Abstract:
A gas sensor including a gas detecting element extending in a longitudinal direction and in which a plurality of ceramic layers are stacked, and wherein a detecting portion is provided at a leading end side of the gas detecting element, the gas detecting element including: a first ceramic layer; a second ceramic layer; a first through-hole conductor; a first peripheral portion; a second through-hole conductor; a second peripheral portion; and an opening all as defined herein, wherein the first peripheral portion and the second peripheral portion respectively have mutually overlapping adhered portions and separated portions opposing one another through a gap continuing to the opening, and a relationship L 1 &gt;S 1  is satisfied, where L 1  represents a maximum length of the adhered portion, and S 1  represents a maximum length of the separated portion.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a gas sensor having a stacked-type gas detecting element in which a plurality of ceramic layers are stacked. 
     2. Description of the Related Art 
     Conventionally, a plate-like gas detecting element is known which extends in a longitudinal direction. A plurality of ceramic layers are stacked to form a plate-like detecting element, and a detecting portion is formed at a leading end side thereof Such gas detecting elements are disclosed, for example, in JP-A-61-134655, JP-A-2001-242129, JP-A-2001-311714 and JP-A-2002-107335. Through holes penetrating the ceramic layers are provided in the gas detecting element. In each of these through holes, a conductor is provided for electrically connecting a lead portion extending from a sensing electrode disposed in the interior of the gas detecting element and an electrode pad disposed on an outer surface of the gas detecting element. 
     Problems to be Solved by the Invention 
       FIGS. 9 to 12  discussed below relate to certain technical problems addressed by the present invention.  FIGS. 9 and 11  represent conductor/ceramic opening and through hole structures conventionally found in plate-like gas detecting elements, whereas  FIGS. 10 and 12  relate to technical problems associated with such structures newly discovered by the present inventors. 
     As shown in  FIG. 9 , among the aforementioned conductors, there is a type in which the conductors are provided on an inner peripheral surface of a connected through hole penetrating a plurality of ceramic layers, and an opening is provided therein. Such an opening  901  is generally fabricated as follows. Namely, unsintered through-hole conductors composed of an unsintered metallizing material, and which are formed into tubular through-hole conductors  903  and  904  after sintering, are formed on the inner peripheral surfaces of through holes  911   c  and  912   c  of ceramic green sheets corresponding to respective ceramic layers  911  and  912 . In addition, unsintered peripheral portions, composed of the unsintered metallizing material and which are formed into annular peripheral portions  905 ,  906 ,  907  and  908  after sintering so as to be connected to the peripheries of respective both ends of the tubular through-hole conductors  903  and  904  in a surrounding manner, are respectively formed on the obverse and reverse surfaces of the ceramic green sheets. 
     Then, these ceramic green sheets are stacked. At this time, the unsintered through-hole conductors of the mutually overlapped ceramic green sheets abut one against another and the unsintered peripheral portions overlap one another. Subsequently, the stacked body of the unsintered ceramic is sintered, whereupon the peripheral portions  905 ,  906 ,  907  and  908  are formed from the unsintered peripheral portions, and the through-hole conductors  903  and  904  are formed from the unsintered through-hole conductors. Thus, the opening  901  formed by the inner peripheral surfaces of the conductors  903  and  904  is formed. 
     The amount of sintering shrinkage differs between the ceramic green sheet and the unsintered metallizing material. For this reason, as shown in  FIG. 10 , there are cases where a gap is produced between the mutually overlapping ceramic layers  911  and  912  at the peripheral portions of the through holes  91   1   c  and  912   c  during sintering due to this difference in sintering shrinkage. If such a gap occurs, a separated portion G 6  is also produced between the mutually overlapping peripheral portions  906  and  907 . As a result, the reliability of electrical connection between the through-hole conductors  903  and  904  can possibly suffer. This is because adhered portions where the peripheral portions  906  and  907  overlap become lost, or the adhered portions become extremely reduced, as shown in the drawing. 
     In another form, there is a type of conductor in which, as shown in  FIG. 11 , a through-hole conductor is provided on an inner peripheral surface of a single through hole, and an opening is formed therein. Such an opening  951  is generally fabricated as follows. Namely, an unsintered through-hole conductor, composed of an unsintered metallizing material and which is formed into a tubular through-hole conductor  953  after sintering, is formed on the inner peripheral surface of a through hole  962   c  of a ceramic green sheet corresponding to a ceramic layer  962 . In addition, unsintered peripheral portions, composed of the unsintered metallizing material and which are formed into annular peripheral portions  955  and  956  after sintering so as to be connected to the peripheries of both ends of the through-hole conductor  953  in a surrounding manner, are respectively formed on the obverse and reverse surfaces of this ceramic green sheet. Meanwhile, an unsintered connection terminal, composed of the unsintered metallizing material and which is formed into a tabular connecting portion  959  after sintering, is formed on the obverse surface of a ceramic green sheet corresponding to a ceramic layer  961 . 
     Then, these ceramic green sheets are stacked. At this time, the unsintered peripheral portion of one ceramic green sheet and the unsintered connection terminal of the other ceramic green sheet overlap one another. Subsequently, the stacked body of the unsintered ceramic is sintered, whereupon the through-hole conductor  953  is formed from the unsintered through-hole conductor, the peripheral portions  955  and  956  are formed from the unsintered peripheral portions, and the opening  951  is formed by the inner peripheral surface of the through-hole conductor  953 . In addition, the connecting portion  959  is formed from the unsintered connection terminal. 
     In this case as well, since the amount of sintering shrinkage differs between the ceramic green sheet and the unsintered metallizing material, there are cases where a gap is produced between the mutually overlapping ceramic layers  961  and  962 . Particularly, this gap is formed at the peripheral portion of the through hole  962   c  during sintering due to this difference in sintering shrinkage, as shown in  FIG. 12 . If such a gap occurs, separated portions G 7  are also produced between the peripheral portion  955  and the connecting portion  959  overlapping one another. As a result, the reliability of electrical connection between the through-hole conductor  953  and the connecting portion  959  can possibly suffer. This is because adhered portions where the peripheral portion  955  and the connecting portion  959  overlap become lost, or the adhered portions become extremely reduced, as shown in the drawing. 
     The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a gas sensor having a gas detecting element having both improved electrical connection reliability within a conductor and between the conductor and other conductors formed in the interior of the element. 
     SUMMARY OF THE INVENTION 
     The above object has been achieved, in accordance with a first aspect of the invention, by providing a gas sensor having a gas detecting element extending in a longitudinal direction and in which a plurality of ceramic layers are stacked, and wherein a detecting portion is formed at a leading end side thereof, the gas detecting element comprising: a first ceramic layer having a first surface and a second surface and having a first through hole penetrating therethrough (i.e., penetrating between the first surface and the second surface); a second ceramic layer having a first surface and a second surface and having a second through hole penetrating therethrough; a first through-hole conductor provided on an inner peripheral surface of the first through hole; a first peripheral portion provided on the second surface of the first ceramic layer at a periphery of the first through hole and connected to the first through-hole conductor; a second through-hole conductor provided on an inner peripheral surface of the second through hole; a second peripheral portion provided on the first surface of the second ceramic layer at a periphery of the second throughhole and connecting the second through-hole conductor and the first peripheral portion; and an opening formed by an inner peripheral surface of the first through-hole conductor and an inner peripheral surface of the second through-hole conductor, wherein the first peripheral portion and the second peripheral portion respectively have mutually overlapping adhered portions and separated portions opposing each other through a gap continuing to the opening, and wherein a relationship L 1 &gt;S 1  is satisfied, where L 1  represents maximum length of the adhered portion, and S 1  represents a maximum length of the separated portion. 
     According to the above-described aspect of the invention, since the first peripheral portion and the second peripheral portion are connected to one another by the adhered portion having a maximum length L 1  greater than the maximum length S 1  of the separated portion, the reliability of the electrical connection between the first through-hole conductor and the second through-hole conductor is high. Accordingly, a highly reliable gas sensor can be made. Furthermore, to further enhance the reliability of electrical connection, L 1  is preferably set to not less than three times S 1 . In addition, the maximum length L 1  of the adhered portion should preferably be set to from 60 μm to 200 μm. 
     The “gas sensor” of the invention may be embodied, for example, as an oxygen sensor, an air-fuel ratio sensor, an NO x  sensor, a CO 2  sensor and the like, as long as the above-described requirements are satisfied. 
     In addition, the term “peripheral portion” means a through-hole conductor which is provided at a periphery of a through hole and is connected to a conductor provided in the through hole, and a circular shape, an elliptical shape, a rectangular shape, or the like can be selected as the shape of the peripheral portion. 
     Further, in the above-described gas sensor, the adhered portion, as viewed in the longitudinal direction, may be longer on a leading end side of the opening than on a base end side of the opening. 
     In a case where the gas detecting element has an elongated shape, a larger space can be secured in the longitudinal direction than in the widthwise direction. For this reason, the first peripheral portion and the second peripheral portion should preferably be formed into an elliptical or rectangular shape which is elongated in the longitudinal direction. In addition, the aforementioned opening is disposed on the base end side of the gas detecting element. Accordingly, if the longitudinal width of the adhered portion is made longer on the leading end side than on the base end side, the opening can be advantageously disposed closer to the base end of the gas detecting element. 
     Further, the above-described gas sensor may preferably further comprise: an electrode pad electrically connected to the first through-hole conductor and provided on an outer surface of the gas detecting element; and a connection terminal abutting against the electrode pad so as to be electrically connected to the electrode pad, wherein an abutment position between the connection terminal and the electrode pad is longitudinally offset from the first peripheral portion and the second peripheral portion. 
     In the portion where the first and second peripheral portions are located, the overall thickness of the gas detecting element increases by the thickness of the overlapping peripheral portions in some instances. Accordingly, if the connection terminal and the electrode pad abut immediately above the first and second peripheral portions, the connection reliability between them is possibly impaired. Accordingly, the abutment position between the connection terminal and the electrode pad is longitudinally offset from the first peripheral portion and the second peripheral portion, thereby making it possible to ensure connection reliability between the connection terminal and the electrode pad. 
     Particularly in the case where the longitudinal width of the adhered portion is set to be longer on the leading end side of the opening than on the base end side thereof, if the abutment position between the connection terminal and the electrode pad, as viewed in the longitudinal direction, is located more on the base end side than the opening, it is possible to easily ensure the connection reliability between the connection terminal and the electrode pad. In addition, according to the above-described structure, the abutment position between the connection terminal and the electrode pad can be easily set close to the opening, so that it is possible to enhance the degree of freedom of design. More preferably, the distance between the opening and the abutment position between the connection terminal and the electrode pad can be set in the range of 30 to 200 μm. 
     In addition, according to another aspect for attaining the above-described object, the present invention provides a gas sensor having a gas detecting element extending in a longitudinal direction and in which a plurality of ceramic layers are stacked, and wherein a detecting portion is provided at a leading end side thereof, the gas detecting element comprising: a first ceramic layer having a first surface and a second surface and having a first through hole penetrating therethrough (i.e., penetrating between the first surface and the second surface); a second ceramic layer having a first surface and a second surface and stacked on a side of the second surface of the first ceramic layer; a first through-hole conductor provided on an inner peripheral surface of the first through hole; a first peripheral portion provided on the second surface of the first ceramic layer at a periphery of the first through hole and connected to the first through-hole conductor; a second connecting portion provided on the first surface of the second ceramic layer and connected to the first peripheral portion so as to close the first through hole; and an opening formed by an inner peripheral surface of the first through-hole conductor, wherein the first peripheral portion and the second connecting portion respectively have mutually overlapping adhered portions and separated portions opposing each other through a gap continuing to the opening, and a relationship L 2 &gt;S 2  is satisfied, where L 2  represents a maximum length of the adhered portion, and S 2  represents a maximum length of the separated portion. 
     According to the above-described aspect of the invention, since the first peripheral portion and the second connecting portion are connected to one another by the adhered portion having a maximum length L 2  greater than the maximum length S 2  of the separated portion, the reliability of the electrical connection between the first through-hole conductor and the second through-hole conductor is high. Accordingly, a highly reliable gas sensor can be made. Furthermore, to further enhance the reliability of electrical connection, L 2  is preferably set to not less than three times S 2 . In addition, the maximum length (maximum adhesion width) L 2  of the adhered portion should preferably be 60 μm to 200 μm. 
     Further, in the above-described gas sensor, the adhered portion, as viewed in the longitudinal direction, may be longer on the leading end side of the opening than on the base end side of the opening. 
     In the case where the gas detecting element has an elongated shape, a larger space can be secured in the longitudinal direction than in the widthwise direction. For this reason, the third connecting portion and the first peripheral portion should preferably be formed into an elliptical or rectangular shape which is elongated in the longitudinal direction. In addition, the aforementioned opening is disposed on the base end side of the gas detecting element. Accordingly, if the longitudinal width of the adhered portion is made longer on the leading end side than on the base end side, the opening can be advantageously disposed closer to the base end of the gas detecting element. 
     The above-described gas sensor may preferably further comprise: an electrode pad electrically connected to the second through-hole conductor and provided on an outer surface of the gas detecting element; and a connection terminal abutting the electrode pad so as to be electrically connected to the electrode pad, wherein an abutment position between the connection terminal and the electrode pad is longitudinally offset from the second connecting portion and the first peripheral portion. 
     In the portion where the first peripheral portion and the second connecting portion are located, the overall thickness of the gas detecting element increases by the thickness of the overlapping peripheral portions in some cases. Accordingly, if the connection terminal and the electrode abut immediately above the first peripheral portion and the second connecting portion, the connection reliability between them is possibly impaired. Accordingly, the abutment position between the connection terminal and the electrode pad is longitudinally offset from the first peripheral portion and the second connecting portion, thereby making it possible to ensure the connection reliability between the connection terminal and the electrode pad. 
     Particularly in the case where the longitudinal width of the adhered portion is set to be longer on the leading end side of the opening than on the base end side thereof, if the abutment position between the connection terminal and the electrode pad, as viewed in the longitudinal direction, is located more on the base end side than the opening, it is possible to easily ensure the connection reliability between the connection terminal and the electrode pad. In addition, according to the above-described structure, the abutment position between the connection terminal and the electrode pad can be easily set close to the opening, so that it is possible to enhance the degree of freedom of design. More preferably, the distance between the opening and the abutment position between the connection terminal and the electrode pad can be set in the range of 30 to 200 μm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal cross-sectional view of an oxygen sensor in accordance with an embodiment of the invention; 
         FIG. 2  is an exploded perspective view of a gas detecting element in accordance with the embodiment; 
         FIG. 3  is an explanatory diagram illustrating a schematic structure of a first opening  206  of the gas detecting element and vicinity thereof in accordance with the embodiment; 
         FIG. 4  is an explanatory diagram illustrating a schematic structure of a second opening  207  of the gas detecting element and vicinity thereof in accordance with the embodiment; 
         FIG. 5  is an explanatory diagram illustrating a schematic structure of an opening  208  of the gas detecting element and vicinity thereof in accordance with the embodiment; 
         FIG. 6  is an explanatory diagram illustrating the first opening  206  where a gap is formed during sintering in the gas detecting element in accordance with the embodiment; 
         FIG. 7  is an explanatory diagram illustrating the second opening  207  where a gap is formed during sintering in the gas detecting element in accordance with the embodiment; 
         FIG. 8  is an explanatory diagram illustrating the opening  208  where a gap is formed during sintering in the gas detecting element in accordance with the embodiment; 
         FIG. 9  is an explanatory diagram illustrating a schematic structure of an opening of a gas detecting element and vicinity thereof in accordance with the conventional art; 
         FIG. 10  is an explanatory diagram illustrating an opening where a gap is formed in the structure of  FIG. 9  during sintering, and relates to a technical problem addressed by the present invention; 
         FIG. 11  is an explanatory diagram illustrating a schematic structure of a through hole of a gas detecting element and vicinity thereof in accordance with the conventional art; and 
         FIG. 12  is an explanatory diagram illustrating a through hole where a gap is formed in the structure of  FIG. 10  during sintering, and relates to a technical problem addressed by the present invention. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
     Reference numerals used to identify various structural features in the drawings include the following.
       100 : oxygen sensor (gas sensor)     139 : connection terminal     200 : gas detecting element     200   a : leading end of the gas detecting element     200   b : base end of the gas detecting element     201 : sensor portion     206 : first opening     207 : second opening     208 : third opening     211 : first solid electrolyte layer (fourth ceramic layer)     211   h : throughhole     213 : first electrode     215 : second electrode     215   e ,  219 : peripheral portion     221 : second solid electrolyte layer (second ceramic layer)     221   h   1 ,  221   h   2 : through hole (second through hole)     222 ,  229 : peripheral portion (second peripheral portion)     223 : third electrode     223   c : connecting portion (second connecting portion)     224 ,  225   c ,  233   e ,  235   e : peripheral portion     226 ,  227 : through-hole conductor (second through-hole conductor)     231 : insulating layer (third ceramic layer)     231   h   1 ,  231   h   2 : through hole     233   d ,  235   d : through-hole conductor     233   f ,  235   f : connecting portion     241 : protective layer (first ceramic layer)     241   h   1 ,  241   h   2 ,  241   h   3 : through hole (first through hole)     243 ,  244 ,  245 : electrode pad     247 ,  248 ,  249 : peripheral portion (first peripheral portion)     251 : heater portion     253 : fifth ceramic layer (ceramic layer)     255 : sixth ceramic layer (ceramic layer)     261 : heater-use outer connection pad     271 ,  272 ,  273 : through-hole conductor (first through-hole conductor)   C 1 , C 2 , C 3 , C 4 , C 5 : adhered portion   G 1 , G 2 , G 3 , G 4 , G 5 : separated portion   L 1 , L 2 : maximum length (width) of the adhered portion   S 1 , S 2 : maximum length (width) of the separated portion   t 1 , t 2 , t 3 : abutment position   

     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, a detailed description will be given of an embodiment of the invention. However, the present invention should not be construed as being limited thereto. 
       FIG. 1  shows an oxygen sensor (gas sensor)  100  in accordance with this embodiment. This oxygen sensor  100  is mounted on an exhaust pipe (not shown) of an automobile to detect the oxygen concentration in exhaust gas. In  FIG. 1 , the lower side of this oxygen sensor  100  is a leading end side, and the upper side thereof is a base end side. This oxygen sensor  100  comprises a gas detecting element  200 , a cylindrical metal shell  103  for holding this gas detecting element  200  in its interior, a protector  125  fitted to a predetermined portion on the leading end side of this metal shell  103 , and a cylindrical casing  131  connected to a predetermined portion on the base end side of this metal shell  103 , among other structural components. 
     Of these, the gas detecting element  200  has a leading end  200   a  and a base end  200   b , and is a rectangular plate-shaped stacked-type element in which a plurality of ceramic layers are stacked, its size being approx. 40 mm long, approx. 5 mm wide, and approx. 1.2 mm thick. As shown in the exploded perspective view of  FIG. 2 , this gas detecting element  200 , when classified into its major components, includes a sensor portion  201  capable of sensing the oxygen concentration and a heater portion  251  capable of heating the sensor portion  201 . In  FIG. 2 , the left side of this gas detecting element  200  is the leading end side, and the right side thereof is the base end side. 
     The sensor portion  201  has an oxygen concentration detection cell  203  and an oxygen pump cell  205 . 
     The oxygen concentration detection cell  203  has a first solid electrolyte layer (fourth ceramic layer)  211  composed of a sintered compact of partially stabilized zirconia. A first electrode  213  is formed on an obverse surface  211   a  of this first solid electrolyte layer  211 , and a second electrode  215  is formed on a reverse surface thereof The first electrode  213  includes of a first electrode portion  213   a  disposed on the leading end side, an elliptical connecting portion  213   c  disposed on the base end side and extending in the longitudinal direction of the gas detecting element  200 , and a lead portion  213   b  connecting the first electrode portion  213   a  and the connecting portion  213   c . Meanwhile, the second electrode  215  includes a second electrode portion  215   a  disposed on the leading end side, an elliptical peripheral portion  215   c  disposed on the base end side and extending in the longitudinal direction, and a lead portion  215   b  connecting the second electrode portion  215   a  and the peripheral portion  215   c.    
     In addition, a through hole  211   h  is provided at a predetermined position on the base end side of the first solid electrolyte layer  211  (see also  FIG. 3 ). Further, a tubular through-hole conductor  217  is formed on the inner peripheral surface of the through hole  211   h . A peripheral portion  215   c  is provided on the reverse surface  211   b  at the periphery of the through hole  211   h  so as to connect to the through-hole conductor  217 . 
     In addition, an elliptical peripheral portion  219  extending in the longitudinal direction is provided on the observe surface  211   a  of the first solid electrolyte layer  211  at the periphery of the through hole  211   h  so as to be connected to the through-hole conductor  217 . 
     Next, a description will be given of the oxygen pump cell  205  (see  FIG. 2 ). The oxygen pump cell  205  comprises a second solid electrolyte layer (second ceramic layer)  221  composed of a sintered compact of partially stabilized zirconia, a third electrode  223  formed on its obverse surface  221   a , and a fourth electrode  225  formed on its reverse surface  221   b . The third electrode  223  includes a third electrode portion  223   a  disposed on the leading end side, an elliptical peripheral portion (second connecting portion)  223   c  disposed on the base end side and extending in the longitudinal direction, and a lead portion  223   b  connecting the third electrode portion  223   a  and the peripheral portion  223   c . Meanwhile, the fourth electrode  225  includes a fourth electrode portion  225   a  disposed on the leading end side, an elliptical peripheral portion  225   c  disposed on the base end side and extending in the longitudinal direction, and a lead portion  225   b  connecting the fourth electrode portion  225   a  and the peripheral portion  225   c.    
     In addition, two through holes (second through holes)  221   h   1  and  221   h   2  are provided at predetermined positions on the base end side of the second solid electrolyte layer  221 . In addition, elliptical peripheral portions (second peripheral portions)  222  and  229  extending in the longitudinal direction are provided on the obverse surface  221   a  of the second solid electrolyte layer  221  at the peripheries of the through holes  221   h   1  and  221   h   2 . Meanwhile, elliptical peripheral portions  224  and  225   c  extending in the longitudinal direction are provided on the reverse surface  221   b  of the second solid electrolyte layer  221  at the peripheries of the through holes  221   h   1  and  221   h   2 . Further, a tubular through-hole conductor  226  is formed on the inner peripheral surface of the through hole  221   h   1  so as to connect the peripheral portion  222  and the peripheral portion  224  (see  FIG. 3 ). In addition, a tubular through-hole conductor (second through-hole conductor)  227  is formed on the inner peripheral surface of the through hole  221   h   2  so as to connect the peripheral portion  229  and the peripheral portion  225   c  (see  FIG. 4 ). 
     Returning to  FIG. 2 , an insulating layer (third ceramic layer)  231  (having an obverse surface  231   a  and a reverse surface  231   b ) whose main constituent is alumina is stacked between the oxygen concentration detection cell  203  and the oxygen pump cell  205  described above. This insulating layer  231  includes an insulating portion  231   f  occupying a major portion thereof and a pair of porous diffusion rate controlling portions  231   g  disposed at predetermined positions on the leading end side. A gas measurement chamber  231   c  of a rectangular shape in plan view is penetratingly formed in the insulating layer  231  at a position corresponding to both the first electrode portion  213   a  of the oxygen concentration detection cell  203  and the fourth electrode portion  225   a  of the oxygen pump cell  205 . This gas measurement chamber  231   c  communicates with the outside through the pair of diffusion rate controlling portions  231   g  at both widthwise end portions of the insulating layer  231 . In this manner, the diffusion rate controlling portions  231   g  are able to control the diffusion at a time when detection gas flows into the gas measurement chamber  231   c.    
     In addition, two elliptical through holes  231   h   1  and  231   h   2  extending in the longitudinal direction are formed on the base end side of the insulating layer  231 . In addition, peripheral portions  233   e  and  235   e  are formed on the obverse surface  231   a  of the insulating layer  231  at the peripheries of the through holes  231   h   1  and  231   h   2 . Additionally, connecting portions  233   f  and  235   f  are overlappingly provided on the peripheral portion  219  and the connecting portion  213   c  which are respectively exposed in the through holes  231   h   1  and  231   h   2 . Further, a through-hole conductor  233   d  whose cross section is U-shaped is formed in the through hole  231   h   1  at a portion on the leading end side of its inner peripheral wall so as to connect the peripheral portion  233   e  and the connecting portion  233   f  (see  FIG. 3 ). Meanwhile, a through-hole conductor  235   d  whose cross section is U-shaped is formed also in the through hole  231   h   2  at a portion on the leading end side of its inner peripheral wall so as to connect the peripheral portion  235   e  and the connecting portion  235   f  (see  FIG. 4  also). 
     Returning to  FIG. 2 , a description will be given of a protective layer (first ceramic layer)  241 . The protective layer  241  whose main constituent is alumina is stacked on the obverse surface  221   a  of the second solid electrolyte layer  221 . This protective layer  241  includes a porous electrode protecting portion  241   e  disposed in correspondence with the third electrode portion  223   a  as well as a reinforcing portion  241   d  occupying the remaining portion. The electrode protecting portion  241   e  covers the third electrode portion  223   a  to prevent and suppress poisoning. In addition, the reinforcing portion  241   d  covers and protects the second solid electrolyte layer  221 . A chamfered portion  200   bc  is formed at the base end of the protective layer  241 . 
     In addition, three through holes (first through holes)  241   h   1 ,  241   h   2  and  241   h   3  are provided on the base end side of the protective layer  241 . Three electrode pads  243 ,  244  and  245 , which extend in the longitudinal direction, are formed on the obverse surface  241   a  of the protective layer  241  at the peripheries of the through holes  241   h   1 ,  241   h   2  and  241   h   3  so as to be juxtaposed in the widthwise direction. Meanwhile, three elliptical peripheral portions (first peripheral portions  247 ,  248  and  249  extending in the longitudinal direction are formed on a reverse surface  241   b  of the protective layer  241  at the peripheries of the through holes  241   h   1 ,  241   h   2  and  241   h   3  in a juxtaposed manner. Further, a through-hole conductor  271  is formed on the inner peripheral surface of the through hole  241   h   1  to connect the electrode pad  243  and the peripheral portion  247  (see  FIG. 3 ). Additionally, a through-hole conductor  272  is formed on the inner peripheral surface of the through hole  241   h   2  to connect the electrode pad  244  and the peripheral portion  248  (see  FIG. 4 ). Furthermore, a tubular through-hole conductor  273  is formed on the inner peripheral surface of the through hole  241   h   3  to connect the electrode pad  245  and the peripheral portion  249  (see  FIG. 5 ). 
     Three openings  206 ,  207  and  208  are formed in the sensor portion  201  of the gas detecting element  200 . 
     Of these, the first opening  206  is formed as the inner peripheral surfaces of the through-hole conductor  217 , the through-hole conductor  233 d, the through hole  231   h   1 , the through-hole conductor  226 , and the through-hole conductor  271  are connected in the thicknesswise direction (see  FIG. 3 ). Meanwhile, the second opening  207  is formed as the inner peripheral surfaces of the through-hole conductor  235   d , the through hole  231   h   2 , the through-hole conductor  227 , and the through-hole conductor  272  are connected in the thicknesswise direction (see  FIG. 4 ). Further, the third opening  208  is formed by the inner peripheral surface of the through-hole conductor  273 . 
     Next, returning to  FIG. 2 , a description will be given of the heater portion  251 . The heater portion  251  includes a fifth ceramic layer  253  and a sixth ceramic layer  255  whose main constituent is alumina; a heating element  257  sandwiched between these ceramic layers; and a pair of heater-use outer connection pads  261  and  262  provided on the base end side of a reverse surface  255   b  of the second ceramic layer  255 . The heating element  257  includes a heating portion  257   a  located on the leading end side; a pair of connecting portions  257   c   1 and  257   c   2  located on the base end side; and a pair of lead portions  257   b   1  and  257   b   2  for connecting the heating portion  257   a  and the connecting portions  257   c   1 and  257   c   2 , respectively. In addition, the sixth ceramic layer  255  has through holes  255   h   1  and  255   h   2  on the base end side. A pair of conductors  259  and  260  are respectively provided on the inner peripheral surfaces of the through holes  255   h   1 and  255   h   2  so as to electrically connect the connecting portions  257   c   1  and  257   c   2  and the heater-use outer connection pads  261  and  262 , respectively. 
     Next, returning to  FIG. 1 , a description will be given of the construction of other portions of the gas sensor  100 . The metal shell  103  is formed of SUS  430 , and has on its outer side an externally threaded portion  105  for installing the gas sensor  100  on the exhaust pipe as well as a hexagonal engaging portion  107  for engaging a tool during the installation. Further, an inner stepped portion  109  protruding radially inwardly is provided on the inner side of the metal shell  103 . This inner stepped portion  109  supports a metal holder  111  for holding the gas detecting element  200 . Further, a ceramic holder  113  and a talc filled layer  115  for locating the gas detecting element  200  in position are disposed on the inner side of this metal holder  111  sequentially from the leading end side. This talc filled layer  115  consists of two layers, a first talc filled layer  116  located on the leading end side and a second talc filled layer  117  located on the base end side. An alumina-made sleeve  119  is disposed on the base end side of the second talc filled layer  117 . The sleeve  119  is formed into a multi-stage cylindrical shape, and the gas detecting element  200  is inserted into its axial hole  119   h . A crimped portion  110  located on the base end side of the metal shell  103  is bent inwardly, thereby pressing the sleeve  119  toward the leading end side of the metal shell  103  by means of a stainless steel-made ring member  121 . 
     In addition, the metallic protector  125  for covering a leading end portion  200   s  of the gas detecting element  200  projecting from the leading end of the metal shell  103  is welded to an outer periphery of the leading end of the metal shell  103 . This protector  125  has a dual structure comprising a bottomed cylindrical outer protector  126  located on the outer side and a bottomed cylindrical inner protector  127  located on the inner side. A plurality of gas inlet holes  126   k  and  127   k  for respectively introducing the exhaust gases into the interior are provided in the outer protector  126  and the inner protector  127 . 
     Meanwhile, the cylindrical casing  131  made of SUS  430  is welded to the base end side of the metal shell  103 . A separator  135  is disposed on the inner side of the casing  131 . The separator  135  is fixed to the casing  131  by means of a holding member  137  interposed between the separator  135  and the casing  131 . In addition, a plurality of connection terminals  139  for electrically connecting to the gas detecting element  200 , as well as a plurality of lead wires  141  whose one ends are electrically connected to these connection terminals  139  and which extend outside the base end side of the gas sensor  100 , are disposed on the separator  135 . In addition, a cylindrical rubber cap  143  for closing a base end-side opening  131   c  of the casing  131  is disposed on the base end side of the separator  135 . The rubber cap  143  is fixed to the casing  131  by crimping the outer periphery of the casing  131  radially inwardly while being fitted to the casing  131 . A plurality of insertion holes  143   h  are provided in the rubber cap  143 , and the aforementioned plurality of lead wires  141  are respectively inserted therein. 
     Next, a description of specific structural portions of the invention will be given with reference to  FIGS. 6 to 8 .  FIGS. 6 to 8  respectively correspond to  FIGS. 3 to 5 , and show the respective structural features in enlarged form. The gas detecting element  200  is formed by first forming an unsintered stacked body by a method such as a conventionally known sheet stacking technique and a conductor paste printing technique, and by subsequently simultaneously sintering the unsintered ceramic green sheet and a conductor paste whose main constituent is platinum and which has been formed between its layers and in the through holes by printing. At the time of sintering, there are cases where, in that portion of the mutually overlapped conductor paste which is adjacent to the through hole, the applied portions of the conductor paste peel off and a gap is produced. This occurs because the amount of sintering shrinkage differs between the ceramic green sheet and the unsintered metallizing material. 
     First, as shown in  FIG. 6 , separated portions G 1  and G 2 , which are located adjacent to the first opening  206  and are respectively opposed to each other with a gap therebetween, are formed between the peripheral portion (first peripheral portion)  247  and the peripheral portion (second peripheral portion)  222  and between the peripheral portion  224  and the peripheral portion  233   e  which mutually overlap. In  FIG. 3 , the separated portions G 1  and G 2  are not shown. 
     However, even if the separated portions G 1  and G 2  are formed, the peripheral portion  247  and the peripheral portion  222  have mutually overlapping adhered portions C 1 , and the peripheral portion  224  and the peripheral portion  233   e  similarly have mutually overlapping adhered portions C 2 . Moreover, a maximum length L 1  of the adhered portions C 1  and C 2  is set to be greater than a maximum length S 1  of the separated portions G 1  and G 2 . Accordingly, sufficient connection reliability is ensured between the peripheral portion  247  and the peripheral portion  222  and between the peripheral portion  224  and the peripheral portion  233   e.    
     It should be noted that the adhered portion C 1  is formed not only on the leading end side of the first opening  206  but also on the base end side thereof. However, in this embodiment, the width of the adhered portion C 1  is set to be longer on the leading end side and very small on the base end side. As a result, as shown in  FIG. 3 , the first opening  206  can be set closer to the base end  200   b  of the gas detecting element  200 . Specifically, the distance between the first opening  206  and the base end  200   b  is preferably set to be not greater than 3 mm. It should be noted that, in this embodiment, the distance between the first opening  206  and the base end  200   b  is set to be approximately 1.5 mm. 
     Because the first opening  206  is thus set close to the base end  200   b , the electrode pad  243  is provided more on the leading end side than the first opening  206 . As shown in  FIG. 3 , an abutment position t 1  between the connection terminal  139  and the electrode pad  243  is located more on the leading end side than such as the peripheral portion  247  including the adhered portion C 1 . Namely, since the connection terminal  139  and the electrode pad  243  are in contact with one another at a portion which avoids an irregularity on the electrode pad  243  caused by the thickness portion of the peripheral portion  247  and the like, the connection reliability between the connection terminal  139  and the electrode pad  243  can be improved. 
     Next, as for the second opening  207 , as shown in  FIG. 7 , separated portions G 3  and G 4 , which are respectively opposed to one another with a gap therebetween, are formed between the peripheral portion (first peripheral portion)  248  and the peripheral portion (second peripheral portion)  229  and between the peripheral portion  225   c  and the peripheral portion  235   e  which mutually overlap. In  FIG. 4 , the separated portions G 3  and G 4  are not shown. 
     However, even if the separated portions G 3  and G 4  are formed, the peripheral portion  248  and the peripheral portion  229  have mutually overlapping adhered portions C 3 , and the peripheral portion  225   c  and the peripheral portion  235   e  similarly have mutually overlapping adhered portions C 4 . Moreover, a maximum length L 1  of the adhered portions C 3  and C 4  is set to be greater than a maximum length S 1  of the separated portions G 3  and G 4 . Accordingly, sufficient connection reliability is ensured between the peripheral portion  248  and the peripheral portion  229  and between the peripheral portion  225   c  and the peripheral portion  235   e.    
     As shown in  FIG. 7 , the adhered portion C 3  is formed not only on the leading end side of the second opening  207  but also on the base end side thereof. However, in this embodiment, the width of the adhered portion C 3  is set to be longer on the leading end side and very small on the base end side. Further, the electrode pad  244  is provided on the base end side of the second opening  207 . As the abutment position t 2  between the connection terminal  139  and the electrode pad  244  is thus located on the base end side where the adhered portion C 3  is shorter (see  FIG. 4 ), it becomes easy to avoid the irregularity occurring on the electrode pad  244  due to the thickness portion of the peripheral portion  248  and the like. In addition, the distance between the second opening  207  and the abutment position t 2  can be made smaller than the distance between the first opening  206  and the abutment position t 1 . Specifically, the distance between the second opening  207  and the abutment position t 2  is 180 μm, while the distance between the first opening  206  and the abutment position t 1  is 220 μm. 
     Next, as for the third opening  208 , as shown in  FIG. 8 , separated portions G 5 , which are opposed to one another with a gap therebetween, are formed between the peripheral portion (first peripheral portion)  249  and the connecting portion (second connecting portion)  223   c  which mutually overlap. In  FIG. 5 , the separated portions  5  are not shown. 
     However, even if the separated portions G 5  are formed, the peripheral portion  249  and the peripheral portion  223   c  have mutually overlapping adhered portions C 5 . Moreover, a maximum length L 2  of the adhered portions C 5  is set to be greater than a maximum length S 2  of the separated portions G 5 . Accordingly, sufficient connection reliability is ensured between the peripheral portion  249  and the peripheral portion  223 . Consequently, it is possible to ensure reliability of the electrical connection between the through-hole conductor (first through-hole conductor)  237  and the connecting portion  223   c.    
     As shown in  FIG. 8 , the adhered portion C 5  is formed not only on the leading end side of the through hole (first through hole)  241   h   3  but also on the base end side thereof However, in this embodiment, the width of the adhered portion C 5  is set to be longer on the leading end side and very small on the base end side. Further, the electrode pad  245  is provided on the base end side of the opening  208 . As the abutment position t 3  between the connection terminal  139  and the electrode pad  245  is thus located on the base end side where the adhered portion C 5  is shorter (see  FIG. 5 ), it becomes easy to avoid the irregularity occurring on the electrode pad  245  due to the thickness portion of the peripheral portion  249  and the like. In addition, the distance between the opening  208  and the abutment position t 3  can be made smaller than the distance between the first opening  206  and the abutment position t 1 . Specifically, the distance between the opening  208  and the abutment position t 3  is 180 μm, while the distance between the first opening  206  and the abutment position t 1  is 220 μm. 
     The aforementioned maximum lengths S 1  of the separated portions G 1  to G 5  are 20 to 55 μm, respectively, while the maximum lengths L 1  of the adhered portions C 1  to C 5  are 60 μm to 200 μm, respectively. The maximum lengths S 1  of the adhered portions C 1  to C 5  are not less than three times the lengths of the separated portions G 1  to G 5 , respectively. 
     In addition, the lengths of the peripheral portions  219 ,  224 ,  222 ,  225   c ,  229 ,  247 ,  248  and  249  are set to be greater on the leading end side than the lengths on the base end side by using their respective through holes as references. Specifically, their lengths on the leading end side are respectively 1.6 mm, and their lengths on the base end side are respectively 0.2 mm. 
     It should further be apparent to those skilled in the art that various changes in form and detail of the invention as shown and described above may be made. It is intended that such changes be included within the spirit and scope of the claims appended hereto. 
     This application is based on Japanese Patent Application JP 2006-195784, filed Jul. 18, 2006, the entire content of which is hereby incorporated by reference, the same as if set forth at length.