EP1590310A1 - Method for production of a b/n/c/si ceramic from a borazine precursor, ceramics made by said method and use of the ceramic made by said method - Google Patents

Abstract

B-tris(silylvinyl)borazine is pyrolysed as a borazine precursor for the production of a B/N/C/Si ceramic. A high-temperature ceramic is obtained by means of a further pyrolysis at higher temperatures after a pre-pyrolysis, which is of high purity and essentially free of pores. The ceramic furthermore contains essentially no oxygen and is particularly suitable as a coating material and for the production of heating elements.

Description

 Process for producing a B / N / C / Si ceramic from a

Borazine precursor, ceramics produced by this process and use of the ceramics produced by this process

The production of ceramics and in particular high-performance ceramics from borazine precursors are known. Nitride and carbonitride ceramics with boron and silicon are particularly important due to their high thermal stability and resistance to oxidation.

WO 02/22625 has made known high temperature stable silicon boron carbide nitride ceramics from silylalkyl borazines as well as processes for their production. The borazines used as precursors each have the structural element Si-C-B-N. To produce a silicon boron carbide nitride ceramic, a silylalkyl borane is pyrolyzed in an inert or ammonia-containing atmosphere at temperatures between -200 ° C and + 2000 ° C. It is then calcined in an inert or ammonia-containing atmosphere at temperatures between 800 ° C and 2000 ° C. Such ceramics are said to have N-Si-C-B-N structural units.

Anja Krummland's dissertation ETH No. 14179 discloses new molecular and polymeric precursors for B / N / C / Si ceramics. B-

Triethinylborazin ran out. Qatar's hydrosilylation produces borazines as precursors. In particular, B-tris (trichlorosilylvinyl) borazine is crosslinked with hydrolyzable SiCl and Si (OR) groups via a sol-gel process and via ammonolysis. The resulting ceramics should be hard, amorphous and homogeneous and stable up to at least 1500 ° C. However, these ceramics usually contain by-products that are difficult to remove. The production of a pure ceramic is therefore comparatively complex. For example, NH ₄ C1 occurs as a by-product. The invention has for its object to provide a method of the type mentioned, which avoids the disadvantages mentioned above. The process should nevertheless enable a comparable high yield.

The method is solved in that the borazine precursor is B-tris (hydrosilylvinyl) borazine (also referred to as B-tris (silylvinyl) borazine) and this is converted into ceramic by pyrolysis. It has surprisingly been found that the ceramic produced by this process is essentially pore-free and shrinks only minimally when converted to a high-temperature ceramic. The borazine precursor used is liquid at room temperature and can therefore be processed particularly well. For example, this borazine precursor can be applied by brushing or by spraying.

The purity of the ceramic obtained is surprisingly high and in pyrolysis there are essentially no other gaseous waste products apart from hydrogen. In addition, the yield is unusually high. A major advantage of the method according to the invention is also seen in the fact that no crosslinking and polymer formation is required. The borazine precursor can be converted directly into ceramic by pyrolysis. A treatment at a higher temperature makes conversion into a high-temperature ceramic easily possible.

It has also been shown, unexpectedly, that the ceramic produced by this process has semiconducting properties. The high conductivity results from the existing double bonds and the extensive freedom from oxygen. The ceramic according to the invention is particularly suitable for producing heating elements which, inter alia, can also be operated without contact and which are particularly air-stable. Another advantageous application is the antistatic coating, for example on the inside of pipes. A static charge can be avoided with such a coating. The coating is also comparatively very stable thermally and chemically.

The invention is explained in more detail below with the aid of examples.

The starting point for the synthesis is triethinylborazine according to formula I. H

C

The synthesis of this compound is known and is disclosed, for example, in EP 0570247 AI. B-tris (E-trichlorosilylvinyl) borazine can be produced from this compound.

Preparation of B-tris (E-trichlorosilylvinyl) borazine (Formula II)

π Reaction equations:

1) n (N ₃ B ₃ H ₃ ) (C≡CH) ₃ + ₃ nHSiCl ₃

2) (N ₃ B ₃ H ₃ ) (CH = CH (SiCl ₃ )) ₃ + (N ₃ B ₃ H ₃ ) (C (SiCl ₃ ) = CH ₂ ) ₃ + isomers triethinylborain 10g, 65.6 mmol

Trichlorosilane 54.2g, 400 mmol

Platinum on carbon (l%) lg, 0.05 mmol Pt

10 g of triethinylborazine are dissolved in 300 ml of toluene and 1 g of Pt / C (1% Pt) is added. Then the trichlorosilane is added via a dropping funnel and stirred vigorously so that the reaction begins. The components react completely at a reaction temperature of about 80 ° C. for 12 hours. The Pt / C is then filtered off and then excess trichlorosilane and the toluene are distilled off. Finally, the reaction product is dried under a high vacuum and is obtained in quantitative yield. The pure presentation / isolation of the compound B-tris (E-trichlorosilylvinyl) borazine takes place via fractional crystallization of the reaction product from hexane at -30 ° C. It is very easy to differentiate between the different isomers using NMR spectra.

Η NMR (C ₆ D ₆ ): 6.85 / 6.22 (3J = 21.3); 4.60 ^π B NMR: 34.0

13 C-NMR: 137.4 / 154 (b)

²⁹ Si NMR: -3.3

This connection is also proven by a single crystal structure analysis.

Representation of B-tris (hydrosilyl vinyl) borazine (formula III or purple to IIIc)

Example 1 :

III purple

Hlb IIIc Reaction equations:

1) 4 (N ₃ B ₃ H ₃ χCH = CH (SiH ₃ )) ₃ + 9 A1H ₄

2) 4 (N ₃ B ₃ H ₃ ) (CH = CH (SiH ₃ )) ₃ + isomers + LiCl + 9ALC1 ₃ B-tris (E-trichlorosilylvinyl) borazinelOg, 17.9 mmol

LiAlH4 1.6g, 42.1 mmol

1.6 g of LiAlH ₄ are suspended in 10 ml of THF and cooled to -20 ° C. Then 10 g of B-tris (E-trichlorosilylvinyl) borazine, dissolved in 80 ml THF, are added dropwise over about 1 hour while maintaining the temperature. After the reaction mixture has warmed to room temperature, stirring is continued for 12 hours. Then 20 ml of hexane are added in order to precipitate excess LiAlH ₄ and part of the AlCl / LiCl. The solid is filtered off and washed with 10 ml of hexane. The filtrate is freed from the solvents and taken up in a mixture of 150 ml hexane and 50 ml toluene. The by-product LiCl is again filtered off from this suspension. The solvents are removed from the filtrate and a colorless, highly viscous liquid is obtained, which solidifies after drying in a high vacuum. The isomers purple to IIIc can be prepared accordingly.

¹ H-NMR (C ₆ D ₆ ): 4.26 / 4.96 / 6.38 / 6.75 (3J _V i _ny ι- _trans = 21.6Hz; 3J _H si- _CH = 2.8HZ) ^π B-NMR: 33.4

¹³ C-NMR: 135 / 153.7 (b) ²⁹ Si-NMR: -63.1 (JSi-H = 199Hz)

Representation of B-tris (hydrosilylvinyl) bora2-ine

Example 2:

Reaction equations:

1) (N ₃ B ₃ H ₃ ) (CH = CH (SiCl ₃ )) ₃ + 9LiHB (Et) ₃

2) (N ₃ B ₃ H ₃ ) (CH = CH (SiH ₃ )) ₃ + isomers + 9LiCl + 9B (Et) ₃ B-tris (E-trichlorosilylvinyl) borazine 10g, 17.9 mmol LiHB (Et) ₃ IM in THF 161.1 mL, 161.1 mmol LiHB (Et) ₃ 10g B-tris (E-trichlorosilylvinyl) borazine are dissolved in 200mL THF and cooled to -50 ° C. 161.1 mL of a 1 molar solution of LiHB (Et) in RHF are added. After the reaction mixture has warmed to room temperature, stirring is continued for 12 hours to complete the reaction. The reaction solution is then concentrated to a volume of about 30 mL, whereupon white LiCl begins to precipitate. 1 OOmL toluene is added to completely precipitate the LiCl. The solid is filtered off and washed with 1 OmL toluene. The solvents and the by-product triethylborane are distilled off from the filtrate in vacuo. The isomers can also be prepared in this way.

Also suitable as precursors, which are converted directly to ceramic by pyrolysis, are the compounds of the formulas IVa-IVd and V to Vd ₅ where R is a hydrogen, alkyl, in particular methyl, a phenyl or amine. The above-mentioned synthesis instructions apply to these compounds, for which methyl or phenyl compounds in each case with PhSi (H) Cl ₂ or CH ₃ Si (H) Cl ₂ , Ph ₂ Si (H) Cl or (CH ₃ ) 2Si ( H) Cl. The subsequent hydrogenation can also be carried out as mentioned above.

The analysis data for B-tris ((phenyl-dihydrosilyl) vinyl) borazine are:

²⁹ Si-NMR: -36.3 / "β-NMR: 31.7 / 1H-NMR: 3.7; 4.9; 6.6; 7.6 / ¹³ C-NMR: 153.2; 139.5; 134.1; 131.9; 132.0; 128.5

The following analysis data were obtained for B-tris ((methyl-dihydrosilyl) vinyl) borazine:

29 Si-NMR: -30.0 / ⁿ B-NMR: 33 / 1H-NMR: 0.6; 3.5; 5.0; 6.8 / ¹³ C-NMR: 151.4; 141.0; 4.6

IVa IVb

IVc IVd

Va vb

Vc Vd The properties of the ceramic and in particular the electrical, thermal and mechanical properties can be changed by the choice of the R groups. It has been shown in particular that a higher proportion of carbon results in a higher conductivity of the ceramic, which can be explained by a higher proportion of double bonds.

The precursor can be mixed with metal compounds in order to obtain a ceramic doped with metal. For this purpose, the precursor is mixed with organometallic compounds in a solvent such as THF, acetonitrile, toluene, benzene or hexane, for example, and then pyrolyzed as mentioned above. Suitable metal compounds have emerged in particular: Fe (CO) ₅ , Fe ₂ (CO); Ferrocene, bis (1,5-cyclooctadiene) - nickel (O), LiHB (CH ₂ CH ₃ ) ₃ , (iBut) ₂ AlH. The metal atoms or clusters can be distributed very homogeneously at the nanoscopic level. Up to 1500 ° C there is an amorphous distribution and a metal content of up to about 3%. Thermal decomposition can be accelerated with UV rays.

FIG. 1 shows an image of a ceramic doped with iron, the iron being visible as black spots which are only up to a few nanometers in size.

A ceramic doped with metal has an increased electrical conductivity. For example, it can be heated inductively via an electrical field without contact. Such a ceramic can be used, for example, to produce a medical implant. Such a doped ceramic can also be used as a magnetic memory. The magnetic information can be erased, for example, thermally or with an alternating field.

The method according to the invention is also suitable for producing ceramic particles which are coated with carbon nanotubes, as shown in FIG. 2. Ni was used as the catalyst for the formation of the nanotubes, which was mixed into the liquid precursor as a suitable soluble compound. The nanotubes are formed in situ on the entire surface of the ceramic during pyrolysis. Very small parts with a high density of nanotubes can be achieved. The nanotubes are suitable, for example, as carriers for catalysts. The high thermal and chemical stability of the ceramic base body is particularly advantageous here.

B-tris (hydrosilylvinyl) borazine is liquid at room temperature after the addition of a small amount of solvent and can be applied as such with a brush or by spraying. The coating properties can be adjusted with a suitable solvent. This is particularly important for the application of thin layers. A suitable tixotropy can also be set here. Different layer thicknesses can be guaranteed exactly. For example, the inside of the tube can be coated with B-tris (hydrosilylvinyl) borazine or solutions thereof.

Pyrolysis of B-tris (hydrosilylvinyl) borazine converts it into a ceramic material in a surprisingly high yield. The ceramic yield was 94% according to the above examples. The pyrolysis is preferably carried out with pre-pyrolysis at a temperature below 1000 ° C. and pyrolysis at a temperature above 1000 ° C. to approximately 2000 ° C. The ceramic material can also be produced by pyrolysis of the isomeric compounds or from an isomer mixture.

A major advantage of this production is seen in the fact that no crosslinking and polymer formation is required. The B-tris (hydrosilylvinyl) borazine is converted directly into ceramic by pyrolysis. In pyrolysis, essentially only escapes as a degradation product

Hydrogen. This results in a ceramic that is essentially non-porous and very dense. The shrinkage is very low during pyrolysis. What is also surprising is the high stability of the ceramic, which can be explained by better networking due to the many double bonds. The comparatively large number of double bonds and extensive freedom from oxygen result in a conductive ceramic. This results in the following preferred uses of the ceramic produced by this process: Due to the high temperature stability and the electrical conductivity, the ceramic is particularly suitable for the production of heating elements and in particular contactless heating elements. The conductivity of the ceramic according to the invention results from the measurement curve in FIG. 3.

Another key advantage is the comparatively high air stability of the ceramic. This air stability is also present at high temperatures.

Another preferred use is the production of ceramic coatings. Due to the comparatively high electrical conductivity, such coatings are antistatic. They are also thermally and chemically stable. In particular, coatings on the inside of pipes can be produced, for example, for the transport of liquids. Coating is carried out with B-tris (hydrosilylvinyl) borazine, for example by brushing or spraying. Solutions of this compound can also be applied. The ceramic can also be added, for example, as a powder to known coating compositions, for example to increase the conductivity. The ceramic is particularly suitable for the production of enamel, since the ceramic retains properties, in particular the conductivity, even at higher temperatures. After application, pyrolysis is carried out.

The properties of the ceramic can be changed by the molecules of the precursor are preferably cross-linked in one or two dimensions by pyrolysis. In particular, the borazine molecules can be crosslinked in a chain-like manner by crystallization, mixed crystals also being conceivable. Such pyrolysis crosslinking means that the ceramic has greater flexibility. This flexibility can be influenced by the degree and type of crosslinking of the pyrolysis and can thus be adapted to the desired properties.

Further uses and applications of the ceramic according to the invention are conceivable. In particular, various semiconductor components can be produced with the ceramic.

Claims

claims 1. A process for producing a B / N / C / Si ceramic from a borazine precursor, characterized in that the borazine precursor is B-tris (hydrosilylvinyl) borazine and this is converted into ceramic by pyrolysis. 2. The method according to claim 1, characterized in that the B-tris (hydrosilylvinyl) borazine is prepared by hydrogenating B-tris (trichlorosilylvinyl) borazine. 3. The method according to claim 2, characterized in that the B-tris (trichlorosilylvinyl) borazine is produced from B-triethinylborazine by hydrosilylation. 4. The method according spoke 1, characterized in that the B-tris (hydrosilyl vinyl) - borazine is pyrolyzed. 5. The method according to any one of claims 1 to 4, characterized in that B-tris (hydrosilylvinyl) borazine is applied in liquid form and is subsequently pyrolyzed. 6. The method according to claim 5, characterized in that B-tris (hydrosilylvinyl) borazine is dissolved in a solvent and adjusted to tixotropy. 7. The method according to claim 5 or 6, characterized in that B-tris (hydrosilylvinyl) borazine or a solution thereof is applied by brushing or spraying and then pyrolyzed. 8. The method according to any one of claims 1 to 7, characterized in that the B-tris (hydrosilylvinyl) borazine is converted into a high-temperature ceramic after pre-pyrolysis at a higher temperature between 1000 ° C and 2000 ° C, in particular 1 100-1300 ° C , 9. The method according to any one of claims 1 to 8, characterized in that the precursor is mixed to produce a doped ceramic with a metal or a metal compound. 10. The method according to any one of claims 1 to 9, characterized in that the molecules of the borazine precursor are cross-linked in one or two dimensions before pyrolysis. 1 1. The method according to claim 1, characterized in that the precursor is B-tris ((phenyl-dihydrosilyl) vinyl) borazine, B-tris ((methyl-dihydrosilyl) vinyl) borazine or an amine. 12. Ceramic produced according to claim 1, characterized in that it is substantially non-porous. 13. Ceramic according to claim 12, characterized in that it is an essentially oxygen-free high-temperature ceramic. 14. Ceramic according to claim 12, characterized in that it is a semiconductor. 15. Ceramic according to claim 12, characterized in that it is doped with metal. 16. Use of the ceramic produced according to claim 1 for the production of a heating element. 17. Use of the ceramic produced according to claim 1 for the production of a coating. 18. Use according to claim 15, characterized in that the coating is antistatic. 19. Use according to claim 15, characterized in that the coating is an inner coating, in particular a tube. 20. Use of the ceramic produced according to claim 1 for the production of a semiconductor. 21. Use of the ceramic produced according to claim 1 as a medical implant. 22. Use according to claim 21, characterized in that the ceramic is doped with metal.