JP2013159538A - Silicon carbide-reacted porous structure and method for manufacturing it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably manufacture a silicon carbide-reacted porous structure by using fibrous carbon and molten silicon.SOLUTION: A carbonized porous structure formed of a phenol resin-derived carbon fiber and silicon are heated at 1,300°C-1,800°C under vacuum or an inert atmosphere to react them by melting and impregnating silicon. The carbon fiber formed of phenol resin is remarkably improved in wettability, and easily reacts with molten silicon and at least the surface is modified to form silicon carbide.

Description

  The present invention relates to a siliconized porous structure made of fibrous silicon carbide and a method for producing the same.

  Since silicon carbide-based ceramics are lightweight and have excellent heat resistance, wear resistance, corrosion resistance, etc., in recent years, for example, high-temperature corrosion-resistant members, heater materials, wear-resistant members, as well as abrasives, grindstones, etc. Widely used in applications. Since this silicon carbide ceramic is mainly manufactured by a sintering technique, it has not yet been put into practical use as a filter-shaped ultralight porous material having a porosity of 90% or more.

Recently, research on such heat-resistant lightweight porous ceramics has begun. For example, Bridgestone Corporation, as a cast iron ceramic foam filter, impregnates a silicon carbide powder slurry into a polyethylene or polyurethane sponge, removes the excess slurry, and dries and fires to obtain a porous silicon carbide structure. The physical properties in the catalog indicate that the porosity is 85% and the apparent specific gravity is about 0.42 g / cm ³ .

However, since the silicon carbide powder slurry is used in the above-described method, there are places where the surplus slurry closes the remaining pores even if the surplus slurry is removed. Basically, it is made by the powder sintering method of ceramics, so the center part of the sponge skeleton part becomes hollow without entering powder, and if the amount of powder attached to the skeleton part is small, the strength becomes low, so the skeleton part It becomes thicker than the sponge skeleton. In addition, the porosity is as low as about 85%, and the apparent specific gravity is as high as about 0.42 g / cm ³ . Also, the pore diameter is as large as about 1 to 5 mm (standard number of cells: 13/25 mm to 6 / mm).

  In a filter through which a large amount of fluid passes, such as DPF (diesel engine exhaust gas filter), pressure loss is particularly important. The pressure loss decreases as the thickness of the filter skeleton decreases. However, when polyurethane sponge or the like is used, the thickness is at least 50 μm, which is not a desirable material in terms of pressure loss.

  Thus, it has been recalled to use fibrous silicon carbide. For example, silicon carbide fibers such as “Nicalon Fiber” (manufactured by Nippon Carbon Co., Ltd.) and “Tyranno Fiber” (manufactured by Ube Industries, Ltd.) are about 10 μm in diameter and are preferable materials from the viewpoint of pressure loss. In addition, it has flexibility as a fiber and is mainly used as a raw material for fiber-reinforced composite materials. However, since an organosilicon compound is a raw material and contains not only carbon and silicon but also other elements, it is not as heat resistant as pure silicon carbide. In addition, in order to use as a filter, it is necessary to form a three-dimensional structure from silicon carbide fibers, but the technology for immobilizing silicon carbide fibers while stably forming pores as filters has not yet been established. .

  Since silicon carbide is a compound of silicon (Si) and carbon (C), studies have been made on silicon carbide fibers by reacting carbon fibers with silicon. As the carbon fiber, polyacrylonitrile (PAN) type and pitch type are known. For example, Japanese Patent Laid-Open No. 06-192917 describes a method for producing silicon carbide fibers by reacting porous carbon fibers with SiO gas. However, the manufacturing method via the gas phase has a problem that the apparatus becomes large and SiO gas reacts with a crucible or the like. There is also a report of developing a Silcomp ceramic composite by reacting carbon fiber bundles with molten silicon (R. L. Mehan, J. Mater. Sci., 13 (1978) 358-366)). In the photograph of this document in which the molten silicon is removed by etching, the carbon fiber reacts with the molten silicon, and there is an appearance of the fiber, but it is an aggregate of cracked crystals of silicon carbide of about 10 μm.

  By the way, the inventors of the present application impregnated a tangible skeleton of a porous structure such as urethane sponge with a slurry made of silicon powder and a resin to produce silicon carbide accompanied by volume reduction of silicon powder and carbon from the porous structure. Porous silicon carbide and residual carbon are generated by the reaction, and silicon is heat-impregnated into the porous skeleton to melt the silicon carbide-based heat-resistant ultralight porous structure with a complex shape. However, it has been found that it can be easily produced while maintaining the shape of the tangible skeleton of the porous structure (Japanese Patent No. 3699992) (Japanese Patent Laid-Open No. 2010-30888).

  However, since the slurry contains silicon powder, when the pore size of the tangible skeleton is reduced to, for example, about 100 μm or less, the silicon powder is difficult to disperse in the tangible skeleton, and the slurry composition may become non-uniform. There is a problem as a manufacturing method. When the slurry does not contain silicon powder, the molten silicon has low wettability at the time of melt impregnation, and the reaction between the molten silicon and the carbon skeleton does not occur.

Japanese Unexamined Patent Publication No. 06-192917 Patent No. 3699992 JP 2010-30888

R.L.Mehan, J.Mater.Sci., 13 (1978) 358-366)

  The present invention has been made in view of the above circumstances, and solves the problem of enabling stable production of a siliconized porous structure with little pressure loss by reaction between fibrous carbon and molten silicon. It should be a challenge.

  One feature of the method for producing a siliconized porous structure of the present invention that solves the above-described problems is that the carbonized porous structure formed from a phenol resin-derived carbon fiber and silicon are subjected to a vacuum or an inert atmosphere. In this method, silicon carbide is produced by heating at a temperature of 1300 ° C. to 1800 ° C., melting and impregnating silicon, and reacting.

  The carbon fiber derived from a phenol resin may be any carbon fiber obtained by spinning and heat-treating a phenol resin. Specifically, a novoloid fiber (trade name: Kynol) obtained by spinning a novolac-type phenol resin, It is a fiber obtained by carbonizing a resol fiber obtained by spinning a resole resin at 900 ° C. to 1300 ° C. in a vacuum or an inert atmosphere. Moreover, the activated carbon fiber which activated this fiber is also contained. Hereinafter, this manufacturing method is referred to as a first manufacturing method.

  Another feature of the method for producing a silicon carbide porous structure of the present invention is that the porous structure formed from phenol resin fibers is heated to 900 ° C. to 1300 ° C. in a vacuum or in an inert atmosphere to be carbonized. The carbonization process for forming the carbonized porous structure, and the carbonized porous structure and silicon are heated at a temperature of 1300 ° C. to 1800 ° C. in a vacuum or in an inert atmosphere, and silicon is melted and impregnated to react. A silicon carbide step of forming silicon carbide to form a silicon carbide porous structure. Hereinafter, this manufacturing method is referred to as a second manufacturing method.

  The features of the silicon carbide porous structure of the present invention produced by the first and second production methods of the present invention are the porous structure having a large number of pores, and the pore walls defining the pores. At least the surface is that the phenol resin is silicon carbide derived from carbonization of the phenol resin and then silicon carbide.

Carbon fibers are roughly classified into PAN-based and pitch-based, and the density of general PAN-based carbon fibers is about 1.8 g / cm ³ , and pitch-based carbon fibers are about 2.2 g / cm ³ . On the other hand, a typical quinol fiber as a carbon fiber formed from a phenol resin has a density of 1.27 g / cm ³ and a carbon content of about 76%. The density of the carbonized fiber is PAN-based or pitch-based carbon fiber. It is 1.4-1.6 g / cm ³ lower than that (Fiber and Industry: Vol. Moreover, since the carbon compound of a phenol resin has an amorphous structure, there are many active sites on the surface.

  The phenol resin is a resin composed only of carbon (C), hydrogen (H), and oxygen (O). Therefore, the carbon fiber formed from the phenol resin has fine pores formed by hydrogen (H) and oxygen (O) bonded to the phenol resin skeleton during the carbonization. It is considered that the low density and the active structure on the surface due to the amorphous structure are affected by this, but as a result of the inventor's research, the carbon fiber formed from the phenol resin is molten silicon. It was revealed that the wettability of the fiber was significantly improved, and it reacted easily with molten silicon to make the fiber surface uniform silicon carbide.

  Moreover, even if molten silicon is impregnated with a woven fabric of carbon fibers formed from phenolic resin, it is independent as with the woven fabric used with each individual fiber, and the surface is uniformly siliconized. ing. Since the reaction in which carbon becomes silicon carbide (SiC) is a reaction in which the volume increases, a dense SiC layer is formed on the carbon fiber surface, and the silicon carbide porous structure of the present invention is resistant to oxidation. Are better.

  Therefore, according to the first and second production methods, since the carbonized porous structure and the molten silicon easily react, the siliconized porous structure can be produced stably.

  And according to the siliconized porous structure of the present invention, at least the surface of the pore walls that define the pores is silicon carbide derived from phenol resin in which the phenol resin is carbonized and then silicon carbide, It has excellent heat resistance and is useful as an ultralight porous material such as a filter.

It is a SEM image of the section of the silicon carbide porous structure concerning the 2nd example of the present invention. It is a SEM image of the section of the silicon carbide porous structure concerning the 2nd example of the present invention.

<First manufacturing method>
In this production method, first, a carbonized porous structure is formed from a carbon fiber derived from a phenol resin.

  There are many known methods for producing phenol resin fibers. For example, a method of heating a resol type phenol resin containing an alkali (earth) metal and curing it in flowing propylene glycol (Japanese Patent Publication No. 48-43570). Publication), a method of heat-treating a cured novolak fiber obtained by subjecting an uncured novolak fiber obtained by melt spinning of a novolak resin with an aldehyde in a non-oxidizing atmosphere (Japanese Patent Laid-Open No. 53-94626), uncured novolak A method of heat-treating a cured novolak resin fiber obtained by adding polyvinyl butyral to a resin and curing it after melt spinning (JP-A-9-13223), reacting phenols and aldehydes in the presence of a basic catalyst The solid resol-type phenolic resin obtained in this way is melted by heat and spun from a spinning nozzle with a traction force of a heated air stream to form a fiber. And a method of insoluble infusible by heating in sexual gas atmosphere (JP-A-9-132818) are known.

  As the carbon fiber derived from phenol resin, commercially available ones such as quinol carbon fiber can be used. Moreover, the carbon fiber formed by carbonizing a phenol resin fiber like the 2nd manufacturing method mentioned later can also be used.

  For example, a woven fabric, a knitted fabric, a nonwoven fabric or the like is formed from the carbon fiber derived from the phenol resin, and these are further laminated or formed into a three-dimensional carbonized porous structure like origami. The shape of the carbonized porous structure is not particularly limited, and can be a shape according to the purpose of using the pores formed between the carbon fibers. For example, a filter-like carbonized porous structure can be formed by laminating or winding in a roll.

  The formed carbonized porous structure is melt-impregnated with silicon, and heated at a temperature of 1300 ° C. to 1800 ° C. in a vacuum or an inert atmosphere to produce a silicon carbide porous structure. This step may be performed by heating the metal silicon to a melting point (about 1410 ° C.) or higher to form molten silicon and impregnating the carbonized porous structure, and it is particularly preferable to perform in a vacuum. The melt-impregnating silicon may be in the form of powder, granules, or lumps. For example, if silicon powder granules are applied to the carbonized porous structure and adhered, and heated to about 1410 ° C or higher in a vacuum or inert atmosphere, the silicon powder will melt and impregnate the carbonized porous structure. Reaction produces a silicon carbide porous structure.

  The ratio of the carbonized porous structure to silicon is preferably an atomic ratio (Si / C) of 0.2 or more, and the entire carbonized porous structure is made silicon carbide by slightly more silicon than the equivalent ratio. Is desirable. Even if the atomic ratio (Si / C) is about 0.2, at least the surface of the carbonized porous structure can be siliconized.

  Even if the silicon is slightly larger than the equivalent ratio and surplus silicon remains, there is no particular problem unless it is so large as to cause problems such as pore clogging. However, when oxidation resistance or corrosion resistance becomes a problem, it is desirable to perform a post-treatment process in which the metal silicon to be exposed is silicon carbide. On the contrary, even if carbon remains inside the fiber, there is no problem as long as the oxidation resistance does not deteriorate.

  In this post-treatment step, the silicon carbide porous structure is impregnated with a slurry containing resins as a carbon source, the excess slurry is removed, and then dried at 70 ° C. for about 12 hours, in a vacuum or in an inert atmosphere. In this case, carbonization is performed by heating to 900 ° C. to 1300 ° C., and at the surface of 1300 ° C. to 1600 ° C., at least the surface of the metal silicon to be exposed is silicon carbide. The temperature during the reaction is desirably 1300 ° C. or higher and lower than the melting point of silicon. Alternatively, excess silicon on the surface may be removed by evaporation in a vacuum at a temperature equal to or higher than the melting point of silicon.

  As the resins contained in the slurry, those having many carbon atoms in the molecule and high wettability to silicon are preferable. Examples of such resins include phenol resins and furan resins. However, when only a resin is included as a carbon source, there may be a small amount of carbon that reacts with silicon, and therefore it is desirable that the slurry contains carbon powder such as carbon black. The amount of the carbon powder in the slurry is not particularly limited as long as it is in a viscosity range capable of impregnating the silicon carbide porous structure as a slurry.

  As the carbon powder added to the slurry, graphite, carbon black or the like can be used, and carbon black is particularly preferable from the viewpoint of reactivity with silicon and dispersibility.

  When the silicon carbide porous structure is impregnated with the slurry, when the carbon powder is not included, it is desirable to apply the slurry thickly in order to increase the amount of the resin. When carbon powder such as carbon black is included, it can be applied relatively thinly depending on the content of carbon powder.

  The silicon carbide porous structure of the present invention thus obtained has silicon carbide at least on its surface such as the surface of the tangible skeleton and the inner surface of the pores. Therefore, since metal silicon is not exposed on the surface, it is excellent in oxidation resistance and corrosion resistance.

<Second production method>
In this production method, a porous structure is first formed from phenol resin fibers formed from a phenol resin. Commercially available phenol resin fibers such as quinol fiber may be used, or phenol resin fibers formed from uncured phenol resin may be used. As the phenol resin, either a resol type or a novolac type can be used.

  A woven fabric, a knitted fabric, a non-woven fabric, or the like is formed from this phenol resin fiber, and these are further laminated or formed into a three-dimensional porous structure like origami. The shape in particular of a porous structure is not restrict | limited, It can be set as the shape according to the objective using the pore formed between the phenol resin fibers. For example, a filter-shaped porous structure can be formed by laminating or winding in a roll.

  The formed porous structure is heated and carbonized at 900 ° C. to 1300 ° C. in a vacuum or an inert atmosphere to form a carbonized porous structure. As the inert atmosphere, an inert gas atmosphere such as argon gas is preferable. In the carbonization process by thermal decomposition of the resin, tar-like substances and vaporized substances are generated, so it is not preferable to carry out in a vacuum.

  The carbonized porous structure thus formed can be made into a silicon carbide porous structure in the same manner as in the first production method. Further, when surplus metal silicon is exposed on the surface, a post-treatment process can be performed in the same manner as in the first manufacturing method.

<Siliconized porous structure of the present invention>
The silicon carbide porous structure of the present invention manufactured by the manufacturing method of the present invention has a large number of pores similar to the tangible skeleton of the carbonized porous structure or organic porous structure formed from carbon fibers. Has a porous structure. Further, the silicon carbide porous structure produced by the production method of the present invention is silicon carbide derived from phenol resin at least on the surface of the pore walls. Therefore, it is excellent in oxidation resistance and corrosion resistance, and is useful as an ultralight porous material such as a filter.

  Hereinafter, embodiments of the present invention will be specifically described with reference to Examples and Comparative Examples.

  A carbon fiber woven fabric made of carbonized novolak-type phenol resin fibers (manufactured by Nihon Kynol, trade name: Kynol activated carbon fiber cloth ACC-507, thickness 0.4 mm) is prepared, and this is referred to as a carbonized porous structure. did. Innumerable pores are formed between the woven yarns.

  Silicon granules were weighed so that the Si / C atomic ratio would be approximately 0.5, 1.0, and 1.5, respectively, and placed on a carbon fiber woven fabric, heated in vacuum at 1450 ° C for 1 hour, A silicon carbide porous structure was obtained. When the obtained silicon carbide porous structure was weighed, the Si / C atomic ratios were 0.69, 1.02, and 1.14, respectively. Each of the obtained silicon carbide porous structures has the same shape as the carbon fiber woven fabric used, and at least the surface of the pore walls (the surface of the weaving yarn) is changed to a uniform greenish color and carbonized. It was silicon. Moreover, it was confirmed that each fiber bundle was separated.

  Prepare a woven fabric made of novolac-type phenolic resin (manufactured by Nihon Kynol, trade name: Kynol fiber cloth 507, thickness 0.4mm), and heat it to 1000 ° C in an argon atmosphere to carbonize porous structure The body. Innumerable pores are formed between the woven yarns.

  Silicon granules are weighed so that the Si / C atomic ratio is approximately 0.5, 1.0, and 1.5, respectively, placed on the obtained carbon fiber woven fabric, and heated in vacuum at 1450 ° C for 1 hour. Three types of silicon carbide porous structures were obtained. When the obtained silicon carbide porous structure was weighed, the Si / C atomic ratios were 0.43, 0.78, and 1.24, respectively. Each of the obtained silicon carbide porous structures has the same shape as the carbon fiber woven fabric used, and at least the surface of the pore walls (the surface of the weaving yarn) is changed to a uniform greenish color and carbonized. It was silicon. Further, it was confirmed that each fiber bundle was separated. The silicon carbide porous structure having a composition of Si / C = 1.24 was found to have a small silicon mass.

  FIG. 1 shows an SEM image of the cross section of the silicon carbide porous structure having a Si / C atomic ratio of 0.43. The black central part of each fiber is carbon fiber, and the gray layer surrounding it is SiC. From this image, it can be seen that the SiC layer is uniformly formed on the surface of each fiber, and the fibers are independent without being joined. The carbon portion has a smooth surface.

  FIG. 2 shows an SEM image of a cross section of the silicon carbide porous structure having a Si / C atomic ratio of 1.24. From the composition analysis, the entire fiber part was SiC. It can also be seen that the fiber part is made of small crystals.

[Comparative Example 1]
A polyacrylonitrile (PAN) -based carbon fiber yarn bundle ("BESFIGHT IM-400" manufactured by Toho Rayon Co., Ltd.) was prepared, and a woven fabric obtained by simply woven this into a plain weave was prepared. This was heat-treated at 1000 ° C. in argon gas to remove the adhering sizing agent.

  Silicon granules were weighed so that the Si / C atomic ratio was approximately 1.5 and placed on the woven fabric, and heated in vacuum at 1450 ° C. for 1 hour to obtain a porous structure.

  When the obtained porous structure was weighed, the Si / C atomic ratio was 1.13. Moreover, although the obtained porous structure is the same shape as the carbon fiber woven fabric used, it can be seen that the woven fabric is entirely silver and covered with silicon. When the carbon fiber bundle was observed, the carbon fiber bundle was covered with silvery silicon near the center of the woven fabric, and each fiber was bonded with molten silicon. Many small silver-colored lumps of silicon were also observed on the surface. The edge part of the woven fabric was partly black and some fiber bundles were not wet with molten silicon.

  In other words, it has been clarified that PAN-based carbon fibers have good wettability with molten silicon but have lower reactivity between silicon and carbon than phenolic resin-based carbon fibers.

[Comparative Example 2]
A plain woven fabric of pitch-based carbon fiber yarn (“Granock cloth 5HS-50A-140” manufactured by Nippon Graphite Fiber Co., Ltd. (woven fabric basis weight: 141.1 g / m ² )) was prepared as a carbonized porous structure. Using this carbonized porous structure, a porous structure was produced in the same manner as in Comparative Example 1.

  When the obtained porous structure was weighed, the Si / C atomic ratio was 1.12. Moreover, although the obtained porous structure has the same shape as the carbon fiber woven fabric used, the silicon melted on the woven fabric is gathered as a lump, so that the silicon does not get wet and most of the carbon structure is carbon. The fiber remained, and no reaction between silicon and carbon fiber was observed.

  Since the silicon carbide porous structure of the present invention contains almost no compound other than silicon carbide, it has excellent heat resistance and is useful as an ultralight porous material such as a filter and a heater.

Claims (4)

  It has a porous structure having a large number of pores, and at least the surface of the pore wall partitioning the pores is silicon carbide derived from phenol resin in which phenol resin is carbonized and then converted into silicon carbide. A silicon carbide porous structure. A method for producing a silicon carbide porous structure according to claim 1,
Silicon carbide by heating a carbonized porous structure formed from phenol resin-derived carbon fibers and silicon at a temperature of 1300 ° C to 1800 ° C in a vacuum or in an inert atmosphere, and then reacting by melting and impregnating silicon. A method for producing a silicon carbide porous structure, characterized in that A method for producing a silicon carbide porous structure according to claim 1,
A carbonization step in which a porous structure formed from a phenol resin fiber is heated at 900 ° C. to 1300 ° C. in a vacuum or under an inert atmosphere and carbonized to form a carbonized porous structure;
The carbonized porous structure and silicon are heated to a temperature of 1300 ° C. to 1800 ° C. in a vacuum or in an inert atmosphere, and silicon carbide is melted and impregnated to react to form silicon carbide. And a siliconization step of forming a silicon carbide porous structure.   The method for producing a siliconized porous structure according to claim 2 or 3, wherein a ratio of the carbonized porous structure to the silicon is 0.2 or more in terms of atomic ratio (Si / C).