Method for producing high purity silicon carbide powder

Abstract

The present invention relates to a method for producing high purity silicon carbide powder, which allows for cost-effective production of high purity silicon carbide powder. This is achieved by filling starch-based packaging chips or expanded starch as organic raw material into an open-top container; placing the container filled with raw material in an oven and gradually heating the packaging chips or expanded starch to a temperature of 2000°C while supplying inert gas or in vacuum, graphitizing the packaging chips or expanded starch into porous graphite flakes; supplying halogen gas such as chlorine or fluorine into the oven to purify the porous graphite flakes at a temperature above 1800°C, forming metal chlorides and removing foreign matter from the porous graphite flakes; converting the porous graphite flakes into powdered silicon carbide by supplying SiO with argon as carrier gas at a temperature above 1,200°C and a pressure of 30 mbar or more.

Description

[Technical Field]

[0001] The present invention relates to a method for producing high purity silicon carbide powder (SiC powder) for use in the manufacture of sintered articles made of silicon carbide or in the manufacture of electronic components. [Background technology]

[0002] It is known that silicon carbide can be produced by sublimation growth from the gas phase without a seed crystal. For this purpose, silicon carbide is grown from a vapor rich in carbon and silicon, so that a single crystal of silicon carbide is formed. This process is carried out at a temperature of about 2500 °C.

[0003] EP 0 403 887 A1 describes a method for producing monocrystalline silicon carbide by sublimation and partial decomposition of crystalline SiC powder and growth on a seed crystal in a reaction vessel under inert gas and a low temperature gradient, in which excess silicon is established in the SiC powder for crystal growth, e.g. by adding elemental silicon.

[0004] The implementation of such a process is rather complicated, and the silicon carbide powder produced in this way may still contain traces of impurities such as heavy metal silicides and carbides, making it unusable for example in electronic components, so that, if possible, the starting material and the produced silicon carbide powder must be subjected to additional chemical cleaning.

[0005] The powdered silicon carbide can then be formed into a compact by hot pressing or otherwise used, whereby boron-containing additives can be mixed into the silicon carbide powder to enhance oxidation or corrosion resistance.

[0006] DE 69019339T2 describes a method for producing silicon carbide by carbothermal reduction. For this purpose, a dispersed reactive mixture of a silicon dioxide source and a carbon source is heated to a sufficiently high temperature in a heating zone at a heating rate of at least 100°C / s, thereby burning off excess carbon and forming a product consisting of at least 80% by weight of silicon carbide crystals having a predetermined size distribution, after treatment with hydrofluoric acid to remove excess silicon dioxide.

[0007] The carbon source is carbon black, acetylene black, a carbohydrate or a starch, and the silicon dioxide source is silicon dioxide, quartz dust or colloidal silicon dioxide.

[0008] DE 19537430 A1 relates to a method for producing high purity silicon carbide powder for producing silicon carbide single crystals, comprising the steps of forming silicon carbide prepared by calcining tetraalkoxysilane, tetraalkoxysilane polymer and silicon dioxide with a carbon starting material in the form of a high purity organic compound in an oxidation-free environment, followed by high temperature treatment to obtain silicon carbide powder.

[0009] DE 69704638T2 describes a method for producing sintered bodies from silicon carbide by heating a homogeneous mixture 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 to 2000-2400°C under high pressure in a non-oxidizing atmosphere.

[0010] Furthermore, DE 10 2008 042 499 A1 describes a method for producing high-purity silicon carbide from carbohydrates and silicon oxide by calcination. To this end, silicon carbide, carbon and / or silicon oxide are reacted in an initial pyrolysis step at low temperatures between 400 and 1400 °C, followed by calcination at high temperatures up to 3000 °C, followed by post-treatment by passive oxidation at temperatures of about 800 °C to produce high-purity silicon carbide. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] EP0403887A1 [Patent Document 2] DE69019339T2 [Patent Document 3] DE19537430A1 [Patent Document 4] DE69704638T2 [Patent Document 5] DE102008042499A1 Summary of the Invention [Problem to be solved by the invention]

[0012] Therefore, the present invention aims to develop a cost-effective method for producing high purity silicon carbide powder.

[0013] The problem underlying the present invention is solved by: - filling starch-based packaging chips or expanded starch as organic raw material into open-top containers; - placing the filled container in an oven and gradually heating the packaging chips or expanded starch in an inert gas supply or under vacuum to a temperature of 2000 ° C, thereby graphitizing the packaging chips or expanded starch into porous graphite flakes; - Supplying halogen gas into the oven to clean the porous graphite flakes at a temperature exceeding 1800°C, forming metal chlorides and removing foreign matter from the porous graphite flakes. Conversion of porous graphite flakes into powdered silicon carbide by sublimation of powdered SiO in the gas phase with argon as carrier gas at temperatures above -1,200 °C and pressures above 30 mbar.

[0014] A particular advantage of the process according to the invention is, on the one hand, the ability to use readily available organic raw materials as starting materials, such as light green packaging chips made by extrusion from corn or potato starch, PVA glue, tallow and water, which, after use as packaging material, would otherwise become waste, can also be used to produce high-purity, finely divided silicon carbide powder.

[0015] Alternatively, pure corn or potato starch, or a mixture of expanded corn or potato starch mixed with PVA glue, can be used as organic ingredients.

[0016] In a first embodiment of the invention, the conversion to silicon carbide is carried out at a temperature of 1520°C.

[0017] Conversion to silicon carbide can be carried out at a pressure of 950 mbar, the actual pressure used mainly affecting the homogeneity and rate of conversion.

[0018] The conversion to silicon carbide is a lengthy process that can take anywhere from 50 to 100 hours, depending on the size of the oven and the amount of material being converted.

[0019] Finally, graphitization can be preceded by a stabilization and homogenization step at a stabilization temperature of 140°C up to 450°C (preferably 250°C) to allow for outgassing of volatiles.

[0020] Stabilization and homogenization of the preformed part can be carried out while the oven is heating up to the stabilization temperature.

[0021] If the SiC powder contains excess carbon, it can be oxidized, i.e., burned, at temperatures above 500°C by adding oxygen.

[0022] Additionally, excess silicon in SiC powder can be removed by treatment with, for example, hydrofluoric acid (HF) or ammonium fluoride (NH4F).

[0023] In certain embodiments of the invention, less than 100% of the porous graphite flakes are converted to SiC powder, and the remaining carbon is oxidized by adding oxygen at above 500° C., leaving pure SiC.

[0024] The present invention will be described in more detail below using exemplary embodiments.

[0025] To produce high-purity silicon carbide powder, commercially available organic packaging chips are preferably used as the starting material. These are based on starch, preferably corn starch or potato starch, and are mixed with PVA (polyvinyl alcohol) or VA (vinyl alcohol) as an adhesive, water, and animal fat, formed into a paste, and extruded using a conventional extruder. Therefore, they are a cost-effective starting material produced on a purely organic basis, and organic packaging chips are colored slightly green, white, or yellow to distinguish them from polystyrene chips.

[0026] These packaging chips are packed as organic raw materials into flat, open-top containers, which are then placed in an oven and gradually heated to a temperature of 2000°C under inert gas or vacuum to graphitize the packaging chips, which produces porous graphite flakes from the packaging chips while simultaneously evaporating volatile components, such as moisture, from the packaging chips.

[0027] Thereafter, halogen gas, mainly consisting of chlorine or fluorine, is introduced into the oven and the porous graphite pieces are washed at a temperature exceeding 1800°C, thereby removing foreign matter from the porous graphite pieces through the formation of metal chlorides.

[0028] This graphitization and cleaning process can take several hours, and the oven must be heated in stages to the graphitization temperature.

[0029] After the cleaning process is complete, the porous graphite flakes are converted into powdered silicon carbide by heating the SiO2 in an oven with argon as the carrier gas at a temperature of 1200°C or higher and a pressure of 30 mbar or higher.

[0030] For this reason, powdered SiO is introduced into the oven before being converted into silicon carbide, and this SiO is sublimated, i.e., turned into gas (gaseous), at temperatures above 1,200°C, converting the carbon into SiC while simultaneously forming CO (carbon monoxide).

[0031] The conversion to silicon carbide is a lengthy process that takes several hours, and can take anywhere from 50 to 100 hours depending on the size of the oven and the amount of material being converted.

[0032] It is understood that instead of organic packaging chips, starch such as corn or potato starch mixed with PVA adhesive can also be used as the organic starting material, and this organic porous starting material can be further processed into silicon carbide as described above.

[0033] The conversion to silicon carbide is preferably carried out at a temperature of 1520° C. and a pressure of 950 mbar.

[0034] If free silicon or carbon is still present after conversion to SiC, the powder can be post-treated above 500 °C to oxidize the free carbon with oxygen. If the proportion of free silicon is too high, post-treatment can also be achieved by etching with hydrofluoric acid or ammonium fluoride.

[0035] Another method for producing pure SiC is to first convert less than 100% porous graphite flakes into SiC powder, and then remove the residual carbon by oxidation.

[0036] However, a suitable, clean, oxygen-resistant oven must be used for this purpose.

[0037] In certain variations of the invention, less than 100% of the porous graphite flakes are converted to SiC powder, and then the remaining residual carbon is oxidized with oxygen addition at above 500° C., leaving pure SiC.

Claims

1. - filling open-top containers with starch-based packaging chips or expanded starch as organic raw materials; - placing the container filled with the raw material in an oven and gradually heating the packaging chips or expanded starch up to a temperature of 2000°C under inert gas supply or in vacuum to graphitize the raw material into porous graphite flakes; - supplying halogen gas into the oven to clean the porous graphite flakes at a temperature above 1800°C to form metal chlorides and remove foreign matter from the porous graphite flakes; and SiO sublimation in a gas phase with argon as a carrier gas for 50 to 100 hours at a temperature above -1,200°C and a pressure of 30 mbar or more, converting the porous graphite flakes into powdered silicon carbide. A method for producing high purity silicon carbide powder.

2. 2. The method according to claim 1, characterized in that pure corn starch or corn starch mixed with PVA adhesive is used as organic raw material.

3. 2. The method of claim 1, wherein the conversion to silicon carbide is carried out at a temperature of 1520°C.

4. 2. The method of claim 1, wherein the conversion to silicon carbide is carried out at a pressure of 950 mbar.

5. 2. The method of claim 1, wherein the heating of the oven to the graphitization temperature is carried out in stages.

6. 6. The method according to claim 1, wherein the SiC powder is oxidized at above 500° C. by supplying oxygen to remove excess carbon.

7. 6. The method according to claim 1, wherein the SiC powder is treated with hydrofluoric acid (HF) or ammonium fluoride (NH4F) to remove excess silicon.

8. 6. The method according to claim 1, wherein less than 100% of the porous graphite flakes are converted into SiC powder, and then the remaining carbon is oxidized above 500° C. by an oxygen supply.