SYNTHESIS, CHARACTERIZATION, AND CERAMIC CONVERSION REACTIONS OF BORAZINE/SILAZANE COPOLYMERS - NEW POLYMERIC PRECURSORS TO SINCB CERAMICS

The first borazine/silazane backbone copolymers derived from the parent borazine, B3N3H6, have been obtained by the thermal condensation of borazine with two silazanes, tris-(trimethylsilylamino)silane (TTS), and 1,1,3,3,5,5-hexamethylcyclotrisilazane (HCT). Both series of copolymers are proposed to contain borazine-boron to silazane-nitrogen linkages. Elemental analyses of the TTS copolymers are consistent with an approximate [[(-B3N3H4)-NH]3SiH](x) structural unit, while the HCT copolymers have compositions ranging from (B3N3H4)(1.00)(N)(1.81)(SiMe(2))(1.57)(H)(1.6) to (B3N3H4)(1.00)(N)(1.07)(SiMe(2))(1.28)(H)(0.9). Molecular weight studies indicate large polydispersities, and when molecular weight data are combined with intrinsic viscosity results, highly branched structures are suggested. Despite their similar compositions, the two series of copolymers yield different types of ceramic materials upon pyrolysis. The TTS copolymers yield BNSi ceramics that are amorphous to 1400 degrees C. Pyrolyses to 1800 degrees C result in further loss of silicon to produce ceramics of variable silicon contents with compositions ranging from B1.00N0.90Si<0.01C<0.01 to B1.00N0.91Si0.14C0.07. While their DRIFT spectra are consistent with the presence of boron nitride, the XRD spectra of these materials showed no crystalline boron-containing species, but instead, peaks due to small amounts of crystalline beta-Si3N4, beta-SiC and elemental Si. The ceramics derived from the HCT copolymers at 1400 degrees C were also amorphous, but showed greater retention of silicon and carbon with typical compositions of B1.0N1.5Si0.4C0.2. Pyrolyses of the HCT copolymers to 1800 degrees C yielded materials with compositions ranging from B1.00N2.58Si0.94Co0.30 to B1.00N0.86Si0.16C0.19. The DRIFT spectra of the ceramics obtained at both temperatures again indicate the presence of boron nitride. XRD spectra, however, show crystalline phases of beta-Si3N4, beta-SiC, and Si, but no diffraction from any boron-containing species.