Patent Publication Number: US-2022236210-A1

Title: Sensor element of gas sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese applications JP2021-008721, filed on Jan. 22, 2021 and JP2022-002292, filed on Jan. 11, 2022, the contents of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a sensor element of a limiting current gas sensor, and, in particular, to a configuration of an electrode provided in an internal space of the sensor element. 
     Description of the Background Art 
     As a device for measuring the concentration of NOx in a measurement gas, such as a combustion gas and an exhaust gas from an internal combustion engine typified by an engine of a vehicle, a NOx sensor including a sensor element including a base formed of oxygen-ion conductive solid electrolyte ceramics, such as zirconia (ZrO 2 ), has been known (see Japanese Patent No. 2885336 and WO 2019/188613, for example). 
     The sensor element (a NOx sensor element) of the NOx sensor includes various electrodes (e.g., a pump electrode, a measurement electrode, and a reference electrode). These electrodes are porous cermet electrodes each formed of a composite material of a noble metal as a catalyst and zirconia as an electrolyte, and having a porous structure including many pores (cavities). As the catalytic noble metal, Pt and Pt with trace amounts of other substances (e.g., Rh and other noble metals) added are used. The NOx sensor element utilizes, at operation thereof, a catalytic reaction of the catalytic noble metal used for the electrodes and oxygen-ion conductivity of zirconia used for the base, and is thus used in a state of being heated to a relatively high sensor element driving temperature (600° C. to 900° C.). 
     The NOx sensor element is heated to the element driving temperature when used, and is at an ambient temperature when not used. A temperature rise and fall between the ambient temperature and the element driving temperature is thus repeated as the NOx sensor element is used continuously. In this repetition, thermal stress is caused in the electrodes provided in the element by a difference in coefficient of thermal expansion between a metal (noble metal) component as a main constituent material and a solid electrolyte underlying the electrodes. 
     In a pump electrode provided in the element, the thermal stress can cause separation of the electrode. Separation of the electrode leads to an abnormal pump current in each pump cell. 
     The occurrence of separation is actually suppressed to some extent by providing each of the electrodes as a cermet of the noble metal and the solid electrolyte, but the abnormal pump current can be generated even if it cannot clearly be said that separation occurs, and this is considered to be presumably because separation occurs at a microscopic level. 
     SUMMARY 
     The present invention relates to a sensor element of a limiting current type gas sensor, and is, in particular, directed to a configuration of an electrode provided in an internal space of the sensor element. 
     According to the present invention, a sensor element of a limiting current type gas sensor includes: a base part containing an oxygen-ion conductive solid electrolyte as a constituent material; at least one internal space into which a measurement gas is introduced; and at least one pump cell including an internal space electrode disposed to face the at least one internal space, an out-of-space pump electrode disposed at a location other than the at least one internal space, and a portion of the base part located between the internal space electrode and the out-of-space pump electrode, wherein the internal space electrode includes a noble metal, the solid electrolyte, and a pore, and, in the internal space electrode, a boundary line length ratio being a ratio of a length of a boundary of a first region and a second region with respect to a length of a boundary of the solid electrolyte and the internal space electrode is 1.1 or more, the first region being formed of the base part or the solid electrolyte contiguous with the base part, the second region being occupied by the noble metal and the pore. 
     A sensor element of a gas sensor in which separation of the internal space electrode is suitably suppressed even if the gas sensor is used continuously is thereby achieved. 
     It is thus an object of the present invention to provide a sensor element of a gas sensor in which separation of an electrode provided in an internal space is more suitably suppressed than that in a conventional gas sensor even if the gas sensor is used continuously. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing one example of a configuration of a gas sensor  100 ; 
         FIG. 2  is a model diagram showing a partial cross section of an inner pump electrode  22  along a thickness direction of an element; 
         FIG. 3  is a diagram further clarifying a boundary BL 1  of a first region RE 1  and a second region RE 2  in the model diagram showing the partial cross section of the inner pump electrode  22  shown in  FIG. 2 ; 
         FIG. 4  is a diagram for describing a way to evaluate the length of the boundary BL 1  of the first region RE 1  and the second region RE 2  targeted for  FIG. 3 ; 
         FIG. 5  is a model diagram showing a partial cross section of an inner pump electrode  22 Z; 
         FIG. 6  is a diagram further clarifying a boundary BL 2  of the first region RE 1  and the second region RE 2  in the model diagram showing the partial cross section of the inner pump electrode  22 Z shown in  FIG. 5 ; 
         FIG. 7  is a flowchart of processing at manufacture of a sensor element  101 ; 
         FIG. 8  is a diagram showing procedures for forming a pattern to eventually be an internal space electrode; and 
         FIG. 9  is a graph showing the rate of change of a pump voltage Vp 0  in a main pump cell  21  with respect to an initial value thereof in an accelerated separation test conducted on the gas sensor  100  of each of Examples 1 to 6 and a conventional example. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     &lt;General Configuration of Gas Sensor&gt; 
       FIG. 1  is a diagram schematically showing one example of a configuration of a gas sensor  100  according to an embodiment. The gas sensor  100  is a limiting current type NOx sensor sensing NOx and measuring the concentration thereof using a sensor element  101 . The gas sensor  100  further includes a controller  110  controlling operation of each part and identifying the NOx concentration based on a NOx current flowing through the sensor element  101 .  FIG. 1  includes a vertical cross-sectional view taken along a longitudinal direction of the sensor element  101 . 
     The sensor element  101  is a planar (elongated planar) element having a structure in which six solid electrolyte layers, namely, a first substrate layer  1 , a second substrate layer  2 , a third substrate layer  3 , a first solid electrolyte layer  4 , a spacer layer  5 , and a second solid electrolyte layer  6  each formed of zirconia (ZrO 2 ) (e.g., yttria stabilized zirconia (YSZ)) as an oxygen-ion conductive solid electrolyte are laminated in the stated order from a bottom side of  FIG. 1 . The solid electrolyte forming these six layers is dense and airtight. A surface on an upper side and a surface on a lower side of each of these six layers in  FIG. 1  are hereinafter also simply referred to as an upper surface and a lower surface, respectively. A part of the sensor element  101  formed of the solid electrolyte as a whole is generically referred to as a base part. 
     The sensor element  101  is manufactured, for example, by performing predetermined processing, printing of circuit patterns, and the like on ceramic green sheets corresponding to the respective layers, then laminating them, and further firing them for integration. 
     Between a lower surface of the second solid electrolyte layer  6  and an upper surface of the first solid electrolyte layer  4  in one leading end portion of the sensor element  101 , a first diffusion control part  11  doubling as a gas inlet  10 , a buffer space  12 , a second diffusion control part  13 , a first internal space  20 , a third diffusion control part  30 , a second internal space  40 , a fourth diffusion control part  60 , and a third internal space  61  are formed adjacent to each other to communicate in the stated order. 
     The buffer space  12 , the first internal space  20 , the second internal space  40 , and the third internal space  61  are spaces (regions) inside the sensor element  101  looking as if they were provided by hollowing out the spacer layer  5 , and having an upper portion, a lower portion, and a side portion respectively defined by the lower surface of the second solid electrolyte layer  6 , the upper surface of the first solid electrolyte layer  4 , and a side surface of the spacer layer  5 . The gas inlet  10  may similarly look as if it was provided by hollowing out the spacer layer  5  at a leading end surface (at the left end in  FIG. 1 ) of the sensor element  101  separately from the first diffusion control part  11 . In this case, the first diffusion control part  11  is formed inside and adjacent to the gas inlet  10 . 
     The first diffusion control part  11 , the second diffusion control part  13 , the third diffusion control part  30 , and the fourth diffusion control part  60  are each provided as two horizontally long slits (whose openings have longitudinal directions perpendicular to the page of  FIG. 1 ). A part extending from the gas inlet  10  to the third internal space  61  is also referred to as a gas distribution part. 
     At a location farther from the leading end than the gas distribution part is, a reference gas introduction space  43  having a side portion defined by a side surface of the first solid electrolyte layer  4  is provided between an upper surface of the third substrate layer  3  and a lower surface of the spacer layer  5 . For example, air is introduced into the reference gas introduction space  43  as a reference gas at measurement of the NOx concentration. 
     An air introduction layer  48  is a layer formed of porous alumina, and the reference gas is introduced into the air introduction layer  48  through the reference gas introduction space  43 . The air introduction layer  48  is formed to cover a reference electrode  42 . 
     The reference electrode  42  is an electrode formed to be sandwiched between the upper surface of the third substrate layer  3  and the first solid electrolyte layer  4 , and the air introduction layer  48  leading to the reference gas introduction space  43  is provided around the reference electrode  42  as described above. As will be described below, an oxygen concentration (oxygen partial pressure) in the first internal space  20  and the second internal space  40  can be measured using the reference electrode  42 . 
     In the gas distribution part, the gas inlet  10  (first diffusion control part  11 ) is a part opening to an external space, and a measurement gas is taken from the external space into the sensor element  101  through the gas inlet  10 . 
     The first diffusion control part  11  is a part providing predetermined diffusion resistance to the taken measurement gas. 
     The buffer space  12  is a space provided to guide the measurement gas introduced through the first diffusion control part  11  to the second diffusion control part  13 . 
     The second diffusion control part  13  is a part providing predetermined diffusion resistance to the measurement gas introduced from the buffer space  12  into the first internal space  20 . 
     In introducing the measurement gas from outside the sensor element  101  into the first internal space  20 , the measurement gas having abruptly been taken into the sensor element  101  through the gas inlet  10  due to pressure fluctuations (pulsation of exhaust pressure in a case where the measurement gas is an exhaust gas of a vehicle) of the measurement gas in the external space is not directly introduced into the first internal space  20  but is introduced into the first internal space  20  after concentration fluctuations of the measurement gas are canceled through the first diffusion control part  11 , the buffer space  12 , and the second diffusion control part  13 . This makes the concentration fluctuations of the measurement gas introduced into the first internal space  20  almost negligible. 
     The first internal space  20  is provided as a space to adjust oxygen partial pressure of the measurement gas introduced through the second diffusion control part  13 . The oxygen partial pressure is adjusted by operation of a main pump cell  21 . 
     The main pump cell  21  is an electrochemical pump cell including an inner pump electrode  22 , an outer (out-of-space) pump electrode  23 , and the second solid electrolyte layer  6  sandwiched between these electrodes. The inner pump electrode  22  has a ceiling electrode portion  22   a  provided on substantially the entire lower surface of a portion of the second solid electrolyte layer  6  facing the first internal space  20 , and the outer pump electrode  23  is provided in a region, on an upper surface of the second solid electrolyte layer  6  (one main surface of the sensor element  101 ), corresponding to the ceiling electrode portion  22   a  to be exposed to the external space. 
     The inner pump electrode  22  is formed on upper and lower solid electrolyte layers (the second solid electrolyte layer  6  and the first solid electrolyte layer  4 ) defining the first internal space  20 . Specifically, the ceiling electrode portion  22   a  is formed on the lower surface of the second solid electrolyte layer  6 , which provides a ceiling surface to the first internal space  20 , and a bottom electrode portion  22   b  is formed on the upper surface of the first solid electrolyte layer  4 , which provides a bottom surface to the first internal space  20 . The ceiling electrode portion  22   a  and the bottom electrode portion  22   b  are connected by a conducting portion (not illustrated) provided on a side wall surface (an inner surface) of the spacer layer  5  forming opposite side wall portions of the first internal space  20 . 
     The ceiling electrode portion  22   a  and the bottom electrode portion  22   b  are provided to be rectangular in plan view. Only the ceiling electrode portion  22   a  or only the bottom electrode portion  22   b  may be provided. 
     The inner pump electrode  22  and the outer pump electrode  23  are each formed as a porous cermet electrode. In particular, the inner pump electrode  22  to be in contact with the measurement gas is formed using a material having a weakened reducing ability with respect to a NOx component in the measurement gas. For example, the inner pump electrode  22  is formed as a cermet electrode of an Au—Pt alloy containing Au of approximately 0.6 wt % to 1.4 wt % and ZrO 2  to have a porosity of 5% to 40% and a thickness of 5 μm to 20 μm. A weight ratio Pt:ZrO 2  of the Au—Pt alloy and ZrO 2  is only required to be approximately 7.0:3.0 to 5.0:5.0. 
     In the present embodiment, the inner pump electrode  22  has a two-layer configuration including a region where pores are not present and a region where pores are present. Details thereof will be described below. 
     On the other hand, the outer pump electrode  23  is formed, for example, as a cermet electrode of Pt or an alloy thereof and ZrO 2  to be rectangular in plan view. 
     The main pump cell  21  can pump out oxygen in the first internal space  20  to the external space or pump in oxygen in the external space to the first internal space  20  by applying, from a variable power supply  24 , a desired pump voltage Vp 0  between the inner pump electrode  22  and the outer pump electrode  23  to allow a main pump current Ip 0  to flow between the inner pump electrode  22  and the outer pump electrode  23  in a positive or negative direction. The pump voltage Vp 0  applied between the inner pump electrode  22  and the outer pump electrode  23  in the main pump cell  21  is also referred to as a main pump voltage Vp 0 . 
     To detect the oxygen concentration (oxygen partial pressure) in an atmosphere in the first internal space  20 , the inner pump electrode  22 , the second solid electrolyte layer  6 , the spacer layer  5 , the first solid electrolyte layer  4 , the third substrate layer  3 , and the reference electrode  42  constitute a main sensor cell  80  as an electrochemical sensor cell. 
     The oxygen concentration (oxygen partial pressure) in the first internal space  20  can be known by measuring electromotive force V 0  in the main sensor cell  80 . 
     Furthermore, the controller  110  performs feedback control of the main pump voltage Vp 0  so that the electromotive force V 0  is constant, thereby to control the main pump current Ip 0 . The oxygen concentration in the first internal space  20  is thereby maintained at a predetermined constant value. 
     The third diffusion control part  30  is a part providing predetermined diffusion resistance to the measurement gas having an oxygen concentration (oxygen partial pressure) controlled by operation of the main pump cell  21  in the first internal space  20 , and guiding the measurement gas to the second internal space  40 . 
     The second internal space  40  is provided as a space to further adjust the oxygen partial pressure of the measurement gas introduced through the third diffusion control part  30 . The oxygen partial pressure is adjusted by operation of an auxiliary pump cell  50 . The oxygen concentration of the measurement gas is adjusted with higher accuracy in the second internal space  40 . 
     After the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space  20 , the auxiliary pump cell  50  further adjusts the oxygen partial pressure of the measurement gas introduced through the third diffusion control part  30  in the second internal space  40 . 
     The auxiliary pump cell  50  is an auxiliary electrochemical pump cell including an auxiliary pump electrode  51 , the outer pump electrode  23  (not limited to the outer pump electrode  23  and only required to be any appropriate electrode outside the sensor element  101 ), and the second solid electrolyte layer  6 . The auxiliary pump electrode  51  has a ceiling electrode portion  51   a  provided on substantially the entire lower surface of a portion of the second solid electrolyte layer  6  facing the second internal space  40 . 
     The auxiliary pump electrode  51  is provided in the second internal space  40  in a similar form to the inner pump electrode  22  provided in the first internal space  20  described previously. That is to say, the ceiling electrode portion  51   a  is formed on the second solid electrolyte layer  6 , which provides a ceiling surface to the second internal space  40 , and a bottom electrode portion  51   b  is formed on the first solid electrolyte layer  4 , which provides a bottom surface to the second internal space  40 . The ceiling electrode portion  51   a  and the bottom electrode portion  51   b  are rectangular in plan view, and are connected by a conducting portion (not illustrated) provided on the side wall surface (inner surface) of the spacer layer  5  forming opposite side wall portions of the second internal space  40 . 
     As with the inner pump electrode  22 , the auxiliary pump electrode  51  is formed using a material having a weakened reducing ability with respect to the NOx component in the measurement gas. 
     In the present embodiment, as with the inner pump electrode  22 , the auxiliary pump electrode  51  has a two-layer configuration including a region where pores are not present and a region where pores are present. 
     The auxiliary pump cell  50  can pump out oxygen in an atmosphere in the second internal space  40  to the external space or pump in oxygen in the external space to the second internal space  40  by applying a desired voltage (an auxiliary pump voltage) Vp 1  between the auxiliary pump electrode  51  and the outer pump electrode  23  under control performed by the controller  110 . 
     To control the oxygen partial pressure in the atmosphere in the second internal space  40 , the auxiliary pump electrode  51 , the reference electrode  42 , the second solid electrolyte layer  6 , the spacer layer  5 , the first solid electrolyte layer  4 , and the third substrate layer  3  constitute an auxiliary sensor cell  81  as an electrochemical sensor cell. 
     The auxiliary pump cell  50  performs pumping using a variable power supply  52  whose voltage is controlled based on electromotive force V 1  detected by the auxiliary sensor cell  81  in accordance with the oxygen partial pressure in the second internal space  40 . The oxygen partial pressure in the atmosphere in the second internal space  40  is thereby controlled to a low partial pressure having substantially no effect on measurement of NOx. 
     At the same time, a resulting auxiliary pump current Ip 1  is used to control the electromotive force in the main sensor cell  80 . Specifically, the auxiliary pump current Ip 1  is input, as a control signal, into the main sensor cell  80 , and, through control of the electromotive force V 0  therein, the oxygen partial pressure of the measurement gas introduced through the third diffusion control part  30  into the second internal space  40  is controlled to have a gradient that is always constant. In use as the NOx sensor, the oxygen concentration in the second internal space  40  is maintained at a constant value of approximately 0.001 ppm by the action of the main pump cell  21  and the auxiliary pump cell  50 . 
     The fourth diffusion control part  60  is a part providing predetermined diffusion resistance to the measurement gas having an oxygen concentration (oxygen partial pressure) controlled by operation of the auxiliary pump cell  50  in the second internal space  40 , and guiding the measurement gas to the third internal space  61 . 
     The third internal space  61  is provided as a space to perform processing concerning measurement of the nitrogen oxide (NOx) concentration of the measurement gas introduced through the fourth diffusion control part  60 . The NOx concentration is measured by operation of a measurement pump cell  41  in the third internal space  61 . The measurement gas having the oxygen concentration adjusted with high accuracy in the second internal space  40  is introduced into the third internal space  61 , so that the NOx concentration can be measured with high accuracy in the gas sensor  100 . 
     The measurement pump cell  41  measures the NOx concentration of the measurement gas in the third internal space  61 . The measurement pump cell  41  is an electrochemical pump cell including a measurement electrode  44 , the outer pump electrode  23 , the second solid electrolyte layer  6 , the spacer layer  5 , and the first solid electrolyte layer  4 . The measurement electrode  44  is provided on an upper surface of a portion of the first solid electrolyte layer  4  facing the third internal space  61  to be separated from the third diffusion control part  30 . 
     The measurement electrode  44  is a porous cermet electrode of a noble metal and a solid electrolyte. For example, the measurement electrode  44  is formed as a cermet electrode of Pt or an alloy of Pt and another noble metal, such as Rh, and ZrO 2  as a constituent material for the sensor element  101 . The measurement electrode  44  also functions as a NOx reduction catalyst to reduce NOx present in the atmosphere in the third internal space  61 . 
     The measurement pump cell  41  can pump out oxygen generated through decomposition of NOx in an atmosphere around the measurement electrode  44 , and detect the amount of generated oxygen as a pump current Ip 2  under control performed by the controller  110 . 
     In the present embodiment, as with the inner pump electrode  22 , the measurement electrode  44  has a two-layer configuration including a region where pores are not present and a region where pores are present. 
     To detect the oxygen partial pressure around the measurement electrode  44 , the second solid electrolyte layer  6 , the spacer layer  5 , the first solid electrolyte layer  4 , the third substrate layer  3 , the measurement electrode  44 , and the reference electrode  42  constitute a measurement sensor cell  82  as an electrochemical sensor cell. A variable power supply  46  is controlled based on electromotive force V 2  detected by the measurement sensor cell  82  in accordance with the oxygen partial pressure around the measurement electrode  44 . 
     NOx in the measurement gas introduced into the third internal space  61  is reduced by the measurement electrode  44  (2NO→N 2 +O 2 ) to generate oxygen. Oxygen as generated is to be pumped by the measurement pump cell  41 , and, in this case, a voltage (measurement pump voltage) Vp 2  of the variable power supply  46  is controlled so that the electromotive force V 2  detected by the measurement sensor cell  82  is constant. The amount of oxygen generated around the measurement electrode  44  is proportional to the NOx concentration of the measurement gas, and thus the NOx concentration of the measurement gas is to be calculated using the pump current Ip 2  in the measurement pump cell  41 . The pump current Ip 2  is hereinafter also referred to as a NOx current Ip 2 . 
     In the case that the measurement electrode  44 , the first solid electrolyte layer  4 , the third substrate layer  3 , and the reference electrode  42  are combined to constitute an oxygen partial pressure detection means as an electrochemical sensor cell, electromotive force in accordance with a difference between the amount of oxygen generated through reduction of a NOx component in the atmosphere around the measurement electrode  44  and the amount of oxygen contained in reference air can be detected, and the concentration of the NOx component in the measurement gas can thereby be determined. 
     The second solid electrolyte layer  6 , the spacer layer  5 , the first solid electrolyte layer  4 , the third substrate layer  3 , the outer pump electrode  23 , and the reference electrode  42  constitute an electrochemical sensor cell  83 , and oxygen partial pressure of the measurement gas outside the sensor can be detected using electromotive force Vref obtained by the sensor cell  83 . 
     The sensor element  101  further includes a heater part  70  playing a role in temperature adjustment of heating the sensor element  101  and maintaining the temperature thereof to enhance oxygen ion conductivity of the solid electrolyte forming the base part. 
     The heater part  70  mainly includes a heater electrode  71 , a heater element  72 , a heater lead  72   a , a through hole  73 , a heater insulating layer  74 , and a heater resistance detection lead, which is not illustrated in  FIG. 1 . A portion of the heater part  70  other than the heater electrode  71  is buried in the base part of the sensor element  101 . 
     The heater electrode  71  is an electrode formed to be in contact with a lower surface of the first substrate layer  1  (the other main surface of the sensor element  101 ). 
     The heater element  72  is a resistive heating element provided between the second substrate layer  2  and the third substrate layer  3 . The heater element  72  generates heat by being powered from a heater power supply, which is not illustrated in  FIG. 1 , outside the sensor element  101  through the heater electrode  71 , the through hole  73 , and the heater lead  72   a , which constitute a current-carrying path. The heater element  72  is formed of Pt, or contains Pt as a main component. The heater element  72  is buried, in a predetermined range of the sensor element  101  in which the gas distribution part is provided, to oppose the gas distribution part in a thickness direction of the element. The heater element  72  is provided to have a thickness of approximately 10 μm to 20 μm. 
     In the sensor element  101 , each part of the sensor element  101  can be heated to a predetermined temperature and the temperature can be maintained by allowing a current to flow through the heater electrode  71  to the heater element  72  to thereby cause the heater element  72  to generate heat. Specifically, the sensor element  101  is heated so that the temperature of the solid electrolyte and the electrodes in the vicinity of the gas distribution part is approximately 700° C. to 900° C. The oxygen ion conductivity of the solid electrolyte forming the base part of the sensor element  101  is enhanced by the heating. A heating temperature of the heater element  72  when the gas sensor  100  is in use (when the sensor element  101  is driven) is referred to as a sensor element driving temperature. 
     A degree of heat generation (heater temperature) of the heater element  72  is grasped by the magnitude of a resistance value (heater resistance) of the heater element  72 . 
     Although not illustrated in  FIG. 1 , a thermal shock resistant protective layer as a single- or multi-porous layer covering the sensor element  101  may further be provided outside in a predetermined range on a side of the one leading end portion (side of the left end in  FIG. 1 ) of the sensor element  101 . The thermal shock resistant protective layer is provided to prevent cracking of the sensor element  101  due to thermal shock caused by moisture contained in the measurement gas adhering to the sensor element  101  and condensing when the gas sensor  100  is in use, and prevent poisoning substances coexisting in the measurement gas from entering into the sensor element  101 . A laminar gap (gap layer) may be formed between the sensor element  101  and the thermal shock resistant protective layer. 
     &lt;Detailed Configuration of Internal Space Electrode&gt; 
     Configurations of the inner pump electrode  22 , the auxiliary pump electrode  51 , and the measurement electrode  44  (hereinafter, simply generically referred to as an internal space electrode) provided in the sensor element  101  having a configuration as described above and forming pump cells will be described in more detail next by taking the bottom electrode portion  22   b  of the inner pump electrode  22  as an example. The bottom electrode portion  22   b  is thus simply referred to as the inner pump electrode  22  in description relating to  FIGS. 2 to 6  below. Description based on  FIGS. 2 to 6 , however, is applicable to the ceiling electrode portion  22   a  and the second solid electrolyte layer  6  on which the ceiling electrode portion  22   a  is provided. 
       FIG. 2  is a model diagram showing a partial cross section of the inner pump electrode  22  along the thickness direction of the element. Dashed lines La and Lb shown in  FIG. 2  are added for ease of understanding. The model diagram is similarly applicable to the auxiliary pump electrode  51  and the measurement electrode  44 . 
     In  FIG. 2 , a portion below the dashed line La is the first solid electrolyte layer  4  formed of a solid electrolyte (zirconia), and the inner pump electrode  22  (more particularly, the bottom electrode portion  22   b  thereof) is provided on the first solid electrolyte layer  4 . A portion above the inner pump electrode  22  is the first internal space  20  (more particularly, a portion of the first internal space  20  not occupied by the inner pump electrode  22 , but hereinafter simply referred to as the first internal space  20  for the sake of convenience). 
     As described above, the inner pump electrode  22  is the porous cermet electrode of the noble metal, such as Pt and an alloy of Pt and Rh and the like, and the solid electrolyte (zirconia). Thus, as shown in  FIG. 2 , the inner pump electrode  22  has a configuration that there coexist a portion formed of a noble metal NM of white in  FIG. 2 , a portion formed of a solid electrolyte SE of gray (or light gray) in  FIG. 2 , and a portion formed of a pore (cavity) CV of black (or dark gray) in  FIG. 2 . As with the solid electrolyte SE, the first solid electrolyte layer  4  is of gray in  FIG. 2 . This is because they are both formed of zirconia. The first internal space  20  is of black in  FIG. 2  as with the pore CV, and this is because they are both spaces or gaps. 
     The inner pump electrode  22  according to the present embodiment, however, does not have a configuration in which the three portions (phases) formed of the noble metal NM, the solid electrolyte SE, and the pore CV randomly coexist as a whole, but has a two-layer configuration including a lower layer (also referred to as a two-phase region)  22 L where only the noble metal NM and the solid electrolyte SE randomly coexist, and the pore CV is not present and an upper layer (also referred to as a three-phase region)  22 U where the noble metal NM, the solid electrolyte SE, and the pore CV randomly coexist. A boundary between the lower layer  22 L and the upper layer  22 U is shown in the dashed line Lb in  FIG. 2 . 
     Viewed another way, the dashed line La shows a lowermost end of a range in which the noble metal NM is present in a thickness direction of the inner pump electrode  22 , and the dashed line Lb shows a lowermost end of a range in which the pore CV is present in the thickness direction of the inner pump electrode  22 . Expressed yet another way, it can be said that the dashed line La is a boundary between a region where the noble metal NM is present and a region where the noble metal NM is not present in the thickness direction of the inner pump electrode  22 , and the dashed line Lb is a boundary between a region where the pore CV is present and a region where the pore CV is not present in the thickness direction of the inner pump electrode  22 . 
     A portion of the solid electrolyte SE forming the inner pump electrode  22  contiguous with the base part formed of the solid electrolyte of the sensor element  101  (a portion contiguous with the base part beyond the dashed line La) is hereinafter particularly referred to as a contiguous region RE. Furthermore, a portion of the contiguous region RE belonging to the lower layer  22 L (a portion located between the dashed line La and the dashed line Lb) is referred to as a lower contiguous region REL, and a portion of the contiguous region RE belonging to the upper layer  22 U (a portion located between the dashed line Lb and the first internal space  20 ) is referred to as an upper contiguous region REU. 
     As described above, the inner pump electrode  22  of the sensor element  101  according to the present embodiment is characterized in that the inner pump electrode  22  as a whole is provided as the porous cermet electrode in which the noble metal NM, the solid electrolyte SE, and the pore CV coexist, but these three phases actually coexist only in the upper layer  22 U provided on a side of an upper surface of the electrode, and a range below the upper layer  22 U is the lower layer  22 L having a two-phase configuration in which the pore CV is not present. 
     In other words, in the inner pump electrode  22 , the boundary (dashed line La) between the base part formed of the solid electrolyte (more particularly, the first solid electrolyte layer  4 ) and the inner pump electrode  22  is different from the boundary (dashed line Lb) between the region where the pore CV is present and the region where the pore CV is not present. 
     &lt;Example of Identification of Locations of Boundaries&gt; 
     A way to identify locations of the respective boundaries (dashed lines La and Lb) when the locations of the boundaries are evaluated will be described next with an example, which are a location of the boundary of the region where the noble metal NM is present and the region where the noble metal NM is not present as the lowermost end of the range in which the noble metal NM is present and a location of the boundary of the region where the pore CV is present and the region where the pore CV is not present as the lowermost end of the range in which the pore CV is present in the thickness direction of the inner pump electrode  22  having a configuration as described above. 
     First, a cross-sectional image of the inner pump electrode  22  along the thickness direction as shown in  FIG. 2  is captured using a scanning electron microscope (SEM) and the like, for example. In this case, an image capturing range is set so that the inner pump electrode  22  is as horizontal as possible in the captured image, and the captured image includes at least the entire range in the thickness direction of the lower layer  22 L of the inner pump electrode  22  and a portion near the boundary between the inner pump electrode  22  and the first solid electrolyte layer  4 . Image capturing magnification is preferably approximately 500× to 1000× in view of the need to clearly identify each of interfaces among the three phases in the captured image. In this case, the cross-sectional image in a range of approximately 100 μm to 200 μm in the longitudinal direction of the element can be obtained. 
     Data of the obtained captured image is then analyzed based on a known image processing method to identify the ranges (all the pixels corresponding to the ranges) in which the noble metal NM, the solid electrolyte SE (and the first solid electrolyte layer  4 ), and the pore CV (and the first internal space  20 ) are present. In a case where the captured image is an SEM image, for example, which phases respective pixels of the captured image correspond to can be identified by a difference in brightness. 
     A coordinate point (pixel location) providing a location of a lowermost end of the noble metal NM in the captured image is then identified based on the data of the captured image. In a case of  FIG. 2 , a point A corresponds to the coordinate point. A straight line passing through the point A and being parallel to a horizontal direction of the captured image is to correspond to the dashed line La showing the lowermost end of the range in which the noble metal NM is present. 
     However, the inner pump electrode  22  might be inclined to no small extent in the captured image, although the inner pump electrode  22  is as horizontal as possible at image capturing. In light of the foregoing, the dashed line La showing the lowermost end of the range in which the noble metal NM is present may be set by identifying, from a range of the noble metal NM in the captured image, the coordinate point (e.g., the point A in  FIG. 2 ) of the location of the lowermost end and a coordinate point (e.g., a point B in  FIG. 2 ) at the second lowest location, correcting the captured image so that a straight line passing through the point A and the point B is horizontal, and determining the straight line passing through the point A and the point B in the corrected captured image as the dashed line La. 
     When the dashed line La is identified, a coordinate point (pixel location) providing a location of a lowermost end of the pore CV in the (corrected) captured image is then identified based on the data of the captured image. In a case of  FIG. 2 , a point C corresponds to the coordinate point. A straight line passing through the point C and being parallel to the horizontal direction of the captured image is to correspond to the dashed line Lb showing the lowermost end of the range in which the pore CV is present. 
     &lt;Boundary Line Length of Solid Electrolyte Region&gt; 
     In the sensor element  101  according to the present embodiment, the contiguous region RE as a portion of the solid electrolyte SE forming the inner pump electrode  22  is contiguous with the base part formed of the solid electrolyte as described above. The majority of the contiguous region RE is the lower contiguous region REL belonging to the lower layer  22 L in which the pore is not present, while a portion of the contiguous region RE is the upper contiguous region REU further contiguous with the lower contiguous region REL and belonging to the upper layer  22 U. 
     Since the pore CV is not present in the lower layer  22 L, portions which are in a lower end portion of the lower layer  22 L and in which the solid electrolyte SE is present constitute the contiguous region RE as a whole. The contiguous region RE is thus significantly formed in the sensor element  101  according to the present embodiment compared with a conventional sensor element in which the pore CV is present up to the lower end portion of the lower layer  22 L. 
     Significant formation of the contiguous region RE in the inner pump electrode  22  means that a boundary of a region formed of the solid electrolyte and contiguous upward from a side of the first solid electrolyte layer  4  in the thickness direction of the element (i.e., a region including the first solid electrolyte layer  4  and the contiguous region RE, hereinafter generically referred to as a first region RE 1 ) and a region above the first region RE 1  occupied by the noble metal NM, the pore CV, and, further, the first internal space  20  (hereinafter generically referred to as a second region RE 2 ) is not along the boundary of the first solid electrolyte layer  4  and the inner pump electrode  22  (i.e., dashed line La), and enters into the inner pump electrode  22  in the thickness direction of the element. 
       FIG. 3  is a diagram further clarifying a boundary BL 1  between the first region RE 1  and the second region RE 2  by showing the first region RE 1  and the second region RE 2  in monochrome in the model diagram showing the partial cross section of the inner pump electrode  22  shown in  FIG. 2 . 
     When the regions are shown in monochrome, the noble metal NM completely surrounded by the solid electrolyte SE, and not in contact with the pore CV may be included in the first region RE 1 . This is because the noble metal NM does not contribute to formation of the boundary BL 1 . On the other hand, the solid electrolyte SE completely surrounded by only the pore CV and the noble metal NM, and not forming the contiguous region RE may be included in the second region RE 2 . This is because the solid electrolyte SE also does not contribute to formation of the boundary BL 1 . 
       FIG. 4  is a diagram for describing a way to evaluate the length (hereinafter, a boundary line length) of the boundary BL 1  of the first region RE 1  and the second region RE 2  targeted for  FIG. 3 . 
     As shown in  FIG. 4 , a polygonal line obtained by taking points located on the boundary BL 1  at predetermined intervals p in a horizontal direction in  FIG. 4  corresponding to the longitudinal direction of the element (a direction of extension of the straight line La in  FIG. 2 ), and sequentially connecting all the points Q( 1 ) to Q(n) using line segments from a side of one end (a side of a left end in  FIG. 4 ) is an approximate line of the boundary BL 1 . The sum of lengths of these line segments is approximately the boundary line length. In the present embodiment, a degree of entry of the boundary BL 1  into the inner pump electrode  22  is evaluated based on the magnitude of the boundary line length (approximately) obtained in this manner. That is to say, the boundary BL 1  of the first region and the second region varies significantly in the thickness direction of the element when the boundary line length has a greater value, and the boundary of the first region and the second region is along the boundary (i.e., dashed line La) of the first solid electrolyte layer  4  and the inner pump electrode  22  when the boundary line length has a smaller value. 
     Furthermore, the value of the boundary line length is closer to a true value when the intervals p have smaller values, but it is practically sufficient to set each of the values of the intervals p to approximately 0.1 μm to 1 μm. 
       FIG. 5  is a model diagram showing a partial cross section of an inner pump electrode  22 Z having a configuration in which three phases randomly coexist in the electrode as a whole shown for comparison.  FIG. 6  is a diagram further clarifying a boundary BL 2  between the first region RE 1  and the second region RE 2  by showing the first region RE 1  and the second region RE 2  in monochrome in the model diagram showing the partial cross section of the inner pump electrode  22 Z shown in  FIG. 5 . 
     A location at which the inner pump electrode  22 Z is disposed in the sensor element is the same as a location at which the inner pump electrode  22  is disposed in the sensor element. A portion of white in  FIG. 5  is the portion formed of the noble metal NM, a portion of gray in  FIG. 5  is the portion formed of the solid electrolyte SE, and a portion of black in  FIG. 5  is the portion formed of the pore CV also in the inner pump electrode  22 Z shown in  FIG. 5 . 
     In the inner pump electrode  22 Z, the boundary between the region where the pore CV is present and the region where the pore CV is not present generally matches the boundary between the base part formed of the solid electrolyte (more particularly, the first solid electrolyte layer  4 ) and the inner pump electrode  22 Z. That is to say, an appreciable portion of the base part is in contact with the pore CV or the noble metal NM. Although the contiguous region RE as a portion of the solid electrolyte SE contiguous with the base part is present, a degree of formation thereof is small as the pore CV is present. 
     Thus, the boundary BL 2  shown in  FIG. 6  almost matches the boundary of the first solid electrolyte layer  4  and the inner pump electrode  22 Z. The boundary line length in the sensor element including the inner pump electrode  22 Z thus has a smaller value than the boundary line length in the sensor element  101  including the inner pump electrode  22  having the two-layer configuration. 
     This suggests that the boundary line length has a value reflecting the configuration of the inner pump electrode  22 . 
     The configuration of the inner pump electrode  22  in which the solid electrolyte contiguous with the first solid electrolyte layer  4  enters into a noble metal portion and the boundary line length has a greater value produces the so-called anchoring effect. The configuration of the inner pump electrode  22  used in the gas sensor  100  according to the present embodiment is thus effective at suppressing separation of the inner pump electrode  22 . 
     More particularly, the anchoring effect can be obtained to some extent when a ratio (boundary line length ratio) of the length of the boundary of the first region RE 1  and the second region RE 2  with respect to the length of the boundary (dashed line La) of the first solid electrolyte layer  4  and the inner pump electrode  22  in a certain section (e.g., from the left end to a right end in  FIG. 4 ) as one is 1.1 or more. 
     For example, the boundary line length ratio of 1.1 or more is suitably obtained when a volume ratio of the solid electrolyte SE in the upper layer  22 U is 20% to 40%, and a volume ratio of the solid electrolyte SE in the lower layer  22 L is 50% to 60%. The boundary line length ratio can be 1.1 or more even if these volume ratios are not satisfied. 
     On the other hand, an upper limit of the boundary line length ratio of 3.0 will suffice in terms of suppression of separation, although it can be said that a greater numerical value is better. In actuality, it is not necessarily easy to form the inner pump electrode  22  in which the boundary line length ratio exceeds 3.0. 
     The second region RE 2  includes the pore CV, and a portion of the boundary of the second region RE 2  and the first region RE 1  formed of the solid electrolyte SE is a boundary of the pore CV and the solid electrolyte SE forming the contiguous region RE. The portion naturally does not contribute to adhesion of the inner pump electrode  22  to the first solid electrolyte layer  4 . A discussion on the anchoring effect and, further, the effect of suppressing separation of the inner pump electrode  22  based on the magnitude of the boundary line length ratio is thus seemingly inappropriate. The inner pump electrode  22 , however, is provided as the porous electrode, and it is thus assumed that the pore CV is present. The boundary line length ratio can sufficiently be used as a relative index to evaluate anchoring effects and separation suppressive effects of such porous electrodes. 
     The likelihood of separation of the inner pump electrode  22  can be evaluated by an accelerated separation test in which turn-on and -off of the gas sensor  100  is repeated many times in air, and a change of the pump voltage Vp 0  in the main pump cell  21  in this case is examined. 
     A temperature rise and fall between an ambient temperature and the element driving temperature by the heater part  70  is repeated when the turn-on and -off of the gas sensor  100  is repeated, and, in this case, thermal expansion and thermal contraction of each of the electrodes of the sensor element  101  repeatedly occur due to a difference in coefficient of thermal expansion between a noble metal component and the solid electrolyte forming the electrode. As a result of thermal stress caused by the repeated thermal expansion and thermal contraction, microscopic separation starts to occur in each of the electrodes. Microscopic separation particularly significantly occurs in the inner pump electrode  22  that can be at the highest temperature in the sensor element  101 . When the separation starts to occur, the value of the pump voltage Vp 0  in the main pump cell  21  changes. The pump voltage Vp 0  significantly changes as separation of the inner pump electrode  22  progresses. The occurrence of separation of the inner pump electrode  22  can thus be grasped by monitoring the pump voltage Vp 0  in the accelerated separation test. 
     Description made so far is targeted mainly for the inner pump electrode  22 , but the model diagram shown in  FIG. 2  is similarly applicable to the auxiliary pump electrode  51  and the measurement electrode  44  as described above. Also for the auxiliary pump electrode  51  and the measurement electrode  44 , the first region RE 1  and the second region RE 2  can be identified, and the anchoring effect can be obtained when the boundary line length ratio of the regions is 1.1 or more (and 3.0 or less). 
     &lt;Process of Manufacturing Sensor Element&gt; 
     A process of manufacturing the sensor element  101  having a configuration and features as described above will be described next. In the present embodiment, a laminated body including green sheets (also referred to as base material tapes) containing zirconia as a ceramic component is formed, and the laminated body is cut and fired to manufacture the sensor element  101 . 
     Description will be made below by taking, as an example, a case where the sensor element  101  including the six layers shown in  FIG. 1  is manufactured. In this case, six green sheets corresponding to the first substrate layer  1 , the second substrate layer  2 , the third substrate layer  3 , the first solid electrolyte layer  4 , the spacer layer  5 , and the second solid electrolyte layer  6  are to be prepared.  FIG. 7  is a flowchart of processing at manufacture of the sensor element  101 . 
     In a case where the sensor element  101  is manufactured, blank sheets (not illustrated) being green sheets having no pattern formed thereon are prepared first (step S 1 ). In a case where the sensor element  101  including the six layers is manufactured, six blank sheets corresponding to the respective layers are prepared. 
     The blank sheets have a plurality of sheet holes used for positioning in printing and lamination. The sheet holes are formed to the blank sheets in advance prior to pattern formation through, for example, punching by a punching machine. Green sheets corresponding to layers in which an internal space is formed also include penetrating portions corresponding to the internal space formed in advance through, for example, punching as described above. The blank sheets corresponding to the respective layers of the sensor element  101  are not required to have the same thickness. 
     After preparation of the blank sheets corresponding to the respective layers, pattern printing and drying are performed on the individual blank sheets (step S 2 ). Specifically, a pattern of various electrodes, a pattern of the heater element  72  and the heater insulating layer  74 , a pattern of internal wiring, which is not illustrated, and the like are formed. 
     Application or placement of a sublimable material to form the first diffusion control part  11 , the second diffusion control part  13 , and the third diffusion control part  30  is also performed at the time of pattern printing. 
     The patterns are printed by applying pastes for pattern formation prepared in accordance with the properties required for respective formation targets onto the blank sheets using known screen printing technology. A known drying means can be used for drying after printing. 
     A method different from a conventional method is used to form, from among these patterns, a pattern to eventually be the internal space electrode (the inner pump electrode  22 , the auxiliary pump electrode  51 , and the measurement electrode  44 ). In this regard, description will be made below. 
     After pattern printing on each of the blank sheets, printing and drying of a bonding paste are performed to laminate and bond the green sheets corresponding to the respective layers (step S 3 ). The known screen printing technology can be used for printing of the bonding paste, and the known drying means can be used for drying after printing. 
     The green sheets to which an adhesive has been applied are then stacked in a predetermined order, and the stacked green sheets are crimped under predetermined temperature and pressure conditions to thereby form a laminated body (step S 4 ). Specifically, crimping is performed by stacking and holding the green sheets as a target of lamination on a predetermined lamination jig, which is not illustrated, while positioning the green sheets at the sheet holes, and then heating and pressurizing the green sheets together with the lamination jig using a lamination machine, such as a known hydraulic pressing machine. The pressure, temperature, and time for heating and pressurizing depend on a lamination machine to be used, and these conditions may be determined appropriately to achieve good lamination. 
     After the laminated body is obtained as described above, the laminated body is cut at a plurality of locations into individual units (referred to as element bodies) of the sensor elements  101  (step S 5 ). 
     The element bodies obtained by cutting are each fired at a firing temperature of approximately 1300° C. to 1500° C. (step S 6 ). The sensor element  101  is thereby manufactured. That is to say, the sensor element  101  is generated by integrally firing the solid electrolyte layers and the electrodes. The firing temperature in this case is preferably 1200° C. or more and 1500° C. or less (e.g., 1400° C.). Integral firing is performed in this manner, so that the electrodes each have sufficient adhesion strength in the sensor element  101 . 
     The sensor element  101  thus obtained is housed in a predetermined housing, and built into the body (not illustrated) of the gas sensor  100 . 
     &lt;Method for Forming Internal Space Electrode&gt; 
     As described above, in the present embodiment, the internal space electrode having a two-layer configuration as described above is provided in the sensor element  101 . Formation of the internal space electrode will be described. 
       FIG. 8  is a diagram showing procedures for forming a pattern to eventually be the internal space electrode. There are two methods for forming the pattern to be the internal space electrode: a manufacturing method A, and a manufacturing method B. In the manufacturing method A, an electrolyte paste containing, as a ceramic component, zirconia as with the green sheets is first applied at a location, on an upper surface of a green sheet to form the first solid electrolyte layer  4 , as a target of formation of each internal space electrode (step S 21 A). The electrolyte paste is a paste obtained by mixing powder of zirconia as the solid electrolyte and an organic component including a binder and the like. 
     Then, a three-component mixed paste is superimposedly applied onto an applied film formed by application of the electrolyte paste (step S 22 ). The three-component mixed paste is herein a paste obtained by mixing powder of the noble metal containing Pt as the main component, powder of zirconia as the ceramic component, powder of a pore forming material as a sublimable material to form the pore CV in the internal space electrode eventually formed, and an organic component including a binder and the like, and is prepared in advance in accordance with a type of the internal space electrode (any of the inner pump electrode  22 , the auxiliary pump electrode  51 , and the measurement electrode  44 ) as a formation target. 
     When application of the three-component mixed paste is completed, a paste applied film (paste-doubled film) where the electrolyte paste and the three-component mixed paste are superimposed is pressed by a predetermined pressing means (step S 23 ). By pressing, some of particles of the noble metal and the pore forming material included in the applied film of the three-component mixed paste enter into the applied film of the electrolyte paste. 
     After that, when firing targeted for an element body is performed as described above, the paste-doubled film is also fired, and volatilization of the organic component and, further, sintering of the noble metal NM and the solid electrolyte SE progress. In a case of the inner pump electrode  22 , for example, the solid electrolyte in the electrolyte paste is integrated with the solid electrolyte in the green sheet, and a portion in which some particles of the noble metal have entered into the electrolyte paste becomes the lower layer  22 L with the progress of sintering. On the other hand, in a portion in which the three-component mixed paste is applied, the pore forming material sublimates with the progress of sintering of the noble metal NM and the solid electrolyte SE to form the pore CV, and the upper layer  22 U in which the noble metal NM, the solid electrolyte SE, and the pore CV coexist is eventually obtained. As a result, the inner pump electrode  22  having a two-layer configuration as shown in  FIG. 2  is formed. The same applies to the auxiliary pump electrode  51  and the measurement electrode  44 . 
     Application thicknesses and application areas of the electrolyte paste and the three-component mixed paste are only required to be set by taking into account contraction by firing and entry of the particles of the noble metal and the pore forming material by pressing so that the thicknesses of the upper layer and the lower layer of the internal space electrode eventually formed and a planar area of the internal space electrode have desired values. For example, a relationship between the application thicknesses and the application areas of the electrolyte paste and the three-component mixed paste and the thicknesses of the upper layer and the lower layer of the internal space electrode and the planar area of the internal space electrode may experimentally be identified in advance. A mixing ratio of the powder of the noble metal, the powder of zirconia as the solid electrolyte, and the powder of the pore forming material of the three-component mixed paste is only required to be set by taking into account entry of the particles of the noble metal and the pore forming material by pressing so that the volume ratios of the noble metal NM, the solid electrolyte SE, and the pore CV in the upper layer (three-phase region) of the internal space electrode eventually formed have desired values. 
     On the other hand, in the manufacturing method B, a two-component mixed paste is applied at the location, on the upper surface of the green sheet to form the first solid electrolyte layer  4 , as the target of formation of the internal space electrode (step S 21 B). The two-component mixed paste is herein a paste obtained by mixing the powder of the noble metal containing Pt as the main component, the powder of zirconia as the ceramic component, and an organic component including a binder and the like, and is prepared in advance in accordance with a type of the internal space electrode (any of the inner pump electrode  22 , the auxiliary pump electrode  51 , and the measurement electrode  44 ) as a formation target. 
     Hereinafter, the three-component mixed paste is superimposedly applied onto an applied film formed by application of the two-component mixed paste (step S 22 ), and, further, a paste applied film (paste-doubled film) where the two-component mixed paste and the three-component mixed paste are superimposed is pressed by a predetermined pressing means (step S 23 ), as in the manufacturing method A. 
     Also in a case of the manufacturing method B, some of particles of the noble metal and the pore forming material included in the applied film of the three-component mixed paste enter into the applied film of the two-component mixed paste by pressing. 
     After that, when firing targeted for the element body is performed as described above, the paste-doubled film is also fired, and volatilization of the organic component and, further, sintering of the noble metal NM and the solid electrolyte SE progress. A portion in which the two-component mixed paste is applied thus becomes the lower layer. On the other hand, in the portion in which the three-component mixed paste is applied, the pore forming material sublimates with the progress of sintering of the noble metal NM and the solid electrolyte SE to form the pore CV, and the upper layer in which the noble metal NM, the solid electrolyte SE, and the pore CV coexist is eventually obtained. Also in this case, as a result, the internal space electrode having the two-layer configuration is formed. 
     Also in the case of the manufacturing method B, the application thicknesses and the application areas of the two-component mixed paste and the three-component mixed paste are only required to be set by taking into account contraction by firing and entry of the particles of the noble metal and the pore forming material by pressing so that the thicknesses of the upper layer and the lower layer of the internal space electrode eventually formed and the planar area of the internal space electrode have desired values. A mixing ratio of the powder of the noble metal and the powder of zirconia as the solid electrolyte of the two-component mixed paste is only required to be set by taking into account entry of the particles of the noble metal and the pore forming material by pressing so that the volume ratios of the noble metal NM and the solid electrolyte SE in the lower layer (two-phase region) of the internal space electrode eventually formed have desired values. Similarly, the mixing ratio of the powder of the noble metal, the powder of zirconia as the solid electrolyte, and the powder of the pore forming material of the three-component mixed paste is only required to be set by taking into account entry of the particles of the noble metal and the pore forming material by pressing so that the volume ratios of the noble metal NM, the solid electrolyte SE, and the pore CV in the upper layer (three-phase region) of the internal space electrode eventually formed have desired values. As in the case of the manufacturing method A, the relationship among these values may experimentally be identified in advance. 
     As described above, according to the present embodiment, the internal space electrode is provided in the internal space of the sensor element of the limiting current type gas sensor to have the two-layer configuration despite that it is provided as the porous cermet electrode as a whole, including the lower layer in which only the noble metal and the solid electrolyte are present and the pore is not present and the upper layer in which the noble metal, the solid electrolyte, and the pore coexist, and thereby the length of the boundary of the region formed of the solid electrolyte and the region occupied by the noble metal and the pore has a greater value, so that separation of the internal space electrode is suitably suppressed. A gas sensor that can be used continuously is thereby achieved. 
     &lt;Modifications&gt; 
     Although the two methods, that is, the manufacturing method A and the manufacturing method B, are shown in the above-mentioned embodiment as the method for forming the pattern to be the internal space electrode, a manufacturing method C may be used in place of these methods. 
     In the manufacturing method C, the electrolyte paste used in the manufacturing method A, the two-component mixed paste used in the manufacturing method B, and the three-component mixed paste used in the manufacturing method A and the manufacturing method B are applied in the stated order at the location, on the upper surface of the green sheet to form the first solid electrolyte layer  4 , as the target of formation of the internal space electrode, and then are pressed by a predetermined pressing means. 
     Also in a case of the manufacturing method C, the application thicknesses and the application areas of the electrolyte paste, the two-component mixed paste, and the three-component mixed paste are only required to be set by taking into account contraction by firing and entry of the particles of the noble metal and the pore forming material by pressing so that the thicknesses of the upper layer and the lower layer of the internal space electrode eventually formed and the planar area of the internal space electrode have desired values. The mixing ratio of the powder of the noble metal and the powder of zirconia as the solid electrolyte of the two-component mixed paste is only required to be set by taking into account entry of the particles of the noble metal and the pore forming material by pressing so that the volume ratios of the noble metal NM and the solid electrolyte SE in the lower layer (two-phase region) of the internal space electrode eventually formed have desired values. Similarly, the mixing ratio of the powder of the noble metal, the powder of zirconia as the solid electrolyte, and the powder of the pore forming material of the three-component mixed paste is only required to be set by taking into account entry of the particles of the noble metal and the pore forming material by pressing so that the volume ratios of the noble metal NM, the solid electrolyte SE, and the pore CV in the upper layer (three-phase region) of the internal space electrode eventually formed have desired values. As in the case of the manufacturing method A and the manufacturing method B, the relationship among these values may experimentally be identified in advance. 
     EXAMPLES 
     Six types of gas sensors  100  (Examples 1 to 6) differing in conditions for manufacturing the inner pump electrode  22  were manufactured as examples, and, for each of the obtained gas sensors  100 , volume ratios of respective phases (the noble metal, the solid electrolyte, and the pore) in each of the upper layer  22 U and the lower layer  22 L of the inner pump electrode  22  and the boundary line length ratio were evaluated. The accelerated separation test was also conducted to evaluate the likelihood of separation of the inner pump electrode  22 . 
     At formation of the inner pump electrodes  22 , the inner pump electrodes  22  were manufactured by two different manufacturing methods, that is, the manufacturing method A and the manufacturing method B, and, for each of the manufacturing methods, the three-component mixed pastes to mainly form the upper layers  22 U had two different weight ratios of the powder of the noble metal, the powder of the solid electrolyte, and the pore forming material. One type of the electrolyte paste was used in the manufacturing method A, and one type of the two-component mixed paste was used in the manufacturing method B. 
     A gas sensor including the inner pump electrode  22 Z formed using the three-component mixed paste as a whole was also manufactured as a conventional example (a method for manufacturing the gas sensor is hereinafter referred to as a conventional manufacturing method), and, on the manufactured gas sensor, the volume ratios of the respective phases (the noble metal, the solid electrolyte, and the pore) of the inner pump electrode  22 Z were evaluated, and the accelerated separation test in air was conducted as with the gas sensors of the examples. 
     Weight ratios (component ratios) of the powder of the noble metal, the powder of the solid electrolyte, and the pore forming material of the pastes used to manufacture the inner pump electrodes  22  of Examples 1 to 6 and the inner pump electrode  22 Z of the conventional example are shown in Table 1 as a list. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 INNER PUMP ELECTRODE PASTE COMPONENT RATIOS (WEIGHT RATIOS) 
               
            
           
           
               
               
               
               
            
               
                   
                 INNER PUMP 
                 FOR UPPER LAYER FORMATION (a:b:c) 
                 FOR LOWER LAYER FORMATION (d:e) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 ELECTRODE 
                 NOBLE 
                   
                 PORE FORMING 
                   
                 NOBLE 
                   
               
               
                   
                 FORMATION 
                 METAL 
                 ELECTROLYTE 
                 MATERIAL 
                   
                 METAL 
                 ELECTROLYTE 
               
               
                   
                 METHOD 
                 a 
                 b 
                 c 
                 USED PASTE 
                 d 
                 e 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 CONVENTIONAL 
                 CONVENTIONAL 
                 10 
                 2 
                 1 
                 NOT USED 
                 — 
                 — 
               
               
                 EXAMPLE 
                 MANUFACTURING 
               
               
                   
                 METHOD 
               
               
                 EXAMPLE 1 
                 MANUFACTURING 
                 10 
                 2 
                 1 
                 ELECTROLYTE 
                 — 
                 10 
               
               
                   
                 METHOD A 
                   
                   
                   
                 PASTE 
               
               
                 EXAMPLE 2 
                 MANUFACTURING 
                 30 
                 10 
                 1 
                   
                 — 
                 10 
               
               
                   
                 METHOD A 
               
               
                 EXAMPLE 3 
                 MANUFACTURING 
                 30 
                 10 
                 1 
                   
                 — 
                 10 
               
               
                   
                 METHOD A 
               
               
                 EXAMPLE 4 
                 MANUFACTURING 
                 10 
                 2 
                 1 
                 TWO- 
                 5 
                 1 
               
               
                   
                 METHOD B 
                   
                   
                   
                 COMPONENT 
               
               
                 EXAMPLE 5 
                 MANUFACTURING 
                 30 
                 10 
                 1 
                 MIXED PASTE 
                 5 
                 1 
               
               
                   
                 METHOD B 
               
               
                 EXAMPLE 6 
                 MANUFACTURING 
                 10 
                 2 
                 1 
                   
                 5 
                 1 
               
               
                   
                 METHOD B 
               
               
                   
               
            
           
         
       
     
     In Table 1, weight ratios of the three-component mixed pastes are shown in columns “FOR UPPER LAYER FORMATION”, and weight ratios of the electrolyte pastes and weight ratios of the two-component mixed pastes are shown in columns “FOR LOWER LAYER FORMATION”. 
     When a, b, and c are respectively weight ratios of the powder of the noble metal, the powder of the solid electrolyte, and the pore forming material of the three-component mixed pastes, and d and e are respectively weight ratios of the powder of the noble metal and the powder of the solid electrolyte of the two-component mixed pastes, the weight ratios of the three-component mixed pastes satisfy an equation a:b:c=10:2:1 (Examples 1, 4, and 6) or an equation a:b:c=30:10:1 (Examples 2, 3, and 5) as shown in Table 1. When the mixing ratio c of the pore forming material of each of the three-component mixed pastes is one, the two-component mixed pastes satisfy an equation (c:)d:e=(1:)5:1 (Examples 4, 5, and 6). Furthermore, as for the weight ratio of the powder of the solid electrolyte of each of the electrolyte pastes, which is expressed as e for the sake of convenience, relative to the mixing ratio c of the pore forming material of each of the three-component mixed pastes, an equation c:e=1:10 (Examples 1, 2, and 3) is satisfied. 
     In each of Examples 1 to 3 in which the inner pump electrode  22  is formed by the manufacturing method A, an intended application thickness of the electrolyte paste is 10 μm, and an intended application thickness of the three-component mixed paste is 15 μM. 
     In each of Examples 4 to 6 in which the inner pump electrode  22  is formed by the manufacturing method B, an intended application thickness of the two-component mixed paste is 5 μm, and the intended application thickness of the three-component mixed paste is 15 μm. 
     In the conventional example in which the inner pump electrode  22 Z is manufactured by the conventional manufacturing method, the three-component mixed paste satisfies the equation a:b:c=10:2:1, and the intended application thickness of the three-component mixed paste is 15 μm. 
     The volume ratios of the respective phases in each of the upper layer  22 U and the lower layer  22 L of the inner pump electrode  22  and results of evaluation of the boundary line length ratio in each of the examples and the conventional example are shown in Table 2. The volume ratios in the inner pump electrode  22 Z of the conventional example are shown in a column “UPPER LAYER” for the sake of convenience. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 INNER PUMP ELECTRODE VOLUME RATIOS (%): 
                   
               
               
                   
                 IN RELATION TO EACH LAYER AS A WHOLE AS 100% 
                 BOUNDARY 
               
            
           
           
               
               
               
               
            
               
                   
                 UPPER LAYER (THREE-PHASE REGION) 
                 LOWER LAYER (TWO-PHASE REGION) 
                 LINE 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 NOBLE METAL P1 
                 ELECTROLYTE P2 
                 PORE P3 
                 NOBLE METAL P4 
                 ELECTROLYTE P5 
                 LENGTH RATIO 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 CONVENTIONAL 
                 40 
                 20 
                 40 
                 — 
                 — 
                 1 
               
               
                 EXAMPLE 
               
               
                 EXAMPLE 1 
                 40 
                 20 
                 40 
                 40 
                 60 
                 1.2 
               
               
                 EXAMPLE 2 
                 45 
                 40 
                 15 
                 45 
                 55 
                 1.4 
               
               
                 EXAMPLE 3 
                 45 
                 40 
                 15 
                 45 
                 55 
                 1.1 
               
               
                 EXAMPLE 4 
                 40 
                 20 
                 40 
                 40 
                 60 
                 2.8 
               
               
                 EXAMPLE 5 
                 45 
                 40 
                 15 
                 40 
                 60 
                 1.7 
               
               
                 EXAMPLE 6 
                 40 
                 20 
                 40 
                 40 
                 60 
                 3.3 
               
               
                   
               
            
           
         
       
     
     In Table 2, P1(%), P2(%), and P3(%) are respectively the volume ratios of the noble metal NM, the solid electrolyte SE, and the pore CV in the upper layer  22 U, and P4(%) and P5(%) are respectively the volume ratios of the noble metal NM and the solid electrolyte SE in the lower layer  22 L. An equation P1+P2+P3=P4+P5=100(%) is satisfied. 
     The volume ratios of the respective phases in each of the inner pump electrodes  22  and the inner pump electrode  22 Z were obtained by capturing a cross-sectional SEM image of the electrode, and performing known image processing on the cross-sectional SEM image. The cross-sectional SEM image was generally ternary valued so that the region where the noble metal NM was present was white, the region where the solid electrolyte SE was present was gray, and the region where the pore or the internal space was present was black, and ratios of the areas (ratios of the cross-sectional areas) of the respective regions were set to the volume ratios. The boundary line length ratio was obtained based on an SEM image of a cross section perpendicular to the thickness direction of the sensor element including the first solid electrolyte layer  4  and the inner pump electrode  22 . Specifically, points on the boundary BL 1  of the first region and the second region were first identified at intervals of 0.1 μm in a horizontal direction of the SEM image corresponding to the longitudinal direction of the element, the sum of the lengths of the line segments obtained by sequentially connecting the identified points were obtained as the boundary line length, and a ratio of the boundary line length with respect to the boundary line length of the conventional example as one was obtained as the boundary line length ratio. 
     The results shown in Table 2 indicate that the inner pump electrode  22  has the two-layer configuration including the upper layer  22 U and the lower layer  22 L in each of Examples 1 to 6. 
     More particularly, values of P1, P2, P3, P4, and P5 are the same in Examples 1, 4, and 6, which differ in the method for manufacturing the inner pump electrode  22 , but have in common that the weight ratios of the three-component mixed pastes satisfy the equation a:b:c=10:2:1. From among these values, the volume ratio of the noble metal NM in each of the upper layer  22 U and the lower layer  22 L satisfies an equation P1=P4=40%. As for the volume ratio of the solid electrolyte SE, the value of P2 in the upper layer  22 U is 20%, whereas the value of P5 in the lower layer  22 L is 60%, which is greater than the value of P2. 
     In contrast, in Examples 2, 3, and 5, which differ in the method for manufacturing the inner pump electrode  22 , but have in common that the weight ratios of the three-component mixed pastes satisfy the equation a:b:c=30:10:1, the values of P1, P2, and P3 are the same, but the values of P4 and P5 in Examples 2 and 3 differ from those in Example 5 by 5%. That is to say, the volume ratio P1 of the noble metal NM in the upper layer  22 U has a value of 45%, and the volume ratio P2 of the solid electrolyte SE in the upper layer  22 U has a value of 40% in each of Examples 2, 3, and 5, but the volume ratio P4 of the noble metal NM in the lower layer  22 L has a value of 45% in each of Examples 2 and 3, and has a value of 40% in Example 5. In response to these values, the volume ratio P5 of the solid electrolyte SE has a value of 55% in each of Examples 2 and 3, and has a value of 60% in Example 5. 
     In each of the examples, however, the volume ratio of the solid electrolyte SE in the lower layer  22 L is greater than the volume ratio of the solid electrolyte SE in the upper layer  22 U. Specifically, the volume ratio of the solid electrolyte SE in the upper layer  22 U is in a range of 20% to 40%, and the volume ratio of the solid electrolyte SE in the lower layer  22 L is in a range of 50% to 60%. 
     The volume ratios P1, P2, and P3 of the respective phases in the inner pump electrode  22 Z of the conventional example are the same as those of Examples 1 and 4 in each of which the three-component mixed paste having the same weight ratios is used. 
     The boundary line length has a greater value in each of Examples 4 to 6 manufactured by the manufacturing method B than in each of the Examples 1 to 3 manufactured by the manufacturing method A. Examples (Examples 2 and 3, and Examples 4 and 6) having the manufacturing method and the weight ratios in the three-component mixed paste in common, however, differ in value of the boundary line length ratio, and a correspondence relationship between the weight ratios and the value of the boundary line length ratio varies depending on the manufacturing method. 
     On the other hand, the accelerated separation test was conducted by repeating the turn-on and -off of the gas sensor  100  in air 100,000 times, and examining the change of the pump voltage Vp 0  in the main pump cell  21  in this case. 
       FIG. 9  is a graph showing the rate of change of the pump voltage Vp 0  in the main pump cell  21  with respect to an initial value thereof in the accelerated separation test conducted on the gas sensor  100  of each of Examples 1 to 6 and the conventional example. In  FIG. 9 , a horizontal axis represents the number of times the turn-on and -off of the gas sensor  100  is repeated, and a vertical axis represents the pump voltage Vp 0 . 
     As shown in  FIG. 9 , in the gas sensor of the conventional example, the pump voltage Vp 0  significantly increases with increasing number of times the turn-on and -off is repeated, and the rate of change of the pump voltage Vp 0  taking into account variation is approximately 60±15% when the number of times the turn-on and -off is repeated reaches 100,000 times, whereas, in the gas sensor of each of Examples 1 to 6, the rate of change of the pump voltage Vp 0  remains at most at 35% when the number of times the turn-on and -off is repeated reaches 100,000 times. This means that significant separation of the inner pump electrode  22 Z occurs in the gas sensor of the conventional example, whereas separation of the inner pump electrode  22  is suppressed in the gas sensor of each of Examples 1 to 6. 
     The above-mentioned results of the accelerated separation test indicate that, when the inner pump electrode  22  of the sensor element of the limiting current type gas sensor is provided to have the two-layer configuration including the lower layer having the two-phase configuration of the noble metal and the solid electrolyte and the upper layer having the three-phase configuration of the noble metal, the solid electrolyte, and the pore, and the ratio of the length of the boundary of the first region RE 1  formed of the solid electrolyte and contiguous upward from the side of the first solid electrolyte layer  4  in the thickness direction of the element and the second region RE 2  above the first region RE 1  occupied by the noble metal NM, the pore CV, and, further, the first internal space  20  with respect to the length of the boundary between the first solid electrolyte layer  4  and the inner pump electrode  22  in the longitudinal direction of the element is set to 1.1 or more, separation of the inner pump electrode  22  when the gas sensor is used continuously can be suppressed. 
     While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.