Method for producing high-purity silicon carbide powder

Abstract

The present invention relates to a method for producing high-purity silicon carbide powder, by which high-purity silicon carbide powder can be produced economically and efficiently. This is achieved by: filling a starch-based packaging chip or expanded starch as an organic raw material into a container with an open top; placing the container filled with the raw material into a furnace, and gradually heating the packaging chip or expanded starch to a temperature of 2000°C, while adding an inert gas or graphitizing the packaging chip and expanded starch into a porous graphite sheet under vacuum; adding a halogen gas (such as chlorine or fluorine) to the furnace at a temperature of >1800°C to purify the porous graphite sheet to remove impurity metals from the porous graphite sheet by forming metal chlorides, and converting the porous graphite sheet into silicon carbide powder by adding SiO with argon as a carrier gas at a pressure of 30 mbar or higher at a temperature of >1200°C.

Description

Technical Field

[0001] The invention relates to a method for producing high-purity silicon carbide powder (SiC powder) for producing sintered products made of silicon carbide or for producing electronic components. Background Art

[0002] As is known, silicon carbide can be produced by sublimation and epitaxial growth, which can be carried out from the gas phase without a seed crystal. To this end, silicon carbide is grown epitaxially from a vapor rich in carbon and silicon, resulting in a silicon carbide single crystal. The process is carried out at a temperature of about 2500 ° C.

[0003] EP 0 403 887 A1 describes a method for producing single-crystalline silicon by sublimation and partial decomposition of crystalline SiC powder and growth on seeds in a reaction vessel under a low temperature gradient and protective gas, wherein an excess of silicon is provided in the SiC powder (for example by addition of elemental silicon) to achieve crystal growth.

[0004] The realization of such a process is rather complex and the silicon carbide powder produced in this way may still contain traces of impurities (e.g. heavy metal silicides or carbides) which are an obstacle to use in, for example, electronic components, which means that, if possible, the starting material or the produced silicon carbide powder has to be subjected to a supplementary chemical cleaning.

[0005] The powdered silicon carbide may then be formed into a shaped article by hot pressing, or used in some other manner, in which case the silicon carbide powder may also be mixed with a boron-containing additive to achieve increased oxidation or corrosion resistance.

[0006] DE 690 19 339 T2 discloses a method for producing silicon carbide by carbothermal reduction. To this end, a dispersed reaction mixture of a silicon dioxide source and a carbon source is heated to a sufficiently high temperature via a heating zone with a heating rate of at least 100° C. / s, after burning off excess carbon and treating with hydrofluoric acid to remove excess silicon dioxide, thereby forming a product consisting of at least 80% by weight of silicon carbide crystals having a specific size distribution.

[0007] The carbon source used is carbon black, acetylene soot, carbohydrates or starch, and the silica source used is silicon dioxide, quartz dust, colloidal silica.

[0008] DE 195 37 430 A1 relates to a method for producing high-purity silicon carbide powder for producing silicon carbide single crystals, the method comprising the steps of forming silicon carbide, which is produced by calcining tetraalkoxysilane, tetraalkoxysilane polymers and silicon dioxide and carbon starting materials in the form of high-purity organic compounds in a non-oxidizing environment, and then performing a high-temperature treatment to obtain silicon carbide powder.

[0009] DE 697 04 638 T2 describes a method for producing a sintered product made of silicon carbide. The sintered product is prepared by heating to 2000° C. to 2400° C. under high pressure and in a non-oxidizing atmosphere to sinter a homogeneous mixture consisting of a silicon source containing at least one liquid silicon source, a carbon source containing at least one liquid organic compound and a polymerization or crosslinking catalyst.

[0010] Furthermore, DE 10 2008 042 499 A1 describes a method for producing high-purity silicon carbide from carbohydrates and silicon oxide by calcination. For this purpose, silicon carbide, carbon and / or silicon oxide are reacted at low temperatures of 400° C. to 1400° C. in a first pyrolysis step and calcined at high temperatures of up to 3000° C., whereby high-purity silicon carbide is formed by passive oxidation post-treatment at a temperature of about 800° C. Summary of the invention

[0011] It is therefore an object of the present invention to provide a method for producing high-purity silicon carbide powder in an economical and efficient manner.

[0012] The object of the present invention is achieved by:

[0013] - Filling starch-based packaging chips or expanded starch as organic raw materials into open-top containers,

[0014] - placing the filled container in a furnace and gradually heating the packaging chips or expanded starch to a temperature of 2000° C. under the addition of a protective gas or under vacuum to graphitize the packaging chips or expanded starch into porous graphite sheets,

[0015] - adding halogen gas to the furnace at a temperature of >1800°C to clean the porous graphite sheet for removing impurity metals from the porous graphite sheet by forming metal chlorides, and

[0016] - Conversion of porous graphite sheets into powdered silicon carbide by sublimation of powdered SiO into the gas phase at temperatures >1200°C and pressures of 30 mbar or more with argon as carrier gas.

[0017] The particular advantages of the process according to the invention can be considered to be, firstly, the ability to use readily available organic raw materials as starting materials. These are light green packaging chips, which are made from corn starch or potato starch, PVA adhesive, grease and water by extrusion and which, after using them as packaging material (which would otherwise be waste), can also be used to produce high-purity fine-grained silicon carbide powder.

[0018] Alternatively, corn starch or potato starch, either pure or already mixed with a PVA binder and already swollen, can also be used as the organic raw material.

[0019] In a first development of the invention, the conversion to silicon carbide is effected at a temperature of 1520° C.

[0020] The conversion to silicon carbide can also be achieved at a pressure of 950 mbar, the pressure used so far having a major influence on the uniformity and speed of the conversion.

[0021] The conversion to silicon carbide is achieved in a long-running process that can last 50 to 100 hours, depending on the size of the furnace and the amount of material being converted.

[0022] Finally, prior to graphitization, a stabilization and homogenization operation may be carried out at a stabilization temperature ranging from 140° C. to no more than 450° C., preferably a temperature of 250° C., in order to allow degassing of volatile substances.

[0023] Stabilization and homogenization of the prefabricated shaped article can be achieved while heating the furnace to a stabilization temperature.

[0024] If the SiC powder contains excess carbon, the excess carbon can be oxidized, ie burned, at >500°C while adding oxygen.

[0025] Furthermore, excess silicon in the SiC powder can be removed, for example, by treatment with hydrofluoric acid (HF) or ammonium fluoride (NH4F).

[0026] In a specific embodiment of the invention, less than 100% of the porous graphite flakes are converted to SiC powder, and the residual carbon is subsequently oxidized at >500°C with the addition of oxygen, thereby retaining pure SiC. DETAILED DESCRIPTION

[0027] The present invention will now be described in more detail with reference to working examples.

[0028] The starting material for producing high-purity silicon carbide powder is preferably a commercially available organic packaging chip, which has been formed based on starch (preferably corn starch or potato starch), mixed with PVA (polyvinyl alcohol) or VA (vinyl alcohol) as a binder and with water and grease to form a paste, and produced by extrusion using a conventional extruder. Therefore, the starting material is economically produced and is a purely organic starting material, and the color of the organic packaging chip is light green, white or yellow so that they can be distinguished from polystyrene chips.

[0029] These packaging chips are filled as organic raw materials into a flat, open-top container, which is then placed in a furnace and gradually heated to a temperature of 2000° C. under the addition of a protective gas or under vacuum to graphitize the packaging chips. Porous graphite sheets are formed from the packaging chips, while volatile components of the packaging chips (e.g., water) escape simultaneously.

[0030] Halogen gas (primarily chlorine or fluorine) is then introduced into the furnace to clean the porous graphite sheet at temperatures >1800°C, thereby removing impurity metals from the porous graphite sheet by forming metal chlorides.

[0031] The graphitization and cleaning process may take several hours, and heating the furnace to the graphitization temperature should be done in steps.

[0032] After the cleaning process is completed, the porous graphite sheets in the furnace are converted into powdered silicon carbide by adding SiO with argon as a carrier gas at a temperature of >1200°C at a pressure of 30 mbar or higher.

[0033] For this purpose, before the conversion to silicon carbide, powdered SiO is introduced into a furnace, where it sublimes (ie becomes gaseous) at >1200° C., with the simultaneous formation of CO (carbon monoxide) during the conversion of carbon into SiC.

[0034] The conversion to silicon carbide is achieved in a long-running process that can last for several hours. Depending on the size of the furnace and the amount of material being converted, the process can last from 50 to 100 hours.

[0035] It should be understood that starch (such as corn starch or potato starch) mixed with a PVA binder can also be used as an organic starting material in place of an organic packaging chip. This porous organic starting material can then be further processed to form silicon carbide as described above.

[0036] The conversion to silicon carbide is preferably effected at a temperature of 1520° C. and a pressure of 950 mbar.

[0037] If free silicon or carbon is still present after conversion to SiC, the powder can be post-treated at temperatures >500°C so that the free carbon is oxidized by oxygen. If the free silicon content is too high, post-treatment can also be performed by etching with hydrofluoric acid or ammonium fluoride.

[0038] Another method of producing pure SiC is to first convert less than 100% of porous graphite flakes into SiC powder and then remove the residual carbon by oxidation.

[0039] However, this will involve the use of a suitable furnace that is clean and oxygen resistant.

[0040] In a particular variant of the invention, less than 100% of the porous graphite flakes are converted to SiC powder and the remaining residual carbon is then oxidized at temperatures >500° C. with the addition of oxygen, thereby retaining pure SiC.

Claims

1. A method for producing high-purity silicon carbide powder, characterized in that: Filling starch-based packaging chips or expanded starch as an organic raw material into an open-top container; Placing the container filled with the raw materials in a furnace, and gradually heating the packaging chips or expanded starch to a temperature of 2000° C. under the addition of a protective gas or under vacuum to graphitize the packaging chips or expanded starch into porous graphite sheets; adding halogen gas to the furnace at a temperature >1800°C to clean the porous graphite sheet for removing impurity metals from the porous graphite sheet by forming metal chlorides, and The porous graphite sheets are converted into powdered silicon carbide by adding SiO with argon as a carrier gas at a temperature >1200°C at a pressure of 30 mbar or higher.

2. The method according to claim 1, characterized in that The organic raw materials used are pure corn starch or corn starch mixed with PVA binder.

3. The method according to claim 1, characterized in that: The conversion to silicon carbide is achieved at a temperature of 1520°C.

4. The method according to claim 1, characterized in that: The conversion to silicon carbide was carried out at a pressure of 950 mbar.

5. The method according to claim 1, characterized in that The conversion to silicon carbide is achieved in a long-running process.

6. The method according to claim 1, characterized in that Heating the furnace to the graphitization temperature is achieved in steps.

7. The method according to any one of claims 1 to 6, characterized in that The SiC powder is oxidized at >500°C with the addition of oxygen to remove excess carbon.

8. The method according to any one of claims 1 to 6, characterized in that SiC powder is treated with hydrofluoric acid (HF) or ammonium fluoride (NH4F) to remove excess silicon.

9. The method according to any one of claims 1 to 6, characterized in that Less than 100% of the porous graphite flakes are converted to SiC powder and the residual carbon is then oxidized at >500°C while adding oxygen.