REACTIONS OF MONOFUNCTIONAL BORANES WITH HYDRIDOPOLYSILAZANE - SYNTHESIS, CHARACTERIZATION, AND CERAMIC CONVERSION REACTIONS OF NEW PROCESSIBLE PRECURSORS TO SINCB CERAMIC MATERIALS

Three new series of processible polymeric precursors (PIN-HPZ, BCP-HPZ , DEB-HPZ) to SiNCB ceramic materials have been synthesized by reactio n of hydridopolysilazane (HPZ) with the monofunctional boranes, pinaco lborane (PIN-H), 1,3-dimethyl-1,3-diaza-2-boracyclopentane (BCP-H), an d 2,4-diethylborazine (DEB-H). Polymers can be prepared with a control lable range of boron contents from similar to 1 to 5%. Spectroscopic a nd chemical studies indicate the boranes are attached to the hydridopo lysilazane backbone via B-N linkages that primarily result from dehydr ocoupling reactions. The isolation of small amounts of trimethylsilane and Me3SiNH-substituted borane side products (i.e., PIN-NHSiMe3, BCP- NHSiMe3, DEB-NHSiMe3) from the polymer reactions, as well as from mode l reactions of the boranes with hexamethyldisilazane, also suggest bor ane reactions at the Si-N bonds of the HPZ backbone lead to some polym er chain cleavage. Consistent with these observations, combined molecu lar weight/infrared spectroscopy studies show that although the polyme rs are modified throughout the molecular weight distribution, the modi fied polymers have lower molecular weights than the starting HPZ, with the highest borane concentrations in the lower molecular weight fract ions. The glass transition temperatures (T-g) of the PIN-HPZ and BCP-H PZ polymers are in the 100-120 degrees C range, while those of the DEB -HPZ polymers decreased to as low as 25 degrees C with increasing modi fication. The polymers each showed regions of thermal stability, thus allowing the formation of PIN-HPZ, BCP-HPZ, and DEB-HPZ polymer fibers by melt spinning. Pyrolysis of these fibers to 1200 degrees C then yi elded SiNCB ceramic fibers. Studies of the polymer to ceramic conversi on reactions showed the modified polymers yield SiNCB ceramics contain ing similar to 1-3% boron at 1400 degrees C, with the highest boron co ntents in the PIN-HPZ derived samples. At 1800 degrees C, the PIN-HPZ derived ceramic exhibited improved thermal stability with up to 23% ni trogen contents. In comparison, the ceramics obtained from unmodified HPZ, BCP-HPZ, and DEB-HPZ retained;less than 4% nitrogen at this tempe rature. While the BCP-HPZ and DEB-HPZ derived ceramics showed crystall ization properties similar to the ceramic obtained from unmodified HPZ , the PIN-HPZ derived ceramic was amorphous to 1600 degrees C and at 1 800 degrees C showed only weak diffraction from beta-SiC.