Silicon-Based Nanostructural Ceramics Derived From Polymer Precursors: Development of Processing, Structure & Property Relationships

摘要

This research program built on the knowledge gained under past AFOSR support and focused on developing additional basic understanding of the relationships between ceramic product and corresponding polymer precursor structures, as a function of processing conditions. The primary objectives for this work were to: (1) demonstrate the use of silicon-based polymeric precursors as sources of nanostructured ceramic phases, including SiC, Si3N4, and SiC/AlN solid solutions; and (2) examine the ambient, as well as high temperature and high pressure behavior of these materials, including crystallization and polytype development, phase stability, densification and sintering characteristics. Poly(methylvinylsilane) converts to C-rich, nanocrystalline Beta-SiC during pyrolysis in argon, and the phase development is time and temperature dependent. Perhydropoly(silazane) provides a miscible source of elemental Si which effectively scavenges the excess C. Blends of the two polymers have increased ceramic char yields, and offer a source of SiC/Si3N4 nanocomposites, especially near the stoichiometric ratio of excess Si to excess C. With blended polymers, crystal grain growth is inhibited below 1600 deg C and sub-stoichiometric ratios, but is enhanced at higher temperatures due to phase separation and decomposition of the perhydropoly(silazane). Poly(aluminosilazanes) are effective single-source polymer precursors to SiC/AIN ceramic products. The molecular-level distribution of Si, C, Al, and N present in the room temperature reaction product is retained in the ceramic. The Si/Al ratio in the starting polymer determines the extent of crystallinity in the ceramic phase. Both 4- and 6-coordinate Al are observed, depending on the starting reactant ratio.