SEM study of virgin and thermally treated samples
The photographs in Fig. 9 depict the microstructure of virgin and thermally treated husk materials. The SEM image of virgin rice husks (Fig. 9 a) shows spherical silica particles of varying form on the organic matrix that consists of cellulose, hemicelluloses and lignin. Furthermore, it is visible that virgin rice husks are compact and do not contain any pore.
The external wall of TRH400 shows the occurrence of a large number of button-like structures with small pores, which were not found in the virgin rice husk particles (Fig. 9 b). The emerging of pores and button-like structures may be caused by the fast removal of volatile organic components from the particle [37]. A cross-section of TRH400 is shown in Fig. 9 (c) and illustrates the presence of pores and channels in the particles with the diameters of about 5–10 μm. The emerging of channels during combustion of rice husk has been discussed earlier in ref. [29]. The interior structure of TRH400 (Fig. 9 d) furthermore indicates the formation of backbone-like structures during combustion [22]. Fig. 9 (e) shows again a cross-section (TRH700) that illustrates the distribution of mesopores andmacropores in the rice husk particles emerged after treatment at 700 °C. The particles underwent drastic changes in this process of high–temperature treatment. Pores increased both in number and size, new types appeared, two or more smaller pores merged into bigger ones, and the surface and volume of pores changed.