Internal combustion engine and manufacturing method therefor

Abstract

In an internal combustion engine in which an anodic oxide film (10) is formed on part or all of a wall surface facing a combustion chamber, the anodic oxide film (10) has a thickness of 30 m to 170 m, the anodic oxide film (10) has first micropores (1a) having a micro-size diameter, nanopores having a nano-size diameter and second micropores (1b) having a micro-size diameter, the first micropores (1a) and the nanopores extending from a surface of the anodic oxide film (10) toward an inside of the anodic oxide film (10) in a thickness direction of the anodic oxide film (10) or substantially the thickness direction, the second micropores (1b) being provided inside the anodic oxide film (10), at least part of the first micropores (1a) and the nanopores are sealed with a seal (2) converted from a sealant (2), and at least part of the second micropores (1b) are not sealed.

Claims

1. An internal combustion engine comprising: an anodic oxide film forming on part or all of an aluminum-based wall surface facing a combustion chamber, wherein an aluminum-based material that forms the aluminum-based wall surface contains Si and Cu as an alloy component, a content of Si in the aluminum-based material is higher than or equal to 5% and less than 20% and a content of Cu in the aluminum-based material is higher than or equal to 0.4% and less than 7%, the anodic oxide film has a thickness of 30 m to 170 m; the anodic oxide film has first micropores having a micro-size diameter, nanopores having a nano-size diameter and second micropores having a micro-size diameter, the first micropores and second micropores have a sectional diameter or maximum size of a range of 1 to 100 m and the nanopores have a sectional diameter or maximum size of a range of 10 to 100 nm, the first micropores and the nanopores extending from a surface of the anodic oxide film toward an inside of the anodic oxide film in a thickness direction of the anodic oxide film or substantially the thickness direction, the second micropores being provided inside the anodic oxide film; the first micropores are cracks extending from the surface of the anodic oxide film to the inside of the anodic oxide film; the second micropores are internal defects not present at the surface of the anodic oxide film but present inside the film; the nanopores are originated from anodizing and are regularly arranged; at least part of the first micropores and the nanopores are sealed with a seal that is converted from a sealant, at least part of the second micropores are not sealed; and the anodic oxide film sealed with the seal has a porosity of 20 to 70%.

2. The internal combustion engine according to claim 1, wherein the seal is made of a substance that includes silica as a main component.

3. The internal combustion engine according to claim 1, wherein the sealant is made of any one of polysiloxane, polysilazane and sodium silicate.

4. The internal combustion engine according to claim 1, wherein the aluminum-based material that forms the aluminum-based wall surface further contains at least one of Mg, Ni, and Fe as the alloy component.

5. A manufacturing method for an internal combustion engine, comprising: a first step of forming an anodic oxide film on part or all of an aluminum-based wall surface facing a combustion chamber, the anodic oxide film having first micropores having a micro-size diameter, nanopores having a nano-size diameter and second micropores having a micro-size diameter, the first micropores and second micropores having a sectional diameter or maximum size of a range of 1 to 100 m and the nanopores having a sectional diameter or maximum size of a range of 10 to 100 nm, the first micropores and the nanopores extending from a surface of the anodic oxide film toward an inside of the anodic oxide film in a thickness direction of the anodic oxide film or substantially the thickness direction, the second micropores being provided inside the anodic oxide film, the anodic oxide film having a thickness of 30 m to 170 m; and a second step of forming the anodic oxide film subjected to sealing in which a sealant is applied to the surface of the anodic oxide film, the sealant penetrates into at least part of the first micropores and the nanopores, the sealant is converted into a seal, at least part of the first micropores and the nanopores are sealed with the seal and at least part of the second micropores are not sealed, wherein an aluminum-based material that forms the aluminum-based wall surface contains Si and Cu as an alloy component, a content of Si in the aluminum-based material is higher than or equal to 5% and less than 20% and a content of Cu in the aluminum-based material is higher than or equal to 0.4% and less than 7%; and the anodic oxide film sealed with the seal has a porosity of 20 to 70%.

6. The manufacturing method according to claim 5, wherein the seal is made of a substance that includes silica as a main component.

7. The manufacturing method according to claim 5, wherein the sealant is made of any one of polysiloxane, polysilazane and sodium silicate.

8. The manufacturing method according to claim 5, wherein the aluminum-based material that forms the aluminum-based wall surface further contains at least one of Mg, Ni, and Fe as the alloy component.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

(2) FIG. 1 is a longitudinal cross-sectional view that schematically shows a state before micropores and nanopores are sealed in an anodic oxide film formed on a wall surface facing a combustion chamber of an internal combustion engine according to an embodiment of the invention;

(3) FIG. 2 is an enlarged view of portion II in FIG. 1;

(4) FIG. 3 is a view in the arrow III direction in FIG. 1;

(5) FIG. 4 is a view of an anodic oxide film according to a reference example, which corresponds to FIG. 1;

(6) FIG. 5 is a view that illustrates an anodic oxide film formed by a manufacturing method for an internal combustion engine according to the embodiment of the invention;

(7) FIG. 6 is a view in the arrow VI direction in FIG. 5;

(8) FIG. 7 is a longitudinal cross-sectional view that schematically shows an internal combustion engine in which the anodic oxide film is formed on all of the wall surface facing the combustion chamber;

(9) FIG. 8A is a schematic view that illustrates the outline of a cooling test;

(10) FIG. 8B is a graph that shows a cooling curve based on the result of the cooling test and a 40 C.-drop time that is derived from the cooling curve;

(11) FIG. 9 is a correlation graph between a fuel economy improvement rate and a 40 C.-drop time in the cooling test;

(12) FIG. 10 is a graph that shows the test result regarding the correlation between a 45 msec achievement porosity and the thickness of the anodic oxide film;

(13) FIG. 11 is a graph that shows the test result regarding the correlation between the thickness of the anodic oxide film and a Vickers hardness;

(14) FIG. 12 is a graph that shows the result of experiment regarding the correlation between the thickness and porosity of the anodic oxide film;

(15) FIG. 13A is an SEM photograph showing the cross-sectional view of Example 2;

(16) FIG. 13B is an SEM photograph showing the cross-sectional view of Comparative Example 3;

(17) FIG. 14A is a TEM photograph showing the plan view of Example 2;

(18) FIG. 14B is an EDX analysis view of the plan view of Example 2;

(19) FIG. 15 is a graph that shows the test result regarding the correlation between the amount of Cu and a porosity in a material forming an aluminum-based wall surface;

(20) FIG. 16 is a graph that shows the test result regarding the correlation between the amount of Si and a porosity in the material forming the aluminum-based wall surface;

(21) FIG. 17A is an SEM photograph showing the cross-sectional view of Comparative Example 4;

(22) FIG. 17B is an SEM photograph showing the cross-sectional view of Comparative Example 6; and

(23) FIG. 17C is an SEM photograph showing the cross-sectional view of Example 4.

DETAILED DESCRIPTION OF EMBODIMENTS

(24) Hereinafter, an internal combustion engine and a manufacturing method therefor according to an embodiment of the invention will be described with reference to the accompanying drawings. In an illustrated example, an anodic oxide film is formed on all of the wall surface facing a combustion chamber of the internal combustion engine. However, the anodic oxide film may be formed only on part of the wall surface facing the combustion chamber, such as only a top face of a piston and only a top surface of a valve.

(25) Embodiment of Internal Combustion Engine and Manufacturing Method Therefor

(26) FIG. 1 and FIG. 5 show the flow diagram of the manufacturing method for an internal combustion engine in the stated order. More specifically, FIG. 1 is a longitudinal cross-sectional view that schematically shows a state before micropores and nanopores are sealed in the anodic oxide film formed on the wall surface facing the combustion chamber of the internal combustion engine according to the invention. FIG. 2 is an enlarged view of portion II in FIG. 1. FIG. 3 is a view in the arrow III direction in FIG. 1.

(27) Initially, an anodic oxide film 1 is formed by applying anodizing to an aluminum-based wall surface B facing the combustion chamber of the internal combustion engine (not shown). That is, the internal combustion engine is mainly formed of an engine block, a cylinder head and a piston. The combustion chamber of the internal combustion engine is defined by a bore face of the cylinder block, a top face of the piston assembled in the bore, a bottom face of the cylinder head and top faces of intake and exhaust valves arranged in the cylinder head. The anodic oxide film to be formed is formed on all of the wall surface facing the combustion chamber.

(28) The aluminum-based wall surface B that constitutes the combustion chamber of the internal combustion engine may be, for example, formed by anodizing aluminum, an aluminum alloy or an aluminized iron-based material. The anodic oxide film that is formed on the wall surface made of aluminum or an aluminum alloy as a base material is an alumite.

(29) As shown in FIG. 1, when the anodic oxide film 1 formed on the surface of the aluminum-based wall surface B that constitutes the wall surface of the combustion chamber is observed microscopically, first micropores 1a (longitudinal cracks), are present on the surface of the anodic oxide film 1, and second micropores 1b (internal defects) are present inside the anodic oxide film 1. The first micropores 1a extend in the thickness direction of the anodic oxide film 1 or substantially the thickness direction and have a micro-size diameter. The second micropores 1b extend in the horizontal direction of the anodic oxide film 1 or substantially the horizontal direction and have a micro-size diameter.

(30) These first micropores 1a and second micropores 1b have a sectional diameter or maximum size of the range of about 1 to 100 m. When not an ordinary aluminum alloy but an aluminum alloy contains at least one of Si, Cu, Mg, Ni, Fe as compared to the ordinary aluminum alloy, the diameter or sectional size of each micropore tends to further increase.

(31) As shown in FIG. 2 and FIG. 3, other than the first and second micropores 1a, 1b, a large number of nano-size small pores (nanopores 1c) are also present inside the anodic oxide film 1. The nanopores 1c, as well as the first micropores 1a, extend in the thickness direction of the anodic oxide film 1 or substantially the thickness direction. The diameter or maximum size of the cross section of each of the nanopores 1c ranges from about 10 to 100 nm.

(32) A manufacturing method for an internal combustion engine according to the embodiment of the invention is intended to form the maximally thin anodic oxide film having an excellent heat insulation property on the wall surface facing the combustion chamber of the internal combustion engine. Specifically, in the manufacturing method, the first micropores 1a and the nanopores 1c facing the surface of the film are sealed with a sealant, but the second micropores 1b present inside the film are not sealed. Thus, the film has a high porosity, so the film having an excellent heat insulation property is manufactured although the film is a thin layer.

(33) Therefore, the thin-layer anodic oxide film 1 having a thickness t of 30 m to 170 m is formed on the surface of the aluminum-based wall surface B facing the combustion chamber by anodizing (first step).

(34) Because the thickness t of the anodic oxide film 1 formed in the first step is small, the length of each first micropore 1a extending in the thickness direction of the film or substantially the thickness direction is also small, so the first micropores 1a are hard to communicate with the second micropores 1b present inside the film. With this configuration, at the time when a sealant is applied in the following second step, the sealant penetrates into the first micropores 1a but does not penetrate into the second micropores 1b. Thus, it is possible to suppress the second micropores 1b from being sealed with the sealant.

(35) FIG. 4 shows an anodic oxide film 1 formed on the surface of the aluminum-based wall surface B and having a thickness t of 300 m or larger.

(36) As the thickness increases, the length of each of the first micropores 1a that are surface cracks also increases. As a result, the first micropores 1a are easy to communicate with the second micropores 1b present inside the film, and there is a high possibility that the sealant applied in the following second step passes through the first micropores 1a and penetrates into the second micropores 1b to seal the second micropores 1b.

(37) Subsequently, in the second step, as shown in FIG. 5 and FIG. 6, a sealant 2 is applied to the first micropores 1a and the nanopores 1c to seal at least part of the first micropores 1a and the nanopores 1c and not to seal the second micropores 1b, not communicating with the first micropores 1a, with the sealant 2 as much as possible. Thus, an anodic oxide film 10 applied to sealing treatment of such a structure that the first micropores 1a and the nanopores 1c are sealed with a seal 2 that is converted from the sealant 2 and the second micropores 1b are not sealed or substantially not sealed is formed.

(38) A method of applying the sealant 2 may be a method of dipping the anodic oxide film into a case in which the sealant 2 is contained, a method of spraying the sealant 2 to the surface of the anodic oxide film, blade coating, spin coating, brush coating, or the like.

(39) The sealant 2 may be polysiloxane, polysilazane, or the like. By using one of these, the sealant 2 is allowed to relatively smoothly penetrate into the small first micropores 1a or the small nanopores 1c, it is possible to convert the sealant 2 into silica at a relatively low temperature, and it is possible to improve the strength of the anodic oxide film 10 after curing of the sealant 2 into a cured product, such as silica glass, having a high hardness.

(40) In this way, because part or all of the micro-size second micropores 1b present inside the formed anodic oxide film 10 are not sealed, the anodic oxide film 10 has a high porosity. Therefore, the anodic oxide film 10 has an excellent heat insulation property although the thickness is small, that is, the thickness ranges from 30 m to 170 m.

(41) FIG. 7 schematically shows an internal combustion engine in which the anodic oxide film 10 is formed on all of the wall surface facing the combustion chamber.

(42) The illustrated internal combustion engine N is intended for a diesel engine, and is roughly formed of a cylinder block. SB, a cylinder head SH, an intake port KP, an exhaust port HP, an intake valve KV, an exhaust valve HV, and a piston PS. A coolant jacket J is formed inside the cylinder block SB. The cylinder head SH is arranged on the cylinder block SB. The intake port KP and the exhaust port HP are defined inside the cylinder head SH. The intake valve KV and the exhaust valve HV are respectively installed at openings of which the intake port KP and the exhaust port HP face the combustion chamber NS so as to be movable up and down. The piston PS is provided so as to be movable up and down through a lower opening of the cylinder block SB. Of course, the internal combustion engine according to the invention may be intended for a gasoline engine.

(43) The component members that constitute the internal combustion engine N all are formed of aluminum or an aluminum alloy (including a high-strength aluminum alloy). Particularly, the aluminum material contains at least one of Si, Cu, Mg, Ni, and Fe as an alloy content, so enlargement in the diameter of each micropore is facilitated, and it is possible to improve the porosity.

(44) Inside the combustion chamber NS defined by the component members of the internal combustion engine N, the anodic oxide film 10 is formed on a wall surface (a cylinder bore face SB, a cylinder head bottom face SH, a piston top face PS, and valve top faces KV, HV) at which these component members face the combustion chamber NS. Swing Characteristic Evaluation Test, Strength Evaluation Test and Results of them

(45) The inventors manufactured a plurality of test pieces obtained by forming the anodic oxide film on base materials, having component compositions shown in the following Table 1 under the condition shown in Table 2, evaluated the swing characteristic of each anodic oxide film by conducting a cooling test and conducting a strength test at the same time, and obtained the correlation among the thickness, swing characteristic and strength of the anodic oxide film.

(46) TABLE-US-00001 TABLE 1 (Each component is indicated in mass %) Component Cu Si Mg Zn Fe Mn Ti Al Alloy 1 0 12.0 0.78 0.11 0.18 <0.01 <0.01 Remainder Alloy 2 0.2 12.0 0.78 0.11 0.18 <0.01 <0.01 Remainder Alloy 3 0.4 12.0 0.78 0.11 0.18 <0.01 <0.01 Remainder Alloy 4 0.8 12.0 0.78 0.11 0.18 <0.01 <0.01 Remainder Alloy 5 0.4 0 0.78 0.11 0.18 <0.01 <0.01 Remainder Alloy 6 0.4 2.0 0.78 0.11 0.18 <0.01 <0.01 Remainder Alloy 7 0.4 5.0 0.78 0.11 0.18 <0.01 <0.01 Remainder

(47) TABLE-US-00002 TABLE 2 Electrolytic Solution Current Solution Temperature ( C.) Density (mA/cm2) 20% Sulfuric Acid 0 60

(48) A method of sealing the pores of the anodic oxide film was performed in such a manner that the anodic oxide film is put in boiled pure water for 30 minutes. At the time of forming the anodic oxide film, the sealant was polysilazane, and a polysilazane 20% solution that uses dibutyl ether as a solvent was produced. A method of applying the sealant was performed in the following manner. The solution was applied with a brush on the entire surface of the anodic oxide film having a selected thickness, the applied solution was dried by warm air in several minutes, then the solution was applied with the brush again (this process was repeated five times), and the resultant product was fired in a firing furnace at 180 C. for 8 hours, thus sealing the micropores and nanopores of the anodic oxide film.

(49) As shown in FIG. 8A, the outline of the swing characteristic evaluation test is as follows. A test piece TP in which the anodic oxide film was applied to one-side face is used. The entire test piece TP is stabilized at about 250 C. by heating the back face (a face to which no anodic oxide film is applied) with high-temperature air jet at 750 C. (Heat in the drawing), a nozzle through which room-temperature jet has been flowing in advance at a predetermined flow rate is moved by a linear motor to in front of the front face (a face to which the anodic oxide film is applied) of the test piece TP, and then cooling is started (this is to provide 25 C. cooling air (Air in the drawing), and high-temperature air jet toward the back face is continued at this time). The temperature of the surface of the anodic oxide film of the test piece TP is measured by a radiation thermometer provided outside, a decrease in the temperature at the time of cooling is measured, and the cooling curve shown in FIG. 8B is created. The cooling test is a test method that simulates the inner wall of the combustion chamber in intake stroke, and is to evaluate the rate of cooing at the heated surface of the heat insulation film. In the case of a heat insulation film having a low thermal conductivity and a low thermal capacity, the rate of rapid cooling tends to increase.

(50) A time required to decrease by 40 C. is read from the created cooling curve, and the heat characteristic of the film is evaluated as a 40 C.-drop time.

(51) On the other hand, according to the inventors, at the time of an experiment, a fuel economy improvement rate of 5% is set as a target value that is achieved by the capability of the anodic oxide film that constitutes the combustion chamber of the internal combustion engine according to the invention. The fuel economy improvement rate of 5% is set as a value that is able to clearly prove improvement in fuel economy and that is not buried as a measurement error and it is possible to reduce NOx by reducing a warm-up time of a NOx reduction catalyst with an increase in exhaust gas temperature. FIG. 9 shows a correlation graph between a fuel economy improvement rate and a 40 C.-drop time in the cooling test, which is identified by the inventors.

(52) According to, the graph, the 40 C.-drop time in the cooling test, corresponding to the fuel economy improvement rate of 5%, is identified as 45 msec, and 45 msec or shorter may be set as an index indicating an excellent swing characteristic.

(53) On the other hand, a micro-Vickers hardness test was employed as the strength test, an evaluation portion was set to the center portion of the anodic oxide film in cross section, and a loaded load was set to 0.025 kg. In measuring the density of the anodic oxide film of the test piece TP, the density of the entire film was measured in accordance with JIS H8688, the porosity of the nanopores was measured by Autosorb, and the porosity of the micropores was obtained by subtracting the porosity of the nanopores from a total porosity calculated from the density. The test result is shown in FIG. 10.

(54) From FIG. 10, the porosity of the anodic oxide film, which satisfies the 40 C.-drop time of 45 msec, is 20% for 30 m thickness of the anodic oxide film. As the thickness increases, the porosity of the anodic oxide film, which satisfies the 40 C.-drop time of 45 msec, decreases.

(55) According to this result, the anodic oxide film that constitutes the internal combustion engine according to the invention has a thickness of 30 m or larger, so the porosity may be defined as 20% or higher.

(56) Hereinafter, the results of the specifications, porosity, Vickers hardness, and the like, of each of test pieces according to Comparative Examples 1 to 5 and Examples 1 to 3 are shown in Table 3. FIG. 11 shows the test results regarding the correlation between the thickness and Vickers hardness of each anodic oxide film. FIG. 12 shows the test results regarding the correlation between the thickness and porosity of each anodic oxide film. FIG. 13A is an SEM photograph of the cross-sectional view of Example 2. FIG. 13B is an SEM photograph of the cross-sectional view of Comparative Example 3. FIG. 14A is a TEM photograph of the plan view of Example 2. FIG. 14B is an EDX analysis view of the plan view of Example 2.

(57) TABLE-US-00003 TABLE 3 Thickness of Anodic Type Cu Si Oxide Film of Content Content Sealed Porosity (m) Alloy (%) (%) Sealant Pores (%) Comparative 10 Alloy 0.8 12 Applied Not-applied 9 Example 1 4 Example 1 30 Alloy 0.8 12 Applied Not-applied 27 4 Example 2 100 Alloy 0.8 12 Applied Not-applied 58 4 Comparative 100 Alloy 0.8 12 Applied Applied 67 Example 2 4 Example 3 170 Alloy 0.8 12 Applied Not-applied 31 4 Comparative 200 Alloy 0.8 12 Applied Not-applied 13 Example 3 4 Comparative 200 Alloy 0.8 12 Applied Applied 18 Example 4 4 Comparative 200 Alloy 0.8 12 Not-applied Applied 77 Example 5 4 Porosity (%) of Anodic Oxide Film Vickers Before Application of After Application of Hardness Sealant Sealant (HV 0.025 kg) Micropores Nanopores Micropores Nanopores Comparative 430 3 15 2 7 Example 1 Example 1 425 22 15 20 7 Example 2 410 55 16 50 7.5 Comparative 290 55 16 51 16 Example 2 Example 3 401 61 16 23 8 Comparative 405 61 15 6 7 Example 3 Comparative 400 61 16 4 14 Example 4 Comparative 230 61 16 61 16 Example 5

(58) According to Table 3, FIG. 11 and FIG. 12, in each of Examples 1 to 3, the Vickers hardness is higher than or equal to 300 HV that is a target value, and the porosity also satisfies 20% or higher.

(59) It has been demonstrated that, in Comparative Example 5 in which no sealant is provided or Comparative Example 2 in which no sealant is impregnated in the anodic oxide film, the hardness of each anodic oxide film is low, and the hardness of each anodic oxide film is ensured because of the fact that the sealant seals the first micropores and the nanopores.

(60) In addition, it has been demonstrated by Comparative Example 1 that the porosity of 20% or higher cannot be achieved when the thickness of the anodic oxide film is smaller than 30 m and, as a result, an excellent swing characteristic in the case where the 40 C.-drop time is shorter than or equal to 45 msec is not satisfied.

(61) Furthermore, it has been demonstrated from FIG. 13B that longitudinal cracks are promoted when the thickness of the anodic oxide film exceeds 170 m, the longitudinal cracks communicate with the internal defects present inside the film, the sealant applied to the surface layer of the anodic oxide film is impregnated into the internal defects and seals the internal defects, with the result that the porosity decreases. It has been confirmed from the EDX analysis view of Example 2 shown in FIG. 14B that Si react in each of the nanopores and polysilazane that is the sealant is impregnated.

(62) Next, the test result that identifies the correlation among a Cu content and an Si content in each alloy and a porosity is shown. The following Table 4 shows the specifications, porosity, Vickers hardness, and the like, of each of test pieces according to Examples 1, 4, 5 and Comparative Examples 6 to 9. FIG. 15 is a graph that shows the test result regarding the correlation between a Cu content and a porosity in the material of forming the aluminum-based wall surface. FIG. 16 is a graph that shows the test result regarding an Si content and a porosity in the material of forming the aluminum-based wall surface. FIG. 17A, FIG. 17B and FIG. 17C are respectively SEM photographs of the cross-sectional views of Comparative Example 4, Comparative Example 6 and Example 4.

(63) TABLE-US-00004 TABLE 4 Thickness of Anodic Type Cu Si Oxide Film of Content Content Sealed Porosity (m) Alloy (%) (%) Sealant Pores (%) Comparative 30 Alloy 0 12 Applied Not-applied 15 Example 6 1 Comparative 30 Alloy 0.2 12 Applied Not-applied 15 Example 7 2 Example 4 30 Alloy 0.4 12 Applied Not-applied 26 3 Example 1 30 Alloy 0.8 12 Applied Not-applied 27 4 Comparative 30 Alloy 0.4 0 Applied Not-applied 15 Example 8 5 Comparative 30 Alloy 0.4 2 Applied Not-applied 17 Example 9 6 Example 5 30 Alloy 0.4 5 Applied Not-applied 27 7 Porosity (%) of Anodic Oxide Film Vickers Before Application of After Application of Hardness Sealant Sealant (HV 0.025 kg) Micropores Nanopores Micropores Nanopores Comparative 420 8 15 8 7 Example 6 Comparative 415 8 15 8 7 Example 7 Example 4 410 19 15 19 7 Example 1 425 22 15 20 7 Comparative 423 8 15 8 7 Example 8 Comparative 410 10 15 10 7 Example 9 Example 5 430 20 15 20 7

(64) It has been demonstrated from the test that film formation of 100 m or larger is not possible because Si interferes with film growth in the case where the Si content is higher than or equal to 20%, and film formation of 100 m or larger is not possible because micropores enlarge due to gas that is generated at Cu in the case where the Cu content is higher than or equal to 7% and it is difficult to form the film.

(65) It has been demonstrated from Table 4 and FIG. 15 that it is possible to enlarge the micropores when the Cu content is higher than or equal to 0.4% and it is possible, to ensure a desired porosity (20% or higher).

(66) It has been demonstrated from Table 4 and FIG. 16 that it is possible to enlarge the micropores when the Si content is higher than or equal to 5% and it is possible to ensure a desired porosity (20% or higher).

(67) It appears from FIG. 17A to FIG. 17C that almost no micropores are present in Comparative Example 4 and a slight amount of micropores are present in Comparative Example 6; whereas a large amount of micropores are present in Example 4, and it is possible to ensure a high porosity.

(68) The embodiment of the invention is described in detail with reference to the accompanying drawings; however, a specific configuration is not limited to the embodiment. The invention also encompasses design changes, and the like, without departing from the scope of the invention.