examples, two types of sensor elements (Comparative Examples 1 and 2) having values of a ratio L1/L0of 85% and 90% and two types of sensor elements (Comparative Examples 3 and 4) each including the leading-end protective layer2not having the extension201awere manufactured. All the comparative examples were manufactured under the same condition as Examples 1 to 5 except for formation of the extension201a.

A heating/cooling cycle test in which heating up and down and an atmosphere change are cyclically repeated was conducted on each of the sensor elements as obtained to evaluate resistance to thermal shock, and whether the leading-end protective layer2was delaminated from the leading end surface101ewas determined after the test (Determination 1).

In the heating/cooling cycle test, a temperature profile of “keeping at 950° C. for five minutes” and then “keeping at 300° C. for five minutes” was set as one cycle of heating up and down, and it was repeated 600 times. Test gas atmosphere was exhaust gas atmosphere with λ=1.1 at 950° C., and was ambient atmosphere at 300° C. X-ray CT was used to determine whether the leading-end protective layer2was delaminated.

A pump current Ip0in the main pump cell P1of each sensor element was measured before and after the heating/cooling cycle test to evaluate the validity of operation of the sensor element. The pump current Ip0was measured under model gas atmosphere including oxygen having an O2concentration of 20.5 mol % and nitrogen as the remainder.

A ratio of a difference value of the pump current Ip0before and after the test to a value of the pump current Ip0before the test was calculated, and whether a prominent sensitivity change occurred before and after the heating/cooling cycle test was determined using the magnitude of the ratio (Determination 2). The magnitude of the value of the pump current Ip0after the test itself was also determined (Determination 3).

The adherence ratio and the results of Determinations 1 to 3 for each sensor element are shown as a list.

As for Determination 1, a cross is marked for the sensor element in which delamination has been identified, and a circle is marked for the sensor element in which delamination has not been identified by the X-ray CT in Table 1.

As for Determination 2, when the ratio is 5% or less, it is determined that the prominent sensitivity change does not occur in the sensor element before and after the heating/cooling cycle test, and a circle is marked in Table 1. When the ratio exceeds 5%, it is determined that the prominent sensitivity change occurs in the sensor element before and after the heating/cooling cycle test, and a cross is marked in Table 1.

As for Determination 3, when the value of the pump current Ip0is 1 mA or more, the pump current Ip0is determined to have a sufficient magnitude, and a circle is marked in Table 1. When the value of the pump current Ip0is less than 1 mA, the pump current Ip0is determined to have an insufficient magnitude, and a cross is marked in Table 1.

In Table 1, the circle is marked in each of Determinations 1 to 3 for the sensor elements in Examples 1 to 5. In contrast, for the sensor elements in Comparative Examples 1 and 2, the cross is marked in Determination 3 while the circle is marked in Determinations 1 and 2, and, for the sensor elements in Comparative Examples 3 and 4, the cross is marked in Determinations 1 and 2 while the circle is marked in Determination 3.

The results shown in Table 1 indicate that, as in the above-mentioned embodiment, in providing the leading-end protective layer to the sensor element, extending the leading-end protective layer from the leading end surface into the gas inlet and fixing the extension to the inner wall surface of the gas inlet is effective in suppressing delamination and, further, detachment of the leading-end protective layer from the leading end surface caused by thermal shock. The results also indicate that the prominent sensitivity change caused by the action of thermal shock does not occur when the delamination and, further, the detachment do not occur. The results also indicate that, in this case, an output from the sensor element is suitably secured when the adherence ratio (from the leading end surface side) to the inner wall surface of the gas inlet has a value in a range of 8% or more and 75% or less.