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  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/90—Carbides
  • C01B32/914—Carbides of single elements
  • C01B32/956—Silicon carbide
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/90—Carbides
  • C01B32/914—Carbides of single elements
  • C01B32/956—Silicon carbide
  • C01B32/963—Preparation from compounds containing silicon
  • C01B32/977—Preparation from organic compounds containing silicon
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/56—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides
  • C04B35/565—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide
  • C04B35/571—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide obtained from Si-containing polymer precursors or organosilicon monomers
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2002/00—Crystal-structural characteristics
  • C01P2002/60—Compounds characterised by their crystallite size
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/30—Particle morphology extending in three dimensions
  • C01P2004/32—Spheres
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/60—Particles characterised by their size
  • C01P2004/62—Submicrometer sized, i.e. from 0.1-1 micrometer
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/60—Particles characterised by their size
  • C01P2004/64—Nanometer sized, i.e. from 1-100 nanometer
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2006/00—Physical properties of inorganic compounds
  • C01P2006/10—Solid density
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2006/00—Physical properties of inorganic compounds
  • C01P2006/12—Surface area

Definitions

• the invention relates to a process for the production of high-purity silicon carbide powder in the submicron range, its use as a sintered material which is suitable for the production of moldings, and a device for carrying out the process.
• Silicon carbide has a number of excellent physical and chemical properties. It is used as an abrasive because of its hardness and as a refractory because of its fire resistance. However, binders are required for the production of molded parts made of silicon carbide. So far, the use of silicon carbide as a high-temperature material has been limited due to premature softening and lack of strength.
• Such powders can be produced by reaction of the elements silicon and carbon, by reduction of silicon dioxide with carbon or gaseous hydrocarbons or by direct gas phase reaction.
• the methods of chemical vapor deposition have proven to be particularly advantageous with regard to the above requirements. This is usually based on volatile silicon halides and hydrocarbons or alkyl silicon halides, which are diluted with a carrier either in a plasma torch at high temperatures (see GB-PS 1 134 782) or in a graphite tube at medium temperatures (see GB-PS 1 174 859) are implemented.
• the reaction of silicon monoxide with hydrocarbons at temperatures above 1400 ° C is known (see GB-PS 960 537).
• DT-PS 1 047 180 a process for the production of crystalline, very pure silicon carbide is described, in which converted into the gas state, alkylated silanes or alkylated halosilanes, the atomic ratio of carbon: silicon of which is equal to 1: 1, directly or indirectly for the formation of silicon carbide be heated. The product obtained is then heated for conversion into a coarsely crystalline carbide at 1800 to 2000 ° C for several hours.
• This known method has the disadvantage that only those alkylsilanes or alkylated halosilanes can be used as starting materials which have the carbon: silicon ratio given above.
• the process is also complicated to perform because of the alkyl must heat silane in a high-frequency electromagnetic field, eg an electron torch; The generation of such electron flares requires complicated devices.
• Silicon carbide powder which can be used for the production of dense moldings, optionally with small additions of boron and carbon or boron and carbon compounds, without external pressure or with external pressure, must have certain properties. These properties are within narrow limits, and there is therefore a need for a method by which silicon carbide powder with certain characteristic properties can be produced.
• the present invention is therefore based on the object of providing a method and a device for producing high-purity silicon carbide powder in the submicron range, the disadvantages of the known methods and devices being avoided.
• the procedure should be simple to perform.
• the starting material should not be restricted to those alkylsilanes which have a carbon: silicon ratio of 1: 1.
• the product should be produced in high yields and in high purity.
• the product should have reproducible properties, it should be able to be used as a sintered material with or without pressure without chemical and thermal aftertreatment and without grinding for the production of moldings.
• the invention relates to a process for the production of high-purity silicon carbide powder in the submicron range, which is suitable as a sintered material for the production of moldings Cleavage of the gaseous alkylsilanes at elevated temperature, which is characterized in that the alkylsilanes, if appropriate together with an inert carrier gas, are passed through a flow reactor, the gaseous alkylsilane is heated in the flow reactor and at a temperature in the range from 450 ° C. to a Temperature as it occurs in the plasma torch, thermally decomposes and then collects the powdery product in a separation chamber.
• the invention further relates to a device for performing the method, which is characterized by a graphite tube with a conical inlet, which is located in a Keisel glass tube, a heating device for heating the graphite tube, cooling devices which are attached to the end of the silica glass tube, a water-cooled gas inlet system and one Separation chamber.
• alkylated silanes or alkylated halosilanes can be used whose carbon: silicon ratio is 1: 1.
• Alkylsilanes or mixtures thereof of the general formula can be used as starting materials in the process according to the invention
• R represents an alkyl group having 1 to 4 carbon atoms, such as a methyl, ethyl, n-butyl, propyl, isopropyl or
• Butyl group, and n can have values from 1 to 4 ..
• methyl and ethylsilanes such as monomethylsilane, dichloromethylsilane, monoethylsilane, diethylsilane, ethylmethylsilane or mixtures of these alkylsilanes.
• the alkyl silanes can e.g. from the large-scale halogen-substituted alkylsüanes (e.g. methyltrichlorosilane) used as intermediates in the manufacture of silicones by reduction with lithium aluminum hydride or sodium hydride in a yield of over 90%.
• alkylsüanes e.g. methyltrichlorosilane
• the alkylsilanes In the absence of oxygen, the alkylsilanes generally have a relatively high thermal stability. Decomposition into solid and gaseous fission products only occurs at temperatures between 400 and 500 ° C. These mostly bright yellow, solid cleavage products of the composition (SiC H 3 ) x , which are formed at low temperatures, can be afterglow at higher temperatures, usually above 1000 ° C, in an inert atmosphere or in vacuum in pure, cubic silicon carbide powder of light yellow Convey color.
• the process according to the invention can be carried out at temperatures in the range from 450 ° C. to a temperature as occurs in the plasma torch.
• the process according to the invention is normally carried out at temperatures in the range from 450 to 2000 ° C. or at temperatures such as occur in the plasma torch. Temperatures generally occur in the plasma torch which are above 3000 ° C. or significantly above.
• the process according to the invention can in principle also be carried out at temperatures in the range from 2000 to 3000 ° C. It is preferably carried out at temperatures of 1000 to 2000 ° C.
• the thermal cleavage of methylsilane takes place, for example, with the liberation of hydrogen in the following stages:
• the powders produced in the temperature range from about 450 to 1000 ° C. can be converted into pure ⁇ -SiC by afterglow, for example at 1600 ° C.
• afterglow for example at 1600 ° C.
• the best sintering results are achieved with powders that have been produced above 1600 ° C and can be used for sintering with additives directly without afterglow.
• Chemical analysis shows a slight excess of silicon in powders with deposition temperatures of 1000 to 1500 ° C, which decreases with increasing temperature.
• An excess of silicon however, has a strong inhibiting effect on the sintering behavior.
• this excess can be removed by evaporation due to the high surface area of the powders.
• the powder which is in the lower temperature range e.g. was produced at a temperature of 450 to 1500 ° C, then subjected to a subsequent annealing treatment at elevated temperature, at a temperature in the range from 1400 to 2000 ° C, in an inert atmosphere or in vacuo.
• the desired sintered material is thus obtained. It was surprising and not obvious that a small particle size sintered material is obtained by this annealing treatment. With knowledge of DT-PS 1 047 180, the person skilled in the art should have expected that a coarsely crystalline carbide is formed by such an annealing treatment, as is described in claim 2 of said patent.
• the alkylsilanes can be thermally decomposed in pure form or in dilution with an inert gas, such as noble gases, for example argon or helium, with nitrogen, hydrogen, mixtures of these gases.
• an inert gas such as noble gases, for example argon or helium
• the decomposition can take place in vacuo, at normal pressure or at elevated pressure, and the ratio of volume unit of gaseous alkylsilane to volume unit of diluent or inert gas can vary within wide ranges.
• Pure alkylsilane can be thermally decomposed or an inert gas can be added to the alkylsilane, for example in a volume ratio of 1: 1 to 1:10. These limits are not critical and can, if necessary be exceeded or fallen below. For reasons of economy, however, it will be less preferred to use excessive amounts of inert gas in the process according to the invention.
• the alkylsilane or a mixture of alkylsuane with or without inert or carrier gas is passed into a tubular reactor and thermally decomposed in the tubular reactor.
• the residence time in the tubular reactor is extremely short. Surprisingly, it was found that residence times in the range from 1 to 5 see are sufficient to give the desired thermal cleavage. At higher temperatures, the residence times can also be less than 1 see, and residence times of 10 -1 see can occur in the plasma torch.
• the advantages of the process for powder production according to the invention lie in an almost 100% yield of silicon carbide and in a broad possibility of influencing the particle size, the specific surface area and the crystallite size by varying the flow parameters and the deposition temperature.
• the specific surface can be varied by increasing the flow rate in the range from 5 to 100 m 2 / g.
• particles with a uniformly narrow particle size distribution are obtained in the process according to the invention.
• the particles are spherical and have a size in the range from ⁇ 0.1 to 1 ⁇ m.
• the size like the specific surface, depends on the deposition conditions.
• the powders obtained at low deposition temperatures prove to be X-ray amorphous; the X-ray reflections of the cubic silicon carbide only become visible above about 1100 ° C.
• the crystallite sizes determined by calculation from the X-ray line distribution are in the range from approximately 50 to 200 ⁇ .
• the powders are of high purity, with average values of metallic impurities below 100 ppm.
• the oxygen content of the powder can be adjusted accordingly tion by carrier gases can be reduced to values of 400 to 1000 ppm, for example 700 ppm.
• the sintering can take place with or without the application of pressure.
• the high-purity silicon carbide powder used to produce the moldings can contain sintering aids.
• silicon carbide powder in the submicron range can also be produced with a boron and carbon content if, together with the alkylsilane, volatile boron and carbon compounds are introduced into the flow reactor in such an amount that the silicon carbide powder obtained is 0.3 to Contains 3 weight percent boron and 0.3 to 3 weight percent carbon.
• the boron can also be added to the silicon carbide in the form of various compounds.
• the usual sintering additives can be added to the silicon carbide. such as boron and B.C. Surprisingly, however, it was found that sintered bodies with particularly good properties can be produced if the silicon carbide is added as a sinter
• B 4 C may also be formed.
• Silicon boron compounds of this type can only be formed by annealing or sintering at higher temperatures (1800 to 2000 ° C) if B and C are added either in the gas phase during powder production or by elementary mixing. It was therefore reasonable to assume that these compounds di add directly as a sintering aid instead of B or BC. Surprisingly, it was found that this is possible and that the direct addition of these compounds makes it possible to meter the sintering additives better.
• the silicon carbide powders produced according to the invention with or without boron and carbon content are suitable for the production of moldings.
• the known sintering additives, such as B and BC can be added to the silicon carbide powder without boron and carbon content, as explained above.
• the compounds mentioned above, namely SiB 4 , SiB 6 , B 12 (C, Si, B) 3 , or B 14 Si or mixtures of these compounds can also be added to it.
• silicon carbide powder with a boron and carbon content is produced according to the invention, then such sintering additives can optionally also be added in the production of moldings, provided that the boron and carbon content of the silicon carbide powder is not sufficient to give a molded body whose boron content is between 0.3 to 3 percent by weight and its carbon content is between 0.3 and 3 percent by weight.
• the invention thus also relates to the use of the high-purity silicon carbide powder produced according to the invention for the production of sintered ceramic bodies.
• DT-OS 2 624 641 and DT-OS 2 449 662 describe the production of sintered ceramic bodies and processes for their production.
• the high-purity silicon carbide powders produced according to the invention can be processed to sintered bodies by these processes known per se.
• the invention further relates to a device for performing the method.
• a suitable apparatus for chemical vapor deposition of silicon carbide powders by thermal cleavage in alkylsuane is shown schematically.
• a graphite tube 1 with. conical inlet is in a silica glass tube 4.
• Several layers of graphite felt 2 provide thermal insulation.
• the graphite tube is heated to a length which is approximately twice as large as the inside diameter from the outside via an induction coil 3 with a high-frequency generator 10.
• the silica glass tube carries water-cooled flanges 5 at both ends.
• the water-cooled gas inlet system 6 which consists of a central nozzle for the reaction gas or the inert diluent gas and an annular gap nozzle for the inert gas and opens into the conical part of the graphite tube.
• the very finely divided silicon carbide powder formed by the silane cleavage passes into the deposition chamber 7, which is provided with the cooling flange 5 at the lower end of the silica glass tube and is provided with a micro-mesh filter 8.
• the gas inlet system and the two cooling flanges are connected in series in a cooling water circuit 9.
• the entire experimental system can be evacuated via a rotary vane pump 11 and is sealed against the atmosphere by a wash bottle 12 with a sealing liquid.
• the gas supply of the alkylsilane takes place from the gas bottle 17 via an adjustable flow meter 16 into the central nozzle.
• the inert diluent gas is supplied from the gas bottle 13 while flowing through an oxygen absorber 14 via two adjustable flow meters 15 into the ring nozzle or, in order to dilute the silane, into the central nozzle.
• the inductively heated graphite reactor can of course also be directly resistance-heated or indirectly heated by radiant heat. Two discharges are also provided for the simultaneous deposition of sintering additives. Hydrocarbons for the separation of additional carbon are introduced via the flow meter 18, a further inlet 19 is used for boron compounds,
• methylsilane as the reaction gas and argon as the inert gas are introduced together under atmospheric pressure.
• the average residence time of the gases in the heating zone is about 1 see.
• the methyl silane is decomposed at different temperatures.
• Example 4 For a pressure sintering test, 4 g of the powder from Example 4 were prepared with the addition of 2 percent by weight aluminum powder with mixing with carbon tetrachloride.
• the powder was pressed in a graphite matrix with an inner diameter of 20 mm at a pressure of 30 N / mm 2 , which resulted in a relative density of 51%.
• the sample was then hot pressed at a temperature of 2000 ° C and a pressure of 35 N / mm 2 under an argon atmosphere. After a holding time of 30 minutes, a density of 3.14 g / cm 3 was reached, corresponding to a relative density of 97.8%.
• the sample was then heated under an argon atmosphere in a graphite resistance furnace at a rate of 800 ° C./h and held at a temperature of 2100 ° C. for 15 minutes. There was a linear shrinkage of 22.8%, the final density reached 3.13 g / cm 3 , corresponding to 97.5% relative density.
• Example 6 A powder amount of Example 6 was pressed according to Example 9 with additions of 0.5 percent by weight of carbon and 0.5 percent by weight of boron to a green density of 1.26 g / cm 3 , corresponding to 39.3% relative density. The subsequent sintering was carried out at a temperature of 2050 ° C and a holding time of 10 minutes. The final density reached was 3.03 g / cm 3 , corresponding to a relative density of 94.4%.
• a powder produced at 1600 ° C with a specific surface area of 21.9 m 2 / g and an oxygen content of 450 ppm was mixed with additions of 1 weight percent SiBg and 1 weight percent C as polyphenylene with the addition of benzene, then dried and isostatically at a pressure of 3 kbar pressed to a density of 1.58 g / cm 3 , corresponding to 49.2% relative density.
• the sintering was carried out at a heating rate of 1450 ° C / h under an argon atmosphere up to a temperature of 2100 ° C; the holding time at this temperature was 16 minutes.
• the density achieved was 2.92 g / cm 3 , corresponding to 91.0% relative density.

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Citations (7)

• 1977
  • 1977-10-04 DE DE19772744636 patent/DE2744636A1/de not_active Withdrawn
• 1978
  • 1978-10-04 JP JP54500031A patent/JPS6225605B2/ja not_active Expired
  • 1978-10-04 DE DE7878900139T patent/DE2862428D1/de not_active Expired
  • 1978-10-04 WO PCT/EP1978/000016 patent/WO1979000178A1/de unknown
• 1979
  • 1979-04-09 EP EP78900139A patent/EP0006921B1/de not_active Expired

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