Polysilanes silazanes

The variety of silicon based polymers is completed by high tech ceramics like silicon nitride and carbide. These materials are produced by pyrolysis of appropriate polymeric precursors such as polysilanes, -carbosilanes, and silazanes. Another important approach is realized by sol-gel processing. 

Industrially produced organo-silicon compounds include monomeric low molecular compounds, almost exclusively belonging to the class of silanes, and oligomers and polymeric compounds including oligomeric silanes, polysilanes, oligomeric silazanes, polysilazanes and particularly oligomeric siloxanes and polysiloxanes (silicones). 

The most commonly used organosilicon compounds are silicones as described above. The others are chlorosilanes, alkoxysilanes, silazanes, fluoroalkylsilanes and polysilanes as shown in Table 8.3 [31]. 

If the direct reaction is carried out at the lowest temperature at which the reaction proceeds, dichlorosilanes, R2SiCl2, are the major products and the other products are R3SiCl and RSiCl3, etc. The chlorosilanes (R SiCl4 , =l-3, R = Me, Ph) are used not only as the raw materials of silicones but also as the starting materials of various kinds of organosilicon compounds such as alkoxy-silanes, silazanes, fluoroalkylsilanes and polysilanes as shown in Table 8.3 [31]. 

Polysilazanes have been shown to be excellent polymeric precursors to amorphous silicon carbonitride (SiCN), silicon nitride, silicon carbide (SiC) and their composites. The actual chemical and phase compositions of the ceramic products depend on the polymer composition and pyrolysis conditions, such as temperature, time and atmosphere. Polymeric silazanes consist of amorphous networks, which transform to amorphous SiCN ceramics by pyrolysis under inert atmosphere at around 1000 C. These ceramic products remain amorphous up to 1400 °C in an inert atmosphere [a.322]. However, at higher temperatures the non-stoichiometric SiCN matrix decomposes, with nitrogen loss, giving the thermodynamically stable phases, namely Si3N4 and SiC. Polysilanes, polycarbosilanes and polysilazanes are commonly used for the preparation of high-performance ceramics such as silicon carbide, silicon nitride and silicon carbonitride. 

---