Patent Application
20080150200
1. Field of the Invention
The present invention relates to a jig for firing a silicon carbide based material, and a method for manufacturing a porous silicon carbide body.
2. Discussion of the Background
Recently, particulates contained in exhaust gases that are discharged from internal combustion engines of vehicles, such as buses and trucks, and construction machines and the like have raised serious problems as contaminants harmful to the environment and the human body.
There have been proposed various ceramic filters capable of capturing particulates in exhaust gases by allowing the exhaust gases to pass through porous ceramics to purify the exhaust gases.
In the conventional manufacture of the porous silicon carbide body of this kind, firstly a silicon carbide powder, a binder and a dispersion medium are mixed to prepare a mixed composition for manufacturing a molded body, and then extrusion-molding and the like is carried out on the mixed composition to manufacture a silicon carbide based molded body.
Next, the obtained silicon carbide based molded body is dried by using a heater and the like, so as to manufacture a dried body of the silicon carbide molded body, which has a certain strength and is easy to deal with.
After the drying process, a degreasing process is carried out by heating the silicon carbide based molded body at a temperature of 300 to 650° C. under an oxygen-containing atmosphere so as to volatilize a solvent, and also to decompose and eliminate a resin component in the components of an organic binder, and further a firing process is carried out by heating the silicon carbide powder at a temperature of 2000 to 2200° C. under an inert gas atmosphere for sintering, thereby manufacturing a porous silicon carbide body.
According to the conventional firing process of a silicon carbide based molded body, first, a plurality of the silicon carbide based molded bodies 32 that have been subjected to the degreasing process are placed in a box-shaped jig 60 with an upper face opened as shown in
In the firing process of a silicon carbide based molded body of this kind, sintering of silicon carbide presumably proceeds as the reaction shown in the following equation (1) proceeds to the right side of the equation (1).
SiO+2C⇄SiC+CO (1)
Here, for the advance of the reaction shown in the reaction equation (1), a firing method carried out by using a firing jig paved with carbon particles, and a firing furnace equipped with an instrument for removing carbon monoxide generated in the firing furnace have been proposed (see, for example, JP-A 2002-226271, JP-A 2002-249385). The contents of JP-A 2002-226271, JP-A 2002-249385 are incorporated herein by reference in their entirety.
A jig for firing a silicon carbide based material according to the present invention is a jig for firing a silicon carbide based material used for placing a silicon carbide based molded body thereon upon firing of the silicon carbide based molded body, wherein a SiO source layer is formed on at least a part of the surface of the jig for firing a silicon carbide based material.
In the jig for firing a silicon carbide based material of the present invention, the thickness of the SiO source layer is desirably about 0.2 mm or more, and the SiO source layer desirably has a thickness of at least about 0.8 mm and at most about 1.6 mm. Moreover, the jig for firing a silicon carbide based material desirably comprises a carbon material.
Also, the SiO source layer is desirably formed by using hydridopolycarbosilane, and the SiO source layer is desirably formed by firing a polymer mainly comprising the hydridopolycarbosilane, and also the SiO source layer is desirably a layer comprising SiC formed by decomposing the hydridopolycarbosilane.
Moreover, the SiO source layer is also desirably formed by using a mixture containing SiC particles and SiO2 particles, and the SiC particles desirably have an average particle diameter of at least about 0.1 μm and at most about 50 μm, and the SiO2 particles desirably have an average particle diameter of at least about 0.1 μm and at most about 200 μm. The SiO source layer is desirably a layer comprising SiC formed by using a mixture including the SiC particles and the SiO2 particles.
Furthermore, the SiO source layer is desirably a layer comprising a recrystallized SiC, and the SiO source layer is desirably a layer comprising a recrystallized SiC formed by firing a material for recrystallization including SiC particles and SiO2 particles under an atmosphere including SiO gas and SiO2 gas.
In the jig for firing a silicon carbide based material according to the present invention, the SiO source layer is desirably a layer comprising a reaction-sintered SiC, and in particular, the SiO source layer is desirably a layer comprising a reaction-sintered SiC formed by firing a mixture including silicon and carbon.
A method for manufacturing a porous silicon carbide body according to the present invention comprises degreasing a pillar-shaped silicon carbide based molded body containing a silicon carbide powder and a binder, and firing the silicon carbide based molded body within a system including a SiO source.
In the method for manufacturing a porous silicon carbide body according to the present invention, the firing process is desirably carried out by placing the silicon carbide based molded body on a jig for firing a silicon carbide based material, and a SiO source layer is desirably formed on at least a part of a surface of the jig for firing a silicon carbide based material.
Moreover, in the method for manufacturing a porous silicon carbide body according to the present invention, the SiO source layer desirably has a thickness of about 0.2 mm or more, and moreover, the SiO source layer desirably has a thickness of at least about 0.8 mm and at most about 1.6 mm. Also, the jig for firing a silicon carbide based material desirably comprises a carbon material.
In the method for manufacturing a porous silicon carbide body according to the present invention, the SiO source layer is desirably formed by using hydridopolycarbosilane, and the SiO source layer is desirably formed by firing a polymer mainly comprising the hydridopolycarbosilane, and moreover, the SiO source layer is desirably a layer comprising SiC formed by decomposing the hydridopolycarbosilane.
In the method for manufacturing a porous silicon carbide body according to the present invention, the SiO source layer is desirably formed by using a mixture including SiC particles and SiO2 particles. The SiC particles desirably have an average particle diameter of at least about 0.1 μm and at most about 50 μm, and the SiO2 particles desirably have an average particle diameter of at least about 0.1 μm and at most about 200 μm, and also, the SiO source layer is desirably a layer comprising SiC formed by using a mixture including the SiC particles and the SiO2 particles.
Moreover, in the method for manufacturing a porous silicon carbide body according to the present invention, the SiO source layer is desirably a layer comprising a recrystallized SiC, and the SiO source layer is desirably a layer comprising a recrystallized SiC formed by firing a material for recrystallization including SiC particles and SiO2 particles under an atmosphere including SiO gas and SiO2 gas.
Furthermore, in the method for manufacturing a porous silicon carbide body according to the present invention, the SiO source layer is desirably a layer comprising a reaction-sintered SiC, and in particular, the SiO source layer is desirably a layer comprising a reaction-sintered SiC formed by firing a mixture including silicon and carbon.
The jig for firing a silicon carbide based material according to one embodiment of the present invention is a jig for firing a silicon carbide based material, which is used for placing a silicon carbide based molded body thereon upon firing of the silicon carbide based molded body, wherein a SiO source layer is formed on at least a part of the surface of the jig for firing a silicon carbide based material.
In the jig for firing a silicon carbide based material according to the embodiment of the present invention, a SiO source layer is formed on at least a part of the surface of the jig for firing a silicon carbide based material, and therefore it may become possible to steadily supply SiO into the firing system upon firing of a silicon carbide based molded body. Accordingly, by using the jig for firing a silicon carbide based material according to the embodiment of the present invention, it may become easier to allow sintering of silicon carbide based molded body to progress steadily.
The method for manufacturing a porous silicon carbide body according to one embodiment of the present invention comprises: degreasing a pillar-shaped silicon carbide based molded body containing a silicon carbide powder and a binder; and firing the silicon carbide based molded body within a system including a SiO source.
By using the method for manufacturing a porous silicon carbide body according to the embodiment of the present invention, a silicon carbide based molded body tends to be certainly sintered, and as a result, a porous silicon carbide body having an almost uniform bending strength tends to be obtained.
First, the following will discuss the jig for firing a silicon carbide based material according to the embodiment of the present invention.
In the jig for firing a silicon carbide based material according to the embodiment of the present invention, a SiO source layer is formed on a part or all of the surface of the jig for firing a silicon carbide based material.
The desirable lower limit of the thickness of the SiO source layer is about 0.2 mm. The thickness of about 0.2 mm or more tends not to cause an insufficient supply of SiO upon manufacturing of a porous silicon carbide body, and as a result, sintering of silicon carbide tends to steadily proceed. Moreover, a ceramic filter using the porous silicon carbide body of this kind tends not to have a high pressure loss or a low bending strength.
The more desirable lower limit of the thickness of the SiO source layer is about 0.8 mm. When the thickness of the SiO source layer is about 0.8 mm or more, it may become possible to certainly manufacture a silicon carbide based fired body having the desired average pore diameter with a low pressure loss and small variation.
On the other hand, the desirable upper limit of the thickness of the SiO source layer is about 1.6 mm. Even if the thickness of the SiO source layer is about 1.6 mm or less, it may become easier to steadily proceed the sintering of a silicon carbide based molded body, and moreover, forming a SiO source layer having a thickness of about 1.6 mm or less tends not to be a complex work and not to require a higher cost as well.
Furthermore, if trying to form a SiO source layer having a thickness of about 1.6 mm or less, warpage tends not to occur in the jig for firing a silicon carbide based material upon forming, and deterioration in the quality of the porous silicon carbide bodies to be manufactured due to warpage occurring in the jig for firing a silicon carbide based material tends not to be caused.
There is no specific limitation on the SiO source layer as long as the SiO source layer is capable of supplying SiO during firing of a silicon carbide based molded body, and examples thereof include a layer formed by using a hydridopolycarbosilane such as allylhydridopolycarbosilane, a layer formed by using a mixture containing SiC particles and SiO2 particles, a layer comprising a recrystallized SiC, a layer comprising a reaction-sintered SiC, and the like.
With respect to a method for forming the SiO source layer, in case where the SiO source layer is a layer formed by using the hydridopolycarbosilane and the like, examples of the method include, a method of applying a polymer consisting mainly of the hydridopolycarbosilane and the like onto the region for forming the SiO source layer in the jig for firing a silicon carbide based material, and then carrying out a drying treatment and a firing treatment, and the like.
Examples of the method for applying the polymer include spray coating, wash coating, brush application, drop application, printing and the like.
With respect to a method for forming the SiO source layer, in case where the SiO source layer is a layer formed by using a mixture containing the SiC particles and the SiO2 particles, examples of the method include a method of applying or placing the mixture containing SiC particles and SiO2 particles on the region for forming the SiO source layer in the jig for firing a silicon carbide based material, and then, carrying out a drying treatment and a firing treatment; a method of coating the region for forming the SiO source layer in the jig for firing a silicon carbide based material with the mixture by using a coating method such as chemical vapor deposition, physical vapor deposition, molten-salt method, nitrogen diffusion method, spraying; and the like.
With regard to the method for applying the mixture, the same methods as those for applying the polymer consisting mainly of hydridopolycarbosilane and the like may be used.
In the mixture containing the SiC particles and the SiO2 particles, the average particle diameter of the SiC particles is desirably at least about 0.1 μm and at most about 50 μm, and more desirably at least about 0.1 μm and at least about 1.0 μm. Moreover, the SiC particles may comprise α-type SiC or β-type SiC, or both of α-type SiC and β-type SiC.
Furthermore, in the mixture containing the SiC particles and the SiO2 particles, the average particle diameter of the SiO2 particle is desirably about 0.1 μm for the lower limit and about 200 μm for the upper limit, and more desirably about 10 μm for the lower limit and about 150 μm for the upper limit. Also, the shape of the SiO2 particles is not particularly limited, and may be a sphere shape or a crushed shape.
When the mixture containing the SiC particles and the SiO2 particles is applied or placed, the mixture may include an organic solvent, if necessary. With this arrangement, application or placement of the mixture can be easily carried out.
Examples of the organic solvent include methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, polyethylene glycol, benzene, an alcohol such as methanol, and the like.
With respect to a method for forming the SiO source layer, in case where the SiO source layer is a layer comprising a recrystallized SiC, an example of the method includes a method of performing a firing treatment, with the material for recrystallization including SiC particles and SiO2 particles being placed on the jig for firing a silicon carbide based material; and a firing apparatus (a firing furnace may also be used) being under an atmosphere including SiO gas or CO gas, so that a layer comprising a recrystallized SiC is formed on the surface of the jig for firing a silicon carbide based material.
The material for recrystallization including the SiC particles and SiO2 particles may be a powder or an agglomerate of the wet mixture, or may be a molded body having an arbitrary shape including a pillar-shaped molded body (honeycomb molded body) in which a number of cells are longitudinally placed in parallel to each other with a cell wall therebetween.
Here, the material for recrystallization including the SiC particles and the SiO2 particles desirably contains an organic binder, and in this case, the content of the organic binder is desirably at least about 1% by weight and at most about 10% by weight of the total amount of the SiC particles and the SiO2 particles. Also, water may be added to the material for crystallization including the SiC particles and the SiO2 particles, if appropriate.
In the material for recrystallization including the SiC particles and the SiO2 particles, the average particle diameter of the SiC particles is desirably at least about 0.1 μm and at most about 50 μm, and more desirably at least about 0.1 μm and at most about 1.0 μm. Also, the SiC particles may be α-type SiC or β-type SiC, or may comprise both of α-type SiC and β-type sic.
Moreover, in the material for recrystallization including the SiC particles and the SiO2 particles, the average particle diameter of the SiO2 particles is desirably about 0.1 μm for the lower limit and about 200 μm for the upper limit, and more desirably about 10 μm for the lower limit and about 150 μm for the upper limit. Moreover, the shape of the SiO2 particles is not particularly limited, and may be a sphere shape or a crushed shape.
When a layer comprising the recrystallized SiC is formed as the SiO source layer, a material for crystallization including SiC particles on which a SiO2 film is formed at the surface is used in place of the material for recrystallization including the SiC particles and the SiO2 particles, and except for above, the same method as those mentioned above may be used, so that the layer comprising a recrystallized SiC may be formed on the surface of the jig for firing a silicon carbide based material.
Here, the material for crystallization including the SiC particles with the surface on which a SiO2 film is formed, may be a powder or an agglomerate of the wet mixture, or may be a molded body having an arbitrary shape (including a honeycomb molded body).
The material for crystallization including the SiC particles with the surface on which a SiO2 film is formed, also desirably contains an organic binder, and the content of the organic binder is desirably at least about 1% by weight and at most about 10% by weight of the amount of the SiC particles with the surface on which a SiO2 film is formed. Furthermore, water may be added to the material, if appropriate.
In the material for crystallization including the SiC particles with the surface on which a SiO2 film is formed, the average particle diameter of the SiC particles is desirably at least about 0.1 μm and at most about 50 μm, and more desirably at least about 0.1 μm and at most about 1.0 μm. The SiC particle may be α-type SiC or β-type SiC, or may comprise both of α-type SiC and β-type SiC, though α-type SiC is desirable.
When a layer comprising a recrystallized SiC is formed by a firing treatment using the material for recrystallization, the firing treatment may be carried out at a temperature of at least about 1400° C. and at most about 2300° C. In addition, the material for recrystallization may be subjected to a drying treatment or a degreasing treatment (at a temperature of at least about 200° C. and at most about 500° C.) prior to the firing treatment.
When the material for recrystallization is placed on the jig for firing a silicon carbide based material, the amount of the material for crystallization and the location for placing the material for recrystallization are not particularly limited.
Also, if the material for recrystallization used here is a material for recrystallization having the same shape as that of the pillar-shaped silicon carbide based molded body in the method for manufacturing a porous silicon carbide body to be described below, it is advantageous in that, without any changes in the manufacturing line used in the method for manufacturing a porous silicon carbide body described below, it may become easier for this manufacturing line to be used for the manufacture of the jig for firing a silicon carbide based material by only changing the starting material.
Furthermore, if a extrusion-molding machine for manufacturing the silicon carbide based molded body and a extrusion-molding machine for manufacturing a material for recrystallization having the same shape as that of the silicon carbide based molded body are installed together, and by sharing other manufacturing line except for these machines, it may also become possible to manufacture the jig for firing a silicon carbide molded body efficiently.
With regard to the method for forming the SiO source layer, in case where the SiO source layer comprises a reaction-sintered SiC, examples of the method include a method of applying or placing a mixture containing Si (silicon) and C (carbon) onto the region for forming the SiO source layer in the jig for firing a silicon carbide based material, and then performing a firing treatment at a temperature of about 1800° C., for example, so that a layer comprising a reaction-sintered SiC is formed, and the like.
Furthermore, in case of the application or the placement of the mixture containing SiC and C, the mixture may contain an organic solvent, if necessary. With this arrangement, the application or the placement of the mixture may be more easily carried out. As for the specific examples of the organic solvent, the same organic solvents as those allowed to be included in the mixture containing the SiC particles and the SiO2 particles may be exemplified.
In the methods for forming the SiO source layer described in the above, it may become possible to adjust the thickness of the SiO source layer by repeating the method as described above for a predetermined number of times.
Also, when a firing treatment is carried out in the respective forming methods as mentioned above, it may also become possible to adjust the thickness of the SiO source layer by repeating only the firing treatment for a predetermined number of times, or by adjusting the time period for the firing treatment.
By using those methods, a SiO source layer tends to be formed on the jig for firing a silicon carbide based material.
To be more specific, in the case where hydridopolycarbosilane is used, presumably, the reaction shown in the following reaction equation (2) proceeds, and thus accordingly, a layer comprising SiC, which functions as a SiO source layer, is formed on the jig for firing a silicon carbide based material. In this method, the thickness of the jig for firing a silicon carbide based material is presumably increased by the thickness of the formed SiO source layer.
When the mixture containing the SiC particles and the SiO2 particles is used, presumably, the reactions shown in the reaction equations (3) and (4) shown below proceed to the right sides of the equations (3) and (4), and thus accordingly a layer comprising SiC, which functions as a SiC source layer is formed on the jig for firing a silicon carbide based material. In this method, since the SiO source layer is formed by the reaction with carbon constituting the jig for firing a silicon carbide based material, presumably, there is almost no change in the thickness of the jig for firing a silicon carbide based material.
2SiO2+SiC⇄3SiO+CO (3)
SiO+2C⇄SiC+CO (4)
Moreover, when the SiC material for recrystallization is used, SiC derived from the material for recrystallization is adhered as a recrystallized SiC to the surface of the jig for firing a silicon carbide based material during the firing treatment, so that it may become possible to form the SiO source layer comprising a recrystallized SiC.
Furthermore, when the mixture containing Si and C is used, a SiC layer is formed on the jig for firing a silicon carbide based material by reaction-sintering, so that it may become possible to form the SiO source layer comprising a reaction-sintered SiC.
The reason why those layers comprising SiC functions as a SiO source layer can be presumably attributed to their capacity to supply SiO in the firing furnace when the reaction shown in the following reaction equation (5) proceeds to the right side of the equation (5) upon firing a silicon carbide based molded body.
SiC+CO⇄SiO+2C (5)
Although the mechanism in which the reaction shown in the reaction equation (5) proceeds is not clear, the mechanism is presumably described as follows.
First, at the stage where the temperature inside the firing furnace is relatively low (at least about 1200° C. and at most about 1400° C.), the reaction shown in the following reaction equation (6) proceeds to the right side of the equation (6), so that SiO and CO are supplied in the firing furnace.
SiO2+C⇄SiO+CO (6)
Here, SiO2 in the reaction equation (6) is supplied from SiO2 included in the silicon carbide material as an impurity, and C is supplied from organic components and the like included in the silicon carbide based molded body. CO thus generated presumably reacts with the layer comprising SiC functioning as the SiO source layer, along with the increase of the temperature in the firing furnace, as shown in the reaction equation (5). As a result, even in the case where a SiO concentration is low at first, as the reaction shown in the reaction equation (5) proceeds to the right side of the reaction equation (5), SiO is supplied in the system. Thus, SiO (and C) required to proceed the reaction shown in the reaction equation (1) to the right side of the reaction equation (1) is (are) presumably generated.
Here, SiC as a sintered body is not generated until it rises to a temperature where the sintering of SiC shown in the reaction equation (1) proceeds, therefore, CO generated in the reaction shown in the reaction equation (6) presumably reacts with the layer comprising SiC functioning as the SiO source layer. Further, in the case where the SiO concentration is low in sintering of SiC, the reaction shown in the reaction equation (1), which inhibits the formation of SiC as the sintered body, proceeds to the left side of the reaction equation (1); however, SiO is supplied due to the reaction shown in the reaction equation (5) by the time of the sintering of SiC, therefore the reaction inhibiting the formation of SiC is presumably suppressed.
Furthermore, even in the case where the temperature in the firing furnace rises so that the sintering of SiC proceeds, a surface area of SiC included in a layer comprising SiC existing on the surface of the jig for firing a silicon carbide based material is larger than that of SiC as a sintered body of the silicon carbide based material, therefore, the reaction shown in the reaction equation (5) presumably tends to occur at the layer comprising SiC. Consequently, in the jig for firing the silicon carbide based material of the present invention, the reaction inhibiting the sintering of SiC in the silicon carbide based molded body presumably tends not to occur.
As for the material constituting the jig for firing a silicon carbide based material, for example, a carbon material and the like may be exemplified. This is because such materials become a carbon source in the reaction shown in the above reaction equation (1), so that it may become possible for sintering of a silicon carbide based molded body to proceed steadily, and also because they are suitable for the formation of the SiO source layer based on the reactions shown in the reaction equations (3) and (4).
The carbon material may be, for example, a porous carbon having pores, dense material and the like.
The shape of the jig for firing a silicon carbide based material is usually a box shape with an upper face opened, as shown in
Furthermore, a notched portion or a through hole may be formed in a part of the jig for firing a silicon carbide based material.
If a through hole or a notched portion as mentioned above is formed, upon piling up the jigs for firing a silicon carbide based material in a plurality of stages to carry out firing of the silicon carbide based molded body, an ambient gas passes through inside the jig for firing a silicon carbide based material, and thus the temperature of the atmosphere surrounding the silicon carbide based molded body placed inside the jig for firing a silicon carbide based material tends to be made almost uniform, regardless of the location of the jig or the location of the silicon carbide based molded body placed inside the jig, and also the concentration of the components such as SiC, SiO and Si in the atmosphere surrounding the silicon carbide based molded body tends to be made uniform, and as a result, it may become easier to fire each of the silicon carbide based molded bodies under uniform conditions.
A carbon powder may be held on the jig for firing a silicon carbide based material.
When the carbon powder is held on the jig for firing a silicon carbide based material, it may become easier to enjoy the following effects.
In the firing process of a silicon carbide based molded body, sintering of silicon carbide proceeds based on the reaction shown in the above reaction equation (1).
Here, as mentioned above, the SiO has its source of supply in impurities in the silicon carbide based molded body or the SiO source layer formed on the jig for firing a silicon carbide based material.
On the other hand, the C (carbon) source in the above reaction equation (1) has its source of supply in carbon (organic component) existing in the silicon carbide based molded body, or carbon constituting the jig for firing a silicon carbide based material.
However, the amount of carbon existing in the silicon carbide based molded body that has been subjected to a degreasing process is so little that the carbon are soon consumed in the reaction shown in the reaction equation (1). Moreover, carbon constituting the jig for firing a silicon carbide based material and the like can be a good source of carbon supply in the reaction shown in the reaction equation (1), however, when a SiO source layer is formed on the surface of the jig, supply of carbon may become difficult in some cases.
As a result, along with the progress of the firing process, the amount of carbon supplied in the reaction shown in the reaction equation (1) is decreased, while in contrast, the concentration of SiO gas is increased, and as a result, between the high concentration of SiO gas and the SiC mainly constituting the silicon carbide based molded body, the reaction shown in the below-mentioned reaction equation (7) presumably proceeds to the right side of the equation (7).
SiO+SiC⇄Si+CO (7)
Furthermore, Si comes to exist in the silicon carbide based molded body during firing, and thus sintering of the silicon carbide based molded body of this kind does not proceed smoothly and, although the silicon carbide particles themselves undergo grain growth, the bond, or what is called necking, among the silicon carbide particles that have undergone grain growth is hardly formed, and as a result, there tends to be variation in the strength of the porous silicon carbide bodies to be manufactured, and thus the strength tends to be deteriorated.
In contrast, when a carbon powder is held on the jig for firing a silicon carbide based material as mentioned above, it may become possible to steadily supply carbon in the reaction shown in the above reaction equation (1), sintering of the silicon carbide based molded body tends to proceed more steadily. Moreover, in the case where a platform member comprising carbon is placed under the silicon carbide based molded body in the firing, carbon is also generated from the platform member, so that, it may become easier to proceed the sintering of the silicon carbide based molded body.
In the jig for firing a silicon carbide based material according to the embodiment of the present invention, a SiO source layer is formed on at least a part of the surface of the jig for firing a silicon carbide based material, and therefore it may become easier to steadily supply SiO into the firing system upon firing of a silicon carbide based molded body. Accordingly, by using the jig for firing a silicon carbide based material according to the embodiment of the present invention, it may become easier to allow sintering of silicon carbide based molded body to progress steadily.
Furthermore, it may become possible for the porous silicon carbide body to be preferably used as a ceramic filter.
Next, the following will discuss the case where a porous silicon carbide body is used as a ceramic filter.
In the ceramic filter 40 as shown in
In other words, as shown in
Those porous silicon carbide bodies 50 of this kind are excellent in heat resistance, and a regenerating treatment thereof and the like are easily carried out, therefore they are used in various large vehicles, vehicles equipped with diesel engine, and the like.
The following description will discuss the method for manufacturing a porous silicon carbide body according to the embodiment of the present invention.
According to the method for manufacturing a porous silicon carbide body according to the embodiment of the present invention, first, a pillar-shaped silicon carbide based molded body comprising a silicon carbide powder and a binder is manufactured. In the present invention, a silicon carbide based molded body refers to a silicon carbide based sintered body containing about 60% by weight or more of silicon carbide, which is obtained after completing a degreasing treatment and a firing treatment, and the silicon carbide based sintered body desirably contains about 96% by weight or more of silicon carbide.
The structure of the silicon carbide based molded body is not particularly limited, and examples thereof include those having a pillar-shaped body in which a number of cells are longitudinally placed in parallel with each other with a cell wall therebetween as mentioned in the background art, and those having a pillar-shaped body with a number of intercommunicating pores inside, and the like. The shape is not particularly limited, and may be, for example, a cylindrical shape, a cylindroid shape, a rectangular pillar shape and the like.
In the following description of one example of the method for manufacturing a porous silicon carbide body according to the embodiment of the present invention, those having a pillar shape in which a number of cells are longitudinally placed in parallel with each other with a cell wall therebetween are used as a silicon carbide based molded body.
Although the particle diameter of the silicon carbide powder is not particularly limited, one that will not undergo shrinkage during the subsequent firing process is preferable. For example, a combination of 100 parts by weight of a powder having an average particle diameter of at least about 0.3 μm and at most about 50 μm, and at least about 5 parts by weight and at most about 65 parts by weight of a powder having an average particle diameter of at least about 0.1 μm and at most about 1.0 μm is preferable for use therein.
The binder is not particularly limited, and examples thereof include methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, polyethylene glycol, and the like.
The preferable blending amount of the binder is normally at least about 1 parts by weight and at most about 10 parts by weight relative to 100 parts by weight of the silicon carbide powder.
The dispersion medium is not particular limited, and examples thereof include alcohol such as methanol, an organic solvent such as benzene, water, and the like.
The dispersion medium is blended in an appropriate amount so that the viscosity of the mixed composition is set in a certain range.
Those silicon carbide powder, binder and dispersion medium are mixed by an attritor and the like, and sufficiently kneaded by a kneader and the like, and then extrusion molded and dried to manufacture a pillar-shaped silicon carbide based molded body containing a silicon carbide powder and a binder.
Here, in the method for manufacturing a porous silicon carbide body according to the present invention, the amount of SiO2 contained as an impurity in the silicon carbide based molded body is not particularly limited, however, if a silicon carbide based molded body in which the amount of the SiO2 is as low as about 0.03% by weight or less is used in particular, sintering tends to be carried out steadily in the subsequent firing process.
After this, degreasing of the silicon carbide based molded body manufactured by the above processes is performed.
In the degreasing process of the silicon carbide based molded body, normally, the silicon carbide based molded body is placed in the jig for firing a silicon carbide based material, and then transported into a degreasing furnace to be heated at a temperature of at least about 30° C. and at most about 650° C. under an oxygen-containing atmosphere.
As a result, the binder and the like are volatilized, and also decomposed and eliminated so that almost only the silicon carbide powder remains.
Upon placing the silicon carbide based molded body in the jig for firing a silicon carbide based material, in order to support the silicon carbide based molded body in a manner to leave a space with the bottom face, the platform members (spacers) 35 may be put on the bottom face of the jig for firing a silicon carbide based material as shown in
Moreover, the platform member (spacer) may be integrally formed in the jig for firing a silicon carbide based material of the present invention. By placing the platform member (spacer), it may become easier to prevent the generation of cracking and the like caused by adherence of the degreased body or sintered body of the silicon carbide based molded body to the bottom face of the jig for firing a silicon carbide based material.
In the firing process, as shown in
Specifically, for example, a method of continuous firing that comprises placing the piled-up body on the supporting table 37, and heating the piled-up body by heaters 31 provided on the upside and the downside of a muffle 34, while allowing the piled-up body to move through the muffle 34, and the like may be used.
This firing process may also be carried out by heating the degreased silicon carbide based molded bodies 32 at a temperature of at least about 1400° C. and at most about 2200° C. under the atmosphere of an inert gas such as nitrogen, argon and the like.
Here, on the supporting table 37, one set of the jigs for firing a silicon carbide based material piled up in a plurality of stages may be placed, or two sets thereof may be placed as shown in
The firing furnace used in this firing process may be a batch-type firing furnace, however, a continuous-type firing furnace is desirable. Because, using a continuous-type firing furnace makes it easier to stabilize the concentrations of SiO gas and CO gas in the furnace at a desired concentration, which is suitable for steadily supplying SiO from the SiC source layer.
In the method for manufacturing a porous silicon carbide body according to the embodiment of the present invention, the firing process is performed within a system including a SiC source.
Specifically, firing is desirably carried out by using the jig for firing a silicon carbide based material according to the embodiment of the present invention, because this tends to allow the steady progression of sintering of the silicon carbide based molded body.
Since a degreased silicon carbide based molded body has a low mechanical strength and is easily broken, it is desirable that the jig for firing a silicon carbide based firing body 10 is allowed to function as a degreasing jig, and after a degreasing process, the jigs for firing a silicon carbide based material 10 also functioning as a degreasing jig are piled up in a plurality of stages and then firing is carried out.
The platform member (spacer) 35 placed on the jig for firing a silicon carbide based material 10 is required to have a heat resistance so as to bear high temperature during firing, and thus the material is desirably those having a heat resistance of this level.
The material of the platform member is desirably those having a relatively high thermal conductivity, and examples thereof include carbon, silicon carbide, aluminum nitride, silicon nitride and the like. Also, carbon cloth is desirably used from the viewpoint of avoiding damage to the porous silicon carbide body.
As mentioned above, in a series of processes from a degreasing process to a firing process, desirably a silicon carbide based molded body is placed on a jig for firing a silicon carbide based material by interposing a platform member (spacer), and then directly subjected to the degreasing process and a firing process. This is because it may become easier to efficiently perform the degreasing process and the firing process, and also it may become easier to prevent the silicon carbide based molded body from being damaged in transfer of the placing and the like.
By using the method for manufacturing a porous silicon carbide body according to the embodiment of the present invention, it may become easier to steadily sinter a silicon carbide based molded body, and as a result, it may become easier to obtain a porous silicon carbide body having an almost uniform bending strength.
Here, with respect to the reaction equation (1), SiO in the reaction equation (1) has its source of supply in SiO2, which is contained as an impurity in the silicon carbide material and reacts with carbon in the firing process to become SiO. For this reason, when the amount of SiO2 contained in the material is high, by concomitantly applying the firing method using the firing jig paved with carbon particles, and the like, it may become possible for sintering of silicon carbide based on the reaction shown in the reaction equation (1) to proceed steadily.
However, when the amount of SiO2 contained as an impurity in the silicon carbide material is low, the supplying amount of SiO becomes low, and therefore sintering based on the reaction equation (1) hardly proceeds even if the firing method using a firing jig paved with carbon particles and the like is used. As a result, the resulting silicon carbide based sintered body tends to have some problems in the quality as follows: pressure loss tends to be high; and bending strength tends to be weak.
In the method for manufacturing a porous silicon carbide body according to the embodiments of the present invention, as a jig for firing a silicon carbide based material, which may make it possible to steadily supply SiO into the firing system, is used during firing of a silicon carbide based molded body, it may become easier to manufacture a porous silicon carbide body which has been certainly sintered.
Application of the porous silicon carbide body thus obtained is not particular limited, and it may be used in a variety of applications. For example, it may be used as a member constituting a catalyst supporting body, a member constituting a ceramic filter, and the like.
Here, in the case where the obtained porous silicon carbide body is used as a ceramic filter, with regard to the pore diameter of the obtained porous silicon carbide body, the lower limit value is desirably about 1 μm, and more desirably about 5 μm, while the upper limit value is desirably about 100 μm, and more desirably about 50 μm. With regard to the porosity, the desirable lower limit value is about 20%, and the desirable upper limit value is about 80%. In the case where the pore diameter and the porosity are in the above-mentioned range, it may become easier to suitably use the porous silicon carbide body obtained by the method for manufacturing the porous silicon carbide body according to the embodiments of the present invention as a ceramic filter.
The following description will discuss the present invention in detail by means of examples; however, the present invention is not intended to be limited by these examples.
The following method was carried out to manufacture a jig for firing a silicon carbide based material on which a SiO source layer using hydridopolycarbosilane was formed.
On the bottom face, that is, on the right face side (the side on which a silicon carbide based molded body is placed), of a previously obtained box-shape jig made of carbon (DSG-332, manufactured by SEC Corp.) with an upper portion opened, a polymer for forming a SiO source layer containing allylhydridopolycarbosilane as a main component (SP-MATRIX Polymer, manufactured by Starfire-Systems Inc.) was applied, and the resulting product was subjected to a process comprising drying at 100° C. for 12 hours followed by firing at 2200° C. for 2.5 hours, repeating the process at the number of the times indicated in Table 1, so that a jig for firing a silicon carbide based material in which a SiO source layer having a thickness of 0.10 to 1.65 mm was formed on the bottom face (right face side) was manufactured.
The thickness of the SiO source layer comprising hydridopolycarbosilane was measured by an electric conductive film thickness measuring instrument.
In this case, the layer comprising SiC functioning as a SiO source layer is formed on the jig for firing a silicon carbide based material according to the reaction equation (2).
The following method was carried out to manufacture a jig for firing a silicon carbide based material on which a SiO source layer using a mixture containing SiC particles and SiO2 particles was formed.
First, a mixture was previously prepared by mixing α-type SiC particles (manufactured by YAKUSHIMA DENKO CO., LTD.) having an average particle diameter of 0.5 μm and SiO2 powders (CS-8, manufactured by Yamakawa Sangyo Co., Ltd.) having an average particle diameter of 140 μm at a weight ratio of 1:2. Next, on the bottom face (right face side) of a previously obtained box-shape jig made of carbon (DSG-332, manufactured by SEC Corp.) with an upper portion opened, 200 g of the previously prepared mixture was applied, and the resulting product was subjected to a firing process at a temperature of 2200° C. for 1.5 hours, repeating the firing process at the times indicated in Table 1, so that a jig for firing a silicon carbide based material in which a SiO source layer comprising a recrystallized SiC having a thickness of 0.07 to 1.68 mm was formed on the bottom face (right face side) was manufactured.
The thickness of the SiO source layer was measured by an electric conductive film thickness measuring instrument.
In this case, the layer comprising a recrystallized SiC functioning as a SiO source layer is formed on the jig for firing a silicon carbide based material according to the reaction equations (2) and (3).
(1) 60% by weight of α-type SiC particles having an average particle diameter of 10 μm, with the surface on which a SiO2 film was formed (SiO2 content: 1% by weight), and 40% by weight of α-type SiC particles having an average particle diameter of 0.5 μm, with the surface on which a SiO2 film was formed (SiO2 content: 4% by weight) were wet-mixed, and to 100 parts by weight of the resulting mixture, 5 parts by weight of an organic binder (methyl cellulose) and 10 parts by weight of water were added and then kneaded to obtain a kneaded product. Next, a small amount of a plasticizer and a lubricant were added to the kneaded product, and further kneaded. Extrusion molding was carried out thereafter to manufacture a silicon carbide based molded body. In the present Examples, this silicon carbide based material was to be used as a material for a recrystallized SiC containing the SiC particles with the surface on which a SiO2 film is formed.
(2) Next, the above-mentioned silicon carbide based molded body was first dried at 100° C. for 3 minutes by using a microwave drier, and further dried at 110° C. for 20 minutes by using a hot-air drier.
(3) After this, ten pieces of the dried silicon carbide based molded bodies were placed on a jig for firing a silicon carbide based material by interposing a platform member 10 made of carbon. Those jigs for firing a silicon carbide molded bodies were piled up in five stages and a plate-shaped lid was placed on the top portion. The two rows of those piled-up bodies were placed on the supporting table.
(4) Next, the jigs in which silicon carbide based molded bodies were placed were transported into a continuous-type degreasing furnace and subjected to a degreasing process by heating at a temperature of 300° C. under an atmosphere of a mixed gas containing 8% oxygen of an air and nitrogen, so that a silicon carbide degreased bodies were manufactured.
The jigs in which the silicon carbide degreased bodies were still placed were transported into a firing apparatus, and subjected to firing treatment at a temperature of 2200° C. for about 3 hours under an argon atmosphere at a normal pressure, repeating the firing treatment at the number of the times indicated in Table 1, so that a SiO source layer comprising the recrystallized SiC having a thickness of 0.08 to 1.74 mm was formed on the bottom face (right face side) of the jig for firing a silicon carbide based material.
The thickness of the SiO source layer was measured by an electric conductive film thickness measuring instrument.
In this case, the layer comprising a recrystallized SiC functioning as a SiO source layer is formed on the jig for firing a silicon carbide based material according to the reaction equation (2).
A jig for firing a silicon carbide based material comprising carbon (DSG-332, manufactured by SEC Corp.) in which a SiO source layer was not formed was prepared.
By using the jigs for firing a silicon carbide based material manufactured in the Examples and the Comparative Example, porous silicon carbide bodies were manufactured by the below-mentioned method, and the characteristics of these porous silicon carbide bodies were evaluated.
(1) 60% by weight of an α-type silicon carbide powder having an average particle diameter of 10 μm and 40% by weight of an α-type silicon carbide powder having an average particle diameter of 0.5 μm were wet-mixed, and to 100 parts by weight of the resulting mixture 5 parts by weight of an organic binder (methyl cellulose) and 10 parts by weight of water were added, and then kneaded to obtain a kneaded product. Next, a small amount of a plasticizer and a lubricant were added to the kneaded product, followed by kneading further, and then extrusion-molded to manufacture a silicon carbide based molded body.
Here, the amount of SiO2 contained as an impurity in the silicon carbide based molded body was 0.03% by weight.
(2) Next, the silicon carbide based molded body was dried by using a microwave drier at a temperature of 100° C. for 3 minutes, and further dried by using a hot-air drier at a temperature of 110° C. for 20 minutes. After that, the dried silicon carbide based molded body was cut, and sealed with a plug paste comprising silicon carbide at the end portion of the cell.
(3) Next, into the jig for firing a silicon carbide based material of the Examples 1 to 29 or the Comparative Example 1, ten pieces of the dried silicon carbide based molded bodies were placed by interposing platform members made of carbon. Those jigs for firing a silicon carbide based material were piled up in five stages, and a plate-shaped lid was placed on the top portion. After that, those two rows of the piled-up bodies were placed on the supporting table.
(4) Next, the above-mentioned jigs in which the silicon carbide molded bodies were still placed were transported into a continuous-type degreasing furnace and subjected to a degreasing process by heating at a temperature of 300° C. under an atmosphere of a mixed gas containing 8% oxygen of an air and nitrogen, so that silicon carbide degreased bodies were manufactured.
The jigs which still kept the silicon carbide degreased bodies placed therein were transported into a firing furnace, and they were subjected to firing treatment at a temperature of 2200° C. for about 3 hours under an argon atmosphere at a normal pressure, so that a quadrangular pillar-shaped porous silicon carbide body was manufactured.
Upon firing the silicon carbide based molded body, first, the reaction shown in the reaction equation (6) proceeds to the right side of the reaction equation (6), so that SiO and CO are generated. SiO2 in the reaction equation (6) has its source in SiO2 and the like included in the silicon carbide based molded body, and C has its source in organic components included in the silicon carbide based molded body, a platform member comprising carbon, a jig for firing a silicon carbide based material and the like. Then, as the generated CO becomes the CO source in the reaction equation (5), the reaction shown in the reaction equation (5) proceeds to the right side of the reaction equation (5), so that SiO is generated. Thus, the sintering reaction of SiC shown in the reaction equation (1) presumably proceeds, with the layer comprising SiC manufactured in Examples functioning as a SiO source layer.
(5) Next, by using a heat-resistance sealing material paste containing 30% by weight of alumina fibers having an average fiber length of 20 μm, 21% by weight of silicon carbide particles having an average particle diameter of 0.6 μm, 15% by weight of silica sol, 5.6% by weight of carboxymethyl cellulose and 28.4% by weight of water, the 16 pieces of the quadrangular pillar-shaped porous silicon carbide bodies were combined to each other (4 pcs.×4 pcs.) in accordance with the above-mentioned method, followed by cutting by using a diamond cutter, a cylindrical-shaped ceramic block with the size of 144 mm in diameter×150 mm in length was manufactured.
After the above process, 23.3% by weight of ceramic fibers made from alumina silicate (shot content: 3%, average fiber length: 100 μm) which served as inorganic fibers, 30.2% by weight of silicon carbide powder having an average particle diameter of 0.3 μm which served as inorganic particles, 7% by weight of silica sol (SiO2 content in the sol: 30% by weight) which served as an inorganic binder, 0.5% by weight of carboxymethyl cellulose which served as an organic binder, and 39% by weight of water were mixed and kneaded to prepare a sealing material paste.
Next, by using the sealing material paste, a sealing material paste layer having a thickness of 1.0 mm was formed on the peripheral portion of the ceramic block. This sealing material paste layer was dried at 120° C. so that a cylindrical-shaped ceramic filter was manufactured.
The ceramic filter manufactured through the above-mentioned processes was evaluated according to the following evaluation test.
(1) Average pore diameter In compliance with JIS R 1655, using a porosimeter (AutoPore III 9405, manufactured by Shimadzu Corp.) to be used in a mercury injection method, ten pieces of porous silicon carbide bodies were cut into a 1 cm cube with respect to each of the center portions of the porous silicon carbide bodies to prepare samples. Then, the pore distribution in the samples was measured on pores with a pore diameter in the range of 0.2 to 500 μm. The average fine pore diameter of each samples here was calculated based on (4V/A). The mean value of the average fine pore diameter of each of the ten samples was determined as average pore diameter. The results are shown in table 1.
The contents of JIS R 1655 are incorporated herein by reference in their entirety.
The initial pressure loss of one of the ceramic filters was measured at a flowing rate of 1000 N m3/hr. The results are shown in Table 1. Here, “N” in the unit means that the data were measured in a standard condition (temperature: 25° C., air pressure: 1 atm)
As shown in Table 1 and
Also, by using a jig for firing a silicon carbide based material having a SiO source layer with a thickness of about 0.2 mm or more, a ceramic filter with a sufficiently low pressure loss tends to be manufactured. It is presumably because sintering of the silicon carbide based molded body proceeded steadily in the firing process. On the other hand, by using a jig for firing a silicon carbide based material having a SiO source layer with the thickness of less than about 0.2 mm (Examples 1 to 3, 20, 21, 25, 26), the pressure loss of the ceramic filter was likely to be a little high. In this relation, an observation was made on the jig for firing a silicon carbide based material having a SiO source layer with the thickness of less than about 0.2 mm, and it was found that the SiO source layer was likely to be formed sparsely, and thus this may be presumed to be the cause.
In the jig for firing a silicon carbide based material with a SiO source layer having a thickness of exceeding about 1.6 mm (Examples 19, 23 and 27), a warpage, though slight, was observed. This is presumably because the thickness of exceeding about 1.6 mm makes it difficult to form the SiO source layer in a uniform thickness, and due to this nonuniformity of the thickness of the SiO source layer, the respective positions supporting the silicon carbide based molded body by interposing the platform members may not be in the same plane, so that the warpage of the silicon carbide based molded body presumably occurs.
Here, the above description discuss the jig for firing a silicon carbide based material in which the SiO source layer is formed by using hydridopolycarbosilane and a recrystallized SiC, it may be presumably possible to obtain the same effects even in the case of using a jig for firing a silicon carbide based material in which the SiO source layer is formed by using the reaction-sintered SiC.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-225341 | Aug 2005 | JP | national |
This application is a continuation application of PCT/JP2006/315421 filed on Aug. 3, 2006, which claims priority of Japanese Patent Application No. 2005-225341 filed on Aug. 3, 2005. The contents of these applications are incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2006/315421 | Aug 2006 | US |
| Child | 11765088 | US |
