JP4478797B2 - Method for producing silicon carbide based porous material - Google Patents

Description

The present invention relates to a method for producing a silicon carbide based porous body having a high specific surface area.

Commercially available activated carbon has a large specific surface area of 1000 to 2000 m ² / g, and is used for various applications such as adsorbents, deodorizers, exhaust gas filters, and catalyst supports for high-temperature chemical reactions. However, activated carbon is inferior in oxidation resistance because it loses pores due to oxidation in nitric acid, and the specific surface area is remarkably reduced, and its use is limited because it burns and disappears at about 600 ° C in air. .
On the other hand, SiC has a high heat-resistant temperature, is excellent in thermal conductivity and electrical conductivity, and has chemical stability. Therefore, SiC is used as various structural materials for high temperatures. In addition, because it can be used in harsh oxidizing atmospheres that were not possible with activated carbon, porous SiC with a large specific surface area is a catalyst carrier, high-temperature gas purification filter, filter for molten metal filtration, microwave absorption heat generation. Used in various fields such as body and breathable insulation. However, since SiC has low sinterability, it has been difficult to produce SiC having a high surface area.

As a method for producing such a SiC porous body, a silicon carbide body is heat-treated at a temperature of 1200 to 1500 ° C. in a non-oxidizing gas atmosphere having an oxygen concentration of 2 to 20 ppm. A manufacturing method is disclosed (for example, refer to Patent Document 1). In the method of Patent Document 1, activated silicon carbide having a high specific surface area can be efficiently produced by subjecting an existing silicon carbide body to a simple heat treatment within a specified range of conditions. In addition, a method for producing activated silicon carbide is disclosed in which silicon carbide is heat-treated in the presence of a catalyst containing a copper compound and / or a potassium compound (see, for example, Patent Document 2). In this Patent Document 2, a part of carbon existing on the surface of silicon carbide is removed by the combustion reaction promoting action of the catalyst, and a large number of fine irregularities are formed on the surface. When silicon is formed. Furthermore, a method for producing SiC ceramics having a high surface area and high strength obtained by adding a metal catalyst to a mixture of carbon and silica to produce porous SiC ceramics and then treating with a mixed acid is disclosed (for example, patents). Reference 3). By the method shown in Patent Document 3, SiC ceramics having high oxidation resistance, high strength and a large surface area can be obtained.
JP 05-306111 A (Claim 1, paragraph [0023]) JP 2003-002625 A (Claim 1, paragraph [0011]) JP 2003-026483 A (Claim 2, paragraph [0006], FIG. 1)

However, the methods of Patent Documents 1 and 2 described above are useful as a post-treatment step because the specific surface area can be increased to some extent by applying to the existing silicon carbide body. Then, it is necessary to strictly control the processing environment at a high processing temperature, and in order to obtain a silicon carbide based porous body having a high specific surface area, a porous body having a certain specific surface area was once manufactured. Later, in order to perform further post-processing, it is necessary to perform a plurality of steps, and the steps are complicated. Furthermore, as shown in FIG. 1, the SiC ceramic obtained by the production method of Patent Document 3 can only be produced having a specific surface area of about 40 m ² / g at the maximum. Not enough to serve.
An object of the present invention is to provide a method for producing a silicon carbide based porous body having a high specific surface area.

As shown in FIG. 1, the invention according to claim 1 is a silicon carbide precursor of polycarbosilane inside the pores of mesoporous silica powder 11 having a mesopore size of several tens of μm and pores of 2 to 50 nm. a step of filling the body solution 12, and removing the solvent component contained in the solution filled by drying the filled powder 13, a packing powder 13 under atmospheric pressure, and held for 1 to 24 hours at 100 to 200 ° C. The step of subjecting the precursor in the pores to infusibilization , the step of firing the filling powder 13 at 1000 to 1200 ° C. for 1 to 2 hours under an inert gas atmosphere, and firing using an etching solution that dissolves silica. by causing than the powder 14 is lost mesoporous silica 11 is formed on the three-dimensional network, and has pores continuous pore structure of the internal network, consists mainly silicon carbide, said silicon Not only carbide include both compounds having a structure of SiC _x O _y oxygen to silicon carbide is bonded portion, and a step of BET specific surface area to obtain a silicon carbide-based porous body of 450~700m ² / g It is a manufacturing method of the silicon carbide type porous body characterized by including.
Note that _x in SiC _x O _y is in the range of 1 to 1.5, and y is in the range of 0 to 0.5.

The invention according to claim 2 is the invention according to claim 1 , wherein the silicon carbide precursor constituting the silicon carbide precursor solution 12 is polycarbosilane, and the solvent is tetrahydrofuran (hereinafter referred to as THF) or. It is a manufacturing method which is toluene.

The silicon carbide based porous material obtained by the production method of the present invention has characteristics such as high heat resistance, excellent thermal conductivity and electrical conductivity, chemical stability, etc. possessed by SiC, and has a high specific surface area. There are advantages.

Next, the best mode for carrying out the present invention will be described with reference to the drawings.
The present inventor used mesoporous silica powder as a mold for silicon carbide based porous material, filled mesopores of this silica powder with a silicon carbide precursor solution, subjected to predetermined treatment, and fired to obtain silica powder. It was found that a silicon carbide based porous body having a high specific surface area can be obtained by forming silicon carbide inside and further removing the silica component of the mesoporous silica powder. The silicon carbide based porous body of the present invention is characterized in that it is formed in a three-dimensional network shape, and the pores inside the network have a continuous pore structure and are mainly composed of silicon carbide. The BET specific surface area of the silicon carbide based porous material of the present invention is 450 to 700 m ² / g. The above range depends on the size of the mesopores of the mesoporous silica powder used as a mold, but if it is less than the lower limit, it is not sufficient for use, and if it exceeds the upper limit, the strength is poor. . The silicon carbide based porous material of the present invention includes not only silicon carbide but also a compound having a SiC _x O _y structure in which oxygen is partially bonded to silicon carbide. In the SiC _x O _y , x is in the range of 1 to 1.5, and y is in the range of 0 to 0.5.

Next, the manufacturing method of the silicon carbide based porous material of the present invention will be described.
First, as shown in FIG. 1, the silicon carbide precursor solution 12 is filled into the pores of the mesoporous silica powder 11. As the mesoporous silica powder 11, those having mesopores having a particle size of about several tens of μm and pores of about 2 to 50 nm are suitable. Moreover, polycarbosilane is mentioned as a silicon carbide precursor which comprises the silicon carbide precursor solution 12, and THF and toluene are mentioned as a solvent, respectively. The silicon carbide precursor solution 12 is filled inside the pores of the mesoporous silica powder 11 by immersing and stirring the mesoporous silica powder 11 in the precursor solution 12. During the filling, it is preferable to deaerate the pores of the silica powder by reducing the pressure. Next, after the filling powder 13 is taken out from the precursor solution and filtered through a filter, the filling powder 13 is dried to remove the solvent component contained in the filled solution. Natural drying is sufficient for drying here.

Subsequently, the filled powder 13 is held at 100 to 200 ° C. for 1 to 24 hours under atmospheric pressure to subject the precursor in the pores to infusibilization. By applying this process, the filled precursor is not melted during firing in the subsequent process. Further, it is possible to lower the proportion of oxygen SiC _x O _y obtained by baking in a step that follows later. If the temperature is lower than 100 ° C., the effect cannot be obtained. If the temperature is higher than 200 ° C., the precursor is excessively oxidized, resulting in a defect that lacks chemical stability. Further, if the holding time is less than 1 hour, a sufficient effect cannot be obtained, and if it exceeds 24 hours, the effect does not change. It is particularly preferable to hold at 150 to 200 ° C. for 12 to 24 hours. Next, the filling powder 13 is fired at 1000 to 1200 ° C. for 1 to 2 hours in an inert gas atmosphere. By this firing, the precursor in the pores of the silica powder becomes silicon carbide or SiC _x O _y in which oxygen is partially bonded to silicon carbide. Examples of the inert gas include nitrogen gas and rare gas. If it is less than 1000 degreeC, the composition in a silicon carbide is not enough, and it becomes a porous body inferior to chemical resistance. Although silicon carbide can be obtained even when the temperature exceeds 1200 ° C., the upper limit is determined from the performance of the firing furnace or the like. Further, if the holding time is less than 1 hour, the composition in the silicon carbide is not sufficient, and the porous body is inferior in chemical resistance. The effect does not change even if it exceeds 2 hours. It is particularly preferable to hold at 1100 to 1200 ° C. for 1 to 2 hours. Furthermore, the silicon carbide based porous body 15 is obtained by eliminating the silica component by dissolving and removing the mesoporous silica 11 from the powder 14 fired using an etching solution for dissolving silica. Examples of the etching solution for eliminating silica include a hydrofluoric acid solution and a saturated NaOH solution.

Next, examples of the present invention will be described in detail together with comparative examples.
<Example 1>
First, mesoporous silica powder (manufactured by Nippon Chemical Industry Co., Ltd .; trade name Sirifam) having a particle size of several tens of μm was prepared. Further, polycarbosilane was added to THF at a predetermined ratio to prepare a silicon carbide precursor solution. Next, the mesoporous silica powder was immersed in the prepared precursor solution and stirred to fill the pores of the mesoporous silica powder with the silicon carbide precursor solution. Next, the filled powder was taken out and filtered through a filter, and then the solvent component contained in the filled solution was removed by naturally drying the filled powder. Subsequently, the filled powder was held at 200 ° C. for 24 hours under atmospheric pressure to subject the precursor in the pores to infusibilization. Next, the filler powder was fired at 1000 ° C. for 1 hour in a nitrogen gas atmosphere, so that the precursor in the pores was silicon carbide. Furthermore, a 50 wt% hydrofluoric acid solution was prepared as an etching solution, and the silica component was eliminated by dissolving and removing powdered mesoporous silica fired with this etching solution to obtain a silicon carbide based porous material.

<Comparative Example 1>
Activated carbon fiber was prepared as a comparative specimen.
<Comparative example 2>
Silicon carbide fibers (manufactured by Nippon Carbon Co., Ltd .; trade name: Nicalon) having an average major axis of 15 μm and an average minor axis of 10 μm were prepared as comparative samples.
<Comparative Example 3>
A β-type silicon carbide powder having an average particle size of 1 μm was prepared as a comparative specimen.

<Comparison test 1>
The following tests were conducted on the porous body of Example 1 and the comparative specimens of Comparative Examples 1 to 3.
(1) The BET specific surface area of the obtained porous material of Example 1 and the prepared comparative samples of Comparative Examples 1 to 3 were measured.
(2) 16 mol / l of concentrated nitric acid was prepared, and the porous body of Example 1 and the comparative sample of Comparative Example 1 were immersed in concentrated nitric acid and subjected to concentrated nitric acid treatment that was held at room temperature for 1 week, and then the BET ratio. The surface area was measured.
(3) After preparing 16 mol / l concentrated nitric acid, the porous body of Example 1 and the comparative sample of Comparative Example 1 were immersed in concentrated nitric acid and treated at 100 ° C. for 3 hours, followed by BET. The specific surface area was measured.
The obtained BET specific surface area results are shown in Table 1, respectively.

  As is clear from Table 1, in test (1), the silicon carbide porous material of Example 1 and the activated carbon fiber of Comparative Example 1 had high specific surface area values. On the other hand, the silicon carbide fiber of Comparative Example 2 that does not have a porous structure and the β-type silicon carbide powder of Comparative Example 3 resulted in low BET specific surface area values. The activated carbon fiber of Comparative Example 1 had a significantly lower BET specific surface area value after the concentrated nitric acid treatment in Test (2) than the BET specific surface area value that was not subjected to the concentrated nitric acid treatment in Test (1). This is thought to be the result of the loss of pores due to oxidation of carbon fibers by concentrated nitric acid treatment. Further, after the concentrated nitric acid treatment in Test (3), all of the activated carbon fibers disappeared, and the BET specific surface area could not be measured. On the other hand, in the silicon carbide based porous material of Example 1, the reason is not clear, but after the concentrated nitric acid treatment in the test (2) and after the concentrated nitric acid treatment in the test (3), the BET specific surface area values were tested. It was higher than the BET specific surface area value of (1) not subjected to concentrated nitric acid treatment.

<Comparison test 2>
X-ray diffraction measurement was performed on the silicon carbide based porous body of Example 1, the silicon carbide based fiber of Comparative Example 2, and the β-type silicon carbide powder of Comparative Example 3. The obtained X-ray diffraction results are shown in FIG.
As can be seen from FIG. 2, the β-type silicon carbide powder of Comparative Example 3 having high crystallinity has a characteristic peak and high crystallinity. On the other hand, the porous body of Example 1 and the silicon carbide fiber of Comparative Example 2 do not have clear peak values, and are considered to have a structure with low crystallinity. Further, the silicon carbide fiber of Comparative Example 2 has a small diffraction intensity of 20 ° to 30 °, whereas the porous body of Example 1 has a high diffraction intensity, and it is presumed that the crystal structures of the two are different.

<Comparison test 3>
Thermogravimetric analysis was performed on the silicon carbide porous material of Example 1 and the activated carbon fiber of Comparative Example 1. Measurement conditions are a temperature rising time of 5 ° C./min in the atmosphere. The obtained thermogravimetric analysis results are shown in FIG.
As is clear from FIG. 3, the activated carbon fiber of Comparative Example 1 suddenly loses weight when the temperature exceeds 500 ° C., and when reaching 600 ° C., the weight reduction rate becomes −100%, and all the measurement samples disappear. did. This disappearance is thought to be due to the burning of the activated carbon fiber. On the other hand, in the silicon carbide based porous material of Example 1, a weight loss of about 20% occurred when the temperature exceeded 500 ° C., but there was no significant weight change even after exceeding 800 ° C. It was found to be a porous body having excellent properties.

  Since the silicon carbide based porous material of the present invention has a large specific surface area and also has nanopores, it can be applied to an adsorbent, a deodorant, an exhaust gas filter, and a catalyst carrier for high-temperature chemical reaction as well as the use of activated carbon. It can be applied to various uses. In addition, by providing conductivity, it can be used as an electrode material for an electric double layer capacitor.

Process drawing which shows the manufacturing method of the silicon carbide type porous body of this invention. The figure which shows the X-ray-diffraction result in the comparative test 2. The figure which shows the thermogravimetric analysis result in the comparative test 3. FIG.

Explanation of symbols

11 Mesoporous silica powder 12 Silicon carbide precursor solution 13 Filled powder 14 Baked powder 15 Silicon carbide based porous body

Claims (2)

Filling a silicon carbide precursor solution (12) of polycarbosilane into pores of mesoporous silica powder (11) having a particle size of several tens of μm and pores having mesopores of 2 to 50 nm ;
Removing the solvent component contained in the filled solution by drying the filled powder (13);
Holding the filled powder (13) at 100 to 200 ° C. under atmospheric pressure for 1 to 24 hours to subject the precursor in the pores to infusibilization ;
Baking the filled powder (13) at 1000 to 1200 ° C. for 1 to 2 hours in an inert gas atmosphere;
By removing the mesoporous silica (11) from the calcined powder (14) using an etching solution that dissolves silica , a three-dimensional network is formed, and pores inside the network have a continuous pore structure. Silicon having a BET specific surface area of 450 to 700 m ² / g , mainly composed of silicon carbide, including not only the silicon carbide but also a compound having a SiC _x O _y structure in which oxygen is partially bonded to silicon carbide. And a step of obtaining a carbide-based porous body. A method for producing a silicon carbide-based porous body.
Note that _x in SiC _x O _y is in the range of 1 to 1.5, and y is in the range of 0 to 0.5. The production method according to claim 1 , wherein the silicon carbide precursor constituting the silicon carbide precursor solution (12) is polycarbosilane, and the solvent is tetrahydrofuran or toluene.

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