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 reaction of hydridopolysilazane (HPZ) with the monofunctional boranes, pinacolborane (PIN-H), 1,3-dimethyl-1,3-diaza-2-boracyclopentane (BCP-H), and 2,4-diethylborazine (DEB-H). Polymers can be prepared with a controllable range of boron contents from similar to 1 to 5%. Spectroscopic and chemical studies indicate the boranes are attached to the hydridopolysilazane backbone via B-N linkages that primarily result from dehydrocoupling 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 model reactions of the boranes with hexamethyldisilazane, also suggest borane reactions at the Si-N bonds of the HPZ backbone lead to some polymer chain cleavage. Consistent with these observations, combined molecular weight/infrared spectroscopy studies show that although the polymers are modified throughout the molecular weight distribution, the modified polymers have lower molecular weights than the starting HPZ, with the highest borane concentrations in the lower molecular weight fractions. The glass transition temperatures (T-g) of the PIN-HPZ and BCP-HPZ 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 modification. 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 yielded SiNCB ceramic fibers. Studies of the polymer to ceramic conversion reactions showed the modified polymers yield SiNCB ceramics containing similar to 1-3% boron at 1400 degrees C, with the highest boron contents in the PIN-HPZ derived samples. At 1800 degrees C, the PIN-HPZ derived ceramic exhibited improved thermal stability with up to 23% nitrogen contents. In comparison, the ceramics obtained from unmodified HPZ, BCP-HPZ, and DEB-HPZ retained;less than 4% nitrogen at this temperature. While the BCP-HPZ and DEB-HPZ derived ceramics showed crystallization properties similar to the ceramic obtained from unmodified HPZ, the PIN-HPZ derived ceramic was amorphous to 1600 degrees C and at 1800 degrees C showed only weak diffraction from beta-SiC.