KR101385570B1 - Manufacturing method of silicon carbide nanofibers from exfoliated graphite and amorphous silica - Google Patents

Abstract

The present invention relates to a method for producing a large amount of high quality silicon carbide nanofibers at a low cost, and more particularly, to prepare expanded graphite by heat-treating graphite subjected to an acid treatment process (step 1); Mixing amorphous silica with an organic solvent to uniformly disperse to prepare a mixed solution (step 2); Mixing the expanded graphite obtained in step 1 with the amorphous silica solution and inserting the amorphous silica powder between the layers of the expanded graphite using a vacuum desiccator (step 3); And drying the composite powder obtained in step 3, followed by heat treatment in a reducing atmosphere to synthesize graphite / silicon carbide nanofibers (step 4); It relates to a method for producing silicon carbide nanofibers, comprising the step (step 5) of removing the graphite by heat-treating the composite powder (graphite / silicon carbide) obtained in step 4 in an oxidizing atmosphere. Silicon carbide nanofibers according to the present invention is a high-quality nanofibers in a linear form to provide a silicon carbide nanofibers that can be manufactured in a low cost and a large amount by using a low cost expanded graphite and amorphous silica. Therefore, it can be applied to various fields such as composite materials and can be usefully used as electrical components such as transistors due to its excellent electrical properties.

Description

 Method for Manufacturing Silicon Carbide Nanofibers Using Expanded Graphite and Amorphous Silica {MANUFACTURING METHOD OF SILICON CARBIDE NANOFIBERS® FROM EXFOLIATED® GRAPHITE AND®AMORPHOUS SILICA}

The present invention relates to a method for producing silicon carbide nanofibers, and more particularly, by inserting amorphous silica between layers of expanded graphite, and then performing a carbon thermal reduction of the mixed powder of expanded graphite and silica at low cost. A method for producing high quality silicon carbide nanofibers in bulk.

Silicon carbide (SiC) has excellent chemical stability at high temperatures, good thermal and thermal stability due to high thermal conductivity and low thermal expansion coefficient, and high mechanical strength and high wear resistance. Due to such excellent characteristics, research is being actively applied to high temperature materials, high temperature semiconductors, semiconductor jig, wear-resistant materials, automotive parts, chemical resistance or chemical parts or electronic parts of chemical plants.

      Among them, silicon carbide nanofibers are generally cylindrical in diameter of several tens to tens of nanometers and have a length of several tens to hundreds of micrometers. The main component is carbon and silicon in an atomic ratio of 1: 1. Depending on the manufacturing method, amorphous silicon oxide may be covered with a thickness of several to several tens of nm. Silicon carbide nanofibers are materials having high strength, excellent chemical stability and excellent electrical properties, like the general silicon carbide (bulk type) described above. Field emission tips (FETs) using nanomaterials, which have been recently researched and developed, require stability at high temperatures during operation.In this respect, silicon carbide nanorods show more stable structural characteristics than carbon nanotubes. It is attracting attention as a field emission device material. Such silicon carbide nanofibers are manufactured by various methods. Conventionally known methods of manufacturing silicon carbide fibers include Chemical Vapor Infiltration (CVI), Polymer Impregnation and Pyrolysis (PIP), Liquid Silicon Infiltration (LSI), High Temperature Pressure sintering (HP, Hot Press) and the like. Chemical vapor deposition is a method of preparing a silicon carbide matrix by infiltrating a gaseous metal organic compound between fibers and pyrolysing to deposit silicon carbide around the fiber. The polymer penetration and pyrolysis method is an organic compound such as polycarbosilane. Is a method of obtaining a silicon carbide matrix phase by mixing with silicon carbide powder to form a slurry and then infiltrating the slurry into the silicon carbide fiber preform and pyrolyzing it. In addition, the molten silicon infiltration method is to prepare a silicon carbide matrix by filling carbon and silicon carbide powder in a preform, melting the silicon, and infiltrating and reacting with carbon. The high-temperature pressurization sintering method uses a slurry in which silicon carbide powder and a sintering aid are mixed. After penetrating the preform, it is sintered under high pressure. However, the chemical vapor deposition method has the disadvantage of long process time, residual pores, and high manufacturing cost.The polymer infiltration and pyrolysis method is difficult to obtain crystalline silicon carbide composed of chemical quantitative ratio, has low thermal conductivity, and reduces volume during thermal decomposition. Since cracks occur, several iterative processes are required. In addition, the molten silicon infiltration method can produce a composite having almost no pores and excellent thermal conductivity, but the heat and radiation resistance characteristics are relatively poor due to the residual of unreacted silicon, and the high temperature pressure sintering method is difficult to sinter the fiber. There is a disadvantage of being damaged.

 Recently, many studies have been conducted to improve the disadvantages of the above known methods. As a specific example, the Republic of Korea Patent No. 1103649 (2012.01.02. Registered) introduced a method of electrospinning a polymer solution prepared by mixing an organosilicon compound, a polymer precursor and a solvent, this method is simple to manufacture Introduced silicon carbide nanofibers have the advantage of having electrical conductivity. In addition, the Republic of Korea Patent No. 0561701 (registered on March 09, 2006) introduced a method of coating the surface of the carbon structure using a transition metal as a metal catalyst, and then reacting the coated carbon structure with a mixture of silicon and silicon dioxide powder. Introduced that there is an advantage that can produce a large amount of carbon nanofiber at low temperature. In addition, the mixture of silica and carbon in Korean Patent Registration No. 0769695 (registered on October 17, 2007), and press-molded to prepare a porous support, the porous support is placed on the top or bottom of the filter and crystallized in an inert atmosphere To produce single crystal silicon carbide nanowires by growing inside the filter, and nickel, iron, cobalt, chromium, lanthanum, titanium, molybdenum, gold or the like in Korean Patent No. 0581005 (registered May 10, 2006). After coating the compound on the substrate in the form of a dispersion coating, heat-treated between 700 ~ 1000 ℃ and agglomerated to a fine size on the surface, HMDS, 1,3-DSB, TMS, TPS or MTS containing silicon and carbon together The method of thermochemical deposition using a single precursor of was introduced. However, the processes for manufacturing the silicon carbide fibers as described above are expensive and high purity raw materials are used and the manufacturing cost is high because the process and the process equipment is complicated and difficult to process. Therefore, the present inventors have developed a process for manufacturing a high-quality silicon carbide nanofibers using expanded graphite and amorphous silica in a simple and low-cost manufacturing process to solve the disadvantages of the conventional manufacturing process of the nanofibers. .

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing high quality silicon carbide nanofibers having a linear dimension in one dimension using expanded graphite as a carbon raw material and amorphous silica as a silicon raw material. Another object of the present invention is to provide a method for producing a large amount of silicon carbide nanofibers at low cost using low-purity expanded graphite and amorphous silica.

In order to achieve the above technical problem, the present invention comprises the steps of heat-treating the graphite subjected to the acid treatment to produce expanded graphite (step 1); Mixing amorphous silica with an organic solvent to uniformly disperse to prepare a mixed solution (step 2); Mixing the expanded graphite and the amorphous silica solution and inserting the amorphous silica powder between the layers of the expanded graphite using a vacuum desiccator (step 3); And heat-treating the expanded graphite powder including amorphous silica in a reducing atmosphere to synthesize a silicon carbide nanofiber / graphite mixture (step 4); It provides a method for producing silicon carbide nanofibers comprising the step (step 5) of producing a silicon carbide nanofibers by heat-treating the silicon carbide / graphite mixture obtained in step 4 in an oxidizing atmosphere.

The present invention also provides a method for mass production of low-cost, high-quality linear linear silicon carbide nanofibers having a diameter of 50-200 nm and a length of 50-200 μm.

One-dimensional linear high-quality silicon carbide nanofibers according to the present invention can be applied to various fields such as composite materials because it can be manufactured in a large amount by using a low-cost raw material and the manufacturing process is simple and excellent electrical properties such as transistors It can also be useful as a component.

1 is a flowchart of a process for producing silicon carbide nanofibers according to the present invention;
2 is a scanning electron micrograph at 5000 magnification after the incorporation of expanded graphite and amorphous silica according to the present invention;
3 is a scanning electron micrograph at 5000 magnification of the silicon carbide nanofibers of Example 1 according to the present invention;
4 is a scanning electron micrograph at 5000 magnification of the silicon carbide nanofibers of Example 2 according to the present invention;
5 is a scanning electron micrograph at 5000 magnification of the silicon carbide nanofibers of Comparative Example 1 according to the present invention;
6 is a scanning electron micrograph at 5000 magnification of the silicon carbide nanofibers of Comparative Example 2 according to the present invention;
7 is an X-ray diffraction analysis of the silicon carbide nanofibers of (b) Example 2 and (a) Comparative Example 1 according to the present invention;
Is a transmission electron micrograph of the silicon carbide nanofibers of Example 2 according to the present invention;

The present invention comprises the steps of heat-treating the graphite subjected to acid treatment to produce expanded graphite (step 1); Mixing amorphous silica with an organic solvent to uniformly disperse to prepare a mixed solution (step 2); Mixing the expanded graphite with the amorphous silica solution and then inserting the amorphous silica powder between the layers of the expanded graphite using a vacuum desiccator (step 3); And heat-treating the expanded graphite powder including amorphous silica in a reducing atmosphere to synthesize graphite / silicon carbide nanofiber mixture (step 4); It provides a method for producing silicon carbide nanofibers comprising the step (step 5) of preparing a pure silicon carbide nanofibers by heat-treating the graphite / silicon carbide mixed powder in an oxidizing atmosphere.

Hereinafter, the method for producing the silicon carbide nanofibers according to the present invention will be described in more detail step by step.

Step 1 is a step of producing expanded graphite by heat treatment of the graphite subjected to the acid treatment process.

Natural graphite powder of 50 ~ 80 mesh size is oxidized at 150 ℃ for 24 hours, intercalated, washed and dried. Intercalation refers to the insertion of an intercalation material between the graphite (carbon) layer and the layer.

In the graphite intercalation, the oxidation of the carbon layer occurs when oxidizing agents such as hydrogen peroxide (H ₂ O ₂ ), potassium permanganate (KMnO ₄ ), and nitric acid (HNO ₃ ) are used. The intercalated graphite starts to expand above 160 ° C. When rapidly heat-treated at 600-1,000 ° C for 1-10 minutes, the particles of graphite expand 80-350 times more in the direction perpendicular to the crystal plane of the graphite particles.

Next, step 2 is to prepare a mixed solution by uniformly dispersing the amorphous silica with an organic solvent.

The content of the solvent to be mixed in the step 2 is not particularly limited, and the content of the amorphous silica to be mixed is preferably added to be 40 to 50% by weight based on the total amount of the expanded graphite and silica. There is a problem that the purity is lowered due to the residual silicon dioxide (SiO ₂ ) that the material is not a linear form of the meandering or irregular form or did not react.

In addition, the solvent is preferably acetone-based organic solvents having a low boiling point in order to absorb the organic solvent of the expanded graphite and to increase the dispersion effect during the process.

Next, the step 3 according to the present invention is to mix the expanded graphite obtained in the step 1 and the amorphous silica solution obtained in the step 2 and then to insert the amorphous silica powder between the layers of the expanded graphite using a vacuum desiccator Step.

In step 3, the expanded graphite and the amorphous silica mixed solution are put in a vacuum desiccator, and the vacuum pressure applied to the expanded graphite and the amorphous silica is maintained for a predetermined time until the solvent inside is boiled. When the solvent is boiled, the particles in the mixed solvent resonate to obtain a high dispersion effect in the process of inserting the amorphous silica powder between the expanded graphite layer.

Next, step 4 according to the present invention is a step of synthesizing the graphite / silicon carbide nanofibers by drying the mixed powder obtained in the step 3 and heat treatment in a reducing atmosphere.

The heat treatment condition is preferably maintained for 5 hours or more at 1400 ~ 1500 ℃ in a reducing atmosphere. If the heat treatment temperature is lower than this or the time is short, as in step 2, the final material is not linear, but the purity is lowered due to the residual silicon dioxide (SiO ₂ ) that is not squiggly or irregularly shaped or not reacted. have.

Next, step 5 according to the invention is a step of removing graphite by heat-treating the composite powder (graphite / silicon carbide) obtained in step 4 in an oxidizing atmosphere.

In the step 5, the heat treatment is preferably performed at 800-1000 ° C. for about 1 hour in an oxidizing atmosphere. If the heat treatment time is too short or long, residual graphite remains or the surface SiO ₂ oxide layer of the silicon carbide nanofibers is thickened to inhibit the characteristics. have.

The present invention also provides a method for mass production of low-cost, high-quality linear linear silicon carbide nanofibers having an average diameter of 100 to 200 nm and a length of 50 to 100 μm.

Therefore, the silicon carbide nanofibers according to the present invention can be applied to various fields such as composite materials and can be usefully used as electrical components such as transistors with excellent electrical properties.

Hereinafter, the embodiment of the present invention will be described in more detail. However, the following examples are merely to illustrate the invention, but the content of the present invention is not limited by the following examples.

(Example 1)

In the case of natural graphite used in the present invention, natural graphite having a particle size of 50 to 80 mesh was used in the experiment. Natural graphite is oxidized by using an intercalation process using sulfuric acid (H ₂ SO ₄ ) as an intercalation material and nitric acid (HNO ₃ ) as an oxidizing agent, followed by rapid heat treatment in a quartz crucible at 900 ° C. for 1 minute. Expanded graphite having a high expansion volume of 200-500 ml / g. In addition, amorphous silica (SiO ₂ ) having a size of several tens of nm to 5 μm or less is mixed with 50% by weight of acetone (500 ml), an organic solvent, based on the total amount of expanded graphite and amorphous silica. Dispersion to prepare a dispersion. The dispersion was mixed with the expanded graphite prepared above, placed in a vacuum desiccator, and kept at a pressure lower than 0.05 MPa for 10 minutes, followed by drying at room temperature. The dry mixed powder was heated at a rate of 5 ° C./min in a reducing atmosphere (25% H ₂ /75% He) and finally heat treated at 1425 ° C. for 6 hours. The heat-treated powder was heated at a rate of 5 ℃ / min in the oxidation atmosphere and finally heat treated at 900 ℃ for 1 hour to obtain silicon carbide nanofibers.

(Example 2)

In the case of natural graphite used in the present invention, natural graphite having a particle size of 50 to 80 mesh was used in the experiment. Natural graphite is oxidized by using an intercalation process using sulfuric acid (H ₂ SO ₄ ) as an intercalation material and nitric acid (HNO ₃ ) as an oxidizing agent, followed by rapid heat treatment in a quartz crucible at 900 ° C. for 1 minute. Expanded graphite having a high expansion volume of 200-500 ml / g. In addition, amorphous silica (SiO ₂ ) having a size of tens of nm to 5 μm or less is mixed with acetone (500 ml), which is an organic solvent, based on the total amount of expanded graphite and amorphous silica, followed by 10 minutes using ultrasonic equipment. Dispersion to prepare a dispersion. The dispersion was mixed with the expanded graphite prepared above, placed in a vacuum desiccator, and maintained at room temperature for 10 minutes by applying vacuum pressure. The dry mixed powder was heated at a rate of 5 ° C./min in a reducing atmosphere (25% H ₂ /75% He) and finally heat treated at 1425 ° C. for 6 hours. The heat-treated powder was heated at a rate of 5 ° C./min in an oxidizing atmosphere and finally heat treated at 900 ° C. for 1 hour to obtain silicon carbide nanofibers.

(Comparative Example 1)

In Example 1, 20% by weight of amorphous silica (SiO ₂ ) was mixed with acetone (500 ml), which is an organic solvent, based on the total amount of expanded graphite and amorphous silica, and then dispersed for 10 minutes using an ultrasonic apparatus to prepare a dispersion. Silicon carbide nanofibers could be produced in the same manner as in Example 1 except for the one.

(Comparative Example 2)

In Example 1, 10% by weight of amorphous silica (SiO ₂ ) was mixed with acetone (500 ml), which is an organic solvent, based on the total amount of expanded graphite and amorphous silica, and then dispersed for 10 minutes using an ultrasonic apparatus to prepare a dispersion. Silicon carbide nanofibers could be produced in the same manner as in Example 1 except for the one.

Experimental Example 1

Morphological Characterization of Silicon Carbide Nanofibers by Initial Silica Content

The initial raw materials (expanded graphite / amorphous silica) content of Examples 1 and 2 and Comparative Examples 1 and 2 according to the present invention are shown in Table 1, respectively. ), And the results are shown in FIGS. 3, 4, and 5.

3, 4, and 5, it can be seen that the silicon carbide nanofibers of Examples 1 and 2 according to the present invention have a linear shape, whereas the silicon carbide nanofibers of Comparative Example 1 having a low content of initial amorphous silica are It can be seen that it has a meandering or irregular shape.

division Initial raw material Expanded Graphite (% by weight) Amorphous silica (% by weight) Sum Example 1 50 50 100 Example 2 60 40 100 Comparative Example 1 80 20 100 Comparative Example 2 90 10 100

Experimental Example 2

Analysis of X-ray Diffraction Characteristics of Silicon Carbide Nanofibers by Initial Silica Content

In order to determine the X-ray diffraction characteristics of the silicon carbide nanofibers prepared according to the processes of Example 2 and Comparative Example 1 according to the present invention, the result was measured by an X-ray diffractometer (XRD) and the results are shown in Example 6 in FIG. 6. (B) and Comparative Example 1 are represented by (a).

Referring to FIG. 6, the X-ray diffraction peak analysis results of Example 2 and Comparative Example 1 both showed a 3C-SiC peak, which is a crystal phase of silicon carbide nanofibers, and were more linear than Comparative Example 1 having a serpentine shape. It can be seen that the peak of 2 has greater intensity.

Experimental Example 3

Crystallographic Characterization of Silicon Carbide Nanofibers by Initial Silica Content

In order to determine the crystallographic characteristics of the silicon carbide nanofibers prepared in Example 1 according to the present invention, the transmission electron microscope (TEM) and the electron diffraction characteristics were measured, and the results are shown in FIG. 7.

Referring to FIG. 7, it can be seen that the silicon carbide nanofibers of Example 1 grow linearly in the [111] crystal plane direction.

Claims (10)

Heat-treating the graphite subjected to the acid treatment to produce expanded graphite (step 1);
Mixing amorphous silica with an organic solvent to uniformly disperse to prepare a mixed solution (step 2);
Mixing the expanded graphite obtained in step 1 with the amorphous silica solution obtained in step 2, and then inserting the amorphous silica powder between the layers of expanded graphite using a vacuum desiccator (step 3); Drying the composite powder obtained in step 3 and then heat treating it in a reducing atmosphere to synthesize graphite / silicon carbide nanofibers (step 4); Heat-treating the graphite / silicon carbide nanofibers obtained in step 4 in an oxidizing atmosphere to remove graphite (step 5); Method for producing silicon carbide nanofibers comprising the The method of claim 1, wherein the acid-treated graphite in step 1 using a quartz crucible is rapidly heat treated at 600 ~ 1000 ℃ for 1 to 10 minutes to produce expanded graphite silicon carbide nanofibers. The method of claim 1, wherein the content of amorphous silica is added to 40 to 50% by weight based on the total amount of expanded graphite and amorphous silica. The method of claim 1, wherein the organic solvent is an acetone-based organic solvent. The method of claim 1, wherein as a method for uniformly inserting the amorphous silica between the layers of expanded graphite, the pressure of the vacuum desiccator is maintained at a low 0.05MPa or less for 10 minutes to manufacture the silicon carbide nanofibers Way.
delete The silicon carbide nanofibers manufacturing method according to claim 1, wherein in step 4, the composite powder composed of expanded graphite and amorphous silica is heat-treated at 1400 to 1500 ° C. for 6 hours in a reducing atmosphere (25% H ₂ /75% He). Way. The method of claim 1, wherein in step 5, the silicon carbide nanofibers / graphite mixture is heat-treated at 800 to 1000 ° C. for at least 1 hour to obtain pure silicon carbide nanofibers. delete delete

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