Abstract:
This invention provides a gas sensing device and gas sensor including a porous portion in which a first porous portion absorbs phosphorus and silicone sufficiently so as to suppress generation of clogging in a second porous portion meeting demands for intensifying the performance and accuracy of a gas sensor, so that the accuracy of detection of air-fuel ratio is further improved.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a gas sensing device and gas sensor for use for combustion control or the like of an internal combustion engine and a manufacturing method thereof. More particularly, the present invention relates to a gas sensing device and gas sensor capable of preventing the detection accuracy of air-fuel ratio from dropping due to poisoning.  
       BACKGROUND OF THE INVENTION  
       [0002]     As a gas sensor which is installed in the exhaust system of an internal combustion engine and used for combustion control of the internal combustion engine by detecting the concentration of oxygen in exhaust gas, conventionally, an oxygen sensor has been well known. This oxygen sensor, for example, comprises a cylindrical main body metal and a sheet-like gas sensing device held by the main body metal. The gas sensoring device comprises a first solid electrolyte layer extending in the length direction, a cell having a first opposing electrode formed on front and rear surfaces on the side of the front end exposed to measuring object gas of the first solid electrolyte layer and a first porous portion overlaid on the cell. One of the opposing electrodes of the cell is disposed in a measuring chamber to which measuring object gas is to be introduced. The first porous portion is provided to control the diffusion rate of the measuring object gas introduced into the measuring chamber.  
         [0003]     Some types of fuel and engine oil used in the internal combustion engine of an automobile or the like contain phosphorous or silicone. When this fuel or engine oil is used, phosphorous or silicone adheres to the surface of the first porous portion so as to close pores in the porous portion so that the first porous portion is clogged. As a result, the diffusion resistance of the first porous portion changes, so that the detection accuracy of the air-fuel ratio of the gas sensor can drop.  
         [0004]     To meet generation of clogging in the porous portion and abnormality in the electrode, providing of a second porous portion for preventing poisoning by phosphorus or silicone between the first porous portion and outside has been known. See, for example, Japanese Paten Application Laid-Open No. 10-221304 to Tsuzuki et al. and U.S. Pat. No. 5,925,814 to Tsuzuki et al. Generation of clogging in the first porous portion can be suppressed by sucking phosphorous or silicone with the second porous portion. Thus, changes in diffusion resistance in the first porous portion can be suppressed to block drop of the detection accuracy of the air-fuel ratio of the gas sensor.  
         [0005]     However, in recent years, higher performance and intensified accuracy of the gas sensor have been demanded and thus, suppressing of clogging generated in the first porous portion by sucking more phosphorous and silicone by means of the second porous portion has been considered important. However, there is a fear that the second porous portion described in the aforementioned patent documents cannot suck phosphorus and silicone sufficiently enough for higher performance and intensified accuracy of the gas sensor. More specifically, the second porous portion of the aforementioned patent documents is so constructed that all measuring object gas is introduced into an introduction passage in order to introduce to a measuring chamber and an interface between the introduction passage and the second porous portion is exposed on an external face of the gas sensing device. In case of such a gas sensing device, when the measuring object gas is introduced into the second porous portion, part of phosphorous and silicone invades into the first porous portion from outside using this interface as a passage without being sucked by the second porous portion. As a result, suppression of clogging generated in the first porous portion is not achieved sufficiently, so that there is a fear that the first porous portion cannot be applied to intensification of performance and accuracy of the gas sensor.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention has been accomplished in views of the conventional problems and an advantage of the invention is a gas sensing device and gas sensor in which phosphorous and silicone can be sucked by the second porous portion thereof securely so as to suppress generation of clogging in the first porous portion thereby meeting a demand for intensifying performance and accuracy of the gas sensor, the gas sensing device being capable of improving the accuracy of detection on air-fuel ratio.  
         [0007]     To achieve the above-described advantage, according to an aspect of the present invention, there is provided a sheet-like gas sensing device comprising a first cell having a first solid electrolyte layer and first opposing electrodes formed on the front and rear faces of the first solid electrolyte layer and a hollow measuring chamber to which gas is introduced through a first porous portion and one of the first opposing electrode faces, wherein at least part of an external face directed to the outermost virtual face connecting outermost faces of the gas sensing device of the first porous portion is located inside the outermost virtual face so that the part is dented from the outermost virtual face, forming a concave portion including the external face, the sheet-like gas sensing device further comprising a second porous portion having a smaller diffusion resistance than the first porous portion with part thereof invading into the concave portion while being in contact with at least an opening edge of the concave portion.  
         [0008]     By disposing the second porous portion at least in contact with the opening edge of the concave portion, an interface (opening edge of the concave portion in the present invention, between an introduction passage and the second porous portion can be prevented from being exposed to the outermost virtual face of the gas sensing device thereby blocking gas from invading into the first porous portion from outside through this interface. Thus, generation of clogging in the first porous portion can be suppressed to improve the accuracy of detection of air-fuel ratio by changes in diffusion resistance of a measuring object gas.  
         [0009]     Further, part of this second porous portion invades into the concave portion. Because part of the second porous portion invades into the concave portion so as to form a wedge-like configuration, the second porous portion can be prevented from being separated from the gas sensing device as compared to a case where the second porous portion is disposed in contact with only the opening edge of the concave portion of the gas sensing device.  
         [0010]     To allow the second porous portion to absorb more phosphorous and silicone, the thickness of the second porous portion needs to be intensified. However, as the thickness of the second porous portion is intensified, the second porous portion is enlarged, thereby leading to tremendous enlargement of the gas sensing device. Usually, when the gas sensing device is heated by a heater so that it is activated, it can detect air-fuel ratio. However, if the gas sensing device is enlarged tremendously, it takes more time for the gas sensing device to be activated (hereinafter, referred to as activation time). As a consequence, there is a fear that the gas sensing device cannot detect air-fuel ratio early. Contrary to this, by introducing part of the second porous portion into the concave portion of the gas sensing device, the thickness of the second porous portion can be secured without enlarging the gas sensing device tremendously, and as a consequence, the second porous portion can absorb more phosphorus and silicone without delaying the activation time of the gas sensing device.  
         [0011]     In the meantime, the diffusion resistance of the second porous portion is set smaller than the diffusion resistance of the first porous portion. This prevents the measuring object gas from being suppressed in diffusion rate by the second porous portion for absorbing phosphorous and silicone thereby blocking the accuracy of detection of air-fuel ratio from dropping.  
         [0012]     The second porous portion may be dented into the concave portion while in contact with only the opening edge of the concave portion or may be dented into the concave portion while covering the entire periphery of the outermost virtual face of the gas sensing device. That is, the second porous portion only need to make contact with the opening edge such that the opening edge of the concave portion is not exposed outside. Further, the outermost virtual face connecting the outermost faces of the gas sensing device mentioned in the present invention refers to a virtual face produced by connecting respective faces located at the outermost side of the sheet-like gas sensor and if speaking in detail, corresponds to a virtual face shown in  FIGS. 4, 6 ,  7  described in embodiments below.  
         [0013]     According to another aspect of the present invention, there is provided the gas sensing device wherein a second cell having a second solid electrolyte layer and second opposing electrodes formed on the front and rear faces of the second solid electrolyte layer is overlaid on the first cell through the first porous portion with one of the second opposing electrodes facing the measuring chamber, the gas sensing device further comprising an insulating layer formed between the first cell and the second cell, which forms the measuring chamber with the first cell, the second cell and the first porous portion.  
         [0014]     By adopting the above-described structure for the gas sensing device, a gas sensing device in which the second porous portion is in contact with the opening edge of the concave portion while part thereof invades into the concave portion can be achieved. Consequently, generation of clogging in the first porous portion can be suppressed so as to improve the accuracy of detection of air-fuel ratio by changes in diffusion resistance of the measuring object gas thereby preventing the second porous portion from being separated.  
         [0015]     In the gas sensing device of the present invention, preferably, the minimum thickness of the second porous portion provided in the concave portion from an external face thereof to an internal face directed to the first porous portion is 130 μm or more.  
         [0016]     By setting the minimum thickness between the external face of the second porous portion provided in the concave portion and the internal face to more than 130 μm, a distance over which the measuring object gas passes through the second porous portion can be increased, so that more phosphorous and silicone can be absorbed by the second porous portion. Consequently, generation of clogging in the first porous portion can be suppressed so as to improve the accuracy of detection of air-fuel ratio by changes in diffusion resistance of the measuring object gas. If the minimum distance of the second porous portion is less than 130 μm, sometimes, the above-described effect cannot be obtained. Although the distance of the second porous portion is preferred to be as long as possible, preferably the maximum thickness between the external face of the second porous portion provided in the concave portion and the internal face is 300 μm or less if considering the activation time of the gas sensing device. In the meantime, the minimum thickness from the external face of the second porous portion provided in the concave portion and the internal face directed to the first porous portion refers to a distance of straight line of an area containing the second porous portion located in the concave portion of the second porous portion.  
         [0017]     Exhaust gas passing through the exhaust pipe of an internal combustion engine contains water droplet or oil droplet and if the water droplet or the like adheres to the gas sensing device when the gas sensor is used, crack may occur in the gas sensing device. If speaking in detail, because the gas sensing device is exposed to exhaust gas (measuring object gas) and heated by a heater when the gas sensor is used, when water droplet or the like makes contact therewith, a large difference in temperature occurs between the portion which the water droplet adheres to and its surrounding, thereby resulting in generation of crack due to thermal shock. As for the portion in which crack occurs due to contact of water droplet, if water droplet adheres to the corner portion extending in the length direction of the gas sensing device, thermal shock is likely to concentrate on that corner portion thereby often causing a crack.  
         [0018]     In the gas sensing device of the present invention, preferably, the second porous portion covers the corner portion in the length direction of the gas sensing device and the thickness of the second porous portion from the corner portion is 20 μm or more. By covering the corner portion in the length direction of the gas sensing device with porous substance based on the fact that the second porous portion is composed of the porous substance, water droplets adhering to the second porous portion penetrate slowly while being diffused into a number of pores, so that the water droplets can be diffused before they reach the corner portion of the gas sensing device. As a consequence, thermal shock generated in the corner portion of the gas sensing device can be suppressed thereby suppressing generation of crack in the gas sensing device.  
         [0019]     Preferably, the thickness of the second porous portion from the outermost virtual face of the gas sensing device is 30 μm or more in order to prevent generation of crack due to wetting and more preferably, 50 μm or more. On the other hand, preferably, the thickness of the second porous portion is 300 μm or less considering the activation time of the gas sensing device.  
         [0020]     In the meantime, the corner portion extending in the length direction mentioned in the present invention refers to a part connecting any one of front and rear faces extending in the length direction with any one of both side faces, of the external faces of the sheet-like gas sensing device. Then, the corner portion is not restricted to the top of a line in which two faces intersect (that is, ridge line) but includes a curved portion which connects two faces with for example, a round configuration. The second porous portion may be formed to cover one or more corner portions. That is, the second porous portion may be formed by selecting one or more of the corner portions likely to be wet considering the installation positions within the gas sensor. The second porous portion may be so constructed to cover not only the first porous portion and corner portion but also the external face of the gas sensing device. A sentence “the thickness of the second porous portion from the corner portion is 20 μm or more” in the present invention means that in a section in the thickness direction of the gas sensing device, a virtual circle having a diameter of 20 μm is formed (included) between the corner portion of the gas sensing device and the surface of the second porous portion.  
         [0021]     In the gas sensing device of the present invention, preferably, the BET specific surface area of the second porous portion is 1.0 m 2 /g or more. If the BET specific surface area of the second porous portion is set to 1.0 m 2 /g or more, the diameter of particles which form the second porous portion becomes smaller, so that the second porous portion can absorb more phosphorous and silicone. Thus, generation of clogging in the first porous portion can be further suppressed to improve the accuracy of detection of air-fuel ratio by changes in diffusion resistance of a measuring object gas. If the BET specific surface area of the second porous portion is less than 1.0 m 2 /g, it is difficult to obtain the above-described effect. In the meantime, the BET specific surface area can be measured according to the BET method.  
         [0022]     In the gas sensing device of the present invention, the first porous portion is overlaid on the first cell and the measuring chamber is defined by the first cell and the first porous portion and further, the shielding layer is provided to be overlaid on the first cell via the first porous portion.  
         [0023]     By adopting the above-described structure for the gas sensing device, a gas sensing device in which the second porous portion is in contact with the opening edge of the concave portion while part thereof invades into the concave portion can be achieved. Consequently, generation of clogging in the first porous portion can be suppressed so as to improve the accuracy of detection of air-fuel ratio by changes in diffusion resistance of the measuring object gas thereby preventing the second porous portion from being separated.  
         [0024]     In the gas sensing device having the above-described structure, the concave portion may be formed at, at least one portion of four portions, both side faces of the gas sensing device, front end face and rear end face and the second porous portion needs to be dented into the concave portion. Further, the concave portion may be formed at all four portions, both side faces of the gas sensing device, front end face and rear end face and the second porous portion may be dented into the concave portion.  
         [0025]     In the gas sensing device of the present invention, preferably, the minimum thickness of the second porous portion formed outside the first porous portion is 130 μm or more in a direction perpendicular to the laminating direction.  
         [0026]     By setting the minimum thickness of the second porous portion formed outside the first porous portion to more than 130 μm, a distance over which the measuring object gas passes through the second porous portion can be increased, so that more phosphorous and silicone can be absorbed by the second porous portion. Consequently, generation of clogging in the first porous portion can be further suppressed so as to improve the accuracy of detection of air-fuel ratio by changes in diffusion resistance of the measuring object gas. If the minimum distance of the second porous portion is less than 130 μm, sometimes, the above-described effect cannot be obtained. Although the distance of the second porous portion is preferred to be as long as possible, preferably the maximum thickness of the second porous portion formed outside the first porous portion is 300 μm or less if considering the activation time of the gas sensing device.  
         [0027]     Exhaust gas passing through the exhaust pipe of an internal combustion engine contains water droplet or oil droplet and if the water droplet or the like adheres to the gas sensing device when the gas sensor is used, crack can occur in the gas sensing device. If speaking in detail, because the gas sensing device is exposed to exhaust gas (measuring object gas) and heated by a heater when the gas sensor is used, when water droplet or the like makes contact therewith, a large difference in temperature occurs between the portion which the water droplet adheres to and its surrounding, thereby resulting in generation of crack due to thermal shock. As for the portion in which crack occurs due to contact of water droplet, if water droplet adheres to the corner portion extending in the length direction of the gas sensing device, thermal shock is likely to concentrate on that corner portion thereby often causing a crack.  
         [0028]     In the gas sensing device of the present invention, preferably, the second porous portion covers the corner portion in the length direction of the gas sensing device and the thickness of the second porous portion from the corner portion is 20 μm or more. By covering the corner portion in the length direction of the gas sensing device with porous substance based on the fact that the second porous portion is composed of the porous substance, water droplets adhering to the second porous portion penetrate slowly while being diffused into a number of pores, so that the water droplets can be diffused before they reach the corner portion of the gas sensing device. As a consequence, thermal shock generated in the corner portion of the gas sensing device can be suppressed thereby suppressing generation of crack in the gas sensing device.  
         [0029]     Preferably, the thickness of the second porous portion from the outermost virtual face of the gas sensing device is 30 μm or more in order to prevent generation of crack due to wetting and more preferably, 50 μm or more. On the other hand, preferably, the thickness of the second porous portion is 300 μm or less considering the activation time of the gas sensing device.  
         [0030]     In the meantime, the corner portion extending in the length direction mentioned in the present invention refers to a part connecting any one of front and rear faces extending in the length direction with any one of both side faces, of the external faces of the sheet-like gas sensing device. Then, the corner portion is not restricted to the top of a line in which two faces intersect (that is, ridge line) but includes a curved portion which connects two faces with for example, a round configuration. The second porous portion may be formed to cover one or more corner portions. That is, the second porous portion may be formed by selecting one or more of the corner portions likely to be wet considering the installation positions within the gas sensor. The second porous portion may be so constructed to cover not only the first porous portion and corner portion but also the external face of the gas sensing device. A sentence “the thickness of the second porous portion from the corner portion is 20 μm or more” in the present invention means that in a section in the thickness direction of the gas sensing device, a virtual circle having a diameter of 20 μm is formed (included) between the corner portion of the gas sensing device and the surface of the second porous portion.  
         [0031]     The gas sensor of the present invention is a gas sensor comprising: a cylindrical main body metal and a gas sensing device held by the main body metal, wherein the gas sensing device is a gas sensing device according to the aspect of the present invention.  
         [0032]     By using the above-described gas sensing device for the gas sensor of the present invention, it can detect air-fuel ratio at an excellent accuracy by changes in diffusion resistance of the measuring object gas. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIG. 1  is a sectional view of a gas sensor  2  of the first embodiment;  
         [0034]      FIG. 2  is an exploded view of a sensing device  4  disposed in the gas sensor  2  of the first embodiment;  
         [0035]      FIG. 3  is a perspective view of the gas sensor  2  of the first embodiment;  
         [0036]      FIG. 4  is a sectional view taken along the line A-A′ of  FIG. 3 ;  
         [0037]      FIG. 5  is an exploded view of a sensing device  204  disposed in a gas sensor  202  of the second embodiment;  
         [0038]      FIG. 6  is a sectional view of a device in the vicinity of a detecting portion of  FIG. 5 ;  
         [0039]      FIG. 7  is a sectional view of a sensing device  804  disposed in a gas sensor  600  of the third embodiment; and  
         [0040]      FIG. 8  is a graph showing a result of poisoning durability test depending on the thickness of a porous portion  30 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0041]     Hereinafter, a gas sensor according to the first embodiment of the present invention will be described with reference to the accompanying drawings. In the first embodiment, a total area liner air-fuel ration sensor  2  including a gas sensing device for detecting specific gas in exhaust gas as a measuring object gas for use in air-fuel ratio feedback control in an internal combustion engine of an automobile and the like, this total area liner air-fuel ration sensor being mounted in an exhaust pipe of the internal combustion engine, will be described.  
         [0042]      FIG. 1  is a sectional view showing the entire structure of the liner air-fuel ration sensor  2  of the first embodiment. The liner air-fuel ration sensor  2  comprises a cylindrical main body metal  38  in which a screw portion  39  for fixing the air-fuel ration sensor  2  to an exhaust pipe is formed on an external surface thereof, a sheet-like gas sensing device  4  extending in an axial direction (length direction of the liner air-fuel ration sensor  2 : vertical direction in the Figure), a cylindrical ceramic sleeve  6  disposed so as to surround the periphery in the diameter direction of the gas sensing device  4 , an insulating contact member  66  disposed such that the inner wall face of a contact through hole  68  passing therethrough in the axial direction surrounds the periphery of the rear end portion of the gas sensing device  4  and five connecting terminals  10  (two pieces indicated in  FIG. 1 ) disposed between the gas sensing device  4  and the insulating contact member  66 .  
         [0043]     The main body metal  38  is so constructed in substantially cylindrical shape having a through hole  54  which goes through in the axial direction and a shelf portion  52  projecting inward in the diameter direction of the through hole  54 . In the main body metal  38 , the front end side (detecting portion  8  described later) of the gas sensing device  4  is disposed outside of the front end side of the through hole  54  while electrode terminal portions  120 ,  121  are disposed outside of the rear end side of the through hole  54 , and passed through the through hole  54  and held. The shelf portion  52  is formed as a tapered face directed inward having an inclination with respect to a plane perpendicular to the axial direction.  
         [0044]     An annular ceramic holder  51 , powder charging layers  53 ,  56  (hereinafter, called talc rings  53 ,  56  depending on a case) and the aforementioned ceramic sleeve  6  are overlaid in this order from a front end side to a rear end side inside the through hole  54  in the main body metal  38 , surrounding the periphery of the diameter direction of the gas sensing device  4 . A caulking packing  57  is disposed between the ceramic sleeve  6  and the rear end portion  40  of the main body metal  38  and a metal holder  58  which holds the talc ring  53  and the ceramic holder  51  so as to maintain air tightness is disposed between the ceramic holder  51  and the shelf portion  52  of the main body metal  38 . The rear end portion  40  of the main body metal  38  is caulked so as to press the ceramic sleeve  6  to the side of the front end through the caulking packing  57 .  
         [0045]     On the other hand, as shown in  FIG. 1 , metal double structured protectors (for example, made of stainless steel) having a plurality of holes, comprising an outside protector  42  and an inside protector  43  are attached to the outer periphery on the front end side (down in  FIG. 1 ) of the main body metal  38  so as to cover the projecting portion of the gas sensing device  4  by welding or the like.  
         [0046]     An outer cylinder  44  is fixed to the outer periphery on the rear end side of the main body metal  38 . A grommet  50  including lead wire passing holes  61  in which five lead wires  46  (only three are expressed in  FIG. 1 ) are passed through, those lead wires being electrically connected to the electrode terminal portions  120 ,  121  of the gas sensing device  4  is disposed at an opening portion on the rear end side (up in  FIG. 1 ) of the outer cylinder  44 .  
         [0047]     The insulating contact member  66  is disposed on the rear end side (up in  FIG. 1 ) of the gas sensing device  4  projecting from the rear end portion  40  of the main body metal  38 . This insulating contact member  66  is disposed around the electrode terminal portions  120 ,  121  formed on the surface on the rear end side of the gas sensing device  4 . The insulating contact member  66  is formed into a cylindrical configuration having a contact through hole  68  which goes through in the axial direction and has a projecting portion  67  projecting outward in the diameter direction from an external surface thereof. The insulating contact member  66  is disposed within the outer cylinder  44  such that the projecting portion  67  makes contact with the outer cylinder  44  via a holding member  69 .  
         [0048]     Next, the gas sensing device  4  which is a major portion of the present invention will be described.  
         [0049]     The gas sensing device  4  is formed into a sheet-like shape extending in the axial direction and a detecting portion  8  is formed on the front end side (down in the figure) which is directed to gas as a measuring object. The electrode terminal portions  120 ,  121  are formed on the front and rear surfaces of an outer surface on the rear end side (up in the figure). In the meantime, the porous portion  30  (see  FIGS. 1, 4 ) is formed in the detecting portion  8 . The connecting terminal  10  is disposed between the gas sensing device  4  and the insulating contact member  66  so that it is electrically connected to the electrode terminal portions  120 ,  121  of the gas sensing device  4 . The connecting terminal  10  is also connected electrically to the lead wire  46  introduced from outside and disposed inside the sensor so as to form a current passage for current flows between an external device which the lead wire is connected to and the electrode terminal portions  120 ,  121 .  
         [0050]     Next, the gas sensing device  4  will be described specifically with reference to  FIGS. 2-4 .  FIG. 2  is an exploded view of the lamination of the gas sensing device  4 ,  FIG. 3  is a perspective view of the gas sensing device and  FIG. 4  is a sectional view taken along the line A-A′ of  FIG. 3 . The porous portion  30  is not indicated in  FIGS. 2, 3  for the reason of the description below.  
         [0051]     As shown in  FIG. 2 , the gas sensing device  4  is comprised of a lamination of a detecting device  300  and a heater  200  and the detecting device  300  is comprised of a lamination of an oxygen concentration detecting cell  130  and an oxygen pump cell  140 .  
         [0052]     The heater  200  comprises a first base body  101  and a second base body  103  composed of mainly alumina and a heating body  102  composed of mainly platinum and sandwiched between the first base body  101  and the second base body  103 . The heating body  102  comprises a heating portion  102   a  located at the front end side and a pair of heater lead portions  102   b  extending in the length direction of the first base body  101  from the heating portion  102   a . Then, the terminals of the heater lead portions  102   b  are connected electrically to electrode terminal portions  120  through heater side through holes  101   a  provided in the first base body  101 .  
         [0053]     The oxygen concentration detecting cell  130  comprises a first solid electrolyte body  105  and first electrode  104  and second electrode  106  formed on both faces of the first solid electrolyte body  105 . The first electrode  104  is comprised of a first electrode portion  104   a  and a first lead portion  104   b  extending in the length direction of the first solid electrolyte body  105  from the first electrode portion  104   a . The second electrode  106  is comprised of a second electrode portion  106   a  and a second lead portion  106   b  extending in the length direction of the first solid electrolyte body  105  from the second electrode portion  106   a .  
         [0054]     The terminal of the first lead portion  104   b  is connected electrically to the electrode terminal portion  121  through a first through hole  105   a  provided in the first solid electrolyte body  105 , a second through hole  107   a  provided in an insulating layer  107  described later, a fourth through hole  109   a  provided in a second solid electrolyte body  109  and a sixth through hole  111   a  provided in protective layer  111 . On the other hand, the terminal of the second lead portion  106   b  is connected electrically to an electrode terminal portion  121  through a third through hole  107   b  provided in an insulating layer  107  described later, a fifth through hole  109   b  provided in the second solid electrolyte body  109  and a seventh through hole  111   b  provided in the protective layer  111 .  
         [0055]     On the other hand, an oxygen pump cell  140  comprises a second solid electrolyte body  109  and third electrode  108  and fourth electrode  110  formed on both faces of the second solid electrolyte body  109 . The third electrode  108  is comprised of a third electrode portion  108   a  and a third lead portion  108   b  extending in the length direction of the second solid electrolyte body  109  from the third electrode portion  108   a . The fourth electrode  110  is comprised of a fourth electrode portion  110   a  and a fourth lead portion  110   b  extending in the length direction of the second solid electrolyte body  109  from the fourth electrode portion  110   a.    
         [0056]     The terminal of the third lead portion  108   b  is connected electrically to the electrode terminal portion  121  through the fifth through hole  109   b  provided in the second solid electrolyte body  109  and the seventh through hole  111   b  provided in the protective layer  111 . On the other hand, the terminal of the fourth lead portion  110   b  is connected electrically to the electrode terminal portion  121  through an eighth through hole  111   c  provided in the protective layer  111  described later. In the meantime, the second lead portion  106   b  and the third lead portion  108   b  are in the same potential through the third through hole  107   b.    
         [0057]     The first solid electrolyte body  105  and the second solid electrolyte body  109  are constituted of partially stabilized zirconia sintered body produced by adding yttria (Y 2 O 3 ) or calcia (CaO) to zirconia (ZrO 2 ) as stabilizer.  
         [0058]     The heating element  102 , first electrode  104 , second electrode  106 , third electrode  108 , fourth electrode  110 , electrode terminal portion  120  and electrode terminal portion  121  may be formed of platinum group element. As the platinum group element preferable for forming these, Pt, Rh, Pd and the like can be mentioned. One of them may be used independently or two or more may be used at the same time.  
         [0059]     More preferably, the heating element  102 , first electrode  104 , second electrode  106 , third electrode  108 , fourth electrode  110 , electrode terminal portion  120  and electrode terminal portion  121  are formed of mainly Pt if considering heat resistance and oxidation resistance. It is more preferable that the heating body  102 , first electrode  104 , second electrode  106 , third electrode  108 , fourth electrode  110 , electrode terminal portion  120  and electrode terminal portion  121  contain ceramic component as well as platinum group element which is a main component. This ceramic component is preferred to of the same component as main material (for example, component mainly constituting the first solid electrolyte body  105  and the second solid electrolyte body  109 ) of a side on which each element is overlaid from viewpoints of fixing.  
         [0060]     The insulating layer  107  is formed between the oxygen pump cell  140  and the oxygen concentration detecting cell  130 . The insulating layer  107  is comprised of an insulating portion  114  and a diffusion rate controlling portion  115 . A gas detecting chamber  107   c  is formed at a position corresponding to the second electrode portion  106   a  and the third electrode portion  108   a  in the insulating portion  114  of this insulating layer  107 . This gas detecting portion  107   c  communicates with outside in the width direction of the insulating layer  107  and a diffusion rate controlling portions  115  which achieve gas diffusion between outside and the gas detecting chamber  107   c  under a predetermined rate controlling condition are disposed in the communicating portions.  
         [0061]     The insulating portion  114  is not restricted to any particular one as long as it is a ceramic sintered body having insulation property and oxide base ceramics such as alumina, mullite or the like can be picked up.  
         [0062]     The diffusion rate controlling portion  115  is a porous body composed of alumina. This diffusion rate controlling portion  115  controls the diffusion rate when a detecting objects gas flows into the gas detecting chamber  107   c.    
         [0063]     The protective layer  111  is formed on the surface of the second solid electrolyte body  109  so as to sandwich the fourth electrode  110 . In this protective layer  111 , porous electrode protecting portion  113   a  which sandwiches the fourth electrode portion  110   a  is inserted into a through hole  112   a  formed in a reinforcing portion  112  which sandwiches the fourth lead portion  110   b.    
         [0064]     As shown in  FIG. 4 , in the diffusion rate controlling portion  115 , its external face  115   a  directed to an outermost virtual face S 1  which connects the outermost faces of the gas sensing device  4  is located inside the outermost virtual face Si. The outermost virtual face S 1  is formed of the external faces of the first base body  101 , second base body  103 , first solid electrolyte body  105 , second solid electrolyte  109 , reinforcing portion  112  and electrode protecting portion  113   a  in the first embodiment and indicated with a solid line or dotted line in  FIG. 4 . Then, a concave portion  116  is formed such that it is dented from the outer most virtual face S 1  toward the external face  115   a  of the diffusion rate controlling portion  115 .  
         [0065]     The porous portion  30  is formed on an opposite side to the gas detecting chamber  107   c  via the diffusion rate controlling portion  115 . More specifically, the porous portion  30  covers the periphery of the outermost virtual face S 1  of the gas sensor device  4  while part thereof invades into the concave portion  116 .  
         [0066]     Because the porous portion  30  is disposed at least such that it makes contact with an opening periphery  116   a  (in the first embodiment, an external face  105   s  of the first solid electrolyte body  105 , an external face  109   s  of the second solid electrolyte body  109 , an external face  107   s  (not shown) of the insulating body  107 ) of the concave portion  116 , the opening edge of the concave portion  116  can be blocked from being exposed to the outermost virtual face S 1 , so that gas can be blocked from invading into the diffusion rate controlling portion  115  through this opening edge from outside. Thus, generation of clogging in the diffusion rate controlling portion  115  can be suppressed to improve the accuracy of detection of air-fuel ratio by changes in diffusion resistance of a measuring object gas.  
         [0067]     Further, part of the porous portion  30  invades into the concave portion  116 . Because part of the porous portion  30  invades into the concave portion  116  so as to form a wedge-like configuration, the porous portion  30  can be prevented from separating from the gas sensing device  4  as compared to a case where a porous portion is disposed to be in contact with only the opening edge  116   a  of the concave portion  116  in the gas sensing device  4 .  
         [0068]     Further, by allowing part of the porous portion  30  to invade into the concave portion  116  in the gas sensing device  4 , the thickness of the porous portion  30  (minimum thickness t 1  described later) of the porous portion  30  can be secured without enlarging the gas sensing device  4  and consequently, the porous portion  30  can absorb phosphorous and silicone more without delaying the activation time of the gas sensing device  4 .  
         [0069]     This porous portion  30  has a smaller diffusion resistance than the diffusion rate controlling portion  115  and the minimum thickness t 1  between the external face of the porous portion  30  provided in the concave portion  116  and the internal face directed to the diffusion rate controlling portion  115  is 150 μm. By setting the thickness of the porous portion  30  larger than 130 μm, a distance over which a measuring object gas passes through the porous portion  30  can be increased, so that the porous portion  30  can absorb phosphorous and silicone more. Accordingly, generation of clogging in the diffusion rate controlling portion  115  can be suppressed to improve the accuracy of detection of air-fuel ratio by changes of the diffusion resistance of the measuring object gas.  
         [0070]     The BET specific surface area of the porous portion  30  is 1.6 m 2 /g. Because the BET specific surface area of the porous portion  30  is set to more than 1.0 m 2 /g, the diameter of particles forming the porous portion  30  is smaller so that the porous portion  30  can absorb more phosphorous and silicone. Consequently, generation of clogging in the diffusion rate controlling portion  115  can be suppressed so as to improve the accuracy of detection of air-fuel ratio by changes in diffusion resistance of a measuring object gas.  
         [0071]     Further, the porous portion  30  covers a corner portion  4 a extending in the length direction of the detecting portion  8  of the gas sensing device  4  and the thickness t 2  of the porous portion  30  from this corner portion  4   a  is 70 μm. By covering the corner portion  4 a in the length direction of the gas sensing device  4  with the porous substance based on the fact that the porous portion  30  is composed of the porous substance, water droplets adhering to the porous portion  30  can be diffused until they reach the corner portion  4   a  of the gas sensing device  4  because they penetrate slowly while being diffused into a number of pores. As a result, a thermal shock generated on the corner portion  4   a  of the gas sensing device  4  can be suppressed thereby preventing crack from being generated in the gas sensing device  4 .  
         [0072]     In the meantime, the oxygen concentration detecting cell  130  of the first embodiment corresponds to “a first cell” in the scope of claim of patent, the first solid electrolyte body  105  corresponds to “a first solid electrolyte layer”, the first electrode  104  and the second electrode  106  correspond to “a first opposing electrode”, the oxygen pump cell  140  corresponds to “a second cell”, the second solid electrolyte body  109  corresponds to “a second solid electrolyte layer”, the third electrode  108  and fourth electrode  110  correspond to a second opposing electrode, the diffusion rate controlling portion  115  corresponds to “a first porous portion”, the insulating portion  114  corresponds to “an insulating body” and the porous portion  30  corresponds to “a second porous portion”.  
         [0073]     Next, the manufacturing method of this gas sensing device  4  will be described.  
         [0074]     A portion before sintering and a portion after sintering will be explained with the same reference numeral. For example, the first solid electrolyte body after sintering and the non-sintered first solid electrolyte body will be explained with same reference numeral  105 .  
         [0075]     First, slurry in which a first raw material powder and a plasticizer were dispersed by wet blending was prepared. The first raw material powder is composed of, for example, alumina powder 97 mass % and silica 3 mass % as sintering adjusting agent. The plasticizer is composed of butyral resin and dibutylphthalate (DBP). After this slurry was formed into a sheet-like matter of 0.4 mm thick according to a sheet forming method using a doctor blade apparatus, this was cut to 140 mm×140 mm so as to obtain the non-sintered reinforcing portion  112 , first non-sintered base body  101 , second non-sintered base body  103  and the non-sintered insulating portion  114  of the non-sintered insulating layer  107 . Then, the through hole  112   a  was formed in the non-sintered reinforcing portion  112 . Additionally, the gas detecting chamber  107  was formed in the non-sintered insulating portion  114 .  
         [0076]     On the other hand, slurry in which the second raw material powder and plasticizer were dispersed by wet blending was prepared. The second raw material powder is composed of, for example, alumina powder 63 mass %, silica 3 mass % as sintering adjusting agent and carbon power 34 mass %. The plasticizer is composed of butyral resin and DBP. Then, the non-sintered electrode protecting portion  113   a  was obtained by using this slurry.  
         [0077]     Further, slurry in which the third raw material powder and plasticizer were dispersed by wet blending was prepared. The third raw material powder is composed of, for example, zirconia powder 97 mass %, 3 mass % of silica (SiO 2 ) powder and alumina powder as sintering adjusting agent. The plasticizer is composed of butyral resin and DBP. The first solid electrolyte body  105  and the second solid electrolyte body  109  were obtained using this slurry.  
         [0078]     Further, slurry in which alumina powder 100 mass % and plasticizer were dispersed by wet blending was prepared. The plasticizer is composed of butyral resin and DBP. The non-sintered diffusion rate controlling portion  115  of the non-sintered insulating layer  107  was obtained using this slurry.  
         [0079]     Then, the first non-sintered base body  101 , non-sintered heating element  102 , second non-sintered base body  103 , first non-sintered electrode  104 , first non-sintered solid electrolyte body  105 , second non-sintered electrode  106 , non-sintered insulating layer  107 , third non-sintered electrode  108 , second non-sintered solid electrolyte body  109 , fourth non-sintered electrode  110 , non-sintered protective layer  111  and the like are overlaid successively from the bottom.  
         [0080]     More specifically, the non-sintered heating element  102  is formed on the first non-sintered base body  101  by screen printing using paste mainly composed of platinum. Then, The second non-sintered base body  103  is overlaid so as to sandwich the non-sintered heating element  102 .  
         [0081]     Then, the first non-sintered electrode  104  was formed on the first non-sintered solid electrolyte body  105 . The first non-sintered electrode  104  is composed of platinum paste constituted of platinum 90 mass % and zirconia power 10 mass %. The first non-sintered electrode  104  is formed according to screen printing method using this platinum paste.  
         [0082]     The first non-sintered electrode  104  was overlaid on the second non-sintered base body  103  and the second non-sintered electrode  106  was formed on the first non-sintered solid electrolyte body  105  by printing. In the meantime, the second non-sintered electrode  106  is made of the same material as the first non-sintered electrode  104 .  
         [0083]     Then, the non-sintered insulating layer  107  was formed on the second non-sintered electrode  106 . More specifically, the non-sintered insulating portion  114  and the non-sintered diffusion rate controlling portion  115  were formed. Paste mainly composed of carbon was printed at a portion which turns to the gas detecting chamber  107   c  after sintering.  
         [0084]     The third non-sintered electrode  108  was printed on the second non-sintered solid electrolyte body  109  and the second non-sintered solid electrolyte body  109  was overlaid on the non-sintered insulating layer  107  in such a manner to sandwich the third non-sintered electrode  108 . Then, the fourth non-sintered electrode  110  was printed on the second non-sintered solid electrolyte body  109 . The third non-sintered electrode  108  and the fourth non-sintered electrode  110  are made of the same material as the first non-sintered electrode  104 . Then, the non-sintered protective layer  111  was overlaid on the fourth non-sintered electrode  110 . In the non-sintered protective layer  111 , the non-sintered electrode protecting portion  113   a  is already inserted into the through hole  112   a  in the non-sintered reinforcing portion  112 .  
         [0085]     These components were pressed at 1 MPa and then cut to a predetermined size, so that 10 pieces of non-sintered laminations were obtained from a single formation.  
         [0086]     After that, resin was diffused by heating from the non-sintered gas sensing device and it was held at a sintering temperature of 1500° C. for an hour and consequently, a lamination for detecting the concentration of oxygen in exhaust gas was sintered.  
         [0087]     Through the sintering process, the first non-sintered electrode  104  turns to the first electrode  104  comprising the first electrode portion  104   a  and the first lead portion  104   b . The first non-sintered solid electrolyte body  105  turns to the first solid electrolyte body  105 . The second non-sintered electrode  106  turns to the second electrode  106  comprising the second electrode portion  106   a  and the second lead portion  106   b . The non-sintered insulating portion  114  of the non-sintered insulating layer  107  turns to the insulating portion  114  and the non-sintered diffusion rate controlling portion  115  of the non-sintered insulating layer  107  turns to the porous diffusion rate controlling portion  115 . The non-sintered insulating layer  107  turns to the insulating layer  107 . The gas detecting chamber  107   c  in the insulating layer  107  communicates with outside through the diffusion rate controlling portions  115  on both sides in the width direction of the insulating portion  114 . The diffusion rate controlling portion  115  realizes gas diffusion between outside and the gas detecting chamber  107   c  under a predetermined diffusion rate controlling condition. The third non-sintered electrode  108  turns to the third electrode  108  comprising the third electrode portion  108   a  and the third lead portion  108   b . The second non-sintered solid electrolyte body  109  turns to the second solid electrolyte body  109 . The fourth non-sintered electrode  110  turns to the fourth electrode  110  comprising the fourth electrode portion  110 a and the fourth lead portion  110   b . The non-sintered reinforcing portion  112  of the non-sintered protective layer  111  turns to the reinforcing portion  112  for protecting the second solid electrolyte body  109  and the non-sintered electrode protecting portion  113   a  of the non-sintered protective layer  111  turns to the porous electrode protecting portion  113   a  for controlling the fourth electrode  110  from poisoning.  
         [0088]     After that, the non-sintered porous portion  30  is formed around the front end side of this lamination. More specifically, slurry was produced of spinel powder, titania and a remnant was produced of alumina sol, and the non-sintered porous portion  30  was formed on the entire periphery on the front end side of the lamination using this slurry. In the meantime, as its formation method, spray, coating and the like may be used. After that, by treating the lamination having this formed non-sintered porous portion  30  with heat for three hours in sintering time under a sintering temperature of 1000° C., the gas sensing device  4  including the porous portion  30  was obtained.  
         [0089]     The gas sensing device  4  produced in the above manufacturing method is inserted into the metal holder  58  and fixed with the ceramic holder  51  and talc ring  53  so as to produce an assembly. After that, this assembly is fixed to the main body metal  38  and the talc ring  56  and the separator  6  were inserted. The rear end side  40  of the main body metal  38  was caulked so as to produce a lower assembly. The outside protector  42  and the inside protector  43  are already installed to the lower assembly. On the other hand, an upper assembly is produced by installing the outer cylinder  44 , insulating contact member  66  and grommet  50  and the like. Then, the lower assembly and the upper assembly are jointed to obtain the gas sensor  2 .  
         [0090]     Hereinafter a gas sensor  202  according to the second embodiment of the present invention will be described with reference to the accompanying drawings. In the meantime, the gas sensor  202  of the second embodiment is different from the gas sensor  2  of the first embodiment in the structure of the gas sensing device  204  and thus, mainly the gas sensing device  204  will be described while description of other components is simplified or omitted.  
         [0091]      FIG. 5  is an exploded view of the lamination of the gas sensing device  204  and  FIG. 6  is a sectional view of the detecting portion of  FIG. 5 .  FIG. 5  does not show the porous portion  30  for the reason of the description below.  
         [0092]     In the gas sensing device  204 , as shown in  FIG. 5 , an oxygen pump cell  500  and a heater  400  are overlaid.  
         [0093]     The heater  400  comprises a first base body  301  and second base body  303 , both composed of mainly alumina and a heating element  302  composed of mainly platinum and sandwiched between the first base body  301  and the second base body  303 . The heating element  302  is comprised of a heating portion  302 a located on the front end side and a pair of heater lead portions  302   b  extending in the length direction of the first base body  301  from the heating portion  302   a . The terminal of the heater lead portion  302   b  is connected electrically to an electrode terminal portion  320  through a heater side through hole  301   a  provided in the first base body  301 .  
         [0094]     The oxygen pump cell  500  comprises a first solid electrolyte body  309  and a first electrode  308  and second electrode  310  formed on both faces of the first solid electrolyte body  309 . The first electrode  308  is comprised of a first electrode portion  308   a  and a first lead portion  308   b  extending in the length direction of the first solid electrolyte body  309  from the first electrode portion  308   a . The second electrode  310  is comprised of a second electrode portion  310   a  and a second lead portion  310   b  extending in the length direction of the first solid electrolyte body  309  from the second electrode portion  310   a . Then, the terminal of the first lead portion  308   b  is connected electrically to an electrode terminal portion  321  through a first through hole  309   a  provided in the first solid electrolyte body  309 .  
         [0095]     The second solid electrolyte body  309  is constituted of partially stabilized zirconia sintered body composed by adding yttria (Y 2 O 3 ) or calcia (CaO) as stabilizer to zirconia (ZrO 2 ).  
         [0096]     The heating element  302 , first electrode  308 , second electrode  310 , electrode terminal portion  320  and electrode terminal portion  321  can be formed of platinum group element. As preferable platinum group element for forming these, Pt, Rh, Pd and the like can be picked up. One of those may be used independently or two or more may be used at the same time.  
         [0097]     The insulating layer  307  is formed between the heater  400  and the oxygen pump cell  500 . The insulating layer  307  is comprised of an insulating portion  307   a  and an atmosphere introduction port  307   b . This atmosphere introduction port  307   b  communicates with outside on the rear end side of the insulating layer  307 . The insulating portion  307   b  is not restricted to any particular one as long as it is a ceramic sintered body having insulation property and for example, oxide base ceramics such as alumina, mullite or the like can be mentioned.  
         [0098]     A gas measuring chamber  307   c  (see  FIG. 6 ) is provided on the surface of the first solid electrolyte body  309  such that it surrounds the second electrode portion  310   a  of the second electrode  310  and the gas measuring chamber  307   c  is covered with the first porous portion  315 . Further, a shielding layer  312  is overlaid on an opposite side to the first solid electrolyte body  309  of the first porous portion  315 . The first porous portion  315  is a porous body composed of alumina.  
         [0099]     As shown in  FIG. 6 , in the first porous portion  315 , its external surface  315   a  facing the outermost virtual face S 2  connecting the outermost faces of the gas sensing device  204  is located inside the outermost virtual face S 2 . In the meantime, according to the second embodiment, the outermost virtual face S 2  is formed of the external surfaces of the first base body  301 , second base body  303 , insulating layer  307 , first solid electrolyte body  309  and shielding layer  312 , referring to a face indicated with a solid line and dotted line in  FIG. 6 . Then, a concave portion  316  is formed such that it is dented from the outermost virtual face S 2  toward the external surface  315   a  of the first porous portion  315 .  
         [0100]     Further, a second porous portion  330  is formed outside the first porous portion  315 . More specifically, the second porous portion  330  covers the periphery of the outermost virtual face S 2  of the gas sensing device  204  while part of them invades into the concave portion  316 .  
         [0101]     By disposing the second porous portion  330  such that it makes at least contact with an opening edge  316   a  (in the second embodiment, external face  309   s  of the first solid electrolyte body  309  and external face  312   s  of the shielding body  312 ), the opening edge of the concave portion  316  can be prevented from being exposed to the outermost virtual face S 2  of the gas sensing device  204  thereby blocking gas from invading into the first porous portion  315  through this opening edge. Thus, generation of clogging in the first porous portion  315  can be suppressed.  
         [0102]     Further, part of this second porous portion  330  invades into the concave portion  316 . Because part of the second porous portion  330  invades into the concave portion  316  so as to form a wedge-like configuration, the second porous portion  330  can be prevented from being separated from the gas sensing device  204  as compared to a case where a second porous portion is disposed to make contact with only the opening edge  316   a  of the concave portion  316  in the gas sensing device  204 .  
         [0103]     Further, by allowing part of the second porous portion  330  to invade into the concave portion  316  in the gas sensing device  204 , the thickness (minimum thickness t 3  described later) of the second porous portion  330  can be secured without enlarging the gas sensing device  204 . As a consequence, the second porous portion  330  can absorb phosphorus and silicone without delaying the activation time of the gas sensing device  204 .  
         [0104]     The minimum thickness t 3  between the external face of the porous portion  330  provided in the concave portion  316  and the internal face facing the first porous portion  315  is 150 μm. By setting the minimum thickness t 3  of the second porous portion  330  to larger than 130 μm, the distance over which a measuring object gas passes through the second porous portion  330  can be increased so that more phosphorous and silicone can be absorbed by the second porous portion  330 .  
         [0105]     The BET specific surface area of the second porous portion  330  is 1.6 m 2 /g. Because the BET specific surface area of the second porous portion  330  is set to more than 1.0 m 2 /g, the diameter of particles forming the porous portion  330  is smaller so that the porous portion  330  can absorb more phosphorous and silicone.  
         [0106]     Further, the porous portion  330  covers a corner portion  204   a  extending in the length direction of the detecting portion  208  of the gas sensing device  204  and the thickness t 4  of the second porous portion  330  from this corner portion  204   a  is 70 μm. Based on the fact that the second porous portion  330  is composed of the porous substance, by covering the corner portion  204   a  in the length direction of the gas sensing device  204  with the porous substance, water droplets adhering to the second porous portion  330  can be diffused until they reach the corner portion  204   a  of the gas sensing device  204  because they penetrate slowly while being diffused into a number of pores. As a result, thermal shock generated on the corner portion  204   a  of the gas sensing device  204  can be suppressed thereby preventing crack from being generated in the gas sensing device  204 .  
         [0107]     The oxygen pump cell  500  of the second embodiment corresponds to “a first cell” in the scope of claim for patent, the first solid electrolyte body  309  corresponds to “a first solid electrolyte layer” and the first electrode  308  and second electrode  310  correspond to “a first opposing electrode”.  
         [0108]     Hereinafter, a gas sensor  600  according to the third embodiment of the present invention will be described with reference to the accompanying drawings. The gas sensor  600  of the third embodiment is different from the first gas sensor  2  only in the structures of the gas sensing device  204  and gas sensing device  804  and mainly the gas sensing device  804  will be described while description of other components is simplified or omitted.  
         [0109]      FIG. 7  is a sectional view of the detecting portion  608  in the gas sensing device  804 . As shown in  FIG. 7 , the gas sensing device  804  comprises a first base body  601  and a second base body  603  formed on the first base body  601  such that it contains cavity internally. Then, a third base body  620  is disposed in the cavity and the third base body  620  contains a heating element  602  composed of mainly platinum.  
         [0110]     An oxygen concentration detecting cell  800  is comprised of a first solid electrolyte body  605  and a first electrode  604  and second electrode  606  formed on both faces of the first solid electrolyte  605 . On the other hand, an oxygen pump  900  is comprised of a second solid electrolyte  609  and a third electrode  608  and fourth electrode  610  formed on both faces of the second solid electrolyte  609 . A through hole  621  is formed in the second solid electrolyte body  609  such that it goes through in the lamination direction.  
         [0111]     An insulating layer  607  is formed between the oxygen pump cell  900  and the oxygen concentration detecting cell  800 . The insulating layer  607  is comprised of an insulating portion  614  and a diffusion rate controlling portion  615 . A gas detecting chamber  607   c  is formed at a position corresponding to the first electrode  604  and third electrode  608  in the insulating portion  614  of the insulating layer  607 . This gas detecting chamber  607   c  communicates with outside through the through hole  621  provided in the second solid electrolyte body  609  and a diffusion rate controlling portion  615  for achieving gas diffusion between outside and the gas detecting chamber  607   c  under a predetermined diffusion rate controlling condition is disposed in this communicating portion.  
         [0112]     An interposing layer  622  is formed between the second base body  602  and the third base body  603  and the oxygen concentration detecting cell  800 . This interposing layer  622  is comprised of an interposing portion  622   a  and an atmosphere introduction port  622   b . This atmosphere introduction port  622   b  communicates with outside on the rear end side of the gas sensing device  804 .  
         [0113]     The first solid electrolyte body  605 , second solid electrolyte body  609 , first base body  601 , second base body  603 , insulating portion  614 , and interposing layer  622  are constituted of partially stabilized zirconia sintered body produced by adding yttria (Y 2 O 3 ) or calcia (CaO) to zirconia (ZrO 2 ) as stabilizer.  
         [0114]     The heating element  602 , first electrode  604 , second electrode  606 , third electrode  608 , and fourth electrode  610  may be formed of platinum group element. As the platinum group element preferable for forming these, Pt, Rh, Pd and the like can be mentioned. One of them may be used independently or two or more may be used at the same time.  
         [0115]     The third base body  620  is not restricted to any particular one as long as it is a ceramic sintered body having insulation property and for example, oxide base ceramics such as alumina, mullite or the like may be picked up. The diffusion rate controlling portion  615  is a porous body composed of alumina. A diffusion rate when the detection gas flows out to the gas detecting chamber  607   c  is controlled by the diffusion rate controlling portion  615 .  
         [0116]     In the diffusion rate controlling portion  615 , an external surface  615   a  directed to the outermost virtual face S 3  connecting the outermost faces of the gas sensing device  804  is located inside the outermost virtual face S 3 . In the meantime, the outermost virtual face S 3  mentioned in the third embodiment is composed of the external surfaces of the first base body  601 , second base body  603 , interposing portion  622 a, first solid electrolyte body  605 , insulating portion  614 , second solid electrolyte body  609  and fourth electrode  610  and indicated with a solid line or dotted line in  FIG. 7 . Then, a concave portion  616  is formed such that it is dented from the outermost virtual face S 3  toward the external surface  615   a  of the diffusion rate controlling portion  615 .  
         [0117]     Further, a porous portion  630  is formed on an opposite side to the gas detecting chamber  607   c  via the diffusion rate controlling portion  615 . More specifically, the porous portion  630  covers the periphery of the outermost virtual face S 3  of the gas sensing device  804  while part thereof invades into the concave portion  616 .  
         [0118]     By disposing the porous portion  630  at least in contact with the opening edge  616   a  (in the third embodiment, an external face  609   s  of the second solid electrolyte body  609 ) of the concave portion  616 , the opening edge of the concave portion  616  can be prevented from being exposed to the outermost virtual face S 3  of the gas sensing device  804 , thereby blocking gas from invading into the diffusion rate controlling portion  615  through this opening edge. Thus, generation of clogging in the diffusion rate controlling portion  615  can be suppressed to improve the accuracy of detection of air-fuel ratio by changes in diffusion resistance of the measuring object gas.  
         [0119]     Further, part of this porous portion  630  invades into the concave portion  616 . Because part of the porous portion  630  invades into the concave portion  616  so as to form a wedge-like configuration, the porous portion  630  can be prevented from being separated from the gas sensing device  804  as compared to a case where a porous portion  630  is disposed to make contact with only the opening edge  616   a  of the concave portion  616  in the gas sensing device  804 .  
         [0120]     Further, by allowing part of the porous portion  630  to invade into the concave portion  616  in the gas sensing device  804 , the thickness (minimum thickness t 5  described later) of the porous portion  630  can be secured without enlarging the gas sensing device  804 . As a consequence, the porous portion  630  can absorb phosphorus and silicone without delaying the activation time of the gas sensing device  804 .  
         [0121]     The porous portion  630  has a smaller diffusion resistance than the diffusion rate controlling portion  615  and the minimum thickness t 5  between the external face of the porous portion  630  provided in the concave portion  616  and the internal face directed to the diffusion rate controlling portion  615  is 150 μm. By setting the minimum thickness t 5  of the porous portion  630  to larger than 130 μm, the distance over which a measuring object gas passes through the porous portion  330  can be increased so that more phosphorous and silicone can be absorbed by the porous portion  630 . Thus, generation of clogging in the diffusion rate controlling portion  615  can be suppressed to improve the accuracy of detection of air-fuel ratio by changes in diffusion resistance of the measuring object gas.  
         [0122]     Further, the BET specific surface area of the porous portion  630  is 1.6 m 2 /g. Because the BET specific surface area of the porous portion  630  is set to more than 1.0 m 2 /g, the diameter of particles forming the porous portion  630  is smaller so that the porous portion  630  can absorb more phosphorous and silicone. Thus, generation of clogging in the diffusion rate controlling portion  615  can be suppressed to improve the accuracy of detection of air-fuel ratio by changes in diffusion resistance of the measuring object gas.  
         [0123]     Further, the porous portion  630  covers a corner portion  804   a  extending in the length direction of the detecting portion  608  of the gas sensing device  804  and the thickness t 6  of the porous portion  630  from this corner portion  804   a  is 70 μm. Based on the fact that the porous portion  630  is composed of the porous substance, by covering the corner portion  804   a  in the length direction of the gas sensing device  804  with the porous substance, water droplets adhering to the porous portion  630  can be diffused until they reach the corner portion  804   a  of the gas sensing device  804  because they penetrate slowly while being diffused into a number of pores. As a result, thermal shock generated on the corner portion  804   a  of the gas sensing device  804  can be suppressed thereby preventing crack from being generated in the gas sensing device  804 .  
         [0124]     The oxygen concentration detecting cell  800  of the third embodiment corresponds to “the first cell” in the scope of claim of patent, the first solid electrolyte body  605  corresponds to “the first solid electrolyte layer”, the first electrode  604  and second electrode  606  correspond to “the first opposing electrode”, the oxygen pump  900  corresponds to “the second cell”, the second solid electrolyte body  609  corresponds to “the second solid electrolyte layer”, the third electrode  608  and fourth electrode  610  correspond to the second opposing electrode, the diffusion rate controlling portion  615  corresponds to “the first porous portion”, the insulating portion  614  corresponds to “the insulating body” and the porous portion  630  corresponds to “the second porous portion”.  
         [0000]     Embodiment  
         [0125]     Next, the effect of the present invention was confirmed in the gas sensing device  4  of the first embodiment.  
         [0126]     First, a lamination was produced in the above-described method. The non-sintered porous portion  30  was formed in this lamination such that the minimum thickness t 1  between the external face of the porous portion  30  and the internal face directed to the diffusion rate controlling portion  115  was 60 μm, 130 μm, 200 μm and 300 μm. By treating the lamination in which the non-sintered porous portion  30  was formed with heat, the gas sensing device  4  was produced. Then, each gas sensing device  4  was installed to the main body metal  38  and outer cylinder  44  and the like so as to produce the gas sensor  2 .  
         [0127]     Poisoning durability test was performed to each gas sensor  2 . In this poisoning durability test, a 1.6-liter inline four-cylinder engine was used and a gas sensor  2  was exposed under a condition of engine revolution number of 3000 rpm (air oil ratio λ=1), exhaust gas temperature of 500° C., P poisoning component (ZnDTP: 1.3 cc/L, calcium sulfonate: 1.0 cc/L), and device temperature of 830° C. and it was picked up after 60 hours so as to measure IP value.  FIG. 8  shows its result.  
         [0128]     The gas sensor  2  in which the thickness t 1  of the porous portion  30  is 60 μm has an IP change rate of −4.0% from  FIG. 8 . Contrary to this, it is evident that the gas sensor  2  in which the thickness t 1  of the porous portion  30  is 130 μm, 200 μm, 300 μm has an excellent detection accuracy of air-fuel ratio because its IP change rate is near −2.0%. That is, it is evident that the gas sensor of the embodiment can absorb much phosphorous and silicone through the porous portion  30  thereby suppressing generation of clogging in the diffusion rate controlling portion  115 .  
         [0129]     The embodiments of the present invention has been described and the present invention is not restricted to the above-described embodiments but may be modified and improved in various ways within a range in which the present invention can be achieved.  
         [0130]     For example, although according to this embodiment, the diffusion rate controlling portion  115  and the first porous portion  315  are in contact with the porous portion  30  and the second porous portion  330 , this embodiment is not restricted to this example, but the diffusion rate controlling portion  115  and the first porous portion  315  may be separated from the porous portion  30  and the second porous portion  330 .  
         [0131]     Although according to this embodiment, the porous portions  30 ,  530  and the second porous portion  330  cover the entire periphery of the gas sensor devices  4 ,  204 ,  804 , this embodiment is not restricted to this example, but they may cover the opening edge of the concave portions  116 ,  316 ,  616 .  
         [0132]     Although according to the first embodiment and second embodiment, two concave portions  116 ,  316  are formed in the width direction of the gas sensor device  4 ,  204 , it is permissible to form any one of them. In the second embodiment, a concave portion  316  may be provided on the front end side or rear end side of the gas sensing device  4 .