Method for producing silicon carbide, and silicon carbide

Abstract

The invention is concerned with the area of materials management and relates to a method which can be used, for example, to produce refractory ceramic. The aim of the invention is to provide a method and silicon carbide, wherein raw silicon carbide is produced in a single production process with high conversion rates and a high yield. This is achieved by a method for producing silicon carbide with a SiC content of more than 98 to 99.9999% by mass, in which silicon- and carbon-containing starting powders having particle sizes with a d95 value of up to 1.5 mm and a proportion of oxygen of no more than 7% by mass are mixed in a ratio of Si : C of 75 : 25 to 65 : 35% by mass and filled into a refractory container with a bulk porosity ranging from 0.33 to 0.50, and the mixture is heat-treated at temperatures of 1400 to 2500 °C in a vacuum or in an oxygen-free atmosphere.

Description

[0001] Process for the production of silicon carbide and silicon carbide

[0002] The invention relates to the fields of materials management, material synthesis and technical ceramics and concerns a process for producing silicon carbide (SiC), in particular raw SiC, which can be used, for example, for producing refractory ceramics for linings in waste incineration plants or in industrial furnaces.

[0003] Silicon carbide (SiC) is a synthetic industrial mineral used in many industries due to its outstanding properties (hardness, high-temperature properties, chemical resistance). Its use in the form of special, highly pure, and narrowly fractionated fine powder grains (0.5 to approximately 250 pm) is particularly important in microelectronics / photovoltaics (wafer sawing), for the production of ballistic protective ceramics for military technology, and in automotive / environmental technology (diesel particulate filters). Its coarser form (2 pm to 2000 pm) is also used as an abrasive material for high-quality surface finishing throughout mechanical engineering.

[0004] SiC powder granules are produced from special raw SiC by grinding, purification, and fractionation. This process produces high and constant amounts of low-quality, poorly usable SiC. For approximately 120 years, raw SiC has been produced using an electrosynthesis process known as the Acheson process (DE 76629 A, DE 85197 A, EG Acheson, Carborundum: Its history, manufacture and uses, J. of the Franklin Institute, Vol. 136, Iss. 3, 1893, pp. 194-203, ISSN 0016-0032). In this process, raw SiC is produced on a technically relevant scale via the carbothermal reduction of SiC>2 with carbon. Quartz sand is typically used as the starting material for SiC>2, and petroleum coke is typically used as the starting material for carbon.

[0005] The carbothermal reduction occurs according to the molecular formula:

[0006] However, in reality, the carbothermal reduction does not proceed completely according to the molecular formula, but depending on the temperature and composition of the starting materials, partial reactions also take place, such as a gas phase reaction of emerging SiO, the formation of SiO2 melts, the reduction of SiO2 to molten Si, the reverse reaction of SiC with SiO2 or SiO.

[0007] Another possibility for producing SiC is to realize the reaction of Si + C -> SiC.

[0008] As a rule, the large-scale production of Si as silicon metal also takes place via a preceding process of carbothermal reduction of SiO2, so that the production process of silicon metal is complex and its use as a raw material for the production of SiC is considered uneconomical and is also relatively unfavorable for environmental reasons due to the very high energy consumption and CO / CO2 emissions of silicon metal production.

[0009] To produce high-quality raw SiC that is well crystallized, has a high purity of > 98% SiC and consists predominantly of the high-temperature modification of SiC, the so-called Alpha SiC, temperatures of > 2000 °C are required in this Acheson process.

[0010] Accordingly, this manufacturing process based on the Acheson process requires high energy input and is therefore tied to the price of electricity and oil (petrol coke as the raw material). It also generates high levels of dust, CO / CO2, and SO2 emissions, which incur additional costs. In addition, due to the necessary high manufacturing temperatures, the process has so far only been carried out discontinuously, which also leads to further technical problems in the equipment used for production, for example, gas evolution or reaction with components of the manufacturing equipment. Discontinuous manufacturing also limits the yield and quality of the raw SiC.

[0011] Alternative, especially continuous, manufacturing processes have not been successful so far, despite many attempts, mostly for economic reasons.

[0012] J.C. McMullen, J. Electrochem. Soc. 104, 1957, 7, 462-465, DOI 10.1149 / 1.2428627 provides an overview of the state of the art in raw SiC production.

[0013] Furthermore, US Pat. No. 2,178,773 A discloses the production of SiC by moving a SiO2-C mixture through an inductively heated furnace. In particular, a briquetted mixture is used, or the mixture is processed in a furnace in refractory containers. Only partially reacted mixtures at lower temperatures can be further reacted with additional heat treatment.

[0014] According to US 2,729,542 A, a raw material mixture for SiC in interconnected containers, which together form a tube, is passed through a tube furnace at a minimum temperature of 1800 °C.

[0015] Using divided containers does not lead to the desired result.

[0016] From DE 1 186447 C a process for the production of silicon carbide is known in which carbon and sand are mixed with an adhesive liquid and continuously passed through a vertical furnace at temperatures up to 2100 °C until 80% conversion is achieved, and then the reaction mixture is cooled and stacked in a second furnace and treated under protective gas at temperatures above 2200 °C.

[0017] This procedure was proposed because the quartz-carbon mixture sinters heavily during silicon carbide production, severely impeding the further movement of the reaction mixture in a furnace. By achieving up to 80% conversion in a vertical furnace, further transport of the reaction mixture is possible. However, a second process step is required for complete conversion.

[0018] The disadvantage of this solution is that a high level of effort is required for grinding and mixing the starting products and for briquetting to stack the reaction material in the second furnace, and safe transport is not guaranteed, even in a vertical furnace.

[0019] WO 95 / 33683 A also describes a two-step process for producing alpha-SiC, using approximately stoichiometric ratios of SiO2 and carbon. The mixture, in pellet form, is converted to almost 100% ß-SiC at temperatures in the range of 1500-1800 °C in a continuously operating rotary kiln. This is then converted to alpha-SiC in a second, conventional discontinuous heat treatment between 1800 and 2300 °C.

[0020] Furthermore, according to DE 30 50 136 C2, a process for producing ß-silicon carbide is known in which very fine carbon-containing powder and very fine silicon dioxide powder are reacted at a temperature of up to 1650 °C in an inert atmosphere, wherein the reaction mixture is conveyed by a screw conveyor through a low-temperature zone, then through a high-temperature zone up to a maximum of 1650 °C and again through a low-temperature zone and this cycle is repeated until the conversion into ß-silicon carbide is complete.

[0021] The disadvantage of this process is that very fine powders are required as starting powder and at least several repetitions of the process are necessary for each reaction product.

[0022] Despite ongoing investigations, no process is known in the state of the art with which a substantially complete conversion of silicon carbide can be achieved in a single continuous process.

[0023] The object of the present invention is to provide a process for the production of silicon carbide and silicon carbide in which crude silicon carbide is produced with high conversion rates and a high yield in a single production process.

[0024] The object is achieved by the invention defined in the claims. Advantageous embodiments are the subject of the dependent claims, whereby the invention also includes combinations of the individual claims in the sense of an AND connection, as long as they are not mutually exclusive.

[0025] In the process according to the invention for producing silicon carbide with an SiC content of more than 98 to 99.9999 wt.%, silicon- and carbon-containing starting powders with particle sizes with a dgs value of up to 1.5 mm and an oxygen content of the starting powder of maximum 7 wt.% are mixed in a Si:C ratio of 75:25 to 65:35 wt.% and filled into at least one refractory container, wherein a bulk porosity of the mixture between 0.33 and 0.50 is set in the container, and subsequently the container with the mixture is heat-treated at temperatures of 1400 to 2500 °C under vacuum or in an oxygen-free atmosphere, and the resulting crude SiC is cooled.

[0026] Advantageously, the silicon-containing starting powders used are

[0027] - consisting of or predominantly of Si particles and / or

[0028] - of or predominantly of metallurgical silicon and / or

[0029] - made from or predominantly from recycled silicon and / or

[0030] - made of or predominantly made of silicon from waste silicon materials and / or

[0031] - from SiC>2, wherein the amount of SiC>2 used is limited by the maximum proportion of oxygen in the silicon and carbon-containing starting powders of 7 wt.%, wherein even more advantageously quartz sands are used as the starting powder in the case of SiCh powder, and / or powder made of metallurgical silicon is used as the starting powder, advantageously with a silicon content of at least 90 wt.%, preferably at least 95 wt.%, even more preferably with at least 97 wt.%, and / or silicon powder from recycled silicon, such as from the recycling of solar cells, is used as the starting powder, and / or silicon powder from waste materials from the silicon industry, such as sawing sludge, filter dust or ground Si single-crystal scrap is used as the starting powder.

[0032] Furthermore, it is advantageous to use powders with a Si content of at least 90 mass% as silicon-containing starting powders.

[0033] Also advantageously, silicon and carbon-containing starting powders with a maximum impurity content of 20 ppm, advantageously < 5 ppm, with particle sizes with a dgs value of up to 1.5 mm and an oxygen content of the starting powders together of a maximum of 7 mass % are mixed in a Si:C ratio of 75:25 to 65:35 mass % and filled into at least one refractory container made of halogen-purified graphite and a bulk porosity of the mixture is set between 0.33 and 0.50 in the container, and then the container with the mixture is heat-treated at temperatures of 1400 to 2500 °C under vacuum or under a high-purity protective gas atmosphere. Also advantageously, silicon and carbon-containing starting powders are used whose oxygen content together is < 4 mass %.

[0034] It is also advantageous if powders made from soot, graphite, coal, particularly in the form of anthracite, coke, such as petroleum coke, pitch coke, coke from lignite or hard coal, recycled carbon, such as so-called thermolysis coke or recovered soot (rCB - recovered carbon black), biochar from wood gasification, or carbon from methane pyrolysis are used as carbon-containing starting powders.

[0035] It is also advantageous if the starting powders are used in a Si : C ratio of 72 : 28 to 68 : 32 mass%.

[0036] It is also advantageous if the starting powders are used with particle sizes with a dgs value of 75 pm to 1.5 mm, advantageously from 500 pm to 800 pm.

[0037] It is also advantageous if, when mixing the starting powders in the at least one first refractory container, a bulk porosity of 0.35 to 0.45 is set.

[0038] It is also advantageous to use fireproof containers made of graphite and / or carbon fiber reinforced carbon (CFC).

[0039] It is also advantageous if, when heating the mixture of the starting powders, a slower heating rate and / or a longer residence time of the mixture at temperatures in the range between 1200 and 1350 °C is realized before the temperature is further increased to the final temperature.

[0040] It is also advantageous if the mixture filled into the refractory container is covered with a layer of carbon-containing powder, whereby the carbon-containing powder of the covering layer is part of the total carbon content in the mixture and amounts to a maximum of 5 mass% thereof.

[0041] It is also advantageous to use fireproof containers with a filling volume of up to 100 liters.

[0042] It is also advantageous if the heat treatment is carried out in a continuous furnace. A variety of fireproof containers are advantageously used.

[0043] And equally advantageously, the heat treatment is carried out at least at temperatures of 2050 to 2350 °C as final temperatures.

[0044] The silicon carbide according to the invention with a SiC content of more than 98 to 99.9999 mass% has a predominant content of isometric and / or congruent SiC particles in predominantly cubic particle shape with an average roundness of 0.5 to 0.8 and essentially convex surfaces and no plate-like portions, wherein the SiC particles are essentially isolated and have no or essentially no closed porosity and a bulk density of the silicon carbide of at least 3.05 g / cm 3 is present.

[0045] Advantageously, silicon carbide contents of more than 98 to 99.95 mass%, advantageously from 99 to 99.999 mass%, are present.

[0046] Also advantageously present as isometric and / or congruent SiC particles are particles which have a ratio of dimensions in all spatial directions of > 0.5 to 1.

[0047] Furthermore, SiC particles are advantageously present with a predominantly cubic particle shape with convex surfaces and no plate-like portions, which have an average roundness of 0.65 to 0.78.

[0048] And also advantageously, the silicon carbide has a bulk density of 3.10 g / cm 3 , advantageously 3.18 g / cm 3 on.

[0049] The solution according to the invention makes it possible for the first time to provide a process for producing silicon carbide in which raw silicon carbide is produced with high conversion rates and a high yield in a single production process.

[0050] This is achieved for the first time with a process for producing silicon carbide with an SiC content of more than 98 to 99.9999 wt.%. In this process, silicon- and carbon-containing starting powders, each with particle sizes with a dgs value of up to 1.5 mm and together with an oxygen content of no more than 7 wt.%, are mixed in a Si:C ratio of 75:25 to 65:35 wt.% and filled into at least one refractory container. Advantageously, the starting powders are used in a Si:C mixing ratio of 72:28 to 68:32 wt.%.

[0051] Advantageously, the silicon-containing starting powders used are

[0052] - consisting of or predominantly of Si particles and / or

[0053] - of or predominantly of metallurgical silicon and / or

[0054] - made from or predominantly from recycled silicon and / or

[0055] - from or predominantly from Si waste materials and / or

[0056] - from SiC>2" where the amount of SiC>2 used is limited by the maximum proportion of oxygen in the silicon and carbon-containing starting powders of 7 wt.%.

[0057] In the case of SiO2 powder, quartz sand is used as the starting powder. Based on the selected silicon- and carbon-containing starting powders and the maximum oxygen content of 7 wt.% of the total starting powder, the maximum possible amount of SiO2 can be easily calculated.

[0058] Powder of metallurgical silicon can also be used as the silicon-containing starting powder, advantageously with a silicon content of at least 90 mass%, preferably at least 95 mass%, even more preferably at least 97 mass%.

[0059] Silicon powder from recycled silicon, such as from the recycling of solar cells, can also be used as a silicon-containing starting powder.

[0060] It is also possible to use silicon powder from waste materials from the silicon industry, such as sawing sludge, filter dust or ground Si single-crystal scrap, as silicon-containing starting powder.

[0061] Furthermore, silicon-containing starting powders with a Si content of at least 90 mass% are advantageously used.

[0062] Powders made from soot, graphite, coal, especially anthracite, coke, such as petroleum coke, pitch coke, coke from lignite or hard coal, recycled carbon, such as so-called thermolysis coke or recovered soot (rCB - recovered carbon black), biochar from wood gasification, or carbon from methane pyrolysis are advantageously used as carbon-containing starting powders.

[0063] The carbonaceous starting powders advantageously have a carbon content of at least 85 mass% carbon, more advantageously at least 90 mass% carbon, and even more advantageously at least 95 mass%. It is also possible to use materials as starting powders that already contain silicon and / or silicon oxides and carbon, such as pyrolyzed rice hulls, or that only contain silicon from grinding sludge in the solar and microelectronics industries, or that already contain silicon carbide, or that contain carbon from organic impurities.

[0064] Advantageously, the proportion of silicon-containing starting powders used predominantly consists of powders made of silicon metal and / or silicon-metal alloys, and only to a lesser extent oxygen-containing silicon compounds, such as SiC>2.

[0065] According to the invention, the mixtures of silicon- and carbon-containing starting powders used may together have a maximum oxygen content of 7 wt.%, which corresponds to a proportion of the mixture of silicon-containing starting powders of a maximum of 20 wt.% SiO2. Advantageously, the mixtures of silicon-containing starting powders have a content of < 4 wt.% oxygen, which corresponds to a proportion of the mixture of silicon-containing starting powders of < 10 wt.% SiO2.

[0066] If the starting powder mixture contains oxygen-containing components, it is advantageous to adjust the Si:C ratio in the mixture by increasing the carbon content by 0.56 mass% and decreasing the Si content by 0.56 mass% for each mass percentage point of oxygen. Therefore, if a mixture with a 4 mass% oxygen content is used, starting from a mixture without oxygen with a Si:C ratio of 70:30, it is advantageous to use a Si:C ratio of 67.8:32.2.

[0067] The mixing of the silicon and carbon-containing starting powders is carried out, for example, using stirrers or in a mill.

[0068] Of particular importance in the process according to the invention is that, in contrast to prior art processes, relatively coarse starting powders can be used. Advantageously, the starting powders have particle sizes with a dgs value of 75 pm to 1.5 mm, advantageously from 500 pm to 800 pm. These particle size values ​​indicate the upper limit of the particle size distribution (D95), at which 95% of the particles in the powder have a particle size up to the upper limit. Such particle size distributions are achieved technically, for example, by sieving or classifying the starting powder. The starting powders have average particle sizes d50 in the range from 150 pm to 1.2 mm, advantageously from 550 pm to 650 pm.In the lower range of the particle size distribution, very fine particles with particle sizes <45 pm may also be present in the starting powders according to the invention, but it is also advantageous that this content of very fine particles in the starting powders is low, since this proportion of very fine particles is unfavorable for the safe handling of the powders during transport, mixing and filling.

[0069] Even coarser particles of the silicon-containing and / or carbon-containing starting powders with a size of >1.5 mm may also be present in isolated cases, up to a maximum of 5% of the particles in the starting powders, but this slows down the reaction of the particles to form SiC and the content of unreacted silicon oxides or carbon in the SiC increases unfavorably.

[0070] Furthermore, according to the invention, contrary to the solutions of the prior art, it is advantageous if the silicon- and carbon-containing starting powders together have a high proportion of silicon-containing starting powders with a low oxygen content, so that a maximum of 7 mass % or less oxygen is present in the starting powders overall. This is advantageous because the reaction of Si+C to SiC is exothermic. In contrast, the carbothermic reduction of SiO2 to SiC is strongly endothermic. Therefore, smaller amounts of energy must be supplied for the exothermic reaction. The high exothermicity in the reaction of Si+C to SiC can, if the reaction is too rapid, lead to technical problems due to local overheating of the starting powder mixture during heat treatment, particularly if the melting temperature of the silicon is exceeded.However, this effect can be controlled by an adapted temperature control of the heat treatment, by realizing part of the reaction to SiC below the melting temperature, for example by slower heating and holding times at temperatures in the range between 1000 and 1350 °C.

[0071] The mixture of starting powders is then poured into at least one fireproof container.

[0072] Advantageously, the at least one fireproof container consists of graphite and / or carbon fiber reinforced carbon (CFC).

[0073] It is also advantageous if carbon-, nitride-, or reaction-bonded SiC, or recrystallized SiC, or silicon carbide ceramics, or CFC containers with a SiC coating are used as container materials. It is particularly important that the mixture of starting powders in the at least one refractory container has a bulk porosity of 0.33 to 0.50, advantageously 0.35 to 0.45.

[0074] The aggregate porosity is defined as

[0075] Mass of the mixture per filled volume of the mixture

[0076] 1 -

[0077] True density of the mixture

[0078] The true density of the mixture can be determined mathematically or experimentally using gas pycnometry if the density of the starting materials and their proportions is known.

[0079] The mass of the mixture can be easily determined by weighing.

[0080] The volume of the containers can be easily determined by calculation for simple container geometries or by filling with liquid and measuring the volume of the liquid for unclear container geometries.

[0081] The containers can be provided with fill level markings to ensure that the same volume of mixture is always added to the containers.

[0082] After the starting powder mixture is filled into at least one refractory container, the container is heat-treated with the mixture at temperatures ranging from 1400 to 2500 °C under vacuum or in an oxygen-free atmosphere. Technical protective gas atmospheres such as argon or nitrogen are used as oxygen-free atmospheres.

[0083] It is particularly advantageous if, when heating the mixture of starting powders, a slower heating rate and / or a longer residence time of the mixture at temperatures in the range between 1200 and 1350 °C is realized before the temperature is further increased to the final temperature.

[0084] Advantageously, the heat treatment of the at least one refractory container with the mixture of starting powders takes place at temperatures between 1650 and 2300 °C.

[0085] The heat treatment can advantageously be carried out in a continuous furnace. It is also advantageous if a plurality of refractory containers are filled with the mixtures of the starting powders and are continuously passed one after the other through a continuous furnace, where they are heat-treated. After the heat treatment of the mixture in the at least one refractory container, the container is cooled, resulting in a reaction mixture that is almost completely converted to SiC. The SiC content of the SiC mixture is essentially > 98 mass %, and only small unconverted portions of Si and C are present in the reaction mixture. The resulting SiC is crude SiC.

[0086] According to the invention, it is also possible to use this process to produce high-purity SiC with very low, unconverted amounts of Si and C, which has further impurities below an amount of 20 ppm or even <5 ppm.

[0087] To achieve this, silicon- and carbon-containing starting powders with a high purity of <20 ppm or less than 5 ppm of impurities with the particle size and aggregate porosity according to the invention are filled into the refractory containers, which are also made of high-purity materials such as halogen-purified graphite, and then heat-treated and cooled according to the process according to the invention. During the process, the use of vacuum or high-purity protective gases is required.

[0088] The heat treatment can advantageously be carried out in a continuous furnace. It is also advantageous if a large number of refractory containers are filled with the mixtures of starting powders and are continuously passed through a continuous furnace, where they are heat-treated.

[0089] After heat treatment, the resulting raw SiC is cooled and can be further processed.

[0090] The adjustment of the aggregate porosity mentioned in the invention in the refractory containers is achieved by filling the mixtures of the starting powders and by a mechanical treatment of the aggregate, for example by vibrating, indenting, pressing, tamping or by a combination of such mechanical treatments, such as vibratory pressing.

[0091] The invention expressly does not envisage that the mixtures be formed into any molded parts before being poured into the containers, and the containers are then filled with them. The resulting compaction cannot achieve the desired aggregate porosity, thus preventing the desired high conversion rates and yields of crude SiC from being achieved. Likewise, the present invention expressly does not envisage that the necessary aggregate porosity be achieved by adding auxiliary materials such as binders or other burnout materials.

[0092] It has been determined according to the invention that if a greater aggregate porosity is set than specified for the respective containers, partial losses of the SiC content occur during SiC production, and thus the yields of raw SiC are significantly lower. In addition, the yield of raw SiC is lower because less material is introduced into the containers.

[0093] Furthermore, it has been found according to the invention that if a lower aggregate porosity is set than specified according to the invention, there is a lower and in particular more inhomogeneous conversion of the starting powders and a strong caking or sintering of SiC particles, which can then no longer be easily separated.

[0094] In addition to the mechanical treatment of the aggregate, the aggregate porosity also depends on the particle size distribution of the starting powder. To achieve the aggregate porosity according to the invention, the particle size distribution can also be varied during the manufacturing process within the limits specified in the invention, which can be determined using simple test series.

[0095] Furthermore, it is very advantageous according to the invention if the fireproof container has a filling volume of up to a maximum of 100 liters.

[0096] It has been shown that under the conditions specified in the invention, a particularly uniform and homogeneous conversion of the starting powders to crude SiC takes place and a significantly higher yield of crude SiC with SiC contents of > 98 mass% is achieved.

[0097] It is also advantageous if the mixture filled into the fireproof container is covered with a layer of carbon-containing powder.

[0098] The enrichment of carbon in a covering layer on the heap in the refractory container promotes the conversion to SiC, since, for example, escaping vaporous Si and / or SiO can react with the carbon of the covering layer to form SiC and does not enter the atmosphere outside the container.

[0099] According to the invention, it is important that the carbon in the cover layer is part of the total carbon content of the mixture in the container and amounts to a maximum of 5 mass% of it. Due to the small amount of carbon in the cover layer, a maximum of 5 mass%, only a thin layer results, the thickness of which depends on the bulk density of the carbon material used. The effect of this thin layer on the aggregate porosity of the mixture is therefore negligible; i.e., the data on aggregate porosity in the first container refer to the aggregate without the thin carbon cover layer.

[0100] However, to calculate the Si:C ratio of the mixture, the amount of carbon in the top layer must be taken into account by adding the amount of carbon in the top layer to the total amount of carbon in the mixture.

[0101] The process according to the invention produces silicon carbide with an SiC content of more than 98 to 99.9999 mass%. This raw SiC is further processed into SiC powder and can then be used, for example, for the production of SiC ceramics or for growing SiC single crystals.

[0102] The silicon carbide according to the invention with a SiC content of more than 98 to 99.9999 mass% has a predominant content of isometric and / or congruent SiC particles in predominantly cubic particle shape with an average roundness of 0.5 to 0.8 and essentially convex surfaces and no plate-like portions, wherein the SiC particles are essentially isolated and have no or essentially no closed porosity and a bulk density of the silicon carbide of at least 3.05 g / cm 3 is present.

[0103] For all information within the scope of the present invention which is stated to be essentially the same, it should be understood that the values ​​deviate only very slightly from the information and that the deviations have been caused by a technical realization or technical processing.

[0104] Advantageously, the silicon carbide according to the invention has a SiC content of more than 98 to 99.95 mass%.

[0105] It is particularly advantageous if the silicon carbide according to the invention has a SiC content of 99 to 99.999 mass%.

[0106] Advantageously, SiC particles are present as isometric and / or congruent SiC particles, which have a ratio of dimensions in all spatial directions of > 0.5 to 1. The determination of isometric particle size, congruent particle shape and roundness can be carried out according to ISO 13322-1:2014-5, Part 1 ISO 13322-2:2021-12 or determined in accordance with DIN EN ISO 14688-1:2020-11.

[0107] Also advantageously, the SiC particles of the silicon carbide according to the invention have a predominantly cubic particle shape with convex surfaces and no plate-like portions and show an average roundness of 0.65 to 0.78.

[0108] The SiC particles according to the invention are rounded, semi-rounded and edge-rounded and have essentially convex surfaces.

[0109] It is also advantageous if the silicon carbide has a bulk density of 3.10 g / cm 3, even more advantageously from 3.18 g / cm 3 , has.

[0110] The bulk density can be determined using the known methods of gas pycnometry. Furthermore, the porosity of the silicon carbide according to the invention can be determined from the bulk density in relation to the theoretical density of the silicon carbide.

[0111] Bulk density

[0112] Porosity [in Vol. -%] = (1 - - ) *100 theoretical density

[0113] The theoretical density of silicon carbide is usually 3.21 g / cm 3 assumed.

[0114] The bulk density of the silicon carbide according to the invention, measured by gas pycnometric methods, is therefore at least 3.05 g / cm 3 , which corresponds to a porosity of <5 vol.%, advantageously at least 3.10 g / cm 3 , which corresponds to a porosity of <3.5 vol.%, more preferably at least 3.18 g / cm 3, which corresponds to a porosity of <1 vol.%.

[0115] The silicon carbide according to the invention exhibits highly isometric and / or congruent, i.e., very uniform and consistent, SiC particles in terms of shape and size. A further feature of the silicon carbide according to the invention is that the SiC particles are only slightly intergrown, with no or essentially no conglomerates present. Likewise, the SiC particles of the silicon carbide according to the invention exhibit no or only very minimal closed porosity.

[0116] The silicon carbide according to the invention can be produced by the process according to the invention for producing silicon carbide.

[0117] For example, X-ray analysis of the raw SiC shows that the silicon carbide according to the invention consists of 100% alpha polytypes, predominantly of the 6H, 4H, and 15R varieties. Furthermore, the crystallite size of the particles is very uniform within the containers, meaning the average crystallite size deviates by only approximately + / - 20% from the mean value. The mean value itself depends on the manufacturing conditions, for example, the temperature, the holding time, and the protective gas atmosphere used during the heat treatment. High temperatures and long holding times result in a coarser powder, and an argon atmosphere also produces coarser powders than a nitrogen atmosphere.

[0118] The silicon carbide according to the invention and produced according to the invention differs from the raw SiC produced by the known Acheson process on the one hand by very high SiC contents of more than 98 mass% and on the other hand the silicon carbide according to the invention and produced according to the invention has very uniform properties, such as the fact that the silicon carbide consists almost exclusively or exclusively of alpha polytypes and has a very uniform crystallite size of the SiC particles.

[0119] Likewise, the raw SiC from the Acheson manufacturing process is known to show very non-uniform properties with regard to SiC content, polytype content, particle size and particle shape within a furnace batch, when viewed over the entire batch process.

[0120] Although SiC crushed from the inner zone of a furnace charge using the Acheson process can also exhibit SiC contents of more than 98 wt. %, this SiC material is highly irregular in terms of particle size and shape. Very large, platelet-shaped and sharp-edged crystals up to 20 mm in size can occur, alongside very small (<200 pm), highly intergrown, and irregular platelet-shaped or even columnar crystals. These intergrowths are so strong that the crystallites also enclose pores and can only be crushed by the application of strong mechanical forces, resulting in a highly irregular and splintery particle shape in the resulting powder.

[0121] Due to the porosity of the intergrowths, the density of the coarsely crushed raw SiC with a particle size between 1 and 3 mm from the Acheson process, measured by gas pycnometry, is far from the pure density of SiC of 3.21 g / cm 3removed and usually has values ​​for the bulk density of less than or equal to 2.97 g / cm 3 , which corresponds to a porosity of 7.5 vol.%, or even only 2.59 g / cm 3 , which corresponds to a porosity of 19 vol.% (EHP Wecht, Feuerfest-Siliconcarbid, Applied Mineralogy Vol 11 , Springer-Verlag 1977, p. 41 and p. 246, ISBN 978-3-7091-7069-4).

[0122] It can be assumed that the properties of the silicon carbide according to the invention arise from the fact that, during production, the inventive aggregate porosity, in particular, prevents excessive sintering and intergrowth of the silicon carbide crystals formed, thus preventing the inclusion of porosity. At the same time, the inventive use of the starting powders and the inventive heat treatment achieve very uniform temperatures and chemical reaction conditions in the refractory containers according to the invention, so that very uniform SiC crystals are formed.

[0123] The invention is explained in more detail below using two exemplary embodiments.

[0124] Example 1

[0125] 1650 kg silicon powder (filter dust from silicon production, Si content 98 wt. %, gas pycnometrically determined density 2.34 g / cm 3, particle size dgs = 1 mm, with a maximum of 10 % of the particles having a particle size of < 45 pm), 125 kg quartz sand (SiO2 content 99 wt. %, gas pycnometrically determined density 2.65 g / cm 3 , particle size dgs = 0.3 mm) and 800 kg petroleum coke (carbon content 89 wt.%, gas pycnometrically determined density 1.40 g / cm 3 , particle size dgs = 0.6 mm, with a maximum of 15% of the particles having a particle size of <45 pm), corresponding to a Si : C ratio of 70.2 : 29.8 and an oxygen content of 2.7 wt.%, are dry mixed in several batches in a drum mixer for 3 h each. The true density of the mixture is 1.94 g / cm 321.8 kg of the mixture are filled into a cuboidal refractory container made of graphite with a length of 350 mm, a width of 350 mm and a height of 200 mm with a wall thickness of 15 mm, and compacted by vibratory tamping so that the powder mixture has a filling height of 180 mm, corresponding to a filling volume of 18432 cm 3 This results in a corresponding aggregate porosity of 0.39. The filled refractory container is transferred into a resistance-heated nitrogen continuous-flow pusher furnace, where it undergoes a heat treatment in a first zone at a temperature of 1300 °C with a residence time of 1 hour, followed by a heat treatment at a maximum temperature of 2350 °C with a residence time of 90 minutes. After passing through the cooling zone, the container is discharged. The slightly sintered but easily separated powder is removed from the refractory container. The amount of powder removed, determined by weighing, is 19.9 kg.

[0126] Subsequently, another 89 identical graphite refractory containers containing the same amount of powder mixture are filled to a height of 180 mm and subjected to the same heat treatment at a maximum temperature of 2350 °C for a residence time of 90 minutes under a nitrogen atmosphere in the continuous furnace. The powder of the reaction mixture taken from the 90 refractory containers is poured into the drum of a roller mixer, where it is homogenized over a period of 2 hours with the addition of 5 mm diameter SiC grinding balls.

[0127] The powder is analyzed according to FEPA Standard 45-1:2011, DIN EN ISO 9286:2023-10, and DIN EN ISO 21068:2024 Part 1-3 and consists of 99.2 wt.% SiC, 0.1 wt.% Si-free, and 0.2 wt.% C-free. The remainder consists of minor impurities such as iron, aluminum, and other elements in trace amounts. X-ray analysis shows a SiC content of 100% polytypes of the alpha modification, predominantly of the 6H polytype. Scanning electron microscopy images reveal an average crystallite size of approximately 160 pm. The SiC particles are predominantly isolated, isometric, and congruent particles in a predominantly cubic shape with an average circularity of 0.7. The bulk density of the powder, determined by gas pycnometric analysis, is 3.12 g / cm³. 3 . In the polished section, almost no closed pores can be seen in the particles.

[0128] Sampling from different areas of the refractory containers (top / middle, bottom / edge) revealed very small compositional deviations of <0.2 wt.%. Both X-ray analysis and SEM investigations showed no differences in polytype content or mean crystallite size.

[0129] Example 2

[0130] 2100 g silicon powder (made from silicon single crystal fragments, Si content 99.9995%, gas pycnometrically determined density 2.34 g / cm 3 , particle size dgs = 0.8 mm, with a maximum of 10% of the particles having a particle size of < 45 pm), 180 g of purified quartz sand (SiO2 content 99.9999 mass%, gas pycnometrically determined density 2.65 g / cm 3 , particle size dgs = 0.8 mm, with a maximum of 12% of the particles having a particle size of < 45 pm) and 900 g of purified graphite powder (carbon content 99.9995 mass%, gas pycnometrically determined true density 1.8 g / cm 3, particle size dgs = 90 pm, 30% <45 pm), are dry mixed in a tumbler mixer for 4 h. The Si : C ratio is 70.8 : 29.2, the oxygen content is 3.0%, and the true density of the mixture is 2.17 g / cm 3 2400 g of the mixture are filled in six layers of 400 g each into a cylindrical refractory container made of halogen-cleaned graphite with a wall thickness of 5 mm, an inner diameter of 185 mm and an inner height of 100 mm, each by light manual pressing, so that the powder filling height is 75 mm, corresponding to a filling volume of 2015 cm 3This results in a corresponding aggregate porosity of 0.45. The container is sealed with a graphite lid and placed in a resistance-heated furnace under high-purity argon, initially at 1270 °C for a 5-hour hold, then further heated to 2300 °C and heat-treated at this temperature for 120 minutes. The furnace cools the powder, which is slightly sintered but easily separated, and is removed from the refractory container as a reaction mixture. The amount of powder removed, determined by weighing, is 2266 g.

[0131] The powder is analyzed according to FEPA Standard 45-1:2011, DIN EN ISO 9286:2023-10, DIN EN ISO 21068:2024 Part 1-3 and consists of >99.8 wt.% SiC, <0.1 wt.% Si-free, and <0.1 wt.% C-free. The total impurity content analyzed by GDMS is 5 ppm. X-ray analysis shows a SiC content of 100% polytypes of the alpha modification, predominantly of the 6H polytype. Scanning electron microscopy images show an average crystallite size of approximately 120 pm. The SiC particles are predominantly isolated, isometric, and congruent particles in a predominantly cubic shape with an average circularity of 0.8. The bulk density of the powder, determined by gas pycnometric analysis, is 3.18 g / cm 3 . In the polished section, almost no closed pores can be seen in the particles.

[0132] When taking samples from different areas of the crucible (top / center of the crucible, bottom / crucible edge), very small deviations occur with regard to the

[0133] Composition. Both the X-ray analysis and the SEM

[0134] Investigations show no differences in polytype content and mean

[0135] Crystallite size.

Claims

Patent claims 1. A process for producing silicon carbide with an SiC content of more than 98 to 99.9999 mass%, in which silicon and carbon-containing starting powders with particle sizes with a dgs value of up to 1.5 mm and an oxygen content of the starting powders of maximum 7 mass% in a ratio of Si:C of 75:25 to 65:35 mass% are mixed and filled into at least one refractory container, wherein a bulk porosity of the mixture between 0.33 and 0.50 is set in the container, and subsequently the container with the mixture is heat-treated at temperatures of 1400 to 2500 °C under vacuum or in an oxygen-free atmosphere, and the resulting crude SiC is cooled.

2. A process according to claim 1, wherein the silicon-containing starting powder is powder - consisting of or predominantly of Si particles and / or - of or predominantly of metallurgical silicon and / or - made from or predominantly from recycled silicon and / or - made of or predominantly made of silicon from waste silicon materials and / or - from SiC>2, whereby the amount of SiC>2 used is limited by the maximum proportion of oxygen in the silicon and carbon-containing starting powders of 7 wt.%.

3. Process according to claim 2, in which essentially quartz sands are used as starting powder in the case of SiCh powder, and / or powder made of metallurgical silicon is used as starting powder, advantageously with a silicon content of at least 90 mass %, preferably at least 95 mass %, even more preferably with at least 97 mass %, and / or silicon powder from recycled silicon, such as from the recycling of solar cells, is used as starting powder, and / or silicon powder from waste materials from the silicon industry, such as sawing sludge, filter dust or ground Si single-crystal scrap is used as starting powder.

4. A process according to claim 1, wherein the silicon-containing starting powder used is powder having a Si content of at least 90 mass%.

5. A method according to claim 1, wherein silicon and carbon-containing starting powders with a maximum content of 20 ppm of impurities, advantageously < 5 ppm of impurities, with particle sizes with a dgs value of up to 1.5 mm and an oxygen content of the starting powders together of a maximum of 7 mass% in a ratio of Si : C from 75 : 25 to 65 : 35 mass% and filled into at least one refractory container made of halogen-purified graphite and a bulk porosity of the mixture between 0.33 and 0.50 is set in the container, and subsequently the container with the mixture is heat-treated at temperatures of 1400 to 2500 °C under vacuum or under a high-purity protective gas atmosphere.

6. Process according to claim 1, in which silicon- and carbon-containing starting powders are used whose oxygen content together is < 4 mass%.

7. A process according to claim 1, wherein the carbon-containing starting powder used is powder made from soot, graphite, coal, in particular in the form of anthracite, coke, such as petroleum coke, pitch coke, coke from lignite or hard coal, recycled carbon, such as so-called thermolysis coke or recovered soot (rCB - recovered carbon black), biochar from wood gasification, or carbon from methane pyrolysis.

8. A process according to claim 1, wherein the starting powders are used in a Si:C ratio of 72:28 to 68:32 mass%.

9. Process according to claim 1, wherein the starting powders are used with particle sizes having a dgs value of 75 pm to 1.5 mm, advantageously from 500 pm to 800 pm.

10. The method according to claim 1, wherein the starting powders are mixed in the at least one first refractory container to achieve a bulk porosity of 0.35 to 0.

45.

11. A method according to claim 1, wherein fireproof containers made of graphite and / or carbon fiber reinforced carbon (CFC) are used.

12. The method according to claim 1, wherein, when heating the mixture of starting powders, a slower heating rate and / or a longer residence time of the mixture at temperatures in the range between 1200 and 1350 °C is realized before the temperature is further increased to the final temperature.

13. The method according to claim 1, wherein the filled mixture in the refractory container is covered with a layer of carbon-containing powder, wherein the carbon-containing powder of the covering layer is a component of the total carbon content in the mixture and amounts to a maximum of 5 mass% thereof.

14. A method according to claim 1, in which fireproof containers with a filling volume of up to 100 liters are used.

15. The method according to claim 1, wherein the heat treatment is carried out in a continuous furnace.

16. A method according to claim 1, wherein a plurality of fireproof containers are used.

17. The method according to claim 1, wherein the heat treatment is carried out at least at temperatures of 2050 to 2350 °C as final temperatures.

18. Silicon carbide with a SiC content of more than 98 to 99.9999 mass%, which has a predominant content of isometric and / or congruent SiC particles in predominantly cubic particle shape with an average roundness of 0.5 to 0.8 and essentially convex surfaces and no plate-like portions, wherein the SiC particles are essentially isolated and have no or essentially no closed porosity and a bulk density of the silicon carbide of at least 3.05 g / cm 3 is present.

19. Silicon carbide according to claim 18, in which silicon carbide contents of more than 98 to 99.95 mass%, advantageously from 99 to 99.999 mass%, are present.

20. Silicon carbide according to claim 18, wherein the SiC particles are present as isometric and / or congruent SiC particles having a ratio of dimensions in all spatial directions of > 0.5 to 1.

21. Silicon carbide according to claim 18, wherein the SiC particles are predominantly cubic in shape with convex surfaces and no plate-like portions, and have an average roundness of 0.65 to 0.

78.

22. Silicon carbide according to claim 18, wherein the silicon carbide has a bulk density of 3.10 g / cm 3 , advantageously of 3.18 g / cm 3 has.

Images