Macroporous Polymer-Derived Ceramic Monoliths for Cryogenic Applications Manufactured by Water-Based Freeze Casting

Abstract

Macroporous SiOC ceramics were prepared by a water-based freeze casting process, using polysiloxanes as precursors and silica sol as water phase and binder. The obtained porous monoliths have anisotropic porous structure and thermal and mechanical properties. The macroporous SiOC aimed at cryogenic applications which involve mass transport and thermal transport processes. The first part of the thesis focuses on manufacturing macroporous monoliths, in which process the surface characteristics of preceramic polymers in terms of hydrophobicity/hydrophilicity were modified to be used in the water-based freeze casting process. Two approaches were chosen for the surface modification. The first approach to modify the wettability of the precursor was pyrolysis of hydrophobic methyl phenyl polysiloxanes (H44) in inert gas at low temperature, by which hybrid ceramic materials (H44-derived filler) were generated. Depending on the pyrolysis temperatures, the surface characteristics can be varied from hydrophobic to hydrophilic. H44-derived fillers obtained by pyrolyzing methyl phenyl polysiloxane at 600 degree Celsius were hydrophilic enough to be used as solid phase in water-based process. The influence of solid loading, freeze temperatures and pyrolysis temperatures on porosity and specific surface areas were investigated. The combination of polymer derived filler materials with freezing casting method resulted in the trimodal pore structure (micro/meso/macropore) at pyrolysis temperature of 600 to 700 degree Celsius. Even at pyrolysis temperature of 1000 degree Celsius, the specific surface area was be as high as 74 square meter per gram. The pore shape can be tailored from lamellar to tubular depending on freezing temperatures. The second approach to modify the wettability was to introduce more hydrophilic groups to the hydrophobic methyl polysiloxane (MK) by cross-linking with (3-Aminopropyl)triethoxysilane (APTES). The molar ratios between MK and APTES and pyrolysis temperature led to different amounts of aminopropyl groups in the cross-linked products, which altered the basicity and hydrophilicity. For both approaches, besides the surface characteristics, surface charges also account for stable suspension to prepare final homogenous monoliths. Filler material prepared with MK: APTES molar ratio of 1:1, pyrolyzed at 600 degree Celsius was applicable for freeze casting considering the wettability and suspension stability. The monolith prepared with MK-APTES derived filler had also a hierarchical micro/meso/macroporous structure. The vapor adsorption indicated that the high content of silica sol improved the hydrophilicity greatly, and pyrolysis temperature also influenced the hydrophilicity to a minor degree. Notably, the silica sol is responsible for the formation of mesopores. The second part of the thesis was to investigate the mechanical and thermal properties of the obtained unidirectional porous SiOC ceramics prepared with MK and H44 at cryogenic and room temperatures. The compressive strength of monoliths was investigated both in air (293 Kelvin) and in liquid nitrogen (77 Kelvin). The influence of both liquid and cryogenic temperature on compressive strength was investigated. The compressive strength of monoliths showed not only anisotropy, but also a significant increase in liquid nitrogen. This increase may be due to the liquid nitrogen trapped inside the porous structures and cryogenic temperature. The linear thermal expansion coefficients (CTE), thermal conductivity and specific heat capacity of porous SiOC, were studied from cryogenic to room temperature. Both monoliths show anisotropic linear expansion coefficients, with the parallel direction having almost twice the shrinkage of the perpendicular direction. The monolith prepared with H44 showed thermal shrinkage twice as much as that prepared with MK and APTES, which might be due to the composition differences and measurement condition. The thermal conductivities of both monoliths made from two precursors showed anisotropic features and similar values. The minimum and maximum values for thermal conductivity are 0.2 and 0.9 Watts per meter per Kelvin. Thermal conductivities and specific heat capacities displayed an upward trend from low temperature to room temperature. It was assumed that the maximum heat conductivities of these materials were determined mainly by the macroporosity and the thermal conductivity of the hybrid material.