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

Abstract Silicon-based ceramics made of Si compounds (SiO 2 , Si 2 N 2 O, Si 3 N 4 , 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 (N 2 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 N 2 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 29 Si nuclei. The formation of Si 3 N 4 and Si 2 N 2 O and the crystallization of SiO 2 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.