EP2507295A1

EP2507295A1 - Method for producing polysilane-polycarbosilane having reduced carbon content and fibers produced therefrom

Info

Publication number: EP2507295A1
Application number: EP10782266A
Authority: EP
Inventors: Juergen Clade; Arne Ruedinger; Dieter Sporn
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2009-11-30
Filing date: 2010-11-22
Publication date: 2012-10-10
Also published as: US20130011675A1; WO2011064174A1; DE102009056371A1

Abstract

The invention relates to a method for producing a polysilane-polycarbosilane copolymer solution from which a ceramic material having a ratio of silicon to carbon in the range of 0.8:1.0 to 1.1:1.0 can be obtained after removal of the solvent and pyrolysis, comprising the following steps: generating a chloric raw polysilane/oligosilane containing hydrocarbon groups by means of disproportioning a methylchlorodisilane or a mixture of a plurality of methylchlorodisilanes of the composition Si₂Me_nCl_6-n, where n = 1-4, wherein the disproportioning takes place by means of a Lewis base as a catalyst, thermally post-cross-linking the raw polysilane/oligosilane into a non-melting polysilane-polycarbosilane copolymer that is soluble in a neutral solvent, and producing said solution by means of dissolving the polysilane-polycarbosilane in a neutral solvent. The invention is characterized in that additional elementary silicon or titanium disilicide is added in one step of said method in a suitable quantity as a powder or in the form of a compound comprising alkyl groups bonded to silicon or to nitrogen, wherein said additive either (a) takes place in that the raw polysilane/oligosilane is generated in the presence of a cross-linking agent, selected from compounds of the formula CI₂R¹Si-R², having a boiling point above 100°C and where R¹ means chlorine, hydrogen, or an alkyl radical having 1 to 4 carbon atoms, and R² is -SiR³ ₃, -NH-SiR³ ₃ or -N(SiR³ ₃₎₂, where -R³ has the same meaning as R¹, or (b) takes place in that powdered silicon or titanium silicide is added to the polysilane-polycarbosilane solution. Green fibers or material in other forms can be produced from the copolymer solution, and can in turn be converted into ceramic silicon carbide materials. Said material can also be used for constructing ceramic matrices.

Description

A process for producing polysilane-polycarbosilanes with reduced

Carbon content and fibers made from these

The present invention relates to polysilane-polycarbosilane copolymers, which are prepared starting from chlorine-containing silanes by targeted thermal treatment and have a significantly reduced carbon content. Ceramics produced by pyrolysis may have a molar ratio of silicon to carbon of nearly 1: 1, i. be almost or completely free of free carbon.

Ceramics of the stoichiometric composition SiC are essential

more stable to oxidation than ceramics with a carbon excess compared to silicon.

Silicon carbide materials are high for their mechanical strengths

Temperatures and their good oxidation resistance known. Therefore, they come for a variety of uses in question, especially in the form of fibers as reinforcing elements in components that are exposed to high temperatures and / or corrosive media.

Polysilanes were first synthesized by Kipping via Wurtz coupling of diphenyldichlorosilane with sodium. By Yajima et al. For example, dodecamethylcyclohexasilane was first used as a raw material for the production of SiC ceramic fibers. For this purpose, the compound in an autoclave using elevated

Temperature and pressure are cross-linked, with a conversion into

Polycarbosilanes takes place (Kumada rearrangement). After extraction of low molecular weight components, an infusible, high molecular weight polycarbosilane powder is obtained. Solutions of this powder in benzene or xylene can after the

Dry spinning process to green fibers are processed, which can be pyrolyzed without prior curing to SiC ceramic fibers. The main disadvantage of this method is the complex synthesis of the starting polymer, the use of alkali metals, reactions in an autoclave and a complicated

Includes extraction process.

In a variant of this process, the use of high pressures in cross-linking and conversion to polycarbosilane is dispensed with, resulting in a fusible material. This can be processed by the melt spinning process into green fibers, which must then be cured by pyrolysis by aging in air at elevated temperature. The resulting ceramic fibers contain Therefore, several mass% of oxygen, which significantly affects their high temperature stability. Both process variants were patented, see US 4,100,233.

Also known are the synthesis of a phenylmethylpolysilane by Wurtz coupling of a mixture of phenylmethyl and Dinnethyldichlorsilan and the synthesis of branched polysilanes by Wurtz coupling of R ₂ SiCl ₂ / RSiCl ₃ mixtures

(R = methyl, ethyl or phenyl). The spinning behavior (melt spinning process) of the obtained polymers was investigated. Numerous other methods for the synthesis of polycarbosilanes have been proposed. Many of these methods are in

WO 2005/108470 listed.

The disproportionation of disilanes with Lewis bases to mono- and polysilanes was discovered in 1953 by Wilkins. The corresponding reaction with methylchlorodisilane mixtures from the Müller-Rochow synthesis was described by Bluestein and by Cooper and Gilbert. Roewer et al. investigated the disproportionation of the methylchlorodisilanes CI ₂ MeSiSiMeCl ₂ , CI ₂ MeSiSiMe ₂ CI, and CIMe ₂ SiSiMe ₂ CI under both heterogeneous and heterogeneous catalysis. In the former case, nitrogen-containing heterocycles, especially N-methylimidazole, were used in the latter nitrogen-containing heterocycles or bis (dimethylamino) phosphoryl groups which are attached to the

Surface of a silicate carrier were bound. Several oligosilanes could be identified in the product mixture. EP 0 610 809 A1 discloses a thermal

Post-treatment of polysilanes for their conversion into polycarbosilanes disclosed; however, this glassy product is at least relatively mild thermal

Treatment (up to 220 ° C) remeltable.

The production of silicon carbide fibers from the polysilanes thus obtained has been described, for. In EP 668 254 B1. However, since the polysilanes are fusible, the green fibers must be cured at elevated temperature prior to pyrolysis with ammonia.

In order to stabilize the shape of green fibers obtained from polycarbosilanes by melt spinning, curing is generally necessary to render the material infusible before pyrolysis. As a rule, this hardening is done by

Treatment with reactive gas. The originally practiced curing with air at elevated temperature has the disadvantage of an increased oxygen input into the fiber, which greatly affects their high temperature stability (fiber damage by outgassing of CO and / or SiO at high temperatures (T. Shimoo et al., J. Ceram Jap., Ed., 102, 952 (1994)). Attempts have therefore been made to reduce the oxygen input during the To reduce green fiber hardening. Lipowitz (US 5,051,215) describes the

Green fiber hardening with NO2 instead of air; In this case, the oxygen uptake of about 10-15% by mass (air hardening) is reduced to <7% by mass. However, a minimum oxygen content of 5-6 mass% is required to avoid fiber bundle bonding. The also proposed curing by irradiation with

high-energy electrons turn with an unwanted

Oxygen input to be connected, which ultimately causes the cure.

A disadvantage of the older method is that the fibers of fusible

Starting materials, as explained above, must be precured by aging in air or with the aid of ammonia at elevated temperatures, which leads to increased, undesirable oxygen levels and other disadvantages. In contrast, fibers of infusible, high molecular weight polycarbosilane powders can be obtained from solutions of these powders in benzene or xylene after the dry-spinning process

Green fibers are processed, which can be pyrolyzed without prior curing to SiC ceramic fibers; However, the path to such infusible powders is costly and cumbersome.

In order to overcome this problem and to arrive at a process which is easy to handle, WO 2005/108470 discloses a process for the preparation of a polysilane-polycarbosilane copolymer solution, from which oxygen-poor ceramic shaped bodies can be produced. The starting material for this solution is inexpensive and easily accessible and can be very easily converted into an infusible material that can be converted after molding without further treatment in the corresponding ceramic material.

The said starting material is polysilanes, which by

Disproportionation of methylchlorodisilane mixtures, as used as a high-boiling fraction in the direct synthesis of methylchlorosilanes (Müller-Rochow process

(US 2,380,995 (1941); R. Müller, Wiss. Z. Techn. Univ. Dresden 12 (1963) 1633) are obtainable with Lewis base catalysts. Preferably, in this disproportionation, a crosslinking aid is added, selected from

Arylhalosilanes and arylhaloboranes. The polysilanes thus obtained (usually referred to as crude polysilanes / oligosilanes) can be conveniently modified by means of a subsequent, targeted thermal treatment in such a way that, although they are difficult or infusible, they are still very soluble in indifferent solvents that they can be further processed in a molding process. Solutions of these materials can be used, for example, for the production of fibers after Use dry spinning or to build ceramic matrices by the liquid phase infiltration method. Polymer fibers made from these

Solutions are available, can be pyrolyzed in the bundle without further form-stabilizing treatment to oxygen-poor SiC ceramic fibers.

However, it is a disadvantage of these materials that their content is relatively high because of the addition of carbonaceous crosslinking agent in the production of carbon: they are pyrolyzed, one obtains ceramics with a ratio of silicon to carbon in the range of about 2: 3. If, however, n-octyltrichlorosilane is used as crosslinking agent, as used, for example, in DE 37 43 373, the octyl radical is split off during pyrolysis, which is why a lower-carbon but porous product is obtained.

The object of the present invention is to provide a method for producing

To provide oxygen-poor or oxygen-free, polysilane-containing polymers in good yield, which can be pyrolyzed to dense ceramics with a ratio of silicon to carbon in the range of 0.8: 1, 0 to 1, 1: 1, 0th This corresponds to an Si content of 44.4 to 52.4 atom%, based on the sum of carbon and silicon. The same starting materials should be used as stated in WO 2005/108470, since these are readily available,

inexpensive starting materials act.

The object is achieved by the proposal to additionally add elemental silicon or titanium disilicide in powder form or a compound containing alkyl groups bound to silicon or to nitrogen in one of the steps of this method.

The invention can be realized in two embodiments:

Either in the preparation of the crude polysilanes / oligosilanes, which incidentally follows the teaching of WO 2005/108470, a crosslinking aid

of the formula (I)

CI ₂ R ¹ Si-R ² (I)

which has a boiling point above 100 ° C and wherein R ^{1 is} chlorine, hydrogen or an alkyl radical having 1 to 4 carbon atoms and R ² -SiR ³ 3, - NH-SiR ³ ₃ or -N (SiR ³ ₃ ) 2 wherein R ^{3 has} the same meaning as R ¹ . Also

Mixtures of these substances with one another or with arylhalosilanes or -boranes such as phenyltrichlorosilane, diphenyldichlorosilane or phenyldichloroborane are possible, provided that the proportion of crosslinking aid of the formula (I) is at least 5 mole%, based on the sum of methylchlorodisilane, Lewis base and crosslinking aid.

It has surprisingly been found that alkyl groups of a crosslinking agent which are bonded to silicon or nitrogen atoms, remain in the pyrolysis in the ceramic, so that one obtains a dense product.

An alternative way of solving the problem is to do one after another

WO 2005/108470 produced polysilane-polycarbosilane copolymer solution so much powdered silicon or titanium disilicate schliesgt that the carbon excess is reacted at high temperatures to silicon carbide and optionally titanium carbide.

Surprisingly, a dense product can also be obtained in this way, because the carbon formed in the subsequent high-temperature treatment reacts off to silicon carbide or optionally in addition to titanium carbide. Other powdered silicon-containing materials such as S1O2 or Si3N, however, have proved to be less suitable because they are reacted in the former case with carbon to SiC and CO, in the latter case to SiC and N ₂ . The case

released gaseous products CO or N ₂ make the resulting ceramic again porous.

In a preferred embodiment of this embodiment, the powdered silicon or titanium disilicide is surface-rendered hydrophobic, for example, prior to addition to the copolymer solution. by exchanging the surface hydroxyl groups for trimethylsilyl ether groups by boiling with trimethylchlorosilane (based on EP 0378785) or the like. In fact, it has been found that the rheological properties of the polymer-silicon or polymer-titanium disilicide mixture, e.g. are important for a fiber spinning, be significantly improved by this measure.

Accordingly, the same starting materials used for polymer production are the same silane / oligosilanes containing chlorine and hydrocarbon groups, which are also specified in WO 2005/108470 A1 as starting materials. These are mixtures of Methylchlorodisilanen with the composition Si2Me _n Cl6 _-n

(n = 1 -4), and preferably those which are obtained as a high-boiling fraction (bp 150-155 ° C) in the "direct synthesis" by Rochow and Müller. The latter usually consist of a mixture of 1, 1, 2,2-tetrachloro-dimethyldisilane and 1, 1, 2-trichloro-trimethyldisilane with less than 10 mol% of other ingredients. The two mentioned disilanes are preferably initially introduced in a molar ratio of 0.5: 1 to 1.5: 1.

The disilane mixtures mentioned are based on e.g. EP 610809 or U.

Herzog et al., Organomet. Chem. 507 (1996) 221 under homogeneous catalysis with a nitrogen-containing Lewis base and - in the first embodiment of the invention - in the presence of the above-mentioned crosslinking aid of the formula (I), preferably at elevated temperature disproportionated, which during the reaction as

Cleavage products obtained monosilane mixtures are continuously distilled off. The reaction temperature is preferably 150 to 300 ° C, more preferably 180 to 250 ° C. The catalyst used is an organic nitrogen compound having Lewis basicity but no N-H function. Preferred catalysts are nitrogen-containing

Heterocycles such as pyridine, quinoline, N-methylpiperidine, N-methylpyrrolidine, N-methylindole or N-methylimidazole. N-methylimidazole is particularly preferred. The amount of catalyst used is preferably 1 to 2% by mass. As a crosslinking aid, 1,1,1-trichlorotrimethyldisilazane is very favorable, the proportion of this or another crosslinking aid of the formula (I) is preferably 5-20% by mass, more preferably 10-15% by mass. The disproportionation takes place, moreover, under the conditions known in the literature; In particular, it is beneficial to keep moisture and oxygen away from the materials through the use of inert gas, such as nitrogen whiff gas, since the product is sensitive to hydrolysis and oxygen.

Optionally, another crosslinking aid may be present, selected from arylhalosilanes and arylhaloboranes, and especially among

Phenyltrichlorosilane, diphenyldichlorosilane and phenyldichloroborane, wherein the proportion of this agent should not exceed 5 mol%, based on the sum of

Methylchlorodisilane, Lewis base and crosslinking aids.

In a particular embodiment, the chlorine content of the thus obtained

Polysilane / oligosilane can be lowered. This is preferably done in one

subsequent step through a chlorine substitution. In this substitution, chlorine is replaced by a nitrogen-containing, chlorine-free substituent, preferably with the aid of amine and / or silylamine compounds as substitution agents, i. Compounds having at least one N-Si moiety or, more preferably, at least one N-H moiety. In a first embodiment of this preferred

Embodiment, these are preferably selected from ammonia and primary or secondary amines. Particularly suitable are amines of the formula HNR ¹ R ² , wherein R ¹ and R ^{2 are} independently hydrogen, optionally with further amino groups substituted alkyl, alkenyl, aryl, arylalkyl, alkylaryl, arylalkenyl, alkenylaryl or (R ³ ) 3Si- [NR ³ -Si (R ³ ) 2] _m where m = 0 to 6, or wherein R ¹ and R ² together are one Alkylene radical having 4 or 5 carbon atoms or -Si (R ³ ) ₂ - [NR ³ -Si (R ³ ) ₂ ] _n - with n = 1 to 6 represent. In a second embodiment, silylamines, in particular silazanes of the formula Si (R ³ ) 3- [NR ³ -Si (R ³ ) 2] _n -R ³ , in which n can be an integer from 1 to 6, are used. Each radical R ³ is the same or different in all cases and means

Hydrogen, alkyl or aryl. In a third, preferred embodiment, the compounds are secondary, cyclic amines, in particular selected from pyrrole, indole, carbazole, pyrazole, piperidine and imidazole. In a fourth

Embodiment, the substitution with a compound of formula N (R ⁴ ) ₃ , wherein R ^{4 has} the meaning (R ³ ) ₃ Si.

The number of amino groups in R ¹ and R ² is not limited, but is preferably 0 to 6, and more preferably 0 to 4. The number of carbon atoms in R ¹ , R ² and R ³ is also not limited, but is preferably 1 to 6 for aliphatic and 5 to 20 for aromatic and aliphatic-aromatic radicals.

More preferably, the amines are selected from ammonia, ethylenediamine, diethylamine, dimethylamine, methylamine, aniline, ethylamine, hexamethyldisilazane, heptamethyldisilazane and tris (trimethylsilyl) amine. Particular preference is given to amines of the abovementioned which carry short-chain alkyl radicals, in particular methyl and ethyl radicals. Dimethylamine is particularly favorable. Secondary amines have the advantage that the resulting polymers carry -NR ₂ groups, that is, are free of NH functions. The advantage is that in the subsequent crosslinking of such substituted polysilane / oligosilanes a polycondensation of amino groups is impossible, which could lead to poor or no longer soluble products, which of course is not desired according to the invention. Nevertheless, silylamines such as disilazanes are also suitable instead of pure amines, since the entry of

Silicon atoms in the substitution no adverse effects for the later

Shaped body or fibers by itself. The substitution with silylamines also has the advantage that the chlorine is not obtained in the form of an ammonium salt, but in the form of trimethylchlorosilane, which is separated by distillation and in the process chain

can be returned.

The chlorine reduction / substitution usually takes place as follows:

The raw material, ie, the hydrocarbon group-carrying / containing raw polysilane / oligosilane obtained by the above-described disproportionation is dissolved in a suitable inert and aprotic solvent. The solvents used are, in particular, aprotic, non-polar solvents, such as aliphatic hydrocarbons (eg n-pentane, n-hexane, cyclohexane, n-heptane, n-octane), halogenated

Hydrocarbons (e.g., methylene chloride, chloroform, carbon tetrachloride,

1, 1, 1-trichloroethane, chlorobenzene) or aromatic hydrocarbons (eg benzene, toluene, o-xylene, sym-mesitylene) in question, further ethereal solvents (eg diethyl ether, diisopropyl ether, tetrahydrofuran, 1, 4-dioxane or a higher or non-symmetric ether). The solvent is preferably a halogen-free

Hydrocarbon, particularly preferably an aromatic hydrocarbon from the group benzene, toluene, o-xylene.

The substitution agent (amine) is added in a molar excess, which is preferably at least 2: 1, based on the bound chlorine atoms in the starting material. The substitution agent is added neat or dissolved in an inert and aprotic solvent as indicated above. The adding can e.g. by dropping; In this case, preferably a temperature between room temperature and the boiling point of the amine or its solution should be maintained. During or after the dropwise addition, a salt is formed which is insoluble in the solvent or, in the case of substitution with silylamines, trimethylchlorosilane. The suspension is allowed to stand for some time, often several hours, or refluxed to the boiling point of the solvent. It is then optionally cooled to room temperature, and if salt has formed, this is filtered off. Subsequently, the solvent and optionally formed trimethylchlorosilane is completely removed, for example in vacuo.

When using an amine which is gaseous during addition to the crude polysilane / oligosilane, e.g. when using ammonia, this can be introduced as gas or either at temperatures below its boiling point in a

Reaction vessel condensed or filled under pressure as a liquid in this, in the case of liquefied amines optionally under dilution with a suitable solvent as indicated above. Subsequently, the starting material, dissolved again in the same solvent as possible, added. After complete addition, the batch is allowed to stand for a similar period of time as described above or refluxed and then worked up as described above. The chlorine content of the starting material thus treated can be reduced by the process step according to the invention at least to not more than 3% by mass, usually less than 1% by mass and generally to less than 0.2% by mass.

The crude polysilane / oligosilane is then subjected to a further thermal treatment, as described in WO 2005/108470, in which on the one hand it is made difficult or infusible by increasing the average molecular weight and, on the other hand, by the rearrangement reactions taking place in this process

Polysilane-polycarbosilane copolymer is transferred. Another inventively intended effect of this thermal treatment is a repeated

Reduction in the content of chemically bound chlorine, if the previous substitution should not have been quantitative.

The thermal aftertreatment usually takes place under atmospheric pressure, whereby it is highly recommended to work under moisture and oxygen exclusion. Therefore, the material is conveniently treated under inert gas, in a particularly advantageous manner under a pure nitrogen atmosphere, wherein the

Temperatures between 250 ° and 500 ° C, preferably between 300 and 450 ° C and most preferably between 300 and 350 ° C increase. The heating is preferably carried out continuously, for example at a rate of 1-5 K / min., Preferably 2-4 K / min. During the reaction, low molecular weight methylsilylamines and z.T. Methylchlorosilylamines are continuously distilled off. The end point of the thermal aftertreatment is noticeable by a steep increase in the torque at the agitator. During the subsequent cooling phase, the last volatiles can be removed in vacuo in a temperature range around 100 ° C. The infusible but soluble copolymer according to the invention can thus be prepared in a single step from the dechlorinated crude polysilane / oligosilane, which usually no further separation steps (extractions, filtrations) are required. By dissolving this copolymer in an inert solvent, a polysilane-polycarbosilane solution according to the invention is obtained.

If it is desired to spin fibers or form other shaped articles from the polysilane-polycarbosilane copolymer prepared according to the invention, the copolymer becomes as follows

WO 2005/108470 known dissolved in an inert organic solvent. The solvents used are, in particular, non-polar solvents, such as aliphatic hydrocarbons (for example n-pentane, n-hexane, cyclohexane, n-heptane, n-octane), aromatic

Hydrocarbons (eg benzene, toluene, o-xylene, sym-mesitylene), halogenated Hydrocarbons (eg methylene chloride, chloroform, carbon tetrachloride, 1,1,1-trichloroethane, chlorobenzene) or ethers (eg diethyl ether, diisopropyl ether,

Tetrahydrofuran, 1, 4-dioxane or a higher or non-symmetrical ether) in question. The solvent is preferably a halogenated or halogen-free hydrocarbon, more preferably a halogen-free aromatic hydrocarbon from the group benzene, toluene, o-xylene.

The proportion of the polysilane-polycarbosilane copolymer in the polymer solution may be adjusted depending on the purpose of use of the solution. If the solution is for the production of fibers by the dry-spinning method, polymer contents of 50-90% by mass, preferably 60-75% by mass, are advantageous. If the solution is to prepare ceramic matrices by the liquid phase infiltration method, the polymer content may be significantly lower due to the low viscosity required, e.g. 20% by mass.

The second embodiment of the invention is limited to those variants in which the polysilane-polycarboxylic acid copolymer is solubilized and does not interfere with a solid content in the solution, e.g. if the solution is to be spun into fibers as mentioned above. In this embodiment, as

Crosslinking aids used in the disproportionation of Methylchlorodisilane not a compound of formula (I), but there are known from WO 2005/108470 crosslinking aids (an arylhalosilane, an arylhaloborane or a mixture of both, in particular arylchlorosilanes such as phenyltrichlorosilane and / or Arylchlorborane such as phenyldichloroborane) used. The further process steps then take place as described above for the first variant, i. with or without

Chlorine reduction. The resulting infusible but soluble copolymer is finally dissolved in an inert solvent such as toluene. The solution is powdered silicon and / or titanium disilicide (usually with a

Particle diameter of about 1 -2 μιτι) added, which was preferably hydrophobicized as described above, to prevent the sedimentation of the added particles and to keep them in suspension. The proportion of silicon or titanium disilicide powder is calculated so that the excess carbon in the subsequent high-temperature treatment to silicon carbide and optionally titanium carbide

is reacted so that the (Si + Ti): C ratio of the resulting ceramic is between 0.8: 1, 0 and 1, 1: 1, 0. The powder content used for this purpose is preferably 20-60% by mass, more preferably 35-50% by mass, based on the copolymer used. The (spinning) solution thus obtained has one for spinning or other Processing processes suitable consistency and this also suitable flow properties.

The polysilane-polycarbosilane copolymer solution according to the invention is generally suitable for the production of ceramic silicon carbide materials having a silicon to carbon ratio in the range from 0.8: 1, 0 to 1, 1: 1, 0. For this purpose, the polysilane-polycarbosilane is converted from said solution into the desired form. Unless the solvent is already evaporated, it is removed and the remaining material is pyrolyzed under inert gas or reducing atmosphere.

In the following, specifically, the production of SiC ceramic fibers from the

Polymer solutions according to the invention described without a

Limitation of the applications of this solution can be seen.

The production of polymer fibers is carried out by the dry spinning process; this is prior art (F. Fourne, "Synthetic Fibers", Carl Hauser Verlag 1995, p 183; V.B. Gupta, V. K. Kothari (ed.), "Manufactured Fiber Technology", Chapman & Hall 1997, p. Preferred parameters for the spinning process are the use of a nozzle package with nozzles diameter 50 to 300 μιτι and a capillary length of 0.2 to 0.5 mm, a shaft temperature of 20 to 50 ° C at a length of 2 m and a take-off speed of 100 up to 300 m / min.

The polymer fibers according to the invention can without previous

form stabilizing treatment are pyrolyzed. The preferred parameters for pyrolysis are a heating rate between 5 and 50 K / min and a

Final temperature of 900 to 1200 ° C. The pyrolysis can be carried out under inert (N ₂ , argon) or reducing (argon / H ₂ , N ₂ / CO, etc.) atmosphere. The preferred

Atmosphere for the pyrolysis is nitrogen or forming gas (argon with 10 vol .-% H ₂ ). As an oven, for example, an electric oven is suitable.

After pyrolysis, the ceramic fibers may be subjected to a further thermal treatment resulting in their compaction and partial or complete

Crystallization leads and improves their mechanical strength.

The thermal treatment is preferably carried out at temperatures between 1500 and 2200 ° C, more preferably between 1700 and 1900 ° C. In the case of production of materials other than fiber form, the

Pyrolysis and / or optionally the thermal treatment can be carried out under the same conditions as described above for the fibers.

The following examples serve for a more detailed description and illustration of the

Invention, without being seen in a limitation of the scope.

example 1

Following EP 502399, 255.4 g of hexamethyldisilazane with 222.0 g

Silicon tetrachloride is added, and the mixture is stirred at 60 ° C for 10 h. The subsequent fractional distillation in vacuo gives 135 g of pure 1,1,1-trichloro-trimethyldisilazane. Another fraction containing 1, 1, 1, 3,3-pentachloro-trimethyltrisilazane as high boilers, can be worked up by a further fractional distillation.

Example 2

Preparation of a crude polysilane / oligosilane

1000 g of a methylchlorodisilane mixture ("disilane fraction" from the Müller-Rochow process, consisting of in each case 45 mol% of CI ₂ MeSiSiMeCl ₂ and Cl ₂ MeSiSiMe ₂ CI and 10 mol% of CIMe ₂ SiSiMe ₂ CI, bp 150-155 ° C) are mixed with 25 g of N-methylimidazole and 100 g of 1,1,1-trichloro-trimethyldisilazane as crosslinking assistant and heated at 0.5 K / min to 180 ° C. In this case, about 450 ml of a distillate are obtained It consists of MeSiCb, Me ₂ SiCl ₂ and Me ₂ CISiSiMe ₂ CI and 153 g of a dark brown, hydrolysis-sensitive crude polysilane / oligosilane solid at room temperature with a chlorine content of about 30% by mass Dissolved 60 wt% solution containing crude polysilane / oligosilane.

Comparative Example 1

Example 2 was repeated except that phenyltrichlorosilane was used instead of 1,1,1-trichloro-trimethyldisilazane. Example 2

Modification of a crude polysilane / oligosilane with liquid methylamine

100 ml of toluene or xylene are placed in a 1 l double-walled three-necked flask with reflux condenser, dropping funnel and KPG stirrer; the double-walled flask is cooled to -30 ° C by means of a cryostat. About 300 ml of methylamine are condensed in and then added dropwise via the dropping funnel 275 g of a 60% solution of the crude polysilane / oligosilane according to Example 2 in toluene or xylene. The methylammonium chloride precipitated after thawing is filtered off via a pressure filter and the filtrate is freed from the solvent in vacuo at 65.degree. The resulting modified polysilane / oligosilane contains less than 0.2% by mass of chlorine (lower

Detection limit).

Example 4

Modification of a crude polysilane / oligosilane with liquid dimethylamine

The modification is analogous to that described in Example 3, but under

Use of dimethylamine instead of methylamine. The modified one

Polysilane / oligosilane contains a maximum of 0.2% by mass of chlorine (lower detection limit).

Example 5

Modification of a crude polysilane / oligosilane with gaseous dimethylamine

In a double-walled vessel, 1.5 l of a 60% strength by weight solution of a crude polysilane / oligosilane according to Example 2 in toluene or xylene are initially charged and cooled to 0 ° C. by means of a cryostat. Via a dip tube, a slow flow of gaseous dimethylamine is introduced below the liquid level. Of the

Volume flow should be adjusted so that the gas is completely absorbed when it enters the liquid; The contents of the reaction vessel should be stirred vigorously. During the reaction, the temperature is measured by means of an internal thermometer; the consumption of dimethylamine is monitored by means of a balance. After introducing the theoretically required amount of dimethylamine, the reaction is terminated; the end can also be recognized by a decrease in the internal temperature. The reaction mixture is filtered through a pressure filter and the filtrate in vacuo at 65 ° C from the solvent freed. The resulting modified polysilane / oligosilane contains less than 0.2 mass% chlorine (lower detection limit).

Comparative Example 2

Example 5 was repeated, but using 1, 5 I of a 60% by mass solution of the crude polysilane / oligosilane according to Comparative Example 1.

Example 6

Preparation of a polysilane-polycarbosilane copolymer by thermal crosslinking

151 g of a polysilane according to Example 2 are heated in a round bottom flask with 3 K / min to 400 ° C and 50 min. kept at this temperature. During the subsequent cooling, the temperature is maintained at 100 ° C. for 1 h, during which time the last volatiles are removed by applying a vacuum. 16 ml of a yellow distillate consisting of various mono-, di- and oligomethyl-chlorosilanes and 108.5 g of a dark brown polysilane-polycarbosilane copolymer are obtained

Comparative Example 3

Example 6 was repeated except that the polysilane according to Comparative Example 1 was thermally crosslinked.

Example 7

Thermal crosslinking of a dimethylamine-modified polysilane / oligosilane

600 g of the modified polysilane / oligosilane from Example 5 is in a

Distillation apparatus slowly heated to a final temperature of about 330 ° C. During the heating, about 200 ml of a yellowish distillate are obtained, which in the

Consists essentially of various dimethylamino-methylmonosilanes; the end point of the cross-linking can be recognized by the solidification of the mass. To

Cooling is the resulting copolymer whose chlorine content now only about

0.5 mass%, dissolved in a solution of about 70% by mass in toluene or xylene. The solution has a suitable viscosity (about 20-40 Pas) in order to be spun into fibers based on the patent application DE 10 2004 04 531 A1. Example 8

Thermal crosslinking of a dimethylamine-modified polysilane / oligosilane and production of a filled with silicon powder spinning mass thereof

Surface modification of silicon powder: 500 g of silicon powder with a

average particle size of <5 μιτι was refluxed in 500 ml of trimethylchlorosilane for 1 h, filtered through a pressure filter, washed with dry n-pentane and dried in vacuo.

600 g of the modified polysilane / oligosilane from Comparative Example 2 were thermally crosslinked as described in Example 7. From the thus obtained solid

Copolymer, whose chlorine content is now only about 0.5% by mass, is prepared about 65% by mass solution in toluene and 45% by mass of the

surface-modified silicon powder, based on the crosslinked copolymer used. The solution has a suitable viscosity (about 20-40 Pas) in order to be spun into fibers based on the patent application DE 10 2004 04 531 A1.

Comparative Example 4

Thermal crosslinking of a dimethylamine-modified polysilane / oligosilane and preparation of a spinning composition thereof

Example 8 is repeated without the addition of surface-modified silicon powder, with the amount of toluene chosen to give the solution a suitable viscosity for spinning.

Example 9

Preparation of polysilane-polycarbosilane copolymer green fibers

The spinning mass obtained according to Example 7 is filled under inert conditions (glove box) in a spinning apparatus consisting of a storage container, a spinning pump and a nozzle package consisting of filter and nozzle plate. The spinning mass is extruded in strand form through the nozzles (diameter 100 μιτι, L / D = 2). The

Polymer threads are dropped after passing through a 40 ° C heated shaft are wound up on a galette. The solvent evaporates in the spinning shaft. About the rotation speed of the galette and the injection speed from the spinnerets, the delay can be varied continuously and thus the

Green fiber diameter can be adjusted.

Example 10

Preparation of polysilane-polycarbosilane green fibers with silicon powder

Example 9 is repeated, but using the silicon powder filled dope of Example 8.

Comparative Example 5

Preparation of polysilane-polycarbosilane green fibers without silicon powder

Example 9 is repeated, but using the dope of the

Comparative Example 4.

Example 11

Production of SiC ceramic fibers

The green fibers prepared according to Example 9 are pyrolyzed in a vertical tube furnace under an inert gas atmosphere (N ₂ ) at a rate of 12 K / min to a final temperature of 1200 ° C. This gives black, shiny fibers with an oxygen content of less than 1 mass%, a Si-C ratio of 1, 0: 1, 0, determined by elemental analysis after appropriate digestion, a diameter of 10-15 μιτι, a tensile strength of 1000-1500 MPa and a modulus of elasticity of about 150-180 GPa. After sintering the fiber at> 2000 ° C., the upper X-ray diagram of FIG. 1 is obtained. It gives no indication

excess (amorphous) carbon. The ceramic therefore consists exclusively of silicon and carbon. Example 12

Production of SiC ceramic fibers

The green fibers prepared according to Example 10 are first pyrolyzed as described in Example 1 1. They are then heated under argon atmosphere at 1500 ° C for 5 min. By this high-temperature treatment, the free carbon in the fibers is reacted with the silicon powder, and the resulting ceramic fibers consist solely of crystallized silicon carbide (atomic ratio of silicon to carbon 1: 1).

Comparative Example 6

Production of SiC ceramic fibers

The green fibers prepared according to Comparative Example 5 are pyrolyzed as described in Example 11. After subsequent sintering to> 2000 ° C gives the

Elemental analysis of Si: C = 1, 0: 1, 68. In the X-ray powder diagram (FIG. 1, bottom), the excess carbon can be recognized batchwise.

Claims

Claims:

1 . A process for producing a polysilane-polycarbosilane copolymer solution from which after removal of the solvent and pyrolysis, a ceramic material having a ratio of silicon to carbon in the range of 0.8: 1, 0 to 1, 1: 1, 0 receive leaves, comprising the following steps:

Producing a chlorine-containing crude polysilane / oligosilane containing hydrocarbon groups by disproportionation of a methylchlorodisilane or a mixture of several methylchlorodisilanes of the composition Si 2 Me _n Cl 6 _-n , where n = 1 -4, the disproportionation being carried out with a Lewis base as catalyst,

thermal post-crosslinking of the crude polysilane / oligosilane to a soluble in indifferent solvents, infusible polysilane-polycarbosilane copolymer, and

the preparation of said solution by dissolving the polysilane

Polycarbosilane in an inert solvent,

characterized in that in one step of this process additionally elemental silicon or titanium disilicide as a powder or a compound containing alkyl groups bound to silicon or to nitrogen is added in an appropriate amount, said additive either

(a) is carried out by reacting the crude polysilane / oligosilane in the presence of a

Crosslinking aid is produced, selected from compounds of the formula

(I)

CI ₂ R ¹ Si-R ² (I)

which have a boiling point above 100 ° C and wherein R ^{1 is} chlorine,

Is hydrogen or an alkyl radical having 1 to 4 carbon atoms and

R ^{2 is} -SiR ³ ₃ , -NH-SiR ³ ₃ or -N (SiR ³ ₃ ) 2, wherein R ^{3 has} the same meaning as R ¹ , or

(b) by adding powdered silicon or titanium disilicide to the polysilane-polycarbosilane solution.

2. The method of claim 1, variant (b), wherein the production of the crude polysilane / oligosilane is produced in the presence of a crosslinking aid selected from arylhalosilanes, arylhaloboranes and mixtures thereof.

3. The method according to claim 1 or 2, characterized in that the

Crosslinking aid in an amount of 5-20 mol%, preferably from 10-15 mol%, based on the molar sum of methylchlorodisilane, Lewis base and crosslinking aid, is present.

5

4. The method according to any one of claims 1 to 3, wherein the chlorine content of the polysilane-polycarbosilane copolymer is lowered by reacting the crude polysilane / oligosilane or the polysilane-polycarbosilane copolymer with a substitution agent, through which chlorine bound therein by io a chlorine-free substituent is replaced.

A process according to claim 4 wherein the substitution agent is selected from compounds having an N-H moiety or an N-Si moiety, most preferably ammonia, primary amines, secondary amines and mixtures thereof.

6. The method according to any one of the preceding claims, characterized in that the thermal post-crosslinking takes place at temperatures of 250 to 500 ° C.

20 7. The method according to claim 6, characterized in that as indifferentes

Solvent is a saturated hydrocarbon from the group of n-pentane, n-hexane, cyclohexane, n-heptane, n-octane, an aromatic hydrocarbon from the group of benzene, toluene, o-xylene, sym-mesitylene, a chlorinated hydrocarbon from the group methylene chloride, Chloroform, carbon tetrachloride, 1, 1, 1 -

25 trichloroethane, chlorobenzene, or an ether from the group diethyl ether,

Diisopropyl ether, tetrahydrofuran, 1, 4-dioxane or a mixture of two or more of these solvents is used.

8. A method of producing green fibers, comprising the steps of:

30 - producing a polysilane-polycarbosilane copolymer solution as claimed in any one of claims 1 to 7, and

Spinning the dissolved polysilane-polycarbosilane copolymer to

Green fibers after the dry-spinning process.

35 9. The method according to claim 8, characterized in that the

Dry spinning process is carried out at a temperature of 20 to 100 ° C at a take-off speed of 20 to 500 m / min.

10. A method of making silicon carbide ceramic materials having a silicon to carbon ratio in the range of 0.8: 1, 0 to 1, 1: 1, 0, comprising the steps of:

Making a polysilane-polycarbosilane copolymer solution as claimed in any one of claims 1 to 7,

Converting the polysilane-polycarbosilane copolymer from this solution into a desired form, and

Pyrolysis of said copolymer under inert gas atmosphere or reducing atmosphere.

1 1. The method of claim 10, wherein the material is fibers, characterized in that the step of converting the polysilane-polycarbosilane copolymer from the corresponding solution to a desired form comprises producing green fibers by the dry-spinning method.

12. The method according to any one of claims 10 or 1 1, characterized in that the pyrolysis at final temperatures of 900 to 1200 ° C with a

Heating rate of 1 to 50 K / min in inert or reducing

Atmosphere is performed.

13. The method according to any one of claims 10 to 12, characterized in that the ceramic silicon carbide material is sintered after pyrolysis at temperatures of 1200-2000 ° C under an inert or reducing atmosphere. 14. Oxygen-poor silicon carbide ceramic fibers having a ratio of silicon to carbon in the range of 0.8: 1, 0 to 1, 1: 1, 0, the fiber diameter between 5 and 50 μιτι, preferably between 10 and 15 μιτι whose tensile strength between 1000 and 1500 MPa and whose elastic modulus is between 150 and 180 GPa, preferably prepared by a process of claims 11 or 12.

5. A method of constructing ceramic matrices, comprising the steps of:

Producing a chlorine-containing, crude polysilane / oligosilane containing hydrocarbon groups by disproportionation of a Methylchlorodisilans or a mixture of several Methylchlorodisilane the composition Si2Me _n Cl6 _-n , wherein n = 1 -4, wherein the disproportionation is carried out with a Lewis base as a catalyst,

thermally postcrosslinking the crude polysilane / oligosilane to a polysilane-polycarbosilane copolymer,

Dissolving the polysilane-polycarbosilane copolymer in an inert solvent, and

Using the dissolved polysilane-polycarbosilane copolymer for the construction of a ceramic matrix by

Liquid-phase infiltration,

characterized in that the crude polysilane / oligosilane is produced in the presence of a crosslinking aid selected from compounds of the formula (I)

CI ₂ R ¹ Si-R ² (I)

which have a boiling point above 100 ° C and wherein R ^{1 is} chlorine,

Is hydrogen or an alkyl radical having 1 to 4 carbon atoms and

R ^{2 is} -SiR ³ ₃ , -NH-SiR ³ ₃ or -N (SiR ³ ₃ ) 2, wherein R ^{3 has} the same meaning as R ¹