Reactions of Monofunctional Boranes with Hydridopolysilazane: Synthesis, Characterization, and Ceramic Conversion Reactions of New Processible Precursors to SiNCB Ceramic Materials

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• 作者：Thomas Wideman ; Enriqueta Cortez ; Edward E. Remsen ; Gregg A. Zank ; Patrick J. Carroll ; and Larry G. Sneddon
• 刊名：Chemistry of Materials
• 出版年：1997
• 出版时间：October 1997
• 年：1997
• 卷：9
• 期：10
• 页码：2218 - 2230
• 全文大小：362K
• 年卷期：v.9,no.10(October 1997)
• ISSN：1520-5002

文摘

Three new series of processible polymeric precursors (PIN-HPZ,BCP-HPZ, DEB-HPZ)to SiNCB ceramic materials have been synthesized by reaction ofhydridopolysilazane (HPZ)with the monofunctional boranes, pinacolborane (PIN-H),1,3-dimethyl-1,3-diaza-2-boracyclopentane (BCP-H), and 2,4-diethylborazine (DEB-H). Polymerscan be prepared witha controllable range of boron contents from ~1 to 5%.Spectroscopic and chemical studiesindicate the boranes are attached to the hydridopolysilazane backbonevia B-N linkagesthat primarily result from dehydrocoupling reactions. Theisolation of small amounts oftrimethylsilane and Me₃SiNH-substituted borane sideproducts (i.e., PIN-NHSiMe₃, BCP-NHSiMe₃, DEB-NHSiMe₃) from the polymerreactions, as well as from model reactions ofthe boranes with hexamethyldisilazane, also suggest borane reactions atthe Si-N bonds ofthe HPZ backbone lead to some polymer chain cleavage. Consistentwith these observations,combined molecular weight/infrared spectroscopy studies show thatalthough the polymersare modified throughout the molecular weight distribution, the modifiedpolymers have lowermolecular weights than the starting HPZ, with the highest boraneconcentrations in thelower molecular weight fractions. The glass transitiontemperatures (T_g) of the PIN-HPZand BCP-HPZ polymers are in the 100-120 C range, while those ofthe DEB-HPZpolymers decreased to as low as 25 C with increasing modification.The polymers eachshowed regions of thermal stability, thus allowing the formation ofPIN-HPZ, BCP-HPZ,and DEB-HPZ polymer fibers by melt spinning. Pyrolysis of thesefibers to 1200 C thenyielded SiNCB ceramic fibers. Studies of the polymer to ceramicconversion reactions showedthe modified polymers yield SiNCB ceramics containing ~1-3% boronat 1400 C, with thehighest boron contents in the PIN-HPZ derived samples. At 1800C, the PIN-HPZ derivedceramic exhibited improved thermal stability with up to 23% nitrogencontents. Incomparison, the ceramics obtained from unmodified HPZ, BCP-HPZ, andDEB-HPZretained less than 4% nitrogen at this temperature. While theBCP-HPZ and DEB-HPZderived ceramics showed crystallization properties similar to theceramic obtained fromunmodified HPZ, the PIN-HPZ derived ceramic was amorphous to 1600C and at 1800 Cshowed only weak diffraction from -SiC.