Patent Publication Number: US-2021179496-A1

Title: Ceramic structured body and sensor element of gas sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of PCT/JP2018/036400, filed on Sep. 28, 2018, 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 structure of an outermost layer of a ceramic structured body, and particularly to suppression of ingress of fluid inside. 
     Description of the Background Art 
     Conventionally, as a gas sensor for determining concentration of a desired gas component in a measurement gas such as exhaust gas from an internal combustion, a gas sensor that includes a sensor element made of an oxygen-ion conductive solid electrolyte, such as zirconia (ZrO 2 ), and including some electrodes on the surface and the inside thereof has been widely known. A sensor element having an elongated planar element shape and including a protective layer (porous protective layer) made up of a porous body on an end portion on a side in which a gas inlet for introducing the measurement gas is provided has already been known (see Japanese Patent No. 4762338 and Japanese Patent No. 5287807, for example). 
     The porous protective layer of the sensor element disclosed in both Japanese Patent No. 4762338 and Japanese Patent No. 5287807 is provided for purpose of preventing so-called water-induced cracking. Herein, the water-induced cracking is a phenomenon that water droplets occurring by condensation of moisture vapor in the measurement gas adhere to the sensor element heated to a high temperature, thus thermal shock in accordance with a local temperature reduction is applied to the sensor element, and the sensor element cracks. 
     Japanese Patent No. 4762338 discloses a sensor element provided with a porous protective layer made up of two layers of a hydrophobic porous protective layer (inner layer) made up of hydrophobic heat-resistant particles having a contact angle with water of 75° or more and a hydrophilic porous protective layer (outer layer) made up of hydrophilic particles having a contact angle with water of 30° or less, thereby intending to prevent the water-induced cracking and also achieve a protection from a poisoned substance included in the measurement gas. 
     However, water enters the hydrophilic outer layer, thus a temperature reduction caused by a water exposure occurs to no small extent in the sensor element. 
     In the meanwhile, Japanese Patent No. 5287807 discloses a sensor element provided with, on an outer surface of a porous diffusion resistance layer, a surface protective layer having a hydrophilic property at normal temperature, having a water-repellent property at high temperature at which a solid electrolyte body is active, and having a surface roughness Ra of 3.0 μm or less with a thickness of 20 μm to 150 μm. 
     In this sensor element, the water-repellent property is developed by Leidenfrost phenomenon, however, a water resistance property (an upper limit of a water-exposure amount with which the water-induced cracking does not occur) remains at approximately 20 μL at most. 
     A sensor element of an oxygen sensor having a bottomed cylindrical element shape and provided with a poisoning prevention layer on a surface thereof also has already been known (see Japanese Patent No. 4440822, for example). 
     However, Japanese Patent No. 4440822 does not describe water-induced cracking at all, but describes that it is necessary for a poisoning prevention layer to have a hole substantially equal to a size distribution of ceramic grains (equal to or larger than 10 μm and equal to or smaller than 50 μm) which are a kind of constituent elements of the poisoning prevention layer. According to the latter condition, there is concern that water enters inside the element from the hole. 
     Furthermore, also known is that lotus effect is developed by a combination of a microstructure and a nanostructure (layered structure) to obtain a high water-repellent property (see “Micro-, nano- and hierarchical structures for superhydrophobicity, self-cleaning and low adhesion”, Bharat Bhushan, Yong Chae Jung, Kerstin Koch, Phil. Trans. R. Soc. A (2009) 367, 1631-1672, for example). 
     However, Bhushan et al. discloses a configuration of obtaining the layered structure using polymer, and does not particularly disclose a formation of the layered structure of ceramic. 
     SUMMARY 
     The present invention is therefore has been made to solve problems as described above, and it is an object of the present invention to provide a technique of appropriately suppressing ingress of water inside in a ceramic structured body such as a sensor element of a gas sensor, for example. 
     According to the present invention, a ceramic structured body includes: a first porous layer in at least a part of an outermost peripheral portion, wherein in the first porous layer, numerous convex parts each having a size of 1.0 μm or less and made up of ceramic microparticles with a diameter of 10 nm to 1.0 μm are discretely formed around numerous ceramic coarse grains each having diameters of 5.0 μm to 40 μm, the respective ceramic coarse grains are connected to each other directly or via the ceramic microparticle, and a degree of porosity is 5% to 50%. 
     Accordingly, the first porous layer has a high water-repellent property by lotus effect, thus appropriately suppressed is ingress of water inside the ceramic structured body in a position where the first porous layer is provided. Moreover, the first porous layer is made of ceramic, thus the ceramic structured body can be used under a high-temperature environment. 
     Another aspect of the present invention, a sensor element of a gas sensor includes: an element base which is a ceramic structured body including a detection part of detecting a target measurement gas component; and a protective layer which is a porous layer provided in at least a part of an outermost peripheral portion of the element base, wherein in the protective layer, numerous convex parts each having a size of 1.0 μm or less and made up of ceramic microparticles with a diameter of 10 nm to 1.0 μm are discretely formed around numerous ceramic coarse grains each having diameters of 5.0 μm to 40 μm, the respective ceramic coarse grains are connected to each other directly or via the ceramic microparticle, and a degree of porosity of the protective layer is 5% to 50%. 
     Accordingly, the protective layer has a high water-repellent property by lotus effect, thus appropriately suppressed is ingress of water inside the sensor element in a position where the protective layer is provided. The protective layer is provided in a portion in which a temperature becomes high when the gas sensor is in use, thus even when water droplets occurring by condensation of moisture vapor adhere to the portion, an occurrence of water-induced cracking in the sensor element is appropriately suppressed. 
     It is therefore an object of the present invention to provide a technique of appropriately suppressing ingress of water inside in a ceramic structured body such as a sensor element of a gas sensor, for example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic external perspective view of a sensor element  10 . 
         FIG. 2  is a schematic diagram illustrating a configuration of a gas sensor  100  including a sectional view taken along a longitudinal direction of the sensor element  10 . 
         FIG. 3  is a diagram schematically illustrating a detail configuration of an inner protective layer  21  and an outer protective layer  22 . 
         FIGS. 4A and 4B  are diagrams for description of an effect of the outer protective layer  22 . 
         FIG. 5  is a diagram illustrating a flow of processing at a manufacture of the sensor element  10 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     &lt;Overview of Sensor Element and Gas Sensor&gt; 
       FIG. 1  is a schematic external perspective view of a sensor element (gas sensor element)  10  as one configuration of a ceramic structured body including a surface structure according to an embodiment of the present invention. In the present embodiment, the ceramic structured body indicates a structure including ceramic as a main constituent material while having constituent element other than a ceramic component (for example, an electrode or an electrical wiring made up of metal, for example) inside or on a surface thereof. 
       FIG. 2  is a schematic diagram illustrating a configuration of a gas sensor  100  including a sectional view taken along a longitudinal direction of the sensor element  10 . The sensor element  10  is a main component of the gas sensor  100  detecting a predetermined gas component in a measurement gas, and measuring concentration thereof. The sensor element  10  is a so-called limiting current gas sensor element. 
     The gas sensor  100  mainly includes a pump cell power supply  30 , a heater power supply  40 , and a controller  50  in addition to the sensor element  10 . 
     As illustrated in  FIG. 1 , the sensor element  10  schematically includes a configuration that a side of one end portion of an elongated planar element base  1  is covered by a porous leading-end protective layer  2 . 
     As illustrated in  FIG. 2 , the element base  1  is a structure mainly made up of an elongated planar ceramic body  101  and includes a main surface protective layer  170  on two main surfaces of the ceramic body  101 , and the sensor element  10  is provided with the leading-end protective layer  2  on an end surface of one leading end portion (a tip end surface  101   e  of the ceramic body  101 ) and on an outer sides of four side surfaces. The four side surfaces of the sensor element  10  (or the element base  1 , or the ceramic body  101 ) other than opposite end surfaces in the longitudinal direction thereof are hereinafter simply referred to as side surfaces of the sensor element  10  (or the element base  1 , or the ceramic body  101 ). 
     The ceramic body  101  is made of ceramic containing, as a main component, zirconia (yttrium stabilized zirconia), which is an oxygen-ion conductive solid electrolyte. Various components of the sensor element  10  are provided outside and inside the ceramic body  101 . The ceramic body  101  having the configuration is dense and airtight. The configuration of the sensor element  10  illustrated in  FIG. 2  is just an example, and a specific configuration of the sensor element  10  is not limited to this configuration. 
     The sensor element  10  illustrated in  FIG. 2  is a so-called serial three-chamber structure type gas sensor element including a first internal chamber  102 , a second internal chamber  103 , and a third internal chamber  104  inside the ceramic body  101 . That is to say, in the sensor element  10 , the first internal chamber  102  communicates, through a first diffusion control part  110  and a second diffusion control part  120 , with a gas inlet  105  opening to the outside on a side of one end portion E 1  of the ceramic body  101  (to be precise, communicating with the outside through the leading-end protective layer  2 ), the second internal chamber  103  communicates with the first internal chamber  102  through a third diffusion control part  130 , and the third internal chamber  104  communicates with the second internal chamber  103  through a fourth diffusion control part  140 . A path from the gas inlet  105  to the third internal chamber  104  is also referred to as a gas distribution part. In the sensor element  10  according to the present embodiment, the distribution part is provided straight along the longitudinal direction of the ceramic body  101 . 
     The first diffusion control part  110 , the second diffusion control part  120 , the third diffusion control part  130 , and the fourth diffusion control part  140  are each provided as two slits vertically arranged in  FIG. 2 . The first diffusion control part  110 , the second diffusion control part  120 , the third diffusion control part  130 , and the fourth diffusion control part  140  provide predetermined diffusion resistance to a measurement gas passing therethrough. A buffer space  115  having an effect of buffering pulsation of the measurement gas is provided between the first diffusion control part  110  and the second diffusion control part  120 . 
     An external pump electrode  141  is provided on an outer surface of the ceramic body  101 , and an internal pump electrode  142  is provided in the first internal chamber  102 . Furthermore, an auxiliary pump electrode  143  is provided in the second internal chamber  103 , and a measurement electrode  145 , which is a detection part of directly detecting a target measurement gas component, is provided in the third internal chamber  104 . In addition, a reference gas inlet  106  which communicates with the outside and through which a reference gas is introduced is provided on a side of the other end portion E 2  of the ceramic body  101 , and a reference electrode  147  is provided in the reference gas inlet  106 . 
     In a case where a target of measurement of the sensor element  10  is NOx in the measurement gas, for example, concentration of a NOx gas in the measurement gas is calculated by a process as described below. 
     First, the measurement gas introduced into the first internal chamber  102  is adjusted to have an approximately constant oxygen concentration by a pumping action (pumping in or out of oxygen) of a main pump cell P 1 , and then introduced into the second internal chamber  103 . The main pump cell P 1  is an electrochemical pump cell including the external pump electrode  141 , the internal pump electrode  142 , and a ceramic layer  101   a  that is a portion of the ceramic body  101  existing between these electrodes. In the second internal chamber  103 , oxygen in the measurement gas is pumped out of the element by a pumping action of an auxiliary pump cell P 2  that is also an electrochemical pump cell, so that the measurement gas is in a sufficiently low oxygen partial pressure state. The auxiliary pump cell P 2  includes the external pump electrode  141 , the auxiliary pump electrode  143 , and a ceramic layer  101   b  that is a portion of the ceramic body  101  existing between these electrodes. 
     The external pump electrode  141 , the internal pump electrode  142 , and the auxiliary pump electrode  143  are each formed as a porous cermet electrode (e.g., a cermet electrode made of ZrO 2  and Pt that contains Au of 1%). The internal pump electrode  142  and the auxiliary pump electrode  143  to be in contact with the measurement gas are each formed using a material having weakened or no reducing ability with respect to a NOx component in the measurement gas. 
     NOx in the measurement gas caused by the auxiliary pump cell P 2  to be in the low oxygen partial pressure state is introduced into the third internal chamber  104 , and reduced or decomposed by the measurement electrode  145  provided in the third internal chamber  104 . The measurement electrode  145  is a porous cermet electrode also functioning as a NOx reduction catalyst that reduces NOx existing in the atmosphere in the third internal chamber  104 . During the reduction or decomposition, a potential difference between the measurement electrode  145  and the reference electrode  147  is maintained constant. Oxygen ions generated by the above-mentioned reduction or composition are pumped out of the element by a measurement pump cell P 3 . The measurement pump cell P 3  includes the external pump electrode  141 , the measurement electrode  145 , and a ceramic layer  101   c  that is a portion of the ceramic body  101  existing between these electrodes. The measurement pump cell P 3  is an electrochemical pump cell pumping out oxygen generated by decomposition of NOx in the atmosphere around the measurement electrode  145 . 
     Pumping (pumping in or out of oxygen) of the main pump cell P 1 , the auxiliary pump cell P 2 , and the measurement pump cell P 3  is achieved, under control performed by the controller  50 , by the pump cell power supply (variable power supply)  30  applying voltage necessary for pumping across electrodes included in each of the pump cells. In a case of the measurement pump cell P 3 , voltage is applied across the external pump electrode  141  and the measurement electrode  145  so that the potential difference between the measurement electrode  145  and the reference electrode  147  is maintained at a predetermined value. The pump cell power supply  30  is typically provided for each pump cell. 
     The controller  50  detects a pump current Ip2 flowing between the measurement   electrode  145  and the external pump electrode  141  in accordance with the amount of oxygen pumped out by the measurement pump cell P 3 , and calculates a NOx concentration in the measurement gas based on a linear relationship between a current value (NOx signal) of the pump current Ip2 and the concentration of decomposed NOx. 
     The gas sensor  100  preferably includes a plurality of electrochemical sensor cells, which are not illustrated, detecting the potential difference between each pump electrode and the reference electrode  147 , and each pump cell is controlled by the controller  50  based on a signal detected by each sensor cell. 
     In the sensor element  10 , the heater  150  is buried in the ceramic body  101 . The heater  150  is provided, below the gas distribution part in  FIG. 2 , over a range from the vicinity of the one end portion E 1  to at least a location of formation of the measurement electrode  145  and the reference electrode  147 . The heater  150  is provided mainly to heat the sensor element  10  to enhance oxygen-ion conductivity of the solid electrolyte forming the ceramic body  101  when the sensor element  10  is in use. More particularly, the heater  150  is provided to be surrounded by an insulating layer  151 . 
     The heater  150  is a resistance heating body made, for example, of platinum. The heater  150  generates heat by being powered from the heater power supply  40  under control performed by the controller  50 . 
     The sensor element  10  according to the present embodiment is heated by the heater  150  when being in use so that the temperature at least in a range from the first internal chamber  102  to the second internal chamber  103  becomes 500° C. or more. In some cases, the sensor element  10  is heated so that the temperature of the gas distribution part as a whole from the gas inlet  105  to the third internal chamber  104  becomes 500° C. or more. These are to enhance the oxygen-ion conductivity of the solid electrolyte forming each pump cell and to desirably demonstrate the ability of each pump cell. In this case, the temperature in the vicinity of the first internal chamber  102 , which becomes the highest temperature, becomes approximately 700° C. to 800° C. 
     In the following description, from among the two main surfaces of the ceramic body  101 , a main surface (or an outer surface of the sensor element  10  having the main surface) which is located on an upper side in  FIG. 2  and on a side where the main pump cell P 1 , the auxiliary pump cell P 2 , and the measurement pump cell P 3  are mainly provided is also referred to as a pump surface, and a main surface (or an outer surface of the sensor element  10  having the main surface) which is located on a lower side in  FIG. 2  and on a side where the heater  150  is provided is also referred to as a heater surface. In other words, the pump surface is a main surface closer to the gas inlet  105 , the three internal chambers, and the pump cells than to the heater  150 , and the heater surface is a main surface closer to the heater  150  than to the gas inlet  105 , the three internal chambers, and the pump cells. 
     A plurality of electrode terminals  160  are provided on the respective main surfaces of the ceramic body  101  on the side of the other end portion E 2  to establish electrical connection between the sensor element  10  and the outside. These electrode terminals  160  are electrically connected to the above-mentioned five electrodes, opposite ends of the heater  150 , and a lead for detecting heater resistance, which is not illustrated, through leads provided inside the ceramic body  101 , which are not illustrated, to have a predetermined correspondence relationship. Application of a voltage from the pump cell power supply  30  to each pump cell of the sensor element  10  and heating by the heater  150  by being powered from the heater power supply  40  are thus performed through the electrode terminals  160 . 
     The sensor element  10  further includes the above-mentioned main surface protective layers  170  ( 170   a,    170   b ) on the pump surface and the heater surface of the ceramic body  101 . The main surface protective layers  170  are layers made of alumina, having a thickness of approximately 5 μm to 30 μm, and including pores with a degree of porosity of approximately 20% to 40%, and are provided to prevent adherence of any foreign matter and poisoned substances to the main surfaces (the pump surface and the heater surface) of the ceramic body  101  and the external pump electrode  141  provided on the pump surface. The main surface protective layer  170   a  on the pump surface thus functions as a pump electrode protective layer for protecting the external pump electrode  141 . 
     In the present embodiment, the degree of porosity is obtained by applying a known image processing method (e.g., binarization processing) to a scanning electron microscope (SEM) image of an evaluation target. 
     The main surface protective layers  170  are provided over substantially all of the pump surface and the heater surface except that the electrode terminals  160  are partially exposed in  FIG. 2 , but this is just an example. The main surface protective layers  170  may locally be provided in the vicinity of the external pump electrode  141  on the side of the one end portion E 1  compared with the case illustrated in  FIG. 2 . 
     &lt;Details of Tip End Protective Layer&gt; 
     In the sensor element  10 , the leading-end protective layer  2  is provided around an outermost peripheral portion in a predetermined range from the one end portion E 1  of the element base  1  having a configuration as described above. The leading-end protective layer  2  is provided to have a thickness of 100 μm to 1000 μm. 
     The leading-end protective layer  2  is provided to surround a portion of the element base  1  in which the temperature becomes high (approximately 700° C. to 800° C. at a maximum) when the gas sensor  100  is in use to thereby securing water resistance property in the portion and suppress the occurrence of cracking (water-induced cracking) of the element base  1  due to thermal shock caused by local temperature reduction upon direct exposure of the portion to water. 
     In addition, the leading-end protective layer  2  is also provided to secure a poisoning resistance property for preventing poisoned substances such as Mg from entering inside the sensor element  10 . 
     As illustrated in  FIG. 2 , in the sensor element  10  according to the present embodiment, the leading-end protective layer  2  is made up of an inner leading-end protective layer (inner protective layer)  21  and an outer leading-end protective layer (outer protective layer)  22 .  FIG. 3  is a diagram schematically illustrating a detail configuration of the inner protective layer  21  and the outer protective layer  22 . 
     The inner protective layer  21  is provided on an outer side of a leading end surface  101   e  on a side of one end portion E 1  and four side surfaces of the element base  1  (an outer periphery of the element base  1  on a side of one end portion E 1 ).  FIG. 2  illustrates a portion  21   a  on a side of the pump surface, a portion  21   b  on a side of the heater surface, and a portion  21   c  on a side of the leading end surface  101   e  in the inner protective layer  21 . 
     As illustrated in  FIG. 3 , the inner protective layer  21  is a porous layer roughly having a configuration that numerous minute spherical pores p are dispersed in a matrix  21   m  including an aggregate made up of ceramic having a grain diameter of 1.0 μm to 10 μm and a binding material made up of ceramic having a grain diameter of 0.01 μm to 1.0 μm with a thickness of 50 μm to 950 μm. A degree of porosity is 20% to 85%. Such a configuration is achieved by a forming method described hereinafter. 
     In the present specification, the grain diameter is defined as a measurement value of a circumcircle of a primary particle which can be visually confirmed in a SEM image of a target evaluation object (measuring points n is equal to or larger than 100). In the case that the primary particle cannot be visually confirmed in a photographing result by a general SEM, the grain diameter may be specified based on an image obtained by a field emission type scanning electron microscope (FE-SEM) or an atomic force microscope (AFM). 
     More specifically, a size of the pore p (pore diameter) is 0.25 μm to 5.0 μm, and a neck diameter of the aggregate is equal to or smaller than 2.0 μm. These are appropriately adjusted by adjusting a particle diameter of a pore forming material used at a time of forming the inner protective layer  21 . In the present specification, the pore diameter is defined as a measurement value of a circumcircle of a primary particle which can be visually confirmed in a SEM image or a FE-SEM image of a target evaluation object (measuring points n is equal to or larger than 100). 
     When the pore diameter is set equal to or smaller than 5.0 μm while keeping the degree of porosity at 20% to 85% as the present embodiment, the minute pores p are uniformly dispersed, thus strength of the inner protective layer  21  is increased. A heat transfer path is miniaturized and thermal conductivity is reduced, thus high thermal insulation is further achieved in the inner protective layer  21 . The high thermal insulation has an effect of further improving the water resistance property of the sensor element  10 . For example, even when there is no difference in the configuration of the outer protective layer  22 , the sensor element  10  in which the inner protective layer  21  has the pore diameter of 5.0 μm or less has water resistance property superior to the sensor element  10  in which the pore diameter is larger than 5.0 μm. 
     Exemplified as a material of the aggregate is an oxide chemically stable in exhaust gas at high temperature such as alumina, spinel, titania, zirconia, magnesia, mullite, or cordierite. A mixture of plural types of oxide is also applicable. 
     Exemplified as a material of the binding material is an oxide chemically stable in exhaust gas at high temperature such as alumina, spinel, titania, zirconia, magnesia, mullite, or cordierite. A mixture of plural types of oxide is also applicable. 
     The inner protective layer  21  also has a role as underlying layer at the time when the outer protective layer  22  is formed with respect to the element base  1 . It is only required that the inner protective layer  21  be formed, on the side surfaces of the element base  1 , at least in a range surrounded by the outer protective layer  22 . 
     The outer protective layer  22  is provided to have a thickness of 50 μm to 950 μm in an outermost peripheral portion of the element base  1  in a predetermined range from the side of the one end portion E 1 . In the case illustrated in  FIG. 2 , the outer protective layer  22  is provided to cover the whole inner protective layer  21  provided on the side of one end portion E 1  (of the ceramic body  101 ) of the element base  1  from an outer side. 
     As illustrated in  FIG. 3 , the outer protective layer  22  has a configuration that numerous coarse grains  22   c  around which numerous minute convex parts made up of microparticles  22   f  are discretely formed are connected to each other directly or via the microparticles  22   f.  Such a configuration is achieved by a forming method described hereinafter. 
     A diameter of the coarse grain  22   c  is 5.0 μm to 40 μm, and a diameter of the microparticle  22   f  is equal to or larger than 10 nm and equal to or smaller than 1.0 μm. A weight ratio of the coarse grain  22   c  to the microparticle  22   f  (coarse grain/microparticle) is 3 to 35. In addition, a size of the convex part (height from a surface of the coarse particle  22   c ) is nano-level of 1.0 μm at most, and is preferably equal to or smaller than 500 nm. An average of intervals between the concave parts is approximately 100 nm to 1000 nm. 
     Exemplified as a material of the coarse grain  22   c  is an oxide chemically stable in exhaust gas at high temperature such as alumina, spinel, titania, zirconia, magnesia, mullite, or cordierite. A mixture of plural types of oxide is also applicable. 
     Exemplified as a material of the microparticle  22   f  is an oxide chemically stable in exhaust gas at high temperature such as alumina, spinel, titania, zirconia, magnesia, mullite, or cordierite. A mixture of plural types of oxide is also applicable. 
     The outer protective layer  22  satisfying these requirements has characteristics as a porous layer in which gas reaching from outside can pass through a gap g appropriately formed between the grains (mainly a gap between the convex parts made up of the microparticles  22   f ). 
     A degree of porosity of the outer protective layer  22  in such a case is preferably 5% to 50%. Furthermore, the degree of porosity of the outer protective layer  22  is preferably smaller than the degree of porosity of the inner protective layer  21 . In such a case, so-called anchoring effect acts between the outer protective layer  22  and the inner protective layer  21  as an underlying layer. Due to the action of the anchoring effect, in the sensor element  10 , delamination of the outer protective layer  22  from the element base  1  caused by a difference in coefficient of thermal expansion between the outer protective layer  22  and the element base  1  is more suitably suppressed when the sensor element  10  is in use. 
     In addition, the outer protective layer  22  has a layered structure of a microstructure and a nanostructure in which the numerous minute convex parts made up of the microparticles  22   f  are formed around the coarse grains  22   c,  thus its layer surface has a high water-repellent property by so-called lotus effect. 
       FIGS. 4A and 4B  are diagrams for description of the lotus effect in the outer protective layer  22 .  FIG. 4A  indicates a case where a water droplet dp having a size of approximately several μm adheres to the surface of the outer protective layer  22  according to the present embodiment, and  FIG. 4B  indicates a case where the similar water droplet dp adheres to a surface of a layer formed of only the coarse grains  22   c  having a size of μm order as with the configuration of a conventional sensor element. 
     Comparing the both cases, in the former case, the water droplets dp mainly have contact with the nanometer-size convex parts formed of the microparticles  22   f.  In contrast, in the latter case, the water droplets dp have contact with the coarse particles  22   c . A contact angle of the former case is larger than a contact angle of the latter case, thus in the latter case, each water droplet dp cannot keep its shape but easily loses the shape, however, in the former case, a surface tension of the water droplet dp is maintained. That is to say, the shape of the water droplet dp is maintained. In other words, the surface of the outer protective layer  22  illustrated in  FIG. 4A  has the excellent water-repellent property. In contrast, the conventional configuration illustrated in  FIG. 4B  has a poor water-repellent property, easily allows the fluid derived from the water droplet dp which has lost its shape to enter inside, and is not preferable. 
     Thus, the sensor element  10  according to the present embodiment having such a water-repellent property appropriately suppresses the ingress of the fluid inside the element from the outer protective layer  22  through the gap g. That is to say, the sensor element  10  according to the present embodiment is excellent in the water resistance property, thereby hardly causing the water-induced cracking compared with the conventional element. 
     When the degree of porosity of the inner protective layer  21  is larger than the degree of porosity of the outer protective layer  22 , the inner protective layer  21  has a higher heat insulation property than the outer protective layer  22  and the main surface protective layer  170 . This configuration also contributes to the improvement of the water resistance property of the sensor element  10 . 
     &lt;Process of Manufacturing Sensor Element&gt; 
     One example of a process of manufacturing the sensor element  10  having a configuration and features as described above will be described next.  FIG. 5  is a flowchart of processing at the manufacture of the sensor element  10 . 
     At the manufacture of the element base  1 , a plurality of blank sheets (not illustrated) being green sheets containing the oxygen-ion conductive solid electrolyte, such as zirconia, as a ceramic component and having no pattern formed thereon are prepared first (Step S 1 ). 
     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 when the sheets are in the form of the blank sheets. Green sheets corresponding to a portion of the ceramic body  101  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 are not required to have the same thickness, and may have different thicknesses in accordance with corresponding portions of the element base  1  eventually formed. 
     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  150  and the insulating layer  151 , a pattern of the electrode terminals  160 , a pattern of the main surface protective layers  170 , a pattern of internal wiring, which is not illustrated, and the like are formed. Application or placement of a sublimable material (vanishing material) for forming the first diffusion control part  110 , the second diffusion control part  120 , the third diffusion control part  130 , and the fourth diffusion control part  140  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. 
     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 (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 out at a plurality of locations to obtain unit bodies eventually becoming the individual element bases  1  (Step S 5 ). 
     The element bodies which have been obtained are then fired at a firing temperature of approximately 1300° C. to 1500° C. (step S 6 ). The element base  1  is thereby manufactured. That is to say, the element base  1  is generated by integrally firing the ceramic body  101  made of the solid electrolyte, the electrodes, and the main surface protective layers  170 . Integral firing is performed in this manner, so that the electrodes each have sufficient adhesion strength in the element base  1 . 
     After the element base  1  is manufactured in the above-mentioned manner, formation of the leading-end protective layer  2  is then performed on the element base  1 . The leading-end protective layer  2  is formed by applying slurry which is prepared in advance for the inner protective layer on a formation target location of the inner protective layer  21  in the element base  1  (Step S 7 ), then applying slurry which is similarly prepared in advance for the outer protective layer on a formation target location of the outer protective layer  22  in the element base  1  (Step S 8 ), and subsequently firing the element base  1  in which the application film is formed in such a manner (Step S 9 ). 
     The materials for slurry for forming the inner protective layer and slurry for forming the outer protective layer are exemplified as follows. 
     A material of the aggregate (the inner protective layer) and a material of the coarse particle (the outer protective layer): an oxide powder chemically stable in exhaust gas at high temperature such as alumina, spinel, titania, zirconia, magnesia, mullite, or cordierite;
         A material of the binding material (the inner protective layer) and a material of the microparticle (the outer protective layer): an oxide powder chemically stable in exhaust gas at high temperature such as alumina, spinel, titania, zirconia, magnesia, mullite, or cordierite;   A pore forming material (only the inner protective layer): it is not particularly designated, but a polymer pore forming material or carbon powder, for example, can be used. For example, acrylic resin, melamine resin, polyethylene particles, polystyrene particles, carbon black powder, or black lead powder can be used;   Binder (common in both layers): there is no particular limitation, but inorganic binder is preferable in terms of improvement of the strength of the inner protective layer  21  obtained by firing. For example, alumina sol, silica gel, or titania sol can be used;   solvent (common in both layers): a general aqueous system or non-aqueous system solvent such as water, ethanol, isopropyl alcohol (IPA) can be used;   A dispersed material (common in both layers): there is no particular limitation, but a material suitable for a solvent may be appropriately added, thus, for example, polycarboxylic system (such as ammonium salt), phosphate ester system, and naphthalene sulfonic acid formalin condensate can be used.       

     In the inner protective layer  21 , the pore diameter can be adjusted by adjusting the particle diameter of the pore forming material, and the degree of porosity can be adjusted by adjusting an amount of the pore forming material. 
     Applicable as a method of applying each slurry are various methods such as dipping coating, spin coating, spray coating, slit die coating, thermal spraying, AD method, and printing method. 
     For example, when slurry is applied by dipping coating, the following conditions are exemplified.
         Viscosity of slurry:
           For forming the outer protective layer: 10 mPa·s to 5000 mPa·s;   For forming the inner protective layer: 500 mPa·s to 7000 mPa·s;   
           Retracting speed: 0.1 mm/s to 10 mm/s;   Drying temperature: room temperature to 300° C.;   Drying time: one minute or more.   Conditions of firing performed after applying slurry are exemplified as follows.   Firing temperature: 800° C. to 1200° C.;   Firing time: 0.5 hours to 10 hours;   Firing atmosphere: atmospheric air.       

     The sensor element  10  obtained by the above procedure is housed in a predetermined housing, and built into the body, which is not illustrated, of the gas sensor  100 . 
     As described above, according to the present embodiment, provided is the protective layer having the layered structure that the numerous ceramic coarse grains around which numerous minute convex parts made up of the ceramic microparticles are discretely formed are connected to each other directly or via the ceramic microparticles on the outermost layer in a portion near the end portion, on the side in which the gas induction inlet is provided, of the sensor element of the gas sensor, thus the protective layer can function as the porous layer, and besides, the surface thereof can have the high water-repellent property by the lotus effect. Achievable by adopting such a configuration is the sensor element appropriately suppressing the ingress of water inside while flowing the gas component inside. 
     Particularly, the part where the layered structure is provided has a high temperature at the time of using the gas sensor (700° C. to 800° C. at a maximum), however, the layered structure is made up of ceramic, thus a particular trouble caused by such a layered structure does not occur at the time of using the gas sensor. That is to say, even when moisture vapor having high temperature is condensed to be water droplets and adheres to the sensor element, the ingress of water inside the sensor element is appropriately suppressed by water-repellent effect. 
     &lt;Modification Example&gt; 
     The above-mentioned embodiments are targeted at a sensor element having three internal chambers, but the sensor element may not necessarily have a three-chamber configuration. That is to say, the configuration that the outer protective layer of the sensor element is a water-repellent layer by the lotus effect is also applicable to a sensor element having two or one internal chamber. 
     In the above-mentioned embodiment, firing is performed after the application of slurry for forming the inner protective layer and slurry for forming the outer protective layer to form the two protective layers at the same time, however, also applicable instead is a configuration that firing is performed once when slurry for forming the inner protective layer is applied to form the inner protective layer, and then firing is performed after slurry for forming the outer protective layer is applied to form the outer protective layer. 
     The configuration that the layered structure in which the numerous minute convex parts made up of the nano-level ceramic microparticles are discretely provided around the micro-level ceramic coarse grains to develop the water-repellent property based on the lotus effect is applicable not only to an elongated planar limiting current sensor element having the above-mentioned configuration, but also to various types of ceramic sensor element regardless of whether or not the water-induced cracking may cause a problem and furthermore, regardless of whether a detection part of detecting a target detection gas component is located inside or located to be exposed outside. Furthermore, the above-mentioned configuration may be applied not only to the sensor element but also an outermost layer of a general ceramic structured body. Obviously, when the outermost layer of the general ceramic structured body is the water-repellent ceramic layer by lotus effect, an underlying layer thereof needs not have a structure as the sensor element. 
     The ceramic structured body of the present invention, that is to say, the ceramic structured body provided with the protective layer having the layered structure that the numerous ceramic coarse grains around which the numerous minute convex parts made up of the ceramic microparticles are discretely formed are connected to each other directly or via the ceramic microparticles on the outermost layer may be used for a purpose other than the sensor element  10 . For example, a ceramic structured body having the above-mentioned protective layer can be used as a setter for firing requiring a high thermal shock resistance property. 
     EXAMPLES 
     Manufactured was the sensor element  10  in which the outer protective layer  22  had the layered structure of the micro-level coarse grains  22   c  and the nano-level microparticles  22   f.    
     Firstly, a powder of alumina planar particles (average particle diameter of 6 μm) as a material of an aggregate and a powder of titania microparticles (average particle diameter of 0.25 μm) as a material of a binding material were weighted so that a weight ratio of them satisfies a coarse particle powder: microparticle powder=1:1 to manufacture slurry for the inner protective layer. These powders, alumina sol as an inorganic binder, acrylic resin as a pore forming material, and ethanol as a solvent were combined by a pot mill to obtain slurry for the inner protective layer. A mixing amount of alumina sol is 10 wt % of a total weight of the alumina powder and the titania powder. 
     A spinel powder (average particle diameter of 20 μm) as a coarse particle powder and a magnesia powder (average particle diameter of 0.05 μm) as a microparticle powder were weighted so that a weight ratio of them satisfies a coarse particle powder: microparticle powder=20:1 to manufacture slurry for the outer protective layer. These powders, alumina sol as an inorganic binder, polycarboxylic ammonium salt as a dispersing agent, and water as a solvent were mixed by a rotating and revolving mixer to obtain slurry for forming the outer protective layer. A mixing amount of alumina sol is 10 wt % of a total weight of the alumina powder and the titania powder. A mixing amount of polycarboxylic ammonium salt is 4 wt % of a weight of the microparticle powder. 
     Slurry for the inner protective layer manufactured in the above-mentioned manner was applied with a thickness of 300 μm to a formation target location of the inner protective layer  21  in the element base  1  which had been manufactured in advance by a known method by dipping coating. Subsequently, the element base  1  was dried for one hour in a drying machine being set to 200° C. 
     Next, slurry for the outer protective layer manufactured in the above-mentioned manner was applied with a thickness of 300 μm to a formation target location of the outer protective layer  22  in the element base  1 , which had been dried, by dipping coating. Subsequently, the element base  1  was dried for one hour in a drying machine being set to 200° C. 
     Finally, the element base  1  was fired for three hours at firing temperature of 1100° C. in the atmosphere to complete the sensor element  10  including the inner protective layer  21  and the outer protective layer  22 . 
     When the outer protective layer  22  of the obtained sensor element  10  was observed by a SEM, confirmed was a configuration that the coarse grains  22   c  around which the numerous minute convex parts made up of the microparticles  22   f  were discretely formed were sintered via the microparticles  22   f.  A size of the convex part is approximately 50 nm to 500 nm, and an interval between the concave parts is approximately 100 nm to 1000 nm. 
     Also confirmed was that the coarse grains  22   c  were spinel and the microparticles  22   f  were magnesia by a constitution analysis using an energy dispersive X-ray spectroscopy (EDS) and an X-ray diffractometer (XRD). 
     That is to say, confirmed was that the sensor element  10  in which the leading-end protective layer was made up of the outer protective layer and the inner protective layer and the outer protective layer had the layered structure made up of the micro-level ceramic coarse grains and the nano-level ceramic microparticles could be manufactured.