Controlling nanostructures of Si-based ceramics via carbothermal reduction of mesoporous silica-carbon nanocomposites

Abstract

Mesoporous carbon-silica (C-SiO2) precursors were synthesized by a sol-gel route with different drying processes. Tetraethoxysilane (TEOS) was used as a silica source and furfuryl alcohol (FA) was a carbon precursor. With a quick evaporation rate, the viscosity of sols increased dramatically and gelated fast; therefore, the chemical precursors of silica and carbon would be mixed homogenously in nanoscale because it’s very hard for poly-furfuryl-alcohol (PFA) oligomers to agglomerate. However, if the sols were dried slowly, PFA oligomers agglomerated during the drying process to form PFA-rich spheres. After calcination in nitrogen gas, the gels dried quickly became homogenous mesoporous silica-carbon nanocomposites, while the gels dried slowly became mesoporous silica-carbon with C-rich spheres due to the conversion of PFA-rich spheres into C-rich spheres. When the homogenous mesoporous C-SiO2 was heated at 1450 °C for 5 hours, SiC nanoparticles formed. Then SiC nuclei were introduced into the mesopores as seeds by infiltration of preceramic precursor polycarbosilane (PCS) prior to the heat-treatment of carbothermal reduction, nanofibers dominated in the final SiC products. It appears that polymer-derived seeds have a substantial effect on both the reaction rate and product morphology in the carbothermal reduction of mesoporous C-SiO2 nanocomposites. The pores of C-SiO2 nanocomposites provide nano channels for the gaseous SiO and CO diffusion in the carbothermal reduction reaction affecting the reaction kinetics, and thus the seeding effect becomes more pronounced. Mesoporous C-SiO2 nanocomposites with undetected C spheres were heated at different stages prior to the final crystallization temperature of SiC. Carbon hemispheres and spheres precipitated from the SiO2-rich matrix; however, the mesoporous property of the whole material was well maintained. By increasing the heating temperature, carbon precipitates became more and more. Above 1350 oC, spheres extruded from the matrix and became SiC rich, but still be amorphous and mesoporous, which due to the reaction between mesoporous carbon precipitate and SiO gas released from the SiO2-rich matrix. Heated at 1400 oC for 5 hours, SiC hollow spheres formed with a shell thickness of 1 μm; whereas solid porous SiC spheres formed after heating at 1400 oC for 10 hours. Therefore, tailoring the heat-treatments of mesoporous C-SiO2 precursors appears to be an effective means in fabricating and controlling nanostructures of porous silicon carbide spheres. When mesoporous C-SiO2 was heated under nitrogen gas at 1400 oC for 10 hours, silicon nitride (Si3N4) microribbons were obtained. By increasing the flow rate of nitrogen gas, the width of as-synthesized Si3N4 ribbons increased while the thickness kept steady. In all experiments, the Si3N4 microribbons chose a lower index direction, [100], to grow longer. However, when mesoporous C-SiO2 were heated under nitrogen gas at 1400 oC, large Si3N4 crystals were formed. With an increasing carbon/silica ratio, the grain size of Si3N4 crystals decreased. The results presented here indicate that mesoporous silica-carbon nanocomposites can be used to control materials synthesis by tailoring the chemistry compositions, pore shapes and pore sizes of their mesoporous precursors. It also reveals that mesoporous structures of carbon and metal oxides could be used to synthesize SiC, Si3N4 and other non-oxides with controlled morphology via carbothermal reduction or carbothermal reduction nitridation reaction.