DE1258850B - Process for the production of pure silicon carbide - Google Patents

Description

Process for making pure silicon carbide The invention relates to a process for the production of pure silicon carbide from the gas phase.

From scientific studies it is known that from condensed (liquid or solid) silicon and CO gas at higher temperatures silicon carbide can arise, which, however, contains oxygen or Si0z.

It is also known to form silicon carbide by reacting SiO gas with solid carbon at about 1400 ° C. or by reacting SiO-CO gas mixtures with gaseous hydrogen-containing carbon compounds, such as methane, acetone. In this case, CO only plays the role of a diluent, since SiO gas is very stable alongside CO at temperatures above 1400 ° C. Apart from that caused by known methods by arise during cooling of the SiO-CO mixtures by disproportionating decay Si, SiO2, C, CO, large amounts of impurities such as carbon black, SiOrest or SiO2.

The elimination of this impurity makes it very difficult because the resulting silicon carbide is extremely finely divided and so easily oxidized.

One has further, on the occasion of scientific investigations, at 1850 ° C a formation of 0.25 mm thick, 1 to 2 mm long needles on a very small scale of silicon carbide from Si gas of very small partial pressure and toluene vapor to the Walls of a graphite tube observed. It was not possible to obtain larger crystals.

It is also known to have pure, well-formed Si carbide crystals Manufacture by Si carbide at 2500 to 2650 ° C in a no local temperature differences having vessel in an essentially resting and not participating in reactions Atmosphere is sublimated by noble gases, HZ or carbon oxide; the supplied Protective gas is only connected to the reaction vessel on one side. This evaporated Inevitably, part of the decomposition formed during the evaporation of the SiC Si vapor and is more or less lost for the process. With that way of working the sublimation process goes very slowly with a very limited yield in front of you.

It is further known by direct synthesis from silicon and Manufacture carbon hexagonal SiC crystals by putting in a one from the outside inwardly running temperature gradient having graphite chamber, which is in an oven heated to 2300 to 2500 ° C, in an inert gas atmosphere or in a carbon dioxide-nitrogen atmosphere with no more than about 1 mole percent nitrogen Silicon evaporates and the steam on perpendicular or almost perpendicular to the temperature gradient arranged carbon surfaces can act. As a result of the siliconization of the applied Carbon surface, the carbon components gradually wear down. The more or less small-crystalline carbon that arises more or less as the carbon crumbles and less valuable part of the formed contaminated with impurities of carbon Si carbide contaminates the desired, more valuable, large crystalline Si carbide.

The invention relates to a process for the production of Si carbide from the gas phase, in which at least one Si-containing, oxygen-free and hydrogen-free Reactants with at least one C-containing reactant at temperatures is implemented above 1400 ° C.

In a known method of this type, Si carbide is produced by gas phase reaction of SiC14 with a halogenated hydrocarbon at 800 to 2500 ° C, in particular 1400 to 2000 ° C.

The invention is based on the object of providing particularly pure Si carbide to manufacture.

This is achieved according to the invention in that the reactants, of which the Si-containing and / or the C-containing are halogen-free, at 1400 to 2350 ° C are implemented.

According to one embodiment of the invention, gaseous silicon and / or oxygen-free, hydrogen-free gaseous Si compounds with gaseous C compounds merged and reacted at 1400 to 2350 ° C, with silicon carbide separates from the gas phase. The procedure here is to use both reactants in a. Reaction vessel introduces and then heated. Preferably but one will proceed in such a way that the reactants, separately or together, z. B. in. Continuous manner, in the reaction vessel heated to the reaction temperature introduces. Of course, the reactants can also be heated and individually then reacted at the reaction temperature.

If one of the reactants consists of a halogen-containing compound, it is advantageous to bring the halogen-free reactants to temperatures above the To heat the reaction temperature and that kept at lower temperatures to add halogen-containing partners. But the reaction temperature of 1400 can be maintained or maintained up to 2350 ° C.

According to a further embodiment of the invention, mixtures of the gaseous reactants (z. B. Si gas and CO-containing gas) at high temperatures, z. B. 2450 to 2800 ° C, formed and at temperatures of 1400 to 2350 ° C, z. B. 1900 to 2350'C, cooled. From the reacting and cooling gas, the silicon carbide which is formed can be removed by methods known for the deposition of solids at high temperatures, e.g. B. in separation chambers, cyclones, on growth surfaces, growth germs, at temperatures above 1400 ° C are deposited.

As a C source for SiC formation in the method of the invention C-containing gases, especially CO gas, and preferably elemental as Si source Si gas, possibly also Si carbide gases, such as SiCZ gas and SiZC gas, and gases containing other Si carbide molecules, possibly mixed with inert gases, used.

These elements can be used as a further Si source or as a C source containing gases, such as. B. S'C14, hydrocarbon gas and CC14 gas, whole or are partly used in the process. The per se known co-use Nothing stands in the way of H2 as a halogen reductor in the process of the invention either.

According to an advantageous embodiment, elementary Si gases and CO-containing gases are assumed which are in equilibrium at temperatures of 2450 to 2800 ° C. in practically essential areas of SiGas - COGas - SiOGas mixtures. The mixture is cooled to 1900 to 2350 ° C. in a continuous or semicontinuous procedure.

The Si-containing or C-containing gases to be used for the present process for the production of pure silicon carbide can be prepared by known processes, e.g. B. be prepared as follows: evaporation of silicon; thermal or chemical decomposition of commercially available, less pure Si carbide; chemical, Si gas-forming reactions, such as. B. after Si02 + SiC @ 2 CO + 3 SiGag at high temperatures using quartz, silica or silicates under some pressures and temperatures that appear thermodynamically suitable to the person skilled in the art, if appropriate using carrier gases, or by allowing SiO gas to act on solid Si carbide; if sufficiently high temperatures are used, it is possible, if desired, to work without a carrier gas or with very small amounts of the same. The generation of Si-containing gas can, for. B. in a known manner in a fluidized bed under normal pressure, negative pressure or positive pressure with or without the aid of carrier gases. Likewise, the method according to the invention of SiC deposition taking place from the gas phase can itself be carried out under normal pressure, underpressure or overpressure. Overpressure in particular generally favors the extent of the SiC deposition according to the invention.

The implementation of the method of the invention is not intended to be reactive Action and contribution of elemental carbon bound. It can even safe, compact carbonaceous materials, such as graphite or possibly Carbon bodies partially or completely converted into Si carbide, as building materials for Pipelines, reaction chambers, separation chambers and sieves are used, because in the method according to the invention the inside of the C-materials under an external one SiC layer and / or under an adjusting CO cushion substantially against destructive decomposition is protected by Si gas and / or SiO gas.

Apart from that, materials are available that are compared to those containing Si0 Gases are fairly stable, such as B. Zr carbide, Ti carbide, mixed carbides as well as borides or nitrides, e.g. B. boron nitride.

When SiC is formed from Si gas and CO, gases containing low levels of Si — O compounds are produced as a by-product. These can be combined with larger amounts of carbon, which may be provided in special rooms, at temperatures of more than 1200 ° C., preferably from more than 1400 to at most about 22.00 ° C., further amounts of Si carbide being deposited. The aforementioned SiO gas removal simultaneously prevents the formation of contaminating SiO2 at lower temperatures, since the SiO content is below its disproportionation limit to SiOs and Si or to SiO2 and SiC. (due to the presence of CO) is depressed. According to the invention, when the reaction system 2 SiGas -E- CO Z-e SiC + si0 is used, it is advantageous to work (at such Si0 partial pressures) that the Si0 partial pressure always does not exceed at least the limit of disproportionation to silicon and SiO.

The SiO gas produced in this embodiment of the invention can also withdrawn or with removed carbon to SiC with evolution of CO implemented.

The SiO gas can also be converted back into SiGas-rich gas by contacting SiC heated to over 2300 to 2600 ° C, according to the reaction of equation 2 Sic, as -f- CO H Si, which runs largely from right to left at high temperatures. C + SiOGas which runs inversely from left to right when the temperature of the gas mixture is sufficiently high, e.g. B. to about 1900'C, is reduced.

The slower the cooling or the flow of the gases, the greater and more regular the separation of the crystals. Depending on the im. Depending on the chosen arrangement, the deposition takes place either as floating crystals, fibers, cotton wool-like shapes and / or as growth crystals on the vessel walls, from where they can be removed. If you mix z. B. the hot Si-containing gas with relatively cool CO gas, so that the resulting gas mixture only has a moderate temperature of z. B. 1950'C , a rapid excretion as floating particles is made possible. The latter can be used e.g. B. separated by settling from the gases. The separation of the formed Si0-containing gases from the formed Si-carbide takes place where the gas mixture is still hot enough and before it has cooled down so far that there is a risk of impurities, such as SiO2, being precipitated by disproportionation of SiO or by whose reaction with CO exists.

Including the Si carbide formed in the process according to the invention of the further Si carbide optionally formed from Si-O gases and carbon can e.g. B. in a known manner in a fluidized bed, for. B. from SiC grains, to be deposited.

If carbon particles are used as the carrier body of the fluidized bed on, it is true that these can also be used to deposit SiC crystals from gases containing Si and CO. From Si0-containing gases, however, as they are z. B. be present after the deposition and possibly already during the SiC deposition, and such carbon grains arise as the latter infiltrates in a known manner with the Si0-containing gases further, but not from the gas phase formed Si carbide, which is very small in crystalline form at temperatures that are not too high is. In the latter case, a completely pure gas phase reaction is not achieved, man but has the advantage that a practically complete deposition of the entire Si content the gases is achieved in a relatively small space in a short time.

If desired, more specific ones can be used for the purpose of obtaining Si carbide Properties according to methods known per se, such as by adding gaseous Substances (such as Al-containing, N-containing, B-containing) in the input gases of the process, cause doping of the Si carbide to be produced with corresponding foreign elements.

Also the crystal form of the silicon carbide produced according to the invention can be influenced. In general, high temperature and very slow rate of crystallization favor the hexagonal shapes, low temperatures (not without exception) the cubic ones Shapes and fibrous, cotton-like shapes and very rapid formation rate on the other hand, the formation of X-ray amorphous silicon carbide.

The implementation of the method of the invention can be per se for the implementation of gas reactions proceeding with the separation of solid substances Known technological methods are carried out at high temperatures, as are those here repeatedly mentioned work-up of the resulting, low Si-O-compounds containing Gases. Example In a heated tube through which inert gas (Ar, He, HZ) flows Si-resistant, expediently fairly gas-tight material (e.g. siliconized graphite or superficially siliconized pyrolytic carbon) is produced by evaporation or evaporation of liquid silicon at 2300 to 2600 ° C an inert gas-Si-gas mixture generated. The heating tube or a narrower one inserted therein can be used as the evaporator tube Serving tube (inner tube). The position of this evaporator can be inclined, horizontal or be vertical and the silicon is in perforated crucibles, boats or the like. are located. The generated gas stream containing Si gas is flowing or resting CO gas mixed. Immediately from the mixed gas Si carbide separates out, which Excretion is enhanced by applying falling temperature of the gas mixture.

The aforementioned oven, for example, is provided with spacious, in suitable extent, warm, large, surrounding the heating pipe and inner pipe Rooms and two larger discharge and discharge openings for gases and for them solids deposited or suspended therein, such as in particular Si carbide, Mistake. Divide in the hottest part of these rooms, which is relatively close to the heating pipe the SiO2-free Si carbide crystals formed from the gas phase hang on the walls and as a solid suspension in the gas space. In the less hot part, below about 1950 to 2050 ° C, the resulting Si0-containing gas mixture is transported through a porous, granular carbon mass led, whereby the Si0 gas with carbon under formation of slowly deeper SiC surface layers reacts practically quantitatively. This increases the risk of SiO2-containing substances if the temperature is further cooled down to an even lower temperature Products through self-disproportionation of the Si0 gas or through reaction with CO gas to deposit, avoided.

The same is done by passing a gas containing CO over an evaporating gas Silicon at high temperatures is a CO-SiGas-SiO equilibrium gas mixture saturated with gaseous Si carbides generated and deposited by cooling to 1900 to 2000 ° C Si carbide from the gas phase.

The table gives the unknown diagram of the total gas equilibrium at a total gas pressure of one atmosphere for the case of the maximum exploitable SiGas limit pressure (physical saturation pressure of silicon) and the maximum Partial pressure on Si carbides, such as SiC2 and Si2C, again. The CO and Si0 equilibrium pressures PCo and Psio were also calculated on the basis of the present invention free energies and equilibrium constants determined.

In the area in which SiO is inconsistent, by the curve Psio upper Limited stability limit, disproportionate SiO. When working in that area z. B. when cooling down and falling below (temperature) or exceeding (pressure) the stability limit, silicon carbide is formed, which is formed by oxygen compounds, z. B. Si02, is heavily contaminated.

The process of the invention does not, of course, include recovery of SiC according to the well-known Acheson process.

Claims (6)

Claims: 1. Process for the production of pure silicon carbide from the gas phase by reaction of at least one Si-containing, oxygen and hydrogen-free reactants with at least one C-containing reactant at temperatures above 1400 ° C, d u r c h e k e n n -z e i c h n e t that the Reaction partners, of which the Si-containing and / or the C-containing halogen-free are to be implemented at 1400 to 2350 ° C. 2. The method according to claim 1, characterized characterized in that silicon and / or Si compounds with C connections mixed below the reaction temperature and then heated to 1400 to 2350 ° C will. 3. The method according to claim 1, characterized in that silicon and / or Si compounds and C compounds mixed and above the reaction temperature then cooled to 1400 to 2350 ° C. 4. The method according to claim 1 and 3, characterized in that one at 2450 to 2800 ° C is essentially in equilibrium located mixture of elemental Si and a CO containing gas ili continuous or semi-continuous operation is cooled to 1900 to 2350 ° C. 5. Procedure according to claims 1 to 4, characterized in that the in the implementation as Low gas mixture containing volatile Si-O compounds formed by-product during or after the SiC deposition above 1200 ° C with carbon as well is converted to Si carbide. 6. The method according to claim 1 and one of the claims 2 to 4, characterized in that the reaction is carried out in a known manner in the presence a dopant is performed. Considered publications: French patent specification No. 1302 077.