C/SiO2 and C/SiC composite foam monoliths from rice husk for thermal insulation and EMI shielding

Preparation of advanced functional materials from agricultural waste by eco-friendly processing route is inevitable for sustainable development. This work demonstrates the development of carbon/silica (C/SiO2) and carbon/silicon carbide (C/SiC) composite foam monoliths of low thermal conductivity, high EMI shielding performance and reasonable compressive strength from rice husk. The C/SiO2 and C/SiC composite foams are obtained by carbonization and subsequent carbothermal reduction, respectively, of rice husk-sucrose composites consolidated by filter-pressing rice husk powder dispersed in sucrose solutions of various concentrations (300-600 g L-1). The amorphous nature of silica in C/SiO2 and the presence of beta-SiC in C/SiC are evidenced from XRD and TEM analysis. The compressive strength and thermal conductivity are depending on the foam density which is tailored by sucrose solution concentration. The compressive strength in the ranges of 0.32-1.67 and 0.19-1.19 MPa are observed for C/SiO2 and C/SiC foams, respectively, with density in the ranges of 0.26-0.37 and 0.18-0.29 g cm(-3). The C/SiO2 and C/SiC exhibited thermal conductivity in the ranges of 0.150-0.205 W m(-1) K-1 and 0.165-0.431 W m(-1) K-1, respectively. The C/SiO2 and C/SiC composite foams show absorption dominated EMI shielding effectiveness in the ranges of 18-38.5 dB and 20-43.7 dB, respectively. The inherent pore channels and corrugated surface structure in rice husk, electrically conducting carbon and dielectric SiO2 and SiC contribute to the total EMI shielding.