Development of Si/C/N/O ceramics from pyrolyzed and heat-treated rice hulls

Silicon-based ceramics made of Si compounds (SiO2, Si2N2O, Si3N4, and SiC) dispersed in a carbon-rich matrix were produced from the thermal decomposition of rice hulls and posterior heat treatments at temperatures varying from 1450 to 1700 °C. The samples were prepared under different atmospheres (N2 or Ar) and using various heating rates. The produced materials were characterized by X-ray diffraction (XRD), solid-state nuclear magnetic resonance (NMR), and thermogravimetric (TG) analysis under oxygen flow. The control of the experimental parameters involved in the thermal processing (heating-rate, final temperature, residence time, and atmosphere) provided the ability to control the chemical composition and morphology of the final products. For samples prepared at 1450 and 1600 °C under N2 with heating rate of 100 °C/min, a highly disordered structure was detected, with weak and broad XRD peaks and large dispersion of chemical shifts for 29Si nuclei. The formation of Si3N4 and Si2N2O and the crystallization of SiO2 were also detected at these intermediate temperatures. On the other hand, all samples showed a trend for the formation of well crystallized β-SiC when heat treated at 1700 °C. The results here presented suggest the possibility of use of rice hulls as a precursor for the preparation of ceramic materials with incorporation of silicon compounds in a predominantly carbonaceous structure.