Synthesis Of Porous SiC Ceramic Fibers From Polymer Precursors

Porous fibers are widely used in catalysis and separation due to large porosity and convenient application. Their development in industry and daily usage are promising. But poor thermal stability, oxidation resistance or low surface area limit the application of existing porous fibers, especially in the fields of high temperature, high humidity and causticity. Polymer-derived SiC fibers have high thermal stability and high oxidation resistance, and have been primarily used in various composites. So porous SiC fibers will meet needs for application in such fields.In this dissertation, 5 kinds of porous SiC fibers were synthesized from polycarbosilane (PCS) precursor. The maximum surface area of porous SiC fibers can be as high as 990 m2/g. In addition, high porosity of semi-pyrolyzed PCS fibers was found and systematically studied.The semi-pyrolyzed PCS fibers demonstrate very high porosity (0.2 cm3/g) and surface area (500 m2/g) when heat treated at temperature range of 300-600℃. This phenomenon was studied by N2 adsorption, small angle X-ray scattering, IR, element analysis, SEM, ESR and so on. It was proposed that the break of small segments at end of PCS chain was the main reason for formation of pores in PCS fibers. It was also found that evaporation of small molecular, such as H2 and CH4, above 600℃, give no obvious porosity to PCS fibers.By air activation, a kind of porous SiC fibers (PF-D) was synthesized, using SiC-C fibers (from precursor fibers composed of PCS and pitch). PF-D was rich in mesopores, with surface area of 280 m2/g. Though carbon content of SiC-C fibers is very high, it is still difficult to be oxidized. So in CO2 case, activation should be proceed beyond the pyrolysis temperature.By KOH/N2 activation, a kind of porous SiC fibers (PF-C) was synthesized, using SiC-C fibers. PF-C was rich in micropores, with surface area of 990 m2/g. By KOH/CO2 activation, a kind of porous SiC fibers (PF-A) was synthesized, using SiC fibers from PCS fibers. PF-A was rich in both mesopores and micropores, with surface area of 680 m2/g.PF-A and PF-C both have"core-shell"structures, with higher carbon content in the shell. It was found that the increase of surface area of fibers was usually accompanied by the increase of carbon content.All these phenomena are related with reaction between SiCxOy phase and KOH. KOH prefer to react with unit composed of Si and O, forming silicate, thus causes the decrease of Si and increase of carbon content. In outer layer of the fibers, more units contain Si reacted with KOH, so silicon content is lower, carbon content and porosity are higher. As extent of activation increases, more Si will be eliminated and more pores formed in fibers.It was found that ZnCl2 can accelerate pyrolysis of PCS, and let PCS decompose and become inorganic SiC fibers at low temperature. ZnCl2 can not give pores to SiC fibers during pyrolysis of PCS fibers, but it can reduce carbon content of SiC fibers.By ZnCl2 treatment and second activation, a kind of micro-porous SiC fibers (PF-A) was synthesized, with surface area of 970 m2/g. If enough ZnCl2 was used, a kind of silica rich SiC fibers can be produced, in which silica phase is enough to form a network. By etch of hydrofluoric acid, micro-porous SiC fibers will be produced.As mixing precursor fibers of PCS and pitch were used, ZnCl2 will produce micropores during pyrolysis of the precursor fibers, thus a kind of micro-porous SiC fibers (PF-E) was produced, with surface area of 305 m2/g. Thermal treatment at 1200℃will increase the thermal stability of fibers, but will cause some decrease of surface area, from 650 m2/g down to 305 m2/g.No"core-shell"structures were found in PF-B and PF-E, and the carbon content of them is close to that of commonly synthesized SiC fibers.Hydrogen adsorption of some synthesized fibers was studied. It was found that high surface area and microporosity are good to hydrogen adsorption. Hydrogen storage of PF-B and PF-SP, at -196℃and 0.1MPa, are 0.69wt% and 1.15wt% respectively.Hydrogen storage of PF-A is higher than that of PF-SP at -196℃and 0.6MPa, reach 0.95wt%. In all, hydrogen adsorption of the fibers is better than carbon nanotubes and silicon carbide nanotubes because of larger surface areas.