New polymeric precursors to boron- and zirconium-modified silicon carbide.

摘要

Silicon carbide based ceramics have properties such as their high strength and thermal stability that make them desirable as high temperature structural materials. However, traditional methodologies for production of ceramic materials are limiting as they do not allow the formation of complex shapes. The goal of the work described in this dissertation was to develop new single-source polymeric precursors to modified silicon carbide ceramics including SiOCB, SiCB, SiOCZr, and SiOCNZr materials for the production of fibers and matrix materials.; Three types of boron-containing polymer precursors to SiCB and SiOCB ceramics were developed. Pinacolborane and diethylborazine-modified polyvinylsiloxanes were synthesized in excellent yields by the RhH(CO)(PPh₃) ₃ catalyzed reactions of polyvinylsiloxane with either pinacolborane or diethylborazine. The 9-borabicyclo[3.3.1 ]nonane-modified allylhydridopolycarbosilane polymers were also synthesized in high yield by the reaction of allylhydridopolycarbosilane with 9-BBN. The degree of polymer substitution in all cases was controlled by varying the reactant ratios. Bulk pyrolysis of the modified polymers produced SiCB ceramics, with boron contents ranging from 0.2 to 0.7% depending upon the percentage of polymer modification. Density and XRD studies of the 1800°C and 2000°C chars showed that the addition of boron, in fact, affected the degree of crystallization and promoted sintering of the ceramic. Additionally, the pinacolborane-modified polyvinylsiloxane polymers were found to be excellent melt-processable, single-source precursors to small diameter SiOCB fibers, while the 9-BBN-modified allyhydridopolycarbosilane polymers can be used for matrix materials.; Three new types of poly(zirconosiloxane), poly(zirconocarbosilane), and poly(zirconosilazane) polymeric precursors were also achieved by the high yield hydrozirconation reactions between polyvinylsiloxane, allylhydridopolycarbosilane, and poly(ureamethylvinyl)silazane and zirconocene chloride hydride. Again the degree of polymer modification was controlled by varying the reactant ratios. All the zirconium-modified polymers had thermal decomposition temperatures similar to their respective parent materials, but that the ceramic yields of the poly(zirconocarbosilane), and poly(zirconosilazane) derived materials were improved over their parents. Bulk pyrolyses of the modified polymers produced SiOCZr, and SiOCNZr ceramic chars with zirconium contents ranging from 0.9 to 2.9%. XRD studies of the ceramics showed that heat resistance and crystal growth were affected by the addition of the zirconium.