DE10050775A1 - Hot tube gas phase pyrolysis of organosilicon compounds to give silicon carbide/nitride powders involves high flow speed and high turbulence mixing with one inflow below precursor decomposition temperature and other above - Google Patents

Abstract

Gas phase production of SiC/SiN powder by hot tube pyrolysis of organo-Si compounds involves high flow speed and high turbulence mixing of (i) a mixture of a precursor vapor and a carrier gas at below the precursor decomposition temperature with (ii) an inert gas (and optionally a reaction gas) at above the precursor decomposition temperature.

Description

The invention relates to a method for producing of silicon carbide-silicon nitride powder from organometallic substances by means of hot tube pyrolysis over the gas phases process by built in a hot tube reactor with a Gas supply Verify gases of different temperatures to be mixed.

Organometallic substances are used in many ways as precursors for the manufacture of materials. These substances are particularly suitable for processes of chemical vapor deposition (English: CVD) for the formation of ceramic layers and coatings, as well as for compact ceramic material, since the subsequent network of the elements involved is pre-shaped in them. For the production of a Si-CN composite ceramic, which is a mixture of SiC and Si ₃ N ₄ phases, mainly organic silicon compounds are used. According to the possibility of evaporating these compounds before thermal decomposition, these compounds can be pyrolyzed in the gas phase. Connections can be used for this, e.g. B. in DE 35 10 264 A1, Yogyo Kyokaishi 95 (1987) or J. of Materials Science 28 (1993). By converting an organic silicon compound into the vapor form and mixing the vapor with a carrier gas and a reactive gas, amorphous fine silicon carbide-silicon nitride powders are formed by heating, which contain varying amounts of silicon, carbon and nitrogen in accordance with the concentration of the reactive gas and represent the suitable raw material for the production of Si-CN composite ceramics.

Known examples of an organic silicon compound used to synthesize an amorphous silicon carbide silicon nitride powder are vaporizable compounds, e.g. B. silazane compounds such as [(CH ₃ ) ₃ Si] ₂ NH, [(CH ₃ ) ₂ SiNH] ₃ , [HSi (CH ₃ ) ₂ ] ₂ NH, [(CH ₃ ) ₃ Si] ₃ NCH ₃ and [ (CH ₃ ) ₂ SiNCH ₃ ] or other cyclic or longer-chain or mixtures of various silazane compounds, aminosilane compounds such as CH ₃ Si (NHCH ₃ ) ₃ , (CH ₃ ) ₂ Si (NHCH ₃ ) ₂ and (CH ₃ ) ₂ Si [ N (CH ₃ ) ₂ ], cyanosilane compounds, such as (C ₆ H ₅ ) ₃ SiCN and (CH ₃ ) ₃ SiCN or alkoxysilane compounds. Examples of the carrier gas are non-oxidizing gases such as nitrogen or argon. Ammonia or hydrogen can be used as gases for regulating the carbon and nitrogen content in the silicon carbide-silicon nitride powders.

The manufacture of silicon carbide silicon nitride powders the gas phase process takes place according to various known Principles that are related to thermal excitation in 3 Classify basic types: plasma, laser and hot tube py rolyse. The present invention relates to the general my energetically and apparatus technically favorable hot tube py rolyse.

A known way of performing the hot tube pyro lysis is achieved by slowly flowing through a gas mixture (organic silicon compound / carrier / reaction gas) by a hot tube in DE 35 16 589 A1, DE 35 10 264 A1 and DE 690 15 882 T2. The gas mixture being so slow flows through the hot tube so that it is at reaction temperature Warm up and bring to powder for implementation can. The flow in the tube from temperature-resistant Material can be led through internals and / or through vertical right or horizontal arrangement of the hot pipe affected to be flowed.

A major disadvantage of this process is the change mutual occurrence of layer-like deposits by CVD Processes and particle precipitation due to slow flows tion processes. This leads to rapid growth in the hot  outer tube, which also detach itself in places. It There are therefore parts of coarse particles in the powder that the other particle size distribution (0.01-5 µm) not too order (called macroparticles). The macro particles have chemical differs from the other powder and physical properties and act in ceramic Material damaging to properties, especially with regard to breakage stigkeit. You can not or only elaborately Powder are removed. Pyrolysis is also limited by an overgrowth of the hot pipe.

Further disadvantages of this invention reside in the difficulty adjustability and reproducibility of properties of the Silicon carbide silicon nitride powder (e.g. particle size ver division, C / N ratio) due to temperature and flow fluctuations.

Another well known way of performing the hot Tube pyrolysis is given with the introduction of the gas mixture (organic silicon compound / carrier / reaction gas) with egg a needle valve entering a hot pipe into a vacuum, as described in Nanostructured Materials 4 (1994). The Ab separation of the resulting silicon carbide silicon nitride Powder takes place at the outlet of the hot tube cold substrate and wipers.

The main disadvantages of this method are very minor Throughputs of just a few milligrams per minute and the necessary high equipment expenditure for the particles divorce. Deposition of layers and No particles in the hot tube in this process either be avoided.

Another well known way of performing the hot Tube pyrolysis is given by blowing in an organic one Silicon compound by means of an ultrasonic nozzle in a mixture  reactive gas and non-oxidizing gas as described in J. of Mat. Science 28 (1993). This creates initially of the hot pipe drop distributions of the organic silicon umverbindung. These are subject to the flow of hot water Pipe of pyrolysis. The separation takes place in a cool powder container.

The main disadvantages of this method are ge wrestle throughput and the lack of adjustability of exact concentration ratios of the organic sili ziumverbindung and the reactive gas to defined leg Flow of the carbon-nitrogen content in the silicon carbide-silicon nitride powder. The particle size also depends The distribution of the resulting powders largely depends on the Conditions under which the existing drop distribution is heated in the hot tube. This has the additional influence unknown unknown parameters on the powder quality ge.

The object of the invention is therefore to develop of a process for the production of silicon carbide Silicon nitride powder made from organic silicon compounds by means of hot tube pyrolysis with stable temperature and flow conditions, the emergence of procedural Excludes macroparticles. The resulting silicon carbide Silicon nitride powder must have the usual particle size External distributions of the primary particles in the range from 0.01 to 1 µm and in suspension measurements of the agglomerates in the area have from 0.1 to 5 µm. In the process, the Konzen ratio of the organic silicon compound to the reactive gas to be changeable to the C / N ratio of the Si silicon carbide-silicon nitride powder defined to set can.

This object is achieved by that described in the claim Procedure solved. Advantageous further refinements of the  Procedures are described in the subclaims.

The inventive method for the production of silicon carbide-silicon nitride powder by hot tube pyrolysis of organic silicon compounds is that a mixture consisting of a vaporized organic silicon compound and a non-oxidizing gas (e.g. N ₂ or Ar) with a gas mixture heated to a predetermined temperature, consisting of a reactive gas (e.g. NH ₃ or H ₂ ) and a non-oxidizing gas, is mixed. The reactive gas is used depending on the concentration to regulate the carbon and nitrogen content in the silicon carbide-silicon nitride powder.

In particular, the process operating under normal pressure consists in supplying a mixture consisting of a vaporous organic silicon compound and a carrier gas (N ₂ ) through an inner tube (feed tube) into an outer hot tube which has a defined set temperature. The gas mixture, consisting of the vaporous organic silicon compound and the carrier gas, is fed out of the inner tube through distributors using a burner-like mixing unit and is thereby mixed turbulently with a carrier / reaction gas mixture supplied in the outer hot tube. The gas mixture in the inner tube is preheated to a temperature above the boiling point, but below the decomposition temperature of the organic silicon compound. The gas mixture supplied in the outer tube has a set temperature above the decomposition temperature of the organic silicon compound, but below 1400 ° C. This can be achieved by positioning the burner-like mixing unit in front of the zone of the highest temperature of the hot tube, so that the temperature on the axial temperature profile of the decomposition temperature of the organic silicon compound does not yet correspond. The axial temperature profile of the hot, outer tube is determined by the arrangement of windings of a resistance heater which are applied to the outside of the hot tube and is intended to represent a rising and falling profile after the zone of highest temperature in its course along the tube axis.

Due to the separate gas supply, the carrier gas / reaction gas mixture via the hot tube separately from the organi silicon compound vapor / carrier gas mixture heated and with the organic present at a lower temperature Silicon compound vapor / carrier gas mixture through the bren ner-like mixing unit with high turbulence guided. The intimate mixing causes the heating of the Vapor of the organic silicon compound at temperatures above its decomposition temperature, so that pyrolysis and Powder formation can take place in a targeted manner.

The hot pipe, the burner-like mixing unit and that Feed pipe are made of temperature-resistant material, preferably aluminum oxide or silicon carbide. The said burner-like mixing unit is so designed leads that the combination of the two process gases ver different temperature according to the Ver drive as fast and intensive as possible, d. H. with high turbu lenz takes place. The constructive shape is advantageous the shape of burners without premix when using Fuel gas and air similar to those in different Indu line applications are used, e.g. B. lance or coin dung mixed burner. Because of the short flame length and the extremely fast mixing is the principle, for example of the swirl burner very cheap.

The procedure will be concluded with the appropriate approval formation of the silicon carbide formed during the pyrolysis Silicon nitride powder. This is done, for example, by air-tight containers with forced deflections or that  Introduce into large volume, also air-sealed Be containers in which the powders can sediment.

The following examples and illustrations serve to explain tion of the invention without being limiting. It demonstrate:

Fig. 1 the principle of supply of the gases to be mixed,

FIG. 2 burner like mixing unit (known in principle construction)

Fig. 3 particle size distribution,

Fig. 4 scanning electron micrograph of the silicon carbide-silicon nitride powder obtained.

1st example

For producing a silicon carbide-silicon nitride powder hexamethyldisilazane ([(CH _{3) 3} Si] ₂ NH) in N ₂ was brought -Trägergasstrom in the described structure (see FIG. 1 and FIG. 2) for conversion. To regulate the carbon content in the powder, NH _{3 was added} in a concentration of 4% by volume, based on the total amount of carrier gas / reaction gas, by the external gas stream. The pyrolysis temperature was 850 ° C. The total volume flow was 308 l / h, of which 32 l / h for the inner feed and 276 l / h for the outer gas flow were omitted. The concentration of hexamethyldisilazane was set at a throughput of 1.5 g / min and was therefore also 4 vol.% In the pyrolysis, based on the total amount of carrier gas / reaction gas. The powder deposited had a carbon content of 18.45% by mass. The nitrogen and silicon contents were determined to be 23.05% by mass and 48.07% by mass, respectively. The hydrogen content was 3.02% by mass.

2nd example

The ratios of the total volume flow and the NH ₃ concentration have been set in accordance with Example 1. The pyrolysis temperature was 950 ° C. and the concentration of hexamethyldisilazane was set at 2.7 g / min at 8% by volume, based on the total amount of carrier gas / reaction gas. The powder produced in this way had a carbon content of 25.01% by mass. The nitrogen and silicon contents were determined here at 16.61% by mass and 48.15% by mass. The hydrogen content was 1.81 mass%.

The powders obtained according to the invention are X-ray amorphous and are agglomerated. Suspension measurements of the agglomerate distribution (laser granulometer) show particle size distributions between 0.2 and 1 µm ( Fig. 3). When sieving with a test sieve of approx. 5 g powder with a sieve mesh size of 5 µm, residues of less than 2% by mass occur. These disintegrate with gentle ultrasonic treatment in the bathroom (e.g. step 1, 1 min) and can also be sieved. In a further suspension measurement, the residue treated in this way approximately shows the above particle size distribution ( FIG. 3). The silicon carbide-silicon nitride powders obtained consist of primary particles in the size of 0.03-0.8 µm. An image of the primary particles of a silicon carbide-silicon nitride powder is shown by means of a scanning electronic recording ( FIG. 4).

Claims (5)

1. A process for the production of silicon carbide-silicon nitride powder by hot tube pyrolysis of organic silicon compounds in the gas phase, characterized in that a mixture consisting of a precursor vapor and a carrier gas and which has a temperature which is below the decomposition temperature of the precursor , with an inert gas or a mixture of an inert gas and a reaction gas, which has a temperature which is above the decomposition temperature of the precursor, is mixed with a high flow rate and high turbulence. 2. The method according to claim 1, characterized in that in order to achieve the high flow speed and door bulence the mixing of the two phases, on the one hand the Precursor vapor and on the other hand contains the inert gas, by means of a device is made, the burner-like construction Features which most advantageously corresponds to the nomenclature speaking swirl burners. 3. The method according to claim 1, characterized in that the Hot tube pyrolysis under normal pressure and in the temperature range from 700 to 1400 ° C is carried out. 4. The method according to claim 1, characterized in that the Concentration of the precursor in the range of 1-50 vol .-%, bezo gene set to the total amount of carrier gas / reaction gas becomes. 5. The method according to claim 1, characterized in that as Reaction gas ammonia and / or hydrogen in the concentration ratio of 0 to 50 vol .-%, based on the total Trä amount of gas / reaction gas.