Patent Publication Number: US-2022214303-A1

Title: Manufacturing method for gas sensor element, gas sensor element, and gas sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2020/017798 filed Apr. 24, 2020, claiming priority from Japanese Patent Application No. 2019-106265, filed Jun. 6, 2019. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for manufacturing a gas sensor element, a gas sensor element, and a gas sensor. 
     2. Description of the Related Art 
     A gas sensor including a gas sensor element is known (see, for example, Patent Literature 1). Such a type of gas sensor detects the gas concentration of a specific gas (for example, oxygen, NO x , etc.) in exhaust gas discharged from various exhaust systems mounted on an automobile or a boiler. Such a gas sensor is used, for example, in a temperature environment of room temperature (for example, 25° C.) to 900° C. or higher. 
     The gas sensor element includes a solid electrolyte and a pair of electrodes provided on both surfaces of the solid electrolyte such that the solid electrolyte is sandwiched therebetween. One of the pair of electrodes is a detection electrode to be exposed to exhaust gas, and the other electrode is a reference electrode to be exposed to a reference gas. Normally, a porous protective layer for protecting the gas sensor element from toxic substances, exhaust condensed water, and the like in exhaust gas is formed on the outer surface of the gas sensor element.
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2017-78679   

     3. Problems Addressed by the Invention 
     During use of the gas sensor, when the gas sensor element in a high temperature state comes into contact with moisture such as condensed water contained in exhaust gas, the gas sensor element may rapidly contract and a crack may occur in the detection electrode and the solid electrolyte. When a crack occurs in the detection electrode and the solid electrolyte, the desired electromotive force is no longer generated between the detection electrode and the reference electrode, resulting in malfunction of the gas sensor element. 
     As described above, the detection electrode in the conventional gas sensor element is normally protected by a porous protective layer, and has durability (water resistance) against a certain amount of moisture. However, it is also assumed that, during use of the gas sensor, the detection electrode of the gas sensor element comes into contact with moisture exceeding the allowable amount for the porous protective layer. Therefore, there is a demand for a gas sensor element having such excellent water resistance that a detection electrode or the like is not damaged even in such a case. 
     SUMMARY OF INVENTION 
     It is therefore an object of the present invention is to provide a method for manufacturing a gas sensor element having excellent water resistance, and a gas sensor element and a gas sensor having excellent water resistance. 
     As a result of extensive research conducted for achieving the above object, the present inventor has found that, in a gas sensor element including a solid electrolyte and an electrode formed on the surface of the solid electrolyte, when the electrode contains monoclinic zirconia, tetragonal/cubic zirconia, and a noble metal and is composed of a single layer, the electrode has excellent water resistance. Thus, the present inventor has been completed. 
     More particularly, the above object has been achieved, in a first aspect (1) of the present invention, by providing a method for manufacturing a gas sensor element including a solid electrolyte and an electrode formed on a surface of the solid electrolyte, the manufacturing method including: a slurry application step of forming a first slurry layer by applying a first slurry containing monoclinic zirconia and tetragonal/cubic zirconia to the surface of the solid electrolyte; a heat treatment step of forming a base layer by heat treating the solid electrolyte having the first slurry layer formed thereon; and a plating step of forming the electrode by plating the base layer using a plating solution containing a noble metal. 
     In a second aspect (2), the present invention provides a method for manufacturing a gas sensor element including a solid electrolyte and an electrode formed on a surface of the solid electrolyte, the method including: a slurry application step of forming a second slurry layer by applying a second slurry containing monoclinic zirconia, tetragonal/cubic zirconia, and a noble metal to the surface of the solid electrolyte; and a heat treatment step of forming the electrode by heat treating the solid electrolyte having the second slurry layer formed thereon. 
     In a preferred embodiment (3) of the method for manufacturing the gas sensor element according to (1) or (2) above, a content of the monoclinic zirconia in the first slurry or the second slurry is not less than 40 mass % and not greater than 90 mass % with respect to 100 mass % of a total amount of the monoclinic zirconia and the tetragonalicubic zirconia. 
     In another preferred embodiment (4) of the method for manufacturing the gas sensor element according to (1) or (2) above, a content of the monoclinic zirconia in the first slurry or the second slurry is not less than 40 mass % and not greater than 70 mass % with respect to 100 mass % of a total amount of the monoclinic zirconia and the tetragonal/cubic zirconia. 
     In a third aspect (5), the present invention provides a gas sensor element including a solid electrolyte and an electrode formed on a surface of the solid electrolyte, wherein the electrode contains monoclinic zirconia, tetragonal/cubic zirconia, and a noble metal, and the electrode is a single layer electrode. 
     In a fourth aspect (6), the present invention provides a gas sensor including: the gas sensor element according to (5) above; and a metal shell holding the gas sensor element. 
     Effects of the Invention 
     According to the present invention, it is possible to provide a method for manufacturing a gas sensor element having excellent water resistance, and a gas sensor element and a gas sensor having excellent water resistance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a configuration of a sensor according to Embodiment 1. 
         FIG. 2  is a cross-sectional view showing a configuration of a gas sensor element. 
         FIG. 3  shows an SEM image of a detection electrode. 
         FIG. 4  shows an SEM image of the detection electrode. 
         FIG. 5  is a flowchart showing a manufacturing process for the gas sensor element according to Embodiment 1. 
         FIG. 6  is an explanatory diagram schematically illustrating the manufacturing process for the gas sensor element according to Embodiment 1. 
         FIG. 7  is a flowchart showing a manufacturing process for a gas sensor element according to Embodiment 2. 
         FIG. 8  is an explanatory diagram schematically showing the manufacturing process for the gas sensor element according to Embodiment 2. 
         FIG. 9  is a cross-sectional view of a gas sensor element according to Embodiment 3 taken along the longitudinal direction thereof. 
         FIG. 10  is a cross-sectional view of the gas sensor element according to Embodiment 3 taken along the width direction thereof. 
         FIG. 11  is an explanatory diagram schematically illustrating a process for dropping a water drop onto a detection electrode of a gas sensor element in a water test. 
         FIG. 12  is an explanatory diagram schematically illustrating a process for checking a crack in a gas sensor element using an insulation meter in the water test. 
         FIG. 13  is a graph showing rich response times of each example and each comparative example. 
         FIG. 14  is a graph showing lean response times of each example and each comparative example. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
     Reference Numerals used to identify various features in the drawings include the following.
           10 : sensor     100 ,  100 A,  100 B: gas sensor element     110 : solid electrolyte     120 : reference electrode     130 ,  130 A,  130 B: detection electrode     132 ,  132 A: base     13 : first slurry layer     14 : base layer     15 : pore     23 : second slurry layer       

     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described in detail with reference to the drawings. However, the present invention should not be construed as being limited thereto. 
     Embodiment 1 
     Embodiment 1 of the present invention will be described with reference to  FIG. 1  to  FIG. 6 .  FIG. 1  is a cross-sectional view showing a configuration of a sensor according to Embodiment 1. The sensor (an example of a gas sensor)  10  is fixed to an exhaust pipe of an internal combustion engine (engine), which is not shown, and measures the concentration of a specific gas contained in exhaust gas as a gas to be measured. Examples of the specific gas include oxygen and NO x . The sensor  10  of the present embodiment measures an oxygen gas concentration. 
       FIG. 1  shows a cross-section of the sensor  10  in an axial line CA direction. The axial line CA is an axial line extending in the longitudinal direction of the sensor  10  at the center of the sensor  10 . Hereinafter, the lower side in the sheet of  FIG. 1  is referred to as a “front side”, and the upper side in the sheet of  FIG. 1  is referred to as a “rear side”. 
     The sensor  10  mainly includes a gas sensor element  100 , a metal shell  200 , a protector  300 , a ceramic heater  150 , an outer casing  410 , a separator  600 , and a grommet  800 . 
     The gas sensor element  100  outputs a signal for detecting the oxygen concentration in exhaust gas. The gas sensor element  100  includes a detection portion  140  to be directed to the exhaust gas, on the front side, and a tubular hole  112  for inserting a connection terminal  510  therein is formed on the rear side in the gas sensor element  100 . The detection portion  140  mainly includes a solid electrolyte  110 , a reference electrode  120  formed on the inner surface of the solid electrolyte  110 , and a detection electrode  130  formed on the outer surface of the solid electrolyte  110 . These components will be described below. The gas sensor element  100  is fixed inside the metal shell  200  in a state where the detection portion  140  projects from the front end of the metal shell  200  and the tubular hole  112  projects from the rear end of the metal shell  200 . In addition, a flange portion  170  is provided at substantially the center of the gas sensor element  100 . 
     The metal shell  200  is mainly used for holding the gas sensor element  100  and mounting the sensor  10  to the exhaust pipe. The metal shell  200  is a tubular metal member that surrounds the periphery of the gas sensor element  100 . The metal shell  200  of the present embodiment is formed from SUS430. 
     A front end portion  240 , a thread portion  210 , a flange portion  220 , a rear end portion  230 , and a crimp portion  252  are formed on the outer periphery of the metal shell  200  in this order from the front side. The front end portion  240  is a portion formed on the front side of the metal shell  200  such that the outer diameter of the metal shell  200  is reduced. The metal shell  200  and the protector  300  are joined together in a state where the front end portion  240  of the metal shell  200  is inserted into the protector  300 . The thread portion  210  is an external thread formed for screwing and mounting the sensor  10  to the exhaust pipe. The flange portion  220  is a portion formed such that the outer shape of the metal shell  200  projects in a polygonal shape toward the outer side in the radial direction. The flange portion  220  is used to engage a tool for mounting the sensor  10  to the exhaust pipe. Therefore, the flange portion  220  is formed in a shape (for example, a hexagon bolt shape) that allows the flange portion  220  to engage the tool. The rear end portion  230  is a portion formed on the rear side of the metal shell  200  such that the outer diameter of the metal shell  200  is reduced. The metal shell  200  and the outer casing  410  are joined together in a state where the rear end portion  230  of the metal shell  200  is inserted into the outer casing  410 . 
     A tubular hole  250  and a step portion  260  are formed on the inner periphery of the metal shell  200 . The tubular hole  250  is a through hole which penetrates the metal shell  200  along the axial line CA. The tubular hole  250  holds the gas sensor element  100  along the axial line CA. The step portion  260  is a portion formed on the front side of the metal shell  200  such that the inner diameter of the metal shell  200  is reduced. A ceramic holder  161  engages the step portion  260  of the metal shell  200  via a packing  159 . Furthermore, the flange portion  170  of the gas sensor element  100  engages the ceramic holder  161  via a packing  160 . Moreover, a seal portion  162 , a ceramic sleeve  163 , and a metal ring  164  are disposed on the rear side of the ceramic holder  161  in the tubular hole  250  of the metal shell  200 . The seal portion  162  is a talc layer formed by filling the tubular hole  250  with talc powder. The seal portion  162  blocks the ventilation between the front side and the rear side in the axial line CA direction in the gap between the gas sensor element  100  and the metal shell  200 . The ceramic sleeve  163  is a tubular insulating member that surrounds the outer periphery of the gas sensor element  100 . The metal ring  164  is a stainless steel flat washer that surrounds the outer periphery of the gas sensor element  100 . 
     In the metal shell  200 , the crimp portion  252  is further formed by bending the opening end on the rear side to the radially inner side (tubular hole  250  side). The seal portion  162  is pressed via the metal ring  164  and the ceramic sleeve  163  by the crimp portion  252 , so that the gas sensor element  100  is fixed inside the metal shell  200 . 
     The protector  300  protects the gas sensor element  100 . The protector  300  is a bottomed cylindrical metal member. The protector  300  is fixed to the front end portion  240  by laser welding so as to surround the periphery of the gas sensor element  100  projecting from the front side of the metal shell  200 . The protector  300  is composed of a double protector including an inner protector  310  and an outer protector  320 . The inner protector  310  and the outer protector  320  are formed with gas introduction holes  311  and  312  and a gas discharge hole  313 , respectively. The gas introduction holes  311  and  312  are through holes formed for introducing exhaust gas to the inner side of the protector  300  (to the gas sensor element  100 ). The gas discharge hole  313  is a through hole for discharging the exhaust gas from the inside of the protector  300  toward the outside of the protector  300 . 
     The ceramic heater  150  raises the temperature of the gas sensor element  100  to a predetermined active temperature to enhance the conductivity of oxygen ions in the detection portion  140  and stabilize operation of the gas sensor element  100 . The ceramic heater  150  is provided within the tubular hole  112  of the gas sensor element  100 . The ceramic heater  150  includes a heating portion  151  and a heater connection terminal  152 . The heating portion  151  is a heat generating resistor formed of a conductor such as tungsten, and generates heat upon receiving power. The heater connection terminal  152  is provided on the rear side of the ceramic heater  150  and is connected to a heater lead wire  590 . The heater connection terminal  152  receives power from the outside via the heater lead wire  590 . 
     The outer casing  410  protects the sensor  10 . The outer casing  410  is a cylindrical metal member that has a through hole along the axial line CA. The rear end portion  230  of the metal shell  200  is inserted into a front end portion  411  of the outer casing  410 . The outer casing  410  and the metal shell  200  are joined together by laser welding. The grommet  800  described below is fitted into a rear end portion  412  of the outer casing  410 . The grommet  800  is fixed to the outer casing  410  by being crimped to the rear end portion  412  of the outer casing  410 . 
     The separator  600  has a substantially cylindrical shape and is formed of an insulating member of alumina or the like. The separator  600  is disposed inside the outer casing  410 . The separator  600  is formed with a separator body  610  and a separator flange portion  620 . The separator body  610  is formed with: a lead wire through hole  630  that penetrates the separator  600  along the axial line CA: and a holding hole  640  that is opened on the front side of the separator  600 . Element lead wires  570  and  580  described below and the heater lead wire  590  are inserted from the rear side of the lead wire through hole  630 . A rear end portion of the ceramic heater  150  is inserted into the holding hole  640 . The inserted ceramic heater  150  is positioned in the axial line CA direction by the rear end surface thereof being brought into contact with the bottom surface of the holding hole  640 . The separator flange portion  620  is a portion formed on the rear side of the separator  600  such that the outer diameter of the separator  600  is increased. The separator flange portion  620  is supported by a holding member  700  disposed in the gap between the outer casing  410  and the separator  600 , thereby fixing the separator  600  inside the outer casing  410 . 
     The grommet  800  is formed from a fluororubber having excellent heat resistance, or the like. The grommet  800  is fitted into the rear end portion  412  of the outer casing  410 . The grommet  800  is formed with: a through hole  820  that penetrates the grommet  800  along the axial line CA in a center portion thereof; and four lead wire insertion holes  810  that penetrate the grommet  800  along the axial line CA around the through hole  820 . A filter unit  900  (a filter and a metal tube) for closing the through hole  820  is disposed in the through hole  820 . 
     The element lead wires  570  and  580  and the heater lead wire  590  are each formed of a conductor coated with an insulating coating made of a resin. Each of rear end portions of the conductors of the element lead wires  570  and  580  and the heater lead wire  590  is electrically connected to a connector terminal provided to a connector. A front end portion of the conductor of the element lead wire  570  is crimped and connected to a rear end portion of an inner connection terminal  520  that is internally fitted on the rear side of the gas sensor element  100 . The inner connection terminal  520  is a conductor that electrically connects the element lead wire  570  and the reference electrode  120  of the gas sensor element  100 . A front end portion of the conductor of the element lead wire  580  is crimped and connected to a rear end portion of an outer connection terminal  530  that is externally fitted on the rear side of the gas sensor element  100 . The outer connection terminal  530  is a conductor that electrically connects the element lead wire  580  and the detection electrode  130  of the gas sensor element  100 . A front end portion of the conductor of the heater lead wire  590  is electrically connected to the heater connection terminal  152  of the ceramic heater  150 . In addition, the element lead wires  570  and  580  and the heater lead wire  590  are inserted into the lead wire through hole  630  of the separator  600  and the lead wire insertion hole  810  of the grommet  800  and extended from the inside of the outer casing  410  toward the outside of the outer casing  410 . 
     The sensor  10  introduces outside air into the tubular hole  112  of the gas sensor element  100  by passing the outside air through the filter unit  900  from the through hole  820  of the grommet  800  and introducing the outside air into the outer casing  410 . The outside air introduced into the tubular hole  112  of the gas sensor element  100  is used as a reference gas that serves as a reference for the sensor  10  (gas sensor element  100 ) to detect oxygen in exhaust gas. In addition, the sensor  10  is configured such that the gas sensor element  100  is exposed to exhaust gas (gas to be measured) by introducing the exhaust gas into the protector  300  through the gas introduction holes  311  and  312  of the protector  300 . Accordingly, the gas sensor element  100  generates an electromotive force corresponding to the difference in oxygen concentration between the reference gas and the exhaust gas as gas to be measured. The electromotive force of the gas sensor element  100  is outputted as a sensor output via the element lead wires  570  and  580  to the outside of the sensor  10 . 
       FIG. 2  is a cross-sectional view showing a configuration of the gas sensor element  100 .  FIG. 2  shows a cross-section of the front side of the gas sensor element  100  in the axial line CA direction. The gas sensor element  100  of the present embodiment includes the solid electrolyte  110 , the reference electrode  120 , the detection electrode  130 , a porous protective layer  180 , and a base layer  190 . 
     The solid electrolyte  110 , together with the reference electrode  120  and the detection electrode  130 , functions as the detection portion  140  that detects the oxygen concentration in exhaust gas. The solid electrolyte  110  extends in the axial line CA direction and is formed in a bottomed tubular shape closed on the front side thereof. The solid electrolyte  110  is made of a material having oxide ion conductivity (oxygen ion conductivity). In the present embodiment, the solid electrolyte  110  is made of zirconia (zirconia oxide: ZrO 2 ) to which a stabilizer is added. In the present embodiment, yttrium oxide (Y 2 O 3 ) is used as the stabilizer. Zirconia to which yttrium oxide is added is also referred to as yttria partially stabilized zirconia. Examples of the stabilizer used for the solid electrolyte  110  include, in addition to yttrium oxide, calcium oxide (CaO), magnesium oxide (MgO), cerium oxide (CeO 2 ), ytterbium oxide (Yb 2 O 3 ), and scandium oxide (Sc 2 O 3 ). 
     The reference electrode  120  is formed on the inner surface of the solid electrolyte  110  and is exposed to the reference gas. The reference electrode  120  is made of a noble metal. In the present embodiment, the reference electrode  120  is made of platinum (Pt). The reference electrode  120  is formed by electroless plating. Examples of the noble metal used for the reference electrode  120  include, in addition to platinum, platinum alloys, other noble metals such as rhodium, and other noble metal alloys. 
     The detection electrode  130  is formed on the outer surface of the solid electrolyte  110  and is exposed to the exhaust gas as the gas to be measured. 
     The porous protective layer  180  protects the gas sensor element  100 . The porous protective layer  180  is formed, for example, from a material that contains one or more ceramic materials selected from the group consisting of alumina, titania, spinel, zirconia, mullite, zircon, and cordierite as a main component and that further contains glass. The porous protective layer  180  is disposed so as to cover the detection electrode  130  via the base layer  190 . The porous protective layer  180  includes: an inner layer  181  disposed so as to cover the detection electrode  130 ; and an outer layer  182  disposed so as to cover the inner layer  181 . The outer layer  182  has a lower porosity than the inner layer  181 . The porous protective layer  180  may be omitted. 
     The base layer  190  improves adhesion of the porous protective layer  180  and protects the detection electrode  130 . The base layer  190  is composed of a sprayed layer of a ceramic material such as spinel, and is a porous protective layer. The base layer  190  is formed so as to cover the detection electrode  130  from the front side of the outer surface of the solid electrolyte  110  to the vicinity of the flange portion  170  where the solid electrolyte  110  outwardly projects. The base layer  190  may be omitted. 
     The detection electrode  130  of the present embodiment contains a noble metal, zirconia or low-stabilizer-content zirconia, and high-stabilizer-content zirconia. In the present embodiment, platinum (Pt) is used as the noble metal. As the noble metal used for the detection electrode  130 , platinum alloys, other noble metals such as rhodium, other noble metal alloys, etc., may be used in addition to platinum. 
     The content (mass %) of the noble metal in the detection electrode  130  is not particularly limited as long as the object of the present invention is not impaired. For example, the noble metal may be contained in a proportion of 75 mass % or greater and preferably 80 mass % or greater, and 90 mass % or less and preferably 85 mass % or less, with respect to the total mass (100 mass %) of the detection electrode  130 . 
     In the detection electrode  130 , the zirconia or the low-stabilizer-content zirconia, and the high-stabilizer-content zirconia are used in combination as a base. The base is used for the purposes of ensuring the adhesiveness of the detection electrode  130  to the solid electrolyte  110  and forming a three-phase interface that reacts with oxygen gas while supporting the noble metal (Pt or the like), for example. 
     The content (mass %) of the base in the detection electrode  130  is not particularly limited as long as the object of the present invention is not impaired. For example, the base may be contained in a proportion of 5 mass % or greater and preferably 10 mass % or greater, and 25 mass % or less and preferably 20 mass % or less, with respect to the total mass (100 mass %) of the detection electrode  130 . 
     The “zirconia” used for the detection electrode  130  is zirconia to which a stabilizer described below is not added, and means pure zirconia (ZrO 2 ) containing no mixture other than inevitable impurities. The crystal structure (under normal temperature conditions) of such zirconia is monoclinic. The monoclinic zirconia undergoes a phase transition at high temperatures (for example, 1200° C.) and changes in volume. 
     The “low-stabilizer-content zirconia” means zirconia to which the stabilizer described below is added in a predetermined proportion such that the zirconia becomes monoclinic under normal temperature conditions. For example, in the case where yttrium oxide (Y 2 O 3 ) is adopted as the stabilizer, the low-stabilizer-content zirconia means zirconia to which the stabilizer is added in a proportion of 4 mol % or less in terms of metal element. For other stabilizers, the addition proportion of the stabilizer can be determined as appropriate by using, for example, a well-known phase diagram or the like. 
     As the stabilizer used for the detection electrode  130 , one or more stabilizers selected from the group consisting of yttrium oxide (Y 2 O 3 ), calcium oxide (CaO), magnesium oxide (MgO), cerium oxide (CeO 2 ), ytterbium oxide (Yb 2 O 3 ), scandium oxide (Sc 2 O 3 ), and strontium oxide (SrO) can be used. In the present embodiment, yttrium oxide (Y 2 O 3 ) is used as the stabilizer for the detection electrode  130 . 
     Each stabilizer shown in the above group contains yttrium (Y), calcium (Cr), magnesium (Mg), cerium (Ce), ytterbium (Yb), scandium (Sc), or strontium (Cr) as a metal element. 
     The crystal structure (under normal temperature conditions) of the low-stabilizer-content zirconia is monoclinic. 
     The “high-stabilizer-content zirconia” means zirconia to which the stabilizer exemplified in the description of the “low-stabilizer-content zirconia” is added in a predetermined proportion such that the zirconia becomes tetragonal or cubic under normal temperature conditions. For example, in the case where yttrium oxide (Y 2 O 3 ) is adopted as the stabilizer, the high-stabilizer-content zirconia means zirconia to which the stabilizer is added in a proportion of greater than 4 mol % and not greater than 20 mol % in terms of metal element. For other stabilizers, the addition proportion of the stabilizer can be determined as appropriate by using, for example, a well-known phase diagram or the like. 
     The crystal structure (under normal temperature conditions) of the high-stabilizer-content zirconia is tetragonal or cubic. 
     As described above, in the detection electrode  130 , the zirconia or the low-stabilizer-content zirconia and the high-stabilizer-content zirconia, which have crystal states different from each other, are used in combination. 
     In the detection electrode  130 , the noble metal (Pt or the like), the zirconia or the low-stabilizer-content zirconia, and the high-stabilizer-content zirconia are mixed with each other and uniformly dispersed. Such a detection electrode  130  is formed as a single layer as a whole. In addition, the detection electrode  130  is continuously formed. 
     In the base of the detection electrode  130 , the zirconia or the low-stabilizer-content zirconia and the high-stabilizer-content zirconia are mixed with each other and uniformly dispersed. 
     In the present description, the zirconia and the low-stabilizer-content zirconia, which are used for the detection electrode  130 , are also collectively referred to as “monoclinic zirconia”. In addition, the high-stabilizer-content zirconia is also referred to as “tetragonal/cubic zirconia”. 
     In the case of the present embodiment, the zirconia is contained as “monoclinic zirconia”. 
     As long as the object of the present invention is not impaired, the detection electrode  130  may contain a component other than the noble metal, the zirconia or the low-stabilizer-content zirconia, and the high-stabilizer-content zirconia which are described above. 
     The content of the zirconia or the low-stabilizer-content zirconia in the detection electrode  130  is preferably not less than 40 mass % and not greater than 90 mass % with respect to 100 mass % of the total of the zirconia or the low-stabilizer-content zirconia and the high-stabilizer-content zirconia. When the above content is within such a range, the detection electrode  130  has excellent water resistance. 
     The content of the zirconia or the low-stabilizer-content zirconia in the detection electrode  130  is preferably not less than 40 mass % and not greater than 90 mass % with respect to 100 mass % of the total of the zirconia or the low-stabilizer-content zirconia and the high-stabilizer-content zirconia. When the above content is within such a range, the detection electrode  130  has excellent water resistance and responsiveness. 
       FIG. 3  and  FIG. 4  each show an SEM image of the detection electrode  130 .  FIG. 3  shows an image obtained by observing, with an SEM (Scanning Electron Microscope), a part of a cross-section obtained by cutting the detection electrode  130  in the thickness direction thereof. At substantially the center of  FIG. 3 , the detection electrode  130  is shown, and the solid electrolyte  110  is shown adjacently on the left side thereof. In addition, in  FIG. 3 , the base layer (sprayed layer)  190  of the porous protective layer  180  is shown adjacently on the right side of the detection electrode  130 . The right-left direction in  FIG. 3  corresponds to the thickness direction of the detection electrode  130 . 
     In the detection electrode  130  shown in  FIG. 3 , the white part represents platinum (noble metal)  131 , and the gray part that is in contact with the platinum  131  represents a base  132 . In the detection electrode  130  shown in  FIG. 3 , the black part represents voids (gaps)  133 . The base  132  of the present embodiment contains the zirconia and the high-stabilizer-content zirconia. As shown in  FIG. 3 , the platinum  131  and the base  132  are mixed with each other and uniformly dispersed, and the detection electrode  130  is formed as a single layer as a whole. 
       FIG. 4  shows an image obtained by observing, with an SEM, the surface of the detection electrode  130  in a state before forming the base layer (sprayed layer)  190  of the porous protective layer  180 . In  FIG. 4 , the white part represents the platinum (noble metal)  131 , the gray part represents the base  132 , and the black part represents the voids (gaps)  133 . As shown in  FIG. 4 , the platinum  131  and the base  132  are mixed with each other and uniformly dispersed, and the detection electrode  130  is formed as a single layer as a whole. 
     The composition ratio of the noble metal, the zirconia or the low-stabilizer-content zirconia, and the high-stabilizer-content zirconia in the detection electrode  130  is determined by elementary analysis using an EPMA (Electron Probe Micro Analyzer). 
     It is speculated that, in the detection electrode  130 , there are many very small cracks (micro cracks) that cannot be confirmed in an SEM image, an EPMA image, or the like. It is speculated that such cracks occur, during phase transition of the zirconia (or the low-stabilizer-content zirconia), at the interface between the zirconia (or the low-stabilizer-content zirconia), which undergoes a phase transition at high temperatures, and the high-stabilizer-content zirconia, which does not undergo a phase transition even at high temperatures, and the like. 
     The detection electrode  130  of the present embodiment has excellent water resistance. As described above, it is speculated that there are many very small cracks (micro cracks) in the detection electrode  130 . It is speculated that, even if such a detection electrode  130  comes into contact with water in a high temperature state (for example, the temperature condition of a water test described below: 500° C. to 700° C.), the occurrence of a large crack is inhibited in the detection electrode  130 . It is speculated that, since a large crack that occurs in the detection electrode  130  propagates to the solid electrolyte  110 , the occurrence of a crack in the solid electrolyte  110  can also be inhibited by inhibiting occurrence of a large crack in the detection electrode  130 . It is speculated that, in the detection electrode  130 , the occurrence of a large crack is inhibited due to the presence of micro cracks even when the detection electrode  130  comes into contact with water. 
     Next, a method for manufacturing the gas sensor element  100  will be described. Here, a manufacturing process for the detection electrode  130  will be mainly described.  FIG. 5  is a flowchart showing a manufacturing process for the gas sensor element  100  according to Embodiment 1, and  FIG. 6  is an explanatory diagram schematically illustrating the contents of the manufacturing process for the gas sensor element  100  according to Embodiment 1. First, as shown in S 1  of  FIG. 5  and ( 6 A) of  FIG. 6 , the solid electrolyte  110  is prepared. Here, the case of forming the detection electrode  130  on one surface (outer surface)  110   a  of the solid electrolyte  110  will be described. 
     Next, as shown in S 2  of  FIG. 5 , a first slurry containing a predetermined amount of zirconia (powder) and a predetermined amount of high-stabilizer-content zirconia (powder) is prepared. In addition to the zirconia and the high-stabilizer-content zirconia, the first slurry contains a predetermined amount of an organic binder (for example, ethyl cellulose), a predetermined amount of a solvent (for example, butyl carbitol acetate), and the like. Moreover, as long as the object of the present invention is not impaired, a known additive such as a viscosity modifier, a pore forming agent, etc., may be added to the first slurry. 
     Subsequently, as shown in S 3  of  FIG. 5  and ( 6 B) of  FIG. 6 , the first slurry is applied to the surface  110   a  of the solid electrolyte  110 , and a layered first slurry layer  13  made of the first slurry is formed on the surface  110   a  of the solid electrolyte  110  (slurry application step). The method for applying the first slurry to the surface  110   a  of the solid electrolyte  110  is not particularly limited as long as the object of the present invention is not impaired, and, for example, dipping or a method using a known coating machine such as a coater is used. 
     After the first slurry layer  13  is formed, a drying step of drying the first slurry layer  13  using a heater or the like may be performed as necessary. 
     Thereafter, as shown in S 4  of  FIG. 5  and ( 6 C) of  FIG. 6 , the zirconia and the high-stabilizer-content zirconia in the first slurry layer  13  are sintered by heat treating (for example, a baking treatment under a temperature condition of 1500° C.) the first slurry layer  13  on the solid electrolyte  110 . In this manner, a base layer  14  for an electrode is formed on the solid electrolyte  110  (heat treatment step). During the heat treatment, the organic binder contained inside the first slurry layer  13  disappears, so that a trace of the disappearance of the organic binder becomes a pore  15 . A plurality of (many) pores  15  are formed in the base layer  14 . 
     The temperature condition in the heat treatment step is not particularly limited as long as the zirconia, the high-stabilizer-content zirconia, and the like in the first slurry layer  13  are sintered, and the pores  15  and the like are formed in the first slurry layer  13 . For example, the heat treatment step is performed in the range of 1200° C. to 1600° C. 
     As shown in S 5  of  FIG. 5  and ( 6 D) of  FIG. 6 , the base layer  14  is plated using a plating solution containing platinum (plating step). For example, in the case of applying platinum by electroless plating, a plating solution containing platinum, a reducing agent, etc., is brought into contact with the base layer  14 , and the platinum is reduced and deposited on the base layer  14 . As a result, the pores  15  of the base layer  14  are filled with the platinum  131 . In this manner, the detection electrode  130  is formed on the solid electrolyte  110 . 
     After the plating, a heat treatment may be performed on the detection electrode  130  formed on the solid electrolyte  110 , as necessary (for example, in the case of flattening the unevenness of the platinum on the surface of the detection electrode  130 ). 
     As described above, the gas sensor element  100  in which the detection electrode  130  is formed on the solid electrolyte  110  can be manufactured. Through a manufacturing process similar to that for the detection electrode  130 , the reference electrode  120  may be formed on the other surface (inner surface) of the solid electrolyte  110 . In addition, the reference electrode  120  may be formed by electroless plating as described above. 
     The method of the present embodiment is a method for manufacturing a gas sensor element including the solid electrolyte  110  and an electrode (the detection electrode  130  or the like) formed on the surface of the solid electrolyte  110 . The manufacturing method includes: a slurry application step of forming the first slurry layer  13  by applying, to the surface of the solid electrolyte  110 , a first slurry containing zirconia or low-stabilizer-content zirconia to which a stabilizer is added in a proportion of 4 mol % or less (in terms of metal element) and high-stabilizer-content zirconia to which a stabilizer is added in a proportion of greater than 4 mol % (in terms of metal element) and not greater than 20 mol % (in terms of metal element); a heat treatment step of forming the base layer  14  by heat treating the solid electrolyte  110  having the first slurry layer  13  formed thereon; and a plating step of forming the electrode (detection electrode  130  or the like) by plating the base layer  14  using a plating solution containing a noble metal. 
     With the method for manufacturing the gas sensor element of the present embodiment, a gas sensor element having excellent water resistance is obtained. 
     In the method for manufacturing the gas sensor element of the present embodiment, the content of the zirconia or the low-stabilizer-content zirconia in the first slurry is preferably not less than 40 mass % and not greater than 90 mass % with respect to 100 mass % of the total amount of the zirconia or the low-stabilizer-content zirconia and the high-stabilizer-content zirconia. When this content is within such a range, the obtained gas sensor element has excellent water resistance. 
     In the method for manufacturing the gas sensor element of the present embodiment, the content of the zirconia or the low-stabilizer-content zirconia in the first slurry is preferably not less than 40 mass % and not greater than 70 mass % with respect to 100 mass % of the total amount of the zirconia or the low-stabilizer-content zirconia and the high-stabilizer-content zirconia. When this content is within such a range, the obtained gas sensor element has excellent water resistance and responsiveness. 
     In the method for manufacturing the gas sensor element of the present embodiment, the first slurry layer is formed using the first slurry containing the organic binder. However, in another embodiment, the pores  15  may be formed in the base layer  14  without using a volatilizing material such as an organic binder. That is, a volatilizing material such as an organic binder is not an essential component. 
     In the present embodiment, a predetermined amount of the monoclinic zirconia is contained in the first slurry. However, in another embodiment, monoclinic low-stabilizer-content zirconia may be contained instead of or together with the zirconia. 
     Embodiment 2 
     Next, a method for manufacturing a gas sensor element  100 A according to Embodiment 2 will be described with reference to  FIG. 7  and  FIG. 8 . The gas sensor element  100 A of the present embodiment can be used instead of the gas sensor element  100  of the sensor  10  of Embodiment 1. In addition, also in the present embodiment, similar to Embodiment 1 described above, a process for manufacturing a detection electrode  130 A will be mainly described.  FIG. 7  is a flowchart showing a process for manufacturing the gas sensor element  100 A according to Embodiment 2, and  FIG. 8  is an explanatory diagram schematically illustrating the contents of the process for manufacturing the gas sensor element  100 A according to Embodiment 2. 
     First, as shown in S 11  of  FIG. 7  and ( 8 A) of  FIG. 8 , a solid electrolyte  110  that is the same as in Embodiment 1 is prepared. Here, similar to Embodiment 1, the case of forming the detection electrode  130 A on one surface (outer surface)  110   a  of the solid electrolyte  110  will be described. 
     Next, as shown in S 12  of  FIG. 7 , a second slurry containing a predetermined amount of zirconia (powder) and predetermined high-stabilizer-content zirconia (powder), and a predetermined amount of platinum (powder) is prepared. In addition to the above, a predetermined amount of a solvent (for example, butyl carbitol acetate), a known additive such as a viscosity modifier, a pore forming agent, etc., may be added to the second slurry. 
     Subsequently, as shown in S 13  of  FIG. 7  and ( 8 B) of  FIG. 8 , the second slurry is applied to the surface  110   a  of the solid electrolyte  110 , and a layered second slurry layer  23  made of the second slurry is formed on the surface  110   a  of the solid electrolyte  110  (slurry application step). In the case of the present embodiment, platinum  131 A is dispersed in the second slurry layer  23 . The method for applying the second slurry to the surface  110   a  of the solid electrolyte  110  is not particularly limited as long as the object of the present invention is not impaired, and, for example, dipping or a method using a known coating machine such as a coater is used. 
     After the second slurry layer  23  is formed, a drying step of drying the second slurry layer  23  using hot air or the like may be performed as necessary. 
     Thereafter, as shown in S 14  of  FIG. 7  and ( 8 C) of  FIG. 8 , the platinum, the zirconia, and the high-stabilizer-content zirconia in the second slurry layer  23  are sintered by heat treating (for example, a baking treatment under a temperature condition of 1500° C.) the second slurry layer  23  on the solid electrolyte  110 . In this manner, the detection electrode  130 A is formed on the solid electrolyte  110  (heat treatment step). The detection electrode  130 A includes a base  132 A containing the zirconia and the high-stabilizer-content zirconia, and the platinum  131 A. 
     The temperature condition in the heat treatment step is not particularly limited as long as the platinum, the zirconia, the high-stabilizer-content zirconia, and the like in the second slurry layer  23  are sintered, and an electrode such as the detection electrode  130 A is formed. For example, the heat treatment step is performed in the range of 1200° C. to 1600° C. 
     As described above, the gas sensor element  100 A in which the detection electrode  130 A is formed on the solid electrolyte  110  can be manufactured. A reference electrode (not shown) may be formed on the other surface (inner surface) of the solid electrolyte  110  through a manufacturing process similar to that for the detection electrode  130 A, or by an electroless plating method or the like. 
     The method of the present embodiment is a method for manufacturing the gas sensor element  110 A which includes the solid electrolyte  110  and an electrode (detection electrode  130 A or the like) formed on the surface  110   a  of the solid electrolyte  110 . The manufacturing method includes: a slurry application step of forming the second slurry layer  23  by applying, to the surface  110   a  of the solid electrolyte  110 , a second slurry containing zirconia or low-stabilizer-content zirconia to which a stabilizer is added in a proportion of 4 mol % or less (in terms of metal element), high-stabilizer-content zirconia to which a stabilizer is added in a proportion of greater than 4 mol % (in terms of metal element) and not greater than 20 mol % (in terms of metal element), and a noble metal; and a heat treatment step of forming the electrode (detection electrode  130 A or the like) by heat treating the solid electrolyte  110  having the second slurry layer  23  formed thereon. 
     With the method for manufacturing the gas sensor element of the present embodiment, a gas sensor element having excellent water resistance is obtained. 
     In the method for manufacturing the gas sensor element of the present embodiment, the content of the zirconia or the low-stabilizer-content zirconia in the second slurry is preferably not less than 40 mass % and not greater than 90 mass % with respect to 100 mass % of the total amount of the zirconia or the low-stabilizer-content zirconia and the high-stabilizer-content zirconia. When this content is within such a range, the obtained gas sensor element has excellent water resistance. 
     In the method for manufacturing the gas sensor element of the present embodiment, the content of the zirconia or the low-stabilizer-content zirconia in the second slurry is preferably not less than 40 mass % and not greater than 70 mass % with respect to 100 mass % of the total of the zirconia or the low-stabilizer-content zirconia and the high-stabilizer-content zirconia. When this content is within such a range, the obtained gas sensor element has excellent water resistance and responsiveness. 
     In the present embodiment, a predetermined amount of the monoclinic zirconia is contained in the second slurry. However, in another embodiment, monoclinic low-stabilizer-content zirconia may be contained instead of or together with the zirconia. 
     Embodiment 3 
     Next, a gas sensor element  100 B according to Embodiment 3 will be described with reference to  FIG. 9  and  FIG. 10 . Unlike the tubular gas sensor elements  100  and  100 A of Embodiments 1 and 2, the gas sensor element  100 B of the present embodiment has a plate shape. The gas sensor element  100 B has a long plate shape as a whole. The gas sensor element  100 B is used in a gas sensor (not shown) for detecting the gas concentration of a specific gas (for example, oxygen, NO x , etc.) in exhaust gas. 
     Similar to the above-mentioned Embodiment 1, etc., the gas sensor in which the gas sensor element  100 B of the present embodiment is used includes known components such as a metal shell. The metal shell is a tubular metal member that surrounds the periphery of the gas sensor element  100 B, and holds the gas sensor element  100 B in a state where the gas sensor element  100 B is housed therein. The metal shell is also used when mounting the gas sensor to an exhaust pipe, for example. 
       FIG. 9  is a cross-sectional view of the gas sensor element  100 B according to Embodiment 3 taken along the longitudinal direction thereof, and  FIG. 10  is a cross-sectional view of the gas sensor element  100 B according to Embodiment 3 taken along the width direction thereof.  FIG. 9  and  FIG. 10  show the front side of the gas sensor element  100 B. 
     The gas sensor element  100 B includes a detection element portion  300 B and a heater portion  200 B stacked thereon. The heater portion B includes: a first base  101 B and a second base  103 B mainly composed of alumina; and a heating element  102 B sandwiched between the first base  101 B and the second base  103 B and mainly composed of platinum. 
     The detection element portion  300 B includes an oxygen concentration detection cell  130 B and an oxygen pump cell  140 B. The oxygen concentration detection cell  130 B is formed of a first solid electrolyte  105 B, and a first electrode  104 B and a second electrode  106 B that are formed on both surfaces of the first solid electrolyte  105 B. The second electrode  106 B includes a second electrode portion  106 Ba facing a hollow measurement chamber  107 Bc described below. 
     The oxygen pump cell  140 B is formed of a second solid electrolyte  109 B, and a third electrode  108 B and a fourth electrode  110 B that are formed on both surfaces of the second solid electrolyte  109 B. The third electrode  108 B includes a third electrode portion  108 Ba facing the hollow measurement chamber  107 Bc described later. The fourth electrode  110 B includes a fourth electrode portion  110 Ba overlapping an electrode protection portion  113 Ba described below. 
     The first solid electrolyte  105 B and the second solid electrolyte  109 B are each composed of, for example, a partially stabilized zirconia sintered body obtained by adding yttria (Y 2 O 3 ) or the like to zirconia (ZrO 2 ). 
     An insulating layer  107 B is formed between the oxygen pump cell  140 B and the oxygen concentration detection cell  130 B. The insulating layer  107 B includes an insulating portion  114 B and a diffusion resistance portion  115 B. The hollow measurement chamber  107 Bc is formed in the insulating portion  114 B of the insulating layer  107 B at a position corresponding to the second electrode portion  106 Ba and the third electrode portion  108 Ba. The measurement chamber  107 Bc communicates with the outside in the width direction of the insulating layer  107 B, and a diffusion resistance portion  115 B that allows for gas diffusion between the outside and the measurement chamber  107 Bc under a predetermined rate-determining condition is disposed at the communication portion of the measurement chamber  107 Bc. 
     The insulating portion  114 B is composed of a ceramic sintered body having an insulation property (for example, an oxide-based ceramic material such as alumina or mullite). The diffusion resistance portion  115 B is composed of, for example, a porous body of alumina. 
     A protective layer  111 B is formed on the surface of the second solid electrolyte  109 B such that the fourth electrode  110 B is sandwiched therebetween. The protective layer  111 B includes, at the position overlapping the fourth electrode portion  110 Ba of the fourth electrode  110 B, the porous electrode protection portion  113 Ba for protecting the fourth electrode portion  110 Ba from poisoning. 
     A porous protective layer  20 B is formed so as to surround the entire periphery of the front side of the gas sensor element  100 B. 
     In the gas sensor element  100 B, the direction and the magnitude of a current flowing between electrodes of the oxygen pump cell  140 B are adjusted such that an electromotive force generated between the electrodes of the oxygen concentration detection cell  130 B becomes a predetermined value (for example, 450 mV). 
     In such a gas sensor element  100 B of the present embodiment, the electrodes that may be directly exposed to exhaust gas outside the sensor are the second electrode portion  106 Ba of the second electrode  106 B, the third electrode portion  108 Ba of the third electrode  108 B, and the fourth electrode portion  110 Ba of the fourth electrode  110 B. 
     Both the second electrode portion  106 Ba of the second electrode  106 B and the third electrode portion  108 Ba of the third electrode  108 B come into contact with exhaust gas that has entered the measurement chamber  107 Bc through the porous protective layer  20 B and the diffusion resistance portion  115 B. In addition, the fourth electrode portion  110 Ba of the fourth electrode  110 B comes into contact with exhaust gas that has passed through the porous protective layer  20 B and the electrode protection portion  113 Ba. That is, moisture such as exhaust condensed water may come into contact with these electrode portions. 
     Therefore, in the gas sensor element  100 B of the present embodiment, the second electrode portion  106 Ba, the third electrode portion  108 Ba, and the fourth electrode portion  110 Ba are made of a material that has water resistance and that is the same as those of the detection electrodes  130  and  130 A of Embodiments 1 and 2 described above. That is, each of such electrodes (electrode portions) contains zirconia or low-stabilizer-content zirconia to which a stabilizer is added in a proportion of 4 mol % or less (in terms of metal element), high-stabilizer-content zirconia to which a stabilizer is added in a proportion of greater than 4 mol % (in terms of metal element) and not greater than 20 mol % (in terms of metal element), and a noble metal, and is composed of a single layer. 
     As described above, an electrode made of a predetermined material having water resistance may be formed on the gas sensor element  100 B that is included in a plate-shaped sensor. 
     EXAMPLES 
     Hereinafter, the present invention will be described in further detail by way of the following Examples. The present invention is not limited to these examples in any way. 
     Example 1 
     A gas sensor element of Example 1 having the same basic configuration as the gas sensor element  100  of the sensor  10  described above was produced. The electrodes (detection electrode and reference electrode) of the gas sensor element were produced by the method described below. 
     First, a predetermined amount of an organic binder (ethyl cellulose) and a predetermined amount of a solvent (butyl carbitol acetate) were mixed while blending a predetermined amount of powdery zirconia (ZrO 2 ) and a predetermined amount of powdery high-stabilizer-content zirconia obtained by adding (doping) yttrium oxide (Y 2 O 3 ) as a stabilizer to zirconia (ZrO 2 ) in a proportion of 4.6 mol % (9.2 mol % in terms of metal element), in proportions (mass %) shown in Table 1, to prepare a slurry. In Table 1, the zirconia is represented as “ZrO 2 ”, and the high-stabilizer-content zirconia is represented as “YSZ (High)”. 
     Subsequently, a solid electrolyte was immersed (dipped) in the slurry to form a slurry layer on the surface of the solid electrolyte. Then, the slurry layer was dried by hot air drying. 
     Next, the slurry layer on the solid electrolyte was heat treated under a temperature condition of 1500° C. to sinter the slurry layer, thereby forming a base layer including a plurality of pores therein. 
     Then, a plating solution containing platinum as a noble metal was prepared, and the platinum was reduced and deposited on the base layer by electroless plating using the plating solution. In this manner, the detection electrode and the reference electrode were formed on the surface (outer surface, inner surface) of the solid electrolyte. 
     In the gas sensor element of Embodiment 1, the detection electrode was exposed, and a porous protective layer and the like were not formed. 
     Examples 2 to 6 and Comparative Examples 1 to 5 
     Gas sensor elements of Examples 2 to 6 and Comparative Examples 1 to 4 were produced in the same manner as Example 1, except that the blending proportions of the zirconia and the high-stabilizer-content zirconia in the slurry were changed to the values shown in Table 1. 
     Water Test 
     A water test was performed on the gas sensor elements of each Example and each Comparative Example according to the procedure described below. First, a predetermined voltage was applied to the ceramic heater in the gas sensor element to cause a heating portion housed inside the front end portion of the gas sensor element to generate heat to reach a specified temperature (initially 500° C.), thereby raising the temperature of the front end portion (detection portion) of the gas sensor element. The front end portion of a gas sensor element  100 T is covered with a detection electrode  130 T. 
       FIG. 11  is an explanatory diagram schematically illustrating a process for dropping a water drop onto the detection electrode  130 T of the gas sensor element  100 T in the water test. As shown in  FIG. 11 , the gas sensor element is disposed such that the longitudinal direction (axial line direction) of the gas sensor element extends horizontally. The detection electrode  130 T has a tubular shape as a whole, and the heating portion is disposed inside the detection electrode  130 T. When the heating portion generates heat, the detection electrode  130 T disposed around the heating portion is heated and the temperature thereof becomes high. With respect to the detection electrode  130 T, a portion surrounding the periphery of the heating portion is heated most and the temperature thereof is increased. In the water test, a specified amount (here, 2 μL) of a water drop  51 T is dropped onto such a portion (highest heat generating portion) X that is heated most and the temperature of which becomes the highest, by using a micro syringe  50 T. Such dropping of a water drop was performed once in total. 
     Thereafter, the heat generation of the heating portion was stopped, and the gas sensor element  100  was allowed to stand to cool until the temperature of the heating portion reached room temperature (25° C.). 
       FIG. 12  is an explanatory diagram schematically illustrating a process for checking a crack in the gas sensor element  100 T using an insulation meter  62 T in the water test. As shown in  FIG. 12 , the insulation meter  62 T was prepared, one terminal of the insulation meter  62 T was connected to the reference electrode (inner electrode) of the gas sensor element  100 T, and another terminal of the insulation meter  62 T was connected to water  61 T contained in a predetermined bath  60 T. Then, the front side of the gas sensor element  100 T was placed in the water  61 T such that the detection electrode  130 T was submerged therein, and it was confirmed whether or not a current flowed between the detection electrode  130 T and the reference electrode. If a crack has formed in the detection electrode  130 T of the gas sensor element  100 T or the like, water seeps into the crack and electricity flows between the detection electrode  130 T and the reference electrode. Therefore, if no current flows between the detection electrode  130 T and the reference electrode, it can be determined that no crack has occurred in the detection electrode  130  and the like. The results are shown in Table 1. 
     If there was no crack in the gas sensor element  100 T, the specified temperature of the heating portion was set sequentially higher as shown in Table 1, and the presence/absence of a crack in the gas sensor element  100 T was checked for each specified temperature. The above water test was performed until a crack occurred in the gas sensor element  100 T. The upper limit of the specified temperature of the heating portion was set to 700° C. In Table 1, the case where a crack occurred in the gas sensor element  100 T as a result of the water test is represented as “x”, and the case where a crack did not occur in the gas sensor element  100 T as a result of the water test is represented as “∘”. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                 SLURRY 
                 ELECTRODE TEMPERATURE 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 PROPORTION 
                 (SPECIFIED TEMPERATURE) ° C. 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 COMPONENT 
                 (wt %) 
                 500 
                 550 
                 575 
                 600 
                 625 
                 650 
                 675 
                 700 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 COMPARATIVE 
                 ZrO 2   
                 0 
                 x 
                   
                   
                   
                   
                   
                   
                   
               
               
                 EXAMPLE 1 
                 YSZ(High) 
                 100 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 COMPARATIVE 
                 ZrO 2   
                 10 
                 x 
                   
                   
                   
                   
                   
                   
                   
               
               
                 EXAMPLE 2 
                 YSZ(High) 
                 90 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 COMPARATIVE 
                 ZrO 2   
                 20 
                 ∘ 
                 x 
                   
                   
                   
                   
                   
                   
               
               
                 EXAMPLE 3 
                 YSZ(High) 
                 80 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 EXAMPLE 1 
                 ZrO 2   
                 40 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                   
                 YSZ(High) 
                 60 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 EXAMPLE 2 
                 ZrO 2   
                 50 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                   
                 YSZ(High) 
                 50 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 EXAMPLE 3 
                 ZrO 2   
                 60 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
                   
               
               
                   
                 YSZ(High) 
                 40 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 EXAMPLE 4 
                 ZrO 2   
                 70 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                   
                 YSZ(High) 
                 30 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 EXAMPLE 5 
                 ZrO 2   
                 80 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                   
                 YSZ(High) 
                 20 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 EXAMPLE 6 
                 ZrO 2   
                 90 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                   
                 YSZ(High) 
                 10 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 COMPARATIVE 
                 ZrO 2   
                 100 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
                   
                   
                   
                   
               
               
                 EXAMPLE 5 
                 YSZ(High) 
                 0 
               
               
                   
               
            
           
         
       
     
     In each gas sensor element used in the water test, the detection electrode is not covered with a porous protective layer or the like, and is exposed. That is, the water test was performed in a much harsher environment than a normal usage environment. As a result of such a water test, as shown in Table 1, in each of the gas sensor elements of Examples 1 to 6, it was confirmed that a crack did not occur in the detection electrode and the like when the specified temperature (electrode temperature) was 625° C. 
     For Examples 1, 2, 5, and 6, it was confirmed that a crack did not occur in the detection electrode and the like even when the specified temperature (electrode temperature) is 700° C. For Example 4, it was confirmed that a crack did not form in the detection electrode and the like when the specified temperature was increased up to 675° C. For Example 3, it was confirmed that a crack did not occur in the detection electrode and the like when the specified temperature was up to 650° C. 
     On the other hand, for Comparative Examples 1 and 2, the occurrence of a crack was confirmed when the specified temperature was 500° C.; for Comparative Example 3, the occurrence of a crack was confirmed when the specified temperature was 550° C.; and for Comparative Example 4, the occurrence of a crack was confirmed when the specified temperature was 575° C. In addition, for Comparative Example 5, the occurrence of a crack was confirmed when the specified temperature was 600° C. 
     Response Test 
     Sensors including the gas sensor elements of each Example and each Comparative Example were produced. The basic configuration of each sensor is the same as that of the sensor  10  described above. In each of the sensors for a response test, a porous protective layer is formed on a detection electrode with a base layer interposed therebetween. 
     Each of the sensors of each Example and each Comparative Example was mounted to an exhaust pipe of an internal combustion engine, and a response time (rich response time) in the case where an air-fuel ratio (A/F) shifted from a lean state to a rich state and a response time (lean response time) in the case where the air-fuel ratio (A/F) shifted from the rich state to the lean state, were measured. 
     The rich response time is the time (msec) until the sensor output reaches 800 mV from 450 mV when the air-fuel ratio shifts from the lean state to the rich state (from λ=1.03 to λ=0.97). 
     The lean response time is the time (msec) until the sensor output reaches 100 mV from 450 mV when the air-fuel ratio shifts from the rich state to the lean state (from λ=0.97 to λ=1.03). 
     The results of the rich response time for each Example and each Comparative Example are shown in  FIG. 13 , and the results of the lean response time for each Example and each Comparative Example are shown in  FIG. 14 . 
       FIG. 13  is a graph showing the results of the rich response times of the sensors of each Example and each Comparative Example. As shown in  FIG. 13 , it was confirmed that, when the blending proportion of the high-stabilizer-content zirconia is high (Comparative Example 1 to Comparative Example 4), the rich response time tends to become longer, and the responsiveness tends to be deteriorated. On the other hand, it was confirmed that, when the blending proportion of the high-stabilizer-content zirconia is low (Examples 1 to 6 and Comparative Example 5), the rich response time becomes shorter, and the responsiveness is excellent. 
       FIG. 14 a    graph showing the results of the lean response times of the sensors of each Example and each Comparative Example. As shown in  FIG. 14 , it was confirmed that, when the blending proportion of the zirconia is high (Examples 5 and 6 and Comparative Example 5), the lean response time tends to become longer, and the responsiveness tends to be deteriorated. On the other hand, it was confirmed that, when the blending proportion of the zirconia is low (Examples 1 to 4 and Comparative Examples 1 to 4), the lean response time becomes shorter, and the responsiveness is excellent. 
     The invention has been described in detail with reference to the above embodiments. However, the invention should not be construed as being limited thereto. 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.