Gas sensor element and gas sensor

There is provided a gas sensor element for detecting the concentration of a specific gas component in gas under measurement, which includes a plate-shaped element body and a porous protection layer. The element body has, at one end portion thereof, a gas sensing portion formed with a solid electrolyte substrate and a pair of electrodes. The porous protection layer has a porous structure formed of ceramic particles and surrounds at least the circumference of the one end portion of the element body. In the present invention, the porous protection layer has an inner region, an intermediate region and an outer region laminated together in order of mention from the element body toward the outside. The intermediate region has a porosity lower than those of the inner and outer regions. There is also provided a gas sensor with such a gas sensor element.

BACKGROUND OF THE INVENTION

The present invention relates to a gas sensor element for detecting the concentration of a specific gas component in gas under measurement such as combustion gas or exhaust gas of an internal combustion engine or a combustor etc. and to a gas sensor using the gas sensor element.

Hereinafter, the terms “front” and “rear” are used with respect to the axial direction of a gas sensor (gas sensor element) for purposes of description. These terms are illustrative and are not intended to limit the scope of the present invention.

There has been used a gas sensor having a gas sensor element for detecting the concentration of a specific gas component such as oxygen in exhaust gas of an internal combustion engine. The gas sensor element includes a plate-shaped element body having, at a front end portion thereof, a gas sensing portion provided with a solid electrolyte substrate and a pair of electrodes. When the front end portion of the gas sensor element in which the gas sensing portion of the element body is located (also referred to as “the sensing end portion of the gas sensor element”) is exposed to the exhaust gas, poisoning substances such as silicon and phosphorus in the exhaust gas may be adhered to the sensing end portion of the gas sensor element. Water content such as condensed water in the exhaust gas or in an exhaust pipe of the internal combustion engine may also be adhered to the sensing end portion of the gas sensor element. At least the sensing end portion of the gas sensor element is thus covered with a porous ceramic protection layer so as to trap poisoning substances and prevent direct contact of water content with the sensing end portion of the gas sensor element. Japanese Laid-Open Patent Publication No. 2003-322632 discloses one such type of porous protection layer having a two-layer structure in which an inner (lower) layer is higher in porosity than an outer (upper) layer. In this protection layer, the inner layer has roughness due to its high porosity and thereby exhibits anchoring effect so as to improve the adhesion of the inner layer to the outer layer. The inner layer also exhibits thermal insulation effects due its high porosity so as to, even when the gas sensor element gets wet with water (water drop becomes adhered to the porous protection layer), prevent heat from being taken away from the gas sensing portion to the outer layer.

SUMMARY OF THE INVENTION

However, the above-disclosed porous protection layer does not attain a sufficient strength of adhesion between the inner and outer layers just by setting the porosity of the inner layer higher than that of the outer layer. The inner and outer layers of the porous protection layer may be separated when the porous protection layer gets wet with water. In general, the likelihood of separation of the inner and outer layers of the porous protection layer increases with the thickness of the porous protection layer. Further, the above-disclosed porous protection layer does not exert sufficient thermal insulation effect so that heat may be taken away from the gas sensing portion when the gas sensor element gets wet with water.

It is therefore an object of the present invention to provide a gas sensor element having a multilayer porous protection layer capable of enhancing an interlaminar adhesion strength while maintaining thermal insulation effect. It is also an object of the present invention to provide a gas sensor using the gas sensor element.

According to one aspect of the present invention, there is provided a gas sensor element for detecting the concentration of a specific gas component in gas under measurement, comprising: a plate-shaped element body having, at one end portion thereof, a gas sensing portion, the gas sensing portion including a solid electrolyte substrate and a pair of electrodes arranged on the solid electrolyte substrate; and a porous protection layer formed of ceramic particles and surrounding at least the circumference of the one end portion of the element body, wherein the porous protection layer has an inner region, an intermediate region and an outer region laminated together in order of mention from the element body toward the outside; and wherein the intermediate region has a porosity lower than those of the inner and outer regions.

In the gas sensor element, the element body may have a heating unit (heater) capable of generating heat upon energization thereof in addition to the element unit; and the porous protection layer may have, in addition to the above-mentioned inner, intermediate and outer regions, any additional region or regions located outside the outer region.

It is preferable that the porosity of the outer region is lower than that of the inner region. It is also preferable that: the outer region contains, as the ceramic particles, rough particles and fine particles smaller in size than the rough particles; the intermediate region contains the same fine particles as those contained in the outer region; and the proportion of the fine particles in the intermediate region is higher than the proportion of the fine particles in the outer region. It is further preferable that the intermediate region contains the same particles as those contained in the inner region. Furthermore, it is preferable that the intermediate region has a thickness smaller than those of the inner and outer regions.

According to another aspect of the present invention, there is provided a gas sensor comprising: the above gas sensor element; and a housing retaining therein the gas sensor element.

The other objects and features of the present invention will also become understood from the following description.

DESCRIPTION OF THE EMBODIMENTS

The present invention will be described in detail below.

As shown inFIG. 1, a gas sensor1according to one exemplary embodiment of the present invention includes a plate-shaped gas sensor element100and a metal shell30(as a housing). By way of example, the gas sensor1is in the form of an oxygen sensor for detecting the concentration of oxygen in exhaust gas (gas under measurement) flowing through e.g. an exhaust pipe of an internal combustion engine in the present embodiment.

The gas sensor element100extends in an axial direction L of the gas sensor1and has a plate-shaped element body in which a sensing unit300and a heating unit (heater)200are laminated together as shown inFIGS. 1 and 2.

The heating unit200includes first and second substrates101and103, a heating member102and terminal pads (also referred to as “heating-unit-side terminal pads”)120. The first and second substrates101and103are arranged in such a manner that a longitudinal direction of the substrate101,103is in agreement with the axial direction L of the gas sensor1. The heating member102is arranged between the first and second substrates101and103and adapted to generate heat upon energization thereof.

As shown inFIG. 2, the heating member102has a heating portion102alocated at a front end side thereof and a pair of lead portions102bextending from the heating portion102aalong the longitudinal direction of the first substrate101. The heating-unit-side terminal pads120are arranged on a main surface of the first substrate101opposite from the heating member102.

Through hole conductors101aare formed through the first substrate101so as to electrically connect ends of the lead portions102bto the heating-unit-side terminal pads120via the respective through hole conductors101a.

In the heating unit200, each of the first and second substrates101and103is formed predominantly of insulating ceramic material such as alumina; and the heating member102is formed of predominantly of platinum-group metal. Specific examples of the platinum-group metal are Pt, Rh and Pd. These platinum-group metals can be used solely or in combination of two or more thereof. In view of the heat resistance and oxidation resistance, it is preferable that the heating member102is formed predominantly of Pt. It is also preferable that the heating member102contains a ceramic component, more preferably the same ceramic component as the main component of the substrate101,103, in view of the adhesion of the heating member102to the substrate101,103.

The sensing unit300includes an oxygen concentration detection cell130and an oxygen pumping cell140laminated to each other.

The oxygen concentration detection cell130has a first solid electrolyte substrate105arranged in such a manner that a longitudinal direction of the first solid electrolyte substrate105is in agreement with the axial direction L of the gas sensor1and first and second electrodes104and106arranged on opposite main surfaces of the first solid electrolyte substrate105. The first electrode104has a first electrode portion104aand a first lead portion104bextending from the first electrode portion104aalong the longitudinal direction of the first solid electrolyte substrate105, whereas the second electrode106has a second electrode portion106aand a second lead portion106bextending from the second electrode portion106aalong the longitudinal direction of the first solid electrolyte substrate105.

The oxygen pumping cell140has a second solid electrolyte substrate109arranged in such a manner that a longitudinal direction of the second solid electrolyte substrate109is in agreement with the axial direction L of the gas sensor1and third and fourth electrodes108and110arranged on opposite main surfaces of the second solid electrolyte substrate109. The third electrode108has a third electrode portion108aand a third lead portion108aextending from the third electrode portion108balong the longitudinal direction of the second solid electrolyte substrate109, whereas the fourth electrode110has a fourth electrode portion110aand a fourth lead portion110aextending from the fourth electrode portion110balong the longitudinal direction of the second solid electrolyte substrate109.

The sensing unit300also includes an insulating layer107arranged between the oxygen concentration detection cell130and the oxygen pumping cell140, a protection layer111arranged on the main surface of the second solid electrolyte substrate109opposite from the oxygen concentration detection cell130and terminal pads (also referred to as “sensing-unit-side terminal pads”)121arranged on a surface of the protection layer111opposite from the oxygen pumping cell140.

A first through hole conductor105a, a second through hole conductor107a, a fourth through hole conductor109aand a sixth through hole conductor111aare formed through the first solid electrolyte substrate105, the insulating layer107, the second solid electrolyte substrate109and the protection layer111, respectively, so as to electrically connect an end of the first lead portion104bto one of the sensing-unit-side terminal pads121via the through hole conductors105a,107a,109band111a. A third through hole conductor107b, a fifth through hole conductor109band a seventh through hole conductor111bare formed through the insulating layer107, the second solid electrolyte substrate109and the protection layer111, respectively, so as to electrically connect an end of the second lead portion106bto another one of the sensing-unit-side terminal pads121via the through hole conductors107b,109band111band to electrically connect an end of the third lead portion108bto the another one of the sensing-unit-side terminal pads121via the though hole conductors109band111b. The second lead portion106band the third lead portion108bare herein kept at the same potential. Further, a eighth through hole conductor111cis formed through the protection layer111so as to electrically connect an end of the fourth lead portion110bto the remaining one of the sensing-unit-side terminal pads121via the through hole conductor111c.

In the sensing unit300, the first and second solid electrolyte substrates105and109are formed of partially stabilized zirconia containing yttria (Y2O3) or calcia (CaO) as a stabilizer; and the first to fourth electrodes104,106,108and110, the terminal pads120and121(also generically called “conducting members”) are formed of platinum-group metal. Specific examples of the platinum-group metal are Pt, Rh and Pd. These platinum-group metals can be used solely or in combination of two or more thereof. In view of the heat resistance and oxidation resistance, it is preferable that the conducting members104,106,108,110,120and121are formed predominantly of Pt. It is also preferable that each of the conducting members104,106,108,110,120and121contains a ceramic component in addition to the platinum-group metal. In this case, the ceramic component of the conducting member104,106,108,110,120,121is preferably the same as (similar to) that of the adjacent structural part to which the conducting member104,106,108,110,120,121is laminated (e.g. the main component of the solid electrolyte substrate105,109) in view of the adhesion of the conducting member104,106,108,110,120,121to the adjacent structural part.

The insulating layer107has an insulating portion114and diffusion limiting portions115. As shown inFIG. 2, a hollow gas detection chamber107cis defined in the insulating portion114of the insulating layer107at a position corresponding to the second and third electrode portions106aand108a. The diffusion limiting portions115are located on both sides of the gas detection chamber107cin a width direction of the insulating layer107so as to provide therethrough gas communication between the gas detection chamber107cand the outside and allow diffusion of the exhaust gas from the outside into the gas detection chamber107cunder predetermined rate-limiting conditions.

There is no particular limitation on the material of the insulating portion114as long as the insulating portion114is in the form of an insulating ceramic sintered body. The insulating portion114is formed of, for example, oxide ceramic material such as alumina or mullite. On the other hand, the diffusion limiting portions115are formed of, for example, porous alumina so as to limit the rate of diffusion of the exhaust gas.

The protection layer111is formed on the main surface of the second solid electrolyte substrate109so as to sandwich the fourth electrode110between the protection layer111and the solid electrolyte substrate109. The protection layer111has a porous electrode protecting portion113acovering the fourth electrode portion110aand thereby protecting the fourth electrode104from poisoning and a reinforcing portion112covering the fourth lead portion110band protecting the solid electrolyte substrate109.

Herein, the oxygen concentration detection cell130(first solid electrolyte substrate105and first and second electrodes104and106) and the gas detection chamber107cconstitutes a gas sensing portion at a front end portion of the sensing unit300(i.e. at a front end portion of the element body of the gas sensor element100) in the present embodiment.

The gas sensor element100is configured to adjust the direction and intensity of electric current flowing between the electrodes108and110of the oxygen pumping cell140in such a manner as to control the voltage (electromotive force) between the electrodes104and106of the oxygen concentration detection cell130to a given value (e.g. 450 mV) and determine the concentration of oxygen in the exhaust gas linearly with the electric current flowing through the oxygen pumping cell140.

The metal shell30is formed of, for example, SUS430 and adapted to retain therein the gas sensor element100, with the front and rear end portions of the element body of the gas sensor element100protruding from the metal shell30. The metal shell30has a male thread portion31for mounting the gas sensor1to the exhaust pipe of the engine and a hexagonal portion32for engagement with a mounting tool at the time of mounting. The metal shell30also has, at an inner surface thereof, a stepped portion33protruding radially inwardly.

A metallic holder34is retained in the metal shell30by the stepped portion33so as to hold therein the gas sensor element100.

A ceramic holder35and a sealing member36are arranged in the metallic holder34, in order of mention from the front side, so as to surround the gas sensor element100. The sealing member36includes a first talc material37located on a front side thereof and a second talc material38located on a rear side thereof and extending over a rear end of the metallic holder34. The first talc material37is compressed into the metallic holder34so as to fix the gas sensor element100in the metallic holder34. The second talc material38is compressed into the metal shell30so as to establish sealing between the outer surface of the gas sensor element100and the inner surface of the metal shell30.

A sleeve39of e.g. alumina is arranged on a rear side of the sealing member36so as to surround the gas sensor element100. The sleeve39has a cylindrical shape including a plurality of stepped portions formed on a radially outer surface thereof and an axial hole39aformed therethrough in the axial direction L so that the gas sensor element100passes through the axial hole39a.

A ring member40of e.g. stainless steel is placed on the stepped portion of the sleeve39. A rear end30aof the metal shell30is bent and crimped radially inwardly so as to push the sleeve39via the ring member40toward the front of the metal shell30.

The protector24is formed with a plurality of gas holes24aand welded to the outer circumference of a front end portion of the metal shell30so as to cover therewith the protruding front end portion of the gas sensor element100. The protector24has a double structure consisting of a bottomed cylindrical outer protector member41having a constant outer diameter and a bottomed cylindrical inner protector member42located in the outer protector member41and having a rear end portion42aand a front end portion42bsmaller in outer diameter than the rear end portion42a.

An outer tube25of e.g. SUS 430 is formed with an enlarged-diameter front end portion25a. This front end portion25ais fitted on and joined by laser welding etc. to a rear end portion of the metal shell30so as to cover therewith the protruding rear end portion of the gas sensor element100.

A separator50is arranged within a rear end portion of the outer tube25and has a protruding portion50aformed on a radially outer surface thereof and an insertion hole50bformed therethrough in the axial direction. Connection terminals16are provided in the insertion hole50band connected to the terminal pads120and121of the gas sensor element100.

A retaining member51is fixed in a gap between the separator50and the outer tube25by crimping the outer tube25radially inwardly with the retaining member51engaged with the protruding portion50aof the separator50.

Lead wires11to15are inserted through the insertion hole50bof the separator50and has front ends connected to the connection terminals16and rear ends connected to an external control device such as ECU via connectors for electrical connection (signal transmission) between the gas sensor element100(sensing unit300and heating200) and the external control device. It is noted that, for purposes of clarity, the wires14and15are not indicated in the drawings. Each of the lead wires11to15has a lead line covered with an insulating resin coating although not shown in detail.

A substantially cylidrical rubber cap52is fixed in a rear open end of the outer tube25by crimping the outer tube25radially inwardly with the rubber cap52inserted in the rear end of the outer tube25, so that the rear end of the outer tube25is closed with the rubber cap52.

In the present embodiment, the gas sensor element100characteristically has a porous protection layer20surrounding the entire circumference of the front end portion of the element body as shown inFIGS. 1 and 3. More specifically, the protection layer20is formed so as to extend in the axial direction L from a front end face of the sensor element body at least to a point rear of the area where the first to fourth electrode portions104a,106a,108aand110aoverlaps and thereby totally cover not only the front end face but also four lateral sides of the front end portion of the sensor element body as shown inFIG. 3in the present embodiment.

As shown inFIGS. 3 to 5, the porous protection layer20has a three-dimensional network structure formed of ceramic particles so as to define a plurality of pores for gas diffusion and includes an inner region21located directly on an outer surface of the sensor element body, an outer region23located so as to cover an outer surface of the inner region21and an intermediate region22located between the inner region21and the outer region23. It is noted that, as the intermediate region22is much smaller in thickness than the inner and outer regions21and23, the intermediate region22is indicated by a line inFIG. 3.

In the present embodiment, the porosity of the intermediate region22is set lower than those of the inner and outer layers21and23. That is, the ceramic particles of the intermediate region22are more closely packed than those of the inner and outer regions21and23so as to increase the number of ceramic particles linking the intermediate region22to the inner and outer regions21and23. The intermediate region22can be thus secured firmly to the inner and outer regions21and23. It is accordingly possible to improve the strength of interlaminar adhesion between the inner region21and the intermediate region22and between the intermediate region22and the outer region23of the porous protection layer20.

As the porosity of the inner region21is set higher than that of the intermediate region22so that the high-porosity inner region21is shielded with the low-porosity intermediate region22, the thermal insulation effect of the inner region21can be increased to, even when the outer region23gets wet with water, prevent heat from being taken away from the sensing unit300(gas sensing portion) to the outer region23.

In addition, it is easier to introduce the exhaust gas (gas under measurement) through the pores of the outer region23and is possible to secure the gas permeability of the porous protection layer20as the porosity of the outer region23is set higher than that of the intermediate region22. It is also possible to trap poisoning substances in the outer region23assuredly while allowing assured penetration of condensed water (water drops) into the outer region23as the poisoning substances and condensed water are difficult to pass through the intermediate region21.

The inner region21, the intermediate region22and the outer region23are herein defined by the following procedure. In the porous protection layer20, the area where the ceramic particles change in material, size, shape etc. is determined as a boundary line of the inner region21and the intermediate region22. The area between where the relatively coarse pores are present and where the relatively coarse pores are not present is then determined as a boundary line of the intermediate region22and the outer region23in parallel with the boundary line of the inner region21and the intermediate region22.

Further, the porosity of the inner region21, the intermediate region22and the outer region23are determined by the following image analysis process. A cross-sectional micrograph (SEM image) of the porous protection layer20is taken as shown inFIGS. 4 and 5. The thus-obtained image is subjected to binarization in a width direction of each of the inner region21, the intermediate region22and the outer region23by commercially available image analysis software, thereby determining the proportion of black area (as indicated by arrows CAand CBinFIG. 4) in the image. In the image, the black area corresponds to the pores; and the white area corresponds to the ceramic particles. It means that, the larger the black area, the higher the porosity. In the case where the image analysis area is larger than the thickness of the intermediate region22, it is feasible to set the image analysis area in such a manner that the whole of the thickness of the intermediate region22and determine the porosity of the intermediate region22based only on the corresponding black area of the image analysis area.

The inner region21is formed by, for example, combining particles of at least one kind of ceramic material selected from the group consisting of alumina, spinel, zirconia, mullite, zircon and cordierite by sintering etc. It is feasible to prepare and sinter a slurry or paste of the ceramic particles and thereby form pores between the ceramic particles of the inner region21. A combustible pore forming material is preferably added to the slurry or paste so that, when the pore forming material is burned out during the sintering, the spaces filled with the pore forming material remains hollow as pores. The inner region21can be thus formed with a low density (high porosity). Examples of the pore forming material are carbon particles, resin beads and organic and inorganic binder particles. Preferably, the inner region21has a porosity of 35 to 70% as determined by the above image analysis process in order to secure good thermal insulation effect. If the porosity of the inner region21is less than 35%, the total pore volume of the inner region21is small so that the thermal insulation effect of the inner region21is decreased. If the porosity of the inner region21exceeds 70%, it is difficult to maintain the structure of the inner region21. Further, the thickness of the inner region21is preferably in the range of 100 to 800 μm.

The outer region23is also formed by, for example, by combining particles of at least one kind of ceramic material selected from the group consisting of alumina, spinel, zirconia, mullite, zircon and cordierite by sintering etc. It is feasible to sinter a slurry or paste containing the ceramic particles and organic or inorganic binder particles so as to burn out the binder particles during the sintering and thereby form pores between the ceramic particles of the outer region23. As the ceramic particles, rough particles and fine particles smaller in size than the rough particles are preferably used in combination. By the use of such ceramic particles, the outer region23can be structured to trap a larger amount of poisoning substances and keep a larger amount of water content penetrating therein. The strength of adhesion between the outer region23and the intermediate region22can also be improved as the fine particles migrate from the outer region23toward the inner region21to constitute the intermediate region22during the sintering as will be explained later in detail. Preferably, the outer region23has a porosity of 10 to 50% as determined by the above image analysis process in order to secure sufficient poisoning substance trapping/water penetrating effect without causing deterioration in gas permeability. If the porosity of the outer region23is less than 10%, it is likely that the outer region23will be clogged with the poisoning substances. If the porosity of the outer region23exceeds 50%, the water may penetrate into the inside of the outer region23so as to thereby cause deterioration in water resistance. Further, the thickness of the outer region23is preferably in the range of 100 to 800 μm.

There is no particular limitation on the relationship between the porosity of the inner region21and the porosity of the outer region23. Although the porosity of the inner region21can be the same as the porosity of the outer region23, it is preferable that the porosity of the outer region23is lower than the porosity of the inner region21so as to effectively trap the poisoning substances in the outer region23and keep water penetrating in the outer region23.

Preferably, the intermediate region21contains the same ceramic particles as those contained in the inner region21and contains the same fine ceramic particles as those contained in the outer region23. When the intermediate region22contains the same ceramic particles as those contained in the inner region21, it is possible to improve the adhesion of the intermediate region22to the inner region21. It is also possible to improve the adhesion of the intermediate region22to the outer region23when the intermediate region22also contains the same fine ceramic particles as those contained in the outer region23. In particular, the proportion of the fine ceramic particles in the intermediate region22is preferably set higher than the proportion of the fine ceramic particles in the outer region23so as to control the porosity of the intermediate region22to be lower than those of the inner and outer regions21and23. It is noted that: the proportion of the fine ceramic particles in the intermediate region22refers to the ratio of the content of the fine ceramic particles in the intermediate region22to the total content of the ceramic particles in the intermediate region22; and the proportion of the fine ceramic particles in the outer region23refers to the ratio of the content of the fine ceramic particles in the outer region23to the total content of the ceramic particles (rough and fine ceramic particles) in the outer region23. The content of the fine particles in each of the intermediate region22and the outer region23can be determined from the amount of the fine particles per unit area in the intermediate region22or outer region23based on the cross-sectional micrograph (SEM image).

The thickness of the intermediate region22is preferably smaller than those of the inner and outer regions21and23as mentioned above in the present embodiment. It is possible by such thickness control to more properly secure the thermal insulation effect of the inner region21and the poisoning substance trapping/water penetrating effect of the outer region23in the porous protection layer20while improving the strength of adhesion between the inner and outer regions21and23by the intermediate region22. More specifically, the thickness of the intermediate region22is preferably in the range of 20 to 80 μm.

For example, the above-mentioned porous protection layer20can be formed by the following procedure.

A slurry for formation of the inner region21(referred to as “inner-region slurry”) and a slurry23xfor formation of the outer region23(referred to as “outer-region slurry”) are first prepared. As mentioned above, a combustible pore forming material is added to the inner-region slurry; and rough ceramic particles231and fine ceramic particles232smaller in size than the rough ceramic particles231are used in the outer-region slurry23x

The inner-region slurry is applied by dipping etc. to the entire circumference of the front end portion of the sensor element body and sintered. As shown inFIG. 6A, the pore forming material is burned out during the sintering to thereby define relatively large pores CAbetween the ceramic particles.

The outer-region slurry23xis next applied by dipping etc. to the above-formed inner coating. When the outer-region slurry23xis applied to the inner coating, some of the fine particles232contained in the outer-region slurry23xbecome embedded into the pores CAof the boundary surface of the inner coating as shown inFIG. 6B. In this state, the outer-region slurry23xis sintered.

During the sintering, the region of the resulting laminated coating where the some of the fine ceramic particles232are embedded in the pores CAof the surface of the inner coating becomes the intermediate region22as shown inFIG. 6C. The intermediate region22can be thus easily formed. Then, the region of the laminated coating located inside the intermediate region22becomes the inner region21; and the region of the laminated coating located outside the intermediate region22becomes the outer region23. Even though the amount of the fine particles present in the area of the outer region23adjacent to the intermediate region22decreases due to the migration of the fine particles, the rough particles remain and constitute a porous body. The outer region23can be thus formed stably with a three-dimensional network structure in which pores CBare defined between the rough and fine particles231and232.

Alternatively, the porous protection layer20may be produced by applying and sintering slurries for formation of the inner region21, the intermediate region22and the outer region23(referred to as “inner-region slurry”, “intermediate-region slurry” and “outer-region slurry”) in order. In this case, it is feasible to apply and sinter the inner-region slurry, apply and sinter the intermediate-region slurry, and then, apply and sinter the outer-region slurry, or feasible to apply the inner-region slurry, the intermediate-region slurry and the outer-region slurry successively, and then, sinter the inner-region slurry, the intermediate-region slurry and the outer-region slurry simultaneously. It is needless to say that, in the case of preparing and applying the inner-region slurry, the intermediate-region slurry and the outer-region slurry separately, the outer-region slurry does not necessarily contain both of rough particles and fine particles.

EXAMPLES

Example

Samples of the plate-shaped gas sensor element100shown inFIGS. 1and2were each produced by forming the porous protection layer20as follows.

A slurry A was prepared as an inner-region slurry by mixing 40 vol % of alumina powder (particle size distribution: D10=0.24 μm, D50=0.40 μm, D90=0.60 μm), 60 vol % of carbon powder (particle size distribution: D10=10.5 μm, D50=20.6 μm, D90=42.2 μm) and 10 vol % of separately prepared alumina sol with ethanol. The prepared slurry A was adjusted to an appropriate viscosity and applied by dipping (immersion) process to the entire circumference (four sides) of the front end portion of the sensor element body (sensing unit300and heating unit200) in such a manner that the coating of the slurry A was 300 μm in thickness. The applied slurry coating was dried in a dryer at 200° C. for several hours, thereby removing excessive organic solvent from the slurry coating. The dried slurry coating was then sintered in the air at 1100° C. for 3 hours.

Further, a slurry B was prepared as an outer-region slurry by mixing 60 vol % of spinel powder (particle size distribution: D10=24.6 μm, D50=44 μm, D90=88 μm), 40% of alumina powder (particle size distribution: D10=0.24 μm, D50=0.40 μm, D90=0.60 μm) and 10 vol % of separately prepared alumina sol with ethanol. The prepared slurry B was adjusted to an appropriate viscosity and applied by dipping (immersion) process to a surface of the above-formed inner coating in such a manner that the coating of the slurry B was 250 μm in thickness. The applied slurry coating was dried in a dryer at 200° C. for several hours, thereby removing excessive organic solvent from the slurry coating. The dried outer slurry coating was then sintered in the air at 1100° C. for 3 hours.

Herein, the particle size distribution of the powder material used in the slurry A, B refers to the cumulative particle size distribution of the particles as measured by laser diffraction scattering where D10, D50 and D90 are particle sizes at 10%, 50% and 90% cumulation from the fine particle side of the cumulative particle size distribution, respectively.

The thus-obtained gas sensor element100with the protection layer20was cut in a direction orthogonal to the axial direction L. A cross-sectional micrograph of the porous protection layer20was then taken by a scanning electron microscope (SEM). The inner region21, the intermediate region22and the outer region23were determined based on the cross-sectional SEM image. Further, each of the porosity of the inner region21, the porosity of the intermediate region22and the porosity of the outer region23was determined based on the cross-sectional SEM image by the above-mentioned image analysis process. The image analysis area was herein 100 μm×100 μm in each image analysis process.

The following water resistance test was performed on the produced samples of the gas sensor element100.

The gas sensor element100was set to 800° C. in the air. In this state, twenty water drops of 3 μl, or 10 μL were successively dropped from above onto a position of the porous protection layer20corresponding to the gas diffusion hole (diffusion limiting portion115). After the dropping, the appearance of the porous protection layer20was observed with a magnifying glass to visually check the occurrence of damage to the porous protection layer20(e.g. separation of the porous protection layer20, crack in the porous protection layer20etc.). Then, the porous protection layer20was peeled off from the element body of the gas sensor element100. The occurrence of crack in the element body of the gas sensor element100was visually checked by so-called “red check”. In Table 1, the test results are indicated in terms of the number of the samples in which the damage occurred to the porous protection layer20and the number of the samples in which the crack occurred in the element body of the gas sensor element100.

Comparative Example

Samples of gas sensor element were produced in the same manner as in Example, except for using a slurry C in place of the slurry B for formation of the porous protection layer. The slurry C was herein prepared by mixing spinel powder (particle size distribution: D10=24.6 μm, D50=44 μm, D90=88 μm) and 10 vol % separately prepared alumina sol with ethanol.

The thus-obtained gas sensor element was subjected to SEM image analysis in the same manner as in Example. It was confirmed by the image analysis that: the porous protection layer consisted of inner and outer layers. There was seen no intermediate region lower in porosity than the inner and outer layers. The reason for the formation of no intermediate region is assumed that the fine alumina particles were contained in the slurry B but were not contained in the slurry C so that, when the slurry C was applied to the inner coating, some of the fine alumina particles did not become embedded and filled in pores of the inner coating.

Further, the produced samples of the gas sensor element were subjected to water resistance test in the same manner as in Example.

The test results are indicated in TABLE 1.

As shown in TABLE 1, there was no damage to the porous protection layer20and no crack in the element body of the gas sensor element100in any of the samples regardless of the water drop volume of the water resistance test in Example. The gas sensor element100of Example had high water resistance.

In Comparative Example, by contrast, more than half of the samples had damage to the porous protection layer and crack in the sensor element body in the case where the water drop volume of the water resistance test was 3 μL; and all of the samples had damage to the porous protection layer and crack in the sensor element body in the case where the water drop volume of the water resistance test was 10 μL. The gas sensor element of Comparative Example was inferior in water resistance. It is assumed that, in the absence of the intermediate region in Comparative Example, separation of the inner and outer layers occurred when the porous protection layer was wetted with water.

The entire contents of Japanese Patent Application No. 2011-035583 (filed on Feb. 22, 2011) and No. 2011-276929 (filed on Dec. 19, 2011) are herein incorporated by reference.

Although the above-mentioned embodiment specifically refers to the oxygen sensor (oxygen sensor element), the present invention is not limited to the above-embodied oxygen sensor (oxygen sensor element). The present invention is applicable to various gas sensors (gas sensor elements) in which a sensing unit has a solid electrolyte substrate and a pair of electrodes. Various modifications and variations of the embodiment described above are possible without departing from the scope of the present invention. For example, the gas sensor (gas sensor element) of the present invention can be embodied as not only an oxygen sensor (oxygen sensor element) for detecting the concentration of O2in gas under measurement but also a NOx sensor (NOx sensor element) for detecting the concentration of NOx in gas under measurement, a HC sensor (HC sensor element) for detecting the concentration of HC in gas sunder measurement and the like. Although the porous protection layer20is formed of ceramic particles in the above embodiment, the porous protection layer20may be formed by mixing ceramic particles with ceramic fibers.

The scope of the invention is defined with reference to the following claims.