Patent Publication Number: US-2015084508-A1

Title: Structural body and electrode structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-197112 filed on Sep. 24, 2013, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a structural body including an insulating body and a conductive material. For example, the present invention relates to a structural body and an electrode structure suitable for use, e.g., in a dielectric-barrier discharge electrode, an ozone generator, or the like. 
     2. Description of the Related Art 
     Heretofore, low-temperature plasma generators described, for example, in Japanese Patent No. 3015268 have been known as a structural body containing an insulating body and a conductive material. 
     Japanese Patent No. 3015268 describes an example of providing structure of one low-temperature plasma generator in the following manner. In particular, a rod-shaped ceramic dielectric body is inserted into a through hole of a rod-shaped ceramic dielectric body extending in the longitudinal direction. Both ends of the conductive body and the dielectric body are integrally joined and sealed with a glass or an inorganic or organic adhesive to form one electrode. A plurality of the electrodes are joined together by line-contact in the ceramic dielectric body. Potentials having different polarities are applied to the adjacent electrodes. 
     SUMMARY OF THE INVENTION 
     However, in the plasma generator described in Japanese Patent No. 3015268, since the adjacent electrodes (to which potentials having different polarities are applied) are joined together by line-contact, supply of fluid such as the air into a space between the adjacent electrodes is not expected. 
     Further, with respect to the electric field distribution contributing to the ozone generation efficiency, the electric field is only generated at a recess having the bottom at the joined portion of surfaces of the adjacent electrodes (surfaces of rod shaped ceramic dielectric bodies). The area of the electric field is small in comparison with the electric field generated in a gap (space) between electrodes which face each other. Therefore, in the example of Japanese Patent No. 3015268, the efficient ozone generation cannot be expected. 
     That is, the above recess is narrow as an area to which oxygen molecules as the ozone raw material are supplied, and the electric field as an energy source of ozone generation cannot be provided efficiently. 
     In an attempt to address the problem, as a possible method to increase the amount of generated ozone, it is suggested to dispose a large number of electrodes that are joined together, and use thick electrodes to increase the effective volume for discharge of electricity (e.g., see FIG. 5 of Japanese Patent No, 3015268). However, in this method, the pressure loss is increased disadvantageously. 
     The present invention has been made to take the problems of this type into consideration, and an object of the present invention is to provide a structural body and an electrode structure in which when fluid passes through the structural body or bodies, the pressure loss in the fluid is reduced, and improvement in the ozone generation efficiency is achieved. 
     [1] According to a first aspect of the present invention, a structural body is provided in fluid, perpendicular to a typical flow direction of the fluid. The structural body includes a cylindrical insulating body having at least one hollow portion and at least one conducting body positioned in the hollow portion of the insulating body. 
     In a cross section of the insulating body having a normal line in an axial direction of the insulating body, a following relationship is satisfied: 
       1.5× Diy≦Dix≦ 15× Diy  
 
     where Dix is a length of the insulating body in the typical flow direction and Diy is a maximum value of a length of the insulating body in a direction perpendicular to the typical flow direction. Preferably, the relationship: 2.0×Diy≦Dix≦10×Diy is satisfied.
 
[2] In the structural body according to the first aspect of the present invention, in the cross section, a following relationship may be satisfied:
 
       1.2× Dcy≦Dcx≦ 12× Dcy  
 
     where Dcx is a length of the conducting body in the typical flow direction and Dcy is a maximum value of a length of the conducting body in the direction perpendicular to the typical flow direction. Preferably, the relationship: 3.0×Dcy≦Dcx≦8.0×Dcy is satisfied.
 
[3] In this case, the insulating body may have a plurality of the hollow portions containing a plurality of the conducting bodies, respectively, and same potential may be applied to each of the conducting bodies.
 
[4] If a plurality of the conducting bodies are arranged in the typical flow direction, the Dcx represents a sum of lengths of the conducting bodies arranged in the typical flow direction.
 
[5] If a plurality of the conducting bodies are arranged in the direction perpendicular to the typical flow direction, and the Dcy represents a maximum value of lengths of the conducting bodies arranged in the direction perpendicular to the typical flow direction.
 
[6] Further, a following relationship may be satisfied:
 
       1.1× Dmn/ 2≦ Lmn≦ 2.0× Dmn/ 2
 
     where Dmn is a sum of typical dimensions of at least a pair of adjacent conducting bodies among the plurality of the conducting bodies and Lmn is a distance between centers of the conducting bodies.
 
Preferably, the relationship: 1.1×Dmn/2≦Lmn≦1.5×Dmn/2 is satisfied.
 
[7] In the cross section, at least one of an upstream end and a downstream end of the insulating body with respect to flow of the fluid may have a shape where a length perpendicular to the typical flow direction is decreased gradually toward a front end of the insulating body.
 
[8] In this case, the at least one of the upstream end and the downstream end may include at least one tapered portion.
 
[9] Further, the front end may have a curved shape configured to satisfy the following relationship:
 
       0.05× Diy≦ 2× Rt≦ 0.7× Diy  
 
     where Rt is a radius of curvature of the curved shape.
 
[10] In the structural body according to the first aspect of the present invention, the insulating body has an upstream end and a downstream end with respect to flow of the fluid, and at least the upstream end may have a shape where a length in a direction perpendicular to the typical flow direction and perpendicular to a longitudinal direction of the insulating body is decreased gradually toward a front end of the insulating body.
 
[11] In this case, the insulating body may include a front surface and a back surface extending in parallel to flow of the fluid, and the front surface may include a tapered surface at the upstream end of the insulating body and the back surface may include a flat surface at the upstream end of the insulating body.
 
[12] Alternatively, the front surface may include a flat surface at the upstream end of the insulating body and the back surface may include a tapered surface at the upstream end of the insulating body.
 
[13] Alternatively, the front surface may include a tapered surface at the upstream end of the insulating body and the back surface may include a tapered surface at the upstream end of the insulating body.
 
[14] In the structural body according to the first aspect of the present invention, the insulating body has an upstream end and a downstream end with respect to flow of the fluid, and at least the downstream end may have a shape where a length in a direction perpendicular to the typical flow direction and perpendicular to a longitudinal direction of the insulating body is decreased gradually toward a front end of the insulating body.
 
[15] In this case, the insulating body may include a front surface and a back surface extending in parallel to flow of the fluid, and the front surface may include a tapered surface at the downstream end of the insulating body and the back surface may include a flat surface at the downstream end of the insulating body.
 
[16] Alternatively, the front surface may include a flat surface at the downstream end of the insulating body and the back surface may include a tapered surface at the downstream end of the insulating body.
 
[17] Alternatively, the front surface may include a tapered surface at the downstream end of the insulating body and the back surface may include a tapered surface at the downstream end of the insulating body.
 
[18] In the structural body according to the first aspect of the present invention, the insulating body and the conducting body may be directly joined together integrally by firing.
 
[19] According to a second aspect of the present invention, a structural body includes a cylindrical insulating body having at least one hollow portion and at least one conducting body positioned in the hollow portion of the insulating body.
 
     In a cross section of the insulating body having a normal line in an axial direction of the insulating body, a following relationship is satisfied: 
       1.5× Diy≦Dix≦ 15× Diy  
 
     where Dix is a length of the insulating body in a first direction, and Diy is a maximum value of a length of the insulating body in a second direction perpendicular to the first direction. 
     In the cross section, a following relationship is satisfied: 
       1.2× Dcy≦Dcx≦ 12× Dcy  
 
     where Dcx is a length of the conducting body in the first direction and Dcy is a maximum value of a length of the conducting body in the second direction.
 
[20] According to a third aspect of the present invention, an electrode structure has at least two structural bodies according to the first aspect or the second aspect of the present invention. Alternating current voltage is applied between the conducting body of one of the structural bodies and the conducting body of another of the structural bodies. The one of the structural bodies and the other of the structural bodies are arranged such that an axial direction of the insulating body is oriented in perpendicular to the typical flow direction.
 
     In the structural body and the electrode structure according to the present invention, when fluid passes through the structural body or bodies, the pressure loss in the fluid is reduced, and improvement in the ozone generation efficiency is achieved. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view with partial omission showing a first structural body; 
         FIG. 1B  is a perspective view with partial omission showing a first electrode structure; 
         FIG. 2A  is a perspective view with partial omission showing a second structural body; 
         FIG. 2B  is a perspective view with partial omission showing a second electrode structure; 
         FIG. 3A  is a view showing an exemplary embodiment model of the second structural body and its operation; 
         FIG. 3B  is a view showing an exemplary reference model and its operation; 
         FIG. 4  is a view showing cross sectional structure at both ends of an insulating body; 
         FIG. 5  is a flow chart showing a first production method of producing the first structural body and the second structural body; 
         FIG. 6A  is a cross sectional view showing a green body prepared in a green body preparation step; 
         FIG. 6B  is a cross sectional view showing a preliminarily-fired body prepared in a preliminarily fired body preparation step; 
         FIG. 6C  is a cross sectional view showing a state where a bulk conducting body is inserted into a hollow portion of the preliminarily fired body in a conducting body insertion step; 
         FIG. 6D  is a cross sectional view showing a structural body prepared in a firing/integration step; 
         FIG. 7  is a flow chart showing a second production method of producing the first structural body and the second structural body; 
         FIG. 8A  is a cross sectional view showing a green body prepared in a green body preparation step; 
         FIG. 8B  is a cross sectional view showing a state where a bulk conducting body is inserted into a hollow portion of the green body in a conducting body insertion step; 
         FIG. 8C  is a cross sectional view showing a structural body prepared in a firing/integration step; 
         FIG. 9A  is a cross sectional view showing a first modified example of the first structural body; 
         FIG. 9B  is a cross sectional view showing a second modified example of the first structural body; 
         FIG. 9C  is a cross sectional view showing a third modified example of the first structural body; 
         FIG. 9D  is a cross sectional view showing a fourth modified example of the first structural body; 
         FIG. 10A  is a cross sectional view showing a fifth modified example of the first structural body; 
         FIG. 10B  is a cross sectional view showing a sixth modified example of the first structural body; 
         FIG. 11A  is a cross sectional view showing a first modified example of the second structural body; 
         FIG. 11B  is a cross sectional view showing a second modified example of the second structural body; 
         FIG. 11C  is a cross sectional view showing a third modified example of the second structural body; 
         FIG. 11D  is a cross sectional view showing a fourth modified example of the second structural body; 
         FIG. 12A  is a cross sectional view showing a fifth modified example of the second structural body; 
         FIG. 12B  is a cross sectional view showing a sixth modified example of the second structural body; 
         FIG. 12C  is a cross sectional view showing a seventh modified example of the second structural body; 
         FIG. 12D  is a cross sectional view showing an eighth modified example of the second structural body; 
         FIG. 13A  is a cross sectional view showing a ninth modified example of the second structural body; 
         FIG. 13B  is a cross sectional view showing a tenth modified example of the second structural body; 
         FIG. 13C  is a cross sectional view showing an eleventh modified example of the second structural body; 
         FIG. 13D  is a cross sectional view showing a twelfth modified example of the second structural body; 
         FIG. 14  is a cross sectional view showing a modified example of the first electrode structure; 
         FIG. 15A  is a cross sectional view showing a first modified example of the second electrode structure; 
         FIG. 15B  is a cross sectional view showing a second modified example of the second electrode structure; 
         FIG. 15C  is a cross sectional view showing a third modified example of the second electrode structure; 
         FIG. 16A  is a cross sectional view showing a fourth modified example of the second electrode structure; 
         FIG. 16B  is a cross sectional view showing a fifth modified example of the second electrode structure; 
         FIG. 16C  is a cross sectional view showing a sixth modified example of the second electrode structure; and 
         FIG. 17  is a cross sectional view showing structure of a pipe for checking pressure losses. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of structural bodies and electrode structures according to the present invention will be described below with reference to  FIGS. 1A to 17 . It should be noted that, in this description, a numeric range of “A to B” includes both the numeric values A and B as the lower limit and upper limit values. 
     A structural body according to a first embodiment (hereinafter referred to as the “first structural body  10 A”) is provided in fluid  12 , in perpendicular to a typical flow direction of the fluid  12  (hereinafter referred to as the typical flow direction x), as can be seen from  FIG. 1A  showing main part of the first structural body  10 A. 
     This first structural body  10 A includes a tubular insulating body  16  having a hollow portion  14 , and a conducting body  18  provided in the hollow portion  14  of the insulating body  16 . The insulating body  16  has a front surface  20   a  and a back surface  20   b  as contact surfaces contacting the fluid  12 . For example, the conducting body  18  has a track shape in cross section, including semi-circular curves at its both ends. 
     The typical flow direction x represents a flow direction at the central portion of the fluid  12  having directivity. This means that the flow direction in the peripheral part of the fluid  12  is not considered since the fluid  12  does not have any directivity in its peripheral part. 
     A structural body according to a second embodiment (hereinafter referred to as the “second structural body  10 B) has substantially the same structure as the above described first structural body  10 A, as can be seen from  FIG. 2A  showing main part of the second structural body  10 B. However, the second structural body  10 B is different from the first structural body  10 A in that the insulating body  16  has a plurality of hollow portions  14  containing conducting bodies  18 , respectively, and the same electric potential is applied to each of the conducting bodies  18 . For example, each of the conducting bodies  18  has a circular shape in cross section. In an example of  FIG. 2A , five hollow portions  14  are formed in the insulating body  16 . These hollow portions  14  contain conducting bodies  18  (five conducting bodies  18 ), respectively. Among the five conducting bodies  18 , a first conducting body  18   a  positioned at an upstream end of the insulating body  16  and a fifth conducting body  18   e  positioned at a downstream end of the insulating body  16  have diameters smaller than those of the other conducting bodies (second conducting bodies  18   b  to fourth conducting bodies  18   d ). 
     Further, the second structural body  10 B satisfies the relationship: L12=L45, L23=L34, and L12&lt;L23 where L12 is the distance between the center of the first conducting body  18   a  and the center of the second conducting body  18   b , L23 is the distance between the center of the second conducting body  18   b  and the center of a third conducting body  18   c , L34 is the distance between the center of the third conducting body  18   c  and the center of the fourth conducting body  18   d , and L45 is the distance between the center of the fourth conducting body  18   d  and the center of the fifth conducting body  18   e . This relationship is merely a non-restrictive example. 
     In the case of using the first structural body  10 A and the second structural body  10 B as an electrode structure or the like, the insulating body  16  may be referred to as the dielectric body for inducing charges. 
     In particular, in the cross section having a normal line in the axial direction z of the insulating body  16 , the embodiment of the present invention satisfies the following relationship: 
       1.5× Diy≦Dix≦ 15× Diy  
 
     where Dix is the length of the insulating body  16  in the typical flow direction x and Diy is the maximum value of the length of the insulating body  16  in the direction y perpendicular to the typical flow direction x. 
     In the structure, since the outer shapes of the first structural body  10 A and the second structural body  10 B are narrow and long with respect to the typical flow direction x, the pressure loss can be reduced. 
     In this regard, in the case where the fluid  12  is used for generating ozone, for example, a raw gas containing air and oxygen may be used. 
     The material for the insulating body  16  may include a single oxide, a single nitride, a composite oxide or a composite nitride material containing one or more substances selected from the group consisting of barium oxide, bismuth oxide, titanium oxide, zinc oxide, neodymium oxide, titanium nitride, aluminum nitride, silicon nitride, alumina, silica, and mullite. 
     The conducting body  18  is preferably made of a material containing a substance selected from the group consisting of molybdenum, tungsten, silver, copper, nickel, and alloys containing at least one of these substances. Examples of such alloys include instar, kovar, inconel (registered trademark), incoloy (registered trademark). 
     The insulating body  16  is preferably made of a ceramic material such as LTCC (Low Temperature Co-fired Ceramics), which can be fired at a temperature lower than the melting point of the conducting body  18 . Specifically, the material for the insulating body  16  preferably includes a single oxide, a single nitride, a composite oxide or a composite nitride material containing one or more substances selected from the group consisting of barium oxide, bismuth oxide, titanium oxide, zinc oxide, neodymium oxide, titanium nitride, aluminum nitride, silicon nitride, alumina, silica, and mullite. 
     Further, in an electrode structure according to the first embodiment using the first structural bodies  10 A (hereinafter referred to as the “first electrode structure  22 A”), as shown in  FIG. 1B , alternating current voltage is applied to the conducting body  18  of one of the first structural bodies  10 A and the conducting body  18  of the other of the first structural bodies  10 A. In each of the one of the first structural bodies  10 A and the other of the first structural bodies  10 A, the axial direction z of the insulating body  16  is perpendicular to the typical flow direction x. In an example of  FIG. 1B , a gap g is formed between front surfaces  20   a  of the first structural bodies  10 A that face each other. It is a matter of course that the back surfaces  20   b  of the first structural bodies  10 A may face each other, or the front surface  20   a  of one of the first structural bodies  10 A and the back surface  20   b  of the other of the first structural bodies  10 A may face each other. 
     Likewise, in an electrode structure according to the second embodiment using the second structural bodies  10 B (hereinafter referred to as the “second electrode structure  22 B”), as shown in  FIG. 2B , alternating current voltage is applied between the conducting body  18  of one of the second structural bodies  10 B and the conducting body  18  of the other of the second structural bodies  103 . In each of the one of the second structural bodies  10 B and the other of the second structural bodies  10 B, the axial direction z of the insulating body  16  is perpendicular to the typical flow direction x. In an example of  FIG. 2B , a gap is formed between front surfaces  20   a  of the second structural bodies  10 B that face each other. It is a matter of course that the back surfaces  20   b  of the second structural bodies  10 B may face each other, or the front surface  20   a  of one of the second structural bodies  103  and the back surface  20   b  of the other of the second structural bodies  10 B may face each other. 
     Further, the first structural body  10 A and the second structural body  10 B satisfy the following relationship: 
       1.2× Dcy≦Dcx≦ 12× Dcy  
 
     where Dcx is the length of the conducting body  18  in the typical flow direction x and Dcy is the maximum value of the length of the conducting body  18  in the direction y perpendicular to the typical flow direction x as shown in  FIG. 1A  and  FIG. 2A . 
     In this regard, as shown in  FIG. 1A , in the first structural body  10 A, the length Dcx represents the maximum value of the conducting body  18  in the typical flow direction x (in this case, the length of the segment in the direction of the central line). As shown in  FIG. 2A , in the second structural body  10 B, the length Dcx represents the sum of the maximum values of the lengths of the respective conducting bodies  18  in the typical flow direction x (in this case, the diameters of the conducting bodies  18 ). In the first structural body  10 A, the maximum value Dcy of the length represents the maximum value of the length of the conducting body  18  in the direction y perpendicular to the typical flow direction x (in this case, the length of the segment in the direction perpendicular to the central line). In the second structural body  10 B, the maximum value Dcy of the length represents the longest dimension among the lengths of the conducting bodies  18  in the direction y perpendicular to the typical flow direction x (in this case, the diameter of the conducting body  18 ). 
     In the structure, in the first structural body  10 A, one long and narrow conducting body  18  can be provided in the typical flow direction x. In the second structural body  10 B, a plurality of conducting bodies  18  can be provided in the typical flow direction x. Therefore, by adopting any of the first electrode structure  22 A and the second electrode structure  22 B, an electrical field can be generated in a planar wide space between the surfaces  20   a . Thus, improvement in the ozone generation efficiency is achieved. 
     In particular, the second structural body  10 B satisfies the following relationship: 
       1.1× Dmn/ 2≦ Lmn≦ 2.0× Dmn/ 2
 
     where Dmn is the sum of the typical dimensions of at least a pair of adjacent conducting bodies  18  among the plurality of conducting bodies  18  and Lmn is the distance between the centers of the conducting bodies  18  as shown in  FIG. 2A . 
     The typical dimensions of at least a pair of adjacent conducting bodies  18  means the respective maximum values of the lengths of the conducting bodies  18  arranged in the typical flow direction x (in this case, the diameters of the conducting bodies  18 ). 
     In this regard, as an example, the adjacent first conducting body  18   a  and second conducting body  18   b , and the adjacent second conducting body  18   b  and third conducting body  18   c  will be considered. The following relationship is present between the sum D12 of the typical dimension D1 of the first conducting body  18   a  and the typical dimension D2 of the second conducting body  18   b  and the distance L12 between the center of the first conducting body  18   a  and the center of the second conducting body  18   b.    
       1.1× D 12/2≦ L 12≦2.0× D 12/2
 
     Likewise, the following relationship is present between the sum D23 of the typical dimension D2 of the second conducting body  18   b  and the typical dimension D3 of the third conducting body  18   c  and the distance L23 between the center of the second conducting body  18   b  and the center of the third conducting body  18   c.    
       1.1× D 23/2≦ L 23≦2.0× D 23/2
 
     The similar relationship is present among the other conducting bodies. 
     In the structure, the adjacent conducting bodies  18  can be provided closely to each other. Therefore, by adopting the second electrode structure  22 B, an electric field can be generated in a planar wide space between the surfaces  20   a . Thus, improvement in the ozone generation efficiency achieved. 
     Next, based on an exemplary embodiment model and an exemplary reference model of the second structural bodies  10 B, the space where the electric field is generated will be considered with reference to  FIGS. 3A and 3B . It should be noted that  FIGS. 3A and 3B  mainly show parts of second structural bodies  10 B where the conducting bodies  18   b  and  18   c  are present (see  FIG. 2A ). Further, in  FIGS. 3A and 3B , though contour lines are smoothly connected at the border between the space and the dielectric body, in reality, the contour lines are bent at this border since the space and the dielectric body have different dielectric constants. 
     Firstly, in the exemplary model of the second structural bodies  10 B, as shown in  FIG. 3A , the sum Dmn (=Dm+Dn) of the typical dimensions of the adjacent conducting bodies  18  and the distance Lmn between the centers of the respective conducting bodies  18  were determined to satisfy the relationship as shown in the above formula. Further, alternating current voltage was applied between the conducting body  18  of one of the second structural bodies  10 B and the conducting body  18  of the other of the second structural bodies  10 B to generate an electric field in a space between these second structural bodies  10 B facing each other. 
     In the exemplary reference model, as shown in  FIG. 33 , two pairs of conducting bodies  18  were prepared. Each pair includes two conducting bodies  18  spaced from each other. The sum Dmn of the typical dimensions of the adjacent conducting bodies  18  and the distance Lmn between the centers of the respective conducting bodies  18  are determined not to satisfy the relationship as shown in the above formula. Further, alternating current voltage was applied between the conducting body  18  of one of the second structural bodies  10 B and the conducting body  18  of the other of the second structural bodies  10 B to generate an electric field in a space between these second structural bodies  10 B facing each other. 
     In the exemplary embodiment model of the second structural bodies  10 B, the conducting bodies  18  are provided closely to each other. In the structure, the distance between an effective space  24  where the electric field is generated between one pair of the two conducting bodies  18   b  (effective space where the electric field is generated for ozone generation) and an effective space  24  where the electric field is generated between the other pair of the two conducting bodies  18   c  is small. As a result, a space  26  between these two effective spaces  24  also functioned as an effective space, and the electric field was effectively generated in a planar wide area as a whole. 
     In contrast, in the exemplary reference model, the conducting bodies  18  are provided remotely from each other in each of the second structural bodies  10 B. In the structure, the distance between an effective space  24  where the electric field is generated between one pair of the two conducting bodies  18   b  and an effective space  24  where the electric field is generated between the other pair of the two conducting bodies  18   c  is long. Consequently, the electric field generated in the space  26  between these two effective spaces  24  is weak. That is, the space  26  did not function like the effective spaces  24 , and the effective electric field was generated in each of the effective spaces  24  locally. 
     Further, as show in  FIG. 4 , the first structural body  10 A and the second structural body  10 B are configured to have a shape where at least at one of both ends of the insulating body  16 , i.e., at least at one of the upstream end  16   a  and the downstream end  16   b  with respect to the flow of the fluid  12 , the length Dy perpendicular to the typical flow direction x is gradually reduced toward a front end  16   t . In this regard, preferably, at least one of the ends  16   a ,  16   b  includes at least one tapered portion  28 . In this structure, the pressure loss can be reduced efficiently. For example, both of the first structural body  10 A and the second structural body  10 B have tapered surfaces  30  at upstream ends of the front surface  20   a  and the back surface  20   b , and have tapered surfaces  30  at their downstream ends of the front surface  20   a  and the back surface  20   b.    
     In particular, by configuring the front end  16   t  of the insulating body  16  to have a curved shape which satisfies the relationship 
       0.05× Diy≦ 2× Rt≦ 0.7× Diy  
 
     where Rt is the radius of curvature of the curved front end  16   t , the pressure loss can be reduced significantly. 
     Next, two methods of producing the first structural body  10 A and the second structural body  10 B (a first production method and a second production method) will be described with reference to  FIGS. 5 to 8C . 
     [First Production Method] 
     As shown in  FIGS. 5 to 6D , the first production method of producing the first structural body  10 A and the second structural body  10 B includes a green-body preparation step S 1  of preparing a green body  34  having a hollow portion  32 , to be formed into the insulating body  16  (see  FIG. 6A ), a preliminarily-fired body preparation step S 2  of degreasing and preliminary-firing the green body  34  to prepare a preliminarily-fired body  38  having a hollow portion  36  (see  FIG. 6B ), a conducting body insertion step S 3  of inserting a bulk body of the conducting body  18  into the hollow portion  36  in the preliminarily-fired body  38 , and a firing/integration step S 4  of firing the preliminarily-fired body  38  together with the conducting body  18  inserted into the hollow portion  36  to produce the first structural body  10 A and the second structural body  10 B (see  FIG. 6D ). 
     In the green-body preparation step S 1 , a starting material slurry is shaped and solidified to prepare the green body. The starting material slurry contains a starting material powder, a dispersion medium, and an organic binder. In addition, the starting material slurry may contain a dispersion aid and a catalyst, as necessary. Specifically, the starting material powder may be a powder of a ceramic containing one or more elements selected from the group consisting of barium, bismuth, titanium, zinc, aluminum, silicon, magnesium, and neodymium. The dispersion medium may be a mixture of an aliphatic polyhydric ester and a polybasic acid ester, or ethylene glycol. The organic binder may be a gelling agent or the like. In a case where the green body  34  has, for example, an extruded shape with the hollow portion  32  (through-hole) as shown in  FIG. 6A , the organic binder may be a substance other than the gelling agent (i.e., a substance that is hardened not by a chemical reaction but only by drying), etc. Of course, in a case where the green body  34  has a shape other than the extruded shape, the gelling agent should preferably be used. In this case, the gelling agent may include a substance that is hardened by a hardening reaction (a chemical reaction such as a urethane reaction). For example, the gelling agent may include a combination of a modified polymethylene polyphenyl polyisocyanate and a polyol. The dispersion medium may be a mixture of a dibasic acid ester. The dispersion aid may be a polycarboxylic acid-based copolymer. The catalyst may be a tertiary amine, and specific examples of the catalyst include 6-dimethylamino-1-hexanol or the like. 
     For example, in the case of preparing the green body  34  having the extruded shape with the through-hole being formed as the hollow portion  32 , the starting material slurry can be preferably shaped by extrusion molding. The inner diameter Da of the hollow portion  32  in the green body  34  is slightly larger than the outer diameter Dc of the conducting body  18 . Therefore, the conducting body  18  can be easily inserted into the hollow portion  32 . 
     In the case of using the extrusion molding, a long body extruded from an extruder is cut into the green bodies  34  having a predetermined length, and successively the green bodies  34  are degreased and preliminarily-fired. Alternatively, a long body extruded from the extruder is cut into the green bodies  34  having a predetermined length while being degreased and preliminarily-fired. Therefore, the steps can be continuously carried out to improve the productivity. 
     Of course, in the case of using the gelling agent in the organic binder, the starting material slurry may be shaped by using a mold having a molding cavity corresponding to the tubular insulating body  16 . In this case, the molding cavity of the mold is filled with the starting material slurry. The starting material slurry is molded into a shape corresponding to the tubular shape of the insulating body  16 . The molded starting material slurry is solidified via the hardening reaction of the gelling agent. The solidified slurry is separated (demolded) from the mold, and then degreased and preliminarily-fired. This process performed by molding the starting material slurry including the starting material powder, the dispersion medium, and the gelling agent, and solidifying the molded slurry via the hardening reaction of the gelling agent to prepare the green body  34 , is known as “a gel casting process”. 
     In the preliminarily-fired body preparation step S 2 , firstly, the shaped green body  34  is degreased and then preliminarily-fired. The degreasing is a treatment for burning to remove an organic component such as a binder from the green body  34 . The green body  34  becomes brittle temporarily by the removal of the binder. The preliminary-firing is a treatment for sintering the brittle green body  34  to some extent to obtain the preliminarily-fired body  38  that is strong enough to handle. It should be noted that the preliminarily-fired body  38  is not brought into a sufficiently-sintered state, and significant firing shrinkage does not occur. More specifically, for example, the green body  34  is preliminarily-fired in an air atmosphere at a temperature of 400° C. to 800° C. for 1 to 8 hours. In view of handling in the following step, the temperature is increased until the firing treatment proceeds to such an extent that the green body  34  can have a sufficient strength (i.e., the preliminarily-fired body  38  is obtained). As described above, the preliminarily-fired body  38  is not significantly shrunk by sintering in this step. Therefore, the inner diameter Db of the hollow portion  36  in the preliminarily-fired body  38  is approximately equal to the inner diameter Da of the hollow portion  32  in the green body  34 , and the conducting body  18  can be easily inserted into the hollow portion  32 . 
     In the conducting body insertion step S 3 , as shown in  FIG. 6C , the solid conducting body  18  itself is inserted into the hollow portion  36  in the preliminarily-fired body  38  obtained in the manner as described above. Though the conducting body  18  is placed at the center of the hollow portion  36  in  FIG. 6C , it is a matter of course that the conducting body  18  may partially contact the inner wall surface of the hollow portion  36  during or after the process of inserting the conducting body  18 . 
     The preliminarily-fired body  38  has a stiffness property. Therefore, the conducting body  18  can be easily inserted into the hollow portion  36  in the preliminarily-fired body  38 , and the preliminarily-fired body  38  can be easily handled. Thus, the conducting body  18  can be automatically inserted using a robot or the like or during transportation of the preliminarily-fired body  38 . For example, the conducting body  18  may be a cylindrical solid made of a metal or cermet material containing molybdenum or a molybdenum alloy. In the following firing step, the preliminarily-fired body  38  is subjected to firing shrinkage, and the conducting body  18  is not shrunk by the firing. Thus, the outer diameter Dc of the conducting body  18  is determined to be smaller than the inner diameter Db of the hollow portion  36  (through-hole) in the preliminarily fired body  38  (see  FIG. 6C ) by the amount of the firing shrinkage of the preliminarily-fired body  38 . By determining the outer diameter Do of the conducting body  18  to be slightly larger than the inner diameter of the green body  34  when it is fired alone, specifically, by a dimension which is larger than 0 μm, and equal to or less than 10 μm, the conducting body  18  and the green body  34  can be tightly adhered together, and combined integrally. 
     In the firing/integration step S 4 , the preliminarily-fired body  38  is fired together with the conducting body  18  inserted into the preliminarily-fired body  38 . For example, the firing is carried out in an oxygen-free atmosphere (such as a nitrogen or argon atmosphere). The oxygen-free atmosphere is not limited to an atmosphere completely free from oxygen, and may be, for example, the following atmosphere (a) or (b): 
     (a) an atmosphere provided by introducing nitrogen or argon into a firing furnace, while discharging air from the firing furnace, to replace the air by the nitrogen or argon; or
 
(b) an atmosphere provided by introducing nitrogen or argon into the firing furnace after vacuating the firing furnace.
 
     In the firing/integration step, the firing temperature is 900° C. to 1600° C., preferably 900° C. to 1050° C. When the firing temperature is within the preferred temperature range, material for the conducting body can be chosen from a wide variety of materials. For example, in the case of using an alumina as the material for the insulating body, the upper limit of the firing temperature is 1600° C. The firing time is 1 to 10 hours. 
     The firing treatment may be carried out while maintaining an atmosphere containing a small amount of oxygen. However, in the case of performing the firing in the oxygen-free atmosphere as described above, it is not necessary to control the atmosphere containing a small amount of oxygen, and the insulating body  16  can be easily sintered while oxidation of the conducting body  18  is prevented. 
     The preliminarily-fired body  38  is shrunk by the firing. As a result, a so-called shrinkage fitting of the conducting body  18  is achieved. Thus, the fired insulating body  16  and the conducting body  18  are firmly joined together integrally. Consequently, the first structural body  10 A and the second structural body  10 B, which contain the insulating body  16  and the conducting body  18  embedded in the hollow portion  14  of the insulating body  16 , is produced. 
     It should be noted that an intermediate layer containing main components of the conducting body  18  (e.g., molybdenum) may be formed at the border portion between the insulating body  16  and the conducting body  18 . This intermediate layer is formed by diffusion of the main components of the conducting body  18  into the insulating body  16  at the time of firing. Further, no pores having the size of 50 μm or more are formed in the insulating body  16  covering the conducting body  18 . If the insulating body  16  has a large porosity expressed in the order of percentage, dielectric breakdown may occur easily due to the voltage applied to ceramics. In the presence of only one closed pore having the size of 50 μm in the insulating body  16  as a whole, dielectric breakdown may occur from the portion of the closed pore to cause arc plasma, and to cause dissolution of ceramics. Ideally, no closed pore should be present. It is desirable that diameters of all of the closed pores dispersed in the material are less than 10 μm. 
     [Second Production Method] 
     As shown in  FIGS. 7 to 8C , the second production method of producing the first structural body  10 A and the second structural body  105  includes a green-body preparation step S 11  of preparing a green body  34  having a hollow portion  32 , to be formed into the insulating body  16  (see  FIG. 8A ), a conducting body insertion step S 12  of inserting a bulk body of the conducting body  18  into the hollow portion  32  in the green body  34 , and a firing/integration step S 13  of firing the green body  34  together with the conducting body  18  inserted into the hollow portion  32  to produce the first structural body  10 A and the second structural body  105 . 
     In the green-body preparation step S 11 , the starting material slurry is shaped and solidified to prepare the green body  34  shown in  FIG. 8A  in the same manner as the green-body preparation step S 1  in the first production method. 
     In the conducting body insertion step S 12 , as shown in  FIG. 8B , the solid conducting body  18  itself is inserted into the hollow portion  32  in the green body  34  obtained in the manner as described above. Though the conducting body  18  is placed at the center of the hollow portion  32  in  FIG. 8B , the conducting body  18  may partially contact the inner wall surface of the hollow portion  32  during or after the process of inserting the conducting body  18 . In the following firing step, the green body  34  is subjected to firing shrinkage, while the conducting body  18  is not shrunk by the firing. Thus, the outer diameter Dc of the conducting body  18  is determined to be smaller than the inner diameter Da of the hollow portion  32  (through-hole) in the green body  34  by the amount of the firing shrinkage of the green body  34 . By determining the outer diameter Dc of the conducting body  18  to be slightly larger than the inner diameter of the green body  34  when it is fired alone, specifically, by a dimension which is larger than 0 μm, and equal to or less than 10 μm, the conducting body  18  and the green body  34  can be tightly adhered together, and combined integrally. 
     In the firing/integration step S 13 , the green body  34  is fired together with the conducting body  18  inserted into the green body  34 . For example, the firing is carried out in a weakly oxidizing atmosphere containing an inert gas such as a humidified nitrogen or argon gas (an atmosphere having a low oxygen partial pressure) at a temperature of 900° C. to 1600° C. (preferably 900° C. to 1050° C.) for 1 to 20 hours. The humidification is achieved by bubbling of the inert gas in water having a temperature of 10° C. to 80° C. The firing is carried out in the weakly oxidizing atmosphere for the following reasons: 
     (1) a certain level of oxidizing atmosphere is required for firing and removing the gelling agent; and
 
(2) the oxygen partial pressure in the oxidizing atmosphere is required to be small in order to prevent excess oxidation of the conducting body  18 .
 
     In the above firing, the green body  34  is subjected to firing shrinkage. As a result, a so-called shrinkage fitting of the conducting body  18  is achieved. Thus, the fired insulating body  16  and the conducting body  18  are firmly joined together. 
     In the first and second production methods described above, in the case of using the gel casting process in the green-body preparation steps S 1  and S 11 , a submicron starting material powder can be used and significantly uniformly distributed in the green body  34 . Therefore, the firing shrinkage ratio can be highly accurately controlled, and a dense sintered body (the insulating body  16 ) can be prepared without defects. The denseness is effective in improving the voltage resistance of the electrode. 
     As for the method of preparing the first structural body  10 A and the second structural body  10 B, instead of adopting the above method, in a possible method, the conducting body  18  and the insulating body  16  are prepared separately, and after the conducting body  18  is inserted into the hollow portion  14  of the insulating body  16 , these components are adhered together using resin or the like. Alternatively, conducting body paste may fill the hollow portion  14  of the insulating body  16 . In the former method, the desired durability cannot be expected at high temperature in terms of heat resistance of resin. In the latter method, it is difficult to form a dense conducting body, and abnormal electrical discharge tends to occur easily. 
     Therefore, as in the cases of the first production method and the second production method described above, it is preferable to insert one or more conducting bodies  18  into hollow portions  36  of the preliminarily-fired body  38 , and thereafter, fire the preliminarily-fired body  38  and the conducting body  18  to directly join these components integrally. 
     Next, modified examples of the first structural body  10 A will be described with reference to  FIGS. 9A to 103 . 
     As shown in  FIG. 9A , a first modified example (10Aa) of the first structural body  10 A is different from the first structural body  10 A in that the conducting body  18  has, e.g., a rectangular shape in cross section, and has a curved shape at its corner portions. 
     As shown in  FIG. 9B , a second modified example (10Ab) of the first structural body  10 A is different from the first structural body  10 A in that the cross sectional shape, in particular, the outer shape of the conducting body  18  is similar to the outer shape of the insulating body  16 . 
     As shown in  FIG. 9C , a third modified example (10Ac) of the first structural body  10 A is different from the first structural body  10 A in that the front surface  20   a  of the insulating body  16  includes tapered surfaces  30  at the upstream end and the downstream end, the back surface  20   b  of the insulating body  16  is a flat surface, and the conducting body  18  has a trapezoidal shape. 
     As shown in  FIG. 9D , a fourth modified example (10Ad) of the first structural body  10 A is different from the first structural body  10 A in that the upstream end of the insulating body  16  has a circular shape, and the upstream end of the conducting body  18  also has a circular shape. 
     As shown in  FIG. 10A , a fifth modified example (10Ae) of the first structural body  10 A is different from the first structural body  10 A in that the conducting body  18  has a circular shape in cross section. 
     As shown in  FIG. 10B , a sixth modified example (10Af) of the first structural body  10 A is different from the first structural body  10 A in that the conducting body  18  has a circular shape in cross section, and the upstream end of the insulating body  16  has a circular shape. 
     Next, modified examples of the second structural body  10 B will be described with reference to  FIGS. 11A to 13D . 
     As shown in  FIG. 11A , a first modified example (10Ba) of the second structural body  10 B is different from the second structural body  10 B in that no conducting bodies  18  are present at the upstream end and the downstream end, and three conducting bodies  18  are present at the central portion. 
     As shown in  FIG. 11B , a second modified example (10Bb) of the second structural body  10 B is different from the first modified example (10Ba) in that two conducting bodies  18  are present at the central portion of the insulating body  16 . 
     As shown in  FIG. 11C , a third modified example (10Bc) of the second structural body  10 B is different from the first modified example (10Ba) in that the front surface  20   a  of the insulating body  16  includes tapered surfaces  30  at the upstream end and the downstream end, and the back surface  20   b  of the insulating body  16  is a flat surface. 
     As shown in  FIG. 11D , a fourth modified example (10Bd) of the second structural body  10 B is different from the first modified example (10Ba) in that the front surface  20   a  of the insulating body  16  includes a tapered surface  30  at its downstream end, and the back surface  20   b  of the insulating body  16  includes a tapered surface  30  at its upstream end. 
     As shown in  FIG. 12A , a fifth modified example (10Be) of the second structural body  10 B is different from the second structural body  10 B in that three conducting bodies  18  are present in the insulating body  16 . 
     As shown in  FIG. 12B , a sixth modified example (10Bf) of the second structural body  10 B is different from the first modified example (10Ba) in that the upstream end of the insulating body  16  has a circular shape. 
     As shown in  FIG. 12C , a seventh modified example (10Bg) of the second structural body  10 B is different from the sixth modified example (10Bf) in that two conducting bodies  18  are present in the insulating body  16 . 
     As shown in  FIG. 12D , an eighth modified example (10Bh) of the second structural body  10 B is different from the seventh modified example (10Bg) in that one conducting body  18  having a small diameter is positioned at the downstream end of the insulating body  16 . 
     As shown in  FIG. 13A , a ninth modified example (10Bi) of the second structural body  10 B is different from the eighth modified example (10Bh) in that one conducting body  18  having a large diameter and one conducting body  18  having a small diameter are present. 
     As shown in  FIG. 13B , a tenth modified example (10Bj) of the second structural body  10 B is different from the second structural body  10 B in that a structural body of an insulating body  16  having a circular outer shape (hereafter referred to as the “circular structural body  40 ”) is provided at the center, and the sixth modified example (10Af) of the first structural body  10 A is provided on each of the left side and right side, i.e., on each of the upstream side and the downstream side of the circular structural body  40  in a symmetrical manner. 
     As shown in  FIG. 13C , an eleventh modified example (10Bk) of the second structural body  10 B is different from the second structural body  10 B in that another circular structural body  40  is provided on the upstream side of the circular structural body  40 , and the sixth modified example (10Af) of the first structural body  10 A is provided on the downstream side of the circular structural body  40 . 
     As shown in  FIG. 13D , a twelfth modified example (10Bl) of the second structural body  10 B is different from the second structural body  10 B in that additional circular structural bodies  40  are provided on the upstream side and on the downstream side of the circular structural body  40 , respectively. 
     Next, a modified example of the first electrode structure  22 A will be described with reference to  FIG. 14 . 
     As shown in  FIG. 14 , a modified example (22Aa) of the first electrode structure  22 A is different from the first electrode structure  22 A in that two structural bodies  10 Ac of the third modified example of the first structural body  10 A (see  FIG. 9C ) are arranged in a linearly symmetrical manner with respect to the typical flow direction x. In the example of  FIG. 14 , the back surfaces  20   b  of the structural bodies  10 Ac of the third modified example face each other. It is a matter of course that front surfaces  20   a  of the structural bodies  10 Ac of the third modified example may face each other, or the front surface  20   a  of one of the structural bodies  10 Ac of the third modified example and the back surface  20   b  of another of the structural bodies  10 Ac of the third modified example may face each other. 
     Next, modified examples of the second electrode structure  22 B will be described with reference to  FIGS. 15A to 16C . 
     As shown in  FIG. 15A , a first modified example (22Ba) of the second electrode structure  22 B is different from the second electrode structure  22 B in that two structural bodies  10 Ba of the first modified example of the second structural body  10 B (see  FIG. 11A ) are arranged in a linearly symmetrical manner with respect to the typical flow direction x. 
     As shown in  FIG. 15B , a second modified example (22Bb) of the second electrode structure  22 B is different from the second electrode structure  22 B in that two structural bodies  10 Bc of the third modified example of the second structural body  10 B (see  FIG. 11C ) are arranged in a linearly symmetrical manner with respect to the typical flow direction x. In the example of  FIG. 15B , the front surfaces  20   a  of the structural bodies  10 Bc of the third modified example face each other. 
     As shown in  FIG. 15C , a third modified example (22Bc) of the second electrode structure  22 B is different from the second modified example (22Bb) in that the back surfaces ( 20 b) face each other. 
     As shown in  FIG. 16A , a fourth modified example (22Bd) of the second electrode structure  22 B is different from the second electrode structure  22 B in that two structural bodies  10 Bd of the fourth modified example of the second structural body  10 B (see  FIG. 11D ) are arranged in a linearly symmetrical manner with respect to the typical flow direction x. In the example of  FIG. 16A , the front surfaces  20   a  of the structural bodies  10 Bd of the fourth modified example face each other. 
     As shown in  FIG. 16B , a fifth modified example (22Be) of the second electrode structure  22 B is different from the fourth modified example (22Bd) in that the back surfaces (2Db) face each other. 
     As shown in  FIG. 16C , a sixth modified example (22Bf) of the second electrode structure  22 B is different from the second electrode structure  22 B in that two structural bodies  10 Bf of the sixth modified example of the second structural body  10 B (see  FIG. 12B ) are arranged in a linearly symmetrical manner with respect to the typical flow direction x. 
     Among these examples, in the first electrode structure  22 A, the second electrode structure  22 B, and the first modified example (22Ba), the second modified example (22Bb), the fifth modified example (22Be), and the sixth modified example (22Bf) of the second electrode structure  22 B, the structural bodies ( 10 A,  10 B) have the tapered surfaces  30  facing each other at their upstream ends. In the structure, the gas is guided along the tapered surfaces  30  to move into the gap between the structural bodies ( 10 A,  10 B) easily. That is, the gas can be supplied into the gap between the structural bodies ( 10 A,  10 B) efficiently. Thus, improvement in the ozone generation efficiency is achieved. 
     Though the first electrode structure  22 A, the second electrode structure  22 B, and their modified examples have been described in connection with the cases where the same type of the structural bodies are arranged to face each other, different types of structural bodies may be provided to face each other. 
     First Embodiment 
     The pressure losses in the structural bodies according to embodiments 1 to 6 and a comparative example 1 were checked. 
     (Method of Checking Pressure Losses) 
     The pressure losses were checked in the following manner. Specifically, as shown in  FIG. 17 , five electrodes (structural body pairs) having the same structure were provided in a pipe having a circular shape in cross section (pipe diameter=60 mm, pipe length=500 mm). The pipe length is a length of the pipe for measuring the pressure loss, and this is a distance for measuring the pressure difference. Further, for development of the flow in the pipe (i.e., for formation of the flow having parabolic velocity distribution in the pipe), segments each having the length of 200 mm were provided on upstream and downstream sides of the pipe. Therefore, the total length including the pipe and the segments on the upstream and downstream sides was 900 mm. Further, the air at room temperature was supplied into the pipe at the flow rate of 250 liter/min. The pressure difference between the inlet and the outlet of the pipe was measured as the pressure loss. 
     The five electrodes (structural body pairs) in the pipe were positioned at the center in the pipe, i.e., at the point remote from the pressure measurement points by 250 mm. The electrodes were arranged at the pitch of 5 mm, and the gap g between the structural bodies of each electrode was 0.5 mm. 
     The details of the structural bodies according to the embodiments 1 to 6 and the comparative example 1 are as follows: 
     Comparative Example 1 
     The structural body according to the comparative example 1 was based on the first structural body  10 A shown in  FIG. 1A , and had the relationship: Dix/Diy=1.0 where Dix was the length of the insulating body  16  in the typical flow direction x and Diy was the maximum value of the length of the insulating body  16  in the direction y perpendicular to the typical flow direction x. 
     Embodiment 1 
     The structural body according to the embodiment 1 was based on the first structural body  10 A shown in  FIG. 1A , and had the relationship: Dix/Diy=1.5 where Dix was the length of the insulating body  16  in the typical flow direction x and Diy was the maximum value of the length of the insulating body  16  in the direction y perpendicular to the typical flow direction x. 
     Embodiments 2 to 6 
     The structural bodies according to the embodiments 2 to 6 had substantially the same structure as the structural body according to the embodiment 1. However, the structural bodies according to the embodiments 2 to 6 were different from the structural body according to the embodiment 1 in that the embodiments 2 to 6 were based on the first structural body  10 A shown in  FIG. 1A , and had values of 2.0, 10.0, 3.3, 3.0, and 5.5, respectively, for the relationship between the length Dix and the maximum value Diy (Dix/Diy). 
     (Evaluation) 
     The pressure losses in the comparative example 1 and the embodiments 1 to 6 were checked. In all of the embodiments 1 to 6, the pressure losses were not more than 200 kPa, and the results were evaluated as suitable. In contrast, in the comparative example 1, the pressure loss exceeded 200 kPa. 
     Second Embodiment 
     In the same manner as in the case of the first embodiment, as shown in  FIG. 17 , five electrodes (structural body pairs) having the same structure were provided in a pipe having a circular shape in cross section (pipe diameter=60 mm, pipe length=500 mm). 
     In each of electrode structures according to embodiments 11 to 22 and a comparative example 2, the ozone generation efficiency was checked. The ozone generation efficiency was measured based on ozone concentration in an exhaust gas at a certain level of power supply and at a certain gas flow rate. 
     (Method of Checking Ozone Generation Efficiency) 
     Firstly, in order to check the ozone generation efficiency, the air was used as a raw fuel gas. The gas flow rate was 2.5 NL/min, and the gas pressure was 0.25 MPa. 
     As the power source for discharging electricity, an alternating current power source capable of outputting electricity at the voltage (amplitude) of ±4 kV, and at the frequency of 20 kHz was used. 
     Under the above conditions, the ozone concentration in the exhaust gas was measured using an ozone concentration monitor (EG-3000D (manufactured by Ebara Jitsugyo Co., Ltd.)). 
     The details of the electrode structures according to the embodiments 11 to 22 and the comparative example 2 are as follows: 
     Embodiment 11 
     As shown in  FIG. 1B , in the electrode structure according to the embodiment 11, the two first electrode structural bodies  10 A were provided to face each other, and alternating current voltage was applied between the conducting body  18  of one of the first structural bodies  10 A and the conducting body  18  of the other of the first structural bodies  10 A. A gap g formed between the first structural bodies  10 A was 0.5 mm. The number of conducting bodies  18  (conducting body number) in each of the first structural bodies  10 A was 1, and the relationship between the maximum value Dcx of the length of the conducting body  18  in the typical flow direction x and the maximum value Dcy of the length of the conducting body  18  in the direction y perpendicular to the typical flow direction x (Dcx/Dcy) had a value of 1.2. The shape of the insulating body  16  was the same as that of the above described embodiment 1. 
     Embodiments 12 to 16 
     Electrode structures according to the embodiments 12 to 16 had the same structure as the electrode structure according to the embodiment 11. However, the electrode structures according to the embodiments 12 to 16 were different from the electrode structure according to the embodiment 11 in that the electrode structures according to the embodiments 12 to 16 had values of 3.0, 12.0, 8.0, 4.0, and 7.0, respectively, for the relationship between the maximum value Dcx and the maximum value Dcy of the length (Dcx/Dcy). Further, the shapes of the insulating bodies  16  of the embodiments 12 to 16 were the same as those of the above described embodiments 2 to 6, respectively. 
     Embodiment 17 
     In the electrode structure according to the embodiment 17, as shown in  FIG. 2B , the two second structural bodies  103  were provided to face each other, and alternating current voltage was applied between the conducting body  18  of one of the second structural bodies  10 B and the conducting body  18  of the other of the second structural bodies  10 B. The gap g between the second structural bodies  10 B was 0.5 mm. The number of the conducting bodies  18  (conducting body number) was 2, and the relationship between the maximum value Dcx and the maximum value Dcy of the length (Dcx/Dcy) had a value of 2.0. Regarding the shape of the insulating body  16 , the relationship between the length Dix and the maximum value Diy (Dix/Diy) had a value of 1.5. 
     Embodiments 18 to 22 
     Electrode structures according to the embodiments 18 to 22 had the same structure as the electrode structure according to the embodiment 17. However, the electrode structures according to the embodiments 18 to 22 were different from the electrode structure according to the embodiment 17 in that the numbers of the conducting bodies  18  in the embodiments 18 to 22 were 6, 12, 6, 3, 7, respectively, and the embodiments 18 to 22 had values of 6.0, 12.0, 6.0, 3.0, and 7.0, respectively, for the relationship between the sum Dcx of the maximum values and the maximum value Dcy (Dcx/Dcy). Further, regarding the shapes of the insulating bodies  16  of the embodiments 18 to 22, the relationship between the length Dix and the maximum value Diy (Dix/Diy) had values of 5.5, 15.0, 3.3, 3.0, and 5.5, respectively. 
     Comparative Example 2 
     The electrode structure according to the comparative example 2 was substantially the same as the electrode structure according to the embodiment 11. However, the electrode structure according to the comparative example 2 was different from the electrode structure according to the embodiment 11 in that the relationship between the sum Dcx of the maximum values and the maximum value Dcy (Dcx/Dcy) had a value of 1.0. Further, regarding the shape of the insulating body  16 , the relationship (Dix/Diy) between the length Dix and the maximum value Diy had a value of 1.0. 
     (Evaluation) 
     As for the ozone generation efficiency, the difference in the ozone generation efficiency in the comparative example 2 and the embodiments 11 to 22 were evaluated relatively, assuming that the ozone generation efficiency in the comparative example 2 was 1.0. The details of the comparative example 2 and the embodiments 11 to 22 and their evaluation results are shown in the following table 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Shape of 
                 Shape of 
                   
               
               
                   
                 Conducting Body 
                 Insulating Body 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Dcx 
                 Dcy 
                 Dcx/ 
                 Dix 
                 Diy 
                 Dix/ 
                   
               
               
                   
                 *1 
                 (mm) 
                 (mm) 
                 Dcy 
                 (mm) 
                 (mm) 
                 Diy 
                 *2 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Comparative 
                 1 
                 0.5 
                 0.5 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
               
               
                 Example 2 
               
               
                 Embodiment 11 
                 1 
                 0.6 
                 0.5 
                 1.2 
                 1.5 
                 1.0 
                 1.5 
                 1.2 
               
               
                 Embodiment 12 
                 1 
                 1.5 
                 0.5 
                 3.0 
                 2.0 
                 1.0 
                 2.0 
                 2.9 
               
               
                 Embodiment 13 
                 1 
                 6.0 
                 0.5 
                 12.0 
                 10.0 
                 1.0 
                 10.0 
                 10.0 
               
               
                 Embodiment 14 
                 1 
                 1.6 
                 0.2 
                 8.0 
                 2.3 
                 0.7 
                 3.3 
                 3.0 
               
               
                 Embodiment 15 
                 1 
                 4.0 
                 1.0 
                 4.0 
                 6.0 
                 2.0 
                 3.0 
                 7.3 
               
               
                 Embodiment 16 
                 1 
                 7.0 
                 1.0 
                 7.0 
                 11.0 
                 2.0 
                 5.5 
                 11.2 
               
               
                 Embodiment 17 
                 2 
                 1.0 
                 0.5 
                 2.0 
                 1.5 
                 1.0 
                 1.5 
                 1.8 
               
               
                 Embodiment 18 
                 6 
                 3.0 
                 0.5 
                 6.0 
                 5.5 
                 1.0 
                 5.5 
                 5.5 
               
               
                 Embodiment 19 
                 12 
                 7.2 
                 0.6 
                 12.0 
                 15.0 
                 1.0 
                 15.0 
                 11.5 
               
               
                 Embodiment 20 
                 6 
                 1.2 
                 0.2 
                 6.0 
                 2.3 
                 0.7 
                 3.3 
                 2.0 
               
               
                 Embodiment 21 
                 3 
                 3.0 
                 1.0 
                 3.0 
                 6.0 
                 2.0 
                 3.0 
                 5.7 
               
               
                 Embodiment 22 
                 7 
                 7.0 
                 1.0 
                 7.0 
                 11.0 
                 2.0 
                 5.5 
                 11.0 
               
               
                   
               
               
                 *1 Conducting Body Number 
               
               
                 *2 Ozone Generation Efficiency 
               
            
           
         
       
     
     As can be seen from the table 1, all of the ozone generation efficiencies in the embodiments 11 to 22 are better than the ozone generation efficiency in the comparative example 2. In particular, as can be seen from the embodiments 13, 16, 19, and 22, the larger the ratio Dcx/Dcy becomes, the higher the efficiency becomes. It is considered that the electric field is generated in a planar wider area, and for this reason, improvement in the ozone generation efficiency is achieved. However, same as the embodiments 14 and 20, even if the ratio Dcx/Dcy is large, in the case where Dix in the shape of the insulating body (maximum value of the length of the insulating body  16  in the typical flow length direction x) is less than 2.5 mm, improvement in the ozone generation efficiency is limited. 
     Third Embodiment 
     The ozone generation efficiencies in the electrode structures according to embodiments 31 to 36 and a reference example 1 were checked. As with the case of the second embodiment, the ozone generation efficiencies were measured based on the ozone concentration in an exhaust gas at a certain level of power supply and at a certain gas flow rate. The method of checking the ozone generation efficiencies is the same as in the case of the second embodiment, and description thereof is omitted. 
     The details of the electrode structures according to the embodiments 31 to 36 and the reference example 1 are as follows: 
     Embodiment 31 
     In the electrode structure according to the embodiment 31, as shown in  FIG. 15A , the two structural bodies  10 Ba of the first modified example of the second structural body  10 B were provided to face each other, and alternating current voltage was applied between the conducting body  18  of one of the structural bodies  10 Ba of the first modified example and the conducting body  18  of the other of the structural bodies  10 Ba of the first modified example. The gap g between the structural bodies  10 Ba of the first modified example was 0.5 mm. The relationship between the sum Dmn of the typical dimensions of the adjacent conducting bodies  18  in the structural bodies  10 Ba of the first modified example and the distance Lmn between the centers of the conducting bodies  18  (Lmn/(Dmn/2)) had a value of 1.1. Further, regarding the shape of the insulating body  16 , the relationship between the length Dix and the maximum value Diy (Dix/Diy) had a value of 4.5. 
     Embodiments 32 to 36 
     Electrode structures according to the embodiments 32 to 36 had the same structure as the electrode structure according to the embodiment 31. However, the electrode structures according to the embodiments 32 to 36 were different from the electrode structure according to the embodiment 31 in that the electrode structures according to the embodiments 32 to 36 had values of 1.5, 2.0, 1.5, 1.1, and 2.0, respectively, for the relationship between the sum Dmn and the distance Lmn between the centers of the conducting bodies  18  (Lmn/(Dmn/2)). Further, regarding the shapes of the insulating bodies  16  of the embodiments 32 to 36, the relationship between the length Dix and the maximum value Diy (Dix/Diy) had values of 4.5, 4.5, 3.3, 3.0, and 3.0, respectively. 
     Reference Example 1 
     Electrode structure according to the reference example 1 had substantially the same structure as the electrode structure according to the embodiment 31. However, the electrode structure according to the reference example 1 was different from the electrode structure according to the embodiment 31 in that the electrode structure according to the embodiment 31 had a value of 3.0 for the relationship between the sum Dmn and the distance Lmn between the centers of the conducting bodies  18  (Lmn/(Dmn/2)). Further, regarding the shape of the insulating body  16 , the relationship between the length Dix and the maximum value Diy (Dix/Diy) had a value of 4.5. 
     (Evaluation) 
     With regard to the ozone generation efficiency, in the same manner as in the case of the second embodiment described above, the differences in the ozone generation efficiencies among the embodiments 31 to 36 and the reference example 1 were evaluated relatively, assuming that the ozone generation efficiency of the embodiment 34 was 1.0. The details of the comparative example 1 and the embodiments 31 to 36 are shown in the following table 2. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Distance Between 
                 Shape of 
                   
               
               
                   
                 Conducting Bodies 
                 Insulating Body 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Lmn 
                 Dmn/2 
                 Lmn/ 
                 Dix 
                 Diy 
                 Dix/ 
                   
               
               
                   
                 (mm) 
                 (mm) 
                 (Dmn/2) 
                 (mm) 
                 (mm) 
                 Diy 
                 *1 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Reference 
                 1.5 
                 0.5 
                 3.0 
                 4.5 
                 1.0 
                 4.5 
                 2.0 
               
               
                 Example 1 
               
               
                 Embodiment 31 
                 0.55 
                 0.5 
                 1.1 
                 4.5 
                 1.0 
                 4.5 
                 2.8 
               
               
                 Embodiment 32 
                 0.75 
                 0.5 
                 1.5 
                 4.5 
                 1.0 
                 4.5 
                 2.7 
               
               
                 Embodiment 33 
                 1.0 
                 0.5 
                 2.0 
                 4.5 
                 1.0 
                 4.5 
                 2.5 
               
               
                 Embodiment 34 
                 0.3 
                 0.2 
                 1.5 
                 2.3 
                 0.7 
                 3.3 
                 1.0 
               
               
                 Embodiment 35 
                 1.1 
                 1.0 
                 1.1 
                 6.0 
                 2.0 
                 3.0 
                 5.8 
               
               
                 Embodiment 36 
                 2.0 
                 1.0 
                 2.0 
                 6.0 
                 2.0 
                 3.0 
                 5.2 
               
               
                   
               
               
                 *1 Ozone Generation Efficiency 
               
            
           
         
       
     
     As can be seen from the table 2, the ozone generation efficiencies in the embodiments 31 to 33, 35, 36 are better than the ozone generation efficiency in the reference example 1. In particular, as can be seen from the results of the reference example 1 and the embodiments 31 to 33, the smaller the distance Lmn between the centers of the conducting bodies  18  becomes, the higher the efficiency becomes. This analysis is applicable to the embodiments 35 and 36 as well. It is considered that the electric field is generated in a planar wider area, and for this reason, improvement in the ozone generation efficiency is achieved. However, as shown in the embodiment 34, it has been found that, even if the ratio Dix/Diy is large, in the case where Dix in the shape of the insulating body (maximum value of the length of the insulating body  16  in the typical flow length direction x) is less than 2.5 mm, improvement in the ozone generation efficiency is limited. It should be noted that if the distance Lmn between the centers is too small, the insulating body  16  between the conducting bodies  18  becomes thin. Therefore, the insulating body  16  is vulnerable to impacts, and the mechanical strength of the insulating body  16  may not be sufficient undesirably. 
     It should be understood that the structural body and the electrode structure according to the present invention are not limited to those of the embodiments described above, and it is a matter of course that various structures can be adopted without deviating the gist of the present invention.