JP2013237015A - Porous body with carbon film and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous body with a carbon film that is improved in coefficient of separation without lowering the transmission speed of a specific component when the separation component is separated from various mixed fluids, and has little characteristic deterioration.SOLUTION: There is provided a porous body 1 with a carbon film that includes a ceramic porous layer 11 and a carbon film 13 including glassy carbon, wherein the carbon film contains a metal element, the transmission rate of polar molecules among separation components in the carbon film is improved by setting the angle of contact between the surface of the carbon film and water to 30° or less, and the transmission rate and coefficient of separation are both high.

Description

  TECHNICAL FIELD The present invention relates to a porous body with a carbon membrane and a method for producing the same, and in particular, a porous body with a carbon membrane useful in techniques such as dehydration concentration of hydrous alcohol, natural gas separation, and isomer separation in a petroleum plant, and a method for producing the same. It is about.

  Conventionally, a filter for separating a specific gas from a mixed gas containing various gases, a filter for removing water from hydrous alcohol, a membrane reactor carrying a catalyst, etc. have been used. The scope of application has expanded, and now these separation and concentration technologies include various combustion engines, food industry, medical equipment, partial replacement for distillation in chemical plants and oil refining plants, as well as solvent recovery and waste disposal. It is also attracting attention in the field.

  For example, a filter for separating hydrogen gas can be used for recovering hydrogen gas from off-gas generated in an oil refining plant, purge gas generated in an ammonia synthesis plant, etc. The application to the removal of carbon dioxide contained in natural gas has been studied for the purpose of preventing corrosion of pipelines. Furthermore, as a filter for separating oxygen, it is applied to medical equipment, sports equipment, and various combustion engines.

  Conventionally, this type of separation membrane has been made of a polymer membrane such as an organic resin, or an inorganic porous membrane such as zeolite, glass or silica membrane. However, polymer membranes generally have a low separation factor and gas permeation rate, and the heat resistance and corrosion resistance of filters are poor. Therefore, they can be used for mixed gases containing acidic and alkaline gases and high-temperature gases. There was a drawback that it was difficult. On the other hand, the inorganic porous membrane has improved heat resistance and corrosion resistance of the filter compared with the polymer membrane, and the gas separation characteristics have been increased, but the water resistance and chemical resistance are not sufficient for application to an actual plant. There was a problem that application was accompanied by a lot of restrictions.

  In recent years, separation membranes made of carbon have attracted particular attention as membranes with greatly improved water resistance and chemical resistance and excellent gas permeation characteristics, and various studies have been made. In particular, a porous body with a carbon film in which the surface of a porous substrate is coated with a carbon film is almost free from restrictions on the strength of the carbon film itself, and the range of means for improving separation characteristics is widened. The body and its means of formation have been proposed.

  Generally, a carbon film is produced by heat-treating an organic film as a precursor in a non-oxidizing atmosphere. In a carbon film containing glassy carbon formed by heat treatment, gas molecules are screened by voids (pores) present in the glassy carbon, and separation characteristics are exhibited. In a carbon membrane, the separation factor can be improved by controlling the pore size to be smaller by performing a heat treatment at a high temperature or the like, but generally the permeation rate decreases as the pore size is reduced. Thus, attempts have been made to improve the separation factor without reducing the transmission rate.

  For example, in Patent Literature 1, hydrophilicity is imparted by immersing the produced carbon film in an acetic acid or citric acid aqueous solution and adsorbing acetic acid molecules or citric acid molecules on the pore walls of the carbon film. It is disclosed that the separation factor can be improved in the separation of water and ethanol.

Japanese Patent No. 4777666

  However, even if the carbon film once formed by heat treatment was given hydrophilicity by immersing the carbon film in an acetic acid or citric acid aqueous solution, the separation of water and an organic solvent such as ethanol was continued. In this case, there was a problem that acetic acid molecules or citric acid molecules were detached from the pore walls of the carbon film over time, and the characteristics deteriorated.

  The present invention has been made in view of the above problems. When separating a specific component, particularly moisture, from various mixed fluids, carbon is improved in the separation factor without reducing the permeation rate of the separated component and has little characteristic deterioration. It aims at providing the porous body with a film | membrane.

  A ceramic porous layer and a carbon film containing glassy carbon, wherein the carbon film contains a metal element, and a contact angle between the surface of the carbon film and water is 30 ° or less. To do.

The method for producing a porous body with a carbon film of the present invention comprises a step of producing a carbon film precursor solution containing a carbon film precursor and a metal compound,
Applying the carbon film precursor solution to the surface of the ceramic porous body to form a film of the carbon film precursor containing a metal compound;
And a step of heat-treating the ceramic porous body on which the coating is formed in a non-oxidizing atmosphere or in a vacuum.

  According to the present invention, when separating a specific component from various mixed fluids, it is possible to provide a porous body with a carbon membrane that improves the separation factor without reducing the permeation rate of the separated component and has little characteristic deterioration. it can.

It is a schematic sectional drawing of the porous body with a carbon film which is one embodiment of the present invention.

  The porous body with a carbon film which is one embodiment of the present invention will be described with reference to FIG. The porous body 1 with a carbon film of the present embodiment has a ceramic porous layer 11 (hereinafter also simply referred to as a porous layer 11) and a carbon film 13 containing glassy carbon. A composite layer 14 containing ceramic particles 15 and a carbonaceous material 16 exists at the interface between the carbon film 13 and the carbon film 13.

  The carbon film 13 contains glassy carbon, and the glassy carbon is defined as carbon having a uniform appearance without having an internal structure such as a grain boundary when observed at an optical microscope level. Is completely different. The glassy carbon referred to in the present specification has a molecular sieving action in which a large number of fine pores are present, and among the fluids to be separated consisting of gas or liquid, those having a small molecular diameter are carbon. It passes through the pores of glassy carbon constituting the membrane 13.

The carbon film 13 in the present embodiment contains a metal element, and the contact angle between the surface of the carbon film 13 and water (hereinafter sometimes simply referred to as a contact angle) is 30 ° or less, so Among the components contained in the fluid, the affinity between polar molecules that can permeate the pores of glassy carbon, such as water molecules and carbon dioxide molecules, and the pore surface of glassy carbon is improved. The moving speed inside the film 13 can be improved. As a result, it is possible to improve both the permeation speed and the separation coefficient of the separation component. Carbon dioxide is usually classified as a nonpolar molecule, but is treated as a polar molecule in this specification because it is soluble in water.

  That is, the contact angle between the surface of the normal carbon film 13 not containing a metal element and water is 90 to 110 °. In the carbon film 13 containing a metal element, the contact angle increases with the content of the metal element. Becomes smaller. This is because the metal element contained in the carbon film 13 is a constituent element of a hydrophilic functional group such as an OM group or a COOM group (M is a metal element contained in the carbon film 13) in the glassy carbon structure. This is considered to promote the movement of polar molecules such as water molecules by the surface diffusion mechanism and capillary condensation mechanism inside the pores of glassy carbon. In particular, when the contact angle is 30 ° or less, the permeation rate of the separation component can be greatly improved even in the carbon membrane 13 having a small pore diameter of glassy carbon and a high separation coefficient.

Even in a normal carbon film 13 that does not contain a metal element, a hydroxyl group (OH), a carboxyl group (COOH), a sulfone group (SO ₃ H), or the like is glassy carbon depending on the material used as the carbon film precursor in the formation process. Although these functional groups are usually decomposed during carbonization by heat treatment, their abundance is small and their contribution to the promotion of migration of separated components is small. Characteristics are not obtained.

  The metal element contained in the carbon film 13 is preferably an alkali metal element or an alkaline earth metal element. Alkali metal elements and alkaline earth metal elements are preferable because they have a high ionization tendency and can easily form hydrophilic functional groups in the structure of glassy carbon and can exist stably. Examples of the alkali metal element include lithium, sodium, and potassium, and examples of the alkaline earth metal element include magnesium, calcium, strontium, and barium. In particular, the metal element contained in the carbon film 13 is preferably sodium because it has a high ionization tendency and can be obtained at a low cost.

  When the metal element is an alkali metal element, the content of the metal element contained in the carbon film 13 is preferably 0.1 to 1.5 atomic% with respect to the carbon atoms in the carbon film 13. By setting it as such a range, generation | occurrence | production of defects, such as a pinhole and a crack, can be suppressed, and the contact angle of the surface of the carbon film 13 and water can be 30 degrees or less. This is because when the content of the alkali metal element is less than 0.1 atomic% with respect to the carbon atom, the functional group to which the alkali metal element present in the structure of the glassy carbon is bonded is small and the carbon film 13 is sufficiently For example, when the metal element is an alkaline earth metal element, the content less than the alkali metal, for example, 0.05 atomic% or more is sufficient for the carbon film 13. It is considered that the effect of imparting hydrophilicity is obtained.

  On the other hand, if an attempt is made to contain an alkali metal element of 1.5 atomic% or more in the carbon film 13, the film formability of the carbon film 13 is deteriorated and pinholes and cracks are likely to occur.

  Note that the content of the metal element in the carbon film may be confirmed by elemental analysis. For example, fluorescent X-ray analysis, wavelength dispersion X-ray spectroscopic analysis (WDS), or the like may be used.

  The thickness of the carbon film 13 is preferably 0.01 to 5 μm, and particularly preferably 0.1 to 3 μm from the viewpoint of suppressing the occurrence of defects such as pinholes and increasing the transmission rate. .

As the material of the porous layer 11, ceramics such as alumina, mullite, cordierite, zirconia, magnesia, silicon carbide, silicon nitride, etc. can be suitably used. By using such ceramics as the material of the porous layer 11, the composite layer 14 and the carbon film 1
3 can improve the thermal expansion difference, heat resistance, mechanical strength, wear resistance, thermal shock resistance, chemical resistance, and corrosion resistance. The porous layer 11 may be a single-layer ceramic porous body or a multilayer structure composed of two or more ceramic porous layers.

  The average particle diameter of the ceramic particles 17 constituting the porous layer 11 is preferably 1 to 10 μm, and more preferably 1 to 5 μm. By setting the average particle size of the ceramic particles 17 in such a range, the mechanical strength of the porous layer 11 can be kept high. The average particle diameter of the ceramic particles 17 constituting the porous layer 11 can be determined by, for example, an intercept method from a cross-sectional photograph of the porous layer 11 by a scanning electron microscope (SEM).

  The average pore diameter of the porous layer 11 is preferably 0.1 to 5 μm, more preferably 0.3 to 3 μm, and the porosity of the porous layer 11 is 30 to 60%, further 30 to 50%. Preferably there is. By setting the average pore diameter and the porosity of the porous layer 11 within such ranges, the permeation rate of the separation component can be increased, and at the same time, the mechanical strength of the porous layer 11 can be maintained high. The average pore diameter and porosity of the porous layer 11 can be obtained by a mercury intrusion method. In the case where a carbon component is present inside the pores of the porous layer 11, a condition that can be removed by cutting out the porous layer 11 from the porous body 1 with a carbon film and oxidizing the carbon component, for example, in the air Then, after removing the carbon component from the inside of the pores of the porous layer 11 by performing a heat treatment at 800 ° C. for about 30 minutes, the average pore diameter may be measured.

  The composite layer 14 is obtained by filling a carbonaceous material 16 into the pores of a ceramic porous layer 12 (hereinafter also simply referred to as a porous layer 12) made of ceramic particles 15. As a material for the ceramic particles 15 included in the composite layer 14, alumina, mullite, cordierite, zirconia, magnesia, silicon carbide, silicon nitride, or the like can be suitably used.

  The average particle diameter of the ceramic particles 15 included in the composite layer 14 is preferably the same as or smaller than the average particle diameter of the ceramic particles 17 constituting the porous layer 11.

  The thickness of the carbon film 13 formed on the upper surface of the composite layer 14 by making the average particle diameter of the ceramic particles 15 equal to or smaller than the average particle diameter of the ceramic particles 17 constituting the porous layer 11. The porous body 1 with a carbon film having a high permeation rate can be obtained. The average particle diameter of the ceramic particles 15 is particularly preferably smaller than 1.0 μm, and more preferably 0.5 μm or less.

  The thickness of the composite layer 14 may be 1 to 500 μm, for example. The thickness of the composite layer 14 is at least twice the average particle size of the ceramic particles 15 from the viewpoint that the carbon film 13 can be prevented from being peeled off due to thermal shock or the like and the reliability with respect to heat resistance can be increased. In view of further improving the permeation rate of the separation component, it is preferably 1 to 100 μm, more preferably 1 to 50 μm.

  The porous layer 11 and the carbon film 13 may be in direct contact with each other without using the composite layer 14.

The manufacturing method of the porous body 1 with a carbon film of this embodiment is demonstrated. First, for example, a porous substrate made of ceramic particles having an average particle diameter of 1 to 10 μm, having an average pore diameter of 0.1 to 5 μm, and a porosity of 30 to 60% is prepared. The carbon film 13 may be formed on the surface of the porous substrate. On one main surface of the porous substrate, for example, an intermediate layer made of ceramic particles having an average particle diameter of 1.0 μm or less is formed. It is preferable to form the carbon film 13 thereon. The intermediate layer is appropriately weighed with a raw material powder made of ceramic particles having an average particle size smaller than 1.0 μm, more preferably 0.5 μm or less, such as alumina, mullite, cordierite, zirconia, magnesia, silicon carbide, silicon nitride. For example, it is dispersed in water using a hydrophilic dispersant, applied onto one main surface of the porous substrate using a coating means such as a dip coating method (dip coating method), dried, and then heat treated. It can be formed by using a method such as. At this time, the ceramic particles constituting the intermediate layer only need to be partially bonded by the neck, and the particle size thereof is approximately equal to the particle size of the raw material powder. In the case where the intermediate layer is formed by a method such as dipping or spin coating, the average particle diameter of the raw material powder of the intermediate layer is preferably 0.02 μm or more from the viewpoint of suppressing particle aggregation.

  The thickness of the intermediate layer may be a thickness that can cover the unevenness present on the surface of the porous substrate with the intermediate layer. From the viewpoint of preventing surface defects such as pinholes from remaining on the intermediate layer and increasing the permeation rate, the thickness of the intermediate layer is 1 of the average particle diameter of the ceramic particles constituting the surface of the porous substrate. It is preferably in the range of ˜50 times, more preferably in the range of 2 to 20 times.

  A carbon film precursor solution containing a metal compound (hereinafter sometimes simply referred to as a precursor solution) is dip coated on at least the intermediate layer side of the porous body composed of the intermediate layer and the porous substrate thus obtained. Then dry it. Examples of metal compounds include phenols, cresol metal salts, benzoic acid, phthalic acid, terephthalic acid, acetic acid metal salts, or metal methoxides, ethoxides, propoxides, and other alkoxides. Examples of the body include those made of aromatic polyimide, polypyrrolone, polyfurfuryl alcohol, polyvinylidene chloride, phenol resin, lignin derivative, wood tar, bamboo tar, and the like. As the solvent, an organic solvent such as tetrahydrofuran, acetone, methanol, ethanol, N-methylpyrrolidone can be preferably used.

  Taking as an example the case of using a phenolic resin as a carbonaceous material precursor and a sodium salt of phenol (sodium carbonate) as a metal compound, the amount of sodium carbonate added is 5 to 40% by mass based on the solid content of the phenolic resin, Furthermore, it is preferable to set it as 10-30 mass%. If the amount of sodium carbonate added is less than 5% by mass, the contact angle between the obtained carbon membrane 13 and water becomes larger than 30 °, and the improvement of separation characteristics is hardly observed. On the other hand, if the amount of sodium carbonate added exceeds 40% by mass, pinholes and cracks are likely to occur in the carbon film.

  The amount of sodium carbonate added exceeds 20% by mass, the contact angle between the carbon membrane 13 and water is below the measurement limit, and a significant effect of improving separation characteristics is observed.

  In addition, when the above-mentioned metal compound is difficult to dissolve in the precursor solution, the solubility of the metal compound can be improved by appropriately mixing a highly polar solvent such as methanol or ethanol.

  The porous body that has undergone the coating process is heat-treated at a temperature of 600 to 800 ° C. in a non-oxidizing atmosphere or in vacuum, whereby the carbonaceous material precursor is carbonized to form glassy carbon, The porous body 1 with a carbon film having the carbon film 13 that is contained and has a contact angle with water of 30 ° or less is obtained.

The carbon film 13 formed in this way is considered to have a large number of hydrophilic functional groups. As a result, the contact angle with water is reduced, and the separation performance is improved. In the case of a normal carbon film 13 that does not contain a metal element, hydrophilic functional groups such as hydroxyl groups (OH), carboxyl groups (COOH), and sulfone groups (SO ₃ H) contained in the carbon film precursor are glassy carbon. Although remaining in the structure, these functional groups are usually decomposed by heat treatment. For example, when heat treated at 650 ° C., the functional group containing O such as a hydroxyl group or a carboxyl group has an O / C ratio of 2 atomic%. The sulfone group is almost 0% in terms of the S / C ratio. In particular, in the carbon membrane 13 in which the separation coefficient is increased by reducing the pore diameter of the glassy carbon by heat treatment at a high temperature, the residual rate is further reduced, and the contribution to the promotion of the movement of the separation component is small. Separation characteristics cannot be obtained.

  On the other hand, in this embodiment, by using a carbon film precursor solution containing a metal compound, the functional group as described above present in the carbon film precursor reacts with the metal compound during heat treatment and contains a metal element. It is considered that a functional group is formed and remains in the glassy carbon structure without being decomposed even in a heat treatment at a high temperature. Such an effect becomes prominent particularly when a metal compound of an alkali metal element or an alkaline earth metal element is used.

  Further, from the viewpoint of the film formability and cost of the carbon film 13, it is preferable to use a phenol resin as the carbon film precursor and an alkali metal salt or alkaline earth metal salt of carboxylic acid as the metal compound.

  The average pore diameter of the intermediate layer is preferably smaller than the average pore diameter of the porous substrate, preferably 0.01 to 0.5 μm, more preferably 0.02 to 0.1 μm. The porosity of the intermediate layer is preferably 30 to 60%, more preferably 30 to 50%. By setting the average pore diameter and porosity of the intermediate layer within such ranges, a thin and uniform carbon film 13 can be formed on the upper surface of the intermediate layer, and the precursor solution can be added to the pores of the intermediate layer as necessary. It can penetrate into the interior and the pores of the porous substrate.

  The thickness of the carbon film 13 depends on the components and composition of the precursor solution and the coating conditions (dipping time and pulling speed) in the coating process, and the pore diameter of the glassy carbon constituting the carbon film 13 depends on the heat treatment conditions (temperature rise). Speed, holding temperature, holding time). By optimizing the thickness and pore diameter of the carbon film 13, various gases and liquids having different molecular diameters can be efficiently separated.

  The porous body 1 with the carbon membrane of the present embodiment thus obtained has high water resistance and chemical resistance, and has a high permeation rate and separation factor for polar molecules, and therefore, dehydration of low concentration alcohol, Excellent separation performance can be exhibited even under severe conditions such as acetic acid dehydration and concentration, carbon dioxide removal from natural gas, and oxygen enrichment.

  First, an alumina porous tube having an outer diameter of 12 mm, an inner diameter of 9 mm, and a length of 100 mm (manufactured by Kyocera) was prepared as a porous substrate. A porous tube having an average pore diameter of 1.2 μm, a porosity of 42%, and an average particle diameter of alumina particles constituting the porous tube was 3.1 μm. Next, an alumina powder having an average particle size of 0.2 μm, which is a raw material for the intermediate layer, was dispersed in water together with polyvinyl alcohol (PVA) to prepare an alumina slurry.

  After the opening at the end of the porous tube was sealed, it was immersed in the prepared alumina slurry and pulled up at a constant rate to form a coating film as an intermediate layer on the outer surface of the porous tube and dried. Thereafter, the entire porous tube was heat-treated at 1100 ° C. to prepare an alumina porous body in which an intermediate layer was formed on the outer surface of the alumina porous tube. The thickness of the intermediate layer obtained from a cross-sectional photograph of this porous alumina body by a scanning electron microscope (SEM) was 12 μm, and the average particle diameter of the alumina particles constituting the intermediate layer was 0.2 μm. The average particle size of the alumina particles was calculated by the intercept method.

Next, the phenol resin powder was dissolved in tetrahydrofuran (THF) as a solvent as a carbon film precursor to prepare a solution having a phenol resin concentration of 25 to 35%. Furthermore, sodium carbonate of the ratio shown in Table 1 with respect to the phenol resin powder in a solution was added and mixed as a metal compound with the produced solution, and it was set as the carbon film precursor solution. In addition, when undissolved sodium carbonate remained, methanol was further added to dissolve all sodium carbonate to obtain a uniform solution. And after immersing the alumina porous body which formed the intermediate | middle layer in the outer surface in the carbon membrane precursor solution and hold | maintaining for 10 to 20 seconds, it pulls up at a speed | rate of 100 mm / min, A carbon membrane precursor solution is made into an intermediate | middle layer shape Was formed and dried at 130 ° C. for 10 minutes. Thereafter, the entire porous body was heat-treated at a temperature shown in Table 1 for 10 minutes in a nitrogen atmosphere to prepare a porous body with a carbon film.

  Note that the carbon film formed on the intermediate layer was made of glassy carbon, and the internal structure such as grain boundaries could not be confirmed for any sample in the observation with an optical microscope.

  The thickness of the carbon film was measured using a scanning electron microscope (SEM) after depositing Pt on the cross section of the produced porous body with a carbon film.

  The contact angle between the surface of the carbon film and water was measured with a contact angle meter DM-501 manufactured by Kyowa Interface Science Co., Ltd. In the present specification, a contact angle of 5 ° was taken as the measurement limit, and all measurement results below that were regarded as 0 °.

  The sodium content in the carbon film was obtained by conducting an elemental analysis of the porous body with the carbon film using wavelength dispersive X-ray spectroscopy (WDS), and calculating the atomic ratio of sodium to the carbon component. In addition, the atomic ratio with respect to the carbon component was also calculated for the oxygen component after removing the alumina component caused by the intermediate layer and the porous tube.

  Table 1 shows the thickness of the carbon film, the contact angle between the surface of the carbon film and water, and the contents of sodium and oxygen in the carbon film.

As an evaluation of the separation characteristics of the produced porous body with a carbon membrane, pervaporation separation measurement of a water / ethanol mixed solution was performed. The supply side (the carbon membrane side of the porous body with the carbon membrane) is at atmospheric pressure and the permeation side (the porous body side of the porous body with the carbon membrane) is in the vacuum, and is outside the carbon membrane of the porous body with the carbon membrane. The water / ethanol mixed solution was permeated to the porous body side using the partial pressure difference between the supply side and the permeation side as a driving force, and the separation coefficient α and permeation rate Q at that time were compared. The feed solution had a water / ethanol (EtOH) ratio of 10/90 (mass%) and a temperature of 75 ° C. The contents (mass%) of ethanol and water on the supply side and the permeation side were measured using a gas chromatograph GC-2014 (Shimadzu Corporation). The separation factor α and the permeation speed Q were calculated using the following equations, and the results are shown in Table 1.

Sample No. 2, 3, 6-11, 13 and 15 contain sodium as a metal element and the contact angle between the carbon film and water is 30 ° or less, and do not contain sodium subjected to the same heat treatment, or contact angle The separation performance was higher than that of samples exceeding 30 °. Also, as the content of sodium, which is a metal element in the carbon membrane, increased, the oxygen content also increased. By containing sodium, a functional group containing sodium and oxygen, for example, ONa group is formed in the structure of glassy carbon, and this functional group remaining in glassy carbon increases according to the sodium content. It is thought that.

DESCRIPTION OF SYMBOLS 1 ... Porous body 11 with a carbon film ... Ceramic porous layer 13 Carbon film 14 Composite layer 15 Ceramic particles 16 contained in the composite layer Carbonaceous material 17 contained in the composite layer・ Ceramic particles constituting the ceramic porous layer

Claims (6)

  A ceramic porous layer and a carbon film containing glassy carbon, wherein the carbon film contains a metal element, and a contact angle between the surface of the carbon film and water is 30 ° or less. Porous body with carbon film.   The porous body with a carbon film according to claim 1, wherein the metal element is an alkali metal element or an alkaline earth metal element.   The porous body with a carbon film according to claim 2, wherein the content of the alkali metal element contained in the carbon film is 0.1 to 1.5 atomic% with respect to the carbon atoms.   The porous body with a carbon film according to claim 2 or 3, wherein the alkali metal element is sodium. Producing a carbon film precursor solution containing a carbon film precursor and a metal compound;
Applying the carbon film precursor solution to the surface of the ceramic porous body to form a film of the carbon film precursor containing a metal compound;
And a step of heat-treating the ceramic porous body on which the coating film is formed in a non-oxidizing atmosphere or in a vacuum.   The method for producing a porous body with a carbon film according to claim 5, wherein the carbon film precursor is a phenol resin, and the metal compound is an alkali metal salt or an alkaline earth metal salt of carboxylic acid.

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