Synthesis and thermal behavior of novel Si-B-C-N ceramic precursors

Several boron-modified polysilazanes of general type {B[C2H4Si(R)NH](3)}(n) (C2H4 = CHCH3 or CH2CH2) were synthesized and their thermal behavior studi ed. In contrast to the known derivatives with R = alkyl or aryl, we describ e ceramic precursors in which the bulky moieties R are substituted with low er weight groups and/or reactive entities. Reactive units enable further cr oss-linking of the polymeric framework and therefore minimize depolymerizat ion during ceramization. The polymer-to-ceramic conversion of all synthesiz ed polymers was monitored by thermogravimetric analysis. Both low molecular weight substituents and/or cross-linking units increase the ceramic yield from 50% (R = CH3) to 83-88%. High-temperature thermogravimetric analysis i n an inert gas atmosphere indicates the ceramics obtained are stable up to similar to 2000 degrees C. XRD studies of the fully amorphous materials poi nt out that, with increasing temperature, formation of alpha-SiC or alpha-S iC/beta-Si3N4 crystalline phases occurs at 1550-1750 degrees C, depending o n the material's composition. The resistance of these novel materials towar d oxidative attack was investigated by TGA in air up to 1700 degrees C and SEM/EDX, indicating that the materials efficiently self-protect toward oxid ation.