Patent Publication Number: US-2021181140-A1

Title: Gas sensor element

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation application of International Application No. PCT/JP2019/031225 filed on Aug. 7, 2019, which is based on and claims the benefit of priority from Japanese Patent Application No. 2018-156179 filed on Aug. 23, 2018. The contents of these applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to a gas sensor element. 
     A gas sensor is disposed in the exhaust pipe of an internal combustion engine, for example, and is used to detect the concentration of a specific gas component, such as the oxygen concentration of the exhaust gas flowing through the exhaust pipe. 
     SUMMARY 
     According to one aspect, the present disclosure provides a gas sensor element comprising 
     an electrolyte layer provided with a holding plate having a placement hole, and a solid electrolyte body disposed in the placement hole, 
     a first insulator laminated on one side of the electrolyte layer, 
     a second insulator laminated on the other side of the electrolyte layer, 
     a measurement gas chamber that is surrounded by the electrolyte layer and the first insulator, and 
     a reference gas chamber that is surrounded by the electrolyte layer and the second insulator, 
     wherein: 
     at least a part of a boundary portion between the placement hole and the solid electrolyte body is sandwiched between a first sandwiching portion of the first insulator and a second sandwiching portion of the second insulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objectives and other objectives, features and advantages of the present disclosure are made clearer by the following detailed description, referring to the appended drawings. In the drawings: 
         FIG. 1  is a cross-sectional view orthogonal to the longitudinal direction of the gas sensor element of the first embodiment, 
         FIG. 2  is a cross-sectional view orthogonal to the width direction of the gas sensor element of the first embodiment, 
         FIG. 3  is a cross-sectional view taken along the line III-III shown in  FIG. 2 , 
         FIG. 4  is an exploded perspective view of respective layers in the gas sensor element of the first embodiment, 
         FIG. 5  is a partial cross-sectional view parallel to the axial direction of the gas sensor of the first embodiment, 
         FIG. 6  is a cross-sectional view orthogonal to the width direction of the gas sensor element of a second embodiment, 
         FIG. 7  is a cross-sectional view taken along the line VII-VII shown in  FIG. 6 , 
         FIG. 8  is an exploded perspective view of respective layers in the gas sensor element of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The inventor of the present disclosure has studied a gas sensor element in which a solid electrolyte body does not readily become displaced from a placement hole. 
     As disclosed in JP 2010-145214 A, a gas sensor is disposed in the exhaust pipe of an internal combustion engine, for example, and is used to detect the concentration of a specific gas component, such as the oxygen concentration of the exhaust gas flowing through the exhaust pipe. The gas sensor described in JP 2010-145214 A includes a holding plate having a placement hole formed therein in the thickness direction, and an electrolyte layer having a solid electrolyte body that is disposed in the placement hole. 
     The gas sensor described in JP 2010-145214 A has a pair of surface alumina layers disposed on respective sides of the holding plate in the thickness direction, at the boundary portion between the placement hole and the solid electrolyte body. It is attempted in that way to prevent the solid electrolyte body from detaching from the alumina sheets. 
     With the gas sensor element described in JP 2010-145214 A, the thickness of the pair of surface alumina layers is relatively small and their strength is low. Hence there is scope for improvement, from the aspect of stability of holding the solid electrolyte body in the placement hole. 
     The present disclosure is intended to provide a gas sensor element in which a solid electrolyte body does not readily become displaced from a placement hole. 
     According to one aspect, the present disclosure provides a gas sensor element comprising 
     an electrolyte layer provided with a holding plate having a placement hole formed in a thickness direction (Z), and a solid electrolyte body disposed in the placement hole and having oxygen ion conductivity, 
     a first insulator laminated on one side of the electrolyte layer, 
     a second insulator laminated on the other side of the electrolyte layer, 
     a measurement gas chamber that is surrounded by the electrolyte layer and the first insulator, and into which a gas (G) to be measured is introduced, and 
     a reference gas chamber that is surrounded by the electrolyte layer and the second insulator, and into which a reference gas (A) is introduced, 
     wherein: 
     at least a part of a boundary portion between the placement hole and the solid electrolyte body is sandwiched between a first sandwiching portion of the first insulator and a second sandwiching portion of the second insulator, 
     the first sandwiching portion is formed, in the thickness direction, at a position overlapping the boundary portion, and is formed, at least in the thickness direction, in the entire region where the measurement gas chamber is disposed, and 
     the second sandwiching portion is formed, in the thickness direction, at a position overlapping the boundary portion, and is formed, at least in the thickness direction, in the entire region where the reference gas chamber is disposed. 
     In the gas sensor element of the above embodiment, at least a part of the boundary portion between the placement hole of the holding plate and the solid electrolyte body is sandwiched between a first sandwiching portion of the first insulator and a second sandwiching portion of the second insulator. The first sandwiching portion is formed on at least the entire region of the measurement gas chamber, in the thickness direction of the holding plate. Furthermore, the second sandwiching portion is formed on least the entire region of the reference gas chamber in the thickness direction. That is, at least a part of the boundary portion is sandwiched between the first sandwiching portion and the second sandwiching portion, which are formed of structures having relatively high rigidity. Hence, since it is easy to ensure rigidity of the first sandwiching portion and the second sandwiching portion which sandwich at least a part of the boundary portion, it is easy to stably hold the solid electrolyte body in the placement hole. 
     As described above, according to the above aspect, it is possible to provide a gas sensor element in which the solid electrolyte body does not easily become displaced from the placement hole. 
     First Embodiment 
     An embodiment of a gas sensor element will be described with reference to  FIGS. 1 to 5 . 
     As shown in  FIGS. 1 and 2 , the gas sensor element  1  of the present embodiment has an electrolyte layer  2 , a first insulator  3 , a second insulator  4 , a measurement gas chamber  5 , and a reference gas chamber  6 . The electrolyte layer  2  includes a holding plate  22  and a solid electrolyte body  21 . The holding plate  22  includes a placement hole  220  formed through the holding plate  22  in the thickness direction thereof (hereinafter, referred to as the Z direction). The solid electrolyte body  21  is disposed in the placement hole  220 . The solid electrolyte body  21  has oxygen ion conductivity. The first insulator  3  is laminated on one side of the electrolyte layer  2 . The second insulator  4  is laminated on the other side of the electrolyte layer  2 . The measurement gas chamber  5  is surrounded by the electrolyte layer  2  and the first insulator  3 . As shown in  FIG. 2 , the gas G to be measured is introduced into the measurement gas chamber  5 . The reference gas chamber  6  is surrounded by the electrolyte layer  2  and the second insulator  4 . The reference gas A is introduced into the reference gas chamber  6 . 
     As shown in  FIGS. 1 and 2 , at least a part of the boundary portion  23  between the placement hole  220  and the solid electrolyte body  21  is sandwiched between a first sandwiching portion  33  of the first insulator  3  and a second sandwiching portion  45  of the second insulator  4 . The first sandwiching portion  33  is formed at a position overlapping the boundary portion  23 , in the Z direction. Furthermore, the first sandwiching portion  33  is formed in at least the entire region where the measurement gas chamber  5  is disposed, in the Z direction. The second sandwiching portion  45  is formed at a position overlapping the boundary portion  23 , in the Z direction. Furthermore, the second sandwiching portion  45  is formed in at least the entire region where the reference gas chamber  6  is disposed, in the thickness direction. This embodiment is described in detail in the following. 
     As shown in  FIG. 5 , a gas sensor  10  having the gas sensor element  1  is disposed in an exhaust pipe (not shown) of an internal combustion engine. The gas sensor  10  Is an A/F sensor which uses the exhaust gas passing through the exhaust pipe as the gas G to be measured and atmospheric air as the reference gas A, for obtaining the oxygen concentration of the gas G to be measured, and obtains the A/F (air-fuel ratio) of the engine based on this oxygen concentration. More specifically, the gas sensor  10  may be an A/F sensor that quantitatively obtains the air-fuel ratio of the engine by utilizing a limiting current characteristic, based on the diffusion rate of the gas G to be measured. 
     As shown in  FIGS. 1, 2, and 4 , the gas sensor element  1  is formed by laminating the first insulator  3 , the electrolyte layer  2 , and the second insulator  4  in the thickness direction, and sintering them. Hereinafter, the stacking direction of the first insulator  3 , the electrolyte layer  2 , and the second insulator  4  is referred to as the Z direction. Furthermore, in the Z direction, the side of the first insulator  3  with respect to the electrolyte layer  2 , is referred to as the Z1 side, and the opposite side, that is, the side of the second insulator  4  with respect to the electrolyte layer  2 , is referred to as the Z2 side. Furthermore, the longitudinal direction of the gas sensor element  1  is referred to as the X direction. In the X direction, the side at which the gas G to be measured is introduced into the gas sensor element  1  is referred to as the tip end side, and the side at which the reference gas A is introduced is referred to as the base end side. The tip end side is referred to as the X1 side, and the base end side is referred to as the X2 side, as appropriate. Furthermore, the direction orthogonal to both the X direction and the Z direction is referred to as the Y direction. The Y direction is the width direction of the gas sensor element  1 . The X direction, the Y direction and the Z direction are orthogonal to one other. 
     As described above, the electrolyte layer  2  includes a holding plate  22  and a solid electrolyte body  21 . The holding plate  22  has a plate shape that is elongated in the X direction and thick in the Z direction. In  FIGS. 2 to 4 , for convenience, the length of the gas sensor element  1  in the X direction is shown as being less than the actual length. 
     As shown in  FIGS. 2 and 4 , a placement hole  220  is formed in a region of the holding plate  22  on the X1 side, in the X direction. The placement hole  220  has a rectangular shape, elongated in the X direction, and is filled with the solid electrolyte body  21 . 
     The solid electrolyte body  21  is formed of a zirconia-based oxide. The solid electrolyte body  21  comprises zirconia (ZrO 2 ) as a main component (that is, containing 50% or more by mass of zirconia) and is formed of a solid electrolyte such as stabilized zirconia or partially stabilized zirconia in which part of the zirconia is replaced by a rare earth metal element or by an alkaline earth metal element, or the like. Part of the zirconia that constitutes the solid electrolyte body  21  may be replaced by yttria (Y 2 O 3 ), scandia (Sc 2 O 3 ) or calcia (CaO). The holding plate  22  is made of a material having higher thermal conductivity than the solid electrolyte body  21 . 
     As shown in  FIGS. 1 and 2  a measurement electrode  11 , exposed to the gas G to be measured that is introduced into the measurement gas chamber  5 , is disposed on a region of the Z1 side surface of the solid electrolyte body  21 , at the X1 side. Furthermore, a reference electrode  12 , exposed to the reference gas A that is introduced into the reference gas chamber  6 , is disposed on a region of the Z2 side surface of the solid electrolyte body  21 , at the X1 side. The measurement electrode  11  and the reference electrode  12  have facing regions  13  which face one another in the Z direction via the solid electrolyte body  21 . Hereinafter, the side that is opposite the X1 side in the X direction is referred to as the X2 side. 
     As shown in  FIG. 4 , each of the measurement electrode  11  and the reference electrode  12  has an electrode lead portion  14  which extends from the facing region  13  toward the X2 side. The pair of electrode lead portions  14  extend to the vicinity of the end part of the gas sensor element  1 , at the X2 side. The pair of electrode lead portions  14  are connected, via through holes that are formed in the second insulator  4 , to a pair of sensor terminals  15  that are formed on the Z2 side surface of the first insulator  3 . The measurement electrode  11  and the reference electrode  12  are electrically connected to the exterior of the gas sensor element  1  via the pair of sensor terminals  15 . 
     The measurement electrode  11  and the reference electrode  12  contain platinum, as a noble metal exhibiting catalytic activity for oxygen, and zirconia-based oxide, which is the same material as that of the solid electrolyte body  21 . Since the measurement electrode  11  and the reference electrode  12  contain the same materials as the solid electrolyte body  21 , when the electrode material in a paste-like form which constitutes the measurement electrode  11  and the reference electrode  12  is printed (coated) on the solid electrolyte body  21  and then sintered, bond strength can be readily ensured between the solid electrolyte body  21 , and the measurement electrode  11  and the reference electrode  12 . 
     As shown in  FIGS. 1, 2, and 4 , the first insulator  3 , which is laminated on the Z1 side surface of the electrolyte layer  2 , has a chamber forming portion  32  and a heater embedding portion  31 , stacked in the Z direction. The chamber forming portion  32  is disposed on the Z1 side of the electrolyte layer  2 . As shown in  FIG. 4 , the chamber forming portion  32  has an insulating spacer  321  which is recessed toward the X2 side, forming a recess  320  at the X1 side, and a diffusion resistor portion  322  disposed such as to close the open end of the recess  320  at the X1 side. 
     The diffusion resistor portion  322  is configured to allow the gas G to be measured to pass at a predetermined diffusion rate. The diffusion resistor portion  322  is formed of a porous metal oxide such as alumina. The exhaust gas passes, as the gas G to be measured, through the diffusion resistor portion  322  and is introduced into the measurement gas chamber  5 . It would be equally possible for the diffusion resistor portion  322  to be, for example, a pinhole, i.e., a small through hole that communicates with the measurement gas chamber  5  and with the exterior of the gas sensor element  1 . The position of the diffusion resistor portion  322  is not limited to that shown, and it may be formed at another position in the measurement gas chamber  5 , such as at the Y side. 
     As shown in  FIG. 2 , the spatial region surrounded by the insulating spacer  321  and the diffusion resistor portion  322  in the recess  320  constitutes the measurement gas chamber  5 , by being enclosed between the electrolyte layer  2  and the heater embedding portion  31  from respective sides in the Z direction. That is, a region is disposed in the Z direction within the chamber forming portion  32 , to form the measurement gas chamber  5 . 
     As shown in  FIG. 3 , the measurement gas chamber  5  is formed such as to fit inside the boundary portion  23  between the placement hole  220  and the solid electrolyte body  21 , as viewed in the Z direction. Together with this, as shown in  FIGS. 1 and 2 , the chamber forming portion  32  is formed such as to cover the entire boundary portion  23  from the Z1 side. As a result, the measurement gas chamber  5  and the reference gas chamber  6  can be readily prevented from communicating with one other via the boundary portion  23  due to a decrease in the airtightness of the boundary portion  23  in the electrolyte layer  2 . In  FIG. 3 , the outer shape of the measurement gas chamber  5  is represented by a two-dot chain line outline, and the outer shape of the reference gas chamber  6  is represented by a broken line outline. 
     As shown in  FIGS. 1 and 2 , the length of the measurement gas chamber  5  in the Z direction is less than the thickness of the electrolyte layer  2  in the Z direction. The measurement gas chamber  5  is formed such as to accommodate at least the facing region  13  of the measurement electrode  11 . In the present embodiment, the measurement gas chamber  5  is formed larger than the facing region  13  of the measurement electrode  11  by the amount of one circumference of the facing region  13 , as viewed in the Z direction. 
     As shown in  FIGS. 1, 2, and 4 , a heater embedding portion  31  is laminated on the Z1 side of the chamber forming portion  32 . The heater embedding portion  31  is formed in the outermost part of the gas sensor element  1  at the Z1 side. The heater embedding portion  31  has a pair of embedding plates  311  stacked in the Z direction and a heater  7  embedded between the pair of embedding plates  311 . That is, the heater  7  is embedded in the first insulator  3 . 
     As shown in  FIG. 4 , the heater  7  has a heat generating portion  71  which generates heat when energized, and a pair of lead portions  72  connected to the heat generating portion  71 . As shown in  FIGS. 1 and 2 , at least a part of the heat generating portion  71  is disposed in a region which overlaps the measurement gas chamber  5  in the Z direction. At least a part of the heat generating portion  71  is disposed such as to overlap the facing region  13  of the measurement electrode  11  and the facing region  13  of the reference electrode  12  in the Z direction. One part of the heat generating portion  71  is disposed at a position overlapping the measurement gas chamber  5  in the Z direction, and the other part is disposed at a position overlapping the embedding plates  311  of the chamber forming portion  32  in the Z direction. A part of the heat generating portion  71  is formed on the X2 side of the measurement gas chamber  5 . That is, a part of the heat generating portion  71  overlaps, in the Z direction, with a part of the embedding plates  311  adjacent to the X2 side of the measurement gas chamber  5 . 
     As shown in  FIG. 4 , the heat generating portion  71  has a shape that meanders between opposing sides in the X direction as it heads toward one side in the Y direction. However, the shape of the heat generating portion  71  is not limited to this, and may have, for example, a shape that meanders between opposing sides in the Y direction as it heads toward one side in the X direction. 
     A pair of lead portions  72  are formed extending from respective ends of the heat generating portion  71 . The lead portions  72  are formed such as to extend to the front of the gas sensor element  1 , at the X2 side. The pair of lead portions  72  are connected, via through holes in the embedding plate  311  at the X1 side, to a pair of heater terminals  16  that are formed on the surface of the embedding plate  311  at the X1 side. The heater  7  is electrically connected to the exterior of the gas sensor element  1  from the heater terminals  16 . 
     The cross-sectional area of the heat generating portion  71  orthogonal to the forming direction is smaller than the cross-sectional area of the lead portions  72  orthogonal to the forming direction. The resistance value of the heat generating portion  71  per unit of length is greater than that of the lead portion  72 . The heat generating portion  71  generates heat by Joule heating when a voltage is applied to the pair of lead portions  72 . The heat that is generated thereby heats and activates the solid electrolyte body  21 . 
     As shown in  FIGS. 1, 2, and 4 , the second insulator  4  which is laminated on the Z2 side surface of the electrolyte layer  2  has a duct forming portion  42  and a supporting portion  43 . The duct forming portion  42  is disposed on the Z2 side surface of the electrolyte layer  2 . The duct forming portion  42  is disposed to form the reference gas chamber  6 , by being disposed at a region where the reference gas chamber is formed in the Z direction. The length of the duct forming portion  42  In the Z direction is greater than that of the measurement gas chamber  5 . The duct forming portion  42  is formed by laminating, in the Z direction, three layers having substantially the same shape, and sintering the layers. 
     As shown in  FIG. 4 , the duct forming portion  42  is formed in a U shape that opens on the X2 side. That is, the duct forming portion  42  has a pair of long side portions  421  extending in the X direction and facing one another in the Y direction, and a short side portion  422  extending in the Y direction which connects the pair of long side portions  421  at the X1 side. As shown in  FIG. 2 , the surface of the short side portion  422  on the X2 side has a shape that is curved such as to head toward the X2 side as it heads toward the Z2 side. The duct forming portion  42  is longer than the measurement gas chamber  5 , in the Z direction. 
     As shown in  FIGS. 1 and 2 , the spatial region inside the duct forming portion  42  that is formed by being surrounded by the electrolyte layer  2 , the duct forming portion  42 , and the supporting portion  43 , constitutes the reference gas chamber  6 . As shown in  FIG. 2 , the reference gas chamber  6  is formed such as to extend to the end of the gas sensor element  1  at the X2 side, and is open to the X2 side. Atmospheric air is introduced into the reference gas chamber  6  as the reference gas A, from the open part of the duct forming portion  42  at the X2 side. 
     As shown in  FIG. 1 , both ends of the reference gas chamber  6  are located inside a pair of first boundary portions  231  of the boundary portion  23 , which face one another in the Y direction. The duct forming portion  42  is configured such as to cover the entire pair of first boundary portions  231 , from the Z2 side. Furthermore, as shown in  FIG. 2 , in the X direction, the end of the reference gas chamber  6  at the X1 side is located inside the pair of second boundary portions  232  of the boundary portion  23  which face one another in the X direction. As can be seen from  FIG. 3 , the duct forming portion (see reference sign  42  in  FIGS. 1, 2, and 4 ) is formed such as to cover, from the Z2 side, all of the second boundary portion  232  which is at the X1 side. Furthermore, as can be seen from  FIG. 3 , of the pair of second boundary portion  232  of the boundary portion  23 , the duct forming portion (see reference sign  42  in  FIGS. 1, 2, and 4 ) is formed such as to cover, from the Z2 side, both ends of the second boundary portion  232  which is at the X2 side. The length of a first boundary portion  231 , as viewed from the Z direction, is greater than that of a second boundary portion  232 . 
     As shown in  FIGS. 1 and 3 , the pair of first boundary portions  231  of the boundary portion  23  are sandwiched at least between the first insulator  3  and the second insulator  4  from both sides in the Z direction. Furthermore as shown in  FIGS. 2 and 3 , of the pair of second boundary portions  232  of the boundary portion  23 , at least all of the second boundary portion  232  which is at the X1 side is sandwiched between the first insulator  3  and the second insulator  4 . Moreover as can be seen from  FIG. 3 , both ends of the second boundary portion  232  which is at the X2 side are sandwiched between the first insulator (see reference sign  3  in  FIGS. 1, 2 and 4 ) and the second insulator  4  (see reference sign  4  in  FIGS. 1, 2 and 4 ). That is, all of the pair of the first boundary portion  231 , all of the second boundary portion  232  which is at the X1 side, and both ends of the second boundary portion  232  which is at the X2 side, are sandwiched between the first insulator  3  and the second insulator  4  in the Z direction. The parts of the chamber forming portion  32  and the heater embedding portion  31  of the first insulator  3  that overlap the boundary portion  23  in the Z direction constitute the first sandwiching portion  33 , and the parts of the duct forming portion  42  and the supporting portion  43  of the second insulator  4  that overlap the boundary portion  23  in the Z direction constitute the second sandwiching portion  45 . The first insulator  3  and the second insulator  4  are respectively disposed such as to straddle the boundary portion  23 , and sandwich the boundary portion  23 . As shown in  FIG. 2 , the diffusion resistor portion  322  constitutes a part of the first sandwiching portion  33 , located on the Z1 side of the second boundary portion  232  at the X1 side. 
     As shown in  FIGS. 1 and 2 , the length of the reference gas chamber  6  in the Z direction is greater than that of the measurement gas chamber  5  in the Z direction. In the present embodiment, the length of the reference gas chamber  6  in the Z direction is three or more times the length of the measurement gas chamber  5  in the Z direction, however the disclosure is not limited to this. The length of the reference gas chamber  6  in the Y direction is slightly greater than that of the reference electrode  12  in the Y direction. The reference electrode  12  is located at the center of the reference gas chamber  6 , in the Y direction. 
     Orthogonal to the X direction, the cross-sectional area of the region in the reference gas chamber  6  on the X2 side of the short side portion  422  is larger than the cross-sectional area of the measurement gas chamber  5  orthogonal to the X direction. Furthermore, the entire reference gas chamber  6  has a larger volume than the entire measurement gas chamber  5 . Due to the fact that the above-mentioned cross-sectional area, length in the in the Z direction, volume, etc. of the reference gas chamber  6  are greater those of the measurement gas chamber  5 , sufficient oxygen for reacting with unburned gas in the measurement electrode  11  can be supplied to the measurement electrode  11 , in the reference gas A from the reference gas chamber  6 . 
     As shown in  FIGS. 1, 2 and 4 , a supporting portion  43  is laminated on the surface of the duct forming portion  42 , at the Z2 side. The supporting portion  43  is formed at the part of the gas sensor element  1  that is farthest toward the Z2 side. The supporting portion  43  encloses the inner space of the duct forming portion  42 , that is, the reference gas chamber  6 , from the Z2 side. 
     In the present embodiment, the holding plate  22 , the insulating spacer  321  in the chamber forming portion  32 , the embedding plates  311 , the duct forming portion  42 , and the supporting portion  43  are made of the same material. Specifically, these are made of alumina (Al 2 O 3 ) which is impermeable to the gas G to be measured. 
     As shown in  FIG. 5 , the part of the gas sensor element  1  which is at the X1 end is provided with a protective layer  101 , to prevent substances that are poisonous to the measurement electrode (see reference sign  11  in  FIGS. 1, 2 , and  4 ), condensed water generated in the exhaust pipe, and the like, from entering the interior of the gas sensor element  1 . The protective layer  101  is formed of a porous ceramic (metal oxide) such as alumina. The porosity of the protective layer  101  is greater than the porosity of the diffusion resistor portion  322 , and the flow rate at which the gas G to be measured can pass through the protective layer  101  is greater than that at which the gas can pass through the diffusion resistor portion  322 . 
     A gas sensor  10  having the gas sensor element  1  of the present embodiment will be described with reference to  FIG. 5 . 
     The gas sensor  10  is configured with the X direction as the axial direction. In other words, the longitudinal direction of the gas sensor element  1  is parallel to the axial direction of the gas sensor  10 . The gas sensor  10  includes the gas sensor element  1 , a first insulator  102 , a housing  103 , a second insulator  104 , and a plurality of contact terminals  105 . The first insulator  102  holds the gas sensor element  1 . The housing  103  holds the first insulator  102 . The second insulator  104  is connected to the first insulator  102 . A plurality of contact terminals  105  are held by the second insulator  104  and are in contact with the sensor terminal  15  and the heater terminal  16  of the gas sensor element  1 . 
     The gas sensor  10  is provided with a tip end cover  106  that is attached to the part of the housing  103  at the X1 side, a second insulator  104  attached to the part of the housing  103  at the X2 side, a base end cover  107  which covers the contact terminals  105  etc., a bush  108  holding lead wires  100  that are connected to the contact terminals  105  on the base end cover  107 , and the like. 
     The tip end cover  106  is disposed such as to be exposed within the exhaust pipe of the internal combustion engine. A part of the gas sensor element  1 , at the X1 end, projects inside the tip end cover  106 . A gas passage hole  106   a  is formed in the tip end cover  106 , for passing exhaust gas as the gas G to be measured. The tip end cover  106  may have a double-wall structure or a single-wall structure. The exhaust gas G to be measured flows into the tip end cover  106  from the gas passage hole  106   a  of the tip end cover  106 , and is led to the measurement electrode  11  by passing through the protective layer  101  and through the diffusion resistor portion  322  of the gas sensor element  1 . 
     The base end cover  107  is disposed external to the exhaust pipe of the internal combustion engine. The base end cover  107  has an air introduction hole  109  formed therein, for introducing atmospheric air as the reference gas A thereinto. A filter  109   a  that blocks the passage of liquid but allows gas to pass through is disposed in the atmospheric air introduction hole  109 . The reference gas A that is introduced into the base end cover  107  from the air introduction hole  109  is guided to the reference electrode  12  through a gap in the base end cover  107  and through the reference gas chamber  6 . 
     The plurality of contact terminals  105  are disposed in the second insulator  104  such as to be respectively connected to the heater terminal  16  and the sensor terminal  15 . Furthermore, the lead wires  100  are connected to the contact terminals  105 . 
     The lead wires  100  are electrically connected to a sensor control device that controls gas detection in the gas sensor  10 . The sensor control device performs electrical control of the gas sensor  10  in cooperation with an engine control device that controls combustion operation in the engine. The sensor control device consists of a measurement circuit for measuring the current flowing between the measurement electrode  11  and the reference electrode  12 , an application circuit for applying a voltage between the measurement electrode  11  and the reference electrode  12 , and an energizing circuit for energizing the heater  7 , etc. It would be equally possible to for the sensor control device to be configured in the engine control device. 
     The action and effects of the present embodiment will next be described. In the gas sensor  10  of the present embodiment, at least a part of the boundary portion  23  between the placement hole  220  of the holding plate  22  and the solid electrolyte body  21  is sandwiched between the first sandwiching portion  33  of the first insulator  3  and the second sandwiching portion  45  of the second insulator  4 . The first sandwiching portion  33  is formed, at least in the Z direction, within the entire region in which the measurement gas chamber  5  is disposed. Furthermore, the second sandwiching portion  45  is formed, at least in the Z direction, within the entire region in which the reference gas chamber  6  is disposed. That is, at least a part of the boundary portion  23  is sandwiched between the first sandwiching portion  33  and the second sandwiching portion  45 , which are composed of structures having relatively high rigidity. Hence it is easy to ensure rigidity of the first sandwiching portion  33  and the second sandwiching portion  45  which sandwich at least a part of the boundary portion  23 , and stably hold the solid electrolyte body  21  in the placement hole  220 . 
     Furthermore, the electrolyte layer  2  has a solid electrolyte body  21  and a holding plate  22 . The holding plate  22  is made of a material having higher thermal conductivity than the solid electrolyte body  21 . Thus, heat conduction of the electrolyte layer  2  overall can readily be enhanced. The heater  7  can therefore easily heat the solid electrolyte body  21  efficiently, and the solid electrolyte body  21  can be activated rapidly. 
     Furthermore, in the present embodiment, since the boundary portion  23  can be held by the chamber forming portion  32 , the heater embedding portion  31 , the duct forming portion  42  and the supporting portion  43 , it is not necessary to dispose sheets on both sides of the boundary portion  23  for the sole purpose of holding the boundary portion  23 , and hence an increase in manufacturing person-hours can be prevented and productivity can be improved. 
     Furthermore at least the entire pair of first boundary portions  231  of the boundary portion  23  are sandwiched between the first sandwiching portion  33  and the second sandwiching portion  45 . Moreover, of the boundary portion  23 , all of the second boundary portion  232  that is on the X1 side of the pair of first boundary portions  231  is sandwiched between the first sandwiching portion  33  and the second sandwiching portion  45 . Hence, the boundary portion  23  can be stably held between the first sandwiching portion  33  and the second sandwiching portion  45 , so that the solid electrolyte body  21  is even more easily prevented from being displaced from the placement hole  220 . 
     Furthermore, the gas sensor element  1  has the heater  7  embedded in the first insulator  3 . As a result, the reference gas chamber  6 , into which the relatively low temperature reference gas A is introduced, is not disposed between the heater  7  and the solid electrolyte body  21 . Hence thermal conductivity between the heater  7  and the solid electrolyte body  21  can readily be enhanced, and the solid electrolyte body  21  can be rapidly activated. 
     As described above, the present embodiment enables a gas sensor element to be provided in which a solid electrolyte body does not readily become displaced from a placement hole. 
     Second Embodiment 
     This embodiment is obtained by changing the shape of the electrolyte layer  2  of the first embodiment, as shown in  FIGS. 6-8 . 
     As shown in  FIGS. 7 and 8 , the holding plate  22  is formed in a U-shape which is open to the X2 side. The region inside the U-shaped holding plate  22  constitutes a placement hole  220 , for disposing the solid electrolyte body  21 . The placement hole  220  is formed up to the end of the holding plate  22  at the X2 side, and is open to the X2 side. 
     As shown in  FIGS. 6 to 8 , the placement hole  220  is filled with the solid electrolyte body  21 . The end of the solid electrolyte body  21  which is at the X2 side is formed such as to extend to the position of the end of the holding plate  22  which is at the X2 side. The end of the solid electrolyte body  21  which is at the X2 side is not in contact with the holding plate  22 , and is exposed from the holding plate  22  at the X2 side. The boundary portion  23  is formed between the placement hole  220  and the solid electrolyte body  21 , other than for the end of the solid electrolyte body  21  which is at the X2 side. 
     In the present embodiment, as shown in  FIGS. 6 and 7 , the entire boundary portion  23  is sandwiched between the first sandwiching portion  33  and the second sandwiching portion  45 . 
     That is, the entire boundary portion  23  is covered from the Z1 side by the chamber forming portion  32 , which constitutes the first sandwiching portion  33 , and is covered from the Z2 side by the duct forming portion  42 , which constitutes the second sandwiching portion  45 . 
     In other respects, this embodiment is identical to the first embodiment. 
     Unless otherwise specified the reference signs used in the second and subsequent embodiments designate the same components, etc., as when used for preceding embodiments. 
     In the present embodiment, the entire boundary portion  23  is sandwiched between the first sandwiching portion  33  and the second sandwiching portion  45 . The boundary portion  23  can thereby be held with greater stability. 
     In other respects, this embodiment provides the same effects as for the first embodiment. 
     Although the present disclosure has been described based on embodiments, it is to be understood that the disclosure is not limited to such embodiments or structures. The scope of the present disclosure also includes various modified examples, and modifications that are within a range of equivalents. In addition, various combinations and forms, including other combinations and forms containing only a single element, or containing more or fewer elements, are included in the category and conceptual scope of the present disclosure. 
     For example, in each of the above embodiments, the gas sensor could be a concentration type of cell, which detects whether the air/fuel ratio of the engine is in an excess-fuel rich state or an excess-air lean state with respect to the stoichiometric air-fuel ratio, where the air/fuel ratio is the mixture ratio of fuel and air in the engine. Alternatively, the gas sensor could be other than an A/F sensor, such as a NOx sensor that detects the NOx concentration in the exhaust gas. When the gas sensor is used as a NOx sensor, a pump electrode and a measurement electrode are provided at the X1 end of the solid electrolyte body, on the surface of the solid electrolyte body which is on the measurement gas chamber side, with the pump electrode being provided for adjusting the oxygen concentration in the measurement gas chamber to be no greater than a predetermined concentration, and the measurement electrode being provided for measuring the NOx concentration. With the gas sensor element in that case, the NOx concentration is obtained from the value of a current, which is in accordance with the NOx concentration in the measurement gas flowing between the measurement electrode and the reference electrode.