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 studied. In contrast to the known derivatives with R = alkyl or aryl, we describe ceramic precursors in which the bulky moieties R are substituted with lower weight groups and/or reactive entities. Reactive units enable further cross-linking of the polymeric framework and therefore minimize depolymerization during ceramization. The polymer-to-ceramic conversion of all synthesized 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 in an inert gas atmosphere indicates the ceramics obtained are stable up to similar to 2000 degrees C. XRD studies of the fully amorphous materials point out that, with increasing temperature, formation of alpha-SiC or alpha-SiC/beta-Si3N4 crystalline phases occurs at 1550-1750 degrees C, depending on the material's composition. The resistance of these novel materials toward oxidative attack was investigated by TGA in air up to 1700 degrees C and SEM/EDX, indicating that the materials efficiently self-protect toward oxidation.