Characterizing the Molecular Structure of Preceramic Polysiloxanes for Freeze Casting of Silicon Oxycarbide Ceramics

Citation

Nandi, Ankita (2024) Characterizing the Molecular Structure of Preceramic Polysiloxanes for Freeze Casting of Silicon Oxycarbide Ceramics. Senior thesis (Major), California Institute of Technology. doi:10.7907/akf6-7631. https://resolver.caltech.edu/CaltechTHESIS:06132024-150611460

Abstract

Preceramic polymers are frequently used as a lower energy intensive precursor for creating ceramics, as they can be transformed into robust ceramics at lower temperatures than is required by traditional processing routes. Additionally, preceramic polymers can be used to produce structures with microstructural variability, such as porosity. Polysiloxanes are one type of preceramic polymer that have been used to create silicon oxycarbide materials. Previous research has utilized polysiloxanes in freeze casting to create porous ceramics, specifically investigating development of different pore morphologies and pyrolysis profiles. However, there has been little exploration into the differing molecular structures of various polysiloxanes impact their behavior through the freeze casting process. Investigating the molecular structure of commonly used proprietary polysiloxane Wacker SILRES® MK has provided some insight into molecular structural changes during the freeze-casting process. These can be used to improve freeze-casting microstructure from another proprietary polysiloxane, Wacker SILRES® H44. MK and H44 were characterized in powder, solution, and post pyrolysis stages of the freeze casting process. Techniques including FTIR-ATR spectroscopy, Raman spectroscopy, NMR spectroscopy, DSC, and SEM imaging were used to determine how to improve the robustness of freeze cast structures made with H44. MK was determined to be a polymethylethoxysiloxane, and H44 to be a polymethylphenylsiloxane. The high energy and high steric strain phenyl groups in H44 require additional energy to facilitate crosslinking during the freezing process for H44. Both MK and H44 converted to silicon oxycarbide upon pyrolysis. Adding crosslinker improved the desired porous microstructure and robustnesss of freeze-cast structures made with H44, as evidenced by SEM imaging. Future exploration into other preceramic polymers should consider the impact of high energy functional groups upon the processing methods to create desired microstructures.

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