Pyrolysis of polysilanes

Pyrolysis of polysilanes played an important role in the discovery of silylene reactions. Through the pyrolysis of alkoxydisilanes in the presence of diphenylacety-lene, Atwell and Weynberg obtained a product regarded as a dimer of dimethylsilylene adduct. 1,2-Shift of a methoxy group in disilanes takes place under relatively mild conditions (Scheme 14.1). ... 

The pyrolysis of polysilanes played an important role in the discovery of silylene reactions1-3,5 and is still widely used for the generation of silylenes in the gas phase. Many such reactions are concerted silylene extrusions in which a substituent migrates from the incipient divalent silicon atom during Si—Si bond cleavage (equation 1). 

The preparation, manufacture, and reactions of SiC have been discussed in detail in Gmelin, as have the electrical, mechanical, and other properties of both crystalline and amorphous of SiC. Silicon carbide results from the pyrolysis of a wide range of materials containing both silicon and carbon but it is manufactured on a large scale by the reduction of quartz in the presence of an excess of carbon (in the form of anthracite or coke), (Scheme 60), and more recently by the pyrolysis of polysilanes or polycarbosUanes (for a review, see Reference 291). Although it has a simple empirical formula, silicon carbide exists in at least 70 different crystalline forms based on either the hexagonal wurtzite (ZnS) structme a-SiC, or the cubic diamond (zinc blende) structme /3-SiC. The structmes differ in the way that the layers of atoms are stacked, with Si being fom-coordinate in all cases. 

The pyrolysis of polysilanes and polycarbosilanes is usually carried out using inert gas (e.g., argon) as pyrolysis atmosphere. A general problem associated with the pyrolytic formation of carbides is the desired stoichiometry of the calcined products in contrast to nitrides, excess carbon carmot be evaporated during calcining it may therefore contaminate the powders obtained as an elemental impurity and thus influences the physical, especially mechanical and electrical, properties of the sintered ceramic bodies. The volatiles evaporated during pyrolytic treatment of carbosilanes to form a network structure are H2 and CH4, and they depend on the structure of the polycarbosilane used (Fig. 2). 

A final comment on the crystallization of SiC from pol)mier derived ACCs is that, more or less, in all papers regarding this problem it is reported that silicon carbide is formed as pSiC, i.e., the polytype 3 C. Our group followed the polytype development of SiC formed by pyrolysis of polysilane in dependence on the pyrolysis temperature [168] and found that, indeed, up to 1800 °C pSiC dominates the XRD, Raman, as well as Si NMR results, but that especially in the case of pyrolysis temperatures in the range 1600-1700 °C non-negligible parts of polytypes of a-SiC also occur which have disappeared again at T = 1800 °C. 

Fig. 10. SEM micrograph of / -SiC whiskers grown during the pyrolysis of polysilanes at low streaming rates of argon. The electron diffraction pattern inserted into the corner reveals the monocrystaHine structure and the growth direction [111]. (Courtesy of B. Ullrich, TU Freiberg, Germany)...

Figure 3.16. Polysilanes are polymers with a silicon chain as a backbone to which organic side groups are attached. Sidegroups R can be aliphatic, cyclic, or unsaturated organic groups (a) Synthesis of polysilanes, (b) Pyrolysis of polysilanes yields silicon carbide ceramics.

P. Larcey. 21st NATAS Conference 1992. The Use of Hyphenated TA-MS in the Study of the Pyrolysis of Polysilane Ceramic Precursor. 

The spectrum of silicon based polymers is enriched by high tech ceramics like silicon nitride and carbide, respectively. These materials are produced by pyrolysis of appropriate polymeric precursors such as polysilanes, polycarbosilanes and polysilazanes (preceramics). These synthetic ceramics display a certain analogy to silicates, having SiC, SiN, or Si(C,N) as structural subunits instead ofSiO. 

Some new aspects in the chemistry of dimethyldisilanes recently reported by Atwell and Weyenburg include the pyrolysis of dimethyldimethoxy-silanes 18 2°) to polysilanes and tetramethyldimethoxydisilane ... 

Materials obtained by pyrolysis of pitch-polysilane blends have been extensively studied as carbon materials containing Si [157-161], For some of these materials, ca. 600mAh/g of Crev for Li insertion, as well as small irreversible capacities and small hysteresis effects, were reported. It has been shown that the materials contain nanodispersion of Si-O-C and Si-O-S-C instead of nanodispersed Si particles [162-165], Furthermore, the oxygen and sulfur contents are proved to be correlated to the irreversible capacity. There is a report about the fabrication of porous Si negative electrodes with 1-D channels, where the usefulness of the fabricated negative electrodes for rechargeable microbatteries is also suggested [166],... 

Larcher D, Mudalige C, George AE, Porter V, Gharghouri M, Dahn JR. Si-containing disordered carbons prepared by pyrolysis of pitch/polysilane blends effect of oxygen and sulfur. Solid State Ionics 1999 122 71-83. 

Baney and co-workers (iO) used the known Si-Si and Si-Cl bond redistribution of methylchlorodisilanes to prepare poly(methylchlorosilane)s. Pyrolysis of these branched polysilane polymers gives nearly stoichiometric amounts of silicon carbide (equation 6). 

As mentioned, the sinterability of pure silicon carbide strongly depends on the properties of the powder used. Powders made by high-temperature routes are predominantly composed of different hexagonal SiC polytypes (e.g., 2H, 4H, and 6H). In contrast, SiC produced by polymeric routes almost exclusively consists of p-SiC. In principle, SiC can be formed by the pyrolysis of either polysilanes or polycarbosilanes. 

By the way, although a dominant majority of papers concerning the formation of amorphous SiC layers describes appUcations of CVD or PVD techniques, there have also been some attempts to use the polymer route for preparing SiC films. Starting from solutions of various polysilanes or polycarbosilanes, frequently films are formed by spin-coating and pyrolyzed under inert atmosphere [215-218]. Of course, such a procedure does not form a part of this section SiC layers via gas phase reactions . However, in this connection it should be mentioned that polysilanes are also applied to form films via evaporation, not only with the aim to build amorphous and/or crystalline SiC films, but also to use special properties of the polysilane films themselves, i.e. without a subsequent pyrolysis of these films. Such amorphous films are characterized by non-linear optical effects [219, 220] and their properties may be controlled by the uniformity of the orientation of polysilane chains which is susceptible to epitaxial influences [221-223]. 

Very fine boron carbide powders of spherical shape and 20-30 nm in size have been prepared by chemical vapor deposition according to (iii). In an Ar-H2-CH2-BCI3 atmosphere a radio frequency plasma produces stoichiometries between Bi5 gC and B3 9C [33, 166]. Also laser-induced pyrolysis of similar gas mixtures with or without acetylene has been employed for the preparation of nano-sized particles [167]. With similar success, composites of B4C and SiC have been produced by the pyrolysis of boron-containing polysilanes [168]. 

Polysilanes have been prepared by the pyrolysis of a variety of silylmetallic compounds (54). 

Like boron fibers, the final SiC monofilament is relatively thick and consists of a core of tungsten of 12.5 pm diameter with a coating of SiC with an outside diameter of 140 pm. To overcome this important drawback, the Japanese team of Professor Yajima introduced an alternative process based on the pyrolysis of an organic silicon-rich polymer (i.e., polycar-bosilane). First the polysilane is synthesized by dechlorinating a halogenated silane with metalhc sodium. The polysilane is then decomposed thermally under pressure to yield the polycarbosilane precursor. After melt spinning, the precursor fiber obtained yields the final SiC monofilament by pyrolysis in air at BOO C. 

As an alternative to thermal decomposition, CVD, and laser-assisted H elimination, the flash pyrolysis of soluble alkylpolysilyne in a vacuum is reported recently. Unless they are under vacuum conditions, polysilanes and polysilyne may convert to silicon carbide (SiC). The flash process, which is a rapid removal of volatile organic substances and H2 gas, enables the control of the dimension of Si structure from 2D to 3D. Flash pyrolysis of alkylpolysilyne in a vacuum above 500 °C leads to the formation of poly- Si with a minimal amount of SiH termini, although the size of the crystals is limited to the range between several pm and several nm. 

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