Method for preparing ultrahigh-purity silicon carbide powder
10106423 ยท 2018-10-23
Assignee
Inventors
Cpc classification
C01P2004/51
CHEMISTRY; METALLURGY
International classification
Abstract
The present invention relates to a method for preparing an ultrahigh-purity silicon carbide powder, more particularly to a method for preparing an ultrahigh-purity silicon carbide granular powder by preparing a gel wherein a silicon compound and a carbon compound are uniformly dispersed via a sol-gel process using a liquid state silicon compound and a solid or liquid state carbon compound of varying purities as raw materials, preparing a silicon dioxide-carbon (SiO.sub.2C) composite by pyrolyzing the prepared gel, preparing a silicon carbide-silicon dioxide-carbon (SiCSiO.sub.2C) composite powder via two-step carbothermal reduction of the prepared silicon dioxide-carbon composite, adding a silicon metal and then conducting carbonization and carbothermal reduction at the same time by heat treating, thereby growing the synthesized silicon carbide particle with an increased yield of the silicon carbide.
Claims
1. A method for preparing a silicon carbide powder, comprising: preparing a gel by mixing a liquid state silicon compound and a carbon compound in liquid state and conducting hydrolysis and gelation via a sol-gel process; preparing a silicon dioxide-carbon composite by pyrolyzing the prepared gel; preparing a silicon carbide-silicon dioxide-carbon composite powder via partial carbothermal reduction of the prepared silicon dioxide-carbon composite under a vacuum at 1300-1400 C.; and preparing a silicon carbide granular powder by adding a silicon metal to the silicon carbide-silicon dioxide-carbon composite powder and facilitating growth of synthesized silicon carbide particles while conducting a direct reaction and an additional carbothermal reduction at 1500-1700 C. with a heating rate of 5-20 C./min.
2. The method for preparing the silicon carbide powder according to claim 1, wherein, in the preparing of the gel, after a liquid sol is formed by mixing and stirring a liquid state silicon compound and a liquid state carbon compound, a gel in which the silicon compound and the carbon compound are mixed is prepared by adding an aqueous acid or base solution that facilitates hydrolysis and gelation of the silicon compound.
3. The method for preparing the silicon carbide powder according to claim 1, wherein the liquid state silicon compound comprises one or more silicon sources selected from the group consisting of silicon mono(C.sub.1-C.sub.4 alkoxide), silicon di(C.sub.1-C.sub.4 alkoxide), silicon tri(C.sub.1-C.sub.4 alkoxide), silicon tetra(C.sub.1-C.sub.4 alkoxide), and polyethyl silicate, and the carbon compound comprises one or more carbon sources selected from the group consisting of sucrose, maltose, lactose, and a phenol resin.
4. The method for preparing the silicon carbide powder according to claim 1, wherein, in the preparing of the gel, the silicon compound and the carbon compound are used with a carbon/silicon (C/Si) molar ratio of 1.6-4.0.
5. The method for preparing the silicon carbide powder according to claim 1, wherein, in the preparing of the gel, the liquid state silicon compound and the liquid state carbon compound are mixed using a Teflon-coated mixing device and the gel comprising the silicon compound, and the carbon compound is prepared by stirring at 50-400 RPM and at 20-60 C. for 6 hours to one week.
6. The method for preparing the silicon carbide powder according to claim 1, wherein, in the preparing of the gel, the gel comprising the silicon compound and the carbon compound is prepared using one or more acid selected from the group consisting of nitric acid, hydrochloric acid, oxalic acid, maleic acid, acrylic acid, acetic acid, and toluenesulfonic acid, or one or more bases selected from the group consisting of ammonia water and hexamethylenetetramine as a catalyst for the hydrolysis and gelation of the silicon compound.
7. The method for preparing the silicon carbide powder according to claim 6, wherein the catalyst for the hydrolysis and gelation of the silicon compound comprises an aqueous solution of an acid or a base, a molar ratio of the acid or base being 0.2 or smaller and a molar ratio of water being 10 or smaller based on the silicon (Si) element in the silicon compound.
8. The method for preparing the silicon carbide powder according to claim 1, wherein, in the preparing of the gel, the prepared gel is pulverized, classified, and dried at 40-150 C. for 1-72 hours to prepare a dried gel in which the silicon compound and the carbon compound are uniformly distributed.
9. The method for preparing the silicon carbide powder according to claim 1, wherein the preparing of the silicon dioxide-carbon composite comprises conducting pyrolysis of the gel under a nitrogen, argon, or vacuum atmosphere at 800-1000 C. for 0.5-5 hours with a heating rate of 1-10 C./min.
10. The method for preparing the silicon carbide powder according to claim 1, wherein the partial carbothermal reduction is performed for 1-5 hours with a heating rate of 1-10 C./min, and the additional carbothermal reduction is performed for 1-10 hours.
11. The method for preparing the silicon carbide powder according to claim 1, wherein the additional carbothermal reduction is performed under an argon or vacuum atmosphere with a heating rate of 1-20 C./min.
12. The method for preparing the silicon carbide powder according to claim 11, wherein, during the reaction, which is between the silicon carbide-silicon dioxide-carbon composite powder and the silicon metal, the silicon carbide granular powder is prepared by changing the heating rate in a range within the 1-20 C./min.
13. The method for preparing the silicon carbide powder according to claim 1, wherein, in the preparing of the silicon carbide granular powder, the silicon metal is added in an amount of 100-500% of moles of unreacted carbon in the silicon carbide-silicon dioxide-carbon composite powder, and the silicon metal comprises either one of a silicon metal powder and a silicon metal piece having a purity of 99.9999 wt % or higher.
14. The method for preparing the silicon carbide powder according to claim 1, wherein the silicon carbide granular powder comprises a -phase silicon carbide powder having an average particle size of 30-200 m, a particle size distribution (d.sub.90/d.sub.10) of 5.0 or smaller, and 1 ppm or less of metal impurities.
15. A method for preparing a silicon carbide powder, comprising: preparing a gel by mixing a liquid state silicon compound and a carbon compound in liquid state and conducting hydrolysis and gelation via a sol-gel process; preparing a silicon dioxide-carbon composite by pyrolyzing the prepared gel; preparing a silicon carbide-silicon dioxide-carbon composite powder via partial carbothermal reduction of the prepared silicon dioxide-carbon composite; and preparing a silicon carbide granular powder by adding a silicon metal to the silicon carbide-silicon dioxide-carbon composite powder and facilitating growth of synthesized silicon carbide particles while simultaneously conducting a direct reaction and an additional carbothermal reduction, wherein the partial carbothermal reduction is performed under a vacuum atmosphere at 1300-1400 C. for 1-5 hours with a heating rate of 1-10 C./min, and the additional carbothermal reduction is performed at 1500-1700 C. for 1-10 hours with a heating rate of 5-20 C./min.
16. A method for preparing a silicon carbide powder, comprising: preparing a gel by mixing a liquid state silicon compound and a carbon compound in liquid state and conducting hydrolysis and gelation via a sol-gel process; preparing a silicon dioxide-carbon composite by pyrolyzing the prepared gel; preparing a silicon carbide-silicon dioxide-carbon composite powder via partial carbothermal reduction of the prepared silicon dioxide-carbon composite; and preparing a silicon carbide granular powder by adding a silicon metal to the silicon carbide-silicon dioxide-carbon composite powder in an amount of 100-500% of moles of unreacted carbon in the silicon carbide-silicon dioxide-carbon composite powder and facilitating growth of synthesized silicon carbide particles while conducting a direct reaction and an additional carbothermal reduction, wherein the silicon metal is a silicon metal powder or a silicon metal piece having a purity of 99.9999 wt % or higher.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1)
(2)
DETAILED DESCRIPTION
(3) Hereinafter, the present invention is described in more detail through exemplary embodiments.
(4) The present invention relates to a method for economically preparing ultrahigh-purity SiC granular powders having a narrow particle size distribution and various particle sizes with a high yield using a SiO.sub.2C composite prepared from a liquid silicon source and a carbon source, which have various purities, via a sol-gel process and a pyrolysis process.
(5) In an exemplary embodiment of the present invention, ultrahigh-purity SiC powders are prepared by a step of preparing a gel using a liquid state silicon compound and a carbon compound in liquid state, a step of preparing a silicon dioxide-carbon (SiO.sub.2C) composite, a step of preparing silicon carbide-silicon dioxide-carbon (SiCSiO.sub.2C) composite powders via partial carbothermal reduction and, finally, a step of preparing SiC granular powders by adding a silicon metal to the prepared silicon carbide-silicon dioxide-carbon (SiCSiO.sub.2C) composite powders and growing the initially synthesized SiC particles while conducting both the direct reaction between the silicon metal and carbon and additional carbothermal reduction at the same time.
(6) In an exemplary embodiment of the present invention, a liquid state silicon compound and a solid state carbon compound, which have various purities, are mixed in liquid state after the solid state carbon compound is dissolved and a gel in which the silicon compound and the carbon compound are uniformly dispersed is prepared via a sol-gel process and then a silicon dioxide-carbon composite is prepared via pyrolysis. After loading the prepared SiO.sub.2C composite in a graphite crucible, SiCSiO.sub.2C composite powders are prepared via a two-step carbothermal reduction in a graphite vacuum furnace. Then, a silicon metal is added to the prepared SiCSiO.sub.2C composite powders, and then the SiCSiO.sub.2C composite powders with the added silicon metal are loaded in the graphite crucible with a high packing density, and ultrahigh-purity -phase SiC granular powders can be synthesized with a high yield via both of a direct reaction between an carbon and the silicon metal and additional carbothermal reduction between an unreacted silicon dioxide and carbon, thereby facilitating growth of the synthesized SiC particles.
(7) In an exemplary embodiment of the present invention, the starting source materials used to synthesize the ultrahigh-purity SiC granular powders may be a silicon compound and a carbon compound containing 0.1 wt % or less of metal impurities, and the silicon metal used as a silicon source material for the direct reaction with carbon in the synthesis of the SiC powders may be specifically silicon metal powders or a silicon metal piece having a purity of 99.9999 wt % or higher.
(8) In an exemplary embodiment of the present invention, the gel is prepared by first mixing the liquid state silicon compound and the carbon compound as the starting source materials in liquid state, next conducting hydrolysis and then gelation via a sol-gel process.
(9) In an exemplary embodiment of the present invention, in the step of preparing the gel, after a liquid sol is formed by mixing and stirring a liquid state silicon compound and a liquid state carbon compound, a gel wherein the silicon compound and the carbon compound are uniformly mixed may be prepared by adding an aqueous solution, as a catalyst, prepared by mixing a predetermined amount of an acid or a base with distilled water so that hydrolysis and gelation of the silicon compound are facilitated.
(10) In an exemplary embodiment of the present invention, the liquid state silicon compound may be one or more silicon source selected from silicon mono(C.sub.1-C.sub.4 alkoxide), silicon di(C.sub.1-C.sub.4 alkoxide), silicon tri(C.sub.1-C.sub.4 alkoxide), silicon tetra(C.sub.1-C.sub.4 alkoxide) and polyethyl silicate and the carbon compound may be one or more carbon source selected from a disaccharide selected from sucrose, maltose and lactose and a phenol resin, and a gel wherein the silicon source and the carbon source are uniformly mixed may be prepared. Specifically, the silicon compound may be silicon tetraethoxide (or tetraethyl orthosilicate) and the carbon compound may be a novolac-type phenol resin.
(11) In an exemplary embodiment of the present invention, when mixing the starting source materials, a carbon/silicon (C/Si) molar ratio may be specifically 1.6-4.0. When the carbon/silicon molar ratio is lower than 1.6, the synthesis yield of SiC decreases rapidly. And, when the carbon/silicon molar ratio exceeds 4.0, the synthesis of small-sized SiC particles is increased due to a reaction between the excess carbon source and the silicon metal, resulting in a broad particle size distribution of the synthesized SiC powders. The amount of the carbon compound may be determined based on the amount of carbon remaining after the pyrolysis and the amount of the silicon atom in the liquid state silicon compound. That is to say, the amount of the carbon compound required for the synthesis of the ultrahigh-purity SiC granular powders is specifically based on the amount of carbon remaining after the pyrolysis of the carbon compound and may be 1.6-4.0 mol per 1 mol of the silicon atom in the liquid state silicon compound. As a solvent used to dissolve the solid carbon source material, an alcohol and/or water may be used in an amount of 10 mol or less per 1 mol of the silicon atom in the liquid state silicon compound.
(12) In an exemplary embodiment of the present invention, when mixing the liquid state silicon compound and the liquid state carbon compound, a Teflon-coated mixing device, which includes low metal impurities, e.g., a Teflon container, a Teflon-coated mixing device, a Teflon-coated appliance, a Teflon-coated impeller, etc., may be used. Specifically, the gel containing both the silicon compound and the carbon compound may be prepared by stirring at 50-400 RPM and at 20-60 C. for 6 hours to one week.
(13) In an exemplary embodiment of the present invention, the gel containing both the silicon compound and the carbon compound may be prepared using one or more acid selected from nitric acid, hydrochloric acid, oxalic acid, maleic acid, acrylic acid, acetic acid and toluenesulfonic acid or one or more base selected from ammonia water and hexamethylenetetramine as a catalyst for the hydrolysis and gelation of the silicon compound.
(14) In an exemplary embodiment of the present invention, the catalyst for the hydrolysis and gelation of the silicon compound may be an aqueous solution of an acid or a base, a molar ratio of the acid or base being 0.2 or smaller, specifically 0.001-0.2, and a molar ratio of water being 10 or smaller, specifically 0.1-10, based on the silicon (Si) element in the silicon compound.
(15) In an exemplary embodiment of the present invention, the gel prepared from the liquid state silicon compound and the liquid state carbon compound as the starting source materials may be pulverized and classified into powders using equipment such as sieve, crusher, and container prepared using a material-including low metals or metal compounds impurities, e.g., Teflon, polyethylene, polyvinyl chloride, etc., and a Teflon-coated sieve. Specifically, the prepared gel may be loaded in a Teflon-coated container or Teflon container and then dried at 40-150 C. for 1 hour or longer (specifically for 1-72 hours) to prepare a dried gel wherein the silicon compound and the carbon compound are uniformly distributed.
(16) In accordance with the present invention, a silicon dioxide-carbon composite is prepared by pyrolyzing the prepared gel.
(17) In an exemplary embodiment of the present invention, after loading the dried gel containing the silicon compound and the carbon compound in a high-purity quartz crucible or a high-purity graphite crucible containing metal impurities other than silicon at low concentrations, a silicon dioxide-carbon (SiO.sub.2C) composite may be prepared by conducting pyrolysis in a high-purity quartz reactor under a nitrogen, argon or vacuum (10 torr or lower) atmosphere at 800-1000 C. for 0.5-5 hours by heating at a rate of 1-10 C./min.
(18) In accordance with the present invention, silicon carbide-silicon dioxide-carbon composite powders are prepared by conducting partial carbothermal reduction of the prepared silicon dioxide-carbon composite.
(19) In an exemplary embodiment of the present invention, after loading the prepared SiO.sub.2C composite in a high-purity graphite crucible containing metal impurities other than silicon at low concentrations, silicon carbide-silicon dioxide-carbon (SiCSiO.sub.2C) composite powders may be prepared by conducting carbothermal reduction in a high-purity graphite vacuum furnace under a vacuum (10 torr or lower) atmosphere at 1300-1400 C. for 1-5 hours by heating at a rate of 1-10 C./min and then further conducting carbothermal reduction at 1500-1700 C. for 1-10 hours by heating at a rate of 5-20 C./min.
(20) In accordance with the present invention, SiC granular powders are prepared by adding a silicon metal to the silicon carbide-silicon dioxide-carbon (SiCSiO.sub.2C) composite powders then conducting both the direct reaction between the silicon metal and carbon and additional carbothermal reduction between a silicon dioxide and carbon, thereby facilitating growth of the synthesized SiC particles.
(21) In an exemplary embodiment of the present invention, after the prepared SiCSiO.sub.2C composite powders are uniformly mixed with a silicon metal and loaded in a graphite crucible with a high packing density, ultrahigh-purity SiC granular powders may be prepared by conducting both a direct reaction between the silicon metal and carbon and carbothermal reduction between the unreacted silicon dioxide and carbon at the same time in a high-purity graphite vacuum furnace under an argon or vacuum (10 torr or lower) atmosphere at 1500-1800 C. by heating at a rate of 1-20 C./min.
(22) In accordance with the present invention, the direct reaction between the silicon metal and the unreacted carbon, carbothermal reduction between the unreacted silicon dioxide and the unreacted carbon, and carbothermal reduction between gaseous silicon monoxide produced from the reaction between the silicon metal and the unreacted silicon dioxide and the unreacted carbon occur in the vacuum furnace and, at the same time, the growth of the synthesized SiC particles occurs. As a result, the ultrahigh-purity -phase SiC granular powders are prepared. Specifically, the silicon metal that may remain in the prepared ultrahigh-purity SiC granular powders may be removed using a mixture of nitric acid and hydrofluoric acid.
(23) In an exemplary embodiment of the present invention, a direct reaction between the silicon metal and carbon and additional carbothermal reduction of the SiCSiO.sub.2C composite powders with the added silicon metal are conducted at the same time. During the reaction between the SiCSiO.sub.2C composite powders and the added silicon metal, ultrahigh-purity SiC granular powders having a narrow particle size distribution and various average particle sizes may be prepared by changing the heating rate in a range from 1 to 20 C./min, the synthesis temperature in a range from 1500 to 1800 C., and heating time in a range from 1 to 5 hours.
(24) In an exemplary embodiment of the present invention, the silicon metal may be added in an amount of 100-500% of the moles of the unreacted carbon in the silicon carbide-silicon dioxide-carbon composite powders, and the silicon metal may be silicon metal powders or a silicon metal piece having a purity of 99.9999 wt % or higher. The SiCSiO.sub.2C composite powders and the silicon metal are uniformly mixed and loaded in a high-purity graphite crucible with a high packing density. After the synthesis reaction, the weight of the synthesized SiC powders may be 90% or greater than the weight of the SiCSiO.sub.2C composite powders.
(25) In an exemplary embodiment of the present invention, the SiC powders may be -phase SiC powders having an average particle size of 30-200 m, with a uniform particle size with a particle size distribution (d.sub.90/d.sub.10) of 5.0 or smaller, and containing 1 ppm or less of metal impurities in the synthesized SiC powders.
(26) In accordance with the present invention, ultrahigh-purity SiC granular powders having various particle sizes and a narrow particle size distribution can be synthesized with a high yield.
(27) As described above, because a liquid state silicon compound and a solid or liquid state carbon compound are used as starting source materials in the method for preparing SiC powders according to the present invention, the starting source materials can be mixed easily without contaminants. In particular, when liquid state starting source materials are used, inclusion of impurities can be prevented during the mixing of the source materials.
(28) And, in accordance with the present invention, ultrahigh-purity SiC granular powders may be prepared using starting source materials having various purities by changing the heat treatment condition.
(29) In the present invention, since, as the starting source materials, the solid carbon compound used as being dissolved in ethanol or methanol or the liquid carbon compound is mixed with the liquid state silicon compound, the carbon compound and the silicon compound can be uniformly mixed in the prepared gel and a high-purity SiO.sub.2C composite wherein the silicon source and the carbon source are uniformly distributed can be prepared through carbonization
(30) Also, in accordance with the present invention, ultrahigh-purity SiC granular powders can be synthesized with a high yield using 60% or greater, specifically 70% or greater, of the volume of a heating zone in a graphite reactor by loading the prepared SiCSiO.sub.2C composite powders together with a silicon metal in a graphite crucible with a high packing density, and then conducting both the carbothermal reduction and direct reaction among the silicon metal, silicon dioxide, and carbon at the same time.
(31) Accordingly, the production cost of the ultrahigh-purity SiC granular powders can be decreased.
(32) The SiC granular powders synthesized according to the present invention have a narrow particle size distribution. In addition, since high-purity SiC granular powders of 100 m or greater can be synthesized at temperatures of 1800 C. or below, although the synthesis of high-purity SiC granular powders was possible only at temperatures of 2200 C. or higher with the existing methods, the operation life time of a high-purity graphite heating element and a graphite insulator in a vacuum graphite furnace can be extended
(33) In addition, in accordance with the present invention, high-purity -phase SiC granular powders of 6N (99.9999 wt. %) or higher may be manufactured by using starting source materials with lower purity than 6N (99.9999 wt. %).
EXAMPLES
(34) The present invention will be described in more detail through examples. However, the scope of this invention is not limited by the examples.
Example 1
(35) For preparation of ultrahigh-purity SiC powders, tetraethyl orthosilicate (TEOS) containing 10 ppm or less of metal impurities was used as a liquid silicon source and a solid phenol resin (novolac-type) containing 100 ppm of metal impurities was used as a carbon source. The amount of the phenol resin was determined such that the C/Si molar ratio based on the silicon source in TEOS was 2.3-3.0 in consideration of the carbon remaining after heat treatment. The phenol resin was dissolved in 4 mol of ethanol based on the silicon element in a Teflon-coated container and stirred at room temperature at 300 RPM using a magnetic stirrer after adding 1 mol of TEOS. The sufficiently mixed TEOS-phenol resin mixture was gelated at room temperature by adding an aqueous nitric acid solution of 0.07 mol of nitric acid and 4 mol of water based on the silicon element. The prepared gel was pulverized in a Teflon container and then dried at 100 C. for 24 hours.
(36) The dried gel was first put in a high-purity graphite crucible, next loaded in a quartz reactor, then heat-treated under a nitrogen gas atmosphere at 900 C. for 3 hours by heating at a rate of 5 C./min to prepare a silicon dioxide-carbon (SiO.sub.2C) composite.
(37) The prepared SiO.sub.2C composite was put in a graphite crucible, next was loaded in a graphite furnace, and then underwent carbothermal reduction during the heat treatment at a rate of 10 C./min under a vacuum atmosphere (10.sup.2 torr) and maintaining temperature at 1400 C. for 2 hours. Then, additional carbothermal reduction was conducted by heating at a rate of 20 C./min and maintaining temperature at 1500 C. for 3 hours to prepare silicon carbide-silicon dioxide-carbon (SiCSiO.sub.2C) composite powders.
(38) The prepared SiCSiO.sub.2C composite powders and silicon metal powders (average particle size: 5 mm, purity: 99.9999 wt % or higher) were uniformly mixed. The mixed powders were put in a high-purity graphite crucible with a high packing density, and then heated to 16001800 C. at the rate of 2 to 10 C./min and maintained at the temperature for 3 hours under vacuum atmosphere to synthesize high purity SiC granular powders. Here, the amount of silicon metal powders, which were mixed into the prepared SiCSiO.sub.2C composite powders, corresponds to a molar ratio of 120-300% based on the moles of unreacted carbon in the SiCSiO.sub.2C composite powders.
Examples 2-4
(39) The C/Si molar ratio of the starting source materials used in the synthesis of the SiC powders in Example 1 was changed to 1.6, 2.5 and 3.0, respectively. To prepare a SiCSiO.sub.2C composite powders, the SiO.sub.2C composite was maintained at 1400 C. for 2 hours and then underwent the carbothermal reduction at 1500 C. for 3 hours by heating at a rate of 20 C./min. After adding a silicon metal corresponding to a molar ratio of 150% based on the moles of unreacted carbon in the SiCSiO.sub.2C composite powders, ultrahigh-purity SiC powders were synthesized by heating at a rate of 10 C./min and maintaining temperature at 1600 C. for 3 hours.
(40) The average particle size of the prepared SiC powders increased from 30 m, to 40 m and 55 m as the C/Si molar ratio in the starting source materials increased from 1.6 to 2.5 and 3.0 respectively. A relatively broad particle size distribution (d.sub.90/d.sub.10) of 3.5-3.9 was obtained for the synthesized SiC powders. XRD analysis revealed that the synthesized SiC powders were crystalline phase SiC. As the C/Si molar ratio increased from 1.6 to 3.0, the synthesis yield increased from 91% to 96% based on the weight of the SiCSiO.sub.2C composite powders. The purity of the synthesized SiC powders was analyzed by GDMS. The synthesized SiC powders had a purity of 99.9999-99.99994 wt %.
Examples 5-6
(41) The C/Si molar ratio of the starting source materials used in the synthesis of the SiC powders in Example 1 was changed to 2.3 and 3.0, respectively. To prepare SiCSiO.sub.2C composite powders, the SiO.sub.2C composite was maintained at 1400 C. for 2 hours and then subjected to carbothermal reduction at 1500 C. for 3 hours by heating at a rate of 20 C./min under vacuum atmosphere. After adding a silicon metal, which corresponds to a molar ratio of 200% based on the moles of unreacted carbon in the SiCSiO.sub.2C composite powders, ultrahigh-purity SiC powders were synthesized by heating at a rate of 10 C./min and maintaining temperature at 1700 C. for 3 hours under vacuum atmosphere.
(42) When observed by SEM, the synthesized SiC powders had a cubic shape. When the C/Si molar ratio of the starting source materials was 2.3, the average particle size and the particle size distribution of the synthesized SiC powders were 60 m and 3.2, respectively. When the C/Si molar ratio of the starting source materials was 3.0, the average particle size and the particle size distribution of the synthesized SiC powders were changed to 75 m and 2.8, respectively. And, the synthesis yield was 92% and 95% based on the weight of the SiCSiO.sub.2C composite powders when the C/Si molar ratio was 2.3 and 3.0, respectively. The purity of the synthesized SiC powders was analyzed by GDMS. The synthesized SiC powders had a purity of 99.99992-99.99996 wt %.
Example 7
(43) The C/Si molar ratio of the starting source materials used in the synthesis of the SiC powders in Example 1 was fixed to 3.0. To prepare SiCSiO.sub.2C composite powders, the SiO.sub.2C composite was maintained at 1400 C. for 2 hours and then underwent carbothermal reduction at 1500 C. for 3 hours by heating at a rate of 20 C./min under vacuum atmosphere. After adding a silicon metal corresponding to a molar ratio of 250% based on the moles of unreacted carbon in the SiCSiO.sub.2C composite powders, ultrahigh-purity SiC powders were synthesized by heating at a rate of 10 C./min and maintaining temperature at 1800 C. for 3 hours under vacuum atmosphere. After etching the residual silicon metal using a mixture of hydrofluoric acid and nitric acid, the synthesized SiC powders were washed with water and dried.
(44) The average particle size and the particle size distribution of the synthesized SiC powders were 110 m and 2.5, respectively, and the synthesis yield was 95% based on the weight of the SiCSiO.sub.2C composite powders. The purity of the synthesized SiC powders was analyzed by GDMS. The synthesized SiC powders had a purity of 99.99999-99.999995 wt %.
Example 8
(45) The C/Si molar ratio of the starting source materials used in the synthesis of the SiC powders in Example 1 was fixed to 3.0. To prepare SiCSiO.sub.2C composite powders, the SiO.sub.2C composite was maintained at 1400 C. for 2 hours and then subjected to carbothermal reduction at 1500 C. for 3 hours by heating at a rate of 20 C./min under vacuum atmosphere. After adding a silicon metal corresponding to a molar ratio of 250% based on the moles of unreacted carbon in the SiCSiO.sub.2C composite powders, ultrahigh-purity SiC powders were synthesized by heating at a rate of 2 C./min and maintaining temperature at 1800 C. for 3 hours under vacuum atmosphere. After etching the residual silicon metal using a mixture of hydrofluoric acid and nitric acid, the synthesized SiC powders was washed with water and dried.
(46) The average particle size and the particle size distribution of the synthesized SiC powders were 140 m and 2.3, respectively, and the synthesis yield was 96% based on the weight of the SiCSiO.sub.2C composite powders. The purity of the synthesized SiC powders was analyzed by GDMS. The synthesized SiC powders had a purity of 99.999992-99.999995 wt %.
Example 9
(47) For preparation of ultrahigh-purity SiC powders, polyethyl silicate containing 1000 ppm of metal impurities was used as a liquid silicon source and a solid phenol resin (novolac-type) containing 100 ppm of metal impurities was used as a carbon source. The C/Si molar ratio of the starting source materials was 3.0. A silicon dioxide-carbon (SiO.sub.2C) composite wherein the silicon source and the carbon are uniformly distributed was prepared in the same manner as in Example 1.
(48) The prepared SiO.sub.2C composite was put in a graphite crucible and, after loading in a graphite furnace, carbothermal reduction was conducted by heating at a rate of 10 C./min under a vacuum atmosphere (10.sup.2 torr) and maintaining temperature at 1400 C. for 4 hours. And then, additional carbothermal reduction was conducted by heating at a rate of 20 C./min and maintaining temperature at 1500 or 1600 C. for 3 hours to prepare silicon carbide-silicon dioxide-carbon (SiCSiO.sub.2C) composite powders.
(49) The prepared SiCSiO.sub.2C composite powders and silicon metal powders (average particle size: 5 mm, purity: 99.9999 wt % or higher) were uniformly mixed. The mixed powders were put in a high-purity graphite crucible with a high packing density, and then heated to 1600 to 1800 C. at the rate of 2 to 10 C./min, and maintained at the temperature for 3 hours under vacuum atmosphere to synthesize high purity SiC granular powders. Here, the amount of silicon metal powders, which were mixed into the prepared SiCSiO.sub.2C composite powders, corresponds to a molar ratio of 200% based on the moles of unreacted carbon in the SiCSiO.sub.2C composite powders.
Example 10
(50) To prepare SiCSiO.sub.2C composite powders, the SiO.sub.2C composite of Example 9 was maintained at 1400 C. for 4 hours and then underwent carbothermal reduction at 1500 C. for 3 hours by heating at a rate of 20 C./min. Ultrahigh-purity SiC powders were synthesized by heating at a rate of 10 C./min and maintaining temperature at 1800 C. for 3 hours.
(51) After first carbothermal reduction and second carbothermal reduction was conducted at 1400 C. and 1500 C. respectively, the prepared SiCSiO.sub.2C composite powders were mixed with silicon metal powders (average particle size: 5 mm, purity: 99.9999 wt % or higher), which correspond to a molar ratio of 200% based on the moles of unreacted carbon in the SiCSiO.sub.2C composite powders. The mixture of SiCSiO.sub.2C composite and the silicon metal was put in a high-purity graphite crucible, and then heated to 1800 C. at the rate of 10 C./min and maintained at the temperature for 3 hours to synthesize high purity SiC granular powders.
(52) The synthesis yield was 96% based on the weight of the SiCSiO.sub.2C composite powders. The average particle size and the particle size distribution of the synthesized SiC powders were 90 m and 3.2, respectively. The purity of the synthesized SiC granular powders was analyzed by GDMS. The synthesized SiC powders had a purity of 99.99992 wt %.
Example 11
(53) To prepare SiCSiO.sub.2C composite powders, the SiO.sub.2C composite of Example 9 was maintained at 1400 C. for 4 hours and then subjected to carbothermal reduction at 1600 C. for 3 hours by heating at a rate of 20 C./min. Ultrahigh-purity SiC powders were synthesized by adding a silicon metal to the prepared SiCSiO.sub.2C composite powders and heating at a rate of 10 C./min and maintaining temperature at 1800 C. for 3 hours under vacuum atmosphere.
(54) After first carbothermal reduction, and second carbothermal reduction was conducted at 1400 C. and 1600 C. respectively. The prepared SiCSiO.sub.2C composite powders and silicon metal powders (average particle size: 5 mm, purity: 99.9999 wt % or higher) were uniformly mixed. The mixed powders were put in a high-purity graphite crucible with a high packing density, and then heated to 1600 to 1800 C. at the rate of 2 to 10 C./min, and maintained at the temperature for 3 hours under vacuum atmosphere to synthesize high purity SiC granular powders. Here, the amount of silicon metal powders, which were mixed to the prepared SiCSiO.sub.2C composite powders, corresponds to a molar ratio of 200% based on the moles of unreacted carbon in the SiCSiO.sub.2C composite powders.
(55) The synthesis yield was 96% based on the weight of the SiCSiO.sub.2C composite powders. The average particle size and the particle size distribution of the synthesized SiC powders were 110 m and 2.4, respectively. The purity of the synthesized SiC granular powders was analyzed by GDMS. The synthesized SiC powders had a purity of 99.99997 wt %.
Example 12
(56) For preparation of ultrahigh-purity SiC powders, tetraethyl orthosilicate (TEOS) having a purity of 99.99% was used as a liquid silicon source and a solid phenol resin (novolac-type) having a purity of 99% was used as a carbon source. The C/Si molar ratio of the starting source materials was 3.0. A silicon dioxide-carbon (SiO.sub.2C) composite wherein the silicon source and the carbon are uniformly distributed was prepared in the same manner as in Example 1.
(57) The prepared SiO.sub.2C composite was put in a graphite crucible and loaded in a graphite furnace, and then carbothermal reduction was conducted by heating at a rate of 10 C./min under a vacuum atmosphere (10.sup.2 torr) and maintaining temperature at 1400 C. for 4 hours. Then, additional carbothermal reduction was conducted by heating at a rate of 20 C./min and maintaining temperature at 1700 C. for 8 hours to prepare silicon carbide-silicon dioxide-carbon (SiCSiO.sub.2C) composite powders.
(58) The prepared SiCSiO.sub.2C composite powders were loaded in a graphite crucible after adding silicon metal powders (average particle size=5 mm, purity=99.9999 wt % or higher), which correspond to a molar ratio of 200% based on the moles of unreacted carbon in the prepared SiCSiO.sub.2C composite powders, and ultrahigh-purity SiC granular powders was synthesized by heating at a rate of 10 C./min and maintaining temperature at 1800 C. for 3 hours.
(59) The synthesis yield of the prepared SiC powders was 96% based on the weight of the SiCSiO.sub.2C composite powders. The average particle size was 90 m and the particle size distribution was relatively broad as 4.6. The purity of the synthesized SiC granular powders was analyzed by GDMS. The synthesized SiC powders had a purity of 99.9999 wt %.
Example 13
(60) For preparation of ultrahigh-purity SiC powders, tetraethyl orthosilicate (TEOS) having a purity of 99% was used as a liquid state silicon source and a solid phenol resin (novolac-type) having a purity of 99.99% was used as a solid state carbon source. The C/Si molar ratio of the starting source materials was 3.0. A silicon dioxide-carbon (SiO.sub.2C) composite wherein the silicon source and the carbon are uniformly distributed was prepared in the same manner as in Example 1.
(61) The prepared SiO.sub.2C composite was put in a graphite crucible and loaded in a graphite furnace, carbothermal reduction was conducted by heating at a rate of 10 C./min under a vacuum atmosphere (10.sup.2 torr) and maintaining temperature at 1400 C. for 4 hours. Then, additional carbothermal reduction was conducted by heating at a rate of 20 C./min and maintaining temperature at 1700 C. for 9 hours to prepare silicon carbide-silicon dioxide-carbon (SiCSiO.sub.2C) composite powders.
(62) The prepared SiCSiO.sub.2C composite powders were loaded in a graphite crucible after adding silicon metal powders (average particle size=5 mm, purity=99.9999 wt % or higher), which correspond to a molar ratio of 200% based on the moles of unreacted carbon in the prepared SiCSiO.sub.2C composite powders, and ultrahigh-purity SiC granular powders were synthesized by heating at a rate of 10 C./min and maintaining temperature at 1800 C. for 3 hours.
(63) The synthesis yield of the prepared SiC powders was 97% based on the weight of the SiCSiO.sub.2C composite powders. The average particle size was 95 m and the particle size distribution (d.sub.90/d.sub.10) was relatively broad as 4.5. The purity of the synthesized SiC granular powders was analyzed by GDMS. The synthesized SiC granular powders had a purity of 99.99993 wt %.
Example 14
(64) For preparation of ultrahigh-purity SiC powders, tetraethyl orthosilicate (TEOS) containing 10 ppm or less of metal impurities was used as a liquid silicon source and a solid phenol resin (novolac-type) containing 1 ppm of metal impurities was used as a carbon source. The C/Si molar ratio of the starting source materials was 3.0. A silicon dioxide-carbon (SiO.sub.2C) composite wherein the silicon source and the carbon are uniformly distributed was prepared in the same manner as in Example 1.
(65) The prepared SiO.sub.2C composite was put in a graphite crucible and, after loading in a graphite furnace, carbothermal reduction was conducted by heating at a rate of 10 C./min under a vacuum atmosphere (10.sup.2 torr) and maintaining temperature at 1400 C. for 1 hour. Then, additional carbothermal reduction was conducted by heating at a rate of 20 C./min and maintaining temperature at 1600 C. for 3 hours to prepare silicon carbide-silicon dioxide-carbon (SiCSiO.sub.2C) composite powders.
(66) The prepared SiCSiO.sub.2C composite powders were loaded in a graphite crucible after adding silicon metal powders (average particle size=5 mm, purity=99.9999 wt % or higher), which correspond to a molar ratio of 200% based on the moles of unreacted carbon in the prepared SiCSiO.sub.2C composite powders, and ultrahigh-purity SiC granular powders were synthesized by heating at a rate of 2 C./min and maintaining temperature at 1800 C. for 3 hours.
(67) The synthesis yield of the prepared SiC powders was 98% based on the weight of the SiCSiO.sub.2C composite powders. The average particle size was 140 m and the particle size distribution (d.sub.90/d.sub.10) was 3.0. The purity of the synthesized SiC granular powders was analyzed by GDMS. The synthesized SiC powders had a purity of 99.999995-99.999997 wt %.
Test Example
(68) The X-ray diffraction pattern of the ultrahigh-purity SiC granular powders prepared in Example 6 from the silicon dioxide-carbon (SiO.sub.2C) composite prepared with the C/Si molar ratio of the starting source materials controlled to be 3.0 was analyzed. The result is shown in
(69) Also, the morphology of the ultrahigh-purity SiC granular powders synthesized in Example 6 was imaged using an electron microscope (200). From the SEM image shown in
(70) The ultrahigh-purity SiC powders prepared by the method for preparing SiC powders according to the present invention can be widely used as a core material for the preparation of high-purity SiC processing components for next-generation semiconductors and LEDs and for the preparation of SiC single crystals for power semiconductors.