Fabrication Of Nano-porous SiC-based Ceramics Using Template Method

SiC-based non-oxide ceramics possess unique properties such as high mechanical strength, high thermal conductivity, excellent thermal stability, resistance toward oxidation and corrosion, and chemical inertness. The nanoporous SiC-based ceramics with these unique properties and tailored pore characteristics can be widely used in high-temperature catalysis, high-temperature membrane, advanced composites, and other fields.In this thesis, highly ordered nano-porous SiC-based ceramics were prepared using low viscous liquid inorganic polymers, allylhydridopolycarbosilane and polyvinylsilazane, as ceramic precursors, through the route of replication of nanoscale structures by a direct-templating process.First, two- and three-dimensional (2-D and 3-D) ordered mesoporous SiC-based ceramics with high surface area (260-470 m~2/g), narrow pore size distribution and high thermal stability were successfully prepared by nanocasting liquid preceramic polymers into the channels of mesoporous silicas (SBA-15 and MCF) and mesoporous carbons (CMK-3 and CMK-8) as templates, followed by processes of curing and pyrolysis to transform polymers to ceramics, and subsequent HF etching or burning in air to remove templates. The obtained SiC-based ceramics are exactly the inverse replicas of their templates and possess similar structural symmetries to their templates, as characterized by small angle X-ray diffraction (XRD), high-resolution transmission electron microscope (TEM) images and nitrogen sorption measurements. Thus, two kinds of ordered mesoporous SiC-based ceramics with reverse mesostructures can be prepared from preceramic polymers by replicating two types of templates (SBA-15 and CMK-3) that are inverse replicas to each other. The trimethylsilylation of silica templates help the preceramic polymer full fill their channels, so it is the key of obtaining highly ordered mesoporous SiC ceramics.Second, highly 3-D ordered macroporous SiC-based ceramics were successfully prepared by using 3-D ordered polystyrene (PS) or poly methyl methacrylate (PMMA) microbeads assemblies as templates, and preceramic polymers as ceramic precursors. The pore size of the obtained macroporous ceramic can be easily controlled by changing the particle size of polymer microbeads in their templates. The ordering and pre-treatment of PS and PMMA microbeads templates and the heating rate in the process of pyrolysis are important impact factors in preparation of 3-D ordered macroporous ceramics with inter-connected pores. Various morphologies such as ordered macroporous structures, macroporous foams and sponges can be selectively fabricated by different designs of templates and preparation procedures.Finally, monolithic continuous macroporous SiC(N)H inorganic-organic hybrid materials micro-fluidic channels were successfully prepared by using packed beds of PS microbeads in micro-channels of poly(dimethyl siloxane) mold as templates, combining the micromolding in capillary (MIMIC) technique.It is proved that the use of carbon and polymer templates can avoid the oxygen contamination from templates to ceramics and the attack to ceramic products from the HF etching, and as a result, reduce the formation of oxidated phase and increase the porous structural thermal stabilities of the ceramic products.In this thesis, the method of "replication by templating process" based on liquid precursors was introduced to the field of fabrication of ordered mesoporous SiC-based ceramics, which made it easy to design and fabricate various SiC-based ceramic nanostructures with controllable morphology and size.